U.S. patent application number 14/912534 was filed with the patent office on 2016-07-14 for aluminum alloy plate having excellent bake hardening responses.
This patent application is currently assigned to KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.). The applicant listed for this patent is KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.). Invention is credited to Yasuhiro ARUGA, Katsushi MATSUMOTO, Hisao SHISHIDO.
Application Number | 20160201168 14/912534 |
Document ID | / |
Family ID | 52628487 |
Filed Date | 2016-07-14 |
United States Patent
Application |
20160201168 |
Kind Code |
A1 |
SHISHIDO; Hisao ; et
al. |
July 14, 2016 |
ALUMINUM ALLOY PLATE HAVING EXCELLENT BAKE HARDENING RESPONSES
Abstract
One of the purposes of the present invention is to provide a
6000-series aluminum alloy plate exhibiting bake hardening (BH)
properties and molding properties after aging at room temperature
for a long period. In one embodiment of the present invention, in a
Sn-containing 6000-series aluminum alloy plate, specific clusters
that greatly contribute to the development of the BH properties,
which are determined by means of a three-dimensional atom probe
field ion microscope, are contained at a predetermined density or
more, and the sizes of the atom clusters that meet the
aforementioned requirement are uniformed to adjust the average
radius of an equivalent circle diameter of each of the clusters to
a value falling within a specified range and reduce the standard
deviation of the radius of the equivalent circle diameter, whereby
the BH properties after aging at room temperature for a long period
can be improved.
Inventors: |
SHISHIDO; Hisao; (Hyogo,
JP) ; MATSUMOTO; Katsushi; (Hyogo, JP) ;
ARUGA; Yasuhiro; (Hyogo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.) |
Hyogo |
|
JP |
|
|
Assignee: |
KABUSHIKI KAISHA KOBE SEIKO SHO
(KOBE STEEL, LTD.)
Kobe-shi, Hyogo
JP
|
Family ID: |
52628487 |
Appl. No.: |
14/912534 |
Filed: |
September 4, 2014 |
PCT Filed: |
September 4, 2014 |
PCT NO: |
PCT/JP2014/073400 |
371 Date: |
February 17, 2016 |
Current U.S.
Class: |
148/439 |
Current CPC
Class: |
C22F 1/043 20130101;
C21D 1/18 20130101; C22F 1/05 20130101; C22F 1/047 20130101; C22C
21/02 20130101; C22C 21/08 20130101 |
International
Class: |
C22C 21/08 20060101
C22C021/08; C21D 1/18 20060101 C21D001/18; C22F 1/043 20060101
C22F001/043; C22C 21/02 20060101 C22C021/02; C22F 1/047 20060101
C22F001/047 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 6, 2013 |
JP |
2013-185197 |
Sep 6, 2013 |
JP |
2013-185198 |
Sep 6, 2013 |
JP |
2013-185199 |
Claims
1. An aluminum alloy sheet, which is an Al--Mg--Si alloy sheet
comprising, in mass %: Mg: 0.2% to 2.0%; Si: 0.3% to 2.0%; Sn:
0.005% to 0.3%; Al; and inevitable impurities, comprising an
aggregate of atoms when measured by a three-dimensional atom probe
field ion microscope, wherein: either or both of a Mg atom and a Si
atom are contained in the aggregate of atoms by a total of 10
pieces or more and, when any atom of the Mg atom and the Si atom
contained therein is used as a reference, a distance between the
atom as the reference and any atom among other atoms adjacent
thereto is 0.75 nm or less; the aggregate of atoms satisfying these
conditions is contained in the aluminum alloy sheet at an average
number density of 2.5.times.10.sup.23 pieces/m.sup.3 or greater and
20.0.times.10.sup.23 pieces/m.sup.3 or less; an average radius of a
circle equivalent diameter of the aggregate of atoms satisfying
these conditions is 1.15 nm or greater and 1.45 nm or less; and a
standard deviation of the radius of the circle equivalent diameter
is 0.45 nm or less.
2. The aluminum alloy sheet according to claim 1, further
comprising at least one of, in mass % Mn: more than 0% and 1.0% or
less, Cu: more than 0% and 1.0% or less, Fe: more than 0% and 1.0%
or less, Cr: more than 0% and 0.3% or less, Zr: more than 0% and
0.3% or less, V: more than 0% and 0.3% or less, Ti: more than 0%
and 0.1% or less, Zn: more than 0% and 1.0% or less, and Ag: more
than 0% and 0.2% or less.
3. An aluminum alloy sheet excellent in bake hardenability, which
is an Al--Mg--Si alloy sheet comprising, in mass %: Mg: 0.2% to
2.0%; Si: 0.3% to 2.0%; Sn: 0.005% to 0.3%; Al; and inevitable
impurities, wherein a ratio (N.sub.cluster/N.sub.total).times.100
of N.sub.cluster to N.sub.total is 1% or greater and 15% or less;
N.sub.total represents a total number of all Mg atoms and Si atoms
measured by a three-dimensional atom probe field ion microscope;
N.sub.cluster represents a total number of all Mg atoms and Si
atoms contained in all aggregates of atoms satisfying conditions
wherein an aggregate of atoms measured by the three-dimensional
atom probe field ion microscope contains either or both of a Mg
atom and a Si atom by a total of 10 pieces or more and, when any
atom of the Mg atom and the Si atom is used as a reference, a
distance between the atom as the reference and any atom among other
atoms adjacent thereto is 0.75 nm or less; and an average radius of
a circle equivalent diameter of the aggregate of atoms is 1.20 nm
or greater and 1.50 nm or less.
4. The aluminum alloy sheet according to claim 3, further
comprising at least one of, in mass % Mn: more than 0% and 1.0% or
less, Cu: more than 0% and 1.0% or less, Fe: more than 0% and 1.0%
or less, Cr: more than 0% and 0.3% or less, Zr: more than 0% and
0.3% or less, V: more than 0% and 0.3% or less, Ti: more than 0%
and 0.1% or less, Zn: more than 0% and 1.0% or less, and Ag: more
than 0% and 0.2% or less.
5. An aluminum alloy sheet excellent in bake hardenability, which
is an Al--Mg--Si alloy sheet comprising, in mass %: Mg: 0.2% to
2.0%; Si: 0.3% to 2.0%; Sn: 0.005% to 0.3%; Al; and inevitable
impurities, comprising an aggregate of atoms when measured by a
three-dimensional atom probe field ion microscope, wherein: either
or both of a Mg atom and a Si atom are contained in the aggregate
of atoms by a total of 10 pieces or more and, when any atom of the
Mg atom and the Si atom contained therein is used as a reference, a
distance between the atom as the reference and any atom among other
atoms adjacent thereto is 0.75 nm or less; and the aggregate of
atoms satisfying these conditions has an average number density of
3.0.times.10.sup.23 pieces/m.sup.3 or greater and
25.0.times.10.sup.23 pieces/m.sup.3 or less and, among the
aggregates of atoms satisfying these conditions, an average
proportion of an aggregate of atoms wherein a ratio (Mg/Si) of a
number of Mg atoms to a number of Si atoms is 1/2 or greater is
0.70 or greater.
6. The aluminum alloy sheet according to claim 5, further
comprising at least one of, in mass % Mn: more than 0% and 1.0% or
less, Cu: more than 0% and 1.0% or less, Fe: more than 0% and 1.0%
or less, Cr: more than 0% and 0.3% or less, Zr: more than 0% and
0.3% or less, V: more than 0% and 0.3% or less, Ti: more than 0%
and 0.1% or less, Zn: more than 0% and 1.0% or less, and Ag: more
than 0% and 0.2% or less.
Description
TECHNICAL FIELD
[0001] The present invention relates to an Al--Mg--Si alloy sheet.
The aluminum alloy sheet referred to in the present invention means
an aluminum alloy sheet that is a rolled sheet such as a hot rolled
sheet or a cold rolled sheet and has been subjected to refining
such as a solution heat treatment and a quenching treatment, but is
not yet subjected to an artificial age hardening treatment such as
a bake hardening treatment. Further, aluminum is hereinafter also
referred to as Al.
BACKGROUND ART
[0002] In recent years, because of environmental awareness and the
like, the society's requirement for weight reduction in a vehicle
such as an automobile has been steadily increasing. In order to
respond to such requirement, as a material for a large body panel
(an outer panel or an inner panel) of an automobile panel such as a
hood, a door, or a roof in particular, instead of a steel material
such as a steel sheet, application of an aluminum alloy material
excellent in formability and bake hardenability, and lighter in
weight has been increasing.
[0003] Among these, for a panel such as an outer panel (outer
sheet) or an inner panel (inner sheet) of a panel structure such as
a hood, a fender, a door, a roof, or a trunk lid of an automobile,
use of an Al--Mg--Si-based AA or JIS 6000 series (hereinafter, also
simply referred to as a 6000 series) aluminum alloy sheet, as a
thin and high strength aluminum alloy sheet, has been studied.
[0004] This 6000 series aluminum alloy sheet contains Si and Mg as
essential elements, and particularly a 6000 series aluminum alloy
with excess Si has a composition having 1 or greater Si/Mg mass
ratio and has excellent age-hardenability. Therefore, it has bake
hardenability (hereinafter, also referred to as bake
hardenability=BH responses or bake hardenability) in which
formability is secured by lowering the proof stress during press
forming or bending, age hardening occurs by heating in an
artificial aging (hardening) treatment at a comparatively low
temperature, such as a bake hardening treatment of a panel after
forming and the like, to improve the proof stress, whereby the
strength required as a panel can be secured.
[0005] In the 6000 series aluminum alloy sheet, the alloy element
amount is comparatively less in comparison to other 5000 series
aluminum alloys and the like where alloy amounts such as Mg amount
are large. Therefore, when the scrap of these 6000 series aluminum
alloy sheets is reused as an aluminum alloy melting material
(melting raw material), an original 6000 series aluminum alloy slab
is easily obtained, and recycling performance is also
excellent.
[0006] On the other hand, as is known well, an outer panel of an
automobile is manufactured by applying combined forming work, such
as stretch forming or bending forming in press forming, to an
aluminum alloy sheet. For example, in a large outer panel such as a
hood or a door, the shape of a formed product is made as an outer
panel by press forming such as stretching, and then joining with an
inner panel is executed by hem work (hemming) of a flat hem and the
like of the outer panel peripheral section to be formed into a
panel structural body.
[0007] Here, the 6000 series aluminum alloy had an advantage of
having excellent BH responses, but had a problem of aging
properties at room temperature, that is, of age hardening during
retention at room temperature for several months after the solution
heat treatment and quenching treatment to increase the strength,
thereby deteriorating formability into a panel, particularly the
bendability. For example, in a case where a 6000 series aluminum
alloy sheet is to be used for an automobile panel, it is typically
placed at room temperature (standing at room temperature) for
approximately 1 to 4 months after the solution heat treatment and
the quenching treatment (after manufacturing) at an aluminum
manufacturer until forming work into a panel at an automobile
manufacturer, and comes to be significantly age hardened (room
temperature aged) during that time. Particularly, in the outer
panel subjected to severe bending, there was such a problem that,
although forming was possible without any problem after the lapse
of 1 month from the manufacturing, cracking occurred in hem working
after the lapse of 3 months. Therefore, in the 6000 series aluminum
alloy sheet for an automobile panel particularly for an outer
panel, it is necessary to suppress room temperature aging over a
comparatively long period of approximately 1 to 4 months.
[0008] Moreover, in a case where such room temperature aging is
great, a problem also occurs in that the BH responses deteriorate
and the proof stress is not improved to the strength required as a
panel by heating during an artificial aging (hardening) treatment
at a comparatively low temperature, such as a bake treatment and
the like of the panel after forming described above.
[0009] Hitherto, from viewpoints of the microstructure of the 6000
series aluminum alloy sheet and particularly the cluster (aggregate
of atoms), various proposals have been made with respect to
improvement of the BH responses and suppression of room temperature
aging. However, in many of those, with respect to the present state
of the cluster (aggregate of atoms) directly affecting the BH
responses and aging properties at room temperature of the 6000
series aluminum alloy sheet, the behavior thereof was merely
indirectly reasoned by analogy.
[0010] Meanwhile, a trial to directly measure and stipulate the
cluster (aggregate of atoms) affecting the BH responses and aging
properties at room temperature of the 6000 series aluminum alloy
sheet has been made.
[0011] In Patent Document 1, out of the clusters (aggregates of
atoms) observed in analysis of the microstructure of a 6000 series
aluminum alloy sheet by using a transmission electron microscope at
one million times, the average number density of the clusters whose
circle equivalent diameter is within the range of 1 to 5 nm has
been stipulated in the range of 4000 to 30000 pieces/.mu.m.sup.2 to
obtain one with excellent BH responses and suppressed room
temperature aging.
[0012] Moreover, Patent Documents 2 and 3 suggest that a 6000
series aluminum alloy sheet in which good BH responses can be
exhibited, even when a vehicle body bake treatment after room
temperature aging is performed, is obtained by controlling
aggregates (clusters) of specific atoms directly measured by a
three-dimensional atom probe field ion microscope. These Patent
Documents describe aggregates of atoms which include either or both
of Mg atoms and Si atoms by a total of 10 pieces or more or 30
pieces or more and, when using any atom of Mg atoms and Si atoms
contained therein as a reference, the distance between the
reference atom and any atom out of other atoms adjacent thereto is
0.75 nm or less.
[0013] In addition, Patent Document 2 describes that aggregates of
atoms satisfying those conditions are contained at an average
number density of 1.0.times.10.sup.5 pieces/.mu.m.sup.3 or
greater.
[0014] Further, Patent Document 3 describes that aggregates of
atoms satisfying those conditions are contained at an average
number density of 5.0.times.10.sup.23 pieces/m.sup.3 or greater and
aggregates of atoms having a size in which the radius of the circle
equivalent diameter as the maximum is 1.5 nm or greater are
contained such that, among the aggregates of atoms satisfying those
conditions, the average number density of aggregates of atoms
having a size in which the radius of the circle equivalent diameter
as the maximum is less than 1.5 nm is regulated to be
10.0.times.10.sup.23 pieces/m.sup.3 or less and a ratio a/b between
an average number density a of aggregates of atoms having a size in
which the radius of the circle equivalent diameter as the maximum
is less than 1.5 nm and an average number density b of aggregates
of atoms having a size in which the radius of the circle equivalent
diameter as the maximum is 1.5 nm or greater is set to 3.5 or
less.
[0015] Meanwhile, as prior patents relating to addition of Sn in
the present invention, a plurality of methods of suppressing room
temperature aging and improving bake hardening by actively adding
Sn to a 6000 series aluminum alloy sheet have been suggested in
addition to Patent Documents 4 and 5. For example, Patent Document
4 describes a method of combining room temperature aging
suppression and bake hardening by restricting a component
relationship between Mg and Si to "-2.0>4Mg-7Si," adding an
appropriate amount of Sn having an effect of suppressing a change
over time, and performing pre-aging after the solution heat
treatment. In addition, Patent Document 5 suggests a method of
improving formability, baking finish properties, and corrosion
resistance by restricting a component relationship between Mg and
Si to "-2.0.ltoreq.4Mg-7Si.ltoreq.1.0," adding Sn that has an
effect of suppressing a change over time and Cu that improves
formability, and performing zinc-based plating.
CITATION LIST
Patent Documents
[0016] Patent Document 1: JP-A-2009-242904
[0017] Patent Document 2: JP-A-2012-193399
[0018] Patent Document 3: JP-A-2013-60627
[0019] Patent Document 4: JP-A-09-249950
[0020] Patent Document 5: JP-A-10-226894
SUMMARY OF INVENTION
Technical Problem
[0021] However, demand for improved fuel efficiency of an
automobile is still high and weight reduction is further
progressed. In this manner, thickness reduction of an aluminum
alloy sheet tends to be required. Meanwhile, in a conventional art
in which the behavior of aggregates (clusters) of atoms is reasoned
by analogy through indirect measurement or in Patent Document 1 in
which the size and the number density of comparatively large
aggregates of atoms which are evaluated by TEM observation are only
controlled, the aggregates of atoms cannot be accurately or
specifically evaluated. For this reason, the aggregates of atoms
cannot be precisely controlled and the 13H responses after room
temperature aging are insufficient. Further, in Patent Documents 2
and 3 in which aggregates (clusters) of specific atoms directly
measured by a transmission electron microscope at one million times
or a three-dimensional atom probe field ion microscope are
controlled, there has been room for improvement of combining good
BH responses after room temperature aging for a long period of time
and preferable workability. The same applies to Patent Documents 4
and 5 in which Sn is actively added to a 6000 series aluminum alloy
sheet.
[0022] In view of the above-described problems, an object of the
present invention is to provide an Al--Si--Mg alloy sheet capable
of exerting good BH responses and preferable workability, even in
the vehicle body bake treatment after room temperature aging for a
long period of time, by means of evaluating aggregates of atoms in
a microstructure in more detail.
Solution to Problem
[0023] In order to achieve the object, the gist of an aluminum
alloy sheet excellent in bake hardenability regarding an aspect of
the present invention (hereinafter, also referred to as a first
aspect of the present invention) is an Al--Mg--Si alloy sheet
containing, in mass %, Mg: 0.2% to 2.0%, Si: 0.3% to 2.0% and Sn:
0.005% to 0.3%, with the remainder being Al and inevitable
impurities, and containing an aggregate of atoms measured by a
three-dimensional atom probe field ion microscope, in which either
or both of an Mg atom and an Si atom are contained in the aggregate
of atoms by a total of 10 pieces or more and, when any atom of the
Mg atom and the Si atom contained therein is used as a reference, a
distance between the atom as the reference and any atom among other
atoms adjacent thereto is 0.75 nm or less, and in which the
aggregate of atoms satisfying these conditions is contained in the
aluminum alloy sheet at an average number density of
2.5.times.10.sup.23 pieces/m.sup.3 or greater and
20.0.times.10.sup.23 pieces/m.sup.3 or less, an average radius of a
circle equivalent diameter of the aggregate of atoms satisfying
these conditions is 1.15 nm or greater and 1.45 nm or less, and a
standard deviation of the radius of the circle equivalent diameter
is 0.45 nm or less.
[0024] Further, in order to achieve the object, the gist of an
aluminum alloy sheet excellent in bake hardenability regarding
another aspect of the present invention (hereinafter, also referred
to as a second aspect of the present invention) is an Al--Mg--Si
alloy sheet containing, in mass %, Mg: 0.2% to 2.0%, Si: 0.3% to
2.0% and Sn: 0.005% to 0.3%, with the remainder being Al and
inevitable impurities, in which a ratio
(N.sub.cluster/N.sub.total).times.100 of N.sub.cluster to
N.sub.total is 1% or greater and 15% or less, in which the
N.sub.total represents a total number of all Mg atoms and Si atoms
measured by a three-dimensional atom probe field ion microscope and
the N.sub.cluster represents a total number of all Mg atoms and Si
atoms contained in all aggregates of atoms satisfying conditions in
which an aggregate of atoms measured by the three-dimensional atom
probe field ion microscope contains either or both of an Mg atom
and an Si atom by a total of 10 pieces or more and, when any atom
of the Mg atom and the Si atom is used as a reference, a distance
between the atom as the reference and any atom among other atoms
adjacent thereto is 0.75 nm or less, and an average radius of a
circle equivalent diameter of the aggregate of atoms is 1.20 nm or
greater and 1.50 nm or less.
[0025] Further, in order to achieve the object, the gist of an
aluminum alloy sheet excellent in bake hardenability regarding
another aspect of the present invention (hereinafter, also referred
to as a third aspect of the present invention) is an Al--Mg--Si
alloy sheet containing, in mass %, Mg: 0.2% to 2.0%, Si: 0.3% to
2.0% and Sn: 0.005% to 0.3%, with the remainder being Al and
inevitable impurities, and containing an aggregate of atoms
measured by a three-dimensional atom probe field ion microscope, in
which either or both of an Mg atom and an Si atom are contained in
the aggregate of atoms by a total of 10 pieces or more and, when
any atom of the Mg atom and the Si atom contained therein is used
as a reference, a distance between the atom as the reference and
any atom among other atoms adjacent thereto is 0.75 nm or less, and
in which the aggregate of atoms satisfying these conditions has an
average number density of 3.0.times.10.sup.23 pieces/m.sup.3 or
greater and 25.0.times.10.sup.23 pieces/m.sup.3 or less and, among
the aggregates of atoms satisfying these conditions, an average
proportion of an aggregate of atoms in which a ratio (Mg/Si) of a
number of Mg atoms to a number of Si atoms is 1/2 or greater is
0.70 or greater.
Advantageous Effects of Invention
[0026] In the first aspect of the present invention, as for the
Al--Mg--Si alloy sheet containing Sn, it was found that, among
aggregates (clusters) of atoms measured by a 3DAP, the average
number density of a specific cluster, which contains Mg atoms or Si
atoms by a specific amount or greater in total as the stipulation
described above and in which the distance between atoms adjacent to
each other which are contained therein is a specific value or less,
is greatly correlated with the BH responses.
[0027] Furthermore, according to the present embodiment, it was
also found that the distribution state of the size of the
aggregates of the specific atoms satisfying these conditions is
important, and the average radius of the circle equivalent
diameters and the standard deviation of radii of the circle
equivalent diameters greatly affect the BH responses.
[0028] That is, in order to improve the BH responses of the
Al--Mg--Si alloy sheet containing Sn, it was found to be required
that the average radius of the circle equivalent diameters of
aggregates of specific atoms is in a specific range of 1.15 nm or
greater and 1.45 nm or less and the standard deviation has a small
value of 0.45 nm or less for the improvement in BH responses.
According to the present aspect, it is possible to provide an
Al--Si--Mg alloy sheet which is capable of exhibiting better BH
responses even in a case of being subjected to long-term room
temperature aging for 100 days.
[0029] In the second aspect of the present invention, as for the
Al--Mg--Si alloy sheet containing Sn, it is premised that, among
aggregates (clusters) of atoms measured by a 3DAP, a great number
of fine clusters in which the distance between the atoms is 0.75 nm
or less are present. In addition, the total amount of Mg and Si
present in these clusters is balanced with the total amount of Mg
and Si which are made into a solid solution in a matrix and the
distribution state of the size of the clusters is controlled as the
average radius of the circle equivalent diameters of the clusters,
thereby improving the BH responses.
[0030] When the total amount of Mg atoms and Si atoms present in a
cluster stipulated as described above is secured after being
balanced with the total amount of Mg and Si which are made into a
solid solution in a matrix, the BH responses can be improved.
Moreover, as an aspect other than being present in the cluster
stipulated in the present aspect or being made into a solid
solution in a matrix, there is a possibility that Mg and Si
contained in the 6000 series aluminum alloy sheet are present by
being contained in a cluster which is coarser than stipulated, or
further coarser precipitate or an intermetallic compound.
Meanwhile, when the total amount of Mg and Si present in the
cluster is controlled after being balanced with the total amount of
Mg and Si being made into a solid solution in a matrix, a coarse
cluster originated by Mg and Si, and further coarser precipitate
and an intermetallic compound can be reduced.
[0031] Further, since the distribution state of the size of the
cluster greatly affects the BH responses, in order to improve the
BH responses of the Al--Mg--Si alloy sheet containing Sn, it is
necessary that the average radius of the circle equivalent
diameters of aggregates of the atoms be adjusted to 1.20 nm or
greater and 1.50 nm or less. By controlling the total amount of Mg
atoms and Si atoms present in clusters and the distribution stated
of the size of clusters, it is possible to provide an Al--Si--Mg
alloy sheet which has excellent formability and is capable of
exhibiting better BH responses even in a case of being subjected to
long-term room temperature aging for 100 days.
[0032] In the third aspect of the present invention, as for the
Al--Mg--Si alloy sheet containing Sn, it is premised that, among
aggregates (clusters) of atoms measured by a 3DAP, a great number
of fine clusters in which the distance between the atoms is 0.75 nm
or less are present. Moreover, the BH responses can be improved by
increasing the proportion of clusters with a great number of Mg
atoms among elements constituting these fine clusters.
[0033] The present inventors found that even the same clusters have
different influences on BH responses depending on the compositions
thereof, and a cluster rich in Si atoms adversely affects BH
responses, while a cluster rich in Mg atoms promotes the BH
responses. For this reason, in the present aspect, among clusters
measured by a 3DAP, the number of clusters in which the distance
between the atoms is small is controlled to be great and among
these clusters, the proportion of clusters having a large number of
Mg atoms is controlled to be high to improve the BH responses.
[0034] In this manner, according to the present aspect, it is
possible to provide an Al--Si--Mg alloy sheet which is capable of
exhibiting better BH responses even in a case of room temperature
aging.
DESCRIPTION OF EMBODIMENTS
[0035] Hereinafter, embodiments of the present invention will be
described in detail for each requirement.
Cluster (Aggregate of Atoms)
[0036] First, the meaning of a cluster referred to in the present
invention will be described. Similar to Patent Documents 2 and 3
described above, the cluster in the present invention indicates an
aggregate (cluster) of atoms measured by a 3DAP described below and
this will be mainly expressed as a cluster in the description
below. In a 6000 series aluminum alloy, it is known that Mg and Si
form an aggregate of atoms which is referred to as a cluster, after
a solution heat treatment and a quenching treatment, during
retention at room temperature or a heat treatment at 50.degree. C.
to 150.degree. C. In this case, the behaviors (properties) of
clusters generated during retention at room temperature and during
a heat treatment at 50.degree. C. to 150.degree. C. are completely
different.
[0037] A cluster formed during retention at room temperature
suppresses precipitation of a GP zone or a .beta.' phase that
increases strength in the subsequent artificial aging or bake
treatment. On the contrary, it is described that a cluster
(alternatively, an Mg/Si cluster) formed at 50.degree. C. to
150.degree. C. promotes precipitation of a GP zone or a .beta.'
phase (for example, described in Yamada et al., Journal of The
Japan Institute of Light Metals, vol. 51, p. 215).
[0038] The paragraphs 0021 to 0025 of Patent Document 1 described
above describe that these clusters are conventionally analyzed by
specific heat measurement, a 3DAP (three-dimensional atom probe) or
the like. At the same time, it also describes that even though the
presence of a cluster itself could be confirmed through observation
in the analysis of a cluster by using a 3DAP, the size or the
number density of the cluster stipulated in the present invention
was still unclear or could be only limitedly measured.
[0039] In the 6000 series aluminum alloy, the analysis of the
cluster by using a 3DAP (three-dimensional atom probe) has been
attempted from the past. However, as described in Patent Document
1, even though the presence of a cluster itself could be confirmed,
the size or the number density of the cluster was unclear. This is
because, among aggregates (clusters) of atoms measured by a 3DAP,
which cluster being greatly correlated with BH responses was
unclear, and which aggregate of atoms being greatly correlated with
BH responses was unclear.
[0040] On the contrary, the present inventors clarified clusters
greatly related to BH responses in Patent Document 2 described
above. That is, it was found that, among clusters measured by a
3DAP, a specific cluster which contains Mg atoms or Si atoms by a
specific amount or greater in total as stipulated above and in
which the distance of atoms adjacent to each other which are
contained therein is a specific value or less, is greatly
correlated with BH responses. In addition, it was found that good
BH responses can be exhibited by increasing the number density of
an aggregate of atoms satisfying these conditions, even in a
vehicle body bake treatment under the condition of a low
temperature for a shortened time after room temperature aging.
[0041] According to Patent Document 2 described above, the presence
of a cluster which contains either or both of Mg atoms and Si atoms
by a total of 30 pieces or more, and in which the distance between
atoms adjacent to each other is 0.75 nm or less improves BH
responses. Further, it is described that an Al--Si--Mg alloy sheet
after room temperature aging is capable of exhibiting better BH
responses when a certain amount or greater of these clusters are
present, even in a case of a vehicle body bake treatment at a low
temperature for a shortened period of time of 150.degree. C. for 20
minutes.
[0042] Meanwhile, as a result of further research, the present
inventors found that the presence of a large amount of the clusters
among clusters measured by a 3DAP certainly improves BH responses,
but the effect of improving the BH responses is not sufficient only
with that. In other words, it was found that the presence of a
large amount of the clusters is a precondition (necessary
condition) of improvement of BH responses, but is not necessarily a
sufficient condition.
[0043] For this reason, the present inventors further filed Patent
Document 3 described above. This is because they found that
clusters containing either or both of Mg atoms and Si atoms have
understandably a difference (distribution) in size thereof, and
thus, there is a great difference in action on the BH responses
depending on the size of the clusters. In other words, the actions
on BH responses depending on the size of the clusters are opposite,
that is, a cluster with a comparatively small size inhibits BH
responses and a cluster with a comparatively large size promotes BH
responses. Based on this, the BH responses can be further improved
by reducing clusters with a comparatively small size and increasing
clusters with a comparatively large size among the above-described
specific clusters. It is considered that even though a cluster with
a comparatively small size disappears during a BH treatment (during
artificial age hardening treatment), it rather inhibits
precipitation, during this BH, of a large cluster which greatly
effects in improvement of strength, thereby degrading the BH
responses. Meanwhile, it is considered that a cluster with
comparatively large size grows during the BH treatment and promotes
precipitation of precipitates during the BH treatment, thereby
improving the BH responses.
[0044] It was found that when an extremely large cluster grows
during the BH treatment, the size thereof becomes extremely large
and this rather leads to degradation of the BH responses and to an
extreme increase in the strength before the BH treatment, and thus
the workability is deteriorated. That is, the clusters having an
optimal size are present in order to improve the BH responses
without deteriorating the workability. It was also found that, even
though the distribution state of the size of aggregates of the
specific atoms is important, the average radius of the circle
equivalent diameters, which is an average size of aggregates of
these specific atoms and standard deviation of radii of the circle
equivalent diameters, greatly affects the BH responses. The present
inventors further filed these contents as an application of
Japanese Patent Application No. 2012-051821 (filed on Mar. 8,
2012). In Japanese Patent Application No. 2012-051821, only
clusters having an optimal size are generated by setting the
average radius of the circle equivalent diameters of clusters to
1.2 nm or greater and 1.5 nm or less and setting the standard
deviation of radii of the circle equivalent diameters to 0.35 nm or
less.
[0045] Based on the subsequent research, related to the first
embodiment of the present invention, it has been found that optimal
ranges, which improve the BH responses, of the average radius of
the circle equivalent diameters which is an average size of
aggregates of the specific atoms and the standard deviation of
radii of the circle equivalent diameters in an Al--Mg--Si alloy
sheet containing Sn are different from those of an Al--Mg--Si alloy
sheet of the prior application which does not contain Sn. That is,
in order to improve the BH responses of the Al--Mg--Si alloy sheet
containing Sn, it has been found to be required that the average
radius of the circle equivalent diameters of aggregates of the
specific atoms is in a specific range of 1.15 nm or greater and
1.45 nm or less and the standard deviation of radii of the circle
equivalent diameters has a small value of 0.45 nm or less for the
improvement in BH responses. In order to improve the BH responses,
it is preferable that only the clusters having an optimal size are
generated, not that the size of aggregates of the specific atoms is
in a broad range from a small value to a great value and unevenness
in size distribution is large. This is intended by that the average
radius of the circle equivalent diameters is 1.15 nm or greater and
1.45 nm or less and the standard deviation of the radii of the
circle equivalent diameters is 0.45 nm or less as stipulated in the
first embodiment of the present invention. In this manner, in the
first embodiment of the present invention, it is possible to
further improve the BH responses of the Al--Mg--Si alloy sheet,
even in a case of long-term retention at room temperature for 100
days with a vehicle body bake treatment being performed.
[0046] Moreover, related to the second embodiment of the present
invention, it has been found that, in the Al--Mg--Si alloy sheet
containing Sn, the balance between the aggregate (cluster) of atoms
and the amount of Mg atoms and Si atoms made into a solid solution
greatly affects the BH responses and the strength after the BH
treatment. That is, the second embodiment of the present invention
is based on the knowledge that the strength before the baking
finish becomes higher and the BH responses can be improved by
controlling the proportions of Mg atoms and Si atoms contained in
the aggregates of atoms satisfying the stipulated conditions and Mg
and Si present in a matrix.
[0047] Further, it has been found that, in the Al--Mg--Si alloy
sheet containing Sn, when an extremely large cluster grows during
the BH treatment, the size thereof becomes extremely large and this
rather leads to degradation of the BH responses and to an extreme
increase in the strength before the BH treatment, and thus the
workability is deteriorated. That is, it also has been found that
clusters having an optimal size are present in order to improve the
BH responses without deteriorating the workability. It also has
been found that, even though the distribution state of the size of
aggregates of the specific atoms is important, the average radius
of the circle equivalent diameters which is an average size of
aggregates of these specific atoms greatly affects the BH
responses. That is, in order to improve the BH responses of the
Al--Mg--Si alloy sheet containing Sn, it is required that the
average radius of the circle equivalent diameters of aggregates of
the specific atoms is in a specific range of 1.20 nm or greater and
1.50 nm or less for the improvement of BH responses.
[0048] Further, related to the third embodiment of the present
invention, it has been found that, in the Al--Mg--Si alloy sheet
containing Sn, even the same clusters have different influences on
BH responses as described above depending on the compositions
thereof, and a cluster rich in Si atoms adversely affects BH
responses, while a cluster rich in Mg atoms promotes the BH
responses. This is the concept of the present embodiment and, for
this reason, in the present embodiment, among clusters measured by
a 3DAP, the number of clusters in which the distance between the
atoms is small is controlled to be great and among these clusters,
the proportion of clusters having a great number of Mg atoms is
controlled to be large, whereby the BH responses can be
improved.
Cluster in First Embodiment of the Present Invention
[0049] Hereinafter, the cluster in the first embodiment of the
present invention will be described.
Stipulation of Cluster of Present Embodiment
[0050] Hereinafter, stipulation of the cluster of the present
embodiment will be described in detail.
[0051] The aluminum alloy sheet in which the cluster is stipulated
in the present embodiment indicates, as described above, a sheet
after a series of refining such as a solution heat treatment, a
quenching treatment and a re-heating treatment are applied thereto
after rolling and indicates a sheet before being subjected to
forming work into a panel by press forming or the like (a sheet
before being subjected to an artificial age hardening treatment
such as a bake hardening treatment). In this case, in order to be
press-formed as the above-described automobile panel or the like,
it is likely to stand at room temperature for a comparatively long
period of time, that is, approximately 1 month to 4 months after
production of the sheet. For this reason, it is preferable that the
microstructure is as stipulated in the present embodiment, even in
a case of a microstructure of the sheet after standing at room
temperature for a long period of time. From this viewpoint, in a
case where the characteristics after long-term room temperature
aging are issues, since it is assumed that the characteristics are
not changed and the microstructure is not changed after room
temperature aging for approximately 100 days, it is more preferable
that the microstructure and characteristics of a sheet after room
temperature aging has sufficiently advanced, that is, after the
above-described series of refining have been carried out, and then
100 or more days have passed are examined and evaluated.
Definition of Cluster of the Present Embodiment
[0052] The microstructure in an arbitrary center part in the
thickness direction of the Al--Mg--Si alloy sheet after being
subjected to refining such as the solution heat treatment or the
quenching treatment before standing at room temperature is measured
by a three-dimensional atom probe field ion microscope. As the
cluster present in the measured microstructure, according to the
present embodiment, first, the cluster contains either or both of
Mg atoms and Si atoms by a total of 10 pieces or more. Further, the
number of pieces of the Mg atoms or the Si atoms contained in the
aggregate of atoms is preferably as large as possible and the upper
limit thereof is not particularly limited. However, from the
manufacturing limit, the upper limit of the number of pieces of Mg
atoms and Si atoms contained in the cluster is approximately 10000
pieces.
[0053] In Patent Document 2 described above, the cluster contains
either or both of Mg atoms and Si atoms by a total of 30 pieces or
more. However, in the present embodiment, as described above, since
clusters with comparatively small size inhibit BH responses, they
are regulated so that the number thereof becomes small. For this
reason, in order to control the clusters with comparatively small
size which need to be regulated to be in a measurable range, it is
stipulated that either or both of Mg atoms and Si atoms are
contained in a total of 10 pieces or more, similar to Patent
Document 3 described above.
[0054] Further in the present embodiment, similar to Patent
Documents 2 and 3 described above, one in which, when any atom of
the Mg atoms and the Si atoms contained in the cluster is used as a
reference, the distance between the atom as the reference and any
atom among other atoms adjacent thereto is 0.75 nm or less is set
as an aggregate (cluster) of atoms stipulated in the present
embodiment (satisfying the stipulation of the present embodiment).
The distance therebetween of 0.75 nm is a numerical value
determined in order to assure the number density of the cluster
with a large size in which the distance between atoms such as Mg
and Si is short and which is effective in improving BH responses at
a low temperature in a short period of time after long-term room
temperature aging, and to regulate the clusters with a small size
to control the number density thereof to be small. The present
inventors have hitherto performed an intensive research on the
relationship between the aluminum alloy sheet capable of exerting
excellent BH responses even in the vehicle body bake treatment
under the conditions of a low temperature for a shortened time and
the aggregate of an atomic level. As a result, it has been found
through experimentation that the high number density of the
aggregates of atoms stipulated by the definition described above
represents the form of the microstructure exerting good BH
responses. Therefore, although the technical implication of the
distance of 0.75 nm between atoms has not been sufficiently
clarified, it is important for the purpose of strictly assuring the
number density of an aggregate of atoms exerting good BH responses
and is a numerical value determined for the purpose.
[0055] The cluster stipulated in the present embodiment contains
both of Mg atoms and Si atoms in most cases. However, the cases
where Mg atoms are contained but Si atoms are not contained, or the
cases where Si atoms are contained but Mg atoms are not contained
are involved. Further, it is not constantly configured only of the
Mg atoms and the Si atoms, and there is a high probability that Al
atoms are additionally contained.
[0056] Moreover, depending on the component composition of the
Al--Mg--Si alloy sheet containing Sn which is the object of the
present embodiment, a case inevitably exists, in which atoms such
as Sn, Fe, Mn, Cu, Cr, Zr, V, Ti, Zn, or Ag contained as alloy
elements or impurities, are contained in the cluster and these
other atoms are counted by 3DAP analysis. However, even when these
other atoms (derived from alloy elements or impurities) are
contained in the cluster, these are lower levels than the total
number of Mg atoms and Si atoms. Therefore, even in a case where
such other atoms are contained in the cluster, those satisfying the
stipulation (condition) function as the cluster of the present
embodiment in the same manner as in the cluster formed of only Mg
atoms and Si atoms. Accordingly, the cluster stipulated in the
present embodiment may contain any other atoms, as long as the
stipulation described above is satisfied.
[0057] In addition, the expression "when any atom of Mg atoms and
Si atoms contained therein is used as a reference, the distance
between the atom as the reference and any atom among other atoms
adjacent thereto is 0.75 mm or less" of the present embodiment
means that all of the Mg atoms and the Si atoms present in the
cluster contain, in the periphery thereof, at least one Mg atom or
Si atom with the distance therebetween being 0.75 nm or less.
[0058] With respect to the stipulation on the distance between
atoms in the cluster of the present embodiment, when any atom of Mg
atoms and Si atoms contained therein is used as a reference, all
distances between the atom as the reference and all atoms of other
atoms adjacent thereto are not necessarily 0.75 nm or less. On the
contrary, all of them may be 0.75 nm or less. In other words, other
Mg atoms or Si atoms whose distance therebetween exceeds 0.75 nm
may be adjacent to each other, and at least one of other Mg atoms
and Si atoms satisfying the stipulated distance (space) may be
present in the periphery of a specific (serving as a reference) Mg
atom or Si atom.
[0059] In a case where one other Mg atom or Si atom adjacent to the
reference atom, which satisfies the stipulated distance is present,
the number of Mg atoms or Si atoms to be counted, that satisfy the
condition of the distance is 2 including the specific (serving as a
reference) Mg atom or Si atom. In the case where two other Mg atoms
or Si atoms adjacent to the reference atom, which satisfy the
stipulated distance is present, the number of Mg atoms or Si atoms
to be counted, that satisfy the condition of the distance is 3
including the specific (serving as a reference) Mg atom or Si
atom.
[0060] The cluster described above is a cluster generated by a
temperature holding treatment after a solution heat treatment and a
stopping of a quenching treatment at a high temperature in refining
after rolling described above and below in detail. That is, the
cluster of the present embodiment is an aggregate of atoms
generated by a temperature holding treatment after a solution heat
treatment and a stopping of a quenching treatment at a high
temperature, and is a cluster which contains either or both of Mg
atoms and Si atoms by a total of 10 pieces or more and, when any
atom of Mg atoms and Si atoms contained therein is used as a
reference, the distance between the atom as the reference and any
atom among other atoms adjacent thereto is 0.75 nm or less.
[0061] Until now, it has been reported that a cluster promoting
precipitation of a GP zone or a .beta.' phase that increases the
strength in artificial aging or a bake treatment is a Mg/Si cluster
as described above, and this cluster is formed by a heat treatment
at 50.degree. C. to 150.degree. C. after the solution heat
treatment and the quenching treatment. Meanwhile, a cluster that
suppresses precipitation of a GP zone or a .beta.' phase in the
artificial aging treatment or the bake treatment is an Si-rich
cluster and the cluster is formed by retention at room temperature
(room temperature aging) after the solution heat treatment and the
quenching treatment (for example, described in Sato, Journal of The
Japan Institute of Light Metals, vol. 56, p. 595).
[0062] However, as a result of the detailed analysis by the present
inventors on the relationship between the strength during the
artificial aging treatment or during the bake treatment and the
cluster, it has been found that a structural factor contributing to
the strength during the artificial aging treatment or during the
bake treatment is not the kind (composition) of the cluster but the
distribution state of the size of the cluster generated by the
refining treatment of a sheet. Further, correspondence of the
distribution state of the size of the clusters to the strength
during the artificial aging treatment or during the bake treatment
has been also clarified only by the analysis with the definition
described above.
[0063] On the other hand, the clusters formed by retention at room
temperature (room temperature aging) have the number of atoms and
the number density of the clusters deviating from the stipulation
of the present embodiment although they are the aggregates of atoms
in measurement by a three-dimensional atom probe field ion
microscope. Therefore, the stipulation of the clusters (aggregates
of atoms) of the present embodiment is also a stipulation
distinguishing the clusters from those formed by retention at room
temperature (room temperature aging) described above and preventing
added (contained) Mg and Si from being consumed by these
clusters.
(Density of Cluster)
[0064] The cluster defined in the present embodiment or the cluster
satisfying the preconditions described above is contained in the
average number density of 2.5.times.10.sup.23 pieces/m.sup.3 or
greater and 20.0.times.10.sup.23 pieces/m.sup.3 or less. When the
average number density of the cluster is much less than
2.5.times.10.sup.23 pieces/m.sup.3, clusters which are extremely
small are newly generated during room temperature aging for a long
period of time, and thus, degradation of BH responses and
deterioration of workability is caused. Meanwhile, when it is much
greater than 20.0.times.10.sup.23 pieces/m.sup.3, the strength
before the BH treatment becomes extremely high, and thus the
workability is deteriorated.
[0065] When the average number density of the clusters stipulated
in the present embodiment is small, the formation amount of the
clusters themselves becomes insufficient, which means that much of
added (contained) Mg and Si are consumed by the clusters formed by
the room temperature aging described above. For this reason, even
though there may be an effect of promoting precipitation of a GP
zone or a phase and improving the BH responses, after standing at
room temperature (room temperature aging) for a long period of
time, improvement of the BH responses remains conventional
approximately 30 MPa to 40 MPa in terms of 0.2% proof stress.
Accordingly, desired better BH responses cannot be secured under
such conditions.
Stipulation of Size Distribution of Clusters of the Present
Embodiment
[0066] On the premise that the above-described predetermined amount
(equal to or greater than the average number density) of the
cluster defined in the present embodiment is allowed to be present,
in the present embodiment, in order to improve the BH responses,
the average radius of the circle equivalent diameters of the
aggregates of atoms satisfying the conditions is 1.15 nm or greater
and 1.45 nm or less, and the standard deviation of radii of the
circle equivalent diameters is 0.45 nm or less, as described
above.
Average Radius E(r) of Circle Equivalent Diameters of Aggregates of
Atoms
[0067] An average radius E(r) (nm) of the circle equivalent
diameters of the aggregates of atoms satisfying the preconditions
described above is represented by "E(r)=(1/n).SIGMA.r." Here, n
represents the number of aggregates of atoms satisfying the
preconditions. r represents the radii (nm) of circle equivalent
diameters of aggregates of respective atoms satisfying the
preconditions.
[0068] First, the size itself of the aggregates of atoms satisfying
the above-described preconditions is important for improving the BH
responses. The aggregates (clusters) of atoms whose average radius
E(r) of the circle equivalent diameters is extremely small
disappear during the BH treatment (during artificial aging
hardening treatment) and suppress precipitation of intermediate
precipitates such as .beta.'' or .beta.' which are highly effective
for improving the strength during the BH treatment, whereby inhibit
the BH responses. Meanwhile, the aggregates (clusters) of atoms
whose average radius E(r) of the circle equivalent diameters is
extremely large already precipitate as intermediate precipitates
such as .beta.'' or .beta.' due to room temperature aging at the
time before the BH treatment (previously or in advance) and rather
increase the strength before the BH treatment, whereby inhibit the
press formability or bendability. When it already precipitate as
intermediate precipitates such as .beta.'' or (V at the time before
the BH treatment, this suppresses the precipitation of the
intermediate precipitates such as a new .beta.'' or .beta.' during
the BH and also leads to inhibition the BH responses. In addition,
.beta.'' and .beta.' are both intermediate precipitated phases and
are both Mg.sub.2Si. However, since the particle structures
(arrangement of atoms) thereof are different from each other and it
is difficult to use separate expressions for them, in a case where
"' (apostrophe)" cannot be used, .beta.' is referred to as a .beta.
prime and .beta.'' is referred to as a .beta. double prime.
[0069] Meanwhile, an aggregate of atoms satisfying the
preconditions as described above, and the size thereof being in a
range of 1.15 nm or greater and 1.45 nm or less by the average
radius E(r) of the circle equivalent diameter precipitates an
intermediate precipitate such as .beta.'' or .beta.' which is
highly effective for improving the strength (which contributes to
improvement of the strength) during the BH treatment. Therefore, it
can have characteristics in which the strength is low and
workability is good at the stage of press forming or bending and
the strength becomes high initially after the BH treatment. For
this reason, the size of the stipulated aggregate of atoms is set
such that the average radius E(r) of the circle equivalent diameter
is 1.15 nm or greater and 1.45 nm or less.
[0070] Standard deviation a of circle equivalent diameters of
aggregates of atoms The standard deviation .sigma. of the circle
equivalent diameters of aggregates of atoms satisfying the
preconditions described above is represented by
".sigma..sup.2=(1/n).SIGMA.[r-E(r)].sup.2" from the average radius
E(r) of the circle equivalent diameters.
[0071] The size of the aggregates of atoms satisfying the
preconditions described above, that is, the average radius E(r) of
the circle equivalent diameters is important, but the standard
deviation of the average radius E(r) of the circle equivalent
diameters which is the average size of the aggregates of specific
atoms satisfying the preconditions described above greatly affects
the BH responses as the distribution state of the size of the
aggregates of atoms. In other words, in order to improve the BH
responses, it is required that the average radius of the circle
equivalent diameters of the aggregates of specific atoms is in the
specific range of 1.15 nm or greater and 1.45 nm or less and the
standard deviation of the radii of the circle equivalent diameters
has a small value of 0.45 nm or less for the improvement of the BH
responses.
[0072] In order to improve the BH responses, it is preferable that
only the clusters having an optimal size are generated, not that
the size of aggregates of the specific atoms is greatly uneven from
a small value to a great value. Such the state where only the
clusters having an optimal size are generated is intended by that
the average radius of the circle equivalent diameters is 1.15 nm or
greater and 1.45 nm or less and the standard deviation of the radii
of the circle equivalent diameters is 0.45 nm or less as described
above. In this manner, in the present embodiment, it is possible to
further improve the BH responses of the Al--Mg--Si alloy sheet,
even in a case where a vehicle body bake treatment is performed
after long-term retention at room temperature.
[0073] In the present embodiment, the size distribution of the
stipulated aggregates of atoms is stipulated by both of the average
radius of the circle equivalent diameters of the stipulated
aggregates of atoms and the standard deviation of the radii of the
circle equivalent diameters, whereby the number or the proportion
of the aggregates (clusters) of atoms having a similar size among
the stipulated aggregates of atoms is increased. In this manner,
the BH responses of the Al--Mg--Si alloy sheet are further
improved, even in a case where a vehicle body bake treatment is
performed after long-term retention at room temperature.
[0074] Even in a case of the clusters stipulated in the above, when
the number of clusters having a small size which inhibit the BH
responses is large, the average radius of the circle equivalent
diameters among the stipulations above becomes less than 1.15 nm,
which is small. In addition, the standard deviation of the radii of
the circle equivalent diameters exceeds 0.45 nm, which is
large.
[0075] Meanwhile, even in a case of the clusters stipulated in the
present embodiment, when the number of clusters having a large size
which inhibit the BH responses is large, the average radius of the
circle equivalent diameters among the stipulations above exceeds
1.45 nm, which is large. In addition, the standard deviation of the
radii of the circle equivalent diameters exceeds 0.45 nm, which is
large.
[0076] The aggregates (clusters) of atoms whose average radius of
the circle equivalent diameters is extremely small disappear during
the BH treatment (during artificial aging hardening treatment) and
suppress precipitation of intermediate precipitates such as
.beta.'' or .beta.' which are highly effective for improving the
strength (contributing to improvement of strength) during the BH
treatment, whereby inhibit the BH responses. Meanwhile, the
aggregates (clusters) of atoms whose average radius of the circle
equivalent diameters is extremely large already precipitate as
intermediate precipitates such as .beta.'' or .beta.' due to room
temperature aging at the time before the BH treatment and increase
the strength before the BH treatment, whereby inhibit the
bendability. When it already precipitate as intermediate
precipitates such as .beta.'' or .beta.' at the time before the BH
treatment, this suppresses the precipitation of the intermediate
precipitates such as a new .beta.'' or .beta.' during the BH and
also leads to inhibition the BH responses.
Cluster in Second Embodiment of the Present Invention
[0077] Hereinafter, the cluster in the second embodiment of the
present invention will be described.
Stipulation of Cluster of Present Embodiment
[0078] Hereinafter, stipulation of the cluster of the present
embodiment will be described in detail.
[0079] The aluminum alloy sheet in which the cluster is stipulated
in the present embodiment indicates, as described above, a rolled
sheet such as a hot rolled sheet or a cold rolled sheet after
refining such as a solution heat treatment and a quenching
treatment is applied thereto and indicates a sheet before being
subjected to forming work into a panel by press forming or the like
(a sheet before being subjected to an artificial age hardening
treatment such as a bake hardening treatment). In this case, in
order to be formed as the above-described automobile member or the
like, it is likely to stand at room temperature for a comparatively
long period of time, that is, approximately 0.5 month to 4 months
after production of the sheet. For this reason, it is preferable
that the microstructure is as stipulated in the present embodiment,
even in a case of a microstructure of the sheet after standing at
room temperature for a long period of time. From this viewpoint, in
a case where the characteristics after long-term room temperature
aging are issues, since it is assumed that the characteristics are
not changed and the microstructure is not changed after room
temperature aging for approximately 100 days, it is more preferable
that the microstructure and characteristics of a sheet after room
temperature aging has sufficiently advanced, that is, after the
above-described series of refining have been carried out, and then
100 or more days have passed are examined and evaluated.
Definition of Cluster of the Present Embodiment
[0080] The microstructure in an arbitrary center part in the
thickness direction of such an aluminum alloy sheet is measured by
a three-dimensional atom probe field ion microscope. As the cluster
present in the measured microstructure, according to the present
embodiment, first, the cluster contains either or both of Mg atoms
and Si atoms by a total of 10 pieces or more. Further, the number
of pieces of the Mg atoms or the Si atoms contained in the
aggregate of atoms is preferably as large as possible and the upper
limit thereof is not particularly limited. However, from the
manufacturing limit, the upper limit of the number of pieces of Mg
atoms and Si atoms contained in the cluster is approximately 10000
pieces.
[0081] In Patent Document 2 described above, the cluster contains
either or both of Mg atoms and Si atoms by a total of 30 pieces or
more. However, in the present embodiment, as described above, since
clusters with comparatively small size inhibit BH responses, they
are regulated so that the number thereof becomes small. For this
reason, in order to control the clusters with comparatively small
size which need to be regulated to be in a measurable range, it is
stipulated that either or both of Mg atoms and Si atoms are
contained in a total of 10 pieces or more, similar to Patent
Document 3 described above.
[0082] Further in the present embodiment, similar to Patent
Documents 2 and 3 described above, one in which, when any atom of
the Mg atoms and the Si atoms contained in the cluster is used as a
reference, the distance between the atom as the reference and any
atom among other atoms adjacent thereto is 0.75 nm or less is set
as an aggregate (cluster) of atoms stipulated in the present
embodiment (satisfying the stipulation of the present embodiment).
The distance therebetween of 0.75 nm is a numerical value
determined in order to assure the number density of the cluster
with a large size in which the distance between atoms such as Mg
and Si is short and which is effective in improving BH responses
after long-term room temperature aging, and to regulate the
clusters with a small size to control the number density thereof to
be small. The present inventors have hitherto performed an
intensive research on the relationship between the aluminum alloy
sheet capable of exerting excellent BH responses even in the
vehicle body bake treatment and the aggregate of an atomic level.
As a result, it has been found through experimentation that the
high number density of the aggregates of atoms stipulated by the
definition described above represents the form of the
microstructure exerting good BH responses. Therefore, although the
technical implication of the distance of 0.75 nm between atoms has
not been sufficiently clarified, it is important for the purpose of
strictly assuring the number density of an aggregate of atoms
exerting good BH responses and is a numerical value determined for
the purpose.
[0083] The cluster stipulated in the present embodiment contains
both of Mg atoms and Si atoms in most cases. However, the cases
where Mg atoms are contained but Si atoms are not contained, or the
cases where Si atoms are contained but Mg atoms are not contained
are involved. Further, it is not constantly configured only of the
Mg atoms and the Si atoms, and there is a high probability that Al
atoms are additionally contained.
[0084] Moreover, depending on the component composition of the
Al--Mg--Si alloy sheet containing Sn which is the object of the
present embodiment, a case inevitably exists, in which atoms such
as Sn, Fe, Mn, Cu, Cr, Zr, V, Ti, Zn, or Ag contained as alloy
elements or impurities, are contained in the cluster and these
other atoms are counted by 3DAP analysis. However, even when these
other atoms (derived from alloy elements or impurities) are
contained in the cluster, these are lower levels than the total
number of Mg atoms and Si atoms. Therefore, even in a case where
such other atoms are contained in the cluster, those satisfying the
stipulation (condition) function as the cluster of the present
embodiment in the same manner as in the cluster formed of only Mg
atoms and Si atoms. Accordingly, the cluster stipulated in the
present embodiment may contain any other atoms, as long as the
stipulation described above is satisfied.
[0085] In addition, the expression "when any atom of Mg atoms and
Si atoms contained therein is used as a reference, the distance
between the atom as the reference and any atom among other atoms
adjacent thereto is 0.75 mm or less" of the present embodiment
means that all of the Mg atoms and the Si atoms present in the
cluster contain, in the periphery thereof, at least one Mg atom or
Si atom with the distance therebetween being 0.75 nm or less.
[0086] With respect to the stipulation on the distance between
atoms in the cluster of the present embodiment, when any atom of Mg
atoms and Si atoms contained therein is used as a reference, all
distances between the atom as the reference and all atoms of other
atoms adjacent thereto are not necessarily 0.75 nm or less. On the
contrary, all of them may be 0.75 nm or less. In other words, other
Mg atoms or Si atoms whose distance therebetween exceeds 0.75 nm
may be adjacent to each other, and at least one of other Mg atoms
and Si atoms satisfying the stipulated distance (space) may be
present in the periphery of a specific (serving as a reference) Mg
atom or Si atom.
[0087] In a case where one other Mg atom or Si atom adjacent to the
reference atom, which satisfies the stipulated distance is present,
the number of Mg atoms or Si atoms to be counted, that satisfy the
condition of the distance is 2 including the specific (serving as a
reference) Mg atom or Si atom. In the case where two other Mg atoms
or Si atoms adjacent to the reference atom, which satisfy the
stipulated distance is present, the number of Mg atoms or Si atoms
to be counted, that satisfy the condition of the distance is 3
including the specific (serving as a reference) Mg atom or Si
atom.
[0088] The cluster described above is a cluster generated by a
temperature holding treatment after a solution heat treatment and a
stopping of a quenching treatment at a high temperature in refining
after rolling described above and below in detail. That is, the
cluster of the present embodiment is an aggregate of atoms
generated by a temperature holding treatment after a solution heat
treatment and a stopping of a quenching treatment at a high
temperature, and is a cluster which contains either or both of Mg
atoms and Si atoms by a total of 10 pieces or more and, when any
atom of Mg atoms and Si atoms contained therein is used as a
reference, the distance between the atom as the reference and any
atom among other atoms adjacent thereto is 0.75 nm or less.
(Amount of Mg and Si in Cluster)
[0089] In the cluster defined in the above-described manner
(satisfying the preconditions) in the present embodiment, the total
amount of Mg atoms and Si atoms which are present in all the
clusters contained in the entirety of the Al--Mg--Si alloy sheet
containing Sn is controlled with regard to the total amount of Mg
and Si contained in the entirety of the aluminum alloy sheet. This
means that the balance between the total amount of Mg atoms and Si
atoms present in the defined clusters and the total amount of Mg
atoms and Si atoms made into a solid solution in a matrix of the
aluminum alloy sheet is suitably controlled. In this manner, the BH
responses can be improved.
[0090] For the purposes of controlling the balance, in the present
embodiment, on the premise that the measurement is performed by
using a three-dimensional atom probe field ion microscope, the
ratio of N.sub.cluster which is the total of all pieces measured
(total amount) of Mg atoms and Si atoms contained in specific
clusters (aggregates of atoms) to N.sub.total which is the total of
all measured pieces of Mg atoms and Si atoms is set to be a
predetermined value.
[0091] That is, the ratio (N.sub.cluster/N.sub.total).times.100 of
N.sub.cluster to N.sub.total is set to fall within a range of 1% or
greater and 15% or less. Here, the ratio of N.sub.cluster to
N.sub.total calculated by (N.sub.cluster/N.sub.total).times.100 is
the average (average ratio) of the values in a plurality of
measurement sites of the center part in the thickness direction of
a test sheet in terms of reproducibility as described in Examples
below.
[0092] By obtaining such a well-balanced microstructure, after 100
days of the retention at room temperature (standing at room
temperature) after a sheet is produced, the strength after the bake
treatment being 200 MPa or greater and the BH responses (difference
in strength between before and after the bake treatment) exceeding
90 MPa can be realized.
[0093] However, such the fact of the correlation between the
microstructure and the BH responses is merely found experimentally
and the mechanism thereof has not been sufficiently elucidated yet.
When the average ratio (N.sub.cluster/N.sub.total).times.100 of
N.sub.cluster to N.sub.total is less than 1%, the amount of Mg and
Si made into a solid solution in the aluminum alloy sheet becomes
larger and thus precipitation strengthening due to the clusters
becomes weak and the strength before the bake treatment becomes
weak due to the limitation of strengthening of the solid solution.
For this reason, the strength after the bake treatment is likely to
be weak inevitably.
[0094] In addition, in a case where the average ratio
(N.sub.cluster/N.sub.total).times.100 of N.sub.cluster to
N.sub.total exceeds 15%, the amount of Mg and Si contained in the
cluster becomes extremely large and the Mg and Si made into a solid
solution in the aluminum alloy sheet becomes small. For this
reason, the number of strengthening phases or) generated during the
artificial aging hardening treatment is decreased and thus the BH
responses are likely to be degraded and the strength after the bake
treatment is likely to be weak.
(Density of Cluster)
[0095] In order to control the average ratio
(N.sub.cluster/N.sub.total).times.100 of N.sub.cluster to
N.sub.total to be in a range of 1% to 15%, it is preferable that
the clusters stipulated in the present embodiment are contained at
an average number density of 2.5.times.10.sup.23 pieces/m.sup.3 or
greater. When the average number density of the clusters is smaller
than 2.5.times.10.sup.23 pieces/m.sup.3, the formation amount of
the clusters themselves becomes insufficient which means that
plenty of added (contained) Mg and Si are consumed by the clusters
formed by the room temperature aging described above. For this
reason, it becomes difficult to control the total amount of Mg and
Si present in the clusters to be 1% or greater and thus the effect
of improving the BH responses is deteriorated after standing at
room temperature (room temperature aging) for a long period of
time. Further, the preferred range of the average number density of
the clusters is the average number density range of
2.5.times.10.sup.23 pieces/m.sup.3 or greater and
20.0.times.10.sup.23 pieces/m.sup.3 or less.
Stipulation of Size Distribution of Clusters of Present
Embodiment
[0096] In the Al--Mg--Si alloy sheet containing Sn of the present
embodiment, the total amount of atoms of Mg and Si in the clusters
is controlled and the average radius of the circle equivalent
diameters of the aggregates of the atoms satisfying the conditions
is controlled to be 1.20 nm or greater and 1.50 nm or less. A
cluster whose size in average radius E(r) of the circle equivalent
diameter falls within a range of 1.20 nm or greater and 1.50 nm or
less among the clusters in which the total amount of Mg atoms and
Si atoms is controlled precipitates as an intermediate precipitate
such as .beta.'' or .beta.' which is highly effective for improving
the strength (which contributes to improvement of the strength)
during the BH treatment. Therefore, the characteristics can be
obtained, in which the strength is low and workability is good at
the stage of press forming or bending and the strength becomes high
initially after the BH treatment.
[0097] An average radius E(r) (nm) of the circle equivalent
diameters of the aggregates of atoms is represented by
"E(r)=(1/n).SIGMA.r." Here, n represents the number of aggregates
of atoms satisfying the preconditions. r represents the radii (nm)
of circle equivalent diameters of aggregates of respective atoms
satisfying the preconditions.
[0098] The aggregates (clusters) of atoms whose average radius E(r)
of the circle equivalent diameters is extremely small disappear
during the BH treatment (during artificial aging hardening
treatment) and suppress precipitation of intermediate precipitates
such as .beta.'' or .beta.' which are highly effective for
improving the strength during the BH treatment, whereby inhibit the
BH responses. Meanwhile, the aggregates (clusters) of atoms whose
average radius E(r) of the circle equivalent diameters is extremely
large already precipitate as intermediate precipitates such as
.beta.'' or .beta.' due to room temperature aging at the time
before the BH treatment (previously or in advance) and rather
increase the strength before the BH treatment, whereby inhibit the
press formability or bendability. When it already precipitate as
intermediate precipitates such as .beta.'' or .beta.' at the time
before the BH treatment, this suppresses the precipitation of the
intermediate precipitates such as a new .beta.'' or .beta.' during
the BH and also leads to inhibition the BH responses. In addition,
.beta.'' and .beta.' are both intermediate precipitated phases and
are both Mg.sub.2Si. However, since the particle structures
(arrangement of atoms) thereof are different from each other and it
is difficult to use separate expressions for them, in a case where
"' (apostrophe)" cannot be used, .beta.' is referred to as a .beta.
prime and .beta.'' is referred to as a .beta. double prime.
[0099] As described above, even in a case of clusters in which the
total amount of Mg atoms and Si atoms is controlled, when the
number of clusters having a small size which inhibit the BH
responses is large, the average radius of the circle equivalent
diameters becomes less than 1.20 nm, which is small, whereby the BH
responses are degraded. Further, even in a case of clusters
stipulated in the present embodiment, when the number of clusters
having a large size which inhibit the BH responses is large, the
average radius of the circle equivalent diameters exceeds 1.50 nm,
which is great, in the stipulated clusters. Therefore, the strength
before the BH treatment becomes extremely high, the press
formability or bendability is degraded, and then BH responses are
degraded.
[0100] Meanwhile, an aggregate of atoms satisfying the
preconditions as described above, and the size thereof being in a
range of 1.20 nm or greater and 1.50 nm or less by the average
radius E(r) of the circle equivalent diameter precipitates an
intermediate precipitate such as .beta.'' or .beta.' which is
highly effective for improving the strength (which contributes to
improvement of the strength) during the BH treatment. Therefore, it
can have characteristics in which the strength is low and
workability is good at the stage of press forming or bending and
the strength becomes high initially after the BH treatment.
Cluster in Third Embodiment of the Present Invention
[0101] Hereinafter, the cluster in the third embodiment of the
present invention will be described.
Stipulation of Cluster of Present Embodiment
[0102] Hereinafter, stipulation of the cluster of the present
embodiment will be described in detail.
[0103] The aluminum alloy sheet in which the cluster is stipulated
in the present embodiment indicates, as described above, a sheet
after a series of refining such as a solution heat treatment, a
quenching treatment and a re-heating treatment are applied thereto
after rolling and indicates a sheet before being subjected to
forming work into a panel by press forming or the like (a sheet
before being subjected to an artificial age hardening treatment
such as a bake hardening treatment).
[0104] In this case, in order to be press-formed as the
above-described automobile panel or the like, it is likely to stand
at room temperature for a comparatively long period of time, that
is, approximately 1 month to 4 months after production of the
sheet. For this reason, it is preferable that the microstructure is
as stipulated in the present embodiment, even in a case of a
microstructure of the sheet after standing at room temperature for
a long period of time. From this viewpoint, in a case where the
characteristics after long-term room temperature aging are issues,
since it is assumed that the characteristics are not changed and
the microstructure is not changed after room temperature aging for
approximately 100 days, it is more preferable that the
microstructure and characteristics of a sheet after room
temperature aging has sufficiently advanced, that is, after the
above-described series of refining have been carried out, and then
100 or more days have passed are examined and evaluated.
Definition of Cluster of the Present Embodiment
[0105] The microstructure in an arbitrary center part in the
thickness direction of such an aluminum alloy sheet is measured by
a three-dimensional atom probe field ion microscope. As the cluster
present in the measured microstructure, according to the present
embodiment, first, the cluster contains either or both of Mg atoms
and Si atoms by a total of 10 pieces or more. Further, the number
of pieces of the Mg atoms or the Si atoms contained in the
aggregate of atoms is preferably as large as possible and the upper
limit thereof is not particularly limited. However, from the
manufacturing limit, the upper limit of the number of pieces of Mg
atoms and Si atoms contained in the cluster is approximately 10000
pieces.
[0106] In Patent Document 2 described above, the cluster contains
either or both of Mg atoms and Si atoms by a total of 30 pieces or
more. However, in the present embodiment, as described above, since
clusters with comparatively small size inhibit BH responses, they
are regulated so that the number thereof becomes small. For this
reason, in order to control the clusters with comparatively small
size which need to be regulated to be in a measurable range, it is
stipulated that either or both of Mg atoms and Si atoms are
contained in a total of 10 pieces or more, similar to Patent
Document 3 described above.
[0107] Further in the present embodiment, similar to Patent
Documents 2 and 3 described above, one in which, when any atom of
the Mg atoms and the Si atoms contained in the cluster is used as a
reference, the distance between the atom as the reference and any
atom among other atoms adjacent thereto is 0.75 inn or less is set
as an aggregate (cluster) of atoms stipulated in the present
embodiment (satisfying the stipulation of the present embodiment).
The distance therebetween of 0.75 nm is a numerical value
determined in order to assure the number density of the cluster
with a large size in which the distance between atoms such as Mg
and Si is short and which is effective in improving BH responses
after long-term room temperature aging, and to regulate the
clusters with a small size to control the number density thereof to
be small. The present inventors have hitherto performed an
intensive research on the relationship between the aluminum alloy
sheet capable of exerting excellent BH responses even in the
vehicle body bake treatment and the aggregate of an atomic level.
As a result, it has been found through experimentation that the
high number density of the aggregates of atoms stipulated by the
definition described above represents the form of the
microstructure exerting good BH responses. Therefore, although the
technical implication of the distance of 0.75 nm between atoms has
not been sufficiently clarified, it is important for the purpose of
strictly assuring the number density of an aggregate of atoms
exerting good BH responses and is a numerical value determined for
the purpose.
[0108] The cluster stipulated in the present embodiment contains
both of Mg atoms and Si atoms in most cases. However, the cases
where Mg atoms are contained but Si atoms are not contained, or the
cases where Si atoms are contained but Mg atoms are not contained
are involved. Further, it is not constantly configured only of the
Mg atoms and the Si atoms, and there is a high probability that Al
atoms are additionally contained.
[0109] Moreover, depending on the component composition of the
Al--Mg--Si alloy sheet containing Sn which is the object of the
present embodiment, a case inevitably exists, in which atoms such
as Sn, Fe, Mn, Cu, Cr, Zr, V, Ti, Zn, or Ag contained as alloy
elements or impurities, are contained in the cluster and these
other atoms are counted by 3DAP analysis. However, even when these
other atoms (derived from alloy elements or impurities) are
contained in the cluster, these are lower levels than the total
number of Mg atoms and Si atoms. Therefore, even in a case where
such other atoms are contained in the cluster, those satisfying the
stipulation (condition) function as the cluster of the present
embodiment in the same manner as in the cluster formed of only Mg
atoms and Si atoms. Accordingly, the cluster stipulated in the
present embodiment may contain any other atoms, as long as the
stipulation described above is satisfied.
[0110] In addition, the expression "when any atom of Mg atoms and
Si atoms contained therein is used as a reference, the distance
between the atom as the reference and any atom among other atoms
adjacent thereto is 0.75 mm or less" of the present embodiment
means that all of the Mg atoms and the Si atoms present in the
cluster contain, in the periphery thereof, at least one Mg atom or
Si atom with the distance therebetween being 0.75 nm or less.
[0111] With respect to the stipulation on the distance between
atoms in the cluster of the present embodiment, when any atom of Mg
atoms and Si atoms contained therein is used as a reference, all
distances between the atom as the reference and all atoms of other
atoms adjacent thereto are not necessarily 0.75 nm or less. On the
contrary, all of them may be 0.75 nm or less. In other words, other
Mg atoms or Si atoms whose distance therebetween exceeds 0.75 nm
may be adjacent to each other, and at least one of other Mg atoms
and Si atoms satisfying the stipulated distance (space) may be
present in the periphery of a specific (serving as a reference) Mg
atom or Si atom.
[0112] In a case where one other Mg atom or Si atom adjacent to the
reference atom, which satisfies the stipulated distance is present,
the number of Mg atoms or Si atoms to be counted, that satisfy the
condition of the distance is 2 including the specific (serving as a
reference) Mg atom or Si atom. In the case where two other Mg atoms
or Si atoms adjacent to the reference atom, which satisfy the
stipulated distance is present, the number of Mg atoms or Si atoms
to be counted, that satisfy the condition of the distance is 3
including the specific (serving as a reference) Mg atom or Si
atom.
[0113] The cluster described above is a cluster generated by a
temperature holding treatment after a solution heat treatment and a
stopping of a quenching treatment at a high temperature in refining
after rolling described above and below in detail. That is, the
cluster of the present embodiment is an aggregate of atoms
generated by a temperature holding treatment after a solution heat
treatment and a stopping of a quenching treatment at a high
temperature, and is a cluster which contains either or both of Mg
atoms and Si atoms by a total of 10 pieces or more and, when any
atom of Mg atoms and Si atoms contained therein is used as a
reference, the distance between the atom as the reference and any
atom among other atoms adjacent thereto is 0.75 nm or less.
[0114] Until now, it has been reported that a cluster promoting
precipitation of a GP zone or a .beta.' phase that increases the
strength in artificial aging or a bake treatment is a Mg/Si cluster
as described above, and this cluster is formed by a heat treatment
at 50.degree. C. to 150.degree. C. after the solution heat
treatment and the quenching treatment. Meanwhile, a cluster that
suppresses precipitation of a GP zone or a .beta.' phase in the
artificial aging treatment or the bake treatment is an Si-rich
cluster and the cluster is formed by retention at room temperature
(room temperature aging) after the solution heat treatment and the
quenching treatment (for example, described in Sato, Journal of The
Japan Institute of Light Metals, vol. 56, p. 595).
[0115] However, in a producing process of a typical aluminum alloy,
since a sheet is left at room temperature typically for 1 month to
4 months (stands at room temperature) after the production thereof
until the forming work into the panel is performed by an automobile
manufacturer as described above, a microstructure is inevitably in
the form in which a Mg--Si cluster generated during production of
the sheet and a cluster rich in Si generated during room
temperature aging coexist and thus it is difficult to generate only
a Mg--Si cluster promoting the BH responses.
[0116] Here, since the present inventors considered that it is
important to control the proportions of the clusters rich in Si
which adversely affect the BH responses and the Mg--Si clusters
which promote the BH responses in order to improve the BH
responses, they evaluated the number density of the clusters and
the components thereof in detail and clarified the form of the
cluster that improves the BH responses.
Stipulation of Composition of Cluster According to Present
Embodiment
[0117] Even in a case of clusters defined in the present embodiment
or clusters satisfying the preconditions have difference influences
on the BH responses depending on the compositions thereof. Clusters
rich in Si atoms adversely affect the BH responses. This is because
the clusters rich in Si are generated during the bake treatment and
a difference in a Mg/Si composition with a strengthening phase such
as .beta.'' or .beta.' that improves the BH responses is
comparatively large and thus generation of strengthening phases
during the bake treatment is not promoted but the generation of
strengthening phases is inhibited.
[0118] Meanwhile, clusters rich in Mg atoms improve the BH
responses. This is because the clusters rich in Mg are generated
during the bake treatment and a Mg/Si composition is comparatively
similar to that in a strengthening phase such as .beta.'' or
.beta.' that improves the BH responses and thus generation of
strengthening phases during the bake treatment is promoted.
[0119] In the present embodiment, among these clusters, the
proportion of the clusters having a large number of Mg atoms is
controlled to be high based on such the relationship in composition
of clusters to improve the BH responses. For this reason, in the
present embodiment, among the aggregates satisfying conditions in
which either or both of an Mg atom and an Si atom are contained by
a total of 10 pieces or more and, when any atom of the Mg atom and
the Si atom contained therein is used as a reference, a distance
between the atom as the reference and any atom among other atoms
adjacent thereto is 0.75 nm or less, the proportion of aggregates
of atoms rich in Mg atoms in which the ratio (Mg/Si) of the number
of Mg atoms to the number of Si atoms is 1/2 or greater is
stipulated as 0.70 or greater. When the proportion of the
aggregates of atoms in which the Mg/Si ratio is 1/2 or greater is
less than 0.70, the number of clusters rich in Si atoms becomes
great and thus the BH responses are likely to be degraded due to
the above-described mechanism.
[0120] Here, the upper limit of the proportion of the aggregates of
atoms in which the Mg/Si ratio is 1/2 or greater is not
particularly limited, but approximately 0.95 is the limit in
manufacturing.
(Density of Cluster)
[0121] The clusters defined or the clusters satisfying the
preconditions as described above are controlled to be contained at
an average number density of 3.0.times.10.sup.23 pieces/m.sup.3 or
greater and 25.0.times.10.sup.23 pieces/m.sup.3 or less in the
present embodiment. When the average number density of the clusters
stipulated in the present embodiment is smaller than
3.0.times.10.sup.23 pieces/m.sup.3, the formation amount of the
clusters themselves becomes insufficient. This means that plenty of
added (contained) Mg and Si are consumed by the clusters formed by
the room temperature aging described above and degradation of the
BH responses and deterioration of workability are thus caused after
standing at room temperature (room temperature aging).
[0122] On the other hand, the upper limit of the average number
density of the clusters is approximately 25.0.times.10.sup.23
pieces/m.sup.3 (approximately 2.5.times.10.sup.24 pieces/m.sup.3)
which is stipulated from the limit in manufacturing.
(Measurement Principle and Measurement Method by 3DAP)
[0123] The measurement principle and the measurement method by a
3DAP of the present invention are disclosed in Patent Documents 2
and 3. That is, the 3DAP (three-dimensional atom probe) is a field
ion microscope (FIM) attached with a time of flight mass
spectrometer. With such the constitution, it is a local analyzer
capable of observing respective atoms on the metal surface by the
field ion microscope and identifying these atoms by time of flight
mass spectrometry. Also, the 3DAP is a means very effective for
structural analysis of the aggregates of atoms because it can
simultaneously analyze the kind and position of atoms emitted from
the sample. Therefore, as described above, it is used for analysis
of the microstructure of a magnetic recording film, an electronic
device, steel material or the like as a widely known technique.
Further, recently, it is also used for determination and the like
of the cluster of the microstructure of an aluminum alloy sheet as
described above.
[0124] The 3DAP utilizes an ionizing phenomenon of sample atoms
themselves under a high electric field which is called electric
field evaporation. When high voltage required by the sample atoms
for electric field evaporation is applied to the sample, atoms are
ionized from the sample surface, pass through a probe hole, and
reach a detector.
[0125] This detector is a position sensitive detector, which
carries out mass spectroscopy of respective ions (identification of
elements that are atomic species), measures the time of flight
until each ion reaches the detector, and can thereby simultaneously
determine the detected position (atomic structural position).
Accordingly, the 3DAP can simultaneously measure the position and
the atomic species of the atom at the tip of the sample, and
therefore has the feature of being able to three-dimensionally
reconstitute and observe the atomic structure of the tip of the
sample. Also, because electric field evaporation takes place in
order from the tip surface of the sample, distribution in the depth
direction of the atoms from the tip of the sample can be examined
with the resolution of an atomic level.
[0126] Because the 3DAP utilizes a high electric field, the sample
to be analyzed should have high electro-conductivity of metal and
the like, and the shape of the sample is generally required to be
ultra fine needle shape with the tip diameter of approximately 100
nm diameter or less than that. Therefore, a sample is taken from
the center part in the sheet thickness direction and the like of an
aluminum alloy sheet that becomes an object to be measured, the
sample is cut and electropolished by a precise cutting apparatus,
and a sample having an ultra fine needle shape tip section for
analysis is manufactured. As a measuring method, "LEAP 3000" made
by Imago Scientific Instruments Corporation is used for example,
high pulse voltage of 1 kV order is applied to the aluminum alloy
sheet sample whose tip is formed into a needle shape, and several
million pieces of atoms are continuously ionized from the tip of
the sample. The ion is detected by the position sensitive type
detector and is applied with pulse voltage. Mass spectroscopy of
the ion (identification of the element that is the atomic species)
is carried out based on the time of flight after the respective
ions come out from the tip of the sample until reaching the
detector.
[0127] Also, utilizing the characteristic that the electric field
evaporation takes place regularly in order from the tip surface of
the sample, a coordinate in the depth direction is properly given
to a two-dimensional map that shows the arrival location of the
ion, and three-dimensional mapping (construction of atomic
structure: atom map in three dimensions) is executed by using an
analytical software "IVAS". Thus, a three-dimensional atom map of
the tip of the sample can be obtained.
[0128] With respect to this three-dimensional atom map, the
aggregate of atoms (cluster) is further analyzed by using a Maximum
Separation Method that is a method for defining an atom belonging
to a precipitate and a cluster. In this analysis, the number of
either or both of Mg atoms and Si atoms (10 pieces or more in
total), the distance (space) between the Mg atom or the Si atom
neighboring each other, and the number of either of the Mg atom or
the Si atom having the predetermined narrow space (0.75 nm or less)
are given as parameters.
[0129] Also, in the first embodiment of the present invention,
either or both of an Mg atom and an Si atom are contained by a
total of 10 pieces or more and, when any atom of the Mg atom and
the Si atom contained therein is used as a reference, a distance
between the atom as the reference and any atom among other atoms
adjacent thereto is 0.75 mu or less, and the cluster satisfying
these conditions is defined to be the aggregate of atoms of the
present embodiment. Then, the dispersion state of the aggregates of
atoms matching this definition is evaluated, and the number density
of the aggregates of atoms is measured and quantified as the
average density per 1 m.sup.3 (pieces/m.sup.3) by averaging for 3
or more measurement samples.
[0130] That is, a maximum rotational radius I.sub.g when the
aggregate of atoms as the measurement object is regarded as a
sphere using the analysis software originally specific to the 3DAP
is acquired by using the following formula of Math. 1.
l g = i = 1 n [ ( x i - x _ ) 2 + ( y i - y _ ) 2 + ( z i - z _ ) 2
] n [ Math . 1 ] ##EQU00001##
[0131] In the formula of Math. 1, I.sub.g represents a rotational
radius automatically calculated by software specific to a
three-dimensional atom probe field ion microscope. x, y, and z
respectively represent an x axis, a y axis, and a z axis which are
invariable in the measuring layout of the three-dimensional atom
probe field ion microscope and x.sub.i, y.sub.i, and z.sub.i
respectively represent the lengths of the x axis, the y axis, and
the z axis and spatial coordinates of Mg atoms and Si atoms
constituting the aggregate of atoms. "x bar" and the like in which
"-" is placed on the top of "x," "y," or "z" also represent the
lengths of the x axis, the y axis, and the z axis, but are
barycentric coordinates of the aggregates of atoms. n represents
the number of Mg atoms and Si atoms constituting the aggregates of
atoms.
[0132] Next, the rotational radius I.sub.g is converted into the
Guinier radius r.sub.G by using the relationship of the following
formula r.sub.G= (5/3)I.sub.g of Math. 2.
r G = 5 3 l g [ Math . 2 ] ##EQU00002##
[0133] The converted Guinier radius r.sub.G is regarded as the
radius of the aggregate of atoms and a circle equivalent diameter r
which is the maximum value of the aggregates of atoms as the
measurement object is calculated. Further, the number n of the
aggregates of atoms satisfying the preconditions is also
calculated. Further, the average number density (pieces/m.sup.3) of
the aggregates of atoms satisfying the preconditions can also be
calculated from the number n.
[0134] Measurements of the clusters with the 3DAP are performed on
10 sites of an arbitrary center part of the Al--Mg--Si alloy sheet
after being subjected to the refining, and the measurement values
(calculated values) are averaged to set an average value stipulated
in the present embodiment.
[0135] Further, the average radius E(r) (nm) of the circle
equivalent diameters of the aggregates of atoms is calculated from
the circle equivalent radius r which is the calculated maximum
value and the number n of the aggregates of atoms satisfying the
preconditions by using the formula "E(r)=(1/n).SIGMA.r" described
above.
[0136] In addition, the standard deviation a of the circle
equivalent diameters of the aggregates of atoms satisfying the
preconditions described above is acquired from the average radius
E(r) of the circle equivalent diameters by using the formula
".sigma..sup.2=(1/n).SIGMA.[r-E(r)].sup.2" described above.
[0137] The calculation formula of the radius of the aggregate of
atoms, and measurement and the conversion method of from the
rotational radius I.sub.g to the Guinier radius r.sub.G use M. K.
Miller: Atom Probe Tomography, (Kluwer Academic/Plenum Publishers,
New York, 2000), p. 184 as a reference. In addition thereto, the
calculation formulae of the radius of aggregates of atoms are
described in many literatures. For example, "(2) Three-dimensional
atom probe analysis" on the page 140 of "Microstructural evolution
in low alloy steels under high dose ion irradiation" (Katsuhiko
Fujii, Koji Fukuya, Tadakatsu Ohkubo, Kazuhiro Hono, et al.)
describes the formula of Math. 1 described above and the conversion
formula to the Guinier radius r.sub.G (in this case, the symbol of
the rotational radius I.sub.g is described as r.sub.G).
[0138] Moreover, in the second embodiment of the present invention,
either or both of an Mg atom and an Si atom are contained by a
total of 10 pieces or more and, when any atom of the Mg atom and
the Si atom contained therein is used as a reference, a distance
between the atom as the reference and any atom among other atoms
adjacent thereto is 0.75 nm or less, and a cluster satisfying these
conditions is claimed to be the aggregate of atoms of the present
embodiment. Then, the dispersion state of the aggregates of atoms
matching this definition is evaluated, and the number density of
the aggregates of atoms is measured and quantified as the average
density per 1 m.sup.3 (pieces/m.sup.3) by averaging for 3 or more
measurement samples.
[0139] In addition, the number N.sub.cluster of Mg atoms and Si
atoms contained in all aggregates of atoms satisfying the
conditions is acquired. Further, the number N.sub.total of all Mg
atoms and Si atoms which are detected by a detector and are
contained in both of a solid solution and aggregates of atoms, that
is, measured by the 3DAP is acquired. Further, the ratio of
N.sub.cluster to N.sub.total is acquired from the formula
"N.sub.cluster/N.sub.total.times.100" and to the average value
(average ratio) is controlled to be 1% or more and 15% or less.
[0140] Furthermore, a maximum rotational radius I.sub.g when the
aggregate of atoms as the measurement object is regarded as a
sphere using the analysis software originally specific to the 3DAP
is acquired by using the above formula of Math. 1.
[0141] Next, the rotational radius I.sub.g is converted into the
Guinier radius r.sub.G by using the relationship of the above
formula of Math. 2, r.sub.G= (5/3)I.sub.g.
[0142] The converted Guinier radius r.sub.G is regarded as the
radius of the aggregate of atoms and a circle equivalent diameter r
which is the maximum value of the aggregates of atoms as the
measurement object is calculated. Further, the number n of the
aggregates of atoms satisfying the preconditions is also
calculated. Further, the average number density (pieces/m.sup.3) of
the aggregates of atoms satisfying the preconditions can also be
calculated from the number n.
[0143] Measurements of the clusters with the 3DAP are performed on
10 sites of an arbitrary center part of the Al--Mg--Si alloy sheet
after being subjected to the refining, and the measurement values
(calculated values) are averaged to set an average value stipulated
in the present embodiment.
[0144] Further, the average radius E(r) (nm) of the circle
equivalent diameters of the aggregates of atoms is calculated from
the circle equivalent radius r which is the calculated maximum
value and the number n of the aggregates of atoms satisfying the
preconditions by using the formula "E(r)=(1/n).SIGMA.r" described
above.
[0145] Moreover, in the third embodiment of the present invention,
either or both of an Mg atom and an Si atom are contained by a
total of 10 pieces or more and, when any atom of the Mg atom and
the Si atom contained therein is used as a reference, a distance
between the atom as the reference and any atom among other atoms
adjacent thereto is 0.75 nm or less, and a cluster satisfying these
conditions is defined to be the aggregate of atoms of the present
embodiment. Then, the dispersion state of the aggregates of atoms
matching this definition is evaluated, and the number density of
the aggregates of atoms is measured and quantified as the average
density per 1 m.sup.3 (pieces/m.sup.3) by averaging for 3 or more
measurement samples.
(Detection Efficiency of Atoms by Using 3DAP)
[0146] Currently, the detection efficiency of atoms by using the
3DAP is limited to approximately 50% of ionized atoms and the
remaining atoms cannot be detected. When the detection efficiency
of atoms by using the 3DAP is improved or the like in the future,
that is, varies greatly, there is a possibility that the
measurement result by the 3DAP of the average number density
(piece/.mu.m.sup.3) of clusters having each size stipulated in the
present invention may vary. Therefore, in order to provide
reproducibility for this measurement, it is preferable that the
detection efficiency of atoms by using the 3DAP is set to
approximately 50%, which is a substantially predetermined
value.
(Chemical Component Composition)
[0147] Next, the chemical component composition of the 6000 series
aluminum alloy sheet will be described below. With respect to the
6000 series aluminum alloy sheet that is the object of the present
invention, various properties such as excellent formability, BH
responses, strength, weldability, and corrosion resistance are
required as a sheet or the like for an outer sheet of an automobile
described above.
[0148] In order to satisfy such requirements, the composition of
the aluminum alloy sheet contains, in mass %, Mg: 0.2% to 2.0%, Si:
0.3% to 2.0% and Sn: 0.005% to 0.3%, with the remainder being Al
and inevitable impurities. Further, all of the % indications of the
content of each element mean mass %. Moreover, in the present
specification, the percentage (mass %) based on the mass is the
same as the percentage (weight %) based on the weight. In addition,
"X % or less (not including 0%)" of the content of each chemical
component is sometimes expressed by "more than 0% and X % or
less."
[0149] The 6000 series aluminum alloy sheet which is the object of
the present invention is preferable to be such a 6000 series
aluminum alloy sheet with excess Si with excellent BH responses in
which the mass ratio Si/Mg of Si to Mg is 1 or greater. The 6000
series aluminum alloy sheet secures the formability by lowering the
proof stress during press forming and bending, and has excellent
age-hardenability (BH responses) with which the proof stress
increases by age hardening through heating during the artificial
aging treatment at a comparatively low temperature such as the bake
treatment of a panel after forming, and required strength can be
secured. Among them, the 6000 series aluminum alloy sheet with
excess Si is excellent in the BH responses in comparison to the
6000 series aluminum alloy sheet in which the mass ratio Si/Mg is
less than 1.
[0150] According to the present invention, elements other than
these Mg and Si are basically impurities or elements that may be
contained, and are set to have the content (allowable amount) of
each element level in conformity with the AA or JIS standard or the
like.
[0151] Even in the present invention, in a case of using, by a
large amount, not only the high purity Al matrix but also the 6000
series alloy, other aluminum alloy scrap materials, a low purity Al
matrix, and the like containing elements other than Mg and Si by a
large amount as additive elements (alloy elements) as the melting
raw material of an alloy from the viewpoint of resources recycling,
other elements described below are inevitably mixed in by a
substantial amount. Moreover, refining itself that daringly reduces
these elements involves cost increase, and inclusion of them to
some extent should be allowed. Further, even when a substantial
amount may be contained, there is an inclusion range not impeding
the object and effects of the present invention.
[0152] Accordingly, in the present invention, inclusion of such
elements described below is allowed in a range of equal to or less
than an upper limit in conformity with the AA or JIS standard or
the like stipulated as described below. Specifically, one kind or
two or more kinds among Mn: 1.0% or less (not including 0%), Cu:
1.0% or less (not including 0%), Fe: 1.0% or less (not including
0%), Cr: 0.3% or less (not including 0%), Zr: 0.3% or less (not
including 0%), V: 0.3% or less (not including 0%), Ti: 0.1% or less
and preferably 0.05% or less (not including 0%), Zn: 1.0% or less
(not including 0%), and Ag: 0.2% or less (not including 0%) may be
further contained in these ranges in addition to the fundamental
composition described above. In a case where these elements are
contained, since the corrosion resistance is likely to be degraded
when the content of Cu is great, the content of Cu is preferably
0.7% or less and more preferably 0.3% or less. Further, when the
contents of Mn, Fe, Cr, Zr, and V are great, comparatively coarse
compounds are likely to be generated and the hem bendability is
thus likely to be degraded. For this reason, the content of Mn is
preferably 0.6% or less and more preferably 0.3% or less and the
each content of Cr, Zr, and V is preferably 0.2% or less and more
preferably 0.1% or less. The inclusion range and the importance, or
the allowable amount of each element in the above-described 6000
series aluminum alloy will be described below.
Si: 0.3% to 2.0%
[0153] Along with Mg, Si is an important element in forming the
cluster stipulated in the present invention. Further, it is an
indispensable element that forms aging precipitates contributing to
strengthening of solid solution and improvement of the strength
during the artificial aging treatment at a low temperature
described above such as the bake treatment to exert
age-hardenability, thereby obtaining the strength (proof stress)
required as an outer panel of an automobile. Further, it is the
most important element for providing the 6000 series aluminum alloy
sheet of the present invention with a combination of various
characteristics such as the total elongation affecting the press
formability. Moreover, in order to exert excellent
age-hardenability in the bake treatment at a lower temperature for
a shorter period of time after forming into a panel, preferred is a
6000 series aluminum alloy composition in which the Si/Mg in mass
ratio is 1.0 or greater and which further contains Si more
excessively high relative to Mg than the generally referred to as
an excessive-Si type.
[0154] When the content of Si is excessively low, since the
absolute amount of Si is insufficient, the cluster stipulated in
the present invention cannot be formed by the stipulated number
density, and the bake hardenability extremely deteriorates.
Further, various properties such as the total elongation required
for respective uses cannot be achieved simultaneously. Meanwhile,
when the content of Si is excessively high, coarse constituents and
precipitates are formed, and bendability, total elongation and the
like extremely deteriorate. Further, the weldability is also
extremely impeded. Therefore, Si is in a range of 0.3% to 2.0%. The
lower limit thereof is more preferably 0.6% and the upper limit
thereof is more preferably 1.4%.
Mg: 0.2% to 2.0%
[0155] Along with Si, Mg is also an important element in forming
the cluster stipulated in the present invention. Further, it is an
indispensable element that forms aging precipitates contributing to
strengthening of solid solution and improvement of the strength
along with Si during the artificial aging treatment such as the
bake treatment to exert age-hardenability, thereby obtaining a
required proof stress as a panel.
[0156] When the content of Mg is excessively low, since the
absolute amount of Mg is insufficient, the cluster stipulated in
the present invention cannot be formed by the stipulated number
density, and the bake hardenability extremely deteriorates.
Therefore, the proof stress required as a panel cannot be obtained.
Meanwhile, when the content of Mg is excessively high, coarse
constituents and precipitates are formed, and bendability, total
elongation and the like extremely deteriorate. Therefore, the
content of Mg is in a range of 0.2% to 2.0%. The lower limit
thereof is more preferably 0.3% and the upper limit thereof is more
preferably 1.0%. Moreover, it is preferable to be such an amount
that the Si/Mg in mass ratio is 1.0 or greater.
Sn: 0.005% to 0.3%
[0157] Sn traps vacancies at room temperature and thereby
suppresses diffusion at room temperature and suppresses generation
of clusters at room temperature. For this reason, Sn has an effect
of reducing As proof stress in both of the initial period (7 days)
of room temperature aging and the late period (100 days) of room
temperature aging and improving the hem workability. Since the
trapped vacancies are released at the high temperature during being
subjected to the baking finish, the diffusion is promoted in
contrast and the BH responses can be improved. For this reason,
even in a case of clusters having the comparable number density, BH
responses can be improved in a case where Sn is contained compared
to a case where Sn is not contained. When the content of Sn is
excessively small, the generation of clusters at room temperature
cannot be suppressed and the number density of the clusters becomes
excessively greater or the average ratio
(N.sub.cluster/N.sub.total).times.100 of N.sub.cluster to
N.sub.total as described above exceeds 15% in some cases. For this
reason, the As proof stress is extremely large after retention at
room temperature for 100 days so that the press formability or the
hem workability is degraded, the number of strengthening phases
(.beta.'') generated during the artificial aging hardening
treatment is decreased, and the BH responses are likely to be
degraded. Therefore, the content of Sn is in a range of 0.005% to
0.3%. The lower limit thereof is more preferably 0.01% and the
upper limit thereof is more preferably 0.2%. The microstructure of
the Al--Si--Mg alloy sheet containing Sn is also different from one
which does not contain Sn, as described below. In this case, even
when Sn is contained in the same manner, since the microstructures
thereof may vary if the production conditions are different from
each other, a microstructure of the present invention, which is
effective in suppressing room temperature aging at a high level and
improving the bake hardening is not necessarily obtained.
(Manufacturing Method)
[0158] Next, a manufacturing method of the aluminum alloy sheet of
the present invention will be described below. With respect to the
aluminum alloy sheet of the present invention, the manufacturing
process itself is an ordinary method or widely known method, and it
is manufactured by casting an aluminum alloy slab having the
above-described 6000 series component composition, performing a
homogenizing heat treatment, subjecting to hot rolling and cold
rolling to obtain a predetermined sheet thickness, and further
subjecting to a refining treatment such as a solution heat
treatment and a quenching treatment.
[0159] However, in order to control the cluster of the present
invention for improving the BH responses during these manufacturing
processes, the solution heat treatment, the quenching treatment,
the suitable quenching (cooling) stop temperature, and retention in
the temperature range thereof are required to be controlled more
appropriately as described below. Further, in other processes,
there are also preferable conditions for controlling the cluster
within the stipulated range of the present invention.
(Melting and Casting Cooling Rate)
[0160] First, in the melting and casting process, the aluminum
alloy molten metal that has been molten so as to be adjusted within
the 6000 series component composition range is casted by properly
selecting a typical melting and casting method such as a continuous
casting method or a semi-continuous casting method (DC casting
method). Here, in order to control the cluster within the
stipulated range of the present invention, it is preferable to make
the average cooling rate in casting as high (quick) as possible at
30.degree. C./min or greater from the liquidus temperature to the
solidus temperature.
[0161] In a case where such temperature (cooling rate) control in a
high temperature range during casting is not performed, the cooling
rate in this high temperature range inevitably becomes slow. In a
case where the average cooling rate in the high temperature range
becomes slow, the amount of the constituents formed coarse in the
temperature range of this high temperature region increases, and
the unevenness of the size or the amount of the constituents in the
sheet thickness direction and the width direction of the slab
increases. As a result, the possibility that the stipulated cluster
cannot be controlled to the range of the present invention
increases.
(Homogenizing Heat Treatment)
[0162] Next, the casted aluminum alloy slab is subjected to the
homogenizing heat treatment prior to hot rolling. The object of the
homogenizing heat treatment (soaking treatment) is homogenization
of the microstructure, that is, to eliminate segregation within the
grains in the microstructure of the slab. The treatment is not
particularly limited as long as it is in the condition for
achieving the object, and can be the treatment typically carried
out once or one step.
[0163] The homogenizing heat treatment temperature is suitably
selected from a range of 500.degree. C. or higher and lower than
the melting point, and the homogenizing time is suitably selected
from 4 hours or longer. When this homogenizing temperature is low,
the segregation within the grains cannot be eliminated
sufficiently, which acts as the fracture origin, and therefore the
stretch flangeability and bendability deteriorate. Even when hot
rolling is started immediately thereafter or hot rolling is started
after cooling and being retained to a suitable temperature, it is
possible to control the number density of the cluster to that
stipulated in the present invention.
[0164] After the homogenizing heat treatment is performed, it is
possible to perform cooling to room temperature with an average
cooling rate of 20.degree. C./h to 100.degree. C./h between
300.degree. C. and 500.degree. C., and performing reheating to
350.degree. C. to 450.degree. C. with an average cooling rate of
20.degree. C./h to 100.degree. C./h, and then to start hot rolling
in this temperature range.
[0165] When the conditions of the average cooling rate after the
homogenizing heat treatment and the reheating rate thereafter are
deviated from, the possibility of formation of the coarse Mg--Si
compounds increases.
(Hot Rolling)
[0166] Hot rolling is constituted of a rough rolling process of a
slab and a finish rolling process according to the thickness of the
sheet to be rolled. In these rough rolling process and finish
rolling process, rolling mills such as a reverse type or a tandem
type are suitably used.
[0167] Here, since burning occurs under the condition in which the
hot rolling (rough rolling) start temperature exceeds the solidus
temperature, hot rolling itself is difficult to carry out. Further,
when the hot rolling start temperature is lower than 350.degree.
C., a load during the hot rolling becomes exceedingly great and
thus the hot rolling itself is difficult to carry out. Therefore,
the hot rolling start temperature is in a range of 350.degree. C.
to the solidus temperature and more preferably in a range of
400.degree. C. to the solidus temperature.
(Annealing of Hot Rolled Sheet)
[0168] Annealing before cold rolling (rough annealing) of the hot
rolled sheet is not necessarily required, but may be performed in
order to further improve the characteristics such as formability by
miniaturizing grains and optimizing the texture.
(Cold Rolling)
[0169] In cold rolling, the above-described hot rolled sheet is
rolled and manufactured into a cold rolled sheet (including a coil)
of a desired final sheet thickness. However, in order to further
miniaturize the grains, the cold rolling ratio is preferably 60% or
greater, and intermediate annealing may be performed between cold
rolling passes with an object similar to that of the rough
annealing described above.
(Solution Heat Treatment and Quenching Treatment)
[0170] After the cold rolling, the solution heat treatment and the
quenching treatment are performed. The solution heat treatment and
the quenching treatment can be performed by heating and cooling by
using a typical continuous heat treatment line and are not
particularly limited. However, because it is preferable to obtain a
sufficient solid solution amount of each element and that the grain
size is finer as described above, it is preferable that the
treatments are performed under the conditions in which heating is
carried out at a solution heat treatment temperature of 520.degree.
C. or more, and equal to or less than the melting temperature, at a
heating rate of 5.degree. C./sec or greater, and then retention is
carried out for 0 second to 10 seconds.
[0171] In addition, from the viewpoint of suppressing formation of
coarse intergranular compounds that deteriorate the formability and
hem workability, it is preferable that the average cooling rate
from the solution heat temperature to the quenching stop
temperature is 3.degree. C./s or greater. When the cooling rate of
the solution heat treatment is low, coarse Mg.sub.2Si and an
element Si are generated during cooling and thus formability is
degraded. Moreover, the amount of solid solution after the solution
heat treatment is reduced and then the BH responses are degraded.
In order to secure this cooling rate, for the quenching treatment,
means and conditions for air cooling by a fan or the like, or water
cooling by mist, spray, immersion or the like are selectively
used.
(Temperature Retaining Treatment after Quenching Stop)
[0172] Here, in the first embodiment of the present invention, the
quenching treatment is performed, not by cooling a sheet after the
solution heat treatment to room temperature but by carrying out the
retention treatment in which cooling (quenching) is stopped in a
temperature range T where the sheet is 80.degree. C. to 130.degree.
C. and the sheet is retained in this temperature range for a
predetermined time t and then it is cooled to room temperature by
using any kind of cooling means such as standing to cool or forced
cooling. The retention (treatment) in a temperature of 80.degree.
C. to 130.degree. C. may be performed by heating or non-heating and
may be isothermal retention or may have a temperature gradient. The
retention time t at this temperature is determined to satisfy the
following formula in relation to the quenching stop temperature
T.
1.6.times.10.sup.4.times.exp[-0.096.times.T]<t<4.3.times.10.sup.5.-
times.exp[-0.097.times.T]
[0173] In the present embodiment, as to the conditions for stopping
the quenching treatment for the purpose of obtaining size
distribution of stipulated predetermined clusters, the relationship
among various quenching stop temperatures, retention times and the
like has been examined in detail. As a result, the size
distribution of clusters during temperature retention after the
quenching stop is greatly affected by the diffusion distance of Mg
or Si. In the analyzed results, it is preferable that the diffusion
distance of Mg or Si is set to a range of 1.3.times.10.sup.-9 m to
6.5.times.10.sup.-9 m.
[0174] The diffusion distance of Mg or Si is expressed by the
following formula of Math. 3. In the formula, D.sub.0 represents a
diffusion coefficient represented by formula of Math. 4 and the
value is 6.2.times.10.sup.-6 (m.sup.2/s). Q represents an
activation energy for diffusion and the value is 11500 (J/mol). R
represents a gas constant and the value is 8.314.
[0175] The quenching stop temperature for obtaining a preferred
range of the diffusion distance of Mg or Si and the temperature
retention time after the quenching stop are re-organized, and the
upper limit and the lower limit of the temperature retention time t
after the quenching stop are determined as shown in the
above-described formula in relation to the quenching stop
temperature T.
D 0 exp [ - Q RT ] t [ Math . 3 ] J mol K [ Math . 4 ]
##EQU00003##
[0176] The size distribution of the predetermined clusters
stipulated in the present embodiment is obtained by the cooling
stop and the temperature retaining treatment at a comparatively
high temperature in the quenching treatment. Further, the average
number density of the aggregates of atoms stipulated in the present
embodiment which is 2.5.times.10.sup.23 pieces/m.sup.3 or greater
and 20.0.times.10.sup.23 pieces/m.sup.3 or less. This is because
most of the aggregates of atoms stipulated in the present
embodiment are formed such that the size thereof is equal or
similar to each other during the processes of cooling stop and the
temperature retaining treatment at a comparatively high temperature
in the quenching treatment.
[0177] In other words, most of the aggregates of atoms which
contain either or both of an Mg atom and an Si atom by a total of
10 pieces or more and in which, when any atom of the Mg atom and
the Si atom contained therein is used as a reference, the distance
between the atom as the reference and any atom among other atoms
adjacent thereto is 0.75 nm or less are formed during the processes
of the cooling stop and the temperature retaining treatment at a
comparatively high temperature in the quenching treatment. Also,
since the size of most of the aggregates of atoms which are
completely formed during the temperature retaining treatment is
equal or similar to each other, the uniformity in size can be
obtained that satisfies the conditions in which the average radius
of the circle equivalent diameters is 1.15 nm or greater and 1.45
nm or less and the standard deviation of radii of the circle
equivalent diameters is 0.45 nm or less.
[0178] Consequently, in the sheet which is subjected to the cooling
stop and the temperature retaining treatment at a comparatively
high temperature in the quenching treatment, the number of clusters
formed by room temperature aging at room temperature after the
temperature retaining treatment is reduced and the room temperature
aging becomes lower. For this reason, press formabilities including
bendability are improved and, even after long-term retention at
room temperature, by performing heating during the artificial aging
treatment at 170.degree. C. for 20 minutes such as the bake
treatment on the panel thereafter, excellent BH responses in which
a difference in 0.2% proof stress before and after BH is 90 MPa or
greater can be obtained.
[0179] Even in a case where the quenching to room temperature is
performed and then the re-heating treatment (annealing treatment)
is performed without stopping the quenching at such a high
temperature as in the related art or a case where the cooling stop
temperature during quenching is room temperature, which is
extremely low, a sheet itself can be manufactured. In this case,
even when clusters whose size is equal or similar to each other,
stipulated in the present embodiment, are formed, there is a
possibility that the absolute number thereof is not small or the
standard deviation becomes greater. Accordingly, a disadvantage
that the stipulation of the cluster of the present embodiment
cannot be satisfied with high reproducibility is generated.
(Cooling after Temperature Retaining Treatment)
[0180] The cooling to room temperature after the temperature
retaining treatment may be standing to cool or forced rapid cooling
by using the cooling means as in the quenching for efficiency in
production. That is, since the clusters whose size is equal or
similar to each other, stipulated in the present embodiment,
completely come out through the temperature retaining treatment,
forced rapid cooling as in a conventional re-heating treatment or
controlling the average cooling rate which is performed in a
complicated manner over several stages is not necessary.
(Re-Heating Treatment)
[0181] Further, in the second embodiment of the present invention,
the re-heating treatment is performed after the solution heat
treatment and the quenching treatment. The re-heating treatment is
carried out in two stages, and the first stage of the treatment is
carried out in a temperature range of 80.degree. C. to 250.degree.
C. which is the attaining temperature (heating temperature) for a
retention time in a range of several seconds to several minutes.
The first stage of cooling after the re-heating treatment may be
standing to cool or forced rapid cooling by using the cooling means
of the solution heat treatment and the quenching treatment for
efficiency in production. Then, when the cooling is finished after
the first stage of re-heating treatment is carried out, within 24
hrs of the retention time at room temperature, the second stage of
the re-heating treatment is carried out in a temperature range of
70.degree. C. to 130.degree. C. which is the attaining temperature
(heating temperature) for a retention time in a range of 3 hrs to
48 hrs. If the retention time at room temperature when the cooling
after the first stage of re-heating treatment is finished exceeds
24 hrs, the room temperature aging excessively proceeds and thus
the effects of the second stage of re-heating treatment is
reduced.
[0182] In a case where the conditions are deviated from such
re-heating treatment conditions, the average content obtained by
summing Mg and Si contained in the aggregates of atoms is difficult
to be set to 10% or greater and 30% or less of the content obtained
by summing Mg and Si contained in the aluminum alloy sheet. For
example, when the attaining temperature of the first stage of
re-heating is lower than 100.degree. C. or the attaining
temperature of the second stage of re-heating is lower than
70.degree. C., Mg--Si clusters promoting the BH responses are not
sufficiently generated. Meanwhile, when the attaining temperature
of the re-heating is excessively high, since an intermetallic
compound phase such as .beta.'' or .beta.' which is different from
the cluster is partially formed, the number density of the cluster
is likely to be lower and thus the BH responses are degraded too
much. Further, due to .beta.'' or .beta.', the formability is
likely to be degraded.
[0183] The cooling to room temperature after the second stage of
re-heating treatment may be standing to cool or forced rapid
cooling by using the cooling means as in the quenching for
efficiency in production. That is, since the clusters whose size is
equal or similar to each other, stipulated in the present
embodiment, completely come out through the temperature retaining
treatment, forced rapid cooling as in a conventional re-heating
treatment or controlling the average cooling rate which is
performed in a complicated manner over several stages is not
necessary.
(Processing with Strain Amount of 0.1% to 5%)
[0184] Moreover, in the third embodiment of the present invention,
in order to further improve the BH responses, it is preferable that
processing with a strain amount of 0.1% to 5% is performed on a
sheet from when the solution heat treatment and the quenching
treatment are finished to when the re-heating treatment as
described below is performed. The means therefor is suitably
selected from leveler straightening, skin pass rolling and the
like. When the processing with a strain amount of 0.1% to 5% is
performed on a sheet from when the solution heat treatment and the
quenching treatment are finished to when the re-heating treatment
is performed, clusters rich in Mg atoms are more easily generated
than clusters rich in Si atoms among the aggregates of atoms
satisfying the stipulated conditions and thus the proportion of
aggregates of atoms in which the Mg/Si ratio is 1/2 or greater is
likely to be set to 0.70 or greater. Meanwhile, when the strain
amount exceeds 5%, which is large, the hem workability is likely to
be degraded. Although the mechanism thereof has many unclear
points, it is estimated as follows. That is, frozen vacancies are
reduced after the solution heat treatment by performing processing
on a sheet with a strain amount of 0.1% to 5% after the solution
heat treatment and thus the diffusion at room temperature is
suppressed. For this reason, it is estimated that the clusters rich
in Si generated at room temperature are unlikely to be generated
and the proportion of aggregates of atoms in which the Mg/Si ratio
is 1/2 or greater is likely to be set to 0.70 or greater.
(Room Temperature Retention)
[0185] Moreover, in order to further improve the BH responses, it
is preferable that the room temperature retention time from when
the solution heat treatment and the quenching treatment are
finished to when the re-heating treatment is started, which
includes the processing with a strain amount of 0.1% to 5%
described above is set to within 24 hours (hr). When the room
temperature retention time is set to be short, the proportion of
aggregates of atoms in which the ratio Mg/Si is 1/2 or greater is
likely to be 0.70 or greater. It is preferable that the room
temperature retention time becomes shorter. The solution heat
treatment, the quenching treatment, and the re-heating treatment
may be continuously performed such that there is almost no interval
therebetween. The lower limit of the time is not particularly
set.
(Re-Heating Treatment)
[0186] It is preferable that the attaining temperature of the
re-heating treatment is in a temperature range of 80.degree. C. to
160.degree. C. and the retention time is in a range of 3 hrs to 100
hrs. When the attaining temperature of the re-heating is 80.degree.
C. or lower or the retention time is shorter than 3 hrs, Mg--Si
clusters promoting the BH responses are not sufficiently generated
and thus the proportion of clusters in which the ratio Mg/Si is 1/2
or greater is likely to be less than 0.70. Meanwhile, in a
condition where the attaining temperature of the re-heating is
higher than 160.degree. C. or the retention time exceeds 100 hrs,
since an intermetallic compound phase such as .beta.'' or .beta.'
which is different from the cluster is partially formed, the number
density of the cluster is likely to be lower and thus the BH
responses are degraded too much. Further, due to .beta.'' or
.beta.', the formability is likely to be degraded.
[0187] The cooling to room temperature after the re-heating
treatment may be standing to cool or forced rapid cooling by using
the cooling means as in the quenching for efficiency in production.
That is, since the clusters whose size is equal or similar to each
other, stipulated in the present embodiment, completely come out
through the temperature retaining treatment, forced rapid cooling
as in a conventional re-heating treatment or controlling the
average cooling rate which is performed in a complicated manner
over several stages is not necessary.
[0188] Hereinafter, the present invention will be described more in
detail with reference to Examples, but the present invention is not
limited to the Examples described below and can be implemented with
modifications being added appropriately within the scope adaptable
to the purposes described above and below, and any of them is to be
included within the technical range of the present invention.
EXAMPLES
[0189] Next, Examples of the present invention will be described.
First, Examples according to the first embodiment of the present
invention will be described. The 6000 series aluminum alloy sheets
with different cluster conditions stipulated in the present
embodiment were distinctively manufactured by performing the
quenching stop at a comparatively high temperature and the
retention treatment at the same temperature during the solution
heat treatment and the quenching treatment, and the BH responses
(bake hardenability) after retention for 7 days and retention for
100 days at room temperature were evaluated. Further, the press
formability and the hem workability as the bend workability were
also evaluated.
[0190] The cluster conditions stipulated in the present embodiment
are the average number density of aggregates of atoms, the average
radius of the circle equivalent diameters, and the standard
deviation of radii of the circle equivalent diameters. The
aggregate of atoms is an aggregate of atoms satisfying the
conditions that either or both of an Mg atom and an Si atom by a
total of 10 pieces or more are contained, in which, when any atom
of the Mg atom and the Si atom contained therein is used as a
reference, the distance between the atom as the reference and any
atom among other atoms adjacent thereto is 0.75 nm or less.
[0191] In addition, the 6000 series aluminum alloy sheets having
the composition shown in Table 1 were distinctively manufactured
variously changing the cooling stop temperature T at a
comparatively high temperature in the quenching treatment after the
solution heat treatment and the retention time t (h) at the cooling
stop temperature as shown in Table 2. Here, in the indication of
the content of each element in Table 1, the indication where the
numerical value in each element is blank indicates that the content
is equal to or less than the detection limit
[0192] The specific manufacturing conditions of the aluminum alloy
sheets are as described below. The aluminum alloy slab of each
composition shown in Table 1 was molten commonly by the DC casting
method. Here, commonly to each Example, the average cooling rate
during casting was set to 50.degree. C./min from the liquidus
temperature to the solidus temperature. Subsequently, the slab was
subjected to a soaking treatment at 540.degree. C. for 4 hours
commonly to each Example, and then hot rough rolling was started.
Further, commonly to each Example, in the finish rolling that
followed, hot rolling was performed to a thickness of 3.5 mm to
obtain a hot rolled sheet (coil). The aluminum alloy sheet after
hot rolling was subjected to rough annealing at 500.degree. C. for
1 minute commonly to each Example and then subjected to cold
rolling at a processing rate of 70% without intermediate annealing
in the middle of cold rolling pass, whereby a cold rolled sheet
(coil) having a thickness of 1.0 mm was obtained commonly to each
Example.
[0193] Further, a refining treatment (T4) was continuously
performed by rewinding and winding the each cold rolled sheet
(coil) in a continuous type heat treatment facility commonly to
each Example. Specifically, the solution heat treatment and the
quenching treatment were performed by heating to the solution heat
treatment temperature shown in Table 2 at an average heating rate
of 10.degree. C./sec up to 500.degree. C. and then immediately
cooling at the average cooling rate shown in Table 2. Here, in each
Example in which the quenching (cooling) was stopped at a high
temperature and the retention treatment was performed at the same
temperature, the quenching and cooling was performed not to room
temperature, but the quenching (cooling) was stopped at the
quenching stop temperature T shown in Table 2 and then the
temperature retention treatment was performed at the temperature
for the retention time t (unit h). The temperature retention
treatment was performed in a retention furnace retained at each
quenching stop temperature in the continuous type heat treatment
facility. Moreover, the temperature retention time (measured
retention time) t of an actual sheet (coil) was determined to
satisfy the formula
"1.6.times.10.sup.4.times.exp[-0.096.times.T]<t<4.3.times.10.sup.5.-
times.exp[-0.097.times.T]" in relation to the quenching stop
temperature T. The retention times of the lower limits and the
upper limits calculated from each quenching stop temperature T by
using the formula, and the retention times (measured retention
time) of an actual sheet (coil) are shown in Table 2 with the unit
h (hour). As a cooling after the temperature retention, in each
Example in which the temperature retention was performed, forced
rapid cooling at a cooling rate of 100.degree. C./S was performed
by using the cooling means as in the quenching.
[0194] A test sheet (blank) was cut out from each final product
sheet after standing at room temperature for 7 days and 100 days
after the refining treatment, and the characteristics of each test
sheet were measured and evaluated. Further, the microstructure
observation by using the 3DAP was only performed on the samples
after 7 days from the refining treatment. These results are shown
in Table 3.
(Cluster)
[0195] First, the microstructure in the center part in the
thickness direction of the test sheet was analyzed by using the
above-described 3DAP method, and the average number density
(.times.10.sup.23 pieces/m.sup.3), the average radius (nm) of the
circle equivalent diameters, and the standard deviation of radii of
the circle equivalent diameters of the clusters stipulated in the
present embodiment were respectively acquired by using the methods
described above. The results thereof are shown in Table 3.
[0196] In Table 2, among the conditions of the clusters stipulated
in the present embodiment, the containing of either or both of an
Mg atom or an Si atom by a total of 10 pieces or more is simply
described as "Mg and Si atoms by 10 pieces or more." Further, when
any atom among the Mg atom and Si atom contained therein is used as
a reference, the distance between the atom as the reference and any
atom among other atoms adjacent thereto being 0.75 nm or less is
simply described as "distance of 0.75 nm or less."
(Bake Hardenability)
[0197] After the refining treatment, 0.2% proof stress (As proof
stress) and total elongation (As total elongation) were acquired by
carrying out a tensile test, as mechanical characteristics of each
of the test sheets after standing at room temperature for 7 days or
100 days. Commonly to each test sheet after being subjected to room
temperature aging for 7 days and room temperature aging for 100
days, 0.2% proof stress (proof stress after BH treatment) of the
test sheet after the artificial aging hardening treatment (after
BH) at 185.degree. C. for 20 minutes was acquired by carrying out
the tensile test. Then, the BH responses of the each test sheet
were evaluated based on a difference (amounts of increase in proof
stress) between the 0.2% proof stresses.
[0198] With respect to the tensile test, No. 5 specimen (25
mm.times.50 mmGL.times.sheet thickness) of JISZ2201 was collected
from each sample sheet to perform the tensile test at room
temperature. The tensile direction of the specimen was set to a
direction orthogonal to the rolling direction. The tensile rate was
set to 5 mm/min until the 0.2% proof stress and 20 mm/min after the
proof stress. The N number of the measurement of the mechanical
characteristics was set to 5, and the average values were
calculated. In addition, with respect to the specimen for
measurement of proof stress after the above-described BH, 2%
pre-strain simulating the press forming of a sheet was applied to
the specimen by using a tensile tester and then the BH treatment
was performed.
(Hem Workability)
[0199] The hem workability was evaluated only for sample sheets
after standing at room temperature for 7 days or 100 days after the
refining treatment. In the test, by using a strip-shaped specimen
having a width of 30 mm, 90.degree. bending of inward bending with
1.0 mm radius by a down flange was performed and, with an inner
having a thickness of 1.0 mm inserted therein, the pre-hem working
of further bending a bent part inward to approximately 130.degree.
in order and the flat hem working of bending by 180.degree. and
allowing an end part to be tightly attached to the inner were
performed.
[0200] The surface state such as occurrence of rough surface, a
minute crack or a large crack of the bent part (edge bent part) of
the flat hem was visually observed and visually evaluated based on
the following criteria.
[0201] 0: without crack and rough surface, 1: slight rough surface,
2: deep rough surface, 3: minute surface crack, 4: linearly
continued surface crack, 5: breakage
[0202] As shown in alloy Nos. 0 to 12 of Table 1 and alloy Nos. 0,
1, 7, 13, and 19 to 27 of Table 2, in each Example of Invention,
the manufacturing and the refining treatment were performed within
the component composition range of the present invention and within
the preferable condition range. Therefore, as shown in Table 2, the
each Example of Invention satisfies the cluster conditions
stipulated in the present embodiment. As a result, in the each
Example of Invention, the BH responses are excellent even after the
room temperature aging for a long period of time after the refining
treatment and even bake-hardened at a low temperature for a short
period of time. In addition, as shown in Table 3, even after the
room temperature aging for a long period of time after the refining
treatment, press formability to an automobile panel is excellent
and the hem workability is also excellent because the As proof
stress is comparatively low. That is, according to the Examples of
Invention, it can be found that an Al--Si--Mg alloy sheet can be
provided, which has better BH responses with a proof stress
difference of 100 MPa or greater and is capable of exerting press
formability or bendability even in a case where the vehicle body
bake treatment was carried out after the long-term room temperature
aging for 100 days.
[0203] In Comparative Examples 2, 8 and 14 of Table 2, Alloy
Examples of Invention 1, 2, and 3 of Table 1 are used. However, in
these Comparative Examples, the cooling rates of the solution heat
treatment are deviated from the preferable condition and
excessively low as shown in Table 2. As a result, since the average
radius and the standard deviation of the cluster stipulated in the
present embodiment are deviated from preferable ranges, and the
room temperature aging is high and particularly the As proof stress
after retention at room temperature for 100 days is comparatively
high, compared to the Examples of Invention which have the same
alloy compositions, the press formability or hem workability to an
automobile panel is degraded and the BH responses are also
degraded.
[0204] In Comparative Examples 3 to 6, 9 to 12, and 15 to 18 of
Table 2, Alloy Examples of Invention 1, 2, and 3 of Table 1 are
used. However, in these Comparative Examples, the temperature
retention treatment condition after the quenching stop is deviated
from a preferable range as shown in Table 2. As a result, since any
of the cluster conditions stipulated in the present embodiment is
deviated from the preferable range and particularly the As proof
stress after retention at room temperature for 100 days is
comparatively high compared to the Examples of Invention which have
the same alloy compositions, the press formability or hem
workability to an automobile panel or the like is degraded or the
BH responses are degraded.
[0205] Further, although Comparative Examples 28 to 37 of Table 2
are manufactured in preferred ranges including the temperature
retention treatment conditions after the quenching stop, alloy Nos.
13 to 22 of Table 1 are used, in which the contents of Mg, Si, and
Sn as indispensable elements are deviated from the ranges of the
present invention or the amount of element impurities is
excessively large. Therefore, in these Comparative Examples 28 to
37, since particularly the As proof stress after retention at room
temperature for 100 days is comparatively high compared to the
Examples of Invention, the press formability or hem workability to
an automobile panel is degraded or the BH responses are degraded as
shown in Table 3. Particularly, in Comparative Example 30 of Table
3 with an excessively small amount of Sn, since the room
temperature aging is not suppressed and the As proof stress after
retention at room temperature for 100 days is excessively high, the
press formability or the hem workability is degraded and the BH
responses are less than 100 MPa in terms of an increase amount of
proof stress, which is not high. Moreover, in Comparative Example
31 with an excessively large amount of Sn, cracks were generated
during hot rolling and thus manufacturing of a sheet itself was not
able to be performed.
[0206] Comparative Example 28 is an alloy 13 of Table 1 and the
amount of Si is excessively small.
[0207] Comparative Example 29 is an alloy 14 of Table 1 and the
amount of Si is excessively large.
[0208] Comparative Example 30 is an alloy 15 of Table 1 and the
amount of Sn is excessively small.
[0209] Comparative Example 31 is an alloy 16 of Table 1 and the
amount of Sn is excessively large.
[0210] Comparative Example 32 is an alloy 17 of Table 1 and the
amount of Fe is excessively large.
[0211] Comparative Example 33 is an alloy 18 of Table 1 and the
amount of Mn is excessively large.
[0212] Comparative Example 34 is an alloy 19 of Table 1 and the
amounts of Cr and Ti are excessively large.
[0213] Comparative Example 35 is an alloy 20 of Table 1 and the
amount of Cu is excessively large.
[0214] Comparative Example 36 is an alloy 21 of Table 1 and the
amount of Zn is excessively large.
[0215] Comparative Example 37 is an alloy 22 of Table 1 and the
amounts of Zr and V are excessively large.
[0216] From the results of Examples described above, it can be
confirmed that all conditions of the cluster stipulated in the
present embodiment are required to be satisfied for the improvement
of BH responses after the room temperature aging for a long period
of time. Further, in order to obtain such cluster conditions and
the BH responses, critical significance or effects of the
requirements of the component composition and preferable
manufacturing conditions in the present embodiment can also be
confirmed.
TABLE-US-00001 TABLE 1 Chemical component of aluminum alloy sheet
Alloy (mass %, remainder Al) Classification No. Mg Si Sn Fe Mn Cr
Zr V Ti Cu Zn Ag Examples of 0 0.6 1.0 0.048 Invention 1 0.6 1.1
0.052 0.2 2 0.4 0.8 0.048 0.2 0.12 0.3 3 0.4 1.2 0.092 0.2 0.21
0.01 4 0.3 1.5 0.050 0.2 0.8 5 0.5 1.3 0.051 0.2 0.7 0.05 6 0.5 0.8
0.207 0.2 0.07 7 0.5 0.9 0.048 0.2 0.22 8 0.6 1.2 0.022 0.2 0.05
0.05 9 1.5 0.5 0.102 0.2 0.1 0.01 10 0.7 1.0 0.050 0.2 0.05 11 0.5
1.2 0.009 0.7 0.6 12 0.5 0.9 0.048 0.2 0.2 0.1 0.1 Comparative 13
1.5 0.2 0.050 0.2 Examples 14 0.4 2.1 0.049 0.2 15 0.6 1.2 0.002
0.2 16 0.6 1.1 0.462 0.2 17 0.4 0.8 0.051 1.3 18 0.6 1.0 0.051 0.2
1.2 0.01 19 0.5 0.8 0.052 0.2 0.4 0.08 20 0.4 0.8 0.050 0.2 1.3 21
0.5 1.0 0.048 0.2 1.2 22 0.5 0.9 0.050 0.2 0.4 0.4 * The columns in
which the numerical values for the elements are blank indicate that
the values are equal to or less than the detection limit.
TABLE-US-00002 TABLE 2 Solution heat treatment and quenching
treatment Temperature retention treatment after quenching stop
Solution heat Average Cooling stop Average Alloy No. temperature
cooling rate temperature T Retention time t cooling rate
Classification No. of Table 1 .degree. C. .degree. C./s .degree. C.
Measured h Lower limit h Upper limit h .degree. C./s Example of
Invention 0 0 540 100 100 8 1 26 100 Example of Invention 1 1 540
100 100 8 1 26 100 Comparative Example 2 1 540 1 100 5 1 26 100
Comparative Example 3 1 540 100 60 100 50 1276 100 Comparative
Example 4 1 540 100 180 0.1 0.0005 0.011 100 Comparative Example 5
1 540 100 100 100 1 26 100 Comparative Example 6 1 540 100 100 0.5
1 26 100 Example of Invention 7 2 540 100 100 8 1 26 100
Comparative Example 8 2 540 1 100 5 1 26 100 Comparative Example 9
2 540 100 60 100 50 1276 100 Comparative Example 10 2 540 100 180
0.1 0.0005 0.011 100 Comparative Example 11 2 540 100 100 100 1 26
100 Comparative Example 12 2 540 100 100 0.5 1 26 100 Example of
Invention 13 3 540 100 100 8 1 26 100 Comparative Example 14 3 540
1 100 5 1 26 100 Comparative Example 15 3 540 100 60 100 50 1276
100 Comparative Example 16 3 540 100 180 0.1 0.0005 0.011 100
Comparative Example 17 3 540 100 100 100 1 26 100 Comparative
Example 18 3 540 100 100 0.5 1 26 100 Example of Invention 19 4 540
10 80 0.5 7 183 100 Example of Invention 20 5 540 100 130 24 0.06 1
100 Example of Invention 21 6 540 100 90 20 3 70 100 Example of
Invention 22 7 540 100 120 1 0.2 4 100 Example of Invention 23 8
540 100 110 2 0.4 10 100 Example of Invention 24 9 540 100 100 4 1
26 100 Example of Invention 25 10 540 100 100 10 1 26 100 Example
of Invention 26 11 540 100 100 8 1 26 100 Example of Invention 27
12 540 100 100 8 1 26 100 Comparative Example 28 13 540 100 100 8 1
26 100 Comparative Example 29 14 540 100 100 8 1 26 100 Comparative
Example 30 15 540 100 100 8 1 26 100 Comparative Example 31 16
Generation of cracks during hot rolling Comparative Example 32 17
540 100 100 8 1 26 100 Comparative Example 33 18 540 100 100 8 1 26
100 Comparative Example 34 19 540 100 100 8 1 26 100 Comparative
Example 35 20 540 100 100 8 1 26 100 Comparative Example 36 21 540
100 100 8 1 26 100 Comparative Example 37 22 540 100 100 8 1 26
100
TABLE-US-00003 TABLE 3 Microstructure of aluminum alloy sheet after
retention at room temperature for 7 days Stipulated cluster (Mg and
Si atoms by Characteristics of aluminum alloy Characteristics of
aluminum alloy 10 pieces or more, sheet after retention at room
sheet after retention at room distance of 0.75 nm or less)
temperature for 7 days temperature for 100 days Average As proof
Increase proof Increase Average radius proof stress amount As proof
stress amount Hem Alloy density Average standard stress after BH of
proof stress after BH of proof work- No. of .times.10.sup.23 radius
deviation 0.2% 0.2% stress Hem 0.2% 0.2% stress a- Classification
No. Table 1 pieces/m.sup.3 nm nm MPa MPa MPa workability MPa MPa
MPa bility Example of 0 0 6.5 1.25 0.29 94 220 126 1 103 211 108 2
Invention Example of 1 1 7.1 1.26 0.28 97 229 132 1 105 223 118 2
Invention Comparative 2 1 9.4 1.42 0.49 123 199 76 3 130 194 64 3
Example Comparative 3 1 12.9 1.13 0.25 112 183 71 3 129 183 54 2
Example Comparative 4 1 8.4 1.51 0.54 128 219 91 3 143 218 75 4
Example Comparative 5 1 23.7 1.35 0.35 152 264 112 4 158 264 106 4
Example Comparative 6 1 1.8 1.23 0.28 71 177 106 1 92 170 78 2
Example Example of 7 2 3.8 1.25 0.29 77 223 146 1 87 218 131 1
Invention Comparative 8 2 7.2 1.46 0.46 122 193 71 3 130 193 63 3
Example Comparative 9 2 8.4 1.12 0.28 100 176 76 1 115 174 59 2
Example Comparative 10 2 5.8 1.52 0.48 118 209 91 2 127 204 77 3
Example Comparative 11 2 20.4 1.34 0.32 142 261 119 3 148 256 108 4
Example Comparative 12 2 1.6 1.23 0.24 68 173 105 1 84 165 81 1
Example Example of 13 3 9.4 1.30 0.30 103 224 121 2 111 221 110 2
Invention Comparative 14 3 14.3 1.43 0.48 137 209 72 4 142 205 63 4
Example Comparative 15 3 16.6 1.13 0.29 106 177 71 1 119 179 60 2
Example Comparative 16 3 9.3 1.52 0.47 131 224 93 4 138 219 81 4
Example Comparative 17 3 25.5 1.35 0.31 158 266 108 4 162 264 102 4
Example Comparative 18 3 2.1 1.24 0.26 69 167 98 1 83 169 86 1
Example Example of 19 4 3.6 1.28 0.36 106 220 114 2 115 217 102 2
Invention Example of 20 5 15.0 1.23 0.27 108 230 122 2 117 220 103
2 Invention Example of 21 6 9.3 1.25 0.27 99 226 127 2 112 224 112
2 Invention Example of 22 7 10.4 1.22 0.26 92 222 130 2 103 216 113
2 Invention Example of 23 8 6.6 1.41 0.34 94 213 119 2 108 217 109
2 Invention Example of 24 9 9.5 1.32 0.29 97 207 110 2 106 209 103
2 Invention Example of 25 10 9.4 1.33 0.28 101 222 121 2 113 224
111 2 Invention Example of 26 11 13.5 1.28 0.27 108 224 116 2 115
221 106 2 Invention Example of 27 12 11.8 1.30 0.27 102 224 122 2
108 216 108 2 Invention Comparative 28 13 1.3 1.26 0.25 77 129 52 1
82 129 47 1 Example Comparative 29 14 4.5 1.23 0.26 115 211 96 4
118 201 83 4 Example Comparative 30 15 16.8 1.28 0.32 132 235 103 2
139 234 95 3 Example Comparative 31 16 Generation of cracks during
hot rolling Example Comparative 32 17 6.1 1.25 0.28 111 218 107 4
118 216 98 4 Example Comparative 33 18 6.4 1.26 0.27 115 218 103 4
122 217 95 4 Example Comparative 34 19 5.8 1.25 0.26 108 219 111 4
117 214 97 4 Example Comparative 35 20 7.5 1.24 0.27 120 224 104 4
126 221 95 4 Example Comparative 36 21 5.1 1.28 0.27 113 220 107 4
121 218 97 4 Example Comparative 37 22 6.6 1.23 0.26 117 226 109 4
129 228 99 4 Example
[0217] Next, Examples according to the second embodiment of the
present invention will be described. The 6000 series aluminum alloy
sheets with different compositions and cluster conditions
stipulated in the present embodiment were distinctively
manufactured by performing the two-stage re-heating treatment
conditions after the completion of the solution heat treatment and
the quenching treatment. Further, the microstructure (cluster) and
the strength, the BH responses (bake hardenability), and the press
formability and the hem workability as the bend workability after
the retention at room temperature for 100 days in the each Example
were also evaluated.
[0218] The cluster conditions are the total amount of Mg atoms and
Si atoms present in the aggregates of atoms, the average radius of
the circle equivalent diameters, and the average number density.
The aggregate of atoms is an aggregate of atoms satisfying the
conditions that either or both of an Mg atom and an Si atom by a
total of 10 pieces or more are contained, in which, when any atom
of the Mg atom and the Si atom contained therein is used as a
reference, the distance between the atom as the reference and any
atom among other atoms adjacent thereto is 0.75 nm or less.
[0219] The specific manufacturing conditions of the aluminum alloy
sheets are as described below. The aluminum alloy slab of each
composition shown in Table 4 was molten commonly by the DC casting
method. Here, commonly to each Example, the average cooling rate
during casting was set to 50.degree. C./min from the liquidus
temperature to the solidus temperature. In the indication of the
content of each element in Table 4 showing the compositions of the
6000 series aluminum alloy sheets of the Examples, the indication
where the numerical value in each element is blank indicates that
the content is equal to or less than the detection limit and the
element is not contained, that is, the value is 0%.
[0220] Subsequently, the slab was subjected to a soaking treatment
at 540.degree. C. for 4 hours commonly to each Example, and then
hot rough rolling was started. Further, commonly to each Example,
in the finish rolling that followed, hot rolling was performed to a
thickness of 3.5 mm to obtain a hot rolled sheet. The aluminum
alloy sheet after hot rolling was subjected to rough annealing at
500.degree. C. for 1 minute commonly to each Example and then
subjected to cold rolling at a processing rate of 70% without
intermediate annealing in the middle of cold rolling pass, whereby
a cold rolled sheet having a thickness of 1.0 mm was obtained
commonly to each Example.
[0221] Moreover, commonly to the each Example, the cold rolled
sheet was subjected to the solution heat treatment in a saltpeter
furnace at 560.degree. C., retained for 10 seconds after reaching
the target temperature, and subjected to the quenching treatment by
water cooling. After the quenching treatment was finished, the
first stage of the preliminary aging treatment was performed at
100.degree. C. to 250.degree. C. under the conditions shown in
Table 5 and water cooling was carried out until the room
temperature. The second stage of the preliminary aging treatment
was then performed at 70.degree. C. to 130.degree. C. and cooling
was carried out by water cooling until the room temperature. Here,
in the present Examples, after the first stage and the second stage
of re-heating treatment, cooling was respectively carried out by
water cooling, but the same microstructure can be obtained even
when the cooling is standing to cool.
[0222] A test sheet (blank) was cut out from each sheet after
standing at room temperature for 100 days from the refining
treatment, and the microstructure and the strength (AS proof
stress) of each test sheet were measured. The microstructure
observation by using the 3DAP was only performed on the samples
after 100 days from the refining treatment. These results are shown
in Table 6.
(Cluster)
[0223] The microstructures on the sections in the thickness
direction of the sheet thickness center parts of the test sheets
after the room temperature aging for 100 days were analyzed by
using the above-described 3DAP method, and the number density
(.times.10.sup.23 pieces/m.sup.3) of the clusters, the average
radius (nm) of the circle equivalent diameters and the ratio of the
total amount N.sub.cluster of the number of all Mg atoms and Si
atoms contained in the clusters to N.sub.total obtained by summing
the number of all measured Mg atoms and Si atoms, stipulated in the
present embodiment, were acquired by using the above-described
analysis methods.
[0224] The results thereof are shown in Table 6. In Table 6, among
the conditions of the clusters stipulated in the present
embodiment, the containing of either or both of an Mg atom or an Si
atom by a total of 10 pieces or more is simply described as "Mg and
Si atoms by 10 pieces or more." Further, when any atom among the Mg
atom and Si atom contained therein is used as a reference, the
distance between the atom as the reference and any atom among other
atoms adjacent thereto being 0.75 nm or less is simply described as
"distance of 0.75 nm or less."
[0225] In the measurement according to the 3DAP method, from a test
sheet having a thickness of 1 mm, three prisms having dimensions of
a length of 30 mm, a width of 1 mm and a thickness of 1 mm were cut
out at intervals of 1 mm in the width direction by using a
precision cutting device, the prism was processed into a thin shape
by performing electrolytic polishing, and then a needle-shaped
sample in which the radius of the tip thereof was 50 nm was
prepared. Accordingly, the measurement site was in the vicinity of
the center portion in the thickness direction. The aluminum alloy
sheet sample formed such that the tip had a needle shape was
measured according to the 3DAP method by using "LEAP3000"
manufactured by Imago Scientific Instruments Corporation. Then, the
number density (.times.10.sup.23 pieces/m.sup.3) of the clusters,
the average radius (nm) of the circle equivalent diameters, and the
ratio of the total amount N.sub.cluster of the number of all Mg
atoms and Si atoms contained in the clusters to N.sub.total
obtained by summing the number of all measured Mg atoms and Si
atoms, of each of the three prisms, were acquired and then
averaged. Therefore, the values in the present Examples are average
values of measurement number N=3. The measured volume according to
the 3DAP method was approximately 1.0.times.10.sup.-22 mm.sup.3 to
1.0.times.10.sup.-21 mm.sup.3.
(Bake Hardenability)
[0226] 0.2% proof stress (As proof stress) and 0.2% proof stress
(proof stress after BH) after the artificial aging hardening
treatment (after BH) at 185.degree. C. for 20 minutes were acquired
as the mechanical characteristics of the each test sheet after the
room temperature aging for 100 days, both by carrying out the
tensile test. Then, the BH responses of the each test sheet were
evaluated based on a difference (amounts of increase in proof
stress) between the 0.2% proof stresses.
[0227] With respect to the tensile test, No. 5 specimen (25
mm.times.50 mmGL.times.sheet thickness) of JISZ2201 was collected
from each sample sheet to perform the tensile test at room
temperature. The tensile direction of the specimen was set to a
direction orthogonal to the rolling direction. The tensile rate was
set to 5 mm/min until the 0.2% proof stress and 20 mm/min after the
proof stress. The N number of the measurement of the mechanical
characteristics was set to 5, and the average values were
calculated. In addition, with respect to the specimen for
measurement of proof stress after the above-described BH, 2%
pre-strain simulating the press forming of a sheet was applied to
the specimen by using a tensile tester and then the BH treatment
was performed.
(Hem Workability)
[0228] The hem workability was evaluated only for sample sheets
after standing at room temperature for 7 days or 100 days after the
refining treatment. In the test, by using a strip-shaped specimen
having a width of 30 mm, 90.degree. bending of inward bending with
1.0 mm radius by a down flange was performed and, with an inner
having a thickness of 1.0 mm inserted therein, the pre-hem working
of further bending a bent part inward to approximately 130.degree.
in order and the flat hem working of bending by 180.degree. and
allowing an end part to be tightly attached to the inner were
performed.
[0229] The surface state such as occurrence of rough surface, a
minute crack or a large crack of the bent part (edge bent part) of
the flat hem was visually observed and visually evaluated based on
the following criteria.
[0230] 0: without crack and rough surface, 1: slight rough surface,
2: deep rough surface, 3: minute surface crack, 4: linearly
continued surface crack, 5: breakage
[0231] As shown in alloy Nos. 23 to 32 of Table 4 and alloy Nos.
38, 39, 45, 51, and 57 to 62 of Table 5, in each Example of
Invention, the manufacturing and the refining treatment were
performed within the component composition range of the present
invention and within the preferable condition range. Therefore, as
shown in Table 6, the each Example of Invention satisfies the
cluster conditions stipulated in the present embodiment. That is,
the cluster satisfying the conditions stipulated in the present
embodiment satisfies the preferable average number density
(3.0.times.10.sup.24 pieces/m.sup.3 or greater) and, in the
cluster, the ratio (N.sub.cluster/N.sub.total).times.100 of
N.sub.cluster to N.sub.total is 1% or greater and 15% or less and
the average radius of the circle equivalent diameters is 1.20 urn
or greater and 1.50 nm or less.
[0232] As a result, in the Examples of Invention, as shown in Table
6, even after the long-term room temperature aging for 100 days or
the like, the BH responses are excellent, press formability to an
automobile panel is excellent because the As proof stress is
comparatively low, and the hem workability is also excellent. That
is, according to the Examples of Invention, it can be found that an
Al--Si--Mg alloy sheet can be provided, which has better BH
responses with a proof stress difference of 100 MPa or greater and
is capable of exerting press formability or bendability even in a
case where the vehicle body bake treatment was carried out after
the long-term room temperature aging for 100 days.
[0233] In Comparative Examples 39 to 44, 46 to 50, and 52 to 56 of
Table 5, Alloy Examples of Invention 24, 25 and 26 of Table 4 are
used. However, in these Comparative Examples, the two-stage
re-heating treatment conditions after the completion of the
solution heat treatment and the quenching treatment are deviated
from the preferable conditions as shown in Table 5.
[0234] In Comparative Examples 40, 46 and 52, only one stage that
is the second stage of the re-heating treatment is performed.
[0235] In Comparative Examples 41, 47 and 53, the re-heating
treatment temperature in the first stage thereof is excessively
low.
[0236] In Comparative Examples 42, 48 and 54, the re-heating
treatment temperature in the first stage thereof is excessively
high.
[0237] In Comparative Examples 43, 49 and 55, the re-heating
treatment temperature in the second stage thereof is excessively
high.
[0238] In Comparative Examples 44, 50 and 56, the re-heating
treatment temperature in the second stage thereof is excessively
low.
[0239] Therefore, in these Comparative Examples, as shown in Table
6, the average radius of the circle equivalent diameters of the
aggregates of atoms is less than 1.20 nm or greater than 1.50 nm,
or the average ratio of Mg atoms and Si atoms contained in the
aggregates of atoms, which is calculated by
N.sub.cluster/N.sub.total.times.100, is less than 1% or greater
than 15%, that is, they are deviated from the stipulation of the
present embodiment. As a result, since the As proof stress after
retention at room temperature for 100 days is comparatively high,
compared to Examples of Invention 39, 45 and 51 which have the same
alloy compositions, the press formability or hem workability to an
automobile panel is degraded or the BH responses are degraded.
[0240] Further, although Comparative Examples 63 to 72 of Table 5
are manufactured in preferred ranges including the refining
treatment, alloy Nos. 33 to 42 of Table 4 are used, and the
contents of Mg, Si, and Cu as indispensable elements are deviated
from the ranges of the present invention or the amount of element
impurities is excessively large. As a result, in these Comparative
Examples, the BH responses or the hem workability is degraded
compared to the Examples of Invention as shown in Table 6.
Particularly, in Comparative Example 65 of Table 6 with an
excessively small amount of Sn, the number density of the cluster
is high and the average ratio (N.sub.cluster/N.sub.total).times.100
of N.sub.cluster to N.sub.total is greater than 15%, which is
excessively high. Therefore, since the room temperature aging is
not suppressed and the As proof stress after retention at room
temperature for 100 days is excessively high, the press formability
or the hem workability is degraded and the BH responses are less
than 100 MPa in terms of an increase amount of proof stress, which
is not high. Moreover, in Comparative Example 66 with an
excessively large amount of Sn, cracks were generated during hot
rolling and thus manufacturing of a sheet itself was not able to be
performed.
[0241] Comparative Example 63 is an alloy 33 of Table 4 and the
amount of Si is excessively small.
[0242] Comparative Example 64 is an alloy 34 of Table 4 and the
amount of Si is excessively large.
[0243] Comparative Example 65 is an alloy 35 of Table 4 and the
amount of Sn is excessively small.
[0244] Comparative Example 66 is an alloy 36 of Table 4 and the
amount of Sn is excessively large.
[0245] Comparative Example 67 is an alloy 37 of Table 4 and the
amount of Fe is excessively large.
[0246] Comparative Example 68 is an alloy 38 of Table 4 and the
amount of Mn is excessively large.
[0247] Comparative Example 69 is an alloy 39 of Table 4 and the
amount of Cu is excessively large.
[0248] Comparative Example 70 is an alloy 40 of Table 4 and the
amount of Cr is excessively large.
[0249] Comparative Example 71 is an alloy 41 of Table 4 and the
amounts of Ti and Zn are excessively large.
[0250] Comparative Example 72 is an alloy 42 of Table 4 and the
amounts of Zr and V are excessively large.
[0251] From the results of Examples described above, it can be
confirmed that conditions of the cluster stipulated in the present
embodiment are required to be satisfied in order for better BH
responses and proof stress after BH to be exerted even in a case
where the strength before the bake treatment becomes higher.
Further, in order to obtain such cluster conditions and the BH
responses, critical significance or effects of the requirements of
the component composition and preferable manufacturing conditions
in the present embodiment can also be confirmed.
TABLE-US-00004 TABLE 4 Chemical component of aluminum alloy sheet
Alloy (mass %, remainder Al) Classification No. Mg Si Sn Fe Mn Cu
Cr Zr V Ti Zn Ag Examples of 23 0.50 1.00 0.051 Invention 24 0.40
1.25 0.051 0.20 25 0.60 0.90 0.050 0.20 0.7 26 0.50 0.80 0.083 0.20
0.2 27 0.80 0.90 0.049 0.20 0.2 0.15 28 0.70 1.10 0.024 0.20 0.05
0.2 0.2 29 0.40 0.85 0.211 0.20 0.07 0.2 30 0.60 1.00 0.090 0.20
0.4 0.1 31 0.35 1.15 0.050 0.20 0.8 0.6 32 1.00 0.55 0.048 0.70 0.1
0.1 Comparative 33 0.80 0.20 0.049 0.20 Examples 34 0.40 2.10 0.049
0.20 35 0.40 1.00 0.002 0.20 36 0.50 1.00 0.353 0.20 37 0.50 0.90
0.050 1.30 38 0.60 1.10 0.049 0.20 1.1 39 0.60 1.10 0.049 0.20 1.2
40 0.50 0.90 0.050 0.20 0.4 41 0.40 0.90 0.051 0.20 0.08 1.2 42
0.60 1.00 0.052 0.20 0.4 0.4 * The columns in which the numerical
values for the elements are blank indicate that the values are
equal to or less than the detection limit.
TABLE-US-00005 TABLE 5 Between first First stage of and second
Second stage of Alloy re-heating treatment stages re-heating
treatment No. of Temperature Retention Retention Temperature
Retention Classification No. Table 4 .degree. C. min min .degree.
C. h Example of 38 23 100 2 60 90 8 Invention Example of 39 24 100
2 60 90 8 Invention Comparative 40 24 -- -- 60 90 8 Example
Comparative 41 24 60 2 60 100 8 Example Comparative 42 24 280 2 60
80 8 Example Comparative 43 24 100 2 60 150 8 Example Comparative
44 24 100 2 60 60 8 Example Example of 45 25 100 2 60 90 8
Invention Comparative 46 25 -- -- 60 90 8 Example Comparative 47 25
60 2 60 100 8 Example Comparative 48 25 280 2 60 80 8 Example
Comparative 49 25 100 2 60 180 8 Example Comparative 50 25 100 2 60
60 8 Example Example of 51 26 100 2 60 90 8 Invention Comparative
52 24 -- -- 15 90 8 Example Comparative 53 26 60 2 15 100 8 Example
Comparative 54 26 280 2 15 80 8 Example Comparative 55 26 100 2 15
180 8 Example Comparative 56 26 100 2 15 60 8 Example Example of 57
27 90 5 15 90 8 Invention Example of 58 28 100 2 5 90 8 Invention
Example of 59 29 200 2 15 90 5 Invention Example of 60 30 100 2 60
80 3 Invention Example of 61 31 200 2 15 100 3 Invention Example of
62 32 200 0.5 15 90 3 Invention Comparative 63 33 100 2 15 90 8
Example Comparative 64 34 100 2 15 90 8 Example Comparative 65 35
100 2 15 90 8 Example Comparative 66 36 Cracks during hot rolling
Example Comparative 67 37 100 2 15 90 8 Example Comparative 68 38
100 2 15 90 8 Example Comparative 69 39 100 2 15 90 8 Example
Comparative 70 40 100 2 15 90 8 Example Comparative 71 41 100 2 15
90 8 Example Comparative 72 42 100 2 15 90 8 Example
TABLE-US-00006 TABLE 6 Microstructure and characteristics of
aluminum alloy sheet after retention at room temperature for 100
days Characteristics of aluminum alloy sheet after Stipulated
cluster (Mg and Si atoms by 10 retention at room temperature for
100 days pieces or more, distance of 0.75 nm or less) 0.2% BH
responses Alloy Average Number As 0.2% proof stress in terms of No.
of radius density N.sub.cluster/N.sub.total .times. proof stress
after BH increase amount Hem Classification No. Table 4 nm
.times.10.sup.23 pieces/m.sup.3 100 MPa MPa of proof stress
workability Example of Invention 38 23 1.26 9.3 6.2 102 210 108 1
Example of Invention 39 24 1.26 11.6 7.1 107 223 116 1 Comparative
Example 40 24 1.18 7.4 4.2 86 175 89 1 Comparative Example 41 24
1.17 6.8 3.9 82 170 88 1 Comparative Example 42 24 1.52 9.0 8.8 138
200 62 3 Comparative Example 43 24 1.48 17.0 15.9 152 205 53 4
Comparative Example 44 24 1.22 1.4 0.8 78 159 81 1 Example of
Invention 45 25 1.24 10.5 6.8 109 222 113 2 Comparative Example 46
25 1.18 9.5 5.4 96 182 86 1 Comparative Example 47 25 1.16 5.8 3.3
93 179 86 1 Comparative Example 48 25 1.51 6.2 6.3 146 203 57 3
Comparative Example 49 25 1.44 18.9 16.5 168 224 56 4 Comparative
Example 50 25 1.21 1.5 0.9 88 167 79 1 Example of Invention 51 26
1.25 7.5 5.5 104 210 106 1 Comparative Example 52 24 1.18 6.2 4.0
85 168 83 1 Comparative Example 53 26 1.18 6.5 4.4 89 166 77 1
Comparative Example 54 26 1.50 7.1 7.7 144 203 59 4 Comparative
Example 55 26 1.46 14.9 15.3 161 223 62 4 Comparative Example 56 26
1.22 1.1 0.8 84 167 83 1 Example of Invention 57 27 1.33 11.9 7.8
122 226 104 2 Example of Invention 58 28 1.34 18.9 11.6 128 239 111
2 Example of Invention 59 29 1.24 4.6 3.6 91 213 122 1 Example of
Invention 60 30 1.25 9.4 5.8 106 210 104 1 Example of Invention 61
31 1.25 7.3 4.6 102 209 107 1 Example of Invention 62 32 1.27 4.5
4.4 109 210 101 1 Comparative Example 63 33 1.26 0.6 0.6 85 138 53
1 Comparative Example 64 34 1.23 5.8 2.2 124 201 77 4 Comparative
Example 65 35 1.33 19.5 15.5 138 236 98 3 Comparative Example 66 36
Cracks during hot rolling Comparative Example 67 37 1.24 6.5 4.3
125 218 93 4 Comparative Example 68 38 1.26 6.6 3.7 128 223 95 4
Comparative Example 69 39 1.25 6.6 3.8 153 248 95 4 Comparative
Example 70 40 1.25 7.3 5.1 122 224 102 4 Comparative Example 71 41
1.27 6.3 4.9 118 210 92 4 Comparative Example 72 42 1.24 5.5 3.3
124 212 88 4
[0252] Next, Examples according to the third embodiment of the
present invention will be described. The 6000 series aluminum alloy
sheets with different compositions and cluster conditions
stipulated in the present embodiment were distinctively
manufactured by the time from when the solution heat treatment and
the quenching treatment were finished to when the re-heating
treatment was started, the processing rate of skin pass rolling
after the completion of the solution heat treatment and the
quenching treatment, and the re-heating treatment conditions.
Further, the BH responses (bake hardenability) after the retention
at room temperature for 100 days in the each Example were
evaluated. Further, the hem workability as the bendability was also
evaluated.
[0253] In the indication of the content of each element in Table 7
showing the compositions of the 6000 series aluminum alloy sheets
of the Examples, the indication where the numerical value in each
element is blank shows that the content is equal to or less than
the detection limit.
[0254] The specific manufacturing conditions of the aluminum alloy
sheets are as described below. The aluminum alloy slab of each
composition shown in Table 7 was molten commonly by the DC casting
method. Here, commonly to each Example, the average cooling rate
during casting was set to 50.degree. C./min from the liquidus
temperature to the solidus temperature. Subsequently, the slab was
subjected to a soaking treatment at 540.degree. C. for 4 hours
commonly to each Example, and then hot rough rolling was started.
Further, commonly to each Example, in the finish rolling that
followed, hot rolling was performed to a thickness of 3.5 mm to
obtain a hot rolled sheet. The aluminum alloy sheet after hot
rolling was subjected to rough annealing at 500.degree. C. for 1
minute commonly to each Example and then subjected to cold rolling
at a processing rate of 70% without intermediate annealing in the
middle of cold rolling pass, whereby a cold rolled sheet having a
thickness of 1.0 mm was obtained commonly to each Example.
[0255] Moreover, commonly to the each Example, the cold rolled
sheet were subjected to the solution heat treatment in a saltpeter
furnace at 550.degree. C., retained for 10 seconds after the
temperature reached the target temperature, and subjected to the
quenching treatment by water cooling. After the quenching treatment
was finished, the skin pass rolling having a strain amount of 0% to
5% shown in Table 8 was immediately performed in a rolling mill,
followed by retaining at room temperature only for the period shown
in Table 8 until the re-heating treatment was started. Thereafter,
the re-heating treatment was performed under the conditions of the
temperature and retention shown in Table 8 by using an atmospheric
annealing furnace and then cooled by water after retention for a
predetermined time.
[0256] A test sheet (blank) was cut out from each final product
sheet after standing at room temperature for 100 days from the
refining treatment, and the characteristics of each test sheet were
measured and evaluated. The microstructure observation by using the
3DAP was only performed on the samples after 100 days from the
refining treatment. These results are shown in Table 9.
(Cluster)
[0257] First, the microstructures in the center parts in the
thickness direction of the test sheets were analyzed by using the
above-described 3DAP method, and the average number density
(.times.10.sup.23 pieces/m.sup.3) and the average proportion of
aggregates of atoms in which the ratio (Mg/Si) of the number of Mg
atoms to the number of Si atoms is 1/2 or greater of the clusters
stipulated in the present embodiment were acquired by using the
methods described above. The results thereof are shown in Table
9.
[0258] Moreover, in Table 9, among the conditions of the clusters
stipulated in the present embodiment, the containing of either or
both of Mg atoms or Si atoms by a total of 10 pieces or more is
simply described as "Mg and Si atoms by 10 pieces or more."
Further, when any atom among the Mg atoms and Si atoms contained
therein is used as a reference, the distance between the reference
atom and any atom among other atoms adjacent to the reference atom
being 0.75 nm or less is simply described as "distance of 0.75 nm
or less."
(Bake Hardenability)
[0259] After the refining treatment, 0.2% proof stress (As proof
stress) was acquired by carrying out a tensile test, as mechanical
characteristic of each of the test sheets after standing at room
temperature for 100 days. Commonly to each test sheet after being
subjected to room temperature aging for 100 days, 0.2% proof stress
(proof stress after BH) of the test sheet after the artificial
aging hardening treatment (after BH) at 185.degree. C. for 20
minutes was acquired by carrying out the tensile test. Then, the BH
responses of the each test sheet were evaluated based on a
difference (amounts of increase in proof stress) between the 0.2%
proof stresses.
[0260] With respect to the tensile test, No. 5 specimen (25
mm.times.50 mmGL.times.sheet thickness) of JISZ2201 was collected
from each sample sheet to perform the tensile test at room
temperature. The tensile direction of the specimen was set to a
direction orthogonal to the rolling direction. The tensile rate was
set to 5 mm/min until the 0.2% proof stress and 20 mm/min after the
proof stress. The N number of the measurement of the mechanical
characteristics was set to 5, and the average values were
calculated. In addition, with respect to the specimen for
measurement of proof stress after the above-described BH, 2%
pre-strain simulating the press forming of a sheet was applied to
the specimen by using a tensile tester and then the BH treatment
was performed.
(Hem Workability)
[0261] The hem workability was evaluated for sample sheets after
standing for 100 days after the refining treatment. In the test, by
using a strip-shaped specimen having a width of 30 mm, 90.degree.
bending of inward bending with 1.0 mm radius by a down flange was
performed and, with an inner having a thickness of 1.0 mm inserted
therein, the pre-hem working of further bending a bent part inward
to approximately 130.degree. in order and the flat hem working of
bending by 180.degree. and allowing an end part to be tightly
attached to the inner were performed.
[0262] The surface state such as occurrence of rough surface, a
minute crack or a large crack of the bent part (edge bent part) of
the flat hem was visually observed and visually evaluated based on
the following criteria.
[0263] 0: without crack and rough surface, 1: slight rough surface,
2: deep rough surface, 3: minute surface crack, 4: linearly
continued surface crack, 5: breakage
[0264] As shown in alloy Nos. 43 to 52 of Table 7 and alloy Nos.
73, 74, 80, 86, and 92 to 97 of Table 8, in each Example of
Invention, the manufacturing and the refining treatment were
performed within the component composition range of the present
invention and within the preferable condition range. Therefore, as
shown in Table 8, the each Example of Invention satisfies the
cluster conditions stipulated in the present embodiment. That is,
the cluster satisfying the conditions stipulated in the present
embodiment satisfies the preferable average number density
(3.0.times.10.sup.24 pieces/m.sup.3 or greater) and, in the
cluster, the average proportion of aggregates of atoms in which the
ratio (Mg/Si) of the number of Mg atoms to the number of Si atoms
is 1/2 or greater is 0.70 or greater.
[0265] As a result, in the Examples of Invention, as shown in Table
9, even after the long-term room temperature aging from the
refining treatment, the BH responses are excellent, press
formability to an automobile panel is excellent because the As
proof stress is comparatively low, and the hem workability is also
excellent. That is, according to the Examples of Invention, it can
be found that an Al--Si--Mg alloy sheet can be provided, which has
better BH responses with a proof stress difference of 100 MPa or
greater and is capable of exerting press formability or bendability
even in a case where the vehicle body bake treatment was carried
out after the long-term room temperature aging for 100 days.
[0266] In Comparative Examples 75, 81 and 87 of Table 8, Alloy
Examples of Invention 44, 45 and 46 of Table 9 are used. However,
in these Comparative Examples, as shown in Table 8, the time from
when the solution heat treatment and the quenching treatment are
finished to when the re-heating treatment is started is extremely
long. As a result, as shown in Table 9, although the average number
density (.times.10.sup.23 pieces/m.sup.3) of the clusters
stipulated in the present embodiment satisfies the stipulation, the
average proportion of aggregates of atoms in which the ratio
(Mg/Si) of the number of Mg atoms to the number of Si atoms is 1/2
or greater is excessively small, and the room temperature aging is
high and particularly the BH responses after the retention at room
temperature for 100 days are degraded compared to Examples of
Invention 74, 80 and 86 which have the same alloy compositions.
[0267] In Comparative Examples 76, 82 and 88 of Table 8, Alloy
Examples of Invention 44, 45 and 46 of Table 9 are used. These
Comparative Examples are manufactured under the preferable
manufacturing conditions except for the skin pass rolling after the
solution heat treatment and the quenching treatment as shown in
Table 8. For this reason, the average number density
(.times.10.sup.23 pieces/m.sup.3) of the clusters stipulated in the
present embodiment satisfies the stipulation. However, since the
skin pass rolling (work) is not performed, as shown in Table 9,
among the aggregates of atoms satisfying these conditions, the
average proportion of aggregates of atoms, in which the ratio
(Mg/Si) of the number of Mg atoms to the number of Si atoms is 1/2
or greater is excessively small. Therefore, the room temperature
aging is high and particularly the BH responses after the retention
at room temperature for 100 days are degraded compared to Examples
of Invention 74, 80 and 86 which have the same alloy
compositions.
[0268] In Comparative Examples 77 to 79, 83 to 85, and 89 to 91 of
Table 8, Alloy Examples of Invention 44, 45 and 46 of Table 7 are
used. However, in these Comparative Examples, the re-heating
treatment conditions are deviated from the preferred ranges as
shown in Table 8. Accordingly, the average number density of the
aggregates of atoms or the average proportion of the aggregates of
atoms, in which the ratio (Mg/Si) of the number of Mg atoms to the
number of Si atoms is 1/2 or greater, is excessively small.
Therefore, the BH responses after the retention at room temperature
for 100 days or the hem workability is degraded compared to
Examples of Invention 74, 80 and 86 which have the same alloy
compositions.
[0269] Further, although Comparative Examples 98 to 107 of Table 9
are manufactured in preferred ranges including the refining
treatment, alloy Nos. 53 to 62 of Table 7 are used, and the
contents of Mg, Si, and Sn as indispensable elements are deviated
from the ranges of the present invention or the amount of element
impurities is excessively large. For this reason, in these
Comparative Examples, particularly the BH responses or the hem
workability after the retention at room temperature for 100 days is
degraded compared to the Examples of Invention as shown in Table 9.
Particularly, in Comparative Example 100 of Table 9 with an
excessively small amount of Sn, since the room temperature aging is
not suppressed and the As proof stress after retention at room
temperature for 100 days is excessively high, the press formability
or the hem workability is degraded and the BH responses are less
than 100 MPa in terms of an increase amount of proof stress, which
is not high. Moreover, in Comparative Example 101 with an
excessively large amount of Sn, cracks were generated during hot
rolling and thus manufacturing of a sheet itself was not able to be
performed.
[0270] Comparative Example 98 is an alloy 53 of Table 7 and the
amount of Si is excessively small.
[0271] Comparative Example 99 is an alloy 54 of Table 7 and the
amount of Si is excessively large.
[0272] Comparative Example 100 is an alloy 55 of Table 7 and the
amount of Sn is excessively small.
[0273] Comparative Example 101 is an alloy 56 of Table 7 and the
amount of Sn is excessively large.
[0274] Comparative Example 102 is an alloy 57 of Table 7 and the
amount of Fe is excessively large.
[0275] Comparative Example 103 is an alloy 58 of Table 7 and the
amount of Mn is excessively large.
[0276] Comparative Example 104 is an alloy 59 of Table 7 and the
amount of Cr is excessively large.
[0277] Comparative Example 105 is an alloy 60 of Table 7 and the
amount of Cu is excessively large.
[0278] Comparative Example 106 is an alloy 61 of Table 7 and the
amounts of Ti and Zn are excessively large.
[0279] Comparative Example 107 is an alloy 62 of Table 7 and the
amounts of Zr and V are excessively large.
[0280] From the results of Examples described above, it can be
confirmed that all conditions of the cluster stipulated in the
present embodiment are required to be satisfied for the improvement
of BH responses after the room temperature aging. Further, in order
to obtain such cluster conditions and the BH responses, critical
significance or effects of the requirements of the component
composition and preferable manufacturing conditions in the present
embodiment can also be confirmed.
TABLE-US-00007 TABLE 7 Chemical component of aluminum alloy sheet
Alloy (mass %, remainder Al) Classification No. Mg Si Sn Fe Mn Cr
Zr V Ti Cu Zn Ag Examples of 43 0.55 1.05 0.050 Invention 44 0.50
1.10 0.049 0.20 45 0.35 1.10 0.049 0.20 0.12 0.2 46 0.60 0.90 0.094
0.20 0.01 0.21 0.01 47 0.30 1.00 0.051 0.20 0.8 48 0.50 1.25 0.048
0.20 0.7 0.2 49 0.50 0.90 0.206 0.20 0.07 0.22 0.1 50 0.40 0.85
0.009 0.20 0.05 0.15 51 0.60 1.15 0.049 0.70 0.05 0.05 0.6 52 0.80
0.55 0.024 0.20 0.1 0.01 Comparative 53 0.80 0.20 0.049 0.20
Examples 54 0.40 2.10 0.051 0.20 55 0.40 1.15 0.003 56 0.40 1.10
0.455 57 0.50 0.90 0.050 1.30 58 0.50 1.10 0.049 0.20 1.1 59 0.45
0.80 0.048 0.20 0.4 60 0.50 0.80 0.049 0.20 1.3 0.01 61 0.40 0.90
0.049 0.20 0.08 1.2 62 0.60 1.00 0.049 0.20 0.4 0.4 * The columns
in which the numerical values for the elements are blank indicate
that the values are equal to or less than the detection limit.
TABLE-US-00008 TABLE 8 Time from completion of solution heat
treatment and quenching treatment Skin pass rolling Re-heating
treatment Alloy No. to start of re-heating treatment processing
rate Temperature Retention No. of Table 7 hr % .degree. C. h
Example of Invention 73 43 1 3 100 8 Example of Invention 74 44 1 3
100 8 Comparative Example 75 44 48 3 100 8 Comparative Example 76
44 1 -- 100 8 Comparative Example 77 44 1 3 60 12 Comparative
Example 78 44 1 3 180 3 Comparative Example 79 44 1 3 100 1 Example
of Invention 80 45 1 3 100 8 Comparative Example 81 45 48 3 100 8
Comparative Example 82 45 1 -- 100 8 Comparative Example 83 45 1 3
60 12 Comparative Example 84 45 1 3 180 3 Comparative Example 85 45
1 3 100 1 Example of Invention 86 46 1 3 100 8 Comparative Example
87 46 48 3 100 8 Comparative Example 88 46 1 -- 100 8 Comparative
Example 89 46 1 3 60 5 Comparative Example 90 46 1 3 180 3
Comparative Example 91 46 1 3 100 1 Example of Invention 92 47 1 3
100 8 Example of Invention 93 48 16 3 100 8 Example of Invention 94
49 1 1 80 16 Example of Invention 95 50 1 5 100 8 Example of
Invention 96 51 1 3 90 12 Example of Invention 97 52 1 3 120 3
Comparative Example 98 53 1 3 100 8 Comparative Example 99 54 1 3
100 8 Comparative Example 100 55 1 3 100 8 Comparative Example 101
56 Generation of cracks during hot rolling Comparative Example 102
57 1 3 100 8 Comparative Example 103 58 1 3 100 8 Comparative
Example 104 59 1 3 100 8 Comparative Example 105 60 1 3 100 8
Comparative Example 106 61 1 3 100 8 Comparative Example 107 62 1 3
100 8
TABLE-US-00009 TABLE 9 Microstructure of aluminum alloy sheet after
retention at room temperature for 100 days Stipulated cluster (Mg
and Si atoms by 10 pieces or Characteristics of aluminum alloy
sheet after retention at more, distance of 0.75 nm or less) room
temperature for 100 days Proportion in which As 0.2% 0.2% proof
stress Increase amount Alloy No. Average density Mg/Si ratio is
proof stress after BH of proof stress Hem Classification No. of
Table 7 .times.10.sup.23 pieces/m.sup.3 1/2 or greater MPa MPa MPa
workability Example of Invention 73 43 7 0.84 114 226 112 2 Example
of Invention 74 44 11 0.83 118 233 115 2 Comparative Example 75 44
15 0.65 121 191 70 2 Comparative Example 76 44 6 0.67 106 196 90 1
Comparative Example 77 44 8 0.62 80 148 68 1 Comparative Example 78
44 2 0.87 146 213 67 4 Comparative Example 79 44 7 0.66 111 192 81
2 Example of Invention 80 45 9 0.81 103 215 112 2 Comparative
Example 81 45 15 0.64 120 186 66 2 Comparative Example 82 45 7 0.65
100 193 93 2 Comparative Example 83 45 6 0.60 77 157 80 1
Comparative Example 84 45 2 0.85 148 208 60 4 Comparative Example
85 45 8 0.62 114 199 85 2 Example of Invention 86 46 9 0.79 116 224
108 2 Comparative Example 87 46 14 0.64 123 195 72 2 Comparative
Example 88 46 8 0.61 106 202 96 2 Comparative Example 89 46 11 0.58
117 179 62 2 Comparative Example 90 46 2 0.80 151 217 66 4
Comparative Example 91 46 8 0.61 115 202 87 2 Example of Invention
92 47 6 0.79 103 209 106 1 Example of Invention 93 48 15 0.72 128
245 117 2 Example of Invention 94 49 9 0.85 106 217 111 2 Example
of Invention 95 50 8 0.84 89 208 119 1 Example of Invention 96 51
16 0.81 124 232 108 2 Example of Invention 97 52 6 0.93 96 202 106
2 Comparative Example 98 53 1 0.88 83 129 46 1 Comparative Example
99 54 15 0.68 120 211 91 4 Comparative Example 100 55 14 0.74 132
230 98 3 Comparative Example 101 56 Generation of cracks during hot
rolling Comparative Example 102 57 8 0.82 125 223 98 4 Comparative
Example 103 58 12 0.78 126 222 96 4 Comparative Example 104 59 4
0.86 128 224 96 4 Comparative Example 105 60 5 0.92 147 251 104 4
Comparative Example 106 61 5 0.78 132 223 91 4 Comparative Example
107 62 9 0.81 128 217 89 4
[0281] The present invention has been described in detail with
reference to specific embodiments, but it is clear for a person
with an ordinary skill in the art that various alterations and
modifications can be added without departing from the spirit and
scope of the present invention.
[0282] Further, the present application is based on Japanese Patent
Application (Japanese Patent Application No. 2013-185197) filed on
Sep. 6, 2013, Japanese Patent Application (Japanese Patent
Application No. 2013-185198) filed on Sep. 6, 2013 and Japanese
Patent Application (Japanese Patent Application No. 2013-185199)
filed on Sep. 6, 2013, and the contents of which are incorporated
herein by reference.
INDUSTRIAL APPLICABILITY
[0283] According to the present invention, it is possible to
provide a 6000 series aluminum alloy sheet having a combination of
the BH responses under the conditions of a low temperature for a
short period of time after room temperature aging for a long period
of time and also the formability after room temperature aging for a
long period of time. Further, it is possible to provide a 6000
series aluminum alloy sheet which is capable of exerting better BH
responses even in a case of room temperature aging in which the
strength before the baking becomes higher. As a result,
applications of the 6000 series aluminum alloy sheet can be
expanded as members and components of transportation machines such
as an automobile, a marine vessel and a vehicle, household electric
appliance, buildings, and structures, and particularly as members
of transportation machines such as an automobile. For example, in
addition to panel materials for an automobile, the sheet is
preferred in a case of being used for pillars such as a center
pillar, arms such as a side arm, and reinforcing materials such as
bumper reinforcement and door beam which are skeleton members or
structural members of an automobile, and also being used as thin
plates for skeleton members or structural members of other than an
automobile.
* * * * *