U.S. patent application number 16/029976 was filed with the patent office on 2019-01-10 for aluminum alloy plate having excellent moldability and bake finish hardening properties.
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 | 20190010581 16/029976 |
Document ID | / |
Family ID | 54240234 |
Filed Date | 2019-01-10 |
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United States Patent
Application |
20190010581 |
Kind Code |
A1 |
SHISHIDO; Hisao ; et
al. |
January 10, 2019 |
ALUMINUM ALLOY PLATE HAVING EXCELLENT MOLDABILITY AND BAKE FINISH
HARDENING PROPERTIES
Abstract
An aluminum alloy sheet excellent in terms of formability and
bake hardenability is provided. The aluminum alloy sheet contains,
in terms of 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 unavoidable impurities. A
differential scanning calorimetry curve of the aluminum alloy sheet
has an endothermic peak in a temperature range of 150 to
230.degree. C. and an exothermic peak in a temperature range of 240
to 255.degree. C. The endothermic peak corresponds to a dissolution
of a Mg--Si cluster and has a peak height of 8 .mu.W/mg or less,
including 0 .mu.W/mg. The exothermic peak corresponds to a
formation of a Mg--Si cluster and has a peak height of 20 .mu.W/mg
or larger.
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.) |
Kobe-shi |
|
JP |
|
|
Assignee: |
KABUSHIKI KAISHA KOBE SEIKO SHO
(KOBE STEEL, LTD.)
Kobe-shi
JP
|
Family ID: |
54240234 |
Appl. No.: |
16/029976 |
Filed: |
July 9, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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15129587 |
Sep 27, 2016 |
|
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PCT/JP2015/058794 |
Mar 23, 2015 |
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16029976 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 21/04 20130101;
C22C 21/06 20130101; C22C 21/02 20130101; C22C 2202/00 20130101;
C22C 21/08 20130101; C22F 1/05 20130101 |
International
Class: |
C22C 21/04 20060101
C22C021/04; C22C 21/08 20060101 C22C021/08; C22C 21/06 20060101
C22C021/06; C22C 21/02 20060101 C22C021/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 2014 |
JP |
2014-074045 |
Mar 31, 2014 |
JP |
2014-074046 |
Claims
1. An aluminum alloy sheet excellent in terms of formability and
bake hardenability, which is an Al--Mg--Si alloy sheet comprising,
in terms of 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 unavoidable impurities,
wherein a differential scanning calorimetry curve of the aluminum
alloy sheet has an endothermic peak in a temperature range of 150
to 230.degree. C., that is an endothermic peak corresponding to a
dissolution of a Mg--Si cluster and that has a peak height of 8
.mu.W/mg or less (including 0 .mu.W/mg), and has an exothermic peak
in a temperature range of 240 to 255.degree. C., that is an
exothermic peak corresponding to a formation of a Mg--Si cluster
and that has a peak height of 20 .mu.W/mg or larger.
2. The aluminum alloy sheet excellent in terms of formability and
bake hardenability according to claim 1, further comprising one
kind or two or more kinds selected from the group consisting of Fe:
more than 0% and 1.0% or less, Mn: 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, Cu: more than 0% and 1.0% or less, Ag: more than 0% and
0.2% or less, and Zn: more than 0% and 1.0% or less.
3. An aluminum alloy sheet excellent in terms of formability and
bake hardenability, which is an Al--Mg--Si alloy sheet comprising,
in terms of mass %, Mg: 0.3 to 1.0%, Si: 0.5 to 1.5% and Sn: 0.005
to 0.3%, with the remainder being Al and unavoidable impurities,
wherein a solid-solution amount of Mg+Si in a solution, separated
by a residue extraction method with hot phenol is 1.0 mass % or
more and 2.0 mass % or less, and wherein atom aggregates observed
with a three-dimensional atom probe field ion microscope satisfy
conditions that either or both of an Mg atom and an Si atom are
contained therein by a total of 10 pieces or more and that, 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
regarding the atom aggregates, an average volume proportion
(.SIGMA.Vi/V.sub.Al).times.100 is in a range of 0.3 to 1.5%, the
average volume proportion (.SIGMA.Vi/V.sub.Al) being a proportion
of the total volume of the atom aggregates, in terms of the total
volume .SIGMA.Vi obtained by summing up volumes of the individual
atom aggregates Vi (=4/37.pi.r.sub.G.sup.3) calculated from a
Guinier radius r.sub.G of the individual atom aggregates each
regarded as a sphere, to a volume V.sub.Al of the aluminum alloy
sheet measured with the three-dimensional atom probe field ion
microscope, wherein an average volume proportion (.SIGMA.Vi.sub.1.5
or more/.SIGMA.Vi).times.100 is 20 to 70%, the average volume
proportion (.SIGMA.Vi.sub.1.5 or more/.SIGMA.Vi) being a proportion
of a total volume .SIGMA.Vi.sub.1.5 or more obtained by summing up
volumes V.sub.1.5 or more of atom aggregates each having the
Guinier radius r.sub.G of 1.5 nm or larger to a total volume of the
atom aggregates .SIGMA.Vi.
4. The aluminum alloy sheet excellent in terms of formability and
bake hardenability according to claim 3, further comprising one
kind or two or more kinds selected from the group consisting of Fe:
more than 0% and 1.0% or less, Mn: more than 0% and 0.4% 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, Cu: more than 0% and 0.4% or less, Ag: more than 0% and
0.2% or less, and Zn: more than 0% and 1.0% 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 a press forming and 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
structure (an outer panel or an inner panel) of an automobile
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 the large body panel structure of an automobile, for
an outer panel (outer sheet) such as a hood, a fender, a door, a
roof, or a trunk lid, use of an Al--Mg--Si-based AA or HS
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] The 6000-series aluminum alloy sheet contains Si and Mg as
essential components. In particular, a 6000-series aluminum alloy
with excess Si has a composition in which the Si/Mg mass ratio is 1
or greater, and has excellent age hardenability. Because of this,
formability for press forming or bending into the outer panels of
automobiles is secured by lowering the proof stress. In addition,
it has such bake hardenability (hereinafter referred to also as BH
response) that it undergoes age hardening upon heating in an
artificial aging (hardening) treatment performed at a relatively
low temperature, such as the baking treatment of formed panels, and
hence improves in proof stress, thereby ensuring the strength
required as a panel.
[0005] On the other hand, as is known well, an outer panel of an
automobile is manufactured by applying combined formings, 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.
[0006] Here, the 6000-series aluminum alloy had an advantage of
having excellent BH response, but had a problem of having aging
properties at room temperature, that is, of age hardening during
retention at room temperature after 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 placed at room temperature
(standing at room temperature) for approximately 1 month after the
solution heat treatment and the quenching treatment (after
manufacturing) at an aluminum manufacturer until forming 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 immediately after manufacturing, cracking occurred in hem
working after the lapse of 1 month. 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 month.
[0007] Moreover, in the case where such room-temperature aging is
great, a problem also occurs in that the BH response 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.
[0008] Hereto, from the standpoint of the structure of 6000-series
aluminum alloy sheets, in particular, the compounds (crystals or
precipitates) formed by elements contained therein, various
proposals have been made on property improvements such as
improvements in formability or BH response and inhibition of
room-temperature aging. Recently, in particular, it has been
proposed to make an attempt to directly examine and control
clusters (aggregates of atoms) which affect the BH response and
room-temperature aging properties of 6000-series aluminum alloy
sheets (Patent Documents 1 to 3).
[0009] Among these, in Patent Document 1, clusters (aggregates of
atoms) which affect BH response and room-temperature aging
properties are analyzed through a direct examination of the
structure of the sheet as such with a transmission electron
microscope at a magnification of one million and, among the
clusters (aggregates of atoms) observed, the average number density
of clusters having a circle equivalent diameter of in the range of
1 to 5 nm is regulated so as to be in a certain range, thereby
attaining excellent BH response and suppressed room-temperature
aging.
[0010] In contrast, in Patent Documents 2 and 3, in place of
directly examining clusters (aggregates of atoms) as in Patent
Document 1, by using a three-dimensional atom probe field ion
microscope and from positional information of atoms of the sheet,
temporarily ionized in a high electric field (electric-field
evaporation), aggregates of atoms are specified, which are defined
in relation to a structure of atoms in the sheet reconstructed by
the analysis. More specifically, atom aggregates are controlled so
that they include ten or more pieces of either or both of Mg atom
and Si atom in total and satisfies a requirement that 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. Namely,
the average number density, size distribution or proportion of atom
aggregates which satisfy these requirements is specified.
[0011] Furthermore, in prior patent documents which are relevant to
the addition of Sn according to the present invention, many methods
have been proposed in which Sn is positively added to a 6000-series
aluminum alloy sheet to suppress room-temperature aging and improve
BH response (bake hardenability). For example, Patent Document 4
proposes a method in which Sn is added in an appropriate amount and
a solution treatment and subsequently preliminary aging are
performed to thereby obtain both of suppressed room-temperature
aging properties and BH response. Patent Document 5 proposes a
method in which Sn and Cu, which improves formability, are added to
improve formability, BH response and corrosion resistance.
PRIOR ART DOCUMENTS
Patent Documents
[0012] Patent Document 1: JP-A-2009-242904 [0013] Patent Document
2: JP-A-2012-193399 [0014] Patent Document 3: JP-A-2013-60627
[0015] Patent Document 4: JP-A-09-249950 [0016] Patent Document 5:
JP-A-10-226894
SUMMARY OF THE INVENTION
Problem that the Invention is to Solve
[0017] However, even in those conventional Al--Mg--Si alloy sheets,
there has still been room for obtaining both of satisfactory
formability and high BH response after long-term room-temperature
aging.
[0018] The various outer panels for automobiles are required, from
the standpoint of design, to attain strain-free, beautiful
curved-surface configurations and character lines. Such
requirements are becoming severer year by year since high-strength
aluminum alloy sheet materials which are difficult to form are
being adopted for the purpose of weight reduction. There is hence a
growing desire in recent years for an aluminum alloy sheet having
even better formability. However, with the conventional structure
control described above, it is impossible to meet such
requirements.
[0019] For example, one cause which renders high-strength aluminum
alloy sheets difficult to apply to such outer panels is the problem
concerning the shapes peculiar to outer panels. Recessed portions
having given depths (protrudent portions, embossed portions) for
attaching devices or members, such as knob mount bases, lamp mount
bases and license (number plate) mount bases, or for drawing wheel
arches are partly provided to outer panels.
[0020] In the cases when such a recessed portion is press-formed
together with consecutive curved surfaces around the recessed
portion shape, face strains are prone to occur and it is difficult
to attain the strain-free, beautiful curved-surface configuration
and character line. Consequently, it is essential for the outer
panels that the occurrence of such face strains should be inhibited
when the raw sheets are formed.
[0021] The problem of such face strains is not a problem only for
those recessed portions (protrudent portions) but a problem common
to automotive panels which partly have a recessed portion
(protrudent portion) that may suffer a face strain, such as a
saddle-shaped portion of a door outer panel, a vertical wall
portion of a front fender, a window corner portion of a rear
fender, a character-line termination portion of a trunk lid or hood
outer panel, and a root portion of a rear fender pillar.
[0022] From the standpoint of attaining improved formability for
inhibiting the occurrence of the face strains to overcome the
conventional problem described above, it is desirable that a sheet
in press forming (having undergone room-temperature aging after
production) should have a 0.2% proof stress reduced to less than
110 MPa. However, in the cases when the proof stress in forming has
been reduced as the above, it is difficult to attain a 0.2% proof
stress of 200 MPa or greater after bake hardening (hereinafter also
referred to as "after BH"), that is, to attain an increase in 0.2%
proof stress through bake hardening of 100 MPa or greater. With the
conventional structure control with a DSC described above, it is
difficult to overcome the problem.
[0023] A first aspect of the present invention has been achieved in
order to overcome the conventional problem. An object thereof
(hereinafter referred to also as first object) is to provide an
aluminum alloy sheet which combines formability and bake
hardenability, that is, which can have, in automotive-panel
forming, a 0.2% proof stress reduced to 110 MPa or less and can
have a 0.2% proof stress after BH of 200 MPa or greater.
[0024] Meanwhile, from the standpoint of attaining improved
formability for inhibiting the occurrence of the face strains to
overcome the conventional problem, it is desirable that a sheet in
press forming (having undergone room-temperature aging after
production) should have, not only a 0.2% proof stress reduced to
110 MPa or less but also a reduced value of yield ratio, which is
the ratio between tensile strength and yield strength [(0.2% proof
stress)/(tensile strength)]. However, in the cases when the proof
stress in forming has been reduced as the above, it is difficult to
attain a 0.2% proof stress of 190 MPa or greater after bake
hardening treatment (hereinafter referred to also as "BH"), that
is, to attain an increase in 0.2% proof stress through bake
hardening of 100 MPa or greater.
[0025] A second aspect of the present invention has been achieved
in order to overcome the conventional problem. An object thereof
(hereinafter referred to also as second object) is to provide an
aluminum alloy sheet which can not only have, in automotive-panel
forming, a 0.2% proof stress reduced to 110 MPa or less and a yield
ratio reduced to less than 0.50 but also have a 0.2% proof stress
after BH of 190 MPa or greater to thereby combines formability and
bake hardenability and attains both an increase in BH response and
a reduction in yield ratio.
Means for Solving the Problem
[0026] The gist of the aluminum alloy sheet according to the first
aspect of the present invention, which is for achieving the first
object and is excellent in terms of formability and bake
hardenability, is an Al--Mg--Si alloy sheet containing, in terms of
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 unavoidable impurities, in which a
differential scanning calorimetry curve of the aluminum alloy sheet
has an endothermic peak in a temperature range of 150 to
230.degree. C., that is an endothermic peak corresponding to a
dissolution of a Mg--Si cluster and that has a peak height of 8
.mu.W/mg or less (including 0 .mu.W/mg), and has an exothermic peak
in a temperature range of 240 to 255.degree. C., that is an
exothermic peak corresponding to a formation of a Mg--Si cluster
and that has a peak height of 20 .mu.W/mg or larger.
[0027] The differential thermal analysis at each of measurement
portions in the aluminum alloy sheet is performed under the same
conditions including a test apparatus of DSC220G, manufactured by
Seiko Instruments Inc., a reference substance of aluminum, a sample
container made of aluminum, temperature increase conditions of
15.degree. C./min, an atmosphere of argon (50 mL/min), and a sample
weight of 24.5 to 26.5 mg. The differential thermal analysis
profile (.mu.W) obtained is divided by the sample weight and
thereby normalized (.mu.W/mg). Thereafter, in the range of 0 to
100.degree. C. in the differential thermal analysis profile, a
region where the differential thermal analysis profile is
horizontal is taken as a reference level of 0, and the height of
exothermic peak from the reference level is measured.
[0028] The gist of the aluminum alloy sheet according to the second
aspect of the present invention, which is for achieving the second
object and is excellent in terms of formability and bake
hardenability, is an Al--Mg--Si alloy sheet containing, in terms of
mass %, Mg: 0.3 to 1.0%, Si: 0.5 to 1.5% and Sn: 0.005 to 0.3%,
with the remainder being Al and unavoidable impurities, in which a
solid-solution amount of Mg+Si in a solution, separated by a
residue extraction method with hot phenol is 1.0 mass % or more and
2.0 mass % or less, and
[0029] in which atom aggregates observed with a three-dimensional
atom probe field ion microscope satisfy conditions that either or
both of an Mg atom and an Si atom are contained therein by a total
of 10 pieces or more and that, 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 regarding the atom
aggregates, an average volume proportion
(.SIGMA.Vi/V.sub.Al).times.100 is in a range of 0.3 to 1.5%, the
average volume proportion (.SIGMA.Vi/V.sub.Al) being a proportion
of the total volume of the atom aggregates, in terms of the total
volume .SIGMA.Vi obtained by summing up volumes of the individual
atom aggregates Vi (=4/3.pi.r.sub.G.sup.3) calculated from a
Guinier radius r.sub.G of the individual atom aggregates each
regarded as a sphere, to a volume V.sub.Al of the aluminum alloy
sheet measured with the three-dimensional atom probe field ion
microscope, in which
[0030] an average volume proportion (.SIGMA.Vi.sub.1.5 or
more/.SIGMA.Vi).times.100 is 20 to 70%, the average volume
proportion (.SIGMA.Vi.sub.1.5 or more/.SIGMA.Vi) being a proportion
of a total volume .SIGMA.Vi.sub.1.5 or more obtained by summing up
volumes V.sub.1.5 or more of atom aggregates each having the
Guinier radius r.sub.G of 1.5 nm or larger to a total volume of the
atom aggregates .SIGMA.Vi.
Effects of the Invention
[0031] With regard to the first aspect, Sn exerts such effects in
the structure of the Al--Mg--Si alloy sheet that, at room
temperature, it captures (traps) atomic holes and thereby inhibits
diffusion of Mg and Si at room temperature, inhibits the strength
from increasing at room temperature and, during the forming of the
sheet into panels, improves the press formability including hem
workability, drawability and punch stretch formability
(hereinafter, this press formability is referred to also as hem
workability as a representative). During an artificial aging
treatment of the panels, such as a baking treatment, it releases
the captured holes and hence in turn enhances the diffusion of Mg
and Si. Consequently, the BH response can be enhanced.
[0032] However, the present inventors have found that the addition
of such Sn poses a new problem due to peculiar properties of Sn.
Specifically, in the cases when Sn is added and a sheet is produced
by an ordinary method, the addition of Sn rather leads, depending
on the production conditions, to a decrease in the amount of Mg--Si
clusters which contribute to strength. There are hence cases where
the addition of Sn results in an insufficient amount of
precipitates which precipitate after a bake hardening treatment,
making it impossible to obtain the strength required as automotive
panels as described above.
[0033] The reason for this is presumed to be because the Sn's
effect of capturing and releasing atomic holes is related with the
fact that the solid-solution amount of Sn in the matrix is
exceedingly small (in an ordinary means, even when the added amount
of Sn is controlled to equal to or less than a theoretical
solid-solution amount, a large proportion thereof crystallizes out
or precipitates as compounds without coming into a solid-solution
state). However, this presumption is uncertain.
[0034] In any case, the addition of Sn itself may become
meaningless unless problems such as the decrease in the amount of
Mg--Si clusters which contribute to strength and the insufficient
amount of precipitates which precipitate after a bake hardening
treatment are overcome, such problems being regarded as side
effects of the addition of Sn.
[0035] Because of this, in the present aspect, the inventors have
ventured to reconsider sheet production processes and contrived
production conditions concerning, for example, a preliminary aging
treatment (reheating treatment) after a solution quenching
treatment as will be described later, so that addition of Sn does
not result in a decrease in the amount of Mg--Si clusters which
contribute to strength or in an insufficient amount of precipitates
which precipitate after a bake hardening treatment.
[0036] The inventors have further discovered that a DSC
(differential scanning calorimetry curve) of this sheet can be
applied as a standard of the structure which can, even when Sn has
been added thereto, prevent the Mg--Si clusters that contribute to
strength from being diminished and increase or ensure the amount of
precipitates that precipitate after a bake hardening treatment.
Specifically, in the present aspect, based on the DSC, an
endothermic peak corresponding to the dissolution of relatively
small Mg--Si clusters, which do not contribute to strength, is
controlled and meanwhile an exothermic peak corresponding to the
formation of relatively large Mg--Si clusters, which contribute to
strength, is enhanced. Thus, Mg--Si clusters that do not contribute
to strength are suppressed and the Mg--Si clusters that contribute
to strength are increased, thereby obtaining desired BH
response.
[0037] As a result, according to the present aspect, it is possible
to provide an aluminum alloy sheet which combines formability and
bake hardenability and which contains Sn and can have a 0.2% proof
stress in automotive-panel forming reduced to 110 MPa or less and
have a 0.2% proof stress after BH of 200 MPa or greater.
[0038] In the second aspect, first in order to ensure formability
of the Al--Mg--Si alloy sheet into the outer panels (hereinafter,
this press formability is referred to also as hem workability as a
representative), the sheet in forming is aimed to have a 0.2% proof
stress reduced to 110 MPa or less and a yield ratio reduced to less
than 0.50.
[0039] Because of this, the solid-solution amount of Mg and Si is
controlled in the present aspect in addition to the alloy
composition including Mg and Si. Furthermore, by adding Sn, the BH
response is enhanced while ensuring the formability. As will be
described later, Sn has an important effect of attaining both an
increase in BH response and a reduction in yield ratio by reducing
the volume proportion of atom aggregates which inhibit the yield
ratio from being reduced, even when the solid-solution amount of
Mg+Si is increased.
[0040] Furthermore, in the present aspect, the size distribution of
atom aggregates observed with a three-dimensional atom probe field
ion microscope is further specified in order to control Mg--Si atom
aggregates, in addition to the means described above, so that the
yield ratio during sheet forming can be reliably reduced to less
than 0.50.
[0041] The term "atom aggregates observed with a three-dimensional
atom probe field ion microscope" here means known atom aggregates
including the measurement methods described in Patent Documents 2
and 3, and does not mean atom aggregates (clusters) observed by
directly examining the sizes and state thereof in the structure of
the sheet with a high-magnification TEM by using the structure of
the sheet as such, as in Patent Document 1. In other words, as in
Patent Documents 2 and 3, those are the atom aggregates in a
three-dimensional structure of atoms (three-dimensional atom map)
obtained by a reconstruction through analysis from the flight times
and positions of atoms of the sheet which have temporarily ionized
in a high electric field (electric-field evaporation) with a
three-dimensional atom probe field ion microscope, as the details
of the measuring method will be described later. Those are the atom
aggregates which are defined to satisfy the given requirements
specified in claim 1 (that is, deemed to be atom aggregates) in the
three-dimensional structure of atoms.
[0042] In the present aspect, in order to reduce a yield ratio to
less than 0.50, as a size distribution of the atom aggregates
observed with a three-dimensional atom probe field ion microscope,
the proportion of atom aggregates that satisfy the requirements
that Mg atom and/or Si atom is contained and the distance between
the atoms are 0.75 nm or less, is regulated so as to be in a
certain range in terms of volume proportion. In addition, the
proportion of relatively large atom aggregates which each have a
Guinier radius r.sub.G of 1.5 nm or larger, among those atom
aggregates, is increased in a certain range in terms of volume
proportion.
[0043] As a result, according to the present aspect, it is possible
to provide an aluminum alloy sheet which combines formability and
bake hardenability and which contains Sn and can not only have, in
automotive-panel forming, a 0.2% proof stress reduced to 110 MPa or
less and a yield ratio reduced to less than 0.50 but also have a
0.2% proof stress after BH of 190 MPa or greater.
BRIEF DESCRIPTION OF THE DRAWING
[0044] FIG. 1 is an explanatory view which shows each DSC of
Examples according to the first aspect.
MODES FOR CARRYING OUT THE INVENTION
(First Aspect)
[0045] The first aspect of the present invention will be explained
below in detail with respect to each requirement.
(Chemical Component Composition)
[0046] First, the chemical component composition of the Al--Mg--Si
(hereinafter referred to also as 6000-series) aluminum alloy sheet
according to the present aspect is explained below. The 6000-series
aluminum alloy sheet targeted by the present aspect, as, for
example, the sheet for the automotive outer panels, is required to
have various properties such as excellent formability, BH response,
strength, weldability, and corrosion resistance. Consequently, such
requirements are also met by means of the composition. In addition,
in the present aspect, Sn is incorporated to suppress the
room-temperature aging of the sheet after production and to reduce
a 0.2% proof stress in the panel forming to 110 MPa or less. Thus,
the formability into automotive panels or the like, which are
particularly problematic in face strains thereof, in automotive
panel structures, can be improved. Simultaneously therewith, a 0.2%
proof stress after bake hardening of 200 MPa or greater is rendered
possible by means of the composition.
[0047] In order to satisfy such requirements, the aluminum alloy
sheet according to the present aspect has a composition which
includes, in terms of 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 unavoidable
impurities. All the content indicated in % of the elements means
that in mass %. In this description, percentage on mass basis (mass
%) is the same as percentage on weight basis (wt %). With respect
to the content of a chemical component, there are cases where "X %
or less (exclusive of 0%)" is expressed by "more than 0% and X % or
less".
[0048] In the present aspect, elements other than Mg, Si and Sn are
impurities or elements which may be contained, and may have
contents (permissible amounts) on levels of the elements in
accordance with the AA or HS standards, etc.
[0049] Namely, there are cases, in the present aspect also, where
not only high-purity Al base metal but also 6000-series alloys
containing elements other than Mg and Si as additive elements
(alloying elements) in large amounts, other aluminum alloy scrap
materials, low-purity Al base metal, and the like are used in large
quantities as melted raw materials for the alloy, from the
standpoint of resource recycling. In such cases, other elements
such as shown below are inevitably included in substantial amounts.
Since refining performed for intentionally diminishing these
elements itself leads to an increase in cost, it is necessary to
accept some degree of inclusion. There are useful content ranges
which permit inclusion of such elements in substantial amounts but
do not inhibit the object or effects of the present aspect.
[0050] Consequently, in the present aspect, inclusion of such
elements shown below is permissible within the range of equal to or
less than the upper limits specified below, which are in accordance
with the AA or JIS standards or the like.
[0051] Specifically, the aluminum alloy sheet may further contain
one kind or two or more kinds selected from the group consisting of
Fe: 1.0% or less (exclusive of 0%), Mn: 1.0% or less (exclusive of
0%), Cr: 0.3% or less (exclusive of 0%), Zr: 0.3% or less
(exclusive of 0%), V: 0.3% or less (exclusive of 0%), Ti: 0.1% or
less (exclusive of 0%), Cu: 1.0% or less (exclusive of 0%), Ag:
0.2% or less (exclusive of 0%), and Zn: 1.0% or less (exclusive of
0%), within those ranges, in addition to the basic composition
shown above.
[0052] In the cases where these elements are contained, the content
of Cu is preferably 0.7% or less and more preferably 0.3% or less,
because Cu is prone to impair the corrosion resistance when
contained in a large amount. Mn, Fe, Cr, Zr, and V are prone to
yield relatively coarse compounds when contained in large amounts,
and are prone to impair the hem workability (hem bendability),
which is addressed by the present aspect. Consequently, the content
of Mn is preferably 0.6% or less and more preferably 0.3% or less,
and the content of each of Cr, Zr and V is preferably 0.2% or less
and more preferably 0.1% or less.
[0053] The content range of each element and the purposes and
permissible amount thereof in the 6000-series aluminum alloy are
explained below in order.
Si: 0.3 to 2.0%
[0054] Si, together with Mg, is an essential element for obtaining
the strength (proof stress) required as automotive panels by
forming aging precipitates which contribute to an improvement in
strength, during an artificial aging treatment such as a baking
treatment, and thus exhibiting an age hardenability. In the case
where the addition amount of Si is too small, the amount of
precipitates after artificial aging is too small and the increase
in strength through baking is too small. Meanwhile, in the case
where the content of Si is too high, the Si forms coarse crystals
with impurity Fe, etc., resulting in a considerable decrease in
formability such as bendability. In addition, too high Si contents
increases not only the strength just after sheet production but
also the room-temperature aging amount after the production,
thereby increases the strength before forming too much, and reduces
the formability into automotive panels or the like, which are
particularly problematic in face strains thereof, in automotive
panel structures. Consequently, the content of Si is regulated so
as to be in the range of 0.3 to 2.0%.
[0055] For attaining an excellent age hardenability in a baking
treatment performed at a lower temperature for a shorter period
after forming into panels, it is preferable to employ a 6000-series
aluminum alloy composition in which Si/Mg is 1.0 or larger in terms
of mass ratio so that Si has been incorporated further excessively
relative to the Mg than in the so-called excess-Si type.
Mg: 0.2 to 2.0%
[0056] Mg, together with Si, is also an important element for
forming the clusters specified in the present aspect. It is an
essential element for obtaining the proof stress required as panels
by forming, together with the Si, aging precipitates which
contribute to an improvement in strength, during an artificial
aging treatment such as a baking treatment, and thus exhibiting an
age hardenability. In the case where the content of Mg is too low,
the amount of precipitates after artificial aging is too small and
the strength after baking is thus too low. Meanwhile, in the case
where the content of Mg is too high, the Mg forms coarse crystals
with impurity Fe, etc., resulting in a considerable decrease in
formability such as bendability. In addition, too high Mg contents
increases not only the strength just after sheet production but
also the room-temperature aging amount after the production,
thereby increases the strength before forming, and reduces the
formability into automotive panels or the like, which are
particularly problematic in face strains thereof, in automotive
panel structures. Consequently, the content of Mg is regulated so
as to be in the range of 0.2 to 2.0%.
Sn: 0.005 to 0.3%
[0057] Sn, at room temperature, has the effect of capturing
(trapping) atomic holes to thereby inhibit room-temperature
diffusion of Mg and Si and inhibit the strength from increasing at
room temperature, and during the forming of the sheet into panels,
improving the press formability including hem workability,
drawability and punch stretch formability (hereinafter, this press
formability is referred to also as hem workability as a
representative). During an artificial aging treatment of the
panels, such as a baking treatment, it releases the captured holes
and hence in turn enhances the diffusion of Mg and Si, thereby
enhancing the BH response. In the case where the content of Sn is
lower than 0.005%, the Sn cannot sufficiently trap holes and is
unable to exhibit the effects thereof. Meanwhile, in the case where
the content of Sn is higher than 0.3%, the Sn segregates at grain
boundaries and it is prone to cause intergranular cracks. A
preferred lower limit of the content of Sn is 0.01%. An upper limit
of the content of Sn is preferably 0.2%, more preferably 0.1% and
further preferably 0.06%.
(Structure)
[0058] The composition described above is employed and furthermore,
in the present aspect, the 6000-series aluminum alloy sheet is made
to have the following structure. In order to ensure high strength
as automotive panels or the like, a DSC of this sheet is used as a
measure for ensuring the amount of precipitates which precipitate
after a bake hardening treatment, and an endothermic peak and an
exothermic peak in specific temperature ranges, which affect, in
particular, the strength before baking and an increase in strength
through the baking are controlled. In other words, a DSC of this
sheet is used to control an endothermic peak and an exothermic peak
in specific temperature ranges, which affect, in particular, the
strength before baking and an increase in strength through the
baking, so that the addition of Sn does not result in a decrease in
the amount of Mg--Si clusters which contribute to strength or
result in insufficient amount of precipitates which precipitate
after a bake hardening treatment.
[0059] More specifically, in the present aspect, based on the DSC,
an endothermic peak corresponding to the dissolution of relatively
small Mg--Si clusters, which do not contribute to strength, is
controlled and meanwhile an exothermic peak corresponding to the
formation of relatively large Mg--Si clusters, which contribute to
strength, is enhanced. Thus, Mg--Si clusters that do not contribute
to strength are suppressed and the Mg--Si clusters that contribute
to strength are increased, thereby obtaining desired BH
response.
[0060] Here, the differential scanning calorimetry curve (DSC) is a
heating curve from solid phase, obtained by measuring the thermal
changes during melting of aluminum alloy sheet after the refining
treatment, by differential thermal analysis performed under the
following conditions.
[0061] Specifically, the differential thermal analysis at each of
measurement portions in the aluminum alloy sheet is performed under
the same conditions including a test apparatus of DSC220G,
manufactured by Seiko Instruments Inc., a reference substance of
aluminum, a sample container made of aluminum, temperature increase
conditions of 15.degree. C./min, an atmosphere of argon (50
mL/min), and a sample weight of 24.5 to 26.5 mg. The differential
thermal analysis profile (.mu.W) obtained is divided by the sample
weight and thereby normalized (.mu.W/mg). Thereafter, in the range
of 0 to 100.degree. C. in the differential thermal analysis
profile, a region where the differential thermal analysis profile
is horizontal is taken as a reference level of 0, and the height of
exothermic peak from the reference level is measured.
[0062] In the present aspect, the number (number density) of Mg--Si
clusters that have a relatively small size and are apt to dissolve
during temperature increase in DSC, which are regarded as Mg--Si
clusters not contributing to strength, is inhibited first. In the
case where the number of such Mg--Si clusters that are apt to
dissolve during temperature increase in DSC increases upon BH, the
number (number density) of Mg--Si clusters that have a relatively
large size and are less apt to dissolve during temperature increase
in DSC, which are regarded as contributive to strength, in turn
decreases upon an artificial age hardening treatment, making it
impossible to increase strength after BH. Specifically, an increase
in 0.2% proof stress of 100 MPa or greater and a strength (0.2%
proof stress) after BH of 200 MPa or greater cannot be obtained,
although this depends on the BH conditions.
[0063] Because of this, in the present aspect, the peak height of
an endothermic peak A in a temperature range of 150 to 230.degree.
C., as an endothermic peak corresponding to the dissolution of
Mg--Si clusters that are apt to dissolve during temperature
increase in DSC and do not contribute to strength, is regulated
(reduced) to 8 .mu.W/mg or less (inclusive of 0 .mu.W/mg).
Consequently, that the endothermic peak in the temperature range of
150 to 230.degree. C. has a peak height of 8 .mu.W/mg indicates a
critical number density which is permissible with respect to the
adverse influence on strength of the Mg--Si clusters having a
relatively small size and not contributing to strength. Although a
sheet in which such Mg--Si clusters having a relatively small size
and not contributing to strength are absent (i.e., the number
density thereof is 0) is difficult to produce due to limitations on
its production, the present aspect includes such the case. Because
of this, the feature in which the peak height of the endothermic
peak A is 8 .mu.W/mg or less involves the case of 0 .mu.W/mg, in
which such Mg--Si clusters having a relatively small size and not
contributing to strength are absent.
[0064] Meanwhile, in the present aspect, Mg--Si clusters which have
a relatively large size and are less apt to dissolve during
temperature increase in DSC and which contribute to strength are
yielded in a large amount to improve the BH response. Because of
this, the peak height of an exothermic peak B in a temperature
range of 240 to 255.degree. C., which corresponds to the formation
of Mg--Si clusters that contribute to strength, is heightened
(increased) to 20 .mu.W/mg or more. Consequently, that the
exothermic peak in the temperature range of 240 to 255.degree. C.
has a peak height of 20 .mu.W/mg indicates a minimum value of the
number density of Mg--Si clusters having a relatively large size
and contributing to strength, the minimum value being necessary for
obtaining the improvement in BH response which is addressed by the
present aspect (an increase in 0.2% proof stress of 100 MPa or
greater and a 0.2% proof stress after BH of 200 MPa or greater)
though it differs by the BH condition. Hence, the higher the number
density, the better, and the larger (higher) the peak height of the
exothermic peak in the temperature range of 240 to 255.degree. C.,
the better. However, in view of limitations on sheet production, an
upper limit thereof is about 80 .mu.W/mg.
[0065] FIG. 1 shows DSCs of three kinds of aluminum alloy sheet in
Examples which will be given later, i.e., Invention Example 8 and
Comparative Example 9 in Table 2 and Comparative Example 25 in
Table 3. Invention Example 8 is indicated by a thick continuous
line, Comparative Example 9 is indicated by a dotted line and
Comparative Example 25 is indicated by a dot-and-dash line.
[0066] In FIG. 1, the DSC of Comparative Example 9 has an
endothermic peak A in the temperature range of 150 to 230.degree.
C., which has a peak height exceeding (larger than) 8 .mu.W/mg as
shown in Table 2, which will be given later, showing that the
number density of Mg--Si clusters having a relatively small size
and not contributing to strength is too high. Meanwhile, the
exothermic peak B in the temperature range of 240 to 255.degree. C.
also has a peak height as high (large) as 20 .mu.W/mg or more,
showing that the number density of Mg--Si clusters having a
relatively large size and contributing to strength is also high.
However, since the number density of the Mg--Si clusters having a
relatively small size and not contributing to strength is too high,
the adverse influences thereof are too greater. Therefore, the
desired BH response (an increase in 0.2% proof stress of 100 MPa or
greater and a 0.2% proof stress after BH of 200 MPa or greater)
cannot be obtained.
[0067] The DSC of Comparative Example 25 in FIG. 1 has an
endothermic peak A in the temperature range of 150 to 230.degree.
C., which has a peak height as low (small) as 8 .mu.W/mg or less as
shown in Table 2, which will be given later, showing that the
number density of Mg--Si clusters having a relatively small size
and not contributing to strength is low. Meanwhile, the exothermic
peak B in the temperature range of 240 to 255.degree. C. also has a
peak height as low (small) as less than 20 .mu.W/mg, showing that
the number density of Mg--Si clusters having a relatively large
size and contributing to strength is also too low. Because of this,
the desired BH response (an increase in 0.2% proof stress of 100
MPa or greater and a 0.2% proof stress after BH of 200 MPa or
greater) cannot be obtained.
[0068] In contrast, the DSC of Invention Example 8 in FIG. 1 has an
endothermic peak A in the temperature range of 150 to 230.degree.
C., which has a peak height as low (small) as 8 .mu.W/mg or less as
shown in Table 2, which will be given later, showing that the
number density of Mg--Si clusters having a relatively small size
and not contributing to strength is low. Meanwhile, the exothermic
peak B in the temperature range of 240 to 255.degree. C. has a peak
height as high (large) as 20 .mu.W/mg or more, showing that the
number density of Mg--Si clusters having a relatively large size
and contributing to strength is high. Because of this, the desired
BH response (an increase in 0.2% proof stress of 100 MPa or greater
and a 0.2% proof stress after BH of 200 MPa or greater) is
obtained.
(Production Process)
[0069] Next, a process for producing the aluminum alloy sheet
according to the present aspect is explained. The aluminum alloy
sheet according to the present aspect is produced through
production steps which themselves are common or known, by
subjecting, after casting, an aluminum alloy slab having the
6000-series component composition to a homogenizing heat treatment,
hot rolling and cold rolling to obtain a given sheet thickness,
followed by a refining treatment such as a solution quenching
treatment.
[0070] However, for obtaining the structure specified with a DSC
according to the present aspect, during those production steps, the
average cooling rate in a quenching treatment after a solution
treatment is controlled and in addition, the conditions for a
preliminary aging treatment after the quenching treatment are
regulated so as to be in a preferred range, as will be described
later. With respect to other steps, there are preferred conditions
for obtaining the structure specified with a DSC according to the
present aspect. Unless such preferred conditions are employed, it
is difficult to obtain the structure specified with a DSC according
to the present aspect.
(Melting and Casting Cooling Rate)
[0071] First, in melting and casting steps, an aluminum alloy
molten metal that has been melted and regulated so as to have a
component composition within the 6000-series composition range is
cast by a suitably selected ordinary melting and casting method,
such as a continuous casting method or a semi-continuous casting
method (DC casting method). Here, it is preferable that the average
cooling rate, during the casting, from the liquidus temperature to
the solidus temperature is as high (quick) as possible at
30.degree. C./min or greater.
[0072] In the 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 is inevitably low. When
an average cooling rate in the high-temperature range is low as the
above, the amount of crystals yielded coarsely in the temperature
range of this high-temperature range is increased and also
unevenness in the size and amount of the crystals along the width
direction and thickness direction of the slab is increased. As a
result, the basic mechanical properties, such as strength and
elongation, which are a prerequisite for the 6000-series aluminum
alloy sheet are reduced.
(Homogenizing Heat Treatment)
[0073] Next, the aluminum alloy slab obtained by casting is
subjected to a homogenizing heat treatment prior to hot rolling.
The purpose of this homogenizing heat treatment (soaking treatment)
is to homogenize the structure, that is, to eliminate segregation
within the grains in the structure of the slab. The conditions are
not particularly limited so long as the purpose is achieved
therewith, and the treatment may be an ordinary one conducted once
or in one stage.
[0074] A homogenizing heat treatment temperature is suitably
selected from the range of 500.degree. C. or more and lower than
the melting point, and a homogenizing time is suitably selected
from the range of 4 hours and longer. In the case where the
homogenizing temperature is low, the segregation within grains
cannot be sufficiently eliminated, and these act as starting points
for fracture, resulting in decreases in stretch flangeability and
bendability. Thereafter, hot rolling may be started immediately.
Alternatively, hot rolling may be started after holding and cooling
to an appropriate temperature.
[0075] After the homogenizing heat treatment has been performed,
cooling to room temperature may be performed so that the average
cooling rate in the range of 300.degree. C. to 500.degree. C. is 20
to 100.degree. C./hr, followed by reheating to 350.degree. C. to
450.degree. C. at an average heating rate of 20 to 100.degree.
C./hr to start hot rolling in this temperature range.
[0076] In the cases when the average cooling rate after the
homogenizing heat treatment and the reheating rate conducted
thereafter do not satisfy those conditions, the possibility of
forming coarse Mg--Si compounds increases, resulting in decreases
in the basic mechanical properties, such as strength and
elongation, that are a prerequisite for the 6000-series aluminum
alloy sheet before exhibiting the effect of Sn.
(Hot Rolling)
[0077] The hot rolling is constituted of a slab rough rolling step
and a finish rolling step in accordance with the thickness of the
plate to be rolled. In these rough rolling step and finish rolling
step, rolling mills such as a reverse type and a tandem type are
suitably used.
[0078] In the cases when the hot-rolling (rough-rolling) start
temperature exceeds the solidus temperature, burning occurs and,
hence, the hot rolling itself is difficult to carry out. Meanwhile,
in the cases when the hot-rolling start temperature is lower than
350.degree. C., the hot-rolling load is too high, rendering the hot
rolling itself difficult. Consequently, the hot-rolling start
temperature is preferably in the range of 350.degree. C. to the
solidus temperature, more preferably in the range of 400.degree. C.
to the solidus temperature.
(Annealing of the Hot-Rolled Plate)
[0079] Annealing (rough annealing) before cold rolling is not
always necessary for the hot-rolled plate. However, it may be
performed in order to further improve properties such as
formability by making the grains smaller and optimizing the
texture.
(Cold Rolling)
[0080] In cold rolling, the hot-rolled sheet is rolled to produce a
cold-rolled sheet (including a coil) having a desired final sheet
thickness. However, for making the grains even smaller, it is
desirable that the total cold rolling ratio should be 60% or
greater regardless of the number of passes.
(Solution Treatment and Quenching Treatment)
[0081] After the cold rolling, a solution treatment is performed,
followed by a treatment for quenching to room temperature. The
solution and quenching treatments may be a heating and a cooling
performed on an ordinary continuous heat treatment line, and are
not particularly limited. However, from the standpoint of obtaining
a sufficient solid-solution amount of each element and because it
is desirable that the grains should be finer as stated above, it is
desirable that the treatments should be conducted under such
conditions of heating at a heating rate of 5.degree. C./sec or
greater to a solution treatment temperature which is 520.degree. C.
or higher and lower than the melting temperature, and then holding
for 0.1 to 10 seconds.
[0082] From the standpoint of suppressing the formation of coarse
intergranular compounds that reduce the formability and hem
workability, it is desirable that the average cooling rate from the
solution treatment temperature to the quenching stop temperature,
which is room temperature, should be 3.degree. C./s or greater. In
the case where the rate of cooling to room temperature after the
solution treatment is too low, coarse Mg--Si and elemental Si are
yielded during the cooling, resulting in impaired formability. In
addition, the solid-solution amount after the solution treatment is
reduced, resulting in a decrease in BH response. In order to secure
that cooling rate, means such as air cooling with fans or water
cooling with mist or spray or by immersion, etc. and conditions
therefor are selected and used for the treatment for quenching to
room temperature.
(Preliminary Aging Treatment: Reheating Treatment)
[0083] After having thus undergone the solution treatment and the
subsequent quenching treatment to be cooled to room temperature,
the cold-rolled sheet is subjected to a preliminary aging treatment
(reheating treatment) within 1 hour. In the case where the
room-temperature holding period from termination of the treatment
for quenching to room temperature to initiation of the preliminary
ageing treatment (initiation of heating) is too long, the small
Mg--Si clusters that do not contribute to strength are yielded in a
large amount as clusters that are apt to dissolve upon
room-temperature aging, making it difficult to suppress a peak
height of the endothermic peak in the temperature range of 150 to
230.degree. C. to 8 .mu.W/mg or less. Consequently, the shorter the
room-temperature holding period, the better. The solution and
quenching treatments and the reheating treatment may be
consecutively performed so that there is substantially no pause
therebetween, and a lower limit of the period is not particularly
determined.
[0084] In this preliminary aging treatment, the rate of temperature
increase to a preliminary-aging temperature and the period of
holding in a preliminary-aging temperature range are regulated. It
is preferable that the temperature increase rate, of these, should
be as high (quick) temperature increase rate as possible at
1.degree. C./s or higher and preferably 5.degree. C./s or higher,
for suppressing the formation of the small Mg--Si clusters not
contributing to strength. In the case where the temperature
increase rate is less than 1.degree. C./s, Mg--Si clusters that are
apt to dissolve during temperature increase in DSC and that do not
contribute to strength are yielded in a large amount, making it
difficult to suppress a peak height of the endothermic peak in the
temperature range of 150 to 230.degree. C. to 8 .mu.W/mg or
less.
[0085] The temperature and holding period in the preliminary aging
treatment is a holding in a temperature range of 60 to 120.degree.
C. for 10 hr or more and 40 hr or less. Here, the holding in the
temperature range of 60 to 120.degree. C. may be a heat treatment
in which the temperature is constant or the temperature is
sequentially changed by temperature increase or annealing, within
that temperature range. In short, the temperature may be
continuously changed by annealing, temperature increase, etc., so
long as it is held in a temperature range of 60 to 120.degree. C.
for that period of 10 hr or more and 40 hr or less.
[0086] In the case where the preliminary-aging temperature is lower
than 60.degree. C. or the holding period is less than 10 hr, the
formation of precipitate nuclei is insufficient and this is prone
to result in a DSC in which an exothermic peak in the range of the
exothermic peak B in the temperature range of 240 to 255.degree. C.
has a peak temperature higher than 255.degree. C. This means that
the amount of Mg--Si clusters having a relatively large size and
contributing to strength decreases, and it becomes impossible to
regulate the exothermic peak B in the temperature range of 240 to
255.degree. C. so as to have a peak height as high (large) as 20
.mu.W/mg or more. As a result, the BH response decreases.
[0087] Meanwhile, in the case where the preliminary-aging
temperature exceeds 120.degree. C. or the holding period exceeds 40
hr, precipitate nuclei are yielded in too large an amount in this
preliminary aging treatment. Because of this, the Mg--Si clusters
having a relatively large size and contributing to strength in turn
decreases, making it impossible to make the exothermic peak B in
the temperature range of 240 to 255.degree. C. have a peak height
as high (large) as 20 .mu.W/mg or more, in a DSC. Consequently,
this also results in a decrease in BH response. In addition,
strength during forming is too high.
[0088] Namely, unless the preliminary aging treatment is regulated
to fall within these preferable conditions, it is difficult to
attain a 0.2% proof stress, in forming of automotive panels,
reduced to 110 MPa or less and a 0.2% proof stress after BH of 200
MPa or greater.
[0089] The second aspect of the present invention will be explained
below in detail with respect to each requirement.
(Chemical Component Composition)
[0090] First, the chemical component composition of the Al--Mg--Si
(hereinafter referred to also as 6000-series) aluminum alloy sheet
according to the present aspect is explained below. The 6000-series
aluminum alloy sheet targeted by the present aspect, as, for
example, the sheet for the automotive outer panels, is required to
have various properties such as excellent formability, BH response,
strength, weldability, and corrosion resistance. Consequently, such
requirements are also met by means of the composition. In addition,
in the present aspect, Sn is incorporated to suppress the
room-temperature aging of the sheet after production, to reduce a
0.2% proof stress in the panel forming to 110 MPa or less and to
reduce a yield ratio to less than 0.50. Thus, the formability into
automotive panels or the like, which are particularly problematic
in face strains thereof, in automotive panel structures, is
improved. Simultaneously therewith, a 0.2% proof stress after bake
hardening of 190 MPa or greater is rendered possible by means of
the composition.
[0091] In order to satisfy such requirements, the aluminum alloy
sheet according to the present aspect has a composition which
includes, in terms of mass %, Mg: 0.3 to 1.0%, Si: 0.5 to 1.5% and
Sn: 0.005 to 0.3%, with the remainder being Al and unavoidable
impurities. All the content indicated in % of the elements means
that in mass %. In this description, percentage on mass basis (mass
%) is the same as percentage on weight basis (wt %). With respect
to the content of a chemical component, there are cases where "X %
or less (exclusive of 0%)" is expressed by "more than 0% and X % or
less".
[0092] In the present aspect, elements other than the Mg, Si and Sn
are impurities or elements which may be contained, and may have
contents (permissible amounts) on levels of the elements in
accordance with the AA or JIS standards, etc.
[0093] Namely, for the same reasons as in the first aspect,
inclusion of such other elements shown below is permissible in the
present aspect within the range of equal to or less than the upper
limits specified below, which are in accordance with the AA or JIS
standards or the like.
[0094] Specifically, the aluminum alloy sheet may further contain
one kind or two or more kinds selected from the group consisting of
Fe: 1.0% or less (exclusive of 0%), Mn: 0.4% or less (exclusive of
0%), Cr: 0.3% or less (exclusive of 0%), Zr: 0.3% or less
(exclusive of 0%), V: 0.3% or less (exclusive of 0%), Ti: 0.1% or
less (exclusive of 0%), Cu: 0.4% or less (exclusive of 0%), Ag:
0.2% or less (exclusive of 0%), and Zn: 1.0% or less (exclusive of
0%), within those ranges, in addition to the basic composition
shown above.
[0095] In the cases where these elements are contained, the content
of Cu is preferably 0.3% or less, because Cu is prone to impair the
corrosion resistance when contained in a large amount. Mn, Fe, Cr,
Zr, and V are prone to yield relatively coarse compounds when
contained in large amounts, and are prone to impair the hem
workability (hem bendability), which is addressed by the present
aspect. Consequently, the content of Mn is preferably 0.35% or
less, and the content of each of Cr, Zr and V is preferably 0.2% or
less and more preferably 0.1% or less.
[0096] The content range of each element and the purposes and
permissible amount thereof in the 6000-series aluminum alloy are
explained below in order.
Si: 0.5 to 1.5%
[0097] Si, together with Mg, is an essential element for obtaining
the strength (proof stress) required as automotive panels by
forming aging precipitates which contribute to an improvement in
strength, during an artificial aging treatment such as a baking
treatment, and thus exhibiting an age hardenability. Furthermore,
solute Si is an element that improves the work hardenability, and
Si, when present in a solid-solution state, has the effect of
lowering the yield ratio, which is a ratio between tensile strength
and yield strength [(0.2% proof stress)/(tensile strength)], less
than 0.50.
[0098] In the case where the content of Si is too low, the amount
of precipitates after an artificial age hardening treatment is too
small, resulting in a decrease in the strength increase due to
baking and resulting in a reduced amount of solute Si and hence in
too large a yield ratio exceeding 0.50. Meanwhile, in the case
where the content of Si is too high, the Si forms coarse crystals
with impurity Fe, etc., resulting in a considerable decrease in
formability such as bendability. In addition, too high Si contents
increases not only the strength just after sheet production but
also the room-temperature aging amount after the production,
thereby increases the strength before forming too much, and reduces
the formability into automotive panels or the like, which are
particularly problematic in face strains thereof, in automotive
panel structures. Consequently, the content of Si is regulated so
as to be in the range of 0.5 to 1.5%.
[0099] For attaining an excellent age hardenability in a baking
treatment performed at a lower temperature for a shorter period
after forming into panels, it is preferable to employ a 6000-series
aluminum alloy composition in which Si/Mg is 1.0 or larger in terms
of mass ratio so that Si has been incorporated further excessively
relative to the Mg than in the so-called excess-Si type.
Mg: 0.3 to 1.0%
[0100] Mg, together with Si, is also an important element for
forming the atom aggregates specified in the present aspect. It is
an essential element for obtaining the proof stress required as
panels by forming, together with the Si, aging precipitates which
contribute to an improvement in strength, during an artificial
aging treatment such as a baking treatment, and thus exhibiting an
age hardenability. Furthermore, solute Mg, like Si, is an element
that improves the work hardenability, and Mg, when present in a
solid-solution state, has the effect of lowering the yield ratio,
which is a ratio between tensile strength and yield strength [(0.2%
proof stress)/(tensile strength)], to less than 0.50.
[0101] In the case where the content of Mg is too low, the amount
of precipitates after an artificial age hardening treatment is too
small, resulting in a decrease in the strength increase due to
baking and resulting in a reduced amount of solute Mg and hence in
too large a yield ratio exceeding 0.50. Meanwhile, in the case
where the content of Mg is too high, the Mg forms coarse crystals
with impurity Fe, etc., resulting in a considerable decrease in
formability such as bendability. In addition, too high Mg contents
increases not only the strength just after sheet production but
also the room-temperature aging amount after the production,
thereby increases the strength before forming, and reduces the
formability into automotive panels or the like, which are
particularly problematic in face strains thereof, in automotive
panel structures. Consequently, the content of Mg is regulated so
as to be in the range of 0.3 to 1.0%.
Sn: 0.005 to 0.3%
[0102] Sn has the important effect of attaining both an increase in
BH response and a reduction in yield ratio by reducing the volume
proportion of atom aggregates which enhance the 0.2% proof stress
in panel forming, even when the solid-solution amount of Mg+Si,
which will be described later, is increased. In general, for
increasing the solid-solution amount of Mg+Si, it is effective to
increase the amount of Mg and/or Si to be contained in the sheet.
However, such increase in Mg and Si contents in the sheet result
not only in an increase in 0.2% proof stress in panel forming but
also in an increase in the volume proportion of atom aggregates
which inhibit yield ratio reduction. It has hence been difficult,
with any of conventional compositions or production processes, to
attain all of an increase in BH response, a reduction in proof
stress and a reduction in yield ratio. In contrast, according to
the present aspect, atom aggregates that inhibit yield ratio
reduction can be diminished even when the solid-solution amount of
Mg+Si is increased to enhance the BH response, by incorporating Sn
in an amount within the range shown above. Thus, all of an increase
in BH response, a reduction in proof stress, and a reduction in
yield ratio can be attained.
[0103] Sn, at room temperature, has the effect of capturing
(trapping) atomic holes to thereby inhibit room-temperature
diffusion of Mg and Si and inhibit the strength increase at room
temperature (room-temperature age hardening), and during the
forming of the sheet into panels, improving the press formability
including hem workability, drawability and punch stretch
formability (hereinafter, this press formability is referred to
also as hem workability as a representative). During an artificial
aging treatment of the panels, such as a baking treatment, it
releases the captured holes and hence in turn enhances the
diffusion of Mg and Si, thereby enhancing the BH response.
[0104] In the case where the content of Sn is lower than 0.005%,
the effects described above, i.e., the effect of lowering the
volume proportion of atom aggregates that inhibit yield ratio
reduction, even when the solid-solution amount of Mg+Si is
increased, and thereby attaining both an increase in BH response
and a reduction in yield ratio and the effect of suppressing
room-temperature age hardening. Meanwhile, in the case where the
content of Sn is higher than 0.3%, the Sn segregates at grain
boundaries and it is prone to cause intergranular cracks. A
preferred lower limit of the content of Sn is 0.01%. An upper limit
of the content of Sn is preferably 0.2%, more preferably 0.1% and
further preferably 0.06%
(Solid-Solution Amount Mg and Si)
[0105] The composition described above is employed. Furthermore, in
the present aspect, the total solid-solution amount of Mg and Si
contained in the sheet (solid-solution amount of Mg+Si) is
increased and ensured so as to be in a specified range of 1.0 mass
% or more and 2.0 mass % or less, in order to enhance the BH
response. In the case where the solid-solution amount of Mg+Si is
less than 1.0 mass %, the BH response cannot be ensured even when
the composition described above is employed. The larger the
solid-solution amount of Mg+Si, the more the BH response improves.
However, due to the composition and production described above,
there are limitations on the contents and solid-solution amount of
Mg and Si. In addition, too high amount of solid-solution pose a
problem in that the volume proportion of the atom aggregates
increases to result in increases in proof stress and yield ratio
during panel forming. An upper limit thereof is hence 2.0 mass
%.
[0106] The solid-solution amount of Mg+Si in a sheet is measured by
dissolving a sample of the sheet to be examined, by a residue
extraction method using hot phenol, separating the solid/liquid by
filtration with a filter having a mesh of 0.1 .mu.m, and regarding
the total content of Mg and Si in the separated solution as the
solid-solution amount of Mg+Si.
[0107] The residue extraction method with hot phenol is performed
specifically in the following manner. First, phenol is introduced
into a decomposition flask and heated, and each sheet sample to be
examined is then transferred to this decomposition flask and
decomposed by heating. Subsequently, benzyl alcohol is added
thereto, followed by performing a suction filtration with the
filter, thereby separating the solid/liquid by filtration. The
solution separated is quantitatively analyzed to determine the
total content of Mg and Si therein. For this quantitative analysis,
use is suitably made of atomic absorption spectrometry (AAS),
inductively coupled plasma optical emission spectroscopy (ICPOES)
or the like. For the suction filtration, a membrane filter having a
diameter of 47 mm and having a mesh (capture particle diameter) of
0.1 .mu.m is used as stated above. This examination and a
calculation are made with respect to three samples obtained from
total of three portions, i.e., one central portion in the
sheet-width direction of the test sheet and two portions located
respectively at both ends of the sheet-width-direction from the
central portion. The solid-solution amount of Mg+Si (mass %) of
these samples are averaged.
(Aggregates of Atoms)
[0108] The composition and structure described above are employed.
In addition, in the present aspect, the structure of the
6000-series aluminum alloy sheet is regulated in the size
distribution of aggregates of Mg and Si atoms observed with a
three-dimensional atom probe filed ion microscope, in order to
reduce a yield ratio to less than 0.50 and to ensure BH response.
Thus, both an increase in BH response and a reduction in yield
ratio are attained not only by the effects of the Sn but also by
regulating the atom aggregates (clusters) present in the structure
of the sheet.
(Definition of Aggregate of Atoms)
[0109] In the present aspect, one which satisfies some conditions
(requirements) specified through an examination and analysis based
on the principle of three-dimensional atom probe field ion
microscope is defined as an aggregate of atoms, as described in the
section Effects. Specifically, defined as an aggregate of atoms is
one which satisfies some conditions (requirements) specified in the
present aspect with respect to a three-dimensional structure of
atoms (three-dimensional atom map) obtained by a reconstruction
through analysis from the flight times and positions of atoms of
the sheet which have temporarily ionized in a high electric field
(electric-field evaporation) with a three-dimensional atom probe
field ion microscope.
[0110] Consequently, the aggregates of atoms specified in the
present aspect are not real atom aggregates (clusters) exist in
6000-series aluminum alloy sheets, such as ones observed by
directly examining the structure of a sheet as such with a
high-magnification TEM (transmission electron microscope) as in
Patent Document 1. However, they correlate deeply with the state in
which the real atom aggregates (clusters) exist in 6000-series
aluminum alloy sheets, such as ones directly observed with a
high-magnification TEM. Because of this, even if the examination of
atom aggregates in the present aspect is indirect or simulative,
the atom aggregates satisfactorily correlate with the state in
which those real atom aggregates (clusters) exist, the state
considerably affecting a reduction in yield ratio and an increase
in BH response. It hence provides a measure for ensuring a
reduction in yield ratio and an increase in BH response by means of
the structure (atom aggregates).
[0111] The sheet to be examined here is a 6000-series aluminum
alloy sheet which has undergone refining such as a solution
treatment and a quenching treatment and which has undergone neither
press forming nor a bake hardening treatment. The structure of
arbitrary central portion in the sheet-thickness direction of this
sheet is examined with a three-dimensional atom probe field ion
microscope.
(Requirements which the Atom Aggregates should Meet)
[0112] Requirements (prerequisites) for being defined (regarded) as
atom aggregates in the present aspect are explained below.
[0113] The requirements which the atom aggregates in the present
aspect should meet are the same as in Patent Documents 2 and 3.
First, either or both of an Mg atom and an Si atom are contained by
a total of 10 pieces or more. Although an upper limit on the number
of the Mg atom and/or Si atom included in the atom aggregate is not
particularly determined, an upper limit of the number of the Mg
atom and/or Si atom included in the atom aggregate is about 10,000
in view of limitations on production.
[0114] Furthermore, the ones are regarded as atom aggregates, in
which 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 distance therebetween of 0.75 nm is a value which
has been experimentally fixed in order that atom aggregates in each
of which the distance between atoms of Mg and/or Si is short and
which each have a size that considerably affects a reduction in
yield ratio and an increase in BH response, and the volume
proportion thereof are specified with satisfactory reproducibility,
although the technical meaning of the value has not been fully
elucidated.
[0115] The atom aggregates specified in the present aspect mostly
are ones each including both of Mg atom and Si atom. However, they
may include one which includes Mg atoms but contains no Si atom and
one which includes Si atoms but contains no Mg atom. Furthermore,
they need not to be composed of Mg atoms and/or Si atoms only, and
it is highly probable that Al atoms are contained besides
these.
[0116] Moreover, depending on the component composition of the
aluminum alloy sheet, there inevitably are cases where atoms
contained as alloying elements or impurities, such as Fe, Mn, Cu,
Cr, Zr, V, Ti, Zn, Ag, etc., are contained in the atom aggregates
and those other atoms are counted by the 3DAP analysis. However,
even in the cases when such other atoms (derived from alloying
elements or impurities) are contained in the atom aggregates, they
are on a low level as compared with the total number of Mg atoms
and Si atoms. Consequently, even in the cases when such other atoms
are contained in the atom aggregates, those meet the limitations
(requirements) function as the atom aggregates according to the
present aspect like the atom aggregates composed of Mg atoms and/or
Si atoms only. Thus, the atom aggregates specified in the present
aspect may contain any other atoms so long as they satisfy the
limitations.
[0117] The wording "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" means that each of all the Mg atoms and
Si atoms present in each aggregate of atoms has at least one Mg
atom or Si atom therearound within a distance of 0.75 nm or
less.
[0118] In the limitation on the distance between atoms in the atom
aggregates according to the present aspect, when any atom of the Mg
atom and the Si atom contained therein is used as a reference, each
of all the distances between the atom as the reference and all
atoms among other atoms adjacent thereto may not be 0.75 nm or
less, and on the contrary, each of them all may be 0.75 nm or less.
In other words, other Mg atom or Si atom may be adjacent at a
distance exceeding 0.75 nm, and it is sufficient that in the
periphery of a specific Mg atom or Si atom (serving as a
reference), at least one Mg atom or Si atom is present which
satisfies the specified distance (spacing).
[0119] In the case where there is one adjacent other Mg atom or Si
atom which satisfies the specified distance, the number of Mg atom
and/or Si atom which satisfy the requirement concerning distance
and which should be counted is 2, including the specific Mg atom or
Si atom (serving as a reference). Meanwhile, in the case where
there are two adjacent other Mg atoms and/or Si atoms which satisfy
the specified distance, the number of Mg atoms and/or Si atoms
which satisfy the requirement concerning distance and which should
be counted is 3, including the specific Mg atom or Si atom (serving
as a reference).
(Regulation of the Atom Aggregates)
[0120] First, in the present aspect, as the total volume of atom
aggregates which satisfy the given requirements explained above,
including the number of Mg atoms and/or Si atoms and the distance
between atoms, the total volume .SIGMA.Vi is determined by summing
up the volumes of the individual atom aggregates Vi
(=4/3.pi.r.sub.G.sup.3) calculated from the Guinier radii r.sub.G
of the individual atom aggregates each regarded as a sphere. Then,
the average volume proportion of this total volume .SIGMA.Vi to the
volume V.sub.Al of the aluminum alloy sheet measured with the
three-dimensional atom probe field ion microscope,
(.SIGMA.Vi/V.sub.Al).times.100, is regulated so as to be in the
range of 0.3 to 1.5%.
[0121] Furthermore, in the present aspect, in addition to the
regulation of the volume proportion of atom aggregates, the average
volume proportion of the total volume .SIGMA.Vi.sub.1.5 or more,
which is the total volume of atom aggregates each having the
Guinier radius r.sub.G of 1.5 nm or larger among the atom
aggregates satisfying those requirements, to the total volume of
the atom aggregates .SIGMA.Vi, (.SIGMA.Vi.sub.or more/.SIGMA.Vi),
is regulated so as to be in the range of 20 to 70%. Namely, the
individual atom aggregates which each satisfy the requirements are
divided at a Guinier radius r.sub.G of 1.5 nm, and the average
volume proportion of the total volume .SIGMA.Vi.sub.1.5 or more
obtained by summing up the volumes V.sub.1.5 or more of the
individual atom aggregates each having a Guinier radius r.sub.G of
1.5 nm or larger to the total volume of the atom aggregates V,
(.SIGMA.Vi.sub.1.5 or more/.SIGMA.Vi).times.100, is regulated so as
to be in the range of 20 to 70%.
[0122] Guinier radius r.sub.G is determined in the following
manner. The individual atom aggregates which each satisfy the
requirements are each regarded as a sphere, and the largest of
values of radius of gyration I.sub.g of each atom aggregate is
taken as the radius of gyration I.sub.g of the atom aggregate. The
Guinier radius r.sub.G is a radius obtained by converting this
radius of gyration I.sub.g by using the equation which will be
described later. The definition of Guinier radius and the method
for calculation thereof which will be described later are known by
Patent Documents 2 and 3.
[0123] Due to those structure regulations in combination with the
compositional regulation, it is possible to make the 6000-series
aluminum alloy sheet have, in automotive-panel forming, a 0.2%
proof stress reduced to 110 MPa or less and a yield ratio reduced
to less than 0.50 and further have a 0.2% proof stress after BH of
190 MPa or greater.
[0124] In the case where the average volume proportion of atom
aggregates which satisfy those requirements,
(.SIGMA.Vi/V.sub.Al).times.100, is less than 0.3%, the absolute
number of relatively large atom aggregates having a Guinier radius
r.sub.G of 1.5 nm or larger and effective for an increase in BH
response and a reduction in yield ratio is insufficient. Because of
this, even when the composition is satisfied, it is impossible to
attain the increase in BH response and reduction in yield ratio.
Meanwhile, in the case where the average volume proportion
(.SIGMA.Vi/V.sub.Al).times.100 exceeds 1.5%, the number of atom
aggregates which satisfy the requirements, including the distance
between atoms being 0.75 nm or less, is too large, making it
impossible to attain reduction in 0.2% proof stress and reduction
in yield ratio in panel forming.
[0125] Furthermore, also in the case where the average volume
proportion (.SIGMA.Vi.sub.1.5 or more/.SIGMA.Vi).times.100 of
relatively large atom aggregates having a Guinier radius r.sub.G of
1.5 nm or larger and effective for an increase in BH response and a
reduction in yield ratio is less than 20%, the absolute number of
these atom aggregates is insufficient and a reduction in yield
ratio cannot be attained even when the composition is satisfied or
even when the average volume proportion of atom aggregates which
satisfy those requirements satisfies the limitation. Meanwhile, the
larger the number or proportion of relatively large atom aggregates
having a Guinier radius r.sub.G of 1.5 nm or larger, the easier the
attainment of a reduction in yield ratio. However, it is difficult,
from the standpoint of production, to increase the average volume
proportion (.SIGMA.Vi.sub.1.5 or more/.SIGMA.Vi).times.100 beyond
70%, and this 70% is determined as an upper limit in view of
limitations on production.
(Principle of Measurement with 3DAP and Method of the Measurement
Therewith)
[0126] The principle of a measurement with a 3DAP and a method of
the measurement therewith are also known from Patent Documents 1 to
3. The 3DAP (three-dimensional atom probe) is configured of a field
ion microscope (FIM) and a time-of-flight mass spectrometer
attached thereto. Due to such configuration, this is a local
analyzer in which individual atoms in a metal surface can be
observed with the field ion microscope and these atoms can be
identified by time-of-flight mass spectrometry. Furthermore, since
the 3DAP can simultaneously analyze the kinds and positions of
atoms emitted from a sample, it can be an exceedingly effective
means for analyzing the structure of atom aggregates. Because of
this, it is used as a known technique for, for example, structural
analysis of magnetic recording films, electronic devices, steel
materials or the like, as stated above. Recently, it is used for
determination or the like of atom aggregates in the structure of an
aluminum alloy sheet, as described above.
[0127] The 3DAP utilizes the phenomenon called electric-field
evaporation, in which atoms of a sample themselves are ionized in a
high electrical field. When a high voltage necessary for causing
atoms of a sample to undergo electric-field evaporation is applied
to the sample, atoms are ionized from the sample surface, and they
pass through the probe hole and reach a detector.
[0128] This detector is a position sensitive detector, which
performs mass spectrometry for individual ions (identification of
elements that are kinds of atom) and measures the time of flight of
each ion to the detector and which can thereby simultaneously
determine the positions detected (atom structure positions).
Consequently, the 3DAP can simultaneously measure the positions and
atom kinds of atoms present at the tip of the sample, and hence has
the feature of being able to three-dimensionally reconstitute and
observe the structure of atoms present in the tip of the sample. In
addition, because electric-field evaporation takes place in order
from the surface of the sample tip, the depth-direction
distribution of atoms from the sample tip can be examined with
atomic-level resolution.
[0129] Since the 3DAP utilizes a high electric field, the sample to
be analyzed is required to have high electroconductivity, like
metals, etc., and the shape of the sample is generally required to
be an ultrafine needle shape having a tip diameter of about 100 nm
or less. Because of this, a sample is taken from, for example, a
central portion in a sheet-thickness direction of an aluminum alloy
sheet to be examined, and this sample is cut with a precise cutting
device and electropolished to produce a sample for analysis which
has an ultrafine needle-shaped tip portion. A measuring method is
as follows. "LEAP 3000", manufactured by Imago Scientific
Instruments Corp., is, for example, used, and a high-pulse voltage
on the order of 1 kV is applied to the aluminum alloy sheet sample
having a tip formed in a needle-shape, thereby continuously ionize
millions of atoms from the sample tip. The ions are detected by the
position sensitive detector. Mass spectrometry of each ion
(identification of the element that is the kind of atom) is
conducted on the basis of the time of flight from the emission of
the individual ion from the sample tip, which is caused by the
pulse-voltage application, to the arrival at the detector.
[0130] Furthermore, the feature in which the electric-field
evaporation takes place regularly in order form the surface of the
sample tip is utilized to suitably give a depth-direction
coordinate to a two-dimensional map which shows ion arrival sites,
and analysis software "IVAS" is used to conduct a three-dimensional
mapping (construction of three-dimensional structure of atoms: atom
map). Thus, a three-dimensional atom map for the sample tip can be
obtained.
[0131] This three-dimensional atom map is further processed by a
maximum separation method, which is a method for defining atoms
belonging to a precipitate or to an atom aggregate, to analyze
aggregates of atoms (atom aggregates). In this analysis, the number
of either or both of Mg atom and Si atom (ten pieces or more in
total), the distance (spacing) between adjacent Mg atoms and/or Si
atoms, and the number of Mg atoms and/or Si atoms which have the
specific narrow spacing (0.75 nm or less) are given as
parameters.
[0132] Then, atom aggregates which satisfy 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, are defined as atom aggregates
according to the present aspect. In addition, the dispersion state
in which atom aggregates according to the definition is evaluated,
and the number density of atom aggregates is quantified by
examining three or more samples and averaging the measured values
in terms of average density per 1 m.sup.3 (pieces/m.sup.3).
[0133] That is, a maximum radius of gyration I.sub.g when each of
the atom aggregates being examined is regarded as a sphere by 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##
[0134] In the formula of Math. 1, I.sub.g represents a radius of
gyration automatically calculated by the software specific to the
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. x.sub.i, y.sub.i and z.sub.i
respectively represent the lengths of the x axis, y axis and z
axis, and are spacial coordinates for the Mg atoms and/or Si atoms
which constitute the atom aggregate. "x bar" and the like in which
"-" is placed on the top of each of "x", "y" and "z" also represent
the lengths of the x, y and z axes, but are barycentric coordinates
for the atom aggregate. n represents the number of Mg atoms and/or
Si atoms which constitute the atom aggregate.
[0135] Next, a maximum of the radius of gyration I.sub.g of each of
the individual atom aggregates is taken as the radius of gyration
I.sub.g of the atom aggregate, and converted to a Guinier radius
r.sub.G by using the relationship r.sub.G= (5/3)I.sub.g of the
following formula of Math. 2. This Guinier radius r.sub.G obtained
by the conversion is regarded as the radius of the atom
aggregate.
r G = 5 3 l g [ Math . 2 ] ##EQU00002##
[0136] On the basis of this, the volumes Vi (=4/3.pi.r.sub.G.sup.3)
of the individual atom aggregates which satisfy those requirements
are summed up to determine the total volume .SIGMA.Vi. Meanwhile,
the volume of the needle-shaped sample which has undergone the
electric-field evaporation (i.e., which has disappeared due to
electric-field evaporation) is taken as the volume V.sub.Al of the
aluminum alloy sheet measured with the three-dimensional atom probe
field ion microscope, and the average volume proportion of the
total volume of the atom aggregates thereto,
(.SIGMA.Vi/V.sub.Al).times.100, is determined. Furthermore, the
average volume proportion of the total volume .SIGMA.Vi.sub.1.5 or
more of atom aggregates each having a Guinier radius r.sub.G of 1.5
nm or larger to the total volume V of the atom aggregates,
(.SIGMA.Vi.sub.1.5 or more/.SIGMA.Vi).times.100, is also
determined. The measurement of each average volume proportion of
atom aggregates with the 3DAP is made on arbitrary ten regions of
central portions in the sheet-thickness direction in the
6000-series aluminum alloy sheet which has undergone the refining,
and the measured values (calculated values) thereof are
averaged.
[0137] The calculation formula for calculating the radius of an
atom aggregate and the methods for measuring and converting the
radius of gyration I.sub.g for Guinier radius r.sub.G are based on
quotations from M. K. Miller: Atom Probe Tomography, (Kluwer
Academic/Plenum Publishers, New York, 2000), p. 184. Calculation
formulae for the radius of an atom aggregate are described in many
documents other than this. For example, "(2) Three-dimensional Atom
Probe Analysis" on 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
including the formula of Math. 1 and the formula for conversion to
Guinier radius r.sub.G (in this document, however, the symbol of
the radius of gyration I.sub.g is described as r.sub.G).
(Efficiency of Atom Detection by 3DAP)
[0138] Currently, the efficiency of the detection of atoms by the
3DAP is about 50% at the most with respect to the ionized atoms,
and the remaining atoms cannot be detected. In the cases when the
efficiency of the detection of atoms by the 3DAP changes
considerably due to, for example, an improvement in the future,
there is a possibility that the results of measurements, with a
3DAP, of the average number density (pieces/.mu.m.sup.3) of each of
atom aggregates of the sizes specified in the present aspect may
vary. Consequently, from the standpoint of conducting the
measurements with reproducibility, it is preferable that the
efficiency of the detection of atoms with a 3DAP should be kept
approximately constant at about 50%.
(Production Process)
[0139] Next, a process for producing the aluminum alloy sheet
according to the present aspect is explained. The aluminum alloy
sheet according to the present aspect is produced through
production steps which themselves are common or known, by
subjecting, after casting, an aluminum alloy slab having the
6000-series component composition to a homogenizing heat treatment,
hot rolling and cold rolling to obtain a given sheet thickness,
followed by a refining treatment such as a solution quenching
treatment.
[0140] However, for obtaining the structure including the atom
aggregates specified with a 3DAP according to the present aspect,
during those production steps, the average cooling rate in a
quenching treatment after a solution treatment is controlled and in
addition, the conditions for a preliminary aging treatment after
the quenching treatment are regulated so as to be in a preferred
range, as will be described later. With respect to other steps,
there are preferred conditions for obtaining the structure
specified in the present aspect. Unless such preferred conditions
are employed, it is difficult to obtain the structure according to
the present aspect.
(Melting and Casting Cooling Rate)
[0141] First, in melting and casting steps, an aluminum alloy
molten metal that has been melted and regulated so as to have a
component composition within the 6000-series composition range is
cast by a suitably selected ordinary melting and casting method,
such as a continuous casting method or a semi-continuous casting
method (DC casting method). Here, it is preferable that the average
cooling rate, during the casting, from the liquidus temperature to
the solidus temperature is as high (quick) as possible at
30.degree. C./min or greater.
[0142] In the 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 is inevitably low. When
an average cooling rate in the high-temperature range is low as the
above, the amount of crystals yielded coarsely in the temperature
range of this high-temperature range is increased and also
unevenness in the size and amount of the crystals along the width
direction and thickness direction of the slab is increased. As a
result, the basic mechanical properties, such as strength and
elongation, which are a prerequisite for the 6000-series aluminum
alloy sheet are reduced.
(Homogenizing Heat Treatment, Hot Rolling, Annealing of Hot-Rolled
Plate, Cold Rolling, and Solution and Quenching Treatments)
[0143] Subsequently, the aluminum alloy slab obtained by casting is
subjected to the treatments of a homogenizing heat treatment, hot
rolling, annealing of the hot-rolled plate (according to need),
cold rolling, and solution and quenching treatments in the same
manners as in the first aspect. The conditions for these treatments
are the same as in the first aspect, and explanations thereon are
omitted here.
(Preliminary Aging Treatment: Reheating Treatment)
[0144] After having thus undergone the solution treatment and the
subsequent quenching treatment to be cooled to room temperature,
the cold-rolled sheet is subjected to a preliminary aging treatment
(reheating treatment) within a period which is as short as possible
and is up to 1 hour (60 minutes).
[0145] In the case where the room-temperature holding period from
termination of the quenching treatment to room temperature to
initiation of the preliminary aging treatment (initiation of
heating) is too long and exceeds 1 hour, it becomes impossible to
regulate the total volume of atom aggregates to 1.5% or less in
terms of average volume proportion, the atom aggregates satisfying
the requirements concerning the number of Mg atoms and/or Si atoms
and the distance between atoms. In addition, relatively large
clusters are less apt to be yielded, making it impossible to
increase the average volume proportion of atom aggregates each
having a Guinier radius r.sub.G of 1.5 nm or larger to the atom
aggregates which satisfy the requirements to 20% or higher. As a
result, the BH response decreases, and a reduction in yield ratio
is also difficult. Consequently, the shorter the room-temperature
holding period, the better. The solution and quenching treatments
and the reheating treatment may be consecutively performed so that
there is substantially no pause therebetween, and a lower limit of
the period is not particularly determined.
[0146] In this preliminary aging treatment, the rate of temperature
increase to a preliminary-aging temperature and the period of
holding in a preliminary-aging temperature range are regulated. It
is preferable that the temperature increase rate, of these, should
be as high (quick) temperature increase rate as possible at
1.degree. C./s or higher, preferably 5.degree. C./s or higher, for
suppressing the formation of small atom aggregates not contributing
to strength. In the case where the temperature increase rate is
less than 1.degree. C./s, small atom aggregates not contributing to
strength are yielded in a large amount, making it impossible to
increase the average volume proportion of atom aggregates each
having a Guinier radius r.sub.G of 1.5 nm or larger to the atom
aggregates which satisfy the requirements to 20% or higher. As a
result, the BH response decreases, and a reduction in yield ratio
is also difficult.
[0147] The temperature and holding period in the preliminary aging
treatment is a holding in a temperature range of 60 to 120.degree.
C. for 10 hr or more and 40 hr or less. Here, the holding in the
temperature range of 60 to 120.degree. C. may be a heat treatment
in which the temperature is constant or the temperature is
sequentially changed by temperature increase or annealing, within
that temperature range. In short, the temperature may be
continuously changed by annealing, temperature increase, etc., so
long as it is held in a temperature range of 60 to 120.degree. C.
for that period of 10 hr or more and 40 hr or less.
[0148] In the case where the preliminary-aging temperature is lower
than 60.degree. C. or the holding period is less than 10 hr, the
formation of precipitate nuclei is insufficient, making it
impossible to increase the average volume proportion of atom
aggregates each having a Guinier radius r.sub.G of 1.5 nm or larger
to the atom aggregates which satisfy the requirements to 20% or
higher. As a result, the BH response decreases.
[0149] Meanwhile, in the case where the preliminary-aging
temperature exceeds 120.degree. C. or the holding period exceeds 40
hr, precipitate nuclei are yielded in too large an amount in this
preliminary aging treatment. Because of this, the amount of atom
aggregates having a relatively large size and contributing to
strength decreases. As a result, the average volume proportion of
atom aggregates which satisfy the requirements increases beyond
1.5%, making it impossible to enable the sheet in forming to have a
yield ratio reduced to less than 0.50.
[0150] Namely, unless the preliminary aging treatment is regulated
to fall within these preferable conditions, it is difficult to
produce a sheet which has, in automotive-panel forming, a 0.2%
proof stress reduced to 110 MPa or less and a yield ratio reduced
to less than 0.50 and further has a 0.2% proof stress after BH of
190 MPa or greater.
EXAMPLES
[0151] The present invention will be explained below in more detail
by reference to Examples. However, the present invention should
not, of course, be construed as being limited by the following
Examples, and can be suitably modified unless the modifications
depart from the gist of the present invention described hereinabove
and hereinafter. All such modifications are included in the
technical range of the present invention.
Examples According to the First Aspect
[0152] Next, Examples according to the first aspect of the present
invention are explained. 6000-series aluminum alloy sheets were
individually produced so as to differ in the structure specified
with a DSC in the present aspect, by changing the conditions for a
preliminary aging treatment performed after solution and quenching
treatments. After a holding at room temperature for 30 days after
the production of the sheets, BH response (bake hardenability), As
proof stress as an index of press formability and hem workability
as bendability are examined and evaluated.
[0153] For individually producing the structure specified with a
DSC, the 6000-series aluminum alloy sheets having the compositions
shown in Table 1 was produced by variously changing conditions such
as the average cooling rate in the quenching treatment after a
solution treatment and the temperature and holding period in the
subsequent preliminary aging treatment as shown in Tables 2 and 3.
With respect to the indications of the contents of elements within
Table 1, a value of the element expressed by a blank indicates that
the content is below a detection limit.
[0154] Specific conditions for aluminum alloy sheet production were
as follows. Slabs of aluminum alloys respectively having the
compositions shown in Table 1 were commonly produced through
casting by the DC casting method. In this casting, the average rate
of cooling from the liquidus temperature to the solidus temperature
was set at 50.degree. C./min in common with all the Examples.
Subsequently, the slabs were subjected to a soaking treatment of
540.degree. C..times.6 hours performed in one stage only, followed
by initiation of hot rough rolling at that temperature, in common
with all the Examples. Thereafter, they were hot-rolled, in the
succeeding finish rolling, to a thickness of 3.5 mm to obtain
hot-rolled sheets, in common with all the Examples. The hot-rolled
aluminum alloy sheets were subjected to rough annealing of
500.degree. C..times.1 minute and then to cold rolling at a
processing rate of 70% without performing intermediate annealing
during the cold-rolling passes, to obtain cold-rolled sheets having
a thickness of 1.0 mm, in common with all the Examples.
[0155] Furthermore, the cold-rolled sheets were each continuously
subjected to a refining treatment (T4) with continuous type heat
treatment facilities while unwinding and winding each sheet, in
common with all the Examples. Specifically, a solution treatment
was performed by heating at an average rate of heating to
500.degree. C. of 10.degree. C./sec and holding for seconds after
the temperature reached a target temperature of 560.degree. C.,
followed by cooling to room temperature by water cooling or air
cooling so as to result in the average cooling rates shown in
Tables 2 and 3. After this cooling and after the subsequent
required periods shown in Table 2 at room temperature, a
preliminary aging treatment was performed by using an atmospheric
furnace and an oil bath and using the temperature increase rates,
reached temperatures, average cooling rates, and holding periods
shown in Tables 2 and 3. As for the cooling after this preliminary
aging treatment, water cooling or gradual cooling (natural cooling)
was conducted in order to change the average rate of cooling.
[0156] From the final product sheets which each had been allowed to
stand at room temperature for 30 days after the refining treatment,
test sheets (blanks) were cut out and the DSC and properties of the
test sheets were examined and evaluated. The results thereof are
shown in Table 3.
(DSC)
[0157] The structure in each of ten portions of the central portion
in the sheet-thickness direction in each test sheet was examined
for the DSC. In the DSC (differential scanning calorimetry curves)
of this sheet, as for the average value for these ten portions, the
peak height (W/mg) of an endothermic peak in the temperature range
of 150 to 230.degree. C. as an endothermic peak corresponding to
the dissolution of Mg--Si clusters not contributing to strength and
the peak height (.mu.W/mg) of an exothermic peak in the temperature
range of 240 to 255.degree. C. as an exothermic peak corresponding
to the formation of Mg--Si clusters contributing to strength were
determined.
[0158] The differential thermal analysis of each of the measurement
portions in each test sheet was performed under the same conditions
including a test apparatus of DSC220G, manufactured by Seiko
Instruments Inc., a reference substance of aluminum, a sample
container made of aluminum, temperature increase conditions of
15.degree. C./min, an atmosphere of argon (50 mL/min), and a sample
weight of 24.5 to 26.5 mg. The differential thermal analysis
profile (.mu.W) obtained was divided by the sample weight and
thereby normalized (.mu.W/mg). Thereafter, in the range of 0 to
100.degree. C. in the differential thermal analysis profile, a
region where the differential thermal analysis profile was
horizontal was taken as a reference level of 0, and the height of
exothermic peak from the reference level was measured. The results
thereof are shown in Tables 2 and 3.
(Bake hardenability)
[0159] The test sheets which had been allowed to stand at room
temperature for 30 days after the refining treatment were each
examined for 0.2% proof stress (As proof stress) as a mechanical
property through a tensile test. Furthermore, these test sheets
were aged at room temperature for 30 days, subsequently subjected
to an artificial age hardening treatment of 170.degree. C..times.20
minutes (after BH), and then examined for 0.2% proof stress (proof
stress after BH) through a tensile test, in common with the test
sheets. The BH response of each test sheet was evaluated on the
basis of the difference between these 0.2% proof stresses (increase
in proof stress).
[0160] With respect to the tensile test, No. 5 specimens (25
mm.times.50 mmGL.times.sheet thickness) according to JIS Z2201 were
cut out of each sample sheet to perform the tensile test at room
temperature. Here, the tensile direction of each specimen was set
so as to be perpendicular to the rolling direction. The tensile
rate was set at 5 mm/min until the 0.2% proof stress and at 20
mm/min after the proof stress. The number N of examinations for
mechanical property was 5, and an average value therefor was
calculated. With respect to the specimens to be examined for the
proof stress after BH, a 2% pre-strain as a simulation of sheet
press forming was given to the specimens by the tensile tester,
followed by performing the BH treatment.
(Hem Workability)
[0161] Hem workability was evaluated only with respect to the test
sheets which had been allowed to stand at room temperature for 30
days after the refining treatment. In the test, strip-shaped
specimens having a width of 30 mm were used and subjected to
90.degree. bending at an inward bending radius of 1.0 mm with a
down flange. Thereafter, an inner plate having a thickness of 1.0
mm was nipped, and the specimen was subjected, in order, to pre-hem
working in which the bent part was further bent inward to
approximately 130.degree. and flat-hem working in which the bent
part was further bent inward to 180.degree. and the end portion was
brought into close contact with the inner plate.
[0162] The surface state, such as the occurrence of rough surface,
a minute crack or a large crack, of the bent part (edge bent part)
of the flat hem was visually examined and visually evaluated on the
basis of the following criteria. In the following criteria, ratings
of 0 to 2 are on an acceptable level, and ratings of 3 and larger
are unacceptable.
[0163] 0, no crack and no rough surface; 1, slight rough surface;
2, deep rough surface; 3, minute surface crack; 4, linearly
continued surface crack.
[0164] Invention Examples Nos. 0, 1, 8, and 13 in Table 2 and Nos.
16 to 24 in Table 3, which employ alloys Nos. 0 to 12 shown in
Table 1, each is not only within the component composition range
according to the present aspect and has been produced under
conditions within preferred ranges but also has undergone the
refining treatment, including the solution quenching treatment and
the preliminary aging treatment, under preferred conditions.
Because of this, these Invention Examples satisfy the DSC
requirements specified in the present aspect, as shown in Tables 2
and 3. That is, in the DSCs of these sheets, the endothermic peak
in the temperature range of 150 to 230.degree. C. as an endothermic
peak corresponding to the dissolution of Mg--Si clusters not
contributing to strength had a peak height of 8 .mu.W/mg or less,
while the exothermic peak in the temperature range of 240 to
255.degree. C. as an exothermic peak corresponding to the formation
of Mg--Si clusters contributing to strength had a peak height of 20
.mu.W/mg or larger.
[0165] As a result, the Invention Examples each show excellent BH
response although the bake hardening is performed after the
refining treatment and subsequent room-temperature aging and is a
treatment conducted at a low temperature for a short period of
time. Furthermore, as shown in Table 3, even after the refining
treatment and subsequent room-temperature aging, they each have a
relatively low As proof stress and hence show excellent press
formability into automotive panels or the like and excellent hem
workability. That is, the Invention Examples, even when having
undergone an automotive-baking treatment after room-temperature
aging, were able to exhibit not only high BH response with a 0.2%
proof stress difference of 100 MPa or greater and a 0.2% proof
stress after BH of 170 MPa or greater but also press formability
with an As 0.2% proof stress of 110 MPa or less and satisfactory
bendability.
[0166] In contrast, Comparative Examples 2 to 7, 9 to 13, 14, and
15 in Table 2, which employed alloy example 1, 2 or 3 in Table 1
like Invention Examples, each have the preliminary aging treatment
conditions outside the preferred ranges, as shown in Table 2. As a
result, they each gave a DSC which was outside the range specified
in the present aspect, and show enhanced room-temperature aging
and, in particular, a relatively high As proof stress after 30-day
room-temperature holding, as compared with the Invention Examples
having the same alloy composition. Because of this, they are poor
in press formability into automotive panels or the like and in hem
workability and are poor also in BH response.
[0167] In Comparative Example 2, the average cooling rate in the
quenching treatment to room temperature performed after the
solution treatment is too low. Because of this, the exothermic peak
B in the temperature range of 240 to 255.degree. C. has a peak
height as low (small) as less than 20 .mu.W/mg, although the
endothermic peak A in the temperature range of 150 to 230.degree.
C. has a peak height of 8 .mu.W/mg or less, showing that the number
density of Mg--Si clusters having a relatively large size and
contributing to strength is low. This is because due to the low
cooling rate in the quenching treatment to room temperature, coarse
Mg.sub.2Si and elemental Si were yielded during the cooling.
Neither the desired press formability with an As 0.2% proof stress
of 110 MPa or less nor satisfactory bendability is obtained. In
addition, the BH response is low.
[0168] In Comparative Examples 3 and 9, the period from the
quenching treatment to room temperature after the solution
treatment to the preliminary aging treatment (initiation of
heating) is too long. Because of this, Mg--Si clusters that are apt
to dissolve during temperature increase in DSC and do not
contribute to strength have been yielded in a large amount, and the
endothermic peak A in the temperature range of 150 to 230.degree.
C. has a peak height higher (larger) than 8 .mu.W/mg, as shown in
FIG. 1. Meanwhile, the exothermic peak B in the temperature range
of 240 to 255.degree. C. has a peak height which also is as high
(large) as 20 .mu.W/mg or more, showing that the number density of
Mg--Si clusters having a relatively large size and contributing to
strength is high. However, since the number density of Mg--Si
clusters having a relatively small size and not contributing to
strength is too high, the adverse influences thereof are too
greater. Therefore, the desired press formability with an As 0.2%
proof stress of 110 MPa or less and satisfactory bendability cannot
be obtained. In addition, the BH response is low.
[0169] In Comparative Examples 4 and 10, the temperature increase
rate in the preliminary aging treatment is too low. Because of
this, Mg--Si clusters that are apt to dissolve during temperature
increase in DSC and do not contribute to strength have undesirably
been yielded in a large amount, and the endothermic peak A in the
temperature range of 150 to 230.degree. C. has a peak height higher
(larger) than 8 .mu.W/mg, as shown in FIG. 1. Meanwhile, the
exothermic peak B in the temperature range of 240-255.degree. C.
has a peak height which also is as high (large) as 20 .mu.W/mg or
more, showing that the number density of Mg--Si clusters having a
relatively large size and contributing to strength is high.
However, since the number density of Mg--Si clusters having a
relatively small size and not contributing to strength is too high,
the adverse influences thereof are too greater. Therefore, the
desired press formability with an As 0.2% proof stress of 110 MPa
or less and satisfactory bendability cannot be obtained. In
addition, the BH response is low.
[0170] In Comparative Examples 5, 11 and 14, the period of holding
in the range of 60 to 120.degree. C. in the preliminary aging
treatment is 1 hour, which is too short. Because of this, Mg--Si
clusters that are apt to dissolve during temperature increase in
DSC and do not contribute to strength have been yielded in a large
amount, and the endothermic peak A in the temperature range of 150
to 230.degree. C. has a peak height higher (larger) than 8
.mu.W/nag, as shown in FIG. 1. Meanwhile, the exothermic peak B in
the temperature range of 240 to 255.degree. C. has a peak height
which also is as high (large) as 20 .mu.W/mg or more, showing that
the number density of Mg--Si clusters having a relatively large
size and contributing to strength is high. However, since the
number density of Mg--Si clusters having a relatively small size
and not contributing to strength is too high, the adverse
influences thereof are too greater. Therefore, the desired press
formability with an As 0.2% proof stress of 110 MPa or less and
satisfactory bendability cannot be obtained. In addition, the BH
response is low.
[0171] In Comparative Examples 6, 12 and 15, the period of holding
in the range of 60 to 120.degree. C. in the preliminary aging
treatment is 48 hours, which is too long. Because of this, the
exothermic peak B in the temperature range of 240 to 255.degree. C.
has a peak height as low (small) as less than 20 .mu.W/mg, showing
that the number density of Mg--Si clusters having a relatively
large size and contributing to strength is low. As a result, the
desired press formability with an As 0.2% proof stress of 110 MPa
or less and satisfactory bendability cannot be obtained. In
addition, the BH response is low.
[0172] In Comparative Example 7, the reached temperature in the
preliminary aging treatment is 130.degree. C., which exceeds the
upper limit of 120.degree. C. and is too high. Because of this, the
amount of Mg--Si clusters having a relatively large size and
contributing to strength has decreased, and the exothermic peak B
in the temperature range of 240 to 255.degree. C. thus has a peak
height as low (small) as less than 20 .mu.W/mg, showing that the
number density of the Mg--Si clusters having a relatively large
size and contributing to strength is low. As a result, the BH
response is low and the As 0.2% proof stress exceeds 110 MPa and is
too high, and press formability and satisfactory bendability cannot
be obtained, too.
[0173] Comparative Examples 25 to 34 in Table 3 have been produced
within preferred ranges, including the conditions for the
preliminary aging treatment. However, since they employed alloys
Nos. 13 to 22 shown in Table 1, the contents of Mg and Si, which
are essential elements, therein are outside the ranges according to
the present aspect or the content of impurity elements therein is
too high. Because of this, these Comparative Examples 24 to 33 each
show, in particular, a relatively high As proof stress after 30-day
room-temperature holding as compared with the Invention Examples,
as shown in Table 3, and hence are poor in press formability into
automotive panels or the like and in hem workability or are poor in
BH response.
[0174] Comparative Example 25 is alloy 13 shown in Table 1, in
which the Si content is too low.
[0175] Comparative Example 26 is alloy 14 shown in Table 1, in
which the Si content is too high.
[0176] Comparative Example 27 is alloy 15 shown in Table 1, in
which the Sn content is too low.
[0177] Comparative Example 28 is alloy 16 shown in Table 1, in
which the Sn content is too high and cracking occurred during the
hot rolling, making the sheet production impossible.
[0178] Comparative Example 29 is alloy 17 shown in Table 1, in
which the Fe content is too high.
[0179] Comparative Example 30 is alloy 18 shown in Table 1, in
which the Mn content is too high.
[0180] Comparative Example 31 is alloy 19 shown in Table 1, in
which the Cr content and Ti content are too high.
[0181] Comparative Example 32 is alloy 20 shown in Table 1, in
which the Cu content is too high.
[0182] Comparative Example 33 is alloy 21 shown in Table 1, in
which the Zn content is too high.
[0183] Comparative Example 34 is alloy 22 shown in Table 1, in
which the Zr content and V content are too high.
[0184] Those results of the Examples establish that, for improving
formability and BH response after room-temperature aging, it is
necessary that all the requirements concerning composition and DSC
specified in the present aspect should be satisfied.
TABLE-US-00001 TABLE 1 Alloy Chemical components of Al--Mg--Si
alloy sheet (mass %; remainder, Al) No. Mg Si Sn Fe Mn Cr Zr V Ti
Cu Zn Ag 0 0.64 0.99 0.040 1 0.58 0.90 0.050 0.2 2 0.40 0.82 0.039
0.2 0.05 0.12 3 0.39 1.18 0.058 0.2 0.2 0.01 4 0.34 1.50 0.097 0.2
0.64 5 0.54 1.31 0.053 0.2 0.22 6 0.55 0.79 0.197 0.2 0.12 7 0.45
0.89 0.042 0.2 0.65 0.05 8 0.64 1.15 0.027 0.2 0.05 0.05 9 1.47
0.53 0.110 0.2 0.3 0.01 10 0.71 1.00 0.055 0.2 0.05 11 0.47 1.23
0.002 0.7 0.6 12 0.55 0.87 0.050 0.2 0.2 0.1 0.1 13 1.53 0.21 0.046
0.2 14 0.40 2.10 0.042 0.2 15 0.58 1.02 0.002 0.2 16 0.60 1.09
0.455 0.2 17 0.38 0.80 0.051 1.3 18 0.65 1.04 0.046 0.2 1.21 0.01
19 0.51 0.80 0.057 0.2 0.44 0.08 20 0.36 0.79 0.044 0.2 1.28 21
0.48 1.01 0.052 0.2 1.23 22 0.49 0.94 0.055 0.2 0.4 0.4 * Field in
which the value for the element is blank indicates below detection
limit.
TABLE-US-00002 TABLE 2 Solution quenching treatment Preliminary
aging Solution Required period Period of treatment Average to
preliminary Temperature Reached holding Average cooling Alloy No.
temperature cooling rate aging increase rate temperature at 60 to
120.degree. C. rate Classification No. in Table 1 .degree. C.
.degree. C./s min .degree. C./s .degree. C. hr .degree. C./s Inv.
Ex. 0 0 540 100 5 20 100 12 100 Inv. Ex. 1 1 540 100 5 20 100 12
100 Com. Ex. 2 1 540 1 5 20 100 12 100 Com. Ex. 3 1 540 100 120 20
100 12 100 Com. Ex. 4 1 540 100 5 0.1 100 12 100 Com. Ex. 5 1 540
100 5 20 100 1 100 Com. Ex. 6 1 540 100 5 20 100 48 100 Com. Ex. 7
1 540 100 5 20 130 12 100 Inv. Ex. 8 2 540 100 5 20 90 12 100 Com.
Ex. 9 2 540 100 80 20 90 12 100 Com. Ex. 10 2 540 100 5 0.1 90 12
100 Com. Ex. 11 2 540 100 5 20 90 3 100 Com. Ex. 12 2 540 100 5 20
90 48 100 Inv. Ex. 13 3 540 100 5 20 100 16 0.05 Com. Ex. 14 3 540
100 5 5 100 3 100 Com. Ex. 15 3 540 100 5 5 100 45 0.02 Structure
of aluminum alloy sheet after 30-day room-temperature holding
Properties of aluminum alloy after Differential scanning
calorimetry curve 30-day room-temperature holding Height of Height
of 0.2% proof endothermic exothermic Exothermic peak B As 0.2%
stress after Proof Alloy No. peak A peak B temperature proof stress
BH stress increase Classification No. in Table 1 .mu.W/mg .mu.W/mg
.degree. C. MPa MPa MPa Hem workability Inv. Ex. 0 0 1.5 48.2 251
103 220 117 1 Inv. Ex. 1 1 0.9 45.3 251 94 226 132 1 Com. Ex. 2 1
2.8 18.5 254 127 209 82 3 Com. Ex. 3 1 13.0 60.3 257 133 201 68 3
Com. Ex. 4 1 8.2 54.1 256 114 206 92 2 Com. Ex. 5 1 8.6 53.3 256 93
183 90 1 Com. Ex. 6 1 1.3 16.2 250 147 262 115 4 Com. Ex. 7 1 1.0
5.3 251 156 224 68 4 Inv. Ex. 8 2 0.6 35.4 251 88 206 118 1 Com.
Ex. 9 2 12.3 60.3 254 135 200 65 3 Com. Ex. 10 2 8.3 63.2 256 112
205 93 2 Com. Ex. 11 2 8.8 58.9 256 90 179 89 1 Com. Ex. 12 2 0.8
15.5 250 144 261 117 4 Inv. Ex. 13 3 2.1 40.6 250 95 216 121 1 Com.
Ex. 14 3 9.1 55.7 256 97 192 95 1 Com. Ex. 15 3 1.7 14.2 250 150
269 119 4
TABLE-US-00003 TABLE 3 Solution quenching treatment Preliminary
aging Solution Required period Period of treatment Average to
preliminary Temperature Reached holding Average cooling Alloy No.
temperature cooling rate aging increase rate temperature at 60 to
120.degree. C. rate Classification No. in Table 1 .degree. C.
.degree. C./s min .degree. C./s .degree. C. hr .degree. C./s Inv.
Ex. 16 4 540 50 5 20 100 12 100 Inv. Ex. 17 5 540 20 5 20 100 12
100 Inv. Ex. 18 6 540 100 15 20 100 12 100 Inv. Ex. 19 7 540 100 5
5 100 12 100 Inv. Ex. 20 8 540 100 5 3 100 12 100 Inv. Ex. 21 9 540
100 5 20 80 12 100 Inv. Ex. 22 10 540 100 5 20 100 8 100 Inv. Ex.
23 11 540 100 5 20 80 27 0.1 Inv. Ex. 24 12 540 100 5 20 70 32 0.1
Com. Ex. 25 13 540 100 5 20 100 12 100 Com. Ex. 26 14 540 100 5 20
100 12 100 Com. Ex. 27 15 540 100 5 20 100 12 100 Com. Ex. 28 16
cracking occurred during hot rolling Com. Ex. 29 17 540 100 5 20
100 12 100 Com. Ex. 30 18 540 100 5 20 100 12 100 Com. Ex. 31 19
540 100 5 20 100 12 100 Com. Ex. 32 20 540 100 5 20 100 12 100 Com.
Ex. 33 21 540 100 5 20 100 12 100 Com. Ex. 34 22 540 100 5 20 100
12 100 Structure of aluminum alloy sheet after 30-day
room-temperature holding Properties of aluminum alloy after
Differential scanning calorimetry curve 30-day room-temperature
holding Height of Height of As 0.2% 0.2% proof endothermic
exothermic Exothermic peak B proof stress after Proof Alloy No. in
peak A peak B temperature stress BH stress increase Classification
No. Table 1 .mu.W/mg .mu.W/mg .degree. C. MPa MPa MPa Hem
workability Inv. Ex. 16 4 2.3 50.2 251 101 213 112 2 Inv. Ex. 17 5
2.2 54.2 251 104 222 118 2 Inv. Ex. 18 6 5.6 42.9 253 103 207 104 1
Inv. Ex. 19 7 2.4 45.0 251 92 208 116 2 Inv. Ex. 20 8 3.0 68.4 252
107 221 114 1 Inv. Ex. 21 9 3.4 47.1 250 98 201 103 1 Inv. Ex. 22
10 4.3 64.9 249 105 215 110 2 Inv. Ex. 23 11 2.5 68.1 248 104 219
115 2 Inv. Ex. 24 12 3.6 57.3 251 84 195 111 1 Com. Ex. 25 13 3.6
6.8 258 78 126 48 1 Com. Ex. 26 14 10.2 53.6 251 125 207 82 4 Com.
Ex. 27 15 1.7 19.5 268 133 255 122 3 Com. Ex. 28 16 cracking
occurred during hot rolling Com. Ex. 29 17 1.3 38.2 251 106 209 103
4 Com. Ex. 30 18 2.2 37.2 251 114 212 98 4 Com. Ex. 31 19 4.0 36.0
251 103 209 106 4 Com. Ex. 32 20 4.3 36.6 250 135 246 111 4 Com.
Ex. 33 21 2.3 34.5 251 112 205 93 4 Com. Ex. 34 22 1.6 39.0 251 108
212 104 4
[0185] Next, Examples according to the second aspect of the present
invention are explained. 6000-series aluminum alloy sheets were
individually produced so as to differ in the structure specified in
the present aspect, by changing the conditions for a preliminary
aging treatment performed after solution and quenching treatments.
After a holding at room temperature for 30 days after the
production of the sheets, BH response (bake hardenability), As
proof stress as an index of press formability and hem workability
as bendability are examined and evaluated.
[0186] For individually producing the structure, the 6000-series
aluminum alloy sheets having the compositions shown in Table 4 was
produced by variously changing conditions such as the average
cooling rate in the quenching treatment after a solution treatment
and the temperature and holding period in the subsequent
preliminary aging treatment as shown in Tables 5 and 6. With
respect to the indications of the contents of elements within Table
4, a value of the element expressed by a blank indicates that the
content is below a detection limit.
[0187] Specific conditions for aluminum alloy sheet production were
as follows. Slabs of aluminum alloys respectively having the
compositions shown in Table 4 were commonly produced through
casting by the DC casting method. In this casting, the average rate
of cooling from the liquidus temperature to the solidus temperature
was set at 50.degree. C./min in common with all the Examples.
Subsequently, the slabs were subjected to a soaking treatment of
540.degree. C..times.6 hours performed in one stage only, and were
then reheated to 500.degree. C. to initiate hot rough rolling, in
common with all the Examples. Thereafter, they were hot-rolled, in
the succeeding finish rolling, to a thickness of 3.5 mm to obtain
hot-rolled sheets, in common with all the Examples. The hot-rolled
aluminum alloy sheets were subjected to rough annealing of
500.degree. C..times.1 minute and then to cold rolling at a
processing rate of 70% without performing intermediate annealing
during the cold-rolling passes, to obtain cold-rolled sheets having
a thickness of 1.0 mm, in common with all the Examples.
[0188] Furthermore, the cold-rolled sheets were each continuously
subjected to a refining treatment (T4) with continuous type heat
treatment facilities while unwinding and winding each sheet, in
common with all the Examples. Specifically, a solution treatment
was performed by heating at an average rate of heating to
500.degree. C. of 10.degree. C./sec and holding for seconds after
the temperature reached a target temperature of 560.degree. C.,
followed by cooling to room temperature by water cooling or air
cooling so as to result in the average cooling rates shown in
Tables 5 and 6. After this cooling and after the subsequent
required periods shown in Table 2 at room temperature, a
preliminary aging treatment was performed by using an atmospheric
furnace and an oil bath and using the temperature increase rates,
reached temperatures, average cooling rates, and holding periods
shown in Tables 5 and 6. As for the cooling after this preliminary
aging treatment, water cooling or gradual cooling (natural cooling)
was conducted in order to change the average rate of cooling.
[0189] From the final product sheets which each had been allowed to
stand at room temperature for 30 days after the refining treatment,
test sheets (blanks) were cut out and the structure and properties
of the test sheets were examined and evaluated. The results thereof
are shown in Tables 5 and 6.
(Structure)
[0190] The solid-solution amount of Mg+Si in the sheet, volume
proportions of atom aggregates determined with a three-dimensional
atom probe field ion microscope, etc. were determined through
measurements and analysis by the measuring methods described above.
In Tables 5 and 6, the average volume proportions (%) of atom
aggregates determined with a three-dimensional atom probe field ion
microscope are abbreviated to "average volume proportions of atom
aggregates determined with 3DAP (%)".
[0191] In the "average volume proportions of atom aggregates" in
Tables 5 and 6, the average volume proportion
(.SIGMA.Vi/V.sub.Al).times.100 of the total volume .SIGMA.Vi of
atom aggregates which satisfied the requirements specified in the
present aspect to the volume V.sub.Al of the needle-shaped sample
which had undergone electric-field evaporation was determined (in
Tables 2 and 3, it is referred to as .SIGMA.vi/V.sub.Al.times.100).
Furthermore, the average volume proportion (.SIGMA.Vi.sub.1.5 or
more/.SIGMA.Vi).times.100 of the total volume .SIGMA.Vi.sub.1.5 or
more of atom aggregates having a Guinier radius r.sub.G of 1.5 nm
or larger to the total volume .SIGMA.Vi of the atom aggregates was
also determined (in Tables 5 and 6, it is referred to as
.SIGMA.Vi.sub.1.5 or more/.SIGMA.Vi.times.100).
(Bake Hardenability)
[0192] The test sheets which had been allowed to stand at room
temperature for 30 days after the refining treatment were each
examined for 0.2% proof stress (As proof stress) as a mechanical
property through a tensile test. Furthermore, these test sheets
were aged at room temperature for 30 days, subsequently subjected
to an artificial age hardening treatment of 170.degree. C..times.20
minutes (after BH), and then examined for 0.2% proof stress (proof
stress after BH) through a tensile test, in common with the test
sheets. The BH response of each test sheet was evaluated on the
basis of the difference between these 0.2% proof stresses (increase
in proof stress).
[0193] With respect to the tensile test, No. 5 specimens (25
mm.times.50 mmGL.times.sheet thickness) according to JIS Z2201 were
cut out of each sample sheet to perform the tensile test at room
temperature. Here, the tensile direction of each specimen was set
so as to be perpendicular to the rolling direction. The tensile
rate was set at 5 mm/min until the 0.2% proof stress and at 20
mm/min after the proof stress. The number N of examination for
mechanical property was 5, and an average value therefor was
calculated. With respect to the specimens to be examined for the
proof stress after BH, a 2% pre-strain as a simulation of sheet
press forming was given to the specimens by the tensile tester,
followed by performing the BH treatment.
(Hem Workability)
[0194] Hem workability was evaluated only with respect to the test
sheets which had been allowed to stand at room temperature for 7
days or 100 days after the refining treatment. In the test,
strip-shaped specimens having a width of 30 mm were used and
subjected to 90.degree. bending at an inward bending radius of 1.0
mm with a down flange. Thereafter, an inner plate having a
thickness of 1.0 mm was nipped, and the specimen was subjected, in
order, to pre-hem working in which the bent part was further bent
inward to approximately 130.degree. and flat-hem working in which
the bent part was further bent inward to 180.degree. and the end
portion was brought into close contact with the inner plate.
[0195] The surface state, such as the occurrence of rough surface,
a minute crack, or a large crack, of the bent part (edge bent part)
of the flat hem was visually examined and visually evaluated on the
basis of the following criteria. In the following criteria, ratings
of 0 to 2 are on an acceptable level, and ratings of 3 and larger
are unacceptable.
[0196] 0, no crack and no rough surface; 1, slight rough surface;
2, deep rough surface; 3, minute surface crack; 4, linearly
continued surface crack.
[0197] Invention Examples Nos. 35, 36, 43, and 48 in Table 5 and
Nos. 51 to 58 in Table 6, which employ alloys Nos. 23 to 34 shown
in Table 4, each is not only within the component composition range
according to the present aspect and has been produced under
conditions within preferred ranges but also has undergone the
refining treatment, including the solution quenching treatment and
the preliminary aging treatment, under preferred conditions.
Because of this, these Invention Examples satisfy the structure
requirements specified in the present aspect, as shown in Tables 5
and 6. That is, the solid-solution amount of Mg+Si is 1.0 mass % or
more and 2.0 mass % or less, the average volume proportion
(.SIGMA.Vi/V.sub.Al).times.100 of the total volume .SIGMA.Vi of
atom aggregates satisfying the requirements specified in the
present aspect to the volume V.sub.Al of the needle-shaped sample
which has undergone electric-field evaporation is in the range of
0.3 to 1.5%, and the average volume proportion .SIGMA.Vi.sub.1.5 or
more/.SIGMA.Vi).times.100 of the total volume .SIGMA.Vi.sub.1.5 or
more of atom aggregates having a Guinier radius r.sub.G of 1.5 nm
or larger to the total volume .SIGMA.Vi of the atom aggregates is
20 to 70%.
[0198] As a result, the Invention Examples each show excellent BH
response although the bake hardening is performed after the
refining treatment and subsequent room-temperature aging and is a
treatment conducted at a low temperature for a short period of
time. Furthermore, as shown in Table 6, even after the refining
treatment and subsequent room-temperature aging, they each have a
relatively low As proof stress and a low yield ratio and hence show
excellent press formability into automotive panels or the like and
excellent hem workability.
[0199] That is, the Invention Examples, even when having undergone
an automotive-baking treatment after room-temperature aging, were
able to exhibit not only high BH response with a 0.2% proof stress
difference of 100 MPa or greater and a 0.2% proof stress after BH
of 190 MPa or greater but also press formability with an As 0.2%
proof stress of 110 MPa or less and a low yield ratio of less than
0.50 and satisfactory bendability. Thus, they have succeeded in
combining formability and bake hardenability and in attaining both
an increase in BH response and a reduction in yield ratio.
[0200] In contrast, Comparative Examples 37 to 42, 44 to 47, 49,
and 50 in Table 5, which employed alloy examples 24, 25 and 26 in
Table 4 like Invention Examples, each have the preliminary aging
treatment condition outside the preferred ranges, as shown in Table
5. As a result, either the solid-solution amount of Mg+Si or the
average volume proportion (.SIGMA.Vi/V.sub.Al).times.100 or the
average volume proportion (.SIGMA.Vi.sub.1.5 or
more/.SIGMA.Vi).times.100 is outside the range specified in the
present aspect. As a result, they show enhanced room-temperature
aging and, in particular, a relatively high As proof stress or an
increased yield ratio after 30-day room-temperature holding, as
compared with the Invention Examples having the same alloy
composition. Because of this, they are poor in press formability
into automotive panels or the like and in hem workability or are
poor in BH response. Thus, they have failed to combine formability
and bake hardenability and to attain both an increase in BH
response and a reduction in yield ratio.
[0201] In Comparative Example 37, the average cooling rate in the
quenching treatment to room temperature performed after the
solution treatment is too low. Because of this, coarse Mg--Si and
elemental Si were yielded during the cooling, resulting in low
formability. In addition, the solid-solution amount of after the
solution treatment is low, and the average volume proportion
(.SIGMA.Vi.sub.1.5 or more/.SIGMA.Vi).times.100 also is less than
20%. Furthermore, the BH response is also low.
[0202] In Comparative Examples 38 and 44, the period from the
quenching treatment to room temperature after the solution
treatment to the preliminary aging treatment (initiation of
heating) is too long. Because of this, the average volume
proportion (.SIGMA.Vi.sub.1.5 or more/.SIGMA.Vi).times.100 is less
than 20% and the BH response is low. A reduction in yield ratio was
also unable to be attained.
[0203] In Comparative Examples 39 and 45, the temperature increase
rate in the preliminary aging treatment is too low. Because of
this, the average volume proportion (.SIGMA.Vi.sub.1.5 or
more/.SIGMA.Vi).times.100 was unable to be increased to 20% or
higher, resulting in low BH response.
[0204] In Comparative Examples 40, 46 and 49, the period of holding
in the range of 60 to 120.degree. C. in the preliminary aging
treatment is 1 hour, which is too short. Because of this, the
formation of precipitate nuclei was insufficient, and the average
volume proportion (.SIGMA.Vi.sub.1.5 or more/.SIGMA.Vi).times.100
was unable to be increased to 20% or higher, resulting in low BH
response.
[0205] In Comparative Examples 41, 47 and 50, the period of holding
in the range of 60 to 120.degree. C. in the preliminary aging
treatment is 48 to 45 hours, which is too long. Because of this,
precipitate nuclei were yielded in too large an amount in the
preliminary aging treatment. As a result, the amount of atom
aggregates having a relatively large size and contributing to
strength has decreased and the average volume proportion
(.SIGMA.Vi/V.sub.Al).times.100 has increased beyond 1.5%, resulting
in a failure in reducing the yield ratio of the sheet during
forming to less than 0.50.
[0206] In Comparative Example 42, the reached temperature in the
preliminary aging treatment is 130.degree. C., which exceeds the
upper limit of 120.degree. C. and is too high. Because of this,
precipitate nuclei were yielded in too large an amount in the
preliminary aging treatment. As a result, the amount of atom
aggregates having a relatively large size and contributing to
strength has decreased and the average volume proportion
(.SIGMA.Vi/V.sub.Al).times.100 has increased beyond 1.5%, resulting
in too high an As proof stress and a failure in reducing the yield
ratio of the sheet during forming to less than 0.50.
[0207] Comparative Examples 59 to 67 in Table 6 have been produced
within preferred ranges, including the conditions for the
preliminary aging treatment. However, since they employed alloys
Nos. 35 to 43 shown in Table 4, the contents of Mg and Si, which
are essential elements, therein are outside the ranges according to
the present aspect or the content of impurity elements therein is
too high. Because of this, Comparative Examples 59 to 67 each show,
in particular, too high an As proof stress and too high a yield
ratio after 30-day room-temperature holding as compared with the
Invention Examples, as shown in Table 6, and hence are poor in
press formability into automotive panels or the like and in hem
workability or are poor in BH response.
[0208] Comparative Example 59 is alloy 35 shown in Table 4, in
which the Si content is too low.
[0209] Comparative Example 60 is alloy 36 shown in Table 4, in
which the Si content is too high.
[0210] Comparative Example 61 is alloy 37 shown in Table 4, in
which the Sn content is too low.
[0211] Comparative Example 62 is alloy 38 shown in Table 4, in
which the Sn content is too high and cracking occurred during the
hot rolling, making the sheet production impossible.
[0212] Comparative Example 63 is alloy 39 shown in Table 4, in
which the Fe content is too high.
[0213] Comparative Example 64 is alloy 40 shown in Table 4, in
which the Mn content is too high.
[0214] Comparative Example 65 is alloy 41 shown in Table 4, in
which the Cr content and Ti content are too high.
[0215] Comparative Example 66 is alloy 42 shown in Table 4, in
which the Zn content is too high.
[0216] Comparative Example 67 is alloy 43 shown in Table 4, in
which the Zr content and V content are too high.
[0217] Those results of the Examples establish that, for improving
formability and BH response after room-temperature aging, it is
necessary that all the requirements concerning composition and
structure specified in the present aspect should be satisfied.
TABLE-US-00004 TABLE 4 Alloy Chemical components of Al--Mg--Si
alloy sheet (mass %; remainder, Al) Classification No. Mg Si Sn Fe
Mn Cr Zr V Ti Cu Zn Ag Inv. Ex. 23 0.64 0.99 0.040 24 0.58 0.90
0.050 0.2 25 0.40 0.82 0.039 0.2 0.05 0.12 26 0.39 1.18 0.058 0.2
0.16 0.01 27 0.36 1.23 0.084 0.2 0.33 28 0.54 1.31 0.053 0.2 0.22
29 0.55 0.79 0.197 0.2 0.12 30 0.45 0.93 0.040 0.2 0.35 0.05 31
0.64 1.15 0.027 0.2 0.05 0.05 32 0.71 0.72 0.055 0.2 0.03 33 0.47
1.23 0.005 0.7 0.6 34 0.55 0.87 0.050 0.2 0.2 0.1 0.1 Com. Ex. 35
0.77 0.45 0.046 0.2 36 0.40 2.10 0.042 0.2 37 0.58 1.02 0.002 0.2
38 0.60 1.09 0.455 0.2 39 0.38 0.80 0.051 1.3 40 0.53 0.98 0.046
0.2 0.78 0.01 41 0.51 0.80 0.057 0.2 0.44 0.08 42 0.48 1.01 0.052
0.2 1.23 43 0.49 0.94 0.055 0.2 0.4 0.4 * Field in which the value
for the element is blank indicates below detection limit.
TABLE-US-00005 TABLE 5 Solution quenching treatment Preliminary
aging Solution Required period Period treatment Average to
preliminary Temperature Reached of holding at Average cooling Alloy
No. in temperature cooling rate aging increase rate temperature 60
to 120.degree. C. rate Classification No. Table 4 .degree. C.
.degree. C./s min .degree. C./s .degree. C. hr .degree. C./s Inv.
Ex. 35 23 540 100 5 20 100 12 100 Inv. Ex. 36 24 540 100 5 20 100
12 100 Com. Ex. 37 24 540 1 5 20 100 12 100 Com. Ex. 38 24 540 100
120 20 100 12 100 Com. Ex. 39 24 540 100 5 0.1 100 12 100 Com. Ex.
40 24 540 100 5 20 100 1 100 Com. Ex. 41 24 540 100 5 20 100 48 100
Com. Ex. 42 24 540 100 5 20 130 12 100 Inv. Ex. 43 25 540 100 5 20
90 12 100 Com. Ex. 44 25 540 100 80 20 90 12 100 Com. Ex. 45 25 540
100 5 0.1 90 12 100 Com. Ex. 46 25 540 100 5 20 90 3 100 Com. Ex.
47 25 540 100 5 20 90 48 100 Inv. Ex. 48 26 540 100 5 20 100 16
0.05 Com. Ex. 49 26 540 100 5 5 100 3 100 Com. Ex. 50 26 540 100 5
5 100 45 0.02 Structure of aluminum alloy sheet after 30-day
room-temperature holding Average volume proportions of atom
aggregates Properties of aluminum alloy sheet after Solid-solution
determined 30-day room-temperature holding amount of with 3DAP
Yield ratio 0.2% Mg and Si (%) As As 0.2% [(proof proof Proof Alloy
Mg Si .SIGMA.Vi/ .SIGMA.Vi.sub.1.5 or more/ tensile proof stress)/
stress stress No. in mass mass Mg + Si V.sub.Al .times. .SIGMA.Vi
.times. strength stress (tensile after BH increase Hem
Classification No. Table 4 % % mass % 100 100 MPa MPa strength)]
MPa MPa workability Inv. Ex. 35 23 0.61 0.89 1.50 0.74 31 221 103
0.466 220 117 1 Inv. Ex. 36 24 0.55 0.80 1.35 0.55 26 207 94 0.455
226 132 1 Com. Ex. 37 24 0.35 0.48 0.83 0.58 8 251 127 0.506 209 82
3 Com. Ex. 38 24 0.55 0.80 1.35 1.54 9 253 133 0.527 201 68 3 Com.
Ex. 39 24 0.55 0.80 1.35 1.15 15 234 114 0.488 206 92 2 Com. Ex. 40
24 0.55 0.80 1.35 0.48 10 211 93 0.441 183 90 1 Com. Ex. 41 24 0.55
0.80 1.35 2.45 45 274 147 0.536 262 115 4 Com. Ex. 42 24 0.55 0.80
1.35 2.73 42 274 156 0.568 224 68 4 Inv. Ex. 43 25 0.38 0.71 1.09
0.42 20 204 88 0.431 206 118 1 Com. Ex. 44 25 0.38 0.71 1.09 1.42 6
264 135 0.512 200 65 3 Com. Ex. 45 25 0.38 0.71 1.09 1.01 9 227 112
0.492 205 93 2 Com. Ex. 46 25 0.38 0.71 1.09 0.44 16 202 90 0.444
179 89 1 Com. Ex. 47 25 0.38 0.71 1.09 2.27 35 271 144 0.532 261
117 4 Inv. Ex. 48 26 0.37 1.02 1.39 0.55 32 207 95 0.458 216 121 1
Com. Ex. 49 26 0.37 1.02 1.39 0.65 17 215 97 0.452 192 95 1 Com.
Ex. 50 26 0.37 1.02 1.39 2.59 51 282 150 0.531 269 119 4
TABLE-US-00006 TABLE 6 Solution quenching treatment Preliminary
aging Solution Required period Period treatment Average cooling to
preliminary Temperature Reached of holding at Average Alloy No. in
temperature rate aging increase rate temperature 60 to 120.degree.
C. cooling rate Classification No. Table 4 .degree. C. .degree.
C./s min .degree. C./s .degree. C. hr .degree. C./s Inv. Ex. 51 27
540 50 5 20 100 12 100 Inv. Ex. 52 28 540 20 5 20 100 12 100 Inv.
Ex. 53 29 540 100 15 20 100 12 100 Inv. Ex. 54 30 540 100 5 5 100
12 100 Inv. Ex. 55 31 540 100 5 3 100 12 100 Inv. Ex. 56 32 540 100
5 20 100 8 100 Inv. Ex. 57 33 540 100 5 20 80 27 0.1 Inv. Ex. 58 34
540 100 5 20 70 32 0.1 Com. Ex. 59 35 540 100 5 20 100 12 100 Com.
Ex. 60 36 540 100 5 20 100 12 100 Com. Ex. 61 37 540 100 5 20 100
12 100 Com. Ex. 62 38 cracking occurred during hot rolling Com. Ex.
63 39 540 100 5 20 100 12 100 Com. Ex. 64 40 540 100 5 20 100 12
100 Com. Ex. 65 41 540 100 5 20 100 12 100 Com. Ex. 66 42 540 100 5
20 100 12 100 Com. Ex. 67 43 540 100 5 20 100 12 100 Structure of
aluminum alloy sheet after 30-day room-temperature holding Average
volume proportions of atom Properties of aluminum alloy sheet after
aggregates determined 30-day room-temperature holding with 3DAP As
Yield ratio Solid-solution amount (%) As 0.2% [(proof 0.2% proof
Proof Alloy of Mg and Si .SIGMA.Vi.sub.1.5 or more/ tensile proof
stress)/ stress stress Hem No. in Mg Si Mg + Si .SIGMA.Vi/V.sub.Al
.times. .SIGMA.Vi .times. strength stress (tensile after BH
increase worka- Classification No. Table 4 mass % mass % mass % 100
100 MPa MPa strength)] MPa MPa bility Inv. Ex. 51 27 0.33 1.08 1.41
0.63 33 202 91 0.450 207 116 2 Inv. Ex. 52 28 0.39 0.88 1.27 0.74
26 223 104 0.467 222 118 2 Inv. Ex. 53 29 0.52 0.70 1.22 0.78 21
223 103 0.462 207 104 1 Inv. Ex. 54 30 0.43 0.71 1.14 0.54 20 204
86 0.422 200 114 2 Inv. Ex. 55 31 0.61 1.02 1.64 0.88 37 224 107
0.477 221 114 1 Inv. Ex. 56 32 0.68 0.62 1.30 0.88 21 216 106 0.491
208 102 2 Inv. Ex. 57 33 0.44 1.05 1.49 0.78 34 224 104 0.464 219
115 2 Inv. Ex. 58 34 0.52 0.78 1.30 0.32 24 201 84 0.417 195 111 1
Com. Ex. 59 35 0.75 0.40 1.15 0.24 13 178 74 0.416 121 47 1 Com.
Ex. 60 36 0.38 1.79 2.17 1.52 66 252 125 0.496 207 82 4 Com. Ex. 61
37 0.56 0.88 1.44 1.77 30 259 133 0.514 255 122 3 Com. Ex. 62 38
cracking occurred during hot rolling Com. Ex. 63 39 0.35 0.69 1.04
0.91 17 219 106 0.483 209 103 4 Com. Ex. 64 40 0.51 0.87 1.38 1.15
28 237 117 0.494 211 94 4 Com. Ex. 65 41 0.49 0.69 1.17 0.79 19 225
103 0.458 209 106 4 Com. Ex. 66 42 0.46 0.90 1.37 1.05 28 232 112
0.483 205 93 4 Com. Ex. 67 43 0.46 0.80 1.26 0.93 24 230 108 0.469
212 104 4
[0218] While the present invention has been described in detail
with reference to specific embodiments thereof, it will be apparent
to one skilled in the art that various changes and modifications
can be made therein without departing from the spirit and scope of
the present invention.
[0219] This application is based on a Japanese patent application
filed on Mar. 31, 2014 (Application No. 2014-074045) and a Japanese
patent application filed on Mar. 31, 2014 (Application No.
2014-074046), the entire contents thereof being incorporated herein
by reference.
INDUSTRIAL APPLICABILITY
[0220] According to the present invention, it is possible to
provide 6000-series aluminum alloy sheets which combine BH response
and formability after room-temperature aging. As a result, the
6000-series aluminum alloy sheets are usable in applications
extended to automotive panels, in particular, outer panels in which
problems may arise concerning the design of beautiful
curved-surface configurations, character lines, etc.
* * * * *