U.S. patent application number 15/128281 was filed with the patent office on 2018-07-05 for aluminum alloy plate having excellent moldability and bake 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 | 20180187293 15/128281 |
Document ID | / |
Family ID | 54240235 |
Filed Date | 2018-07-05 |
United States Patent
Application |
20180187293 |
Kind Code |
A1 |
SHISHIDO; Hisao ; et
al. |
July 5, 2018 |
ALUMINUM ALLOY PLATE HAVING EXCELLENT MOLDABILITY AND BAKE
HARDENING PROPERTIES
Abstract
The purpose of the present invention is to provide an aluminum
alloy plate capable of having a 0.2% proof stress during molding of
no more than 110 MPa and a 0.2% proof stress after BH of at least
170 MPa. The present invention pertains to an aluminum alloy plate
including, in mass %, 0.2%-1.0% Mg and 0.2%-1.0% Si, fulfilling
{(Mg content)+(Si content)}.ltoreq.1.2%, having a 20-50 .mu.W/mg
high exothermic peak within a temperature range of 230-330.degree.
C. in a differential scanning calorimetry curve, and having both
excellent moldability and excellent bake hardening properties.
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: |
54240235 |
Appl. No.: |
15/128281 |
Filed: |
March 23, 2015 |
PCT Filed: |
March 23, 2015 |
PCT NO: |
PCT/JP2015/058795 |
371 Date: |
September 22, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 21/06 20130101;
C22C 21/02 20130101; C22C 21/08 20130101; C22F 1/05 20130101 |
International
Class: |
C22C 21/02 20060101
C22C021/02; C22C 21/08 20060101 C22C021/08; C22F 1/05 20060101
C22F001/05 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 2014 |
JP |
2014-074044 |
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: 0.2-1.0% and Si: 0.2-1.0% and satisfying
{(Mg content)+(Si content)}.ltoreq.1.2%, with the remainder being
Al and unavoidable impurities, wherein a differential scanning
thermal analysis curve of the aluminum alloy sheet has, in a
temperature range of 230-330.degree. C., only one exothermic peak
(i) or only two exothermic peaks (ii) having a temperature
difference between the peaks of 50.degree. C. or less, and wherein
the exothermic peak (i) or the peak having a higher peak height of
the exothermic peaks (ii) has a height in a range of 20-50
.mu.W/mg.
2. The aluminum alloy sheet excellent in terms of formability and
bake hardenability according to claim 1, further comprising one
element or two or more elements selected from the group consisting
of Fe: more than 0% and 0.5% or less, Mn: more than 0% and 0.3% or
less, Cr: more than 0% and 0.3% or less, Zr: more than 0% and 0.1%
or less, V: more than 0% and 0.1% or less, Ti: more than 0% and
0.1% or less, Cu: more than 0% and 0.5% or less, Ag: more than 0%
and 0.1% or less, and Zn: more than 0% and 0.5% 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 JIS
6000-series (hereinafter, also simply referred to as a 6000-series)
aluminum alloy sheet, as a thin and high strength aluminum alloy
sheet, has been studied.
[0004] 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 case 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, there also is a case 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, in order to cope with such decreases in the
formability and BH response of 6000-series aluminum alloy sheets
due to room-temperature aging, various proposals have been made on
methods for regulating Mg--Si clusters which are formed in the
sheets during room-temperature standing after refining (after
solution and quenching treatments). Among these proposed methods is
a technique in which such Mg--Si clusters are controlled by means
of endothermic peaks and exothermic peaks of a differential
scanning thermal analysis curve (also called a differential
scanning calorimetry curve; hereinafter referred to also as DSC) of
the 6000-series aluminum alloy sheet.
[0009] For example, Patent Documents 1 and 2 propose that the
formation amount of Mg--Si clusters that inhibit room-temperature
aging and suppress low-temperature age hardenability, in
particular, Si/hole clusters (GPI), is regulated. In these
techniques, for regulating the formation amount of GPI, it is
regulated that the T4 material (after solution treatment and
subsequent natural aging) gives a DSC which has no endothermic peak
in the temperature range of 150-250.degree. C., corresponding to
the dissolution of GPI. In these techniques, a low-temperature heat
treatment of holding at 70-150.degree. C. for about 0.5-50 hours is
performed after a solution treatment and quenching to room
temperature, in order to inhibit or control the formation of the
GPI.
[0010] Patent Document 3 proposes a 6000-series aluminum alloy
sheet with excess Si which, after a refining treatment including
solution and quenching treatments of this aluminum alloy sheet,
gives a DSC in which an endothermic peak in the temperature range
of 150-250.degree. C. and corresponds to a dissolution of Si/hole
clusters (GPI) has a minus height of 1,000 .mu.W or less and an
exothermic peak in the temperature range of 250-300.degree. C. and
corresponds to a precipitation of Mg/Si clusters (GPII) has a plus
height of 2,000 .mu.W or less. This aluminum alloy sheet, after
having undergone room-temperature aging for at least 4 months after
the refining treatment, has the properties in which a proof stress
is in the range of 110-160 MPa, a difference in proof stress with
the one just after the refining treatment is 15 MPa or less, an
elongation is 28% or greater, and a proof stress, as measured after
application of a 2% strain thereto and a subsequent low-temperature
aging treatment of 150.degree. C..times.20 minutes, is 180 MPa or
greater.
[0011] Patent Document 4 proposes that a 6000-series aluminum alloy
sheet is set to give, after a refining treatment, a DSC in which an
exothermic peak in the temperature range of 100-200.degree. C. has
a height W1 of 50 .mu.W or larger and a ratio of a height W2 of an
exothermic peak in the temperature range of 200 to 300.degree. C.
to the exothermic-peak height W1, (W2/W1), is 20.0 or less, in
order to obtain BH response in a bake hardening treatment performed
at a low temperature for a short period.
[0012] The document states that the exothermic peak W1 corresponds
to the precipitation of GP zones serving as nucleus formation sites
of .beta.'' (Mg.sub.2Si phase) in an artificial age hardening
treatment, and that the higher the W1 peak height, the more the GP
zones serving as nucleus formation sites of .beta.'' in an
artificial age hardening treatment have already been formed and
secured in the sheet after refining. It states that as a result,
the .beta.'' grows rapidly in a bake hardening treatment after
forming, thereby attaining an improvement in BH response. It states
that the exothermic peak W2, on the other hand, corresponds to a
precipitation peak of the .beta.'' itself, and that the height of
this exothermic peak W2 is made as small as possible in order to
reduce the proof stress of the sheet to be formed to less than 135
MPa and to thereby ensure formability.
[0013] Patent Document 5 proposes that three exothermic-peak
heights (three portions) in a DSC in specific temperature ranges
and particularly affect BH response are selected and regulated to
enhance the BH response (bake hardenability). The three exothermic
peaks are peak A at 230-270.degree. C., peak B at 280-320.degree.
C. and peak C at 330-370.degree. C. In the proposed method, the
height of the peak B is regulated to 20 .mu.W/mg or larger and the
peak ratio (A/B) and the peak ratio (C/B) are regulated to 0.45 or
less and 0.6 or less, respectively, thereby attaining an increase
in 0.2% proof stress, through an artificial hardening treatment of
170.degree. C..times.20 minutes after application of a 2% strain,
of 100 MPa or greater.
PRIOR ART DOCUMENTS
Patent Documents
Patent Document 1: JP-A-10-219382
Patent Document 2: JP-A-2000-273567
Patent Document 3: JP-A-2003-27170
Patent Document 4: JP-A-2005-139537
Patent Document 5: JP-A-2013-167004
SUMMARY OF THE INVENTION
Problem that the Invention is to Solve
[0014] The various outer panels for automobiles are required to
attain strain-free, beautiful curved-surface configurations and
character lines, from the standpoint of design. However, since
higher-strength aluminum alloy sheet materials are being adopted
for the purpose of weight reduction and this results in
difficulties in forming, it is becoming difficult year by year to
meet such requirements. There is hence a growing desire in recent
years for a high-strength aluminum alloy sheet having even better
formability. However, with the above-mentioned conventional
structure controls with a DSC, it is difficult to meet such
requirements.
[0015] For example, one cause which renders high-strength aluminum
alloy sheets difficult to apply to outer panels is 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.
[0016] 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, application of high-strength
aluminum alloy sheets to the outer panels has a problem in that it
is necessary to obtain a high-strength aluminum alloy sheet which
has improved formability and is inhibited from suffering face
strains.
[0017] The problem concerning such face strains is not 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 wind corner portion of a rear fender, a character-line
termination portions of a trunk lid or hood outer panel, and a root
portion of a rear fender pillar.
[0018] From the standpoint of attaining improved formability for
inhibiting the occurrence of the face strains to overcome the
problem described above, it is desirable that a sheet in press
forming, which has 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 170 MPa or greater after bake hardening (hereinafter also
referred to as "after BH") and to attain an increase in 0.2% proof
stress through bake hardening of 70 MPa or greater. As described
above, with conventional structure controls with a DSC disclosed in
Patent Documents 1 to 5, it is difficult to overcome the
problem.
[0019] The present invention has been achieved in order to overcome
the problem described above. An object thereof 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 170 MPa or greater.
Means for Solving the Problem
[0020] The present inventors diligently made investigations and, as
a result, have discovered that an aluminum alloy sheet which
combines formability and bake hardenability can be obtained by
adopting a specific composition and specific exothermic peaks in
the DSC for an Al--Mg--Si alloy sheet, which contains Mg and Si.
The present invention has been thus completed.
[0021] The gist of the aluminum alloy sheet of the present
invention, which is excellent in terms of formability and bake
hardenability, is an Al--Mg--Si alloy sheet containing, in terms of
mass %, Mg: 0.2-1.0% and Si: 0.2-1.0% and satisfying {(Mg
content)+(Si content)}.ltoreq.1.2%, with the remainder being Al and
unavoidable impurities, in which a differential scanning thermal
analysis curve of the aluminum alloy sheet has, in a temperature
range of 230-330.degree. C., only one exothermic peak (i) or only
two exothermic peaks (ii) having a temperature difference between
the peaks of 50.degree. C. or less, and in which the exothermic
peak (i) or the peak having a higher peak height of the exothermic
peaks (ii) has a height in a range of 20-50 .mu.W/mg.
[0022] The differential thermal analysis at each of measurement
portions in the 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 24.5-26.5 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 0-100.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.
[0023] The aluminum alloy sheet excellent in terms of formability
and bake hardenability may further contain one element or two or
more elements selected from the group consisting of Fe: more than
0% and 0.5% or less, Mn: more than 0% and 0.3% or less, Cr: more
than 0% and 0.3% or less, Zr: more than 0% and 0.1% or less, V:
more than 0% and 0.1% or less, Ti: more than 0% and 0.1% or less,
Cu: more than 0% and 0.5% or less, Ag: more than 0% and 0.1% or
less, and Zn: more than 0% and 0.5% or less.
Effects of the Invention
[0024] According to the present invention, the contents of Mg and
Si, which are major elements of an Al--Mg--Si alloy sheet, are
regulated to be relatively low, thereby enabling a 0.2% proof
stress in forming of the sheet, which has been produced and then
subjected to room-temperature aging, to be reduced to 110 MPa or
less. Consequently, it can have improved formability when applied
to automotive panels or the like, which are particularly
problematic in face strains thereof, in automotive panel
structures.
[0025] In addition, the thermal properties (structure) in the DSC
of the aluminum alloy sheet are regulated. As a result, an
increased strength which includes a 0.2% proof stress after BH of
170 MPa or greater and an increase in 0.2% proof stress of 70 MPa
or greater, which is useful as automotive panels can be ensured.
The regulation of thermal properties (structure) in the DSC
provides a measure for ensuring the amount of precipitates which
precipitate after a bake hardening treatment.
[0026] Due to such regulation of composition and structure, an
aluminum alloy sheet which combines formability and bake
hardenability can be provided merely with a basic composition of
Al--Mg--Si alloys, without the need of newly adding any additive
element or without the need of giving a large modification to
ordinary production processes.
BRIEF DESCRIPTION OF THE DRAWING
[0027] FIG. 1 is a view which shows DSCs of the aluminum alloy
sheets of some examples in the Examples.
MODES FOR CARRYING OUT THE INVENTION
[0028] Modes for carrying out the present invention will be
specifically explained below with respect to each requirement. In
this description, "mass %" has the same meaning as "wt %".
(Chemical Component Composition)
[0029] First, the chemical component composition of the Al--Mg--Si
(hereinafter referred to also as 6000-series) aluminum alloy sheet
(hereinafter also referred to simply as "aluminum alloy sheet")
according to the present invention is explained below.
[0030] The 6000-series aluminum alloy sheet targeted by the present
invention, as, for example, a 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 invention, the
contents of Mg and Si, which are major elements, are regulated so
as to be relatively low, thereby reducing a 0.2% proof stress in
forming of the sheet, which has been produced and then subjected to
room-temperature aging, 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 170 MPa or greater is rendered
possible by means of composition.
[0031] In order to satisfy such requirements, the aluminum alloy
sheet has a composition which contains, in terms of mass %, Mg:
0.2-1.0% and Si: 0.2-1.0% and satisfies {(Mg content)+(Si
content)}.ltoreq.1.2%, with the remainder being Al and unavoidable
impurities. In this description, all the content indicated in % of
the elements means that in mass %. Furthermore, the "-" in each
content means that the content is equal to or more than the lower
limit value but is equal to or less than the upper limit value.
[0032] In the present invention, elements other than Mg, Si and Al
basically are impurities or elements which may be contained. The
contents of such other elements are the contents (permissible
amounts) on levels in accordance with the AA or JIS standards,
etc., or are on levels below such standards. Namely, there are
cases, in the present invention 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 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 an inclusion of some degree of amount. There
are content ranges which do not defeat or lessen the object or
effects of the present invention, even if included in substantial
amounts.
[0033] Consequently, in the present invention, examples of the
other elements which may be contained in the aluminum alloy include
the following elements. The permissible contents thereof are within
the ranges of equal to or less than the upper limits according to
the AA or JIS standards or the like, and are as shown below.
[0034] Specifically, the aluminum alloy sheet may further contain
one element or two or more elements selected from the group
consisting of Fe: 0.5% or less (exclusive of 0%), Mn: 0.3% or less
(exclusive of 0%), Cr: 0.3% or less (exclusive of 0%), Zr: 0.1% or
less (exclusive of 0%), V: 0.1% or less (exclusive of 0%), Ti: 0.1%
or less (exclusive of 0%), Cu: 0.5% or less (exclusive of 0%), Ag:
0.1% or less (exclusive of 0%), and Zn: 0.5% or less (exclusive of
0%), within those ranges.
[0035] In this description, the expression "exclusive of 0%" has
the same meaning as that the content is "higher than 0%".
[0036] The content range of each element and the purposes and
permissible amount thereof in the 6000-series aluminum alloy sheet
according to the present invention are explained below.
Si: 0.2-1.0%
[0037] Si, together with the Mg, is an essential element for
obtaining the strength (proof stress) required as automotive panels
because it forms aging precipitates which contribute to an
improvement in strength, during an artificial aging treatment such
as a baking treatment, and thus exhibits an age hardenability. In
the case where the content of Si is too low, the amount of aging
precipitates after an artificial aging treatment is too small,
resulting in too small an increase in strength after baking.
Meanwhile, in the case where the content of Si is too high, not
only the strength of the sheet just after production but also the
amount of room-temperature aging after the production are
increased, resulting in too high a strength before forming. Because
of this, the formability into automotive panels or the like, which
are particularly problematic in face strains thereof, in automotive
panel structures, is reduced. In addition, coarse crystals and
precipitates are formed, resulting in a considerable decrease in
bendability. A preferred upper limit of the content of Si is
0.8%.
[0038] 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: 0.2-1.0%
[0039] Mg is also an essential element for obtaining the proof
stress required as panels, since it forms, together with the Si,
aging precipitates which contribute to an improvement in strength,
and thus exhibits an age hardenability. In the case where the
content of Mg is too low, the precipitate amount of precipitates
after an artificial aging treatment is too small, resulting in too
small an increase in strength after baking. Meanwhile, in the case
where the content of Mg is too high, not only the strength of the
sheet just after production but also the amount of room-temperature
aging after the production are increased, resulting in too high a
strength before forming. Because of this, the formability into
automotive panels or the like, which are particularly problematic
in face strains thereof, in automotive panel structures, is
reduced. A preferred upper limit of the content of Mg is 0.8%.
{(Mg Content)+(Si Content)}.ltoreq.1.2%
[0040] {(Mg content)+(Si content)}, which is the total content of
Mg and Si, as the structure of the 6000-series aluminum alloy sheet
before forming, considerably affects exothermic peaks present in
the temperature range of 230-330.degree. C. in the DSC of this
aluminum alloy sheet.
[0041] On the assumption that the appropriate production process
which will be described later is used, by regulating {(Mg
content)+(Si content)} to 1.2% or less, in the case where there
exist only two exothermic peaks (ii) in the temperature range of
230-330.degree. C., the difference in temperature between the peaks
of the two exothermic peaks (ii) can be 50.degree. C. or less and
the one having a higher peak height can have a peak height in the
range of 20-50 .mu.W/mg. Meanwhile, in the case where there exists
only one exothermic peak (i) in that temperature range, this
exothermic peak (i) can have a height in the range of 20-50
.mu.W/mg.
[0042] Consequently, it is preferred that {(Mg content)+(Si
content)} is as small as possible. However, since there essentially
are minimum necessary Mg and Si amounts for exhibiting basic
performances as a sheet, a lower limit of {(Mg content)+(Si
content)} is determined by the minimum contents of these each. From
this standpoint, a lower limit of {(Mg content)+(Si content)} is
preferably 0.6% or higher.
[0043] Meanwhile, in the case where {(Mg content)+(Si content)} is
too high above 1.2%, it is difficult to regulate the DSC exothermic
peaks so as to fall within the specified ranges, even if the
appropriate production process which will be described later is
used. Specifically, in the case where there are two exothermic
peaks in the temperature range of 230-330.degree. C., these two
exothermic peaks cannot have a temperature difference between the
peaks of 50.degree. C. or less. In the case where there is only one
exothermic peak in that temperature range, this exothermic peak
cannot have a height in the range of 20-50 .mu.W/mg. Because of
this, it is difficult to attain both a reduction in strength during
forming (before baking) and an enhancement in increase in strength
through paint baking. Consequently, an upper limit of {(Mg
content)+(Si content)} is 1.2% or less and preferably 1.0% or
less.
(Differential Scanning Thermal Analysis Curve, Differential
Scanning Calorimetry Curve, DSC)
[0044] The composition described above is employed. Furthermore, in
the present invention, peaks in the DSC of the aluminum alloy sheet
are regulated as a measure for ensuring the amount of precipitates
which precipitate after a bake hardening treatment, in order to
ensure high strength as automotive panels or the like.
Specifically, a structure is configured in which two exothermic
peaks, which have conventionally been present in the temperature
range of 230-330.degree. C. apart from each other, are present so
as to near to each other (with a reduced temperature difference)
and to overlap each other. This makes it possible to attain a 0.2%
proof stress in forming into automotive panels reduced to 110 MPa
or less and to attain a 0.2% proof stress after bake hardening of
170 MPa or greater.
[0045] 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 of the sheet, by differential thermal analysis performed
under the following conditions.
[0046] 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.
[0047] In the DSC, according to conventional techniques, there are
two exothermic peaks .beta.'' and .beta.' in the range of
230-330.degree. C., existing apart from each other so as to have a
large temperature difference (distance) between the peaks. In the
present invention, the structure of the aluminum alloy sheet has
been specified so that the two exothermic peaks are located near to
each other (with a reduced temperature difference therebetween) and
to overlap each other. Specifically, in a DSC of the aluminum alloy
sheet, in the temperature range of 230-330.degree. C., there is
only one exothermic peak (i) or there are only two exothermic peaks
(ii), having the difference in temperature between the peaks of
50.degree. C. or less. Moreover, the only one exothermic peak (i),
or the exothermic peak having a larger (higher) peak height of the
only two exothermic peaks (ii) has a height in the range of 20-50
.mu.W/mg.
[0048] In 6000-series aluminum alloy sheets, various precipitate
phases are yielded, depending on aging temperatures, such as
clusters, GP zones, strengthening phase 1 (.beta.''), strengthening
phase 2 (.beta.'), and equilibrium phase (Mg.sub.2Si). It is
presumed that for enhancing the strength after baking (artificial
aging treatment), it is effective to yield .beta.'' and .beta.',
among those phases, during the baking. However, the 6000-series
aluminum alloy sheet of the present invention, in which the
contents of Mg and Si have been regulated so as to be relatively
low in order to make the sheet have, in forming after
room-temperature aging, a 0.2% proof stress reduced to 110 MPa or
less, considerably differs in the appearing behavior (appearing
temperature) of the strengthening phase 1 (.beta.'') and
strengthening phase 2 (.beta.') upon BH (artificial aging
treatment), from ordinary 6000-series aluminum alloy sheets having
relatively high Mg and Si contents.
[0049] The changes in the appearing behavior of .beta.'' and
.beta.' upon BH (upon baking treatment) can be simulated with DSC.
This is a base of specifying the structure in the present invention
by means of DSC.
[0050] A simulation with DSC of the appearing behavior of .beta.''
and .beta.' upon BH shows that in the case of, for example,
ordinary 6000-series aluminum alloy sheets having relatively high
Mg and Si contents, the exothermic peaks assigned to .beta.'' and
.beta.' are present more widely apart from each other in the range
of 230-330.degree. C. More specifically, a conventional exothermic
peak assigned to .beta.'' is mostly present around 240-260.degree.
C., which is the lower-temperature former half of that temperature
range. Meanwhile, a conventional exothermic peak assigned to
.beta.' is present around 310-320.degree. C., which is the
higher-temperature latter half of that temperature range, and they
have existed in a state that the difference in temperature between
the peaks of .beta.'' and .beta.' has been larger than 50.degree.
C.
[0051] Such state of conventional exothermic peaks is a
representative example, and that appearing behavior of the
exothermic peaks varies widely, as a matter of course, depending on
the composition of the sheet and production conditions. For
example, there are cases where a DSC has three exothermic peaks
(three portions) regarding BH response and they are respectively
called peak A at 230-270.degree. C., peak B at 280-320.degree. C.
and peak C at 330-370.degree. C., as in Patent Document 5.
[0052] In contrast, when a simulation with DSC of the appearing
behavior of .beta.'' and .beta.' upon BH is similarly made with
respect to the 6000-series aluminum alloy sheet of the present
invention, in which the contents of Mg and Si are relatively low,
it can be seen that the exothermic peaks assigned to .beta.'' and
.beta.' are characterized in that the positions where the
exothermic peaks appear (peak positions) and the distance between
the peaks (temperature difference) are nearer to each other
(overlapping), as compared with those ordinary 6000-series aluminum
alloy sheets. There also is a feature in which this phenomenon
occurs as a result of changing the conditions for sheet production,
in particular, the conditions for a preliminary aging treatment
performed after solution and quenching treatments.
[0053] In a 6000-series aluminum alloy sheet of the present
invention having relatively low Mg and Si contents, when produced
by an ordinary process, exothermic peaks of .beta.'' and .beta.'
exist in the wide temperature range of 230-330.degree. C. as two
separate peaks, the distance between whose peaks is 50.degree. C.
or larger in terms of temperature difference, like ordinary
6000-series aluminum alloy sheets having relatively high Mg and Si
contents. As typical examples thereof, the DSC indicated by the
broken line shown in FIG. 1, which will be described later, and
Comparative Example 19 in Table 2 in the Examples.
[0054] On the other hand, it has been found that in the cases when
a production process is modified to perform the refining after
rolling of the sheet so that the conditions for a preliminary aging
treatment after solution and quenching treatments are changed, the
exothermic peaks of .beta.'' and .beta.' appear so that the peaks
thereof overlap each other (are located near to each other), with
the difference in temperature between the peaks being as small as
less than 50.degree. C.
[0055] According to the finding made by the present inventors, the
appearing temperature of the exothermic peak assigned to .beta.''
(also called first or former-half peak) shifts from the position
(temperature) around 250-260.degree. C. of low temperature to a
position (temperature) around 270-290.degree. C. of high
temperature. Meanwhile, the appearing temperature of the exothermic
peak assigned to .beta.' (also called second or latter-half peak)
shifts from the position (temperature) around 300-310.degree. C. of
high temperature to a position (temperature) around 290-300.degree.
C. of low temperature.
[0056] It has been found that in the cases when the exothermic
peaks assigned to .beta.'' and .beta.' have appeared so that the
peaks are located near to each other or overlap each other, with
the temperature difference between the peaks being as small as less
than 50.degree. C., then an amount of artificial-aging precipitates
which serve to enhance the proof stress after BH can be ensured.
Namely, by regulating the exothermic peaks assigned to .beta.'' and
.beta.' so as to be located near to each other or overlap each
other, the 0.2% proof stress in panel forming can be reduced to 110
MPa or less and, simultaneously therewith, the 0.2% proof stress of
the panel after BH can be increased to 170 MPa or greater. In
contrast, in the case where those two exothermic peaks have the
difference in temperature between the peaks as large as more than
50.degree. C., those properties cannot be exhibited.
[0057] One of the features of the present invention is that the
state in which the exothermic peaks assigned to .beta.'' and
.beta.' overlap each other has been specified as above.
Specifically, the 6000-series aluminum alloy sheet gives a DSC in
which only two (only two in total) exothermic peaks, i.e., a
lower-temperature-side exothermic peak assigned to .beta.'' and a
higher-temperature-side exothermic peak assigned to .beta.', that
have a difference in temperature between the peaks of 50.degree. C.
or less, preferably 30.degree. C. or less, are present in the
temperature range of 230-330.degree. C., preferably in the
temperature range of 250-320.degree. C., and in which the height of
either exothermic peak of these, which has a larger (higher) peak
height is in the range of 20-50 .mu.W/mg. In the case where the
lower-temperature-side exothermic peak assigned to .beta.'' and the
higher-temperature-side exothermic peak assigned to .beta.' are
located nearer to each other to overlap each other so that the
difference in temperature between these peaks cannot be recognized
(measured), i.e., in the case where it is deemed that there is only
one so-called synthesized (superposed) exothermic peak in the
temperature range of 230-330.degree. C., then the height of this
exothermic peak is in the range of 20-50 .mu.W/mg.
[0058] In the present invention, in the case where only two
exothermic peaks having a difference in temperature between the
peaks of 50.degree. C. or less, preferably 30.degree. C. or less,
are present in the temperature range of 230-330.degree. C.,
preferably in the temperature range of 250-320.degree. C., it is
preferable that the exothermic peak assigned to .beta.'' should be
present around 270-290.degree. C. as a lower-temperature-side first
or former-half peak. It is also preferable that the exothermic peak
assigned to .beta.' should be present around 290-300.degree. C. as
a higher-temperature-side second or latter-half peak. Furthermore,
the difference in temperature between the peaks of these exothermic
peaks is 50.degree. C. or less, and the height of the exothermic
peak, which has a higher peak height of these exothermic peaks is
in the range of 20-50 .mu.W/mg. Examples thereof are the thick
continuous line among the DSCs shown in FIG. 1, which will be
described later, and Invention Examples 0, 1, 16, 17, 19, 21, etc.
shown in Table 2 in the Examples.
[0059] Meanwhile, the thin continuous line among the DSCs shown in
FIG. 1, which will be described later, and Invention Examples 5, 6,
12, 15, 18, 20, etc. shown in Table 2 in the Examples are the case
where a lower-temperature-side exothermic peak assigned to .beta.''
and a higher-temperature-side exothermic peak assigned to .beta.'
more overlap each other to render the difference in temperature
between these peaks unrecognizable and, hence, there is only one
synthesized exothermic peak in the temperature range of
230-330.degree. C., preferably in the temperature range of
270-300.degree. C.
[0060] Also important for ensuring the BH response is, of course,
the height of an exothermic peak which indicates the amount of
artificial-aging precipitates in BH. Namely, in the case where
there are two exothermic peaks in the temperature range of
230-330.degree. C., the height (.mu.W/mg) of the exothermic peak
assigned to .beta.' (appearing around about 300.degree. C. in
Invention Examples in the Examples, which will be described later),
which is the exothermic peak having a larger peak height and
contributing to BH response, is regulated so as to be in the range
of 20-50 .mu.W/mg.
[0061] Meanwhile, in the case where there is only one exothermic
peak in the temperature range of 230-330.degree. C., that is, in
the case where the exothermic peak assigned to .beta.'' (the first
or former-half peak, preferably appearing around 270-290.degree.
C.) and the exothermic peak assigned to .beta.' (the second or
latter-half peak, preferably appearing around 290-300.degree. C.)
overlap each other to form only one synthesized exothermic peak,
the height of this exothermic peak is regulated so as to be in the
range of 20-50 .rho.W/mg.
[0062] Thus, it is possible to reduce the proof stress in panel
forming to 110 MPa or lower and to attain a proof stress after BH
of 170 MPa or greater. In other words, aging precipitates of
.beta.'' and .beta.' which are yielded during BH can be ensured in
such an amount that a proof stress after BH of 170 MPa or greater
is brought about. In the case where the heights of those exothermic
peaks are smaller than the lower limit of, or are larger than the
upper limit of, the range of 20-50 .mu.W/mg, this means that the
amount of the desired aging precipitates of such as .beta.'' and
.beta.', which have influences on BH response through a bake
hardening treatment is too small or too large and such precipitates
are unable to be yielded in the desired amount. Because of this, it
is inevitably impossible to attain both a reduction in proof stress
in panel forming to 110 MPa or less and a control of a proof stress
after BH to 170 MPa or greater.
(Production Process)
[0063] Next, a process for producing the aluminum alloy sheet
according to the present invention is explained. The aluminum alloy
sheet according to the present invention 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.
[0064] However, for obtaining the structure specified with a DSC
according to the present invention, during those production steps,
the conditions for a preliminary aging treatment after the solution
and quenching treatments 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 invention. Unless
such preferred conditions are employed, it is difficult to obtain
the structure specified with a DSC according to the present
invention.
(Melting and Casting Cooling Rate)
[0065] 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, in order to regulate the clusters
so as to be in the range specified in the present invention, 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.
[0066] 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, it is highly probable that the specified clusters cannot be
regulated so as to be in the ranges according to the present
invention.
(Homogenizing Heat Treatment)
[0067] 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.
[0068] 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. When hot rolling is thereafter started immediately or
when hot rolling is started after holding and cooling to an
appropriate temperature, control within the number density of the
clusters specified in the present invention can be achieved.
[0069] 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./hour, followed by reheating to 350.degree. C. to
450.degree. C. at an average heating rate of 20 to 100.degree.
C./hour to start hot rolling in this temperature range.
[0070] 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.
(Hot Rolling)
[0071] 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.
[0072] 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)
[0073] 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)
[0074] 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 cold rolling ratio should be 60% or greater.
Intermediate annealing may be performed between cold-rolling passes
for the same purpose as in the rough annealing.
(Solution Treatment and Quenching Treatment)
[0075] 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 0.1-10 seconds.
[0076] 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./sec or greater.
In the case where the average rate of cooling to room temperature
after the solution treatment is too low, coarse Mg.sub.2Si 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 quenching treatment.
(Preliminary Aging Treatment: Reheating Treatment)
[0077] 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, clusters that
are prone to dissolve upon room-temperature aging are yielded,
making it impossible to form the exothermic peaks, as a
prerequisite, specified with a DSC according to the present
invention. 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.
[0078] In this preliminary aging treatment, it is important that
periods of holding both in the relatively higher-temperature-side
range of 80-120.degree. C. and in the relatively
lower-temperature-side range of 60-40.degree. C. should be ensured.
Thus, the exothermic peaks specified with a DSC according to the
present invention are formed.
[0079] Here, the higher-temperature-side range of 80-120.degree. C.
and the lower-temperature-side range of 60-40.degree. C. may be
divided into stages, e.g., in two stages, in terms of temperature,
or may be regulated so that the temperature changes continuously.
Furthermore, the temperature holding in the higher-temperature-side
range may be a heat treatment in which a constant temperature
within that temperature range is maintained or in which the
temperature is gradually changed within that temperature range by
temperature increase. Meanwhile, the temperature holding in the
lower-temperature-side range may be a heat treatment in which a
constant temperature within that temperature range is maintained or
in which the temperature is gradually changed within that
temperature range by temperature decrease. In short, the
temperature may be continuously changed by temperature increase,
temperature decrease (annealing), etc., so long as the temperature
is held in each of the temperature ranges for the necessary holding
period. The temperature holding in the higher-temperature-side and
in the lower-temperature-side may be a heat treatment of
consecutive two stages in which the temperature is divided into
stages, or may be heat treatment in which the holding temperature
is kept constant within each of the specified temperature ranges or
may be a continuous heat treatment in which temperature increase,
temperature decrease, natural cooling, etc are suitably combined
within each of the specified temperature ranges. The cooling after
the preliminary aging treatment may be natural cooling or rapid
cooling.
[0080] The period of holding in the higher-temperature-side range
of 80-120.degree. C. in the former half is preferably regulated to
5-40 hours including the time period during which the sheet is held
in the temperature range of 80-120.degree. C. in the temperature
increase of the sheet. Meanwhile, the period of holding in the
lower-temperature-side range of 60-40.degree. C. in the latter half
is preferably regulated to 20-300 hours including the period of
temperature decrease from the holding in the
higher-temperature-side range or the time period during which the
sheet is held in the temperature range of 60-40.degree. C. in the
cooling such as natural cooling or rapid cooling.
[0081] In the case where those temperatures are too low or those
holding periods are too short, similar to in the case where no
preliminary aging treatment is performed, the structure according
to the present invention specified with a DSC is less apt to be
obtained, and no exothermic peak appears in the temperature range
of 230-330.degree. C. or, even if two exothermic peaks appear, the
temperature difference between the peaks exceeds 50.degree. C. or
the specified exothermic peak height exceeds 50 .mu.W/mg.
[0082] Conversely, also in the case where those temperatures are
too high or those holding periods are too long, the structure
according to the present invention specified with a DSC is less apt
to be obtained, and no exothermic peak appears in the temperature
range of 230-330.degree. C. or the specified exothermic peak height
exceeds 50 .mu.W/mg.
Examples
[0083] 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.
[0084] Examples according to 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
invention, 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.
[0085] For the individual producing, the 6000-series aluminum alloy
sheets having the compositions shown in Table 1 was produced by
variously changing conditions such as the temperature and holding
period in the preliminary aging treatment after the solution and
quenching treatments as shown in Table 2. 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.
[0086] 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, 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.
[0087] 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 5 seconds after
the temperature reached a target temperature of 540.degree. C.,
followed by cooling to room temperature by performing water cooling
at an average cooling rate of 100.degree. C./sec. After this
cooling, a preliminary aging treatment was performed in two stages
of the higher-temperature-side range and the lower-temperature-side
range, using the temperatures (.degree. C.) and holding periods
(hr) shown in Table 2. Specifically, this two-stage preliminary
aging treatment was performed by holding at the given temperature
for the given period by using an oil bath, as the
higher-temperature-side range, and thereafter, by holding at the
given temperature for the given period by using a thermostatic
oven, as the lower-temperature-side range, followed by annealing
(natural cooling).
[0088] In the preliminary aging treatment, the period of holding in
the higher-temperature-side range included the time period during
which the sheet was held in the temperature range of 80-120.degree.
C. in the temperature increase of the sheet. The period of holding
in the lower-temperature-side range included the temperature
decrease from the holding in the higher-temperature-side range or
the time period during which the sheet was held in the temperature
range of 60-40.degree. C. in the cooling by natural cooling.
[0089] 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 2.
(DSC)
[0090] 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 thermal analysis
curves) of this sheet, as for the average value for these ten
portions, the exothermic peaks present in the temperature range of
230-330.degree. C. were examined. Specifically, in the cases when
two exothermic peaks were present, the difference in temperature
(.degree. C.) between these exothermic peaks and the peak height
(.mu.W/mg) of the exothermic peak having a higher peak height were
determined. In the cases when only one exothermic peak was present,
the height (.mu.W/mg) of this exothermic peak was determined.
[0091] 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)
[0092] 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).
[0093] 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)
[0094] 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.
[0095] 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.
[0096] 0, no crack and no rough surface; 1, slight rough surface;
2, deep rough surface; 3, minute surface crack; 4, linearly
continued surface crack.
[0097] As shown by alloys Nos. 0 to 9 in Table 1 and Nos. 0, 1, 5,
6, 12, and 15 to 21 in Table 2, the Invention Examples each not
only have a component composition within the range according to the
present invention and have been produced under conditions within
preferred ranges but also have undergone the refining treatment,
including the preliminary aging treatment, under conditions within
preferred ranges. Because of this, these Invention Examples satisfy
the DSC requirements specified in the present invention, as shown
in Table 2. That is, these sheets each gave a DSC which had only
one or only two exothermic peaks in the temperature range of
230-330.degree. C. and in which when only two exothermic peaks were
present, then the difference in temperature between the peaks was
50.degree. C. or less and the exothermic-peak height of one having
a higher exothermic-peak height was in the range of 20-50 .mu.W/mg.
Furthermore, when only one exothermic peak was present, the height
of this exothermic peak was in the range of 20-50 .mu.W/mg.
[0098] In Table 2, as for the peak height in the case where only
two exothermic peaks were present in the temperature range of
230-330.degree. C., the peak appeared around 300.degree. C. had a
larger peak height than the peak appeared on the lower-temperature
side, in both Invention Examples and Comparative Examples.
Consequently, the peak height (W/mg) of this exothermic peak was
determined.
[0099] 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 2, 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 70 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.
[0100] In contrast, Comparative Examples 2 to 4, 7 to 11, 13, and
14 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 invention, 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.
[0101] In Comparative Examples 2 and 9, among these, the period
from the solution treatment and the quenching treatment to room
temperature to the preliminary aging treatment (initiation of
heating) is 120 minutes, which is too long. Because of this, Mg--Si
clusters that do not contribute to strength have been yielded in a
large amount. Although the two exothermic peaks present in the
temperature range of 230-330.degree. C. have a difference in
temperature between the peaks of 50.degree. C. or less, the
exothermic-peak height exceeds 50 .mu.W/mg.
[0102] In Comparative Example 3, the period of holding in the
higher-temperature-side range in the preliminary aging treatment is
48 hours, which is too long. Because of this, the one exothermic
peak present in the temperature range of 230-330.degree. C. has too
small a height less than 20 .mu.W/mg.
[0103] In Comparative Examples 4, 11 and 14, the period of holding
in the lower-temperature-side range in the preliminary aging
treatment is 2 hours, which is too short. Because of this, although
the two exothermic peaks present in the temperature range of
230-330.degree. C. have a difference in temperature between the
peaks of 50.degree. C. or less, the exothermic-peak height exceeds
50 .mu.W/mg, or in the case where one exothermic peak is present in
the temperature range of 230-330.degree. C., this exothermic peak
has a height exceeding 50 .mu.W/mg.
[0104] In Comparative Examples 10 and 13, the period of holding in
the higher-temperature-side range in the preliminary aging
treatment is 2 hours, which is too short. Because of this, in the
case where one exothermic peak is present in the temperature range
of 230-330.degree. C., this exothermic peak has a height exceeding
50 .mu.W/mg.
[0105] In Comparative Example 7, the temperature in the
higher-temperature-side range in the preliminary aging treatment is
70.degree. C., which is too low. Because of this, although the two
exothermic peaks present in the temperature range of
230-330.degree. C. have a difference in temperature between the
peaks of 50.degree. C. or less, the higher exothermic peak has a
height exceeding 50 .mu.W/mg.
[0106] In Comparative Example 8, the temperature in the
higher-temperature-side range in the preliminary aging treatment is
130.degree. C., which is too high. Because of this, in the case
where one exothermic peak is present in the temperature range of
230-330.degree. C., this exothermic peak has a height less than 20
.mu.W/mg.
[0107] Comparative Examples 22 to 30 in Table 2 have been produced
under preferred conditions, including the conditions for the
preliminary aging treatment. However, since they employed alloys
Nos. 10 to 18 shown in Table 1, the contents of Mg and Si, which
are essential elements, therein are outside the ranges according to
the present invention or the content of impurity elements therein
is too high. Because of this, these Comparative Examples 22 to 30
each show, in particular, a relatively too high As proof stress
after 30-day room-temperature holding as compared with the
Invention Examples, as shown in Table 2. They hence are poor in
press formability into automotive panels or the like and in hem
workability or are poor in BH response. The compositions of
Comparative Examples 22 to 30 are described in detail below.
[0108] Comparative Example 22 is alloy 10 shown in Table 1, in
which the Si content is too low.
[0109] Comparative Example 23 is alloy 12 shown in Table 1, in
which the Mg+Si content is too high.
[0110] Comparative Example 24 is alloy 11 shown in Table 1, in
which the Si content is too high and the Mg+Si content is too
high.
[0111] Comparative Example 25 is alloy 13 shown in Table 1, in
which the Fe content is too high.
[0112] Comparative Example 26 is alloy 14 shown in Table 1, in
which the Mn content is too high.
[0113] Comparative Example 27 is alloy 15 shown in Table 1, in
which the Cr and Ti contents are too high.
[0114] Comparative Example 28 is alloy 16 shown in Table 1, in
which the Cu content is too high.
[0115] Comparative Example 29 is alloy 17 shown in Table 1, in
which the Zn content is too high.
[0116] Comparative Example 30 is alloy 18 shown in Table 1, in
which the Zr and V contents are too high.
[0117] DSCs selected from those of the Invention Examples and
Comparative Examples are shown in FIG. 1. In FIG. 1, the thick
continuous line indicates Invention Example 1, the thin continuous
line indicates Invention Example 12 and the broken line indicates
Comparative Example 23.
[0118] In the DSC of Invention Example 1, a first exothermic peak
of .beta.'' appears around 270.degree. C. and a second exothermic
peak of .beta.' appears around 300.degree. C. near the first peak,
and the difference in temperature between these peaks is 27.degree.
C. as shown in Table 2, which is 50.degree. C. or less as
specified.
[0119] In the DSC of Invention Example 12, a first exothermic peak
of .beta.'' and a second exothermic peak of .beta.' overlap each
other to form one synthesized peak. This synthesized peak appears
around 290.degree. C. and, as shown in Table 2, has a peak height
of 35.9 .mu.W/mg, which is in the range of 20-50 .mu.W/mg.
[0120] In contrast, in the DSC of Comparative Example 23, a first
exothermic peak of .beta.'' appears around 260.degree. C. and a
second exothermic peak of .beta.' appears around 310.degree. C.,
and the difference in temperature between these peaks is 53.degree.
C. as shown in Table 2, which exceeds the specified temperature of
50.degree. C.
[0121] Those results of the Examples support 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 invention should be satisfied.
TABLE-US-00001 TABLE 1 Alloy Chemical components of Al--Mg--Si
alloy sheet (mass %; remainder, Al) No. Mg Si Mg + Si Fe Cu Mn Cr
Zr V Ti Zn Ag 0 0.40 0.60 1.00 1 0.40 0.60 1.00 0.4 2 0.32 0.65
0.97 0.2 0.12 3 0.34 0.58 0.92 0.2 0.12 0.05 4 0.38 0.45 0.83 0.2
0.3 5 0.48 0.52 1.00 0.2 0.2 6 0.54 0.45 0.99 0.2 0.05 0.06 7 0.28
0.67 0.95 0.2 0.07 0.07 8 0.36 0.49 0.85 0.2 0.08 0.4 9 0.54 0.61
1.15 0.2 0.2 10 0.66 0.15 0.81 0.2 11 0.45 1.03 1.48 0.2 12 0.40
0.91 1.31 0.2 13 0.38 0.66 1.04 0.71 14 0.65 0.41 1.06 0.2 0.72
0.01 15 0.35 0.80 1.15 0.2 0.4 0.13 16 0.41 0.62 1.03 0.2 0.88 17
0.31 0.58 0.89 0.2 0.95 18 0.36 0.72 1.08 0.2 0.4 0.4
TABLE-US-00002 TABLE 2 Preliminary aging treatment
Higher-temperature- Lower-temperature- Required side range side
range period to (80-120.degree. C.) (60-40.degree. C.) preliminary
Holding Holding Alloy No. aging Temperature period Temperature
period Classification No. in Table 1 min .degree. C. hr .degree. C.
hr Inv. Ex. 0 0 5 100 12 50 24 Inv. Ex. 1 1 5 100 12 50 24 Com. Ex.
2 1 120 100 12 50 24 Com. Ex. 3 1 5 100 48 50 24 Com. Ex. 4 1 5 100
12 50 2 Inv. Ex. 5 2 5 100 12 50 24 Inv. Ex. 6 2 5 110 5 50 24 Com.
Ex. 7 2 5 70 12 50 24 Com. Ex. 8 2 5 130 12 50 24 Com. Ex. 9 2 120
100 12 50 24 Com. Ex. 10 2 5 100 2 50 24 Com. Ex. 11 2 5 100 12 50
2 Inv. Ex. 12 3 5 100 12 50 240 Com. Ex. 13 3 5 100 2 50 240 Com.
Ex. 14 3 5 100 12 50 2 Inv. Ex. 15 4 5 100 12 50 24 Inv. Ex. 16 4 5
80 30 50 24 Inv. Ex. 17 5 5 90 12 50 24 Inv. Ex. 18 6 15 100 12 50
24 Inv. Ex. 19 7 5 100 20 50 24 Inv. Ex. 20 8 5 100 12 40 240 Inv.
Ex. 21 9 5 90 12 60 24 Com. Ex. 22 10 5 100 12 50 24 Com. Ex. 23 12
5 100 12 50 24 Com. Ex. 24 11 5 100 12 50 24 Com. Ex. 25 13 5 100
12 50 24 Com. Ex. 26 14 5 100 12 50 24 Com. Ex. 27 15 5 100 12 50
24 Com. Ex. 28 16 5 100 12 50 24 Com. Ex. 29 17 5 100 12 50 74 Com.
Ex. 30 18 5 100 12 50 24 Structure of aluminum alloy sheet after
30-day room-temperature holding Exothermic peaks at 230-330.degree.
C. in differential scanning thermal analysis curve Height of
First-peak Second-peak Peak Alloy No. Number higher peak
temperature temperature temperature Classification No. in Table 1
of peaks .mu.W/mg .degree. C. .degree. C. difference Inv. Ex. 0 0 2
40.5 273 301 28 Inv. Ex. 1 1 2 41.9 273 300 27 Com. Ex. 2 1 2 63.4
268 294 26 Com. Ex. 3 1 1 16.8 295 -- -- Com. Ex. 4 1 2 54.8 271
297 26 Inv. Ex. 5 2 1 33.6 290 -- -- Inv. Ex. 6 2 1 28.1 291 -- --
Com. Ex. 7 2 2 56.8 272 299 27 Com. Ex. 8 2 1 12.2 295 -- -- Com.
Ex. 9 2 2 54.5 271 301 30 Com. Ex. 10 2 1 54.4 286 -- -- Com. Ex.
11 2 1 52.1 296 -- -- Inv. Ex. 12 3 1 35.9 290 -- -- Com. Ex. 13 3
1 57.2 287 -- -- Com. Ex. 14 3 1 54.7 295 -- -- Inv. Ex. 15 4 1
35.6 295 -- -- Inv. Ex. 16 4 2 42.1 270 299 29 Inv. Ex. 17 5 2 40.1
274 301 27 Inv. Ex. 18 6 1 43.7 290 -- -- Inv. Ex. 19 7 2 27.5 273
301 28 Inv. Ex. 20 8 1 34.8 292 -- -- Inv. Ex. 21 9 2 38.2 271 297
26 Com. Ex. 22 10 1 10.4 296 -- -- Com. Ex. 23 12 2 56.0 258 311 53
Com. Ex. 24 11 2 38.7 258 312 54 Com. Ex. 25 13 2 43.1 271 298 27
Com. Ex. 26 14 1 35.8 291 -- -- Com. Ex. 27 15 2 44.5 269 296 27
Com. Ex. 28 16 2 39.2 273 300 27 Com. Ex. 29 17 1 38.3 290 -- --
Com. Ex. 30 18 2 40.2 271 298 27 Properties of aluminum alloy after
30-day room-temperature holding As 0.2% 0.2% proof Proof stress
Alloy No. proof stress stress after BH increase Hem Classification
No. in Table 1 MPa MPa MPa workability Inv. Ex. 0 0 105 195 90 2
Inv. Ex. 1 1 103 195 92 2 Com. Ex. 2 1 108 162 54 2 Com. Ex. 3 1
123 211 88 3 Com. Ex. 4 1 88 166 78 1 Inv. Ex. 5 2 98 182 84 1 Inv.
Ex. 6 2 107 185 78 2 Com. Ex. 7 2 105 166 61 2 Com. Ex. 8 2 136 184
48 3 Com. Ex. 9 2 103 154 51 1 Com. Ex. 10 2 85 161 76 1 Com. Ex.
11 2 87 163 76 1 Inv. Ex. 12 3 106 184 78 2 Com. Ex. 13 3 102 166
64 2 Com. Ex. 14 3 86 166 80 1 Inv. Ex. 15 4 107 179 72 2 Inv. Ex.
16 4 105 182 77 2 Inv. Ex. 17 5 98 175 77 1 Inv. Ex. 18 6 102 176
74 1 Inv. Ex. 19 7 108 194 86 2 Inv. Ex. 20 8 106 179 73 2 Inv. Ex.
21 9 105 187 82 1 Com. Ex. 22 10 71 114 43 1 Com. Ex. 23 12 137 249
112 3 Com. Ex. 24 11 146 249 103 4 Com. Ex. 25 13 107 191 84 4 Com.
Ex. 26 14 121 202 81 4 Com. Ex. 27 15 118 211 93 4 Com. Ex. 28 16
132 223 91 3 Com. Ex. 29 17 102 180 78 4 Com. Ex. 30 18 117 201 84
4
[0122] While the present invention has been described in detail and
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. This application is based on a Japanese
patent application filed on Mar. 31, 2014 (Application No.
2014-074044), the contents thereof being incorporated herein by
reference.
INDUSTRIAL APPLICABILITY
[0123] 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.
* * * * *