U.S. patent application number 15/295455 was filed with the patent office on 2017-04-27 for aluminum alloy sheet having good formability.
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, Hisao Shishido.
Application Number | 20170114431 15/295455 |
Document ID | / |
Family ID | 58558411 |
Filed Date | 2017-04-27 |
United States Patent
Application |
20170114431 |
Kind Code |
A1 |
Aruga; Yasuhiro ; et
al. |
April 27, 2017 |
ALUMINUM ALLOY SHEET HAVING GOOD FORMABILITY
Abstract
Provided is a 6000-series aluminum alloy sheet having good
formability for an automotive body panel, which can be manufactured
without greatly varying an existing composition or an existing
manufacturing condition. The amount of solute Si and the amount of
solute Cu in an Al--Mg--Si aluminum alloy sheet are increased in a
balanced manner, so that the sheet has a dislocation density within
a specific range when tensile deformation in a low-strain region is
applied to the sheet. This suppresses localization of dislocations
introduced into a material due to tensile deformation during press
forming into an automotive body panel, and allows dislocations to
be evenly multiplied from the low-strain region to a high-strain
region. Consequently, uneven deformation is suppressed during
forming into the automotive body panel, and good work hardenability
is exhibited.
Inventors: |
Aruga; Yasuhiro; (Kobe-shi,
JP) ; Shishido; Hisao; (Moka-shi, 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: |
58558411 |
Appl. No.: |
15/295455 |
Filed: |
October 17, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 21/02 20130101;
C22C 21/08 20130101 |
International
Class: |
C22C 21/08 20060101
C22C021/08; C22C 21/02 20060101 C22C021/02 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 21, 2015 |
JP |
2015-207061 |
Claims
1. An aluminum alloy sheet having good formability, comprising an
Al--Mg--Si aluminum alloy sheet that contains, by mass percent, Si:
0.30 to 2.0%, Mg: 0.20 to 1.5%, Cu: 0.05 to 1.0%, Mn: 1.0% or less
(not including 0%), and Fe: 1.0% or less (not including 0%), the
remainder consisting of Al and inevitable impurities, wherein an
amount of solute Si is 0.30 to 2.0% and an amount of solute Cu is
0.05 to 1.0% in a solution of the aluminum alloy sheet, the
solution being separated by a hot-phenol residue extraction method,
and when tensile deformation with a strain of 5% is applied to the
aluminum alloy sheet in a rolling direction of the aluminum alloy
sheet, dislocation density in a rolled surface of the aluminum
alloy sheet is 6.0.times.10.sup.14 to 12.times.10.sup.14 m.sup.-2
in average, the dislocation density being measured by X-ray
diffraction.
2. The aluminum alloy sheet according to claim 1, wherein the
aluminum alloy sheet further contains one or more of Cr: 0.3% or
less (not including 0%), Zr: 0.3% or less (not including 0%), V:
0.3% or less (not including 0%), Ti: 0.1% or less (not including
0%), Zn: 1.0% or less (not including 0%), Ag: 0.2% or less (not
including 0%), and Sn: 0.15% or less (not including 0%).
3. The aluminum alloy sheet according to claim 1, wherein a yield
ratio is 0.56 or less, the yield ratio being defined by a ratio of
0.2% proof stress to tensile strength (0.2% proof stress/tensile
strength) of the aluminum alloy sheet, and a total elongation is
26% more.
Description
BACKGROUND
[0001] The present invention relates to an Al--Mg--Si aluminum
alloy sheet having good formability. The aluminum alloy sheet
described in the invention refers to a rolled sheet such as a
hot-rolled sheet or a cold-rolled sheet, which has been subjected
to tempering such as solution treatment and quenching, but has not
been bent into a component to be used or has not been subjected to
paint-bake hardening. Hereinafter, aluminum may be referred to as
Al.
[0002] Recently, a social demand for weight saving of vehicles has
increased more and more out of consideration for the global
environment. To meet such a demand, a more lightweight aluminum
alloy material having good formability and paint-bake hardenability
(bake hardenability, hereinafter may be referred to as BH property)
is increasingly used as a vehicle material in place of a steel
material such as a steel sheet.
[0003] AA or JIS 6000-series Al--Mg--Si (hereinafter, may be simply
referred to as 6000-series) aluminum alloy sheet is typically
exemplified as an aluminum alloy sheet for a large automotive body
panel such as an outer panel and an inner panel of an automobile.
The 6000-series aluminum alloy sheet has a composition
indispensably containing Si and Mg, which maintains formability due
to its low proof stress (low strength) during forming, but is
increased in proof stress (strength) through heating during
artificial aging (hardening) such as paint-bake hardening of a
formed panel so as to have a required strength, i.e., has good
paint-bake hardenability.
[0004] In design, the automotive outer panel must achieve a
beautiful curved-surface configuration and a beautiful character
line without distortion or wrinkles even if a corner or a character
line has a sharpened or complicated shape. The automotive inner
panel must also achieve a beautiful curved-surface configuration
without distortion or wrinkles even if a designed concavo-convex
shape becomes deeper (higher) or complicated in relation to the
outer panel.
[0005] Such a demand for high formability becomes strict every year
along with expanded use of the aluminum alloy sheet as a
material.
[0006] However, it is considerably difficult to achieve such good
formability required for the automotive body panel application
without greatly varying a typical (existing) alloy composition
range, a typical manufacturing process, or a typical manufacturing
condition of the 6000-series aluminum alloy sheet that is a
material less workable than a steel sheet material.
[0007] In this regard, as well known, there have been suggested
many approaches for controlling a composition or a microstructure
to improve formability or strength characteristics of the material
6000-series aluminum alloy sheet for the body panel, the approaches
including control of grain size, control of a texture, and control
of atom cluster.
[0008] Such suggested approaches for microstructure control include
various approaches such as control of the amount of solute Mg,
control of the amount of solute Si, control of the amount of solute
Cu, and control of dislocation density.
[0009] For example, Japanese Unexamined Patent Application
Publication No. 2008-174797 suggests that the amount of solute Si
is defined to be 0.55 to 0.80 mass %, the amount of solute Mg is
defined to be 0.35 to 0.60 mass %, and a ratio of the amount of
solute Si to the amount of solute Mg is defined to be 1.1 to 2 in
order to produce a 6000-series aluminum alloy sheet that has good
normal-temperature stability and is less likely to be deteriorated
in material properties such as bake hardenability (BH property)
through room temperature aging.
[0010] Japanese Unexamined Patent Application Publication No.
2008-266684 suggests a warm-forming 6000-series aluminum alloy
sheet having good BH property, which has an amount of solute Cu of
0.01 to 0.7%, the amount being measured by a residue extraction
method, and an average grain size of 10 to 50 .mu.m.
[0011] T. Masuda; S. Hirosawa; Z. Horita; K. Matsuda Experimental
and Computational Studies of Competitive Precipitation Behavior
Observed in an Al--Mg--Si Alloy with High Dislocation Density and
Ultrafine-Grained Microstructures, J. Japan Inst. Metals. 2011,
75(5), pp. 283-290 suggests that a microscopic structural parameter
(dislocation density, grain size) as an optimum combination of
dislocation strengthening or refining strengthening and
precipitation strengthening is predicted to further increase
strength of a 6000-series aluminum alloy sheet.
[0012] It is described that a specimen, which is prepared by
performing cold rolling or HPT processing as one giant straining
process on a 6000-series aluminum alloy sheet, is examined in
dislocation density, and an unprocessed material has a dislocation
density of about 10.sup.11 m.sup.-2, and a cold-rolled material
subjected to a reduction of 30% (equivalent strain 0.36) has a
dislocation density of about 10.sup.14 m.sup.-2.
[0013] The dislocation density is measured by a cross analysis
method using five view fields in a 100,000.times.TEM photograph
with a fringe of equal thickness method.
SUMMARY
[0014] In such existing techniques, control of the amount of a
solute element or control of dislocation density is performed to
specifically improve strength characteristics of the 6000-series
aluminum alloy sheet. Hence, although formability is naturally
considered to be improved, such formability is still at a level of
common hem bendability or press formability. That is, it is not
intended to achieve severe and good formability as required for the
recent automotive body panel.
[0015] Hence, the fact is that there have been only known measures
to achieve such severe and good formability required for the
automotive body panel application, such as a reduction in load
during forming by modifying a panel design or a forming condition,
or a reduction in strength during forming of the 6000-series
aluminum alloy sheet.
[0016] An object of the invention, which has been given to solve
such a problem, is to provide a 6000-series aluminum alloy sheet
having good formability for the automotive body panel, the aluminum
alloy sheet being manufactured without greatly varying a
composition or manufacturing condition of the existing 6000-series
aluminum alloy sheet.
[0017] To achieve the object, an aluminum alloy sheet having good
formability of the invention is summarized by an Al--Mg--Si
aluminum alloy sheet that contains, by mass percent, Si: 0.30 to
2.0%, Mg: 0.20 to 1.5%, Cu: 0.05 to 1.0%, Mn: 1.0% or less (not
including 0%), and Fe: 1.0% or less (not including 0%), the
remainder consisting of Al and inevitable impurities, in which the
amount of solute Si is 0.30 to 2.0% and the amount of solute Cu is
0.05 to 1.0% in a solution of the aluminum alloy sheet, the
solution being separated by a hot-phenol residue extraction method,
and when tensile deformation with a strain of 5% is applied to the
aluminum alloy sheet in a rolling direction of the aluminum alloy
sheet, dislocation density in a rolled surface of the aluminum
alloy sheet is 6.0.times.10.sup.14 to 12.times.10.sup.14 m.sup.-2
in average, the dislocation density being measured by X-ray
diffraction.
[0018] The invention is intended to increase the amount of solute
Si and the amount of solute Cu in the 6000-series aluminum alloy
sheet, and suppress localization of dislocations introduced into a
material due to tensile deformation during forming into an
automotive body panel, and thus uniformly (relatively highly)
multiply dislocations from a low strain region to a high strain
region of the tensile deformation.
[0019] This makes it possible to suppress uneven deformation from
the high strain region to rupture in press forming into the
automotive body panel, so that good work hardenability can be
exhibited.
[0020] However, an important index is the amount of dislocation
density in the sheet in the low strain region of tensile
deformation to simulate actual forming into the automotive body
panel in order to allow such a mechanism of solute Si or solute Cu
to be securely exhibited, and securely achieve good formability for
the automotive body panel.
[0021] In other words, it has been found that an increase in the
amount of solute Si or solute Cu alone is not enough, and the
amount of dislocation density in the sheet is also satisfied in the
low strain region of tensile deformation, thereby good formability
for the automotive body panel can be achieved.
[0022] It has been also found that the amount of dislocation
density in the low strain region during forming (during tensile
deformation) into an actual automotive body panel can be simulated
by dislocation density at application of tensile deformation with a
strain of 5% in a rolling direction of a material sheet, and thus
the two dislocation densities correlate with each other.
[0023] Specifically, it is a necessary condition that the amount of
solute Si and the amount of solute Cu in a material sheet are
increased in a balanced manner, and it is a sufficient condition
that when tensile deformation with a strain of 5% is applied to the
material sheet in a rolling direction of the material sheet, the
sheet has a predetermined dislocation density. Good formability for
the automotive body panel can be achieved by satisfying such two
conditions.
[0024] In addition, the good formability provided by such controls
can be advantageously achieved without greatly varying an existing
aluminum alloy composition or an existing manufacturing
condition.
DETAILED DESCRIPTION
[0025] Hereinafter, an embodiment of the invention is specifically
described for each of requirements.
Chemical Composition
[0026] A chemical composition of the Al--Mg--Si (hereinafter, may
be referred to as 6000-series) aluminum alloy sheet of the
invention is now described. The invention also satisfies the
requirements for the properties required for the body panel in
terms of the composition, the properties including good
formability, a BH property, strength, weldability, and corrosion
resistance. In such a case, however, it is also assumed that the
existing composition and the existing manufacturing condition are
not greatly varied.
[0027] To meet such a challenge in terms of the composition, the
composition of the 6000-series aluminum alloy sheet contains, by
mass percent, Si: 0.30 to 2.0%, Mg: 0.20 to 1.5%, Cu: 0.05 to 1.0%,
Mn: 1.0% or less (not including 0%), and Fe: 1.0% or less (not
including 0%), the remainder consisting of Al and inevitable
impurities.
[0028] In addition, the composition may contain one or more of Cr:
0.3% or less (not including 0%), Zr: 0.3% or less (not including
0%), V: 0.3% or less (not including 0%), Ti: 0.1% or less (not
including 0%), Zn: 1.0% or less (not including 0%), Ag: 0.2% or
less (not including 0%), and Sn: 0.15% or less (not including
0%).
[0029] The content range, and the meaning, and/or the acceptable
amount of each element of the 6000-series aluminum alloy sheet are
now described. The percentage representing the content of each
element refers to mass percent.
Si: 0.30 to 2.0%
[0030] Si is an indispensable element to provide strength (proof
stress) required for the outer panel of a vehicle, which, with Mg,
exhibits solid-solution strengthening, and forms Mg--Si
precipitates that contribute to an increase in strength during
artificial aging such as paint-bake treatment and thus exhibits
artificial aging hardenability (BH property).
[0031] The solute Si suppresses localization of dislocations
introduced into the material during press forming into the
automotive body panel, and has an effect of evenly multiplying
dislocations from the low strain region to the high strain region
of tensile deformation. This makes it possible to suppress uneven
deformation from the high strain region to rupture in press
forming, so that large elongation and good work hardenability can
be exhibited.
[0032] If the Si content is too small, the amount of solute Si
decreases, leading to a reduction in elongation during press
forming or deterioration in work hardenability. This results in a
decrease in amount of dislocation multiplication after application
of tensile deformation with a strain of 5%. In addition, since
production of the Mg--Si precipitates becomes insufficient, the BH
property is deteriorated, leading to a significant reduction in
strength after paint-bake treatment.
[0033] If the Si content is too large, coarse particles and
precipitates are formed, and a large crack occurs in the sheet
during hot rolling.
[0034] Consequently, the Si content is within a range from 0.30 to
2.0%. The preferred lower limit of Si is 0.50%, and the preferred
upper limit thereof is 1.5%.
Mg: 0.20 to 1.5%
[0035] Mg is also an indispensable element to provide proof stress
required for the panel, which, with Si, exhibits solid-solution
strengthening, and forms Mg--Si precipitates that contribute to an
increase in strength during artificial aging such as paint-bake
treatment and thus exhibits artificial aging hardenability (BH
property).
[0036] As with the solute Si, the solute Mg suppresses localization
of dislocations introduced into the material during press forming
into the automotive body panel, and exhibits an effect of evenly
multiplying dislocations from the low strain region to the high
strain region of tensile deformation. This suppresses uneven
deformation from the high strain region to rupture in press
forming, allowing large elongation and good work hardenability to
be exhibited.
[0037] If the Mg content is too small, the amount of solute Mg
decreases, leading to deterioration in work hardenability. This
results in a decrease in amount of dislocation multiplication after
application of tensile deformation with a strain of 5%. In
addition, since production of the Mg--Si precipitates becomes
insufficient, the BH property is deteriorated, leading to a
reduction in strength after paint-bake treatment.
[0038] If the Mg content is too large, coarse particles and
precipitates are formed, and a large crack occurs in the sheet
during hot rolling.
[0039] Consequently, the Mg content is within a range from 0.20 to
1.5%. The preferred lower limit of Mg is 0.30%, and the preferred
upper limit thereof is 1.2%.
Cu: 0.05 to 1.0%
[0040] Cu contributes to an increase in strength and improvement in
formability. As with the solute Si, solute Cu improves work
hardenability, and improves a balance between strength and
formability.
[0041] If the Cu content is less than 0.05%, the effect of Cu is
small. In addition, the amount of solute Cu is also insufficient,
and the effect of the solute Cu is also insufficient.
[0042] If the Cu content exceeds 1.0%, filiform corrosion
resistance after painting and stress corrosion cracking resistance
are significantly deteriorated. Hence, the Cu content is preferred
to be 0.8% or less for an application in which corrosion resistance
is important.
Mn: 1.0% or Less (not Including 0%)
[0043] Mn increases strength of an aluminum alloy through
solid-solution strengthening and a grain refinement effect.
However, if Mn is excessively contained to exceed 1.0%, the amount
of Al--Mn intermetallic compounds increases and fracture origins
are caused, and thus elongation easily decreases. When a low strain
of about 5% is applied to the sheet, dislocations are localized
around the Al--Mn intermetallic compounds, and thus work
hardenability is also deteriorated.
[0044] Consequently, the Mn content is 1.0% or less (not including
0%), preferably 0.80% or less (not including 0%).
Fe: 1.0% or Less (not Including 0%)
[0045] Fe forms Al--Fe intermetallic compounds in an aluminum
alloy. Hence, if the Fe content increases, the amount of such
compounds increases and fracture origins are caused, and thus
elongation easily decreases. In addition, each Al--Fe intermetallic
compound often includes Si, and thus the amount of solute Si is
decreased in correspondence to the amount of Si captured by the
intermetallic compound.
[0046] Fe is contaminated into an aluminum alloy as a bullion
impurity, and the content of Fe increases with an increase in
amount of aluminum alloy scrap (ratio relative to aluminum
bullion); hence, the smaller the Fe content, the better. However,
decreasing the Fe content to the detection limit or lower leads to
an increase in cost; hence, a certain level of Fe content must be
allowed.
[0047] Consequently, the Fe content is 1.0% or less (not including
0%), preferably 0.5% or less (not including 0%).
Other Elements
[0048] In addition, the invention allows the composition to further
contain one or more of Cr: 0.3% or less (not including 0%), Zr:
0.3% or less (not including 0%), V: 0.3% or less (not including
0%), Ti: 0.1% or less (not including 0%), Zn: 1.0% or less (not
including 0%), Ag: 0.2% or less (not including 0%), and Sn: 0.15%
or less (not including 0%).
[0049] Such elements in common exhibit an effect of increasing
strength of a sheet, and thus can be considered to be equieffective
in increasing strength. However, specific mechanisms of the effect
are in common on the one hand, but are naturally different on the
other hand.
[0050] As with Mn, each of Cr, Zr, and V forms dispersed particles
(dispersed phase) during homogenization. Such dispersed particles
have an effect of preventing grain boundary migration after
recrystallization, and refine grains.
[0051] Ti, with B, forms particles, which serve as nuclei of
recrystallized grains, and thus prevents coarsening of grains, and
refines the grains.
[0052] Each of Zn and Ag is useful for improving artificial aging
performance (BH property), and exhibits an effect of promoting
precipitation of a compound phase such as a GP zone into a grain
boundary of a sheet microstructure under a condition of relatively
low temperature and relatively short artificial aging.
[0053] Sn captures atomic vacancies and thus suppresses diffusion
of Mg or Si at room temperature, and thus suppresses an increase in
strength at room temperature (room temperature aging), and releases
the captured vacancies and thus promotes diffusion of Mg or Si,
leading to an effect of improving the BH property.
[0054] However, if the content of each of such elements is too
large, coarse compounds are formed, making it difficult to
manufacture the sheet. Furthermore, this reduces strength,
formability such as bendability, and corrosion resistance.
Consequently, when each of such elements is contained, the content
of the element is equal to or lower than the upper limit.
Microstructure
[0055] Assuming such an alloy composition, the invention also
defines a microstructure of the sheet in order to improve
formability, the microstructure including the amount of solute Si,
the amount of solute Cu, and dislocation density as described
below.
Amount of Solute Si and Amount of Solute Cu
[0056] For automotive body panel application, the amount of solute
Si and the amount of solute Cu in the 6000-series aluminum alloy
sheet have been mainly controlled to improve strength
characteristics as described in Japanese Unexamined Patent
Application Publication Nos. 2008-174797 and 2008-266684.
[0057] On the other hand, in the invention, formability into the
automotive body panel is improved by increasing the amount of
solute Si and the amount of solute Cu in a balanced manner.
[0058] The inventors have not found the case where the amount of
solute Si and the amount of solute Cu in the 6000-series aluminum
alloy sheet for the automotive body panel application are
controlled to improve formability.
[0059] A defined range of each of the amount of solute Si and the
amount of solute Cu and the meaning of the range are now
described.
Amount of Solute Si 0.30 to 2.0%
[0060] A larger amount of solute Si reduces stacking-fault energy
of aluminum alloy in conjunction with solute Cu, and suppresses
localization of dislocations introduced into a material during
tensile deformation, e.g., during press forming into an automotive
body panel, and thus evenly multiplies dislocations from the low
strain region to the high strain region of tensile deformation. As
a result, work hardenability is improved, a yield ratio is
decreased, and elongation increases.
[0061] If the amount of solute Si is less than 0.30%, the effect is
insufficient even if the amount of solute Cu is satisfied.
[0062] The upper limit of the amount of solute Si is substantially
equal to the upper limit of the Si content.
[0063] As with the solute Si, solute Mg also improves work
hardenability, reduces a yield ratio, and increases elongation.
[0064] However, while control of the amount of solute Si is
complicated and important because Si is precipitated together with
Al--Fe or Al--Mn intermetallic compounds, the amount of solute Mg
is relatively easily controlled because Mg is mainly precipitated
with Si.
[0065] Furthermore, since fluctuation of the amount of solute Mg
shows the same behavior or tendency as that of the amount of solute
Si, if only the amount of solute Si is measured and controlled to
satisfy the definition, the amount of solute Mg necessarily falls
within a preferred range, and thus the amount of solute Mg is not
necessary to be measured and controlled.
[0066] In the invention, therefore, the amount of solute Mg is not
particularly defined while functions and effects of the solute Mg
are expected.
Amount of Solute Cu 0.05 to 1.0%
[0067] The amount of solute Cu is also important in addition to the
amount of solute Si. As with solute Si, a larger amount of solute
Cu also improves work hardenability, decreases a yield ratio, and
increases elongation, and thus improves a balance between strength
and formability.
[0068] If the amount of solute Cu is less than 0.05%, the effect is
insufficient even if the amount of solute Si is satisfied.
[0069] The upper limit of the amount of solute Cu is substantially
equal to the upper limit of the amount of added Cu.
Dislocation Density
[0070] To allow the above-described mechanism of solute Si or
solute Cu to be securely exhibited to securely achieve good
formability for the automotive body panel, it is necessary not only
to control the amount of solute Si and the amount of solute Cu, but
also to control the amount of dislocation density in the sheet in
the low strain region during forming into the actual automotive
body panel.
[0071] Such an amount of dislocation density in the low strain
region can be reproducibly measured by dislocation density in a
sheet to which tensile deformation with a strain of 5% is applied
in a rolling direction of the sheet to simulate press forming into
an actual automotive body panel.
[0072] Hence, in the invention, a sheet, which satisfies the
composition, the amount of solute Si, and the amount of solute Cu,
is subjected to a tensile test simulating the press forming into an
actual automotive body panel. Dislocation density of the sheet, to
which the tensile deformation with a strain of 5% is (has been)
applied, is controlled into a range from 6.0.times.10.sup.14 to
12.times.10.sup.14 m.sup.-2.
[0073] The dislocation density is determined through measurement by
X-ray diffraction of a microstructure of a rolled surface (rolled
plane) of the sheet to which tensile deformation with a strain of
5% has been applied in a rolling direction of the sheet.
[0074] Dislocations are multiplied evenly (relatively highly) in
the above-described range in the low strain region in which a
strain of about 5% is shown at the tensile test. Uneven deformation
is thus suppressed from the subsequent high strain region to
rupture, and good work hardenability (a decrease in yield ratio, an
increase in elongation) is exhibited.
[0075] The dislocation density of lower than 6.0.times.10.sup.14
m.sup.-2 suggests that dislocations are less likely to be
multiplied, i.e., work hardenability is bad. This causes early
rupture in the high strain region, leading to deterioration of
formability.
[0076] Conversely, the dislocation density of higher than
12.times.10.sup.14 m.sup.-2 decreases dislocations that can be
introduced and accumulated in the subsequent high strain region;
hence, formability is also not improved.
[0077] Consequently, when (after) tensile deformation of 5% is
applied in a rolling direction of the sheet, dislocation density is
defined to be within a range from 6.0.times.10.sup.14 to
12.times.10.sup.14 m.sup.-2, preferably 7.0.times.10.sup.14 to
11.times.10.sup.14 m.sup.-2 in average.
[0078] As described in J. Japan Inst. Metals. 2011, 75(5), pp.
283-290, a typical 6000-series aluminum alloy sheet, to which
tensile deformation with a strain of 5% is not applied unlike in
the invention, has a dislocation density of only about 10.sup.11
m.sup.-2 in an unprocessed state (solution-treated material) while
simple comparison is difficult because such a value is measured by
a different method (measured with TEM with 100,000 magnifications).
The sheet has a dislocation density of only about 10.sup.14
m.sup.-2 while being subjected to cold rolling with a reduction of
30% (equivalent strain 0.36).
[0079] In contrast, in the invention, tensile deformation with a
low strain of only 5% is merely applied to a solution-treated
cold-rolled sheet, thereby the dislocation density of
6.0.times.10.sup.14 to 12.times.10.sup.14 m.sup.-2 can be
introduced, the dislocation density exceeding dislocation density
applied by cold rolling in J. Japan Inst. Metals. 2011, 75(5), pp.
283-290.
[0080] This is because of the increase in each of the amount of
solute Si and the amount of solute Cu in the invention. This means
that the dislocation density defined by the invention cannot be
introduced without such an increase. Furthermore, this means that a
mechanism of strain or dislocation density introduced into a
material is completely different between tensile deformation during
forming into an automotive body panel and cold rolling of a sheet
as in J. Japan Inst. Metals. 2011, 75(5), pp. 283-290.
[0081] The technical idea of the invention, i.e., the idea of
increasing the amount of solute Si and the amount of solute Cu is
given only after recognizing the relationship between formability
into the automotive body panel and the amount of solute Si or the
amount of solute Cu.
[0082] The technical idea of controlling the amount of dislocation
density in a sheet is also given only after noting dislocation
density introduced into a material due to tensile deformation,
particularly dislocation density in the low strain region during
forming into an automotive body panel, for example.
[0083] Furthermore, the understanding that a dislocation density as
high as 6.0.times.10.sup.14 to 12.times.10.sup.14 m.sup.-2 can be
introduced only by applying tensile deformation with a low strain
of only 5% to a solution-treated material (unprocessed material) is
given only after getting the technical idea and confirming the idea
through an actual test.
[0084] Hence, even if known examples including J. Japan Inst.
Metals. 2011, 75(5), pp. 283-290 have noted influence of
dislocation density in a sheet on properties such as strength of
the sheet, or even if there is a known example where sheet strength
is increased by increasing the amount of solute Si or the amount of
solute Cu, the configuration of the invention is not easily
obtained.
Measurement Method of Dislocation Density
[0085] Dislocation density is generally measured by, for example, a
transmission electron microscope as in J. Japan Inst. Metals. 2011,
75(5), pp. 283-290. In the invention, dislocation density is
measured more simply and reproducibly by X-ray diffraction.
[0086] A region dense with linear or streaky dislocations (a cell
wall or shear band) in a dislocation is difficult to be determined
by the transmission electron microscope, and may cause measurement
error for obtaining dislocation density .rho.. On the other hand,
X-ray diffraction is advantageous in that errors are decreased even
for such a forest dislocation because the dislocation density .rho.
is calculated from half value widths of diffraction peaks from
various faces of a texture as described later.
[0087] In a microstructure of a sheet into which dislocations are
introduced by applying plastic deformation through cold rolling or
a tensile test, lattice distortion occurs around a dislocation. In
addition, a low-angle grain boundary or a cell structure is
developed with dislocation arrangement. When such a dislocation and
a domain structure associated with the dislocation are taken from
an X-ray diffraction pattern, a distinctive spread or shape
corresponding to a diffraction index appears in a diffraction peak.
Dislocation density can be determined through analysis (line
profile analysis) of the diffraction peak shape (line profile).
[0088] Specifically, first, a JIS Z2201 No. 5 test specimen (25
mm.times.50 mm gage length (GL).times.thickness) is taken as a test
sample sheet from a tempered cold-rolled sheet according to a
procedure of a tensile test, and the test specimen is stretched at
room temperature in a rolling direction as a tensile direction.
This is to simulate a dislocation density of a sheet in a low
strain region during forming into an actual automotive body panel,
and tensile deformation with a strain of 5% is applied as the low
strain region.
[0089] A microstructure of a rolled surface (rolled plane) of the
test specimen, to which tensile deformation with a strain of 5% is
applied, is subjected to X-ray diffraction to obtain half value
widths of diffraction peaks from the faces (bearing faces) of
(111), (200), (220), (311), (400), (331), (420), and (422) as major
orientations of a texture of a surficial portion of the sheet (test
specimen). The half value width of the diffraction peak of each
face increases with an increase in dislocation density .rho.. The
rolled surface to be measured by X-ray diffraction of the test
specimen, to which tensile deformation with a strain of 5% is
applied, may be left as it is, or may be washed without
etching.
[0090] Subsequently, lattice distortion (crystal distortion)
.epsilon. is obtained from the half value widths of the diffraction
peaks from the faces by the Williamson-Hall method, and then the
dislocation density .rho. can be calculated by the following
formula.
.rho.=16.1.epsilon..sup.2/b.sup.2
[0091] where .rho. is dislocation density, e is lattice distortion,
and b is magnitude of Burgers vector.
[0092] Moreover, 2.8635.times.10.sup.-10 m is used as the magnitude
of Burgers vector.
[0093] The Williamson-Hall method is a known line profile analysis
that is generally used to determine dislocation density or grain
size from a relationship between a plurality of half value widths
of diffraction and a plurality of diffraction angles. Such a series
of ways to determine dislocation density by X-ray diffraction are
also well known. The invention generally refers to the series of
ways to determine dislocation density by X-ray diffraction as
"dislocation density measured by X-ray diffraction".
Index of Good Work Hardenability (Good Formability)
[0094] An index (guideline) of achievement of good work
hardenability (good formability) by the control of the composition
and the microstructure includes yield ratio and elongation.
[0095] A low yield ratio and large elongation support better
formability for the automotive body panel without a forming test of
a sheet with a small test specimen or without a forming test of a
sheet into an actual automotive body panel.
[0096] Specifically, the index (guideline) of achievement of good
formability is that a yield ratio, which is defined by a ratio of
0.2% proof stress to tensile strength (0.2% proof stress/tensile
strength), of an aluminum alloy sheet is 0.56 or less, and total
elongation is 26% or more as supported by Example described
later.
[0097] If the yield ratio is excessively high to extend 0.56, or if
the total elongation is excessively small, less than 26%, the good
work hardenability or good formability for the automotive body
panel cannot be achieved.
Manufacturing Method
[0098] A method of manufacturing the aluminum alloy sheet of the
invention is now described.
[0099] The aluminum alloy sheet of the invention is manufactured by
a common or known manufacturing process, in which an aluminum alloy
slab having the 6000-series composition is casted, and is then
subjected to homogenization, hot rolling, and cold rolling in order
and thus formed into a sheet having a predetermined thickness, and
then the sheet is subjected to tempering such as solution
hardening.
[0100] In such a manufacturing process, however, as described
later, a soaking condition, a hot finish rolling condition, and a
solution condition, and a quenching condition are each adjusted to
be within a preferred range in order to securely and reproducibly
provide the microstructure (the amount of solute Si and the amount
of solute Cu, or dislocation density) defined by the invention.
Cooling Rate in Melting and Casting
[0101] In a melting-and-casting step, molten metal of aluminum
alloy, which is melted and adjusted to be within the 6000-series
composition range, is casted by an appropriately selected common
melting-and-casting process such as a continuous casting process
and a semi-continuous casting process (DC casting process). The
average cooling rate during casting is preferably controlled to be
as high (fast) as possible, i.e., 30.degree. C./min or more from
the liquidus temperature to the solidus temperature in order to
control the microstructure (the amount of solute Si and the amount
of solute Cu, or dislocation density) within the range defined by
the invention.
[0102] If such temperature (cooling rate) control in a high
temperature region during casting is not performed, the cooling
rate in the high temperature region inevitably becomes lower. If
the cooling rate in the high temperature region thus becomes lower,
an increased amount of coarse particles are produced within the
temperature range of the high temperature region, leading to a
decrease in amount of solute Si and in amount of solute Cu in the
slab. As a result, it is difficult to control the microstructure to
be within the range of the invention.
Homogenization
[0103] Subsequently, the casted aluminum alloy slab is subjected to
homogenization prior to hot rolling. The homogenization (soaking)
is important for sufficient solid solution of Si and Mg in addition
to homogenization of a microstructure (eliminating segregation in a
grain of a slab microstructure) as a common purpose. Any
homogenization condition including common onetime or one-stage
treatment may be used without limitation as long as such purposes
are achieved.
[0104] Homogenization temperature is 500 to 560.degree. C., and
homogenization (holding) time is appropriately selected from a
range of 1 hr or more to sufficiently dissolve Si and Cu. If the
homogenization temperature is low, the amount of solute Si or Cu
cannot be provided, and the microstructure (the amount of solute Si
and the amount of solute Cu) defined by the invention cannot be
produced even by pre-aging (reheating) after solution hardening as
described later. In addition, segregation in the grain cannot be
sufficiently eliminated, which serves as an origin of fracture,
leading to deterioration in formability.
[0105] After the homogenization, the slab is hot-rolled, in which
the temperature of the slab is not decreased to 500.degree. C. or
lower before start of hot rough rolling after the homogenization in
order to provide the amount of solute Si and the amount of solute
Cu.
[0106] If temperature of the slab is decreased to 500.degree. C. or
lower before start of rough rolling, Si or Cu is precipitated. It
is therefore more difficult to provide a certain amount of solute
Si or solute Cu to form the microstructure defined by the
invention.
Hot Rolling
[0107] Hot rolling includes a rough rolling step for the slab and a
finish rolling step depending on thickness of the sheet to be
rolled. A reverse-type or tandem-type rolling mill is appropriately
used for the rough rolling step or the finish rolling step.
[0108] During rolling from start to finish of hot rough rolling, it
is necessary to maintain the amount of solute Si and the amount of
solute Mg without lowering temperature to 450.degree. C. or
lower.
[0109] If minimum interpass temperature of a rough rolled sheet is
lowered to 450.degree. C. or lower due to, for example, increased
rolling time, Mg--Si compounds are easily precipitated, and thus
the amount of solute Si and the amount of solute Cu are decreased.
It is therefore more difficult to provide a certain amount of
solute Si or solute Cu to form the microstructure defined by the
invention.
[0110] After such hot rough rolling, the slab is subjected to hot
finish rolling with finish temperature in a range from 300 to
360.degree. C.
[0111] If finish temperature of the hot finish rolling is extremely
low, lower than 300.degree. C., a rolling load increases and
productivity is reduced. On the other hand, if the finish
temperature of the hot finish rolling is increased to form a
recrystallized structure while a large amount of worked structure
is not left, the finish temperature of more than 360.degree. C.
causes the Mg--Si compounds to be easily precipitated, leading to a
decrease in each of the amount of solute Si and the amount of
solute Cu. It is therefore difficult to provide the amount of
solute Si or solute Cu to form the microstructure defined by the
invention.
[0112] An average cooling rate from the material (sheet)
temperature immediately after finish of the hot finish rolling to
the material temperature of 150.degree. C. is controlled to at
least 5.degree. C./hr.
[0113] If the average cooling rate is lower than 5.degree. C./hr, a
large amount of Mg--Si precipitates are produced during such
cooling, and the amount of solute Si in a product sheet is
decreased.
[0114] Hence, the average cooling rate immediately after finish of
the hot finish rolling is preferably higher, and is at least
5.degree. C./hr or higher, preferably 8.degree. C./hr or
higher.
Annealing of Hot-Rolled Sheet
[0115] Although annealing (heat treatment) of the hot-rolled sheet
before cold rolling is not necessary, the annealing may be
performed.
Cold Rolling
[0116] In cold rolling, the hot-rolled sheet is rolled and formed
into a cold-rolled sheet (including a coil) having a desired final
thickness. Cold reduction is desirably 30% or more to further
refine the grains. In addition, intermediate annealing may be
performed between cold rolling passes for the same purpose as that
of the heat treatment.
Solution Treatment and Quenching
[0117] The cold-rolled sheet is subjected to solution treatment and
subsequent quenching to room temperature. The solution hardening
may be performed using a typical continuous heat treatment
line.
[0118] However, to provide a sufficient solid-solution amount of
each element such as Mg and Si, the cold-rolled sheet is preferably
heated to a solution treatment temperature of 550.degree. C. or
higher and equal to or lower than the melting point and held at the
temperature for 10 sec or more, and then cooled with a preferred
average cooling rate of 20.degree. C./sec or more from such holding
temperature to 100.degree. C.
[0119] If the temperature is lower than 550.degree. C., or if the
holding time is shorter than 10 see, reversion of Cu-containing
Al--Mn, Al--Fe, or Mg--Si compounds, which have been produced
before the solution treatment, is insufficient, and the amount of
solute Si and the amount of solute Cu are decreased.
[0120] If the average cooling rate is less than 20.degree. C./sec,
Mg--Si precipitates are mainly produced during cooling and thus the
amount of solute Si is decreased. Consequently, the amount of
solute Si is also difficult to be provided. To achieve such a
cooling rate, cooling methods such as air cooling with a fan and
water cooling with mist, spray, or dipping, and conditions are
selectively used for the quenching.
Pre-Aging: Reheating
[0121] Pre-aging is performed after such solution treatment and
quenching as necessary. The pre-aging has a small influence on the
amount of solute Si or the amount of solute Cu, and is selectively
performed if improvement in BH property is necessary, for
example.
[0122] The pre-aging (reheating), if performed, is preferably
performed within one hour after the sheet is subjected to the
quenching and cooled to room temperature.
[0123] If room-temperature holding time from finish of the
room-temperature quenching to start of pre-aging (start of
reheating) is too long, an Mg--Si cluster that does not contribute
to the BH property is formed due to room-temperature aging, and an
Mg--Si cluster having a good balance of Mg and Si, which
contributes to the BH property, is less likely to be increased.
Hence, the shorter the room-temperature holding time, the better.
That is, the solution treatment and quenching may be followed by
the reheating with substantially no time difference, and
lower-limit time is not specifically set.
[0124] In the pre-aging, holding time ranging from 60 to
120.degree. C. is 10 to 40 hr. This results in formation of the
Mg--Si cluster having a good balance of Mg and Si.
[0125] Although the invention is now described in detail with
Example, the invention should not be limited thereto, and
modifications or alterations thereof may be made within the scope
without departing from the gist described before and later, all of
which are included in the technical scope of the invention.
Example
[0126] Example of the invention is now described. 6000-Series
aluminum alloy sheets, which had different compositions as shown in
Table 1 and different microstructures as shown in Table 2, each
microstructure including the amount of solute Si, the amount of
solute Cu, and dislocation density after application of tensile
deformation of 5%, were appropriately manufactured under different
manufacturing conditions.
[0127] Each of the manufactured sheets was held for 10 days at room
temperature (subjected to room-temperature aging), and then the
amount of solute Si, the amount of solute Cu, dislocation density
after application of tensile deformation of 5%, 0.2% proof stress,
tensile strength, a yield ratio (0.2% proof stress/tensile
strength), and total elongation were measured and evaluated. Table
2 also shows results of those. Table 2 is the rest of Table 1, and
respective alloy numbers in Table 1 correspond to alloy numbers in
Table 2.
[0128] In the specific appropriate manufacturing method, the
6000-series aluminum alloy sheets having the chemical compositions
as shown in Table 1 were manufactured under different manufacturing
conditions as shown in Table 2, each manufacturing condition
including soaking temperature, minimum interpass temperature of a
rough rolled sheet in hot rough rolling (shown as minimum
temperature in Table 2), finish temperature of hot finish rolling,
average cooling rate from material (sheet) temperature immediately
after finish of hot finish rolling to material temperature of
150.degree. C., and holding temperature and average cooling rate of
solution treatment.
[0129] In representation of the content of each element in Table 1,
representation with no numerical value for each element indicates
that the content of the element is equal to or lower than the
detection limit.
[0130] Specific manufacturing conditions of the aluminum alloy
sheets were as follows. Aluminum alloy slabs having the
compositions shown in Table 1 were in common melted by a DC casting
process. This melting was in common performed such that the average
cooling rate during the casting was 50.degree. C./min from the
liquidus temperature to the solidus temperature. Subsequently, the
slabs were in common soaked for six hours, and were then subjected
to hot rough rolling at the temperature. Table 2 also shows the
minimum (pass) temperature of that hot rough rolling.
[0131] The slabs were in common subjected to subsequent hot finish
rolling such that the slabs were hot-rolled into a thickness of 2.5
mm with finish temperatures and the average cooling rates (.degree.
C./hr) after finish of the rolling as shown in Table 2, and thus
the slabs were formed into hot-rolled sheets.
[0132] The hot-rolled aluminum alloy sheets were in common
subjected to heat treatment of 500.degree. C..times.1 min, and was
then cold-rolled with a reduction of 50% without process annealing
between cold rolling passes, and were thus formed into cold-rolled
sheets 1.0 mm in thickness.
[0133] Furthermore, the cold-rolled sheets were in common
continuously subjected to tempering (T4) while being rewound and
wound up in continuous heat treatment equipment. Specifically, the
solution treatment was performed in such a manner that each of the
cold-rolled sheets was heated to the target temperature (holding
temperature) listed in Table 2 with an average heating rate of
50.degree. C./sec below 500.degree. C., and then the cold-rolled
sheets were in common held at the target temperature for 20 sec and
then water-cooled to room temperature at the average cooling rates
(.degree. C./sec) listed in Table 2.
[0134] Test sample sheets (blanks) were cut from each final product
sheet that was left at room temperature for 10 days after such
tempering, and the amount of solute Si and the amount of solute Cu,
a microstructure defined by dislocation density, and mechanical
properties of each test sample sheet were measured and evaluated.
Table 2 shows results of those.
Measurement of Amount of Solute Si and Amount of Solute Cu
[0135] The amount of solute Si and the amount of solute Cu in each
of the test sample sheets were measured by the hot-phenol residue
extraction method as follows: A sample to be measured was
dissolved, solid and liquid were separated and classified through
filtration separation with a filter having a mesh of 0.1 .mu.m, and
the content of Si and the content of Cu in the separated solution
were measured as the amount of solute Si and the amount of solute
Cu, respectively.
[0136] The hot-phenol residue extraction method was specifically
performed as follows. First, phenol was put into a decomposition
flask and heated, and then each test sample sheet to be measured
was transferred into the decomposition flask and thermally
decomposed. Subsequently, benzyl alcohol was added, and solid and
liquid were separated and classified by suction filtration with the
filter, and the content of Si and the content of Cu in the
separated solution were each quantitatively analyzed.
[0137] The atomic absorption analysis (AAS) or the
inductively-coupled plasma emission spectrometry (ICP-OES) was
appropriately used for the quantitative analysis.
[0138] A 47 mm diameter membrane filter having a mesh (collection
particle size) of 0.1 .mu.m as described above was used for the
suction filtration.
[0139] Such measurement and calculation were performed for each of
three samples taken at three points in total including one point in
the center in a sheet width direction and two points at both ends
in the sheet width direction from the center of the test sample
sheet, and the amounts (mass %) of each of solute Si and solute Cu
in the samples were averaged and defined as the amount of each of
solute Si and solute Cu in the sheet.
Measurement of Dislocation Density
[0140] Tensile deformation with a strain of 5% was applied to each
of the test sample sheets (sampled test specimens) in a rolling
direction according to the above-described procedure, and
dislocation density (.times.10.sup.14 m.sup.-2) in a rolled surface
was measured by X-ray diffraction under the above-described
specific condition. The measurement was performed for each of
appropriate five points on each test sample sheet, and an average
of the dislocation densities at the five points was defined as
average dislocation density (.times.10.sup.14 m.sup.-2)
Tensile Test
[0141] The tensile test of each test sample sheet was performed at
room temperature with a JIS Z2201 No. 5 test specimen (25
mm.times.50 mm gage length (GL).times.thickness) taken from each
test sample sheet. The tensile direction of the test specimen was a
direction parallel to the rolling direction. The tensile speed was
5 mm/min below the 0.2% proof stress, and was 20 mm/min at or above
the 0.2% proof stress. The number N of times of measurement of each
of the mechanical properties was three, and an average of the
measured values was calculated for each property.
[0142] For each test sample sheet, 0.2% proof stress, tensile
strength, a yield ratio (0.2% proof stress/tensile strength), and
total elongation were calculated.
[0143] As shown in Tables 1 and 2, inventive examples 1 to 11 are
each within a range of the chemical composition of the invention,
and are each manufactured within the range of the preferred
condition.
[0144] In the inventive examples, therefore, as shown in Table 2,
the amount of solute Si is 0.30 to 2.0% and the amount of solute Cu
is 0.05 to 1.0% in the solution separated by the hot-phenol residue
extraction method, and when tensile deformation with a strain of 5%
is applied to the sheet in a rolling direction of the sheet,
dislocation density in a rolled surface of the sheet is
6.0.times.10.sup.14 to 12.times.10.sup.14 m.sup.-2 in average, the
dislocation density being measured by X-ray diffraction, as defined
by the invention.
[0145] As a result, as shown in Table 2, each inventive example
exhibits a yield ratio of 0.56 or less, the yield ratio being
defined by a ratio of 0.2% proof stress to tensile strength (0.2%
proof stress/tensile strength), and total elongation of 26% or more
even after room-temperature aging, i.e., has a good formability
acceptable for the automotive body panel.
[0146] On the other hand, although comparative examples 12 to 16 in
Table 2 are each manufactured within a preferred condition range,
they use the alloy Nos. 12 to 16, respectively, in each of which
the content of at least one of Si, Mg, Cu, Mn, and Fe is out of the
range of the invention.
[0147] Hence, as shown in Table 2, such comparative examples are
each bad in formability compared with the inventive examples, in
which one of the amount of solute Si, the amount of solute Cu, and
average dislocation density in the low strain region is out of the
range defined by the invention, and furthermore the yield ratio
exceeds 0.56, or total elongation is less than 26%. Consequently,
the comparative examples are unacceptable for the automotive body
panel.
[0148] The comparative example No. 12 corresponds to alloy 12 in
Table 1, in which the content of Mg is excessively small.
[0149] The comparative example No. 13 corresponds to alloy 13 in
Table 1, in which the content of Si is excessively small.
[0150] The comparative example No. 14 corresponds to alloy 14 in
Table 1, in which the content of Cu is excessively small.
[0151] The comparative example No. 15 corresponds to alloy 15 in
Table 1, in which the content of Mn is excessively large.
[0152] The comparative example No. 16 corresponds to alloy 16 in
Table 1, in which the content of Fe is excessively large.
[0153] Comparative examples 17 to 21 in Table 2 each use the alloy
within the range of the invention as shown in Table 1. However, as
shown in Table 2, such comparative examples are each out of a
preferred manufacturing condition including soaking temperature,
minimum temperature of hot rough rolling, finish temperature of hot
finish rolling, average cooling rate (.degree. C./hr) after finish
of the hot finish rolling, and holding temperature and average
cooling rate (.degree. C./sec) of solution treatment.
[0154] As a result, at least one of the amount of solute Si, the
amount of solute Cu, and average dislocation density in the low
strain region is out of the range defined by the invention, the
yield ratio exceeds 0.56, or total elongation is less than 26%,
unlike the inventive examples. Consequently, the comparative
examples are unacceptable for the automotive body panel.
[0155] In the comparative example 17, the soaking temperature and
the minimum temperature of hot rough rolling are each excessively
low. Hence, the amount of solute Si and the amount of solute Cu are
each excessively small to be below the lower limit, and the average
dislocation density in the low strain region is also excessively
low. Consequently, the yield ratio exceeds 0.56 and the total
elongation is less than 26%, leading to bad formability.
[0156] In the comparative example 18, the minimum temperature of
hot rough rolling and the finish temperature of hot finish rolling
are each excessively low. Hence, the amount of solute Si and the
amount of solute Cu are each excessively small to be below the
lower limit, and the average dislocation density in the low strain
region is also excessively low. Consequently, the yield ratio
exceeds 0.56 and the total elongation is less than 26%, leading to
bad formability.
[0157] In the comparative example 19, the average cooling rate
(.degree. C./hr) after finish of the hot finish rolling is
excessively low. Hence, the amount of solute Si is excessively
small to be below the lower limit, and the average dislocation
density in the low strain region is also excessively low.
Consequently, the yield ratio exceeds 0.56 and the total elongation
is less than 26%, leading to bad formability.
[0158] In the comparative example 20, the holding temperature of
solution treatment is excessively low. Hence, the amount of solute
Si and the amount of solute Cu are each excessively small to be
below the lower limit, and the average dislocation density in the
low strain region is also excessively low. Consequently, the yield
ratio exceeds 0.56 and the total elongation is less than 26%,
leading to bad formability.
[0159] In the comparative example 21, average cooling rate
(.degree. C./sec) after solution treatment is excessively low.
Hence, the amount of solute Si is excessively small to be below the
lower limit, and the average dislocation density in the low strain
region is also excessively low. Consequently, although the amount
of solute Cu satisfies the definition, the yield ratio exceeds 0.56
and the total elongation is less than 26%, leading to bad
formability.
[0160] These results of the Example support the meaning of
satisfying all the requirements of the composition and the
microstructure defined by the invention, the requirements being to
produce a 6000-series aluminum alloy sheet having good formability
for the automotive body panel without greatly varying the existing
composition or manufacturing condition.
TABLE-US-00001 TABLE 1 Chemical composition of aluminum alloy sheet
(mass %, the remainder: Al) No. Si Mg Cu Mn Fe Cr Zr V Ti Zn Ag Sn
1 1.0 0.45 0.20 0.07 0.17 2 0.50 0.65 0.08 0.08 0.18 3 1.5 0.43
0.19 0.07 0.17 0.05 4 0.96 0.47 0.20 0.08 0.15 0.10 5 0.98 0.30
0.24 0.07 0.16 0.10 0.03 0.05 6 1.1 0.45 0.18 0.07 0.20 0.12 7 0.83
0.25 0.41 0.19 0.10 0.20 8 0.36 0.95 0.80 0.50 0.08 0.20 0.15 9 1.7
1.4 0.06 0.05 0.28 0.20 10 0.95 0.63 0.25 0.78 0.11 0.07 11 1.0 1.2
0.90 0.32 0.45 0.70 0.05 12 0.67 0.16 0.18 0.08 0.15 0.05 0.30 0.03
13 0.27 1.0 0.44 0.81 0.33 0.08 0.05 14 1.1 0.72 0.02 0.28 0.21
0.10 15 1.3 0.80 0.13 1.3 0.17 0.10 0.05 16 0.52 0.75 0.21 0.11 1.2
0.10 0.05 0.10 17 0.60 0.65 0.13 0.08 0.18 18 0.60 0.65 0.13 0.08
0.18 19 0.60 0.65 0.13 0.08 0.18 20 0.60 0.65 0.13 0.08 0.18 21
0.60 0.65 0.13 0.08 0.18
TABLE-US-00002 TABLE 2 Manufactoring condition of aluminum alloy
sheet Hot finish rolling Average cooling rate Aluminum alloy sheet
held for 10 days at room temperature (.degree. C./hr) Average from
dislocation Hot material density Property rough temperature
Solution (.times.10.sup.14 m.sup.-2) Yield Soaking rolling
immediately treatment after ratio Soaking Minimum Finish after
Holding Average Solute tensile 0.2% (proof temper- temper- temper-
finish of temper- cooling amount deformation Proof Tensile stress/
Elon- ature ature ature rolling to ature rate (mass %) of 5%
strength strength tensile gation Classification No. (.degree. C.)
(.degree. C.) (.degree. C.) 150.degree. C. (.degree. C.) (.degree.
C./sec) Si Cu (.times.10.sup.14 m.sup.-2) (MPa) (MPa) strength (%)
Inventive 1 540 470 330 10 570 30 0.85 0.19 9.0 135 250 0.54 31
example 2 550 480 340 6 560 25 0.46 0.07 7.2 125 240 0.52 27 3 500
480 330 10 570 25 0.82 0.14 8.8 128 232 0.55 29 4 540 450 310 10
560 30 0.83 0.18 8.9 133 242 0.55 30 5 540 470 360 20 560 25 0.84
0.22 9.0 138 249 0.55 30 6 530 460 320 10 550 20 0.87 0.16 9.3 117
240 0.49 32 7 560 480 350 15 570 30 0.73 0.36 8.2 124 244 0.51 31 8
530 470 320 20 560 25 0.31 0.69 6.3 130 256 0.51 28 9 510 460 310
10 550 30 1.1 0.07 11 145 265 0.55 29 10 520 470 330 15 560 25 0.78
0.22 8.5 137 253 0.54 31 11 520 460 320 10 550 40 0.75 0.78 8.3 139
264 0.53 30 Comparative 12 540 470 330 10 570 30 0.51 0.15 5.8 85
150 0.57 26 example 13 540 470 330 10 570 30 0.26 0.35 5.5 119 218
0.55 24 14 540 470 330 10 570 30 0.82 0.02 8.6 145 256 0.57 27 15
540 470 330 10 570 30 1.0 0.09 13 150 260 0.58 24 16 540 470 330 10
570 30 0.25 0.18 5.5 91 166 0.55 24 17 480 420 300 10 560 30 0.27
0.03 5.7 118 205 0.58 23 18 500 430 280 10 560 30 0.28 0.04 5.8 120
211 0.57 23 19 530 470 320 3 560 25 0.28 0.08 5.8 124 217 0.57 24
20 540 470 330 10 530 30 0.25 0.03 5.5 119 205 0.58 23 21 540 470
330 10 570 15 0.27 0.06 5.6 122 215 0.57 24
[0161] According to the invention, a 6000-series aluminum alloy
sheet having good formability for the automotive body panel can be
produced without greatly varying the existing composition or
manufacturing condition. As a result, use of the 6000-series
aluminum alloy sheet for the automotive body panel can be
expanded.
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