U.S. patent application number 14/767096 was filed with the patent office on 2015-12-31 for aluminum alloy sheet for structural material.
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 | 20150376742 14/767096 |
Document ID | / |
Family ID | 51536850 |
Filed Date | 2015-12-31 |
United States Patent
Application |
20150376742 |
Kind Code |
A1 |
MATSUMOTO; Katsushi ; et
al. |
December 31, 2015 |
ALUMINUM ALLOY SHEET FOR STRUCTURAL MATERIAL
Abstract
Disclosed is a 7xxx-series aluminum alloy sheet having a
specific chemical composition and produced by a common procedure.
The aluminum alloy sheet has a good balance between a Zn content
and a Mg content while having a lower Zn content so as to have a
strength retained at high level. The aluminum alloy sheet has a
microstructure having a specific endothermic peak temperature and a
specific maximum height of exothermic peak(s) in a differential
scanning calorimetric curve, where the curve is plotted after
natural aging of the produced sheet. The aluminum alloy sheet can
thereby have a high strength, satisfactory formability, and good
corrosion resistance which are required for structural
components.
Inventors: |
MATSUMOTO; Katsushi;
(Kobe-shi, JP) ; ARUGA; Yasuhiro; (Kobe-shi,
JP) ; SHISHIDO; Hisao; (Kobe-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.) |
Kobe-shi, Hyogo |
|
JP |
|
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi, Hyogo
JP
|
Family ID: |
51536850 |
Appl. No.: |
14/767096 |
Filed: |
March 12, 2014 |
PCT Filed: |
March 12, 2014 |
PCT NO: |
PCT/JP2014/056567 |
371 Date: |
August 11, 2015 |
Current U.S.
Class: |
420/532 |
Current CPC
Class: |
C22C 21/08 20130101;
C22C 21/06 20130101; C22F 1/047 20130101; C22F 1/00 20130101; C22F
1/002 20130101; C22C 21/10 20130101; C22F 1/053 20130101 |
International
Class: |
C22C 21/10 20060101
C22C021/10; C22F 1/00 20060101 C22F001/00; C22F 1/047 20060101
C22F001/047; C22C 21/08 20060101 C22C021/08; C22F 1/053 20060101
C22F001/053 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 14, 2013 |
JP |
2013-051608 |
Claims
1. An aluminum alloy sheet for structural components, the aluminum
alloy sheet being an Al--Zn--Mg alloy sheet and comprising: in
terms of a chemical composition in mass percent, Zn in a content of
3.0% to 6.0%; Mg in a content of 1.5% to 4.5%; and Cu in a content
of 0.05% to 0.5%, the Zn content [Zn] and the Mg content [Mg] meet
a condition as specified by: [Zn].gtoreq.-0.3[Mg]+4.5, with the
remainder consisting of Al and inevitable impurities, wherein, when
the sheet is subjected sequentially to solution treatment,
quenching, and natural aging, and a differential scanning
calorimetric curve is plotted after the natural aging, a highest
endothermic peak temperature is 130.degree. C. or lower, and a
maximum height of exothermic peak(s) in a temperature range of
200.degree. C. to 300.degree. C. is 50 .mu.W/mg or more in the
differential scanning calorimetric curve, and a work hardening
coefficient n (10% to 20%) is 0.22 or more.
2. The aluminum alloy sheet for structural components according to
claim 1, wherein the aluminum alloy sheet further comprises at
least one element selected from the group consisting of: in mass
percent, Zr in a content of 0.05% to 0.3%; Mn in a content of 0.1%
to 1.5%; Cr in a content of 0.05% to 0.3%; and Sc in a content of
0.05% to 0.3%.
3. The aluminum alloy sheet for structural components according to
claim 1, wherein the aluminum alloy sheet further comprises in mass
percent, Ag in a content of 0.01% to 0.2%.
4. The aluminum alloy sheet for structural components according to
claim 1, wherein the Zn content [Zn] and the Mg content [Mg] in the
aluminum alloy sheet meet a condition as specified by formula:
[Zn].gtoreq.-0.5[Mg]+5.75, and wherein a 0.2% yield strength after
artificial aging treatment is 400 MPa or more.
Description
TECHNICAL FIELD
[0001] The present invention relates to an aluminum alloy sheet for
structural components (structural materials), which aluminum alloy
sheet has better workability, excellent corrosion resistance, and a
high strength. As used herein the term "aluminum alloy sheet"
refers to a rolled sheet that is prepared by producing a rolled
sheet by rolling, subjecting the rolled sheet sequentially to
solution treatment and quenching, thereafter subjecting the rolled
sheet to natural aging for two weeks or longer and is before
forming into a structural component and before artificial aging
treatment.
BACKGROUND ART
[0002] A social demand for weight reduction of automobile bodies
has been increasingly made in consideration typically of the global
environment. To meet the demand, aluminum alloy materials are
applied to some of automobile body parts so as to replace part of
iron/steel materials such as steel sheets. The target automobile
body parts are exemplified by panels such as outer panels and inner
panels typically of hoods, doors, and roofs; and reinforcers such
as bumper reinforcement (bumper R/F) and door beams.
[0003] For further weight reduction of the automobile bodies,
however, the aluminum alloy materials should be applied also to
automobile structural components such as frames and pillars. This
is because these structural components particularly contribute to
weight reduction. However, these automobile structural components
require higher strength as compared with the automobile panels. For
example, the automobile structural components require a 0.2% yield
strength of 350 MPa or more. In this regard, JIS or AA 6xxx-series
aluminum alloy sheets used as the automobile panels excel in
formability, strength, and corrosion resistance and can be
satisfactorily recycled because of having low-alloy chemical
compositions. The 6xxx-series aluminum alloy sheets, however, are
not anywhere near the high strength even when their chemical
compositions are controlled and even when they are produced under
controlled temper refining (e.g., solution treatment and quenching,
and further artificial aging treatment) conditions.
[0004] This causes such high-strength automobile structural
components to employ JIS or AA 7xxx-series aluminum alloy sheets
that are used for the reinforcers requiring high strength at
similar level. However, the material 7xxx-series aluminum alloys as
being Al--Zn--Mg alloys, have inferior general corrosion
resistance. In addition, these alloys achieve a high strength by
allowing Zn--Mg precipitates MgZn.sub.2 to be distributed in a high
density and may thereby cause stress corrosion cracking
(hereinafter also simply referred to as "SCC"). To prevent this,
the alloys are forced to be subjected to over-aging and are used in
actual use at a 0.2% yield strength of about 300 MPa and have
diluted features or properties as high-strength alloys.
[0005] As possible solutions to this, various proposals have been
made on chemical composition control and on microstructure control
(e.g., precipitates control) so as to provide 7xxx-series aluminum
alloys having a strength and SCC resistance both at satisfactory
levels.
[0006] The chemical composition control is representatively
exemplified by a technique disclosed in Patent Literature (PTL 1).
The technique relates to a 7xxx-series aluminum alloy extrusion and
utilizes Mg added in excess of the stoichiometric ratio for
MgZn.sub.2, where such excess Mg contributes to a higher strength.
The stoichiometric ratio refers to the proportions of Zn and Mg to
form MgZn.sub.2 in just proportion. Specifically, Mg is added in
excess of the stoichiometric ratio for MgZn.sub.2 so as to reduce
the MgZn.sub.2 amount and thereby allows the 7xxx-series aluminum
alloy extrusion to have a higher strength without deterioration in
SCC resistance.
[0007] The microstructure control (e.g., precipitates control) is
representatively exemplified by a technique disclosed in PTL 2.
According to this technique, a 7xxx-series aluminum alloy extrusion
after artificial aging treatment is controlled so that precipitates
having a particle diameter of 1 to 15 nm are present in grains in a
number density of 1000 to 10000 per square micrometer so as to have
a smaller potential difference between the inside of a grain and
the grain boundary, where the number density is determined by
observation with a transmission electron microscope (TEM). Thus,
the 7xxx-series aluminum alloy extrusion has better SCC
resistance.
[0008] Though not all are exemplified, there are many other
techniques relating to chemical composition control and
microstructure control (e.g., precipitates control) on 7xxx-series
aluminum alloy extrusions so as to have a strength and SCC
resistance both at satisfactorily levels. This is because a large
number of 7xxx-series aluminum alloy extrusions are in actual use.
In contrast, there are very few conventional techniques relating to
chemical composition control and microstructure control (e.g.,
precipitates control) on 7xxx-series aluminum alloy sheets, because
a small number of the 7xxx-series aluminum alloy sheets are in
actual use.
[0009] For example, PTL 3 proposes a technique relating to a
structural component including a clad sheet prepared by
weld-bonding 7xxx-series aluminum alloy sheets with each other.
According to the technique, precipitates after artificial aging
treatment are controlled to be present in the form of spheres
having a diameter of 50 angstroms or less in a predetermined amount
so as to offer a higher strength. The literature, however, does not
disclose the SCC resistance performance at all and fails to
describe corrosion resistance data in working examples.
[0010] PTL 4 discloses a technique relating to a 7xxx-series
aluminum alloy sheet prepared by rapidly solidifying a molten
metal, cold-rolling the workpiece, and subjecting the workpiece to
artificial aging treatment. According to this technique,
precipitates in grains are controlled to have a size in terms of
equivalent circle diameter of 3.0 .mu.m or less and an average area
fraction of 4.5% or less so as to have a higher strength and better
elongation. The equivalent circle diameter and the average area
fraction are measured with an optical microscope at 400-fold
magnification, where the size is calculated as the diameter of a
circle having an equivalent area.
[0011] Although few, there are some proposals relating to metallic
texture control in a sheet. For example, PTL 5 and PTL 6 disclose
techniques relating to a 7xxx-series sheet for structural
components. According to the techniques, an ingot is forged and
then repeatedly rolled in a warm working region to have a refined
microstructure in order to have a higher strength and better SCC
resistance. The techniques are intended to refine the
microstructure to thereby restrain large angle grain boundaries
having a misorientation of 20.degree. or more so as to have a
metallic texture including small angle grain boundaries having a
misorientation of 3.degree. to 10.degree. in an amount of 25% or
more. This is because such large angle grain boundaries cause the
potential difference between the grain boundary and the grain
inside, where the potential difference in turn causes SCC
resistance deterioration. However, the repeated warm rolling
operations are performed because hot rolling and cold rolling by a
common procedure fail to give a metallic texture including small
angle grain boundaries in an amount of 25% or more. The techniques
therefore significantly differ in process from the common procedure
and are not considered to be practical techniques to produce
sheets.
CITATION LIST
Patent Literature
[0012] PTL 1: Japanese Unexamined Patent Application Publication
(JP-A) No. 2011-144396
[0013] PTL 2: JP-A No. 2010-275611
[0014] PTL 3: JP-A No. Hei9(1997)-125184
[0015] PTL 4: JP-A No. 2009-144190
[0016] PTL 5: JP-A No. 2001-335874
[0017] PTL 6: JP-A No. 2002-241882
SUMMARY OF INVENTION
Technical Problem
[0018] As described above, various proposals relating to chemical
composition control and microstructure control (e.g., precipitates
or metallic texture control) of 7xxx-series aluminum alloys have
been made in the field of extrusions so as to offer a strength and
SCC resistance both at satisfactory levels. However, under present
circumstances, there are few proposals relating to rolled sheets
produced by a common procedure typically by soaking an ingot and
subjecting the soaked ingot to hot rolling and cold rolling, except
for the special rolling or production methods such as the use of
clad sheet, rapid solidification, and warm rolling.
[0019] From the rolled sheets, the extrusions are quite different
in production processes such as hot working process and are
significantly different in the resulting microstructure such as
grains and precipitates. Typically, the extrusions include fibrous
grains that are elongated in the extrusion direction, whereas the
rolled sheets basically include equiaxial grains as the grains. For
these reasons, it is unknown whether the proposals relating to
chemical composition control and microstructure control (e.g.,
precipitates control) in the extrusions are applicable as intact to
7xxx-series aluminum alloy sheets and to automobile structural
components including the 7xxx-series aluminum alloy sheets and
effectively contribute to both a higher strength and better SCC
resistance really. Namely, this still remains in the realm of
expectation unless being actually verified.
[0020] Under present circumstances, there is still no effective
means, are many unknown points, and is room for clarification
relating to microstructure control techniques of the 7xxx-series
aluminum alloy sheets produced by a common procedure so as to offer
a strength and SCC resistance both at satisfactorily levels. In
addition, the 7xxx-series aluminum alloy sheets require a lower Zn
content from the viewpoints of strength and corrosion resistance,
because the presence of Zn causes a less noble potential that is
involved in the general corrosion resistance. However, with a
decreasing Zn content, the 7xxx-series aluminum alloy sheets have a
lower strength, although having better corrosion resistance and
having better processability such as bendability which is a
property necessary for the structural components. This is
inconsistent with the target high strength and constitutes a
technical difficulty.
[0021] In consideration of the above-described problems, it is an
object of the present invention to provide a 7xxx-series aluminum
alloy sheet for structural components such as automobile
components, which aluminum alloy sheet is provided as a rolled
sheet produced according to the common procedure, has a strength
and processability both at satisfactory levels, and still has
excellent corrosion resistance.
Solution to Problem
[0022] The present invention provides, to achieve the object, an
aluminum alloy sheet for structural components. The aluminum alloy
sheet is an Al--Zn--Mg alloy sheet and contains, in terms of a
chemical composition in mass percent, Zn in a content of 3.0% to
6.0%, Mg in a content of 1.5% to 4.5%, and Cu in a content of 0.05%
to 0.5%, where the Zn content [Zn] and the Mg content [Mg] meet a
condition as specified by: [Zn].gtoreq.-0.3[Mg]+4.5, with the
remainder consisting of Al and inevitable impurities. When the
sheet is subjected sequentially to solution treatment, quenching,
and natural aging, and a differential scanning calorimetric curve
is plotted after the natural aging, a highest endothermic peak
temperature is 130.degree. C. or lower and a maximum height of
exothermic peak(s) in a temperature range of 200.degree. C. to
300.degree. C. is 50 .mu.W/mg or more in the differential scanning
calorimetric curve, and a work hardening coefficient n (10% to 20%)
is 0.22 or more.
Advantageous Effects of Invention
[0023] As used herein the term "aluminum alloy sheet" refers to a
7xxx-series aluminum alloy sheet that is a sheet produced by
rolling and is produced by a common procedure including soaking an
ingot, subjecting the soaked ingot sequentially to hot rolling and
cold rolling to give a cold-rolled sheet, and further subjecting
the cold-rolled sheet to a temper refining treatment (T4 in a
temper designation) such as solution treatment and quenching. In
other words, the term "aluminum alloy sheet" does not include
sheets produced by such a special rolling method as to subjecting
an ingot to forging and to repeat warm rolling operations many
times, as in the techniques disclosed in PTL 5 and PTL 6.
[0024] As the aluminum alloy sheet according to the present
invention, a 7xxx-series aluminum alloy sheet produced in the above
manner and undergoing natural aging is controlled to have a
specific microstructure. In addition, the aluminum alloy sheet is,
as a material aluminum alloy sheet, processed or formed into an
intended structural component. Accordingly, the term "aluminum
alloy sheet" as used herein refers to a sheet that is produced in
the above manner and undergoes natural aging (standing at room
temperature) after production, but is before being formed into an
intended structural component and before being subjected to
artificial aging treatment.
[0025] The present inventors prepared a 7xxx-series aluminum alloy
sheet having a chemical composition with a lower Zn content for
better corrosion resistance and a higher Mg content for a certain
strength and analyzed the microstructure of the 7xxx-series
aluminum alloy sheet after natural aging based on a differential
scanning calorimetric curve. As a result, the present inventors
have found that dusters formed by natural aging in the 7xxx-series
aluminum alloy sheet having the chemical composition have a
different chemical composition and act in a different manner from a
7xxx-series aluminum alloy sheet having a high Zn content.
[0026] Specifically, the present inventors have found that dusters
(atomic dusters) formed by natural aging in the 7xxx-series
aluminum alloy sheet having a lower Zn content contributes not only
to artificial aging properties (BH response) after forming into an
intended structural component, but also to ductility (work
hardening properties) necessary upon forming into the structural
component. The control of these clusters therefore contributes not
only to better corrosion resistance, but also to a better balance
between strength and ductility (formability) and can provide a
7xxx-series aluminum alloy sheet that is used for structures
(structural components), is produced as a rolled sheet by a common
procedure, has a strength and processability both at satisfactory
levels, and still excels in corrosion resistance such as SCC
resistance.
[0027] It is difficult, however, to quantitatively specify the
dusters (atomic dusters) with microstructural factors such as
chemical composition, size, and/or density, because such
microstructural factors of the dusters cannot be directly
determined at the present time using a common observation device
such as a SEM or TEM.
[0028] For this reason, at the present invention, the
microstructure of the 7xxx-series aluminum alloy sheet after
production and after natural aging is indirectly specified and
controlled by analysis based on a differential scanning
calorimetric curve. More specifically, the 7xxx-series aluminum
alloy sheet has better work hardening properties (as ductility)
with a decreasing temperature of an endothermic peak in the
differential scanning calorimetric curve analysis, where the
temperature of endothermic peak corresponds to re-dissolution of
clusters formed by natural aging. In contrast, precipitates after
artificial aging are formed in a larger amount and a higher
strength is obtained with an increasing maximum height of
exothermic peak(s) in the temperature range of 200.degree. C. to
300.degree. C., where the maximum height of exothermic peaks
corresponds to the precipitates after artificial aging
treatment.
[0029] For specifying dusters formed by natural aging, the
differential scanning calorimetric curve in the present invention
is measured not for a sheet immediately after the temper refining
treatment and before undergoing natural aging, but for a sheet
after natural aging (standing at room temperature) for
approximately 2 weeks or longer as a rough reference, but before
forming into a structural component and before artificial aging
treatment.
DESCRIPTION OF EMBODIMENTS
[0030] Embodiments of the present invention will be specifically
illustrated for individual factors or conditions below.
[0031] Aluminum Alloy Chemical Composition
[0032] Initially, the chemical composition of the aluminum alloy
sheet according to the present invention, and reasons for
specifying the contents of individual elements will be illustrated
below. All percentages of individual element contents are by
mass.
[0033] The chemical composition of the aluminum alloy sheet
according to the present invention is specified as such a
precondition that a rolled sheet produced by a common procedure can
have a strength and processability both at satisfactory levels and
can still have satisfactory corrosion resistance, where these
properties are required properties as automobile components and
other structural components that are intended uses in the present
invention. For the reason, the 7xxx-series Al--Zn--Mg--C alloy in
the present invention has such a chemical composition as to have a
lower Zn content for better corrosion resistance and a higher Mg
content for a strength at certain level.
[0034] From this viewpoint, the aluminum alloy sheet according to
the present invention has a chemical composition including, in mass
percent, Zn in a content of 3.0% to 6.0%, Mg in a content of 1.5%
to 4.5%, and Cu in a content of 0.05% to 0.5%, where the Zn content
[Zn] and the Mg content [Mg] meet a condition specified by formula:
[Zn].gtoreq.-0.3[Mg]+4.5, with the remainder consisting of Al and
inevitable impurities. In addition to the elements, the aluminum
alloy sheet may further contain at least one transition element
selected from the group consisting of Zr in a content of 0.05% to
0.3%, Mn in a content of 0.1% to 1.5%, Cr in a content of 0.05% to
0.3%, and Sc in a content of 0.05% to 0.3%. The aluminum alloy
sheet may further selectively contain Ag in a content of 0.01% to
0.2% in addition to, or instead of the at least one transition
element.
[0035] Zn: 3.0% to 6.0%
[0036] Zinc (Zn) acts as an essential alloy element, forms dusters
with Mg upon natural aging of the produced sheet after temper
refining, and contributes to better work hardening properties and
better processability into the structural component. This element
also forms precipitates upon artificial aging after forming into
the structural component and contributes to a higher strength. The
aluminum alloy sheet, if having a Zn content less than 3.0%, may
have an insufficient strength after artificial aging. However, the
aluminum alloy sheet, if having an excessively high Zn content
greater than 6.0%, may often suffer from grain-boundary corrosion
and have inferior corrosion resistance due to an increased amount
of grain-boundary precipitate MgZn.sub.2. To prevent these, the Zn
content is controlled in the present invention to be relatively
low. The Zn content is therefore controlled to 3.0% or more and
preferably 3.5% or more in terms of lower limit, and is controlled
to 6.0% or less, and preferably 4.5% or less in terms of upper
limit.
[0037] Mg: 1.5% to 4.5%
[0038] Magnesium (Mg) acts as an essential alloy element, forms
dusters with Zn upon natural aging of the produced sheet after
temper refining, and contributes to better work hardening
properties. This element also forms precipitates upon artificial
aging after forming into the structural component and contributes
to a higher strength. The Mg content is set as relatively high in
the present invention, because the Zn content is controlled to be
relatively low. The aluminum alloy sheet, if having a Mg content
less than 1.5 percent by mass, may have inferior work hardening
properties. However, the aluminum alloy sheet, if having a Mg
content greater than 4.5 percent by mass, may have inferior rolling
properties and higher susceptibility to SCC. To prevent these, the
Mg content may be controlled to 1.5% or more, and preferably 2.5%
or more in terms of lower limit; and controlled to 4.5% or less in
terms of upper limit.
[0039] Balance Formula Between Zn and Mg
[0040] According to the present invention, it is important to
control not only the Zn and Mg contents, but also the balance
between the Zn content [Zn] (in mass percent) and the Mg content
[Mg] (in mass percent) so as to allow Zn and Mg to surely
contribute to a higher strength. The balance control is performed
so that [Zn] and [Mg] meet the balance formula:
[Zn].gtoreq.-0.3[Mg]+4.5, and preferably meet the balance formula:
[Zn].gtoreq.-0.5[Mg]+5.75.
[0041] The aluminum alloy sheet, as meeting the balance formula:
[Zn].gtoreq.-0.3[Mg]+4.5 and when being produced by an
after-mentioned preferred production method, may allow the
resulting structural component after artificial aging treatment to
have a 0.2% yield strength of 350 MPa or more. The aluminum alloy
sheet, when meeting the balance formula: [Zn].gtoreq.-0.5[Mg]+5.75
and when being produced by the preferred production method, may
allow the structural component after artificial aging treatment to
have a 0.2% yield strength of 400 MPa or more.
[0042] The aluminum alloy sheet, if having contents of Zn and Mg
meeting a condition as specified by: [Zn]<-0.3[Mg]+4.5, may
possibly cause the structural component after artificial aging
treatment to fail to have a 0.2% yield strength of 350 MPa or more,
even when the individual element contents fall within the specific
ranges, or even when the sheet is produced by the preferred
production method. Likewise, the aluminum alloy sheet, if having
contents of Zn and Mg meeting a condition as specified by:
[Zn]<-0.5[Mg]+5.75, may possibly cause the structural component
after artificial aging treatment to fail to have a 0.2% yield
strength of 400 MPa or more.
[0043] Cu: 0.05% to 0.5%
[0044] Copper (Cu) effectively restrains SCC susceptibility of the
Al--Zn--Mg alloy, contributes to better SCC resistance, and still
contributes to better general corrosion resistance. Cu, if
contained in a content less than 0.05%, may not so effectively
contribute to better SCC resistance and better general corrosion
resistance. In contrast, Cu, if contained in a content greater than
0.5%, may contrarily adversely affect properties such as rolling
properties and weldability. To prevent these, the Cu content may be
controlled to 0.05% or more in terms of lower limit and may be
controlled to 0.5% or less, and preferably 0.4% or less in terms of
upper limit.
[0045] At least one selected from Zr in a content of 0.05% to 0.3%,
Mn in a content of 0.1% to 1.5%, Cr in a content of 0.05% to 0.3%,
and Sc in a content of 0.05% to 0.3%
[0046] The transition elements Zr, Mn, Cr, and Sc induce grain
refinement of the ingot and of the final product, thereby
contribute to a higher strength, and may be selectively added
according to necessity. In an embodiment, the aluminum alloy sheet
contains at least one of these elements. In this embodiment, the
aluminum alloy sheet, if containing at least one of Zr, Mn, Cr, and
Sc in a content each lower than the lower limit, may have a lower
strength due to the insufficient content. In contrast, the aluminum
alloy sheet, if containing at least one of Zr, Mn, Cr, and Sc each
in a content greater than the upper limit, may have lower
elongation due to the formation of coarse precipitates. To prevent
these, the aluminum alloy sheet, when containing at least one of
these elements, may contain at least one selected from the group
consisting of Zr in a content of 0.05% to 0.3%, Mn in a content of
0.1% to 1.5%, Cr in a content of 0.05% to 0.3%, and Sc in a content
of 0.05% to 0.3%; and preferably contains at least one selected
from the group consisting of Zr in a content of 0.08% to 0.2%, Mn
in a content of 0.2% to 1.0%, Cr in a content of 0.1% to 0.2%, and
Sc in a content of 0.1% to 0.2%.
[0047] Ag: 0.01% to 0.2%
[0048] Silver (Ag) effectively allows precipitates to be formed
densely and finely through artificial aging after forming into the
structural component and contributes to a higher strength, because
the precipitates contribute to such higher strength. The aluminum
alloy sheet may selectively contain this element as needed. Ag, if
contained in a content less than 0.01%, may not so effectively
contribute to a higher strength. In contrast, Ag, if contained in a
content greater than 0.2%, may have saturated effects and cause
higher cost. To prevent these, the Ag content may be controlled
within the range of 0.01% to 0.2%.
[0049] Other Elements
[0050] Other elements than those mentioned above are basically
inevitable impurities. Such inevitable impurity elements may be
assumed (accepted) to be contained in the aluminum alloy sheet
typically by using aluminum alloy scrap as molten raw materials in
addition to pure aluminum ingots. While assuming (accepting) the
contamination of the impurity elements, the elements may be
contained in contents within ranges as specified by Japanese
Industrial Standards for 7xxx-series alloys. Typically, Ti and B
are impurities for a rolled sheet, but effectively contribute to
grain refinement of the ingot. Ti may be contained in a content of
0.2% or less, and preferably 0.1% or less in terms of upper limit;
and B may be contained in a content of 0.05% or less, and
preferably 0.03% or less in terms of upper limit. Fe and Si do not
affect properties of the aluminum alloy rolled sheet according to
the present invention and may be contained therein as long as Fe in
a content of 0.5% or less and Si in a content of 0.5% or less.
[0051] Microstructure
[0052] On the precondition of having the chemical composition, the
7xxx-series aluminum alloy sheet according to the present invention
has a microstructure as follows. Assume that the sheet is
sequentially subjected to solution treatment and quenching, further
subjected to natural aging for 2 weeks or longer as a rough
reference, and subjected to differential scanning calorimetry to
plot a differential scanning calorimetric curve. In the curve, a
highest endothermic peak temperature is 130.degree. C. or lower,
and a maximum height of exothermic peak(s) in the temperature range
of 200.degree. C. to 300.degree. C. is 50 .mu.W/mg or more.
[0053] The highest endothermic peak temperature corresponds to
re-dissolution of dusters formed upon natural aging of the sheet.
With a decreasing highest endothermic peak temperature, the dusters
have decreasing thermal stability (increasing susceptibility to
decomposition) and become more susceptible to decomposition not
only by heat, but also by cutting of dislocations upon plastic
deformation. The dusters, when having decreasing stability as
above, may less impede the movement of dislocations and less cause
strain concentration upon plastic deformation, such as forming of
the sheet into a structural component. An index for the duster
stability is a highest endothermic peak temperature of 130.degree.
C. or lower. When the microstructure meets the condition, the
dusters have low stability (are unstable) and contributes to better
work hardening properties and to a work hardening coefficient n
(10% to 20%) of 0.22 or more.
[0054] In contrast, with increasing stability of the dusters, the
highest endothermic peak temperature rises higher than 130.degree.
C., the work hardening properties are lowered, and a work hardening
coefficient n (10% to 20%) of 0.22 or more is not obtained. This is
because the dusters, if having increasing stability, may impede the
movement of dislocations upon plastic deformation such as forming
of the sheet into a structural component, but, once the
dislocations cut the dusters and begin moving, the dislocation
movement concentrates on the slip plane. This impedes strain
concentration and impairs work hardening properties.
[0055] The maximum height of exothermic peak(s) in the temperature
range of 200.degree. C. to 300.degree. C. corresponds to the
precipitation of precipitates (artificial-aging precipitates) upon
artificial aging, where the precipitates contribute to a higher
strength. Accordingly, with an increasing maximum height of
exothermic peaks, artificial-aging precipitates are formed in a
larger amount (in a higher number density) and contribute to a
still higher strength. An index for this is a maximum height of
exothermic peaks of 50 .mu.W/mg or more in the temperature range of
200.degree. C. to 300.degree. C. If the maximum height of
exothermic peaks is less than 50 .mu.W/mg, the structural component
after artificial aging treatment may highly possibly fail to have a
0.2% yield strength of 350 MPa or more.
[0056] Work Hardening Properties
[0057] The 7xxx-series aluminum alloy sheet according to the
present invention may have better processability as having the
chemical composition and the microstructure and when being produced
by the preferred production method. In particular, the work
hardening coefficient n (10% to 20%) is specified in the present
invention so as to surely have bending formability, where the
bending may be used in forming into a structural component.
Specifically, assume that a 7xxx-series aluminum alloy sheet having
the chemical composition and the microstructure is produced by the
preferred production method. Further assume that this sheet is
sequentially subjected to solution treatment, quenching, and
natural aging for a duration of 2 weeks or longer as a rough
reference. In this case, a work hardening coefficient n (10% to
20%) is controlled to 0.22 or more.
[0058] The work hardening is a phenomenon in which a workpiece has
increased hardness by plastic deformation upon the application of
stress typically by forming. The work hardening is also called
"strain hardening". In the work hardening, the workpiece has
increasing resistance and increasing hardness with proceeding
deformation by forming. A property value for the work hardening
acts as an index for processability and called a coefficient "n".
The "coefficient n" refers to a coefficient n when the relation
between stress .sigma. and strain .epsilon. in a plastic region at
a yield point or more are approximated. The approximation may be
performed according to the Voce equation that is suitable for
aluminum. With an increasing coefficient n, the workpiece may have
increasing susceptibility to work hardening and undergo hardening
in a portion receiving plastic deformation by forming such as
bending. This may allow other portions around the hardened portion
to be readily deformable and may contribute to better
processability typically in bending. In contrast, with a decreasing
coefficient n, the workpiece may more resist work hardening. In
this case, a portion receiving plastic deformation at first and
receiving maximum stress may not be hardened, but undergo more
plastic deformation, and become constricted and susceptible to
rupture, resulting in inferior processability typically in
bending.
[0059] Production Method
[0060] A method for producing the 7xxx-series aluminum alloy sheet
according to the present invention will be specifically illustrated
below.
[0061] The 7xxx-series aluminum alloy sheet according to the
present invention can be produced by a production method including
common production processes. Specifically, a material is subjected
to common production processes such as casting (direct chill
casting (DC casting) or continuous casting), soaking, and hot
rolling to give an aluminum alloy hot-rolled sheet having a
thickness of 1.5 to 5.0 mm. Next, the aluminum alloy hot-rolled
sheet is cold-rolled into a cold-rolled sheet having a thickness of
3 mm or less. In this process, one or more process annealing
operations may be selectively performed before or during the cold
rolling.
[0062] Melting and Casting Cooling Rate
[0063] Initially, a molten aluminum alloy adjusted to have a
chemical composition within the 7xxx-series chemical composition is
prepared by melting, and cast into an ingot by a common casting
technique in a melting-casting process. The casting technique may
be selected as appropriate typically from continuous casting and
semi-continuous casting (DC casting).
[0064] Soaking
[0065] Next, the cast aluminum alloy ingot is subjected to soaking
in advance of hot rolling. The soak ng (homogenization heat
treatment) is performed in order to homogenize the microstructure,
i.e., to remove or mitigate the segregation in grains in the ingot
microstructure.
[0066] The soaking in the present invention is preferably performed
by a two-stage or double soaking process. The two-stage or double
soaking is preferred to offer better work hardening properties when
the aluminum alloy sheet is sequentially subjected to the temper
refining treatment and natural aging and is then formed into a
structural component. The two-stage or double soaking is also
preferred to offer a higher strength of the resulting structural
component after forming and after artificial aging. In the
two-stage soaking, a workpiece after first-stage soaking is cooled
not down to 200.degree. C. or lower, but down to a cooling end
temperature of higher than 200.degree. C., held at that
temperature, and subjected to hot rolling as being held at that
temperature or as being reheated to a higher temperature. In
contrast in the double soaking, a workpiece after first soaking is
once cooled down to a temperature of 200.degree. C. or lower
(including room temperature), reheated, held at the reheating
temperature for a predetermined time, and subjected to hot
rolling.
[0067] In the two-stage or double soaking process, the first-stage
or first soaking is intended to finely disperse transition element
compounds and to perform refinement of such compounds that affect
the formability of the aluminum alloy sheet into a structural
component; and the second-stage or second soaking is intended to
accelerate the solid-solution (dissolution) of Zn, Mg, and Cu and
to offer better work hardening properties after natural aging and a
higher strength after artificial aging.
[0068] To achieve the intentions, the first-stage or first soaking
temperature may be controlled to be from 400.degree. C. to
450.degree. C., and preferably from 400.degree. C. to 440.degree.
C. The heating and holding of the ingot at a temperature within the
range allows zirconium (Zr) compounds and compounds including any
of Mn, Cr, and Sc to be finely dispersed. The first-stage or first
soaking, if performed at a temperature lower than 400.degree. C.,
may fail to give sufficiently effective refinement and may fail to
contribute to better work hardening properties after natural aging.
In contrast, the first-stage or first soaking, if performed at a
temperature higher than 450.degree. C., may cause the compounds to
coarsen and may also fail to contribute to better work hardening
properties after natural aging. Holding in the first-stage or first
soaking may be performed for a time (holding time) of about 1 to
about 8 hours.
[0069] The second-stage or second soaking temperature may be
controlled in the range of 450.degree. C. to the solidus
temperature, preferably in the range of 470.degree. C. to the
solidus temperature. The heating and holding of the ingot at a
temperature within the range may accelerate the dissolution of Zn,
Mg, and Cu and contribute to a higher strength upon artificial
aging after the solution treatment. The second-stage or second
soaking, if performed at a temperature lower than 450.degree. C.,
may fail to give sufficient dissolution of these elements and may
fail to contribute to better work hardening properties after
natural aging and a higher strength after artificial aging. In
contrast, the second-stage or second soaking, if performed at a
temperature higher than the solidus temperature, may cause partial
melting to impair mechanical properties. To prevent this, the
temperature is controlled to be equal to or lower than the solidus
temperature in terms of upper limit. The holding in the
second-stage or second soaking may be performed for a time (holding
time) of from about 1 to about 8 hours.
[0070] Hot Rolling
[0071] The hot rolling, if started at a temperature higher than the
solidus temperature, may cause burning and may be performed with
difficulty in itself. In contrast, the hot rolling, if started at a
temperature lower than 350.degree. C., may require an excessively
large load and may be performed with difficulty in itself. To
prevent these, the hot rolling may be performed at a start
temperature selected within the range of 350.degree. C. to the
solidus temperature and may give a hot-rolled sheet having a
thickness of about 2 to about 7 mm. The hot-rolled sheet may be
subjected to, but not necessarily, annealing (heat treatment) in
advance of cold rolling.
[0072] Cold Rolling
[0073] In the cold rolling, the hot-rolled sheet is rolled to give
a cold-rolled sheet (including a coil) having a desired final
thickness of from about 1 to about 3 mm. The workpiece may be
subjected to process annealing between passes of cold rolling.
[0074] Solution Treatment
[0075] The workpiece (cold-rolled sheet) after cold rolling is
subjected to solution treatment as temper refining. The solution
treatment may be performed as heating and cooling in a common
continuous heat treatment line, and the procedure of which is not
limited. However, the solution treatment is preferably performed by
heating the workpiece to a solution treatment temperature of
450.degree. C. to the solidus temperature, and more preferably
480.degree. C. to 550.degree. C., and holding the workpiece for a
duration in the range of from 2 or 3 seconds to 30 minutes after
the temperature reaches the predetermined solution treatment
temperature. This is preferred for obtaining sufficient amounts of
individual elements as solutes and for grain refinement.
[0076] Cooling (temperature fall) after the solution treatment may
be performed at any average cooling rate not critical. The cooling
after the solution treatment may be performed by forced cooling
means or by quenching the workpiece in warm water at a temperature
of room temperature to 100.degree. C. The forced cooling means may
be selected from, alone or in combination, air cooling means such
as fan; and water cooling means such as mist, spray, and immersion.
In this connection, the solution treatment may be basically
performed only once. However, typically when natural age hardening
excessively proceeds, solution treatment and/or reversion treatment
may be performed again under the preferred conditions so as to
temporarily cancel the excessively proceeded natural age hardening.
This is preferred for surely offering formability into an
automobile component.
[0077] In an embodiment, the aluminum alloy sheet according to the
present invention is used as a material, formed into an automobile
component, and assembled as the automobile component. In another
embodiment, the aluminum alloy sheet is formed into an automobile
component, then further subjected to artificial aging treatment,
and used as an automobile component or automobile body.
[0078] Artificial Aging Treatment
[0079] The 7xxx-series aluminum alloy sheet according to the
present invention, after being formed into a structural component,
may be subjected to artificial aging treatment so as to have a
desired strength as the structural component such as an automobile
component, namely, a strength in terms of a 0.2% yield strength of
350 MPa or more, and preferably 400 MPa or more. The artificial
aging treatment is preferably performed after the forming of the
material 7xxx-series aluminum alloy sheet into an automobile
component. This is because the 7xxx-series aluminum alloy sheet, if
subjected to artificial aging treatment before forming, has a
higher strength, but offers lower formability, and may resist
forming when the target automobile component has a certain
complicated shape.
[0080] The artificial aging treatment may be performed under
conditions (e.g., temperature and time) that can be freely
determined within common artificial aging treatment conditions (T6
or T7) depending on the desired strength, the strength of the
material 7xxx-series aluminum alloy sheet, and/or the degree of
proceeding of natural aging. Exemplary artificial aging treatment
conditions are as follows. The artificial aging treatment, when
performed as single-stage aging, may be performed as temper aging
at 100.degree. C. to 150.degree. C. for 12 to 36 hours (including
the over-aging region). The treatment, when performed as two-stage
aging, may be performed so that a first heat treatment is performed
at a temperature of from 70.degree. C. to 100.degree. C. for 2
hours or longer, and a second heat treatment is performed at a
temperature of from 100.degree. C. to 170.degree. C. for 5 hours or
longer (including the overaging region).
EXAMPLES
[0081] A series of 7xxx-series aluminum alloy cold-rolled sheets
having Al--Zn--Mg--Cu chemical compositions as given in Tables 1
and 2 below was produced so as to have different microstructures as
analyzed by the DSC curve. Each of the produced cold-rolled sheets
was sequentially subjected to solution treatment, quenching, and
natural aging. A DSC curve of the resulting sample was measured and
plotted to measure a highest endothermic peak temperature and a
maximum height of exothermic peak(s) in the temperature range of
200.degree. C. to 300.degree. C. A work hardening coefficient n
(10% to 20%) was also measured. In addition, each sample was
subjected to artificial aging treatment and was examined to
evaluate general corrosion resistance and mechanical properties
such as strength. The results are indicated in Tables 3 and 4
below.
[0082] The microstructures of the cold-rolled sheets were
controlled mainly by varying soaking conditions as given in Tables
3 and 4. Specifically, commonly in each sample, a 7xxx-series
molten aluminum alloy having the chemical composition given in
Tables 1 and 2 was subjected to DC casting and yielded an ingot of
a size of 45 mm thick by 220 mm wide by 145 mm long. The ingot was
subjected to two-stage soaking or double soaking under conditions
given in Tables 3 and 4. In the two-stage soaking, the sample was
subjected to first-stage soaking, cooled down to 250.degree. C.,
the cooling was once stopped at that temperature, and the sample
was reheated to and held at a second-stage soaking temperature, and
cooled down to a hot rolling start temperature, followed by
starting of hot rolling. In the double soaking, the sample was
subjected to first soaking, once cooled down to room temperature,
reheated to and held at a second soaking temperature, and cooled
down to a hot rolling start temperature, followed by starting of
hot rolling. In the case in Tables 3 and 4 where soaking was
performed only once, the sample was not subjected to reheating (for
second soaking) after once cooling, but was held at the soaking
temperature for the soaking time by a common procedure, and then
cooled down to a hot rolling start temperature, followed by
starting of hot rolling.
[0083] After the soaking process as above, each sample was
subjected to hot rolling at a start temperature given in Tables 3
and 4 and yielded a hot-rolled sheet having a thickness t of 5 mm.
The hot-rolled sheet was subjected to heat treatment, in which the
hot-rolled sheet was held at 500.degree. C. for 30 seconds and
cooled by forced wind cooling. The sample was then subjected to
cold rolling to a thickness t of 2 mm to give a cold-rolled sheet.
Commonly in each sample, the cold-rolled sheet was subjected to
solution treatment at 500.degree. C. for one minute. After the
solution treatment, the sample workpiece was cooled by forced wind
cooling down to room temperature and yielded a T4 tempered aluminum
alloy sheet. Commonly in each sample, the aluminum alloy sheet
after the solution treatment was subjected to natural aging for 2
weeks, and sheet-like test specimens were sampled from the
resulting workpiece and subjected to DSC measurement and a tensile
test. Individual properties were examined in manners as
follows.
[0084] DSC Measurement (Differential Thermal Analysis
[0085] The DSC measurement (differential thermal analysis) was
performed under the same conditions as follows commonly in each
sample.
[0086] Test equipment: DSC220C supplied by Seiko Instruments
Inc.,
[0087] Reference material: pure aluminum,
[0088] Specimen container: pure aluminum,
[0089] Temperature rise condition: 15.degree. C./min,
[0090] Atmosphere (in the specimen container): argon gas (at a gas
flow rate of 50 ml/min), and
[0091] Test specimen weight: 24.5 to 26.5 mg.
[0092] In the differential thermal analysis, test specimens were
sampled from the aluminum alloy sheet after natural aging at ten
points essentially including a tip portion, a central portion, and
a rear-end portion in the longitudinal direction of the sheet.
Measured values of the ten test specimens were each averaged.
[0093] The differential thermal analysis profiles (.mu.W) obtained
for each sample was divided by the test specimen weight to be
standardized (.mu.W/mg). A region where the differential thermal
analysis profile leveled off in the section of 0.degree. C. to
100.degree. C. in the differential thermal analysis profile was
defined as a reference level of zero (0). Of exothermic peaks in
the temperature range of 200.degree. C. to 300.degree. C., the
height of a highest exothermic peak was measured as the maximum
height of exothermic peaks from the reference level.
[0094] The aluminum alloy sheet after natural aging was subjected
to artificial aging treatment at 90.degree. C. for 3 hr and at
140.degree. C. for 8 hr as T6 treatment. A sheet-like test specimen
was sampled from a central portion of the aluminum alloy sheet
after the artificial aging treatment, on which mechanical
properties and corrosion resistance were examined in manners as
follows. Results of these are also each indicated in Tables 3 and
4.
[0095] Mechanical Properties
[0096] Commonly in each sample, sheet-like test specimens were each
subjected to room temperature tensile test in a direction
perpendicular to the rolling direction to measure a 0.2% yield
strength (MPa) and a total elongation (%) as mechanical properties.
The room temperature tensile test was performed in accordance with
JIS Z2241 (1980) at room temperature 20.degree. C. The test was
performed at a constant tensile speed of 5 mm/minute until the test
specimen was broken.
[0097] Work Hardening Coefficient n
[0098] The work hardening coefficient n was measured in the
following manner. The sheet-like test specimen after the artificial
aging treatment was processed into a JIS No. 5 tensile test
specimen with a gauge length of 50 mm and subjected to a room
temperature tensile test in a direction perpendicular to the
rolling direction. A true stress and a true strain were calculated
from the endpoint of yield elongation, plotted on a logarithmic
scale with the abscissa indicating the strain and the ordinate
indicating the stress. The gradient of a straight line indicated by
measurement points was calculated between two points of nominal
strain of 10% and 20%, and this was defined as the work hardening
coefficient n (10% to 20%).
[0099] Intergranular Corrosion Susceptibility
[0100] To evaluate general corrosion resistance, the sheet-like
test specimens after the artificial aging treatment (three test
specimens per sample) were each subjected to an grain-boundary
corrosion susceptibility test in accordance with former JIS-W1103.
The test was performed under conditions as follows. Each test
specimen was immersed in an aqueous nitric acid solution (30
percent by mass) at room temperature for one minute, immersed in an
aqueous sodium hydroxide solution (5 percent by mass) at 40.degree.
C. for 20 seconds, and immersed in an aqueous nitric acid solution
(30 percent by mass) at room temperature for one minute so as to
clean the surface of the test specimen. The test specimen was
applied with a current at a current density of 1 mA/cm.sup.2 for 24
hours while being immersed in an aqueous sodium chloride solution
(5 percent by mass), then raised and retrieved from the solution,
cut to give a cross section, and the cross section was polished.
The polished cross section was observed with an optical microscope
to measure a corrosion depth from the specimen surface. The
observation and measurement was performed at 100-fold
magnification. A sample having a corrosion depth of 200 .mu.m or
less was evaluated as being corroded slightly and indicated with
".largecircle.". A sample having a corrosion depth greater than 200
.mu.m was evaluated as being corroded significantly and indicated
with "x".
[0101] As apparent from Tables 1 and 3, each of examples (samples
according to the present invention) had aluminum alloy chemical
compositions within the range as specified in the present invention
and were produced under conditions within the preferred production
conditions.
[0102] The resulting sheet was sequentially subjected to solution
treatment, quenching, natural aging, and differential scanning
calorimetry to plot a differential scanning calorimetric (DSC). In
the DSC curve, a highest endothermic peak temperature was
130.degree. C. or lower, and a maximum height of exothermic peak(s)
in the temperature range of 200.degree. C. to 300.degree. C. was 50
.mu.W/mg or more. Thus, the examples met the conditions for
microstructure as analyzed based on the DSC curve.
[0103] The examples therefore had a high work hardening coefficient
n (10% to 20%) of 0.22 or more even after natural aging, had
excellent ductility, and offered excellent processability
(formability) into a structural component. In addition, the
examples had excellent bake hardening response and a high strength
even after natural aging and offered satisfactory corrosion
resistance.
[0104] Of these examples, examples having a chemical composition
meeting the condition: [Zn].gtoreq.-0.3[Mg]+4.5 had a 0.2% yield
strength of 350 MPa or more after the artificial aging treatment;
and Examples 2, 4, 6, 8 to 12, 16 to 19, and 21 having a chemical
composition further meeting the condition:
[Zn].gtoreq.-0.5[Mg]+5.75 in addition to the former condition had a
0.2% yield strength of 400 MPa or more after the artificial aging
treatment.
[0105] In contrast, each of comparative examples had an alloy
chemical composition out of the range specified in the present
invention and/or was produced under conditions out of the preferred
ranges. The comparative examples did not have workability and a
strength both at satisfactory levels, as in Tables 2 and 4.
[0106] Comparative Example 22 to 25 in Table 4 had Zn and Mg
contents each within the specified range, but met neither of the
conditions: [Zn].gtoreq.-0.3[Mg]+4.5 and [Zn].gtoreq.-0.5[Mg]+5.75
(see Alloy Nos. 22 to 25 in Table 2). These samples did not meet
the conditions for microstructure as analyzed by the DSC curve
although they were produced under the preferred production
conditions including soaking conditions. The samples had a 0.2%
yield strength of less than 350 MPa after the artificial aging
treatment, although having a work hardening coefficient n (10% to
20%) of 0.22 or more after natural aging. Thus, the samples did not
have workability and a strength both at satisfactory levels.
[0107] Comparative Example 26 in Table 4 had a Zn content less than
the lower limit (see Alloy No. 26 in Table 2). Comparative Example
27 had a Zn content greater than the upper limit (see Alloy No. 27
in Table 2). Comparative Example 28 had a Cu content less than the
lower limit (see Alloy No. 28 in Table 2). Comparative Example 29
had a Cu content greater than the upper limit (see Alloy No. 29 in
Table 2). These comparative examples, although produced under the
preferred production conditions including soaking conditions, did
not meet the conditions for microstructure as analyzed by the DSC
curve, had a work hardening coefficient n (10% to 20%) of less than
0.22 after natural aging, and did not have workability and a
strength both at satisfactory levels. In addition, Comparative
Example 27 had an excessively high Zn content, and Comparative
Example 28 had an excessively low Cu content. Both the comparative
examples had poor corrosion resistance.
[0108] In Table 4, Comparative Examples 30 to 32 employed Alloy No.
2 in Table 1, namely, the aluminum alloy for use in the present
invention, but were produced under conditions out of the preferred
production condition ranges. Comparative Example 30 underwent a
single soaking (corresponding to second soaking). Comparative
Example 31 underwent first soaking at an excessively low soaking
temperature. Comparative Example 32 underwent second soaking at an
excessively low soaking temperature. Accordingly, these comparative
examples underwent soaking under conditions out of the preferred
range and did not meet the conditions for microstructure as
analyzed by the DSC curve, had a work hardening coefficient n (10%
to 20%) of less than 0.22 after natural aging, or had a 0.2% yield
strength of less than 350 MPa after the artificial aging treatment.
They did not have workability and a strength both at satisfactory
levels.
[0109] Comparative Example 33 in Table 4 had Zn and Mg contents
each within the specified range, but met neither of the conditions:
[Zn].gtoreq.-0.3[Mg]+4.5 and [Zn].gtoreq.-0.5[Mg]+5.75 (see Alloy
No. 33 in Table 2). This sample did not meet the conditions for
microstructure as analyzed by the DSC curve, although produced
under the preferred production conditions including soaking
conditions. The sample thereby had a work hardening coefficient n
(10% to 20%) on the order of 0.22 after natural aging, but a 0.2%
yield strength of less than 350 MPa after the artificial aging
treatment, and did not have workability and a strength both at
satisfactory levels.
[0110] Comparative Examples 34, 35, and 36 in Table 4 each had a Mg
content lower than the lower limit (see Alloy Nos. 31, 32, and 33
in Table 2). These samples therefore failed to meet the conditions
for microstructure as analyzed by the DSC curve even though they
met both the conditions: [Zn].gtoreq.-0.3[Mg]+4.5 and
[Zn].gtoreq.-0.5[Mg]+5.75, and/or even though they were produced
under the preferred production conditions including soaking
conditions. As a result, the samples had a work hardening
coefficient n (10% to 20%) on the order of 0.21 to 0.22 after
natural aging, but a 0.2% yield strength of less than 350 MPa after
the artificial aging treatment, and did not have workability and a
strength both at satisfactory levels.
[0111] Comparative Examples 37, 38, and 39 in Table 4 had a Zn
content greater than the upper limit (see Alloy Nos. 34, 35, and 36
in Table 2). The samples therefore failed to meet the conditions
for microstructure as analyzed by the DSC curve even though they
met both the conditions: [Zn].gtoreq.-0.3[Mg]+4.5 and
[Zn].gtoreq.-0.5[Mg]+5.75 and/or even though they were produced
under the preferred production conditions including soaking
conditions. As a result, the samples had a work hardening
coefficient n (10% to 20%) on the order of 0.21 after natural aging
and did not have workability and a strength both at satisfactory
levels. In addition, these comparative examples also had poor
corrosion resistance due to the excessively high Zn content.
[0112] Comparative Examples 40 to 43 in Table 4 had a Mg content
greater than the upper limit (see Alloy Nos. 37 to 40 in Table 2).
The samples failed to meet the conditions for microstructure as
analyzed by the DSC curve even though they met both the conditions
[Zn].gtoreq.-0.3[Mg]+4.5 and [Zn].gtoreq.-0.5[Mg]+5.75 and/or even
though they were produced under the preferred production conditions
including soaking conditions. As a result, Comparative Examples 40
and 41 having a relatively high Zn content had a low work hardening
coefficient n (10% to 20%) on the order of 0.21 after natural aging
and did not have workability and a strength both at satisfactory
levels. Comparative Examples 42 and 43 having a relatively low Zn
content had a work hardening coefficient n (10% to 20%) on the
order of 0.22 after natural aging, but a 0.2% yield strength of
less than 350 MPa after the artificial aging treatment, and did not
have workability and a strength both at satisfactory levels. These
comparative examples also had poor corrosion resistance due to the
excessively high Mg content.
[0113] These results support critical significance of the
individual conditions specified in the present invention so as to
allow the aluminum alloy sheets according to the present invention
to have all of high strength, good ductility (formability), and
satisfactory SCC resistance.
TABLE-US-00001 TABLE 1 Aluminum alloy chemical composition in mass
percent (the remainder: Al) [Zn] .gtoreq. -0.3 [Zn] .gtoreq. -0.5
Category Number Zn Mg [Mg] + 4.5 [Mg] + 5.75 Cu Ag Zr Mn Cr Sc Si
Fe Ti Examples 1 4.5 2.0 .largecircle. X 0.17 -- -- -- -- -- 0.08
0.10 0.02 2 4.5 2.5 .largecircle. .largecircle. 0.17 -- -- -- -- --
0.08 0.10 0.02 3 4.9 1.5 .largecircle. X 0.17 -- -- -- -- -- 0.08
0.10 0.02 4 4.9 2.8 .largecircle. .largecircle. 0.17 -- -- -- -- --
0.08 0.10 0.02 5 3.5 3.5 .largecircle. X 0.20 -- 0.15 -- -- -- 0.08
0.10 0.02 6 3.5 4.5 .largecircle. .largecircle. 0.20 -- -- 0.90 --
-- 0.08 0.10 0.02 7 3.2 4.5 .largecircle. X 0.18 -- -- -- 0.20 --
0.08 0.10 0.02 8 6.0 1.5 .largecircle. .largecircle. 0.18 -- -- --
-- 0.15 0.08 0.10 0.02 9 4.5 2.7 .largecircle. .largecircle. 0.50
0.10 -- -- -- -- 0.08 0.10 0.02 10 5.0 2.5 .largecircle.
.largecircle. 0.50 0.02 0.07 -- 0.05 -- 0.08 0.15 0.02 11 4.8 2.8
.largecircle. .largecircle. 0.20 -- 0.14 0.20 -- -- 0.30 0.20 0.10
12 3.8 4.0 .largecircle. .largecircle. 0.18 -- 0.15 -- -- -- 0.20
0.40 0.02 13 4.1 1.5 .largecircle. X 0.17 -- 0.25 -- -- -- 0.08
0.10 0.02 14 3.7 3 .largecircle. X 0.17 -- -- 0.15 -- -- 0.08 0.10
0.02 15 3.4 4.5 .largecircle. X 0.17 -- -- -- 0.25 -- 0.08 0.10
0.02 16 5 1.5 .largecircle. .largecircle. 0.17 -- -- -- -- 0.02
0.08 0.10 0.02 17 6 2.5 .largecircle. .largecircle. 0.17 -- -- --
-- 0.25 0.08 0.10 0.02 18 4 4.5 .largecircle. .largecircle. 0.17
0.15 -- -- -- -- 0.08 0.10 0.02 19 6 4.5 .largecircle.
.largecircle. 0.17 -- 0.10 -- 0.07 -- 0.08 0.10 0.02 20 4.2 3
.largecircle. X 0.17 -- 0.03 0.08 0.05 -- 0.08 0.10 0.02 21 4.3 3
.largecircle. .largecircle. 0.17 0.08 -- -- -- 0.07 0.08 0.10
0.02
TABLE-US-00002 TABLE 2 Aluminum alloy chemical composition in mass
percent (the remainder: Al) [Zn] .gtoreq. -0.3 [Zn] .gtoreq. -0.5
Category Number Zn Mg [Mg] + 4.5 [Mg] + 5.75 Cu Ag Zr Mn Cr Sc Si
Fe Ti Comparative 22 3.8 1.5 X X 0.17 -- -- -- -- -- 0.08 0.10 0.02
Examples 23 3.0 4.5 X X 0.17 -- -- -- -- -- 0.08 0.10 0.02 24 3.3
3.8 X X 0.17 -- -- -- -- -- 0.08 0.10 0.02 25 3.6 2.0 X X 0.17 --
-- -- -- -- 0.08 0.10 0.02 26 2.8 6.0 .largecircle. .largecircle.
0.18 -- -- -- -- -- 0.08 0.10 0.02 27 6.1 1.2 .largecircle.
.largecircle. 0.18 -- -- -- -- -- 0.08 0.10 0.02 28 4.6 2.3
.largecircle. .largecircle. 0.03 -- -- -- -- -- 0.08 0.10 0.02 29
4.6 2.3 .largecircle. .largecircle. 0.80 -- -- -- -- -- 0.08 0.10
0.02 30 3.5 3.2 X X 0.20 -- -- -- -- -- 0.08 0.10 0.02 31 4 1.45 X
X 0.17 -- -- -- -- -- 0.08 0.10 0.02 32 5 1.4 .largecircle. X 0.17
-- -- -- -- -- 0.08 0.10 0.02 33 5.9 1.45 .largecircle.
.largecircle. 0.17 -- -- -- -- -- 0.08 0.10 0.02 34 6.1 1.55
.largecircle. .largecircle. 0.17 -- -- -- -- -- 0.08 0.10 0.02 35
6.1 2.5 .largecircle. .largecircle. 0.17 -- -- -- -- -- 0.08 0.10
0.02 36 6.1 4.5 .largecircle. .largecircle. 0.17 -- -- -- -- --
0.08 0.10 0.02 37 5.9 4.55 .largecircle. .largecircle. 0.17 -- --
-- -- -- 0.08 0.10 0.02 38 4 4.55 .largecircle. .largecircle. 0.17
-- -- -- -- -- 0.08 0.10 0.02 39 3.5 4.55 .largecircle. X 0.17 --
-- -- -- -- 0.08 0.10 0.02 40 3.2 4.55 .largecircle. X 0.17 -- --
-- -- -- 0.08 0.10 0.02
TABLE-US-00003 TABLE 3 Aluminum alloy sheet Soaking Hot after
natural aging (T4) First Second rolling DSC curve Soaking Soaking
Start Highest Maximum height of Alloy temper- Soaking temper-
Soaking temper- endothermic exothermic peaks in No. in Soaking
ature time ature time ature peak temperature 200.degree.
C.-300.degree. C. Category Number Table 1 pattern (.degree. C.) hr
(.degree. C.) hr (.degree. C.) (.degree. C.) .mu.W/mg Examples 1 1
Double 410 6 480 4 350 127 59.2 2 2 Double 410 6 480 4 350 128 66.1
3 3 Double 430 4 500 2 410 129 55.7 4 4 Double 430 4 500 2 410 129
68.5 5 5 Double 400 5 510 2 400 125 57.8 6 6 Double 400 5 510 2 400
126 63.5 7 7 Two-stage 420 4 470 6 370 126 60.9 8 8 Two-stage 410 2
520 4 360 130 52.6 9 9 Two-stage 400 6 460 8 400 128 68.7 10 10
Double 425 6 500 6 350 129 69.4 11 11 Two-stage 420 4 490 3 420 128
68.2 12 12 Double 405 6 470 2 380 125 62.2 13 13 Double 425 5 490 3
410 128 55.8 14 14 Two-stage 410 6 500 4 420 126 52.3 15 15
Two-stage 430 4 470 6 380 126 59.8 16 16 Double 415 8 510 3 430 129
59.6 17 17 Double 420 7 475 4 355 130 68.2 18 18 Two-stage 430 6
505 8 400 127 58.5 19 19 Double 400 6 480 7 410 129 61.3 20 20
Two-stage 410 4 490 6 420 126 59.6 21 21 Two-stage 425 8 485 4 390
128 62.2 Aluminum alloy sheet after natural aging (T4) Aluminum
alloy sheet Formability Mechanical after artificial aging Work
properties treatment (T6) hardening 0.2% Elonga- 0.2% General
coefficient n Yield strength tion Yield strength corrosion Overall
Category Number (10~20%) MPa (%) MPa resistance evaluation Examples
1 0.248 247 23 385 .circleincircle. .largecircle. 2 0.229 259 23
423 .circleincircle. .largecircle. 3 0.224 244 23 360 .largecircle.
.largecircle. 4 0.231 261 23 439 .largecircle. .largecircle. 5
0.243 260 23 372 .circleincircle. .largecircle. 6 0.26 265 24 404
.circleincircle. .largecircle. 7 0.261 261 24 395 .circleincircle.
.largecircle. 8 0.238 242 23 420 .largecircle. .largecircle. 9
0.252 261 23 429 .circleincircle. .largecircle. 10 0.247 263 23 432
.largecircle. .largecircle. 11 0.257 258 26 433 .largecircle.
.largecircle. 12 0.242 263 24 401 .circleincircle. .largecircle. 13
0.227 231 23 371 .circleincircle. .largecircle. 14 0.238 248 23 368
.circleincircle. .largecircle. 15 0.287 261 24 395 .circleincircle.
.largecircle. 16 0.228 248 23 400 .largecircle. .largecircle. 17
0.242 240 24 443 .largecircle. .largecircle. 18 0.259 267 24 408
.circleincircle. .largecircle. 19 0.267 270 23 433 .largecircle.
.largecircle. 20 0.232 260 23 396 .circleincircle. .largecircle. 21
0.233 264 23 425 .circleincircle. .largecircle.
TABLE-US-00004 TABLE 4 Aluminum alloy sheet Soaking Hot after
natural aging (T4) First Second rolling DSC curve Soaking Soaking
Start Highest Maximum height of Alloy temper- Soaking temper-
Soaking temper- endothermic peak exothermic peaks in No. in Soaking
ature time ature time ature temperature 200.degree. C.-300.degree.
C. Category Number Table 2 pattern (.degree. C.) hr (.degree. C.)
hr (.degree. C.) (.degree. C.) .mu.W/mg Comparative 22 22 Double
430 4 460 4 340 129 40.3 examples 23 23 Double 420 4 470 4 400 128
49.4 24 24 Double 400 4 460 4 420 128 48.6 25 25 Two-stage 410 4
480 4 350 128 38.5 26 26 Double 410 4 480 3 410 132 48.7 27 27
Two-stage 430 4 460 4 380 134 61.8 28 28 Double 410 4 460 4 430 133
63.4 29 29 Double 420 4 450 4 420 136 67.1 30 2 Single 400 4 -- --
380 121 46.8 31 2 Double 380 4 450 4 350 134 44.8 32 2 Double 410 4
430 4 400 135 47.8 33 30 Two-stage 410 5 450 3 355 127 49.3 34 31
Double 400 3 460 2 380 129 46.4 35 32 Two-stage 420 4 475 4 370 131
49.5 36 33 Two-stage 405 4 460 4 400 132 48.8 37 34 Double 430 2
460 2 420 135 52.9 38 35 Double 440 4 480 1 385 134 67.2 39 36
Two-stage 420 3 450 4 360 133 69.1 40 37 Double 440 2 470 2 390 132
60.2 41 38 Double 410 3 455 3 400 131 49.2 42 39 Two-stage 430 4
470 2 395 130 47.9 43 40 Double 410 2 460 4 370 129 46.1 Aluminum
alloy sheet after natural aging (T4) Aluminum alloy sheet
Formability Mechanical after artificial aging Work properties
treatment (T6) hardening 0.2% Elonga- 0.2% General coefficient n
Yield strength tion Yield strength corrosion Overall Category
Number (10~20%) MPa (%) MPa resistance evaluation Comparative 22
0.221 206 22 247 .largecircle. X examples 23 0.259 243 24 321
.largecircle. X 24 0.244 254 23 303 .largecircle. X 25 0.246 217 22
235 .largecircle. X 26 0.217 278 24 318 .largecircle. X 27 0.213
240 22 401 X X 28 0.216 255 22 402 X X 29 0.203 259 21 418
.largecircle. X 30 0.229 211 24 277 .largecircle. X 31 0.211 246 20
267 .largecircle. X 32 0.209 243 20 272 .largecircle. X 33 0.224
246 21 318 .largecircle. X 34 0.225 230 21 269 .largecircle. X 35
0.219 245 20 320 .largecircle. X 36 0.217 243 20 317 .largecircle.
X 37 0.211 244 19 373 X X 38 0.213 246 20 415 X X 39 0.216 249 20
430 X X 40 0.217 247 20 396 X X 41 0.218 243 20 314 X X 42 0.22 248
20 281 X X 43 0.221 244 20 272 X X
[0114] While the present invention has been particularly described
with reference to specific embodiments thereof it is obvious to
those skilled in the art that various changes and modifications may
be made without departing from the spirit and scope of the present
invention.
[0115] The present application is based on Japanese Patent
Application No. 2013-051608 filed on Mar. 14, 2013, the entire
contents of which are incorporated herein by reference.
INDUSTRIAL APPLICABILITY
[0116] As described above, the present invention can provide
7xxx-series aluminum alloy sheets that may be used for automobile
components and have a strength, formability, and corrosion
resistance all at satisfactory levels. The present invention is
therefore suitably applicable to automobile structural components
such as frames and pillars which contribute to body weight
reduction; as well as to structural components for other uses.
* * * * *