Note: Descriptions are shown in the official language in which they were submitted.
CA 02941997 2016-09-08
a
DESCRIPTION
Title of the Invention
ALUMINUM ALLOY PLATE HAVING EXCELLENT MOLDABILITY AND BAKE
HARDENING PROPERTIES
Technical Field
[0001]
The present invention relates to an Al-Mg-Si alloy sheet. The aluminum alloy
sheet referred to in the present invention means an aluminum alloy sheet that
is a rolled
sheet such as a hot rolled sheet or a cold rolled sheet and has been subjected
to refining
such as a solution heat treatment and a quenching treatment, but is not yet
subjected to a
press forming and a bake hardening treatment. Further, aluminum is hereinafter
also
referred to as Al.
Background Art
[0002]
In recent years, because of environmental awareness and the like, the
society's
requirement for weight reduction in a vehicle such as an automobile has been
steadily
increasing. In order to respond to such requirement, as a material for a large
body panel
structure (an outer panel or an inner panel) of an automobile instead of a
steel material
such as a steel sheet, application of an aluminum alloy material excellent in
formability
and bake hardenability and lighter in weight has been increasing.
[0003]
Among the large body panel structure of an automobile, for an outer panel
(outer
sheet) such as a hood, a fender, a door, a roof, or a trunk lid, use of an Al-
Mg-Si-based AA
or JIS 6000-series (hereinafter, also simply referred to as a 6000-series)
aluminum alloy
sheet, as a thin and high strength aluminum alloy sheet, has been studied.
[0004]
The 6000-series aluminum alloy sheet contains Si and Mg as essential
components. In particular, a 6000-series aluminum alloy with excess Si has a
composition in which the Si/Mg mass ratio is 1 or greater, and has excellent
age
hardenability. Because of this, formability for press forming or bending into
the outer
panels of automobiles is secured by lowering the proof stress. In addition, it
has such
bake hardenability (hereinafter referred to also as BH response) that it
undergoes age
hardening upon heating in an artificial aging (hardening) treatment performed
at a
relatively low temperature, such as the baking treatment of formed panels, and
hence
improves in proof stress, thereby ensuring the strength required as a panel.
[0005]
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CA 02941997 2016-09-08
On the other hand, as is known well, an outer panel of an automobile is
manufactured by applying combined formings, such as stretch forming or bending
forming
in press forming, to an aluminum alloy sheet. For example, in a large outer
panel such as
a hood or a door, the shape of a formed product is made as an outer panel by
press forming
such as stretching, and then joining with an inner panel is executed by hem
work
(hemming) of a flat hem and the like of the outer panel peripheral section to
be formed into
a panel structural body.
[0006]
Here, the 6000-series aluminum alloy had an advantage of having excellent BH
response, but had a problem of having aging properties at room temperature,
that is, of age
hardening during retention at room temperature after solution heat treatment
and
quenching treatment to increase the strength, thereby deteriorating
formability into a panel,
particularly the bendability. For example, in a case where a 6000-series
aluminum alloy
sheet is to be used for an automobile panel, it is placed at room temperature
(standing at
room temperature) for approximately 1 month after the solution heat treatment
and the
quenching treatment (after manufacturing) at an aluminum manufacturer until
forming into
a panel at an automobile manufacturer, and comes to be significantly age
hardened (room-
temperature aged) during that time. Particularly, in the outer panel subjected
to severe
bending, there was such a case that, although forming was possible without any
problem
immediately after manufacturing, cracking occurred in hem working after the
lapse of 1
month. Therefore, in the 6000-series aluminum alloy sheet for an automobile
panel,
particularly for an outer panel, it is necessary to suppress room-temperature
aging over a
comparatively long period of approximately 1 month.
[0007]
Moreover, in the case where such room-temperature aging is great, there also
is a
case that the BH response deteriorate and the proof stress is not improved to
the strength
required as a panel by heating during an artificial aging (hardening)
treatment at a
comparatively low temperature, such as a bake treatment and the like of the
panel after
forming described above.
[0008]
Hereto, in order to cope with such decreases in the formability and BH
response
of 6000-series aluminum alloy sheets due to room-temperature aging, various
proposals
have been made on methods for regulating Mg-Si clusters which are formed in
the sheets
during room-temperature standing after refining (after solution and quenching
treatments).
Among these proposed methods is a technique in which such Mg-Si clusters are
controlled
by means of endothermic peaks and exothermic peaks of a differential scanning
thermal
analysis curve (also called a differential scanning calorimetry curve;
hereinafter referred to
also as DSC) of the 6000-series aluminum alloy sheet.
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CA 02941997 2016-09-08
[0009]
For example, Patent Documents 1 and 2 propose that the formation amount of
Mg-Si clusters that inhibit room-temperature aging and suppress low-
temperature age
hardenability, in particular, Si/hole clusters (GPI), is regulated. In these
techniques, for
regulating the formation amount of GPI, it is regulated that the T4 material
(after solution
treatment and subsequent natural aging) gives a DSC which has no endothermic
peak in
the temperature range of 150-250 C, corresponding to the dissolution of GPI.
In these
techniques, a low-temperature heat treatment of holding at 70-150 C for about
0.5-50
hours is performed after a solution treatment and quenching to room
temperature, in order
to inhibit or control the formation of the GPI.
[0010]
Patent Document 3 proposes a 6000-series aluminum alloy sheet with excess Si
which, after a refining treatment including solution and quenching treatments
of this
aluminum alloy sheet, gives a DSC in which an endothermic peak in the
temperature range
of 150-250 C and corresponds to a dissolution of Si/hole clusters (GPI) has a
minus height
of 1,000 1.1W or less and an exothermic peak in the temperature range of 250-
300 C and
corresponds to a precipitation of Mg/Si clusters (GPII) has a plus height of
2,000 or
less. This aluminum alloy sheet, after having undergone room-temperature aging
for at
least 4 months after the refining treatment, has the properties in which a
proof stress is in
the range of 110-160 MPa, a difference in proof stress with the one just after
the refining
treatment is 15 MPa or less, an elongation is 28% or greater, and a proof
stress, as
measured after application of a 2% strain thereto and a subsequent low-
temperature aging
treatment of 150 C x 20 minutes, is 180 MPa or greater.
[0011]
Patent Document 4 proposes that a 6000-series aluminum alloy sheet is set to
give, after a refining treatment, a DSC in which an exothermic peak in the
temperature
range of 100-200 C has a height W1 of 50 p.W or larger and a ratio of a height
W2 of an
exothermic peak in the temperature range of 200 to 300 C to the exothermic-
peak height
Wl, (W2/W1), is 20.0 or less, in order to obtain BH response in a bake
hardening
treatment performed at a low temperature for a short period.
[0012]
The document states that the exothermic peak W1 corresponds to the
precipitation
of GP zones serving as nucleus formation sites of f5" (Mg2Si phase) in an
artificial age
hardening treatment, and that the higher the W1 peak height, the more the GP
zones
serving as nucleus formation sites of (V' in an artificial age hardening
treatment have
already been formed and secured in the sheet after refining. It states that as
a result, the
p" grows rapidly in a bake hardening treatment after forming, thereby
attaining an
improvement in BH response. It states that the exothermic peak W2, on the
other hand,
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CA 02941997 2016-09-08
corresponds to a precipitation peak of the P" itself, and that the height of
this exothermic
peak W2 is made as small as possible in order to reduce the proof stress of
the sheet to be
formed to less than 135 MPa and to thereby ensure formability.
[0013]
Patent Document 5 proposes that three exothermic-peak heights (three portions)
in
a DSC in specific temperature ranges and particularly affect BR response are
selected and
regulated to enhance the BR response (bake hardenability). The three
exothermic peaks
are peak A at 230-270 C, peak B at 280-320 C and peak Cat 330-370 C. In the
proposed
method, the height of the peak B is regulated to 20 1.1.W/mg or larger and the
peak ratio
(A/B) and the peak ratio (C/B) are regulated to 0.45 or less and 0.6 or less,
respectively,
thereby attaining an increase in 0.2% proof stress, through an artificial
hardening treatment
of 170 C x 20 minutes after application of a 2% strain, of 100 MPa or greater.
Prior Art Documents
Patent Documents
[0014]
Patent Document 1: JP-A-10-219382
Patent Document 2: JP-A-2000-273567
Patent Document 3: JP-A-2003-27170
Patent Document 4: JP-A-2005-139537
Patent Document 5: JP-A-2013-167004
Summary of the Invention
Problem that the Invention is to Solve
[0015]
The various outer panels for automobiles are required to attain strain-free,
beautiful curved-surface configurations and character lines, from the
standpoint of design.
However, since higher-strength aluminum alloy sheet materials are being
adopted for the
purpose of weight reduction and this results in difficulties in forming, it is
becoming
difficult year by year to meet such requirements. There is hence a growing
desire in
recent years for a high-strength aluminum alloy sheet having even better
formability.
However, with the above-mentioned conventional structure controls with a DSC,
it is
difficult to meet such requirements.
[0016]
For example, one cause which renders high-strength aluminum alloy sheets
difficult to apply to outer panels is the shapes peculiar to outer panels.
Recessed portions
having given depths (protrudent portions, embossed portions) for attaching
devices or
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members, such as knob mount bases, lamp mount bases and license (number plate)
mount
bases, or for drawing wheel arches are partly provided to outer panels.
[0017]
In the cases when such a recessed portion is press-formed together with
consecutive curved surfaces around the recessed portion shape, face strains
are prone to
occur and it is difficult to attain the strain-free, beautiful curved-surface
configuration and
character line. Consequently, application of high-strength aluminum alloy
sheets to the
outer panels has a problem in that it is necessary to obtain a high-strength
aluminum alloy
sheet which has improved formability and is inhibited from suffering face
strains.
[0018]
The problem concerning such face strains is not for those recessed portions
(protrudent portions) but a problem common to automotive panels which partly
have a
recessed portion (protrudent portion) that may suffer a face strain, such as a
saddle-shaped
portion of a door outer panel, a vertical wall portion of a front fender, a
wind corner
portion of a rear fender, a character-line termination portions of a trunk lid
or hood outer
panel, and a root portion of a rear fender pillar.
[0019]
From the standpoint of attaining improved formability for inhibiting the
occurrence of the face strains to overcome the problem described above, it is
desirable that
a sheet in press forming, which has undergone room-temperature aging after
production,
should have a 0.2% proof stress reduced to less than 110 MPa. However, in the
cases
when the proof stress in forming has been reduced as the above, it is
difficult to attain a
0.2% proof stress of 170 MPa or greater after bake hardening (hereinafter also
referred to
as "after BH") and to attain an increase in 0.2% proof stress through bake
hardening of 70
MPa or greater. As described above, with conventional structure controls with
a DSC
disclosed in Patent Documents 1 to 5, it is difficult to overcome the problem.
[0020]
The present invention has been achieved in order to overcome the problem
described above. An object thereof is to provide an aluminum alloy sheet which
combines formability and bake hardenability, that is, which can have, in
automotive-panel
forming, a 0.2% proof stress reduced to 110 MPa or less and can have a 0.2%
proof stress
after BH of 170 MPa or greater.
Means for Solving the Problem
[0021]
The present inventors diligently made investigations and, as a result, have
discovered that an aluminum alloy sheet which combines formability and bake
hardenability can be obtained by adopting a specific composition and specific
exothermic
5
peaks in the DSC for an Al-Mg-Si alloy sheet, which contains Mg and Si. The
present
invention has been thus completed.
[0022]
The gist of the aluminum alloy sheet of the present invention, which is
excellent
in terms of formability and bake hardenability, is an Al-Mg-Si alloy sheet
containing, in
terms of mass %, Mg: 0.2-1.0% and Si: 0.2-1.0% and satisfying {(Mg
content)+(Si
content)}<1.2%, with the remainder being Al and unavoidable impurities, in
which a
differential scanning thermal analysis curve of the aluminum alloy sheet has,
in a
temperature range of 230-330 C, only one exothermic peak (i) or only two
exothermic
peaks (ii) having a temperature difference between the peaks of 50 C or less,
and in
which the exothermic peak (i) or the peak having a higher peak height of the
exothermic
peaks (ii) has a height in a range of 20-50 RW/mg.
The differential thermal analysis at each of measurement portions in the sheet
is
performed under the same conditions including a test apparatus of DSC220G,
manufactured by Seiko Instruments Inc., a reference substance of aluminum, a
sample
container made of aluminum, temperature increase conditions of 15 C/min, an
atmosphere of argon (50 mL/min), and a sample weight of 24.5-26.5 mg. The
differential
thermal analysis profile ( W) obtained is divided by the sample weight and
thereby
normalized (W/mg). Thereafter, in the range of 0-100 C in the differential
thermal
analysis profile, a region where the differential thermal analysis profile is
horizontal is
taken as a reference level of 0, and the height of exothermic peak from the
reference level
is measured.
[0023]
The aluminum alloy sheet excellent in terms of formability and bake
hardenability
may further contain one element or two or more elements selected from the
group
consisting of Fe: more than 0% and 0.5% or less, Mn: more than 0% and 0.3% or
less,
Cr: more than 0% and 0.3% or less, Zr: more than 0% and 0.1% or less, V: more
than 0%
and 0.1% or less, Ti: more than 0% and 0.1% or less, Cu: more than 0% and 0.5%
or less,
Ag: more than 0% and 0.1% or less, and Zn: more than 0% and 0.5% or less.
In yet another aspect, the present invention provides an aluminum alloy sheet,
which is an Al-Mg-Si alloy sheet comprising, in terms of mass %, Mg: 0.2-1.0%
and Si: 0.2-1.0% and satisfying {(Mg content)+(Si content)}<1.2%, with the
remainder
being Al and unavoidable impurities, wherein a differential scanning
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thermal analysis curve of the aluminum alloy sheet has, in a temperature range
of 230-
330 C, only one exothermic peak (i) or only two exothermic peaks (ii) having a
temperature difference between the peaks of 50 C or less, wherein the
exothermic peak
(i) or the peak having a higher peak height of the exothermic peaks (ii) has a
height in a
range of 20-500W/tug, wherein the differential scanning thermal analysis curve
is
obtained by performing a differential thermal analysis with a reference
substance of
aluminum, a sample container made of aluminum, a temperature increase
condition of
about 1 5 C/min., an atmosphere of argon at about 50mL/min. and a sample
weight of
24.5 mg to 26.5 mg, and wherein the sheet is selected to have, in automotive-
panel
forming, a 0.2% proof stress reduced to 110 MPa or less and a 0.2% proof
stress after
bake hardening of 170 MPa or greater.
Effects of the Invention
[0024]
According to the present invention, the contents of Mg and Si, which are major
elements of an Al-Mg-Si alloy sheet, are regulated to be relatively low,
thereby enabling
a 0.2% proof stress in forming of the sheet, which has been produced and then
subjected
to room-temperature aging, to be reduced to 110 MPa or less. Consequently, it
can have
improved formability when applied to automotive panels or the like, which are
particularly problematic in face strains thereof, in automotive panel
structures.
[0025]
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CA 02941997 2016-09-08
In addition, the thermal properties (structure) in the DSC of the aluminum
alloy
sheet are regulated. As a result, an increased strength which includes a 0.2%
proof stress
after BH of 170 MPa or greater and an increase in 0.2% proof stress of 70 MPa
or greater,
which is useful as automotive panels can be ensured. The regulation of thermal
properties (structure) in the DSC provides a measure for ensuring the amount
of
precipitates which precipitate after a bake hardening treatment.
[0026]
Due to such regulation of composition and structure, an aluminum alloy sheet
which combines formability and bake hardenability can be provided merely with
a basic
composition of Al-Mg-Si alloys, without the need of newly adding any additive
element or
without the need of giving a large modification to ordinary production
processes.
Brief Description of the Drawing
[0027]
[Fig. 1] Fig. 1 is a view which shows DSCs of the aluminum alloy sheets of
some
examples in the Examples.
Modes for Carrying Out the Invention
[0028]
Modes for carrying out the present invention will be specifically explained
below
with respect to each requirement. In this description, "mass %" has the same
meaning as
[0029]
(Chemical Component Composition)
First, the chemical component composition of the Al-Mg-Si (hereinafter
referred
to also as 6000-series) aluminum alloy sheet (hereinafter also referred to
simply as
"aluminum alloy sheet") according to the present invention is explained below.
The 6000-series aluminum alloy sheet targeted by the present invention, as,
for
example, a sheet for the automotive outer panels, is required to have various
properties
such as excellent formability, BH response, strength, weldability, and
corrosion resistance.
Consequently, such requirements are also met by means of the composition. In
addition,
in the present invention, the contents of Mg and Si, which are major elements,
are
regulated so as to be relatively low, thereby reducing a 0.2% proof stress in
forming of the
sheet, which has been produced and then subjected to room-temperature aging,
to 110 MPa
or less. Thus, the formability into automotive panels or the like, which are
particularly
problematic in face strains thereof, in automotive panel structures, can be
improved.
Simultaneously therewith, a 0.2% proof stress after bake hardening of 170 MPa
or greater
is rendered possible by means of composition.
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[0030]
In order to satisfy such requirements, the aluminum alloy sheet has a
composition
which contains, in terms of mass %, Mg: 0.2-1.0% and Si: 0.2-1.0% and
satisfies {(Mg
content)+(Si content)}1.2%, with the remainder being Al and unavoidable
impurities. In
this description, all the content indicated in % of the elements means that in
mass %.
Furthermore, the "-" in each content means that the content is equal to or
more than the
lower limit value but is equal to or less than the upper limit value.
[00311
In the present invention, elements other than Mg, Si and Al basically are
impurities or elements which may be contained. The contents of such other
elements are
the contents (permissible amounts) on levels in accordance with the AA or JIS
standards,
etc., or are on levels below such standards. Namely, there are cases, in the
present
invention also, where not only high-purity Al base metal but also 6000-series
alloys
containing elements other than Mg and Si as additive elements (alloying
elements) in large
amounts, other aluminum alloy scrap materials, low-purity Al base metal, and
the like are
used in large quantities as melted raw materials for the alloy, from the
standpoint of
resource recycling. In such cases, other elements shown below are inevitably
included in
substantial amounts. Since refining performed for intentionally diminishing
these
elements itself leads to an increase in cost, it is necessary to accept an
inclusion of some
degree of amount. There are content ranges which do not defeat or lessen the
object or
effects of the present invention, even if included in substantial amounts.
[0032]
Consequently, in the present invention, examples of the other elements which
may
be contained in the aluminum alloy include the following elements. The
permissible
contents thereof are within the ranges of equal to or less than the upper
limits according to
the AA or JIS standards or the like, and are as shown below.
Specifically, the aluminum alloy sheet may further contain one element or two
or
more elements selected from the group consisting of Fe: 0.5% or less
(exclusive of 0%),
Mn: 0.3% or less (exclusive of 0%), Cr: 0.3% or less (exclusive of 0%), Zr:
0.1% or less
(exclusive of 0%), V: 0.1% or less (exclusive of 0%), Ti: 0.1% or less
(exclusive of 0%),
Cu: 0.5% or less (exclusive of 0%), Ag: 0.1% or less (exclusive of 0%), and
Zn: 0.5% or
less (exclusive of 0%), within those ranges.
In this description, the expression "exclusive of 0%" has the same meaning as
that
the content is "higher than 0%".
[0033]
The content range of each element and the purposes and permissible amount
thereof in the 6000-series aluminum alloy sheet according to the present
invention are
explained below.
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[0034]
Si: 0.2-1.0%
Si, together with the Mg, is an essential element for obtaining the strength
(proof
stress) required as automotive panels because it forms aging precipitates
which contribute
to an improvement in strength, during an artificial aging treatment such as a
baking
treatment, and thus exhibits an age hardenability. In the case where the
content of Si is
too low, the amount of aging precipitates after an artificial aging treatment
is too small,
resulting in too small an increase in strength after baking. Meanwhile, in the
case where
the content of Si is too high, not only the strength of the sheet just after
production but also
the amount of room-temperature aging after the production are increased,
resulting in too
high a strength before forming. Because of this, the formability into
automotive panels or
the like, which are particularly problematic in face strains thereof, in
automotive panel
structures, is reduced. In addition, coarse crystals and precipitates are
formed, resulting
in a considerable decrease in bendability. A preferred upper limit of the
content of Si is
0.8%.
[0035]
For attaining an excellent age hardenability in a baking treatment performed
at a
lower temperature for a shorter period after forming into panels, it is
preferable to employ
a 6000-series aluminum alloy composition in which Si/Mg is 1.0 or larger in
terms of mass
ratio so that Si has been incorporated further excessively relative to the Mg
than in the so-
called excess-Si type.
[0036]
Mg: 0.2-1.0%
Mg is also an essential element for obtaining the proof stress required as
panels,
since it forms, together with the Si, aging precipitates which contribute to
an improvement
in strength, and thus exhibits an age hardenability. In the case where the
content of Mg is
too low, the precipitate amount of precipitates after an artificial aging
treatment is too
small, resulting in too small an increase in strength after baking. Meanwhile,
in the case
where the content of Mg is too high, not only the strength of the sheet just
after production
but also the amount of room-temperature aging after the production are
increased, resulting
in too high a strength before forming. Because of this, the formability into
automotive
panels or the like, which are particularly problematic in face strains
thereof, in automotive
panel structures, is reduced. A preferred upper limit of the content of Mg is
0.8%.
[0037]
.. {(Mg content)+(Si content)) .1.2%
[(Mg content)+(Si content)), which is the total content of Mg and Si, as the
structure of the 6000-series aluminum alloy sheet before forming, considerably
affects
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exothermic peaks present in the temperature range of 230-330 C in the DSC of
this
aluminum alloy sheet.
[0038]
On the assumption that the appropriate production process which will be
described later is used, by regulating {(Mg content)+(Si content)} to 1.2% or
less, in the
case where there exist only two exothermic peaks (ii) in the temperature range
of 230-
330 C, the difference in temperature between the peaks of the two exothermic
peaks (ii)
can be 50 C or less and the one having a higher peak height can have a peak
height in the
range of 20-50 .LW/mg. Meanwhile, in the case where there exists only one
exothermic
peak (i) in that temperature range, this exothermic peak (i) can have a height
in the range
of 20-50 gW/mg.
[0039]
Consequently, it is preferred that {(Mg content)+(Si content)} is as small as
possible. However, since there essentially are minimum necessary Mg and Si
amounts
for exhibiting basic performances as a sheet, a lower limit of {(Mg
content)+(Si content)}
is determined by the minimum contents of these each. From this standpoint, a
lower limit
of {(Mg content)+(Si content)} is preferably 0.6% or higher.
[0040]
Meanwhile, in the case where {(Mg content)+(Si content)) is too high above
1.2%, it is difficult to regulate the DSC exothermic peaks so as to fall
within the specified
ranges, even if the appropriate production process which will be described
later is used.
Specifically, in the case where there are two exothermic peaks in the
temperature range of
230-330 C, these two exothermic peaks cannot have a temperature difference
between the
peaks of 50 C or less. In the case where there is only one exothermic peak in
that
temperature range, this exothermic peak cannot have a height in the range of
20-50
uW/mg. Because of this, it is difficult to attain both a reduction in strength
during
forming (before baking) and an enhancement in increase in strength through
paint baking.
Consequently, an upper limit of {(Mg content)+(Si content)} is 1.2% or less
and preferably
1.0% or less.
[0041]
(Differential scanning thermal analysis curve, differential scanning
calorimetry curve,
DSC)
The composition described above is employed. Furthermore, in the present
invention, peaks in the DSC of the aluminum alloy sheet are regulated as a
measure for
ensuring the amount of precipitates which precipitate alter a bake hardening
treatment, in
order to ensure high strength as automotive panels or the like. Specifically,
a structure is
configured in which two exothermic peaks, which have conventionally been
present in the
temperature range of 230-330 C apart from each other, are present so as to
near to each
CA 02941997 2016-09-08
other (with a reduced temperature difference) and to overlap each other. This
makes it
possible to attain a 0.2% proof stress in forming into automotive panels
reduced to 110
MPa or less and to attain a 0.2% proof stress after bake hardening of 170 MPa
or greater.
[0042]
Here, the differential scanning calorimetry curve (DSC) is a heating curve
from
solid phase, obtained by measuring the thermal changes during melting of
aluminum alloy
sheet after the refining treatment of the sheet, by differential thermal
analysis performed
under the following conditions.
Specifically, the differential thermal analysis at each of measurement
portions in
the aluminum alloy sheet is performed under the same conditions including a
test apparatus
of DSC220G, manufactured by Seiko Instruments Inc., a reference substance of
aluminum,
a sample container made of aluminum, temperature increase conditions of 15
C/min, an
atmosphere of argon (50 mL/min), and a sample weight of 24.5 to 26.5 mg. The
differential thermal analysis profile (i.tW) obtained is divided by the sample
weight and
thereby normalized (1W/mg). Thereafter, in the range of 0 to 100 C in the
differential
thermal analysis profile, a region where the differential thermal analysis
profile is
horizontal is taken as a reference level of 0, and the height of exothermic
peak from the
reference level is measured.
[0043]
In the DSC, according to conventional techniques, there are two exothermic
peaks
13" and 13' in the range of 230-330 C, existing apart from each other so as to
have a large
temperature difference (distance) between the peaks. In the present invention,
the
structure of the aluminum alloy sheet has been specified so that the two
exothermic peaks
are located near to each other (with a reduced temperature difference
therebetween) and to
overlap each other. Specifically, in a DSC of the aluminum alloy sheet, in the
temperature range of 230-330 C, there is only one exothermic peak (i) or there
are only
two exothermic peaks (ii), having the difference in temperature between the
peaks of 50 C
or less. Moreover, the only one exothermic peak (i), or the exothermic peak
having a
larger (higher) peak height of the only two exothermic peaks (ii) has a height
in the range
of 20-50 1.1W/mg.
[0044]
In 6000-series aluminum alloy sheets, various precipitate phases are yielded,
depending on aging temperatures, such as clusters, GP zones, strengthening
phase 1 (13"),
strengthening phase 2 (fv), and equilibrium phase (Mg2Si). It is presumed that
for
enhancing the strength after baking (artificial aging treatment), it is
effective to yield 13"
and 13', among those phases, during the baking. However, the 6000-series
aluminum alloy
sheet of the present invention, in which the contents of Mg and Si have been
regulated so
as to be relatively low in order to make the sheet have, in forming after room-
temperature
11
CA 02941997 2016-09-08
aging, a 0.2% proof stress reduced to 110 MPa or less, considerably differs in
the
appearing behavior (appearing temperature) of the strengthening phase 1 (13")
and
strengthening phase 2 (13') upon BH (artificial aging treatment), from
ordinary 6000-series
aluminum alloy sheets having relatively high Mg and Si contents.
[0045]
The changes in the appearing behavior of 13" and 13' upon BFI (upon baking
treatment) can be simulated with DSC. This is a base of specifying the
structure in the
present invention by means of DSC.
[0046]
A simulation with DSC of the appearing behavior of [3" and 13' upon BH shows
that in the case of, for example, ordinary 6000-series aluminum alloy sheets
having
relatively high Mg and Si contents, the exothermic peaks assigned to [3" and
[3' are present
more widely apart from each other in the range of 230-330 C. More
specifically, a
conventional exothermic peak assigned to [3" is mostly present around 240-260
C, which is
the lower-temperature former half of that temperature range. Meanwhile, a
conventional
exothermic peak assigned to 13' is present around 310-320 C, which is the
higher-
temperature latter half of that temperature range, and they have existed in a
state that the
difference in temperature between the peaks of f3" and 13' has been larger
than 50 C.
[0047]
Such state of conventional exothermic peaks is a representative example, and
that
appearing behavior of the exothermic peaks varies widely, as a matter of
course, depending
on the composition of the sheet and production conditions. For example, there
are cases
where a DSC has three exothermic peaks (three portions) regarding BH response
and they
are respectively called peak A at 230-270 C, peak B at 280-320 C and peak C at
330-
370 C, as in Patent Document 5.
[0048]
In contrast, when a simulation with DSC of the appearing behavior of 13" and
[3'
upon BH is similarly made with respect to the 6000-series aluminum alloy sheet
of the
present invention, in which the contents of Mg and Si are relatively low, it
can be seen that
the exothermic peaks assigned to p" and [3' are characterized in that the
positions where the
exothermic peaks appear (peak positions) and the distance between the peaks
(temperature
difference) are nearer to each other (overlapping), as compared with those
ordinary 6000-
series aluminum alloy sheets. There also is a feature in which this phenomenon
occurs as
a result of changing the conditions for sheet production, in particular, the
conditions for a
preliminary aging treatment performed after solution and quenching treatments.
[0049]
In a 6000-series aluminum alloy sheet of the present invention having
relatively
low Mg and Si contents, when produced by an ordinary process, exothermic peaks
of p"
12
CA 02941997 2016-09-08
and 13' exist in the wide temperature range of 230-330 C as two separate
peaks, the
distance between whose peaks is 50 C or larger in terms of temperature
difference, like
ordinary 6000-series aluminum alloy sheets having relatively high Mg and Si
contents.
As typical examples thereof, the DSC indicated by the broken line shown in
Fig. 1, which
.. will be described later, and Comparative Example 19 in Table 2 in the
Examples.
[0050]
On the other hand, it has been found that in the cases when a production
process is
modified to perform the refining after rolling of the sheet so that the
conditions for a
preliminary aging treatment after solution and quenching treatments are
changed, the
exothermic peaks of13" and 13' appear so that the peaks thereof overlap each
other (are
located near to each other), with the difference in temperature between the
peaks being as
small as less than 50 C.
[0051]
According to the finding made by the present inventors, the appearing
temperature
of the exothermic peak assigned to 13" (also called first or former-half peak)
shifts from the
position (temperature) around 250-260 C of low temperature to a position
(temperature)
around 270-290 C of high temperature. Meanwhile, the appearing temperature of
the
exothermic peak assigned to 13' (also called second or latter-half peak)
shifts from the
position (temperature) around 300-310 C of high temperature to a position
(temperature)
around 290-300 C of low temperature.
[0052]
It has been found that in the cases when the exothermic peaks assigned to and
13' have appeared so that the peaks are located near to each other or overlap
each other, with
the temperature difference between the peaks being as small as less than 50 C,
then an
amount of artificial-aging precipitates which serve to enhance the proof
stress after BH can
be ensured. Namely, by regulating the exothermic peaks assigned tor." and [31
so as to be
located near to each other or overlap each other, the 0.2% proof stress in
panel forming can
be reduced to 110 MPa or less and, simultaneously therewith, the 0.2% proof
stress of the
panel after BH can be increased to 170 MPa or greater. In contrast, in the
case where
.. those two exothermic peaks have the difference in temperature between the
peaks as large
as more than 50 C, those properties cannot be exhibited.
[0053]
One of the features of the present invention is that the state in which the
exothermic peaks assigned to 13" and [3' overlap each other has been specified
as above.
Specifically, the 6000-series aluminum alloy sheet gives a DSC in which only
two (only
two in total) exothermic peaks, i.e., a lower-temperature-side exothermic peak
assigned to
fr and a higher-temperature-side exothermic peak assigned to 13', that have a
difference in
temperature between the peaks of 50 C or less, preferably 30 C or less, are
present in the
13
CA 02941997 2016-09-08
temperature range of 230-330 C, preferably in the temperature range of 250-320
C, and in
which the height of either exothermic peak of these, which has a larger
(higher) peak
height is in the range of 20-50 filV/mg. In the case where the lower-
temperature-side
exothermic peak assigned to p" and the higher-temperature-side exothermic peak
assigned
to IT are located nearer to each other to overlap each other so that the
difference in
temperature between these peaks cannot be recognized (measured), i.e., in the
case where
it is deemed that there is only one so-called synthesized (superposed)
exothermic peak in
the temperature range of 230-330 C, then the height of this exothermic peak is
in the range
of 20-50 MW/mg.
[0054]
In the present invention, in the case where only two exothermic peaks having a
difference in temperature between the peaks of 50 C or less, preferably 30 C
or less, are
present in the temperature range of 230-330 C, preferably in the temperature
range of 250-
320 C, it is preferable that the exothermic peak assigned to p" should be
present around
270-290 C as a lower-temperature-side first or former-half peak. It is also
preferable that
the exothermic peak assigned to p' should be present around 290-300 C as a
higher-
temperature-side second or latter-half peak. Furthermore, the difference in
temperature
between the peaks of these exothermic peaks is 50 C or less, and the height of
the
exothermic peak, which has a higher peak height of these exothermic peaks is
in the range
of 20-50 SW/mg. Examples thereof are the thick continuous line among the DSCs
shown
in Fig. 1, which will be described later, and Invention Examples 0, 1, 16, 17,
19, 21, etc.
shown in Table 2 in the Examples.
[0055]
Meanwhile, the thin continuous line among the DSCs shown in Fig. 1, which will
be described later, and Invention Examples 5, 6, 12, 15, 18, 20, etc. shown in
Table 2 in the
Examples are the case where a lower-temperature-side exothermic peak assigned
to 13" and
a higher-temperature-side exothermic peak assigned to f3' more overlap each
other to
render the difference in temperature between these peaks unrecognizable and,
hence, there
is only one synthesized exothermic peak in the temperature range of 230-330 C,
preferably
in the temperature range of 270-300 C.
[0056]
Also important for ensuring the BH response is, of course, the height of an
exothermic peak which indicates the amount of artificial-aging precipitates in
BH.
Namely, in the case where there are two exothermic peaks in the temperature
range of 230-
330 C, the height (AW/mg) of the exothermic peak assigned to 13' (appearing
around about
300 C in Invention Examples in the Examples, which will be described later),
which is the
exothermic peak having a larger peak height and contributing to BH response,
is regulated
so as to be in the range of 20-50 JAW/mg.
14
CA 02941997 2016-09-08
[0057]
Meanwhile, in the case where there is only one exothermic peak in the
temperature range of 230-330 C, that is, in the case where the exothermic peak
assigned to
13" (the first or former-half peak, preferably appearing around 270-290 C) and
the
exothermic peak assigned to p' (the second or latter-half peak, preferably
appearing around
290-300 C) overlap each other to form only one synthesized exothermic peak,
the height
of this exothermic peak is regulated so as to be in the range of 20-50 W/mg.
[0058]
Thus, it is possible to reduce the proof stress in panel forming to 110 MPa or
lower and to attain a proof stress after BH of 170 MPa or greater. In other
words, aging
precipitates of [3" and 13' which are yielded during BH can be ensured in such
an amount
that a proof stress after BH of 170 MPa or greater is brought about. In the
case where the
heights of those exothermic peaks are smaller than the lower limit of, or are
larger than the
upper limit of, the range of 20-50 pW/mg, this means that the amount of the
desired aging
precipitates of such as f3" and which have influences on BH response through a
bake
hardening treatment is too small or too large and such precipitates are unable
to be yielded
in the desired amount. Because of this, it is inevitably impossible to attain
both a
reduction in proof stress in panel forming to 110 MPa or less and a control of
a proof stress
after BH to 170 MPa or greater.
[0059]
(Production Process)
Next, a process for producing the aluminum alloy sheet according to the
present
invention is explained. The aluminum alloy sheet according to the present
invention is
produced through production steps which themselves are common or known, by
subjecting, after casting, an aluminum alloy slab having the 6000-series
component
composition to a homogenizing heat treatment, hot rolling and cold rolling to
obtain a
given sheet thickness, followed by a refining treatment such as a solution
quenching
treatment.
[0060]
However, for obtaining the structure specified with a DSC according to the
present invention, during those production steps, the conditions for a
preliminary aging
treatment after the solution and quenching treatments are regulated so as to
be in a
preferred range, as will be described later. With respect to other steps,
there are preferred
conditions for obtaining the structure specified with a DSC according to the
present
invention. Unless such preferred conditions are employed, it is difficult to
obtain the
structure specified with a DSC according to the present invention.
[0061]
(Melting and casting cooling rate)
CA 02941997 2016-09-08
First, in melting and casting steps, an aluminum alloy molten metal that has
been
melted and regulated so as to have a component composition within the 6000-
series
composition range is cast by a suitably selected ordinary melting and casting
method, such
as a continuous casting method or a semi-continuous casting method (DC casting
method).
Here, in order to regulate the clusters so as to be in the range specified in
the present
invention, it is preferable that the average cooling rate, during the casting,
from the
liquidus temperature to the solidus temperature is as high (quick) as possible
at 30 C/min
or greater.
[0062]
In the case where such temperature (cooling rate) control in a high-
temperature
range during casting is not performed, the cooling rate in this high-
temperature range is
inevitably low. When an average cooling rate in the high-temperature range is
low as the
above, the amount of crystals yielded coarsely in the temperature range of
this high-
temperature range is increased and also unevenness in the size and amount of
the crystals
along the width direction and thickness direction of the slab is increased. As
a result, it is
highly probable that the specified clusters cannot be regulated so as to be in
the ranges
= according to the present invention.
[0063]
(Homogenizing heat treatment)
Next, the aluminum alloy slab obtained by casting is subjected to a
homogenizing
heat treatment prior to hot rolling. The purpose of this homogenizing heat
treatment
(soaking treatment) is to homogenize the structure, that is, to eliminate
segregation within
the grains in the structure of the slab. The conditions are not particularly
limited so long
as the purpose is achieved therewith, and the treatment may be an ordinary one
conducted
once or in one stage.
[0064]
A homogenizing heat treatment temperature is suitably selected from the range
of
500 C or more and lower than the melting point, and a homogenizing time is
suitably
selected from the range of 4 hours and longer. In the case where the
homogenizing
temperature is low, the segregation within grains cannot be sufficiently
eliminated, and
these act as starting points for fracture, resulting in decreases in stretch
flangeability and
bendability. When hot rolling is thereafter started immediately or when hot
rolling is
started after holding and cooling to an appropriate temperature, control
within the number
density of the clusters specified in the present invention can be achieved.
[0065]
After the homogenizing heat treatment has been performed, cooling to room
temperature may be performed so that the average cooling rate in the range of
300 C to
16
CA 02941997 2016-09-08
500 C is 20 to 100 C/hour, followed by reheating to 350 C to 450 C at an
average heating
rate of 20 to 100 C/hour to start hot rolling in this temperature range.
[0066]
In the cases when the average cooling rate after the homogenizing heat
treatment
and the reheating rate conducted thereafter do not satisfy those conditions,
the possibility
of forming coarse Mg-Si compounds increases.
[0067]
(Hot rolling)
The hot rolling is constituted of a slab rough rolling step and a finish
rolling step
in accordance with the thickness of the plate to be rolled. In these rough
rolling step and
finish rolling step, rolling mills such as a reverse type and a tandem type
are suitably used.
[0068]
In the cases when the hot-rolling (rough-rolling) start temperature exceeds
the
solidus temperature, burning occurs and, hence, the hot rolling itself is
difficult to carry
out. Meanwhile, in the cases when the hot-rolling start temperature is lower
than 350 C,
the hot-rolling load is too high, rendering the hot rolling itself difficult.
Consequently, the
hot-rolling start temperature is preferably in the range of 350 C to the
solidus temperature,
more preferably in the range of 400 C to the solidus temperature.
[0069]
(Annealing of the hot-rolled plate) ,
Annealing (rough annealing) before cold rolling is not always necessary for
the
hot-rolled plate. However, it may be performed in order to further improve
properties
such as formability by making the grains smaller and optimizing the texture.
[0070]
(Cold rolling)
In cold rolling, the hot-rolled sheet is rolled to produce a cold-rolled sheet
(including a coil) having a desired final sheet thickness. However, for making
the grains
even smaller, it is desirable that the cold rolling ratio should be 60% or
greater.
Intermediate annealing may be performed between cold-rolling passes for the
same
purpose as in the rough annealing.
[0071]
(Solution treatment and quenching treatment)
After the cold rolling, a solution treatment is performed, followed by a
treatment
= for quenching to room temperature. The solution and quenching treatments
may be a
heating and a cooling performed on an ordinary continuous heat treatment line,
and are not
particularly limited. However, from the standpoint of obtaining a sufficient
solid-solution
amount of each element and because it is desirable that the grains should be
finer as stated
above, it is desirable that the treatments should be conducted under such
conditions of
17
CA 02941997 2016-09-08
heating at a heating rate of 5 C/sec or greater to a solution treatment
temperature which is
520 C or higher and lower than the melting temperature, and then holding for
0.1-10
seconds.
[0072]
From the standpoint of suppressing the formation of coarse intergranular
compounds that reduce the formability and hem workability, it is desirable
that the average
cooling rate from the solution treatment temperature to the quenching stop
temperature,
which is room temperature, should be 3 C/sec or greater. In the case where the
average
rate of cooling to room temperature after the solution treatment is too low,
coarse Mg2Si
and elemental Si are yielded during the cooling, resulting in impaired
formability. In
addition, the solid-solution amount after the solution treatment is reduced,
resulting in a
decrease in BH response. In order to secure that cooling rate, means such as
air cooling
with fans or water cooling with mist or spray or by immersion, etc. and
conditions therefor
are selected and used for the quenching treatment.
[0073]
(Preliminary aging treatment: reheating treatment)
After having thus undergone the solution treatment and the subsequent
quenching
treatment to be cooled to room temperature, the cold-rolled sheet is subjected
to a
preliminary aging treatment (reheating treatment) within 1 hour. In the case
where the
room-temperature holding period from termination of the treatment for
quenching to room
temperature to initiation of the preliminary ageing treatment (initiation of
heating) is too
long, clusters that are prone to dissolve upon room-temperature aging are
yielded, making
it impossible to form the exothermic peaks, as a prerequisite, specified with
a DSC
according to the present invention. Consequently, the shorter the room-
temperature
holding period, the better. The solution and quenching treatments and the
reheating
treatment may be consecutively performed so that there is substantially no
pause
therebetween, and a lower limit of the period is not particularly determined.
[0074]
In this preliminary aging treatment, it is important that periods of holding
both in
the relatively higher-temperature-side range of 80-120 C and in the relatively
lower-
temperature-side range of 60-40 C should be ensured. Thus, the exothermic
peaks
specified with a DSC according to the present invention are formed.
[0075]
Here, the higher-temperature-side range of 80-120 C and the lower-temperature-
side range of 60-40 C may be divided into stages, e.g., in two stages, in
terms of
temperature, or may be regulated so that the temperature changes continuously.
Furthermore, the temperature holding in the higher-temperature-side range may
be a heat
treatment in which a constant temperature within that temperature range is
maintained or in
18
=
CA 02941997 2016-09-08
which the temperature is gradually changed within that temperature range by
temperature
increase. Meanwhile, the temperature holding in the lower-temperature-side
range may
be a heat treatment in which a constant temperature within that temperature
range is
maintained or in which the temperature is gradually changed within that
temperature range
by temperature decrease. In short, the temperature may be continuously changed
by
temperature increase, temperature decrease (annealing), etc., so long as the
temperature is
held in each of the temperature ranges for the necessary holding period. The
temperature
holding in the higher-temperature-side and in the lower-temperature-side may
be a heat
treatment of consecutive two stages in which the temperature is divided into
stages, or may
be heat treatment in which the holding temperature is kept constant within
each of the
specified temperature ranges or may be a continuous heat treatment in which
temperature
increase, temperature decrease, natural cooling, etc are suitably combined
within each of
the specified temperature ranges. The cooling after the preliminary aging
treatment may
be natural cooling or rapid cooling.
[0076]
The period of holding in the higher-temperature-side range of 80-120 C in the
former half is preferably regulated to 5-40 hours including the time period
during which
the sheet is held in the temperature range of 80-120 C in the temperature
increase of the
sheet. Meanwhile, the period of holding in the lower-temperature-side range of
60-40 C
in the latter half is preferably regulated to 20-300 hours including the
period of temperature
decrease from the holding in the higher-temperature-side range or the time
period during
which the sheet is held in the temperature range of 60-40 C in the cooling
such as natural
cooling or rapid cooling.
[0077]
In the case where those temperatures are too low or those holding periods are
too
short, similar to in the case where no preliminary aging treatment is
performed, the
structure according to the present invention specified with a DSC is less apt
to be obtained,
and no exothermic peak appears in the temperature range of 230-330 C or, even
if two
exothermic peaks appear, the temperature difference between the peaks exceeds
50 C or
the specified exothermic peak height exceeds 50 W/mg.
[0078]
Conversely, also in the case where those temperatures are too high or those
holding periods are too long, the structure according to the present invention
specified with
a DSC is less apt to be obtained, and no exothermic peak appears in the
temperature range
of 230-330 C or the specified exothermic peak height exceeds 50 ,W/mg.
Examples
[0079]
19
CA 02941997 2016-09-08
The present invention will be explained below in more detail by reference to
Examples. However, the present invention should not, of course, be construed
as being
limited by the following Examples, and can be suitably modified unless the
modifications
depart from the gist of the present invention described hereinabove and
hereinafter. All
such modifications are included in the technical range of the present
invention.
[0080]
Examples according to the present invention are explained. 6000-series
aluminum alloy sheets were individually produced so as to differ in the
structure specified
with a DSC in the present invention, by changing the conditions for a
preliminary aging
treatment performed after solution and quenching treatments. After a holding
at room
temperature for 30 days after the production of the sheets, BH response (bake
hardenability), As proof stress as an index of press formability and hem
workability as
bendability are examined and evaluated.
[0081]
For the individual producing, the 6000-series aluminum alloy sheets having the
compositions shown in Table 1 was produced by variously changing conditions
such as the
temperature and holding period in the preliminary aging treatment after the
solution and
quenching treatments as shown in Table 2. With respect to the indications of
the contents
of elements within Table 1, a value of the element expressed by a blank
indicates that the
content is below a detection limit.
[0082]
Specific conditions for aluminum alloy sheet production were as follows. Slabs
of aluminum alloys respectively having the compositions shown in Table 1 were
commonly produced through casting by the DC casting method. In this casting,
the
average rate of cooling from the liquidus temperature to the solidus
temperature was set at
50 C/min in common with all the Examples. Subsequently, the slabs were
subjected to a
soaking treatment of 540 C x 6 hours, followed by initiation of hot rough
rolling at that
temperature, in common with all the Examples. Thereafter, they were hot-
rolled, in the
succeeding finish rolling, to a thickness of 3.5 mm to obtain hot-rolled
sheets, in common
with all the Examples. The hot-rolled aluminum alloy sheets were subjected to
rough
annealing of 500 C x 1 minute and then to cold rolling at a processing rate of
70% without
performing intermediate annealing during the cold-rolling passes, to obtain
cold-rolled
sheets having a thickness of 1.0 mm, in common with all the Examples.
[0083]
Furthermore, the cold-rolled sheets were each continuously subjected to a
refining
treatment (T4) with continuous type heat treatment facilities while unwinding
and winding
each sheet, in common with all the Examples. Specifically, a solution
treatment was
performed by heating at an average rate of heating to 500 C of 10 C/sec and
holding for 5
CA 02941997 2016-09-08
seconds after the temperature reached a target temperature of 540 C, followed
by cooling
to room temperature by performing water cooling at an average cooling rate of
100 C/sec.
After this cooling, a preliminary aging treatment was performed in two stages
of the
higher-temperature-side range and the lower-temperature-side range, using the
temperatures ( C) and holding periods (hr) shown in Table 2. Specifically,
this two-stage
preliminary aging treatment was performed by holding at the given temperature
for the
given period by using an oil bath, as the higher-temperature-side range, and
thereafter, by
holding at the given temperature for the given period by using a thermostatic
oven, as the
lower-temperature-side range, followed by annealing (natural cooling).
[0084]
In the preliminary aging treatment, the period of holding in the higher-
temperature-side range included the time period during which the sheet was
held in the
temperature range of 80-120 C in the temperature increase of the sheet. The
period of
holding in the lower-temperature-side range included the temperature decrease
from the
holding in the higher-temperature-side range or the time period during which
the sheet was
held in the temperature range of 60-40 C in the cooling by natural cooling.
[0085]
From the final product sheets which each had been allowed to stand at room
temperature for 30 days after the refining treatment, test sheets (blanks)
were cut out and
the DSC and properties of the test sheets were examined and evaluated. The
results
thereof are shown in Table 2.
[0086]
(DSC)
The structure in each of ten portions of the central portion in the sheet-
thickness
direction in each test sheet was examined for the DSC. In the DSC
(differential scanning
thermal analysis curves) of this sheet, as for the average value for these ten
portions, the
exothermic peaks present in the temperature range of 230-330 C were examined.
Specifically, in the cases when two exothermic peaks were present, the
difference in
temperature ( C) between these exothermic peaks and the peak height (0//mg) of
the
exothermic peak having a higher peak height were determined. In the cases when
only
one exothermic peak was present, the height (RW/mg) of this exothermic peak
was
determined.
[0087]
The differential thermal analysis of each of the measurement portions in each
test
sheet was performed under the same conditions including a test apparatus of
DSC220G,
manufactured by Seiko Instruments Inc., a reference substance of aluminum, a
sample
container made of aluminum, temperature increase conditions of 15 C/min, an
atmosphere
of argon (50 mi./min), and a sample weight of 24.5 to 26.5 mg. The
differential thermal
21
CA 02941997 2016-09-08
analysis profile (uW) obtained was divided by the sample weight and thereby
normalized
(uW/mg). Thereafter, in the range of 0 to 100 C in the differential thermal
analysis
profile, a region where the differential thermal analysis profile was
horizontal was taken as
a reference level of 0, and the height of exothermic peak from the reference
level was
measured. The results thereof are shown in Tables 2 and 3.
[0088]
(Bake hardenability)
The test sheets which had been allowed to stand at room temperature for 30
days
after the refining treatment were each examined for 0.2% proof stress (As
proof stress) as a
.. mechanical property through a tensile test. Furthermore, these test sheets
were aged at
room temperature for 30 days, subsequently subjected to an artificial age
hardening
treatment of 170 C x 20 minutes (after BH), and then examined for 0.2% proof
stress
(proof stress after BH) through a tensile test, in common with the test
sheets. The BH
response of each test sheet was evaluated on the basis of the difference
between these 0.2%
proof stresses (increase in proof stress).
[0089]
With respect to the tensile test, No. 5 specimens (25 mm x 50 mmGL x sheet
thickness) according to ITS Z2201 were cut out of each sample sheet to perform
the tensile
test at room temperature. Here, the tensile direction of each specimen was set
so as to be
perpendicular to the rolling direction. The tensile rate was set at 5 mm/min
until the 0.2%
proof stress and at 20 mm/min after the proof stress. The number N of
examinations for
mechanical property was 5, and an average value therefor was calculated. With
respect to
the specimens to be examined for the proof stress after BH, a 2% pre-strain as
a simulation
of sheet press forming was given to the specimens by the tensile tester,
followed by
performing the BH treatment.
[0090]
(Hem workability)
Hem workability was evaluated only with respect to the test sheets which had
been allowed to stand at room temperature for 30 days after the refining
treatment. In the
test, strip-shaped specimens having a width of 30 mm were used and subjected
to 90
bending at an inward bending radius of 1.0 mm with a down flange. Thereafter,
an inner
plate having a thickness of 1.0 mm was nipped, and the specimen was subjected,
in order,
to pre-hem working in which the bent part was further bent inward to
approximately 130
and flat-hem working in which the bent part was further bent inward to 180
and the end
portion was brought into close contact with the inner plate.
[0091]
The surface state, such as the occurrence of rough surface, a minute crack or
a
large crack, of the bent part (edge bent part) of the flat hem was visually
examined and
22
CA 02941997 2016-09-08
visually evaluated on the basis of the following criteria. In the following
criteria, ratings
of 0 to 2 are on an acceptable level, and ratings of 3 and larger are
unacceptable.
0, no crack and no rough surface; 1, slight rough surface; 2, deep rough
surface; 3,
minute surface crack; 4, linearly continued surface crack.
[0092]
As shown by alloys Nos. 0 to 9 in Table 1 and Nos. 0, 1, 5, 6, 12, and 15 to
21 in
Table 2, the Invention Examples each not only have a component composition
within the
range according to the present invention and have been produced under
conditions within
preferred ranges but also have undergone the refining treatment, including the
preliminary
aging treatment, under conditions within preferred ranges. Because of this,
these
Invention Examples satisfy the DSC requirements specified in the present
invention, as
shown in Table 2. That is, these sheets each gave a DSC which had only one or
only two
exothermic peaks in the temperature range of 230-330 C and in which when only
two
exothermic peaks were present, then the difference in temperature between the
peaks was
50 C or less and the exothermic-peak height of one having a higher exothermic-
peak
height was in the range of 20-50 W/mg. Furthermore, when only one exothermic
peak
was present, the height of this exothermic peak was in the range of 20-50
1AW/mg.
[0093]
In Table 2, as for the peak height in the case where only two exothermic peaks
were present in the temperature range of 230-330 C, the peak appeared around
300 C had
a larger peak height than the peak appeared on the lower-temperature side, in
both
Invention Examples and Comparative Examples. Consequently, the peak height
( W/mg) of this exothermic peak was determined.
[0094]
As a result, the Invention Examples each show excellent BH response although
the bake hardening is performed after the refining treatment and subsequent
room-
temperature aging and is a treatment conducted at a low temperature for a
short period of
time. Furthermore, as shown in Table 2, even after the refining treatment and
subsequent
room-temperature aging, they each have a relatively low As proof stress and
hence show
excellent press formability into automotive panels or the like and excellent
hem
workability. That is, the Invention Examples, even when having undergone an
automotive-baking treatment after room-temperature aging, were able to exhibit
not only
high BH response with a 0.2% proof stress difference of 70 MPa or greater and
a 0.2%
proof stress after BH of 170 MPa or greater but also press formability with an
As 0.2%
proof stress of 110 MPa or less and satisfactory bendability.
[0095]
In contrast, Comparative Examples 2 to 4, 7 to 11, 13, and 14 in Table 2,
which
employed alloy example 1,2 or 3 in Table 1 like Invention Examples, each have
the
23
=
CA 02941997 2016-09-08
preliminary aging treatment conditions outside the preferred ranges, as shown
in Table 2.
As a result, they each gave a DSC which was outside the range specified in the
present
invention, and show enhanced room-temperature aging and, in particular, a
relatively high
As proof stress after 30-day room-temperature holding, as compared with the
Invention
Examples having the same alloy composition. Because of this, they are poor in
press
formability into automotive panels or the like and in hem workability and are
poor also in
BH response.
[0096]
In Comparative Examples 2 and 9, among these, the period from the solution
treatment and the quenching treatment to room temperature to the preliminary
aging
treatment (initiation of heating) is 120 minutes, which is too long. Because
of this, Mg-Si
clusters that do not contribute to strength have been yielded in a large
amount. Although
the two exothermic peaks present in the temperature range of 230-330 C have a
difference
in temperature between the peaks of 50 C or less, the exothermic-peak height
exceeds 50
g_tW/mg.
[0097]
In Comparative Example 3, the period of holding in the higher-temperature-side
range in the preliminary aging treatment is 48 hours, which is too long.
Because of this,
the one exothermic peak present in the temperature range of 230-330 C has too
small a
height less than 20 4W/mg.
[0098]
In Comparative Examples 4, 11 and 14, the period of holding in the lower-
temperature-side range in the preliminary aging treatment is 2 hours, which is
too short.
Because of this, although the two exothermic peaks present in the temperature
range of
230-330 C have a difference in temperature between the peaks of 50 C or less,
the
exothermic-peak height exceeds 50 11W/mg, or in the case where one exothermic
peak is
present in the temperature range of 230-330 C, this exothermic peak has a
height
exceeding 50 AW/mg.
[0099]
In Comparative Examples 10 and 13, the period of holding in the higher-
temperature-side range in the preliminary aging treatment is 2 hours, which is
too short.
Because of this, in the case where one exothermic peak is present in the
temperature range
of 230-330 C, this exothermic peak has a height exceeding 50 W/mg.
[0100]
In Comparative Example 7, the temperature in the higher-temperature-side range
in the preliminary aging treatment is 70 C, which is too low. Because of this,
although
the two exothermic peaks present in the temperature range of 230-330 C have a
difference
24
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CA 02941997 2016-09-08
in temperature between the peaks of 50 C or less, the higher exothermic peak
has a height
exceeding 50 4W/mg.
[0101]
In Comparative Example 8, the temperature in the higher-temperature-side range
in the preliminary aging treatment is 130 C, which is too high. Because of
this, in the
case where one exothermic peak is present in the temperature range of 230-330
C, this
exothermic peak has a height less than 20 ilW/mg.
[0102]
Comparative Examples 22 to 30 in Table 2 have been produced under preferred
conditions, including the conditions for the preliminary aging treatment.
However, since
they employed alloys Nos. 10 to 18 shown in Table 1, the contents of Mg and
Si, which are
essential elements, therein are outside the ranges according to the present
invention or the
content of impurity elements therein is too high. Because of this, these
Comparative
Examples 22 to 30 each show, in particular, a relatively too high As proof
stress after 30-
day room-temperature holding as compared with the Invention Examples, as shown
in
Table 2. They hence are poor in press formability into automotive panels or
the like and
in hem workability or are poor in BH response. The compositions of Comparative
Examples 22 to 30 are described in detail below.
[0103]
Comparative Example 22 is alloy 10 shown in Table 1, in which the Si content
is
too low.
Comparative Example 23 is alloy 12 shown in Table 1, in which the Mg+Si
content is too high.
Comparative Example 24 is alloy 11 shown in Table 1, in which the Si content
is
too high and the Mg+Si content is too high.
Comparative Example 25 is alloy 13 shown in Table 1, in which the Fe content
is
too high.
Comparative Example 26 is alloy 14 shown in Table 1, in which the Mn content
is
too high.
Comparative Example 27 is alloy 15 shown in Table 1, in which the Cr and Ti
contents are too high.
Comparative Example 28 is alloy 16 shown in Table 1, in which the Cu content
is
too high.
Comparative Example 29 is alloy 17 shown in Table 1, in which the Zn content
is
too high.
Comparative Example 30 is alloy 18 shown in Table 1, in which the Zr and V
contents are too high.
[0104]
CA 02941997 2016-09-08
DSCs selected from those of the Invention Examples and Comparative Examples
are shown in Fig. 1. In Fig. 1, the thick continuous line indicates Invention
Example 1,
the thin continuous line indicates Invention Example 12 and the broken line
indicates
Comparative Example 23.
[0105]
In the DSC of Invention Example 1, a first exothermic peak of f3" appears
around
270 C and a second exothermic peak of' appears around 300 C near the first
peak, and
the difference in temperature between these peaks is 27 C as shown in Table 2,
which is
50 C or less as specified.
[0106]
In the DSC of Invention Example 12, a first exothermic peak of p" and a second
exothermic peak of ly overlap each other to form one synthesized peak. This
synthesized
peak appears around 290 C and, as shown in Table 2, has a peak height of 35.9
11W/mg,
which is in the range of 20-50 nW/mg.
[0107]
In contrast, in the DSC of Comparative Example 23, a first exothermic peak of
(3"
appears around 260 C and a second exothermic peak of IT appears around 310 C,
and the
difference in temperature between these peaks is 53 C as shown in Table
2,which exceeds
the specified temperature of 50 C.
[0108]
Those results of the Examples support that, for improving formability and BH
response after room-temperature aging, it is necessary that all the
requirements concerning
composition and DSC specified in the present invention should be satisfied.
26
[0109]
[Table 1]
Alloy Chemical components of Al-Mg-Si alloy sheet (mass%;
remainder, Al)
No. Mg Si Mg+Si Fe Cu Mn Cr Zr V Ti Zn Ag
0 0.40 0.60 1.00
.
1 0.40 0.60 1.00 0.4
2 0.32 0.65 0.97 0.2 0.12
.
3 0.34 0.58 0.92 0.2 0.12 0.05
4 0.38 0.45 0.83 0.2 0.3
0.48 0.52 1.00 0.2 0.2
6 0.54 0.45 0.99 0.2 0.05
0.06
7 0.28 0.67 0.95 0.2 0.07
0.07 9
_
8 0.36 0.49 0.85 0.2 0.08
0.4 2
9 0.54 0.61 1.15 0.2 0.2
it
.-
0.66 0.15 0.81 0.2
11 0.45 1.03 1.48 0.2
12 0.40 0.91 1.31 0.2
i
13 0.38 0.66 1.04 0.71
2
14 0.65 0.41 1.06 0.2 0.72
0.01
0.35 0.80 1.15 0.2 0.4 0.13
_
16 0.41 0.62 1.03 0.2 0.88 _
17 0.31 ' 0.58 0.89 0.2
0.95
18 0.36 0.72 1.08 0.2 0.4 '
0.4
27
. =
CA 02941997 2016-09-08
. e
. .
[0110]
[Table 2]
Preliminary aging treatment
Required
Higher-temperature- Lower-temperature-
period to
side range side range
Classifi- Alloy No. preliminary(80-120 C) No. (80-120 C) (60-
40 C)
cation in Table 1 aging
Holding Holding
Temperature .. period
Temperature
in m period
C hr 'V hr
Inv. Ex. 0 0 5 100 12 50 24
Inv. Ex. 1 1 5 100 12 50 24
Com. Ex. 2 1 120 100 12 50 24
Com. Ex. 3 1 5 100 48 50 24
Com. Ex. 4 1 5 100 12 50 2
Inv. Ex. 5 2 5 100 12 50 24
Inv. Ex. 6 2 5 110 5 50 24
Com. Ex. 7 2 5 70 12 50 24
Com. Ex. 8 2 5 130 12 50 24
Com. Ex. 9 2 120 100 12 50 24
Com. Ex. 10 2 5 100 2 _ 50 24
Com. Ex. 11 2 5 100 12 50 2
Inv. Ex. 12 3 5 100 12 50 240
Com. Ex. 13 3 5 100 2 50 240
Com. Ex. 14 3 5 100 12 50 2
Inv. Ex. 15 4 5 100 12 50 24
Inv. Ex. 16 4 5 80 30 50 24
Inv. Ex. 17 5 5 90 12 50 24
Inv. Ex. 18 6 15 100 12 50 24
Inv. Ex. 19 7 5 100 20 50 24
Inv. Ex. 20 8 5 100 12 40 240
Inv. Ex. 21 9 5 90 12 60 24 ,
Com. Ex. 22 10 5 100 12 50 24
Corn. Ex. ._, 23 12 5 100 12 50 24
Corn. Ex. _ 24 11 5 100 12 50 24
Com. Ex. 25 13 5 100 12 50 24
Com. Ex. 26 14 5 100 12 50 24
Conn. Ex. 27 15 5 100 12 50 24
Corn. Ex. 28 16 5 100 12 50 24 _
Corn. Ex. 29 17 5 100 12 50 24
Corn. Ex. 30 18 5 100 12 50 24
28
,
CA 02941997 2016-09-08
. .
(Table 2 Continued)
Structure of aluminum alloy sheet after
30-day room-temperature holding
Exothermic peaks at 230-330 C in differential scanning
Classifi- Alloy No.
No. thermal analysis curve
cation in Table 1
N b Height of First-peak Second-peak Peak
umer
higher peak temperature temperature temperature
of peaks
IAW/mg C C
difference
Inv. Ex. 0 0 2 40.5 273 301 28
Inv. Ex. 1 1 2 41.9 273 300 27
Corn. Ex. 2 1 2 63.4 268 294 26
Corn. Ex. 3 1 1 16.8 295 - -
Corn. Ex. 4 1 2 54.8 271 297 26
Inv. Ex. 5 2 1 33.6 290 - -
Inv. Ex. 6 2 1 28.1 , 291 - -
Corn. Ex. 7 2 2 56.8 272 299 27
Corn. Ex. 8 2 1 12.2 295 - -
Com. Ex. 9 2 2 54.5 271 301 30
Corn. Ex. 10 2 I 54.4 286 - -
Corn. Ex. 11 2 1 52.1 296 - -
Inv. Ex. 12 3 1 35.9 290 - -
Corn. Ex. 13 3 1 57.2 287 - -
Corn. Ex. 14 3 1 54.7 295 - -
Inv. Ex. 15 4 1 35.6 295 - -
Inv. Ex. 16 4 2 42.1 270 299 29
Inv. Ex. 17 5 2 40.1 274 301 27
4
Inv. Ex. 18 6 1 43.7 290 - -
Inv. Ex. 19 7 2 27.5 273 301 28
Inv. Ex. 20 8 1 34.8 292 , - -
Inv. Ex. 21 9 2 38.2 271 297 26
_
Com. Ex. 22 10 I 10.4 296 - -
Corn. Ex. 23 12 2 56.0 258 311 53
Com. Ex. 24 11 2 38.7 258 312 54
Com. Ex. 25 13 2 43.1 271 298 27
Corn. Ex. 26 14 1 35.8 291 - -
Corn. Ex. 27 15 2 44.5 269 296 27
Corn. Ex. 28 16 2 39.2 273 300 27
Com. Ex. 29 17 1 38.3 290 - -
Corn. Ex. 30 18 2 40.2 271 298 27
29
f
(Table 2 Continued)
Properties of aluminum alloy after
30-day room-temperature holding
Classifi- Alloy No. As 0.2% 0.2% proof Proof stress Hem
cation No.in Table 1 Proof stress stress after BH increase workability
MPa MPa MPa
Inv. Ex. 0 0 105 195 90 2
Inv. Ex. 1 1 103 195 92 2
Com. Ex. 2 1 108 162 54 2
Com. Ex. 3 1 123 211 88 3
Com. Ex. 4 1 88 166 , 78 1
Inv. Ex. 5 2 98 182 84 1
Inv. Ex. 6 2 107 185 78 2
Com. Ex. 7 2 105 166 61 2
Com. Ex. 8 2 136 184 48 3
Com. Ex. 9 2 103 154 51 1
Com. Ex. 10 2 85 161 76 1
Corn. Ex. 11 2 87 163 76 1
Inv. Ex. 12 3 106 184 78 2
Com. Ex. 13 3 102 166 64 2
Corn. Ex. 14 3 86 166 80 1
Inv. Ex. 15 4 107 179 72 2
Inv. Ex. 16 4 105 182 77 2
Inv. Ex. 17 5 98 175 77 1
Inv. Ex. 18 6 102 176 74 1
Inv. Ex. 19 7 108 194 86 2
Inv. Ex. 20 8 106 179 73 2
Inv. Ex. 21 9 105 187 82 I
Com. Ex. 22 10 71 114 43 1
Com. Ex. 23 12 137 249 112 3
Com. Ex. 24 11 146 249 103 4
Corn. Ex. 25 13 107 191 84 4
Com. Ex. 26 14 121 202 81 4
-
Com. Ex. 27 15 118 211 93 4
Com. Ex. 28 16 132 223 91 3
Com. Ex. 29 17 102 180 I 78 4
Corn. Ex; 30 18 117 201 1 84 4
[0111]
While the present invention has been described in detail and with reference to
specific embodiments thereof, it will be apparent to one skilled in the art
that various changes
and modifications can be made therein without departing from the spirit and
scope of the
present invention.
CA 2941997 2018-03-12
(
Industrial Applicability
[0112]
According to the present invention, it is possible to provide 6000-series
aluminum
alloy sheets which combine BH response and formability after room-temperature
aging. As
a result, the 6000-series aluminum alloy sheets are usable in applications
extended to
automotive panels, in particular, outer panels in which problems may arise
concerning the
design of beautiful curved-surface configurations, character lines, etc.
31
CA 2941997 2018-03-12
