U.S. patent application number 10/582272 was filed with the patent office on 2007-09-13 for method for producing al-mg-si alloy sheet excellent in bake-hardenability and hemmability.
This patent application is currently assigned to Nippon Light Metal Company, Ltd.. Invention is credited to Pizhi Zhao.
Application Number | 20070209739 10/582272 |
Document ID | / |
Family ID | 34675076 |
Filed Date | 2007-09-13 |
United States Patent
Application |
20070209739 |
Kind Code |
A1 |
Zhao; Pizhi |
September 13, 2007 |
Method for producing Al-Mg-Si alloy sheet excellent in
bake-hardenability and hemmability
Abstract
[PROBLEMS] To provide a method for producing an aluminum alloy
sheet excellent in bake-hardenability and hemmability at a low cost
by the employment of a very short production process. [MEANS FOR
SOLVING PROBLEMS] A method for producing an aluminum alloy sheet,
which comprises providing an aluminum alloy melt having a chemical
composition, in wt %, that Mg: 0.30 to 1.00%, Si: 0.30 to 1.20%,
Fe: 0.05 to 0.50%, Mn: 0.05 to 0.50%, Ti: 0.005 to 0.10%,
optionally further one or more of Cu: 0.05 to 0.70% and Zr: 0.05 to
0.40%, and the balance: Al and inevitable impurities, casting the
alloy melt into a slab having a thickness of 5 to 15 mm by the twin
belt casting method with a cooling speed at 1/4 of the thickness of
the slab of 40 to 150.degree. C./s, winding up a coil, subjecting
the coil to a homogenizing treatment, cooling the resultant coil to
a temperature of 250.degree. C. or lower with a cooling speed of
500.degree. C./hr or more, followed by cold rolling, and then
subjecting the resulting product to a solution treatment.
Inventors: |
Zhao; Pizhi; (Ihara-gun,
JP) |
Correspondence
Address: |
MCKENNA LONG & ALDRIDGE LLP
1900 K STREET, NW
WASHINGTON
DC
20006
US
|
Assignee: |
Nippon Light Metal Company,
Ltd.
2-20, Higashi-Shinagawa 2-chome
Shinagawa-ku, Tokyo
JP
1408628
|
Family ID: |
34675076 |
Appl. No.: |
10/582272 |
Filed: |
December 13, 2004 |
PCT Filed: |
December 13, 2004 |
PCT NO: |
PCT/JP04/18581 |
371 Date: |
March 28, 2007 |
Current U.S.
Class: |
148/551 |
Current CPC
Class: |
C22C 21/08 20130101;
B22D 11/0605 20130101; C22C 21/02 20130101; B22D 11/124 20130101;
C22F 1/05 20130101 |
Class at
Publication: |
148/551 |
International
Class: |
C22F 1/04 20060101
C22F001/04 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 11, 2003 |
JP |
2003-413885 |
Claims
1. A method of producing aluminum alloy sheets excelling in
bake-hardenability and hemmability, comprising steps of casting, by
means of a twin-belt casting method, an alloy melt comprising
0.30-1.00 wt % of Mg, 0.30-1.20 wt % of Si, 0.05-0.50 wt % of Fe,
0.05-0.50 wt % of Mn and 0.005-0.10 wt % of Ti, optionally further
comprising at least one of 0.05-0.70 wt % of Cu or 0.05-0.40 wt %
of Zr, the remainder consisting of Al and unavoidable impurities,
to form a 5-15 mm thick slab at a cooling rate of 40-150.degree.
C./s at a quarter-thickness of the slab; winding into a coil;
subjecting to a homogenization treatment by inserting the coil into
a batch furnace, heating to 520-580.degree. C. at a heating rate of
at least 30.degree. C./h, then holding at that temperature for 2-24
hours; cooling to 250.degree. C. or less at a cooling rate of at
least 500.degree. C./h; cold rolling; then subjecting to a solution
treatment by heating to 530-560.degree. C. at a heating rate of at
least 10.degree. C./s in a continuous annealing line, and holding
for 30 seconds or less.
2. (canceled)
3. (canceled)
4. A method in accordance with claim 1, comprising steps, after
said solution treatment, of cooling to room temperature at a
cooling rate of at least 10.degree. C./s, then subjecting to a
restoration treatment by holding for 30 seconds or less at
260-300.degree. C. in a continuous annealing furnace, and cooling
to room temperature at a cooling rate of at least 10.degree.
C./s.
5. A method in accordance with claim 1, comprising steps, after
said solution treatment, of water-cooling to 250.degree. C. or less
at a cooling rate of at least 10.degree. C./s, then air-cooling to
60-100.degree. C. at a cooling rate of 1-20.degree. C./s, coiling
up, and subjecting to a preliminary ageing treatment by cooling to
room temperature.
6. A method in accordance with claim 1, comprising steps, after
said solution treatment, of cooling to room temperature at a
cooling rate of at least 10.degree. C./s, then subjecting to a
restoration treatment by holding for 30 seconds or less at
260-300.degree. C. in a continuous annealing furnace, cooling to
60-100.degree. C. at a cooling rate of at least 1.degree. C./s,
coiling up, and subjecting a preliminary ageing treatment by
cooling to room temperature.
7. A method in accordance with claim 1 further comprising the step
of, after said homogenization treatment, removing the coil from the
batch furnace and forcibly cooling while unwinding the coil.
Description
TECHNICAL FIELD
[0001] The present invention relates to a production method for
obtaining an Al--Mg--Si alloy sheet that is abundant in hemmability
while simultaneously having a high age-hardening ability, by
casting a thin slab by continuous casting of an Al--Mg--Si alloy,
performing a homogenization treatment, then cold rolling, and
performing a solution treatment in a continuous annealing furnace
as needed. According to the present method, it is possible to
produce, at a low cost as compared to the conventional art, rolled
sheets of Al--Mg--Si alloy that are suitable for forming by
bending, press forming and the like of automotive parts, household
appliances and the like.
BACKGROUND ART
[0002] Al--Mg--Si alloys have the property of increasing in
strength when heat is applied during processes such as coating
after forming, so that they are well-suited for use in automotive
panels or the like. Furthermore, the production of sheets of the
alloys by continuous casting and rolling has been proposed to
reduce costs by improved productivity.
[0003] For example, Japanese Patent Application, First Publication
No. S62-207851 discloses an aluminum alloy sheet for forming and
method of production thereof, obtained by continuous casting of an
aluminum alloy melt comprising 0.4-2.5% Si, 0.1-1.2% Mg and one or
more among 1.5% or less of Cu, 2.5% or less of Zn, 0.3% or less of
Cr, 0.6% or less of Mn and 0.3% or less of Zr, to form a 3-15 mm
thick slab, cold rolling, then performing a solution treatment and
quenching, characterized in that the maximum size of intermetallic
compounds in the matrix is 5 .mu.m or less.
[0004] Japanese Patent Application, First Publication No.
H10-110232 discloses an Al--Mg--Si alloy sheet, obtained by
preparing a direct cast rolled sheet of Al alloy comprising
0.2-3.0% Si and 0.2-3.0% Mg, containing one or more of 0.01-0.5%
Mn, 0.01-0.5% Cr, 0.01-0.5% Zr and 0.001-0.5% Ti, and further
containing 0-2.5% Cu, 0-0.20% Sn and 0-2.0% Zn, with Fe being
limited to 1.0% or less and the remainder consisting of Al and
unavoidable impurities, and further cold rolling, characterized in
that the maximum crystal size in the metallic portion of the sheet
is 100 .mu.m or less and the maximum length of continuous
Mg.sub.2Si compounds on the surface layer portion is 50 .mu.m or
less.
[0005] Additionally, Japanese Patent Application, First Publication
No. 2001-262264 proposes an Al--Mg--Si alloy sheet excelling in
ductility and bendability, the aluminum alloy comprising 0.1-2.0%
Si, 0.1-2.0% Mg, 0.1-1.5% Fe or one or more further elements chosen
from among 2% or less of Cu, 0.3% or less of Cr, 1.0% or less of
Mn, 0.3% or less of Zr, 0.3% or less of V, 0.03% or less of Ti,
1.5% or less of Zn and 0.2% or less of Ag, wherein the maximum size
of intermetallic compounds is 5 .mu.m or less, the maximum aspect
ratio is 5 or less and the average crystal grain size is 30 .mu.m
or less.
[0006] Patent Document 1: Japanese Patent Application, First
Publication No. S62-207851
[0007] Patent Document 2: Japanese Patent Application, First
Publication No. Hi0-110232
[0008] Patent Document 3: Japanese Patent Application, First
Publication No. 2001-262264
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0009] Alloy sheets that are used as outer panels in automotive
body sheets or the like require exceptional hemmability and
bake-hardenability. For this reason, Al--Mg--Si alloy sheets that
excel in bendability and age-harden when heated have been sought.
However, sheets produced by continuous casting and rolling have the
drawbacks of poor hemmability and insufficient bake-hardenability
after coating.
[0010] The problem to be solved by the present invention is to
obtain, at a low cost, an Al--Mg--Si alloy sheet for forming that
suppresses GP zones that are deposited during natural ageing when
left at room temperature, achieves a high level of bake-hardening
due to a reinforcement phase being quickly deposited upon heating
during coating and baking, while simultaneously having abundant
bendability.
MEANS FOR SOLVING THE PROBLEMS
[0011] A thin slab of Al--Mg--Si alloy is continuously cast by a
twin-belt casting machine, the cast thin slab is directly wound,
subjected to a homogenization treatment under appropriate
conditions, and cold rolled, then combined with a solution
treatment in a continuous annealing furnace as needed, thereby
fragmenting the compounds and raising the hemmability while
simultaneously enabling the procedure to be considerably shortened.
Furthermore, microsegregation is reduced by a homogenization
treatment, and the cooling rate after the homogenization treatment
is raised, thereby reducing the deposition of Mg.sub.2Si while
cooling, to obtain an aluminum sheet for automotive body sheets
with excellent bake-hardenabiltiy and hemmability after a final
anneal.
[0012] The present invention which solves the above problem relates
to a method of producing aluminum alloy sheets characterized by
winding into thin slabs, subjecting to a homogenization treatment,
cold rolling, then subjecting to a solution treatment.
Specifically, as recited in claim 1, it is a method of producing
aluminum alloy sheets excelling in bake-hardenability and
hemmability, comprising steps of casting, by means of a twin-belt
casting method, an alloy melt comprising 0.30-1.00 wt % of Mg,
0.30-1.20 wt % of Si, 0.05-0.50 wt % of Fe, 0.05-0.50 wt % of Mn
and 0.005-0.10 wt % of Ti, optionally further comprising at least
one of 0.05-0.70 wt % of Cu or 0.05-0.40 wt % of Zr, the remainder
consisting of Al and unavoidable impurities, to form a 5-15 mm
thick slab at a cooling rate of 40-150.degree. C./s at a
quarter-thickness of the slab; winding into a coil; subjecting to a
homogenization treatment; cooling to 250.degree. C. or less at a
cooling rate of at least 500.degree. C./h; cold rolling; then
subjecting to a solution treatment (invention according to claim
1).
[0013] In the above production method, the homogenization treatment
preferably involves heating to 520-580.degree. C. at a heating rate
of at least 30.degree. C./h in a batch furnace, then holding at
that temperature for 2-24 hours (invention according to claim
2).
[0014] The solution treatment preferably involves heating to
530-560.degree. C. at a heating rate of at least 10.degree. C./s in
a continuous annealing line, and holding for 30 seconds or less
(invention according to claim 3).
[0015] Furthermore, in the invention according to claim 3 mentioned
above, the solution treatment may be followed by steps of cooling
to room temperature at a cooling rate of at least 10.degree. C./s,
then subjecting to a restoration treatment by holding for 30
seconds or less at 260-300.degree. C. in a continuous annealing
furnace, and cooling to room temperature at a cooling rate of at
least 10.degree. C./s (invention according to claim 4).
[0016] Alternatively, in the invention according to claim 3
mentioned above, the solution treatment may be followed by steps of
water-cooling to 250.degree. C. or less at a cooling rate of at
least 10.degree. C./s, then air-cooling to 60-100.degree. C. at a
cooling rate of 1-20.degree. C./s, coiling up, and subjecting to a
preliminary ageing treatment by cooling to room temperature
(invention according to claim 5).
[0017] Alternatively, in the invention according to claim 3
mentioned above, the solution treatment may be followed by steps of
cooling to room temperature at a cooling rate of at least
10.degree. C./s, then subjecting to a restoration treatment by
holding for 30 seconds or less at 260-300.degree. C. in a
continuous annealing furnace, cooling to 60-100.degree. C. at a
cooling rate of at least 1.degree. C./s, coiling up, and subjecting
to a preliminary ageing treatment by cooling to room temperature
(invention according to claim 6).
EFFECTS OF THE INVENTION
[0018] According to the aluminum alloy sheet production method of
the present invention, it is possible to obtain an aluminum alloy
sheet with exceptional hemmability and bake-hardenability.
Additionally, this production method is capable of obtaining an
aluminum alloy sheet in an extremely short procedure and at low
cost.
BEST MODE FOR CARRYING OUT THE INVENTION
[0019] The present invention relates to a method of producing a
rolled sheet of Al--Mg--Si alloy, characterized by casting a thin
slab by a twin-belt casting method, winding the slab directly onto
a coil, subjecting to a homogenization treatment, then cold
rolling, and further subjecting to a solution treatment.
[0020] In the present invention, an alloy melt consisting of the
aforementioned composition is cast into a slab 5-15 mm thick at a
cooling rate of 40-150.degree. C./s at a quarter thickness of the
slab, using a twin-belt casting method, and after winding into a
coil, it is subjected to a homogenization treatment and cooled to
250.degree. C. or less at a cooling rate of at least 500.degree.
C./s, then cold rolled, and subsequently subjected to a solution
treatment.
[0021] The twin-belt casting method is a method of casting thin
slabs by pouring a melt between water-cooled rotating belts that
oppose each other from above and below, so as to harden the melt by
cooling through the belt surfaces. In the present invention, slabs
that are 5-15 mm thick are cast by the twin-belt casting method. If
the slab thickness exceeds 15 mm, it becomes difficult to wind the
thin slabs into coils, and if the slab thickness is less than 5 mm,
there is a loss in productivity and it becomes difficult to cast
the thin slabs.
[0022] By casting a slab 5-15 mm thick using the twin-belt casting
method, it is possible to make the cooling rate 40-150.degree. C./s
at a quarter thickness of the slab. The cooling rate is computed by
measuring the DAS (Dendrite Arm Spacing) by a line intersection
method from observations of the microstructure in the slab at
quarter thickness. When the cooling rate is less than 40.degree.
C./s, the cast structure formed in the central portion of the slab
during hardening becomes coarse, thus reducing the hemmability,
while if the cooling rate exceeds 150.degree. C./s, Al--Fe--Si
crystals and Al--(Fe.Mn)--Si crystals become 1 .mu.m or less and
the size of recrystallized grains becomes coarse at 30 .mu.m or
more.
[0023] After winding a thin slab, this coil is subjected to a
homogenization treatment under appropriate conditions to fragment
the Al--Fe--Si crystals and Al--(Fe.Mn)--Si crystals that have an
adverse effect on hemmability, thus improving the hemmability.
Furthermore, it is possible to obtain thin slabs in a state where
relatively small Mg.sub.2Si crystals that reside in the cast
structure are completely dissolved into the matrix, thus raising
the effectiveness of the solid solution treatment after the cold
rolling process.
[0024] The reason that the cooling after the homogenization
treatment is performed at a rate of at least 500.degree. C./s and
to 250.degree. C. or less is in order to suppress the deposition of
relatively coarse Mg.sub.2Si as much as possible, and to dissolve
the Mg and Si into the matrix in an oversaturated state.
[0025] After winding the thin slab, the coil is inserted into a
batch furnace, and heated at a rate of at least 30.degree. C./h to
520-580.degree. C., at which temperature it is held for 2-24 hours
to perform a homogenization treatment, after which the coil may be
extracted from the batch furnace and forcibly air-cooled to room
temperature at a cooling rate of at least 500.degree. C./h. This
cooling can be performed, for example, by a fan while unwinding the
coil.
[0026] The reason the heating rate to the homogenization
temperature is limited to at least 30.degree. C./h for the
homogenization treatment following winding of the thin slab is that
if the heating rate is less than 30.degree. C./h, at least 16 hours
will be required to reach the predetermined homogenization
temperature, thus raising costs.
[0027] The reason the homogenization temperature is within the
range of 520-580.degree. C. is that if the temperature is less than
520.degree. C., the fragmentation of Al--Fe--Si crystals and
Al--(Fe.Mn)--Si crystals is inadequate, and not enough to dissolve
the Mg.sub.2Si that crystallized during casting into the matrix,
and if the temperature exceeds 580.degree. C., the metals with low
melting points will melt and cause burning.
[0028] Additionally, the reason that the homogenization treatment
time is set to within the range of 2-24 hours is because if the
treatment time is less than 2 hours, the fragmentation of
Al--Fe--Si crystals and Al--(Fe.Mn)--Si crystals is inadequate, and
not enough to dissolve the Mg.sub.2Si that crystallized during
casting into the matrix, and if the treatment time exceeds 24
hours, the Mg.sub.2Si that crystallized during casting is
well-dissolved into the matrix, and the Mg and Si become saturated,
resulting in cost increases.
[0029] The invention is characterized by further cold rolling this
coil and performing a solution treatment. This solution treatment
is preferably performed in a normal continuous annealing line
(CAL).
[0030] A continuous annealing line (CAL) is an installation for
performing continuous solution treatments and the like of coils,
characterized by comprising inductive heating devices for
performing heat treatments, water tanks for water-cooling, air
nozzles for air-cooling, and the like.
[0031] As for the solution treatment, it should preferably be
performed by heating at a rate of at least 10.degree. C./s to
530-560.degree. C. by means of a continuous annealing line, and
holding for 30 seconds or less.
[0032] The reason the heating rate to the solution treatment
temperature is limited to at least 10.degree. C./s in the solution
treatment is that if the heating rate is less than 10.degree. C./s,
the coil advancing speed becomes too slow, as a result of which the
processing time becomes long and the cost mounts.
[0033] The reason the solution treatment temperature is set to be
within the range of 530-560.degree. C. is that if the temperature
is less than 530.degree. C., it is not sufficient to cause
Mg.sub.2Si that crystallized while casting or precipitated while
being cooled after homogenization to be dissolved into the matrix,
and if the temperature exceeds 560.degree. C., the metals with low
melting points will melt and cause burning.
[0034] Additionally, the reason the solution treatment time is
restricted to be within 30 seconds is that in the case of treatment
times exceeding 30 seconds, Mg.sub.2Si that crystallized while
casting or precipitated while being cooled after homogenization is
well-dissolved into the matrix, and the Mg and Si become saturated,
thereby slowing the coil advancement speed, as a result of which
the processing time is increased and the costs mount.
[0035] The invention is characterized by cooling to room
temperature at a rate of at least 10.degree. C./s after the
solution treatment. The reason the cooling rate after the solution
treatment is at least 10.degree. C./s is that if the cooling rate
is less than 10.degree. C./s, Si is deposited in the crystal grain
boundary during the cooling step, thus reducing the
hemmability.
[0036] After performing the aforementioned homogenization treatment
on the thin slab, it is further cold rolled, subjected to a
solution treatment and cooled to room temperature at a rate of at
least 10.degree. C./s, and after the coil is left at room
temperature, it may be held for 30 seconds or less at
260-300.degree. C. in a continuous annealing line, then cooled to
room temperature at 10.degree. C./s.
[0037] This solution treatment and restoration treatment are
preferably performed in a normal continuous annealing line. A
continuous annealing line (CAL) is an installation for performing
continuous solution treatments and the like of coils, characterized
by comprising inductive heating devices for performing heat
treatments, water tanks for water-cooling, air nozzles for
air-cooling, and the like. Due to the restoration treatment, it is
possible to re-dissolve GP zones that appear due to natural ageing
when left at room temperature after a solution treatment, thus
enabling adequate strength to be obtained after heating for coating
and baking.
[0038] Additionally, in order to obtain adequate strength after
heating for coating and baking, it is left at room temperature
after the solution treatment and subjected to a restoration
treatment at 260-300.degree. C. If the restoration treatment
temperature is less than 260.degree. C., adequate
bake-hardenability cannot be obtained, and if it exceeds
300.degree. C., the hemmability is reduced.
[0039] The reason the time over which the restoration treatment
temperature is held is restricted to within 30 seconds is that if
the treatment time exceeds 30 seconds, it is not possible to
adequately re-dissolve the GP zones that appear due to natural
ageing when left at room temperature after the solution treatment,
in addition to which the coil advancement speed is too slow, as a
result of which the treatment time is long and the costs mount.
[0040] After performing the aforementioned homogenization treatment
on the thin slab, it can be further cold rolled, subjected to a
heat solution treatment in a continuous annealing line,
water-cooled to 250.degree. C. or less at a cooling rate (first
cooling rate) of at least 10.degree. C./s, then air-cooled to
60-100.degree. C. at a cooling rate (second cooling rate) of
1-20.degree. C./s, coiled up and cooled to room temperature.
[0041] This heat solution treatment and subsequent cooling are
preferably performed in a normal continuous annealing line (CAL).
During this heat solution treatment and subsequent cooling, a heat
treatment (preliminary ageing) can be performed to evenly generate
nuclei for .beta.'' deposition in the matrix, to obtain adequate
strength after heating for coating and baking.
[0042] After subjecting the thin slab to a homogenization treatment
and further cold rolling, it may be subjected to a solution
treatment by heating to 530-560.degree. C. at a rate of at least
10.degree. C./s, then holding for 30 seconds or less, then cooled
to room temperature at a rate of at least 10.degree. C./s,
thereafter subjected to a restoration treatment by holding within a
range of 260-300.degree. C. for 30 seconds, then cooled to
60-100.degree. C. at a cooling rate of at least 1.degree. C./s,
coiled up and subjected to a preliminary ageing treatment by
cooling to room temperature.
[0043] This solution treatment and subsequent cooling, and
restoration treatment and subsequent cooling are preferably
performed in a normal continuous annealing line (CAL). With this
production method, not only is it possible to re-dissolve GP zones
that appear due to natural ageing when left at room temperature
after the solution treatment, but it is also possible to perform a
heat treatment (preliminary ageing) to generate nuclei for .beta.''
deposition, thus further improving the resistance after coating and
baking.
[0044] Next, the significance of the alloy ingredients of the
present invention and the reasons for their limitations shall be
explained. The essential element Mg is dissolved in the matrix
after the heat solution treatment, and is deposited as a
reinforcing phase together with Si upon heating for coating and
baking, thereby improving the strength. The reason the Mg content
is limited to 0.30-1.00 wt % is that the effect is small if less
than 0.30 wt %, and if more than 1.00 wt %, the hemmability after
the solution treatment is reduced. A more preferable range for the
Mg content is 0.30-0.70 wt %.
[0045] The essential element Si is deposited together with Mg as an
intermediary phase of Mg.sub.2Si known as .beta.'' or an analogous
reinforcing phase upon being heated for coating and baking, thereby
increasing the strength. The reason the Si content is limited to
0.30-1.20 wt % is that if less than 0.30 wt %, its effects are
minimal, and if more than 1.20 wt %, the hemmability is reduced
after the heat solution treatment. A more preferable range of Si
content is 0.60-1.20 wt %.
[0046] The essential element Fe, when coexisting with Si and Mn,
generates many Al--Fe--Si crystals and Al--(Fe.Mn)--Si crystals of
a size of 5 .mu.m or less upon casting, so that re-crystallized
nuclei are increased, as a result of which the recrystallized
grains are refined and sheets of exceptional formability are
obtained. If the Fe content is less than 0.05 wt %, the effects are
not very remarkable. If it exceeds 0.50 wt %, coarse Al--Fe--Si
crystals and Al--(Fe.Mn)--Si crystals are formed upon casting, thus
not only reducing the hemmability but also reducing the amount of
Si dissolved in the thin slabs, as a result of which the
bake-hardenability of the final sheets is reduced. Therefore, the
preferable range of Fe content is 0.05-0.50 wt %. A more preferable
range of Fe content is 0.05-0.30 wt %.
[0047] The essential element Mn is added as an element to refine
the re-crystallized grains. By keeping the size of the
re-crystallized grains relatively small at 10-25 .mu.m, it is
possible to form sheets with exceptional formability. If the Mn
content is less than 0.05 wt %, the effect is not adequate, and if
it exceeds 0.50 wt %, coarse Al--Fe--Si crystals and
Al--(Fe.Mn)--Si crystals are formed upon casting, thus not only
reducing the hemmability but also reducing the amount of Si
dissolved in the thin slabs, as a result of which the
bake-hardenability of the final sheets is reduced. Therefore, the
preferable range of Mn content is 0.05-0.50 wt %. A more preferable
range of Mn content is 0.05-0.30 wt %.
[0048] The essential element Ti will not inhibit the effects of the
present invention if it is contained at 0.10 wt % or less, and it
can function as a crystal grain refiner for the thin slabs, so as
to reliably prevent casting defects of the slabs such as cracks or
the like. If the Ti content is less than 0.005 wt %, the effects
are not adequate, and if the Ti content exceeds 0.10 wt %, coarse
intermetallic compounds such as TiAl.sub.3 and the like are formed
during casting, thus greatly reducing the hemmability. Therefore,
the preferable range of Ti content is 0.005-0.10 wt %. A more
preferable range for the Ti content is 0.005-0.05 wt %.
[0049] The optional element Cu is an element that promotes
age-hardening and raises the bake-hardenability. If the Cu content
is less than 0.05 wt %, the effect is small, and if it exceeds 0.70
wt %, the yield strength of the sheets becomes high after a
preliminary ageing treatment, and not only does the hemmability
decrease, but the reduction in corrosion resistance is also marked.
Therefore, the Cu content is preferably within a range of 0.05-0.70
wt %. The Cu content is more preferably 0.10-0.60 wt %.
[0050] The optional element Zr is added as an element for refining
the re-crystallized grains. If the Zr content is less than 0.05 wt
%, the effect is not adequate, and if it exceeds 0.40 wt %, coarse
Al--Zr crystals are created during slab casting, thus reducing the
hemmability. Therefore, the Zr content is preferably within a range
of 0.05-0.40 wt %. The Zr content is more preferably within a range
of 0.05-0.30 wt %.
[0051] As explained above, the present invention allows an
Al--Mg--Si alloy sheet for use in automotive body sheets having
exceptional bake-hardenablitiy and hemmability after a final anneal
to be produced at low cost. While a restoration treatment or
high-temperature winding is required to suppress natural ageing as
with conventional methods, the steps such as facing, hot rolling
and the like that precede these steps can be largely simplified,
thus greatly reducing the total production cost.
[0052] Herebelow, the best modes of the present invention shall be
described using examples.
EXAMPLE 1
[0053] In the below-given examples, the samples after cold rolling
are not coils but all cut sheets. Therefore, in order to simulate
the step of continuous annealing of a coil in a continuous
annealing line (CAL), a solution treatment of the samples in a salt
bath and a cold water quench or 85.degree. C. water quench were
employed.
[0054] After degassing melts having the compositions shown in Table
1, they were cast into slabs 7 mm thick by means of a twin-belt
casting method. The DAS (Dendrite Arm Spacing) was measured by an
intersection method from observation of the microstructures at a
quarter-thickness of the slab, and the cooling rate 75.degree. C./s
was computed. A predetermined homogenization treatment was
performed on the slabs which were then cooled to room temperature
at a predetermined cooling rate, and cold rolled to form sheets of
1 mm thickness. Next, solution treatments were performed on these
cold rolled sheets in a salt bath, and they were either 1) quenched
in 85.degree. C. water and immediately inserted into an annealer
with a predetermined atmospheric temperature to perform a heat
treatment under predetermined conditions, or 2) quenched in cold
water, left at room temperature for 24 hours, then subjected to a
heat treatment under predetermined conditions. Furthermore, in
order to simulate automobile coating steps, they were held for one
week at room temperature after the heat treatment, and measured for
0.2% yield strength, further baked at 180.degree. C. for 30
minutes, and again measured for 0.2% yield strength.
[0055] The difference in yield strength before and after the baking
treatment was taken as the bake-hardenability, and those exceeding
80 MPa were judged to have excellent bake-hardenability. In order
to simulate hemmability, the sheets prior to baking were
preliminarily warped by 5%, then bent into a U shape using a jig
having a radius r=0.5 mm, then 1 mm thick spacers were inserted and
they were bent 180.degree.. Those which did not crack were ranked
.largecircle. and those which cracked were ranked X. The detailed
sheet production steps and evaluation results are shown in Table
2-6.
[0056] [Table 1] TABLE-US-00001 TABLE 1 Alloy Composition (wt %)
Alloy No. Mg Si Fe Mn Cu Ar Ti A 0.5 0.7 0.2 0.2 -- -- 0.02 B 0.5
0.8 0.2 0.2 -- -- 0.02 C 0.6 0.8 0.2 0.2 -- -- 0.02 D 0.5 1 0.2 0.2
0.5 -- 0.02 E 0.5 0.8 0.2 0.2 -- 0.15 0.02 F 0.4 1.2 0.2 0.2 0.1 --
0.02
[0057] Table 2 shows the results for cases in which the
homogenization conditions and cooling rate after the homogenization
treatment were changed. After the homogenization treatment, the
slabs were cold rolled to a thickness of 1 mm, these cold rolled
sheets were subjected to a solution treatment by holding for 15
seconds at a predetermined temperature by means of a salt bath,
then quenched with 85.degree. C. water, and immediately inserted
into an annealer with an atmospheric temperature of 85.degree. C.
to perform a preliminary ageing of 8 hours. Those falling within
the scope of conditions of the present invention (1-7) had
exceptional bake-hardenability and hemmability. Those that did not
undergo a homogenization treatment (8, 10) had poor
bake-hardenability and hemmability. Additionally, those which had a
slow cooling rate after the homogenization treatment had poor
bake-hardenability (9).
[0058] [Table 2] TABLE-US-00002 TABLE 2 Cooling Rate after
Homogenization and Bake-Hardenability/Hemmability Homogenization
Treatment Cast Type/ Heating Holding Holding Cooling Alloy Slab
Thick. Rate Temp Time Rate ID No. (mm) (.degree. C./h) (.degree.
C.) (h) (.degree. C./h) Present 1 A twin-belt/7 30 560 5 1500
Invention 2 B twin-belt/7 50 560 6 1700 3 B twin-belt/7 50 550 5
500 4 C twin-belt/7 30 530 10 1000 5 D twin-belt/7 40 530 10 1000 6
E twin-belt/7 40 530 10 1000 7 F twin-belt/7 50 550 6 1000 Comp. 8
A twin-belt/7 None Example 9 B twin-belt/7 50 560 6 250 10 B
twin-belt/7 None Cold Roll Sol. Yield Str. Bake- Sheet Treat.
Prelim. before/after Hard. ID Thick. Temp. Ageing Baking (Mpa)
(MPa) Hem. Present 1 1 mm 550.degree. C. 85.degree. C. .times. 8 h
100/192 92 .largecircle. Invention 2 1 mm 550.degree. C. 85.degree.
C. .times. 8 h 110/210 100 .largecircle. 3 1 mm 530.degree. C.
85.degree. C. .times. 8 h 95/175 80 .largecircle. 4 1 mm
540.degree. C. 85.degree. C. .times. 8 h 107/209 102 .largecircle.
5 1 mm 550.degree. C. 85.degree. C. .times. 8 h 122/221 99
.largecircle. 6 1 mm 550.degree. C. 85.degree. C. .times. 8 h
115/213 98 .largecircle. 7 1 mm 550.degree. C. 85.degree. C.
.times. 8 h 117/208 91 .largecircle. Comp. 8 1 mm 550.degree. C.
85.degree. C. .times. 8 h 110/158 48 X Example 9 1 mm 550.degree.
C. 85.degree. C. .times. 8 h 90/145 55 .largecircle. 10 1 mm
550.degree. C. 85.degree. C. .times. 8 h 92/160 68 X
[0059] Table 3 shows the results when the temperatures/times of the
homogenization treatment are changed. After the homogenization
treatment, the slabs were cold rolled to a thickness of 1 mm, these
cold rolled sheets were subjected to a solution treatment by
holding for 15 seconds at a predetermined temperature by means of a
salt bath, then quenched in 85.degree. C. water and immediately
entered into an annealer with an atmospheric temperature of
85.degree. C. to perform a preliminary ageing of 8 hours. Those
falling within the scope of conditions of the present invention
(11-14) had exceptional bake-hardenability and hemmability. Those
that had a low homogenization temperature (15) or had a short
holding time (16) had poor bake-hardenability and hemmability.
[0060] [Table 3] TABLE-US-00003 TABLE 3 Homogenization
Temperature/Time and Bake-Hardenabilit/Hemmability Homogenization
Treatment Cast Type/ Heating Holding Holding Cooling Alloy Slab
Thick. Rate Temp Time Rate ID No. (mm) (.degree. C./h) (.degree.
C.) (h) (.degree. C./h) Present 11 B twin-belt/7 30 560 5 1500
Invention 12 B twin-belt/7 50 560 6 1500 13 C twin-belt/7 50 550 5
1500 14 C twin-belt/7 30 530 10 1500 Comp. 15 B twin-belt/7 50 500
6 1500 Example 16 B twin-belt/7 50 560 1 1500 Cold Roll Sol. Yield
Str. Bake- Sheet Treat. Prelim. before/after Hard. ID Thick. Temp.
Ageing Baking (Mpa) (MPa) Hem. Present 11 1 mm 550.degree. C.
85.degree. C. .times. 8 h 110/210 100 .largecircle. Invention 12 1
mm 550.degree. C. 85.degree. C. .times. 8 h 111/213 103
.largecircle. 13 1 mm 530.degree. C. 85.degree. C. .times. 8 h
107/209 102 .largecircle. 14 1 mm 540.degree. C. 85.degree. C.
.times. 8 h 112/215 103 .largecircle. Comp. 15 1 mm 550.degree. C.
85.degree. C. .times. 8 h 95/165 70 X Example 16 1 mm 550.degree.
C. 85.degree. C. .times. 8 h 100/175 75 X
[0061] Table 4 shows the results when the homogenization conditions
and restoration conditions were changed. After the homogenization
treatment, the slabs were cold rolled to a thickness of 1 mm, these
cold rolled sheets are subjected to a solution treatment by holding
for 15 seconds at a predetermined temperature by means of a salt
bath, then quenched in cold water, and after leaving at room
temperature for 24 hours, subjected to a restoration treatment by
holding for 15 seconds at a predetermined temperature. Those
falling within the scope of conditions of the present invention
(17-20) had exceptional bake-hardenability and hemmability. Those
that had a low restoration temperature (reheating temperature) (21)
had poor bake-hardenability. Those whose restoration temperature
(reheating temperature) was too high (22) had poor hemmability.
Furthermore, even when the restoration conditions were within the
scope of the present invention, those in which the homogenization
temperature was low (23) or the holding time was short (24) had
poor hemmability. Those in which the cooling rate after the
homogenization treatment was slow (25) had poor
bake-hardenability.
[0062] [Table 4] TABLE-US-00004 TABLE 4 Homogenization
Method/Reheat Temperature and Bake-Hardenability/Hemmability
Homogenization Treatment Cast Type/ Heating Holding Holding Cooling
Alloy Slab Thick. Rate Temp Time Rate ID No. (mm) (.degree. C./h)
(.degree. C.) (h) (.degree. C./h) Present 17 B twin-belt/7 30 560 5
1500 Invention 18 B twin-belt/7 50 560 6 2000 19 C twin-belt/7 50
550 5 1000 20 C twin-belt/7 30 530 10 2500 Comp. 21 B twin-belt/7
50 560 6 1500 Example 22 B twin-belt/7 50 560 6 1500 23 B
twin-belt/7 50 500 6 500 24 B twin-belt/7 50 560 1 1000 25 B
twin-belt/7 50 560 6 200 Cold Roll Sol. Yield Str. Bake- Sheet
Treat. Prelim. before/after Hard. ID Thick. Temp. Ageing Baking
(Mpa) (MPa) Hem. Present 17 1 mm 550.degree. C. 270 110/210 100
.largecircle. Invention 18 1 mm 550.degree. C. 270 111/213 103
.largecircle. 19 1 mm 530.degree. C. 290 107/209 102 .largecircle.
20 1 mm 540.degree. C. 290 112/215 103 .largecircle. Comp. 21 1 mm
550.degree. C. 240 95/170 75 .largecircle. Example 22 1 mm
550.degree. C. 310 127/229 102 X 23 1 mm 550.degree. C. 290 97/197
100 X 24 1 mm 550.degree. C. 280 90/160 70 X 25 1 mm 550.degree. C.
290 95/145 50 .largecircle.
[0063] Table 5 shows the results when the homogenization conditions
and cooling pattern after the solution treatment were changed. The
cooling rate after the solution treatment was divided into two
stages, with the cooling rate from the solution temperature to an
intermediate temperature being defined as the first cooling rate
and the cooling rate from the intermediate temperature to the
coil-up temperature being defined as the second cooling rate. After
the homogenization treatment, the slabs were cold rolled to a
thickness of 1 mm, and these cold rolled sheets were subjected to a
solution treatment by holding for 15 seconds at a predetermined
temperature by means of a salt bath, after which they were cooled
to the intermediate temperature at the first cooling rate, then
cooled to the coil-up temperature at the second cooling rate, and
thereafter cooled to room temperature at 5.degree. C./h.
[0064] Those falling within the scope of the present invention
(26-28) had exceptional bake-hardenability and hemmability. Those
in which the first cooling rate after the solution treatment was
slow (29), those in which the second cooling rate was slow (31) or
those in which the intermediate temperature was too high (30) had
poor hemmability. Those in which the coil-up temperature was too
low (32) had poor bake-hardenability. Conversely, those in which
the coil-up temperature was too high (33) had poor hemmability.
Furthermore, those in which the homogenization treatment
temperature was too low (34) or the holding time was too short (35)
had poor hemmability. Those in which the cooling rate after the
homogenization treatment was too slow (36) had poor
bake-hardenability.
[0065] [Table 5] TABLE-US-00005 TABLE 5 Homogenization
Method/Coil-up Temperature and Bake-Hardenability/Hemmability
Homogenization Treatment Cast Type/ Heating Holding Holding Cooling
Alloy Slab Thick. Rate Temp Time Rate ID No. (mm) (.degree. C./h)
(.degree. C.) (h) (.degree. C./h) Present 26 B twin-belt/7 30 560 5
1500 Invention 27 B twin-belt/7 50 560 6 2000 28 B twin-belt/7 50
550 5 1000 Comp. 29 B twin-belt/7 50 560 6 1500 Example 30 B
twin-belt/7 50 560 6 1500 31 B twin-belt/7 50 560 6 1500 32 B
twin-belt/7 50 560 6 1500 33 B twin-belt/7 50 560 6 2000 34 B
twin-belt/7 50 500 6 1000 35 B twin-belt/7 50 560 1 1000 36 B
twin-belt/7 50 560 6 200 Cold Sol. First Sec. Coil Roll Treat. Cool
Int. Cool Up YS b/a Bake- Sheet Tem. Temp Temp Temp Temp Bak. Hard.
ID Thick. (.degree. C.) (.degree. C.) (.degree. C.) (.degree. C.)
(.degree. C.) (Mpa) (MPa) Hem Present 26 1 mm 550 100 200 20 85
110/210 101 .largecircle. Invention 27 1 mm 550 100 200 20 70
105/207 102 .largecircle. 28 1 mm 530 100 200 20 90 101/211 100
.largecircle. Comp. 29 1 mm 550 5 200 20 80 106/201 95 X Example 30
1 mm 550 100 300 20 80 101/197 96 X 31 1 mm 550 100 250 1 80
102/198 96 X 32 1 mm 550 100 200 20 50 112/165 53 .largecircle. 33
1 mm 550 100 200 15 110 130/240 110 X 34 1 mm 550 100 200 20 85
97/197 100 X 35 1 mm 550 100 200 20 85 104/194 90 X 36 1 mm 550 100
200 20 80 89/134 45 .largecircle.
[0066] Table 6 shows the results when the restoration treatment
temperature (reheating temperature) after the solution treatment
and coil-up temperature were changed. After the homogenization
treatment, the slabs were cold rolled to a thickness of 1 mm, these
cold rolled sheets are subjected to a solution treatment by holding
for 15 seconds at a predetermined temperature by means of a salt
bath, then quenched in cold water, and after leaving at room
temperature for 24 hours, held for 15 seconds at a predetermined
temperature (preheating temperature) and cooled to a predetermined
coil-up temperature at 10.degree. C./s, then further cooled to room
temperature at 10.degree. C./h. Those falling within the scope of
conditions of the present invention (37-40) had exceptional
bake-hardenability and hemmability. Those in which the restoration
treatment temperature (reheating temperature) was too high (41) had
poor hemmability. Those in which the restoration treatment
temperature (reheating temperature) was too low (42) had reduced
bake-hardenability. Those in which the coil-up temperature was too
low (43) had poor bake-hardenability. Those in which the coil-up
temperature was too high (44) had poor hemmability.
[0067] [Table 6] TABLE-US-00006 TABLE 6 Reheat Temperature/Coil-up
Temperature and Bake-Hardenability/Hemmability Sol. Reheat Coil Up
Yield Str. Bake- Alloy Treat. Temp Temp before/after Hard. ID No.
Tem. (.degree. C.) (.degree. C.) (.degree. C.) Baking (Mpa) (MPa)
Hem. Present 37 B 550 270 85 121/231 110 .largecircle. Invention 38
B 550 270 90 125/237 114 .largecircle. 39 B 530 290 70 117/228 111
.largecircle. 40 B 540 290 80 119/231 112 .largecircle. Comp. 41 B
550 320 85 124/234 110 X Example 42 B 550 250 80 111/198 87
.largecircle. 43 B 550 260 40 110/185 75 .largecircle. 44 B 550 290
120 131/249 118 X Homogenization: 550.degree. C. .times. 6 h
Cooling Rate after Homogenization: 1000.degree. C./h
INDUSTRIAL APPLICABILITY
[0068] According to the present invention, rolled sheets of
Al--Mg--Si alloy that are suitable for forming by bending, press
forming and the like of automotive parts, household appliances and
the like can be produced at a low cost relative to the conventional
art.
* * * * *