Note: Descriptions are shown in the official language in which they were submitted.
CA 02551599 2006-06-23
Specification
[Name of the Invention] Manufacturing Method for AI-Mg-Si Aluminum Alloy
Sheets
with Excellent Bake Hardenability
[Technical Field]
[OOOI)
The present invention relates to a manufacturing method for AI-Mg-Si alloy
sheets. The
present invention concerns a manufacturing method for Al-Mg-Si aluminum alloy
sheets,
characterized in that a molten aluminum alloy metal containing a predetermined
amount
of Mg, Si as essential elements besides AI, and additionally in some cases a
predetermined
amount of Fe, Cu, Mn, and Cr is used, and during continuous casting of this,
casting so that
the average cooling rate at the time of solidification is 20 degrees C or
above, and making
the temperature of the ingot at the time it is taken out of the casting
machine 250 degrees C
or below, or cooling the ingot so that the temperature of the ingot is 250
degrees C or below
within two minutes after the molten metal is poured into the casting machine,
and further,
after then having done rolling to the final sheet thickness by cold rolling
only and without
homogenization or hot rolling, solution treatment is done in a continuous
annealing
furnace. The AI-Mg-Si alloy sheet obtained by the present invention has
excellent bake
hardenability, so that by making use of this property, it may be utilized
widely for vehicles
such as cars, outer panels for household electrical appliances and the like,
building
materials and the like.
[Background Art]
[0002)
For example, cold-rolled steel has been used conventionally for automobile
panel materials.
However, recently, as a measure for reducing the weight of automobile bodies
with the aim
of improving fuel efficiency and reducing exhaust gases, a trend towards using
aluminum
alloy materials that are lightweight and have high specific strength, and have
excellent
metal forming processability is rapidly increasing. Among these, as an
aluminum alloy
sheet for automobiles that is often used with a paint coating for aesthetic
improvement,
AI-Mg-Si alloy, with its excellent bake hardenability, is getting attention,
and its practical
realization is being advanced in some quarters.
[0003]
However, the method that has generally been implemented in the past as a
manufacturing
for aluminum alloy sheets is the method whereby after scalping and
homogenization, the
CA 02551599 2006-06-23
processes such as hot rolling, cold rolling, and annealing are sequentially
performed on an
ingot produced by a semi-continuous casting method. Conventional aluminum
alloy sheets
produced through such processes, since their press formability is excellent,
and their bake
hardenability is also excellent, were sufficient for the requirements of
customers.
[0004]
However, the requirements of customers in recent years have become more
stringent, and
not only is there a tendency to seek higher strength in order to achieve
weight reduction,
but also further improvements in formability and bake hardenability
are=required, and
further, the requirements of cost reduction by improving productivity are
further
mcreasmg.
[0005]
As relatively new aluminum alloy sheet manufacturing technologies that meet
these
requirements, methods whereby after an ingot is made by continuous casting,
this is
immediately sent to the rolling process such as hot rolling and cold rolling
are performed
(herebelow called the continuous/direct rolling method) and scalping and
homogenization
are omitted are being considered (JP-A S55-27497, examined patent application
publication
S62-54182 and the like). According to these methods, in addition to promoting
cost
reduction by the omission of scalping and homogenization, since the solute
elements in a
super-saturated solid solution are not precipitated during homogenization,
there is the
advantage that the strength of aluminum alloy sheets is improved due to solid
solution
strengthening.
[Patent Citation 1] JP-A S55-27497
[Patent Citation 2] Examined Patent Application Publication S62-54182
[0006]
A method has been proposed whereby, during the production of an aluminum alloy
sheet
using molten aluminum alloy, hot rolling is done after continuous casting, and
cold-rolling
is further done, the precipitation of super-saturated solute elements during
the sequence of
the processes of continuous casting, hot rolling, cold rolling, and
intermediate annealing in
particular is reduced as much as possible, and the strength of the final cold
rolling product
is increased, and bake hardenability and press formability are improved
markedly (JP-A
H7-252616). This method uses a molten aluminum alloy containing specific
amounts of
alloy elements such as Mg, Mn, and Si in particular, and manufactures an AI-Mg-
Si alloy
sheet by hot rolling after continuous casting, and further performing cold
rolling, but at
that time, by regulating the cooling rate during continuous casting and after
hot rolling,
and additionally by controlling the heat processing conditions after the cold
rolling that is
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subsequently performed, an Al-Mg-Si alloy sheet with improved press
formability and
bake hardenability and the like is obtained.
[Patent Citation 3] JP-A H7-252616
[0007]
Continuous casting methods utilized in such continuous casting/direct rolling
methods that
are currently practically realized are the water cooled continuous casting
method (a
continuous casting method whereby continuous casting slabs that are formed
into sheets
and come out of a stationary type water cooled continuous casting mold are
directly cooled
and solidified), a twin roll casting method developed at Hunter Engineering (a
continuous
casting method whereby molten metal is supplied between a pair of rotating
cooled rollers,
and cooling and solidification is done between said rollers), a belt type
continuous casting
method developed at Hazelett (a method whereby molten metal is supplied
between two
movable belt-shaped cooling members, and casting in the shape of a sheet
continuously
while cooling and solidifying between said belts), a block-type continuous
casting method
developed at Swiss Aluminum (a method whereby molten metal is supplied between
two
movable block-shaped cooling members, and casting in the shape of a sheet
while cooling
and solidifying between said blocks), and the like.
[Disclosure of the Invention]
[Problem to be Overcome by the Invention]
[0008]
However, in the continuous casting/direct rolling methods that are presently
practically
realized, intermediate annealing is done at a relatively low temperature in
the range of 350
to 500 degrees C in order to prevent process cracking, but a problem arises in
that
precipitation of super-saturated solute elements occurs during the
intermediate annealing
process, and inhibits strengthening of the final cold rolling product.
Additionally, in the
methods described above, that is, methods wherein molten aluminum alloy are
used for
which the contained amount of alloy elements such as Mg, Mn, and Si in
particular are
specified, and during manufacturing of an AI-Mg-Si alloy sheet by hot rolling
after
continuous casting, and further cold rolling, by regulating the cooling rate
during
continuous casting and after hot rolling, and additionally by controlling the
heat
processing conditions after the cold rolling that is subsequently performed,
an Al-Mg-Si
alloy sheet with improved press formability and bake hardenability and the
like is obtained,
there is a problem in that hot rolling after the continuous casting, and heat
processing after
the cold rolling is needed, making the cost high so that the advantages of
continuous
casting cannot be utilized. Furthermore, in the obtained aluminum alloy sheet,
the press
formability and the bake hardenability leave room for further improvement.
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[Means for Solving the Problems]
(0009)
The present invention was made with attention to the problems with the
conventional art
described above, and concerns a manufacturing method for Al-Mg-Si aluminum
alloy
sheets with excellent bake hardenability, characterized in that during twin
belt casting of
Al-Mg-Si aluminum, casting is done at an average cooling rate of 20 degrees C
per second
or above at the time of solidification, and the temperature of the ingot when
coming out of
the casting machine is 250 degrees C or below, and additionally, the ingot is
cooled so that
the ingot temperature is 250 degrees C or below within 2 minutes from pouring
the molten
metal into the casting machine, and further, after subsequently rolling to the
final sheet
thickness by cold rolling and without homogenization or hot rolling, solution
treatment is
done in a continuous annealing furnace.
[0010]
The first invention for solving the abovementioned problems is a manufacturing
method
for Al-Mg-Si aluminum alloy sheets with excellent bake hardenability, the main
points
being that a molten Al-Mg-Si aluminum alloy comprising Mg: 0.3 - 1.0 wt%, Si:
0.3 - 1.5
wt%, Cu: 1.0 wt% or below (including 0%), Fe: 1.2 wt% or below (including 0%),
and
according to need, containing Mn: 0.1 - 0.7 wt% and/or Cr: 0.1 - 0.3 %, and
the remnant
being AI is twin belt cast at an average cooling rate of 20 degrees C per
second or above at
the time of solidification, and at this time, the temperature of the ingot
coming out of the
casting machine is 250 degrees C or below, and subsequently rolling is done to
the final
sheet thickness by cold rolling and without homogenization or hot rolling, and
solution
treatment is done in a continuous annealing furnace.
[0011]
The second invention for solving the abovementioned problems is a
manufacturing method
for Al-Mg-Si aluminum alloy sheets with excellent bake hardenability, the main
points
being that a molten Al-Mg-Si aluminum alloy comprising Mg: 0.3 - 1.0 wt%, Si:
0.3 - I.5
wt%, Cu: 1.0 wt% or below (including 0%), Fe: 1.2 wt% or below (including 0%),
and
according to need, containing Mn: 0.1 - 0.7 wt% and/or Cr: 0.1 - 0.3 %, and
the remnant
being A1 is twin belt cast at an average cooling rate of 20 degrees C per
second or above at
the time of solidification, and at this time, the ingot is cooled so that the
ingot temperature
is 250 degrees C or below within 2 minutes of pouring molten metal into the
casting
machine, and after this rolling is done to the final sheet thickness by cold
rolling and
without homogenization or hot rolling, and solution treatment is done in a
continuous
annealing furnace.
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[0012]
The reason for making the average cooling rate 20 degrees C or above is that
if the average
cooling rate is less than 20 degrees C per second, coarse MgzSi readily
precipitates during
solidification, and this coarse Mg2Si is difficult to be dissolved into the
matrix sufficiently
during solution treatment with a continuous annealing furnace, so as a result,
the bake
hardenability is inferior.
[0013]
The reason for making the temperature of the ingot when coming out of the
casting
machine 250 degrees C or below is that if said temperature is above 250
degrees C, since
MgzSi precipitates during the cooling process of the ingot, the temperature
and time
needed for the solution treatment of the final sheet with a continuous
annealing furnace
increases, and as a result, bake hardenability is inferior.
[0014]
The reason for not doing homogenization or hot rolling is that even if the
precipitation of
MgzSi is suppressed during the casting and cooling processes, since MgzSi
precipitates
again during homogenization or hot rolling, it becomes difficult to be
dissolved into the
matrix sufficiently during solution treatment, and as a result, the bake
hardenability is
inferior.
[0015]
The reason for cooling the ingot to 250 degrees C or below within 2 minutes of
pouring the
molten metal is that if 2 minutes is passed, the precipitation of MgzSi
occurs, and it
becomes difficult to dissolve this MgzSi into the matrix during solution
treatment of the
final sheet with a continuous annealing furnace, and as a result, bake
hardenability is
inferior.
[0016]
In order to make the temperature of the ingot when it comes out of the casting
machine 250
degrees C or below, it is necessary to take away approximately 2200 MJ of heat
or more
from the ingot inside the casting machine for every 1 m3 of the volume of the
ingot. In the
case of casting an ingot with a width of 1 m and a sheet thickness of 1 cm in
a casting
machine with an effective cooling length of 1 m, at a casting speed of 8 m per
minute, this
corresponds to casting with an average removed heat flow density of 3.0 MW/m2
inside the
casting machine.
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[0017]
By making the temperature of the ingot after casting 250 degrees C or below,
or by cooling
the ingot so that it is 250 degrees C or below within 2 minutes from pouring
the molten
metal, and further, by rolling to the final sheet thickness using only cold
rolling, without
subsequent homogenization or hot rolling, it is possible to reduce the
precipitation of
coarse MgzSi; and during subsequent solution treatment with a continuous
annealing
furnace, MgzSi easily dissolves into the matrix. By doing so, in combination
with having
adjusted the composition of the Al-Mg-Si alloy appropriately, strengthening of
the
cold-rolling product is achieved, and proof stress after the subsequent baking
treatment is
increased, and further, an Al-Mg-Si alloy sheet is realized that is also
markedly superior in
press formability.
[0018]
Herebelow, the manufacturing conditions.prescribed for the present invention
including
composition of the AI-Mg-Si alloy and cooling conditions during continuous
casting and
after heat rolling shall be explained in detail. First, the reason for
prescribing the
composition of the Al-Mg-Si alloy used in the present invention shall be
explained.
[0019]
Mg (0.3 -1.0 wt%) is an element that forms MgzSi and contributes to
strengthening, and it is
necessary to include 0.3 wt% or above in order to secure the strength
necessary for outer
panel materials as described above. However, if the content is too high, this
reduces
formability, so that it is also necessary to keep the content at 1.0 wt% or
below. A more
preferable lower bound for Mg is 0.4 wt%, and a more preferable upper bound is
0.8 wt%.
[0020]
Si (0.3 - 1.5 wt%) is an element that forms MgzSi with the abovementioned Mg,
and
contributes to strengthening, and in order to effectively realize the effects
of its addition, it
is necessary to include 0.3% or above. However, if the content is too high,
there is an
adverse effect on press formability, so that it is also necessary to keep the
content at 1.5 wt%
or below. A more preferable lower bound for Si is 0.6%, and a more preferable
upper
bound is 1.2 wt%. As described above, in the present invention, Mg and Si form
aggregates (clusters) of MgzSi composition called a G. F: zone, or an
intermediate layer
within the aluminum alloy, and are important elements that contribute to
hardening by
baking treatment.
[0021)
Cu (1.0 wt% or below) is not absolutely necessary, but it has a precipitation
strengthening
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effect, so that it is desirable to include proactively in cases where the
demands for strength
are high. However, if the content is too high, adverse effects will appear, so
that it should
be kept at 1.0 wt% or below. Considering the balance between strength and
formability, a
more preferable Cu content is in the range of 0.4 - 0.9 wt%.
[0022]
Fe (1.2 wt°I° or below) is also not absolutely necessary, but it
has the effect of increasing
strength, so that it is desirable to include proactively in cases where the
demands for
strength are high. However, if the content is too high, adverse effects will
appear, so that
it should be kept at 1.2 wt% or below. Considering the balance between
strength and
formability, a more preferable Fe content is in the range of 0.1- 0.5
wt°!°.
[0023]
Mn (0.1 - 0.7 wt%) is an element that is effective as a solid solution
strengthening element
and a recrystallized grain refinement element, and in order to effectively
realize these
effects, 0.1 wt% or above must be included. However, if the content is too
high, due to the
increase in the amount of Mn that cannot be dissolved into a solid solution, a
tendency to
worsen the formability appears, so that it must be kept at 0.7 wt% or below.
[0024]
Cr (0.1- 0.3 wt%) has an effect as a recrystallized grain refinement element,
and in order to
effectively realize these effects, a greater amount than the lower bound must
be included.
However, if the content surpasses the abovementioned upper bound,
intermetallic
compounds are generated and adverse effects appear. Considering these points,
a
desirable content for Cr is in the range of 0.1- 0.3 wt%.
[0025)
The components making up the remnant of the aluminum alloy in the present
invention
are A1 and unavoidable impurities, and examples of unavoidable impurities are
Ni, Zn, Zr,
V, Ti, Li and the like, but as long as these are in unavoidable impurity
amounts, they will
not be a particular obstacle for securing the properties intended for the
present invention.
Next, the conditions for continuous casting using the abovementioned Al-Mg-Si
alloy, and
cold rolling, shall be explained.
[0026]
If the average cooling rate at the time of solidification during continuous
casting is
prescribed in the above manner, the amount of Al-Fe-Si intermetallic compounds
in the
continuous cast structure decrease due to forced solid dissolution, and
additionally, the size
CA 02551599 2006-06-23
of said AI-Fe-Si intermetallic compounds are refined to an average size of
approximately 2
pm or below, and the press formability and bake hardenability are markedly
increased.
However, when the average cooling rate at the time of solidification during
continuous
casting is below the abovementioned rate, the amount of intermetallic
compounds
precipitated increases, and additionally their size becomes coarse, and not
only does
satisfactory press formability become unobtainable, but bake hardenability
also becomes
inferior.
[0027]
Additionally, after the abovementioned continuous casting, by making the
temperature of
the ingot 250 degrees C or below when it subsequently comes out of the casting
machine,
or by cooling the ingot so that the ingot temperature becomes 250 degrees C or
below
within 2 minutes after molten metal is poured into the casting machine, and
further, by
subsequently utilizing rapid cooling whereby rolling is done to the final
thickness by cold
rolling without homogenization or hot rolling, the precipitation of super-
saturated solute
components during ingot cooling is suppressed and the amount of super-
saturated solid
solution is maintained, and sheets that have excellent bake hardenability may
be
manufactured. Incidentally, if the temperature of the ingot after casting
surpasses 250
degrees C, precipitation of super-saturated solute components occurs, and
sheets with
inferior bake hardenability are manufactured.
[0028]
After achieving the final sheet thickness by cold-rolling, solution treatment
is performed in
a temperature range of 530 - 570 degrees C in a continuous annealing furnace,
and after
quenching with hot or cold water, preliminary aging treatment is done. The
reason for
prescribing the solution treatment temperature at this time in the above
manner is to
suppress the precipitation of solute elements during solution treatment and
maintain a
sufficient super-saturated solute amount, and increasing strength, to increase
bake
hardenability by increasing the amount of solute elements. Incidentally, if
the
temperature of the solution treatment is below 530 degrees C, the improvement
effect on
bake hardenability also becomes insufficient. On the other hand, if the
temperature
surpasses 570 degrees C, the recrystallized grains become coarser, and
additionally,
burning due to eutectic melting occurs, and press formability is worsened.
[0029)
Additionally, after the abovementioned solution treatment, and after quenching
with hot or
cold water, by continuing on to perform preliminary heat treatment, an AI-Mg-
Si alloy
sheet with extremely excellent press formability and bake hardenability is
obtainable. The
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CA 02551599 2006-06-23
conditions for quenching and aging heat treatment are not particularly
restricted, but as
preferable conditions, the condition for quenching is hot water quenching, and
the
condition for aging heat treatment are approximately 10 minutes to 8 hours at
60 - 200
degrees C.
[0030]
The present invention, in addition to specifying the composition of an AI-Mg-
Si alloy as
described above, has the characteristic that during continuous casting using
said molten
alloy, casting is done so that the average cooling rate at the time of
solidification is 20
degrees C or above, and the temperature of the ingot at the time it is taken
out of the
casting machine is made to be 250 degrees C or below, or the ingot is cooled
so that the
temperature of the ingot is 250 degrees C or below within two minutes after
the molten
metal is poured into the casting machine, and subsequently rolling to the
final sheet
thickness by cold rolling only and without homogenization or hot rolling, and
prescribing
the conditions for solution treatment in a continuous annealing furnace, and
there are no
specific restrictions on other conditions, but if other preferable conditions
and the like are
to be explained, then they are as follows.
[0031]
The present invention is characterized by continuous casting so that the ingot
temperature
is 250 degrees C or below, or cooling a continuously cast slab to 250 degrees
C or below,
rolling this up once, then rolling to the final sheet thickness by only cold
rolling and
without homogenization or hot rolling, and prescribing the conditions for
solution
treatment with a continuous annealing furnace, and due to this, compared to
the method
whereby after rolling up once after continuous casting, cooling is done and
then
homogenization or hot rolling is further done, the heat loss is small, and it
is also effective
for increasing productivity.
[0032]
Further, in carrying out the present invention, an aluminum alloy manufactured
sheet is
manufactured, by continuously manufacturing sheet-shaped slabs of a thickness
of
approximately 4 - 15 mm normally by continuous casting, by cold rolling this
to a
thickness of 0.1 - 1 mm after having rolled this up, and further performing
solution
treatment with a continuous annealing furnace and preliminary aging.
Incidentally, as the
continuous casting method utilized here, the aforementioned water-cooled type
continuous
casting method, twin roll type continuous casting method, belt type continuous
casting
method, block type continuous casting method, and the like may be selected as
appropriate
and utilized.
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[Best Mode for Carrying Out the Invention]
(0033]
Next, embodiments of the present example shall be shown, but the present
invention is not
restricted to the embodiments described below, and it is of course possible to
carry out by
adding changes as seen fit that fit within the range of the points of the
present invention,
and these shall also be included within the technical scope of the present
invention.
[Embodiment 1]
[0034]
Embodiment 1
An ingot of thickness 1 cm was cast on a twin belt casting machine under the
following
conditions:
Effective cooling length of casting machine: 1 m
Casting rate: 8 m/min
Molten metal pouring temperature: 700 degrees C
Composition: AI, Mg:0.6wt%, Si:0.8wt%, Fe:0.2wt%, Mn:0.2wt%, Ti:0.01wt%
[0035]
By changing the average removed heat flow density inside the casting machine,
ingots with
differing ingot temperatures immediately after casting were obtained.
Subsequently, after cold-rolling to a 1 mm sheet, and performing a solution
treatment of
545 degrees C x 15 seconds -~ hot water quenching, preliminary aging of 85
degrees C x 8
hours was done, and T4 material was made. Additionally, for T6 material, after
naturally
aging T4 material for 1 week, 170 degrees C x 30 minutes of artificial aging
was done. In
order to evaluate bake hardenability, the proof stress of the T4 and T6
materials were
measured, and the difference in the proof stress between T4 and T6 materials
was taken to
be the bake hardenability. Additionally, the target for superior bake
hardenability was 100
MPa or above. Further, in order to see the effects of performing or not
performing
homogenization or hot rolling, the bake hardenability of sheets processed by
homogenization or hot rolling was measured as comparative examples.
[0036]
[Table 1]
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Number 1 2 3 . 4
Average removed heat flow
density
inside the casting machine3.3 3.3 3.3 2.8
(MW/m2)
Ingot temperatures
immediately after casting197 197 197 330
(C)
Homogenization NO YES NO NO
Hot rolling NO NO YES NO
T4-YS
110 105 106 109
(MPa)
T6-YS
215 180 190 198
(MPa)
Bake hardenability 105 75 84 91
Assessment o x x x
Present ComparativComparativComparativ
example a examplea examplea example
Homogenization: 560 degrees C x 6 hours maintained --> furnace cooling
Hot rolling: After raising temperature to 560 degrees C, hot rolling is done
to 4 mm with
the rolling start temperature at 550 degrees C. After this, cold rolling was
done to 1 mm.
[0037)
Embodiment 2
An ingot with thickness 1 cm was cast on a twin belt casting machine under the
following
conditions:
Effective cooling length of casting machine: 1 m
Casting rate: 8 m/min
Molten metal pouring temperature: 700 degrees C
Composition: Al, Mg:0.6wt%, Si:0.8wt%, Fe:0.2wt%, Mn:0.2wt%, Ti: O.Olwt%
[0038)
A cooling device was attached at the exit opening of the casting machine so
that the ingot
may be cooled immediately after casting. When the cooling device was running,
the
CA 02551599 2006-06-23
temperature of the ingots, which were 357 degrees C immediately after casting,
by passing
through the cooling device, were reduced to 230 degrees C within 2 minutes
after pouring
molten metal into the casting machine. In contrast, when the cooling device
was not
running, the temperature of the ingots 2 minutes after molten metal was poured
into the
casting machine was still hot at 330 degrees. After this, after cold-rolling
to a 1 mm sheet,
and performing a solution treatment of 545 degrees C x 15 seconds -~ hot water
quenching,
preliminary aging of 85 degrees C x 8 hours was done, and T4 material was
made.
Additionally, for T6 material, after naturally aging T4 material for 1 week,
170 degrees C x
30 minutes of artificial aging was done. In order to evaluate bake
hardenability, the proof
stress of the T4 and T6 materials were measured, and the difference in the
proof stress
between T4 and T6 materials was taken to be the bake hardenability. The target
for
superior bake hardenability was 100 MPa or above.
[Table 2]
Number 1 2
Average removed heat
flow density
inside the casting machine2.7 2.7
(MW/m2)
Ingot temperatures
immediately after casting357 357
(C)
Cooling device YES NO
Ingot temperatures
230 330
2 minutes after pouring
molten metal
T4-YS
108 107
(MPa)
T6-YS
211 181
(MPa)
Bake hardenability 103 74
Assessment o
Present Comparativ
example a example
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