Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
Al-Mg ALLOY SHEET WITH EXCELLENT FORMABILITY
AT HIGH TEMPERATURES AND HIGH SPEEDS
AND METHOD OF PRODUCTION OF SAME
TECHNICAL FIELD
The present invention relates to an Al-Mg alloy
sheet with excellent formability at high temperatures and
high speeds and a method of production of the same.
BACKGROUND ART
An Al-Mg alloy is light and excellent in strength
and corrosion resistance, so is being proposed as an
automobile sheet material or other worked or formed
material. However, its elongation at room temperature is
low, therefore there is the problem that an Al-Mg alloy
cannot be formed into a complex shape by cold working.
For this reason, an A1-Mg-based superplastic alloy
suppressing the recrystallization at the time of hot
working to reduce the size of the crystal grains and
obtaining an elongation of several 100% in a high
temperature region of for example 500 to 550°C as been
developed and is being used for various applications.
A conventional Al-Mg-based superplastic alloy
manifests its superplasticity at a slow forming speed
(strain rate)~of 10-4 to 10-3/sec and requires a long time,
therefore is low in productivity when applied to ordinary
press forming and is not practical.
Therefore, an aluminum alloy sheet able to give a
sufficient elongation even with high forming speed of a
strain rate of for example 0.1/sec or more in the high
temperature region for hot working, that is, 100 times or
more than that of the prior art, and able to suppress
occurrence of cavities at the time of forming has been
developed.
For example, Japanese Unexamined Patent Publication
(Kokai) No. 10-259441 proposes an aluminum alloy sheet
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with excellent superplastic formability at high speeds
and having a reduced amount of cavities after forming
characterized in that it contains 3.0-8.Oo (wt%, same
below) of Mg, 0.21-0.500 of Cu, and 0.001-0.10 of Ti,
contains as impurities Fe to 0.060 or less and Si to
0.060 or less, and the balance of Al and impurities and
has an average crystal grain size of 20 to 200 Vim.
In the prior art, however, in order to achieve a
good high temperature high speed formability in the
finally obtained sheet product, there is the problem that
it is necessary to go through many processes such as
large slab casting by semi-continuous casting, surface
scalping, soaking, hot rolling, cold rolling,
intermediate annealing, final rolling, and final
annealing and so the cost increases.
Further, a large slab has a slow cooling speed at
the time of casting of for example about 1 to 10 or so
°C/sec, therefore the intermetallic compounds of A1-Fe-Si,
Al6Mn, etc. become coarse of several tens of ~m or more.
Even in the final sheet product after the soaking, hot
rolling, cold rolling, annealing, etc., coarse
intermetallic compounds of 10 ~tm or more still remain.
Cavities easily occur due to peeling at the interface
between the intermetallic compounds and matrix at the
time of high temperature forming. As a countermeasure for
this, the method of suppressing the contents of Fe and Si
to 0.10 or less is employed, but it is necessary to use
expensive high purity metal for this, so there was the
problem that the cost rose in the end.
DISCLOSURE OF THE INVENTION
An object of the present invention is to provide an
aluminum alloy sheet solving the above problems of the
prior art, not requiring the use of high purity metal
accompanied with higher cost, improving the formability
at high temperatures and high speeds, and reducing the
cavities after forming and a method of production of the
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same.
To attain the above object, according to the present
invention, there is provided an aluminum alloy sheet with
excellent formability at high temperatures and high
speeds with a reduced amount of cavities after forming
characterized in that it consists of:
Mg: 2.0-8.Owto,
Si: 0.06-0.2wto,
Fe: 0.1-0.5wto,
Mn: 0.1-0.5wto, and
the balance of Al and unavoidable impurities,
wherein
a density of an inter-metallic compound having an
equivalent circle diameter of 1 to 5 ~,m is 5000/mm2 or
more and an average crystal grain size is 20 ~,m or less.
In order to achieve the above object, according to
the present invention, there is further provided a method
of production of an aluminum alloy sheet of the present
invention with excellent formability at high temperatures
and high speeds with a reduced amount of cavities after
forming characterized in that it comprises the steps of:
preparing an alloy melt having a composition of the
aluminum alloy sheet of the present invention,
casting the alloy melt by a twin belt casting
machine at a cooling rate of 20 to 150°C/sec at the
location of 1/4 of the slab thickness during casting to
form a slab having a thickness of 5 to 15 mm,
subsequently rewinding up the slab as a coil,
cold rolling the slab taken out from the coil with a
cold rolling reduction of 70 to 960, and
performing annealing for heating the obtained cold
rolled sheet at a rate of temperature rise of 5°C/sec or
more to 420 to 500°C.
The aluminum alloy sheet of the present invention
defines ranges of the chemical composition and
microstructure and disperses the inter-metallic compounds
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uniformly and finely so as to improve the formability at
high temperatures and high speeds by the increased
fineness of the crystal grains without requiring any high
purity metal and reduce the cavities after forming.
Further, the method of production of the present
invention secures a high cooling rate at the time of
casting by twin belt casting, restricts the cold rolling
reduction, and limits the annealing conditions after the
cold rolling so as to realize a uniform fine dispersion
of the inter-metallic compounds and increased fineness of
the crystal grains.
By using the aluminum alloy sheet of the present
invention, a high grade formed product is obtained, the
forming time is shortened, and the productivity is
l5 enhanced.
BEST MODE FOR WORKING THE INVENTION
The reasons for the limitation of the chemical
composition of the alloy in the present invention will be
explained next. The "%" representing the chemical
composition in the present description means "wt%" unless
particularly indicated otherwise.
[Mg: 2.0-8.Oo]
Mg is an element improving the strength. In order to
manifest this effect, it is necessary to set the Mg
content to 2.0o or more. However, if the Mg content
exceeds 8.0o, the castability of a thin slab is lowered.
Accordingly, the Mg content is limited to 2.0 to 8.0a. If
stressing the castability, preferably the upper limit of
the Mg content is further limited to 6.0o or less.
[Si: 0. 06-0.2 0]
Si is precipitated as fine particles of Al-Fe-Si-
based, Mg2Si, and other inter-metallic compounds at the
time of casting and functions as a nucleus generating
site of recrystallization at the time of annealing after
cold rolling. Accordingly, the larger the number of
particles of these inter-metallic compounds, the larger
the number of generated recrystallized nucleii and as a
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result the larger number of fine recrystallized grains
formed. Further, the fine particles of the inter-metallic
compounds pin the grain boundaries of the generated
recrystallized grains and suppress growth due to merging
of crystal grains to stably maintain the fine
recrystallized grains.
In order to manifest these effects, it is necessary
to make the Si content 0.060 or more. However, if the Si
content exceeds 0.20, the tendency of the precipitated
inter-metallic compounds to become coarser becomes
stronger, so the formation of cavities is promoted at the
time of high temperature deformation. Accordingly, the Si
content is limited to 0.06 to 0.20. The preferred range
is 0.07 to 0.150.
In general, Si is regarded as an impurity element to
be eliminated in the same way as the following Fe, but in
the present invention, conversely a suitable amount of Si
is made present in order to increase the fineness of the
recrystallized grains as described above. Accordingly,
high purity metal is not needed and there is no
accompanying rise in cost.
[Fe: 0.1-0.50]
Fe is precipitated as fine grains of Al-Fe-Si-based
or other inter-metallic compounds at the time of casting
and functions as a nuclei generating site of
recrystallization at the time of annealing after cold
rolling. Accordingly, the larger the number of particles
of these inter-metallic compounds, the larger the number
of the generated recrystallized nucleii and as a result
the larger the number of fine recrystallized grains
formed. Further, the fine particles of the inter-metallic
compounds pin the grain boundaries of the generated
recrystallized grains and suppress the growth due to
merger of crystal grains to stably maintain the fine
recrystallized grains. In order to manifest this effect,
it is necessary to make the Fe content 0.10 or more.
However, if the Fe content exceeds 0.50, the tendency of
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the precipitated inter-metallic compounds to become
coarser becomes stronger, so the occurrence of cavities
is promoted at the time of high. temperature deformation.
Accordingly, the Fe content is limited to 0.1 to 0.50. A
preferred range is 0.1 to 0.30.
In general, Fe is regarded as an impurity element to
be eliminated in the same way as the above Si, but in the
present invention, conversely a suitable amount of Fe is
made present in order to increase the fineness of the
recrystallized grains as described above. Accordingly,
high purity metal is not needed and there is no
accompanying rise in cost.
[Mn: 0.1-0.5%]
Mn is an element increasing the fineness of the
recrystallized grains. In order to manifest this effect,
it is necessary to make the Mn content 0.10 or more.
However, if the Mn content exceeds 0.50, a coarse Al-
(Fe~Mn)-Si-based inter-metal compound is formed, and the
occurrence of cavities is promoted at the time of high
temperature deformation. Accordingly, the Mn content is
limited to 0.1 to 0.5%. Particularly, when stressing the
prevention of occurrence of cavities; preferably the
upper limit of the Mn content is further restricted to
0.3a.
[Optional ingredient Cu: 0.1-0.50]
In the present invention, Cu can be added within a
range of 0.1-0.5o in order to improve the strength of the
aluminum alloy sheet. In order to obtain precipitation
hardening effect sufficiently, it is necessary to make
the amount of addition of Cu 0.10 or more. However if the
amount of addition of Cu exceeds 0.5o, the castability is
lowered. When stressing the castability, preferably the
upper limit of the amount of addition of Cu is further
restricted to 0.30 or less.
[Optional ingredients Zr and Cr: 0.1-0.40]
In the present invention, in order to assist the
increased fineness of the recrystallized grains, at least
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one type of Zr and Cr can be incorporated within a range
of 0.1-0.40. Zr and Cr are elements for increasing the
fineness of the recrystallized grains. In order to
manifest this effect, it is necessary to make the amounts
of addition of both the Zr and Cr 0.10 or more. However,
if the amounts of addition exceed 0.40, coarse inter-
metallic compounds are formed at the time of the casting,
and the occurrence of cavities is promoted at the time of
high temperature deformation. Particularly, when
stressing the prevention of the occurrence of cavities,
preferably the upper limits of the amounts of addition
are further restricted to 0.20 or less.
[Other elements]
In the present invention, in order to increase the
fineness of the casting structure, Ti can be added within
a range of 0.001-0.150. In order to manifest this effect,
it is necessary to make the amount of addition of Ti
0.0010 or more. However, if the amount of addition of Ti
exceeds 0.150, a coarse compound such as TiAl3 is
generated, the formability at a high temperature is
deteriorated, and the occurrence of cavities is promoted.
A preferred range is 0.006-0.100.
Next, the reasons for the limitation of the
microstructure of the alloy sheet in the present
invention will be explained.
[Density of inter-metallic compounds having
equivalent circle diameters of 1 to 5 ~m of 5000/mm2 or
more]
The present invention utilizes the fine inter-
metallic compound particles as (1) the recrystallized
grain nuclei generating sites and (2) means for pinning
the grain boundaries of the recrystallized grains and
generates finer recrystallized grains by the annealing
after the cold rolling. The fine grain structure obtained
by this gives a high elongation at the time of
deformation at high temperatures and high speeds, whereby
the formability at high temperatures and high speeds is
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enhanced.
In order to obtain the above effect, the inter-
metallic compound having the equivalent circle diameter
of 1 to 5 ~,m,must be present in a density of 5000/mm2 or
more. As the inter-metallic compound, as already
mentioned, inter-metallic compounds such as Al-(Fe~Mn)-
Si-based compounds, Mg2Si, and Al6Mn are precipitated
during casting. In order to manifest the effects of the
above (1) and (2) by these inter-metallic compounds, the
equivalent circle diameter must be 1 to 5 ~,m. If the
equivalent circle diameter is less than 1 Vim., the
particles are too small to manifest the effects of (1)
and (2) described above. Conversely, if it exceeds 5 Vim,
cavities are easily generated at the time of deformation
at high temperatures and high speeds, and the strength
and elongation after the shaping are lowered.
The inter-metallic compounds having the size within
the above described range must be present at a density of
5000/mm2 or more.
If the density is less than 5000/mm2, the
recrystallized grain diameter at the time of the
annealing exceeds 20 Vim, and the elongation at the time
of high temperature deformation.is lowered.
[Average crystal grain diameter of 20 ~tm or less]
In the alloy sheet of the present invention, the
average crystal grain diameter is made 20 ~m or less. If
the average crystal grain diameter exceeds 20 ~,m, the
elongation at the time of the high temperature
deformation is lowered.
The reasons for the limitation of conditions of the
method of production of the present invention will be
explained next.
[Slab having thickness of 5 to 15 mm cast by twin
~ belt casting and taken up in the form of a coil]
The twin belt casting method is a continuous casting
method injecting a melt into a mould of a pair of water
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cooled rotating belts facing each other from one end in
the vertical direction, solidifying the melt by the
cooling from the belt surfaces to form the slab, pulling
out the formed slab from the other end of the mould, and
taking it up in the form of a coil.
In the present invention, the thickness of the slab
cast by this twin belt casting method is made 5 to 15 mm.
When the thickness is within this range, a high
solidification speed can be secured even at the center
portion of the sheet thickness, therefore a uniform
casting structure can be easily formed. Simultaneously,
with the composition of the present invention, it is
possible to easily suppress the generation of coarse
inter-metallic compounds and it becomes easy to control
the average grain size of the recrystallized grains in
the final sheet product to 20 ~,m or less. The above
described slab thickness range is also suitable from the
viewpoint of the twin belt casting.
Namely, if the slab thickness is less than 5 mm, the
amount of the aluminum alloy melt passing through the
casting machine per unit time becomes too small, so the
twin belt casting becomes difficult. If the slab
thickness exceeds 15 mm, it becomes difficult to rewind
it up as a coil.
[Cooling rate at time of casting of 20 to 150°C/sec]
In the method of production of the present
invention, a slab having a thickness of 5 to 15 mm is
cast by twin belt casting. At that time, in order to
cause the precipitation of inter-metallic compounds
having the equivalent circle diameter of 1 to 5 ~m
prescribed for the alloy of the present invention with a
density of 5000/mm2 or more, the cooling rate at the
location of 1/4 of the slab thickness during the casting
is made 20 to 150°C/sec. In the aluminum alloy of the
present invention, the inter-metallic compounds such as
the Al-(Fe~Mn)-Si-based compounds and Mg2Si are
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precipitated at the time of the casting. If the cooling
rate is less than 20°C/sec, these inter-metallic compounds
become coarse and the compounds exceeding 5 ~tm increase.
Conversely, if the cooling rate exceeds 150°C/sec, the
inter-metallic compounds become finer and the compounds
less than 1 ~m increase. In the end, in either case, the
density of the inter-metallic compounds having the
equivalent circle diameter of 1 to 5 ~m becomes less than
5000/mm2 and the nuclei of the recrystallized grains
become fewer at the time of the final annealing (CAL), so
the recrystallized grains become coarse.
[Cold rolling with cold rolling reduction of 70 to
960]
Accumulation of dislocation occurring due to the
plastic working by the cold rolling around the
intermetallic compounds is indispensable for forming the
fine recrystallization structure at the time of the final
annealing. If the cold rolling reduction is less than
700, the accumulation of the dislocations becomes
insufficient and a fine recrystallization structure
cannot be obtained. If the cold rolling reduction exceeds
96%, edge cracks occur during the cold rolling, so cold
rolling becomes difficult.
[Annealing for heating to 420 to 500°C at a rate of
temperature rise of 5°C/sec or more]
In the present invention, the above annealing is
conducted as the final annealing after the cold rolling.
This is generally conducted by the continuous annealing,
but it is not particularly necessary to limit the
annealing to this.
The annealing temperature of the final annealing is
made a range of 420 to 500°C. If the temperature is less
than 420°C, the energy required for recrystallization is
insufficient, therefore the recrystallization becomes
insufficient and a fine recrystallization structure
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cannot be obtained. However, if it exceeds~500°C, the
recrystallized grain diameter exceeds 20 ~tm, and the fine
recrystallization structure cannot be obtained.
The heating rate to the annealing temperature is
made 5°C/sec or more. If the temperature is slowly
elevated by a rate less than 5°C/sec, the recrystallized
grains become coarse, so the fine recrystallization
structure cannot be obtained.
Finally, the forming of the aluminum alloy sheet of
the present invention is preferably conducted at a
temperature of 400-500°C. If the forming temperature is
less than 400°C, a sufficient elongation cannot be
obtained. If the forming temperature exceeds 550°C, the
coarsening of the crystal grains occurs. Further, burning
occurs in an alloy having a high Mg content within the
range of the present invention, and the elongation is
lowered. The strain rate at the time of the shaping is
preferably 0.1/sec or more. If the strain rate is less
than 0.1/sec, the coarsening of the crystal grains occurs
during the forming, so a drop of the elongation is
induced.
Examples
Aluminum alloy melts having the compositions shown
in Table 1 were cast by the twin belt casting method to
form slabs having thicknesses of 7 to 9 mm. Each slab was
cold rolled down to a thickness of 1 mm and annealed at
450°C, then test pieces prescribed in JIS H7501 were cut
out and measured for elongation after a tensile test.
Further, cross-sections of broken samples were polished,
then the area ratios of the cavities (cavity ratios) were
measured by an image analyzer. The production process and
characteristics are shown in Table 2.
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Table 1: Alloy Composition (wto)
Alloy Mg Mn Fe Si Cu Zr
A 3.1 0.3 0.12 0.07 - -
B 5.2 0.3 0.15 0.10 - -
C 7.1 0.4 0.10 0.09 - -
D 3.2 0.2 0.12 0.07 0.3 -
E 3.2 0.2 0.12 0.07 - 0.2
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Sheets obtained by cold rolling thin slabs cast by a
twin belt casting machine (products of the present
invention, Sample Nos. 1 to 7), as apparent also from the
alloy compositions of Table 1, irrespective of the fact
that the Fe contents were 0.10 or more and the Si
contents were 0.060 or more in all samples, had densities
of the inter-metallic compounds having equivalent circle
diameters of 1 to 5 ~,m of 5000/mm~ or more and crystal
grain sizes of 20 ~.m or less. For this reason, the
elongations at the tensile temperature of 500°C were good
ones of 2000 or more and also the cavity ratios after the
high temperature tension were good ones of the range of
0.15-0.270 or less than 1%.
A sheet obtained by cold rolling a thin slab cast by
a twin roll casting machine (comparative example, Sample
No. 8) had a large number of very fine intermetallic
compounds having equivalent circle diameters less than 1
~.m since the cooling rate at the time of the casting was
a relatively high 300°C/sec, therefore the density of~the
inter-metallic compounds having an equivalent circle
diameter of 1 to 5 ~.m in the final sheet became less than
5000/mm2 or coarse exceeding the crystal grain size of 20
~.m or more. For this reason, the cavity ratio after the
high temperature tension was a relatively low good one of
0.120, but the elongation at the tensile temperature of
500°C was a poor 80o.
A sheet obtained by soaking an ordinary slab cast by
a DC casting machine, then hot rolling the slab down to a
thickness of 7 mm, then cold rolling (comparative
example, Sample No. 9) had a cooling rate at the time of
the casting of a relatively slow 5°C/sec, therefore
intermetallic compounds having an equivalent circle
diameter exceeding 5 ~,m were generated, therefore the
density of intermetallic compounds having an equivalent
circle diameter of 1 to 5 ~m in the final sheet became
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less than 5000/mm2, and the crystal grains became slightly
coarse exceeding 20 Vim. For this reason, the cavity ratio
after the high temperature tensile test was a poor high
1.50, while the elongation at the tensile temperature of
500°C was a poor 1600.
A sheet obtained by cold rolling a thin slab cast by
a twin belt casting machine down to the sheet thickness
of 2 mm, then intermediate annealing the slab at 350°C,
then cold rolling down to 1 mm (comparative example,
Sample No. 10) had a density of inter-metallic compounds
of an equivalent circle diameter of 1 to 5 ~tm in the
final sheet of 5000/mm2 or more, but the cold rolling
reduction before the final annealing was a low one of
less than 700, therefore the crystal grains became
slightly coarse exceeding the crystal grain size of 20
~,m. The elongation at the tensile temperature of 500°C was
a poor one of less than 2000.
A sheet obtained by cold rolling a thin slab cast by
a twin belt casting machine (comparative example, Sample
No. 11) had a density of inter-metallic compounds having
an equivalent circle diameter of 1 to 5 ~m in the final
sheet of 5000/mm2 or more and a crystal grain size of 20
~m or less. However, the tensile temperature in the
tensile test was a relatively low 350°C, therefore the
elongation was a poor one of less than 2000.
A sheet obtained by cold rolling a thin slab cast by
a twin belt casting machine (comparative example, Sample
No. 12) had a density of inter-metallic compounds having
an equivalent circle diameter of 1 to 5 ~tm in the final
sheet of 5000/mm2 or more and a crystal grain size of 20
~.m or less. However, the tensile speed in the tensile
test was a relatively slow 0.01/sec, therefore the cavity
ratio after the high temperature tension was also a poor
1.8o and the elongation at the tensile temperature of
500°C was a poor one of less than 2000.
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INDUSTRIAL APPLICABILITY
According to the present invention, aluminum alloy
sheet with excellent formability at high temperatures and
high speeds with a reduced amount of cavities after the
forming and the method of production of the same are
provided.,
