U.S. patent application number 12/746127 was filed with the patent office on 2010-12-09 for aluminum alloy sheet for motor vehicle and process for producing the same.
This patent application is currently assigned to Nippon Light Metal Co., Ltd.. Invention is credited to Toshiya Anami, Akira Goto, Hitoshi Kazama, Kazumitsu Mizushima, Kunihiro Yasunaga, Pizhi Zhao.
Application Number | 20100307645 12/746127 |
Document ID | / |
Family ID | 40951821 |
Filed Date | 2010-12-09 |
United States Patent
Application |
20100307645 |
Kind Code |
A1 |
Zhao; Pizhi ; et
al. |
December 9, 2010 |
ALUMINUM ALLOY SHEET FOR MOTOR VEHICLE AND PROCESS FOR PRODUCING
THE SAME
Abstract
An aluminum alloy sheet for motor vehicles is produced by
casting a melt, containing 3.0-3.5 mass % Mg, 0.05-0.3 mass % Fe,
0.05-0.15 mass % Si, and less than 0.1 mass % Mn, a balance
substantially being inevitable impurities and Al, into a slab
having a thickness of 5 to 15 mm in a twin-belt caster so that
cooling rate at 1/4 depth of thickness of the slab is 20 to
200.degree. C./sec; winding the cast thin slab into a coiled thin
slab subjected to cold rolling with a roll having a surface
roughness of 0.2 to 0.7 .mu.m Ra at a cold rolling reduction of 50
to 98%; subjecting the cold rolled sheet to final annealing either
continuously in a CAL at a holding temperature of 400 to
520.degree. C. or in a batch annealing furnace at a holding
temperature of 300 to 400.degree. C.; and subjecting the resulting
sheet to straightening with a leveler.
Inventors: |
Zhao; Pizhi; (Shizuoka,
JP) ; Anami; Toshiya; (Shizuoka, JP) ;
Mizushima; Kazumitsu; (Aichi, JP) ; Goto; Akira;
(Saitama, JP) ; Kazama; Hitoshi; (Saitama, JP)
; Yasunaga; Kunihiro; (Saitama, JP) |
Correspondence
Address: |
HESLIN ROTHENBERG FARLEY & MESITI PC
5 COLUMBIA CIRCLE
ALBANY
NY
12203
US
|
Assignee: |
Nippon Light Metal Co.,
Ltd.
Tokyo
JP
|
Family ID: |
40951821 |
Appl. No.: |
12/746127 |
Filed: |
February 6, 2008 |
PCT Filed: |
February 6, 2008 |
PCT NO: |
PCT/JP2008/000161 |
371 Date: |
August 20, 2010 |
Current U.S.
Class: |
148/552 ;
148/437; 148/440 |
Current CPC
Class: |
C22C 21/06 20130101;
B21B 27/005 20130101; B21B 2003/001 20130101; B21B 3/00 20130101;
C22F 1/047 20130101; B21B 1/22 20130101; B22D 11/124 20130101; B22D
11/0605 20130101; C22F 1/04 20130101; C22C 21/08 20130101 |
Class at
Publication: |
148/552 ;
148/437; 148/440 |
International
Class: |
C22F 1/04 20060101
C22F001/04; C22C 21/00 20060101 C22C021/00; C22C 21/06 20060101
C22C021/06 |
Claims
1. An aluminum alloy sheet for motor vehicles excellent in press
formability, resistance to surface roughening and shape fixability,
the sheet comprising: 3.0-3.5 mass % Mg, 0.05-0.3 mass % Fe,
0.05-0.15 mass % Si, and further a limited amount of less than 0.1
mass % Mn; and a balance substantially being inevitable impurities
and Al; the sheet having an intermetallic compound maximum size of
5 .mu.m or less by circle-equivalent diameter in a region at 1/4
depth of the sheet thickness, an average recrystallized grain size
of 15 .mu.m or less, a surface roughness of 0.2-0.6 .mu.m Ra, a
yield strength of 145 MPa or less, and a tensile strength of 225
MPa or more.
2. The aluminum alloy sheet for motor vehicles excellent in press
formability, resistance to surface roughening and shape fixability
according to claim 1, wherein the aluminum alloy sheet has a punch
stretch forming height of 29 mm or more.
3. The aluminum alloy sheet for motor vehicles excellent in press
formability, resistance to surface roughening and shape fixability
according to claim 1, wherein the aluminum alloy sheet further
comprises 0.001-0.1% Ti.
4. A process for producing an aluminum alloy sheet for motor
vehicles excellent in press formability, resistance to surface
roughening and shape fixability, the process comprising: casting a
melt comprising 3.0-3.5 mass % Mg, 0.05-0.3 mass % Fe, 0.05-0.15
mass % Si, further a limited amount of less than 0.1 mass % Mn, and
a balance substantially being inevitable impurities and Al, into a
thin slab having a thickness of 5 to 15 mm in a twin-belt caster so
that the cooling rate at 1/4 depth of a thickness of the thin slab
is 20 to 200.degree. C./sec; winding the cast thin slab into a
coiled thin slab; subjecting the coiled thin slab to cold rolling
with a roll having a surface roughness of 0.2 to 0.7 .mu.m Ra at a
cold rolling reduction of 50 to 98% to produce a cold rolled thin
sheet; subjecting the cold rolled thin sheet to final annealing
continuously in a CAL at a holding temperature of 400 to
520.degree. C.; and subjecting the resulting sheet to straightening
with a leveler.
5. The process for producing the aluminum alloy sheet for motor
vehicles excellent in press formability, resistance to surface
roughening and shape fixability according to claim 4, wherein the
cold rolled thin sheet is subjected to final annealing in a batch
annealing furnace at a holding temperature of 300 to 400.degree.
C.
6. The aluminum alloy sheet for motor vehicles excellent in press
formability, resistance to surface roughening and shape fixability
according to claim 2, wherein the aluminum alloy sheet further
comprises 0.001-0.1% Ti.
Description
TECHNICAL FIELD
[0001] The present invention relates to an aluminum alloy sheet for
motor vehicles and a process for producing the same, particularly
to an aluminum alloy sheet suitable for forming a body sheet for
motor vehicles and the like and a method for producing the
same.
BACKGROUND ART
[0002] Heretofore, cold rolled steel sheets have been mainly used,
for example, for automobile outer panels. However, in accordance
with the requirements for weight reduction of automobile bodies,
the use of aluminum alloy sheets such as an Al--Mg-based alloy
sheet and an Al--Mg--Si-based alloy sheet has been studied
recently. Particularly, the Al--Mg-based alloy sheet has been
proposed as a body sheet for motor vehicles because it is excellent
in strength, formability and corrosion resistance.
[0003] Heretofore, as a process for producing such an aluminum
alloy sheet, there has been employed a process including casting a
slab by DC casting, face milling both surfaces of the slab,
homogenizing the face-milled slab in a soaking furnace, and
subjecting the homogenized face-milled slab to hot rolling, cold
rolling, intermediate annealing, cold rolling, and final annealing
to finish it to a predetermined sheet thickness (refer to Patent
Document 1).
[0004] On the other hand, there has been proposed a process
including continuously casting a thin slab with a belt caster,
directly winding the resulting thin slab into a coil, subjecting
the coiled thin slab to cold rolling and final annealing to finish
it to a predetermined sheet thickness. For example, there is
disclosed a process for producing an aluminum alloy sheet for motor
vehicles excellent in press formability and stress corrosion
cracking resistance (Patent Document 2). This process comprises
preparing a melt comprising 3.3-3.5 wt. % Mg and 0.1-0.2 wt. % Mn
and further comprising at least one of 0.3 wt. % or less Fe and
0.15 wt. % or less Si, a balance being ordinary impurities and Al;
casting the melt into a thin slab having a thickness of 5 to 10 mm
in a twin-belt caster at a speed of 5 to 15 m/min so that the
cooling rate at 1/4 depth of the thickness of the thin slab is 40
to 90.degree. C./sec; winding the resulting thin slab into a roll;
cold rolling the rolled thin slab with a roll having a surface
roughness of 0.2 to 0.7 .mu.m Ra; and annealing the cold rolled
thin sheet.
[0005] However, in the above process, since 0.1-0.2 wt. % Mn is
contained in the chemical composition of the melt for the purpose
of refining the recrystallized grains and the solidification
cooling rate is relatively fast, the size of intermetallic
compounds such as Al--(Fe.Mn)--Si is reduced to resulting in
excellent formability. On the other hand, there is a problem that,
since the amount of dissolved Mn in the matrix is excessively high,
yield strength is higher and spring back after forming is
increased.
[0006] In order to solve this problem, for example, a so-called
stabilization treatment is proposed (Patent Document 3) in which a
continuously cast and rolled sheet of an aluminum alloy containing
3-6 wt. % Mg is subjected to annealing treatment followed by
straightening, heated at a predetermined temperature of 240 to
340.degree. C. for 1 hour or more, and then slowly cooled.
Patent Document 1: Japanese Patent No. 3155678
[0007] Patent Document 2: International Publication No. WO
2006/011242
Patent Document 3: Japanese Patent Laid-Open No. 11-80913
DISCLOSURE OF THE INVENTION
[0008] In order to solve the problems as described above, the
present invention has employed a process for producing an aluminum
alloy sheet for motor vehicles excellent in press formability,
resistance to surface roughening and shape fixability, the process
comprising: casting a melt, comprising 3.0-3.5 mass % Mg, 0.05-0.3
mass % Fe, 0.05-0.15 mass % Si, and further a limited amount of
less than 0.1 mass % Mn, a balance substantially being inevitable
impurities and Al, into a thin slab having a thickness of 5 to 15
mm in a twin-belt caster so that the cooling rate at 1/4 depth of
the thickness of the thin slab is 20 to 200.degree. C./sec; winding
the cast thin slab into a coil; subjecting the coiled thin slab to
cold rolling with a roll having a surface roughness of 0.2 to 0.7
.mu.m Ra at a cold rolling reduction of 50 to 98%; subjecting the
cold rolled thin sheet to final annealing continuously in a CAL at
a holding temperature of 400 to 520.degree. C.; and then subjecting
the resulting sheet to straightening with a leveler. Alternatively,
the cold rolled thin sheet may be subjected to final annealing in a
batch annealing furnace at a holding temperature of 300 to
400.degree. C.
[0009] Employing such a production process has made it possible to
provide an aluminum alloy sheet for motor vehicles excellent in
press formability, resistance to surface roughening and shape
fixability, the sheet comprising 3.0-3.5 mass % Mg, 0.05-0.3 mass %
Fe, 0.05-0.15 mass % Si, and further a limited amount of less than
0.1 mass % Mn, a balance substantially being inevitable impurities
and Al, wherein the sheet has an intermetallic compound maximum
size of 5 .mu.m or less by circle-equivalent diameter in a region
at 1/4 depth of the sheet thickness, an average recrystallized
grain size of 15 .mu.m or less, a surface roughness of 0.2-0.6
.mu.m Ra, a yield strength of 145 MPa or less, and a tensile
strength of 225 MPa or more.
[0010] According to the present invention, an Al--Mg-based alloy
sheet excellent in formability and shape fixability can be produced
without subjecting a continuous cast and rolled sheet to
stabilization treatment.
BEST MODE FOR CARRYING OUT THE INVENTION
[0011] The reasons why the chemical composition of the alloy has
been limited in the present invention will be described below.
Herein, "%" indicating chemical composition means "% by mass",
unless otherwise specified.
[3.0-3.5% Mg]
[0012] Mg is an element which increases strength by solid solution
strengthening effect. If the Mg content is less than 3.0%, this
effect cannot be exhibited, and tensile strength will be reduced.
If the Mg content exceeds 3.5%, yield strength will be excessively
high to result in reduction of shape fixability.
[0.05-0.3% Fe]
[0013] Fe is crystallized as fine grains of intermetallic compounds
such as an Al--Fe--Si-based compound during casting and functions
as a nucleation site of recrystallization during annealing after
cold rolling. Therefore, the number of recrystallized nuclei to be
produced will be increased with an increase in the number of grains
of these intermetallic compounds, resulting in formation of a large
number of fine recrystallized grains. Moreover, the fine grains of
the intermetallic compounds have an effect of pinning the grain
boundaries of produced recrystallized grains to suppress the growth
of crystal grains by the coalescence thereof to stably maintain the
fine recrystallized grains. For exhibiting this effect, the Fe
content needs to be 0.05% or more. However, if the Fe content
exceeds 0.3%, the intermetallic compounds crystallized tend to be
coarser, which leads to formation of voids with these intermetallic
compounds as starting point during forming, resulting in inferior
formability. Therefore, the Fe content is limited to 0.05 to 0.3%.
A preferred range is from 0.05 to 0.25%.
[0.05-0.15% Si]
[0014] Si is crystallized as fine grains of intermetallic compounds
such as an Al--Fe--Si-based compound during casting and functions
as a nucleation site of recrystallization during annealing after
cold rolling. Therefore, the number of recrystallized nuclei to be
produced will be increased with an increase in the number of grains
of these intermetallic compounds, resulting in formation of a large
number of fine recrystallized grains. Moreover, the fine grains of
the intermetallic compounds have an effect of pinning the grain
boundaries of produced recrystallized grains to suppress the growth
of crystal grains by the coalescence thereof to stably maintain the
fine recrystallized grains. For exhibiting this effect, the Si
content needs to be 0.05% or more. However, if the Si content
exceeds 0.15%, the intermetallic compounds crystallized tend to be
coarser, which leads to formation of voids with these intermetallic
compounds as starting point during forming, resulting in inferior
formability. Therefore, the Si content is limited to 0.05 to 0.15%.
A preferred range is from 0.05 to 0.1%.
[Less than 0.1% Mn]
[0015] When Mn content is 0.1% or more, the solidification cooling
rate during casting is high. This high solidification cooling rate
increases the amount of dissolved Mn in the matrix, which
excessively increases the yield strength of the final sheet to
result in the reduction of shape fixability. Further, the Mn
content is preferably limited to less than 0.08%, more preferably
to less than 0.06%.
[0.001-0.1% Ti as an Optional Component]
[0016] In the present invention, Ti is preferably contained in the
range of 0.001 to 0.1% for refining crystal grains of an ingot. For
exhibiting this effect, the Ti content needs to be 0.001% or more.
However, if the Ti content exceeds 0.1%, coarse intermetallic
compounds such as TiAl.sub.3 will be produced, leading to formation
of voids during forming, which reduces formability. A more
preferred range of the Ti content is from 0.001 to 0.05%. Ti may be
added as a master alloy such as Al-10% Ti or may be added as a
grain-refining agent (rod hardener) such as Al-5% Ti-1% B and
Al-10% Ti-1% B.
[0.0005-0.01% B as an Optional Component]
[0017] In the present invention, B is preferably contained in the
range of 0.0005 to 0.01% for refining crystal grains of an ingot.
When B coexists with Ti, B has the effect of producing nuclei
(TiBx) which serve as starting points for forming .alpha.Al grains
in the melt. A more preferred range of the B content is from 0.0005
to 0.005%. B may be added as a master alloy such as Al-5% B or may
be added as a grain-refining agent (rod hardener) such as Al-5%
Ti-1% B and Al-10% Ti-1% B.
[0018] The process for producing an aluminum alloy sheet according
to the present invention is not limited to the procedures to be
described below. The process includes casting conditions and a
final annealing condition, whose significance and reasons for
limitation will be described below.
[Casting Conditions of the Thin Slab]
[0019] The twin-belt casting process is a continuous casting
process in which a melt is poured between two water-cooled rotating
belts vertically facing each other and cooled from the belt
surfaces to be solidified to form a slab, and the slab is
continuously pulled out from the assembly of the belts opposite to
the side where the melt is poured and wound into a coil.
[0020] In the present invention, the thickness of the slab to be
cast is preferably from 5 to 15 mm. If the thickness of the thin
slab is less than 5 mm, the amount of aluminum passing through the
casting machine per unit time will be too small to cast the slab.
Conversely, if the thickness exceeds 15 mm, the slab cannot be
wound with a roll. Therefore, the thickness of the slab is limited
to the range of 5 to 15 mm. This range of thickness allows a
solidification cooling rate at 1/4 depth of the thickness of the
slab during casting of 20 to 200.degree. C./sec, which allows the
control of the intermetallic compounds maximum size to 5 .mu.m or
less by circle-equivalent diameter.
[Surface Roughness of the Cold Rolling Roll of 0.2-0.7 .mu.m
Ra]
[0021] The surface roughness of the cold rolling roll is limited to
0.2-0.7 .mu.m Ra in order to adjust the surface roughness of the
finally annealed sheet. Since the shape of the roll surface is
transferred to the rolled sheet surface in the cold rolling step,
the surface roughness of the finally annealed sheet is 0.2-0.6
.mu.m Ra. When the surface roughness of the finally annealed sheet
is in the range of 0.2-0.6 .mu.m Ra, the surface shape of the final
sheet will act as a micro pool for uniformly holding a low
viscosity lubricating oil used during forming, thus providing a
sheet excellent in press formability. Note that the surface
roughness of the cold rolling roll is preferably 0.3-0.7 .mu.m Ra,
and in this case, the surface roughness of the finally annealed
sheet is 0.3-0.6 .mu.m Ra. The surface roughness of the cold
rolling roll is more preferably 0.4-0.7 .mu.m Ra, and in this case,
the surface roughness of the finally annealed sheet is 0.4-0.6
.mu.m Ra.
[Intermetallic Compound Maximum Size of 5 .mu.m or Less by
Circle-Equivalent Diameter]
[0022] With respect to the intermetallic compounds in the
metallographic structure in the region at 1/4 depth of the
thickness of the aluminum alloy sheet according to the present
invention, the maximum size by circle-equivalent diameter is
limited to 5 .mu.m or less. Thus, very fine intermetallic compounds
are dispersed in the matrix, so that the movement of dislocation in
the aluminum sheet during forming thereof is suppressed to enhance
the tensile strength thereof by solid solution strengthening effect
by Mg and provide a sheet excellent in formability.
[Average Recrystallized Grain Size of 15 .mu.m or Less]
[0023] The average recrystallized grain size in the region at 1/4
depth of the thickness of the finally annealed sheet is limited to
15 .mu.m or less. If this is exceeded, the level difference
produced in the crystal grain boundaries during the deformation of
material will be excessively large, and the orange peel after
deformation will be remarkable, thus reducing the resistance to
surface roughening.
[The Reason for Limiting the Cold Rolling Reduction to 50-98%]
[0024] The rolling reduction during cold rolling is preferably from
50 to 98%. The dislocation generated by the plastic working by
rolling is accumulated around the above fine crystallized products.
Therefore, the dislocation is necessary to obtain a fine
recrystallized structure during final annealing. If the rolling
reduction during cold rolling is less than 50%, the accumulation of
dislocation will not be enough to obtain a fine recrystallized
structure. If the rolling reduction during cold rolling exceeds
98%, edge cracks during rolling will be remarkable, and the yield
will be reduced. A more preferred cold rolling reduction is in the
range of 55 to 96%.
[Final Annealing Conditions in a Continuous Annealing Furnace]
[0025] The temperature of the final annealing in a continuous
annealing furnace is limited to 400 to 520.degree. C. If the
temperature is less than 400.degree. C., the energy required for
recrystallization will be insufficient. Therefore, a fine
recrystallized structure cannot be obtained. If the holding
temperature exceeds 520.degree. C., the growth of recrystallized
grains will be remarkable, and the average recrystallized grain
size will exceed 15 .mu.m, resulting in reduction of formability
and resistance to surface roughening.
[0026] The holding time of the continuous annealing is preferably
within 5 minutes. If the holding time of the continuous annealing
exceeds 5 minutes, the growth of recrystallized grains will be
remarkable, and the average recrystallized grain size will exceed
15 .mu.m, resulting in reduction of formability and resistance to
surface roughening.
[0027] Regarding the heating rate and cooling rate during the
continuous annealing treatment, the heating rate is preferably
100.degree. C./min or more. If the heating rate during the
continuous annealing treatment is less than 100.degree. C./min, a
fine recrystallized structure will not be obtained and formability
and resistance to surface roughening will be reduced.
[Final Annealing Conditions in a Batch Furnace]
[0028] The temperature of the final annealing in a batch furnace is
limited to 300 to 400.degree. C. If the temperature is less than
300.degree. C., the energy required for recrystallization will be
insufficient. Therefore, a fine recrystallized structure cannot be
obtained. If the holding temperature exceeds 400.degree. C., the
growth of recrystallized grains will be remarkable, and the average
size of recrystallized grains will exceed 15 .mu.m, resulting in
reduction of formability and resistance to surface roughening.
[0029] The holding time of the final annealing in a batch furnace
is not particularly limited, but it is preferably 1 to 8 hours. If
it is less than 1 hour, the coil may not be uniformly heated. If
the holding time exceeds 8 hours, the average size of
recrystallized grains will exceed 15 .mu.m, and formability and
resistance to surface roughening will be reduced.
[Straightening with a Leveler]
[0030] Since the sheet is deformed by thermal strain after the
final annealing, it is subjected to straightening such as
repetitive bending with a leveler roll in the state of a coil or a
sheet to correct the shape and restore the flatness. This
straightening enables the sheet to obtain a predetermined tensile
strength and yield strength, thus providing an aluminum alloy sheet
excellent in formability, resistance to surface roughening and
shape fixability.
Examples
[0031] Hereinafter, Examples according to the present invention
will be described as compared with Comparative Examples. A melt
each having a chemical composition shown in Table 1 (alloy A, B, C,
D, E, F, I) was degassed and settled, and the resulting melt was
then fed to a twin-belt caster to continuously cast a thin slab
having a thickness of 10 mm, which was directly wound into a coil.
Similarly, a melt having the chemical composition shown in Table 1
(alloy G) was degassed and settled, and the resulting melt was then
subjected to DC casting process to cast a slab of 1000 mm
(width).times.500 mm (thickness).times.4000 mm (length). The slab
was subjected to face milling of both surfaces thereof and then
subjected to homogenization of 450.degree. C..times.8 hours in a
soaking furnace followed by hot rolling to produce a hot-rolled
sheet having a thickness of 6 mm, which was wound into a coil.
Similarly, a melt having the chemical composition shown in Table 1
(alloy H) was degassed and settled, and the resulting melt was then
fed to a twin-belt caster to continuously cast a thin slab having a
thickness of 6 mm, which was directly wound into a coil.
TABLE-US-00001 TABLE 1 Chemical composition of alloy Composition
(mass %) Alloy symbol Mg Mn Fe Si A 3.35 0.00 0.2 0.08 B 3.25 0.06
0.2 0.08 C 3.75 0.05 0.2 0.08 D 2.50 0.07 0.2 0.08 E 3.45 0.20 0.2
0.08 F 4.00 0.30 0.2 0.08 G 3.35 0.00 0.2 0.08 H 3.35 0.00 0.2 0.08
I 3.25 0.06 0.2 0.08
[0032] Next, these thin slabs and the hot rolled sheets were cold
rolled with cold rolling rolls which were finished to a
predetermined surface roughness (0.6 .mu.m, 1.0 .mu.m Ra) to form
sheets having a thickness of 1 mm. Then, these sheets were passed
through a CAL to undergo continuous annealing at a holding
temperature of 460.degree. C. Further, the finally annealed sheets
were passed through a leveler to undergo straightening to remove
thermal strain therefrom followed by cutting to obtain test
specimens. Note that Table 2 shows production conditions of the
test specimens in each production step in Examples and Comparative
Examples.
TABLE-US-00002 TABLE 2 Production conditions Cold rolling Cooling
roll surface Alloy Casting process/ rate roughness Ra Thickness
Annealing symbol thickness (mm) (.degree. C./s) Hot rolling (.mu.m)
(mm) temperature Example 1 A Twin belt/10 100 None 0.6 1.0
460.degree. C. Example 2 B Twin belt/10 78 None 0.6 1.0 460.degree.
C. Comparative C Twin belt/10 85 None 0.6 1.0 460.degree. C.
Example 1 Comparative D Twin belt/10 75 None 0.6 1.0 460.degree. C.
Example 2 Comparative E Twin belt/10 76 None 0.6 1.0 460.degree. C.
Example 3 Comparative F Twin belt/10 74 None 0.6 1.0 460.degree. C.
Example 4 Comparative G DC/500 3 6 mm 0.6 1.0 460.degree. C.
Example 5 Comparative H Twin roll/6 300 None 0.6 1.0 460.degree. C.
Example 6 Comparative I Twin belt/10 78 None 1.0 1.0 460.degree. C.
Example 7
[0033] Next, these test specimens were evaluated for the
recrystallized grain size, the intermetallic compound maximum size
by circle-equivalent diameter, surface roughness, 0.2% yield
strength (0.2% YS), tensile strength (UTS), elongation (EL), and
punch stretch height.
[0034] The recrystallized grain size (D) of a test specimen was
measured by an cross-cut method. The test specimen was cut,
embedded in a resin, polished, and subjected to anodic coating in
an aqueous fluoroboric acid solution to apply an anodic oxide film
to the surface of the section of the test specimen. A photograph
(200 times) of grains in the section of the test specimen was taken
with a polarizing microscope. On the photograph, three lines were
drawn both in the vertical direction and in the horizontal
direction. The number (n) of crystal grain boundaries crossing
these lines was counted. The average value (D) of the grain sizes
determined by dividing the total length (L) of the lines by (n-1)
was defined as the average recrystallized grain size of the test
specimen. The intermetallic compound maximum size by
circle-equivalent diameter was measured with an image analyzer
(trade name: LUZEX).
D=L/(n-1)
[0035] The surface roughness of the test specimen was measured
using a surface roughness meter according to JIS B0601, wherein the
direction of measurement was perpendicular to the rolling
direction; the measurement region was 4 mm; and the cutoff was 0.8
mm. The resulting surface roughness was defined as the average
roughness Ra. Note that surface roughness of the roll was measured
in the same manner as in the measurement of the surface roughness
of the test specimen using a surface roughness meter according to
JIS B0601, wherein the direction of measurement was in the
transverse direction of the roll; the measurement region was 4 mm;
and the cutoff was 0.8 mm. The resulting surface roughness was
defined as the average roughness Ra.
[0036] The punch stretch height was measured using the following
die assembly and indicates the critical forming height at
break.
(Punch: 100 mm in diameter, shoulder R: 50 mm, die: 105 mm in
diameter, shoulder R: 4 mm)
[0037] The resistance to surface roughening was evaluated at three
stages (.largecircle.: excellent, .DELTA.: a little poor, X: poor)
by visually observing the surface condition near the broken part of
the test piece after the tensile test.
[0038] The results of Examples and Comparative Examples measured as
described above are shown in Table 3.
TABLE-US-00003 TABLE 3 Evaluation results of properties Evaluation
of Average size of Maximum size Surface Punch resistance
recrystallized of crystallized roughness YS UTS stretch to surface
grains (.mu.m) products (.mu.m) Ra (.mu.m) (Mpa) (Mpa) EL (%)
height (mm) roughening Example 1 12 3.7 0.35 133 234 28 30
.largecircle. Example 2 11 3.5 0.41 134 233 27 30 .largecircle.
Comparative 10 4 0.37 146 248 27 29 .largecircle. Example 1
Comparative 11 4.2 0.42 121 209 26 30 .largecircle. Example 2
Comparative 9 4.1 0.39 148 244 27 29 .largecircle. Example 3
Comparative 8 4.5 0.38 155 265 27 28 .largecircle. Example 4
Comparative 25 15 0.45 120 224 28 27 .DELTA. Example 5 Comparative
54 2 0.35 115 222 26 26 X Example 6 Comparative 13 3.7 0.80 132 232
28 28 .largecircle. Example 7
[0039] In Examples 1 and 2, the Mg content is proper, and in
addition, the Mn content is suppressed to less than 0.1%. As a
result, the test specimens in Examples 1 and 2 are excellent in
shape fixability since they have a yield strength of 145 MPa or
less; they are excellent in resistance to surface roughening since
they have fine recrystallized grains; and they are excellent in
formability to an extent of a punch stretch height of 29 mm or more
since they have fine intermetallic compounds and have a proper
surface roughness of 0.35 and 0.41 .mu.m, respectively.
[0040] On the other hand, in Comparative Example 1, since the Mg
content is as high as 3.75%, the 0.2% yield strength is excessively
increased to result in reduction of shape fixability. In
Comparative Example 2, since the Mg content is as low as 2.5%, both
the tensile strength and elongation are insufficient.
[0041] In Comparative Example 3, the Mg content is proper, but the
Mn content is as high as 0.2%. As a result, the 0.2% yield strength
is excessively increased to result in reduction of shape
fixability.
[0042] In Comparative Example 4, since the Mg content and the Mn
content are as high as 4.0% and 0.3%, respectively, the 0.2% yield
strength is excessively increased to result in reduction of shape
fixability.
[0043] In Comparative Example 5, since the solidification cooling
rate during the slab casting by a DC casting process is low, the
maximum size of the intermetallic compounds is excessively large,
and the recrystallized grain size is also excessively large. As a
result, the tensile strength is reduced, and the resistance to
surface roughening and punch stretch formability are also
reduced.
[0044] In Comparative Example 6, since the solidification cooling
rate of the cast rolled sheet by a twin-roll process is high, the
number of the intermetallic compounds which serve as the nuclei of
recrystallized grains during the final annealing is insufficient,
and the number of the intermetallic compounds having so-called
pinning effect that prevents the motion of the grain boundaries of
recrystallized grains is also insufficient, thereby excessively
increasing the size of the recrystallized grains. As a result, the
tensile strength and elongation are insufficient, and the
resistance to surface roughening and punch stretch formability are
reduced.
[0045] In Comparative Examples 7, the cold rolling roll has a
surface roughness of 1.0 .mu.m Ra, and the test specimen has a
surface roughness of 0.8 .mu.m Ra. As a result, the punch stretch
height is 28 mm, indicating a reduced formability.
* * * * *