BACKGROUND OF THE INVENTION
Field of the Invention:
[0001] This invention relates to an aluminum alloy sheet suitable for a material to form
an auto body sheet or the like, a method for manufacturing the same and a method for
forming the same, and more particularly, to an aluminum alloy sheet, which can be
formed under low-viscosity lubrication condition only by subjecting a sheet surface
to mill finish with an ordinary reduction roll without any special shot dull finish,
a method for manufacturing such an aluminum alloy sheet and a method for forming such
an aluminum alloy sheet.
Description of the Prior Art:
[0002] Conventionally, cold rolled steel sheets have been mainly used for auto body sheets.
However, use of Al-Mg, Al-Mg-Si or like aluminum alloy sheets have been recently examined
upon demand for an auto body to make more lightweight. The auto body sheets have been
generally formed by using a low-viscosity lubricant to facilitate subsequent degreasing
and cleaning steps. It is well known that a so-called shot dull finish material (surface
roughness Ra is approximately in the range of 0.6 to 1.0) is excellent in formability
under low-viscosity lubrication condition. The surface of the shot dull finish material
is roughened for micro pools which are adapted to hold a lubricant on the sheet surface
in contact with a mold because of a coarse sheet surface configuration.
[0003] Incidentally, the surface of a reduction roll is roughened by hitting sand, steel
balls or the like against the roll surface or patterning the roll surface with fine
irregularities by use of a laser or the like (such a roughened surface is called a
shot surface), and the roll surface pattern is transferred to the sheet by rolling
to provide a material having a roughened surface. This material is termed a shot dull
finish material.
[0004] In case of applying the above shot dull finish material to an aluminum rolled sheet,
the roll surface pattern should be transferred to the sheet surface by subjecting
the sheet to skin pass rolling with a roll having a shot surface in a final cold rolling
pass. Therefore, the manufacture of the conventional shot dull finish material requires
one extra pass, and the shot working on the roll is highly expensive. In addition,
it is necessary to hold the roll at all times, and a frequent roll exchange is required
during rolling. Accordingly, there is caused a problem in an increase of cost.
[0005] An object of the present invention is to overcome the above problems, and more specifically,
to provide an aluminum alloy sheet, which permits improvement in formability by forming
micro pools on the sheet surface at a forming process under lubrication condition
using a low-viscosity lubricant of 20 or less cSt in case of mill finish with an ordinary
reduction roll, a method for manufacturing such an aluminum alloy sheet and a method
for forming such an aluminum alloy sheet.
SUMMARY OF THE INVENTION
[0006] In order to overcome the above problems, the first invention relates to an aluminum
alloy sheet used for an auto body sheet and containing 2.0 to 8.0 wt.% of Mg, one
or two or more elements selected from a group consisting of 1.5 wt.% or less of Fe,
1.0 wt.% or less of Mn, 0.3 wt.% or less of Cr and 0.3 wt.% or less of Zr, and the
remainder consisting of inevitable impurities and aluminum, wherein the surface layer
in a former step is 70 to 300 µm in recrystallized grain size, the finally-annealed
surface layer in a final finish sheet is 10 to 50 µm in recrystallized grain size
and 0.5 µm or less in surface roughness Ra (Ra ≦ 0.5 µm), and the surface roughness
Ra in case of applying 10% of stretch is set to be 0.8 µm or above (Ra ≧ 0.8 µm).
[0007] The second invention relates to a method for manufacturing the above aluminum alloy
sheet used for an auto body sheet, comprising the steps of hot-rolling an aluminum
alloy slab having the above composition, subsequently cold-rolling the hot-rolled
sheet at a rolling reduction of 20 % or less, then subjecting the cold-rolled sheet
to intermediate annealing at 320 to 550°C, then cold-rolling the annealed sheet as
final-cold rolling with a roll having a roll roughness Ra of 0.5 µm or less (Ra ≦
0.5 µm), and subjecting the resultant sheet to final annealing at a heating rate of
3.0°C/sec or above so as to hold at 450 to 550°C for 120 sec or less, wherein the
intermediate-annealed surface layer is 70 to 300 µm in recrystallized grain size,
the finally-annealed surface layer in a final finish sheet is 10 to 50 µm in recrystallized
grain size and 0.5 µm or less in surface roughness Ra (Ra ≦ 0.5 µm), and the surface
roughness Ra in case of applying 10 % of stretch is set to be 0.8 µm or above (Ra
≧ 0.8 µm).
[0008] The third invention relates to another method for manufacturing the above aluminum
alloy sheet for an auto body sheet, comprising the steps of hot-rolling an aluminum
alloy slab having the above composition on condition that the hot-rolling end temperature
reaches 250 to 300°C, subsequently annealing the hot-rolled sheet at 320 to 550°C,
then cold-rolling the annealed sheet as final-cold rolling with a roll having a roll
roughness Ra of 0.5 µm or less (Ra ≦ 0.5 µm), and subjecting the resultant sheet to
final annealing at a heating rate of 3.0°C/sec or above so as to hold at 450 to 550°C
for 120 sec or less, wherein the intermediate-annealed surface layer is 70 to 300
µm in recrystallized grain size, the finally-annealed surface layer in a final finish
sheet is 10 to 50 µm in recrystallized grain size and 0.5 µm or less in surface roughness
Ra (Ra ≦ 0.5 µm), and the surface roughness Ra in case of applying 10% of stretch
is set to be 0.8 µm or above (Ra ≧ 0.8 µm).
[0009] The fourth present invention relates to a further method for manufacturing the above
aluminum alloy sheet for an auto body sheet, comprising the steps of hot-rolling an
aluminum alloy slab having the above composition on conditions of hot-rolling end
pass that a rolling reduction is set to be 20 % or less, and a hot-rolling end temperature
reaches 400°C or above, subsequently cold-rolling the hot-rolled sheet as final-cold
rolling with a roll having a roll roughness Ra of 0.5 µm or less (Ra ≦ 0.5 µm), and
then subjecting the resultant sheet to final annealing at a heating rate of 3.0°C/sec
or above so as to hold at 450 to 550°C for 120 sec or less, wherein the hot-rolled
surface layer is 70 to 300 µm in recrystallized grain size, the finally-annealed surface
layer in a final finish sheet is 10 to 50 µm in recrystallized grain size and 0.5
µm or less in surface roughness Ra (Ra ≦ 0.5 µm), and the surface roughness Ra in
case of applying 10% of stretch is set to be 0.8 µm or above (Ra ≧ 0.8 µm).
[0010] The fifth present invention relates to a method for forming the aluminum alloy sheet
for an auto body sheet according to claim 1, comprising the step of forming the aluminum
alloy sheet with a low-viscosity lubricant having viscosity of 20 cSt or less on condition
that a maximum equivalent strain value of a sliding part reaches 0.06 or above.
[0011] The aluminum alloy sheet (the above first invention) of the present invention can
be manufactured through some special steps (the above second to fourth inventions)
such as to adjust the recrystallized grain size of the surface layer in the former
step (after intermediate annealing or hot rolling) and that of the surface layer in
the final finish sheet. In addition, at the time of deforming, particularly, drawing
the material, intergranular steps are caused for grains to roughen the sheet surface.
Thus, even the surface of a mill finish material can be roughened similarly to that
of a shot dull finish material, and the intergranular steps serve as micro pools to
permit improvement in formability under low-viscosity lubrication condition.
[0012] Next will be described why the composition of the aluminum alloy employed in the
present invention is limited as described above.
[0013] Mg permits an increase of strength through solid solution and also permits an increase
of work hardenability to increase ductility, thus resulting in contribution to the
improvement in formability. The amount of Mg to be added is limited to 2.0 to 8.0
wt. % for the following reasons. If the amount of Mg to be added is less than 2.0
wt. %, it is of little effect. On the other hand, when the amount exceeds 8.0 wt.
%, hot workability is degraded to result in an increase in cost for manufacture.
[0014] Elements of Fe, Mn, Cr, Zr or the like are effective in increasing the strength.
However, if the amounts of Fe, Mn, Cr and Zr to be added respectively exceed 1.5 wt.
%, 1.0 wt. %, 0.3 wt. % and 0.3 wt. %, the ductility is lowered.
[0015] Cu, Si and Ti or the like are contained as impurities. As long as the contents of
Cu, Si and Ti are respectively 0.5 wt. % or less, 0.2 wt. % or less and 0.2 wt. %
or less, these elements do not hinder the effects of the present invention.
[0016] Next will be described with respect to the aluminum alloy sheet as the first invention
why the surface layer in the former step is 70 to 300 µm in recrystallized grain size,
the finally-annealed surface layer in the final finish sheet is 10 to 50 µm in recrystallized
grain size and 0.5 µm or less in surface roughness Ra (Ra ≦ 0.5 µm), and the surface
roughness Ra in case of applying 10 % of stretch is set to be 0.8 µm or above (Ra
≧ 0.8 µm).
[0017] In the sheet of the present invention, even if the recrystallization is caused in
final annealing, the recrystallized texture in the former step, i.e., the intermediate-stage
step among the whole rolling steps, specifically the intermediate annealing or hot
rolling step still affects the final finish sheet. When such a sheet is formed, the
intergranular steps are caused depending on a difference in orientation of recrystallized
grains in the former step, in addition to a difference in orientation of final recrystallized
grains, thus resulting in an increase of roughness. Therefore, since a micro pool
is formed on a roughened recess to hold a low-viscosity lubricant, the formability
can be improved.
[0018] A description will be given of the relation among recrystallized grains in the former
step, recrystallized grains in the final finish sheet and the intergranular steps
with reference to drawings for easy understanding about the fact described above.
[0019] Fig. 1(a) shows coarse recrystallized grains (I.G) in the former step, and Fig. 1(b)
shows the coarse recrystallized grains elongated in the rolling direction by cold
rolling. Fig. 1(c) shows the state, in which the cold-rolled texture of Fig. 1(b)
is subjected to final annealing to produce finely final-recrystallized grains (F.G)
in the recrystallized grains (I.G) in the former step.
[0020] Fig. 2 is an enlarged-scale sectional view of a surface layer portion showing the
state of intergranular steps caused when an aluminum alloy sheet having the texture
shown in Fig. 1(c) is formed. In the drawing, a portion A shows the intergranular
steps caused by the coarse recrystallized grains I.G, and a portion B shows the intergranular
steps caused by the coarse recrystallized grains F.G. In case of forming, the intergranular
steps at the portion A serve as micro pools.
[0021] The recrystallized grain size in the former step is limited to 70 to 300 µm for the
following reasons. If the size is less than 70 µm, the above effects can not be sufficiently
obtained. On the other hand, if the size exceeds 300 µm, orange peel is caused to
pose a problem in external appearance of the finish sheet, even though the sufficient
effects can be obtained. The preferable range of the size is 70 to 150 µm.
[0022] The recrystallized grain size of the finally-annealed sheet is limited to 10 to 50
µm for the following reasons. If the size is less than 10 µm, Luders band is remarkably
caused during forming. On the other hand, if the size exceeds 50 µm, the orange peel
is caused depending on a use to pose a problem in surface quality. The preferable
range of the size is 20 to 50 µm.
[0023] Incidentally, since the present invention closely relates to the grain size of the
surface layer, the recrystallized grain size of the surface layer is adjusted in the
former step and in the final finish sheet.
[0024] Further, according to the present invention, the surface roughness Ra as the center
average roughness is set to be 0.5 µm or less (Ra ≦ 0.5 µm) so as to discriminate
between the surface state of the sheet as a mill finish material according to the
present invention and that of a shot dull finish material. Incidentally, the surface
roughness of the sheet prior to the forming is approximately equal to that of a normal
rolled material.
[0025] The extent of the intergranular steps caused with deformation is regulated on condition
that the surface roughness Ra in case of applying 10 % of stretch is set to be 0.8
µm or above (Ra ≧ 0.8 µm). If Ra is less than 0.8 µm in the state of a roughened surface
when applying 10 % of stretch, the size of resultant micro pool is too small to obtain
sufficient effects on improvement in formability.
[0026] With the constitution of the sheet as described above, it is possible to form the
sheet similarly to the conventional shot dull finish material.
[0027] Next will be described why the manufacturing conditions are determined as described
above in the method (the second to fourth inventions) for manufacturing the aluminum
alloy sheet according to the present invention.
[0028] The aluminum alloy sheet of the present invention is manufactured on normal casting
and solid-solution conditions and using a normal roll for cold rolling.
[0029] The method for manufacture in the second invention comprises the steps of hot-rolling
the aluminum alloy slab at 350 to 450°C according to a normal process, subsequently
cold-rolling the hot-rolled sheet at a rolling reduction of 20 % or less, and then
subjecting the cold-rolled sheet to intermediate annealing at 320 to 550°C, wherein
the recrystallized grain size of the intermediate-annealed surface layer is adjusted
to 70 to 300 µm. These recrystallized grains are contained in the surface layer of
the sheet as a final product together with the grains having the final grain size
of 10 to 50 µm, and the individual grains cause the intergranular steps on the sheet
surface at the time of deformation.
[0030] The intermediate annealing temperature is set to be 320 to 550°C for the following
reasons. If this temperature is less than 320°C, no recrystallization is caused. On
the other hand, if this temperature exceeds 550°C, it is liable to occur melting.
[0031] In the method of manufacture according to the third invention, when the hot rolling
end temperature is in the range of 250 to 300°C, no recrystallization is caused during
hot rolling. In this case, accumulated dislocation caused by hot rolling remains,
and the recrystallization is caused by applying the intermediate annealing at 320
to 550°C.
[0032] In the method for manufacture according to the fourth invention, a hot rolling end
pass is performed on conditions that the rolling reduction is set to be 20 % or less
and the hot-rolling end temperature reaches 400°C or above. Under these conditions,
the recrystallization is caused by the self annealing effect during hot rolling, and
therefore, no intermediate annealing is required. In this case, the conditions of
the hot rolling end pass are regulated such that the rolling reduction is set to be
20% or less and the hot-rolling end temperature reaches 400°C or above for the following
reasons. If the rolling reduction exceeds 20 %, the surface is not sufficiently roughened
due to excessive recrystallizing force. On the other hand, if the hot-rolling end
temperature is less than 400°C, it is less liable to cause the recrystallization by
self annealing.
[0033] Then, the recrystallized texture obtained by the method of manufacture according
to the second to fourth inventions as described above is subjected to cold rolling.
The cold rolling is performed as the final cold rolling using a normal roll having
a roughness Ra of 0.5 µm or less (Ra ≦ 0.5 µm). Subsequently, the final annealing
is performed at a heating rate of 3.0/sec or above so as to hold at 450 to 550°C for
120 sec or less.
[0034] When the final annealing is performed at the heating rate of 3.0/sec or above to
hold at 450 to 550°C for 120 sec or less, a high heating rate is obtained and the
grains are oriented at random to cause larger intergranular steps depending on the
difference in the slip direction of each grain at the time of deformation of the material.
[0035] Incidentally, the cooling rate is preferably 3.0°C/sec or above from the viewpoint
of the prevention of occurrence of Luders band.
[0036] The fifth invention relates to a method for forming the aluminum alloy sheet manufactured
as described above. The use of the low-viscosity lubricant having the viscosity of
20cSt or less is regulated in forming for the following reasons. If the viscosity
of lubricant exceeds 20cSt, the removal of lubricant after the forming of the sheet
is made difficult in the subsequent degreasing and cleaning steps.
[0037] Further, the forming conditions are determined such that the maximum equivalent strain
value of the sliding part reaches 0.06 or above for the following reasons. When the
sliding part of a mold product is only deformed to be less than 0.06 in equivalent
strain, the unsatisfactory intergranular steps are caused to fulfill no effect on
improvement in formability. Specifically, in case of forming at a draw ratio exceeding
2.4, for instance, the deformation of a flange up to breakage is as small as approximately
0.04 in equivalent strain. As a result, even with the use of the material of the present
invention, an increase of roughness is small. Thus, it is not possible to attain formability
largely exceeding that of the conventional mill finish material. Accordingly, the
sheet in this case is inferior to the shot dull finish material.
[0038] Incidentally, the equivalent strain in this case is expressed by the following formula.

ε₁ : true strain in length direction
ε₂ : true strain in width direction
ε₃ : true strain in thickness direction
BRIEF DESCRIPTION OF THE DRAWINGS
[0039]
Fig. 1 is a view for explaining a metal texture of a surface layer portion according
to the present invention, in which Fig. 1(a) shows coarse recrystallized grains in
the former step, Fig. 1(b) shows the coarse recrystallized grains elongated in the
rolling direction by cold rolling, and Fig. 1(c) shows the state of recrystallized
grains of a final sheet in case of subjecting the cold-rolled sheet to final annealing;
and
Fig. 2 is an enlarged-scale sectional view of a surface layer portion showing the
state of intergranular steps caused when an aluminum alloy sheet having the texture
shown in Fig. 1(c) is formed.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0040] Hereinafter will be described preferred embodiments of the present invention in comparison
with comparative examples and conventional examples.
[0041] Table 1 shows an aluminum alloy composition used in the embodiments, comparative
examples and conventional examples.
[0042] An aluminum alloy having the composition shown in Table 1 was homogenized according
to a normal process, and subsequently subjected to hot rolling, cold rolling (with
a roll having Ra of 0.4 µm in final cold rolling), intermediate annealing and final
annealing according to manufacturing conditions shown in Tables 2 and 3 to provide
a sheet material having a thickness of 1 mm.
[0043] Incidentally, Nos. 1 to 9 in Table 2 show aluminum alloy sheets (Claim 1) as embodiments
of the present invention manufactured according to a method (Claims 2 to 4). Nos.
10 to 18 in Table 3 show aluminum alloy sheets as comparative examples manufactured
according to the conditions without the range of the method of the present invention,
and Nos. 19 and 20 show shot dull finish sheets as conventional examples.
[0044] With respect to the above sheet materials, the recrystallized grain size in the former
step, the grain size in the final finish sheet, Ra of a blank sheet and Ra of a sheet
after applying 10 % of stretch were measured. The results are shown in Tables 2 and
3. The measurement of Ra was done in the direction normal to the rolling direction
of the sheet.
[0045] A deep drawing test was made as a forming test to measure the drawable height. In
this case, cylindrical deep drawing at a draw ratio of 1.94 with a punch having a
diameter of 33mm was done on conditions that force against wrinkling is set to be
1000 Kgf and a low-viscosity anti-rust lubricant having the viscosity of 5 cSt is
used. In this case, the equivalent strain in an R portion of die serving as a sliding
part was 0.07. These forming conditions correspond to those of the invention as defined
in Claim 5.
[0046] Further, the external appearance of the formed sheet was evaluated as follows. ⓞ
represents "satisfactory", ○ represents "no defective", Δ represents "somewhat defective",
and × represents "Luders or orange peel". These results are shown in Tables 2 and
3.
TABLE I
COMPOSITION No. |
CHEMICAL COMPONENTS (wt %) |
|
Mg |
Fe |
Mn |
Cr |
Ti |
Zr |
Cu |
Si |
Al |
A |
2.5 |
0.25 |
0.07 |
0.20 |
0.05 |
- |
0.04 |
0.10 |
REMAINDER |
B |
4.5 |
0.13 |
0.22 |
0.01 |
0.06 |
- |
0.03 |
0.16 |
REMAINDER |
C |
4.5 |
0.13 |
0.07 |
0.03 |
0.02 |
0.02 |
0.28 |
0.11 |
REMAINDER |
D |
4.5 |
0.08 |
0.07 |
0.01 |
0.04 |
- |
0.05 |
0.06 |
REMAINDER |
E |
4.5 |
0.35 |
0.22 |
0.01 |
0.06 |
- |
0.03 |
0.16 |
REMAINDER |
F |
4.5 |
1.03 |
0.22 |
0.01 |
0.06 |
- |
0.03 |
0.16 |
REMAINDER |
G |
5.0 |
0.35 |
0.22 |
0.01 |
0.06 |
- |
0.03 |
0.16 |
REMAINDER |
H |
5.9 |
0.03 |
- |
- |
- |
- |
0.02 |
0.04 |
REMAINDER |
I |
7.8 |
0.06 |
- |
0.04 |
0.01 |
- |
0.01 |
0.02 |
REMAINDER |

[0047] As is apparent from Tables 1, 2 and 3, the aluminum alloy sheets shown in Nos. 1,
2, 4, 5 and 6 manufactured on conditions corresponding to Claims 1 and 2, those shown
in Nos. 3, 7 and 8 manufactured on conditions corresponding to Claims 1 and 3 and
that shown in No. 9 manufactured on conditions corresponding to Claims 1 and 4 are
excellent in formability, and the external appearance of each formed sheet is satisfactory.
[0048] On the other hand, with respect to the aluminum alloy sheet shown in No. 10 as the
comparative example, the conditions in the former step for recrystallization are outside
the range of the present invention. Thus, in case of forming such an aluminum alloy
sheet with the same alloy composition as those described above, since the recrystallized
grain size is small in the former step, Ra after applying 10% of stretch is small
and the deep drawing height is small as well. With respect to the aluminum alloy sheet
shown in No. 17, the conditions in the former step are within the range of the present
invention. However, since the final annealing conditions are outside the range of
the present invention, Ra after applying 10% of stretch is small, and the external
appearance of the formed sheet is unsatisfactory. With respect to the aluminum alloy
sheet shown in No. 18, the conditions in the former step and the final annealing conditions
are both outside the range of the present invention, and therefore, the large final
grain size is obtained to cause the orange peel for the sheet after forming.
[0049] With respect to the aluminum alloy sheets shown in Nos. 11 to 15 and 16, the conditions
in the former step are outside the range of the present invention. Thus, even though
the final annealing is performed, one of the deep drawing height and the external
appearance of the formed sheet is inferior.
[0050] Accordingly, it is found that the aluminum alloy sheet of the present invention is
equal in formability to and more excellent in external appearance of the formed sheet
than those of the shot dull finish sheets shown in Nos. 19 and 20 as the conventional
examples.
[0051] As has been described above, the aluminum alloy sheet of the present invention has
the formability as high as that of the conventional shot dull finish sheet, and is
more excellent in external appearance of the formed sheet than that of the conventional
shot dull finish sheet. In addition, it is possible to supply a low-cost material,
and the remarkable effects on industry can be expected.