EP0690142A1

EP0690142A1 - Aluminium alloy sheet for auto body sheet, method for manufacturing same and method for forming same

Info

Publication number: EP0690142A1
Application number: EP95401319A
Authority: EP
Inventors: Tetsushi Habu; Yoichiro Bekki; Kunihiro C/O K.K. Honda Yasunaga; Noboru C/O K.K. Honda Hayashi
Original assignee: Honda Motor Co Ltd; Furukawa Electric Co Ltd
Current assignee: Honda Motor Co Ltd; Furukawa Electric Co Ltd
Priority date: 1994-06-09
Filing date: 1995-06-07
Publication date: 1996-01-03

Abstract

The sheet is provided as a material subjected to mill finish with an ordinary reduction roll and is equal in formability under low - viscosity lubrication condition to that of a conventional shot dull finish material. Specifically, there is provided 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 a surface layer in a former step is 70 to 300 µm in recrystallized grain size, the 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). A method for manufacturing such an aluminum alloy sheet and a method for forming such an aluminum alloy sheet are also disclosed.

Description

BACKGROUND OF THE INVENTION

Field of the Invention:

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:

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.
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.
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.
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

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).
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).
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).
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).
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.
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.
Next will be described why the composition of the aluminum alloy employed in the present invention is limited as described above.
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.
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.
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.
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).
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.
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.
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.
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.
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.
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.
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.
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.
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.
With the constitution of the sheet as described above, it is possible to form the sheet similarly to the conventional shot dull finish material.
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.
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.
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.
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.
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.
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.
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.
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.
Incidentally, the cooling rate is preferably 3.0°C/sec or above from the viewpoint of the prevention of occurrence of Luders band.
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.
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.
Incidentally, the equivalent strain in this case is expressed by the following formula. $equivalent strain = \sqrt{\frac{2}{3} {(ε}_{1}^{2} {+ ε}_{2}^{2} {+ ε}_{3}^{2})}$
ε₁ : true strain in length direction
ε₂ : true strain in width direction
ε₃ : true strain in thickness direction

BRIEF DESCRIPTION OF THE DRAWINGS

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

Hereinafter will be described preferred embodiments of the present invention in comparison with comparative examples and conventional examples.
Table 1 shows an aluminum alloy composition used in the embodiments, comparative examples and conventional examples.
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.
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.
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.
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.

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

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.
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.
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.
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.
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.

Claims

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, characterized in that:
a surface layer in a former step is 70 to 300 µm in recrystallized grain size;
the 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).
A method for manufacturing an aluminum alloy sheet for an auto body sheet, comprising the steps of:
hot-rolling an aluminum alloy slab having the composition as defined in claim 1;
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 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 to hold at 450 to 550°C for 120 sec or less;
wherein a surface layer of the intermediate-annealed sheet is 70 to 300 µm in recrystallized grain size;
the 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).
A method for manufacturing an aluminum alloy sheet for an auto body sheet, comprising the steps of:
hot-rolling an aluminum alloy slab having the composition as defined in claim 1 on conditions that a 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 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 to hold at 450 to 550°C for 120 sec or less;
wherein a surface layer of the intermediate-annealed sheet is 70 to 300 µm in recrystallized grain size;
the surface layer of 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).
A method for manufacturing an aluminum alloy sheet for an auto body sheet, comprising the steps of:
hot-rolling an aluminum alloy slab having the composition as defined in claim 1 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 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°/sec or above to hold at 450 to 550°C for 120 sec or less;
wherein a surface layer in the hot-rolled sheet is 70 to 300 µm in recrystallized grain size;
the surface layer of 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).
A method for forming an aluminum alloy sheet for an auto body sheet, comprising the step of forming the aluminum alloy sheet as defined in claim 1 with a low-viscosity lubricant having viscosity of 20 cSt or less on conditions that a maximum equivalent strain value of a sliding part reaches 0.06 or above .