US20070217943A1

US20070217943A1 - Al-Mg Alloy Sheet with Excellent Formability at High Temperatures and High Speeds and Method of Production of Same

Info

Publication number: US20070217943A1
Application number: US11/578,908
Authority: US
Inventors: Pizhi Zhao; Kazuhiro Shiozawa
Original assignee: Nippon Light Metal Co Ltd
Current assignee: Nippon Light Metal Co Ltd
Priority date: 2004-04-23
Filing date: 2005-04-15
Publication date: 2007-09-20
Also published as: JP4534573B2; EP1737995A1; TWI310789B; WO2005103313A1; KR20060135849A; TW200540280A; CN100519797C; JP2005307300A; CA2563789A1; CN1946861A

Abstract

To provide an aluminum alloy sheet with excellent formability at high temperatures and high speeds with a reduced amount of cavities after forming and a method of production of the same. An aluminum alloy sheet consisting of 2.0-8.0 wt % of Mg, 0.06-0.2 wt % of Si, 0.1-0.5 wt % of Fe, 0.1-0.5 wt % of Mn, and the balance of Al and unavoidable impurities, wherein a density of inter-metallic compounds having an equivalent circle diameter of 1 to 5 μm is 5000/mm²or more and an average crystal grain size is 20 μm or less. A method of production of an aluminum alloy sheet comprising the steps of casting an alloy melt having the above described composition by a twin belt casting machine at a cooling rate of 20 to 150° C./sec at the location of ¼ of the slab thickness during the 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 at a cold rolling reduction of 70 to 96%, and performing annealing heating the obtained cold rolled sheet at a heating rate of 50° C./sec or more to 420 to 500° C.

Description

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 Al—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 10⁻⁴to 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 with excellent superplastic formability at high speeds and having a reduced amount of cavities after forming characterized in that it contains 3.0-8.0% (wt %, same below) of Mg, 0.21-0.50% of Cu, and 0.001-0.1% of Ti, contains as impurities Fe to 0.06% or less and Si to 0.06% or less, and the balance of Al and impurities and has an average crystal grain size of 20 to 200 μm.
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 Al—Fe—Si, Al₆Mn, etc. become coarse of several tens of μm or more. Even in the final sheet product after the soaking, hot rolling, cold rolling, annealing, etc., coarse intermetallic compounds of 10 μm 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 0.1% 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 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: 2.0-8.0 wt %,
Si: 0.06-0.2 wt %,
Fe: 0.1-0.5 wt %,
Mn: 0.1-0.5 wt %, 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 μm is 5000/mm²or more and an average crystal grain size is 20 μm 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 ¼ 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 96%, 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 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 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: 2.0-8.0%

Mg is an element improving the strength. In order to manifest this effect, it is necessary to set the Mg content to 2.0% or more. However, if the Mg content exceeds 8.0%, the castability of a thin slab is lowered. Accordingly, the Mg content is limited to 2.0 to 8.0%. If stressing the castability, preferably the upper limit of the Mg content is further limited to 6.0% or less.

Si: 0.06-0.2%

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 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 0.06% or more. However, if the Si content exceeds 0.2%, 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 0.2%. The preferred range is 0.07 to 0.15%.
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: 0.1-0.5%

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 0.1% or more. However, if the Fe content exceeds 0.5%, the tendency of 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 0.5%. A preferred range is 0.1 to 0.3%.
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 0.1% or more. However, if the Mn content exceeds 0.5%, 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 0.3%.

Optional Ingredient Cu: 0.1-0.5%

In the present invention, Cu can be added within a range of 0.1-0.5% 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 0.1% or more. However if the amount of addition of Cu exceeds 0.5%, the castability is lowered. When stressing the castability, preferably the upper limit of the amount of addition of Cu is further restricted to 0.3% or less.

Optional Ingredients Zr And Cr: 0.1-0.4%

In the present invention, in order to assist the increased fineness of the recrystallized grains, at least one type of Zr and Cr can be incorporated within a range of 0.1-0.4%. 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 0.1% or more. However, if the amounts of addition exceed 0.4%, 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 0.2% 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 0.001-0.15%. In order to manifest this effect, it is necessary to make the amount of addition of Ti 0.001% or more. However, if the amount of addition of Ti exceeds 0.15%, a coarse compound such as TiAl₃is generated, the formability at a high temperature is deteriorated, and the occurrence of cavities is promoted. A preferred range is 0.006-0.10%.
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 μm of 5000/mm²Or 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 enhanced.
In order to obtain the above effect, the inter-metallic compound having the equivalent circle diameter of 1 to 5 μm must be present in a density of 5000/mm²or 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 μm. If the equivalent circle diameter is less than 1 μm, the particles are too small to manifest the effects of (1) and (2) described above. Conversely, if it exceeds 5 μm, 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/mm²or more.
If the density is less than 5000/mm², the recrystallized grain diameter at the time of the annealing exceeds 20 μm, and the elongation at the time of high temperature deformation is lowered.

Average Crystal Grain Diameter of 20 μm Or Less

In the alloy sheet of the present invention, the average crystal grain diameter is made 20 μm or less. If the average crystal grain diameter exceeds 20 μm, 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 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 μm 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 μm prescribed for the alloy of the present invention with a density of 5000/mm²or more, the cooling rate at the location of ¼ 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 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 μm increase. Conversely, if the cooling rate exceeds 150° C./sec, the inter-metallic compounds become finer and the compounds less than 1 μm increase. In the end, in either case, the density of the inter-metallic compounds having the equivalent circle diameter of 1 to 5 μm becomes less than 5000/mm²and 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 96%

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 70%, 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 cannot be obtained. However, if it exceeds 500° C., the recrystallized grain diameter exceeds 20 μm, 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.

TABLE 1

Alloy Composition

(wt %)

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

TABLE 2


Process and Characteristics

Inter-

Final

Cold

Crystal

met.

Slab

Cooling

Sheet

Inter.

sheet

roll.

grain

comp.

Tensile

Cavity

Sample

thick.

Rate

thick.

anneal

thick.

reduction

size

dens.

temp.

speed

Elong.

ratio

No.

Alloy

(mm)

(° C./sec)

(mm)

(° C.)

(mm)

(%)

(μm)

(/mm²)

(° C.)

(/sec)

(%)

Remarks

1	A	8	75	—	—	1	88	10	6233	500	0.5	231	0.23	Inv.
2	B	9	73	—	—	1	89	7	7501	500	0.5	252	0.27	Inv.
3	C	7	78	—	—	1	86	8	6145	500	0.5	270	0.19	Inv.
4	D	8	75	—	—	1	88	9	6345	500	0.5	243	0.24	Inv.
5	E	8	75	—	—	1	88	7	6433	500	0.5	255	0.25	Inv.
6	C	7	78	—	—	1	86	8	6145	450	0.5	250	0.17	Inv.
7	C	7	78	—	—	1	86	8	6145	500	0.25	201	0.15	Inv.
8	A	5	300	—	—	1	88	68	2574	500	0.5	80	0.12	Comp.
														TRC
9	A	400	5	Hot rolled	—	1	86	25	2890	500	0.5	160	1.5	Comp.
				sheet thick.:										DC
				7 mm
10	A	8	75	2	350	1	50	23	6844	500	0.5	101	0.54	Comp.
11	A	8	75	—	—	1	88	10	6233	350	0.5	89	0.24	Comp.
12	A	8	75	—	—	1	88	10	6233	500	0.01	138	1.8	Comp.

Note)
The recrystallized grains were measured by the cross-cut method.
The cooling rate was calculated from DAS measurement results at ¼ thickness of cast slab.

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 0.1% or more and the Si contents were 0.06% or more in all samples, had densities of the inter-metallic compounds having equivalent circle diameters of 1 to 5 μm of 5000/mm²or more and crystal grain sizes of 20 μm or less. For this reason, the elongations at the tensile temperature of 500° C. were good ones of 200% or more and also the cavity ratios after the high temperature tension were good ones of the range of 0.15-0.27% 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 μm 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 μm in the final sheet became less than 5000/mm²or coarse exceeding the crystal grain size of 20 μm or more. For this reason, the cavity ratio after the high temperature tension was a relatively low good one of 0.12%, but the elongation at the tensile temperature of 500° C. was a poor 80%.
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 μm were generated, therefore the density of intermetallic compounds having an equivalent circle diameter of 1 to 5 μm in the final sheet became less than 5000/mm², and the crystal grains became slightly coarse exceeding 20 μm. For this reason, the cavity ratio after the high temperature tensile test was a poor high 1.5%, while the elongation at the tensile temperature of 500° C. was a poor 160%.
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 μm in the final sheet of 5000/mm²or more, but the cold rolling reduction before the final annealing was a low one of less than 70%, therefore the crystal grains became slightly coarse exceeding the crystal grain size of 20 μm. The elongation at the tensile temperature of 500° C. was a poor one of less than 200%.
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 μm in the final sheet of 5000/mm²or more and a crystal grain size of 20 μm 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 200%.
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 μm in the final sheet of 5000/mm²or more and a crystal grain size of 20 μm 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 1.8% and the elongation at the tensile temperature of 500° C. was a poor one of less than 200%.

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.

Claims

1. 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: 2.0-8.0 wt %,

Si: 0.06-0.2 wt %,

Fe: 0.1-0.5 wt %,

Mn: 0.1-0.5 wt %, 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 mm is 5000/mm2 or more and an average crystal grain size is 20 mm or less.

2. An aluminum alloy sheet as set forth in claim 1, characterized by further containing Cu: 0.1 to 0.5 wt %.

3. An aluminum alloy sheet as set forth in claim 1, characterized by further containing at least one of Zr: 0.1 to 0.4 wt % and Cr: 0.1 to 0.4 wt %.

4. An aluminum alloy sheet as set forth in claim 1, characterized in that an elongation during tensile deformation at a strain rate of 0.1 to 1.0/sec in a temperature region of 400 to 550° C. is at least 200%.

5. An aluminum alloy sheet as set forth in claim 4, characterized in that a cavity ratio in a cross-section after breakage due to the tensile deformation is not more than 1%.

6. A method of production of an aluminum alloy sheet with excellent formability at high temperatures and high speeds with a reduced amount of cavities after forming as set forth in any one of claim 1, characterized in that said method comprises the steps of:

preparing an alloy melt having a composition as set forth in any one of claims 1 to 3,

casting the alloy melt by a twin belt casting machine at a cooling rate of 20 to 150° C./sec at the location of ¼ 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 96%, and

performing annealing for heating the obtained cold rolled sheet at a heating rate of 5° C./sec or more to 420 to 500° C.