CN100366761C

CN100366761C - Warm rolling method

Info

Publication number: CN100366761C
Application number: CNB2004800136487A
Authority: CN
Inventors: 鸟塚史郎; 村松荣次郎; 井上忠信; 长井寿
Original assignee: National Institute for Materials Science
Current assignee: National Institute for Materials Science
Priority date: 2003-05-20
Filing date: 2004-05-20
Publication date: 2008-02-06
Anticipated expiration: 2024-05-20
Also published as: KR100749381B1; CN1791688A; EP1642988A1; JP4221497B2; KR20060035603A; US20060191613A1; TWI266659B; EP1642988B1; TW200510084A; JP2004346420A; EP1642988A4; WO2004104235A1

Abstract

A multi-directional warm-rolling method for manufacturing a superfine grain steel material with a superfine crystal grain structure of 3 mum or smaller in average grain size. When rolling of two passes or more is performed for a steel material in the rolling temperature range of 350 to 800 1/2 C, a rolling by an oval shape hole die and a rolling by the other shape hole die are performed at least one time so that a large amount of strain can be introduced into the material by a simple means with less section reduction rate and less number of passes. Steel materials having the superfine crystal grain structure and excellent strength and ductility can be manufactured by this method.

Description

Warm rolling method

Technical Field

The present invention relates to a novel warm rolling method for producing a superfine-grained steel material having a superfine grain structure with a grain size of 3 μm or less and excellent in strength and plasticity.

Background

Since the ultrafine grained steel can significantly increase the strength without adding alloying elements and can significantly lower the plastic-brittle transition temperature, the present inventors have studied the method of warm rolling (document 1) and the method of multidirectional working (document 2) in order to industrially realize the ultrafine grained steel, and have invented a method of warm rolling (document 1) and a method of cold rolling (document 2) in multiple passes (パス).

If multidirectional warm rolling can be facilitated, wider use of ultrafine grain steel can be achieved, however, the present inventors found that this is not so easy in the course of research.

One technical difficulty is that more than a certain degree of strain needs to be created in the material. For example, the critical strain is 1.5 to 2.3, preferably about 3, but when the strain is 3, the reduction rate of the fracture surface is 95%, and large deformation processing is required. When a round bar having a diameter of 10mm is to be obtained as a final product, warm working from 45mm is required, and in order to generate such large strain in a warm temperature region having large deformation resistance, the billet must be increased in size, and the number of passes is increased in any case.

Therefore, if a large strain can be generated in the material with a smaller reduction ratio of the cross section or the number of passes, the ultrafine structure can be obtained more easily, and there are many advantages such as improvement of the rolling efficiency from an industrial point of view.

Heretofore, the inventors have proposed a method of multi-directional rolling in an anvil (document 2) and a 2-directional reduction rolling technique. However, the multidirectional processing is a method capable of efficiently generating a large strain, but the processing from at least 2 directions involves some technical difficulties.

Document 1: japanese patent laid-open No. 2000-309850

Document 2: japanese patent laid-open No. 2001-240912

Disclosure of Invention

The present invention has been made under the above circumstances, and an object of the present invention is to further develop the findings obtained by the previous studies by the present inventors, to provide a novel multidirectional warm rolling method capable of generating large strain in a material at a smaller reduction ratio of a cross section or the number of passes by a simpler means, and to provide a method for producing a steel material having an ultrafine grain structure obtained thereby and being excellent in strength and plasticity.

The present invention has been made to solve the above problems, and the 1 st aspect of the present invention is to provide a warm rolling method for producing an ultra-fine grain steel material having an ultra-fine grain structure with an average grain diameter of 3 μm or less; the method is characterized in that: when rolling is performed in 2 or more passes in a temperature range of 350 to 800 ℃ relative to a steel material, at least 1 pass of rolling with an oval pass and rolling with passes of other shapes are performed, and when rolling is performed with an oval pass, the maximum minor axis length of the steel material after rolling is 75% or less of the length of the opposite side of the billet before rolling, and plastic strain of 1.5 or more is generated in at least 50% by volume of the area inside the steel material by the oval pass rolling and the rolling with passes of other shapes. The second step of providing a warm rolling method, the warm rolling method being characterized by: rolling of the oval pass is followed by rolling of the pass of other shapes.

Further, regarding the above method, the 3 rd aspect provides a warm rolling method characterized by: the hole patterns with other shapes are square or round hole patterns.

The fifth aspect of the present invention provides a warm rolling method, which is based on any one of the warm rolling methods described above, and further has the following features: in the times of main track: in N, when N is more than 2, rolling of oval hole patterns with the maximum number of times of N/2 or more is carried out; the warm rolling method is characterized in that, on the basis of any one of the warm rolling methods, the warm rolling method comprises the following steps: carrying out continuous 2-pass rolling; the 6 th, provide a warm rolling method, the warm rolling method has such characteristic: in 2-pass rolling with pass shapes of ellipse and square, the reduction rate of the cross section of the square pass rolled from the blank is more than or equal to 20 percent; and 7, providing a warm rolling method, wherein the warm rolling method is also characterized in that: in the rolling of a combination of 2 passes of rolling in which the pass shape is oval and square, the reduction in cross section is 40% or more for the 2 passes of rolling and 60% or more for the 3 passes of rolling.

Further, according to the present invention, in addition to any one of the warm rolling methods described above, there is provided a warm rolling method characterized by: comprises a rolling step in which the maximum minor axis length of the material after rolling with the oval pass is 75% or less of the length of the opposite sides of the billet before rolling with the oval pass; a warm rolling method according to claim 9 is characterized in that, in addition to any one of the warm rolling methods described above: generating a plastic strain of 1.5 or more in at least a region of 50% by volume inside the material; a warm rolling method is provided, which has the following features: generating plastic strain 2 or more in a region of 90% by volume or more inside the steel material; the 11 th, provide a warm rolling method, the characteristic of the warm rolling method lies in: the rolling condition parameter Z represented by the following formula (1) is not less than 11 (when the microstructure immediately before rolling is ferrite, bainite, martensite, pearlite, or the like, and the crystal structure of Fe is bcc) or not less than 20 (when the microstructure immediately before rolling is austenite and the crystal structure of Fe is fcc).

Epsilon: strain of

t: time from start to end of rolling(s)

T: the rolling temperature (. Degree. C., in the case of multi-pass rolling, the average temperature of the rolling temperature in each pass)

Q: 254,000 was used when the microstructure immediately before rolling had ferrite, bainite, martensite, or pearlite as the matrix phase, and 300,000 was used when austenite was used as the matrix phase.

12, providing a warm rolling method, the warm rolling method is characterized in that: the reduction rate of the section of the initial blank and the section after final rolling is less than or equal to 90 percent; the 13 th, provide a warm rolling method, the characteristic of the warm rolling method lies in: manufacturing ultra-fine grain steel with the average grain diameter of the C section or the L section being less than or equal to 3 mu m; 14, providing a warm rolling method, the warm rolling method is characterized in that: an ultra-fine grain steel having an average grain diameter of 1 μm or less in C-section or L-section is produced.

The present invention having the above-described features is completed based on a new finding obtained by the inventors' study. That is, as a conventional method for producing a steel bar, a method of pass rolling is generally known in which rolling is performed using a roll having a grooved roll, and the shape of the grooved roll is roughly classified into a square shape (square, rhombus), an oval shape, and a circular shape. By performing the pass (カリバ i, grooved roll) rolling in the warm temperature region, the ultrafine grain ferrite main structure can be obtained by the multi-pass rolling (document 1). It has also been found that the use of the oval hole pattern is effective for equiaxial formation of ferrite grain shapes in the L-section (section parallel to the longitudinal direction of the bar) of the bar steel.

The inventors have conducted extensive studies and found that a large strain can be generated in a material even at a small reduction ratio of a cross section by performing rolling in a suitable temperature region in combination with an oval groove and other types of grooves such as a square groove and a circular groove (カリバ).

Drawings

Fig. 1 is a diagram showing the pass of example 1.

Fig. 2 is a photograph showing a C-section of the rolled steel bar.

Fig. 3 is a grid diagram of a blank.

Fig. 4 is a graph showing the plastic strain after 1 pass and ellipse.

Fig. 5 is a graph showing plastic strain after 2 passes and square hole patterns.

Fig. 6 is a graph showing plastic strain after 3 passes and ellipse.

Fig. 7 is a graph showing plastic strain after 4 passes and square hole patterns.

Fig. 8 is a graph showing plastic strain after 5 passes and ellipse.

Fig. 9 is a graph showing plastic strain after 6 passes and round holes.

Fig. 10 is an SEM (scanning electron microscope) image showing the structure after 2 passes and square pass.

Fig. 11 is an SEM image showing the structure after 4 passes and square hole type.

FIG. 12 is an SEM image of the structure of examples 2 to 4.

Fig. 13 is a diagram showing a hole pattern.

Fig. 14 is a photograph showing a C-section of the rolled steel bar.

Fig. 15 is an SEM image of the tissue.

FIG. 16 is an SEM image of the structure of comparative example 1.

Fig. 17 is a graph showing the relationship between the parameter Z and the average particle diameter.

Detailed Description

The present invention has the above-described features, and embodiments thereof will be described below.

The warm rolling method of the present invention combines the oval hole pattern rolling and the other types of hole patterns rolling as described above, thereby producing a steel material having an ultra-fine grain structure with an average grain size of 3 μm or less. The grooved rolls used for this rolling are an oval grooved roll and a different type of grooved roll.

Here, in the oval grooved roll, the hole shape formed by the upper mold and the lower mold is not circular, and can be said to have a circular flattened shape. As other kinds of hole patterns combined with the oval hole pattern, a square shape, a diamond shape, a circular shape, or various shapes similar to these shapes can be used.

In the present invention, as a warm rolling method for producing an ultra-fine grain steel material having an ultra-fine grain structure with an average grain diameter of 3 μm or less, oval pass rolling and pass rolling of other shapes are performed at least 1 time or more when rolling is performed in 2 or more passes in a temperature range of 350 to 800 ℃ relative to the steel material.

Actually, as a preferable mode, rolling of a pass of another shape is performed after rolling of a pass of an oval shape, or in the total number of passes: in N, when N > 2, rolling of an oval pass is carried out 2 times or more and N/2 times or less at the maximum, or continuous 2 passes of rolling and the like are carried out.

For example, in the case of combining the oval pass and the square pass, the total pass may include 2 or more passes of rolling in which the pass shape is an oval-square combination, or square rolling may be performed in the middle of an oval-square combination as in the oval-square-oval-square combination, or elliptical-square-oval-square 4 passes, or elliptical-square-elliptical-square 6 passes. In this case, the square shape may be a circle, a rhombus, or the like.

In the rolling method of the present invention, the microscopic local azimuthal difference caused by the large strain obtained by warm working is the origin of ultrafine grains, and the dislocation density in the grains is decreased during the working process or during the recovery process after the working process, and the grain boundary is formed to form an ultrafine grain structure. However, recovery is insufficient at a low temperature, and therefore, a processed structure having a high residual dislocation density is obtained. On the other hand, when the temperature is too high, the grains are coarsened by discontinuous recrystallization or normal grain growth, and an ultrafine grain structure of 3 μm or less cannot be obtained. For this purpose, the rolling temperature is limited to 350 ℃ to 800 ℃.

Further, in the present invention, ultrafine grains are generated from flattened working grains by warm working, which increases as the strain increases, but in order to obtain a structure consisting substantially entirely of ultrafine grains, a strain of at least 1.5 is required.

More specifically, by causing a plastic strain of 1.5 or more and even 2 or more in at least a region of 50% by volume inside the material, ultrafine grains can be formed in the region. Preferably, the plastic strain of 2 or more is generated in a region of 90% by volume or more in the material, whereby an ultrafine grain region can be formed in the region.

The larger the strain generated, the larger the azimuth difference angle between the fine grains. That is, the large angle grain boundaries increase. When the strain 3 can be generated, there is a sufficient proportion of high angle grain boundaries among the grain boundaries of fine grains. Therefore, if the strain region of 3 or more is 50% or more, preferably 80% or more of the total cross section, a steel bar having excellent mechanical properties can be obtained.

In addition, processing strains are applied from at least two directions in combination with the processing from another direction constituting an angle of about 90 ° with the main direction of the pressing, whereby the orientation of the ultrafine grains can be dispersed to increase the proportion of high angle grain boundaries.

The inventors have previously studied and found that the average particle size of ultrafine grains formed by hot working depends on the working temperature and strain rate. The grain size is refined as the rolling condition parameter Z of the above formula (1) increases as a function of the working temperature and strain rate. In order to obtain a structure having an average grain size of 1 μm or less, it is necessary to set the rolling condition parameter Z to a certain critical value or more. From the experimental results of 1-pass large strain compression working using small-sized samples, it was found that the critical value was about 11 in the case of iron (ferrite, bainite, martensite, pearlite, etc.) of bcc structure and about 20 in the case of fcc structure (austenite) (fig. 17).

The strain (. Epsilon.) of the formula (1) may be a true strain which is a strain convenient in industry. For example, when the initial area of the steel bar is So and the area of C section after rolling is S, the reduction ratio R is used

R＝(So-S)/So (2)

And (4) showing. Thus, true strain e is used

e＝-Ln(1-R)

And (4) showing. In addition, instead of the true strain, a strain calculated by a finite element method (for example, "finite element method entry" such as Jia san Lang, spring sea (co-published strain): 1990, 3, and 15) may be used.

TABLE 1

Procedure for calculation of plastic strain

1 obtaining a stress-strain curve corresponding to the processing temperature of the material

Preparation for finite element method calculation

(1) Making grids for processed objects

(2) Determining contact condition friction coefficient =0.3 coulomb condition

(3) Determine the stress-strain curve and the material physical property value

3 is calculated according to the conditions of (1) to (3) by a general finite element code such as ABAQUS. The plastic strain ε is expressed as the following equation, and each strain increment is calculated according to the general finite element code.

dε _x dε _y dε _z : increment of strain of x, y, z

dγ _xy dγ _yz dγ _zx : increment of shear strain

In the warm rolling method of the present invention, as is clear from the above, it is preferable to set the rolling conditions so that the parameter Z is not less than 11 (bcc structure) or not less than 20 (fcc structure).

In addition, in the present invention, as a preferable form, a case where the reduction of area is 20% or more by 2-pass rolling in which the pass shape is oval and square in 2-pass rolling of oval and square pass rolling of the billet, a case where the reduction of area is 40% or more by 2-pass rolling in which the pass shape is oval and square in combination and 60% or more by 3-pass rolling in combination, and a case where a rolling step in which the maximum minor axis length of the material after oval pass rolling is 70% or less of the length of the pair of sides of the billet before oval rolling are included can be exemplified.

The composition of the steel material to which the warm rolling method of the present invention is applicable is not limited at all because the mechanism of achieving high strength by phase transformation is not utilized, and the addition of alloy elements for improving strength is not required, and therefore, the composition of the steel material is not limited, and for example, steel materials having a wide composition range such as steel types free from phase transformation, such as ferrite single-phase steel and austenite single-phase steel, can be used. More specifically, a composition in which no alloying element is added, that is, a composition in which, in terms of weight percent,

c: more than or equal to 0.001 percent and less than or equal to 1.2 percent

Si: more than or equal to 0.1 percent and less than or equal to 2 percent

Mn: more than or equal to 0.1 percent and less than or equal to 3 percent

P: less than or equal to 0.2 percent

S: : less than or equal to 0.2 percent

Al: less than or equal to 1.0 percent

N: less than or equal to 0.02 percent

The total content of Cr, mo, cu and Ni is less than or equal to 30 percent

The total content of Nb, ti and V is less than or equal to 0.5 percent

B: less than or equal to 0.01 percent

The balance being Fe and unavoidable impurities. Of course, the alloying elements such as Cr, mo, cu, ni, nb, ti, V, and B may be added in an amount exceeding the above range, if necessary, or may be completely excluded.

The following is a more detailed description according to the examples. Of course, the invention is not limited by the following examples.

Examples

Table 2 below shows the chemical composition (the remainder being Fe) of the test steels used in the examples.

TABLE 2

Chemical composition of test Steel (% by mass)

	C	Si	Mn	P	S	Al
	C	Si	Mn	P	S	Al	a	0.15	0.3	1.5	0.01	0.001	0.03
b	0.11	0.3	0.5	0.02	0.005	0.03	a	0.15	0.3	1.5	0.01	0.001	0.03

< example 1>

A24 mm X24 mm steel bar having the composition of Table 2a and a ferrite + pearlite structure with an average ferrite grain size of 5 μm was subjected to rolling at a rolling temperature of 520 to 450 ℃ using a 6-pass (カリバ one) of the pass shown in FIG. 1. The outline of the hole pattern size (mm) in fig. 1 is shown in table 3 below.

TABLE 3

	Long shaft	Short shaft	Radius of curvature
	Long shaft	Short shaft	Radius of curvature	1 pass ellipse	54	12	64
3 pass ellipse	41	9	49	1 pass ellipse	54	12	64
3 pass ellipse	41	9	49	5 pass ellipse	19	10	12
6 pass ellipse		Diameter: 12		5 pass ellipse	19	10	12

Fig. 2 shows the change in cross-sectional shape and the reduction in cross-sectional area in each pass of rolling. The reduction rate of the cross section of a 24 × 24mm square bar in the 1 st pass through the oval pass rolling was 37%, the reduction rate of the cross section in the 2 nd pass through the square pass rolling was 21%, the reduction rate of the cross section in the 3 rd pass through the oval pass rolling was 15%, the reduction rate of the cross section in the 4 th pass through the square pass rolling was 24%, the reduction rate of the cross section in the 5 th pass through the oval pass rolling was 13%, and the reduction rate of the cross section in the 6 th pass through the round pass rolling was 12%. The reduction in area from the billet to the 17mm square bar of the 2 nd pass was 44%, the reduction in area from the billet to the 13mm square bar of the 4 th pass was 71%, and the reduction in area from the billet to the 12.5mm round bar of the 6 th pass was 80%.

Fig. 3 to 9 show plastic strain distributions inside the material calculated by the finite element method. As is clear from fig. 5, in the 2-pass rolling of the oval-square shape, a region exceeding the plastic strain of 1.5 was already present in the material. The area ratio was 75%. As shown in fig. 6, the region having a plastic strain of 2 or more after 3 passes of the oval-square-oval rolling accounts for 92% of the whole, and the region having a plastic strain of 3 or more after 4 passes of the oval-square-oval-square rolling accounts for 95% of the whole, as shown in fig. 7. In addition, when the oval-round rolling of fig. 9 is performed, the plastic strain is 3 or more in the 100% region.

It was found that although the reduction of the cross section after 2 passes was about 44% (when the reduction of the cross section R was simply converted into the true strain e, e =0.67 was obtained from e = -Ln (1-R/100)), about 71% after 4 passes (when the reduction of the cross section was simply converted into the true strain, 1.23), and about 80% after 6 passes (when the reduction of the cross section was simply converted into the true strain, 1.61), a large plastic strain was generated inside the material. This is because rolling by combining the oval hole pattern and the square hole pattern generates a significantly larger strain than that calculated from a simple reduction ratio of the cross section.

Fig. 10 and 11 show SEM photographs of the tissue. Fine ferrite grains of 1 μm or less are generated in the portions (1) and (2) of fig. 10 corresponding to fig. 5, and no fine grains are generated in the portion (3). As is clear from the microstructure photograph of FIG. 11 corresponding to FIG. 7, substantially the entire region is constituted of an ultrafine microstructure of ultrafine ferrite grains of 1 μm or less.

Table 4 shows the mechanical properties of the material 13mm by 13mm after 4 passes (example 1). The properties of the 24mm × 24mm rod before rolling are also shown in comparison (comparative example 2). The 13mm x 13mm rod had a yield strength about 2 times that of the 24mm x 24mm rod and had an absorption energy of 118J at-120 c without brittle failure at liquid nitrogen temperature.

TABLE 4

	Ferrite grain (μm)	Yield strength (MPa)	Tensile strength (MPa)	Plastic brittleness Transition temperature (℃)	Absorbed energy (J) -120℃	Center dimension Hardness in durometer (-)
	Ferrite grain (μm)	Yield strength (MPa)	Tensile strength (MPa)	Plastic brittleness Transition temperature (℃)	Absorbed energy (J) -120℃	Center dimension Hardness in durometer (-)	Example 1	0.5	840	850	-196＞	118	290
Example 5	0.6	800	810	-196＞	80	270-310	Example 1	0.5	840	850	-196＞	118	290
Example 5	0.6	800	810	-196＞	80	270-310	Comparative example 2	5	460	580	-40	0

< examples 2 to 4>

A24 mm X24 mm steel bar having a composition shown in Table 2a and a ferrite + pearlite structure with an average ferrite grain size of 5 μm was subjected to 2-pass rolling using passes shown in (1) and (2) in FIG. 1 at rolling temperatures of 400 ℃, 600 ℃ and 700 ℃. Fig. 12 (a), (b), and (c) show SEM structures of the central portion (portion corresponding to (1) in fig. 10) of the steel bar, and fine ferrite grains having average grain sizes of 0.5, 1, and 1.5 μm were obtained.

< example 5>

The bar steel having the composition shown in Table 2b and a ferrite + pearlite structure with an average ferrite grain size of 20 μm was subjected to pass rolling at a rolling temperature of 450 to 500 ℃ using a pass type shown in FIG. 13 for a diameter of 15 mm. Table 5 shows the dimensions of the hole pattern. Fig. 14 shows the change in the cross-sectional shape and the reduction ratio of the cross-section in each rolling pass. Fig. 15 shows an SEM photograph of the structure after 6 passes, but the structure was composed of a fine ferrite grain structure even if the reduction rate of the cross section was about 74%. As for the mechanical properties, excellent properties of Vickers hardness of 270 to 310 and tensile strength of 800MPa or more were obtained as shown in Table 4.

TABLE 5 [mm]

	Long shaft	Short shaft	Radius of curvature
	Long shaft	Short shaft	Radius of curvature	1 pass ellipse	31	6.8	38
3 pass ellipse	27	5.3	35.9	1 pass ellipse	31	6.8	38
3 pass ellipse	27	5.3	35.9	5-pass ellipse	15	6.5	10.7
6 pass ellipse		Diameter: 8		5-pass ellipse	15	6.5	10.7

< comparative example 1>

A steel bar 24mm × 24mm having the composition shown in Table 2a and a ferrite + pearlite structure with an average ferrite grain size of 5 μm was subjected to 7 pass rolling with a reduction rate of 70% (strain 1.2) at a rolling temperature of 500 ℃ until the steel bar became 13mm × 13mm using the pass shown in FIG. 1. Rolling with oval passes is not included. As shown in the SEM photograph of fig. 16, no fine crystal grains were generated in the central portion of the steel bar.

< comparative example 2>

After heating a steel bar having a diameter of 115mm and having a composition shown in Table 2a to 900 ℃, pass rolling with a reduction of 94% (strain 3.1) was performed at a rolling temperature of 870 to 850 ℃ using a square pass until the rolling became 24mm × 24mm. Rolling with oval passes is not involved. The average particle size was 5 μm, and no fine crystal grains were formed. The mechanical properties are shown in Table 4, and the yield strength and tensile strength are 460 MPa and 580MPa, respectively.

Possibility of industrial utilization

As described above in detail, according to the present invention, it is possible to provide a novel multidirectional warm rolling method which can form a large strain in a material with a smaller reduction rate of a cross section or the number of passes by a simpler means, and further, to provide a method for manufacturing a steel material having an ultrafine grain structure obtained thereby and excellent in strength and plasticity.

Claims

1. A warm rolling method for producing an ultrafine-grained steel material having an ultrafine-grained structure with an average grain size of 3 μm or less; the method is characterized in that: when rolling is performed in 2 or more passes in a temperature range of 350 to 800 ℃ relative to a steel material, at least 1 pass of rolling with an oval pass and rolling with a pass having another shape are performed, and when rolling is performed with an oval pass, the maximum minor axis length of the steel material after rolling is 75% or less of the length of the opposite side of the billet before rolling, and plastic strain of 1.5 or more is generated in at least 50% by volume of the area inside the steel material by the oval pass rolling and the rolling with the pass having another shape.

2. Warm rolling method according to claim 1, characterized in that: the plastic strain 2 or more is generated in a region of 90% by volume or more inside the steel material.

3. Warm rolling method according to claim 1 or 2, characterized in that: rolling of the oval pass is followed by rolling of passes of other shapes.

4. Warm rolling method according to claim 1 or 2, characterized in that: the hole patterns with other shapes are square or round hole patterns.

5. Warm rolling method according to claim 1 or 2, characterized in that: in the total number of passes: in N, when N is more than 2, rolling of oval pass is carried out at 2 times or more and at most N/2 times or less.

6. Warm rolling method according to claim 3, characterized in that: continuous 2-pass rolling was performed.

7. Warm rolling process according to claim 1 or 2, characterized in that: an ultra-fine grain steel having an average grain diameter of 3 μm or less in C-section or L-section is produced.

8. Warm rolling method according to claim 7, characterized in that: the average grain diameter of the ultra-fine grain steel is less than or equal to 1 μm.