JPH045727B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention is characterized in that the steel plate (strip) has an easy magnetization axis <001> orientation in the longitudinal direction and the direction perpendicular to the longitudinal direction, and a {100} plane appears on the rolled surface (Miller index The present invention relates to a method of manufacturing a so-called bidirectional electrical steel sheet composed of {100}<001>) crystal grains. (Prior Art) Conventionally, typically, the method described in U.S. Pat.
Unidirectional electrical steel sheets with low iron loss values are manufactured. This unidirectional electrical steel sheet has {100}
It is composed of crystal grains with a <001> orientation (Miller index: {100}<001>), which is an axis of easy magnetization only in the rolling direction (longitudinal direction of the steel sheet). Unidirectional electrical steel sheets with excellent magnetic properties only in the rolling direction (for example, _B10 value in the rolling direction:
1.92Tesla, B ₁₀ in the direction perpendicular to the rolling direction
value: 1.45Tesla), the bidirectional electrical steel sheet is
It has easy magnetization axes both in the longitudinal direction of the steel plate (strip) and in the direction perpendicular to the longitudinal direction of the steel plate (strip), and has excellent magnetic properties in both directions.
It is particularly advantageous to use it as an iron core material for large rotating machines. On the other hand, in the field of small rotating machines, cold-rolled non-oriented electrical steel sheets that are not highly concentrated in the axis of easy magnetization are generally used; It is extremely effective in terms of equipment miniaturization and efficiency. As mentioned above, bidirectional electrical steel sheets have superior magnetic properties compared to unidirectional electrical steel sheets, and although their commercialization has been long-awaited, to date they have not been produced as industrial products. It has not yet been reached. On a laboratory scale, the following two types of manufacturing methods for bidirectional electrical steel sheets have been announced, but both have problems as manufacturing processes on an industrial scale. One of the prior art is the technology disclosed in Japanese Patent Publication No. 37-7110. This technique involves subjecting the material to high-temperature annealing in an atmosphere containing a polar gas, such as hydrogen sulfide.
This is a method of selectively forming <001> oriented grains. However, this technique requires strict control of the atmosphere on the surface of the steel plate, making it unsuitable for mass production processes. Another prior art is the method disclosed in Japanese Patent Publication No. 35-2657 by Satoru Taguchi et al.
This technique is a so-called "cross cold rolling method" in which cold rolling is performed in one direction and then cold rolling is performed in a direction perpendicular to the rolling direction. In this technology,
In the "cross cold rolling method", after the first cold rolling process, the strip is cut into a certain length to form a steel plate (sheet), and in this state, the second cold rolling process is carried out in a direction that crosses the first cold rolling direction. or the cut sheet is rotated 90 degrees so that the edges of the strip at the time of the first cold rolling form an abutting part and then welded to form a strip. , a method is adopted in which the second cold rolling is performed continuously. However, when using these techniques, there are problems for manufacturers in that the process is complicated and it is difficult to obtain a steel plate with a uniform shape. On the other hand, for the user side, when the material is supplied in the form of sheets, the punching operation is extremely inefficient, and when the material is supplied in the form of strip coils, there are welding points at regular intervals. ,
Since the magnetism of that part is poor, it is necessary to remove the part. For the reasons mentioned above, bidirectional electrical steel sheets according to the prior art have not been used as industrial products at all. In addition to the problems mentioned above, there are important reasons why the technology disclosed in Japanese Patent Publication No. 35-2657 cannot become mainstream in industry. That is, according to this cross cold rolling method, relatively high magnetization characteristics ( _B10 value) can be obtained.
However, since it does not have excellent magnetic properties that are commensurate with the high cost due to the complexity of its manufacturing method, it cannot compete with conventional unidirectional electrical steel sheets. In particular, the magnetization characteristics ( _B10 value) of recent unidirectional electrical steel sheets have rapidly progressed since the invention of the technology disclosed in Japanese Patent Publications No. 15644/1972 and No. 13469/1982. ₁₀ ≧1.89Tesla is also standardized by JIS, and products with a _B10 value of around 1.92Tesla are commercially available. Under such circumstances, it is necessary that bidirectional electrical steel sheets also have magnetization properties comparable to those of the unidirectional electrical steel sheets. As a means to improve the magnetization characteristics of bidirectional electrical steel sheets,
Publication No. 8213 proposed a method of annealing a hot-rolled material and then cold-rolling it in mutually orthogonal directions, but the magnetization properties obtained even with this method cannot necessarily be said to be sufficient. . The iron core material has excellent core loss characteristics in addition to the above-mentioned magnetization characteristics. (The iron loss value W/Kg is small). In order to improve iron loss characteristics,
Increasing the _B10 value and reducing the thickness of the product are particularly effective in the field of unidirectional electrical steel sheets.
It is standardized by JIS up to a thickness of 0.23mm.
However, with such a thin steel plate, {100}
It is extremely difficult to obtain <001> oriented grains, and in both the processes disclosed in Japanese Patent Publication No. 35-2657 and Japanese Patent Publication No. 38-8213, the final plate thickness is
The limit is 0.30mm. Therefore, as an improved technology,
A technique disclosed in Japanese Patent Publication No. 35-17208 has been proposed, but in this technique cold rolling and annealing are added, making the manufacturing cost extremely high. In addition, the magnetization properties ( _B10 value) of the resulting product are less than 1.85 Tesla, and the final plate thickness is also
It stops at 0.294mm. (Problems to be Solved by the Invention) The present invention provides a product that can be manufactured consistently in the form of a strip, and that the magnetic properties of the product are uniform in the length direction of the strip. A method for manufacturing a bidirectional electrical steel sheet with high magnetic properties, which has no parts, has excellent thickness uniformity and excellent shape (flatness), and can be efficiently and continuously punched when processed into an iron core. It was made for the purpose of providing. Another object of the present invention is to provide a method for producing a thin product plate with a thickness of less than 0.30 mm and a high magnetic flux density. (Means for Solving the Problems) The features of the present invention are as follows: (1) By weight, Si: 0.8 to 5.2%, acid-soluble Al: 0.008%
~0.048%, remainder: steel plate consisting essentially of Fe,
First, cold rolling is applied to apply a reduction rate of 40 to 80% in the longitudinal direction of the material, and then cross cold rolling is applied to apply a reduction rate of 30 to 70% in a direction crossing the rolling direction in the cold rolling. Further, the material is rolled in the same direction as the first longitudinal cold rolling.
After performing cold rolling applying a rolling reduction of %,
After being annealed for a short time in the temperature range of 750-1000℃,
A method for manufacturing a high magnetic flux density bidirectional electrical steel sheet having uniform magnetic properties in the longitudinal direction, characterized by performing high-temperature finish annealing in a temperature range of 900 to 1200°C. (2) By weight, Si: 0.8-5.2%, acid-soluble Al: 0.008
~0.048%, remainder: steel plate consisting essentially of Fe
After annealing in a temperature range of 750 to 1200°C for 30 seconds to 30 minutes, first cold rolling is applied to apply a reduction rate of 40 to 80% in the longitudinal direction of the material, and then the rolling direction in the cold rolling is 30~ in the direction that intersects with
After cross cold rolling with a rolling reduction of 70% and further cold rolling with a rolling reduction of 5 to 33% in the same direction as the initial cold rolling in the longitudinal direction of the material, the temperature is 750 to 1000℃. A method for producing a high magnetic flux density bidirectional electrical steel sheet having uniform magnetic properties in the longitudinal direction, which comprises annealing for a short time in a temperature range of 100 to 1200 °C, followed by high-temperature finish annealing in a temperature range of 900 to 1200 °C. (3) After completion of cold rolling, a temperature increase stage before the onset of secondary recrystallization in the process of short-time annealing in the temperature range of 750 to 1000°C or in the process of high-temperature finish annealing in the temperature range of 900 to 1200°C. The method according to claim 1 or 2, wherein the steel plate (strip) is nitrided so that the increase in the N content of the material is 0.002 to 0.06% in terms of total N amount. It is in. The present invention will be explained in detail below. In the process of the present invention, as a means for cold rolling the material in the strip form after the first cold rolling in a direction perpendicular to the direction of the first cold rolling, Japanese Patent Publication No. 62-45007 describes There is a cross cold rolling method disclosed. According to this cold rolling method, relatively high magnetization characteristics (Tesla indicated by the value of _B10 ) can be obtained, but there are problems with the shape of the material (strip) after rolling. It has not yet been put to practical use as a cold rolling method for producing bidirectional electrical steel sheets. Firstly, in this cold rolling method, the material is rolled intermittently, so the plate thickness at the boundary between each pass becomes thicker, resulting in uneven thickness in the longitudinal direction of the material, which causes laminated steel There is a problem with the core material. Second, by applying stress in the material width direction,
Bending stress occurs within the material plane at the boundary between the deformed part and the undeformed part, and undulations (ear waves) occur at the side edges of the material, causing the shape (flatness) of the laminated iron core material to change. cannot be secured. After the cross cold rolling, the inventors performed a continuous rolling process in the form of rolling rolls in a direction perpendicular to the rolling direction, that is, in the same direction as the initial cold rolling direction,
Moreover, by limiting the rolling reduction ratio to 5 to 33%, it is possible to make the thickness of the strip uniform in the longitudinal direction of the material, and to maintain a good shape (flatness) of the material (strip) after rolling, and to improve the quality of the final product. It has been found that the obtained magnetic flux density can be improved. C: 0.053%, Si: 3.2%, Mn: 0.080%, S:
0.023%, Al: 0.033%, N: 0.0075%, balance: A 2.3 mm thick hot rolled plate consisting essentially of Fe was heated at 1100°C.
After annealing for 2 minutes at a temperature of In 1987, special public service
- The above-mentioned first method by the means disclosed in Publication No. 45007.
It was cold rolled (cross rolled) to a thickness of 0.55 mm in a direction perpendicular to the direction of the second cold rolling to form a strip coil. Furthermore, continuous cold rolling was performed in the same direction as the first cold rolling using a rolling mill with the same rolling roll type as in the first cold rolling, applying a rolling reduction of 5 to 50%. . In addition, as a comparison material, a material that was not subjected to cold rolling using the rolling reduction ratio of 5 to 50% was also subjected to the treatment process after cold rolling. These cold-rolled sheets were heated at 820℃×5 in a wet hydrogen atmosphere.
After decarburization annealing for 1 minute, MgO was applied and 1200
High temperature finish annealing was performed at ℃ for 20 hours. The magnetic flux density, variation in plate thickness in the longitudinal direction, and shape (flatness) of the obtained product are shown in FIG. As is clear from Fig. 1, when the post-processing process is completed without cross-cold rolling, there is a large variation in the thickness in the longitudinal direction of the product, and the undulations (ear waves) on both side edges of the product have not disappeared. It did not become a product. In contrast, the material according to the present invention, which is subjected to cold rolling of 5% or more in the same direction as the first cold rolling direction after cross cold rolling, does not have the above problem at all. However, it can be seen that when a rolling reduction of more than 33% is applied in cold rolling after cross cold rolling, the magnetic flux density deteriorates rapidly. Next, one feature of the present invention is that after cross cold rolling and subsequent cold rolling applying a rolling reduction of 5 to 33% in the same direction as the first cold rolling, a final annealing process is performed. By introducing a certain amount of nitrogen from the surface of the steel sheet until the appearance of {100}<001> oriented grains (secondary recrystallization),
The advantage is that preferential growth of {100}<001> oriented grains is promoted to obtain a bidirectional electrical steel sheet with high magnetic flux density. {100} in the final annealing process after final cold rolling
The means for introducing a certain amount of nitrogen from the surface of the steel sheet until the appearance of <001> oriented grains is not particularly limited.
For example, a method of nitriding a steel sheet in an atmosphere with nitriding ability during short-time annealing for decarburization after final cold rolling, or the development of {100}<001> oriented grains in the final annealing process. A method can be applied in which the temperature of the steel plate is raised up to 100% in an atmosphere capable of nitriding. If the object of the final annealing is a strip coil and the strip coil is large, it is difficult for nitrogen to penetrate between the layers of the strip, and the steel plate may not be sufficiently nitrided. It is desirable to take measures such as adding metal nitrides or ammonides, which release nitrogen during the final annealing process, to the annealing separator applied to the strip surface prior to final annealing. In the present invention, the nitrogen introduced into the steel plate is
Presumably, fine precipitates such as AlN, Si ₃ N ₄ , (Al, Si)N, etc. promote the preferential growth of {100}<001> oriented grains, but the truth of this is not clear. The treatment for increasing the amount of nitrogen into the steel sheet, which characterizes the present invention, is carried out either during the decarburization annealing, in additional annealing after the decarburization annealing, or in the temperature raising process of the final annealing. Here, additional annealing after decarburization annealing is performed in an ammonia-containing atmosphere for various times, and the magnetic flux density of the product is shown when the amount of nitrogen in the steel sheet is varied. The treatment process at that time was as follows. C: 0.055%, Si: 3.23%, acid soluble Al: 0.028,
TotalN: 0.0073%, balance: Fe and unavoidable impurities.
After being annealed for 1 minute, it is cold rolled in the same direction as the hot rolling at a reduction rate of 65%, and then in a direction crossing the cold rolling direction (substantially perpendicular direction).
Cold rolling is performed applying a rolling reduction of 60%, and further cold rolling is performed to 0.20 mm in the same direction as the initial cold rolling using a rolling mill with the same rolling roll type as the initial cold rolling. did. The thus obtained cold-rolled sheet was subjected to a wet hydrogen atmosphere.
Decarburization annealing was performed at 810°C for 90 seconds. The nitrogen content of the material after this decarburization annealing is 0.0075%, the same as that in the raw material, and is not nitrided at this stage. The material after decarburization annealing was additionally annealed for (10 to 180) seconds at 550° C. in an atmosphere containing 10% NH ₃ to be nitrided. The material obtained in this way is used as an annealing separator.
After applying MgO and drying it (25% N ₂ , 75% H ₂ ), it was heated to 1200°C in a 100% H ₂ atmosphere for 20 min.
A time purification annealing was performed. Figure 2 shows the relationship between the _B10 value of the obtained product and the increase in nitrogen content due to additional annealing (nitriding treatment of the steel plate) performed before final annealing. As is clear from Figure 2, if nitrogen enrichment treatment is not performed, secondary recrystallization will not occur and the magnetic flux density (B ₁₀ value) will decrease.
is low. On the other hand, if the amount of nitrogen is increased too much, the crystal grains of the product will become extremely large, the frequency of appearance of grains with orientations other than {100}<001> will increase, and the B ₁₀ value will decrease. Even though the nitrogen increase is within the range of 0.002 to 0.060% and the product plate thickness is 0.20 mm, which is thin and has not been disclosed so far, a product with a high _B10 value of 1.88 Tesla or more can be obtained. Increase in amount from 0.0060 to 0.0200%
A product with the highest magnetic flux density is obtained in the range of . Other ways to increase the amount of nitrogen other than the above include adding metal nitrides or ammonides to the annealing separator to release nitrogen during the final annealing process and nitriding the steel sheet, or adding nitriding to the atmosphere during decarburization annealing. It is possible to adopt a method of processing the material in a way that gives it the ability to do so, but there is no need to be particular about it. However, even if the material is subjected to nitrogen increasing treatment before cold rolling, it has no effect, and only when nitrogen increasing is performed during the annealing process after cold rolling, there is an effect. Next, embodiments of the present invention will be described. After the hot-rolled sheet is pickled, its longitudinal direction is 40
Cold rolling applying a rolling reduction of ~80% and then in the form of a strip in a direction perpendicular to the direction of said first cold rolling, according to the method disclosed in Japanese Patent Publication No. 62-45007. It is then cross-cold-rolled and further rolled in a mill of the type with ordinary rolling rolls in the same direction as the first cold-rolling.
It is subjected to cold rolling applying a rolling reduction of %. Although it is possible to increase the magnetic flux density of the product by annealing hot-rolled sheets for a short period of 30 to 30 minutes at a temperature range of 750 to 1200°C, it increases manufacturing costs, so it is difficult to achieve the desired level of magnetic flux density. It is preferable to decide whether or not to use the short-time annealing process depending on the balance. In the case of a process of short-time annealing of a hot-rolled sheet, the short-time annealing is followed by the first cold rolling in the longitudinal direction. As the material before cold rolling, a normal hot rolled silicon steel plate can be used. Further, a steel plate obtained by continuously casting molten steel, for example, a continuously cast ribbon having a thickness of 1.5 to 3.0 mm can also be used. The ingredients contained in the material include Si: 0.8-5.2%,
Acid-soluble Al: 0.008 to 0.048%, remainder: Fe and unavoidable impurities, with these as essential components and no other limitations. When the Si content exceeds 5.2%, the material tends to crack during cold rolling, making rolling impossible. On the other hand, the smaller the Si content, the better in terms of increasing the magnetic flux density of the product, but if material α→r transformation occurs during high-temperature finish annealing, the orientation of the crystal will be destroyed. The lower limit of Si content is 0.8%. By adding acid-soluble Al in a range of 0.008 to 0.048%, the magnetic flux density of the product increases. In particular, when acid-soluble Al is in the range of 0.018 to 0.036% and subjected to the nitrogen increasing treatment described below, the magnetic flux density (B ₁₀ value) becomes 1.92 Tesla or higher, which is unprecedentedly high. The remainder other than the above is Fe and unavoidable impurities. The molten steel consisting of the above components is produced by casting → hot rolling, or by continuously casting the molten steel directly into a thin ribbon.
Cold rolling is performed immediately or after a short annealing step. After cold rolling, the material is heated to 750~750°C in a wet hydrogen atmosphere to remove the minute amount of C that is normally contained in steel.
Decarburization annealing is performed for a short time in a temperature range of 1000℃. Next, the steel plate (strip) is coated either during the decarburization annealing, in additional annealing after the decarburization annealing, or in the temperature raising process in high-temperature finish annealing.
Perform nitrogen enrichment treatment in the range of 0.002 to 0.060%. After applying an annealing separator to the material thus obtained and drying it, high-temperature finish annealing is performed in a temperature range of 900 to 1200°C. (Example) Example 1 C: 0.05%, Si: 3.1%, Mn: 0.08%, S:
Contains 0.018%, N: 0.0072%, acid-soluble Al:
Two types of 1.8 mm thick hot rolled sheets containing 0.005% and 0.029% were annealed for a short time at 1070°C for 2 minutes, then cold rolled to 0.68 mm in the longitudinal direction of the material, and then formed into strips. Using the rolling method disclosed in Japanese Patent Publication No. 62-45007, cross cold rolling was performed in a direction perpendicular to the direction of the first cold rolling until the plate thickness was 0.23 mm. . Thereafter, continuous cold rolling was performed in the same direction as the first cold rolling using a conventional cold rolling mill to a thickness of 0.20 mm. This cold-rolled sheet was decarburized and annealed at 810° C. for 90 seconds in a wet hydrogen atmosphere. Next, after applying MgO containing 5% MnN as an annealing separator, the temperature was raised in an atmosphere of 25% _N2 + 75% _H2 , and then heated at high temperature for 20 hours in an atmosphere of 100% _H2 . Finish annealing was performed. Table 1 shows the B ₁₀ values of the products obtained. 1st
As is clear from the table, a product with a high _B10 value can be obtained using the 0.029% Al material within the range of the present invention.

[Table] Example 2 C: 0.048%, Si: 2.0%, Mn: 0.14%, S:
A 2.0 mm hot-rolled plate containing 0.012%, acid-soluble Al: 0.032%, and N: 0.0035% was annealed at 1120°C for 2 minutes, and then cold rolled to 0.70 mm in the longitudinal direction of the material. However, it remained in the form of a strip in 1986.
-By the method disclosed in Publication No. 45007,
0.23mm in the direction perpendicular to the first cold rolling direction
Cross cold rolling was performed until the thickness reached 0.20 mm, and then cold rolling was performed in the same direction as the first cold rolling direction using a conventional cold rolling mill to a thickness of 0.20 mm. The obtained cold-rolled sheet was decarburized and annealed in a wet hydrogen atmosphere at 810°C for 90 seconds, and then MnN was contained as an annealing separator in an amount of 0%, 2%, 5%, and 10%, respectively.
After applying MgO, the temperature was raised in an atmosphere of N ₂ : 10% + H ₂ : 90%, and high temperature finish annealing was performed in an atmosphere of H ₂ : 100%. The _B10 value of the product obtained in this way and the steel plate that was analyzed after heating was stopped (cut off) at the stage of 900°C during the heating process in the N ₂ : 10% + H ₂ : 90% atmosphere. The total N amount is shown in Table 2.

[Table] As is clear from Table 2, during annealing separation
If there is no MnN addition and the steel sheet has a small amount of nitrogen added,
_B10 value is low. In comparison, steel sheets in which MnH was added during annealing separation and an appropriate amount of nitrogen was added to the steel sheet had a high B ₁₀ value. Example 3 Among the two types of hot rolled sheets in Example 1, the acid-soluble one
Regarding 1.8 mm thick hot rolled sheets with the same composition as the one containing Al: 0.029%, one was as hot rolled, the other was hot rolled sheet annealed at 950℃ x 2 minutes, and the other One was a hot rolled sheet annealed at 1120°C for 2 minutes. The obtained materials were cold-rolled to a thickness of 0.78 mm in the same direction as the hot-rolling, and then, in the form of a strip, were subjected to the first cold rolling by the method disclosed in Japanese Patent Publication No. 62-45007. Cross cold rolling was performed in the direction perpendicular to the direction of inter-rolling to a thickness of 0.35 mm. Next, continuous cold rolling was performed in the same direction as the first cold rolling to a thickness of 0.30 mm using a rolling mill with the same roll type as a normal cold rolling mill. The cold-rolled sheet thus obtained was placed in a wet hydrogen atmosphere.
After decarburizing annealing at 810℃×90 seconds, after applying MgO containing 10% MnN as an annealing separator,
The temperature was raised in an atmosphere of N ₂ : 25% + H ₂ : 75%, and H ₂ :
High temperature finish annealing in 100% atmosphere. The B ₁₀ values of the obtained products are shown in Table 3. Table 3 shows that when the material is subjected to hot-rolled plate annealing, a product with a particularly high _B10 value can be obtained, which is equivalent to or higher than that of current grain-oriented electrical steel sheets in the cold rolling direction.
A bidirectional electrical steel sheet with a B value of ₁₀ could be obtained.

[Table] (Effects of the invention) Since this invention is configured and operated as described above, the _B10 value in the cold rolling direction of the currently highest level unidirectional electrical steel sheet is equal to or higher than that of the present invention. _B10 value in both directions, extremely small thickness deviation in the longitudinal direction of the product, and shape (flatness)
This makes it possible to industrially produce a bidirectional electrical steel sheet in the form of a strip.

[Brief explanation of drawings]

Figure 1a is a diagram showing the relationship between rolling reduction and strip shape when cold rolling is performed in the same direction as the first cold rolling direction (third cold rolling) after cross cold rolling. Figure b shows the relationship between the rolling reduction and the magnetic flux density ( _B10 value) of the product when cold rolling is performed in the same direction as the first cold rolling direction (third cold rolling) after cross cold rolling. Figure 2 is a diagram showing the _B10 value in the first cold rolling direction and the cross cold rolling direction of the product obtained by the present invention in relation to the amount of nitrogen added to the steel plate. be.

Claims (1)

[Claims] 1. By weight, Si: 0.8-5.2%, acid-soluble Al: 0.008
~0.048%, remainder: steel plate consisting essentially of Fe,
First, cold rolling is applied to apply a reduction rate of 40 to 80% in the longitudinal direction of the material, and then cross cold rolling is applied to apply a reduction rate of 30 to 70% in a direction crossing the rolling direction in the cold rolling. Furthermore, after performing cold rolling in the same direction as the initial cold rolling in the longitudinal direction of the material, applying a reduction rate of 5 to 33%, and then annealing for a short time in the temperature range of 750 to 1000 °C, A method for producing a high magnetic flux density bidirectional electrical steel sheet having uniform magnetic properties in the longitudinal direction, characterized by high-temperature finish annealing in a temperature range of ~1200°C. 2 By weight, Si: 0.8-5.2%, acid-soluble Al: 0.008
~0.048%, remainder: steel plate consisting essentially of Fe,
After annealing in a temperature range of 750 to 1200°C for 30 seconds to 30 minutes, first cold rolling is applied in the longitudinal direction of the material at a reduction rate of 40 to 80%, and then the rolling direction in the cold rolling is cross cold rolling applying a rolling reduction of 30 to 70% in the direction intersecting the
After performing cold rolling applying a rolling reduction of 750~
After being annealed for a short time in the temperature range of 1000℃, 900~
A method for manufacturing a high magnetic flux density bidirectional electrical steel sheet having uniform magnetic properties in the longitudinal direction, which is characterized by high-temperature finish annealing in a temperature range of 1200°C. 3 After the completion of cold rolling, the material is 3. The method according to claim 1 or 2, wherein the steel plate (strip) is nitrided so that the increase in the N content is 0.002 to 0.06% in terms of total N amount.