JP3397293B2 - Manufacturing method of ultra high magnetic flux density unidirectional electrical steel sheet - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an ultrahigh magnetic flux density unidirectional electrical steel sheet having a highly developed {110} <001> orientation integration degree used as an iron core of a transformer or the like.
Concerning the law .

[0002]

2. Description of the Related Art Unidirectional electrical steel sheets are mainly used as iron core materials for transformers and other electrical equipment, and are required to have excellent magnetic properties such as excitation characteristics and iron loss characteristics. The value that shows the excitation characteristics is usually 800 A /
Magnetic flux density B in a magnetic field of m (this is shown below as B ₈ )
Is used. W _17/50 (iron loss per unit kg when magnetized to 1.7 T at a frequency of 50 Hz) is used as a representative value representing the iron loss characteristic.

The magnetic flux density is an important controlling factor of the iron loss characteristics, and generally speaking, the higher the magnetic flux density is, the better the iron loss is. However, if the magnetic flux density becomes too high, the secondary recrystallized grains become large and the abnormal eddy current loss becomes large, which may deteriorate the iron loss. On the other hand, iron loss can be improved by controlling the magnetic domains regardless of the secondary recrystallized grains.

The unidirectional electrical steel sheet is subjected to secondary recrystallization during finish annealing in the manufacturing process to cause {11
0}, so-called Goss having <001> in the rolling direction
Obtained by developing tissue. Among them, B
Those with excellent excitation characteristics of ₈ ≧ 1.88T are called high magnetic flux density grain-oriented electrical steel sheets.

Typical methods for producing a high magnetic flux density unidirectional electrical steel sheet include Japanese Patent Publication No. 40-15644 and Japanese Patent Publication No. 51-13469 by Taguchi et al. In the former, MnS and AlN were used as the main inhibitors that caused the secondary recrystallization of the Goss structure.
In the latter, MnS, MnSe, Sb, etc. are used. Products based on the above patents are currently produced on a global scale. According to Japanese Examined Patent Publication No. 40-15644, the manufacturing method is characterized by performing hot-rolled sheet annealing and then performing cold rolling once at a cold rolling rate of 80 to 95%.

Further, it is required that a coating having an electrically insulating property is formed on the surface of the grain-oriented electrical steel sheet. In addition to the role of maintaining insulation, this coating also has a role of applying tension to the steel sheet to reduce iron loss. Therefore, it is extremely important to form this coating uniformly.

The coating of the high magnetic flux density unidirectional electrical steel sheet has a two-stage structure of a primary coating and a secondary coating. Among them, the primary coating is obtained by reacting SiO2 formed on the surface of the steel sheet with the annealing separating agent applied thereafter in the decarburizing annealing in the manufacturing process. Generally annealing separator are used those composed mainly of MgO, reacts with SiO ₂ at the time of finish annealing Mg
₂ SiO ₄ becomes the primary coating.

By the way, recently, by Takashima et al., B ₈ ≧
An ultra-high magnetic flux density grain-oriented electrical steel sheet having an extremely excellent excitation characteristic of 1.95T has been reported. As a typical example thereof, JP-A-6-89805 can be cited. Further, as a typical example of the manufacturing method thereof, JP-A-6-8817
No. 1 publication is mentioned. All of them are characterized by containing Bi in the slab, but the others are not different from the manufacturing method described in Japanese Patent Publication No. 40-15644 by Taguchi et al.

[0009]

However, there are many cases in which the product does not become a product due to deterioration of the adhesion of the primary coating, which is considered to be caused by the inclusion of Bi in the steel, and defective formation of the primary coating.

The present invention avoids such a problem, enables stable production of a primary coating of a grain-oriented electrical steel sheet having an extremely high magnetic flux density, and improves deterioration of adhesion of the primary coating and failure to form the primary coating. To aim.

[0011]

The gist of the present invention is as follows. (1) C: 0.03 to 0.15% by weight, Si: 2.5 to
4.0%, Mn: 0.02 to 0.30%, one or both of S and Se: 0.005 to 0.04
0%, acid-soluble Al: 0.015 to 0.040%, N: 0.0030 to 0.0150%, Bi: 0.00
A slab containing 0. 05 to 0.05%, with the balance being Fe and unavoidable impurities, is used as a starting material, and the hot-rolled sheet produced by heating this slab and then hot-rolling the sheet is annealed after the hot-rolled sheet is finished and cold-rolled. Or cold-rolling including intermediate annealing, or hot-rolled sheet annealing followed by multiple cold-rolling including intermediate annealing to finish the product sheet thickness, decarburize annealing, and then apply MgO to the steel sheet surface. A method for producing a grain-oriented electrical steel sheet, comprising applying an annealing separator as a main component and finish annealing, characterized in that the water content of the annealing separator coating film at the start of finish annealing is 0.3 to 3%. Manufacturing method of ultra-high magnetic flux density grain-oriented electrical steel sheet.

( 2 ) C: 0.03 to 0.15% by weight, Si: 2.5 to
4.0%, Mn: 0.02 to 0.30%, one or both of S and Se: 0.005 to 0.04
0%, acid-soluble Al: 0.015 to 0.040%, N: 0.0030 to 0.0150%, Sn: 0.05 to 0.50%, Cu: 0.01
.About.0.10%, Bi: 0.0005 to 0.05%, with the balance being Fe and unavoidable impurities as a starting material, the superhigh magnetic flux density according to (1) above. Method for manufacturing grain-oriented electrical steel sheet.

( 3 ) By weight%, C: 0.03 to 0.15%, Si: 2.5 to
4.0%, Mn: 0.02 to 0.30%, one or both of S and Se: 0.005 to 0.04
0%, acid-soluble Al: 0.015 to 0.040%, N: 0.0030 to 0.0150%, one or both of Sb and Mo: 0.0030 to 0.
3%, Bi: 0.0005 to 0.05%, with the balance being Fe and unavoidable impurities as a starting material, an ultrahigh magnetic flux density directional electromagnetic field according to (1) above. Steel plate manufacturing method.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. As a result of various studies to further stably obtain a primary coating of a so-called ultra-high magnetic flux density unidirectional electrical steel sheet, the present inventor has found that unidirectional electromagnetic radiation containing Bi-containing MnS and AlN as main inhibitors. Product sheet thickness after heating as a slab starting material for steel sheets and then hot rolling, finishing cold rolling after annealing hot rolled sheet, or multiple cold rolling including intermediate annealing, or multiple cold rolling including intermediate annealing after annealing hot rolled sheet In the method for producing an ultra-high magnetic flux density grain-oriented electrical steel sheet in which decarburization annealing is applied, an annealing separator is applied, and then finish annealing is performed in an annealing separator containing MgO as a main component at the start of finishing annealing. By making the water content of 0.3 to 3%, it was possible to stably manufacture the primary coating of ultra-high magnetic flux density unidirectional electrical steel sheet with extremely high magnetic flux density.

First, the component conditions of the present invention will be described. If C is less than 0.03%, crystal grains grow abnormally during slab heating prior to hot rolling, and secondary recrystallization defects called linear fine grains occur in the product, which is not preferable. On the other hand, if the content exceeds 0.15%, decarburization annealing after cold rolling requires a long time for decarburization, which is not economical, and the decarburization tends to be incomplete, resulting in a magnetic aging called magnetic aging in the product. It is not preferable because it causes defects.

Si is an extremely effective element for increasing the electrical resistance of steel and reducing the eddy current loss that constitutes a part of iron loss, but if it is less than 2.5%, the eddy current loss of the product cannot be suppressed. . Further, if it exceeds 4.0%, the workability is remarkably deteriorated and cold rolling at room temperature becomes difficult, which is not preferable.

Mn is an important element that forms MnS and MnSe called inhibitors that influence secondary recrystallization. If it is less than 0.02%, the absolute amount of MnS or the like necessary for causing secondary recrystallization is insufficient, which is not preferable.
On the other hand, if it exceeds 0.30%, not only is it difficult to form a solid solution during heating of the slab, but also the precipitation size during hot rolling tends to become coarse, and the optimum size distribution as an inhibitor is impaired, which is not preferable.

S and Se are important elements forming Mn and MnS and / or MnSe described above. 0.005
If it deviates from the range of 0.040%, a sufficient inhibitory effect cannot be obtained, so it was decided to add in this range.

Acid-soluble Al is a main inhibitor constituent element for high magnetic flux density grain-oriented electrical steel sheet, and is 0.0
If it is less than 15%, the amount is insufficient and the inhibitor strength is insufficient, which is not preferable. On the other hand, if it exceeds 0.040%, AlN precipitated as an inhibitor becomes coarse, and as a result, the inhibitor strength is lowered, which is not preferable.

N is an important element forming AlN and the acid-soluble Al. If it deviates from the above range, a sufficient inhibitory effect cannot be obtained, so 0.0030 to 0.01
It should be limited to 50%.

Further, Sn is effective as an element for stably obtaining secondary recrystallization of a thin product, and also has an action of reducing the secondary recrystallization particle size, so it is added if necessary. In order to obtain this effect, it is necessary to add 0.05% or more, and if it exceeds 0.50%, the action is saturated, so from the viewpoint of cost increase, it is limited to 0.50% or less.

Since Cu is effective as a primary coating improving element for Sn-added steel, it is added if necessary. 0.
If it is less than 01%, the effect is small, and if it exceeds 0.10%, the magnetic flux density of the product decreases, which is not preferable.

Sb and Mo are effective as elements for stably obtaining secondary recrystallization of thin products, so Sb and Mo are added as necessary. To obtain this effect, 0.0030%
It is necessary to add the above, and if it exceeds 0.30%, its action will be saturated, so 0.30 from the viewpoint of cost increase.
% Or less.

Bi is an essential element in the slab which is the starting material in the stable production of the ultrahigh magnetic flux density grain-oriented electrical steel sheet of B ₈ ≧ 1.92T according to the present invention. That is,
It has the effect of improving the magnetic flux density. If it is less than 0.0005%, the effect is not sufficiently obtained, and if it exceeds 0.05%, not only the effect of improving the magnetic flux density is saturated, but also cracks occur at the end of the hot rolled coil, which is not preferable. .

Next, a method for improving adhesion and improving iron loss by the stable production of the primary coating of the present invention will be described. The molten steel for producing an ultra-high magnetic flux density unidirectional electrical steel sheet having the components adjusted as described above is cast by a usual method. There is no particular limitation on the casting method. Then, it is rolled into a hot rolled coil by ordinary hot rolling.

Subsequently, the product sheet thickness is finished by hot-rolled sheet annealing followed by finish cold-rolling, a plurality of cold-rolled sheets including intermediate annealing, or a plurality of cold-rolled sheets including hot-rolled sheet annealing and intermediate annealing. In the annealing before finish cold rolling, the crystal structure is homogenized and AlN precipitation is controlled.

Then, after the continuous decarburization annealing and before the finish annealing, an annealing separating agent containing MgO as a main component is applied.

This annealing separator is generally applied as a slurry to a steel sheet, and then dried and finish annealing is carried out. In the present invention, the water content of the annealing separator coating film at the start of finish annealing is 0.3 to 0.3%. It is characterized by being 3%.

After the finish annealing, continuous strain relief annealing / secondary coating application and baking are performed. Further, magnetic domain subdivision processing such as laser irradiation and groove formation is performed if necessary.

Up to now, no particular reference has been made to the formation of the primary coating in the production of ultrahigh magnetic flux density grain-oriented electrical steel sheet. For example, in Japanese Patent Laid-Open No. 6-88171,
Not mentioned at all. Nevertheless, the primary coating may be defective in some cases, and the current situation is that stable production has not been achieved.

However, after decarburization annealing, an annealing separator containing MgO as a main component is applied, and the water content of the annealing separator at the start of finish annealing is set to 0.3 to 3%.
It was found that the degree of 10} <001> orientation integration is extremely important for the stable formation of the primary coating of the grain-oriented electrical steel sheet, which is extremely excellent.

In FIG. 1, C: 0.074%, Si: 3.26
%, Mn: 0.08%, S: 0.027%, acid-soluble A
1: 0.025%, N: 0.0080%, Bi: 0
A slab containing 0.00500% is subjected to decarburization annealing in a normal process, and then an annealing separator containing MgO as a main component is applied at 6 g / m ² per one side of the steel sheet, and the water content at the start of finish annealing is 0.1%. ˜5% and Bi at the time of post-process treatment
The content, the water content of the annealing separator at the start of finish annealing, and the primary film rating are shown. The score was decided as follows. 20
If the product does not peel off even if it is bent along a round bar with a diameter of 30 mm, it is B if the product does not peel even if it is bent along a round bar with a diameter of 30 mm, and if the product is bent along a round bar with a diameter of 30 mm. The case of peeling was designated as C. That is, the score A is the best, followed by B and C.

From FIG. 1, when the Bi content is less than 5 ppm, the water content of the annealing separator at the start of finish annealing is 0.3.
The primary coating score is A even if less than 3% or more than 3%,
It can be seen that when the Bi content is 5 ppm or more, the primary film rating is 0.3 to 3% and B and C are outside this range.

FIG. 2 shows C: 0.080%, Si: 3.2.
8%, Mn: 0.08%, S: 0.024%, acid-soluble Al: 0.025%, N: 0.0088%, Bi: 0.
A slab containing 0087% is subjected to decarburization annealing in a normal process, and then an annealing separator containing MgO as a main component is applied at 6 g / m ² per side of the steel sheet, and the water content at the start of finish annealing is 0.1 to 5 %, And shows the water content, the primary coating amount, and the iron loss W _17/50 when the finish annealing at 1200 ° C., the secondary coating film coating, and the magnetic domain control by laser irradiation are performed. It can be seen that by setting the water content to 0.3 to 3%, the amount of primary coating is 2.5 g / m ² or more, so that the applied tension is sufficient and excellent iron loss is obtained.

The main reasons for this are as follows. Finish annealing is generally performed in a coil shape, but Mg
When the oxidization degree increases more than necessary due to the water introduced by the annealing separator containing O as a main component, a low melting point composite oxide is formed between the Bi-based oxide and the Si-based oxide or the Al-based oxide. However, defective primary film formation occurs. On the contrary, if the degree of oxidation caused by the water introduced by the annealing separator containing MgO as the main component falls below the required level, the formation reaction of Mg ₂ SiO ₄ which is the main compound of the primary coating does not proceed and the primary coating formation failure occurs. .

However, it is considered that the primary film formation was stably performed by appropriately controlling the water content of the annealing separator at the start of finish annealing. The water content of the annealing separator coating at the start of finish annealing is determined by the temperature of the sheet after drying after applying the slurry annealing separator to the steel sheet, the elapsed time until the start of finishing annealing after drying, and the MgO Ig-loss. Etc. For example, if the plate temperature at the time of applying and drying the slurry-like annealing separator is low, the water content increases. Further, when the elapsed time after the start of finish annealing after drying becomes long, the annealing separator coating film tends to absorb moisture and increase the water content. Therefore, it is important to control the water content appropriately so that the water content at the start of finish annealing falls within the range of the present invention.

Various methods have been described so far for stably forming a primary coating in the production of a high magnetic flux density unidirectional electrical steel sheet. For example, JP-A-59-56582
Japanese Patent Laid-Open No. 62-54085, Japanese Patent Laid-Open No. 2-2
No. 59017 is cited. These all define the components in the annealing separator, the amount of the annealing separator applied, and the properties of the surface oxide layer formed by decarburization annealing.

On the other hand, the present invention focuses on Bi in the base iron, which has a great influence on the formation of the primary coating, and forms a low melting point composite oxide whose oxide is a Si-based oxide or an Al-based oxide. This is a big difference in that it is considered that the formation of the primary coating film is caused by this. The behavior is to be controlled by the water content of the coating film for the annealing separator at the start of finish annealing, which is completely different from the prior art.

[0039]

EXAMPLES [Example 1] C: 0.074%, Si: 3.26%, Mn: 0.08
%, S: 0.027%, acid-soluble Al: 0.025%,
A slab containing N: 0.0080% and Bi: 0.0087% was heated at 1360 ° C. and immediately hot rolled to give a hot rolled coil having a thickness of 2.3 mm. Then, the obtained hot-rolled coil has 10
It was annealed at 80 ° C., cold rolled once to a thickness of 0.220 mm, and then decarburized annealed at 850 ° C.

Thereafter, MgO containing 3% of TiO ₂ was added.
After the annealing separator is applied to the steel sheet, the water content is set to 0.1 to 4.8% by changing the sheet temperature at the time of drying.
The final annealing was performed at 0 ° C., the secondary coating was applied, and the magnetic domains were controlled by laser irradiation. After the finish annealing, the primary coating amount was measured, and the iron loss was measured after controlling the magnetic domains by laser irradiation. Table 1 shows the amount of primary coating and the iron loss W _17/50 .

[0041]

[Table 1]

As is clear from Table 1, the water content is 0.3.
At ~ 3%, a good primary coating amount of 2.5 g / m ² or more is formed. In addition, 0.70 W / k
An extremely excellent iron loss of g or less is obtained.

Example 2 C: 0.075%, Si: 3.22%, Mn: 0.08
%, S: 0.027%, acid-soluble Al: 0.025%,
A slab containing N: 0.0080% and Bi: 0.046% was heated at 1360 ° C. and immediately hot rolled to give a hot rolled coil having a thickness of 2.3 mm. The picked hot-rolled coil was pickled 1.6
It was pre-cold rolled to 0 mm and annealed at 1000 ° C. to 0.200 mm. After that, 10% of TiO ₂ was added to MgO, and Na ₂ B ₄ O was added.
After applying the annealing separator containing 0.6% of ₇ added, the water content at the start of finish annealing is set to 0.1 to 5.0% by changing the plate temperature during drying, and the finish annealing is performed at 1200 ° C. , The secondary coating was applied, and the magnetic domain was controlled by laser irradiation. After the finish annealing, the primary coating film was measured, and the core loss was measured after controlling the magnetic domains by laser irradiation.

Primary coating rating, primary coating amount and iron loss W _17/50
Is shown in Table 2. The score was decided as follows. 20 mm
If the product does not peel off even if it is bent along a round bar,
The case where the product was not peeled off even if the product was bent along the 30 mm diameter round bar was designated as B, and the case where the product was peeled off when bent along the 30 mm diameter round bar was designated as C. That is, the score A is the best, followed by B and C.

[0045]

[Table 2]

As is clear from Table 2, the water content is 0.3.
-3%, a primary coating amount of 2.5 g / m ² or more and a primary coating adhesion rating A are obtained. By this,
An extremely excellent iron loss of 0.70 W / kg or less is obtained.

Example 3 C: 0.078%, Si: 3.30%, Mn: 0.08
%, S: 0.025%, acid-soluble Al: 0.029%,
N: 0.0084%, Sn: 0.14%, Cu: 0.0
A slab containing 0.0122% Bi added to molten steel containing 60% was heated at 1350 ° C. and immediately hot-rolled to 2.0.
A hot rolled coil having a thickness of mm was used. Next, the obtained hot rolled coil was annealed at 1050 ° C., cold rolled twice to a thickness of 0.220 mm, and then decarburized and annealed at 840 ° C.

After that, TiO ₂ was 8% and Sb ₂ (S
After the MgO annealing separator containing 0.1% of O ₄ ) ₃ is applied to the steel sheet, the water content at the start of finish annealing is set to 0.1 to 5.3% by changing the sheet temperature during drying. 118
Finish annealing was performed at 0 ° C. After washing with water, the primary coating film was measured. Table 3 shows the primary coating adhesion rating and the primary coating amount. The rating is the same as the evaluation method of Example 2.

[0049]

[Table 3]

As is clear from Table 3, the water content is 0.3.
At 3%, the primary coating adhesion rating A is obtained with a good primary coating amount of 2.5 g / m ² or more.

Example 4 C: 0.078%, Si: 3.30%, Mn: 0.08
%, Se: 0.025%, acid-soluble Al: 0.025
%, N: 0.0084%, Sb: 0.022%, Mo:
A slab containing 0.014% and Bi: 0.0080% was heated at 1350 ° C. and immediately hot rolled to give a hot rolled coil having a thickness of 2.3 mm. The obtained hot-rolled coil was cold-rolled twice including intermediate annealing at 1000 ° C. to a thickness of 0.220 mm, and then 860
Decarburization annealing was performed at ℃.

Then, after applying the MgO annealing separator containing 10% of TiO _{2 and} 0.3% of SrSO ₄ to the steel sheet,
By changing the plate temperature during drying, the water content at the start of finish annealing was set to 0.1 to 5.5%, and finish annealing was performed at 1200 ° C. After washing with water, the primary coating film was measured. Table 4 shows the primary coating adhesion rating and the primary coating amount. The rating is the same as the evaluation method of Example 2.

[0053]

[Table 4]

As is clear from Table 4, the water content is 0.3.
At 3%, the primary coating adhesion rating A is obtained with a good primary coating amount of 2.5 g / m ² or more.

[0055]

EFFECT OF THE INVENTION A hot rolled coil obtained from a slab for unidirectional electrical steel sheet containing Bi added is subjected to finish cold rolling after hot rolled sheet annealing, or a plurality of cold rolled sheets including intermediate annealing, or after hot rolled sheet annealing. After finishing the product sheet thickness by multiple cold rolling including intermediate annealing, decarburizing annealing, after applying the annealing separator, finish annealing is started in the manufacturing method of super high magnetic flux density unidirectional electrical steel sheet. By setting the moisture content of the annealing separator containing MgO as the main component at 0.3 to 3%, a super-high magnetic flux density unidirectional electrical steel sheet having a good primary coating can be obtained, and after the magnetic domain subdivision treatment. The iron loss characteristics of are extremely excellent and can be said to be industrially very valuable and useful.

[Brief description of drawings]

FIG. 1 is a chart showing the relationship between the Bi content, the water content of the annealing separator at the start of finish annealing, and the primary coating adhesion rating.

FIG. 2 shows the water content of the annealing separator at the start of finish annealing, the amount of primary coating, and the iron loss W after controlling the magnetic domains by laser irradiation.
It is a chart showing the relationship with _17/50 .

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI H01F 1/16 H01F 1/16 B (72) Inventor Kazutoshi Takeda 1 Fuji-machi, Hirohata-ku, Himeji-shi, Hyogo Nippon Steel Corporation Hirohata Works Ltd. (72) Inventor Kento Abe No. 1 Fuji-machi, Hirohata-ku, Himeji City, Hyogo Prefecture Nippon Steel Corporation Hirohata Works (56) Reference JP-A-8-283865 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C21D 8/12 C21D 9/46 501 C22C 38/00-38/60 H01F 1/16-1/18

Claims (3)

(57) [Claims] 1. By weight%, C: 0.03 to 0.15%, Si: 2.5 to 4.0%, Mn: 0.02 to 0.30%, one or both of S and Se: 0.005-0.04
0%, acid-soluble Al: 0.015 to 0.040%, N: 0.0030 to 0.0150%, Bi: 0.0005 to 0.05%, with the balance being Fe and inevitable impurities. A slab is used as a starting material, and the slab is heated and then hot-rolled to produce a hot-rolled sheet, which is subjected to hot-rolled sheet annealing and then finish cold-rolling, or multiple cold-rolling including intermediate annealing, or hot-rolled sheet. A unidirectional electrical steel sheet that is finish-annealed by finishing the product sheet thickness by performing multiple cold-rolling including intermediate annealing and decarburizing-annealing after annealing, and then applying an annealing separator containing MgO as a main component to the steel sheet surface. The method for producing an ultra-high magnetic flux density unidirectional electrical steel sheet according to the above method, wherein the water content of the annealing separator coating film is 0.3 to 3% at the start of finish annealing. 2. By weight%, C: 0.03 to 0.15%, Si: 2.5 to 4.0%, Mn: 0.02 to 0.30%, one or both of S and Se: 0.005-0.04
0%, acid-soluble Al: 0.015 to 0.040%, N: 0.0030 to 0.0150%, Sn: 0.05 to 0.50%, Cu: 0.01 to 0.10%, Bi The method for producing an ultrahigh magnetic flux density grain-oriented electrical steel sheet according to claim 1, wherein the starting material is a slab containing 0.0005 to 0.05% and the balance being Fe and inevitable impurities. 3. By weight%, C: 0.03 to 0.15%, Si: 2.5 to 4.0%, Mn: 0.02 to 0.30%, one or both of S and Se: 0.005-0.04
0%, acid-soluble Al: 0.015 to 0.040%, N: 0.0030 to 0.0150%, one or both of Sb and Mo: 0.0030 to 0.
3%, Bi: 0.0005 to 0.05%, and a balance of Fe and unavoidable impurities as a starting material is a slab as a starting material. Manufacturing method.