KR20140060725A

KR20140060725A - Non-oriented electrical steel steet and manufacturing method for the same

Info

Publication number: KR20140060725A
Application number: KR1020120127371A
Authority: KR
Inventors: 김재훈; 한동현
Original assignee: 주식회사 포스코
Priority date: 2012-11-12
Filing date: 2012-11-12
Publication date: 2014-05-21

Abstract

The present invention relates to a non-oriented electrical steel sheet and a method of manufacturing the same. The steel sheet comprises 2.5 to 3.5% of Si, 0.5 to 3.5% of Mn, 0.5 to 3.5% of Cr, 0.8% At least one of P: not more than 0.02% (excluding 0%), 0.0008 to 0.003% of Mg, 0.025% or less of Cu (excluding 0%) as a base component, 0.01 to 0.08% of Sn and 0.005 to 0.05% of Sb A non-oriented electrical steel sheet comprising 0.01 to 0.1% of Sn + Sb, satisfying Si + Al + Mn / 2 + Cr / 2: 4.1 to 5.9%, and the balance Fe and other inevitably added impurities; A manufacturing method thereof is disclosed.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-oriented electrical steel sheet,

The present invention relates to a non-oriented electrical steel sheet and a method of manufacturing the same, and more particularly, to a non-oriented electrical steel sheet which improves magnetic properties by controlling Cr by addition of Si, Al and Mn, and a manufacturing method thereof.

Recently, technologies to convert existing internal combustion engine vehicles into hybrid electric vehicles (HEVs) or electric vehicles (EVs) have been rapidly developed as measures to reduce fossil fuel shortages and greenhouse gas emissions. These hybrid vehicles or electric vehicles are vehicles that convert some or all of the drive system into electric motors to reduce the consumption of gasoline or diesel fuel, which is a conventional internal combustion engine fuel, while achieving better fuel economy.

The motors used in such automobiles must produce a large torque at low speeds or accelerations, and at high speeds at constant speeds and at high speeds. Therefore, the nonoriented electric steel sheet, which is an iron core material, should have a large magnetic flux density characteristic at low speed rotation and have a low iron loss at high frequency. The measurement range of high-frequency iron loss varies depending on the driving characteristics of the vehicle, but a value of 800 Hz or more is mainly used in automotive non-oriented electrical steel plates rotating at high speed.

In order to improve the high-frequency iron loss in the electric steel sheet, it is necessary to increase the resistivity of the material. In order to increase the resistivity of the electric steel sheet used for commercial use, alloying elements such as Si, Al and Mn are mainly added. However, in the case of addition of such alloying elements, when the Si is 3.4% It was impossible to roll and the inherent resistance for obtaining high-frequency iron loss could not exceed 60 μΩ · cm. However, a higher value is preferable in an electric steel sheet for an automobile drive motor rotating at a high speed.

Therefore, in order to improve the high-frequency iron loss by further increasing the intrinsic resistance of the material, and to have processability that can be produced in a conventional electric steel sheet production process, elements other than Si, Al and Mn should be considered as additional elements. From this point of view, there are technologies that have focused on the intrinsic resistance of Cr. For example, Japanese Patent Application Laid-Open No. 8-47235 discloses a technique for increasing the resistivity by adding Cr at a content of 9% or more. However, there is a problem that Ti, which is disadvantageous to magnetism, is added and high-frequency magnetism is heated. In addition, Japanese Patent Application Laid-Open No. 11-343544 discloses an Fe-Cr-Si alloy excellent in high resistivity and workability, but still lacks high-frequency iron loss for application to automobile drive motors. In Japanese Patent Application Laid-Open No. 2004-339603, Cu was added in Fe-Cr-Si steel to achieve both high strength and high frequency iron loss, but Cu aging took a long time and productivity was extremely low.

Therefore, it is necessary to provide an electric steel sheet technology having excellent high-frequency iron loss by obtaining 60 ~ 80 μΩ · cm which is the inherent resistance value which can not be obtained until now, while having processability that can be made by ordinary electric steel sheet process by appropriately adding Cr.

In order to solve the above-mentioned problems, the present invention relates to a method of manufacturing an electric steel sheet, which comprises adding Cr in addition to Si, Al, and Mn and having an intrinsic resistance value of 60 to 80 占占㎝ m, An electric steel sheet and a manufacturing method thereof.

In one or more embodiments of the present invention, it is preferable that the steel sheet contains 2.5 to 3.5% of Si, 0.5 to 3.5% of Mn, 0.5 to 3.5% of Cr, 0.8% or less of Al (excluding 0% (Excluding 0%), Mg: 0.0008 to 0.003%, Cu: not more than 0.025% (excluding 0%), at least one of Sn: 0.01 to 0.08% and Sb: 0.005 to 0.05% An unoriented electrical steel sheet which is composed of + Sb: 0.01 to 0.1%, Si + Al + Mn / 2 + Cr / 2: 4.1 to 5.9%, and the balance Fe and other inevitably added impurities can be provided.

In one or more embodiments of the present invention, in one or more embodiments of the present invention, it is preferable that Si: 2.5 to 3.5%, Mn: 0.5 to 3.5%, Cr: 0.5 to 3.5%, Al: 0.8% , At least one of Sn: 0.01 to 0.08% and Sb: 0.005 to 0.05% is added as a base component, and the content of Mg is 0.0008 to 0.003% and Cu is 0.025% or less (excluding 0% , Sn + Sb: 0.01 to 0.1%, Si + Al + Mn / 2 + Cr / 2: 4.1 to 5.9%, and the balance Fe and other inevitably added impurities; Heating the slab at a temperature ranging from 1,100 to 1,250 ° C; Hot rolling the heated slab to produce a hot rolled sheet; Annealing the hot-rolled sheet in the temperature range of 850 to 1,150 캜, omitting the hot-rolled sheet, and pickling the hot-rolled sheet; Cold-rolling the annealed hot-rolled sheet to produce a cold-rolled sheet; And finally annealing the cold-rolled steel sheet.

And the final annealing is performed in a temperature range of 750 to 1,100 ° C.

Further, it satisfies the relational expression of Si + Al: 2.5 to 3.5% and satisfies the relational expression of Cr + Mn: 1.2 to 6.8%.

The inevitably added impurities may include C, S, N, and Ti, and the content thereof is 0.004 wt% or less.

According to the embodiment of the present invention, a non-oriented electrical steel sheet suitable for high-speed rotation can be manufactured because Si, Al, Mn, and Cr can be optimally set and produced by a normal process,

Further, by controlling the component system, it is possible to produce an electrical steel sheet having a specific resistance of 60 to 80 mu OMEGA.

Advantages and features of the present invention and methods of achieving them will become apparent with reference to the embodiments described in detail below. However, it is to be understood that the present invention is not limited to the disclosed embodiments, but may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. It is intended that the disclosure of the present invention be limited only by the terms of the appended claims.

In the examples according to the present invention, a non-oriented electrical steel sheet excellent in high frequency magnetism and good in workability was prepared by adding Cr as well as Si, Al and Mn as elements added to steel, and controlling the ratio of these elements.

According to the embodiment of the present invention, the non-oriented electrical steel sheet excellent in workability and remarkably improved in high frequency magnetic properties comprises 2.5 to 3.5% of Si, 0.5 to 3.5% of Mn, 0.5 to 3.5% of Cr, 0.5 to 3.5% (Excluding 0%), P: not more than 0.02% (excluding 0%), Mg: 0.0008 to 0.003%, Cu: not more than 0.025% : 0.005 to 0.05%, or a combination of Sn and Sb 2, and contains 0.01 to 0.1% of Sn + Sb, and Si + Al + Mn / 2 + Cr / 2 of 4.1 to 5.9% , Si + Al: 2.5 to 3.5%, and Cr + Mn: 1.2 to 6.8%, and the balance Fe and other inevitably added impurities. According to the above composition, it is possible to produce an electrical steel sheet having an intrinsic resistance of 60 to 80 mu OMEGA.

In addition to the above impurity elements, inevitably incorporated impurities such as C, S, N and Ti may be included.

In the examples according to the present invention, the following empirical formula was used for the relationship between the component (%) and the specific resistance (rho).

ρ = 13.25 + 11.3 (Al + Si + Cr / 2 + Mn / 2)

In the embodiment according to the present invention, the specific resistance value is higher than 35A210 according to JIS standard, and specifically, it has a value of 60 to 80 mu OMEGA .cm. This is because the inherent resistance value of the electric steel sheet must be high in order to obtain good characteristics in high-speed rotation, and the low eddy resistance leads to a large eddy current loss, which causes the motor efficiency to deteriorate rapidly.

Therefore, it is necessary to adjust the range of the intrinsic resistance within the range of 60 to 80 [micro] [Omega] -cm and to provide an electric steel sheet suitable for a hybrid motor vehicle or a drive motor of an electric vehicle.

First, the composition of the steel sheet and the reason for limiting the addition ratio between components in the examples according to the present invention will be described. Unless otherwise stated, the unit of component content is percent by weight (wt%).

Si: 2.5 to 3.5 wt%

When Si is added in an amount of less than 2.5%, the effect of improving the high-frequency iron loss is insufficient. When the Si content exceeds 3.5%, the hardness of the material increases, The content of Si is limited to the above range in the embodiment according to the present invention.

Al: 0.8 wt% or less (excluding 0%)

Al increases the resistivity of the material and lowers the iron loss. However, when Al exceeds 0.8%, the property of the mold flux is changed during the continuous casting, and the continuous casting becomes difficult due to the continuous casting. Further, at the time of annealing, the surface oxidation is promoted and the precipitates are formed by binding with Cr to deteriorate the magnetic properties. Therefore, the content of Al in the embodiment of the present invention is limited to the above range.

In addition, in the embodiment according to the present invention, the Si + Al content is limited to 2.5 to 3.5%. If the content is less than 2.5%, the effect of improving the high frequency iron loss is small. The productivity is lowered. Therefore, the content of Si + Al in the embodiment of the present invention is limited to the above range.

Mn: 0.5 to 3.5 wt%

When Mn is added in an amount of less than 0.5%, there is no effect of improving the high-frequency iron loss. When Mn exceeds 3.5%, the formation of [111] So that the content of Mn is limited to the above range in the examples according to the present invention.

Cr: 0.5 to 3.5 wt%

Cr has an advantage of increasing the resistivity of the material to improve the iron loss and not to increase the strength of the material. In order to secure high-frequency iron loss, it is required to add at least 0.5%. When it exceeds 3.5%, the magnetic flux density is rapidly reduced due to development of unfavorable texture of magnetism, so that the content of Cr is limited to the above range in the embodiment of the present invention.

In addition, in the embodiment according to the present invention, Cr + Mn is limited to 1.2 to 6.8%. If less than 1.2% is added, the effect of improving high-frequency iron loss is small, and if it exceeds 6.8% The range according to the embodiment of the present invention is limited to the above range.

Further, in the embodiment according to the present invention, Si + Al + Mn / 2 + Cr / 2 is limited to 4.1 to 5.9%, which should be 4.1% or more so that the resistivity is higher than that of a conventional non- The content of Si + Al + Mn / 2 + Cr / 2 in the embodiment according to the present invention is reduced in the range of the above-mentioned range .

P: 0.02 wt% or less (excluding 0%)

In the case of many alloying elements such as Si, Al, Mn, and Cr, P is mostly segregated in the grain boundaries to lower the toughness of the material, thereby lowering the productivity and the saturation. Therefore, The content of P is limited to the above range.

Sn: 0.01 to 0.08 wt%

Sn is segregated on the surface and grain boundaries of the steel sheet and inhibits surface oxidation during annealing and improves the texture. When it is added in an amount of less than 0.01%, Sn is not effective. When it exceeds 0.08%, Sn is segregated in grain boundaries, The productivity is lowered compared to the improvement in magnetic properties, so that the content of Sn is limited to the above range in the examples according to the present invention.

Sb: 0.005 to 0.05 wt%

Sb segregates on the surface and grain boundaries of the steel sheet and inhibits surface oxidation during annealing and improves the texture. When Sb is added in an amount of less than 0.005%, it is not effective. When it is added in excess of 0.05% The toughness is lowered and the productivity against magnetic improvement is lowered. Therefore, the content of Sb in the examples according to the present invention is limited to the above range.

In the embodiment according to the present invention, the total amount of Sn and Sb, [Sn] + [Sb] is limited to 0.01 to 0.1% because the surface and the magnetic improvement are most effective in this range. If the total amount of Sn and Sb exceeds 0.1%, the fraction of fine inclusions is increased to deteriorate the magnetic properties.

Mg: 0.0008 to 0.0030 wt%

Mg is combined with Sn or Sb to form Mg ₃ Sb ₂ and Mg ₂ Sn to form a fine dispersed phase. Therefore, it is necessary to control the ratio with Sn and Sb. Mg is advantageous to the magnetism as it is not present in the steel, but it is essentially introduced into the product due to the influence of the refractory during the manufacturing process. Therefore, the lower limit is set to 0.0008% in consideration of the amount of Mg.

Therefore, it is necessary to optimize the steelmaking and performance manufacturing process, increase the purity of various raw materials such as alloying elements, and control it to 0.0030% or less so as not to affect grain growth. If it exceeds 0.030%, the Mg ₃ Sb ₂ or Mg ₂ Sn dispersed phase is formed in the steel to have a diameter of 5 to 15 nm, thereby dislocating the magnet. Therefore, the content of Mg is limited to the above range in the examples of the present invention.

Cu: 0.025 wt% or less (excluding 0%)

Cu is limited to 0.025% or less because it forms an emulsion by binding with S in the steel or forms a fine dispersed phase by itself to lower the grain growth.

In addition to the above impurity elements, inevitably incorporated impurities such as C, S, N, and Ti may be included in embodiments according to the present invention. C causes self-aging, S and N form sulphides and nitrides, respectively, to dislocate grain growth, and Ti promotes the growth of [111] texture, which is an undesirable crystal orientation in the nonoriented electrical steel sheet. Is limited to 0.004% or less. And more preferably 0.003% or less.

Hereinafter, a method for manufacturing a non-oriented electrical steel sheet according to an embodiment of the present invention will be described.

First, in terms of% by weight, at least one of Si: 2.5 to 3.5%, Mn: 0.5 to 3.5%, Cr 0.5 to 3.5%, Al: 0.8% or less (excluding 0%), P: Of Sn and 0.01 to 0.08% of Sn or 0.005 to 0.05% of Sb, or a combination of two kinds of Sn and Sb species added in an amount of 0.0008 to 0.003% and Cu: 0.025% or less (excluding 0% , Sn + Sb: 0.01 to 0.1%, Si + Al + Mn / 2 + Cr / 2: 4.1 to 5.9%, Si + Al: 2.5 to 3.5% and Cr + Mn: 1.2 to 6.8% The remainder Fe and other inevitably added impurities is prepared, and the slab is heated to a temperature of 1,100 to 1,250 DEG C, followed by hot rolling while hot rolling is performed at 800 DEG C or higher.

The hot-rolled hot-rolled sheet is annealed in the temperature range of 850 to 1,150 占 폚 or omitted, pickled, cold-rolled at a reduction ratio of 70 to 95%, cold- To produce a non-oriented electrical steel sheet excellent in magnetic properties. Such a manufacturing method is the same as a general electric steel sheet manufacturing process, and is excellent in productivity and high in resistivity, so that it is suitable as a motor core for high-speed rotation.

Hereinafter, a method of manufacturing the non-oriented electrical steel sheet according to the present invention will be described in more detail.

In the embodiment according to the present invention, it is preferable to use one having a high purity of the alloy element in order to minimize the content of impurities in the steelmaking step. After the molten steel whose component composition is controlled as described above is solidified in the continuous casting step to produce a slab, the slab is charged into a heating furnace and reheated to a temperature of 1,100 ° C to 1,250 ° C. If the slab is heated to a temperature exceeding 1,250 캜, the inclusions which deteriorate magnetism may be reused and may be finely precipitated after hot rolling, so that the slab is heated at a temperature of 1,250 캜 or lower in the embodiment of the present invention.

After the slab is reheated, the hot rolling is performed. In the hot rolling in the embodiment according to the present invention, the hot rolling is performed at a temperature of 800 DEG C or higher.

The hot-rolled hot-rolled sheet is annealed at a temperature of 850 to 1,150 ° C to increase the crystal orientation favorable to magnetism. If the annealing temperature of the hot-rolled sheet is less than 850 ° C, the structure does not grow or grows finely, If the annealing temperature of the hot-rolled sheet exceeds 1,150 DEG C, the magnetic properties are rather deteriorated and the rolling workability may be deteriorated due to the deformation of the plate-like phase, so that the annealing temperature range of the hot- It is limited to 850 ~ 1,150 ℃. More preferably, the annealing temperature of the hot-rolled sheet is 950 to 1,150 ° C.

Thereafter, the hot rolled sheet is pickled, cold rolled at a reduction ratio of 70 to 95% to have a predetermined thickness, and an electric steel sheet used for a hybrid automobile or electric automobile has a thickness of 0.35 mm 0.2 mm.

If the final annealing temperature is less than 750 캜, recrystallization does not sufficiently occur. If the final annealing temperature exceeds 1,110 캜, the crystal grain diameter becomes too large and high-frequency iron loss is caused to heat. The final annealing temperature in the examples according to the invention is limited to the above range. The final annealing is preferably carried out at a temperature of 900 to 1,050 DEG C in order to obtain a grain size of 50 to 150 mu m.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Were vacuum-melted in a laboratory to produce ingots having the compositions shown in Table 1 below. All the impurities C, S, N and Ti of the material were controlled to 0.003% or less. Each material was heated to 1,150 캜 and hot-rolled at 850 캜 to produce a hot-rolled sheet having a thickness of 2.0 mm. The hot-rolled hot-rolled sheet was annealed at 1,100 ° C for 4 minutes and pickled. Thereafter, the sheet was cold-rolled to a thickness of 0.30 mm, and final annealing was performed at the respective temperatures shown in Table 2 for 38 seconds. The magnetic properties were determined by an average value in the rolling direction and the vertical direction using a single sheet tester and are shown in Table 2 below.

Steel grade Si Al Mn Cr P Sn Sb C S N Ti ±% ±% ±% ±% ±% ±% ±% ±% ±% ±% ±% One 3.4 0.8 0.5 0 0.01 0.03 - 0.0025 0.0025 0.0022 0.0023 2 2.5 0.8 0.5 One 0.01 0.03 - 0.0018 0.0026 0.0021 0.0018 3 2.5 0.8 1.5 2 0.01 - - 0.0023 0.0025 0.0023 0.0018 4 2.5 0.8 1.5 1.5 0.01 - 0.03 0.0021 0.0024 0.0018 0.0021 5 3.0 0.8 2 1.5 0.01 0.03 0.03 0.0024 0.0021 0.0018 0.0018 6 3.0 0.8 2 2 0.01 0.03 0.03 0.0022 0.0024 0.0022 0.0021 7 2.7 0.5 1.5 3 0.01 0.03 - 0.0024 0.0022 0.0018 0.0014 8 2.7 0.5 3 3 0.01 0.03 - 0.0019 0.0019 0.0023 0.0014 9 2.7 0.5 4 2 0.01 0.03 0.03 0.0023 0.0019 0.0022 0.0018 10 2.7 1.0 2 3 0.01 0.03 0.03 0.0022 0.0021 0.0021 0.0023 11 2.7 0.5 2 3 0.01 0.03 0.03 0.0021 0.0022 0.0018 0.0021 12 2.3 0.5 2.5 4 0.01 0.03 0.03 0.0020 0.0024 0.0021 0.0023 13 2.3 0.5 2.5 3.5 0.01 - 0.03 0.0018 0.0025 0.0018 0.0022

Steel grade
Si + Al Mn + Cr Si + Al + Mn / 2 + Cr / 2 Resistivity Final annealing temperature Hardness Grain size W10 / 400 W10 / 800 W10 / 1000 B50 Remarks
±% ±% ±% μΩ · cm ℃ Hv1 ㎛ W / Kg W / Kg W / Kg T One 4.2 0.5 4.45 64 980 228 101 14.5 43.1 61.8 1.65 Comparative Example 2 3.3 1.5 4.05 59 950 195 68 14.3 41.5 59.4 1.65 Comparative Example 3 3.3 3.5 5.05 70 990 195 95 13.8 40.4 57.8 1.61 Comparative Example 4 3.3 3 4.8 67 980 193 80 13.0 38.0 54.9 1.65 Honor 5 3.8 3.5 5.55 76 990 225 75 13.5 38.4 55.6 1.62 Comparative Example 6 3.8 4 5.8 79 990 230 70 13.2 38.6 56.2 1.62 Comparative Example 7 3.2 4.5 5.45 75 990 210 85 12.8 37.6 54.3 1.64 Honor 8 3.2 6 6.2 83 1020 215 70 12.0 37.1 53.6 1.63 Honor 9 3.2 6 6.2 83 1020 225 65 13.2 38.3 54.2 1.61 Comparative Example 10 3.7 5 6.2 83 1020 210 45 13.8 38.5 54.6 1.60 Comparative Example 11 3.2 5 5.7 78 1000 202 75 12.3 37.1 53.1 1.63 Honor 12 2.8 6.5 6.05 82 020 228 55 13.2 38.4 55.3 1.62 Comparative Example 13 2.8 6 5.8 79 1010 218 85 12.5 37.1 54.2 1.64 Honor

In the case of the steel types 4, 7, 8, 11, and 13 belonging to the range of the present invention, the intrinsic resistance component was optimally set, and the workability was good. On the other hand, the high-frequency iron loss is low due to low intrinsic resistance of 1 and 2, and the flux density is low because no special element such as Sn or Sb is added. Steel types 5, 6, and 9 are too hard to produce because of the normal rolling process in order to heat the rolling properties. In the steel type 10, the inclusion in the material increases due to the increase of the Al content, and the magnetic properties are poor due to the low grain growth. Therefore, if it is outside of the scope of the present invention, it is understood that there is a lack of magnetization in crystal grain growth, or a hardening of the material, which makes it difficult to manufacture in a normal process.

And an electrical steel sheet prepared as above is less than 13W / kg iron loss (W _10/400) on the basis of the sheet thickness 0.3mm, the iron loss _{(W10 / 800)} is 38W / kg or less, the iron loss (W _10/1000) is 55 W / kg or less, and a magnetic flux density (B50) of 1.63 T or more.

While the present invention has been described in connection with certain exemplary embodiments, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined by the appended claims.

It is therefore to be understood that the above-described embodiments are illustrative in all aspects and not restrictive. The scope of the present invention is defined by the appended claims rather than the detailed description, and all changes or modifications derived from the meaning and scope of the claims and their equivalents should be interpreted as being included in the scope of the present invention .

Claims

(Excluding 0%), P: 0.02% or less (excluding 0%), Mg: 0.0008 to 0.5% 0.003% and Cu: 0.025% or less (excluding 0%) as a base component, at least one of Sn: 0.01 to 0.08% and Sb: 0.005 to 0.05%
Sn + Sb: 0.01 to 0.1%
Si + Al + Mn / 2 + Cr / 2: 4.1 to 5.9%
And the balance Fe and other inevitably added impurities.

The method according to claim 1,
Si + Al: 2.5 to 3.5%.

3. The method according to claim 1 or 2,
Cr + Mn: 1.2 to 6.8%.

The method of claim 3,
Wherein the inevitably added impurities may include C, S, N, and Ti, and the content thereof is 0.004 wt% or less, respectively.

(Excluding 0%), P: 0.02% or less (excluding 0%), Mg: 0.0008 to 0.5% At least one of Sn: 0.01 to 0.08% and Sb: 0.005 to 0.05%, Sn + Sb: 0.01 to 0.1%, Si + Al + Mn / 2 + Cr / 2: 4.1 to 5.9%, and the balance Fe and other inevitably added impurities;
Heating the slab at a temperature ranging from 1,100 to 1,250 ° C;
Hot rolling the heated slab to produce a hot rolled sheet;
Annealing the hot-rolled sheet in the temperature range of 850 to 1,150 캜, omitting the hot-rolled sheet, and pickling the hot-rolled sheet;
Cold-rolling the annealed hot-rolled sheet to produce a cold-rolled sheet; And
And finally annealing the cold rolled steel sheet.

6. The method of claim 5,
Si + Al: 2.5 to 3.5%. &Lt; / RTI >

The method according to claim 5 or 6,
Cr + Mn: 1.2 to 6.8%. &Lt; RTI ID = 0.0 > 11. < / RTI >

8. The method of claim 7,
Wherein the final annealing is performed in a temperature range of 750 to 1,100 ° C.

9. The method of claim 8,
Wherein the inevitably added impurities may include C, S, N, and Ti, and the content thereof is 0.004 wt% or less.