Argon Oxygen Decarburization Refining Method For Molten Austenitic Stainless Steel

Abstract

An argon oxygen decarburization (AOD) refining method for molten austenitic stainless steel includes, preparing molten austenitic stainless steel in an electric arc furnace, pouring the molten austenitic stainless steel into an AOD refining furnace by adjusting a carbon concentration of the molten austenitic stainless steel to 2.0 wt % to 2.5 wt %, decarburizing the poured molten austenitic stainless steel by blowing oxygen (O.sub.2) and argon (Ar) thereinto, and reduction-decarburizing the decarburized molten austenitic stainless steel by blowing argon (Ar) thereinto.

Description

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the priority and benefit of Korean Patent Application No. 10-2015-0183587, filed on Dec. 22, 2015, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

[0002] The present disclosure relates to an argon oxygen decarburization (AOD) refining method for molten austenitic stainless steel.

BACKGROUND ART

[0003] Stainless steel is commonly manufactured using a strip casting method, and thus, manufacturing costs may be reduced, in comparison with a slab casting method. There are also the advantages of suppressing a precipitation phase by rapid solidification, and having an excellent slab internal quality due to inclusion refinement or the like. As a result, demand therefor has been increasing.

[0004] However, to improve workability of a final product of a stainless steel grade, levels of carbon (C) and nitrogen (N) in molten steel should be managed to be low.

[0005] By managing levels of carbon (C) and nitrogen (N) to be low, yield strength of stainless steel can be reduced and formability thereof can be improved. When yield strength of stainless steel is reduced, a springback phenomenon is reduced and workability such as bending or the like is increased. Therefore, stainless steel can be used for various purposes in an electronic product such as an air conditioner pipe, or the like.

[0006] In a case of ferritic stainless steel formed using vacuum oxygen decarburization (VOD), since the atmosphere is controlled to have a low partial pressure by vacuum equipment, decarburizing efficiency is improved due to O.sub.2 and Ar blowing, ingress of air is blocked, and nitrogen is controlled. Therefore, in the case of ferritic stainless steel, the content of carbon and nitrogen in steel can be significantly reduced.

[0007] On the other hand, in the case of austenitic stainless steel formed using only argon oxygen decarburization (AOD), there may be limitations to reducing the content of carbon and nitrogen.

[0008] (Prior art document) Patent Document 1: Korea Patent Application No. 2009-0128466

DISCLOSURE

Technical Problem

[0009] An aspect of the present disclosure may provide an argon oxygen decarburization (AOD) refining method for molten austenitic stainless steel and, more particularly, to an argon oxygen decarburization (AOD) refining method capable of reducing carbon and nitrogen in molten austenitic stainless steel in AOD refinement.

[0010] On the other hand, the objective of the present disclosure is not limited to the above description. The objective of the present disclosure maybe understood from the content of the present specification. Those skilled in the art have no difficulty in understanding additional objectives of the present disclosure.

Technical Solution

[0011] According to an aspect of the present disclosure, an argon oxygen decarburization (AOD) refining method for molten austenitic stainless steel includes: preparing molten austenitic stainless steel in an electric arc furnace; pouring the molten austenitic stainless steel into an AOD refining furnace by adjusting a carbon concentration of the molten austenitic stainless steel to 2.0 wt % to 2.5 wt %; decarburizing the poured molten austenitic stainless steel by blowing oxygen (O.sub.2) and argon (Ar) thereinto; and reduction-decarburizing the decarburized molten austenitic stainless steel by blowing argon (Ar) thereinto.

[0012] The reduction-decarburizing may be performed under conditions in which a flow rate of Ar is 50 Nm.sup.3/min to 55 Nm.sup.3/min.

[0013] The decarburizing may be performed by gradually reducing a flow rate of the oxygen and gradually increasing a flow rate of the argon.

[0014] An initial flow rate of the oxygen may be 140 Nm.sup.3/min to 170 Nm.sup.3/min.

[0015] A nitrogen concentration of the molten austenitic stainless steel after the reduction-decarburizing maybe 75 ppm or less.

[0016] A component of the molten austenitic stainless steel after the reduction-decarburizing includes, by wt %, C: 0.003% to 0.16%, Si: 0.2% to 0.7%, Mn: 1.0% to 5.0%, P: 0.03% or less, S: 0.02% or less, Cr: 16% to 18%, Ni: 7% to 9%, Mo: 0.001% to 0.200%, N: 75 ppm by weight or less, iron (Fe) as a remainder thereof, and other unavoidable impurities.

[0017] In addition, the solution to the problems described above does not list all features of the present disclosure. Various features, advantages, and effects of the present disclosure can be understood in more detail with reference to the following specific embodiments.

Advantageous Effects

[0018] According to an exemplary embodiment in the present disclosure, an AOD refining method capable of reducing carbon and nitrogen of molten austenitic stainless steel in AOD refining, may be provided.

DESCRIPTION OF DRAWINGS

[0019] FIG. 1 is a schematic diagram illustrating a conventional stainless steel manufacturing process.

[0020] FIG. 2 is a graph illustrating a change in yield strength according to the content of C+N in austenitic stainless steel (304J1).

[0021] FIG. 3 is a graph illustrating a change in a critical carbon concentration according to CO partial pressure in stainless steel in which the content of Cr is 18%.

[0022] FIG. 4 is a schematic diagram illustrating a conventional denitrification reaction mechanism.

[0023] FIG. 5 is a graph comparing AOD tapping nitrogen concentrations for each casting number according to a result in which an inventive example and a comparative example are applied to actual refining.

[0024] FIG. 6 is a graph comparing AOD tapping carbon concentrations for each casting number according to a result in which an inventive example and a comparative example are applied to actual refining.

BEST MODE FOR INVENTION

[0025] Hereinafter, exemplary embodiments of the present disclosure will be described. The disclosure may, however, be exemplified in many different forms and should not be construed as being limited to the specific embodiments set forth herein. Embodiments of the present disclosure are also provided to more fully describe the present disclosure to those skilled in the art.

[0026] The inventors recognize that there may be limitations to reducing the content of carbon and nitrogen on austenitic stainless steel formed using only argon oxygen decarburization (AOD), and have conducted research in order to solve this problem.

[0027] As a result, in the case that conditions of an AOD process are properly controlled, it is confirmed that carbon and nitrogen concentrations are efficiently reduced in molten steel, thereby completing an exemplary embodiment.

[0028] FIG. 1 is a schematic diagram illustrating a conventional process of manufacturing stainless steel. Molten metal melted in an electric arc furnace (EAF), that is, molten metal in EAF, is tapped to a charging ladle, and the charging ladle is tilted to remove a portion of slag floating on an upper part of the molten metal. In addition, after residual remaining slag is removed, the molten metal in EAF is poured into a refining furnace. In molten steel, in order to remove carbon in an argon oxygen decarburization (AOD) refining furnace, oxygen and argon gases are blown into the molten steel to perform decarburization. The molten steel passes through a further reduction process for chrome and iron oxides generated in the case of the decarburization. In a ladle treatment (LT) process for adjusting molten steel, fine component adjustment, molten steel temperature homogenization, and bottom bubbling (B/B) for improving a quality of molten steel may be performed.

[0029] To improve workability of a final product, levels of carbon (C) and nitrogen (N) in molten steel are required to be low. When yield strength of stainless steel is reduced, a springback phenomenon is reduced and workability such as bending or the like is increased. Therefore, it has the advantage of being used for various purposes in an electronic product such as an air conditioner pipe or the like. In detail, an influence of nitrogen (N) with respect to material softening is significantly great, whereby management with respect to nitrogen is important.

[0030] FIG. 2 illustrates a change in yield strength according to the content of C+N in austenitic stainless steel 304J1. When the content of C+N is reduced by 100 ppm, yield strength tends to be reduced by about 6 MPa to 7 MPa. When yield strength is maintained at a level of about 200 MPa or less, a material has further softening properties. Therefore, the material may be applied to a part requiring high softening properties such as an air conditioner pipe. In detail, carbon is required to be tapped after being removed through a sufficient decarburizing operation during an AOD process, and nitrogen is required to be tapped after being sufficiently removed through denitrification promoting and absorption preventing operations during an AOD process.

[0031] In addition, KA4 to KA7 denote a sample number (No.). KA6 and KA7 are values obtained through an argon oxygen decarburization (AOD) process. KA4 and KA5 are values obtained through a vacuum oxygen decarburization (VOD) process. The AOD process is confirmed to be limited to reducing the concentration of C+N.

[0032] Hereinafter, an AOD refining method for molten austenitic stainless steel according to an aspect of an exemplary embodiment will be described in detail.

[0033] An AOD refining method for molten austenitic stainless steel according to an aspect of an exemplary embodiment includes: preparing molten austenitic stainless steel in an electric arc furnace; pouring the molten austenitic stainless steel into an argon oxygen decarburization (AOD) refining furnace by adjusting a carbon concentration of the molten austenitic stainless steel to 2.0 wt % to 2.5 wt %; decarburizing the poured molten austenitic stainless steel by blowing oxygen (O.sub.2) and argon (Ar); and reduction-decarburizing the decarburized molten austenitic stainless steel by blowing argon (Ar).

[0034] Molten Steel Inputting Operation

[0035] An electric arc furnace, molten austenitic stainless steel is prepared, and a carbon concentration of the molten austenitic stainless steel is adjusted to 2.0 wt % to 2.5 wt % to pour the molten austenitic stainless steel into an argon oxygen decarburization (AOD) refining furnace.

[0036] To remove nitrogen dissolved in steel during AOD refining, a nitrogen gas emission capacity upwards a ladle is required to be improved. A conventional denitrification reaction mechanism is illustrated in FIG. 4.

[0037] 1) Each nitrogen atom inside a liquid is moved in an unspecified direction, but the entirety thereof is moved toward an interface. 2) The nitrogen atom moved toward the interface is absorbed at the interface. 3) Nitrogen atoms absorbed at the interface collide with each other. 4) The collided nitrogen atoms become nitrogen molecules (N.sub.2), and are moved from an interface of a liquid layer to an interface of a gas layer. 5) N.sub.2 moved toward the interface of the gas layer becomes a gas to be moved toward a vapor layer.

[0038] In general, operation 4) in which a nitrogen molecule (N.sub.2) is generated in a denitrification reaction is a rate limiting operation. In detail, an operation in which a nitrogen atom is moved to a reaction interface to generate a molecule at a low nitrogen concentration is the rate limiting operation, and thus, a CO, CO.sub.2 or Ar bubble serves to move a nitrogen atom toward a reaction interface, thereby promoting molecule generation.

[0039] To move a nitrogen atom toward a reaction interface by a CO, CO.sub.2 or Ar bubble, a generation amount of CO gas generated by blowing oxygen through a top lance in an upper part of a refining furnace and through a tuyere pipe in a lower part thereof, during refining decarburization is required to be increased.

[0040] CO gas generated by a chemical reaction in steel rises to an interface. In this case, a gas flow, in which nitrogen atoms inside molten steel are absorbed by the CO gas and the CO gas rises to an interface, is generated. As a result, a nitrogen atom is moved to an interfacial layer to remove nitrogen. To this end, a carbon concentration of molten steel is adjusted to 2.0 wt % to 2.5 wt % higher than a conventional carbon concentration to pour the molten steel into an argon oxygen decarburization (AOD) refining furnace, thereby increasing an amount of CO gas to reduce nitrogen in steel.

[0041] In a case in which the carbon concentration is less than 2.0 wt %, a generated amount of CO gas is insufficient, whereby a nitrogen reduction effect may be insufficient. On the other hand, in a case in which the carbon concentration exceeds 2.5 wt %, the carbon concentration is significantly high when tapping, whereby yield strength of a final product is increased. Thus, it is preferable that the carbon concentration is 2.0 wt % to 2.5 wt %.

[0042] As an electric arc furnace tapping carbon concentration is increased, a larger amount of CO gas may be generated as described later. Decarburizing is performed from a refining furnace initial carbon concentration to a critical carbon concentration for each operation by direct decarburizing caused by the reaction of [C]+[O]=CO(g) and indirect decarburizing caused by the reaction of Cr.sub.2O.sub.3+3[C]=2[Cr]+3CO (g), due to O.sub.2 injected through a top lance in the upper part of a refining furnace. When a carbon concentration reaches a critical carbon concentration and decarburizing efficiency is decreased, an injected concentration of oxygen is adjusted to increase decarburizing efficiency, and thus, a carbon concentration is gradually reduced, thereby lowering a critical carbon concentration in a next operation. In general, through a first blowing operation to a fifth blowing operation or further in a decarburizing operation, a flow rate of oxygen is gradually reduced and a flow rate of argon is gradually increased to perform decarburization, and thus, carbon may be effectively removed while oxidation of chrome is suppressed. Therefore, after AOD refining, when tapping, the content of carbon in molten steel may be lowered.

[0043] Decarburizing Operation

[0044] Oxygen (O.sub.2) and argon (Ar) are blown into the poured molten austenitic stainless steel to decarburize the molten austenitic stainless steel.

[0045] Decarburization refining in stainless steel denotes that decarburization is effectively performed while oxidation of Cr having high oxidative power while being relatively expensive is suppressed. In general, in a decarburization reaction of molten steel containing Cr, oxygen blown into molten steel oxidizes Cr first, and decarburization is performed by the medium of an oxide thereof. Specific reactions proceed according to Relational expressions.

[C]+[O]=CO(g)

2[Cr]+3[O]=Cr.sub.2O.sub.3

Cr.sub.2O.sub.3+3[C]=2[Cr]+3CO(g)

K = a [ Cr ] 2 P CO 3 a Cr 2 O 3 a [ C ] 3 ##EQU00001## log K=-40,990/T+25.83

Here, a.sub.i: activity of I component, K: equilibrium constant and Pco: partial pressure of CO gas.

[0046] In this case, the decarburization operation may be performed by gradually reducing a flow rate of the oxygen, and gradually increasing a flow rate of the argon.

[0047] Referring to FIG. 3, a graph illustrating a change in a critical carbon concentration according to CO partial pressure in stainless steel in which the content of Cr is 18%, a critical carbon concentration may be expressed as a function of CO partial pressure, activity of Cr (concentration of Cr), and a temperature. As seen in FIG. 3, at the higher temperature, the lower CO partial pressure, and the lower chrome concentration, a critical carbon concentration is lowered.

[0048] In other words, as carbon is removed from molten steel, an equilibrium chrome concentration is reduced. Chrome in the molten steel does not present above an equilibrium concentration, and thus, chrome is oxidized at high speed. When a temperature is increased or CO partial pressure is reduced, an equilibrium carbon concentration is lowered at the same chrome concentration. Thus, while oxidation of chrome is suppressed, carbon may be effectively removed.

[0049] Thus, a method for lowering CO partial pressure is to significantly increase efficiency of CO gas reaction by adjusting an amount of O.sub.2 reacting with carbon by properly controlling a ratio of O.sub.2 and Ar gases, and optimizing emission of CO gas upwardly by adjusting an injection amount of Ar.

[0050] In a decarburizing operation, decarburization is performed by gradually reducing a flow rate of oxygen and gradually increasing a flow rate of argon, thereby effectively lowering CO partial pressure. Thus, as an equilibrium carbon concentration is lowered at the same chrome concentration, carbon may be effectively removed while oxidation of chrome is suppressed.

[0051] For example, as described below, in a first blowing operation to a fifth blowing operation in a decarburization operation, decarburization is performed by gradually reducing a flow rate of oxygen and gradually increasing a flow rate of argon, thereby effectively removing carbon while oxidation of chrome is suppressed.

[0052] First blowing operation O.sub.2:Ar=150 Nm.sup.3/min :20 Nm.sup.3/min, critical carbon concentration 0.35 wt %

[0053] Second blowing operation O.sub.2:Ar=60 Nm.sup.3/min :20 Nm.sup.3/min, critical carbon concentration 0.20 wt %

[0054] Third blowing operation O.sub.2:Ar=45 Nm.sup.3/min :45 Nm.sup.3/min, critical carbon concentration 0.10 wt %

[0055] Fourth blowing operation O.sub.2:Ar=20 Nm.sup.3/min :60 Nm.sup.3/min, critical carbon concentration 0.05 wt %

[0056] Fifth blowing operation O.sub.2:Ar=12 Nm.sup.3/min :48 Nm.sup.3/min, critical carbon concentration 0.01 wt %

[0057] In addition, to lower Pco according to [C]% during blowing, an amount of inert gas is required to be increased. For inert gas, Ar may be used, but relatively inexpensive N.sub.2 may be used as long as it is no defective in terms of quality. However, in an exemplary embodiment, to lower yield strength and increase formability and workability, the content of C+N is required to be significantly reduced. Thus, Ar is used for inert gas.

[0058] It is preferable that an initial flow rate of the oxygen is 140 Nm.sup.3/min to 170 Nm.sup.3/min.

[0059] In a case in which an initial flow rate of the oxygen is less than 140 Nm.sup.3/min, a problem in which decarburizing efficiency is low may occur. In a case in which an initial flow rate of the oxygen exceeds 170 Nm.sup.3/min, oxygen may remain in molten steel to generate an oxide (inclusion), thereby causing degradation of a quality.

[0060] Reduction-Decarburization Operation

[0061] Argon (Ar) is blown into the decarburized molten austenitic stainless steel to perform reduction-decarburization, so as to further reduce chrome and iron oxides generated in the decarburization operation.

[0062] In this case, the reduction-decarburization may be performed under conditions in which a flow rate of Ar is 50 Nm.sup.3/min to 55 Nm.sup.3/min. Ar gas in reduction-decarburization serves to move a nitrogen atom to a reaction interface as described above.

[0063] As a flow rate of Ar is controlled to 50 Nm.sup.3/min to 55 Nm.sup.3/min, higher than a conventional flow rate thereof, a nitrogen atom is absorbed onto a surface of bubble of Ar, inert gas, to move a nitrogen atom to a reaction interface, thereby promoting molecularization of nitrogen to effectively reduce nitrogen.

[0064] In a case in which a flow rate of Ar is less than 50 Nm.sup.3/min, a nitrogen reduction effect is insufficient.

[0065] As described above, Ar injection is the principle of removing nitrogen by flow of a bubble in an upward direction by allowing nitrogen molecules contained in molten steel to be easily absorbed in Ar bubble while Ar is injected through a pipe in a lower part of an AOD refining furnace. In a case in which a flow rate of Ar is excessively high, as the flow of bubble coming up from the lower part is strong, molten steel in an upper part thereof and slag covering the molten steel are exposed to atmosphere, rather a nitrogen adsorption phenomenon in which atmospheric nitrogen is dissolved in steel may occur. Thus, it is preferable that an upper limit of a flow rate of Ar is 55 Nm.sup.3/min.

[0066] In addition, a nitrogen concentration in molten steel after the reduction-decarburization operation may be 75 ppm or less, more preferably, 70 ppm or less, further more preferably, 65 ppm or less.

[0067] In addition, a component of the molten steel after the reduction-decarburization operation, includes, by wt %, C: 0.003% to 0.16%, Si: 0.2% to 0.7%, Mn: 1.0% to 5.0%, P: 0.03% or less, S: 0.02% or less, Cr: 16% to 18%, Ni: 7% to 9%, Mo: 0.001% to 0.200%, N: 75 ppm by weight or less, iron (Fe) as a remainder thereof, and other unavoidable impurities.

Embodiments of Invention

[0068] Hereinafter, the present disclosure will be described in more detail byway of embodiments. It should be noted, however, that the embodiments described below are intended to describe the present disclosure in more detail and not to limit the scope of the present disclosure, because the scope of the present disclosure is determined by the matters described in the claims and the matters reasonably inferred therefrom.

Embodiment 1

[0069] AOD refining is performed to obtain molten steel including, by wt %, C: 0.003% to 0.16%, Si: 0.2% to 0.7%, Mn: 1.0% to 5.0%, P: 0.03% or less, S: 0.02% or less, Cr: 16% to 18%, Ni: 7% to 9%, Mo: 0.001% to 0.200%, N: 75 ppm by weight or less, iron (Fe) as a remainder thereof, and other unavoidable impurities, and a carbon concentration of poured molten steel and an Ar flow rate of reduction-decarburization in Table 1 are applied to AOD refine molten austenitic stainless steel.

[0070] Hereinafter, ADO tapping nitrogen and a carbon concentration are measured to be described in Table 1.

TABLE-US-00001 TABLE 1 AOD poured molten steel AOD tapping AOD tapping carbon Ar flow rate of nitrogen carbon concentration reduction-decarburization concentration concentration Classification (wt %) (Nm.sup.3/min) (ppm by weight) (ppm by weight) Comparative 1.2 45 83 68 Example 1 Comparative 1.4 45 82 65 Example 2 Inventive 2.0 45 71 55 Example 1 Inventive 2.5 45 66 52 Example 2 Inventive 2.5 55 53 49 Example 3

[0071] In Inventive Examples 1 to 3 satisfying control conditions of an exemplary embodiment, AOD tapping nitrogen and carbon concentrations are confirmed to be reduced in comparison with Comparative Examples 1 and 2.

Embodiment 2

[0072] To compare a method in an exemplary embodiment with a conventional method, results that the method in an exemplary embodiment and a conventional method are applied to actual refining are illustrated in FIGS. 5 and 6. At week 1 to week 7, a carbon concentration of poured molten steel is 1.4 wt %, and an Ar flow rate of reduction-decarburization is 45 Nm.sup.3/min. At week 8 to week 14, a carbon concentration thereof is 2.5 wt %, and an Ar flow rate of reduction-decarburization is 55 Nm.sup.3/min.

[0073] At week 1 to week 7 to which a conventional refining method is applied, a nitrogen concentration of 70 ppm or more and a carbon concentration of 60 ppm or more are shown, but at week 8 to week 14 to which a refining method according to an exemplary embodiment is applied, nitrogen and carbon concentrations are confirmed to be clearly reduced.

Embodiment 3

[0074] In addition, to confirm a change in an initial O.sub.2 concentration in a decarburization operation, an additional experiment under conditions in Table 2 is performed, and a carbon concentration in a first AOD blowing operation is described in Table 2. Here, an initial carbon concentration of AOD poured molten steel is equal to 2.0 wt %.

TABLE-US-00002 TABLE 2 First blowing First blowing operation operation First blowing AOD molten steel AOD molten steel operation injection injection carbon O.sub.2 flow rate Ar flow rate concentration Classification (Nm.sup.3/min) (Nm.sup.3/min) (wt %) Comparative 130 10 0.41 Example 1 Comparative 135 10 0.40 Example 2 Inventive 140 10 0.38 Example 1 Inventive 150 10 0.36 Example 2 Inventive 150 20 0.35 Example 3

[0075] As seen in Table 2, when an initial O.sub.2 flow rate in a decarburization operation, that is, a first blowing operation AOD molten steel injection oxygen flow rate is 140 Nm.sup.3/min or more, a decarburizing effect is confirmed to be further increased.

[0076] While exemplary embodiments have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present invention as defined by the appended claims.

Claims

1. An argon oxygen decarburization (AOD) refining method for molten austenitic stainless steel comprising: preparing molten austenitic stainless steel in an electric arc furnace; pouring the molten austenitic stainless steel into an AOD refining furnace by adjusting a carbon concentration of the molten austenitic stainless steel to 2.0 wt % to 2.5 wt %; decarburizing the poured molten austenitic stainless steel by blowing oxygen (O.sub.2) and argon (Ar) thereinto; and reduction-decarburizing the decarburized molten austenitic stainless steel by blowing argon (Ar) thereinto.

2. The AOD refining method for molten austenitic stainless steel of claim 1, wherein the reduction-decarburizing is performed under conditions in which a flow rate of the argon is 50 Nm.sup.3/min to 55 Nm.sup.3/min.

3. The AOD refining method for molten austenitic stainless steel of claim 1, wherein the decarburizing is performed by gradually reducing a flow rate of the oxygen and by gradually increasing a flow rate of the argon.

4. The AOD refining method for molten austenitic stainless steel of claim 3, wherein an initial flow rate of the oxygen is 140 Nm.sup.3/min to 170 Nm.sup.3/min.

5. The AOD refining method for molten austenitic stainless steel of claim 1, wherein a nitrogen concentration of the molten austenitic stainless steel after the reduction-decarburizing is 75 ppm or less.

6. The AOD refining method for molten austenitic stainless steel of claim 1, wherein a component of the molten austenitic stainless steel after the reduction-decarburizing includes, by wt %, C: 0.003% to 0.16%, Si: 0.2% to 0.7%, Mn: 1.0% to 5.0%, P: 0.03% or less, S: 0.02% or less, Cr: 16% to 18%, Ni: 7% to 9%, Mo: 0.001% to 0.200%, N: 75 ppm by weight or less, iron (Fe) as a remainder thereof, and other unavoidable impurities.