Non-quenched and tempered steel wire with improved workability and toughness and method for manufacturing same

Abstract

The present invention relates to a non-quenched and tempered steel wire having improved workability and toughness and a method for manufacturing the same. The non-quenched and tempered steel wire rod according to the present invention comprises: 0.3 wt% to 0.5 wt% of C; from 0.4% to 0.9% by weight of Si; 0.5 wt% to 1.2 wt% of Mn; up to 0.02% by weight of P; 0.01 wt% to 0.05 wt% of S; from 0.015 wt% to 0.05 wt% of a soluble Al; from 0.1 wt% to 0.3 wt% of Cr; and 0.007 to 0.02 wt% of N, with the remainder being Fe and unavoidable impurities, in which the microstructure contains ferrite and pearlite, and satisfies the following relational expression 1. [relational expression 1] [N]-[Al] / 1.93 < = 0.009.

Description

Technical Field

[0001] The present disclosure relates to a non-quenched and tempered steel wire having excellent workability and impact toughness and a method for manufacturing the same, and more particularly, to a non-quenched and tempered steel wire suitable for use as a material for automobile or machine parts and a method for manufacturing the same. Background Art

[0002] Unlike quenched and tempered steels that obtain a specific level of strength and toughness through quenching and tempering (QT) heat treatment, the QT heat treatment process is omitted in non-quenched and tempered steels. Therefore, non-quenched and tempered steels are not only economically advantageous by reducing heat treatment costs, simplifying processes to shorten delivery times and improving productivity, but are also environmentally friendly by reducing CO2 produced by operating furnaces during heat treatment. At the beginning of development, non-quenched and tempered steels were only applied to parts that did not require high toughness due to their relatively poor toughness relative to quenched and tempered steels. However, with the recent increase in demand for environmental feasibility and cost reduction, the demand for improving the toughness of non-quenched and tempered steels is increasing. In addition, since a cutting process is usually performed to obtain the final shape of a component, machinability is also required. Typically, a large amount of MnS is produced by adding S to improve machinability, resulting in a problem of reduced toughness of the product. Summary of the invention

[0003] Technical issues

[0004] The present disclosure provides a non-quenched and tempered steel wire rod and a method for manufacturing the same, which has impact toughness and provides wear resistance to a cutting tool by adjusting the microstructure, for example, through AlN grain boundary shot peening and cryogenic rolling or by obtaining a sufficient fraction of ferrite, which is a soft phase, in order to improve the impact toughness inferior to that of conventional quenched and tempered steel.

[0005] Technical Solution

[0006] A non-quenched and tempered steel wire having improved workability and impact toughness according to one embodiment of the present disclosure includes, in terms of weight percentage (wt%), 0.3% to 0.5% C, 0.4% to 0.9% Si, 0.5% to 1.2% Mn, 0.02% or less P, 0.01% to 0.05% S, 0.015% to 0.05% soluble Al, 0.1% to 0.3% Cr, 0.007% to 0.020% N, and the balance of Fe and inevitable impurities, wherein the microstructure includes ferrite and pearlite, and satisfies the following relational expression 1.

[0007] [Relational expression 1] [N] - [Al] / 1.93 ≤ 0.009

[0008] A method for manufacturing a non-quenched and tempered steel wire having improved workability and impact toughness according to an embodiment of the present disclosure includes: reheating a steel piece at a temperature of 950° C. to 1100° C., the steel piece comprising, in terms of weight percentage (wt%), 0.3% to 0.5% C, 0.4% to 0.9% Si, 0.5% to 1.2% Mn, 0.02% or less P, 0.01% to 0.05% S, 0.015% to 0.05% soluble Al, 0.1% to 0.3% of Cr, 0.007% to 0.020% of N, and the balance of Fe and inevitable impurities; finish rolling the reheated steel piece at a temperature of 750° C. to 850° C. into a steel wire rod; and coiling and cooling the steel wire rod, wherein the cooling performed after the coiling includes a process of cooling the steel wire rod to 400° C. at an average cooling rate greater than 0.1° C. / second but not greater than 5.0° C. / second, and the steel wire rod satisfies Relational Expression 1.

[0009] Beneficial Effects

[0010] In a non-quenched and tempered steel wire with improved machinability and impact toughness according to an embodiment of the present disclosure, Al is combined with N to form AlN nitrides that suppress the growth of grain boundaries during heating and refine the grains, thereby improving impact toughness. In addition, by adjusting the area fraction of ferrite in a region from the center of the steel wire to a quarter diameter or more from the surface to 20% to 40%, the impact toughness is further improved. The deterioration of impact toughness can be minimized, and machinability, especially wear resistance of cutting tools, can be obtained by reducing the size of MnS, which improves machinability but may deteriorate impact toughness. Therefore, even after omitting heat treatment, the steel wire can be applied to materials for automobiles or machine parts that require machinability and impact toughness. DETAILED DESCRIPTION

[0011] A non-quenched and tempered steel wire having improved workability and impact toughness according to one embodiment of the present disclosure includes, in terms of weight percentage (wt%), 0.3% to 0.5% C, 0.4% to 0.9% Si, 0.5% to 1.2% Mn, 0.02% or less P, 0.01% to 0.05% S, 0.015% to 0.05% soluble Al, 0.1% to 0.3% Cr, 0.007% to 0.02% N, and the balance of Fe and inevitable impurities, wherein the microstructure includes ferrite and pearlite, and satisfies the following relational expression 1.

[0012] [Relational expression 1] [N]-[A1] / 1.93≤0.009.

[0013] Embodiments of the invention

[0014] This specification does not describe all elements of the embodiments of the present disclosure, and detailed descriptions known in the art or redundant descriptions of substantially identical configurations may be omitted. In addition, unless otherwise indicated, the term "comprising" an element does not exclude other elements, but may also include another element. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. Hereinafter, the present disclosure will be described in detail.

[0015] The present inventors have studied a method for providing a steel wire having workability and impact toughness from various angles, and found that workability and toughness can be obtained without heat treatment by appropriately controlling the composition of alloy elements and the microstructure of the steel wire, thereby completing the present disclosure.

[0016] A non-quenched and tempered steel wire having improved workability and impact toughness according to one embodiment of the present disclosure includes, in terms of weight percentage (wt%), 0.3% to 0.5% C, 0.4% to 0.9% Si, 0.5% to 1.2% Mn, 0.02% or less P, 0.01% to 0.05% S, 0.015% to 0.05% soluble Al, 0.1% to 0.3% Cr, 0.007% to 0.02% N, and the balance of Fe and inevitable impurities, wherein the microstructure includes ferrite and pearlite, and satisfies the following relational expression 1.

[0017] [Relational expression 1] [N] - [Al] / 1.93 ≤ 0.009

[0018] Hereinafter, the reason for numerically limiting the content of alloy elements in the embodiment of the present disclosure will be described. Hereinafter, unless otherwise specified, the unit is weight %.

[0019] The C content is 0.3% to 0.5%.

[0020] Carbon (C) is an element for improving the strength of the steel wire rod. In order to obtain the above-mentioned effect, C is preferably contained in an amount of 0.3% or more. However, an excessive C content may deteriorate toughness and workability, and thus the upper limit of the C content may be controlled to 0.5%.

[0021] The content of Si is 0.4% to 0.9%.

[0022] Silicon (Si), an element effective as a deoxidizer, is used to improve strength. When the Si content is less than 0.4%, the above effect may not be obtained. When the Si content exceeds 0.9%, the deformation resistance of the steel increases rapidly due to solid solution strengthening, resulting in deterioration of cold workability, and therefore, the upper limit of the Si content can be controlled to 0.9%.

[0023] The content of Mn is 0.5% to 1.2%.

[0024] Manganese (Mn) is an element effective as a deoxidizer and a desulfurizer. When the Mn content is less than 0.5%, the above effects may not be obtained. When the Mn content exceeds 1.2%, the strength of the steel increases excessively, so that the deformation resistance of the steel increases rapidly, resulting in deterioration of cold workability, and therefore, the upper limit of the Mn content can be controlled to 1.2%.

[0025] The content of P is 0.02% or less.

[0026] Phosphorus (P), as an impurity inevitably contained in steel, is a main causative element that segregates into grain boundaries to cause deterioration of toughness and reduction in delayed fracture resistance. Therefore, it is preferable to control the P content as low as possible. Theoretically, it is preferable to control the P content to 0%, but P is inevitably contained therein during the manufacturing process. Therefore, it is important to control the upper limit, and in the present disclosure, the upper limit of the P content can be controlled to 0.02%.

[0027] The content of S is 0.01% to 0.05%.

[0028] Sulfur (S), as a main causative element that segregates into grain boundaries, causes significant deterioration of ductility in steel and formation of emulsions, impairs delayed fracture resistance and stress relaxation, is an impurity that is inevitably contained in steel during the manufacturing process. However, as in the present disclosure, S can be actively used to improve workability. Since S combines with Mn to form MnS that improves workability, considering that the S content is effective for improving the workability of steel without significantly impairing the toughness of steel, the S content is controlled within the range of 0.01% to 0.05% in the present disclosure.

[0029] The content of soluble Al is 0.015% to 0.05%.

[0030] Soluble Al is an element effective as a deoxidizer. Soluble Al may be included in an amount of 0.015% to obtain the above-mentioned effect. However, in the case where the Al content exceeds 0.05%, difficulties may occur during the casting process due to Al oxides. Therefore, the upper limit of the Al content may be controlled to 0.05% in the present disclosure.

[0031] The content of Cr is 0.1% to 0.3%.

[0032] Chromium (Cr) is an element for promoting the transformation of ferrite and pearlite during hot rolling. In addition, Cr does not increase the strength of steel too much, reduces the amount of solid solution of C by precipitating carbides, and helps to reduce dynamic deformation aging caused by solid solution of carbon. In the case where the Cr content is less than 0.1%, the above effect may not be obtained, and in the case where the C content exceeds 0.3%, the strength of the steel is excessively increased to rapidly increase the deformation resistance of the steel, resulting in deterioration of cold workability. Therefore, the upper limit of the Cr content can be controlled to 0.3%.

[0033] The content of N is 0.007% to 0.02%.

[0034] N is an essential element for achieving the effect of improving impact toughness by reducing the grain size through the formation of nitrides with Al. In the case where the N content is less than 0.007%, it is difficult to obtain a sufficient amount of nitrides, resulting in a reduction in the production of AlN precipitates, and the toughness desired in the present disclosure cannot be obtained. In the case where the N content exceeds 0.02%, the solid solution of N that does not exist as a nitride increases to deteriorate the toughness and ductility of the steel wire. Therefore, in the present disclosure, the upper limit of the N content can be controlled to 0.02%.

[0035] The remaining component of the non-quenched and tempered steel wire of the present disclosure is iron (Fe). However, the non-quenched and tempered steel wire may contain other impurities introduced during the common industrial manufacturing process of steel. Since impurities are known to any technician in the manufacturing field, impurities are not specifically mentioned in the present disclosure.

[0036] The non-quenched and tempered steel wire according to one embodiment of the present disclosure may satisfy relational expressions 1 and 2. In relational expressions 1 and 2, [Al], [N], [C], [S], [Mn], and [Si] respectively represent the contents (weight %) of the elements.

[0037] [Relational Expression 1] (Impact toughness)

[0038] [N]-[Al] / 1.93≤0.009

[0039] Relational expression 1 is an expression related to toughness. According to the present disclosure, AlN is formed by adding high contents of N and Al. Since fine AlN precipitates in the steel inhibit the growth of grains, the particles are refined to improve the impact toughness of the non-quenched and tempered steel wire according to the present disclosure. In order to exhibit the above-mentioned effect, it is preferred to control the [N]-[Al] / 1.93 ratio to 0.009 or less. In the case where the [N]-[Al] / 1.93 ratio exceeds 0.009, a large amount of N added in a state of insufficient Al exists in the steel without being combined with Al, and thus the impact toughness may be deteriorated.

[0040] [Relational expression 2] -23[C]+[Si](5-2[Si])-4[Mn]+104[S]+3≥0 (machinability)

[0041] Relational expression 2 is an expression related to tool wear in machinability. Generally, the addition of S leads to the formation of MnS, which acts as a stress concentrator during the cutting process to deteriorate cutting resistance and lubricates to improve the life of the tool. However, since tool wear is accelerated in the case of an increase in hardness due to the addition of alloying elements, they should be considered together. Relational expression 2 reflects these influences in a complex manner, and when the value is 0 or more, tool wear is not serious.

[0042] The non-quenched and tempered steel wire according to one embodiment of the present disclosure includes ferrite and pearlite as microstructures, wherein the area fraction of ferrite measured in a region from the center to a quarter diameter or more from the surface of the steel wire satisfies a range of 20% to 40%.

[0043] In the non-quenched and tempered steel wire according to one embodiment of the present disclosure, the area fraction of the formed AlN may be 0.03% or more.

[0044] In the non-quenched and tempered steel wire according to one embodiment of the present disclosure, the size of the formed AlN may be 150 nm or less.

[0045] Furthermore, in the non-quenched and tempered steel wire according to one embodiment of the present disclosure, the number of carbonitrides having an average equivalent circle diameter of 100 nm or less per unit area may be 2 pieces / μm 2 or more.

[0046] Furthermore, the tensile strength of the non-quenched and tempered steel material according to the present disclosure may be 700 MPa or higher.

[0047] Furthermore, the yield strength of the non-quenched and tempered steel material according to the present disclosure may be 350 MPa to 450 MPa.

[0048] In addition, the non-quenched and tempered steel material according to the present disclosure may have a yield ratio of 0.45 to 0.65.

[0049] In addition, the impact toughness of the non-quenched and tempered steel material according to the present disclosure can be 60 J / cm 2 or larger.

[0050] In addition, the product of the tensile strength and the impact toughness of the non-quenched and tempered steel material according to the present disclosure may be 30,000 to 60,000.

[0051] Hereinafter, a method for manufacturing a non-quenched and tempered steel wire rod according to one embodiment of the present disclosure will be described.

[0052] The non-quenched and tempered steel wire rod having improved workability and impact toughness according to the present disclosure may be manufactured in various methods, and the manufacturing method thereof is not particularly limited. However, the steel wire rod may be manufactured according to the following method.

[0053] A method for manufacturing a non-quenched and tempered steel wire rod having improved workability and impact toughness according to an embodiment of the present disclosure includes: reheating a steel piece, the steel piece comprising, in weight percentage (wt%), 0.3% to 0.5% C, 0.4% to 0.9% Si, 0.5% to 1.2% Mn, 0.02% or less P, 0.01% to 0.05% S, 0.015% to 0.05% soluble Al, 0.1% to 0.3% Cr, 0.007% to 0.020% N, and the balance of Fe and inevitable impurities; hot rolling the reheated steel piece into a steel wire rod; and coiling and cooling the steel wire rod,

[0054] wherein cooling performed after coiling includes a process of cooling the steel wire rod to 400° C. at an average cooling rate greater than 0.1° C. / sec but not greater than 5.0° C. / sec, and the steel wire rod satisfies the following relational expression 1.

[0055] [Relational expression 1] [N] - [A1] / 1.93 ≤ 0.009

[0056] Hereinafter, each process of the manufacturing method will be described in more detail.

[0057] First, a large square billet satisfying the above alloy element composition is heated and rolled into a small square billet.

[0058] Reheating process

[0059] The reheating process, as a process of reheating the rolled billet, is a process for reducing the rolling load when rolling the steel wire rod. In this regard, the reheating may be performed at a temperature of 950°C to 1,100°C. At a reheating temperature below 950°C, the rolling load may increase, resulting in difficulty in the manufacturing method. On the contrary, at a reheating temperature above 1,100°C, AlN formed in the steel piece may reform a solid solution during heating, so that the area fraction of AlN becomes less than 0.03%, thereby significantly reducing the grain refinement effect.

[0060] The process of rolling steel wire

[0061] In the process of rolling the steel wire rod, the reheated steel piece is hot rolled into the steel wire rod.

[0062] In this case, the finishing temperature of hot rolling may be 750 to 850° C. At a finishing temperature below 750° C., rolling load may increase, and at a finishing temperature above 850° C., grains may coarsen, making it difficult to obtain high toughness desired in the present disclosure.

[0063] Winding process

[0064] A process of winding the steel wire manufactured as described above into a coil shape may be performed. In this case, the winding temperature may be 750°C to 850°C. Since the temperature of the steel wire obtained by finish rolling may be increased due to phase transformation heating, the temperature of the steel wire immediately before winding may be higher than the final rolling temperature. In this case, the steel wire may be coiled after cooling to the coiling temperature, or may be coiled without a separate cooling process, depending on the temperature increased by heating. At a coiling temperature below 750°C, the martensite generated in the surface layer during cooling may not be recovered due to double rows, and tempered martensite may be formed, resulting in a problem of increasing the possibility of inducing surface defects during the drawing process. On the contrary, at a coiling temperature above 850°C, a thick scale may be formed on the surface of the steel wire, so that surface defects may easily occur during the removal of the scale, and productivity may be deteriorated due to the increase in cooling time in the subsequent cooling process.

[0065] Cooling process

[0066] The coiled steel wire may be cooled, and in this case, the cooling process may be performed by air cooling or controlled cooling after hot forging at an average cooling rate of more than 0.1°C / sec but not more than 5.0°C / sec to 400°C. When cooled to 400°C at an average cooling rate lower than 0.1°C / sec after coiling, the desired strength may not be obtained due to excessive formation of proeutectoid ferrite. At an average cooling rate higher than 5°C / sec, a low-temperature structure such as martensite may be formed, and thus toughness and workability may be deteriorated.

[0067] {Example}

[0068] A bloom having an alloy element composition shown in Table 1 was heated at 1200°C for 4 hours and rolled into a billet at a finishing temperature of 1,100°C. Then, the billet was heated at 1090°C for 90 minutes, finished rolled at 800°C, coiled at 780°C, and cooled into a steel wire having a diameter of 26 mm. Steel wires (Table 1) containing the components of Inventive Steels 1 to 7 and Comparative Steels 1 to 7 were manufactured, and the tensile strength, impact toughness, wear depth of cutting tools, and area fractions of ferrite and AlN of the steel wire samples were measured and are shown in Table 2 below.

[0069] Here, the room temperature tensile strength was measured at the center of a non-quenched and tempered steel sample at 25°C, and the room temperature impact toughness of a sample having a U-shaped notch (based on a standard sample, 10×10×55 mm) was measured at 25°C using a Charpy impact energy value obtained by a Charpy impact test.

[0070] In addition, in order to evaluate the machinability, a steel wire having a diameter of 26 mm was processed into a cold drawn bar (CD bar) having a diameter of 24 mm at a reduction rate of 14.8%. The degree of tool wear was evaluated by using a CNC lathe. After a turning operation was performed on a CD bar having a diameter of 24 mm to a diameter of 15 mm and a length of 20 mm, the degree of tool wear was evaluated. In this case, cutting was performed under the conditions of a cutting rate of 100 mm / min, a feed rate of 0.1 mm / rev, and a cutting depth of 1.0 mm by using cutting oil, and a metal ceramic tool with a chip breaker was used as a cutting tool. After continuously processing 300 parts having the above-mentioned shape, the tool wear depth was evaluated by measuring the depth of the tool face wear, and a depth exceeding 0.2 mm was evaluated as poor, and a depth not exceeding 0.2 mm was evaluated as good. In order to measure and quantify the area fraction of extremely fine AlN having a size of 100 nm or less, a transmission electron microscope sample was prepared and observed at a magnification of 100,000x using a replica method, and then the arithmetic mean of the AlN area fraction was obtained from 50 images using image analysis software.

[0071] [Table 1]

[0072]

[0073]

[0074] [Table 2]

[0075]

[0076]

[0077] *Area fraction of ferrite is measured in the area from the center of the steel wire to one quarter diameter or more from the surface.

[0078] As shown in Tables 1 and 2, the steel wires of Examples 1 to 7 satisfying all chemical compositions, relational expressions, and manufacturing conditions provided in the present disclosure have the desired 60 J / cm 2 or greater impact toughness, 700MPa or greater tensile strength and good cutting tool wear characteristics.

[0079] In contrast, the steel wires of Comparative Examples 1 to 10, which do not satisfy all the conditions suggested by the present disclosure, have poor properties in one or more of tensile strength, impact toughness, and cutting tool wear depth.

[0080] Specifically, Comparative Steel 1 of Comparative Example 1 having a low C content cannot satisfy the recommended tensile strength of 700 MPa or more, and Comparative Examples 2 and 3 having excessive Si and excessive Mn, respectively, have insufficient impact toughness. In addition, Comparative Examples 4 and 5 having Al and N contents lower than those suggested in the present disclosure cannot satisfy the recommended AlN area fraction of 0.03% or more, and thus obtain low impact toughness values. Although Comparative Examples 6 and 7 satisfy the chemical compositions suggested in the present disclosure, relational expressions 1 and 2 are not satisfied, resulting in low impact toughness and poor cutting tool wear. Although Comparative Examples 8 to 10 also satisfy the chemical compositions suggested in the present disclosure, the heating temperature of the steel piece is high, and the average cooling rate range to 400°C during cooling after rolling is not satisfied, so that the impact toughness and tensile strength are outside the target values ​​or the cutting tool wear is poor.

[0081] While the present disclosure has been particularly described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the present disclosure.

[0082] Industrial Applicability

[0083] According to the present disclosure, a non-quenched and tempered steel wire rod having improved impact toughness and workability can be provided while solving environmental issues and reducing costs, and thus the present disclosure has industrial applicability.

Claims

1. A non-quenched and tempered steel wire rod having improved workability and impact toughness, comprising, in weight percentage (wt%): 0.3% to 0.5% C, 0.4% to 0.9% Si, 0.5% to 1.2% Mn, 0.02% or less P, 0.01% to 0.05% S, 0.015% to 0.05% soluble Al, 0.1% to 0.3% Cr, 0.007% to 0.02% N, and the balance Fe and unavoidable impurities, The microstructure contains ferrite and pearlite, and The following relation expression 1 is satisfied: [Relational expression 1] [N]-[Al] / 1.93≤0.

009.

2. The non-quenched and tempered steel wire according to claim 1, wherein the following relational expression 2 is satisfied: [Relational Expression 2]-23[C]+[Si](5-2[Si])-4[Mn]+104[S]+3≥0.

3. The non-quenched and tempered steel wire rod according to claim 1, wherein the area fraction of ferrite measured in a region from the center to a quarter diameter or more from the surface of the steel wire rod is 20% to 40%. 4 . The non-quenched and tempered steel wire material according to claim 1 , wherein the area fraction of AlN formed is 0.03% or more. 5 . The non-quenched and tempered steel wire material according to claim 1 , wherein a size of AlN formed is 150 nm or less.

6. The non-quenched and tempered steel wire rod according to claim 1, wherein the number of carbonitrides having an average equivalent circle diameter of 100 nm or less per unit area is 2 pieces / μm 2 or more.

7. The non-quenched and tempered steel wire rod according to claim 1, wherein the tensile strength is 700 MPa or more.

8. The non-quenched and tempered steel wire rod according to claim 1, wherein the yield strength is 350 MPa to 450 MPa.

9. The non-quenched and tempered steel wire rod according to claim 1, wherein the yield ratio is 0.45 to 0.

65.

10. The non-quenched and tempered steel wire according to claim 1, wherein the room temperature impact toughness is 60 J / cm 2 or larger.

11. The non-quenched and tempered steel wire rod according to claim 1, wherein the product of tensile strength and room temperature impact toughness is 30,000 to 60,000.

12. A method for producing a non-quenched and tempered steel wire having improved workability and impact toughness, the method comprising: reheating a steel piece at a temperature of 950° C. to 1100° C., the steel piece comprising, in terms of weight percentage (wt%), 0.3% to 0.5% C, 0.4% to 0.9% Si, 0.5% to 1.2% Mn, 0.02% or less P, 0.01% to 0.05% S, 0.015% to 0.05% soluble Al, 0.1% to 0.3% Cr, 0.007% to 0.02% N, and the balance of Fe and unavoidable impurities; Finish rolling the reheated steel piece at a temperature of 750° C. to 850° C. into a steel wire rod; as well as Coiling and cooling the steel wire; The cooling performed after the coiling includes a process of cooling the steel wire to 400° C. at an average cooling rate greater than 0.1° C. / sec but not greater than 5.0° C. / sec, and the steel wire satisfies the following relational expression 1: [Relational expression 1] [-N]-[Al] / 1.93≤0.

009.

13. The method according to claim 12, wherein the following relational expression 2 is satisfied: [Relational Expression 2]-23[C]+[Si](5-2[Si])-4[Mn]+104[S]+3≥0.

14. The method of claim 12, wherein the area fraction of ferrite is measured in a region from the center to a quarter diameter or more from the surface of the steel wire. 15 . The method according to claim 12 , wherein an area fraction of AlN formed during the reheating is 0.03% or more. 16 . The method according to claim 12 , wherein a size of AlN formed during the reheating is 150 nm or less.

17. The method according to claim 12, wherein the number of carbonitrides having an average equivalent circular diameter of 100 nm or less per unit area is 2 pieces / μm 2 or more.

18. The method of claim 12, wherein the tensile strength is 700 MPa or more.

19. The method of claim 12, wherein the yield strength is 350 MPa to 450 MPa.

20. The method of claim 12, wherein the yield ratio is 0.45 to 0.

65.

21. The method according to claim 12, wherein the impact toughness is 60 J / cm 2 or higher.

22. The method of claim 12, wherein the product of tensile strength and impact toughness is 30,000 to 60,000.

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