Non-quenched and tempered steel wire rod for hot forging having excellent workability and impact 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, in weight percent: C: 0.3% to 0.5%; si: 0.4% to 0.9%; mn: 0.5% to 1.2%; 0.02% or less of P; 0.01% to 0.05% of S; a soluble A1: from 0.01% to 0.05%; cr: 0.1% to 0.3%; 0.01% to 0.02% of Ti; ca: 0.0005% to 0.002%; n: 0.007% to 0.02%; and the remainder consisting of Fe and unavoidable impurities, and including ferrite and pearlite as a microstructure wherein the following relational expression 1 is satisfied, and the area fraction of MnS satisfies 0.10% to 0.60%. [Relational Expression 1] 20 < = [Mn] / [S] < = 70.

Description

Technical Field

[0001] The present disclosure relates to a non-quenched and tempered steel wire having improved 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 mechanical 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 the 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 poor toughness relative to that of 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 part, 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 relates to a non-quenched and tempered steel wire having excellent workability and impact toughness by improving toughness inferior to that of conventional quenched and tempered steel and by adding high contents of S and N without additional heat treatment, and a method for manufacturing the same.

[0005] Technical Solution

[0006] A non-quenched and tempered steel wire rod with improved workability and impact toughness according to one embodiment of the present disclosure contains, 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, 0.01% to 0.05% soluble Al, 0.1% to 0.3% Cr, 0.01% to 0.02% Ti, 0.0005% to 0.002% Ca, 0.007% to 0.02% N, and the balance of Fe and inevitable impurities, and contains ferrite and pearlite as microstructures, wherein the following relational expression 1 is satisfied, and the area fraction of MnS satisfies the range of 0.10% to 0.60%.

[0007] [Relational expression 1] 20≤[Mn] / [S]≤70

[0008] According to one embodiment of the present disclosure, in the non-quenched and tempered steel wire with improved workability and impact toughness, the number density of MnS may be 70 pieces / mm 2 or more, and the aspect ratio of MnS may be 40 or less.

[0009] According to one embodiment of the present disclosure, a non-quenched and tempered steel wire having improved workability and impact toughness may have a tensile strength of 700 MPa or more, a yield strength of 350 MPa to 500 MPa, and a yield strength ratio of 0.45 to 0.65.

[0010] According to one embodiment of the present disclosure, a non-quenched and tempered steel wire having improved processability and impact toughness may have a strength of 60 J / cm 2 or greater impact toughness and 45000MPa·J / cm 2 or greater value of tensile strength x impact toughness.

[0011] 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 comprises: reheating a steel piece at a temperature range of 950° C. to 1120° 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, 0.01% to 0.05% soluble Al, 0.1% to 0.3% Cr, 0.01% to 0.02% Ti, 0.0005% or less S ...0005% or less S, 0.1% to 0.3% Cr, 0.01% to 0.02% Ti, 0.0005% or less S, 0.0005% or less S, 0.0005% or less S, 0.0005% or less S, 0.0005% or less S, 0.0005 % to 0.002% of Ca, 0.007% to 0.02% of N, and the balance of Fe and inevitable impurities, and containing ferrite and pearlite as microstructures; 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 to 400° C. at an average cooling rate of 0.1° C. / sec to 5.0° C. / sec, wherein the steel wire rod contains ferrite and pearlite as microstructures, satisfies Relational Expression 1, and the area fraction of MnS is 0.10% to 0.60%.

[0012] Beneficial Effects

[0013] In the non-quenched and tempered steel wire rod with improved workability and impact toughness according to one embodiment of the present disclosure, Ti and Al are combined with N to form nitrides such as TiN and AlN, and such nitrides interfere with the growth of grain boundaries to reduce the grain size, thereby improving toughness. In addition, Ca-based oxides generated by the addition of Ca serve as nuclei for the formation of MnS and suppress the elongation of MnS during rolling to improve workability and toughness. Therefore, even if heat treatment is omitted, the steel wire rod can be applied to materials for automobiles or mechanical parts that require both workability and toughness. DETAILED DESCRIPTION

[0014] A non-quenched and tempered steel wire rod with improved workability and impact toughness according to one embodiment of the present disclosure contains, 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, 0.01% to 0.05% soluble Al, 0.1% to 0.3% Cr, 0.01% to 0.02% Ti, 0.0005% to 0.002% Ca, 0.007% to 0.02% N, and the balance of Fe and inevitable impurities, and contains ferrite and pearlite as microstructures, wherein the following relational expression 1 is satisfied, and the area fraction of MnS satisfies the range of 0.10% to 0.60%.

[0015] [Relational expression 1] 20≤[Mn] / [S]≤70

[0016] Embodiments of the invention

[0017] This specification does not describe all the elements of the embodiments of the present disclosure, and can omit the detailed description known in the art or the redundant description of substantially the same configuration. In addition, unless otherwise specified, the term "comprising" an element does not exclude other elements, but may also include other elements. 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.

[0018] The present inventors have examined methods for providing a steel wire having workability and impact toughness from various viewpoints, and have found that workability and toughness can be obtained by appropriately controlling the composition of alloy elements and the microstructure of the steel wire without additional heat treatment, thereby completing the present disclosure.

[0019] A non-quenched and tempered steel wire rod with improved workability and impact toughness according to one embodiment of the present disclosure contains, 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, 0.01% to 0.05% soluble Al, 0.1% to 0.3% Cr, 0.01% to 0.02% Ti, 0.0005% to 0.002% Ca, 0.007% to 0.02% N, and the balance of Fe and inevitable impurities, and contains ferrite and pearlite as microstructures, wherein the following relational expression 1 is satisfied, and the area fraction of MnS is 0.10% to 0.60%.

[0020] [Relational expression 1] 20≤[Mn] / [S]≤70

[0021] Hereinafter, the reason for numerically limiting the content of the alloying elements in the embodiment of the present disclosure will be described. Hereinafter, unless otherwise specified, the unit is weight %.

[0022] The C content is 0.3% to 0.5%.

[0023] 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%.

[0024] The content of Si is 0.4% to 0.9%.

[0025] Silicon (Si), as 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. Therefore, the upper limit of the Si content can be controlled to 0.9%.

[0026] The content of Mn is 0.5% to 1.2%.

[0027] Manganese (Mn) is an element effective as a deoxidizer and 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. Therefore, the upper limit of the Mn content can be controlled to 1.2%.

[0028] The content of Cr is 0.1% to 0.3%.

[0029] 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 in steel, 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 Cr content exceeds 0.3%, the strength of the steel is excessively increased, so that the deformation resistance of the steel increases rapidly, resulting in deterioration of cold workability. Therefore, the upper limit of the Cr content can be controlled to 0.3%.

[0030] The content of P is 0.02% or less.

[0031] Phosphorus (P), as an impurity inevitably contained in steel, is a main causative element that segregates into grain boundaries as a deterioration in toughness and a decrease 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 the upper limit of the P content can be controlled to 0.02% in the present disclosure.

[0032] The content of S is 0.01% to 0.05%.

[0033] Sulfur (S), as a main causative element that significantly deteriorates ductility due to segregation into grain boundaries and deteriorates delayed fracture resistance and stress relaxation due to the formation of emulsions in steel, 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 workability without significantly impairing the toughness of the steel, the S content is controlled within the range of 0.01% to 0.05% in the present disclosure.

[0034] The content of soluble Al is 0.01% to 0.05%.

[0035] Soluble Al is an element effective as a deoxidizer. Soluble Al may be contained in an amount of 0.01% or more to obtain the above-mentioned effect. However, in the case where the Al content exceeds 0.05%, difficulties may occur during the manufacturing process due to Al oxides generated during the casting process. Therefore, the upper limit of the Al content may be controlled to 0.05% in the present disclosure.

[0036] The content of Ti is 0.01% to 0.02%.

[0037] Titanium (Ti) is an element that plays a major role in improving the toughness of steel by forming TiN precipitates during the solidification process of steel to suppress the growth of austenite grains during the heating and hot rolling processes of the steel slab to reduce the grain size of the final structure. In the case where the Ti content is less than 0.01%, it is difficult to obtain a sufficient amount of TiN precipitates to suppress the migration of austenite grain boundaries. On the contrary, in the case where the Ti content exceeds 0.02%, coarse titanium nitride may be formed, which deteriorates the toughness instead, and therefore the upper limit of the Ti content may be controlled to 0.02% in the present disclosure.

[0038] The content of Ca is 0.0005% to 0.002%.

[0039] Ca is an essential element for achieving the effect of improving workability and impact toughness by reducing the aspect ratio of MnS. The addition of Ca promotes the formation of oxides that serve as nuclei for the formation of MnS to suppress the elongation of MnS when rolling the steel wire and maintain a low aspect ratio. The low aspect ratio of MnS not only improves workability, but also suppresses the deterioration of toughness by reducing the anisotropy of the microstructure. However, Ca should be added in an amount of 0.0005% or more to obtain the above-mentioned effect, but a Ca content exceeding 0.002% may cause difficulties in the manufacturing process. Therefore, the upper limit of the Ca content is controlled to 0.002%.

[0040] The content of N is 0.007% to 0.02%.

[0041] N is an essential element for achieving the effect of improving impact toughness by reducing the grain size through the formation of nitrides with Ti and Al. When the N content is less than 0.007%, it is difficult to obtain a sufficient amount of nitrides, resulting in a reduction in the generation of precipitates of Al, Ti, etc., and the toughness desired in the present disclosure cannot be obtained. When 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%.

[0042] 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 incorporated during the common industrial manufacturing process of steel. The type and content of impurities are not specifically mentioned in the present disclosure, as they are known to any person skilled in the art of manufacturing.

[0043] The non-quenched and tempered steel wire rod according to one embodiment of the present disclosure may satisfy the following Relational Expression 1. In Relational Expression 1, [S] and [Mn] respectively represent contents (weight %) of elements.

[0044] [Relational Expression 1] 20≤[Mn] / [S]≤70 (machinability)

[0045] Relational expression 1 is an expression related to machinability. According to the present disclosure, MnS is formed by adding high contents of S and Mn. MnS, as an elongated inclusion, has a shape and orientation that is elongated in the rolling direction, and significantly improves the machinability of the medium carbon non-quenched and tempered steel wire rod of the present disclosure. However, when an impact is applied thereto, MnS acts as the starting point of the crack and its propagation path, thereby deteriorating the impact toughness. When the [Mn] / [S] ratio is less than 20, machinability can be met, but impact toughness may be deteriorated. When the [Mn] / [S] ratio exceeds 70, machinability may be insufficient. Therefore, in the present disclosure, the [Mn] / [S] ratio can be controlled to be 20 to 70, preferably 30 to 60.

[0046] Furthermore, in the non-quenched and tempered steel material according to one embodiment of the present disclosure, the area fraction of MnS may be 0.10% to 0.60%, preferably 0.15% to 0.50%, and more preferably 0.15% to 0.45%.

[0047] In addition, in the non-quenched and tempered steel material according to one embodiment of the present disclosure, the number density of MnS may be 70 pieces / mm 2 or larger, preferably 80 / mm 2 or more, and more preferably 90 pieces / mm 2 As the density of MnS increases, MnS acts as a stress concentration source during cutting to reduce cutting resistance, thereby improving machinability. For this reason, it is necessary to make the density of MnS at least 70 pieces / mm 2 .

[0048] Furthermore, in the non-quenched and tempered steel material according to one embodiment of the present disclosure, the aspect ratio of MnS may be 40 or less, preferably 30 or less, and more preferably 20 or less. In this case, when the aspect ratio of MnS is greater than 40, impact toughness may be rapidly reduced.

[0049] In addition, the tensile strength of the non-quenched and tempered steel wire according to one embodiment of the present disclosure may be 700 MPa or more.

[0050] In addition, the non-quenched and tempered steel wire according to one embodiment of the present disclosure may have a yield strength of 350 MPa to 500 MPa.

[0051] In addition, the non-quenched and tempered steel wire rod according to one embodiment of the present disclosure may have a yield ratio of 0.45 to 0.65.

[0052] In addition, the impact toughness of the non-quenched and tempered steel wire according to one embodiment of the present disclosure may be 60 J / cm 2 or larger.

[0053] In addition, the value of tensile strength×impact toughness of the non-quenched and tempered steel wire according to one embodiment of the present disclosure may be 45000 MPa·J / cm 2 or larger.

[0054] Hereinafter, a method for manufacturing a non-quenched and tempered steel wire rod according to one embodiment of the present disclosure will be described.

[0055] A non-quenched and tempered steel wire with improved workability and impact toughness according to an embodiment of the present disclosure comprises: reheating a steel piece at a temperature range of 950° C. to 1120° C., the steel piece comprising, by 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, 0.01% to 0.05% soluble Al, 0.1% to 0.3% Cr, 0.01% to 0.02% Ti, 0.0005% to 0.01% S, 0.02 ... 0.002% of Ca, 0.007% to 0.02% of N, and the balance of Fe and inevitable impurities, and containing ferrite and pearlite as microstructures; 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 to 400° C. at an average cooling rate of 0.1° C. / sec to 5.0° C. / sec, wherein the steel wire rod contains ferrite and pearlite as microstructures, satisfies Relational Expression 1, and the area fraction of MnS is 0.10% to 0.60%.

[0056] [Relational expression 1] 20≤[Mn] / [S]≤70

[0057] Hereinafter, each process of the manufacturing method will be described in more detail.

[0058] First, a bloom satisfying the composition of the above-mentioned alloy elements is heated and rolled into a billet.

[0059] Reheating process

[0060] The reheating process, as a step 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 1,120°C. At a reheating temperature below 950°C, the rolling load may increase, resulting in a difficult manufacturing method. On the contrary, at a reheating temperature above 1,120°C, all of the AlN finely formed in the steel piece may reform into a solid solution during heating, thereby significantly reducing the grain refining effect.

[0061] The process of rolling steel wire

[0062] In the process of rolling the steel wire rod, the reheated steel piece is hot rolled into the steel wire rod.

[0063] 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, while at a finishing temperature above 850° C., grains may coarsen, so that high toughness desired in the present disclosure may not be obtained.

[0064] Winding process

[0065] 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 an additional 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 cannot be recovered due to the recovered heat, and tempered martensite is formed, causing 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 descaling, and productivity may be deteriorated due to the increase in cooling time in the subsequent cooling process.

[0066] Cooling process

[0067] The coiled steel wire may be cooled, and in this case, the cooling process may be performed to 400° C. by air cooling or controlled cooling at an average cooling rate range of 0.1° C. / sec to 5.0° C. / sec after hot forging. 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 generated, so that toughness and workability may be deteriorated.

[0068] Example

[0069] A bloom having a composition of alloy elements shown in Table 1 was heated at 1,200° C. for 4 hours and rolled into a billet at a finishing temperature of 1,100° C. Subsequently, the billet was heated under the temperature conditions shown in Table 2 below for 90 minutes, finish-rolled at 800° C., coiled at 780° C., and cooled into a steel wire having a diameter of 26 mm under the temperature conditions of Table 2 below. Steel wires (Table 1) comprising the compositions of Inventive Steels 1 to 7 and Comparative Steels 1 to 6 were manufactured, and the workability, tensile strength, and impact toughness of the specimens of the steel wires were measured.

[0070] Here, the room temperature tensile strength was measured at the center of a specimen of non-quenched and tempered steel at 25°C, and the room temperature impact toughness of a specimen having a U-shaped notch (based on a standard sample, 10 mm×10 mm×55 mm) was measured at 25°C using a Charpy impact energy value obtained by a Charpy impact test.

[0071] 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 machinability was evaluated by using a CNC lathe, and the degree of tool wear was evaluated after the turning operation was performed until the CD bar having a diameter of 24 mm was reduced to a diameter of 15 mm. 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. As a cutting tool, a metal ceramic tool with a chip breaker was used. After continuously processing 300 parts having the above-mentioned shape, the wear depth of the tool was obtained by measuring the depth of the side wear surface. A wear depth greater than 0.2 mm was evaluated as poor, and a wear depth of 0.2 mm or less was evaluated as good.

[0072] Furthermore, the area fraction of MnS, the number density of MnS, and the aspect ratio of MnS were evaluated by obtaining 20 images of the L cross section of each wire at a magnification of 200× using an optical microscope and analyzing the images by image analysis software.

[0073] [Table 1]

[0074]

[0075] [Table 2]

[0076]

[0077]

[0078] [Table 3]

[0079]

[0080]

[0081] As shown in Tables 1 to 3, the steel wires of Examples 1 to 7 satisfying all of the chemical compositions, relational expressions, and area fractions, number densities, aspect ratios, and manufacturing conditions of MnS provided in the present disclosure satisfy a tensile strength of 700 MPa or more, a yield strength of 350 MPa to 500 MPa, a yield ratio of 0.45 to 0.65, a strength of 45000 MPa·J / cm 2 or greater tensile strength × impact toughness value, 60J / cm 2 or greater impact toughness and good processability.

[0082] In contrast, the steel wires of Comparative Examples 1 to 9 that do not satisfy one or more conditions provided in the present disclosure have at least one poor property selected from tensile strength, impact toughness, a value of tensile strength×impact toughness, and workability.

[0083] Specifically, Comparative Example 1, which does not satisfy the C content range suggested by the present disclosure, exhibits poor tool wear due to high strength, and Comparative Examples 2 and 3 exhibit low impact toughness due to excessive Si and Mn. In addition, Comparative Example 4 exhibits poor impact toughness due to insufficient grain refinement effect caused by insufficient Ti content. Comparative Example 5 exhibits reduced impact toughness due to a higher aspect ratio of MnS, and Comparative Example 6 exhibits poor tool wear due to insufficient area fraction and density due to failure to satisfy relational expression (1). Although Comparative Examples 7 to 9 satisfy the chemical composition, the toughness is poor or the target strength is not satisfied because the heating temperature and the cooling rate are outside the suggested range.

[0084] 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 therein without departing from the spirit and scope of the present disclosure.

[0085]

Industrial Applicability

[0086] According to the present disclosure, a non-quenched and tempered steel wire rod having both workability and impact toughness and a method for manufacturing the same can be provided without additional heat treatment, 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 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, 0.01% to 0.05% soluble Al, 0.1% to 0.3% Cr, 0.01% to 0.02% Ti, 0.0005% to 0.002% Ca, 0.007% to 0.02% N, and the balance of Fe and unavoidable impurities, and containing ferrite and pearlite as microstructures, wherein the following relational expression 1 is satisfied and the area fraction of MnS satisfies the range of 0.10% to 0.60%: [Relational expression 1]20≤[Mn] / [S]≤70.

2. The non-quenched and tempered steel wire according to claim 1, wherein the number density of MnS is 70 pieces / mm 2 or larger.

3. The non-quenched and tempered steel wire rod according to claim 1, wherein the aspect ratio of MnS is 40 or less.

4. The non-quenched and tempered steel wire rod according to claim 1, wherein the tensile strength is 700 MPa or more. The non-quenched and tempered steel wire rod according to claim 1 , wherein the yield strength is 350 MPa to 500 MPa. The non-quenched and tempered steel wire rod according to claim 1 , wherein the yield ratio is 0.45 to 0.

65.

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

8. The non-quenched and tempered steel wire according to claim 1, wherein the value of tensile strength x impact toughness is 45000 MPa·J / cm 2 or larger.

9. 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 range of 950° C. to 1120° 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, 0.01% to 0.05% soluble Al, 0.1% to 0.3% Cr, 0.01% to 0.02% Ti, 0.0005% to 0.002% Ca, 0.007% to 0.02% N, and the balance of Fe and inevitable impurities, and comprising ferrite and pearlite as a microstructure; Finish rolling the reheated steel piece at a temperature of 750° C. to 850° C. into a steel wire rod; as well as The steel wire is coiled and cooled, wherein the cooling performed after the coiling includes a process of cooling to 400° C. at an average cooling rate of 0.1° C. / sec to 5.0° C. / sec, wherein the steel wire rod contains ferrite and pearlite as microstructures, satisfies relational expression 1, and the area fraction of MnS is 0.10% to 0.60%: [Relational expression 1]20≤[Mn] / [S]≤70.

10. The method according to claim 9, wherein the number density of MnS is 70 pieces / mm 2 or larger.

11. The method according to claim 9, wherein the aspect ratio of MnS is 40 or less.

12. The method of claim 9, wherein the coiling temperature is 750°C to 850°C.

Images