Wire rod, bolt and manufacturing method thereof

Abstract

A wire rod, according to an embodiment of the present invention, comprises, in weight %: 0.20% to 0.35% of C; 0.05% to 0.50% of Si; 0.30-0.60% of Mn; 1.00% to 1.60% of Cr; 0.02% to 0.10% of Al; 0.02% to 0.05% of Ti; and 0.0005% to 0.0100% of B, the remainder being Fe and other impurities, wherein formula (1) is satisfied, and after hot rolling the microstructure comprises, by area fraction, 30% to 60% pearlite and 40% to 70% ferrite. Formula (1): 5.9 ≤ 9.2x[C] + 1.2x(0.9x[Mn] + 2.1x[Cr]) ≤ 6.3 (where [C], [Mn] and [Cr] signify the weight % of each element).

Description

Wire rods, bolts and their manufacturing methods

[0001] The present invention relates to wires, bolts and a method for manufacturing the same.

[0002] The existing JS-SCM440 steel, which is a raw material for bolts and other components used in fastening wind towers, requires spheroidization heat treatment for cold forging, and the addition of expensive Mo elements reduces its economic feasibility. Meanwhile, the JS-SCr420B steel, which can be cold forged without post-rolling heat treatment, has low C and Cr contents, which reduces strength after hot-dip galvanizing, necessitating a lower tempering temperature in the QT heat treatment process. This, in turn, reduces low-temperature (-40°C) impact toughness.

[0003] The purpose of the present invention to solve the above-described problem is to improve the cold forging property of a hot-rolled material by forming fine carbides during tempering without spheroidizing heat treatment.

[0004] In addition, the present invention aims to provide a wire rod, a bolt, and a method for manufacturing the same, which can simultaneously secure cold forging properties and Vickers hardness.

[0005] The problems to be solved by the present invention are not limited to the problems mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the description below.

[0006] According to one embodiment of the present invention, a wire rod includes, in wt%, C: 0.20% to 0.35%, Si: 0.05% to 0.50%, Mn: 0.30% to 0.60%, Cr: 1.00% to 1.60%, Al: 0.02% to 0.10%, Ti: 0.02% to 0.05%, and B: 0.0005% to 0.0100%, with the remainder being Fe and other impurities, and satisfies the following formula (1), and after hot rolling, the microstructure includes, in terms of area fraction, pearlite 30% to 60% and ferrite 40% to 70%.

[0007] Equation (1): 5.90 ≤ 9.2x[C] + 1.2x(0.9x[Mn] + 2.1x[Cr]) ≤ 6.30

[0008] (Here, [C], [Mn], and [Cr] represent the weight percent of each element)

[0009] In addition, the wire according to one embodiment of the present invention can satisfy the following equation (2).

[0010] Equation (2): 0.99 ≤ 25x[Ti] + 124x[B] ≤ 1.36

[0011] (Here, [Ti] and [B] represent the weight percent of each element)

[0012] Additionally, the wire according to one embodiment of the present invention may have an average austenite grain size of 25 ㎛ or less.

[0013] In addition, the wire according to one embodiment of the present invention may have a Vickers hardness of 180 HV or less.

[0014] A method for manufacturing a wire rod according to one embodiment of the present invention comprises the steps of heating a billet, which contains, in wt%, C: 0.20% to 0.35%, Si: 0.05% to 0.50%, Mn: 0.30% to 0.60%, Cr: 1.00% to 1.60%, Al: 0.02% to 0.10%, Ti: 0.02% to 0.05%, and B: 0.0005% to 0.0100%, with the remainder being Fe and other impurities, to 950°C to 1200°C; hot-rolling it at 850°C to 950°C; coiling it at 750°C to 850°C; and cooling it to room temperature at a cooling rate of 0.1°C / s to 0.5°C / s.

[0015] Equation (1): 5.90 ≤ 9.2x[C] + 1.2x(0.9x[Mn] + 2.1x[Cr]) ≤ 6.30

[0016] (Here, [C], [Mn], and [Cr] represent the weight percent of each element)

[0017] In addition, in a method for manufacturing a wire rod according to another embodiment of the present invention, the microstructure of the wire rod after the hot rolling may include 30% to 60% of pearlite and 40% to 70% of ferrite in terms of area fraction.

[0018] In addition, in a method for manufacturing a wire according to another embodiment of the present invention, the average grain size of austenite can be controlled to 25 ㎛ or less in the cooling step.

[0019] In addition, in a method for manufacturing a wire according to another embodiment of the present invention, the spheroidizing heat treatment after the hot rolling may be omitted.

[0020] A bolt according to one embodiment of the present invention comprises, in wt%, C: 0.20% to 0.35%, Si: 0.05% to 0.50%, Mn: 0.30% to 0.60%, Cr: 1.00% to 1.60%, Al: 0.02% to 0.10%, Ti: 0.02% to 0.05%, and B: 0.0005% to 0.0100%, with the remainder being Fe and other impurities, and satisfies the following formula (1), and after quenching heat treatment and hot-dip galvanizing, the microstructure comprises tempered martensite of 90% to 99% in area fraction, bainite of 1% to 10%, and retained austenite of 1% or less.

[0021] Equation (1): 5.90 ≤ 9.2x[C] + 1.2x(0.9x[Mn] + 2.1x[Cr]) ≤ 6.30

[0022] (Here, [C], [Mn], and [Cr] represent the weight percent of each element)

[0023] In addition, a bolt according to another embodiment of the present invention can satisfy the following equation (2).

[0024] Equation (2): 0.99 ≤ 25x[Ti] + 124x[B] ≤ 1.36

[0025] (Here, [Ti] and [B] represent the weight percent of each element)

[0026] In addition, a bolt according to another embodiment of the present invention may have a bolt body having a diameter of 15 mm to 40 mm and a Vickers hardness of 320 HV or more.

[0027] Additionally, a bolt according to another embodiment of the present invention may have a low-temperature (-40°C) impact toughness of 50 J or more.

[0028] A method for manufacturing a bolt according to one embodiment of the present invention comprises the steps of: providing a wire according to the present invention; drawing the wire with a drawing reduction of 20% or less; heating at 850°C to 950°C and then quenching at a temperature of 20°C to 80°C; performing a tempering heat treatment at 450°C to 550°C for 3000 seconds to 10000 seconds; and hot-dip galvanizing at a temperature of 500°C to 550°C, wherein the microstructure includes tempered martensite of 90% to 99% in area fraction, bainite of 1% to 10%, and retained austenite of 1% or less.

[0029] According to one embodiment of the present invention, the wire has a Vickers hardness of 180 HV or less, thereby improving cold forging properties, and thus cold forging is possible without spheroidizing heat treatment.

[0030] In addition, a bolt according to one embodiment of the present invention may have a Vickers hardness of 320 HV or more and an impact toughness at -40°C of 50 J or more.

[0031] Preferred embodiments of the present invention are described below. However, the embodiments of the present invention may be modified in various ways, and the technical concept of the present invention is not limited to the embodiments described below. Furthermore, the embodiments of the present invention are provided to more fully explain the present invention to those of ordinary skill in the art.

[0032] The terminology used in this application is solely for the purpose of describing specific examples. Therefore, for example, singular expressions include plural expressions unless the context clearly dictates otherwise. Additionally, it should be noted that terms such as "comprise" or "have" used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the presence of other features, steps, functions, components, or combinations thereof.

[0033] Meanwhile, unless otherwise defined, all terms used herein should be considered to have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Therefore, unless explicitly defined herein, specific terms should not be interpreted in an overly idealistic or formal sense. For example, singular expressions herein include plural expressions unless the context clearly indicates otherwise.

[0034] In addition, the terms "about", "substantially", etc. in this specification are used in the sense of or close to the numerical value when manufacturing and material tolerances inherent to the meanings mentioned are presented, and are used to prevent unscrupulous infringers from unfairly using the disclosure contents in which exact or absolute numerical values ​​are mentioned to aid in the understanding of the present invention.

[0035] The basic principle of the present invention is to limit the C content to minimize the increase in Vickers hardness after hot rolling, limit the Mn content to reduce the solid solution strengthening effect of hot rolled material, and minimize the Si content to secure cold forgeability. In addition, B is added to secure hardenability during quenching heat treatment, and Cr is added to suppress the decrease in strength after hot-dip galvanizing at high temperatures of 500°C or higher, thereby contributing to improving the softening resistance of tempering.

[0036] First, the wire of the present invention will be described.

[0037] According to one embodiment of the present invention, a wire rod includes, in wt%, C: 0.20% to 0.35%, Si: 0.05% to 0.50%, Mn: 0.30% to 0.60%, Cr: 1.00% to 1.60%, Al: 0.02% to 0.10%, Ti: 0.02% to 0.05%, and B: 0.0005% to 0.0100%, with the remainder being Fe and other impurities, and satisfies the following formula (1), and after hot rolling, the microstructure includes, in terms of area fraction, pearlite 30% to 60% and ferrite 40% to 70%.

[0038] Equation (1): 5.90 ≤ 9.2x[C] + 1.2x(0.9x[Mn] + 2.1x[Cr]) ≤ 6.30

[0039] (Here, [C], [Mn], and [Cr] represent the weight percent of each element)

[0040] Below, the reasons for limiting the composition range of each alloy element are explained. Unless otherwise specified, the unit is weight percent.

[0041] The C (carbon) content can be 0.20% to 0.35%.

[0042] C is the element that most effectively increases the strength of a material. However, at a content of less than 0.20%, hardenability is insufficient during QT heat treatment. Furthermore, exceeding 0.35% can excessively increase the tensile strength during cooling after rolling of the wire rod, and reduce the life of the forging die during cold forging, thereby increasing the cost. Preferably, the C content is 0.21 to 0.34%.

[0043] The content of Si (silicon) can be 0.05% to 0.50%.

[0044] Silicon (Si) is not only useful for deoxidation of steel but also effective in securing strength through solid solution strengthening, but it is an element that deteriorates cold forgeability. When Si is contained at a content of less than 0.05%, strength is not sufficiently secured through deoxidation and solid solution strengthening of the steel. When Si is contained at a content of more than 0.50%, cold forgeability due to solid solution strengthening may be deteriorated, making it undesirable. Preferably, the Si content may be 0.06% to 0.29%. More preferably, the Si content may be 0.16% to 0.22%.

[0045] The content of Mn (manganese) can be 0.30% to 0.60%.

[0046] Manganese (Mn) is an alloying element that enhances the hardenability of steel, thereby securing strength. It also plays a role in increasing rollability and reducing brittleness. If Mn is added at less than 0.30%, it is difficult to secure sufficient strength. If Mn is added at more than 0.60%, hard structures are likely to develop during cooling after hot rolling, and a large amount of MnS inclusions may be generated, potentially reducing fatigue properties. Preferably, the Mn content is 0.31% to 0.48%.

[0047] The content of Cr (chromium) can be 1.00% to 1.60%.

[0048] Cr, together with Mn, is an element that is effective in improving hardenability and securing hardness. It can be added in amounts of 1.00% or more. However, if the content is excessive, there is a problem of coarse carbide being formed, so the upper limit may be limited to 1.60%. Preferably, the content of Cr may be greater than 1.10% and less than 1.49%.

[0049] The content of Al (aluminum) can be 0.02% to 0.10%.

[0050] Al is widely used as a deoxidizer in the steelmaking process and is effective in refining austenite grains by AlN formed by reacting with nitrogen. If added below 0.02%, the number of nitrogen compounds is insufficient, reducing the grain refinement effect. If added exceeding 0.10%, the formation of non-metallic inclusions such as alumina is excessive, aggravating the occurrence of defects in the steel, so it needs to be limited. Preferably, it can be more than 0.02% and less than 0.04%.

[0051] The content of Ti (titanium) can be 0.02% to 0.05%.

[0052] Ti combines with nitrogen introduced into the steel to form titanium carbonitride, thereby preventing boron from combining with nitrogen. When the titanium content is less than 0.02%, it is difficult to utilize the effect of boron because the nitrogen introduced during the steelmaking process is not sufficient to form titanium carbonitride. When it exceeds 0.05%, coarse carbonitride is formed, causing microcracks to occur and lowering the resistance to delayed fracture. Preferably, the Ti content may be more than 0.02% and less than 0.04%.

[0053] The content of B (boron) can be 0.0005% to 0.0100%.

[0054] B is an element that improves hardenability. If the content of B is less than 0.0005%, it is difficult to expect the effect of improving hardenability, and if it exceeds 0.0100%, Fe is present at the grain boundaries. 23 (CB)6 carbide is formed, which is not desirable because it causes embrittlement of austenite grain boundaries. Preferably, the B content may be greater than 0.0005% and less than or equal to 0.0090%. More preferably, the B content may be 0.0018% to 0.0022%.

[0055] In addition to the above composition, the wire rod according to one embodiment of the present invention must satisfy the above formula (1), and by satisfying formula (1) of 5.90 or more and 6.30 or less, sufficient Cr-based fine carbides are secured, so that the wire rod secures a Vickers hardness of 180 HV or less while improving cold forgeability, thereby enabling cold forging without spheroidizing heat treatment. In addition, by satisfying formula (1), when formula (1) is satisfied, a Cr-based carbide ratio of 90% to 95% is satisfied in the final bolt. That is, when formula (1) is satisfied and the Cr-based carbide ratio is 90% to 95%, a Vickers hardness of 320 HV or more can be secured.

[0056] In addition, the wire according to one embodiment of the present invention can satisfy the following equation (2).

[0057] Equation (2): 0.99 ≤ 25x[Ti] + 124x[B] ≤ 1.36

[0058] (Here, [Ti] and [B] represent the weight percent of each element)

[0059] Equation (2) is an equation related to Ti and B, and Ti is also used as an element that dissolves nitrogen to secure the amount of free boron in boron steel. Therefore, in order to secure the amount of free boron (free B) for grain boundary strengthening or hardenability improvement through boron, and to improve the hardenability and impact toughness of the steel by fixing nitrogen and precipitating fine TiN to refine the grains, Equation (2) must satisfy 0.99 to 1.36. When Equation (2) is less than 0.99, the contents of Ti and B are insufficient, and when it exceeds 1.36, coarse carbonitrides are formed. Therefore, if the range of Equation (2) is not satisfied, the impact toughness of the final bolt at -40℃ cannot satisfy 50J or more.

[0060] In addition, the wire according to one embodiment of the present invention may have an average austenite grain size of 25 μm or less. Preferably, it may be 1 μm to 24 μm.

[0061] In addition, the wire according to one embodiment of the present invention may have a Vickers hardness of 180 HV or less. If the Vickers hardness of the wire exceeds 180 HV, the cold forgeability of the wire is poor, and thus a spheroidizing heat treatment is required to address this. The Vickers hardness may preferably be 150 HV to 180 HV.

[0062] Next, the manufacturing method of the wire is explained.

[0063] A method for manufacturing a wire rod according to one embodiment of the present invention comprises the steps of heating a billet, which contains, in wt%, C: 0.20% to 0.35%, Si: 0.05% to 0.50%, Mn: 0.30% to 0.60%, Cr: 1.00% to 1.60%, Al: 0.02% to 0.10%, Ti: 0.02% to 0.05%, and B: 0.0005% to 0.0100%, with the remainder being Fe and other impurities, to 950°C to 1200°C; hot-rolling the billet at 850°C to 950°C; coiling the billet at 750°C to 850°C; and cooling the billet to room temperature at a cooling rate of 0.1°C / s to 0.5°C / s.

[0064] Equation (1): 5.90 ≤ 9.2x[C] + 1.2x(0.9x[Mn] + 2.1x[Cr]) ≤ 6.30

[0065] (Here, [C], [Mn], and [Cr] represent the weight percent of each element)

[0066] In addition, in a method for manufacturing a wire according to another embodiment of the present invention, after hot rolling, the microstructure may include 30% to 60% pearlite and 40% to 70% ferrite in terms of area fraction.

[0067] In addition, in the method for manufacturing a wire rod according to another embodiment of the present invention, the average austenite grain size can be controlled to 25 ㎛ or less in the cooling step. When the cooling rate after coiling satisfies 0.1°C / s to 0.5°C / s, the average austenite grain size is controlled to 25 ㎛ or less, thereby preventing cracks from occurring inside the final bolt after cold forging without spheroidizing heat treatment since the subsequent wire rod tensile strength is in the range of 180 HV or less.

[0068] If the cooling rate after coiling exceeds 0.5°C / s, the wire Vickers hardness exceeds 180 HV, which can lead to microcracks within the final bolt after cold forging. To prevent this, pre-spheroidization heat treatment is necessary. If the cooling rate is less than 0.1°C / s, the cooling rate becomes excessively slow, which can lead to decreased productivity due to the delayed cooling time.

[0069] In addition, in the method for manufacturing a wire rod according to another embodiment of the present invention, the spheroidizing heat treatment after the rolling can be omitted. By satisfying the range of the above formula (1) according to the present invention, the Vickers hardness of the wire rod can be controlled to a range of 180 HV or less through an appropriate ratio of Cr-based carbides, and thus, cracking can be prevented during subsequent cold forging without the spheroidizing heat treatment.

[0070] Next, a bolt manufactured using the wire of the present invention is described.

[0071] A bolt according to one embodiment of the present invention comprises, in wt%, C: 0.20% to 0.35%, Si: 0.05% to 0.50%, Mn: 0.30% to 0.60%, Cr: 1.00% to 1.60%, Al: 0.02% to 0.10%, Ti: 0.02% to 0.05%, and B: 0.0005% to 0.0100%, with the remainder being Fe and other impurities, and satisfies the following formula (1), and after quenching heat treatment and hot-dip galvanizing, the microstructure comprises tempered martensite of 90% to 99% in area fraction, bainite of 1% to 10%, and retained austenite of 1% or less.

[0072] Equation (1): 5.90 ≤ 9.2x[C] + 1.2x(0.9x[Mn] + 2.1x[Cr]) ≤ 6.30

[0073] (Here, [C], [Mn], and [Cr] represent the weight percent of each element)

[0074] In addition, a bolt according to another embodiment of the present invention can satisfy the following equation (2).

[0075] Equation (2): 0.99 ≤ 25x[Ti] + 124x[B] ≤ 1.36

[0076] (Here, [Ti] and [B] represent the weight percent of each element)

[0077] In addition, a bolt according to another embodiment of the present invention may have a bolt body diameter of 15 mm to 40 mm and a Vickers hardness of 320 HV or more. Preferably, it may be 320 HV to 400 HV.

[0078] Additionally, a bolt according to another embodiment of the present invention may have a low-temperature (-40°C) impact toughness of 50 J or more.

[0079] A method for manufacturing a bolt according to one embodiment of the present invention comprises the steps of: providing a wire according to the present invention; drawing the wire at a drawing reduction rate of 20% or less; heating at 850°C to 950°C and then quenching in oil at 20°C to 80°C; performing a tempering heat treatment at 450°C to 550°C for 3000 seconds to 10000 seconds; and hot-dip galvanizing at a temperature of 500°C to 550°C, wherein the microstructure includes tempered martensite of 90% to 99% in area fraction, bainite of 1% to 10%, and retained austenite of 1% or less.

[0080] When trying to control a bolt with a diameter of 15 mm to 40 mm in the body with a Vickers hardness of 320 HV or higher using general steel, a tempering heat treatment temperature of 450°C to 550°C is preferable.

[0081] Specifically, a wire having the above-described alloy composition, microstructure, and formula (1) and having an average austenite grain size of 25 ㎛ or less is drawn at a drawing reduction rate of 20% or less, heated to a temperature between 850°C and 950°C, quenched in oil at 20°C to 80°C, and tempered at 450°C to 550°C for 3,000 seconds to 10,000 seconds, after which the bolt can secure a microstructure of 90% to 99% tempered martensite, 1% to 10% bainite, and 1% or less of retained austenite.

[0082] That is, cold forging is possible without spheroidizing heat treatment of the wire rod, and by subsequent tempering at high temperatures, the thin film-shaped carbides that are mainly formed at the grain boundaries of old austenite are suppressed and spheroidized, and fine carbides are dispersed and distributed inside and outside the grain boundaries, so that the final bolt with a diameter of 15 mm to 40 mm in the body portion that has undergone hot-dip galvanizing at 500 to 550°C can secure high strength and high impact toughness of 320 HV or more and low-temperature (-40°C) impact toughness of 50 J or more.

[0083] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0084] {Example}

[0085] A billet having an alloy composition as shown in Table 1 below was heated to 950°C to 1200°C, hot-rolled at 850°C to 950°C, and coiled at 800°C. The cooling rate after coiling was set to 0.1°C / s to 0.5°C / s.

[0086] Alloy composition (wt%) CSiMnCrTiBAl Inventive material 10.240.220.381.420.030.00210.03 Inventive material 20.330.160.351.100.030.00210.03 Inventive material 30.220.220.491.360.040.00180.03 Inventive material 40.340.190.301.120.030.00220.03 Inventive material 50.210.170.311.490.030.00190.03 Comparative material 10.190.220.321.210.040.00170.03 Comparative material 20.410.190.331.110.030.00180.03Comparative 30.320.140.211.250.030.00160.03Comparative 40.340.250.350.910.040.00300.03Comparative 50.510.220.411.210.030.00180.03Comparative 60.220.220.371.420.030.00060.03Comparative 70.250.220.351.420.040.00350.03Comparative 80.230.220.381.420.030.00520.03

[0087] Table 2 below shows the microstructure and properties of the wire rod. In the case of cold forging, if the Vickers hardness of the wire rod exceeds 180 HV and thus the cold forging is poor, it is marked as X. If the cold forging is good due to the Vickers hardness of 180 HV or less, it is marked as O. In addition, in the case of the Vickers hardness of the wire rod and bolt, each specimen was measured using a Vickers hardness tester.

[0088] Formula (1) Formula (2) Vickers hardness (HV) Cold forging composition Inventive material 16.20 1.01 169 O Inventive material 26.19 1.01 177 O Inventive material 35.98 1.22 167 O Inventive material 46.27 1.02 178 O Inventive material 56.02 0.99 166 O Comparative material 15.14 1.21 163 O Comparative material 26.93 0.97 225 X Comparative material 36.32 0.94 186 X Comparative material 45.80 1.37 178 O Comparative material 58.18 0.97 233 X Comparative material 66.00 0.82 165 O Comparative material 76.26 1.43 158 O Comparative material 86.10 1.39 162 O

[0089] Equation (1): 5.9 ≤ 9.2x[C] + 1.2x(0.9x[Mn] + 2.1x[Cr]) ≤ 6.3Equation (2): 0.99 ≤ 25x[Ti] + 124x[B] ≤ 1.36

[0090] Table 3 below shows the measured values ​​of austenite grain size according to the cooling rate during the cooling stage of the wire rod. In addition, the presence or absence of microcracks in the final bolts produced by the manufacturing method below is shown. If cracks occurred, they were marked as O, and if no cracks occurred, they were marked as X. The average austenite grain size of the wire rod was measured at three random locations according to the ASTM E 112 standard and calculated as the average value.

[0091] In addition, the presence or absence of cracks was confirmed by using a delayed fracture simulation method that observed the presence or absence of microcracks in the screw thread, which is a stress concentration area, before and after immersing the target steel plate in a 5% hydrochloric acid + 95% distilled water solution for 10 minutes after the final product was heat treated.

[0092] Cooling rate (℃ / s)Average austenite grain size (㎛)Presence of microcracksInventor 10.155XInventor 20.2422XInventor 30.4216XInventor 40.2820XInventor 50.4524XComparative 10.084XComparative 20.036XComparative 30.057XComparative 40.079XComparative 50.0811XComparative 62.528OComparative 73.236OComparative 84.440O

[0093] The bolts were manufactured by drawing the above wire with a reduction in area of ​​20% or less, heating it at 940°C for 3600 seconds, and then quenching it in oil at 70°C. After that, a tempering heat treatment was performed at 450°C to 550°C for 3000 seconds to 10000 seconds, and then hot-dip galvanizing was performed at 500°C to 550°C to manufacture the bolts. The impact toughness of the bolts was measured by processing a V-notch specimen according to the ASTM E23 standard and performing a Charpy impact test at a low temperature (-40°C), and the results are shown in Table 4 below. In addition, the Cr-based carbide ratio of the bolts was measured using an image analyzer according to the ASTM E 552 standard method.

[0094] Bolt classification formula (1) Formula (2) Vickers hardness (HV) Low temperature (-40℃) Impact toughness (J) Cr carbide ratio (%) Inventive material 16.20 1.013326893.3 Inventive material 26.19 1.013465594.2 Inventive material 35.98 1.223227692.5 Inventive material 46.27 1.023525794.8 Inventive material 56.02 0.99 3258290.2 Comparative material 15.14 1.212807582.5 Comparative material 26.93 0.97 3612595.3 Comparative material 36.32 0.94 3114596.6 Comparative material 45.80 1.373024388.2 Comparative material 58.180.973722298.5 Comparative material 66.000.823243294.5 Comparative material 76.261.433541892.8 Comparative material 86.101.393621593.6

[0095] Inventive examples 1 to 5 satisfy all of the alloy composition, austenite average grain size, and the ranges of formula (1) and formula (2) of the wire rod according to the present invention, so that the final bolt can have a Vickers hardness of 320 HV or more and a low-temperature (-40°C) impact toughness of 50 J or more. On the other hand, it can be confirmed that comparative examples 1 to 5, which do not satisfy the composition, austenite average grain size, or formula (1) or formula (2), have a Vickers hardness of less than 320 HV or a low-temperature (-40°C) impact toughness of less than 50 J.

[0096] Furthermore, in the case of comparative materials 6 to 8, the alloy composition according to the present invention and the range of formula (1) were satisfied, but formula (2) was not satisfied. Specifically, in the case of comparative material 6, the value of formula (2) was less than 0.99, which did not secure a sufficient content of free boron, resulting in inferior impact toughness, and in the case of comparative materials 7 and 8, the value of formula (2) exceeded 1.36, resulting in the generation of coarse carbonitrides. Accordingly, it was confirmed that comparative materials 6 to 8 had inferior impact toughness at -40°C of less than 50 J.

[0097] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible within the scope and spirit of the claims set forth below.

Claims

1. Containing, by weight%, C: 0.20% to 0.35%, Si: 0.05% to 0.50%, Mn: 0.30% to 0.60%, Cr: 1.00% to 1.60%, Al: 0.02% to 0.10%, Ti: 0.02% to 0.05%, and B: 0.0005% to 0.0100%, the remainder being Fe and other impurities, and satisfying the following formula (1): A wire rod having a microstructure after hot rolling, which contains, in terms of area fraction, 30 to 60% pearlite and 40 to 70% ferrite. Equation (1): 5.90 ≤ 9.2x[C] + 1.2x(0.9x[Mn] + 2.1x[Cr]) ≤ 6.30 (Here, [C], [Mn], and [Cr] represent the weight% of each element) 2. In claim 1, A pre-existing material satisfying the following equation (2). Equation (2): 0.99 ≤ 25x[Ti] + 124x[B] ≤ 1.36 (Here, [Ti] and [B] represent the weight% of each element) 3. In claim 1, A wire having an average austenite grain size of 25 ㎛ or less.

4. In claim 1, The above wire has a Vickers hardness of 180 HV or less.

5. Containing, by weight%, C: 0.20% to 0.35%, Si: 0.05% to 0.50%, Mn: 0.30% to 0.60%, Cr: 1.00% to 1.60%, Al: 0.02% to 0.10%, Ti: 0.02% to 0.05%, and B: 0.0005% to 0.0100%, the remainder being Fe and other impurities, A step of heating a billet satisfying the following equation (1) to 950°C to 1200°C; A hot rolling step at 850℃ to 950℃; Step of coiling at 750℃ to 850℃; and A method for manufacturing a wire rod, comprising a step of cooling to room temperature at a cooling rate of 0.1°C / s to 0.5°C / s. Equation (1): 5.90 ≤ 9.2x[C] + 1.2x(0.9x[Mn] + 2.1x[Cr]) ≤ 6.30 (Here, [C], [Mn], and [Cr] represent the weight% of each element) 6. In claim 5, A method for manufacturing a wire rod, wherein the microstructure of the wire rod after the hot rolling comprises an area fraction of 30 to 60% pearlite and 40 to 70% ferrite.

7. In claim 5, A method for manufacturing a wire rod, wherein the average austenite grain size in the above cooling step is controlled to 25 ㎛ or less.

8. In claim 5, A method for manufacturing a wire rod, wherein the spheroidizing heat treatment after the above hot rolling can be omitted.

9. Containing C: 0.20% to 0.35%, Si: 0.05% to 0.50%, Mn: 0.30% to 0.60%, Cr: 1.00% to 1.60%, Al: 0.02% to 0.10%, Ti: 0.02% to 0.05%, and B: 0.0005% to 0.0100%, the remainder being Fe and other impurities, and satisfying the following formula (1): A bolt having a microstructure comprising an area fraction of 90 to 99% tempered martensite, 1 to 10% bainite, and 1% or less of retained austenite after quenching heat treatment and hot-dip galvanization. Equation (1): 5.90 ≤ 9.2x[C] + 1.2x(0.9x[Mn] + 2.1x[Cr]) ≤ 6.30 (Here, [C], [Mn], and [Cr] represent the weight% of each element) 10. In claim 9, A bolt satisfying the following equation (2). Equation (2): 0.99 ≤ 25x[Ti] + 124x[B] ≤ 1.36 (Here, [Ti] and [B] represent the weight% of each element) 11. In claim 9, A bolt having a body diameter of 15 to 40 mm and a Vickers hardness of 320 HV or higher.

12. In claim 9, Bolts having a low temperature (-40℃) impact toughness of 50J or more.

13. A step of preparing the precursors of claims 1 to 4; A step of freshening the above raw material with a freshness reduction rate of 20% or less; A step of heating at 850℃ to 950℃ and then quenching at a temperature of 20℃ to 80℃; A step of tempering heat treatment at 450℃ to 550℃ for 3000 seconds to 10000 seconds; and Comprising a step of hot-dip galvanizing at a temperature of 500°C to 550°C, A method for manufacturing a bolt, wherein the microstructure comprises tempered martensite of area fraction of 90% to 99%, bainite of area fraction of 1% to 10%, and retained austenite of 1% or less.