Wire, bolt and method for manufacturing the same

Abstract

According to one embodiment of this disclosure, the wire comprises, by weight percent: 0.20% to 0.35% C, 0.05% to 0.50% Si, 0.30% to 0.60% Mn, 1.00% to 1.60% Cr, 0.02% to 0.10% Al, 0.02% to 0.05% Ti, and 0.0005% to 0.0100% B, with the remainder being Fe and other impurities, wherein the wire satisfies formula (1), and the microstructure after hot rolling 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 (wherein [C], [Mn] and [Cr] refer to the weight percent of each element).

Description

Technical Field

[0001] This disclosure relates to wires, bolts, and methods for manufacturing them. Background Technology

[0002] The existing JS-SCM440 steel, used as raw material for bolts used to fasten wind towers, requires spheroidizing heat treatment for cold forging, which reduces its economic viability due to the addition of expensive Mo. Meanwhile, the problem with JS-SCr420B steel, which can be cold forged without heat treatment after wire rolling, is that its strength decreases after hot-dip galvanizing due to its low C and Cr content, necessitating a lower tempering temperature during QT heat treatment, which reduces its low-temperature (-40°C) impact toughness. Summary of the Invention

[0003] Technical issues

[0004] The purpose of this disclosure in order to address the aforementioned problems is to improve the cold forgeability of hot-rolled materials by forming fine carbides during tempering, even without spheroidizing heat treatment.

[0005] Furthermore, this disclosure provides wires, bolts, and methods for manufacturing them that can simultaneously ensure cold forgeability and Vickers hardness.

[0006] The problems to be addressed by this disclosure are not limited to those mentioned above, and those skilled in the art will clearly understand from the following description other problems not mentioned.

[0007] Technical solution

[0008] According to one embodiment of this disclosure, the wire contains, by weight percentage (wt%), 0.20% to 0.35% carbon (C), 0.05% to 0.50% silicon (Si), 0.30% to 0.60% manganese (Mn), 1.00% to 1.60% chromium (Cr), 0.02% to 0.10% aluminum (Al), 0.02% to 0.05% titanium (Ti), and 0.0005% to 0.0100% boron (B), the remainder being iron (Fe), and unavoidable impurities, satisfying the following formula (1), and by area fraction comprising 30% to 60% pearlite and 40% to 70% ferrite as the microstructure after hot rolling.

[0009] Formula (1): 5.90 ≤ 9.2x[C] + 1.2x(0.9x[Mn] + 2.1x[Cr]) ≤ 6.30

[0010] (Where, [C], [Mn] and [Cr] refer to the weight percentage of each element).

[0011] Furthermore, the wire according to one embodiment of this disclosure can satisfy the following formula (2).

[0012] Equation (2): 0.99 ≤ 25x[Ti] + 124x[B] ≤ 1.36

[0013] (Where, [Ti] and [B] refer to the weight percentage of each element).

[0014] Furthermore, in one embodiment of the wire according to this disclosure, the average austenite grain size of the wire may be 25 μm or less.

[0015] Furthermore, in one embodiment of the wire according to this disclosure, the Vickers hardness of the wire may be 180 HV or lower.

[0016] A method for manufacturing wire according to one embodiment of the present disclosure includes: heating a billet to 950°C to 1200°C, the billet comprising, by weight percent (wt%), 0.20% to 0.35% carbon (C), 0.05% to 0.50% silicon (Si), 0.30% to 0.60% manganese (Mn), 1.00% to 1.60% chromium (Cr), 0.02% to 0.10% aluminum (Al), 0.02% to 0.05% titanium (Ti), and 0.0005% to 0.0100% boron (B), the remainder being iron (Fe) and unavoidable impurities, and satisfying the following formula (1); hot rolling at 850°C to 950°C; coiling at 750°C to 850°C; and cooling to room temperature at a cooling rate of 0.1°C / sec to 0.5°C / sec.

[0017] Formula (1): 5.90 ≤ 9.2x[C] + 1.2x(0.9x[Mn] + 2.1x[Cr]) ≤ 6.30

[0018] (Where, [C], [Mn] and [Cr] refer to the weight percentage of each element).

[0019] Furthermore, in another embodiment of the method for manufacturing wire according to this disclosure, the microstructure of the wire after hot rolling may contain 30% to 60% pearlite and 40% to 70% ferrite in area fraction.

[0020] Furthermore, in another embodiment of the method for manufacturing wire according to this disclosure, the average austenite grain size can be controlled to 25 μm or less during cooling.

[0021] Furthermore, in another embodiment of the method for manufacturing wire according to this disclosure, the method is carried out after hot rolling without spheroidizing heat treatment.

[0022] According to one embodiment of this disclosure, the bolt contains, by weight percentage (wt%), 0.20% to 0.35% carbon (C), 0.05% to 0.50% silicon (Si), 0.30% to 0.60% manganese (Mn), 1.00% to 1.60% chromium (Cr), 0.02% to 0.10% aluminum (Al), 0.02% to 0.05% titanium (Ti), and 0.0005% to 0.0100% boron (B), with the remainder being iron (Fe) and unavoidable impurities, satisfying the following formula (1), and by area fraction containing 90% to 99% tempered martensite, 1% to 10% bainite, and 1% or less retained austenite as the microstructure after quenching heat treatment and hot-dip galvanizing.

[0023] Formula (1): 5.90 ≤ 9.2x[C] + 1.2x(0.9x[Mn] + 2.1x[Cr]) ≤ 6.30

[0024] (Where, [C], [Mn] and [Cr] refer to the weight percentage of each element).

[0025] Furthermore, the bolt according to another embodiment of this disclosure can satisfy the following formula (2).

[0026] Equation (2): 0.99 ≤ 25x[Ti] + 124x[B] ≤ 1.36

[0027] (Where, [Ti] and [B] refer to the weight percentage of each element).

[0028] Furthermore, in another embodiment of the bolt according to this disclosure, the diameter of the bolt body portion can be from 15 mm to 40 mm, and the Vickers hardness can be 320 HV or higher.

[0029] Furthermore, according to another embodiment of this disclosure, the bolt's low-temperature (-40°C) impact toughness can be 50 J or higher.

[0030] A method for manufacturing bolts according to one embodiment of this disclosure includes: preparing wire according to this disclosure; drawing the wire with a draw reduction ratio of 20% or less; heating at 850°C to 950°C and then quenching at a temperature of 20°C to 80°C; 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 comprises, by area fraction, 90% to 99% tempered martensite, 1% to 10% bainite, and 1% or less retained austenite.

[0031] Beneficial effects

[0032] The wire according to one embodiment of this disclosure ensures a Vickers hardness of 180 HV or less, thereby improving cold forging capability and making cold forging possible without spheroidizing heat treatment.

[0033] Furthermore, a bolt according to one embodiment of this disclosure may have a Vickers hardness of 320 HV or higher and an impact toughness of 50 J or higher at -40°C. Detailed Implementation

[0034] Preferred embodiments of this disclosure will be described below. However, embodiments of this disclosure may be modified in various other ways, and the technical spirit of this disclosure is not limited to the embodiments described below. Furthermore, embodiments of this disclosure are provided to provide a more complete description of this disclosure to those skilled in the art.

[0035] The terms used in this application are for the purpose of describing specific instances only. Therefore, for example, singular expressions include plural expressions unless the context explicitly indicates that they must be singular. Furthermore, it should be noted that terms such as “comprising” or “having” as 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 presuppose the presence of other features, steps, functions, components, or combinations thereof.

[0036] Furthermore, unless otherwise defined, all terms used herein should be considered to have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure pertains. Therefore, unless explicitly defined herein, certain terms should not be interpreted in an overly ideal or formal sense. For example, unless the context clearly indicates an exception, singular expressions herein include plural expressions.

[0037] Furthermore, in this specification, the terms “about,” “substantially,” etc., are used to mean that the manufacturing and material tolerances inherent in the meaning are at or close to that value, and are used to prevent malicious infringers from unfairly using this disclosure (where precise or absolute values ​​are mentioned to aid in understanding this disclosure).

[0038] The basic principle of this disclosure is to limit the carbon content to minimize the increase in Vickers hardness after hot rolling, limit the manganese content to reduce the solid solution strengthening effect of hot-rolled materials, and minimize the silicon content to ensure cold forgeability. Furthermore, the addition of boron (B) ensures hardenability during quenching heat treatment, and the addition of chromium (Cr) suppresses strength reduction after hot-dip galvanizing at 500°C or higher, thereby contributing to improved softening resistance during tempering.

[0039] First, let's describe the cable used in this disclosure.

[0040] According to one embodiment of this disclosure, the wire contains, by weight percentage (wt%), 0.20% to 0.35% carbon (C), 0.05% to 0.50% silicon (Si), 0.30% to 0.60% manganese (Mn), 1.00% to 1.60% chromium (Cr), 0.02% to 0.10% aluminum (Al), 0.02% to 0.05% titanium (Ti), and 0.0005% to 0.0100% boron (B), the remainder being iron (Fe), and unavoidable impurities, satisfying the following formula (1), and by area fraction comprising 30% to 60% pearlite and 40% to 70% ferrite as the microstructure after hot rolling.

[0041] Formula (1): 5.90 ≤ 9.2x[C] + 1.2x(0.9x[Mn] + 2.1x[Cr]) ≤ 6.30

[0042] (Where, [C], [Mn] and [Cr] refer to the weight percentage of each element).

[0043] The reasons for limiting the composition range of each alloying element are described below. Unless otherwise stated, all units below are by weight%.

[0044] The C (carbon) content can be from 0.20% to 0.35%.

[0045] Carbon (C) is the element that most effectively increases material strength, and at contents below 0.20%, it is insufficient to ensure hardenability during QT heat treatment. Furthermore, contents exceeding 0.35% can excessively increase tensile strength during cooling after wire rolling and may reduce die life during cold forging, thus increasing costs. Preferably, the C content can be from 0.21% to 0.34%.

[0046] The Si content can be from 0.05% to 0.50%.

[0047] Si is effective not only in deoxidizing steel but also in ensuring strength through solid solution strengthening, but it is also an element that degrades cold forgeability. A Si content of less than 0.05% is insufficient to ensure strength through deoxidation and solid solution strengthening, and a content exceeding 0.50% is undesirable because solid solution strengthening may result in poor cold forgeability. Preferably, the Si content can be from 0.06% to 0.29%. More preferably, the Si content can be from 0.16% to 0.22%.

[0048] The Mn (manganese) content can be from 0.30% to 0.60%.

[0049] Mn is an alloying element that helps ensure strength by improving the hardenability of steel and plays a role in increasing rollability and reducing brittleness. When Mn is added in amounts less than 0.30%, it is difficult to ensure sufficient strength, and when added in amounts exceeding 0.60%, hard structures may form during cooling after hot rolling, and a large number of MnS inclusions may be generated, which may reduce fatigue properties. Preferably, the Mn content can be from 0.31% to 0.48%.

[0050] The Cr (chromium) content can be from 1.00% to 1.60%.

[0051] Cr is an effective element in improving hardenability and ensuring hardness when used with Mn, and can be added in amounts of 1.00% or more. However, excessive amounts may lead to the formation of coarse carbides, so the upper limit can be limited to 1.60%. Preferably, the Cr content can be greater than 1.10% and 1.49% or less.

[0052] The Al (aluminum) content can be from 0.02% to 0.10%.

[0053] Al is widely used as a deoxidizer in steelmaking processes and is effective in refining austenite grains due to the AlN formed by its reaction with N. When added in amounts less than 0.02%, the amount of nitrogen compounds is insufficient, thus reducing the grain refining effect. Furthermore, when added in amounts exceeding 0.10%, excessive formation of non-metallic inclusions, such as alumina, occurs, exacerbating defects in the steel. Therefore, it is necessary to limit the content. Preferably, the content can be greater than 0.02% and 0.04% or less.

[0054] The Ti (titanium) content can be from 0.02% to 0.05%.

[0055] Ti combines with nitrogen introduced into the steel to form titanium carbonitrides, thereby preventing boron from combining with nitrogen. When the titanium content is below 0.02%, it is difficult to utilize the effect of boron because it is insufficient to form titanium carbonitrides from the nitrogen introduced during the steelmaking process, and when the content exceeds 0.05%, coarse carbonitrides are formed, leading to microcracks and deterioration of resistance to delayed fracture. Preferably, the Ti content can be greater than 0.02% and 0.04% or less.

[0056] The content of boron (B) can be from 0.0005% to 0.0100%.

[0057] Boron (B) is an element that improves hardenability. When the B content is less than 0.0005%, the improvement in hardenability is difficult to predict, and when the content exceeds 0.0100%, Fe forms at the grain boundaries. 23(CB)6 carbides, which lead to brittleness at austenite grain boundaries, are undesirable. Preferably, the content of boron (B) can be greater than 0.0005% and less than 0.0090%. More preferably, the content of boron (B) can be from 0.0018% to 0.0022%.

[0058] In addition to satisfying the above composition, the wire according to one embodiment of this disclosure shall also satisfy Equation (1), and a value of 5.90 or greater and 6.30 or less in Equation (1) is sufficient to ensure fine Cr-based carbides, thereby ensuring a Vickers hardness of 180 HV or less while improving cold forgeability, thus enabling cold forging without spheroidizing heat treatment. Furthermore, satisfying Equation (1) ensures a 90% to 95% Cr-based carbide ratio in the final bolt. That is, when Equation (1) is satisfied and the Cr-based carbide ratio is 90% to 95%, a Vickers hardness of 320 HV or higher can be ensured.

[0059] Furthermore, the wire according to one embodiment of this disclosure can satisfy the following formula (2).

[0060] Equation (2): 0.99 ≤ 25x[Ti] + 124x[B] ≤ 1.36

[0061] (Where, [Ti] and [B] refer to the weight percentage of each element).

[0062] Equation (2) involves Ti and B, and Ti is also used as an element to solidify nitrogen to ensure the amount of free boron in boron steel. Therefore, in order to fix nitrogen to ensure the amount of free boron (free B), thereby achieving grain boundary strengthening or hardenability improvement through boron, and to improve the hardenability and impact toughness of steel by grain refinement through the precipitation of fine TiN, Equation (2) should satisfy 0.99 to 1.36. When Equation (2) is less than 0.99, the content of Ti and B becomes insufficient, and when it exceeds 1.36, coarse carbonitrides are formed. Therefore, the range that does not satisfy Equation (2) makes it impossible for the final bolt to meet the impact toughness of 50 J or higher at -40°C.

[0063] Furthermore, the average austenite grain size of the wire according to one embodiment of this disclosure can be 25 μm or less. Preferably, it can be from 1 μm to 24 μm.

[0064] Furthermore, according to one embodiment of this disclosure, the Vickers hardness of the wire can be 180 HV or lower. When the Vickers hardness of the wire exceeds 180 HV, the cold forgeability of the wire deteriorates, thus requiring spheroidizing heat treatment to address this issue. Preferably, the Vickers hardness can be between 150 HV and 180 HV.

[0065] Next, the method for manufacturing the wire will be described.

[0066] A method for manufacturing wire according to one embodiment of the present disclosure includes: heating a billet to 950°C to 1200°C, the billet comprising, by weight percent (wt%), 0.20% to 0.35% carbon (C), 0.05% to 0.50% silicon (Si), 0.30% to 0.60% manganese (Mn), 1.00% to 1.60% chromium (Cr), 0.02% to 0.10% aluminum (Al), 0.02% to 0.05% titanium (Ti), and 0.0005% to 0.0100% boron (B), the remainder being iron (Fe) and unavoidable impurities, and satisfying the following formula (1); hot rolling at 850°C to 950°C; coiling at 750°C to 850°C; and cooling to room temperature at a cooling rate of 0.1°C / sec to 0.5°C / sec.

[0067] Formula (1): 5.90 ≤ 9.2x[C] + 1.2x(0.9x[Mn] + 2.1x[Cr]) ≤ 6.30

[0068] (Where, [C], [Mn] and [Cr] refer to the weight percentage of each element).

[0069] Furthermore, in another embodiment of the method for manufacturing wire according to this disclosure, the microstructure after hot rolling may contain 30% to 60% pearlite and 40% to 70% ferrite in area fraction.

[0070] Furthermore, in another embodiment of the method for manufacturing wire according to this disclosure, the average austenite grain size can be controlled to 25 μm or less during cooling. A cooling rate following coiling at 0.1°C / s to 0.5°C / s controls the average austenite grain size to 25 μm or less, resulting in a Vickers hardness of the wire subsequently in the range of 180 HV or less, thereby preventing cracks from forming inside the final bolt after cold forging without spheroidizing heat treatment.

[0071] When the cooling rate after coiling exceeds 0.5°C / s, the Vickers hardness of the wire exceeds 180 HV, leading to microcracks inside the final bolt after cold forging. Therefore, pre-treatment with spheroidizing heat treatment is necessary to prevent this. When the cooling rate is less than 0.1°C / s, the cooling rate becomes too slow, which may reduce productivity due to the delay in cooling time.

[0072] Furthermore, in another embodiment of the method for manufacturing wire according to this disclosure, spheroidizing heat treatment can be omitted after rolling. Satisfying the range of formula (1) according to this disclosure enables the Vickers hardness of the wire to be controlled within the range of 180 HV or less by means of an appropriate ratio of Cr-based carbides, thereby preventing cracking during subsequent cold forging even without spheroidizing heat treatment.

[0073] Next, a bolt manufactured using wire according to this disclosure will be described.

[0074] According to one embodiment of this disclosure, the bolt contains, by weight percentage (wt%), 0.20% to 0.35% carbon (C), 0.05% to 0.50% silicon (Si), 0.30% to 0.60% manganese (Mn), 1.00% to 1.60% chromium (Cr), 0.02% to 0.10% aluminum (Al), 0.02% to 0.05% titanium (Ti), and 0.0005% to 0.0100% boron (B), with the remainder being iron (Fe) and unavoidable impurities, satisfying the following formula (1), and by area fraction containing 90% to 99% tempered martensite, 1% to 10% bainite, and 1% or less retained austenite as the microstructure after quenching heat treatment and hot-dip galvanizing.

[0075] Formula (1): 5.90 ≤ 9.2x[C] + 1.2x(0.9x[Mn] + 2.1x[Cr]) ≤ 6.30

[0076] (Where, [C], [Mn] and [Cr] refer to the weight percentage of each element).

[0077] Furthermore, the bolt according to another embodiment of this disclosure can satisfy the following formula (2).

[0078] Equation (2): 0.99 ≤ 25x[Ti] + 124x[B] ≤ 1.36

[0079] (Where, [Ti] and [B] refer to the weight percentage of each element).

[0080] Furthermore, in another embodiment of the bolt according to this disclosure, the diameter of the bolt body portion can be from 15 mm to 40 mm, and the Vickers hardness can be 320 HV or higher. Preferably, it can be from 320 HV to 400 HV.

[0081] Furthermore, according to another embodiment of this disclosure, the bolt's low-temperature (-40°C) impact toughness can be 50 J or higher.

[0082] A method for manufacturing bolts according to one embodiment of the present disclosure includes: preparing wire according to the present disclosure; drawing the wire with a draw reduction ratio of 20% or less; heating at 850°C to 950°C and then quenching in oil at 20°C to 80°C; 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 comprises, by area fraction, 90% to 99% tempered martensite, 1% to 10% bainite, and 1% or less retained austenite.

[0083] When bolts with a body diameter of 15 mm to 40 mm are controlled to a Vickers hardness of 320 HV or higher in the case of ordinary steel, a temperature of 450°C to 550°C is preferred for tempering heat treatment.

[0084] Specifically, after drawing wires with a drawing reduction ratio of 20% or less that meet the above alloy composition, microstructure and formula (1) and have an average austenite grain size of 25 μm or less, heating them at a temperature of 850°C to 950°C, quenching them in oil at 20°C to 80°C, and tempering them at 450°C to 550°C for 3000 seconds to 10000 seconds, the bolts can ensure a microstructure of 90% to 99% tempered martensite, 1% to 10% bainite and 1% or less retained austenite.

[0085] That is, cold forging is possible without spheroidizing heat treatment of the wire, and the subsequent tempering at high temperature suppresses the thin film-type carbides generated mainly at the original austenite grain boundaries and disperses the spheroidized fine carbides and distributes them inside and outside the grain boundaries. This allows the final bolts with a body diameter of 15 mm to 40 mm, which have undergone hot-dip galvanizing at 500°C to 550°C, to ensure high strength and high impact toughness with a Vickers hardness of 320 HV or higher and a low-temperature (-40°C) impact toughness of 50 J or higher.

[0086] The present disclosure will be described in more detail below through examples. However, it should be noted that the following examples are for illustrative purposes only and are not intended to limit the scope of the disclosure. This is because the scope of the present disclosure is determined by the matters described in the scope of the claims and matters reasonably inferred therefrom.

[0087] {Example}

[0088] A billet having the alloy composition shown in Table 1 below is heated to 950°C to 1200°C, then hot-rolled at 850°C to 950°C, and coiled at 800°C. The cooling rate after coiling is 0.1°C / s to 0.5°C / s.

[0089] [Table 1]

[0090]

[0091] Table 2 below shows the microstructure and physical properties of the wire. For cold forgeability, a value of X indicates poor cold forgeability due to a Vickers hardness exceeding 180 HV, while O indicates good cold forgeability based on a Vickers hardness of 180 HV or lower. Furthermore, the Vickers hardness of the wire and bolts was measured using a Vickers hardness tester.

[0092] [Table 2]

[0093]

[0094] Equation (1): 5.9 ≤ 9.2x[C] + 1.2x(0.9x[Mn] + 2.1x[Cr]) ≤ 6.3 Equation (2): 0.99 ≤ 25x[Ti] + 124x[B] ≤ 1.36

[0095] Table 3 below shows the measured austenite grain size based on the cooling rate during the wire cooling process. Additionally, the presence or absence of microcracks in the final bolt is shown by the following manufacturing methods. The presence of cracks is indicated by O, and the absence of cracks by X. The average austenite grain size of the wire was measured at three random locations according to ASTM E 112 and calculated as the average value.

[0096] Furthermore, the presence or absence of cracks was determined by a delayed fracture simulation method: after the heat treatment of the final product and after fastening to the target steel plate, before / after immersion in a 5% hydrochloric acid + 95% distilled water solution for 10 minutes, the presence or absence of microcracks in the threads, which are stress concentration points, was observed.

[0097] [Table 3]

[0098]

[0099] To manufacture the bolts, wire was drawn with a draw reduction ratio of 20% or less, heated at 940°C for 3600 seconds, then immersed in oil at 70°C and quenched. Subsequently, it underwent tempering heat treatment at 450°C to 550°C for 3000 to 10000 seconds, followed by hot-dip galvanizing at 500°C to 550°C to produce the bolts. The impact toughness of the bolts was measured by machining V-notch specimens according to ASTM E23 and performing Charpy impact tests at a low temperature (-40°C), and is shown in Table 4 below. Furthermore, the Cr-based carbide ratio of the bolts was measured using an image analyzer according to the ASTM E 552 standard method.

[0100] [Table 4]

[0101]

[0102] Invention steels 1 to 5 satisfy all ranges of the alloy composition, average austenite grain size, and formulas (1) and (2) of the wire according to this disclosure, such that the final bolt can have a Vickers hardness of 320 HV or higher and a low-temperature (-40°C) impact toughness of 50 J or higher. On the other hand, it can be determined that comparative examples 1 to 5, which do not satisfy the composition, average austenite grain size, 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.

[0103] Furthermore, in the case of comparative steels 6 to 8, the alloy composition and range of formula (1) according to this disclosure are satisfied, but formula (2) is not satisfied. Specifically, in the case of comparative steel 6, the value of formula (2) is less than 0.99, thus insufficient free boron content cannot be ensured, resulting in poor impact toughness; and in the cases of comparative steels 7 and 8, the value of formula (2) exceeds 1.36, resulting in the formation of coarse carbonitrides. Therefore, comparative steels 6 to 8 are determined to be poor due to their impact toughness being less than 50 J at -40°C.

[0104] Exemplary embodiments of the present disclosure have been described above, but the present disclosure is not limited thereto, and those skilled in the art will understand that various changes and modifications are possible without departing from the concept and scope of the appended claims.

Claims

1. A wire comprising, by weight percentage (wt%), 0.20% to 0.35% carbon (C), 0.05% to 0.50% silicon (Si), 0.30% to 0.60% manganese (Mn), 1.00% to 1.60% chromium (Cr), 0.02% to 0.10% aluminum (Al), 0.02% to 0.05% titanium (Ti), and 0.0005% to 0.0100% boron (B), the balance being iron (Fe), and unavoidable impurities, wherein the following formula (1) is satisfied, and The microstructure after hot rolling contains 30% to 60% pearlite and 40% to 70% ferrite by area fraction. Formula (1): 5.90 ≤ 9.2x[C] + 1.2x(0.9x[Mn] + 2.1x[Cr]) ≤ 6.30 (in, [C], [Mn], and [Cr] refer to the weight percentage of each element.

2. The wire according to claim 1, wherein the following formula (2) is satisfied: Equation (2): 0.99 ≤ 25x[Ti] + 124x[B] ≤ 1.36 (in, [Ti] and [B] refer to the weight percentage of each element.

3. The wire according to claim 1, wherein the average austenite grain size of the wire is 25 μm or less.

4. The wire according to claim 1, wherein the Vickers hardness of the wire is 180 HV or lower.

5. A method for manufacturing wire, comprising: The billet is heated to 950°C to 1200°C, the billet containing, by weight percentage (wt%), 0.20% to 0.35% carbon (C), 0.05% to 0.50% silicon (Si), 0.30% to 0.60% manganese (Mn), 1.00% to 1.60% chromium (Cr), 0.02% to 0.10% aluminum (Al), 0.02% to 0.05% titanium (Ti) and 0.0005% to 0.0100% boron (B), the remainder being iron (Fe) and unavoidable impurities, and satisfying the following formula (1); Hot-rolled at 850℃ to 950℃; Winding at 750°C to 850°C; and Cool to room temperature at a cooling rate of 0.1℃ / second to 0.5℃ / second. Formula (1): 5.90 ≤ 9.2x[C] + 1.2x(0.9x[Mn] + 2.1x[Cr]) ≤ 6.30 (Where, [C], [Mn] and [Cr] refer to the weight percentage of each element).

6. The method of claim 5, wherein the microstructure of the wire after hot rolling comprises 30% to 60% pearlite and 40% to 70% ferrite by area fraction.

7. The method of claim 5, wherein the average austenite grain size is controlled to 25 μm or less during the cooling process.

8. The method of claim 5, wherein the method is performed after the hot rolling without spheroidizing heat treatment.

9. A bolt comprising, by weight percentage (wt%), 0.20% to 0.35% carbon (C), 0.05% to 0.50% silicon (Si), 0.30% to 0.60% manganese (Mn), 1.00% to 1.60% chromium (Cr), 0.02% to 0.10% aluminum (Al), 0.02% to 0.05% titanium (Ti), and 0.0005% to 0.0100% boron (B), the remainder being iron (Fe) and unavoidable impurities, wherein the following formula (1) is satisfied, and The microstructure after quenching heat treatment and hot-dip galvanizing contains, by area fraction, 90% to 99% tempered martensite, 1% to 10% bainite, and 1% or less retained austenite. Formula (1): 5.90 ≤ 9.2x[C] + 1.2x(0.9x[Mn] + 2.1x[Cr]) ≤ 6.30 (in, [C], [Mn], and [Cr] refer to the weight percentage of each element.

10. The bolt according to claim 9, wherein the following formula (2) is satisfied. Equation (2): 0.99 ≤ 25x[Ti] + 124x[B] ≤ 1.36 (in, [Ti] and [B] refer to the weight percentage of each element.

11. The bolt according to claim 9, wherein the diameter of the bolt body portion is 15 mm to 40 mm, and the Vickers hardness is 320 HV or higher.

12. The bolt according to claim 9, wherein the low-temperature (-40°C) impact toughness is 50 J or higher.

13. A method for manufacturing bolts, comprising: Prepare wires according to any one of claims 1 to 4; The wire is drawn with a draw reduction ratio of 20% or less; Heating at 850°C to 950°C, followed by quenching at 20°C to 80°C; Tempering heat treatment at 450℃ to 550℃ for 3000 to 10000 seconds; as well as Hot-dip galvanizing is performed at a temperature of 500℃ to 550℃. The microstructure contains 90% to 99% tempered martensite, 1% to 10% bainite, and 1% or less retained austenite by area fraction.

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