Steel wire and coil spring

Abstract

A steel wire according to the present disclosure has a circular cross-section that is perpendicular to the longitudinal direction, and contains, in mass%, 0.40-0.60% of C, 1.40-3.00% of Si, 0.10-1.50% of Mn, 0.15-1.50% of Cr, 0.050% or less of Al, 0.015% or less of P, 0.015% or less of S, 0.010-0.100% of Ti, 0.0010-0.0060% of B, 0.0070% or less of N, and 0.0030% or less of O, while satisfying formula (1) and formula (2). In addition, the martensite area ratio is 95% or more, the ratio of the edge dislocation in the dislocation density is 30% or less, and the tensile strength is 1,950 MPa or more.　(1): Ti-3.5×N≥0, (2): 0.15×Si-12×B>0.20. In formula (1) and formula (2), the content in mass% of the corresponding element is assigned to each element symbol.

Description

Steel wire and coil springs

[0001] The present invention relates to a steel wire and a coil spring, and more particularly to a steel wire suitable as a material for a suspension spring, and a coil spring manufactured using the steel wire as a material.

[0002] Coil springs, such as suspension springs for automobiles, are manufactured using steel wire as a raw material. Specifically, coil springs are manufactured by cold coiling the steel wire and then performing stress relief annealing. Here, the steel wire is a linear steel material obtained by wire drawing using a die, as defined in ISO 6929:2013.

[0003] Coil springs are often used in outdoor environments. Therefore, fatigue cracks may occur due to corrosion. To suppress fatigue cracks caused by corrosion, it has been proposed to adjust the chemical composition of the coil spring. However, it is difficult to suppress fatigue cracks by adjusting the chemical composition of alloying elements, etc. In order to suppress the occurrence of fatigue cracks, it is effective to achieve both high strength and high toughness.

[0004] Japanese Patent Laid-Open Publication No. 2014-5532 (Patent Document 1) proposes a steel wire for suspension springs with improved mechanical properties. The steel wire disclosed in this document contains, by mass%, 0.40 to 0.65% C, 1.0 to 3.0% Si, 0.6 to 2.0% Mn, 0.015% or less P, 0.015% S, and 0.001 to 0.10% Al, with the balance being Fe and impurities. The steel wire has an area fraction of tempered martensite of 70% or more and a prior austenite grain size number of 10.0 or more. This document states that high strength can be obtained by adjusting the microstructure as described above.

[0005] JP 2014-5532 A

[0006] Tomoda et al., "Recent Progress in Neutron and X-ray Diffraction Line Profile Analysis," Iron and Steel, Vol. 103 (2017) No. 2, pp. 73-85 Takagi et al., "Evaluation of Dislocation Density in Cold-Worked Low-Carbon Ferritic Steel," Iron and Steel, Vol. 104 (2018) No. 11, pp. 683-688

[0007] However, in the steel wire disclosed in Patent Document 1, specific means for achieving both high strength and high toughness have not been fully considered.

[0008] An object of the present disclosure is to provide a steel wire and a coil spring that can achieve both high strength and high toughness.

[0009] The steel wire of the present disclosure is a steel wire having a circular cross section perpendicular to the longitudinal direction, and contains, in mass %, C: 0.40 to 0.60%, Si: 1.40 to 3.00%, Mn: 0.10 to 1.50%, Cr: 0.15 to 1.50%, Al: 0.050% or less, P: 0.015% or less, S: 0.015% or less, Ti: 0.010 to 0.100%, B: 0.0010 to 0.0060%, N: 0.0070% or less, O: 0.0030% or less, Mo: 0 to 1.00%, Ni: 0. The steel wire contains 0.050% or less of Cu, 0.050% or less of Cu, 0.050% or less of Nb, 0.050% or less of V, 0.050% or less of Sn, 0.050% or less of Ca, 0.0050% or less of Mg, 0.0050% or less of Sb, 0.050% or less of As, 0.050% or less of Zr, 0.050% or less of Bi, 0.050% or less of Se, 0.100% or less of Te, and 0.09% or less of Pb, with the balance consisting of Fe and impurities, and satisfies formulas (1) and (2). The microstructure at a depth of R / 2 in the radial direction from the surface of the steel wire, where R is the radius of the cross section, has a martensite area fraction of 95% or more. In the steel wire, the ratio of edge dislocations in the dislocation density is 30% or less, and the tensile strength is 1950 MPa or more. Ti - 3.5 × N ≧ 0 (1) 0.15 × Si - 12 × B > 0.20 (2) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) and (2).

[0010] The coil spring of the present disclosure includes a spirally wound steel wire containing, by mass%, 0.40 to 0.60% C, 1.40 to 3.00% Si, 0.10 to 1.50% Mn, 0.15 to 1.50% Cr, 0.050% or less Al, 0.015% or less P, 0.015% or less S, 0.010 to 0.100% Ti, 0.0010 to 0.0060% B, 0.0070% or less N, 0.0030% or less O, 0 to 1.00% Mo, 0 to 1.00% Ni, and 0 to 1.00% Cu. The steel wire contains 0.00%, Nb: 0-0.100%, V: 0-0.50%, Sn: 0-0.100%, Ca: 0-0.0050%, Mg: 0-0.0050%, Sb: 0-0.050%, As: 0-0.050%, Zr: 0-0.050%, Bi: 0-0.050%, Se: 0-0.100%, Te: 0-0.050%, and Pb: 0-0.09%, with the balance consisting of Fe and impurities, and satisfies formulas (1) and (2). The steel wire has a circular cross section perpendicular to the longitudinal direction. The martensite area ratio is 95% or more in the microstructure at a depth of R / 2 from the surface of the steel wire in the radial direction, where R is the radius of the cross section. In the steel wire, the ratio of edge dislocations in the dislocation density is 30% or less, and the tensile strength is 1950 MPa or more. Ti - 3.5 × N ≧ 0 (1) 0.15 × Si - 12 × B > 0.20 (2) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) and (2).

[0011] The steel wire and coil spring of the present disclosure can achieve both high strength and high toughness.

[0012] The present inventors first investigated steel wires that could achieve both high strength and high toughness from the viewpoint of chemical composition, and as a result, they found that the chemical composition was, in mass %, C: 0.40 to 0.60%, Si: 1.40 to 3.00%, Mn: 0.10 to 1.50%, Cr: 0.15 to 1.50%, Al: 0.050% or less, P: 0.015% or less, S: 0.015% or less, Ti: 0.010 to 0.100%, B: 0.0010 to 0.0060%, N: 0.0070% or less, O: 0.0030% or less, Mo: 0 to 1.00%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, The inventors considered that a steel wire containing Nb: 0 to 0.100%, V: 0 to 0.50%, Sn: 0 to 0.100%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Sb: 0 to 0.050%, As: 0 to 0.050%, Zr: 0 to 0.050%, Bi: 0 to 0.050%, Se: 0 to 0.100%, Te: 0 to 0.050%, and Pb: 0 to 0.09%, with the balance being Fe and impurities, could potentially achieve both high strength and high toughness.

[0013] However, even when the chemical composition of the steel wire is within the above-mentioned range, it is still sometimes difficult to achieve both high strength and high toughness. Therefore, the present inventors conducted further studies and obtained the following findings.

[0014] Solute B segregates at the grain boundaries and strengthens them. This increases the strength of the steel wire and also increases its toughness. However, B bonds with N in the steel wire to form BN. If BN is formed, the amount of solute B in the steel wire decreases. Therefore, in order to suppress the decrease in the amount of solute B, the chemical composition of the steel wire is made to satisfy formula (1). Ti-3.5×N≧0 (1)

[0015] In this case, the steel wire contains enough Ti to bond with all of the N in the steel wire. The affinity between N and Ti is higher than the affinity between N and B. Therefore, the solute B can be prevented from bonding with N, and a sufficient amount of solute B can be secured. As a result, both the strength and toughness of the steel wire can be increased.

[0016] However, even if the above-mentioned chemical composition and formula (1) are satisfied, it is still sometimes difficult to achieve both high strength and high toughness. Therefore, the present inventors further investigated means for achieving both high strength and high toughness from the viewpoint of the microstructure.

[0017] Here, the inventors focused on dislocations. In steel wires and coil springs made from steel wires, dislocation structures affect strength and toughness. Specifically, among dislocations, screw dislocations exist across multiple crystal orientation planes. Therefore, when an external force is applied, screw dislocations tend to move in multiple crystal orientations and can activate multiple slip systems. On the other hand, edge dislocations exist only on one specific crystal orientation plane. Therefore, when an external force is applied, the slip systems that edge dislocations can activate are limited. Therefore, it is difficult for edge dislocations to disperse the energy applied by an external force, unlike screw dislocations.

[0018] Therefore, if the proportion of edge dislocations in a steel wire is higher than the proportion of screw dislocations, it is thought that even if high strength is obtained, toughness is likely to decrease. On the other hand, if the proportion of edge dislocations is low, it is thought that high strength can be maintained by screw dislocations, and high toughness can also be easily maintained.

[0019] Based on the above results, the inventors further investigated the dislocation structure in steel wire. As a result, the inventors found that high strength and high toughness can be achieved simultaneously if the chemical composition further satisfies formula (2) and the proportion of edge dislocations in the dislocation density is 30% or less. 0.15 × Si - 12 × B > 0.20 (2)

[0020] During the manufacturing process of steel wire, the higher the tempering temperature, the easier it is for edge dislocations to move and disappear. On the other hand, if the tempering temperature is high, the strength of the steel wire is more likely to decrease. Si increases tempering softening resistance. In other words, Si can suppress the decrease in strength even when the tempering temperature is high. Therefore, a higher Si content can reduce edge dislocations while maintaining high strength even when the tempering temperature is high. Furthermore, B suppresses dislocation movement. Therefore, if the B content is too high relative to the Si content, the reduction of edge dislocations is suppressed. By satisfying formula (2), the proportion of edge dislocations in the dislocation density can be reduced.

[0021] When the content of each element in the chemical composition is within the above-mentioned range, formula (1) and formula (2) are satisfied, and further, the rate of edge dislocations in the dislocation density is set to 30% or less, a steel wire and a coil spring that can achieve both high strength and high toughness can be obtained.

[0022] The steel wire and coil spring of this embodiment, which have been completed based on the above technical concept, have the following configuration.

[0023] The steel wire of this embodiment is a steel wire having a circular cross section perpendicular to the longitudinal direction, and contains, in mass %, C: 0.40 to 0.60%, Si: 1.40 to 3.00%, Mn: 0.10 to 1.50%, Cr: 0.15 to 1.50%, Al: 0.050% or less, P: 0.015% or less, S: 0.015% or less, Ti: 0.010 to 0.100%, B: 0.0010 to 0.0060%, N: 0.0070% or less, O: 0.0030% or less, Mo: 0 to 1.00%, Ni: The steel wire contains 0 to 1.00%, Cu: 0 to 1.00%, Nb: 0 to 0.100%, V: 0 to 0.50%, Sn: 0 to 0.100%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Sb: 0 to 0.050%, As: 0 to 0.050%, Zr: 0 to 0.050%, Bi: 0 to 0.050%, Se: 0 to 0.100%, Te: 0 to 0.050%, and Pb: 0 to 0.09%, with the balance being Fe and impurities, and satisfies formulas (1) and (2). The microstructure at a depth of R / 2 in the radial direction from the surface of the steel wire, where R is the radius of the cross section, has a martensite area fraction of 95% or more. In the steel wire, the ratio of edge dislocations in the dislocation density is 30% or less, and the tensile strength is 1950 MPa or more. Ti - 3.5 × N ≧ 0 (1) 0.15 × Si - 12 × B > 0.20 (2) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) and (2).

[0024] The steel wire contains, in mass %, Mo: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Cu: 0.01 to 1.00%, Nb: 0.001 to 0.100%, V: 0.01 to 0.50%, Sn: 0.001 to 0.100%, Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, Sb : 0.001 to 0.050%, As: 0.001 to 0.050%, Zr: 0.001 to 0.050%, Bi: 0.001 to 0.050%, Se: 0.001 to 0.100%, Te: 0.001 to 0.050%, and Pb: 0.001 to 0.09%.

[0025] The coil spring of this embodiment includes a spirally wound steel wire containing, by mass%, 0.40 to 0.60% C, 1.40 to 3.00% Si, 0.10 to 1.50% Mn, 0.15 to 1.50% Cr, 0.050% or less Al, 0.015% or less P, 0.015% or less S, 0.010 to 0.100% Ti, 0.0010 to 0.0060% B, 0.0070% or less N, 0.0030% or less O, 0 to 1.00% Mo, 0 to 1.00% Ni, and 0 to 1.00% Cu. The steel wire contains 0.00%, Nb: 0-0.100%, V: 0-0.50%, Sn: 0-0.100%, Ca: 0-0.0050%, Mg: 0-0.0050%, Sb: 0-0.050%, As: 0-0.050%, Zr: 0-0.050%, Bi: 0-0.050%, Se: 0-0.100%, Te: 0-0.050%, and Pb: 0-0.09%, with the balance consisting of Fe and impurities, and satisfies formulas (1) and (2). The steel wire has a circular cross section perpendicular to the longitudinal direction. The martensite area ratio is 95% or more in the microstructure at a depth of R / 2 from the surface of the steel wire in the radial direction, where R is the radius of the cross section. In the steel wire, the ratio of edge dislocations in the dislocation density is 30% or less, and the tensile strength is 1950 MPa or more. Ti - 3.5 × N ≧ 0 (1) 0.15 × Si - 12 × B > 0.20 (2) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) and (2).

[0026] In the coil spring, the steel wire contains, in mass %, Mo: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Cu: 0.01 to 1.00%, Nb: 0.001 to 0.100%, V: 0.01 to 0.50%, Sn: 0.001 to 0.100%, Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.005 0%, Sb: 0.001 to 0.050%, As: 0.001 to 0.050%, Zr: 0.001 to 0.050%, Bi: 0.001 to 0.050%, Se: 0.001 to 0.100%, Te: 0.001 to 0.050%, and Pb: 0.001 to 0.09%.

[0027] The steel wire and the coil spring of this embodiment will be described in detail below.

[0028] [Features of the Steel Wire of the Present Embodiment] The steel wire of the present embodiment has a circular cross section perpendicular to the longitudinal direction. The steel wire of the present embodiment satisfies the following features. Hereinafter, unless otherwise specified, % for elements means % by mass. (Feature 1) The chemical composition is, in mass%, C: 0.40 to 0.60%, Si: 1.40 to 3.00%, Mn: 0.10 to 1.50%, Cr: 0.15 to 1.50%, Al: 0.050% or less, P: 0.015% or less, S: 0.015% or less, Ti: 0.010 to 0.100%, B: 0.0010 to 0.0060%, N: 0.0070% or less, O: 0.0030% or less, Mo: 0 to 1.00%, Ni: 0 to 1.00% , Cu: 0-1.00%, Nb: 0-0.100%, V: 0-0.50%, Sn: 0-0.100%, Ca: 0-0.0050%, Mg: 0-0.0050%, Sb: 0-0.050%, As: 0-0.050%, Zr: 0-0.050%, Bi: 0-0.050%, Se: 0-0.100%, Te: 0-0.050%, and Pb: 0-0.09%, with the balance consisting of Fe and impurities. (Feature 2) The above chemical composition further satisfies formula (1) and formula (2). Ti - 3.5 x N ≧ 0 (1) 0.15 x Si - 12 x B > 0.20 (2) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1) and formula (2). (Feature 3) In the microstructure at a depth of R / 2 in the radial direction from the surface of the steel wire, where R is the radius of the cross section, the martensite area fraction is 95% or more. (Feature 4) The proportion of edge dislocations in the dislocation density is 30% or less. (Feature 5) The tensile strength is 1950 MPa or more. Features 1 to 5 will be explained below.

[0029] [(Feature 1) Chemical Composition] The chemical composition of the steel wire of this embodiment contains the following elements in the following amounts.

[0030] C: 0.40 to 0.60% Carbon (C) increases the strength of the steel wire and the strength of the coil spring manufactured using the steel wire as a raw material. If the C content is less than 0.40%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content exceeds 0.60%, the proportion of edge dislocations in the dislocation density becomes excessively high. In this case, the toughness of the steel wire and coil spring decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the C content is 0.40 to 0.60%. The preferred lower limit of the C content is 0.41%, more preferably 0.42%, and even more preferably 0.43%. The preferred upper limit of the C content is 0.58%, more preferably 0.57%, and even more preferably 0.56%.

[0031] Si: 1.40 to 3.00% Silicon (Si) increases temper softening resistance and reduces the proportion of edge dislocations in the dislocation density. If the Si content is less than 1.40%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content exceeds 3.00%, decarburization is promoted during hot rolling or heat treatment in the steel wire manufacturing process. In this case, the strength of the steel wire and coil spring decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Si content is 1.40 to 3.00%. A preferred lower limit of the Si content is 1.45%, more preferably 1.50%, and even more preferably 1.55%. A preferred upper limit of the Si content is 2.80%, more preferably 2.70%, even more preferably 2.60%, and even more preferably 2.50%.

[0032] Mn: 0.10 to 1.50% Manganese (Mn) increases the strength of steel wires and coil springs. If the Mn content is less than 0.10%, the above effects cannot be sufficiently obtained, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content exceeds 1.50%, center segregation is promoted during casting in the steel wire manufacturing process. Therefore, even if the contents of other elements are within the ranges of this embodiment, the toughness of the steel wires and coil springs decreases. Therefore, the Mn content is 0.10 to 1.50%. A preferred lower limit of the Mn content is 0.15%, more preferably 0.20%, and even more preferably 0.25%. A preferred upper limit of the Mn content is 1.48%, more preferably 1.45%, even more preferably 1.30%, even more preferably 1.20%, even more preferably 1.10%, and even more preferably 1.00%.

[0033] Cr: 0.15 to 1.50% Chromium (Cr) refines cementite and increases the strength of steel wires and coil springs. If the Cr content is less than 0.15%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Cr content exceeds 1.50%, center segregation is promoted during casting in the steel wire manufacturing process. Therefore, even if the contents of other elements are within the ranges of this embodiment, the toughness of the steel wires and coil springs decreases. Therefore, the Cr content is 0.15 to 1.50%. A preferred lower limit of the Cr content is 0.20%, more preferably 0.30%, and even more preferably 0.40%. A preferred upper limit of the Cr content is 1.40%, more preferably 1.35%, even more preferably 1.20%, and even more preferably 1.10%.

[0034] Al: 0.050% or less Aluminum (Al) deoxidizes steel during the steelmaking process in the steel wire manufacturing process. Furthermore, Al combines with N to form AlN, reducing the amount of solute N. This inhibits B, described below, from combining with N, enhancing the grain boundary strengthening effect of solute B. Even if even a small amount of Al is contained, the above effect can be achieved to some extent. However, if the Al content exceeds 0.050%, coarse oxides are formed. In this case, the toughness of the steel wire and coil spring decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Al content is 0.050% or less. The preferred lower limit of the Al content is greater than 0%, more preferably 0.001%, even more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the Al content is 0.040%, even more preferably 0.030%, even more preferably 0.025%, and even more preferably 0.020%. The above-mentioned Al content is the total Al content (mass %).

[0035] P: 0.015% or less Phosphorus (P) is an unavoidable impurity. In other words, the P content is greater than 0%. If the P content exceeds 0.015%, P segregates excessively in the prior austenite grain size, embrittling the grain boundaries. Therefore, even if the contents of other elements are within the ranges of this embodiment, the toughness of the steel wire and coil spring decreases. Therefore, the P content is 0.015% or less. The P content is preferably as low as possible. However, excessive reduction of the P content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the P content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the P content is 0.014%, more preferably 0.012%, and even more preferably 0.010%.

[0036] S: 0.015% or less Sulfur (S) is an unavoidable impurity. In other words, the S content is greater than 0%. If the S content exceeds 0.015%, excessive sulfides are generated. Sulfides act as starting points for corrosion. Therefore, even if the contents of other elements are within the ranges of this embodiment, the corrosion resistance of the steel wire or coil spring decreases. Therefore, the S content is 0.015% or less. The S content is preferably as low as possible. However, excessive reduction of the S content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the S content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the S content is 0.014%, more preferably 0.013%, and even more preferably 0.012%.

[0037] Ti: 0.010 to 0.100% Titanium (Ti) combines with N to form TiN, reducing the amount of solute N. Reducing the amount of solute N suppresses the formation of BN. As a result, the grain boundary strengthening effect of solute B is enhanced. If the Ti content is less than 0.010%, the above effect cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ti content exceeds 0.100%, coarse TiN is formed. In this case, the toughness of the steel wire and coil spring decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ti content is 0.010 to 0.100%. The preferred lower limit of the Ti content is 0.015%, more preferably 0.020%, and even more preferably 0.025%. The preferred upper limit of the Ti content is 0.090%, more preferably 0.085%, even more preferably 0.080%, and even more preferably 0.070%.

[0038] B: 0.0010 to 0.0060% Boron (B) improves hardenability and makes it easier to form martensite. This increases the strength of the steel wire and coil spring. B also strengthens grain boundaries through solid solution, increasing the toughness of the steel wire and coil spring. If the B content is less than 0.0010%, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the B content exceeds 0.0060%, the above effects saturate. Therefore, the B content is 0.0010 to 0.0060%. A preferred lower limit of the B content is 0.0012%, more preferably 0.0015%, and even more preferably 0.0018%. A preferred upper limit of the B content is 0.0055%, more preferably 0.0050%, and even more preferably 0.0045%.

[0039] N: 0.0070% or less N is an unavoidable impurity. That is, the N content is greater than 0%. If the N content exceeds 0.0070%, even if the contents of other elements are within the ranges of this embodiment, N combines with B to form excessive BN, significantly reducing the grain boundary strengthening effect of solute B. Therefore, the N content is 0.0070% or less. The N content is preferably as low as possible. However, excessive reduction of the N content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the N content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the N content is 0.0065%, more preferably 0.0060%, and even more preferably 0.0050%.

[0040] O: 0.0030% or less O is an unavoidably contained impurity. In other words, the O content is greater than 0%. If the O content exceeds 0.0030%, excessive coarse oxides are generated. In this case, even if the contents of other elements are within the ranges of this embodiment, the toughness of the steel wire and coil spring decreases. Therefore, the O content is 0.0030% or less. The O content is preferably as low as possible. However, excessive reduction of the O content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the O content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the O content is 0.0025%, more preferably 0.0020%, and even more preferably 0.0015%.

[0041] The balance of the chemical composition of the steel wire of this embodiment contains Fe and impurities. Preferably, the balance of the chemical composition of the steel wire consists of Fe and impurities. Here, the impurities refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, when industrially manufacturing a steel wire, and are acceptable within a range that does not adversely affect the steel wire of this embodiment.

[0042] [Optional Elements] The chemical composition of the steel wire of this embodiment may further contain one or more elements selected from the group consisting of Mo: 0-1.00%, Ni: 0-1.00%, Cu: 0-1.00%, Nb: 0-0.100%, V: 0-0.50%, Sn: 0-0.100%, Ca: 0-0.0050%, Mg: 0-0.0050%, Sb: 0-0.050%, As: 0-0.050%, Zr: 0-0.050%, Bi: 0-0.050%, Se: 0-0.100%, Te: 0-0.050%, and Pb: 0-0.09%. All of these elements are optional and may not be included. Each element will be described below.

[0043] [First Group (Mo, Ni, Cu, Nb, V, and Sn)] The chemical composition of the steel wire of the present embodiment may contain one or more elements selected from the group consisting of Mo, Ni, Cu, Nb, V, and Sn, instead of a portion of Fe. All of these elements increase the strength of the steel wire and the coil spring.

[0044] Mo: 0 to 1.00% Molybdenum (Mo) is an optional element and does not necessarily need to be contained. That is, the Mo content may be 0%. When contained, that is, when the Mo content exceeds 0%, Mo improves the hardenability of the steel material and increases the strength of the steel wire and coil spring. Even if even a small amount of Mo is contained, the above effects can be obtained to some extent. However, if the Mo content exceeds 1.00%, the strength becomes excessively high. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability decreases. Therefore, the Mo content is 0 to 1.00%. The preferred lower limit of the Mo content is 0.01%, more preferably 0.03%, even more preferably 0.05%, even more preferably 0.07%, and even more preferably 0.10%. The preferred upper limit of the Mo content is 0.90%, even more preferably 0.80%, even more preferably 0.70%, and even more preferably 0.60%.

[0045] Ni: 0 to 1.00% Nickel (Ni) is an optional element and does not necessarily need to be contained. In other words, the Ni content may be 0%. When contained, that is, when the Ni content exceeds 0%, Ni improves the hardenability of the steel material and increases the strength of the steel wire and coil spring. Even if even a small amount of Ni is contained, the above effects can be obtained to some extent. However, if the Ni content exceeds 1.00%, the strength becomes excessively high. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability decreases. Therefore, the Ni content is 0 to 1.00%. The preferred lower limit of the Ni content is 0.01%, more preferably 0.03%, even more preferably 0.05%, even more preferably 0.07%, and even more preferably 0.10%. The preferred upper limit of the Ni content is 0.90%, even more preferably 0.80%, even more preferably 0.70%, and even more preferably 0.60%.

[0046] Cu: 0 to 1.00% Copper (Cu) is an optional element and does not necessarily need to be contained. That is, the Cu content may be 0%. When contained, that is, when the Cu content exceeds 0%, Cu improves the hardenability of the steel material and increases the strength of the steel wire and coil spring. Even if even a small amount of Cu is contained, the above effects can be obtained to some extent. However, if the Cu content exceeds 1.00%, the strength becomes excessively high. Therefore, even if the contents of other elements are within the ranges of this embodiment, the cold workability decreases. Therefore, the Cu content is 0 to 1.00%. The preferred lower limit of the Cu content is 0.01%, more preferably 0.03%, even more preferably 0.05%, even more preferably 0.07%, and even more preferably 0.10%. The preferred upper limit of the Cu content is 0.90%, even more preferably 0.80%, even more preferably 0.70%, and even more preferably 0.60%.

[0047] Nb: 0 to 0.100% Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When Nb is contained, that is, when the Nb content exceeds 0%, Nb forms precipitates and refines crystal grains. As a result, the strength and toughness of the steel wire and coil spring are increased. Even if even a small amount of Nb is contained, the above effects can be obtained to some extent. However, if the Nb content exceeds 0.100%, the strength increases excessively. In this case, even if the contents of other elements are within the ranges of this embodiment, the cold workability decreases. Therefore, the Nb content is 0 to 0.100%. The preferred lower limit of the Nb content is 0.001%, more preferably 0.003%, even more preferably 0.010%, and even more preferably 0.020%. The preferred upper limit of the Nb content is 0.090%, even more preferably 0.080%, and even more preferably 0.070%.

[0048] V: 0 to 0.50% Vanadium (V) is an optional element and may not be contained. That is, the V content may be 0%. When V is contained, that is, when the V content exceeds 0%, V forms precipitates and refines crystal grains. As a result, the strength and toughness of the steel wire and coil spring are increased. Even if even a small amount of V is contained, the above effect can be obtained to some extent. However, if the V content exceeds 0.50%, the strength increases excessively. In this case, even if the contents of other elements are within the ranges of this embodiment, the cold workability decreases. Therefore, the V content is 0 to 0.50%. The preferred lower limit of the V content is 0.01%, more preferably 0.03%, even more preferably 0.05%, and even more preferably 0.08%. The preferred upper limit of the V content is 0.45%, even more preferably 0.40%, and even more preferably 0.30%.

[0049] Sn: 0 to 0.100% Tin (Sn) is an optional element and does not necessarily need to be contained. That is, the Sn content may be 0%. When Sn is contained, that is, when the Sn content exceeds 0%, Sn suppresses coarsening of crystal grains and increases the strength of the steel wire or coil spring. Even if even a small amount of Sn is contained, the above effect can be obtained to some extent. However, if the Sn content exceeds 0.100%, the steel material becomes excessively embrittled. Therefore, even if the contents of other elements are within the ranges of this embodiment, the hot workability and cold workability deteriorate. Therefore, the Sn content is 0 to 0.100%. The preferred lower limit of the Sn content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The upper limit of the Sn content is preferably 0.090%, more preferably 0.080%, even more preferably 0.070%, even more preferably 0.060%, and still more preferably 0.050%.

[0050] [Second Group (Ca and Mg)] The chemical composition of the steel wire of the present embodiment may contain one or more elements selected from the group consisting of Ca and Mg, instead of a portion of Fe. Any of these elements improves the hot workability and cold workability of the steel material that is the raw material for the steel wire during the manufacturing process of the steel wire.

[0051] Ca: 0 to 0.0050% Calcium (Ca) is an optional element and does not necessarily need to be contained. That is, the Ca content may be 0%. When Ca is contained, that is, when the Ca content exceeds 0%, Ca refines MnS in the steel material and reduces the aspect ratio of the MnS. This improves the hot workability and cold workability of the steel material. Even if even a small amount of Ca is contained, the above effects can be achieved to some extent. However, if the Ca content exceeds 0.0050%, coarse oxides are formed. In this case, even if the contents of other elements are within the ranges of this embodiment, the cold workability will be reduced. Therefore, the Ca content is 0 to 0.0050%. The preferred lower limit of the Ca content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The preferred upper limit of the Ca content is 0.0040%, and even more preferably 0.0030%.

[0052] Mg: 0 to 0.0050% Magnesium (Mg) is an optional element and does not necessarily need to be contained. That is, the Mg content may be 0%. When Mg is contained, that is, when the Mg content exceeds 0%, Mg refines MnS in the steel material and reduces the aspect ratio of the MnS. This improves the hot workability and cold workability of the steel material. Even if even a small amount of Mg is contained, the above effects can be achieved to some extent. However, if the Mg content exceeds 0.0050%, coarse oxides are formed. In this case, even if the contents of other elements are within the ranges of this embodiment, the cold workability will be reduced. Therefore, the Mg content is 0 to 0.0050%. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The preferred upper limit of the Mg content is 0.0040%, and even more preferably 0.0030%.

[0053] [Third Group (Sb and As)] The chemical composition of the steel wire of the present embodiment may contain one or more elements selected from the group consisting of Sb and As, instead of a portion of Fe. Any of these elements suppresses decarburization of the steel material that is the raw material for the steel wire during the manufacturing process of the steel wire.

[0054] Sb: 0 to 0.050% Antimony (Sb) is an optional element and may not be contained. In other words, the Sb content may be 0%. When Sb is contained, that is, when the Sb content exceeds 0%, Sb suppresses decarburization of the steel material, which is the raw material for the steel wire, during the steel wire manufacturing process. Even if even a small amount of Sb is contained, the above effect can be obtained to some extent. However, if the Sb content exceeds 0.050%, Sn segregates excessively. In this case, even if the contents of other elements are within the ranges of this embodiment, the hot workability and cold workability of the steel material will be reduced. Therefore, the Sb content is 0 to 0.050%. The preferred lower limit of the Sb content is 0.001%, more preferably 0.010%, and even more preferably 0.020%. The upper limit of the Sb content is preferably 0.045%, more preferably 0.040%, even more preferably 0.035%, even more preferably 0.030%, and still more preferably 0.025%.

[0055] As: 0 to 0.050% Arsenic (As) is an optional element and may not be contained. In other words, the As content may be 0%. When As is contained, that is, when the As content exceeds 0%, As suppresses decarburization of the steel material, which is the raw material for the steel wire, during the manufacturing process of the steel wire. Even if even a small amount of As is contained, the above effect can be obtained to some extent. However, if the As content exceeds 0.050%, As segregates excessively. In this case, even if the contents of other elements are within the ranges of this embodiment, the hot workability and cold workability of the steel material will be reduced. Therefore, the As content is 0 to 0.050%. The preferred lower limit of the As content is 0.001%, more preferably 0.010%, and even more preferably 0.020%. The upper limit of the As content is preferably 0.045%, more preferably 0.040%, even more preferably 0.035%, even more preferably 0.030%, and still more preferably 0.025%.

[0056] [Fourth Group (Zr, Bi, Se, Te, and Pb)] The chemical composition of the steel wire of the present embodiment may further contain one or more elements selected from the group consisting of Zr, Bi, Se, Te, and Pb in place of a portion of Fe. Any of these elements improves the cold workability of the steel material that is the raw material for the steel wire during the manufacturing process of the steel wire.

[0057] Zr: 0 to 0.050% Zirconium (Zr) is an optional element and does not necessarily need to be contained. That is, the Zr content may be 0%. When Zr is contained, that is, when the Zr content exceeds 0%, Zr dissolves in sulfides, modifying the morphology of the sulfides and improving the cold workability of the steel. Even if even a small amount of Zr is contained, the above effect can be achieved to some extent. However, if the Zr content exceeds 0.050%, coarse oxides are formed. In this case, even if the contents of other elements are within the ranges of this embodiment, the hot workability and cold workability of the steel will be reduced. Therefore, the Zr content is 0 to 0.050%. The preferred lower limit of the Zr content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the Zr content is 0.040%, more preferably 0.030%, and even more preferably 0.020%.

[0058] Bi: 0 to 0.050% Bismuth (Bi) is an optional element and does not necessarily need to be contained. In other words, the Bi content may be 0%. When Bi is contained, that is, when the Bi content exceeds 0%, Bi improves the cold workability of the steel material. Even if even a small amount of Bi is contained, the above effect can be obtained to some extent. However, if the Bi content exceeds 0.050%, the hot workability of the steel material will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the Bi content is 0 to 0.050%. The preferred lower limit of the Bi content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the Bi content is 0.040%, more preferably 0.030%, and even more preferably 0.020%.

[0059] Se: 0 to 0.100% Selenium (Se) is an optional element and does not necessarily need to be contained. That is, the Se content may be 0%. When Se is contained, that is, when the Se content exceeds 0%, Se improves the cold workability of the steel material. Even if even a small amount of Se is contained, the above effect can be obtained to some extent. However, if the Se content exceeds 0.100%, the hot workability of the steel material will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the Se content is 0 to 0.100%. A preferred lower limit of the Se content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. A preferred upper limit of the Se content is 0.090%, more preferably 0.070%, and even more preferably 0.050%.

[0060] Te: 0 to 0.050% Tellurium (Te) is an optional element and does not necessarily need to be contained. That is, the Te content may be 0%. When Te is contained, that is, when the Te content exceeds 0%, Te improves the cold workability of the steel material. Even if even a small amount of Te is contained, the above effect can be obtained to some extent. However, if the Te content exceeds 0.050%, the hot workability of the steel material decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Te content is 0 to 0.050%. A preferred lower limit of the Te content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. A preferred upper limit of the Te content is 0.040%, more preferably 0.030%, and even more preferably 0.020%.

[0061] Pb: 0 to 0.09% Lead (Pb) is an optional element and does not necessarily need to be contained. That is, the Pb content may be 0%. When Pb is contained, that is, when the Pb content exceeds 0%, Pb improves the cold workability of the steel material. Even if even a small amount of Pb is contained, the above effect can be obtained to some extent. However, if the Pb content exceeds 0.09%, the hot workability of the steel material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Pb content is 0 to 0.09%. The preferred lower limit of the Pb content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Pb content is 0.08%, more preferably 0.07%, and even more preferably 0.06%.

[0062] [(Feature 2) Regarding Formulas (1) and (2)] The chemical composition of the steel wire of this embodiment further satisfies Formulas (1) and (2): Ti - 3.5 x N ≥ 0 (1) 0.15 x Si - 12 x B > 0.20 (2) Here, the content of the corresponding element in mass % is substituted for each element symbol in Formulas (1) and (2).

[0063] [Regarding Formula (1)] F1 is defined as follows: F1 = Ti - 3.5 × N F1 corresponds to the left side of Formula (1). As described above, Ti combines with N to form TiN. This allows solute N to combine with solute B to form BN, thereby preventing a reduction in solute B. This enhances the grain boundary strengthening effect of solute B. As a result, the toughness of steel wires and coil springs is improved. Considering the atomic weights of Ti and N, in order to bond all of the solute N in the steel as TiN, the Ti content must be 3.5 times the N content, in mass %. When F1 is 0 or more, the Ti content is 3.5 times or more the N content. Therefore, the solute N can be sufficiently fixed as TiN by Ti. The preferred lower limit of F1 is 0.01, more preferably 0.02, and even more preferably 0.03.

[0064] [Regarding Equation (2)] F2 is defined as follows: F2 = 0.15 × Si - 12 × B F2 corresponds to the left side of Equation (2). F2 is an index for reducing the proportion of edge dislocations in the dislocation density. Dislocations (edge ​​dislocations and screw dislocations) multiply during quenching in the steel wire manufacturing process. On the other hand, dislocations become more likely to move during tempering in the manufacturing process. At this time, edge dislocations present in one crystal orientation plane are less likely to move than screw dislocations present across different crystal orientation planes. However, because edge dislocations exist in one crystal orientation plane, they are more likely to collide with other edge dislocations present in the same crystal orientation plane and be annihilated. On the other hand, screw dislocations exist across different crystal orientation planes as described above. Therefore, even during tempering, screw dislocations are more likely to move than edge dislocations, but are less likely to be annihilated.

[0065] The higher the tempering temperature, the more likely edge dislocations are to disappear. On the other hand, if the tempering temperature is high, the strength of the steel wire is more likely to decrease. Si increases tempering softening resistance. In other words, Si can suppress the decrease in strength even when the tempering temperature is high. Therefore, a higher Si content can reduce edge dislocations while maintaining high strength even when the tempering temperature is high. Furthermore, B suppresses dislocation movement. Therefore, if the B content is too high compared to the Si content, the reduction of edge dislocations is suppressed.

[0066] If F2 is more than 0.20, the temper softening resistance is sufficiently high and the restriction of dislocation movement due to the B content is also reduced. Therefore, the proportion of edge dislocations in the steel wire can be reduced. The lower limit of F2 is preferably 0.25, more preferably 0.27, even more preferably 0.29, and even more preferably 0.30. The upper limit of F2 is not particularly limited. However, in the case of a chemical composition that satisfies Feature 1, the upper limit of F2 is, for example, 0.44.

[0067] [(Feature 3) Regarding the Microstructure of the Steel Wire] In the microstructure of the steel wire of this embodiment at a depth of R / 2 in the radial direction from the surface, the martensite area ratio is 95% or more. Here, martensite also includes tempered martensite and bainite. The remainder other than martensite consists of one or more types selected from the group consisting of pro-eutectoid ferrite and pearlite. The preferred lower limit of the martensite area ratio is 97%, and more preferably 98%. The martensite area ratio may be 100%.

[0068] [Method for Measuring Martensite Area Ratio] The martensite area ratio of a steel wire is determined by the following method.

[0069] Three test pieces are taken from a cross section perpendicular to the axial direction of the steel wire, with the radius of the cross section being R, at a depth of R / 2 in the radial direction from the surface of the steel wire. The R / 2 depth position refers to the center position of the radius (R) in a circular cross section perpendicular to the axial direction (longitudinal direction) of the steel wire. The cross section perpendicular to the axial direction (longitudinal direction) of the steel wire is used as the observation surface. The observation surface of the test piece is mirror-polished. The mirror-polished observation surface is etched for 5 minutes at room temperature using an alcohol solution of 5 mass% trinitrophenol. Ten observation fields including the R / 2 depth position are selected from the etched observation surface. Secondary electron images are taken and observed for each observation field using a scanning electron microscope (SEM). The field area is 100 μm × 100 μm (magnification 1000x). SEM observation allows martensite, pro-eutectoid ferrite, and pearlite to be distinguished as follows: Regions with low brightness where no substructure is observed within the grains are determined to be pro-eutectoid ferrite. Regions with a mixture of low brightness regions (ferrite) and high brightness regions (cementite) are determined to be pearlite. A structure containing a lath-like substructure is determined to be martensite. Note that when the lath-like substructure contains high brightness particles (cementite), it is also determined to be martensite (tempered martensite or bainite).

[0070] The total area of ​​martensite in the observation field is determined. The ratio of the determined total area of ​​martensite to the total area of ​​the observation field is defined as the martensite area ratio (%). The arithmetic mean value of the martensite area ratios obtained for the three test specimens is defined as the martensite area ratio (%) of the steel wire.

[0071] [(Feature 4) Edge Dislocation Ratio] In the steel wire of this embodiment, the ratio of edge dislocations in the dislocation density is 30% or less. In steel wires and coil springs made from steel wires, dislocation structures affect strength and toughness. Specifically, among dislocations, screw dislocations exist across multiple crystal orientation planes. Therefore, when an external force is applied, screw dislocations tend to move to multiple crystal orientations and can activate multiple slip systems. On the other hand, edge dislocations exist only on one specific crystal orientation plane. Therefore, when an external force is applied, the slip systems that edge dislocations can activate are limited. Therefore, it is difficult for edge dislocations to disperse the energy applied by an external force, as is the case with screw dislocations.

[0072] Therefore, if the proportion of edge dislocations is higher than the proportion of screw dislocations in a steel wire, toughness is likely to decrease even if high strength is obtained. On the other hand, if the proportion of edge dislocations is low, high strength can be maintained by screw dislocations, and high toughness can also be easily maintained.

[0073] If Features 1 to 3 are satisfied and the ratio of edge dislocations to the dislocation density is 30% or less, the number of edge dislocations is sufficiently small relative to the number of screw dislocations, thereby achieving both high strength and high toughness.

[0074] The upper limit of the edge dislocation ratio is preferably 28%, more preferably 26%, even more preferably 24%, and even more preferably 22%. The lower limit of the edge dislocation ratio is not particularly limited. The lower limit of the edge dislocation ratio is, for example, 10%, for example, 14%.

[0075] [Method for Measuring Edge Dislocation Ratio] The edge dislocation ratio can be determined by the following method.

[0076] A test piece is taken from the steel wire, with the observation surface being a cross section parallel to the axial direction (longitudinal direction) of the steel wire. The center of the observation surface is located on the central axis of the steel wire. To remove processing strain from the observation surface, electrolytic polishing is performed using 10% by volume of perchloric acid (acetic acid solvent). An XRD line profile is obtained using an X-ray diffractometer for any measurement point on the central axis of the steel wire on the observation surface after electrolytic polishing. For XRD, a Cu-Kα source is used, the acceleration voltage is 45 kV, and the acceleration current is 200 mA. Measurements are also performed with a diffraction angle (2θ) ranging from 30 to 150°, at 0.02° intervals, and for 0.6 seconds per point. The diffraction peaks are separated into Kα1 and Kα2, and the integrated intensity of the Kα1 diffraction peak is used. Calibration of the peak position corresponding to the diffraction angle is performed by using a Si standard plate to confirm that the diffraction peak position of a specific surface is not misaligned with the reference position. Regarding the half-value width, LaB ６ (lanthanum hexaboride) is used as a standard sample, and the width of the device is measured in advance to perform correction.

[0077] From the obtained line profile, the half-widths ΔH of the diffraction peaks on the (111), (200), (211), (220), (310) and (222) diffraction planes were calculated. ｈｋｌ Here, the half width ΔK ｈｋｌ The half-width ΔH of the diffraction peak in the {hkl} diffraction plane is ｈｋｌ Using the above, the edge dislocation fraction S is calculated by the modified Williamson-Hall method as follows:

[0078] When discussing the elastic properties of the crystal plane {hkl}, the orientation parameter H defined by formula (I) ２ is used. ２ = (h ２ k ２ +k ２ l ２ +l ２ h ２ ) / (h ２ +k ２ +l ２ ) ２(I) The average contrast factor C used in the modified Williamson-Hall method is defined by the formula (II): C = 0.285(1-qH ２ ) (II)

[0079] The edge dislocation fraction F can be determined using q from formula (III): F = (qs - q) / (qs - qe) (III) In formula (III), qe is the value when all dislocations in pure iron are edge dislocations, and qe = 1.2. qs is the value when all dislocations in pure iron are screw dislocations, and qs = 2.8.

[0080] Here, q is calculated by the modified Williamson-Hall equation defined by equation (IV). ｈｋｌ = α + βK ｈｋｌ √(C) + γ(K ｈｋｌ ) ２ C (IV) Here, the diffraction angle in the line profile of the {hkl} diffraction plane obtained by XRD is θ ｈｋｌ (°), half width is ω ｈｋｌ (rad), the wavelength of the X-ray is λ, the crystallite size is D, and the amount of strain around the dislocation line is ε ｋ Then, ΔK in formula (IV) ｈｋｌ and K. ｈｋｌ is defined by formula (V) and formula (VI). ｈｋｌ = 2 sin(θ ｈｋｌ +ω ｈｋｌ / 2)-2sin(θ ｈｋｌ -ω ｈｋｌ / 2) / λ=0.9 / D+ε ｋ K ｈｋｌ (V) K ｈｋｌ = 2 sin θ ｈｋｌ / λ (VI)

[0081] In formula (IV), β and γ are material constants. Here, γ is a very small value compared to β and α. Therefore, the third term on the right side of formula (IV) is ignored and formula (VII) is used. ΔK ｈｋｌ = α + βK ｈｋｌ √(C) (VII)

[0082] Here, α in equation (VII) is moved to the left side and squared to derive equation (VIII).ｈｋｌ -α) ２ / (K ｈｋｌ ) ２ = β ２ 0.285 (1-qH ２ ) (VIII)

[0083] Formula (VIII) is the left-hand side value and H ２ When the value of α is optimal, the left side and H ２ The linear relationship between α and α is optimal. Therefore, an appropriate value is substituted for α to find the fitting index, which indicates the accuracy of the linear relationship, and the relationship between the value of α and the fitting index is organized to find the α that provides the best linear relationship. q is found using the obtained α and formula (VIII). The obtained q is substituted into formula (III) to find the edge dislocation fraction F. The derivation of q shown above is also described in Non-Patent Document 1 and Non-Patent Document 2.

[0084] [(Feature 5) Tensile Strength] The steel wire of this embodiment further has a tensile strength of 1950 MPa or more. If the steel wire satisfies Features 1 to 4 and further has a tensile strength of 1950 MPa or more, it is possible for the steel wire to have both high strength and high toughness.

[0085] The lower limit of the tensile strength is preferably 1960 MPa, more preferably 1970 MPa, and even more preferably 1980 MPa. The upper limit of the tensile strength is not particularly limited. The upper limit of the tensile strength is, for example, 2400 MPa, for example, 2300 MPa, or for example, 2250 MPa.

[0086] [Method for Measuring Tensile Strength] The tensile strength of a steel wire is determined by the following method. In accordance with JIS Z 2241:2022, a No. 14A tensile test specimen with a parallel portion diameter of 6 mm is taken from the steel wire of each test number. If the diameter of the circular cross section perpendicular to the longitudinal direction of the steel wire is smaller than 6 mm and a tensile test specimen cannot be taken from the steel specimen, the parallel portion diameter D is reduced to match the actual material dimensions, and a proportional test specimen is taken that is geometrically similar. Using the taken test specimen, a tensile test in accordance with JIS Z 2241:2022 is performed at room temperature in air to determine the tensile strength (MPa).

[0087] [Effects of the Steel Wire of the Present Embodiment] The steel wire of the present embodiment satisfies Features 1 to 5. Therefore, the steel wire of the present embodiment can achieve both high strength and high toughness.

[0088] [Uses of the Steel Wire of the Present Embodiment] The steel wire of the present embodiment is widely applicable to applications requiring high strength and high toughness. The steel wire of the present embodiment is particularly suitable as a material for coil springs, typified by suspension springs. As described above, the cross section perpendicular to the longitudinal direction of the steel wire is circular. Here, "circular" includes not only a perfect circle but also an approximately circular shape. An approximately circular shape refers to a cross section that is circular if the difference between the maximum and minimum values ​​of a line segment passing through the center of gravity of the cross section and having both end points tangent to the outer edge of the cross section (i.e., the diameter of the circle) is within 10.0% of the maximum value. Note that when the cross section is elliptical, the R / 2 depth position defined in Feature 3 is the center position of the radius of the major axis from the surface of the steel wire. Note that the diameter of the steel wire is not particularly limited. The lower limit of the diameter of the steel wire is, for example, 3 mm, e.g., 5 mm. The upper limit of the diameter of the steel wire is, for example, 30 mm, e.g., 25 mm.

[0089] [Example of a method for manufacturing a steel wire according to this embodiment] An example of a method for manufacturing a steel wire according to this embodiment will be described. The method for manufacturing a steel wire described below is one example for manufacturing the steel wire according to this embodiment. Therefore, a steel wire having the above-described configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing a steel wire according to this embodiment.

[0090] An example of a method for manufacturing a steel wire according to this embodiment includes the following steps: (Step 1) Material preparation step (Step 2) Blooming step (Step 3) Wire rod manufacturing step (Step 4) Wire drawing step (Step 5) Quenching and tempering step Steps 1 to 3 are used to manufacture wire rod, which is the raw material for steel wire. Steps 4 and 5 are used to manufacture steel wire using the wire rod as the raw material. Each step will be described below.

[0091] [(Step 1) Material Preparation Step] In the material preparation step, a material for the steel wire of this embodiment is prepared. Specifically, molten steel having a chemical composition that satisfies Feature 1 is produced. The refining method is not particularly limited, and a well-known method may be used. For example, molten pig iron produced by a well-known method is subjected to refining (primary refining) in a converter. The molten steel tapped from the converter is subjected to well-known secondary refining. Through the above steps, molten steel having a chemical composition that satisfies Feature 1 is produced.

[0092] The produced molten steel is used to produce a material by a well-known casting method. For example, an ingot may be produced by an ingot casting method using the molten steel. Alternatively, a bloom may be produced by a continuous casting method using the molten steel. By the above methods, a material (ingot or bloom) is produced.

[0093] [(Step 2) Blooming Step] In the blooming step, the material (ingot or bloom) prepared in the material preparation step is bloomed to produce a billet. In the blooming step, the material is first heated using a heating furnace by a well-known method. The heating temperature is not particularly limited. Any well-known temperature will suffice. The heating temperature is, for example, 1100 to 1300°C.

[0094] The heated material is rolled using a blooming mill, or a blooming mill and a continuous rolling mill, to produce a billet. Specifically, the heated material is reverse rolled using the blooming mill to produce a billet. If a well-known continuous rolling mill is located downstream of the blooming mill, the billet after blooming may be further subjected to tandem rolling using the continuous rolling mill to produce a smaller billet. The produced billet is allowed to cool (air-cooled) to room temperature before the finish rolling process.

[0095] [(Step 3) Wire Rod Manufacturing Step] In the wire rod manufacturing step, hot rolling is performed on the billet produced in the blooming step to produce wire rod. In the wire rod manufacturing step, the billet is first heated using a heating furnace. The heating temperature is not particularly limited and may be any known heating temperature. The heating temperature is, for example, 950 to 1250°C. The heated billet is then hot rolled using a continuous rolling mill to produce wire rod. The continuous rolling mill includes multiple rolling stands arranged in a row from upstream to downstream. Each rolling stand includes a pair of work rolls. Each work roll is formed with a caliber, and the calibers of the pair of work rolls form a groove. The finish rolling temperature in hot rolling is not particularly limited, but is, for example, 950 to 850°C.

[0096] The wire rod produced by hot rolling is cooled. The wire rod is coiled at a temperature of 750 to 900°C. The cooling rate after coiling is not particularly limited. For example, the average cooling rate from 700 to 600°C is set to 0.6 to 20.0°C / sec.

[0097] [(Step 4) Wiredrawing Step] In the wiredrawing step, the produced wire rod is subjected to wiredrawing. The wiredrawing may be only a primary wiredrawing, or multiple wiredrawing steps such as secondary wiredrawing may be performed. When multiple wiredrawing steps are performed, a well-known patenting treatment may be performed after each wiredrawing step and before the next wiredrawing step.

[0098] [(Step 5) Quenching and Tempering Step] In the quenching and tempering step, the steel wire after the wire drawing step is subjected to the following quenching and tempering steps. In this embodiment, the quenching and tempering are performed using a high-frequency heating device.

[0099] [Quenching process] In the quenching process, the steel wire after wire drawing is subjected to induction quenching. At this time, the quenching temperature is set to 850 to 1050°C, and the holding time at the quenching temperature is set to 5 to 200 seconds. After the holding time has elapsed, the steel wire is water-cooled.

[0100] [Tempering Process] Induction tempering is performed on the steel wire after quenching. In this embodiment, the first tempering process is performed, followed by the second tempering process. That is, in this process, two-stage tempering (first tempering process and second tempering process) is performed. Induction tempering is performed in both the first tempering process and the second tempering process. Induction tempering has an extremely fast heating rate up to the tempering temperature compared to tempering using a heat treatment furnace. In the first tempering process and the second tempering process, the heating rate up to the tempering temperature in induction tempering is, for example, 30°C / second or more. By performing the two-stage tempering process, the proportion of edge dislocations in the steel wire is reduced. The tempering process is performed using an induction heating device.

[0101] Specifically, the tempering step satisfies the following conditions: (Condition 1) In the first tempering step, the tempering temperature T1 is set to 250 to 300° C. (Condition 2) In the second tempering step, the tempering temperature T2 is set to 400 to 600° C. Each condition will be explained below.

[0102] [(Condition 1) Conditions for the First Tempering Step] In the first tempering step, tempering is performed in a lower temperature range than in the second tempering step. In the first tempering step, the tempering temperature T1 is set to 250 to 300°C. In this temperature range, cementite is not generated, but ε carbide, a precursor of cementite, is generated. Although ε carbide hinders screw dislocations, it is less likely to hinder edge dislocations. Therefore, in the first tempering step, edge dislocations are more mobile and disappear preferentially than screw dislocations. If the tempering temperature T1 exceeds 300°C, screw dislocations also move as easily as edge dislocations. Therefore, the proportion of edge dislocations in the steel wire after the second tempering step becomes excessive. Therefore, the tempering temperature T1 is set to 250 to 300°C. Note that if the tempering temperature T1 is less than 250°C, ε carbide is not generated in sufficient amounts. In this case, edge dislocations are also not sufficiently reduced. The holding time at the tempering temperature T1 is not particularly limited, and is, for example, 5 to 30 seconds.

[0103] [(Condition 2) Conditions for the Second Tempering Step] The second tempering step is carried out after the first tempering step. In the second tempering step, the tempering temperature is increased to the cementite formation temperature range to adjust the strength and toughness of the steel wire. If the second tempering temperature T2 is less than 400°C, an excessive number of ε carbides and dislocations remain in the crystal grains. In this case, the steel wire cannot obtain sufficient toughness. If the second tempering temperature T2 exceeds 600°C, the cementite coarsens and the dislocations are excessively reduced. In this case, the steel wire cannot obtain sufficient strength. Therefore, the tempering temperature T2 is set to 400 to 600°C. The holding time at the tempering temperature T2 is not particularly limited. The holding time at the tempering temperature T1 is, for example, 5 to 15 seconds.

[0104] The first tempering step and the second tempering step may be performed continuously or intermittently. For example, in the first tempering step, the steel wire may be induction tempered at a tempering temperature T1 of 250 to 300° C., and then the second tempering step may be performed continuously by increasing the tempering temperature from T1 to a tempering temperature T2 of 400 to 600° C. After the first tempering step, the temperature of the steel wire may be lowered to room temperature, and then the steel wire may be reheated and the second tempering step may be performed.

[0105] The steel wire of this embodiment is manufactured by the above manufacturing steps.

[0106] [Coil Spring of the Present Embodiment] The coil spring of the present embodiment includes a steel wire wound in a spiral shape. As described above, the cross section perpendicular to the axial direction (longitudinal direction) of the steel wire is circular. The coil spring is manufactured using the steel wire of the present embodiment described above as a material. Therefore, the steel wire constituting the coil spring satisfies Features 1 to 5 described above. As a result, the coil spring can achieve both high strength and high toughness.

[0107] [Method for Manufacturing Coil Spring] An example of a method for manufacturing a coil spring according to the present embodiment is as follows. The example of a method for manufacturing a coil spring according to the present embodiment includes the following steps: (Step 6) Cold coiling step (Step 7) Strain relief annealing step Each step will be described below.

[0108] [(Step 6) Cold Coiling Step] In the cold coiling step, the steel wire of this embodiment is cold coiled (cold coiling) to produce an intermediate steel material for a coil spring. Cold coiling is performed using a well-known coiling device. The coiling device, for example, includes multiple conveying roller sets, a wire guide, multiple coil forming jigs (coiling pins), and a core bar having a semicircular cross section. The conveying roller set includes a pair of rollers facing each other. The multiple conveying roller sets are arranged in a row. Each conveying roller set sandwiches the steel wire between a pair of rollers and conveys the steel wire in the direction of the wire guide. The steel wire passes through the wire guide. The steel wire emerging from the wire guide is bent into an arc shape by multiple coiling pins and core bars, and formed into a coil-shaped intermediate steel material.

[0109] [(Step 7) Stress relief annealing process] In the stress relief annealing process, annealing is performed to remove residual stress generated in the intermediate steel material by the cold coiling process. The treatment temperature (annealing temperature) in the annealing process is, for example, 300 to 500°C. The holding time at the annealing temperature is not particularly limited, but is, for example, 10 to 60 minutes. After the holding time has elapsed, the intermediate steel material is allowed to cool or slowly cooled to room temperature.

[0110] The intermediate steel material after the stress relief annealing process is subjected to shot peening. The intermediate steel material after shot peening is subjected to setting to adjust the shape of the coil spring. The coil spring can be manufactured through the above manufacturing processes.

[0111] The effects of the steel wire of this embodiment will be described more specifically with reference to examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the steel wire of this embodiment. Therefore, the steel wire of this embodiment is not limited to this one example of conditions.

[0112] Steel wires having the chemical compositions shown in Table 1 (Tables 1A to 1C) were produced by the following production method.

[0113]

[0114]

[0115]

[0116] Specifically, blooms were produced by continuous casting using molten steel having the chemical composition shown in Table 1. The blooms were subjected to a blooming process. In the blooming process, the blooms were heated at 1200°C for 3 hours, and then hot rolled using a blooming mill and a continuous rolling mill to produce billets.

[0117] The produced billet was used to carry out a wire rod production process. In the wire rod production process, the heating temperature was 1150 to 1200°C, and the holding time was 1.5 hours. A wire rod having a diameter of 13 mm was produced by hot rolling. The finish rolling temperature in the wire rod production process was 910°C. The wire rod was coiled at a temperature of 750 to 900°C. The average cooling rate from 700°C to 600°C was 0.6 to 20.0°C / sec.

[0118] The produced wire rod was subjected to a wire drawing process to produce a steel wire. In the wire drawing process, a steel wire with a diameter of 12 mm was produced. The steel wire after the wire drawing process was subjected to a quenching and tempering process using a high-frequency heating device. In the quenching process, the quenching temperature was 980°C, and the holding time at the quenching temperature was 10 seconds. After the holding time had elapsed, the steel wire was subjected to water quenching by fountain cooling. The steel wire after the quenching process was subjected to a first tempering process and a second tempering process. The first tempering temperature T1 (°C) in the first tempering process and the second tempering temperature T2 (°C) in the second tempering process were as shown in Table 2. The holding time at the first tempering temperature T1 was 10 seconds, and the holding time at the second tempering temperature T2 was 10 seconds. In both the first tempering process and the second tempering process, the heating rate up to the tempering temperature in induction tempering was 30°C / second. The first tempering step and the second tempering step were carried out consecutively.

[0119]

[0120] Steel wires with each test number were manufactured by the above manufacturing process.

[0121] [Evaluation Tests] The following tests were carried out on the manufactured steel wires with each test number: (Test 1) Martensite Area Fraction Measurement Test (Test 2) Edge Dislocation Ratio Measurement Test (Test 3) Strength Evaluation Test (Test 4) Toughness Evaluation Test Tests 1 to 4 will be described below.

[0122] [(Test 1) Martensite Area Ratio Measurement Test] The martensite area ratio (%) of the steel wire of each test number was determined based on the method described in the above [Method for measuring martensite area ratio]. The obtained martensite area ratios are shown in Table 2.

[0123] [(Test 2) Edge dislocation ratio measurement test] The edge dislocation ratio F (%) of the steel wire of each test number was determined based on the method described in the above-mentioned [Method for measuring edge dislocation ratio]. The obtained edge dislocation ratios are shown in Table 2.

[0124] [(Test 3) Strength Evaluation Test] In accordance with JIS Z 2241:2022, No. 14A tensile test pieces with a parallel portion diameter of 6 mm were taken from the steel wires of each test number. Using the taken test pieces, a tensile test in accordance with JIS Z 2241:2022 was carried out at room temperature in the air to determine the tensile strength (MPa). The obtained tensile strengths are shown in Table 2.

[0125] [(Test 4) Toughness Evaluation Test] In accordance with JIS Z 2242:2023, a 2 mm-U notch impact test specimen (subsize, width 5 mm) with a length of 55 mm was taken from the steel wire of each test number. Using the taken test specimen, a Charpy impact test was carried out in the air at room temperature in accordance with JIS Z 2242:2023, and the Charpy impact value (J / cm ２ The Charpy impact values ​​obtained are shown in Table 2.

[0126] The strength-toughness balance was evaluated by the following method: the tensile strength (MPa) obtained in Test 3 and the Charpy impact value (J / cm) obtained in Test 4. ２ ) was used to determine the strength-toughness balance B defined by the following formula: B = (tensile strength / 2000) × (Charpy impact value / 50) When the strength-toughness balance B was 1.0 or more, it was determined that both high strength and high toughness were achieved (an excellent strength-toughness balance was obtained).

[0127] [Evaluation Results] Referring to Tables 1 and 2, the steel wires of test numbers 1 to 32 satisfied characteristics 1 to 5. Therefore, the B value was 1.0 or more, and an excellent balance of strength and toughness was obtained.

[0128] On the other hand, in test number 33, the Ti content and B content were low. Therefore, both F1 and F2 were too low. As a result, the edge dislocation ratio was high, the B value was low, and a sufficient balance of strength and toughness was not obtained.

[0129] In test number 34, the C content was too high, resulting in a low B value and an insufficient balance of strength and toughness.

[0130] In test number 35, the C content was too low, which resulted in a too low B value and a sufficient balance of strength and toughness.

[0131] In test number 36, the Si content was too low, and F2 did not satisfy formula (2). Therefore, the edge dislocation ratio was too high. As a result, the B value was low, and a sufficient balance of strength and toughness was not obtained.

[0132] In test number 37, the B content was low, and therefore the B value was low, and a sufficient balance of strength and toughness was not obtained.

[0133] In test numbers 38 and 39, F1 was too low, so the B value was too low and a sufficient balance of strength and toughness was not obtained.

[0134] In test numbers 40 and 41, F2 was too low, and therefore the proportion of edge dislocations was too high. As a result, the B value was low and a sufficient balance of strength and toughness was not obtained.

[0135] In test numbers 42 and 43, the tempering temperature T1 was too high, which resulted in an excessively high proportion of edge dislocations. As a result, the B value was low and a sufficient balance of strength and toughness was not achieved.

[0136] In test numbers 44 and 45, the tempering temperature T1 was too low, which resulted in an excessively high rate of edge dislocations, resulting in a low B value and an insufficient balance of strength and toughness.

[0137] In test numbers 46 and 47, the tempering temperature T2 was too high, and therefore the tensile strength was too low.

[0138] In test numbers 48 and 49, the tempering temperature T2 was too low, which resulted in an excessively high proportion of edge dislocations. As a result, the B value was low and a sufficient balance of strength and toughness was not achieved.

[0139] In test numbers 50 and 51, the second tempering step was not performed (shown as "-" in the tempering temperature T2 column in Table 2). Therefore, the edge dislocation ratio was too high. As a result, the B value was low and a sufficient balance of strength and toughness was not obtained.

[0140] Coil springs were manufactured using the steel wires of test numbers 1 to 5 in Table 1 as raw materials. Specifically, the steel wires were subjected to cold coiling to manufacture intermediate steel materials. A stress relief annealing process was carried out on the intermediate steel materials. The annealing temperature was 300 to 500°C, and the holding time was 10 to 60 minutes. Shot peening and setting were carried out on the intermediate steel materials after the stress relief annealing process to manufacture coil springs. The manufactured coil springs were subjected to the above-mentioned tests 1 to 4. The results are shown in Table 3.

[0141]

[0142] Referring to Table 3, all of the coils of Test No. 1 to Test No. 5 satisfied Features 1 to 5. Therefore, the B value was 1.0 or more, and an excellent balance of strength and toughness was obtained.

[0143] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure.

Claims

1. A steel wire having a circular cross section perpendicular to the longitudinal direction, containing, in mass%, C: 0.40 to 0.60%, Si: 1.40 to 3.00%, Mn: 0.10 to 1.50%, Cr: 0.15 to 1.50%, Al: 0.050% or less, P: 0.015% or less, S: 0.015% or less, Ti: 0.010 to 0.100%, B: 0.0010 to 0.0060%, N: 0.0070% or less, O: 0.0030% or less, Mo: 0 to 1.00%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, Nb: 0 to 0.100%, V: 0 to 0.50%, A steel wire comprising: Sn: 0-0.100%, Ca: 0-0.0050%, Mg: 0-0.0050%, Sb: 0-0.050%, As: 0-0.050%, Zr: 0-0.050%, Bi: 0-0.050%, Se: 0-0.100%, Te: 0-0.050%, and Pb: 0-0.09%, with the balance being Fe and impurities; satisfying formulas (1) and (2); wherein, where R is the radius of the cross section, in a microstructure at a depth of R / 2 in the radial direction from the surface of the steel wire, a martensite area fraction is 95% or more; a proportion of edge dislocations in the dislocation density is 30% or less; and a tensile strength is 1950 MPa or more. Ti-3.5×N≧0 (1) 0.15×Si-12×B>0.20 (2) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) and (2).

2. A steel wire according to claim 1, comprising, in mass%, Mo: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Cu: 0.01 to 1.00%, Nb: 0.001 to 0.100%, V: 0.01 to 0.50%, Sn: 0.001 to 0.100%, Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, Sb: 0.001 to 0.050%, As: 0.001 to 0.050%, Zr: 0.001 to 0.050%, Bi: 0.001 to 0.050%, Se: 0.001 to 0.100%, A steel wire containing one or more elements selected from the group consisting of Te: 0.001 to 0.050%, and Pb: 0.001 to 0.09%.

3. A steel wire wound in a spiral shape, the steel wire having, in mass%, C: 0.40 to 0.60%, Si: 1.40 to 3.00%, Mn: 0.10 to 1.50%, Cr: 0.15 to 1.50%, Al: 0.050% or less, P: 0.015% or less, S: 0.015% or less, Ti: 0.010 to 0.100%, B: 0.0010 to 0.0060%, N: 0.0070% or less, O: 0.0030% or less, Mo: 0 to 1.00%, Ni: 0 to 1.00%, Cu: 0 to 1.00%, Nb: 0 to 0.100%, V: 0 to 0.50%, Sn: 0 to 0.100%, 1. A coil spring comprising: Ca: 0-0.0050%, Mg: 0-0.0050%, Sb: 0-0.050%, As: 0-0.050%, Zr: 0-0.050%, Bi: 0-0.050%, Se: 0-0.100%, Te: 0-0.050%, and Pb: 0-0.09%, with the balance being Fe and impurities; and satisfying formulas (1) and (2), the steel wire has a circular cross section perpendicular to the longitudinal direction, the cross section having a radius of R, and a martensite area ratio of 95% or more in a microstructure at a depth of R / 2 radially from the surface of the steel wire, the proportion of edge dislocations in the dislocation density being 30% or less, and the tensile strength being 1950 MPa or more. Ti-3.5×N≧0 (1) 0.15×Si-12×B>0.20 (2) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) and (2).

4. A coil spring according to claim 3, wherein the steel wire contains, in mass %, Mo: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Cu: 0.01 to 1.00%, Nb: 0.001 to 0.100%, V: 0.01 to 0.50%, Sn: 0.001 to 0.100%, Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, Sb: 0.001 to 0.050%, As: 0.001 to 0.050%, Zr: 0.001 to 0.050%, Bi: 0.001 to 0.050%, Se: 0.001 to 0.100%, A coil spring comprising one or more elements selected from the group consisting of Te: 0.001 to 0.050%, and Pb: 0.001 to 0.09%.

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