Steel wire

Abstract

To provide a steel wire having high tensile strength and high endurance ratio.SOLUTION: A steel wire according to the present invention has a chemical composition containing, by mass%, C: 0.70 to less than 1.10%, Si: 0.10 to 1.00%, Mn: 0.20 to 1.00%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, O: 0.0050% or less, and N: 0.0100% or less, with the balance being Fe and an impurity, and has a tensile strength of 1600MPa or more, and a ratio RS of an average microstrain in a surface layer of the steel wire to an average microstrain in a cross section parallel to an axial direction of the steel wire and including a center line is 1.05 or more.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to steel wire. [Background technology]

[0002] Steel wires are used in applications such as beads for automobile tires, wire ropes for bridges, wire ropes for elevators, PC steel wires, and hard steel wires for power transmission. High tensile strength is required for the steel wires used in these applications. However, in the environments in which steel wires are used, they are subjected to repeated changes in the stress state. Therefore, steel wires are required to have not only high tensile strength but also a high durability ratio (= fatigue strength / tensile strength).

[0003] A steel wire that can obtain high tensile strength and high durability ratio is proposed in Japanese Patent Laid-Open Publication No. 6-49592 (Patent Document 1).

[0004] The wire rod disclosed in Patent Document 1, which is the raw material for steel wire, contains, by weight, 0.80 to 1.10% C, 0.25% or less Si, 0.45% or less Mn, 0.010% or less P, 0.010% or less S, 0.15 to 0.35% Cr, 0.0040% or less N, and (0.0005 to 0.01) × Cr [%] + 0.001% B, with the remainder consisting of Fe and unavoidable impurities. The inclusion of B in this wire rod promotes the growth of cementite in pearlite. This suppresses the occurrence of microdefects at the ferrite-cementite interface during the wiredrawing process in the steel wire manufacturing process. Patent Document 1 also describes that, as a result, the tensile strength and durability ratio of steel wires manufactured using the wire rod as a raw material are improved. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 6-49592 [Non-patent literature]

[0006] [Non-Patent Document 1] Takagi et al., "Correlation of parameters obtained by direct-fitting method and modified Williamson-Hall method for cold-worked iron", Iron and Steel, Vol. 105 (2019) No. 6, pp. 648-654 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the tensile strength and durability ratio of the steel wire may be increased by other means different from the means proposed in Patent Document 1.

[0008] An object of the present disclosure is to provide a steel wire having high tensile strength and high durability ratio. [Means for solving the problem]

[0009] The steel wire according to the present disclosure comprises: The chemical composition is, in mass%, C: 0.70 to less than 1.10% Si: 0.10 to 1.00%, Mn: 0.20 to 1.00%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, O: 0.0050% or less, and N: 0.0100% or less, the balance being Fe and impurities, The tensile strength is 1600 MPa or more, The ratio R of the average microstrain in the surface layer of the steel wire to the average microstrain in the cross section parallel to the axial direction of the steel wire and including the central axis S is 1.05 or more.

[0010] The steel wire according to the present disclosure comprises: The chemical composition is, in mass%, C: 0.70 to less than 1.10% Si: 0.10 to 1.00%, Mn: 0.20 to 1.00%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, O: 0.0050% or less, and N: 0.0100% or less, Further, it contains one or more selected from the group consisting of Group 1 and Group 2, the balance being Fe and impurities, The tensile strength is 1600 MPa or more, The ratio R of the average microstrain in the surface layer of the steel wire to the average microstrain in the cross section parallel to the axial direction of the steel wire and including the central axis S is 1.05 or more. [Group 1] Cr: 0.50% or less, V: 0.10% or less, Cu: 0.50% or less, Ni: 0.50% or less, Sn: 0.100% or less, Mo: 0.20% or less Nb: 0.100% or less, Ti: 0.100% or less, B: 0.0050% or less, Co: 0.50% or less, and W: 0.20% or less, one or more selected from the group consisting of [Group 2] Ca: 0.0050% or less, Mg: 0.0050% or less, Zr: 0.010% or less, and Rare earth elements: 0.005% or less, one or more selected from the group consisting of [Effects of the Invention]

[0011] The steel wire of the present disclosure provides high tensile strength and high durability ratio. [Brief explanation of the drawings]

[0012] [Figure 1]FIG. 1 is a perspective view of a test piece for measuring the average microstrain in a cross section parallel to the axial direction of a steel wire and including the central axis in measuring the average microstrain ratio RS. [Figure 2] FIG. 2 is a perspective view of a test piece for measuring the average microstrain in the surface layer of a steel wire in measuring the average microstrain ratio RS. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present inventors first investigated steel wires having high tensile strength and high durability ratio from the viewpoint of chemical composition, and as a result, found that the chemical composition is, in mass %, C: 0.70 to less than 1.10%, Si: 0.10 to 1.00%, Mn: 0.20 to 1.00%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, O: 0.0050% or less, N: 0.0100% or less, Cr: 0 to 0.50%, V: 0 to 0.10%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Sn: 0 to 0.100%, It was thought that a steel wire containing Mo: 0-0.20%, Nb: 0-0.100%, Ti: 0-0.100%, B: 0-0.0050%, Co: 0-0.50%, W: 0-0.20%, Ca: 0-0.0050%, Mg: 0-0.0050%, Zr: 0-0.010%, and rare earth elements: 0-0.005%, with the remainder being Fe and impurities, would provide high tensile strength and a high durability ratio.

[0014] However, although a steel wire having the above chemical composition can achieve a tensile strength of 1600 MPa or more, it sometimes fails to achieve a high durability ratio. Therefore, the present inventors investigated the cause of this and found the following:

[0015] The present inventors first observed the cross sections of steel wires that did not achieve a high durability ratio. As a result, it was found that in steel wires with a low durability ratio, cracks originated on the surface of the steel wire. Therefore, the present inventors thought that the durability ratio of the steel wire could be increased by increasing the strength of the surface layer of the steel wire, and investigated the following points.

[0016] A large number of dislocations are introduced into steel wires during wire drawing. As a result, high strength is achieved. However, the dislocations introduced into steel wires may be smaller in the surface layer than in the interior. This is thought to be because the dislocations in the surface layer disappear due to the heat generated during wire drawing. In this case, even if the steel wire obtains tensile strength due to the dislocations in the interior, cracks will occur in the surface layer, which has fewer dislocations than the interior, if the steel wire is subjected to repeated fatigue. This reduces the durability ratio of the steel wire. Therefore, the inventors focused on the microstrain in the surface layer of steel wires as a means of increasing the durability ratio, and considered increasing the microstrain in the surface layer more than the microstrain in the interior. As a result, the inventors discovered the following: Microstrain is an index that represents the amount of dislocations in a steel wire.

[0017] The inventors have determined that the ratio R of the average microstrain of the surface layer to the average microstrain of the cross section parallel to the axial direction of the steel wire and including the central axis of the steel wire S It has been found that if the value of 1.05 or more, high tensile strength and high durability ratio can be obtained.

[0018] The steel wire of this embodiment has been completed based on the above technical concept and has the following configuration.

[0019] The steel wire of the first configuration is The chemical composition is, in mass%, C: 0.70 to less than 1.10% Si: 0.10 to 1.00%, Mn: 0.20 to 1.00%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, O: 0.0050% or less, and N: 0.0100% or less, the balance being Fe and impurities, The tensile strength is 1600 MPa or more, The ratio R of the average microstrain of the surface layer of the steel wire to the average microstrain of the cross section parallel to the axial direction of the steel wire and including the central axis S is 1.05 or more.

[0020] The steel wire of the second configuration is The chemical composition is, in mass%, C: 0.70 to less than 1.10% Si: 0.10 to 1.00%, Mn: 0.20 to 1.00%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, O: 0.0050% or less, and N: 0.0100% or less, Further, it contains one or more selected from the group consisting of Group 1 and Group 2, the balance being Fe and impurities, The tensile strength is 1600 MPa or more, The ratio R of the average microstrain of the surface layer of the steel wire to the average microstrain of the cross section parallel to the axial direction of the steel wire and including the central axis S is 1.05 or more. [Group 1] Cr: 0.50% or less, V: 0.10% or less, Cu: 0.50% or less, Ni: 0.50% or less, Sn: 0.100% or less, Mo: 0.20% or less Nb: 0.100% or less, Ti: 0.100% or less, B: 0.0050% or less, Co: 0.50% or less, and W: 0.20% or less, one or more selected from the group consisting of [Group 2] Ca: 0.0050% or less, Mg: 0.0050% or less, Zr: 0.010% or less, and Rare earth elements: 0.005% or less, one or more selected from the group consisting of

[0021] The third configuration of steel wire is A steel wire of a second configuration, The chemical composition includes the first group.

[0022] The fourth configuration of steel wire is A steel wire of the second or third configuration, The chemical composition includes the second group.

[0023] The steel wire according to this embodiment will be described in detail below. Note that "%" regarding elements means "mass %" unless otherwise specified.

[0024] [Features of the steel wire of this embodiment] The steel wire of this embodiment has the following features. (Feature 1) The chemical composition is, in mass%, C: 0.70 to less than 1.10%, Si: 0.10 to 1.00%, Mn: 0.20 to 1.00%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, O: 0.0050% or less, N: 0.0100% or less, Cr: 0 to 0.50%, V: 0 to 0.10%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, It contains Sn: 0-0.100%, Mo: 0-0.20%, Nb: 0-0.100%, Ti: 0-0.100%, B: 0-0.0050%, Co: 0-0.50%, W: 0-0.20%, Ca: 0-0.0050%, Mg: 0-0.0050%, Zr: 0-0.010%, and rare earth elements: 0-0.005%, with the balance being Fe and impurities. (Feature 2) The tensile strength is 1600 MPa or more. (Feature 3) The ratio R of the average microstrain in the surface layer of the steel wire to the average microstrain in the cross section parallel to the axial direction of the steel wire and including the central axis S is 1.05 or more. Features 1 to 3 will be explained below.

[0025] [(Feature 1) Chemical composition] The chemical composition of the steel wire of this embodiment contains the following elements.

[0026] C: 0.70 to less than 1.10% Carbon (C) increases the strength of the steel wire. If the C content is less than 0.70%, 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 C content is 1.10% or more, pro-eutectoid cementite is excessively formed in the wire rod that is the raw material for the steel wire, and therefore, even if the contents of other elements are within the ranges of this embodiment, the wire drawability of the wire rod is reduced. Therefore, the C content is 0.70 to less than 1.10%. The lower limit of the C content is preferably 0.74%, more preferably 0.78%, and even more preferably 0.80%. The upper limit of the C content is preferably 1.09%, more preferably 1.05%, and even more preferably 1.03%.

[0027] Si: 0.10 to 1.00% Silicon (Si) increases the strength of the steel wire. If the Si 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 Si content exceeds 1.00%, Si dissolves excessively in the ferrite, and the wire drawability of the wire rod that is the raw material for the steel wire deteriorates. Therefore, the Si content is 0.10 to 1.00%. The lower limit of the Si content is preferably 0.13%, more preferably 0.15%, and even more preferably 0.18%. The upper limit of the Si content is preferably 0.95%, more preferably 0.90%, and even more preferably 0.80%.

[0028] Mn: 0.20 to 1.00% Manganese (Mn) improves hardenability and increases the strength of the steel wire. If the Mn content is less than 0.20%, the above effects cannot be sufficiently obtained. On the other hand, if the Mn content exceeds 1.00%, Mn segregates in the wire rod that is the raw material for steel wire. In this case, martensite or bainite is formed in the region where Mn segregates, and the wire drawability of the wire rod deteriorates. Therefore, the Mn content is 0.20 to 1.00%. The lower limit of the Mn content is preferably 0.25%, more preferably 0.30%, and even more preferably 0.40%. The upper limit of the Mn content is preferably 0.95%, more preferably 0.90%, and even more preferably 0.80%.

[0029] P:0.020% or less Phosphorus (P) is an impurity. That is, the lower limit of the P content is more than 0%. If the P content exceeds 0.020%, P segregates at the grain boundaries, embrittling the grain boundaries. Therefore, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel wire decreases. Therefore, the P content is 0.020% or less. The lower the P content, the better. However, excessive reduction in the P content increases production costs. Therefore, taking into consideration normal industrial production, the lower limit of the P content is preferably 0.001%, more preferably 0.003%, and even more preferably 0.004%. The upper limit of the P content is preferably 0.018%, more preferably 0.015%, and even more preferably 0.010%.

[0030] S: 0.020% or less Sulfur (S) is an impurity. That is, the lower limit of the S content is more than 0%. If the S content exceeds 0.020%, S segregates at the grain boundaries, embrittling the grain boundaries. Therefore, even if the contents of other elements are within the ranges of this embodiment, the strength of the steel wire decreases. Therefore, the S content is 0.020% or less. The S content is preferably as low as possible. However, excessive reduction of the S content increases production costs. Therefore, taking into consideration normal industrial production, the lower limit of the S content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%. The upper limit of the S content is preferably 0.018%, more preferably 0.015%, and even more preferably 0.010%.

[0031] Al: 0.080% or less Aluminum (Al) is an impurity. That is, the lower limit of the Al content is more than 0%. If the Al content exceeds 0.080%, excessive Al oxides are generated in the wire rod, which is the raw material for steel wire. In this case, the wire drawability of the wire rod is reduced. Therefore, the Al content is 0.080% or less. The Al content is preferably as low as possible. However, excessive reduction in the Al content increases production costs. Therefore, taking into consideration normal industrial production, the lower limit of the Al content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%. The upper limit of the Al content is preferably 0.079%, and more preferably 0.070%.

[0032] O: 0.0050% or less Oxygen (O) is an impurity. That is, the lower limit of the O content is more than 0%. If the O content exceeds 0.0050%, coarse oxides are generated in the steel wire, which may become the starting point of cracks. Therefore, even if the contents of other elements are within the ranges of this embodiment, the fatigue strength of the steel wire decreases. Therefore, the O content is 0.0050% or less. The O content is preferably as low as possible. However, excessive reduction in the O content increases production costs. Therefore, taking into consideration normal industrial production, the lower limit of the O content is preferably 0.0001%, and more preferably 0.0003%. The upper limit of the O content is preferably 0.0045%, and more preferably 0.0040%.

[0033] N: 0.0100% or less Nitrogen (N) is an impurity. In other words, the lower limit of the N content is more than 0%. If the N content exceeds 0.0100%, excessive nitrides are formed. In this case, the wire drawability of the wire rod that is the raw material for steel wire is reduced. Therefore, the N content is 0.0100% or less. The N content is preferably as low as possible. However, excessive reduction in the N content increases production costs. Therefore, taking into consideration normal industrial production, the lower limit of the N content is preferably 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, and even more preferably 0.0030%. The upper limit of the N content is preferably 0.0090%, more preferably 0.0070%, and even more preferably 0.0050%.

[0034] The balance of the chemical composition of the steel wire according to the present embodiment is composed of Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of the steel wire, and are acceptable within a range that does not adversely affect the steel wire according to the present embodiment.

[0035] [About 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 a first group and a second group, in place of a portion of Fe. [Group 1] Cr: 0.50% or less, V: 0.10% or less, Cu: 0.50% or less, Ni: 0.50% or less, Sn: 0.100% or less, Mo: 0.20% or less Nb: 0.100% or less, Ti: 0.100% or less, B: 0.0050% or less, Co: 0.50% or less, and W: 0.20% or less, one or more selected from the group consisting of [Group 2] Ca: 0.0050% or less, Mg: 0.0050% or less, Zr: 0.010% or less, and Rare earth elements: 0.005% or less, one or more selected from the group consisting of These optional elements will be explained below.

[0036] [Group 1: Cr, V, Cu, Ni, Sn, Mo, Nb, Ti, B, Co and W] The chemical composition of the steel wire of this embodiment may further contain the above-mentioned elements of Group 1 in place of a portion of Fe. These elements are optional elements, and all of them increase the strength of the steel wire. Each element of Group 1 will be described below.

[0037] Cr:0.50% or less Chromium (Cr) is an optional element and may not be contained, that is, the Cr content may be 0%. When contained, that is, when the Cr content is more than 0%, Cr improves the hardenability of the wire rod that is the raw material for the steel wire and increases the strength of the steel wire. Even if even a small amount of Cr is contained, the above effects can be obtained to some extent. On the other hand, if the Cr content exceeds 0.50%, Cr segregates in the wire rod. In this case, bainite or martensite is formed in the region where Cr segregates. Therefore, even if the contents of other elements are within the ranges of this embodiment, the wire drawability of the wire rod is reduced. Therefore, the Cr content is 0 to 0.50%, and if contained, it is 0.50% or less. The lower limit of the Cr content is preferably 0.01%, more preferably 0.05%, and even more preferably 0.08%. The upper limit of the Cr content is preferably 0.45%, more preferably 0.40%, and even more preferably 0.30%.

[0038] V: 0.10% or less 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 improves the hardenability and increases the strength of the steel wire. Even if even a small amount of V is contained, the above effects can be obtained to some extent. On the other hand, if the V content exceeds 0.10%, the wire drawability of the wire rod that is the raw material for the steel wire decreases. Therefore, the V content is 0 to 0.10%, and if V is contained, it is 0.10% or less. The lower limit of the V content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%. The upper limit of the V content is preferably 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0039] Cu: 0.50% or less Copper (Cu) is an optional element and may not be contained, that is, the Cu content may be 0%. When contained, that is, when the Cu content is more than 0%, Cu improves the hardenability of the wire material from which the steel wire is made and increases the strength of the steel wire. Even if even a small amount of Cu is contained, the above effects can be obtained to some extent. On the other hand, if the Cu content exceeds 0.50%, the hardness of the wire becomes excessively high, and therefore the wire drawability of the wire deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0 to 0.50%, and if contained, it is 0.50% or less. The lower limit of the Cu content is preferably 0.01%, more preferably 0.05%, and even more preferably 0.08%. The upper limit of the Cu content is preferably 0.45%, more preferably 0.40%, and even more preferably 0.30%.

[0040] Ni: 0.50% or less Nickel (Ni) is an optional element and may not be contained, that is, the Ni content may be 0%. When Ni is contained, that is, when the Ni content exceeds 0%, Ni delays the transformation of pro-eutectoid cementite and grain boundary ferrite during cooling from the austenite temperature range in the steel wire manufacturing process. This causes a fine lamellar structure of pearlite to be formed. As a result, the strength of the steel wire is increased. Even if even a small amount of Ni is contained, the above effects can be obtained to some extent. On the other hand, if the Ni content exceeds 0.50%, the hardenability of the wire rod used as the raw material for the steel wire becomes excessively high. In this case, bainite is formed in the wire rod. Therefore, even if the contents of other elements are within the ranges of this embodiment, the wire drawability of the wire rod decreases. Therefore, the Ni content is 0 to 0.50%, and when Ni is contained, it is 0.50% or less. The lower limit of the Ni content is preferably 0.01%, more preferably 0.03%, and even more preferably 0.10%. The upper limit of the Ni content is preferably 0.48%, more preferably 0.45%, and even more preferably 0.40%.

[0041] Sn: 0.100% or less Tin (Sn) is an optional element and may not be contained, that is, the Sn content may be 0%. When Sn is contained, that is, when the Sn content is more than 0%, Sn suppresses ferrite decarburization that occurs during hot working and suppresses a decrease in the strength of the steel wire. As a result, the strength of the steel wire is increased. Even if even a small amount of Sn is contained, the above effects can be obtained to some extent. On the other hand, if the Sn content exceeds 0.100%, Sn segregates at grain boundaries, causing red embrittlement, which reduces the ductility of the steel wire even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sn content is 0 to 0.100%, and if Sn is contained, it is 0.100% or less. The lower limit of the Sn content is preferably 0.001%, more preferably 0.010%, and even more preferably 0.020%. The upper limit of the Sn content is preferably 0.090%, more preferably 0.080%, and even more preferably 0.070%.

[0042] Mo: 0.20% or less Molybdenum (Mo) is an optional element and may not 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 wire rod that is the raw material for the steel wire and increases the strength of the steel wire. Even if even a small amount of Mo is contained, the above effects can be obtained to some extent. On the other hand, if the Mo content exceeds 0.20%, the hot workability of the steel material deteriorates in the steel wire manufacturing process even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mo content is 0 to 0.20%, and if contained, it is 0.20% or less. The lower limit of the Mo content is preferably 0.01%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the Mo content is preferably 0.18%, more preferably 0.15%, and even more preferably 0.10%.

[0043] Nb: 0.100% or less 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 is more than 0%, Nb forms precipitates, which are carbides and / or carbonitrides, and increases the strength of the steel wire. Even if even a small amount of Nb is contained, the above effects can be obtained to some extent. On the other hand, if the Nb content exceeds 0.100%, the hot workability of the steel material deteriorates in the manufacturing process of the steel wire even if the contents of other elements are within the ranges of this embodiment. Therefore, the Nb content is 0 to 0.100%, and if Nb is contained, it is 0.100% or less. The lower limit of the Nb content is preferably 0.001%, more preferably 0.010%, and even more preferably 0.020%. The upper limit of the Nb content is preferably 0.090%, more preferably 0.080%, and even more preferably 0.070%.

[0044] Ti:0.100% or less Titanium (Ti) is an optional element and may not be contained, that is, the Ti content may be 0%. When contained, that is, when the Ti content is more than 0%, Ti forms precipitates of carbides and / or nitrides, thereby increasing the strength of the steel wire. Even if even a small amount of Ti is contained, the above effects can be obtained to some extent. On the other hand, if the Ti content exceeds 0.100%, the hot workability of the steel material deteriorates in the manufacturing process of the steel wire even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ti content is 0 to 0.100%, and if contained, it is 0.100% or less. The lower limit of the Ti content is preferably 0.001%, and more preferably 0.010%. The upper limit of the Ti content is preferably 0.090%, and more preferably 0.080%.

[0045] B: 0.0050% or less Boron (B) is an optional element and may not be contained, that is, the B content may be 0%. When B is contained, that is, when the B content exceeds 0%, B improves the hardenability of the wire material that is the raw material for the steel wire and increases the strength of the steel wire. Even if even a small amount of B is contained, the above effects can be obtained to some extent. On the other hand, if the B content exceeds 0.0050%, the hot workability of the steel material deteriorates in the manufacturing process of the steel wire even if the contents of other elements are within the ranges of this embodiment. Therefore, the B content is 0 to 0.0050%, and if B is contained, it is 0.0050% or less. The lower limit of the B content is preferably 0.0001%, and more preferably 0.0010%. The upper limit of the B content is preferably 0.0040%, and more preferably 0.0030%.

[0046] Co:0.50% or less Cobalt (Co) is an optional element and may not be contained, that is, the Co content may be 0%. When Co is contained, that is, when the Co content exceeds 0%, Co improves the hardenability of the wire material that is the raw material for the steel wire and increases the strength of the steel wire. Even if even a small amount of Co is contained, the above effects can be obtained to some extent. On the other hand, if the Co content exceeds 0.50%, the hardenability of the wire rod used as the raw material for the steel wire becomes excessively high. In this case, bainite is formed in the wire rod. Therefore, even if the contents of other elements are within the ranges of this embodiment, the wire drawability of the wire rod decreases. Therefore, the Co content is 0 to 0.50%, and if Co is contained, it is 0.50% or less. The lower limit of the Co content is preferably 0.01%, more preferably 0.05%, and even more preferably 0.08%. The upper limit of the Co content is preferably 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[0047] W: 0.20% or less Tungsten (W) is an optional element and may not be contained, that is, the W content may be 0%. When W is contained, that is, when the W content exceeds 0%, W improves the hardenability of the wire material that is the raw material for the steel wire and increases the strength of the steel wire. Even if even a small amount of W is contained, the above effects can be obtained to some extent. On the other hand, if the W content exceeds 0.20%, the hot workability of the steel material deteriorates in the manufacturing process of the steel wire even if the contents of other elements are within the ranges of this embodiment. Therefore, the W content is 0 to 0.20%, and if W is contained, it is 0.20% or less. The lower limit of the W content is preferably 0.01%, more preferably 0.03%, and even more preferably 0.05%. The upper limit of the W content is preferably 0.18%, more preferably 0.16%, and even more preferably 0.14%.

[0048] [Group 2: Ca, Mg, Zr and rare earth elements (REM)] The chemical composition of the steel wire of the present embodiment may further contain the above-mentioned elements of Group 2 in place of a portion of Fe. These elements are optional elements, and all of them improve the wire drawability of the wire rod that is the raw material for the steel wire.

[0049] Ca:0.0050% or less Calcium (Ca) is an optional element and may not be contained, that is, the Ca content may be 0%. When contained, that is, when the Ca content is more than 0%, Ca reduces hard alumina-based inclusions and improves the wire drawability of the wire rod. Even if even a small amount of Ca is contained, the above effects can be obtained to some extent. On the other hand, if the Ca content exceeds 0.0050%, coarse oxides are formed in the wire rod, and in this case, the wire drawability of the wire rod is reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ca content is 0 to 0.0050%, and if contained, it is 0.0050% or less. The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The upper limit of the Ca content is preferably 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%.

[0050] Mg: 0.0050% or less Magnesium (Mg) is an optional element and may not be contained, that is, the Mg content may be 0%. When Mg is contained, that is, when the Mg content is more than 0%, Mg forms fine oxides. The fine oxides refine the microstructure and improve the wire drawability of the wire rod. Even if even a small amount of Mg is contained, the above effects can be obtained to some extent. On the other hand, if the Mg content exceeds 0.0050%, coarse oxides are formed in the wire rod, and in this case, the wire drawability of the wire rod is reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mg content is 0 to 0.0050%, and if Mg is contained, it is 0.0050% or less. The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The upper limit of the Mg content is preferably 0.0040%, more preferably 0.0030%, and even more preferably 0.0020%.

[0051] Zr: 0.010% or less Zirconium (Zr) is an optional element and may not be contained, that is, the Zr content may be 0%. When contained, that is, when the Zr content is more than 0%, Zr forms fine oxides. The fine oxides refine the microstructure and improve the wire drawability of the wire rod. Even if even a small amount of Zr is contained, the above effects can be obtained to some extent. On the other hand, if the Zr content exceeds 0.010%, coarse oxides are formed in the wire rod, and in this case, the wire drawability of the wire rod is reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the Zr content is 0 to 0.010%, and if contained, it is 0.010% or less. The lower limit of the Zr content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%. The upper limit of the Zr content is preferably 0.009%, more preferably 0.008%, and even more preferably 0.007%.

[0052] Rare earth elements: 0.005% or less Rare earth elements (REM) are optional elements and may not be contained, i.e., the REM content may be 0%. When REM is contained, that is, when the REM content is more than 0%, REM fixes S, thereby improving the wire drawability of the wire rod. Even if even a small amount of REM is contained, the above effect can be obtained to some extent. On the other hand, if the REM content exceeds 0.005%, coarse oxides are formed in the wire rod, and in this case, the wire drawability of the wire rod is reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the REM content is 0 to 0.005%, and if contained, it is 0.005% or less. The lower limit of the REM content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.003%. The upper limit of the REM content is preferably 0.004%, and more preferably 0.003%.

[0053] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc), which has atomic number 21, yttrium (Y), which has atomic number 39, and the lanthanides lanthanum (La), which has atomic number 57, to lutetium (Lu), which has atomic number 71. In addition, the REM content in this specification refers to the total content of these elements.

[0054] [(Feature 2) Tensile strength] Furthermore, the steel wire of this embodiment has a tensile strength of 1600 MPa or more. Although the steel wire of this embodiment has such a high tensile strength, it also has a high durability ratio by satisfying Features 1 and 3.

[0055] The lower limit of the tensile strength is preferably 1650 MPa, and more preferably 1700 MPa. The upper limit of the tensile strength is not particularly limited, but is, for example, 2600 MPa, and preferably 2500 MPa.

[0056] [Method for measuring tensile strength] In this embodiment, the tensile strength can be determined by the following method. A tensile test is performed in air at room temperature (20±15°C) in accordance with JIS Z 2241:2022 to obtain the tensile strength (MPa). The tensile test specimen is taken so that the longitudinal direction is the axial direction of the steel wire. The test is performed three times, and the arithmetic mean value is defined as the tensile strength (MPa) of the steel wire.

[0057] (Feature 3) Average micro strain ratio R S About

[0058] In the steel wire of this embodiment, the ratio R of the average microstrain in the surface layer of the steel wire to the average microstrain in a cross section parallel to the axial direction of the steel wire and including the central axis of the steel wire is further S The ratio R of the average microstrain in the surface layer of the steel wire to the average microstrain in the cross section parallel to the axial direction of the steel wire and including the central axis is 1.05 or more. S is herein referred to as the average microstrain ratio R S In the following description, a cross section parallel to the axial direction of the steel wire and including the central axis is also referred to as an L cross section.

[0059] As mentioned above, a large number of dislocations are introduced into steel wires during wire drawing, resulting in high strength. However, the dislocations introduced into the steel wire may be smaller in the surface layer than in the interior due to the influence of heat generated during wire drawing. In this case, even if the tensile strength of the steel wire is obtained from the internal dislocations, cracks will occur in the surface layer, which has fewer dislocations than the interior, if the steel wire is subjected to repeated fatigue. This reduces the durability ratio of the steel wire. Therefore, the ratio R of the average microstrain in the surface layer of the steel wire to the average microstrain in the L cross section of the steel wire is S The surface layer is defined as the region extending from the surface of the steel wire to a depth of 10 μm in the radial direction.

[0060] Average microstrain ratio R S If the average microstrain ratio R is less than 1.05, the average microstrain in the surface layer of the steel wire is too small compared to the average microstrain in the L-section of the steel wire. As a result, even if the steel wire satisfies Features 1 and 2, a high durability ratio cannot be obtained. On the other hand, S If the ratio is 1.05 or more, the average microstrain of the surface layer of the steel wire is sufficiently larger than the average microstrain of the L cross section of the steel wire. As a result, a high durability ratio can be obtained, provided that the steel wire satisfies Features 1 and 2. Therefore, the average microstrain ratio R S is 1.05 or greater.

[0061] Average microstrain ratio R SThe lower limit of is preferably 1.10, more preferably 1.15, and even more preferably 1.20. Average microstrain ratio R S The upper limit of is not particularly limited, and in consideration of normal industrial production, it is, for example, 1.70, and more preferably 1.60.

[0062] [Average microstrain ratio R S Measurement method] Average microstrain ratio R of steel wire S is calculated by the following method.

[0063] First, we will explain how to determine the average microstrain of the L cross section of a steel wire. Referring to Figure 1, test pieces 1 to 3 were taken from three random locations on the steel wire, with the cross section parallel to the axial direction of the steel wire and including the central axis as the observation surface. Each test piece had a length L1 in the axial direction of the test piece of 20 mm. The observation surface of each test piece was wet-polished, followed by chemical polishing using a mixed solution of hydrogen peroxide and hydrofluoric acid. The mixed solution was, for example, an aqueous solution containing 29 to 34% hydrogen peroxide by mass and 2 to 3% hydrofluoric acid by mass. After chemical polishing, the test pieces were arranged without gaps, with the observation surfaces facing the same direction, as shown in Figure 1. The test pieces were arranged so that the long sides of the rectangular observation surface of each test piece were parallel to each other. Furthermore, the test pieces were arranged so that the short sides of the rectangular observation surface of each test piece were aligned in a straight line.

[0064] The X-ray diffraction intensity of the observation surface of each test piece arranged in the above manner is measured using an X-ray diffractometer. The observation area measured with the X-ray diffractometer is a rectangular area of ​​3D (mm) x 20 (mm), where D is the diameter of the steel wire, and is measured so that the observation surfaces of the three test pieces are included. The diffraction intensity profile obtained here is considered to be the average of the diffraction intensity profiles on the observation surface of each test piece. In the following explanation, this diffraction intensity profile will be referred to as the "diffraction intensity profile L."

[0065] Next, we will explain how to determine the average microstrain in the surface layer of a steel wire. Referring to Figure 2, the steel wire is cut perpendicular to the axial direction, and three cylindrical test pieces (test pieces 4 to 6) are taken from the steel wire. The length L2 of the test pieces in the axial direction is 20 mm. The circumferential surface of each test piece is used as the observation surface.

[0066] As shown in Figure 2, the test pieces are arranged without any gaps. The test pieces are arranged so that their axial directions are parallel. Furthermore, the test pieces are arranged so that the end faces of each piece are aligned in the same straight line.

[0067] The X-ray diffraction intensity of the observation surface of each test piece arranged in the above manner is measured using an X-ray diffractometer. The observation area measured with the X-ray diffractometer is a rectangular area of ​​3D (mm) x 20 (mm), which is measured so as to include the observation surfaces of the three test pieces. The diffraction intensity profile obtained here is considered to be the average of the diffraction intensity profiles on the observation surface of each test piece. In the following explanation, this diffraction intensity profile will be referred to as the "diffraction intensity profile S."

[0068] The radiation source of the X-ray diffraction device is Cu-Kα, and X-rays are irradiated by the focusing method with an acceleration voltage of 50 kV and an acceleration current of 300 mA. Based on each obtained diffraction intensity profile, the average microstrain of the L cross section and the average microstrain of the surface layer are determined by the DF method (Direct Fitting method) described in Non-Patent Document 1.

[0069] The method for calculating the average microstrain using the DF method is as follows. The diffraction intensity profiles obtained by X-ray diffraction measurement are analyzed to determine the diffraction angle 2θ and half-width β (rad) for the (111), (200), (211), (220), (310), and (222) diffraction planes. From the diffraction angle 2θ and half-width β (rad), K and ΔK for each diffraction plane, as defined by the following equation, are calculated. K=2×sinθ / λ (a) ΔK=β×cosθ / λ (b) In the formulas (a) and (b), λ is the wavelength λ (nm) of the X-rays used in the X-ray diffraction measurement. Regarding the relationship between K and ΔK, the Williamson-Hall equation shown in equation (c) is known. ΔK=α+ε×K (c) Here, α is a value dependent on the crystal size, and ε represents the microstrain.

[0070] Equation (c) shows that the relationship between K and ΔK (hereinafter referred to as the WH plot) is linear. However, in reality, the WH plot becomes less linear and more irregularly distributed due to the influence of elastic anisotropy of each crystal plane. Therefore, the DF method uses ω as an index of elastic anisotropy and corrects the Williamson-Hall equation in equation (c) using the following equation: ΔK=α+ε×(K / ω) (d) Specifically, an appropriate value is introduced for ω in equation (d), and the fitting index is obtained for a plot with K / ω on the horizontal axis and ΔK on the vertical axis (hereinafter referred to as the corrected WH plot). Here, the fitting index is an index that indicates the accuracy of the linear relationship between K / ω and ΔK. The relationship between the ω value and the fitting index is organized to determine the ω at which the fitting index is maximized (the best linear relationship is obtained). The microstrain ε is obtained based on the corrected WH plot when the ω that gives the best linear relationship is substituted. The microstrain ε is a value rounded off to the fourth decimal place.

[0071] Based on the above-mentioned method for calculating microstrain ε, the microstrain obtained from the diffraction intensity profile L is defined as the average microstrain of the L cross section of the steel wire. Furthermore, based on the above-mentioned method for calculating microstrain ε, the microstrain obtained from the diffraction intensity profile S is defined as the average microstrain of the surface layer of the steel wire. The ratio R of the average microstrain of the surface layer of the steel wire to the average microstrain of the L cross section of the steel wire obtained by the above-mentioned method is S Ask for.

[0072] [Effects of the steel wire of this embodiment] The steel wire of this embodiment satisfies Features 1 to 3. Therefore, a high durability ratio can be obtained while maintaining high tensile strength. Preferably, the steel wire of this embodiment has a durability ratio of 0.30 or more when the tensile strength is 1600 MPa or more.

[0073] The lower limit of the durability ratio is preferably 0.31, and more preferably 0.32. The upper limit of the durability ratio is not particularly limited. However, when the contents of each element in the chemical composition of the steel wire are within the ranges of this embodiment, the upper limit of the durability ratio is, for example, 0.60, and more preferably 0.50.

[0074] [Applications to which the steel wire of this embodiment can be applied] The steel wire of this embodiment can be widely used in applications requiring high tensile strength and a high durability ratio, such as bead wire, steel cord, bridge wire, and wire rope. In the microstructure of the steel wire of this embodiment, the area ratio of pearlite is 90% or more, and preferably, the area ratio of pearlite in the microstructure is 95%.

[0075] [Method for observing the microstructure of the steel wire according to this embodiment] The microstructure observation and the area ratio of pearlite of the steel wire of this embodiment are measured by the following methods.

[0076] A test piece is taken from the steel wire. The cross section of the surface of the test piece that is parallel to the axial direction of the steel wire and includes the central axis is used as the observation surface. The observation surface is mirror-polished. The mirror-polished observation surface is etched with picral to reveal the structure. On the etched observation surface, four locations located at a depth of D / 2 in the radial direction from the surface corresponding to the outer circumferential surface of the steel wire and arranged at a pitch of 100 μm parallel to the axial direction of the steel wire are used as the observation field. The observation field is observed at a magnification of 2000 times using a field emission scanning electron microscope (FE-SEM). A photographic image of each field is generated. The size of each field is, for example, 50 μm × 40 μm.

[0077] Based on the contrast of the photographic images of each field of view, each structure (ferrite, proeutectoid cementite, pearlite, bainite, and martensite) is identified. For example, for pearlite, a structure having a lamellar structure can be distinguished as pearlite. The total area (μm 2 ) is calculated. The area ratio (%) of pearlite is calculated based on the total area of ​​the four fields of view and the total area of ​​pearlite.

[0078] It is known that the microstructure of steel wires used for applications such as beads for automobile tires, steel cords, wire ropes for bridges, wire ropes for elevators, PC steel wires, and power transmission wires, as well as wire rods used as raw materials for such steel wires, is substantially a pearlite structure.

[0079] [One example of the method for manufacturing the steel wire according to this embodiment] An example of a method for manufacturing the steel wire of this embodiment will be described. The method for manufacturing the steel wire described below is one example for manufacturing the steel wire of this embodiment. Therefore, the 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 the method for manufacturing the steel wire of this embodiment.

[0080] An example of the method for manufacturing the steel wire of this embodiment includes the following steps. (Process 1) Material preparation process (Process 2) Blooming rolling process (Process 3) Finishing rolling process (Process 4) Wire drawing process (Step 5) Chemical polishing process A wire rod that is a raw material for a steel wire is manufactured through steps 1 to 3. A steel wire is manufactured from the wire rod as a raw material through steps 4 and 5. Each step will be described below.

[0081] [(Process 1) Material preparation process] 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.

[0082] 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.

[0083] [(Process 2) Blossom rolling process] In the blooming process, the material (ingot or bloom) prepared in the material preparation process is subjected to blooming to produce a billet. In the blooming process, the material is first heated in 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, 1000 to 1200°C.

[0084] The heated material is rolled using a blooming mill, or a blooming mill and a continuous rolling mill, to produce billets. Specifically, the heated material is reverse rolled using the blooming mill to produce billets. If a well-known continuous rolling mill is located downstream of the blooming mill, the billets after blooming may be further subjected to tandem rolling using the continuous rolling mill to produce smaller billets. The produced billets are allowed to cool (air-cool) to room temperature before the finish rolling process.

[0085] [(Process 3) Finishing rolling process] In the finish rolling process, the billet produced in the blooming process is subjected to finish rolling to produce wire rod. In the finish rolling process, first, the billet produced in the blooming process is heated in a heating furnace. The heating temperature is 900 to 1150°C.

[0086] The heated billet is subjected to finish rolling (continuous rolling) using a continuous rolling mill to produce wire rod. The continuous rolling mill includes a plurality of 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 finish rolling (the temperature at the end of finish rolling) is 800 to 1000°C.

[0087] The cooling conditions after the finish rolling are not particularly limited, and the cooling after the finish rolling is carried out by, for example, air blast cooling, mist cooling, water cooling, or the like.

[0088] [(Process 4) Wire drawing process] In the wiredrawing process, the produced wire rod is subjected to multiple wiredrawing processes to produce a steel wire. In the multiple wiredrawing processes, a well-known patenting treatment may be performed after each wiredrawing process and before the next wiredrawing process. In the wiredrawing process, the following conditions 1 and 2 are satisfied. (Condition 1) The area reduction rate of the final skin-pass drawing is 3 to 7%. (Condition 2) When the diameter before the initial wiredrawing is d0 and the diameter after the final wiredrawing is d1, the amount of strain S defined by the following formula is 1.2 to 3.0. S = 2 × ln(d0 / d1) Conditions 1 and 2 will be explained below.

[0089] [Regarding Condition 1] In the wire drawing process in this embodiment, the area reduction rate of the final pass skin pass wire drawing is 3 to 7%. In the wire drawing process, increasing the area reduction rate introduces many dislocations. As a result, high strength is obtained. However, increasing the area reduction rate makes it easier for processing heat to occur in the surface layer of the steel wire. In this case, the processing heat makes it easier for dislocations to move within the crystal grains in the surface layer of the steel wire, which can lead to dislocation annihilation. If such annihilation increases in the surface layer of the steel wire, the average microstrain in the surface layer of the steel wire decreases. If skin-pass drawing with an area reduction rate of 3 to 7% is performed in the final pass of the wire drawing process, processing heat in the surface layer of the steel wire can be suppressed and dislocations in the surface layer are less likely to annihilate, thereby increasing the average microstrain in the surface layer of the steel wire.

[0090] If the area reduction rate in the final pass exceeds 7%, processing heat is likely to occur in the surface layer of the steel wire. In this case, dislocations in the surface layer of the steel wire are more likely to move within the crystal grains, leading to annihilation of dislocations in the surface layer. This reduces the average microstrain in the surface layer of the steel wire. As a result, the ratio R of the average microstrain in the surface layer of the steel wire to the average microstrain in the L cross section of the steel wire S is less than 1.05. On the other hand, if the area reduction rate in the final pass is less than 3%, the compressive force perpendicular to the longitudinal direction in skin-pass wire drawing will not be applied evenly. In this case, uneven force will be applied to the steel wire after the final pass, which may damage the coil shape of the steel wire after winding or deteriorate the surface properties of the steel wire. As a result, the quality of the steel wire as a product will not be met. Therefore, the area reduction rate in the final pass of skin-pass wire drawing should be 3 to 7%.

[0091] The area reduction rate (%) is calculated using the following formula. Area reduction rate = (1 - area of ​​cross section perpendicular to the longitudinal direction of the steel wire after wire drawing / area of ​​cross section perpendicular to the longitudinal direction of the wire before wire drawing) x 100 The reduction rate shall be an integer. In other words, the reduction rate shall be an integer obtained by rounding off the first decimal place. [Regarding Condition 2] In the wire drawing process in this embodiment, the strain amount S is 1.2 to 3.0. If the strain amount S is less than 1.2, the amount of dislocations in the steel wire is too small. In this case, sufficient tensile strength cannot be obtained. On the other hand, if the strain amount S is more than 3.0, the amount of dislocations in the steel wire is too large. In this case, the wire drawability of the wire material used for the steel wire is reduced, and wire drawing of the wire cannot be performed. Therefore, the strain amount S in the wire drawing process is 1.2 to 3.0. Note that the strain amount S is the total strain amount from multiple wire drawing processes, that is, the cumulative strain amount. The value of the strain amount S is also to one decimal place. In other words, the strain amount S is the value obtained by rounding off the first decimal place.

[0092] [(Step 5) Chemical polishing process] In the chemical polishing step, the steel wire after wire drawing is subjected to chemical polishing by immersing the steel wire in an acid solution to dissolve the surface of the steel wire. Specifically, when the diameter of the steel wire is D, the drawn steel wire is immersed in an acid solution to dissolve the steel wire to a depth of 0.016D to 0.050D from the surface in the radial direction. The acid solution is, for example, a mixed solution of hydrogen peroxide and hydrofluoric acid. The mixed solution is, for example, an aqueous solution containing 29 to 34% hydrogen peroxide by mass and 2 to 3% hydrofluoric acid by mass. The temperature of the acid solution in the acid solution bath is adjusted to room temperature, and the steel wire is immersed in the acid solution bath. The immersion time of the steel wire is not particularly limited as long as the steel wire can be dissolved to a depth of 0.016D to 0.050D from the surface in the radial direction. However, considering normal industrial production, the immersion time is, for example, 10 to 40 seconds. After immersion in the acid solution, the steel wire is rinsed with water and dried. The conditions for rinsing and drying are not particularly limited.

[0093] In the above-mentioned wiredrawing process, the average microstrain in the surface layer of the steel wire is increased by performing wiredrawing that satisfies conditions 1 and 2. However, even if conditions 1 and 2 are satisfied, a certain amount of processing heat is generated by the wiredrawing process, and dislocations in the surface layer of the steel wire may be annihilated. Therefore, in order to further increase the average microstrain in the surface layer of the steel wire, it is effective to remove the region including the surface layer of the steel wire where dislocations have annihilated. Specifically, by performing the above-mentioned chemical polishing process, the region from the surface of the steel wire after wiredrawing to a depth of 0.016D to 0.050D in the radial direction is removed.

[0094] If the conditions 1 and 2 in the wire drawing process are satisfied and the above-mentioned chemical polishing process is also carried out, the average microstrain in the surface layer of the steel wire will increase. As a result, the ratio R of the average microstrain in the surface layer of the steel wire to the average microstrain in the L cross section of the steel wire will be S is 1.05 or more.

[0095] The steel wire after the chemical polishing step may be subjected to a heat treatment, such as a bluing treatment, a hot-dip plating treatment, or a thermal diffusion treatment for electroplating such as brass plating.

[0096] The steel wire of this embodiment is manufactured through the above steps. The above-described manufacturing method is a preferred example of the manufacturing method of the steel wire according to this embodiment. Therefore, the steel wire having the above-described configuration may be manufactured by a manufacturing method other than the above-described manufacturing method. In short, the manufacturing method is not particularly limited as long as it can manufacture the steel wire of this embodiment having the above-described configuration. [Example]

[0097] 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.

[0098] [Material preparation process] Steel wires having the chemical compositions shown in Tables 1A and 1B were produced by the following method.

[0099] [Table 1A]

[0100] [Table 1B]

[0101] The produced blooms were subjected to a blooming process to produce billets. Specifically, the blooms were heated to 1100°C using a heating furnace. The heated blooms were rolled using a blooming mill and a continuous rolling mill to produce billets. The billets produced in the blooming process were allowed to cool to room temperature.

[0102] The produced billet was subjected to a finish rolling process. Specifically, the billet was heated to 950°C using a heating furnace. The heated billet was subjected to finish rolling (continuous rolling) using a continuous rolling mill to produce wire rod. The finish rolling temperature was 800 to 1000°C. After finish rolling, cooling was carried out by air blast cooling.

[0103] The produced wire rod was subjected to a wire drawing process to produce steel wire. The area reduction rate (%) of the skin-pass wire drawing in the final pass for each test number is shown in Table 2. Furthermore, the diameter d0 (mm) of the steel wire before wire drawing, the diameter d1 (mm) of the steel wire after wire drawing, and the amount of strain S for each test number are also shown in Table 2. The average microstrain ratio R S In the examples marked with "-" in the durability ratio and the average micro strain ratio R S and durability ratio could not be measured.

[0104] [Table 2]

[0105] A chemical polishing process was performed on the steel wire after wire drawing. Specifically, the steel wire after wire drawing was immersed in an acid solution to dissolve the steel wire to a depth of 0.016D to 0.050D in the radial direction from the surface of the steel wire. The acid solution was an aqueous solution containing 29 to 34% by mass of hydrogen peroxide and 2 to 3% by mass of hydrofluoric acid. The temperature of the acid solution in the acid solution bath was adjusted to room temperature, and the steel wire was immersed in the acid solution bath. The immersion time was 10 to 40 seconds. For each test number, if the chemical polishing process was performed, a "○" was entered in the "Chemical polishing process performed" column of Table 2, and if the chemical polishing process was not performed, an "×" was entered in the "Chemical polishing process performed" column of Table 2. The steel wire after the chemical polishing process was washed with water and dried.

[0106] Steel wires of each test number were manufactured by the above manufacturing process. The microstructure of each steel wire was measured based on the method described in the above-mentioned "Method for Observing the Microstructure of a Steel Wire According to the Present Embodiment." As a result, the area ratio of pearlite in each steel wire of each test number was 90% or more.

[0107] [About the evaluation test] The following evaluation tests were carried out on the manufactured steel wires with each test number. (Test 1) Tensile strength measurement test (Test 2) Average microstrain ratio R S Measurement test (Test 3) Durability ratio evaluation test Each test will be explained below.

[0108] [(Test 1) Tensile strength measurement test] The tensile strength of the steel wire of each test number was determined based on the method described above in [Method for measuring tensile strength]. The obtained tensile strengths are shown in the "Tensile strength (MPa)" column in Table 2.

[0109] [(Test 2) Average microstrain ratio R S Measurement test The above-mentioned [average microstrain ratio R S Based on the method described in [Measurement method for the strain ratio R of the steel wire of each test number], S The average microstrain ratio RS The average micro strain ratio R S " column.

[0110] [(Test 3) Durability Ratio Evaluation Test] The durability ratio of the steel wire of each test number was determined by the following method. A test piece with a length of 400 mm was taken from an arbitrary position of the steel wire of each test number. The fatigue strength (MPa) was determined using a Nakamura type rotating bending fatigue tester. The test conditions were room temperature and in air, with a span distance (length of the evaluation section) of 100 mm, a rotation speed of 3000 rpm, and a maximum number of repetitions of 1.0 x 10 7 The maximum stress among the test pieces of each test number that endured the maximum number of cycles was taken as the fatigue strength (MPa) of the steel wire of each test number.

[0111] The durability ratio of the steel wire of each test number was calculated using the following formula based on the fatigue strength (MPa) of the steel wire of each test number obtained and the tensile strength (MPa) of the steel wire of each test number obtained in Test 1. The obtained durability ratios are shown in the "Durability Ratio" column in Table 2. Durability ratio = fatigue strength (MPa) / tensile strength (MPa)

[0112] [Test Results] Referring to Table 1A, Table 1B and Table 2, the steel wires of test numbers 1 to 28 satisfied features 1 to 3. Therefore, the tensile strength was 1600 MPa or more, and the durability ratio was 0.30 or more, and high tensile strength and high durability ratio were obtained.

[0113] On the other hand, in test numbers 29 and 30, the area reduction rate of the final skin-pass drawing was too high. Therefore, the average microstrain ratio R S As a result, a high durability ratio was not obtained.

[0114] In test numbers 31 and 32, the area reduction rate of the final skin-pass drawing was too low. This resulted in the coil shape of the steel wire being damaged after winding, and the surface quality of the steel wire also deteriorated. As a result, the steel wire did not meet the quality standards for the product.

[0115] In test numbers 33 and 34, the chemical polishing process was not performed. Therefore, the average microstrain ratio R S As a result, a high durability ratio was not obtained.

[0116] 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, The chemical composition, in mass%, is C: 0.70 to less than 1.10% Si: 0.10-1.00%, Mn: 0.20-1.00%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, O: 0.0050% or less, and N: 0.0100% or less, the balance being Fe and impurities; The tensile strength is 1600 MPa or more, The ratio R of the average microstrain of the surface layer of the steel wire to the average microstrain of the cross section parallel to the axial direction of the steel wire and including the central axis Ｓ is 1.05 or more, Steel wire.

2. A steel wire, The chemical composition, in mass%, is C: 0.70 to less than 1.10% Si: 0.10-1.00%, Mn: 0.20-1.00%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, O: 0.0050% or less, and N: 0.0100% or less, Further, the composition contains one or more selected from the group consisting of Group 1 and Group 2, the balance being Fe and impurities; The tensile strength is 1600 MPa or more, The ratio R of the average microstrain of the surface layer of the steel wire to the average microstrain of the cross section parallel to the axial direction of the steel wire and including the central axis Ｓ is 1.05 or more, Steel wire. [Group 1] Cr: 0.50% or less, V: 0.10% or less, Cu: 0.50% or less, Ni: 0.50% or less, Sn: 0.100% or less, Mo: 0.20% or less, Nb: 0.100% or less, Ti: 0.100% or less, B: 0.0050% or less, Co: 0.50% or less, and W: 0.20% or less, one or more selected from the group consisting of [Group 2] Ca: 0.0050% or less, Mg: 0.0050% or less, Zr: 0.010% or less, and Rare earth elements: 0.005% or less, one or more selected from the group consisting of

3. The steel wire according to claim 2, the chemical composition contains the first group; Steel wire.

4. The steel wire according to claim 2, The chemical composition contains the second group. Steel wire.

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