Steel wire

Abstract

To provide a steel wire having high tensile strength and 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, the steel wire having a tensile strength equal to or higher than 1600MPa. 1.50 5.00 1.80 4.50, A ratio RB of the pearlite block width HS to the pearlite block width HC is 0.90 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Steel wire is used in applications such as beads for automobile tires, wire ropes for bridges, wire ropes for elevators, PC steel wire, and hard steel wire for power transmission wires. High tensile strength is required for steel wires used in these applications. Meanwhile, in the environments in which steel wires are used, they are subjected to repeated changes in the stress state. Therefore, high fatigue strength is required for steel wires. However, above a certain level of tensile strength, fatigue strength may not increase relative to the high tensile strength. 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 Summary of the Invention [Problem to be solved by the invention]

[0006] 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.

[0007] 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]

[0008] 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, In a cross section including the central axis of the steel wire, The pearlite block width HC at the center of the steel wire is 1.80 to 5.00 μm, The pearlite block width HS of the surface layer of the steel wire is 1.50 to 4.50 μm, The ratio R of the perlite block width HS to the perlite block width HC B is less than 0.90.

[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, 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, In a cross section including the central axis of the steel wire, The pearlite block width HC at the center of the steel wire is 1.80 to 5.00 μm, The pearlite block width HS of the surface layer of the steel wire is 1.50 to 4.50 μm, The ratio R of the perlite block width HS to the perlite block width HC B is less than 0.90. [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]

[0010] The steel wire of the present disclosure provides high tensile strength and durability ratio. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a schematic diagram for explaining a method for measuring the ratio RB of the pearlite block width HS to the pearlite block width HC of a steel wire. DETAILED DESCRIPTION OF THE INVENTION

[0012] 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.

[0013] However, although a steel wire having the above chemical composition can obtain high tensile strength, it sometimes fails to obtain a high durability ratio. Therefore, the present inventors investigated the cause of this and found the following.

[0014] The present inventors first observed the cross sections of steel wires that had high tensile strength but did not have a high durability ratio. As a result, it was found that in steel wires with low durability ratios, cracks originated on the surface of the steel wire. Therefore, the present inventors investigated means for suppressing cracks in the surface layer of the steel wire.

[0015] The microstructure of a steel wire having the above chemical composition is mainly pearlite. Here, the inventors considered the boundaries where the crystal orientation difference of pearlite is 8° or more in a cross section (L cross section) including the central axis of the steel wire to be the boundaries of pearlite blocks. Furthermore, the distance between the boundaries of adjacent pearlite blocks in the radial direction of the steel wire was defined as the pearlite block width. When the inventors observed the pearlite block width of steel wires in which cracks occurred in the surface layer, the pearlite block width of the surface layer or the pearlite block width of the central and surface layers in the L cross section of the steel wire was large. Furthermore, when the pearlite block width of the central and surface layers was large, high tensile strength was not obtained. Therefore, the inventors thought that by reducing the pearlite block width of the central and surface layers in the L cross section of the steel wire, it may be possible to achieve both high tensile strength and a high durability ratio of the steel wire.

[0016] However, simply reducing the width of the pearlite blocks in the center and surface layers of the L-shaped cross section of the steel wire may result in cracks occurring in the surface layer of the steel wire, resulting in a decrease in the durability ratio. Therefore, the present inventors further investigated the cause of this and found the following:

[0017] The inventors further observed the pearlite block width of steel wires in which cracks had occurred in the surface layer and found that the pearlite block width (pearlite block width HS) of the surface layer was larger than the pearlite block width (pearlite block width HC) of the center of the L-shaped cross section of the steel wire. On the other hand, when the pearlite blocks of steel wires in which a high durability ratio was obtained were observed, it was found that the pearlite block width HS of the surface layer was fine compared to the pearlite block width HC of the center of the steel wire. Therefore, the inventors thought that if the ratio of the pearlite block width HS of the surface layer of the steel wire to the pearlite block width HC of the center of the steel wire was lowered, it might be possible to achieve both high tensile strength and a high durability ratio of the steel wire.

[0018] Based on the above-mentioned results of the investigation, the inventors have determined that, in a cross section including the central axis of the steel wire, the pearlite block width HC in the center of the steel wire is 1.80 to 5.00 μm, the pearlite block width HS in the surface layer of the steel wire is 1.50 to 4.50 μm, and the ratio R of the pearlite block width HS to the pearlite block width HC is B It has been found that if the value is 0.90 or less, high tensile strength and high durability ratio can be obtained.

[0019] The steel wire of this embodiment has been completed based on the above technical concept and has the following configuration.

[0020] 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, In a cross section including the central axis of the steel wire, The pearlite block width HC at the center of the steel wire is 1.80 to 5.00 μm, The pearlite block width HS of the surface layer of the steel wire is 1.50 to 4.50 μm, The ratio R of the perlite block width HS to the perlite block width HC B is less than 0.90.

[0021] 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, In a cross section including the central axis of the steel wire, The pearlite block width HC at the center of the steel wire is 1.80 to 5.00 μm, The pearlite block width HS of the surface layer of the steel wire is 1.50 to 4.50 μm, The ratio R of the perlite block width HS to the perlite block width HC B is less than 0.90. [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

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

[0023] The fourth configuration of steel wire is A steel wire of the second or third configuration, The chemical composition includes the second group.

[0024] The steel wire according to this embodiment will be described in detail below. Note that "%" regarding elements means "mass %" unless otherwise specified.

[0025] [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) In a cross section including the central axis of the steel wire, the pearlite block width HC in the center of the steel wire is 1.80 to 5.00 μm, the pearlite block width HS in the surface layer of the steel wire is 1.50 to 4.50 μm, and the ratio R of the pearlite block width HS to the pearlite block width HC B is less than 0.90. Features 1 to 3 will be explained below.

[0026] [(Feature 1) Chemical composition] The chemical composition of the steel wire of this embodiment contains the following elements.

[0027] 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%.

[0028] 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%.

[0029] 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%.

[0030] 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%.

[0031] 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%.

[0032] 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%.

[0033] 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%.

[0034] 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%.

[0035] 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.

[0036] [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.

[0037] [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 first group elements 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 the first group will be described below.

[0038] 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 material from which the steel wire is made 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%.

[0039] 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 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 V is contained, the above effects can be obtained to some extent. On the other hand, if the V content exceeds 0.10%, the hardness of the wire rod increases excessively, and therefore the wire drawability of the wire rod decreases even if the contents of other elements are within the ranges of this embodiment. 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%.

[0040] 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 rod increases excessively, and therefore the wire drawability of the wire rod decreases 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%.

[0041] 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%.

[0042] 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%.

[0043] 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 manufacturing process of the steel wire 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%.

[0044] 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%.

[0045] 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 that are 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%.

[0046] 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%.

[0047] 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%.

[0048] 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%.

[0049] [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.

[0050] 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%.

[0051] 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%.

[0052] 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%.

[0053] 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%.

[0054] 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.

[0055] [Microstructure of the steel wire according to this embodiment] In the microstructure of the steel wire of this embodiment, the area fraction of pearlite is 90% or more. Preferably, the area fraction of pearlite in the microstructure is 95%. It is well known to those skilled in the art that the area fraction of pearlite in a steel wire having the above-mentioned chemical composition is 90% or more.

[0056] [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.

[0057] 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.

[0058] 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.

[0059] [(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.

[0060] 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.

[0061] [Method for measuring tensile strength] In this embodiment, the tensile strength can be determined by the following method.

[0062] In accordance with JIS Z 2241:2022, a tensile test is conducted in air at room temperature (20±15°C) 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 conducted three times, and the arithmetic mean value is the tensile strength (MPa) of the steel wire.

[0063] (Feature 3) Pearlite block width ratio R B About In the steel wire of this embodiment, in a cross section including the central axis of the steel wire, the pearlite block width HC at the center of the steel wire is 1.80 to 5.00 μm, the pearlite block width HS at the surface layer of the steel wire is 1.50 to 4.50 μm, and the ratio R of the pearlite block width HS to the pearlite block width HC is B The ratio R of the pearlite block width HS to the pearlite block width HC is 0.90 or less. B is the pearlite block width ratio R B It is also called.

[0064] As described above, the boundaries where the difference in crystal orientation of pearlite is 8° or more in the cross section (L cross section) including the central axis of the steel wire were considered to be the boundaries of pearlite blocks. Furthermore, the distance between the boundaries of adjacent pearlite blocks in the radial direction of the steel wire was defined as the pearlite block width. By reducing the pearlite block width HS in the surface layer of the steel wire, cracking in the surface layer of the steel wire can be suppressed. On the other hand, by reducing the pearlite block width HC in the center of the steel wire, the strength of the steel wire increases. However, if the pearlite block width HC in the center of the steel wire is excessively small, the strength of the center of the steel wire will increase excessively. If the strength of the center of the steel wire increases excessively, the durability ratio of the steel wire will decrease.

[0065] While the pearlite block widths in the center and surface of the steel wire are refined, the ratio R of the pearlite block width HS in the surface of the steel wire to the pearlite block width HC in the center of the steel wire is B The surface layer is defined as the region extending from the surface of the steel wire to a depth of 20 μm in the radial direction.

[0066] Perlite block width ratio R B If the pearlite block width ratio R exceeds 0.90, the pearlite block width HS at the surface of the steel wire becomes excessively coarse compared to the pearlite block width HC at the center of the steel wire. As a result, even if a steel wire satisfies Features 1 and 2, a high durability ratio cannot be obtained. On the other hand, if the pearlite block width ratio R B If the ratio is 0.90 or less, the pearlite block width HS in the surface layer of the steel wire becomes finer than the pearlite block width HC in the center 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 pearlite block width ratio R B is less than 0.90.

[0067] Perlite block width ratio R B The upper limit of is preferably 0.85, more preferably 0.80, and even more preferably 0.75. Perlite block width ratio R B The lower limit of is not particularly limited, and in consideration of normal industrial production, it is, for example, 0.50, and more preferably 0.60.

[0068] [Pearlite block width ratio R B Measurement method] Pearlite block width ratio R of steel wire B is calculated by the following method.

[0069] When the diameter of the steel wire is D, test pieces (D × 20 mm) including a cross section including the central axis of the steel wire are taken from any five positions on the steel wire. Here, the cross section including the central axis of the steel wire (L cross section) refers to a cross section that includes the central axis of the steel wire and is parallel to the central axis. For each test specimen, the L cross section is used as the observation surface. The observation surface of each test specimen is polished with colloidal silica abrasive. The polished observation surface is observed at a magnification of 10,000x using an FE-SEM in the center portion including the D / 2 position of the steel wire and the surface layer including a position 1 μm deep from the surface of the steel wire. In the following description, the observation area in the center is defined as observation area AC, and the observation area in the surface layer is defined as observation area AS. For example, an FE-SEM JSM-7100F manufactured by JEOL Ltd. is used. Photographic images of the observation areas AC and AS of each test specimen are generated. The size of the observation areas AC and AS is 8 μm × 8 μm. At this time, the observation areas AC and AS of each test specimen are selected as observation areas where the area ratio of pearlite is 90% or more.

[0070] Using the FE-SEM, a crystal orientation map is measured for the specified observation areas AC and AS of each test piece using the electron backscattering diffraction (EBSD) method installed on the FE-SEM. For the EBSD method, the tilt angle of the sample stage is set to 70°, the acceleration voltage is 15 kV, the probe current is 15 mA, the working distance (WD) is 15 mm, and the measurement step is 30 nm. For crystal orientation analysis, a TSL OIM Analysis 7 manufactured by TSL Solutions, Inc. is used, for example.

[0071] By analyzing the crystal orientation, boundaries surrounded by boundaries with a crystal orientation misorientation of 8° or more are identified as pearlite blocks. Figure 1 is a schematic diagram showing the results of binarization processing of a crystal orientation map with a crystal orientation misorientation threshold of 8° or more. Referring to Figure 1, the L direction and T direction in the rectangular region of the crystal orientation map are defined. The L direction is parallel to the axial direction of the steel wire, and the T direction is parallel to the radial direction of the steel wire. The solid line in Figure 1 indicates a boundary with a crystal orientation misorientation of 8° or more, and is defined as boundary 1. The pearlite block width in observation area AC and observation area AS is measured using the following method.

[0072] The length in the L direction in observation area AC and observation area AS is 8 μm, and the length in the T direction is 8 μm. First, in the rectangular area of ​​the crystal orientation map, line segments (line segments 10 to 70) that are parallel to the T direction and spaced at a pitch of 1 μm in the L direction are arranged. Next, the intersections of line segments 10 to 70 with boundary 1 are identified. Furthermore, all distances between adjacent intersections (hereinafter also referred to as inter-intersection distances) are identified. Of the inter-intersection distances, the maximum inter-intersection distance (maximum inter-intersection distance MD) is identified. Here, boundary 1 whose length PL parallel to the L direction is 1.0 μm or less is excluded from the intersections with line segments 10 to 70.

[0073] The maximum inter-intersection distance MD in the observation areas AC and AS of each test piece is identified. The arithmetic mean of the maximum inter-intersection distances MD in the five observation areas AC obtained is defined as the central pearlite block width (μm). Furthermore, the arithmetic mean of the maximum inter-intersection distances MD in the five observation areas AS obtained is defined as the surface pearlite block width (μm). The ratio R of the surface pearlite block width to the central pearlite block width obtained B Ask for.

[0074] [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.

[0075] 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.

[0076] [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.

[0077] 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.

[0078] [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.

[0079] 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 (Step 4) Heat treatment step (Process 5) Wire drawing process (Step 6) Chemical polishing process A wire rod that is a raw material for a steel wire is manufactured through steps 1 to 4. A steel wire is manufactured from the wire rod as a raw material through steps 5 and 6. Each step will be described below.

[0080] [(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.

[0081] 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.

[0082] [(Process 2) Blooming 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.

[0083] 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.

[0084] [(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, the billet produced in the blooming process is first heated in a heating furnace. The heating temperature is 1000 to 1150°C, and the holding time is 30 to 120 minutes.

[0085] The heated billet is subjected to finish rolling (continuous rolling) 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 temperature before finish rolling in the finish rolling process is 850 to 950°C. After finish rolling, coiling is performed at 750 to 880°C.

[0086] The cooling conditions after coiling are not particularly limited. Cooling after finish rolling is performed by, for example, water cooling. The temperature of the cooled wire rod is set to a temperature equal to or higher than the molten salt temperature in the first molten salt bath in the next step.

[0087] [(Step 4) Heat treatment step] In the heat treatment process, the wire rod after the finish rolling process is subjected to heat treatment. The heat treatment includes immersion in two molten salt baths. The molten salt temperatures are different in each stage. In the following explanation, the first stage molten salt bath is also referred to as the first molten salt bath, and the second stage molten salt bath is also referred to as the second molten salt bath. In the heat treatment process, the pearlite block width ratio R of the steel wire is B Preferably, the heat treatment step satisfies conditions 1 and 2. (Condition 1) The temperature of the molten salt in the first molten salt bath is 520 to 600°C. (Condition 2) The temperature of the molten salt in the second molten salt tank is 610 to 660°C. Conditions 1 and 2 will be explained below.

[0088] [Regarding Condition 1] In the heat treatment step of this embodiment, the temperature of the molten salt in the first molten salt bath is 520 to 600°C. The temperature of the molten salt in the first molten salt bath affects the pearlite block width in the surface layer of the wire rod. If the temperature of the molten salt in the first molten salt bath is too low, the pearlite block width in the center of the wire rod becomes too fine. In this case, even if condition 2 and condition 3 described later are satisfied, the pearlite block width ratio R of the steel wire manufactured from the wire rod will be too small. BOn the other hand, if the temperature of the molten salt in the first molten salt bath is too high, the pearlite block width in the surface layer of the wire rod becomes coarse. In this case, even if the conditions 2 and 3 are satisfied, the pearlite block width ratio R of the steel wire manufactured from the wire rod may be too large. B If the temperature of the molten salt in the first molten salt bath is 520 to 600°C, and conditions 2 and 3 are satisfied, the ratio R of the pearlite block width of the steel wire manufactured from the wire rod as a raw material is B The immersion time in the first molten salt bath is preferably 10 seconds or more.

[0089] [Regarding Condition 2] In the heat treatment step of this embodiment, the temperature of the molten salt in the second molten salt bath is 610 to 660°C. The temperature of the molten salt in the second molten salt bath affects the pearlite block width at the center of the wire rod. If the temperature of the molten salt in the second molten salt bath is too low, the pearlite block width at the center of the wire rod becomes small. In this case, even if conditions 1 and 3 are satisfied, the pearlite block width ratio R of the steel wire manufactured from the wire rod will be too small. B exceeds 0.90. On the other hand, if the temperature of the molten salt in the second molten salt bath is too high, the pearlite block width at the center of the wire rod becomes too coarse. In this case, even if conditions 1 and 3 are satisfied, the pearlite block width HC at the center of the steel wire manufactured from the wire rod exceeds 5.00 μm. As a result, the tensile strength of the steel wire decreases. If the temperature of the molten salt in the second molten salt bath is 610 to 660°C, assuming that conditions 1 and 3 are satisfied, the pearlite block width ratio R B The immersion time in the second molten salt bath is preferably 10 seconds or more. Furthermore, the total immersion time in the first and second molten salt baths is preferably 30 to 90 seconds.

[0090] [(Process 5) Wire drawing process] In the wiredrawing process, the produced wire rod is subjected to a well-known wiredrawing process to produce a steel wire. The wiredrawing process may be performed only once or multiple times. When performing multiple wiredrawing processes, a well-known patenting process may be performed after the wiredrawing process and before the next wiredrawing process. The area reduction rate in each pass in the wiredrawing process is not particularly limited, but is, for example, 18 to 25%. Note that oxide scale may be removed from the wire rod before the wiredrawing process. Furthermore, the wire rod may be subjected to a lubrication process before the wiredrawing process.

[0091] [(Step 6) Chemical polishing process] In the chemical polishing step, chemical polishing is performed on the steel wire after the wiredrawing step. In the chemical polishing step, the steel wire is immersed in an acidic solution to dissolve the surface layer of the steel wire after the wiredrawing step. Preferably, the chemical polishing step satisfies Condition 3. (Condition 3) The steel wire after wire drawing is immersed in an acidic solution to dissolve the steel wire to a depth of 0.016D to 0.050D from the surface in the radial direction.

[0092] [Regarding condition 3] 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] During the wire drawing process, processing heat is generated in the surface layer of the steel wire. If processing heat is generated, the pearlite blocks in the surface layer of the steel wire are likely to recrystallize. In this case, the width of the pearlite blocks in the surface layer of the steel wire becomes coarse. As a result, the pearlite block width ratio R of the steel wire B exceeds 0.90. Therefore, the ratio R of the pearlite block width of the steel wire B In order to improve the wire drawing strength, it is effective to remove the region including the surface layer of the steel wire where pearlite blocks have become coarse due to recrystallization. Specifically, by carrying out 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 to 3 of the heat treatment process for the wire are satisfied and the above-mentioned chemical polishing process is further carried out, the pearlite block width ratio R of the steel wire can be B is 0.90 or less.

[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 1000 to 1150°C using a heating furnace and held at that temperature for 30 to 120 minutes. The heated billet was subjected to finish rolling (continuous rolling) using a continuous rolling mill to produce a wire rod. The temperature before finish rolling was 850 to 950°C. After finish rolling, coiling was carried out at 750 to 880°C. After coiling, cooling was carried out by water cooling.

[0103] The wire rods after the finish rolling process were subjected to a heat treatment process. The molten salt temperature in the first molten salt bath in the heat treatment process for each test number is shown in the "molten salt temperature in the first molten salt bath (°C)" column of Table 2, and the molten salt temperature in the second molten salt bath is shown in the "molten salt temperature in the second molten salt bath (°C)" column of Table 2.

[0104] The heat-treated wire rod was subjected to a wiredrawing process to produce steel wire. The area reduction rate for each pass in the wiredrawing process for each test number was 18 to 25%. The initial wire diameter (wire diameter before wiredrawing) and final wire diameter (wire diameter after wiredrawing) for each test number in the wiredrawing process were as shown in the "Initial Wire Diameter (mm)" and "Final Wire Diameter (mm)" columns in Table 2. The steel wire after wiredrawing was subjected to a chemical polishing process. Specifically, the steel wire after wiredrawing was immersed in an acidic 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 acidic solution was an aqueous solution containing 29 to 34% hydrogen peroxide by mass and 2 to 3% hydrofluoric acid by mass. The temperature of the acidic solution in the acidic solution bath was adjusted to room temperature, and the steel wire was immersed in the acidic solution bath. The immersion time was 10 to 40 seconds. For each test number, if the chemical polishing process was performed, "◯" was entered in the "Chemical polishing process performed" column in Table 2, and if the chemical polishing process was not performed, "×" was entered in the "Chemical polishing process performed" column in Table 2. After the chemical polishing process, the steel wire was washed with water and dried.

[0105] [Table 2]

[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) Pearlite block width ratio R B 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) Pearlite block width ratio R B Measurement test The above-mentioned [ratio of perlite block width R B Based on the method described in [Measurement method of the pearlite block width R of the steel wire of each test number], B The pearlite block width HC at the center of the obtained steel wire is shown in the "Pearlite block width HC (μm)" column in Table 2, the pearlite block width HS at the surface layer of the steel wire is shown in the "Pearlite block width HS (μm)" column in Table 2, and the pearlite block width ratio R B The "pearlite block width ratio R" in Table 2 B " 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 Nos. 1 to 23 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 Nos. 24 and 25, the temperature of the molten salt in the first molten salt bath was too low. B exceeded 0.90. As a result, a high durability ratio could not be obtained.

[0114] In Test Nos. 26 and 27, the molten salt temperature in the first molten salt bath was too high. B exceeded 0.90. As a result, a high durability ratio could not be obtained.

[0115] In test numbers 28 and 29, the temperature of the molten salt in the second molten salt tank was too low. B exceeded 0.90. As a result, a high durability ratio could not be obtained.

[0116] In test numbers 30 and 31, the molten salt temperature in the second molten salt bath was too high, which caused the pearlite block width HC to exceed the upper limit, resulting in a tensile strength of less than 1600 MPa.

[0117] In test numbers 32 and 33, the chemical polishing process was not performed. Therefore, the pearlite block width ratio R B exceeded 0.90. As a result, a high durability ratio could not be obtained.

[0118] 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, In a cross section including the central axis of the steel wire, The pearlite block width HC at the center of the steel wire is 1.80 to 5.00 μm, The pearlite block width HS of the surface layer of the steel wire is 1.50 to 4.50 μm, The ratio R of the perlite block width HS to the perlite block width HC Ｂ is 0.90 or less, 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, In a cross section including the central axis of the steel wire, The pearlite block width HC at the center of the steel wire is 1.80 to 5.00 μm, The pearlite block width HS of the surface layer of the steel wire is 1.50 to 4.50 μm, The ratio R of the perlite block width HS to the perlite block width HC Ｂ is 0.90 or less, 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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