Wire rod, steel wire, steel rope, and method for manufacturing steel rope

Abstract

The present invention provides: a wire rod having a chemical composition containing, in mass%, 0.80 to 1.10% of C, 0.10 to 1.50% of Si, 0.10 to 1.00% of Mn, 0.030% or less of P, 0.030% or less of S, 0.0120% or less of N, 0.0100% or less of O, 0.005 to 0.070% of Al, and 0.02 to 0.20% of Mo; and a steel wire manufactured from the wire rod, the total area ratio of a ferritic structure and a martensitic structure in the center of a cross section is 5% or less, the average Vickers hardness H in the cross section, the surface layer Vickers hardness Hs, and the center Vickers hardness Hc satisfy the relationships of formulae (1)-(3) ([C] is the content of C in mass% contained in the wire), and the wire diameter is 7.0 mm or more. In addition, the steel wire rope is formed by bundling the steel wires. 480 * [C] <-40 lt >; hlt; 480 * [C] + 40... (1) 0 < = Hs-Hlt; (2) 0 < = H-Hclt; 30... (3).

Description

Technical Field

[0001] The present invention relates to a wire rod, a steel wire, a steel rope and a method for manufacturing the steel rope. Background Art

[0002] Steel wires for bridge cables and steel ropes are manufactured by, for example, subjecting a high-carbon wire rod to a sorbitizing treatment to form a pearlite structure, drawing the wire, and plating the wire to form a steel wire.

[0003] For example, bridge cables are required to have higher strength in order to increase the degree of freedom in cable design, and the steel wires used are also required to have higher strength.

[0004] For example, Patent Document 1 discloses a high-strength PC steel wire having a tensile strength of 2000 MPa or more, which contains, in mass %, 0.9-1.2% C, 0.01-1.5% Si, 0.2-1.5% Mn, 0.001-0.05% Al, and 0.0005-0.010% N, with the remainder being Fe and unavoidable impurities. Furthermore, the PC steel wire comprises 90% or more of drawn pearlite and 10% or less of ferrite and bainite, has a tensile strength of 2000 MPa or more, and, when the wire diameter of the PC steel wire is denoted as D, a ratio (Hv table / Hv inner) of the surface Hv hardness (Hv table) of a region (surface portion) 0.1D from the surface of the PC steel wire to the internal Hv hardness (Hv inner) of a region (interior portion) inner than the surface portion is 1.1 or less.

[0005] However, as the strength increases, the torsional characteristics decrease, which becomes a technical problem.

[0006] As a steel wire for the purpose of improving torsional properties, for example, Patent Document 2 discloses a plated steel wire for PWS having excellent torsional properties. The steel wire contains, in mass %, 0.8-1.1% C, 0.8-1.3% Si, 0.3-0.8% Mn, 0.001-0.006% N, and 0.0004-0.0060% B. Furthermore, the steel wire contains 0.0002% or more of solid-solubilized B, one or two of 0.005-0.1% Al and 0.005-0.1% Ti, with the remainder being Fe and unavoidable impurities. Furthermore, in a portion from the surface to a depth of 50 μm, the area ratio of non-pearlite structure is 10% or less, and the total area ratio of non-pearlite structure in the entire cross section is 5% or less. Furthermore, the surface is plated with a coating weight of 300-500 g / m2. 2 galvanized.

[0007] Patent Document 1: Japanese Patent Application Laid-Open No. 2009-280836

[0008] Patent Document 2: International Publication No. 2008 / 093466 Summary of the Invention

[0009] Technical problem solved by the invention

[0010] For steel wires used in bridge cables and the like, there is a need for the development of steel wires having high strength and excellent torsional properties, and wire materials suitable for producing steel wires having such properties.

[0011] Therefore, an object of the present invention is to provide a wire rod suitable for producing a steel wire having high strength and excellent torsional properties by wire drawing; a steel wire having high strength and excellent torsional properties; a steel rope; and a method for producing a steel rope.

[0012] Technical means to solve the problem

[0013] The technical means for solving the problem include the following methods.

[0014] <1> A steel wire having the following chemical composition:

[0015] Contains by mass%

[0016] C: 0.80~1.10%,

[0017] Si: 0.10~1.50%,

[0018] Mn: 0.10~1.00%,

[0019] P: 0.030% or less,

[0020] S: 0.030% or less,

[0021] N: 0.0120% or less,

[0022] O: 0.0100% or less,

[0023] Al: 0.005~0.070%,

[0024] Mo: 0.02~0.20%,

[0025] Cr: 0~1.00%,

[0026] Cu: 0~0.80%,

[0027] Sn: 0~0.50%,

[0028] Ni: 0~0.50%,

[0029] V: 0~0.15%,

[0030] Ti: 0~0.050%,

[0031] Nb: 0~0.050%,

[0032] B: 0~0.0040%,

[0033] REM: 0~0.030%,

[0034] Mg: 0~0.0040%,

[0035] Ca: 0~0.0040%,

[0036] Zr: 0~0.030%,

[0037] W: 0~0.10%,

[0038] Te: 0~0.030%, and

[0039] Sb: 0~0.030%;

[0040] The rest is Fe and impurities.

[0041] The total area ratio of the ferrite structure and martensite structure in the central part within 1.0 mm from the center in the cross section perpendicular to the longitudinal direction is less than 5%, and the remaining part is a mixed structure containing ferrite and cementite.

[0042] When the content of C in the wire rod expressed in mass % is defined as [C], the average Vickers hardness H in the cross section satisfies the formula (1),

[0043] 480×[C]-40 <H<480×[C]+40···(1)

[0044] The surface Vickers hardness Hs and the average Vickers hardness H at a depth of 0.5 to 1.0 mm from the surface satisfy the relationship of formula (2).

[0045] 0≤Hs-H<30 ···(2)

[0046] The central Vickers hardness Hc and the average Vickers hardness H within 1.0 mm from the center satisfy the relationship of formula (3),

[0047] 0≤H-Hc<30 ···(3)

[0048] The wire diameter is 7.0mm or more.

[0049] <2> according to <1> The wire, wherein

[0050] The chemical composition comprises, by mass%, at least one or more chemical substances selected from the following:

[0051] Cr: 0.03~1.00%,

[0052] Cu: 0.01~0.80%,

[0053] Sn: 0.001~0.50%,

[0054] Ni: 0.01~0.50%, and

[0055] V: 0.01~0.15%.

[0056] <3> according to <1> or <2> The wire, wherein

[0057] The chemical composition comprises, by mass%, one or more chemical substances selected from the following:

[0058] Ti: 0.002~0.050%,

[0059] Nb: 0.002~0.050%,

[0060] B: 0.0003~0.0040%,

[0061] REM: 0.002~0.030%,

[0062] Mg: 0.0002~0.0040%,

[0063] Ca: 0.0002~0.0040%,

[0064] Zr: 0.002~0.030%,

[0065] W: 0.02~0.10%,

[0066] Te: 0.001~0.030%, and

[0067] Sb: 0.001~0.030%.

[0068] <4> A steel wire having the following chemical composition:

[0069] Contains by mass%

[0070] C: 0.80~1.10%,

[0071] Si: 0.10~1.50%,

[0072] Mn: 0.10~1.00%,

[0073] P: 0.030% or less,

[0074] S: 0.030% or less,

[0075] N: 0.0120% or less,

[0076] O: 0.0100% or less,

[0077] Al: 0.005~0.070%,

[0078] Mo: 0.02~0.20%,

[0079] Cr: 0~1.00%,

[0080] Cu: 0~0.80%,

[0081] Sn: 0~0.50%,

[0082] Ni: 0~0.50%,

[0083] V: 0~0.15%,

[0084] Ti: 0~0.050%,

[0085] Nb: 0~0.050%,

[0086] B: 0~0.0040%,

[0087] REM: 0~0.030%,

[0088] Mg: 0~0.0040%,

[0089] Ca: 0~0.0040%,

[0090] Zr: 0~0.030%,

[0091] W: 0~0.10%,

[0092] Te: 0~0.030%, and

[0093] Sb: 0~0.030%;

[0094] The rest is Fe and impurities.

[0095] The total area ratio of the ferrite structure and the martensite structure in the central portion within 1.0 mm from the center in a cross section parallel to the longitudinal direction and passing through the central axis is 5% or less, and the remainder is a mixed structure containing ferrite and cementite.

[0096] The average Vickers hardness h in the cross section perpendicular to the longitudinal direction is 450~620,

[0097] The surface Vickers hardness hs and the average Vickers hardness h at a depth of 0.2~0.5mm from the surface satisfy the relationship of formula (4).

[0098] 0≤hs-h<30 ···(4)

[0099] The central Vickers hardness hc and the average Vickers hardness h within 1.0 mm from the center of the vertical cross section satisfy the relationship of formula (5),

[0100] 0≤h-hc<30 ···(5).

[0101] <5> according to <4> The steel wire, wherein

[0102] The chemical composition comprises, by mass%, at least one or more chemical substances selected from the following:

[0103] Cr: 0.03~1.00%,

[0104] Cu: 0.01~0.80%,

[0105] Sn: 0.001~0.50%,

[0106] Ni: 0.01~0.50%, and

[0107] V: 0.01~0.15%.

[0108] <6> according to <4> or <5> The steel wire, wherein

[0109] The chemical composition comprises, by mass%, one or more chemical substances selected from the following:

[0110] Ti: 0.002~0.050%,

[0111] Nb: 0.002~0.050%,

[0112] B: 0.0003~0.0040%,

[0113] REM: 0.002~0.030%,

[0114] Mg: 0.0002~0.0040%,

[0115] Ca: 0.0002~0.0040%,

[0116] Zr: 0.002~0.030%,

[0117] W: 0.02~0.10%,

[0118] Te: 0.001~0.030%, and

[0119] Sb: 0.001~0.030%.

[0120] <7> according to <4> ~ <6> The steel wire according to any one of the above aspects is plated on its surface.

[0121] <8> A steel rope that is made by bundling multiple <4> ~ <7> The steel wire described in any one of the above.

[0122] <9> A method for manufacturing a steel rope, comprising:

[0123] Multiple roots <4> ~ <7> The process of bundling the steel wires to form a steel rope.

[0124] Effects of the Invention

[0125] According to the present invention, there can be provided a wire rod suitable for producing a steel wire having high strength and excellent torsional properties by wire drawing; a steel wire having high strength and excellent torsional properties; a steel rope; and a method for producing a steel rope. BRIEF DESCRIPTION OF THE DRAWINGS

[0126] [ Figure 1 ] is a diagram showing an example of a SEM photograph of each structure of a wire rod.

[0127] [ Figure 2 ] is a diagram showing an example of a SEM photograph of the ferrite structure and martensite structure of a steel wire. DETAILED DESCRIPTION

[0128] An embodiment as an example of the present invention will be described.

[0129] It should be noted that in this specification, the “surface” or “surface portion” of the wire refers to a range of 0.5 to 1.0 mm in depth from the surface (peripheral surface) of the wire in a cross section perpendicular to the longitudinal direction of the wire (a region with a depth of 0.5 mm or more and 1.0 mm or less from the surface), and the “center portion” refers to a range within 1.0 mm from the center of the wire in a cross section perpendicular to the longitudinal direction of the wire.

[0130] On the other hand, the "surface layer" or "surface portion" of the steel wire refers to a range of 0.2 to 0.5 mm in depth from the surface (outer peripheral surface) of the steel wire in a cross section parallel to or perpendicular to the longitudinal direction of the steel wire (a region with a depth of 0.2 mm to 0.5 mm from the surface), and the "center portion" refers to a range within 1.0 mm from the center of the steel wire in a cross section parallel to or perpendicular to the longitudinal direction of the steel wire.

[0131] Furthermore, the “central axis” refers to an imaginary line that passes through the center point of a cross section perpendicular to the axial direction (longitudinal direction) of the wire rod or steel wire and extends in the axial direction.

[0132] In this specification, a cross section perpendicular to the longitudinal direction of a wire rod or a steel wire may be referred to as a "cross section," and a cross section parallel to the longitudinal direction and including the central axis may be referred to as a "longitudinal section."

[0133] In this specification, a numerical range expressed using "~" means a range that includes the numerical values ​​described before and after the "~" as the lower limit and upper limit. In addition, a numerical range in which "exceeds" or "less than" is indicated with respect to the numerical values ​​described before and after the "~" means a range that does not include these numerical values ​​as the lower limit or upper limit.

[0134] In the numerical ranges described in stages in this specification, the upper limit value of a numerical range in a certain stage may be replaced by the upper limit value of a numerical range described in another stage, or may be replaced by a value shown in Examples.

[0135] In the numerical ranges described in stages in this specification, the lower limit value of a numerical range in a certain stage may be replaced by the lower limit value of a numerical range described in another stage, or may be replaced by a value described in Examples.

[0136] The content of an element in a chemical composition may be simply expressed as "amount" (for example, C amount, Si amount, etc.).

[0137] Regarding the content of elements in the chemical composition, “%” means “mass %”.

[0138] When the content of an element in a chemical composition is described as "0 to" as a lower limit, it means that the element is an optional element and may not be included.

[0139] Furthermore, the term “process” encompasses not only independent processes but also processes that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved.

[0140] [Wire]

[0141] The wire rod of the present invention has the following chemical composition:

[0142] Contains by mass%

[0143] C: 0.80~1.10%,

[0144] Si: 0.10~1.50%,

[0145] Mn: 0.10~1.00%,

[0146] P: 0.030% or less,

[0147] S: 0.030% or less,

[0148] N: 0.0120% or less,

[0149] O: 0.0100% or less,

[0150] Al: 0.005~0.070%, and

[0151] Mo: 0.02~0.20%;

[0152] Optionally include:

[0153] Cr: 0~1.00%,

[0154] Cu: 0~0.80%,

[0155] Sn: 0~0.50%,

[0156] Ni: 0~0.50%,

[0157] V: 0~0.15%,

[0158] Ti: 0~0.050%,

[0159] Nb: 0~0.050%,

[0160] B: 0~0.0040%,

[0161] REM: 0~0.030%,

[0162] Mg: 0~0.0040%,

[0163] Ca: 0~0.0040%,

[0164] Zr: 0~0.030%,

[0165] W: 0~0.10%,

[0166] Te: 0~0.030%, and

[0167] Sb: 0~0.030%;

[0168] The remainder is Fe and impurities.

[0169] In addition, the wire rod of the present invention has a total area ratio of ferrite and martensite in the central portion within 1.0 mm from the center in a cross section perpendicular to the longitudinal direction (sometimes referred to as "area ratio of ferrite and martensite" in the present invention) of 5% or less, and the remainder is a mixed structure containing ferrite and cementite.

[0170] When the content of C in the wire rod expressed in mass % is defined as [C], the average Vickers hardness (sometimes referred to as "average hardness" in the present invention) H in the cross section satisfies the formula (1),

[0171] 480×[C]-40 <H<480×[C]+40···(1)

[0172] The surface Vickers hardness Hs at a depth of 0.5 to 1.0 mm from the surface (sometimes referred to as "surface hardness" in the present invention) and the average Vickers hardness H satisfy the relationship of formula (2),

[0173] 0≤Hs-H<30 ···(2)

[0174] The central Vickers hardness (sometimes referred to as "center hardness" in the present invention) Hc within 1.0 mm from the center and the average Vickers hardness H satisfy the relationship of formula (3),

[0175] 0≤H-Hc<30 ···(3)

[0176] The wire diameter is 7.0mm or more.

[0177] The inventors of the present invention discovered the wire rod and steel wire of the present invention through the following studies.

[0178] In order to carry out wire drawing and obtain given wire diameter and intensity and well keep torsional characteristic, preferably the mixed organization comprising ferrite and cementite such as pearlite, bainite of work hardening ability excellence is used as main organization.When the organization (ferrite organization, martensite organization) that hardly contains cementite is mixed, torsional characteristic reduces.It should be noted that, if pearlite and bainite hardness are equal, the characteristic after wire drawing is similar, is also difficult to distinguish from photograph as organization, therefore pearlite and bainite are not distinguished in the present invention.

[0179] In addition, in steel wires used with a wire diameter of 5.0mm to 7.0mm, such as steel wires for bridges, the wire diameter of the wire rods often exceeds 7.0mm. In the case of thick wire rods with a wire diameter exceeding 7.0mm, it is difficult to uniformly form pearlite and bainite structures in the cross section. There is a difference between the pearlite transformation temperature and the bainite transformation temperature in the surface layer with a high cooling rate and the center portion with a low cooling rate. Therefore, the surface layer tends to harden compared to the center portion. The hardness distribution formed by the wire rod continues after the wire drawing process, and the surface layer also becomes a hard distribution in the steel wire. In the torsion test, the strain is concentrated on the surface layer, so the torsion characteristics become unfavorable in the hard surface distribution.

[0180] Therefore, the inventors of the present invention studied how to sufficiently delay the phase transformation so that even in the center portion, where the cooling rate is slow, pearlite or bainite transformation can occur at a temperature comparable to that of the surface layer. The inventors of the present invention conducted repeated studies and found that Mo has the effect of delaying the phase transformation. The addition of Mo improves the uniformity of the hardness distribution in the cross section of the wire (a cross section perpendicular to the longitudinal direction of the wire). Furthermore, they confirmed that steel wire containing a predetermined amount of Mo exhibits excellent torsional properties even with a tensile strength of 2050 MPa or more.

[0181] <Chemical Composition>

[0182] Hereinafter, the chemical composition (content of each element) of the wire material of the present invention will be described.

[0183] C: 0.80~1.10%

[0184] C is a component necessary for increasing the tensile strength of a wire rod and a steel wire obtained by wire drawing.

[0185] When the C content is less than 0.80%, the tensile strength is insufficient.

[0186] On the other hand, if the C content of the wire rod is too high, the wire rod becomes hardened and the torsional properties deteriorate. If the C content of the wire rod exceeds 1.10%, it is difficult to suppress the formation of proeutectoid cementite, and the target torsional properties cannot be achieved even if other requirements are met.

[0187] From the perspective of tensile strength, the C content is preferably 0.85% or more, and particularly 0.90% or more, exhibiting better properties. On the other hand, from the perspective of torsional properties, the C content is preferably 1.05% or less, and particularly 1.00% or less, exhibiting better properties.

[0188] Si: 0.10~1.50%

[0189] Si is a component effective in improving the tensile strength of a wire rod and a steel wire obtained by wire drawing.

[0190] When the Si content of the wire rod is less than 0.10%, the effect of containing Si cannot be fully obtained.

[0191] On the other hand, if the Si content of the wire rod exceeds 1.50%, it becomes difficult to suppress the formation of a martensite structure that is a hard phase, and even if other requirements are satisfied, the target torsional characteristics cannot be obtained.

[0192] From the perspective of tensile strength, the Si content is preferably 0.50% or more, and particularly 0.70% or more, exhibiting better properties. On the other hand, from the perspective of torsional properties, the Si content is preferably 1.40% or less, and particularly 1.30% or less, exhibiting better properties.

[0193] Mn: 0.10~1.00%

[0194] Mn is a component that is effective in increasing the tensile strength of wire rods and steel wires obtained after wire drawing. Mn also has the function of fixing S in steel as MnS, thereby suppressing hot brittleness.

[0195] When the Mn content of the wire rod is less than 0.10%, the effect of including Mn cannot be fully obtained.

[0196] On the other hand, if the wire rod contains more than 1.00% of Mn, it becomes difficult to suppress the formation of a martensite structure that is a hard phase, and even if other requirements are satisfied, the target torsional characteristics cannot be obtained.

[0197] From the viewpoint of tensile strength, the Mn content is preferably 0.25% or more. On the other hand, from the viewpoint of torsional properties, the Mn content is preferably 0.80% or less.

[0198] P: 0.030% or less

[0199] P is an element that segregates at the grain boundaries of the wire rod and degrades the torsional characteristics.

[0200] When the P content of the wire rod is 0.030% or less, a decrease in torsional characteristics is suppressed, and other requirements are also satisfied, thereby achieving the target torsional characteristics.

[0201] The upper limit of the P content is preferably 0.025%, more preferably 0.020% or less. It should be noted that the lower limit of the P content is not limited and can be 0% (i.e., not containing any P). From the perspective of reducing the cost of P removal, it can exceed 0% and can also be 0.001% or more.

[0202] S: 0.030% or less

[0203] S is an element that reduces torsional characteristics.

[0204] When the S content of the wire rod is 0.030% or less, the target torsional characteristics can be obtained by satisfying other requirements.

[0205] The upper limit of the S content is preferably 0.020%. It should be noted that the lower limit of the S content is not limited, and from the perspective of reducing the cost of desulfurization, it may be more than 0% or may be 0.001% or more.

[0206] N: 0.0120% or less

[0207] N is an element that reduces torsional characteristics.

[0208] When the N content of the wire rod is 0.0120% or less, the target torsional characteristics can be obtained by satisfying other requirements.

[0209] The preferred upper limit of the N content is 0.0100%, and the more preferred upper limit is 0.0070%. It should be noted that the lower limit of the N content is not limited, and from the perspective of reducing refining costs, it can exceed 0% and can also be 0.0001% or more.

[0210] O: 0.0100% or less

[0211] O is an element that easily forms oxide inclusions in the wire rod.

[0212] When the O content of the wire rod is 0.0100% or less, the coarsening of oxide inclusions is suppressed, and a decrease in torsional characteristics can be suppressed.

[0213] The preferred upper limit of the O content is 0.0070%, and the more preferred upper limit is 0.0050%. It should be noted that the lower limit of the O content is not limited, and from the perspective of reducing refining costs, it can exceed 0% and can also be 0.0001% or more.

[0214] Al: 0.005%~0.070%

[0215] Al is an element having a deoxidizing effect and is necessary to reduce the amount of oxygen in the wire.

[0216] When the Al content of the wire rod is less than 0.005%, it is difficult to obtain the effect of containing Al.

[0217] On the other hand, Al is an element that easily forms hard oxide inclusions. If the Al content of the wire exceeds 0.070%, coarse oxide inclusions are likely to be significantly formed, significantly reducing the drawability.

[0218] From the perspective of deoxidation effect, the Al content is preferably 0.010% or more, and may be 0.020% or more. On the other hand, from the perspective of wire drawing workability, the Al content is preferably 0.050% or less, and may be 0.040% or less.

[0219] Mo: 0.02~0.20%

[0220] Mo is an element that can delay pearlite and bainite transformations in trace amounts, and is particularly effective in increasing the strength of the center portion of wires with a wire diameter of 7.0 mm or greater. It also increases the tensile strength of the steel wire obtained after wire drawing.

[0221] When the Mo content of the wire rod is less than 0.02%, the effect of including Mo cannot be fully obtained.

[0222] On the other hand, if the Mo content of the wire rod exceeds 0.20%, it becomes difficult to suppress the formation of a martensite structure that is a hard phase, and even if other requirements are satisfied, the target torsional characteristics cannot be obtained.

[0223] From the perspective of tensile strength, the Mo content is preferably 0.025% or more, and particularly 0.03% or more, exhibiting better properties. On the other hand, from the perspective of torsional properties, the Mo content is preferably 0.15% or less, and particularly 0.12% or less, exhibiting better properties.

[0224] The wire rod of the present invention may also contain one or more of Cr, Cu, Ni, Sn, V, Ti, Nb, B, REM, Mg, Ca, Zr, W, Te, and Sb as optional elements to replace a portion of Fe. These optional elements may not be included (i.e., 0%), or they may be included within the following ranges. The lower limit of the content when these optional elements are included may exceed 0%. For example, one or more selected from Cr, Cu, Sn, Ni, and V may be included, and one or more selected from Ti, Nb, B, REM, Mg, Ca, Zr, W, Te, and Sb may be included.

[0225] Cr: 0~1.00%

[0226] The inclusion of Cr is optional.

[0227] Cr has the effect of increasing the tensile strength of the wire rod and the steel wire obtained after wire drawing. In order to stably obtain this effect, the Cr content is preferably set to 0.03% or more.

[0228] When the Cr content exceeds 1.00%, it becomes difficult to suppress the formation of a martensite structure which is a hard phase, and the torsional properties deteriorate.

[0229] Therefore, when actively including Cr in the wire rod, the Cr content is preferably within the range of 0.03 to 1.00%. Alternatively, it may be preferably 0.85% or less, and more preferably 0.10 to 0.70%.

[0230] Cu: 0~0.80%

[0231] The inclusion of Cu is optional.

[0232] Cu has the effect of improving the corrosion resistance of the wire rod and the steel wire obtained after wire drawing. In order to stably obtain this effect, the Cu content is preferably set to 0.01% or more.

[0233] On the other hand, even if the Cu content of the wire exceeds 0.80%, the effect is saturated.

[0234] Therefore, when Cu is actively included in the wire rod, the Cu content is preferably within the range of 0.01 to 0.80%, and more preferably 0.05 to 0.60%.

[0235] Sn: 0~0.50%

[0236] The inclusion of Sn is optional.

[0237] Sn improves the corrosion resistance of wire rods and steel wires obtained after wire drawing. To stably achieve this effect, the Sn content is preferably set to 0.005% or more.

[0238] On the other hand, even if the Sn content of the wire exceeds 0.50%, the effect is saturated.

[0239] Therefore, when Sn is actively included in the wire rod, the Sn content is preferably within the range of 0.001 to 0.50%, and more preferably 0.005 to 0.40%.

[0240] Ni: 0~0.50%

[0241] The inclusion of Ni is optional.

[0242] Ni has the effect of improving the corrosion resistance of the wire rod and the steel wire obtained after wire drawing. In order to stably obtain this effect, the Ni content is preferably set to 0.01% or more.

[0243] On the other hand, even if the Ni content of the wire material exceeds 0.50%, the effect is saturated.

[0244] Therefore, when Ni is actively included in the wire rod, the Ni content is preferably within the range of 0.01 to 0.50%, and more preferably 0.05 to 0.40%.

[0245] V: 0~0.15%

[0246] The inclusion of V is optional.

[0247] V has the effect of increasing the tensile strength of the wire rod and the steel wire obtained after wire drawing. In order to stably obtain this effect, the V content of the wire rod is preferably set to 0.01% or more.

[0248] On the other hand, when the V content of the wire rod exceeds 0.15%, the torsional characteristics deteriorate.

[0249] Therefore, when V is actively contained in the wire rod, the V content of the wire rod is preferably 0.02% to 0.15%, more preferably 0.03% to 0.13%, and even more preferably 0.05% to 0.12%.

[0250] Ti: 0~0.050%

[0251] The inclusion of Ti is optional.

[0252] Ti forms carbides or carbonitrides in the wire, improving torsional properties. To achieve this effect, the Ti content in the wire is preferably 0.002% or more.

[0253] On the other hand, when the Ti content of the wire rod exceeds 0.050%, coarse carbides or carbonitrides are likely to be formed, and the torsional characteristics are deteriorated.

[0254] Therefore, when Ti is actively contained in the wire rod, the Ti content of the wire rod is preferably 0.002 to 0.050%, and more preferably 0.005 to 0.030%.

[0255] Nb: 0~0.050%

[0256] The inclusion of Nb is optional.

[0257] Nb forms carbides or carbonitrides in the wire, improving torsional properties. To achieve this effect, the Nb content in the wire is preferably 0.002% or more.

[0258] On the other hand, when the Nb content of the wire rod exceeds 0.050%, coarse carbides or carbonitrides are likely to be formed, and the torsional characteristics are deteriorated.

[0259] Therefore, when Nb is actively contained in the wire rod, the Nb content of the wire rod is preferably 0.002 to 0.050%, and more preferably 0.005 to 0.030%.

[0260] B: 0~0.0040%

[0261] The inclusion of B is optional.

[0262] B has the effect of suppressing ferrite structure and improving torsional properties. To achieve this effect, the B content of the wire rod is preferably 0.0003% or more.

[0263] On the other hand, when the B content of the wire rod exceeds 0.0040%, coarse carbides are likely to be formed, and the torsional characteristics are deteriorated.

[0264] Therefore, when B is actively contained in the wire rod, the B content of the wire rod is preferably 0.0003 to 0.0040%, and more preferably 0.0006 to 0.0030%.

[0265] REM: 0~0.030%

[0266] The inclusion of REM is optional.

[0267] If REM is included, high torsional properties can be more stably exhibited. To achieve this effect, the REM content of the wire rod is preferably 0.002% or more.

[0268] On the other hand, if the REM content of the wire rod exceeds 0.030%, the effect is saturated.

[0269] Therefore, when REM is actively included, the REM content of the wire rod is preferably 0.002 to 0.030%.

[0270] It should be noted that REM refers to a total of 17 elements including Sc, Y, and lanthanum-based elements. The REM content refers to the content of a single REM element and to the total content of the REM elements when two or more REM elements are present.

[0271] Mg: 0~0.0040%

[0272] The inclusion of Mg is optional.

[0273] If Mg is included, high torsional properties can be more stably exhibited. To achieve this effect, the Mg content of the wire rod is preferably set to 0.0002% or more.

[0274] On the other hand, if the Mg content of the wire exceeds 0.0040%, the effect is saturated.

[0275] Therefore, when Mg is actively contained in the wire rod, the Mg content of the wire rod is preferably 0.0002 to 0.0040%.

[0276] Ca: 0~0.0040%

[0277] The inclusion of Ca is optional.

[0278] If Ca is included, high torsional properties can be more stably exhibited. To achieve this effect, the Ca content of the wire rod is preferably 0.0002% or more.

[0279] On the other hand, if the Ca content of the wire rod exceeds 0.0040%, the effect is saturated.

[0280] Therefore, when Ca is actively contained in the wire rod, the Ca content of the wire rod is preferably 0.0002 to 0.0040%.

[0281] Zr: 0~0.030%

[0282] The inclusion of Zr is optional.

[0283] If Zr is included, high torsional properties can be exhibited more stably. To achieve this effect, the Zr content of the wire rod is preferably set to 0.002% or more.

[0284] On the other hand, when the Zr content of the wire rod exceeds 0.030%, coarse carbides or carbonitrides are likely to be formed, and the torsional characteristics are degraded.

[0285] Therefore, when Zr is actively contained in the wire rod, the Zr content of the wire rod is preferably 0.002 to 0.030%.

[0286] W: 0~0.10%

[0287] The inclusion of W is optional.

[0288] High torsional properties can be more stably exhibited if W is contained. To achieve this effect, the W content of the wire rod is preferably 0.02% or more.

[0289] On the other hand, if the W content of the wire exceeds 0.10%, the effect is saturated.

[0290] Therefore, when W is actively contained in the wire rod, the W content of the wire rod is preferably 0.02 to 0.10%.

[0291] Te: 0~0.030%

[0292] The inclusion of Te is optional.

[0293] If Te is included, high torsional properties can be exhibited more stably. To achieve this effect, the Te content of the wire rod is preferably 0.001% or more.

[0294] On the other hand, when the Te content of the wire exceeds 0.030%, the effect is saturated.

[0295] Therefore, when Te is actively contained in the wire rod, the Te content of the wire rod is preferably 0.001 to 0.030%.

[0296] Sb: 0~0.030%

[0297] The inclusion of Sb is optional.

[0298] If Sb is included, high torsional properties can be more stably exhibited. To achieve this effect, the Sb content of the wire rod is preferably 0.001% or more.

[0299] On the other hand, if the Sb content of the wire exceeds 0.030%, the effect is saturated.

[0300] Therefore, when Sb is actively contained in the wire rod, the Sb content of the wire rod is preferably 0.001 to 0.030%.

[0301] <Metallic Structure>

[0302] Next, the metal structure of the wire rod of the present invention will be described.

[0303] (Total area ratio of ferrite and martensite in the center of the cross section)

[0304] The total area ratio of the ferrite structure and martensite structure in the center of the cross section of the wire rod (the cross section perpendicular to the longitudinal direction) is 5% or less. This reduces the torsional properties of the steel wire after wire drawing. If the total area ratio of the ferrite structure and martensite structure in the center of the wire rod is 5% or less, the total area ratio of the ferrite structure and martensite structure including the surface layer is low, so the center is used as a representative for measurement. The total area ratio of the ferrite structure and martensite structure in the center of the cross section of the wire rod can be 4% or less, or 2% or less, or 1% or less. It should be noted that one of the ferrite structure and martensite structure may be included, or both may not be included. The same applies to the structure in the longitudinal cross section of the steel wire described below.

[0305] <Hardness>

[0306] Next, the hardness of the wire material of the present invention will be described.

[0307] (Average Vickers hardness H)

[0308] The average Vickers hardness H in the cross section of the wire rod of the present invention satisfies the formula (1).

[0309] 480×[C]-40 <H<480×[C]+40···(1)

[0310] The average Vickers hardness H in the cross section depends on the mass % carbon content [C] of the wire rod. If the average Vickers hardness is less than "480 × [C] - 40", it is difficult to stably impart the required tensile strength (e.g., 2050 MPa or more) to the steel wire after wire drawing.

[0311] On the other hand, if the average value of the Vickers hardness in the cross section exceeds "480×[C]+40", micro cracks are likely to occur during wire drawing, and even if other requirements are met, it is impossible to obtain a tensile strength of 2050 MPa or more as a steel wire, and it is impossible to achieve the target number of twists of 12 or more in the torsion test in the following examples.

[0312] The average Vickers hardness H may be in a range satisfying the following formula (1A) or in a range satisfying the following formula (1B).

[0313] 480×[C]-30 <H<480×[C]+30 ···(1A)

[0314] 480×[C]-20 <H<480×[C]+20 ···(1B)

[0315] (Relationship between surface Vickers hardness Hs and average Vickers hardness H)

[0316] In the case of the wire rod of the present invention, the surface Vickers hardness Hs at a depth of 0.5 to 1.0 mm from the surface and the average Vickers hardness H satisfy the relationship of formula (2):

[0317] 0≤Hs-H<30 ···(2)

[0318] When the surface Vickers hardness Hs is 30 or more higher than the average Vickers hardness H, micro cracks are likely to occur during a torsion test, and the target torsion test results cannot be obtained even if other requirements are met.

[0319] On the other hand, it is industrially difficult to make the surface layer Vickers hardness Hs lower than the average Vickers hardness H for a typical rolled wire rod.

[0320] The relationship between the surface layer Vickers hardness Hs and the average Vickers hardness H may be in a range satisfying the following formula (2A) or in a range satisfying the following formula (2B).

[0321] 0≤Hs-H<25···(2A)

[0322] 0≤Hs-H<20 ···(2B)

[0323] (Relationship between center Vickers hardness Hc and average Vickers hardness H)

[0324] The central Vickers hardness Hc within 1.0 mm from the center in the cross section of the wire rod of the present invention and the average Vickers hardness H satisfy the relationship of formula (3).

[0325] 0≤H-Hc<30 ···(3)

[0326] When the center Vickers hardness Hc is smaller than the average Vickers hardness H by 30 or more, it is difficult to stably impart the tensile strength (for example, 2050 MPa or more) required for the steel wire after wire drawing.

[0327] It is industrially difficult to increase the center Vickers hardness Hc to a higher value than the average Vickers hardness H in conventional rolled wire rods.

[0328] The relationship between the center Vickers hardness Hc and the average Vickers hardness H may be in a range satisfying the following formula (3A) or in a range satisfying the following formula (3B).

[0329] 0≤H-Hc<25···(3A)

[0330] 0≤H-Hc<20 ···(3B)

[0331] <Wire diameter>

[0332] The wire diameter of the wire rod of the present invention is 7.0 mm or more. If the wire diameter of the wire rod is less than 7.0 mm, the effects of the present invention cannot be obtained.

[0333] On the other hand, the upper limit of the wire diameter of the wire rod of the present invention is not particularly limited, but it is difficult to industrially produce a wire rod having a wire diameter exceeding 20 mm by a normal production process.

[0334] The wire diameter of the wire material of the present invention can be 7.5-18.0 mm, or 8.0-16.0 mm.

[0335] [Steel Wire]

[0336] Next, the steel wire of the present invention will be described.

[0337] The steel wire of the present invention has the following chemical composition:

[0338] Contains by mass%

[0339] C: 0.80~1.10%,

[0340] Si: 0.10~1.50%,

[0341] Mn: 0.10~1.00%,

[0342] P: 0.030% or less,

[0343] S: 0.030% or less,

[0344] N: 0.0120% or less,

[0345] O: 0.0100% or less,

[0346] Al: 0.005~0.070%, and

[0347] Mo: 0.02~0.20%;

[0348] Optionally include:

[0349] Cr: 0~1.00%,

[0350] Cu: 0~0.80%,

[0351] Sn: 0~0.50%,

[0352] Ni: 0~0.50%,

[0353] V: 0~0.15%,

[0354] Ti: 0~0.050%,

[0355] Nb: 0~0.050%,

[0356] B: 0~0.0040%,

[0357] REM: 0~0.030%,

[0358] Mg: 0~0.0040%,

[0359] Ca: 0~0.0040%,

[0360] Zr: 0~0.030%,

[0361] W: 0~0.10%,

[0362] Te: 0~0.030%, and

[0363] Sb: 0~0.030%;

[0364] The remainder is Fe and impurities.

[0365] Furthermore, in a cross section parallel to the longitudinal direction and passing through the central axis of the steel wire of the present invention, the combined area ratio of the ferrite structure and the martensite structure in the central portion is 5% or less, and the remainder is a mixed structure containing ferrite and cementite.

[0366] The average Vickers hardness h in the cross section perpendicular to the length direction is 450~620.

[0367] The surface Vickers hardness hs and the average Vickers hardness h at a depth of 0.2~0.5mm from the surface satisfy the relationship of formula (4).

[0368] 0≤hs-h<30 ···(4)

[0369] The central Vickers hardness hc and the average Vickers hardness h within 1.0 mm from the center in the vertical cross section satisfy the relationship of formula (5).

[0370] 0≤h-hc<30 ···(5)

[0371] When the wire rod of the present invention is drawn within a drawing strain range of 1.0 to 2.5 and the average Vickers hardness is controlled to 450 to 620, the difference (hs-h) between the surface Vickers hardness and the average Vickers hardness and the difference (h-hc) between the center Vickers hardness and the average Vickers hardness are each less than 30. By thus achieving a hardness distribution in which the surface is not too hard and the center is not too soft relative to the average hardness, a steel wire having high strength and excellent torsional properties can be obtained.

[0372] The steel wire of the present invention may be plated with zinc or a zinc alloy, or subjected to heat input through degreasing. That is, the steel wire of the present invention also includes steel wire with a plated surface (plated wire). It should be noted that when the steel wire of the present invention has a plating layer, the chemical composition, metal structure, and hardness in the present invention refer to the values ​​of the steel portion.

[0373] The steel wire of the present invention may also contain the same optional elements as those in the chemical composition of the aforementioned wire rod.

[0374] The wire diameter of the steel wire of the present invention is not particularly limited, and may be, for example, 3.0 to 8.0 mm, or 4.0 to 7.5 mm.

[0375] <Measurement Method>

[0376] Next, the area ratio of the metal structure and the hardness of the wire rod and steel wire of the present invention are described. It should be noted that the values ​​in the following examples are measured based on the following measurement methods.

[0377] In the following description, the diameter of the wire rod or steel wire to be measured is referred to as D.

[0378] (Measurement of Area Ratio of Wire Material Structure)

[0379] Scanning electron microscopy (SEM) was performed at 2.7 × 10 -3 mm 2 Five images were taken of the center and 200 μm above, below, and to the left and right of the center in a cross section of an area (0.045 mm long and 0.060 mm wide). Note that before SEM observation, the cross section was mirror-polished and then etched with picrol to reveal the metal structure.

[0380] Figure 1 (A) to (D) represent pearlite, martensite, and ferrite, respectively, as an example of a mixed structure in which ferrite and cementite are mixed. Figure 1(C) is marked as "ferrite"), an example of a mixed structure including ferrite and cementite. Ferrite and martensite are structures that do not contain cementite, and the mixed structure including ferrite and cementite is pearlite, bainite, etc., and cementite is mixed in ferrite. For example, in (C), there is a pearlite structure (an example of a mixed structure of ferrite and cementite) as a layered structure of ferrite (blackened part) and cementite (whitish part), and there is no cementite in the arrow part, which is different from the surrounding pearlite structure, so it is judged to be a ferrite structure. On the other hand, in (A), the two arrow parts are structures in which ferrite exists more than cementite, and the split cementite exists in ferrite, which is judged to be a mixed structure of ferrite and cementite. The area ratio (%) of ferrite and martensite is measured in each SEM photograph. Ferrite and martensite, which are structures that do not contain cementite, do not need to be distinguished between the two. Specifically, after printing each SEM image on paper, a transparent sheet such as an OHP (Over Head Projector) sheet was superimposed on the paper, and the structure (ferrite structure and martensite structure) without cementite was colored. Then, the transparent sheet colored on the ferrite structure and martensite structure was analyzed by image analysis to measure the total area ratio of the ferrite structure and martensite structure. The area per field of view was set to 2.7×10 -3 mm 2 (0.045 mm in length and 0.060 mm in width), and image analysis software (e.g., LUZEX AP manufactured by NIRECO) was used for image analysis. The average value was calculated from the total area ratio of the ferrite structure and martensite structure in the five images, and the average value was used as the total area ratio of the ferrite structure and martensite structure of the wire rod.

[0381] (Measurement of Area Ratio of Steel Wire Structure)

[0382] Scanning electron microscopy (SEM) was used to -4 mm 2 Five images of the center of the longitudinal section (0.015 mm in length, 0.02 mm in width) were taken at intervals of 200 μm. Note that before SEM observation, the longitudinal section of the steel wire was mirror-polished and then etched with picrol to visualize the metal structure.

[0383] Figure 2(A), (B) represent an example of a SEM photo of a longitudinal section of a steel wire comprising a ferrite structure or a martensite structure and a mixed structure comprising ferrite and cementite. In addition, (a) is a diagram in which the arrow portion of (A) is amplified, and (b) is a diagram in which the arrow portion of (B) is amplified. Ferrite and martensite are tissues that do not contain cementite, and the mixed structure comprising ferrite and cementite is a tissue obtained by drawing pearlite, bainite, etc., with cementite mixed in ferrite. The area ratio (%) of ferrite and martensite is measured in each SEM photo. Ferrite and martensite as tissues that do not contain cementite do not need to be distinguished from each other. Specifically, after each SEM image is printed on paper, a transparent sheet such as an OHP (Over Head Projector) sheet is overlapped on paper to color ferrite and martensite. Then, the colored transparent sheet is analyzed by image analysis to thereby measure the total area ratio of ferrite and martensite. The area of ​​each field of view is set to 3×10 -4 mm 2 The image was analyzed using image analysis software (e.g., LUZEX AP manufactured by NIRECO). The average area ratio of the ferrite and martensite structures in the five images was calculated and used as the total area ratio of the ferrite and martensite structures of the steel wire.

[0384] (Determination of wire hardness)

[0385] Using a Vickers hardness tester, an indenter was pressed into the cross section of the wire rod with a load of 1 kgf.

[0386] Starting at a depth of 0.5 mm from the surface (outer circumference) of the wire rod, make dots at 0.5 mm intervals toward the center. Make dots in four directions at 90° intervals.

[0387] The depth within 0.5 to 1.0 mm from the surface was defined as the surface layer, and the average value of the Vickers hardness at a total of 8 points punched at distances of 0.5 mm and 1.0 mm from the surface was defined as the surface Vickers hardness Hs.

[0388] On the other hand, the range of 1.0 mm from the center (central axis of the wire) in the cross section is defined as the central portion, and the hardness of the points punched at the center and at distances of 0.5 mm and 1.0 mm from the center is averaged over a total of 9 points as the central Vickers hardness Hc.

[0389] In addition, the average value of the Vickers hardness measured at the center of the entire cross section, that is, the cross section and at intervals of 0.5 mm from the surface to the center (excluding the area less than 0.3 mm from the center) is used as the average Vickers hardness. For example, if the wire diameter is 12 mm, the average value of the Vickers hardness measured at 1 point in the center and 11 points between the position 0.5 mm from the surface and the position 0.5 mm from the center × 4 directions = 44 points, a total of 45 points, is used as the average Vickers hardness H. In addition, for example, in the case of a wire with a wire diameter of 8.4 mm, the average value of the hardness measured at 1 point in the center and 7 points between 0.5 mm and 3.5 mm from the surface (the position 4.0 mm from the surface is 0.2 mm from the center, so it is excluded) × 4 directions = 28 points, a total of 29 points, is used as the average Vickers hardness H.

[0390] (Determination of the hardness of steel wire)

[0391] Using a Vickers hardness tester, an indenter was pressed into the cross section of the steel wire with a load of 1 kgf.

[0392] Starting at a depth of 0.2 mm from the surface (outer periphery) of the steel wire, dot the wire at 0.2 mm intervals toward the center. Dot the wire in four directions at 90° intervals. For plated steel wire, start at a depth of 0.2 mm from the surface of the steel portion.

[0393] The average Vickers hardness of points punched within a depth of 0.2 to 0.5 mm from the surface of the wire was taken as the surface Vickers hardness hs, and the average hardness of points punched within 1 mm from the center was taken as the center Vickers hardness hc.

[0394] The average Vickers hardness is the average value of the Vickers hardness measured at the center of the cross section and at intervals of 0.2 mm from the surface toward the center (excluding areas less than 0.2 mm from the center). For example, in the case of a 5.4 mm diameter steel wire, the average value of the hardness measured at 49 locations (1 location at the center and 12 locations between 0.2 mm and 2.4 mm from the surface (excluding the location 2.6 mm from the surface, which is 0.1 mm from the center) x 4 directions = 48 locations) is the average Vickers hardness h.

[0395] (Torsion test of steel wire)

[0396] The number of times the wire broke was evaluated, and five wires were tested for each test, and the minimum value was evaluated.

[0397] The distance between the chucks was 100×wire diameter D. The rotation speed was 20 rpm.

[0398] It should be noted that the term "wire breakage" herein includes not only the entire wire breakage but also local cracking of the wire. Specifically, when a local crack occurs in the wire during the test, the number of twists at that point in time is used for evaluation.

[0399] (Tensile test)

[0400] The wire used for the tensile test can be straightened. The wire length is set to 340 mm, the distance between the chucks is set to 200 mm, and the tensile test is performed at a stroke speed of 10 mm / min.

[0401] The diameter of the wire was measured in two orthogonal directions using a vernier caliper at the center of the wire's length and the average value was taken. The tensile strength was calculated as the cross-sectional area (mm) calculated from the maximum load (N) during the tensile test / the diameter of the wire. 2 ).

[0402] Tensile testing and diameter measurement of steel wire can be performed in the same manner as for wire rods. For plated wire, the plating is stripped and tensile testing is performed on the steel wire alone. Plating stripping can be performed chemically, such as by immersion in hydrochloric acid, or physically, such as by polishing.

[0403] <Methods for producing wire rods and steel wires>

[0404] The method for producing the wire rod and the steel wire of the present invention is not particularly limited. Hereinafter, an example of a suitable production method will be described.

[0405] (Casting of Slabs)

[0406] For example, a method for producing a cast slab having the above chemical composition is as follows: in the case of continuous casting, after melting in a converter, the molten steel is thoroughly electromagnetically stirred, and then rolled during solidification to obtain a cast slab.

[0407] (Rolling of Cast Slabs)

[0408] The cast slab is heated to 1200-1250°C and then rolled to obtain a steel sheet.

[0409] (Rolling of Wire Rod)

[0410] The steel sheet is then rolled to obtain a rolled wire rod. The heating temperature of the steel sheet is 1020°C to 1080°C.

[0411] When heating at a temperature lower than 1020°C, the reaction force becomes large and rolling becomes difficult.

[0412] On the other hand, if the temperature exceeds 1080°C, the austenite structure coarsens, and during immersion in a molten salt described below, untransformed austenite may remain and martensite may be mixed.

[0413] The outlet temperature of finishing rolling (finishing rolling temperature) is 900~1000℃.

[0414] It is industrially difficult to lower the finishing rolling temperature to below 900°C.

[0415] On the other hand, if the finish rolling temperature exceeds 1000° C., the austenite structure coarsens, and untransformed austenite may remain during immersion in a molten salt, and martensite may be mixed therein.

[0416] (Cooling before coiling)

[0417] After finishing rolling, the steel is cooled to 800°C~900°C (temperature before coiling) by water cooling or air cooling.

[0418] When the temperature is lower than 800°C, the temperature drops excessively before immersion in the molten salt, pearlite transformation starts from the surface layer, and the surface layer softens.

[0419] When the temperature exceeds 900°C, the austenite structure coarsens, and during the subsequent immersion in a molten salt, untransformed austenite may remain and martensite may be mixed.

[0420] (Pre-cooling of molten salt)

[0421] Cooling is performed before immersion in the molten salt.

[0422] Before immersion in the molten salt, the steel is cooled at a rate of 10°C / second or more to a temperature of 740°C to 800°C (temperature before molten salt).

[0423] When the temperature is lower than 740℃, pearlite transformation starts from the surface layer, softening the surface layer. Coarse pearlite reduces the torsional properties.

[0424] When the temperature exceeds 800°C, the temperature of the molten salt rises and pearlite transformation cannot proceed at the target temperature.

[0425] Furthermore, when the cooling rate is lower than 10°C / s, the torsional properties are degraded due to the precipitation of proeutectoid cementite. The upper limit of the cooling rate is not particularly limited, but from a technical point of view, it can be 50°C / s or lower.

[0426] (Molten salt immersion)

[0427] After cooling to a temperature of 740°C to 800°C, the steel is immersed in a molten salt to transform into pearlite or bainite.

[0428] The molten salt bath was set at 540°C to 570°C, and the immersion time was 60 seconds to 120 seconds.

[0429] When the temperature of the molten salt is lower than 540° C., the ferrite structure increases. On the other hand, when the temperature of the molten salt exceeds 570° C., the cooling rate inside the wire rod is insufficient, causing softening.

[0430] If the immersion time in the molten salt is less than 60 seconds, untransformed austenite may remain and martensite may be mixed in. If the immersion time is longer than 120 seconds, cementite may spheroidize after transformation, resulting in a decrease in strength.

[0431] (Wire drawing)

[0432] The wire rod produced through the above steps can be subjected to wire drawing to obtain a steel wire having high strength and excellent torsional properties.

[0433] The wire drawing strain ε due to the wire drawing process is expressed by the following formula.

[0434] ε=Ln(D0 / D) 2

[0435] D0 is the diameter before wire drawing, and D is the diameter after wire drawing. ε is preferably set to be greater than 1.0 and less than 2.5. When the wire drawing strain is less than 1.0, it is difficult to show strength. When it is greater than 2.5, the processing rate is too large and the torsional properties are reduced. It should be noted that it is preferred to perform surface lubrication treatment such as zinc phosphate film or borax film before wire drawing.

[0436] The above process allows the production of high-strength steel wire with excellent torsional properties. It should be noted that post-treatments such as plating and degreasing can also be performed after wire drawing. Degreasing and zinc or zinc alloy plating are preferably performed between 400°C and 530°C. Below 400°C, the steel becomes hardened due to age hardening, and torsional properties deteriorate. Above 530°C, softening due to spheroidization of cementite becomes significant, resulting in insufficient strength.

[0437] <Purpose>

[0438] The steel wire obtained by drawing the wire rod of the present invention is not particularly limited in its uses and is suitable for various applications requiring high strength and torsional properties, such as steel wire for bridge cables and various steel ropes. The wire rod of the present invention is suitable as a raw material for steel wire used in these applications. For example, a plurality of steel wires of the present invention can be bundled to form a steel rope (strand). Alternatively, a plurality of zinc-plated steel wires of the present invention can be bundled to form a steel rope.

[0439] Example

[0440] The following examples will further describe the wire rod and steel wire of the present invention. However, these examples do not limit the wire rod and steel wire of the present invention.

[0441] <Example 1>

[0442] Steel materials having the chemical compositions (unit: mass %) shown in Table 1 were prepared, and wire rods were produced using the methods (conditions) shown in Table 2. Note that "-" in Table 1 indicates that the element content is at an impurity level and can be considered substantially absent. This also applies to "-" in Table 4 below. The remainder of the chemical compositions in Tables 1 and 4 consists of Fe and impurities.

[0443] [Table 1]

[0444]

[0445] [Table 2]

[0446]

[0447] The wire rods having a wire diameter of 10.0 to 14.0 mm produced by the above-described method were each measured for the total area ratio of the ferrite structure and the martensite structure at the center, the Vickers hardness, and the tensile strength by the above-described method.

[0448] In addition, the wire was drawn. If the wire had a wire diameter of 5.0 mm, that is, a wire with a wire diameter of 14.0 mm, a steel wire with a wire drawing strain of 2.06 was prepared. After aging at 450°C for 30 seconds, the Vickers hardness and tensile strength were measured by the above methods, and the number of torsion was measured by a torsion test.

[0449] When the tensile strength of the steel wire was 2050 MPa or more, it was judged that the tensile strength was good.

[0450] In addition, the number of times until the steel wire broke in the torsion test was 12 times or more and was evaluated as good (mark A), and the number of times less than 12 times was evaluated as poor (mark B).

[0451] The measurement results are shown in Table 3A and Table 3B.

[0452] [Table 3A]

[0453]

[0454] [Table 3B]

[0455]

[0456] The wire rods No. 1 to 4 were obtained by wire drawing to obtain steel wires having high strength (2050 MPa or more) and excellent torsional properties.

[0457] In Nos. 10 and 11, ferrite or martensite structures were mixed in the wire rods, and the area ratio of the ferrite or martensite structure was high, resulting in wire breakage during the wire drawing process.

[0458] The surface hardness and average hardness of the wire rod No. 12 were low, and the tensile strength of the steel wire after the wire drawing process was insufficient.

[0459] In No. 13, the wire rod had a mixed structure of ferrite or martensite, and the area ratio of the ferrite or martensite structure was high, resulting in wire breakage during the wire drawing process.

[0460] The surface hardness and average hardness of the wire rod No. 14 were low, and the tensile strength of the steel wire after wire drawing was insufficient.

[0461] In No. 15, the average hardness was low due to pearlite transformation at high temperature, and the tensile strength of the steel wire after wire drawing was insufficient.

[0462] In No. 16, due to the temperature difference between the surface and the interior of the wire rod, the difference between the average hardness of the wire rod and the hardness of the center portion was large, and the torsional characteristics of the steel wire after wire drawing were insufficient.

[0463] In No. 17, the wire rod had a mixed structure of ferrite or martensite, and the area ratio of the ferrite or martensite structure was high, resulting in wire breakage during the wire drawing process.

[0464] In No. 18, the average hardness of the wire rod was low due to pearlite transformation at high temperature, and the tensile strength of the steel wire after wire drawing was insufficient.

[0465] In No. 19, the wire rod had a mixed structure of ferrite or martensite, and the area ratio of the ferrite or martensite structure was high, resulting in wire breakage during the wire drawing process.

[0466] <Example 2>

[0467] Steel materials having the chemical compositions shown in Table 4 (unit: mass %) were prepared, and wire rods were manufactured under the conditions of Production Method A shown in Table 2.

[0468] [Table 4]

[0469]

[0470] The produced wire rods were measured for the total area ratio of the ferrite structure and the martensite structure, the Vickers hardness, and the tensile strength by the above-mentioned methods.

[0471] Furthermore, the wire rod was drawn to prepare a steel wire with a diameter of 5.0 mm. The total area ratio of the ferrite structure and martensite structure in the center, Vickers hardness, and tensile strength were measured by the above methods, and the number of torsion was measured by a torsion test.

[0472] The measurement results are shown in Table 5A and Table 5B.

[0473] [Table 5A]

[0474]

[0475] [Table 5B]

[0476]

[0477] The wire rods Nos. 20 to 33 satisfy the requirements of the present invention and, through wire drawing, provide steel wires having high strength (2050 MPa or more) and excellent torsional properties.

[0478] In No. 40, the amount of C was too small, and the tensile strength of the steel wire after wire drawing was insufficient.

[0479] In No. 41, the amount of C was too high, so the average hardness relative to the amount of C was small, and the torsional characteristics of the steel wire after wire drawing were insufficient.

[0480] In No. 42, the amount of Si was too high, so the area ratios of ferrite and martensite structures in the wire rod were high, and wire breakage occurred during the wire drawing process.

[0481] In No. 43, the Mn content was too high, resulting in a high area ratio of ferrite and martensite in the wire rod, causing wire breakage during wire drawing. In No. 44, the Mo content was too low, resulting in a low average hardness relative to the C content. The difference between the average hardness and the hardness in the center, as well as the difference between the surface hardness and the average hardness, was large, resulting in insufficient tensile strength and torsional properties of the wire after wire drawing.

[0482] No. 45 has a large amount of Mo, so martensite is mixed. The area ratio of the martensite structure in the wire rod is high, the average hardness is low, and wire breakage occurs during wire drawing.

[0483] In No. 46, the amount of Al was too high, and wire breakage occurred during the wire drawing process.

[0484] While the wire rod, etc. of the present invention has been described above, the wire rod, etc. of the present invention are not limited to the aforementioned embodiments and examples. For example, the method for manufacturing the wire rod of the present invention is not limited to the method of immersing the wire rod in a molten salt after winding. The scope of the present invention also includes the following methods: heating the wire rod, immersing the wire rod in a lead bath, and producing the wire rod of the present invention by lead sorbitization to form a metal structure; drawing the wire rod to produce the steel wire of the present invention; and bundling the steel wires to produce a steel rope (cable).

Claims

1. A wire having the following chemical composition: Contains by mass% C：0.80~1.10%、 Si: 0.10~1.50%, Mn: 0.10~1.00%, P: 0.030% or less, S: 0.030% or less, N: 0.0120% or less, O: 0.0100% or less, Al：0.005~0.070%、 Mo: 0.02~0.20%, Cr：0~1.00%、 Cu: 0~0.80%, Sn: 0~0.50%, Ni: 0~0.50%, V：0~0.15%、 Ti: 0~0.050%, Nb: 0~0.050%, B：0~0.0040%、 REM: 0~0.030%, Mg: 0~0.0040%, Ca: 0~0.0040%, Zr：0~0.030%、 W：0~0.10%、 Te: 0~0.030%, and Sb: 0~0.030%; The rest is Fe and impurities. In the wire rod, the total area ratio of the ferrite structure and the martensite structure in the central portion within 1.0 mm from the center in a cross section perpendicular to the longitudinal direction is 5% or less, and the remainder is a mixed structure containing ferrite and cementite. When the content of C in the wire rod expressed in mass % is defined as [C], the average Vickers hardness H in the cross section satisfies the formula (1), 480×[C]-40 <H<480×[C]+40 ···(1) The surface Vickers hardness Hs and the average Vickers hardness H at a depth of 0.5 to 1.0 mm from the surface satisfy the relationship of formula (2). 0≤Hs-H<30 ···(2) The central Vickers hardness Hc and the average Vickers hardness H within 1.0 mm from the center satisfy the relationship of formula (3), 0≤H-Hc<30 ···(3) The wire diameter is 7.0mm or more.

2. The wire according to claim 1, wherein The chemical composition comprises, by mass%, at least one or more chemical substances selected from the following: Cr：0.03~1.00%、 Cu: 0.01~0.80%, Sn: 0.001~0.50%, Ni: 0.01~0.50%, and V：0.01~0.15%。 3. The wire according to claim 1 or 2, wherein The chemical composition comprises, by mass%, one or more chemical substances selected from the following: Ti: 0.002~0.050%, Nb: 0.002~0.050%, B：0.0003~0.0040%、 REM: 0.002~0.030%, Mg: 0.0002~0.0040%, Ca: 0.0002~0.0040%, Zr：0.002~0.030%、 W：0.02~0.10%、 Te: 0.001~0.030%, and Sb: 0.001~0.030%.

4. A steel wire having the following chemical composition: Contains by mass% C：0.80~1.10%、 Si: 0.10~1.50%, Mn: 0.10~1.00%, P: 0.030% or less, S: 0.030% or less, N: 0.0120% or less, O: 0.0100% or less, Al：0.005~0.070%、 Mo: 0.02~0.20%, Cr：0~1.00%、 Cu: 0~0.80%, Sn: 0~0.50%, Ni: 0~0.50%, V：0~0.15%、 Ti: 0~0.050%, Nb: 0~0.050%, B：0~0.0040%、 REM: 0~0.030%, Mg: 0~0.0040%, Ca: 0~0.0040%, Zr：0~0.030%、 W：0~0.10%、 Te: 0~0.030%, and Sb: 0~0.030%; The rest is Fe and impurities. The total area ratio of the ferrite structure and the martensite structure in the central portion within 1.0 mm from the center in a cross section parallel to the longitudinal direction and passing through the central axis is 5% or less, and the remainder is a mixed structure containing ferrite and cementite. The average Vickers hardness h in the cross section perpendicular to the longitudinal direction is 450~620, The surface Vickers hardness hs and the average Vickers hardness h at a depth of 0.2~0.5mm from the surface satisfy the relationship of formula (4). 0≤hs-h<30 ···(4) The central Vickers hardness hc and the average Vickers hardness h within 1.0 mm from the center of the vertical cross section satisfy the relationship of formula (5), 0≤h-hc<30 ···(5).

5. The steel wire according to claim 4, wherein The chemical composition comprises, by mass%, at least one or more chemical substances selected from the following: Cr：0.03~1.00%、 Cu: 0.01~0.80%, Sn: 0.001~0.50%, Ni: 0.01~0.50%, and V：0.01~0.15%。 6. The steel wire according to claim 4 or 5, wherein The chemical composition comprises, by mass%, one or more chemical substances selected from the following: Ti: 0.002~0.050%, Nb: 0.002~0.050%, B：0.0003~0.0040%、 REM: 0.002~0.030%, Mg: 0.0002~0.0040%, Ca: 0.0002~0.0040%, Zr：0.002~0.030%、 W：0.02~0.10%、 Te: 0.001~0.030%, and Sb: 0.001~0.030%. 7 . The steel wire according to claim 4 , wherein the surface of the steel wire is plated.

8. A steel rope formed by bundling a plurality of steel wires according to any one of claims 4 to 7.

9. A method for manufacturing a steel rope, comprising: A step of bundling a plurality of the steel wires according to any one of claims 4 to 7 to form a steel rope.

Images