Wire material

Abstract

Provided is a wire material having excellent wire drawing workability. In the wire material according to the present embodiment, in a circular cross-section perpendicular to the axial direction in the wire material, when the average value of the true lamellar spacing at a depth position of 0.5 mm from the surface of the wire material is designated as LaveS (nm), and the maximum value of the true lamellar spacing is designated as LmaxS (nm), LmaxS / LaveS is 1.70 or less. In the circular cross-section, when the full width at half maximum of the true lamellar spacing at a depth position of 0.5 mm from the surface of the wire material is designated as FWHMs (nm), and the full width at half maximum of the true lamellar spacing at the center position of the wire material is designated as FWHMc (nm), FWHMc / FWHMs is 1.10 to 2.00.

Description

wire rod

[0001] The present disclosure relates to a wire rod, and more particularly to a wire rod suitable for use as a material for steel cords and bead wires for automobile tires, wires for bridges, and the like.

[0002] Steel wires such as steel cords made from wire rod are manufactured, for example, by the following manufacturing method. Wire rod with a wire diameter (diameter) of approximately 4 to 6 mm is prepared by hot rolling and then controlled cooling. The wire rod is subjected to a primary wiredrawing process to obtain a steel wire with a wire diameter of 3 to 4 mm. After the primary wiredrawing process, an intermediate patenting process is performed, followed by a secondary wiredrawing process to obtain a steel wire with a wire diameter of 1 to 2 mm. This steel wire is then subjected to a final patenting process. After the final patenting process, the steel wire is brass plated and then subjected to a final wet wiredrawing process to obtain a steel wire with a wire diameter of 0.15 to 0.40 mm. Furthermore, multiple steel wires are twisted together to obtain a stranded steel wire. Steel cords are manufactured through the above steps. Bead wires, bridge wires, etc. are also manufactured by patenting hot-rolled wire rods, drawing them, and then twisting them into a stranded steel wire.

[0003] When wire rods are drawn once or multiple times, wire breakage is more likely to occur if the amount of drawing is large. In order to prevent such breakage, a heat treatment called intermediate heat treatment is usually performed on the wire rod (or steel wire) before the wire drawing. However, from the viewpoint of reducing manufacturing costs and improving productivity, there is a demand for omission of the intermediate heat treatment. Therefore, there is a demand for further improvement in wire drawability of wire rods.

[0004] Techniques for improving wiredrawability have been proposed in International Publication No. 2018 / 117157 (Patent Document 1) and International Publication No. 2016 / 021556 (Patent Document 2).

[0005] The wire rod disclosed in Patent Document 1 has a chemical composition, in mass%, of 0.90 to 1.20% C, 0.10 to 1.00% Si, 0.20 to 0.80% Mn, 0.10 to 0.40% Cr, 0 to 0.002% Al, 0 to 0.002% Ti, 0 to 0.0050% N, 0 to 0.020% P, 0 to 0.010% S, 0 to 0.0040% O, 0 to 0.20% Mo, and 0 to 0.0030% B, with the balance being Fe and impurities. The average value of %Mn + 2 × %Cr throughout the wire rod is 0.50 to 1.00%. The metal structure is pearlite at an area fraction of 90% or more, with the remainder including one or more of ferrite, cementite, and bainite, with the cementite area fraction being less than 3%. The maximum grain size of TiN is less than 15 μm. As measured on a cross section perpendicular to the longitudinal direction of the wire, the maximum value of %Mn + 2 × %Cr in a region where the S content and O content are less than 1% in the central portion, which is a region from the central axis of the wire to 1 / 10 of the diameter of the wire, is 2.0 times or less the average value of %Mn + 2 × %Cr throughout the wire. As measured on a cross section perpendicular to the longitudinal direction of the wire, the ratio of the maximum value to the minimum value of %Mn + 2 × %Cr in a region where the S content and O content are less than 1% in the outer peripheral portion, which is a region from the outer edge of the central portion to a depth of 0.1 mm into the surface of the wire, is 2.0 or less. Here, %Mn and %Cr represent the contents of Mn and Cr in mass%. In this wire rod, the wire drawability is improved by adjusting the chemical composition, the metal structure mainly composed of pearlite, the size of TiN, and the segregation of Mn and Cr.

[0006] The wire rod disclosed in Patent Document 2 has a chemical composition, in mass%, of 0.70 to 1.20% C, 0.10 to 1.2% Si, 0.10 to 1.0% Mn, 0.001 to 0.012% P, 0.001 to 0.010% S, and 0.0010 to 0.0050% N, with the balance consisting of Fe and impurities. In this wire rod, in a cross section perpendicular to the longitudinal direction, the area ratio of pearlite is 95% to 100%, the average block grain size of pearlite is 10 μm to 30 μm, and the standard deviation of the block grain size is 20 μm or less. Furthermore, when C (%), Si (%), and Mn (%) are the contents of C, Si, and Mn, respectively, in mass%, and Ceq. is calculated using the following formula (1), the tensile strength is 760 × Ceq. + 255 MPa or more (760 × Ceq.). +325 MPa or less, and the reduction of area in a tensile test is -65 × Ceq. +96(%) or more, and the standard deviation of the reduction of area is 6% or less. Ceq. = C(%) + Si(%) / 24 + Mn(%) / 6 Formula (1) In this wire rod, the pearlite block size is adjusted to improve wiredrawability.

[0007] Furthermore, in Japanese Patent Laid-Open No. 2012-126955 (Patent Document 3), unlike Patent Documents 1 and 2, wiredrawability is improved by adjusting the lamellar spacing of pearlite. Specifically, the wire rod disclosed in Patent Document 3 contains, by mass%, 0.68 to 0.86% C, 0.05 to 0.5% Si, and 0.1 to 0.8% Mn, with the remainder being Fe and unavoidable impurities. The microstructure of this wire rod has 95 area % or more pearlite, the average lamellar spacing L of this pearlite is in the range of 150 to 300 nm, the region of the pearlite lamellae having a spacing of 120 nm or less is 30% or less, and the standard deviation of the lamellar spacing is 50 nm or less. Furthermore, the average nodule diameter D of this pearlite is 40 μm or less, and the average nodule diameter D and the average lamellar spacing L satisfy the relationship D<−0.1×L+60.

[0008] International Publication No. 2018 / 117157 International Publication No. 2016 / 021556 Japanese Patent Application Laid-Open No. 2012-126955

[0009] The techniques disclosed in Patent Documents 1 to 3 can also improve the drawability of wire rods. However, other techniques may also be used to improve the drawability of wire rods.

[0010] An object of the present invention is to provide a wire rod that is excellent in wire drawability.

[0011] The wire rod of the present disclosure has a chemical composition, in mass %, of C: 0.70 to 1.20%, Si: 0.10 to 1.50%, Mn: 0.10 to 1.00%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, N: 0.0010 to 0.0100%, O: 0.0050% or less, Cr: 0 to 0.50%, Co: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Mo: 0. The wire rod has a circular cross section perpendicular to its axial direction, and the average true lamellar spacing at a depth of 0.5 mm from the surface of the wire rod is defined as LaveS (nm), and the maximum true lamellar spacing at a depth of 0.5 mm from the surface of the wire rod is defined as LmaxS (nm), where LmaxS / LaveS is 1.70 or less. In the circular cross section, when the full width at half maximum of the true lamellar spacing at a depth of 0.5 mm from the surface of the wire is defined as FWHMs (nm) and the full width at half maximum of the true lamellar spacing at the center position of the wire is defined as FWHMc (nm), FWHMc / FWHMs is 1.10 to 2.00.

[0012] The wire rod of the present invention is excellent in wire drawability.

[0013] Fig. 1 is a schematic diagram for explaining apparent pearlite lamellar spacing and true pearlite lamellar spacing in microstructure observation. Fig. 2 is a schematic diagram showing an example of an observation surface when a cross section perpendicular to the axial direction of a wire rod is used as the observation surface. Fig. 3 is a schematic diagram in which the observation field in Fig. 2 is divided into a plurality of minute sections. Fig. 4 is a schematic diagram of the minute sections in Fig. 3. Fig. 5 is a schematic diagram of another minute section different from Fig. 4. Fig. 6 is a schematic diagram of another minute section different from Figs. 4 and 5.

[0014] The present inventors first investigated the chemical composition of wire rod suitable for use as a material for steel cords, bead wires, bridge wires, etc. As a result, the chemical composition was found to be, in mass %, C: 0.70 to 1.20%, Si: 0.10 to 1.50%, Mn: 0.10 to 1.00%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, N: 0.0010 to 0.0100%, O: 0.0050% or less, Cr: 0 to 0.50%, Co: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Mo: 0 to 0.20%, B: 0 to 0.005%, W ... : 0 to 0.20%, Ti: 0 to 0.10%, Nb: 0 to 0.10%, Sn: 0 to 0.10%, As: 0 to 0.050%, Sb: 0 to 0.050%, V: 0 to 0.10%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Zr: 0 to 0.010%, and rare earth elements: 0 to 0.0050%, with the balance being Fe and impurities, it was considered that the steel would be suitable as a material for steel cords, bead wires, bridge wires, and the like.

[0015] The present inventors further investigated means for improving wiredrawability in wire rods satisfying the above-mentioned chemical composition. Here, the present inventors focused on the lamellar spacing of pearlite, rather than the inclusions and pearlite block size adjusted in Patent Documents 1 and 2. Patent Document 3 determines the "apparent lamellar spacing" of pearlite observed in microstructural observation of a two-dimensional cross section of a wire rod. Usually, microstructural observation is performed on a two-dimensional observation surface. With reference to FIG. 1 , when observing pearlite containing lamellar ferrite 1 and lamellar cementite 2 on an observation surface 10, the distance λ , which is the minimum distance between adjacent lamellar cementite 2 in pearlite on the observation surface 10, which is a two-dimensional plane, is used.ａ is the apparent lamellar spacing λ ａ On the other hand, the distance λ between adjacent lamellar cementite 2 in pearlite in three-dimensional space is defined as ｔ is the "true lamellar spacing λ ｔ " is defined as:

[0016] As shown in Figure 1, the apparent lamellar spacing λ ａ varies greatly depending on the angle Φ between the observation plane 10, which is a two-dimensional plane, and the extending direction of the lamellar ferrite 1 and lamellar cementite 2. Therefore, in many cases, the apparent lamellar spacing λ ａ is the true lamellar spacing λ ｔ In Patent Document 3, the apparent lamellar spacing λ in the observation plane is ａ A local region with a small lamellar spacing is selected from the above, and the apparent lamellar spacing of that local region is determined. However, it is thought that not only such a local region affects the wiredrawability, but that a wider range of lamellar spacing also affects the wiredrawability. From the above study results, the inventors have determined that the apparent lamellar spacing λ ａ Instead, the true lamellar spacing λ in a wide range ｔ It was thought that the distribution of influences the wire drawability.

[0017] Therefore, the present inventors have proposed that the true lamellar spacing λ in the wire, which has not been investigated until now, can be determined by the following method. ｔ We found that it is possible to determine the distribution of

[0018] [True lamellar spacing λ ｔ Regarding the method for measuring the distribution of lamellar spacing λ in the wire ｔ The method for measuring the distribution of lamellar spacing is carried out in the following three steps: (Step 1) Obtaining an observation image using a scanning electron microscope (SEM); (Step 2) Measuring the apparent lamellar spacing distribution; and (Step 3) Measuring the true lamellar spacing distribution. ａ As described above, the true lamellar spacing λ means the distance between adjacent lamellar cementite particles observed in a two-dimensional plane SEM observation image. ｔmeans the distance between the actual lamellar cementite plates in three-dimensional space. ｔ does not appear on the observation surface 10 and therefore cannot be observed in the SEM observation image. Therefore, in this embodiment, a mathematical approach is used to calculate the apparent lamellar spacing λ ａ Based on the true lamellar spacing λ ｔ Each step is explained below.

[0019] [(Step 1) Obtaining SEM Observation Images] The wire is cut perpendicular to the axial direction (longitudinal direction), and the resulting cross section (cross section perpendicular to the axial direction of the wire) is used as the observation surface. FIG. 2 is a schematic diagram showing an example of the observation surface. The observation surface 10 is mirror-polished. Within 5 minutes of mirror-polishing, the observation surface 10 is etched using picral for approximately 7 seconds to reveal the microstructure on the observation surface 10. After the etching, SEM observation is performed at four observation fields SA in the surface region and four observation fields CA in the central region of the observation surface 10, with an acceleration voltage of 15 kV and a magnification of 2000x to obtain SEM images. Specifically, as shown in FIG. 2, four rectangular observation fields SA, each centered at a depth of 0.5 mm from the surface, with 5120 pixels in the circumferential direction and 3840 pixels in the radial direction of the observation surface 10, are selected at 90° intervals around the center point C1 of the observation surface 10. The area of ​​the observation field SA is a rectangle with a circumferential dimension of 60 μm (5120 pixels) and a radial dimension of 45 μm (3840 pixels). Hereinafter, the four observation fields SA will be referred to as observation fields SA of the surface layer region.

[0020] Furthermore, four observation fields CA are arbitrarily selected from the central region 20, which is a circular region centered on the center C1 of the observation surface 10 and having a radius of D / 8 (D is the diameter of the wire). Each observation field CA is a rectangle with 5120 pixels in the circumferential direction and 3840 pixels in the radial direction, and the area of ​​each observation field CA is a rectangle with 60 μm (5120 pixels) in the circumferential direction and 45 μm (3840 pixels) in the radial direction. The center position of each observation field CA is D / 16 in the radial direction from the center point C1. Hereinafter, the four observation fields CA will be referred to as observation fields CA of the central region. The SEM device used is, for example, a JSM-7100F manufactured by JEOL Ltd.

[0021] [(Step 2) Apparent lamellar spacing λ ａ Measurement of the distribution of apparent lamellar spacing λ obtained from two-dimensional planar SEM images in the surface region and the central region using an observation field SA of the surface region and an observation field CA of the central region. ａ Find the distribution of .

[0022] As shown in Fig. 3, in each SEM image (observation field SA, observation field CA), the SEM image is divided into minute sections 30 of 50 pixels x 50 pixels (0.586 µm x 0.586 µm). Note that since the SEM image is 5120 pixels x 3840 pixels, when the SEM image is divided into minute sections 30, there will be areas that do not fit into 50 pixels x 50 pixels. For these areas, the apparent lamellar spacing λ ａ are excluded from the measurement.

[0023] 4 is a schematic diagram showing an example of the minute compartments 30. In each minute compartment, the apparent lamellar spacing λ ａ is calculated by the following method. As shown in Fig. 4, among the line segments that pass through the center P0 of the minute compartment and have the maximum number of intersections with lamellar cementite 2, a line segment SG is determined in which L0 = {length of line segment / (number of intersections between the line segment and lamellar cementite - 1)} is the smallest. Here, the length of the line segment is the length between the two most distant intersections P1 and P2 of the intersections between the line segment and lamellar cementite 2. L0 of the determined line segment is calculated by the apparent lamellar spacing λ in the minute compartment 30. ａ In the example of FIG. 4, the number of intersections between the line segment SG and the lamellar cementite is "4".

[0024] As shown in Fig. 5, among the plurality of lamellar cementite 2 in the minute compartment 30, the number ratio NR of lamellar cementite 2 in which both end points of the linear lamellar cementite 2 are not in contact with the sides of the minute compartment 30 exceeds 50%. More specifically, in Fig. 5, among the seven lamellar cementite 2 in the minute compartment 30, there are six lamellar cementite 2 in which both end points are not in contact with the sides of the minute compartment 30, so the number ratio NR is 86% (= 6 / 7 x 100). In this way, when the number ratio NR of lamellar cementite 2 in which both end points of the lamellar cementite 2 are not in contact with the sides of the minute compartment 30 exceeds 50%, the minute compartment 30 has an apparent lamellar spacing λ ａ 6, the number ratio NR is 43% (= 3 / 7 × 100), so the minute section 30 in FIG. 6 is the measurement target.

[0025] By the above method, the apparent lamellar spacing λ in the surface region ａ distribution and apparent lamellar spacing λ in the central region ａ Find the distribution of .

[0026] [(Procedure 3) Measurement of true lamellar spacing distribution] As described above, the apparent lamellar spacing λ obtained from a two-dimensional SEM image is ａ The distribution of the true lamellar spacing λ in the wire ｔ Therefore, the apparent lamellar spacing λ obtained in step 2 is different from the distribution of ａ The distribution of is fitted with equation (a) to obtain the true lamellar spacing λ ｔ Here, the apparent lamellar spacing λ ａ Distribution of and true lamellar spacing λ ｔ This is an equation showing the relationship between the distribution of

[0027]

[0028] In formula (a), λ ａ is the apparent lamellar spacing, λ ｔ is the true lamellar spacing, ρ(λ ａ ) is the apparent lamellar spacing distribution, G(λ ｔ ) is the true lamellar spacing λｔ The distribution of G(λ ｔ ) is a Gaussian distribution function, and is shown in equation (b), where A, B, and C are fitting parameters.

[0029]

[0030] As mentioned above, the apparent lamellar spacing λ ａ depends on the lamellar orientation angle Φ shown in Figure 1. Therefore, the apparent lamellar spacing λ ａ From the true lamellar spacing λ ｔ As shown in Figure 1, the apparent lamellar spacing λ ａ and the true lamellar spacing λ ｔ satisfies the formula (1).

[0031]

[0032] Furthermore, Φ-space and λ ａ The spatial relationship is shown in equation (2): ρ(Φ)dΦ=ρ(λ ａ ) dλ ａ (2)

[0033] where ρ(Φ) and ρ(λ ａ ) is the probability density function. The apparent lamellar spacing λ ａ is irregular (random) with respect to the observation surface 10, so ρ(λ ａ ) is expressed by equation (3). ａ )=const. (3)

[0034] Based on the formulas (1) to (3), the true lamellar spacing λ ｔ If does not have a distribution, the apparent lamellar spacing λ ａ is shown in equation (4).

[0035]

[0036] Here, the true lamellar spacing λ for Eq. (4) ｔ The distribution G(λ ｔ ), the apparent lamellar spacing λ ａ The distribution of is expressed as formula (a), where the true lamellar spacing λ ｔ The distribution G(λ ｔ) is assumed to be normally distributed.

[0037] Apparent lamellar spacing λ in the surface region ａ By fitting the distribution of with equation (a), the true lamellar spacing λ ｔ The average value LaveS (nm) (= B), the true lamellar spacing λ ｔ The maximum value LmaxS (nm) (= B + 2.33C), the true lamellar spacing λ ｔ Similarly, the apparent lamellar spacing λ in the central region is calculated. ａ By fitting the distribution of with equation (a), the true lamellar spacing λ ｔ The full width at half maximum FWHMc of the distribution of

[0038] The relationship between the distribution of lamellar spacing obtained by the above method and the reduction of area obtained by performing a tensile test on the wire was investigated. As a result, the lamellar spacing obtained by the above method (true lamellar spacing λ ｔ As a result of further investigation, the inventors have found that, when the average lamellar spacing at a depth of 0.5 mm from the surface of a wire in a circular cross section perpendicular to the axial direction of the wire is defined as LaveS (nm) and the maximum lamellar spacing is defined as LmaxS (nm), wiredrawability is improved if LmaxS / LaveS is 1.70 or less.

[0039] However, even when a wire rod satisfies the above chemical composition and has an LmaxS / LaveS ratio of 1.70 or less, there are cases in which excellent wiredrawability cannot be obtained. Therefore, the present inventors conducted further studies and obtained the following findings.

[0040] As described above, in the surface layer of a circular cross section perpendicular to the axial direction of the wire rod, the narrower the distribution range of lamellar spacing, the higher the wiredrawability. However, in the central part of a circular cross section perpendicular to the axial direction of the wire rod, a relatively wide distribution range of lamellar spacing results in excellent work hardenability. The reason for this is unclear, but the following is thought to be the case.

[0041] When a wire is drawn, strain is introduced into the surface layer of the wire. This causes the surface layer to work harden. Here, to improve wiredrawability, it is preferable that work hardening proceed uniformly in the circular cross section. If work hardening proceeds non-uniformly, variations in hardness occur in the circular cross section. In this case, local cracks occur in areas of low hardness in the circular cross section, and propagate, leading to fracture. On the other hand, if work hardening proceeds uniformly, variations in hardness are less likely to occur in the circular cross section. In this case, the occurrence of local cracks is suppressed, and as a result, wiredrawability can be improved.

[0042] To ensure uniform work hardening in the circular cross section, it is effective to make the distribution range of lamellar spacing in the central region of the circular cross section wider than the distribution range of lamellar spacing in the surface region. In other words, it is effective for the lamellar spacing in the central region to vary more than the lamellar spacing in the surface region. In this case, work hardening in the surface layer is more likely to progress to the central region, resulting in a more uniform hardness distribution in the circular cross section. As a result, excellent wiredrawability is obtained in the wire.

[0043] Based on the above findings, the inventors investigated the relationship between the distribution of lamellar spacing in the surface region of a circular cross section, the distribution of lamellar spacing in the central region, and wiredrawability. As a result, they found that, when the full width at half maximum of the lamellar spacing at a position 0.5 mm deep from the surface of a wire rod in a circular cross section is defined as FWHMs (nm), and the full width at half maximum of the lamellar spacing at the center position of the wire rod is defined as FWHMc (nm), excellent wiredrawability can be obtained if FWHMc / FWHMs is 1.10 to 2.00.

[0044] In addition, even if the minute compartments 30 having a number ratio NR of more than 50% as shown in FIG. 5 are excluded from the measurement target by steps 1 to 3, if the minute compartments 30 having a number ratio NR of 50% or less as shown in FIGS. 4 and 6 are measured, the true lamellar spacing λ ｔ The average value LaveS (nm), the maximum value LmaxS (nm), and the full width at half maximum FWHMs (nm), and the true lamellar spacing λ in the central region ｔ The full width at half maximum (FWHMc) (nm) of the wire shows a good correlation with wire drawability.

[0045] The gist of the wire rod of the present invention, which was completed based on the above findings, is as follows.

[0046] The wire rod of the first configuration has a chemical composition, in mass %, of C: 0.70 to 1.20%, Si: 0.10 to 1.50%, Mn: 0.10 to 1.00%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, N: 0.0010 to 0.0100%, O: 0.0050% or less, Cr: 0 to 0.50%, Co: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Mo: The wire rod has a circular cross section perpendicular to its axial direction, and the average true lamellar spacing at a depth of 0.5 mm from the surface of the wire rod is defined as LaveS (nm), and the maximum true lamellar spacing at a depth of 0.5 mm from the surface of the wire rod is defined as LmaxS (nm), where LmaxS / LaveS is 1.70 or less. In the circular cross section, when the full width at half maximum of the true lamellar spacing at a depth of 0.5 mm from the surface of the wire is defined as FWHMs (nm) and the full width at half maximum of the true lamellar spacing at the center position of the wire is defined as FWHMc (nm), FWHMc / FWHMs is 1.10 to 2.00.

[0047] The wire rod of the second configuration is the wire rod of the first configuration, and has a chemical composition, in mass %, of one or more elements selected from the group consisting of Cr: 0.01 to 0.50%, Co: 0.01 to 0.50%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Mo: 0.01 to 0.20%, B: 0.001 to 0.005%, W: 0.01 to 0.20%, Ti: 0.01 to 0.10%, Nb: 0.01 to 0.10%, Sn: 0.01 to 0.10%, As: 0.001 to 0.050%, Sb: 0.001 to 0.050%, and V: 0.01 to 0.10%.

[0048] The wire rod of the third configuration has a chemical composition, in mass %, of C: 0.70 to 1.20%, Si: 0.10 to 1.50%, Mn: 0.10 to 1.00%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, N: 0.0010 to 0.0100%, O: 0.0050% or less, Cr: 0 to 0.50%, Co: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0-0.50%, Mo: 0-0.20%, B: 0-0.005%, W: 0-0.20%, Ti: 0-0.10%, Nb: 0-0.10%, Sn: 0-0.10%, V: 0-0.10%, Ca: 0-0.0050%, Mg: 0-0.0050%, Zr: 0-0.010%, and rare earth elements: 0-0.0050%, with the balance consisting of Fe and impurities. Furthermore, when the average true lamellar spacing at a depth of 0.5 mm from the surface of the wire in a circular cross section perpendicular to the axial direction of the wire is LaveS (nm) and the maximum true lamellar spacing at a depth of 0.5 mm from the surface of the wire is LmaxS (nm), LmaxS / LaveS is 1.70 or less. In the circular cross section, when the full width at half maximum of the true lamellar spacing at a depth of 0.5 mm from the surface of the wire is defined as FWHMs (nm) and the full width at half maximum of the true lamellar spacing at the center position of the wire is defined as FWHMc (nm), FWHMc / FWHMs is 1.10 to 2.00.

[0049] The wire rod of the fourth configuration is the wire rod of the third configuration, and has a chemical composition, in mass %, of one or more elements selected from the group consisting of Cr: 0.01 to 0.50%, Co: 0.01 to 0.50%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%, Mo: 0.01 to 0.20%, B: 0.001 to 0.005%, W: 0.01 to 0.20%, Ti: 0.01 to 0.10%, Nb: 0.01 to 0.10%, Sn: 0.01 to 0.10%, and V: 0.01 to 0.10%.

[0050] The wire rod of the fifth configuration is a wire rod of the first to fourth configurations, and has a chemical composition, in mass %, of one or more elements selected from the group consisting of Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, Zr: 0.001 to 0.010%, and rare earth elements: 0.0001 to 0.0050%.

[0051] The wire rod according to this embodiment will be described in detail below. Note that "%" regarding elements means mass % unless otherwise specified.

[0052] [Features of the Wire Rod of the Present Embodiment] The wire rod of the present embodiment includes the following features 1 to 3. (Feature 1) The chemical composition, in mass %, is: C: 0.70 to 1.20%, Si: 0.10 to 1.50%, Mn: 0.10 to 1.00%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, N: 0.0010 to 0.0100%, O: 0.0050% or less, Cr: 0 to 0.50%, Co: 0 to 0.50%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, Mo: 0 to 0.2 0%, B: 0-0.005%, W: 0-0.20%, Ti: 0-0.10%, Nb: 0-0.10%, Sn: 0-0.10%, As: 0-0.050%, Sb: 0-0.050%, V: 0-0.10%, Ca: 0-0.0050%, Mg: 0-0.0050%, Zr: 0-0.010%, and rare earth elements: 0-0.0050%, with the balance consisting of Fe and impurities. (Feature 2) When the average true lamellar spacing at a depth of 0.5 mm from the surface of the wire in a circular cross section perpendicular to the axial direction of the wire is LaveS (nm) and the maximum true lamellar spacing at a depth of 0.5 mm from the surface of the wire is LmaxS (nm), LmaxS / LaveS is 1.70 or less. (Feature 3) When the full width at half maximum of the true lamellar spacing at a depth of 0.5 mm from the surface of the wire in the circular cross section is FWHMs (nm) and the full width at half maximum of the true lamellar spacing at the center of the wire is FWHMc (nm), FWHMc / FWHMs is 1.10 to 2.00. Features 1 to 3 will be described below.

[0053] [(Feature 1) Chemical Composition] The chemical composition of the wire rod of this embodiment contains the following elements.

[0054] C: 0.70 to 1.20% Carbon (C) increases the strength of the wire rod. If the C content is less than 0.70%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content exceeds 1.20%, even if the contents of other elements are within the ranges of this embodiment, pro-eutectoid cementite is excessively formed. In this case, the wire drawability of the wire rod is reduced. Therefore, the C content is 0.70 to 1.20%. The preferred lower limit of the C content is 0.72%, more preferably 0.78%, even more preferably 0.80%, and even more preferably 0.82%. The preferred upper limit of the C content is 1.18%, more preferably 1.12%, even more preferably 1.08%, and even more preferably 1.05%.

[0055] Si: 0.10 to 1.50% Silicon (Si) increases the strength of wire rod. Furthermore, Si deoxidizes steel during the steelmaking process in the wire rod manufacturing process. If the Si content is less than 0.10%, the above-mentioned effect cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content exceeds 1.50%, Si segregates within the wire rod that serves as the raw material for steel wire, even if the contents of other elements are within the ranges of this embodiment. In this case, bainite forms in the region where Si segregates, reducing the wire drawability of the wire rod. Therefore, the Si content is 0.10 to 1.50%. The preferred lower limit of the Si content is 0.15%, more preferably 0.20%, and even more preferably 0.25%. The preferred upper limit of the Si content is 1.42%, more preferably 1.35%, even more preferably 1.00%, even more preferably 0.90%, and even more preferably 0.80%.

[0056] Mn: 0.10 to 1.00% Manganese (Mn) improves the hardenability of steel and increases the strength of wire rod. Mn also fixes S in steel, improving the hot workability of wire rod. If the Mn content is less than 0.10%, the above effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content exceeds 1.00%, Mn segregates within the wire rod, even if the contents of other elements are within the ranges of this embodiment. In this case, bainite forms in the region where Mn segregates, reducing the drawability of the wire rod. Therefore, the Mn content is 0.10 to 1.00%. The preferred lower limit of the Mn content is 0.12%, more preferably 0.20%, and even more preferably 0.25%. The preferred upper limit of the Mn content is 0.90%, more preferably 0.85%, even more preferably 0.80%, and even more preferably 0.70%.

[0057] P: 0.020% or less Phosphorus (P) is an impurity. If the P content exceeds 0.020%, P segregates at grain boundaries even if the contents of other elements are within the ranges of this embodiment. As a result, the grain boundaries become embrittled, resulting in a decrease in the wire drawability of the wire rod. Therefore, the P content is 0.020% or less. The lower the P content, the more preferable it is. However, excessive reduction in the P content increases production costs. Therefore, considering normal industrial production, the preferred lower limit of the P content is more than 0%, more preferably 0.001%, even more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit of the P content is 0.018%, even more preferably 0.016%, and even more preferably 0.015%.

[0058] S: 0.020% or less Sulfur (S) is an impurity. If the S content exceeds 0.020%, S segregates at grain boundaries even if the contents of other elements are within the ranges of this embodiment. Furthermore, excessive coarse MnS is generated. This reduces the wire drawability of the wire rod. 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 manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the S content is more than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the S content is 0.018%, even more preferably 0.016%, even more preferably 0.014%, even more preferably 0.010%, and even more preferably 0.008%.

[0059] Al: 0.080% or less Aluminum (Al) deoxidizes steel. Even if even a small amount of Al is contained, the above effect can be obtained to some extent. However, if the Al content exceeds 0.080%, excessive coarse oxides are generated even if the contents of other elements are within the ranges of this embodiment. In this case, the wire drawability of the wire rod deteriorates. Therefore, the Al content is 0.080% or less. The preferred lower limit of the Al content is more than 0%, more preferably 0.001%, even more preferably 0.003%, and even more preferably 0.0007%. The preferred upper limit of the Al content is 0.075%, even more preferably 0.070%, even more preferably 0.065%, and even more preferably 0.0060%.

[0060] N: 0.0010 to 0.0100% Nitrogen (N) fixes dislocations during wire drawing, thereby increasing the strength of the steel wire after wire drawing. If the N content is less than 0.0010%, the above effect cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the N content exceeds 0.0100%, excessive nitrides are formed in the wire, even if the contents of other elements are within the ranges of this embodiment. In this case, the wire drawability of the wire deteriorates. Therefore, the N content is 0.0010 to 0.0100%. The preferred lower limit of the N content is 0.0015%, more preferably 0.0020%, and even more preferably 0.0025%. The preferred upper limit of the N content is 0.0090%, more preferably 0.0080%, even more preferably 0.0070%, and even more preferably 0.0060%.

[0061] O: 0.0050% or less Oxygen (O) is an impurity. If the O content exceeds 0.0050%, even if the contents of other elements are within the ranges of this embodiment, excessive coarse oxides are generated in the wire rod. In this case, the wire drawability of the wire rod is reduced. Therefore, the O content is 0.0050% or less. The O content is preferably as low as possible. However, excessive reduction of the O content increases production costs. Therefore, considering normal industrial production, the preferred lower limit of the O content is more than 0%, more preferably 0.0001%, even more preferably 0.0003%, and even more preferably 0.0005%. The preferred upper limit of the O content is 0.0045%, even more preferably 0.0040%, even more preferably 0.0030%, and even more preferably 0.0020%.

[0062] The balance of the chemical composition of the wire rod according to this 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 or scrap or the manufacturing environment during industrial production of the wire rod, and are unintentionally contained, but are acceptable within a range that does not adversely affect the wire rod according to this embodiment.

[0063] [Optional Elements] The chemical composition of the wire rod of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Cr: 0-0.50%, Co: 0-0.50%, Cu: 0-0.50%, Ni: 0-0.50%, Mo: 0-0.20%, B: 0-0.005%, W: 0-0.20%, Ti: 0-0.10%, Nb: 0-0.10%, Sn: 0-0.10%, As: 0-0.050%, Sb: 0-0.050%, V: 0-0.10%, Ca: 0-0.0050%, Mg: 0-0.0050%, Zr: 0-0.010%, and rare earth elements: 0-0.0050%. All of these elements are optional. Each optional element will be described below.

[0064] [First group: Cr, Co, Cu, Ni, Mo, B, W, Ti, Nb, Sn, As, Sb, and V] The chemical composition of the wire rod of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Cr, Co, Cu, Ni, Mo, B, W, Ti, Nb, Sn, As, Sb, and V. These elements are optional elements, and all of them improve the hardenability of the steel material.

[0065] Cr: 0 to 0.50% Chromium (Cr) is an optional element and does not necessarily need to be contained. That is, the Cr content may be 0%. When chromium is contained, that is, when the Cr content exceeds 0%, Cr improves the hardenability of the steel material and increases the strength of the wire rod. Even if even a small amount of Cr is contained, the above effects can be obtained to some extent. However, if the Cr content exceeds 0.50%, Cr segregates within the wire rod even if the contents of other elements are within the ranges of this embodiment. In this case, bainite forms in the region where Cr segregates, reducing the wire drawability of the wire rod. Therefore, the Cr content is 0 to 0.50%. The preferred lower limit of the Cr content is 0.01%, more preferably 0.05%, and even more preferably 0.08%. The preferred upper limit of the Cr content is 0.45%, more preferably 0.40%, even more preferably 0.35%, and even more preferably 0.30%.

[0066] Cobalt (Co) is an optional element and does not necessarily need to 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 steel material and increases the strength of the wire rod. Even if even a small amount of Co is contained, the above effects can be obtained to some extent. However, if the Co content exceeds 0.50%, even if the contents of other elements are within the ranges of this embodiment, the hardness of the wire rod increases excessively, and the wire drawability of the wire rod decreases. Therefore, the Co content is 0 to 0.50%. The preferred lower limit of the Co content is 0.01%, more preferably 0.05%, and even more preferably 0.08%. The preferred upper limit of the Co content is 0.45%, more preferably 0.40%, even more preferably 0.35%, even more preferably 0.30%, and even more preferably 0.20%.

[0067] Cu: 0 to 0.50% Copper (Cu) is an optional element and does not necessarily need to be contained. That is, the Cu content may be 0%. When contained, that is, when the Cu content exceeds 0%, Cu improves the hardenability of the steel material and increases the strength of the wire rod. Even if even a small amount of Cu is contained, the above effects can be obtained to some extent. However, if the Cu content exceeds 0.50%, even if the contents of other elements are within the ranges of this embodiment, the hardness of the wire rod increases excessively, and the wire drawability of the wire rod decreases. Therefore, the Cu content is 0 to 0.50%. The preferred lower limit of the Cu content is 0.01%, more preferably 0.05%, even more preferably 0.08%, and even more preferably 0.15%. The preferred upper limit of the Cu content is 0.45%, more preferably 0.40%, even more preferably 0.35%, even more preferably 0.30%, and even more preferably 0.25%.

[0068] Ni: 0 to 0.50% Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When contained, that is, when the Ni content exceeds 0%, Ni improves the hardenability of the steel material. Ni also delays the transformation of pro-eutectoid cementite and grain boundary ferrite during cooling from the austenite temperature range in the wire rod manufacturing process. This results in the formation of fine pearlite lamellae. As a result, the strength of the steel wire manufactured using the wire rod as a raw material is increased while sufficiently maintaining the wire rod drawability. Even if even a small amount of Ni is contained, the above effects can be achieved to some extent. However, if the Ni content exceeds 0.50%, the hardenability of the steel material becomes excessively high. In this case, even if the contents of other elements are within the ranges of this embodiment, bainite will form in the wire rod, reducing the wire rod drawability. Therefore, the Ni content is 0 to 0.50%. The lower limit of the Ni content is preferably 0.01%, more preferably 0.05%, and even more preferably 0.08%. The upper limit of the Ni content is preferably 0.45%, more preferably 0.40%, more preferably 0.35%, even more preferably 0.30%, and even more preferably 0.25%.

[0069] Mo: 0 to 0.20% Molybdenum (Mo) is an optional element and does not necessarily need to be contained. That is, the Mo content may be 0%. When contained, that is, when the Mo content exceeds 0%, Mo improves the hardenability of the wire rod and increases its strength. Even if even a small amount of Mo is contained, the above effects can be obtained to some extent. However, if the Mo content exceeds 0.20%, the hot workability of the steel material decreases during the wire rod manufacturing process, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Mo content is 0 to 0.20%. The preferred lower limit of the Mo content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit of the Mo content is 0.18%, more preferably 0.16%, even more preferably 0.14%, even more preferably 0.12%, and even more preferably 0.10%.

[0070] B: 0 to 0.005% Boron (B) is an optional element and does not necessarily need to be contained. In other words, the B content may be 0%. When contained, that is, when the B content exceeds 0%, B improves the hardenability of the wire rod and increases its strength. Even if even a small amount of B is contained, the above effects can be obtained to some extent. However, if the B content exceeds 0.005%, the hot workability of the steel material decreases in the wire rod manufacturing process even if the contents of other elements are within the ranges of this embodiment. Therefore, the B content is 0 to 0.005%. The preferred lower limit of the B content is 0.001%, and more preferably 0.002%. The preferred upper limit of the B content is 0.004%, more preferably 0.003%, and even more preferably 0.002%.

[0071] W: 0 to 0.20% Tungsten (W) is an optional element and does not necessarily need to 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 rod and increases its strength. Even if even a small amount of W is contained, the above effects can be obtained to some extent. However, if the W content exceeds 0.20%, the hot workability of the steel material decreases during the wire rod manufacturing process, even if the contents of other elements are within the ranges of this embodiment. Therefore, the W content is 0 to 0.20%. The preferred lower limit of the W content is 0.01%, more preferably 0.03%, even more preferably 0.05%, and even more preferably 0.08%. The preferred upper limit of the W content is 0.18%, even more preferably 0.16%, even more preferably 0.14%, even more preferably 0.12%, and even more preferably 0.10%.

[0072] Ti: 0 to 0.10% Titanium (Ti) is an optional element and does not necessarily need to be contained. That is, the Ti content may be 0%. When contained, that is, when the Ti content exceeds 0%, Ti improves the hardenability of the wire rod and increases its strength. Even if even a small amount of Ti is contained, the above effects can be obtained to some extent. However, if the Ti content exceeds 0.10%, the hot workability of the steel material decreases during the wire rod manufacturing process, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ti content is 0 to 0.10%. The preferred lower limit of the Ti content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Ti content is 0.09%, more preferably 0.08%, even more preferably 0.07%, and even more preferably 0.05%.

[0073] Nb: 0 to 0.10% Niobium (Nb) is an optional element and does not necessarily need to be contained. That is, the Nb content may be 0%. When contained, that is, when the Nb content exceeds 0%, Nb improves the hardenability of the wire rod and increases its strength. Even if even a small amount of Nb is contained, the above effects can be obtained to some extent. However, if the Nb content exceeds 0.10%, the hot workability of the steel material decreases in the wire rod manufacturing process even if the contents of other elements are within the ranges of this embodiment. Therefore, the Nb content is 0 to 0.10%. The preferred lower limit of the Nb content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Nb content is 0.09%, more preferably 0.08%, even more preferably 0.07%, and even more preferably 0.05%.

[0074] Sn: 0 to 0.10% Tin (Sn) is an optional element and does not necessarily need to be contained. That is, the Sn content may be 0%. When contained, that is, when the Sn content exceeds 0%, Sn suppresses ferrite decarburization that occurs during hot working and suppresses a decrease in the strength of the wire rod. Even if even a small amount of Sn is contained, the above effect can be obtained to some extent. However, if the Sn content exceeds 0.10%, even if the contents of other elements are within the ranges of this embodiment, Sn segregates to grain boundaries and red embrittlement occurs. In this case, the hot workability of the wire rod is reduced. Therefore, the Sn content is 0 to 0.10%. The preferred lower limit of the Sn content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Sn content is 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0075] As: 0 to 0.050% Arsenic (As) is an optional element and does not necessarily need to be contained. In other words, the As content may be 0%. When contained, that is, when the As content exceeds 0%, As increases the strength of the wire rod. Even if even a small amount of As is contained, the above effect can be obtained to some extent. However, if the As content exceeds 0.050%, the hot workability of the wire rod deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the As content is 0 to 0.050%. The preferred lower limit of the As content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the As content is 0.049%, more preferably 0.040%, and even more preferably 0.030%.

[0076] Sb: 0 to 0.050% Antimony (Sb) is an optional element and does not necessarily need to be contained. That is, the Sb content may be 0%. When contained, that is, when the Sb content exceeds 0%, Sb increases the strength of the wire rod. Even if even a small amount of Sb is contained, the above effect can be obtained to some extent. However, if the Sb content exceeds 0.050%, the hot workability of the wire rod will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sb content is 0 to 0.050%. The preferred lower limit of the Sb content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit of the Sb content is 0.049%, more preferably 0.040%, and even more preferably 0.030%.

[0077] V: 0 to 0.10% Vanadium (V) is an optional element and does not necessarily need to be contained. That is, the V content may be 0%. When vanadium is contained, that is, when the V content exceeds 0%, V improves the hardenability of the wire rod and increases its strength. Even if even a small amount of V is contained, the above effects can be obtained to some extent. However, if the V content exceeds 0.10%, 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%. The preferred lower limit of the V content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the V content is 0.09%, more preferably 0.08%, and even more preferably 0.07%.

[0078] [Second Group: Ca, Mg, Zr, and Rare Earth Elements (REM)] The chemical composition of the wire rod of this embodiment may further contain one or more elements selected from the group consisting of Ca, Mg, Zr, and rare earth elements (REM), in place of a portion of Fe. These elements are optional elements, and all of them increase the ductility of the wire rod.

[0079] Ca: 0 to 0.0050% Calcium (Ca) is an optional element and may not be contained. In other words, the Ca content may be 0%. When contained, that is, when the Ca content exceeds 0%, Ca reduces hard alumina-based inclusions and improves the ductility of the wire rod. Even if even a small amount of Ca is contained, the above effect can be obtained to some extent. However, if the Ca content exceeds 0.0050%, excessive coarse oxides are generated even if the contents of other elements are within the ranges of this embodiment. In this case, the wire drawability of the wire rod is reduced. Therefore, the Ca content is 0 to 0.0050%. The preferred lower limit of the Ca content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0008%. The upper limit of the Ca content is preferably 0.0045%, more preferably 0.0040%, even more preferably 0.0035%, even more preferably 0.0030%, even more preferably 0.0025%, and even more preferably 0.0020%.

[0080] Mg: 0 to 0.0050% Magnesium (Mg) is an optional element and may not be contained. In other words, the Mg content may be 0%. When contained, that is, when the Mg content exceeds 0%, Mg forms fine oxides. The fine oxides refine the structure of the wire rod and increase the ductility of the wire rod. Even if even a small amount of Mg is contained, the above effect can be obtained to some extent. However, if the Mg content exceeds 0.0050%, excessive coarse oxides are formed even if the contents of other elements are within the ranges of this embodiment. In this case, the wire drawability of the wire rod is reduced. Therefore, the Mg content is 0 to 0.0050%. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0008%. The upper limit of the Mg content is preferably 0.0045%, more preferably 0.0040%, even more preferably 0.0035%, even more preferably 0.0030%, and still more preferably 0.0025%.

[0081] Zr: 0 to 0.010% Zirconium (Zr) is an optional element and does not necessarily need to be contained. That is, the Zr content may be 0%. When contained, that is, when the Zr content exceeds 0%, Zr forms fine oxides. The fine oxides refine the structure of the wire rod and increase the ductility of the wire rod. Even if even a small amount of Zr is contained, the above effect can be obtained to some extent. However, if the Zr content exceeds 0.010%, even if the contents of other elements are within the ranges of this embodiment, excessive coarse oxides are formed. In this case, the wire drawability of the wire rod is reduced. Therefore, the Zr content is 0 to 0.010%. The preferred lower limit of the Zr content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit of the Zr content is 0.009%, more preferably 0.008%, and even more preferably 0.007%.

[0082] Rare Earth Elements: 0 to 0.0050% Rare earth elements (REM) are optional elements and may not be contained. That is, the REM content may be 0%. When contained, that is, when the REM content exceeds 0%, REM forms fine sulfides and renders S harmless. Therefore, the ductility of the wire rod is improved. Even if even a small amount of REM is contained, the above effect can be obtained to some extent. However, if the REM content exceeds 0.0050%, even if the contents of other elements are within the ranges of this embodiment, excessive coarse oxides are formed. In this case, the wire rod drawability is reduced. Therefore, the REM content is 0 to 0.0050%. The preferred lower limit of the REM content is 0.0001%, more preferably 0.0005%, even more preferably 0.0008%, even more preferably 0.0013%, and even more preferably 0.0018%. The upper limit of the REM content is preferably 0.0045%, more preferably 0.0040%, even more preferably 0.0035%, and still more preferably 0.0030%.

[0083] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanoids lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In addition, in this specification, the REM content refers to the total content of these elements.

[0084] [(Feature 2) Regarding LmaxS / LaveS] Furthermore, in the wire of this embodiment, when the average true lamellar spacing at a depth of 0.5 mm from the surface of the wire in a circular cross section perpendicular to the axial direction of the wire is LaveS (nm) and the maximum true lamellar spacing is LmaxS (nm), LmaxS / LaveS is 1.70 or less. If LmaxS / LaveS is 1.70 or less in the surface layer of the wire, the wire drawability of the wire is improved.

[0085] The upper limit of LmaxS / LaveS is preferably 1.65, more preferably 1.60, even more preferably 1.55, and even more preferably 1.50. The lower limit of LmaxS / LaveS is not particularly limited. The lower limit of LmaxS / LaveS is, for example, 1.20, for example, 1.10, or for example, 1.00.

[0086] [(Feature 3) Regarding FWHMc / FWHMs] Furthermore, in the wire of the present embodiment, when the full width at half maximum of the true lamellar spacing at a depth of 0.5 mm from the surface of the wire in the circular cross section is FWHMs (nm), and the full width at half maximum of the true lamellar spacing at the center of the wire is FWHMc (nm), FWHMc / FWHMs is 1.10 to 2.00.

[0087] If FWHMc / FWHMs is 1.10 to 2.00, the distribution range of lamellar spacing in the central region of the circular cross section of the wire is sufficiently wider than the distribution range of lamellar spacing in the surface layer. Therefore, work hardening in the surface layer is more likely to progress toward the center, and the hardness distribution in the circular cross section becomes more uniform. As a result, the wiredrawability of the wire is improved.

[0088] The lower limit of FWHMc / FWHMs is preferably 1.12, more preferably 1.15, even more preferably 1.20, and even more preferably 1.25. The upper limit of FWHMc / FWHMs is preferably 1.90, more preferably 1.80, even more preferably 1.70, and even more preferably 1.65.

[0089] [Method of Measuring LaveS, LmaxS, FWHMc, and FWHMs] LaveS, LmaxS, FWHMc, and FWHMs are measured by the above-mentioned [True Lamellar Spacing λ ｔ The measurement method for the distribution of [method for measuring the distribution of the chromatic aberration] can be carried out by carrying out steps 1 to 3.

[0090] [Effects of the Wire Rod of the Present Embodiment] The wire rod of the present embodiment satisfies Features 1 to 3. Therefore, the wire rod of the present embodiment can obtain excellent wiredrawability by specifying the distribution state of the lamellar spacing.

[0091] [Microstructure of Wire Rod of Present Embodiment] In the microstructure of the wire rod of this embodiment, the pearlite area ratio is 95% or more. That is, the microstructure of the wire rod of this embodiment is substantially composed of pearlite. The microstructure other than pearlite is composed of, for example, one or more types selected from the group consisting of ferrite, pro-eutectoid cementite, bainite, and martensite.

[0092] [Method for Measuring Pearlite Area Ratio] The pearlite area ratio in the microstructure of the wire rod of this embodiment is determined by the following method.

[0093] A test specimen is taken with a cross section parallel to the central axis of the wire as the observation surface. The observation surface of the test specimen is mirror-polished. Within 5 minutes of mirror-polishing, the observation surface is etched using picral for approximately 7 seconds to reveal the microstructure on the observation surface. From the observation surface after etching, four rectangular observation fields are selected, each including the central axis of the wire and measuring 50 μm in the radial direction and 40 μm in the axial direction. The four observation fields are arranged at a pitch of 100 μm in the axial direction. The central axis of the wire is positioned at the radial center of the observation field. Each observation field is observed at 2000x magnification using a field emission scanning electron microscope (FE-SEM), and a photographic image is generated.

[0094] Based on the contrast and morphology of the photographic image of each observation field, the pearlite structure and other structures (ferrite, pro-eutectoid cementite, bainite, and martensite) are identified. A striped structure consisting of low-brightness black ferrite and high-brightness white cementite in the observation field is recognized as a pearlite structure. Although it is not necessary to distinguish between structures other than pearlite, for example, a region of black crystal grains in which no substructure is observed within the grains is a ferrite structure.

[0095] The pearlite area ratio (%) is calculated based on the total area of ​​pearlite identified in the four observation fields and the total area of ​​the four observation fields. The pearlite area ratio (%) is calculated by rounding the obtained value to the nearest integer.

[0096] [Applications to which the wire rod of the present embodiment can be applied] The wire rod of the present embodiment can be widely applied, for example, as a material for steel wires produced by wire drawing. The steel wires can be applied, for example, to bead wires, steel cords, bridge wires, wire ropes, etc.

[0097] [Cross-sectional shape perpendicular to the axial direction of the wire rod of this embodiment] The cross-section perpendicular to the axial direction of the wire rod of this embodiment is, for example, circular. The wire diameter of the wire rod of this embodiment is not particularly limited. The wire diameter (diameter) of the wire rod is, for example, 3.0 to 8.0 mm. The wire diameter (diameter) of a steel wire manufactured using the wire rod of this embodiment as a raw material is, for example, 0.8 to 3.0 mm. The above-mentioned "circular shape" not only refers to a perfect circle, but also includes, for example, a circular shape that satisfies the deviation in diameter (0.64 mm or less) specified in Table 3 of 7.b) of JIS G 3506 (2017) "Hard Steel Wire Rod."

[0098] [Example of a method for manufacturing a wire rod according to this embodiment] An example of a method for manufacturing a wire rod according to this embodiment will be described. The method for manufacturing a wire rod described below is one example for manufacturing the wire rod according to this embodiment. Therefore, the wire rod having the above-described configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing a wire rod according to this embodiment.

[0099] An example of the method for manufacturing wire rod according to this embodiment includes the following steps: (Step 1) Material preparation step (Step 2) Blooming rolling step (Step 3) Finish rolling step Each step will be described below.

[0100] [(Step 1) Material Preparation Step] In the material preparation step, a material for the wire rod 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.

[0101] 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.

[0102] [(Step 2) Blooming Step] In the blooming step, the material (ingot or bloom) prepared in the material preparation step is bloomed to produce a billet.

[0103] 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.

[0104] The heated material is hot rolled (rough 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.

[0105] In the blooming process, the cooling conditions for the billet after hot rolling are as follows: (Condition 1) The cooling rate at 700 to 670°C is 0.50°C / second or less.

[0106] In the blooming process, pearlite is formed in the billet in the temperature range of 700 to 670°C after hot rolling. If the cooling rate at 700 to 670°C exceeds 0.50°C / sec, the cooling rate is too fast. In this case, the difference between the lamellar spacing of pearlite in the central region of the produced wire rod and the lamellar spacing of pearlite in the surface layer is not large enough. As a result, the FWHMc / FWHMs ratio becomes less than 1.10.

[0107] If the cooling rate at 700 to 670°C in the blooming process is slow cooled at 0.50°C / sec or less, Mn segregation (center segregation) is appropriately formed in the center of the billet. This center segregation reduces the true lamellar spacing λ in the central region of the wire rod in the finish rolling process. ｔ becomes wider, and the lamellar spacing λ ｔ As a result, FWHMc / FWHMs falls within an appropriate range.

[0108] [(Step 3) Finish Rolling Step] In the finish rolling step, the billet produced in the blooming step is finish rolled to produce a wire rod. The finish rolling step includes the following steps: (Step 31) Heating Step (Step 32) Rolling Step The heating step and rolling step in the finish rolling step will be described below.

[0109] [(Step 31) Heating Step] In the heating step, the billet produced in the blooming step is heated in a heating furnace at a heating temperature of 900 to 1150°C.

[0110] [(Step 32) Rolling Step] In the rolling step, the billet heated in the heating step is subjected to finish rolling (continuous rolling) using a continuous rolling mill to produce a wire rod. The continuous rolling mill includes a plurality of rolling stands arranged in a line 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.

[0111] The rolling process further satisfies the following conditions: (Condition 2) The finish rolling temperature is set to 910 to 1150°C. (Condition 3) Water cooling is started within 0.5 seconds after the final finish rolling. Conditions 2 and 3 will be explained below.

[0112] [(Condition 2) Finish Rolling Temperature] As mentioned above, the finish rolling temperature means the surface temperature (°C) of the steel material (wire rod) at the entry side of the rolling stand that applied the final reduction to the steel material in the continuous rolling mill. The finish rolling temperature can be measured with a thermometer installed at the entry side of the rolling stand that applied the final reduction. If the finish rolling temperature is 910 to 1150°C, the true lamellar spacing λ of the central region of the billet being finish rolled will be ｔ does not change significantly due to finish rolling. Therefore, the true lamellar spacing λ in the central region ｔ can maintain a wide variation.

[0113] [(Condition 3) Water Cooling Immediately After Final Finish Rolling] In a wire rod immediately after final finish rolling, the austenite grains are finer in the surface layer where the amount of strain is large. However, if the high temperature state continues thereafter, the austenite grains in the surface layer grow, and the austenite grains in the axial cross section of the wire rod become uniform. In this case, the difference between the lamellar spacing in the surface layer and the lamellar spacing in the central region does not become sufficiently large. Furthermore, if the cooling time immediately after rolling is excessively long, the variation in the lamellar spacing in the surface layer increases.

[0114] If water cooling is started within 0.5 seconds after final finish rolling, the austenite grains in the surface layer can be easily maintained at a fine size. As a result, the difference between the lamellar spacing in the surface layer and the lamellar spacing in the central region becomes sufficiently large, and FWHMc / FWHMs becomes 1.10 or more. Furthermore, since the surface layer is water-cooled immediately after final finish rolling, the austenite grains in the surface layer become finer and pearlite transformation proceeds at a high speed. Therefore, the variation in lamellar spacing in the surface layer can be suppressed overall. As a result, LmaxS / LaveS becomes 1.70 or less. The coiling temperature of the wire rod is 700°C or higher.

[0115] The wire rod after the finish rolling is coiled. The coiled wire rod is further cooled by air blast cooling. In this embodiment, a heating step such as rapid heating is not performed until the coiled wire rod reaches room temperature.

[0116] The wire rod of this embodiment is manufactured by the above manufacturing process.

[0117] [Method for manufacturing steel wire] The method for manufacturing a hard steel wire using the wire rod of this embodiment as a raw material is a well-known manufacturing method. The steel wire is, for example, a steel cord. The method for manufacturing a steel wire using the wire rod of this embodiment is, for example, as follows: Oxide scale is removed from the wire rod, and a lubrication treatment is performed. The lubricated wire rod is subjected to dry wire drawing to manufacture a steel wire. A well-known patenting treatment is performed on the steel wire after dry wire drawing, and a well-known plating treatment is performed. Furthermore, wet wire drawing is performed. The steel wire is manufactured by the above manufacturing steps.

[0118] In the wire rod of this embodiment, the variation in lamellar spacing is suppressed to a certain extent, while the ratio of the variation in the surface layer to the central region is increased to a certain extent, resulting in sufficient wiredrawability in the steel wire after dry wiredrawing.

[0119] The effects of the wire rod of this embodiment will be explained more specifically using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the wire rod of this embodiment. Therefore, the wire rod of this embodiment is not limited to this one example of conditions.

[0120] Wires having the chemical compositions shown in Tables 1-1 and 1-2 were produced by the following method.

[0121] [Material Preparation Step] Materials (blooms) having the chemical compositions shown in Tables 1-1 and 1-2 were produced.

[0122]

[0123]

[0124] [Blooming Process] The blooms thus produced were subjected to a blooming process to produce billets. Specifically, the blooms were heated to 1,100°C using a heating furnace. The heated blooms were then hot rolled (roughly rolled) using a blooming mill and a continuous rolling mill to produce billets. After hot rolling, the cooling rates CR (°C / sec) at 700 to 670°C were as shown in Table 2.

[0125]

[0126] [Finish Rolling Step] The produced billets were subjected to a finish rolling step. Specifically, the billets of each test number were heated to 950 to 1150°C using a heating furnace. The heated billets were subjected to finish rolling (continuous rolling) using a continuous rolling mill to produce wire rods. The finish rolling temperatures (°C) in the finish rolling were as shown in Table 2.

[0127] In Test Nos. 1 to 41, the time t from immediately after final finish rolling to the start of water cooling was within 0.5 seconds (shown as "T (True)" in the "Time t≦0.5 seconds" column in Table 2). On the other hand, in Test Nos. 42 to 45, the time t from immediately after final finish rolling to the start of water cooling exceeded 0.5 seconds (shown as "F (False)" in the "Time t≦0.5 seconds" column in Table 2). Thereafter, water cooling was carried out until the temperature of the wire rod reached 800°C. The coiling temperature was 700°C or higher. The wire rod after coiling was subjected to air blast cooling and cooled to room temperature without reheating or the like. A wire rod having a wire diameter of 5.5 mm was produced by the above manufacturing process.

[0128] The produced wire rods were subjected to removal of oxide scale from the wire rods under the same conditions for each test number, and then subjected to a lubrication treatment (bonder treatment). The lubrication-treated wire rods were subjected to dry wire drawing to produce steel wires with a wire diameter of 1.5 mm. In the wire drawing, a die with a die angle of 10° (full angle) was used, and the wire drawing speed was 10 m / min.

[0129] [Regarding Evaluation Tests] The following evaluation tests of wire rods (Tests 1 and 2) and steel wires (Test 3) were carried out on the manufactured wire rods and steel wires of each test number: (Test 1) Pearlite area ratio measurement test (Test 2) LaveS, LmaxS, FWHMc, and FWHMs measurement test (Test 3) Twisting test Each test will be described below.

[0130] [(Test 1) Pearlite Area Ratio Measurement Test] The pearlite area ratio (%) of the wire rod of each test number was determined based on the method described in the above [Method for measuring pearlite area ratio]. The results are shown in Table 2. In all test numbers, the pearlite area ratio was 95% or more. The remainder other than pearlite was one or more types selected from the group consisting of ferrite, pro-eutectoid cementite, bainite, and martensite.

[0131] [(Test 2) Measurement Test of LaveS, LmaxS, FWHMc and FWHMs] For the wire rods of each test number, the true lamellar spacing λ was measured based on the method described in the above [Method for measuring LaveS, LmaxS, FWHMc and FWHMs]. ｔThe LaveS (nm), LmaxS (nm), FWHMc and FWHMs were determined by carrying out steps 1 to 3 of the method for measuring the distribution of [Table 2]. The results are shown in Table 2.

[0132] [(Test 3) Twist Test] A twist test was performed on the steel wire of each test number as follows. First, the tensile strength of the steel wire of each test number was determined by the following method. The steel wire was cut to a length of 340 mm and then straightened to form a straight wire. A tensile test was performed at room temperature in the air, with chucks at 70 mm above and below the straight wire and a chuck distance of 200 mm, to determine the tensile strength TS (MPa). Furthermore, test specimens for the twist test were taken from the steel wire of each test number. The chuck distance of the test specimen was set to the wire diameter d × 100 mm. Then, 50 mm from the end of the test specimen was gripped. A load of 0.1 × TS was applied to the test specimen, and the twist test was performed. The twist speed was 20 rpm. A twist test was performed on three test specimens for each test number. The number of twists until final breakage was determined, and the arithmetic average of the three twists obtained was calculated. Based on the obtained arithmetic mean value TC of the number of twists and the tensile strength TS, the following evaluation value was calculated: Evaluation value = TC × TS If the evaluation value was 70,000 or more, it was determined that excellent wiredrawability was obtained (indicated by "E (Excellent)" in the "TC × TS ≧ 70,000" column in Table 2). On the other hand, if the evaluation value was less than 70,000, it was determined that sufficient wiredrawability was not obtained (indicated by "B (Bad)" in the "TC × TS ≧ 70,000" column in Table 2).

[0133] [Test Results] The test results are shown in Table 2. Referring to Tables 1-1, 1-2 and 2, the wire rods of test numbers 1 to 37 satisfied characteristics 1 to 3. As a result, the evaluation value was 70,000 or more, and excellent wiredrawability was obtained.

[0134] On the other hand, in test numbers 38 to 41, the cooling rate under condition 1 was too fast. As a result, the FWHMc / FWHMs was less than 1.10. As a result, the evaluation value was less than 70,000, and sufficient wiredrawability was not obtained.

[0135] In test numbers 42 to 45, the time t from immediately after final finish rolling to the start of water cooling exceeded 0.5 seconds. As a result, LmaxS / LaveS exceeded 1.70. In addition, FWHMc / FWHMs was less than 1.10. As a result, the evaluation value was less than 70,000, and sufficient wiredrawability was not obtained.

[0136] 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 wire rod having a chemical composition, in mass%, of C: 0.70-1.20%, Si: 0.10-1.50%, Mn: 0.10-1.00%, P: 0.020% or less, S: 0.020% or less, Al: 0.080% or less, N: 0.0010-0.0100%, O: 0.0050% or less, Cr: 0-0.50%, Co: 0-0.50%, Cu: 0-0.50%, Ni: 0-0.50%, Mo: 0-0.20%, B: 0-0.005%, W: 0-0.20%, Ti: 0-0.10%, Nb: 0-0.10%, Sn: 0-0.10%, As: 0-0.050%, Sb: 0-0.050%, V: 0-0.10%, Ca: 0-0.0050%, Mg: 0-0.0050%, Zr: 0-0.010%, and rare earth elements: 0-0.0050%, with the balance being Fe and impurities, wherein, in a circular cross section perpendicular to the axial direction of the wire, an average value of true lamellar spacing at a depth of 0.5 mm from the surface of the wire is LaveS (nm) and a maximum value of the true lamellar spacing is LmaxS (nm), LmaxS / LaveS is 1.70 or less, A wire in which, in the circular cross section, a full width at half maximum of the true lamellar spacing at a depth of 0.5 mm from the surface of the wire is FWHMs (nm) and a full width at half maximum of the true lamellar spacing at a center position of the wire is FWHMc (nm), in which FWHMc / FWHMs is 1.10 to 2.

00.

2. A wire according to claim 1, wherein the chemical composition contains, in mass%, one or more elements selected from the group consisting of Cr: 0.01-0.50%, Co: 0.01-0.50%, Cu: 0.01-0.50%, Ni: 0.01-0.50%, Mo: 0.01-0.20%, B: 0.001-0.005%, W: 0.01-0.20%, Ti: 0.01-0.10%, Nb: 0.01-0.10%, Sn: 0.01-0.10%, As: 0.001-0.050%, Sb: 0.001-0.050%, and V: 0.01-0.10%.

3. A wire according to claim 1 or 2, wherein the chemical composition contains, in mass%, one or more elements selected from the group consisting of Ca: 0.0001-0.0050%, Mg: 0.0001-0.0050%, Zr: 0.001-0.010%, and rare earth elements: 0.0001-0.0050%.