Wire material

Abstract

A wire rod that is excellent in drawability is provided. In a wire rod of the present embodiment, in a circular cross section perpendicular to an axial direction of the wire rod, when an average value of true interlamellar spacing at a position at a depth of 0.5 mm from the surface of the wire rod is designated as LaveS (nm) and a maximum value of the true interlamellar spacing is designated as LmaxS (nm), LmaxS / LaveS is 1.70 or less, and in the circular cross section, when a full width at half maximum of the true interlamellar spacing at a position at a depth of 0.5 mm from the surface of the wire rod is designated as FWHMs (nm) and a full width at half maximum of the true interlamellar spacing at a center position of the wire rod is designated as FWHMc (nm), FWHMc / FWHMs is 1.10 to 2.00.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a wire rod, and more particularly relates to a wire rod that is suitable as a starting material for steel cords or bead wires of automobile tires or for bridge cables and the like.BACKGROUND ART

[0002] A steel wire such as a steel cord for which a wire rod is used as a starting material is produced, for example, by the following production method. A wire rod having a diameter (diameter) of about 4 to 6 mm, which has been subjected to controlled cooling after hot rolling, is prepared. The wire rod is subjected to primary wire drawing to be made into a steel wire having a diameter of 3 to 4 mm. After the primary wire drawing, an intermediate patenting treatment is performed, and then secondary wire drawing is performed to obtain a steel wire having a diameter of 1 to 2 mm. The obtained steel wire is subjected to a final patenting treatment. After undergoing the final patenting, the steel wire is subjected to brass plating, and a final wet wire drawing is then performed to obtain a steel wire having a diameter of 0.15 to 0.40 mm. In addition, a plurality of the steel wires are twisted together by a stranding process to be made into a stranded steel wire. A steel cord is produced by the above process. Further, wires for bead, wires for bridge and the like are produced by subjecting wire rods after hot rolling to a patenting treatment, subjecting the resultant steel wires to wire drawing, and thereafter making the obtained steel wires into a stranded steel wire.

[0003] When a wire rod is subjected to wire drawing once or multiple times, if the amount of working in the wire drawing is large, wire fracture will easily occur. To suppress the occurrence of such kinds of wire fracture, usually the wire rod (or steel wire) is subjected to a heat treatment referred to as an "intermediate heat treatment" before the wire drawing. However, from the viewpoint of reducing the production cost and improving productivity, there is a demand to omit the intermediate heat treatment. Therefore, there is a need to further improve the drawability of wire rods.

[0004] Techniques for improving drawability of wire rods have been proposed in International Application Publication No. WO2018 / 117157 (Patent Literature 1) and International Application Publication No. WO2016 / 021556 (Patent Literature 2).

[0005] Patent Literature 1 discloses a wire rod having a chemical composition containing, in mass%, C: 0.90 to 1.20%, Si: 0.10 to 1.00%, Mn: 0.20 to 0.80%, Cr: 0.10 to 0.40%, Al: 0 to 0.002%, Ti: 0 to 0.002%, N: 0 to 0.0050%, P: 0 to 0.020%, S: 0 to 0.010%, O: 0 to 0.0040%, Mo: 0 to 0.20%, and B: 0 to 0.0030%, with the balance being Fe and impurities. An average value of % Mn+2× % Cr over the entirety of the wire rod is 0.50% to 1.00%. The microstructure consists of, by area fraction, pearlite in an amount of 90% or more, with the balance being any one or more types selected from ferrite, cementite, and bainite, of which the area fraction of cementite is less than 3%. The maximum grain size of TiN is less than 15 µm. The maximum value of % Mn+2× % Cr measured on a cutting surface having a right angle to the longitudinal direction of the wire rod in a region where both the content of S and the content of O are less than 1% in a central portion which is a region from the central axis of the wire rod to 1 / 10 of the diameter of the wire rod is 2.0 times or less than the average value of % Mn+2× % Cr over the entirety of the wire rod. The ratio of the maximum value to the minimum value of % Mn+2× % Cr measured on the cutting surface having the right angle to the longitudinal direction of the wire rod in a region where both the content of S and the content of O are less than 1% in an outer circumferential portion, which is a region from an outer edge of the central portion to a depth of 0.1 mm from the surface of the wire rod, is 2.0 or less. Here, "% Mn" and "% Cr" represent the amounts of Mn and Cr in percent by mass, respectively. In this wire rod, drawability is improved by adjusting the chemical composition, the microstructure which is mainly composed of pearlite, the size of TiN, and segregation of Mn and Cr.

[0006] Patent Literature 2 discloses a wire rod having a chemical composition containing, in mass%, C: 0.70 to 1.20%, Si: 0.10 to 1.2%, Mn: 0.10 to 1.0%, P: 0.001 to 0.012%, S: 0.001 to 0.010%, and N: 0.0010 to 0.0050%, with the balance being Fe and impurities. In this wire rod, in a cross section perpendicular to the longitudinal direction, the area fraction of pearlite is 95% or more and 100% or less, an average pearlite block size is 10 µm to 30 µm, and the standard deviation of the pearlite block size is 20 µm or less. In addition, when C (%), Si (%), and Mn (%) are taken as representing the contents in percent by mass of C, Si, and Mn, respectively, and Ceq. is determined using Formula (1) below, the tensile strength is 760×Ceq.+255 MPa or more and 760×Ceq.+325 MPa or less, 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

[0007] In this wire rod, drawability is improved by adjusting the pearlite block size.

[0008] Further, in Japanese Patent Application Publication No. 2012-126955 (Patent Literature 3), unlike Patent Literature 1 and Patent Literature 2, drawability is improved by adjusting the interlamellar spacing of pearlite. Specifically, Patent Literature 3 discloses a wire rod containing, in mass%, each of C: 0.68 to 0.86%, Si: 0.05 to 0.5%, and Mn: 0.1 to 0.8%, with the balance being Fe and unavoidable impurities. The microstructure of this wire rod includes pearlite in an amount of 95% by area or more, and an average interlamellar spacing L of the pearlite is within the range of 150 to 300 nm, a region of lamellae having a spacing of 120 nm or less among the lamellae of the pearlite is 30% or less, and the standard deviation of the interlamellar spacing is 50 nm or less. In addition, an average nodule diameter D of the pearlite is 40 µm or less, and the average nodule diameter D and the aforementioned average interlamellar spacing L satisfy the relation D < -0.1×L+60.CITATION LISTPATENT LITERATURE

[0009] Patent Literature 1: International Application Publication No. WO2018 / 117157 Patent Literature 2: International Application Publication No. WO2016 / 021556 Patent Literature 3: Japanese Patent Application Publication No. 2012-126955 SUMMARY OF INVENTIONTECHNICAL PROBLEM

[0010] The drawability of a wire rod can be improved by the techniques disclosed in Patent Literature 1 to Patent Literature 3. However, the drawability of a wire may also be improved by another technique.

[0011] An objective of the present invention is to provide a wire rod that is excellent in drawability.SOLUTION TO PROBLEM

[0012] A wire rod of the present disclosure has a chemical composition consisting of, 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 metal: 0 to 0.0050%, with the balance being Fe and impurities. In addition, in a circular cross section perpendicular to an axial direction of the wire rod, when an average value of true interlamellar spacing at a position at a depth of 0.5 mm from a surface of the wire rod is designated as LaveS (nm) and a maximum value of the true interlamellar spacing is designated as LmaxS (nm), LmaxS / LaveS is 1.70 or less. In the circular cross section, when a full width at half maximum of the true interlamellar spacing at a position at a depth of 0.5 mm from the surface of the wire rod is designated as FWHMs (nm) and a full width at half maximum of the true interlamellar spacing at a center position of the wire rod is designated as FWHMc (nm), FWHMc / FWHMs is 1.10 to 2.00.ADVANTAGEOUS EFFECT OF INVENTION

[0013] The wire rod of the present invention is excellent in drawability.BRIEF DESCRIPTION OF DRAWINGS

[0014] [FIG. 1] FIG. 1 is a schematic diagram for describing an apparent interlamellar spacing of pearlite and a true interlamellar spacing of pearlite in microstructure observation. [FIG. 2] FIG. 2 is a schematic diagram illustrating an example of an observation surface in a case where a cross section perpendicular to an axial direction of a wire rod is adopted as an observation surface. [FIG. 3] FIG. 3 is a schematic diagram in which an observation field in FIG. 2 has been separated into a plurality of micro-sections. [FIG. 4] FIG. 4 is a schematic diagram of a micro-section illustrated in FIG. 3. [FIG. 5] FIG. 5 is a schematic diagram of another micro-section, which is different from FIG. 4. [FIG. 6] FIG. 6 is a schematic diagram of another micro-section, which is different from FIG. 4 and FIG. 5. DESCRIPTION OF EMBODIMENTS

[0015] The present inventors first conducted studies regarding the chemical composition of a wire rod that is suitable as a starting material for steel cords and bead wires, and bridge cables and the like. As a result, the present inventors have considered that if a wire rod has a chemical composition consisting of, 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 metal: 0 to 0.0050%, with the balance being Fe and impurities, the wire rod will be suitable as a starting material for steel cords and bead wires, and bridge cables and the like.

[0016] The present inventors also investigated means for improving drawability in a wire rod satisfying the chemical composition described above. Here, rather than focusing on inclusions or a pearlite block size that are adjusted in the techniques disclosed in Patent Literature 1 and Patent Literature 2, the present inventors focused on the interlamellar spacing of pearlite. In Patent Literature 3, an "apparent interlamellar spacing" of pearlite observed when observing the microstructure of a two-dimensional cross section of a wire rod is determined.

[0017] Usually, microstructure observation is performed on a two-dimensional observation surface. Referring to FIG. 1, on an observation surface 10, when observing pearlite that includes ferrite lamellae 1 and cementite lamellae 2, a distance λ a that is the minimum distance between adjacent cementite lamellae 2 in pearlite on the observation surface 10 that is a two-dimensional plane is defined as an "apparent interlamellar spacing λ a ".

[0018] On the other hand, a distance λ t that is the minimum distance between adjacent cementite lamellae 2 in pearlite in a three-dimensional space is defined as a "true interlamellar spacing λ t ".

[0019] As illustrated in FIG. 1, the apparent interlamellar spacing λ a varies greatly depending on an angle Φ formed between the observation surface 10, which is a two-dimensional plane, and the extending direction of the ferrite lamellae 1 and the cementite lamellae 2. Consequently, in many cases the value of the apparent interlamellar spacing λ a will be different from the value of the true interlamellar spacing λ t . According to Patent Literature 3, among apparent interlamellar spacings λ a on an observation surface, a local region where the interlamellar spacing is small is selected, and the apparent interlamellar spacing of the local region is determined. However, it is conceivable that rather than the drawability being affected only by such kind of local region, the drawability is affected by the interlamellar spacing over a wider range. Based on the results of the investigations described above, the present inventors have considered that the distribution of the true interlamellar spacing λ t over a wide range affects drawability, and not the apparent interlamellar spacing λ a .

[0020] The present inventors have discovered that if the following method is used, the distribution state of the true interlamellar spacing λ t in a wire rod, which had not been investigated heretofore, can be determined.[Regarding method for measuring distribution of true interlamellar spacing λ t ]

[0021] The method for measuring the distribution of the interlamellar spacing λ t in a wire rod is carried out by performing the following three steps. (Step 1) Acquisition of scanning electron microscope (SEM) observation images (Step 2) Measurement of apparent interlamellar spacing distribution (Step 3) Measurement of true interlamellar spacing distribution

[0022] Here, as mentioned above, the term "apparent interlamellar spacing λ a " means the distance between adjacent cementite lamellae observed in an SEM micrograph that is a two-dimensional plane. The term "true interlamellar spacing λ t " means the actual distance between lamellar cementite plates in a three-dimensional space. Because the true interlamellar spacing λ t does not appear on the observation surface 10, the true interlamellar spacing λ t cannot be observed in an SEM micrograph. Therefore, in the present embodiment, a mathematical approach is used to calculate the true interlamellar spacing λ t based on the apparent interlamellar spacing λ a . Hereunder, each step will be described.[(Step 1) Acquisition of SEM micrographs]

[0023] The wire rod is cut perpendicular to the axial direction (longitudinal direction) thereof, and a cross section (cross section perpendicular to the axial direction of the wire rod) thereof is adopted as an observation surface. FIG. 2 is a schematic diagram illustrating an example of the observation surface. The observation surface 10 is subjected to mirror polishing. Within 5 minutes after the mirror polishing, the observation surface 10 is etched for about 7 seconds using picral to reveal the microstructure on the observation surface 10. On the observation surface 10 after etching, four observation fields SA in a surface layer region and four observation fields CA in a central region are subjected to SEM observation at a magnification of 2000× with an acceleration voltage of 15 kv to obtain SEM images. Specifically, as illustrated in FIG. 2, on the observation surface 10, rectangular observation fields SA are selected at four locations at a pitch of 90° around a center point C1 of the observation surface 10, with each of the observation fields SA being centered on a position at a depth of 0.5 mm from the surface and consisting of 5120 pixels in the circumferential direction × 3840 pixels in the radial direction of the observation surface 10. The area of each observation field SA is to be a rectangle with dimensions of 60 µm (5120 pixels) in the circumferential direction and 45 µm (3840 pixels) in the radial direction. Hereinafter, the four observation fields SA are referred to as "observation fields SA of the surface layer region".

[0024] In addition, observation fields CA at an arbitrary four locations are selected from a central region 20 that is a circular region centered on the center C1 of the observation surface 10 and which has a radius of D / 8 (D represents the diameter of the wire rod). Each observation field CA is to be a rectangle consisting of 5120 pixels in the circumferential direction × 3840 pixels in the radial direction, and the area of each observation field CA is to be a rectangle with dimensions of 60 µm (5120 pixels) in the circumferential direction and 45 µm (3840 pixels) in the radial direction. Note that, the center position of each observation field CA is to be a position located at D / 16 in the radial direction from the center point C1. Hereinafter, the four observation fields CA are referred to as "observation fields CA of the central region".

[0025] Note that, for example, an apparatus with the trade name "JSM-7100F" manufactured by JEOL Ltd. is used as the SEM apparatus.[(Step 2) Measurement of distribution of apparent interlamellar spacing λ a ]

[0026] The observation fields SA in the surface layer region and the observation fields CA in the central region are used to determine the distribution of the apparent interlamellar spacing λ a obtained from the SEM images of the two-dimensional plane in the surface layer region and the central region.

[0027] As illustrated in FIG. 3, for the respective SEM images (observation fields SA and observation fields CA), the SEM image is divided into micro-sections 30 of 50 pixels × 50 pixels (0.586 µm × 0.586 µm). Note that, since the SEM image is 5120 pixels × 3840 pixels, when the SEM image is divided into the micro-sections 30, regions arise that are not composed of 50 pixels × 50 pixels. These regions are excluded from the targets for measurement of the apparent interlamellar spacing λ a .

[0028] FIG. 4 is a schematic diagram illustrating one example of the micro-section 30. In each micro-section, the apparent interlamellar spacing λ a is determined by the following method. As illustrated in FIG. 4, among line segments that pass through a center P0 of the micro-section and which have the largest number of intersection points with the cementite lamellae 2, a line segment SG for which L0 = {length of line segment / (number of intersection points between the relevant line segment and the cementite lamellae -1)} is the smallest value is determined. Here, the length of the line segment is taken as the length between the two intersection points P1 and P2 that are furthest apart among the intersection points between the relevant line segment and the cementite lamellae 2. L0 of the determined line segment is taken as the apparent interlamellar spacing λ a (nm) in the relevant micro-section 30. In the example in FIG. 4, the number of intersection points between the line segment SG and the cementite lamellae is "4".

[0029] Note that, as illustrated in FIG. 5, among the plurality of cementite lamellae 2 within the micro-section 30, a numerical proportion NR of cementite lamellae 2 which are linear cementite lamellae 2 in which both of the endpoints do not contact a side of the micro-section 30 is more than 50%. More specifically, in FIG. 5, among seven cementite lamellae 2 within the micro-section 30, there are six cementite lamellae 2 in which both endpoints do not contact a side of the micro-section, and therefore the numerical proportion NR is 86% (= 6 / 7 × 100). In a case such as this in which, among a plurality of cementite lamellae 2 within the micro-section 30, the numerical proportion NR of cementite lamellae 2 in which both endpoints do not contact a side of the micro-section 30 is more than 50%, the relevant micro-section 30 is excluded from the targets for measurement of the apparent interlamellar spacing λ a . Note that, in FIG. 6, the numerical proportion NR is 43% (= 3 / 7×100). Therefore, the micro-section 30 in FIG. 6 is taken as a measurement target.

[0030] The distribution of the apparent interlamellar spacing λ a in the surface layer region and the distribution of the apparent interlamellar spacing λ a in the central region are determined by the above method.[(Step 3) Measurement of true interlamellar spacing distribution]

[0031] As mentioned above, the distribution of the apparent interlamellar spacing λ a obtained from an SEM image that is a two-dimensional plane is different from the distribution of the true interlamellar spacing λ t in a wire rod. Therefore, the distribution of the true interlamellar spacing λ t is determined by applying the distribution of the apparent interlamellar spacing λ a determined in step 2 to Formula (a) for fitting. Here, Formula (a) is a formula showing the relation between the distribution of the apparent interlamellar spacing λ a and the distribution of the true interlamellar spacing λ t . [Math 1] ρ λ a = ∫ 0 λ a G λ t λ t 2 λ a 2 λ a 2 − λ t 2 dλ t

[0032] In Formula (a), λ a represents the apparent interlamellar spacing, λ t represents the true interlamellar spacing, ρ(λ a ) represents the distribution of the apparent interlamellar spacing, and G(λ t ) represents the distribution of the true interlamellar spacing λ t . G(λ t ) is the Gauss distribution function which is expressed by Formula (b). A, B, and C in Formula (b) are fitting parameters. [Math 2] G λ t = A × exp − λ t − B 2 2 C 2

[0033] As mentioned above, the apparent interlamellar spacing λ a depends on an angle of lamellar orientation Φ illustrated in FIG. 1. Therefore, it is necessary to convert from the apparent interlamellar spacing λ a to the true interlamellar spacing λ t . As illustrated in FIG. 1, the apparent interlamellar spacing λ a and the true interlamellar spacing λ t satisfy Formula (1). [Math 3] λ a = λ t cos ϕ

[0034] In addition, the relation between the Φ space and the λa space is expressed by Formula (2). ρ Φ dΦ = ρ λ a dλ a

[0035] Here, ρ(Φ) and ρ(λ a ) are probability density functions. Since the apparent interlamellar spacing λ a is irregular (random) with respect to the observation surface 10, ρ(λ a ) is expressed by Formula (3). ρ λ a = const .

[0036] Based on Formula (1) to Formula (3), in a case where the true interlamellar spacing λ t has no distribution, the apparent interlamellar spacing λ a is expressed by Formula (4). [Math 4] ρ λ a = 2 π λ t λ a 2 − λ t 2

[0037] Here, when the distribution G(λ t ) of the true interlamellar spacing λ t is taken into consideration with respect to Formula (4), the distribution of the apparent interlamellar spacing λ a is expressed as shown in Formula (a). Here, let us assume that the distribution G(λ t ) of the true interlamellar spacing λ t is a normal distribution.

[0038] By applying the distribution of the apparent interlamellar spacing λ a in the surface layer region to Formula (a) for fitting, an average value LaveS (nm) (= B) of the true interlamellar spacing λ t , a maximum value LmaxS (nm) (= B+2.33C) of the true interlamellar spacing λ t , and a full width at half maximum FWHMs (= 2.35×C) of the distribution of the true interlamellar spacing λ t are determined. Similarly, by applying the distribution of the apparent interlamellar spacing λ a in the central region to Formula (a) for fitting, a full width at half maximum FWHMc of the distribution of the true interlamellar spacing λ t is determined.

[0039] The present inventors investigated the relation between the distribution state of interlamellar spacing obtained by the above method and a reduction of area obtained by performing a tensile test on a wire rod. As a result, it was found that the sharper the distribution of the interlamellar spacing (true interlamellar spacing λ t ) obtained by the above method was, in other words, the narrower the distribution range of the interlamellar spacing was, the higher the reduction of area became. As a result of further investigation, the present inventors have discovered that, in a circular cross section perpendicular to the axial direction in a wire rod, when an average value of the interlamellar spacing at a position at a depth of 0.5 mm from the surface of the wire rod is designated as LaveS (nm) and a maximum value of the interlamellar spacing is designated as LmaxS (nm), if LmaxS / LaveS is 1.70 or less, the drawability will improve.

[0040] However, even in the case of wire rods having a chemical composition satisfying the chemical composition described above and in which LmaxS / LaveS was 1.70 or less, there were still cases where excellent drawability could not be obtained. Therefore, the present inventors conducted further studies. As a result, the present inventors obtained the following finding.

[0041] In a circular cross section perpendicular to the axial direction of a wire rod, at the outer layer, as described above, the narrower the distribution range of the interlamellar spacing is, the more the drawability improves. However, with respect to the central portion in the circular cross section perpendicular to the axial direction of the wire rod, in contrast, excellent work hardening properties are obtained when the distribution range of the interlamellar spacing is wide to a certain extent. Although the reason for this is unclear, the following reason is conceivable.

[0042] When subjecting a wire rod to drawing, strain is introduced into the outer layer of the wire rod. This causes work hardening of the outer layer. Here, in order to improve drawability, it is preferable that work hardening progresses uniformly in the circular cross section. If work hardening progresses in a non-uniform manner, variations in hardness will occur within the circular cross section. In such case, cracks will occur and propagate locally from regions where the hardness is low in the circular cross section, which will lead to a fracture. On the other hand, if work hardening progresses uniformly, variations in hardness will not occur within the circular cross section. In such case, the occurrence of local cracks will be suppressed, and as a result the drawability will improve.

[0043] In order to cause work hardening to progress uniformly in the circular cross section, it is effective to make the distribution range of the interlamellar spacing in the central region of the circular cross section wider than the distribution range of the interlamellar spacing in the surface layer region. In other words, it is effective for the interlamellar spacing in the central region to vary more than the interlamellar spacing in the surface layer region. In such case, work hardening of the outer layer will easily progress to the central region, and the hardness distribution in the circular cross section will become more uniform. As a result, excellent drawability will be obtained in the wire rod.

[0044] Based on the above finding, the present inventors investigated the relation between the distribution of the interlamellar spacing in the surface layer region of the circular cross section, the distribution of the interlamellar spacing in the central region, and the drawability. As a result, the present inventors have discovered that, in the circular cross section, when a full width at half maximum of the interlamellar spacing at a position at a depth of 0.5 mm from the surface of the wire rod is designated as FWHMs (nm) and a full width at half maximum of the interlamellar spacing at the center position of the wire rod is designated as FWHMc (nm), if FWHMc / FWHMs is 1.10 to 2.00, excellent drawability will be obtained.

[0045] Note that, even when the micro-sections 30 in which the numerical proportion NR is more than 50% as illustrated in FIG. 5 are excluded from the targets to be measured by step 1 to step 3, as long as the micro-sections 30 in which the numerical proportion NR is 50% or less as illustrated in FIG. 4 and FIG. 6 are taken as the measurement targets, the determined average value LaveS (nm), maximum value LmaxS (nm), and full width at half maximum FWHMs (nm) of the true interlamellar spacing λ t in the surface layer region and the full width at half maximum FWHMc (nm) of the true interlamellar spacing λ t in the central region will exhibit a good correlation with drawability.

[0046] The gist of the wire rod of the present invention, which was completed based on the findings described above, is as follows.

[0047] A wire rod according to a first aspect has a chemical composition consisting of, 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 metal: 0 to 0.0050%, with the balance being Fe and impurities. In addition, in a circular cross section perpendicular to an axial direction of the wire rod, when an average value of true interlamellar spacing at a position at a depth of 0.5 mm from a surface of the wire rod is designated as LaveS (nm) and a maximum value of the true interlamellar spacing is designated as LmaxS (nm), LmaxS / LaveS is 1.70 or less. In the circular cross section, when a full width at half maximum of the true interlamellar spacing at a position at a depth of 0.5 mm from the surface of the wire rod is designated as FWHMs (nm) and a full width at half maximum of the true interlamellar spacing at a center position of the wire rod is designated as FWHMc (nm), FWHMc / FWHMs is 1.10 to 2.00.

[0048] A wire rod according to a second aspect is in accordance with the wire rod of the first aspect, wherein the chemical composition contains one or more elements selected from a group consisting of, in mass%, 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%.

[0049] A wire rod according to a third aspect has a chemical composition consisting of, 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%, V: 0 to 0.10%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, Zr: 0 to 0.010%, and rare earth metal: 0 to 0.0050%, with the balance being Fe and impurities. In addition, in a circular cross section perpendicular to an axial direction of the wire rod, when an average value of true interlamellar spacing at a position at a depth of 0.5 mm from a surface of the wire rod is designated as LaveS (nm) and a maximum value of the true interlamellar spacing is designated as LmaxS (nm), LmaxS / LaveS is 1.70 or less. In the circular cross section, when a full width at half maximum of the true interlamellar spacing at a position at a depth of 0.5 mm from the surface of the wire rod is designated as FWHMs (nm) and a full width at half maximum of the true interlamellar spacing at a center position of the wire rod is designated as FWHMc (nm), FWHMc / FWHMs is 1.10 to 2.00.

[0050] A wire rod according to a fourth aspect is in accordance with the wire rod of the third aspect, wherein the chemical composition contains one or more elements selected from a group consisting of, in mass%, 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%.

[0051] A wire rod according to a fifth aspect is in accordance with the wire rod according to any one of the first to fourth aspects, wherein the chemical composition contains one or more elements selected from a group consisting of, in mass%, Ca: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, Zr: 0.001 to 0.010%, and rare earth metal: 0.0001 to 0.0050%.

[0052] Hereunder, the wire rod according to the present embodiment will be described in detail. Note that, the symbol "%" in relation to elements means mass percent unless otherwise stated.[Features of wire rod of present embodiment]

[0053] The wire rod of the present embodiment includes the following feature 1 to feature 3.(Feature 1)

[0054] The chemical composition consists of, 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 metal: 0 to 0.0050%, with the balance being Fe and impurities.(Feature 2)

[0055] In a circular cross section perpendicular to the axial direction of the wire rod, when an average value of true interlamellar spacing at a position at a depth of 0.5 mm from the surface of the wire rod is designated as LaveS (nm) and a maximum value of the true interlamellar spacing is designated as LmaxS (nm), LmaxS / LaveS is 1.70 or less.(Feature 3)

[0056] In the circular cross section, when a full width at half maximum of the true interlamellar spacing at a position at a depth of 0.5 mm from the surface of the wire rod is designated as FWHMs (nm) and a full width at half maximum of the true interlamellar spacing at a center position of the wire rod is designated as FWHMc (nm), FWHMc / FWHMs is 1.10 to 2.00.

[0057] Feature 1 to feature 3 are described hereunder.[(Feature 1) Regarding chemical composition]

[0058] The chemical composition of the wire rod of the present embodiment contains the following elements.C: 0.70 to 1.20%

[0059] Carbon (C) increases the strength of the wire rod. If the content of C is less than 0.70%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

[0060] On the other hand, if the content of C is more than 1.20%, even if the contents of other elements are within the range of the present embodiment, pro-eutectoid cementite will excessively form. In such case, the drawability of the wire rod will decrease.

[0061] Therefore, the content of C is 0.70 to 1.20%.

[0062] A preferable lower limit of the content of C is 0.72%, more preferably is 0.78%, further preferably is 0.80%, and further preferably is 0.82%.

[0063] A preferable upper limit of the content of C is 1.18%, more preferably is 1.12%, further preferably is 1.08%, and further preferably is 1.05%.Si: 0.10 to 1.50%

[0064] Silicon (Si) increases the strength of the wire rod. In addition, Si deoxidizes the steel in a steelmaking process during the process of producing the wire rod. If the content of Si is less than 0.10%, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

[0065] On the other hand, if the content of Si is more than 1.50%, even if the contents of other elements are within the range of the present embodiment, Si will segregate inside the wire rod that will serve as a starting material for a steel wire. In such case, bainite will form in regions where Si segregated, and the drawability of the wire rod will decrease.

[0066] Therefore, the content of Si is 0.10 to 1.50%.

[0067] A preferable lower limit of the content of Si is 0.15%, more preferably is 0.20%, and further preferably is 0.25%.

[0068] A preferable upper limit of the content of Si is 1.42%, more preferably is 1.35%, further preferably is 1.00%, further preferably is 0.90%, and further preferably is 0.80%.Mn: 0.10 to 1.00%

[0069] Manganese (Mn) increases hardenability of the steel material and thereby increases the strength of the wire rod. In addition, Mn fixes S in the steel material, and thereby increases the hot workability of the wire rod. If the content of Mn is less than 0.10%, the aforementioned advantageous effects will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

[0070] On the other hand, if the content of Mn is more than 1.00%, even if the contents of other elements are within the range of the present embodiment, Mn will segregate inside the wire rod. In such case, bainite will form in regions where Mn segregated, and the drawability of the wire rod will decrease.

[0071] Therefore, the content of Mn is 0.10 to 1.00%.

[0072] A preferable lower limit of the content of Mn is 0.12%, more preferably is 0.20%, and further preferably is 0.25%.

[0073] A preferable upper limit of the content of Mn is 0.90%, more preferably is 0.85%, further preferably is 0.80%, and further preferably is 0.70%.P: 0.020% or less

[0074] Phosphorus (P) is an impurity. If the content of P is more than 0.020%, P will segregate to grain boundaries even if the contents of other elements are within the range of the present embodiment. Consequently, the grain boundaries will become brittle and the drawability of the wire rod will decrease.

[0075] Therefore, the content of P is 0.020% or less.

[0076] The content of P is preferably as low as possible. However, excessively reducing the content of P will increase the production cost. Therefore, when ordinary industrial production is taken into consideration, a preferable lower limit of the content of P is more than 0%, more preferably is 0.001%, further preferably is 0.003%, and further preferably is 0.005%.

[0077] A preferable upper limit of the content of P is 0.018%, more preferably is 0.016%, and further preferably is 0.015%.S: 0.020% or less

[0078] Sulfur (S) is an impurity. If the content of S is more than 0.020%, S will segregate to grain boundaries even if the contents of other elements are within the range of the present embodiment. In addition, coarse MnS will excessively form. Consequently, the drawability of the wire rod will decrease.

[0079] Therefore, the content of S is 0.020% or less.

[0080] The content of S is preferably as low as possible. However, excessively reducing the content of S will increase the production cost. Therefore, when ordinary industrial production is taken into consideration, a preferable lower limit of the content of S is more than 0%, more preferably is 0.001%, further preferably is 0.002%, and further preferably is 0.003%.

[0081] A preferable upper limit of the content of S is 0.018%, more preferably is 0.016%, further preferably is 0.014%, further preferably is 0.010%, and further preferably is 0.008%.Al: 0.080% or less

[0082] Aluminum (Al) deoxidizes the steel. If even a small amount of Al is contained, the aforementioned advantageous effect will be obtained to a certain extent. However, if the content of Al is more than 0.080%, coarse oxides will excessively form even if the contents of other elements are within the range of the present embodiment. In such case, the drawability of the wire rod will decrease.

[0083] Therefore, the content of Al is 0.080% or less.

[0084] A preferable lower limit of the content of Al is more than 0%, more preferably is 0.001%, further preferably is 0.003%, and further preferably is 0.0007%.

[0085] A preferable upper limit of the content of Al is 0.075%, more preferably is 0.070%, further preferably is 0.065%, and further preferably is 0.0060%.N: 0.0010 to 0.0100%

[0086] Nitrogen (N) is locking dislocations when the wire rod is subjected to wire drawing, and increases the strength of the steel wire obtained after the wire drawing. If the content of N is less than 0.0010%, the aforementioned advantageous effect will not be sufficiently obtained even if the contents of other elements are within the range of the present embodiment.

[0087] On the other hand, if the content of N is more than 0.0100%, even if the contents of other elements are within the range of the present embodiment, nitrides will excessively form in the wire rod. In such case, the drawability of the wire rod will decrease.

[0088] Therefore, the content of N is 0.0010 to 0.0100%.

[0089] A preferable lower limit of the content of N is 0.0015%, more preferably is 0.0020%, and further preferably is 0.0025%.

[0090] A preferable upper limit of the content of N is 0.0090%, more preferably is 0.0080%, further preferably is 0.0070%, and further preferably is 0.0060%.O: 0.0050% or less

[0091] Oxygen (O) is an impurity. If the content of O is more than 0.0050%, coarse oxides will excessively form in the wire rod even if the contents of other elements are within the range of the present embodiment. In such case, the drawability of the wire rod will decrease.

[0092] Therefore, the content of O is 0.0050% or less.

[0093] The content of O is preferably as low as possible. However, excessively reducing the content of O will increase the production cost. Therefore, when ordinary industrial production is taken into consideration, a preferable lower limit of the content of O is more than 0%, more preferably is 0.0001%, further preferably is 0.0003%, and further preferably is 0.0005%.

[0094] A preferable upper limit of the content of O is 0.0045%, more preferably is 0.0040%, further preferably is 0.0030%, and further preferably is 0.0020%.

[0095] The balance of the chemical composition of the wire rod according to the present embodiment is Fe and impurities. Here, the term "impurities" in the chemical composition means substances which are mixed in from ore and scrap used as raw material or from the production environment or the like when industrially producing the wire rod, and which are substances that are not intentionally contained but are permitted within a range that does not adversely affect the wire rod according to the present embodiment.[Regarding optional elements]

[0096] The chemical composition of the wire rod of the present embodiment may further contain, in lieu of a part of Fe, one or more elements selected from a group consisting of 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 metal: 0 to 0.0050%. Each of these elements is an optional element. Hereunder, each optional element will be described.[First group: Cr, Co, Cu, Ni, Mo, B, W, Ti, Nb, Sn, As, Sb, and V]

[0097] The chemical composition of the wire rod of the present embodiment may further contain, in lieu of a part of Fe, one or more elements selected from a group consisting of Cr, Co, Cu, Ni, Mo, B, W, Ti, Nb, Sn, As, Sb, and V. These elements are optional elements, and each of these elements increases the hardenability of the steel material.Cr: 0 to 0.50%

[0098] Chromium (Cr) is an optional element, and does not have to be contained. That is, the content of Cr may be 0%.

[0099] When contained, that is, when the content of Cr is more than 0%, Cr increases the hardenability of the steel material and thereby increases the strength of the wire rod. If even a small amount of Cr is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0100] However, if the content of Cr is more than 0.50%, Cr will segregate inside the wire rod even if the contents of other elements are within the range of the present embodiment. In such case, bainite will form in regions where Cr segregated, and the drawability of the wire rod will decrease.

[0101] Therefore, the content of Cr is 0 to 0.50%.

[0102] A preferable lower limit of the content of Cr is 0.01%, more preferably is 0.05%, and further preferably is 0.08%.

[0103] A preferable upper limit of the content of Cr is 0.45%, more preferably is 0.40%, further preferably is 0.35%, and further preferably is 0.30%.Co: 0 to 0.50%

[0104] Cobalt (Co) is an optional element, and does not have to be contained. That is, the content of Co may be 0%.

[0105] When contained, that is, when the content of Co is more than 0%, Co increases the hardenability of the steel material and thereby increases the strength of the wire rod. If even a small amount of Co is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0106] However, if the content of Co is more than 0.50%, even if the contents of other elements are within the range of the present embodiment, the hardness of the wire rod will excessively increase and the drawability of the wire rod will decrease.

[0107] Therefore, the content of Co is 0 to 0.50%.

[0108] A preferable lower limit of the content of Co is 0.01%, more preferably is 0.05%, and further preferably is 0.08%.

[0109] A preferable upper limit of the content of Co is 0.45%, more preferably is 0.40%, further preferably is 0.35%, further preferably is 0.30%, and further preferably is 0.20%.Cu: 0 to 0.50%

[0110] Copper (Cu) is an optional element, and does not have to be contained. That is, the content of Cu may be 0%.

[0111] When contained, that is, when the content of Cu is more than 0%, Cu increases the hardenability of the steel material and thereby increases the strength of the wire rod. If even a small amount of Cu is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0112] However, if the content of Cu is more than 0.50%, even if the contents of other elements are within the range of the present embodiment, the hardness of the wire rod will excessively increase and the drawability of the wire rod will decrease.

[0113] Therefore, the content of Cu is 0 to 0.50%.

[0114] A preferable lower limit of the content of Cu is 0.01%, more preferably is 0.05%, further preferably is 0.08%, and further preferably is 0.15%.

[0115] A preferable upper limit of the content of Cu is 0.45%, more preferably is 0.40%, further preferably is 0.35%, further preferably is 0.30%, and further preferably is 0.25%.Ni: 0 to 0.50%

[0116] Nickel (Ni) is an optional element, and does not have to be contained. That is, the content of Ni may be 0%.

[0117] When contained, that is, when the content of Ni is more than 0%, Ni increases the hardenability of the steel material. In addition, Ni delays transformation of pro-eutectoid cementite and grain boundary ferrite during cooling from the austenite temperature range in the process for producing the wire rod. This causes pearlite lamellae to be finely formed. As a result, while sufficiently maintaining the drawability of the wire rod, the strength of a steel wire produced using the wire rod as a starting material is increased. If even a small amount of Ni is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0118] However, if the content of Ni is more than 0.50%, the hardenability of the steel material will be excessively high. In such case, even if the contents of other elements are within the range of the present embodiment, bainite will form inside the wire rod and the drawability of the wire rod will decrease.

[0119] Therefore, the content of Ni is 0 to 0.50%.

[0120] A preferable lower limit of the content of Ni is 0.01%, more preferably is 0.05%, and further preferably is 0.08%.

[0121] A preferable upper limit of the content of Ni is 0.45%, more preferably is 0.40%, further preferably is 0.35%, further preferably is 0.30%, and further preferably is 0.25%.Mo: 0 to 0.20%

[0122] Molybdenum (Mo) is an optional element, and does not have to be contained. That is, the content of Mo may be 0%.

[0123] When contained, that is, when the content of Mo is more than 0%, Mo increases the hardenability of the wire rod and thereby increases the strength of the wire rod. If even a small amount of Mo is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0124] However, if the content of Mo is more than 0.20%, even if the contents of other elements are within the range of the present embodiment, the hot workability of the steel material will decrease in the process for producing the wire rod.

[0125] Therefore, the content of Mo is 0 to 0.20%.

[0126] A preferable lower limit of the content of Mo is 0.01%, more preferably is 0.03%, and further preferably is 0.05%.

[0127] A preferable upper limit of the content of Mo is 0.18%, more preferably is 0.16%, further preferably is 0.14%, further preferably is 0.12%, and further preferably is 0.10%.B: 0 to 0.005%

[0128] Boron (B) is an optional element, and does not have to be contained. That is, the content of B may be 0%.

[0129] When contained, that is, when the content of B is more than 0%, B increases the hardenability of the wire rod and thereby increases the strength of the wire rod. If even a small amount of B is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0130] However, if the content of B is more than 0.005%, even if the contents of other elements are within the range of the present embodiment, the hot workability of the steel material will decrease in the process for producing the wire rod.

[0131] Therefore, the content of B is 0 to 0.005%.

[0132] A preferable lower limit of the content of B is 0.001%, and more preferably is 0.002%.

[0133] A preferable upper limit of the content of B is 0.004%, more preferably is 0.003%, and further preferably is 0.002%.W: 0 to 0.20%

[0134] Tungsten (W) is an optional element, and does not have to be contained. That is, the content of W may be 0%.

[0135] When contained, that is, when the content of W is more than 0%, W increases the hardenability of the wire rod and thereby increases the strength of the wire rod. If even a small amount of W is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0136] However, if the content of W is more than 0.20%, even if the contents of other elements are within the range of the present embodiment, the hot workability of the steel material will decrease in the process for producing the wire rod.

[0137] Therefore, the content of W is 0 to 0.20%.

[0138] A preferable lower limit of the content of W is 0.01%, more preferably is 0.03%, further preferably is 0.05%, and further preferably is 0.08%.

[0139] A preferable upper limit of the content of W is 0.18%, more preferably is 0.16%, further preferably is 0.14%, further preferably is 0.12%, and further preferably is 0.10%.Ti: 0 to 0.10%

[0140] Titanium (Ti) is an optional element, and does not have to be contained. That is, the content of Ti may be 0%.

[0141] When contained, that is, when the content of Ti is more than 0%, Ti increases the hardenability of the wire rod and thereby increases the strength of the wire rod.

[0142] If even a small amount of Ti is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0143] However, if the content of Ti is more than 0.10%, even if the contents of other elements are within the range of the present embodiment, the hot workability of the steel material will decrease in the process for producing the wire rod.

[0144] Therefore, the content of Ti is 0 to 0.10%.

[0145] A preferable lower limit of the content of Ti is 0.01%, more preferably is 0.02%, and further preferably is 0.03%.

[0146] A preferable upper limit of the content of Ti is 0.09%, more preferably is 0.08%, further preferably is 0.07%, and further preferably is 0.05%.Nb: 0 to 0.10%

[0147] Niobium (Nb) is an optional element, and does not have to be contained. That is, the content of Nb may be 0%.

[0148] When contained, that is, when the content of Nb is more than 0%, Nb increases the hardenability of the wire rod and thereby increases the strength of the wire rod. If even a small amount of Nb is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0149] However, if the content of Nb is more than 0.10%, even if the contents of other elements are within the range of the present embodiment, the hot workability of the steel material will decrease in the process for producing the wire rod.

[0150] Therefore, the content of Nb is 0 to 0.10%.

[0151] A preferable lower limit of the content of Nb is 0.01%, more preferably is 0.02%, and further preferably is 0.03%.

[0152] A preferable upper limit of the content of Nb is 0.09%, more preferably is 0.08%, further preferably is 0.07%, and further preferably is 0.05%.Sn: 0 to 0.10%

[0153] Tin (Sn) is an optional element, and does not have to be contained. That is, the content of Sn may be 0%.

[0154] When contained, that is, when the content of Sn is more than 0%, Sn suppresses ferrite decarburization that occurs during hot working, and suppresses a reduction in the strength of the wire rod. If even a small amount of Sn is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0155] However, if the content of Sn is more than 0.10%, even if the contents of other elements are within the range of the present embodiment, Sn will segregate to grain boundaries and hot shortness will occur. In such case, the hot workability of the wire rod will decrease.

[0156] Therefore, the content of Sn is 0 to 0.10%.

[0157] A preferable lower limit of the content of Sn is 0.01%, more preferably is 0.02%, and further preferably is 0.03%.

[0158] A preferable upper limit of the content of Sn is 0.09%, more preferably is 0.08%, and further preferably is 0.07%.As: 0 to 0.050%

[0159] Arsenic (As) is an optional element, and does not have to be contained. That is, the content of As may be 0%.

[0160] When contained, that is, when the content of As is more than 0%, As increases the strength of the wire rod. If even a small amount of As is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0161] However, if the content of As is more than 0.050%, the hot workability of the wire rod will decrease even if the contents of other elements are within the range of the present embodiment.

[0162] Therefore, the content of As is 0 to 0.050%.

[0163] A preferable lower limit of the content of As is 0.001%, more preferably is 0.005%, and further preferably is 0.010%.

[0164] A preferable upper limit of the content of As is 0.049%, more preferably is 0.040%, and further preferably is 0.030%.Sb: 0 to 0.050%

[0165] Antimony (Sb) is an optional element, and does not have to be contained. That is, the content of Sb may be 0%.

[0166] When contained, that is, when the content of Sb is more than 0%, Sb increases the strength of the wire rod. If even a small amount of Sb is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0167] However, if the content of Sb is more than 0.050%, the hot workability of the wire rod will decrease even if the contents of other elements are within the range of the present embodiment.

[0168] Therefore, the content of Sb is 0 to 0.050%.

[0169] A preferable lower limit of the content of Sb is 0.001%, more preferably is 0.005%, and further preferably is 0.010%.

[0170] A preferable upper limit of the content of Sb is 0.049%, more preferably is 0.040%, and further preferably is 0.030%.V: 0 to 0.10%

[0171] Vanadium (V) is an optional element, and does not have to be contained. That is, the content of V may be 0%.

[0172] When contained, that is, when the content of V is more than 0%, V increases the hardenability of the wire rod and thereby increases the strength of the wire rod. If even a small amount of V is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0173] However, if the content of V is more than 0.10%, the drawability of the wire rod will decrease even if the contents of other elements are within the range of the present embodiment.

[0174] Therefore, the content of V is 0 to 0.10%.

[0175] A preferable lower limit of the content of V is 0.01%, more preferably is 0.02%, and further preferably is 0.03%.

[0176] A preferable upper limit of the content of V is 0.09%, more preferably is 0.08%, and further preferably is 0.07%.[Second group: Ca, Mg, Zr, and rare earth metal (REM)]

[0177] The chemical composition of the wire rod of the present embodiment may further contain, in lieu of a part of Fe, one or more elements selected from a group consisting of Ca, Mg, Zr, and rare earth metal (REM). These elements are optional elements, and each of these elements increases the ductility of the wire rod.Ca: 0 to 0.0050%

[0178] Calcium (Ca) is an optional element, and does not have to be contained. That is, the content of Ca may be 0%.

[0179] When contained, that is, when the content of Ca is more than 0%, Ca reduces hard alumina-based inclusions and thereby increases the ductility of the wire rod. If even a small amount of Ca is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0180] However, if the content of Ca is more than 0.0050%, coarse oxides will excessively form even if the contents of other elements are within the range of the present embodiment. In such case, the drawability of the wire rod will decrease.

[0181] Therefore, the content of Ca is 0 to 0.0050%.

[0182] A preferable lower limit of the content of Ca is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0008%.

[0183] A preferable upper limit of the content of Ca is 0.0045%, more preferably is 0.0040%, further preferably is 0.0035%, further preferably is 0.0030%, further preferably is 0.0025%, and further preferably is 0.0020%.Mg: 0 to 0.0050%

[0184] Magnesium (Mg) is an optional element, and does not have to be contained. That is, the content of Mg may be 0%.

[0185] When contained, that is, when the content of Mg is more than 0%, Mg forms fine oxides. The fine oxides refine the microstructure of the wire rod, and thereby increase the ductility of the wire rod. If even a small amount of Mg is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0186] However, if the content of Mg is more than 0.0050%, coarse oxides will excessively form even if the contents of other elements are within the range of the present embodiment. In such case, the drawability of the wire rod will decrease.

[0187] Therefore, the content of Mg is 0 to 0.0050%.

[0188] A preferable lower limit of the content of Mg is 0.0001%, more preferably is 0.0005%, and further preferably is 0.0008%.

[0189] A preferable upper limit of the content of Mg is 0.0045%, more preferably is 0.0040%, further preferably is 0.0035%, further preferably is 0.0030%, and further preferably is 0.0025%.Zr: 0 to 0.010%

[0190] Zirconium (Zr) is an optional element, and does not have to be contained. That is, the content of Zr may be 0%.

[0191] When contained, that is, when the content of Zr is more than 0%, Zr forms fine oxides. The fine oxides refine the microstructure of the wire rod, and thereby increase the ductility of the wire rod. If even a small amount of Zr is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0192] However, if the content of Zr is more than 0.010%, coarse oxides will excessively form even if the contents of other elements are within the range of the present embodiment. In such case, the drawability of the wire rod will decrease.

[0193] Therefore, the content of Zr is 0 to 0.010%.

[0194] A preferable lower limit of the content of Zr is 0.001%, more preferably is 0.002%, and further preferably is 0.003%.

[0195] A preferable upper limit of the content of Zr is 0.009%, more preferably is 0.008%, and further preferably is 0.007%.Rare earth metal: 0 to 0.0050%

[0196] Rare earth metal (REM) is an optional element, and does not have to be contained. In other words, the content of REM may be 0%.

[0197] When contained, that is, when the content of REM is more than 0%, REM forms fine sulfides and thereby renders S harmless. Therefore, the ductility of the wire rod increases. If even a small amount of REM is contained, the aforementioned advantageous effect will be obtained to a certain extent.

[0198] However, if the content of REM is more than 0.0050%, coarse oxides will excessively form even if the contents of other elements are within the range of the present embodiment. In such case, the drawability of the wire rod will decrease.

[0199] Therefore, the content of REM is 0 to 0.0050%.

[0200] A preferable lower limit of the content of REM is 0.0001%, more preferably is 0.0005%, further preferably is 0.0008%, further preferably is 0.0013%, and further preferably is 0.0018%.

[0201] A preferable upper limit of the content of REM is 0.0045%, more preferably is 0.0040%, further preferably is 0.0035%, and further preferably is 0.0030%.

[0202] In the present description, the term "REM" means one or more elements selected from a group consisting of scandium (Sc) which is the element with atomic number 21, yttrium (Y) which is the element with atomic number 39, and the elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71 that are lanthanoids. Further, in the present description, the term "content of REM" means the total content of these elements.[(Feature 2) Regarding LmaxS / LaveS]

[0203] In the wire rod of the present embodiment, in addition, in a circular cross section perpendicular to the axial direction of the wire rod, when an average value of true interlamellar spacing at a position at a depth of 0.5 mm from the surface of the wire rod is designated as LaveS (nm) and a maximum value of the true interlamellar spacing is designated as LmaxS (nm), LmaxS / LaveS is 1.70 or less. In the outer layer of the wire rod, if LmaxS / LaveS is 1.70 or less, the drawability of the wire rod will improve.

[0204] A preferable upper limit of LmaxS / LaveS is 1.65, more preferably is 1.60, further preferably is 1.55, and further preferably is 1.50.

[0205] The lower limit of LmaxS / LaveS is not particularly limited. The lower limit of LmaxS / LaveS is, for example, 1.20, or for example 1.10, or for example 1.00.[(Feature 3) Regarding FWHMc / FWHMs]

[0206] In the wire rod of the present embodiment, in addition, in the circular cross section, when a full width at half maximum of the true interlamellar spacing at a position at a depth of 0.5 mm from the surface of the wire rod is designated as FWHMs (nm) and a full width at half maximum of the true interlamellar spacing at the center position of the wire rod is designated as FWHMc (nm), FWHMc / FWHMs is 1.10 to 2.00.

[0207] If FWHMc / FWHMs is 1.10 to 2.00, the distribution range of the interlamellar spacing in the central region of the circular cross section of the wire rod will be sufficiently wider than the distribution range of the interlamellar spacing in the outer layer. Therefore, work hardening of the outer layer will easily progress to the central portion, and the hardness distribution in the circular cross section will become more uniform. As a result, the drawability of the wire rod will improve.

[0208] A preferable lower limit of FWHMc / FWHMs is 1.12, more preferably is 1.15, further preferably is 1.20, and further preferably is 1.25.

[0209] A preferable upper limit of FWHMc / FWHMs is 1.90, more preferably is 1.80, further preferably is 1.70, and further preferably is 1.65.[Method for measuring LaveS, LmaxS, FWHMc, and FWHMs]

[0210] LaveS, LmaxS, FWHMc, and FWHMs can be determined by performing step 1 to step 3 of the method described above in the section [Regarding method for measuring distribution of true interlamellar spacing λ t ].[Advantageous effect of wire rod of present embodiment]

[0211] The wire rod of the present embodiment satisfies feature 1 to feature 3. Therefore, in the wire rod of the present embodiment, excellent drawability is obtained by regulating the distribution state of the interlamellar spacing.[Microstructure of wire rod of present embodiment]

[0212] In the microstructure of the wire rod of the present embodiment, the pearlite area fraction is 95% or more. In other words, the microstructure of the wire rod of the present embodiment is substantially composed of pearlite. In the microstructure, the microstructure other than pearlite is, for example, one or more types selected from a group consisting of ferrite, pro-eutectoid cementite, bainite, and martensite.[Method for measuring pearlite area fraction]

[0213] The pearlite area fraction in the microstructure of the wire rod of the present embodiment is determined by the following method.

[0214] A test specimen in which a cross section parallel to the central axis of the wire rod is adopted as an observation surface is taken. The observation surface of the test specimen is subjected to mirror polishing. Within 5 minutes after the mirror polishing, the observation surface is etched for about 7 seconds using picral to reveal the microstructure on the observation surface. On the observation surface after etching, four rectangular observation fields which each include the central axis of the wire rod and which each have dimensions of 50 µm in the radial direction and 40 µm in the axial direction are selected. The four observation fields are arranged at a pitch of 100 µm in the axial direction. Note that, the central axis of the wire rod is disposed at the center in the radial direction of the observation fields.

[0215] Each observation field is observed at a magnification of 2000× using a Field Emission Scanning Electron Microscope (FE-SEM), and photographic images are generated.

[0216] Based on the contrast and morphology in the photographic images of the respective observation fields, the pearlite microstructure and the structures other than pearlite (ferrite, pro-eutectoid cementite, bainite, and martensite) are identified. In the observation fields, a striped structure composed of black ferrite where the brightness is low and white cementite where the brightness is high is recognized as pearlite microstructure. Although it is not necessary to distinguish structures other than pearlite from each other, for example, a region that is a black grain and in which a substructure is not observed within the grain is ferrite microstructure.

[0217] The pearlite area fraction (%) is calculated based on the total area of pearlite recognized in the four observation fields, and the total area of the four observation fields. An integer obtained by rounding off decimals of the determined value is taken as the area fraction (%) of pearlite.[Applicable uses of wire rod of present embodiment]

[0218] The wire rod of the present embodiment can be widely applied, for example, as a starting material for a steel wire that is produced by wire drawing. The steel wire can be applied, for example, as a bead wire, a steel cord, a bridge cables, a wire rope or the like.[Shape of cross section perpendicular to axial direction of wire rod of present embodiment]

[0219] A cross section perpendicular to the axial direction of the wire rod of the present embodiment is, for example, a circular shape. The diameter of the wire rod of the present embodiment is not particularly limited. The diameter (diameter) of the wire rod is, for example, 3.0 to 8.0 mm. Note that, the diameter (diameter) of a steel wire produced using the wire rod of the present embodiment as a starting material is, for example, 0.8 to 3.0 mm. Note that, the aforementioned term "circular shape" includes not only a perfect circle, but also includes, for example, a circular shape satisfying the condition with respect to out-of-round (0.64 mm or less) specified in Table 3 of 7.b) of JIS G 3506 (2017) "High carbon steel wire rods".[One example of method for producing wire rod of present embodiment]

[0220] One example of a method for producing the wire rod of the present embodiment will now be described. The method for producing a wire rod described hereunder is one example for producing the wire rod of the present embodiment. Accordingly, a wire rod composed as described above may also be produced by a production method other than the production method described hereunder. However, the production method described hereunder is a preferable example of a method for producing the wire rod of the present embodiment.

[0221] One example of a method for producing the wire rod of the present embodiment includes the following processes. (Process 1) Starting material preparation process (Process 2) Blooming process (Process 3) Finish rolling process

[0222] Each process will be described hereunder.[(Process 1) Starting material preparation process]

[0223] In the starting material preparation process, a starting material for the wire rod of the present embodiment is prepared. Specifically, a molten steel having a chemical composition that satisfies feature 1 is produced. The refining method is not particularly limited, and it suffices to use a well-known method. For example, molten iron produced by a well-known method is subjected to refining (primary refining) using a converter. Molten steel tapped from the converter is subjected to a well-known secondary refining. A molten steel having a chemical composition that satisfies feature 1 is produced by the above process.

[0224] The produced molten steel is used to produce a starting material by a well-known casting process. For example, an ingot may be produced by an ingot-making process using the molten steel. Further, a bloom may be produced by a continuous casting process using the molten steel. A starting material (ingot or bloom) is produced by the above method.[(Process 2) Blooming process]

[0225] In the blooming process, the starting material (ingot or bloom) prepared in the starting material preparation process is subjected to blooming to produce a billet.

[0226] In the blooming process, first, the starting material is heated using a heating furnace by a well-known method. The heating temperature is not particularly limited. It suffices that the heating temperature is a well-known temperature. The heating temperature is, for example, 1000 to 1200°C.

[0227] After being heated, the starting material is subjected to hot rolling (rough rolling) using a blooming mill, or a blooming mill and a continuous mill, to produce a billet. Specifically, the heated starting material is subjected to reverse rolling using a blooming mill to produce a billet. In a case where a well-known continuous mill is arranged downstream of the blooming mill, the continuous mill may be used to further perform tandem rolling on the billet obtained after performing blooming, to thereby produce a billet of an even smaller size.

[0228] In the blooming process, the cooling conditions for the billet after hot rolling are as follows.(Condition 1) The cooling rate from 700 to 670°C is to be 0.50°C / sec or less.

[0229] In the blooming process, pearlite forms in the billet in the temperature range of 700 to 670°C after hot rolling. If the cooling rate from 700 to 670°C is more than 0.50°C / sec, the cooling rate will be too fast. In such case, a difference between the interlamellar spacing of pearlite in the central region and the interlamellar spacing of pearlite in the outer layer of the produced wire rod will not be very large. As a result, FWHMc / FWHMs will be less than 1.10.

[0230] If the cooling rate from 700 to 670°C in the blooming process is made slow cooling at 0.50°C / sec or less, Mn segregation (central segregation) will be appropriately formed at the central portion of the billet. As a result of forming the central segregation, the true interlamellar spacing λ t will widen at the central region of the wire rod in the finish rolling process, and the distribution of the interlamellar spacing λ t will become broad. As a result, FWHMc / FWHMs will fall within an appropriate range.[(Process 3) Finish rolling process]

[0231] In the finish rolling process, the billet produced in the blooming process is subjected to finish rolling to produce a wire rod. The finish rolling process includes the following processes.(Process 31) Heating process(Process 32) Rolling process

[0232] The heating process and rolling process in the finish rolling process are described hereunder.[(Process 31) Heating process]

[0233] In the heating process, the billet produced in the blooming process is heated using a heating furnace. The heating temperature is to be 900 to 1150°C.[(Process 32) Rolling process]

[0234] In the rolling process, the billet heated in the heating process is subjected to finish rolling (continuous rolling) using a continuous mill to produce a wire rod. The continuous mill includes a plurality of roll stands arranged in a row from upstream to downstream. Each stand includes a pair of work rolls. A caliber is formed in each work roll, and grooves are formed with the calibers of each pair of work rolls.

[0235] In addition, in the rolling process the following conditions are satisfied.

[0236] (Condition 2) The rolling finishing temperature is to be 910 to 1150°C.

[0237] (Condition 3) Water cooling is to be started within 0.5 seconds immediately after the final finish rolling.

[0238] Condition 2 and condition 3 are described hereunder.[(Condition 2) Regarding rolling finishing temperature]

[0239] As mentioned above, the term "rolling finishing temperature" means the surface temperature (°C) of the steel material (wire rod) on the entrance side of the roll stand where the final rolling reduction is applied to the steel material by the continuous mill. It is possible to measure the rolling finishing temperature with a thermometer provided on the entrance side of the roll stand where the final rolling reduction is applied.

[0240] If the rolling finishing temperature is 910 to 1150°C, the true interlamellar spacing λ t in the central region of the billet that undergoes the finish rolling will not fluctuate significantly due to the finish rolling. Therefore, a state in which the true interlamellar spacing λ t in the central region is widely dispersed can be maintained.[(Condition 3) Regarding water cooling immediately after final finish rolling]

[0241] In the wire rod immediately after the final finish rolling, austenite grains are refined more in the outer layer where the amount of strain is large. However, thereafter, if the high temperature state is continued, the austenite grains in the outer layer will grow, and the austenite grains in a cross section in the axial direction of the wire rod will be uniformized. In such case, the difference between the interlamellar spacing in the outer layer and the interlamellar spacing in the central region will not be sufficiently large. Furthermore, if the time for which cooling is allowed immediately after rolling is excessively long, variations in the interlamellar spacing in the outer layer will increase.

[0242] If water cooling is started within 0.5 seconds immediately after the final finish rolling, it will be easy to maintain the austenite grains in a fine state in the outer layer. As a result, the difference between the interlamellar spacing in the outer layer and the interlamellar spacing in the central region will be sufficiently large, and FWHMc / FWHMs will be 1.10 or more. In addition, because the outer layer is subjected to water cooling immediately after the final finish rolling, the austenite grains in the outer layer are refined and pearlite transformation proceeds at a fast rate. Therefore, variations in the interlamellar spacing in the outer layer can be suppressed overall. As a result, LmaxS / LaveS will be 1.70 or less. Note that, the coiling temperature of the wire rod is 700°C or more.

[0243] Note that, the wire rod is coiled after finish rolling. After coiling, the wire rod is cooled further by performing air blast cooling. In the present embodiment, a heating process such as rapid heating is not performed during a period until the wire rod reaches normal temperature after coiling.

[0244] The wire rod of the present embodiment is produced by the above production process.[Method for producing steel wire]

[0245] A method for producing a hard steel wire using the wire rod of the present embodiment as a starting material is a well-known production method. The steel wire is, for example, a steel cord or the like. A method for producing the steel wire using the wire rod of the present embodiment is, for example, as follows. Oxide scale of the wire rod is removed, and a lubrication treatment is performed. After being subjected to the lubrication treatment, the wire rod is subjected to dry wire drawing to produce a steel wire. The steel wire obtained after the dry wire drawing is subjected to a well-known patenting treatment, and then a well-known plating treatment is performed. In addition, wet wire drawing is performed. A steel wire is produced by the above production process.

[0246] In the wire rod of the present embodiment, while suppressing variations in the interlamellar spacing to a certain extent, the ratio between the variations in the outer layer and the variations in the central region is large to a certain extent. As a result, sufficient drawability is obtained in the steel wire after the dry wire drawing.EXAMPLES

[0247] The advantageous effect of the wire rod of the present embodiment will now be described more specifically by way of examples. The conditions adopted in the following examples are one example of conditions adopted for confirming the feasibility and advantageous effect of the wire rod of the present embodiment. Accordingly, the wire rod of the present embodiment is not limited to this one example of conditions.

[0248] Wire rods having the chemical compositions shown in Table 1-1 and Table 1-2 were produced by the following method.[Starting material preparation process]

[0249] Starting material (blooms) having the chemical compositions shown in Table 1-1 and Table 1-2 were produced.[Table 1-1]

[0250] TABLE1-1Test NumberChemical Composition (unit is mass percent; balance is Fe and impurities)CSiMnPsAlNO10.720.170.500.0100.0020.0320.00340.001520.820.180.490.0100.0020.0260.00390.000830.990.110.490.0100.0040.0340.00330.000941.190.190.470.0090.0040.0270.00330.001650.810.180.490.0100.0040.0330.00390.001060.821.480.260.0180.0080.0070.00480.004670.800.200.480.0100.0040.0330.00310.000881.020.500.410.0150.0080.0070.00310.004590.920.450.260.0170.0070.0080.00350.0046100.820.210.450.0160.0070.0090.00450.0043110.801.420.470.0090.0040.0260.00350.0006120.810.480.220.0100.0040.0300.00370.0009130.710.170.110.0100.0050.0780.00360.0011140.800.180.910.0090.0040.0700.00310.0009150.810.420.350.0160.0080.0090.00410.0043160.810.190.490.0090.0040.0510.00380.0010171.180.980.470.0100.0040.0780.00390.0015180.720.230.460.0170.0060.0090.00430.0041191.180.920.480.0100.0040.0320.00870.0012200.810.340.540.0150.0070.0080.00410.0036210.810.190.690.0100.0040.0260.00910.0006220.720.200.690.0090.0040.0280.00910.0010230.940.120.500.0180.0080.0310.00460.0040240.840.310.340.0160.0090.0080.00370.0045250.820.110.480.0100.0050.0340.00980.0010260.810.190.120.0090.0050.0330.00320.0011270.820.190.990.0100.0040.0280.00340.0013280.810.180.910.0100.0040.0330.00370.0007290.810.340.490.0100.0050.0270.00400.0010300.800.550.490.0090.0040.0280.00320.0012310.700.190.500.0110.0050.0370.00360.0017320.810.110.470.0110.0030.0280.00140.0010330.970.870.470.0130.0050.0170.00250.0010341.180.780.450.0070.0040.0220.00540.0018350.790.180.120.0080.0050.0360.00500.0015360.960.510.460.0150.0070.0080.00460.0042370.910.320.250.0150.0060.0080.00390.0048380.800.600.200.0100.0040.0310.00360.0010390.790.170.490.0080.0040.0460.00920.0013400.800.510.500.0100.0050.0380.00240.0009410.800.170.470.0180.0030.0310.00290.0013420.820.160.510.0120.0190.0380.00160.0012430.810.170.500.0120.0030.0760.00380.0010441.170.490.490.0090.0060.0310.00400.0030451.180.900.460.0100.0030.0160.00450.0015 [Table 1-2]

[0251] TABLE1-2Test NumberChemical Composition (unit is mass percent; balance is Fe and impurities)CrCoCuNiMoBWTiNbSnAsSbVCaMgZrREM1-----------------2-----------------3-----------------4-----------------5-----------------6-----------------70.10----------------80.28----------------90.12----------------100.49----------------11-0.15---------------12--0.21--------------13---0.11-------------14----0.12------------15----0.13------------16-----0.003-----------17------0.12----------18------0.18----------19-------0.03---------20-------0.04---------21--------0.07--------22---------0.08-------23----------0.049------24-----------0.048-----25------------0.05----26-------------0.0015---27--------------0.0030--28---------------0.008-29----------------0.0040300.20-0.200.120.09----0.08-------31-0.12----0.110.01---------32-------------0.00100.0020--33---------------0.0030.0020340.31----0.002--------0.0041--35----0.02----------0.005-36-0.450.500.45-------------37----0.18---0.05---0.090.0040---38-----------------390.21----------------40-----------------41-------------0.0030---42-----------------43----0.12------------44-----------------45--------------0.0020-- [Blooming process]

[0252] The produced blooms were subjected to a blooming process to produce billets. Specifically, each bloom was heated to 1100°C using a heating furnace. After heating, the bloom was subjected to hot rolling (rough rolling) using a blooming mill and a continuous mill to produce a billet. After hot rolling, the cooling rate CR (°C / sec) from 700 to 670°C was as shown in Table 2.[Table 2]

[0253] TABLE2Test NumberBlooming ProcessFinish Rolling ProcessPearlite Area Fraction (%)LaveS (nm)LmaxS (nm)LmaxS / LaveSFWHMc (nm)FWHMs (nm)FWHMc / FWHMsTC× TS≥ 70000RemarksCondition 1Condition 2Condition 3Cooling Rate CRRolling FinishingTime t ≤ 0.5 Secs(°C / sec)Temperature (°C)10.351092T971131471.3024181.33EInventive Example20.36962T951181621.3730221.36EInventive Example30.35965T961251781.4230231.30EInventive Example40.35972T961181551.3127191.42EInventive Example50.38944T961111461.3223191.21EInventive Example60.37956T96951541.6226161.63EInventive Example70.35926T951231801.4628211.33EInventive Example80.381046T951211571.3023191.21EInventive Example90.371068T96981211.2327181.50EInventive Example100.351106T951211621.3425171.47EInventive Example110.351119T97801151.4416121.33EInventive Example120.35951T971061411.3323171.35EInventive Example130.38998T961181631.3827211.29EInventive Example140.351075T951201581.3230221.36EInventive Example150.411045T951171591.3626161.63EInventive Example160.351069T971131631.4425191.32EInventive Example170.45916T97891321.4817151.13EInventive Example180.42999T961161601.3824161.50EInventive Example190.461032T96841171.3919171.12EInventive Example200.311005T981181801.5326181.44EInventive Example210.451148T971141611.4123191.21EInventive Example220.401051T951171601.3729231.26EInventive Example230.451002T961211581.3128161.75EInventive Example240.32972T981251681.3427151.80EInventive Example250.441132T961131601.4223201.15EInventive Example260.45944T961111641.4823191.21EInventive Example270.48940T961071581.4822191.16EInventive Example280.19937T971261841.4633211.57EInventive Example290.181115T961251691.3535221.59EInventive Example300.201093T971211661.3733241.38EInventive Example310.211057T971291711.3332231.39EInventive Example320.201095T961281871.4638271.41EInventive Example330.191019T951131681.4927191.42EInventive Example340.181020T951181681.4228191.47EInventive Example350.161126T961271661.3131201.55EInventive Example360.38975T951131531.3527181.50EInventive Example370.46946T971191781.5025181.39EInventive Example380.611088T95851241.4617171.00BComparative Example390.631012T97941311.3918171.06BComparative Example400.701112T96821131.3813140.93BComparative Example410.70922T97911251.3716170.94BComparative Example420.341000F961252231.7820201.00BComparative Example430.351133F971172061.7620220.91BComparative Example440.35912F961111961.7719210.90BComparative Example450.381093F96881511.7219181.06BComparative Example [Finish rolling process]

[0254] The produced billet was subjected to a finish rolling process. Specifically, the billet of each test number was heated to 950 to 1150°C using a heating furnace.

[0255] The heated billet was subjected to finish rolling (continuous rolling) using a continuous mill to produce a wire rod. The rolling finishing temperature (°C) in the finish rolling was as shown in Table 2.

[0256] In Test Nos. 1 to 41, a time t from immediately after the final finish rolling until starting water cooling was 0.5 seconds or less (indicated by "T (True)" in the column "Time t ≤ 0.5 Secs" in Table 2). On the other hand, in Test Nos. 42 to 45, the time t from immediately after the final finish rolling until starting water cooling was more than 0.5 seconds (indicated by "F (False)" in the column "Time t ≤ 0.5 Secs" in Table 2). Thereafter, water cooling was performed until the temperature of the wire rod became 800°C. Note that, the coiling temperature was 700°C or more. After performing coiling, the wire rod was subjected to air blast cooling and was cooled to normal temperature without being subjected to reheating or the like. Wire rods having a diameter of 5.5 mm were produced by the above production process.

[0257] For each of the produced wire rods, the oxide scale was removed and a lubrication treatment (bonderizing) was performed under the same conditions for each test number. After the lubrication treatment, the wire rod was subjected to dry wire drawing to produce a steel wire having a diameter of 1.5 mm. In the wire drawing, a die with a die approach angle of 10° (full angle) was used, and the wire running rate was set to 10 m / min.[Regarding evaluation tests]

[0258] The produced wire rod and steel wire of each test number were subjected to the following wire rod evaluation tests (Tests 1 and 2) and steel wire evaluation test (Test 3). (Test 1) Pearlite area fraction measurement test (Test 2) Test to measure LaveS, LmaxS, FWHMc, and FWHMs (Test 3) Torsion test

[0259] Each test will be described hereunder.[(Test 1) Pearlite area fraction measurement test]

[0260] The pearlite area fraction (%) in the wire rod of each test number was determined based on the method described above in the section [Method for measuring pearlite area fraction]. The obtained results are shown in Table 2. In each test number, the pearlite area fraction was 95% or more. Note that, the balance other than pearlite was one or more types selected from a group consisting of ferrite, pro-eutectoid cementite, bainite, and martensite.[(Test 2) Test to measure LaveS, LmaxS, FWHMc, and FWHMs]

[0261] For the wire rod of each test number, LaveS (nm), LmaxS (nm), FWHMc, and FWHMs were determined by performing step 1 to step 3 described above in the section [Regarding method for measuring distribution of true interlamellar spacing λ t ] based on the method described above in the section [Method for measuring LaveS, LmaxS, FWHMc, and FWHMs]. The obtained results are shown in Table 2.[(Test 3) Torsion test]

[0262] A torsion test was performed in the following manner on the steel wire of each test number.

[0263] 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 thereafter was straightened to make a straight wire. Portions of 70 mm in length at the top and bottom of the straight wire, respectively, were gripped by chucks so that the chuck-to-chuck distance was 200 mm, and a tensile test was performed at normal temperature in air to determine the tensile strength TS (MPa).

[0264] In addition, test specimens for a torsion test were taken from the steel wire of each test number. For each test specimen, the chuck-to-chuck distance was set to a distance corresponding to the diameter d × 100 mm. Then, a length of 50 mm of an end portion of the test specimen was gripped. A load of 0.1×TS was applied to the test specimen, and a torsion test was performed. The torsion speed was set to 20 rpm. For each test number, a torsion test was performed on three test specimens. The number of twists until the final fracture was determined, and the arithmetic average value of the number of twists determined for the three test specimens was calculated. The following evaluation value was calculated based on the determined arithmetic average value TC of the number of twists, and the tensile strength TS. Evaluation value = TC × TS

[0265] If the evaluation value was 70000 or more, it was determined that excellent drawability was obtained (described as "E (Excellent)" in the column "TC×TS ≥ 70000" in Table 2). On the other hand, if the evaluation value was less than 70000, it was determined that sufficient drawability was not obtained (described as "B (Bad)" in the column "TC×TS ≥ 70000" in Table 2).[Test results]

[0266] The test results are shown in Table 2. Referring to Table 1-1, Table 1-2, and Table 2, the wire rods of Test Nos. 1 to 37 satisfied feature 1 to feature 3. As a result, the evaluation value was 70000 or more, and thus excellent drawability was obtained.

[0267] On the other hand, in Test Nos. 38 to 41, the cooling rate of condition 1 was too fast. Therefore, FWHMc / FWHMs was less than 1.10. As a result, the evaluation value was less than 70000, and thus sufficient drawability was not obtained.

[0268] In Test Nos. 42 to 45, the time t from immediately after the final finish rolling until starting water cooling was more than 0.5 seconds. Therefore, LmaxS / LaveS was more than 1.70. Further, FWHMc / FWHMs was less than 1.10. As a result, the evaluation value was less than 70000, and thus sufficient drawability was not obtained.

[0269] An embodiment of the present disclosure has been described above. However, the embodiment described above is merely an example for carrying out the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiment, and can be implemented by appropriately modifying the above-described embodiment within a range not departing from the gist thereof.

Claims

1. A wire rod having a chemical composition comprising of, 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 metal: 0 to 0.0050%, with the balance comprising of Fe and impurities, wherein: in a circular cross section perpendicular to an axial direction of the wire rod, when an average value of true interlamellar spacing at a position at a depth of 0.5 mm from a surface of the wire rod is designated as LaveS (nm) and a maximum value of the true interlamellar spacing is designated as LmaxS (nm), LmaxS / LaveS is 1.70 or less; and in the circular cross section, when a full width at half maximum of the true interlamellar spacing at a position at a depth of 0.5 mm from the surface of the wire rod is designated as FWHMs (nm) and a full width at half maximum of the true interlamellar spacing at a center position of the wire rod is designated as FWHMc (nm), FWHMc / FWHMs is 1.10 to 2.00.

2. The wire rod according to claim 1, wherein the chemical composition contains one or more elements selected from a group consisting of, in mass%, 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%.

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

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