wire

Abstract

A wire having a chemical composition of C:0.10~0.60%, Si:0.01~0.45%, Mn:0.30~1.00%, P:0.030% or less, S:0.001~0.050%, Al:0.001~0.080%, Cr:0.85~1.50%, N:0.0010~0.0200%, O:0.004% or less, with the remainder being Fe and impurities, wherein, when the diameter is D, the metallic structure at 1 / 4D contains bainite by area ratio of 96% or more, with the remainder being martensite when the area ratio of bainite is less than 100%, and the X-ray diffraction pattern measured using an X-ray stress diffractometer at 1 / 4D of the cross-section in the longitudinal direction using Cr-Kα rays has a full width at half maximum of 2.20° or more of the diffraction peak of the (211) plane, and the aperture is 30.0% or more.

Description

Technical Field

[0001] This disclosure relates to wire rods.

Background Art

[0002] Wire rods for cold forging of low alloy steel containing Mn and Cr have high deformation resistance and low ductility. Therefore, during cold forging, mold wear and damage may occur, and processing cracks may occur in the formed parts. To improve these problems, a method of spheroidizing annealing the hot-rolled wire rod to soften it has been implemented. In recent years, for the purpose of reducing component manufacturing costs and enhancing component functionality, the shape of components has become more complex. Therefore, the steel materials used for cold forging are required to be soft and have extremely high ductility.

[0003] When the content of Cr or Mo is high, sufficient workability cannot be obtained by one-time spheroidizing annealing, and it is manufactured by performing two-time spheroidizing annealing. However, an increase in the number of annealing times leads to an increase in manufacturing costs and CO2 emissions. Therefore, a technology for simplifying spheroidizing annealing is required.

[0004] In Patent Document 1, for example, as a steel for mechanical structures for cold working that can be sufficiently softened even when the spheroidizing treatment time is shortened at a relatively low spheroidizing annealing temperature of about 750 °C, it has a predetermined chemical composition, the area ratio of primary ferrite is 10% or more and 70% or less, and contains one or more selected from the group consisting of bainite, martensite, and pearlite, and the dislocation density is 3.5×10 14 m -2 or more, a steel for mechanical structures for cold working has been proposed.

[0005] Patent Document 2 describes a steel wire containing alloying elements and having excellent cold forgeability, having a predetermined chemical composition, in which 95% or more of the metallic structure consists of ferrite and spherical carbides, the ferrite having an average particle size of 10.0 to 30.0 μm, the spherical carbides having an average aspect ratio of 2.5 or less for spherical carbides with an equivalent circle diameter of 0.1 μm or more, and when the carbon content (mass%) in the steel wire is expressed as [C], the number of spherical carbides with an equivalent circle diameter of 0.1 μm or more is 1.5 × 10⁻¹⁰. 6 ×[C]~7.0×10 6 ×[C] pieces / mm 2 A steel wire has been proposed.

[0006] Patent Document 3 proposes a steel wire material that enables shortening of the spheroidizing annealing time, achieves improved processing performance and reduced deformation resistance in cold forging after spheroidizing treatment, and realizes excellent cold forgeability. This material contains C: 0.005 to 0.6 mass%, has 10 area percent or more of pseudo-pearlite, 75 area percent or less of bainite, 60 area percent or less of ferrite, and satisfies the relationship (pseudo-pearlite area % + bainite area % + ferrite area %) ≥ 90 area percent.

[0007] Patent Document 4 proposes a steel wire having a predetermined chemical composition and cementite as its internal structure, wherein at least 80% of the cementite particles in a cross section perpendicular to the longitudinal direction of the wire have a minor diameter of 0.1 μm or less and an aspect ratio of 2.0 or less, determined by the ratio of major to minor diameters, in order to achieve stable wire drawing and cold forging performance.

[0008] Patent Document 5 proposes a wire that can achieve reduced deformation resistance and improved ductility after spheroidizing annealing, which contains Cr: 0.85~1.50%, etc., and satisfies a specific relationship between the content of C, Si, Mn, Cr, Cu, Mo, and V, and whose metallic structure contains at least bainite and martensite, and whose total area ratio of bainite, martensite, and ferrite measured in a cross section perpendicular to the longitudinal direction of the wire is 98% or more, the area ratio of ferrite is less than 5%, the area ratio of martensite satisfies a specific relationship, the average equivalent circle diameter of the bainite block is 15 μm or less, the bainite block satisfies a specific relationship, and the tensile strength satisfies a specific relationship.

[0009] Patent Document 1: International Publication No. 2022 / 181272 Patent Document 2: Patent No. 7151885 Patent Document 3: Japanese Unexamined Patent Publication No. 2006-225701 Patent Document 4: International Publication No. 2015 / 141840 Patent Document 5: Japanese Unexamined Patent Publication No. 2020-186446 [Overview of the project] [Problems that the invention aims to solve]

[0010] The present disclosure aims to provide a wire rod that, when manufacturing steel wire by drawing and annealing a low-alloy steel wire rod containing Mn and Cr, has the same strength as when annealing is performed twice, even with only one annealing cycle, and also has good workability. [Means for solving the problem]

[0011] The above problems will be solved by the following means. <1> In mass%, C: 0.10~0.60%, Si: 0.01~0.45%, Mn: 0.30~1.00%, P: 0.030% or less, S: 0.001~0.050%, Al: 0.001~0.080%, Cr: 0.85~1.50%, N: 0.0010~0.0200%, O: 0.004% or less, Mo: 0~1.00%, V: 0~0.50%, Nb: 0~0.050%, Ti: 0~0.100%, Cu: 0~0.40%, Ni: 0~0.30%, Sn: 0~0.10%, B: 0~0.0200%, Ca: 0~0.0050%, Mg: 0~0.0050%, The remainder has a chemical composition consisting of Fe and impurities. When the diameter of the wire is D, the metallographic structure in a cross section perpendicular to the length of the wire, at a depth of 1 / 4D from the surface of the wire, contains bainite in an area ratio of 96% or more and 100% or less, and the remainder when the area ratio of bainite is less than 100% is martensite. In the 1 / 4D portion of the cross-section parallel to the longitudinal direction, which includes the central axis of the wire, the X-ray diffraction pattern was measured using an X-ray stress diffractometer with Cr-Kα rays, and the half-width of the diffraction peak of the (211) plane was 2.20° or more. A wire material with an aperture of 30.0% or more. <2> In mass%, Mo: 0.01~1.00%, V: 0.01~0.50%, Nb: 0.001~0.050%, and Ti: 0.005~0.100%, One or more selected from the group consisting of including, <1> The wire material described above. <3> In mass%, Cu: 0.02~0.40%, Ni: 0.02~0.30%, Sn: 0.01~0.10%, and B: 0.0003~0.0200% One or more selected from the group consisting of The wire rod according to <1> or <2>, comprising At <4> mass%, Ca: 0.0001 to 0.0050%, and Mg: 0.0001 to 0.0050% One or two selected from the group consisting of The wire rod according to any one of <1> to <3>, comprising <5> The wire rod according to any one of <1> to <4>, having a tensile strength of 1200 MPa or less <6> After wire drawing the wire rod in the range of a reduction ratio of 30% to 50%, heating is carried out at a heating rate of 150 °C / hour as an annealing process, held at 720 °C for 12 hours, and slowly cooled, and the tensile strength of the annealed material obtained is 500 ± 15 MPa and the drawing is 80.0 ± 5.0%. The wire rod according to any one of <1> to <5>

[0012] According to the present disclosure, when manufacturing a steel wire by subjecting a low alloy steel wire rod containing Mn and Cr to wire drawing and annealing treatment, even if the number of annealing times is 1, it has the same strength as when annealing is performed twice, and a wire rod capable of manufacturing a steel wire with good workability is provided.

Brief Description of the Drawings

[0013] [Figure 1A] It is a SEM image showing an example of the structure obtained by etching a 1 / 4D part of the wire rod with picral. [Figure 1B] It is a diagram in which martensite is marked in the SEM image shown in Fig. 1A. [Figure 2] It is a schematic diagram showing an example of the half-value width of the diffraction peak in the X-ray diffraction pattern measured by an X-ray stress diffractometer at the 1 / 4D part of the wire rod. [Figure 3] It is a schematic diagram for explaining the region for measuring ferrite grains in the steel wire.

Embodiments for Carrying Out the Invention

[0014] An example of an embodiment of this disclosure will be described. The wire rods relating to this disclosure particularly concern wire rods that can be made into high-strength and highly ductile (workable) steel wires after drawing and annealing, and are suitable for use as materials for machine parts such as bolts, screws, and nuts formed by cold forging or rolling. The wire rods covered in this disclosure also include "bar-in-coils" which are hot-rolled wire rods wound into a coil shape.

[0015] In this disclosure, a numerical range represented by "~" means a range that includes the numbers written before and after "~" as the lower and upper limits, respectively. When numbers preceded or followed by "~" are preceded by "greater than" or "less than," the numerical range refers to a range that does not include those numbers as the lower or upper limit. The elemental content in a chemical composition is sometimes expressed as elemental amount (e.g., amount of C, amount of Si, etc.). In chemical composition, the percentage "%" indicates "mass percent". When the chemical composition of an element is described as "0~", it means that the element does not need to be included. The term "process" includes not only independent processes, but also any process that cannot be clearly distinguished from other processes, as long as its intended purpose is achieved. The "surface" of a wire refers to its "outer surface." A "C-section" refers to a cross-section perpendicular to the longitudinal direction of the wire. An "L-section" refers to a cross-section parallel to the longitudinal direction of a wire or steel wire. The notation "1 / 4D" means that, when the diameter of the wire is D, the position is at a depth of 1 / 4 times the diameter D, moving from the surface toward the central axis (radially), and is synonymous with "D / 4".

[0016] To improve workability after spheroidizing annealing, it is effective to improve ductility (crack suppression, reduction of area) and reduce deformation resistance (strength). To improve ductility, it is necessary to suppress coarse spheroidal cementite, which is the starting point of fracture during plastic deformation, and it is advisable to use bainite as the microstructure before spheroidizing annealing. By using bainite, in which carbides are more uniformly dispersed compared to ferrite and pearlite, the dispersion of cementite after annealing can be made uniform and fine. However, if the cementite is too fine, the deformation resistance will increase.

[0017] The inventors of this disclosure focused on coarsening the ferrite grain size to reduce deformation resistance. Coarsening the ferrite grain size is effective in reducing deformation resistance. The inventors of this disclosure found that the deformation resistance of a wire mainly composed of a bainite structure obtained by isothermal transformation treatment at a temperature of less than 500°C in a molten salt bath (primary holding process / secondary holding process) is further reduced by drawing and spheroidizing annealing to point A1 or lower. It is thought that the bainite structure treated with isothermal transformation treatment at a temperature of less than 500°C has a high half-width of the X-ray diffraction peak of the (211) plane, which is a function of dislocation density, and that this acted as a driving force for recrystallization, resulting in coarser ferrite grains. It is presumed that when the isothermal transformation temperature is high (above 500°C) or the tempering temperature is high (above 500°C), alloying elements such as Mo concentrate at the grain boundaries, delaying recrystallization during spheroidizing annealing and hindering the coarsening of the ferrite grain size.

[0018] <Chemical composition> The chemical composition (steel component) of the wire rod relating to this disclosure is, in mass%, C: 0.10~0.60%, Si: 0.01~0.45%, Mn: 0.30~1.00%, P: 0.030% or less, S: 0.001~0.050%, Al: 0.001~0.080%, Cr: 0.85~1.50%, N: 0.0010~0.0200%, O: 0.004% or less, with the remainder consisting of Fe and impurities. Furthermore, the wire material relating to this disclosure may contain, in place of a portion of Fe, one or more substances selected from the groups A, B, and C described below. [Group A] Mo: 0~1.00%, V: 0~0.50%, Nb: 0~0.050%, and Ti: 0~0.100% One or more selected from the group consisting of [Group B] Cu: 0~0.40%, Ni: 0~0.30%, Sn: 0~0.10%, and B: 0~0.0200% One or more selected from the group consisting of [Group C] Ca: 0~0.0050%, and Mg: 0~0.0050% In other words, the elements in groups A to C are arbitrary elements, and these elements do not have to be included in the wire rod relating to this disclosure, but if they are included, they shall be within the above range. The following describes each element in the wire material related to this disclosure.

[0019] C: 0.10~0.60% Carbon (C) is added to ensure the strength of the machine part. If the C content is less than 0.10%, it is difficult to achieve the necessary strength for a machine part. On the other hand, if the C content exceeds 0.60%, ductility, toughness, and cold forgeability deteriorate. Therefore, the C content should be between 0.10% and 0.60%. The preferred range for C content that balances high strength with ductility, toughness, and cold workability is 0.15% to 0.50%.

[0020] Si: 0.01~0.45% Si (silicon) functions as a deoxidizing element, imparts hardenability, improves tempering softening resistance, and is an effective element for providing the necessary strength to machine parts. These effects are insufficient when the Si content is less than 0.01%. When the Si content exceeds 0.45%, the ductility and toughness of machine parts deteriorate, and the deformation resistance of the steel wire increases, degrading its cold forgeability. Therefore, the Si content should be between 0.01% and 0.45%. A preferred Si content range is 0.03% to 0.35%. A more preferred Si content range is 0.05% to 0.30%.

[0021] Mn: 0.30~1.00% Mn is an essential element for imparting hardenability and the necessary strength to machine parts. Below 0.30% Mn content, the effect is insufficient. Above 1.00% Mn content, the toughness of machine parts deteriorates, and the deformation resistance of the steel wire increases, degrading its cold forgeability. Therefore, the Mn content should be between 0.30% and 1.00%. A preferred Mn content range is 0.35% to 0.90%. A more preferred Mn range is 0.40% to 0.85%.

[0022] P:0.030% or less P is present in the wire as an impurity. Since P segregates at the grain boundaries of machine parts after quenching and tempering, degrading toughness, it is desirable to reduce its content. For this reason, the upper limit of P content should be 0.030%. A preferred upper limit of P content is 0.020%. A more preferred upper limit of P content is 0.015% or less. The lower limit of P content should ideally be 0% (i.e., not present), but from the viewpoint of reducing the cost of removing P, it may be greater than 0% (or 0.0001% or more).

[0023] S: 0.001~0.050% S is present in the wire as sulfides such as MnS. These sulfides improve the machinability of the steel wire. If the S content exceeds 0.050%, it deteriorates the cold forgeability of the steel wire and also degrades the toughness of machine parts after quenching and tempering. For this reason, the upper limit for the S content is set at 0.050%. The preferred upper limit for the S content is 0.040%. A more preferred upper limit for the S content is 0.030%. The lower limit for the S content is 0.001% or higher, from the viewpoint of reducing the cost of desulfurization.

[0024] Al: 0.001~0.080% Al functions as a deoxidizing element and also forms AlN, which refines austenite grains and improves the toughness of machine parts. Furthermore, it fixes dissolved nitrogen, suppressing dynamic strain aging and reducing deformation resistance. These effects are insufficient when the Al content is less than 0.001%. When the Al content exceeds 0.080%, the effects saturate and manufacturability may decrease. Therefore, the Al content should be between 0.001% and 0.080%. A preferred Al content range is 0.010% to 0.060%. A more preferred Al content range is 0.020% to 0.050%.

[0025] Cr: 0.85~1.50% Cr is an essential element for improving hardenability and imparting the necessary strength to machine parts. Furthermore, the presence of Cr causes the carbides to become spherical after annealing, improving cold workability. Below 0.85% Cr content, the effect is insufficient. Above 1.50% Cr content, the spheroidization time of the carbides becomes long, increasing manufacturing costs and increasing the deformation resistance of the steel wire, thus degrading cold forgeability. Therefore, the Cr content should be between 0.85% and 1.50%. The preferred range for Cr content is 0.87% to 1.40%. A more preferred range for Cr content is 0.90% to 1.30%.

[0026] N: 0.0010~0.0200% N forms nitrides with Al, Ti, Nb, V, etc., which refines the austenite grains and improves the toughness of machine parts. If the N content is less than 0.0010%, the amount of nitride precipitate is insufficient, and the effect is not obtained. If the N content exceeds 0.0200%, the deformation resistance of the steel wire increases due to dynamic strain aging by solid-solution N, degrading the workability. Therefore, the N content should be between 0.0010 and 0.0200%. The preferred range for the N content is 0.0020 to 0.0080%. A more preferred range for the N content is between 0.0030 and less than 0.0050%.

[0027] O: 0.004% or less Oxygen (O) is inevitably present in wire materials and exists as an oxide of Al, Ti, etc. High Oxygen content leads to the formation of coarse oxides, which reduces the fatigue strength of machine parts. Therefore, the Oxygen content should be limited to 0.004% or less.

[0028] The wire material relating to this disclosure may contain one or more materials selected from the groups A, B, and C below, for the purpose of improving the properties described below. [Group A] Mo: 0.01~1.00%, V: 0.01~0.50%, Nb: 0.001~0.050%, and Ti: 0.005~0.100%, One or more selected from the group consisting of [Group B] Cu: 0.02~0.40%, Ni: 0.02~0.30%, Sn: 0.01~0.10%, and B: 0.0003~0.0200% One or more selected from the group consisting of [Group C] Ca: 0.0001~0.0050%, and Mg: 0.0001~0.0050% One or two selected from the group consisting of

[0029] Mo: 0~1.00% Mo improves hardenability and provides the necessary strength to machine parts. If the Mo content exceeds 1.00%, the alloy cost increases, and the deformation resistance of the steel wire rises, degrading its cold forgeability. Therefore, when Mo is included, the Mo content should be between 0 and 1.00%. The preferred range for Mo content is 0.01 to 0.90%, and 0.05 to 0.80%.

[0030] V: 0~0.50% V has the effect of increasing the strength of machine parts by precipitating carbide VC. If the amount of V exceeds 0.50%, the alloy cost increases. Therefore, when V is included, it is preferable to keep the amount of V between 0 and 0.50%. The preferred range for the amount of V is 0.01 to 0.45%, and more preferably 0.05 to 0.40%.

[0031] Nb: 0~0.050% Nb has several effects, including increasing the strength of machine parts by precipitating carbides and nitrides, improving toughness by refining austenite grains, and reducing deformation resistance by reducing dissolved nitrogen. When the amount of Nb exceeds 0.050%, the effects saturate and cold forgeability may deteriorate. Therefore, when Nb is included, it is preferable to keep the amount of Nb between 0 and 0.050%. The preferred range for the amount of Nb is 0.001 to 0.030%. More preferably, it is 0.005 to 0.020%.

[0032] Ti: 0~0.100% Ti functions as a deoxidizing element and also has the effect of improving the toughness of machine parts by forming nitrides and carbides, refining austenite crystal grains, promoting the formation of solid solution B, improving hardenability, and fixing solid solution N, suppressing dynamic strain aging and reducing deformation resistance. When the amount of Ti exceeds 0.100%, these effects saturate and coarse oxides or nitrides are formed, which may degrade the fatigue strength of machine parts. Therefore, when Ti is included, it is preferable to keep the amount of Ti between 0 and 0.100%. The preferred range for the amount of Ti is 0.005 to 0.050%. A more preferred range for the amount of Ti is 0.008 to 0.030%, and even more preferably 0.010 to 0.025%.

[0033] Cu: 0~0.40% Cu improves hardenability, precipitates finely, and provides the necessary strength for machine parts while also improving corrosion resistance. If the Cu content exceeds 0.40%, hot ductility deteriorates and surface defects are more likely to occur. Therefore, when Cu is included, it is preferable to keep the Cu content between 0% and 0.40%. The preferred range for Cu content is 0.02% to 0.30%, and more preferably 0.05% to 0.25%.

[0034] Ni: 0~0.30% Ni improves hardenability and provides the necessary strength to machine parts. When the Ni content exceeds 0.30%, the alloy cost increases. Therefore, when Ni is included, it is preferable to keep the Ni content between 0% and 0.30%. The preferred range for Ni content is 0.02% to 0.25%, and more preferably 0.05% to 0.20%.

[0035] Sn: 0~0.10% Sn has the effect of improving corrosion resistance. If the amount of Sn exceeds 0.10%, ductility decreases and cold workability deteriorates, so it should be limited to 0.10% or less. When Sn is included, the amount of Sn should be between 0 and 0.10%. The preferred range for the amount of Sn is 0.01 to 0.08%.

[0036] B: 0~0.0200% B, as solid solution B, segregates at grain boundaries, improving hardenability and providing the necessary strength to machine parts. If the amount of B exceeds 0.0200%, carbides may form at grain boundaries, degrading wire drawing workability. Therefore, when B is included, the amount of B should be between 0 and 0.0200%. The preferred range for the amount of B is 0.0003 to 0.0100%. A more preferred range for the amount of B is 0.0005 to 0.0040%, and even more preferably 0.0010 to 0.0030%.

[0037] Ca: 0~0.0050% Ca is added for the purpose of deoxidizing. These elements have the effect of making oxides finer and improving fatigue strength. If the amount added exceeds 0.0050%, the effect saturates and coarse oxides are formed, which may degrade the fatigue properties. Therefore, when Ca is included, the amount of Ca should be greater than 0 to 0.0050%. A preferred amount of Ca is 0.0001 to 0.0030%. More preferably it is 0.0005 to 0.0020%, and even more preferably 0.0008 to 0.0015%.

[0038] Mg: 0~0.0050% Mg is added for the purpose of deoxidizing. These elements have the effect of making oxides finer and improving fatigue strength. If the amount added exceeds 0.0050%, the effect saturates and coarse oxides are generated, which may degrade the fatigue properties. Therefore, when Mg is included, the amount of Mg should be between 0 and 0.0050%. A preferred amount of Mg is 0.0001 to 0.0040%. More preferably, it is 0.0005 to 0.0035%.

[0039] Remainder: Fe and impurities In the chemical composition of the wire material relating to this disclosure, the remainder consists of Fe and impurities. Here, "impurities" refer to components present in the raw materials or components introduced during the manufacturing process, and not components that were intentionally included. Furthermore, even if a component is intentionally included, impurities also include components present in amounts that do not affect the performance of the steel wire.

[0040] [Metal structure] Next, the microstructure of the wire related to this disclosure will be described. The wire rod according to this disclosure, when the diameter of the wire rod is D, has a metal structure in the 1 / 4D portion (1 / 4D) of the wire rod, perpendicular to the length direction of the wire rod, where the metal structure is located at a depth of 1 / 4D from the surface of the wire rod, containing 96% or more bainite by area ratio. The area ratio of the metal structure in the 1 / 4D portion of the C section may be 100% bainite, i.e., it may contain no other structures, or it may contain a total of 4% or less martensite. It is preferable that the metal structure does not contain pearlite, as the presence of pearlite reduces the ductility of the steel wire after annealing and deteriorates its cold forgeability.

[0041] <Baynight> The microstructure in the 1 / 4D portion of the wire according to this disclosure is mainly bainite. The area ratio of bainite in the 1 / 4D portion is 96% or more, preferably 97% or more, and more preferably 98% or more. In this disclosure, bainite, like pearlite, contains a ferrite phase (α) and a cementite phase (Fe3C). However, pearlite has a structure in which the ferrite phase and the cementite phase are alternately and continuously layered, while bainite has a structure in which lath (needle-like substructure) is contained within the grains and granular or needle-like carbides are dispersed.

[0042] <Martensite> In the wire according to this disclosure, if the area ratio of martensite in the 1 / 4D portion exceeds 4%, the strength of the wire increases and the reduction in draw size decreases, making it more prone to breakage during wire drawing. Therefore, the area ratio of martensite in the 1 / 4D portion is preferably 4% or less, more preferably 3% or less, and even more preferably 2% or less. Note that the metal structure of the wire according to this disclosure does not necessarily have to contain martensite. That is, the area ratio of martensite may be 0%.

[0043] (Methods for measuring tissue) The microstructure of the wire related to this disclosure shall be measured by the following method. The area percentages (area %) of bainite and martensite are determined by the following procedure. First, the C-section of the wire to be measured (hereinafter sometimes referred to as "the object") is mirror-polished, and then etched with picral (5% picric acid + 95% ethanol solution) to reveal the microstructure. Next, assuming the diameter of the object is D, four regions measuring 80 μm in the depth direction and 120 μm in the circumferential direction are identified at a depth of 1 / 4D from the surface of the object, at 90° intervals in the circumferential direction. Then, using a Hitachi High-Technologies Corporation field emission scanning electron microscope (SEM), microstructure images are taken at these four locations at a magnification of 2000x under the conditions of 2560 × 1920 pixels, an acceleration voltage of 15 kV, and secondary electron imaging. Figure 1A is an SEM image showing an example of the microstructure obtained by etching the 1 / 4D portion of the wire with picral. Figure 1B is a diagram showing the SEM image in Figure 1A with martensite marked. In this disclosure, in the captured microstructure images, areas with weak corrosion in both nital and picral were identified as ferrite, and a structure in which cementite with a (long axis length) / (short axis length) of 5.0 or more was continuously layered alternately with ferrite, and no granular or needle-shaped cementite was found between the layers, was defined as pearlite. Bainite was defined as a structure in which lath was contained within the granules, and granular or needle-shaped carbides were dispersed between or within the lath, or a structure in which fragmented cementite was arranged in rows.

[0044] Furthermore, each bainite and / or martensite tissue in the captured tissue photographs is visually marked (see Figure 1B), and the area of ​​each tissue region is determined by image analysis (software: LUZEX_AP, a compact general-purpose image processing and analysis system manufactured by Nireco). This operation is performed on at least two samples, and the average value is calculated, with this average value being used as the area % of each tissue in this disclosure. If it is difficult to distinguish between bainite and martensite, the observation site is identified by an indentation, the specimen is etched with picral and a micrograph is taken, then it is repolished and etched with nital (5% nitric acid + 95% ethanol solution) to reveal the microstructure. A micrograph of the same area is taken using an SEM at a magnification of 2000x. Areas that are etched with nital but etched weakly with picral are identified as martensite, and the areas of each microstructure are visually marked using the method described above, and the area percentage is determined by image analysis.

[0045] (FHAW of the diffraction peak of the (211) plane in the X-ray diffraction pattern) The wire according to this disclosure has a half-width at half maximum (FWHM) of the diffraction peak of the (211) plane of the X-ray diffraction pattern measured at a 1 / 4D portion (a portion at a depth of 0.25 times the diameter D of the wire in the direction from the surface of the wire toward the center of the wire) of a cross section (L section) that includes the central axis C of the wire and is parallel to the central axis C (length direction), which is 2.20° or more, preferably 2.60° or more. By having a high FWHM of 2.20° or more at the 1 / 4D portion, the recrystallization rate of ferrite grains during annealing after wire drawing can be increased. As a result, coarse ferrite grains can be formed, and the tensile strength of the steel wire can be lowered. (211) There is no particular upper limit to the width at half maximum of the surface diffraction peak, but if the width at half maximum is too high, the wire is more likely to break during drawing, so it is preferable that it be 3.20° or less. The (211) plane diffraction peak half-width was measured using a Rigaku Auto Mate X-ray stress diffractometer under the following conditions: X-ray: Cr-Kα line, tube velocity voltage: 40kV, tube current: 40mA, diffraction plane: α(211) plane. The X-rays used for the measurement were separated into Kα1 and Kα2 lines of the Cr-Kα line by the measuring device, and the Kα1 line was used for the measurement. Figure 2 schematically shows an example of the half-width of the diffraction peak in the X-ray spectrum (X-ray diffraction pattern) measured by the X-ray stress diffractometer.

[0046] (Aperture) The wire material according to this disclosure has a draw ratio of 30.0% or more. A draw ratio of 30.0% or more makes it possible to draw the wire without breakage in the subsequent wire drawing process. The draw ratio of the wire material according to this disclosure is preferably 35.0% or more, and more preferably 40.0% or more. To perform the wire drawing test, the wire is cut to a length of 340 mm, straightened, and made into a straight rod. 200 mm is the length between the chucks (test length), and 70 mm above and below are chucked, and a tensile test is performed. In the tensile test described above, the reduction of area (%) of the wire is calculated using the following formula. Reduced area (%) = (Cross-sectional area before tensile test - Cross-sectional area of ​​the fractured portion during the tensile test) / Cross-sectional area before tensile test × 100

[0047] (Tensile strength) The tensile strength of the wire according to this disclosure is not particularly limited, but is preferably 1200 MPa or less. If the tensile strength of the wire exceeds 1200 MPa, it is prone to breakage during the wire drawing process before annealing; therefore, the tensile strength of the wire is preferably 1200 MPa or less, and more preferably 1150 MPa or less. If the tensile strength of the wire rod according to this disclosure is too low, the softening after annealing will be insufficient, and the cold forgeability will deteriorate. For this reason, the tensile strength of the wire rod according to this disclosure is preferably 900 MPa or higher, and more preferably 950 MPa or higher. The tensile strength of the wire was measured using a 9A test specimen according to JIS Z2241:2011, and performed a tensile test according to the test method of JIS Z2241:2011.

[0048] (Average grain size of ferrite after wire drawing and annealing) The wire rod according to this disclosure preferably has an average ferrite particle size of 20 μm or more after drawing and annealing. This is because if the average ferrite particle size is less than 20 μm, the strength will be high but the ductility (workability) will be insufficient. The average ferrite particle size of the steel wire can be measured by the following method.

[0049] (Method for measuring the average particle size of ferrite) The particle size of ferrite after drawing and annealing the wire according to this disclosure is measured by the following method. -Method for measuring ferrite grains- The average particle size of ferrite grains can be measured by electron backscattering diffraction (EBSD). For the analysis, a field emission scanning electron microscope (SEM) manufactured by Hitachi High-Technologies Corporation, a CCD camera manufactured by TSL Corporation, and TSL Corporation's OIM Analysis software were used. Specifically, as shown in Figure 3, the crystal orientation of bcc-Fe is measured at each measurement point within a region of 500 μm in the depth direction (radial direction) and 500 μm in the direction of the central axis, i.e., regions of 500 μm square indicated by A1, A2, and A3 in Figure 3, using an acceleration voltage of 20 kV, an irradiation current of 20 nA, and a measurement step of 1.0 μm. Here, boundaries with an orientation difference of 15 degrees or more are defined as ferrite grain boundaries. Regions of 5 pixels or more enclosed by these ferrite grain boundaries are defined as ferrite grains. The ferrite particle size was calculated using the method of Johnson-Saltikov (see "Metrometric Morphology," Uchida Rokakuho, published July 30, 1972, original work: RTDeHoff, FNRhiness, p. 189). This was performed on two samples, and the average of the average particle sizes measured in a total of six measurement areas was taken as the average particle size of the ferrite grains.

[0050] [Method of manufacturing wire] An example of a method for manufacturing wire according to this disclosure will be described. The method for manufacturing wire according to this disclosure is not particularly limited, but it can be suitably manufactured by a method including a heating step, a hot rolling step, a winding step, a cooling step, a primary holding step (cooling bath), and a secondary holding step (constant temperature heat treatment or tempering: constant temperature bath). Specifically, a steel billet consisting of predetermined components that satisfy the aforementioned chemical composition is heated to 1050-1250°C, held in the furnace for 90 minutes or more, then hot-rolled at a finish rolling temperature of 750-1000°C, and wound into a ring shape at 780-820°C. After winding, the material is cooled to 400-495°C at an average cooling rate of 30-250°C / s. Subsequently, the mixture is held at T1: 400~495℃ for 20~100 seconds (primary holding process). Furthermore, a secondary holding process is performed at T2: 400~495℃ for 35~100 seconds. Specifically, a primary holding process is performed at T1: 400~495℃ for t1: 20~100 seconds, and a secondary holding process is performed at T1≦T2<495℃ for t2: 35~100 seconds, so that the cumulative holding time (t1+t2) of the primary and secondary holding processes is 60~200 seconds. The wire can then be obtained by water cooling. The following describes each step in detail.

[0051] (Heating process) In the heating process, a steel billet having the component composition of the wire rod according to the present disclosure is heated to 1050-1250°C and held for 90 minutes or more. If the heating temperature is less than 1050°C or the holding time is less than 90 minutes, the solid solution of carbides will be insufficient, resulting in insufficient ductility of the wire rod and making it prone to breakage. On the other hand, if the heating temperature exceeds 1250°C, the crystal grains become coarse, the workability of the wire rod deteriorates, and it becomes prone to breakage during the wire drawing process.

[0052] (Hot rolling process) In the hot rolling process, heated steel billets are hot-rolled at a finish rolling temperature of 750 to 1000°C. If the finish rolling temperature is below 750°C, the grain size becomes uneven, reducing the ductility of the wire rod and making it prone to breakage during the wire drawing process. If the finish rolling temperature exceeds 1000°C, the grains become coarse, degrading the workability of the wire rod and making it prone to breakage during the wire drawing process. The finish rolling temperature refers to both the entry temperature for finish rolling and the surface temperature of the wire rod immediately after rolling. In other words, hot rolling should be performed so that both the entry temperature for finish rolling and the surface temperature of the wire rod immediately after rolling are within the range of 750 to 1000°C.

[0053] (Winding process) The hot-rolled wire is wound into a ring shape at a winding temperature of 770-820°C. If the winding temperature is below 770°C, the (211) plane diffraction peak half-width of the wire decreases, and the ductility after drawing and annealing deteriorates. If the winding temperature exceeds 820°C, the crystal grains become coarser, and the processability of the wire deteriorates. The winding temperature refers to the surface temperature of the wire immediately after it is placed on the conveyor when winding the hot-rolled wire.

[0054] In the cooling process, the wire, wound into a ring shape after hot rolling and heated to 770-820°C, is cooled to 400-495°C at an average cooling rate of 30-250°C / s. If the average cooling rate is less than 30°C / s, a ferrite and pearlite structure will form, preventing the acquisition of the microstructure described in this disclosure, and resulting in reduced reduction of area after annealing. Achieving an average cooling rate exceeding 250°C / s increases manufacturing costs. Note that the average cooling rate refers to the average cooling rate at the surface of the wire. For example, the hot-rolled wire can be wound into a ring shape and cooled by air cooling or wind cooling to achieve the above average cooling rate.

[0055] (Primary holding process) Afterward, the material is held in a molten salt bath at 400-495°C for 20-100 seconds. If the holding temperature is below 400°C, the amount of martensite increases, resulting in higher wire strength and reduced draw, which makes the wire more prone to breakage during the wire drawing process. Furthermore, if the holding temperature exceeds 495°C, the transformation completion time in the secondary holding process becomes significantly longer, and untransformed portions remain after the primary and secondary holding processes. These untransformed portions can cause wire breakage during the wire drawing process.

[0056] (Secondary holding process) The material is held in a molten salt bath at 400-495°C for 35-100 seconds. If the holding temperature is below 400°C, the amount of martensite increases, resulting in higher wire strength and reduced draw, making the wire more prone to breakage during the wire drawing process. Furthermore, if the holding temperature exceeds 495°C, the transformation completion time in the secondary holding process becomes significantly longer, and untransformed portions remain after the primary and secondary holding processes. These untransformed portions can cause wire breakage during the wire drawing process.

[0057] -Cumulative holding time between the primary and secondary holding processes- The cumulative holding time (t1+t2) of the primary holding time t1 and the secondary holding time t2 should be between 60 and 200 seconds. If the cumulative holding time is less than 60 seconds, the bainite transformation will be incomplete, resulting in untransformed portions. These untransformed portions can cause wire breakage during the wire drawing process. If the cumulative holding time at 400-495°C exceeds 200 seconds, the bainite transformation is complete, and further heat treatment for longer periods will increase costs.

[0058] Through the above process, the wire rod according to this disclosure can be suitably manufactured. The wire diameter D of the wire rod according to this disclosure is not particularly limited, but from the viewpoint of manufacturability, it is preferably 5.5 mm to 21.0 mm, and more preferably 9.5 to 16.0 mm.

[0059] [Method of manufacturing steel wire] When manufacturing steel wire using the wire rod relating to this disclosure, a method including a wire drawing process and an annealing (spheroidizing annealing) process may be used.

[0060] (Wire drawing process) After the secondary holding process, tempering is performed as needed, and the wire rod, cooled to room temperature, is drawn to a total reduction ratio of, for example, 30-50%. Drawing promotes the spheroidization of carbides during the annealing process and also promotes the growth of ferrite grains. If the total reduction ratio during drawing is less than 20%, these effects are insufficient, and the cold workability deteriorates. On the other hand, if the total reduction ratio during drawing exceeds 50%, the effects saturate, and the steel wire diameter becomes smaller, limiting its applications. The diameter of the steel wire produced by drawing the wire material relating to this disclosure is not particularly limited, but is preferably 2.5 to 18.0 mm, and more preferably 4.5 to 16.0 mm.

[0061] (Annealing process) After wire drawing, it is preferable to perform an annealing process, for example, by holding the wire at 650°C or higher but less than 750°C for 3 hours or more and then cooling it. If the annealing temperature is below 650°C, ferrite grain growth takes time, increasing manufacturing costs. On the other hand, if the annealing temperature is 750°C or higher, the ferrite grains become finer, and the strength increases. Furthermore, if the holding time during the annealing process is less than 3 hours, the cementite grains become finer, resulting in a deterioration of cold workability.

[0062] As described above, by using the wire material according to this disclosure, a steel wire with good workability can be obtained even if the number of spheroidizing annealing steps after drawing is reduced to one. For example, when the wire rod according to this disclosure is drawn to a reduction ratio of 30% to 50%, and then subjected to an annealing process in which the temperature is increased at a heating rate of 150°C / hour and held at 720°C for 12 hours to cool slowly, it is possible to obtain an annealed material with a tensile strength of 500±15MPa and a reduction of 80.0±5.0%. [Examples]

[0063] The wires relating to this disclosure will be described in more detail below with reference to examples. However, these examples are not intended to limit the wires relating to this disclosure.

[0064] Steel billets of grades A to M having the chemical compositions shown in Table 1 were manufactured. In Table 1, blank spaces indicate that the component (element) was intentionally omitted, and the remainder consists of Fe and impurities. Underlined text in each table indicates that it is outside the scope of this disclosure.

[0065] [Table 1]

[0066] Using the manufactured steel billets, wire rods were produced by heating, hot rolling, winding, cooling, primary holding, and secondary holding processes under the conditions shown in Table 2. Some of the wire rods were tempered. This resulted in the production of wire rods (wire diameter: 10 mm).

[0067] [Table 2]

[0068] For each manufactured wire, the area ratio of the metallic structure and the (211)-plane diffraction peak half-width were measured in the 1 / 4D section using the method described above. Tensile strength and reduction of area were also measured using the tensile test described above. The results are shown in Table 3.

[0069] [Table 3]

[0070] A wire rod with a diameter of 10.0 mm, test number 24, was manufactured and subjected to wire drawing and two spheroidizing annealing processes under the conditions shown in Table 4 (conventional conditions) to produce steel wire. Steel wire was produced by drawing and performing one spheroidizing annealing on test numbers 1, 3-23, with a diameter of 10.0 mm, under the conditions shown in Tables 5 and 6. The heating rate to the annealing temperature was 150°C / hour.

[0071] [Table 4]

[0072] [Table 5]

[0073] The average ferrite particle size of the steel wire manufactured as described above was measured using the method described previously. Furthermore, tensile tests were performed on these steel wires to evaluate their mechanical properties. Similar to the tensile tests for the wire rods, the tensile tests were conducted using JIS Z2241:2011 9A specimens, following the JIS Z2241:2011 test method. Specifically, linear specimens cut from the steel wire to a length of 390 mm were used, with a gauge length of 100 mm and a crosshead speed of 10 mm / min. The tests were performed at room temperature in air, and the tensile strength and reduction of area were measured. Tensile tests were performed on three steel wires for each test number, and the average value was used. Table 6 shows the evaluation results. A tensile strength of 430-560 MPa and a reduction of area of ​​75.0% or more were judged as having high strength and "good" workability. If either the tensile strength or reduction of area was outside the above range, it was judged as "poor". Table 6 also shows the number of spheroidizing annealing cycles.

[0074] [Table 6]

[0075] Based on the results above, wire rods that meet the requirements of this disclosure exhibited good workability even when manufactured as steel wire with only one spheroidizing annealing step after wire drawing. In tests 1, 7-12, and 16, the wire rods were drawn to a reduction ratio of 30% to 50%, and then annealed at a heating rate of 150°C / hour, held at 720°C for 12 hours, and slowly cooled. The resulting annealed material had a tensile strength of 500±15MPa and a reduction of 80.0±5.0%. In tests 3 and 6, the tempering heat treatment temperature after the secondary holding process (isothermal transformation) was too high, resulting in a small (211) plane diffraction peak full width at half maximum (dislocation density) and insufficient aperture. In test number 17, the heating temperature before rolling was too high, resulting in insufficient bainite area ratio and reduction of area, which caused wire breakage during the drawing process. Test number 18 showed that the heating temperature before rolling was too low, resulting in insufficient bainite area ratio and reduction of area, which caused wire breakage during the drawing process. Test number 19 showed that the finishing rolling temperature was too high, resulting in insufficient drawing and wire breakage during the wire drawing process. Test No. 20 showed insufficient reduction in size after annealing due to an excessively low finishing rolling temperature, resulting in a low (211) plane diffraction peak half-width. In test number 21, the temperature during the primary holding process was too low, resulting in insufficient bainite area ratio and reduction, which caused wire breakage during the drawing process. In test number 22, the temperature during the secondary holding process was too high, resulting in insufficient bainite area ratio and insufficient reduction of diameter after drawing and annealing. In test number 23, the cumulative time for the primary and secondary holding processes was too short, resulting in insufficient bainite area ratio and insufficient reduction of diameter after drawing and annealing. Test number 24 uses a conventional manufacturing method. The material was not held in a molten salt bath, and slow cooling was performed after winding. A two-phase structure of ferrite and pearlite was formed, and spheroidization was insufficient in a short time. Therefore, even after two annealing processes, the reduction in size after spheroidization annealing was insufficient. [Industrial applicability]

[0076] The applications of the wire rod relating to this disclosure are not limited, but it can be suitably used, for example, as structural steel for machine parts such as bolts, screws, and nuts. The wire rod relating to this disclosure is extremely useful in industry because, after wire drawing, a steel wire with excellent workability can be manufactured by a single annealing process. Using the wire material relating to this disclosure, it is possible to shorten the spheroidizing annealing time and achieve excellent cold forging properties after the spheroidizing treatment.

Claims

1. In mass percent, C: 0.10-0.60%, Si: 0.01-0.45%, Mn: 0.30-1.00%, P: 0.030% or less, S: 0.001-0.050%, Al: 0.001-0.080%, Cr: 0.85-1.50%, N: 0.0010-0.0200%, O: 0.004% or less, Mo: 0-1.00%, V: 0 to 0.50%, Nb: 0 to 0.050%, Ti: 0 to 0.100%, Cu: 0 to 0.40%, Ni: 0 to 0.30%, Sn: 0 to 0.10%, B: 0 to 0.0200%, Ca: 0-0.0050%, Mg: 0 to 0.0050%, The remainder has a chemical composition consisting of Fe and impurities. When the diameter of the wire is D, the metal structure in a cross section perpendicular to the length of the wire, at a depth of 1 / 4D from the surface of the wire, contains bainite in an area ratio of 96% or more and 100% or less, and the remainder when the area ratio of bainite is less than 100% is martensite. In the 1 / 4D portion of the cross-section parallel to the longitudinal direction, including the central axis of the wire, the X-ray diffraction pattern was measured using an X-ray stress diffractometer with Cr-Kα rays, and the half-width of the diffraction peak of the (211) plane was 2.20° or more and 3.20° or less. A wire material with an aperture of 30.0% or more.

2. In mass percent, Mo: 0.01-1.00%, V: 0.01-0.50%, Nb: 0.001–0.050%, and Ti: 0.005-0.100%, One or more selected from the group consisting of The wire material according to claim 1, including the wire material described in claim 1.

3. In mass percent, Cu: 0.02-0.40%, Ni: 0.02-0.30%, Sn: 0.01–0.10%, and B: 0.0003-0.0200% One or more selected from the group consisting of The wire material according to claim 1, including the wire material described in claim 1.

4. In mass percent, Ca: 0.0001 to 0.0050%, and Mg: 0.0001-0.0050% One or two selected from the group consisting of The wire material according to claim 1, including the wire material described in claim 1.

5. A wire rod according to any one of claims 1 to 4, wherein the tensile strength is 1200 MPa or less.

6. The wire according to any one of claims 1 to 4, wherein, after drawing the wire to a reduction ratio of 30% to 50%, an annealing process is performed, in which the temperature is raised at a heating rate of 150°C / hour, and the material is held at 720°C for 12 hours and then slowly cooled to obtain an annealed material, the resulting annealed material has the characteristics of having a tensile strength of 500 ± 15 MPa and a reduction of 80.0 ± 5.0%.

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