Wire rod, steel wire, and machine component
Patent Information
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Filing Date
- 2025-11-28
- Publication Date
- 2026-05-22
Abstract
Description
Wire rods, steel wires, and machine parts
[0001] The present disclosure relates to wire rods, steel wires, and machine parts.
[0002] High-strength materials are required for machine parts such as steel bolts and screws in order to reduce their weight and size. Conventionally, these machine parts are manufactured by softening a steel wire rod for machine structures containing C, Mn, Cr, Mo, etc. through spheroidizing annealing, then forming it into a predetermined shape through cold heading or rolling, and then quenching and tempering (thermal refining) to impart strength. On the other hand, the heat treatments of spheroidizing annealing and quenching and tempering increase manufacturing costs and CO ２ In order to increase the discharge amount, non-tempered bolts are known that omit these heat treatments and instead increase the strength of wire rod by adjusting the composition or by rapid cooling, and then perform wire drawing to impart strength to the wire (see, for example, Patent Documents 1 to 3).
[0003] Patent Document 1: Japanese Unexamined Patent Publication No. 2-166229 Patent Document 2: Patent No. 3806602 Patent Document 3: Patent No. 3816721
[0004] Non-heat treated bolts are made by cold forging steel wire that has been strengthened by wire drawing, so as the strength of the steel wire increases, the mold becomes more susceptible to damage and wear during forming, and the product becomes more susceptible to processing cracks, making it difficult to apply to high-strength bolts. Furthermore, with conventional non-heat treated bolts, the lower neck portion of the bolt is stronger than the shank due to work hardening. This poses a problem of reduced toughness in the lower neck portion.
[0005] An object of the present disclosure is to provide a wire rod, a steel wire, and a mechanical component that can be used to manufacture mechanical components that have high strength and excellent formability in cold forging, and in which local increases in strength during forming are suppressed.
[0006] The above-mentioned problems are solved by the following means. <1> In mass%, C: 0.30 to 0.50%, Si: 0.03 to 1.50%, Mn: 0.50 to 2.00%, P: 0.050% or less, S: 0.050% or less, Al: 0.005 to 0.080%, N: 0.0010 to 0.0150%, O: 0.003% or less, Ti: 0 to 0.050%, B: 0 to 0.0050%, Cr: 0 to 1.50%, Mo: 0 to 0.50%, V: 0 to 0.20%, Nb: 0 to 0.050%, Cu: 0 to 0.50%, Ni: 0 to 0.30%, Sn: 0 to 0.10%, The wire has a chemical composition consisting of Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, and the balance being Fe and impurities, wherein, when a diameter of the wire is D and a content of C in mass % is [C], in a cross section perpendicular to the longitudinal direction of the wire, a metal structure at a ¼D position where a depth from the surface of the wire is ¼D contains 70% or more of bainite in terms of area ratio and 10% or less of martensite, and an aspect ratio F(AS) and an average circular equivalent diameter F(r) [μm] of cementite at a position 50 μm deep from the surface of the wire and at the ¼D position satisfy the following formulas (1) and (2), respectively: F(AS)≦5.2×[C]+0.4 (1) F(r)≦0.62×[C]+0.1 (2) The wire has a tensile strength F(TS) [MPa] satisfying the following formula (3), F(TS)≧−150×[C]+900 (3) A wire rod, in which a value obtained by dividing the standard deviation of Vickers hardness by the average value of Vickers hardness in each of a cross section perpendicular to and a cross section parallel to the longitudinal direction of the wire rod is 0.15 or less. <2> The wire rod according to <1>, wherein the chemical composition is, in mass%, and further includes one or two elements selected from the following Group A: [Group A] Ti: 0.002 to 0.050%, and B: 0.0002 to 0.0050% <3> The wire rod according to <1> or <2>, wherein the chemical composition is, in mass%, one or two or more elements selected from the following Group B: [Group B] Cr: 0.02 to 1.50%, Mo: 0.02 to 0.50%, V: 0.02 to 0.20%, and Nb: 0.001 to 0.050%. <4> The wire rod according to any one of <1> to <3>, wherein the chemical composition includes, in mass%, one or more elements selected from the following Group C:[Group C] Cu: 0.02 to 0.50%, Ni: 0.02 to 0.30%, Sn: 0.002 to 0.10%, Ca: 0.0005 to 0.0050%, and Mg: 0.0005 to 0.0050% <5> In mass%, C: 0.30 to 0.50%, Si: 0.03 to 1.50%, Mn: 0.50 to 2.00%, P: 0.050% or less, S: 0.050% or less, Al: 0.005 to 0.080%, N: 0.0010 to 0.0150%, O: 0.003% or less, Ti: 0 to 0.050%, B: 0 to 0.0050%, a steel wire having a chemical composition of Cr: 0 to 1.50%, Mo: 0 to 0.50%, V: 0 to 0.20%, Nb: 0 to 0.050%, Cu: 0 to 0.50%, Ni: 0 to 0.30%, Sn: 0 to 0.10%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, with the balance being Fe and impurities, wherein, when a diameter of a steel wire is Dm and a content of C in mass % is [C], in a cross section perpendicular to the longitudinal direction of the steel wire, a metal structure at a position 1 / 4Dm deep from the surface of the steel wire contains 70% or more bainite by area ratio and 10% or less martensite, and a cementite aspect ratio F(AS) and an average circle equivalent diameter F(r) [μm] at a position 50 μm deep from the surface of the steel wire and at the position ¼ Dm deep satisfy the following formulas (4) and (5): F(AS)≦5.2×[C]+0.4 (4) F(r)≦0.62×[C]+0.1 (5). The tensile strength F(TS) [MPa] and reduction of area F(RA) [%] of the steel wire satisfy the following formulas (6) and (7): F(TS)≧680×[C]+660 (6) F(RA)≧−10×[C]+70 (7). A steel wire in which a value obtained by dividing the standard deviation of Vickers hardness by the average value of Vickers hardness in each of cross sections perpendicular to and parallel to the longitudinal direction of the steel wire is 0.15 or less. <6> The steel wire according to <5>, wherein the chemical composition is, in mass%, and further includes one or two elements selected from the following Group A. [Group A] Ti: 0.002 to 0.050%, and B: 0.0002 to 0.0050% <7> The steel wire according to <5> or <6>, wherein the chemical composition is, in mass%, one or two or more elements selected from the following Group B:[Group B] Cr: 0.02 to 1.50%, Mo: 0.02 to 0.50%, V: 0.02 to 0.20%, and Nb: 0.001 to 0.050%. <8> The steel wire according to any one of <5> to <7>, wherein the chemical composition includes, in mass%, one or more elements selected from the following Group C: [Group C] Cu: 0.02 to 0.50%, Ni: 0.02 to 0.30%, Sn: 0.002 to 0.10%, Ca: 0.0005 to 0.0050%, and Mg: 0.0005 to 0.0050% <9> A mechanical part including a shaft portion, comprising, in mass%, C: 0.30 to 0.50%, Si: 0.03 to 1.50%, Mn: 0.50 to 2.00%, P: 0.050% or less, S: 0.050% or less, Al: 0.005 to 0.080%, N: 0.0010 to 0.0150%, O: 0.003% or less, Ti: 0 to 0.050%, B: 0 to 0.0050%, a chemical composition consisting of Cr: 0 to 1.50%, Mo: 0 to 0.50%, V: 0 to 0.20%, Nb: 0 to 0.050%, Cu: 0 to 0.50%, Ni: 0 to 0.30%, Sn: 0 to 0.10%, Ca: 0 to 0.0050%, Mg: 0 to 0.0050%, with the balance being Fe and impurity elements, wherein, when the diameter of the shaft portion is dm and the content of C in mass % is [C], in a cross section perpendicular to the longitudinal direction of the shaft portion, a metallographic structure at a 1 / 4 dm position that is 1 / 4 dm deep from the surface of the shaft portion contains 70% or more bainite by area ratio and 10% or less martensite, and a cementite aspect ratio F(AS) and an average circle equivalent diameter F(r) [μm] at a position 50 μm deep from the surface of the shaft portion and at the 1 / 4 dm position satisfy the following formulas (8) and (9): F(AS)≦5.2×[C]+0.4 (8) F(r)≦0.62×[C]+0.1 (9). The tensile strength F(TS) [MPa] and reduction of area F(RA) [%] of the shaft portion satisfy the following formulas (10) and (11): F(TS)≧680×[C]+660 (10) F(RA)≧−10×[C]+70 (11). A mechanical component wherein a value obtained by dividing the standard deviation of Vickers hardness by the average value of Vickers hardness in each of cross sections perpendicular to and parallel to the longitudinal direction of the shaft portion is 0.15 or less.<10> The mechanical component according to <9>, wherein the chemical composition includes, in mass %, one or two elements selected from the following Group A: [Group A] Ti: 0.002 to 0.050%, and B: 0.0002 to 0.0050% <11> The mechanical component according to <9> or <10>, wherein the chemical composition includes, in mass %, one or two or more elements selected from the following Group B: [Group B] Cr: 0.02 to 1.50%, Mo: 0.02 to 0.50%, V: 0.02 to 0.20%, and Nb: 0.001 to 0.050% <12> The mechanical component according to any one of <9> to <11>, wherein the chemical composition includes, in mass %, one or two or more elements selected from the following Group C: [Group C] Cu: 0.02-0.50%, Ni: 0.02-0.30%, Sn: 0.002-0.10%, Ca: 0.0005-0.0050%, and Mg: 0.0005-0.0050%.
[0007] According to the present disclosure, there are provided a wire rod, a steel wire, and a mechanical component that can be used to manufacture a mechanical component having high strength and excellent formability in cold forging, and in which local increases in strength during forming are suppressed.
[0008] Fig. 1 is a diagram showing an example of an SEM photograph of a mainly bainite structure; Fig. 2 is an example of an SEM photograph showing a structure including pearlite and ferrite; Fig. 3 is an example of an SEM photograph showing a structure including martensite; Fig. 4 is a flow chart showing a procedure for determining the structure fraction; Fig. 5 is a schematic diagram showing measurement positions of Vickers hardness in a cross section parallel to the longitudinal direction of a wire rod;
[0009] An embodiment serving as an example of the present disclosure will be described. In this disclosure, a numerical range expressed using "to" means a range that includes the numerical values before and after "to" as the lower and upper limits. However, when the numerical values before and after "to" are followed by "greater than" or "less than," the numerical range does not include these numerical values as the lower or upper limit. The content of an element in a chemical composition may be expressed by adding "amount" to the element symbol (e.g., C content, Si content, etc.). Regarding the content of an element in a chemical composition, "%" means "mass %." When the content of an element in a chemical composition is described as "0 to," this means that the element may not be included. The term "process" includes not only an independent process, but also processes that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved. The "surface" of a wire rod means the "outer peripheral surface." The "central axis" of a wire rod means an imaginary line that passes through the center point of a cross section perpendicular to the longitudinal direction of the wire rod and extends in the longitudinal direction (axial direction). A cross section parallel to the length direction of the wire means a cross section that is parallel to the longitudinal direction of the wire and includes the central axis. "1 / 4D" is synonymous with "D / 4". "Average value" in this disclosure means a calculated average.
[0010] The inventors of the present disclosure have conducted extensive research into work hardening and strength increase (which means the same thing) of non-thermal refined wire rod, and have made the following findings regarding suppression of the amount of strength increase. By controlling the cementite distribution in bainite at the wire rod stage (reducing the aspect ratio and the equivalent circle diameter), the constraint on dislocations by cementite is weakened, and the amount of strength increase is reduced. This makes it possible to reduce the amount of work hardening when the steel wire is forged. This can be achieved by controlling the dispersion state of cementite by reducing the carbon content and performing bainite transformation and low-temperature tempering. However, since this does not provide sufficient strength, low-temperature bainite transformation and low-temperature tempering are used to ensure strength.
[0011] If the hardness variation in a cross section perpendicular to the length direction at the wire rod stage (where the wire diameter is D and the positions are 0, 90, 180, and 270° from the surface at a depth of D / 4) is small, the hardness variation in the underneck when processed into a bolt will be small. Also, if the diameter of the shank of the bolt is dm, if the hardness variation at the position dm / 4 from the surface of the shank within an axial length of 100 mm is large, the hardness variation between the shank and the underneck will be large. This will be a factor in reducing the underneck toughness.
[0012] The wire rod needs to be high strength. One method of increasing the reduction in area during wire drawing is to increase the strength of the bolt, but if the reduction in area is too large, the size is limited and there is a risk of wire breakage and hardness variations. In order to achieve high strength without increasing the reduction in area, it is necessary to increase the strength of the wire rod.
[0013] Based on these findings, the researchers conducted extensive research and found that steel wire for untempered bolts, manufactured at an area reduction rate of 20 to 50% using wire rod with a specific chemical composition, metal structure, and Vickers hardness, has excellent cold forgeability, and enables the production of bolts with a small difference in strength between the under-neck portion and the shank after bolt head formation and excellent under-neck toughness.
[0014] <Wire Rod> The wire rod according to the present disclosure was discovered based on the above findings. The chemical composition, metal structure, mechanical properties, manufacturing method, etc. of the wire rod according to the present disclosure will be described below.
[0015] [Chemical composition] The chemical composition of the wire rod according to the present disclosure contains, in mass %, as essential elements: C: 0.30 to 0.50%, Si: 0.03 to 1.50%, Mn: 0.50 to 2.00%, P: 0.050% or less, S: 0.050% or less, Al: 0.005 to 0.080%, and N: 0.0010 to 0.0150%, O: 0.003% or less, with the balance consisting of Fe and impurity elements. The wire rod according to the present disclosure may contain other optional elements as necessary.
[0016] C: 0.30 to 0.50% C is contained to ensure the strength required for mechanical parts. If the C content is less than 0.30%, it is difficult to ensure the strength required for mechanical parts. On the other hand, if the C content exceeds 0.50%, ductility, toughness, and cold forgeability deteriorate. Therefore, the C content is set to 0.30 to 0.50%. The lower limit of the C content is 0.30%, preferably 0.32%, and more preferably 0.34%, and the upper limit of the C content is 0.50%, preferably 0.49%, and more preferably 0.48%.
[0017] Si: 0.03 to 1.50% Si functions as a deoxidizing element and is effective in imparting the necessary strength to mechanical components. If the Si content is less than 0.03%, these effects are insufficient. If the Si content exceeds 1.50%, the ductility and toughness of the mechanical component deteriorate, and the deformation resistance of the steel wire increases, deteriorating cold forgeability. Therefore, the Si content is set to 0.03 to 1.50%. The preferred Si content is 0.05 to 1.00%, and more preferably 0.10 to 0.60%.
[0018] Mn: 0.50 to 2.00% Mn is an element necessary for imparting the necessary strength to mechanical components. If the Mn content is less than 0.50%, the effect is insufficient. If the Mn content exceeds 2.00%, the toughness of the mechanical components deteriorates and the deformation resistance of the steel wire increases, deteriorating cold forgeability. Therefore, the Mn content is set to 0.50 to 2.00%. The preferred Mn content is 0.70 to 1.50%, and more preferably 0.90 to 1.20%.
[0019] P: 0.050% or less P is contained in the wire rod as an impurity. P segregates at the grain boundaries of mechanical components and deteriorates toughness, so it is desirable to reduce the P content. Therefore, the upper limit of the P content is set to 0.050%. A preferred upper limit of the P content is 0.020%, and a more preferred upper limit is 0.015% or less. Note that the lower limit of the P content is preferably 0% (i.e., no P is contained), but from the viewpoint of reducing the dephosphorization cost, it may be more than 0% (or 0.0001% or more).
[0020] S: 0.050% or less S is contained in the wire rod as sulfides such as MnS. If the S content exceeds 0.050%, the cold heading property of the steel wire is deteriorated and the toughness of the mechanical parts is deteriorated. Therefore, the upper limit of the S content is set to 0.050%. A preferable upper limit of the S content is 0.030%. A more preferable upper limit is 0.015%. Note that the lower limit of the S content may be more than 0% (or 0.001% or more) from the viewpoint of reducing desulfurization costs.
[0021] Al: 0.005 to 0.080% Al functions as a deoxidizing element, and also forms AlN to refine crystal grains and improve the toughness of mechanical components. It also has the effect of fixing solute N, suppressing dynamic strain aging, and reducing deformation resistance. If the Al content is less than 0.005%, these effects are insufficient. If the Al content exceeds 0.080%, the effects saturate and manufacturability may decrease. Therefore, the Al content is set to 0.005 to 0.080%. The preferred Al content is 0.010 to 0.065%, and more preferably 0.020 to 0.055%.
[0022] N: 0.0010 to 0.0150% N forms nitrides with Al, Ti, Nb, V, etc., which refines crystal grains and improves the toughness of machine parts. If the N content is less than 0.0010%, the amount of nitride precipitation is insufficient and the effect cannot be obtained. If the N content exceeds 0.0150%, the deformation resistance of the steel wire increases due to dynamic strain aging caused by solute N, deteriorating workability. Therefore, the N content is set to 0.0010 to 0.0150%. The N content is preferably 0.0020 to 0.0100%, and more preferably 0.0025 to 0.0060%.
[0023] O: 0.003% or less O is an impurity and is inevitably contained in steel. If the O content exceeds 0.003%, coarse oxides may form, which may reduce fatigue strength, so the O content is limited to 0.003% or less. The preferred upper limit of the O content is 0.002%.
[0024] Balance: Fe and impurity elements In the chemical composition of the wire rod according to the present disclosure, the balance is Fe and impurity elements (sometimes referred to as "impurities" as appropriate in the present disclosure). Here, impurities refer to components contained in raw materials or components mixed in during the manufacturing process, but not intentionally added. Furthermore, impurities also include components that are intentionally added but contained in amounts that do not affect the performance of a steel wire obtained by wiredrawing the wire rod according to the present disclosure.
[0025] Furthermore, the wire rod according to the present disclosure may contain, in mass %, one or more elements selected from the group consisting of the following Groups A to C, in place of a portion of Fe. Note that the elements in the following Groups A to C are optional elements, and these elements may not be included, i.e., may be 0%.
[0026] [Group A] One or two selected from the group consisting of Ti: 0.050% or less, and B: 0.0050% or less
[0027] Ti: 0.050% or less Ti functions as a deoxidizing element and also has the effect of facilitating the formation of bainite. If the Ti content exceeds 0.050%, these effects become saturated and coarse oxides or nitrides are formed, which may deteriorate the fatigue strength of mechanical parts. Therefore, if Ti is contained, the Ti content should be 0.002 to 0.050%. The preferred Ti content is 0.003 to 0.050%, and more preferably 0.007 to 0.040%.
[0028] B: 0.0050% or less B has the effect of facilitating the formation of bainite. If the B content exceeds 0.0050%, carbides are formed at grain boundaries, which may deteriorate wiredrawability. Therefore, when B is contained, the B content should be 0.0002 to 0.0050%. The preferred B content is 0.0003 to 0.0050%, and more preferably 0.0008 to 0.0030%.
[0029] [Group B] One or more elements selected from the group consisting of Cr: 1.50% or less, Mo: 0.50% or less, V: 0.20% or less, and Nb: 0.050% or less.
[0030] Cr: 1.50% or less Cr is an element necessary for imparting the necessary strength to mechanical components. If the Cr content exceeds 1.50%, the martensite fraction of the wire rod increases, degrading wiredrawability and increasing the deformation resistance of the steel wire, degrading cold forgeability. Therefore, the Cr content is set to 1.50% or less. The preferred Cr content is 0.02 to 0.80%, and more preferably 0.10 to 0.50%.
[0031] Mo: 0.50% or less Mo has the effect of imparting the necessary strength to mechanical parts. If the Mo content exceeds 0.50%, the alloy cost increases, and the deformation resistance of the steel wire increases, deteriorating cold forgeability. Therefore, if Mo is contained, the Mo content should be 0.02 to 0.50%. The preferred Mo content is 0.03 to 0.35%, and more preferably 0.05 to 0.25%.
[0032] V: 0.20% or less V has the effect of precipitating carbides and nitrides to increase the strength of mechanical parts. If the V content exceeds 0.20%, the alloy cost increases. Therefore, if V is contained, the V content should be 0.02 to 0.20%. The preferred V content range is 0.01 to 0.15%.
[0033] Nb: 0.050% or less Nb has the effect of increasing the strength of machine parts by precipitating carbides and nitrides, the effect of improving toughness by refining crystal grains, and the effect of reducing solute N and reducing deformation resistance. If the Nb content exceeds 0.050%, the effect saturates and cold forgeability may deteriorate. Therefore, if Nb is contained, the Nb content should be 0.002 to 0.050%. A preferable Nb content is 0.001 to 0.040%. A more preferable Nb content is 0.005 to 0.030%.
[0034] [Group C] One or more elements selected from the group consisting of Cu: 0.50% or less, Ni: 0.30% or less, Sn: 0.10% or less, Ca: 0.0050% or less, and Mg: 0.0050% or less.
[0035] Cu: 0.50% or less Cu precipitates finely in the ferrite structure, imparting the necessary strength to mechanical components and improving corrosion resistance. If the Cu content exceeds 0.50%, hot ductility deteriorates and surface defects are more likely to occur. Therefore, if Cu is contained, the Cu content should be 0.02 to 0.50%. The preferred Cu content is 0.02 to 0.30%.
[0036] Ni: 0.30% or less Ni has the effect of improving corrosion resistance. If the Ni content exceeds 0.30%, the alloy cost increases. Therefore, if Ni is contained, the Ni content should be 0.02 to 0.30%. The Ni content is preferably 0.03 to 0.28%, and more preferably 0.05 to 0.25%.
[0037] Sn: 0.10% or less Sn has the effect of improving corrosion resistance. When Sn is contained, the Sn content is preferably 0.002% or more. However, if the Sn content exceeds 0.10%, the ductility decreases and the cold workability deteriorates, so the Sn content is limited to 0.10% or less. The preferred upper limit of the Sn content is 0.08%.
[0038] Ca: 0.0050% or less Ca is contained as a deoxidizing element. Ca has the effect of refining oxides and improving fatigue strength. When Ca is contained, the Ca content is preferably 0.0005% or more. However, if the Ca content exceeds 0.0050%, the ductility decreases and the cold workability deteriorates, so it is limited to 0.0050% or less. The preferred upper limit of the Ca content is 0.0040%, and the more preferred upper limit is 0.0030%.
[0039] Mg: 0.0050% or less Mg is contained as a deoxidizing element. Mg has the effect of refining oxides and improving fatigue strength. When Mg is contained, the Mg content is preferably 0.0005% or more. However, if the Mg content exceeds 0.0050%, the ductility decreases and the cold workability deteriorates, so the Mg content is limited to 0.0050% or less. The preferred upper limit of the Mg content is 0.0040%, and the more preferred upper limit is 0.0030%.
[0040] [Metal Structure of Wire Rod] Next, the metal structure of the wire rod according to the present disclosure will be described.
[0041] (Bainite Area Ratio) In the metal structure of the wire rod according to the present disclosure, the area ratio of bainite at a position ¼D deep from the surface (outer peripheral surface) of the wire rod in a cross section (C cross section) perpendicular to the longitudinal direction of the wire rod is 70% or more. If the bainite area ratio is less than 70%, the workability of mechanical parts will deteriorate. The bainite area ratio is preferably 80% or more, and more preferably 90% or more.
[0042] (Martensite Area Ratio) In the metal structure of the wire rod according to the present disclosure, the area ratio of martensite at a depth of ¼D from the surface (outer peripheral surface) of the wire rod in a cross section (C cross section) perpendicular to the longitudinal direction of the wire rod is 10% or less. If the martensite area ratio is more than 10%, the workability of mechanical parts will deteriorate. The martensite area ratio is preferably 5% or less, and more preferably 0%.
[0043] (Aspect ratio F(AS) and average equivalent circle diameter F(r) of cementite) In the metal structure of the wire rod according to the present disclosure, when the diameter of the wire rod is D and the content of C in mass % is [C], the aspect ratio F(AS) and average equivalent circle diameter F(r) [μm] of cementite at a position 50 μm deep and a position ¼D deep from the surface of the wire rod satisfy the following formulas (1) and (2): F(AS)≦5.2×[C]+0.4 (1) F(r)≦0.62×[C]+0.1 (2)
[0044] If the aspect ratio F(AS) of cementite is greater than 5.2 × [C] + 0.4, the increase in strength of the bolt head will be significant, and if the average equivalent circle diameter F(r) is greater than 0.62 × [C] + 0.1, the ductility of the wire rod will decrease, causing cracks when the bolt is forged. The aspect ratio F(AS) of cementite is preferably 2.00 or less, and more preferably 1.80 or less. The average equivalent circle diameter F(r) of cementite is preferably 0.35 or less, and more preferably 0.30 or less. The lower limit of the average equivalent circle diameter F(r) of cementite is not particularly limited, but is preferably 0.10 or more from the viewpoint of suppressing a decrease in ductility due to fine dispersion of cementite.
[0045] (Method for Measuring the Metallic Structure of Wire Rod) For the structural observation, two samples were taken from positions 50 mm apart in the axial direction of the wire rod. The cross sections (C sections) perpendicular to the central axis of each wire rod sample were mirror-polished, and then etched with picral to reveal the structure at a depth of 50 μm and a ¼D position from the wire rod surface. First, the C section of the wire rod to be measured was mirror-polished and then etched with picral (5% picric acid + 95% ethanol solution) to reveal the structure. Next, at four positions 50 μm deep and ¼D positions from the wire rod surface, spaced 90° apart in the circumferential direction, a 20 μm x 20 μm or larger region at each position was photographed at 5000x magnification using a field emission scanning electron microscope (FE-SEM). From the viewpoint of measurement accuracy, cementite with a circle equivalent diameter of 0.05 μm or larger was measured, and the image size was 2560 x 1920.
[0046] The metal structure may include bainite, pearlite, martensite, and ferrite. Of these, martensite and ferrite do not contain carbides within the grains. The bainite structure and pearlite structure each include cementite and ferrite. The pearlite structure is a structure in which cementite and ferrite are alternately arranged approximately parallel, while the bainite structure is a structure in which cementite and ferrite other than pearlite are mixed. FIG. 1 is an example of an SEM photograph (15 kV, 10,000x magnification) showing a bainite structure. Note that FIG. 1 shows an example of a bainite structure, and does not limit the measurement magnification for the area fraction of the bainite structure in the present disclosure. Furthermore, FIGS. 2 and 3 are each example of an SEM photograph (15 kV, 5,000x magnification) including a structure other than bainite. In the photographed structural photograph, structures other than bainite (structures not containing intragranular carbides and pearlite structures) are visually marked, and the area of the region of structures other than bainite is determined by image analysis (software name: Nireco's small general-purpose image processing analysis system LUZEX_AP). Note that this operation is performed by measuring and calculating two samples, averaging these values, and subtracting the area percentage of structures other than bainite from the total area (100%) to determine the area percentage of bainite in the present disclosure.
[0047] FIG. 4 is a flow chart showing the procedure for determining the microstructure fraction (area fraction). Specifically, the observation position of the sample (position ¼D deep from the surface of the wire rod) is identified by indentation, and picral corrosion is performed. As shown in FIGS. 2 and 3, the regions with weak corrosion are designated as ferrite ( FIG. 2 ) and martensite ( FIG. 3 ). The area fraction of martensite is determined by etching with picral, taking a microstructure photograph at 5000x magnification using an SEM, re-polishing, and etching with nital (5% nitric acid + 95% ethanol solution) to reveal the microstructure. A microstructure photograph of the same location is taken at 1000x magnification using an SEM. The image size of the microstructure photograph is 2560 × 1920 pixels. Regions that are etched with nital but weakly corroded by picral are determined as martensite, and regions that are weakly corroded by both nital and picral are determined as ferrite. The regions of each microstructure are visually marked using the method described above, and the area percentage is determined by image analysis. In the area where corrosion with picral was severe, pearlite was defined as a structure in which ferrite and pearlite were alternately layered and did not contain granular or acicular cementite between the layers (see Figure 2). Bainite was defined as a structure containing laths within the grains and with granular or acicular carbides dispersed between and within the laths, or a structure in which fragmented cementite was arranged in rows, as shown in Figure 1, and the area ratio of each structure was calculated.
[0048] The aspect ratio F(AS) of cementite was determined by observing the wire at a depth of 50 μm and a depth of 1 / 4D from the surface as follows. The particles revealed by corrosion with picral were cementite, and the aspect ratio of the cementite was determined by image analysis (software name: ImageJ image processing software). The magnification was 5000x, and the image size was 2560 × 1920 pixels. To ensure measurement accuracy, cementite with a major axis (maximum length) of 0.1 μm or more was measured, and cementite that reached the edge of the image was excluded. The maximum distance between two lines parallel to the major axis that contact the cementite was defined as the minor axis. Each cementite was approximated as an ellipse, and the ratio of the major axis to the minor axis (major axis / minor axis) was defined as the aspect ratio. This procedure was performed on two samples, and the average value was calculated.
[0049] The average equivalent circle diameter F(r) of cementite is determined in the same manner as the aspect ratio. The particles revealed by etching with picral are cementite, and the equivalent circle diameter of cementite is determined by image analysis (software name: image processing software ImageJ). This operation was performed by measuring and calculating two samples, and the average value was calculated.
[0050] [Mechanical Properties of Wire Rod] (Tensile Strength of Wire Rod) The wire rod according to the present disclosure has a tensile strength F(TS) [MPa] that satisfies the following formula (3): F(TS)≧−150×[C]+900 (3) When the C content in mass % is [C], the wire rod according to the present disclosure has a tensile strength of −150×[C]+900 [MPa] or more, thereby achieving the tensile strength required for a bolt. The tensile strength of the wire rod according to the present disclosure is preferably 800 [MPa] or more.
[0051] The tensile test of the wire rod was performed using a 14A test piece of JIS Z2241:2011 in accordance with the test method of JIS Z2241:2011, and the tensile strength (TS), 0.05% proof stress, and reduction of area were measured. The tensile test piece was obtained by machining the wire rod immediately after straightening. The tensile test was performed on three samples, and the average value was used.
[0052] (Vickers hardness of wire rod) In the wire rod according to the present disclosure, the value obtained by dividing the standard deviation of Vickers hardness by the average value of Vickers hardness (sometimes referred to in the present disclosure as "Vickers hardness (standard deviation / average value)" or simply "(standard deviation / average value)") in each of the cross sections perpendicular to the length direction of the wire rod (C cross section) and parallel to the length direction of the wire rod (L cross section) is 0.15 or less. When the Vickers hardness (standard deviation / average value) in each cross section is 0.15 or less, an increase in strength of the bolt neck portion of a mechanical part manufactured from the steel wire after wire drawing is suppressed. In addition, workability when forming into a mechanical part is improved. The (standard deviation / average value) is preferably 0.10 or less, more preferably 0.05 or less.
[0053] The hardness test is carried out by mirror-polishing the C-section of the wire, and using a Vickers hardness tester, measurements are taken at four points at a depth of 1.0 mm from the surface of the wire, rotated at 90° intervals in the circumferential direction of the wire, four points at a depth of D / 4 (where D is the diameter of the wire), rotated at 90° intervals in the circumferential direction of the wire, and one point at the center, for a total of nine points, and the average value and standard deviation are calculated. １ , x ２ ,...x ｎ When this is done, the average value X ａｖｅ and standard deviation s were calculated using the following formula: ａｖｅ = (1 / n) × Σx ｎ s=((1 / n)×Σ(x ｎ -X ａｖｅ ) ２ ) （１／２） The test force for Vickers hardness measurement is 9.8 N, and the measurement is performed according to the method of JIS Z2244-1:2020.
[0054] As shown in FIG. 5, the Vickers hardness of the L cross section of the wire was measured at nine locations at a depth of ¼D from the surface of the wire, spaced 100 μm apart along the longitudinal direction of the wire, and at nine locations at a depth of ½D, spaced 100 μm apart along the longitudinal direction of the wire. The average value and standard deviation of the 18 locations were then calculated using the above formula.
[0055] (Diameter of Wire Rod) The wire diameter D of the wire rod according to the present disclosure is not particularly limited, but is preferably 5.5 to 22.0 mm, and more preferably 6.0 to 22.0 mm.
[0056] [Wire Rod Manufacturing Method] An example of a wire rod manufacturing method according to the present disclosure will be described. A steel billet consisting of predetermined components satisfying the aforementioned chemical composition is heated to 1050 to 1250°C, held in a furnace for 90 minutes or more, then hot rolled at an entry temperature of 750 to 850°C for finish rolling, and wound into a ring shape at 820 to 940°C. After winding, the steel is subjected to primary cooling to 750°C at an average cooling rate of 2 to 15°C / s. This is followed by secondary cooling from 750°C to 500°C at an average cooling rate of 25°C / s or more. This is followed by tertiary cooling from 500°C to 460°C at an average cooling rate of 20°C / s or less. Thereafter, the wire is held at 400 to 460°C for 15 to 100 seconds (primary holding), and if necessary, the temperature is increased from 400 to 530°C at an average temperature increase rate of 10°C / s or more, held at 400 to 530°C for 30 to 200 seconds (secondary holding), and then water-cooled to obtain a wire.
[0057] If the heating temperature is less than 1050°C or the heating furnace time is less than 90 minutes, the solid solution of carbides becomes insufficient, resulting in a decrease in the ductility of the wire rod. If the heating temperature exceeds 1250°C, the surface hardness of the wire rod decreases due to decarburization. If the entry temperature of finish rolling exceeds 850°C, γ grains (austenite grains) become coarse and non-uniform, reducing the ductility of the wire rod. Not only does this result in a deterioration in workability, but the standard deviation of the hardness of the wire rod also increases, resulting in a deterioration in the neck toughness of mechanical parts. If the entry temperature is less than 750°C, the γ grain size becomes non-uniform, resulting in a large standard deviation of the hardness of the wire rod. If the coiling temperature exceeds 940°C, the γ grains become coarse and non-uniform, reducing the ductility of the wire rod. Not only does this result in a deterioration in workability, but it also results in a large standard deviation of the hardness of the wire rod also, resulting in a deterioration in the neck toughness of mechanical parts. If the coiling temperature is less than 820°C, the area ratio of bainite decreases, resulting in a decrease in the ductility of the wire rod. After coiling, if the primary cooling rate to 750°C is less than 2°C / s, the gamma grains become coarse, reducing the ductility of the wire and degrading workability. If the rate exceeds 15°C / s, the recovery and recrystallization of the gamma grains are delayed, resulting in mixed grains. This results in non-uniform gamma grain size, a larger standard deviation in the hardness of the wire, and reduced neck toughness of mechanical parts. If the secondary cooling rate from 750°C to 500°C is less than 25°C / s, the area ratio of ferrite and pearlite increases, reducing the tensile strength of the wire. If the tertiary cooling rate from 500°C to 460°C exceeds 20°C / s, the standard deviation in the hardness of the wire increases, resulting in reduced neck toughness of bolts. If the primary holding time at 400-460°C is less than 15 seconds, the hardness variation of the wire increases, reducing the neck toughness of mechanical parts. If the primary holding time exceeds 100 seconds, the manufacturing cost increases. When the temperature is increased to a secondary holding temperature to grow cementite after the primary holding, if the average heating rate from 400°C to 530°C is less than 10°C / s, the hardness of the wire rod will vary greatly and the neck toughness of the machine part will deteriorate. If the secondary holding temperature is less than 400°C, the ductility of the machine part will deteriorate, and if it exceeds 530°C, the cementite will coarsen and the strength will decrease. If the secondary holding time is less than 30 seconds, the ductility of the wire rod will decrease. If the secondary holding time exceeds 200 seconds, carbides will coarsen and the reduction of area of the steel wire after wiredrawing will decrease.
[0058] The wire rod according to the present disclosure is drawn at an area reduction rate of 20 to 50% to form a steel wire, which is then cold-headed and rolled to form a mechanical part such as a bolt. If the drawing area reduction rate is less than 20%, the under-neck toughness of the mechanical part will decrease. If the drawing area reduction rate exceeds 50%, the workability of the mechanical part will deteriorate.
[0059] A steel wire manufactured using the wire rod of the present disclosure has high strength and drawing ability, and is excellent in increasing the strength and formability of bolts. A non-heat treated bolt manufactured using this steel wire can obtain high bolt cold workability and excellent under-neck toughness even if a bluing treatment (heat treatment) after forming is omitted. The steel wire and mechanical parts according to the present disclosure will be described below. The chemical composition of the steel wire and mechanical parts according to the present disclosure is the same as the chemical composition of the wire rod described above, and will not be described below.
[0060] <Steel Wire> The steel wire according to the present disclosure is obtained by wiredrawing the above-mentioned wire rod, and plastic flow is observed in the surface layer of the longitudinal cross section of the steel wire (a cross section parallel to the longitudinal direction of the steel wire). The metallographic structure and mechanical properties of the steel wire according to the present disclosure will be described below.
[0061] [Metal structure of steel wire] (Bainite area ratio) In the steel wire according to the present disclosure, when the diameter of the steel wire is Dm, the metal structure at a ¼ Dm position, which is a depth from the surface of the steel wire in a cross section perpendicular to the longitudinal direction of the steel wire, contains bainite at an area ratio of 70% or more. If the bainite area ratio is less than 70%, the workability of mechanical parts will deteriorate. The bainite area ratio is preferably 80% or more, and more preferably 90% or more.
[0062] (Martensite Area Ratio) In the metal structure of the steel wire according to the present disclosure, the area ratio of martensite at a position ¼ Dm deep from the surface (outer peripheral surface) of the steel wire in a cross section (C cross section) perpendicular to the longitudinal direction of the steel wire is 10% or less. If the martensite area ratio is more than 10%, the workability of mechanical parts will deteriorate. The martensite area ratio is preferably 5% or less, and more preferably 0%.
[0063] (Aspect ratio F(AS) and average equivalent circle diameter F(r) of cementite) In the steel wire according to the present disclosure, the aspect ratio F(AS) and average equivalent circle diameter F(r) [μm] of cementite at a position 50 μm and a position ¼ Dm deep from the surface of the steel wire satisfy the following formulas (4) and (5): F(AS)≦5.2×[C]+0.4 (4) F(r)≦0.62×[C]+0.1 (5) When the aspect ratio F(AS) and average equivalent circle diameter F(r) of cementite at a position 50 μm and a position ¼ Dm deep from the surface of the steel wire satisfy the above relationships, high cold workability and excellent under-neck toughness of the mechanical parts can be obtained when non-thermal refined mechanical parts are manufactured from this steel wire.
[0064] The metallographic structure of the steel wire can be measured in the same manner as the metallographic structure of the wire rod described above.
[0065] [Mechanical Properties of Steel Wire] (Tensile Strength and Reduction of Area of Steel Wire) The steel wire according to the present disclosure has a tensile strength F(TS) [MPa] and a reduction of area F(RA) [%] that satisfy the following formulas (6) and (7): F(TS)≧680×[C]+660 (6) F(RA)≧−10×[C]+70 (7) When the tensile strength F(TS) and the reduction of area F(RA) of the steel wire satisfy the above relationships, when a non-tempered mechanical part is manufactured from this steel wire, the cold workability is improved and sufficient strength for the mechanical part can be obtained.
[0066] From the viewpoint of achieving both cold workability and tensile strength when made into a mechanical part, it is preferable that the tensile strength F(TS) is 1000 MPa or more and the reduction of area F(RA) is 70% or more.
[0067] The tensile strength F(TS) and reduction of area F(RA) of the steel wire according to the present disclosure can be measured in accordance with the test method of JIS Z2241:2011 using a No. 14A test piece of JIS Z2241:2011, similar to the tensile test of the wire rod described above.
[0068] (Vickers hardness of steel wire) In the steel wire according to the present disclosure, the value obtained by dividing the standard deviation of Vickers hardness by the average value of Vickers hardness (standard deviation / average value) in each of a cross section perpendicular to the longitudinal direction of the steel wire (C cross section) and a cross section parallel to the longitudinal direction of the steel wire (L cross section) is 0.15 or less. When (standard deviation / average value) is 0.15 or less, the workability when forming into mechanical parts is improved, and the under-neck toughness of the mechanical parts is also increased. It is preferably 0.10 or less, more preferably 0.05 or less. The hardness test of the steel wire can be performed in the same manner as the hardness test of the wire rod described above, and the average value and standard deviation can be determined in the same manner.
[0069] (Diameter of Steel Wire) The wire diameter Dm of the steel wire according to the present disclosure is not particularly limited, but may be, for example, 2.0 to 18.0 mm, or 4.0 to 16.0 mm.
[0070] <Mechanical component> The mechanical component according to the present disclosure is obtained by cold heading and rolling the above-mentioned steel wire and processing it into the shape of a mechanical component such as a bolt. The chemical composition of the mechanical component according to the present disclosure is the same as that of the wire rod described above. The mechanical component according to the present disclosure includes a shaft portion, and the shaft portion has the following metal structure and mechanical properties.
[0071] [Metal structure of shank portion] (Bainite area ratio) When the diameter of the shank portion is dm, the metal structure at a 1 / 4 dm position, which is 1 / 4 dm deep from the surface of the shank in a cross section (C cross section) perpendicular to the longitudinal direction of the shank portion, contains bainite at an area ratio of 70% or more. If the bainite area ratio is less than 70%, the workability of the mechanical component will deteriorate. The bainite area ratio is preferably 80% or more, and more preferably 90% or more.
[0072] (Martensite Area Ratio) In the metal structure of the shaft portion of the mechanical component according to the present disclosure, the area ratio of martensite at a position ¼ dm deep from the surface (outer peripheral surface) of the shaft portion in a cross section (C cross section) perpendicular to the longitudinal direction of the shaft portion is 10% or less. If the martensite area ratio is more than 10%, the workability of the mechanical component deteriorates. The martensite area ratio is preferably 5% or less, and more preferably 0%.
[0073] (Aspect ratio F(AS) and average equivalent circle diameter F(r) of cementite) In the shank portion of the mechanical component according to the present disclosure, the aspect ratio F(AS) and average equivalent circle diameter F(r) [μm] of cementite at a position 50 μm and a position 1 / 4 dm deep from the surface of the shank portion satisfy the following formulas (8) and (9): F(AS)≦5.2×[C]+0.4 (8) F(r)≦0.62×[C]+0.1 (9) When the aspect ratio F(AS) and average equivalent circle diameter F(r) of cementite at the position 50 μm and a position 1 / 4 dm deep from the surface of the shank portion satisfy the above relationships, high cold workability and excellent under-neck toughness can be obtained.
[0074] The metal structure of the shank can be measured in the same manner as the metal structure of the wire rod described above. In this disclosure, when the mechanical part is a bolt, the "shank" refers to the part that is 1.0 mm or more away from the bearing surface.
[0075] [Mechanical Properties of Shank Portion] In the mechanical component according to the present disclosure, when the C content in mass % is [C], the tensile strength F(TS) [MPa] and reduction of area F(RA) [%] of the shaft portion satisfy the following formulas (10) and (11): F(TS)≧680×[C]+660 (10) F(RA)≧−10×[C]+70 (11) The shaft portion having such tensile strength and reduction of area allows the mechanical component to exhibit high strength.
[0076] The tensile test can be carried out using a No. 14A test piece of JIS Z2241:2011, in the same manner as the tensile test of wire rod described above, in accordance with the test method of JIS Z2241:2011. Note that, for example, if all or most of the portion below the neck of a bolt is a threaded portion, the threaded portion is regarded as the shank and the test piece is taken.
[0077] (Vickers hardness of shank portion) In the mechanical component according to the present disclosure, the value obtained by dividing the standard deviation of Vickers hardness by the average value of Vickers hardness (standard deviation / average value) in each of a cross section perpendicular to the longitudinal direction of the shank (C cross section) and a cross section parallel to the longitudinal direction of the shank (L cross section) is 0.15 or less. When (standard deviation / average value) is 0.15 or less, the workability when forming into a mechanical component manufactured from the steel wire after wire drawing is improved. It is preferably 0.10 or less, more preferably 0.05 or less. The hardness test of the shank portion can be performed in the same manner as the hardness test of the wire rod described above, and the average value and standard deviation can be determined in the same manner.
[0078] (Diameter of Shank Portion) The diameter dm of the shaft portion of the mechanical component according to the present disclosure is not particularly limited, but may be, for example, 2.0 to 18.0 mm, or 4.0 to 16.0 mm.
[0079] Examples of the wire rod, steel wire, and mechanical component according to the present disclosure will be described below. Note that the following examples do not limit the wire rod, steel wire, and mechanical component according to the present disclosure.
[0080] 1. Test Steel Composition <Production and Evaluation of Wire Rod> Steel billets having the chemical compositions shown in Table 1 were prepared. In Table 1, underlines indicate compositions outside the scope of the present disclosure, and blank spaces indicate that the content of the corresponding element is 0% in terms of the significant figures (numbers down to the least significant digit) described above. For example, a blank space in the Mo column means that the Mo content is 0.00%. The remainder in Table 1 is Fe and impurities.
[0081]
[0082] 2. Production of Wire Rods Wire rods were produced using the steel billets shown in Table 1 under the conditions shown in Table 2.
[0083]
[0084] 3. Wire Rod Properties The obtained wire rods were subjected to microstructural observation, tensile testing, and hardness measurement. Table 3 shows the structure and mechanical properties of the wire rods for each test number. Note that "F" in "remaining structure" means ferrite structure, and "P" means pearlite structure. In Table 3 and subsequent tables, underlines indicate that the results are outside the scope of the present disclosure.
[0085]
[0086] For microstructural observation, a cross section (C cross section) perpendicular to the central axis of the wire was mirror-polished, and then etched with picral. The 50 μm and ¼ D positions were observed. The area ratio of the microstructure was calculated by observing the ¼ D position. As shown in FIG. 4, observations were made after etching with picral and then after etching with nital. Tensile tests were performed in air at room temperature using linear test specimens prepared by cutting the steel wire to a length of 390 mm, with a gauge length of 100 mm and a crosshead speed of 10 mm / min. The tensile strength (TS) was measured. The tensile test was performed on three steel wires of each test number, and the average value was used. For the hardness test, a cross section perpendicular to the central axis of the wire (C cross section) and a cross section including the central axis and parallel to the central axis (L cross section) were mirror-polished. Using a Vickers hardness tester, measurements were taken at four locations at a depth of 1.0 mm from the surface of the wire, rotated at 90° intervals around the circumferential direction of the wire, four locations at a depth of D / 4, where D is the diameter of the wire, rotated at 90° intervals around the circumferential direction of the wire, and one location at the center, for a total of nine locations, and the average value and standard deviation were calculated. The test force used for the Vickers hardness measurement was 9.8 N. For the L cross section, measurements were taken at nine locations at 10 μm intervals along the length of the wire, at a depth of D / 4 and a depth of D / 2, as shown in FIG. 5, for a total of 18 locations, and the average value and standard deviation were calculated.
[0087] 4. Production of Steel Wire and Properties of Steel Wire Steel wires were obtained by wiredrawing the wire rods shown in Table 3 at the total area reduction rates shown in Table 4. Tensile tests were carried out on the obtained steel wires in air at room temperature to measure the tensile strength (TS) and reduction of area (RA).
[0088]
[0089] The steel wires in Table 4 were cold-formed and rolled to form bolts, which were then galvanized and baked at 200°C to obtain bolts. Tensile test specimens were taken from the shanks of the resulting bolts, and tensile tests were conducted to measure the tensile strength (TS). The cold forgeability of the bolts was evaluated by checking for cracks in the bolt collar after cold forging. The under-neck ductility of the bolts was evaluated by a head impact test specified in JIS B1051:2014 to check for cracks in the transition between the shank and the head. If any one of the following conditions was met, the bolt was judged to be "fail": cracks in the collar, cracks in the transition, or insufficient tensile strength (less than 680 × [C] + 660 MPa). If none of these conditions was met, the bolt was judged to be "good." The results are shown in Table 5.
[0090]
[0091] The wire rods of test numbers 1 to 14 and 31 to 35 that met the requirements of the present disclosure were excellent in formability during bolt processing after wire drawing, and the bolts obtained had high strength even without heat treatment after bolt processing, and also had high strength at the transition between the shank and the head.
[0092] Test numbers 15 to 30 and 36 did not meet the requirements of the present disclosure, and cracks in the collar portion or transition portion were confirmed by cold forging or head impact testing of the bolts, or in the absence of such cracks or cracks, the tensile strength was insufficient.
[0093] The disclosure of Japanese Patent Application No. 2024-095412, filed on June 12, 2024, is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards mentioned herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated herein by reference.
Claims
1. In mass percent, C: 0.30-0.50%, Si: 0.03 to 1.50%, Mn: 0.50-2.00%, P: 0.050% or less, S: 0.050% or less, Al: 0.005-0.080%, N: 0.0010-0.0150%, O: 0.003% or less, Ti: 0 to 0.050%, B: 0 to 0.0050%, Cr: 0-1.50%, Mo: 0 to 0.50%, V: 0 to 0.20%, Nb: 0 to 0.050%, Cu: 0 to 0.50%, Ni: 0 to 0.30%, Sn: 0 to 0.10%, Ca: 0-0.0050%, Mg: 0 to 0.0050%, The chemical composition consists of Fe and impurities as the remainder. When the diameter of the wire is D and the C content by mass is [C], in a cross section perpendicular to the length direction of the wire, the microstructure at the 1 / 4D position, where the depth from the surface of the wire is 1 / 4D, contains 70% or more bainite by area and 10% or less martensite. The aspect ratio F(AS) and mean circle equivalent diameter F(r) [μm] of the cementite at a depth of 50 μm from the surface of the wire and at the 1 / 4D position satisfy the following equations (1) and (2): F(AS)≦5.2×[C]+0.4 (1) F(r)≦0.62×[C]+0.1 (2) The tensile strength F(TS) [MPa] of the aforementioned wire satisfies the following formula (3): F(TS)≧-150×[C]+900 (3) A wire in which the value obtained by dividing the standard deviation of the Vickers hardness by the mean value of the Vickers hardness in both the cross section perpendicular to the length direction and the cross section parallel to the length direction of the wire is 0.15 or less.
2. The wire according to claim 1, wherein the chemical composition comprises, in mass%, one or two selected from group A below. [Group A] Ti: 0.002–0.050%, and B: 0.0002-0.0050%
3. The wire according to claim 1, wherein the chemical composition comprises, in mass%, one or more selected from group B below. [Group B] Cr: 0.02-1.50%, Mo: 0.02-0.50%, V: 0.02-0.20%, and Nb: 0.001-0.050%
4. The wire according to claim 2, wherein the chemical composition comprises, by mass%, one or more selected from group B below. [Group B] Cr: 0.02-1.50%, Mo: 0.02-0.50%, V: 0.02-0.20%, and Nb: 0.001-0.050%
5. The wire according to any one of claims 1 to 4, wherein the chemical composition comprises, in mass%, one or more selected from group C below. [Group C] Cu: 0.02 to 0.50%, Ni: 0.02-0.30%, Sn: 0.002 to 0.10%, Ca: 0.0005 to 0.0050%, and Mg: 0.0005-0.0050%
6. In mass percent, C: 0.30-0.50%, Si: 0.03 to 1.50%, Mn: 0.50-2.00%, P: 0.050% or less, S: 0.050% or less, Al: 0.005-0.080%, N: 0.0010-0.0150%, O: 0.003% or less, Ti: 0 to 0.050%, B: 0 to 0.0050%, Cr: 0-1.50%, Mo: 0 to 0.50%, V: 0 to 0.20%, Nb: 0 to 0.050%, Cu: 0 to 0.50%, Ni: 0 to 0.30%, Sn: 0 to 0.10%, Ca: 0-0.0050%, Mg: 0 to 0.0050%, The chemical composition consists of Fe and impurities as the remainder. When the diameter of the steel wire is Dm and the carbon content by mass is [C], the microstructure at the 1 / 4Dm position, where the depth from the surface of the steel wire is 1 / 4Dm in a cross section perpendicular to the length of the steel wire, contains 70% or more bainite by area and 10% or less martensite. The aspect ratio F(AS) and mean circle equivalent diameter F(r) [μm] of the cementite at a depth of 50 μm from the surface of the steel wire and at the 1 / 4 Dm position satisfy the following equations (4) and (5): F(AS)≦5.2×[C]+0.4 (4) F(r)≦0.62×[C]+0.1 (5) The tensile strength F(TS) [MPa] and reduction of area F(RA) [%] of the steel wire satisfy the following formulas (6) and (7): F(TS)≧680×[C]+660 (6) F(RA)≧-10×[C]+70 (7) A steel wire in which the value obtained by dividing the standard deviation of the Vickers hardness by the mean value of the Vickers hardness in both a cross section perpendicular to the length direction of the steel wire and a cross section parallel to the length direction of the steel wire is 0.15 or less.
7. The steel wire according to claim 6, wherein the chemical composition comprises, in mass%, one or two selected from group A below. [Group A] Ti: 0.002–0.050%, and B: 0.0002-0.0050%
8. The steel wire according to claim 6, wherein the chemical composition comprises, in mass%, one or more selected from group B below. [Group B] Cr: 0.02-1.50%, Mo: 0.02-0.50%, V: 0.02-0.20%, and Nb: 0.001-0.050%
9. The steel wire according to claim 7, wherein the chemical composition comprises, by mass%, one or more selected from group B below. [Group B] Cr: 0.02-1.50%, Mo: 0.02-0.50%, V: 0.02-0.20%, and Nb: 0.001-0.050%
10. The steel wire according to any one of claims 6 to 9, wherein the chemical composition comprises, by mass%, one or more selected from group C below. [Group C] Cu: 0.02 to 0.50%, Ni: 0.02-0.30%, Sn: 0.002 to 0.10%, Ca: 0.0005 to 0.0050%, and Mg: 0.0005-0.0050%
11. A mechanical part including a shaft, In mass percent, C: 0.30-0.50%, Si: 0.03 to 1.50%, Mn: 0.50-2.00%, P: 0.050% or less, S: 0.050% or less, Al: 0.005-0.080%, N: 0.0010-0.0150%, O: 0.003% or less, Ti: 0 to 0.050%, B: 0 to 0.0050%, Cr: 0-1.50%, Mo: 0 to 0.50%, V: 0 to 0.20%, Nb: 0 to 0.050%, Cu: 0 to 0.50%, Ni: 0 to 0.30%, Sn: 0 to 0.10%, Ca: 0-0.0050%, Mg: 0 to 0.0050%, The chemical composition consists of Fe and impurity elements as the remainder. When the diameter of the shaft portion is dm and the C content in mass% is [C], the microstructure at the 1 / 4dm position, where the depth from the surface of the shaft portion is 1 / 4dm in a cross section perpendicular to the longitudinal direction of the shaft portion, contains 70% or more bainite by area ratio and 10% or less martensite. The aspect ratio F(AS) and mean circle equivalent diameter F(r) [μm] of the cementite at a depth of 50 μm from the surface of the shaft and at the 1 / 4 dm position satisfy the following equations (8) and (9): F(AS)≦5.2×[C]+0.4 (8) F(r)≦0.62×[C]+0.1 (9) The tensile strength F(TS) [MPa] and reduction of area F(RA) [%] of the shaft portion satisfy the following formulas (10) and (11): F(TS)≧680×[C]+660 (10) F(RA)≧-10×[C]+70 (11) A machine part in which the value obtained by dividing the standard deviation of the Vickers hardness by the average value of the Vickers hardness in both the cross section perpendicular to the longitudinal direction of the shaft and the cross section parallel to the shaft is 0.15 or less.
12. The mechanical part according to claim 11, wherein the chemical composition comprises, in mass%, one or two selected from group A below. [Group A] Ti: 0.002–0.050%, and B: 0.0002-0.0050%
13. The mechanical part according to claim 11, wherein the chemical composition comprises, in mass%, one or more selected from group B below. [Group B] Cr: 0.02-1.50%, Mo: 0.02-0.50%, V: 0.02-0.20%, and Nb: 0.001-0.050%
14. The mechanical part according to claim 12, wherein the chemical composition comprises, by mass%, one or more selected from group B below. [Group B] Cr: 0.02-1.50%, Mo: 0.02-0.50%, V: 0.02-0.20%, and Nb: 0.001-0.050%
15. The mechanical part according to any one of claims 11 to 14, wherein the chemical composition comprises, in mass%, one or more selected from the following group C. [Group C] Cu: 0.02 to 0.50%, Ni: 0.02-0.30%, Sn: 0.002 to 0.10%, Ca: 0.0005 to 0.0050%, and Mg: 0.0005-0.0050%