Steel material and component for mechanical structure

Abstract

Provided is a steel material having excellent weld cracking resistance and excellent machinability, and being capable of exhibiting high fatigue strength when used as a component for a mechanical structure. The steel material according to the present disclosure has the chemical composition described in the description, wherein: Fn defined by formula (1) is 0.45-1.05; the number density of fine sulfide particles which are sulfide particles having an equivalent circle diameter of 0.1-2.0 μm is 550 particles per 1 mm2 or more, and the number density of coarse sulfide particles which are sulfide particles having an equivalent circle diameter of more than 2.0 μm is 55 particles per 1 mm2 or less at a position of 0.08R depth from the surface of the steel material; and the number density of the fine sulfide particles is less than 550 particles per 1 mm2, and the number density of the coarse sulfide particles is more than 55 particles per 1 mm2 at the position of the center of gravity.

Description

Steel materials and machine structural parts

[0001] The present disclosure relates to steel materials and machine structural components, and more particularly to steel materials that are used as raw materials for machine structural components, and machine structural components.

[0002] High fatigue strength is required for machine structural parts, such as crankshafts, used in automobiles, construction vehicles, industrial machinery, and the like.

[0003] A technique for increasing the fatigue strength of machine structural parts is disclosed in, for example, Japanese Patent Laid-Open Publication No. 2008-169411 (Patent Document 1).

[0004] The steel material disclosed in Patent Document 1, which is used as a raw material for machine structural parts, contains, in mass %, C: 0.15 to 0.55%, Si: 0.01 to 2.0%, Mn: 0.01 to 2.5%, Cu: 0.01 to 2.0%, Ni: 0.01 to 2.0%, Cr: 0.01 to 2.5%, Mo: 0.01 to 3.0%, and a total amount of at least one element selected from the group consisting of V and W: 0.01 to 1.0%, with the balance consisting of Fe and unavoidable impurities. This steel is soaked at 1010°C to 1050°C, cooled to 500°C to 550°C at a cooling rate of 200°C / min or more, subsequently cooled to 150°C or less at a cooling rate of 100°C / min or more, and then heated in a temperature range of 550°C to 700°C. After this, the LMP that gives the maximum HRC hardness at room temperature is 17.66 or more. In this document, the LMP that gives the maximum HRC hardness at room temperature after heat treatment under the above conditions is set to 17.66 or more, thereby improving softening resistance. This improves the fatigue properties of machine structural parts.

[0005] Japanese Patent Application Laid-Open No. 2008-169411

[0006] In order to improve the fatigue strength of machine structural parts, surface hardening treatment is sometimes performed on the machine structural parts.

[0007] Induction hardening is one of the various surface hardening processes. Induction hardening allows hardening only in the required areas. Induction hardening involves heating to a high temperature and then cooling to form a hardened layer on the surface of the steel material. Induction hardening can achieve a greater hardened layer depth and higher fatigue strength than other surface hardening processes such as soft nitriding.

[0008] The induction hardening treatment applied to a machine structural part will be described below, taking a crankshaft as an example. When the machine structural part is a crankshaft, induction hardening is applied to, for example, the fillet R portion of the crankshaft in order to increase the fatigue strength of the crankshaft. In this case, a hardened layer is formed on the surface of the fillet R portion.

[0009] In order to increase the hardened layer depth, the heating temperature during induction hardening can be increased by increasing the output of high-frequency power. However, when induction hardening is performed at high temperatures, the heating temperature tends to become excessively high at the edge of the machine structural component. For example, if the machine structural component is a crankshaft, the heating temperature tends to become excessively high at the edge of the crank web or the edge of the counterweight. In particular, if the temperature rise rate during induction hardening is fast, the heating temperature tends to become excessively high at the edge of the crank web or the edge of the counterweight.

[0010] For example, if the heating temperature during induction hardening becomes excessively high, reaching 1350°C or higher, a portion of the steel material may melt and crack. Hereinafter, such cracks will be referred to as "fusion cracks" in this specification. It is preferable to be able to prevent the occurrence of such fusion cracks. In other words, steel materials are required to have excellent resistance to fusion cracks.

[0011] Furthermore, steel materials used as raw materials for machine structural parts are subjected to cutting during the manufacturing process of the machine structural parts. At this time, excellent machinability may be required, particularly in the interior of the steel material. For example, if the machine structural part is a crankshaft, processing such as drilling is performed on the center portions of both end faces. Therefore, excellent machinability is required in the steel material, especially in the interior of the steel material.

[0012] In the above-mentioned Patent Document 1, the resistance to melting cracking and machinability of the steel material are not considered.

[0013] An object of the present disclosure is to provide a steel material and a machine structural component that have excellent melting crack resistance and excellent machinability, and that, when used as a material for machine structural components, enable the machine structural components to obtain high fatigue strength.

[0014] The steel material of the present disclosure has a chemical composition, in mass%, of C: more than 0.30 to 0.60%, Si: 0.01 to 0.90%, Mn: 0.50 to 1.70%, P: 0.030% or less, S: 0.015 to 0.080%, Cr: 0.01 to 1.30%, Al: more than 0.010 to 0.090%, N: 0.0250% or less, O: 0.0050% or less, V: 0 to 0.200%, Sn: 0 to 0.1000%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Pb: 0 to 0.09%, Bi: 0 to 0.25%, Mg: 0 to 0.010%. 0%, Ti: 0 to 0.0400%, Nb: 0 to 0.0500%, W: 0 to 0.4000%, Zr: 0 to 0.2000%, Ca: 0 to 0.0100%, Te: 0 to 0.0100%, B: 0 to 0.0050%, rare earth elements: 0 to 0.0100%, Co: 0 to 0.0100%, Se: 0 to 0.0100%, In: 0 to 0.0100%, Mo: 0 to 0.30%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, and the balance being Fe and impurities, and Fn defined by formula (1) is 0.45 to 1.05. Fn=C+(Si / 10)+(Mn / 5)-(5S / 7)+(5Cr / 22)+1.65V (1) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1), and when an element is not contained, "0" is substituted for the corresponding element symbol. In the steel material of the present disclosure, when the length of the shortest line segment among the line segments connecting the center of gravity of the cross section perpendicular to the axial direction of the steel material and the surface of the steel material is defined as R, the number density of fine sulfide particles, which are sulfide particles with a circle equivalent diameter of 0.1 to 2.0 μm, at a depth position of 0.08R from the surface of the steel material on the shortest straight line is 550 particles / mm ２ or more, and the number density of coarse sulfide particles, which are sulfide particles having a circle equivalent diameter of more than 2.0 μm, is 55 particles / mm ２ Furthermore, at the position of the center of gravity of the cross section, the number density of the fine sulfide particles is 550 particles / mm ２ and the number density of coarse sulfide particles is less than 55 particles / mm２ It's super.

[0015] A machine structural component according to the present disclosure has a chemical composition, in mass %, of a part of the machine structural component, of C: more than 0.30 to 0.60%, Si: 0.01 to 0.90%, Mn: 0.50 to 1.70%, P: 0.030% or less, S: 0.015 to 0.080%, Cr: 0.01 to 1.30%, Al: more than 0.010 to 0.090%, N: 0.0250% or less, O: 0.0050% or less, V: 0 to 0.200%, Sn: 0 to 0.1000%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Pb: 0 to 0.09%, Bi: 0 to 0.25%. , Mg: 0 to 0.0100%, Ti: 0 to 0.0400%, Nb: 0 to 0.0500%, W: 0 to 0.4000%, Zr: 0 to 0.2000%, Ca: 0 to 0.0100%, Te: 0 to 0.0100%, B: 0 to 0.0050%, rare earth elements: 0 to 0.0100%, Co: 0 to 0.0100%, Se: 0 to 0.0100%, In: 0 to 0.0100%, Mo: 0 to 0.30%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, and the balance is Fe and impurities, and Fn defined by formula (1) is 0.45 to 1.05. Fn=C+(Si / 10)+(Mn / 5)-(5S / 7)+(5Cr / 22)+1.65V (1) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1), and when an element is not contained, "0" is substituted for the corresponding element symbol. In some of the machine structural components of the present disclosure, when the length of the shortest line segment among the line segments connecting the center of gravity of the cross section perpendicular to the axial direction of the machine structural component and the surface of the machine structural component is defined as R, the number density of fine sulfide particles, which are sulfide particles with a circle equivalent diameter of 0.1 to 2.0 μm, is 550 particles / mm at a depth position of 0.08R from the surface of the shortest line segment. ２ or more, and the number density of coarse sulfide particles, which are sulfide particles having a circle equivalent diameter of more than 2.0 μm, is 55 particles / mm ２ In addition, the number density of the fine sulfide particles at the position of the center of gravity of the cross section is 550 particles / mm ２ and the number density of coarse sulfide particles is less than 55 particles / mm ２ It's super.

[0016] The steel material of the present disclosure has excellent resistance to melting cracking and excellent machinability, and when used as a material for machine structural parts, the machine structural parts can obtain high fatigue strength. The machine structural parts of the present disclosure have high fatigue strength.

[0017] FIG. 1A is a schematic diagram showing an example of a cross section perpendicular to the axial direction of a steel material of this embodiment. FIG. 1B is a schematic diagram showing another example of a cross section perpendicular to the axial direction of a steel material of this embodiment, different from FIG. 1A. FIG. 2 is a cross-sectional view parallel to the axial direction of a part of a crankshaft, which is an example of a machine structural part. FIG. 3 is a schematic diagram of a microstructure in a fusion crack evaluation test in an example. FIG. 4 is a schematic diagram of a microstructure different from FIG. 3 in a fusion crack evaluation test in an example. FIG. 5 is a side view of a fatigue test piece used in a fatigue strength evaluation test in an example.

[0018] The present inventors first investigated the chemical composition of a steel material that, when used as a material for machine structural parts, increases the fatigue strength of the machine structural parts. As a result, the inventors have found that the composition is, in mass%, C: more than 0.30 to 0.60%, Si: 0.01 to 0.90%, Mn: 0.50 to 1.70%, P: 0.030% or less, S: 0.015 to 0.080%, Cr: 0.01 to 1.30%, Al: more than 0.010 to 0.090%, N: 0.0250% or less, O: 0.0050% or less, V: 0 to 0.200%, Sn: 0 to 0.1000%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Pb: 0 to 0.09%, Bi: 0 to 0.25%, Mg: 0 to 0.0100%, Ti: 0 to 0.0400%, Nb: 0 to 0. 0.0500%, W: 0-0.4000%, Zr: 0-0.2000%, Ca: 0-0.0100%, Te: 0-0.0100%, B: 0-0.0050%, rare earth elements: 0-0.0100%, Co: 0-0.0100%, Se: 0-0.0100%, In: 0-0.0100%, Mo: 0-0.30%, Cu: 0-0.50%, Ni: 0-0.50%, and the balance being Fe and impurities, it was considered that when a machine structural part is manufactured using this steel as a raw material, the machine structural part may have excellent fatigue strength.

[0019] Subsequently, the inventors investigated means for improving the machinability of steel materials in which the content of each element in the chemical composition is within the above-mentioned range. As a result, the inventors discovered that by setting Fn, defined by formula (1), between 0.45 and 1.05, excellent fatigue strength can be obtained in machine structural parts, and the machinability of the steel material from which machine structural parts are made can be improved. Fn = C + (Si / 10) + (Mn / 5) - (5S / 7) + (5Cr / 22) + 1.65V (1)

[0020] The present inventors further investigated means for improving the resistance of steel to melt cracking during induction hardening.

[0021] The carbon content affects the occurrence of fusion cracks in steel during induction hardening. Specifically, carbon segregating at grain boundaries lowers the melting point at the grain boundaries. As a result, fusion cracks are more likely to occur. Therefore, fine sulfides are generated in the steel. Fine sulfide particles have a pinning effect, which suppresses the coarsening of crystal grains (austenite grains) in the steel during induction hardening. If the crystal grains can be kept fine during induction hardening, the reduction in grain boundary area can be suppressed. If the reduction in grain boundary area can be suppressed and a certain amount of grain boundary area can be secured, the concentration of carbon segregating at the grain boundaries per unit area decreases. As a result, the occurrence of fusion cracks is suppressed.

[0022] On the other hand, coarse sulfide particles improve the machinability of the steel. Considering the above, it seems difficult to simultaneously obtain excellent fusion cracking resistance and excellent machinability in a steel having the above-mentioned chemical composition.

[0023] However, the present inventors have considered that excellent fusion crack resistance and excellent machinability can be simultaneously obtained by varying the number density of fine sulfide particles and the number density of coarse sulfide particles depending on the region of the steel material. Specifically, fusion cracks are likely to occur in the surface layer region of the steel material. Therefore, it is sufficient to ensure that excellent fusion crack resistance is obtained in the surface layer region of the steel material. On the other hand, fusion cracks are less likely to occur in the internal region of the steel material. Therefore, it is sufficient to ensure that high machinability is obtained in the internal region of the steel material.

[0024] Based on the above technical concept, we investigated and examined the relationship between the number density of fine sulfide particles and the number density of coarse sulfide particles in the surface layer region and the internal region of steel, and the fusion cracking resistance and machinability of the steel. As a result, we found that in steel that satisfies the above chemical composition and has an Fn of 0.45 to 1.05, the number density of fine sulfide particles in the surface layer region is 550 particles / mm ２ The number density of coarse sulfide particles is 55 particles / mm ２ The number density of fine sulfide particles in the inner region is 550 particles / mm ２ less than 55 particles / mm ２ The present inventors have found that if the steel sheet has a hardness of 0.15 or more, excellent resistance to melting cracking and excellent machinability can be simultaneously satisfied, and further, when the steel sheet is used as a material for machine structural parts, excellent fatigue strength can be obtained in the machine structural parts.

[0025] The steel material and machine structural component according to this embodiment, which have been completed based on the above technical concept, have the following configuration.

[0026] The steel material of the first embodiment has a chemical composition, in mass %, of C: more than 0.30 to 0.60%, Si: 0.01 to 0.90%, Mn: 0.50 to 1.70%, P: 0.030% or less, S: 0.015 to 0.080%, Cr: 0.01 to 1.30%, Al: more than 0.010 to 0.090%, N: 0.0250% or less, O: 0.0050% or less, V: 0 to 0.200%, Sn: 0 to 0.1000%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Pb: 0 to 0.09%, Bi: 0 to 0.25%, and Mg: 0 to 0.01%. 00%, Ti: 0 to 0.0400%, Nb: 0 to 0.0500%, W: 0 to 0.4000%, Zr: 0 to 0.2000%, Ca: 0 to 0.0100%, Te: 0 to 0.0100%, B: 0 to 0.0050%, rare earth elements: 0 to 0.0100%, Co: 0 to 0.0100%, Se: 0 to 0.0100%, In: 0 to 0.0100%, Mo: 0 to 0.30%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, and the balance being Fe and impurities, and Fn defined by formula (1) is 0.45 to 1.05. Fn=C+(Si / 10)+(Mn / 5)-(5S / 7)+(5Cr / 22)+1.65V (1) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1), and when an element is not contained, "0" is substituted for the corresponding element symbol. Furthermore, in this steel material, when the length of the shortest line segment among the line segments connecting the center of gravity of the cross section perpendicular to the axial direction of the steel material and the surface of the steel material is defined as R, the number density of fine sulfide particles, which are sulfide particles with a circle equivalent diameter of 0.1 to 2.0 μm, at a depth position of 0.08R from the surface of the steel material on the shortest straight line is 550 particles / mm ２ or more, and the number density of coarse sulfide particles, which are sulfide particles having a circle equivalent diameter of more than 2.0 μm, is 55 particles / mm ２ Furthermore, at the position of the center of gravity of the cross section, the number density of the fine sulfide particles is 550 particles / mm ２ and the number density of coarse sulfide particles is less than 55 particles / mm ２ It's super.

[0027] The steel material of the second embodiment is the steel material of the first embodiment, and has a chemical composition, in mass %, of V: ​​0.001 to 0.200%, Sn: 0.0001 to 0.1000%, Sb: 0.0001 to 0.0500%, As: 0.0001 to 0.0500%, Pb: 0.01 to 0.09%, Bi: 0.01 to 0.25%, Mg: 0.0001 to 0.0100%, Ti: 0.0001 to 0.0400%, Nb: 0.0001 to 0.0500%, W: 0.0001 to 0.4000%, Zr: 0. 0.0001-0.2000%, Ca: 0.0001-0.0100%, Te: 0.0001-0.0100%, B: 0.0001-0.0050%, rare earth elements: 0.0001-0.0100%, Co: 0.0001-0.0100%, Se: 0.0001-0.0100%, In: 0.0001-0.0100%, Mo: 0.01-0.30%, Cu: 0.01-0.50%, Ni: 0.01-0.50%.

[0028] A machine structural component of a first embodiment has a chemical composition in a part of the machine structural component, in mass %, of C: more than 0.30 to 0.60%, Si: 0.01 to 0.90%, Mn: 0.50 to 1.70%, P: 0.030% or less, S: 0.015 to 0.080%, Cr: 0.01 to 1.30%, Al: more than 0.010 to 0.090%, N: 0.0250% or less, O: 0.0050% or less, V: 0 to 0.200%, Sn: 0 to 0.1000%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Pb: 0 to 0.09%, and Bi: 0 to 0.25%. %, Mg: 0 to 0.0100%, Ti: 0 to 0.0400%, Nb: 0 to 0.0500%, W: 0 to 0.4000%, Zr: 0 to 0.2000%, Ca: 0 to 0.0100%, Te: 0 to 0.0100%, B: 0 to 0.0050%, rare earth elements: 0 to 0.0100%, Co: 0 to 0.0100%, Se: 0 to 0.0100%, In: 0 to 0.0100%, Mo: 0 to 0.30%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, and the remainder being Fe and impurities, and Fn defined by formula (1) is 0.45 to 1.05. Fn=C+(Si / 10)+(Mn / 5)-(5S / 7)+(5Cr / 22)+1.65V (1) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1), and when an element is not contained, "0" is substituted for the corresponding element symbol. In some of the machine structural components, when the length of the shortest line segment among the line segments connecting the center of gravity of the cross section perpendicular to the axial direction of the machine structural component and the surface of the machine structural component is defined as R, the number density of fine sulfide particles, which are sulfide particles with a circle equivalent diameter of 0.1 to 2.0 μm, at a depth position of 0.08R from the surface of the shortest line segment is 550 particles / mm ２ or more, and the number density of coarse sulfide particles, which are sulfide particles having a circle equivalent diameter of more than 2.0 μm, is 55 particles / mm ２ Furthermore, at the position of the center of gravity of the cross section, the number density of the fine sulfide particles is 550 particles / mm ２ and the number density of coarse sulfide particles is less than 55 particles / mm ２ It's super.

[0029] A machine structural component of a second embodiment is the machine structural component of the first embodiment, wherein the chemical composition is, in mass %, V: 0.001 to 0.200%, Sn: 0.0001 to 0.1000%, Sb: 0.0001 to 0.0500%, As: 0.0001 to 0.0500%, Pb: 0.01 to 0.09%, Bi: 0.01 to 0.25%, Mg: 0.0001 to 0.0100%, Ti: 0.0001 to 0.0400%, Nb: 0.0001 to 0.0500%, W: 0.0001 to 0.4000%. %, Zr: 0.0001 to 0.2000%, Ca: 0.0001 to 0.0100%, Te: 0.0001 to 0.0100%, B: 0.0001 to 0.0050%, rare earth elements: 0.0001 to 0.0100%, Co: 0.0001 to 0.0100%, Se: 0.0001 to 0.0100%, In: 0.0001 to 0.0100%, Mo: 0.01 to 0.30%, Cu: 0.01 to 0.50%, Ni: 0.01 to 0.50%.

[0030] A machine structural component of a third aspect is the machine structural component of the first or second aspect, wherein the machine structural component is a crankshaft and the part is a rear flange.

[0031] The steel material of this embodiment will be described in detail below. Unless otherwise specified, "%" regarding elements means mass %.

[0032] [Features of the Steel Material of the Present Embodiment] The steel material of the present embodiment satisfies the following features 1 to 4. (Feature 1) The chemical composition is, in mass %, C: more than 0.30 to 0.60%, Si: 0.01 to 0.90%, Mn: 0.50 to 1.70%, P: 0.030% or less, S: 0.015 to 0.080%, Cr: 0.01 to 1.30%, Al: more than 0.010 to 0.090%, N: 0.0250% or less, O: 0.0050% or less, V: 0 to 0.200%, Sn: 0 to 0.1000%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Pb: 0 to 0.09%, Bi: 0 to 0.25%, M 0-0.0100%, Ti: 0-0.0400%, Nb: 0-0.0500%, W: 0-0.4000%, Zr: 0-0.2000%, Ca: 0-0.0100%, Te: 0-0.0100%, B: 0-0.0050%, rare earth elements: 0-0.0100%, Co: 0-0.0100%, Se: 0-0.0100%, In: 0-0.0100%, Mo: 0-0.30%, Cu: 0-0.50%, Ni: 0-0.50%, and the balance consisting of Fe and impurities. (Feature 2) Fn defined by formula (1) is 0.45-1.05. Fn=C+(Si / 10)+(Mn / 5)-(5S / 7)+(5Cr / 22)+1.65V (1) Here, the content of the corresponding element in mass % is substituted for each element symbol in formula (1), and when an element is not contained, "0" is substituted for the corresponding element symbol. (Feature 3) When the length of the shortest straight line among the line segments connecting the center of gravity of the cross section perpendicular to the axial direction of the steel material and the surface of the steel material is defined as R, the number density of fine sulfide particles, which are sulfide particles with a circle equivalent diameter of 0.1 to 2.0 μm, is 550 particles / mm at a depth position of 0.08R from the surface of the steel material on the shortest straight line. ２ or more, and the number density of coarse sulfide particles, which are sulfide particles having a circle equivalent diameter of more than 2.0 μm, is 55 particles / mm ２ (Feature 4) At the position of the center of gravity of the cross section, the number density of the fine sulfide particles is 550 particles / mm ２ and the number density of coarse sulfide particles is less than 55 particles / mm ２ Features 1 to 4 will be explained below.

[0033] [(Feature 1) Chemical Composition] The chemical composition of the steel material of this embodiment contains the following elements.

[0034] C: More than 0.30% to 0.60% Carbon (C) increases the hardness of machine structural components manufactured using steel as a raw material, thereby improving the fatigue strength of the machine structural components. If the C content is 0.30% or less, the above effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, C lowers the melting point of the steel. Therefore, if the C content exceeds 0.60%, even if the contents of other elements are within the ranges of this embodiment, fusion cracks are likely to occur in the steel when induction hardening is performed on the steel in the manufacturing process of machine structural components made from the steel. Therefore, the C content is more than 0.30% to 0.60%. The preferred lower limit of the C content is 0.31%, more preferably 0.35%, even more preferably 0.37%, and even more preferably 0.38%. The preferred upper limit of the C content is 0.55%, even more preferably 0.50%, and even more preferably 0.45%.

[0035] Si: 0.01 to 0.90% Silicon (Si) deoxidizes steel during the steelmaking process. Si also increases the hardness of machine structural components manufactured using steel as a raw material, thereby improving the fatigue strength of the machine structural components. If the Si content is less than 0.01%, the above effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, Si has a weak affinity with C. Therefore, if the Si content exceeds 0.90%, C is more likely to segregate at grain boundaries than within grains where Si is dissolved, even if the contents of other elements are within the ranges of this embodiment. As a result, when induction hardening is performed on steel during the manufacturing process of machine structural components made from steel, melting cracks are more likely to occur in the steel. Therefore, the Si content is 0.01 to 0.90%. The preferred lower limit of the Si content is 0.02%, more preferably 0.05%, even more preferably 0.08%, and even more preferably 0.10%. The upper limit of the Si content is preferably 0.80%, more preferably 0.75%, even more preferably 0.70%, even more preferably 0.65%, even more preferably 0.55%, and even more preferably 0.50%.

[0036] Mn: 0.50 to 1.70% Manganese (Mn) deoxidizes steel during the steelmaking process. Mn also improves the hardenability of steel. As a result, machine structural components manufactured using steel are harder and have higher fatigue strength. Furthermore, Mn has a strong affinity with C. Therefore, during heating, C remains within the grains where Mn is dissolved. This suppresses segregation of C to grain boundaries. As a result, when induction hardening is performed on steel in the manufacturing process of machine structural components made from steel, the occurrence of melting cracks is suppressed. Furthermore, Mn combines with S to form Mn sulfides. Therefore, Mn can suppress the formation of coarse FeS. As a result, the hot workability of the steel is improved. If the Mn content is less than 0.50%, the above effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, Mn lowers the melting point of the steel. Therefore, if the Mn content exceeds 1.70%, even if the contents of other elements are within the ranges of this embodiment, the steel will have reduced resistance to melting cracking when induction hardened in the manufacturing process for machine structural components made from the steel. Furthermore, if the Mn content exceeds 1.70%, even if the contents of other elements are within the ranges of this embodiment, the hardness of the steel will be excessively increased. In this case, the machinability of the steel will be insufficient. Therefore, the Mn content is 0.50 to 1.70%. The preferred lower limit of the Mn content is 0.70%, more preferably 0.80%, even more preferably 0.85%, and even more preferably 0.90%. The preferred upper limit of the Mn content is 1.65%, even more preferably 1.60%, even more preferably 1.55%, even more preferably 1.50%, even more preferably 1.48%, even more preferably 1.45%, even more preferably 1.43%, and even more preferably 1.40%.

[0037] P: 0.030% or less Phosphorus (P) is an impurity. P segregates at grain boundaries and lowers the melting point of steel. Therefore, when induction hardening is performed on steel during the manufacturing process of machine structural parts made from steel, melting cracks are likely to occur in the steel. Therefore, the P content is 0.030% or less. The lower the P content, the more preferable it is. However, excessive reduction in the P content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the P content is more than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit of the P content is 0.028%, more preferably 0.026%, even more preferably 0.023%, and even more preferably 0.020%.

[0038] S: 0.015 to 0.080% Sulfur (S) forms sulfides, improving the machinability of steel. Furthermore, the formation of fine sulfides reduces the likelihood of fusion cracks. If the S content is less than 0.015%, the above effects are not fully achieved, even if the contents of other elements are within the ranges of this embodiment. However, S lowers the melting point of steel. Therefore, if the S content exceeds 0.080%, fusion cracks are more likely to occur in the steel when induction hardening is performed in the manufacturing process of machine structural components made from the steel, even if the contents of other elements are within the ranges of this embodiment. Therefore, the S content is 0.015 to 0.080%. The preferred lower limit of the S content is 0.020%, more preferably 0.025%, even more preferably 0.030%, and even more preferably 0.035%. The preferred upper limit of the S content is 0.075%, even more preferably 0.070%, and even more preferably 0.060%.

[0039] Cr: 0.01 to 1.30% Chromium (Cr) improves the hardenability of steel. Therefore, the hardness of machine structural components manufactured using the steel increases, thereby improving the fatigue strength of the machine structural components. If the Cr content is less than 0.01%, the above effects are not fully achieved, even if the contents of other elements are within the ranges of this embodiment. However, if the Cr content exceeds 1.30%, the steel does not achieve sufficient machinability, even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cr content is 0.01 to 1.30%. The preferred lower limit of the Cr content is 0.03%, more preferably 0.05%, even more preferably 0.10%, and even more preferably 0.15%. The preferred upper limit of the Cr content is 1.20%, more preferably 1.00%, even more preferably 0.60%, and even more preferably 0.39%.

[0040] Al: More than 0.010% to 0.090% Aluminum (Al) deoxidizes steel. If the Al content is 0.010% or less, the above effect cannot be sufficiently achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content exceeds 0.090%, Al forms coarse oxides even if the contents of other elements are within the ranges of this embodiment. The coarse oxides reduce the fatigue strength of machine structural components manufactured using the steel material. Therefore, the Al content is more than 0.010% to 0.090%. A preferred lower limit of the Al content is 0.011%, more preferably 0.015%, and even more preferably 0.020%. A preferred upper limit of the Al content is 0.060%, more preferably 0.050%, and even more preferably 0.045%.

[0041] N: 0.0250% or less Nitrogen (N) forms nitrides and / or carbonitrides during the manufacturing process of machine structural components made from steel, thereby precipitation-strengthening the steel. As a result, the fatigue strength of machine structural components manufactured from steel is increased. Even if even a small amount of N is contained, the above effect can be achieved to some extent, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the N content exceeds 0.0250%, the hot workability of the steel deteriorates, even if the contents of other elements are within the ranges of this embodiment. Therefore, the N content is 0.0250% or less. The preferred lower limit of the N content is greater than 0%, more preferably 0.0001%, even more preferably 0.0005%, even more preferably 0.0010%, even more preferably 0.0020%, even more preferably 0.0030%, and even more preferably 0.0040%. The upper limit of the N content is preferably 0.0200%, more preferably 0.0190%, even more preferably 0.0170%, even more preferably 0.0150%, even more preferably 0.0130%, and even more preferably 0.0100%.

[0042] O: 0.0050% or less Oxygen (O) is an impurity. O forms oxides in steel and reduces the fatigue strength of machine structural parts manufactured using steel as a raw material. Therefore, the O content is 0.0050% or less. The O content is preferably as low as possible. However, excessive reduction of the O content increases production costs. Therefore, considering normal industrial production, the preferred lower limit of the O content is more than 0%, more preferably 0.0001%, and even more preferably 0.0002%. The preferred upper limit of the O content is 0.0030%, more preferably 0.0025%, even more preferably 0.0020%, even more preferably 0.0015%, and even more preferably 0.0012%.

[0043] The balance of the chemical composition of the steel material according to this embodiment is composed of Fe and impurities. Here, the impurities refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of the steel material, and are acceptable within a range that does not adversely affect the steel material according to this embodiment.

[0044] [Optional Elements] The chemical composition of the steel material of this embodiment may further include, in place of a portion of Fe, V: 0 to 0.200%, Sn: 0 to 0.1000%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Pb: 0 to 0.09%, Bi: 0 to 0.25%, Mg: 0 to 0.0100%, Ti: 0 to 0.0400%, Nb: 0 to 0.0500%, W: 0 to 0.4000%, Zr: 0 to 0.2 000%, Ca: 0-0.0100%, Te: 0-0.0100%, B: 0-0.0050%, rare earth elements: 0-0.0100%, Co: 0-0.0100%, Se: 0-0.0100%, In: 0-0.0100%, Mo: 0-0.30%, Cu: 0-0.50%, and Ni: 0-0.50% may be contained. These elements are optional elements. Each optional element will be described below.

[0045] [First Group: V] The chemical composition of the steel material of this embodiment may further contain 0 to 0.200% V in place of a portion of Fe. These elements are optional elements and increase the fatigue strength of machine structural parts.

[0046] V: 0 to 0.200% Vanadium (V) is an optional element and does not necessarily need to be contained. That is, the V content may be 0%. When vanadium (V) is contained, that is, when the V content exceeds 0%, V precipitates in ferrite in the steel during the manufacturing process of machine structural components made from the steel. This increases the hardness of ferrite in the steel. As a result, the fatigue strength of the machine structural components is increased. Even if even a small amount of V is contained, the above effects can be obtained to some extent. However, if the V content exceeds 0.200%, the above effects saturate and the manufacturing costs increase. Therefore, the V content is 0 to 0.200%. The preferred lower limit of the V content is 0.001%, more preferably 0.003%, even more preferably 0.005%, even more preferably 0.007%, even more preferably 0.010%, even more preferably 0.015%, and even more preferably 0.020%. The upper limit of the V content is preferably 0.180%, more preferably 0.160%, even more preferably 0.140%, even more preferably 0.120%, even more preferably 0.100%, even more preferably 0.080%, even more preferably 0.050%, and even more preferably 0.049%.

[0047] [Regarding Group 2: Sn, Sb, As, Pb, and Bi] The chemical composition of the steel material of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Sn: 0.0001 to 0.1000%, Sb: 0.0001 to 0.0500%, As: 0.0001 to 0.0500%, Pb: 0.01 to 0.09%, and Bi: 0.01 to 0.25%. All of these elements are optional elements, and all of them improve the machinability of the steel material. Each element will be described below.

[0048] Sn: 0 to 0.1000% Tin (Sn) is an optional element and does not necessarily need to be contained. In other words, the Sn content may be 0%. If Sn is contained, that is, if the Sn content exceeds 0%, Sn segregates at the interface between the matrix and inclusions, embrittling the steel. This improves the machinability of the steel. Even if even a small amount of Sn is contained, the above effect can be achieved to some extent. However, if the Sn content exceeds 0.1000%, excessive Sn segregates even if the contents of other elements are within the ranges of this embodiment. In this case, the hot workability of the steel deteriorates. Therefore, the Sn content is 0 to 0.1000%. The preferred lower limit of the Sn content is 0.0001%, more preferably 0.0010%, and even more preferably 0.0020%. The upper limit of the Sn content is preferably 0.0500%, more preferably 0.0100%, even more preferably 0.0090%, even more preferably 0.0080%, even more preferably 0.0070%, even more preferably 0.0060%, even more preferably 0.0050%, and even more preferably 0.0040%.

[0049] Sb: 0 to 0.0500% Antimony (Sb) is an optional element and does not necessarily need to be contained. That is, the Sb content may be 0%. When contained, that is, when the Sb content exceeds 0%, Sb segregates at the interface between the matrix and inclusions, embrittling the steel. This improves the machinability of the steel. Even if even a small amount of Sb is contained, the above effect can be achieved to some extent. However, if the Sb content exceeds 0.0500%, Sb will segregate excessively even if the contents of other elements are within the ranges of this embodiment. In this case, the hot workability of the steel will be reduced. Therefore, the Sb content is 0 to 0.0500%. The preferred lower limit of the Sb content is 0.0001%, more preferably 0.0010%, and even more preferably 0.0020%. The upper limit of the Sb content is preferably 0.0400%, more preferably 0.0300%, even more preferably 0.0200%, even more preferably 0.0100%, even more preferably 0.0080%, and even more preferably 0.0060%.

[0050] As: 0 to 0.0500% Arsenic (As) is an optional element and does not necessarily need to be contained. In other words, the As content may be 0%. If arsenic is contained, that is, if the As content exceeds 0%, As segregates at the interface between the matrix and inclusions, embrittling the steel. This improves the machinability of the steel. Even if even a small amount of As is contained, the above effect can be achieved to some extent. However, if the As content exceeds 0.0500%, excessive As segregation occurs even if the contents of other elements are within the ranges of this embodiment. In this case, the hot workability of the steel deteriorates. Therefore, the As content is 0 to 0.0500%. The preferred lower limit of the As content is 0.0001%, more preferably 0.0010%, and even more preferably 0.0020%. The upper limit of the As content is preferably 0.0100%, more preferably 0.0070%, even more preferably 0.0060%, even more preferably 0.0050%, and still more preferably 0.0040%.

[0051] Pb: 0 to 0.09% Lead (Pb) is an optional element and does not necessarily need to be contained. That is, the Pb content may be 0%. If Pb is contained, that is, if the Pb content exceeds 0%, Pb generates Pb particles in the matrix, embrittling the steel. This improves the machinability of the steel. Even if even a small amount of Pb is contained, the above effect can be achieved to some extent. However, if the Pb content exceeds 0.09%, excessive Pb particles will be generated even if the contents of other elements are within the ranges of this embodiment. In this case, the hot workability of the steel will be reduced. Therefore, the Pb content is 0 to 0.09%. The preferred lower limit of the Pb content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Pb content is 0.08%, more preferably 0.07%, even more preferably 0.06%, and even more preferably 0.05%.

[0052] Bi: 0 to 0.25% Bismuth (Bi) is an optional element and does not necessarily need to be contained. That is, the Bi content may be 0%. When Bi is contained, that is, when the Bi content exceeds 0%, Bi generates Bi particles in the matrix, embrittling the steel. This improves the machinability of the steel. Even if even a small amount of Bi is contained, the above effect can be achieved to some extent. However, if the Bi content exceeds 0.25%, excessive Bi particles will be generated even if the contents of other elements are within the ranges of this embodiment. In this case, the hot workability of the steel will be reduced. Therefore, the Bi content is 0 to 0.25%. The preferred lower limit of the Bi content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit of the Bi content is 0.20%, more preferably 0.09%, even more preferably 0.06%, and even more preferably 0.05%.

[0053] [Third Group: Mg] The chemical composition of the steel material of this embodiment may further contain Mg: 0 to 0.0100% instead of a portion of Fe.

[0054] Mg: 0 to 0.0100% Magnesium (Mg) is an optional element and does not necessarily need to be present. In other words, the Mg content may be 0%. When present, that is, when the Mg content exceeds 0%, Mg deoxidizes the steel. Even if even a small amount of Mg is present, the above effect can be achieved to some extent. However, when the Mg content exceeds 0.0100%, Mg forms coarse oxides even when the contents of other elements are within the ranges of this embodiment. The coarse oxides reduce the fatigue strength of machine structural components manufactured using the steel material. Therefore, the Mg content is 0 to 0.0100%. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The preferred upper limit of the Mg content is 0.0050%, more preferably 0.0045%, and even more preferably 0.0040%.

[0055] [Fourth Group: Ti, Nb, W, and Zr] The chemical composition of the steel material of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ti: 0-0.0400%, Nb: 0-0.0500%, W: 0-0.4000%, and Zr: 0-0.2000%. These elements are optional elements, and all form precipitates and refine the crystal grains in the steel material through a pinning effect, thereby improving the toughness of machine structural parts manufactured using the steel material. Each element will be described below.

[0056] Ti: 0 to 0.0400% Titanium (Ti) is an optional element and does not necessarily need to be contained. In other words, the Ti content may be 0%. When titanium is contained, that is, when the Ti content exceeds 0%, Ti forms precipitates (carbides and / or carbonitrides). These precipitates refine the grains of the steel material through a pinning effect. This increases the toughness of machine structural components. Even if even a small amount of Ti is contained, the above effect can be obtained to some extent. However, if the Ti content exceeds 0.0400%, the above effect saturates and manufacturing costs increase. Therefore, the Ti content is 0 to 0.0400%. The preferred lower limit of the Ti content is 0.0001%, more preferably 0.0010%, even more preferably 0.0050%, and even more preferably 0.0080%. The upper limit of the Ti content is preferably 0.0290%, more preferably 0.0200%, further preferably 0.0175%, and still further preferably 0.0150%.

[0057] Nb: 0 to 0.0500% Niobium (Nb) is an optional element and does not necessarily need to be contained. That is, the Nb content may be 0%. When contained, that is, when the Nb content exceeds 0%, Nb, like Ti, forms precipitates to refine the grains of the steel material and increase the toughness of machine structural components. Even if even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content exceeds 0.0500%, the above effect saturates and manufacturing costs increase. Therefore, the Nb content is 0 to 0.0500%. The preferred lower limit of the Nb content is 0.0001%, more preferably 0.0010%, even more preferably 0.0050%, and even more preferably 0.0080%. The preferred upper limit of the Nb content is 0.0200%, even more preferably 0.0175%, and even more preferably 0.0150%.

[0058] W: 0 to 0.4000% Tungsten (W) is an optional element and does not necessarily need to be contained. In other words, the W content may be 0%. When contained, that is, when the W content exceeds 0%, W, like Ti, forms precipitates to refine the grains of the steel material and increase the toughness of machine structural components. Even if even a small amount of W is contained, the above effects can be obtained to some extent. However, if the W content exceeds 0.4000%, the above effects saturate and manufacturing costs increase. Therefore, the W content is 0 to 0.4000%. The preferred lower limit of the W content is 0.0001%, more preferably 0.0050%, and even more preferably 0.0500%. The preferred upper limit of the W content is 0.3500%, more preferably 0.3000%, and even more preferably 0.2000%.

[0059] Zr: 0 to 0.2000% Zirconium (Zr) is an optional element and does not necessarily need to be contained. In other words, the Zr content may be 0%. When contained, that is, when the Zr content exceeds 0%, Zr, like Ti, forms precipitates to refine the grains of the steel material and improve the toughness of machine structural components. Even if even a small amount of Zr is contained, the above effects can be obtained to some extent. However, if the Zr content exceeds 0.2000%, the above effects saturate and manufacturing costs increase. Therefore, the Zr content is 0 to 0.2000%. The preferred lower limit of the Zr content is 0.0001%, more preferably 0.0010%, even more preferably 0.0020%, and even more preferably 0.0050%. The preferred upper limit of the Zr content is 0.1500%, even more preferably 0.1000%, even more preferably 0.0500%, and even more preferably 0.0100%.

[0060] [Group 5: Ca, Te, B, and Rare Earth Elements (REM)] The chemical composition of the steel material of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Ca: 0-0.0100%, Te: 0-0.0100%, B: 0-0.0050%, and rare earth elements: 0-0.0100%. These elements are optional elements, and all of them improve the machinability of the steel material. Each element will be described below.

[0061] Ca: 0 to 0.0100% Calcium (Ca) is an optional element and does not necessarily need to be contained. In other words, the Ca content may be 0%. When contained, that is, when the Ca content exceeds 0%, Ca improves the machinability of the steel. Even if even a small amount of Ca is contained, the above effect can be obtained to some extent. However, if the Ca content exceeds 0.0100%, coarse oxides are formed even if the contents of other elements are within the ranges of this embodiment. The coarse oxides reduce the fatigue strength of machine structural components manufactured using the steel. Therefore, the Ca content is 0 to 0.0100%. The preferred lower limit of the Ca content is 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, and even more preferably 0.0015%. The upper limit of the Ca content is preferably 0.0085%, more preferably 0.0070%, further preferably 0.0050%, and still further preferably 0.0030%.

[0062] Te: 0 to 0.0100% Tellurium (Te) is an optional element and does not necessarily need to be contained. In other words, the Te content may be 0%. When contained, that is, when the Te content exceeds 0%, Te improves the machinability of the steel. Even if even a small amount of Te is contained, the above effect can be obtained to some extent. However, if the Te content exceeds 0.0100%, the hot workability of the steel will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the Te content is 0 to 0.0100%. The preferred lower limit of the Te content is 0.0001%, more preferably 0.0003%, and even more preferably 0.0010%. The preferred upper limit of the Te content is 0.0090%, more preferably 0.0085%, even more preferably 0.0080%, and even more preferably 0.0040%.

[0063] B: 0 to 0.0050% Boron (B) is an optional element and does not necessarily need to be contained. In other words, the B content may be 0%. When contained, that is, when the B content exceeds 0%, B bonds with N to form BN, improving the machinability of the steel. B also segregates at grain boundaries, contributing to grain boundary strengthening and increasing the fatigue strength of machine structural components manufactured using the steel. Even if even a small amount of B is contained, the above effect can be achieved to some extent. However, if the B content exceeds 0.0050%, the hot workability of the steel will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the B content is 0 to 0.0050%. The preferred lower limit of the B content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the B content is 0.0040%, more preferably 0.0035%, and even more preferably 0.0030%.

[0064] Rare earth elements (REM): 0 to 0.0100% Rare earth elements (REM) are optional elements and may not be present. That is, the REM content may be 0%. When REM is present, that is, when the REM content exceeds 0%, REM improves the machinability of the steel material. Even if even a small amount of REM is present, the above effect can be achieved to some extent. However, if the REM content exceeds 0.0100%, the hot workability of the steel material will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the REM content is 0 to 0.0100%. The preferred lower limit of the REM content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the REM content is 0.0090%, more preferably 0.0080%, even more preferably 0.0070%, and even more preferably 0.0040%.

[0065] In this specification, REM refers to one or more elements selected from the group consisting of scandium (Sc), which has atomic number 21, yttrium (Y), which has atomic number 39, and the lanthanides lanthanum (La), which has atomic number 57, to lutetium (Lu), which has atomic number 71. In this specification, the REM content refers to the total content of these elements.

[0066] [Group 6: Co, Se, and In] The chemical composition of the steel material of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Co: 0 to 0.0100%, Se: 0 to 0.0100%, and In: 0 to 0.0100%. These elements are optional elements, and all of them suppress decarburization of the steel material. Each element will be described below.

[0067] Cobalt (Co) is an optional element and may not be contained. That is, the Co content may be 0%. When Co is contained, that is, when Co exceeds 0%, Co suppresses decarburization of the steel material during the manufacturing process. Even if even a small amount of Co is contained, the above effect can be obtained to some extent. However, if the Co content exceeds 0.0100%, the hot workability of the steel material will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the Co content is 0 to 0.0100%. The preferred lower limit of the Co content is 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, and even more preferably 0.0030%. The preferred upper limit of the Co content is 0.0090%, even more preferably 0.0080%, and even more preferably 0.0070%.

[0068] Se: 0 to 0.0100% Selenium (Se) is an optional element and does not necessarily need to be contained. That is, the Se content may be 0%. When contained, that is, when Se exceeds 0%, Se suppresses decarburization of the steel material during the manufacturing process. Even if even a small amount of Se is contained, the above effect can be obtained to some extent. However, if the Se content exceeds 0.0100%, the hot workability of the steel material will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the Se content is 0 to 0.0100%. The preferred lower limit of the Se content is 0.0001%, more preferably 0.0010%, and even more preferably 0.0020%. The preferred upper limit of the Se content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0069] In: 0 to 0.0100% Indium (In) is an optional element and does not necessarily need to be contained. That is, the In content may be 0%. When contained, that is, when In exceeds 0%, In suppresses decarburization of the steel material during the manufacturing process. Even if even a small amount of In is contained, the above effect can be obtained to some extent. However, if the In content exceeds 0.0100%, the hot workability of the steel material will deteriorate even if the contents of other elements are within the ranges of this embodiment. Therefore, the In content is 0 to 0.0100%. The preferred lower limit of the In content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit of the In content is 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0070] [Seventh Group: Mo, Cu, and Ni] The chemical composition of the steel material of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of Mo: 0-0.30%, Cu: 0-0.50%, and Ni: 0-0.50%. These elements are optional elements, and all of them increase the fatigue strength of machine structural parts manufactured using the steel material as a raw material. Each element will be described below.

[0071] Mo: 0 to 0.30% Molybdenum (Mo) is an optional element and does not necessarily need to be contained. That is, the Mo content may be 0%. When contained, that is, when the Mo content exceeds 0%, Mo enhances the fatigue strength of machine structural components manufactured using the steel material. Even if even a small amount of Mo is contained, the above effect can be obtained to some extent. However, if the Mo content exceeds 0.30%, even if the contents of other elements are within the ranges of this embodiment, the hardness of the steel material becomes excessively high, and the hot workability of the steel material decreases. Therefore, the Mo content is 0 to 0.30%. The preferred lower limit of the Mo content is 0.01%, more preferably 0.05%, and even more preferably 0.10%. The preferred upper limit of the Mo content is 0.20%, more preferably 0.17%, and even more preferably 0.15%.

[0072] Cu: 0 to 0.50% Copper (Cu) is an optional element and does not necessarily need to be contained. In other words, the Cu content may be 0%. When contained, that is, when the Cu content exceeds 0%, Cu enhances the fatigue strength of machine structural components. Even if even a small amount of Cu is contained, the above effect can be obtained to some extent. However, if the Cu content exceeds 0.50%, melt cracks are likely to occur during induction hardening even if the contents of other elements are within the ranges of this embodiment. Therefore, the Cu content is 0 to 0.50%. The preferred lower limit of the Cu content is 0.01%, and more preferably 0.02%. The preferred upper limit of the Cu content is 0.20%, more preferably 0.10%, and even more preferably 0.05%.

[0073] Ni: 0 to 0.50% Nickel (Ni) is an optional element and does not necessarily need to be contained. In other words, the Ni content may be 0%. When contained, that is, when the Ni content exceeds 0%, Ni enhances the fatigue strength of machine structural components. Even if even a small amount of Ni is contained, the above effect can be obtained to some extent. However, if the Ni content exceeds 0.50%, melting cracks are likely to occur during induction hardening even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ni content is 0 to 0.50%. The preferred lower limit of the Ni content is 0.01%, and more preferably 0.02%. The preferred upper limit of the Ni content is 0.20%, more preferably 0.10%, and even more preferably 0.05%.

[0074] [(Feature 2) Regarding Fn] Furthermore, in the steel material of this embodiment, Fn, as defined by formula (1), is 0.45 to 1.05. Fn=C+(Si / 10)+(Mn / 5)-(5S / 7)+(5Cr / 22)+1.65V (1) Here, the content of the corresponding element in mass% is substituted for each element symbol in formula (1). When an element is not contained, "0" is substituted for the corresponding element symbol.

[0075] Fn is an index of the fatigue strength of a machine structural part manufactured using a steel material as a raw material and the machinability of the steel material. If Fn is less than 0.45, sufficient fatigue strength cannot be obtained in a machine structural part manufactured using the steel material, even if the steel material satisfies Features 1, 3, and 4. On the other hand, if Fn exceeds 1.05, the machinability of the steel material decreases, even if the steel material satisfies Features 1, 3, and 4. Therefore, Fn is 0.45 to 1.05. The preferred lower limit of Fn is 0.50, more preferably 0.55, and even more preferably 0.60. The preferred upper limit of Fn is 1.00, more preferably 0.95, and even more preferably 0.90.

[0076] [(Feature 3) 0.08R depth position D ０．０８ＲIn the steel material of this embodiment, when the length of the shortest straight line among the line segments connecting the center of gravity of the cross section and the surface of the steel material in a cross section perpendicular to the axial direction of the steel material is defined as R, the number density of fine sulfide particles, which are sulfide particles having a circle equivalent diameter of 0.1 to 2.0 μm, is 550 particles / mm at a depth position of 0.08R from the surface of the steel material of the shortest line segment. ２ or more, and the number density of coarse sulfide particles, which are sulfide particles having a circle equivalent diameter of more than 2.0 μm, is 55 particles / mm ２ The following is the result.

[0077] The axial direction of a steel material refers to the central axis of the steel material, and means the longitudinal direction of the steel material. The axial direction also refers to the elongation direction or rolling direction of the steel material. The elongation direction or rolling direction can be determined based on metal flow. For example, the elongation direction or rolling direction can be determined based on the direction in which segregation zones or inclusions extend the most in multiple cross sections of the steel material. The center of gravity of a cross section refers to the position of the center of gravity when the cross section is considered to be a thin plate-like object. In this embodiment, it is preferable that the center of gravity of the cross section is located within the cross section.

[0078] 1A and 1B are schematic diagrams showing an example of a cross section perpendicular to the axial direction of a steel material according to this embodiment. As an example of a cross-sectional shape, FIGS. 1A and 1B show an example of a cross section perpendicular to the axial direction of the steel material having a rotationally symmetric shape. However, the cross section perpendicular to the axial direction of the steel material is not limited to a rotationally symmetric shape. A rotationally symmetric shape refers to a shape that overlaps with the original shape when rotated 360° / N around the center of gravity in the cross section. Here, N is an integer of 2 or more. Examples of rotationally symmetric shapes include a circle, a square, a rectangle, or a regular (2M) polygon with M being 3 or more. Examples of regular (2M) polygons with M being 3 or more include a regular hexagon or a regular octagon. When the rotationally symmetric shape is a rectangle or a regular polygon, the corners of the rectangle or regular polygon may be rounded. The terms "circular, square, rectangular, and regular polygonal" are concepts that include not only strict geometric shapes (circular, square, rectangular, and regular polygonal), but also shapes that are roughly the same.

[0079] 1A is a cross-sectional view of a steel material in which the cross section perpendicular to the axial direction is circular. FIG. 1B is a cross-sectional view of a steel material in which the cross section perpendicular to the axial direction is square. Referring to FIGS. 1A and 1B, in a cross section 10 perpendicular to the axial direction of the steel material, the center of gravity D of the cross section 10 is １．００Ｒ The length of the shortest line segment connecting the center of gravity D and the surface of the steel material is defined as R. １．００Ｒ Among the line segments connecting the center of gravity D and the surface of the steel material, the shortest line segment is the radius. １．００Ｒ Among the line segments connecting the steel surface and the steel surface, the shortest line segment is the center of the width of the steel surface (one side in the cross section) and the center of gravity D １．００Ｒ is the line segment connecting

[0080] In cross section 10, the center of gravity D １．００Ｒ Among the line segments connecting the surface of the steel material and the surface of the steel material, the shortest line segment is ０．００Ｒ The position of 0.08R depth from the starting point is "0.08R depth position D ０．０８Ｒ " (See Figures 1A and 1B).

[0081] In the steel material of this embodiment, the depth position D is 0.08R. ０．０８Ｒ In the above, the number density of fine sulfide particles, which are sulfide particles having a circle equivalent diameter of 0.1 to 2.0 μm, is 550 particles / mm ２ or more, and the number density of coarse sulfide particles, which are sulfide particles having a circle equivalent diameter of more than 2.0 μm, is 55 particles / mm ２ The following is the result.

[0082] As shown in FIGS. 1A and 1B, the surface D of the steel material ０．００Ｒ From 0.08R depth position D ０．０８Ｒ The region ranging from is defined as the surface layer region SA. The surface layer region SA is a region where the temperature rises due to high-frequency induction heating when induction hardening is performed in the manufacturing process of a machine structural part made of steel material. Therefore, the surface layer region SA is required to have sufficient resistance to fusion cracking. Therefore, the surface layer region SA of the steel material is required to have sufficient resistance to fusion cracking.

[0083] As described above, during heating in induction hardening, fine sulfide particles, which are sulfide particles with a circle equivalent diameter of 0.1 to 2.0 μm, suppress coarsening of crystal grains in the surface layer region SA due to the pinning effect. If the number density of fine sulfide particles in the surface layer region SA is increased, the pinning effect can maintain the crystal grains in a fine state. Therefore, reduction in grain boundary area is suppressed, and the occurrence of fusion cracks is suppressed. 0.08R depth position D in the surface layer region SA ０．０８Ｒ In this case, the number density of fine sulfide particles is 550 particles / mm ２ If the above condition is met, the crystal grains can be kept fine during heating in induction hardening, and as a result, the steel material can have sufficient resistance to melting cracks.

[0084] On the other hand, the influence of coarse sulfide particles on the fusion cracking resistance of steel is small. Furthermore, if the number density of coarse sulfide particles is high, the number density of fine sulfide particles is correspondingly low. Therefore, it is preferable that the number density of coarse sulfide particles is low in the surface layer region SA. 0.08R depth position D in the surface layer region SA ０．０８Ｒ In this case, the number density of coarse sulfide particles is 55 particles / mm ２ If the thickness is less than 100 μm, a sufficient amount of fine sulfide particles are likely to be generated in the surface layer region SA, and as a result, the steel material can have sufficient resistance to fusion cracking.

[0085] Therefore, 0.08R depth position D ０．０８Ｒ The number density of fine sulfide particles at ２ The number density of coarse sulfide particles is 55 particles / mm ２ The following is the result.

[0086] 0.08R depth position D ０．０８Ｒ The preferred lower limit of the number density of fine sulfide particles is 560 particles / mm ２ and more preferably 570 pieces / mm ２ and more preferably 580 pieces / mm ２ and more preferably 590 pieces / mm ２ and 600 pieces / mm ２ and more preferably 650 pieces / mm ２ and more preferably 680 pieces / mm ２and more preferably 700 pieces / mm ２ 0.08R depth position D ０．０８Ｒ When the steel material satisfies the characteristics 1 and 2, the upper limit of the number density of the fine sulfide particles at the depth position D ０．０８Ｒ The upper limit of the number density of fine sulfide particles in ２ and more preferably 1000 pieces / mm ２ and more preferably 900 pieces / mm ２ is.

[0087] 0.08R depth position D ０．０８Ｒ The preferred upper limit of the number density of coarse sulfide particles is 50 particles / mm ２ and more preferably 45 pieces / mm ２ and more preferably 40 pieces / mm ２ and more preferably 35 pieces / mm ２ and more preferably 30 pieces / mm ２ 0.08R depth position D ０．０８Ｒ The number density of the coarse sulfide particles at the depth position D is preferably low. ０．０８Ｒ The preferred lower limit of the number density of coarse sulfide particles is 2 particles / mm ２ and more preferably 1 piece / mm ２ and more preferably 0 pieces / mm ２ more preferably 0 pieces / mm ２ is.

[0088] (Feature 4) Center of gravity position D １．００Ｒ 1A and 1B, the position of the center of gravity in the cross section 10 is referred to as the "center of gravity position D １．００Ｒ " is defined as

[0089] In the steel material of this embodiment, the center of gravity position D １．００Ｒ In the above, the number density of fine sulfide particles, which are sulfide particles having a circle equivalent diameter of 0.1 to 2.0 μm, is 550 particles / mm ２ and the number density of coarse sulfide particles, which are sulfide particles having an equivalent circle diameter of more than 2.0 μm, is 55 particles / mm ２ It's super.

[0090] As shown in FIG. 2, the center of gravity position D １．００Ｒ The circular area having the center at the position D is defined as the inner area CA. The outer edge of the inner area CA is not particularly limited, but for example, ０．６５Ｒ The inner area CA of the steel material is less susceptible to heat during induction hardening than the surface area SA. Therefore, the inner area CA does not require resistance to melting cracking compared to the surface area SA. On the other hand, the inner area CA may be subjected to cutting in the manufacturing process of machine structural parts made from steel material. In this case, the inner area CA is required to have sufficient machinability.

[0091] During cutting, the coarse sulfide particles become the starting points for peeling off chips from the steel body. Therefore, the coarse sulfide particles improve the machinability of the steel. Therefore, it is preferable that the number density of the coarse sulfide particles in the inner area CA is high. The center of gravity position D in the inner area CA １．００Ｒ In this case, the number density of coarse sulfide particles is 55 particles / mm ２ If the hardness is greater than 100%, sufficient machinability can be obtained in the inner area CA of the steel material.

[0092] On the other hand, the influence of fine sulfide particles on the machinability of steel is low. Furthermore, if the number density of fine sulfide particles is high, the number density of coarse sulfide particles will be low accordingly. Therefore, it is preferable that the number density of fine sulfide particles is low in the inner region CA. The center of gravity position D in the inner region CA １．００Ｒ In this case, the number density of fine sulfide particles is 550 particles / mm ２ If the Cr content is less than 100%, coarse sulfide particles are likely to be generated in the inner region CA, and as a result, sufficient machinability is obtained in the steel material.

[0093] Therefore, the center of gravity position D １．００Ｒ The number density of fine sulfide particles at ２ The number density of coarse sulfide particles is less than 55 particles / mm ２ It's super.

[0094] Center of gravity position D １．００Ｒ The preferred upper limit of the number density of fine sulfide particles is 540 particles / mm ２ and more preferably 530 pieces / mm ２and more preferably 520 pieces / mm ２ and more preferably 510 pieces / mm ２ and more preferably 500 pieces / mm ２ and more preferably 450 pieces / mm ２ and more preferably 400 pieces / mm ２ and more preferably 350 pieces / mm ２ and more preferably 300 pieces / mm ２ Center of gravity position D １．００Ｒ The number density of the fine sulfide particles at the center of gravity position D is preferably low. １．００Ｒ The preferred lower limit of the number density of fine sulfide particles is 250 particles / mm ２ and more preferably 200 pieces / mm ２ and more preferably 150 pieces / mm ２ and more preferably 50 pieces / mm ２ and more preferably 0 pieces / mm ２ is.

[0095] Center of gravity position D １．００Ｒ The preferred lower limit of the number density of coarse sulfide particles is 60 particles / mm ２ and more preferably 70 pieces / mm ２ and more preferably 80 pieces / mm ２ and more preferably 85 pieces / mm ２ Center of gravity position D １．００Ｒ However, when the steel material satisfies the characteristics 1 and 2, the upper limit of the number density of the coarse sulfide particles at the center of gravity position D １．００Ｒ The upper limit of the number density of coarse sulfide particles is, for example, 200 particles / mm ２ and more preferably 150 pieces / mm ２ is.

[0096] [0.08R depth position D ０．０８Ｒ Regarding the method for measuring the number density of fine sulfide particles and coarse sulfide particles at a depth position D of 0.08R ０．０８Ｒ The number density of the fine sulfide particles and the number density of the coarse sulfide particles can be measured by the following method.

[0097] In a cross section parallel to the axial direction of the steel material and including the central axis of the steel material, 0.08R depth position D ０．０８Ｒ Of the collected test specimens, a cross section that is parallel to the axial direction of the steel material and includes the central axis of the steel material is used as the observation surface.

[0098] The observation surface is mirror-polished. The mirror-polished observation surface is observed at a magnification of 500 times using a scanning electron microscope (SEM). Specifically, a 0.08R depth position D is positioned at the center of a rectangular field of view of 405 μm × 405 μm. ０．０８Ｒ The total area of ​​the observed fields is 20.0 mm. ２ is.

[0099] Based on the backscattered electron images of each field of view obtained by SEM observation, the number density of coarse sulfide particles and fine sulfide particles is examined using a well-known particle analysis method of image analysis. Specifically, particles in the steel are identified based on the interface between the steel matrix and the particles. In the backscattered electron image, the color tone of each pixel is set to 256 gradations. When generating the backscattered electron image, the contrast is adjusted so that the matrix steel structure (bright contrast) and the particles (dark contrast) are sufficiently distinguished. After adjustment, the intermediate gradation between the peak gradation of bright contrast and the peak gradation of dark contrast is set as the threshold. The particles referred to here are inclusions or precipitates. Image analysis is performed to determine the circle-equivalent diameter of the identified particles. Specifically, the area of ​​each identified particle is determined. The diameter of a circle having the same area as the determined area is defined as the circle-equivalent diameter (μm) of the particle.

[0100] Among the particles observed in the backscattered electron image obtained by the SEM observation, particles having a circle-equivalent diameter of 0.1 to 2.0 μm are subjected to point analysis of all detectable elements in the particle composition using an energy dispersive X-ray analyzer (EDX: Energy Dispersive X-ray spectroscopy) equipped in the SEM. Specifically, an X-ray profile of all elements in the particle is obtained by EDX. From the obtained X-ray profile of all elements, Si, Mn, N, O, S, Cr, Ti, Nb, Ca, Mg, Zr, Al, V, Ce, La, Na, K, and Cl are quantified. The total mass of these elements is taken as 100%, and the mass% of the corresponding element is calculated. As a result of the quantification, particles having an S content of 5% or more by mass and an Mn content of 5% or more by mass are identified as fine sulfide particles. Furthermore, among particles observed in a backscattered electron image obtained by SEM observation, particles having a circle-equivalent diameter of more than 2.0 μm and having an S content of 5% by mass or more and an Mn content of 5% by mass or more as a result of point analysis of the particle composition using EDX are identified as coarse sulfide particles. The acceleration voltage for EDX analysis is 20 kV.

[0101] The total number of fine sulfide particles identified in each field of view and the total area (20.0 mm2) of the multiple fields of view constituting the above observation area were calculated. ２ ) and based on 0.08R depth position D ０．０８Ｒ The number density of fine sulfide particles (particles / mm ２ Similarly, the total number of coarse sulfide particles identified in each visual field and the total area (20.0 mm ２ ) and based on 0.08R depth position D ０．０８Ｒ The number density of coarse sulfide particles (particles / mm ２ ) is required.

[0102] [Center of gravity position D １．００Ｒ Method for measuring the number density of fine sulfide particles and coarse sulfide particles at center of gravity position D １．００Ｒ The number density of the fine sulfide particles and the number density of the coarse sulfide particles can be measured by the following method.

[0103] In a cross section parallel to the axial direction of the steel material and including the central axis of the steel material, the center of gravity position D １．００ＲOf the collected test specimens, a cross section that is parallel to the axial direction of the steel material and includes the central axis of the steel material is used as the observation surface.

[0104] The observation surface is mirror-polished. The mirror-polished observation surface is observed at a magnification of 500 times using an SEM. Specifically, the center of gravity position D is set at the center of a rectangular field of view of 405 μm × 405 μm. １．００Ｒ The total area of ​​the observed fields is 20.0 mm. ２ is.

[0105] Based on the backscattered electron images of each field of view obtained by SEM observation, the number density of coarse sulfide particles and fine sulfide particles is examined using a well-known particle analysis method of image analysis. Specifically, particles in the steel are identified based on the interface between the steel matrix and the particles. In the backscattered electron image, the color tone of each pixel is set to 256 gradations. When generating the backscattered electron image, the contrast is adjusted so that the matrix steel structure (bright contrast) and the particles (dark contrast) are sufficiently distinguished. After adjustment, the intermediate gradation between the peak gradation of bright contrast and the peak gradation of dark contrast is set as the threshold. The particles referred to here are inclusions or precipitates. Image analysis is performed to determine the circle-equivalent diameter of the identified particles. Specifically, the area of ​​each identified particle is determined. The diameter of a circle having the same area as the determined area is defined as the circle-equivalent diameter (μm) of the particle.

[0106] Among the particles observed in the backscattered electron image obtained by the SEM observation, particles with a circle-equivalent diameter of 0.1 to 2.0 μm are subjected to point analysis of all detectable elements in the particle composition using an EDX equipped in the SEM. Specifically, an X-ray profile of all elements in the particle is obtained by EDX. From the obtained X-ray profile of all elements, Si, Mn, N, O, S, Cr, Ti, Nb, Ca, Mg, Zr, Al, V, Ce, La, Na, K, and Cl are quantified. The total mass of these elements is taken as 100%, and the mass% of the corresponding element is calculated. As a result of the quantification, particles with an S content of 5% or more by mass and an Mn content of 5% or more by mass are identified as coarse sulfide particles. The acceleration voltage for EDX analysis is 20 kV.

[0107] The total number of fine sulfide particles identified in each field of view and the total area (20.0 mm2) of the multiple fields of view constituting the above observation area were calculated. ２ ) and the center of gravity position D １．００Ｒ The number density of fine sulfide particles (particles / mm ２ Similarly, the total number of coarse sulfide particles identified in each visual field and the total area (20.0 mm ２ ) and the center of gravity position D １．００Ｒ The number density of coarse sulfide particles (particles / mm ２ ) is required.

[0108] The upper limit of the equivalent circle diameter of the coarse sulfide particles is not particularly limited. However, excessively coarse sulfides may become the starting point for cracks during processing such as hot working. Therefore, the preferred upper limit of the equivalent circle diameter of the particles containing coarse particles is 20.0 μm, more preferably 18.0 μm.

[0109] [0.08R depth position D ０．０８Ｒ Center of gravity position D １．００Ｒ In the steel material of this embodiment, the number density ratio NDR1 of fine sulfides at the depth position D ０．０８Ｒ The center of gravity position D for the number density of fine sulfides at １．００Ｒ The ratio of the number density of fine sulfides at 0.08R depth position D ０．０８Ｒ 1.00R depth position D １．００Ｒ The number density ratio NDR1 of fine sulfides at the steel sheet is defined as NDR1. In the steel sheet of this embodiment, the number density ratio NDR1 is preferably low.

[0110] Preferably, the number density ratio NDR1 is 0.65 or less. In this case, excellent fusion crack resistance is more effectively obtained in the surface layer region SA, and excellent machinability is more effectively obtained in the inner region CA. A preferred upper limit of the number density ratio NDR1 is 0.62, more preferably 0.60, even more preferably 0.55, even more preferably 0.50, and even more preferably 0.45. There is no particular restriction on the lower limit of the number density ratio NDR1. However, when the steel material satisfies Features 1 to 4, the number density ratio NDR1 at the depth position D of 0.08R is 0.08R. ０．０８Ｒ Center of gravity position D１．００Ｒ The lower limit NDR1 of the number density ratio of fine sulfides in this case is, for example, 0.10, more preferably 0.20, and even more preferably 0.30.

[0111] [0.08R depth position D ０．０８Ｒ Center of gravity position D １．００Ｒ In the steel material of this embodiment, the number density ratio NDR2 of coarse sulfides at the depth position D ０．０８Ｒ The center of gravity position D for the number density of coarse sulfides at １．００Ｒ The ratio of the number density of coarse sulfides at the depth position D ０．０８Ｒ 1.00R depth position D １．００Ｒ The number density ratio NDR2 of coarse sulfides at the steel sheet is defined as NDR2. In the steel sheet of this embodiment, the number density ratio NDR2 is preferably high.

[0112] Preferably, the number density ratio NDR2 is 1.40 or more. In this case, excellent fusion crack resistance is more effectively obtained in the surface layer region SA, and excellent machinability is more effectively obtained in the inner region CA. A preferred lower limit of the number density ratio NDR2 is 1.45, more preferably 1.50, even more preferably 1.55, even more preferably 1.60, and even more preferably 1.65. There is no particular upper limit to the number density ratio NDR2. However, when the steel material satisfies Features 1 to 4, the 0.08R depth position D ０．０８Ｒ Center of gravity position D １．００Ｒ The upper limit NDR2 of the number density ratio of coarse sulfides in this case is, for example, 10.00, more preferably 9.00, even more preferably 8.00, and even more preferably 7.00.

[0113] [Effects of the Steel Material of the Present Embodiment] As described above, the steel material of the present embodiment satisfies Features 1 to 4. Therefore, the steel material simultaneously exhibits excellent resistance to fusion cracking and excellent machinability. Furthermore, machine structural parts manufactured using the steel material exhibit high fatigue strength.

[0114] [Preferred uses and shapes of the steel material of this embodiment] The steel material of this embodiment can be widely applied, for example, as a material for machine structural parts. The steel material of this embodiment is particularly suitable when induction hardening or cutting is performed in the manufacturing process of machine structural parts. However, even if one or more of the induction hardening and cutting processes are not performed, the steel material of this embodiment can be applied as a material for machine structural parts. Note that even if it is not used as a material for machine structural parts, it can be widely applied in fields where fusion crack resistance or machinability is required.

[0115] As described above, in the steel material of this embodiment, the shape of the cross section perpendicular to the axial direction of the steel material is not particularly limited. The cross section may have, for example, a rotationally symmetric shape. However, the cross section is not limited to a rotationally symmetric shape.

[0116] The steel material of this embodiment may be, for example, a steel bar or a wire rod. As defined in JIS G 0203 (2023) No. 2501, a steel bar is a rod-shaped steel material manufactured by hot rolling or hot forging.

[0117] The wire rod is a steel material that is rolled into a rod shape and wound into a coil shape, as defined in JIS G 0203 (2023) No. 2511.

[0118] The steel material of this embodiment may be a billet. The billet refers to a steel material obtained by rolling or forging an ingot (steel ingot) or a bloom, or a steel material (cast piece) obtained by continuous casting. In a cross section perpendicular to the axial direction of the steel material, the center of gravity D １．００Ｒ When the length of the shortest line segment among the line segments connecting the surface of the steel material and the base is defined as R, 2R is, for example, 10 to 400 mm.

[0119] [Manufacturing Method] An example of a method for manufacturing a steel material according to this embodiment will be described. A steel material satisfying Features 1 to 4 may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of a method for manufacturing a steel material according to this embodiment.

[0120] An example of the method for manufacturing a steel material according to this embodiment includes the following steps. Note that step 3 is an optional step and does not need to be performed. (Step 1) Refining step (Step 2) Casting step (Step 3) Hot working step Each step will be described below.

[0121] [(Step 1) Refining Step] In the refining step, molten steel is produced having a chemical composition that satisfies the above-mentioned Features 1 and 2. The refining step may be carried out by a known process.

[0122] [(Step 2) Casting Step] The casting step includes the following steps: (Step 21) Continuous Casting Step (Step 22) Immersion and Heating Step (Step 23) Slab Reduction Step Each step will be explained below. [(Step 21) Continuous Casting Step] In the casting step, first, a slab is produced using a continuous casting device. In the continuous casting step, the cooling rate satisfies the following condition: (Condition 1) When the solidification cooling rate at a depth of 15 mm from the surface in a cross section perpendicular to the longitudinal direction of the slab is defined as the surface solidification cooling rate, the surface solidification cooling rate is 550°C / min or more.

[0123] [Condition 1: Surface Layer Solidification and Cooling Rate] Sulfides crystallize or precipitate during the continuous casting process. If the solidification and cooling rate in the surface layer of the slab during the continuous casting process is sufficiently fast, the sulfides are less likely to coarsen in the surface layer of the slab. In this case, the number density of fine sulfides in the surface layer region SA of the produced steel can be increased, and the number density of coarse sulfides can be sufficiently reduced. In this embodiment, the cooling rate of the slab in the mold of the continuous casting device is adjusted so that the solidification and cooling rate at a depth of 15 mm from the surface of the produced slab is 550°C / min or more.

[0124] Here, the cooling rate from the liquidus temperature to the solidus temperature within the temperature range of the steel in the casting process is defined as the solidification cooling rate (°C / min). As described above, the solidification cooling rate at a depth of 15 mm from the surface of the slab is defined as the "surface solidification cooling rate." The surface solidification cooling rate is determined by the following method.

[0125] A test piece is taken from a cross section (transverse cross section) perpendicular to the longitudinal direction of a slab produced by continuous casting, including a position 15 mm deep from the surface of the slab. For example, if the cross section perpendicular to the longitudinal direction of the slab is rectangular, a test piece is taken from the center of the width of the surface of the slab (or the center of the width of the surface corresponding to the long side if the cross section is rectangular). The surface of the test piece corresponding to the cross section (transverse cross section) perpendicular to the longitudinal direction of the slab is used as the observation surface. Within the observation surface, a 5 mm x 5 mm region centered at a position 15 mm deep from the surface of the slab is used as the observation region. The dendrite secondary arm spacing is measured at 10 locations within the observation region, and the arithmetic mean value is defined as λ2 (μm). The measured dendrite secondary arm spacing λ2 (μm) is used to calculate the surface solidification cooling rate V (°C / min). V = (λ2 / 770) －１／０．４１

[0126] The region of the slab at a depth of 15 mm from the surface solidifies while passing through the mold of the continuous casting machine. Therefore, the surface solidification cooling rate is adjusted by the cooling mechanism in the mold. The upper limit of the surface solidification cooling rate is not particularly limited. The lower limit of the surface solidification cooling rate is preferably 600°C / min or more, and more preferably 700°C / min or more.

[0127] [(Step 22) Immersion and Heating Step] The slab produced in the continuous casting step is cut with a torch. The temperature of the surface layer of the slab cut with the torch and the temperature of the internal region below the surface layer are both high, at 600°C or higher. In the immersion and reheating step, the slab cut with the torch in a high-temperature state is first immersed in a water tank for a predetermined time. In other words, the slab cut with the torch is not allowed to cool to room temperature, but is instead immersed in a high-temperature state. At this time, the surface layer of the slab is rapidly cooled, and the surface layer temperature drops to 500°C or lower. On the other hand, the temperature in the internal region of the slab does not drop as much, and is maintained at a temperature higher than that of the surface layer.

[0128] After immersion, the slab is heated in a heating furnace. During this process, a reverse transformation from ferrite to austenite occurs in the surface layer, resulting in finer grains. This promotes the formation of fine sulfides in the surface layer and suppresses the formation of coarse sulfides. Meanwhile, the internal region of the slab is maintained at a high temperature even during immersion and is heated in the heating furnace. This promotes the growth of sulfides in the internal region of the slab, reducing the number density of fine sulfides. The immersion and heating processes satisfy the following conditions: (Condition 2) The surface temperature T0 of the slab immediately before immersion is set to 600 to 850°C. (Condition 3) The immersion time t0 is set to 5 to 50 seconds. (Condition 4) The heating temperature T1 in the heating furnace is set to 1000 to 1200°C, and the holding time t1 at the heating temperature T1 is set to 30 to 120 minutes. Each condition is explained below.

[0129] [Condition 2: Regarding the surface temperature T0 of the slab immediately before the start of immersion] If the surface temperature T0 of the slab is less than 600°C, the temperature of the slab before immersion is too low. In this case, sulfides cannot grow sufficiently in the internal region. On the other hand, if the surface temperature T0 exceeds 850°C, the temperature of the slab before immersion is too high. In this case, even if immersion is performed, the surface layer temperature of the slab does not drop sufficiently. As a result, sulfides grow excessively in the surface layer of the slab. Therefore, the surface temperature T0 is set to 600 to 850°C.

[0130] [Condition 3: Immersion time t0] If the immersion time t0 is too short, the temperature of the surface layer of the slab will not drop sufficiently even after immersion. As a result, sulfides will grow excessively in the surface layer of the slab. On the other hand, if the immersion time t0 is too long, the temperature of the internal region of the slab will drop excessively. In this case, sulfides will not grow sufficiently in the internal region. Therefore, the immersion time t0 is 5 to 50 seconds.

[0131] [Condition 4: Heating time T1 and holding time t1 in the heating furnace] If the heating temperature T1 in the heating furnace after immersion is low or the holding time t1 is short, sulfides do not grow sufficiently in the inner region of the slab. On the other hand, if the heating temperature T1 is high or the holding time t1 is long, sulfides grow excessively in the surface layer of the slab. Therefore, the heating temperature T1 is 1000 to 1200°C, and the holding time t1 is 30 to 120 minutes.

[0132] [(Step 23) Slab Reduction Step] The heated slab is hot worked to produce a billet. The cumulative reduction rate is not particularly limited, but is, for example, 30 to 60%.

[0133] [(Step 3) Hot Working Step] The hot working step is an optional step. That is, the hot working step may or may not be performed. When the hot working step is performed, the hot working step involves hot working the slab produced in the casting step to produce a steel material.

[0134] The hot working process may, for example, consist solely of a well-known billet production process (blooming process), or may include a well-known billet production process and a well-known product production process (bar rolling process, wire rod rolling process) performed after the billet production process. The hot working process may also consist solely of a well-known product production process. In the billet production process, for example, a billet is produced from a heated slab or steel ingot by blooming, or blooming and hot rolling using a continuous rolling mill after blooming. In the product production process, for example, a steel material (bar or wire rod) is produced by hot rolling the heated billet using a well-known continuous rolling mill. The heating temperature in the billet production process is, for example, 1000 to 1300°C. The heating temperature in the product production process is, for example, 1000 to 1300°C.

[0135] In the above-described hot working step, steel is manufactured by hot rolling. However, steel may be manufactured by other hot working methods than hot rolling. For example, steel may be manufactured by hot forging instead of hot rolling. Steel may also be manufactured by performing both hot rolling and hot forging. Even when hot forging is performed in the hot working step, the heating temperature is, for example, 1000 to 1300°C.

[0136] The steel material of this embodiment is manufactured by the above manufacturing steps. As mentioned above, the hot working step may be omitted. In other words, the steel material of this embodiment may be a cast product (bill).

[0137] [Regarding the Machine Structural Component] The machine structural component of this embodiment is manufactured using the steel material of this embodiment described above as a raw material. Referring to Fig. 2, the machine structural component includes a component body 100 extending in the axial direction. An example of the machine structural component is a crankshaft shown in Fig. 2. However, the machine structural component is not limited to a crankshaft. Examples of the machine structural component include knuckles, shafts, hubs, gears, and suspension components.

[0138] The crankshaft, which is an example of a machine structural component of this embodiment, includes a fillet R portion 1 and an edge portion 2 of a crank web 4 or a counterweight 5. The crankshaft further includes a crank pin 6, a crank journal 7, and a rear flange 3.

[0139] At least a part of the mechanical structural component has a low degree of processing in the manufacturing process of the mechanical structural component. In other words, at least a part of the mechanical structural component retains the structure of the steel material from which the mechanical structural component is made. In the case where the mechanical structural component is the crankshaft shown in FIG. 2 , such a part of the mechanical structural component is, for example, the rear flange 3.

[0140] At least a portion of the machine structural component of this embodiment has the same chemical composition as the steel material of this embodiment described above. That is, the chemical composition of this portion is, in mass %, C: more than 0.30 to 0.60%, Si: 0.01 to 0.90%, Mn: 0.50 to 1.70%, P: 0.030% or less, S: 0.015 to 0.080%, Cr: 0.01 to 1.30%, Al: more than 0.010 to 0.090%, N: 0.0250% or less, O: 0.0050% or less, V: 0 to 0.200%, Sn: 0 to 0.1000%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Pb: 0 to 0.09%, and Bi: 0 to 0. 25%, Mg: 0-0.0100%, Ti: 0-0.0400%, Nb: 0-0.0500%, W: 0-0.4000%, Zr: 0-0.2000%, Ca: 0-0.0100%, Te: 0-0.0100%, B: 0-0.0050%, rare earth elements: 0-0.0100%, Co: 0-0.0100%, Se: 0-0.0100%, In: 0-0.0100%, Mo: 0-0.30%, Cu: 0-0.50%, Ni: 0-0.50%, and the balance is Fe and impurities.

[0141] Note that a portion of the machine structural component of this embodiment may be manufactured by induction hardening. In this case, a quench-hardened layer is formed on the surface layer of the portion of the machine structural component. In this case, in the machine structural component of this embodiment, the portion of the machine structural component has a quench-hardened layer formed on the surface layer and a core portion deeper than the quench-hardened layer. The quench-hardened layer is a configuration well known to those skilled in the art. The quench-hardened layer is made of martensite. Note that a quench-hardened layer may be formed on the surface layer of portions of the machine structural component other than the above-mentioned portion.

[0142] In addition, the size and number density (number / mm) of sulfide particles in machine structural parts can be improved by heat treatment such as induction hardening. ２ ) does not change.

[0143] In some of the machine structural components of this embodiment, Fn, as defined by formula (1), is 0.45 to 1.05. Fn=C+(Si / 10)+(Mn / 5)-(5S / 7)+(5Cr / 22)+1.65V (1)

[0144] Furthermore, in some of the mechanical structural components, in a cross section perpendicular to the axial direction of the mechanical structural component, the center of gravity D １．００Ｒ The length of the shortest line segment connecting the surface of the rotationally symmetric part to the surface of the rotationally symmetric part is defined as R. In this case, the surface of the shortest line segment ０．００Ｒ Similarly, the position of 0.08R depth starting from "0.08R depth position D ０．０８Ｒ ". The position of the center of gravity in the cross section is defined as "center of gravity position D １．００Ｒ " is defined as

[0145] In this case, 0.08R depth position D ０．０８Ｒ In the above, the number density of fine sulfide particles, which are sulfide particles having a circle equivalent diameter of 0.1 to 2.0 μm, is 550 particles / mm ２ or more, and the number density of coarse sulfide particles, which are sulfide particles having a circle equivalent diameter of more than 2.0 μm, is 55 particles / mm ２ The center of gravity position D １．００Ｒ In this case, the number density of fine sulfide particles is 550 particles / mm ２ and the number density of coarse sulfide particles is less than 55 particles / mm ２ It's super.

[0146] In addition, when it is difficult to determine the axial direction of a machine structural part, the elongation direction or rolling direction of the machine structural part is regarded as the axial direction. The elongation direction or rolling direction of the machine structural part can be determined based on the metal flow patterns in multiple cross sections of the machine structural part. For example, the elongation direction or rolling direction of the machine structural part can be determined based on the direction in which the segregation band or inclusion extends the most in multiple cross sections of the machine structural part.

[0147] The machine structural component of this embodiment satisfies the above-mentioned characteristics. Therefore, in the manufacturing process of the machine structural component, excellent fusion crack resistance and excellent machinability can be simultaneously obtained. For example, if an intermediate product having the shape shown in FIG. 2 is manufactured using the steel material of this embodiment as a raw material and the fillet R portion 1 of the intermediate product is induction hardened, the occurrence of cracks due to excessive heating of the edge portion 2 is suppressed, and excellent fusion crack resistance is obtained. Furthermore, the machinability of the internal region of the intermediate product can also be improved. Furthermore, high fatigue strength can be obtained in the machine structural component.

[0148] [Method for Manufacturing a Machine Structural Component] The method for manufacturing a machine structural component according to this embodiment is well known, and is, for example, as follows.

[0149] The steel material of this embodiment is hot worked to produce an intermediate product in the shape of a machine structural part (e.g., a crankshaft). The hot working is, for example, hot forging. The produced intermediate product is allowed to cool in the atmosphere.

[0150] The intermediate product after cooling is subjected to cutting to cut the intermediate product into a predetermined shape. The intermediate product after cutting is subjected to a known heat treatment. For example, the intermediate product is subjected to known induction hardening (tempering is omitted), or known induction hardening and known tempering. Alternatively, the intermediate product is subjected to full hardening, and then induction hardening or soft nitriding treatment is performed after full hardening. A machine structural part is manufactured through the above steps.

[0151] The effects of the steel material of this embodiment will be explained more specifically using examples. The conditions in the following examples are one example of conditions adopted to confirm the feasibility and effects of the steel material of this embodiment. Therefore, the steel material of this embodiment is not limited to this one example of conditions.

[0152] Steel materials were produced having the chemical compositions shown in Table 1 (Tables 1A to 1F). In Table 1 (Tables 1A to 1F), "-" means that the corresponding element was not intentionally included.

[0153]

[0154]

[0155]

[0156]

[0157]

[0158]

[0159] Specifically, a 270-ton converter was used to produce molten steel through a well-known refining process. A slab was produced from the molten steel by a continuous casting method. Conditions 1 to 4 for each test number are shown in Table 2 (Table 2A and Table 2B). The cumulative reduction rate in the slab reduction step of the casting process was 30 to 60%. Through the above process, a billet was produced, the cross section of which perpendicular to the axial direction was rectangular and measured 162 mm x 162 mm.

[0160]

[0161]

[0162] The produced billet was subjected to hot working. Specifically, the billet was subjected to hot forging equivalent to finish rolling to produce a steel material (steel bar) having a diameter of 50 mm and a circular cross section.

[0163] [Evaluation Tests] The following evaluation tests were carried out on the steel materials with each test number. (Test 1) 0.08R depth position D ０．０８Ｒ Test 2: Measurement of the number density of fine sulfide particles and coarse sulfide particles at the center of gravity D １．００Ｒ (Test 3) Melting crack evaluation test (Test 4) Machinability evaluation test (drill life test) (Test 5) Fatigue strength evaluation test (rotating bending fatigue test) Each evaluation test will be described below.

[0164] [(Test 1) 0.08R depth position D ０．０８Ｒ For each test number, the steel material was subjected to the above-mentioned [0.08R depth position D ０．０８Ｒ [Method for measuring the number density of fine sulfide particles and coarse sulfide particles at a depth of 0.08R D ０．０８Ｒ The number density of fine sulfide particles (particles / mm ２ ) and 0.08R depth position D ０．０８Ｒ The number density of coarse sulfide particles (particles / mm ２ The results are shown in Table 3 (Table 3A and Table 3B).

[0165]

[0166]

[0167] [(Test 2) Center of gravity position D １．００Ｒ For each test number, the steel material was subjected to the above-mentioned [measurement test of the number density of fine sulfide particles and coarse sulfide particles at the center of gravity position D １．００Ｒ [Method for measuring the number density of fine sulfide particles and coarse sulfide particles in a sample] １．００Ｒ The number density of fine sulfide particles (particles / mm ２ ) and center of gravity position D １．００Ｒ The number density of coarse sulfide particles (particles / mm ２ The results are shown in Table 3.

[0168] [(Tests 3 to 5) Melting crack evaluation test, machinability evaluation test, and fatigue strength evaluation test] [Manufacturing of simulated intermediate machine structural parts] A heat treatment simulating hot forging in the manufacturing process of machine structural parts was carried out using the steel material of each test number as the raw material. Specifically, the steel material was heated and held at 1100°C for 30 minutes. The steel material was then allowed to cool in the atmosphere. Hereinafter, the steel material subjected to the above heat treatment will be referred to as a "simulated intermediate machine structural part (or simply a simulated intermediate part)." The simulated intermediate machine structural part was a steel bar (round steel) with a diameter of 50 mm.

[0169] [(Test 3) Melting crack evaluation test] A test piece having a width of 10 mm, a thickness of 3 mm, and a length of 100 mm was prepared by machining from a region including the surface region SA of the simulated intermediate product of a mechanical structural component. The thickness direction of the test piece was the radial direction of the simulated intermediate product, and the longitudinal direction of the test piece was parallel to the longitudinal direction of the simulated intermediate product.

[0170] A hardening test simulating induction hardening was performed on the test specimen using a thermal cycle tester manufactured by Fuji Electric Industrial Co., Ltd. Specifically, the test specimen was heated to 1390°C at a temperature increase rate of 100°C / sec. The test specimen was then held at 1390°C for 15 seconds. Thereafter, the test specimen was water-cooled.

[0171] After water cooling, the test piece was cut in a direction perpendicular to the longitudinal direction at the center of the longitudinal direction of the test piece. The cut surface was used as the observation surface. The observation surface was mechanically polished. The mechanically polished observation surface was corroded with picral reagent. The entire corroded observation surface was observed under an optical microscope at 400x magnification, and the presence or absence of melting cracks was visually confirmed.

[0172] When a clearly corroded region (corroded region) having a length of 5 μm or more was observed at the grain boundary on the observed surface, it was determined that fusion cracks had occurred. An example of a clearly corroded region having a width of 5 μm or more at the grain boundary is the region indicated by reference numeral 15 in FIG. 3. On the other hand, when no corroded region was observed at the grain boundary, as in FIG. 4, it was determined that fusion cracks had been suppressed. The evaluation results of fusion cracks are shown in the "fusion cracks" column in Table 3. When fusion cracks occurred, it was indicated as "NG." When fusion cracks were suppressed, it was indicated as "OK."

[0173] [(Test 4) Machinability Evaluation Test (Drill Life Test)] A test piece for the machinability evaluation test was cut out from a simulated intermediate product of a machine structural part. Specifically, the inner area CA (center of gravity position D) of a cross section perpendicular to the longitudinal direction of the simulated intermediate product with a diameter of 50 mm was １．００Ｒ The outer edge is at a depth of 0.65R. ０．６５Ｒ A drill was used to drill holes at any position in the area corresponding to the circular area (circular area defined as ). A drill with model number SD3.0 manufactured by Fujikoshi Corporation was used as the tool. The drilling conditions were a feed rate per revolution of 0.25 mm / rev and a drilling depth of 9 mm per hole. The lubricant was a water-soluble cutting oil.

[0174] Drilling was performed under the above drilling conditions to evaluate the machinability of the steel material. The maximum cutting speed VL1000 (m / min) was used as an evaluation index. The maximum cutting speed VL1000 is the maximum cutting speed of a drill capable of drilling a hole with a cumulative hole depth of 1000 mm.

[0175] Based on the maximum cutting speed VL1000, machinability was evaluated as follows: VL1000 is 20 m / min or more: Excellent machinability ("OK") VL1000 is less than 20 m / min: Poor machinability ("NG") The "Cutting ability" column in Table 3 shows the evaluation results of "OK" and "NG".

[0176] (Test 5) Fatigue Strength Evaluation Test (Rotating Bending Fatigue Test) Fatigue strength was evaluated by the following test method using fatigue test specimens that were assumed to be machine structural parts manufactured from steel materials.

[0177] A fatigue test specimen shown in FIG. 5 was prepared from a simulated intermediate product of a machine structural component. The fatigue test specimen was a round bar specimen, with a diameter D1 of the parallel portion being 8 mm and a diameter D2 of the grip portion being 12 mm. The overall length of the specimen was 80 mm, and the length of the grip portion was 25 mm. The fatigue test specimen was prepared by machining from the R / 2 position (i.e., the center position of the radius) of a cross section perpendicular to the longitudinal direction of the simulated intermediate product of a machine structural component. The longitudinal direction of the fatigue test specimen was parallel to the longitudinal direction of the simulated intermediate product. It is common technical knowledge known to those skilled in the art that if the rotating bending fatigue strength of the test specimen before induction hardening is sufficiently high, the rotating bending fatigue strength of the test specimen after induction hardening will also be sufficiently high.

[0178] The parallel portion of the fatigue test specimen was subjected to finish polishing to adjust the surface roughness. Specifically, the centerline average roughness (Ra) of the surface was set to 3.0 μm or less and the maximum height (Rmax) was set to 9.0 μm or less, in accordance with JIS B 0601:2001.

[0179] Using the fatigue test specimens, an Ono-type rotating bending fatigue test was carried out at room temperature (23°C) in an air atmosphere at a rotation speed of 3600 rpm in both directions. Fatigue tests were carried out on multiple test specimens by varying the stress applied. ７ The highest stress at which the specimen did not break after cycling was taken as the fatigue strength (MPa).

[0180] If the fatigue strength obtained was 250 MPa or more, it was determined that sufficient fatigue strength was obtained. The results of the fatigue strength evaluation are shown in the "Fatigue strength" column in Table 3. A fatigue strength of 250 MPa or more was rated "OK," and a fatigue strength of less than 250 MPa was rated "NG."

[0181] [Test Results] Tables 1 to 3 show the test results.

[0182] Referring to Tables 1 to 3, the steels of test numbers 1 to 57 had appropriate chemical compositions and satisfied formula (1). Furthermore, the manufacturing conditions were also appropriate. Therefore, the steels of each test number satisfied features 1 to 4. As a result, no fusion cracking was observed, and excellent fusion cracking resistance was obtained. Furthermore, the maximum cutting speed VL1000 of the steels was 20 m / min or more, and excellent machinability was obtained. Furthermore, the fatigue strength of the steels was 250 MPa or more, and the fatigue strength of the steels was high.

[0183] On the other hand, in test numbers 58 and 59, Fn was less than 0.45, and therefore the fatigue strength of the steel material was low.

[0184] In test numbers 60 and 61, Fn exceeded 1.05, and therefore the machinability of the steel material was poor.

[0185] In test numbers 62 and 63, the surface solidification cooling rate V was too slow. ０．０８Ｒ The number density of the coarse sulfides was too high and the number density of the fine sulfides was too low, resulting in insufficient resistance to melting cracking.

[0186] In test numbers 64 and 65, the surface temperature T0 of the slab just before the start of immersion was too low. １．００Ｒ The number density of coarse sulfides was too low and the number density of fine sulfides was too high in the surface layer. As a result, sufficient machinability was not obtained in the inner region compared to the surface layer.

[0187] In test numbers 66 and 67, the surface temperature T0 of the slab just before the start of immersion was too high. ０．０８Ｒ The number density of the coarse sulfides was too high and the number density of the fine sulfides was too low, resulting in insufficient resistance to melting cracking.

[0188] In test numbers 68 and 69, the immersion time t0 was too short. ０．０８Ｒ The number density of the coarse sulfides was too high and the number density of the fine sulfides was too low, resulting in insufficient resistance to melting cracking.

[0189] In test numbers 70 and 71, the immersion time t0 was too long.１．００Ｒ The number density of coarse sulfides was too low and the number density of fine sulfides was too high in the surface layer. As a result, sufficient machinability was not obtained in the inner region compared to the surface layer.

[0190] In test numbers 72 and 73, the heating temperature T1 in the heating furnace after immersion was too low. １．００Ｒ The number density of coarse sulfides was too low and the number density of fine sulfides was too high in the surface layer. As a result, sufficient machinability was not obtained in the inner region compared to the surface layer.

[0191] In test numbers 74 and 75, the heating temperature T1 in the heating furnace after immersion was too high. ０．０８Ｒ The number density of the coarse sulfides was too high and the number density of the fine sulfides was too low, resulting in insufficient resistance to melting cracking.

[0192] In test numbers 76 and 77, the holding time t1 in the heating furnace after immersion was too short. １．００Ｒ The number density of coarse sulfides was too low and the number density of fine sulfides was too high in the surface layer. As a result, sufficient machinability was not obtained in the inner region compared to the surface layer.

[0193] In test numbers 78 and 79, the holding time t1 in the heating furnace after immersion was too long. ０．０８Ｒ The number density of the coarse sulfides was too high and the number density of the fine sulfides was too low, resulting in insufficient resistance to melting cracking.

[0194] In test numbers 80 and 81, the immersion step was not performed. ０．０８Ｒ The number density of the coarse sulfides was too high and the number density of the fine sulfides was too low, resulting in insufficient resistance to melting cracking.

[0195] The embodiments of the present invention have been described above. However, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and carried out without departing from the spirit of the present invention.

[0196] 1 Fillet R portion 2 Edge portion of crankshaft 3 Rear flange of crankshaft 15 Corrosion area (melting crack)

Claims

1. A steel material having a chemical composition, in mass%, of C: over 0.30 to 0.60%, Si: 0.01 to 0.90%, Mn: 0.50 to 1.70%, P: 0.030% or less, S: 0.015 to 0.080%, Cr: 0.01 to 1.30%, Al: over 0.010 to 0.090%, N: 0.0250% or less, O: 0.0050% or less, V: 0 to 0.200%, Sn: 0 to 0.1000%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Pb: 0 to 0.09%, Bi: 0 to 0.25%, Mg: 0 to 0.0100%, Ti: 0 to 0.0400%, Nb: 0 to 0.0500%, W: 0 to 0.4000%, Zr: 0 to 0.2000%, Ca: 0 to 0.0100%, Te: 0 to 0.0100%, B: 0 to 0.0050%, rare earth elements: 0 to 0.0100%, Co: 0 to 0.0100%, Se: 0 to 0.0100%, In: 0 to 0.0100%, Mo: 0 to 0.30%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, and the balance being Fe and impurities, Fn as defined in formula (1) is 0.45 to 1.05, In a cross section perpendicular to the axial direction of the steel material, when the length of the shortest line segment among the line segments connecting the center of gravity of the cross section and the surface of the steel material is defined as R, at a depth position of 0.08R from the surface of the steel material on the shortest line segment, the number density of fine sulfide particles, which are sulfide particles having a circle equivalent diameter of 0.1 to 2.0 μm, is 550 particles / mm ２ or more, and the number density of coarse sulfide particles, which are sulfide particles having an equivalent circle diameter of more than 2.0 μm, is 55 particles / mm ２ or less, and the number density of the fine sulfide particles at the position of the center of gravity of the cross section is 550 particles / mm ２ the number density of the coarse sulfide particles is less than 55 particles / mm ２ A steel material that is greater than 1.65V. Fn=C+(Si / 10)+(Mn / 5)-(5S / 7)+(5Cr / 22)+1.65V (1) Here, the content of the corresponding element in mass% is substituted for each element symbol in formula (1), and when an element is not contained, "0" is substituted for the corresponding element symbol.

2. A steel material according to claim 1, wherein the chemical composition is, in mass%, V: 0.001 to 0.200%, Sn: 0.0001 to 0.1000%, Sb: 0.0001 to 0.0500%, As: 0.0001 to 0.0500%, Pb: 0.01 to 0.09%, Bi: 0.01 to 0.25%, Mg: 0.0001 to 0.0100%, Ti: 0.0001 to 0.0400%, Nb: 0.0001 to 0.0500%, W: 0.0001 to 0.4000%, Zr: 0.0001 to 0.2000%, Ca: 0.0001 to 0.0100%, A steel material containing one or more elements selected from the group consisting of Te: 0.0001 to 0.0100%, B: 0.0001 to 0.0050%, rare earth elements: 0.0001 to 0.0100%, Co: 0.0001 to 0.0100%, Se: 0.0001 to 0.0100%, In: 0.0001 to 0.0100%, Mo: 0.01 to 0.30%, Cu: 0.01 to 0.50%, and Ni: 0.01 to 0.50%.

3. A machine structural component, wherein a part of the machine structural component has a chemical composition, in mass%, of C: over 0.30 to 0.60%, Si: 0.01 to 0.90%, Mn: 0.50 to 1.70%, P: 0.030% or less, S: 0.015 to 0.080%, Cr: 0.01 to 1.30%, Al: over 0.010 to 0.090%, N: 0.0250% or less, O: 0.0050% or less, V: 0 to 0.200%, Sn: 0 to 0.1000%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Pb: 0 to 0.09%, Bi: 0 to 0.25%, Mg: 0 to 0.0100%, Ti: 0 to 0.0400%, Nb: 0 to 0.0500%, W: 0 to 0.4000%, Zr: 0 to 0.2000%, Ca: 0 to 0.0100%, Te: 0 to 0.0100%, B: 0 to 0.0050%, rare earth elements: 0 to 0.0100%, Co: 0 to 0.0100%, Se: 0 to 0.0100%, In: 0 to 0.0100%, Mo: 0 to 0.30%, Cu: 0 to 0.50%, Ni: 0 to 0.50%, and the balance being Fe and impurities, Fn as defined in formula (1) is 0.45 to 1.05, In a part of the machine structural component, when the length of the shortest line segment among the line segments connecting the center of gravity of the cross section perpendicular to the axial direction of the machine structural component and the surface of the machine structural component is defined as R, at a depth position of 0.08R from the surface of the shortest line segment, the number density of fine sulfide particles, which are sulfide particles having a circle equivalent diameter of 0.1 to 2.0 μm, is 550 particles / mm ２ or more, and the number density of coarse sulfide particles, which are sulfide particles having an equivalent circle diameter of more than 2.0 μm, is 55 particles / mm ２ At the position of the center of gravity, the number density of the fine sulfide particles is 550 particles / mm or less. ２ the number density of the coarse sulfide particles is less than 55 particles / mm ２ A machine structural part having a mass percent content of the corresponding element, where Fn=C+(Si / 10)+(Mn / 5)-(5S / 7)+(5Cr / 22)+1.65V (1) where, for each element symbol in formula (1), the content of the corresponding element in mass percent is substituted, and when an element is not contained, "0" is substituted for the corresponding element symbol.

4. A machine structural part according to claim 3, wherein the chemical composition is, in mass %, V: 0.001 to 0.200%, Sn: 0.0001 to 0.1000%, Sb: 0.0001 to 0.0500%, As: 0.0001 to 0.0500%, Pb: 0.01 to 0.09%, Bi: 0.01 to 0.25%, Mg: 0.0001 to 0.0100%, Ti: 0.0001 to 0.0400%, Nb: 0.0001 to 0.0500%, W: 0.0001 to 0.4000%, Zr: 0.0001 to 0.2000%, Ca: 0.0001 to 0.0100%, A machine structural part containing one or more elements selected from the group consisting of Te: 0.0001 to 0.0100%, B: 0.0001 to 0.0050%, rare earth elements: 0.0001 to 0.0100%, Co: 0.0001 to 0.0100%, Se: 0.0001 to 0.0100%, In: 0.0001 to 0.0100%, Mo: 0.01 to 0.30%, Cu: 0.01 to 0.50%, and Ni: 0.01 to 0.50%.

5. A machine structural component according to claim 3 or 4, wherein the machine structural component is a crankshaft, and the part is a rear flange.

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