steel

Abstract

To provide a steel material excellent in machinability, capable of preventing a fusion crack at high frequency induction hardening, and having an excellent fatigue strength when used as a component for machine structure.SOLUTION: A steel material includes, by mass%, over 0.30 to 0.60% C, 0.01 to 0.55% Si, 0.50 to 1.65% Mn, 0.050% or less P, 0.010 to 0.200% S, 0.0001 to 0.0050% Bi, 0.0001 to 0.0050% Ca, 0.001 to 0.005% Al, 0.0030 to 0.0250% N, and 0.0030% or less O, and satisfies an expression (1) in a specification. In the steel material, a number density of fine sulfide is 20 / mm2 or more, a number density of coarse Bi particles is 2.00 / mm2, and a number density of a specific oxide is 0.1 / mm2 or more.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a steel material, and more particularly to a steel material that is used as a raw material for machine structural parts. [Background technology]

[0002] High fatigue strength is required for machine structural parts used in crankshafts of automobiles and construction vehicles, and therefore, surface hardening treatment is sometimes performed on machine structural parts to improve fatigue strength.

[0003] Among various surface hardening processes, induction hardening can harden only the necessary areas. Furthermore, because induction hardening involves heating at high temperatures followed by cooling, it can achieve a deeper hardened layer and higher fatigue strength than other surface hardening processes such as soft nitriding. For this reason, induction hardening is often used for machine structural parts. For example, to improve the fatigue strength of crankshafts, which are one type of machine structural part, a technology has been put into practical use in which the fillet R portion 1 shown in Figure 1 is hardened by induction hardening.

[0004] In recent years, there has been a demand for further improvements in the fatigue strength of machine structural components. In order to increase the hardened layer depth using induction hardening, the heating temperature can be increased by increasing the output of high-frequency power during induction hardening. 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, in the case of a crankshaft shown in FIG. 1, the edge corresponds to the portion indicated by reference symbol 2). This is particularly likely to occur when the temperature rise rate during induction hardening is rapid. For example, if the heating temperature during induction hardening becomes excessively high, reaching 1350°C or higher, the surface or interior of the steel may melt, causing cracks. Hereinafter, such cracks are referred to as "fusion cracks" in this specification. It is preferable to suppress the occurrence of fusion cracks in machine structural components. In other words, suppression of fusion cracks is required when induction hardening is performed.

[0005] When manufacturing a machine structural part using steel material that will be the material for the machine structural part, the machine structural part is manufactured, for example, by the following method. First, hot forging is performed to manufacture an intermediate product of the machine structural part that has a shape similar to the machine structural part. Then, cutting is performed on the intermediate product. The intermediate product after cutting is subjected to induction hardening. The intermediate product after induction hardening is further subjected to finishing processing (cutting or grinding) to manufacture the machine structural part. In other words, when manufacturing a machine structural part using steel material that will be the material for the machine structural part, cutting or grinding may be performed twice, once before induction hardening and once after induction hardening. Therefore, the steel material that will be the material for the machine structural part is required to have excellent machinability, both in the intermediate product before induction hardening and in the intermediate product after induction hardening.

[0006] Steel materials used as raw materials for machine structural parts are disclosed in, for example, Japanese Patent Application Laid-Open No. 2017-082299 (Patent Document 1), Japanese Patent Application Laid-Open No. 2003-226934 (Patent Document 2), and Japanese Patent Application Laid-Open No. 2004-91886 (Patent Document 3).

[0007] The product member disclosed in Patent Document 1 contains 0.4-0.7% C, 0.25% or less Si, 0.5-2.6% Mn, 0.050% or less P, 0.005-0.020% S, 0.01-0.06% Nb, 0.010-0.050% Al, 0.005-0.025% N, and 0.003% or less O, with the balance being Fe and unavoidable impurities, and has a chemical composition that satisfies formula (1). Here, formula (1) is 17.0≦25.9C+6.35Mn≦27.2. During the manufacturing process of this product member, the induction hardening and tempering conditions are appropriately controlled. As a result, the hardened material after induction hardening and tempering has a structure at a reference position corresponding to a depth of 20 μm from a position corresponding to the surface of the final product component, which contains martensite and a volume fraction of 8 to 20% retained austenite. Furthermore, the cutting conditions for the hardened material are appropriately controlled. As a result, in the structure at the reference position of this product component, the volume fraction of retained austenite is 12% or less, the retained austenite reduction rate Δγ calculated from the retained austenite volume fraction before cutting (RI) and the retained austenite volume fraction after cutting (RF) using formula (A) is 40% or more, and the arithmetic mean roughness Ra of the surface is 0.8 μm or less. Here, formula (A) is Δγ = (RI - RF) / RI × 100. In this document, the chemical composition of the product component, the structure and hardness after induction hardening, and the structure and arithmetic mean roughness after cutting are controlled. This improves the fatigue strength and machinability of the product component.

[0008] The steel for machine structural use disclosed in Patent Document 2 has a chemical composition containing 0.05-0.8% C, 0.01-2.5% Si, 0.1-3.5% Mn, 0.01-0.2% S, 0.001-0.020% Al, 0.0005-0.02% Ca, 0.0005-0.01% O, and 0.001-0.04% N, and further containing one or two of 0.002-0.020% Ti and 0.002-0.040% Zr, with the balance being unavoidable impurities and Fe. The steel for machine structural use disclosed in Patent Document 3 has the same chemical composition as the steel for machine structural use of Patent Document 2, except for the upper limits of the Si, Ti, and Zr contents. In the steel for machine structural use of Patent Document 3, the upper limit of the Si content is 2.0%, the upper limit of the Ti content is 0.010%, and the upper limit of the Zr content is 0.025%. In the steel for machine structural use of Patent Document 2, the area occupied by sulfide-based inclusions containing 1.0% or more by weight of Ca, which are in contact with oxide-based inclusions with a CaO content of 0.2 to 62% by weight, is within a field area of ​​3.5 mm 2 2.0x10 -4 mm 2 That's all. The steel for machine structural use of Patent Document 3 further restricts the oxide-based inclusions of the steel for machine structural use of Patent Document 2 in that they have a melting point of 1500 to 1750°C. The steel for machine structural use of Patent Document 3 further restricts the Ca content of the sulfide-based inclusions of the steel for machine structural use of Patent Document 2 in that they have a Ca content of 1 to 45 wt%. In Patent Documents 2 and 3, the MnS inclusions are finely dispersed, thereby improving machinability. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-082299 [Patent Document 2] Japanese Patent Application Laid-Open No. 2003-226934 [Patent Document 3] Japanese Patent Application Laid-Open No. 2004-91886 Summary of the Invention [Problem to be solved by the invention]

[0010] However, although Patent Documents 1 to 3 above at least consider machinability and fatigue strength, they do not consider the prevention of melting cracks during induction hardening.

[0011] An object of the present invention is to provide a steel material which has excellent machinability, can suppress melting cracks during induction hardening, and has excellent fatigue strength when used for machine structural parts. [Means for solving the problem]

[0012] The steel material according to the present invention is In mass%, C: more than 0.30~0.60%, Si: 0.01 to 0.55%, Mn: 0.50-1.65% P: 0.050% or less, S: 0.010~0.200%, Bi: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, Al: 0.001 to 0.005%, N: 0.0030 to 0.0250%, and O: 0.0030% or less, The balance is Fe and impurities. Assuming that the content of each element is within the above range, formula (1) is satisfied, In the steel material, The number density of fine sulfides with a circle equivalent diameter of 0.2 to less than 1.0 μm is 20 pieces / mm 2 That's all, The number density of coarse Bi particles with a circle equivalent diameter of 5 μm or more is 2.00 particles / mm 2 is as follows: When oxide-based inclusions containing Bi and Ca and having an equivalent circle diameter of 1 μm or more are defined as specific oxides, the number density of the specific oxides is 0.1 particles / mm 2 That's all. Ca / Al≧0.18 (1) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in mass %.

[0013] The steel material according to the present invention is In mass%, C: more than 0.30~0.60%, Si: 0.01 to 0.55%, Mn: 0.50-1.65% P: 0.050% or less, S: 0.010~0.200%, Bi: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, Al: 0.001 to 0.005%, N: 0.0030 to 0.0250%, and O: 0.0030% or less, Further, it contains one or more selected from the group consisting of Groups 1 to 4, The balance is Fe and impurities. Assuming that the content of each element is within the above range, formula (1) is satisfied, In the steel material, The number density of fine sulfides with a circle equivalent diameter of 0.2 to less than 1.0 μm is 20 pieces / mm 2 That's all, The number density of coarse Bi particles with a circle equivalent diameter of 5 μm or more is 2.00 particles / mm 2 is as follows: When oxide-based inclusions containing Bi and Ca and having an equivalent circle diameter of 1 μm or more are defined as specific oxides, the number density of the specific oxides is 0.1 particles / mm 2 That's all. [Group 1] V: 0.400% or less, Ti: 0.050% or less, Nb: 0.050% or less, W: 0.400% or less, and Zr: 0.0100% or less, one or more selected from the group consisting of [Group 2] Mg: 0.0100% or less Te: 0.0100% or less B: 0.0050% or less Sn: 0.0100% or less, and Rare earth elements: 0.0100% or less, one or more selected from the group consisting of [Group 3] Co:0.0100% or less, Se: 0.0100% or less, and Sb: 0.0100% or less, one or more selected from the group consisting of [Group 4] Cr: 0.30% or less, Mo: 0.30% or less Cu: 0.50% or less, and Ni: 0.50% or less, one or more selected from the group consisting of Ca / Al≧0.18 (1) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in mass %. [Effects of the Invention]

[0014] The steel material of the present invention has excellent machinability, can suppress melting cracks during induction hardening, and has excellent fatigue strength when used as a machine structural part. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a front view showing a part of a crankshaft, which is a machine structural part. [Figure 2] FIG. 2 is a schematic diagram of the microstructure of a steel material outside the scope of the present invention after it was heated to 1350° C. or higher at a heating rate of 100° C. / sec, held at that temperature for 15 seconds, and then water-cooled. [Figure 3] FIG. 3 is a schematic diagram of the microstructure of the steel material of this embodiment after it is heated to 1350° C. or higher at a heating rate of 100° C. / sec, held at that temperature for 15 seconds, and then water-cooled. [Figure 4] FIG. 4 is a schematic diagram of a rotating bending fatigue test specimen taken from the intermediate product of each simulated machine structural part. DETAILED DESCRIPTION OF THE INVENTION

[0016] The present inventors first investigated the chemical composition of a steel material that has excellent machinability and excellent fatigue strength when used as a machine structural part. As a result, the present inventors concluded that a steel material that has a chemical composition, by mass%, of C: more than 0.30 to 0.60%, Si: 0.01 to 0.55%, Mn: 0.50 to 1.65%, P: 0.050% or less, S: 0.010 to 0.200%, Ca: 0.0001 to 0.0050%, Al: 0.001 to 0.005%, N: 0.0030 to 0.0250%, and O: 0.0030% or less, and if any optional elements are contained, further contains one or more selected from the group consisting of the above-mentioned Groups 1 to 4, with the balance being Fe and impurities, may have excellent machinability and excellent fatigue strength when used as a machine structural part.

[0017] Next, the inventors investigated means for suppressing fusion cracks during induction hardening in steel materials in which the content of each element in the chemical composition is within the above-mentioned range. First, the inventors observed the microstructure of the area where fusion cracks occurred during induction hardening in order to identify the cause of fusion cracks in the steel material. As a result, decarburization did not occur in the area where fusion cracks occurred. On the other hand, fusion cracks did not occur in the area where decarburization occurred.

[0018] From these results, the inventors considered that the C content affects the occurrence of fusion cracks in steel materials during induction hardening. Specifically, the inventors considered that C segregating at grain boundaries makes fusion cracks more likely to occur. Therefore, the inventors investigated means for suppressing the segregation of C at grain boundaries.

[0019] The present inventors focused on sulfides formed in steel. Specifically, they thought that if the sulfides in steel were fine, it would be possible to suppress fusion cracks in the steel during induction hardening. The reasons for this are believed to be as follows: Fine sulfides suppress the coarsening of austenite (γ) grains in steel during induction hardening due to their pinning effect. If the sulfides are fine, the pinning effect is enhanced. If γ grains are maintained fine during induction hardening, the grain boundary area of ​​the γ grains increases. If the grain boundary area increases, the concentration of C segregating at the grain boundaries per unit area decreases. As a result, the occurrence of fusion cracks in the steel during induction hardening is suppressed.

[0020] Next, the present inventors investigated means for refining sulfides formed in a steel material. As a result, they found that fine sulfides can be formed in a steel material by further adding 0.0001 to 0.0050% Bi to the above-mentioned chemical composition. Specifically, most of the sulfides formed in a steel material crystallize in the molten steel before solidification or during solidification. The solidification structure of a steel material generally has a dendrite form. During solidification, solute elements such as Mn and S tend to concentrate in the interdendritic regions of dendrites. Therefore, sulfides such as MnS crystallize between the dendrites. By adding 0.0001 to 0.0050% Bi, the interdendritic spacing of dendrites becomes shorter. Therefore, the sulfides crystallizing between the dendrites can be refined.

[0021] From the above, we thought that by adding an appropriate amount of Bi and ensuring a certain number density of fine sulfides formed in the steel, it would be possible to sufficiently suppress melt cracks during induction hardening. Therefore, we further investigated and considered the number density of fine sulfides that would fully demonstrate this effect. As a result, we found that in steel with the above chemical composition, the number density of fine sulfides with a circle equivalent diameter of less than 0.2 to 1.0 μm was 20 / mm 2 The present inventors have found that if the above conditions are met, it is possible to sufficiently suppress melting cracks during induction hardening.

[0022] However, as a result of further investigation by the present inventors, it was found that although the inclusion of Bi suppresses the occurrence of melting cracks during induction hardening, it may accelerate tool wear in the finishing process (cutting or grinding) after induction hardening. Therefore, the present inventors investigated the cause of this. As a result, the present inventors obtained the following new findings.

[0023] When bismuth is added to steel to prevent melting cracks, coarse bismuth particles with a circle-equivalent diameter of 5 μm or more may be generated. An intermediate product after induction hardening is harder than an intermediate product before induction hardening. Therefore, when cutting or grinding an intermediate product after induction hardening (hereinafter also referred to as cutting, etc.), the heat generated during cutting or other processing tends to be greater than when cutting an intermediate product before induction hardening. Coarse bismuth particles have a low melting point. Therefore, if coarse bismuth particles are present in an intermediate product after induction hardening, the heat generated during cutting or other processing melts the coarse bismuth particles and reacts with cutting or other tools. This causes localized embrittlement of the cutting or other tools. As a result, the wear of cutting or other tools is accelerated during processes such as cutting an intermediate product after induction hardening.

[0024] Based on the above findings, the present inventors have considered the possibility of suppressing the generation of coarse Bi particles to suppress tool wear during cutting of induction-hardened intermediate products. However, it has been found that even if coarse Bi particles are suppressed, tool wear during cutting of induction-hardened intermediate products may not be sufficiently suppressed. The present inventors have therefore investigated the cause of this. As a result, the present inventors have obtained the following new findings.

[0025] In order to suppress the wear of cutting tools, etc., it is effective to generate oxides containing Ca (hereinafter also referred to as Ca-containing oxides) in steel. The Ca-containing oxides in steel soften due to the heat generated during cutting and other processes. Therefore, the Ca-containing oxides adhere to and accumulate on the surface of cutting tools, etc. If the Ca-containing oxides accumulate on the tools, the wear of the cutting tools, etc. is suppressed.

[0026] Therefore, the present inventors have investigated means for promoting the formation of Ca-containing oxides in steel materials, and as a result, the present inventors have concluded that if a steel material in which the contents of each element in the chemical composition are within the above-mentioned ranges further satisfies formula (1), it may be possible to sufficiently suppress tool wear in processes such as cutting of intermediate products after induction hardening. Ca / Al≧0.18 (1) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in mass %.

[0027] However, simply generating Ca-containing oxides in a steel material may not be enough to suppress the wear of tools for cutting or the like. Specifically, even if a sufficient amount of Ca-containing oxides is generated in a steel material, the Ca-containing oxides may not be sufficiently deposited on the surface of a tool for cutting or the like. Therefore, the present inventors further investigated means for sufficiently depositing Ca-containing oxides on the surface of a tool for cutting or the like. As a result, it was found that a Ca-containing oxide containing Bi, that is, an oxide-based inclusion containing Bi and Ca, can increase the amount of Ca-containing oxide deposited on the surface of a tool for cutting or the like.

[0028] Oxide-based inclusions containing Bi and Ca include Ca-containing oxides and Bi particles. As mentioned above, Bi particles have a low melting point. Therefore, Bi particles are more likely to soften than Ca-containing oxides due to heat generated during machining, such as cutting, of intermediate products after induction hardening, which generates a large amount of heat. In the case of oxide-based inclusions containing Bi and Ca, the softened Bi particles promote adhesion between the Ca-containing oxide and a cutting tool. As a result, the amount of Ca-containing oxide that accumulates on the surface of a cutting tool increases.

[0029] Based on the above study results, the present inventors have considered that, in a steel material having the above chemical composition, by satisfying the above formula (1), suppressing the number density of coarse Bi particles in the steel material as much as possible, and further ensuring a certain number density of oxide-based inclusions containing Bi and Ca, it may be possible to sufficiently suppress tool wear in processes such as cutting after induction hardening. Therefore, further research and investigation was carried out into the number density of coarse Bi particles and the number density of oxide-based inclusions containing Bi and Ca that will fully demonstrate these effects. As a result, it was found that, in a steel material having the above chemical composition, the number density of fine sulfides having an equivalent circle diameter of less than 0.2 to 1.0 μm is 20 / mm 2 On the premise that the above formula (1) is satisfied and the number density of coarse Bi particles with a circle equivalent diameter of 5 μm or more is 2.00 particles / mm 2 When the specific oxide is defined as an oxide-based inclusion containing Bi and Ca, the specific oxide has a number density of 0.1 particles / mm 2 The present inventors have found that if the above conditions are met, wear of tools for cutting and the like can be sufficiently suppressed in the process of cutting the intermediate product after induction hardening.

[0030] The steel material according to this embodiment, which has been completed based on the above findings, has the following configuration.

[0031] [1] A steel material, In mass%, C: more than 0.30~0.60%, Si: 0.01 to 0.55%, Mn: 0.50-1.65% P: 0.050% or less, S: 0.010~0.200%, Bi: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, Al: 0.001 to 0.005%, N: 0.0030 to 0.0250%, and O: 0.0030% or less, The balance is Fe and impurities. Assuming that the content of each element is within the above range, formula (1) is satisfied, In the steel material, The number density of fine sulfides with a circle equivalent diameter of 0.2 to less than 1.0 μm is 20 pieces / mm 2 That's all, The number density of coarse Bi particles with a circle equivalent diameter of 5 μm or more is 2.00 particles / mm 2 is as follows: When oxide-based inclusions containing Bi and Ca and having an equivalent circle diameter of 1 μm or more are defined as specific oxides, the number density of the specific oxides is 0.1 particles / mm 2 That's all. Steel material. Ca / Al≧0.18 (1) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in mass %.

[0032] [2] A steel material, In mass%, C: more than 0.30~0.60%, Si: 0.01 to 0.55%, Mn: 0.50-1.65% P: 0.050% or less, S: 0.010~0.200%, Bi: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, Al: 0.001 to 0.005%, N: 0.0030 to 0.0250%, and O: 0.0030% or less, Further, it contains one or more selected from the group consisting of Groups 1 to 4, The balance is Fe and impurities. Assuming that the content of each element is within the above range, formula (1) is satisfied, In the steel material, The number density of fine sulfides with a circle equivalent diameter of 0.2 to less than 1.0 μm is 20 pieces / mm 2 That's all, The number density of coarse Bi particles with a circle equivalent diameter of 5 μm or more is 2.00 particles / mm2 is as follows: When oxide-based inclusions containing Bi and Ca and having an equivalent circle diameter of 1 μm or more are defined as specific oxides, the number density of the specific oxides is 0.1 particles / mm 2 That's all. Steel material. [Group 1] V: 0.400% or less, Ti: 0.050% or less, Nb: 0.050% or less, W: 0.400% or less, and Zr: 0.0100% or less, one or more selected from the group consisting of [Group 2] Mg: 0.0100% or less Te: 0.0100% or less B: 0.0050% or less Sn: 0.0100% or less, and Rare earth elements: 0.0100% or less, one or more selected from the group consisting of [Group 3] Co:0.0100% or less, Se: 0.0100% or less, and Sb: 0.0100% or less, one or more selected from the group consisting of [Group 4] Cr: 0.30% or less, Mo: 0.30% or less Cu: 0.50% or less, and Ni: 0.50% or less, one or more selected from the group consisting of Ca / Al≧0.18 (1) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in mass %.

[0033] [3] [2] The steel material according to containing the first group, Steel material.

[0034] [4] [2] or [3] steel material, containing the second group, Steel material.

[0035] [5] The steel material according to any one of [2] to [4], containing the third group, Steel material.

[0036] [6] The steel material according to any one of [2] to [5], containing the fourth group, Steel material.

[0037] The steel material of this embodiment will be described in detail below. "%" relating to elements means mass % unless otherwise specified.

[0038] [Features of the steel material of this embodiment] The steel material of this embodiment satisfies the following characteristics 1 to 5. (Feature 1) The content of each element in the chemical composition is within the range of this embodiment. (Feature 2) The chemical composition satisfies formula (1). Ca / Al≧0.18 (1) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in mass %. (Feature 3) In the steel material, the number density of fine sulfides with a circle equivalent diameter of 0.2 to less than 1.0 μm is 20 pieces / mm 2 That's all. (Feature 4) In the steel material, the number density of coarse Bi particles with a circle equivalent diameter of 5 μm or more is 2.00 particles / mm 2 The following is the result. (Feature 5) In steel materials, when oxide-based inclusions containing Bi and Ca and having an equivalent circle diameter of 1 μm or more are defined as specific oxides, the number density of the specific oxides is 0.1 pieces / mm 2 That's all. Features 1 to 5 will be explained below.

[0039] [(Feature 1) Chemical composition] The chemical composition of the steel material of this embodiment contains the following elements.

[0040] C: More than 0.30~0.60% Carbon (C) increases the hardness of machine structural parts manufactured from steel materials, thereby increasing the fatigue strength of the machine structural parts. If the C content is 0.30% or less, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content exceeds 0.60%, C segregates at the grain boundaries even if the contents of other elements are within the ranges of this embodiment. In this case, the C concentration at the grain boundaries increases. As the C concentration increases, the melting point decreases. As a result, melting cracks are more likely to occur during induction hardening. Therefore, the C content is more than 0.30% to 0.60%. The lower limit of the C content is preferably 0.35%, more preferably 0.36%, and even more preferably 0.38%. The upper limit of the C content is preferably 0.55%, more preferably 0.50%, and even more preferably 0.45%.

[0041] Si: 0.01 to 0.55% Silicon (Si) deoxidizes steel during the steelmaking process. Si also increases the hardness of machine structural parts, thereby increasing their fatigue strength. If the Si content is less than 0.01%, the above effects cannot be sufficiently obtained, even if the contents of other elements are within the ranges of this embodiment. On the other hand, Si has a weak affinity for C. Therefore, if the Si content exceeds 0.55%, even if the contents of other elements are within the ranges of this embodiment, C is more likely to segregate at grain boundaries during heating than within the grains where Si is dissolved. As a result, melting cracks are more likely to occur during induction hardening. Therefore, the Si content is 0.01 to 0.55%. The lower limit of the Si content is preferably 0.02%, more preferably 0.05%, and even more preferably 0.08%. The upper limit of the Si content is preferably 0.50%, more preferably 0.40%, even more preferably 0.35%, and still more preferably 0.30%.

[0042] Mn: 0.50 to 1.65% Manganese (Mn) deoxidizes steel during the steelmaking process. Mn also increases the hardness of machine structural parts and increases their fatigue strength. Mn also has a strong affinity with C. Therefore, during heating, C remains within the grains where Mn is dissolved. This suppresses the segregation of C to grain boundaries, and the occurrence of melting cracks during induction hardening. If the Mn content is less than 0.50%, the above effects cannot be sufficiently obtained, 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 material. Therefore, if the Mn content exceeds 1.65%, melting cracks are likely to occur during induction hardening, even if the contents of other elements are within the ranges of this embodiment. If the Mn content exceeds 1.65%, the hardness of the steel material will also increase excessively. As a result, the machinability of the intermediate product before induction hardening will decrease when it is cut. Therefore, the Mn content is 0.50 to 1.65%. The lower limit of the Mn content is preferably 0.55%, more preferably 0.60%, and even more preferably 0.70%. The upper limit of the Mn content is preferably 1.60%, more preferably 1.55%, even more preferably 1.45%, and still more preferably 1.40%.

[0043] P:0.050% or less Phosphorus (P) is an impurity. P segregates at grain boundaries. As a result, P lowers the melting point of steel, making it more susceptible to melting cracks during induction hardening. Therefore, the P content should be 0.050% or less. The P content is preferably as low as possible. However, excessive reduction in the P content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the P content is preferably more than 0%, more preferably 0.001%, and even more preferably 0.002%. The upper limit of the P content is preferably 0.030%, more preferably 0.025%, even more preferably 0.020%, and still more preferably 0.015%.

[0044] S: 0.010~0.200% Sulfur (S) forms sulfides, which improve the machinability when cutting an intermediate product before induction hardening. S also suppresses wear of tools, such as cutting tools, in processes such as cutting the intermediate product after induction hardening. In other words, S improves the machinability when cutting the intermediate product after induction hardening. S also forms fine sulfides, which suppresses melt cracking during induction hardening. If the S content is less than 0.010%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, S lowers the melting point of the steel material, so if the S content exceeds 0.200%, 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 S content is 0.010 to 0.200%. The lower limit of the S content is preferably 0.012%, more preferably 0.015%, even more preferably 0.018%, and still more preferably 0.020%. The upper limit of the S content is preferably 0.150%, more preferably 0.095%, and even more preferably 0.070%.

[0045] Bi: 0.0001 to 0.0050% Bismuth (Bi) refines the solidification structure in steel. This refines sulfides. As a result, melt cracks during induction hardening are suppressed. Bi also improves machinability when cutting intermediate products before induction hardening. Bi may also produce oxide-based inclusions (specific oxides) containing Bi and Ca. In this case, wear of cutting tools is suppressed in processes such as cutting the intermediate products after induction hardening. In other words, machinability is improved when cutting the intermediate products after induction hardening. If the Bi content is less than 0.0001%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Bi content exceeds 0.0050%, even if the contents of other elements are within the ranges of this embodiment, coarse Bi particles are generated, and therefore, tool wear during cutting or other processes of the intermediate product after induction hardening is not sufficiently suppressed. Therefore, the Bi content is 0.0001 to 0.0050%. The lower limit of the Bi content is preferably 0.0003%, more preferably 0.0005%, and even more preferably 0.0010%. The upper limit of the Bi content is preferably 0.0048%, more preferably 0.0045%, and even more preferably 0.0040%.

[0046] Ca: 0.0001 to 0.0050% Calcium (Ca) generates Ca-containing oxides. Among the Ca-containing oxides, Ca generates oxide-based inclusions (specific oxides) containing Bi and Ca and having a circle-equivalent diameter of 1 μm or more. Therefore, wear of cutting tools is suppressed in processes such as cutting the induction-hardened intermediate product. In other words, the machinability of the induction-hardened intermediate product is improved when cutting it. If the Ca content is less than 0.0001%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ca content exceeds 0.0050%, even if the contents of other elements are within the ranges of this embodiment, excessive CaS is generated. As a result, the machinability of the intermediate product before induction hardening is reduced when it is cut. Furthermore, if excessive CaS is generated, wear of cutting tools, etc., cannot be sufficiently suppressed in processes such as cutting the intermediate product after induction hardening. Therefore, the Ca content is 0.0001 to 0.0050%. The lower limit of the Ca content is preferably 0.0005%, more preferably 0.0010%, and even more preferably 0.0015%. The upper limit of the Ca content is preferably 0.0030%, more preferably 0.0025%, and even more preferably 0.0020%.

[0047] Al: 0.001 to 0.005% Aluminum (Al) deoxidizes steel during the steelmaking process, and if the Al content is less than 0.001%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Al content exceeds 0.005%, even if the contents of other elements are within the ranges of this embodiment, excessive Al2O3 is generated. In this case, the generation of Ca-containing oxides is suppressed. Therefore, the specific oxides are not generated sufficiently. As a result, in processes such as cutting the intermediate product after induction hardening, wear of cutting tools cannot be sufficiently suppressed. Therefore, the Al content is 0.001 to 0.005%. The preferred lower limit of the Al content is 0.002%. The upper limit of the Al content is preferably 0.004%, and more preferably 0.003%.

[0048] N: 0.0030~0.0250% Nitrogen (N) forms nitrides and / or carbonitrides during the cooling process after hot working in the manufacturing process of machine structural parts, thereby precipitation strengthening the steel material. As a result, the fatigue strength of the machine structural parts is improved. If the N content is less than 0.0030%, the above effects cannot be sufficiently obtained 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 material will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the N content is 0.0030 to 0.0250%. The lower limit of the N content is preferably 0.0035%, more preferably 0.0040%, and even more preferably 0.0050%. The upper limit of the N content is preferably 0.0200%, more preferably 0.0180%, even more preferably 0.0150%, even more preferably 0.0120%, and even more preferably 0.0080%.

[0049] O: 0.0030% or less Oxygen (O) is an impurity. O forms oxides in steel and reduces the fatigue strength of machine structural parts. Therefore, the O content is limited to 0.0030% or less. The O content is preferably as low as possible. However, excessive reduction of the O content increases production costs. Therefore, taking into consideration normal industrial production, the lower limit of the O content is preferably more than 0%, more preferably 0.0005%, and even more preferably 0.0008%. The upper limit of the O content is preferably 0.0025%, more preferably 0.0020%, and even more preferably 0.0015%.

[0050] 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 or scrap or the manufacturing environment when industrially manufacturing the steel material, but are not intentionally contained and are allowed to a range that does not adversely affect the steel material according to this embodiment.

[0051] [Optional elements] The steel material of this embodiment may further contain one or more elements selected from the group consisting of first to fourth groups in place of a portion of Fe. [Group 1] V: 0.400% or less, Ti: 0.050% or less, Nb: 0.050% or less, W: 0.400% or less, and Zr: 0.0100% or less, one or more selected from the group consisting of [Group 2] Mg: 0.0100% or less Te: 0.0100% or less B: 0.0050% or less Sn: 0.0100% or less, and Rare earth elements: 0.0100% or less, one or more selected from the group consisting of [Group 3] Co:0.0100% or less, Se: 0.0100% or less, and Sb: 0.0100% or less, one or more selected from the group consisting of [Group 4] Cr: 0.30% or less, Mo: 0.30% or less Cu: 0.50% or less, and Ni: 0.50% or less, one or more selected from the group consisting of The first to fourth groups will be explained below.

[0052] [Group 1 (V, Ti, Nb, W and Zr)] V, Ti, Nb, W and Zr are optional elements, and all of them form precipitates to increase the toughness of machine structural parts.

[0053] V:0.400% or less Vanadium (V) is an optional element and may not be contained, that is, the V content may be 0%. When V is contained, that is, when the V content exceeds 0%, V forms carbides and / or carbonitrides during the cooling process of the hot working step in the manufacturing process of the machine structural component, thereby refining the crystal grains. This increases the toughness of the machine structural component. Even if even a small amount of V is contained, the above effect can be obtained to some extent. However, if the V content exceeds 0.400%, the above effects become saturated and the manufacturing cost increases even if the contents of other elements are within the ranges of this embodiment. Therefore, the V content is 0 to 0.400%, and if V is contained, it is 0.400% or less. The lower limit of the V content is preferably 0.001%, more preferably 0.010%, even more preferably 0.050%, and still more preferably 0.100%. The upper limit of the V content is preferably 0.300%, more preferably 0.200%, and even more preferably 0.150%.

[0054] Ti: 0.050% or less Titanium (Ti) is an optional element and may not be contained, that is, the Ti content may be 0%. When Ti is contained, that is, when the Ti content is more than 0%, Ti forms carbides and / or carbonitrides during the cooling process of the hot working step in the manufacturing process of the machine structural part, thereby refining the crystal grains. This increases the toughness of the machine structural part. 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 0.050%, the above effects become saturated and the manufacturing cost increases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ti content is 0 to 0.050%, and if contained, it is 0.050% or less. The lower limit of the Ti content is preferably 0.001%, more preferably 0.005%, even more preferably 0.011%, and still more preferably 0.021%. The upper limit of the Ti content is preferably 0.040%, more preferably 0.030%, and even more preferably 0.025%.

[0055] Nb: 0.050% or less Niobium (Nb) is an optional element and may not be contained, that is, the Nb content may be 0%. When Nb is contained, that is, when the Nb content is more than 0%, Nb forms carbides and / or carbonitrides during the cooling process of the hot working step in the manufacturing process of machine structural parts, thereby refining the crystal grains. This increases the toughness of the machine structural parts. 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 0.050%, the above effects become saturated and the manufacturing cost becomes high even if the contents of other elements are within the ranges of this embodiment. Therefore, the Nb content is 0 to 0.050%, and when Nb is contained, it is 0.050% or less. The lower limit of the Nb content is preferably 0.001%, more preferably 0.010%, and even more preferably 0.020%. The upper limit of the Nb content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.030%.

[0056] W:0.400% or less Tungsten (W) is an optional element and may not be contained, that is, the W content may be 0%. When W is contained, that is, when the W content is more than 0%, W generates carbides and / or carbonitrides during the cooling process of the hot working step in the manufacturing process of the machine structural component, thereby refining the crystal grains. This increases the toughness of the machine structural component. Even if even a small amount of W is contained, the above effect can be obtained to some extent. However, if the W content exceeds 0.400%, the above effects become saturated and the manufacturing cost increases even if the contents of other elements are within the ranges of this embodiment. Therefore, the W content is 0 to 0.400%, and when W is contained, it is 0.400% or less. The lower limit of the W content is preferably 0.001%, more preferably 0.010%, even more preferably 0.050%, and still more preferably 0.100%. The upper limit of the W content is preferably 0.300%, more preferably 0.200%, and even more preferably 0.150%.

[0057] Zr: 0.0100% or less Zirconium (Zr) is an optional element and may not be contained, that is, the Zr content may be 0%. When Zr is contained, that is, when the Zr content is more than 0%, Zr forms carbides and / or carbonitrides during the cooling process of the hot working step in the manufacturing process of machine structural parts, thereby refining the crystal grains. This increases the toughness of the machine structural parts. Even if even a small amount of Zr is contained, the above effect can be obtained to some extent. However, if the Zr content exceeds 0.0100%, the above effects become saturated and the manufacturing cost increases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Zr content is 0 to 0.0100%, and if contained, it is 0.0100% or less. The lower limit of the Zr content is preferably 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The upper limit of the Zr content is preferably 0.0050%, more preferably 0.0030%, and even more preferably 0.0019%.

[0058] [Group 2 (Mg, Te, B, Sn, and rare earth elements)] Mg, Te, B, Sn, and rare earth elements are optional elements that all improve the machinability of the intermediate product before induction hardening. These elements also all improve the machinability of the intermediate product after induction hardening.

[0059] Mg: 0.0100% or less Magnesium (Mg) is an optional element and may not be contained, that is, the Mg content may be 0%. When contained, that is, when the Mg content is more than 0%, Mg improves the machinability of the intermediate product before induction hardening. Mg also improves the machinability of the intermediate product after induction hardening. Even if even a small amount of Mg is contained, the above effects can be obtained to some extent. However, if the Mg content exceeds 0.0100%, even if the contents of other elements are within the ranges of this embodiment, Mg generates coarse oxides, which reduce the fatigue strength of machine structural parts manufactured using the steel material. Therefore, the Mg content is 0 to 0.0100%, and if Mg is contained, it is 0.0100% or less. The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0005%. The upper limit of the Mg content is preferably 0.0050%, more preferably 0.0045%, and even more preferably 0.0040%.

[0060] Te: 0.0100% or less Tellurium (Te) is an optional element and may not be contained, that is, the Te content may be 0%. When contained, that is, when the Te content is more than 0%, Te improves the machinability of the intermediate product before induction hardening. Te also improves the machinability of the intermediate product after induction hardening. Even if even a small amount of Te is contained, the above effects can be obtained to some extent. However, if the Te content exceeds 0.0100%, the hot workability deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Te content is 0 to 0.0100%, and if contained, it is 0.0100% or less. The lower limit of the Te content is preferably 0.0001%, more preferably 0.0003%, and even more preferably 0.0010%. The upper limit of the Te content is preferably 0.0090%, more preferably 0.0080%, and even more preferably 0.0040%.

[0061] B: 0.0050% or less Boron (B) is an optional element and may not be contained, that is, the B content may be 0%. When contained, that is, when the B content exceeds 0%, B improves the machinability of the intermediate product before induction hardening. B also improves the machinability of the intermediate product after induction hardening. Even if even a small amount of B is contained, the above effects can be obtained to some extent. However, if the B content exceeds 0.0050%, the hot workability deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the B content is 0 to 0.0050%, and if B is contained, it is 0.0050% or less. The lower limit of the B content is preferably 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The upper limit of the B content is preferably 0.0040%, more preferably 0.0035%, and even more preferably 0.0030%.

[0062] Sn: 0.0100% or less Tin (Sn) is an optional element and may not be contained, that is, the Sn content may be 0%. When Sn is contained, that is, when the Sn content is more than 0%, Sn improves the machinability of the intermediate product before induction hardening. Sn also improves the machinability of the intermediate product after induction hardening. Even if even a small amount of Sn is contained, the above effects can be obtained to some extent. However, if the Sn content exceeds 0.0100%, the hot workability deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sn content is 0 to 0.0100%, and when Sn is contained, the Sn content is 0.0100% or less. The lower limit of the Sn content is preferably 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The upper limit of the Sn content is preferably 0.0090%, more preferably 0.0070%, and even more preferably 0.0050%.

[0063] Rare earth elements: 0.0100% or less Rare earth elements (REM) are optional elements and may not be contained, that is, the REM content may be 0%. When REM is contained, that is, when the REM content is greater than 0%, REM improves the machinability of the intermediate product before induction hardening. REM also improves the machinability of the intermediate product after induction hardening. Even if even a small amount of REM is contained, the above effects can be obtained to some extent. However, if the REM content exceeds 0.0100%, the hot workability will be reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the REM content is 0 to 0.0100%, and if contained, it is 0.0100% or less. The lower limit of the REM content is preferably 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The upper limit of the REM content is preferably 0.0090%, more preferably 0.0050%, and even more preferably 0.0030%.

[0064] 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 addition, the REM content in this specification refers to the total content of these elements.

[0065] [Group 3 (Co, Se, and Sb)] Co, Se, and Sb are optional elements, and all of them suppress decarburization of the steel material.

[0066] Co:0.0100% or less 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 the Co content exceeds 0%, Co suppresses decarburization of the steel material during hot working. 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 deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Co content is 0 to 0.0100%, and when Co is contained, it is 0.0100% or less. The lower limit of the Co content is preferably 0.0001%, more preferably 0.0005%, even more preferably 0.0010%, and still more preferably 0.0030%. The upper limit of the Co content is preferably 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0067] Se:0.0100% or less Selenium (Se) is an optional element and may not be contained, that is, the Se content may be 0%. When Se is contained, that is, when the Se content is more than 0%, Se suppresses decarburization of the steel material during hot working. Even if even a small amount of Se is contained, the above effect can be obtained to some extent. However, when the Se content exceeds 0.0100%, the hot workability deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Se content is 0 to 0.0100%, and if contained, it is 0.0100% or less. The lower limit of the Se content is preferably 0.0001%, more preferably 0.0010%, and even more preferably 0.0020%. The upper limit of the Se content is preferably 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0068] Sb: 0.0100% or less Antimony (Sb) is an optional element and may not be contained, that is, the Sb content may be 0%. When Sb is contained, that is, when the Sb content exceeds 0%, Sb suppresses decarburization of the steel material during hot working. Even if even a small amount of Sb is contained, the above effect can be obtained to some extent. However, if the Sb content exceeds 0.0100%, the hot workability deteriorates even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sb content is 0 to 0.0100%, and if contained, it is 0.0100% or less. The lower limit of the Sb content is preferably 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The upper limit of the Sb content is preferably 0.0090%, more preferably 0.0080%, and even more preferably 0.0070%.

[0069] [Group 4 (Cr, Mo, Cu, and Ni)] Cr, Mo, Cu, and Ni are optional elements, and all of them increase the fatigue strength of machine structural parts.

[0070] Cr:0.30% or less Chromium (Cr) is an optional element and may not be contained, that is, the Cr content may be 0%. When contained, that is, when the Cr content is more than 0%, Cr increases the fatigue strength of machine structural parts. Even if even a small amount of Cr is contained, the above effect can be obtained to some extent. However, if the Cr content exceeds 0.30%, the hardness of the steel material will be excessively increased even if the contents of other elements are within the ranges of this embodiment. As a result, the machinability of the steel material will be reduced when cutting an intermediate product before induction hardening. Cr lowers the melting point of the steel material. Therefore, if the Cr content exceeds 0.30%, melting cracks will be more likely to occur during induction hardening. Therefore, the Cr content is 0 to 0.30%, and if contained, it is 0.30% or less. The lower limit of the Cr content is preferably 0.02%, more preferably 0.05%, even more preferably 0.10%, and still more preferably 0.15%. The upper limit of the Cr content is preferably 0.28%, more preferably 0.26%, and even more preferably 0.24%.

[0071] Mo: 0.30% or less Molybdenum (Mo) is an optional element and may not be contained, that is, the Mo content may be 0%. When contained, that is, when the Mo content is more than 0%, Mo increases the fatigue strength of machine structural parts. 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%, the hardness of the steel material increases excessively even if the contents of other elements are within the ranges of this embodiment, resulting in a decrease in hot workability. Therefore, the Mo content is 0 to 0.30%, and if contained, it is 0.30% or less. The lower limit of the Mo content is preferably 0.01%, more preferably 0.05%, and even more preferably 0.10%. The upper limit of the Mo content is preferably 0.19%, more preferably 0.17%, and even more preferably 0.15%.

[0072] Cu: 0.50% or less Copper (Cu) is an optional element and may not be contained, that is, the Cu content may be 0%. When Cu is contained, that is, when the Cu content is more than 0%, Cu increases the fatigue strength of machine structural parts. Even if even a small amount of Cu is contained, the above effect can be obtained to some extent. However, Cu, like Si, promotes the occurrence of melting cracks during induction hardening, so if the Cu 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 Cu content is 0 to 0.50%, and if contained, it is 0.50% or less. The lower limit of the Cu content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%. The upper limit of the Cu content is preferably 0.20%, more preferably 0.10%, and even more preferably 0.05%.

[0073] Ni: 0.50% or less Nickel (Ni) is an optional element and may not be contained, that is, the Ni content may be 0%. When contained, that is, when the Ni content is more than 0%, Ni increases the fatigue strength of machine structural parts. Even if even a small amount of Ni is contained, the above effect can be obtained to some extent. However, like Si and Cu, Ni promotes the occurrence of melting cracks during induction hardening, so 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%, and when Ni is contained, it is 0.50% or less. The lower limit of the Ni content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%. The upper limit of the Ni content is preferably 0.20%, more preferably 0.10%, and even more preferably 0.05%.

[0074] [Method for measuring the chemical composition of steel] The chemical composition of the steel material of this embodiment can be measured by a known elemental analysis method in accordance with JIS G0321:2017. Specifically, chips are collected from the interior of the steel material to a depth of 1 mm or more from the surface using a drill. The collected chips are dissolved in acid to obtain a solution. The solution is subjected to ICP-AES (Inductively Coupled Plasma Atomic Emission Spectrometry) to perform elemental analysis of the chemical composition. The C content and S content are determined by a known high-frequency combustion method (combustion-infrared absorption method). The N content is determined by a known inert gas fusion-thermal conductivity method. The O content is determined by a known inert gas fusion-infrared absorption method.

[0075] The content of each element is determined by rounding off the measured value to the nearest significant digit based on the significant digits specified in this embodiment. For example, the C content of the steel material in this embodiment is determined to be a value up to two decimal places. Therefore, the C content is determined to be a value up to two decimal places obtained by rounding off the measured value to two decimal places. Similarly, the contents of other elements other than the C content of the steel material of this embodiment are also determined by rounding off the measured value to the smallest digit specified in this embodiment, and this is the content of the element. Rounding off means rounding down if the fraction is less than 5, and rounding up if the fraction is 5 or more.

[0076] [(Feature 2) Regarding Formula (1)] The chemical composition of the steel material of this embodiment, assuming that Feature 1 is satisfied, further satisfies formula (1). Ca / Al≧0.18 (1) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in mass %.

[0077] The formula for fn1 is fn1 = Ca / Al. fn1 is an index of the number density of Ca-containing oxides generated in steel. Ca-containing oxides in steel adhere to and accumulate on the surface of cutting tools during machining, such as cutting, of intermediate products after induction hardening. The Ca-containing oxides that adhere to cutting tools suppress tool wear. In other words, they improve machinability when cutting intermediate products after induction hardening. The deoxidation process using Ca in the refining process promotes the generation of Ca-containing oxides. However, steel also contains Al. Therefore, Al reacts with oxygen in the steel to generate Al2O3. Therefore, suppressing the generation of Al2O3 is effective in increasing the amount of Ca-containing oxides generated in steel. Specifically, it is effective to include a sufficient amount of Ca relative to the Al content.

[0078] Even if the content of each element in the steel is within the range of this embodiment, if fn1 is less than 0.18, the Ca content is insufficient relative to the Al content in the steel. Therefore, the generation of Ca-containing oxides is suppressed and the generation of Al2O3 is promoted. In this case, the generation of specific oxides is suppressed. Therefore, in processes such as cutting the intermediate product after induction hardening, tool wear cannot be sufficiently suppressed. In other words, the machinability of the intermediate product when cutting it after induction hardening is poor. Therefore, fn1 is 0.18 or more. The lower limit of fn1 is preferably 0.50, more preferably 0.75, and even more preferably 1.00. The upper limit of fn1 is not particularly limited, but is preferably 5.00, more preferably 4.50, and even more preferably 4.00. The numerical value of fn1 is a value obtained by rounding off to two decimal places.

[0079] [(Feature 3) Number density of fine sulfides] In the steel material of this embodiment, on the premise that the chemical composition satisfies Features 1 and 2, the number density of fine sulfides having a circle equivalent diameter of less than 0.2 to 1.0 μm (hereinafter simply referred to as fine sulfides) is 20 particles / mm 2 The number density of fine sulfides is 20 / mm 2 If the above condition is met, the occurrence of melting cracks during induction hardening is suppressed.

[0080] In this specification, fine sulfides refer to sulfides having an equivalent circle diameter of 0.2 to less than 1.0 μm. In the method for measuring the number density of fine sulfides described below, inclusions having an equivalent circle diameter of 0.2 to 1.0 μm are defined as fine sulfides if they have a sulfur (S) content of 3% or more by mass and Mn is detected by an energy dispersive X-ray spectrometry (EDX). Examples of sulfides include MnS. The sulfides may contain Ca and / or Fe in addition to S and Mn. The sulfides may contain CaS and / or FeS in addition to MnS, for example. The fine sulfides may exist alone in a steel material without adhering to or in contact with other particles (precipitates or inclusions). The fine sulfides may exist in a steel material while adhering to or in contact with other particles. When fine sulfides are present attached to or in contact with other particles, if the circle-equivalent diameter calculated from the total area of ​​a particle formed by the sulfide and the other particles is less than 0.2 to 1.0 μm, the particle is determined to be one fine sulfide.

[0081] As mentioned above, fine sulfides pin the γ grain boundaries. If the circle equivalent diameter of the fine sulfides is less than 0.2 to 1.0 μm, the pinning effect of the γ grain boundaries is enhanced. If the γ grains are kept fine during induction hardening, the grain boundary area of ​​the γ grains increases. If the grain boundary area increases, the concentration of C segregating at the grain boundaries decreases. As a result, the occurrence of fusion cracks is suppressed. If the number density of the fine sulfides is 20 particles / mm 2 If it is less than this, the above effects cannot be obtained sufficiently. Therefore, the density of fine sulfides is 20 particles / mm 2 That's all. The preferable lower limit of the number density of fine sulfides is 25 pieces / mm 2 and more preferably 30 pieces / mm 2 and more preferably 35 pieces / mm 2 and more preferably 40 pieces / mm 2 is. The upper limit of the number density of fine sulfides is not particularly limited, but is preferably 200 pieces / mm 2 and more preferably 180 pieces / mm 2 is.

[0082] [Measurement of number density of fine sulfides] The number density of fine sulfides can be measured by the following method. A sample is taken from the R / 2 position (the center position of the line connecting the central axis of the steel and the outer surface in the longitudinal section of the steel) of a cross section (longitudinal section) parallel to the longitudinal direction of the steel (steel bar). Of the surfaces of the taken sample, the surface corresponding to the longitudinal section of the steel is used as the observation surface. After mirror polishing the observation surface, the mirror-polished observation surface is observed at 500x magnification using a scanning electron microscope (SEM). The total observation area is 32 mm 2 Let's say.

[0083] Based on the backscattered electron image obtained by SEM observation, the number density of fine sulfides is examined using a well-known particle analysis method based on image analysis. Specifically, the inclusions and precipitates in the steel are identified based on the interfaces between the matrix of the steel and the inclusions and precipitates. Image analysis is performed to calculate the circle-equivalent diameters of the inclusions and precipitates. Here, the circle-equivalent diameter refers to the diameter of a circle when the area of ​​each inclusion and precipitate is converted into a circle having the same area.

[0084] The components of the obtained inclusions and precipitates having a circle equivalent diameter of less than 0.2 to 1.0 μm are analyzed using an EDX equipped in an SEM. In this embodiment, the beam diameter of the EDX used for the component analysis is not particularly limited as long as it can analyze the components of the inclusions and precipitates to be analyzed. When the EDX elemental analysis result shows that the S content is 3% by mass or more and that Mn is also contained, the inclusion is defined as a fine sulfide. The acceleration voltage during EDX analysis is 20 kV.

[0085] Identify fine sulfides using the method above. Total 32mm 2 Based on the total number of fine sulfides identified in the observation area, the number of fine sulfides per unit area (pieces / mm 2 ) is found.

[0086] [(Feature 4) Number density of coarse Bi particles] In the steel material of this embodiment, on the premise that Features 1 and 2 are satisfied, the number density of coarse Bi particles having a circle equivalent diameter of 5 μm or more (hereinafter simply referred to as coarse Bi particles) is 2.00 particles / mm 2 The number density of coarse Bi particles is 2.00 particles / mm 2 If the thickness is equal to or less than this, wear of cutting tools can be suppressed in steps such as cutting the induction-hardened intermediate product, which means that the machinability of the induction-hardened intermediate product is improved when it is cut.

[0087] In this specification, coarse Bi particles refer to Bi particles with an equivalent circle diameter of 5 μm or more. In the method for measuring the number density of coarse Bi particles described below, inclusions with an equivalent circle diameter of 5 μm or more and a Bi content of 70% or more by mass detected by EDX are defined as coarse Bi particles. Coarse Bi particles may exist alone in a steel material without adhering to or contacting other particles (precipitates or inclusions). Coarse Bi particles may exist in a steel material adhering to or contacting other particles. When coarse Bi particles exist adhering to or contacting other particles, if the equivalent circle diameter calculated from the total area of ​​a particle formed by the Bi particle and the other particles is 5 μm or more, the particle is considered to be a single coarse Bi particle. There is no particular upper limit to the equivalent circle diameter of coarse Bi particles, but in the chemical composition of this embodiment, the coarse Bi particles are at most 50 μm or less.

[0088] When Bi is contained in steel, coarse Bi particles with a circle equivalent diameter of 5 μm or more may be generated. These coarse Bi particles may accelerate the wear of cutting tools during processes such as cutting intermediate products after induction hardening. If the number of coarse Bi particles is 2.00 / mm 2 If it exceeds this value, the above effect cannot be obtained sufficiently. Therefore, in the steel material of this embodiment, the number density of coarse Bi particles is 2.00 particles / mm 2 The following is the result. The density of coarse Bi particles is 0 particles / mm 2 However, excessive reduction in the number density of the coarse Bi particles increases the production cost. Therefore, in consideration of normal industrial production, the preferred lower limit of the number density of the coarse Bi particles is 0.01 particles / mm 2 and more preferably 0.03 pieces / mm 2 is. The preferred upper limit of the number density of coarse Bi particles is 1.00 particles / mm 2 and more preferably 0.50 pieces / mm 2 and more preferably 0.20 pieces / mm 2 It is preferable that the number density of the coarse Bi particles is as low as possible.

[0089] [Measurement of the number density of coarse Bi particles] The number density of coarse Bi particles can be measured by the following method: A sample is collected in the same manner as in the method for measuring the number density of fine sulfides, and the observation surface is observed with an SEM.

[0090] Based on the backscattered electron images obtained by SEM observation, the number density of coarse Bi particles is examined using a well-known particle analysis method based on image analysis. Specifically, the inclusions and precipitates in the steel are identified based on the interfaces between the matrix of the steel and the inclusions and precipitates. Image analysis is performed to calculate the circle-equivalent diameters of the inclusions and precipitates. Here, the circle-equivalent diameter refers to the diameter of a circle when the area of ​​each inclusion and precipitate is converted into a circle with the same area.

[0091] The components of the obtained inclusions and precipitates having a circle equivalent diameter of 5 μm or more are analyzed using an EDX equipped in an SEM. In this embodiment, the beam diameter of the EDX used for the component analysis is not particularly limited as long as it can analyze the components of the inclusions and precipitates to be analyzed. If the EDX elemental analysis result shows that the Bi content is 70% by mass or more, the inclusion is defined as a coarse Bi particle. The acceleration voltage during EDX analysis is 20 kV.

[0092] Identify large Bi particles using the above method. Total 32mm 2 Based on the total number of coarse Bi particles identified in the observation area, the number of coarse Bi particles per unit area (particles / mm 2 ) is found.

[0093] [(Feature 5) Number density of specific oxides] In the steel material of this embodiment, on the premise that the chemical composition satisfies Features 1 and 2, when oxide-based inclusions having a circle equivalent diameter of 1 μm or more and containing Bi and Ca are defined as specific oxides, the number density of the specific oxides is 0.1 pieces / mm 2 The density of specific oxides is 0.1 particles / mm 2If the above is the case, wear of tools for cutting, etc. can be suppressed in the process of cutting the induction-hardened intermediate product, etc. In other words, the machinability when cutting the induction-hardened intermediate product is improved.

[0094] In this specification, the term "specific oxide" refers to a Ca-containing oxide having an equivalent circle diameter of 1 μm or more and containing Bi, i.e., an oxide-based inclusion containing Bi and Ca. The Ca-containing oxide is, for example, an inclusion containing CaO. The Ca-containing oxide may contain Al2O3 and SiO2 in addition to CaO. A Ca-containing oxide is defined as one in which the oxygen (O) content is 3% or more by mass and the Ca content is 1% or more by mass as detected by EDX. Among Ca-containing oxides, a Ca-containing oxide having an equivalent circle diameter of 1 μm or more and a Bi content of more than 0 and less than 70% by mass as detected by EDX is defined as the specific oxide. The specific oxide may exist alone in a steel material without adhering to or contacting particles (precipitates or inclusions) other than Bi particles. The specific oxide may exist in a steel material by adhering to or contacting particles other than Bi particles. When the specific oxide is present attached to or in contact with particles other than Bi particles, if the circle-equivalent diameter calculated from the total area of ​​a particle formed by the specific oxide and the other particles is 1 μm or more, the particle is determined to be one specific oxide. There is no particular upper limit to the circle-equivalent diameter of the specific oxide, but in the case of the chemical composition of this embodiment, the specific oxide is at most 10 μm or less.

[0095] Ca-containing oxides accumulate on the surface of cutting tools during processes such as cutting of intermediate products after induction hardening, thereby suppressing tool wear. When Ca-containing oxides containing Bi, i.e., oxide-based inclusions containing Bi and Ca, are present, Bi particles with a low melting point promote the accumulation of Ca-containing oxides on tools. As a result, tool wear can be further suppressed. If the size of the oxide-based inclusions containing Bi and Ca is 1 μm or more in equivalent circle diameter, tool wear can be further suppressed. In other words, specific oxides in steel suppress tool wear during processes such as cutting of intermediate products after induction hardening. When the number density of specific oxides is 0.1 particles / mm 2 If it is less than this, the above effects cannot be obtained sufficiently. Therefore, in the steel material of this embodiment, the number density of the specific oxide is 0.1 pieces / mm 2 That's all. The preferred lower limit of the specific oxide density is 0.4 pieces / mm 2 and more preferably 0.8 pieces / mm 2 and more preferably 1.2 pieces / mm 2 and more preferably 2.0 pieces / mm 2 is. The number density of the specific oxide is preferably as high as possible. There is no particular upper limit to the number density of the specific oxide, but it is preferably 10.0 particles / mm 2 is.

[0096] [Measurement of the number density of specific oxides] The number density of a specific oxide can be measured by the following method: A sample is taken in the same manner as in the method for measuring the number density of fine sulfides, and the observation surface is observed by SEM.

[0097] Based on the backscattered electron image obtained by SEM observation, the number density of specific oxides is examined using a well-known particle analysis method based on image analysis. Specifically, the inclusions and precipitates in the steel material are identified based on the interface between the matrix of the steel material and the inclusions and precipitates. Image analysis is performed to calculate the circle-equivalent diameters of the inclusions and precipitates. Here, the circle-equivalent diameter refers to the diameter of a circle when the area of ​​each inclusion and precipitate is converted into a circle having the same area.

[0098] The components of the obtained inclusions and precipitates having a circle equivalent diameter of 1 μm or more are analyzed using an EDX equipped in an SEM. In this embodiment, the beam diameter of the EDX used for the component analysis is not particularly limited as long as it can analyze the components of the inclusions and precipitates to be analyzed. If the EDX elemental analysis results show that the oxygen (O) content is 3% or more by mass, the Ca content is 1% or more by mass, and the Bi content is more than 0 and less than 70% by mass, the inclusion is defined as a specific oxide. The acceleration voltage during EDX analysis is 20 kV.

[0099] Identify specific oxides using the above method. Total 32mm 2 Based on the total number of specific oxides identified in the observation area, the number of specific oxides per unit area (pieces / mm 2 ) is found.

[0100] [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.

[0101] 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 necessarily have to be performed. (Process 1) Refining process (Process 2) Casting process (Process 3) Hot processing process Each step will be described below.

[0102] [(Process 1) Refining process] In the refining process, molten steel having the above-mentioned chemical composition is produced. The refining process includes a primary refining process and a secondary refining process. In the primary refining process, molten pig iron produced by a known method is refined in a converter. Specifically, oxygen is blown onto the molten pig iron to remove carbon. In the secondary refining process, elements are added to adjust the composition, and molten steel having the chemical composition of the steel material of this embodiment is produced. Specifically, after the primary refining process, deoxidation is performed on the molten steel tapped from the converter. After deoxidation, slag removal is performed. After slag removal, secondary refining is performed. For example, combined refining is performed as the secondary refining process. For example, first, refining is performed using an LF (Ladle Furnace) or VAD (Vacuum Arc Degassing). Then, RH (Ruhrstahl-Hausen) vacuum degassing is performed. In the refining process using the LF or VAD and the RH vacuum degassing process, the composition of elements other than Bi is adjusted while the molten steel is stirred. After the RH vacuum degassing process, Bi is added using a wire, and the molten steel is then stirred to adjust the Bi content. The secondary refining process satisfies the following conditions: (Condition 1) The time t0 from when Bi is added to the molten steel until stirring is completed in the secondary refining step is set to more than 5 minutes and less than 50 minutes. (Condition 2) The stirring power density ε of the molten steel after adding Bi to the molten steel is set to 10 to 100 W / t, where the stirring power density ε (W / t) is defined by the following formula (A): ε=0.0285×Q×T / W×LOG(1+513.5×Z / V1) (A) Here, the flow rate (NL / min) of gas injected into the ladle containing molten steel is substituted for Q in equation (A). The temperature of the molten steel (K) is substituted for T. The mass of the molten steel (t) is substituted for W. The depth of the molten steel in the ladle (m) is substituted for Z. The degree of vacuum (torr) in the atmosphere containing the molten steel being stirred is substituted for V1. Conditions 1 and 2 will be explained below.

[0103] [Condition 1: Time t0] In the secondary refining step, the time from adding Bi to the molten steel until the end of stirring in the secondary refining step is more than 5 minutes and less than 50 minutes. If the time from adding Bi until the end of stirring in the secondary refining step is more than 5 minutes and less than 50 minutes, Bi is sufficiently diffused in the molten steel. Therefore, during cooling in the casting step described below, Bi particles with an equivalent circle diameter of less than 5 μm (hereinafter also referred to as fine Bi particles) are sufficiently generated. The structure in the steel can be sufficiently refined during solidification. As a result, fine sulfides can be sufficiently generated. If fine Bi particles are sufficiently generated, specific oxides can also be sufficiently generated.

[0104] On the other hand, if the time from the addition of Bi until the end of stirring in the secondary refining process is 5 minutes or less, Bi does not diffuse sufficiently in the molten steel. As a result, an excessive number of coarse Bi particles are generated. If the time from the addition of Bi until the end of stirring in the secondary refining process is 50 minutes or more, fine Bi particles tend to aggregate. As a result, the number density of fine Bi particles decreases. As a result, the structure in the steel cannot be sufficiently refined during solidification. As a result, fine sulfides are not generated sufficiently. If the number density of fine Bi particles decreases, specific oxides cannot be generated sufficiently. Therefore, the time from the addition of Bi in the secondary refining process until the end of stirring in the secondary refining process is more than 5 minutes but less than 50 minutes.

[0105] The upper limit of the time from the addition of Bi to the end of stirring in the secondary refining step is preferably 40 minutes, more preferably 30 minutes, and the lower limit of the time from the addition of Bi to the end of stirring in the secondary refining step is preferably 10 minutes, more preferably 20 minutes.

[0106] After the addition of Bi, the temperature of the molten steel is 1510 to 1630°C until the stirring is completed in the secondary refining step.

[0107] [Condition 2: Stirring power density ε] The stirring power density ε of the molten steel after adding Bi to the molten steel is 10 to 100 W / t. If the stirring power density ε of the molten steel after adding Bi is 10 to 100 W / t, Bi is sufficiently diffused in the molten steel. Therefore, fine Bi particles are sufficiently generated during cooling in the casting process described below. The structure in the steel can be sufficiently refined during solidification. As a result, fine sulfides can be sufficiently generated. If fine Bi particles are sufficiently generated, specific oxides can also be sufficiently generated.

[0108] If the stirring power density ε of the molten steel after adding Bi to the molten steel is less than 10 W / t, Bi does not diffuse sufficiently in the molten steel. As a result, an excessive number of coarse Bi particles are generated. On the other hand, if the stirring power density ε of the molten steel after adding Bi to the molten steel exceeds 100 W / t, fine Bi particles tend to aggregate. As a result, the number density of fine Bi particles decreases. As a result, the structure in the steel cannot be sufficiently refined during solidification. As a result, fine sulfides are not generated sufficiently. If the number density of fine Bi particles decreases, specific oxides cannot be generated sufficiently.

[0109] [(Process 2) Casting process] In the casting process, molten steel is used to produce slabs (or blooms) or steel ingots by well-known casting methods, such as continuous casting and ingot casting.

[0110] [(Process 3) Hot processing process] 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 or steel ingot produced in the casting step to produce the steel material of this embodiment. The steel material of this embodiment is, for example, a steel bar. The hot working step may be, for example, hot rolling or hot forging. More specifically, when hot rolling is performed in the hot working step, for example, it may be only a rough rolling step, or it may include a rough rolling step and a finish rolling step. The rough rolling step is, for example, blooming. The finish rolling step is, for example, finish rolling using a continuous rolling mill. In the continuous rolling mill, for example, horizontal stands each having a pair of horizontal rolls and vertical stands each having a pair of vertical rolls are alternately arranged in a row. The heating temperature in the rough rolling step and the finish rolling step is, for example, 1000 to 1300°C.

[0111] The steel material may be manufactured by hot forging after hot rolling. When hot forging is performed in the hot working step, the heating temperature is 1000 to 1300°C.

[0112] The steel material of this embodiment is manufactured by the above manufacturing steps. As described above, the hot working step may be omitted from this manufacturing method. That is, the steel material of this embodiment may be a cast product (a slab or a steel ingot). Alternatively, the steel material of this embodiment may be manufactured by carrying out a hot working step.

[0113] [Mechanical manufacturing method for machine structural parts] As described above, the steel material of this embodiment is used as a material for machine structural parts. Examples of machine structural parts include parts for automobiles and construction vehicles. Examples of machine structural parts include crankshafts.

[0114] A machine structural part using the steel material of this embodiment as a raw material is manufactured, for example, by the following manufacturing method.

[0115] First, the steel material of this embodiment is hot worked to manufacture an intermediate product having a rough shape for a machine structural component. Hot working is, for example, hot forging. The manufactured intermediate product is cut into a predetermined shape by machining. The cut intermediate product is then induction hardened. After induction hardening, tempering is performed as necessary. If tempering is performed, finishing (cutting or grinding) is performed on the tempered intermediate product. If tempering is not performed, finishing is performed on the induction hardened intermediate product. Through the above steps, a machine structural component is manufactured.

[0116] In the steel material of this embodiment, the content of each element in the chemical composition is within the range of this embodiment, and formula (1) is satisfied. Furthermore, the number density of fine sulfides having an equivalent circle diameter of 0.2 to less than 1.0 μm is 20 pieces / mm 2 or more, and the number density of coarse Bi particles with a circle equivalent diameter of 5 μm or more is 2.00 particles / mm 2 When the specific oxide is defined as an oxide-based inclusion containing Bi and Ca, the specific oxide has a number density of 0.1 particles / mm 2 That's all. Therefore, when manufacturing machine structural parts using the steel material of this embodiment as a raw material, the occurrence of melting cracks is suppressed even when induction hardening is performed. Furthermore, when manufacturing machine structural parts using the steel material of this embodiment as a raw material, the machinability of the intermediate product before induction hardening and the intermediate product after induction hardening is high. Furthermore, machine structural parts manufactured using the steel material of this embodiment as a raw material have excellent fatigue strength. [Example]

[0117] 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.

[0118] Steel materials having the chemical compositions shown in Tables 1 to 4 were manufactured.

[0119] [Table 1]

[0120] [Table 2]

[0121] [Table 3]

[0122] [Table 4]

[0123] Specifically, a 70-ton converter was used to carry out the refining process (primary refining process and secondary refining process). In the primary refining process, molten pig iron produced by a known method was refining in a converter under the same conditions. After the primary refining process, the molten steel tapped from the converter was deoxidized. After deoxidation, a slag removal process was carried out. After the slag removal process, a LF refining process was carried out, followed by an RH vacuum degassing process. During the LF refining process and the RH vacuum degassing process, the components of elements other than Bi were adjusted while the molten steel was stirred. After the RH vacuum degassing process, Bi was added using a wire to adjust the Bi component. The time t0 (min) until the end of stirring after Bi addition was as shown in Tables 5 and 6. Furthermore, the stirring power density ε (W / t) during stirring was as shown in Tables 5 and 6. After the addition of Bi, the temperature of the molten steel was 1510 to 1630°C until the end of stirring in the secondary refining process.

[0124] [Table 5]

[0125] [Table 6]

[0126] A bloom was produced by continuous casting. After heating, the bloom was rolled into a billet. After heating the billet to 1250°C, it was hot forged into a steel bar with a diameter of 55 mm.

[0127] [Manufacturing intermediate parts for simulated mechanical structures] The manufactured steel was subjected to a heat treatment simulating the hot forging process used to manufacture machine structural parts from steel. Specifically, the steel was heated to 1100°C and held at that temperature for 30 minutes. The steel was then allowed to cool in the atmosphere, producing intermediate simulated machine structural parts. The intermediate simulated machine structural parts were steel bars with a diameter of 55 mm.

[0128] [Evaluation test] The steel material of each test number was subjected to number density measurements of fine sulfides, coarse Bi particles, and specific oxides. The intermediate products of the simulated machine structural parts of each test number were subjected to a fusion crack evaluation test, a machinability evaluation test, and a fatigue strength evaluation test.

[0129] [Measurement of the number density of fine sulfides, coarse Bi particles, and specific oxides] The number density of fine sulfides was measured by the following method. A sample was taken from the R / 2 position of a cross section (longitudinal cross section) parallel to the longitudinal direction of the steel material (steel bar) of each test number. Of the surfaces of the taken samples, the surface corresponding to the longitudinal cross section of the steel material was used as the observation surface. After mirror polishing the observation surface, the mirror-polished observation surface was observed at a magnification of 500 times using an SEM. The total observation area was 32 mm 2 It was decided.

[0130] Based on the backscattered electron images obtained by SEM observation, the number density of fine sulfides was investigated using a well-known image analysis particle analysis method. Specifically, the inclusions and precipitates in the steel were identified based on the interfaces between the steel matrix and the inclusions and precipitates. Image analysis was performed to calculate the circle equivalent diameters of the inclusions and precipitates.

[0131] The components of the obtained inclusions and precipitates with a circle equivalent diameter of 0.2 to less than 1.0 μm were analyzed using an EDX equipped in an SEM. The beam diameter of the EDX used for the component analysis was appropriately adjusted so that the components of the inclusions and precipitates to be analyzed could be analyzed. In the EDX elemental analysis results, if the S content was 3% or more by mass and Mn was also contained, the inclusion was defined as a fine sulfide. The acceleration voltage during EDX analysis was 20 kV.

[0132] Fine sulfides were identified using the above method. A total of 32 mm 2 Based on the total number of fine sulfides identified in the observation area, the number of fine sulfides per unit area (pieces / mm 2 The number density of the fine sulfides obtained is shown in Tables 5 and 6 as "Number density of fine sulfides (pieces / mm 2 ) column.

[0133] The number densities of coarse Bi particles and specific oxides were measured in the same manner as in the measurement of the number density of fine sulfides. The components of inclusions and precipitates with a circle equivalent diameter of 5 μm or more obtained by SEM observation were analyzed using EDX equipped in the SEM. The beam diameter of the EDX used for component analysis was adjusted appropriately so that the components of the inclusions and precipitates to be analyzed could be analyzed. In the results of EDX elemental analysis, if the Bi content was 70% or more by mass, the inclusion was defined as a coarse Bi particle. Coarse Bi particles were identified using the above method. A total of 32 mm 2 Based on the total number of coarse Bi particles identified in the observation area, the number of coarse Bi particles per unit area (particles / mm 2 The number density of the obtained coarse Bi particles is shown in Tables 5 and 6 as "Number density of coarse Bi particles (particles / mm 2 ) column.

[0134] The components of inclusions and precipitates with a circle equivalent diameter of 1 μm or more obtained by SEM observation were analyzed using EDX equipped in the SEM. The beam diameter of the EDX used for component analysis was adjusted appropriately so that the components of the inclusions and precipitates to be analyzed could be analyzed. In the results of elemental analysis by EDX, if the oxygen (O) content was 3% or more by mass, the Ca content was 1% or more by mass, and the Bi content was more than 0 and less than 70% by mass, the inclusion was defined as a specific oxide. The specific oxide was identified by the above method. A total of 32 mm 2 Based on the total number of specific oxides identified in the observation area, the number of specific oxides per unit area (pieces / mm 2 The number density of the specific oxides obtained is shown in Tables 5 and 6 as "Number density of specific oxides (pieces / mm 2 ) column.

[0135] [Lapse crack evaluation test] A test piece measuring 10 mm in width, 3 mm in thickness, and 10 mm in length was machined from the R / 2 position of the cross section perpendicular to the longitudinal direction of the intermediate simulated mechanical structural component of each test number. The longitudinal direction of the test piece was parallel to the longitudinal direction of the intermediate simulated mechanical structural component. The central axis of the test piece, which was parallel to the longitudinal direction, also coincided with the R / 2 position.

[0136] A simulated induction hardening test was performed on the test specimen using a test device (product name "Thermal Cycle Test Device") manufactured by Fuji Electric Industrial Co., Ltd. Specifically, the test specimen was heated to 1350°C at a heating rate of 100°C / sec using a high-frequency coil. The test specimen was then held at 1350°C for 15 seconds. Thereafter, the test specimen was water-cooled.

[0137] After water cooling, the cross section (observation surface) perpendicular to the longitudinal direction of the test piece was mechanically polished. The mechanically polished observation surface was corroded with picral reagent. The corroded observation surface was observed under an optical microscope at 400x magnification, and the presence or absence of melt cracks was visually confirmed. The observation surface was 250 μm × 400 μm.

[0138] When a clearly corroded area (corroded area) with a width of 5 μm or more was observed at the grain boundary of the structure of the observed surface, it was determined that a fusion crack had occurred. An area with a clearly corroded area with a width of 5 μm or more at the grain boundary means, for example, an area such as fusion crack 10 in Figure 2. On the other hand, when no corroded area was observed at the grain boundary, as in Figure 3, it was determined that a fusion crack had not occurred. The evaluation results of fusion cracks are shown in the "fusion cracks" column in Tables 5 and 6. Cases where no fusion cracks occurred were marked with "Good", and cases where fusion cracks occurred were marked with "Poor".

[0139] [Machinability evaluation test] The following two types of tests were conducted to evaluate machinability: A drill life test was conducted to evaluate the machinability of intermediate products before induction hardening. A tool wear suppression evaluation test was conducted to evaluate the machinability of intermediate products after induction hardening.

[0140] [Machinability evaluation of intermediate products before induction hardening (drill life test)] Machinability evaluation test pieces were cut from the intermediate simulated machine structural components of each test number. Specifically, a drill hole was drilled 14 mm from the outer surface of a cross section perpendicular to the longitudinal direction of the intermediate simulated machine structural components, 55 mm in diameter. Fujikoshi Corporation SD3.0 drill was used as the tool, with a feed rate of 0.25 mm / rev and a drilling depth of 9 mm per hole. Water-soluble cutting oil was used as the lubricant. The drill was drilled under the above conditions, and the machinability of the steel was evaluated. The maximum cutting speed VL1000 (m / min) was used as the evaluation index. The maximum cutting speed VL1000 is the cutting speed of a drill capable of drilling a 1000 mm long hole. A maximum cutting speed VL1000 of 15 m / min or greater was considered to have high machinability. A maximum cutting speed VL1000 of less than 15 m / min was considered to have low machinability. The results of the machinability evaluation are shown in the "Before induction hardening" column of the "Machinability" column in Tables 5 and 6. High machinability was marked with "Good", and low machinability was marked with "Poor".

[0141] [Evaluation of machinability of intermediate products after induction hardening (test to evaluate tool wear prevention)] A tool wear suppression evaluation test was conducted as follows. A 55 mm diameter simulated intermediate machine structural component was machined to produce a round bar test specimen. The round bar test specimen was cylindrical, 35 mm in diameter, and 300 mm long. A simulated induction hardening test was conducted on the round bar test specimen. Specifically, the round bar test specimen was heated at a frequency of 100 kHz for 2.0 seconds. A 5-15% diluted aqueous quenching coolant was then sprayed onto the round bar test specimen to cool it down. After cooling, the round bar test specimen was tempered to produce a test specimen for machinability evaluation simulating the intermediate component after induction hardening. The tempering temperature was 150°C, and the holding time at the tempering temperature was 2 hours.

[0142] Machinability evaluation test pieces (hereinafter referred to as test pieces), which simulated intermediate products after induction hardening, were turned using a general-purpose lathe. Specifically, turning was performed on the test pieces with each test number under the following conditions. The cutting tool used was a CBN sintered tool. The CBN sintered tool had a TiAlN-based ceramic coating on the surface of a sintered material whose main component was CBN particles and whose binder was ceramic. The cutting speed was 150 m / min, the feed rate was 0.4 mm / rev, and the depth of cut was 0.1 mm. Turning was performed wet using water-soluble cutting oil.

[0143] Under the above-mentioned turning conditions, one test piece was subjected to one pass of cutting. Turning was repeated for multiple test pieces until the total cutting time reached 10 minutes. After that, the flank wear (μm) of the cutting tool was measured.

[0144] When the obtained flank wear amount was 40 μm or less, it was determined that tool wear was sufficiently suppressed. When the obtained flank wear amount was more than 40 μm, it was determined that tool wear was not sufficiently suppressed. The results of the evaluation of tool wear suppression are shown in the "After induction hardening" column of the "Machinability" column in Tables 5 and 6. When tool wear was sufficiently suppressed and machinability was high, it was marked with "Good", and when tool wear was not sufficiently suppressed and machinability was low, it was marked with "Poor".

[0145] [Fatigue strength evaluation test (rotating bending fatigue test)] Rotating bending fatigue test specimens were taken from the manufactured intermediate products of the simulated mechanical structural components. Figure 4 is a schematic diagram of the rotating bending fatigue test specimens taken from each intermediate product of the simulated mechanical structural components. The diameter of the parallel part of the rotating bending fatigue test specimen was 8 mm, and the diameter of the gripping part was 12 mm. The intermediate products of the simulated mechanical structural components were machined to produce the rotating bending fatigue test specimens. The rotating bending fatigue test specimens were taken from the R / 2 position of the cross section perpendicular to the longitudinal direction of the intermediate product of the simulated mechanical structural component. The longitudinal direction of the rotating bending fatigue test specimen was parallel to the longitudinal direction of the intermediate product of the simulated mechanical structural component. The central axis parallel to the longitudinal direction of the rotating bending fatigue test specimen corresponded to the R / 2 position of the intermediate product of the simulated mechanical structural component. It is common technical knowledge known to those skilled in the art that if the rotating bending fatigue strength of a test specimen taken from an intermediate product of a simulated mechanical structural part before induction hardening is sufficiently high, the rotating bending fatigue strength of a test specimen taken from the intermediate product of a simulated mechanical structural part after induction hardening will also be sufficiently high.

[0146] The parallel part of the rotating bending 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 within 3.0 μm, and the maximum height (Rmax) was set to within 9.0 μm.

[0147] Using the rotary bending fatigue test specimens that had been subjected to finish polishing, an Ono-type rotary bending fatigue test was conducted at room temperature (23°C) in an air atmosphere at a rotation speed of 3600 rpm, with alternating swings. Fatigue tests were conducted on multiple test specimens with different applied stresses, and the fatigue test results were 10 7 The highest stress that did not cause fracture after cycling was taken as the fatigue strength (MPa).

[0148] If the fatigue strength obtained was 300 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 of Tables 5 and 6. A fatigue strength of 300 MPa or more was marked "Good", and a fatigue strength of less than 300 MPa was marked "Poor".

[0149] [Test Results] The test results are shown in Tables 5 and 6. Referring to Tables 1 to 6, the steels with test numbers 1 to 47 had appropriate chemical compositions, satisfied formula (1), and the time until the end of stirring after adding Bi was also appropriate. Therefore, the number density of fine sulfides in the steels with each test number was 20 particles / mm 2 The number density of coarse Bi particles is 2.00 particles / mm 2 The number density of the specific oxide is 0.1 pieces / mm or less. 2 As a result, the occurrence of fusion cracks was sufficiently suppressed. In a machinability evaluation test on the intermediate product before induction hardening, the maximum cutting speed VL1000 was 15 m / min or more, and the machinability of the intermediate product before induction hardening was high. Furthermore, in a machinability evaluation test on the intermediate product after induction hardening, the amount of flank wear was 40 μm or less. In other words, tool wear was sufficiently suppressed, and the machinability of the intermediate product after induction hardening was high. Furthermore, the fatigue strength was 300 MPa or more, and was high.

[0150] On the other hand, in test number 48, the C content was too high, which caused melting cracks.

[0151] In test number 49, the C content was too low, which resulted in low fatigue strength.

[0152] In test number 50, the Si content was too high, which caused melting cracks.

[0153] In test number 51, the Si content was too low, which resulted in low fatigue strength.

[0154] Test No. 52 had too high a Mn content, which resulted in fusion cracking. Furthermore, the machinability of the intermediate product before induction hardening was poor.

[0155] In test number 53, the Mn content was too low, which resulted in fusion cracking and low fatigue strength.

[0156] In test number 54, the P content was too high, which caused melting cracks.

[0157] In test number 55, the S content was too high, which caused melting cracks.

[0158] In test number 56, the S content was too low. Therefore, the number density of fine sulfides was low. As a result, fusion cracks occurred. Furthermore, the machinability of the intermediate product before and after induction hardening was poor.

[0159] Test No. 57 had too high a Cr content, which resulted in fusion cracking. Furthermore, the machinability of the intermediate product before induction hardening was poor.

[0160] In test number 58, the Bi content was too high. Therefore, the number density of coarse Bi particles was too high. As a result, the machinability of the intermediate product after induction hardening was poor.

[0161] In test number 59, the Bi content was too low. As a result, the number density of fine sulfides was low. Furthermore, the number density of specific oxides was also low. As a result, fusion cracks occurred. Furthermore, the machinability of the intermediate product before and after induction hardening was poor.

[0162] In test number 60, the Ca content was too high, and as a result, the machinability of the intermediate product before induction hardening and the intermediate product after induction hardening was low.

[0163] In test number 61, the Ca content was too low, so the density of specific oxides was 0.1 particles / mm 2 As a result, the machinability of the intermediate product after induction hardening was low.

[0164] In test number 62, the Al content was too high, so the density of specific oxides was 0.1 particles / mm 2 As a result, the machinability of the intermediate product after induction hardening was low.

[0165] In test number 63, the oxygen (O) content was too high, which resulted in low fatigue strength.

[0166] In test numbers 64 and 65, fn1 was too low. In other words, formula (1) was not satisfied. Therefore, the number density of the specific oxide was 0.1 particles / mm 2 As a result, the machinability of the intermediate product after induction hardening was low.

[0167] In test numbers 66 and 67, the time t0 (min) from the addition of Bi to the end of stirring in the refining process was too short. As a result, the number density of coarse Bi particles was 2.00 particles / mm 2 As a result, the machinability of the intermediate product after induction hardening was low.

[0168] In test numbers 68 and 69, the time t0 (min) from the addition of Bi to the end of stirring in the refining process was too long. As a result, the number density of fine sulfides was 20 / mm 2 The density of specific oxides is less than 0.1 particles / mm 2 As a result, fusion cracks occurred. Furthermore, the machinability of the intermediate product after induction hardening was low.

[0169] In test numbers 70 and 71, the power stirring density ε was too small in the refining process. As a result, the number density of coarse Bi particles was 2.00 particles / mm 2 As a result, the machinability of the intermediate product after induction hardening was low.

[0170] In test numbers 72 and 73, the power stirring density ε was too large in the refining process. Therefore, the number density of fine sulfides was 20 particles / mm 2 The density of specific oxides is less than 0.1 particles / mm 2 As a result, fusion cracks occurred. Furthermore, the machinability of the intermediate product after induction hardening was low.

[0171] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and can be implemented by appropriately modifying the above-described embodiments within the scope of the present disclosure. [Explanation of symbols]

[0172] 1 Fillet R part 2. Crankshaft edge 10 Melting cracks

Claims

1. A steel material, In mass%, C: more than 0.30 to 0.60%, Si: 0.01 to 0.55%, Mn: 0.50-1.65%, P: 0.050% or less, S: 0.010-0.200%, Bi: 0.0001 to 0.0050%, Ca: 0.0001-0.0050%, Al: 0.001-0.005%, N: 0.0030 to 0.0250%, and O: 0.0030% or less, The balance is Fe and impurities. On the premise that the content of each element is within the above range, formula (1) is satisfied, In the steel material, The number density of fine sulfides with a circle equivalent diameter of 0.2 to less than 1.0 μm is 20 pieces / mm ２ That's all, The number density of coarse Bi particles with a circle equivalent diameter of 5 μm or more is 2.00 particles / mm ２ is as follows: When oxide-based inclusions containing Bi and Ca and having an equivalent circle diameter of 1 μm or more are defined as specific oxides, the number density of the specific oxides is 0.1 particles / mm ２ That's all. Steel material. Ca / Al≧0.18 (1) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in mass %.

2. A steel material, In mass%, C: more than 0.30 to 0.60%, Si: 0.01 to 0.55%, Mn: 0.50-1.65%, P: 0.050% or less, S: 0.010-0.200%, Bi: 0.0001 to 0.0050%, Ca: 0.0001-0.0050%, Al: 0.001-0.005%, N: 0.0030 to 0.0250%, and O: 0.0030% or less, Further, it contains one or more selected from the group consisting of Groups 1 to 4, The balance is Fe and impurities. On the premise that the content of each element is within the above range, formula (1) is satisfied, In the steel material, The number density of fine sulfides with a circle equivalent diameter of 0.2 to less than 1.0 μm is 20 pieces / mm ２ That's all, The number density of coarse Bi particles with a circle equivalent diameter of 5 μm or more is 2.00 particles / mm ２ is as follows: When oxide-based inclusions containing Bi and Ca and having an equivalent circle diameter of 1 μm or more are defined as specific oxides, the number density of the specific oxides is 0.1 particles / mm ２ That's all. Steel material. [Group 1] V: 0.400% or less, Ti: 0.050% or less, Nb: 0.050% or less, W: 0.400% or less, and Zr: 0.0100% or less, one or more selected from the group consisting of [Group 2] Mg: 0.0100% or less Te: 0.0100% or less B: 0.0050% or less Sn: 0.0100% or less, and Rare earth elements: 0.0100% or less, one or more selected from the group consisting of [Group 3] Co: 0.0100% or less, Se: 0.0100% or less, and Sb: 0.0100% or less, one or more selected from the group consisting of [Group 4] Cr: 0.30% or less, Mo: 0.30% or less, Cu: 0.50% or less, and Ni: 0.50% or less, one or more selected from the group consisting of Ca / Al≧0.18 (1) Here, each element symbol in formula (1) is substituted with the content of the corresponding element in mass %.

3. The steel material according to claim 2, containing the first group, Steel material.

4. The steel material according to claim 2, containing the second group, Steel material.

5. The steel material according to claim 2, containing the third group, Steel material.

6. The steel material according to claim 2, containing the fourth group Steel material.

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