Machine structural component

Abstract

To provide a machine structural component having an excellent rolling fatigue life under a hydrogen generating environment.SOLUTION: A machine structural component according to the present embodiment includes a quench hardened layer formed at least in a surface layer, has a chemical composition described in the description, satisfies Formulae (1) to (3), has a number concentration of MC-type carbides in the surface layer of 5.0 pieces / μ 100nm or more, the MC-type carbides having equivalent circle diameters of 20 to m2 and having a Mo concentration of 5% or more in atom%, and has a prior austenite grain size in the surface layer of 6.0 μm or less. (Si + Ni) / Cr ≥ 1.30 (1) Mo + V ≥ 0.30 (2) 1.70 ≤ 2.5 * Mn + Cr ≤ 3.90 (3) Here, a content (mass%) of a corresponding element is substituted for each symbol of an element in Formula (1) to Formula (3).SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a machine structural component including a hardened surface layer. [Background technology]

[0002] Mining machinery, construction machinery, automobiles, and the like use machine structural parts made of steel. Examples of machine structural parts include bearing parts. The steel material used for machine structural parts such as bearing parts is exemplified by SUJ2, as specified in JIS G 4805 (2019). Machine structural parts are typically manufactured using steel as the raw material through the following manufacturing process: Hot forging is performed on the raw steel material, and cutting is further performed as necessary to produce an intermediate product with a desired shape. Heat treatment is performed on the intermediate product to adjust the hardness and microstructure of the steel. Examples of heat treatment include quenching and tempering, carburizing, or carbonitriding. Machine structural parts are manufactured through the above manufacturing process.

[0003] To increase the fatigue life of a machine structural component manufactured using steel with a high C content of 0.75% or more, quenching and tempering may be performed as a heat treatment during the manufacturing process of the machine structural component. The quenching and tempering process hardens at least the surface layer of the machine structural component, thereby improving the fatigue life of the machine structural component.

[0004] A technique for increasing the fatigue life of machine structural parts manufactured by carrying out such quenching and tempering treatment is proposed in Japanese Patent Laid-Open Publication No. 2012-132094 (Patent Document 1).

[0005] The bearing material disclosed in Patent Document 1 contains, in mass%, C: 0.80 to 1.20%, Si: 0.15 to 0.70%, Mn: 0.80% or less, Cr: 0.50 to 2.00%, P: 0.020% or less, S: 0.0020% or less, Al: 0.005 to 0.025%, O: 0.0007% or less, and N: 0.0040% or less, with the balance being Fe and unavoidable impurities. 2 If so, (length x width)1 / 2 The total number of oxide-based non-metallic inclusions and sulfide-containing oxide-based non-metallic inclusions with an average diameter of 3 μm or more calculated by 2 The total number of oxide-based non-metallic inclusions and sulfide-containing oxide-based non-metallic inclusions with an average diameter of 10 μm or more is 100 or less per 1000 mm 2 The number of particles per steel sheet is two or less, and further, 90% or more of the oxide-based non-metallic inclusions and sulfide-containing oxide-based non-metallic inclusions having an average diameter of 3 μm or more have a magnesium oxide concentration of 5% by mass or less. Patent Document 1 describes that an excellent rolling contact fatigue life can be obtained by controlling the chemical composition of the steel material and the shape, number, and composition of the oxide-based non-metallic inclusions and sulfide-containing oxide-based non-metallic inclusions as described above. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-132094 Summary of the Invention [Problem to be solved by the invention]

[0007] Incidentally, among machine structural parts, there are machine structural parts that are used in an environment where lubricating oil circulates, such as bearing parts applied to drive parts such as transmissions.

[0008] Recently, in order to improve fuel efficiency, the viscosity of lubricating oil has been reduced to reduce frictional resistance and transmission resistance, and the amount of circulating lubricating oil used has been reduced. As a result, in the operating environment of mechanical structural parts, such as bearing parts, lubricating oil is more likely to decompose during use, generating hydrogen. When hydrogen is generated in the operating environment, it penetrates into the mechanical structural parts from the outside. The penetrated hydrogen causes structural changes in part of the microstructure of the mechanical structural parts. The structural changes during use of the mechanical structural parts cause cracks (peeling) in the parts near the surface of the mechanical structural parts, reducing the rolling fatigue life of the mechanical structural parts. Hereinafter, in this specification, the environment in which hydrogen, which causes structural changes, is generated, is referred to as the "hydrogen-generating environment." Mechanical structural parts used in a hydrogen-generating environment are required to have an excellent rolling fatigue life in the hydrogen-generating environment.

[0009] Patent Document 1 does not consider the rolling fatigue life of machine structural parts manufactured from steel materials in a hydrogen generating environment.

[0010] An object of the present disclosure is to provide a machine structural component that can achieve an excellent rolling fatigue life in a hydrogen generating environment. [Means for solving the problem]

[0011] The machine structural component according to the present disclosure comprises: When a region from the surface of the machine structural component to a depth of 200 μm is defined as a surface layer, the component has at least a quench-hardened layer formed on the surface layer, The chemical composition of the machine structural part is, in mass%, C: 0.75~1.20%, Si: 0.05 to 0.80% Mn: 0.50 to 1.50% P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50% Mo: 0.06 to 0.35%, V: 0.09~0.40%, Ni: over 0.30 to 1.50% Al: 0.005 to 0.100%, N: 0.030% or less, and O: 0.0015% or less, the balance being Fe and impurities, Formulas (1) to (3) are satisfied, In the surface layer, the number density of MC type carbides having a circle equivalent diameter of 20 to 100 nm and an Mo concentration of 5% or more in atomic % is 5.0 particles / μm 2 That's all, The prior austenite grain size in the surface layer is 6.0 μm or less. (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

[0012] The machine structural component according to the present disclosure comprises: When a region from the surface of the machine structural component to a depth of 200 μm is defined as a surface layer, the component has at least a quench-hardened layer formed on the surface layer, The chemical composition of the machine structural part is, in mass%, C: 0.75~1.20%, Si: 0.05 to 0.80% Mn: 0.50 to 1.50% P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50% Mo: 0.06 to 0.35%, V: 0.09~0.40%, Ni: over 0.30 to 1.50% Al: 0.005 to 0.100%, N: 0.030% or less, and O: 0.0015% or less, Further, it contains one or more selected from the group consisting of Groups 1 to 4, the balance being Fe and impurities, Formulas (1) to (3) are satisfied, In the surface layer, the number density of MC type carbides having a circle equivalent diameter of 20 to 100 nm and an Mo concentration of 5% or more in atomic % is 5.0 particles / μm 2 That's all, The prior austenite grain size in the surface layer is 6.0 μm or less. [Group 1] Ti: 0.050% or less, and Nb: 0.050% or less, one or more selected from the group consisting of [Group 2] B: 0.0050% or less, and Cu: 1.50% or less, one or more selected from the group consisting of [Group 3] Sn: 0.100% or less [Group 4] Ca: 0.0050% or less, and Mg: 0.0050% or less, one or more selected from the group consisting of (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol. [Effects of the Invention]

[0013] The machine structural component according to the present disclosure can achieve an excellent rolling fatigue life in a hydrogen generating environment. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a side view of a test piece (small roller test piece) for a roller pitting fatigue test manufactured as a part for simulating a machine structure. [Figure 2] Figure 2 is a schematic diagram of the roller pitting fatigue test. [Figure 3] FIG. 3 is a front view of the large roller test piece in FIG. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present inventors have conducted research into machine structural parts that can achieve excellent rolling fatigue life in a hydrogen generating environment.

[0016] The rolling contact fatigue life in a hydrogen generating environment is reduced by the occurrence of hydrogen-induced spalling on the surface of a machine structural component. Therefore, the inventors investigated the causes of hydrogen-induced spalling in a hydrogen generating environment.

[0017] In a hydrogen-generating environment, spalling on the surface of a machine structural component is thought to occur through the following mechanism: As described above, when hydrogen is generated in a hydrogen-generating environment, the generated hydrogen penetrates into the machine structural component. When the penetrated hydrogen aggregates on the surface layer of the machine structural component, it promotes cracking along the prior austenite grain boundaries. When prior austenite grain boundary cracking occurs, a structural change occurs starting from the crack, and white structure is formed. As a result, spalling is caused on the surface along the white structure. In this way, the rolling fatigue life of the machine structural component is reduced.

[0018] As described above, hydrogen-induced spalling in a hydrogen-generating environment occurs when hydrogen penetrates into mechanical structural components (Cause 1), and the penetrated hydrogen agglomerates (Cause 2), causing prior austenite grain boundary cracking (Cause 3).

[0019] First, the inventors focused on factor 1 and investigated means for suppressing hydrogen penetration. If hydrogen penetration into a mechanical structural component can be suppressed, hydrogen will not aggregate within the mechanical structural component. Therefore, the inventors investigated means for suppressing hydrogen penetration in a hydrogen-generating environment from the perspective of the chemical composition of the mechanical structural component. As a result, the inventors obtained the following findings.

[0020] In a hydrogen generating environment, Si and Ni are elements that suppress the penetration of hydrogen into mechanical structural components. On the other hand, Cr is an element that promotes the penetration of hydrogen into mechanical structural components. Therefore, by ensuring a certain amount of Si content and Ni content in the chemical composition and keeping the Cr content as low as possible, it is possible to suppress the penetration of hydrogen in a hydrogen generating environment.

[0021] Based on the above findings, the present inventors have investigated the chemical composition of machine structural parts. As a result, it was found that a machine structural part having a chemical composition, in mass%, of C: 0.75 to 1.20%, Si: 0.05 to 0.80%, Mn: 0.50 to 1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50%, Mo: 0.06 to 0.35%, V: 0.09 to 0.40%, Ni: more than 0.30% to 1.50%, Al: 0.005 to 0.100%, N: 0.030% or less, and O: 0.0015% or less, and if any optional element is contained, further containing one or more elements selected from the group consisting of the above-mentioned Groups 1 to 4 in place of a portion of Fe, with the remainder consisting of Fe and impurities, and further satisfying formula (1), has an increased rolling contact fatigue life in a hydrogen generating environment. (Si+Ni) / Cr≧1.30 (1)

[0022] In order to further increase the rolling fatigue life in a hydrogen generation environment, the present inventors have also investigated suppression of factors 2 and 3. As a result, the present inventors have further discovered the following.

[0023] As described above, in order to suppress the aggregation of the penetrated hydrogen (factor 2), it is effective to disperse fine precipitates and trap hydrogen. In this specification, fine precipitates refer to precipitates with an equivalent circle diameter of 20 to 100 nm. Here, Mo and V combine with C to precipitate MC-type carbides. MC-type carbides are finer than other precipitates such as cementite, and have a high ability to trap hydrogen that has penetrated into machine structural components. To increase the amount of MC-type carbides formed in the surface layer, it is preferable to increase the total amount of Mo and V. Therefore, the present inventors investigated the relationship between the Mo and V contents in the chemical composition and the rolling contact fatigue life in a hydrogen generation environment. As a result, it was found that if the chemical composition further satisfies formula (2), the rolling contact fatigue life in a hydrogen generation environment can be further extended. Mo+V≧0.30 (2)

[0024] Mechanical structural components have a quench-hardened layer at least on the surface. The quench-hardened layer is a layer hardened by quenching and is mainly composed of martensite. However, the quench-hardened layer of a mechanical structural component contains not only martensite but also a certain amount of retained austenite. The retained austenite, like precipitates, traps hydrogen that has penetrated into the mechanical structural component. Therefore, if an appropriate amount of retained austenite is contained in the quench-hardened layer, the aggregation of hydrogen that has penetrated into the mechanical structural component can be further suppressed.

[0025] On the other hand, retained austenite may undergo stress-induced martensite transformation due to external forces during use of machine structural components. If hydrogen is present nearby during stress-induced martensite transformation, excessive hydrogen will aggregate at the interface between the retained austenite and stress-induced martensite. This promotes the formation of white structure at the interface between the retained austenite and stress-induced martensite. As a result, the rolling contact fatigue life in a hydrogen-generating environment will actually decrease.

[0026] The amount of retained austenite contained in the quench-hardened layer of a machine structural component can be adjusted by the Mn and Cr contents in the chemical composition of the machine structural component. Therefore, the inventors investigated the relationship between the Mn and Cr contents in the chemical composition and the rolling fatigue life in a hydrogen generating environment. As a result, it was found that if the chemical composition further satisfies formula (3), the rolling fatigue life in a hydrogen generating environment can be further extended. 1.70≦2.5×Mn+Cr≦3.90 (3)

[0027] Prior austenite grain boundary cracking (factor 3) can be suppressed more easily as the prior austenite grains become finer. Therefore, by reducing the prior austenite grain size in the surface layer, spalling on the surface of machine structural components can be suppressed. As a result, the rolling fatigue life in a hydrogen generating environment is improved.

[0028] Induction hardening, for example, is an effective way to refine the prior austenite grains in the surface layer of a machine structural component. In particular, machine structural components whose chemical composition satisfies the above-mentioned formula (2) have fine MC carbides dispersed in the surface layer. Therefore, it seems that induction hardening can be expected to sufficiently refine the prior austenite grains through the pinning effect. However, even when induction hardening is performed on machine structural components having the above-mentioned chemical composition, there have been cases in which excellent rolling contact fatigue life cannot be achieved in a hydrogen-generating environment. The present inventors observed the microstructure of the surface layer of machine structural components that did not achieve excellent rolling contact fatigue life and found that the fine MC carbides dissolved during induction hardening, preventing a sufficient pinning effect. As a result, the prior austenite grains in the surface layer were not sufficiently refined.

[0029] The present inventors conducted research and investigation focusing on the concentration of each element contained in fine MC carbides in order to suppress dissolution of fine MC carbides during induction hardening. As a result, it was found that if the Mo concentration in atomic % in MC carbides is 5% or more, dissolution during induction hardening can be suppressed even if the particle size is fine (20 to 100 nm).

[0030] Therefore, the inventors further investigated the relationship between the number density of fine MC carbides with an atomic % Mo concentration of 5% or more, the prior austenite grain size in the surface layer, and the rolling contact fatigue life in a hydrogen generating environment. As a result, it was found that in the surface layer of a machine structural part having the above-mentioned chemical composition, the number density of MC carbides with an equivalent circle diameter of 20 to 100 nm and an atomic % Mo concentration of 5% or more was 5.0 particles / μm 2 It was found that, if the above conditions are met, it is possible to achieve both the effect of trapping intruded hydrogen by MC-type carbides and the pinning effect of induction hardening. In other words, in this case, even for machine structural parts with the above-mentioned chemical composition, the prior austenite grain size in the surface layer can be adjusted to 6.0 μm or less by induction hardening. As a result, all of the above-mentioned factors 1 to 3 are suppressed, and an excellent rolling contact fatigue life is obtained in a hydrogen-generating environment.

[0031] The machine structural component of this embodiment has been completed based on the above technical concept, and has the following configuration.

[0032] The machine structural component of the first configuration comprises: When a region from the surface of the machine structural component to a depth of 200 μm is defined as a surface layer, the component has at least a quench-hardened layer formed on the surface layer, The chemical composition of the machine structural part is, in mass%, C: 0.75~1.20%, Si: 0.05 to 0.80% Mn: 0.50 to 1.50% P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50% Mo: 0.06 to 0.35%, V: 0.09~0.40%, Ni: over 0.30 to 1.50% Al: 0.005 to 0.100%, N: 0.030% or less, and O: 0.0015% or less, the balance being Fe and impurities, Formulas (1) to (3) are satisfied, In the surface layer, the number density of MC type carbides having a circle equivalent diameter of 20 to 100 nm and an Mo concentration of 5% or more in atomic % is 5.0 particles / μm 2 That's all, The prior austenite grain size in the surface layer is 6.0 μm or less. (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

[0033] The second configuration of the machine structural component is When a region from the surface of the machine structural component to a depth of 200 μm is defined as a surface layer, the component has at least a quench-hardened layer formed on the surface layer, The chemical composition of the machine structural part is, in mass%, C: 0.75~1.20%, Si: 0.05 to 0.80% Mn: 0.50 to 1.50% P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50% Mo: 0.06 to 0.35%, V: 0.09~0.40%, Ni: over 0.30 to 1.50% Al: 0.005 to 0.100%, N: 0.030% or less, and O: 0.0015% or less, Further, it contains one or more selected from the group consisting of Groups 1 to 4, the balance being Fe and impurities, Formulas (1) to (3) are satisfied, In the surface layer, the number density of MC type carbides having a circle equivalent diameter of 20 to 100 nm and an Mo concentration of 5% or more in atomic % is 5.0 particles / μm 2 That's all, The prior austenite grain size in the surface layer is 6.0 μm or less. [Group 1] Ti: 0.050% or less, and Nb: 0.050% or less, one or more selected from the group consisting of [Group 2] B: 0.0050% or less, and Cu: 1.50% or less, one or more selected from the group consisting of [Group 3] Sn: 0.100% or less [Group 4] Ca: 0.0050% or less, and Mg: 0.0050% or less, one or more selected from the group consisting of (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

[0034] The third configuration of mechanical structural parts is: A machine structural component having a second configuration, The chemical composition of the mechanical structural part contains the first group.

[0035] The fourth configuration of mechanical structural parts is: A machine structural part having the second or third configuration, The chemical composition of the mechanical structural part contains the second group.

[0036] The fifth component of the mechanical structural parts is: A machine structural component having any one of the second to fourth configurations, The chemical composition of the machine structural part contains the third group.

[0037] The sixth component of the mechanical structural parts is: A machine structural part having any one of the second to fifth configurations, The chemical composition of the machine structural part contains the fourth group.

[0038] The machine structural component of this embodiment will be described in detail below. Note that "%" for elements means mass % unless otherwise specified.

[0039] [Configuration of the machine structural component of this embodiment] The mechanical structural component of this embodiment refers to a component manufactured by performing quenching and tempering. In this specification, the surface layer is defined as the region from the surface of the mechanical structural component to a depth of 200 μm. The mechanical structural component of this embodiment has a quench-hardened layer formed at least on the surface layer.

[0040] The quench-hardened layer is a layer hardened by quenching and is mainly composed of martensite. The quench-hardened layer has a Vickers hardness of 600 HV or more according to JIS Z 2244-1 (2020). A mechanical structural part is determined to have a quench-hardened layer when the Vickers hardness at a depth of 200 μm from the surface of the part is 600 HV or more.

[0041] [Vickers hardness measurement method] The Vickers hardness at a depth of 200 μm from the surface of a machine structural part can be measured by the following method. A test piece is taken with a cross section perpendicular to any surface of the machine structural component as the observation surface. The size of the observation surface is not particularly limited as long as it includes a position 200 μm deep from any surface of the machine structural component. The observation surface is surface polished. After mirror polishing, the Vickers hardness is measured at three arbitrary points 200 μm deep from the surface using a Vickers hardness test in accordance with JIS Z 2244-1 (2020). The test force is 9.8 N. The arithmetic mean of the Vickers hardness values ​​obtained at the three points is calculated. If the arithmetic mean value is 600 HV or higher, the machine structural component is determined to have a hardened layer.

[0042] [Features of the machine structural component of this embodiment] The machine structural component of this embodiment includes the following features. (Feature 1) The chemical composition, in mass%, is C: 0.75 to 1.20%, Si: 0.05 to 0.80%, Mn: 0.50 to 1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50%, Mo: 0.06 to 0.35%, V: 0.09 to 0.40%, Ni: more than 0.30 to 1.50%, Al: 0.005 to 0.100%, N: 0.030% or less, and O: 0.0015% or less, with the remainder being Fe and impurities. When the optional elements are contained, the chemical composition contains, in mass %, C: 0.75 to 1.20%, Si: 0.05 to 0.80%, Mn: 0.50 to 1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50%, Mo: 0.06 to 0.35%, V: 0.09 to 0.40%, Ni: more than 0.30% to 1.50%, Al: 0.005 to 0.100%, N: 0.030% or less, and O: 0.0015% or less, and further contains one or more elements selected from the group consisting of the above-mentioned first to fourth groups, with the balance being Fe and impurities. (Feature 2) The chemical composition satisfies formulas (1) to (3). (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (3). (Feature 3) In the surface layer, the number density of MC type carbides with a circle equivalent diameter of 20 to 100 nm and an Mo concentration of 5% or more in atomic percent is 5.0 pieces / μm 2 That's all. (Feature 4) The prior austenite grain size in the surface layer is 6.0 μm or less. Each feature will be explained below.

[0043] [(Feature 1) Chemical composition] The chemical composition of the machine structural component of this embodiment contains the following elements.

[0044] C: 0.75 to 1.20% Carbon (C) combines with Mo and V to form MC-type carbides. These MC-type carbides trap hydrogen that penetrates into machine structural components when used in a hydrogen-generating environment. This suppresses the aggregation of the penetrated hydrogen and the exfoliation on the surface of the machine structural component. As a result, the rolling fatigue life of the machine structural component in a hydrogen-generating environment is improved. If the C content is less than 0.75%, 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 1.20%, the steel material used for machine structural parts may become too hard, resulting in reduced machinability and the formation of coarse precipitates. These coarse precipitates are likely to become crack initiation sites when machine structural parts are used in a hydrogen-generating environment. Therefore, the rolling fatigue life of machine structural parts in a hydrogen-generating environment may be reduced. Therefore, the C content is 0.75 to 1.20%. The lower limit of the C content is preferably 0.80%, more preferably 0.85%, and even more preferably 0.90%. The upper limit of the C content is preferably 1.18%, more preferably 1.15%, and further preferably 1.10%.

[0045] Si: 0.05 to 0.80% Silicon (Si) increases the temper softening resistance of machine structural components. Si also inhibits hydrogen penetration into machine structural components, thereby increasing the rolling fatigue life of machine structural components in a hydrogen-generating environment. If the Si content is less than 0.05%, 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 Si content exceeds 0.80%, the steel material used as the raw material for machine structural parts becomes too hard, and the machinability of the steel material decreases. Therefore, the Si content is 0.05 to 0.80%. The lower limit of the Si content is preferably 0.08%, more preferably 0.10%, and even more preferably 0.15%. The upper limit of the Si content is preferably 0.75%, more preferably 0.70%, and even more preferably 0.65%.

[0046] Mn: 0.50 to 1.50% Manganese (Mn) improves the hardenability of machine structural components. As a result, the strength of the machine structural components increases. Therefore, the rolling fatigue life of the machine structural components in a hydrogen generating environment increases. 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, if the Mn content exceeds 1.50%, the hardness of the steel material used to make machine structural parts becomes too high, and the machinability of the steel material decreases. Therefore, the Mn content is 0.50 to 1.50%. The lower limit of the Mn content is preferably 0.55%, more preferably 0.60%, and even more preferably 0.65%. The upper limit of the Mn content is preferably 1.45%, more preferably 1.40%, even more preferably 1.35%, even more preferably 1.30%, and still more preferably 1.20%.

[0047] P:0.015% or less Phosphorus (P) is an impurity. P segregates at grain boundaries and reduces grain boundary strength. If the P content exceeds 0.015%, even if the contents of other elements are within the ranges of this embodiment, P will segregate excessively at grain boundaries. In this case, the grain boundary strength will decrease. As a result, the rolling fatigue life of machine structural parts in a hydrogen generating environment will decrease. Therefore, the P content is 0.015% 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%, even more preferably 0.002%, and even more preferably 0.003%. The upper limit of the P content is preferably 0.012%, more preferably 0.010%, and even more preferably 0.008%.

[0048] S: 0.005% or less Sulfur (S) is an impurity. S generates sulfide-based inclusions. Coarse sulfide-based inclusions are likely to become the starting point for cracks during use of machine structural components in a hydrogen-generating environment. If the S content exceeds 0.005%, the sulfide-based inclusions become coarse. As a result, even if the contents of other elements are within the ranges of this embodiment, the rolling fatigue life of machine structural components in a hydrogen-generating environment will be reduced. Therefore, the S content is 0.005% or less. The S content is preferably as low as possible. However, excessive reduction in the S content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the S content is preferably more than 0%, more preferably 0.001%, and even more preferably 0.002%. The upper limit of the S content is preferably 0.004%, and more preferably 0.003%.

[0049] Cr: 0.05 to less than 0.50% Chromium (Cr) improves the hardenability of machine structural components. As a result, the strength of the machine structural components increases. Therefore, the rolling fatigue life of the machine structural components in a hydrogen generating environment increases. If the Cr content is less than 0.05%, 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, as described above, Cr promotes the penetration of hydrogen into machine structural components. If the Cr content is 0.50% or more, even if the contents of other elements are within the ranges of this embodiment, the penetration of hydrogen cannot be sufficiently suppressed during use of the machine structural component in a hydrogen generating environment. As a result, the rolling fatigue life of the machine structural component in a hydrogen generating environment is reduced. Therefore, the Cr content is 0.05 to less than 0.50%. The lower limit of the Cr content is preferably 0.08%, more preferably 0.10%, and even more preferably 0.15%. The upper limit of the Cr content is preferably 0.49%, more preferably 0.48%, even more preferably 0.47%, even more preferably 0.46%, even more preferably 0.45%, even more preferably 0.44%, and even more preferably 0.43%.

[0050] Mo: 0.06 to 0.35% Molybdenum (Mo) forms MC-type carbides together with C and V. These MC-type carbides trap hydrogen that penetrates into machine structural components when the components are used in a hydrogen-generating environment. This suppresses the aggregation of the penetrated hydrogen and the exfoliation on the surface of the machine structural components. Furthermore, MC-type carbides with a high Mo concentration have high thermal stability. This suppresses the dissolution of MC-type carbides during induction hardening, enhancing the pinning effect of the MC-type carbides. As a result, the rolling fatigue life of machine structural components in a hydrogen-generating environment is improved. If the Mo content is less than 0.06%, 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 Mo content exceeds 0.35%, even if the contents of other elements are within the ranges of this embodiment, the hardness of the steel material used for machine structural parts becomes excessively high, and therefore the machinability of the steel material decreases. Therefore, the Mo content is 0.06 to 0.35%. The lower limit of the Mo content is preferably 0.08%, more preferably 0.12%, and even more preferably 0.16%. The upper limit of the Mo content is preferably 0.33%, more preferably 0.30%, and further preferably 0.28%.

[0051] V: 0.09 to 0.40% Vanadium (V) forms MC-type carbides together with C and Mo. These MC-type carbides trap hydrogen that penetrates into the machine structural component when it is used in a hydrogen-generating environment. This suppresses the aggregation of the penetrated hydrogen and the exfoliation on the surface of the machine structural component. As a result, the rolling fatigue life of the machine structural component in a hydrogen-generating environment is improved. If the V content is less than 0.09%, 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 V content exceeds 0.40%, coarse MC carbides may be generated. These coarse MC carbides are likely to become the starting point for cracks when machine structural components are used in a hydrogen-generating environment. Therefore, even if the contents of other elements are within the ranges of this embodiment, the rolling fatigue life of machine structural components in a hydrogen-generating environment may be reduced. If the V content exceeds 0.40%, the machinability of the steel material used to make the machine structural components may also be reduced. Therefore, the V content is 0.09 to 0.40%. The lower limit of the V content is preferably 0.10%, more preferably 0.12%, even more preferably 0.14%, and still more preferably 0.16%. The upper limit of the V content is preferably 0.38%, more preferably 0.37%, and even more preferably 0.35%.

[0052] Ni: Over 0.30 to 1.50% Nickel (Ni) inhibits the penetration of hydrogen into machine structural components in a hydrogen generating environment. As a result, the rolling fatigue life of machine structural components in a hydrogen generating environment is increased. If the Ni content is 0.30% or less, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ni content exceeds 1.50%, the steel material used as the raw material for machine structural parts becomes too hard, and the machinability of the steel material decreases. Therefore, the Ni content is more than 0.30% to 1.50%. The lower limit of the Ni content is preferably 0.31%, more preferably 0.35%, even more preferably 0.40%, even more preferably 0.45%, and even more preferably 0.50%. The upper limit of the Ni content is preferably 1.40%, more preferably 1.30%, even more preferably 1.20%, and still more preferably 1.10%.

[0053] Al: 0.005 to 0.100% Aluminum (Al) deoxidizes steel during the steelmaking process. Furthermore, Al combines with N to form AlN. As a result, the machinability of steel materials used as raw materials for machine structural components is improved. If the Al content is less than 0.005%, 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 0.100%, even if the contents of other elements are within the ranges of this embodiment, coarse clustered oxides will be generated. The coarse clustered oxides will become the starting points for cracks when the machine structural component is used in a hydrogen generating environment. Therefore, even if the contents of other elements are within the ranges of this embodiment, the rolling fatigue life of the machine structural component in a hydrogen generating environment will be reduced. Therefore, the Al content is 0.005 to 0.100%. The lower limit of the Al content is preferably 0.008%, and more preferably 0.010%. The upper limit of the Al content is preferably 0.080%, more preferably 0.070%, and even more preferably 0.060%.

[0054] N: 0.030% or less Nitrogen (N) is an impurity. N dissolves in the steel material used to make machine structural parts, reducing the hot workability of the steel material. If the N content exceeds 0.030%, the hot workability of the steel material will be significantly reduced even if the contents of other elements are within the ranges of this embodiment. Therefore, the N content is 0.030% or less. The N content is preferably as low as possible. However, excessive reduction in the N content increases production costs. Therefore, taking into consideration normal industrial production, the lower limit of the N content is preferably more than 0%, more preferably 0.001%, and even more preferably 0.002%. The upper limit of the N content is preferably 0.028%, more preferably 0.025%, and even more preferably 0.020%.

[0055] O: 0.0015% or less Oxygen (O) is an impurity. O combines with other elements to form coarse oxides (including coarse oxides due to clustering). These coarse oxides become the starting points for cracks in a hydrogen generating environment. This reduces the rolling fatigue life of machine structural components in a hydrogen generating environment. If the O content exceeds 0.0015%, the rolling fatigue life of machine structural components in a hydrogen generating environment will be significantly reduced, even if the contents of other elements are within the ranges of this embodiment. Therefore, the O content is 0.0015% 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 0.0001%, even more preferably 0.0003%, and even more preferably 0.0005%. The upper limit of the O content is preferably 0.0013% or less, more preferably 0.0011%, and even more preferably 0.0009%.

[0056] The balance of the chemical composition of the machine structural component of this embodiment is Fe and impurities. Here, the impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of the machine structural component, and are acceptable within a range that does not adversely affect the machine structural component of this embodiment.

[0057] [About optional elements] The chemical composition of the machine structural component 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] Ti: 0.050% or less, and Nb: 0.050% or less, one or more selected from the group consisting of [Group 2] B: 0.0050% or less, and Cu: 1.50% or less, one or more selected from the group consisting of [Group 3] Sn: 0.100% or less [Group 4] Ca: 0.0050% or less, and Mg: 0.0050% or less, one or more selected from the group consisting of These elements are optional and may not be contained. These optional elements will be described below.

[0058] [Group 1: Ti and Nb] The chemical composition of the machine structural component of this embodiment may further contain one or more elements selected from the group consisting of Ti and Nb in place of a portion of Fe. These elements are optional elements, and both form precipitates, thereby increasing the strength of the machine structural component through precipitation strengthening.

[0059] 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 Ti precipitates such as carbides, nitrides, and carbonitrides. Ti precipitates increase the strength of machine structural parts through precipitation strengthening. Even if Ti is contained even a small amount, the above effects can be obtained to some extent. However, if the Ti content exceeds 0.050%, coarse precipitates may be formed, which reduces the rolling fatigue life of machine structural parts in a hydrogen generating environment 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 0.002%, and even more preferably 0.004%. The upper limit of the Ti content is preferably 0.045%, more preferably 0.040%, even more preferably 0.035%, and still more preferably 0.030%.

[0060] 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 Nb precipitates such as carbides, nitrides, and carbonitrides. Nb precipitates increase the strength of machine structural parts through precipitation strengthening. Even if even a small amount of Nb is contained, the above effects can be obtained to some extent. However, if the Nb content exceeds 0.050%, coarse precipitates may be formed, which reduces the rolling fatigue life of machine structural parts in a hydrogen generating environment 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 0.005%, and even more preferably 0.010%. The upper limit of the Nb content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0061] [Group 2: B and Cu] The chemical composition of the machine structural component of this embodiment may further contain one or more elements selected from the group consisting of B and Cu in place of a portion of Fe. These elements are optional elements, and both improve the hardenability of the machine structural component and increase the strength of the machine structural component.

[0062] 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 is more than 0%, B improves the hardenability of machine structural parts. Furthermore, it suppresses the grain boundary segregation of P. As a result, the strength of machine structural parts is increased. As a result, the rolling fatigue life of machine structural parts in a hydrogen generating environment is improved. 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%, even if the contents of other elements are within the ranges of this embodiment, the hardness of the steel material used to make machine structural parts becomes excessively high, which reduces the machinability of the steel material. Therefore, the B content is 0 to 0.0050%, and if B is contained, the B content is 0.0050% or less. The lower limit of the B content is preferably 0.0001%, more preferably 0.0002%, even more preferably 0.0005%, and still more preferably 0.0010%. The upper limit of the B content is preferably 0.0040%, more preferably 0.0030%, even more preferably 0.0020%, and still more preferably 0.0010%.

[0063] Cu:1.50% 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 improves the hardenability of the machine structural parts and increases the strength of the machine structural parts. As a result, the rolling fatigue life of the machine structural parts in a hydrogen generating environment is improved. Even if even a small amount of Cu is contained, the above effects can be obtained to some extent. However, if the Cu content exceeds 1.50%, even if the contents of other elements are within the ranges of this embodiment, the hardness of the steel material used for machine structural parts becomes excessively high, and therefore the machinability of the steel material decreases. Therefore, the Cu content is 0 to 1.50%, and if contained, it is 1.50% or less. The lower limit of the Cu content is preferably 0.01%, more preferably 0.05%, even more preferably 0.10%, and still more preferably 0.30%. The upper limit of the Cu content is preferably 1.40%, more preferably 1.30%, even more preferably 1.20%, and still more preferably 1.10%.

[0064] [Group 3: Sn] The chemical composition of the machine structural component of this embodiment may further contain Sn in place of a portion of Fe. Sn: 0.100% or less Tin (Sn) is an optional element and may not be contained, that is, the Sn content may be 0%. When contained, that is, when the Sn content is more than 0%, Sn improves the machinability of the steel material that is used as the raw material for machine structural parts. Even if even a small amount of Sn is contained, the above effect can be obtained to some extent. However, if the Sn content exceeds 0.100%, the hot workability of the steel material used to make machine structural parts will decrease even if the contents of other elements are within the ranges of this embodiment. Therefore, the Sn content is 0 to 0.100%, and if Sn is contained, it is 0.100% or less. The lower limit of the Sn content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.005%. The upper limit of the Sn content is preferably 0.090%, more preferably 0.080%, even more preferably 0.060%, even more preferably 0.040%, even more preferably 0.030%, and even more preferably 0.025%.

[0065] [Group 4: Ca and Mg] The chemical composition of the machine structural component of this embodiment may further contain one or more elements selected from the group consisting of Ca and Mg in place of a portion of Fe. These elements are optional elements, and all of them refine sulfides and increase the rolling fatigue life of the machine structural component in a hydrogen generating environment.

[0066] Ca:0.0050% or less Calcium (Ca) is an optional element and may not be contained, that is, the Ca content may be 0%. When contained, that is, when the Ca content is greater than 0%, Ca refines the sulfides contained in the machine structural parts. Furthermore, Ca promotes the spheroidization of the sulfides. As a result, the rolling fatigue life of the machine structural parts in a hydrogen generating environment is improved. Even if even a small amount of Ca is contained, the above effects can be obtained to some extent. However, if the Ca content exceeds 0.0050%, even if the contents of other elements are within the ranges of this embodiment, coarse Ca oxides may be formed in the machine structural component, which reduces the rolling fatigue life of the machine structural component in a hydrogen generating environment. Therefore, the Ca content is 0 to 0.0050%, and if contained, it is 0.0050% or less. The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0002%, even more preferably 0.0005%, and still more preferably 0.0010%. The upper limit of the Ca content is preferably 0.0040%, more preferably 0.0035%, and even more preferably 0.0030%.

[0067] Mg: 0.0050% or less Magnesium (Mg) is an optional element and may not be contained, that is, the Mg content may be 0%. When Mg is contained, that is, when the Mg content is more than 0%, Mg refines the sulfides contained in the machine structural parts. Furthermore, Mg promotes the spheroidization of the sulfides. As a result, the rolling fatigue life of the machine structural parts in a hydrogen generating environment is improved. 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 0.0050%, coarse Mg oxides may be formed in the machine structural component even if the contents of other elements are within the ranges of this embodiment, which reduces the rolling fatigue life of the machine structural component in a hydrogen generating environment. Therefore, the Mg content is 0 to 0.0050%, and if Mg is contained, it is 0.0050% or less. The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0002%, even more preferably 0.0005%, even more preferably 0.0010%, and even more preferably 0.0015%. The upper limit of the Mg content is preferably 0.0040%, more preferably 0.0035%, and even more preferably 0.0030%.

[0068] [(Feature 2) Equation (1) to Equation (3)] The chemical composition of the machine structural component of this embodiment further satisfies formulas (1) to (3). (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol. The formulas (1) to (3) will be explained below.

[0069] [Formula (1)] F1 is defined as (Si + Ni) / Cr. As described above, Si and Ni suppress the penetration of hydrogen into mechanical structural components in a hydrogen generating environment. On the other hand, Cr promotes the penetration of hydrogen into mechanical structural components in a hydrogen generating environment. Therefore, from the viewpoint of suppressing the penetration of hydrogen into mechanical structural components in a hydrogen generating environment, a high F1 is preferable.

[0070] If F1 is less than 1.30, even if the machine structural part satisfies Features 1, 3, and 4 and also satisfies Equations (2) and (3), an excellent rolling fatigue life cannot be obtained in a hydrogen generating environment.

[0071] If F1 is 1.30 or more, a machine structural part can obtain an excellent rolling fatigue life in a hydrogen generating environment, provided that the machine structural part satisfies Features 1, 3, and 4 and also satisfies Equations (2) and (3).

[0072] The lower limit of F1 is preferably 1.32, more preferably 1.34, even more preferably 1.36, even more preferably 1.38, and even more preferably 1.40. Although the upper limit of F1 is not particularly limited, when the chemical composition satisfies Feature 1, the upper limit of F1 is 46.00. The upper limit of F1 is preferably less than 8.00, more preferably 7.99, even more preferably 7.80, even more preferably 7.50, even more preferably 7.00, and even more preferably 6.50. Note that F1 is the value obtained by rounding off the obtained numerical value to two decimal places (i.e., the value to one decimal place).

[0073] [Formula (2)] It is defined as F2 = Mo + V. F2 is an index of the total amount of Mo and V required to obtain a sufficient amount of MC type carbides in the surface layer of a machine structural part.

[0074] As mentioned above, Mo and V combine with C in the surface layer to form MC-type carbides. MC-type carbides trap hydrogen that penetrates into machine structural parts when they are used in a hydrogen-generating environment. As a result, the rolling fatigue life of machine structural parts in a hydrogen-generating environment is improved. Therefore, a sufficient amount of MC-type carbides is required to trap the invading hydrogen.

[0075] If F2 is 0.30 or more, the Mo content and V content sufficient to form a sufficient amount of MC type carbides are ensured. Therefore, on the premise that the machine structural part satisfies Feature 1, the machine structural part can satisfy Feature 3. As a result, if the machine structural part further satisfies Formula (1), Formula (3), and Feature 4, it can obtain an excellent rolling contact fatigue life in a hydrogen generating environment.

[0076] The lower limit of F2 is preferably 0.31, more preferably 0.32, even more preferably 0.33, even more preferably 0.34, and even more preferably 0.35. The upper limit of F2 is not particularly limited, but when the chemical composition satisfies Feature 1, the upper limit of F2 is 0.75. The upper limit of F2 is preferably 0.73, and more preferably 0.71. Note that F2 is the value obtained by rounding off the obtained numerical value to two decimal places (i.e., the value to one decimal place).

[0077] [Formula (3)] It is defined as F3 = 2.5 × Mn + Cr. F3 is an index that represents the amount of retained austenite contained in the hardened layer of a machine structural part. Mn and Cr reduce the amount of retained austenite contained in the hardened layer. In other words, the higher F3, the less retained austenite is contained in the hardened layer.

[0078] As mentioned above, the retained austenite contained in the quench-hardened layer may be transformed into stress-induced martensite by external force during use of a machine structural component. In this case, excessive hydrogen aggregation at the interface between the retained austenite and stress-induced martensite and the formation of white structure are promoted. If F3 is 1.70 or more, the amount of retained austenite contained in the quench-hardened layer is sufficiently reduced. As a result, excessive hydrogen aggregation and the formation of white structure are suppressed.

[0079] On the other hand, retained austenite traps hydrogen that has penetrated into machine structural components. Therefore, if an appropriate amount of retained austenite is contained in the quench-hardened layer, it can suppress the aggregation of hydrogen that has penetrated into machine structural components. If F3 is 3.90 or less, the quench-hardened layer contains a sufficient amount of retained austenite to suppress the aggregation of hydrogen. Therefore, if F3 is 1.70 to 3.90, the machine structural part satisfies Features 1, 3, and 4, and also satisfies Formulas (1) and (2), and therefore an excellent rolling fatigue life can be obtained in a hydrogen generating environment.

[0080] A more preferable lower limit of F3 is 1.72, even more preferably 1.80, even more preferably 1.90, and even more preferably 2.00. A more preferable upper limit of F3 is 3.78, even more preferably 3.70, even more preferably 3.50, and even more preferably 3.00. Note that F3 is the value obtained by rounding off the obtained numerical value to two decimal places (i.e., the value to one decimal place).

[0081] [(Feature 3) Number density ND of MC type carbides with a circle equivalent diameter of 20 to 100 nm and Mo concentration of 5% or more in atomic percent] The machine structural component of this embodiment further has a surface layer in which the number density of MC type carbides having a circle equivalent diameter of 20 to 100 nm and an Mo concentration of 5% or more in atomic percent is 5.0 particles / μm 2 Here, the number density of MC type carbides in the surface layer, which have a circle equivalent diameter of 20 to 100 nm and an Mo concentration of 5% or more in atomic %, is defined as ND (number / μm 2 ) is defined as

[0082] As described above, fine MC carbides with an equivalent circle diameter of 20 to 100 nm can trap hydrogen that penetrates into machine structural components and suppress hydrogen aggregation. Furthermore, if the Mo concentration in atomic percent in the MC carbides is 5% or more, the thermal stability of the MC carbides is enhanced. Therefore, when machine structural components are induction hardened, dissolution of the MC carbides is suppressed, and the pinning effect of the MC carbides is enhanced. As a result, induction hardening makes it easier to refine prior austenite grains in the surface layer. Therefore, by increasing the number density ND of MC carbides with an equivalent circle diameter of 20 to 100 nm and an Mo concentration in atomic percent of 5% or more in the surface layer, it is possible to achieve both the hydrogen trapping effect and the pinning effect of the MC carbides.

[0083] Number density ND is 5.0 pieces / μm 2 If the above conditions are met, the hydrogen trapping effect and pinning effect of the MC carbides can be sufficiently obtained, and therefore, provided that the machine structural part satisfies Features 1, 2, and 4, it can obtain an excellent rolling contact fatigue life in a hydrogen generating environment.

[0084] The preferred lower limit of the number density ND is 6.0 pieces / μm 2 and more preferably 8.0 particles / μm 2 is. The upper limit of the number density ND is not particularly limited, but in consideration of normal industrial production, it is, for example, 50.0 pieces / μm 2 is.

[0085] [Method for measuring number density ND] The number density ND (number / μm) of MC type carbides in the surface layer, which have a circle equivalent diameter of 20 to 100 nm and an Mo concentration of 5% or more in atomic percent, 2 ) can be measured in the following way: A test piece is taken whose observation surface is 200 μm deep from the surface of the mechanical structural component. The size of the test piece is not particularly limited, as long as the observation surface includes a 100 μm x 100 μm measurement area centered at a depth of 200 μm. The observation surface is parallel to the depth direction from the surface of the mechanical structural component. After embedding the test piece in resin, the observation surface is mirror-polished. The mirror-polished observation surface is etched for 10 seconds with a 4% nital etching solution. Carbon is vapor-deposited on the etched observation surface. The test piece is placed in a hydrochloric acid-methanol solution and a potential of 7.5 V relative to the reference potential is applied for 1 minute. The observation surface is then washed with distilled water to release the carbon film, and sampled with a Cu mesh.

[0086] The sampled carbon film is observed using a transmission electron microscope (TEM). The TEM observation is performed at a magnification of 50,000x, with 10 non-overlapping observation fields of 3.0 μm x 3.0 μm within the measurement area described above. All particles with a circular equivalent diameter of 20 to 100 nm within the observation field are identified. Quantitative analysis is then performed on the identified particles with a circular equivalent diameter of 20 to 100 nm using energy dispersive X-ray spectroscopy (EDS) attached to the TEM. The EDS analysis identifies all particles with a circular equivalent diameter of 20 to 100 nm that have an atomic percent Mo concentration of 5% or more and a V concentration of 15% or more. In machine structural parts whose chemical composition satisfies Features 1 and 2, particles with a circle equivalent diameter of 20 to 100 nm, an Mo concentration of 5% or more, and a V concentration of 15% or more in atomic percent are all considered to be MC type carbides. Based on the total number of particles identified in all observation fields and the total area of ​​all observation fields, the number density of the particles (particles / μm 2 ) is found.

[0087] Here, the deeper the position from the surface of the machine structural component, the lower the number density of MC type carbides having a circle equivalent diameter of 20 to 100 nm and an Mo concentration in atomic % of 5% or more. Therefore, in this embodiment, the number density of the particles determined from a plurality of observation fields in an observation region centered at a depth of 200 μm is calculated as the number density ND (particles / μm 2 ) is considered. The number density ND is the value obtained by rounding off the obtained numerical value to one decimal place (that is, the value to one decimal place).

[0088] [(Feature 4) Prior austenite grain size D in the surface layer] Furthermore, in the machine structural component of this embodiment, the prior austenite grain size in the surface layer is 6.0 μm or less, where the prior austenite grain size in the surface layer is defined as D (μm).

[0089] As described above, spalling on the surface of a machine structural component in a hydrogen generating environment begins with prior austenite grain boundary cracking. Here, if the prior austenite grain size D in the surface layer is reduced, prior austenite grain boundary cracking in the surface layer can be suppressed. As a result, spalling on the surface of the machine structural component can be suppressed, and the rolling fatigue life can be extended.

[0090] Induction hardening can be cited as an example of a means for refining the prior austenite grain size D in the surface layer of a machine structural component. The machine structural component of this embodiment satisfies Feature 3, and therefore a sufficient pinning effect can be obtained. Therefore, induction hardening can further refine the prior austenite grain size D in the surface layer.

[0091] If the prior austenite grain size D in the surface layer is 6.0 μm or less, prior austenite grain boundary cracking in the surface layer is sufficiently suppressed. As a result, provided that the machine structural component satisfies Features 1 to 3, it can obtain an excellent rolling contact fatigue life in a hydrogen generating environment.

[0092] There is no particular restriction on the lower limit of the prior austenite grain size D, but in consideration of normal industrial production, it is, for example, 1.0 μm. The upper limit of the prior austenite grain size D is preferably 5.5 μm, and more preferably 5.0 μm.

[0093] [Method for measuring prior austenite grain size D] The prior austenite grain size D in the surface layer can be measured by the following method. A test piece is taken whose observation surface extends 200 μm from the surface of the mechanical structural component. The size of the test piece is not particularly limited, as long as the observation surface includes a 150 μm × 150 μm observation area centered at a depth of 200 μm. The observation surface is parallel to the depth direction from the surface of the mechanical structural component. The observation surface is mirror-polished. The mirror-polished observation surface is immersed in a mixed solution of picric acid and ethanol (4 g of picric acid per 100 ml of ethanol) for 5 minutes to reveal the prior austenite grain boundaries. An optical microscope is then used to observe the 150 μm × 150 μm observation area, and the prior austenite grain size is calculated using the intercept method. Specifically, a straight line with a total length L is drawn in the 150 μm × 150 μm observation area, and the number of crystal grains nL that intersect this line is determined. The total length L of the line is determined so that the number of crystal grains nL is 10 or more. Based on the total length L and the number of crystal grains obtained nL, the intercept length (= L / nL) is calculated. The intercept lengths are calculated for any five straight lines. The arithmetic mean value is calculated from the intercept lengths obtained for any five straight lines.

[0094] Here, the deeper the position from the surface of the machine structural component, the larger the prior austenite grain size becomes. Therefore, in this embodiment, the arithmetic mean value of the intercept lengths determined in the observation region centered at a depth of 200 μm is regarded as the prior austenite grain size D (μm) in the surface layer. The prior austenite grain size D in the surface layer is a value obtained by rounding off the obtained numerical value to one decimal place (i.e., the value to one decimal place).

[0095] [Effects of the machine structural component of this embodiment] The machine structural component of this embodiment satisfies Features 1 to 4. Therefore, an excellent rolling fatigue life can be obtained in a hydrogen generating environment.

[0096] [Use of the machine structural part of this embodiment] The machine structural component of this embodiment can be widely applied to fields requiring a long rolling fatigue life. The machine structural component is, for example, a bearing component. The bearing component refers to a rolling bearing component. The bearing component is, for example, a raceway ring, a washer, or a rolling element. The raceway ring may be an inner ring or an outer ring, and the washer may be a shaft washer, a housing washer, a central washer, or an aligning housing washer. The raceway ring and the washer are not particularly limited as long as they have a raceway surface. The rolling element may be a ball or a roller. Examples of rollers include cylindrical rollers, rod rollers, needle rollers, tapered rollers, and convex rollers. It should be noted that the machine structural component of this embodiment can also be applied to applications other than bearing components. Examples of applications other than bearing components include gears, constant velocity joints, and ball screws.

[0097] [Mechanical manufacturing method for machine structural parts] An example of a manufacturing method for a machine structural component of this embodiment will be described. The manufacturing method for a machine structural component described below is one example for manufacturing the machine structural component of this embodiment. Therefore, a machine structural component having the above-described configuration 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 manufacturing method for a machine structural component of this embodiment.

[0098] An example of a method for manufacturing a machine structural component according to this embodiment includes the following steps. (Process 1) Steel material preparation process (Process 2) Hot processing process (Step 3) Spheroidizing annealing step (Process 4) Rough processing process (Step 5) Deep hardening process (Process 6) High-frequency hardening process Each step will be described below.

[0099] [(Process 1) Steel material preparation process] In the steel preparation step, a steel material to be used as the material for the machine structural component of this embodiment is prepared. Specifically, molten steel having a chemical composition that satisfies Features 1 and 2 is produced. The refining method is not particularly limited, and any known method may be used. For example, molten pig iron produced by a known method is subjected to refining (primary refining) in a converter. The molten steel tapped from the converter is subjected to known secondary refining. In the secondary refining, alloy elements are added to the molten steel to adjust the composition, and molten steel having a chemical composition that satisfies Features 1 and 2 is produced.

[0100] The molten steel produced by the above-described refining method is used to produce a material by a known casting method. For example, an ingot may be produced by an ingot casting method using the molten steel. Alternatively, a bloom may be produced by a continuous casting method using the molten steel. The produced material (ingot or bloom) is subjected to known hot working to produce a steel material of a predetermined shape. For example, hot forging may be performed as the hot working. Alternatively, finish rolling using a continuous rolling mill may be performed after blooming or hot forging. The continuous rolling mill has a plurality of rolling stands arranged in a row. The heating temperature during hot working is not particularly limited, but is, for example, 1000 to 1300°C. The shape of the steel material produced in this way is, for example, a steel bar.

[0101] [(Process 2) Hot processing process] In the hot working step, hot working is performed on the steel material of this embodiment to manufacture an intermediate product having a predetermined shape. The hot working is, for example, hot forging. In the hot working step, the steel material is c3 After the steel is heated to a temperature above this point for a specified time, it is processed. After hot processing, the intermediate product is cooled to room temperature.

[0102] The hot working process satisfies the following conditions: (Condition 1) The holding time t1 at a heating temperature of 1100°C or higher is 30 minutes or longer. (Condition 2) The processing end temperature is 1050°C or higher, and the average cooling rate R from 1050°C to 850°C is more than 2.0°C / sec. Conditions 1 and 2 will be explained below.

[0103] [(Condition 1) Holding time t1 at a heating temperature of 1100°C or higher] Fine MC carbides with equivalent circle diameters of 20 to 100 nm mainly precipitate during cooling from the heating temperature in the hot working process. Specifically, precipitation of MC carbides begins in the temperature range of 1,050°C to 850°C, and the MC carbides grow during cooling to 760°C. If coarse MC carbides are formed before cooling from the heating temperature in the hot working process, the number density of the fine MC carbides formed during cooling decreases. Furthermore, in this case, the Mo solid solution concentration in the grains decreases, so the Mo concentration in the fine MC carbides also decreases. As a result, it is difficult to obtain a sufficient number of fine MC carbides with a Mo concentration of 5% or more in atomic percent. In other words, to increase the number density of fine MC carbides with a Mo concentration of 5% or more, it is preferable to fully dissolve the coarse MC carbides during heating in the hot working process.

[0104] In the hot working process, the coarse MC carbides are dissolved by heating the intermediate product to 1100°C or higher. If the holding time t1 at a heating temperature of 1100°C or higher is 30 minutes or longer, the coarse MC carbides are sufficiently dissolved. As a result, the Mo solid solution concentration within the grains is also sufficiently increased, and the manufactured machine structural parts can satisfy Feature 3. Therefore, in the hot working process, the holding time t1 at a heating temperature of 1100°C or higher is set to 30 minutes or longer. The upper limit of the retention time t1 is not particularly limited, and in consideration of normal industrial production, the upper limit of the retention time t1 is, for example, 300 minutes. The upper limit of the heating temperature of the intermediate product is not particularly limited, and is, for example, 1300°C in consideration of normal industrial production.

[0105] [(Condition 2) Average cooling rate R from 1050°C to 850°C] As mentioned above, MC carbides start to precipitate in the temperature range from 1050°C to 850°C. However, in this temperature range, the driving force for Mo precipitation is insufficient, so MC carbides with a low Mo concentration, mainly composed of V, precipitate preferentially. Therefore, in order to increase the number density of MC carbides with a Mo concentration of 5% or more in atomic percent, it is preferable to increase the cooling rate from 1050°C to 850°C to suppress precipitation in this temperature range.

[0106] If the processing end temperature is 1050°C or higher and the average cooling rate R from 1050°C to 850°C exceeds 2.0°C / sec, the precipitation of MC carbides in that temperature range is sufficiently suppressed, and the number density of MC carbides with a high Mo concentration that precipitate at 850°C or lower can be increased. As a result, the manufactured machine structural parts can satisfy Feature 3. Therefore, in the hot processing step, the processing end temperature is set to 1050°C or higher, and the average cooling rate R from 1050°C to 850°C exceeds 2.0°C / sec. A preferred lower limit of the average cooling rate R is 2.1°C / sec. There is no particular upper limit to the average cooling rate R. In consideration of normal industrial production, the upper limit of the average cooling rate R is, for example, 6.0° C. / sec.

[0107] [(Step 3) Spheroidizing annealing step] In the spheroidizing annealing process, spheroidizing annealing is performed on the intermediate product after the hot working process. In spheroidizing annealing, coarse cementite is spheroidized to improve the workability of the intermediate product. In the spheroidizing annealing process, the intermediate product is heated to a heating temperature T (°C) and held for a specified time, after which the intermediate product is slowly cooled to 600°C in a furnace. The intermediate product is then air-cooled to room temperature.

[0108] In the spheroidizing annealing process, it is necessary to dissolve most of the cementite present in the machine structural component. However, if the heating temperature T is less than 760°C, the cementite cannot be sufficiently dissolved. On the other hand, in order to spheroidize the cementite, it is necessary to leave some of the cementite as nuclei for spheroidization. If the heating temperature T exceeds 820°C, the cementite present in the machine structural component will be completely dissolved. As a result, the pearlite structure will be regenerated, and the cementite will not be sufficiently spheroidized. For these reasons, the heating temperature T in the spheroidizing annealing process is set to 760 to 820°C.

[0109] The spheroidizing annealing process satisfies the following conditions. (Condition 3) The holding time t2 at the heating temperature T (°C) is 300 minutes or less. Condition 3 will be explained below.

[0110] [(Condition 3) Holding time t2 at heating temperature T (℃)] As described above, the heating temperature T in the spheroidizing annealing step is 760 to 820°C. In this temperature range, the MC carbides precipitated in the hot working step undergo Ostwald ripening. In order to increase the number density of fine MC carbides, it is preferable that the holding time t2 (minutes) at the heating temperature T (°C) is short.

[0111] If the holding time t2 at the heating temperature T (°C) is 300 minutes or less, excessive Ostwald ripening of MC carbides is suppressed. Therefore, the number density of fine MC carbides is sufficiently increased. As a result, the manufactured machine structural parts can satisfy Feature 3. Therefore, in the spheroidizing annealing process, the holding time t2 at the heating temperature T (°C) is set to 300 minutes or less. The lower limit of the retention time t2 is not particularly limited, and in consideration of normal industrial production, the lower limit of the retention time t2 is, for example, 100 minutes. A preferred upper limit of the retention time t2 is 240 minutes.

[0112] [(Process 4) Rough processing process] In the rough processing step, the intermediate product after the spheroidizing annealing step is roughly processed into a shape close to the final shape of the machine structural component. The rough processing may be, for example, cutting or cold processing such as cold forging.

[0113] [(Step 5) Deep hardening process] In the through-hardening process, the intermediate product after the rough machining process is subjected to well-known through-hardening (overall hardening) to manufacture a machine structural part having a hardened layer formed at least on the surface. In the deep quenching, the intermediate product is carried into the heat treatment furnace and c3 The intermediate product is heated and held at a temperature equal to or higher than the quenching temperature. The quenching temperature is preferably 820 to 900° C. Thereafter, the intermediate product is carried out of the heat treatment furnace and quenched. The quenching method is not particularly limited, but examples include water cooling and oil cooling.

[0114] The machine structural part after deep quenching may be subjected to tempering treatment, for example, by holding the deep quenched intermediate product within a temperature range of 100 to 200°C for a predetermined time.

[0115] [(Process 6) High-frequency hardening process] In the induction hardening process, induction hardening is performed on intermediate products after the deep hardening process. In the induction hardening process, the surface layer of the machine structural parts is hardened by high-frequency heating. c3 The temperature is raised to a temperature higher than the induction hardening point, and then cooled. In this case, the surface layer of the machine structural component is transformed into austenite, and then transformed into a hard structure mainly composed of martensite. This increases the strength of the machine structural component. Furthermore, the prior austenite grain size in the surface layer is refined compared to before the induction hardening process. In particular, since the machine structural component of this embodiment satisfies Feature 3, a sufficient pinning effect is obtained, and the prior austenite grain size in the surface layer is likely to be further refined. Therefore, the machine structural component can satisfy Feature 4 by using a well-known induction hardening process.

[0116] Induction hardening is a process in which the surface of a machine structural part is hardened by high-frequency heating. c3There are no particular limitations as long as the material can be heated to a temperature above this point. That is, the induction hardening treatment can be performed using a known induction heating device and a known cooling device. For example, a circular induction heating device may be used as the induction heating device, and a circular cooling device may be used as the cooling device. Furthermore, the induction hardening treatment may be performed two or more times.

[0117] The machine structural parts after induction hardening are subjected to tempering treatment, for example, at a tempering temperature of 150 to 250° C. and for a holding time at the tempering temperature of 30 to 150 minutes.

[0118] Through the above steps, a machine structural component having Features 1 to 4 is manufactured. Note that the machine structural component after the tempering treatment in the induction hardening step may be subjected to a finishing process. The finishing process may be, for example, polishing. [Example]

[0119] Steel materials having the chemical compositions shown in Table 1 (Table 1A and Table 1B) were manufactured as materials for machine structural parts by the following method. Note that the steel material with test number 29 was a reference steel material, and had a chemical composition equivalent to SUJ2 specified in JIS G 4805 (2019).

[0120] [Table 1A]

[0121] [Table 1B]

[0122] Specifically, a 100 kg ingot was produced by vacuum melting. The produced ingot was hot forged to produce a steel material (steel bar) with a diameter of 60 mm. The heating temperature of the ingot before hot forging was 1000 to 1300°C. The steel material after hot forging was air-cooled to room temperature.

[0123] From the produced steel material, several test pieces (small roller test pieces) for roller pitting fatigue tests were produced as simulated machine structural parts, as shown in Figure 1. The numbers in Figure 1 indicate dimensions (unit: mm). "φ" in the figure means diameter.

[0124] Specifically, a hot working process was carried out on the steel material of each test number. In the hot working process, the steel material of each test number was heated at a heating temperature of 1100 to 1200°C. At this time, the holding time t1 (minutes) at a heating temperature of 1100°C or higher was as shown in Table 2. After heating, the steel material was hot worked (hot forged) with a working end temperature of 1050°C or higher to produce a steel bar with a diameter of 35 mm. The steel bar after hot working was air-cooled to room temperature. At this time, the average cooling rate R (°C / sec) from 1050°C to 850°C was as shown in Table 2.

[0125] [Table 2]

[0126] A spheroidizing annealing process was carried out on steel bars with a diameter of 35 mm. In the spheroidizing annealing process, the steel bars of each test number were held at a heating temperature T (°C) for t2 minutes, and then rapidly cooled to 720°C. Note that the heating temperature T was 800°C for all test numbers. Thereafter, they were cooled to 600°C at a rate of 10°C / hour, and then air-cooled to room temperature. Here, the holding time t2 (minutes) for each test number was as shown in Table 2. The steel bars after the spheroidizing annealing process were subjected to cutting to produce intermediate products with the shape shown in Figure 1.

[0127] The manufactured intermediate products were subjected to a deep quenching process. In the deep quenching process, the quenching temperature was 840°C for test numbers 1 to 26 and 29, and 760°C for test numbers 27 and 28. For all test numbers, the holding time at the quenching temperature was 30 minutes, and the intermediate products were cooled in oil at 130°C. The intermediate products after oil cooling for each test number were tempered at a tempering temperature of 180°C for a holding time of 180 minutes.

[0128] The intermediate product after the deep hardening process was subjected to an induction hardening process. In the induction hardening process, the surface of the intermediate product was heated to 900°C using a circular high-frequency heating device. After that, the surface of the intermediate product was rapidly cooled to room temperature using a circular cooling device. The intermediate product after the induction hardening process was subjected to a tempering process. In the tempering process, the intermediate product was held at 180°C for 120 minutes, and then air-cooled to room temperature.

[0129] Using the above manufacturing process, simulated mechanical structural parts (small roller test pieces) of each test number were manufactured. The Vickers hardness at a depth of 200 μm from the surface of each simulated mechanical structural part was measured according to the method described above in [Vickers Hardness Measurement Method]. As a result, the Vickers hardness at a depth of 200 μm from the surface of all simulated mechanical structural parts of all test numbers was 600 HV or higher. In other words, all simulated mechanical structural parts of all test numbers had a quench-hardened layer at least on the surface.

[0130] [Evaluation test] The following evaluation tests were carried out on the simulated machine structural parts with each test number. (Test 1) Number density ND measurement test (Test 2) Prior austenite grain size D measurement test (Test 3) Rolling fatigue life evaluation test under hydrogen generation environment Tests 1 to 3 will be explained below.

[0131] [(Test 1) Number density ND measurement test] Based on the method described in the above [Method for measuring number density ND], the number density ND (numbers / μm 2 The number density ND (number / μm 2 ) are shown in Table 2.

[0132] [(Test 2) Prior austenite grain size D measurement test] Based on the method described above in [Method for measuring prior austenite grain size D], the prior austenite grain size D (μm) in the surface layer of the simulated machine structural component of each test number was measured. The obtained prior austenite grain size D (μm) for each test number is shown in Table 2.

[0133] [(Test 3) Rolling fatigue life evaluation test under hydrogen generation environment] The rolling fatigue life in a hydrogen generating environment was evaluated using small roller test pieces, which were simulated machine structural parts of each test number. Specifically, the following roller pitting fatigue test was carried out.

[0134] FIG. 2 is a schematic diagram of the roller pitting fatigue test. As shown in FIG. 2, the large roller test piece 100 was pressed against the small roller test piece 200 with the surface pressure described below, while the small roller test piece 200 was rotated. The small roller test piece 200 used was one prepared by the method described above (a test piece for roller pitting fatigue test). The large roller test piece 100 had the shape shown in FIG. 3. The numbers in FIG. 3 indicate dimensions (unit: mm). "R150" in the figure indicates that the radius of curvature of the outer circumferential surface was 150 mm.

[0135] The large roller test specimen 100 was prepared by the following method. An intermediate product having a chemical composition equivalent to SUJ2 specified in JIS G 4805 (2019) and the shape shown in Figure 3 was prepared. The intermediate product was quenched and tempered. For quenching, the intermediate product was held at 840°C for 30 minutes, and then oil-cooled in 130°C oil. For tempering, the intermediate product was held at 160°C for 180 minutes, and then air-cooled. The surface of the tempered intermediate product was polished to prepare the large roller test specimen 100 shown in Figure 3. The diameter of the large roller test specimen 100 was 130 mm.

[0136] In order to simulate a hydrogen generation environment, a hydrogen charging treatment was carried out on the small roller test piece 200. Specifically, the small roller test piece 200 was immersed in a 3% NaCl + 3 g / L ammonium thiocyanate (NH4SCN) aqueous solution for 96 hours with a cathodic current density of 0.80 mA / cm 2A constant current controlled to 1000 V was generated to add hydrogen to the small roller test piece 200. The temperature of the aqueous solution during storage was set to 20° C. The hydrogen charging process was carried out under the above conditions.

[0137] The large roller test piece 100 was pressed against the small roller test piece 200 after hydrogen charging treatment. The Hertzian stress surface pressure at the contact area between the small roller test piece 200 and the large roller test piece 100 was constant at 3000 MPa. The peripheral speed directions of both roller test pieces at the contact area were set to the same direction, and the test was performed by rotating them at a slip ratio of -40% (the peripheral speed at the contact area of ​​the large roller test piece 100 was 40% higher than that of the small roller test piece 200). The oil temperature of the ATF (automatic transmission lubricating oil) supplied as lubricating oil to the contact area was 90°C. The test was discontinued after 50 million cycles (5.0 × 10 7 Roller pitting fatigue tests were conducted under the above conditions, and the test results were plotted on a Weibull distribution. The L10 life, which indicates a 10% probability of failure, was then taken as the rolling fatigue life.

[0138] Based on the obtained rolling contact fatigue life, the rolling contact fatigue life in a hydrogen generation environment was evaluated as follows: Using test number 29, whose chemical composition meets the SUJ2 standard of JIS G 4805 (2019), as the standard, the rolling contact fatigue life ratio for each test number was calculated using the following formula. Rolling contact fatigue life ratio = (rolling contact fatigue life of each test number) / (rolling contact fatigue life of test number 29) Based on the obtained rolling fatigue life ratio, the rolling fatigue life in a hydrogen generating environment was evaluated as follows. Evaluation E (Excellent): Rolling fatigue life ratio ≥ 1.50, or no peeling up to 50 million repeated loads Rating B (Bad): Rolling fatigue life ratio < 1.50 When the rating was E, it was determined that an excellent rolling fatigue life was obtained. On the other hand, when the rating was B, it was determined that an excellent rolling fatigue life was not obtained. The evaluation results are shown in the "Rolling Contact Fatigue Life" column in Table 2.

[0139] [Test Results] Referring to Table 1 (Table 1A and Table 1B) and Table 2, the simulated machine structural parts of test numbers 1 to 10 satisfied features 1 to 4. Therefore, in the rolling fatigue life evaluation test in a hydrogen generation environment, an excellent rolling fatigue life was obtained.

[0140] On the other hand, in test numbers 11 and 12, the Cr content was too high, and therefore an excellent rolling fatigue life was not obtained in the rolling fatigue life evaluation test in a hydrogen generation environment.

[0141] In test numbers 13 and 14, F1 was too low, and therefore an excellent rolling fatigue life was not obtained in the rolling fatigue life evaluation test in a hydrogen generation environment.

[0142] In test numbers 15 and 16, F2 was too low. Therefore, the number density ND was too low. As a result, the prior austenite grain size D was also too large. As a result, an excellent rolling fatigue life was not obtained in the rolling fatigue life evaluation test in a hydrogen generation environment.

[0143] In test numbers 17 and 18, F3 was too low, and therefore an excellent rolling fatigue life was not obtained in the rolling fatigue life evaluation test in a hydrogen generation environment.

[0144] In test numbers 19 and 20, F3 was too high, and therefore an excellent rolling fatigue life was not obtained in the rolling fatigue life evaluation test in a hydrogen generation environment.

[0145] In test numbers 21 and 22, the holding time t1 in the hot working process was too short. Therefore, the number density ND was too low. As a result, the prior austenite grain size D was also too large. As a result, an excellent rolling fatigue life was not obtained in the rolling fatigue life evaluation test in a hydrogen generation environment.

[0146] In test numbers 23 and 24, the average cooling rate R in the hot working process was too slow. As a result, the number density ND was too low. As a result, the prior austenite grain size D was also too large. As a result, an excellent rolling fatigue life was not obtained in the rolling fatigue life evaluation test in a hydrogen generation environment.

[0147] In test numbers 25 and 26, the holding time t2 in the spheroidizing annealing process was too long. Therefore, the number density ND was too low. As a result, the prior austenite grain size D was also too large. As a result, an excellent rolling fatigue life was not obtained in the rolling fatigue life evaluation test in a hydrogen generation environment.

[0148] In test numbers 27 and 28, F2 was too low. Therefore, the number density ND was too low. As a result, although the prior austenite grain size D was sufficiently small because the quenching temperature in the deep quenching process was low, an excellent rolling fatigue life was not obtained in the rolling fatigue life evaluation test in a hydrogen generation environment.

[0149] 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.

Claims

1. A machine structural part, When a region from the surface of the machine structural component to a depth of 200 μm is defined as a surface layer, the component has a quench-hardened layer formed at least on the surface layer, The chemical composition of the machine structural part is, in mass%, C: 0.75-1.20%, Si: 0.05-0.80%, Mn: 0.50 to 1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50% Mo: 0.06-0.35%, V: 0.09-0.40%, Ni: more than 0.30 to 1.50%; Al: 0.005-0.100%, N: 0.030% or less, and O: 0.0015% or less, the balance being Fe and impurities; Formulas (1) to (3) are satisfied, In the surface layer, the number density of MC type carbides having a circle equivalent diameter of 20 to 100 nm and an Mo concentration of 5% or more in atomic % is 5.0 particles / μm ２ That's all, The prior austenite grain size in the surface layer is 6.0 μm or less. Mechanical structural parts. (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

2. A machine structural part, When a region from the surface of the machine structural component to a depth of 200 μm is defined as a surface layer, the component has a quench-hardened layer formed at least on the surface layer, The chemical composition of the machine structural part is, in mass%, C: 0.75-1.20%, Si: 0.05-0.80%, Mn: 0.50 to 1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50% Mo: 0.06-0.35%, V: 0.09-0.40%, Ni: more than 0.30 to 1.50%; Al: 0.005-0.100%, N: 0.030% or less, and O: 0.0015% or less, Further, it contains one or more selected from the group consisting of Groups 1 to 4, the balance being Fe and impurities; Formulas (1) to (3) are satisfied, In the surface layer, the number density of MC type carbides having a circle equivalent diameter of 20 to 100 nm and an Mo concentration of 5% or more in atomic % is 5.0 particles / μm ２ That's all, The prior austenite grain size in the surface layer is 6.0 μm or less. Mechanical structural parts. [Group 1] Ti: 0.050% or less, and Nb: 0.050% or less, one or more selected from the group consisting of [Group 2] B: 0.0050% or less, and Cu: 1.50% or less, one or more selected from the group consisting of [Group 3] Sn: 0.100% or less [Group 4] Ca: 0.0050% or less, and Mg: 0.0050% or less, one or more selected from the group consisting of (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

3. The machine structural component according to claim 2, The chemical composition of the mechanical structural component contains the first group. Mechanical structural parts.

4. The machine structural component according to claim 2, The chemical composition of the mechanical structural component contains the second group. Mechanical structural parts.

5. The machine structural component according to claim 2, The chemical composition of the mechanical structural part contains the third group. Mechanical structural parts.

6. The machine structural component according to claim 2, The chemical composition of the mechanical structural component contains the fourth group. Mechanical structural parts.

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