Carburized machine structural component

Abstract

To provide a carburized machine structural component having an excellent rolling fatigue life under a hydrogen generating environment.SOLUTION: A carburized mechanical structural component according to the present embodiment includes a carburized hardened layer formed at least in a surface layer, and a core portion inside the carburized hardened layer, wherein the core portion has a chemical composition containing, by mass%, C: 0.10 to 0.45%, Si: 0.05 to 0.80%, Mn: 0.40 to 1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to 0.80%, Mo: 0.05 to 0.35%, V: 0.06 to 0.40%, Al: 0.005 to 0. 100%, N: 0.030% or less, and O: 0.0015% or less, with the balance being Fe and impurities, in the surface layer, the number densities of MC-type carbides having circle-equivalent diameters of 20 to 300nm and N concentrations of 10% or more in atom% are 5.0 pieces / μ m2 or more, and prior austenite grain sizes in the surface layer are 6.0 μm or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a carburized machine structural component, which is a machine structural component that has been subjected to a carburizing treatment. [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] When an improvement in fatigue life is required for a machine structural component, carburizing may be performed as a heat treatment during the manufacturing process of the machine structural component. In carburizing, a carburized hardened layer is formed on the surface of the steel material, hardening the surface layer of the steel material. This improves the fatigue life of the machine structural component. In this specification, a machine structural component that has been subjected to carburizing is also referred to as a "carburized machine structural component."

[0004] A technique for increasing the fatigue life of carburized machine structural parts is proposed in Japanese Patent Laid-Open Publication No. 8-49057 (Patent Document 1).

[0005] The rolling bearing disclosed in Patent Document 1 uses a steel material for at least one of the raceways and rolling elements containing 0.1-0.7 wt% C, 0.5-3.0 wt% Cr, 0.3-1.2 wt% Mn, 0.3-1.5 wt% Si, and 3 wt% or less Mo, and further containing 0.8-2.0 wt% V. An intermediate product formed from this material is carburized to set the carbon concentration of the bearing surface to 0.8-1.5 wt% and the V / C concentration ratio of the bearing surface to 1-2.5. Patent Document 1 states that V carbides are formed on the surface of this rolling bearing, increasing hardness at high temperatures and thereby extending the rolling contact fatigue life.

[0006] 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.

[0007] 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 components, such as bearing components, 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 component from the outside. The penetrated hydrogen causes structural changes in part of the microstructure of the mechanical structural component. Structural changes during use of the mechanical structural component cause peeling on the surface of the mechanical structural component, reducing the rolling fatigue life of the mechanical structural component. 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 components used in a hydrogen-generating environment are required to have an excellent rolling fatigue life in the hydrogen-generating environment.

[0008] A technology for increasing the fatigue life in a hydrogen generating environment is proposed in Japanese Patent Laid-Open Publication No. 2008-280583 (Patent Document 2).

[0009] The case-hardening steel disclosed in Patent Document 2 has a composition, in mass%, of 0.1 to 0.4% C, 0.5% or less Si, 1.5% or less Mn, 0.03% or less P, 0.03% or less S, 0.3 to 2.5% Cr, 0.1 to 2.0% Mo, 0.1 to 2.0% V, 0.050% or less Al, 0.0015% or less O, 0.025% or less N, V ​​+ Mo: 0.4 to 3.0%, and the balance being Fe and unavoidable impurities. This case-hardening steel is a carburized steel, in which the surface layer C concentration after carburizing is 0.6 to 1.2%, the surface hardness is HRC 58 or more but less than 64, and the number ratio of fine V-based carbides with a particle size of less than 100 nm is 80% or more among the V-based carbides in the surface layer. Patent Document 2 states that by trapping hydrogen that has penetrated into mechanical structural parts in fine V-based carbides on the surface, it is possible to suppress the reduction in fatigue life caused by hydrogen embrittlement in a hydrogen-generating environment. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 8-49057 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-280583 Summary of the Invention [Problem to be solved by the invention]

[0011] However, the rolling fatigue life of carburized machine structural parts in a hydrogen generating environment may be increased by a means different from that of the steel material disclosed in Patent Document 2.

[0012] An object of the present disclosure is to provide a carburized machine structural component that can achieve an excellent rolling fatigue life in a hydrogen generating environment. [Means for solving the problem]

[0013] The carburized machine structural component according to the present disclosure comprises: When the surface layer is defined as a region from the surface of the carburized machine structural component to a depth of 200 μm, a carburized hardened layer formed at least on the surface layer, a core portion located inside the carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.10~0.45%, Si: 0.05 to 0.80% Mn: 0.40 to 1.50% P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to 0.80%, Mo: 0.05 to 0.35% V: 0.06 to 0.40%, Al: 0.005 to 0.100%, N: 0.030% or less, and O: 0.0015% or less, the balance being Fe and impurities, In the surface layer, the number density of MC type carbides having a circle equivalent diameter of 20 to 300 nm and an N concentration of 10% 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.

[0014] The carburized machine structural component according to the present disclosure comprises: When the surface layer is defined as a region from the surface of the carburized machine structural component to a depth of 200 μm, a carburized hardened layer formed at least on the surface layer, a core portion located inside the carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.10~0.45%, Si: 0.05 to 0.80% Mn: 0.40 to 1.50% P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to 0.80%, Mo: 0.05 to 0.35% V: 0.06 to 0.40%, 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, In the surface layer, the number density of MC type carbides having a circle equivalent diameter of 20 to 300 nm and an N concentration of 10% 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, Cu: 0.40% or less, and Ni: 0.30% 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 [Effects of the Invention]

[0015] The carburized 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]

[0016] [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 simulated carburized machine structural part. [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

[0017] The present inventors have conducted research into carburized machine structural parts that can provide excellent rolling fatigue life in a hydrogen generating environment.

[0018] The rolling contact fatigue life in a hydrogen generating environment is reduced by the occurrence of spalling due to hydrogen on the surface of carburized machine structural components. Therefore, the inventors investigated the causes of spalling due to hydrogen in a hydrogen generating environment.

[0019] In a hydrogen-generating environment, spalling on the surface of carburized machine structural components is thought to occur via the following mechanism: As described above, when hydrogen is generated in a hydrogen-generating environment, the generated hydrogen penetrates into the carburized machine structural component. When the penetrated hydrogen condenses on the surface layer of the carburized 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 carburized machine structural component is reduced.

[0020] In order to suppress the aggregation of the invaded hydrogen, it is effective to disperse fine precipitates and trap hydrogen. In this specification, fine precipitates refer to precipitates with a circle-equivalent diameter of 20 to 300 nm. Here, Mo and V combine with carbon that has penetrated into the surface layer of a machine structural component during the carburizing process in the manufacturing process 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 the carburized machine structural component. In other words, by including sufficient amounts of Mo and V in the chemical composition of the carburized machine structural component, the amount of MC-type carbides formed in the surface layer can be increased. As a result, the aggregation of the invaded hydrogen is suppressed.

[0021] Based on the above findings, the present inventors have investigated the chemical composition of the core portion of a carburized machine structural component that has a carburized hard layer formed at least on the surface and a core portion located deeper than the carburized hard layer. As a result, it was found that if the chemical composition of the core contains, in mass %, C: 0.10 to 0.45%, Si: 0.05 to 0.80%, Mn: 0.40 to 1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to 0.80%, Mo: 0.05 to 0.35%, V: 0.06 to 0.40%, Al: 0.005 to 0.100%, N: 0.030% or less, and O: 0.0015% or less, and if any optional elements are contained, further contains one or more elements selected from the group consisting of the above-mentioned Groups 1 to 4 in place of part of the Fe, with the remainder consisting of Fe and impurities, the rolling fatigue life in a hydrogen-generating environment will be increased.

[0022] The present inventors have focused on the microstructure of the surface layer in order to further increase the rolling fatigue life in a hydrogen generating environment.

[0023] The finer the prior austenite grains, the more effectively they suppress prior austenite grain boundary cracking. In other words, by reducing the prior austenite grain size in the surface layer, spalling on the surface of carburized machine structural components can be suppressed. As a result, the rolling contact fatigue life in a hydrogen generating environment is improved.

[0024] Induction hardening, for example, is an effective method for refining prior austenite grains in the surface layer of carburized machine structural components. In particular, carburized machine structural components with a core having the aforementioned chemical composition have fine MC carbides dispersed in the surface layer. Therefore, induction hardening is expected to sufficiently refine the prior austenite grains through the pinning effect. However, even when induction hardening is performed on carburized machine structural components with a core having the aforementioned chemical composition, there are cases in which the rolling contact fatigue life in a hydrogen generating environment is not sufficiently improved. Therefore, when the microstructure of the surface layer of carburized machine structural components that did not sufficiently improve the rolling contact fatigue life in a hydrogen generating environment was observed, it was found that the fine MC carbides dissolved during induction hardening, resulting in an insufficient pinning effect. As a result, the prior austenite grain size in the surface layer was not sufficiently refined.

[0025] 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 N concentration in atomic % in MC carbides is 10% or more, dissolution during induction hardening can be suppressed even if the particle size is fine (20 to 300 nm).

[0026] Therefore, the inventors further investigated the relationship between the number density of fine MC carbides with an N concentration of 10% or more in atomic percent, the prior austenite grain size in the surface layer, and the rolling fatigue life in a hydrogen generating environment. As a result, it was found that in the surface layer of a carburized machine structural part whose core has the above-mentioned chemical composition, the number density of MC carbides with an equivalent circle diameter of 20 to 300 nm and an N concentration of 10% or more in atomic percent 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 if the core of a carburized machine structural part has 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, an excellent rolling contact fatigue life can be obtained in a hydrogen-generating environment.

[0027] The carburized machine structural part of this embodiment has been completed based on the above technical concept and has the following configuration.

[0028] The carburized machine structural component of the first configuration is When the surface layer is defined as a region from the surface of the carburized machine structural component to a depth of 200 μm, a carburized hardened layer formed at least on the surface layer, a core portion located inside the carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.10~0.45%, Si: 0.05 to 0.80% Mn: 0.40 to 1.50% P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to 0.80%, Mo: 0.05 to 0.35% V: 0.06 to 0.40%, Al: 0.005 to 0.100%, N: 0.030% or less, and O: 0.0015% or less, the balance being Fe and impurities, In the surface layer, the number density of MC type carbides having a circle equivalent diameter of 20 to 300 nm and an N concentration of 10% 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.

[0029] The second configuration of the carburized mechanical structural part is When the surface layer is defined as a region from the surface of the carburized machine structural component to a depth of 200 μm, a carburized hardened layer formed at least on the surface layer, a core portion located inside the carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.10~0.45%, Si: 0.05 to 0.80% Mn: 0.40 to 1.50% P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to 0.80%, Mo: 0.05 to 0.35% V: 0.06 to 0.40%, 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, In the surface layer, the number density of MC type carbides having a circle equivalent diameter of 20 to 300 nm and an N concentration of 10% 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, Cu: 0.40% or less, and Ni: 0.30% 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

[0030] The third configuration of carburized mechanical structural parts is A carburized machine structural component of a second configuration, The chemical composition of the core contains the first group.

[0031] The fourth configuration of carburized mechanical structural parts is: A carburized machine structural part having the second or third configuration, The chemical composition of the core contains the second group.

[0032] The fifth configuration of carburized mechanical structural parts is: A carburized machine structural part having any one of the second to fourth configurations, The chemical composition of the core contains the third group.

[0033] The sixth configuration of carburized mechanical structural parts is: A carburized machine structural part having any one of the second to fifth configurations, The chemical composition of the core contains the fourth group.

[0034] The carburized machine structural component of this embodiment will be described in detail below. Note that "%" for elements means mass % unless otherwise specified.

[0035] [Configuration of the carburized machine structural component of this embodiment] The carburized mechanical structural component of this embodiment refers to a mechanical structural component that has been carburized. In this specification, the surface layer is defined as the region from the surface of the carburized mechanical structural component to a depth of 200 μm. The carburized mechanical structural component of this embodiment has a carburized hardened layer and a core portion located deeper than the carburized hardened layer.

[0036] A carburized hardened layer is a layer that has been hardened by the penetration of carbon during carburizing. The carburized hardened layer is formed from the surface of a carburized mechanical structural part to a predetermined depth that includes at least the surface layer. Here, if the carbon concentration at a depth of 200 μm from the surface of a carburized mechanical structural part is 0.50% by mass or more, it is determined that a carburized hardened layer has formed at least in the surface layer. The microstructure of the carburized hardened layer is primarily composed of martensite.

[0037] The core is the area deeper than the carburized hard layer, and is an area that is not affected by the penetration and diffusion of C caused by the carburizing treatment. It is a technical matter well known to those skilled in the art that the carburized hard layer and the core can be distinguished by well-known microstructural observation.

[0038] [Method for measuring carbon concentration at a depth of 200 μm from the surface] The carbon concentration at a depth of 200 μm from the surface is determined using an electron probe microanalyzer (EPMA). Specifically, a test piece is taken from the carburized machine structural part, with the observation surface covering a depth of 200 μm from the surface. The size of the test piece is not particularly limited, as long as the observation surface includes a 50 μm x 50 μm measurement area centered at a depth of 200 μm. The observation surface is parallel to the depth direction from the surface of the carburized machine structural part.

[0039] The carbon concentration (mass%) is measured on the observation surface of the obtained test piece using an EPMA. Specifically, point analysis is performed at 5 μm intervals in the above-mentioned measurement area of ​​the observation surface. The arithmetic mean value of the carbon concentration (mass%) at each measurement point obtained in the 11 x 11 point analysis is defined as the carbon concentration (mass%) at a depth of 200 μm from the surface. Note that the EPMA point analysis is performed with an acceleration voltage of 15 kV, a probe current of 500 nA, and an electron beam diameter of 3 μm.

[0040] [Characteristics of the carburized machine structural part of this embodiment] The carburized machine structural component of this embodiment has the following features. (Feature 1) The chemical composition of the core is, in mass %, C: 0.10 to 0.45%, Si: 0.05 to 0.80%, Mn: 0.40 to 1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to 0.80%, Mo: 0.05 to 0.35%, V: 0.06 to 0.40%, 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 of the core contains, in mass %, C: 0.10 to 0.45%, Si: 0.05 to 0.80%, Mn: 0.40 to 1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to 0.80%, Mo: 0.05 to 0.35%, V: 0.06 to 0.40%, 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 consisting of Fe and impurities. (Feature 2) In the surface layer, the number density of MC type carbides with a circle equivalent diameter of 20 to 300 nm and an N concentration of 10% or more in atomic percent is 5.0 pieces / μm 2 That's all. (Feature 3) The prior austenite grain size in the surface layer is 6.0 μm or less. Each feature will be explained below.

[0041] [(Feature 1) Chemical composition] The chemical composition of the core of the carburized machine structural component of this embodiment contains the following elements.

[0042] C: 0.10 to 0.45% Carbon (C) improves the hardenability of carburized machine structural components. As a result, the strength of the carburized machine structural components is increased. Furthermore, C combines with Mo and V to form MC-type carbides during the carburizing process in the manufacturing process. These MC-type carbides trap hydrogen that has penetrated into the carburized machine structural components when used in a hydrogen-generating environment. This suppresses the aggregation of the penetrated hydrogen and the spalling on the surface of the carburized machine structural components. As a result, the rolling fatigue life of the carburized machine structural components in a hydrogen-generating environment is improved. If the C content is less than 0.10%, the above effects cannot be sufficiently obtained, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content exceeds 0.45%, the steel material used for carburized machine structural parts becomes too hard, resulting in reduced machinability and the formation of coarse precipitates. These coarse precipitates are likely to become crack initiation sites when carburized machine structural parts are used in hydrogen-generating environments. This may result in a reduced rolling fatigue life of carburized machine structural parts in hydrogen-generating environments. Therefore, the C content is 0.10 to 0.45%. The lower limit of the C content is preferably 0.12%, more preferably 0.14%, even more preferably 0.16%, even more preferably 0.18%, and even more preferably 0.23%. The upper limit of the C content is preferably 0.44%, more preferably 0.43%, even more preferably 0.42%, and still more preferably 0.39%.

[0043] Si: 0.05 to 0.80% Silicon (Si) improves the hardenability of carburized machine structural components. Si also increases the temper softening resistance of the carburized hardened layer. Si also inhibits the penetration of hydrogen into carburized machine structural components, thereby increasing the rolling fatigue life of carburized machine structural components in hydrogen-generating environments. If the Si content is less than 0.05%, the above effects cannot be fully achieved, even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Si content exceeds 0.80%, the steel material used to make the carburized 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%.

[0044] Mn: 0.40 to 1.50% Manganese (Mn) improves the hardenability of carburized machine structural components. As a result, the strength of the carburized machine structural components increases. This increases the rolling fatigue life of the carburized machine structural components in a hydrogen-generating environment. If the Mn content is less than 0.40%, 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 the carburized machine structural parts becomes too high, and the machinability of the steel material decreases. Therefore, the Mn content is 0.40 to 1.50%. The lower limit of the Mn content is preferably 0.45%, more preferably 0.50%, even more preferably 0.55%, and still more preferably 0.60%. 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%.

[0045] 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 specified in 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 carburized machine structural components in a hydrogen-generating environment will decrease. Therefore, the P content is 0.015% or less. The lower the P content, the better. However, excessively reducing 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%.

[0046] 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 carburized machine structural components in a hydrogen-generating environment. If the S content exceeds 0.005%, the sulfide-based inclusions become coarse even if the contents of other elements are within the ranges of this embodiment. As a result, the rolling fatigue life of carburized machine structural components in a hydrogen-generating environment decreases. Therefore, the S content is 0.005% or less. The S content is preferably as low as possible. However, excessively reducing the S content increases the production cost. 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 less than 0.005%, more preferably 0.004%, and even more preferably 0.003%.

[0047] Cr: 0.05 to 0.80% Chromium (Cr) improves the hardenability of carburized machine structural parts. As a result, the strength of the carburized machine structural parts increases. Therefore, the rolling fatigue life of the carburized machine structural parts 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, Cr promotes the penetration of hydrogen into carburized machine structural components. If the Cr content exceeds 0.80%, even if the contents of other elements are within the ranges of this embodiment, it will not be possible to sufficiently suppress the penetration of hydrogen during use of the carburized machine structural component in a hydrogen generating environment. As a result, the rolling contact fatigue life of the carburized machine structural component in a hydrogen generating environment will be reduced. Therefore, the Cr content is 0.05 to 0.80%. 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 0.70%, more preferably 0.60%, even more preferably less than 0.50%, even more preferably 0.49%, even more preferably 0.45%, and even more preferably 0.40%.

[0048] Mo: 0.05 to 0.35% Molybdenum (Mo) forms MC-type carbides together with C and V during the carburizing treatment in the manufacturing process. These MC-type carbides trap hydrogen that penetrates into the carburized machine structural component when it is used in a hydrogen-generating environment. This suppresses the aggregation of the penetrated hydrogen and the spalling on the surface of the carburized machine structural component. As a result, the rolling fatigue life of the carburized machine structural component in a hydrogen-generating environment is improved. If the Mo 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 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 to make the carburized machine structural parts becomes excessively high, resulting in a decrease in the machinability of the steel material. Therefore, the Mo content is 0.05 to 0.35%. The lower limit of the Mo content is preferably 0.06%, more preferably 0.08%, even more preferably 0.12%, and still more preferably 0.16%. The upper limit of the Mo content is preferably 0.33%, more preferably 0.30%, and further preferably 0.28%.

[0049] V: 0.06 to 0.40% Vanadium (V) forms MC-type carbides together with C and Mo during the carburizing treatment in the manufacturing process. These MC-type carbides trap hydrogen that penetrates into the carburized machine structural component when it is used in a hydrogen-generating environment. This suppresses the aggregation of the penetrated hydrogen and the spalling on the surface of the carburized machine structural component. As a result, the rolling fatigue life of the carburized machine structural component in a hydrogen-generating environment is improved. If the V 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 V content exceeds 0.40%, coarse MC-type carbides may be formed. These coarse MC-type carbides are likely to become the starting point for cracks when carburized 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 carburized machine structural components in a hydrogen-generating environment may be reduced. Furthermore, if the V content exceeds 0.40%, the machinability of the steel material from which the carburized machine structural components are made may be reduced. Therefore, the V content is 0.06 to 0.40%. The lower limit of the V content is preferably 0.08%, more preferably 0.10%, even more preferably 0.12%, even more preferably 0.14%, and even 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%.

[0050] 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 the steel material used to make carburized machine structural parts is improved. If the Al content is less than 0.005%, the above effects cannot be fully achieved even if the contents of other elements are within the ranges of this embodiment. On the other hand, 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 form. The coarse clustered oxides will become the starting points for cracks when the carburized 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 contact fatigue life of the carburized 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%.

[0051] N: 0.030% or less Nitrogen (N) concentrates in MC carbides and increases their thermal stability. This suppresses the dissolution of MC carbides during induction hardening, enhancing the pinning effect of the MC carbides. As a result, the prior austenite grains in the surface layer of carburized machine structural parts after induction hardening are refined, improving the rolling fatigue life in a hydrogen generating environment. Even if only a small amount of N is included, the above effects can be achieved to some extent. Therefore, the N content is set to be greater than 0%. On the other hand, if the N content exceeds 0.030%, the hot workability of the steel material used to make the carburized machine structural parts 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 lower limit of the N content is preferably 0.001%, and more preferably 0.010%. The upper limit of the N content is preferably 0.028%, more preferably 0.025%, and even more preferably 0.020%.

[0052] 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 carburized machine structural components in a hydrogen generating environment. If the O content exceeds 0.0015%, the rolling fatigue life of carburized 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, excessively reducing the O content increases the production cost. Therefore, in consideration of 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%.

[0053] The remainder of the chemical composition of the core of the carburized machine structural component of this embodiment is composed of 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 from the manufacturing environment, during industrial production of the carburized machine structural component, and are acceptable within a range that does not adversely affect the carburized machine structural component of this embodiment.

[0054] [About optional elements] The chemical composition of the core of the carburized 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, Cu: 0.40% or less, and Ni: 0.30% 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.

[0055] [Group 1: Ti and Nb] The chemical composition of the core of the carburized 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 part of Fe. These elements are optional elements, and both form precipitates, thereby increasing the strength of the carburized machine structural component through precipitation strengthening.

[0056] 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 carburized 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 form, which reduces the rolling fatigue life of the carburized machine structural component 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%.

[0057] 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 carburized 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 form, which reduces the rolling fatigue life of the carburized machine structural component 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%.

[0058] [Group 2: B, Cu and Ni] The chemical composition of the core of the carburized machine structural component of this embodiment may further contain one or more elements selected from the group consisting of B, Cu, and Ni in place of a portion of Fe. These elements are optional, and all of them improve the hardenability and strength of the carburized machine structural component.

[0059] 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 carburized machine structural parts. Furthermore, it suppresses the grain boundary segregation of P. As a result, the strength of carburized machine structural parts is increased. As a result, the rolling fatigue life of carburized machine structural parts in a hydrogen generating environment is extended. 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 the carburized machine structural parts becomes excessively high, resulting in a decrease in 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%.

[0060] Cu: 0.40% or less Copper (Cu) is an optional element and may not be contained, that is, the Cu content may be 0%. When Cu is present, that is, when the Cu content is greater than 0%, Cu improves the hardenability and strength of the carburized machine structural parts. As a result, the rolling fatigue life of the carburized machine structural parts in a hydrogen generating environment is improved. Even if even a small amount of Cu is present, the above effects can be obtained to some extent. However, if the Cu content exceeds 0.40%, even if the contents of other elements are within the ranges of this embodiment, the hardness of the steel material used to make the carburized machine structural component becomes excessively high, resulting in a decrease in the machinability of the steel material. Therefore, the Cu content is 0 to 0.40%, and if contained, it is 0.40% or less. The lower limit of the Cu content is preferably 0.01%, and more preferably 0.02%. The upper limit of the Cu content is preferably 0.30%, more preferably 0.25%, and even more preferably 0.20%.

[0061] Ni: 0.30% or less Nickel (Ni) is an optional element and may not be contained, that is, the Ni content may be 0%. When contained, that is, when the Ni content is greater than 0%, Ni improves the hardenability of carburized machine structural parts and increases their strength. As a result, the rolling fatigue life of carburized machine structural parts in hydrogen-generating environments is improved. Even if even a small amount of Ni is contained, the above effects can be achieved to some extent. However, if the Ni content exceeds 0.30%, even if the contents of other elements are within the ranges of this embodiment, the hardness of the steel material used to make the carburized machine structural parts becomes excessively high, and as a result, the machinability of the steel material decreases. Therefore, the Ni content is 0 to 0.30%, and if Ni is contained, it is 0.30% or less. The lower limit of the Ni content is preferably 0.01%, and more preferably 0.02%. The upper limit of the Ni content is preferably 0.25%, and more preferably 0.20%.

[0062] [Group 3: Sn] The chemical composition of the core of the carburized 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 Sn is contained, that is, when the Sn content is more than 0%, Sn improves the machinability of the steel material that is used to make carburized 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 the carburized 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%.

[0063] [Group 4: Ca and Mg] The chemical composition of the core of the carburized 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, and all of them refine sulfides and increase the rolling fatigue life of the carburized machine structural component in a hydrogen-generating environment.

[0064] 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 carburized machine structural parts. Furthermore, Ca promotes the spheroidization of the sulfides. As a result, the rolling fatigue life of the carburized 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%, coarse Ca oxides may form in the carburized 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 carburized 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%.

[0065] 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 present, that is, when the Mg content is greater than 0%, Mg refines the sulfides contained in the carburized machine structural parts. Furthermore, Mg promotes the spheroidization of the sulfides. As a result, the rolling fatigue life of the carburized machine structural parts in a hydrogen generating environment is improved. Even if even a small amount of Mg is present, the above effects can be achieved to some extent. However, if the Mg content exceeds 0.0050%, coarse Mg oxides may form in the carburized 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 carburized 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%.

[0066] [(Feature 2) Number density ND of MC type carbides with a circle equivalent diameter of 20 to 300 nm and an N concentration of 10% or more in atomic percent] The carburized machine structural part 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 300 nm and an N concentration of 10% or more in atomic percent is 5.0 particles / μm 2Here, the number density of MC type carbides in the surface layer, which have a circle equivalent diameter of 20 to 300 nm and an N concentration of 10% or more in atomic %, is defined as ND (number / μm 2 ) is defined as

[0067] As described above, fine MC carbides with an equivalent circle diameter of 20 to 300 nm can trap hydrogen that penetrates into carburized machine structural components and suppress hydrogen aggregation. Furthermore, if the N concentration in atomic percent of the MC carbides is 10% or more, the thermal stability of the MC carbides is enhanced. Therefore, when carburized machine structural components are induction hardened, the 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 the 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 300 nm and an N concentration in atomic percent of 10% or more in the surface layer, it is possible to achieve both the hydrogen trapping effect and the pinning effect of the MC carbides.

[0068] Number density ND is 5.0 pieces / μm 2 If the above conditions are met, the hydrogen trapping and pinning effects of the MC carbides are sufficiently obtained, and therefore, provided that the carburized machine structural part satisfies Features 1 and 3, it can obtain an excellent rolling contact fatigue life in a hydrogen generating environment.

[0069] 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.

[0070] [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 300 nm and an N concentration of 10% 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 carburized mechanical structural part. 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 carburized mechanical structural part. 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 immersed 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.

[0071] The sampled carbon film is observed using a transmission electron microscope (TEM). 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 300 nm within the observation field are identified. Quantitative analysis is then performed on the identified particles with a circular equivalent diameter of 20 to 300 nm using energy dispersive X-ray spectroscopy (EDS) attached to the TEM. EDS analysis is used to identify all particles with a circular equivalent diameter of 20 to 300 nm that have an N concentration of 10% or more and a V concentration of 15% or more in atomic percent. In carburized machine structural parts whose core chemical composition satisfies characteristic 1, particles with a circle equivalent diameter of 20 to 300 nm, an N concentration of 10% or more in atomic percent, and a V concentration of 15% or more 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.

[0072] Here, the deeper the position from the surface of the carburized machine structural component, the lower the number density of MC type carbides having a circle equivalent diameter of 20 to 300 nm and an N concentration in atomic % of 10% 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).

[0073] [(Feature 3) Prior austenite grain size D in the surface layer] Furthermore, the carburized machine structural component of this embodiment has a prior austenite grain size in the surface layer of 6.0 μm or less, where the prior austenite grain size in the surface layer is defined as D (μm).

[0074] As mentioned above, spalling on the surface of carburized machine structural components in a hydrogen-generating environment begins with prior austenite grain boundary cracking. Here, by reducing the prior austenite grain size D in the surface layer, prior austenite grain boundary cracking in the surface layer can be suppressed. As a result, spalling on the surface of carburized machine structural components can be suppressed, and the rolling fatigue life can be extended.

[0075] 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 carburized machine structural component. The carburized machine structural component of this embodiment satisfies Feature 2, and therefore achieves a sufficient pinning effect. Therefore, induction hardening can further refine the prior austenite grain size D in the surface layer.

[0076] 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, carburized machine structural parts can achieve excellent rolling contact fatigue life in a hydrogen generating environment, provided that they satisfy Features 1 and 2.

[0077] 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.

[0078] [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 carburized 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 carburized mechanical structural component. The observation surface is mirror-polished. After mirror polishing, the 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. The observation area is then observed using an optical microscope, 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.

[0079] Here, the deeper the position from the surface of the carburized 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).

[0080] [Effects of the carburized machine structural parts of this embodiment] The carburized machine structural component of this embodiment satisfies Features 1 to 3. Therefore, an excellent rolling fatigue life can be obtained in a hydrogen generating environment.

[0081] [Use of carburized machine structural parts according to this embodiment] The carburized machine structural component of this embodiment is widely applicable to fields requiring a long rolling fatigue life. The carburized machine structural component is suitable, for example, as a carburized bearing component. The bearing component refers to a rolling bearing component. Examples of bearing components include a raceway ring, a washer, and rolling elements. 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 elements may be balls or rollers. Examples of rollers include cylindrical rollers, rod rollers, needle rollers, tapered rollers, and convex rollers. It should be noted that the carburized machine structural component of this embodiment is naturally applicable to applications other than bearing components. Examples of applications other than bearing components include gears, constant velocity joints, and ball screws.

[0082] [Manufacturing method for carburized machine structural parts] An example of a method for manufacturing a carburized machine structural component of this embodiment will be described below. The method for manufacturing a carburized machine structural component described below is one example for manufacturing a carburized machine structural component of this embodiment. Therefore, a carburized 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 method for manufacturing a carburized machine structural component of this embodiment.

[0083] An example of the method for manufacturing the carburized machine structural component of this embodiment includes the following steps. (Process 1) Steel material preparation process (Process 2) Hot processing process (Process 3) Carburizing process (Process 4) High-frequency hardening process Each step will be described below.

[0084] [(Process 1) Steel material preparation process] In the steel preparation step, a steel material is prepared as the raw material for the carburized machine structural component of this embodiment. Specifically, molten steel having a chemical composition that satisfies Feature 1 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 Feature 1 is produced.

[0085] 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.

[0086] [(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 holding the steel at a heating temperature equal to or higher than the above point for a predetermined time, the steel is processed. The hot-worked intermediate product is cooled to room temperature. If necessary, the hot-worked intermediate product is cut. Note that the hot-worked or cut intermediate product may be subjected to a known heat treatment. Examples of known heat treatments include normalizing and / or spheroidizing.

[0087] 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.

[0088] [(Condition 1) Holding time t1 at a heating temperature of 1100°C or higher] In order to increase the number density of MC carbides with an N concentration of 10% or more in atomic percent, it is necessary to maintain a sufficient amount of N in solid solution within the grains until just before the carburizing process, in which the MC carbides precipitate. By heating the intermediate to 1100°C or higher, the N-containing precipitates and inclusions in the intermediate dissolve, increasing the N solid solution concentration within the grains. Therefore, the longer the holding time t1 (minutes) at a heating temperature of 1100°C or higher, the better. There is no particular upper limit to the heating temperature of the intermediate, but considering normal industrial production, it is, for example, 1300°C.

[0089] If the holding time t1 at a heating temperature of 1100°C or higher is 30 minutes or longer, the solid solution concentration of N within the grains will be sufficiently increased. As a result, the manufactured carburized machine structural parts will be able to satisfy Feature 2. 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. In consideration of normal industrial production, the upper limit of the retention time t1 is, for example, 300 minutes or less.

[0090] [(Condition 2) Average cooling rate R from 1050°C to 850°C] In the temperature range of 850 to 1050°C, the formation of N-containing precipitates and the growth of N-containing precipitates and inclusions are promoted. As a result, the amount of N dissolved in the grains decreases. As mentioned above, it is necessary to maintain a sufficient amount of N dissolved in the grains until just before the carburizing process, so the faster the average cooling rate R (°C / sec) from 1050°C to 850°C, the better.

[0091] 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 / s, the decrease in the solid solution concentration of N within the grains can be sufficiently suppressed. As a result, the manufactured carburized machine structural parts can satisfy Feature 2. 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 / s. 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.

[0092] [(Process 3) Carburizing process] In the carburizing process, the intermediate product is subjected to carburizing treatment to produce a carburized machine structural component having a carburized hardened layer formed at least on the surface. The carburizing treatment includes carburizing and quenching and tempering. In the carburizing and quenching, the intermediate product is subjected to A in an atmosphere containing a carburizing gas with a predetermined carbon potential. c3 The carburized and quenched intermediate product is heated to and held at a temperature above this point, and then rapidly cooled. In the tempering process, the carburized and quenched intermediate product is held for a predetermined time within a temperature range of, for example, 100 to 200°C. Here, the carburizing and quenching gas refers to the well-known endothermic quenching gas (RX gas). RX gas is a gas produced by mixing a hydrocarbon gas such as butane or propane with air and passing it through a heated Ni catalyst to cause a reaction, and is a mixed gas containing CO, H2, N2, etc. Note that well-known sub-zero treatment may be performed on the carburized and quenched intermediate product or the tempered carburized machine structural component.

[0093] The carburizing process meets the following conditions: (Condition 3) In carburizing and quenching, the holding time t2 at a heating temperature of 930° C. or higher in an atmosphere with a carbon potential of 0.80% or higher is set to 20 to 300 minutes. Condition 3 will be explained below.

[0094] [(Condition 3) Holding time t2 at a heating temperature of 930°C or higher in an atmosphere with a carbon potential of 0.80% or higher] During carburizing and quenching in the carburizing process, carbon that penetrates into the surface layer bonds with Mo and V to form MC carbides. If the hot working process satisfies the above conditions 1 and 2, MC carbides with a sufficiently high N concentration can be obtained.

[0095] However, if the holding time t2 at a heating temperature of 930°C or higher in an atmosphere with a carbon potential of 0.80% or higher is less than 20 minutes during carburizing and quenching, fine MC carbides are not sufficiently generated. On the other hand, if the holding time t2 at a heating temperature of 930°C or higher in an atmosphere with a carbon potential of 0.80% or higher exceeds 300 minutes, Ostwald ripening of the resulting MC carbides is excessively promoted, resulting in a decrease in the number density of fine MC carbides.

[0096] If the holding time t2 at a heating temperature of 930°C or higher in an atmosphere with a carbon potential of 0.80% or higher is 20 to 300 minutes, the number density of fine MC carbides is sufficiently increased. As a result, the manufactured carburized machine structural parts can satisfy Feature 2. Therefore, in carburizing and quenching, the holding time t2 at a heating temperature of 930°C or higher in an atmosphere with a carbon potential of 0.80% or higher is set to 20 to 300 minutes. A preferred lower limit of the retention time t2 is 40 minutes. A preferred upper limit of the retention time t2 is 180 minutes.

[0097] [(Process 4) High-frequency hardening process] In the induction hardening process, high-frequency heating is used to harden the surface of carburized machine structural parts. c3The carburized machine structural component is heated to a temperature higher than the induction hardening point and then cooled. In this case, the surface layer of the carburized machine structural component transforms to austenite, and then transforms into a hard structure mainly composed of martensite. This increases the strength of the carburized machine structural component. Furthermore, the prior austenite grain size in the surface layer is refined compared to before the induction hardening process. In particular, the carburized machine structural component of this embodiment satisfies Feature 2, so a sufficient pinning effect is obtained, and the prior austenite grain size in the surface layer is likely to be further refined. Therefore, the carburized machine structural component can satisfy Feature 3 by using a well-known induction hardening process.

[0098] The induction hardening process involves heating the surface of carburized machine structural parts to A c3 There are no particular limitations as long as the material can be heated to a temperature above this point. That is, the induction hardening process 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. The induction hardening process may also be repeated two or more times.

[0099] The carburized machine structural component after the induction hardening process may be 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.

[0100] Furthermore, the carburized machine structural component after the induction hardening process or tempering process may be subjected to finishing, such as polishing.

[0101] By the above steps, a carburized machine structural part having Features 1 to 3 is manufactured. [Example]

[0102] Steel materials having the chemical compositions shown in Table 1 (Table 1A and Table 1B) were manufactured as materials for carburized machine structural parts by the following method. Note that the steel material with test number 25 was a reference steel material, and had a chemical composition equivalent to SCM420 specified in JIS G 4053 (2016).

[0103] [Table 1A]

[0104] [Table 1B]

[0105] 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.

[0106] From the produced steel, several test pieces (small roller test pieces) for roller pitting fatigue tests were produced as simulated carburized machine structural parts, as shown in Figure 1. The numbers in Figure 1 indicate dimensions (unit: mm). "φ" in the figure means diameter.

[0107] Specifically, a hot working process was performed 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 hot worked steel bar was air-cooled to room temperature. At this time, the average cooling rate R (°C / second) from 1050°C to 850°C was as shown in Table 2. The produced steel bar with a diameter of 35 mm was normalized at a heating temperature of 925°C for a holding time of 60 minutes, and the normalized steel bar was air-cooled to room temperature. The 35 mm diameter steel bar was then machined (cut) to produce an intermediate product with the shape shown in Figure 1.

[0108] [Table 2]

[0109] The manufactured intermediate products were subjected to a carburizing process. The carburizing process began with carburizing and quenching. Specifically, for test numbers 1 to 20 and 23 to 25, the intermediate products were heated at 930°C for t2 minutes in an atmosphere with a carbon potential of 0.80 to 1.00%. The holding time t2 (minutes) for each test number is shown in Table 2. Subsequently, the intermediate products were heated at 900°C for 30 minutes in an atmosphere with a carbon potential of 0.80%. After that, they were oil-quenched in 60°C oil. Meanwhile, for test numbers 21 and 22, the intermediate products were heated at 900°C for 90 minutes in an atmosphere with a carbon potential of 0.80%. After that, they were oil-quenched in 60°C oil. In other words, for test numbers 21 and 22, the holding time t2 at a heating temperature of 930°C or higher in an atmosphere with a carbon potential of 0.80% or higher was 0 minutes.

[0110] After oil cooling, the test pieces with each test number were tempered at a tempering temperature of 180°C for a holding time of 120 minutes. After the holding time had elapsed, the intermediate products were air-cooled. Using the above gas carburizing method, carburized intermediate products with each test number were manufactured.

[0111] The manufactured carburized intermediate products were subjected to an induction hardening process. In the induction hardening process, the surface layer of the carburized intermediate product was heated to 900°C using a circular high-frequency heating device. After that, the surface layer of the carburized intermediate product was rapidly cooled to room temperature using a circular cooling device. After the induction hardening process, the carburized intermediate product was subjected to a tempering process. In the tempering process, the carburized intermediate product was held at 180°C for 120 minutes, and then air-cooled to room temperature. After that, 50 μm was removed from the surface of the carburized intermediate product by grinding.

[0112] Using the above manufacturing process, simulated carburized machine structural parts (small roller test pieces) of each test number were manufactured. The carbon concentration at a depth of 200 μm from the surface of each simulated carburized machine structural part was measured using the method described above in [Method for measuring carbon concentration at a depth of 200 μm from the surface]. As a result, the carbon concentration at a depth of 200 μm from the surface of all simulated carburized machine structural parts of all test numbers was 0.50% by mass or more. In other words, all simulated carburized machine structural parts of all test numbers had a carburized hardened layer at least on the surface.

[0113] [Evaluation test] The following evaluation tests were carried out on the simulated carburized 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.

[0114] [(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.

[0115] [(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 carburized machine structural component for each test number was measured. The prior austenite grain size D (μm) obtained for each test number is shown in Table 2.

[0116] [(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 carburized machine structural parts with each test number. Specifically, the following roller pitting fatigue test was carried out.

[0117] 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.

[0118] 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.

[0119] 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 2 A 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.

[0120] 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.

[0121] 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 25, whose chemical composition meets the SCM420 standard of JIS G 4053 (2016), as the standard, the rolling contact fatigue life ratio for each test number was calculated using the following formula. Rolling fatigue life ratio = (rolling fatigue life of each test number) / (rolling fatigue life of test number 25) 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.

[0122] [Test Results] Referring to Table 1 (Table 1A and Table 1B) and Table 2, the simulated carburized machine structural parts of test numbers 1 to 16 satisfied features 1 to 3. Therefore, in the rolling fatigue life evaluation test in a hydrogen generation environment, an excellent rolling fatigue life was obtained.

[0123] On the other hand, in test numbers 17 and 18, 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.

[0124] In test numbers 19 and 20, 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.

[0125] In test numbers 21 and 22, the holding time t2 in the carburizing process was too short. Therefore, although the prior austenite grain size D was small, the number density ND was too low. As a result, an excellent rolling fatigue life was not obtained in the rolling fatigue life evaluation test in a hydrogen generation environment.

[0126] In test numbers 23 and 24, the holding time t2 in the carburizing process was too long. 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.

[0127] 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 carburized machine structural part, When the surface layer is defined as a region from the surface of the carburized machine structural component to a depth of 200 μm, a carburized hard layer formed at least on the surface layer, a core portion located inside the carburized hardened layer, The chemical composition of the core is, in mass %, C: 0.10-0.45%, Si: 0.05-0.80%, Mn: 0.40-1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.05-0.80%, Mo: 0.05-0.35%, V: 0.06-0.40%, Al: 0.005-0.100%, N: 0.030% or less, and O: 0.0015% or less, the balance being Fe and impurities; In the surface layer, the number density of MC type carbides having a circle equivalent diameter of 20 to 300 nm and an N concentration of 10% 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. Carburized mechanical structural parts.

2. A carburized machine structural part, When the surface layer is defined as a region from the surface of the carburized machine structural component to a depth of 200 μm, a carburized hard layer formed at least on the surface layer, a core portion located inside the carburized hardened layer, The chemical composition of the core is, in mass %, C: 0.10-0.45%, Si: 0.05-0.80%, Mn: 0.40-1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.05-0.80%, Mo: 0.05-0.35%, V: 0.06-0.40%, 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; In the surface layer, the number density of MC type carbides having a circle equivalent diameter of 20 to 300 nm and an N concentration of 10% 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. Carburized 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, Cu: 0.40% or less, and Ni: 0.30% 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

3. The carburized machine structural component according to claim 2, the chemical composition of the core portion contains the first group; Carburized mechanical structural parts.

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

5. The carburized machine structural component according to claim 2, The chemical composition of the core portion contains the third group. Carburized mechanical structural parts.

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

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