Steel and carburized mechanical structural component

Abstract

To provide a steel with superior rolling fatigue life in a hydrogen generation environment.SOLUTION: A steel according to an embodiment comprises, in mass%, C: 0.10-0.45%, Si: 0.05-0.80%, Mn: 0.40-1.50%, P: 0.015% or less, S: less than 0.005%, Cr: 0.05-0.50% (excluding 0.50%), Mo: 0.05-0.35%, V: 0.06-0.40%, Ni: 0.30-1.50% (excluding 0.30%), Al: 0.005-0.100%, N: 0.030% or less, and O: 0.0015% or less, with the balance being Fe and impurities, satisfying formula (1)-formula (3): (Si+Ni) / Cr≥1.30 (1), Mo+V≥0.30 (2) and 0.30<Mo / V≤4.00 (3), where, the element symbols in formulae (1) to (3) are substituted with the content (mass%) of the corresponding elements.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present disclosure relates to a steel material that can be used as a material for a carburized machine structural component, which is a machine structural component that has been subjected to a carburizing treatment, and to a carburized machine structural component. [Background technology]

[0002] Mining machinery, construction machinery, automobiles, etc. use machine structural parts made of steel. Machine structural parts are, for example, bearing parts. The steel material used for machine structural parts such as bearing parts is typified by SUJ2 specified in JIS G 4805 (2008). Machine structural parts are usually manufactured using steel as a material in the following manufacturing process. Hot forging is performed on the steel material, and cutting is further performed as necessary to manufacture an intermediate product of a desired shape. Heat treatment is performed on the intermediate product to adjust the hardness and microstructure of the steel. The heat treatment is, for example, quenching and tempering, carburizing treatment, or carbonitriding treatment. Machine structural parts are manufactured by the above manufacturing process.

[0003] When an improvement in fatigue life is required for a machine structural part, a carburizing treatment is performed as a heat treatment in the above-mentioned manufacturing process of the machine structural part. In the carburizing treatment, a hardened layer (carburized layer) is formed on the surface layer of the steel material, and the surface layer of the steel material is hardened. This improves the fatigue life of the machine structural part. In this specification, a machine structural part that has been subjected to a carburizing treatment is also called a "carburized machine structural part."

[0004] A technique for improving 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 steel as a material for at least one of the raceways and rolling elements, containing 0.1-0.7% by weight of C, 0.5-3.0% by weight of Cr, 0.3-1.2% by weight of Mn, 0.3-1.5% by weight of Si, 3% by weight or less of Mo, and 0.8-2.0% by weight of V. An intermediate product formed using the material is carburized to set the carbon concentration of the bearing surface to 0.8-1.5% by weight, and the V / C concentration ratio of the bearing surface to 1-2.5. Patent Document 1 describes that V carbides are generated on the surface of this rolling bearing, increasing the hardness at high temperatures, thereby increasing the rolling 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 lubricating oil used for circulation has been reduced. Therefore, in the use environment of machine structural parts, such as bearing parts, lubricating oil is easily decomposed during use, generating hydrogen. When hydrogen is generated in the use environment, hydrogen penetrates into the machine structural parts from the outside. The penetrated hydrogen causes structural changes in part of the microstructure of the machine structural parts. The structural changes during use of the machine structural parts cause cracks (peeling) in the surface vicinity of the machine structural parts, reducing the rolling fatigue life of the machine 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." Machine structural parts used in a hydrogen generating environment are required to have an excellent rolling fatigue life in the hydrogen generating environment.

[0008] A technique for improving fatigue life in a hydrogen generating environment is proposed in Japanese Patent Application Laid-Open No. 2008-280583 (Patent Document 2).

[0009] The case-hardened steel disclosed in Patent Document 2 has a composition, in mass%, of C: 0.1 to 0.4%, Si: 0.5% or less, Mn: 1.5% or less, P: 0.03% or less, S: 0.03% or less, Cr: 0.3 to 2.5%, Mo: 0.1 to 2.0%, V: 0.1 to 2.0%, Al: 0.050% or less, O: 0.0015% or less, N: 0.025% or less, V + Mo: 0.4 to 3.0%, and the balance Fe and unavoidable impurities. This case-hardened steel is a carburized steel, in which the surface layer C concentration after carburization is 0.6 to 1.2%, the surface hardness is HRC58 or more and less than 64, and the number ratio of fine V-based carbides with a grain size of less than 100 nm among the V-based carbides in the surface layer is 80% or more. Patent Document 2 describes 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 decrease 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] JP 2008-280583 A Summary of the Invention [Problem to be solved by the invention]

[0011] However, the rolling fatigue life of carburized machine structural components in a hydrogen generating environment may be improved by a means other than that of the steel material disclosed in Patent Document 2.

[0012] An object of the present disclosure is to provide a steel material and a carburized machine structural component that can be used as a raw material to obtain a sufficient rolling fatigue life in a hydrogen generating environment. [Means for solving the problem]

[0013] The steel material according to the present disclosure comprises, in mass%, C: 0.10-0.45%, Si: 0.05 to 0.80%, Mn: 0.40-1.50%, P: 0.015% or less, S: less than 0.005%, Cr: 0.05 to less than 0.50% Mo: 0.05-0.35%, V: 0.06-0.40%, 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, Satisfy formulas (1) to (3). (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 0.30 <Mo / V≦4.00 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

[0014] The steel material according to the present disclosure comprises, in mass%, C: 0.10-0.45%, Si: 0.05 to 0.80%, Mn: 0.40-1.50%, P: 0.015% or less, S: less than 0.005%, Cr: 0.05 to less than 0.50% Mo: 0.05-0.35%, V: 0.06-0.40%, 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, Satisfying formulas (1) to (3). [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) 0.30 <Mo / V≦4.00 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

[0015] The carburized machine structural part according to the present disclosure comprises: A hardened layer; A core portion is provided inside the hardened layer, The chemical composition of the core is, in mass%, C: 0.10-0.45%, Si: 0.05 to 0.80%, Mn: 0.40-1.50%, P: 0.015% or less, S: less than 0.005%, Cr: 0.05 to less than 0.50% Mo: 0.05-0.35%, V: 0.06-0.40%, 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, Satisfying formulas (1) to (3). (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 0.30 <Mo / V≦4.00 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

[0016] The carburized machine structural part according to the present disclosure comprises: A hardened layer; A core portion is provided inside the hardened layer, The chemical composition of the core is, in mass%, C: 0.10-0.45%, Si: 0.05 to 0.80%, Mn: 0.40-1.50%, P: 0.015% or less, S: less than 0.005%, Cr: 0.05 to less than 0.50% Mo: 0.05-0.35%, V: 0.06-0.40%, 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, Satisfying formulas (1) to (3). [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) 0.30 <Mo / V≦4.00 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol. Effect of the Invention

[0017] In the steel material according to the present disclosure, a sufficient rolling fatigue life can be obtained in a carburized machine structural component manufactured using the steel material as a raw material in a hydrogen generating environment. The carburized machine structural component according to the present disclosure has a sufficient rolling fatigue life in a hydrogen generating environment. [Brief description of the drawings]

[0018] [Figure 1] FIG. 1 is a side view of a roller pitting fatigue test specimen (small roller test specimen) used in a rolling fatigue life evaluation test. [Diagram 2] FIG. 2 is a schematic diagram of the roller pitting fatigue test. [Diagram 3] FIG. 3 is a front view of the large roller test piece in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] The present inventors have conducted a study on a steel material that can provide a sufficient rolling fatigue life in a hydrogen generating environment when used as a raw material to manufacture a carburized machine structural part.

[0020] The rolling fatigue life in a hydrogen generating environment is reduced by the occurrence of hydrogen-induced cracking (flaking) in the surface vicinity of carburized machine structural components. Therefore, the present inventors have investigated the cause of hydrogen-induced cracking (flaking) in a hydrogen generating environment.

[0021] In a hydrogen generating environment, cracks (flaking) in the near-surface areas of carburized machine structural components are 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 carburized machine structural components. When the penetrated hydrogen condenses, it causes a structural change and white structure is formed. When this structural change occurs in the near-surface areas of a carburized machine structural component, cracks (flaking) occur in the near-surface areas and the rolling fatigue life is reduced.

[0022] As described above, cracks (spalling) caused by hydrogen in a hydrogen generating environment occur when hydrogen penetrates into a carburized machine structural component (factor 1) and the penetrated hydrogen coagulates (factor 2). In the above-mentioned Patent Document 2, fine V precipitates are formed on the surface layer of the machine structural component, and the fine V precipitates trap the penetrated hydrogen, thereby suppressing the coagulation of the penetrated hydrogen and suppressing the occurrence of cracks. In other words, Patent Document 2 proposes a means that focuses on Factor 2.

[0023] Here, the present inventors focused on factor 1 and investigated means for suppressing hydrogen penetration. If hydrogen penetration into carburized machine structural components could be suppressed, hydrogen would not aggregate within the carburized machine structural components. Therefore, the present inventors investigated means for suppressing hydrogen penetration in a hydrogen generating environment from the viewpoint of chemical composition. As a result, the present inventors obtained the following findings.

[0024] Among the elements in the above-mentioned chemical composition, Si and Ni are elements that suppress the penetration of hydrogen into steel (carburized machine structural parts) in a hydrogen generating environment. On the other hand, Cr is an element that promotes the penetration of hydrogen into steel (carburized machine structural parts) in a hydrogen generating environment. Therefore, by ensuring a certain level of Si and Ni content in the chemical composition of the steel and keeping the Cr content as low as possible, it is possible to suppress the penetration of hydrogen in a hydrogen generating environment.

[0025] Based on the above findings, the present inventors have investigated the chemical composition of steel materials. As a result, it was considered that if a steel material 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: less than 0.005%, Cr: 0.05 to less than 0.50%, Mo: 0.05 to 0.35%, V: 0.06 to 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, and if any optional element is contained, further containing one or more elements selected from the group consisting of the above-mentioned first to fourth groups in place of a portion of Fe, with the balance being Fe and impurities, then a carburized machine structural part manufactured using the steel material can have a sufficient rolling fatigue life in a hydrogen generating environment.

[0026] However, even in the case of steel materials satisfying the above-mentioned chemical composition, there have been cases where carburized machine structural components manufactured using the steel materials do not obtain a sufficient rolling fatigue life in a hydrogen generating environment. Therefore, the present inventors have further investigated and examined the relationship between the content of each element in the chemical composition and the rolling fatigue life in a hydrogen generating environment. As a result, the present inventors have obtained the following findings.

[0027] The ratio of three elements (Si, Ni, and Cr) that affect the penetration of hydrogen into steel in a hydrogen generating environment affects the rolling fatigue life in the hydrogen generating environment. Specifically, it is defined as F1 = (Si + Ni) / Cr. If F1 is 1.30 or more, the sum of the Si content and the Ni content is sufficiently large relative to the Cr content. In this case, the penetration of hydrogen in the hydrogen generating environment can be sufficiently suppressed. Therefore, from the viewpoint of factor 1, it is possible to increase the rolling fatigue life in the hydrogen generating environment. Therefore, F1 is set to a range that satisfies formula (1). (Si+Ni) / Cr≧1.30 (1)

[0028] In order to improve the rolling fatigue life in a hydrogen generating environment, it is necessary to consider not only factor 1 but also factor 2. Forming precipitates is an effective way to trap the invaded hydrogen. Mo and V form precipitates in carburized machine structural parts made of steel material and trap the invaded hydrogen. Therefore, the total amount of Mo and V content correlates with the amount of precipitates.

[0029] F2 is defined as F2 = Mo + V. If F2 is 0.30 or more, a sufficient amount of precipitates can be generated to trap the absorbed hydrogen. Therefore, F2 is set to a range that satisfies formula (2). Mo+V≧0.30 (2)

[0030] Mo and V form finely dispersed composite precipitates during carburizing and quenching in the manufacturing process of carburized machine structural parts made of steel. If the Mo content or V content is too high compared to the other, the composite precipitates are not formed sufficiently, and as a result, the rolling fatigue life in a hydrogen generating environment is reduced.

[0031] F3 is defined as Mo / V. If F3 is 0.30 or less, the V content is too high relative to the Mo content. If F3 exceeds 4.00, the Mo content is too high relative to the V content. Therefore, fine composite precipitates are not generated sufficiently. On the other hand, if F3 is more than 0.30 to 4.00, a sufficient amount of fine composite precipitates can be generated in a carburized machine structural part made of a steel material, provided that the steel material has the above-mentioned chemical composition and satisfies formulas (1) and (2), and the rolling fatigue life in a hydrogen generating environment can be increased. Therefore, F3 is set to a range that satisfies formula (3). 0.30 <Mo / V≦4.00 (3)

[0032] The steel material and carburized machine structural part of the present embodiment have been completed based on the above technical concept, and have the following configuration.

[0033] [1] In mass%, C: 0.10-0.45%, Si: 0.05 to 0.80%, Mn: 0.40-1.50%, P: 0.015% or less, S: less than 0.005%, Cr: 0.05 to less than 0.50% Mo: 0.05-0.35%, V: 0.06-0.40%, 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, Satisfying formula (1) to formula (3), Steel material. (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 0.30 <Mo / V≦4.00 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

[0034] [2] In mass%, C: 0.10-0.45%, Si: 0.05 to 0.80%, Mn: 0.40-1.50%, P: 0.015% or less, S: less than 0.005%, Cr: 0.05 to less than 0.50% Mo: 0.05-0.35%, V: 0.06-0.40%, 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, Satisfying formula (1) to formula (3), Steel material. [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) 0.30 <Mo / V≦4.00 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

[0035] [3] [2] The steel material according to the present invention, Containing the first group, Steel material.

[0036] [4] The steel material according to [2] or [3], Containing the second group, Steel material.

[0037] [5] The steel material according to any one of [2] to [4], Containing the third group, Steel material.

[0038] [6] The steel material according to any one of [2] to [5], Containing the fourth group, Steel material.

[0039] [7] A hardened layer; A core portion is provided inside the hardened layer, The chemical composition of the core is, in mass%, C: 0.10-0.45%, Si: 0.05 to 0.80%, Mn: 0.40-1.50%, P: 0.015% or less, S: less than 0.005%, Cr: 0.05 to less than 0.50% Mo: 0.05-0.35%, V: 0.06-0.40%, 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, Satisfying formula (1) to formula (3), Carburized mechanical structural parts. (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 0.30 <Mo / V≦4.00 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

[0040] [8] A hardened layer; A core portion is provided inside the hardened layer, The chemical composition of the core is, in mass%, C: 0.10-0.45%, Si: 0.05 to 0.80%, Mn: 0.40-1.50%, P: 0.015% or less, S: less than 0.005%, Cr: 0.05 to less than 0.50% Mo: 0.05-0.35%, V: 0.06-0.40%, 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, Satisfying formula (1) to formula (3), 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, 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) 0.30 <Mo / V≦4.00 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

[0041] [9] A carburized machine structural part according to [8], Containing the first group, Carburized mechanical structural parts.

[0042]

[10] A carburized machine structural part according to [8] or [9], Containing the second group, Carburized mechanical structural parts.

[0043]

[11] A carburized machine structural part according to any one of [8] to

[10] , Containing the third group, Carburized mechanical structural parts.

[0044]

[12] A carburized machine structural part according to any one of [8] to

[11] , Containing the fourth group, Carburized mechanical structural parts.

[0045] The steel material according to this embodiment and the carburized machine structural part manufactured using the steel material will be described in detail below. Note that "%" for elements means mass % unless otherwise specified.

[0046] [Features of the steel material of this embodiment] The steel material of this embodiment includes the following features. (Feature 1) The chemical composition, in mass%, is C: 0.10 to 0.45%, Si: 0.05 to 0.80%, Mn: 0.40 to 1.50%, P: 0.015% or less, S: less than 0.005%, Cr: 0.05 to less than 0.50%, Mo: 0.05 to 0.35%, V: 0.06 to 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, with the remainder being Fe and impurities. When the optional elements are contained, the chemical composition 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: less than 0.005%, Cr: 0.05 to less than 0.50%, Mo: 0.05 to 0.35%, V: 0.06 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) 0.30 <Mo / V≦4.00 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol. Each feature will be explained below.

[0047] [(Feature 1) Chemical composition] The chemical composition of the steel material of this embodiment contains the following elements.

[0048] C: 0.10-0.45% Carbon (C) enhances the hardenability of steel material. Therefore, it enhances the strength of carburized machine structural parts manufactured from steel material. Furthermore, C combines with Mo and V to form composite precipitates during the carburizing process in the process of manufacturing carburized machine structural parts from steel material. These composite precipitates trap hydrogen that has penetrated into the steel material when the carburized machine structural parts are used in a hydrogen generating environment. Therefore, the change in the microstructure due to the penetrated hydrogen is suppressed, and the peeling of the surface vicinity part of the carburized machine structural parts caused by the changed microstructure is suppressed. As a result, the rolling fatigue life of the carburized machine structural parts in a hydrogen generating environment is increased. If the C content is less than 0.10%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the C content exceeds 0.45%, the steel becomes too hard, which may reduce machinability or produce coarse precipitates. The coarse precipitates are likely to become the starting point of cracks when carburized machine structural parts are used in a hydrogen generating environment. As a result, the rolling fatigue life of carburized machine structural parts in a hydrogen generating environment may be reduced. 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%, still more preferably 0.16%, still more preferably 0.18%, and still more preferably 0.23%. The upper limit of the C content is preferably 0.44%, more preferably 0.43%, further preferably 0.42%, and further preferably 0.39%.

[0049] Silicon: 0.05 to 0.80% Silicon (Si) improves the hardenability of steel. Si also improves the temper softening resistance of the hardened layer of a carburized machine structural part manufactured using steel as a raw material. Si also improves the rolling fatigue life of a carburized machine structural part 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 becomes too hard and the machinability of the steel 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 further preferably 0.15%. The upper limit of the Si content is preferably 0.75%, more preferably 0.70%, and further preferably 0.65%.

[0050] Mn: 0.40-1.50% Manganese (Mn) improves the hardenability of steel and the strength of the core of carburized machine structural parts manufactured from steel. Therefore, the rolling fatigue life of carburized machine structural parts in a hydrogen generating environment is increased. 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 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%, further preferably 0.55%, and further preferably 0.60%. The upper limit of the Mn content is preferably 1.45%, more preferably 1.40%, still more preferably 1.35%, still more preferably 1.30%, and still more preferably 1.20%.

[0051] P:0.015% or less Phosphorus (P) is an unavoidably contained impurity. In other words, the P content is more than 0%. P segregates at grain boundaries and reduces grain boundary strength. If the P content exceeds 0.015%, P will segregate excessively at grain boundaries even if the contents of other elements are within the ranges of this embodiment. In this case, the grain boundary strength will decrease. As a result, the rolling fatigue life of carburized machine structural parts in a hydrogen generating environment will decrease. Therefore, the P content is 0.015% or less. The lower the P content, the more preferable it is. However, if the P content is reduced too much, the manufacturing cost will increase. Therefore, in consideration of normal industrial production, the lower limit of the P content is preferably 0.001%, 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 further preferably 0.008%.

[0052] S: Less than 0.005% Sulfur (S) is an impurity that is inevitably contained. In other words, the S content is more than 0%. S generates sulfide-based inclusions. Coarse sulfide-based inclusions are likely to become the starting point of cracks during use of carburized machine structural components in a hydrogen generating environment. If the S content is 0.005% or more, 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 the carburized machine structural components in a hydrogen generating environment is reduced. Therefore, the S content is less than 0.005%. The lower the S content, the better. However, if the S content is reduced too much, the manufacturing cost will increase. Therefore, in consideration of normal industrial production, the lower limit of the S content is preferably 0.001%, and more preferably 0.002%. The upper limit of the S content is preferably 0.004%, and more preferably 0.003%.

[0053] Cr: 0.05 to less than 0.50% Chromium (Cr) improves the hardenability of steel and increases the strength of the core of carburized machine structural parts manufactured from steel material. Therefore, the rolling fatigue life of carburized machine structural parts in a hydrogen generating environment is increased. If the Cr content is less than 0.05%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, as described above, Cr promotes the penetration of hydrogen into steel materials. 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 into steel materials cannot be sufficiently suppressed during use of carburized machine structural components in a hydrogen generating environment. As a result, the rolling fatigue life of the carburized machine structural components 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 further preferably 0.15%. The upper limit of the Cr content is preferably 0.49%, more preferably 0.48%, more preferably 0.47%, more preferably 0.46%, more preferably 0.45%, more preferably 0.44%, and more preferably 0.43%.

[0054] Mo: 0.05 to 0.35% Molybdenum (Mo) forms composite precipitates together with C and V during carburization in the process of manufacturing carburized machine structural parts using steel as a raw material. These composite precipitates trap hydrogen that penetrates into the steel when the carburized machine structural parts are used in a hydrogen generating environment. This suppresses the formation of white structure due to the penetrated hydrogen, and suppresses peeling of the surface vicinity of the carburized machine structural parts due to the formation of white structure. As a result, the rolling fatigue life of the carburized machine structural parts 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 range 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 becomes excessively high, and therefore the machinability of the steel material decreases. 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%, further preferably 0.12%, and further preferably 0.16%. The upper limit of the Mo content is preferably 0.33%, more preferably 0.30%, and further preferably 0.28%.

[0055] V: 0.06~0.40% Vanadium (V) forms composite precipitates together with C and Mo during carburization treatment in the process of manufacturing carburized machine structural parts using steel as a raw material. These composite precipitates trap hydrogen that penetrates into the steel when the carburized machine structural parts are used in a hydrogen generating environment. Therefore, the formation of white structure due to the penetrated hydrogen is suppressed, and peeling of the surface vicinity part of the carburized machine structural parts caused by the formation of white structure is suppressed. As a result, the rolling fatigue life of the carburized machine structural parts in a hydrogen generating environment is increased. If the V content is less than 0.06%, the above effect cannot be sufficiently obtained even if the contents of other elements are within the range of this embodiment. On the other hand, if the V content exceeds 0.40%, coarse precipitates may be generated. The coarse precipitates are likely to become the starting points of cracks when the carburized machine structural part 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 carburized machine structural part in a hydrogen generating environment may be reduced. If the V content exceeds 0.40%, the machinability of the steel material is further 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%, more preferably 0.12%, 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 further preferably 0.35%.

[0056] Ni: over 0.30 to 1.50% Nickel (Ni) improves the hardenability of steel and increases the strength of the core of carburized machine structural parts manufactured using steel as a material. Ni also inhibits the penetration of hydrogen into the steel (carburized machine structural parts) in a hydrogen generating environment. As a result, the rolling fatigue life of the carburized machine structural parts in a hydrogen generating environment is improved. If the Ni content is 0.30% or less, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ni content exceeds 1.50%, the steel becomes too hard and the machinability of the steel 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%, more preferably 0.40%, 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%, further preferably 1.20%, and further preferably 1.10%.

[0057] Al: 0.005 to 0.100% Aluminum (Al) deoxidizes steel in the steelmaking process. Furthermore, Al combines with N in the steel to form AlN, suppressing the deterioration of the machinability of the steel due to solute N. 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 are generated. The coarse clustered oxides 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 fatigue life of the carburized machine structural component in a hydrogen generating environment is 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 further preferably 0.060%.

[0058] N: 0.030% or less Nitrogen (N) is an unavoidably contained impurity. In other words, the N content is more than 0%. N dissolves in the steel material and reduces the hot workability of the steel material. If the N content exceeds 0.030%, the hot workability of the steel material is 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, if the N content is reduced too much, the manufacturing cost increases. Therefore, in consideration of normal industrial production, the lower limit of the N content is preferably 0.001%, and more preferably 0.002%. The upper limit of the N content is preferably 0.028%, more preferably 0.025%, and further preferably 0.020%.

[0059] O: 0.0015% or less Oxygen (O) is an impurity that is inevitably contained. In other words, the O content is more than 0%. O combines with other elements in the steel to form coarse oxides (including coarse oxides due to clustering). The coarse oxides become the starting points for cracks in a hydrogen generating environment. As a result, the rolling fatigue life of carburized machine structural parts in a hydrogen generating environment is reduced. If the O content exceeds 0.0015%, the rolling fatigue life of carburized machine structural parts in a hydrogen generating environment is 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, if the O content is reduced too much, the manufacturing cost increases. Therefore, in consideration of normal industrial production, the lower limit of the O content is preferably 0.0001%, 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 further preferably 0.0009%.

[0060] The balance of the chemical composition of the steel material according to the present embodiment is composed of Fe and impurities. Here, the impurities in the chemical composition refer to those that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of the steel material, are unintentionally contained, and are permissible within a range that does not adversely affect the steel material according to the present embodiment.

[0061] [About optional elements] The chemical composition of the steel material of this embodiment may further contain, in place of a portion of Fe, one or more elements selected from the group consisting of first to fourth groups. [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 optional elements are all optional elements and may not be contained. These optional elements will be described below.

[0062] [Group 1: Ti and Nb] The chemical composition of the steel material of this embodiment may further contain one or more elements selected from the group consisting of Ti and Nb, instead of a part of Fe. These elements are optional elements, and all of them form precipitates, and by precipitation strengthening, increase the strength of the carburized machine structural part manufactured using the steel material as a material.

[0063] 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 contained, that is, when the Ti content is more than 0%, Ti forms Ti precipitates such as carbides, nitrides, and carbonitrides. The Ti precipitates increase the strength of carburized machine structural parts manufactured from steel materials through precipitation strengthening. Even if even a small amount of Ti is contained, the above effect can be obtained to some extent. However, if the Ti content exceeds 0.050%, coarse precipitates may form, and in this case, the rolling fatigue life of the carburized machine structural part in a hydrogen generating environment decreases 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 further preferably 0.004%. The upper limit of the Ti content is preferably 0.045%, more preferably 0.040%, further preferably 0.035%, and further preferably 0.030%.

[0064] 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. The Nb precipitates increase the strength of carburized machine structural parts manufactured from steel materials through precipitation strengthening. Even if even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content exceeds 0.050%, coarse precipitates may form, and in this case, the rolling fatigue life of the carburized machine structural part in a hydrogen generating environment decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the Nb content is 0 to 0.050%, and if contained, it is 0.050% or less. The lower limit of the Nb content is preferably 0.001%, more preferably 0.005%, and further preferably 0.010%. The upper limit of the Nb content is preferably 0.045%, more preferably 0.040%, and further preferably 0.035%.

[0065] [Group 2: B and Cu] The chemical composition of the steel material of the present 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 all of them improve the hardenability and strength of the steel material.

[0066] 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 the steel, suppresses the grain boundary segregation of P, and increases the strength of the steel. As a result, the rolling fatigue life of carburized machine structural parts manufactured using the steel in a hydrogen generating environment is improved. Even if even a small amount of B is contained, the above effects can be obtained to a certain 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 becomes excessively high, and therefore the machinability of the steel material decreases. 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%, still 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%, still more preferably 0.0020%, and still more preferably 0.0010%.

[0067] 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 contained, that is, when the Cu content is more than 0%, Cu improves the hardenability of the steel material and increases its strength. As a result, it improves the rolling fatigue life of carburized machine structural parts manufactured from the steel material in a hydrogen generating environment. Even if even a small amount of Cu is contained, the above effects can be obtained to a certain 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 becomes excessively high, and 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%, and more preferably 0.02%. The upper limit of the Cu content is preferably 1.40%, and more preferably 1.30%, and even more preferably 1.20%, and even more preferably 1.10%.

[0068] [Group 3: Sn] The chemical composition of the steel material of the present embodiment may further contain Sn instead 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. 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 decreases 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 contained, it is 0.100% or less. The lower limit of the Sn content is preferably 0.001%, more preferably 0.002%, and further preferably 0.005%. The upper limit of the Sn content is preferably 0.090%, more preferably 0.080%, more preferably 0.060%, more preferably 0.040%, more preferably 0.030%, and more preferably 0.025%.

[0069] [Group 4: Ca and Mg] The chemical composition of the steel material 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 the sulfides in the steel material and increase the rolling fatigue life of carburized machine structural parts manufactured using the steel material in a hydrogen generating environment.

[0070] 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 more than 0%, Ca refines the sulfides in the steel. Furthermore, Ca promotes the spheroidization of the sulfides in the steel. As a result, the rolling fatigue life of carburized machine structural parts manufactured from the steel in a hydrogen generating environment is improved. Even if even a small amount of Ca is contained, the above effects can be obtained to a certain 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 steel material, which reduces the rolling fatigue life of the carburized machine structural parts 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%, further preferably 0.0005%, and further preferably 0.0010%. The upper limit of the Ca content is preferably 0.0040%, more preferably 0.0035%, and further preferably 0.0030%.

[0071] 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 contained, that is, when the Mg content is over 0%, Mg refines the sulfides in the steel. Furthermore, Mg promotes the spheroidization of the sulfides in the steel. As a result, the rolling fatigue life of carburized machine structural parts manufactured from the steel in a hydrogen generating environment is improved. Even if even a small amount of Mg is contained, the above effects can be obtained to a certain extent. However, if the Mg content exceeds 0.0050%, even if the contents of other elements are within the ranges of this embodiment, coarse Mg oxides may be formed in the steel material, which reduces the rolling fatigue life of the carburized machine structural parts in a hydrogen generating environment. Therefore, the Mg content is 0 to 0.0050%, and if contained, it is 0.0050% or less. The lower limit of the Mg content is preferably 0.0001%, more preferably 0.0002%, more preferably 0.0005%, more preferably 0.0010%, and more preferably 0.0015%. The upper limit of the Mg content is preferably 0.0040%, more preferably 0.0035%, and further preferably 0.0030%.

[0072] [(Feature 2) Regarding formulas (1) to (3)] The chemical composition of the steel material of this embodiment further satisfies formulas (1) to (3). (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 0.30 <Mo / V≦4.00 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol. Formulas (1) to (3) will be explained below.

[0073] [Regarding formula (1)] F1 is defined as (Si+Ni) / Cr. As described above, Si and Ni suppress the penetration of hydrogen into steel materials in a hydrogen generating environment. On the other hand, Cr promotes the penetration of hydrogen into steel materials in a hydrogen generating environment. Therefore, from the viewpoint of suppressing the penetration of hydrogen into steel materials in a hydrogen generating environment, it is preferable that F1 is high.

[0074] If F1 is less than 1.30, even if the steel satisfies characteristic 1 and also satisfies formulas (2) and (3), a sufficient rolling fatigue life cannot be obtained in a carburized machine structural part in a hydrogen generating environment.

[0075] If F1 is 1.30 or more, a sufficient rolling fatigue life can be obtained in a carburized machine structural part in a hydrogen generating environment, provided that the steel satisfies Characteristic 1 and also satisfies Equations (2) and (3).

[0076] The lower limit of F1 is preferably 1.32, more preferably 1.34, more preferably 1.36, more preferably 1.38, and even more preferably 1.40. Although a preferable upper limit of F1 is not particularly limited, when the steel material satisfies the characteristic 1, the upper limit of F1 is 46.00. A preferable upper limit of F1 is less than 8.00, more preferably 7.99, more preferably 7.80, more preferably 7.50, more preferably 7.00, and 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).

[0077] [Regarding formula (2)] Defined as F2 = Mo + V. F2 is an index of the amount of composite precipitates that trap hydrogen that penetrates into steel during use in a hydrogen generating environment in carburized machine structural parts manufactured from steel.

[0078] As mentioned above, during the carburizing and quenching process in the manufacturing process of carburized machine structural parts made of steel, Mo and V combine with C to form complex precipitates. These complex precipitates trap hydrogen that penetrates into the steel (carburized machine structural parts) when the carburized machine structural parts are used in a hydrogen generating environment, thereby improving the rolling fatigue life of the carburized machine structural parts in a hydrogen generating environment. Therefore, a sufficient amount of these complex precipitates is required to trap the invaded hydrogen.

[0079] If F2 is 0.30 or more, the Mo content and V content are sufficient to generate a sufficient amount of complex precipitates. Therefore, assuming that the steel material satisfies Characteristic 1 and also satisfies Equations (1) and (3), excellent rolling fatigue life can be obtained in carburized machine structural parts in a hydrogen generating environment.

[0080] The lower limit of F2 is preferably 0.31, more preferably 0.32, more preferably 0.33, more preferably 0.34, and even more preferably 0.35. Although there are no particular limitations on the preferred upper limit of F2, when the steel material satisfies characteristic 1, the upper limit of F2 is 0.75. The preferred upper limit of F2 is 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).

[0081] [Regarding formula (3)] Defined as F3 = Mo / V, F3 is an index relating to the hydrogen trapping ability of composite precipitates that trap hydrogen that penetrates into steel during use in a hydrogen generating environment in carburized machine structural parts manufactured from steel.

[0082] Mo and V form finely dispersed composite precipitates. If either the Mo content or the V content is higher than the other, the composite precipitates are not formed sufficiently. If F3 is 0.30 or less, the V content is too high relative to the Mo content. If F3 exceeds 4.00, the Mo content is too high relative to the V content. In these cases, even if F2 is 0.30 or more, fine composite precipitates are not formed sufficiently. Therefore, sufficient rolling fatigue life cannot be obtained in carburized machine structural parts in a hydrogen generating environment.

[0083] When F3 is greater than 0.30 and less than 4.00, fine composite precipitates are generated in sufficient quantities, provided that the steel satisfies characteristic 1 and formulas (1) and (2). Therefore, excellent rolling fatigue life can be obtained in carburized machine structural parts in a hydrogen generating environment.

[0084] The lower limit of F3 is preferably 0.31, more preferably 0.33, more preferably 0.36, more preferably 0.39, more preferably 0.42, and more preferably 0.44. The upper limit of F3 is preferably 3.95, more preferably 3.90, more preferably 3.80, more preferably 3.70, and even more preferably 3.50. 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).

[0085] [Effects of the steel material according to this embodiment] The steel material of this embodiment satisfies Features 1 and 2. Therefore, in a carburized machine structural part manufactured using the steel material as a raw material, a sufficient rolling fatigue life can be obtained in a hydrogen generating environment.

[0086] [About the microstructure of steel] The microstructure of the steel material according to this embodiment is not particularly limited. In this embodiment, as described above, a sufficient rolling fatigue life can be obtained in a hydrogen generating environment in a carburized machine structural component manufactured using a steel material as a material. This effect is a characteristic required for a carburized machine structural component manufactured using a steel material as a material. Usually, during hot working in the manufacturing process of a carburized machine structural component made of a steel material, the steel material is subjected to A c3 When the steel material is heated to or above the austenite point, the microstructure of the steel material is transformed into austenite. Therefore, the steel material according to the present embodiment has the above-mentioned effects regardless of the microstructure.

[0087] [Shape of steel material in this embodiment] The steel material in this embodiment is a steel bar or a wire rod. The steel bar or wire rod is a steel material that extends in a rod shape. The steel material may be wound in a coil shape or may be cut to a predetermined length.

[0088] [Use of the steel material according to this embodiment] The steel material of this embodiment satisfies Features 1 and 2, and thus has a sufficient rolling fatigue life in a hydrogen generating environment. Therefore, the steel material of this embodiment can be used as a material for machine structural parts used in automobiles, industrial machines, etc. In particular, the steel material of this embodiment is suitable as a material for carburized machine structural parts manufactured by carburizing treatment. Carburized machine structural parts are, for example, carburized bearing parts.

[0089] [Steel manufacturing method] An example of a method for manufacturing the steel material of this embodiment will be described. The method for manufacturing the steel material described below is an example for manufacturing the steel material of this embodiment. Therefore, the steel material having the above-mentioned 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 the steel material of this embodiment. In this embodiment, a method for manufacturing a steel bar will be described as an example of a steel material.

[0090] An example of a method for manufacturing a steel material according to this embodiment includes the following steps. (Process 1) Material preparation process (Process 2) Hot processing process Each step will be described below.

[0091] [(Process 1) Material preparation process] In the material preparation step, a material for the steel material 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 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.

[0092] The molten steel produced by the above-mentioned refining method is used to produce a material by a known casting method. For example, the molten steel is used to produce an ingot by an ingot casting method. The molten steel may also be used to produce a bloom or billet by a continuous casting method. The material (ingot, bloom, or billet) is produced by the above-mentioned method.

[0093] [(Process 2) Hot processing process] The produced material is hot worked to produce a steel material. In the hot working step, one or more hot workings are usually performed. When hot working is performed multiple times, the first hot working may be, for example, rolling using blooming or hot forging, and the next and subsequent hot workings may be rolling using a continuous rolling mill. The continuous rolling mill has a plurality of rolling stands arranged in a row. The steel material after the hot working is cooled to room temperature. A billet may be produced by blooming and rolling using a continuous rolling mill, and then the billet may be reheated and further finished rolling using a continuous rolling mill may be performed to produce a steel material of a desired size. Alternatively, the steel material may be produced from the material only by hot forging. The heating temperature of the material during hot working is not particularly limited, but is, for example, 1000 to 1300°C.

[0094] [Carburized machine structural parts according to this embodiment] The carburized machine structural component of this embodiment means a machine structural component that has been carburized. The carburized machine structural component of this embodiment is manufactured using the steel material of this embodiment as a material. The carburized machine structural component of this embodiment has a hardened layer and a core portion located inside the hardened layer. The hardened layer is formed on the surface of the carburized machine structural parts, and is a layer that is hardened by the penetration of C by the carburizing process. The hardened layer is formed from the surface of the carburized machine structural parts to a specified depth. The core is a portion located inside the hardened 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 hardened layer and the core can be distinguished by well-known microstructural observation.

[0095] The carburized machine structural component of the present embodiment includes the following feature 3 and feature 4. (Feature 3) 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: less than 0.005%, Cr: 0.05 to less than 0.50%, Mo: 0.05 to 0.35%, V: 0.06 to 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, 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: less than 0.005%, Cr: 0.05 to less than 0.50%, Mo: 0.05 to 0.35%, V: 0.06 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 4) The chemical composition of the core further satisfies formulas (1) to (3). (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 0.30 <Mo / V≦4.00 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

[0096] The action of each element in Feature 3 is the same as the action of the corresponding element described in Feature 1 of the steel material of the present embodiment. Moreover, the technical meaning of formulas (1) to (3) in Feature 4 is the same as the content described in Feature 2 of the steel material of the present embodiment.

[0097] [Effects of the carburized machine structural parts according to this embodiment] As described above, the carburized machine structural component of this embodiment includes Features 3 and 4. Therefore, the carburized machine structural component of this embodiment has a sufficient rolling fatigue life in a hydrogen generating environment.

[0098] [Applications of the carburized machine structural parts of this embodiment] The carburized machine structural parts are widely applicable to fields requiring rolling fatigue life. The carburized machine structural parts are suitable as carburized bearing parts, for example. The bearing parts refer to parts of rolling bearings. The bearing parts are, for example, races, races, rolling elements, etc. The races may be inner or outer races, and the races may be shaft races, housing races, central races, or aligning housing races. The races and races are not particularly limited as long as they are members having a raceway surface. The rolling elements may be balls or rollers. The rollers are, for example, cylindrical rollers, rod rollers, needle rollers, tapered rollers, convex rollers, etc.

[0099] [Method of manufacturing carburized machine structural parts according to the present embodiment] The carburized machine structural component of this embodiment is manufactured using the above-mentioned steel material of this embodiment as a material by, for example, the following manufacturing method. (Process 3) Hot processing process (Process 4) Carburizing process Each step will be described below.

[0100] [(Process 3) 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, the well-known hot forging. In the hot working step, the steel material is c3 After heating to a temperature above the temperature limit, the steel is processed. Therefore, the microstructure of the steel is reset during the heating step of the hot working process. The heating temperature is a known temperature, for example, 1000 to 1300°C. The intermediate product after the hot working is cooled to room temperature. If necessary, the intermediate product after the hot working is cut. The intermediate product after the hot working or the cutting process may be subjected to a known heat treatment. The known heat treatment is, for example, a normalizing treatment and / or a spheroidizing treatment.

[0101] [(Process 4) Carburizing process] In the carburizing process, the intermediate product is subjected to a known carburizing treatment to manufacture a carburized machine structural part. The carburizing treatment includes carburizing quenching and tempering. In the carburizing quenching, the intermediate product is heated in an atmosphere containing a known carburizing gas. c3 After heating and holding at a temperature equal to or higher than the transformation point, the product is quenched. 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 means a well-known endothermic transformation gas (RX gas). RX gas is a gas in which a hydrocarbon gas such as butane or propane is mixed with air and reacted by passing through a heated Ni catalyst, and is a mixed gas containing CO, H2, N2, etc. Note that the well-known sub-zero treatment may be performed on the intermediate product after carburizing and quenching, or the carburized machine structural part after tempering.

[0102] By the above steps, a carburized machine structural part having features 3 and 4 is manufactured. EXAMPLES

[0103] Steel materials having the chemical compositions shown in Table 1 (Tables 1-1 and 1-2) were produced by the following method.

[0104] [Table 1-1]

[0105] [Table 1-2]

[0106] Specifically, a 100 kg ingot was produced by vacuum melting. The produced ingot was subjected to hot forging to produce a steel material (steel bar) having 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. Through the above manufacturing process, the steel material (steel bar having a diameter of 60 mm) of each test number was produced. The manufactured steel was subjected to a rolling fatigue life evaluation test in a hydrogen generation environment.

[0107] [Rolling fatigue life evaluation test under hydrogen generation environment] A test piece for roller pitting fatigue test (small roller test piece) was prepared as a simulated carburized machine structural part of each test number, and the rolling fatigue life was investigated by the following method. From the steel material (steel bar with a diameter of 60 mm) with each test number, several intermediate small roller test pieces for the rolling fatigue life evaluation test shown in Figure 1 were machined. The numbers in Figure 1 indicate the dimensions (unit: mm). "φ" in the figure means the diameter.

[0108] Specifically, the steel material (steel bar with a diameter of 60 mm) of each test number was heated at a heating temperature of 1200°C and a holding time of 30 minutes. Then, hot processing (hot forging) was performed with a finishing temperature of 950°C or higher to produce a steel bar with a diameter of 35 mm. The steel bar after hot processing was air-cooled to room temperature. The manufactured steel bar with a diameter of 35 mm was normalized at a heating temperature of 925°C and a holding time of 60 minutes, and the normalized steel bar was air-cooled to room temperature. Then, the steel bar with a diameter of 35 mm was machined (cut) to process an intermediate product of a test piece for a roller pitting fatigue test. The intermediate product was subjected to a carburizing treatment (carburizing, quenching, and tempering) to produce a plurality of small roller test pieces shown in Figure 1 for each test number.

[0109] The carburizing process was carried out in the following manner. The intermediate specimens of each test number were heated at 930°C for 80 minutes in an atmosphere with a carbon potential Cp1 of 1.00%. Next, they were heated at 930°C for 60 minutes in an atmosphere with a carbon potential Cp2 of 0.80%. After that, they were heated at 900°C for 30 minutes and quenched in oil at 60°C. The test specimens after oil quenching were tempered at a tempering temperature of 180°C for a holding time of 120 minutes. After the holding time had elapsed, the intermediate specimens were air-cooled. By using the above gas carburizing method, the C concentration on the surface of the small roller test specimens was adjusted to about 0.80 mass%.

[0110] (Roller pitching fatigue test) The following roller pitting fatigue test was carried out using the small roller test pieces thus prepared. 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 a 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 above method (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.

[0111] The large roller test piece 100 was prepared by the following method. An intermediate product having a chemical composition equivalent to SUJ2 specified in JIS G 4805 (2019) and a shape shown in FIG. 3 was prepared. The intermediate product was quenched and tempered. In quenching, the intermediate product was held at 840°C for 30 minutes, and then oil-cooled with oil at 130°C. In tempering, the intermediate product was held at 160°C for 180 minutes, and then air-cooled. The surface of the intermediate product after tempering was polished to prepare the large roller test piece 100 shown in FIG. 3. The diameter of the large roller test piece 100 was 130 mm.

[0112] To simulate the hydrogen generation environment, a hydrogen charging process was performed on the small roller test piece 200. Specifically, the small roller test piece 200 was immersed in an aqueous solution of 3% NaCl + 3 g / L ammonium thiocyanate (NH4SCN) and held for 96 hours. The temperature of the aqueous solution during holding was set to 20°C. The hydrogen charging process was performed under the above conditions. The small roller test piece 200 after the hydrogen charging process was used to perform the roller pitting fatigue test shown below.

[0113] In the roller pitting fatigue test, the large roller test piece 100 was pressed against the small roller test piece 200 with various Hertz stress surface pressures. The test was performed by rotating the small roller test piece 200 with the circumferential speed directions of both roller test pieces in the same direction at the contact area and with a slip ratio of -40% (the large roller test piece 100 had a circumferential speed at the contact area that was 40% higher than that of the small roller test piece 200). The oil temperature of the ATF (automatic transmission lubricant) supplied as lubricant to the contact area was 90°C, and the maximum surface pressure of the contact stress between the large roller test piece 100 and the small roller test piece 200 was 3000 MPa. The test was discontinued after 50 million cycles (5.0 x 10 7 Roller pitting fatigue tests were carried out 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 taken as the rolling fatigue life.

[0114] Based on the obtained rolling fatigue life, the rolling fatigue life in a hydrogen generation environment was evaluated as follows. The rolling fatigue life of the steel with test number 37, which meets the SCM420 standard of JIS G 4053:2016, was taken as the rolling fatigue life of the reference steel. The ratio of the rolling fatigue life of the steel with each test number to the rolling fatigue life of the reference steel was taken as the rolling fatigue life ratio. In other words, the rolling fatigue life ratio was calculated by the following formula. Rolling fatigue life ratio = Rolling fatigue life of steel with each test number / Rolling fatigue life of reference steel (test number 37) 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 after 50 million repeated loads Rating B (Bad): Rolling fatigue life ratio < 1.50 In the case of evaluation E, it was determined that a sufficient rolling fatigue life was obtained. On the other hand, in the case of evaluation B, it was determined that a sufficient rolling fatigue life was not obtained. The evaluation results are shown in the "Rolling Contact Fatigue Life" column in Table 2.

[0115] [Table 2]

[0116] [Test Results] With reference to Table 1 (Table 1-1 and Table 1-2) and Table 2, the steel materials of test numbers 1 to 22 satisfied characteristics 1 and 2, and the small roller test pieces simulating carburized machine structural parts satisfied characteristics 3 and 4. Therefore, a sufficient rolling fatigue life was obtained in the rolling fatigue life evaluation test in a hydrogen generation environment.

[0117] On the other hand, in test numbers 23 and 24, the Ni content was too low, and therefore a sufficient rolling fatigue life was not obtained in the rolling fatigue life evaluation test in a hydrogen generation environment.

[0118] In test numbers 25 and 26, the S content was too high. Therefore, in the rolling fatigue life evaluation test in a hydrogen generation environment, a sufficient rolling fatigue life was not obtained.

[0119] In test numbers 27 and 28, the Cr content was too high, and therefore a sufficient rolling fatigue life was not obtained in the rolling fatigue life evaluation test in a hydrogen generation environment.

[0120] In test numbers 29 and 30, F1 was too low, and therefore a sufficient rolling fatigue life was not obtained in the rolling fatigue life evaluation test in a hydrogen generation environment.

[0121] In test numbers 31 and 32, F2 was too low, and therefore a sufficient rolling fatigue life was not obtained in the rolling fatigue life evaluation test in a hydrogen generation environment.

[0122] In test numbers 33 and 34, F3 was too low. Therefore, in the rolling fatigue life evaluation test in the hydrogen generation environment, a sufficient rolling fatigue life was not obtained.

[0123] In test numbers 35 and 36, F3 was too high. Therefore, in the rolling fatigue life evaluation test in the hydrogen generation environment, a sufficient rolling fatigue life was not obtained.

[0124] 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 without departing from the spirit of the present disclosure.

Claims

1. In mass percent, C: 0.10-0.45%, Si: 0.05-0.80%, Mn: 0.40-1.50%, P: 0.015% or less, S: less than 0.005%, Cr: 0.05 to less than 0.50% Mo: 0.05-0.35%, V: 0.06-0.40%, 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 is Fe and impurities, Satisfying formulas (1) to (3); Steel material. (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 0.30<Mo / V≦4.00 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

2. In mass percent, C: 0.10-0.45%, Si: 0.05-0.80%, Mn: 0.40-1.50%, P: 0.015% or less, S: less than 0.005%, Cr: 0.05 to less than 0.50% Mo: 0.05-0.35%, V: 0.06-0.40%, 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, the composition contains one or more selected from the group consisting of Groups 1 to 4, The balance is Fe and impurities, Satisfying formulas (1) to (3); Steel material. [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) 0.30<Mo / V≦4.00 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

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

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

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

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

7. A hardened layer; A core portion is provided inside the 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: less than 0.005%, Cr: 0.05 to less than 0.50% Mo: 0.05-0.35%, V: 0.06-0.40%, 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 is Fe and impurities, Satisfying formulas (1) to (3); Carburized mechanical structural parts. (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 0.30<Mo / V≦4.00 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

8. A hardened layer; A core portion is provided inside the 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: less than 0.005%, Cr: 0.05 to less than 0.50% Mo: 0.05-0.35%, V: 0.06-0.40%, 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, the composition contains one or more selected from the group consisting of Groups 1 to 4, The balance is Fe and impurities, Satisfying formulas (1) to (3); 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, 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) 0.30<Mo / V≦4.00 (3) Here, the content (mass %) of each element in formulas (1) to (3) is substituted for the corresponding element symbol.

9. The carburized machine structural part according to claim 8, Containing the first group, Carburized mechanical structural parts.

10. The carburized machine structural part according to claim 8, Containing the second group, Carburized mechanical structural parts.

11. The carburized machine structural part according to claim 8, Containing the third group, Carburized mechanical structural parts.

12. The carburized machine structural part according to claim 8, Containing the fourth group Carburized mechanical structural parts.

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