Steel material and machine structural part

Abstract

To provide a steel material that has sufficient rolling contact fatigue life in an environment where hydrogen is generated.SOLUTION: A steel material according to the present embodiment contains, in mass%, C: 0.75-1.20%, Si: 0.05-0.80%, Mn: 0.50-1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.05% to less than 0.50%, Mo: 0.06-0.35%, V: 0.10-0.40%, Ni: more than 0.30% to 1.50%, Al: 0.005-0.100%, N: 0.030% or less, and O: 0.0015% or less, with the balance being Fe and impurities, and satisfies formulas (1) to (3). (Si+Ni) / Cr≥1.30 (1), Mo+V≥0.30 (2), 1.70≤2.5×Mn+Cr≤3.90 (3). In formulas (1) to (3), each element symbol is substituted with the content of the corresponding element in mass%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present disclosure relates to a steel material that can be used as a material for machine structural components that have been subjected to quenching and tempering treatment, and to a machine structural component that includes a quench-hardened layer on its surface. [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 performed as necessary to produce an intermediate product with a desired shape. Heat treatment is performed on the intermediate product to adjust the hardness and microstructure of the steel. Examples of heat treatment include quenching and tempering, carburizing, or carbonitriding. Machine structural parts are manufactured through the above manufacturing process.

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

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

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

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

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

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

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

[0010] An object of the present disclosure is to provide a steel material and a machine structural component manufactured using the steel material as a raw material, which have a sufficient rolling fatigue life in a hydrogen generating environment. [Means for solving the problem]

[0011] The steel material according to the present disclosure comprises, in mass%, C: 0.75~1.20%, Si: 0.05 to 0.80% Mn: 0.50 to 1.50% P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50% Mo: 0.06 to 0.35%, V: 0.10~0.40%, Ni: over 0.30 to 1.50% Al: 0.005 to 0.100%, N: 0.030% or less, and O: 0.0015% or less, the balance being Fe and impurities, Formulas (1) to (3) are satisfied. (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (3).

[0012] The steel material according to the present disclosure comprises, in mass%, C: 0.75~1.20%, Si: 0.05 to 0.80% Mn: 0.50 to 1.50% P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50% Mo: 0.06 to 0.35%, V: 0.10~0.40%, Ni: over 0.30 to 1.50% Al: 0.005 to 0.100%, N: 0.030% or less, and O: 0.0015% or less, Further, it contains one or more selected from the group consisting of Groups 1 to 4, the balance being Fe and impurities, Formulas (1) to (3) are satisfied. [Group 1] Ti: 0.050% or less, and Nb: 0.050% or less, one or more selected from the group consisting of [Group 2] B: 0.0050% or less, and Cu: 1.50% or less, one or more selected from the group consisting of [Group 3] Sn: 0.100% or less [Group 4] Ca: 0.0050% or less, and Mg: 0.0050% or less, one or more selected from the group consisting of (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (3).

[0013] The machine structural component according to the present disclosure comprises: A hardened layer is formed on at least the surface layer. The chemical composition of the machine structural part is, in mass%, C: 0.75~1.20%, Si: 0.05 to 0.80% Mn: 0.50 to 1.50% P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50% Mo: 0.06 to 0.35%, V: 0.10~0.40%, Ni: over 0.30 to 1.50% Al: 0.005 to 0.100%, N: 0.030% or less, and O: 0.0015% or less, the balance being Fe and impurities, Formulas (1) to (3) are satisfied. (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (3).

[0014] The machine structural component according to the present disclosure comprises: A hardened layer is formed on at least the surface layer. The chemical composition of the machine structural part is, in mass%, C: 0.75~1.20%, Si: 0.05 to 0.80% Mn: 0.50 to 1.50% P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50% Mo: 0.06 to 0.35%, V: 0.10~0.40%, Ni: over 0.30 to 1.50% Al: 0.005 to 0.100%, N: 0.030% or less, and O: 0.0015% or less, Further, it contains one or more selected from the group consisting of Groups 1 to 4, the balance being Fe and impurities, Formulas (1) to (3) are satisfied. [Group 1] Ti: 0.050% or less, and Nb: 0.050% or less, one or more selected from the group consisting of [Group 2] B: 0.0050% or less, and Cu: 1.50% or less, one or more selected from the group consisting of [Group 3] Sn: 0.100% or less [Group 4] Ca: 0.0050% or less, and Mg: 0.0050% or less, one or more selected from the group consisting of (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (3). [Effects of the Invention]

[0015] In the steel material according to the present disclosure, machine structural components manufactured using the steel material as a raw material can have a sufficient rolling fatigue life in a hydrogen generating environment. The machine structural components according to the present disclosure have a sufficient 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 used in a rolling fatigue life evaluation test. [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 investigated steel materials that can be used as raw materials to manufacture machine structural parts and that can provide a sufficient rolling fatigue life in a hydrogen generating environment.

[0018] The rolling fatigue life in a hydrogen generating environment is reduced by the occurrence of cracks (flaking) due to hydrogen in the vicinity of the surface of a machine structural component. Therefore, the inventors investigated the causes of cracks (flaking) due to hydrogen in a hydrogen generating environment.

[0019] In a hydrogen generating environment, cracking (flaking) in the near-surface portion of a mechanical structural component is thought to occur through the following mechanism: As described above, when hydrogen is generated in a hydrogen generating environment, the generated hydrogen penetrates into the mechanical structural component. When the penetrated hydrogen condenses, it causes a structural change and white structure is formed. When this structural change occurs in the near-surface portion of a mechanical structural component, cracking (flaking) occurs in the near-surface portion, reducing the rolling fatigue life.

[0020] As described above, cracks (peeling) caused by hydrogen in a hydrogen-generating environment occur when hydrogen penetrates into mechanical structural components (factor 1) and the penetrated hydrogen condenses (factor 2).

[0021] Here, the present inventors focused on factor 1 and investigated means for suppressing hydrogen penetration. If hydrogen penetration into a machine structural component can be suppressed, hydrogen will not aggregate within the machine structural component. Therefore, the present inventors investigated means for suppressing hydrogen penetration in a hydrogen generation environment from the perspective of chemical composition. As a result, the present inventors obtained the following findings.

[0022] Among the elements in the above-mentioned chemical composition, Si and Ni are elements that suppress the penetration of hydrogen into steel (machine structural parts) in a hydrogen generating environment. On the other hand, Cr is an element that promotes the penetration of hydrogen into steel (machine structural parts) in a hydrogen generating environment. Therefore, by ensuring a certain level of Si content 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.

[0023] 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: 0.75 to 1.20%, Si: 0.05 to 0.80%, Mn: 0.50 to 1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50%, Mo: 0.06 to 0.35%, V: 0.10 to 0.40%, Ni: more than 0.30% to 1.50%, Al: 0.005 to 0.100%, N: 0.030% or less, and O: 0.0015% or less, and if any optional element is contained, further containing one or more elements selected from the group consisting of the above-mentioned Groups 1 to 4 in place of a portion of Fe, with the balance consisting of Fe and impurities, then sufficient rolling contact fatigue life can be obtained in a hydrogen generating environment in machine structural parts manufactured using the steel material.

[0024] However, even when steel materials satisfy the above-mentioned chemical composition, there have been cases where machine structural parts manufactured using the steel materials do not achieve sufficient rolling contact fatigue life in a hydrogen generating environment. Therefore, the present inventors further investigated and investigated the relationship between the content of each element in the chemical composition and the rolling contact fatigue life in a hydrogen generating environment. As a result, the present inventors have obtained the following findings.

[0025] The ratio of the contents of three elements (Si, Ni, and Cr) that affect the penetration of hydrogen into steel in a hydrogen generating environment affects the rolling contact fatigue life in the hydrogen generating environment. Specifically, F1 is defined as (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, hydrogen penetration in the hydrogen generating environment can be sufficiently suppressed. Therefore, from the perspective of factor 1, the rolling contact fatigue life in the hydrogen generating environment can be extended. Therefore, F1 is set to a range that satisfies formula (1). (Si+Ni) / Cr≧1.30 (1)

[0026] To improve the rolling contact 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 intruded hydrogen. Mo and V form precipitates in machine structural parts made of steel, trapping the intruded hydrogen. Therefore, the total amount of Mo and V content correlates with the amount of precipitates.

[0027] 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)

[0028] Mechanical structural components manufactured from steel materials have a quench-hardened layer at least on the surface. The quench-hardened layer is a layer hardened by quenching and is primarily composed of martensite. However, the quench-hardened layer of a mechanical structural component contains not only martensite but also a certain amount of retained austenite. During use of the mechanical structural component, the retained austenite may undergo stress-induced martensite transformation due to external forces. If hydrogen is present nearby during stress-induced martensite transformation, excessive hydrogen coagulates at the interface between the retained austenite and stress-induced martensite, making it more likely to become a starting point for crack initiation. Furthermore, the formation of white structure is promoted at the interface between the retained austenite and stress-induced martensite.

[0029] On the other hand, retained austenite traps hydrogen that has penetrated into machine structural components, so if an appropriate amount of retained austenite is contained in the quench-hardened layer, it is possible to suppress the aggregation of hydrogen that has penetrated into machine structural components.

[0030] The amount of retained austenite contained in the quench-hardened layer of a machine structural component can be adjusted by the Mn and Cr contents of the steel material from which the machine structural component is made. It is defined as F3 = 2.5 × Mn + Cr. If F3 is 1.70 or more, the amount of retained austenite contained in the quench-hardened layer is sufficiently small. As a result, excessive hydrogen aggregation at the interface between the retained austenite and deformation-induced martensite and the formation of white structure are suppressed. On the other hand, if F3 is 3.90 or less, a sufficient amount of retained austenite is contained in the quench-hardened layer to suppress hydrogen aggregation. Therefore, F3 should be in a range that satisfies formula (3). 1.70≦2.5×Mn+Cr≦3.90 (3)

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

[0032] The first structural steel is In mass%, C: 0.75~1.20%, Si: 0.05 to 0.80% Mn: 0.50 to 1.50% P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50% Mo: 0.06 to 0.35%, V: 0.10~0.40%, Ni: over 0.30 to 1.50% Al: 0.005 to 0.100%, N: 0.030% or less, and O: 0.0015% or less, the balance being Fe and impurities, Formulas (1) to (3) are satisfied. (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (3).

[0033] The second structural steel is In mass%, C: 0.75~1.20%, Si: 0.05 to 0.80% Mn: 0.50 to 1.50% P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50% Mo: 0.06 to 0.35%, V: 0.10~0.40%, Ni: over 0.30 to 1.50% Al: 0.005 to 0.100%, N: 0.030% or less, and O: 0.0015% or less, Further, it contains one or more selected from the group consisting of Groups 1 to 4, the balance being Fe and impurities, Formulas (1) to (3) are satisfied. [Group 1] Ti: 0.050% or less, and Nb: 0.050% or less, one or more selected from the group consisting of [Group 2] B: 0.0050% or less, and Cu: 1.50% or less, one or more selected from the group consisting of [Group 3] Sn: 0.100% or less [Group 4] Ca: 0.0050% or less, and Mg: 0.0050% or less, one or more selected from the group consisting of (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (3).

[0034] The third component of steel is A steel material of a second configuration, Contains the first group.

[0035] The fourth structural steel is A steel material having the second or third configuration, Contains the second group.

[0036] The fifth structural steel is A steel material having any one of the second to fourth configurations, Contains the third group.

[0037] The sixth structural steel is A steel material having any one of the second to fifth configurations, Contains the fourth group.

[0038] The seventh structural steel is A steel material having any one of the second to sixth configurations, In mass%, Cu: Contains 0.05 to 1.50%.

[0039] The machine structural component of the first configuration comprises: A hardened layer is formed on at least the surface layer. The chemical composition of the machine structural part is, in mass%, C: 0.75~1.20%, Si: 0.05 to 0.80% Mn: 0.50 to 1.50% P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50% Mo: 0.06 to 0.35%, V: 0.10~0.40%, Ni: over 0.30 to 1.50% Al: 0.005 to 0.100%, N: 0.030% or less, and O: 0.0015% or less, the balance being Fe and impurities, Formulas (1) to (3) are satisfied. (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (3).

[0040] The second configuration of the machine structural component is A hardened layer is formed on at least the surface layer. The chemical composition of the machine structural part is, in mass%, C: 0.75~1.20%, Si: 0.05 to 0.80% Mn: 0.50 to 1.50% P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50% Mo: 0.06 to 0.35%, V: 0.10~0.40%, Ni: over 0.30 to 1.50% Al: 0.005 to 0.100%, N: 0.030% or less, and O: 0.0015% or less, Further, it contains one or more selected from the group consisting of Groups 1 to 4, the balance being Fe and impurities, Formulas (1) to (3) are satisfied. [Group 1] Ti: 0.050% or less, and Nb: 0.050% or less, one or more selected from the group consisting of [Group 2] B: 0.0050% or less, and Cu: 1.50% or less, one or more selected from the group consisting of [Group 3] Sn: 0.100% or less [Group 4] Ca: 0.0050% or less, and Mg: 0.0050% or less, one or more selected from the group consisting of (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (3).

[0041] The third configuration of mechanical structural parts is: A machine structural component having a second configuration, Contains the first group.

[0042] The fourth configuration of mechanical structural parts is: A machine structural part having the second or third configuration, Contains the second group.

[0043] The fifth component of the mechanical structural parts is: A machine structural part having any one of the second to fourth configurations, Contains the third group.

[0044] The sixth component of the mechanical structural parts is: A machine structural part having any one of the second to fifth configurations, Contains the fourth group.

[0045] The seventh configuration of mechanical structural parts is: A machine structural part having any one of the second to sixth configurations, In mass%, Cu: Contains 0.05 to 1.50%.

[0046] The steel material according to this embodiment and the machine structural parts manufactured using the steel material will be described in detail below. Note that "%" regarding elements means mass % unless otherwise specified.

[0047] [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: 0.75 to 1.20%, Si: 0.05 to 0.80%, Mn: 0.50 to 1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50%, Mo: 0.06 to 0.35%, V: 0.10 to 0.40%, Ni: more than 0.30 to 1.50%, Al: 0.005 to 0.100%, N: 0.030% or less, and O: 0.0015% or less, with the remainder being Fe and impurities. When the optional elements are contained, the chemical composition contains, in mass %, C: 0.75 to 1.20%, Si: 0.05 to 0.80%, Mn: 0.50 to 1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50%, Mo: 0.06 to 0.35%, V: 0.10 to 0.40%, Ni: more than 0.30% to 1.50%, Al: 0.005 to 0.100%, N: 0.030% or less, and O: 0.0015% or less, and further contains one or more elements selected from the group consisting of the above-mentioned first to fourth groups, with the balance being Fe and impurities. (Feature 2) The chemical composition satisfies formulas (1) to (3). (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (3). Each feature will be explained below.

[0048] [(Feature 1) Chemical composition] The chemical composition of the steel material of this embodiment contains the following elements.

[0049] C: 0.75 to 1.20% Carbon (C) combines with Mo and V to form precipitates. These precipitates trap hydrogen that penetrates into the steel when the machine structural component is used in a hydrogen-generating environment. This suppresses changes in the microstructure due to the penetrated hydrogen, and prevents peeling near the surface of the machine structural component due to the changed microstructure. As a result, the rolling fatigue life of the machine structural component in a hydrogen-generating environment is improved. If the C content is less than 0.75%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the C content exceeds 1.20%, the steel may become too hard, resulting in reduced machinability or the formation of coarse precipitates. The coarse precipitates are likely to become crack initiation sites when machine structural components are used in a hydrogen-generating environment. Therefore, even if the contents of other elements are within the ranges of this embodiment, the rolling fatigue life of machine structural components in a hydrogen-generating environment will be reduced. Therefore, the C content is 0.75 to 1.20%. The lower limit of the C content is preferably 0.80%, more preferably 0.85%, and even more preferably 0.90%. The upper limit of the C content is preferably 1.18%, more preferably 1.15%, and further preferably 1.10%.

[0050] Si: 0.05 to 0.80% Silicon (Si) increases the temper softening resistance of machine structural parts manufactured from steel. Si also increases the rolling fatigue life of machine structural parts 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%, even if the contents of other elements are within the ranges of this embodiment, the steel material becomes too hard, and therefore the machinability of the steel material deteriorates. 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%.

[0051] Mn: 0.50 to 1.50% Manganese (Mn) improves the hardenability of steel and the strength of machine structural parts manufactured using the steel. Therefore, the rolling fatigue life of machine structural parts in a hydrogen generating environment is increased. If the Mn content is less than 0.50%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mn content exceeds 1.50%, the hardness of the steel material becomes too high, and the machinability of the steel material decreases. Therefore, the Mn content is 0.50 to 1.50%. The lower limit of the Mn content is preferably 0.55%, more preferably 0.60%, and even more preferably 0.65%. The upper limit of the Mn content is preferably 1.45%, more preferably 1.40%, even more preferably 1.35%, even more preferably 1.30%, and still more preferably 1.20%.

[0052] 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%, P segregates excessively at grain boundaries. In this case, the grain boundary strength decreases. As a result, even if the contents of other elements are within the ranges of this embodiment, the rolling fatigue life of machine structural parts in a hydrogen generating environment decreases. Therefore, the P content is 0.015% or less. The P content is preferably as low as possible. However, excessive reduction in the P content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the P content is preferably more than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The upper limit of the P content is preferably 0.012%, more preferably 0.010%, and even more preferably 0.008%.

[0053] S: 0.005% or less Sulfur (S) is an impurity. S generates sulfide-based inclusions. Coarse sulfide-based inclusions are likely to become the starting point for cracks during use of machine structural components in a hydrogen-generating environment. If the S content exceeds 0.005%, the sulfide-based inclusions become coarse. As a result, even if the contents of other elements are within the ranges of this embodiment, the rolling fatigue life of machine structural components in a hydrogen-generating environment will be reduced. Therefore, the S content is 0.005% or less. The S content is preferably as low as possible. However, excessive reduction in the S content increases production costs. Therefore, in consideration of normal industrial production, the lower limit of the S content is preferably more than 0%, more preferably 0.001%, and even more preferably 0.002%. The upper limit of the S content is preferably 0.004%, and more preferably 0.003%.

[0054] Cr: 0.05 to less than 0.50% Chromium (Cr) improves the hardenability of steel and the strength of machine structural parts manufactured from steel. Therefore, the rolling fatigue life of machine structural parts in a hydrogen generating environment is extended. If the Cr content is less than 0.05%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, as described above, Cr promotes the penetration of hydrogen into steel materials (machine structural components). If the Cr content is 0.50% or more, even if the contents of other elements are within the ranges of this embodiment, the penetration of hydrogen into the steel material cannot be sufficiently suppressed during use of the machine structural component in a hydrogen generating environment. As a result, the rolling fatigue life of the machine structural component in a hydrogen generating environment is reduced. Therefore, the Cr content is 0.05 to less than 0.50%. The lower limit of the Cr content is preferably 0.08%, more preferably 0.10%, and even more preferably 0.15%. The upper limit of the Cr content is preferably 0.49%, more preferably 0.48%, even more preferably 0.47%, even more preferably 0.46%, even more preferably 0.45%, even more preferably 0.44%, and even more preferably 0.43%.

[0055] Mo: 0.06 to 0.35% Molybdenum (Mo) forms precipitates together with C and V during quenching in the process of manufacturing machine structural components using steel as a raw material. These precipitates trap hydrogen that penetrates into the steel when the machine structural component is used in a hydrogen-generating environment. This suppresses the formation of white structure due to the penetrated hydrogen, and also suppresses peeling near the surface of the machine structural component due to the formation of white structure. As a result, the rolling fatigue life of the machine structural component in a hydrogen-generating environment is improved. If the Mo content is less than 0.06%, the above effects cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Mo content exceeds 0.35%, even if the contents of other elements are within the ranges of this embodiment, the hardness of the steel material becomes excessively high, and therefore the machinability of the steel material deteriorates. Therefore, the Mo content is 0.06 to 0.35%. The lower limit of the Mo content is preferably 0.08%, more preferably 0.12%, and even more preferably 0.16%. The upper limit of the Mo content is preferably 0.33%, more preferably 0.30%, and further preferably 0.28%.

[0056] V: 0.10 to 0.40% Vanadium (V) forms precipitates together with C and Mo during quenching in the process of manufacturing machine structural components using steel as a raw material. These precipitates trap hydrogen that penetrates into the steel when the machine structural component is used in a hydrogen-generating environment. This suppresses the formation of white structure due to the penetrated hydrogen, and also suppresses peeling near the surface of the machine structural component due to the formation of white structure. As a result, the rolling fatigue life of the machine structural component in a hydrogen-generating environment is improved. If the V content is less than 0.10%, these effects cannot be 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 precipitates may be formed. The coarse precipitates are likely to become the starting point for cracks when machine structural parts are used in a hydrogen generating environment. Therefore, even if the contents of other elements are within the ranges of this embodiment, the rolling fatigue life of machine structural parts in a hydrogen generating environment may be reduced. Furthermore, if the V content exceeds 0.40%, the machinability of the steel material decreases. Therefore, the V content is 0.10 to 0.40%. The lower limit of the V content is 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 even more preferably 0.35%.

[0057] Ni: Over 0.30 to 1.50% Nickel (Ni) inhibits the penetration of hydrogen into steel materials (machine structural components) in a hydrogen generating environment. As a result, the rolling fatigue life of the machine structural components in a hydrogen generating environment is improved. If the Ni content is 0.30% or less, the above effect cannot be sufficiently obtained even if the contents of other elements are within the ranges of this embodiment. On the other hand, if the Ni content exceeds 1.50%, even if the contents of other elements are within the ranges of this embodiment, the steel material becomes too hard, and therefore the machinability of the steel material decreases. Therefore, the Ni content is more than 0.30% to 1.50%. The lower limit of the Ni content is preferably 0.31%, more preferably 0.35%, even more preferably 0.40%, even more preferably 0.45%, and even more preferably 0.50%. The upper limit of the Ni content is preferably 1.40%, more preferably 1.30%, even more preferably 1.20%, and still more preferably 1.10%.

[0058] Al: 0.005 to 0.100% Aluminum (Al) deoxidizes steel during the steelmaking process. Furthermore, Al combines with N in the steel to form AlN, which suppresses 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%, coarse clustered oxides are generated. The coarse clustered oxides become the starting points for cracks when the machine structural component is used in a hydrogen generating environment. Therefore, even if the contents of other elements are within the ranges of this embodiment, the rolling contact fatigue life of the 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 even more preferably 0.060%.

[0059] N: 0.030% or less Nitrogen (N) is an impurity. N dissolves in steel and reduces the hot workability of the steel. If the N content exceeds 0.030%, the machinability of the steel significantly decreases even if the contents of other elements are within the ranges of this embodiment. Therefore, the N content is 0.030% or less. The N content is preferably as low as possible. However, excessive reduction in the N content increases production costs. Therefore, taking into consideration normal industrial production, the lower limit of the N content is preferably more than 0%, more preferably 0.001%, and even more preferably 0.002%. The upper limit of the N content is preferably 0.028%, more preferably 0.025%, and even more preferably 0.020%.

[0060] O: 0.0015% or less Oxygen (O) is an impurity. O combines with other elements in the steel to form coarse oxides (including coarse oxides due to clustering). These coarse oxides become the starting points for cracks in a hydrogen generating environment. This reduces the rolling fatigue life of machine structural parts in a hydrogen generating environment. If the O content exceeds 0.0015%, the rolling fatigue life of machine structural parts in a hydrogen generating environment will be significantly reduced, even if the contents of other elements are within the ranges of this embodiment. Therefore, the O content is 0.0015% or less. The O content is preferably as low as possible. However, excessive reduction of the O content increases production costs. Therefore, taking into consideration normal industrial production, the lower limit of the O content is preferably more than 0%, more preferably 0.0001%, even more preferably 0.0003%, and even more preferably 0.0005%. The upper limit of the O content is preferably 0.0013% or less, more preferably 0.0011%, and even more preferably 0.0009%.

[0061] 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 substances that are mixed in from raw materials such as ore and scrap, or the manufacturing environment, during industrial production of the steel material, and are acceptable within a range that does not adversely affect the steel material according to the present embodiment.

[0062] [About optional elements] The chemical composition of the steel material of this embodiment may further contain one or more elements selected from the group consisting of first to fourth groups, in place of a portion of Fe. [Group 1] Ti: 0.050% or less, and Nb: 0.050% or less, one or more selected from the group consisting of [Group 2] B: 0.0050% or less, and Cu: 1.50% or less, one or more selected from the group consisting of [Group 3] Sn: 0.100% or less [Group 4] Ca: 0.0050% or less, and Mg: 0.0050% or less, one or more selected from the group consisting of These elements are optional and may not be contained. These optional elements will be described below.

[0063] [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 in place of a portion of Fe. These elements are optional elements, and both form precipitates, thereby enhancing the strength of machine structural parts manufactured using the steel material as a raw material through precipitation strengthening.

[0064] 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. Ti precipitates increase the strength of machine structural parts manufactured from steel materials through precipitation strengthening. Even if even a small amount of Ti is contained, the above effects can be obtained to some extent. However, if the Ti content exceeds 0.050%, coarse precipitates may be formed, which reduces the rolling fatigue life of machine structural parts in a hydrogen generating environment even if the contents of other elements are within the ranges of this embodiment. Therefore, the Ti content is 0 to 0.050%, and if contained, it is 0.050% or less. The lower limit of the Ti content is preferably 0.001%, more preferably 0.002%, and even more preferably 0.004%. The upper limit of the Ti content is preferably 0.045%, more preferably 0.040%, even more preferably 0.035%, and still more preferably 0.030%.

[0065] Nb: 0.050% or less Niobium (Nb) is an optional element and may not be contained, that is, the Nb content may be 0%. When Nb is contained, that is, when the Nb content is more than 0%, Nb forms Nb precipitates such as carbides, nitrides, and carbonitrides. Nb precipitates increase the strength of machine structural parts manufactured using steel as a raw material through precipitation strengthening. Even if even a small amount of Nb is contained, the above effects can be obtained to some extent. However, if the Nb content exceeds 0.050%, coarse precipitates may be formed, which reduces the rolling fatigue life of machine structural parts in a hydrogen generating environment even if the contents of other elements are within the ranges of this embodiment. Therefore, the Nb content is 0 to 0.050%, and when Nb is contained, it is 0.050% or less. The lower limit of the Nb content is preferably 0.001%, more preferably 0.005%, and even more preferably 0.010%. The upper limit of the Nb content is preferably 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0066] [Group 2: B and Cu] The chemical composition of the steel material of this embodiment may further contain one or more elements selected from the group consisting of B and Cu in place of a portion of Fe. These elements are optional elements, and all of them improve the hardenability of the steel material and increase the strength of machine structural parts manufactured using the steel material as a raw material.

[0067] 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 B is 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 strength of machine structural parts manufactured using the steel is increased. Even if even a small amount of B is contained, the above effects can be obtained to some extent. However, if the B content exceeds 0.0050%, the hardness of the steel material will be excessively high even if the contents of other elements are within the ranges of this embodiment, resulting in a decrease in the machinability of the steel material. Therefore, the B content is 0 to 0.0050%, and when 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%.

[0068] 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, the strength of machine structural parts manufactured using the steel material is increased. Even if even a small amount of Cu is contained, the above effects can be obtained to some extent. Furthermore, when the Cu content is 0.05% or more, the retained austenite contained in machine structural parts manufactured using the steel as a raw material is stabilized, thereby increasing the rolling fatigue life of the machine structural parts in a hydrogen generating environment. However, if the Cu content exceeds 1.50%, the toughness of the steel material is significantly reduced even if the contents of other elements are within the ranges of this embodiment, resulting in a reduction in the rolling fatigue life of machine structural parts in a hydrogen generating environment. Therefore, the Cu content is 0 to 1.50%, and if contained, it is 1.50% or less. The lower limit of the Cu content is preferably 0.01%, more preferably 0.05%, even more preferably 0.10%, and still more preferably 0.30%. The upper limit of the Cu content is preferably 1.40%, more preferably 1.30%, even more preferably 1.20%, and still more preferably 1.10%.

[0069] [Group 3: Sn] The chemical composition of the steel material of this embodiment may further contain Sn in place of a portion of Fe. Sn: 0.100% or less Tin (Sn) is an optional element and may not be contained, that is, the Sn content may be 0%. When contained, that is, when the Sn content is more than 0%, Sn improves the machinability of the steel material. 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 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%.

[0070] [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 sulfides in the steel material and increase the rolling fatigue life of machine structural parts manufactured using the steel material in a hydrogen-generating environment.

[0071] 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 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 some extent. However, if the Ca content exceeds 0.0050%, coarse Ca oxides may be formed in the steel material, which may reduce the rolling fatigue life of machine structural parts in a hydrogen generating environment, even if the contents of other elements are within the ranges of this embodiment. Therefore, the 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%.

[0072] Mg: 0.0050% or less Magnesium (Mg) is an optional element and may not be contained, that is, the Mg content may be 0%. When Mg is contained, that is, when the Mg content is more than 0%, Mg refines the sulfides in the steel. Furthermore, Mg promotes the spheroidization of the sulfides in the steel. As a result, the rolling fatigue life of 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 some extent. However, if the Mg content exceeds 0.0050%, coarse Mg oxides may be formed in the steel material, which may reduce the rolling fatigue life of machine structural parts in a hydrogen generating environment, even if the contents of other elements are within the ranges of this embodiment. Therefore, the 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%.

[0073] [(Feature 2) Regarding Equations (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) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (3). The formulas (1) to (3) will be explained below.

[0074] [Regarding formula (1)] F1 is defined as (Si + Ni) / Cr. As described above, Si and Ni suppress the penetration of hydrogen into steel in a hydrogen generating environment. On the other hand, Cr promotes the penetration of hydrogen into steel in a hydrogen generating environment. Therefore, from the viewpoint of suppressing the penetration of hydrogen into steel in a hydrogen generating environment, a high F1 is preferable.

[0075] If F1 is less than 1.30, even if the steel satisfies Feature 1 and also satisfies Equations (2) and (3), sufficient rolling fatigue life cannot be obtained in machine structural parts in a hydrogen generating environment.

[0076] If F1 is 1.30 or more, a sufficient rolling fatigue life can be obtained for machine structural parts in a hydrogen generating environment, provided that the steel satisfies Feature 1 and also satisfies Equations (2) and (3).

[0077] The lower limit of F1 is preferably 1.32, more preferably 1.34, even more preferably 1.36, even more preferably 1.38, and even more preferably 1.40. Although a preferred upper limit of F1 is not particularly limited, when the steel material satisfies Feature 1, the upper limit of F1 is 46.00. A preferred upper limit of F1 is less than 8.00, more preferably 7.99, even more preferably 7.80, even more preferably 7.50, even more preferably 7.00, and even more preferably 6.50. Note that F1 is the value obtained by rounding off the obtained numerical value to two decimal places (i.e., the value to one decimal place).

[0078] [Regarding formula (2)] F2 is defined as F2 = Mo + V. F2 is an index of the amount of complex precipitates that trap hydrogen that penetrates into steel during use in a hydrogen-generating environment in machine structural parts manufactured from steel.

[0079] As mentioned above, during quenching in the manufacturing process of machine structural parts made from steel, Mo and V combine with C to form complex precipitates. These complex precipitates trap hydrogen that penetrates into the steel (machine structural parts) when the machine structural parts are used in a hydrogen-generating environment, thereby increasing the rolling fatigue life of the machine structural parts in the hydrogen-generating environment. Therefore, a sufficient amount of these complex precipitates is required to trap the penetrated hydrogen.

[0080] If F2 is 0.30 or more, the Mo content and V content are sufficient to form a sufficient amount of complex precipitates, and therefore, provided that the steel satisfies Feature 1 and also satisfies Equations (1) and (3), it is possible to obtain an excellent rolling contact fatigue life for machine structural parts in a hydrogen generating environment.

[0081] The lower limit of F2 is preferably 0.31, more preferably 0.32, even more preferably 0.33, even more preferably 0.34, and even more preferably 0.35. Although there are no particular limitations on the preferred upper limit of F2, when the steel material satisfies Feature 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).

[0082] [Regarding formula (3)] It is defined as F3 = 2.5 × Mn + Cr. F3 is an index that represents the amount of retained austenite contained in the quench-hardened layer of a machine structural part manufactured using steel as a raw material. Mn and Cr reduce the amount of retained austenite contained in the quench-hardened layer. In other words, the higher the F3, the less retained austenite there is in the quench-hardened layer.

[0083] As mentioned above, the retained austenite contained in the quench-hardened layer may be transformed into stress-induced martensite by external force during use of a machine structural component. In this case, excessive hydrogen aggregation at the interface between the retained austenite and stress-induced martensite and the formation of white structure are promoted. If F3 is 1.70 or more, the amount of retained austenite contained in the quench-hardened layer is sufficiently reduced. As a result, excessive hydrogen aggregation and the formation of white structure are suppressed.

[0084] On the other hand, retained austenite traps hydrogen that has penetrated into machine structural components. Therefore, if an appropriate amount of retained austenite is contained in the quench-hardened layer, it can suppress the aggregation of hydrogen that has penetrated into machine structural components. If F3 is 3.90 or less, the quench-hardened layer contains a sufficient amount of retained austenite to suppress the aggregation of hydrogen. Therefore, if F3 is 1.70 to 3.90, the steel satisfies Characteristic 1, and on the premise that formulas (1) and (2) are satisfied, excellent rolling fatigue life can be obtained in machine structural parts in a hydrogen generating environment.

[0085] The lower limit of F3 is preferably 1.72, more preferably 1.80, even more preferably 1.90, and even more preferably 2.00. The upper limit of F3 is preferably 3.78, more preferably 3.70, even more preferably 3.50, and even more preferably 3.00. Note that F3 is the value obtained by rounding off the obtained numerical value to two decimal places (i.e., the value to one decimal place).

[0086] [Effects of the steel material of this embodiment] The steel material of this embodiment satisfies Features 1 and 2. Therefore, machine structural parts manufactured using the steel material as a raw material can obtain a sufficient rolling fatigue life in a hydrogen generating environment.

[0087] [Preferable form of steel material according to this embodiment] Preferably, the steel material of this embodiment includes the following feature 3 in addition to the above-mentioned features 1 and 2. (Feature 3) The chemical composition contains, in mass %, Cu: 0.05 to 1.50%. Feature 3 will be explained below.

[0088] [(Feature 3) Cu content] When the chemical composition of a steel contains 0.05 to 1.50% Cu, the retained austenite contained in the quench-hardened layer of a machine structural component is stabilized. Therefore, deformation-induced martensitic transformation during use of the machine structural component is suppressed. As a result, excessive hydrogen aggregation at the interface between the retained austenite and deformation-induced martensite and the formation of white structure are further suppressed. In this way, the rolling contact fatigue life of the machine structural component in a hydrogen-generating environment can be further improved. Therefore, the steel material of this embodiment preferably contains Cu: 0.05 to 1.50%.

[0089] [About the microstructure of steel] The microstructure of the steel material of this embodiment is not particularly limited. In this embodiment, as described above, a machine structural part manufactured using a steel material as a raw material can obtain a sufficient rolling fatigue life in a hydrogen generating environment. This effect is a characteristic required for a machine structural part manufactured using a steel material as a raw material. Usually, during hot working in the manufacturing process of a machine structural part made of a steel material, the steel material is subjected to A c3 When the steel material is heated to above this temperature, the microstructure of the steel material is transformed into austenite. Therefore, the steel material of this embodiment can achieve the above-mentioned effects regardless of the microstructure.

[0090] [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.

[0091] [Use of the steel material according to this embodiment] Machine structural parts manufactured using the steel material of this embodiment as a raw material can achieve a sufficient rolling fatigue life in a hydrogen generating environment. Therefore, the steel material of this embodiment can be used as a raw material for machine structural parts used in automobiles, industrial machinery, etc. In particular, the steel material of this embodiment is suitable as a raw material for machine structural parts manufactured by quenching and tempering. Machine structural parts manufactured by quenching and tempering are, for example, bearing parts. Note that the steel material of this embodiment can also be used for applications other than as a raw material for machine structural parts.

[0092] [Steel manufacturing method] An example of a method for manufacturing a steel material according to this embodiment will be described. The method for manufacturing a steel material described below is an example for manufacturing the steel material according to this embodiment. Therefore, a steel material 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 steel material according to this embodiment. In this embodiment, a method for manufacturing a steel bar will be described as an example of a steel material.

[0093] An example of the method for manufacturing the 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.

[0094] [(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. Preferably, molten steel that satisfies Features 1, 2, and also 3 is produced. The refining method is not particularly limited, and a well-known method may be used. For example, molten pig iron produced by a well-known method is subjected to refining (primary refining) in a converter. The molten steel tapped from the converter is subjected to well-known secondary refining. In the secondary refining, alloy elements are added to the molten steel to adjust the composition.

[0095] The molten steel produced by the above-described refining method is used to produce a material by a well-known casting method. For example, an ingot is produced by an ingot casting method using the molten steel. Alternatively, a bloom or billet may be produced by a continuous casting method using the molten steel. The material (ingot, bloom, or billet) is produced by the above-described method.

[0096] [(Process 2) Hot processing process] The produced material is hot worked to produce steel. In the hot working process, one or more hot working steps are usually performed. When hot working is performed multiple times, the first hot working step may be, for example, rolling using blooming or hot forging, and subsequent hot working steps may be rolling using a continuous rolling mill. The continuous rolling mill has multiple rolling stands arranged in a row. The steel material after 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 subjected to finish rolling using a continuous rolling mill to produce steel of a desired size. Alternatively, steel may be produced from the material by hot forging alone. The heating temperature of the material during hot working is not particularly limited, but is, for example, 1000 to 1300°C.

[0097] [Regarding the machine structural component of this embodiment] In this embodiment, the machine structural component refers to a component manufactured by quenching and tempering steel material. The machine structural component is, for example, a bearing component. The bearing component refers to a rolling bearing component. The bearing component is, for example, a raceway ring, a washer, a rolling element, etc. 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 are components having a raceway surface. The rolling element may be a ball or a roller. The roller may be, for example, a cylindrical roller, a rod roller, a needle roller, a tapered roller, a convex roller, etc.

[0098] The machine structural component of this embodiment is manufactured using the steel material of this embodiment as a raw material, and has a quench-hardened layer at least on the surface.

[0099] The quench-hardened layer is a layer hardened by quenching and is mainly composed of martensite. The quench-hardened layer has a Vickers hardness of 600 HV or more according to JIS Z 2244-1 (2020). A mechanical structural part is determined to have a quench-hardened layer when the Vickers hardness at a depth of 100 μm from the surface of the part is 600 HV or more.

[0100] [Vickers hardness measurement method] The Vickers hardness at a depth of 100 μm from the surface of a machine structural part can be measured by the following method. A test piece is taken with a cross section perpendicular to any surface of the machine structural component as the observation surface. The size of the observation surface is not particularly limited as long as it includes a position 100 μm deep from any surface of the machine structural component. The observation surface is surface polished. After mirror polishing, the Vickers hardness is determined at three arbitrary points 100 μm from the surface using a Vickers hardness test in accordance with JIS Z 2244-1 (2020). The test force is 9.8 N. The arithmetic mean of the Vickers hardness values obtained at the three points is calculated. If the arithmetic mean value is 600 HV or higher, the machine structural component is determined to have a hardened layer.

[0101] [Features of the machine structural component of this embodiment] The machine structural component of this embodiment includes the following features 4 and 5. (Feature 4) The chemical composition, in mass%, is C: 0.75 to 1.20%, Si: 0.05 to 0.80%, Mn: 0.50 to 1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50%, Mo: 0.06 to 0.35%, V: 0.10 to 0.40%, Ni: more than 0.30 to 1.50%, Al: 0.005 to 0.100%, N: 0.030% or less, and O: 0.0015% or less, with the remainder being Fe and impurities. When the optional elements are contained, the chemical composition contains, in mass %, C: 0.75 to 1.20%, Si: 0.05 to 0.80%, Mn: 0.50 to 1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50%, Mo: 0.06 to 0.35%, V: 0.10 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 5) The chemical composition satisfies formulas (1) to (3). (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (3).

[0102] The function of each element in Feature 4 is the same as the function of the corresponding element described in Feature 1 of the steel material of this embodiment. Moreover, the technical significance of formulas (1) to (3) in Feature 5 is the same as that described in Feature 2 of the steel material of this embodiment.

[0103] [Effects of the machine structural component of this embodiment] As described above, the machine structural component of this embodiment includes Features 4 and 5. Therefore, the machine structural component of this embodiment has a sufficient rolling fatigue life in a hydrogen generating environment.

[0104] [Preferred embodiment of the machine structural component of this embodiment] Preferably, the machine structural component of this embodiment includes the following Feature 6 in addition to the above-mentioned Features 4 and 5. The technical significance of Feature 6 is the same as that explained in Feature 3 of the steel material of this embodiment. (Feature 6) The chemical composition contains, in mass %, Cu: 0.05 to 1.50%.

[0105] [Method for manufacturing a machine structural component according to this embodiment] The machine structural component of this embodiment is manufactured using the above-described steel material of this embodiment as a raw material, for example, by the following manufacturing method. (Process 3) Hot processing process (Step 4) Quenching and tempering process Each step will be described below.

[0106] [(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 this point, the steel is processed. Therefore, the microstructure of the steel is reset during heating in the hot processing step. The heating temperature is a known temperature, for example, 1000 to 1300°C. The intermediate product after hot processing is cooled to room temperature. If necessary, the intermediate product after hot processing is subjected to cutting processing. Note that the intermediate product after hot processing or cutting processing may be subjected to known heat treatment. Known heat treatments include, for example, normalizing and / or spheroidizing.

[0107] [(Process 4) Quenching and tempering process] In the quenching and tempering process, the manufactured intermediate product is subjected to well-known quenching and tempering to manufacture a machine structural part. In quenching, the intermediate product is A c3 The intermediate product is heated to and maintained at a temperature equal to or higher than the transformation point, and then rapidly cooled. In tempering, the quenched intermediate product is maintained at a temperature within a range of, for example, 100 to 200°C for a predetermined period of time. Note that the quenched intermediate product or the tempered machine structural part may be subjected to a well-known sub-zero treatment.

[0108] Through the above steps, the machine structural component of this embodiment is manufactured. [Example]

[0109] Steel materials having the chemical compositions shown in Table 1 (Table 1-1 and Table 1-2) were produced by the following method.

[0110] [Table 1-1]

[0111] [Table 1-2]

[0112] 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. After hot forging, the steel material was air-cooled to room temperature. Through the above production process, the steel material (steel bar with 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.

[0113] [Rolling fatigue life evaluation test under hydrogen generation environment] Test pieces (small roller test pieces) for roller pitting fatigue tests, which are simulated machine structural parts for each test number, were prepared, 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 dimensions (unit: mm). "φ" in the figure means diameter.

[0114] Specifically, the steel material (60 mm diameter steel bar) of each test number was heated at a heating temperature of 1200°C for a holding time of 30 minutes. Then, hot working (hot forging) was performed at a finishing temperature of 950°C or higher to produce a 35 mm diameter steel bar. The hot-worked steel bar was air-cooled to room temperature. The produced 35 mm diameter steel bar was subjected to spheroidizing annealing. In the spheroidizing annealing, the 35 mm diameter steel bar was held at 800°C for 3 hours, then rapidly cooled to 720°C, and then cooled to 600°C at a cooling rate of 10°C / hour. After cooling to 600°C at the above cooling rate, it was air-cooled to room temperature. The 35 mm diameter steel bar was machined (cut) to produce an intermediate small roller test piece. The intermediate products were subjected to quenching and tempering treatment, and a plurality of small roller test pieces (simulated mechanical structural parts) shown in Figure 1 were prepared for each test number.

[0115] The quenching and tempering process was performed as follows. The intermediate products of each test number were quenched. The quenching temperature was 840°C, the holding time at the quenching temperature was 30 minutes, and the products were cooled in oil at 130°C. After oil cooling, the intermediate products were tempered at 180°C for 180 minutes. Vickers hardness was measured at three arbitrary points 100 μm deep from the surface of the 26 mm diameter small roller test piece (simulated mechanical structural part) according to the Vickers hardness measurement method described above. As a result, the arithmetic mean value of Vickers hardness was 600 HV or more for all small roller test pieces (simulated mechanical structural part), and all small roller test pieces (simulated mechanical structural part) of each test number had a quench-hardened layer.

[0116] (Roller pitching fatigue test) The following roller pitting fatigue test was carried out using the prepared small roller test pieces. 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.

[0117] 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 subjected to quenching and tempering under the same conditions as the small roller test specimen 200. 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.

[0118] To simulate the hydrogen generation environment, a hydrogen charging treatment was performed 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 and held for 96 hours. The temperature of the aqueous solution during holding was set to 20°C. The hydrogen charging treatment was performed under the above conditions. The small roller test piece 200 after the hydrogen charging treatment was used to carry out the roller pitting fatigue test described below.

[0119] In the roller pitching fatigue test, the large roller test piece 100 was pressed against the small roller test piece 200 with various Hertzian stress surface pressures. The test was performed by rotating the test piece 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 temperature of the ATF (automatic transmission lubricating oil) 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 7Roller 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.

[0120] 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 24, which meets the SUJ2 standard of JIS G 4805 (2019), 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 using the following formula. Rolling fatigue life ratio = Rolling fatigue life of steel material with each test number / Rolling fatigue life of reference steel material (test number 24) 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 ≥ 10.00, or no peeling up to 50 million repeated loads Evaluation G (Good): 1.50≦rolling fatigue life ratio<10.00 Rating B (Bad): Rolling fatigue life ratio < 1.50 In the cases of evaluations E and G, 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.

[0121] [Table 2]

[0122] [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 12 satisfied characteristics 1 and 2. Therefore, the small roller test pieces simulating machine structural parts satisfied characteristics 4 and 5. As a result, a sufficient rolling fatigue life was obtained in a rolling fatigue life evaluation test in a hydrogen generation environment.

[0123] The steel materials of test numbers 9 to 12 further satisfied feature 3 in addition to features 1 and 2. Therefore, the small roller test pieces simulating machine structural parts satisfied feature 6 in addition to features 4 and 5. As a result, an even better rolling fatigue life was obtained in the rolling fatigue life evaluation test in a hydrogen generation environment (rating E).

[0124] On the other hand, the Cr content was too high in test numbers 13 and 14. As a result, a sufficient rolling fatigue life was not obtained in the rolling fatigue life evaluation test in a hydrogen generation environment.

[0125] In test number 15, the Cu 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.

[0126] In test numbers 16 and 17, 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.

[0127] In test numbers 18 and 19, 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.

[0128] In test numbers 20 and 21, F3 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.

[0129] In test numbers 22 and 23, F3 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.

[0130] 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. In mass%, C: 0.75-1.20%, Si: 0.05-0.80%, Mn: 0.50 to 1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50% Mo: 0.06-0.35%, V: 0.10-0.40%, Ni: more than 0.30 to 1.50%; Al: 0.005-0.100%, N: 0.030% or less, and O: 0.0015% or less, the balance being Fe and impurities; Formulas (1) to (3) are satisfied. Steel material. (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (3).

2. In mass%, C: 0.75-1.20%, Si: 0.05-0.80%, Mn: 0.50 to 1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50% Mo: 0.06-0.35%, V: 0.10-0.40%, Ni: more than 0.30 to 1.50%; Al: 0.005-0.100%, N: 0.030% or less, and O: 0.0015% or less, Further, it contains one or more selected from the group consisting of Groups 1 to 4, the balance being Fe and impurities; Formulas (1) to (3) are satisfied. 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) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (3).

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. The steel material according to any one of claims 2 to 6, In mass%, Cu: Contains 0.05 to 1.50%; Steel material.

8. A machine structural part, A hardened layer is formed on at least the surface layer. The chemical composition of the machine structural part is, in mass%, C: 0.75-1.20%, Si: 0.05-0.80%, Mn: 0.50 to 1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50% Mo: 0.06-0.35%, V: 0.10-0.40%, Ni: more than 0.30 to 1.50%; Al: 0.005-0.100%, N: 0.030% or less, and O: 0.0015% or less, the balance being Fe and impurities; Formulas (1) to (3) are satisfied. Mechanical structural parts. (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (3).

9. A machine structural part, A hardened layer is formed on at least the surface layer. The chemical composition of the machine structural part is, in mass%, C: 0.75-1.20%, Si: 0.05-0.80%, Mn: 0.50 to 1.50%, P: 0.015% or less, S: 0.005% or less, Cr: 0.05 to less than 0.50% Mo: 0.06-0.35%, V: 0.10-0.40%, Ni: more than 0.30 to 1.50%; Al: 0.005-0.100%, N: 0.030% or less, and O: 0.0015% or less, Further, it contains one or more selected from the group consisting of Groups 1 to 4, the balance being Fe and impurities; Formulas (1) to (3) are satisfied. Mechanical structural parts. [Group 1] Ti: 0.050% or less, and Nb: 0.050% or less, one or more selected from the group consisting of [Group 2] B: 0.0050% or less, and Cu: 1.50% or less, one or more selected from the group consisting of [Group 3] Sn: 0.100% or less [Group 4] Ca: 0.0050% or less, and Mg: 0.0050% or less, one or more selected from the group consisting of (Si+Ni) / Cr≧1.30 (1) Mo+V≧0.30 (2) 1.70≦2.5×Mn+Cr≦3.90 (3) Here, the content of the corresponding element in mass % is substituted for each element symbol in formulas (1) to (3).

10. The machine structural component according to claim 9, containing the first group, Mechanical structural parts.

11. The machine structural component according to claim 9, containing the second group, Mechanical structural parts.

12. The machine structural component according to claim 9, containing the third group, Mechanical structural parts.

13. The machine structural component according to claim 9, containing the fourth group Mechanical structural parts.

14. A machine structural component according to any one of claims 9 to 13, In mass%, Cu: Contains 0.05 to 1.50%; Mechanical structural parts.

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