MECHANICAL COMPONENT AND METHOD FOR MANUFACTURING A MECHANICAL COMPONENT

Abstract

A spur gear (1) as a mechanical component is made of steel containing 0.12 mass % or more and 0.28 mass % or less of carbon, 0.15 mass % or more and 0.70 mass % or less of silicon, 0.20 mass % or more and 0.95 mass % or less of manganese, 0.85 mass % or more and 1.90 mass % or less of chromium, with the remainder being iron and unavoidable impurities, and has a martensitic structure and includes a carburized layer (31) arranged to form at least a part of a surface and having a higher carbon concentration than the other part. The carburized layer (31) includes a composite oxide film (40) arranged to form the surface and having a thickness of 1 µm or more and 25 µm or less and containing magnetite and hematite.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a mechanical component and a method of manufacturing the mechanical component.

[0002] This application claims priority based on Japanese Patent Application No. 2023-034868, filed on March 7, 2023, the contents of which are hereby incorporated by reference. STATE OF THE ART

[0003] In a mechanical component made of steel, a carburized layer with a higher carbon concentration than other parts can be formed to form a surface from the perspective of improving strength. The mechanical component with a carburized layer and quench-hardened exhibits high hardness in the surface layer portion. On the other hand, the area outside the carburized layer exhibits low hardness and excellent toughness. As a result, the mechanical component with the carburized layer can achieve both strength and toughness.

[0004] The carburized layer is generally formed by gas carburizing. In gas carburizing, carbon (C) is formed on the surface of the mechanical component through a reaction in which two molecules of carbon monoxide (CO) are converted into one molecule of carbon dioxide (CO2), and the carbon is then added to the surface of the mechanical component. Gas carburizing is an excellent treatment method because it can be carried out at a low cost. However, since the above reaction produces carbon dioxide, a problem arises that a large amount of carbon dioxide is released during the carburizing treatment.

[0005] Another well-known carburizing process is vacuum carburizing. In vacuum carburizing, the mechanical component is heated in a reduced-pressure atmosphere (e.g., at a pressure of 1 kPa or less), and carbon generated by the decomposition reaction of hydrocarbons such as acetylene (C2H2) on the surface of the mechanical component is introduced onto the surface of the mechanical component. Vacuum carburizing generally requires higher equipment costs than gas carburizing, but has the advantages of shortening treatment time, reducing production costs, and avoiding the emission of large amounts of carbon dioxide.Steel suitable for vacuum carburizing, mechanical components with excellent properties due to the carburized layer formed by vacuum carburizing, and the like have been proposed (see, for example, JP 2022-080369 A (Patent Document 1), WO 2020 / 144830 (Patent Document 2), and JP 2006-183095 A (Patent Document 3)). LIST OF CITATIONS Patent literature Patent document 1: JP 2022-080369 A Patent Document 2: WO 2020 / 144830 Patent Document 3: JP 2006-183095 A SUMMARY OF THE INVENTIONTechnical Problem

[0006] As mentioned in the above-mentioned patent documents, a mechanical component with a carburized layer formed by vacuum carburization has the characteristic of suppressing the formation of a grain boundary oxide layer on the surface compared to a mechanical component with a carburized layer formed by gas carburization. It is generally believed that the properties such as fatigue strength (service life) of the mechanical component are improved by suppressing the formation of a grain boundary oxide layer.

[0007] However, the investigations of the present inventors have revealed that in a mechanical component in which the carburized layer is formed by vacuum carburizing, a phenomenon (white layer peeling) may occur in which a white layer forms on the surface and starts peeling off in a comparatively short time compared with a mechanical component in which the carburized layer is formed by gas carburizing.

[0008] An object of the present disclosure is to provide a mechanical component having improved durability by suppressing the occurrence of peeling of the white layer, and a method for manufacturing the mechanical component. Solution to the problem

[0009] A mechanical component according to a first aspect of the present disclosure is made of steel containing 0.12 mass % or more and 0.28 mass % or less of carbon (C), 0.15 mass % or more and 0.70 mass % or less of silicon (S), 0.20 mass % or more and 0.95 mass % or less of manganese (Mn), and 0.85 mass % or more and 1.90 mass % or less of chromium (Cr), the remainder being iron and unavoidable impurities, and the steel has a martensitic structure. The mechanical component includes a carburized layer arranged to form at least a part of a surface and having a higher carbon concentration than the other part.The carburized layer includes a composite oxide film arranged to form the surface and having a thickness of 1 µm or more and 25 µm or less and containing magnetite (Fe3O4) and hematite (Fe2O3).

[0010] A mechanical component according to a second aspect of the present disclosure is made of steel containing 0.12 mass % or more and 0.28 mass % or less of carbon, 0.15 mass % or more and 0.70 mass % or less of silicon, 0.20 mass % or more and 0.95 mass % or less of manganese, 0.85 mass % or more and 1.90 mass % or less of chromium, and at least one selected from the group consisting of 0.15 mass % or more and 0.45 mass % or less of molybdenum (Mo), 0.01 mass % or more and 2.00 mass % or less of nickel (Ni), and 0.04 mass % or more and 0.08 mass % or less of niobium (Nb), the balance being iron and unavoidable impurities, and the steel has a martensitic structure.The mechanical component includes a carburized layer arranged to form at least a portion of a surface and having a higher carbon concentration than the other portion. The carburized layer includes a composite oxide film arranged to form the surface and having a thickness of 1 μm or more and 25 μm or less, and containing magnetite and hematite.

[0011] A method for manufacturing a mechanical component according to a first aspect of the present disclosure includes a step of preparing a steel material, a step of obtaining a compact, a step of forming a composite oxide film, a step of forming a carburized layer, and a step of quench-hardening the compact. In the step of preparing a steel material, a steel material is provided containing 0.12 mass % or more and 0.28 mass % or less of carbon, 0.15 mass % or more and 0.70 mass % or less of silicon, 0.20 mass % or more and 0.95 mass % or less of manganese, and 0.85 mass % or more and 1.90 mass % or less of chromium, with the remainder being iron and unavoidable impurities. In the step of obtaining a compact, the compact is obtained by molding the steel material.In the step of forming a composite oxide film, the compact is heated in an oxidizing atmosphere, and a composite oxide film containing magnetite and hematite is formed on the surface of the compact. In the step of forming a carburized layer, the compact on which the composite oxide film has been formed is heated in a carburizing atmosphere at a pressure of 1 kPa or less, thereby forming a carburized layer on the surface of the compact. The carburized layer has a higher carbon concentration than the other part and a thickness greater than the composite oxide film. In the step of quench-hardening the compact, the compact with the composite oxide film and the carburized layer formed thereon is quenched.

[0012] A method for manufacturing a mechanical component according to a second aspect of the present disclosure includes a step of preparing a steel material, a step of obtaining a compact, a step of forming a composite oxide film, a step of forming a carburized layer, and a step of quench-hardening the compact.In the step of providing a steel material, a steel material is provided containing 0.12 mass % or more and 0.28 mass % or less of carbon, 0.15 mass % or more and 0.70 mass % or less of silicon, 0.20 mass % or more and 0.95 mass % or less of manganese, 0.85 mass % or more and 1.90 mass % or less of chromium, and at least one selected from the group consisting of 0.15 mass % or more and 0.45 mass % or less of molybdenum, 0.01 mass % or more and 2.00 mass % or less of nickel, and 0.04 mass % or more and 0.08 mass % or less of niobium, with the remainder being iron and unavoidable impurities. In the step of obtaining a compact, the compact is obtained by molding the steel material.In the step of forming a composite oxide film, the compact is heated in an oxidizing atmosphere, and a composite oxide film containing magnetite and hematite is formed on the surface of the compact. In the step of forming a carburized layer, the compact on which the composite oxide film has been formed is heated in a carburizing atmosphere at a pressure of 1 kPa or less, thereby forming a carburized layer on the surface of the compact. The carburized layer has a higher carbon concentration than the other part and a thickness greater than the composite oxide film. In the step of quench-hardening the compact, the compact with the composite oxide film and the carburized layer formed thereon is quenched. ADVANTAGEOUS EFFECTS OF THE INVENTION

[0013] According to the mechanical component and the method for manufacturing the mechanical component, it is possible to provide a mechanical component and a method for manufacturing the mechanical component in which durability is improved by suppressing the occurrence of peeling of the white layer. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic perspective view illustrating the appearance of a spur gear according to a first embodiment. Fig. 2 is a schematic cross-sectional view illustrating a cross-sectional structure of the spur gear according to the first embodiment. Fig. 3 is a schematic cross-sectional view illustrating the structure of a carburized layer according to the first embodiment. Fig. 4 is a flowchart illustrating a method of manufacturing the spur gear according to the first embodiment. Fig. 5 is a diagram illustrating the heat treatment performed in the method of manufacturing the spur gear according to the first embodiment. Fig. 6 is a schematic cross-sectional view illustrating a structure of a carburized layer of a spur gear according to a second embodiment. Fig. 7 is a flowchart illustrating a method of manufacturing the spur gear according to the second embodiment. Fig. 8 is a diagram illustrating the heat treatment performed in the method of manufacturing the spur gear according to the second embodiment. Fig. 9 is a schematic cross-sectional view illustrating a structure of a carburized layer of a spur gear according to a third embodiment. Fig. 10 is a flowchart illustrating the method for manufacturing the spur gear according to the third embodiment. Fig. 11 is a flowchart illustrating a method of manufacturing a spur gear according to the fourth embodiment. Fig. 12 is a diagram illustrating the heat treatment performed in the method of manufacturing the spur gear according to the fourth embodiment. Fig. 13 is a schematic perspective view illustrating the appearance of a bevel pinion. Fig. Figure 14 is a graph showing a relationship between the thickness of a composite oxide film and the occurrence of a white layer. Fig. Figure 15 is a graph showing the results of X-ray diffraction analysis of the composite oxide film. Fig. Figure 16 is a graph showing the results of X-ray diffraction analysis of an oxide film formed by gas carburizing. Fig. Figure 17 is a diagram illustrating a procedure for a tooth root flexural fatigue test. Fig. Figure 18 is a graph showing the results of a surface compressive strength test. Fig. Figure 19 is a graph showing the results of the tooth root flexural fatigue test. Fig. Figure 20 is a graph showing the results of a tooth root impact bending test. Fig. Figure 21 is a graph showing the results of a Charpy impact test. DESCRIPTION OF EMBODIMENTS Brief description of the embodiment

[0014] A mechanical component according to a first aspect of the present disclosure is made of steel containing 0.12 mass % or more and 0.28 mass % or less of carbon (C), 0.15 mass % or more and 0.70 mass % or less of silicon (S), 0.20 mass % or more and 0.95 mass % or less of manganese (Mn), and 0.85 mass % or more and 1.90 mass % or less of chromium (Cr), the remainder being iron and unavoidable impurities, and the steel has a martensitic structure. The mechanical component includes a carburized layer arranged to form at least a part of a surface and having a higher carbon concentration than the other part.The carburized layer includes a composite oxide film arranged to form the surface and having a thickness of 1 µm or more and 25 µm or less and containing magnetite (Fe3O4) and hematite (Fe2O3).

[0015] A mechanical component according to a second aspect of the present disclosure is made of steel containing 0.12 mass % or more and 0.28 mass % or less of carbon, 0.15 mass % or more and 0.70 mass % or less of silicon, 0.20 mass % or more and 0.95 mass % or less of manganese, 0.85 mass % or more and 1.90 mass % or less of chromium, and at least one selected from the group consisting of 0.15 mass % or more and 0.45 mass % or less of molybdenum (Mo), 0.01 mass % or more and 2.00 mass % or less of nickel (Ni), and 0.04 mass % or more and 0.08 mass % or less of niobium (Nb), the balance being iron and unavoidable impurities, and the steel has a martensitic structure.The mechanical component includes a carburized layer arranged to form at least a portion of a surface and having a higher carbon concentration than the other portion. The carburized layer includes a composite oxide film arranged to form the surface and having a thickness of 1 μm or more and 25 μm or less, and containing magnetite and hematite.

[0016] The present inventors investigated a measure to suppress the occurrence of white layer peeling in a mechanical component having a carburized layer formed by vacuum carburization. As a result, the inventors found that the occurrence of white layer peeling can be suppressed by forming a composite oxide film containing magnetite and hematite with a predetermined thickness or more, specifically, a thickness of 1 μm or more, to form the surface of the carburized layer. The reason for suppressing the occurrence of white layer peeling is not limited to the following points, but the following can be considered as a reason, for example.

[0017] It is believed that the peeling of the white layer occurs as follows. First, a new surface is formed on the surface of the mechanical component through contact with another component or the like. When lubricating oil comes into contact with the new surface, the hydrocarbons contained in the lubricating oil are decomposed, and the hydrogen generated by the decomposition enters the surface layer of the mechanical component, leading to the formation of a white layer. The white layer serves as a starting point for cracks in the mechanical component, causing damage (peeling of the white layer) to occur on the surface of the mechanical component within a short period of time.

[0018] In contrast, the occurrence of peeling of the white layer can be suppressed by forming a composite oxide film containing magnetite and hematite with a predetermined thickness or more (particularly 1 μm or more) to form the surface of the carburized layer. The reasons for this include, for example, that the presence of the composite oxide film suppresses the formation of the above-described new surface, and that the composite oxide film is appropriately removed at a contact part with another mechanical component when it is newly installed in a machine and put into operation, thereby compensating for a shape error of the mechanical component (improving the initial fitting accuracy). It is known that an oxide film (grain boundary oxide layer) forming a surface is also formed in a carburized layer formed by gas carburization.However, the oxide layer formed during gas carburizing is an oxide layer consisting of a single phase of magnetite and is different from the composite oxide film described above.

[0019] One of the most important advantages of using vacuum carburizing to form a carburized layer is that no oxide film (grain boundary oxide layer) is formed in the surface layer part. In contrast, the present inventors found that the white layer peeling can be suppressed by deliberately forming a composite oxide film containing magnetite and hematite on the surface of the carburized layer. To sufficiently exhibit the white layer peeling suppression function, the thickness of the composite oxide film must be 1 μm or more. On the other hand, if the thickness of the composite oxide film exceeds 25 μm, the above-described function is saturated, while the bending strength, torsional strength, and the like of the mechanical component are reduced, and the manufacturing cost increases. Therefore, the thickness of the composite oxide film must be 25 μm or less.

[0020] In the mechanical component according to the present disclosure, the carburized layer includes a composite oxide film arranged to form a surface, having a thickness of 1 μm or more and 25 μm or less, and containing magnetite and hematite. As a result, the mechanical component according to the present disclosure can provide a mechanical component with improved durability by suppressing the occurrence of peeling of the white layer.

[0021] The following describes the reasons for limiting the composition of the steel constituting the mechanical component of the present disclosure to the above-mentioned range. Carbon: 0.12 mass percent or more and 0.28 mass percent or less

[0022] Carbon content has a significant impact on the hardness of steel with a martensitic structure (quench-hardened steel). To ensure sufficient hardness of a base part, which is a region other than the carburized layer, and to shorten the carburizing treatment time required to provide sufficient carbon content in the carburized layer, the carbon content must be 0.12 mass percent or more. On the other hand, if the carbon content of the base part is high, the toughness of the mechanical component decreases. To ensure sufficient toughness, the carbon content must be 0.28 mass percent or less. Silicon: 0.15 mass percent or more and 0.70 mass percent or less

[0023] Silicon contributes to improving hardenability and temper softening resistance. To ensure such a function, the silicon content must be 0.15 mass percent or more. On the other hand, processability, such as machinability, tends to decrease if the silicon content is too high. To enable easy processing, the silicon content must be 0.70 mass percent or less. From a processability perspective, the silicon content is preferably 0.35 mass percent or less. From a hardenability perspective and temper softening resistance, the silicon content is preferably 0.45 mass percent or more. Manganese: 0.20 mass percent or more and 0.95 mass percent or less

[0024] Manganese contributes to improving hardenability. To ensure this function, the manganese content must be 0.20 mass percent or more. On the other hand, if the manganese content is too high, the amount of retained austenite remaining after quenching tends to increase. To properly control the amount of retained austenite, the manganese content must be 0.95 mass percent or less. From the perspective of hardenability, the manganese content is preferably 0.40 mass percent or more. To better control retained austenite, the manganese content is preferably 0.40 mass percent or less. Chromium: 0.85 mass percent or more and 1.90 mass percent or less

[0025] Chromium is an element that improves hardenability. To ensure sufficient hardenability, the chromium content must be 0.85 mass percent or more. On the other hand, if the chromium content is too high, toughness decreases. Therefore, the chromium content must be 1.90 mass percent or less. Chromium contributes to the refinement of carbides. To sufficiently ensure this function, the chromium content is preferably 1.70 mass percent or more. On the other hand, to ensure toughness, the chromium content is preferably 1.30 mass percent or less. Inevitable contamination

[0026] In addition to the components intentionally added during the manufacturing process, elements other than those mentioned above may be contained as unavoidable impurities in the steel constituting the mechanical component. For example, oxygen (O) is reduced as much as possible by deoxidation treatment, but it is unavoidably contained in the steel. Oxygen can form non-metallic inclusions in the steel and can adversely affect the properties of mechanical components. Therefore, the oxygen content is preferably 20 ppm or less. It is also preferable to reduce the other impurity elements as much as possible within a reasonable manufacturing cost. The total amount of unavoidable impurities is preferably 1.00 mass% or less. Molybdenum: 0.15 mass percent or more and 0.45 mass percent or less

[0027] Molybdenum does not necessarily have to be added intentionally. However, it contributes to improving hardenability and temper softening resistance. To achieve this, the molybdenum content is preferably 0.15 mass percent or more. On the other hand, molybdenum is an expensive element, and excessive addition leads to increased costs. To avoid unnecessary cost increases, the molybdenum content is preferably 0.45 mass percent or less. Nickel: 0.01 mass percent or more and 2.00 mass percent or less

[0028] Nickel does not necessarily have to be added intentionally. However, nickel contributes to improving toughness. To achieve this function, the nickel content is preferably 0.01 mass percent or more. On the other hand, nickel is an expensive element, and excessive addition leads to increased costs. To avoid unnecessary cost increases, the nickel content is preferably 2.00 mass percent or less. From a cost perspective, the nickel content is preferably 0.75 mass percent or less, more preferably 0.25 mass percent or less. On the other hand, from a toughness perspective, the nickel content is preferably 0.35 mass percent or more, and more preferably 1.55 mass percent or more. Niobium: 0.04 mass percent or more and 0.08 mass percent or less

[0029] Niobium does not necessarily have to be added intentionally. However, it does contribute to the refinement of crystal grains. To achieve this, the niobium content is preferably 0.04 mass percent or more. On the other hand, if the niobium addition exceeds 0.08 mass percent, no additional effect is achieved because the aforementioned effect is saturated. Therefore, the niobium content is preferably 0.08 mass percent or less.

[0030] In the mechanical component described above, the carburized layer can have a thickness of 500 µm or more. This design can provide sufficient strength to the mechanical component.

[0031] For the mechanical component described above, the maximum carbon concentration in the thickness direction of the carburized layer can be 0.6 mass percent or more. This design makes it easy to impart sufficient hardness to the surface layer portion of the mechanical component.

[0032] In the carburized layer of the mechanical component described above, the maximum carbon concentration in the thickness direction can be 0.8 mass percent or more and 1.2 mass percent or less. In the carburized layer, the maximum grain size of carbides in a cross-section perpendicular to the surface can be 1 μm or less. In the carburized layer, the grain size number defined in JIS G0551 can be 12 or more. As described above, a high level of strength and toughness can be achieved by setting a high maximum carbon concentration of the carbides, dispersing fine carbides in the carbides, and refining the crystal grains. In the carburized layer, the area ratio of carbide in a cross-section perpendicular to the surface can be 1% or more and 10% or less.In the carburized layer, the average grain size of the carbides in a cross-section perpendicular to the surface can be 1 µm or less.

[0033] In the carburized layer of the mechanical component, the maximum carbon concentration in the thickness direction can be 1.1 mass % or more and 1.8 mass % or less. In the carburized layer, the maximum grain size of the carbide in a cross section perpendicular to the surface can be 25 μm or less. In the carburized layer, the grain size number defined in JIS G0551 can be 11 or more. As described above, both high surface damage resistance (e.g., pitting resistance) and high toughness can be achieved by setting the maximum carbon concentration of the carburized layer high and refining the crystal grains. In the carburized layer, the area ratio of the carbide in a cross section perpendicular to the surface can be 3% or more and 30% or less.

[0034] In the carburized layer of the mechanical component, the maximum nitrogen concentration in the thickness direction can be 0.7 mass percent or more and 1.2 mass percent or less. By infiltrating nitrogen into the carburized layer and ensuring an appropriate amount of retained austenite, pitting corrosion resistance is significantly improved. In the carburized layer, the maximum amount of retained austenite in the thickness direction can be 50 volume percent or more and 70 volume percent or less.

[0035] The mechanical component may be a component that is a component of a transmission, axle, final drive, or swing device of a work machine. The mechanical component of the present disclosure, whose improved durability is enhanced by suppressing the occurrence of white layer peeling, is suitable for these components.

[0036] A method for manufacturing a mechanical component according to a first aspect of the present disclosure includes a step of preparing a steel material, a step of obtaining a compact, a step of forming a composite oxide film, a step of forming a carburized layer, and a step of quench-hardening the compact. In the step of preparing a steel material, a steel material is provided containing 0.12 mass % or more and 0.28 mass % or less of carbon, 0.15 mass % or more and 0.70 mass % or less of silicon, 0.20 mass % or more and 0.95 mass % or less of manganese, and 0.85 mass % or more and 1.90 mass % or less of chromium, with the remainder being iron and unavoidable impurities. In the step of obtaining a compact, the compact is obtained by molding the steel material.In the step of forming a composite oxide film, the compact is heated in an oxidizing atmosphere, and a composite oxide film containing magnetite and hematite is formed on the surface of the compact. In the step of forming a carburized layer, the compact on which the composite oxide film has been formed is heated in a carburizing atmosphere at a pressure of 1 kPa or less, thereby forming a carburized layer on the surface of the compact. The carburized layer has a higher carbon concentration than the other part and a thickness greater than the composite oxide film. In the step of quench-hardening the compact, the compact with the composite oxide film and the carburized layer formed thereon is quenched.

[0037] A method for manufacturing a mechanical component according to a second aspect of the present disclosure includes a step of preparing a steel material, a step of obtaining a compact, a step of forming a composite oxide film, a step of forming a carburized layer, and a step of quench-hardening the compact.In the step of providing a steel material, a steel material is provided containing 0.12 mass % or more and 0.28 mass % or less of carbon, 0.15 mass % or more and 0.70 mass % or less of silicon, 0.20 mass % or more and 0.95 mass % or less of manganese, 0.85 mass % or more and 1.90 mass % or less of chromium, and at least one selected from the group consisting of 0.15 mass % or more and 0.45 mass % or less of molybdenum, 0.01 mass % or more and 2.00 mass % or less of nickel, and 0.04 mass % or more and 0.08 mass % or less of niobium, with the remainder being iron and unavoidable impurities. In the step of obtaining a compact, the compact is obtained by molding the steel material.In the step of forming a composite oxide film, the compact is heated in an oxidizing atmosphere, and a composite oxide film containing magnetite and hematite is formed on the surface of the compact. In the step of forming a carburized layer, the compact on which the composite oxide film has been formed is heated in a carburizing atmosphere at a pressure of 1 kPa or less, thereby forming a carburized layer on the surface of the compact. The carburized layer has a higher carbon concentration than the other part and a thickness greater than the composite oxide film. In the step of quench-hardening the compact, the compact with the composite oxide film and the carburized layer formed thereon is quenched.

[0038] According to the method for manufacturing a mechanical component according to the present disclosure, the above-described mechanical component of the present invention can be easily manufactured.

[0039] In the method for manufacturing a mechanical component according to the present disclosure, the step of forming the composite oxide film and the step of forming the carburized layer can be alternately repeated multiple times. This allows the carbon introduced into the compact in the carburized layer forming step to diffuse into the interior of the compact in the subsequent step of forming the composite oxide film. As a result, the formation of the composite oxide film and the formation of the carburized layer can be performed efficiently.

[0040] In the method for manufacturing a mechanical component according to the present disclosure, in the step of forming the carburized layer, the carburized layer may be formed such that the maximum carbon concentration in the thickness direction is 0.8 mass % or more and 1.2 mass % or less.The method for manufacturing a mechanical component may further include, prior to the step of quench-hardening the compact, a step of cooling the compact with the carburized layer formed thereon from a temperature range of the A1 transformation point or higher to a temperature range below the A1 transformation point, thereby converting the carburized layer into a pearlitic structure, and a step of heating the compact with the carburized layer converted into a pearlitic structure to the temperature range of the A1 transformation point or higher and then cooling the compact to a temperature range below the A1 transformation point, thereby spherodizing the carbides contained in the carburized layer and refining the crystal grains of the carburized layer.After the quench-hardening step of the compact, the maximum grain size of the carbides in the carburized layer in a cross-section perpendicular to the surface of the compact can be 1 μm or less, and the grain size number of the carburized layer, defined in JIS G0551, can be 12 or more. This can easily manufacture a mechanical component that achieves high levels of both strength and toughness. In the carburized layer after the quench-hardening step of the compact, the area ratio of the carbide in a cross-section perpendicular to the surface can be 1% or more and 10% or less. In the carburized layer after the quench-hardening step of the compact, the average grain size of the carbides in a cross-section perpendicular to the surface can be 1 μm or less.

[0041] In the method for manufacturing a mechanical component according to the present disclosure, in the step of forming the carburized layer, the carburized layer may be formed such that the maximum carbon concentration in the thickness direction is 1.1 mass % or more and 1.8 mass % or less.The method for producing a mechanical component may further include, prior to the step of quench-hardening the compact, a step of cooling the compact with the carburized layer formed thereon to a temperature range below the A1 transformation point, thereby converting the carburized layer into a pearlitic structure, and a step of heating the compact with the carburized layer converted into a pearlitic structure to the temperature range of the A1 transformation point or higher and then cooling the compact to a temperature range below the A1 transformation point, thereby spherodizing the carbides contained in the carburized layer.After the quench-hardening step of the compact, the maximum grain size of the carbides in the carburized layer in a cross-section perpendicular to the surface of the compact can be 25 μm or less, and the grain size number of the carburized layer, defined in JIS G0551, can be 11 or more. Accordingly, a mechanical component can be easily manufactured that exhibits both high resistance to surface damage (e.g., pitting resistance) and high toughness. After the quench-hardening step of the compact, the area ratio of the carbides in the carburized layer in a cross-section perpendicular to the surface can be 3% or more and 30% or less.

[0042] The method for manufacturing a mechanical component according to the present disclosure may further include heating the compact in a nitriding atmosphere prior to quench-hardening the compact, thereby infiltrating nitrogen into the carburized layer. Accordingly, the mechanical component having the nitrogen-containing carburized layer can be easily manufactured. Examples of specific embodiments

[0043] Specific embodiments of the mechanical component of the present disclosure will be described below with reference to the drawings. In the following drawings, like components or equivalent components are denoted by the same reference numerals, and descriptions thereof will not be repeated. First embodiment

[0044] First, with reference to the Fig. 1 to 3, a spur gear according to a first embodiment is described, which is an example of a mechanical component according to the present disclosure. Fig. 1 is a schematic perspective view illustrating the appearance of the spur gear according to a first embodiment. Fig. 2 is a schematic cross-sectional view illustrating a cross-sectional structure of the spur gear according to the first embodiment. Fig. 3 is a schematic cross-sectional view illustrating the structure of a carburized layer according to the first embodiment. Fig. 2 illustrates a cross section perpendicular to the central axis A of Fig. 1. Fig. Figure 3 illustrates the structure of the steel near the surface of the carburized layer.

[0045] With reference to Fig. 1, a spur gear 1 in the first embodiment is a gear that is a component of a planetary gear mechanism. The spur gear 1 has an annular shape (hollow cylindrical shape). A through hole 2 is formed in the spur gear 1 along the central axis. On an outer peripheral surface 10 of the spur gear 1, a plurality of teeth 11 are formed circumferentially over the entire area, which mesh with teeth of a sun gear and an outer gear of the planetary gear. An inner peripheral surface 20 surrounding the through hole 2 has a cylindrical shape. A shaft of a carrier of the planetary gear mechanism is inserted into the through hole 2.The spur gear 1 can be used as a component which, for example, forms part of an axle drive or a swivel device of a hydraulic excavator, a gearbox or an axle drive of a bulldozer, a gearbox or an axle drive of a dump truck or similar work machines.

[0046] The spur gear 1 is made of steel with a martensitic structure containing 0.12 mass percent or more and 0.28 mass percent or less of carbon, 0.15 mass percent or more and 0.70 mass percent or less of silicon, 0.20 mass percent or more and 0.95 mass percent or less of manganese, and 0.85 mass percent or more and 1.90 mass percent or less of chromium, the remainder being iron and unavoidable impurities.The spur gear 1 may be made of steel having a martensitic structure, the steel containing 0.12 mass percent or more and 0.28 mass percent or less of carbon, 0.15 mass percent or more and 0.70 mass percent or less of silicon, 0.20 mass percent or more and 0.95 mass percent or less of manganese, 0.85 mass percent or more and 1.90 mass percent or less of chromium, and at least one selected from the group consisting of 0.15 mass percent or more and 0.45 mass percent or less of molybdenum, 0.01 mass percent or more and 2.00 mass percent or less of nickel, and 0.04 mass percent or more and 0.08 mass percent or less of niobium, the remainder being iron and unavoidable impurities.

[0047] With reference to Fig. 2, the spur gear 1 includes a carburized layer 31 forming the outer peripheral surface 10 with the teeth 11 formed thereon and having a higher carbon concentration than a base part 32 forming the other part. The thickness t A The thickness of the carburized layer 31 may be, for example, 500 µm or more or 800 µm or more. The maximum carbon concentration in the thickness direction of the carburized layer 31 (the direction perpendicular to the outer peripheral surface 10) may be 0.6 mass percent or more. The carburized layer 31 is formed over the entire area of ​​the outer peripheral surface 10. The carburized layer 31 is formed to cover the entire surface of the teeth 11.

[0048] With reference to Fig. 3, the carburized layer 31 has a structure in which a large number of original austenite crystal grains 50 are distributed throughout. The boundary between the adjacent original austenite crystal grains 50 is a crystal grain boundary 51. In the present embodiment, an oxidized grain boundary region 41 is formed, which is a region into which oxygen is infiltrated from the outer peripheral surface 10 along the crystal grain boundary 51. The oxidized grain boundary region 41 is composed of a composite oxide containing magnetite and hematite. In the present application, the part of the surface layer in which the oxidized grain boundary region 41 composed of the composite oxide is formed is defined as a composite oxide film 40. The thickness t Bof the composite oxide film 40 is 1 μm or more and 25 μm or less. That is, the carburized layer 31 in the present embodiment includes the composite oxide film arranged to form the outer peripheral surface 10, that is, the surface of the spur gear 1, which has a thickness of 1 μm or more and 25 μm or less and contains magnetite and hematite. The grain size number of the carburized layer 31 defined by JIS G0551 (hereinafter also simply referred to as "grain size number") is, for example, 8. The maximum carbon concentration in the thickness direction of the carburized layer 31 is, for example, 0.68 mass % or more and 0.80 mass % or less. Carbides (cementites) are substantially not dispersed in the carburized layer 31.The state in which cementites are substantially non-dispersed here refers to a state in which, for example, when examining ten square areas with a side length of 20 µm in a cross section of a carburized layer at a magnification of 5000 times using an SEM (scanning electron microscope), the number of cementite particles found is one or less.

[0049] In the spur gear 1, which is the mechanical component of the present embodiment, the carburized layer 31 includes the composite oxide film 40 arranged to form the outer peripheral surface 10, that is, the surface of the spur gear 1, which has a thickness of 1 μm or more and 25 μm or less and contains magnetite and hematite. As a result, the spur gear 1 of the present embodiment is a mechanical component with improved durability by suppressing the occurrence of delamination.

[0050] An example of a method for manufacturing the spur gear 1, which is the mechanical component of the present embodiment, will be described below. Fig. 4 is a flowchart schematically illustrating a method for manufacturing a spur gear. Referring to Fig. 4, in the method for manufacturing the spur gear 1 of the present embodiment, a steel material manufacturing step is first performed as step S10. In step S10, a steel material made of steel is provided, which forms the spur gear 1 of the present embodiment.

[0051] Specifically, a steel material is provided which contains 0.12 mass percent or more and 0.28 mass percent or less of carbon, 0.15 mass percent or more and 0.70 mass percent or less of silicon, 0.20 mass percent or more and 0.95 mass percent or less of manganese, and 0.85 mass percent or more and 1.90 mass percent or less of chromium, the remainder being iron and unavoidable impurities.In step S10, a steel material may be provided which contains 0.12 mass % or more and 0.28 mass % or less of carbon, 0.15 mass % or more and 0.70 mass % or less of silicon, 0.20 mass % or more and 0.95 mass % or less of manganese, 0.85 mass % or more and 1.90 mass % or less of chromium and at least one selected from the group consisting of 0.15 mass % or more and 0.45 mass % or less of molybdenum, 0.01 mass % or more and 2.00 mass % or less of nickel, and 0.04 mass % or more and 0.08 mass % or less of niobium, with the remainder being iron and unavoidable impurities. As the steel material provided in process S10, for example, a steel according to JIS standard such as SCr420H, SCM415H, SCM418H, SCM420H, SCM425H, SCM822H, SNCM220H, SNCM420H and the like can be used.

[0052] Subsequently, a forming step is performed as step S20. In step S20, the steel material provided in step S10 is formed into a compact. Specifically, the steel material provided in step S10 is processed to produce a compact having the shape of the spur gear 1. For example, hot forging, cold forging, cutting, and the like can be performed as the process in step S20.

[0053] Subsequently, an oxide film formation step is performed in step S30. In step S30, the compact produced in step S20 is heated in an oxidizing atmosphere, thereby forming the composite oxide film 40 containing magnetite and hematite on the surface (outer peripheral surface 10) of the compact (spur gear 1).

[0054] Subsequently, the carburizing step is performed as step S40. In step S40, the compact (spur gear 1) on which the composite oxide film 40 was formed in step S30 is heated in a carburizing atmosphere at a pressure of 1 kPa or less, so that the carburized layer 31 having a higher carbon concentration than the base part 32, which is the other part, is formed on the surface (outer peripheral surface 10) of the spur gear 1 with a thickness thicker than the composite oxide film 40.

[0055] Subsequently, a diffusion step is performed as step S50. In step S50, the compact (spur gear 1) on which the carburized layer 31 was formed in step S40 is maintained at a temperature range of the A1 transformation point or higher, so that carbon infiltrated into the vicinity of the surface of the spur gear 1 in step S40 diffuses into the interior. As a result, the carbon concentration in the carburized layer 31 is equalized and the thickness of the carburized layer 31 is increased.

[0056] Subsequently, the quenching step is performed as step S60. In this step S60, the compact (spur gear 1) on which the composite oxide film 40 was formed in step S30 and the carburized layer 31 was formed in step S40 is quenched. Specifically, the spur gear 1 on which the composite oxide films 40 and the carburized layers 31 were formed is quenched by cooling (rapid cooling) from the temperature range of the A1 transformation point or higher to a temperature range below an MS point.

[0057] Hereinafter, details of specific examples of steps S30 to S60 will be described with reference to Fig. 5 described. Fig. Figure 5 is a diagram illustrating the heat treatment performed in the method for manufacturing the spur gear according to the first embodiment. Fig. 5 corresponds to the horizontal axis of time. Time progresses to the right on the horizontal axis. In Fig. 5, the vertical axis corresponds to the heating temperature of the compact (spur gear 1). The heating temperature increases with increasing position on the vertical axis.

[0058] With reference to the Fig. 5 and Fig. 4, the oxide film forming step is first carried out as step S30. In particular, as shown in Fig. 5 illustrates, first, the spur gear 1 is heated to temperature T1 at time t1. The temperature T1 corresponds to the temperature of the A1 transformation point (727 °C) or higher, and may be a temperature of 930 °C or higher and 980 °C or lower, for example, 950 °C. At this time, the spur gear 1 is heated in an oxidizing atmosphere containing at least one of oxygen (O2) and carbon dioxide (CO2) and having a moisture content below the dew point, and is maintained at temperature T1 from time t1 to time t2. Dry air, for example, can be used as the oxidizing atmosphere. The period from time t1 to time t2 can be, for example, 120 minutes. The atmospheric pressure can be 100 kPa or less, for example, 40 kPa. As a result, the composite oxide film 40 containing magnetite and hematite is formed on the surface of the spur gear 1 (the surface of the tooth 11).More specifically, oxygen infiltrates along the austenite grain boundary of the steel of which the spur gear 1 is made, and the oxidized grain boundary region 41 is formed, which consists of a composite oxide containing magnetite and hematite.

[0059] Subsequently, the carburizing step is carried out as step S40. In particular, as shown in Fig. As illustrated in Figure 5, the spur gear 1 is heated in a carburizing atmosphere at a pressure of 1 kPa or less for a period from time t2 to time t3, thereby forming the carburized layer 31. For example, an acetylene atmosphere of hydrocarbons can be used as the carburizing atmosphere. The period from time t2 to time t3 may be shorter than the period from time t1 to time t2, and may be, for example, 10 minutes.

[0060] In the present embodiment, step S30 is subsequently executed again. In the present embodiment, step S30 for forming the composite oxide film and step S40 for forming the carburized layer are alternately repeated several times (more precisely, four times). In particular, as shown in Fig. 5, the spur gear 1 is maintained at the same temperature T1 in the same atmosphere as the period from time t1 to time t2 for a period from time t3 to time t4. Accordingly, oxygen continues to infiltrate along the austenite grain boundary of the steel constituting the spur gear 1, and the thickness of the composite oxide film 40 increases. The time from time t3 to time t4 may be shorter than the time from time t1 to time t2, for example, five minutes. At this time, the carbon that has infiltrated into the vicinity of the surface of the spur gear 1 during the period from time t2 to time t3 diffuses into the spur gear 1. This increases the thickness of the carburized layer 31 and equalizes the carbon concentration in the thickness direction of the carburized layer. That is, step S40 performed after step S30 also functions as a carbon diffusion step.

[0061] Subsequently, step S40 is executed again. In particular, as shown in Fig. As illustrated in Figure 5, the spur gear 1 is maintained at the same temperature T1 in the same atmosphere as during the period from time t2 to time t3 during a period from time t4 to time t5. As a result, further carbon is infiltrated from the surface of the spur gear 1, and the carburized layer 31 is formed. The period from time t4 to time t5 may be the same as the period from time t2 to time t3, for example, 10 minutes.

[0062] Subsequently, step S30 is executed again. In particular, as shown in Fig. As illustrated in Figure 5, the spur gear 1 is maintained at the same temperature T1 in the same atmosphere as the period from time t3 to time t4 during a period from time t5 to time t6. Accordingly, oxygen continues to infiltrate from the surface of the spur gear 1, thereby increasing the thickness of the composite oxide film 40. The period from time t5 to time t6 may be longer than the period from time t3 to time t4 and may be shorter than the period from time t1 to time t2, for example, 20 minutes.

[0063] Subsequently, step S40 is executed again. In particular, as shown in Fig. As illustrated in Figure 5, the spur gear 1 is maintained at the same temperature T1 in the same atmosphere as during the period from time t2 to time t3 for a period from time t6 to time t7. As a result, further carbon is infiltrated from the surface of the spur gear 1, and the carburized layer 31 is formed. The period from time t6 to time t7 may be the same as the period from time t2 to time t3, for example, 10 minutes.

[0064] Subsequently, step S30 is executed again. In particular, as shown in Fig. As illustrated in Figure 5, the spur gear 1 is maintained at the same temperature T1 in the same atmosphere as the period from time t3 to time t4 during a period from time t7 to time t8. Accordingly, oxygen continues to infiltrate from the surface of the spur gear 1, thereby increasing the thickness of the composite oxide film 40. The period from time t7 to time t8 may be longer than the period from time t5 to time t6 and may be shorter than the period from time t1 to time t2, for example, 50 minutes.

[0065] Subsequently, step S40 is executed again. In particular, as shown in Fig. 5, the spur gear 1 is maintained at the same temperature T1 in the same atmosphere as the period from time t2 to time t3 for a period from time t8 to time t9. As a result, further carbon is infiltrated from the surface of the spur gear 1, and the carburized layer 31 is formed. The period from time t8 to time t9 may be shorter than the period from time t2 to time t3, for example, four minutes. The carburizing step from time t8 to time t9 serves as a compensation step for the decarburization that occurred in the oxide film formation step from time t7 to time t8, that is, it serves as a recarburization step.

[0066] Subsequently, step S50 is executed. In particular, the temperature during the period from time t9 to time t 10maintained at the same temperature T1 as in the period from time t8 to time t9, in an inert gas atmosphere (for example, an argon gas atmosphere or a nitrogen gas atmosphere). During this period, the carbon that has penetrated near the surface of the spur gear 1 diffuses into the spur gear 1. This increases the thickness of the carburized layer 31 and equalizes the carbon concentration in the thickness direction of the carburized layer. The period from time t9 to time t 10 can be shorter than the period from time t8 to time t9 and can be, for example, one minute.

[0067] Subsequently, step S60 is executed. More specifically, as shown in Fig. 5, during a period from time t 10 until time t 11The temperature of the spur gear 1 is cooled from temperature T1 to temperature T2, for example to 850 °C, which is lower than temperature T1 and higher than the A1 transformation point. Subsequently, the spur gear 1 is cooled for a period from time t 11 until time t 12 held at temperature T2 and then cooled to the MS point temperature or below (rapid cooling). As a result, the spur gear 1 is quenched and the structure of the steel from which the spur gear 1 is made assumes a martensitic structure.

[0068] With reference to Fig. 4, a tempering step is performed as step S70. In this step S70, the compact (spur gear 1) quenched in step S60 is subjected to a tempering treatment. More specifically, the spur gear 1 is heated to a temperature below the A1 transformation point, for example, to a temperature range of 150°C or higher and 200°C or lower (for example, 160°C), and then cooled to room temperature. The spur gear 1 of the present embodiment can be manufactured using the above procedure. Second embodiment

[0069] A second embodiment, which is another embodiment of the present disclosure, will be described. The spur gear 1 of the second embodiment has basically the same structure as the spur gear 1 of the first embodiment and achieves the same effects. However, the spur gear 1 of the second embodiment differs from that of the first embodiment mainly in the structure of the carburized layer 31. The differences from the first embodiment will be described below.

[0070] Fig. Fig. 6 is a schematic cross-sectional view illustrating a structure of a carburized layer of a spur gear according to the second embodiment. Referring to Fig. 6, in the carburized layer 31 of the spur gear 1 in the second embodiment, the maximum carbon concentration in the thickness direction of the carburized layer 31 is 0.8 mass percent or more and 1.2 mass percent or less. Numerous carbides 61 (cementite) are dispersed in the carburized layer 31. The maximum grain size of the carbide 61 in the cross section of the carburized layer 31 (cross section in Fig. 6) perpendicular to the outer peripheral surface 10, which is the surface of the spur gear 1, is 1 µm or less. Furthermore, in the carburized layer 31, the grain size number is 12 or more. The area ratio of the carbides 61 in the cross section of Fig. 6 can be 1% or more and 10% or less. The average grain size of carbides 61 in the cross section of Fig. 6 may be 1 µm or less. The area ratio, maximum grain size, and average grain size of the carbide 61 in the carburized layer 31 can be determined, for example, by using an SEM to measure the area ratio, maximum grain size, and average grain size of cementite obtained when 10 square areas each with a side of 20 µm are observed at a magnification of 5000 times in the cross section of the carburized layer.

[0071] As the steel constituting the spur gear 1, for example, steel with a higher chromium content than in the first embodiment can be used. As the steel constituting the spur gear 1 of the second embodiment, for example, steel containing 0.22 mass percent or more and 0.26 mass percent or less of carbon, 0.45 mass percent or more and 0.70 mass percent or less of silicon, 0.20 mass percent or more and 0.40 mass percent or less of manganese, 1.70 mass percent or more and 1.90 mass percent or less of chromium, 0.04 mass percent or more and 0.08 mass percent or less of niobium, with the remainder being iron and unavoidable impurities, can be used.

[0072] As described above, the spur gear 1 of the second embodiment is a mechanical component with high strength and toughness by setting the maximum carbon concentration of the carburized layer 31 high, dispersing the fine carbides 61 in an appropriate area ratio in the carburized layer 31, and refining the original austenite crystal grains 50.

[0073] A method for manufacturing the spur gear 1 according to the second embodiment is described below. The spur gear 1 of the second embodiment can be manufactured using basically the same procedure as the spur gear 1 of the first embodiment. However, the manufacturing method for the spur gear 1 of the second embodiment differs in part from that of the first embodiment in order to obtain the characteristic structure of the carburized layer 31. The differences from the first embodiment are described below.

[0074] Fig. 7 is a flowchart illustrating the method for manufacturing the spur gear according to the second embodiment. Fig. 8 is a diagram illustrating the heat treatment performed in the method of manufacturing the spur gear according to the second embodiment. Fig. 7 and Fig. 8 correspond to Fig. 4 and Fig. 5 in the first embodiment.

[0075] With reference to Fig. 7, first, as in the first embodiment, the step of preparing the steel material is performed as step S10. In the method for manufacturing the spur gear in the second embodiment, a steel material may be prepared containing, for example, 0.22 mass % or more and 0.26 mass % or less of carbon, 0.45 mass % or more and 0.70 mass % or less of silicon, 0.20 mass % or more and 0.40 mass % or less of manganese, 1.70 mass % or more and 1.90 mass % or less of chromium, 0.04 mass % or more and 0.08 mass % or less of niobium, with the remainder being iron and unavoidable impurities.

[0076] Subsequently, step S20 is executed in the same manner as in the first embodiment. Then, steps S30 to S50 (until time t 10 in Fig. 8) in the same way as in the first embodiment, see Fig. 7 and Fig. 8. Here, in step S40, the carburizing time is set longer than in the first embodiment, thereby forming the carburized layer 31 with a higher carbon concentration than in the first embodiment. Specifically, the carburized layer 31 is formed such that the maximum carbon concentration in the thickness direction is 0.8 mass % or more, 1.2 mass % or less.

[0077] Subsequently, a pearlitization step is performed as step S51. In this step, the compact (spur gear 1) on which the carburized layer 31 was formed is cooled from a temperature range of the transformation point A1 or higher to a temperature range below the transformation point A1. This converts the carburized layer 31 into a pearlitic structure. More specifically, with reference to Fig. 8, is during a period from time t 10 until time t 21 the spur gear 1 is cooled from the temperature T1 of the A1 transformation point or higher to the temperature T3 below the A1 transformation point and until the time t 22 The temperature T3 can, for example, be 650 °C.

[0078] Subsequently, a step for refining carbides and grains is performed as step S52. In this step S52, the compact (spur gear 1) having the carburized layer 31 with a pearlitic structure is heated to the temperature range of the A1 transformation point or higher and then cooled to a temperature range below the A1 transformation point, thereby spherodizing the carbides contained in the carburized layer 31 and refining the grains of the carburized layer 31. More specifically, with reference to Fig. 8, is during a period from time t 22 until time t 23 the spur gear 1 is heated from temperature T3, which is a temperature below the A1 transformation point, to temperature T4, which is a temperature of the A1 transformation point or higher, and until time t 24Temperature T4 can be lower than temperature T1, for example, 810 °C. This causes the lamellar carbides that make up the pearlitic structure to be divided, and a large number of fine carbide nuclei are formed. Furthermore, the structure of the steel is re-austenitized at temperature T4, which allows the formation of fine austenite crystal grains. Subsequently, the spur gear 1 is subjected to a period from time t 24 until time t 25 cooled to temperature T3 and kept at time t 26 As a result, the nuclei grow due to the precipitated carbides, and numerous fine spherical carbides are formed. Furthermore, a steel structure with fine, original austenite crystal grains 50 can be obtained.

[0079] In the present embodiment, the spur gear 1 is rotated during a period from time t 26 until time t27 heated again from temperature T3 to temperature T4 and is kept at time t 28 This causes the remaining lamellar carbides to be divided and numerous fine carbide nuclei to form. Furthermore, the structure of the steel is re-austenitized at temperature T4, allowing the formation of fine austenite crystal grains. Subsequently, the spur gear 1 is subjected to a grinding operation for a period from time t 28 until time t 29 cooled to temperature T5 and kept at time t 30 The temperature T5 can be higher than the temperature T3 and lower than the A1 transformation point, for example, 700 °C. As a result, the nuclei grow due to the precipitated carbides, and numerous fine spherical carbides are also formed. Furthermore, a steel structure with fine, original austenite crystal grains 50 can be obtained.

[0080] Subsequently, the quenching step is carried out as step S60. In step S60, the compact (spur gear 1) subjected to the steps up to step S52 is quenched. In particular, as shown in Fig. 8, the spur gear 1 during a period from time t 30 until time t 31 from temperature T5 to temperature T2. Temperature T2 can be higher than temperature T5 and lower than temperature T1, for example 850 °C. Spur gear 1 is then heated for a period from time t 31 until time t 32 held at temperature T2 and then cooled to the MS point temperature or below (rapid cooling). As a result, the spur gear 1 is quenched and the structure of the steel from which the spur gear 1 is made assumes a martensitic structure.

[0081] At this time, in the present embodiment, the maximum grain size of the carbides 61 in the carburized layer 31 in the cross section perpendicular to the outer peripheral surface 10, which is the surface of the compact (spur gear 1), is 1 μm or smaller. Furthermore, in the carburized layer 31, the grain size number is 12 or more. Furthermore, the area ratio of the carbides 61 in the carburized layer 31 in the cross section perpendicular to the outer peripheral surface 10 can be set to 1% or more and 10% or less, and the average grain size of the carbides 61 can be set to 1 μm or smaller.

[0082] Thereafter, the tempering step is performed as step S70 in the same manner as in the first embodiment. The spur gear 1 of the present embodiment can be manufactured using the above procedure. Third embodiment

[0083] A third embodiment, which is yet another embodiment of the present disclosure, will be described. The spur gear 1 of the third embodiment has basically the same structure as the spur gear 1 of the first embodiment and achieves the same effects. However, the spur gear 1 of the third embodiment differs from that of the first embodiment mainly in the structure of the carburized layer 31. The differences from the first embodiment will be described below.

[0084] Fig. 9 is a schematic cross-sectional view illustrating a structure of a carburized layer of a spur gear according to the third embodiment. Referring to Fig. 9, in the carburized layer 31 of the spur gear 1 in the third embodiment, the maximum carbon concentration in the thickness direction of the carburized layer 31 is 1.1 mass percent or more and 1.8 mass percent or less. Numerous carbides 61 (cementite) are dispersed in the carburized layer 31. The maximum grain size of the carbide 61 in the cross section of the carburized layer 31 (cross section in Fig. 9) perpendicular to the outer peripheral surface 10, which is the surface of the spur gear 1, is, for example, 25 µm or less. Furthermore, in the carburized layer 31, the grain size number is 11 or more. As the steel of the spur gear 1, for example, steel containing 0.35 mass percent or more of molybdenum, such as steel of JIS standard SCM822H, can be used.

[0085] As described above, by setting a high maximum carbon concentration of the carburized layer 31 and refining the original austenite crystal grains 50, the spur gear 1 of the third embodiment is a mechanical component that has both surface damage resistance (e.g., pitting resistance) and high toughness. In the carburized layer 31, the area ratio of the carbides 61 in the cross section perpendicular to the outer peripheral surface 10 can be 3% or more and 30% or less.

[0086] A method for manufacturing the spur gear 1 according to the third embodiment is described below. The spur gear 1 of the third embodiment can be manufactured using basically the same procedure as the spur gear 1 of the second embodiment. However, the manufacturing method for the spur gear 1 of the third embodiment differs in part from that of the second embodiment in order to obtain the characteristic structure of the carburized layer 31. The differences from the second embodiment are described below.

[0087] Fig. 10 is a flowchart illustrating the method for manufacturing the spur gear according to the third embodiment. Fig. 10 is a diagram showing Fig. 7 in the second embodiment.

[0088] With reference to Fig. 10, the step of preparing the steel material is first performed as step S10, as in the second embodiment. In the method for manufacturing the spur gear according to the third embodiment, for example, a steel member manufactured according to JIS standard SCM822H can be provided.

[0089] Subsequently, step S20 is carried out in the same manner as in the first and second embodiments. Then, as shown in the Fig. 10 and Fig. 7, steps S30 to S51 are performed in the same manner as in the second embodiment. Here, in step S40, the carburizing time is set even longer than in the second embodiment, thereby forming the carburized layer 31 with a higher carbon concentration than in the first and second embodiments. Specifically, the carburized layer 31 is formed such that the maximum carbon concentration in the thickness direction is 1.1 mass % or more and 1.8 mass % or less.

[0090] Subsequently, a carbide spherodization step is performed as step S53. In this step S53, the compact (spur gear 1) with the carburized layer 31 having a pearlitic structure is heated to a temperature range of the A1 transformation point or higher and then cooled to a temperature range below the A1 transformation point, whereby the carbides contained in the carburized layer 31 are spherodized. In particular, as shown in Fig. 8, the heating and cooling as in the period from time t 22 until time t 30in the second embodiment. As a result, the lamellar carbides constituting the pearlitic structure are divided, a large number of carbide nuclei are formed, and a large number of spherical carbides are formed by the growth of the nuclei. Since the carbon content of the carburized layer 31 is higher than in the second embodiment, the area ratio of the carbides 61 at this time is higher than in the second embodiment, and a steel structure in which the carbides 61 have a large grain size can be obtained. Furthermore, as in the case of the second embodiment, a steel structure having fine original austenite crystal grains 50 can be obtained.

[0091] Thereafter, the quenching step is performed as step S60 in the same manner as in the second embodiment. As a result, the spur gear 1 is hardened by quenching, and the structure of the steel constituting the spur gear 1 assumes a martensitic structure.

[0092] At this time, in the present embodiment, the maximum grain size of the carbides 61 in the carburized layer 31 in the cross section perpendicular to the outer peripheral surface 10, which is the surface of the compact (spur gear 1), is 25 μm or less. Furthermore, in the carburized layer 31, the grain size number is 11 or more. In the carburized layer 31, the area ratio of the carbides 61 in the cross section of the carburized layer 31 perpendicular to the outer peripheral surface 10 may be 3% or more and 30% or less.

[0093] Thereafter, the tempering step is performed as step S70 in the same manner as in the first and second embodiments. The spur gear 1 of the present embodiment can be manufactured using the above procedure. Fourth embodiment

[0094] A fourth embodiment, which is yet another embodiment of the present disclosure, will be described. The spur gear 1 of the fourth embodiment has basically the same structure as the spur gear 1 of the first embodiment and achieves the same effects. However, the spur gear 1 of the fourth embodiment differs from that of the first embodiment in that the carburized layer 31 contains nitrogen. The differences from the first embodiment will be described below.

[0095] With reference to Fig. 3, in the carburized layer 31 of the spur gear 1 in the fourth embodiment, the maximum nitrogen concentration in the thickness direction is 0.7 mass percent or more and 1.2 mass percent or less. Thus, by infiltrating nitrogen into the carburized layer 31 and ensuring an appropriate amount of retained austenite, pitting corrosion resistance can be significantly improved. In the carburized layer 31, the maximum amount of retained austenite in the thickness direction can be 50 volume percent or more and 70 volume percent or less.

[0096] A method for manufacturing the spur gear 1 according to the fourth embodiment will be described below. The spur gear 1 of the fourth embodiment can be manufactured using basically the same process as the spur gear 1 of the first embodiment. However, the manufacturing method for the spur gear 1 of the fourth embodiment differs from that of the first embodiment in part in introducing nitrogen into the carburized layer 31. The differences from the first embodiment will be described below.

[0097] Fig. 11 is a flowchart illustrating the method for manufacturing the spur gear according to the fourth embodiment. Fig. 12 is a diagram illustrating the heat treatment performed in the method of manufacturing the spur gear according to the fourth embodiment. Fig. 11 and Fig. 12 correspond Fig. 4 and Fig. 5 in the first embodiment.

[0098] With reference to Fig. 11, the step of preparing the steel element is first performed as step S10, as in the first embodiment. In the method for manufacturing the spur gear of the fourth embodiment, a steel material made of steel with the same component composition as in the first embodiment is prepared.

[0099] Subsequently, step S20 is executed in the same manner as in the first embodiment. Then, as shown in Fig. 11 and Fig. 12, the steps S30 to S40 (up to time t9 in Fig. 12) in the same manner as in the first embodiment.

[0100] Subsequently, a nitriding step is carried out as step S54. In step S54, the compact (spur gear 1) is heated in a nitriding atmosphere to infiltrate the carburized layer 31 with nitrogen. In particular, as shown in Fig. 12, the compact (spur gear 1) on which the carburized layer 31 was formed, from time t9 to time t 41 at temperature T1 in a nitriding atmosphere. A mixed gas of nitrogen (N2) and ammonia (NH3), for example, can be used as the nitriding atmosphere. The atmospheric pressure can be 100 kPa or less and can be, for example, 40 kPa. This causes nitrogen to infiltrate into the carburized layer 31.

[0101] Subsequently, the spur gear 1 is rotated during a period from time t 41 until time t 42 from temperature T1 to temperature T2. Then, from time t42 until time t 43 maintained at temperature T2 in the nitriding atmosphere. This further infiltrates nitrogen into the carburized layer 31.

[0102] Subsequently, the quenching step is carried out as step S60. In step S60, the compact (spur gear 1) subjected to the steps up to step S54 is quenched. In particular, the spur gear 1, which has been subjected to the quenching process up to time t 43 kept in the nitriding atmosphere, cooled to the temperature of the MS point or below (rapidly cooled), see Fig. 12. As a result, the spur gear 1 is hardened by quenching and the structure of the steel from which the spur gear 1 is made assumes a state with a martensitic structure.

[0103] Thereafter, the tempering step is performed as step S70 in the same manner as in the first embodiment. The spur gear 1 of the present embodiment can be manufactured using the above procedure. Fifth embodiment

[0104] Hereinafter, as a fifth embodiment, an example in which the mechanical component of the present disclosure is applied to a bevel pinion will be described. Fig. 13 is a schematic perspective view illustrating the appearance of a bevel pinion.

[0105] With reference to Fig. 13, a bevel pinion 4 in the fifth embodiment includes a shaft portion 5 and a gear portion 6. The shaft portion 5 includes a spline portion 5A on the side of one end portion 5B. The spline portion 5B is a portion in which axially extending groove portions are formed on the outer peripheral surface at regular intervals over the entire circumference. The gear portion 6 is connected to one end portion of the shaft portion 5 on the side opposite to the one end portion 5A. The gear portion 6 has a truncated cone shape. In the area corresponding to the outer peripheral surface of the truncated cone shape (taper surface), spiral teeth 6A are formed over the entire circumference. The bevel pinion 4 can be used, for example, as an axle component of a work machine such as a wheel loader or a dump truck.

[0106] The bevel pinion 4 of the present embodiment is made of steel similar to that of the spur gear 1 of the first to fourth embodiments. The bevel pinion 4 is arranged to form an outer peripheral surface of the gear part 6 and includes a carburized layer having a higher carbon concentration than other parts, as in the spur gear 1 of the first to fourth embodiments. As in the spur gear 1 in the first to fourth embodiments, the carburized layer includes a composite oxide film arranged to form the outer peripheral surface of the gear part 6, which has a thickness of 1 μm or more and 25 μm or less and which contains magnetite and hematite.

[0107] The bevel pinion 4 of the present embodiment has a configuration similar to that of the first to fourth embodiments described above, and is thus a mechanical component with improved durability by suppressing the occurrence of peeling of the white layer. The bevel pinion 4 of the present embodiment can be manufactured using the same procedure as the spur gear 1 of the first to fourth embodiments. Examples(1) Effect of peeling off the white layer of a composite oxide film

[0108] To confirm the effect of suppressing the peeling of the white layer of the composite oxide film in the mechanical component of the present disclosure, an experiment was conducted. The experimental procedure is as follows.

[0109] First, a compact in the shape of a test gear was prepared, and heat treatment was performed using the same method as in steps S30 to S70 described in the first embodiment, thereby manufacturing a test gear. At this time, the conditions of the oxide film formation step (S30) were changed to manufacture test gears in which the thickness of the composite oxide film was varied between 2 and 8 μm. For comparison, a test gear without a composite oxide film (the thickness of the composite oxide film was 0) was also manufactured by omitting the oxide film formation step (S30).

[0110] A gear (large gear) with a larger diameter than the test gear was prepared separately. Then, the test gear and the large gear were meshed, the test gear was driven, and the gears were driven until pitching (material wear) occurred on the surface of the test gear while the large gear was driven. The rotational speed of the test gear was 2000 rpm, and the surface pressure in the contact area between the gears was 220 to 260 kgf / mm. 2 To promote the formation of a white layer, the temperature of the lubricating oil was also set higher than under normal application conditions. After the test, the damaged part was examined, and the occurrence rate of the white layer was calculated. The test results are shown in Fig. 14 shown.

[0111] In Fig. 14, the horizontal axis corresponds to the thickness of the composite oxide film containing magnetite and hematite. The vertical axis corresponds to the occurrence of the white layer. With reference to Fig. 14, the peeling frequency of the white layer is 100% for the test gear without a composite oxide film (the thickness of the composite oxide film is 0). On the other hand, it can be seen that the peeling frequency of the white layer is suppressed in the test gear in which the thickness of the composite oxide film is 1 μm or more. Specifically, it can be seen that the occurrence frequency of a white layer is 70% or less when the thickness of the composite oxide film is 2 μm or more, 10% or less when the thickness of the composite oxide film is 6 μm or more, and 5% when the thickness of the composite oxide film is 8 μm or more. From this, it can be concluded that the thickness of the composite oxide film is effective when it is 1 μm or more, and that it is more preferable to set the thickness to 2 μm or more, 6 μm or more, and further 8 μm or more. (2) Confirmation of the composition of the composite oxide film

[0112] An experiment was conducted to confirm the composition of the composite oxide film included in the mechanical component of the present disclosure. The experimental procedure is as follows.

[0113] First, a steel compact was prepared, and heat treatment was performed in the same manner as in steps S30 to S70 described in the first embodiment, thereby preparing a sample of the example.

[0114] On the other hand, for comparison, a similarly manufactured steel compact was subjected to gas carburizing and quenching (sample of the comparative example). The gas carburizing conditions were a carburizing time of 180 minutes and a diffusion time of 120 minutes. Subsequently, thin-film X-ray diffraction (XRD) analysis was performed on the samples of the example and the comparative example to identify oxides contained in the oxide films formed on the surfaces of the samples.

[0115] Fig. Figure 15 is a graph showing the results of X-ray diffraction analysis of the composite oxide film in the sample of Example. Fig. Figure 16 is a graph showing the results of X-ray diffraction analysis of an oxide film formed by gas carburization in a sample of Comparative Example. Referring to Fig. 15, it is confirmed that a composite oxide film of magnetite and hematite is formed in the sample of Example. On the other hand, referring to Fig. 16, the presence of hematite in the sample of Comparative Example is not confirmed and it is confirmed that an oxide film consisting of a single magnetite phase is formed. (3) Confirmation of the effect of fine carbide

[0116] An experiment was conducted to confirm the superiority of the mechanical component of the second embodiment (the mechanical component in which fine carbides are dispersed in the carburized layer). The experimental procedure is as follows.

[0117] A test gear was manufactured in the same manner as described in (1) above, following the procedure of steps S10 to S70 of the second embodiment (Example). For comparison, a test gear was also manufactured in which the heat treatment was changed to gas carburizing (Comparative Example). The gas carburizing conditions were a carburizing time of 180 minutes and a diffusion time of 120 minutes. Steel A and Steel B in Table 1 were used as the steels constituting the test gears of the Example and Comparative Example. Components not listed in Table 1 are iron and unavoidable impurities. Subsequently, a large gear was separately manufactured in the same manner as in (1) above, and by varying the surface pressure applied to the tooth surface, the number of engagements until pitting corrosion occurred was examined at each surface pressure (surface compressive strength test). [Table 1] (C) Si Mn Cr Mo Nb Stahl A 0,24 0,50 0,30 1,80 - 0,04 Stahl B 0,18 0,25 0,80 1,00 0,15 -

[0118] Furthermore, the test gears 100 of the above-described example and the comparative example were subjected to a tooth root bending fatigue strength test. Fig. Figure 17 is a diagram illustrating a procedure for a tooth root flexural fatigue test. Referring to Fig. 17, a test device 200 includes a fixing device 201 and a pivoting device 202. In a state where the fixing device 201 is in contact with a tooth 111 of the test gear 100 of each example and comparative example, the pivoting device 202 is brought into contact with another tooth 111, and the pivoting device 202 is pivoted along an arrow α, thereby repeatedly applying a bending load to the tooth root surface of the tooth 111. Subsequently, it was examined how many times the load was repeated until the tooth 111 was broken. The vibration frequency of the vibration device 202 was set to 5 Hz, and the load applied to the tooth 111 was adjusted to a range of 100 to 240 kgf / mm 2 set. If at the time the number of load repetitions reaches 4.0 × 10 6 was, no breakage occurred, the test was stopped.

[0119] Furthermore, an impact load was applied to the tooth root of the test gear of the above-described example and comparative example, and the minimum load until fracture was examined (tooth root impact test). The test was conducted at room temperature. Furthermore, test pieces were prepared according to the procedure of steps S10 to S70 of the second embodiment, and Charpy impact tests (see JIS Standard Z2242) were performed. The shape of the test piece had a 10R notch. The test was conducted at normal temperature (room temperature) and low temperature (-40°C).

[0120] The test results are described below. Fig. Figure 18 is a graph showing the results of a surface compressive strength test. In Fig. 18, the horizontal axis corresponds to the number of engagements of the teeth and the vertical axis to the average surface pressure exerted on the tooth surface. With reference to Fig. 18 confirms that when pitching occurs at the same number of engagements, the average surface pressure exerted on the test gear of the example is higher than the average surface pressure exerted on the test gear of the comparative example. For example, the load at which pitching occurs when the number of engagements is 7.0 × 10 7 is 1.28 in Example 1, while the load is 1.00 in Comparative Example 1. It is confirmed that the mechanical component of the second embodiment, in which fine carbides are dispersed in the carburized layer, has better pitting corrosion resistance than the mechanical component produced by conventional gas carburizing.

[0121] Fig. Figure 19 is a graph showing the results of the tooth root flexural fatigue test. In Fig. 19, the horizontal axis corresponds to the number of loading repetitions and the vertical axis to the tooth root bending load. In Fig. 19, a data point is marked with an arrow corresponding to a test condition where, at the time when the number of load repetitions was 4.0 × 10 6 times and the test was stopped, no fracture occurred. The vertical axis of Fig. 19 represents a relative value with respect to the load at a point where a curve drawn along the test results of the comparative example intersects a vertical line corresponding to the number of load repetitions of 4.0 × 10 6 which is defined as 1.00.

[0122] With reference to Fig. 19, it is confirmed that when the fracture occurs at the tooth root at the same number of repetitions, the tooth root bending load applied to the test gear of the example is higher than that applied to the test gear of the comparative example. For example, the bending load at which the number of load repetitions is 4.0 × 10 6 achieved without fracture in Example 1.15, while the bending stress in Comparative Example 1.00. Therefore, the mechanical component of the second embodiment, in which fine carbides are dispersed in the carburized layer, exhibits higher fatigue strength against tooth root bending stress than a mechanical component produced by conventional gas carburizing.

[0123] Fig. Figure 20 is a graph showing the results of the tooth root impact bending test. The breaking load, which represents the vertical axis of Fig. 20 is given as a relative value, with the breaking load of the comparative example being 1.00. With reference to Fig. 20, the ultimate load in the tooth root impact bending test of the example is approximately 20% higher than that of the comparative example. Therefore, the mechanical component of the second embodiment, in which fine carbides are dispersed in the carburized layer, exhibits higher strength against the tooth root impact bending load than a mechanical component manufactured by conventional gas carburizing.

[0124] Fig. Figure 21 is a graph showing the results of the Charpy impact test. The values ​​indicated by the vertical axis in Fig. The breaking load shown in Figure 21 is given by a relative value, with the breaking load of the comparative example being 1.00 for both normal temperature and low temperature conditions. With reference to Fig.21, the fracture load in the Charpy impact test of the example is significantly higher than that of the comparative example. This shows that the mechanical component of the second embodiment, in which fine carbides are dispersed in the carburized layer, exhibits excellent toughness due to the effect of grain refinement, compared to the mechanical component produced by conventional gas carburizing.

[0125] In the above-described embodiment, the spur gear as a component of the transmission, the final drive, or the swing mechanism of the work machine, and the bevel pinion as a component of the axle were described as examples of the mechanical component of the present disclosure. However, the application of the mechanical component of the present disclosure is not limited to this. For example, the mechanical component of the present disclosure is also applicable to a shaft that is a component of a final drive or a swing mechanism of a work machine, a bevel gear or a ring gear that is a component of an axle, and the like.

[0126] It is to be understood that the embodiment and example disclosed herein are exemplary in all respects and not restrictive in any way. The scope of the present invention is not limited to the description given above, but is defined by the claims and is intended to include all modifications within the meaning and scope of the claims. List of reference symbols

[0127] 1 Spur gear 2 Through hole 4 Bevel pinion 5 Shaft part 5A End part 5B Spline part 6 Gear part 6A Tooth 10 Outer peripheral surface 11 Tooth 20 Inner peripheral surface 31 Carburized layer 32 Base part 40 Composite oxide film 41 Oxidized grain boundary region 50 Originally austenitic grains 51 Grain boundary 61 Carbide 100 Test gear 111 Tooth 200 Test fixture 201 Fixture 202 Swing fixture A Central axis t A Thickness t B thickness QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] JP 2023-034868

[0002] JP 2022-080369 A

[0005] WO 2020 / 144830

[0005] JP 2006-183095 A

[0005]

Claims

[1] A mechanical component made of steel containing 0.12 mass percent or more and 0.28 mass percent or less of carbon, 0.15 mass percent or more and 0.70 mass percent or less of silicon, 0.20 mass percent or more and 0.95 mass percent or less of manganese, and 0.85 mass percent or more and 1.90 mass percent or less of chromium, the remainder being iron and unavoidable impurities, and having a martensitic structure, the mechanical component including a carburized layer arranged to form at least a part of a surface and having a higher carbon concentration than the other part, wherein the carburized layer includes a composite oxide film arranged to form the surface and having a thickness of 1 µm or more and 25 µm or less and containing magnetite and hematite. [2] A mechanical component made of steel containing 0.12 mass percent or more and 0.28 mass percent or less of carbon, 0.15 mass percent or more and 0.70 mass percent or less of silicon, 0.20 mass percent or more and 0.95 mass percent or less of manganese, 0.85 mass percent or more and 1.90 mass percent or less of chromium, and at least one selected from the group consisting of 0.15 mass percent or more and 0.45 mass percent or less of molybdenum, 0.01 mass percent or more and 2.00 mass percent or less of nickel, 0.04 mass percent or more and 0.08 mass percent or less of niobium, the remainder being iron and unavoidable impurities, and having a martensitic structure, the mechanical component including a carburized layer arranged so thatthat it forms at least part of a surface and has a higher carbon concentration than the other part, wherein, the carburized layer includes a composite oxide film arranged to form the surface and having a thickness of 1 µm or more and 25 µm or less and containing magnetite and hematite. [3] A mechanical component according to claim 1 or 2, wherein the carburized layer has a thickness of 500 µm or more. [4] The mechanical component according to claim 1 or 2, wherein the maximum carbon concentration in the thickness direction of the carburized layer is 0.6 mass% or more. [5] Mechanical component according to claim 1 or 2, wherein in the carburized layer, the maximum carbon concentration in the thickness direction is 0.8 mass% or more and 1.2 mass% or less, and a maximum grain size of the carbides in a cross-section perpendicular to the surface is 1 µm or less, and a grain size number defined in JIS G0551 is 12 or more. [6] Mechanical component according to claim 1 or 2, wherein in the carburized layer, the maximum carbon concentration in the thickness direction is 1.1 mass% or more and 1.8 mass% or less, and a maximum grain size of the carbides in a cross-section perpendicular to the surface is 25 µm or less, and a grain size number defined in JIS G0551 is 11 or more. [7] Mechanical component according to claim 1 or 2, wherein in the carburized layer, a maximum nitrogen concentration in the thickness direction is 0.7 mass% or more and 1.2 mass% or less. [8] Mechanical component according to claim 1 or 2, wherein the mechanical component is a component part of a transmission, an axle, an axle drive or a swivel device of a work machine. [9] A method for manufacturing a mechanical component, comprising: Providing a steel material containing 0.12 mass percent or more and 0.28 mass percent or less of carbon, 0.15 mass percent or more and 0.70 mass percent or less of silicon, 0.20 mass percent or more and 0.95 mass percent or less of manganese, and 0.85 mass percent or more and 1.90 mass percent or less of chromium, the remainder being iron and unavoidable impurities; Shaping the steel material to obtain a compact; Heating the compact in an oxidizing atmosphere, whereby a composite oxide film containing magnetite and hematite is formed on a surface of the compact; Heating the compact on which the composite oxide film is formed in a carburizing atmosphere at a pressure of 1 kPa or less, thereby forming on the surface of the compact a carburized layer having a higher carbon concentration than the other part and having a thickness greater than the composite oxide film; and Quench hardening of the compact with the composite oxide film and carburized layer formed thereon. [10] A method for manufacturing a mechanical component, comprising: Providing a steel material containing 0.12 mass % or more and 0.28 mass % or less of carbon, 0.15 mass % or more and 0.70 mass % or less of silicon, 0.20 mass % or more and 0.95 mass % or less of manganese, 0.85 mass % or more and 1.90 mass % or less of chromium, and at least one selected from the group consisting of 0.15 mass % or more and 0.45 mass % or less of molybdenum, 0.01 mass % or more and 2.00 mass % or less of nickel, and 0.04 mass % or more and 0.08 mass % or less of niobium, the balance being iron and unavoidable impurities. Shaping the steel material to obtain a compact; Heating the compact in an oxidizing atmosphere, whereby a composite oxide film containing magnetite and hematite is formed on a surface of the compact; Heating the compact on which the composite oxide film is formed in a carburizing atmosphere at a pressure of 1 kPa or less, thereby forming on the surface of the compact a carburized layer having a higher carbon concentration than the other part and having a thickness greater than the composite oxide film; and Quench hardening of the compact with the composite oxide film and carburized layer formed thereon. [11] A method for manufacturing a mechanical component according to claim 9 or 10, wherein the forming of the composite oxide film and the forming of the carburized layer are alternately repeated a plurality of times. [12] A method for manufacturing a mechanical component according to claim 9 or 10, wherein in the formation of the carburized layer, the carburized layer is formed such that a maximum carbon concentration in the thickness direction is 0.8 mass% or more and 1.2 mass% or less, the method further comprising: prior to quench-hardening the compact, cooling the compact on which the carburized layer has been formed from a temperature range of an A1 transformation point or higher to a temperature range below the A1 transformation point, thereby transforming the carburized layer into a pearlitic structure; and Heating the compact with the carburized layer formed into a pearlitic structure to the temperature range of the A1 transformation point or higher and then cooling the compact to a temperature range below the A1 transformation point, whereby carbides contained in the carburized layer are spherodized and crystal grains of the carburized layer are refined, wherein after quench hardening of the compact, the maximum grain size of the carbides in the carburized layer in a cross-section perpendicular to the surface of the compact is 1 µm or less, and the grain size number of the carburized layer defined in JIS G0551 is 12 or more. [13] A method for manufacturing a mechanical component according to claim 9 or 10, wherein in the formation of the carburized layer, the carburized layer is formed such that a maximum carbon concentration in the thickness direction is 1.1 mass% or more and 1.8 mass% or less; the method further comprising: prior to quench-hardening the compact, cooling the compact on which the carburized layer is formed to a temperature range below the Al transformation point, thereby transforming the carburized layer into a pearlitic structure; and Heating the compact with the carburized layer formed into the pearlitic structure to the temperature range of the A1 transformation point or higher and then cooling the compact to a temperature range below the A1 transformation point, whereby carbides contained in the carburized layer are spherodized, wherein after quench hardening of the compact, the maximum grain size of the carbides in the carburized layer in a cross-section perpendicular to the surface of the compact is 25 µm or less, and the grain size number of the carburized layer defined in JIS G0551 is 11 or more. [14] A method of manufacturing a mechanical component according to claim 9 or 10, further comprising: Before quench hardening, the compact is heated in a nitriding atmosphere, whereby nitrogen is infiltrated into the carburized layer.

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