Machine component and machine component manufacturing method

Abstract

A spur gear (1), which is a machine component, is composed of a steel having a martensitic structure, and is provided with a carburized layer (31) that is disposed so as to constitute at least a part of the surface and has a higher carbon concentration than the other parts. The steel contains 0.12% by mass or more and 0.28% by mass or less of carbon, 0.15% by mass or more and 0.70% by mass or less of silicon, 0.20% by mass or more and 0.95% by mass or less of manganese, and 0.85% by mass or more and 1.90% by mass or less of chromium, with the balance being iron and unavoidable impurities. The carburized layer (31) includes a composite oxide film (40) that is disposed so as to constitute a surface, has a thickness of 1 [mu] m to 25 [mu] m, and includes magnetite and hematite.

Description

Technical Field

[0001] The present invention relates to a mechanical component and a method for manufacturing the mechanical component.

[0002] This application claims priority based on Japanese Patent Application No. 2023-034868, filed on March 7, 2023, and incorporates by reference all the contents described in that Japanese application. Background Art

[0003] For the mechanical parts made of steel, from the viewpoint of improving strength, sometimes the carburized layer with carbon concentration higher than other parts is formed on the surface. The hardness of the surface layer of the mechanical parts with carburized layer and quenched hardening is high. On the other hand, the hardness of the region outside the carburized layer is low, and has excellent toughness. As a result, the mechanical parts with carburized layer can take into account both strength and toughness.

[0004] Carburized layer is generally formed by gas carburizing.In gas carburizing, the carbon monoxide (CO) of 2 molecules is changed into the carbon dioxide (CO2) of 1 molecule on the surface of mechanical parts ) the carbon (C) generated in the reaction is supplied to the surface of mechanical parts.From the viewpoint that can implement carburizing at low cost, gas carburizing is an excellent processing method.On the other hand, owing to can generate carbon dioxide in the above-mentioned reaction, therefore there is the problem that can discharge a large amount of carbon dioxide in carburizing treatment.

[0005] As other carburizing methods, known vacuum carburizing.In vacuum carburizing, mechanical parts are heated in decompression (such as pressure 1kPA or less) gas atmosphere, acetylene (C2H2) and other hydrocarbons are decomposed in the reaction of the carbon generated by the surface of mechanical parts is supplied to the surface of mechanical parts. Vacuum carburizing is compared with gas carburizing, and although equipment cost increases usually, it has the advantage of being able to shorten processing time, reduce manufacturing cost, avoid the discharge of a large amount of carbon dioxide. Proposed steel suitable for vacuum carburizing, carburized layer is formed by vacuum carburizing and there is the mechanical parts etc. of excellent characteristics (for example, with reference to Japanese Patent Laid-Open No. 2022-080369 Gazette (Patent Document 1), International Publication No. 2020 / 144830 (Patent Document 2) and Japanese Patent Laid-Open No. 2006-183095 Gazette (Patent Document 3)).

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Application Laid-Open No. 2022-080369

[0009] Patent Document 2: International Publication No. 2020 / 144830

[0010] Patent Document 3: Japanese Patent Application Laid-Open No. 2006-183095 Summary of the Invention

[0011] Problems to be solved by the invention

[0012] As also mentioned in the aforementioned patent document, mechanical components with a carburized layer formed by vacuum carburizing have the characteristic of suppressing the formation of a grain boundary oxide layer on the surface, compared to mechanical components with a carburized layer formed by gas carburizing. Generally speaking, suppressing the formation of a grain boundary oxide layer improves the fatigue strength (lifespan) and other properties of the mechanical component.

[0013] However, according to the research of the present inventors, for mechanical parts with a carburized layer formed by vacuum carburizing, a white layer sometimes peels off from the surface in a shorter time (white layer peeling) compared to mechanical parts with a carburized layer formed by gas carburizing.

[0014] One object of the present invention is to provide a mechanical component having improved durability by suppressing the occurrence of white layer peeling, and a method for manufacturing the mechanical component.

[0015] Solutions to the Problem

[0016] A mechanical component according to a first aspect of the present disclosure is composed of steel, and includes a carburized layer disposed so as to constitute at least a portion of a surface and having a higher carbon concentration than other portions. The steel contains: carbon (C) of 0.12% to 0.28% by mass, silicon (Si) of 0.15% to 0.70% by mass, manganese (Mn) of 0.20% to 0.95% by mass, and chromium (Cr) of 0.85% to 1.90% by mass, with the remainder being iron and unavoidable impurities, the steel having a martensitic structure. The carburized layer includes a composite oxide film disposed so as to constitute the surface, having a thickness of 1 μm to 25 μm, and containing magnetite (Fe₃O₄) and hematite (Fe₂O₃).

[0017] A second aspect of the present disclosure relates to a mechanical component comprising steel, comprising a carburized layer disposed so as to constitute at least a portion of a surface thereof and having a carbon concentration higher than that of other portions thereof, the steel containing at least one element selected from the group consisting of 0.12% by mass to 0.28% by mass of carbon, 0.15% by mass to 0.70% by mass of silicon, 0.20% by mass to 0.95% by mass of manganese, 0.85% by mass to 1.90% by mass of chromium, 0.15% by mass to 0.45% by mass of molybdenum (Mo), 0.01% by mass to 2.00% by mass of nickel (Ni), and 0.04% by mass to 0.08% by mass of niobium (Nb), with the remainder being iron and unavoidable impurities, the steel having a martensitic structure. The carburized layer comprises a composite oxide film disposed so as to constitute the surface thereof, having a thickness of 1 μm to 25 μm, and containing magnetite and hematite.

[0018] The manufacturing method of the mechanical component of the first aspect of the present disclosure comprises: a process of preparing steel, a process of obtaining a molded body, a process of forming a composite oxide film, a process of forming a carburized layer, and a process of quenching and hardening the molded body. In the process of preparing the steel, a steel containing 0.12% by mass and 0.28% by mass of carbon, 0.15% by mass and 0.70% by mass of silicon, 0.20% by mass and 0.95% by mass of manganese, and 0.85% by mass and 1.90% by mass of chromium, with the remainder being iron and unavoidable impurities, is prepared. In the process of obtaining the molded body, the molded body is obtained by molding the steel. In the process of forming the composite oxide film, a composite oxide film containing magnetite and hematite is formed on the surface of the molded body by heating the molded body in an oxidizing gas atmosphere. In the step of forming a carburized layer, the molded body having the composite oxide film formed thereon is heated in a carburizing gas atmosphere at a pressure of 1 kPA or less, thereby forming a carburized layer having a greater carbon concentration than other portions of the molded body on its surface. In the step of quenching and hardening the molded body, the molded body having the composite oxide film and the carburized layer formed thereon is quenched and hardened.

[0019] The second aspect of the present disclosure is a method for manufacturing a mechanical component, comprising the steps of preparing a steel material, obtaining a formed body, forming a composite oxide film, forming a carburized layer, and quenching and hardening the formed body. In the step of preparing the steel material, a steel material containing at least one element selected from the following elements is prepared: 0.12% by mass to 0.28% by mass of carbon, 0.15% by mass to 0.70% by mass of silicon, 0.20% by mass to 0.95% by mass of manganese, 0.85% by mass to 1.90% by mass of chromium, 0.15% by mass to 0.45% by mass of molybdenum, 0.01% by mass to 2.00% by mass of nickel, and 0.04% by mass to 0.08% by mass of niobium, with the remainder being iron and unavoidable impurities. In the step of obtaining the formed body, the formed body is obtained by forming the steel material. In the step of forming a composite oxide film, a composite oxide film comprising magnetite and hematite is formed on the surface of the molded body by heating the molded body in an oxidizing gas atmosphere. In the step of forming a carburized layer, the molded body having the composite oxide film formed thereon is heated in a carburizing gas atmosphere at a pressure of 1 kPA or less, thereby forming a carburized layer having a greater thickness than the composite oxide film and a higher carbon concentration than other portions of the molded body on the surface of the molded body. In the step of quenching and hardening the molded body, the molded body having the composite oxide film and the carburized layer formed thereon is quenched and hardened.

[0020] Effects of the Invention

[0021] According to the above-described mechanical component and method for manufacturing a mechanical component, it is possible to provide a mechanical component and method for manufacturing a mechanical component in which durability is improved by suppressing the occurrence of white layer peeling. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a schematic perspective view showing the appearance of the spur gear according to the first embodiment.

[0023] Figure 2 This is a schematic cross-sectional view showing the cross-sectional structure of the spur gear according to the first embodiment.

[0024] Figure 3 This is a schematic cross-sectional view showing the structure of the carburized layer according to the first embodiment.

[0025] Figure 4 This is a flowchart showing an outline of the method for manufacturing a spur gear according to the first embodiment.

[0026] Figure 5 These are diagrams for explaining the heat treatment performed in the method for manufacturing the spur gear according to the first embodiment.

[0027] Figure 6 This is a schematic cross-sectional view showing the structure of a carburized layer according to the second embodiment.

[0028] Figure 7 This is a flowchart showing an outline of a method for manufacturing a spur gear according to the second embodiment.

[0029] Figure 8 It is a diagram for explaining the heat treatment performed in the method for manufacturing a spur gear according to the second embodiment.

[0030] Figure 9 This is a schematic cross-sectional view showing the structure of a carburized layer according to the third embodiment.

[0031] Figure 10 This is a flowchart showing an outline of a method for manufacturing a spur gear according to a third embodiment.

[0032] Figure 11 This is a flowchart showing an outline of a method for manufacturing a spur gear according to a fourth embodiment.

[0033] Figure 12 It is a diagram for explaining the heat treatment performed in the method for manufacturing a spur gear according to the fourth embodiment.

[0034] Figure 13 It is a schematic perspective view showing the appearance of a bevel pinion gear.

[0035] Figure 14 This is a graph showing the relationship between the thickness of the composite oxide film and the incidence rate of the white layer.

[0036] Figure 15 It is a graph showing the results of X-ray diffraction analysis of the composite oxide film.

[0037] Figure 16 This is a diagram showing the results of X-ray diffraction analysis of an oxide film formed by gas carburizing.

[0038] Figure 17 This is a diagram for explaining the method of tooth root bending fatigue test.

[0039] Figure 18 It is a graph showing the results of the surface pressure strength test.

[0040] Figure 19 It is a graph showing the results of a tooth root bending fatigue test.

[0041] Figure 20 It is a graph showing the results of the tooth root bending impact test.

[0042] Figure 21 It is a graph showing the results of the Charpy impact test.

[0043] Explanation of symbols

[0044] 1 Spur gear, 2 Through hole, 4 Bevel gear, 5 Shaft, 5A End, 5B Spline, 6 Gear, 6A Tooth, 10 Outer surface, 11 Tooth, 20 Inner surface, 31 Carburized layer, 32 Base, 40 Composite oxide film, 41 Grain boundary oxidation area, 50 Prior austenite grain, 51 Grain boundary, 61 Carbide, 100 Test gear, 111 Tooth, 200 Test apparatus, 201 Fixture, 202 Swing fixture, A Center axis, t A Thickness, t B thickness. DETAILED DESCRIPTION

[0045] [Overview of Embodiments]

[0046] A mechanical component according to a first aspect of the present disclosure is composed of steel, and includes a carburized layer disposed so as to constitute at least a portion of a surface and having a higher carbon concentration than other portions. The steel contains: carbon (C) of 0.12% to 0.28% by mass, silicon (Si) of 0.15% to 0.70% by mass, manganese (Mn) of 0.20% to 0.95% by mass, and chromium (Cr) of 0.85% to 1.90% by mass, with the remainder being iron and unavoidable impurities, the steel having a martensitic structure. The carburized layer includes a composite oxide film disposed so as to constitute the surface, having a thickness of 1 μm to 25 μm, and containing magnetite (Fe₃O₄) and hematite (Fe₂O₃).

[0047] A second aspect of the present disclosure relates to a mechanical component comprising steel, comprising a carburized layer disposed so as to constitute at least a portion of a surface thereof and having a carbon concentration higher than that of other portions thereof, the steel containing at least one element selected from the group consisting of 0.12% by mass to 0.28% by mass of carbon, 0.15% by mass to 0.70% by mass of silicon, 0.20% by mass to 0.95% by mass of manganese, 0.85% by mass to 1.90% by mass of chromium, 0.15% by mass to 0.45% by mass of molybdenum (Mo), 0.01% by mass to 2.00% by mass of nickel (Ni), and 0.04% by mass to 0.08% by mass of niobium (Nb), with the remainder being iron and unavoidable impurities, the steel having a martensitic structure. The carburized layer comprises a composite oxide film disposed so as to constitute the surface thereof, having a thickness of 1 μm to 25 μm, and containing magnetite and hematite.

[0048] The present inventors have studied methods for suppressing white layer peeling in mechanical components with a carburized layer formed by vacuum carburizing. They have discovered that white layer peeling can be suppressed by forming a composite oxide film containing magnetite and hematite with a predetermined thickness or greater, more specifically, a thickness of 1 μm or greater, on the surface of the carburized layer. The reasons for suppressing white layer peeling are not limited to the following, but are considered to be, for example, the following reasons.

[0049] White layer delamination is believed to occur as follows. First, a new surface forms on the surface of a mechanical component due to contact with other components. When lubricating oil contacts this new surface, hydrocarbons contained in the lubricating oil decompose, and the hydrogen generated by this decomposition infiltrates the surface of the mechanical component, forming a white layer. Cracks form in the mechanical component, starting from this white layer, causing damage to the surface (white layer delamination) within a short period of time.

[0050] In contrast, by forming a composite oxide film having a thickness of at least a given thickness (specifically, at least 1 μm) and containing magnetite and hematite on the surface of the carburized layer, the occurrence of white layer peeling can be suppressed. The reasons for this are, for example, that the presence of the composite oxide film suppresses the generation of the above-mentioned new surface; that the composite oxide film is appropriately removed from the contact portion with other mechanical parts when the mechanical parts are newly assembled into the machine and begin operation, thereby absorbing the shape error of the mechanical parts (improving initial running-in), etc. In addition, it is known that an oxide film (grain boundary oxide layer) is also formed in the carburized layer formed by gas carburizing in a manner constituting the surface. However, the oxide film formed in gas carburizing is an oxide film composed of a single phase of magnetite, which is different from the above-mentioned composite oxide film.

[0051] Generally, one of the most important advantages of using vacuum carburizing in the formation of the carburized layer is that no oxide film (grain boundary oxide layer) is formed on the surface. In contrast, the present inventors have found that by intentionally forming a composite oxide film comprising magnetite and hematite on the surface of the carburized layer, the occurrence of white layer peeling can be suppressed. In order to fully exert the function of suppressing the occurrence of white layer peeling, the thickness of the composite oxide film needs to be 1 μm or more. On the other hand, when the thickness of the composite oxide film exceeds 25 μm, the above-mentioned function is saturated, and the bending strength, torsional strength, etc. of the mechanical parts are reduced, and the manufacturing cost increases. Therefore, the thickness of the composite oxide film needs to be set to 25 μm or less.

[0052] In the mechanical component disclosed herein, the carburized layer comprises a composite oxide film, which is arranged to form the surface, has a thickness of 1 μm to 25 μm, and contains magnetite and hematite. As a result, the mechanical component disclosed herein can provide a mechanical component with improved durability by suppressing the occurrence of white layer peeling.

[0053] The reasons for limiting the chemical composition of the steel constituting the machine component of the present disclosure to the above-mentioned range will be described below.

[0054] Carbon: 0.12 mass% or more and 0.28 mass% or less

[0055] Carbon content has a great influence on the hardness of steel (through quenching hardened steel) with martensitic structure. From the viewpoint of ensuring the sufficient hardness of the region outside the carburized layer, that is, the base portion, and suppressing the carburizing treatment time for making the carburized layer a sufficient amount of carbon, carbon content needs to be set to more than 0.12 mass %. On the other hand, when the carbon content of the base portion increases, the toughness of the mechanical parts decreases. From the viewpoint of ensuring sufficient toughness, carbon content needs to be set to below 0.28 mass %.

[0056] Silicon: 0.15 mass% or more and 0.70 mass% or less

[0057] Silicon contributes to improved hardenability and temper softening resistance. To ensure these functions, the silicon content needs to be 0.15% by mass or more. On the other hand, excessive silicon content tends to reduce machinability and other workability. To facilitate machining, the silicon content needs to be 0.70% by mass or less. To prioritize workability, the silicon content is preferably 0.35% by mass or less. To prioritize hardenability and temper softening resistance, the silicon content is preferably 0.45% by mass or more.

[0058] Manganese: 0.20 mass% or more and 0.95 mass% or less

[0059] Manganese contributes to improved hardenability. To ensure this function, the manganese content needs to be 0.20% by mass or more. On the other hand, excessive manganese content tends to increase the amount of retained austenite after quenching. To appropriately control the amount of retained austenite, the manganese content needs to be 0.95% by mass or less. To prioritize hardenability, the manganese content is preferably 0.40% by mass or less. To more appropriately control the amount of retained austenite, the manganese content is preferably 0.40% by mass or less.

[0060] Chromium: 0.85 mass% or more and 1.90 mass% or less

[0061] Chromium is an element that improves hardenability. To ensure sufficient hardenability, the Cr content needs to be at least 0.85 mass%. On the other hand, excessive Cr content reduces toughness. Therefore, the Cr content needs to be at most 1.90 mass%. Chromium has the function of easily refining carbides. To fully ensure this function, the chromium content is preferably at least 1.70 mass%. On the other hand, to prioritize toughness, the chromium content is preferably at most 1.30 mass%.

[0062] Unavoidable impurities

[0063] In addition to the components intentionally added in the manufacturing process, as unavoidable impurities, the steel constituting the mechanical parts sometimes contains elements other than those mentioned above. For example, although oxygen (O) is reduced as much as possible through deoxidation treatment, it is inevitably contained in the steel. Oxygen forms non-metallic inclusions in the steel, which may adversely affect the properties of the mechanical parts. Therefore, the oxygen content is preferably set to less than 20 ppm. Other impurity elements are also preferably reduced as much as possible within the range of appropriate manufacturing costs. The total amount of unavoidable impurities is preferably set to less than 1.00 mass%.

[0064] Molybdenum: 0.15 mass% or more and 0.45 mass% or less

[0065] Molybdenum is not an element that must be added intentionally. However, it contributes to improved hardenability and temper softening resistance. To achieve these properties, the molybdenum content is preferably set to 0.15% by mass or more. On the other hand, molybdenum is an expensive element, and adding it excessively will lead to increased costs. To avoid excessively increasing costs, the molybdenum content is preferably set to 0.45% by mass or less.

[0066] Nickel: 0.01 mass% or more and 2.00 mass% or less

[0067] Nickel is not an element that must be added intentionally. However, nickel contributes to the improvement of toughness. In order to obtain such a function, the nickel content is preferably set to 0.01% by mass or more. On the other hand, nickel is an expensive element. If added in excess, it will lead to an increase in cost. From the viewpoint of not excessively increasing the cost, the nickel content is preferably set to 2.00% by mass or less. From the viewpoint of emphasizing cost reduction, the nickel content is preferably set to 0.75% by mass or less, and more preferably set to 0.25% by mass or less. On the other hand, from the viewpoint of emphasizing the improvement of toughness, the nickel content is preferably set to 0.35% by mass or more, and more preferably set to 1.55% by mass or more.

[0068] Niobium: 0.04 mass% or more and 0.08 mass% or less

[0069] Niobium is not an element that must be intentionally added. However, it contributes to grain refinement. To achieve this function, the niobium content is preferably set to 0.04% by mass or greater. On the other hand, even if niobium is added in amounts exceeding 0.08% by mass, the aforementioned effect is saturated. Therefore, the niobium content is preferably set to 0.08% by mass or less.

[0070] In the above-mentioned machine component, the carburized layer may have a thickness of 500 μm or more. With this configuration, sufficient strength can be easily imparted to the machine component.

[0071] In the above-mentioned machine component, the maximum carbon concentration in the thickness direction of the carburized layer may be 0.6 mass % or more. With this configuration, it is easy to impart sufficient hardness to the surface layer portion of the machine component.

[0072] In the carburized layer of the above-mentioned mechanical parts, the maximum carbon concentration in the thickness direction can be more than 0.8 mass % and less than 1.2 mass %. In the carburized layer, the maximum particle size of the carbide in the cross section perpendicular to the surface can be less than 1 μm. In the carburized layer, the grain size number specified in JIS G0551 can be more than 12. In this way, by setting the maximum carbon concentration of the carburized layer to be high, and dispersing fine carbides in the carburized layer, and refining the grains, it is possible to take into account both strength and toughness at a high level. In the carburized layer, the area ratio of the carbide in the cross section perpendicular to the surface can be more than 1% and less than 10%. In the carburized layer, the average particle size of the carbide in the cross section perpendicular to the surface can be less than 1 μm.

[0073] In the carburized layer of above-mentioned mechanical parts, the maximum carbon concentration in the thickness direction can be more than 1.1 mass % and less than 1.8 mass %.In the carburized layer, the maximum particle size of the carbide in the cross section perpendicular to the surface can be less than 25 μm.In the carburized layer, the crystal grain size numbering specified by JIS G0551 can be more than 11.Like this, by setting the maximum carbon concentration of the carburized layer higher, and making grain refinement, it is possible to take into account durability (such as pitting resistance) and toughness for surface damage with a high level.In the carburized layer, the area ratio of the carbide in the cross section perpendicular to the surface can be more than 3% and less than 30%.

[0074] In the carburized layer of the mechanical component, the maximum nitrogen concentration in the thickness direction can be 0.7% by mass or more and 1.2% by mass or less. By allowing nitrogen to enter the carburized layer, an appropriate amount of retained austenite is ensured, and pitting corrosion resistance is greatly improved. In the carburized layer, the maximum retained austenite in the thickness direction can be 50% by volume or more and 70% by volume or less.

[0075] The mechanical component may be a transmission, an axle, a final drive, or a swing mechanism constituting a working machine. The mechanical component of the present disclosure, which has improved durability by suppressing the occurrence of white layer peeling, is suitable as such a component.

[0076] The manufacturing method of the mechanical component of the first aspect of the present disclosure comprises: a process of preparing steel, a process of obtaining a molded body, a process of forming a composite oxide film, a process of forming a carburized layer, and a process of quenching and hardening the molded body. In the process of preparing the steel, the following steel is prepared, wherein the steel contains: carbon of not less than 0.12 mass % and not more than 0.28 mass %, silicon of not less than 0.15 mass % and not more than 0.70 mass %, manganese of not less than 0.20 mass % and not more than 0.95 mass %, and chromium of not less than 0.85 mass % and not more than 1.90 mass %, with the remainder being iron and unavoidable impurities. In the process of obtaining the molded body, the molded body is obtained by molding the steel. In the process of forming the composite oxide film, a composite oxide film containing magnetite and hematite is formed on the surface of the molded body by heating the molded body in an oxidizing gas atmosphere. In the step of forming a carburized layer, the molded body having the composite oxide film formed thereon is heated in a carburizing gas atmosphere at a pressure of 1 kPA or less, thereby forming a carburized layer having a greater carbon concentration than other portions of the molded body on its surface. In the step of quenching and hardening the molded body, the molded body having the composite oxide film and the carburized layer formed thereon is quenched and hardened.

[0077] The manufacturing method of the mechanical component of the second aspect of the present disclosure comprises: a process of preparing a steel material, a process of obtaining a formed body, a process of forming a composite oxide film, a process of forming a carburized layer, and a process of quenching and hardening the formed body. In the process of preparing the steel material, the following steel material is prepared, wherein the steel material contains at least one selected from the following elements: 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, 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 process of obtaining the formed body, the formed body is obtained by forming the steel material. In the step of forming a composite oxide film, a composite oxide film comprising magnetite and hematite is formed on the surface of the molded body by heating the molded body in an oxidizing gas atmosphere. In the step of forming a carburized layer, the molded body having the composite oxide film formed thereon is heated in a carburizing gas atmosphere at a pressure of 1 kPA or less, thereby forming a carburized layer having a greater thickness than the composite oxide film and a higher carbon concentration than other portions of the molded body on the surface of the molded body. In the step of quenching and hardening the molded body, the molded body having the composite oxide film and the carburized layer formed thereon is quenched and hardened.

[0078] According to the method for manufacturing a mechanical component disclosed in the present invention, the mechanical component of the present invention described above can be easily manufactured.

[0079] In the disclosed method for manufacturing a mechanical component, the steps of forming a composite oxide film and forming a carburized layer can be repeated multiple times, alternating between the steps. This allows the carbon introduced into the molded body during the carburized layer formation step to diffuse into the molded body during the subsequent composite oxide film formation step. Consequently, the composite oxide film and carburized layer formation can be performed efficiently.

[0080] In the manufacturing method of the mechanical component disclosed above, a carburized layer can be formed in a process of forming the carburized layer in such a manner that the maximum carbon concentration in the thickness direction is 0.8 mass % or more and 1.2 mass % or less. For the manufacturing method of the mechanical component, before the process of quenching and hardening the formed body, the following steps can be further included: cooling the formed body with the carburized layer from a temperature range above the A1 transformation point to a temperature range below the A1 transformation point so that the carburized layer forms a pearlite structure; and heating the formed body with the carburized layer formed with pearlite structure to a temperature range above the A1 transformation point, and then cooling it to a temperature range below the A1 transformation point, thereby spheroidizing the carbides contained in the carburized layer and refining the grains of the carburized layer. After the process of quenching and hardening the formed body, the maximum particle size of the carbides in the carburized layer in a cross section perpendicular to the surface of the formed body is 1 μm or less, and the grain size number of the carburized layer specified in JIS G0551 can be 12 or more. Thus, it is possible to easily manufacture mechanical components that have both high strength and toughness. In the carburized layer after the formed body is quenched and hardened, the area ratio of carbides in a cross section perpendicular to the surface can be 1% or more and 10% or less. In the carburized layer after the formed body is quenched and hardened, the average particle size of carbides in a cross section perpendicular to the surface can be 1 μm or less.

[0081] In the manufacturing method of the mechanical component of the present disclosure, a carburized layer can be formed in a process of forming a carburized layer in a manner such that the maximum carbon concentration in the thickness direction is 1.1% by mass or more and 1.8% by mass or less. For the manufacturing method of the mechanical component, before the process of quenching and hardening the formed body, the process may further include: cooling the formed body with the carburized layer to a temperature region less than the A1 transformation point so that the carburized layer forms a pearlite structure; and heating the formed body with the carburized layer formed with pearlite structure to a temperature region greater than the A1 transformation point, cooling it to a temperature region less than the A1 transformation point, thereby spheroidizing the carbides contained in the carburized layer. After the process of quenching and hardening the formed body, the maximum particle size of the carbides in the carburized layer in the cross section perpendicular to the surface of the formed body is less than 25 μm, and the grain size number of the carburized layer specified in JIS G0551 can be 11 or more. Thus, it is possible to easily manufacture a mechanical component that has both high durability (such as pitting resistance) and toughness against surface damage. After the formed body is subjected to the step of quench-hardening, the area ratio of carbides in the carburized layer in a cross section perpendicular to the surface may be 3% or more and 30% or less.

[0082] The method for manufacturing a mechanical component disclosed herein may further include, before the step of quench-hardening the formed body, a step of heating the formed body in a nitriding gas atmosphere to introduce nitrogen into the carburized layer. This facilitates the manufacture of the mechanical component having a carburized layer containing nitrogen.

[0083] [Examples of specific embodiments]

[0084] Next, specific embodiments of the mechanical component disclosed herein will be described with reference to the accompanying drawings. In the following drawings, the same or corresponding parts are denoted by the same reference numerals, and their description will not be repeated.

[0085] (Implementation 1)

[0086] First, refer to Figures 1 to 3 , a spur gear according to Embodiment 1, which is an example of a mechanical component of the present disclosure, will be described. Figure 1 This is a schematic perspective view showing the appearance of the spur gear according to the first embodiment. Figure 2 This is a schematic cross-sectional view showing the cross-sectional structure of the spur gear according to the first embodiment. Figure 3 This is a schematic cross-sectional view showing the structure of the carburized layer according to the first embodiment. Figure 2 Shown with Figure 1 A cross section perpendicular to the central axis A. Figure 3 The structure of the steel near the surface constituting the carburized layer is shown.

[0087] Reference Figure 1 The spur gear 1 of embodiment 1 is a gear constituting a planetary gear mechanism. The spur gear 1 has a circular ring shape (hollow cylindrical shape). A through hole 2 is formed in the spur gear 1 and passes through along the center axis. A plurality of teeth 11 that mesh with the teeth of the sun gear and the external gear of the planetary gear mechanism are formed on the outer peripheral surface 10 of the spur gear 1 over the entire circumferential area. The inner peripheral surface 20 surrounding the through hole 2 has a cylindrical shape. The shaft of the planetary gear carrier of the planetary gear mechanism is inserted into the through hole 2. The spur gear 1 can be used as a component constituting a main reducer or a swing mechanism of a hydraulic excavator as a working machine, a transmission or a main reducer of a bulldozer, a transmission or a main reducer of a dump truck, etc., for example.

[0088] The spur gear 1 is made of steel having a martensitic structure, the steel containing 0.12 mass % to 0.28 mass % carbon, 0.15 mass % to 0.70 mass % silicon, 0.20 mass % to 0.95 mass % manganese, and 0.85 mass % to 1.90 mass % chromium, with the balance being iron and unavoidable impurities. The spur gear 1 can be made of steel having a martensitic structure, wherein the steel contains at least one element selected from the following: 0.12 mass% to 0.28 mass% carbon, 0.15 mass% to 0.70 mass% silicon, 0.20 mass% to 0.95 mass% manganese, 0.85 mass% to 1.90 mass% chromium, 0.15 mass% to 0.45 mass% molybdenum, 0.01 mass% to 2.00 mass% nickel, and 0.04 mass% to 0.08 mass% niobium, with the remainder being iron and unavoidable impurities.

[0089] Reference Figure 2 The spur gear 1 includes a carburized layer 31 having a higher carbon concentration than the base portion 32 as the other portion, and the carburized layer 31 is arranged so as to constitute the outer peripheral surface 10 on which the teeth 11 are formed. The thickness t of the carburized layer 31 is A For example, the thickness may be 500 μm or greater, or 800 μm or greater. The maximum carbon concentration of the carburized layer 31 in the thickness direction (perpendicular to the outer peripheral surface 10) may be 0.6 mass % or greater. The carburized layer 31 is formed over the entire area of ​​the outer peripheral surface 10. The carburized layer 31 is formed so as to completely cover the surface of the tooth 11.

[0090] Reference Figure 3 , the carburized layer 31 has a structure covered with a large number of original austenite grains 50. The boundary between adjacent original austenite grains 50 is a grain boundary 51. In this embodiment, a grain boundary oxidation region 41 is formed, and the grain boundary oxidation region 41 is a region where oxygen enters from the outer peripheral surface 10 along the grain boundary 51. The grain boundary oxidation region 41 is composed of a composite oxide containing magnetite and hematite. In this application, the surface portion where the grain boundary oxidation region 41 composed of the composite oxide is formed is defined as a composite oxide film 40. The thickness t of the composite oxide film 40 BIt is greater than 1 μm and less than 25 μm. That is, the carburized layer 31 in the present embodiment includes a composite oxide film containing magnetite and hematite, and the composite oxide film is configured in a manner that constitutes the outer peripheral surface 10 as the surface of the spur gear 1, and has a thickness of greater than 1 μm and less than 25 μm. The grain size number specified in JIS G0551 of the carburized layer 31 (hereinafter 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, greater than 0.68 mass % and less than 0.80 mass %. Carbide (cementite) is not substantially dispersed in the carburized layer 31. Here, the state in which cementite is substantially not dispersed refers to a state in which, for example, when 10 square areas with a side length of 20 μm are investigated at 5000 times using an SEM (Scanning Electron Microscope) in the cross section of the carburized layer, the number of cementites found is less than 1.

[0091] In the spur gear 1, a mechanical component according to this embodiment, the carburized layer 31 includes a composite oxide film 40 containing magnetite and hematite. This composite oxide film 40 is arranged so as to constitute the outer peripheral surface 10, which is the surface of the spur gear 1, and has a thickness of 1 μm to 25 μm. As a result, the spur gear 1 according to this embodiment becomes a mechanical component with improved durability by suppressing the occurrence of white layer peeling.

[0092] Next, an example of a method for manufacturing the spur gear 1 as the mechanical component according to the present embodiment will be described. Figure 4 Flowchart showing the outline of the method for manufacturing a spur gear. Figure 4 In the method for manufacturing the spur gear 1 of the present embodiment, first, a steel material preparation step is performed as step S10. In this step S10, a steel material composed of steel constituting the spur gear 1 of the present embodiment is prepared.

[0093] Specifically, a steel material is prepared, the steel material containing 0.12 mass % to 0.28 mass % carbon, 0.15 mass % to 0.70 mass % silicon, 0.20 mass % to 0.95 mass % manganese, and 0.85 mass % to 1.90 mass % chromium, with the balance being iron and unavoidable impurities. In step S10, a steel material containing at least one element selected from the group consisting of 0.12% to 0.28% carbon, 0.15% to 0.70% silicon, 0.20% to 0.95% manganese, 0.85% to 1.90% chromium, 0.15% to 0.45% molybdenum, 0.01% to 2.00% nickel, and 0.04% to 0.08% niobium, with the remainder being iron and unavoidable impurities, can be prepared. For example, JIS standard SCr420H, SCM415H, SCM418H, SCM420H, SCM425H, SCM822H, SNCM220H, and SNCM420H can be used as the steel material prepared in step S10.

[0094] Next, a forming step is performed as step S20. In step S20, the steel material prepared in step S10 is formed into a formed body. Specifically, the steel material prepared in step S10 is processed to produce a formed body having the shape of the spur gear 1. Processing in step S20 can include hot forging, cold forging, cutting, and the like.

[0095] Next, an oxide film forming step is performed as step S30. In step S30, the molded body produced in step S20 is heated in an oxidizing gas atmosphere to form a composite oxide film 40 containing magnetite and hematite on the surface (outer peripheral surface 10) of the molded body (spur gear 1).

[0096] Next, a carburizing step is performed as step S40. In step S40, the formed body (spur gear 1) on which the composite oxide film 40 has been formed in step S30 is heated in a carburizing gas atmosphere at a pressure of 1 kPA or less. This forms a carburized layer 31 on the surface (outer peripheral surface 10) of the spur gear 1, having a greater thickness than the composite oxide film 40 and having a higher carbon concentration than that of the base portion 32, which is the other portion.

[0097] Next, a diffusion step is performed as step S50. In step S50, the formed body (spur gear 1), on which the carburized layer 31 was formed in step S40, is held at a temperature above the A1 transformation point. This allows the carbon that has entered the vicinity of the surface of the spur gear 1 in step S40 to diffuse inward. This equalizes the carbon concentration in the carburized layer 31 and increases the thickness of the carburized layer 31.

[0098] Next, a quenching step is performed as step S60. In step S60, the formed body (spur gear 1) on which the composite oxide film 40 is formed in step S30 and the carburized layer 31 is formed in step S40 is quench-hardened. Specifically, the spur gear 1 on which the composite oxide film 40 and the carburized layer 31 are formed is quench-hardened by cooling (quenching) the spur gear 1 from a temperature range above the A1 transformation point to a temperature range below the MS point.

[0099] Here, refer to Figure 5 A specific example of steps S30 to S60 will be described in detail. Figure 5 : is a figure for explaining the heat treatment performed in the manufacturing method of the spur gear of embodiment 1. Figure 5 In , the horizontal axis corresponds to time. Time passes as you move to the right on the horizontal axis. Figure 5 In FIG. 1 , the vertical axis corresponds to the heating temperature of the molded body (spur gear 1). As the vertical axis moves upward, the heating temperature increases.

[0100] Reference Figure 5 and Figure 4 First, as step S30, an oxide film forming step is performed. Specifically, Figure 5 As shown, the spur gear 1 is first heated to temperature T1 before reaching time t1. Temperature T1 is a temperature above the A1 transformation point (727°C), and can be set to a temperature above 930°C and below 980°C, for example, 950°C. At this time, the spur gear 1 is heated in an oxidizing gas atmosphere containing at least one of oxygen (O2) and carbon dioxide (CO2) and having a moisture content less than the dew point, and is maintained at temperature T1 from time t1 to t2. As the oxidizing gas atmosphere, dry air can be used, for example. The time from time t1 to t2 can be set to, for example, 120 minutes. The pressure of the gas atmosphere can be set to below 100 kPA, for example, 40 kPA. As a result, a composite oxide film 40 containing magnetite and hematite can be formed on the surface of the spur gear 1 (the surface of the tooth 11). More specifically, oxygen enters along the austenite grain boundaries of the steel constituting the spur gear 1, forming a grain boundary oxidation region 41 composed of a composite oxide containing magnetite and hematite.

[0101] Next, as step S40, a carburizing step is performed. Specifically, Figure 5As shown, from time t2 to t3, the spur gear 1 is heated in a carburizing gas atmosphere at a pressure of 1 kPA or less, thereby forming a carburized layer 31. The carburizing gas atmosphere can be, for example, an acetylene gas atmosphere, which is a hydrocarbon. The time from time t2 to t3 can be shorter than that from time t1 to t2, for example, 10 minutes.

[0102] Then, in this embodiment, step S30 is performed again. In this embodiment, step S30 as a process for forming a composite oxide film and step S40 as a process for forming a carburized layer can be alternately repeated multiple times (specifically, 4 times). Specifically, as Figure 5 As shown, from time t3 to t4, the spur gear 1 is maintained at the same temperature T1 in the same gas atmosphere as from time t1 to t2. As a result, oxygen further infiltrates along the austenite grain boundaries of the steel constituting the spur gear 1, increasing the thickness of the composite oxide film 40. The time from time t3 to t4 can be shorter than that from time t1 to t2, for example, 5 minutes. At this time, the carbon that infiltrated near the surface of the spur gear 1 from time t2 to t3 diffuses into the interior of the spur gear 1. As a result, the thickness of the carburized layer 31 increases, and the carbon concentration throughout the thickness of the carburized layer is evened out. That is, step S30, performed after step S40, also functions as a carbon diffusion step.

[0103] Then, the step S40 is performed again. Specifically, Figure 5 As shown, from time t4 to t5, spur gear 1 is maintained at the same temperature T1 in the same atmosphere as from time t2 to t3. This allows carbon to further penetrate the surface of spur gear 1, forming a carburized layer 31. The time from time t4 to t5 can be set to the same time as from time t2 to t3, for example, 10 minutes.

[0104] Then, the step S30 is performed again. Specifically, Figure 5 As shown, from time t5 to t6, the spur gear 1 is maintained at the same temperature T1 in the same gas atmosphere as from time t3 to t4. As a result, oxygen further enters from the surface of the spur gear 1, increasing the thickness of the composite oxide film 40. The time from time t5 to t6 can be set to be longer than from time t3 to t4 and shorter than from time t1 to t2, for example, 20 minutes.

[0105] Then, the step S40 is performed again. Specifically, Figure 5 As shown, from time t6 to t7, spur gear 1 is maintained at the same temperature T1 in the same gas atmosphere as from time t2 to t3. This allows carbon to further penetrate the surface of spur gear 1, forming a carburized layer 31. The time from time t6 to t7 can be set to the same time as from time t2 to t3, for example, 10 minutes.

[0106] Then, the step S30 is performed again. Specifically, Figure 5 As shown, from time t7 to t8, the spur gear 1 is maintained at the same temperature T1 in the same gas atmosphere as from time t3 to t4. As a result, oxygen further enters from the surface of the spur gear 1, increasing the thickness of the composite oxide film 40. The time from time t7 to t8 can be set to be longer than from time t5 to t6 and shorter than from time t1 to t2, for example, to 50 minutes.

[0107] Then, the step S40 is performed again. Specifically, Figure 5 As shown, from time t8 to t9, spur gear 1 is maintained at the same temperature T1 in the same atmosphere as from time t2 to t3. This allows carbon to further penetrate the surface of spur gear 1, forming a carburized layer 31. The time from time t8 to t9 can be shorter than that from time t2 to t3, for example, 4 minutes. The carburizing step from time t8 to t9 functions as a process to compensate for the decarburization that occurred during the oxide film formation step from time t7 to t8, i.e., as a recarburization step.

[0108] Next, step S50 is performed. Specifically, from time t9 to time t 10 The temperature T1 is maintained at the same temperature as that from time t8 to t9 in an inert gas atmosphere (e.g., an argon atmosphere or a nitrogen atmosphere). At this time, the carbon that has entered the vicinity of the surface of the spur gear 1 diffuses into the interior of the spur gear 1. As a result, the thickness of the carburized layer 31 increases, and the carbon concentration in the thickness direction of the carburized layer is equalized. From time t9 to t 10 The time can be set to be shorter than the time from time t8 to t9, for example, to 1 minute.

[0109] Next, step S60 is performed. Specifically, Figure 5 As shown, at time t 10 to t 11 The temperature of the spur gear 1 is cooled from temperature T1 to temperature T2, which is lower than temperature T1 and higher than the A1 phase transition point, for example, to 850°C. Then, at time t 11 to t 12 After being kept at the temperature T2 for a period of time, the spur gear 1 is cooled (quenched) to a temperature below the MS point. Thus, the spur gear 1 is quench-hardened, and the structure of the steel constituting the spur gear 1 becomes a state having a martensite structure.

[0110] Next, refer to Figure 4As step S70, a tempering step is performed. In step S70, the formed body (spur gear 1) hardened by quenching in step S60 is tempered. Specifically, the spur gear 1 is heated to a temperature below the A1 transformation point, for example, a temperature range of 150°C to 200°C (e.g., 160°C), and then cooled to room temperature. The above steps produce the spur gear 1 of this embodiment.

[0111] (Implementation Method 2)

[0112] Embodiment 2, another embodiment of the present disclosure, will now be described. The spur gear 1 of Embodiment 2 has essentially the same structure as the spur gear 1 of Embodiment 1 and achieves the same effects. However, the spur gear 1 of Embodiment 2 differs from that of Embodiment 1 primarily in the structure of the carburized layer 31. The differences from Embodiment 1 will be described below.

[0113] Figure 6 : is a schematic cross-sectional view showing the structure of the carburized layer of the spur gear according to the second embodiment. Figure 6 In the carburized layer 31 of the spur gear 1 of the second embodiment, the maximum carbon concentration in the thickness direction of the carburized layer 31 is 0.8 mass % or more and 1.2 mass % or less. A large amount of carbides 61 (cementite) are dispersed in the carburized layer 31. The cross section of the carburized layer 31 perpendicular to the outer peripheral surface 10 as the surface of the spur gear 1 ( Figure 6 The maximum grain size of the carbide 61 in the cross section (of the embodiment) is 1 μm or less. In addition, in the carburized layer 31, the grain size number is 12 or more. Figure 6 The area ratio of the carbide 61 in the cross section may be 1% or more and 10% or less. Figure 6 The average particle size of the carbides 61 in the cross section can be 1 μm or less. Here, the area ratio, maximum particle size, and average particle size of the carbides 61 in the carburized layer 31 can be determined, for example, by using an SEM to examine 10 square areas with a side length of 20 μm in the cross section of the carburized layer at 5000 times magnification, and measuring the area ratio, maximum particle size, and average particle size of the cementite found in this case.

[0114] As the steel constituting the spur gear 1, for example, steel having a higher chromium content than that in the case of 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% to 0.26 mass% of carbon, 0.45 mass% to 0.70 mass% of silicon, 0.20 mass% to 0.40 mass% of manganese, 1.70 mass% to 1.90 mass% of chromium, 0.04 mass% to 0.08 mass% of niobium, with the balance being iron and unavoidable impurities can be used.

[0115] Thus, by setting the maximum carbon concentration of the carburized layer 31 high, dispersing the fine carbides 61 in the carburized layer 31 at an appropriate area ratio, and refining the prior austenite grains 50, the spur gear 1 of the second embodiment becomes a mechanical component having both high strength and toughness.

[0116] Next, a method for manufacturing the spur gear 1 according to the second embodiment will be described. The spur gear 1 according to the second embodiment can be manufactured using essentially the same method as the spur gear 1 according to the first embodiment. However, the manufacturing method of the spur gear 1 according to the second embodiment differs partially from that of the first embodiment in order to obtain the structure of the characteristic carburized layer 31 described above. The differences from the first embodiment will be described below.

[0117] Figure 7 This is a flowchart showing an outline of a method for manufacturing a spur gear according to the second embodiment. Figure 8 It is a diagram for explaining the heat treatment performed in the method for manufacturing a spur gear according to the second embodiment. Figure 7 and Figure 8 are respectively corresponding to the embodiment 1 Figure 4 and Figure 5 Picture.

[0118] Reference Figure 7 First, a steel material preparation step is performed as step S10 in the same manner as in the first embodiment. In the method for manufacturing a spur gear according to the second embodiment, for example, steel containing 0.22% by mass to 0.26% by mass of carbon, 0.45% by mass to 0.70% by mass of silicon, 0.20% by mass to 0.40% by mass of manganese, 1.70% by mass to 1.90% by mass of chromium, 0.04% by mass to 0.08% by mass of niobium, and the balance consisting of iron and unavoidable impurities can be prepared.

[0119] Next, step S20 is carried out in the same manner as in embodiment 1. Figure 7 and Figure 8 , and the steps S30 to S50 (to Figure 8 Time t 10 Here, in step S40, by making the carburizing time longer than in embodiment 1, a carburized layer 31 having a higher carbon concentration than in embodiment 1 can be formed. Specifically, the carburized layer 31 is formed so that the maximum carbon concentration in the thickness direction is 0.8 mass % or more and 1.2 mass % or less.

[0120] Next, as step S51, a pearlite forming step is performed. In this step, the formed body (spur gear 1) with the carburized layer 31 formed thereon is cooled from a temperature range above the A1 transformation point to a temperature range below the A1 transformation point. As a result, the carburized layer 31 becomes a pearlite structure. Specifically, refer to Figure 8 , at time t 10 to t 21 During this time, the spur gear 1 is cooled from a temperature T1 which is a temperature higher than the A1 transformation point to a temperature T3 which is a temperature lower than the A1 transformation point, and is kept at this temperature until time t 22 The temperature T3 can be set to 650° C., for example.

[0121] Next, as step S52, a carbide and grain refinement step is performed. In this step S52, the formed body (spur gear 1) having the carburized layer 31 formed into a pearlite structure is heated to a temperature range above the A1 transformation point, and then cooled to a temperature range below the A1 transformation point. As a result, the carbides contained in the carburized layer 31 are spheroidized and the grains of the carburized layer 31 are refined. Specifically, referring to Figure 8 , at time t 22 to t 23 During this time, the spur gear 1 is heated from a temperature T3 which is a temperature lower than the A1 phase transition point to a temperature T4 which is a temperature higher than the A1 phase transition point, and is kept at this temperature until time t 24 The temperature T4 can be set to a temperature lower than the temperature T1, for example, it can be set to 810 ° C. As a result, the layered carbides constituting the pearlite structure are cut off, forming a large number of fine carbide nuclei. In addition, by austenitizing the steel structure again at the temperature T4, fine austenite grains are obtained. Then, at the time t 24 to t 25 During this time, the spur gear 1 cools to temperature T3 and maintains it until time t 26 As a result, the nuclei grow due to the precipitated carbides, forming a large number of fine spherical carbides. In addition, a steel structure having fine prior austenite grains 50 can be obtained.

[0122] In this embodiment, at time t 26 to t 27 During this time, the spur gear 1 is heated from temperature T3 to temperature T4 again and maintained until time t 28 As a result, the remaining layered carbides are cut off, and a large number of fine carbide nuclei are further formed. In addition, the structure of the steel is austenitized again at temperature T4, thereby obtaining fine austenite grains. Then, at time t 28 to t 29 During this time, the spur gear 1 cools to temperature T5 and maintains it until time t 30Temperature T5 can be set to a temperature higher than temperature T3 and lower than the A1 transformation point, for example, 700°C. As a result, the precipitated carbides cause the nuclei to grow, further forming a large number of fine spherical carbides. Furthermore, a steel structure having fine prior austenite grains 50 can be obtained.

[0123] Next, as step S60, a quenching step is performed. In this step S60, the formed body (spur gear 1) that has been processed up to step S52 is quenched and hardened. Specifically, refer to Figure 8 , at time t 30 to t 31 The spur gear 1 is heated from temperature T5 to temperature T2. Temperature T2 can be set to a temperature higher than temperature T5 and lower than temperature T1, for example, it can be set to 850°C. Then, at time t 31 to t 32 After being kept at the temperature T2 for a period of time, the spur gear 1 is cooled (quenched) to a temperature below the MS point. Thus, the spur gear 1 is quench-hardened, and the structure of the steel constituting the spur gear 1 becomes a state having a martensite structure.

[0124] In this embodiment, the maximum particle size of the carbides 61 within the carburized layer 31 in a cross section perpendicular to the outer peripheral surface 10, which is the surface of the formed body (spur gear 1), is 1 μm or less. Furthermore, the grain size number of the carburized layer 31 is 12 or greater. Alternatively, the area ratio of the carbides 61 within the carburized layer 31 in a cross section perpendicular to the outer peripheral surface 10 may be set to 1% or more and 10% or less, and the average particle size of the carbides 61 may be set to 1 μm or less.

[0125] Then, as step S70 , a tempering step can be performed in the same manner as in the case of Embodiment 1. Through the above steps, the spur gear 1 of this embodiment can be manufactured.

[0126] (Implementation 3)

[0127] Embodiment 3, another embodiment of the present disclosure, will now be described. The spur gear 1 of Embodiment 3 has essentially the same structure as the spur gear 1 of Embodiment 1, and achieves the same effects. However, the spur gear 1 of Embodiment 3 differs from that of Embodiment 1 primarily in the structure of the carburized layer 31. The differences from Embodiment 1 will be described below.

[0128] Figure 9 : is a schematic cross-sectional view showing the structure of the carburized layer of the spur gear of the third embodiment. Figure 9In the carburized layer 31 of the spur gear 1 of the third embodiment, the maximum carbon concentration in the thickness direction of the carburized layer 31 is 1.1 mass % or more and 1.8 mass % or less. A large amount of carbides 61 (cementite) are dispersed in the carburized layer 31. The cross section of the carburized layer 31 perpendicular to the outer peripheral surface 10 as the surface of the spur gear 1 ( Figure 9 The maximum grain size of the carbide 61 in the cross section (of the carburized layer 31) is, for example, 25 μm or less. Furthermore, the grain size number in the carburized layer 31 is 11 or greater. The steel constituting the spur gear 1 may be, for example, steel containing 0.35% by mass or more of molybdenum, such as JIS standard SCM822H.

[0129] By thus setting the maximum carbon concentration of the carburized layer 31 high and refining the prior austenite grains 50, the spur gear 1 of the third embodiment becomes a mechanical component that achieves both high levels of durability against surface damage (e.g., pitting resistance) and toughness. In the carburized layer 31, the area ratio of carbides 61 in a cross section perpendicular to the outer peripheral surface 10 can be 3% or more and 30% or less.

[0130] Next, a method for manufacturing the spur gear 1 according to the third embodiment will be described. The spur gear 1 according to the third embodiment can be manufactured using essentially the same method as the spur gear 1 according to the second embodiment. However, the method for manufacturing the spur gear 1 according to the third embodiment partially differs from that of the second embodiment in order to obtain the structure of the characteristic carburized layer 31 described above. The differences from the second embodiment will be described below.

[0131] Figure 10 This is a flowchart showing an outline of a method for manufacturing a spur gear according to a third embodiment. Figure 10 This corresponds to implementation 2. Figure 7 Picture.

[0132] Reference Figure 10 First, a steel material preparation step is performed as step S10 in the same manner as in Embodiment 2. In the method for manufacturing a spur gear according to Embodiment 3, a steel material made of, for example, JIS standard SCM822H can be prepared.

[0133] Next, step S20 is performed in the same manner as in Embodiments 1 and 2. Figure 10 and Figure 7 , steps S30 to S51 are performed in the same manner as in the case of embodiment 2. Here, in step S40, by further extending the carburizing time compared to embodiment 2, a carburized layer 31 having a higher carbon concentration than in embodiments 1 and 2 can be formed. Specifically, the carburized layer 31 is formed so that the maximum carbon concentration in the thickness direction is 1.1 mass % or more and 1.8 mass % or less.

[0134] Next, as step S53, a carbide spheroidization step is performed. In this step S53, the formed body (spur gear 1) having the carburized layer 31 formed into a pearlite structure is heated to a temperature range above the A1 transformation point, and then cooled to a temperature range below the A1 transformation point, thereby spheroidizing the carbides contained in the carburized layer 31. Specifically, referring to Figure 8 , and the time t of implementation mode 2 22 to t 30 The heating and cooling are repeated in the same manner. Thus, the layered carbides constituting the pearlite structure are truncated, forming a large number of carbide nuclei, which grow to form a large number of spherical carbides. At this time, the carbon content of the carburized layer 31 is higher than in the second embodiment, so the area ratio of the carbides 61 is higher than in the second embodiment, and a steel structure with a large particle size of the carbides 61 can be obtained. Furthermore, as in the second embodiment, a steel structure with fine prior austenite grains 50 can be obtained.

[0135] Then, as step S60 , a quenching step is performed in the same manner as in the case of Embodiment 2. Thus, the spur gear 1 is quench-hardened, and the structure of the steel constituting the spur gear 1 becomes a state having a martensitic structure.

[0136] In this embodiment, the maximum particle size of the carbides 61 within the carburized layer 31 in a cross section perpendicular to the outer peripheral surface 10, which is the surface of the formed body (spur gear 1), is 25 μm or less. Furthermore, the grain size number of the carburized layer 31 is 11 or greater. 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.

[0137] Then, as step S70 , a tempering step is performed in the same manner as in the cases of Embodiments 1 and 2. Through the above steps, the spur gear 1 of the present embodiment can be manufactured.

[0138] (Implementation 4)

[0139] Embodiment 4, another embodiment of the present disclosure, will now be described. The spur gear 1 of Embodiment 4 has essentially the same structure as the spur gear 1 of Embodiment 1 and achieves the same effects. However, the spur gear 1 of Embodiment 4 differs from Embodiment 1 in that the carburized layer 31 contains nitrogen. The differences from Embodiment 1 will be described below.

[0140] Reference Figure 3In the carburized layer 31 of the spur gear 1 according to the fourth embodiment, the maximum nitrogen concentration in the thickness direction is 0.7% to 1.2% by mass. This allows nitrogen to enter the carburized layer 31, ensuring an appropriate amount of retained austenite, thereby significantly improving pitting corrosion resistance. The maximum retained austenite in the thickness direction of the carburized layer 31 can be 50% to 70% by volume.

[0141] Next, a method for manufacturing the spur gear 1 of the fourth embodiment will be described. The spur gear 1 of the fourth embodiment can be manufactured using essentially the same method as the spur gear 1 of the first embodiment. However, the manufacturing method of the spur gear 1 of the fourth embodiment differs partially from that of the first embodiment in order to introduce nitrogen into the carburized layer 31. The differences from the first embodiment will be described below.

[0142] Figure 11 This is a flowchart showing an outline of a method for manufacturing a spur gear in a fourth embodiment. Figure 12 It is a diagram for explaining the heat treatment performed in the method for manufacturing a spur gear in the fourth embodiment. Figure 11 and Figure 12 This corresponds to the first embodiment. Figure 4 and Figure 5 Picture.

[0143] Reference Figure 11 First, a steel material preparation step is performed as step S10 in the same manner as in the first embodiment. In the method for manufacturing a spur gear according to the fourth embodiment, a steel material made of steel having the same component composition as in the first embodiment is prepared.

[0144] Next, step S20 is carried out in the same manner as in embodiment 1. Figure 11 and Figure 12 , and the steps S30 to S40 (to Figure 12 until time t9).

[0145] Next, as step S54, a nitriding step is performed. In this step S54, the formed body (spur gear 1) is heated in a nitriding gas atmosphere to allow nitrogen to enter the carburized layer 31. Specifically, refer to Figure 12 , from time t9 to t 41 The formed body (spur gear 1) with the carburized layer 31 formed thereon is maintained at a temperature T1 in a nitriding gas atmosphere. For example, a mixed gas of nitrogen (N2) and ammonia (NH3) can be used as the nitriding gas atmosphere. The pressure of the gas atmosphere can be set to 100 kPA or less, for example, 40 kPA. This allows nitrogen to enter the carburized layer 31.

[0146] Then, at time t 41to t 42 During this time, the spur gear 1 cools from temperature T1 to temperature T2. Then, at time t 42 to t 43 Meanwhile, the spur gear 1 is kept at the temperature T2 in the nitriding gas atmosphere. As a result, nitrogen further enters the carburized layer 31 .

[0147] Next, as step S60, a quenching step is performed. In this step S60, the formed body (spur gear 1) that has been processed up to step S54 is quenched and hardened. Specifically, refer to Figure 12 , until time t 43 The spur gear 1, which has been held in a nitriding gas atmosphere until now, is cooled (quenched) to a temperature below the MS point. This causes the spur gear 1 to be quench-hardened, and the structure of the steel constituting the spur gear 1 becomes a martensitic structure.

[0148] Then, as step S70 , a tempering step is performed in the same manner as in the case of Embodiment 1. Through the above steps, the spur gear 1 of this embodiment can be manufactured.

[0149] (Implementation 5)

[0150] Next, as a fifth embodiment, an example in which the mechanical component of the present disclosure is applied to a bevel pinion will be described. Figure 13 It is a schematic perspective view showing the appearance of a bevel pinion gear.

[0151] Reference Figure 13 , the small bevel gear 4 of embodiment 5 includes a shaft portion 5 and a gear portion 6. The shaft portion 5 includes a spline portion 5B on the side of one end portion 5A. The spline portion 5B is a portion in which grooves extending in the axial direction are formed at equal intervals throughout the entire circumference on the outer circumference. The gear portion 6 is connected to the end of the shaft portion 5 on the opposite side of the one end portion 5A. The gear portion 6 has a truncated cone shape. Teeth 6A are formed in a spiral shape throughout the entire circumference in an area corresponding to the outer circumference of the truncated cone (conical surface). The small bevel gear 4 can be used, for example, as a component constituting an axle of a wheel loader or a dump truck as a working machine.

[0152] The bevel pinion 4 of this embodiment is made of the same steel as the spur gear 1 of Embodiments 1 to 4. Like the spur gear 1 of Embodiments 1 to 4, the bevel pinion 4 includes a carburized layer, which is arranged to form the outer peripheral surface (surface) of the gear portion 6 and has a higher carbon concentration than other parts. Like the spur gear 1 of Embodiments 1 to 4, the carburized layer includes a composite oxide film containing magnetite and hematite, which is arranged to form the outer peripheral surface of the gear portion 6 and has a thickness of 1 μm to 25 μm.

[0153] The bevel pinion 4 of this embodiment has the same structure as the above-mentioned embodiments 1 to 4, and is a mechanical component with improved durability by suppressing the occurrence of white layer peeling. The bevel pinion 4 of this embodiment can be manufactured by the same steps as the spur gear 1 of the above-mentioned embodiments 1 to 4.

[0154] Example

[0155] (1) Effect of composite oxide film on white layer peeling

[0156] An experiment was conducted to confirm the effect of the composite oxide film of the present invention in suppressing white layer peeling in the mechanical component. The experimental procedure is as follows.

[0157] First, a molded body having the shape of a test gear was prepared and heat treated in the same manner as steps S30 to S70 described in the first embodiment. Test gears were produced. By varying the conditions of the oxide film formation step ( S30 ), test gears were produced with varying composite oxide film thicknesses between 2 and 8 μm. For comparison, the oxide film formation step ( S30 ) was omitted, resulting in a test gear without a composite oxide film (composite oxide film thickness of 0).

[0158] Then, a gear (large gear) with a larger diameter than the test gear was prepared. The test gear was then meshed with the large gear and driven, with the large gear driven, until pitting occurred on the surface of the test gear. The test gear was rotated at 2000 rpm and the contact pressure between the gears was set at 220-260 kgf / mm. 2 In addition, in order to facilitate the occurrence of white layer, the temperature of the lubricating oil is set higher than the normal use conditions. Then, after the test is completed, the damaged part is studied and the occurrence rate of white layer is calculated. The test results are as follows Figure 14 shown.

[0159] exist Figure 14 In the figure, the horizontal axis corresponds to the thickness of the composite oxide film containing magnetite and hematite. The vertical axis corresponds to the incidence of the white layer. Figure 14 In the test gears without a composite oxide film (composite oxide film thickness of 0), the incidence of white layer peeling was 100%. In contrast, in the test gears with a composite oxide film thickness of 1μm or greater, white layer peeling was clearly suppressed. Specifically, it was found that by setting the composite oxide film thickness to 2μm or greater, the white layer incidence rate was 70% or less, by setting it to 6μm or greater, the white layer incidence rate was 10% or less, and by setting it to 8μm or greater, the white layer incidence rate was 5%. This indicates that the composite oxide film thickness is effective when it is 1μm or greater, and is more preferably 2μm or greater, 6μm or greater, and even more preferably 8μm or greater.

[0160] (2) Confirmation of the composition of the composite oxide film

[0161] An experiment was conducted to confirm the composition of the composite oxide film on the machine component of the present disclosure. The experimental procedure is as follows.

[0162] First, a steel formed body was made and heat-treated by the same steps as steps S30 to S70 described in the first embodiment to prepare samples of the embodiment. On the other hand, for comparison, the steel formed body was subjected to gas carburizing and quenching (sample of the comparative example). The gas carburizing conditions were set to a carburizing time of 180 minutes and a diffusion time of 120 minutes. Then, thin film XRD (X-ray Diffraction) analysis was performed on the samples of the embodiment and the comparative example to identify the oxides contained in the oxide film formed on the surface of the sample.

[0163] Figure 15 It is a graph showing the results of X-ray diffraction analysis of the composite oxide film in the sample of the Example. Figure 16 : is a graph showing the results of X-ray diffraction analysis of an oxide film in a sample of a comparative example formed by gas carburization. Figure 15 In the samples of the examples, it was confirmed that a composite oxide film containing magnetite and hematite was formed. Figure 16 In the sample of the comparative example, the presence of hematite was not confirmed, but the formation of an oxide film consisting of a single phase of magnetite was confirmed.

[0164] (3) Confirmation of the effect of fine carbides

[0165] An experiment was conducted to confirm the superiority of the mechanical component (a mechanical component having fine carbides dispersed in a carburized layer) according to the second embodiment. The experimental procedure is as follows.

[0166] According to the steps S10 to S70 of the above-mentioned embodiment 2, a test gear (Example) was produced in the same manner as in the above (1). For comparison, a test gear (Comparative Example) was produced in which the heat treatment was changed to gas carburizing. The conditions for gas carburizing were set as a carburizing time of 180 minutes and a diffusion time of 120 minutes. Steel A and steel B in Table 1 were used as steels for the test gears constituting the Example and the Comparative Example, respectively. In Table 1, the components other than the components shown are iron and unavoidable impurities. Then, a large gear was prepared in the same manner as in the above (1), and the surface pressure applied to the tooth surface was changed, and the number of meshing times until pitting occurred under each surface pressure was investigated (surface pressure strength test).

[0167]

[0168] Furthermore, tooth root bending fatigue was applied to the test gears 100 of the above-described embodiment and comparative example. Figure 17 This figure explains the tooth root bending fatigue test method. Figure 17 The test device 200 includes a fixed fixture 201 and a swing fixture 202. While the fixed fixture 201 is in contact with one tooth 111 of the test gear 100 of the embodiment and the comparative example, the swing fixture 202 is brought into contact with the other teeth 111 and the swing fixture 202 is swung along the arrow α, thereby repeatedly applying bending stress to the root of the tooth 111. Then, the number of repetitions of stress until the tooth 111 is broken was investigated. The frequency of the swing of the swing fixture 202 was set to 5 Hz, and the load applied to the tooth 111 was set to 100~240 kgf / mm 2 It should be noted that when the number of stress repetitions is 4.0×10 6 If no damage occurs at the next time point, the test is terminated.

[0169] In addition, an impact bending load was applied to the tooth roots of the test gears of the above-mentioned embodiments and comparative examples, and the minimum load that causes breakage was investigated (tooth root bending impact test). The test was carried out at room temperature. In addition, a test piece was produced according to the steps S10 to S70 of the above-mentioned embodiment 2, and a Charpy impact test was carried out (refer to JIS standard Z2242). The shape of the test piece was a 10R notch. In addition, the test was carried out at normal temperature (room temperature) and low temperature (-40°C).

[0170] The test results are described below. Figure 18 is a graph showing the results of the surface pressure strength test. Figure 18 In the figure, the horizontal axis corresponds to the number of tooth engagements, and the vertical axis corresponds to the average surface pressure applied to the tooth surface. Figure 18 It was confirmed that when pitting occurred at the same number of meshing times, the average surface pressure applied to the test gear of the embodiment was higher than the average surface pressure applied to the test gear of the comparative example. 7 The load at which pitting corrosion occurs was 1.28 for the Example, with the Comparative Example being 1.00. This confirms that the mechanical component of Embodiment 2, in which fine carbides are dispersed in the carburized layer, exhibits superior pitting corrosion resistance compared to mechanical components manufactured by conventional gas carburizing.

[0171] Figure 19 This is a graph showing the results of a tooth root bending fatigue test. Figure 19 In the equation, the horizontal axis corresponds to the number of stress repetitions, and the vertical axis corresponds to the tooth root bending stress. Figure 19 In the case of stress repetition number of 4.0×10 6The data points corresponding to the test conditions where no damage occurred at the time point and the test was terminated are marked with arrows. Figure 19 The vertical axis is the number of stress repetitions 4.0×10 6 The curve drawn based on the test results of the comparative example is shown by setting the stress at the point where the corresponding vertical lines intersect as a relative value of 1.00.

[0172] Reference Figure 19 , it was confirmed that when the tooth root was damaged with the same number of repetitions, the tooth root bending stress applied to the test gear of the embodiment was higher than that applied to the test gear of the comparative example. For example, for the gear without damage and with the stress repetition number reaching 4.0×10 6 The bending stress of the Example reached 1.15, with the Comparative Example being 1.00. This confirms that the mechanical component of Embodiment 2, in which fine carbides are dispersed in the carburized layer, exhibits superior durability against tooth root bending stress compared to mechanical components manufactured by conventional gas carburization.

[0173] Figure 20 It is a graph showing the results of the tooth root bending impact test. Figure 20 The breakage load on the vertical axis is expressed as a relative value with the breakage load of the comparative example being set to 1.00. Figure 20 Regarding the breakage load in the tooth root bending impact test, the Example exceeded the Comparative Example by approximately 20%. This confirms that the mechanical component of Embodiment 2, in which fine carbides are dispersed in the carburized layer, is superior in strength to tooth root bending impact loads compared to mechanical components manufactured by conventional gas carburization.

[0174] Figure 21 This is a graph showing the results of the Charpy impact test. Figure 20 The breakage load on the vertical axis is expressed as a relative value with the breakage load of the comparative example being set to 1.00. Figure 21 The fracture load in the Charpy impact test was significantly greater in the Example than in the Comparative Example. This confirms that the mechanical component of Embodiment 2, in which fine carbides are dispersed in the carburized layer, has superior toughness compared to mechanical components manufactured by conventional gas carburization due to the effect of grain refinement.

[0175] It should be noted that, while the above embodiments illustrate the mechanical components disclosed herein as examples of transmissions, final reducers, or spur gears constituting a working machine's swing mechanism, and bevel pinion gears constituting an axle, the mechanical components disclosed herein are not limited thereto. For example, the mechanical components disclosed herein can also be applied to shafts constituting a final reducer or swing mechanism of a working machine, and bevel gears or ring gears constituting an axle.

[0176] It should be understood that the embodiments and examples disclosed herein are illustrative in all respects and are non-restrictive in any respect. The scope of the present invention is not limited by the above description but by the claims, and includes all modifications within the meaning and scope equivalent to the claims.

Claims

1. A mechanical component comprising steel having a martensitic structure, wherein the steel comprises: 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% to 1.90 mass% of chromium, The balance consists of iron and unavoidable impurities. The mechanical component includes a carburized layer that is arranged so as to constitute at least a portion of the surface and has a higher carbon concentration than other portions. The carburized layer includes a composite oxide film that is arranged to constitute the surface, has a thickness of 1 μm to 25 μm, and contains magnetite and hematite.

2. A mechanical component comprising steel having a martensitic structure, wherein the steel contains at least one selected from the following elements: 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, 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% to 0.08 mass% of niobium, The balance consists of iron and unavoidable impurities. The mechanical component includes a carburized layer that is arranged so as to constitute at least a portion of the surface and has a higher carbon concentration than other portions. The carburized layer includes a composite oxide film that is arranged to constitute the surface, has a thickness of 1 μm to 25 μm, and contains magnetite and hematite.

3. The 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 of the carburized layer in the thickness direction is 0.6 mass % or more.

5. The 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, The maximum particle size of carbides in a cross section perpendicular to the surface is 1 μm or less, The crystal grain size number specified in JIS G0551 is 12 or more.

6. The 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, The maximum particle size of carbides in a cross section perpendicular to the surface is 25 μm or less, The crystal grain size number specified in JIS G0551 is 11 or more.

7. The mechanical component according to claim 1 or 2, wherein: In the carburized layer, The maximum nitrogen concentration in the thickness direction is 0.7 mass % or more and 1.2 mass % or less.

8. The mechanical component according to claim 1 or 2, wherein: The mechanical component is a component constituting a transmission, an axle, a final drive, or a swing mechanism of the working machine.

9. A method for manufacturing a mechanical component, comprising: A step of preparing a steel material comprising: 0.12 mass % to 0.28 mass % carbon, 0.15 mass % to 0.70 mass % silicon, 0.20 mass % to 0.95 mass % manganese, and 0.85 mass % to 1.90 mass % chromium, the balance being iron and unavoidable impurities; a step of forming the steel material into a formed body; forming a composite oxide film containing magnetite and hematite on the surface of the molded body by heating the molded body in an oxidizing gas atmosphere; a step of heating the molded body having the composite oxide film formed thereon in a carburizing gas atmosphere at a pressure of 1 kPA or less to form a carburized layer having a greater carbon concentration than other portions on the surface of the molded body and having a thickness greater than that of the composite oxide film; and A step of quenching and hardening the formed body having the composite oxide film and the carburized layer formed thereon.

10. A method for manufacturing a mechanical component, comprising: A step of preparing a steel material containing at least one element selected from the group consisting of 0.12 mass % to 0.28 mass % of carbon, 0.15 mass % to 0.70 mass % of silicon, 0.20 mass % to 0.95 mass % of manganese, 0.85 mass % to 1.90 mass % of chromium, 0.15 mass % to 0.45 mass % of molybdenum, 0.01 mass % to 2.00 mass % of nickel, and 0.04 mass % to 0.08 mass % of niobium, with the balance being iron and unavoidable impurities; a step of forming the steel material into a formed body; forming a composite oxide film containing magnetite and hematite on the surface of the molded body by heating the molded body in an oxidizing gas atmosphere; a step of heating the molded body having the composite oxide film formed thereon in a carburizing gas atmosphere at a pressure of 1 kPA or less to form a carburized layer having a greater carbon concentration than other portions on the surface of the molded body and having a thickness greater than that of the composite oxide film; and A step of quenching and hardening the formed body having the composite oxide film and the carburized layer formed thereon.

11. The method for manufacturing a mechanical component according to claim 9 or 10, wherein: The step of forming the composite oxide film and the step of forming the carburized layer are alternately repeated a plurality of times.

12. The method for manufacturing a mechanical component according to claim 9 or 10, wherein: In the step of forming the carburized layer, the carburized layer is formed so that the maximum carbon concentration in the thickness direction is 0.8 mass % or more and 1.2 mass % or less. Before the step of quenching and hardening the formed body, the method further comprises: The step of cooling the formed body having the carburized layer formed thereon from a temperature range equal to or higher than the A1 transformation point to a temperature range lower than the A1 transformation point to form a pearlite structure in the carburized layer; as well as The step of heating the formed body having the carburized layer formed with pearlite structure to a temperature range of not less than the A1 transformation point and then cooling the formed body to a temperature range of less than the A1 transformation point, thereby spheroidizing carbides contained in the carburized layer and refining the grains of the carburized layer. After the molded body is quenched and hardened, the maximum particle size of carbides in the carburized layer in a cross section perpendicular to the surface of the molded body is 1 μm or less, and the carburized layer has a grain size number of 12 or greater as specified in JIS G0551.

13. The method for manufacturing a mechanical component according to claim 9 or 10, wherein: In the step of forming the carburized layer, the carburized layer is formed so that the maximum carbon concentration in the thickness direction is 1.1 mass % or more and 1.8 mass % or less. Before the step of quenching and hardening the formed body, the method further comprises: The step of cooling the formed body having the carburized layer formed thereon to a temperature range lower than the A1 transformation point to form a pearlite structure in the carburized layer; and The step of heating the formed body having the carburized layer formed with pearlite structure to a temperature range of not less than the A1 transformation point and then cooling it to a temperature range of less than the A1 transformation point to spheroidize carbides contained in the carburized layer, After the molded body is quenched and hardened, the maximum particle size of carbides in the carburized layer in a cross section perpendicular to the surface of the molded body is 25 μm or less, and the grain size number of the carburized layer specified in JIS G0551 is 11 or more.

14. The method for manufacturing a mechanical component according to claim 9 or 10, wherein: Before the step of quenching and hardening the formed body, the method further comprises: A step of heating the formed body in a nitriding atmosphere to allow nitrogen to enter the carburized layer.

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