Carburized parts

Abstract

Provide a carburized component that can obtain high surface fatigue strength and high bending fatigue strength. 【Solution means】The carburized component of the present disclosure has the chemical composition described in the specification, satisfies formula (1), the average grain size number of the prior austenite grains on the surface layer is 11.0 or more, the compressive residual stress on the surface layer is 800 MPa or more, and the lattice constant a of the retained austenite on the surface layer y (Å) satisfies formula (2), the Vickers hardness at a depth of 0.2 mm starting from the surface is HV 0.2 (HV), and when the Vickers hardness at a depth of 1.0 mm starting from the surface is HV 1.0 (HV), the average hardness gradient G defined by formula (3) is -400 HV / mm or more. -11×Si + 4.2×Mn + 12×Cr > 5.0 (1) 200×a y +0.314×Si - 0.240×Mn - 714.400 < 7.000 (2) G = (HV 1.0 -HV 0.2 ) / 0.8 (3)

Description

Technical Field

[0001] The present invention relates to carburized parts.

Background Art

[0002] In mechanical parts used in power sources such as engines of automobiles and industrial machines, electric motors, etc. and power transmission mechanisms, wear occurs due to impact loads and sliding applied. Furthermore, mechanical parts are subjected to bending stress during use. Therefore, mechanical parts for these applications are required to have high surface fatigue strength and high bending fatigue strength. In order to obtain high surface fatigue strength and high bending fatigue strength, carburized parts are often used for mechanical parts used in these applications. Carburized parts are obtained by carburizing steel materials. By the carburizing treatment, a carburized hardened layer is formed on the surface of the carburized parts. This carburized hardened layer provides high surface fatigue strength and high bending fatigue strength. The carburizing treatment includes a carburizing quenching process and a tempering process.

[0003] A technique for increasing the surface fatigue strength and bending fatigue strength of carburized parts has been proposed in Japanese Patent Application Laid-Open No. 2023-163968 (Patent Document 1).

[0004] The carburized parts disclosed in Patent Document 1 include a hardened layer which is a carburized layer and a core part inside the hardened layer, and the chemical composition of the core part is, in mass%, C: 0.10 to 0.25%, Si: 0.60 to 1.20%, Mn: 0.86 to 1.20%, P: 0.030% or less, S: 0.005 to 0.030%, Cr: 1.20 to 1.75%, Al: 0.010 to 0.060%, N: 0.003 to 0.020%, and O: 0.0020% or less, and satisfies formula (1) (76 - 28×Si + 37×Mn + 3×Cr ≤ 90.0), and formula (2) (1.0 ≤ Al / N ≤ 3.0), and the balance consists of Fe and impurities. In this carburized part, furthermore, the average C concentration in the region from the surface to a depth of 50 μm is 0.65% or more, and the average surface hardness at a depth of 50 μm from the surface is 600 HV or more, whereby high surface fatigue strength and high bending fatigue strength are obtained, as described in Patent Document 1. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2023-163968 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, high surface fatigue strength and high bending fatigue strength may be obtained in the carburized part by means other than those proposed in Patent Document 1.

[0007] The purpose of this disclosure is to provide a carburized component that can obtain high surface fatigue strength and high bending fatigue strength. [Means for solving the problem]

[0008] The carburized parts of this disclosure are When the region from the surface to a depth of 0.2 mm starting from the surface is defined as the surface layer, The carburized hardened layer formed on the surface, It comprises a core portion located inside the aforementioned carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.10~0.40%, Si: 0.05~1.00%, Mn: 0.30~1.00%, P: Less than 0.030% S: Less than 0.030% Cr: 0.80~2.50%, Al: 0.010~0.100%, N: 0.003~0.030%, and, Contains O: 0.0030% or less, The remainder consists of Fe and impurities. Satisfying equation (1), The average particle size number of the prior austenite grains in the aforementioned surface layer is 11.0 or higher. The compressive residual stress in the aforementioned surface layer is 800 MPa or more. The lattice constant a of retained austenite in the aforementioned surface layer y (Å) satisfies equation (2), The Vickers hardness at a depth of 0.2 mm from the aforementioned surface is HV 0.2 (HV) is defined as the Vickers hardness at a depth of 1.0 mm from the surface. 1.0 When (HV) is used, the mean hardness gradient G defined by equation (3) is -400HV / mm or greater. -11×Si + 4.2×Mn + 12×Cr > 5.0 (1) 200 × a y +0.314 × Si - 0.240 × Mn - 714.400 < 7.000 (2) G=(HV 1.0 -HV 0.2 ) / 0.8 (3) Here, the content (mass%) of the corresponding element is substituted for each element symbol in equations (1) and (2).

[0009] The carburized parts of this disclosure are When the region from the surface to a depth of 0.2 mm starting from the surface is defined as the surface layer, The carburized hardened layer formed on the surface, It comprises a core portion located inside the aforementioned carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.10~0.40%, Si: 0.05~1.00%, Mn: 0.30~1.00%, P: Less than 0.030% S: Less than 0.030% Cr: 0.80~2.50%, Al: 0.010~0.100%, N: 0.003~0.030%, and, Contains O: 0.0030% or less, Furthermore, it contains one or more substances selected from the groups consisting of Group 1 to Group 3, The remainder consists of Fe and impurities. Satisfying equation (1), The average grain size number of the prior austenite grains in the surface layer is 11.0 or more, the compressive residual stress in the surface layer is 800 MPa or more, the lattice constant a of the retained austenite in the surface layer y (Å) satisfies the formula (2),<00001​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​​

[0011] [Figure 1] Figure 1 is a schematic diagram showing the relationship between the strength distribution from the surface to the depth direction of a carburized component and the shear stress distribution applied to the carburized component during use. [Figure 2] Figure 2 is a side view of a test specimen (small roller test specimen) used for roller pitting fatigue testing in surface fatigue strength evaluation tests. [Figure 3] Figure 3 is a schematic diagram of a roller pitting fatigue test. [Figure 4] Figure 4 is a front view of the large roller test specimen shown in Figure 3. [Figure 5] Figure 5 is a side view of the Ono-type rotary bending fatigue test specimen used in bending fatigue strength evaluation tests. [Modes for carrying out the invention]

[0012] The inventors of this invention investigated carburized parts that can obtain high surface fatigue strength and high bending fatigue strength.

[0013] As described above, increasing the strength of the carburized hardened layer formed on the surface of a carburized part increases the surface fatigue strength and bending fatigue strength of the carburized part. Therefore, the inventors investigated means to increase the strength of the carburized hardened layer from the viewpoint of chemical composition. As a result, the inventors found that the chemical composition of the core of the carburized part, in mass%, is C: 0.10~0.40%, Si: 0.05~1.00%, Mn: 0.30~1.00%, P: less than 0.030%, S: less than 0.030%, Cr: 0.80~2.50%, Al: 0.010~0.100%, N: 0.003~0.030%, O: 0.0030% or less, Mo: 0~0.80%, Ti: 0~0. It was hypothesized that if a material containing 100% of the following elements—Nb: 0-0.100%, V: 0-0.50%, B: 0-0.0100%, Cu: 0-0.50%, Ni: 0-0.50%, Sn: 0-0.015%, Ca: 0-0.0100%, and Mg: 0-0.0100% or less, with the remainder being Fe and impurities, and satisfying the following formula (1), then high surface fatigue strength and high bending fatigue strength can be obtained in carburized parts. -11×Si + 4.2×Mn + 12×Cr > 5.0 (1) Here, the content (mass%) of the corresponding element is substituted for each element symbol in equation (1).

[0014] Equation (1) is a formula for improving carburizability in the carburizing process. Mn and Cr enhance carburizability. On the other hand, if Si is present in excessive amounts, it reduces carburizability. Therefore, by appropriately adjusting the Mn content, Cr content, and Si content, carburizability can be improved. If the core of the carburized part satisfies the above chemical composition and equation (1), the strength of the carburized hardened layer of the carburized part will increase. However, even when the strength of the carburized hardened layer was sufficient, high surface fatigue strength and high bending fatigue strength could not be obtained in carburized parts. Therefore, the inventors investigated the reasons why high surface fatigue strength and high bending fatigue strength could not be obtained in carburized parts. As a result, the inventors obtained the following findings.

[0015] The inventors focused on the microstructure of carburized parts. They found that in carburized parts that did not achieve high surface fatigue strength and high bending fatigue strength, cracks were occurring and propagating along the prior austenite grain boundaries in the carburized hardened layer. The inventors hypothesized that by refining the prior austenite grains in the carburized hardened layer of carburized parts, stress concentration at the prior austenite grain boundaries could be reduced, potentially suppressing the occurrence and propagation of cracks due to surface fatigue and bending fatigue. Here, in carburized parts whose core satisfies the above-mentioned chemical composition and formula (1), a carburized hardened layer is formed in a region at least from the surface of the carburized part to a depth of 0.2 mm. Therefore, the inventors hypothesized that if the average grain size number of the prior austenite grains in the region from the surface of the carburized part to a depth of 0.2 mm is 11.0 or higher, high surface fatigue strength and high bending fatigue strength can be obtained in the carburized part. The region from the surface of the carburized part to a depth of 0.2 mm is defined as the surface layer.

[0016] Furthermore, the inventors hypothesized that increasing the compressive residual stress on the surface of a carburized part would increase its surface fatigue strength and bending fatigue strength. When the compressive residual stress on the surface of a carburized part increases, the residual stress acts in a direction that suppresses crack initiation and propagation on the surface. Therefore, the surface fatigue strength and bending fatigue strength of the carburized part increase. Accordingly, the inventors hypothesized that if the compressive residual stress on the surface of a carburized part is 800 MPa or higher, high surface fatigue strength and high bending fatigue strength can be obtained.

[0017] Furthermore, the inventors focused on retained austenite on the surface of carburized parts. Hereinafter, retained austenite will also be referred to as "retained γ". When an external force is applied to a carburized part during use, retained γ undergoes a work-induced transformation into martensite. This increases the hardness of the surface of the carburized part during use. As a result, the surface fatigue strength and bending fatigue strength of the carburized part increase. Therefore, the inventors investigated the factors that contribute to the improvement of surface fatigue strength and bending fatigue strength due to the work-induced transformation of retained γ. As a result, the inventors obtained the following findings.

[0018] As described above, carburizing treatment is performed during the manufacturing process of carburized parts. As a result, the carbon content dissolved in the residual gamma on the surface of the carburized parts increases. The carbon in the residual gamma stabilizes it. Therefore, if the carbon content in the residual gamma increases, processing-induced transformation during use is suppressed. Consequently, in order to allow the residual gamma on the surface to undergo processing-induced transformation appropriately during use, it is necessary to reduce the carbon content in the residual gamma.

[0019] Here, the carbon content in residual γ correlates with the lattice constant of residual γ. When carbon is dissolved in residual γ, it penetrates into the interlattice positions of the fcc structure of residual γ. In this case, the fcc structure of residual γ expands. As a result, the lattice constant of residual γ increases. In other words, if the carbon content in residual γ increases, the lattice constant of residual γ increases. Therefore, the inventors considered that the carbon content in residual γ could be reduced by adjusting the lattice constant of residual γ on the surface of the carburized component.

[0020] Furthermore, the inventors investigated factors influencing the stabilization of residual γ from the viewpoint of chemical composition. As a result, it was found that, from the viewpoint of stabilizing residual γ, Si and Mn in the chemical composition of the core of the carburized part tend to affect the mechanical stability of residual γ. Si tended to inhibit the work-induced transformation of residual γ, while Mn tended to promote the work-induced transformation of residual γ.

[0021] Based on the above findings, the lattice constant of residual γ is a y When (Å), the lattice constant a y The appropriate relationship between Si content and Mn content was investigated. As a result, it was found that high surface fatigue strength can be obtained for carburized parts if the following equation (2) is satisfied. 200 × a y +0.314 × Si - 0.240 × Mn - 714.400 < 7.000 (2) Here, the content (mass%) of the corresponding element is substituted for each element symbol in equation (2). However, even with carburized parts that met the above conditions, high bending fatigue strength was still sometimes not achieved. Therefore, the inventors conducted further investigations and studies. As a result, the inventors obtained the following findings.

[0022] Observation of carburized parts that did not achieve high bending fatigue strength revealed that cracks were occurring and propagating at a depth greater than the surface layer of the carburized part (hereinafter referred to as the depth beyond the surface). The inventors then further investigated the cause of crack occurrence and propagation. The inventors focused on the shear stress applied to the carburized parts during use. Here, shear stress refers to the shear stress component of the Hertz contact stress generated in the depth direction from the contact surface in parts that are in contact with each other, such as gears. Figure 1 is a schematic diagram showing the relationship between the strength (Vickers hardness) distribution from the surface to the depth direction of a carburized part and the shear stress distribution applied to the carburized part during use. Referring to Figure 1, the dashed line S1 is the shear stress distribution applied during use. The solid lines V1 and V2 are the strength distributions in the depth direction of two different types of carburized parts. Region A11 shows the carburized hardened layer of the carburized part.

[0023] In region A11, both the carburized component represented by solid line V1 and the carburized component represented by solid line V2 exhibit strengths higher than the shear stress indicated by dashed line S1. In this case, the carburized hardened layer shown in region A11 suppresses the initiation and propagation of cracks due to shear stress. On the other hand, the strength of the carburized components represented by solid lines V1 and V2 gradually decreases as the depth increases from the carburized hardened layer. In particular, the decrease in strength is more pronounced in the carburized component represented by solid line V1 compared to the carburized component represented by solid line V2.

[0024] Here, the carburized part indicated by the solid line V1 has a depth of D C As you move towards the depth, the strength of the carburized component becomes less than the shear stress. In this case, depth D C Starting from the vicinity, cracks initiate and propagate due to shear stress. As a result, the bending fatigue strength of the carburized part decreases. In other words, in the carburized part represented by the solid line V1, the strength of the carburized hardened layer is sufficient, but the strength in the core is insufficient, and crack initiation and propagation cannot be adequately suppressed.

[0025] On the other hand, in the carburized part represented by the solid line V2, the strength of the carburized part is greater than the shear stress. In this case, the initiation and propagation of cracks due to shear stress are suppressed not only in the carburized hardened layer but also in the core of the carburized part. As a result, the bending fatigue strength of the carburized part is increased. In other words, the inventors considered that if the slope of the decrease in strength from the surface to the depth direction of the carburized part (hardness gradient) is gentle, it may be possible to suppress the initiation and propagation of cracks in the carburized part due to shear stress. Therefore, the inventors investigated the hardness gradient of the carburized part as a means to increase the bending fatigue strength of the carburized part. As a result, the inventors determined that the Vickers hardness at a depth of 0.2 mm starting from the surface of the carburized part is HV 0.2 (HV) is defined as the Vickers hardness at a depth of 1.0 mm from the surface. 1.0 When the hardness is (HV), we found that if the mean hardness gradient G, defined by equation (3) below, is -400HV / mm or higher, then high bending fatigue strength can be obtained in carburized parts. G=(HV 1.0 -HV 0.2 ) / 0.8 (3)

[0026] The carburized component of this embodiment was completed based on the above technical concept and has the following configuration.

[0027] The carburized component of the first configuration is When the region from the surface to a depth of 0.2 mm starting from the surface is defined as the surface layer, The carburized hardened layer formed on the surface, It comprises a core portion located inside the aforementioned carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.10~0.40%, Si: 0.05~1.00%, Mn: 0.30~1.00%, P: Less than 0.030% S: Less than 0.030% Cr: 0.80~2.50%, Al: 0.010~0.100%, N: 0.003~0.030%, and, Contains O: 0.0030% or less, The remainder consists of Fe and impurities. Satisfying equation (1), The average particle size number of the prior austenite grains in the aforementioned surface layer is 11.0 or higher. The compressive residual stress in the aforementioned surface layer is 800 MPa or more. The lattice constant a of retained austenite in the aforementioned surface layer y (Å) satisfies equation (2), The Vickers hardness at a depth of 0.2 mm from the aforementioned surface is HV 0.2 (HV) is defined as the Vickers hardness at a depth of 1.0 mm from the surface. 1.0 When (HV) is used, the mean hardness gradient G defined by equation (3) is -400HV / mm or greater. -11×Si + 4.2×Mn + 12×Cr > 5.0 (1) 200 × a y +0.314 × Si - 0.240 × Mn - 714.400 < 7.000 (2) G=(HV 1.0-HV 0.2 ) / 0.8 (3) Here, the content (mass%) of the corresponding element is substituted for each element symbol in equations (1) and (2).

[0028] The carburized component of the second configuration is When the region from the surface to a depth of 0.2 mm starting from the surface is defined as the surface layer, The carburized hardened layer formed on the surface, It comprises a core portion located inside the aforementioned carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.10~0.40%, Si: 0.05~1.00%, Mn: 0.30~1.00%, P: Less than 0.030% S: Less than 0.030% Cr: 0.80~2.50%, Al: 0.010~0.100%, N: 0.003~0.030%, and, Contains O: 0.0030% or less, Furthermore, it contains one or more substances selected from the groups consisting of Group 1 to Group 3, The remainder consists of Fe and impurities. Satisfying equation (1), The average particle size number of the prior austenite grains in the aforementioned surface layer is 11.0 or higher. The compressive residual stress in the aforementioned surface layer is 800 MPa or more. The lattice constant a of retained austenite in the aforementioned surface layer y (Å) satisfies equation (2), The Vickers hardness at a depth of 0.2 mm from the aforementioned surface is HV 0.2 (HV) is defined as the Vickers hardness at a depth of 1.0 mm from the surface. 1.0 When (HV) is used, the mean hardness gradient G defined by equation (3) is -400HV / mm or greater. [Group 1] Mo: 0.80% or less Ti: 0.100% or less, Nb: 0.100% or less, V: 0.50% or less, B: 0.0100% or less Cu: 0.50% or less, and, Select one or more from the group consisting of Ni: 0.50% or less. [Group 2] Sn: 0.015% or less [Group 3] Ca: 0.0100% or less, and One or more selected from the group consisting of Mg: 0.0100% or less. -11×Si + 4.2×Mn + 12×Cr > 5.0 (1) 200 × a y +0.314 × Si - 0.240 × Mn - 714.400 < 7.000 (2) G=(HV 1.0 -HV 0.2 ) / 0.8 (3) Here, the content (mass%) of the corresponding element is substituted for each element symbol in equations (1) and (2).

[0029] The third component, the carburized part, A carburized component of the second configuration, The chemical composition of the core contains the first group.

[0030] The carburized component of the fourth configuration is, A carburized component having a second or third configuration, The chemical composition of the core contains the second group.

[0031] A fifth component is a carburized part, A carburized component having one of the second to fourth configurations, The chemical composition of the core contains the third group.

[0032] The carburized parts according to this embodiment will be described in detail below. Unless otherwise specified, the "%" in relation to elements refers to "mass%".

[0033] [Configuration of the carburized component in this embodiment] In this embodiment, the carburized part refers to a carburized part that has undergone carburizing treatment. As described above, the carburizing treatment includes a carburizing and quenching process and a tempering process. Furthermore, the carburizing treatment includes both carburizing and carbonitriding. Therefore, in this specification, the carburized hardened layer is a concept that also includes a carbonitriding hardened layer. Note that the carburizing treatment refers to gas carburizing. Furthermore, in this specification, the region from the surface of the carburized part to a depth of 0.2 mm is defined as the surface layer. The carburized part in this embodiment comprises a carburized hardened layer and a core portion located inside the carburized hardened layer.

[0034] The carburized hardened layer is a layer that has hardened due to the penetration of carbon (C) during the carburizing process. The carburized hardened layer is formed from the surface of the carburized part to a predetermined depth, including at least the surface layer. Here, the microstructure of the carburized hardened layer consists mainly of martensite.

[0035] The carbon content of the surface layer of the carburized part in this embodiment is 0.70% or more. The upper limit of the carbon content of the surface layer of the carburized part is not particularly limited, but for example it is 1.20%.

[0036] The core is the portion inside the carburized hardened layer and is an area unaffected by the penetration and diffusion of carbon due to the carburizing process. The chemical composition of the core is the same as that of the steel material used to make the carburized part. The hardness of the core is lower than that of the carburized hardened layer. It is a well-known technical matter to those skilled in the art that the carburized hardened layer and the core can be easily distinguished by contrast through well-known microstructural observation.

[0037] [Method for measuring the carbon content of the surface layer] The carbon content of the surface layer of the carburized hardened layer can be measured by the following method. The carburized part is cut perpendicular to the surface of the carburized hardened layer. The cut surface is mirror-polished. Line analysis based on an electron probe microanalyzer (EPMA) is performed on the mirror-polished cut surface. Specifically, point analysis is performed at 100 measurement points at 2 μm intervals in the depth direction, up to a depth of 0.2 mm from the surface of the steel part, to obtain the C concentration (mass%) at each measurement point. The arithmetic mean of the obtained C concentrations is defined as the C concentration (mass%) of the surface layer of the carburized hardened layer. For the EPMA line analysis, the acceleration voltage is 15 kV, the irradiation current is 400 nA, and the electron beam diameter is 2 μm.

[0038] [Features of the carburized parts in this embodiment] The carburized component of this embodiment has the following features: (Feature 1) The chemical composition of the core is as follows (by mass%): C: 0.10-0.40%, Si: 0.05-1.00%, Mn: 0.30-1.00%, P: less than 0.030%, S: less than 0.030%, Cr: 0.80-2.50%, Al: 0.010-0.100%, N: 0.003-0.030%, O: 0.0030% or less, Mo: 0 It contains approximately 0.80% of the following: Ti: 0-0.100%, Nb: 0-0.100%, V: 0-0.50%, B: 0-0.0100%, Cu: 0-0.50%, Ni: 0-0.50%, Sn: 0-0.015%, Ca: 0-0.0100%, and Mg: 0-0.0100% or less, with the remainder being Fe and impurities. (Feature 2) The equation (1) is satisfied. -11×Si + 4.2×Mn + 12×Cr > 5.0 (1) Here, the content (mass%) of the corresponding element is substituted for each element symbol in equation (1). (Feature 3) The average particle size number of the prior austenite grains in the surface layer is 11.0 or higher. (Feature 4) The compressive residual stress at the surface is 800 MPa or higher. (Feature 5) Lattice constant a of retained austenite in the surface layer y (Å) satisfies equation (2). 200 × a y +0.314 × Si - 0.240 × Mn - 714.400 < 7.000 (2) Here, the content (mass%) of the corresponding element is substituted for each element symbol in equation (2). (Feature 6) The Vickers hardness at a depth of 0.2 mm from the surface is defined as HV. 0.2 (HV) is defined as the Vickers hardness at a depth of 1.0 mm from the surface. 1.0 When (HV) is used, the mean hardness gradient G defined by equation (3) is -400HV / mm or greater. G=(HV 1.0 -HV 0.2 ) / 0.8 (3) Features 1 through 6 are explained below.

[0039] [(Feature 1) Regarding chemical composition] C: 0.10~0.40% Carbon (C) enhances hardenability and increases the surface fatigue strength and bending fatigue strength of carburized parts. If the C content is less than 0.10%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the carbon content exceeds 0.40%, even if the content of other elements is within the range of this embodiment, the hardness of the steel material used for the carburized parts will become excessively high. As a result, the hot workability of the steel material will decrease. Therefore, the C content is 0.10-0.40%. The preferred lower limit for the C content is 0.13%, more preferably 0.18%, and even more preferably 0.20%. The preferred upper limit for the C content is 0.38%, more preferably 0.35%, and even more preferably 0.30%.

[0040] Si: 0.05~1.00% Silicon (Si) enhances hardenability. Furthermore, Si increases the resistance of parts to softening during surface fatigue. As a result, the surface fatigue strength and bending fatigue strength of carburized parts are increased. If the Si content is less than 0.05%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Si content exceeds 1.00%, even if the content of other elements is within the range of this embodiment, it excessively increases the chemical potential of carbon on the steel surface in the gas carburizing temperature range. In this case, the penetration of carbon into the steel is suppressed during the gas carburizing process. As a result, the hardness of the surface layer of the carburized part decreases, and the surface fatigue strength and bending fatigue strength decrease. Furthermore, if the Si content exceeds 1.00%, the hot workability of the steel material used for the carburized part decreases. Therefore, the Si content is 0.05 to 1.00%. The preferred lower limit for the Si content is 0.10%, more preferably 0.15%, and even more preferably 0.20%. The preferred upper limit for the Si content is 0.95%, more preferably 0.90%, and even more preferably 0.80%.

[0041] Mn: 0.30~1.00% Manganese (Mn) enhances hardenability. Furthermore, Mn increases resistance to softening in surface fatigue. As a result, the surface fatigue strength and bending fatigue strength of carburized parts are increased. If the Mn content is less than 0.30%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Mn content exceeds 1.00%, the hot workability of the steel material used for the carburized parts will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Mn content is 0.30-1.00%. The preferred lower limit for the Mn content is 0.35%, more preferably 0.40%, and even more preferably 0.45%. The preferred upper limit for the Mn content is 0.95%, more preferably 0.90%, and even more preferably 0.80%.

[0042] P: Less than 0.030% Phosphorus (P) is an impurity. If the P content is 0.030% or higher, P segregates excessively at the grain boundaries, reducing the grain boundary strength. As a result, even if the content of other elements is within the range of this embodiment, the surface fatigue strength and bending fatigue strength of the carburized parts will decrease. Therefore, the P content is less than 0.030%. A low phosphorus (P) content is preferable. However, excessive reduction of the P content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the P content is greater than 0%, more preferably 0.001%, even more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the P content is 0.025%, more preferably 0.020%, and even more preferably 0.017%.

[0043] S: Less than 0.030% Sulfur (S) is an impurity. If the S content is 0.030% or higher, coarse sulfide inclusions will form. Therefore, even if the content of other elements is within the range of this embodiment, the surface fatigue strength and bending fatigue strength of the carburized part will decrease. Therefore, the sulfur content is less than 0.030%. A low sulfur (S) content is preferable. However, excessive reduction of the S content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the S content is greater than 0%, more preferably 0.001%, and even more preferably 0.002%. The preferred upper limit for the S content is 0.025%, more preferably 0.020%, and even more preferably 0.017%.

[0044] Cr: 0.80~2.50% Chromium (Cr) enhances hardenability. Furthermore, Cr increases the resistance of parts to softening during surface fatigue. As a result, the surface fatigue strength and bending fatigue strength of carburized parts are increased. If the Cr content is less than 0.80%, the above effects cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Cr content exceeds 2.50%, even if the content of other elements is within the range of this embodiment, it excessively lowers the chemical potential of carbon on the steel surface in the gas carburizing temperature range. In this case, carbon penetrates and diffuses excessively into the steel during the gas carburizing process. As a result, a large amount of soft retained austenite is formed on the surface of the carburized part, reducing the bending fatigue strength of the carburized part. Furthermore, if the Cr content exceeds 2.50%, the hot workability of the steel material used for the carburized part decreases. Therefore, the Cr content is between 0.80% and 2.50%. The preferred lower limit of the Cr content is 0.85%, more preferably 0.90%, even more preferably 1.00%, even more preferably 1.10%, even more preferably 1.20%, and even more preferably 1.30%. The preferred upper limit for the Cr content is 2.40%, more preferably 2.30%, more preferably 2.20%, more preferably 2.10%, more preferably 2.00%, more preferably 1.90%, more preferably 1.80%, and more preferably 1.70%.

[0045] Al: 0.010~0.100% Aluminum (Al) combines with nitrogen to form AlN, and the pinning effect suppresses grain coarsening during heating in the carburizing process. As a result, the surface fatigue strength and bending fatigue strength of the carburized parts are increased. If the Al content is less than 0.010%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the Al content exceeds 0.100%, clustered coarse oxides will be formed even if the content of other elements is within the range of this embodiment. These clustered coarse oxides reduce the surface fatigue strength and bending fatigue strength of the carburized parts. Therefore, the Al content is between 0.010% and 0.100%. The preferred lower limit for the Al content is 0.015%, more preferably 0.020%, and even more preferably 0.025%. The preferred upper limit for the Al content is 0.090%, more preferably 0.080%, and even more preferably 0.070%.

[0046] N: 0.003~0.030% Nitrogen (N) strengthens carburized parts by forming nitrides and / or carbonitrides during cooling after hot working in the manufacturing process. As a result, the surface fatigue strength and bending fatigue strength of the carburized parts are increased. If the N content is less than 0.003%, the above effect cannot be fully obtained, even if the content of other elements is within the range of this embodiment. On the other hand, if the N content exceeds 0.030%, the hot workability of the steel material used for the carburized parts will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the N content is 0.003 to 0.030%. The preferred lower limit for the N content is 0.004%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the N content is 0.028%, more preferably 0.026%, and even more preferably 0.020%.

[0047] O: 0.0030% or less Oxygen (O) is an impurity. O combines with other elements to form coarse oxide inclusions. These coarse oxide inclusions become the starting point for fatigue in carburized parts. If the O content exceeds 0.0030%, the surface fatigue strength and bending fatigue strength of the carburized part will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the O content is 0.0030% or less. A low oxygen content is preferable. However, excessive reduction of the oxygen content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the oxygen content is greater than 0%, more preferably 0.0001%, even more preferably 0.0002%, and even more preferably 0.0003%. The preferred upper limit for the O content is 0.0028% or less, more preferably 0.0025%, and even more preferably 0.0020%.

[0048] The remainder of the chemical composition of the carburized part in this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are introduced during the industrial manufacture of the carburized part from raw materials such as ore, scrap, or the manufacturing environment, and are acceptable within a range that does not adversely affect the carburized part in this embodiment.

[0049] [About Optional Elements] The chemical composition of the carburized component in this embodiment may further contain one or more elements selected from the group consisting of Groups 1 to 3 in place of a portion of Fe. [Group 1] Mo: 0.80% or less Ti: 0.100% or less, Nb: 0.100% or less, V: 0.50% or less, B: 0.0100% or less Cu: 0.50% or less, and, Select one or more from the group consisting of Ni: 0.50% or less. [Group 2] Sn: 0.015% or less [Group 3] Ca: 0.0100% or less, and One or more selected from the group consisting of Mg: 0.0100% or less. The following describes these arbitrary elements.

[0050] [Group 1: Mo, Ti, Nb, V, B, Cu and Ni] The chemical composition of the core of the carburized part in this embodiment may further include one or more elements selected from the group consisting of Mo, Ti, Nb, V, B, Cu, and Ni, instead of a portion of Fe. These elements are arbitrary and all increase the strength of the carburized part.

[0051] Mo: 0.80% or less Molybdenum (Mo) is an optional element and does not need to be included. In other words, the Mo content may be 0%. When present, i.e., when the Mo content is greater than 0%, Mo enhances hardenability and increases the strength of carburized parts. Even a small amount of Mo will provide some of the above effects. However, if the Mo content exceeds 0.80%, the hardness of the steel material used for the carburized parts becomes excessively high. As a result, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material decreases. Therefore, the Mo content is between 0 and 0.80%, and if present, the Mo content is 0.80% or less. The preferred lower limit for the Mo content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit for the Mo content is 0.75%, more preferably 0.70%, and even more preferably 0.65%.

[0052] Ti:0.100% or less Titanium (Ti) is an optional element and does not need to be included. In other words, the Ti content may be 0%. If Ti is present, that is, if the Ti content is greater than 0%, the Ti forms Ti precipitates such as Ti carbides and Ti carbonitrides. These Ti precipitates suppress grain coarsening during carburizing due to their pinning effect. Therefore, they increase the strength of the carburized parts. Even if only a small amount of Ti is present, the above effect can be obtained to some extent. However, if the Ti content exceeds 0.100%, the Ti precipitates become coarser. In this case, the coarsening of the crystal grains during the carburizing process cannot be sufficiently suppressed. Therefore, even if the content of other elements is within the range of this embodiment, the surface fatigue strength and bending fatigue strength of the carburized part will decrease. Therefore, the Ti content is between 0 and 0.100%, and if present, the Ti content is 0.100% or less. The preferred lower limit of the Ti content is 0.001%, more preferably 0.010%, and even more preferably 0.020%. The preferred upper limit for the Ti content is 0.090%, more preferably 0.085%, and even more preferably 0.080%.

[0053] Nb: 0.100% or less Niobium (Nb) is an optional element and does not need to be included. In other words, the Nb content may be 0%. If Nb is present, that is, if the Nb content is greater than 0%, the Nb forms Nb precipitates such as Nb carbides and Nb carbonitrides. These Nb precipitates suppress grain coarsening during carburizing due to their pinning effect. Therefore, they increase the strength of the carburized parts. Even if only a small amount of Nb is present, the above effect can be obtained to some extent. However, if the Nb content exceeds 0.100%, the Nb precipitates become coarser. In this case, the coarsening of the crystal grains during the carburizing process cannot be sufficiently suppressed. Therefore, even if the content of other elements is within the range of this embodiment, the surface fatigue strength and bending fatigue strength of the carburized part will decrease. Therefore, the Nb content is between 0 and 0.100%, and if present, the Nb content is 0.100% or less. The preferred lower limit of the Nb content is 0.001%, more preferably 0.010%, and even more preferably 0.020%. The preferred upper limit for the Nb content is 0.090%, more preferably 0.085%, and even more preferably 0.080%.

[0054] V: 0.50% or less Vanadium (V) is an optional element and may not be present. In other words, the V content may be 0%. When V is present, that is, when the V content is greater than 0%, V forms V precipitates such as V carbides and V carbonitrides. These V precipitates suppress grain coarsening during carburizing due to their pinning effect. Therefore, they increase the strength of the carburized parts. Even if only a small amount of V is present, the above effect can be obtained to some extent. However, if the V content exceeds 0.50%, the hardness of the steel material used for the carburized parts becomes excessively high. As a result, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material decreases. Therefore, the V content is 0-0.50%, and if present, the V content is 0.50% or less. The preferred lower limit for the V content is 0.01%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit for the V content is 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[0055] B: 0.0100% or less Boron (B) is an optional element and does not need to be included. In other words, the B content may be 0%. When present, i.e., when the B content is greater than 0%, B enhances the hardenability of the carburized parts. Furthermore, B suppresses grain boundary segregation of P. As a result, the strength of the carburized parts increases. Even a small amount of B present will provide some of the above effects. However, if the B content exceeds 0.0100%, the hardness of the steel material used for the carburized parts becomes excessively high. As a result, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material decreases. Therefore, the B content is between 0 and 0.0100%, and if present, the B content is 0.0100% or less. The preferred lower limit for the B content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0005%. The preferred upper limit for the B content is 0.0095%, more preferably 0.0090%, and even more preferably 0.0085%.

[0056] Cu: 0.50% or less Copper (Cu) is an optional element and does not need to be included. In other words, the Cu content may be 0%. When present, i.e., when the Cu content is greater than 0%, Cu enhances hardenability and increases the strength of carburized parts. Even a small amount of Cu will provide some degree of the above effect. However, if the Cu content exceeds 0.50%, the hardness of the steel material used for the carburized parts becomes excessively high. As a result, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material decreases. Therefore, the Cu content is between 0 and 0.50%, and if present, the Cu content is 0.50% or less. The preferred lower limit for the Cu content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit for the Cu content is 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[0057] Ni: 0.50% or less Nickel (Ni) is an optional element and does not need to be included. In other words, the Ni content may be 0%. When present, i.e., when the Ni content is greater than 0%, Ni enhances hardenability and increases the strength of carburized parts. Even a small amount of Ni will provide some of the above effects. However, if the Ni content exceeds 0.50%, the hardness of the steel material used for the carburized parts becomes excessively high. As a result, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material decreases. Therefore, the Ni content is between 0 and 0.50%, and if present, the Ni content is 0.50% or less. The preferred lower limit for the Ni content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit for the Ni content is 0.45%, more preferably 0.40%, and even more preferably 0.35%.

[0058] [Group 2: Sn] The chemical composition of the core of the carburized component in this embodiment may further include Sn instead of some of the Fe.

[0059] Sn: 0.015% or less Tin (Sn) is an optional element and does not need to be included. In other words, the Sn content may be 0%. If present, i.e., if the Sn content is greater than 0%, Sn improves the machinability of the steel material used for carburized parts. Even a small amount of Sn will provide some degree of the above effect. However, if the Sn content exceeds 0.015%, the hardness of the steel material used for the carburized parts becomes excessively high. As a result, even if the content of other elements is within the range of this embodiment, the hot workability of the steel material decreases. Therefore, the Sn content is 0-0.015%, and if present, the Sn content is 0.015% or less. The preferred lower limit for the Sn content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the Sn content is 0.013%, more preferably 0.010%, and even more preferably 0.008%.

[0060] [Group 3: Ca and Mg] The chemical composition of the core of the carburized part in this embodiment may further include one or more elements selected from the group consisting of Ca and Mg, instead of a portion of Fe. These elements are arbitrary and all improve the hot workability of the steel material used to make the carburized part.

[0061] Ca:0.0100% or less Calcium (Ca) is an optional element and does not need to be included. In other words, the Ca content may be 0%. When calcium is present, i.e., when the calcium content is greater than 0%, the calcium neutralizes sulfur in the steel by fixing it as sulfide, thereby improving the hot workability of the steel. Even a small amount of calcium can provide the above effect to some extent. However, if the Ca content exceeds 0.0100%, the oxides in the steel become coarser. Therefore, even if the content of other elements is within the range of this embodiment, the surface fatigue strength and bending fatigue strength of the carburized parts will decrease. Therefore, the Ca content is between 0 and 0.0100%, and if present, the Ca content is 0.0100% or less. The preferred lower limit for the Ca content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%. The preferred upper limit for the Ca content is 0.0095%, more preferably 0.0090%, and even more preferably 0.0085%.

[0062] Mg: 0.0100% or less Magnesium (Mg) is an optional element and does not need to be included. In other words, the Mg content may be 0%. When magnesium is present, i.e., when the Mg content is greater than 0%, the Mg neutralizes the sulfur in the steel by fixing it as sulfides, thereby improving the hot workability of the steel. Even a small amount of Mg can provide some degree of the above effect. However, if the Mg content exceeds 0.0100%, the oxides in the steel become coarser. Therefore, even if the content of other elements is within the range of this embodiment, the surface fatigue strength and bending fatigue strength of the carburized parts will decrease. Therefore, the Mg content is between 0 and 0.0100%, and if present, the Mg content is 0.0100% or less. The preferred lower limit of the Mg content is 0.0001%, more preferably 0.0002%, and even more preferably 0.0003%. The preferred upper limit for the Mg content is 0.0095%, more preferably 0.0090%, and even more preferably 0.0085%.

[0063] [Regarding (Feature 2) Equation (1)] The carburized component of this embodiment further satisfies formula (1). -11×Si + 4.2×Mn + 12×Cr > 5.0 (1) Here, the content (mass%) of the corresponding element is substituted for each element symbol in equation (1).

[0064] Fn1 is defined as follows: Fn1 = -11 × Si + 4.2 × Mn + 12 × Cr

[0065] Fn1 is an index for enhancing the carburizing properties in the core chemical composition of a carburized part that satisfies characteristic 1. As described above, a carburized hardened layer is formed on the surface of the carburized part. If the carburizing properties of the carburized part are increased, the strength of the carburized hardened layer of the carburized part increases. Here, Mn and Cr are elements that enhance carburizing properties. On the other hand, Si is an element that reduces carburizing properties when present in excess. Therefore, if the relationship between the content of Si, Mn, and Cr is appropriate, the carburizing properties will increase. The relationship between the content of Si, Mn, and Cr is defined by Fn1. If the value of Fn1 is appropriate, the surface fatigue strength and bending fatigue strength of the carburized part will increase.

[0066] If Fn1 is 5.0 or less, the carburizing properties of the carburized part will not be sufficient. As a result, the strength of the carburized hardened layer of the carburized part will decrease. Consequently, high surface fatigue strength and high bending fatigue strength cannot be obtained for the carburized part.

[0067] If Fn1 exceeds 5.0, the carburizing properties of the carburized part increase. As a result, the strength of the carburized hardened layer of the carburized part increases. Consequently, assuming that features 1 and 3 to 6 are satisfied, high surface fatigue strength and high bending fatigue strength can be obtained in the carburized part.

[0068] A preferred lower limit for Fn1 is 6.0, more preferably 7.0, more preferably 8.0, more preferably 9.0, more preferably 10.0, more preferably 12.0, and more preferably 14.0. The upper limit of Fn1 is not particularly limited. If the chemical composition of the core of the carburized component satisfies feature 1, the upper limit of Fn1 is, for example, 28.0. Fn1 is the value obtained by rounding the second decimal place of the calculated number to the nearest tenth.

[0069] [(Feature 3) Regarding the average particle size number of the old austenite grains in the surface layer] The carburized component of this embodiment further has an average particle size number of prior austenite grains on the surface of 11.0 or higher. In this specification, this average particle size number is also referred to as "average particle size number GN".

[0070] As mentioned above, crack initiation and propagation can occur at the prior austenite grain boundaries on the surface of carburized parts. In this case, the surface fatigue strength and bending fatigue strength of the carburized parts decrease. By refining the prior austenite grains on the surface of the carburized parts, crack initiation and propagation at the prior austenite grain boundaries can be suppressed. As a result, the surface fatigue strength and bending fatigue strength of the carburized parts increase.

[0071] If the average grain size number GN is less than 11.0, the prior austenite grains on the surface of the carburized part have not been sufficiently refined. Therefore, the initiation and propagation of cracks at the prior austenite grain boundaries on the surface of the carburized part cannot be suppressed. As a result, high surface fatigue strength cannot be obtained for the carburized part.

[0072] If the average grain size number GN is 11.0 or higher, the prior austenite grains on the surface of the carburized part will be of an appropriate size. Therefore, the initiation and propagation of cracks at the prior austenite grain boundaries on the surface of the carburized part can be suppressed. As a result, assuming that features 1, 2, and 4 to 6 are satisfied, high surface fatigue strength and high bending fatigue strength can be obtained in the carburized part.

[0073] The preferred lower limit for the average particle size number GN is 11.2, more preferably 11.5, and even more preferably 12.0. There is no particular upper limit to the average particle size number GN. Considering typical industrial production, it is, for example, 12.9, more preferably 12.7, and even more preferably 12.5. Note that the average particle size number GN is the value obtained by rounding the second decimal place of the calculated value to the first decimal place.

[0074] [Method for measuring average particle size number GN] In this embodiment, the average particle size number GN of prior austenite grains in the surface layer can be determined by the following method. A test specimen was taken from the carburized component, including the region from the surface down to a depth of 0.2 mm. The size of the test specimen was not particularly limited, as long as the region from the surface down to a depth of 0.2 mm was included in the observation area. After mirror polishing the observation area, etching was performed with a saturated picric acid aqueous solution. In the region from the surface down to a depth of 0.2 mm of the etched observation area, 10 arbitrary fields of view were observed with an optical microscope (400x magnification) to identify the prior austenite grains. Each measurement field of view was 200 μm × 200 μm. For the identified prior austenite grains, the grain size numbers of the 10 fields of view were determined by the sectioning method in accordance with JIS G 0551 (2020), and their arithmetic mean was calculated.

[0075] [(Feature 4) Regarding compressive residual stress in the surface layer] The carburized component of this embodiment further has a compressive residual stress of 800 MPa or more at the surface.

[0076] As described above, if the compressive residual stress on the surface of a carburized component increases, the residual stress acts in a direction that suppresses the initiation and propagation of cracks on the surface. In this case, the surface fatigue strength and bending fatigue strength of the carburized component increase.

[0077] If the compressive residual stress is less than 800 MPa, the compressive residual stress at the surface is too low. Therefore, the residual stress does not act in a way that suppresses crack initiation and propagation at the surface. As a result, high surface fatigue strength cannot be obtained in carburized parts.

[0078] If the compressive residual stress is 800 MPa or higher, the compressive residual stress in the surface layer is sufficient. Therefore, crack initiation and propagation in the surface layer can be suppressed. As a result, assuming that features 1 to 3, 5 and 6 are satisfied, high surface fatigue strength and high bending fatigue strength can be obtained in the carburized parts.

[0079] The preferred lower limit of the compressive residual stress is 820 MPa, more preferably 850 MPa, and even more preferably 880 MPa. The upper limit of compressive residual stress is not particularly limited. Considering typical industrial production, it is, for example, 1200 MPa, more preferably 1150 MPa, and even more preferably 1100 MPa. The compressive residual stress is an integer value obtained by rounding the calculated value to the first decimal place.

[0080] [Method for measuring compressive residual stress] In this embodiment, the compressive residual stress (MPa) at the surface can be determined by the following method. A test specimen is taken from an arbitrary surface location of the carburized component, with a cross-section including the depth direction from that surface. The compressive residual stress is measured on the test specimen by X-ray diffraction. The AutoMATE micro-area X-ray stress analyzer manufactured by Rigaku Corporation is used as the X-ray diffractometer. A Cr tube is used as the light source. The tube voltage is 40kV, the tube current is 40mA, and the collimator diameter is 1.0mm. The compressive residual stress is measured at each depth position at 0.05 mm intervals from the surface of the specimen. The compressive residual stress is measured at each depth position at 0.05 mm intervals from the surface of the specimen down to a depth of 0.20 mm. The residual stress on the surface of the carburized part is determined by X-ray diffraction after irradiation with X-rays without electropolishing. The residual stress at depths of 0.05 mm, 0.10 mm, 0.15 mm, and 0.20 mm from the surface is determined by electropolishing to expose the measurement surface at each depth position, and then determined by X-ray diffraction. The arithmetic mean of the compressive residual stresses obtained at each measurement point is taken as the compressive residual stress at the surface (MPa).

[0081] [Regarding (Feature 5) Equation (2)] The carburized component of this embodiment further has a lattice constant a of retained austenite on the surface. y (Å) satisfies equation (2). 200 × a y +0.314 × Si - 0.240 × Mn - 714.400 < 7.000 (2) Here, the content (mass%) of the corresponding element is substituted for each element symbol in equation (2).

[0082] Fn2 is defined as follows: Fn2 = 200 × a y +0.314 × Si - 0.240 × Mn - 714.400

[0083] Fn2 is an index indicating the ease with which residual γ undergoes work-induced transformation on the surface of a carburized part. As mentioned above, when external force is applied during use of a carburized part, residual γ undergoes work-induced transformation into martensite. This increases the hardness of the surface of the carburized part during use. As a result, the surface fatigue strength and bending fatigue strength of the carburized part increase.

[0084] The carbon in residual γ stabilizes it. Therefore, by adjusting the carbon content in residual γ to an appropriate range, the stabilization of residual γ is suppressed. Here, the carbon content in residual γ correlates with the lattice constant of residual γ. Therefore, the lattice constant a of residual γ at the surface is... y By adjusting (Å) to an appropriate range, the stabilization of residual γ is suppressed.

[0085] Furthermore, the Si content in the chemical composition of the core of the carburized component stabilizes residual γ. Therefore, limiting the Si content to some extent is effective in making residual γ more susceptible to work-induced transformation. Furthermore, the Mn content in the chemical composition of the core of the carburized component facilitates work-induced transformation of residual γ. Therefore, increasing the Mn content to some extent is effective in making residual γ more susceptible to work-induced transformation. Lattice constant a of residual γ y If the relationship between Si content and Mn content is appropriate, residual γ on the surface becomes more susceptible to processing-induced transformation, increasing the surface fatigue strength and bending fatigue strength of the carburized part.

[0086] If Fn2 is 7.000 or greater, the lattice constant a of the residual γ is... y The relationship between Si content and Mn content is not appropriate. Therefore, residual γ on the surface is less likely to undergo processing-induced transformation. As a result, high surface fatigue strength cannot be obtained in carburized parts.

[0087] If Fn2 is less than 7.000, the lattice constant a of the residual γ yThis results in an appropriate relationship between Si content and Mn content. Consequently, residual γ on the surface becomes more susceptible to processing-induced transformation. As a result, assuming that features 1 to 4 and feature 6 are satisfied, high surface fatigue strength and high bending fatigue strength can be obtained in the carburized parts.

[0088] A preferred upper limit for Fn2 is 6.800, more preferably 6.600, even more preferably 6.400, even more preferably 6.200, even more preferably 6.000, and even more preferably 5.800. The lower limit of Fn2 is not particularly limited. Considering typical industrial production, it is, for example, 2.000, more preferably 2.500, and even more preferably 3.000. Fn2 is the value obtained by rounding the fourth decimal place of the calculated number to the third decimal place.

[0089] [Lattice constant a of residual γ] y [Measurement Method] In this embodiment, the lattice constant a of residual γ in the surface layer y (Å) can be found by the following method. A test specimen is taken from an arbitrary surface location of the carburized component, with a cross-section including the depth direction from that surface. The lattice constant of residual gamma is measured on the test specimen by X-ray diffraction. The AutoMATE micro-area X-ray stress measurement device manufactured by Rigaku Corporation is used as the X-ray diffractometer. A Cr tube is used as the light source. The tube voltage is 40kV, the tube current is 40mA, and the collimator diameter is 1.0mm. The diffraction peak angle θ of the fcc in the (220) plane at each depth position at 0.05 mm intervals from the surface position of the test specimen. hkl The diffraction peak angle θ is determined. hkl Therefore, the lattice constant of the residual γ is determined by equations (4) and (5) below. λ = 2d hkl sinθ hkl (4) d hkl =a y / √( h 2 +k 2 +l 2 ) (5) Here, λ in equation (4) represents the wavelength of the characteristic X-ray. Substitute λ = 2.2897 (Å) for λ, which is the wavelength of the CrKα line. In equation (5), h, k, and l represent the Miller indices (hkl). Substitute (220) for h, k, and l, which are the Miller indices of fcc at each depth position at a 0.05 mm pitch.

[0090] The diffraction peaks of fcc are measured at 0.05 mm intervals from the surface of the test specimen up to 0.20 mm, and the lattice constant of residual γ is calculated. The diffraction peaks on the surface of the carburized part are determined by X-ray diffraction after irradiation with X-rays without electropolishing. The diffraction peaks at depths of 0.05 mm, 0.10 mm, 0.15 mm, and 0.20 mm from the surface are determined by electropolishing to expose the measurement surface at each depth, and then determined by X-ray diffraction. The lattice constant of residual γ is calculated from the diffraction peaks obtained at each measurement point, and the arithmetic mean of these is the lattice constant a of residual γ at the surface. y Let (Å) be the value.

[0091] [(Feature 6) Regarding the average hardness gradient G] The carburized part of this embodiment further has a Vickers hardness of HV at a depth of 0.2 mm from the surface. 0.2 (HV) is defined as the Vickers hardness at a depth of 1.0 mm from the surface. 1.0 When (HV) is used, the mean hardness gradient G defined by equation (3) is -400HV / mm or greater. G=(HV 1.0 -HV 0.2 ) / 0.8 (3)

[0092] The strength of a carburized part in the surface layer (the region from the surface of the carburized part to a depth of 0.2 mm) affects the surface fatigue strength of the carburized part. When a carburized part satisfies the above-mentioned characteristics 1 to 5, the strength of the surface layer increases, and thus the surface fatigue strength increases. On the other hand, the strength tends to decrease more easily in the deeper parts of the carburized part compared to the surface. If the strength in the deeper parts of the carburized part decreases excessively, cracks are more likely to occur and propagate in the deeper parts. In this case, the bending fatigue strength of the carburized part will not increase. If the slope of the decrease in strength from the surface of the carburized part in the depth direction (hardness gradient) is gentle, the occurrence and propagation of cracks in the carburized part due to shear stress can be suppressed. Vickers hardness HV at a depth of 0.2 mm from the surface of the carburized part. 0.2 (HV) and Vickers hardness HV at a depth of 1.0 mm from the surface of the carburized part. 1.0 Based on (HV), the bending fatigue strength of the carburized part is increased by adjusting the required average hardness gradient G.

[0093] If the mean hardness gradient G is less than -400 HV / mm, the Vickers hardness at a depth of 1.0 mm from the surface of the carburized part is too low. As a result, cracks are more likely to occur and propagate in deeper layers of the carburized part than in the surface layer. Consequently, high bending fatigue strength cannot be obtained in the carburized part.

[0094] If the average hardness gradient G is -400 HV / mm or higher, the Vickers hardness at a depth of 1.0 mm from the surface of the carburized part is sufficiently high. Therefore, crack initiation and propagation in the deeper layers of the carburized part are suppressed. As a result, assuming that features 1 to 5 are satisfied, high surface fatigue strength and high bending fatigue strength can be obtained in the carburized part.

[0095] The preferred lower limit of the mean hardness gradient G is -390 HV / mm, more preferably -380 HV / mm, more preferably -370 HV / mm, more preferably -360 HV / mm, more preferably -350 HV / mm, and more preferably -340 HV / mm. The upper limit of the average hardness gradient G is not particularly limited. Considering typical industrial production, it is, for example, -200 HV / mm, more preferably -210 HV / mm, and even more preferably -220 HV / mm. The average hardness gradient G is an integer value obtained by rounding the calculated value to the first decimal place.

[0096] [Method for measuring the average hardness gradient G] In this embodiment, the average hardness gradient G can be determined by the following method. First, in the carburized part, the Vickers hardness HV at a depth of 0.2 mm starting from the surface. 0.2 (HV), and the Vickers hardness at a depth of 1.0 mm from the surface is HV 1.0 (HV) is calculated using the following method. A test specimen is taken from the surface of the carburized component, including a point 0.2 mm deep. The size of the test specimen is not particularly limited, as long as the observation surface includes the point 0.2 mm deep from the surface. Furthermore, a test specimen is taken from the surface of the carburized component, including a point 1.0 mm deep from the surface. The size of the test specimen is not particularly limited, as long as the observation surface includes the point 1.0 mm deep from the surface. A Vickers hardness test will be performed at five arbitrary points at a depth of 0.2 mm from the surface and at five arbitrary points at a depth of 1.0 mm from the surface, in accordance with JIS Z 2244 (2024). The test force will be 0.98 N. The average of the values ​​obtained at the five arbitrary points will be used as the Vickers hardness HV. 0.2 (HV), and Vickers hardness HV 1.0 (HV) is defined as (HV). The obtained Vickers hardness HV 0.2 , and Vickers hardness HV 1.0 Based on this, substitute into equation (3) above to find the mean hardness gradient G.

[0097] [Effects of the carburized parts in this embodiment] The carburized component of this embodiment satisfies features 1 to 6. Therefore, high surface fatigue strength and high bending fatigue strength can be obtained.

[0098] [Applications of the carburized parts of this embodiment] The carburized parts of this embodiment can be widely applied to applications requiring high surface fatigue strength and high bending fatigue strength. They are particularly suitable as gears used in automobiles, construction vehicles, and the like.

[0099] [Manufacturing method for carburized parts] An example of a method for manufacturing the carburized component of this embodiment will be described. The method for manufacturing the carburized component described below is just one example for manufacturing the carburized component of this embodiment. Therefore, a carburized component having the above-described configuration may be manufactured by a manufacturing method other than the one described below. However, the manufacturing method described below is a preferred example of a method for manufacturing the carburized component of this embodiment.

[0100] An example of a method for manufacturing the carburized part of this embodiment includes the following steps. (Process 1) Steel material preparation process (Process 2) Hot working process (Step 3) Carburizing process (Process 4) High-frequency induction hardening process (Step 5) Shot Peening Process The following describes each step.

[0101] [(Process 1) Steel material preparation process] In the steel preparation process, the steel material to be used for the carburized parts of this embodiment is prepared. Specifically, molten steel whose chemical composition satisfies features 1 and 2 is produced. The refining method is not particularly limited, and any well-known method may be used. For example, molten iron produced by a well-known method is refined in a converter (primary refining). A well-known secondary refining is performed on the molten steel tapped from the converter. In secondary refining, alloying elements are added to the molten steel to adjust its composition and produce molten steel having a chemical composition that satisfies features 1 and 2.

[0102] Using the molten steel produced by the refining method described above, a bloom is manufactured by a continuous casting method. The manufactured bloom is then subjected to a well-known hot working method to produce steel material. For example, hot forging may be performed as the hot working method. Alternatively, the billet may be reheated after bract rolling or hot forging, and then finish rolling may be performed on the reheated billet using a continuous rolling mill. The continuous rolling mill is equipped with multiple rolling stands arranged in a row. The heating temperature during hot working is not particularly limited, but is, for example, 1000 to 1300°C. The shape of the steel material produced in this way is, for example, a steel bar or wire rod.

[0103] [(Process 2) Hot working process] In the hot working process, the manufactured steel material is subjected to hot working to produce an intermediate product with a predetermined shape. Hot working is, for example, hot forging. The heating temperature in the hot working process is, for example, 1000 to 1300°C. The intermediate product is then cooled to room temperature after hot working. If necessary, machining may be performed on the intermediate product after hot working. In other words, machining is an optional process.

[0104] [(Step 3) Carburizing process] In the carburizing process, the intermediate product after the soaking process is subjected to carburizing to manufacture carburized parts. The carburizing process includes a carburizing and quenching process and a tempering process. Furthermore, the carburizing and quenching process includes a carburizing process and a diffusion process. In the carburizing and quenching process, the intermediate product is subjected to A in an atmosphere containing a carburizing modified gas with a predetermined carbon potential. c3The material is heated at a heating temperature T1 (°C) above 1.5°C, held for a heating time t1 (hours), and then rapidly cooled. Here, the heating temperature T1 is the heating temperature in the carburizing and diffusion processes, and the heating time t1 is the total time for the carburizing and diffusion processes. In the tempering process, the carburized and quenched intermediate product is held for a predetermined time in a temperature range of, for example, 100 to 200°C. Here, the carbon potential of the carburizing and quenching gas is not particularly limited, but for example, it is 0.80 to 1.10% in the carburizing process and 0.70 to 1.00% in the diffusion process. The carburizing and quenching gas refers to the well-known endothermic quenching gas (RX gas). RX gas is a gas produced by mixing hydrocarbon gases such as butane and propane with air and reacting them by passing them through a heated Ni catalyst, and is a mixed gas containing CO, H2, N2, etc. The carburizing process satisfies the following conditions 1 and 2.

[0105] (Condition 1) The heating temperature T1 (°C) and heating time t1 (hours) of the carburizing process satisfy equation (A). (7.2 × 10 9 ×α×t1) 0.5 >0.35 (A) Here, the following values ​​are substituted for α in equation (A): α = 4.7 × 10 -5 ×exp(-1.55×10 5 / (8.31 × (T1 + 273))) (Condition 2) The average cooling rate v1 from the heating temperature T1 (°C) to 300°C in the carburizing process shall be 6.0°C / second or higher. Conditions 1 and 2 are explained below.

[0106] [Regarding Condition 1] FnA is defined as follows: FnA=(7.2×10 9 ×α×t1) 0.5 FnA is an index for increasing the strength of a carburized part in the deeper layers than the surface. In the carburizing process, as described above, the intermediate product is heated at a heating temperature T1 and held for a heating time t1 in an atmosphere containing a carburizing modified gas with a predetermined carbon potential, and then rapidly cooled to increase the strength of the surface layer of the carburized part. Here, the strength of the deeper layers than the surface of the carburized part is adjusted by adjusting the heating temperature T1 and heating time t1. If the heating temperature T1 is too low and / or the heating time t1 is too short, the strength of the deeper layers than the surface of the carburized part will not increase. Therefore, by adjusting the heating temperature T1 and heating time t1 to an appropriate range, the strength of the deeper layers than the surface of the carburized part can be increased.

[0107] If FnA is 0.35 or less, the heating temperature T1 is too low and / or the heating time t1 is too short. As a result, the strength of the deeper layers of the carburized part does not increase beyond the surface layer. Consequently, even if conditions 2 and 3 described later are met, the average hardness gradient G of the carburized part will be less than -400 HV / mm. Therefore, the FnA value in the carburizing process is greater than 0.35.

[0108] [Regarding Condition 2] As described above, the intermediate product is heated to a heating temperature T1 (°C) and held for a heating time t1 in an atmosphere containing a carburizing modified gas with a predetermined carbon potential, and then rapidly cooled. Here, if the average cooling rate v1 (°C / sec) from the heating temperature T1 to 300°C is excessively slow, carbon tends to become concentrated in the residual γ on the surface of the carburized part. In this case, the lattice constant a of the residual γ is y (Å) becomes excessively large. As a result, the residual γ on the surface becomes stable, and the residual γ is less likely to undergo processing-induced transformation. If the average cooling rate v1 (℃ / sec) increases, the lattice constant a of the residual γ on the surface of the carburized part increases. y This is reduced.

[0109] If the average cooling rate v1 is less than 6.0°C / second, the average cooling rate v1 is too slow. Therefore, the lattice constant a of residual γ on the surface of the carburized part is... yThis becomes excessively large. As a result, even if condition 1 and condition 3 (described later) are met, Fn2 will be 7.000 or higher. Therefore, the average cooling rate v1 in the carburizing process is 6.0°C / second or higher.

[0110] [(Step 4) High-frequency induction hardening process] In the high-frequency induction hardening process, the surface layer of the carburized part is heated to a heating temperature T2 (°C) by high-frequency heating, and then hardened (water-cooled). The heating temperature T2 is A c3 The temperature is set higher than the point. In this case, the surface layer of the carburized part transforms into austenite, and then into a hard structure mainly composed of martensite. As a result, the strength of the carburized part increases.

[0111] High-frequency induction hardening can be carried out using a well-known high-frequency heating device and a well-known cooling device. For example, an annular high-frequency heating device may be used as the high-frequency heating device, and an annular cooling device may be used as the cooling device. Furthermore, the high-frequency induction hardening process may be repeated two or more times. The high-frequency induction hardening process satisfies the following condition 3.

[0112] (Condition 3) In the high-frequency induction hardening process, the heating temperature T2 is set to 830-1100°C.

[0113] [Regarding Condition 3] In the high-frequency induction hardening process, as described above, the surface layer of the carburized part after carburizing treatment is A c3 The temperature is raised above a certain point. After the surface layer of the carburized part transforms into austenite, it transforms into a hard structure mainly composed of martensite. If the heating temperature T2 in the induction hardening process is too high, the crystal grains will grow excessively. In this case, the prior austenite grains on the surface layer of the carburized part after induction hardening will become coarser. If the heating temperature T2 is between 830 and 1100°C, excessive crystal grain growth is suppressed. Therefore, the average grain size number GN of the prior austenite grains on the surface layer of the carburized part after induction hardening will be 11.0 or higher.

[0114] If the heating temperature T2 exceeds 1100°C, the crystal grains will grow excessively. As a result, even if conditions 1 and 2 are met, the average grain size number GN of the prior austenite grains on the surface of the carburized part after induction hardening will be less than 11.0.

[0115] The lower limit of the heating temperature T2 is not particularly limited, but considering typical industrial production, the lower limit of the heating temperature T2 is 830°C. Therefore, the heating temperature T2 in the high-frequency induction hardening process is 830 to 1100°C.

[0116] For carburized parts after the high-frequency induction hardening process, a tempering treatment may be performed. In the tempering treatment, for example, the tempering temperature may be set to 150 to 250°C, and the holding time at the tempering temperature may be set to 30 to 150 minutes.

[0117] [(Step 5) Shot Peening Process] In the shot peening process, shot peening is performed on the surface of the carburized part after the induction hardening process. This imparts compressive residual stress to the surface layer of the carburized part. Shot peening can be carried out using well-known methods. For example, a projectile with a diameter of 0.01 to 1.50 mm is used for shot peening. The projectile can be, for example, cut wire or shot granules with a diameter of 1.00 mm or less, and well-known materials can be used. Preferably, the projection conditions are an arc height of 0.3 mm or more and a coverage of 300% or more. Shot peening can be performed multiple times by changing the projectile and projection conditions. By performing the above shot peening, the compressive residual stress on the surface layer becomes 800 MPa or more.

[0118] Furthermore, finishing processes may be performed on the carburized parts after the shot peening process. Finishing processes include, for example, polishing.

[0119] Through the above process, a carburized part containing features 1 to 6 is manufactured. [Examples]

[0120] Steel materials with the chemical compositions shown in Table 1 (Table 1A and Table 1B) were manufactured for each test number as the material for the carburized parts.

[0121] [Table 1A]

[0122] [Table 1B]

[0123] Blooms were produced by continuous casting using molten steel. The produced blooms were heated and subjected to bloc rolling and subsequent continuous rolling to produce 160 mm x 160 mm billets. The heating temperature of the cast slabs during bloc rolling was 1000°C to 1300°C. Furthermore, the produced billets were heated again and then subjected to finish rolling to produce steel bars (steel bars) with a diameter of 50 mm. The heating temperature of the billets at this time was 1000°C to 1300°C. The produced steel bars were cooled to room temperature in the atmosphere. Through the above manufacturing process, steel bars with a diameter of 50 mm for each test number were produced.

[0124] [Evaluation Test] The following evaluation tests were conducted using the manufactured steel materials. (Test 1) Measurement test of average particle size number GN of old austenite grains (Test 2) Compression Residual Stress Measurement Test (Experiment 3) Lattice constant a of residual γ y Measurement test (Test 4) Average hardness gradient G measurement test (Test 5) Surface fatigue strength evaluation test (Test 6) Bending fatigue strength evaluation test The following describes each test.

[0125] [(Test 1) Measurement test of average particle size number GN of old austenite grains] Simulated carburized parts were manufactured from the steel materials (50mm diameter steel bars) of each test number. Specifically, the steel materials of each test number were heated at a heating temperature of 1200°C for a holding time of 30 minutes. After that, hot working (hot forging) was performed at a finishing temperature of 950°C or higher, and then cooled in the air to obtain 35mm diameter steel bars (intermediate products).

[0126] A carburizing process was performed on intermediate samples of each test number produced. In the carburizing process, carburizing and quenching were performed first. Specifically, in the carburizing and quenching process, the carbon potential in the carburizing process was set to 0.80-1.00%, and the carbon potential in the diffusion process was set to 0.80-0.90%. In the carburizing and quenching process, the intermediate samples were heated at a heating temperature T1 (°C) for a heating time t1 (hours). After that, they were oil-cooled in 60°C oil. Here, the average cooling rate from the heating temperature T1 (°C) to 300°C was the average cooling rate v1. The heating temperature T1, heating time t1, FnA, and average cooling rate v1 are shown in Table 2.

[0127] [Table 2]

[0128] Intermediate samples for each test number, after oil cooling, were tempered at a temperature of 180°C for a holding time of 120 minutes. After the holding time, the intermediate samples were air-cooled. Using the above carburizing method, carburized intermediate samples for each test number were produced.

[0129] The manufactured carburized intermediate product underwent a high-frequency induction hardening process. In the high-frequency induction hardening process, the surface layer of the carburized intermediate product was heated to a heating temperature T2 (°C) using an annular high-frequency heating device. Subsequently, the surface layer of the carburized intermediate product was rapidly cooled to room temperature using an annular cooling device. After the high-frequency induction hardening process, the carburized intermediate product underwent a tempering treatment. In the tempering treatment, the carburized intermediate product was held at 180°C for 120 minutes and then air-cooled to room temperature. The heating temperature T2 is shown in Table 2.

[0130] By the above manufacturing process, simulated carburized parts of test numbers 25 and 26 were manufactured. The simulated carburized parts of test numbers 1 to 24 further underwent a shot peening process. In the shot peening process, a projectile with a diameter of 1.00 mm was used. The projectile was steel shot. The shot peening conditions were set such that the arc height was 0.3 mm or more and the coverage was 300%. When the shot peening process was carried out, the column "Shot Peening Process" in Table 2 was marked as "Performed", and when the shot peening process was not carried out, the column "Shot Peening Process" in Table 2 was marked as "Not Performed".

[0131] In addition, the C content at a depth of 0.2 mm from the surface of the simulated carburized parts of each test number was measured based on the method described in the above [Measurement Method of C Content in the Surface Layer]. As a result, the C content at a depth of 0.2 mm from the surface of the simulated carburized parts of each test number was 0.75 - 0.95% by mass. That is, the simulated carburized parts of each test number had a carburized hardened layer on the surface layer.

[0132] For the simulated carburized parts of each test number, the average grain size number GN of the prior austenite grains was determined based on the method described in [Measurement Method of Average Grain Size Number GN]. The obtained results are shown in the column "Average Grain Size Number GN" in Table 3.

[0133]

Table 3

[0134] [(Test 2) Compressive Residual Stress Measurement Test] From the steel materials (round bars with a diameter of 50 mm) of each test number, simulated carburized parts were manufactured under the same conditions as in the above [(Test 1) Average Grain Size Number GN Measurement Test of Prior Austenite Grains]. Based on the method described in the above [Measurement Method of Compressive Residual Stress], the compressive residual stress (MPa) at the surface layer of the simulated carburized parts of each test number was measured. The obtained results are shown in the column "Compressive Residual Stress (MPa)" in Table 3.

[0135] [(Test 3) Lattice Constant a of Residual γ y Measurement Test] From the steel materials (round bars with a diameter of 50 mm) of each test number, simulated carburized parts were manufactured under the same conditions as the above [(Test 1) Measurement test of the average grain size number GN of old austenite grains]. Based on the method described in the above [Measurement method of the lattice constant a of retained γ y of the simulated carburized parts of each test number was measured. The obtained results are shown in the column of "a y (Å)" in Table 3. y (Å)" in Table 3.

[0136] [(Test 4) Measurement test of the average hardness gradient G From the steel materials (round bars with a diameter of 50 mm) of each test number, simulated carburized parts were manufactured under the same conditions as the above [(Test 1) Measurement test of the average grain size number GN of old austenite grains]. Based on the method described in the above [Measurement method of the average hardness gradient G], the average hardness gradient G of the simulated carburized parts of each test number was measured. The obtained results are shown in the column of "Average hardness gradient G (HV / mm)" in Table 3.

[0137] [(Test 5) Surface fatigue strength evaluation test From the steel materials (round bars with a diameter of 50 mm) of each test number, test pieces (small roller test pieces) for the roller pitching fatigue test for surface fatigue strength evaluation shown in Fig. 2 were processed. The numerical values in Fig. 2 indicate dimensions (unit: mm). "φ" in the figure means diameter.

[0138] Specifically, the steel material for each test number was heated to a heating temperature of 1200°C for a holding time of 30 minutes. After that, it was hot-worked (hot forged) by setting the finishing temperature to 950°C or higher, and then cooled in air to obtain a 35 mm diameter steel bar. The steel bars for each test number that were manufactured were machined (cut) to produce intermediate small roller test pieces. For the intermediate pieces for each test number, a carburizing process and a high-frequency induction hardening process were carried out under the same conditions as in the above-mentioned [(Test 1) Average grain size number GN measurement test of prior austenite grains]. A shot peening process was also carried out on the small roller test pieces for test numbers 1 to 24. On the other hand, the shot peening process was not carried out on the small roller test pieces for test numbers 25 and 26. Six small roller test pieces (simulated carburized parts) were manufactured for each test number under the above conditions.

[0139] The following roller pitting fatigue tests were conducted using the prepared small roller test specimens. Figure 3 is a schematic diagram of the roller pitting fatigue test. As shown in Figure 3, the small roller test piece 200 was rotated while the large roller test piece 100 was pressed against it with the surface pressure described later. The small roller test piece 200 was made using the method described above. The large roller test piece 100 had the shape shown in Figure 4. The numbers in Figure 4 indicate dimensions (in mm). "R150" in the figure indicates that the radius of curvature of the outer surface was 150 mm.

[0140] Large roller test specimen 100 was prepared by the following method. An intermediate product having a chemical composition equivalent to SUJ2 as specified in JIS G 4805 (2019) and the shape shown in Figure 4 was prepared. The intermediate product was subjected to quenching and tempering. For quenching, the intermediate product was held at 840°C for 30 minutes, and then oil-cooled in 130°C oil. For tempering, the intermediate product was held at 160°C for 180 minutes, and then air-cooled. After tempering, the intermediate product was surface-polished to produce the large roller test specimen 100 shown in Figure 4. The diameter of the large roller test specimen 100 was 130 mm.

[0141] In the roller pitting fatigue test, a large roller specimen 100 was pressed against a small roller specimen 200 with surface pressures of various Hertz stresses. The peripheral speed direction of both roller specimens at the contact point was set to the same direction, and the test was conducted by rotating them with a slip ratio of -40% (the peripheral speed at the contact point of the large roller specimen 100 was 40% higher than that of the small roller specimen 200). The oil temperature of the ATF (automatic transmission fluid) supplied as lubricant to the contact point was 90°C, and the maximum surface pressure of the contact stress between the large roller specimen 100 and the small roller specimen 200 was set to 4000 MPa. The number of test cycles was limited to 10 million (10 7 The number of cycles was set as follows: For each test number, six test specimens were subjected to 10 million cycles, and the highest stress at which no pitting occurred was defined as the surface fatigue strength (MPa).

[0142] Based on the obtained surface fatigue strength, the following evaluation was performed. Using steel materials whose chemical composition meets the SCr420 standard described in JIS G 4053 (2023), small roller test specimens (reference specimens) for roller pitting fatigue testing were prepared using a general manufacturing process, namely normalizing, specimen processing, eutectoid carburizing in a gas carburizing furnace, and low-temperature tempering. Six reference specimens were prepared, and the same roller pitting fatigue test as described above was performed using these reference specimens. After 10 million cycles, the highest stress at which pitting did not occur was defined as the surface fatigue strength (MPa) of the reference specimen. If the surface fatigue strength of the small roller test specimen for each test number was 1.20 times or more the surface fatigue strength of the standard test specimen, it was evaluated as "E (Excellent)," indicating that excellent surface fatigue strength was obtained (indicated as "E" in the "Surface Fatigue Strength" column in Table 3). On the other hand, if the surface fatigue strength of the small roller test specimen for each test number was less than 1.20 times the surface fatigue strength of the standard test specimen, it was evaluated as "B (Bad)," indicating that excellent surface fatigue strength was not obtained (indicated as "B" in the "Surface Fatigue Strength" column in Table 3).

[0143] [(Test 6) Bending fatigue strength evaluation test] Ono-type rotary bending fatigue test specimens, simulating carburized parts, were prepared from the steel material (50 mm diameter steel bar) of each test number. The shape of the Ono-type rotary bending fatigue test specimen is shown in Figure 5. The numbers in Figure 5 represent dimensions (in mm). In Figure 5, "φ" means diameter. "R1" means that the radius of curvature of the notch bottom is 1 mm.

[0144] Specifically, the steel material for each test number was heated to a heating temperature of 1200°C for a holding time of 30 minutes. After that, it was hot-worked (hot forged) by finishing at a temperature of 950°C or higher, and then cooled in air to obtain steel bars with a diameter of 35 mm. Intermediate specimens for the Ono-type rotary bending fatigue test were processed by machining (cutting) of the steel bars produced for each test number.

[0145] For the intermediate samples of each test number, a carburizing process and a high-frequency induction hardening process were carried out under the same conditions as in the above-mentioned [(Test 1) Average particle size number GN measurement test of the old austenite grains]. Shot peening was further carried out on the Ono-type rotary bending fatigue test specimens of test numbers 1 to 24. On the other hand, shot peening was not carried out on the Ono-type rotary bending fatigue test specimens of test numbers 25 and 26. Six Ono-type rotary bending fatigue test specimens (simulated carburized parts) were manufactured for each test number under the above conditions.

[0146] Using Ono-type rotary bending fatigue test specimens for each test number, Ono-type rotary bending fatigue tests were performed in accordance with the "Rotational Bending Fatigue Test Method for Metallic Materials" specified in JIS Z 2274:1978. The fatigue tests were conducted by varying the stress applied to each test specimen, and the results were obtained after 10 million cycles (10 7 After repeating the process several times, the highest stress at which fracture did not occur was defined as the bending fatigue strength (MPa). In the Ono rotary bending fatigue test, the rotation speed was set to 3000 rpm, and the stress ratio was set to bidirectional.

[0147] Based on the obtained bending fatigue strength, the following evaluation was performed. Using steel materials whose chemical composition meets the SCr420 standard described in JIS G 4053 (2023), Ono-type rotary bending fatigue test specimens (reference specimens) were prepared using a general manufacturing process, namely normalizing, specimen processing, eutectoid carburizing in a gas carburizing furnace, and low-temperature tempering. Six reference specimens were prepared, and the same Ono-type rotary bending fatigue test as described above was performed using these reference specimens. After 10 million cycles, the highest stress at which fracture did not occur was defined as the bending fatigue strength (MPa) of the reference specimen. If the bending fatigue strength of the Ono-type rotary bending fatigue test specimen for each test number was 1.60 times or more the bending fatigue strength of the reference test specimen, it was evaluated as "E (Excellent)," indicating that excellent bending fatigue strength was obtained (indicated as "E" in the "Bending Fatigue Strength" column in Table 3). On the other hand, if the bending fatigue strength of the Ono-type rotary bending fatigue test specimen for each test number was less than 1.60 times the bending fatigue strength of the reference test specimen, it was evaluated as "B (Bad)," indicating that excellent bending fatigue strength was not obtained (indicated as "B" in the "Bending Fatigue Strength" column in Table 3).

[0148] [Test Results] Referring to Tables 1 (Tables 1A and 1B), 2, and 3, the simulated carburized parts for test numbers 1 to 14 satisfied features 1 to 6. Therefore, the surface fatigue strength was 1.20 times or more that of the reference test specimen, indicating high surface fatigue strength. Furthermore, the bending fatigue strength was 1.60 times or more that of the reference test specimen, indicating high bending fatigue strength.

[0149] On the other hand, in test numbers 15 and 16, Fn1 was too low. As a result, high surface fatigue strength could not be obtained for the carburized parts.

[0150] In tests 17 and 18, Fn2 was too high. As a result, the high surface fatigue strength of the carburized parts could not be obtained.

[0151] In tests 19 and 20, the FnA value during the carburizing process was too low. As a result, the mean hardness gradient G was less than -400 HV / mm. Consequently, high bending fatigue strength could not be obtained in the carburized parts.

[0152] In Tests Nos. 21 and 22, the average cooling rate v1 in the carburizing process was too slow. As a result, Fn2 became 7.000 or more. Consequently, a high surface fatigue strength of the carburized parts could not be obtained.

[0153] In Tests Nos. 23 and 24, the heating temperature T2 in the induction hardening process was too high. As a result, the average grain size number GN became less than 11.0. Consequently, a high surface fatigue strength of the carburized parts could not be obtained.

[0154] In Tests Nos. 25 and 26, the shot peening process was not carried out. As a result, the compressive residual stress became less than 800 MPa. Consequently, a high surface fatigue strength of the carburized parts could not be obtained.

[0155] The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and implemented without departing from the spirit thereof.

Claims

1. Carburized parts, When the region from the surface to a depth of 0.2 mm starting from the surface is defined as the surface layer, The carburized hardened layer formed on the surface, It comprises a core portion located inside the aforementioned carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.10-0.40%, Si: 0.05-1.00%, Mn: 0.30-1.00%, P: Less than 0.030% S: Less than 0.030% Cr: 0.80-2.50%, Al: 0.010-0.100%, N: 0.003–0.030%, and, O: Contains 0.0030% or less, The remainder consists of Fe and impurities. Satisfying equation (1), The average particle size number of the prior austenite grains in the aforementioned surface layer is 11.0 or higher. The compressive residual stress in the aforementioned surface layer is 800 MPa or more. The lattice constant a of retained austenite in the surface layer ｙ (Å) satisfies equation (2), The Vickers hardness at a depth of 0.2 mm from the aforementioned surface is defined as HV ０．２ (HV) is defined as the Vickers hardness at a depth of 1.0 mm from the surface. １．０ When (HV) is used, the average hardness gradient G defined by equation (3) is -400 HV / mm or more. Carburized parts. -11 × Si + 4.2 × Mn + 12 × Cr > 5.0 (1) 200×a ｙ +0.314×Si-0.240×Mn-714.400<7.000 (2) G=(HV) １．０ -HV ０．２ ) / 0.8 (3) Here, the content (mass%) of the corresponding element is substituted for each element symbol in equations (1) and (2).

2. Carburized parts, When the region from the surface to a depth of 0.2 mm starting from the surface is defined as the surface layer, The carburized hardened layer formed on the surface, It comprises a core portion located inside the aforementioned carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.10-0.40%, Si: 0.05-1.00%, Mn: 0.30-1.00%, P: Less than 0.030% S: Less than 0.030% Cr: 0.80-2.50%, Al: 0.010-0.100%, N: 0.003–0.030%, and, O: Contains 0.0030% or less, Furthermore, it contains one or more selected from the groups consisting of Group 1 to Group 3, The remainder consists of Fe and impurities. Satisfying equation (1), The average particle size number of the prior austenite grains in the aforementioned surface layer is 11.0 or higher. The compressive residual stress in the aforementioned surface layer is 800 MPa or more. The lattice constant a of retained austenite in the surface layer ｙ (Å) satisfies equation (2), The Vickers hardness at a depth of 0.2 mm starting from the said surface is HV ０．２ (HV), and the Vickers hardness at a depth of 1.0 mm starting from the said surface is HV １．０ (HV), when the average hardness gradient G defined by formula (3) is not less than -400 HV / mm Carburized parts. [Group 1] Mo: 0.80% or less Ti: 0.100% or less, Nb: 0.100% or less, V: 0.50% or less, B: 0.0100% or less Cu: 0.50% or less, Ni: One or more selected from the group consisting of 0.50% or less. [Group 2] Sn: 0.015% or less [Group 3] Ca: 0.0100% or less, One or more selected from the group consisting of Mg: 0.0100% or less. -11 × Si + 4.2 × Mn + 12 × Cr > 5.0 (1) 200×a ｙ +0.314×Si-0.240×Mn-714.400<7.000 (2) G=(HV) １．０ -HV ０．２ ) / 0.8 (3) Here, the content (mass%) of the corresponding element is substituted for each element symbol in equations (1) and (2).

3. A carburized part according to claim 2, The chemical composition of the core contains the first group, Carburized parts.

4. A carburized part according to claim 2, The chemical composition of the core contains the second group, Carburized parts.

5. A carburized part according to claim 2, The chemical composition of the core contains the third group, Carburized parts.

Images