Carburized parts

Abstract

To provide a carburized component that exhibits high surface fatigue strength and high bending fatigue strength. [Solution] The carburized part of the present disclosure has a core chemical composition in mass%, containing 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, with the remainder being Fe and impurities, satisfying formula (1), the average particle size number of prior austenite grains in the region from the surface of the carburized hardened layer to a depth of 0.2 mm is 11.0-13.0, and when surface analysis of the Cr content is performed on the surface of the oxide film using an electron beam microanalyzer, the area ratio of the region where the Cr content is 4.0 times or more the Cr content of the core is 80% or more in a 2.0 mm × 2.0 mm measurement area. -11×Si + 4.2×Mn + 12×Cr > 5.0 (1)

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 applied and sliding. Further, 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 part. This carburized hardened layer increases the surface fatigue strength and the bending fatigue strength. Note that 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 that 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. The present invention relates to carburized parts.

Prior Art 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 Carburized hardened layer, The oxide film formed on the carburized hardened layer, 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 grain size number of prior austenite grains in the region from the surface of the carburized hardened layer to a depth of 0.2 mm is 11.0 to 13.0. When surface analysis of the Cr content was performed on the surface of the oxide film using an electron beam microanalyzer, the area ratio of the region with a Cr content of 4.0 times or more than the Cr content of the core was 80% or more within a 2.0 mm × 2.0 mm measurement area. -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).

[0009] The carburized parts of this disclosure are Carburized hardened layer, The oxide film formed on the carburized hardened layer, 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 prior austenite grains in the region from the surface of the carburized hardened layer to a depth of 0.2 mm is 11.0 to 13.0. When surface analysis of the Cr content was performed on the surface of the oxide film using an electron beam microanalyzer, the area ratio of the region with a Cr content of 4.0 times or more than the Cr content of the core was 80% or more within a 2.0 mm × 2.0 mm measurement area. [Group 1] Mo: 0.80% or less Ti: 0.100% or less, Nb: 0.100% or less, V: less than 0.50%, and one or more selected from the group consisting of B: less than 0.0100% [Group 2] Cu: less than 0.50% Ni: less than 0.50%, and one or more selected from the group consisting of Sn: less than 0.015% [Group 3] Ca: less than 0.0100%, and one or more selected from the group consisting of Mg: less than 0.0100% -11×Si + 4.2×Mn + 12×Cr > 5.0 (1) Here, the content (mass%) of the corresponding element is substituted into each element symbol in formula (1). [Advantages of the Invention]

[0010] The carburized part of the present disclosure can obtain high surface fatigue strength and high bending fatigue strength. [Brief Description of the Drawings]

[0011] [Figure 1] Figure 1 is a side view of a test piece (small roller test piece) for a roller pitching fatigue test used in a surface fatigue strength evaluation test. [Figure 2] Figure 2 is a schematic diagram of a roller pitching fatigue test. [Figure 3] Figure 3 is a front view of the large roller test piece in Figure 2. [Figure 4] Figure 4 is a side view of a小野式 (Ono type) rotating bending fatigue test piece used in a bending fatigue strength evaluation test. [Modes for Carrying Out the Invention]

[0012] The inventors of the present invention studied carburized parts that can obtain high surface fatigue strength and high bending fatigue strength.

[0013] As described above, the formation of a carburized hardened layer on the surface of a carburized part increases its surface fatigue strength and bending fatigue strength. In other words, the inventors considered that increasing the strength of the carburized hardened layer could potentially increase 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 refining the prior austenite grains in the carburized hardened layer of carburized parts could potentially increase surface fatigue strength and bending fatigue strength. On the other hand, if the prior austenite grains in the carburized hardened layer of carburized parts are excessively refined, the hardenability in the region of the carburized hardened layer decreases. In this case, the refinement of the prior austenite grains suppresses crack propagation. However, the decrease in hardenability in the region of the carburized hardened layer reduces the surface strength of the carburized part. As a result, cracks are more likely to occur on the surface of the carburized part, and the surface fatigue strength of the carburized part decreases. In a carburized component 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 component to a depth of 0.2 mm. Therefore, the inventors considered that by making the prior austenite grains in the region from the surface of the carburized component to a depth of 0.2 mm appropriate in size, high surface fatigue strength and high bending fatigue strength of the carburized component could be obtained.

[0016] Further investigation revealed that if the core of the carburized part satisfies the above-mentioned chemical composition and formula (1), and if the average grain size number of prior austenite grains in the region from the surface of the carburized part to a depth of 0.2 mm is 11.0 to 13.0, then high bending fatigue strength can be obtained in the carburized part. However, the surface fatigue strength of carburized parts was still sometimes insufficient. Therefore, the inventors further investigated and examined means to improve surface fatigue strength.

[0017] Here, in order to increase the surface fatigue strength of carburized parts, the inventors focused on the conformability of the carburized parts. Here, "conformability" refers to the characteristic of parts that come into contact with each other, such as gears, where the surface becomes smooth and friction is reduced when sliding or pressure is applied during contact, resulting in smooth operation. If conformability is increased, excessive pressure can be suppressed from being applied to the surface of the carburized parts. Therefore, it is thought that the surface fatigue strength of the carburized parts will increase. Based on the above considerations, the inventors investigated means to improve the conformability of carburized parts. As a result, it was found that if a chromium-based oxide film (hereinafter referred to as the chromium oxide film) is formed on the surface of the carburized parts, the conformability of the carburized parts is improved. The inventors believe the reason for this is as follows.

[0018] The surface of a carburized part has fine irregularities. When a load is applied to the surface of a carburized part, if the carburized part has high conformability, the surface irregularities will deform and become smooth. In this case, the friction generated on the surface of the carburized part is reduced, and the surface fatigue strength of the carburized part is increased. When a chromium oxide film is formed on the surface of a carburized part, the chromium oxide film is softer than the carburized hardened layer. Therefore, when the carburized part comes into contact with other parts, the surface irregularities of the carburized part are more likely to deform and become smooth, increasing the conformability of the carburized part.

[0019] The inventors quantitatively investigated the Cr oxide film formed on the surface of carburized parts and examined specific means to increase the surface fatigue strength of carburized parts. As a result, the inventors found that when surface analysis of the Cr content was performed on the surface using an electron beam microanalyzer, if the area ratio of the region where the Cr content is 4.0 times or more than the Cr content of the core is 80% or more within a 2.0 mm × 2.0 mm measurement area, a Cr oxide film is formed on the surface of the carburized part, and the surface fatigue strength of the carburized part is increased.

[0020] The carburized component of this embodiment was completed based on the above technical concept and has the following configuration.

[0021] The carburized component of the first configuration is Carburized hardened layer, The oxide film formed on the carburized hardened layer, 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 grain size number of prior austenite grains in the region from the surface of the carburized hardened layer to a depth of 0.2 mm is 11.0 to 13.0. When surface analysis of the Cr content was performed on the surface of the oxide film using an electron beam microanalyzer, the area ratio of the region with a Cr content of 4.0 times or more than the Cr content of the core was 80% or more within a 2.0 mm × 2.0 mm measurement area. -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).

[0022] The carburized component of the second configuration is Carburized hardened layer, The oxide film formed on the carburized hardened layer, 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 prior austenite grains in the region from the surface of the carburized hardened layer to a depth of 0.2 mm is 11.0 to 13.0. When surface analysis of the Cr content was performed on the surface of the oxide film using an electron beam microanalyzer, the area ratio of the region with a Cr content of 4.0 times or more than the Cr content of the core was 80% or more within a 2.0 mm × 2.0 mm measurement area. [Group 1] Mo: 0.80% or less Ti: 0.100% or less, Nb: 0.100% or less, V: 0.50% or less, and, B: Select one or more from the group consisting of 0.0100% or less. [Group 2] Cu: 0.50% or less, Ni: 0.50% or less, and, One or more selected from the group consisting of 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) Here, the content (mass%) of the corresponding element is substituted for each element symbol in equation (1).

[0023] The third component, the carburized part, A carburized component of the second configuration, The chemical composition of the core contains the first group.

[0024] 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.

[0025] 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.

[0026] The carburized parts according to this embodiment will be described in detail below. Unless otherwise specified, the "%" in relation to elements refers to "mass%".

[0027] [Configuration of the carburized component in this embodiment] In this embodiment, the carburized part refers to a carburized part that has undergone carburizing treatment. In this specification, carburizing treatment includes carburizing treatment and carbonitriding treatment. Therefore, in this specification, the carburized hardened layer is a concept that also includes a carbonitriding hardened layer. Note that carburizing treatment refers to gas carburizing treatment. The carburized part in this embodiment comprises a carburized hardened layer, an oxide film formed on the carburized hardened layer, and a core portion located inside the carburized hardened layer. Here, in this specification, the region from the surface of the carburized hardened layer to a depth of 0.2 mm is defined as the surface layer.

[0028] The carburized layer is a layer formed when carbon penetrates the surface of the steel material from the outside during the carburizing process, causing it to harden. The carburized layer is formed from the surface of the carburized layer to a predetermined depth, including at least the surface layer. Here, the microstructure of the carburized layer consists mainly of martensite.

[0029] The carbon content of the surface layer of the carburized part in this embodiment is 0.50% or more. The upper limit of the carbon content of the surface layer of the carburized part is not particularly limited, but is, for example, 1.20%.

[0030] 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.

[0031] [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 on the cut surface at 200 measurement points at 1 μm intervals in the depth direction, up to a depth of 0.2 mm from the surface of the steel part, and the carbon concentration (mass%) is obtained at each measurement point. The arithmetic mean of the obtained carbon concentrations is defined as the carbon concentration (mass%) of the surface layer of the carburized hardened layer. For the EPMA point analysis, the acceleration voltage is 15 kV, the irradiation current is 30 nA, and the electron beam diameter is 1 μm.

[0032] [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 grain size number of prior austenite grains in the region from the surface of the carburized hardened layer to a depth of 0.2 mm is 11.0 to 13.0. (Feature 4) When surface analysis of the Cr content of the oxide film was performed using an electron beam microanalyzer, the area ratio of regions with a Cr content of 4.0 times or more than the Cr content of the core was 80% or more within a 2.0 mm × 2.0 mm measurement area. Features 1 through 4 are explained below.

[0033] [(Feature 1) Regarding chemical composition] The chemical composition of the core of the carburized component in this embodiment contains the following elements:

[0034] C: 0.10~0.40% Carbon (C) increases the strength of carburized parts and enhances their surface fatigue strength and bending fatigue strength. 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 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%.

[0035] Si: 0.05~1.00% Silicon (Si) enhances hardenability and increases the surface fatigue strength and bending fatigue strength of carburized parts. 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%, the 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 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%.

[0036] Mn: 0.30~1.00% Manganese (Mn) enhances hardenability and increases the surface fatigue strength and bending fatigue strength of carburized parts. 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 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%.

[0037] P: Less than 0.030% Phosphorus (P) is an impurity. If the P content is 0.030% or higher, P will excessively segregate 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%.

[0038] S: Less than 0.030% Sulfur (S) is an impurity. If the S content is 0.030% or higher, coarse sulfide inclusions will be formed. 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%.

[0039] Cr: 0.80~2.50% Chromium (Cr) enhances hardenability and increases the surface fatigue strength and bending fatigue strength of carburized parts. 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%, the 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 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%.

[0040] 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%.

[0041] N: 0.003~0.030% Nitrogen (N) combines with Ti, Al, V, and Nb in steel to form nitrides and carbonitrides, 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 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 above effect saturates 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%.

[0042] 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%, more preferably 0.0025%, and even more preferably 0.0020%.

[0043] 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.

[0044] [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, and, B: Select one or more from the group consisting of 0.0100% or less. [Group 2] Cu: 0.50% or less, Ni: 0.50% or less, and, One or more selected from the group consisting of 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. The following describes these arbitrary elements.

[0045] [Group 1: Mo, Ti, Nb, V, and B] 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, and B in place of some of the Fe. These elements are arbitrary and all increase the strength of the carburized part.

[0046] 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 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%.

[0047] 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%.

[0048] 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%.

[0049] 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 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%.

[0050] 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 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%.

[0051] [Group 2: Cu, Ni, and Sn] The chemical composition of the core of the carburized component in this embodiment may further include one or more elements selected from the group consisting of Cu, Ni, and Sn in place of a portion of Fe. These elements are arbitrary and all enhance the corrosion resistance of the carburized component.

[0052] 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 the corrosion resistance 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 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%.

[0053] 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 the corrosion resistance of carburized parts. Even a small amount of Ni will provide some degree of the above effect. 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 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%.

[0054] 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 enhances the corrosion resistance of carburized parts. Even a small amount of Sn present 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 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%.

[0055] [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 workability of the steel material used to make the carburized part.

[0056] 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, that is, when the calcium content is greater than 0%, the calcium neutralizes sulfur in the steel by fixing it as sulfides, thereby improving the 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%.

[0057] 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, that is, when the Mg content is greater than 0%, the Mg neutralizes the sulfur in the steel by fixing it as sulfides, thereby improving the workability of the steel. Even if only a small amount of Mg is present, the above effect can be obtained to some extent. 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%.

[0058] [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).

[0059] Fn1 is defined as follows: Fn1 = -11 × Si + 4.2 × Mn + 12 × Cr

[0060] 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.

[0061] 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.

[0062] 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, 3, and 4 are satisfied, high surface fatigue strength and high bending fatigue strength can be obtained in the carburized part.

[0063] 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 part 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.

[0064] [(Feature 3) Regarding the average particle size number of the prior austenite grains in the surface layer] In this embodiment, the carburized component further has an average particle size number of prior austenite grains of 11.0 to 13.0 in the region from the surface of the carburized hardened layer to a depth of 0.2 mm. In this specification, this average particle size number is also referred to as "average particle size number GN".

[0065] As described above, cracks may initiate and propagate at the prior austenite grain boundaries in the carburized hardened layer of a carburized part. In this case, the surface fatigue strength and bending fatigue strength of the carburized part decrease. By refining the prior austenite grains on the surface of the carburized part, 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 part increase. On the other hand, if the prior austenite grains are excessively refined, the hardenability of the surface of the carburized part decreases. In this case, crack propagation is suppressed by refining the prior austenite grains. However, the decrease in hardenability of the surface of the carburized part reduces the surface strength of the carburized part. As a result, cracks are more likely to occur on the surface of the carburized part, and the surface fatigue strength of the carburized part decreases.

[0066] 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 and high bending fatigue strength cannot be obtained in the carburized part.

[0067] If the average grain size number GN exceeds 13.0, the prior austenite grains on the surface of the carburized part are excessively refined. As a result, the hardenability of the surface of the carburized part decreases. Consequently, high surface fatigue strength cannot be obtained in the carburized part.

[0068] If the average grain size number GN is between 11.0 and 13.0, 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 are satisfied, high surface fatigue strength and high bending fatigue strength can be obtained in the carburized part.

[0069] The preferred lower limit for the average particle size number GN is 11.2, more preferably 11.5, and even more preferably 12.0. The preferred upper limit for the average particle size number GN is 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.

[0070] [Method for measuring average particle size number GN] In this embodiment, the average particle size number 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 to a depth of 0.2 mm within the carburized hardened layer. The size of the test specimen was not particularly limited, as long as the region from the surface 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 to a depth of 0.2 mm of the etched observation area, 10 arbitrary fields of view were observed with an optical microscope (2000x magnification) to identify prior austenite grains. Each measurement field of view was 140 μm × 10⁵ 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.

[0071] [(Feature 4) Regarding the Cr content on the surface of the oxide film] Furthermore, in the carburized component of this embodiment, when surface analysis of the Cr content of the oxide film surface is performed using an electron beam microanalyzer, the area ratio of the region where the Cr content is 4.0 times or more than the Cr content of the core is 80% or more within a 2.0 mm × 2.0 mm measurement area. In this specification, this area ratio is also referred to as the "Cr oxide film area ratio AR".

[0072] As mentioned above, the surface of a carburized part has fine irregularities. When a load is applied to the surface of a carburized part, if the carburized part has high conformability, the surface irregularities will disappear and the surface will become smooth. In this case, the friction generated on the surface of the carburized part is reduced, and the surface fatigue strength of the carburized part increases. When a chromium oxide film is formed on the surface of a carburized part, the strength decreases in the area where the chromium oxide film is formed. As a result, the surface irregularities of the carburized part become easier to remove, and the conformability of the carburized part increases.

[0073] If the Cr oxide film area ratio (AR) is less than 80%, a sufficient Cr oxide film has not formed on the surface of the carburized part. As a result, the conformability of the carburized part decreases. Consequently, high surface fatigue strength cannot be obtained for the carburized part.

[0074] If the Cr oxide film area ratio (AR) is 80% or higher, a Cr oxide film is formed on the surface of the carburized part. This improves the conformability of the carburized part. As a result, assuming that features 1 to 3 are satisfied, high surface fatigue strength and high bending fatigue strength can be obtained in the carburized part.

[0075] The preferred lower limit for the Cr oxide film area ratio AR is 81%, more preferably 83%, and even more preferably 85%. There is no particular upper limit to the Cr oxide film area ratio (AR). Considering typical industrial production, for example, the upper limit of the Cr oxide film area ratio (AR) is 98%.

[0076] [Method for measuring the Cr oxide film area ratio (AR)] In this embodiment, the Cr content (%) on the surface of the oxide film can be determined by the following method. The Cr content on the surface of the oxide film is determined by EPMA. Specifically, a test specimen is taken that includes the surface of the oxide film as the observation surface. The size of the test specimen is not particularly limited as long as it includes a measurement area of ​​2.0 mm × 2.0 mm on the observation surface.

[0077] The Cr content (mass%) is measured on the observation surface of the obtained test specimen using EPMA. Specifically, point analysis is performed at 5 μm intervals in a 2.0 mm × 2.0 mm measurement area on the observation surface corresponding to the surface of the oxide film. The arithmetic mean of the Cr content (mass%) at each measurement point obtained from the 400 × 400 point analysis is defined as the Cr content (mass%) on the surface of the oxide film. The Cr content on the surface of the oxide film is the value obtained by rounding the third decimal place of the obtained value to the second decimal place. From the obtained Cr content (mass%) in the measurement area, the area where the Cr content is 4.0 times or more the Cr content of the core is identified. The Cr oxide film area ratio AR (%) of the area where the Cr content in the measurement area is 4.0 times or more the Cr content of the core is calculated. The Cr oxide film area ratio AR is the integer value obtained by rounding the first decimal place of the obtained value. For EPMA surface analysis, the acceleration voltage is set to 15kV, the irradiation current to 500nA, the electron beam diameter to 5μm, and the measurement pitch to 5μm.

[0078] [Effects of the carburized parts in this embodiment] The carburized component of this embodiment satisfies features 1 to 4. Therefore, high surface fatigue strength and high bending fatigue strength can be obtained.

[0079] [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.

[0080] [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.

[0081] 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) Soaking process (Step 4) Carburizing process (Step 5) High-frequency induction hardening process The following describes each step.

[0082] [(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.

[0083] 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.

[0084] [(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.

[0085] [(Step 3) Soaking process] In the soaking process, the intermediate product after hot working is soaked at a heating temperature T1 (°C) and a heating time t1 (hours). This soaking is carried out in a heat treatment furnace under an air atmosphere. This soaking may also be carried out in a heat treatment furnace where carburizing is performed, provided it is under an air atmosphere. The soaking process satisfies the following conditions:

[0086] (Condition 1) The heating temperature T1 (°C) and heating time t1 (hours) in the soaking process satisfy equation (A). 21300≦T1×(log(t1)+20)×(1+0.05×Si+0.1×Cr) (A) Here, in equation (A), Si is substituted with the Si content (mass%) in the core of the carburized component, and Cr is substituted with the Cr content (mass%) in the core of the carburized component. Also, log means common logarithm. Condition 1 will be explained below.

[0087] [Regarding Condition 1] FnA is defined as follows: FnA=T1×(log(t1)+20)×(1+0.05×Si+0.1×Cr) FnA is an indicator for forming a Cr oxide film on the surface of a carburized part. In the soaking process, the surface of the carburized part is oxidized by heating at a heating temperature T1 and heating time t1 in an atmospheric environment. Here, Si and Cr are components that are easily oxidized by heating in an atmospheric environment. In order to preferentially form a Cr oxide film on the surface of a carburized part, it is necessary to appropriately adjust the relationship between the heating temperature T1 and heating time t1, as well as the Si and Cr content. For this reason, the above-mentioned FnA is defined. By increasing FnA, a sufficient Cr oxide film is formed on the surface of the carburized part.

[0088] If FnA is less than 21300, the surface of the carburized part will not be sufficiently oxidized. As a result, a sufficient Cr oxide film will not form on the surface of the carburized part. Consequently, even if condition 2 described later is met, the Cr oxide film area ratio AR of the carburized part will be less than 80%.

[0089] If the FnA value during the soaking process is 21,300 or higher, a sufficient Cr oxide film will be formed on the surface of the carburized part. As a result, assuming that condition 2 described later is met, the Cr oxide film area ratio AR of the carburized part will be 80% or higher. There is no particular upper limit to the FnA value during the soaking process, but considering normal industrial production, it is 23,000 or lower.

[0090] [(Step 4) Carburizing process] In the carburizing process, a well-known carburizing treatment is performed on the intermediate product after the soaking process 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. c3 After heating and holding above a certain temperature, the material is rapidly cooled. 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 1.00% in the carburizing process and 0.80 to 0.90% 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. Furthermore, a well-known sub-zero treatment may be performed on the intermediate product after carburizing and quenching, or on the carburized part after tempering.

[0091] [(Step 5) 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 c3The 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.

[0092] 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 must satisfy the following conditions.

[0093] (Condition 2) In the high-frequency induction hardening process, the heating temperature T2 is set to 830-1100°C.

[0094] [Regarding Condition 2] 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. This causes the surface layer of the carburized part to transform into austenite, and then 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.

[0095] If the heating temperature T2 is less than 830°C, the crystal grains become excessively refined. As a result, even if condition 1 is met, the average grain size number GN of the prior austenite grains on the surface of the carburized part after induction hardening exceeds 13.0.

[0096] If the heating temperature T2 exceeds 1100°C, the crystal grains will grow excessively. As a result, even if condition 1 is 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. Therefore, the heating temperature T2 in the high-frequency induction hardening process is 830 to 1100°C.

[0097] 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.

[0098] Furthermore, finishing processes may be performed on carburized parts after the high-frequency induction hardening process or tempering treatment. Finishing processes include, for example, polishing.

[0099] Through the above process, a carburized part containing features 1 to 4 is manufactured. [Examples]

[0100] 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.

[0101] [Table 1A]

[0102] [Table 1B]

[0103] 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 150 mm x 150 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.

[0104] [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) Cr oxide film area ratio AR measurement test (Test 3) Surface fatigue strength evaluation test (Test 4) Bending fatigue strength evaluation test The following describes each test.

[0105] [(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, the finishing temperature was raised to 950°C or higher for hot working (hot forging), and then cooled in the air to obtain 35mm diameter steel bars (intermediate products).

[0106] A soaking process was performed on the intermediate samples of each test number produced. In the soaking process, heating was carried out at a heating temperature T1 (°C) and a heating time t1 (hours). The atmosphere in the heating furnace for each test number was as shown in Table 2. Next, a carburizing process was performed on the soaked intermediate product. In the carburizing process, carburizing quenching was performed first. Specifically, the intermediate product was heated at 950°C for 240 minutes in an atmosphere with a carbon potential of 0.80-1.00%. Subsequently, it was heated at 900°C for 30 minutes in an atmosphere with a carbon potential of 0.80%. After that, it was oil-cooled in 60°C oil. The heating temperature T1, heating time t1, and FnA are shown in Table 2.

[0107] [Table 2]

[0108] 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.

[0109] 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.

[0110] The simulated carburized parts for each test number were manufactured using the above manufacturing process. The carbon content at a depth of 0.2 mm from the surface of the carburized hardened layer of the simulated carburized parts for each test number was measured based on the method described in [Method for measuring carbon content of the surface layer] above. As a result, the carbon content at a depth of 0.2 mm from the surface of the carburized hardened layer of the simulated carburized parts for each test number was 0.50 to 1.20% by mass. In other words, the simulated carburized parts for each test number had a carburized hardened layer on their surface.

[0111] For each simulated carburized component of the test number, the average particle size number GN of the prior austenite grains was determined based on the method described in [Method for Measuring Average Particle Size Number GN]. The results obtained are shown in the "Average Particle Size Number GN" column of Table 2.

[0112] [(Test 2) Cr oxide film area ratio AR measurement test] Simulated carburized parts were manufactured from the steel material (50 mm diameter steel bar) of each test number under the same conditions as in the above-mentioned [(Test 1) Measurement Test of Average Grain Size Number GN of Former Austenite Grains]. Based on the method described in the above-mentioned [Method for Measuring Cr Oxide Film Area Ratio AR], the Cr oxide film area ratio AR (%) on the surface of the oxide film of the simulated carburized parts of each test number was measured. The results obtained are shown in the "Cr Oxide Film Area Ratio AR (%)" column in Table 2.

[0113] [(Test 3) Surface Fatigue Strength Evaluation Test] From the steel material (50 mm diameter steel bar) of each test number, test specimens for roller pitting fatigue testing to evaluate surface fatigue strength (small roller test specimens) were processed as shown in Figure 1. The numbers in Figure 1 indicate dimensions (in mm). "φ" in the figure represents the diameter.

[0114] 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, the material was hot-worked (hot forged) by raising the finishing temperature to 950°C or higher, and then cooled in the air to obtain steel bars with a diameter of 35 mm. The steel bars for each test number that were manufactured were machined (cut) to produce intermediate small roller test pieces. The intermediate pieces were subjected to a soaking process, a carburizing process, and a high-frequency induction hardening process under the same conditions as in the above-mentioned [(Test 1) Average grain size number GN measurement test of prior austenite grains]. Ten small roller test pieces (simulated carburized parts) were prepared for each test number after the high-frequency induction hardening process.

[0115] The following roller pitting fatigue tests were conducted using the prepared small roller test specimens. Figure 2 is a schematic diagram of the roller pitting fatigue test. As shown in Figure 2, 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 3. The numbers in Figure 3 indicate dimensions (in mm). "R150" in the figure indicates that the radius of curvature of the outer surface was 150 mm.

[0116] 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 3 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 3. The diameter of the large roller test specimen 100 was 130 mm.

[0117] 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, 10 test specimens were subjected to 10 million cycles, and the highest stress at which pitting did not occur was defined as the surface fatigue strength (MPa). If the obtained surface fatigue strength was 2500 MPa or higher, it was evaluated as "E (Excellent)," indicating that excellent surface fatigue strength had been obtained (indicated as "E" in the "Surface Fatigue Strength" column in Table 2). On the other hand, if the obtained surface fatigue strength was less than 2500 MPa, it was evaluated as "B (Bad)," indicating that excellent surface fatigue strength had not been obtained (indicated as "B" in the "Surface Fatigue Strength" column in Table 2).

[0118] [(Test 4) 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 4. The numbers in Figure 4 represent dimensions (in mm). In Figure 4, "φ" means diameter. "R1" means that the radius of curvature of the notch bottom is 1 mm.

[0119] 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.

[0120] For intermediate samples of each test number, a soaking process, a carburizing process, and an induction hardening process were performed under the same conditions as in the above-mentioned [(Test 1) Average grain size number GN measurement test of the old austenite grains]. Ten Ono-type rotary bending fatigue test specimens (simulated carburized parts) were prepared for each test number after the induction hardening process.

[0121] Ono-type rotary bending fatigue tests were performed using Ono-type rotary bending fatigue test specimens for each test number. The fatigue tests were conducted by varying the stress applied to each specimen, and the results were obtained after 10 million cycles (10 7 After repeating the test 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. If the obtained bending fatigue strength was 850 MPa or higher, it was evaluated as "E (Excellent)," indicating that excellent bending fatigue strength had been obtained (indicated as "E" in the "Bending Fatigue Strength" column in Table 2). On the other hand, if the obtained bending fatigue strength was less than 850 MPa, it was evaluated as "B (Bad)," indicating that excellent bending fatigue strength had not been obtained (indicated as "B" in the "Bending Fatigue Strength" column in Table 2).

[0122] [Test Results] Referring to Table 1 (Tables 1A and 1B) and Table 2, the simulated carburized parts for test numbers 1 to 14 satisfied features 1 to 4. Therefore, a surface fatigue strength of 2500 MPa or higher was achieved, indicating high surface fatigue strength. Furthermore, a bending fatigue strength of 850 MPa or higher was achieved, indicating high bending fatigue strength.

[0123] On the other hand, in test numbers 15 and 16, Fn1 was too low. As a result, high surface fatigue strength and high bending fatigue strength were not obtained for the carburized parts.

[0124] In tests 17 and 18, the soaking process was conducted under a nitrogen atmosphere. As a result, the Cr oxide film area ratio (AR) was less than 80%. Consequently, high surface fatigue strength could not be obtained for the carburized parts.

[0125] In tests 19 and 20, the FnA level during the soaking process was too low. As a result, the Cr oxide film area ratio (AR) was less than 80%. Consequently, high surface fatigue strength could not be obtained in the carburized parts.

[0126] In tests 21 and 22, the heating temperature T2 during the high-frequency induction hardening process was too low. As a result, the average grain size number GN exceeded 13.0. Consequently, high surface fatigue strength could not be obtained for the carburized parts.

[0127] In tests 23 and 24, the heating temperature T2 during the high-frequency induction hardening process was too high. As a result, the average grain size number GN was less than 11.0. Consequently, high surface fatigue strength and high bending fatigue strength were not obtained for the carburized parts.

[0128] The embodiments of this disclosure have been described above. However, the embodiments described above are merely examples for implementing this disclosure. Therefore, this disclosure is not limited to the embodiments described above, and the embodiments described above can be modified as appropriate without departing from the spirit of this disclosure.

Claims

1. Carburized parts, Carburized hardened layer, The oxide film formed on the carburized hardened layer, 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 grain size number of prior austenite grains in the region from the surface of the carburized hardened layer to a depth of 0.2 mm is 11.0 to 13.

0. When surface analysis of the Cr content of the oxide film was performed on the surface using an electron beam microanalyzer, the area ratio of the region where the Cr content is 4.0 times or more than the Cr content of the core was 80% or more within a 2.0 mm × 2.0 mm measurement area. 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).

2. Carburized parts, Carburized hardened layer, The oxide film formed on the carburized hardened layer, 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 grain size number of prior austenite grains in the region from the surface of the carburized hardened layer to a depth of 0.2 mm is 11.0 to 13.

0. When surface analysis of the Cr content of the oxide film was performed on the surface using an electron beam microanalyzer, the area ratio of the region where the Cr content is 4.0 times or more than the Cr content of the core was 80% or more within a 2.0 mm × 2.0 mm measurement area. 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: One or more selected from the group consisting of 0.0100% or less. [Group 2] Cu: 0.50% or less, Ni: 0.50% or less, One or more selected from the group consisting of 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) Here, the content (mass%) of the corresponding element is substituted for each element symbol in equation (1).

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.

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