Carburized steel parts

Abstract

To provide carburized steel parts with excellent surface fatigue strength. [Solution] The carburized steel part of the present disclosure has a core chemical composition in mass%, containing C: 0.14-0.30%, Si: 0.10-0.80%, Mn: 0.10-2.00%, P: 0.015% or less, S: 0.025% or less, Cr: 0.36-2.00%, Mo: 0.01-0.50%, Al: 0.005-0.100%, N: 0.0020-0.0300%, and O: 0.0015% or less. In the carburized hardened surface region, the total area ratio AR of carbides is 1.5% or more, and the average carbon concentration [C] ave Fn1, defined using (mass%), is 0.65 or higher, and the Si content, Mo content, and the solid solution Cr concentration [Cr] in the carburized hardened surface region are... sol The value of Fn2, defined by equation (2) using (mass%), is 3.7 or greater. Fn1=([C] ave -0.067AR) (1) Fn2 = 5Si + 2Mo + 10[Cr] sol (2)

Description

[Technical Field]

[0001] This disclosure relates to steel parts, and more particularly to carburized steel parts having a carburized hardened layer formed on the surface by a carburizing and quenching treatment. [Background technology]

[0002] Structural components are used in automobiles, construction vehicles, mining machinery, etc. Examples of structural components include gears, shafts, and bearings. As materials for structural components, structural alloy steels such as SCr420, SCM420, and SNCM420, as specified in JIS G 4053 (2016), are used.

[0003] Structural machine parts made from these steel materials are manufactured, for example, through the following process: Hot working (hot forging) is performed on the raw steel material, followed by machining as needed to produce an intermediate product of the desired shape. Surface hardening heat treatment (quenching and tempering, or carburizing and quenching, etc.) is performed on the intermediate product to adjust its hardness and microstructure. Structural machine parts are manufactured through the above process.

[0004] The mechanical structural components used in the above-mentioned applications require high surface fatigue strength. Carburizing and quenching is a known method for increasing the surface fatigue strength of mechanical structural components. In this specification, carburizing and quenching includes carbonitriding and quenching. Carburizing and quenching forms a hardened layer (carburized layer or carbonitriding layer) on the surface of the mechanical structural component. This hardened layer increases the surface fatigue strength of the mechanical structural component. In the following description, mechanical structural components that have undergone carburizing and quenching are referred to as carburized steel components. Recently, with the trend towards weight reduction in automobiles and other vehicles, there is a demand for further improvement in the surface fatigue strength of carburized steel components.

[0005] A technique for enhancing the surface fatigue strength of carburized steel parts is disclosed in Japanese Patent Application Laid-Open No. 2005-68453 (Patent Document 1). The carburized steel parts disclosed in Patent Document 1 contain C: 0.15 to 0.40%, Si: 0.50 to 1.50%, Mn: 0.20 to 1.50%, Cr: 0.50 to 1.50%, and Mo: 0.05 to 0.50%, and contain at least one or more elements selected from the group consisting of Ni: 0.50 to 3.50%, Ti: 0.03 to 0.20%, Nb: 0.03 to 0.15%, and Al: 0.01 to 0.10%, with P: 0.010% or less, and the balance being composed of a steel material of Fe and inevitable impurities, and the carbon concentration at the rolling part on the part surface is 0.8 to 1.2%.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, the surface fatigue strength may be enhanced by other means other than Patent Document 1.

[0008] The object of the present disclosure is to provide carburized steel parts having excellent surface fatigue strength.

Means for Solving the Problems

[0009] The carburized steel component of the present disclosure includes a carburized hardened layer and a core part inside the carburized hardened layer. The chemical composition of the core part contains, by mass%, C: 0.14 to 0.30%, Si: 0.10 to 0.80%, Mn: 0.10 to 2.00%, P: 0.015% or less, S: 0.025% or less, Cr: 0.36 to 2.00%, Mo: 0.01 to 0.50%, Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, and the balance consists of Fe and impurities. In the carburized hardened surface layer region from the surface of the carburized hardened layer to a depth of 100 μm, the total area ratio AR of carbides is 1.5% or more. In the carburized hardened surface layer region, Fn1 defined by formula (1) using the average carbon concentration [C] ave (mass%) and the total area ratio AR (%) is 0.65 or more. The Fn2 defined by formula (2) using the Si content and Mo content in mass% in the chemical composition of the core part and the solid solution Cr concentration [Cr] sol (mass%) in the carburized hardened surface layer region is 3.7 or more. Fn1 = ([C] ave - 0.067AR) (1) Fn2 = 5Si + 2Mo + 10[Cr] sol (2)

[0010] The carburized steel component of the present disclosure includes a carburized hardened layer and a core part inside the carburized hardened layer. The chemical composition of the core part contains, by mass%, C: 0.14 to 0.30%, Si: 0.10 to 0.80%, Mn: 0.10 to 2.00%, P: 0.015% or less, S: 0.025% or less, Cr: 0.36 to 2.00%, Mo: 0.01 to 0.50%, Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, and O: 0.0015% or less, and further contains one or more selected from the group consisting of Group 1 to Group 4, and the balance consists of Fe and impurities. In the carburized hardened surface layer region from the surface of the carburized hardened layer to a depth of 100 μm, the total area ratio AR of carbides is 1.5% or more. In the carburized hardened surface layer region, Fn1 defined by formula (1) using the average carbon concentration [C] ave (mass%) and the total area ratio AR (%) is 0.65 or more. The Si content and Mo content in mass% in the chemical composition of the core part and the solid solution Cr concentration [Cr]sol The value of Fn2, defined by equation (2) using (mass%), is 3.7 or greater. [Group 1] One or more elements selected from the group consisting of Cu: 0.20% or less, Ni: 0.35% or less, and B: 0.0050% or less. [Group 2] One or more elements selected from the group consisting of V: ​​0.15% or less, Nb: 0.100% or less, Ti: 0.100% or less, and W: 0.15% or less. [Group 3] Select one or more elements from the group consisting of Ca: 0.0050% or less and Mg: 0.010% or less. [Group 4] One or more elements selected from the group consisting of Te: 0.050% or less, Bi: 0.100% or less, Pb: 0.09% or less, Sn: 0.050% or less, and Sb: 0.050% or less. Fn1=([C] ave -0.067AR) (1) Fn2 = 5Si + 2Mo + 10[Cr] sol (2) [Effects of the Invention]

[0011] The carburized steel components of this disclosure provide excellent surface fatigue strength. [Brief explanation of the drawing]

[0012] [Figure 1] Figure 1 is a side view of a small roller test specimen used for surface fatigue strength evaluation testing. [Figure 2] Figure 2 is a schematic diagram of the roller pitting test performed in the surface fatigue strength evaluation test. [Figure 3] Figure 3 is a front view of the large roller test specimen shown in Figure 2. [Modes for carrying out the invention]

[0013] The inventors first examined carburized steel parts capable of obtaining excellent surface fatigue strength from the perspective of chemical composition. As a result, if the chemical composition of the core part is, in mass%, C: 0.14 to 0.30%, Si: 0.10 to 0.80%, Mn: 0.10 to 2.00%, P: 0.015% or less, S: 0.025% or less, Cr: 0.36 to 2.00%, Mo: 0.01 to 0.50%, Al: 0.005 to 0.100%, N: 0.0020 to 0.0300%, O: 0.0015% or less, Cu: 0 to 0.20%, Ni: 0 to 0.35%, B: 0 to 0.0050%, V: 0 to 0.15%, Nb: 0 to 0.100%, Ti: 0 to 0.100%, W: 0 to 0.15%, Ca: 0 to 0.0050%, Mg: 0 to 0.010%, Te: 0 to 0.050%, Bi: 0 to 0.100%, Pb: 0 to 0.09%, Sn: 0 to 0.050%, and Sb: 0 to 0.050%, and the balance is composed of Fe and impurities, it is considered that excellent surface fatigue strength may be obtained.

[0014] Therefore, in carburized steel parts having a core part with the above chemical composition, the means for increasing the surface fatigue strength was further examined. As a result, the inventors obtained the following findings.

[0015] If the strength and hardness of the carburized hardened layer of the carburized steel part are increased, the surface fatigue strength of the carburized steel part will increase. When carbides are formed in the carburized hardened layer, the hardness of the carbides is higher than the hardness of the base (matrix) of the carburized hardened surface layer. Therefore, if carbides are formed in the carburized hardened layer and further the total area ratio of the carbides in the carburized hardened layer is increased, the strength and hardness of the carburized hardened layer will increase. Furthermore, if the strength of the base (matrix) itself of the carburized hardened layer can also be increased, the strength and hardness of the carburized hardened layer will further increase.

[0016] As a result of further examination based on the above findings, the inventors found that if the total area ratio AR of the carbides in the carburized hardened layer is 1.5% or more and further Fn1 defined by the formula (1) is 0.65 or more, the strength and hardness of the carburized hardened layer will increase sufficiently and the surface fatigue strength will increase. Fn1 = ([C] ave - 0.067AR) (1) Here, in equation (1) [C] ave For , the average carbon concentration (mass%) in the carburized hardened layer is substituted, and for AR, the total area percentage AR(%) of carbides in the carburized hardened layer is substituted.

[0017] However, even when the total surface area ratio (AR) of carbides in the carburized hardened layer was increased and Fn1 was set to 0.65% or higher, sufficient surface fatigue strength was still not obtained in some cases. Therefore, the inventors conducted further investigations. As a result, the inventors obtained the following findings.

[0018] When carburized steel components are used as mechanical structural parts such as gears, shafts, and bearings, these components come into contact with or slide against other mechanical structural parts during use. This contact or sliding generates heat in the carburized layer of the carburized steel component. This heat generation can reduce the strength and hardness of the carburized layer, resulting in a decrease in the surface fatigue strength of the carburized steel component.

[0019] To obtain high surface fatigue strength even when heat is generated, it is effective to increase the tempering softening resistance of the matrix of the carburized hardened layer. The chemical composition described above contains Si, Mo, and Cr, which are elements that increase tempering softening resistance. Therefore, it seems that sufficient tempering softening resistance can be obtained in the carburized hardened layer if the above chemical composition is satisfied.

[0020] However, investigations revealed that the solid-solution Cr concentration in the matrix of the carburized layer can be significantly lower than the Cr content in the aforementioned chemical composition. Cr can be dissolved not only in the matrix of the carburized layer but also in the carbides. Therefore, to increase the tempering softening resistance of the carburized layer of carburized steel parts, it is effective to ensure a sufficient solid-solution Cr concentration [Cr]sol, in addition to the Si and Mo content in the matrix of the carburized layer.

[0021] Based on the above findings, further investigation revealed that if Fn2, as defined by equation (2), is set to 3.7 or higher in the carburized hardened layer, excellent surface fatigue strength can be obtained in the carburized hardened surface region. Fn2 = 5Si + 2Mo + 10[Cr] sol (2) Here, the Si and Mo in equation (2) are substituted with the mass percent of the Si and Mo content in the chemical composition of the core. Also, [Cr] sol The solid solution Cr concentration (mass%) in the carburized hardened surface region is substituted into this value.

[0022] Based on the above technical concept, the carburized steel component of this embodiment has the following configuration.

[0023] The first configuration of the carburized steel part comprises a carburized hardened layer and a core portion located inside the carburized hardened layer. The chemical composition of the core portion, in mass%, contains C: 0.14~0.30%, Si: 0.10~0.80%, Mn: 0.10~2.00%, P: 0.015% or less, S: 0.025% or less, Cr: 0.36~2.00%, Mo: 0.01~0.50%, Al: 0.005~0.100%, N: 0.0020~0.0300%, and O: 0.0015% or less, with the remainder being Fe and impurities. In the carburized hardened surface region from the surface of the carburized hardened layer to a depth of 100 μm, the total area ratio AR of carbides is 1.5% or more. In the carburized hardened surface region, the average carbon concentration [C] ave The Fn1 defined by formula (1) using (mass%) and total area ratio AR(%) is 0.65 or greater. The Si content and Mo content in mass% of the chemical composition of the core, and the solid solution Cr concentration [Cr] in the carburized hardened surface region. sol The value of Fn2, defined by equation (2) using (mass%), is 3.7 or greater. Fn1=([C] ave -0.067AR) (1) Fn2 = 5Si + 2Mo + 10[Cr] sol (2)

[0024] The second configuration of the carburized steel part comprises a carburized hardened layer and a core portion located inside the carburized hardened layer. The chemical composition of the core portion, in mass%, contains C: 0.14-0.30%, Si: 0.10-0.80%, Mn: 0.10-2.00%, P: 0.015% or less, S: 0.025% or less, Cr: 0.36-2.00%, Mo: 0.01-0.50%, Al: 0.005-0.100%, N: 0.0020-0.0300%, and O: 0.0015% or less, and further contains one or more elements selected from groups 1 to 4, with the remainder being Fe and impurities. In the carburized hardened surface region from the surface of the carburized hardened layer to a depth of 100 μm, the total area ratio AR of carbides is 1.5% or more. In the carburized and hardened surface region, the average carbon concentration [C] ave The Fn1 defined by formula (1) using (mass%) and total area ratio AR(%) is 0.65 or greater. The Si content and Mo content in mass% of the chemical composition of the core, and the solid solution Cr concentration [Cr] in the carburized hardened surface region. sol The value of Fn2, defined by equation (2) using (mass%), is 3.7 or greater. [Group 1] One or more elements selected from the group consisting of Cu: 0.20% or less, Ni: 0.35% or less, and B: 0.0050% or less. [Group 2] One or more elements selected from the group consisting of V: ​​0.15% or less, Nb: 0.100% or less, Ti: 0.100% or less, and W: 0.15% or less. [Group 3] Select one or more elements from the group consisting of Ca: 0.0050% or less and Mg: 0.010% or less. [Group 4] One or more elements selected from the group consisting of Te: 0.050% or less, Bi: 0.100% or less, Pb: 0.09% or less, Sn: 0.050% or less, and Sb: 0.050% or less. Fn1=([C] ave -0.067AR) (1) Fn2 = 5Si + 2Mo + 10[Cr] sol (2)

[0025] The third carburized steel component is the same as the second carburized steel component, and the chemical composition of the core contains the first group.

[0026] The fourth carburized steel component is a carburized steel component of the second or third component, wherein the chemical composition of the core contains the second group.

[0027] The fifth configuration of the carburized steel component is a carburized steel component having one of the second to fourth configurations, wherein the chemical composition of the core contains the third group.

[0028] The sixth configuration of the carburized steel component is a carburized steel component having one of the second to fifth configurations, wherein the chemical composition of the core contains the fourth group.

[0029] The seventh component of the carburized steel is a gear, which is a carburized steel component of any one of the first to sixth components.

[0030] The carburized steel parts of this embodiment will be described in detail below. Unless otherwise specified, the "%" in relation to elements refers to mass percentage.

[0031] [Configuration of the carburized steel component in this embodiment] The carburized steel component of this embodiment comprises a carburized hardened layer and a core. The carburized hardened layer is formed on the surface of carburized steel parts. This layer is formed when carbon penetrates the surface of the steel during the carburizing and quenching process, resulting in hardening. Therefore, the carbon concentration in the carburized hardened layer is higher than the carbon content in the core's chemical composition. The carburized hardened layer extends from the surface of the carburized steel part to a predetermined depth. The microstructure of the carburized hardened layer consists mainly of martensite.

[0032] The carbon concentration in the carburized hardened layer of the carburized steel part of this embodiment is not particularly limited as long as it is higher than the carbon content of the core. For example, the carbon concentration in the carburized hardened layer is 0.60% or more by mass. The preferred lower limit of the carbon concentration in the carburized hardened layer is 0.70%, and more preferably 0.75%. The upper limit of the carbon concentration in the carburized hardened layer is not particularly limited, but for example, it is 1.50%.

[0033] The core is the region inside the carburized hardened layer and is substantially unaffected by the intrusion and diffusion of carbon due to the carburizing and quenching process. As mentioned above, the carbon content (mass%) in the core is lower than the carbon concentration (mass%) in the carburized hardened layer. In carburized steel parts, the carburized hardened layer and the core can be easily distinguished by microstructural observation, which is well known to those skilled in the art.

[0034] The carburized steel component of this embodiment, having the above configuration, satisfies the following characteristics. (Feature 1) The chemical composition of the core is as follows (by mass%): C: 0.14~0.30%, Si: 0.10~0.80%, Mn: 0.10~2.00%, P: 0.015% or less, S: 0.025% or less, Cr: 0.36~2.00%, Mo: 0.01~0.50%, Al: 0.005~0.100%, N: 0.0020~0.0300%, O: 0.0015% or less, Cu: 0~0.20%, Ni: 0~0 It contains 0.35%, B: 0-0.0050%, V: 0-0.15%, Nb: 0-0.100%, Ti: 0-0.100%, W: 0-0.15%, Ca: 0-0.0050%, Mg: 0-0.010%, Te: 0-0.050%, Bi: 0-0.100%, Pb: 0-0.09%, Sn: 0-0.050%, and Sb: 0-0.050%, with the remainder being Fe and impurities. (Feature 2) In the carburized surface region of the carburized layer, from the surface to a depth of 100 μm, the total area ratio (AR) of carbides is 1.5% or more. (Feature 3) In the carburized and hardened surface region, the average carbon concentration [C] ave Fn1, defined by equation (1) using (mass%) and total area ratio AR(%), is 0.65 or greater. Fn1=([C] ave -0.067AR) (1) (Feature 4) The Si and Mo content in mass percent of the chemical composition of the core, and the solid solution Cr concentration [Cr] in the carburized hardened surface region. sol The value of Fn2, defined by equation (2) using (mass%), is 3.7 or greater. Fn2 = 5Si + 2Mo + 10[Cr] sol (2) Features 1 through 4 are explained below.

[0035] [(Feature 1) Regarding chemical composition] The chemical composition of the core of the carburized steel component in this embodiment contains the following elements:

[0036] C: 0.14~0.30% Carbon (C) enhances the hardenability of the steel material used for carburized steel parts, and increases the surface fatigue strength and bending fatigue strength of the carburized steel parts. If the C content is less than 0.14%, 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 0.30%, coarse precipitates may form in the core of the carburized steel part. In this case, even if the content of other elements is within the range of this embodiment, the bending fatigue strength of the carburized steel part will decrease. Therefore, the C content is 0.14-0.30%. The preferred lower limit of the C content is 0.16%, more preferably 0.18%, even more preferably 0.20%, and even more preferably 0.22%. The preferred upper limit for the C content is 0.28%, more preferably 0.27%, more preferably 0.26%, and still more preferably 0.25%.

[0037] Si: 0.10~0.80% Silicon (Si) enhances the hardenability of steel materials and increases the surface fatigue strength and bending fatigue strength of carburized steel parts. Furthermore, Si increases the tempering softening resistance of the carburized hardened layer of carburized steel parts, thereby increasing the surface fatigue strength of the carburized steel parts. If the Si 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 Si content exceeds 0.80%, an excessive grain boundary oxide layer may form after gas carburizing. In this case, even if the content of other elements is within the range of this embodiment, the bending fatigue strength of the carburized steel part will decrease. Therefore, the Si content is 0.10-0.80%. The preferred lower limit for the Si content is 0.15%, more preferably 0.20%, and even more preferably 0.25%. The preferred upper limit for the Si content is 0.75%, more preferably 0.70%, and even more preferably 0.65%.

[0038] Mn: 0.10~2.00% Manganese (Mn) enhances the hardenability of steel materials and increases the surface fatigue strength and bending fatigue strength of carburized steel parts. If the Mn 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 Mn content exceeds 2.00%, even if the content of other elements is within the range of this embodiment, the formation of Mn oxides is promoted on the surface of the steel during gas carburizing. As a result, an excessive grain boundary oxide layer may be formed in the carburized steel parts. In this case, even if the content of other elements is within the range of this embodiment, the bending fatigue strength of the carburized steel parts decreases. Therefore, the Mn content is 0.10-2.00%. The preferred lower limit of the Mn content is 0.20%, more preferably 0.30%, and even more preferably 0.40%. The preferred upper limit for the Mn content is 1.90%, more preferably 1.80%, even more preferably 1.70%, and even more preferably 1.60%.

[0039] P:0.015% or less Phosphorus (P) is an impurity. If the P content exceeds 0.015%, even if the content of other elements is within the range of this embodiment, P will excessively segregate at the grain boundaries, reducing the grain boundary strength. As a result, the surface fatigue strength and bending fatigue strength of the carburized steel parts will decrease. Therefore, the P content is 0.015% or less. 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 0.012%, more preferably 0.010%, and even more preferably 0.008%.

[0040] S: 0.025% or less Sulfur (S) is an impurity. If the S content exceeds 0.025%, coarse sulfide inclusions will form. Even if the content of other elements is within the range of this embodiment, the bending fatigue strength of the carburized steel parts will decrease. Therefore, the sulfur content is 0.025% or less. 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 0.020%, more preferably 0.010%, and even more preferably 0.005%.

[0041] Cr: 0.36~2.00% Chromium (Cr) enhances the hardenability of steel materials and increases the strength of carburized steel parts. Furthermore, Cr dissolves into the carburized hardened layer of carburized steel parts, increasing the tempering softening resistance of the carburized hardened layer. Through these actions, Cr increases the surface fatigue strength of carburized steel parts. If the Cr content is less than 0.36%, 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 chromium content exceeds 2.00%, the hardness of the steel becomes excessively high. In this case, the machinability of the steel decreases. Furthermore, if the chromium content exceeds 2.00%, the raw material cost of the steel also increases. Therefore, the Cr content is between 0.36% and 2.00%. The preferred lower limit for the Cr content is 0.38%, more preferably 0.40%, and even more preferably 0.45%. The preferred upper limit for the Cr content is 1.90%, more preferably 1.80%, more preferably 1.70%, more preferably 1.60%, and more preferably 1.50%.

[0042] Mo: 0.01~0.50% Molybdenum (Mo) enhances the hardenability of steel materials and increases the strength of carburized steel parts. Furthermore, Mo dissolves into the carburized hardened layer of carburized steel parts, increasing the tempering softening resistance of the carburized hardened layer. Through these actions, Mo increases the surface fatigue strength of carburized steel parts. If the Mo content is less than 0.01%, 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 Mo content exceeds 0.50%, the hardness of the steel becomes excessively high. In this case, the machinability of the steel decreases. Furthermore, if the Mo content exceeds 0.50%, the raw material cost of the steel also increases. Therefore, the Mo content is 0.01-0.50%. The preferred lower limit for the Mo content is 0.02%, more preferably 0.03%, and even more preferably 0.05%. The preferred upper limit for the Mo content is 0.48%, more preferably 0.45%, and even more preferably 0.40%.

[0043] Al: 0.005~0.100% Aluminum (Al) combines with nitrogen in the steel to form AlN, and the pinning effect suppresses grain coarsening during heating in the gas carburizing process. As a result, the surface fatigue strength and bending fatigue strength of the carburized steel parts are increased. If the Al content is less than 0.005%, 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%, coarse oxides will be formed even if the content of other elements is within the range of this embodiment. These coarse oxides reduce the bending fatigue strength of the carburized steel parts. Therefore, the Al content is between 0.005% and 0.100%. The preferred lower limit for the Al content is 0.008%, and more preferably 0.010%. The preferred upper limit for the Al content is 0.080%, more preferably 0.070%, and even more preferably 0.060%.

[0044] N: 0.0020~0.0300% Nitrogen (N) combines with Al in the steel to form AlN, and the pinning effect suppresses grain coarsening during heating in the gas carburizing process. As a result, the surface fatigue strength and bending fatigue strength of the carburized steel parts are increased. If the N content is less than 0.0020%, 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 0.0300%, the hot workability of the steel material used for the carburized steel parts will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the N content is between 0.0020% and 0.0300%. The preferred lower limit for the N content is 0.0025%, more preferably 0.0030%, and even more preferably 0.0035%. The preferred upper limit for the N content is 0.0280%, more preferably 0.0250%, and even more preferably 0.0200%.

[0045] O: 0.0015% or less Oxygen (O) is an unavoidable impurity. In other words, the O content is greater than 0%. O combines with other elements in the steel to form coarse oxide inclusions. Coarse oxide inclusions reduce the bending fatigue strength of carburized steel parts. If the O content exceeds 0.0015%, the bending fatigue strength of the carburized steel parts will decrease significantly, even if the content of other elements is within the range of this embodiment. Therefore, the O content is 0.0015% 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 0.0003%, and even more preferably 0.0005%. The preferred upper limit for the O content is 0.0013% or less, more preferably 0.0011%, and even more preferably 0.0009%.

[0046] The remainder of the chemical composition of the core of the carburized steel part in this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from the raw materials such as ore, scrap, or the manufacturing environment during the industrial production of the steel material that forms the basis of the carburized steel part, and are acceptable within a range that does not adversely affect the carburized steel part in this embodiment.

[0047] [About Optional Elements] The chemical composition of the core of the carburized steel part in this embodiment may further include one or more substances selected from the group consisting of groups 1 to 4, instead of a portion of Fe. [Group 1] One or more elements selected from the group consisting of Cu: 0.20% or less, Ni: 0.35% or less, and B: 0.0050% or less. [Group 2] One or more elements selected from the group consisting of V: ​​0.15% or less, Nb: 0.100% or less, Ti: 0.100% or less, and W: 0.15% or less. [Group 3] Select one or more elements from the group consisting of Ca: 0.0050% or less and Mg: 0.010% or less. [Group 4] One or more elements selected from the group consisting of Te: 0.050% or less, Bi: 0.100% or less, Pb: 0.09% or less, Sn: 0.050% or less, and Sb: 0.050% or less. These elements are optional and do not need to be included. The elements in groups 1 through 4 are described below.

[0048] [Group 1: Cu, Ni, and B] The chemical composition of the steel material in this embodiment may further contain one or more elements selected from the group consisting of Cu, Ni, and B in place of a portion of Fe. These elements are arbitrary and all enhance the surface fatigue strength and bending fatigue strength of the carburized steel parts.

[0049] Cu:0.20% 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 hardenability of steel and increases the surface fatigue strength and bending fatigue strength of carburized steel parts. Even a small amount of Cu will provide some of the above effects. However, if the Cu content exceeds 0.20%, even if the content of other elements is within the range of this embodiment, the hardness of the steel material used for the carburized steel parts will become excessively high, and the machinability of the steel will decrease. Therefore, the Cu content is 0-0.20%, and if present, it is 0.20% or less. The preferred lower limit for the Cu content is 0.01%, more preferably 0.02%, and even more preferably 0.05%. The preferred upper limit for the Cu content is 0.18%, more preferably 0.15%, and even more preferably 0.10%.

[0050] Ni: 0.35% 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 hardenability of steel materials and increases the surface fatigue strength and bending fatigue strength of carburized steel parts. Even a small amount of Ni will provide some of the above effects. However, if the Ni content exceeds 0.35%, even if the content of other elements is within the range of this embodiment, the hardness of the steel becomes excessively high, and the machinability of the steel material used for the carburized steel parts decreases. Therefore, the Ni content is 0-0.35%, and if present, it is 0.35% or less. The preferred lower limit for the Ni content is 0.01%, more preferably 0.02%, and even more preferably 0.05%. The preferred upper limit for the Ni content is 0.32%, more preferably 0.30%, and even more preferably 0.25%.

[0051] B: 0.0050% 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 steel materials and increases the surface fatigue strength and bending fatigue strength of carburized steel parts. Even a small amount of B present will provide some of the above effects. However, if the B content exceeds 0.0050%, the hardness of the steel becomes excessively high, even if the content of other elements is within the range of this embodiment. As a result, the machinability of the steel material used for the carburized steel parts decreases. Therefore, the B content is between 0 and 0.0050%, and if present, it is 0.0050% or less. The preferred lower limit for the B content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0008%. The preferred upper limit for the B content is 0.0040%, more preferably 0.0035%, and even more preferably 0.0030%.

[0052] [Group 2: V, Nb, Ti, and W] The chemical composition of the steel material in this embodiment may further contain one or more elements selected from the group consisting of V, Nb, Ti, and W in place of a portion of Fe. These elements are arbitrary and all form precipitates to increase the surface fatigue strength and bending fatigue strength of the carburized steel parts.

[0053] V: 0.15% 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. V precipitates increase the surface fatigue strength and bending fatigue strength of carburized steel 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 0.15%, the hardness of the steel becomes excessively high, even if the content of other elements is within the range of this embodiment. As a result, the machinability of the steel material used for the carburized steel parts decreases. Therefore, the V content is 0-0.15%, and if present, the V content is 0.15% or less. The preferred lower limit for the V content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit for the V content is 0.14%, more preferably 0.12%, and even more preferably 0.10%.

[0054] 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%, Nb forms Nb precipitates such as Nb carbides and Nb carbonitrides. Nb precipitates increase the surface fatigue strength and bending fatigue strength of carburized steel 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, even if the content of other elements is within the range of this embodiment, the bending fatigue strength of the carburized steel part decreases. Therefore, the Nb content is between 0 and 0.100%, and if present, it is 0.100% or less. The preferred lower limit of the Nb content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the Nb content is 0.097%, more preferably 0.080%, even more preferably 0.060%, and even more preferably 0.040%.

[0055] 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%. When Ti is present, that is, when the Ti content is greater than 0%, Ti forms Ti precipitates such as Ti carbides and Ti carbonitrides. Ti precipitates increase the surface fatigue strength and bending fatigue strength of carburized steel 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, even if the content of other elements is within the range of this embodiment, the bending fatigue strength of the carburized steel part decreases. Therefore, the Ti content is between 0 and 0.100%, and if present, it is 0.100% or less. The preferred lower limit for the Ti content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the Ti content is 0.095%, more preferably 0.080%, and even more preferably 0.050%.

[0056] W: 0.15% or less Tungsten (W) is an optional element and does not need to be included. In other words, the W content may be 0%. When W is present, that is, when the W content is greater than 0%, W forms W precipitates such as W carbides and W carbonitrides. These W precipitates increase the surface fatigue strength and bending fatigue strength of carburized steel parts through a pinning effect. Even if only a small amount of W is present, the above effect can be obtained to some extent. However, if the W content exceeds 0.15%, the W precipitates become coarser. In this case, even if the content of other elements is within the range of this embodiment, the bending fatigue strength of the carburized steel part decreases. Therefore, the W content is 0-0.15%, and if present, it is 0.15% or less. The preferred lower limit of the W content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit for the W content is 0.14%, more preferably 0.10%, and even more preferably 0.07%.

[0057] [Group 3: Regarding Ca and Mg] The chemical composition of the steel material in this embodiment may further include Ca and Mg in place of some of the Fe. These elements are arbitrary and both contribute to the refinement and spheroidization of sulfides, thereby increasing the bending fatigue strength of the carburized steel parts.

[0058] Ca: 0.0050% or less Calcium (Ca) is an optional element and may not be present. In other words, the Ca content may be 0%. When calcium is present, i.e., when the calcium content is greater than 0%, calcium refines the sulfides in the steel. Furthermore, calcium promotes the spheroidization of sulfides in the steel. As a result, the bending fatigue strength of carburized steel parts made from steel increases. Even a small amount of calcium can provide some of the above effects. However, if the Ca content exceeds 0.0050%, the formation of coarse Ca oxides in the steel is promoted. In this case, even if the content of other elements is within the range of this embodiment, the bending fatigue strength of the carburized steel part will decrease. Therefore, the Ca content is between 0 and 0.0050%, and if present, it is 0.0050% or less. The preferred lower limit for the Ca content is 0.0001%, more preferably 0.0005%, and even more preferably 0.0010%. The preferred upper limit for the Ca content is 0.0045%, more preferably 0.0040%, and even more preferably 0.0035%.

[0059] Mg: 0.010% 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 Mg is present, i.e., when the Mg content is greater than 0%, Mg refines the sulfides in the steel. Furthermore, Mg promotes the spheroidization of sulfides in the steel. Therefore, it increases the bending fatigue strength of carburized steel parts made from steel. Even a small amount of Mg can provide some of the above effects. However, if the Mg content exceeds 0.010%, the formation of coarse Mg oxides in the steel is promoted. In this case, even if the content of other elements is within the range of this embodiment, the bending fatigue strength of the carburized steel part will decrease. Therefore, the Mg content is 0-0.010%, and if present, it is 0.010% or less. The preferred lower limit of the Mg content is 0.001%, more preferably 0.002%, and even more preferably 0.003%. The preferred upper limit for the Mg content is 0.009%, more preferably 0.008%, and even more preferably 0.007%.

[0060] [Group 4: Te, Bi, Pb, Sn, and Sb] The chemical composition of the steel material in this embodiment may further contain one or more elements selected from the group consisting of Te, Bi, Pb, Sn, and Sb in place of a portion of Fe. These elements are arbitrary and all enhance the machinability of the steel material.

[0061] Te: 0.050% or less Tellurium (Te) is an optional element and does not need to be included. In other words, the Te content may be 0%. If present, i.e., if the Te content is greater than 0%, Te improves the machinability of the steel material used for carburized steel parts. Even a small amount of Te will provide some degree of the above effect. However, if the Te content exceeds 0.050%, the hot workability of the steel material used for the carburized steel parts will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Te content is between 0 and 0.050%, and if present, it is 0.050% or less. The preferred lower limit for the Te content is 0.001%, more preferably 0.003%, and even more preferably 0.005%. The preferred upper limit for the Te content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0062] Bi:0.100% or less Bismuth (Bi) is an optional element and does not need to be included. In other words, the Bi content may be 0%. When present, i.e., when the Bi content is greater than 0%, Bi improves the machinability of steel. Even a small amount of Bi will provide some degree of the above effect. However, if the Bi content exceeds 0.100%, the hot workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Bi content is between 0 and 0.100%, and if present, it is 0.100% or less. The preferred lower limit of the Bi content is 0.001%, more preferably 0.002%, more preferably 0.005%, and still more preferably 0.010%. The preferred upper limit for the Bi content is 0.095%, more preferably 0.090%, even more preferably 0.080%, and even more preferably 0.070%.

[0063] Pb: 0.09% or less Lead (Pb) is an optional element and does not need to be included. In other words, the Pb content may be 0%. When Pb is present, that is, when the Pb content is greater than 0%, Pb improves the machinability of steel. Even if only a small amount of Pb is present, the above effect can be obtained to some extent. However, if the Pb content exceeds 0.09%, the hot workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Pb content is 0-0.09%, and if present, the Pb content is 0.09% or less. The preferred lower limit of the Pb content is 0.01%, more preferably 0.02%, and even more preferably 0.03%. The preferred upper limit for the Pb content is 0.08%, more preferably 0.07%, even more preferably 0.06%, and even more preferably 0.05%.

[0064] Sn: 0.050% or less Tin (Sn) is an optional element and does not need to be included. In other words, the Sn content may be 0%. When present, i.e., when the Sn content is greater than 0%, Sn improves the machinability of steel. Even a small amount of Sn present will provide some degree of the above effect. However, if the Sn content exceeds 0.050%, the hot workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Sn content is 0-0.050%, and if present, the Sn content is 0.050% or less. The preferred lower limit for the Sn content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the Sn content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0065] Sb: 0.050% or less Antimony (Sb) is an optional element and does not need to be included. In other words, the Sb content may be 0%. When present, i.e., when the Sb content is greater than 0%, Sb improves the machinability of steel. Even a small amount of Sb can provide some degree of this effect. However, if the Sb content exceeds 0.050%, the hot workability of the steel material will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the Sb content is between 0 and 0.050%, and if present, the Sb content is 0.050% or less. The preferred lower limit for the Sb content is 0.001%, more preferably 0.005%, and even more preferably 0.010%. The preferred upper limit for the Sb content is 0.045%, more preferably 0.040%, and even more preferably 0.035%.

[0066] [(Feature 2) Regarding the total area ratio AR (%) of carbides in the carburized and hardened surface region] In the carburized steel part of this embodiment, the region from the surface of the carburized hardened layer to a depth of 100 μm is referred to as the "carburized hardened surface region." The total area ratio AR of carbides in the carburized hardened surface region is 1.5% or more.

[0067] The total area ratio (AR) of carbides in the carburized surface region contributes to improving surface fatigue strength. A higher total area ratio of carbides increases the surface fatigue strength of the carburized steel component. If the total area ratio (AR) of carbides in the carburized surface region is 1.5% or higher, the strength and hardness of the carburized layer are sufficiently increased. Assuming that the other features 1, 3, and 4 are also satisfied, excellent surface fatigue strength can be obtained in the carburized steel component.

[0068] The preferred lower limit for the total area ratio (AR) of carbides in the carburized and hardened surface region is 1.8%, more preferably 2.0%, more preferably 2.2%, and still more preferably 2.4%.

[0069] There is no particular upper limit to the total area ratio (AR) of carbides in the carburized and hardened surface region. For example, the upper limit for the total area ratio (AR) of carbides is 20.0%.

[0070] [Method for measuring the total area ratio (AR) of carbides in the carburized and hardened surface region] The total area ratio AR of carbides in the carburized and hardened surface region is determined by the following method. A test specimen is taken that has a cross-section parallel to the normal direction of the surface of the carburized steel part. This cross-section of the test specimen is designated as the observation surface. The test specimen is taken such that the observation surface includes the surface of the carburized steel part and also includes an area extending at least 1 mm in depth from the surface.

[0071] The observation surface is mirror-polished, and then etched using a 5% picral etching solution (etching solution containing 5g of picric acid per 100mL of ethanol). A field emission scanning electron microscope (FE-SEM) is used to generate a secondary electron image (SEM image) of the etched observation surface. The observation field of view is a 50μm × 50μm rectangle within the region from the surface of the carburized steel part to a depth of 100μm in the direction normal to the surface.

[0072] A rectangular field of view is observed at a magnification of 5000x, and a SEM image is generated. Particles are identified in the SEM image based on contrast. The equivalent circle diameter of each identified particle is determined. Among the identified particles, those with an equivalent circle diameter of 0.1 μm or more are identified as the carbide material being measured.

[0073] The total area of ​​the identified carbides is determined. The area of ​​the carbides can be determined by a well-known image analysis method. The total area ratio AR (%) of the carbides is calculated based on the total area of ​​the carbides within the observation field and the area of ​​the observation field (50 μm × 50 μm). The total area ratio AR is rounded to the first decimal place by rounding the second decimal place of the obtained value.

[0074] [(Feature 3) About Fn1] The average carbon concentration in the carburized hardened surface region of the carburized steel part of this embodiment is the average carbon concentration [C]. ave (Defined as mass%). Furthermore, the total area ratio of carbides in the carburized and hardened surface region is defined as total area ratio AR(%). At this time, the average carbon concentration [C] ave The Fn1, defined by equation (1) using (mass%) and the total area ratio AR(%) of the carbides, is 0.65 or greater. Fn1=([C] ave -0.067AR) (1)

[0075] Fn1 is an index that indicates the strength of the matrix, which is the region of the carburized hardened layer other than the carbides, among the components that contribute to the surface fatigue strength of the carburized hardened surface layer. If Fn1 is less than 0.65, even if the total area ratio AR of carbides in the carburized hardened surface region is 1.5% or more, sufficient surface fatigue strength cannot be obtained because the strength of the matrix of the carburized hardened layer is insufficient. If Fn1 is 0.65 or more, the strength of the matrix of the carburized hardened layer is sufficiently high. Therefore, assuming that features 1, 2, and 4 are satisfied, excellent surface fatigue strength can be obtained in carburized steel parts.

[0076] The preferred lower limit of Fn1 is 0.67 or higher, more preferably 0.70, even more preferably 0.80, and even more preferably 0.90. The upper limit of Fn1 is not particularly limited. For example, the upper limit of Fn1 could be 2.00 or 1.60.

[0077] [Average carbon concentration [C] ave Measurement method] Average carbon concentration [C] in the carburized and hardened surface region ave (Mass %) is measured by the following method. A test specimen is taken that has a cross-section parallel to the normal direction of the surface of the carburized steel part. This cross-section of the test specimen is designated as the observation surface. The test specimen is taken such that the observation surface includes the surface of the carburized steel part and also includes an area extending at least 1 mm in depth from the surface.

[0078] The observation surface is polished to a mirror finish. Of the mirror-polished observation surface, the region from the surface to a depth of 100 μm is defined as the carburized hardened surface region. Within the carburized hardened surface region, an arbitrary rectangular region measuring 90 μm in the depth direction and 10 μm in the direction perpendicular to the depth direction is defined as the C concentration measurement region. Three arbitrary rectangular regions are selected within the carburized hardened surface region.

[0079] Within each rectangular region, line analysis is performed using an electron probe microanalyzer (EPMA) to measure the carbon concentration (mass%). Within each rectangular region, an electron beam is irradiated from the surface side of the carburized steel part at 3 μm intervals in the depth direction, and the carbon concentration is measured at a total of 31 points. For the EPMA line analysis, the acceleration voltage is set to 15 kV, the irradiation current to 500 nA, and the electron beam diameter to 3 μm.

[0080] The arithmetic mean of all C concentrations obtained from the three rectangular regions is used to determine the average carbon concentration [C] in the carburized and hardened surface region. ave (Assuming mass %). Note that the average carbon concentration [C] ave (Mass %) is the value obtained by rounding the third decimal place of the obtained arithmetic mean to the second decimal place.

[0081] [(Feature 4) About Fn2] In the carburized steel part of this embodiment, the Si content and Mo content in mass percent of the chemical composition of the core, and the solid solution Cr concentration [Cr] of the carburized hardened surface region are further specified. sol The value of Fn2, defined by equation (2) using (mass%), is 3.7 or greater. Fn2 = 5Si + 2Mo + 10[Cr] sol (2)

[0082] Fn2 is an index that indicates the tempering softening resistance of the matrix of the carburized steel layer, which is one of the components that contribute to the surface fatigue strength of the carburized steel surface layer. When carburized steel parts are used, the carburized steel layer of the carburized steel part comes into contact with other components. At this time, heat is generated in the carburized steel layer due to contact. It is preferable that the strength of the matrix of the carburized steel layer does not decrease even when such heat is generated. If Fn2 is 3.7 or higher, the strength of the matrix of the carburized steel layer is sufficiently maintained even when heat is generated due to contact. As a result, excellent surface fatigue strength can be obtained in carburized steel parts.

[0083] A preferred lower limit for Fn2 is 4.0, more preferably 4.2, even more preferably 4.4, even more preferably 4.6, and even more preferably 4.8. There is no particular upper limit to Fn2. For example, the upper limit of Fn2 could be 30.0, 26.0, or 24.0.

[0084] [Solid solution Cr concentration in the carburized and hardened surface region [Cr]] sol [Measurement Method] Solid solution Cr concentration in the carburized and hardened surface region [Cr] sol (By mass %) can be calculated using the following method. A test specimen containing the surface with the carburized hardened layer is taken from a carburized steel part. The surface of the test specimen other than the surface with the carburized hardened layer (the cut surface) is coated with an insulating resin. Constant current electrolysis is performed on the sample with the coated cut surface using a 10% AA system solution (a solution containing 10% acetylacetone, 1% tetramethylammonium chloride, and 89% methanol solution by volume fraction).

[0085] In constant current electrolysis, the current density is 30 mA / cm² at room temperature. 2 The specimen is held in place, and the region from the surface to a depth of 100 μm is electrolyzed. The tolerance range for the depth position is ±20 μm (i.e., 100 μm ± 20 μm from the surface). The depth of the electrolyzed region is determined by the specific gravity of the specimen, which is 7.8 g / cm³. 3 The constant current electrolysis is performed by measuring the mass difference (decrease) (g) of the sample before and after constant current electrolysis, and by measuring the surface area of ​​the test specimen (excluding the cut surface coated with insulating resin). After constant current electrolysis, the test specimen is immersed in an alcohol solution and then ultrasonically cleaned to remove any deposits from the surface of the test specimen.

[0086] The 10% AA solution used for constant current electrolysis and the alcohol solution used for subsequent ultrasonic cleaning are filtered by suction using a 0.2 μm mesh filter to extract the residue. Elemental chemical analysis is performed on the extracted residue using ICP-AES. Specifically, the residue is dissolved in acid to obtain a solution. Elemental chemical analysis is performed on the solution using ICP-AES to obtain the Cr mass A (g) in the residue. The obtained Cr mass A (g), the mass B (g) of the test specimen dissolved by constant current electrolysis, and the Cr concentration contained in the steel of the test specimen are used: [Cr]st Based on (mass%), the solid solution Cr concentration is: [Cr] sol We can find it using the following formula. [Cr] sol =[Cr] st -A / B×100

[0087] [Effects of the carburized steel parts of this embodiment] The carburized steel component of this embodiment satisfies features 1 to 4. Therefore, the carburized steel component of this embodiment provides excellent surface fatigue strength.

[0088] [Applications of the carburized steel parts of this embodiment] The carburized steel parts of this embodiment are widely applicable as structural components for machinery used in automobiles, industrial machinery, and the like. While the carburized steel parts are not particularly limited, examples include gears, shafts, and bearings. Furthermore, the carburized steel parts of this embodiment are also applicable to uses other than structural components for machinery.

[0089] [Manufacturing method for carburized steel parts according to this embodiment] An example of a manufacturing method for the carburized steel parts of this embodiment will be described. Carburized steel parts satisfying features 1 to 4 may be manufactured by methods other than those described below. However, the manufacturing method described below is a preferred example of a manufacturing method for the carburized steel parts of this embodiment.

[0090] An example of a method for manufacturing the carburized steel part of this embodiment includes the following steps. (Process 1) Steel material preparation process (Process 2) Hot working process (Step 3) Gas carburizing and quenching process The following describes each step.

[0091] [(Process 1) Steel material preparation process] In the material preparation process, steel materials to be used as raw materials for the structural components of the carburizing machine of this embodiment are prepared. Specifically, molten steel having a chemical composition that satisfies Feature 1 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 subjected to refining 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 Feature 1.

[0092] Using the molten steel produced by the refining method described above, a material is manufactured by a well-known casting method. For example, a bloom is produced using the molten steel by a continuous casting method. A well-known hot working method is then performed on the bloom to produce a steel material of a predetermined shape. For example, after heating the bloom, a billet or steel material is produced by hot forging. Alternatively, after heating the bloom, a billet is produced by hot forging, bract rolling, and / or hot rolling using a continuous rolling mill. In this case, the produced billet is reheated and a steel material is produced by finish rolling using a continuous rolling mill. The heating temperatures for the bloom and billet mentioned above are not particularly limited, but are, for example, 1000 to 1300°C. The steel produced by hot working is then cooled to room temperature.

[0093] [(Process 2) Hot working process] In the hot working process, the prepared steel material is subjected to hot working to produce an intermediate product having 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 cooled to room temperature after hot working. If necessary, machining may be performed on the intermediate product after hot working.

[0094] [(Step 3) Gas carburizing and quenching process] In the gas carburizing and quenching process, intermediate products after hot working or machining are subjected to gas carburizing and quenching to manufacture carburized steel parts. The gas carburizing and quenching process consists of two gas carburizing and quenching steps. Specifically, the gas carburizing and quenching process includes a first gas carburizing and quenching step, a second gas carburizing and quenching step, and a tempering step. The second gas carburizing and quenching step is performed after the first gas carburizing and quenching step. The tempering step is performed after the second gas carburizing and quenching step.

[0095] In each of the first and second gas carburizing and quenching processes, the intermediate product is heated and held in an atmosphere containing a well-known carburizing and modification gas, and then rapidly cooled. The carburizing and modification gas refers to a well-known endothermic modification 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 carbon potential Cp is not particularly limited, but for example, it is set to 0.60 to 2.00. In the tempering process, the carburized and quenched intermediate product is tempered at 150-200°C.

[0096] [Manufacturing conditions in the gas carburizing and quenching process] The gas carburizing and quenching process further satisfies the following manufacturing conditions. (Condition 1) In the first carburizing and quenching process, the heating temperature T1 (°C) is set to be equal to or greater than FnA as defined by formula (A). FnA=764+15Si+5Mn+40Cr+10Mo (A) Here, each element symbol in equation (A) is substituted with the corresponding element content in mass percent of the chemical composition of the core (the steel material). (Condition 2) In the second carburizing and quenching process, the heating temperature T2 (°C) is set to be greater than or equal to FnB as defined by formula (B), and less than or equal to FnC as defined by formula (C). FnB = 760 × 10Cr (B) FnC=894+15Si+5Mn+40Cr+10Mo (C) Here, the elemental symbols in equations (B) and (C) are substituted with the corresponding elemental content in mass percent of the chemical composition of the core (the steel material). The following explains each condition.

[0097] [Regarding Condition 1] In the first gas carburizing and quenching process, heating temperature T1 is the heating temperature (°C) in the carburizing and diffusion processes. If heating temperature T1 is less than FnA, the heating temperature in the first gas carburizing and quenching process is too low. In this case, even if the second gas carburizing and quenching process is carried out, the total area ratio AR of carbides in the carburized hardened layer of the manufactured carburized steel part will be excessively low, or Fn1 will be excessively low. Therefore, heating temperature T1 (°C) should be set to be FnA or higher.

[0098] [Regarding Condition 2] In the second gas carburizing and quenching process, the heating temperature T2 is the heating temperature (°C) in the carburizing and diffusion processes. If the heating temperature T2 is less than FnB, Cr is excessively distributed to the carbides, and the amount of dissolved Cr decreases excessively. As a result, Fn2 becomes excessively low. On the other hand, if the heating temperature T2 exceeds FnC, the formation of carbides becomes excessively low. As a result, the total area ratio AR of carbides in the carburized hardened layer of the manufactured carburized steel part becomes excessively low. Therefore, the heating temperature T2 (°C) should be set to be between FnB and FnC.

[0099] The carburized machine structural component of this embodiment is manufactured through the above process. If necessary, the steel material may be subjected to normalizing or spheroidizing annealing before the hot working process. [Examples]

[0100] Steel materials having the chemical compositions shown in Table 1 (Tables 1A and 1B) were manufactured.

[0101] [Table 1A]

[0102] [Table 1B]

[0103] In Table 1, a "-" indicates that the corresponding element was intentionally omitted.

[0104] Blooms were produced by continuous casting using molten steel. The produced blooms were heated and subjected to bloc rolling and subsequent hot rolling in a continuous rolling mill to produce 160mm x 160mm billets. The heating temperature of the blooms during bloc rolling was 1000°C to 1300°C. Furthermore, the produced billets were heated again and then subjected to finish rolling in a continuous rolling mill to produce steel bars. The heating temperature of the billets during finish rolling was 1000°C to 1300°C. The produced steel bars were air-cooled to room temperature. Through the above manufacturing process, steel bars with a diameter of 50mm were produced.

[0105] The manufactured steel material was subjected to hot working and gas carburizing and quenching processes to produce carburized steel parts (small roller test pieces) with the shape shown in Figure 1. Figure 1 is a side view of the manufactured small roller test piece. The numbers in Figure 1 represent dimensions (in mm). "φ" in Figure 1 represents the diameter.

[0106] Specifically, the steel material for each test number (50 mm diameter steel bar) was heated to a heating temperature of 1200°C, and then hot forged to produce a 35 mm diameter steel bar. The manufactured steel bars were allowed to cool to room temperature in the atmosphere. Machining was performed on the 35 mm diameter steel bars to produce intermediate products with the shape shown in Figure 1. Gas carburizing and quenching treatment was then performed on the intermediate products for each test number.

[0107] Specifically, the manufactured intermediate product was subjected to a first gas carburizing and quenching process, a second gas carburizing and quenching process, and a tempering process in sequence. The heating temperature T1 (°C) in the first gas carburizing and quenching process is shown in Table 2. The holding time at heating temperature T1 was 90 to 900 minutes. The heating temperature T2 in the second gas carburizing and quenching process is shown in Table 2. The holding time at heating temperature T2 was 10 to 300 minutes. The carbon potential CP in both the first and second gas carburizing and quenching processes was set to 0.80 to 1.80. In both the first and second gas carburizing and quenching processes, quenching was performed by oil quenching at 130°C. The tempering temperature in the tempering process was 150 to 200°C for all test numbers.

[0108] [Table 2]

[0109] Table 2 also shows FnA to FnC. Through the manufacturing process described above, multiple carburized steel parts (small roller test pieces 1) for each test number were produced for each test number.

[0110] [About the evaluation test] The following evaluation tests were performed on the manufactured carburized steel parts. (Test 1) Measurement test of the total area ratio (AR) of carbides (Test 2) Average carbon concentration [C] ave Measurement test (Test 3) Solid solution Cr concentration [Cr] sol Measurement test (Test 4) Surface fatigue strength evaluation test The following explains Exams 1 through 4.

[0111] [(Test 1) Measurement test of the total area ratio (AR) of carbides] In accordance with the method described in [Method for measuring the total area ratio AR of carbides in the carburized surface region] above, the total area ratio AR (%) of carbides in the carburized surface region of the carburized steel parts for each test number was determined. The measurement site was the carburized surface region of the cylindrical part with a diameter of 26.0 mm as shown in Figure 1. The obtained total area ratios AR (%) are shown in Table 3.

[0112] [Table 3]

[0113] [(Test 2) Average carbon concentration [C] ave [Measurement Test] The above-mentioned average carbon concentration [C] ave The average carbon concentration [C] in the carburized surface region of the carburized steel part for each test number was determined in accordance with the method described in [Measurement Method]. ave The (mass %) was calculated. The measurement site was the carburized and hardened surface region of the cylindrical section with a diameter of 26.0 mm, as shown in Figure 1. The obtained average carbon concentration [C] ave Table 3 shows the (mass%) and Fn1 values.

[0114] [(Test 3) Solid solution Cr concentration [Cr] sol [Measurement test] The above-mentioned [Solid solution Cr concentration in the carburized hardened surface region [Cr] sol In accordance with the method described in [Measurement Method], the solid solution Cr concentration [Cr] in the carburized hardened surface region of the carburized steel part for each test number was measured. sol The (mass %) was calculated. The measurement site was the carburized hardened surface region of the cylindrical section with a diameter of 26.0 mm, as shown in Figure 1. The obtained solid solution Cr concentration [Cr] sol Table 3 shows the (mass%) and Fn2 values.

[0115] [(Test 4) Surface Fatigue Strength Evaluation Test] Using the carburized steel parts (small roller test piece 1) for each test number, the roller pitting test (two-cylinder rolling fatigue test) shown in Figure 2 was performed to evaluate the surface fatigue strength of the carburized steel parts. The roller pitting test machine "RP201" manufactured by Komatsu Engineering Co., Ltd. was used as the testing machine.

[0116] Specifically, referring to Figure 2, the small roller test piece 1, which is a carburized steel part, was pressed against the large roller test piece 2 while the small roller test piece 1 was rotated. The large roller test piece 2 was prepared by the following method.

[0117] An intermediate product with the shape shown in Figure 3 was machined from a cylindrical steel material with a diameter of 140 mm having a chemical composition equivalent to SUJ2 as specified in JIS G 4805 (2019). The numbers in Figure 3 represent dimensions (in mm). In Figure 3, "φ" means the diameter at the center of the thickness. In Figure 3, "R150" means that the crowning radius of the outer surface is 150 mm. The intermediate product underwent quenching and tempering processes. In the quenching process, the intermediate product was heated and held at 830°C for 60 minutes in an atmosphere with a carbon potential Cp of 0.6%, and then rapidly cooled in oil at 130°C. After the quenching process, the intermediate product underwent tempering. The tempering temperature was set to 180°C, and the holding time at the tempering temperature was set to 120 minutes. After the holding time, it was cooled in the air. A large roller test piece 2 with the shape shown in Figure 3 was produced using the above manufacturing process.

[0118] The cylindrical portion of the small roller test piece 1, with a diameter of 26.0 mm, was rolled in contact with the center of the outer surface of the large roller test piece 2 (the outer portion with a diameter of 130 mm). The contact pressure was set to a Hertz pressure of 2.0 to 3.7 GPa. The rotation speed of the small roller test piece 1 was set to 2000 rpm. The peripheral speed direction of both roller test pieces (small roller test piece 1 and large roller test piece 2) at the contact point was set to the same direction, and the slip ratio was set to -40% (the peripheral speed at the contact point of the large roller test piece 2 was 40% higher than that of the small roller test piece 1). During the test, lubricating oil was supplied to the contact area between the small roller test piece 1 and the large roller test piece 2. The lubricating oil was automatic transmission fluid, with an oil temperature of 90°C and an oil flow rate of 2.0 L / min. The number of cycles to terminate the roller pitting test was 2.0 × 10⁻⁶. 7 The number of cycles was set to 2.0 × 10 in the small roller test specimen 1, where no fatigue delamination (pitting damage) occurred. 7The maximum surface pressure (GPa) reached was defined as the fatigue limit (GPa) of the small roller test specimen 1. Pitching damage was detected using a vibration meter installed in the testing machine. After vibration detection, the rotation of both the small roller test specimen 1 and the large roller test specimen 2 was stopped, and the presence or absence of pitting damage in the small roller test specimen 1 was visually checked. If pitting damage was confirmed, the number of repetitions at the time of vibration detection was defined as the number of repetitions at which pitting damage occurred. If no pitting damage was confirmed, the roller pitting test was repeated from the beginning using a different small roller test specimen 1 at that surface pressure.

[0119] The fatigue limit ratio was defined as the ratio of the fatigue limit of the carburized steel component at each test number to the fatigue limit of the reference carburized steel component at test number REF. In other words, the fatigue limit ratio was calculated using the following formula. Fatigue limit ratio = Fatigue limit (GPa) for the relevant test number / Fatigue limit (GPa) for test number REF

[0120] If the obtained fatigue limit ratio was 1.20 or higher, it was determined that excellent surface fatigue strength had been obtained (indicated as "E (Excellent)" in the "Surface Fatigue Strength" column in Table 3). On the other hand, if the fatigue limit ratio was less than 1.20, it was determined that excellent surface fatigue strength had not been obtained (indicated as "B (Bad)" in the "Surface Fatigue Strength" column in Table 3).

[0121] [Evaluation Results] Referring to Tables 1 to 3, in test numbers 1 to 18, the carburized steel parts met characteristics 1 to 4. Therefore, the fatigue limit ratio was 1.20 or higher, and excellent surface fatigue strength was obtained.

[0122] In tests 19 and 20, the first gas carburizing and quenching process was not performed. As a result, the total area ratio (AR) of carbides was too low. Consequently, the fatigue limit ratio was less than 1.20, and sufficient surface fatigue strength could not be obtained.

[0123] In tests 21 and 22, the heating temperature T1 in the first gas carburizing and quenching process was too low. As a result, Fn1 was too low. Consequently, the fatigue limit ratio was less than 1.20, and sufficient surface fatigue strength could not be obtained.

[0124] In tests 23 and 24, the heating temperature T2 in the second gas carburizing and quenching process was too low. As a result, Fn2 was too low. Consequently, the fatigue limit ratio was less than 1.20, and sufficient surface fatigue strength could not be obtained.

[0125] In tests 25 and 26, the heating temperature T2 in the second gas carburizing and quenching process was too high. As a result, the total area ratio AR of carbides was too low. Consequently, the fatigue limit ratio was less than 1.20, and sufficient surface fatigue strength could not be obtained.

[0126] 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 steel parts, Carburized hardened layer, It comprises a core portion located inside the aforementioned carburized hardened layer, The aforementioned core portion is The chemical composition is expressed in mass percent. C: 0.14-0.30%, Si: 0.10-0.80%, Mn: 0.10-2.00%, P: 0.015% or less, S: 0.025% or less, Cr: 0.36-2.00%, Mo: 0.01-0.50%, Al: 0.005-0.100%, N: 0.0020 to 0.0300%, and, O: Contains 0.0015% or less, The remainder consists of Fe and impurities. In the carburized surface region of the carburized layer, from the surface to a depth of 100 μm, the total area ratio (AR) of carbides is 1.5% or more. In the carburized and hardened surface region, the average carbon concentration [C] ａｖｅ Fn1, defined by formula (1) using (mass%) and the total area ratio AR (%), is 0.65 or greater. The Si content and Mo content in mass percent of the aforementioned chemical composition, and the solid solution Cr concentration [Cr] in the carburized hardened surface region. ｓｏｌ The Fn2 defined by equation (2) using (mass%) is 3.7 or greater. Carburized steel parts. Fn1=([C] ａｖｅ -0.067AR) (1) Fn2=5Si+2Mo+10[Cr] ｓｏｌ (2)

2. Carburized steel parts, Carburized hardened layer, It comprises a core portion located inside the aforementioned carburized hardened layer, The aforementioned core portion is The chemical composition is expressed in mass percent. C: 0.14-0.30%, Si: 0.10-0.80%, Mn: 0.10-2.00%, P: 0.015% or less, S: 0.025% or less, Cr: 0.36-2.00%, Mo: 0.01-0.50%, Al: 0.005-0.100%, N: 0.0020 to 0.0300%, and, O: Contains 0.0015% or less, Furthermore, it contains one or more selected from the groups consisting of groups 1 to 4, The remainder consists of Fe and impurities. In the carburized surface region of the carburized layer, from the surface to a depth of 100 μm, the total area ratio (AR) of carbides is 1.5% or more. In the carburized and hardened surface region, the average carbon concentration [C] ａｖｅ Fn1, defined by formula (1) using (mass%) and the total area ratio AR (%), is 0.65 or greater. The Si content and Mo content in mass percent of the aforementioned chemical composition, and the solid solution Cr concentration [Cr] in the carburized hardened surface region. ｓｏｌ The Fn2 defined by equation (2) using (mass%) is 3.7 or greater. Carburized steel parts. [Group 1] Cu: 0.20% or less, Ni: 0.35% or less, B: One or more selected from the group consisting of 0.0050% or less. [Group 2] V: 0.15% or less, Nb: 0.100% or less, Ti: 0.100% or less, One or more selected from the group consisting of W: 0.15% or less. [Group 3] Ca: 0.0050% or less, One or more selected from the group consisting of Mg: 0.010% or less. [Group 4] Te: 0.050% or less, Bi: 0.100% or less, Pb: 0.09% or less, Sn: 0.050% or less, Sb: One or more selected from the group consisting of 0.050% or less. Fn1=([C] ａｖｅ -0.067AR) (1) Fn2=5Si+2Mo+10[Cr] ｓｏｌ (2)

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

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

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

6. A carburized steel part according to claim 2, The aforementioned chemical composition contains the fourth group, Carburized steel parts.

7. A carburized steel part according to any one of claims 1 to 6, The aforementioned carburized steel component is a gear. Carburized steel parts.

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