Carburized parts

Abstract

To provide a carburized component that exhibits excellent bending fatigue strength. [Solution] The carburized component of the present disclosure has a core chemical composition in mass%, containing C: 0.10-0.40%, Si: greater than 1.00%-2.00%, Mn: 0.30-1.00%, P: less than 0.030%, S: less than 0.030%, Cr: less than 0.01-0.80%, Al: 0.010-0.100%, N: 0.003-0.030%, and O: 0.0030% or less, with the remainder being Fe and impurities, satisfying formula (1A), and having an average particle size number of prior austenite grains of 9.7-13.0 in the region from the surface to a depth of 3.0 mm, and a standard deviation of prior austenite grain size of 1.00 μm or less. 45 × Mn + 25 × Cr < 60.0 (1A)

Description

Technical Field

[0001] The present invention relates to carburized parts.

Background Art

[0002] Mechanical parts used in power sources such as engines and electric motors of automobiles and industrial machines and power transmission mechanisms are repeatedly subjected to bending stress due to loads such as impacts received during use. Therefore, high bending fatigue strength is required for mechanical parts for these applications.

[0003] Mechanical parts are usually manufactured from steel materials through the following manufacturing processes. Hot forging is performed on the steel material used as the raw material, and further machining is performed as necessary to produce an intermediate product having a desired shape. Heat treatment is performed on the intermediate product to adjust the hardness and microstructure of the steel material. When improvement of the bending fatigue strength of mechanical parts is required, carburizing and quenching treatment may be performed as the above heat treatment. The bending stress applied to mechanical parts is usually maximum at the surface layer. And cracks that cause bending fatigue failure often progress starting from the surface. In contrast, in carburizing and quenching treatment, a carburized hardened layer can be formed on the surface layer of the steel material to harden the surface layer of the steel material. As a result, the bending fatigue strength of mechanical parts is improved. In this specification, a mechanical part subjected to carburizing and quenching treatment is also referred to as a "carburized part".

[0004] In recent years, for carburized parts used in the above applications, with the increase in the transmission torque of mechanical parts due to higher output and the miniaturization of mechanical parts, even higher bending fatigue strength has been required. A technique for further enhancing the bending fatigue strength of carburized parts is proposed, for example, in Japanese Patent Laid-Open No. 8-092690 (Patent Document 1).

[0005] The carburized part disclosed in Patent Document 1 is made of steel that contains, by mass%, C: 0.10-0.30%, Mn: 0.3-2.0%, Cr: 1.20% or less, Al: 0.01-0.06%, N: 0.0040-0.0200%, B: 0.0050% or less, with the remainder being Fe and unavoidable impurities, where Si is 0.3% or less, P is 0.03% or less, and S is 0.03% or less. Furthermore, the carbon concentration on the surface of the carburized hardened layer is 0.5-1.0% by weight, the austenite grain size of the carburized hardened layer is JIS grain size number 9 or higher, and the core hardness is Hv350 or higher. Patent Document 1 states that this provides excellent bending fatigue strength. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 8-092690 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, excellent bending fatigue strength may be obtained in the carburized part by means other than those proposed in Patent Document 1.

[0008] The objective of this disclosure is to provide a carburized component that exhibits excellent bending fatigue strength. [Means for solving the problem]

[0009] The carburized parts of this disclosure are A carburized hardened layer formed on the surface, It comprises a core portion located inside the aforementioned carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.10~0.40%, Si: more than 1.00%~2.00%, Mn: 0.30~1.00%, P: Less than 0.030% S: Less than 0.030% Cr: 0.01-0.80% 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 (1A), In the region from the surface of the carburized component to a depth of 3.0 mm, the average particle size number of the prior austenite grains is 9.7 to 13.0, and the standard deviation of the prior austenite grain size is 1.00 μm or less. 45 × Mn + 25 × Cr < 60.0 (1A) Here, the content (mass%) of the corresponding element is substituted for each element symbol in equation (1A).

[0010] The carburized parts of this disclosure are A carburized hardened layer formed on the surface, It comprises a core portion located inside the aforementioned carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.10~0.40%, Si: more than 1.00%~2.00%, Mn: 0.30~1.00%, P: Less than 0.030% S: Less than 0.030% Cr: 0.01-0.80% 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 (1B), In the region from the surface of the carburized component to a depth of 3.0 mm, the average particle size number of the prior austenite grains is 9.7 to 13.0, and the standard deviation of the prior austenite grain size is 1.00 μm or less. [Group 1] Mo: less than 0.80%, Ti: less than 0.100%, Nb: less than 0.100%, V: less than 0.50%, and B: less than 0.0100%, one or more selected from the group consisting of [Group 2] Cu: less than 0.50%, Ni: less than 0.50%, and Sn: less than 0.015%, one or more selected from the group consisting of [Group 3] Ca: less than 0.0100%, and Mg: less than 0.0100%, one or more selected from the group consisting of 45×Mn + 40×V + 25×Cr + 25×Mo + 20×Ni + 10×Cu < 60.0 (1B) Here, for each element symbol in formula (1B), the content in mass % of the corresponding element is substituted. When the element is not contained, "0" is substituted for the corresponding element symbol.

Advantages of the Invention

[0011] The carburized part of the present disclosure can obtain excellent bending fatigue strength.

Brief Description of the Drawings

[0012] [Figure 1] FIG. 1 is a side view of a小野 type rotating bending fatigue test piece used in the bending fatigue strength evaluation test.

Modes for Carrying Out the Invention

[0013] The inventors of the present invention have conducted various studies on means for increasing the bending fatigue strength of carburized parts.

[0014] It should be noted that "小野式" in the original text might be a specific name or term in Japanese, and since there is no direct equivalent in English, it is left as it is. If it is a well-known technical term with an established English translation, it should be replaced accordingly.The inventors first investigated means to increase the bending fatigue strength of carburized parts from the perspective of chemical composition. As a result, the inventors found that in a carburized part comprising a carburized hardened layer formed on the surface and a core portion inside the carburized hardened layer, the chemical composition of the core portion is C: 0.10~0.40%, Si: greater than 1.00%~2.00%, Mn: 0.30~1.00%, P: less than 0.030%, S: less than 0.030%, Cr: less than 0.01~0.80%, Al: 0.010~0.100%, N: 0.003~0.030%, O: 0.0 It was thought that excellent bending fatigue strength could be obtained if the material contained less than 0.30%, with Mo: 0-0.80%, 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%, with the remainder being Fe and impurities, and satisfying the following formula (1B). 45×Mn+40×V+25×Cr+25×Mo+20×Ni+10×Cu<60.0 (1B) Here, in equation (1B), each element symbol is substituted with the mass percentage content of the corresponding element, and if an element is not present, "0" is substituted for the corresponding element symbol.

[0015] Equation (1B) is an equation to suppress excessive improvement in austenite stability. As mentioned above, carburizing and quenching treatment is performed in the manufacturing process of carburized parts. In the process of carburizing and quenching, the steel material that will be used for the carburized parts is A c3After being heated and held at a temperature above 15°C, the material is rapidly cooled. As a result, the microstructure of the carburized part consists mainly of martensite. On the other hand, the microstructure of the carburized part may contain retained austenite. Retained austenite is softer than martensite. Here, Mn, V, Cr, Mo, Ni, and Cu contained in the carburized part enhance the stability of the austenite. Increased austenite stability leads to an increase in the amount of retained austenite in the carburized part manufactured through the carburizing and quenching process. Furthermore, retained austenite in the carburized part may undergo work-induced martensitic transformation due to external forces applied during use of the carburized part. However, if the stability of retained austenite is high, the occurrence of work-induced martensitic transformation is also suppressed. As a result, the bending fatigue strength of the carburized part decreases. Conversely, the inventors believe that the bending fatigue strength of the carburized part can be increased by limiting the content of the aforementioned elements that enhance austenite stability so that the chemical composition of the core satisfies equation (1B).

[0016] However, even with carburized parts having the above-mentioned chemical composition, excellent bending fatigue strength was sometimes not obtained. Therefore, the inventors focused on the prior austenite grain size of the carburized parts and conducted further investigations.

[0017] In carburized parts that did not achieve excellent bending fatigue strength, cracks were observed to occur and propagate along the prior austenite grain boundaries. To suppress crack initiation, it is effective to refine the prior austenite grains in the microstructure of the carburized part. As mentioned above, the bending stress applied to a carburized part during use is maximum at the surface. However, in reality, significant bending stress is also applied in a region approximately 3.0 mm deep from the surface of the carburized part. Therefore, especially in carburized parts where the surface is sufficiently hardened, cracks may occur at a location deeper than the surface and propagate down to the surface. As will be described later, in this specification, the region from the surface of the carburized part to a depth of 0.2 mm is referred to as the "surface layer." Furthermore, the region from a depth of 0.2 mm from the surface of the carburized part to a depth of 3.0 mm is referred to as the "semi-surface layer." To increase the bending fatigue strength of carburized parts, it is preferable that prior austenite grains are refined not only in the surface layer where the carburized hardened layer is formed, but also in the subsurface layer where large bending stresses are applied.

[0018] On the other hand, the finer the prior austenite grains in a carburized part, the lower its hardenability. In the subsurface layer, the influence of carbon penetration and diffusion from the surface due to carburizing is smaller compared to the surface layer. Also, because the subsurface layer is an internal region compared to the surface layer, the heating and cooling rates during heat treatment are slower. Therefore, if the prior austenite grains in the subsurface layer are excessively fine, martensitic transformation does not proceed sufficiently in the subsurface layer of a carburized part whose core has the chemical composition described above. As a result, the bending fatigue strength of the carburized part actually decreases. Therefore, the inventors considered that the bending fatigue strength of a carburized part can be increased by appropriately adjusting the average grain size number of prior austenite grains in the region including not only the surface layer but also the subsurface layer of the carburized part.

[0019] Furthermore, the inventors also focused on the distribution of prior austenite grain size. In carburized parts, the prior austenite grain size generally increases from the surface to the interior. In addition, variations in heating temperature during the manufacturing process of carburized parts can lead to the formation of localized fine-grained or coarse-grained structures. Regions with coarse prior austenite grain size have lower strength compared to regions with fine prior austenite grain size. Regions with excessively fine prior austenite grain size also have reduced hardenability and low strength, as described above. These low-strength regions are prone to yielding under bending stress. If some regions yield locally, the stress applied to the surrounding regions increases further. As a result, yielding progresses in a chain reaction from regions with coarse or excessively fine prior austenite grain size, promoting bending fatigue fracture. In other words, even if the average grain size number of prior austenite grains in the region including the surface and subsurface of the carburized part is appropriate, if there is a large variation in the prior austenite grain size in that region, excellent bending fatigue strength cannot be obtained.

[0020] Based on the above findings, the inventors further investigated the relationship between the average particle size number and the standard deviation of the prior austenite grain size in the region from the surface to a depth of 3.0 mm, including the surface and subsurface layers of the carburized part, and the bending fatigue strength of the carburized part. As a result, the inventors found that in a carburized part having the above-described configuration, if the average particle size number of the prior austenite grain is 9.7 to 13.0 and the standard deviation of the prior austenite grain size is 1.00 μm or less in the region from the surface to a depth of 3.0 mm, excellent bending fatigue strength can be obtained.

[0021] The carburized component of this embodiment was completed based on the above technical concept and has the following configuration.

[0022] The carburized component of the first configuration is A carburized hardened layer formed on the surface, It comprises a core portion located inside the aforementioned carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.10~0.40%, Si: more than 1.00%~2.00%, Mn: 0.30~1.00%, P: Less than 0.030% S: Less than 0.030% Cr: 0.01-0.80% 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 (1A), In the region from the surface of the carburized component to a depth of 3.0 mm, the average particle size number of the prior austenite grains is 9.7 to 13.0, and the standard deviation of the prior austenite grain size is 1.00 μm or less. 45 × Mn + 25 × Cr < 60.0 (1A) Here, the content (mass%) of the corresponding element is substituted for each element symbol in equation (1A).

[0023] The carburized component of the second configuration is A carburized hardened layer formed on the surface, It comprises a core portion located inside the aforementioned carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.10~0.40%, Si: more than 1.00%~2.00%, Mn: 0.30~1.00%, P: Less than 0.030% S: Less than 0.030% Cr: 0.01-0.80% 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 (1B), In the region from the surface of the carburized component to a depth of 3.0 mm, the average particle size number of the prior austenite grains is 9.7 to 13.0, and the standard deviation of the prior austenite grain size is 1.00 μm or less. [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, and One or more selected from the group consisting of Mg: 0.0100% or less. 45×Mn+40×V+25×Cr+25×Mo+20×Ni+10×Cu<60.0 (1B) Here, in equation (1B), each element symbol is substituted with the mass percentage content of the corresponding element, and if an element is not present, "0" is substituted for the corresponding element symbol.

[0024] The third component, the carburized part, A carburized component of the second configuration, The chemical composition of the core contains the first group.

[0025] 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.

[0026] 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.

[0027] The carburized parts according to this embodiment will be described in detail below. Unless otherwise specified, the "%" in relation to elements refers to "mass%".

[0028] [Configuration of the carburized component in this embodiment] In this embodiment, a carburized part refers to a mechanical part that has undergone carburizing and quenching treatment. Carburizing and quenching treatment is, for example, vacuum carburizing and quenching treatment. In this specification, carburizing and quenching treatment includes carbonitriding and quenching treatment. Therefore, in this specification, the concept of a carburized hardened layer also includes a carbonitriding hardened layer. Furthermore, in this specification, the surface layer is defined as the region from the surface of the carburized part to a depth of 0.2 mm. The carburized component of this embodiment comprises a carburized hardened layer and a core portion located inside the carburized hardened layer.

[0029] [Carburized hardened layer] The carburized hardened layer is a layer formed when carbon penetrates the surface of the steel material from the outside during the carburizing and quenching process, causing it to harden. The carburized hardened layer is formed from the surface of the carburized part to a predetermined depth, including at least the surface layer. Here, if the carbon concentration at a depth of 0.2 mm from the surface of the carburized part is 0.50% or more by mass, it is determined that a carburized hardened layer has been formed on the surface. There is no particular upper limit to the carbon content of the surface layer of the carburized part, but for example, it is 1.20%. The microstructure of the carburized hardened layer consists mainly of martensite.

[0030] [Method for measuring C concentration at a depth of 0.2 mm from the surface] The carbon (C) concentration at a depth of 0.2 mm from the surface of the carburized component is determined using an electron probe microanalyzer (EPMA). Specifically, a test specimen is taken that includes the observation surface at a depth of 0.2 mm from the surface of the carburized component. The size of the test specimen is not particularly limited, as long as the observation surface includes a measurement area of ​​50 μm × 50 μm with the 0.2 mm depth position at its center. The observation surface should be parallel to the depth direction from the surface of the carburized component.

[0031] The carbon (C) concentration (mass%) is measured on the observation surface of the obtained test specimen using EPMA. Specifically, point analysis is performed at 5 μm intervals within the measurement area of ​​the observation surface. The arithmetic mean of the carbon concentration (mass%) at each measurement point obtained from the 11 x 11 point analysis is defined as the carbon concentration (mass%) at a depth of 0.2 mm from the surface. For the EPMA point analysis, the acceleration voltage is set to 15 kV, the irradiation current to 500 nA, and the electron beam diameter to 3 μm.

[0032] [Core] The core is the portion inside the carburized hardened layer and is unaffected by the penetration and diffusion of carbon due to the carburizing process. The hardness of the core is lower than that of the carburized hardened layer. The microstructure of the core consists mainly of martensite.

[0033] [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: C: 0.10-0.40%, Si: greater than 1.00%-2.00%, Mn: 0.30-1.00%, P: less than 0.030%, S: less than 0.030%, Cr: less than 0.01-0.80%, Al: 0.010-0.100%, N: 0.003-0.030%, O: 0.0030% or less, Mo: It contains 0-0.80% of the following elements: 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%, with the remainder being Fe and impurities. (Feature 2) The chemical composition of the core satisfies formula (1B). 45×Mn+40×V+25×Cr+25×Mo+20×Ni+10×Cu<60.0 (1B) Here, in equation (1B), each element symbol is substituted with the mass percentage content of the corresponding element, and if an element is not present, "0" is substituted for the corresponding element symbol. (Feature 3) In the region from the surface of the carburized component to a depth of 3.0 mm, the average grain size number of the prior austenite grains is between 9.7 and 13.0. (Feature 4) In the region from the surface of the carburized component to a depth of 3.0 mm, the standard deviation of the prior austenite grain size is 1.00 μm or less. Features 1 through 4 are explained below.

[0034] [(Feature 1) Chemical Composition] The chemical composition of the core of the carburized component in this embodiment contains the following elements:

[0035] C: 0.10~0.40% Carbon (C) increases the strength of carburized parts and enhances their 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%.

[0036] Si: More than 1.00%~2.00% Silicon (Si) enhances hardenability and increases the bending fatigue strength of carburized parts. If the Si content is 1.00% or less, 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 2.00%, the machinability 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 between 1.00% and 2.00%. The preferred lower limit of the Si content is 1.01%, more preferably 1.10%, and even more preferably 1.20%. The preferred upper limit for the Si content is 1.90%, more preferably 1.80%, and even more preferably 1.70%.

[0037] Mn: 0.30~1.00% Manganese (Mn) enhances hardenability and increases the 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%.

[0038] P: Less than 0.030% Phosphorus (P) is an impurity. If the P content is 0.030% or higher, P segregates excessively at the grain boundaries, reducing the grain boundary strength. As a result, even if the content of other elements is within the range of this embodiment, the bending fatigue strength of the carburized part decreases. 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 0.029%, more preferably 0.025%, even more preferably 0.020%, and even more preferably 0.017%.

[0039] S: Less than 0.030% Sulfur (S) is an impurity. If the S content is 0.030% or higher, coarse sulfide inclusions will form. As a result, even if the content of other elements is within the range of this embodiment, the 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 0.029%, more preferably 0.025%, even more preferably 0.020%, and even more preferably 0.017%.

[0040] Cr: 0.01-0.80% Chromium (Cr) enhances hardenability and increases the bending fatigue strength of carburized parts. If the Cr 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 Cr content is 0.80% or more, 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 less than 0.01-0.80%. The preferred lower limit for the Cr content is 0.03%, more preferably 0.05%, and even more preferably 0.07%. The preferred upper limit for the Cr content is 0.79%, more preferably 0.75%, and even more preferably 0.70%.

[0041] 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 bending fatigue strength of the carburized parts is 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 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%.

[0042] N: 0.003~0.030% Nitrogen (N) combines with Ti, Al, V, and Nb in the steel to form nitrides and carbonitrides, and the pinning effect suppresses grain coarsening during heating in the carburizing and quenching process. As a result, the bending fatigue strength of the carburized parts is 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. 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%.

[0043] 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 failure in carburized parts. If the O content exceeds 0.0030%, the bending fatigue strength of the carburized part will decrease, even if the content of other elements is within the range of this embodiment. Therefore, the O content is 0.0030% or less. A low oxygen content is preferable. However, excessive reduction of the oxygen content increases manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the oxygen content is greater than 0%, more preferably 0.0001%, even more preferably 0.0002%, and even more preferably 0.0003%. The preferred upper limit for the O content is 0.0028% or less, more preferably 0.0025%, and even more preferably 0.0020%.

[0044] The remainder of the chemical composition of the core of the carburized part in this embodiment consists of Fe and impurities. Here, impurities in the chemical composition refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment during the industrial production of the carburized part, and are acceptable within a range that does not adversely affect the carburized part in this embodiment.

[0045] [About Optional Elements] 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 Groups 1 to 3, instead 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, and One or more selected from the group consisting of Mg: 0.0100% or less. The following describes these arbitrary elements.

[0046] [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 bending fatigue strength of the carburized part.

[0047] 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 bending fatigue 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%.

[0048] 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, i.e., when the Ti content is greater than 0%, 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 bending fatigue strength of the carburized parts. Even a small amount of Ti present can provide some degree of the above effect. 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 bending fatigue strength of the carburized part decreases. 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%.

[0049] 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%. When Nb is present, i.e., when the Nb content is greater than 0%, Nb forms Nb precipitates such as Nb carbides and Nb carbonitrides. These Nb precipitates suppress grain coarsening during carburizing due to their pinning effect. As a result, they increase the bending fatigue strength of the carburized parts. Even a small amount of Nb can provide some degree of the above effect. 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 bending fatigue strength of the carburized part decreases. 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%.

[0050] 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 bending fatigue strength of 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%.

[0051] 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 bending fatigue strength of the carburized parts increases. Even a small amount of B is present to some extent to obtain 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%.

[0052] [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.

[0053] 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%.

[0054] 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%.

[0055] 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%.

[0056] [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.

[0057] 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 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%.

[0058] 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 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 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%.

[0059] [(Feature 2) Formula (1B)] The carburized component of this embodiment further satisfies formula (1B) in the chemical composition of its core. 45×Mn+40×V+25×Cr+25×Mo+20×Ni+10×Cu<60.0 (1B) Here, in equation (1B), each element symbol is substituted with the mass percentage content of the corresponding element, and if an element is not present, "0" is substituted for the corresponding element symbol. Furthermore, if the chemical composition of the core does not contain any arbitrary element, formula (1B) is equivalent to the following formula (1A). In other words, if the chemical composition of the core does not contain any arbitrary element, the chemical composition of the core only needs to satisfy formula (1A) instead of formula (1B). 45 × Mn + 25 × Cr < 60.0 (1A) Here, the content (mass%) of the corresponding element is substituted for each element symbol in equation (1A).

[0060] Fn1A and Fn1B are defined as follows: Fn1A = 45 × Mn + 25 × Cr Fn1B=45×Mn+40×V+25×Cr+25×Mo+20×Ni+10×Cu

[0061] Fn1A and Fn1B are indicators of austenite stability. As mentioned above, Mn, V, Cr, Mo, Ni, and Cu enhance austenite stability. In other words, the higher the Fn1A and Fn1B values, the more stable the austenite formed in the carburized part. Increased austenite stability leads to an increase in the amount of retained austenite in the carburized part manufactured through carburizing and quenching. It also suppresses the occurrence of work-induced martensitic transformation during use of the carburized part. As a result, the bending fatigue strength of the carburized part decreases. If Fn1A and Fn1B are 60.0 or higher, the austenite stability is excessively high. In this case, even if the carburized part satisfies features 1, 3, and 4, excellent bending fatigue strength cannot be obtained.

[0062] On the other hand, if Fn1A and Fn1B are less than 60.0, the austenite stability is sufficiently reduced. In this case, assuming that the carburized component satisfies features 1, 3, and 4, excellent bending fatigue strength can be obtained.

[0063] The lower limits of Fn1A and Fn1B are not particularly limited, but are, for example, 13.8 when the carburized part satisfies feature 1. Preferred lower limits for Fn1A and Fn1B are 20.0, more preferably 25.0, and even more preferably 30.0. The preferred upper limits for Fn1A and Fn1B are 57.0, more preferably 54.0, even more preferably 51.0, and even more preferably 48.0. Note that Fn1A and Fn1B are values ​​obtained by rounding the second decimal place of the calculated number to the first decimal place.

[0064] [(Feature 3) Average particle size number of old austenite grains] In this embodiment, the carburized component further has an average particle size number of prior austenite grains of 9.7 to 13.0 in the region from the surface of the carburized component to a depth of 3.0 mm. In this specification, this average particle size number is also referred to as "average particle size number GN". The region from the surface of the carburized component to a depth of 3.0 mm consists of a surface layer and a subsurface layer (the region from a depth of 0.2 mm from the surface of the carburized component to a depth of 3.0 mm).

[0065] As mentioned above, the bending stress applied to carburized parts during use is greatest in the surface layer, but is also somewhat large in the subsurface layer. Therefore, cracks may occur not only in the surface layer but also in the subsurface layer. To increase the bending fatigue strength of carburized parts, it is preferable to sufficiently refine the prior austenite grain size in both the surface and subsurface layers to suppress the occurrence of cracks. Here, if the average grain size number GN is less than 9.7, the prior austenite grain size in the surface and subsurface layers has not been sufficiently refined. In this case, even if the carburized part satisfies features 1, 2, and 4, excellent bending fatigue strength cannot be obtained.

[0066] On the other hand, the finer the prior austenite grains in a carburized part, the lower its hardenability. In the first place, the subsurface layer is less hardenable than the surface layer. If the average grain size number GN exceeds 13.0, the prior austenite grain size in the surface and subsurface layers is excessively fine. Therefore, martensitic transformation has not progressed sufficiently, especially in the subsurface layer. In this case, even if the carburized part satisfies features 1, 2, and 4, excellent bending fatigue strength cannot be obtained.

[0067] If the average grain size number GN is between 9.7 and 13.0, the prior austenite grain size in the surface and subsurface layers is within an appropriate range. Therefore, it is possible to suppress the initiation of cracks that cause bending fatigue fracture while suppressing the decrease in hardenability. In this case, assuming that the carburized part satisfies features 1, 2, and 4, excellent bending fatigue strength can be obtained.

[0068] The preferred lower limit for the average particle size number GN is 10.0, more preferably 10.2, and even more preferably 10.4. The preferred upper limit for the average particle size number GN is 12.8, more preferably 12.6, and even more preferably 12.4.

[0069] [(Feature 4) Standard deviation of the old austenite grain size] The carburized component of this embodiment further has a standard deviation of 1.00 μm or less in the region from the surface of the carburized component to a depth of 3.0 mm. In this specification, this standard deviation is also referred to as "standard deviation σ".

[0070] Even if a carburized component satisfies characteristic 3 above and the average grain size number GN of the prior austenite grain is within an appropriate range, if the variation in the prior austenite grain is large, there may be regions with coarse or excessively fine prior austenite grains. In these regions, the strength decreases locally. Therefore, these regions preferentially yield due to bending fatigue. If some regions in the carburized component locally yield, the stress applied to the surrounding regions increases further. As a result, yielding progresses in a chain reaction from the regions with coarse or excessively fine prior austenite grains, promoting bending fatigue failure. If the standard deviation σ exceeds 1.00 μm, the variation in prior austenite grain in the surface and subsurface layers is excessively large. In this case, even if the carburized component satisfies characteristics 1 to 3, excellent bending fatigue strength cannot be obtained.

[0071] If the standard deviation σ is 1.00 μm or less, the variation in prior austenite grain size in the surface and subsurface layers is sufficiently small. Therefore, chain-like yielding starting from localized low-strength regions can be suppressed. In this case, assuming that the carburized component satisfies features 1 to 3, excellent bending fatigue strength can be obtained.

[0072] The lower limit of the standard deviation σ is not particularly limited, but is preferably 0 μm. On the other hand, considering typical industrial production, the lower limit of the standard deviation σ is, for example, 0.40 μm, or for example, 0.45 μm. A preferred upper limit for the standard deviation σ is 0.95 μm, more preferably 0.90 μm, and even more preferably 0.85 μm.

[0073] [Measurement method for average particle size number GN and standard deviation σ] In this embodiment, the average particle size number GN and standard deviation σ in the region from the surface of the carburized component to a depth of 3.0 mm can be determined by the following method. Ten test specimens were taken from the carburized part, each containing an observation surface with a cross-section parallel to the depth direction at an arbitrary position on the surface. The size of the observation surface is not particularly limited, as long as it includes the surface of the carburized part and the area from the surface to a depth of 3.0 mm. After mirror polishing the observation surface, etching was performed with a saturated picric acid aqueous solution. The etched observation surface was observed using an optical microscope at an observation magnification of 2000x. Specifically, seven observation points were set starting from an arbitrary position on the surface of the carburized part at depths of 0.1 mm, 0.5 mm, 1.0 mm, 1.5 mm, 2.0 mm, 2.5 mm, and 3.0 mm. An observation field of view of 140 μm × 105 μm centered on each measurement point was observed at the above magnification. The grain size number G was determined in each observation field of view based on the cutting method in accordance with JIS G 0551 (2020). The same procedure was performed for all test specimens. The average grain size number GN is defined as the arithmetic mean of the grain size numbers G obtained from all 70 observation fields. The average grain size number GN is rounded to one decimal place by rounding the second decimal place of the obtained value.

[0074] Furthermore, the average particle size M (μm) of the prior austenite grains is calculated from the grain size number G of each observation field based on the following equations (I) and (II). The standard deviation (μm) is then calculated using the average particle size M of the 70 prior austenite grains obtained. The obtained value is defined as the standard deviation σ. Note that the standard deviation σ is the value obtained by rounding the third decimal place of the obtained value to two decimal places. n=2 G+3 (I) M = 1000 / (n 1 / 2 ) (II)

[0075] [Effects of the carburized parts in this embodiment] The carburized component of this embodiment satisfies features 1 to 4. Therefore, excellent bending fatigue strength can be obtained.

[0076] [Applications of the carburized parts of this embodiment] The carburized parts of this embodiment can be widely applied to applications requiring excellent bending fatigue strength. They are particularly suitable as gears used in automobiles, construction vehicles, and the like.

[0077] [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.

[0078] 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) Vacuum carburizing and quenching process (Step 4) Furnace heating and re-quenching process (Process 5) Tempering process The following describes each step.

[0079] [(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.

[0080] Using the molten steel produced by the refining method described above, a material is manufactured by a well-known casting method. For example, an ingot may be manufactured using the ingot-making method with the molten steel. Alternatively, a bloom may be manufactured using the continuous casting method with the molten steel. A well-known hot working method is performed on the manufactured material (ingot or bloom) to produce a steel material of a predetermined shape. For example, hot forging may be performed as part of the hot working. Alternatively, after performing bloc rolling or hot forging, finish rolling may be performed 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 for example, it is 1000 to 1300°C. The shape of the steel material manufactured in this way may be, for example, a steel bar or wire rod.

[0081] [(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.

[0082] [(Step 3) Vacuum Carburizing and Quenching Process] In the vacuum carburizing and quenching process, the intermediate product after the hot working process is subjected to vacuum carburizing and quenching treatment. The vacuum carburizing and quenching process includes the following steps: (Step 31) Heating step (Step 32) Soaking process (Step 33) Carburizing process (Step 34) Diffusion process (Process 35) Hardening process

[0083] [(Step 31) Heating step] In the heating process, the intermediate product is placed in a heat treatment furnace and heated to the carburizing temperature. At this time, the furnace pressure is set to, for example, 10 Pa or less. The carburizing temperature is, for example, 900 to 1100°C.

[0084] [(Step 32) Soaking process] In the soaking process, the intermediate product after the heating process is held at the carburizing temperature for a predetermined time to perform the soaking treatment. The furnace pressure during the soaking treatment may be 10 Pa or less, or a nitrogen gas atmosphere of 1000 Pa or less may be maintained by simultaneously introducing nitrogen gas and evacuating with a vacuum pump. The holding time in the soaking process is, for example, 30 to 60 minutes.

[0085] [(Step 33) Carburizing process] In the carburizing process, the intermediate product after the soaking process is held at the carburizing temperature in a carburizing gas atmosphere for a predetermined time. A well-known carburizing gas is used in the carburizing process. The carburizing gas is, for example, a hydrocarbon gas such as acetylene, propane, or ethylene. The carburizing gas pressure is set to a predetermined gas pressure depending on the type of carburizing gas. When acetylene is used as the carburizing gas, the carburizing gas pressure is, for example, 10 to 1000 Pa. When propane is used as the carburizing gas, the carburizing gas pressure is, for example, 200 to 3000 Pa. The holding time in the carburizing process is, for example, 30 to 240 minutes.

[0086] [(Step 34) Diffusion process] In the diffusion process, the intermediate product after the carburizing process is held at the carburizing temperature for a predetermined time. The furnace pressure in the diffusion process may be 100 Pa or less to remove residual gases from the carburizing process. Alternatively, a nitrogen gas atmosphere of 1000 Pa or less may be created by simultaneously introducing nitrogen gas and evacuating with a vacuum pump. The holding time in the diffusion process is, for example, 30 to 120 minutes.

[0087] [(Process 35) Hardening Process] In the quenching process, the intermediate product after the diffusion process is subjected to quenching treatment. Specifically, the intermediate product, heated to the carburizing temperature, is rapidly cooled by immersing it in a cooling bath containing oil or water, which is a cooling medium.

[0088] The quenching process in the vacuum carburizing and quenching process satisfies the following conditions: (Condition 1) The average cooling rate v1 from the carburizing temperature to 300°C shall be 5.0°C / second or higher.

[0089] [(Condition 1) Average cooling rate v1] In the intermediate product after the diffusion process, the solid solution concentration of carbon (C) is particularly high near the surface due to the intrusion and diffusion of C from the surface. If the average cooling rate from the carburizing temperature to 300°C is slow during the quenching process, the supersaturated C precipitates as carbides. If a large amount of precipitated carbides is present in the intermediate product after the quenching process, an excessively fine-grained structure is formed in the subsequent furnace heating and re-quenching process, with the carbides acting as nucleation sites. As a result, the average particle size number GN in the surface and near-surface layers becomes large. Furthermore, since the amount of carbide formation depends on the solid solution concentration of C increased by the carburizing treatment, variations occur in the surface and near-surface layers depending on the distance from the surface. Consequently, variations also occur in the prior austenite particle size obtained after the furnace heating and re-quenching process. As a result, the standard deviation σ becomes large.

[0090] If the average cooling rate v1 from the carburizing temperature to 300°C is less than 5.0°C / second, too much carbide precipitates during the cooling process. This greatly promotes the formation of excessive fine-grained structures. As a result, the average particle size number GN exceeds 13.0. Furthermore, the standard deviation σ exceeds 1.00 μm. On the other hand, if the average cooling rate v1 from the carburizing temperature to 300°C is 5.0°C / second or higher, the precipitation of carbide during the cooling process can be sufficiently suppressed. Therefore, the formation of excessive fine-grained structures can also be suppressed. In this case, assuming other conditions in the manufacturing process are met, the carburized part can satisfy features 3 and 4. Therefore, the average cooling rate v1 from the carburizing temperature to 300°C should be 5.0°C / second or higher. There is no particular upper limit to the average cooling rate v1 from the carburizing temperature to 300°C, but considering typical industrial production, it is, for example, 100°C / second.

[0091] [(Step 4) Furnace heating and re-quenching process] As described above, quenching is also performed in the vacuum carburizing and quenching process, and a microstructure consisting mainly of martensite is obtained. However, in the vacuum carburizing and quenching process, in order to promote the penetration and diffusion of carbon from the surface, the intermediate product is treated with A for a long time from the heating process to the diffusion process. c3It is necessary to maintain a temperature above 10°C. Therefore, at the end of the vacuum carburizing and quenching process, the prior austenite grain size in the surface and subsurface layers is not sufficiently refined. Thus, in the furnace re-quenching process, the intermediate product after the vacuum carburizing and quenching process is heated again in a heat treatment furnace and quenched. Specifically, the intermediate product after the vacuum carburizing and quenching process is brought into the heat treatment furnace and held at a heating temperature T1 (°C) for t1 minutes. After the holding time, the intermediate product is removed from the heat treatment furnace and rapidly cooled by oil cooling or water cooling. In this way, a martensitic structure with a finer prior austenite grain size than that after the vacuum carburizing and quenching process can be obtained. Note that the furnace re-quenching process may be performed only once or repeated twice.

[0092] The furnace heating and re-quenching process must satisfy the following conditions. If the furnace heating and re-quenching process is repeated twice, the following conditions only need to be met during the second furnace heating and re-quenching process. (Condition 2) The heating temperature T1 should be 900°C or higher. (Condition 3) The holding time t1 at heating temperature T1 should be 90 minutes or less.

[0093] [(Condition 2) Heating temperature T1] Intermediate carburized parts whose core chemical composition satisfies features 1 and 2 are A c3 The score is high. Therefore, if the heating temperature T1 is low, recrystallization of austenite grains does not occur sufficiently, especially in the subsurface layer. As a result, even if the furnace heating and re-quenching process is performed, regions where the prior austenite grain size is not sufficiently refined remain. Also, if the heating temperature T1 is low, regions with excessively fine prior austenite grain sizes may occur in the surface layer. As a result, the variation in prior austenite grain size in the surface and subsurface layers becomes large.

[0094] If the heating temperature T1 is less than 900°C, the variation in prior austenite grain size in the surface and subsurface layers becomes excessively large. As a result, the standard deviation σ exceeds 1.00 μm. On the other hand, if the heating temperature T1 is 900°C or higher, the variation in prior austenite grain size in the surface and subsurface layers can be sufficiently suppressed. In this case, assuming that other conditions in the manufacturing process are met, the carburized part can satisfy characteristic 4. Therefore, the heating temperature T1 should be 900°C or higher. There is no particular upper limit to the heating temperature T1, but considering typical industrial production, it is, for example, 1000°C.

[0095] [(Condition 3) Retention time t1] If the holding time t1 at heating temperature T1 is long, the growth of austenite grains during heating is promoted. As a result, the grain size of the prior austenite grains after quenching also increases. If the holding time t1 exceeds 90 minutes, the growth of austenite grains in the surface and subsurface layers is excessively promoted. As a result, the average grain size number GN becomes less than 9.7. On the other hand, if the holding time t1 is 90 minutes or less, the excessive growth of austenite grains is sufficiently suppressed. In this case, assuming that other conditions in the manufacturing process are met, the carburized part can satisfy characteristic 3. Therefore, the holding time t1 at heating temperature T1 should be 90 minutes or less. The lower limit of the holding time t1 is not particularly limited, but considering typical industrial production, it is, for example, 20 minutes.

[0096] [(Step 5) Tempering process] In the tempering process, the intermediate product after the furnace heating and re-quenching process is subjected to tempering treatment. The tempering temperature is, for example, 130 to 200°C. The holding time at the tempering temperature is, for example, 60 to 180 minutes.

[0097] The carburized component of this embodiment is manufactured through the above process. [Examples]

[0098] Steel materials with the chemical compositions shown in Tables 1A and 1B were manufactured for each test number as the material for the carburized parts.

[0099] [Table 1A]

[0100] [Table 1B]

[0101] Specifically, a bloom was produced by continuous casting using molten steel. The produced bloom was heated and subjected to bloc rolling and subsequent finish rolling to produce a 150 mm x 150 mm billet. The heating temperature of the cast slab during bloc rolling was 1000°C to 1300°C. Furthermore, the produced billet was heated again and then hot-rolled to produce a 50 mm diameter steel bar. The heating temperature of the billet at this time was 1000°C to 1300°C. The produced steel bar was cooled to room temperature in the atmosphere. Through the above manufacturing process, steel bars of each test number with a diameter of 50 mm were produced.

[0102] [Evaluation Test] The following evaluation tests were conducted using the manufactured steel materials. (Test 1) Measurement test of average particle size number GN and standard deviation σ (Test 2) Bending fatigue strength evaluation test The following describes each test.

[0103] [(Test 1) Measurement test of average particle size number GN and standard deviation σ] Simulated carburized parts were manufactured from steel materials (50 mm 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 diameter was reduced to 35 mm by hot working (hot forging) with a finishing temperature of 950°C or higher, and then cooled in the air. In this way, steel bars (intermediate products) with a diameter of 35 mm were obtained.

[0104] A vacuum carburizing and quenching process was performed on the intermediate samples of each test number produced. In the vacuum carburizing and quenching process, a heating process was first performed. In the heating process, the furnace pressure was kept below 10 Pa, and the intermediate samples were heated to a carburizing temperature of 950°C. After the heating process, a soaking process was performed. In the soaking process, the intermediate samples were held at a carburizing temperature of 950°C for 60 minutes. After the soaking process, a carburizing process was performed. In the carburizing process, acetylene was supplied to the furnace as the carburizing gas. The carburizing gas pressure in the carburizing process was kept below 1000 Pa. In the carburizing process, the samples were held at a carburizing temperature of 950°C for 40 minutes. After the carburizing process, a diffusion process was performed. In the diffusion process, the carburizing gas pressure was kept below 5 hPa. In the diffusion process, the samples were held at a carburizing temperature of 950°C for 70 minutes. After the diffusion process, a quenching process was performed. In the quenching process, the intermediate product was removed from the furnace and rapidly cooled in 130°C oil. The average cooling rate v1 from the carburizing temperature (950°C) to 300°C during the quenching process is shown in Table 2.

[0105] [Table 2]

[0106] Intermediate samples following the vacuum carburizing and quenching process were subjected to one furnace re-quenching process. In the furnace re-quenching process, each intermediate sample with a test number was held at a heating temperature T1 (°C) for t1 minutes, and then rapidly cooled in 130°C oil. The heating temperature T1 (°C) and holding time t1 (minutes) are shown in Table 2.

[0107] For each test number's intermediate sample after the furnace heating and re-quenching process, a tempering process was performed at a tempering temperature of 180°C and a holding time of 120 minutes. After the holding time, the intermediate samples were air-cooled.

[0108] Through the above process, simulated carburized parts for each test number were manufactured. The C concentration at a depth of 0.2 mm from the surface of each simulated carburized part for each test number was measured based on the method described in [Method for measuring C concentration at a depth of 0.2 mm from the surface] above. As a result, the C concentration at a depth of 0.2 mm from the surface was 0.50% or higher by mass for all simulated carburized parts for all test numbers. In other words, a carburized hardened layer was formed on the surface of all simulated carburized parts for all test numbers.

[0109] For each simulated carburized component with a test number, the average particle size number GN and standard deviation σ in the region from the surface to a depth of 3.0 mm were determined based on the method described in [Method for measuring average particle size number GN and standard deviation σ] above. The results are shown in Table 2.

[0110] [(Test 2) 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 1. The numbers in Figure 1 represent dimensions (in mm). In Figure 1, "φ" means diameter. "R2" means that the radius of curvature of the notch bottom is 2 mm.

[0111] Specifically, the steel material for each test number was heated to a heating temperature of 1200°C for a holding time of 30 minutes. Subsequently, 35 mm diameter steel bars were manufactured by hot working (hot forging) with a finishing temperature of 950°C or higher. After hot working, the steel bars for each test number were machined (cut) to produce intermediate specimens of the Ono-type rotary bending fatigue test piece with the shape shown in Figure 1.

[0112] For intermediate samples of each test number, the following processes were carried out: vacuum carburizing and quenching, furnace heating and re-quenching, and tempering, under the same conditions as described in [(Test 1) Measurement test of average particle size number GN and standard deviation σ]. Through these processes, six Ono-type rotary bending fatigue test specimens (simulated carburized parts) were produced for each test number.

[0113] 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 several cycles, 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 880 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 880 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).

[0114] [Test Results] Referring to Tables 1A, 1B, and 2, the simulated carburized parts for test numbers 1 to 14 met features 1 to 4. Therefore, excellent bending fatigue strength was obtained.

[0115] On the other hand, in test number 15, Fn1A was too high. As a result, excellent bending fatigue strength could not be obtained.

[0116] In test number 16, Fn1B was too high. As a result, excellent bending fatigue strength was not obtained.

[0117] In tests 17 and 18, the average cooling rate v1 during the quenching process in the vacuum carburizing and quenching process was too slow. As a result, the average grain size number GN was too large, and the standard deviation σ was also too large. Consequently, excellent bending fatigue strength could not be obtained.

[0118] In tests 19 and 20, the heating temperature T1 in the furnace heating and re-quenching process was too low. As a result, the standard deviation σ was too large. Consequently, excellent bending fatigue strength could not be obtained.

[0119] In tests 21 and 22, the holding time t1 at heating temperature T1 during the furnace heating and re-quenching process was too long. As a result, the average grain size number GN was too small. Consequently, excellent bending fatigue strength could not be obtained.

[0120] 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, A carburized hardened layer formed on the surface, It comprises a core portion located inside the aforementioned carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.10-0.40%, Si: more than 1.00% to 2.00%, Mn: 0.30-1.00%, P: Less than 0.030% S: Less than 0.030% Cr: 0.01% to less than 0.80% 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 (1A), In the region from the surface of the carburized part to a depth of 3.0 mm, the average particle size number of the prior austenite grains is 9.7 to 13.0, and the standard deviation of the prior austenite grain size is 1.00 μm or less. Carburized parts. 45×Mn+25×Cr<60.0 (1A) Here, the content (mass %) of the corresponding element is substituted for each element symbol in equation (1A).

2. Carburized parts, A carburized hardened layer formed on the surface, It comprises a core portion located inside the aforementioned carburized hardened layer, The chemical composition of the core is, in mass%, C: 0.10-0.40%, Si: more than 1.00% to 2.00%, Mn: 0.30-1.00%, P: Less than 0.030% S: Less than 0.030% Cr: 0.01% to less than 0.80% 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 (1B), In the region from the surface of the carburized part to a depth of 3.0 mm, the average particle size number of the prior austenite grains is 9.7 to 13.0, and the standard deviation of the prior austenite grain size is 1.00 μm or less. 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. 45×Mn+40×V+25×Cr+25×Mo+20×Ni+10×Cu<60.0 (1B) Here, in formula (1B), each element symbol is substituted with the mass percentage content of the corresponding element, and if an element is not present, "0" is substituted for the corresponding element symbol.

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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