Steel gear and method for manufacturing steel gear

Abstract

To provide a steel gear capable of reducing the coefficient of friction.SOLUTION: A steel gear (1) includes: a carburization quench-hardened layer formed on a front surface including tooth surfaces of a plurality of gear teeth (GT); and a core part being a part except the carburization quench-hardened layer. In the carburization quench-hardened layer, when defining an area having a depth of 10 μm from the tooth surface and a length of 50 μm in a tooth trace direction in a cross-section including a normal direction of the tooth surface and the tooth trace direction of the gear tooth (GT) as an outermost layer rectangle area, at least a part of the carburization quench-hardened layer has 7.0% or more of a {203} crystal orientation area ratio obtained by crystal orientation analysis in the normal direction in the outermost layer rectangle area.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present disclosure relates to a steel gear and a method for manufacturing a steel gear, and more particularly, to a steel gear including a carburized hardened layer on the surface layer and a method for manufacturing a steel gear.

Background Art

[0002] Steel gears are used as components of power units such as automobile engines and motors, for example. Many of these steel gears are made of steel.

[0003] Steel gears used for the above applications are required to have high fatigue strength. Carburizing treatment is known as a means for increasing the fatigue strength of these steel gears. Carburizing treatment includes gas carburizing treatment and vacuum carburizing treatment. In this specification, gas carburizing treatment includes not only gas carburizing treatment but also gas carbonitriding treatment. Vacuum carburizing treatment includes not only vacuum carburizing treatment but also vacuum carbonitriding treatment.

[0004] A carburized hardened layer is formed on the surface layer of the carburized steel gear. The carburized hardened layer increases the hardness of the surface layer of the steel gear. Therefore, the fatigue strength of the steel gear is increased.

[0005] Steel parts (hereinafter also referred to as carburized steel parts) having increased fatigue strength by carburizing treatment have been proposed in, for example, Japanese Patent Application Laid-Open No. 2019-026899 (Patent Document 1) and Japanese Patent Application Laid-Open No. 2019-031745 (Patent Document 2).

[0006] The carburized steel part disclosed in Patent Document 1 has a chemical composition based on SCR420. Further, in the chemical composition of this carburized steel part, the Si, Cu, Ni, and Cr contents are adjusted so as to have a predetermined relationship. Further, the carbide area ratio of the carburized hardened layer on the surface layer is adjusted to 5 to 40%. It is described in Patent Document 1 that this suppresses the amount of retained austenite in the carburized layer of the carburized steel part and increases the fatigue strength.

[0007] The carburized steel part disclosed in Patent Document 2 has a predetermined chemical composition. Furthermore, the grain size number of the prior austenite on the surface of the carburized steel part, the surface carbon concentration, and the effective hardened layer depth are adjusted to a predetermined range. Patent Document 2 states that this results in high surface fatigue strength. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2019-026899 [Patent Document 2] Japanese Patent Publication No. 2019-031745 [Overview of the project] [Problems that the invention aims to solve]

[0009] Incidentally, there is a growing demand for further improvements in automobile fuel efficiency. If energy loss in the transmission can be reduced, further improvements in fuel efficiency can be achieved. One type of energy loss in the transmission is friction loss during power transmission. Reducing this friction loss can reduce energy loss.

[0010] To reduce frictional losses in power transmission, it is effective to reduce the coefficient of friction (static friction coefficient and kinetic friction coefficient) of the steel gears involved in power transmission. Here, the static friction coefficient is a coefficient proportional to the frictional force that acts to hinder the movement of a steel gear when it starts to rotate or perform other movements. The kinetic friction coefficient is a coefficient proportional to the frictional force that acts to hinder the movement of a steel gear while it is rotating. If the coefficient of friction (static friction coefficient and kinetic friction coefficient) can be reduced, the static friction force and kinetic friction force can be reduced. As a result, frictional losses in power transmission can be reduced.

[0011] The object of this disclosure is to provide a steel gear capable of reducing the coefficient of friction and a method for manufacturing a steel gear. [Means for solving the problem]

[0012] The steel gear according to this disclosure has the following configuration:

[0013] A carburized hardened layer formed on the surface including the tooth surfaces of multiple teeth, It includes the core portion which is the part other than the carburized hardened layer, In the carburized hardened layer, when the region extending from the tooth surface to a depth of 10 μm and with a length of 50 μm in the tooth trace direction is defined as the outermost rectangular region, In at least a portion of the carburized hardened layer, The area ratio of the {203} crystal orientation obtained by the crystal orientation analysis in the normal direction of the outermost rectangular region is 7.0% or more. steel gears.

[0014] The method for manufacturing steel gears according to this disclosure includes the following steps.

[0015] The above-mentioned method for manufacturing steel gears, An intermediate product preparation step for preparing an intermediate product comprising a carburized hardened layer formed on the surface including the tooth surfaces of multiple teeth, and a core portion which is the part other than the carburized hardened layer, wherein the arithmetic mean roughness Ra of the surface of the carburized hardened layer conforms to JIS B 0601:2013 is 0.05 to 2.00 μm, and the volume fraction of retained austenite in the carburized hardened layer is 10.0 to 40.0%, The process includes a step of adjusting the crystal orientation of the outermost layer, wherein a steel gear tool, which includes a plurality of tool teeth having tooth surfaces harder than the tooth surfaces of the teeth of the intermediate product and meshing with a plurality of teeth of the intermediate product, is rotated while being pressed against the intermediate product at 8.5 to 15.0 GPa, thereby sliding the tool teeth in the direction of the tooth traces of the teeth of the intermediate product to plastically deform the outermost layer of the carburized hardened layer, and the area ratio of the {203} crystal orientation obtained by crystal orientation analysis in the normal direction in the outermost layer rectangular area of ​​the carburized hardened layer is set to 7.0% or more in at least a portion of the carburized hardened layer. A method for manufacturing steel gears. [Effects of the Invention]

[0016] In the steel gear according to the present disclosure, it is possible to reduce the friction coefficient. The method for manufacturing the steel gear according to the present disclosure can manufacture the above-described steel gear.

Brief Description of the Drawings

[0017] [Figure 1] FIG. 1 is a perspective view of the steel gear of the present embodiment. [Figure 2] FIG. 2 is a perspective view of the teeth in FIG. 1. [Figure 3] FIG. 3 is a diagram showing an example of orientation mapping in the outermost surface rectangular region shown in FIG. 2. [Figure 4A] FIG. 4A is a diagram showing the relationship between the area ratio of the {203} crystal orientation in the outermost surface rectangular region of the steel gear and the static friction coefficient. [Figure 4B] FIG. 4B is a diagram showing the relationship between the area ratio of the {203} crystal orientation in the outermost surface rectangular region of the steel gear and the dynamic friction coefficient. [Figure 5] FIG. 5 is a schematic diagram of an apparatus (outer surface plastic processing apparatus) for performing outer surface plastic processing. [Figure 6] FIG. 6 is a schematic diagram of a block-on-ring test. [Figure 7] FIG. 7 is a diagram showing an example of a graph of the friction coefficient obtained in the rotation test after the second time in the block-on-ring test.

Embodiments for Carrying Out the Invention

[0018] The inventors of the present invention studied means for reducing the friction coefficient (static friction coefficient and dynamic friction coefficient) of the steel gear. First, the inventors of the present invention considered that if a carburized hardened layer was formed on the surface including the tooth surface of the steel gear by performing carburizing treatment and the surface hardness of the steel gear was increased, the friction coefficient would be suppressed. In this case, the tooth surface on which the carburized hardened layer was formed contacts and operates with other steel gears.

[0019] However, simply increasing the surface hardness of the steel gear could not sufficiently reduce the friction coefficient. Therefore, the inventors of the present invention studied other means for reducing the friction coefficient.

[0020] The inventors hypothesized that the orientation of the crystals at the outermost layer of the carburized hardened layer of a steel gear might affect the coefficient of friction. Therefore, the inventors investigated and examined the relationship between the orientation of the crystals at the outermost layer of the carburized hardened layer and the coefficient of friction.

[0021] Specifically, the inventors defined the carburized hardened layer region in a steel gear, extending 10 μm from the tooth surface to a length of 50 μm in the tooth trace direction, as the "outermost rectangular region." They then investigated the relationship between the area ratio of the {203} crystal orientation obtained by crystal orientation analysis in the normal direction of the outermost rectangular region and the friction coefficient of the steel gear. As a result, the inventors found that the higher the area ratio of the {203} crystal orientation in the outermost rectangular region, the more the friction coefficient is suppressed. Further investigation revealed that, for the first time, if the area ratio of the {203} crystal orientation in the outermost rectangular region is 7.0% or higher, the friction coefficient (static friction coefficient and kinetic friction coefficient) can be sufficiently suppressed.

[0022] The steel gear of this embodiment was completed based on the above-described technical concept and has the following configuration.

[0023] [1] A carburized hardened layer formed on the surface including the tooth surfaces of multiple teeth, It includes the core portion which is the part other than the carburized hardened layer, In the carburized hardened layer, when the region extending from the tooth surface to a depth of 10 μm and with a length of 50 μm in the tooth trace direction is defined as the outermost rectangular region, In at least a portion of the carburized hardened layer, The area fraction of the {203} crystal orientation obtained by the crystal orientation analysis in the normal direction of the outermost rectangular region is 7.0% or more. steel gears.

[0024] [2] [1] The steel gear described above, The area fraction of the {203} crystal orientation is 10.0% or more. steel gears.

[0025] [3] [1] The steel gear described above, The area fraction of the {203} crystal orientation is 12.5% ​​or more. steel gears.

[0026] [4] A method for manufacturing steel gears as described in any one of items [1] to [3], An intermediate product preparation step for preparing an intermediate product comprising a carburized hardened layer formed on the surface including the tooth surfaces of multiple teeth, and a core portion which is the part other than the carburized hardened layer, wherein the arithmetic mean roughness Ra of the surface of the carburized hardened layer conforms to JIS B 0601:2013 is 0.05 to 2.00 μm, and the volume fraction of retained austenite in the carburized hardened layer is 10.0 to 40.0%, The process includes a step of adjusting the crystal orientation of the outermost layer, wherein a steel gear tool, which includes a plurality of tool teeth having tooth surfaces harder than the tooth surfaces of the teeth of the intermediate product and meshing with a plurality of teeth of the intermediate product, is rotated while being pressed against the intermediate product at 8.5 to 15.0 GPa, thereby sliding the tool teeth in the direction of the tooth traces of the teeth of the intermediate product to plastically deform the outermost layer of the carburized hardened layer, and the area ratio of the {203} crystal orientation obtained by crystal orientation analysis in the normal direction in the outermost layer rectangular area of ​​the carburized hardened layer is set to 7.0% or more in at least a portion of the carburized hardened layer. A method for manufacturing steel gears.

[0027] [5] A method for manufacturing steel gears as described in [4], The aforementioned intermediate product preparation process is: The processing steps for working with steel materials, The process includes a heat treatment step in which the processed steel material is subjected to carburizing and tempering to form the carburized hardened layer on the surface of the steel material, and the volume fraction of retained austenite in the carburized hardened layer is set to 10.0 to 40.0%. A method for manufacturing steel gears.

[0028] [6] A method for manufacturing steel gears as described in [5], The aforementioned intermediate product preparation process further, The process includes a surface roughness adjustment step to adjust the surface roughness of the carburized hardened layer of the steel material after the heat treatment step, thereby producing an intermediate product having an arithmetic mean surface roughness Ra of the carburized hardened layer in accordance with JIS B 0601:2013 of 0.05 to 2.00 μm. A method for manufacturing steel gears.

[0029] The steel gear according to this embodiment will be described in detail below.

[0030] [Structure of steel gears] Figure 1 is a schematic diagram of the steel gear of this embodiment. Referring to Figure 1, the steel gear 1 comprises a plurality of teeth GT. The plurality of teeth GT are arranged around the rotation axis C.

[0031] The steel gear 1 further includes a carburized hardened layer and a core. In other words, the steel gear 1 has undergone a carburizing treatment. The carburized hardened layer is formed on the surface layer including the tooth surfaces of the teeth GT of the steel gear 1. The core is the part of the steel gear 1 other than the carburized hardened layer.

[0032] The carburized layer and the core have different microstructures. Specifically, the microstructure of the carburized layer consists mainly of martensite, while the core substantially contains no martensite. Therefore, it is well known to those skilled in the art that the presence or absence of the carburized layer can be determined by performing a well-known microstructural observation.

[0033] As stated above, in this specification, "carburizing treatment" includes both carburizing treatment and carbonitriding treatment. Therefore, in this specification, "carburized hardened layer" is a concept that also includes a carbonitriding hardened layer. Carburizing treatment includes gas carburizing treatment and vacuum carburizing treatment. In other words, gas carburizing treatment includes not only gas carburizing treatment but also gas carbonitriding treatment. Vacuum carburizing treatment includes not only vacuum carburizing treatment but also vacuum carbonitriding treatment.

[0034] [Chemical composition of steel gear 1] The steel gear 1 is made of steel. The chemical composition of the steel constituting the steel gear 1 is not particularly limited. For example, the chemical composition of the steel constituting the steel gear 1 contains 90.0% or more Fe. For example, the chemical composition of the steel constituting the steel gear 1 contains 90.0% or more Fe, 0.05 to 0.28% C, 0.10 to 2.00% Si, and 0.30 to 2.00% Mn.

[0035] Preferably, the chemical composition of the steel constituting the steel gear 1 of this embodiment contains C: 0.05-0.28%, Si: 0.10-2.00%, Mn: 0.30-2.00%, P: less than 0.030%, S: less than 0.030%, Ni: 0-3.50%, Cr: 0-2.50%, Mo: 0-0.60%, Cu: 0-0.50%, Al: 0.001-0.100%, N: 0.0250% or less, O: 0.0050% or less, V: 0-0.200%, Nb: 0-0.100%, Ti: 0-0.200%, B: 0-0.0050%, and Ca: 0-0.0050%, with the remainder being Fe and impurities. The preferred upper limit for the Si content is 1.90%, and more preferably 1.85%. The preferred upper limit for the Cr content is 2.30%, and more preferably 2.10%.

[0036] The chemical composition of the steel constituting the steel gear 1 of this embodiment may, for example, satisfy any of SMn420, SCr420, SCM420, SNC415, and SNCM420 as specified in JIS G4053:2016. Alternatively, for example, SMnC420, SCr415, SCM415, SCM418, SCM421, SCM425, SCM822, SNC815, and SNCM as specified in JIS G4053:2016 may be used. 2 The steel may satisfy any of the following: 20, SNCM415, SNCM616, and SNCM815. The chemical composition of the steel constituting the steel gear 1 of this embodiment may be any known composition as described above.

[0037] In at least a portion of the carburized hardened layer of the steel gear 1 of this embodiment, the area ratio of the {203} crystal orientation obtained by crystal orientation analysis in the normal direction in the outermost rectangular area is 7.0% or more. This point will be explained below.

[0038] Figure 2 is a perspective view of tooth GT in Figure 1. Referring to Figure 2, assume that a carburized hardened layer is formed on the surface of tooth GT, including the tooth surface TS. In this case, an arbitrary rectangular region with a depth of 10 μm from the tooth surface TS and a length of 50 μm in the tooth trace direction, within a cross section including the normal Y1 and tooth trace direction X1 at any position on the tooth surface TS, is defined as the "outermost rectangular region" 20. The outermost rectangular region 20 is included in the carburized hardened layer of the steel gear 1.

[0039] In the outermost rectangular region 20, a crystal orientation analysis of the normal vector N20 of the outermost rectangular region 20 is performed to obtain an orientation mapping in the outermost rectangular region 20. Figure 3 shows an example of the orientation mapping in the outermost rectangular region 20. Referring to Figure 3, the black areas in the outermost rectangular region 20 are regions with the {203} crystal orientation. In the obtained orientation mapping, the area fraction of the {203} crystal orientation is determined.

[0040] Figure 4A shows the relationship between the area ratio of the {203} crystal orientation in the outermost rectangular region 20 of the carburized hardened layer of steel gear 1 and the rate of change of static friction coefficient (%), which is the ratio of the static friction coefficient to the static friction coefficient of a conventional steel gear. The rate of change of static friction coefficient is defined by the following formula. Change rate of static friction coefficient = Static friction coefficient of the steel gear in question / Static friction coefficient of the conventional steel gear × 100 Figure 4B shows the relationship between the area ratio of the {203} crystal orientation in the outermost rectangular region 20 of the carburized hardened layer of steel gear 1 and the rate of change of the kinetic friction coefficient (%), which is the ratio of the kinetic friction coefficient to the kinetic friction coefficient of a conventional steel gear. The rate of change of the kinetic friction coefficient is defined by the following formula. Change rate of dynamic friction coefficient = Dynamic friction coefficient of the steel gear in question / Dynamic friction coefficient of the conventional steel gear × 100 Figures 4A and 4B were created using the rate of change of static friction coefficient and the rate of change of dynamic friction coefficient obtained from the block-on-ring test described later.

[0041] Referring to Figures 4A and 4B, the friction coefficient (static friction coefficient, kinetic friction coefficient) does not change significantly even as the area ratio of the {203} crystal orientation increases, until the area ratio of the {203} crystal orientation, obtained by crystal orientation analysis of the normal N20 in the outermost rectangular area 20 of the carburized hardened layer of the steel gear 1, reaches 7.0%. On the other hand, when the area ratio of the {203} crystal orientation exceeds 7.0%, the friction coefficient (static friction coefficient, kinetic friction coefficient) decreases significantly as the area ratio of the {203} crystal orientation increases. In other words, the graphs in Figures 4A and 4B have an inflection point near 7.0% of the area ratio of the {203} crystal orientation.

[0042] Therefore, in the carburized hardened layer of the steel gear 1, if the area ratio of the {203} crystal orientation obtained by crystal orientation analysis in the outermost rectangular region 20, which is 10 μm deep from the tooth surface TS and has a length of 50 μm in the tooth trace direction, is 7.0% or more, the friction coefficient can be sufficiently suppressed. In other words, if the area ratio of the {203} crystal orientation is 7.0% or more in at least a part of the steel gear 1, the friction coefficient will be suppressed.

[0043] The preferred lower limit of the area fraction of the {203} crystal orientation is 10.0%, more preferably 12.5%, more preferably 15.0%, more preferably 20.0%, and more preferably 25.0%. The upper limit of the area fraction of the {203} crystal orientation is not particularly limited. For example, the upper limit of the area fraction of the {203} crystal orientation is 80.0%, more preferably 70.0%, more preferably 65.0%, more preferably 60.0%, more preferably 50.0%, more preferably 40.0%, and more preferably 35.0%.

[0044] [Method for measuring the area fraction of the {203} crystal orientation in the outermost rectangular region 20] The area ratio of the {203} crystal orientation obtained by crystal orientation analysis of the normal N20 in the outermost rectangular region 20 of the carburized hardened layer of the steel gear 1 is determined by electron backscatter diffraction (EBSD) using the following method.

[0045] As shown in Figure 2, in the carburized hardened layer of the steel gear 1, a test specimen is taken from a cross-section that includes the tooth surface TS, the tooth trace direction Y1 of the tooth surface TS, and the outermost rectangular region 20 with a depth D20 of 10 μm and a length L20 in the tooth trace direction of 50 μm. The size of the test specimen is not particularly limited as long as it includes the outermost rectangular region 20.

[0046] Of the multiple surfaces of the specimen, the surface containing the outermost rectangular region 20 is defined as the observation surface. The observation surface is polished to a mirror finish. From the mirror-polished observation surface, an arbitrary outermost rectangular region 20 (a rectangular region including the tooth surface TS of the carburized hardened layer, with a depth D20 of 10 μm from the tooth surface TS and a length L20 of 50 μm in the tooth trace direction) is selected. EBSD measurement is performed on the selected outermost rectangular region 20. In the EBSD measurement, the acceleration voltage is set to 15 kV, the irradiation current to 25 nA, and the irradiation interval to 0.04 μm. The direction of incidence of the electron beam is tilted 70° from the normal direction N20 of the outermost rectangular region 20. The EBSD measurement provides information on the position of each measurement point within the outermost rectangular region 20 (hereinafter referred to as position information) and information on the crystal orientation at the measurement point (hereinafter referred to as orientation information). An orientation mapping is created based on the obtained position information and orientation information. In the generated orientation mapping, the region of the {203} crystal orientation is identified. The allowable orientation difference is set to 10°. Furthermore, data with a Confidence Index (CI value) greater than 0.1 are selected.

[0047] Orientation mapping makes it possible to distinguish and indicate regions with specific crystal orientations, as shown in Figure 3. The black regions in Figure 3 represent regions with the {203} crystal orientation within the outermost rectangular region 20. Using the obtained orientation mapping, the area fraction of the {203} crystal orientation is determined. Specifically, the ratio of the total area of ​​regions with the {203} crystal orientation to the total area of ​​regions with a CI value greater than 0.1 in the outermost rectangular region 20 is defined as the area fraction (%) of the {203} crystal orientation. By this method, the area fraction of the region showing the {203} crystal orientation in the outermost rectangular region 20 can be measured. Orientation mapping can be performed on a computer using, for example, well-known analysis software (OIM Analysis Ver. 7.3.1: manufactured by TSL Solutions Co., Ltd.).

[0048] [Effect of Steel Gear 1] In the steel gear 1 having the above configuration, in at least a portion of the carburized hardened layer, the area ratio of the {203} crystal orientation obtained by crystal orientation analysis in the normal direction in the outermost rectangular area is 7.0% or more. Therefore, the coefficient of friction (static friction coefficient and kinetic friction coefficient) in that portion of the steel gear 1 is low. As a result, friction losses in power sources such as engines and powertrains in which the steel gear 1 is used can be reduced, contributing to improved fuel efficiency.

[0049] In this embodiment, the steel gear 1 may contain a carburized hardened layer over its entire surface, or it may contain a carburized hardened layer over only a portion of its surface. In other words, the steel gear 1 contains a carburized hardened layer over at least a portion of its surface.

[0050] Furthermore, in the steel gear 1 of this embodiment, the area ratio of the {203} crystal orientation obtained by crystal orientation analysis in the normal direction in the outermost rectangular area may be 7.0% or more in at least a part of the carburized hardened layer, or the area ratio of the {203} crystal orientation obtained by crystal orientation analysis in the normal direction in the outermost rectangular area may be 7.0% or more in the entire carburized hardened layer.

[0051] [Example of steel gear 1] Figure 1 shows a spur gear as an example of a steel gear 1. However, the steel gear 1 is not limited to a spur gear. The steel gear 1 may be, for example, a helical gear, a screw gear, or a spigot gear, where the tooth traces are within tolerance relative to the axis of rotation C. The steel gear 1 may also be a bevel gear, where the teeth are arranged on a conical surface. Examples of bevel gears include straight-hinged bevel gears, helical bevel gears, and hypoid gears. The steel gear 1 may further be an internal-hinged gear.

[0052] [Manufacturing method for steel gear 1] An example of a method for manufacturing the steel gear 1 according to this embodiment will be described. The method for manufacturing the steel gear 1 described below is just one example of how to manufacture the steel gear 1 according to this embodiment. Therefore, the steel gear 1 having the above-described configuration may be manufactured by other manufacturing methods other than the method described below. However, the manufacturing method described below is a preferred example of how to manufacture the steel gear 1 according to this embodiment.

[0053] An example of a method for manufacturing steel gears according to this embodiment includes the following steps. (Process 1) Intermediate product preparation process (Step 2) Outermost layer crystal orientation adjustment step The following describes each step.

[0054] [(Process 1) Intermediate product preparation process] In the intermediate product preparation process, an intermediate product is prepared. The intermediate product has a shape similar to the final steel gear product. Specifically, the intermediate product includes a carburized hardened layer formed on the surface, including the tooth surfaces of multiple teeth, and a core. The intermediate product may be supplied by a third party. Alternatively, the intermediate product may be manufactured and prepared.

[0055] The intermediate product to be prepared further has the following components: (A) The arithmetic mean roughness Ra of the carburized hardened layer surface is 0.05 to 2.00 μm, in accordance with JIS B 0601:2013. (B) The volume fraction of retained austenite in the carburized hardened layer is 10.0 to 40.0%.

[0056] [(A) Regarding the arithmetic mean roughness Ra of the carburized hardened layer surface] The surface roughness of the carburized hardened layer of the intermediate product affects the area ratio of the {203} crystal orientation in the outermost rectangular area 20 of the steel gear 1 after the outermost crystal orientation adjustment process described later. If the arithmetic mean roughness Ra of the surface of the carburized hardened layer of the intermediate product is less than 0.05 μm, even if the intermediate product has composition (B), the area ratio of the {203} crystal orientation in the outermost rectangular area 20 of the steel gear 1 after the outermost crystal orientation adjustment process will be less than 7.0%. Therefore, the arithmetic mean roughness Ra of the surface of the carburized hardened layer of the intermediate product should be 0.05 μm or more.

[0057] On the other hand, there is no particular upper limit to the surface roughness of the carburized hardened layer of the intermediate product. However, if the surface roughness of the carburized hardened layer of the intermediate product becomes excessively rough, cracks may occur on the surface of the steel gear 1 after the outermost crystal orientation adjustment process. Therefore, the arithmetic mean surface roughness Ra of the carburized hardened layer of the intermediate product should be 2.00 μm or less.

[0058] [(B) Regarding the volume fraction of retained austenite in the carburized hardened layer] The volume fraction of retained austenite in the carburized hardened layer of the intermediate product affects the area fraction of the {203} crystal orientation in the outermost rectangular region 20 of the steel gear 1. If the volume fraction of retained austenite is less than 10.0% or more than 40.0%, the area fraction of the {203} crystal orientation in the outermost rectangular region 20 of the steel gear 1 will be less than 7.0% even after performing the outermost crystal orientation adjustment process. Therefore, the volume fraction of retained austenite in the carburized hardened layer of the intermediate product having configuration (A) is set to 10.0 to 40.0%. In this case, by performing the outermost crystal orientation adjustment process, the area fraction of the {203} crystal orientation in the outermost rectangular region 20 of the steel gear 1 can be set to 7.0% or more.

[0059] An intermediate product having the above configuration can be manufactured, for example, in the following manufacturing process. (Step 11) Processing process (Step 12) Heat treatment process (Step 13) Surface roughness adjustment step The following describes each step.

[0060] [(Step 11) Processing process] In the manufacturing process, the steel material that will become the steel gear is processed to shape it into a form close to the final product, the steel gear.

[0061] The processing method for the steel material may be any well-known method. For example, the steel material may be hot-worked to form a predetermined shape. Examples of hot-working methods include hot forging and hot rolling. The steel material may also be cold-worked to form a predetermined shape. Examples of cold-working methods include cold forging and cold drawing. The steel material may also be machined to form a predetermined shape. After hot-working or cold-working the steel material, further machining may be performed to form a predetermined shape.

[0062] [(Step 12) Heat Treatment Process] In the heat treatment process, the processed steel material is subjected to heat treatment to form a carburized hardened layer on the surface of the intermediate product. Specifically, the processed steel material is subjected to carburizing and tempering. In this example of a manufacturing process, the carburizing process includes a carburizing process and a quenching process. In other words, the heat treatment process includes the following steps: (Step 121) Carburizing process (Process 122) Hardening process (Step 123) Tempering process By performing a heat treatment process, the volume fraction of retained austenite in the carburized hardened layer of the intermediate product is adjusted to 10.0-40.0%. Each process is described below.

[0063] [(Step 121) Carburizing process] In the carburizing process, one of the following treatments is performed: gas carburizing, gas carbonitriding, vacuum carburizing, or vacuum carbonitriding. The carburizing process increases the carbon concentration of the surface layer of the intermediate product compared to the carbon concentration of the core. In the following explanation, gas carburizing and gas carbonitriding will be collectively referred to as "gas carburizing." Similarly, vacuum carburizing and vacuum carbonitriding will be collectively referred to as "vacuum carburizing."

[0064] [Gas carburizing treatment] The carburizing process in gas carburizing treatment includes, in chronological order, a heating process, a gas carburizing process, and a diffusion process.

[0065] In the heating process, the steel material charged into the heat treatment furnace is heated to its carburizing temperature. The carburizing temperature in the heating process is, for example, 830 to 1100°C.

[0066] The gas carburizing process is performed after the heating process. In the gas carburizing process, the steel material is held for a predetermined time (holding time) at a predetermined carbon potential Cp1 and carburizing temperature. The carbon potential Cp1 in the carburizing process is, for example, 0.5 to 1.2%, and the holding time at the carburizing temperature is, for example, 60 to 240 minutes.

[0067] The diffusion process is performed after the gas carburizing process. In the diffusion process, the steel material is held for a predetermined time (holding time) at a predetermined carbon potential Cp2 and carburizing temperature. In the diffusion process, the carbon that penetrated the steel material in the carburizing process is diffused within the steel material. The carbon potential Cp2 in the diffusion process is, for example, 0.5 to 1.2%. The holding time at the carburizing temperature in the diffusion process is, for example, 30 to 90 minutes. The carbon potential Cp2 in the diffusion process may be lower than or the same as the carbon potential Cp1 in the carburizing process.

[0068] The gas carburizing process and the diffusion process may be performed once each, or they may be performed multiple times alternately.

[0069] [Vacuum carburization treatment] The carburizing process in vacuum carburizing treatment includes, in chronological order, a heating process, a vacuum carburizing process, and a diffusion process.

[0070] In the heating process, the steel material charged into the heat treatment furnace is heated to its carburizing temperature. The carburizing temperature in the heating process is, for example, 830 to 1100°C. In the heating process, the furnace is further evacuated or depressurized. For example, the pressure inside the furnace is reduced to 1 kPa or less.

[0071] The vacuum carburizing process is performed after the heating process. In the vacuum carburizing process, a hydrocarbon gas is introduced into the furnace under vacuum or reduced pressure, and the steel material is held at the carburizing temperature for a predetermined time (holding time). The gas introduced in the vacuum carburizing process is not particularly limited as long as it is a hydrocarbon gas. Examples of hydrocarbon gases include acetylene and propane. The holding time at the carburizing temperature is not particularly limited. For example, the holding time at the carburizing temperature is 5 to 120 minutes. By performing carburizing under vacuum or reduced pressure, the concentration of carbon (C) that penetrates the surface layer of the steel material can be increased compared to gas carburizing.

[0072] The diffusion process is performed after the vacuum carburizing process. In the diffusion process, the material is held at the carburizing temperature for a predetermined time (holding time) without introducing hydrocarbon gases into the furnace. The pressure inside the furnace during the diffusion process may be the same as that during the vacuum carburizing process. Alternatively, the pressure inside the furnace during the diffusion process may be reduced to a lower level than that during the vacuum carburizing process (e.g., 100 Pa or less) to remove residual gases from the vacuum carburizing process. The holding time at the carburizing temperature during the diffusion process is not particularly limited, but is, for example, 5 to 150 minutes.

[0073] The vacuum carburizing process and the diffusion process may be performed once each, or they may be performed alternately multiple times.

[0074] [(Process 122) Hardening Process] The quenching process is carried out after the carburizing process. In the quenching process, the known quenching method is performed on the steel material after the carburizing process. Specifically, in the quenching process, the steel material after the carburizing process is A r3 The steel is held at a quenching temperature of 1.5°C or higher. Then, the steel is rapidly cooled and quenched.

[0075] The holding time at the quenching temperature is not particularly limited, but is, for example, 30 to 60 minutes. The quenching temperature is preferably lower than the carburizing temperature. The cooling method during the quenching process is oil cooling or water cooling. Specifically, the steel material, held at the quenching temperature, is rapidly cooled by immersion in a cooling bath containing oil or water as the cooling medium. The temperature of the oil or water used as the cooling medium is, for example, room temperature to 200°C. Sub-zero treatment may also be performed as needed.

[0076] [(Step 123) Tempering process] The tempering process is performed after the quenching process. In the tempering process, the steel material after the quenching process is subjected to a well-known tempering treatment. The tempering temperature is, for example, 100 to 200°C. The holding time at the tempering temperature is, for example, 90 to 150 minutes.

[0077] As described above, in the heat treatment process, the volume fraction of retained austenite in the carburized hardened layer of the intermediate product is adjusted to 10.0-40.0%. The volume fraction of retained austenite can be adjusted to 10.0-40.0% by appropriately adjusting the carbon potential Cp1 in the gas carburizing process, the carbon potential Cp2 in the diffusion process, the carburizing temperature in the gas carburizing and vacuum carburizing processes, the holding time at the carburizing temperature, the holding time at the carburizing temperature in the diffusion process, and the temperature of the cooling medium (water or oil) in the quenching process.

[0078] [(Step 13) Surface roughness adjustment step] In the surface roughness adjustment process, the surface roughness of the carburized hardened layer of the intermediate product is adjusted to a predetermined roughness by machining, grinding, or shot peening the steel material after the heat treatment process. Specifically, in the surface roughness adjustment process, the arithmetic mean roughness Ra on the surface of the carburized hardened layer of the intermediate product before the outermost crystal orientation adjustment process is adjusted to a range of 0.05 to 2.00 μm. The preferred lower limit of the arithmetic mean roughness Ra is 0.10 μm, more preferably 0.15 μm, still more preferably 0.20 μm, and still more preferably 0.25 μm. The preferred upper limit of the arithmetic mean roughness Ra is 1.50 μm. The arithmetic mean roughness Ra is measured in accordance with JIS B 0601:2013.

[0079] Cutting and grinding processes can be carried out using well-known methods. Similarly, shot peening can be carried out using well-known methods. Although not particularly limited, shot peening may be performed using shot particles with a diameter of 0.7 mm or less and under conditions of an arc height of 0.4 mm or more.

[0080] In this manufacturing process example, the surface roughness of the carburized hardened layer of the intermediate product before the crystal orientation adjustment process is deliberately made rough through the surface roughness adjustment process. This makes it possible to increase the area ratio of the {203} crystal orientation in the outermost crystal orientation adjustment process.

[0081] Furthermore, if the arithmetic mean roughness Ra of the carburized hardened layer surface of the intermediate product after the heat treatment process is between 0.05 and 2.00 μm, the surface roughness adjustment process may be omitted.

[0082] Through the above process, an intermediate product is prepared that comprises a carburized hardened layer and a core, with an arithmetic mean surface roughness Ra of 0.05 to 2.00 μm and a volume fraction of retained austenite in the carburized hardened layer of 10.0 to 40.0%.

[0083] [Method for measuring the volume fraction of retained austenite in the carburized hardened layer of an intermediate product, and the arithmetic mean surface roughness Ra of the intermediate product] The volume fraction of retained austenite in the intermediate product, and the arithmetic mean surface roughness Ra of the intermediate product, can be measured by the following method.

[0084] [Volume fraction of retained austenite] The volume fraction of retained austenite in the carburized hardened layer of the intermediate product is determined by X-ray diffraction. Specifically, a sample containing the carburized hardened layer is taken from the intermediate product. X-ray diffraction is performed on the surface of the carburized hardened layer of the sample to obtain the integrated intensity ratio of the diffraction peaks of the (211) plane of the bcc structure and the (220) plane of the fcc structure. Based on the obtained diffraction intensity ratio, the volume fraction (%) of retained austenite is determined. A Cr tube is used as the light source. The voltage of the light source is set to 40kV and the current to 40mA.

[0085] [Arithmetic mean surface roughness Ra] The arithmetic mean roughness Ra of the carburized hardened surface of the intermediate product will be measured in accordance with the measurement method specified in JIS B 0601:2013. Specifically, 10 arbitrary locations will be selected as measurement points on the surface of the carburized hardened layer of the intermediate product. At each measurement point, the arithmetic mean roughness Ra will be measured over an evaluation length extending in the axial direction L. The reference length (cutoff wavelength) will be 0.25 mm if the arithmetic mean roughness Ra is between 0.02 and 0.10 μm, and 0.80 mm if the arithmetic mean roughness Ra is greater than 0.10 to 2.00 μm. Furthermore, the evaluation length will be 5 times the reference length (cutoff wavelength). The arithmetic mean roughness Ra will be measured using a stylus-type roughness meter, with a measurement speed of 0.2 mm / sec. Of the 10 arithmetic mean roughness values ​​Ra obtained, the arithmetic mean roughness Ra (μm) is defined as the arithmetic mean mean roughness Ra (μm) of the six values ​​remaining after excluding the largest, second largest, smallest, and second smallest arithmetic mean roughness values ​​Ra.

[0086] [(Step 2) Outermost layer crystal orientation adjustment step] In the outermost crystal orientation adjustment process, in the outermost rectangular region of the carburized hardened layer of an intermediate product in which the arithmetic mean roughness Ra of the surface of the carburized hardened layer is 0.05 to 2.00 μm in accordance with JIS B 0601:2013 and the volume fraction of retained austenite in the carburized hardened layer is 10.0 to 40.0%, the area fraction of the {203} crystal orientation obtained by crystal orientation analysis in the normal direction of the outermost rectangular region is increased.

[0087] Specifically, a steel gear tool with multiple tool teeth arranged in a grid is prepared. The hardness of the tooth surface of the tool teeth is made higher than the hardness of the tooth surface of the intermediate product. Multiple tool teeth of the steel gear tool are meshed with multiple teeth of the intermediate product, and by rotating it while pressing it against the intermediate product in the direction of the intermediate product at a force of 8.5 to 15.0 GPa, the tool teeth are made to slide in the direction of the tooth trace of the intermediate product, causing plastic deformation of the outermost layer of the carburized hardened layer of the intermediate product, and the area ratio of the {203} crystal orientation obtained by crystal orientation analysis in the normal direction in the outermost rectangular area of ​​the carburized hardened layer is made to be 7.0% or more. This point will be explained below.

[0088] Figure 5 is a schematic diagram of a device for performing surface plastic deformation (surface plastic deformation device). Referring to Figure 5, the surface plastic deformation device 30 comprises a rotating shaft 31 and a steel gear tool 32. The rotating shaft 31 is positioned on the central axis of the steel gear tool 32 and is fixed to the steel gear tool 32. The rotating shaft 31 rotates around the rotation axis C2 of the steel gear tool 32, thereby rotating the steel gear tool 32.

[0089] The steel gear tool 32 includes helical tool teeth 33. The helical tool teeth 33 have tool teeth formed in a helical shape around the rotation axis C2. The central axis of the helical tool teeth 33 coincides with the central axis C2 of the steel gear tool 32. The helical tool teeth 33 have a shape that meshes with the teeth TG of the intermediate product 100. As described above, the hardness of the tooth surface of the helical tool teeth 33 is higher than the hardness of the tooth surface of the teeth TG of the intermediate product 100. The helical tool teeth 33 may be made of, for example, carbide or diamond. The surface of the helical tool teeth 33 may be coated with DLC or the like.

[0090] As the steel gear tool 32 rotates around the rotation axis C2, a load is applied in the direction F1 by a pressurizing mechanism (not shown). As a result, the helical tool teeth 33 of the steel gear tool 32 are pressed against the teeth TG of the intermediate part 100 as it rotates.

[0091] Surface plastic deformation using the surface plastic deformation apparatus 30 is performed as follows. First, the helical tool teeth 33 of the steel gear tool 32 of the surface plastic deformation apparatus 30 are engaged with the teeth TG of the intermediate product 100. Then, while pressing the steel gear tool 32 against the intermediate product 100, the steel gear tool 32 is rotated around the rotation axis C2. Because the helical tool teeth 33 and the intermediate product 100 are engaged, the rotation of the steel gear tool 32 causes the intermediate product 100 to rotate around the rotation axis C1.

[0092] By rotating the steel gear tool 32 while pressing it against the intermediate product 100, the helical tool teeth 33 slide in the direction of the tooth trace of the tooth TG of the intermediate product 100. As a result, the carburized hardened layer near the tooth surface of tooth TG is subjected to pressure in the direction of the tooth trace, and the outermost layer of the carburized hardened layer undergoes plastic deformation. At this time, crystal orientation rotation occurs in the outermost layer of the carburized hardened layer due to plastic deformation.

[0093] In surface plastic deformation using the surface plastic deformation apparatus 30, the following conditions must be met during the process. (C) In surface plastic deformation, the steel gear tool 32 is pressed against the intermediate product 100 with a pressure of 8.5 to 15.0 GPa. Hereinafter, the above pressure will be referred to as the "pressing force".

[0094] If the pressing force is less than 8.5 GPa, even when using intermediate products having configurations (A) and (B), the area ratio of the {203} crystal orientation in the outermost rectangular region 20 of the final steel gear 1 will be less than 7.0%. If the pressing force is 8.5 GPa or more, the area ratio of the {203} crystal orientation in the final layer rectangular region 20 of the final steel gear 1 will be 7.0% or more. The higher the pressing force, the more the plastic deformation of the outermost layer of the carburized hardened layer of the intermediate product is promoted. Therefore, the preferred lower limit of the pressing force is 10.0 GPa, and more preferably 11.5 GPa. On the other hand, if the pressing force is too high, cracks may occur on the surface of the steel gear 1. Therefore, the upper limit of the pressing force is 15.0 GPa. Therefore, in this embodiment, the pressing force is set to 8.5 to 15.0 GPa.

[0095] If surface plastic deformation is performed on an intermediate product having the configurations of (A) and (B) above under the conditions of (C), the concentration of {203} crystal orientations increases in the normal direction N20 of the outermost rectangular area 20 of the carburized hardened layer of the steel gear 1. As a result, the area ratio of {203} crystal orientations in the outermost rectangular area 20 becomes 7.0% or more.

[0096] The steel gear 1 of this embodiment is manufactured by the manufacturing method described above. The above manufacturing method is just one example of a manufacturing method for the steel gear 1 of this embodiment. Therefore, as long as the steel gear 1 has the above-described configuration, the steel gear 1 of this embodiment may be manufactured by other manufacturing methods. [Examples]

[0097] The effects of one embodiment of the steel gear 1 of this embodiment will be described in more detail below with reference to examples. The conditions in the following examples are just one example of conditions adopted to confirm the feasibility and effects of the steel gear 1 of this embodiment. Therefore, the steel gear 1 of this embodiment is not limited to this one example of conditions.

[0098] Steel materials having the chemical composition shown in Table 1 were prepared.

[0099] [Table 1]

[0100] The shape of the steel material was that of a spur gear, as shown in Figure 1.

[0101] The steel samples corresponding to each test number in Table 1 were subjected to the carburizing treatment (heat treatment) shown in Table 2.

[0102] [Table 2]

[0103] In Table 2, "Gas" in the "Carburizing Type" column means that gas carburizing was performed as a heat treatment process under the following conditions. "Vacuum" means that vacuum carburizing was performed as a heat treatment process under the following conditions.

[0104] [Gas carburizing treatment] Gas carburizing treatment was carried out under the following conditions: The steel material was charged into a heat treatment furnace and heated to 930°C for 60 minutes. Subsequently, the gas carburizing process was carried out. Specifically, it was held at a carburizing temperature of 930°C for 100 minutes. At this time, the carbon potentials Cp1 and Cp2 were adjusted to 0.6 to 1.2. After the gas carburizing process, a diffusion process was carried out. In the diffusion process, it was held at the same carburizing temperature as the gas carburizing process for 60 minutes. The volume fraction of retained austenite in the carburized hardened layer of the intermediate product was adjusted by adjusting the carbon potentials Cp1 and Cp2 within the above range.

[0105] Following the diffusion process, a quenching process was carried out. The quenching process was performed immediately after the diffusion process in the same furnace used for the gas carburizing treatment. During the quenching process, the steel material was held at 860°C for 30 minutes. After that, the steel material was oil-cooled using 60°C oil.

[0106] After the quenching process, the steel material underwent a tempering process. In the tempering process, the steel material was held at 180°C for 120 minutes. After that, the steel material was allowed to cool in the atmosphere.

[0107] [Vacuum carburization treatment] Vacuum carburizing treatment was carried out under the following conditions: The steel material was charged into a heat treatment furnace and heated to 930°C for 60 minutes. After that, the pressure inside the furnace was reduced to 1 kPa. After the pressure reduction, the vacuum carburizing process was carried out. Specifically, while introducing acetylene gas into the furnace, the material was held at a carburizing temperature of 930°C for 20 to 120 minutes. After the vacuum carburizing process, a diffusion process was carried out. In the diffusion process, with the introduction of acetylene gas stopped, the material was held at 930°C for 60 to 150 minutes under reduced pressure. In the above processes, the holding time in the vacuum carburizing process and the holding time in the diffusion process were adjusted to control the volume fraction of retained austenite in the carburized hardened layer of the intermediate product.

[0108] Following the diffusion process, a quenching process was carried out. The quenching process was performed immediately after the diffusion process in the same furnace used for the vacuum carburizing treatment. During the quenching process, the steel material was held at 860°C for 30 minutes. After that, the steel material was oil-cooled using 60°C oil.

[0109] After the quenching process, the steel material underwent a tempering process. In the tempering process, the steel material was held at 180°C for 120 minutes. After that, the steel material was allowed to cool in the atmosphere.

[0110] A surface roughness adjustment process was performed on the steel material after carburizing. Specifically, the surface roughness of the steel material was adjusted by grinding or turning.

[0111] Through the above process, intermediate samples for each test number were manufactured.

[0112] [Measurement test of the volume fraction of retained austenite and the arithmetic mean surface roughness Ra of intermediate samples for each test number after the surface roughness adjustment process] The volume fraction of retained austenite and the arithmetic mean surface roughness Ra of the intermediate samples for each test number after the surface roughness adjustment process were determined by the following method.

[0113] [Volume fraction of retained austenite] The volume fraction of retained austenite in the carburized hardened layer of the intermediate product was determined by X-ray diffraction. Specifically, samples containing the carburized hardened layer were taken from the intermediate product of each test number. X-ray diffraction was performed on the surface of the carburized hardened layer of the samples to obtain the integrated intensity ratio of the diffraction peaks of the (211) plane of the bcc structure and the (220) plane of the fcc structure. Based on the obtained diffraction intensity ratio, the volume fraction (%) of retained austenite was determined. For X-ray diffraction, a RIGAK Corporation product name: RINT-2500HL / PC was used. A Cr tube was used as the light source. The light source voltage was 40kV and the current was 40mA. The obtained volume fraction of retained austenite is shown in the "Retained γ (volume %)" column of Table 2.

[0114] [Arithmetic mean surface roughness Ra] The arithmetic mean roughness Ra of the carburized hardened surface of the intermediate sample for each test number was measured in accordance with the measurement method specified in JIS B 0601:2013. Specifically, 10 arbitrary locations were selected as measurement points on the surface of the carburized hardened layer of the intermediate sample. At each measurement point, the arithmetic mean roughness Ra was measured over an evaluation length extending in the axial direction L. The reference length (cutoff wavelength) was set to 0.25 mm when the arithmetic mean roughness Ra was between 0.02 and 0.10 μm, and to 0.80 mm when the arithmetic mean roughness Ra was greater than 0.10 to 2.00 μm. Furthermore, the evaluation length was set to 5 times the reference length (cutoff wavelength). The arithmetic mean roughness Ra was measured using a stylus-type roughness meter, and the measurement speed was set to 0.2 mm / sec. Of the 10 arithmetic mean roughness values ​​Ra obtained, the arithmetic mean mean roughness Ra was defined as the arithmetic mean mean roughness Ra of the six values ​​remaining after excluding the largest, second largest, smallest, and second smallest arithmetic mean roughness Ra.

[0115] A contact-type surface roughness meter (product name: Surftest SJ-301) manufactured by Mitutoyo Corporation was used. The obtained arithmetic mean roughness Ra (μm) is shown in the "Ra (μm)" column of Table 2.

[0116] A surface crystal orientation adjustment process was performed on the intermediate samples after the surface roughness adjustment process. Specifically, surface plastic deformation was performed on the intermediate samples of each test number using the surface plastic deformation apparatus 30 shown in Figure 5. A steel gear tool 32 having tool teeth made of cemented carbide was used as the steel gear tool 32. The pressing force during surface plastic deformation was as shown in the "Pressing Force (GPa)" column of Table 2.

[0117] Furthermore, for the conventional steel gear (hereinafter referred to as the "reference steel gear") that serves as the standard for the rate of change of static friction coefficient and the rate of change of dynamic friction coefficient, the vacuum carburizing treatment described above was performed on the steel material of test number C in Table 1, and then normal grinding was carried out. The outermost crystal orientation adjustment process was not performed. The final product, a steel gear (spur gear), was manufactured using the above manufacturing process.

[0118] [Evaluation Test] The following evaluation tests were performed on the steel gears of each test number.

[0119] [Area fraction measurement test for {203} crystal orientation in the outermost layer region] For each test number of steel gear, a specimen was taken from the carburized hardened layer, including the tooth surface and a rectangular outermost region with a depth of 10 μm from the tooth surface and a length of 50 μm in the tooth trace direction, within a cross-section that includes the tooth surface and the tooth trace direction. Of the multiple surfaces of the specimen, the surface including the rectangular outermost region was defined as the observation surface.

[0120] The observation surface was polished to a mirror finish. From the mirror-polished observation surface, an arbitrary outermost rectangular region (a rectangular region including the tooth surface of the carburized hardened layer, with a depth of 10 μm from the tooth surface and a length of 50 μm in the tooth trace direction) was selected. EBSD measurement was performed on the selected outermost rectangular region. In the EBSD measurement, the acceleration voltage was set to 15 kV and the irradiation interval to 0.04 μm. The incident direction of the electron beam was tilted 70° from the normal direction N20 of the outermost rectangular region 20. Based on the positional and orientation information obtained from the EBSD measurement, an axial orientation mapping of the steel gear was created. In the created orientation mapping, the region of the {203} crystal orientation was identified. At this time, the allowable orientation difference was set to 10°. In addition, data with a reliability index CI value greater than 0.1 were adopted.

[0121] Using the obtained orientation mapping, the area fraction of the {203} crystal orientation was determined. Specifically, the ratio of the area of ​​the {203} crystal orientation region to the total area of ​​the region with a CI value greater than 0.1 in the outermost layer region was defined as the area fraction (%) of the {203} crystal orientation. The obtained area fractions of the {203} crystal orientation are shown in the "{203} Area Fraction (%)" column of Table 2.

[0122] [Maximum static friction coefficient measurement test] For each steel gear with a given test number, a block-on-ring test was performed to determine the maximum static friction coefficient.

[0123] Figure 6 is a schematic diagram of the block-on-ring test. Referring to Figure 6, the block-on-ring test machine 200 is equipped with a bath 201 containing lubricating oil 202 and a ring test piece 203. The lubricating oil 202 has a kinematic viscosity of 5.4 mm at 100°C. 2 I used commercially available engine oil with a / s rating.

[0124] The material of ring test specimen 203 was SAEO1 as specified by the Society of Automotive Engineers' industrial standards. The outer diameter D of ring test specimen 203 was 34.99 mm. The width W of ring test specimen 203 was 8.74 mm.

[0125] Block test specimens 300 were prepared from the teeth of the steel gears corresponding to each test number. Of the block test specimens 300, the surface that contacts the circumferential surface of the ring test specimen 203 (referred to as the contact surface) was the tooth surface of the steel gear.

[0126] As shown in Figure 6, the lower part of the ring test piece 203 was immersed in the lubricating oil 202 in the bathtub 201. Then, the block test piece 300 was placed above the ring test piece 203. At this time, the block test piece 300 was positioned so that its contact surface faced the circumferential surface of the ring test piece 203, and the direction of the tooth traces was parallel to the rotation axis of the ring test piece 203.

[0127] After completing the above preparations, steps 1 through 4 were repeated 10 times. Step 1: A load P of 100N is applied to the block test specimen 300 from above downwards, 300 was pressed against the circumferential surface of the ring test piece 203. Step 2: The state in step 1 was held for 30 seconds in order to allow the lubricating oil 202 to be discharged from between the contact surface of the block test piece 300 and the circumferential surface of the ring test piece 203. Step 3: The ring test piece 203 was started to rotate at a sliding speed of 0.1 m / s (55 rpm) and then rotated for 30 seconds. Step 4: After rotating for 30 seconds, the load P was removed. Then, the rotation of the ring test piece 203 was stopped.

[0128] During steps 1 to 4, the force F applied to the block test specimen 300 was measured using a load cell. The coefficient of friction μ(-) was then calculated using the following formula. F=μP The relationship between the obtained coefficient of friction μ and the test time was determined. Figure 7 shows an example of a graph of the coefficient of friction for the second and subsequent rotation tests. In the graph in Figure 7, the horizontal axis is time and the vertical axis is the coefficient of friction. Referring to Figure 7, the peak of the coefficient of friction during ring rotation (within the circular area in the figure) was defined as the static friction coefficient. The arithmetic mean of the static friction coefficients obtained from the second to the tenth tests was defined as the static friction coefficient (-) for each test number.

[0129] In the first test, the static friction coefficient obtained was significantly higher than that obtained in the second to tenth tests because the lubricating oil had not sufficiently permeated the ring test piece 203. Therefore, the static friction coefficient obtained in the first test was excluded from consideration.

[0130] The static friction coefficient of a reference steel gear that did not undergo the outermost crystal orientation adjustment process was defined as the static friction coefficient of the reference steel gear. The percentage change in the static friction coefficient of the steel gear for each test number was calculated using the following formula. Change rate of static friction coefficient = Static friction coefficient of the corresponding test number / Static friction coefficient of the reference steel gear × 100

[0131] [Dynamic friction coefficient measurement test] For each steel gear with a given test number, a block-on-ring test similar to the static friction coefficient measurement test was performed to determine the dynamic friction coefficient.

[0132] In the block-on-ring test shown in Figure 6, the kinematic viscosity of the lubricating oil 202, the outer diameter D of the ring test piece 203, the width W of the ring test piece 203, the material of the block test piece 300, and the contact surface of the block test piece 300 that contacts the circumferential surface of the ring test piece 203 were all the same as in the static friction coefficient measurement test.

[0133] As shown in Figure 6, the lower part of the ring test piece 203 was immersed in the lubricating oil 202 in the bathtub 201. Then, the block test piece 300 was placed above the ring test piece 203. At this time, the block test piece 300 was positioned so that its contact surface faced the circumferential surface of the ring test piece 203, and the direction of the tooth traces was parallel to the rotation axis of the ring test piece 203.

[0134] After completing the above preparations, steps 1 through 3 were carried out. Step 1: The rotation of the ring test piece 203 was started at a sliding speed of 1.0 m / sec (550 rpm). Step 2: A load P of 300N is applied to the block test specimen 300 from above downwards, 300 was pressed against the circumferential surface of the ring test piece 203. Step 3: After rotating for 300 seconds, the load P was removed. Then, the rotation of the ring test piece 203 was stopped.

[0135] During steps 1 to 3, the force F applied to the block test specimen 300 was measured using a load cell. The coefficient of friction μ(-) was then calculated using the following formula. F=μP The arithmetic mean of the friction coefficient μ obtained during a 300-second rotation, excluding the peak, was defined as the kinetic friction coefficient (-) for each test number.

[0136] The dynamic friction coefficient of a reference steel gear that did not undergo the outermost crystal orientation adjustment process was defined as the dynamic friction coefficient of the reference steel gear. The percentage change in the dynamic friction coefficient of the steel gear for each test number was calculated using the following formula. Change rate of dynamic friction coefficient = Dynamic friction coefficient of the corresponding test number / Dynamic friction coefficient of the reference steel gear × 100

[0137] [Test Results] Tests 1-20 showed appropriate arithmetic mean roughness Ra of the carburized hardened layer and volume fraction of retained austenite in the carburized hardened layer of the intermediate product before the outermost crystal orientation adjustment process. Furthermore, the pressing force in the outermost crystal orientation adjustment process was appropriate. As a result, the area fraction of the {203} crystal orientation obtained by crystal orientation analysis in the normal direction of the outermost rectangular area of ​​the manufactured steel gear was 7.0% or more. The rate of change of the static friction coefficient was low at less than 90.0%, and the rate of change of the dynamic friction coefficient was also low at less than 80.0%, indicating that the friction coefficient was sufficiently reduced.

[0138] On the other hand, in tests 21 and 22, the volume fraction of retained austenite in the carburized hardened layer of the intermediate product before the outermost crystal orientation adjustment process was too low. As a result, the area fraction of the {203} crystal orientation in the outermost rectangular region of the steel gear was less than 7.0%. Consequently, the rate of change of the static friction coefficient was high at over 90.0%, and the rate of change of the dynamic friction coefficient was also high at over 80.0%, indicating that the friction coefficient was not sufficiently reduced.

[0139] In tests 23 and 24, the volume fraction of retained austenite in the carburized hardened layer of the intermediate product before the outermost crystal orientation adjustment process was too high. As a result, the area fraction of the {203} crystal orientation in the outermost rectangular region of the steel gear was less than 7.0%. Consequently, the rate of change of the static friction coefficient was high at over 90.0%, and the rate of change of the dynamic friction coefficient was also high at over 80.0%, indicating that the friction coefficient was not sufficiently reduced.

[0140] In tests 25 and 26, the arithmetic mean roughness Ra of the carburized hardened layer surface of the intermediate product before the outermost crystal orientation adjustment process was too low. As a result, the area ratio of the {203} crystal orientation in the outermost rectangular region of the steel gear was less than 7.0%. Consequently, the rate of change of the static friction coefficient was high at over 90.0%, and the rate of change of the dynamic friction coefficient was also high at over 80.0%, indicating that the friction coefficient was not sufficiently reduced.

[0141] In tests 27 and 28, the load F1 during the outermost crystal orientation adjustment process was too low. As a result, the area ratio of the {203} crystal orientation in the outermost rectangular region of the steel gear was less than 7.0%. Consequently, the rate of change of the static friction coefficient was high (over 90.0%), and the rate of change of the dynamic friction coefficient was also high (over 80.0%), resulting in insufficient reduction of the friction coefficient.

[0142] 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. A carburized hardened layer formed on the surface including the tooth surfaces of multiple teeth, It includes the core portion which is the part other than the carburized hardened layer, In the carburized hardened layer, when the region extending from the tooth surface to a depth of 10 μm and with a length of 50 μm in the tooth trace direction is defined as the outermost rectangular region, In at least a portion of the carburized hardened layer, The area ratio of the {203} crystal orientation obtained by the crystal orientation analysis in the normal direction of the outermost rectangular region is 7.0% or more. steel gears.

2. A steel gear according to claim 1, The area fraction of the {203} crystal orientation is 10.0% or more. steel gears.

3. A steel gear according to claim 1, The area fraction of the {203} crystal orientation is 12.5% ​​or more. steel gears.

4. A method for manufacturing a steel gear according to any one of claims 1 to 3, An intermediate product preparation step for preparing an intermediate product comprising a carburized hardened layer formed on the surface including the tooth surfaces of multiple teeth, and a core portion which is the part other than the carburized hardened layer, wherein the arithmetic mean roughness Ra of the surface of the carburized hardened layer in accordance with JIS B 0601:2013 is 0.05 to 2.00 μm, and the volume fraction of retained austenite in the carburized hardened layer is 10.0 to 40.0%, The process includes a step of adjusting the crystal orientation of the outermost layer, wherein a steel gear tool, which includes a plurality of tool teeth having tooth surfaces harder than the tooth surfaces of the teeth of the intermediate product and meshing with a plurality of teeth of the intermediate product, is rotated while being pressed against the intermediate product at 8.5 to 15.0 GPa, thereby sliding the tool teeth in the direction of the tooth traces of the teeth of the intermediate product to plastically deform the outermost layer of the carburized hardened layer, and the area ratio of the {203} crystal orientation obtained by crystal orientation analysis in the normal direction in the outermost layer rectangular area of ​​the carburized hardened layer is set to 7.0% or more in at least a portion of the carburized hardened layer. A method for manufacturing steel gears.

5. A method for manufacturing a steel gear according to claim 4, The aforementioned intermediate product preparation process is: The processing steps for working with steel materials, The process includes a heat treatment step in which the processed steel material is subjected to carburizing and tempering to form the carburized hardened layer on the surface of the steel material, and the volume fraction of retained austenite in the carburized hardened layer is set to 10.0 to 40.0%. A method for manufacturing steel gears.

6. A method for manufacturing a steel gear according to claim 5, The aforementioned intermediate product preparation process further, The process includes a surface roughness adjustment step to adjust the surface roughness of the carburized hardened layer of the steel material after the heat treatment step, thereby producing an intermediate product having an arithmetic mean surface roughness Ra of the carburized hardened layer in accordance with JIS B 0601:2013 of 0.05 to 2.00 μm. A method for manufacturing steel gears.

Images