Bearing parts

Abstract

To provide a bearing component having improved rolling fatigue life.SOLUTION: A bearing component is a rolling element used for a rocker arm bearing, a shaft used for the rocker arm bearing, or a shaft used for a planetary gear mechanism bearing. The bearing component has a hardened layer on the surface. The hardening layer includes a plurality of martensite crystal grains. A ratio of the total area of the martensite crystal grains in the hardening layer is 70% or more. The martensite crystal grains are divided into a first group and a second group. A smallest crystal grain size of the martensite crystal grains belonging to the first group is larger than a largest crystal grain size of the martensite crystal grains belonging to the second group. A value obtained by dividing the total area of the martensite crystal grains belonging to the first group by the total area of the martensite crystal grains is 0.5 or more. A value obtained by dividing the total area of the martensite crystal grains belonging to the first group excluding the martensite crystal grains having the smallest crystal grain size belonging to the first group by the total area of the martensite crystal grains is less than 0.5.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to bearing components, and more particularly to a rolling element used in a rocker arm bearing, a shaft used in a rocker arm bearing, and a shaft used in a planetary gear mechanism bearing. [Background technology]

[0002] The rolling fatigue life of bearing parts is improved by carbonitriding the surfaces of the bearing parts (the raceway surfaces of the inner and outer rings and the rolling surfaces of the rolling elements) as described in Patent Document 1 (Japanese Patent No. 5592540). Also, the rolling fatigue life of rolling bearings is improved by refining prior austenite grains on the surfaces of the bearing parts as described in Patent Document 2 (Japanese Patent No. 3905430). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5592540 [Patent Document 2] Patent No. 3905430 Summary of the Invention [Problem to be solved by the invention]

[0004] The steel used for bearing parts is generally quenched. That is, a quench-hardened layer, the main constituent structure of which is martensite, is formed on the surface of the bearing part. However, it has not been known how the state of martensite crystal grains affects the rolling fatigue life of bearing parts.

[0005] The rolling elements or shafts of rocker arm bearings may be locally subjected to high surface pressure due to the influence of mounting errors, load bias, and the like. In addition, since rocker arm bearings are used inside engines, foreign matter may be mixed into the lubricant, and the lubricant may deteriorate. Furthermore, since rocker arm bearings are full complement bearings, interference between the rolling elements (rollers), skew with respect to the rollers, and insufficient supply of lubricant between the raceway and the rollers may occur. Due to these factors, the rolling fatigue life of the rolling elements or shafts of rocker arm bearings may become shorter than expected, and therefore it is desirable to improve the rolling fatigue life. Similarly, it is desirable to improve the rolling fatigue life of shafts used in planetary gear mechanism bearings.

[0006] The present invention has been made in consideration of the above-mentioned problems in the prior art. More specifically, the present invention provides a bearing component having an improved rolling fatigue life. [Means for solving the problem]

[0007] A bearing part according to a first aspect of the present invention is a rolling element used in a rocker arm bearing, a shaft used in a rocker arm bearing, or a shaft used in a planetary gear mechanism bearing. The bearing part has a hardened layer on its surface. The hardened layer includes a plurality of martensite grains. The ratio of the total area of ​​the martensite grains in the hardened layer is 70 percent or more. The martensite grains are divided into a first group and a second group. The minimum grain size of the martensite grains belonging to the first group is greater than the maximum grain size of the martensite grains belonging to the second group. The value obtained by dividing the total area of ​​the martensite grains belonging to the first group by the total area of ​​the martensite grains is 0.5 or more. The value obtained by dividing the total area of ​​the martensite grains belonging to the first group, excluding the martensite grains belonging to the first group with the smallest grain size, by the total area of ​​the martensite grains is less than 0.5. The average grain size of the martensite grains belonging to the first group is 0.97 μm or less. The steel used is SUJ2, a high carbon chromium bearing steel specified by the JIS standard.

[0008] A bearing part according to a second aspect of the present invention is a rolling element used in a rocker arm bearing, a shaft used in a rocker arm bearing, or a shaft used in a planetary gear mechanism bearing. The bearing part has a hardened layer on its surface. The hardened layer includes a plurality of martensite grains. The ratio of the total area of ​​the martensite grains in the hardened layer is 70 percent or more. The martensite grains are divided into a first group and a second group. The minimum grain size of the martensite grains belonging to the first group is greater than the maximum grain size of the martensite grains belonging to the second group. The value obtained by dividing the total area of ​​the martensite grains belonging to the first group by the total area of ​​the martensite grains is 0.5 or more. The value obtained by dividing the total area of ​​the martensite grains belonging to the first group, excluding the martensite grains belonging to the first group with the smallest grain size, by the total area of ​​the martensite grains is less than 0.5. The average grain size of the martensite grains belonging to the first group is 0.97 μm or less. The average aspect ratio of the martensite grains belonging to the first group is 2.57 or less.

[0009] A bearing part according to a third aspect of the present invention is a rolling element used in a rocker arm bearing, a shaft used in a rocker arm bearing, or a shaft used in a planetary gear mechanism bearing. The bearing part has a hardened layer on its surface. The hardened layer includes a plurality of martensite grains. The ratio of the total area of ​​the martensite grains in the hardened layer is 70 percent or more. The martensite grains are divided into a third group and a fourth group. The minimum grain size of the martensite grains belonging to the third group is greater than the maximum grain size of the martensite grains belonging to the fourth group. The value obtained by dividing the total area of ​​the martensite grains belonging to the third group by the total area of ​​the martensite grains is 0.7 or more. The value obtained by dividing the total area of ​​the martensite grains belonging to the third group, excluding the martensite grains belonging to the third group with the smallest grain size, by the total area of ​​the martensite grains is less than 0.7. The average grain size of the martensite grains belonging to the third group is 0.75 μm or less. The steel used is SUJ2, a high carbon chromium bearing steel specified by the JIS standard.

[0010] A bearing part according to a fourth aspect of the present invention is a rolling element used in a rocker arm bearing, a shaft used in a rocker arm bearing, or a shaft used in a planetary gear mechanism bearing. The bearing part has a hardened layer on its surface. The hardened layer includes a plurality of martensite grains. The ratio of the total area of ​​the martensite grains in the hardened layer is 70 percent or more. The martensite grains are divided into a third group and a fourth group. The minimum grain size of the martensite grains belonging to the third group is greater than the maximum grain size of the martensite grains belonging to the fourth group. The value obtained by dividing the total area of ​​the martensite grains belonging to the third group by the total area of ​​the martensite grains is 0.7 or more. The value obtained by dividing the total area of ​​the martensite grains belonging to the third group, excluding the martensite grains belonging to the third group with the smallest grain size, by the total area of ​​the martensite grains is less than 0.7. The average grain size of the martensite grains belonging to the third group is 0.75 μm or less. The average aspect ratio of the martensite grains belonging to the third group is 2.45 or less.

[0011] In the above-mentioned bearing component, the hardness of the quench-hardened layer on the surface may be 730 Hv or more.

[0012] In the bearing component, the hardened layer may contain nitrogen. An average nitrogen concentration of the hardened layer between the surface and a position 10 μm away from the surface may be 0.15 mass percent or more. In the bearing component, the hardened layer may include a plurality of austenite grains. A volume ratio of the austenite grains in the hardened layer may be 30 percent or less. Effect of the Invention

[0013] According to the bearing components according to the first to fourth aspects of the present invention, the rolling fatigue life can be improved. [Brief description of the drawings]

[0014] [Figure 1] FIG. 2 is a cross-sectional view of a bearing 10. [Diagram 2] 2 is an enlarged cross-sectional view of a rolling element 13 in the vicinity of an outer circumferential surface 13a. [Diagram 3] 2 is an enlarged cross-sectional view of the shaft 11 in the vicinity of the outer circumferential surface 11a. [Figure 4] 3A to 3C are process diagrams showing a manufacturing method of the rolling element 13. [Diagram 5] 2 is a schematic diagram of a planetary gear mechanism 30. FIG. [Figure 6] FIG. [Figure 7] 2 is an enlarged cross-sectional view of shaft 21 in the vicinity of outer circumferential surface 21a. [Figure 8] EBSD image of a cross section of sample 1. [Figure 9] EBSD image of the cross section of sample 2. [Figure 10] EBSD image of the cross section of sample 3. [Figure 11] 1 is a graph showing the relationship between the average grain size of martensite crystal grains and the rolling fatigue life. [Figure 12] 1 is a graph showing the relationship between the average aspect ratio of martensite crystal grains and the rolling fatigue life. [Figure 13] 1 is a graph showing the relationship between maximum contact pressure and indentation depth. [Figure 14] 1 is a graph showing the relationship between the average grain size of martensite crystal grains and the static load capacity. [Figure 15] 1 is a graph showing the relationship between the average aspect ratio of martensite grains and static load capacity. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The details of the embodiment will be described with reference to the drawings. In the following drawings, the same or corresponding parts are given the same reference numerals, and the overlapping description will not be repeated.

[0016] (First embodiment) The configuration of a rocker arm bearing according to a first embodiment (hereinafter referred to as "bearing 10") will be described below.

[0017] <Schematic configuration of bearing 10> FIG. 1 is a cross-sectional view of a bearing 10. As shown in FIG. 1, the bearing 10 has a shaft 11, an outer ring 12, and rolling elements 13. The shaft 11, the outer ring 12, and the rolling elements 13 are made of steel. More specifically, the shaft 11, the outer ring 12, and the rolling elements 13 are made of bearing steel. The shaft 11, the outer ring 12, and the rolling elements 13 are preferably made of high carbon chromium bearing steel SUJ2 defined in the JIS standard (JIS G 4805:2008). The shaft 11, the outer ring 12, and the rolling elements 13 may be made of high carbon chromium bearing steel SUJ3 defined in the JIS standard, 52100 defined in the ASTM standard, 100Cr6 defined in the DIN standard, or GCr5 (GCr15) defined in the GB standard.

[0018] The shaft 11 has an outer peripheral surface 11a. The outer peripheral surface 11a is a raceway surface (surface that comes into contact with the rolling elements 13). The shaft 11 has, for example, a cylindrical shape. The shaft 11 may be solid or hollow. The shaft 11 has a central axis A1. The shaft 11 has a first end 11b and a second end 11c in the axial direction (direction along the central axis A1). The second end 11c is an end opposite to the first end 11b. The shaft 11 is fixed to a rocker arm 14 at the first end 11b and the second end 11c. The rocker arm 14 is pushed and moved by a cam (not shown) to swing. The swinging of the rocker arm 14 causes the shaft 11 to rotate around the central axis A1.

[0019] The outer ring 12 has an annular (ring-shaped) shape. The outer ring 12 has a top surface 12a, a bottom surface 12b, an inner peripheral surface 12c, and an outer peripheral surface 12d. The top surface 12a and the bottom surface 12b form end surfaces of the outer ring 12 in the axial direction. The bottom surface 12b is the surface opposite the top surface 12a in the axial direction.

[0020] The inner peripheral surface 12c and the outer peripheral surface 12d extend in the circumferential direction (a direction along a circumference centered on the central axis A1). The inner peripheral surface 12c faces the central axis A1, and the outer peripheral surface 12d faces the opposite side to the central axis A1. In other words, the outer peripheral surface 12d is the opposite surface to the inner peripheral surface 12c in the radial direction (a direction passing through the central axis A1 and perpendicular to the central axis A1). The outer ring 12 is disposed such that the inner peripheral surface 12c faces the outer peripheral surface 11a. The inner peripheral surface 12c forms a raceway surface.

[0021] The rolling element 13 has a cylindrical shape extending along the axial direction. Specifically, the rolling element 13 is a needle roller. The rolling element 13 has an outer peripheral surface 13a. The outer peripheral surface 13a forms a rolling surface. The rolling element 13 is disposed between the shaft 11 and the outer ring 12 such that the outer peripheral surface 13a contacts the outer peripheral surface 11a and the inner peripheral surface 12c. This allows the shaft 11 to be supported rotatably around the central axis A1. The bearing 10 does not have a cage. In other words, the bearing 10 is a full complement bearing.

[0022] <Detailed configuration of the rolling element 13> Fig. 2 is an enlarged cross-sectional view of the rolling element 13 near the outer circumferential surface 13a. As shown in Fig. 2, the rolling element 13 has a quench-hardened layer 15 on the outer circumferential surface 13a. The quench-hardened layer 15 is a layer hardened by quenching. The quench-hardened layer 15 includes a plurality of martensite crystal grains.

[0023] When the deviation between the crystal orientation of a first martensite grain and the crystal orientation of a second martensite grain adjacent to the first martensite grain is 15° or more, the first martensite grain and the second martensite grain are different martensite grains, whereas when the deviation between the crystal orientation of a first martensite grain and the crystal orientation of a second martensite grain adjacent to the first martensite grain is less than 15°, the first martensite grain and the second martensite grain constitute a single martensite grain.

[0024] The hardened layer 15 has a martensite phase as a main structural component. More specifically, the ratio of the total area of ​​martensite grains in the hardened layer 15 is 70 percent or more. The ratio of the total area of ​​martensite grains in the hardened layer 15 may be 80 percent or more.

[0025] The hardened layer 15 is made up of martensite grains, austenite grains, ferrite grains, and cementite (Fe 3 C) contains crystal grains. The volume ratio of the austenite crystal grains in the hardened layer 15 is preferably 30 percent or less. The volume ratio of the austenite crystal grains in the hardened layer 15 is more preferably 20 percent or more. The volume ratio of the austenite crystal grains in the hardened layer 15 is measured by an X-ray diffraction method. More specifically, the volume ratio of the austenite crystal grains in the hardened layer 15 is calculated based on the ratio between the X-ray diffraction intensity of the austenite phase and the X-ray diffraction intensity of other phases contained in the hardened layer 15.

[0026] The martensite grains are divided into a first group and a second group. The minimum grain size of the martensite grains belonging to the first group is greater than the maximum grain size of the martensite grains belonging to the second group.

[0027] The value obtained by dividing the total area of ​​the martensite grains belonging to the first group by the total area of ​​the martensite grains (the sum of the total area of ​​the martensite grains belonging to the first group and the total area of ​​the martensite grains belonging to the second group) is 0.5 or more.

[0028] The value obtained by dividing the total area of ​​the martensite grains belonging to the first group, excluding the martensite grains belonging to the first group having the smallest grain size, by the total area of ​​the martensite grains is less than 0.5.

[0029] Another way of looking at this is that the martensite grains are assigned to group 1 in order of increasing grain size, and the assignment to group 1 ends when the total area of ​​the martensite grains assigned to group 1 up to that point is at least 0.5 times the total area of ​​the martensite grains, and the remaining martensite grains are assigned to group 2.

[0030] The average grain size of the martensite grains belonging to the first group is 0.97 μm or less. Preferably, the average grain size of the martensite grains belonging to the first group is 0.90 μm or less. More preferably, the average grain size of the martensite grains belonging to the first group is 0.85 μm or less.

[0031] The aspect ratio of the martensite grains belonging to the first group is 2.57 or less. Preferably, the aspect ratio of the martensite grains belonging to the first group is 2.50 or less. More preferably, the aspect ratio of the martensite grains belonging to the first group is 2.45 or less.

[0032] The martensite grains may be divided into a third group and a fourth group, and the minimum grain size of the martensite grains in the third group is greater than the maximum grain size of the martensite grains in the fourth group.

[0033] The value obtained by dividing the total area of ​​the martensite grains belonging to the third group by the total area of ​​the martensite grains (the sum of the total area of ​​the martensite grains belonging to the third group and the total area of ​​the martensite grains belonging to the fourth group) is 0.7 or more.

[0034] The value obtained by dividing the total area of ​​the martensite grains belonging to the third group, excluding the martensite grains belonging to the third group having the smallest grain size, by the total area of ​​the martensite grains is less than 0.7.

[0035] Another way of looking at this is that the martensite grains are assigned to group 3 in order of increasing grain size, and the assignment to group 3 ends when the total area of ​​the martensite grains assigned to group 3 so far is at least 0.7 times the total area of ​​the martensite grains, and the remaining martensite grains are assigned to group 4.

[0036] The average grain size of the martensite grains belonging to the third group is 0.75 μm or less. Preferably, the average grain size of the martensite grains belonging to the third group is 0.70 μm or less. More preferably, the average grain size of the martensite grains belonging to the third group is 0.65 μm or less.

[0037] The aspect ratio of the martensite grains belonging to the third group is 2.45 or less. Preferably, the aspect ratio of the martensite grains belonging to the third group is 2.40 or less. More preferably, the aspect ratio of the martensite grains belonging to the third group is 2.35 or less.

[0038] The average grain size of the martensite grains belonging to the first group (third group) and the aspect ratio of the martensite grains belonging to the first group (third group) are measured by using the EBSD (Electron Backscattered Diffraction) method.

[0039] More specifically, the process is as follows. First, a cross-sectional image of the hardened layer 15 is taken based on the EBSD method (hereinafter, referred to as an "EBSD image"). The EBSD image is taken so as to include a sufficient number (20 or more) of martensite grains. Boundaries between adjacent martensite grains are identified based on the EBSD image. Second, the area and shape of each martensite grain displayed in the EBSD image are calculated based on the identified boundaries of the martensite grains.

[0040] More specifically, the equivalent circle diameter of each martensite grain displayed in the EBSD image is calculated by calculating the square root of the value obtained by dividing the area of each martensite grain displayed in the EBSD image by π / 4.

[0041] Based on the equivalent circle diameter of each martensite grain calculated as described above, among the martensite grains displayed in the EBSD image, the martensite grains belonging to the first group (third group) are determined. The value obtained by dividing the total area of the martensite grains belonging to the first group (third group) among the martensite grains displayed in the EBSD image by the total area of the martensite grains displayed in the EBSD image is the value obtained by dividing the total area of the martensite grains belonging to the first group (third group) by the total area of the martensite grains.

[0042] Based on the equivalent circle diameter of each martensite grain calculated as described above, the martensite grains displayed in the EBSD image are classified into a first group and a second group (classified into a third group and a fourth group). The value obtained by dividing the sum of the equivalent circle diameters of the martensite grains displayed in the EBSD image classified into the first group (third group) by the number of the martensite grains displayed in the EBSD image classified into the first group (third group) is the average grain size of the martensite grains belonging to the first group (third group).

[0043] From the shape of each martensite grain displayed in the EBSD image, the shape of each martensite grain displayed in the EBSD image is approximated as an ellipse by the least squares method. This ellipse approximation by the least squares method is performed according to the method described in S. Biggin and DJ Dingley, Journal of Applied Crystallography, (1977)10, 376-378. In this ellipse shape, the aspect ratio of each martensite grain displayed in the EBSD image is calculated by dividing the major axis dimension by the minor axis dimension. The sum of the aspect ratios of the martensite grains displayed in the EBSD images classified in the first group (third group) divided by the number of martensite grains displayed in the EBSD images classified in the first group (third group) is regarded as the average aspect ratio of the martensite grains belonging to the first group (third group).

[0044] The quench-hardened layer 15 contains nitrogen. The average nitrogen concentration of the quench-hardened layer 15 between the outer peripheral surface 13a and a position 10 μm away from the outer peripheral surface 13a is preferably 0.05 mass percent or more. This average nitrogen concentration may be 0.10 mass percent or more. This average nitrogen concentration is, for example, 0.20 mass percent or less. The average nitrogen concentration is measured using an EPMA (Electron Probe Micro Analyzer). This average nitrogen concentration is preferably measured at the axial center position of the rolling surface (a position where a virtual line that passes through the center in the direction along the central axis of the rolling element 13 and is perpendicular to the central axis intersects with the outer peripheral surface 13a). The penetration depth of nitrogen in the outer peripheral surface 13a at the axial center position of the rolling surface is preferably 0.2 mm or more. The penetration depth of nitrogen is the depth until the concentration of nitrogen measured using the EPMA becomes 0 mass percent.

[0045] The hardness of the quench-hardened layer 15 on the outer peripheral surface 13a is preferably 730 Hv or more. The hardness of the quench-hardened layer 15 on the outer peripheral surface 13a is measured in accordance with the JIS standard (JJS Z 2244:2009).

[0046] <Detailed configuration of axis 11> Fig. 3 is an enlarged cross-sectional view of shaft 11 near outer circumferential surface 11a. As shown in Fig. 3, shaft 11 has a hardened layer 16 on outer circumferential surface 11a. The configuration of hardened layer 16 is similar to the configuration of hardened layer 15.

[0047] More specifically, the hardened layer 16 includes a plurality of martensite grains. The ratio of the total area of ​​the martensite grains in the hardened layer 16 is 70 percent or more (preferably 80 percent or more).

[0048] The martensite grains in the hardened layer 16 are divided into a first group and a second group. The value obtained by dividing the total area of ​​the martensite grains belonging to the first group by the total area of ​​the martensite grains is 0.5 or more. The value obtained by dividing the total area of ​​the martensite grains belonging to the first group, excluding the martensite grains belonging to the first group having the smallest grain size, by the total area of ​​the martensite grains is less than 0.5.

[0049] The average grain size of the martensite grains belonging to the first group is 0.97 μm or less (preferably 0.90 μm or less, more preferably 0.85 μm or less), and the aspect ratio of the martensite grains belonging to the first group is 2.57 or less (preferably 2.50 or less, more preferably 2.45 or less).

[0050] The martensite grains in the hardened layer 16 may be divided into a third group and a fourth group. The value obtained by dividing the total area of ​​the martensite grains belonging to the third group by the total area of ​​the martensite grains is 0.7 or more. The value obtained by dividing the total area of ​​the martensite grains belonging to the third group, excluding the martensite grains belonging to the third group having the smallest grain size, by the total area of ​​the martensite grains is less than 0.7.

[0051] The average grain size of the martensite grains belonging to the third group is 0.75 μm or less (preferably 0.70 μm or less, more preferably 0.65 μm or less).The aspect ratio of the martensite grains belonging to the third group is 2.45 or less (preferably 2.40 or less, more preferably 2.35 or less).

[0052] The quench hardened layer 16 contains nitrogen. The average nitrogen concentration of the quench hardened layer 16 between the outer peripheral surface 11a and a position 10 μm away from the outer peripheral surface 11a is preferably 0.05 mass percent or more. This average nitrogen concentration may be 0.10 mass percent or more. This average nitrogen concentration is, for example, 0.20 mass percent or less. This average nitrogen concentration is preferably measured at the axial center position of the rolling surface (a position where a virtual line that passes through the center in the direction along the central axis of the rolling element 13 and is perpendicular to the central axis intersects with the outer peripheral surface 11a). The penetration depth of nitrogen in the outer peripheral surface 11a at the axial center position of the rolling surface is preferably 0.2 mm or more. The hardness of the quench hardened layer 16 on the outer peripheral surface 11a is preferably 730 Hv or more. In addition, the volume ratio of austenite grains in the quench hardened layer 16 is preferably 30 percent or less (preferably 20 percent or more and 30 percent or less).

[0053] A method for manufacturing the rolling elements 13 will now be described. Fig. 4 is a process diagram showing a manufacturing method of the rolling element 13. As shown in Fig. 4, the manufacturing method of the rolling element 13 includes a preparation step S1, a carbo-nitriding step S2, a first tempering step S3, a quenching step S4, a second tempering step S5, and a post-treatment step S6.

[0054] In the preparation step S1, a cylindrical workpiece to be processed that becomes the rolling elements 13 is prepared by going through a carbo-nitriding step S2, a first tempering step S3, a quenching step S4, a second tempering step S5, and a post-treatment step S6. In the preparation step S1, first, the workpiece is cut. In the preparation step S1, second, the workpiece is cold forged or cold heading. In the preparation step S1, third, cutting is performed as necessary to make the shape of the workpiece closer to the shape of the rolling elements 13.

[0055] In the carbonitriding step S2, first, the workpiece is heated to a first temperature or higher, thereby performing carbonitriding treatment on the workpiece. The first temperature is the A temperature of the steel constituting the workpiece. 1 The temperature is equal to or higher than the transformation point. In the carbo-nitriding step S2, secondly, the workpiece is cooled. This cooling is performed so that the temperature of the workpiece becomes equal to or lower than the Ms transformation point.

[0056] In the first tempering step S3, tempering is performed on the workpiece. The first tempering step S3 is performed by holding the workpiece at a second temperature for a first time. The second temperature is A 1 The second temperature is a temperature lower than the transformation point. The second temperature is, for example, 160° C. or more and 200° C. or less. The first time is, for example, 1 hour or more and 4 hours or less.

[0057] In the quenching step S4, quenching is performed on the workpiece. In the quenching step S4, first, the workpiece is heated to a third temperature. The third temperature is the A temperature of the steel constituting the workpiece. 1 The third temperature is a temperature equal to or higher than the transformation point. The third temperature is preferably lower than the first temperature. Secondly, in the quenching step S4, the workpiece is cooled. This cooling is performed so that the temperature of the workpiece becomes equal to or lower than the Ms transformation point.

[0058] In the second tempering step S5, tempering is performed on the workpiece. The second tempering step S5 is performed by holding the workpiece at a fourth temperature for a second time. The fourth temperature is A 1 The fourth temperature is a temperature lower than the transformation point. The fourth temperature is, for example, 160° C. or more and 200° C. or less. The second time is, for example, 1 hour or more and 4 hours or less. The quenching step S4 and the second tempering step S5 may be repeated multiple times.

[0059] In the post-treatment step S6, the workpiece is subjected to post-treatment. In the post-treatment step S6, for example, the workpiece is washed, and the surface of the workpiece is subjected to machining such as grinding and polishing. In this manner, the rolling element 13 is manufactured.

[0060] The method of manufacturing the shaft 11 is similar to the method of manufacturing the rolling elements 13, and therefore a detailed description thereof will be omitted here.

[0061] The effects of the rolling elements 13 and the shaft 11 will be explained below. When considering material fracture using the weakest link model, the areas with relatively low strength, i.e., martensite grains with a relatively large grain size, have a large effect on material fracture. In the hardened layers 15 and 16, the average grain size of the martensite grains belonging to the first group (third group) is 0.97 μm or less (0.75 μm or less). Therefore, in the rolling elements 13 and shaft 11, even if the martensite grains belonging to the first group (third group) with relatively large grains are fine grains, the rolling fatigue strength and static load capacity are improved.

[0062] As the average aspect ratio of martensite grains decreases, the shape of the martensite grains becomes closer to a sphere, and stress concentration is less likely to occur. Therefore, when the average aspect ratio of martensite grains belonging to the first group (third group) is 2.57 or less (2.45 or less), the rolling fatigue strength and static load capacity can be further improved.

[0063] By having the volume ratio of austenite crystal grains in the hardened layers 15 and 16 be 30 percent or less, it is possible to suppress a decrease in hardness at the outer peripheral surfaces 13a and 11a while maintaining the dimensional stability of the rolling elements 13 and shaft 11.

[0064] Second embodiment The configuration of a planetary gear mechanism bearing (hereinafter referred to as "bearing 20") according to the second embodiment will be described below.

[0065] <Configuration of the planetary gear mechanism 30> Fig. 5 is a schematic diagram of the planetary gear mechanism 30. As shown in Fig. 5, the planetary gear mechanism 30 has a ring gear 31 (internal gear), a sun gear 32 (sun gear), and a plurality of pinion gears 33 (planetary gears).

[0066] The ring gear 31 has a ring-like (annular) shape. The ring gear 31 has an inner peripheral surface 31a. Internal teeth 31b are formed on the inner peripheral surface 31a. The sun gear 32 has a disk-like shape. The sun gear 32 has an outer peripheral surface 32a. External teeth 32b are formed on the outer peripheral surface 32a. The sun gear 32 is disposed at the center of the ring gear 31. The pinion gear 33 has a disk-like shape. The pinion gear 33 has an outer peripheral surface 33a. External teeth 33b are formed on the outer peripheral surface 33a. The pinion gear 33 is disposed between the ring gear 31 and the sun gear 32, and the external teeth 33b mesh with the internal teeth 31b and the external teeth 32b.

[0067] <Schematic configuration of bearing 20> Fig. 6 is a cross-sectional view of a bearing 20. As shown in Fig. 6, the bearing 20 has a shaft 21, rolling elements 22, and a cage 23. The shaft 21 and rolling elements 22 are made of steel. For example, the shaft 21 and rolling elements 22 are made of high carbon chromium bearing steel SUJ2 defined in the JIS standard. The shaft 21 and rolling elements 22 may also be made of high carbon chromium bearing steel SUJ3 defined in the JIS standard, 52100 defined in the ASTM standard, 100Cr6 defined in the DIN standard, or GCr5 (GCr15) defined in the GB standard.

[0068] The shaft 21 has a cylindrical shape extending along the central axis A2. The shaft 21 has an outer peripheral surface 21a. The outer peripheral surface 21a is a raceway surface. The shaft 21 is inserted into a through hole 33c formed in the pinion gear 33. The outer peripheral surface 21a faces the inner wall surface of the through hole 33c.

[0069] The shaft 21 has a first end 21b and a second end 21c in the axial direction (direction along the central axis A2). The second end 21c is the end opposite the first end 21b in the axial direction. The shaft 21 is fixed to a carrier 34 at the first end 21b and the second end 21c. The orbital motion of the pinion gear 33 is input and output from the carrier 34.

[0070] An oil passage 21d is formed inside the shaft 21. The oil passage 21d has a supply port 21da that opens at the second end 21c and a discharge port 21db that opens at the outer circumferential surface 21a. The lubricant supplied from the supply port 21da passes through the oil passage 21d and is discharged from the discharge port 21db. As a result, the lubricant is supplied to the periphery of the rolling element 22.

[0071] The rolling element 22 extends along the axial direction. The rolling element 22 has a cylindrical shape. The rolling element 22 is, for example, a needle roller. The rolling element 22 has an outer peripheral surface 22a. The outer peripheral surface 22a forms a rolling surface. The rolling element 22 is disposed between the shaft 21 and the pinion gear 33 such that the outer peripheral surface 22a contacts the outer peripheral surface 21a and the inner wall surface of the through hole 33c. As a result, the pinion gear 33 is supported by the bearing 20 so as to be rotatable around the central axis A2.

[0072] The cage 23 holds the rolling elements 22 so as to keep the spacing between the rolling elements 22 in the circumferential direction (the direction along the circumference passing through the central axis A2) within a certain range.

[0073] <Detailed configuration of axis 21> Fig. 7 is an enlarged cross-sectional view of shaft 21 near outer circumferential surface 21a. As shown in Fig. 7, shaft 21 has a hardened layer 24 on outer circumferential surface 21a. The configuration of hardened layer 24 is similar to the configuration of hardened layer 15.

[0074] The hardened layer 24 includes a plurality of martensite grains. The ratio of the total area of ​​the martensite grains in the hardened layer 24 is 70 percent or more (preferably 80 percent or more).

[0075] The martensite grains in the hardened layer 24 are divided into a first group and a second group. The value obtained by dividing the total area of ​​the martensite grains belonging to the first group by the total area of ​​the martensite grains is 0.5 or more. The value obtained by dividing the total area of ​​the martensite grains belonging to the first group, excluding the martensite grains belonging to the first group having the smallest grain size, by the total area of ​​the martensite grains is less than 0.5.

[0076] The average grain size of the martensite grains belonging to the first group is 0.97 μm or less (preferably 0.90 μm or less, more preferably 0.85 μm or less), and the aspect ratio of the martensite grains belonging to the first group is 2.57 or less (preferably 2.50 or less, more preferably 2.45 or less).

[0077] The martensite grains in the hardened layer 24 may be divided into a third group and a fourth group. The value obtained by dividing the total area of ​​the martensite grains belonging to the third group by the total area of ​​the martensite grains is 0.7 or more. The value obtained by dividing the total area of ​​the martensite grains belonging to the third group, excluding the martensite grains belonging to the third group having the smallest grain size, by the total area of ​​the martensite grains is less than 0.7.

[0078] The average grain size of the martensite grains belonging to the third group is 0.75 μm or less (preferably 0.70 μm or less, more preferably 0.65 μm or less).The aspect ratio of the martensite grains belonging to the third group is 2.45 or less (preferably 2.40 or less, more preferably 2.35 or less).

[0079] The quench hardened layer 24 contains nitrogen. The average nitrogen concentration of the quench hardened layer 24 between the outer peripheral surface 21a and a position 10 μm away from the outer peripheral surface 21a is preferably 0.05 mass percent or more. This average nitrogen concentration may be 0.10 mass percent or more. This average nitrogen concentration is, for example, 0.20 mass percent or less. This average nitrogen concentration is preferably measured at the axial center position of the rolling surface (a position where a virtual line that passes through the center in the direction along the central axis of the rolling element 22 and is perpendicular to the central axis intersects with the outer peripheral surface 21a). The penetration depth of nitrogen in the outer peripheral surface 21a at the axial center position of the rolling surface is preferably 0.2 mm or more. The hardness of the quench hardened layer 24 on the outer peripheral surface 21a is preferably 730 Hv or more. In addition, the volume ratio of austenite grains in the quench hardened layer 24 is preferably 30 percent or less (preferably 20 percent or more and 30 percent or less).

[0080] The manufacturing method of shaft 21 and the effects of shaft 21 are similar to those of rolling elements 13 and rolling elements 13, and therefore a detailed description thereof will be omitted here.

[0081] A rolling fatigue test and a static load capacity test carried out to confirm the effects of the rolling elements 13, the shaft 11 and the shaft 21 will be described below.

[0082] <Test material> Samples 1, 2, and 3 were used for the rolling fatigue test and static load capacity test. Samples 1 and 2 were made of SUJ2. Sample 3 was made of SCM435, a chromium-molybdenum steel defined in the JIS standard (JIS G 4053:2016).

[0083] Sample 1 was prepared by carrying out the same heat treatment as for the rolling element 13 (shaft 11 or shaft 21). More specifically, in the preparation of Sample 1, the first temperature was set to 850°C, the second temperature was set to 180°C, the third temperature was set to 810°C, and the fourth temperature was set to 180°C. For Samples 2 and 3, the quenching step S4 and the second tempering step S5 were not carried out. In the preparation of Sample 2, the first temperature was set to 850°C, and the second temperature was set to 180°C. In the preparation of Sample 3, the first temperature was set to 930°C, and the second temperature was set to 170°C. The heat treatment conditions for Samples 1 to 3 are shown in Table 1.

[0084] [Table 1]

[0085] In addition, in Samples 1 to 3, the ratio of the total area of ​​austenite crystal grains at a position 50 μm away from the surface was 20 percent or more and 30 percent or less, the nitrogen concentration at the surface was 0.15 mass percent or more and 0.20 mass percent or less, and the hardness at the surface was 730 Hv.

[0086] In sample 1, the martensite grains in the first group had an average grain size of 0.80 μm and an average aspect ratio of 2.41. In sample 1, the martensite grains in the third group had an average grain size of 0.64 μm and an average aspect ratio of 2.32.

[0087] In sample 2, the average grain size of the martensite grains belonging to the first group was 1.11 μm, and the average aspect ratio of the martensite grains belonging to the first group was 3.00. In sample 2, the average grain size of the martensite grains belonging to the third group was 0.84 μm, and the average aspect ratio of the martensite grains belonging to the third group was 2.77.

[0088] In sample 3, the average grain size of the martensite grains belonging to the first group was 1.81 μm, and the average aspect ratio of the martensite grains belonging to the first group was 3.38. In sample 2, the average grain size of the martensite grains belonging to the third group was 1.28 μm, and the average aspect ratio of the martensite grains belonging to the third group was 3.04.

[0089] The measurement results of the average grain size and the average aspect ratio of the martensite grains for Samples 1 to 3 are shown in Table 2.

[0090] [Table 2]

[0091] Fig. 8 is an EBSD image of a cross section of Sample 1. Fig. 9 is an EBSD image of a cross section of Sample 2. Fig. 10 is an EBSD image of a cross section of Sample 3. As shown in Figs. 8 to 10, it can be seen that martensite grains are refined in Sample 1 compared to Samples 2 and 3.

[0092] <Rolling fatigue test conditions> For the rolling fatigue test, inner rings, outer rings and tapered rollers were prepared using Sample 1 and Sample 3, and tapered roller bearings were fabricated using these. The rolling fatigue test was conducted under conditions of an inner ring rotation speed of 3000 rpm and a maximum contact pressure of 2.6 GPa. Lubrication in the rolling fatigue test was performed using hot water bath lubrication with turbine oil VG56. Hard gas atomized powder was mixed into this turbine oil at a ratio of 0.2 g / l. The test conditions for the rolling fatigue test are shown in Table 3. The rolling fatigue test was conducted on tapered roller bearings fabricated using six pieces of Sample 1 and tapered roller bearings fabricated using six pieces of Sample 3.

[0093] [Table 3]

[0094] <Static load capacity test conditions> For the static load capacity test, flat plate members were produced using Samples 1 to 3. The static load capacity test was carried out by pressing a silicon nitride ceramic ball against the mirror-finished surface of the flat plate member to obtain the relationship between the maximum contact pressure and the indentation depth. The static load capacity was evaluated based on the maximum contact pressure when the value obtained by dividing the indentation depth by the diameter of the ceramic ball reached 1 / 10,000 (when the value obtained by dividing the indentation depth by the diameter of the ceramic ball and multiplying it by 10,000 reached 1).

[0095] <Rolling fatigue test results> In the tapered roller bearing prepared using sample 1, L 50 On the other hand, in the tapered roller bearing prepared using sample 3, the life (50 percent failure life) was 50.4 hours. 50 The rolling fatigue life was 31.2 hours. Thus, when a tapered roller bearing was made using Sample 1, the rolling fatigue life was improved by more than twice as much as when a tapered roller bearing was made using Sample 3. The test results are shown in Table 4.

[0096] [Table 4]

[0097] Fig. 11 is a graph showing the relationship between the average grain size of martensite grains and the rolling fatigue life. Fig. 12 is a graph showing the relationship between the average aspect ratio of martensite grains and the rolling fatigue life. In Fig. 11, the horizontal axis represents the average grain size (unit: μm) of martensite grains, and the vertical axis represents the rolling fatigue life L 50 In Fig. 12, the horizontal axis represents the average aspect ratio of martensite grains, and the vertical axis represents the rolling fatigue life L 50 (Unit: hours).

[0098] As shown in Fig. 11 and Fig. 12, the rolling fatigue life L 50 The smaller the average grain size of the martensite grains belonging to the first group (third group), the more improved the property, and the smaller the average aspect ratio of the martensite grains belonging to the first group (third group), the more improved the property.

[0099] <Static load capacity test results> Fig. 13 is a graph showing the relationship between the maximum contact pressure and the indentation depth. In Fig. 13, the horizontal axis represents the maximum contact pressure (unit: GPa), and the vertical axis represents the indentation depth divided by the diameter of the ceramic ball × 10 4 13, the curve corresponding to Sample 1 had a larger maximum contact pressure value when the vertical axis value was 1 than the curves corresponding to Sample 2 and Sample 3. That is, the static load capacity value of Sample 1 was larger than that of Sample 2 and Sample 3.

[0100] Fig. 14 is a graph showing the relationship between the average grain size of martensite grains and the static load capacity. Fig. 15 is a graph showing the relationship between the average aspect ratio of martensite grains and the static load capacity. In Fig. 14, the horizontal axis represents the average grain size of martensite grains (unit: μm), and the vertical axis represents the static load capacity (unit: GPa). In Fig. 15, the horizontal axis represents the average aspect ratio of martensite grains, and the vertical axis represents the static load capacity (unit: GPa).

[0101] As shown in Figures 14 and 15, the static load capacity is improved as the average grain size of the martensite grains belonging to the first group (third group) becomes smaller, and is improved as the average aspect ratio of the martensite grains belonging to the first group (third group) becomes smaller. Considering this together with the results shown in Figures 11 and 12, when the average grain size of the martensite grains belonging to the first group (third group) is 0.97 μm or less (0.75 μm or less) and the average aspect ratio of the martensite grains belonging to the first group (third group) is 2.57 or less (2.45 or less), the rolling fatigue life L 50 (That is, the rolling fatigue life L 50 ) rolling fatigue life L 50 It is possible to achieve a static load capacity of 5.3 GPa or more.

[0102] These test results experimentally demonstrated that the presence of the hardened layer 15 (hardened layer 16, hardened layer 24) improves the rolling fatigue strength and static load capacity of the rolling element 13 (shaft 11, shaft 21).

[0103] Although the embodiment of the present invention has been described above, the above-mentioned embodiment can be modified in various ways. The scope of the present invention is not limited to the above-mentioned embodiment. The scope of the present invention is defined by the claims, and is intended to include all modifications within the meaning and scope of the claims. [Industrial Applicability]

[0104] The above-described embodiment is particularly advantageously applicable to the rolling elements of the rocker arm bearing, the shaft of the rocker arm bearing, and the shaft of the planetary gear mechanism bearing.

Explanation of Reference Numerals

[0105] 10 Bearing, 11 Shaft, 11a Outer peripheral surface, 11b First end, 11c Second end, 12 Outer ring, 12a Upper surface, 12b Bottom surface, 12c Inner peripheral surface, 12d Outer peripheral surface, 13 Rolling element, 13a Outer peripheral surface, 14 Rocker arm, 15 Case-hardened layer, 16 Case-hardened layer, 20 Bearing, 21 Shaft, 21a Outer peripheral surface, 21b First end, 21c Second end, 21d Oil passage, 21da Supply port, 21db Discharge port, 22 Rolling element, 22a Outer peripheral surface, 23 Retainer, 24 Case-hardened layer, 30 Planetary gear mechanism, 31 Ring gear, 31a Inner peripheral surface, 31b Internal teeth, 32 Sun gear, 32a Outer peripheral surface, 32b External teeth, 33 Pinion gear, 33a Outer peripheral surface, 33b External teeth, 33c Through hole, 34 Carrier, A1, A2 Central axis, S1 Preparation process, S2 Carbonitriding process, S3 First tempering process, S4 Hardening process, S5 Second tempering process, S6 Post-treatment process.

Claims

1. A steel bearing part having a hardened layer on its surface, The bearing part is a rolling element used in a rocker arm bearing, a shaft used in a rocker arm bearing, or a shaft used in a planetary gear mechanism bearing, The hardened layer includes a plurality of martensite grains, The ratio of the total area of ​​the martensite grains in the hardened layer is 70% or more, The martensite grains are divided into a first group and a second group, the minimum value of the grain size of the martensite grains belonging to the first group is greater than the maximum value of the martensite grains belonging to the second group; a value obtained by dividing the total area of ​​the martensite grains belonging to the first group by the total area of ​​the martensite grains is 0.5 or more; a value obtained by dividing the total area of ​​the martensite grains belonging to the first group, excluding the martensite grains having the smallest grain size in the first group, by the total area of ​​the martensite grains is less than 0.5; The average grain size of the martensite grains belonging to the first group is 0.97 μm or less, The bearing part, wherein the steel is high carbon chromium bearing steel SUJ2 as defined by the JIS standard.

2. A steel bearing part having a hardened layer on its surface, The bearing part is a rolling element used in a rocker arm bearing, a shaft used in a rocker arm bearing, or a shaft used in a planetary gear mechanism bearing, The hardened layer includes a plurality of martensite grains, The ratio of the total area of ​​the martensite grains in the hardened layer is 70% or more, The martensite grains are divided into a first group and a second group, the minimum value of the grain size of the martensite grains belonging to the first group is greater than the maximum value of the martensite grains belonging to the second group; a value obtained by dividing the total area of ​​the martensite grains belonging to the first group by the total area of ​​the martensite grains is 0.5 or more; a value obtained by dividing the total area of ​​the martensite grains belonging to the first group, excluding the martensite grains having the smallest grain size in the first group, by the total area of ​​the martensite grains is less than 0.5; The average grain size of the martensite grains belonging to the first group is 0.97 μm or less, The average aspect ratio of the martensite grains belonging to the first group is 2.57 or less.

3. A steel bearing part having a hardened layer on its surface, The bearing part is a rolling element used in a rocker arm bearing, a shaft used in a rocker arm bearing, or a shaft used in a planetary gear mechanism bearing, The hardened layer includes a plurality of martensite grains, The ratio of the total area of ​​the martensite grains in the hardened layer is 70% or more, The martensite grains are divided into a third group and a fourth group, the minimum value of the grain size of the martensite grains belonging to the third group is greater than the maximum value of the martensite grains belonging to the fourth group; a value obtained by dividing the total area of ​​the martensite grains belonging to the third group by the total area of ​​the martensite grains is 0.7 or more; a value obtained by dividing the total area of ​​the martensite grains belonging to the third group, excluding the martensite grains having the smallest grain size in the third group, by the total area of ​​the martensite grains is less than 0.7; The average grain size of the martensite grains belonging to the third group is 0.75 μm or less, The bearing part, wherein the steel is high carbon chromium bearing steel SUJ2 as defined by the JIS standard.

4. A steel bearing part having a hardened layer on its surface, The bearing part is a rolling element used in a rocker arm bearing, a shaft used in a rocker arm bearing, or a shaft used in a planetary gear mechanism bearing, The hardened layer includes a plurality of martensite grains, The ratio of the total area of ​​the martensite grains in the hardened layer is 70% or more, The martensite grains are divided into a third group and a fourth group, the minimum value of the grain size of the martensite grains belonging to the third group is greater than the maximum value of the martensite grains belonging to the fourth group; a value obtained by dividing the total area of ​​the martensite grains belonging to the third group by the total area of ​​the martensite grains is 0.7 or more; a value obtained by dividing the total area of ​​the martensite grains belonging to the third group, excluding the martensite grains having the smallest grain size in the third group, by the total area of ​​the martensite grains is less than 0.7; The average grain size of the martensite grains belonging to the third group is 0.75 μm or less, The average aspect ratio of the martensite grains belonging to the third group is 2.45 or less.

5. The bearing component according to any one of claims 1 to 4, wherein the hardness of the quenching and hardening layer on the surface is 730 Hv or more.

6. The quenching and hardening layer contains nitrogen, The bearing component according to any one of claims 1 to 5, wherein the average nitrogen concentration of the quenching and hardening layer between the surface and a position where the distance from the surface is 10 μm is 0.15 mass percent or more.

7. The quenching and hardening layer contains a plurality of austenite crystal grains, The bearing component according to any one of claims 1 to 6, wherein the volume ratio of the austenite crystal grains in the quenching and hardening layer is 30 percent or less.

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