Rolling bearing race

Abstract

To provide a bearing ring of a rolling bearing, in which rolling fatigue characteristic of a raceway surface can be improved and furthermore creep resistance of an anti-raceway surface can be improved.SOLUTION: A bearing ring of a rolling bearing is made of hardened steel, and comprises an inner circumferential surface and an outer circumferential surface. One of the inner circumferential surface and the outer circumferential surface provides a raceway surface. The other of the inner circumferential surface and the outer circumferential surface provides an anti-raceway surface. A retained austenite amount in the steel of the anti-raceway surface is smaller than a retained austenite amount in the steel of the raceway surface. Difference between the retained austenite amount in the steel of the raceway surface and the retained austenite amount in the steel of the anti-raceway surface is 3 vol. % or more. A minimum value of the compressive residual stress of the raceway surface is 100 MPa or more.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a raceway of a rolling bearing. [Background technology]

[0002] For example, Patent Document 1 (JP 2017-187104 A) describes an inner ring of a rolling bearing. The inner ring in Patent Document 1 is made of hardened steel. The inner ring described in Patent Document 1 has an inner layer portion inside the inner ring and a surface layer portion surrounding the entire periphery of the inner layer. The surface layer portion is present not only on the outer peripheral surface side including the raceway surface, but also on the inner peripheral surface side (opposite raceway surface side). The amount of retained austenite in the steel in the surface layer portion is greater than the amount of retained austenite in the steel in the inner layer portion. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2017-187104 A Summary of the Invention [Problem to be solved by the invention]

[0004] As described above, in the inner ring described in Patent Document 1, the surface layer is present not only on the outer peripheral surface side but also on the inner peripheral surface side, so that the inner diameter expands over time, loosening the fit with the shaft and risking creep. Also, in the inner ring described in Patent Document 1, the minimum value of compressive residual stress on the raceway surface is low, leaving room for improvement in the rolling fatigue characteristics of the raceway surface.

[0005] The present invention has been made in view of the above-mentioned problems of the prior art. More specifically, the present invention provides a raceway of a rolling bearing capable of improving the rolling fatigue characteristics of the raceway surface and improving the creep resistance of the anti-raceway surface. [Means for solving the problem]

[0006] The raceway of the rolling bearing of the present invention is made of hardened steel and has a surface having an inner peripheral surface and an outer peripheral surface. One of the inner peripheral surface and the outer peripheral surface includes a raceway surface. The other of the inner peripheral surface and the outer peripheral surface is an anti-raceway surface. The amount of retained austenite in the steel at the anti-raceway surface is less than the amount of retained austenite in the steel at the raceway surface. The difference between the amount of retained austenite in the steel at the raceway surface and the amount of retained austenite in the steel at the anti-raceway surface is 3 volume percent or more. The minimum value of compressive residual stress in the raceway surface is 100 MPa or more.

[0007] In the raceway of the above-mentioned rolling bearing, an average amount of retained austenite in the steel may be 10 volume percent or less.

[0008] In the raceway of the rolling bearing, a nitrided layer may be formed on the surface. The average amount of retained austenite in the steel may be 20 percent or less.

[0009] In the raceway of the above-mentioned rolling bearing, the hardness of the steel on the raceway surface and the hardness of the steel on the anti-raceway surface may be 700 Hv or more.

[0010] In the raceway of the above-mentioned rolling bearing, the amount of retained austenite in the steel on the anti-raceway surface may be 5 volume percent or less.

[0011] In the raceway of the above-mentioned rolling bearing, the steel may be high carbon chromium bearing steel SUJ2 defined by the JIS standard. Effect of the Invention

[0012] According to the race of the rolling bearing of the present invention, it is possible to improve the rolling fatigue characteristics of the raceway surface and at the same time improve the creep resistance of the anti-raceway surface. [Brief description of the drawings]

[0013] [Figure 1] FIG. [Diagram 2]FIG. 2 is a cross-sectional view taken along line II-II in FIG. [Diagram 3] FIG. 4 is a cross-sectional view of an inner ring 10 according to a modified example. [Figure 4] 3A to 3C are process diagrams showing a manufacturing method of the inner ring 10. [Diagram 5] FIG. 11 is a schematic plan view for explaining the tempering step S3. [Figure 6] FIG. 11 is a schematic cross-sectional view for explaining the tempering step S3. [Figure 7] 11 is a graph showing a simulation result regarding the relationship between the heating time by the heating coil 30 and the temperatures at the inner circumferential surface 20c and the outer circumferential surface 20d. [Figure 8] 13 is a graph showing a simulation result of the heating temperature of the outer circumferential surface 20d when the heating temperature of the inner circumferential surface 20c is changed. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] 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.

[0015] (Configuration of raceways of rolling bearing according to embodiment) The configuration of the raceway of the rolling bearing according to the embodiment will be described below.

[0016] The raceway of the rolling bearing according to the embodiment is, for example, the inner ring of a deep groove ball bearing (hereinafter, referred to as "inner ring 10"). However, the raceway of the rolling bearing according to the embodiment is not limited to this. The raceway of the rolling bearing according to the embodiment may be the outer ring of a deep groove ball bearing, or may be a raceway of a rolling bearing other than a deep groove ball bearing.

[0017] The inner ring 10 is made of hardened steel. That is, the steel contains martensite grains and retained austenite grains. The steel may contain grains other than martensite grains and retained austenite grains (e.g., ferrite grains and carbide grains). The steel is, for example, SUJ2, a high carbon chromium bearing steel defined in the JIS standard (JIS G 4805:2008).

[0018] Fig. 1 is a plan view of the inner ring 10. Fig. 2 is a cross-sectional view taken along line II-II in Fig. 1. As shown in Fig. 1 and Fig. 2, the inner ring 10 has an annular shape. The inner ring 10 has a central axis A.

[0019] The inner ring 10 has a first end face 10a and a second end face 10b, an inner circumferential surface 10c, and an outer circumferential surface 10d. The first end face 10a, the second end face 10b, the inner circumferential surface 10c, and the outer circumferential surface 10d are sometimes collectively referred to as the surface of the inner ring 10.

[0020] The first end face 10a and the second end face 10b constitute end faces in a direction (hereinafter referred to as the "axial direction") along the central axis A. The second end face 10b is the opposite face to the first end face 10a in the axial direction.

[0021] The inner peripheral surface 10c extends in a direction along a circumference centered on the central axis A (hereinafter referred to as the "circumferential direction"). The inner peripheral surface 10c faces the central axis A. The inner peripheral surface 10c is continuous with the first end face 10a and the second end face 10b. The inner ring 10 is fitted onto a shaft (not shown) at the inner peripheral surface 10c.

[0022] The outer peripheral surface 10d extends in the circumferential direction. The outer peripheral surface 10d faces the opposite side to the central axis A. In other words, the outer peripheral surface 10d is the opposite surface of the inner peripheral surface 10c in a direction perpendicular to the central axis A and passing through the central axis A (hereinafter referred to as the "radial direction").

[0023] The outer peripheral surface 10d has a raceway surface 10da. The raceway surface 10da of the outer peripheral surface 10d is recessed toward the inner peripheral surface 10c. The raceway surface 10da has an arc shape in a cross-sectional view passing through the central axis A. The raceway surface 10da is a surface that contacts a rolling element (not shown). The counter raceway surface is a surface that is opposite the raceway surface 10da in the radial direction. In the inner ring 10, the inner peripheral surface 10c is the counter raceway surface.

[0024] The amount of retained austenite on inner peripheral surface 10c, which is the opposite raceway surface, is less than the amount of retained austenite on raceway surface 10da. The average amount of retained austenite in the steel constituting inner ring 10 is preferably 10 volume percent or less. "The average amount of retained austenite in the steel constituting inner ring 10" is a value obtained by integrating, along the circumferential direction, the distribution curve of the amount of retained austenite obtained by measuring multiple points equally spaced along the radial direction of inner ring 10 between raceway surface 10da and inner peripheral surface 10c, and dividing the result by the cross-sectional area of ​​the raceway (inner ring 10) parallel to the circumferential direction.

[0025] The difference between the amount of retained austenite on the inner circumferential surface 10c and the amount of retained austenite on the raceway surface 10da is preferably 3 volume percent or more.The amount of retained austenite on the inner circumferential surface 10c is preferably 5 volume percent or less.

[0026] The amount of retained austenite in the steel constituting the inner ring 10 is measured by X-ray diffraction. More specifically, the amount of retained austenite is obtained by comparing the intensities of the diffraction peaks of each phase obtained by irradiating with X-rays.

[0027] The minimum value of the compressive residual stress at the raceway surface 10da is 100 MPa or more. In an area that is 0.2 mm or less away from the raceway surface 10da, the compressive residual stress is preferably 100 MPa or less. The residual stress at the raceway surface 10da is measured by an X-ray diffraction method. More specifically, the residual stress at the raceway surface 10da is measured based on the change in the diffraction peak angle when the raceway surface 10da is irradiated with X-rays.

[0028] The hardness of the raceway surface 10da is higher than that of the inner peripheral surface 10c. The hardness of the raceway surface 10da and the inner peripheral surface 10c is preferably 700 Hv or more. The hardness of the raceway surface 10da and the inner peripheral surface 10c is measured according to the Vickers hardness test method defined in the JIS standard (JIS Z 2244:2009).

[0029] <Modification> Fig. 3 is a cross-sectional view of an inner ring 10 according to a modified example. As shown in Fig. 3, a nitriding layer 10e may be formed on the surface of the inner ring 10. The nitrogen concentration in the steel located in the nitriding layer 10e is higher than the nitrogen concentration in the steel located outside the nitriding layer 10e. The nitrogen concentration in the steel is measured by an EPMA (Electron Probe Micro Analyzer).

[0030] When the nitrided layer 10e is formed on the surface of the inner ring 10, the average amount of retained austenite in the steel constituting the inner ring 10 is preferably 20 volume percent or less.

[0031] (Method of manufacturing races of rolling bearing according to embodiment) A method for manufacturing the inner ring 10 will now be described.

[0032] Fig. 4 is a process diagram showing a manufacturing method of the inner ring 10. As shown in Fig. 4, the manufacturing method of the inner ring 10 includes a preparation step S1, a quenching step S2, a tempering step S3, and a post-treatment step S4. In the preparation step S1, an annular processed member 20 that will become the inner ring 10 by passing through the quenching step S2, the tempering step S3, and the post-treatment step S4 is prepared.

[0033] In addition, when the nitrided layer 10e is formed on the surface of the inner ring 10, a nitriding treatment is performed on the surface of the workpiece 20 prior to the quenching step S2. The nitriding treatment is performed, for example, in an atmospheric gas containing nitrogen (e.g., ammonia (NH 3 The heating is performed by holding the workpiece 20 in a heating gas at a predetermined temperature for a predetermined period of time.

[0034] In the quenching step S2, quenching is performed on the workpiece 20. The quenching step S2 includes a heating step S21 and a cooling step S22. In the heating step S21, the workpiece 20 is quenched. 1 The temperature is heated to a temperature above the A point and held for a specified period of time. 1 The point is a temperature at which the ferrite in the steel starts to transform into austenite. By carrying out the heating step S21, austenite grains are generated in the steel constituting the workpiece 20.

[0035] The cooling step S22 is performed after the heating step S21. In the cooling step S22, the workpiece 20 is cooled to a temperature equal to or lower than the Ms point. The Ms point is the temperature at which transformation from austenite to martensite starts. Therefore, by the cooling step S22, some of the austenite grains in the steel constituting the workpiece 20 become martensite grains.

[0036] In the cooling step S22, the temperature is lowered below the Ms point, and cooled to a temperature equal to or near the Mf point. The Mf point is the temperature at which the transformation from austenite to martensite is completed. That is, in the cooling step S22, a so-called sub-zero treatment (deep cooling treatment) is performed. As a result, the amount of retained austenite in the steel constituting the workpiece 20 is considerably reduced.

[0037] The tempering step S3 is carried out after the quenching step S2. In the tempering step S3, the workpiece 20 is tempered.

[0038] Fig. 5 is a schematic plan view for explaining the tempering step S3. Fig. 6 is a schematic cross-sectional view for explaining the tempering step S3. As shown in Fig. 5 and Fig. 6, the heating in the tempering step S3 is performed by, for example, induction heating.

[0039] More specifically, the heating coil 30 is rotated in the circumferential direction along the inner peripheral surface 20c of the workpiece 20 to inductively heat the inner peripheral surface 20c. While the inner peripheral surface 20c is being heated by the heating coil 30, the outer peripheral surface 20d of the workpiece 20 is cooled by a cooling liquid such as water sprayed from the spray unit 31.

[0040] Fig. 7 is a graph showing the results of a simulation of the relationship between the heating time by the heating coil 30 and the temperatures at the inner circumferential surface 20c and the outer circumferential surface 20d. In Fig. 7, the horizontal axis represents the heating time by the heating coil 30 (unit: seconds), and the vertical axis represents the temperatures at the inner circumferential surface 20c and the outer circumferential surface 20d (unit: °C). The simulation of Fig. 7 was performed under the conditions that the heating temperature of the inner circumferential surface 20c is 420 °C, the outer circumferential surface 20d is water-cooled, and the distance between the inner circumferential surface 20c and the outer circumferential surface 20d is 3 mm. As shown in Fig. 7, in the tempering process S3, the heating temperature of the outer circumferential surface 20d is lower than the heating temperature of the inner circumferential surface 20c.

[0041] FIG. 8 is a graph showing the results of a simulation of the heating temperature of the outer peripheral surface 20d when the heating temperature of the inner peripheral surface 20c is changed. In FIG. 8, the horizontal axis indicates the heating temperature of the inner peripheral surface 20c (unit: °C), and the vertical axis indicates the heating temperature of the outer peripheral surface 20d (unit: °C). The simulation of FIG. 8 was performed under the same conditions as the simulation of FIG. 7, except that the heating temperature of the inner peripheral surface 20c was changed. As shown in FIG. 8, the heating temperature of the outer peripheral surface 20d is a linear expression of the heating temperature of the inner peripheral surface 20c. If the heating temperature of the inner peripheral surface 20c is x and the heating temperature of the outer peripheral surface 20d is y, then y=a×x+b (a is a positive number less than 1, and b is a positive number) (hereinafter, this formula will be referred to as "Formula 1").

[0042] For example, as described in Japanese Patent Application Laid-Open No. 10-102137, the volume ratio of the retained austenite in the steel constituting the workpiece 20 after the tempering step S3 (M 1 ) is the volume ratio (M 0 ), heating temperature (T) and heating time (t) are used to calculate M 1 =M 0 ×{A×exp(-Q / RT)×t n} (A, Q, and n are constants, and R is a gas constant) (hereinafter, this formula will be referred to as "Formula 2").

[0043] Therefore, by appropriately adjusting the heating temperature and heating time of the inner surface 20c by the heating coil 30, the heating temperature of the outer surface 20d can be appropriately adjusted, and accordingly, the volume ratio of retained austenite on the inner surface 20c and the volume ratio of retained austenite on the outer surface 20d can be appropriately adjusted.

[0044] For example, as described in a reference (Inoue Takeshi, "New Tempering Parameters and Their Application to a Tempering Integration Method Along a Continuous Heating Curve", Iron and Steel, 66, 10 (1980), 1533), the hardness (Hv) of the steel constituting the workpiece 20 after the tempering step S3 is calculated as Hv=c×logt+d / T+e (c, d, and e are constants) using the heating time (t) and heating temperature (T). Therefore, the hardness of the inner circumferential surface 20c can be appropriately adjusted by appropriately adjusting the heating temperature and heating time of the inner circumferential surface 20c by the heating coil 30.

[0045] In the post-treatment step S4, post-treatment is performed on the workpiece 20. This post-treatment includes grinding the workpiece 20, cleaning the workpiece 20, etc. The manufacturing process for the inner ring 10 is thus completed.

[0046] (Effects of the rolling bearing according to the embodiment) The effects of the inner ring 10 will be described below.

[0047] In the inner ring 10, the amount of retained austenite on the anti-raceway surface (inner circumferential surface 10c) is less than the amount of retained austenite on the raceway surface 10da, so the dimensional change of the inner circumferential surface 10c caused by the transformation of the retained austenite into martensite over time is small. As a result, the fit of the inner ring 10 with the shaft is less likely to loosen, and the creep resistance of the anti-raceway surface can be improved.

[0048] In the inner ring 10, the amount of retained austenite on the inner circumferential surface 10c is less than the amount of retained austenite on the raceway surface 10da (from another perspective, the amount of retained austenite reduced on the inner circumferential surface 10c is greater than the amount of retained austenite reduced on the raceway surface 10da), and therefore the shrinkage on the inner circumferential surface 10c side after completion of the tempering process S3 is greater than that on the raceway surface 10da side.

[0049] Due to this difference in the amount of shrinkage, compressive residual stress acts on the raceway surface 10da. In the inner ring 10, the difference between the amount of retained austenite on the inner circumferential surface 10c and the amount of retained austenite on the raceway surface 10da is 3 volume percent or more, so a large compressive residual stress (specifically, a minimum value of 100 MPa or more) acts on the raceway surface 10da. Therefore, the inner ring 10 can improve the rolling fatigue characteristics of the raceway surface 10da.

[0050] When the average amount of retained austenite in the steel constituting the inner ring 10 is 10 volume percent or less (20 volume percent or less when the nitriding layer 10e is formed), the transformation of the retained austenite into martensite over time is less likely to occur, and creep resistance can be further improved.

[0051] By performing sub-zero treatment in the cooling step S22, the average amount of retained austenite in the steel constituting the inner ring 10 can be set to 10 volume percent or less (20 volume percent or less when the nitriding layer 10e is formed). If the average amount of retained austenite in the steel is reduced before the tempering step S3, the time required for the tempering step S3 can be shortened and the heating temperature of the inner circumferential surface 10c in the tempering step S3 can be reduced. As a result, hardness can be maintained not only in the raceway surface 10da but also in the inner circumferential surface 10c.

[0052] <Experimental Example> Samples 1 to 4 were prepared as samples for the experiment. Samples 1 to 4 are annular members formed of SUJ2 as defined by the JIS standard. For Samples 1 and 2, no nitriding treatment was performed on the surfaces. For Samples 3 and 4, nitriding treatment was performed on the surfaces. For Samples 1 and 3, sub-zero treatment was not performed in the cooling step S22, whereas for Samples 2 and 4, sub-zero treatment was performed in the cooling step S22. For Samples 1 to 4, the tempering step S3 was performed.

[0053] As shown in Table 1, the amount of retained austenite on the raceway surface of Sample 1 was 11 volume percent. On the other hand, the amount of retained austenite on the raceway surface of Sample 2 was 7 volume percent. Also, the amount of retained austenite on the raceway surface of Sample 3 was 31 volume percent, while the amount of retained austenite on the raceway surface of Sample 4 was 16 volume percent.

[0054] Since the amount of retained austenite in the steel constituting the processed component 20 decreases from the raceway surface to the anti-raceway surface by the tempering process S3, by performing sub-zero treatment in the cooling process S22, the average amount of retained austenite in the steel constituting the inner ring 10 can be made 10 volume percent or less (20 volume percent or less if a nitriding layer 10e is formed).

[0055] [Table 1]

[0056] A tempering process S3 was carried out on Sample 2. At this time, the heating temperature on the inner peripheral surface side of Sample 2 was set to 300° C., and the outer peripheral surface side of Sample 2 was water-cooled. The heating time was set based on Equations 1 and 2 so that the difference between the amount of retained austenite on the outer peripheral surface and the amount of retained austenite on the inner peripheral surface was 3 volume percent.

[0057] As shown in Table 2, in Sample 2 before the tempering process S3, residual tensile stress acted on the outer peripheral surface, but in Sample 2 after the tempering process S3, compressive residual stress of 100 MPa or more was generated at a position up to 0.2 mm away from the outer peripheral surface. This experimentally demonstrated that a compressive residual stress of 100 MPa or more is generated on raceway surface 10da when the difference between the amount of retained austenite on inner peripheral surface 10c and the amount of retained austenite on raceway surface 10da is 3 volume percent or more.

[0058] [Table 2]

[0059] 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]

[0060] The above-described embodiment is particularly advantageously applied to the races of rolling bearings. [Explanation of symbols]

[0061] 10 Inner ring, 10a 1st end surface, 10b 2nd end surface, 10c Inner surface, 10d Outer surface, 10da raceway surface, 10e Nitriding layer, 20 Part to be processed, 20c Inner surface, 20d Outer surface, 30 Heating coil, 31 Injection part, A center shaft, S1 Preparation process, S2 Quenching process, S3 Tempering process, S4 Post-processing process, S21 heating process, S22 cooling process.

Claims

1. A raceway of a rolling bearing, the raceway is made of hardened steel and has a surface having an inner circumferential surface and an outer circumferential surface; one of the inner circumferential surface and the outer circumferential surface includes a raceway surface, the other of the inner circumferential surface and the outer circumferential surface is a counter-orbital surface, an amount of retained austenite in the steel at the anti-raceway surface is less than an amount of retained austenite in the steel at the raceway surface; a difference between an amount of retained austenite in the steel at the raceway surface and an amount of retained austenite in the steel at the anti-raceway surface is 3 volume percent or more; the minimum value of the compressive residual stress on the raceway surface is 100 MPa or more; the hardness of the steel at the raceway surface and the hardness of the steel at the anti-raceway surface are 700 Hv or more; A nitriding layer is formed on the surface, A raceway of a rolling bearing, wherein the average amount of retained austenite in the steel is 20 volume percent or less.

2. 2. The raceway of a rolling bearing according to claim 1, wherein the amount of retained austenite in the steel at the anti-raceway surface is 5 volume percent or less.

3. 3. The raceway of a rolling bearing according to claim 1, wherein the steel is high carbon chromium bearing steel SUJ2 defined in the JIS standard.

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