Rolling member and rolling bearing

Abstract

To provide a rolling member capable of improving dimension stability.SOLUTION: A rolling member has a surface, and is made of steel having been hardened and tempered, and a nitrogen concentration at the surface comprising a nitriding layer formed on the surface, is 0.20 mass% or more. A carbon concentration at the surface is 0.70 mass% or more and 1.5 mass% or less. A hardness at the surface is 820 Hv or more and 1000 Hv or less. An amount of retained austenite on the surface is 20 volume% or less.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a rolling member and a rolling bearing. [Background technology]

[0002] Rolling bearings used in the spindles of machine tools, etc., are required to have high dimensional stability because they are directly related to the machining accuracy of the machine tools. The raceway members and rolling elements that make up the rolling bearings are made of steel that has been quenched and tempered. In other words, the metal structure of the steel that makes up the raceway members and rolling elements contains retained austenite.

[0003] When the temperature of a rolling bearing rises during use, the retained austenite decomposes, and the volume expansion that accompanies the decomposition of the retained austenite causes dimensional changes in the raceway members and rolling elements. The dimensional changes in the raceway members and rolling elements can cause creep, an increase in contact pressure due to a reduction in the gap between the raceway surface and the rolling elements, early damage due to the increase in contact pressure, and the generation of abnormal noise and vibration.

[0004] Furthermore, the deterioration of the dimensional stability of rolling bearings is caused not only by structural changes but also by wear and surface damage (smearing, peeling, etc.).

[0005] Japanese Patent Laid-Open Publication No. 2001-099163 (Patent Document 1) describes a raceway of a rolling bearing, and Japanese Patent Laid-Open Publication No. 2006-322017 (Patent Document 2) describes a rolling element of a rolling bearing.

[0006] Japanese Patent Application Publication No. 2004-232858 (Patent Document 3) describes a raceway of a rolling bearing. Japanese Patent Application Publication No. 2004-339575 (Patent Document 4) describes a raceway of a rolling bearing. The raceway described in Patent Document 4 has its surface hardened by shot peening.

[0007] Japanese Patent Publication No. 2017-088958 (Patent Document 5), Japanese Patent Publication No. 2022-073906 (Patent Document 6), and Japanese Patent Publication No. 2022-108242 (Patent Document 7) describe rolling members. In the rolling members described in Patent Documents 5, 6, and 7, a molybdenum-based, vanadium-based, or chromium-molybdenum-vanadium-based nitride is precipitated on the surface layer. Japanese Patent Publication No. 2016-108616 (Patent Document 8) describes a raceway of a rolling bearing. In the raceway of Patent Document 8, a silicon-manganese-based nitride is precipitated on the surface layer. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2001-099163 A [Patent Document 2] JP 2006-322017 A [Patent Document 3] JP 2004-232858 A [Patent Document 4] JP 2004-339575 A [Patent Document 5] JP 2017-088958 A [Patent Document 6] Patent Publication No. 2022-073906 [Patent Document 7] JP 2022-108242 A [Patent Document 8] JP 2016-108616 A Summary of the Invention [Problem to be solved by the invention]

[0009] The raceway described in Patent Document 1 has a low surface hardness (less than 752 Hv), which raises concerns about reduced dimensional stability due to wear and surface damage.The rolling element described in Patent Document 2 has a large amount of retained austenite (more than 20 volume percent), which raises concerns about reduced dimensional stability due to decomposition of the retained austenite during use.

[0010] The dimensional change rate of the raceway described in Patent Document 3 is large even though the constant temperature holding conditions are low temperature and short time. The raceway described in Patent Document 4 requires a process other than heat treatment to improve the hardness of the surface. The rolling members described in Patent Documents 5, 6 and 7 require the addition of molybdenum or vanadium to the steel constituting the rolling members, which leads to an increase in the cost of steel.

[0011] In rolling bearings for machine tools, ceramic balls made of silicon nitride or the like are sometimes used as rolling elements. In the raceway of Patent Document 8, the surface energy of silicon-manganese nitride is close to that of silicon nitride, so that surface damage is likely to occur when such ceramic balls are used as rolling elements.

[0012] The present invention has been made in view of the above-mentioned problems of the prior art. More specifically, the present invention provides a rolling member capable of improving dimensional stability. [Means for solving the problem]

[0013] The rolling contact member of the present invention has a surface, is made of steel that has been hardened and tempered, and has a nitriding layer formed on the surface. The nitrogen concentration on the surface is 0.20 mass percent or more. The carbon concentration on the surface is 0.70 mass percent or more and 1.5 mass percent or less. The hardness on the surface is 820 Hv or more and 1000 Hv or less. The amount of retained austenite on the surface is 20 volume percent or less. The dislocation density of the retained austenite on the surface and the dislocation density of the martensite on the surface are each 2.0×10 14 m-2 Above 1.3×10 15 m -2 It is more than that. Effect of the Invention

[0014] The rolling elements of the present invention make it possible to improve the dimensional stability. [Brief description of the drawings]

[0015] [Figure 1] 1 is a cross-sectional view of a rolling bearing 100. FIG. [Diagram 2] 3A to 3C are manufacturing process diagrams of the rolling bearing 100. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] An embodiment of the present invention will be described with reference to the drawings. In the following drawings, the same or corresponding parts are given the same reference characters, and overlapping descriptions will not be repeated. The rolling bearing according to the embodiment is referred to as a rolling bearing 100.

[0017] (Configuration of rolling bearing 100) The configuration of the rolling bearing 100 will be described below.

[0018] The rolling bearing 100 is, for example, a rolling bearing for a machine tool. However, the application of the rolling bearing 100 is not limited to this. The rolling bearing 100 may be a rolling bearing for an electric axle. The electric axle is a unit with a three-axis structure composed of, for example, a drive motor, a reducer, an inverter, etc. Rolling bearings for electric axles are required to have dimensional stability as well as foreign matter resistance and high load capacity.

[0019] The rolling bearing 100 may be a rolling bearing for an electric brake. The electric brake is a unit that includes, for example, a motor, a reduction gear, a ball screw, a cylinder, a control device, etc. In addition to dimensional stability, rolling bearings for electric brakes are required to have foreign matter resistance and high load capacity.

[0020] The rolling bearing 100 may be a rolling bearing for an electric compressor. An electric compressor is a unit that cools the interior of a vehicle, as well as a battery and on-board electronic devices that tend to become hot. In addition to dimensional stability, rolling bearings for electric compressors are required to have resistance to foreign matter and a high load capacity.

[0021] This embodiment is also applicable to mechanical parts other than rolling bearings. For example, it is also applicable to transmissions, electric actuators, positioning devices, electric jacks, servo cylinders, mechanical presses, electric servo presses, electric push-button steering, ball screws used in electric injection molding machines, gears, axes and shafts for electric axles, ball screws for electric brakes, etc. These mechanical parts are also required to have dimensional stability as well as foreign matter resistance and high load capacity. The ball screw is a mechanical part composed of a shaft, a nut (outer ring), rolling elements (balls) arranged between the raceway surface of the shaft and the raceway surface of the nut, a tube, a top, an end cap, etc.

[0022] Fig. 1 is a cross-sectional view of a rolling bearing 100. As shown in Fig. 1, the rolling bearing 100 has an inner ring 10, an outer ring 20, a plurality of rolling elements 30, and a cage 40. The central axis of the inner ring 10 is defined as the central axis A. The direction of the central axis A is defined as the axial direction. The direction along the circumference centered on the central axis A when viewed along the axial direction is defined as the circumferential direction. The direction passing through the central axis A and perpendicular to the central axis A is defined as the radial direction.

[0023] The inner ring 10 is annular and extends in the circumferential direction. The inner ring 10 has a width surface 10a, a width surface 10b, an inner diameter surface 10c, and an outer diameter surface 10d. The width surfaces 10a and 10b are end surfaces of the inner ring 10 in the axial direction. The width surface 10a faces one side in the axial direction (the right side in FIG. 1). The width surface 10b is the surface opposite the width surface 10b in the axial direction, and faces the other side in the axial direction (the left side in FIG. 1).

[0024] The inner diameter surface 10c and the outer diameter surface 10d extend in the circumferential direction. The inner diameter surface 10c faces inward in the radial direction. The outer diameter surface 10d faces outward in the radial direction. In other words, the outer diameter surface 10d is the opposite surface to the inner diameter surface 10c in the radial direction. One end and the other end in the axial direction of the inner diameter surface 10c are connected to the width surface 10a and the width surface 10b, respectively. One end and the other end in the axial direction of the outer diameter surface 10d are connected to the width surface 10a and the width surface 10b, respectively.

[0025] The inner ring 10 is fitted to a shaft (not shown) at its inner diameter surface 10c. The outer diameter surface 10d has a raceway surface 10da. The raceway surface 10da contacts the rolling elements 30. The raceway surface 10da extends in the circumferential direction. The outer diameter surface 10d is recessed at the raceway surface 10da towards the inner diameter surface 10c. The raceway surface 10da is located at the center of the outer diameter surface 10d in the axial direction. The raceway surface 10da has, for example, a partial arc shape in a cross section perpendicular to the circumferential direction.

[0026] The outer ring 20 is annular and extends in the circumferential direction. The outer ring 20 has a width surface 20a, a width surface 20b, an inner diameter surface 20c, and an outer diameter surface 20d. The width surfaces 20a and 20b are end surfaces of the outer ring 20 in the axial direction. The width surface 20a faces one side in the axial direction (the right side in FIG. 1). The width surface 20b is the surface opposite the width surface 20b in the axial direction, and faces the other side in the axial direction (the left side in FIG. 1).

[0027] The inner diameter surface 20c and the outer diameter surface 20d extend in the circumferential direction. The inner diameter surface 20c faces inward in the radial direction. The outer diameter surface 20d faces outward in the radial direction. In other words, the outer diameter surface 20d is the opposite surface to the inner diameter surface 20c in the radial direction. One end and the other end in the axial direction of the inner diameter surface 20c are connected to the width surface 20a and the width surface 20b, respectively. One end and the other end in the axial direction of the outer diameter surface 20d are connected to the width surface 20a and the width surface 20b, respectively.

[0028] The outer ring 20 is fitted to a housing (not shown) at its outer diameter surface 20d. The inner diameter surface 20c has a raceway surface 20ca. The raceway surface 20ca contacts the rolling element 30. The raceway surface 20ca extends in the circumferential direction. The inner diameter surface 20c is recessed toward the outer diameter surface 20d at the raceway surface 20ca. The raceway surface 20ca is located at the center of the inner diameter surface 20c in the axial direction. The raceway surface 20ca is, for example, partially arc-shaped in a cross section perpendicular to the circumferential direction. The outer ring 20 is disposed radially outside the inner ring 10 so that the raceway surface 20ca faces the raceway surface 10da with a gap therebetween in the radial direction.

[0029] The rolling elements 30 are spherical. The rolling elements 30 are disposed between the raceway surface 10da and the raceway surface 20ca. The rolling elements 30 have a surface 30a. The surface 30a is in contact with the raceway surface 10da and the raceway surface 20ca. The rolling elements 30 are arranged in the circumferential direction. The cage 40 is disposed between the outer diameter surface 10d and the inner diameter surface 20c. The cage 40 holds the rolling elements 30 such that the interval between two adjacent rolling elements 30 is within a certain range.

[0030] The inner ring 10, the outer ring 20, and the rolling elements 30 are made of hardened and tempered steel. The steel constituting the inner ring 10, the outer ring 20, and the rolling elements 30 preferably has a composition shown in Table 1. Note that SUJ2, a high carbon chromium bearing steel defined in the JIS standard, falls within the composition range shown in Table 1.

[0031] [Table 1]

[0032] A nitriding layer 50 is formed on the surface of the inner ring 10, the surface of the outer ring 20, and the surface (surface 30a) of the rolling elements 30. In the nitriding layer 50, nitrogen is dissolved in the steel. In addition, in the nitriding layer 50, chromium-based nitrides are precipitated in the steel. The portions of the inner ring 10, the outer ring 20, and the rolling elements 30 that are located farther from the surface than the nitriding layer 50 are referred to as the core portion 60. In other words, the portions of the inner ring 10, the outer ring 20, and the rolling elements 30 other than the nitriding layer 50 are the core portion 60. In the core portion 60, nitrogen is not dissolved in the steel, and chromium-based nitrides are not precipitated in the steel.

[0033] In a cross-sectional view perpendicular to the surfaces of the inner ring 10, the outer ring 20, and the rolling element 30, the nitrided layer 50 contains chromium nitrides having a grain size of 1.0 μm or less and a grain size of 100 μm or less. 2 It is preferable that a total of five or more chromium-based nitrides are present per one well, and the area ratio of the chromium-based nitrides in the nitrided layer 50 is 0.95 percent or more and 10 percent or less.

[0034] The area ratio of chromium-based nitrides is calculated by taking a cross-sectional image of the nitrided layer 50 at a magnification of 4000 times using a field emission scanning electron microscope (FE-SEM), binarizing the cross-sectional image, and performing image processing on the binarized cross-sectional image. The cross-sectional images of the nitrided layer 50 are taken from three or more fields of view, and the area ratio is calculated as the average value of the multiple cross-sectional images.

[0035] The particle size of each chromium-based nitride is obtained by obtaining the area of ​​each chromium-based nitride using the same method as above, and then multiplying the square root of the value obtained by dividing the area by π by 2. 2 The number of permeabilities is calculated by counting the number of chromium-based nitrides with a grain size of 1.0 μm or less in a cross-sectional image identified using the same method as above, and using the area of ​​the cross-sectional image.

[0036] The nitrogen concentration on the surface of the inner ring 10, the surface of the outer ring 20, and the surface of the rolling elements 30 is 0.20 mass percent or more. The nitrogen concentration on the surface of the inner ring 10, the surface of the outer ring 20, and the surface of the rolling elements 30 may be 0.30 mass percent or more. The carbon concentration on the surface of the inner ring 10, the surface of the outer ring 20, and the surface of the rolling elements 30 may be 0.70 mass percent or more and 1.5 mass percent or less. The carbon concentration on the surface of the inner ring 10, the surface of the outer ring 20, and the surface of the rolling elements 30 may be 0.70 mass percent or more and 0.95 mass percent or less. The above nitrogen concentration and nitrogen concentration are measured using an EPMA (Electron Probe Micro Analyzer).

[0037] The hardness of the surface of the inner ring 10, the surface of the outer ring 20, and the surface of the rolling elements 30 is 820 Hv or more and 1000 Hv or less. The hardness of the surface of the inner ring 10, the surface of the outer ring 20, and the surface of the rolling elements 30 is measured by the Vickers hardness test method specified in the JIS standard (JIS Z 2244:2009). The load during hardness measurement is 300 g, and the average value of hardness measured at three or more points is used.

[0038] In cases where it is difficult to measure the hardness of the surfaces of the inner ring 10, the outer ring 20, and the rolling elements 30, hardness measurements are made at positions in the cross section that are 50 μm deep from the surfaces of the inner ring 10, the outer ring 20, and the rolling elements 30. The hardness measured in this manner is also 820 Hv or more and 1000 Hv or less.

[0039] The amount of retained austenite on the surface of the inner ring 10, the surface of the outer ring 20, and the surface of the rolling elements 30 is 20 volume percent or less. The amount of retained austenite in the core portion 60 is, for example, 0.1 volume percent or more and 9 volume percent or less. The amount of retained austenite is measured using an X-ray diffraction method. A chrome tube type X-ray diffraction apparatus is used for this purpose. In the chrome tube type X-ray diffraction apparatus, the wavelength of Cr-Kα radiation is 2.29093×10 -10m, the tube voltage is 30 kV, the tube current is 10 mA, and the collimator size is 2 mm × 2 mm. When measuring the amount of retained austenite in the core portion 60, the test piece is preferably electrolytically polished so that the retained austenite does not undergo processing-induced transformation.

[0040] The dislocation density of the retained austenite on the surface of the inner ring 10, the surface of the outer ring 20, and the rolling element 30 is 2.0×10 14 m -2 The dislocation density of martensite on the surface of the inner ring 10, the surface of the outer ring 20, and the rolling element 30 is 1.3×10 15 m -2 The dislocation density of the retained austenite in the inner ring 10, the outer ring 20, and the core 60 of the rolling element 30 is preferably 4.5×10 14 m -2 That's all.

[0041] The dislocation density of the retained austenite and the dislocation density of the martensite are measured using a cobalt tube type X-ray diffractometer. More specifically, first, the X-ray profiles of the austenite and the martensite are measured using a cobalt tube type X-ray diffractometer. In this case, the wavelength of the Co-Kα ray in the cobalt tube type X-ray diffractometer is set to 1.7889×10 -10 m, the tube voltage is 40 kV, the tube current is 50 mA, and the collimator size is 1 mm in diameter. The X-ray profiles of austenite and martensite are measured within a 2θ range of 30° to 135°.

[0042] Secondly, after Rietveld analysis, the half-widths of the peaks in the X-ray profiles of martensite and austenite obtained by X-ray diffraction are separated into crystallite size and strain. Thirdly, the separated crystallite size and strain are applied to the Williamson-Hall equation to obtain the dislocation densities of martensite and austenite. The Williamson-Hall equation defines ρ as the dislocation density (unit: m -2 ), ε is the strain, and b is the length of the Burgers vector (b = 0.25 × 10-9 m), then ρ = 14.4 × ε 2 / b 2 It is expressed as:

[0043] In the X-ray profile of martensite obtained by X-ray diffraction, the peaks of the {110}, {200}, {211}, and {220} planes are measured. In the X-ray profile of austenite, the peaks of the {111}, {200}, {220}, {311}, and {222} planes are measured. The Rietveld analysis is performed in the above in order to reduce the influence of the {200} plane of martensite and the {200} plane of austenite, which have different elastic moduli.

[0044] The dimensional change rate of the inner ring 10, the outer ring 20, and the rolling element 30 after holding at 160°C for 2500 hours is 40×10 -5 The dimensional change rate of the inner ring 10, the outer ring 20, and the rolling element 30 after holding at 160° C. for 2500 hours is 15×10 -5 The rate of dimensional change is calculated by subtracting the dimensions of the inner ring 10 (outer ring 20, rolling elements 30) before holding from the dimensions of the inner ring 10 (outer ring 20, rolling elements 30) after holding, and dividing the result by the dimensions of the inner ring 10 (outer ring 20, rolling elements 30) before holding.

[0045] (Method of manufacturing rolling bearing 100) A method for manufacturing the rolling bearing 100 will now be described.

[0046] Fig. 2 is a manufacturing process diagram of the rolling bearing 100. As shown in Fig. 2, the manufacturing method of the rolling bearing 100 includes a preparation step S1, a nitriding step S2, a quenching step S3, a cooling step S4, a tempering step S5, a post-treatment step S6, and an assembly step S7.

[0047] In the preparation step S1, a workpiece is prepared. The workpiece for forming the inner ring 10 or the outer ring 20 is annular. The workpiece for forming the rolling element 30 is spherical. The workpiece is made of steel having the composition shown in Table 1.

[0048] In the nitriding step S2, a nitriding treatment is performed on the workpiece. The nitriding treatment of the workpiece is performed by heating and holding the workpiece in an atmosphere gas containing a nitrogen source. The heating temperature and the nitrogen concentration in the atmosphere gas in the nitriding step S2 are set so that a compound layer is not formed on the surface of the workpiece. By performing the nitriding step S2, nitrogen penetrates from the surface of the workpiece to the inside. As a result, nitrogen is dissolved in the workpiece in the surface layer of the workpiece, and chromium-based nitrides are precipitated in the surface layer of the workpiece. The nitriding step S2 is performed so that nitrogen reaches a position inside the workpiece that will become the surfaces of the inner ring 10, the outer ring 20, and the rolling element 30 after the post-treatment step S6 is performed. In the nitriding step S2, the carbon concentration in the surface layer of the workpiece may be adjusted to be lower than the carbon concentration in the steel constituting the workpiece by adjusting the components of the atmosphere gas. This adjustment is preferably performed so that the carbon concentration at the position that will become the surface of the workpiece after the post-treatment step S6 is 0.70 mass percent or more and 0.95 mass percent or less. If the carbon concentration is lower than that of the base material, the amount of undissolved carbides decreases. As a result, the chromium in the undissolved carbides diffuses into the base material, making it easier for chromium-based nitrides to precipitate.

[0049] In the quenching step S3, the workpiece is quenched. The quenching of the workpiece is performed by heating the workpiece to a temperature equal to or higher than the A1 transformation point of the steel constituting the workpiece and holding the temperature at that temperature. SThe quenching step S3 is performed by cooling the steel to a temperature below the transformation point. By performing the quenching step S3, martensite and retained austenite are generated in the steel constituting the workpiece. After the quenching step S3 is performed, the workpiece may be heated again to a temperature above the A1 transformation point to repeat the quenching step S3. By performing the quenching step S3 multiple times, the crystal grains become finer, improving the effect of the cooling step S4.

[0050] In the cooling step S4, sub-zero treatment is performed on the member to be processed. In the cooling step S4, cryo-treatment (ultra-sub-zero treatment) may be performed on the member to be processed. In the sub-zero treatment, the member to be processed is cooled to a temperature above -100°C and below room temperature. In the cryo-treatment, the member to be processed is cooled to a temperature below -100°C. By performing the cooling step S4, a part of the retained austenite in the steel constituting the member to be processed is transformed into martensite. Note that a low-temperature tempering step or a cleaning step may be performed before the cooling step S4 to prevent cracking.

[0051] In the tempering step S5, the workpiece is tempered. The workpiece is tempered by heating it to a temperature below the A1 transformation point of the steel that constitutes the workpiece. More specifically, the workpiece is tempered by heating it to a temperature of about 180°C. In the post-treatment step S6, the surface of the workpiece is machined, such as by grinding and polishing. In this manner, the inner ring 10, the outer ring 20, and the rolling elements 30 are manufactured.

[0052] When the workpiece is heated at 180°C or higher in the tempering step S5, the higher the heating temperature, the lower the dislocation density of martensite and the lower the hardness. On the other hand, when the cooling step S4 is performed, the dislocation density of martensite is less likely to be reduced by heating in the tempering step S5, so that although the hardness decreases with increasing heating temperature, a higher hardness than usual can be obtained.

[0053] In an assembly step S7, the inner ring 10, the outer ring 20 and the rolling elements 30 are assembled together with the cage 40. In this manner, the rolling bearing 100 is manufactured.

[0054] In the above, an example has been described in which the nitriding layer 50 is formed on all of the surfaces of the inner ring 10, the outer ring 20, and the rolling elements 30, but it is sufficient that the nitriding layer 50 is formed on at least one of the surfaces of the inner ring 10, the outer ring 20, and the rolling elements 30.

[0055] (Effect of Rolling Bearing 100) The effects of the rolling bearing 100 will be described below.

[0056] As a measure to suppress the dimensional change over time in steel raceways and rolling elements that have been quenched and tempered, it is conceivable to reduce the amount of retained austenite by tempering at high temperatures. However, although tempering at high temperatures makes it possible to suppress the dimensional change over time as the amount of retained austenite decreases, the hardness of the steel on the surfaces of the raceways and rolling elements decreases. If the hardness of the steel on the surface decreases, there is a concern that the resistance to surface damage will decrease.

[0057] The inner ring 10, the outer ring 20, and the rolling elements 30 are subjected to sub-zero treatment or cryo-treatment, thereby reducing the amount of retained austenite in the steel. Since the inner ring 10, the outer ring 20, and the rolling elements 30 are subjected to sub-zero treatment or cryo-treatment, the dislocation density of the martensite in the steel is increased, and as a result, the dislocation density of the retained austenite in the steel is also increased. Therefore, in the steel constituting the inner ring 10, the outer ring 20, and the rolling elements 30, the retained austenite is surrounded by martensite with high dislocation density, and even if the retained austenite in the steel decomposes due to the temperature rise associated with the use of the rolling bearing 100, the volume expansion associated with the decomposition is restrained by the surrounding martensite with high dislocation density, so that dimensional changes are unlikely to occur. In this way, the inner ring 10, the outer ring 20, and the rolling elements 30 are ensured to have dimensional stability against the temperature rise associated with use.

[0058] The inner ring 10, the outer ring 20, and the rolling elements 30 have not been tempered at high temperatures, and the decomposition of martensite on the surfaces of the inner ring 10, the outer ring 20, and the rolling elements 30 progresses only slightly. In addition, the nitrogen concentration on the surfaces of the inner ring 10, the outer ring 20, and the rolling elements 30 is 0.2 mass percent or more, and the hardness of the surfaces of the inner ring 10, the outer ring 20, and the rolling elements 30 is increased by solid solution strengthening. As a result, the hardness of the steel on the surfaces of the inner ring 10, the outer ring 20, and the rolling elements 30 is 820 Hv or more and 1000 Hv or less. Therefore, the inner ring 10, the outer ring 20, and the rolling elements 30 have dimensional stability against surface damage and wear.

[0059] Note that the surface of the rolling element 30 may be given a high residual compressive stress by the pressurizing process, and indentations are less likely to form on it than on the surfaces of the raceways (inner ring 10, outer ring 20). Therefore, even if only the raceways (inner ring 10, outer ring 20) are used as the rolling members according to this embodiment, the surface damage resistance of the rolling bearing 100 is improved.

[0060] (Evaluation of dimensional stability) To evaluate the dimensional stability, Samples A to J were prepared. SUJ2, a high carbon chromium bearing steel specified in the JIS standard, was used for Samples A to J. As shown in Table 2, the surface properties and core properties were changed for Samples A to J.

[0061] [Table 2]

[0062] The nitrogen concentration and precipitation state of chromium-based nitrides on the surface of each sample were adjusted by changing the ratio of gas components in the atmosphere, the heating time and the holding temperature in the nitriding step S2. Also, the dislocation density of the retained austenite on the surface of each sample, the dislocation density of martensite on the surface, the amount of retained austenite on the surface and the dislocation density of the retained austenite in the core 60 were adjusted by changing the cooling temperature and holding time in the sub-zero treatment or cryo-treatment.

[0063] In Sample A and Samples D to G, the nitriding step S2 was performed, followed by the quenching step S3. In addition, in Sample A and Samples D to G, a cryo-treatment was then performed as a cooling step S4, and tempering was performed at 180°C in a tempering step S5. In Sample A and Samples D to G, the surface material was adjusted by adjusting the grinding allowance of the test piece. Therefore, the amount of retained austenite and the dislocation density of the retained austenite in the core portion 60 were approximately the same in Sample A and Samples D to G.

[0064] For Samples B and C, the nitriding step S2 was performed, followed by the quenching step S3. For Samples B and C, a cryo-treatment was then performed as the cooling step S4, followed by tempering at 180° C. in the tempering step S5. For Samples B and C, the amount of undissolved carbide was adjusted by adjusting the concentration of the atmospheric gas in the nitriding step S2.

[0065] In sample H, the quenching step S3 was performed under standard conditions, and then cryo-treatment was performed as the cooling step S4. In addition, in sample H, tempering at 180°C was performed thereafter in the tempering step S5. In sample I, the quenching step S3 was performed under standard conditions, and then tempering at 180°C was performed in the tempering step S5. In sample J, the nitriding step S2 was performed, and then the quenching step S3 was performed. In addition, in sample J, tempering at 180°C was performed in the tempering step S5.

[0066] <Wear resistance test> In the wear resistance test, each sample was shaped like a block with a length of 20 mm, a width of 10 mm, and a height of 5 mm. The surface of each sample, 20 mm long and 10 mm wide, was mirror-polished to an arithmetic mean roughness of 0.005 μm. In the wear resistance test, a rotating cylindrical member was pressed against the surface of each sample, 20 mm long and 10 mm wide, and a load of 50 N was applied. The dimensions of the cylindrical member were an outer diameter of 40 mm, a width of 10 mm, and a minor curvature of 60 mm. The material of the cylindrical member was SUJ2, the hardness of the surface was 760 Hv, and the arithmetic mean roughness of the surface was 0.01 μm. The maximum contact pressure between the cylindrical member and each sample was 0.5 GPa. The cylindrical member and each sample were lubricated with Mobil Velocity oil No. 3 (ISO VG2) using a felt pad.

[0067] The rotation speed of the cylindrical member was 24.5 revolutions per minute, and the cylindrical member was rotated for 60 minutes. After rotating the cylindrical member for 60 minutes, the maximum depth of the wear scar on the surface of the sample was measured with a laser microscope to evaluate the wear resistance of each sample. The results of the wear resistance test are shown in Table 3.

[0068] [Table 3]

[0069] As shown in Table 3, the wear depth was 0.8 μm or less in Samples A to E and Sample J, and the wear resistance was good. In addition, in Samples A to E and Sample J, the chromium nitride particles having a particle size of 1.0 μm or less were 100 μm in thickness. 2 The number of particles per square meter was 5 or more, and the area ratio of chromium-based nitrides was 0.95 percent or more. Samples H and J were subjected to cryo-treatment but not nitriding, so their wear resistance was not improved. No particular relationship was found between the area ratio of undissolved carbides and wear resistance.

[0070] <Evaluation test for dimensional changes over time> In the test to evaluate the dimensional change over time, each sample was in the shape of a ring with an inner diameter of 54 mm, an outer diameter of 60 mm, and a width of 15 mm. In the test to evaluate the dimensional change over time, each sample was kept at a constant temperature of 160°C for 2500 hours, and the dimensional change rate of the outer diameter was measured before and after the temperature holding. The results of the test to evaluate the dimensional change over time are shown in Table 4.

[0071] [Table 4]

[0072] As shown in Table 4, for samples A to H, the dimensional change rate after 2500 hours of constant temperature holding at 160°C was 40×10 -5 In particular, Sample A, Sample B, and Sample D to Sample G had high dislocation densities in martensite and retained austenite, and exhibited lower dimensional change rates after isothermal holding.

[0073] Condition A is that the nitrogen concentration at the surface is 0.2 mass percent or more. Condition B is that the hardness at the surface is 820 Hv or more. Condition C is that the amount of retained austenite at the surface is less than 20 volume percent. Condition D is that the dislocation density of martensite at the surface is 2.0×10 14 m -2 The condition D is that the dislocation density of martensite on the surface is 1.3×10 15 m -2 The above condition is defined as condition E.

[0074] Samples A to E satisfied all of conditions A to E. On the other hand, Samples F to J did not satisfy at least one of conditions A to E. Samples A to E were both good in abrasion resistance and dimensional change rate after constant temperature maintenance. On the other hand, Samples F to J were not good in either abrasion resistance or dimensional change rate after constant temperature maintenance. From this comparison, it was experimentally clarified that by satisfying conditions A to E, both dimensional stability caused by abrasion and surface damage and dimensional stability against temperature rise associated with use are ensured.

[0075] 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. [Explanation of symbols]

[0076] 10 Inner ring, 10a,10b width surface, 10c inner diameter surface, 10d outer diameter surface, 10da raceway surface, 20 outer ring, 20a,20b width surface, 20c inner diameter surface, 20ca raceway surface, 20d outer diameter surface, 30 rolling element, 30a surface, 40 cage, 50 nitrided layer, 60 core, 100 Rolling bearing, A center shaft, S1 preparation process, S2 nitriding process, S3 quenching process, S4 cooling process, S5 tempering process, S6 post-treatment process, S7 assembly process.

Claims

1. A rolling member made of hardened and tempered steel having a surface, A nitriding layer is formed on the surface, The nitrogen concentration at the surface is 0.20 mass percent or more, the carbon concentration on the surface is equal to or greater than 0.70 mass percent and equal to or less than 1.5 mass percent, The hardness of the surface is 820 Hv or more and 1000 Hv or less, The amount of retained austenite on the surface is 20 volume percent or less, The dislocation density of the retained austenite on the surface and the dislocation density of the martensite on the surface are each 2.0×10 １４ m －２ Above and 1.3 x 10 １５ m －２ The rolling members are as described above.

2. Chromium nitrides are precipitated in the nitriding layer, In a cross-sectional view perpendicular to the surface, the chromium-based nitrides having a particle size of 1.0 μm or less are present in the nitriding layer at a concentration of 100 μm or less. ２ 2. The rolling member according to claim 1, wherein a total of five or more chromium-based nitrides are present per one surface, and an area ratio of said chromium-based nitrides in said nitrided layer is 0.95% or more and 10% or less.

3. Further comprising a core portion located farther from the surface than the nitriding layer, The dislocation density of the retained austenite in the core is 4.5×10 １４ m －２ 2. The rolling member according to claim 1, wherein the above-mentioned configuration is adopted.

4. Further comprising a core portion located farther from the surface than the nitriding layer, The dislocation density of the retained austenite in the core is 4.5×10 １４ m －２ 3. The rolling member according to claim 2, wherein the above is true.

5. A track member; a rolling element disposed in contact with the raceway member, A rolling bearing, wherein at least one of the raceway member and the rolling elements is the rolling member according to any one of claims 1 to 4.

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