Bearing parts and rolling bearings

Abstract

To provide a bearing component improved in durability.SOLUTION: A bearing component has a surface and is made of steel. The bearing component comprises a surface layer portion which is a region at a distance up to 20 μm from the surface. The steel contains carbon in an amount from 0.70 mass% to 1.10 mass% inclusive, silicon in an amount from 0.15 mass% to 0.35 mass% inclusive, manganese in an amount from 0.30 mass% to 0.60 mass% inclusive, chromium in an amount from 1.30 mass% to 1.60 mass% inclusive, vanadium in an amount of 0.50 mass% or less, and molybdenum in an amount of 0.50 mass% or less, with the remaining portion being iron and unavoidable impurities. The steel in the surface layer portion has martensite block grains and a precipitate. The precipitate is a nitride whose main constituent is chromium or vanadium, or a carbonitride whose main constituent is chromium or vanadium.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[0002] Patent Document 1 (Japanese Patent No. 5489111) describes a bearing component. The bearing component described in Patent Document 1 is formed by subjecting a steel workpiece to nitriding, quenching, and tempering. The bearing component described in Patent Document 1 has high wear resistance because cementite is dispersed in the steel surface layer.

[0003] Patent Document 2 (Japanese Patent No. 6023422) describes a bearing component. The bearing component described in Patent Document 2 is formed by subjecting a steel workpiece to nitriding, quenching, and tempering. The bearing component described in Patent Document 2 has a large amount of retained austenite in the steel surface layer, and therefore has high durability against indentation-initiated flaking under lubrication conditions containing foreign matter. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5489111 [Patent Document 2] Patent No. 6023422 Summary of the Invention [Problem to be solved by the invention]

[0005] As a result of extensive research, the present inventors have found that there is room for further improvement in durability of the bearing component described in Patent Document 1 and the bearing component described in Patent Document 2. The present invention provides a bearing component and a rolling bearing with improved durability. [Means for solving the problem]

[0006] The bearing component of the present invention has a surface and is made of steel. The bearing component has a surface layer portion, which is a region up to 20 μm away from the surface. The steel contains 0.70 to 1.10 mass percent carbon, 0.15 to 0.35 mass percent silicon, 0.30 to 0.60 mass percent manganese, 1.30 to 1.60 mass percent chromium, 0.50 mass percent or less vanadium, and 0.50 mass percent or less molybdenum, with the remainder being iron and unavoidable impurities. The steel in the surface layer portion has martensite block grains and precipitates. The precipitates are nitrides containing chromium or vanadium as the main component, or carbonitrides containing chromium or vanadium as the main component. In the steel in the surface layer portion, the average grain size of the martensite block grains at a comparative area ratio of 30% is 2.0 μm or less.

[0007] In the above-mentioned bearing component, the steel may contain 0.90 to 1.10 percent by mass of carbon, 0.20 to 0.30 percent by mass of silicon, 0.40 to 0.50 percent by mass of manganese, 1.40 to 1.60 percent by mass of chromium, 0.20 to 0.30 percent by mass of vanadium, and 0.10 to 0.30 percent by mass of molybdenum, with the remainder being iron and unavoidable impurities.

[0008] In the bearing component, the area ratio of precipitates in the steel of the surface layer portion may be 2.0% or more.

[0009] In the bearing component, the maximum particle size of precipitates in the steel surface layer may be 0.5 μm or less.

[0010] In the bearing component, the steel of the surface layer portion may further contain cementite, and the maximum grain size of the cementite in the steel of the surface layer portion may be 1.5 μm or less.

[0011] In the bearing component, the nitrogen concentration in the steel of the surface layer may be 0.15 mass percent or more.

[0012] In the bearing component, the volume ratio of retained austenite in the steel at a position 50 μm away from the surface may be 15% or more.

[0013] In the above-described bearing component, the hardness of the steel may be 58 HRC or more at a position 50 μm away from the surface.

[0014] In the bearing component, the volume ratio of retained austenite in the steel at a position 50 μm away from the surface may be 25% or more and 35% or less, and the hardness of the steel at a position 50 μm away from the surface may be 58 HRC or more and 64 HRC or less.

[0015] A rolling bearing according to the present invention comprises an inner ring, an outer ring, and rolling elements, at least one of which is the above-described bearing component. [Effects of the Invention]

[0016] The bearing component of the present invention and the rolling bearing of the present invention have improved durability. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 2 is a cross-sectional view of the inner ring 10. [Figure 2] FIG. 2 is an enlarged view of II in FIG. [Figure 3] 3A to 3C are process diagrams showing a manufacturing method of the inner ring 10. [Figure 4] FIG. 1 is a cross-sectional view of a rolling bearing 100. [Figure 5] 10 is a graph showing measurement results of nitrogen concentration and carbon concentration in the vicinity of the raceway surface of the bearing washer of Sample 1. [Figure 6] 10 is a graph showing measurement results of nitrogen concentration and carbon concentration in the vicinity of the raceway surface of the bearing washer of Sample 2. [Figure 7] 10 is an SEM image of the surface layer of the race of Sample 1. [Figure 8] This is an SEM image of the surface layer of the race of Sample 2. [Figure 9] EBSD phase map of the surface layer of the race of Sample 1. [Figure 10] EBSD phase map of the surface layer of the race of Sample 2. [Figure 11] EBSD phase map of the surface layer of the race of Sample 3. [Figure 12] 1 is a bar graph showing the average grain size of martensite block grains in the surface layer portions of the bearing races of Samples 1 to 3. [Figure 13] 1 is a graph showing the results of a rolling fatigue life test. DETAILED DESCRIPTION OF THE INVENTION

[0018] The details of the embodiments of the present invention will be described with reference to the drawings. Here, the same or corresponding parts are designated by the same reference numerals, and redundant description will not be repeated.

[0019] The bearing component according to the embodiment is, for example, an inner ring 10 of a rolling bearing. In the following, the inner ring 10 will be described as an example of the bearing component according to the embodiment. However, the bearing component according to the embodiment is not limited to this. The bearing component according to the embodiment may also be an outer ring of a rolling bearing or a rolling element of a rolling bearing.

[0020] (Configuration of inner ring 10) FIG. 1 is a cross-sectional view of inner ring 10. As shown in FIG. 1, inner ring 10 is ring-shaped. The central axis of inner ring 10 is designated as central axis A. Inner ring 10 has width surfaces 10a, 10b, an inner peripheral surface 10c, and an outer peripheral surface 10d. Width surfaces 10a, 10b, inner peripheral surface 10c, and outer peripheral surface 10d form the surface of inner ring 10.

[0021] In the following, the direction of the central axis A will be referred to as the axial direction. Also, in the following, the direction along the circumference centered on the central axis A when viewed along the axial direction will be referred to as the circumferential direction. Furthermore, in the following, the direction perpendicular to the axial direction will be referred to as the radial direction.

[0022] The width surface 10a and the width surface 10b are end surfaces in the axial direction of the inner ring 10. The width surface 10b is the surface opposite to the width surface 10a in the axial direction.

[0023] The inner peripheral surface 10c extends in the circumferential direction. The inner peripheral surface 10c faces the central axis A. One axial end of the inner peripheral surface 10c is continuous with the width surface 10a, and the other axial end is continuous with the width surface 10b. The inner peripheral surface 10c of the inner ring 10 is fitted onto a shaft (not shown).

[0024] The outer peripheral surface 10d extends in the circumferential direction. The outer peripheral surface 10d faces away from the central axis A. That is, the outer peripheral surface 10d is the opposite surface of the inner peripheral surface 10c in the radial direction. One axial end of the outer peripheral surface 10d is continuous with the width surface 10a, and the other axial end is continuous with the width surface 10b.

[0025] The outer peripheral surface 10d has a raceway surface 10da. The raceway surface 10da extends in the circumferential direction. The raceway surface 10da of the outer peripheral surface 10d is recessed toward the inner peripheral surface 10c. In a cross-sectional view, the raceway surface 10da has a partially circular shape. The raceway surface 10da is located at the center of the outer peripheral surface 10d in the axial direction. The raceway surface 10da is a part of the outer peripheral surface 10d that comes into contact with the rolling elements (not shown in FIG. 1).

[0026] The inner ring 10 is made of steel. More specifically, the inner ring 10 is made of steel that has been hardened and tempered. The steel that constitutes the inner ring 10 contains 0.70 to 1.10 percent by mass of carbon, 0.15 to 0.35 percent by mass of silicon, 0.30 to 0.60 percent by mass of manganese, 1.30 to 1.60 percent by mass of chromium, 0.50 to 0.50 percent by mass of vanadium, and 0.50 to 0.50 percent by mass of molybdenum. In this steel, the molybdenum content is 0.01 percent by mass or more, and the vanadium content is 0.01 percent by mass or more.

[0027] The carbon content of the steel constituting the inner ring 10 is 0.70 mass percent or more to improve hardness, and the carbon content of the steel constituting the inner ring 10 is 1.10 mass percent or less to suppress quench cracking.

[0028] The silicon content in the steel of the inner ring 10 is 0.15 mass percent or more in order to increase temper softening resistance and improve workability. The silicon content in the steel of the inner ring 10 is 0.35 mass percent or less because an excessive silicon content actually reduces workability.

[0029] The manganese content in the steel of the inner ring 10 is 0.30 mass percent or more to ensure hardenability, and the manganese content in the steel of the inner ring 10 is 0.60 mass percent or less because an excessive amount of manganese increases manganese-based non-metallic inclusions in the steel.

[0030] The chromium content of the steel constituting the inner ring 10 is 1.30 mass percent or more to ensure hardenability and to form nitrides and carbonitrides, while the chromium content of the steel constituting the inner ring 10 is 1.60 mass percent or less to inhibit the formation of coarse precipitates.

[0031] Vanadium is included in the steel that constitutes the inner ring 10 to refine nitrides and carbonitrides, and the vanadium content in the steel that constitutes the inner ring 10 is 0.50 mass percent or less to suppress the increase in cost that accompanies the addition of vanadium.

[0032] Molybdenum is included in the steel of the inner ring 10 to refine nitrides and carbonitrides and to improve hardenability. The molybdenum content of the steel of the inner ring 10 is 0.50 mass percent or less to suppress the increase in cost associated with the addition of molybdenum.

[0033] The steel constituting the inner ring 10 may contain 0.90 mass percent to 1.10 mass percent carbon, 0.20 mass percent to 0.30 mass percent silicon, 0.40 mass percent to 0.50 mass percent manganese, 1.40 mass percent to 1.60 mass percent chromium, 0.20 mass percent to 0.30 mass percent vanadium, and 0.10 mass percent to 0.30 mass percent molybdenum. The remainder of the steel constituting the inner ring 10 is iron and unavoidable impurities.

[0034] FIG. 2 is an enlarged view of II in FIG. 1. As shown in FIG. 2, the region of inner ring 10 up to a distance of 20 μm from the surface constitutes surface layer portion 11. The surface of inner ring 10 has been subjected to, for example, nitriding treatment. As a result, the nitrogen concentration in the steel of surface layer portion 11 is, for example, 0.15 mass percent or more. The nitrogen concentration in the steel of surface layer portion 11 is preferably 0.20 mass percent or more and 0.30 mass percent or less. The nitrogen concentration in the steel of surface layer portion 11 is measured using an EPMA (Electron Probe Micro Analyzer).

[0035] Precipitates are dispersed in the steel of the surface layer portion 11. The precipitates are nitrides containing chromium or vanadium as the main component, or carbonitrides containing chromium or vanadium as the main component.

[0036] The nitride containing chromium (vanadium) as the main component is a nitride of chromium (vanadium) or a nitride in which some of the chromium (vanadium) sites are substituted with an alloy element other than chromium (vanadium).

[0037] In carbonitrides containing chromium (vanadium) as the main component, some of the carbon sites in chromium (vanadium) carbide are substituted with nitrogen. The chromium (vanadium) sites of carbonitrides containing chromium (vanadium) as the main component may be substituted with an alloying element other than chromium (vanadium).

[0038] The area ratio of precipitates in the steel of the surface layer portion 11 is preferably 2.0% or less. The maximum grain size of precipitates in the steel of the surface layer portion 11 is preferably 0.5 μm or less.

[0039] The area ratio and maximum particle size of precipitates in the steel of the surface layer portion 11 are measured by the following method. First, a cross-sectional image (hereinafter referred to as "SEM image") is obtained using a scanning electron microscope (SEM) of a cross section of the inner ring 10 including the surface layer portion 11. The magnification when obtaining this SEM image is 15,000 times.

[0040] Second, the acquired SEM image is subjected to image processing. More specifically, because the precipitates appear white in the SEM image, the area of ​​each white area in the SEM image and the total area are calculated by image processing.

[0041] The total area of ​​the white parts in the SEM image is considered to be the area ratio of precipitates in the steel of the surface layer portion 11. The square root of the maximum area of ​​each white part in the SEM image divided by π / 4 is considered to be the maximum grain size of precipitates in the steel of the surface layer portion 11.

[0042] Cementite (FeC) may be further dispersed in the steel of the surface layer portion 11. Some of the iron sites in the cementite may be substituted with alloying elements, and some of the carbon sites in the cementite may be substituted with nitrogen. The maximum grain size of cementite in the steel of the surface layer portion 11 is preferably 1.5 μm or less.

[0043] The maximum grain size of cementite in the steel of the surface layer portion 11 is measured by the following method. First, an SEM image is acquired of a cross section of the inner ring 10 including the surface layer portion 11. The magnification for acquiring this SEM image is 15,000 times. Second, the acquired SEM image is subjected to image processing. More specifically, since cementite appears as an oval gray in the SEM image, the area of ​​each oval gray portion in the SEM image is calculated by image processing. Then, the square root of the value obtained by dividing the maximum area of ​​each oval gray portion in the SEM image by π / 4 is regarded as the maximum grain size of cementite in the steel of the surface layer portion 11.

[0044] The volume ratio of retained austenite in the steel at a position 50 μm away from the surface of inner ring 10 is preferably 15 percent or more. The volume ratio of retained austenite in the steel at a position 50 μm away from the surface of inner ring 10 is even more preferably 25 percent or more and 35 percent or less.

[0045] The volume ratio of retained austenite in steel is measured by X-ray diffraction, that is, the volume ratio of retained austenite in steel is calculated by comparing the integrated intensity of the diffraction peak in X-ray diffraction of austenite with the integrated intensity of the diffraction peak in X-ray diffraction of a phase other than austenite.

[0046] The hardness of the steel at a position 50 μm away from the surface of the inner ring 10 is preferably 58 HRC or greater. The hardness of the steel at a position 50 μm away from the surface of the inner ring 10 is more preferably 58 HRC or greater and 64 HRC or less. The hardness of the steel is measured in accordance with the Rockwell hardness testing method defined in the JIS standard (JIS Z 2245:2016).

[0047] The steel of the surface layer 11 has martensite block grains. The difference in crystal orientation between two adjacent martensite block grains at the grain boundary is 15° or more. From another perspective, even if there is a location where the crystal orientation is misaligned, if the difference in crystal orientation is less than 15°, the location is not considered to be a grain boundary of martensite block grains. The grain boundary of the martensite block grains is determined by EBSD (Electron Back Scattered Diffraction) method.

[0048] In the steel of the surface layer portion 11, the average grain size of the martensite block grains is 2.0 μm or less when the comparative area ratio is 30%. In the steel of the surface layer portion 11, the average grain size of the martensite block grains is preferably 1.5 μm or less when the comparative area ratio is 50%.

[0049] The average grain size of the martensite block grains at a comparative area ratio of 30 percent (50 percent) is measured by the following method. First, a cross-section of the inner ring 10 including the surface layer portion 11 is observed. At this time, the martensite block grains included in the observation field are identified by the EBSD method. This observation field is an area of ​​50 μm × 45 μm. Second, the area of ​​each of the martensite block grains included in the observation field is analyzed from the crystal orientation data obtained by the EBSD method.

[0050] Third, the areas of the martensite block grains included in the observation field are added up in descending order of area. This addition is continued until the area reaches 30 percent (50 percent) of the total area of ​​the martensite block grains included in the observation field. The circle-equivalent diameter is calculated for each of the martensite block grains that have been added up. This circle-equivalent diameter is the square root of the area of ​​the martensite block grain divided by π / 4. The average circle-equivalent diameter of the martensite block grains that have been added up is considered to be the average diameter of the martensite block grains when the comparison area ratio is 30 percent (50 percent).

[0051] (Manufacturing method of inner ring 10) FIG. 3 is a process diagram showing a method for manufacturing the inner ring 10. As shown in FIG. 3, the method for manufacturing the inner ring 10 includes a preparation step S1, a nitriding step S2, a quenching step S3, a tempering step S4, and a post-treatment step S5. The nitriding step S2 is performed after the preparation step S1. The quenching step S3 is performed after the nitriding step S2. The tempering step S4 is performed after the quenching step S3. The post-treatment step S5 is performed after the tempering step S4.

[0052] In the preparation step S1, a workpiece to be processed is prepared. The workpiece is a ring-shaped member made of the same steel as the inner ring 10.

[0053] In the nitriding step S2, the surface of the workpiece is subjected to a nitriding treatment by holding the workpiece in an atmosphere containing a nitrogen source (e.g., ammonia) at a temperature equal to or higher than the A1 transformation point of the steel constituting the workpiece.

[0054] In the quenching step S3, the workpiece is quenched. The quenching is performed by holding the workpiece at a temperature equal to or higher than the A1 transformation point of the steel constituting the workpiece, and then quenching the M of the steel constituting the workpiece. SThis is done by quenching to a temperature below the transformation point. The heating temperature in the quenching step S3 is preferably equal to or lower than the heating temperature in the nitriding step S2. The quenching step S3 may be performed twice. The heating temperature in the second quenching step S3 is preferably lower than the heating temperature in the first quenching step S3. This allows fine and abundant precipitates to be dispersed in the surface layer of the workpiece.

[0055] In the tempering step S4, the workpiece is tempered. Tempering is performed by holding the workpiece at a temperature below the A1 transformation point of the steel that constitutes the workpiece. In the post-treatment step S5, the surface of the workpiece is machined (grinded, polished), cleaned, and the like. As a result, the inner ring 10 having the structure shown in Figures 1 and 2 is formed.

[0056] In addition, since the precipitates are dispersed finely and in large amounts in the steel of the surface layer portion 11, the martensite block grains are less likely to grow large, and therefore the average grain size of the martensite block grains in the steel of the surface layer portion 11 at a comparative area ratio of 30% is 2.0 μm or less.

[0057] (The effect of inner circle 10) In the inner ring 10, the martensite block grains are refined in the steel of the surface layer 11 so that the average grain size at a comparative area ratio of 30 percent is 2.0 μm or less. As a result, the surface layer 11 of the inner ring 10 has high toughness, improving the shear resistance of the surface of the inner ring 10 that comes into contact with the rolling elements (specifically, the raceway surface 10da). In this way, the inner ring 10 has improved durability.

[0058] When the area ratio of precipitates in the steel of the surface layer 11 is 2.0 percent or more, i.e., when precipitates are dispersed at a high density in the steel of the surface layer 11, the shear resistance of the surface of the inner ring 10 that comes into contact with the rolling element (specifically, the raceway surface 10da) is improved, thereby further improving durability.

[0059] When the maximum grain size of precipitates in the steel of surface layer portion 11 is 0.5 μm, the precipitates are densely and finely dispersed in the steel of surface layer portion 11, improving the wear resistance and toughness and further improving the durability of inner ring 10. When the maximum grain size of cementite in the steel of surface layer portion 11 is 1.5 μm or less, the wear resistance and toughness of inner ring 10 are further improved due to the fine dispersion of cementite.

[0060] When the volume fraction of retained austenite in the steel at a position 50 μm from the surface of inner ring 10 is 15 percent or more (25 percent to 35 percent), durability against indentation-initiated spalling in an environment containing foreign matter is improved. When the hardness of the steel at a position 50 μm from the surface of inner ring 10 is 58 HRC or more (58 HRC to 64 HRC), the wear resistance of inner ring 10 is further improved.

[0061] (Rolling bearing according to an embodiment) A rolling bearing according to an embodiment (referred to as "rolling bearing 100") will be described below.

[0062] Fig. 4 is a cross-sectional view of rolling bearing 100. As shown in Fig. 4, rolling bearing 100 is a deep groove ball bearing. However, rolling bearing 100 is not limited to this. Rolling bearing 100 may be, for example, a thrust ball bearing. Rolling bearing 100 has an inner ring 10, an outer ring 20, rolling elements 30, and a cage 40.

[0063] The outer ring 20 has a width surface 20a, a width surface 20b, an inner peripheral surface 20c, and an outer peripheral surface 20d. The surface of the outer ring 20 is composed of the width surface 20a, the width surface 20b, the inner peripheral surface 20c, and the outer peripheral surface 20d.

[0064] The width surface 20a and the width surface 20b are end surfaces in the axial direction of the outer ring 20. The width surface 20b is the surface opposite to the width surface 20a in the axial direction.

[0065] The inner peripheral surface 20c extends in the circumferential direction. The inner peripheral surface 20c faces the central axis A. One axial end of the inner peripheral surface 20c is continuous with the width surface 20a, and the other axial end is continuous with the width surface 20b. The outer ring 20 is disposed so that the inner peripheral surface 20c faces the outer peripheral surface 10d.

[0066] The inner peripheral surface 20c has a raceway surface 20ca. The raceway surface 20ca extends in the circumferential direction. The raceway surface 20ca of the inner peripheral surface 20c is recessed toward the outer peripheral surface 20d. In a cross-sectional view, the raceway surface 20ca has a partially circular shape. The raceway surface 20ca is located at the center of the inner peripheral surface 20c in the axial direction. The raceway surface 20ca is a part of the inner peripheral surface 20c that comes into contact with the rolling elements 30.

[0067] The outer peripheral surface 20d extends in the circumferential direction. The outer peripheral surface 20d faces away from the central axis A. In other words, the outer peripheral surface 20d is the opposite surface to the inner peripheral surface 20c in the radial direction. One end of the outer peripheral surface 20d in the axial direction is continuous with the width surface 20a, and the other end of the outer peripheral surface 20d in the axial direction is continuous with the width surface 20b. The outer ring 20 is fitted into a housing (not shown) at the outer peripheral surface 20d.

[0068] The rolling elements 30 are spherical. The rolling elements 30 are disposed between the outer peripheral surface 10d (raceway surface 10da) and the inner peripheral surface 20c (raceway surface 20ca). The cage 40 is ring-shaped and disposed between the outer peripheral surface 10d and the inner peripheral surface 20c. The cage 40 holds the rolling elements 30 so that the distance between two adjacent rolling elements 30 in the circumferential direction is within a certain range.

[0069] The outer ring 20 and the rolling elements 30 may be formed from the same steel as the inner ring 10. Furthermore, the surface layer portion of the outer ring 20 (region up to 20 μm away from the surface of the outer ring 20) and the surface layer portion of the rolling elements 30 (region up to 20 μm away from the surface of the rolling elements 30) may have the same configuration as the surface layer portion 11.

[0070] (Rolling fatigue life test) In order to confirm the effects of the bearing component according to the embodiment, a rolling fatigue life test was carried out. Samples 1, 2, and 3 were used in the rolling fatigue life test. Samples 1 to 3 are thrust ball bearings of model number 51106 specified in the JIS standard.

[0071] In Sample 1, the races (inner and outer rings) were made of the first steel material. In Samples 2 and 3, the races were made of the second steel material. The compositions of the first and second steel materials are shown in Table 1. As shown in Table 1, the components of the first and second steel materials are almost identical except for the molybdenum and vanadium contents. The second steel material corresponds to SUJ2, a high-carbon chromium bearing steel specified in the JIS standard.

[0072] [Table 1]

[0073] Fig. 5 is a graph showing the measurement results of the nitrogen concentration and carbon concentration near the raceway surface of the bearing washer of Sample 1. Fig. 6 is a graph showing the measurement results of the nitrogen concentration and carbon concentration near the raceway surface of the bearing washer of Sample 2. The horizontal axis of Fig. 5 and Fig. 6 represents the distance from the raceway surface (unit: mm), and the vertical axis of Fig. 5 and Fig. 6 represents the carbon or nitrogen concentration (unit: mass percent). As shown in Fig. 5 and Fig. 6, the bearing washer surfaces of Samples 1 and 2 were subjected to nitriding treatment. The heating temperature during this nitriding treatment was 850°C. On the other hand, the bearing washer surface of Sample 3 was not subjected to nitriding treatment.

[0074] Table 2 shows the nitrogen concentrations in the surface layer (region up to 20 μm away from the raceway surface) of the races of Samples 1 to 3. As shown in Table 2, the nitrogen concentration in the steel of the surface layer of the races of Samples 1 and 2 was 0.3 percent or more and 0.5 percent or less. The nitrogen concentration in the steel of the surface layer of the race of Sample 3 was 0.0 percent.

[0075] [Table 2]

[0076] The bearing washer of Samples 1 to 3 was subjected to quenching and tempering. The heating temperature during quenching was 850°C. The heating temperature during tempering was 180°C. The heating time during tempering was 2 hours.

[0077] Figure 7 is an SEM image of the surface layer of the bearing washer of Sample 1. Figure 8 is an SEM image of the surface layer of the bearing washer of Sample 2. In the SEM images of Figures 7 and 8, the white areas are precipitates, and the oval gray areas are cementite.

[0078] As shown in Table 3, the area ratio of precipitates in the steel of the surface layer of the washer of Sample 1 was 2.7 percent. As shown in Table 3, the area ratio of precipitates in the steel of the surface layer of Sample 2 was 1.6 percent. In other words, precipitates were dispersed at a higher density in the surface layer of the washer of Sample 1 compared to the surface layer of the washer of Sample 2. This comparison revealed that by adding 0.5 mass percent or less of vanadium and molybdenum, precipitates were dispersed at a higher density in the steel of the surface layer of the washer.

[0079] The maximum particle size of precipitates in the surface layer of the washer of Sample 1 was 0.5 μm. The maximum particle size of precipitates in the surface layer of the washer of Sample 2 was 1.1 μm. In other words, the precipitates were more finely dispersed in the surface layer of the washer of Sample 1 compared to the surface layer of the washer of Sample 2. This comparison revealed that by adding 0.5 mass percent or less of vanadium and molybdenum, precipitates were densely and finely dispersed in the steel in the surface layer of the washer.

[0080] [Table 3]

[0081] As shown in Table 4, the maximum grain size of cementite was 1.5 μm or less in the surface layer portions of the races of Sample 1 and Sample 2. The maximum grain size of cementite in the surface layer portion of the race of Sample 3 exceeded 1.5 μm.

[0082] [Table 4]

[0083] As shown in Table 5, in Samples 1 and 2, the volume ratio of retained austenite in the steel at a position 50 μm away from the raceway surface was 15 percent or more. In Sample 3, the volume ratio of retained austenite in the steel at a position 50 μm away from the raceway surface was less than 15 percent. In Samples 1 to 3, the hardness of the steel at a position 50 μm away from the raceway surface was 58 HRC or more.

[0084] [Table 5]

[0085] Fig. 9 is an EBSD phase map of the surface layer portion of the bearing washer of Sample 1. Fig. 10 is an EBSD phase map of the surface layer portion of the bearing washer of Sample 2. Fig. 11 is an EBSD phase map of the surface layer portion of the bearing washer of Sample 3. In Figs. 9 to 11, martensite block grains are shown in white. Fig. 12 is a bar graph showing the average grain size of the martensite block grains in the surface layer portions of the bearing washer of Samples 1 to 3. The vertical axis of the graph in Fig. 12 is the average grain size (unit: μm) of the martensite block grains.

[0086] 9 to 12, in the surface layer portion of the bearing washer of Sample 1, the average grain size of the martensite block grains was 2.0 μm or less when the comparative area ratio was 30 percent. On the other hand, in the surface layer portions of the bearing washer of Sample 2 and Sample 3, the average grain size of the martensite block grains was greater than 2.0 μm when the comparative area ratio was 30 percent.

[0087] In the surface layer portion of the race of Sample 1, the average grain size of the martensite block grains at a comparative area ratio of 50 percent was 1.5 μm or less. On the other hand, in the surface layer portions of the races of Sample 2 and Sample 3, the average grain size of the martensite block grains at a comparative area ratio of 50 percent exceeded 1.5 μm.

[0088] Figure 13 is a graph showing the results of the rolling fatigue life test. The horizontal axis of the graph in Figure 13 represents life (unit: hours), and the vertical axis of the graph in Figure 13 represents the cumulative failure probability (unit: percentage). The rolling fatigue life test was conducted under the conditions shown in Table 6. That is, the maximum contact pressure between the rolling element and the washer was set to 2.3 GPa, the washer was rapidly accelerated and decelerated between 0 rpm and 2500 rpm, and the lubricant used was polyglycol oil mixed with pure water.

[0089] [Table 6]

[0090] As shown in FIG. 13 and Table 7, Sample 1 exhibited a better rolling contact fatigue life than Sample 2. More specifically, the L of Sample 1 10 The lifespan (lifespan at which the cumulative failure probability reaches 10%) is L for sample 3. 10 The L of sample 2 is 2.7 times the lifespan. 10 The lifespan is L for sample 3 10 This was 2.1 times the lifespan.

[0091] As described above, in the surface layer portion of the bearing washer of Sample 1, the average grain size of martensite grains was 2.0 μm or less when the comparative area ratio was 30 percent. On the other hand, in the surface layer portions of the bearing washer of Sample 2 and Sample 3, the average grain size of martensite grains was greater than 2.0 μm when the comparative area ratio was 30 percent. This comparison makes it clear that the bearing component according to the embodiment has improved durability.

[0092] [Table 7]

[0093] Furthermore, as described above, in the surface layer portion of the washer of Sample 1, the precipitates were finer and more densely dispersed than in the surface layer portion of the washer of Sample 2. This comparison reveals that the durability of the bearing component according to the embodiment can be further improved by setting the area ratio and maximum grain size of the precipitates in the surface layer portion to 2.0 percent or more and 0.5 μm or less, respectively.

[0094] Also, L of sample 2 10 The lifespan is L for sample 3. 10 As mentioned above, the maximum grain size of cementite in the surface layer of the race of Sample 2 was 1.5 μm or less, while the maximum grain size of cementite in the surface layer of the race of Sample 3 exceeded 1.5 μm. Also, in the race of Sample 2, the volume ratio of retained austenite at a position 50 μm away from the raceway surface was 15 percent or more, while in the race of Sample 3, the volume ratio of retained austenite at a position 50 μm away from the raceway surface was less than 15 percent.

[0095] This comparison revealed that the durability of bearing components can be improved by setting the maximum grain size of cementite in the surface layer of the bearing component to 1.5 μm or less and by setting the volume ratio of retained austenite to 15 percent or more at a position 50 μm away from the surface of the bearing component.

[0096] Although the embodiments of the present invention have been described above, the above embodiments can be modified in various ways. Furthermore, the scope of the present invention is not limited to the above embodiments. 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]

[0097] This embodiment is particularly advantageously applied to a bearing component and a rolling bearing having the same. [Explanation of symbols]

[0098] 10 inner ring, 10a, 10b width, 10c inner circumference, 10d outer circumference, 10da raceway, 11 surface, 20 outer ring, 20a, 20b width, 20c inner circumference, 20ca raceway, 20d outer circumference, 30 rolling elements, 40 cage, 100 rolling bearing, A center shaft, S1 Preparation process, S2 nitriding process, S3 quenching process, S4 tempering process, S5 post-treatment process.

Claims

1. A steel bearing component having a surface, a surface layer portion that is a region up to 20 μm away from the surface, The steel contains 0.70 to 1.10 percent by weight of carbon, 0.15 to 0.35 percent by weight of silicon, 0.30 to 0.60 percent by weight of manganese, 1.30 to 1.60 percent by weight of chromium, 0.01 to 0.50 percent by weight of vanadium, and 0.01 to 0.50 percent by weight of molybdenum, with the balance being iron and inevitable impurities; The steel of the surface layer portion has martensite block grains and precipitates, At the grain boundary between two adjacent martensite block grains, the crystal orientation is shifted by 15° or more, the precipitate is a nitride containing chromium or vanadium as a main component, or a carbonitride containing chromium or vanadium as a main component, In the steel of the surface layer portion, the average grain size of the martensite block grains at a comparative area ratio of 30% is 2.0 μm or less, an area ratio of the precipitates in the steel of the surface layer portion is 2.0% or more; the maximum particle size of the precipitates in the steel of the surface layer portion is 0.5 μm or less, The steel of the surface layer portion further has cementite, In the steel of the surface layer portion, the maximum grain size of the cementite is 1.5 μm or less, The nitrogen concentration in the steel in the surface layer portion is 0.15 mass percent or more, The volume ratio of retained austenite in the steel at a position 50 μm away from the surface is 15% or more, A bearing component, wherein the hardness of the steel is 58 HRC or more at a position 50 μm away from the surface.

2. 2. A bearing component according to claim 1, wherein the steel contains 0.90 to 1.10 percent by mass of carbon, 0.20 to 0.30 percent by mass of silicon, 0.40 to 0.50 percent by mass of manganese, 1.40 to 1.60 percent by mass of chromium, 0.20 to 0.30 percent by mass of vanadium, 0.10 to 0.30 percent by mass of molybdenum, and the remainder being iron and unavoidable impurities.

3. At a position 50 μm away from the surface, the volume ratio of retained austenite in the steel is 25% or more and 35% or less, 3. The bearing component according to claim 1, wherein the hardness of the steel at a position 50 μm away from the surface is 58 HRC or more and 64 HRC or less.

4. With inner circle, The outer ring and a rolling element; A rolling bearing, wherein at least one of the inner ring, the outer ring, and the rolling elements is the bearing component according to any one of claims 1 to 3.

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