JP7731202B2 - Bearing parts and rolling bearings - Google Patents

Description

The present invention relates to bearing components and rolling bearings.

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

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, making it highly resistant to indentation-initiated flaking when lubricated with foreign matter.

Patent No. 5489111 Patent No. 6023422

After extensive research, the inventors have found that the bearing components described in Patent Document 1 and Patent Document 2 have room for further improvement in durability. The present invention provides bearing components and rolling bearings with improved durability.

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 to 0.50 mass percent vanadium, and 0.50 to 0.50 mass percent molybdenum, with the balance being iron and unavoidable impurities. The steel in the surface layer portion contains martensite block grains and precipitates. The precipitates are nitrides primarily composed of chromium or vanadium, or carbonitrides primarily composed of chromium or vanadium. In the steel in the surface layer portion, the average grain size of the martensite block grains at a comparative area ratio of 30 percent is 2.0 μm or less.

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

In the above-mentioned bearing component, the area ratio of precipitates in the steel surface layer may be 2.0 percent or more.

In the above-mentioned bearing component, the maximum particle size of precipitates in the steel surface layer may be 0.5 μm or less.

In the above-mentioned bearing component, the steel in the surface layer may further contain cementite. The maximum grain size of the cementite in the steel in the surface layer may be 1.5 μm or less.

In the above-mentioned bearing component, the nitrogen concentration in the steel of the surface layer may be 0.15 mass percent or more.

In the above-mentioned bearing component, the volume ratio of retained austenite in the steel at a position 50 μm away from the surface may be 15 percent or more.

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

In the above-mentioned bearing component, the volume ratio of retained austenite in the steel at a position 50 μm away from the surface may be 25 percent or more and 35 percent or less. At a position 50 μm away from the surface, the hardness of the steel may be 58 HRC or more and 64 HRC or less.

The rolling bearing of the present invention comprises an inner ring, an outer ring, and rolling elements. At least one of the inner ring, outer ring, and rolling elements is a bearing component as described above.

The bearing components and rolling bearings of the present invention provide improved durability.

FIG. 2 is a cross-sectional view of the inner ring 10. FIG. 2 is an enlarged view of II in FIG. 3A to 3C are process diagrams showing a manufacturing method of the inner ring 10. FIG. 1 is a cross-sectional view of a rolling bearing 100. 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. 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. 10 is an SEM image of a surface layer portion of a bearing washer of Sample 1. 10 is an SEM image of a surface layer portion of a bearing washer of Sample 2. 10 is a phase map of EBSD in a surface layer portion of the bearing washer of Sample 1. 10 is a phase map of EBSD in the surface layer portion of the bearing washer of Sample 2. 10 is a phase map of EBSD in the surface layer portion of the bearing washer of Sample 3. 1 is a bar graph showing the average grain size of martensite block grains in the surface layer portions of the bearing washer of Samples 1 to 3. 1 is a graph showing the results of a rolling fatigue life test.

Details of embodiments of the present invention will be described with reference to the drawings. Here, the same or corresponding parts will be designated by the same reference symbols, and overlapping descriptions will not be repeated.

The bearing component according to the embodiment is, for example, the 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 the outer ring of a rolling bearing or the rolling element of a rolling bearing.

(Configuration of inner ring 10)
Fig. 1 is a cross-sectional view of an inner ring 10. As shown in Fig. 1, the inner ring 10 is ring-shaped. The central axis of the inner ring 10 is designated as central axis A. The inner ring 10 has a width surface 10a, a width surface 10b, an inner peripheral surface 10c, and an outer peripheral surface 10d. The width surface 10a, the width surface 10b, the inner peripheral surface 10c, and the outer peripheral surface 10d constitute the surface of the inner ring 10.

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 of a circle 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.

Width surface 10a and width surface 10b are the end surfaces of the inner ring 10 in the axial direction. Width surface 10b is the surface opposite width surface 10a in the axial direction.

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 connected to the width surface 10a, and the other axial end is connected to the width surface 10b. The inner ring 10 is fitted onto a shaft (not shown) at the inner peripheral surface 10c.

The outer peripheral surface 10d extends in the circumferential direction. The outer peripheral surface 10d faces away from the central axis A. In other words, 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.

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 the part of the outer peripheral surface 10d that contacts the rolling elements (not shown in FIG. 1).

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.

The carbon content of the steel making up the inner ring 10 is 0.70 mass percent or more to improve hardness. The carbon content of the steel making up the inner ring 10 is 1.10 mass percent or less to suppress quench cracking.

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

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

The chromium content of the steel making up the inner ring 10 is 1.30 mass percent or more to ensure hardenability and to allow the formation of nitrides and carbonitrides. The chromium content of the steel making up the inner ring 10 is 1.60 mass percent or less to prevent the formation of coarse precipitates.

Vanadium is included in the steel that makes up the inner ring 10 to refine the nitrides and carbonitrides. The vanadium content of the steel that makes up the inner ring 10 is limited to 0.50 mass percent or less to prevent the increase in cost that would accompany the addition of vanadium.

Molybdenum is included in the steel that makes up the inner ring 10 to refine nitrides and carbonitrides and to improve hardenability. The molybdenum content of the steel that makes up the inner ring 10 is limited to 0.50 mass percent or less to prevent the increase in cost that would accompany the addition of molybdenum.

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

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

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

A nitride whose main component is chromium (vanadium) is a nitride of chromium (vanadium) or a nitride in which some of the chromium (vanadium) sites have been replaced by an alloying element other than chromium (vanadium).

A carbonitride whose main component is chromium (vanadium) is a chromium (vanadium) carbide in which some of the carbon sites have been replaced with nitrogen. The chromium (vanadium) sites of a carbonitride whose main component is chromium (vanadium) may be replaced with an alloying element other than chromium (vanadium).

The area ratio of precipitates in the steel of the surface layer 11 is preferably 2.0 percent or less. The maximum particle size of precipitates in the steel of the surface layer 11 is preferably 0.5 μm or less.

The area ratio and maximum particle size of precipitates in the steel of the surface layer 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) at a cross section of the inner ring 10 including the surface layer 11. The magnification used to obtain this SEM image is 15,000x.

Second, the acquired SEM image is subjected to image processing. More specifically, since precipitates appear white in the SEM image, the area of each white area in the SEM image and the total area are calculated through image processing.

The total area of the white areas in the SEM image is considered to be the area ratio of precipitates in the steel of the surface layer 11. The square root of the maximum area of each white area in the SEM image divided by π/4 is considered to be the maximum particle size of precipitates in the steel of the surface layer 11.

Cementite ( _Fe3C ) 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.

The maximum grain size of cementite in the steel of the surface layer 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 11. The magnification for acquiring this SEM image is 15,000x. Second, the acquired SEM image is subjected to image processing. More specifically, since cementite appears as oval gray areas in the SEM image, the area of each oval gray area in the SEM image is calculated by image processing. The square root of the maximum area of each oval gray area in the SEM image divided by π/4 is then considered to be the maximum grain size of cementite in the steel of the surface layer 11.

At a position 50 μm away from the surface of the inner ring 10, the volume ratio of retained austenite in the steel is preferably 15 percent or more. It is even more preferable that at a position 50 μm away from the surface of the inner ring 10, the volume ratio of retained austenite in the steel is 25 percent or more and 35 percent or less.

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 peaks in X-ray diffraction of austenite with the integrated intensity of the diffraction peaks in X-ray diffraction of phases other than austenite.

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. It is even more preferable that the hardness of the steel at a position 50 μm away from the surface of the inner ring 10 be 58 HRC or greater and 64 HRC or less. The hardness of the steel is measured in accordance with the Rockwell hardness test method defined in the JIS standard (JIS Z 2245:2016).

The steel in 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°, that location is not considered to be a martensite block grain boundary. The grain boundaries of martensite block grains are determined using EBSD (Electron Back Scattered Diffraction) analysis.

In the steel of the surface layer 11, the average grain size of the martensite block grains at a comparative area ratio of 30 percent is 2.0 μm or less. In the steel of the surface layer 11, the average grain size of the martensite block grains at a comparative area ratio of 50 percent is preferably 1.5 μm or less.

The average grain size of 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 11, is observed. At this time, the martensite block grains included in the observation field are identified using the EBSD method. This observation field is an area of 50 μm x 45 μm. Second, the area of each martensite block grain included in the observation field is analyzed from the crystal orientation data obtained by the EBSD method.

Third, the areas of the martensite block grains included in the observation field are added together 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 equivalent circle diameter is calculated for each of the martensite block grains that have been added together. This equivalent circle diameter is the square root of the area of the martensite block grain divided by π/4. The average value of the equivalent circle diameters of the martensite block grains that have been added together is considered to be the average diameter of the martensite block grains when the comparison area ratio is 30 percent (50 percent).

(Method for manufacturing the 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.

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

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.

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 rapidly cooling it to a temperature equal to or lower than the _M2S transformation point of the steel constituting the workpiece. 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.

In the tempering step S4, the workpiece is tempered by holding the workpiece at a temperature below the A1 transformation point of the steel that makes up the workpiece. In the post-treatment step S5, the surface of the workpiece is machined (grinded, polished), cleaned, etc. By performing the above steps, the inner ring ₁₀ having the structure shown in Figures 1 and 2 is formed.

Finely dispersing a large amount of precipitates in the steel of the surface layer 11 makes it difficult for the martensite block grains to grow large, so the average grain size of the martensite block grains in the steel of the surface layer 11 at a comparative area ratio of 30 percent is 2.0 μm or less.

(Effect of inner ring 10)
In the inner ring 10, the martensite block grains are refined so that the average grain size at a comparative area ratio of 30 percent is 2.0 μm or less in the steel of the surface layer 11. As a result, the toughness of the surface layer 11 of the inner ring 10 is increased, 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.

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 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 elements (specifically, the raceway surface 10da) is improved, thereby further improving durability.

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

When the volume ratio of retained austenite in the steel at a position 50 μm from the surface of the inner ring 10 is 15 percent or more (25 percent or more and 35 percent or less), 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 the inner ring 10 is 58 HRC or more (58 HRC or more and 64 HRC or less), the wear resistance of the inner ring 10 is further improved.

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

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

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.

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

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 connected to the width surface 20a, and the other axial end is connected to the width surface 20b. The outer ring 20 is positioned so that the inner peripheral surface 20c faces the outer peripheral surface 10d.

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 the part of the inner peripheral surface 20c that contacts the rolling elements 30.

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 of the inner peripheral surface 20c in the radial direction. One axial end of the outer peripheral surface 20d is continuous with the width surface 20a, and the other axial end is continuous with the width surface 20b. The outer ring 20 is fitted into a housing (not shown) at the outer peripheral surface 20d.

The rolling elements 30 are spherical. The rolling elements 30 are arranged 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 arranged 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.

The outer ring 20 and rolling elements 30 may be made of the same steel as the inner ring 10. In addition, the surface layer of the outer ring 20 (the region up to 20 μm from the surface of the outer ring 20) and the surface layer of the rolling elements 30 (the region up to 20 μm from the surface of the rolling elements 30) may have the same configuration as the surface layer 11.

(Rolling fatigue life test)
In order to confirm the effects of the bearing component according to the embodiment, a rolling fatigue life test was conducted. 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.

In Sample 1, the races (inner and outer rings) were made from the first steel material. In Samples 2 and 3, the races were made from 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.

Figure 5 is a graph showing the measurement results of nitrogen and carbon concentrations near the raceway surface of the bearing washer for Sample 1. Figure 6 is a graph showing the measurement results of nitrogen and carbon concentrations near the raceway surface of the bearing washer for Sample 2. The horizontal axis of Figures 5 and 6 represents the distance from the raceway surface (unit: mm), and the vertical axis of Figures 5 and 6 represents the carbon or nitrogen concentration (unit: mass percent). As shown in Figures 5 and 6, nitriding was performed on the bearing washer surface for Samples 1 and 2. The heating temperature during this nitriding was 850°C. On the other hand, no nitriding was performed on the bearing washer surface for Sample 3.

Table 2 shows the nitrogen concentration in the surface layer (region up to 20 μm from the raceway surface) of the races of Samples 1 to 3. As shown in Table 2, the nitrogen concentration in the steel in 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 in the surface layer of the race of Sample 3 was 0.0 percent.

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.

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

As shown in Table 3, the area ratio of precipitates in the steel 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 surface layer of Sample 2 was 1.6 percent. In other words, precipitates were more densely dispersed in the steel surface layer of the washer of Sample 1 than in the steel surface layer of the washer of Sample 2. This comparison reveals that by adding 0.5 mass percent or less of vanadium and molybdenum, precipitates are more densely dispersed in the steel surface layer of the washer.

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

As shown in Table 4, the maximum cementite grain size in the surface layer of the races of Samples 1 and 2 was 1.5 μm or less. In the surface layer of the race of Sample 3, the maximum cementite grain size exceeded 1.5 μm.

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.

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

As shown in Figures 9 to 12, in the surface layer portion of the washer of Sample 1, the average grain size of the martensite block grains at a comparative area ratio of 30 percent was 2.0 μm or less. On the other hand, in the surface layer portion of the washer of Sample 2 and Sample 3, the average grain size of the martensite block grains at a comparative area ratio of 30 percent exceeded 2.0 μm.

In the surface layer of the washer 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 of the washer 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.

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 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.

As shown in Figure 13 and Table 7, Sample 1 exhibited a better rolling contact fatigue life than Sample 2. More specifically, the _L10 life (life at which the cumulative failure probability becomes 10 percent) of Sample 1 was 2.7 times that of Sample ₃ , and the _L10 life of Sample 2 was 2.1 times that of Sample ₃ .

As described above, in the surface layer portion of the 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 washer of Samples 2 and 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.

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 particle size of precipitates in the surface layer portion to 2.0 percent or more and 0.5 μm or less, respectively.

Furthermore, the _L10 life of Sample 2 was longer than the _L10 life of Sample 3. As described above, the maximum grain size of cementite in the surface layer portion of the bearing washer of Sample 2 was 1.5 μm or less, while the maximum grain size of cementite in the surface layer portion of the bearing washer of Sample 3 exceeded 1.5 μm. Furthermore, in the bearing washer 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 bearing washer of Sample 3, the volume ratio of retained austenite at a position 50 μm away from the raceway surface was less than 15 percent.

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

Although an embodiment of the present invention has been described above, the above embodiment can be modified in various ways. Furthermore, the scope of the present invention is not limited to the above embodiment. The scope of the present invention is defined by the claims, and is intended to include all modifications that are equivalent in meaning to and within the scope of the claims.

This embodiment is particularly advantageously applicable to bearing components and rolling bearings that include the same.

10 inner ring, 10a, 10b width surface, 10c inner peripheral surface, 10d outer peripheral surface, 10da raceway surface, 11 surface layer, 20 outer ring, 20a, 20b width surface, 20c inner peripheral surface, 20ca raceway surface, 20d outer peripheral surface, 30 rolling element, 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 (4)

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 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, 0.01 to 0.50 percent by mass of vanadium, 0.01 to 0.50 percent by mass of molybdenum, and 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 . The 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, and 0.10 to 0.30 percent by mass of molybdenum, with the remainder being iron and unavoidable impurities. 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. 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 .