Rolling elements and rolling bearings

Abstract

The rolling element is made of quenched steel with a contact surface. The rolling element has a surface layer in the area from the contact surface to a depth of 20 μm. The steel contains 0.70 mass% to 1.10 mass% carbon, 0.15 mass% to 0.35 mass% silicon, 0.30 mass% to 0.60 mass% manganese, 1.30 mass% to 1.60 mass% chromium, 0.01 mass% to 0.50 mass% molybdenum, and 0.01 mass% to 0.50 mass% vanadium, with the remainder consisting of iron and unavoidable impurities. The nitrogen content in the surface layer is 0.2 mass% to 0.8 mass%.

Description

Technical Field

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

[0002] Conventionally, rolling elements, such as raceways and rolling elements, of rolling bearings are made of high-carbon chromium bearing steel (SUJ2, SUJ3, etc.) specified in the JIS standard (JIS G 4805:2008). Furthermore, to improve the durability of the rolling element surface, nitriding treatment has been performed on the surface layer.

[0003] In recent years, the operating environment of rolling bearings has become increasingly harsh. Therefore, simply forming rolling elements from ordinary steel materials such as SUJ2 and SUJ3 and nitriding their surfaces may not provide sufficient durability.

[0004] In the rolling bearings described in Patent Document 1 (Japanese Patent No. 3873741) and Patent Document 2 (Japanese Patent No. 5372316), by forming the rolling elements with steel materials containing a large amount of silicon (Si) and manganese (Mn), it is expected that the wear resistance and seizure resistance will be improved in an environment accompanied by sliding contact and when the lubricating oil dries up.

[0005] In the rolling bearing described in Patent Document 3 (Japanese Patent Application Laid-Open No. 2000-282178), a large amount of chromium (Cr) is added to a steel material used for a rolling element to form a passive film on the surface, thereby suppressing hydrogen intrusion into the steel.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent No. 3873741

[0009] Patent Document 2: Japanese Patent No. 5372316

[0010] Patent Document 3: Japanese Patent Application Laid-Open No. 2000-282178 Summary of the Invention

[0011] Technical problem to be solved by the invention

[0012] However, considering the recent demand for improved durability of rolling element members, the rolling bearings described in Patent Documents 1 to 3 may have insufficient durability of the rolling element members depending on the intended use.

[0013] For example, in the rolling bearing described in Patent Document 3, carbides in the steel may become coarse due to the addition of chromium to the steel material. The coarse carbides may become a stress concentration source and may become the starting point of premature separation.

[0014] Furthermore, the passive film inhibits hydrogen diffusion into the steel, but it also promotes hydrogen absorption. If the rolling bearing described in Patent Document 3 is used intermittently, hydrogen dissipates when stopped. Therefore, the passive film effectively prevents premature delamination by slowing hydrogen intrusion into the steel. However, if the rolling bearing described in Patent Document 3 is used continuously, a large amount of hydrogen is absorbed by the passive film, increasing the amount of hydrogen intrusion into the steel and making premature delamination more likely to occur.

[0015] In the future, it is expected that rolling bearings that operate continuously without human intervention will increase. However, the durability of existing rolling bearings, including the rolling bearing described in Patent Document 3, is insufficient for such applications.

[0016] The present invention has been made based on the above-mentioned problems of the prior art. More specifically, the present invention provides a rolling member and a rolling bearing that can achieve a longer life.

[0017] Technical solutions used to solve technical problems

[0018] A rolling element according to one embodiment of the present invention is made of quenched steel having a contact surface. The rolling element has a surface portion in the area from the contact surface to a depth of 20 μm. The steel contains 0.70% to 1.10% carbon, 0.15% to 0.35% silicon, 0.30% to 0.60% manganese, 1.30% to 1.60% chromium, 0.01% to 0.50% molybdenum, and 0.01% to 0.50% vanadium, with the remainder being iron and unavoidable impurities. The nitrogen content in the surface portion is 0.2% to 0.8% by mass.

[0019] In the rolling element described above, the steel may contain not less than 0.90 mass% and not more than 1.10 mass% carbon, not less than 0.20 mass% and not more than 0.30 mass% silicon, not less than 0.40 mass% and not more than 0.50 mass% manganese, not less than 1.40 mass% and not more than 1.60 mass% chromium, not less than 0.20 mass% and not more than 0.30 mass% molybdenum, and not less than 0.20 mass% and not more than 0.30 mass% vanadium, with the remainder being iron and unavoidable impurities. The nitrogen content in the surface layer may be not less than 0.3 mass% and not more than 0.5 mass%.

[0020] In the rolling element, the surface layer may have a thickness of 100 μm in a cross-section perpendicular to the contact surface.2 The steel may contain a total of 60 or more precipitates with a particle size of 0.50 μm or less, and the total area ratio of the precipitates in the surface layer may be 1% or more and 10% or less. The volume fraction of retained austenite at a depth of 50 μm from the contact surface may be 20% or more and 40% or less. The hardness at a depth of 50 μm from the contact surface may be 653 Hv or more and 800 Hv or less.

[0021] In the rolling element, the surface layer may have a thickness of 100 μm in a cross-section perpendicular to the contact surface. 2 The steel may contain a total of 80 or more precipitates with a particle size of 0.50 μm or less, and the total area ratio of the precipitates in the surface layer may be 2% or more and 7% or less. The volume fraction of retained austenite at a depth of 50 μm from the contact surface may be 25% or more and 35% or less. The hardness at a depth of 50 μm from the contact surface may be 653 Hv or more and 800 Hv or less.

[0022] A rolling bearing according to one aspect of the present invention includes a raceway member and a rolling element arranged in contact with the raceway member. At least one of the raceway member and the rolling element is the above-mentioned rolling element.

[0023] Effects of the Invention

[0024] According to one aspect of the present invention, the rolling member and the rolling bearing can achieve a longer life.

[0025] BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is a cross-sectional view of the rolling bearing 100 .

[0027] Figure 2 1 and 2 are process diagrams illustrating a method for manufacturing the rolling bearing 100 .

[0028] Figure 3 Graph showing the distribution of carbon and nitrogen contents in the surface layer portions of the inner ring and outer ring of Sample 1.

[0029] Figure 4 This is a graph showing the distribution of the carbon and nitrogen contents in the surface layer portions of the inner and outer rings of Sample 2.

[0030] Figure 5 These are representative cross-sectional FE-SEM images of the surface layer portions of the inner and outer rings of Sample 1.

[0031] Figure 6 These are representative cross-sectional FE-SEM images of the surface layer portions of the inner and outer rings of Sample 2.

[0032] Figure 7 It is a graph showing the results of a rolling fatigue life test.

[0033] Figure 8 is a cross-sectional view of the inner ring 210 .

[0034] Figure 9 yes Figure 8 Magnified view of position IX in FIG.

[0035] Figure 10 1 and 2 are process diagrams illustrating a method for manufacturing the inner ring 210 .

[0036] Figure 11 It is a cross-sectional view of the rolling bearing 200 .

[0037] Figure 12 Graphs showing the results of measuring the nitrogen concentration and carbon concentration near the raceway surface of the washer of Sample 4.

[0038] Figure 13 Graphs showing the results of measuring the nitrogen concentration and carbon concentration near the raceway surface of the washer of Sample 5.

[0039] Figure 14 This is an SEM image of the surface layer portion of the gasket of Sample 4.

[0040] Figure 15 This is an SEM image of the surface layer portion of the gasket of Sample 5.

[0041] Figure 16 This is an EBSD phase image of the surface layer of the gasket of Sample 4.

[0042] Figure 17 This is an EBSD phase diagram of the surface layer of the gasket of Sample 5.

[0043] Figure 18 This is an EBSD phase diagram of the surface layer of the gasket of Sample 6.

[0044] Figure 19 It is a bar graph showing the average grain size of the martensite block grains in the surface layer portion of the washers of Samples 4 to 6.

[0045] Figure 20 It is a graph showing the results of a rolling fatigue life test. DETAILED DESCRIPTION

[0046] The embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the following drawings, the same or corresponding parts are marked with the same reference numerals and the description thereof will not be repeated.

[0047] (Configuration of Rolling Bearing of First Embodiment)

[0048] The following describes the structure of a rolling bearing according to a first embodiment (hereinafter referred to as "rolling bearing 100"). Rolling bearing 100 is, for example, a one-way thrust ball bearing with a flat raceway. However, rolling bearing 100 is not limited to this.

[0049] The rolling bearing 100 is used, for example, in a vehicle (fuel cell vehicle, electric vehicle, etc.) for a transaxle, a vehicle's transmission (continuously variable transmission, etc.), or a vehicle's motor (for a drive unit, for a transmission). The rolling bearing 100 can be used in a hydrogen pressure reducing valve or a hydrogen circulator for a fuel cell vehicle. The rolling bearing 100 can also be used in electrical components of the vehicle or auxiliary machinery of the vehicle (electromagnetic clutches, fan coupling devices, intermediate pulleys, electric fan motors, compressors, etc. for alternators, automotive air conditioners).

[0050] Rolling bearing 100 may be a wheel hub bearing. Rolling bearing 100 may be used in machine tools (for spindles, etc.), wind turbines (for speed increasers, etc.), railway vehicles (for axles, drive units, main motors, etc.), construction machinery (for axles, etc.), papermaking machines, or transmissions. However, the applications of rolling bearing 100 are not limited to these.

[0051] Figure 1 is a cross-sectional view of the rolling bearing 100. Figure 1 As shown, the rolling bearing 100 has a center axis A. Figure 1 FIG shows a cross-sectional view of a rolling bearing 100 taken along a center axis A. The rolling bearing 100 includes a raceway member (raceway ring or washer) and rolling elements. In the rolling bearing 100, the raceway members are an inner ring (bearing shaft ring) 10 and an outer ring (bearing seat ring) 20, and the rolling elements are balls 30. The rolling bearing 100 also includes a retainer 40.

[0052] The inner ring 10 has an annular (ring-shaped) shape and includes a first surface 10a, a second surface 10b, an inner peripheral surface 10c, and an outer peripheral surface 10d.

[0053] First surface 10a and second surface 10b form end surfaces along the direction of central axis A (hereinafter referred to as the "axial direction"). Second surface 10b is the axially opposing surface of first surface 10a. First surface 10a includes a raceway surface 10aa. First surface 10a is recessed within raceway surface 10aa toward second surface 10b. In cross-section, raceway surface 10aa has a partially circular arc shape. Raceway surface 10aa is the surface that contacts ball 30 and constitutes the contact surface of inner ring 10.

[0054] The inner peripheral surface 10c is a surface facing the central axis A. One end of the inner peripheral surface 10c in the axial direction is continuous with the first surface 10a, and the other end in the axial direction is continuous with the second surface 10b.

[0055] The outer peripheral surface 10d is a surface facing the opposite side from the central axis A. In other words, the outer peripheral surface 10d is a surface facing the inner peripheral surface 10c in a direction (hereinafter referred to as the "radial direction") perpendicular to the central axis A. One end of the outer peripheral surface 10d in the axial direction is connected to the first surface 10a, and the other end in the axial direction is connected to the second surface 10b.

[0056] The outer ring 20 has an annular shape and includes a first surface 20a, a second surface 20b, an inner peripheral surface 20c, and an outer peripheral surface 20d.

[0057] The first surface 20a and the second surface 20b constitute axial end surfaces. The outer ring 20 is arranged so that the first surface 20a faces the first surface 10a. The second surface 20b is the axially opposing surface of the first surface 20a. The first surface 20a has a raceway surface 20aa. The first surface 20a is recessed toward the second surface 20b within the raceway surface 20aa. In cross-section, the raceway surface 20aa has a partially circular arc shape. The raceway surface 20aa is the surface that contacts the ball 30 and constitutes the contact surface of the outer ring 20.

[0058] The inner peripheral surface 20c is a surface facing the central axis A. One end of the inner peripheral surface 20c in the axial direction is connected to the first surface 20a, and the other end in the axial direction is connected to the second surface 20b.

[0059] The outer peripheral surface 20d is a surface facing the opposite side from the central axis A. In other words, the outer peripheral surface 20d is a surface facing the inner peripheral surface 20c in a direction (hereinafter referred to as "radial direction") perpendicular to the central axis A. One end of the outer peripheral surface 20d in the axial direction is connected to the first surface 20a, and the other end in the axial direction is connected to the second surface 20b.

[0060] Ball 30 has a spherical shape. There are multiple balls 30. Ball 30 is positioned between first surface 10a and first surface 20a. More specifically, ball 30 is positioned between track surface 10aa and track surface 20aa. Ball 30's surface contacts track surface 10aa and track surface 20aa. In other words, the surface of ball 30 is the contact surface.

[0061] The retainer 40 retains the balls 30. The retainer 40 retains the balls 30 in a direction along the circumference centered on the central axis A (hereinafter referred to as "circumferential direction") so that the interval between two adjacent balls 30 is within a certain range.

[0062] <Steel used in track members and rolling elements>

[0063] The inner ring 10, outer ring 20, and balls 30 are formed from steel having the composition shown in Table 1 (hereinafter referred to as the "first composition"). Alternatively, the inner ring 10, outer ring 20, and balls 30 may be formed from steel having the composition shown in Table 2 (hereinafter referred to as the "second composition"). The steel constituting the inner ring 10, outer ring 20, and balls 30 is quenched. It is sufficient that at least one of the inner ring 10, outer ring 20, and balls 30 is formed from steel having the first composition (second composition).

[0064] [Table 1]

[0065]

[0066] Unit: mass%

[0067] [Table 2]

[0068]

[0069] Unit: mass%

[0070] Carbon (C) affects the hardness of the contact surface after quenching (the surface of the track surface 10aa, the track surface 20aa and the ball 30). When the carbon content in the steel is less than 0.70% by mass, it is difficult to ensure sufficient hardness on the contact surface. In addition, when the carbon content in the steel is less than 0.70% by mass, it is necessary to supplement the carbon content in the surface through carburizing treatment, etc., which becomes the main reason for the decrease in production efficiency and increase in manufacturing costs. When the carbon content in the steel is greater than 1.10% by mass, cracks (quenching cracks) may occur during quenching. Therefore, the carbon content in the steel of the first composition is set to be greater than 0.70% by mass and less than 1.10% by mass.

[0071] Silicon is added to ensure workability before deoxidation and nitriding during steelmaking. If the silicon content in the steel is less than 0.15% by mass, the steel's resistance to tempering softening is insufficient. As a result, the hardness of the contact surface may decrease due to tempering after quenching or the increase in temperature during use of the rolling bearing 100. If the silicon content in the steel exceeds 0.35% by mass, the steel becomes too hard, and the tool life of the cutting tools used to machine the inner ring 10 (outer ring 20, ball 30) may be shortened. Furthermore, this leads to an increase in the material cost of the steel. Therefore, the silicon content in the steel of the first composition is set to not less than 0.15% by mass and not more than 0.35% by mass.

[0072] Manganese is added to ensure the steel's hardenability and hardness. If the manganese content in the steel is less than 0.30 mass%, it is difficult to ensure the steel's hardenability and hardness. If the manganese content in the steel exceeds 0.60 mass%, the steel becomes too hard, potentially shortening the tool life of the cutting tools used to machine the inner ring 10 (outer ring 20, ball 30). This also increases the material cost of the steel. Therefore, the manganese content in the steel of the first composition is set to between 0.30 mass% and 0.60 mass%.

[0073] Chromium is added to ensure the hardenability of the steel and to ensure hardness by forming fine precipitates during nitriding. When the chromium content in the steel is less than 1.30% by mass, it is difficult to ensure the hardenability and hardness of the steel. When the chromium content in the steel is greater than 1.60% by mass, the precipitates become coarse and may become the starting point of fatigue failure. In addition, this leads to an increase in the material cost of the steel. Therefore, the chromium content in the steel of the first composition is set to 1.30% by mass or more and 1.60% by mass or less.

[0074] Molybdenum is added to ensure the hardenability of the steel and to ensure hardness by forming fine precipitates during nitriding. Molybdenum also has a strong affinity for carbon, so it precipitates as undissolved carbides in the steel during nitriding. These undissolved molybdenum carbides serve as precipitation nuclei during quenching, so molybdenum increases the amount of precipitates after quenching.

[0075] When the molybdenum content in the steel is less than 0.01% by mass, it is difficult to ensure the steel's hardenability and hardness. When the molybdenum content in the steel exceeds 0.50% by mass, the precipitates become coarse and may become the starting point of fatigue failure. This also leads to an increase in the material cost of the steel. Therefore, the molybdenum content in the steel of the first composition is set to 0.01% by mass or more and 0.50% by mass or less.

[0076] Vanadium is added to ensure the hardenability of the steel and to ensure hardness by forming fine precipitates during nitriding. When the vanadium content in the steel is less than 0.01% by mass, it is difficult to ensure the hardenability and hardness of the steel. When the vanadium content in the steel exceeds 0.50% by mass, the precipitates become coarse and may become the starting point of fatigue failure. This also leads to an increase in the material cost of the steel. Therefore, the vanadium content in the steel of the first composition is set to 0.01% by mass or more and 0.50% by mass or less.

[0077] <Surface Layer 50>

[0078] like Figure 1As shown, the inner ring 10, outer ring 20, and ball 30 have a surface portion 50 on their surfaces. Surface portion 50 is the region from the surface of the inner ring 10, outer ring 20, and ball 30 to a depth of 20 μm. Surface portion 50 only needs to be formed on at least the contact surfaces of the inner ring 10, outer ring 20, and ball 30. Furthermore, surface portion 50 only needs to be formed on at least one of the inner ring 10, outer ring 20, and ball 30.

[0079] The surface portion 50 is formed by nitriding. The nitrogen content in the surface portion 50 is 0.2% by mass or more and 0.8% by mass or less. If the nitrogen content in the surface portion 50 is less than 0.2% by mass, it is difficult to ensure the durability of the contact surface. If the nitrogen content in the surface portion 50 is greater than 0.8% by mass, the time required for nitriding treatment becomes longer and the manufacturing cost increases. Therefore, the nitrogen content in the surface portion 50 is set to 0.2% by mass or more and 0.8% by mass or less. The nitrogen content in the surface portion 50 is preferably 0.3% by mass or more and 0.5% by mass or less.

[0080] The nitrogen content in the surface layer portion 50 can be measured by an electron probe micro analyzer (EPMA: Electron Probe Micro Analyzer).

[0081] In the cross-sectional view perpendicular to the contact surface, the surface layer portion 50 preferably has a thickness of 100 μm. 2 There are more than 60 precipitates with a particle size of 0.50 μm or less in total. In the cross-sectional view perpendicular to the contact surface, it is more preferable that the surface layer 50 has a particle size of 0.50 μm or less. 2 A total of 80 or more precipitates having a particle size of 0.50 μm or less were present.

[0082] The total area ratio of the precipitates in the surface portion 50 in a cross-sectional view perpendicular to the contact surface is preferably 1% to 10%. More preferably, the total area ratio of the precipitates in the surface portion 50 in a cross-sectional view perpendicular to the contact surface is 2% to 7%.

[0083] Precipitates in the surface portion 50 include, for example, carbonitrides and nitrides. Carbonitrides include iron carbides, compounds in which carbon in iron carbides is replaced by nitrogen, and compounds in which iron in iron carbides is replaced by alloying elements other than iron. Nitrides are iron nitrides. Precipitates in the surface portion 50 may be carbides, carbonitrides, or nitrides of alloying elements contained in the steel.

[0084] The precipitates in the steel of the surface layer portion 50 may be nitrides mainly composed of chromium or vanadium or carbonitrides mainly composed of chromium or vanadium.

[0085] The nitride containing chromium (vanadium) as a main component is a nitride of chromium (vanadium) or a nitride in which a part of the chromium (vanadium) sites in the nitride are substituted with an alloy element other than chromium (vanadium).

[0086] Chromium (vanadium)-based carbonitrides are carbonitrides in which some of the carbon sites in chromium (vanadium) carbides are replaced by nitrogen. The chromium (vanadium) sites in chromium (vanadium)-based carbonitrides may be replaced by alloying elements other than chromium (vanadium).

[0087] The area ratio of the precipitates was calculated by obtaining a cross-sectional image of the surface portion 50 at a magnification of 5000x using a field emission scanning electron microscope (FE-SEM). This cross-sectional image was binarized and then image processed. Cross-sectional images of the surface portion 50 were obtained in three or more fields of view, and the area ratio was calculated using the average value of these multiple cross-sectional images.

[0088] The area of ​​each precipitate is obtained by the same method as above, and the square root of the value obtained by dividing the area by π is multiplied by 2 to obtain the particle size of each precipitate.

[0089] <Retained Austenite in Steel>

[0090] The volume fraction of retained austenite in the steel constituting the inner ring 10, outer ring 20, and ball 30 is preferably 20% to 40% at a depth of 50 μm from the contact surface. This improves the durability of the contact surface (particularly in environments with foreign matter contamination). However, if the volume fraction of retained austenite is less than 25%, the durability of the contact surface may be insufficient. Furthermore, if the volume fraction of retained austenite exceeds 35%, degradation associated with the decomposition of the retained austenite may occur.

[0091] The volume ratio of retained austenite in the steel constituting the inner ring 10 , the outer ring 20 , and the ball 30 is more preferably 25% or more and 35% or less at a depth of 50 μm from the contact surface.

[0092] The amount of retained austenite in the steel at a depth of 50 μm from the contact surface was measured by X-ray diffraction. More specifically, the amount of retained austenite in the steel at a depth of 50 μm from the contact surface was measured using MSF-3M manufactured by Rigaku Corporation.

[0093] <Hardness at a depth of 50 μm from the contact surface>

[0094] The hardness of the inner ring 10, outer ring 20, and ball 30 at a depth of 50 μm from the contact surface is preferably 653 Hv or higher and 800 Hv or lower. If the hardness at a depth of 50 μm from the contact surface is less than 653 Hv, the durability of the contact surface may be insufficient. On the other hand, if the hardness at a depth of 50 μm from the contact surface is greater than 800 Hv, ductility and toughness may decrease, potentially leading to premature damage.

[0095] The hardness of the inner ring 10, outer ring 20, and ball 30 at a depth of 50 μm from the contact surface was measured using the Vickers hardness test method specified in the JIS standard (JIS Z 2244:2009). The load used during the measurement was 300 gf.

[0096] <Martensite Block in Surface Layer 50>

[0097] The steel in the surface layer 50 comprises martensite block grains. The crystal orientation difference between two adjacent martensite block grains at the grain boundary is 15° or greater. Explaining this point from another perspective, even if there are areas with deviated crystal orientation, if the crystal orientation difference is less than 15°, these areas are not considered to be crystal boundaries of martensite block grains. The grain boundaries of martensite block grains are determined using the EBSD (Electron Back Scattered Diffraction) method.

[0098] In the steel of the surface portion 50, the average grain size of the martensite massive grains at a comparative area ratio of 30% is 2.0 μm or less. In the steel of the surface portion 50, the average grain size of the martensite massive grains at a comparative area ratio of 50% is preferably 1.5 μm or less.

[0099] The average grain size of the martensite block grains at a comparative area ratio of 30% (50%) is measured by the following method. First, a cross-sectional observation is performed in a cross-section of the inner ring 10 including the surface portion 50. At this time, the martensite block grains contained in the observation field are specifically observed using the EBSD method. This observation field is set to an area of ​​50μm×35μm. Second, based on the crystal orientation data obtained by the EBSD method, the respective areas of the martensite block grains contained in the observation field are analyzed.

[0100] Third, the areas of the martensite block grains included in the observation field are accumulated in order from large to small. This accumulation is performed until it reaches 30% (50%) of the total area of ​​the martensite block grains included in the observation field. The equivalent circular diameter is calculated for each martensite block grain that is the object of the above accumulation. The equivalent circular diameter is the square root of the value obtained by dividing the area of ​​the martensite block grain by π / 4. The average value of the equivalent circular diameters of the martensite block grains that are the object of the above accumulation is regarded as the average grain size of the martensite block grains when the comparison area ratio is 30% (50%).

[0101] (Method for Manufacturing Rolling Bearing According to First Embodiment)

[0102] The following describes a method for manufacturing a rolling bearing.

[0103] Figure 2 1 is a process diagram showing a method for manufacturing the rolling bearing 100. Figure 2 As shown, the method for manufacturing the rolling bearing 100 includes a preparation step S1, a heat treatment step S2, a finishing step S3, and an assembly step S4. The heat treatment step S2 is performed after the preparation step S1. The finishing step S3 is performed after the heat treatment step S2. The assembly step S4 is performed after the finishing step S3.

[0104] In preparation step S1, a workpiece is prepared for heat treatment step S2 and finishing step S3. This workpiece is an annular member for forming inner ring 10 and outer ring 20, or a spherical member for forming ball 30. This workpiece is made of steel of the first or second composition.

[0105] The heat treatment step S2 includes a heating step S21, a cooling step S22, and a tempering step S23. In the heating step S21, the workpiece is maintained at a temperature above the A1 transformation point for a predetermined time. Furthermore, in the heating step S21, the workpiece is nitrided. This nitriding is performed by heating and maintaining the workpiece in an atmosphere containing a nitrogen source gas (e.g., ammonia).

[0106] The cooling process S22 is performed after the heating process S21. In the cooling process S22, the workpiece is cooled to M s The temperature is lower than the transformation point. This cooling is performed by, for example, oil cooling. The tempering step S23 is performed after the cooling step S22. In the tempering step S23, the workpiece is held at a temperature lower than the A1 transformation point for a predetermined time.

[0107] In the finishing step S3, the workpiece is finished (grinded and polished) and cleaned. In this way, the inner ring 10, outer ring 20 and ball 30 are prepared. In the assembly step S4, the inner ring 10, outer ring 20 and ball 30 are assembled together with the retainer 40. Figure 1 A rolling bearing 100 of the structure shown.

[0108] (Effects of the Rolling Bearing of the First Embodiment)

[0109] Next, the effects of the rolling bearing 100 will be described.

[0110] In rolling bearing 100, because the contact surfaces of the inner ring 10, outer ring 20, and ball 30 are nitrided with a surface layer 50, cracks and other damage are less likely to occur on the contact surfaces (and immediately below). Furthermore, because the inner ring 10, outer ring 20, and ball 30 in rolling bearing 100 are formed from steel of the first or second composition, fine precipitates, etc., are precipitated in the surface layer 50, ensuring the hardness of the contact surfaces (and immediately below). These precipitates are also prevented from becoming stress concentration sources (or starting points for cracks).

[0111] In the rolling bearing 100, because fine precipitates are precipitated on the surface portion 50, the hardness of the contact surface is ensured, thereby suppressing the formation of new metal surfaces on the contact surface. Therefore, in the rolling bearing 100, hydrogen is not easily generated on the contact surface. In addition, in the rolling bearing 100, because fine precipitates are precipitated on the surface portion 50, the vicinity of the precipitates becomes a hydrogen capture site, and the amount of hydrogen intrusion into the surface portion 50 is reduced. Therefore, early peeling damage caused by hydrogen embrittlement is not easily generated in the rolling bearing 100. As described above, according to the rolling bearing 100, the life of the rolling bearing can be extended.

[0112] When the martensite block grains in the steel of the surface portion 50 are refined to an average grain size of 2.0 μm or less at a comparative area ratio of 30%, the increased toughness of the surface portion 50 improves the shear resistance of the contact surfaces (specifically, the raceway surfaces 10da and 20ca, and the surfaces of the balls 30). Consequently, in this case, the durability of the rolling bearing 100 can be further improved.

[0113] Example

[0114] An embodiment of the rolling bearing 100 will be described below.

[0115] <Sample>

[0116] As rolling bearing samples, sample 1, sample 2, and sample 3 were prepared. Sample 1, sample 2, and sample 3 were JIS standard 51106 type one-way thrust ball bearings having an inner diameter of 30 mm, an outer diameter of 47 mm, and a width of 11 mm.

[0117] The inner and outer rings of Sample 1 were made of the steel compositions shown in Table 3. The compositions shown in Table 3 were within the ranges of the first and second compositions. The inner and outer rings of Sample 2 and Sample 3 were made of the steel compositions shown in Table 4. The compositions shown in Table 4 were within the composition ranges for SUJ2 specified in the JIS standard, but outside the ranges of the first and second compositions. The balls of Samples 1, 2, and 3 were made of stainless steel (SUS440C).

[0118] [Table 3]

[0119]

[0120] Unit: mass%

[0121] [Table 4]

[0122]

[0123] Unit: mass%

[0124] Heat treatment step S2 was performed on the inner and outer rings of Sample 1 and Sample 2. Heat treatment step S2 was not performed on the inner and outer rings of Sample 3. More specifically, the inner and outer rings of Sample 3 were quenched and tempered, but not nitrided.

[0125] Figure 3 Graph showing the distribution of carbon and nitrogen contents in the surface layer portions of the inner ring and outer ring of Sample 1. Figure 4 This is a graph showing the distribution of the carbon and nitrogen contents in the surface layer portions of the inner and outer rings of Sample 2. Figure 3 and Figure 4 In the graph, the horizontal axis is the distance from the surface (in mm), and the vertical axis is the carbon and nitrogen content (in mass %). Figure 3 、 Figure 4 As shown in Table 5, the nitrogen content in the surface layers of the inner and outer rings of Sample 1 and Sample 2 was 0.3% to 0.5% by mass. On the other hand, as shown in Table 5, the nitrogen content in the surface layers of the inner and outer rings of Sample 3 was 0% by mass (nitrogen-free).

[0126] As shown in Table 5, the total area ratio of precipitates in the surface layers of the inner and outer rings of Sample 1 was 2.2% to 7.0%. The total area ratio of precipitates in the surface layers of the inner and outer rings of Sample 2 was 1.2% to 4.0%. The total area ratio of precipitates in the surface layers of the inner and outer rings of Sample 3 was 0.07% to 0.24%.

[0127] As shown in Table 5, the number of precipitates in the surface layer of the inner and outer rings of Sample 1 is 2 The total number of precipitates is 66 or more and 425 or less. In the surface layer of the inner and outer rings of Sample 2, the number of precipitates is 100 μm. 2 The total number of precipitates is 29 or more and 81 or less. In the surface layer of the inner and outer rings of sample 3, the number of precipitates is 100 μm. 2 The total number of items is more than 8 and less than 50.

[0128] [Table 5]

[0129]

[0130]

[0131] Figure 5 These are representative cross-sectional FE-SEM images of the surface layer portions of the inner and outer rings of Sample 1. Figure 6 The following are representative cross-sectional FE-SEM images of the surface layer of the inner and outer rings of Sample 2. Figure 5 As shown in Figure 1, in the surface layer of the inner and outer rings of Sample 1, the precipitates are miniaturized (almost all of the precipitates have a particle size of 0.5 μm or less). Figure 6 As shown, in the surface layer portions of the inner and outer rings of Sample 2, the precipitates were not miniaturized (almost all of the precipitates had a particle size greater than 0.5 μm).

[0132] As shown in Table 6, the volume ratio of retained austenite at a depth of 50 μm from the contact surface in the inner and outer rings of Sample 1 was 29.8% to 30.8%. The volume ratio of retained austenite at a depth of 50 μm from the contact surface in the inner and outer rings of Sample 2 was 30.2% to 31.4%. The volume ratio of retained austenite at a depth of 50 μm from the contact surface in the inner and outer rings of Sample 3 was 9.7% to 11.5%.

[0133] As shown in Table 6, the hardness of the inner and outer rings of Sample 1 at a depth of 50 μm from the contact surface is 755 Hv to 759 Hv. The hardness of the inner and outer rings of Sample 2 at a depth of 50 μm from the contact surface is 749 Hv to 758 Hv. The hardness of the inner and outer rings of Sample 3 at a depth of 50 μm from the contact surface is 735 Hv to 765 Hv.

[0134] [Table 6]

[0135]

[0136] <Rolling fatigue life test>

[0137] Rolling fatigue life tests were conducted on Samples 1, 2, and 3. As shown in Table 7, the rolling fatigue life tests were conducted under the following conditions: a maximum contact surface pressure of 2.3 GPa, rapid acceleration and deceleration between 0 rpm and 2500 rpm, and lubrication using a mixture of polyethylene glycol and pure water.

[0138] Figure 7 It is a graph showing the results of a rolling fatigue life test. Figure 7 In the figure, the horizontal axis is the lifespan (in hours) and the vertical axis is the cumulative probability of damage (in %). Figure 7 As shown in Table 7, L 10 When comparing the rolling fatigue life (the time it takes for the cumulative probability of failure to reach 10%), the rolling fatigue life of Sample 2 was longer than that of Sample 3. This comparison also experimentally demonstrated that nitriding treatment under conditions where the nitrogen content in the surface layer 50 is between 0.2 mass % and 0.8 mass % can improve the rolling fatigue life.

[0139] like Figure 7 As shown in Table 7, L 10 When comparing the rolling fatigue life of sample 1 and sample 2, the rolling fatigue life was longer than that of sample 2. This comparison also experimentally demonstrated that forming at least one of inner ring 10, outer ring 20, and ball 30 from the steel of the first composition resulted in fine dispersion of precipitates in surface layer 50, thereby improving the rolling fatigue life.

[0140] [Table 7]

[0141] <![CDATA[L 10 Lifespan]]> Sample 1 2.7 times that of sample 3 Sample 2 2.1 times that of sample 3

[0142] <Hydrogen Intrusion Characteristics>

[0143] The hydrogen intrusion characteristics of the surface layers of the track components (inner and outer rings) of Samples 1 and 3 were evaluated using the following method. First, the track components of Samples 1 and 3 were heated from room temperature to 400°C before the rolling fatigue life test, and the amount of hydrogen released from the track components of Samples 1 and 3 before the rolling fatigue life test was measured. Second, the track components of Samples 1 and 3 were heated from room temperature to 400°C after 50 hours of the rolling fatigue life test, and the amount of hydrogen released from the track components of Samples 1 and 3 after 50 hours of the rolling fatigue life test was measured.

[0144] In Sample 3, the ratio of hydrogen release before and after the rolling fatigue life test (i.e., the value obtained by dividing the hydrogen release after the rolling fatigue life test by the hydrogen release before the rolling fatigue life test) was 3.2. On the other hand, in Sample 1, the ratio of hydrogen release before and after the rolling fatigue life test was 0.9. This comparison experimentally demonstrated that forming the surface layer 50 on the contact surface can suppress hydrogen intrusion into the surface layer 50 and thus prevent premature delamination caused by hydrogen embrittlement.

[0145] (Second embodiment)

[0146] The bearing component of the second embodiment is, for example, the inner ring 210 of a rolling bearing. The following description uses the inner ring 210 as an example of the bearing component of the second embodiment. However, the bearing component of the second embodiment is not limited to this. The bearing component of the second embodiment may also be the outer ring of a rolling bearing or a rolling element of a rolling bearing.

[0147] (Configuration of Inner Ring 210)

[0148] Figure 8 is a cross-sectional view of the inner ring 210. Figure 8 As shown, inner ring 210 is annular. The central axis of inner ring 210 is central axis A1. Inner ring 210 has wide surface 210a, wide surface 210b, inner circumferential surface 210c, and outer circumferential surface 210d. Wide surface 210a, wide surface 210b, inner circumferential surface 210c, and outer circumferential surface 210d constitute the surface of inner ring 210.

[0149] Hereinafter, the direction of the central axis A1 is referred to as the axial direction. Also, hereafter, the direction along the circumference of the circle centered on the central axis A1 when viewed along the axial direction is referred to as the circumferential direction. Also, hereafter, the direction orthogonal to the axial direction is referred to as the radial direction.

[0150] The wide surface 210a and the wide surface 210b are end surfaces in the axial direction of the inner ring 210. The wide surface 210b is an axially opposing surface to the wide surface 210a.

[0151] The inner circumferential surface 210c extends circumferentially. It faces the central axis A1. One axial end of the inner circumferential surface 210c is connected to the wide surface 210a, and the other axial end is connected to the wide surface 210b. The inner ring 210 is fitted onto a shaft (not shown) at the inner circumferential surface 210c.

[0152] The outer peripheral surface 210d extends circumferentially. It faces the side opposite the central axis A1. In other words, the outer peripheral surface 210d is the radially opposite surface of the inner peripheral surface 210c. One axial end of the outer peripheral surface 210d is connected to the wide surface 210a, and the other axial end is connected to the wide surface 210b.

[0153] The outer peripheral surface 210d has a track surface 210da. The track surface 210da extends in the circumferential direction. The outer peripheral surface 210d is recessed toward the inner peripheral surface 210c in the track surface 210da. From a cross-sectional perspective, the track surface 210da is partially circular. The track surface 210da is located in the center of the outer peripheral surface 210d in the axial direction. The track surface 210da is in contact with the rolling element ( Figure 1 A portion of the outer peripheral surface 210d that contacts (not shown).

[0154] Inner ring 210 is made of steel. More specifically, it is quenched and tempered steel. The steel comprising inner ring 210 contains 0.70% to 1.10% carbon, 0.15% to 0.35% silicon, 0.30% to 0.60% manganese, 1.30% to 1.60% chromium, 0.50% to 0.50% vanadium, and 0.50% to 0.50% molybdenum. The molybdenum content of this steel is 0.01% to 0.01% and the vanadium content is 0.01% to 0.01% by mass.

[0155] The carbon content of the steel constituting the inner ring 210 is 0.70 mass % or more to improve hardness, and 1.10 mass % or less to suppress quenching cracking.

[0156] The silicon content in the steel constituting the inner ring 210 is 0.15 mass % or more to improve temper softening resistance and workability, and 0.35 mass % or less to improve workability when the silicon content is excessive.

[0157] The manganese content in the steel constituting the inner ring 210 is 0.30 mass % or more to ensure hardenability, and 0.60 mass % or less because an excess amount of manganese increases manganese-based non-metallic inclusions in the steel.

[0158] The chromium content in the steel constituting the inner ring 210 is 1.30 mass % or more to ensure hardenability and form nitrides and carbonitrides, and 1.60 mass % or less to suppress the formation of coarse precipitates.

[0159] The steel constituting the inner ring 210 contains vanadium to refine nitrides and carbonitrides. The vanadium content in the steel constituting the inner ring 210 is set at 0.50 mass % or less to suppress the cost increase associated with the addition of vanadium.

[0160] The steel constituting the inner ring 210 contains molybdenum to refine nitrides and carbonitrides and improve hardenability. The molybdenum content in the steel constituting the inner ring 210 is set at 0.50 mass % or less to suppress the cost increase associated with the addition of molybdenum.

[0161] The steel comprising inner ring 210 may contain at least 0.90 mass% and no more than 1.10 mass% carbon, at least 0.20 mass% and no more than 0.30 mass% silicon, at least 0.40 mass% and no more than 0.50 mass% manganese, at least 1.40 mass% and no more than 1.60 mass% chromium, at least 0.20 mass% and no more than 0.30 mass% vanadium, and at least 0.10 mass% and no more than 0.30 mass% molybdenum. The remainder of the steel comprising inner ring 210 is iron and unavoidable impurities.

[0162] Figure 9 yes Figure 8 The enlarged view of IX in FIG. Figure 9 As shown, the area of ​​inner ring 210 up to 20 μm from the surface is surface portion 211. The surface of inner ring 210 is subjected to, for example, nitriding. As a result, the nitrogen concentration in the steel of surface portion 211 reaches, for example, 0.15 mass% or higher. The nitrogen concentration in the steel of surface portion 211 is preferably between 0.20 mass% and 0.30 mass%. The nitrogen concentration in the steel of surface portion 211 is measured using an EPMA (Electron Probe Micro Analyzer).

[0163] Precipitates are dispersed in the steel of the surface layer portion 211. The precipitates are nitrides containing chromium or vanadium as a main component or carbonitrides containing chromium or vanadium as a main component.

[0164] The nitride containing chromium (vanadium) as a main component is a nitride of chromium (vanadium) or a nitride in which a part of the chromium (vanadium) sites in the nitride are substituted with an alloy element other than chromium (vanadium).

[0165] Chromium (vanadium)-based carbonitrides are carbonitrides in which some of the carbon sites in chromium (vanadium) carbides are replaced by nitrogen. The chromium (vanadium) sites in chromium (vanadium)-based carbonitrides may be replaced by alloying elements other than chromium (vanadium).

[0166] In the steel of the surface layer portion 211 , the area ratio of precipitates is preferably 2.0% or less. In the steel of the surface layer portion 211 , the maximum grain size of precipitates is preferably 0.5 μm or less.

[0167] The area ratio and maximum grain size of the precipitates in the steel of the surface portion 211 were measured using the following method. First, a cross-sectional image (hereinafter referred to as a "SEM image") was obtained using a SEM (Scanning Electron Microscope) of the cross section of the inner ring 210 including the surface portion 211. The SEM image was obtained at a magnification of 15,000x.

[0168] Second, the acquired SEM image was processed. More specifically, since the precipitates appear white in the SEM image, the individual areas and the total area of ​​the white portions in the SEM image were calculated by image processing.

[0169] The total area of ​​the white portions in the SEM image can be regarded as the area ratio of the precipitates in the steel of the surface portion 211. The square root of the value obtained by dividing the maximum area of ​​each white portion in the SEM image by π / 4 is regarded as the maximum particle size of the precipitates in the steel of the surface portion 211.

[0170] Cementite (Fe3C) may be dispersed in the steel of the surface layer 211. Iron sites in the cementite may be partially substituted with alloying elements, and carbon sites in the cementite may be partially substituted with nitrogen. The maximum particle size of the cementite in the steel of the surface layer 211 is preferably 1.5 μm or less.

[0171] The maximum particle size of cementite in the steel of the surface portion 211 is determined by the following method. First, a SEM image is obtained in the cross section of the inner ring 210 including the surface portion 211. The magnification when obtaining the SEM image is set to 15,000 times. Second, the obtained SEM image is image processed. More specifically, in the SEM image, cementite appears as an elliptical gray, so the respective areas of the elliptical gray parts in the SEM image are calculated by image processing. Next, the square root of the value obtained by dividing the maximum value of the area of ​​each elliptical gray part in the SEM image by π / 4 is regarded as the maximum particle size of cementite in the steel of the surface portion 211.

[0172] The volume ratio of retained austenite in the steel at a distance of 50 μm from the surface of the inner ring 210 is preferably 15% or more. More preferably, the volume ratio of retained austenite in the steel at a distance of 50 μm from the surface of the inner ring 210 is 25% or more and 35% or less.

[0173] The volume fraction of retained austenite in steel is measured by X-ray diffraction. Specifically, the volume fraction 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 phases other than austenite.

[0174] The hardness of the steel at a distance of 50 μm from the surface of the inner ring 210 is preferably 58 HRC or higher. More preferably, the hardness of the steel at a distance of 50 μm from the surface of the inner ring 210 is 58 HRC or higher and 64 HRC or lower. The hardness of the steel is measured according to the Rockwell hardness test method specified in the JIS standard (JIS Z 2245:2016).

[0175] The steel in the surface layer 211 has martensite block grains. The crystal orientation difference between two adjacent martensite block grains at the grain boundary is 15° or greater. Explaining this from another perspective, even if there are areas with deviated crystal orientation, if the crystal orientation difference is less than 15°, these areas are not considered to be crystal boundaries of martensite block grains. The grain boundaries of martensite block grains are determined using the EBSD (Electron Back Scattered Diffraction) method.

[0176] The maximum grain size of the martensite massive grains in the steel of the surface layer portion 211 is 5.0 μm or less. The maximum grain size of the martensite massive grains in the steel of the surface layer portion 211 is measured by the following method.

[0177] First, a cross-sectional observation is performed on the cross-section of the inner ring 210 including the surface portion 211. At this time, the martensite block grains contained in the observation field are identified using the EBSD method. The observation field is set to an area of ​​50μm×35μm. Second, based on the crystal orientation data obtained by the EBSD method, the respective areas of the martensite block grains contained in the observation field are analyzed. The square root of the value obtained by dividing the maximum value of the respective areas of the martensite block grains contained in the observation field by π / 4 is regarded as the maximum grain size of the martensite block grains in the steel of the surface portion 211.

[0178] In the steel of the surface portion 211 , the average grain size of the martensite massive grains at a comparative area ratio of 30% is preferably 2.0 μm or less. In the steel of the surface portion 211 , the average grain size of the martensite massive grains at a comparative area ratio of 50% is preferably 1.5 μm or less.

[0179] The average grain size of the martensite block grains at a comparative area ratio of 30% (50%) was determined by the following method. First, a cross-sectional observation was performed on the cross-section of the inner ring 210 including the surface portion 211. At this time, the martensite block grains contained in the observation field were identified using the EBSD method. This observation field was set to an area of ​​50μm×35μm. Second, based on the crystal orientation data obtained by the EBSD method, the individual areas of the martensite block grains contained in the observation field were analyzed.

[0180] Third, the areas of the martensite block grains included in the observation field are accumulated in order from large to small. This accumulation is performed until it reaches 30% (50%) of the total area of ​​the martensite block grains included in the observation field. The equivalent circular diameter is calculated for each martensite block grain that is the object of the above accumulation. The equivalent circular diameter is the square root of the value obtained by dividing the area of ​​the martensite block grain by π / 4. The average value of the equivalent circular diameters of the martensite block grains that are the object of the above accumulation is regarded as the average grain size of the martensite block grains when the comparison area ratio is 30% (50%).

[0181] The maximum value of the {011} plane orientation density of the martensite block grains in the steel of the surface portion 211 is 3.25 times or less random. The maximum value of the {011} plane orientation density of the martensite block grains in the steel of the surface portion 211 is measured by the following method.

[0182] First, a cross-sectional observation was performed on the cross-section of the inner ring 210 including the surface portion 211. At this time, the martensite block grains contained in the field of view were specifically observed using the EBSD method. The field of view was set to an area of ​​50 μm × 35 μm. Second, based on the crystal orientation data obtained by the EBSD method, the crystal orientation density distribution of the {011} plane of the martensite block grains was analyzed according to the method described in HJ Bunge, Mathematische Methodender Texturanalyse, Akademie-Verlag (1969) using spherical harmonic functions. The highest crystal orientation density value within this crystal orientation density distribution was regarded as the maximum value of the crystal orientation density of the {011} plane.

[0183] The smaller the maximum value of the crystal orientation density of the {011} planes of the martensite block grains in the steel of the surface portion 211 , the higher the randomness of the formation orientation of the {011} planes of the martensite block grains in the steel of the surface portion 211 .

[0184] (Method of Manufacturing Inner Ring 210)

[0185] Figure 10 2 is a process diagram showing a method for manufacturing the inner ring 210. Figure 10 As shown, the method for manufacturing the inner ring 210 includes a preparation step S11, a nitriding step S12, a quenching step S13, a tempering step S14, and a post-processing step S15. The nitriding step S12 is performed after the preparation step S11. The quenching step S13 is performed after the nitriding step S12. The tempering step S14 is performed after the quenching step S13. The post-processing step S15 is performed after the tempering step S14.

[0186] In the preparation step S11 , a workpiece is prepared. The workpiece is an annular member formed of the same steel as the inner ring 210 .

[0187] In the nitriding step S12, the surface of the workpiece is nitrided by holding the workpiece in an atmosphere containing a nitrogen source (eg, ammonia) at a temperature equal to or higher than the Al transformation point of the steel constituting the workpiece.

[0188] In the quenching step S13, the workpiece is quenched. The workpiece is kept at a temperature above the Al transformation point of the steel constituting the workpiece and then rapidly cooled to the M transformation point of the steel constituting the workpiece. S The quenching is performed at a temperature below the phase transformation point. The heating and holding temperature in the quenching step S13 is preferably lower than the heating and holding temperature in the nitriding step S12. The quenching step S13 can be performed twice. The heating and holding temperature in the second quenching step S13 is preferably lower than the heating and holding temperature in the first quenching step S13. This allows the precipitates to be finely and abundantly dispersed in the surface layer of the workpiece.

[0189] In the tempering step S14, the workpiece is tempered. The workpiece is tempered by keeping it at a temperature lower than the A1 transformation point of the steel constituting the workpiece. In the post-processing step S15, the surface of the workpiece is subjected to mechanical processing (grinding, lapping) and cleaning. Figure 1 and Figure 9 The inner ring 210 of the structure is shown.

[0190] Furthermore, because the precipitates are finely and abundantly dispersed in the steel of the surface portion 211, the orientation of the martensite block grains is likely to be less biased toward a specific orientation, resulting in a smaller maximum value of the {011} plane crystal orientation density of the martensite block grains in the steel of the surface portion 211. Furthermore, because the precipitates are finely and abundantly dispersed in the steel of the surface portion 211, the martensite block grains are less likely to grow larger, resulting in a maximum grain size of 5.0 μm or less in the steel of the surface portion 211.

[0191] (Effect of inner ring 210)

[0192] In the steel of the surface layer 211 of the inner ring 210, the martensite block grains are refined to a maximum grain size of 5.0 μm or less. Furthermore, the formation orientation of the martensite block grains is randomized, with the maximum crystal orientation density of the {011} plane being 3.25 times or less. As a result, the increased toughness of the surface layer 211 of the inner ring 210 improves the shear resistance of the surface of the inner ring 210 (specifically, the raceway surface 210da) that contacts the rolling elements. Consequently, the inner ring 210 improves durability.

[0193] When the area ratio of the precipitates in the steel of the surface portion 211 is 2.0% or more (i.e., when the precipitates are dispersed at a high density in the steel of the surface portion 211), the shear resistance of the surface of the inner ring 210 (specifically, the raceway surface 210da) in contact with the rolling element is improved, thereby further improving the durability.

[0194] When the maximum particle size of the precipitates in the steel of surface portion 211 is 0.5 μm, the precipitates are finely dispersed at a high density in the steel of surface portion 211, thereby improving wear resistance and toughness, further enhancing the durability of inner ring 210. When the maximum particle size of the cementite in the steel of surface portion 211 is 1.5 μm or less, the fine dispersion of the cementite further improves the wear resistance and toughness of inner ring 210.

[0195] In the steel of the surface portion 211, the martensite block grains are refined so that the average grain size is less than 2.0 μm (1.5 μm) when the comparative area ratio is 30% (50%), thereby improving the toughness in the surface portion 211 and further improving the durability of the inner ring 210.

[0196] When the volume ratio of retained austenite in the steel at a distance of 50 μm from the surface of inner ring 210 is 15% or greater (25% to 35%), durability against indentation-initiated peeling in an environment with foreign matter is improved. When the hardness of the steel at a distance of 50 μm from the surface of inner ring 210 is 58 HRC or greater (58 HRC to 64 HRC), wear resistance of inner ring 210 is further improved.

[0197] (Rolling Bearing of Second Embodiment)

[0198] Hereinafter, a rolling bearing according to a second embodiment (hereinafter referred to as “rolling bearing 200 ”) will be described.

[0199] Figure 11 is a cross-sectional view of the rolling bearing 200. Figure 11 As shown, rolling bearing 200 is a deep groove ball bearing. However, rolling bearing 200 is not limited to this. Rolling bearing 200 may be, for example, a thrust ball bearing. Rolling bearing 200 includes an inner ring 210, an outer ring 220, rolling elements 230, and a retainer 240.

[0200] The outer ring 220 has a wide surface 220a, a wide surface 220b, an inner peripheral surface 220c, and an outer peripheral surface 220d. The surface of the outer ring 220 is composed of the wide surface 220a, the wide surface 220b, the inner peripheral surface 220c, and the outer peripheral surface 220d.

[0201] The wide surface 220a and the wide surface 220b are end surfaces in the axial direction of the outer ring 220. The wide surface 220b is an axially opposing surface to the wide surface 220a.

[0202] The inner circumferential surface 220c extends circumferentially. It faces the central axis A1. One axial end of the inner circumferential surface 220c is connected to the wide surface 220a, and the other axial end is connected to the wide surface 220b. The outer ring 220 is positioned so that the inner circumferential surface 220c faces the outer circumferential surface 210d.

[0203] The inner circumferential surface 220c includes a raceway surface 220ca. Raceway surface 220ca extends circumferentially. The inner circumferential surface 220c is recessed toward the outer circumferential surface 220d within the raceway surface 220ca. In cross-section, raceway surface 220ca is partially circular. Raceway surface 220ca is axially centered within the inner circumferential surface 220c. Raceway surface 220ca is the portion of the inner circumferential surface 220c that contacts the rolling element 230.

[0204] The outer circumferential surface 220d extends circumferentially. It faces the side opposite the central axis A1. That is, the outer circumferential surface 220d radially opposes the inner circumferential surface 220c. One axial end of the outer circumferential surface 220d is connected to the wide surface 220a, and the other axial end is connected to the wide surface 220b. The outer ring 220 is mated to a shaft (not shown) at the outer circumferential surface 220d.

[0205] Rolling elements 230 are spherical. They are positioned between outer circumferential surface 210d (track surface 210da) and inner circumferential surface 220c (track surface 220ca). Retainer 240 is annular and positioned between outer circumferential surface 210d and inner circumferential surface 220c. Retainer 240 retains the rolling elements 230, ensuring that the spacing between circumferentially adjacent rolling elements 230 remains within a certain range.

[0206] The outer ring 220 and rolling elements 230 can be formed from the same steel as the inner ring 210. The surface portion of the outer ring 220 (the area up to 20 μm from the surface of the outer ring 220) and the surface portion of the rolling elements 230 (the area up to 20 μm from the surface of the rolling elements 230) can have the same structure as the surface portion 211.

[0207] (Rolling fatigue life test)

[0208] In order to confirm the effect of the bearing component of the second embodiment, a rolling contact fatigue life test was conducted. The rolling contact fatigue life test used samples 4, 5, and 6. Samples 4 to 6 are thrust ball bearings of model 51106 specified in the JIS standard.

[0209] In Sample 4, the washers (inner and outer rings) were made of the first steel material. In Samples 5 and 6, the washers were made of the second steel material. The compositions of the first and second steel materials are shown in Table 8. As shown in Table 8, the first and second steel materials have essentially the same composition, 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.

[0210] [Table 8]

[0211]

[0212] Figure 12 Graphs showing the results of measuring the nitrogen concentration and carbon concentration near the raceway surface of the washer of Sample 4. Figure 13 Graphs showing the results of measuring the nitrogen concentration and carbon concentration near the raceway surface of the washer of Sample 5. Figure 12 and Figure 13 The horizontal axis is the distance from the track surface (unit: mm), Figure 12 and Figure 13 The vertical axis represents the concentration of carbon or nitrogen (unit: mass %).

[0213] like Figure 12 and Figure 13 As shown, the surface of the gasket was nitrided in Samples 4 and 5. The heating and holding temperature during the nitriding treatment was set at 850° C. On the other hand, the surface of the gasket was not nitrided in Sample 6.

[0214] Table 9 shows the nitrogen concentration in the surface layer (the region up to 20 μm from the raceway surface) of washers from Samples 4 to 6. As shown in Table 9, the nitrogen concentration in the steel in the surface layer of washers from Samples 4 and 5 was 0.3% or higher and 0.5% or lower. The nitrogen concentration in the steel in the surface layer of the washer from Sample 6 was 0.0%.

[0215] [Table 9]

[0216] Nitrogen concentration in the surface layer (mass %) Sample 4 0.3-0.5 Sample 5 0.3-0.5 Sample 6 0.0

[0217] Washers of Samples 4 to 6 were quenched and tempered. The heating and holding temperature during quenching was 850° C. The heating and holding temperature during tempering was 180° C. The heating and holding time during tempering was 2 hours.

[0218] Figure 14 This is an SEM image of the surface layer portion of the gasket of Sample 4. Figure 15 This is an SEM image of the surface layer portion of the gasket of Sample 5. Figure 14 and Figure 15 In the SEM image, the white part is the precipitate and the oval gray part is the cementite.

[0219] As shown in Table 10, the area ratio of precipitates in the steel in the surface layer of the gasket of Sample 4 was 2.7%. As shown in Table 10, the area ratio of precipitates in the steel in the surface layer of Sample 5 was 1.6%. In other words, the precipitates were dispersed at a higher density in the surface layer of the gasket of Sample 4 than in the surface layer of the gasket of Sample 5. This comparison demonstrates that the addition of 0.5 mass % or less of vanadium and molybdenum results in a higher density of precipitates dispersed in the steel in the surface layer of the gasket.

[0220] In the surface portion of the gasket of Sample 4, the maximum particle size of the precipitates was 0.5 μm. In the surface portion of the gasket of Sample 5, the maximum particle size of the precipitates was 1.1 μm. In other words, the precipitates were more finely dispersed in the surface portion of the gasket of Sample 4 than in the surface portion of the gasket of Sample 5. This comparison demonstrates that the addition of 0.5 mass % or less of vanadium and molybdenum results in high-density and fine dispersion of precipitates in the steel of the surface portion of the gasket.

[0221] [Table 10]

[0222] Area ratio of precipitates in the surface layer (%) Maximum particle size of precipitates in the surface layer (μm) Sample 4 2.7 0.5 Sample 5 1.6 1.1

[0223] As shown in Table 11, the maximum cementite grain size was 1.5 μm or less in the surface layer of the gaskets of Samples 4 and 5. The maximum cementite grain size was larger than 1.5 μm in the surface layer of the gasket of Sample 6.

[0224] [Table 11]

[0225] Maximum particle size of cementite in the surface layer (μm) Sample 4 1.01 Sample 5 1.11 Sample 6 2.10

[0226] As shown in Table 12, in Samples 4 and 5, the volume ratio of retained austenite in the steel was 15% or greater at a distance of 50 μm from the raceway surface. In Sample 6, the volume ratio of retained austenite in the steel was less than 15% at a distance of 50 μm from the raceway surface. In all samples 4 to 6, the hardness of the steel was 58 HRC or greater at a distance of 50 μm from the raceway surface.

[0227] [Table 12]

[0228]

[0229] Figure 16 This is the EBSD phase diagram of the surface layer of the gasket of sample 4. Figure 17 This is the EBSD phase diagram of the surface layer of the gasket of sample 5. Figure 18 This is the EBSD phase diagram of the surface layer of the gasket of sample 6. Figures 16 to 18 In the sample, the martensite blocky grains appear white.

[0230] like Figures 16 to 18As shown in Table 13, in the surface portion of the washer of Sample 4, the maximum value of the crystal orientation density of the {011} planes of the martensite block grains was less than 3.25 times random. On the other hand, in the surface portion of the washer of Sample 4, the maximum value of the crystal orientation density of the {011} planes of the martensite block grains was greater than 3.25 times random.

[0231] [Table 13]

[0232] Maximum value of crystal orientation density of martensite block grains (times random) Sample 4 3.16 Sample 5 3.32 Sample 6 3.82

[0233] like Figures 16 to 18 As shown in Table 14, the maximum grain size of the martensite block grains in the surface portion of the washer of Sample 4 was 5.0 μm or less. On the other hand, the maximum grain size of the martensite block grains in the surface portion of the washers of Samples 5 and 6 was greater than 5.0 μm.

[0234] [Table 14]

[0235] Maximum grain size of martensite block grains (μm) Sample 4 4.4 Sample 5 6.6 Sample 6 5.2

[0236] Figure 19 It is a bar graph showing the average grain size of the martensite massive grains in the surface layer portion of the washers of Samples 4 to 6. Figure 19 The vertical axis of the graph represents the average grain size of martensite block grains (unit: μm).

[0237] like Figure 19 As shown in the figure, in the surface portion of the gasket of Sample 4, the average grain size of the martensite block grains at a comparative area ratio of 30% is 2.0 μm or less. On the other hand, in the surface portion of the gaskets of Samples 5 and 6, the average grain size of the martensite block grains at a comparative area ratio of 30% is greater than 2.0 μm.

[0238] In the surface portion of the gasket of Sample 4, the average grain size of the martensite block grains at a comparative area ratio of 50% was 1.5 μm or less. On the other hand, in the surface portion of the gaskets of Samples 5 and 6, the average grain size of the martensite block grains at a comparative area ratio of 50% was greater than 1.5 μm.

[0239] Figure 20 It is a graph showing the results of a rolling fatigue life test. Figure 20 The horizontal axis of the graph represents lifespan (unit: hours). Figure 13 The vertical axis of the graph represents the cumulative probability of breakage (unit: %). The rolling fatigue life test was conducted under the conditions shown in Table 15. Specifically, the maximum contact surface 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 was a mixture of pure water and polyethylene glycol oil.

[0240] [Table 15]

[0241] Maximum contact surface pressure 2.3GPa Speed Rapid acceleration and deceleration between 0 rpm and 2500 rpm Lubricant Polyethylene glycol + pure water

[0242] like Figure 20 As shown in Table 16, Sample 4 exhibited a rolling fatigue life superior to that of Sample 5. More specifically, the L 10 The lifespan (lifespan at which the cumulative probability of failure is 10%) is L for sample 6. 10 2.7 times the lifespan of sample 5. 10 The lifespan is L of sample 6 10 2.1 times the lifespan.

[0243] As described above, in the surface portion of the washer of Sample 4, the maximum value of the crystal orientation density of the {011} planes of the martensite grains was 3.25 times or less random, and the maximum grain size of the martensite grains was 5.0 μm or less. On the other hand, in the surface portions of the washers of Samples 5 and 6, the maximum value of the crystal orientation density of the {011} planes of the martensite grains was greater than 3.25 times random, and the maximum grain size of the martensite grains was greater than 5.0 μm. This comparison demonstrates that the bearing component of the second embodiment exhibits improved durability.

[0244] [Table 16]

[0245] <![CDATA[Lifetime ratio relative to Sample 6 10 > Sample 4 2.7 Sample 5 2.1 Sample 6 1.0

[0246] Furthermore, as described above, in the surface portion of the washer of Sample 4, the precipitates are finely dispersed at a higher density than in the surface portion of the washer of Sample 5. This comparison demonstrates that the durability of the bearing component of the second embodiment is further improved by setting the area ratio and maximum particle size of the precipitates in the surface portion to 2.0% or more and 0.5 μm or less, respectively.

[0247] In addition, the L of sample 5 10 The lifespan is longer than that of sample 6. 10 Long service life. As described above, the maximum cementite grain size in the surface layer of the gasket of Sample 5 is 1.5 μm or less, while the maximum cementite grain size in the surface layer of the gasket of Sample 6 is greater than 1.5 μm. Furthermore, in the gasket of Sample 5, the volume ratio of retained austenite at a distance of 50 μm from the raceway surface is 15% or more, while in the gasket of Sample 6, the volume ratio of retained austenite at a distance of 50 μm from the raceway surface is less than 15%.

[0248] This comparison shows that the durability of the bearing component is improved by setting the maximum grain size of cementite in the surface layer of the bearing component to 1.5 μm or less and the volume ratio of retained austenite at a position 50 μm from the surface of the bearing component to 15% or more.

[0249] <Hydrogen Intrusion Characteristics>

[0250] The hydrogen intrusion characteristics of the surface layers of gaskets of Samples 4 and 6 were evaluated using the following methods. First, the gaskets of Samples 4 and 6 were heated from room temperature to 400°C before the rolling contact fatigue life test, and the amount of hydrogen released from these gaskets was measured. Second, the gaskets of Samples 4 and 6 were heated from room temperature to 400°C after 50 hours of the rolling contact fatigue life test, and the amount of hydrogen released from these gaskets was measured after 50 hours of the rolling contact fatigue life test.

[0251] In Sample 6, the ratio of hydrogen release before and after the rolling fatigue life test (i.e., the value obtained by dividing the hydrogen release after the rolling fatigue life test by the hydrogen release before the rolling fatigue life test) was 3.2. On the other hand, in Sample 4, the ratio of hydrogen release before and after the rolling fatigue life test was 0.9. This comparison experimentally demonstrated that forming the surface layer 211 on the contact surface can suppress hydrogen intrusion into the surface layer 211 and thus prevent premature delamination caused by hydrogen embrittlement.

[0252] The embodiments of the present invention have been described above, but the above embodiments may be modified in various ways. In addition, the scope of the present invention is not limited to the above embodiments. The scope of the present invention is indicated by the claims, and is intended to include all changes within the meaning and scope equivalent to the claims.

[0253] Possibility of industrial application

[0254] The above-described embodiment can be particularly advantageously applied to raceway members such as inner rings and outer rings, rolling elements such as balls, and rolling bearings using the same.

[0255] Explanation of symbols

[0256] 10 Inner ring, 10a first surface, 10aa raceway surface, 10b second surface, 10c inner circumference, 10d outer circumference, 20 Outer ring, 20a first surface, 20aa raceway surface, 20b second surface, 20c inner circumference, 20d outer circumference, 30 Ball, 40 Cage, 50 Surface layer, 100 Rolling bearing, A Center shaft, S1 Preparation process, S2 Heat treatment process, S3 Finishing process, S4 Assembly process, S21 Heating process, S22 Cooling process, S23 Tempering process Sequence: 210 inner ring, 210a, 210b wide surface, 210c inner circumference, 210d outer circumference, 210da raceway surface, 211 surface layer, 220 outer ring, 220a, 220b wide surface, 220c inner circumference, 220ca raceway surface, 220d outer circumference, 230 rolling element, 240 retainer, 200 rolling bearing, A1 center shaft, S11 preparation process, S12 nitriding process, S13 quenching process, S14 tempering process, S15 post-treatment process.

Claims

1. A bearing component, which is a bearing component made of steel having a surface, wherein It has a region up to 20 μm away from the surface, that is, a surface layer portion, The steel contains 0.70 mass % to 1.10 mass % of carbon, 0.15 mass % to 0.35 mass % of silicon, 0.30 mass % to 0.60 mass % of manganese, 1.30 mass % to 1.60 mass % of chromium, 0.50 mass % to 0.50 mass % of vanadium, and 0.50 mass % to 0.50 mass % of molybdenum, with the remainder being iron and unavoidable impurities. The steel in the surface layer portion has martensite block grains and precipitates, The precipitate is a nitride mainly composed of chromium or vanadium or a carbonitride mainly composed of chromium or vanadium, In the steel in the surface layer, the maximum grain size of the martensite block grains is 5.0 μm or less. In the steel in the surface layer portion, the crystal orientation density of the {011} planes of the martensite block grains is 3.25 times random or less.

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

3. The bearing component according to claim 1, wherein: The area ratio of the precipitates in the steel in the surface layer portion is 2.0% or more.

4. The bearing component according to claim 1, wherein: In the steel in the surface layer portion, the maximum particle size of the precipitates is 0.5 μm or less.

5. The bearing component according to claim 1, wherein: The steel in the surface layer further comprises cementite, In the steel in the surface layer portion, the maximum grain size of the cementite is 1.5 μm or less.

6. The bearing component according to claim 1, wherein: In the steel in the surface layer portion, the average grain size of the martensite massive grains when the comparative area ratio is 30% is 2.0 μm or less.

7. The bearing component according to claim 1, wherein: In the steel in the surface layer portion, the average grain size of the martensite block grains when the comparative area ratio is 50% is 1.5 μm or less.

8. The bearing component according to claim 1, wherein: The nitrogen concentration in the steel in the surface layer portion is 0.15 mass % or more.

9. The bearing component according to claim 1, wherein: The volume ratio of retained austenite in the steel at a position 50 μm away from the surface is 15% or more.

10. 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 higher.

11. The bearing component according to any one of claims 1 to 10, wherein: The volume ratio of retained austenite in the steel at a position 50 μm away from the surface is 25% or more and 35% or less, The hardness of the steel at a position 50 μm away from the surface is 58 HRC or more and 64 HRC or less.

12. A rolling bearing comprising an inner ring, an outer ring and rolling elements. At least one of the inner ring, the outer ring, and the rolling element is the bearing component according to any one of claims 1 to 11.

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