Rolling member and rolling bearing

Abstract

To provide a long-life rolling member for hydrogen application devices, and a rolling bearing including the same.SOLUTION: A rolling member for hydrogen application devices has a surface and is made of steel. The rolling member includes a surface layer portion on the surface. The steel contains carbon, silicon, manganese, chromium, molybdenum, and vanadium, with the remainder consisting of iron and unavoidable impurities. The surface layer portion contains nitrogen dissolved in iron. In a second region of the surface layer portion having a depth of 100 μm or less from the surface, the volume ratio of retained austenite is less than 20%. In a first region, at least one of a first precipitate and a second precipitate is dispersed as a precipitate. The first precipitate and the second precipitate occupy, in the first region, a region whose area ratio is at least 2.0% of the first region.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to rolling members and rolling bearings.

Background Art

[0002] In recent years, due to energy conservation, the use of hydrogen gas as a power source has been promoted, and the development of fuel cell vehicles, hydrogen gas engines, etc. has been advanced. In these hydrogen utilization devices, hydrogen gas used as fuel or water generated by a chemical reaction may leak from gaps in pipes or the like. The leaked hydrogen or water adheres to rolling parts included in the device, and hydrogen intrusion may cause hydrogen embrittlement of the rolling member, shortening the life of the device. Therefore, from the viewpoint of suppressing damage caused by hydrogen brittleness of rolling parts, it is necessary to increase the strength of the rolling member.

[0003] Conventionally, as technologies for increasing the strength of rolling parts, for example, the technologies described in the following patent documents have been proposed.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0005] Each of the above documents discloses a technique for suppressing peeling caused by hydrogen embrittlement. However, with the disclosed techniques of each of the above documents, it is difficult to extend the life when used in hydrogen utilization devices.

[0006] The present invention has been made in view of the above problems. Its object is to provide a long-life rolling part in a hydrogen utilization device and a rolling bearing including the same.

Means for Solving the Problems

[0007] The rolling member according to the present disclosure is a steel rolling member having a surface. The rolling member is for a hydrogen utilization device. The rolling member includes a surface layer portion on the surface. The steel contains 0.80% by mass or more and 1.10% by mass or less of carbon, 0.15% by mass or more and 0.50% by mass or less of silicon, 0.30% by mass or more and 0.70% by mass or less of manganese, 1.30% by mass or more and 1.60% by mass or less of chromium, 0.10% by mass or more and 0.50% by mass or less of molybdenum, and 0.12% by mass or more and 0.50% by mass or less of vanadium, with the balance being iron and unavoidable impurities. The surface layer portion contains a solid solution of nitrogen in iron. The structure in the first region of the surface layer portion with a depth from the surface of 5 μm or less is composed of a structure other than cementite. In the second region of the surface layer portion with a depth from the surface of 100 μm or less, the volume ratio of the retained austenite amount is less than 20%. In the first region, as precipitates, at least one of a first precipitate mainly composed of at least one of chromium and vanadium and a second precipitate mainly composed of at least one of manganese and silicon is dispersed. In the first region, the total area of the first precipitate and the second precipitate together is 2.0% or more of the area of the first region.

Advantages of the Invention

[0008] According to the present disclosure, it is possible to provide a long-life rolling part in a hydrogen utilization device and a rolling bearing including the same.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Mode for Carrying Out the Invention

[0010] The details of the embodiments of the present invention will be described with reference to the drawings. In the following drawings, the same or corresponding parts are denoted by the same reference numerals, and redundant descriptions will not be repeated.

[0011] (Configuration of the rolling bearing according to the embodiment) The configuration of the rolling bearing (hereinafter referred to as "rolling bearing 100") according to the embodiment will be described below. The rolling bearing 100 is, for example, a thrust ball bearing. However, the rolling bearing 100 is not limited thereto. The rolling bearing 100 may be, for example, a deep groove ball bearing, an angular ball bearing, a cylindrical roller bearing, a tapered roller bearing, or a self-aligning roller bearing.

[0012] The rolling bearing 100 is for hydrogen utilization equipment. The hydrogen utilization equipment is, for example, a ball valve or a compressor for a hydrogen station. The type of the compressor is not particularly limited. For example, the compressor may be any of a reciprocating (reciprocating), rotary (screw), centrifugal, or axial flow type. The hydrogen utilization equipment may be a high-pressure hydrogen pressure reducing valve or a hydrogen circulation pump for a fuel cell vehicle.

[0013] FIG. 1 is a cross-sectional view of the rolling bearing 100. As shown in FIG. 1, the rolling bearing 100 has a central axis A. FIG. 1 shows a cross-sectional view of the rolling bearing 100 in a cross-section along the central axis A. The rolling bearing 100 has an orbital member (orbital ring or raceway plate) and rolling elements. In the rolling bearing 100, the orbital members are the inner ring 10 and the outer ring 20. The rolling bearing 100 has rolling elements 30. The rolling bearing 100 further has a cage 40.

[0014] The inner ring 10 has an annular (ring-shaped) configuration. The inner ring 10 has a first surface 10a, a second surface 10b, an inner peripheral surface 10c, and an outer peripheral surface 10d.

[0015] The first surface 10a and the second surface 10b constitute end surfaces in a direction along the central axis A (hereinafter referred to as the "axial direction"). The second surface 10b is the opposite surface of the first surface 10a in the axial direction. The first surface 10a has a raceway surface 10aa. The first surface 10a is recessed on the second surface 10b side at the raceway surface 10aa. In a cross-sectional view, the raceway surface 10aa has a partial arc shape. The raceway surface 10aa is a surface that contacts the rolling element 30 and constitutes the contact surface of the inner ring 10.

[0016] The inner peripheral surface 10c is a surface facing the central axis A side. The inner peripheral surface 10c is continuous with the first surface 10a at one end in the axial direction and is continuous with the second surface 10b at the other end in the axial direction.

[0017] The outer peripheral surface 10d is a surface facing the side opposite to the central axis A. That is, the outer peripheral surface 10d is the opposite surface of the inner peripheral surface 10c in a direction orthogonal to the central axis A (hereinafter referred to as the "radial direction"). The outer peripheral surface 10d is continuous with the first surface 10a at one end in the axial direction and is continuous with the second surface 10b at the other end in the axial direction.

[0018] The outer ring 20 has a ring-shaped configuration. The outer ring 20 has a first surface 20a, a second surface 20b, an inner peripheral surface 20c, and an outer peripheral surface 20d.

[0019] The first surface 20a and the second surface 20b constitute end surfaces in the axial direction. The outer ring 20 is arranged such that the first surface 20a faces the first surface 10a. The second surface 20b is the opposite surface of the first surface 20a in the axial direction. The first surface 20a has a raceway surface 20aa. The first surface 20a is recessed on the second surface 20b side at the raceway surface 20aa. In a cross-sectional view, the raceway surface 20aa has a partial arc shape. The raceway surface 20aa is a surface that contacts the rolling element 30 and constitutes the contact surface of the outer ring 20.

[0020] The inner peripheral surface 20c is the surface facing the central axis A side. The inner peripheral surface 20c is continuous with the first surface 20a at one end in the axial direction and is continuous with the second surface 20b at the other end in the axial direction.

[0021] The outer peripheral surface 20d is the surface facing the side opposite to the central axis A. That is, the outer peripheral surface 20d is the opposite surface of the inner peripheral surface 20c in the direction orthogonal to the central axis A (hereinafter referred to as the "radial direction"). The outer peripheral surface 20d is continuous with the first surface 20a at one end in the axial direction and is continuous with the second surface 20b at the other end in the axial direction.

[0022] The rolling element 30 has a spherical shape. The number of rolling elements 30 is plural. The rolling element 30 is disposed between the first surface 10a and the first surface 20a. More specifically, the rolling element 30 is disposed between the raceway surface 10aa and the raceway surface 20aa. The rolling element 30 contacts the raceway surface 10aa and the raceway surface 20aa on its surface. That is, the surface of the rolling element 30 is the contact surface.

[0023] The cage 40 holds the rolling elements 30. The cage 40 holds the rolling elements 30 such that the interval between two adjacent rolling elements 30 is within a certain range in the direction along the circumference centered on the central axis A (hereinafter referred to as the "circumferential direction").

[0024] The inner ring 10, the outer ring 20, and the rolling elements 30 are made of steel that has been quenched and tempered. The steel constituting the inner ring 10, the steel constituting the outer ring 20, and the steel constituting the rolling elements 30 have the compositions shown in Table 1.

[0025]

Table 1

[0026] Carbon affects the hardness of steel on the surface of rolling elements (inner ring 10, outer ring 20, and rolling elements 30) after quenching. When the carbon content in the steel is less than 0.80% by mass, it is difficult to ensure sufficient hardness on the surface of the rolling elements. When the carbon content in the steel is less than 0.80% by mass, it is necessary to supplement the carbon content on the surface of the rolling elements by carburizing treatment or the like, which becomes a factor in the reduction of production efficiency and the increase in manufacturing cost. On the other hand, when the carbon content in the steel exceeds 1.10% by mass, there is a risk of cracking (quench cracking) during quenching. Therefore, in the composition shown in Table 1, the carbon content is set to be 0.80% by mass or more and 1.10% by mass or less. Among them, it is more preferable that the carbon content in the steel is 0.90% by mass or more and 1.10% by mass or less.

[0027] Silicon is added for deoxidation during steel refining and ensuring workability before nitriding treatment. When the silicon content in the steel is less than 0.15% by mass, the tempering softening resistance becomes insufficient. As a result, due to tempering after quenching or temperature rise during the use of the rolling bearing 100, the hardness on the surface of the rolling elements may decrease. Also, in this case, the workability during machining of the rolling elements becomes insufficient.

[0028] When the silicon content in the steel exceeds 0.50% by mass, the steel becomes too hard and the workability during machining of the rolling elements rather decreases. Also, in this case, the material cost of the steel increases. Therefore, in the composition shown in Table 1, the silicon content is set to be 0.15% by mass or more and 0.50% by mass or less. Among them, it is more preferable that the silicon content in the steel is 0.20% by mass or more and 0.30% by mass or less.

[0029] Manganese is added to ensure the hardenability and hardness of steel. When the manganese content in the steel is less than 0.30% by mass, it is difficult to ensure the hardenability of the steel. When the manganese content in the steel exceeds 0.70% by mass, manganese-based non-metallic inclusions, which are impurities, will increase. Therefore, in the composition shown in Table 1, the manganese content is set to be 0.30% by mass or more and 0.70% by mass or less. Among them, it is more preferable that the manganese content in the steel is 0.40% by mass or more and 0.50% by mass or less.

[0030] Chromium is added to ensure the hardenability of steel and to form fine precipitates (nitrides, carbonitrides) during the nitriding treatment. When the chromium content in the steel is less than 1.30% by mass, it is difficult to ensure the hardenability of the steel and to sufficiently form fine precipitates. When the chromium content in the steel exceeds 1.60% by mass, the material cost of the steel will increase. In addition, when the chromium content in the steel exceeds 1.60% by mass, there is a concern about a decrease in the lifespan due to the formation of coarse precipitates. Therefore, in the composition shown in Table 1, the chromium content is set to be 1.30% by mass or more and 1.60% by mass or less. Among them, it is more preferable that the chromium content in the steel is 1.40% by mass or more and 1.60% by mass or less.

[0031] Molybdenum is added to ensure the hardenability of steel and to form fine precipitates during the nitriding treatment. Since molybdenum has a strong affinity for carbon, it precipitates as undissolved carbides in the steel during the nitriding treatment. Since these undissolved carbides of molybdenum serve as precipitation nuclei during hardening, molybdenum increases the amount of precipitates after hardening.

[0032] When the molybdenum content in the steel is less than 0.10% by mass, it is difficult to ensure the hardenability of the steel and to sufficiently form fine precipitates. When the molybdenum content in the steel exceeds 0.50% by mass, the material cost of the steel increases. Therefore, in the composition shown in Table 1, the molybdenum content is set to be 0.10% by mass or more and 0.50% by mass or less. Among them, it is more preferable that the molybdenum content in the steel is 0.20% by mass or more and 0.30% by mass or less.

[0033] Vanadium is added to ensure the hardenability of the steel and to form fine precipitates during the nitriding treatment. When the vanadium content in the steel is less than 0.12% by mass, it is difficult to ensure the hardenability of the steel and to sufficiently form fine precipitates. When the vanadium content in the steel exceeds 0.50% by mass, the material cost of the steel increases. Therefore, in the composition shown in Table 1, the vanadium content is set to be 0.12% by mass or more and 0.50% by mass or less. Among them, it is more preferable that the vanadium content in the steel is 0.20% by mass or more and 0.30% by mass or less.

[0034] The inner ring 10, the outer ring 20, and the rolling elements 30 are made of steel that has been quenched and tempered. The steel constituting the inner ring 10, the steel constituting the outer ring 20, and the steel constituting the rolling elements 30 are composed of iron and inevitable impurities for the remainder other than the above-mentioned respective materials. That is, the above-mentioned steel contains phosphorus, sulfur, nitrogen, and oxygen as inevitable impurities. However, the steel may not contain phosphorus, sulfur, nitrogen, and oxygen. Further, the steel may further contain aluminum as an inevitable impurity.

[0035] The surfaces of the inner ring 10, the outer ring 20, and the rolling elements 30 are nitrided, whereby a surface layer portion 50 is formed. A region of the surface layer portion 50 where the distance from the surface is 5 μm or less is defined as the first region. A region of the surface layer portion 50 where the distance from the surface is 100 μm or less is defined as the second region. The distance from the surface of the surface layer portion 50 is measured along a direction orthogonal to the surface from an arbitrary point on the surface. The first region is a part of the second region. By performing the nitriding treatment, the surface layer portion 50 contains a solid solution of iron and nitrogen. Here, the solid solution refers to a state in which nitrogen has penetrated into the iron lattice.

[0036] The steel constituting the inner ring 10, the steel constituting the outer ring 20, and the steel constituting the rolling elements 30 have martensite block grains in the surface layer portion 50. The crystal orientation difference between two adjacent martensite block grains is 15° or more at the grain boundary. In other words, even if there is a location where the crystal orientation is misaligned, if the crystal orientation difference is less than 15°, that location is not regarded as the crystal grain boundary of the martensite block grains. The grain boundary of the martensite block grains is determined by the EBSD (Electron Back Scattered Diffraction) method.

[0037] In the first region, cementite has disappeared. That is, the first region does not contain cementite. In other words, the first region is composed of a structure other than cementite. Specifically, the structure in the first region consists of martensite, retained austenite, the first precipitate, and the second precipitate. The disappearance of cementite in the steel is confirmed by the absence of a peak in the carbon amount attributable to cementite using an EPMA (Electron Probe Micro Analyzer). This will be described with reference to FIG. 2.

[0038] FIG. 2 is a graph showing the relationship between the distance from the steel raceway surface constituting the rolling member and the concentrations of carbon and nitrogen. Referring to FIG. 2, the horizontal axis of the graph indicates the distance from the raceway surface of the rolling member such as the inner ring 10 in units of (mm). The vertical axis of the graph in FIG. 2 indicates the concentrations of carbon and nitrogen at each position in units of (mass %). The values on the vertical axis of the graph in FIG. 2 are measured using EPMA.

[0039] From FIG. 2, particularly in the first region where the distance from the raceway surface is 0.005 mm or less, no peak where the carbon concentration rapidly increases appears. From this, it can be seen that there is no cementite in the first region. That is, the structure constituting the first region of the surface layer portion 50 is a structure other than cementite. The conditions of EPMA during the measurement in FIG. 2 were such that the acceleration voltage of the electron beam irradiated to the object was 15 kV, and the spot diameter, which is the diameter of the cross-section orthogonal to the traveling direction of the electron beam, was 2 μm. Further, the measurement was performed for 1 second every 2 μm. Since there is no cementite in the first region, the number density of the first precipitate or the second precipitate in the first region increases.

[0040] In the second region, the volume ratio of the retained austenite amount is less than 20%. Specifically, it is as follows. In the second region, the retained austenite amount is less than 20% with respect to the total amount obtained by adding the retained austenite and the structure other than the retained austenite such as martensite. The amount of retained austenite and the amount of martensite in the second region are measured by an X-ray analyzer.

[0041] In the steel constituting the inner ring 10, the steel constituting the outer ring 20, and the steel constituting the rolling elements 30, precipitates are dispersed in the surface layer portion 50, particularly in the first region and the second region. The precipitates may be first precipitates mainly composed of at least one of chromium and vanadium. The first precipitates may contain only one of chromium and vanadium, or may contain both. Alternatively, the precipitates may be second precipitates mainly composed of at least one of manganese and silicon. The second precipitates may contain only one of manganese and silicon, or may contain both. The precipitates dispersed in the steel may be only one of the first precipitates and the second precipitates, or may be both. In any case, the above-mentioned precipitates are nitrides or carbonitrides.

[0042] Nitrides mainly composed of chromium (vanadium) are nitrides of chromium (vanadium) or those in which a part of the sites of chromium (vanadium) in the nitrides are substituted by alloying elements other than chromium (vanadium). Carbonitrides mainly composed of chromium (vanadium) are those in which a part of the sites of carbon in the carbides of chromium (vanadium) are substituted by nitrogen. The sites of chromium (vanadium) in the carbonitrides mainly composed of chromium (vanadium) may be substituted by alloying elements other than chromium (vanadium). Similarly, nitrides mainly composed of manganese (silicon) are nitrides of manganese (silicon) or those in which a part of the sites of manganese (silicon) in the nitrides are substituted by alloying elements other than manganese (silicon). Carbonitrides mainly composed of manganese (silicon) are those in which a part of the sites of carbon in the carbides of manganese (silicon) are substituted by nitrogen. The sites of manganese (silicon) in the carbonitrides mainly composed of manganese (silicon) may be substituted by alloying elements other than manganese (silicon).

[0043] In the first region, the total area of the first precipitate and the second precipitate combined is 2.0% or more of the area of the first region. In other words, in the first region, the first precipitate and the second precipitate are present (arranged) in a region covering 2.0% or more of the surface of the surface layer portion 50. Stated yet another way, in the first region, the first precipitate and the second precipitate occupy a region with an area ratio of 2.0% or more within the first region. The average value of the area ratio occupied by at least either the first precipitate or the second precipitate in the entire surface of the surface layer portion 50 of each rolling member (inner ring 10, outer ring 20, and rolling elements 30) in the first region may be 2.0% or more. The area ratio of the precipitate in the first region is measured by the following method.

[0044] The average area ratio of the precipitate is calculated by acquiring a cross-sectional image of the second region at a magnification of 5000 times using a field emission scanning electron microscope (FE-SEM), binarizing the cross-sectional image, and performing image processing on the binarized cross-sectional image. The above cross-sectional image is acquired in three or more fields of view, and the average area ratio is the average value of the area ratios of the precipitates obtained from those multiple cross-sectional images.

[0045] In the first region, preferably, the maximum grain size of the martensite block grains is 5.0 μm or less. The maximum grain size of the martensite block grains in the first region is measured by the following method.

[0046] First, cross-sectional observation is performed on a cross-section including the first region. At this time, the martensite block grains included in the observation field of view are identified by the EBSD method. This observation field of view is a region observed at a magnification of 1500 times. Second, from the crystal orientation data obtained by the EBSD method, the area of each of the martensite block grains included in the observation field of view is analyzed. The square root of the value obtained by dividing the maximum value of the area of each of the martensite block grains included in the observation field of view by π / 4 is the maximum grain size of the martensite block grains in the first region.

[0047] In the first region, preferably, the maximum particle size of at least one of the first precipitate and the second precipitate is 2.0 μm or less. The maximum particle size of the precipitate in the first region is measured by the following method.

[0048] The particle size of each precipitate is obtained by using the same method as the above (method for calculating the average area ratio of the precipitate) to obtain the area of each precipitate, and calculating the square root of the value obtained by dividing the area by π / 4. Then, the largest of the obtained particle sizes of the precipitates is taken as the maximum particle size of the precipitate in the second region.

[0049] Furthermore, from FIG. 2, it can be seen that the concentration of nitrogen contained in the first region (measured using EPMA) is 0.3% by mass or more. Also, the concentration of nitrogen contained in the first region is 0.6% by mass or more. Also, the compressive residual stress in the first region is preferably 80 MPa or more. The compressive residual stress in the first region is measured by the X-ray diffraction method.

[0050] (Manufacturing method of the rolling bearing 100) Hereinafter, the manufacturing method of the rolling bearing 100 will be described.

[0051] FIG. 3 is a manufacturing process diagram of the rolling bearing 100. As shown in FIG. 3, the manufacturing method of the rolling bearing 100 mainly includes a preparation step S1, a nitriding treatment step S2, a decarburization treatment step S3, a quenching step S4, a tempering step S5, a post-treatment step S6, and an assembly step S7. The manufacturing method of the rolling bearing 100 may further include a quenching step different from the quenching step S4, and may further include a tempering step different from the tempering step S5. A quenching step different from the quenching step S4 is hereinafter referred to as a secondary quenching. A tempering step different from the tempering step S5 is hereinafter referred to as a secondary tempering.

[0052] In the preparation step S1, a member to be processed is prepared. When forming the inner ring 10 and the outer ring 20, an annular member is prepared as the member to be processed, and when forming the rolling element 30, a spherical member is prepared. This member to be processed is formed of steel having the composition shown in Table 1.

[0053] In the nitriding treatment step S2, nitriding treatment is performed on the surface of the member to be processed. This nitriding treatment is carried out by holding the member to be processed at a temperature equal to or higher than the A1 transformation point for a predetermined time in an atmosphere gas containing a gas serving as a nitrogen source (for example, ammonia gas).

[0054] The decarburization treatment step S3 may be performed in parallel with the nitriding treatment step S2. Decarburization is a phenomenon in which carbon is lost from the surface of steel. The decarburization treatment is a treatment in which the steel is heated by reducing the activity of carbon in the atmosphere gas to a value at which cementite in the steel disappears. Therefore, when the carbon concentration in the steel is high, the decarburization treatment is performed. When the carbon concentration in the steel is low in advance, the decarburization treatment step S3 may not be performed.

[0055] When performing the decarburization treatment step S3 while performing the nitriding treatment step S2, the flow rate of the enriching gas is adjusted to adjust the activity of carbon in the atmosphere gas.

[0056] In the quenching step S4, quenching of the member to be processed is performed. This quenching is carried out by holding the member to be processed at a temperature equal to or higher than the A1 transformation point for a predetermined time and then cooling the member to be processed to a temperature equal to or lower than the Ms transformation point. The Ms transformation point tends to be lower as the nitrogen concentration in solid solution is higher. Therefore, the Ms point is higher in the deep part (the part away from the surface) of the member to be processed with a low nitrogen concentration, and the expansion accompanying the martensite transformation during quenching precedes in the deep part of the member to be processed. Then, when the martensite transformation occurs in the surface layer of the member to be processed with a high nitrogen concentration as the cooling progresses, the already hardened deep part of the member to be processed becomes a resistance to the expansion accompanying the martensite transformation in the surface layer of the member to be processed, and the surface layer part of the member to be processed cannot expand completely and reaches room temperature in a state where the lattice spacing is compressed. Therefore, a compressive stress remains in the surface layer part of the member to be processed.

[0057] In the tempering step S5, tempering of the member to be processed is performed. This tempering is performed by holding the member to be processed at a temperature below the A1 transformation point for a predetermined time. Also, depending on the tempering only once, the amount of retained austenite in the second region may not sufficiently decrease. In this case, it is preferable to perform secondary tempering, which is tempering again. Note that secondary tempering may not be performed. When performing tempering again, hereinafter, the first tempering step may be described as the primary tempering step S5, and the second (again) tempering step may be described as the secondary tempering step S5A.

[0058] When performing secondary tempering, in one or both of the primary tempering and the secondary tempering, heat treatment (high-temperature tempering) at a temperature higher than the normal tempering conditions may be performed. As the high-temperature tempering, for example, heating at 230°C for 2 hours may be performed. However, the heating temperature and time in the high-temperature tempering are appropriately adjusted from the viewpoint of adjusting the amount of retained austenite to a desired value while maintaining the required hardness of the steel material. Therefore, the above heating temperature and time in the high-temperature tempering are merely examples and are not limited thereto.

[0059] Between the quenching step S4 and the tempering step S5, only either the secondary quenching step or the sub-zero step may be performed, or both of these may be performed in this order. However, the secondary quenching step and the sub-zero step may not be performed. In the secondary quenching step, the member to be processed is held at a temperature equal to or higher than the A1 transformation point for a predetermined time. However, in the secondary quenching step, it is preferable that the heating temperature is lower than that of the aforementioned quenching step S4. The sub-zero step is a heat treatment performed to transform retained austenite in the steel into martensite. In the sub-zero step, the steel is cooled to a temperature lower than room temperature and soaked at that temperature. Note that when no further change in the structure is required, the secondary quenching step and the sub-zero step may be omitted.

[0060] Either the tempering process (high-temperature tempering) or the sub-zero process can reduce the amount of retained austenite in the steel. That is, while maintaining the required hardness of the steel, the amount of retained austenite is adjusted by the sub-zero process, the primary tempering process, and the secondary tempering process.

[0061] In the post-treatment process S6, finishing processes (grinding and polishing) and cleaning are performed on the workpiece member. As a result, the inner ring 10, the outer ring 20, and the rolling elements 30 are formed. In the assembly process S7, the inner ring 10, the outer ring 20, and the rolling elements 30 are assembled together with the cage 40. Thus, the rolling bearing 100 having the structure shown in FIG. 1 is manufactured.

[0062] (Effect of the rolling bearing 100) Next, the operating effects of the rolling bearing 100 will be described. Here, hydrogen exists in the form of diffusible hydrogen and non-diffusible hydrogen. Diffusible hydrogen is hydrogen that can diffuse from the trap site into the steel matrix under the use conditions of the rolling element. Non-diffusible hydrogen is hydrogen that cannot diffuse from the trap site into the steel matrix under the use conditions of the rolling element. The diffusion of hydrogen into the matrix causes embrittlement by hydrogen, which is not preferable. Therefore, the following explanation is based on the premise that it is preferable that the number of trap sites for diffusible hydrogen is small and the number of trap sites for non-diffusible hydrogen is large.

[0063] In the rolling bearing 100, the inner ring 10, the outer ring 20, and the rolling elements 30 are formed of steel having the composition shown in Table 1. The steel having the composition shown in Table 1 has a low content of expensive alloying elements. Therefore, according to the rolling bearing 100, the steel material cost can be suppressed. The rolling element of the present embodiment is based on the alloy composition of SUJ2, which is a type of high-carbon chromium bearing steel material described in JIS G 4805, and only a trace amount of molybdenum and vanadium are added. Thereby, while suppressing the cost of the steel material, the hardenability of the high-carbon steel can be enhanced.

[0064] The rolling member of the present embodiment has only trace amounts of molybdenum and vanadium added to steel. By subjecting it to nitriding treatment, the surface layer portion 50 contains a solid solution of iron and nitrogen. In addition to the trace addition of molybdenum and vanadium, by performing nitriding treatment, fine precipitates are dispersed at a high density in the surface layer portion 50. As a result, the formation of a fresh metal surface on the raceway surface of the rolling member is suppressed. Therefore, it is difficult for hydrogen to be generated on the raceway surface in the surface layer portion 50. Further, the fine precipitates dispersed at a high density in the surface layer portion 50 of the steel cause the vicinity of the precipitates to become a strong hydrogen trap site. For this reason, the hydrogen that has invaded the surface layer portion 50 is trapped and rendered harmless, and the amount of diffusible hydrogen in the surface layer portion 50 decreases. For this reason, the rolling member can have a long life.

[0065] Specifically, by adding trace amounts of vanadium or the like and performing nitriding treatment, at least one of the first precipitate and the second precipitate, which are hard fine precipitates, is dispersed in the steel, particularly in the surface layer portion 50. The dispersion of the fine precipitates increases the trap sites for non-diffusible hydrogen in the surface layer portion 50. Further, the dispersion precipitation of the fine precipitates improves the wear resistance of the steel. The fresh steel surface serves as a catalyst for the reaction in which the lubricant decomposes to generate hydrogen. For this reason, since it is difficult to form a fresh steel surface, the amount of hydrogen intrusion in the surface layer portion 50 decreases. Therefore, in the rolling bearing 100, early peeling due to hydrogen embrittlement is less likely to occur.

[0066] The rolling member of the present embodiment has a small amount of retained austenite in the second region of the surface layer portion 50 due to tempering at a high temperature. The volume ratio of the amount of retained austenite in the second region of the surface layer portion 50 is less than 20%.

[0067] Retained austenite has a lower thermal decomposition temperature than martensite. For this reason, when the operating temperature of the rolling member is high and the amount of hydrogen trapped in the retained austenite is large, a part of the hydrogen trapped in the retained austenite can become diffusible hydrogen. The life due to hydrogen embrittlement depends on the amount of diffusible hydrogen. For this reason, it is important to reduce the retained austenite that can become a hydrogen trap site that promotes an increase in diffusible hydrogen.

[0068] In this embodiment, since the amount of retained austenite is small, the amount of diffusible hydrogen that causes hydrogen embrittlement can be reduced. As a result, the rolling members can have a long life.

[0069] Also, in the rolling members of this embodiment, tempering at a high temperature reduces the dislocation density of martensite in the steel (particularly in the second region of the surface layer portion 50). Specifically, the dislocation density of martensite in the second region of the surface layer portion 50 is preferably reduced to, for example, 2.0×10 15 m -2 or less. The rolling members of this embodiment reduce dislocations, which are trap sites for diffusible hydrogen, by tempering at a high temperature. As described above, trap sites for diffusible hydrogen can easily diffuse the hydrogen trapped therein and may cause hydrogen embrittlement to progress. Therefore, from the viewpoint of preventing the diffusion of hydrogen and the resulting embrittlement, it is preferable that there are few trap sites for diffusible hydrogen.

[0070] The dislocation density of martensite depends on the temperature and time of tempering performed on the steel. Therefore, a reduction in the dislocation density of martensite in the steel occurs when tempering (high-temperature tempering) is performed at a temperature higher than 180°C and for a longer time, which are normal tempering conditions, for 2 hours. Therefore, the effect due to the reduction in the dislocation density of martensite can be obtained only when such high-temperature tempering is performed. The effect here is to suppress the early peeling of the contact surface due to hydrogen brittleness and extend the life of the rolling members.

[0071] In the rolling elements included in the rolling bearing 100, the structure existing in the first region is a structure other than cementite. Cementite existing at the grain boundaries of steel promotes crack propagation at the grain boundaries. Therefore, if cementite disappears, crack propagation at the grain boundaries can be suppressed. Accordingly, fatigue failure of the rolling elements can be suppressed. Further, due to decarburization, the solid solution ratio of nitrogen, which makes a large contribution to solid solution strengthening, to iron increases, so the fatigue strength of the steel material is improved. Furthermore, due to the decarburization treatment, the precipitation amount of fine precipitates existing in the surface layer portion 50 increases. Since the vicinity of this fine precipitate becomes a strong trap site for hydrogen, the hydrogen that has invaded the surface layer portion 50 is rendered harmless, and the amount of diffusible hydrogen in the surface layer portion 50 decreases. Therefore, in the rolling bearing 100 subjected to the decarburization treatment, early peeling due to hydrogen embrittlement is less likely to occur.

[0072] In the first region of the rolling elements included in the rolling bearing 100, it is preferable that the maximum grain size of the martensite block grains is 5.0 μm or less. Thereby, the effect that the dislocation movement causing hydrogen embrittlement is suppressed can be obtained.

[0073] In the first region of the rolling elements included in the rolling bearing 100, it is preferable that the maximum grain size of at least one of the first precipitate and the second precipitate is 2.0 μm or less. The refinement of the precipitates improves the wear resistance of the steel surface, increases the trapping amount of hydrogen invading from the steel surface, and promotes the harmlessness of the invaded hydrogen. Therefore, the refinement of the precipitates suppresses damage caused by hydrogen embrittlement and contributes to the long life of the rolling elements.

[0074] (Application example of rolling elements) The rolling elements according to the embodiment are used in the ball valve 200. FIG. 4 is an enlarged cross-sectional view of the ball valve 200. As shown in FIG. 4, the ball valve 200 has a body 210, a seat retainer 220, a ball 230, stems 231 and 232, and a sliding bearing 240.

[0075] The seat retainer 220 is disposed inside the body 210. An internal space 220a, a flow path 220b, and a flow path 220c are formed in the seat retainer 220. The flow paths 220b and 220c are connected to the internal space 220a. The ball 230 is disposed in the internal space 220a. The wall surface of the internal space 220a contacts the surface of the ball 230 at the seal portion 220aa.

[0076] The stem 231 and the stem 232 are respectively connected to the upper end and the lower end of the ball 230. When the stem 231 and the stem 232 rotate around the central axis, the flow paths 220b and 220c are connected through a through hole (not shown) formed in the ball 230. The stem 231 and the stem 232 are passed through through holes formed in the body 210 and the seat retainer 220. Hydrogen flows through the flow path 220b, the flow path 220c, and the through hole formed in the ball 230.

[0077] The sliding bearing 240 is cylindrical and is attached to the body 210 on the outer peripheral surface. The sliding bearing 240 rotatably supports the stem 231 (stem 232). The sliding bearing 240 is a rolling member according to the embodiment. That is, the sliding bearing 240 is made of steel having a composition shown in Table 1, for example, and a surface layer portion 50 is formed on the contact surface.

[0078] (Application example of rolling bearing) FIG. 5 is a cross-sectional view of the hydrogen circulation pump 300. The hydrogen circulation pump 300 includes a motor housing 310, a pump housing 320, a rotating shaft 331 and a rotating shaft 332, a motor stator 341 and a motor rotor 342, a gear 351 and a gear 352, a rotor 361 and a rotor 362, a rolling bearing 371, a rolling bearing 372, a rolling bearing 373, a rolling bearing 374, a rolling bearing 375, and a rolling bearing 376.

[0079] The motor housing 310 is attached to the pump housing 320. One end side of the rotating shaft 331 is disposed within the motor housing 310, and the other end side of the rotating shaft 331 is disposed within the pump housing 320. One end and the other end of the rotating shaft 331 are rotatably supported by a rolling bearing 371 disposed within the motor housing 310 and a rolling bearing 372 disposed within the pump housing 320, respectively. The rotating shaft 331 is rotatably supported by rolling bearings 373 and 374 disposed within the pump housing 320 between one end and the other end.

[0080] The rotating shaft 332 is disposed within the pump housing 320. One end of the rotating shaft 332 is rotatably supported by a rolling bearing 375 disposed within the pump housing 320. The rotating shaft 332 is rotatably supported by a rolling bearing 376 disposed within the pump housing 320 at a position away from one end.

[0081] The motor stator 341 is disposed within the motor housing 310. The motor rotor 342 is attached to the rotating shaft 331 so as to face the motor stator 341. The rotating shaft 331 is rotated by the motor stator 341 and the motor rotor 342. A gear 351 and a gear 352 are attached to the rotating shaft 331 and the rotating shaft 332, respectively. The rotation of the rotating shaft 331 is transmitted to the rotating shaft 332 by the gear 351 and the gear 352. Note that the gear 351 is between the rolling bearing 373 and the rolling bearing 374, and the gear 352 is between the rolling bearing 375 and the rolling bearing 376.

[0082] Inside the pump housing 320, a pump chamber 320a is formed. Inside the pump chamber 320a, a rotor 361 and a rotor 362 are arranged. The rotor 361 and the rotor 362 are respectively attached to a rotating shaft 331 and a rotating shaft 332. As the rotor 361 rotates with the rotation of the rotating shaft 331 and the rotor 362 rotates with the rotation of the rotating shaft 332, hydrogen is inhaled into the pump chamber 320a and hydrogen is discharged from the pump chamber 320a.

[0083] The rolling bearings 371, 372, 373, and 375 are deep groove ball bearings. The rolling bearings 374 and 375 are double row angular contact ball bearings. The rolling bearings 371, 372, 373, 374, 375, and 376 are rolling bearings according to the embodiment. That is, in the rolling bearings 371, 372, 373, 374, 375, and 376, the raceway member and the rolling elements are made of steel having a composition shown in Table 1 for example, and a surface layer portion 50 is formed on the contact surface.

Example

[0084] A test (hereinafter referred to as "First Test") was conducted to examine the relationship between the amount of retained austenite in steel and the amount of hydrogen traps. The steel applied to the test was the first steel or the second steel as shown in Table 2. The first steel was within the composition range shown in Table 1, and the second steel was not within the composition range shown in Table 1. Although not shown in Table 2, both the first steel and the second steel in Table 2 contain, as inevitable impurities, 0.005 mass% or more and 0.050 mass% or less of aluminum, 0.020 mass% or less of phosphorus, 0.010 mass% or less of sulfur, 0.015 mass% or less of nitrogen, and 0.005 mass% or less of oxygen.

[0085]

Table 2

[0086] The samples used in the first test are of two types (Sample 1 and Sample 2). These samples were generally formed according to the manufacturing method described in the above "Manufacturing Method of the Rolling Bearing 100" column. However, since the types of heat treatments actually performed for each sample are slightly different, they are shown in Table 3 below.

[0087]

Table 3

[0088] Also, after these samples were ground, which corresponds to the post-treatment process S6 in FIG. 3, they had the characteristics such as the material shown in Table 4 below.

[0089]

Table 4

[0090] The nitrogen concentration and the compressive residual stress in Table 4 are those in the first region, and the precipitate area ratio is also that in the first region. The precipitate area ratio indicates the area ratio of the region occupied by the precipitates in the first region. Here, the precipitate is at least one of the first precipitate mainly composed of at least one of chromium and vanadium and the second precipitate mainly composed of at least one of manganese and silicon.

[0091] As shown in Tables 3 and 4, the decarburization treatment was performed only on Sample 1. The nitriding treatment was performed only on Sample 1. The secondary quenching and the secondary tempering were not performed on either Sample 1 or Sample 2.

[0092] In the nitriding treatment process S2, the decarburization treatment process S3, and the quenching process S4 of Sample 1, after holding at 850 °C for 6 hours in the endothermic type conversion gas and ammonia gas, oil quenching was performed. In the quenching process S4 of Sample 2, after holding at 850 °C for 1 hour in the endothermic type conversion gas, oil quenching was performed. In the primary tempering process S5, holding was performed at 180 °C for 2 hours.

[0093] Table 5 below shows the volume ratio of the retained austenite amount in the second region of Samples 1 and 2 formed as described above in units of %, and shows the ratio of the amount of hydrogen trapped in each sample.

[0094]

Table 5

[0095] Table 5 shows the amount of hydrogen trapped inside the retained austenite and at the interface (outermost edge) of the retained austenite. Table 5 shows the ratio of the amount of hydrogen in Sample 1 when the amount of hydrogen in Sample 2 is taken as 1. The combined amount of hydrogen trapped inside and at the interface (outermost edge) is hereinafter referred to as the amount of hydrogen trapped in the retained austenite. The larger this value, the more hydrogen is trapped in the retained austenite in the structure. The amount of hydrogen in Table 5 is the ratio of the initial amount of hydrogen trapped in the retained austenite in the steel by the heat treatment carried out under the conditions shown in Table 3. More specifically, the ratio of the amount of hydrogen in Table 5 indicates the ratio of the integrated amount of hydrogen released when the steel is heated from room temperature to 400°C.

[0096] From Table 5, it was found that if the amount of retained austenite in the surface layer portion (second region) is large, the amount of hydrogen trapped increases. Also, it was found that there is a correlation between the amount of retained austenite in the surface layer portion (second region) and the amount of hydrogen trapping sites.

[0097] As described above, a part of the hydrogen trapped in the retained austenite can become diffusible hydrogen. For this reason, the retained austenite can cause damage due to hydrogen embrittlement to the rolling member. Therefore, it can be said that it is more preferable that the amount of retained austenite is small and the amount of hydrogen trapped therein is small than when the amount of retained austenite is large and the amount of hydrogen trapped therein is large.

[0098] The conditions of each sample shown in Table 3 and Table 4 are merely the conditions adjusted to output the volume ratio of the retained austenite amount as shown in Table 5. Therefore, the relationship between the results of the first test shown in Table 5 and the conditions shown in Table 3 and Table 4 will not be considered here.

Example

[0099] As described above, if the wear resistance of steel is improved, hydrogen intrusion is less likely to occur. A test (hereinafter referred to as the "second test") was conducted to evaluate the relationship between the forming conditions of steel and its wear resistance. The steel applied to this test was the first steel or the second steel in Table 2, similar to Example 1.

[0100] Three types of samples (Sample 3, Sample 4, and Sample 5) were used in the second test. The heat treatment conditions of these samples were as shown in the following Table 6.

[0101]

Table 6

[0102] In addition, after these samples were ground, which corresponded to the post-treatment process S6 in FIG. 3, they had the characteristics of the material and the like shown in the following Table 7.

[0103]

Table 7

[0104] The target areas and definitions of each item in Table 7 are the same as those in Table 4 of Example 1. No secondary quenching was performed on any of Samples 3, 4, and 5. Nitrocarburizing treatment and secondary tempering were only performed on Samples 3 and 4.

[0105] In the nitriding treatment step S2, decarburization treatment step S3, and quenching step S4 for Samples 3 and 4, after holding at 850°C for 6 hours in endothermic conversion gas and ammonia gas, oil quenching was performed. In the quenching step S4 for Sample 5, after holding at 850°C for 1 hour in endothermic conversion gas, oil quenching was performed. In the primary tempering step S5, holding was performed at 180°C for 2 hours. In the secondary tempering step S5A performed on Samples 3 and 4, holding was performed at 230°C for 2 hours.

[0106] All of Samples 3, 4, and 5 shown in Tables 6 and 7 have a low volume ratio of retained austenite, less than 20%. Therefore, similar to Sample 2, Samples 3, 4, and 5 have a small amount of hydrogen trapped in retained austenite. Thus, all of Samples 3, 4, and 5 can suppress hydrogen embrittlement caused by hydrogen trapped in retained austenite becoming diffusible hydrogen.

[0107] The wear test was performed under the conditions shown in Table 8. The results of the wear test are shown in Table 9. The evaluation of wear resistance was performed by observing the wear scar after the test with a laser microscope and calculating the wear volume. As shown in Table 9, it was found that the wear resistance of Sample 3 is superior compared to Samples 4 and 5. Also, Sample 4 was significantly superior to at least Sample 5.

[0108] When the wear resistance of the surface is high, as a result, hydrogen intrusion from the surface to the surface layer part 50 is suppressed, and the occurrence of hydrogen embrittlement is suppressed. Therefore, it is less likely that early peeling due to hydrogen brittleness will occur in Sample 3 compared to Samples 4 and 5.

[0109]

Table 8

[0110]

Table 9

[0111] From the results in Table 9, Samples 3 and 4, to which a trace amount of molybdenum and vanadium were added and which were further subjected to nitriding treatment, have better wear resistance than Sample 5 to which these treatments were not applied. If the precipitate occupies an area ratio of 1.0% or more in the first region, the wear resistance is excellent. Among them, if the precipitate occupies an area ratio of 2.0% or more in the first region, the wear resistance is particularly excellent.

[0112] The area ratio of the precipitate corresponds to the density of the trap sites of non-diffusible hydrogen. That is, there is a correlation between the size of the area ratio of the precipitate and the size of the density of the trap sites of non-diffusible hydrogen. In the first region, if the total area of the first precipitate and the second precipitate together is 2.0% or more of the area of the first region, the density of the trap sites of non-diffusible hydrogen can be sufficiently increased.

[0113] Also, the concentration of nitrogen contained in the first region is preferably 0.3 mass% or more. In this way, excellent wear resistance can be obtained as shown by Samples 3 and 4. The compressive residual stress in the first region is preferably 80 MPa or more. In this way, excellent wear resistance can be obtained as shown by Samples 3 and 4.

[0114] For example, when the compressive residual stress is 80 MPa or more, the shear deformation due to dislocation movement is relatively less likely to occur compared to the case where there is no residual stress. Therefore, the progress of hydrogen embrittlement of the material is suppressed.

[0115] Among the above samples, Sample 3 in particular satisfies all the conditions considered preferable in the embodiments of the present case. Specifically, Sample 3 combines all of the hydrogen intrusion suppression effect due to the improvement of wear resistance, the hydrogen intrusion suppression effect due to the addition of molybdenum and vanadium and nitriding treatment, and the hydrogen embrittlement suppression effect due to the low volume ratio of retained austenite. Furthermore, Sample 3 has a hydrogen embrittlement suppression effect due to an appropriate value of compressive residual stress. Therefore, if a rolling member for a hydrogen utilization device is manufactured under the conditions of Sample 3, the rolling parts can be significantly extended in service life when used in these devices.

[0116] (Supplementary Note) The above-described embodiments include the following configurations.

[0117] <Supplementary Note 1> A rolling member made of steel having a surface, The rolling member, is provided with a surface layer portion on the surface, The steel contains 0.80% by mass or more and 1.10% by mass or less of carbon, 0.15% by mass or more and 0.50% by mass or less of silicon, 0.30% by mass or more and 0.70% by mass or less of manganese, 1.30% by mass or more and 1.60% by mass or less of chromium, 0.10% by mass or more and 0.50% by mass or less of molybdenum, and 0.12% by mass or more and 0.50% by mass or less of vanadium, with the balance being iron and inevitable impurities, The surface layer portion contains a solid solution of iron and nitrogen, The structure in the first region of the surface layer portion where the depth from the surface is 5 μm or less is a structure other than cementite, In the second region of the surface layer portion where the depth from the surface is 100 μm or less, the volume ratio of the retained austenite amount is less than 20%, In the first region, at least one of a first precipitate mainly composed of at least one of chromium and vanadium and a second precipitate mainly composed of at least one of manganese and silicon is dispersed as a precipitate, A rolling member in which the first precipitate and the second precipitate occupy a region having an area ratio of 2.0% or more in the first region.

[0118] <Supplementary Note 2> The rolling member according to Supplementary Note 1, wherein in the first region, the maximum grain size of the martensite block grains is 5.0 μm or less.

[0119] <Supplementary Note 3> The rolling member according to Supplementary Note 1 or 2, wherein in the first region, the maximum grain size of at least one of the first precipitate and the second precipitate is 2.0 μm or less.

[0120] <Appended Note 4> The rolling member according to any one of appended notes 1 to 3, wherein the concentration of nitrogen contained in the first region is 0.3% by mass or more.

[0121] <Appended Note 5> The rolling member according to any one of appended notes 1 to 4, wherein the compressive residual stress in the first region is 80 MPa or more.

[0122] <Appended Note 6> The rolling member according to any one of appended notes 1 to 5, wherein the rolling member is used in a hydrogen utilization device.

[0123] <Appended Note 7> A rolling bearing comprising: A raceway member; Rolling elements arranged in contact with the raceway member, wherein the rolling bearing is for a hydrogen utilization device, and at least one of the raceway member and the rolling elements is the rolling member according to any one of appended notes 1 to 6.

[0124] <Appended Note 8> A steel rolling member having a surface, wherein the rolling member is for a hydrogen utilization device, the surface is provided with a surface layer portion, the steel contains 0.80% by mass or more and 1.10% by mass or less of carbon, 0.15% by mass or more and 0.50% by mass or less of silicon, 0.30% by mass or more and 0.70% by mass or less of manganese, 1.30% by mass or more and 1.60% by mass or less of chromium, 0.10% by mass or more and 0.50% by mass or less of molybdenum, and 0.12% by mass or more and 0.50% by mass or less of vanadium, with the balance being iron and inevitable impurities, the surface layer portion contains a solid solution of nitrogen in iron, the structure in the first region of the surface layer portion with a depth from the surface of 5 μm or less is a structure other than cementite, in the second region of the surface layer portion with a depth from the surface of 100 μm or less, the volume ratio of the retained austenite amount is less than 20%. In the first region, at least one of a first precipitate mainly composed of at least one of chromium and vanadium and a second precipitate mainly composed of at least one of manganese and silicon is dispersed as a precipitate. In the first region, the first precipitate and the second precipitate occupy a region having an area ratio of 2.0% or more in the first region. A rolling member.

[0125] <Appendix 9> In the first region, the maximum grain size of martensite blocks is 5.0 μm or less. The rolling member according to Appendix 8.

[0126] <Appendix 10> In the first region, the maximum grain size of at least one of the first precipitate and the second precipitate is 2.0 μm or less. The rolling member according to Appendix 8 or 9.

[0127] <Appendix 11> The concentration of nitrogen contained in the first region is 0.3% by mass or more. The rolling member according to any one of Appendices 8 to 10.

[0128] <Appendix 12> The compressive residual stress in the first region is 80 MPa or more. The rolling member according to any one of Appendices 8 to 11.

[0129] <Appendix 13> A rolling bearing, An orbital member, And a rolling element disposed in contact with the orbital member. The rolling bearing is for a hydrogen utilization device, At least one of the orbital member and the rolling element is the rolling member according to any one of Appendices 8 to 12. A rolling bearing.

[0130] Although the embodiments and examples of the present disclosure have been described as above, it is also possible to variously modify the above-described embodiments. Further, the scope of the present disclosure is not limited to the above-described embodiments and examples. The scope of the present disclosure is indicated by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

Explanation of Reference Numerals

[0131] 10 inner ring, 10a first surface, 10aa raceway surface, 10b second surface, 10c inner peripheral surface, 10d outer peripheral surface, 20 outer ring, 20a first surface, 20aa raceway surface, 20b second surface, 20c inner peripheral surface, 20d outer peripheral surface, 30 rolling element, 40 cage, 50 surface layer portion, 100 rolling bearing, 200 ball valve, 210 body, 220 seat retainer, 220a internal space, 220aa seal portion, 220b, 220c flow path, 230 ball, 231, 232 stem, 240 sliding bearing, 300 hydrogen circulation pump, 310 motor housing, 320 pump housing, 320a pump chamber, 331, 332 rotating shaft, 341 motor stator, 342 motor rotor, 351, 352 gear, 361, 362 rotor, 371, 372, 373, 374, 375, 376 rolling bearing, A central axis.

Claims

1. A steel rolling element having a surface, wherein the rolling element has a surface layer on the surface, the steel contains 0.80% by mass or more and 1.10% by mass or less of carbon, 0.15% by mass or more and 0.50% by mass or less of silicon, 0.30% by mass or more and 0.70% by mass or less of manganese, 1.30% by mass or more and 1.60% by mass or less of chromium, 0.10% by mass or more and 0.50% by mass or less of molybdenum, and 0.12% by mass or more and 0.50% by mass or less of vanadium, with the balance being iron and unavoidable impurities, the surface layer contains a solid solution of nitrogen in iron, the structure in the first region of the surface layer with a depth from the surface of 5 μm or less is a structure other than cementite, in the second region of the surface layer with a depth from the surface of 100 μm or less, the volume ratio of the retained austenite amount is less than 20%, in the first region, at least one of a first precipitate mainly composed of at least one of chromium and vanadium and a second precipitate mainly composed of at least one of manganese and silicon is dispersed as a precipitate, a rolling element in which, in the first region, the first precipitate and the second precipitate occupy a region having an area ratio of 2.0% or more in the first region.

2. The rolling element according to claim 1, wherein in the first region, the maximum grain size of the martensite block grains is 5.0 μm or less.

3. The rolling element according to claim 1 or 2, wherein in the first region, the maximum grain size of at least one of the first precipitate and the second precipitate is 2.0 μm or less.

4. The rolling element according to claim 1 or 2, wherein the nitrogen concentration contained in the first region is 0.3% by mass or more.

5. The rolling element according to claim 1 or 2, wherein the compressive residual stress in the first region is 80 MPa or more.

6. The rolling element according to claim 1 or 2, wherein the rolling element is used in a hydrogen utilization device.

7. A rolling bearing, comprising a raceway member, and rolling elements arranged in contact with the raceway member, wherein the rolling bearing is for a hydrogen utilization device, and at least one of the raceway member and the rolling elements is the rolling element according to claim 1 or 2.

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