JP7777968B2 - Rolling parts and rolling bearings - Google Patents

Description

The present invention relates to rolling components and rolling bearings. More specifically, the present invention relates to rolling components and rolling bearings for hydrogen-utilizing equipment.

Patent Document 1 (Japanese Patent No. 3990212) describes a rolling component. The rolling component described in Patent Document 1 is made of SUJ2, a high-carbon chromium bearing steel specified in the JIS standard. The bearing component described in Patent Document 1 is formed by nitriding, quenching, and tempering. The bearing component described in Patent Document 1 has high wear resistance due to the dispersion of cementite and other materials in the steel.

Patent Document 2 (JP 2000-234145 A) describes a rolling part. The rolling part described in Patent Document 2 is made of steel, and the amount of retained austenite in the steel is high on the surface. As a result, the rolling part described in Patent Document 2 is less likely to develop indentation-initiated flaking when used in an environment where foreign matter is likely to be mixed in.

Patent No. 3990212 Japanese Patent Application Laid-Open No. 2000-234145

However, the rolling parts described in Patent Document 1 and Patent Document 2 leave room for improvement in terms of durability under harsh usage environments, particularly environments exposed to hydrogen. More specifically, the rolling parts described in Patent Document 1 and Patent Document 2 may have insufficient durability under harsh usage environments due to dimensional changes associated with the decomposition of retained austenite and hydrogen embrittlement associated with hydrogen penetration from the surface.

The present invention was made in consideration of the problems with the prior art described above. More specifically, the present invention provides a rolling component that can suppress dimensional changes associated with the decomposition of retained austenite and hydrogen embrittlement associated with hydrogen penetration from the surface.

A rolling element according to a first aspect of the present invention has a surface and is made of steel. The rolling element has a surface layer portion that is a region extending from the surface to a depth of 20 μm. The rolling element is for use in hydrogen-utilizing equipment. The steel contains 0.70 to 1.10 mass percent carbon, 0.15 to 0.35 mass percent silicon, 0.30 to 0.60 mass percent manganese, 1.30 to 1.60 mass percent chromium, 0.01 to 0.50 mass percent molybdenum, and 0.01 to 0.50 mass percent vanadium, with the balance consisting of iron and inevitable impurities. The nitrogen concentration in the steel in the surface layer portion is 0.2 mass percent or greater. Precipitates primarily composed of chromium or vanadium are present in the steel in the surface layer portion. The hardness of the steel at a distance of 50 μm from the surface is 64 HRC or greater. The amount of retained austenite in the steel at a distance of 50 μm from the surface is less than 20 volume percent.

A rolling element according to a second aspect of the present invention has a surface and is made of steel. The rolling element has a surface layer portion that is a region up to 20 μm deep from the surface. The rolling element is for use in hydrogen-utilizing equipment. The steel contains 0.70 to 1.10 mass percent carbon, 0.15 to 0.35 mass percent silicon, 0.30 to 0.60 mass percent manganese, 1.30 to 1.60 mass percent chromium, 0.01 to 0.50 mass percent molybdenum, and 0.01 to 0.50 mass percent vanadium, with the balance consisting of iron and inevitable impurities. The nitrogen concentration in the steel in the surface layer portion is 0.2 mass percent or more. Precipitates primarily composed of chromium or vanadium are precipitated in the steel in the surface layer portion. The average grain size of martensite block grains at the top 50 percent area ratio in the surface layer is 1.3 μm or less. The hardness of the steel at a position 50 μm from the surface is 64 HRC or greater. The amount of retained austenite in the steel at a position 50 μm from the surface is less than 25 volume percent.

In the above rolling part, the steel may contain 0.90 to 1.10 percent by mass of carbon, 0.20 to 0.30 percent by mass of silicon, 0.40 to 0.50 percent by mass of manganese, 1.40 to 1.60 percent by mass of chromium, 0.20 to 0.30 percent by mass of molybdenum, and 0.20 to 0.30 percent by mass of vanadium, with the remainder consisting of iron and unavoidable impurities.

In the above rolling component, the maximum particle size of the precipitates may be 1.0 μm or less. In the above rolling component, the average area ratio of the precipitates may be 2.0 percent or more. In the above rolling component, the hardness of the steel at a position 50 μm from the surface may be 65.5 HRC or more.

The rolling bearing according to the present invention comprises an inner ring, an outer ring, and rolling elements. The rolling bearing is for use in hydrogen-utilizing equipment. At least one of the inner ring, outer ring, and rolling elements is a rolling component as described above.

The rolling components and rolling bearings of the present invention can suppress dimensional changes associated with the decomposition of retained austenite and hydrogen embrittlement associated with hydrogen penetration from the surface.

FIG. 1 is a cross-sectional view of a rolling bearing 100. FIG. 2 is an enlarged cross-sectional view of the ball valve 200. FIG. 3 is a cross-sectional view of a hydrogen circulation pump 300. 3A to 3C are process diagrams showing a manufacturing method of the rolling bearing 100. 10A to 10C are process diagrams showing a modified example of the method for manufacturing the rolling bearing 100. 1 is a cross-sectional SEM image of Sample 1 near the raceway surface. 10 is a cross-sectional SEM image of Sample 3 near the raceway surface. 1 is a phase map of a cross section of Sample 1 near the orbital plane by EBSD. 1 is a phase map of a cross section of Sample 2 near the orbital plane by EBSD. 10 is a phase map of a cross section of Sample 3 near the orbital plane by EBSD. 10 is a phase map of a cross section of Sample 4 near the orbital plane by EBSD. 1 is a graph showing the average grain size of martensite block grains in steel in a region up to a depth of 20 μm from the raceway surface of Samples 1 to 4.

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

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

The rolling bearing 100 is for use in hydrogen-utilizing equipment. The hydrogen-utilizing equipment is, for example, a ball valve or compressor for a hydrogen station. The type of compressor is not particularly limited. For example, the compressor may be of a reciprocating type (reciprocating), rotary type (screw), centrifugal type, or axial type. The hydrogen-utilizing equipment may also be a high-pressure hydrogen pressure reducing valve or hydrogen circulation pump for a fuel cell vehicle. The rolling bearing 100 may be used in any application where it is exposed to hydrogen.

Figure 1 is a cross-sectional view of a rolling bearing 100. As shown in Figure 1, the rolling bearing 100 has a central axis A. Figure 1 shows a cross section parallel to and passing through the central axis A. The rolling bearing 100 has an inner ring 10, an outer ring 20, multiple rolling elements 30, and a cage 40. The inner ring 10 and the outer ring 20 are ring-shaped. The rolling elements 30 are balls (spherical).

The direction along the central axis A is the axial direction. The direction passing through the central axis A and perpendicular to the central axis A is the radial direction. The direction along the circumference of a circle centered on the central axis A is the circumferential direction.

The inner ring 10 has a first end face 10a, a second end face 10b, an inner peripheral surface 10c, and an outer peripheral surface 10d. The first end face 10a, the second end face 10b, the inner peripheral surface 10c, and the outer peripheral surface 10d form the surface of the inner ring 10. The first end face 10a and the second end face 10b are the end faces of the inner ring 10 in the axial direction. The second end face 10b is the surface opposite the first end face 10a.

The inner peripheral surface 10c extends in the circumferential direction. The inner peripheral surface 10c faces the central axis A. Although not shown, the inner ring 10 is fitted to a shaft at the inner peripheral surface 10c. The inner peripheral surface 10c is continuous with the first end face 10a at one axial end and with the second end face 10b at the other axial end.

The outer peripheral surface 10d extends in the circumferential direction. The outer peripheral surface 10d faces away from the central axis A. In other words, the outer peripheral surface 10d is the opposite surface of the inner peripheral surface 10c in the radial direction. The outer peripheral surface 10d is connected to the first end surface 10a at one axial end and to the second end surface 10b at the other axial end.

The outer peripheral surface 10d has a raceway surface 10da. The raceway surface 10da is the portion of the outer peripheral surface 10d that contacts the rolling elements 30. The raceway surface 10da extends in the circumferential direction. The raceway surface 10da is located in the center of the outer peripheral surface 10d in the axial direction. In a cross-sectional view, the raceway surface 10da has a partial arc shape that is recessed toward the inner peripheral surface 10c.

The outer ring 20 has a first end face 20a, a second end face 20b, an inner peripheral surface 20c, and an outer peripheral surface 20d. The first end face 20a, the second end face 20b, the inner peripheral surface 20c, and the outer peripheral surface 20d form the surface of the outer ring 20. The outer ring 20 is disposed radially outward of the inner ring 10, with the inner peripheral surface 20c facing the outer peripheral surface 10d at a distance.

The first end face 20a and the second end face 20b are end faces of the outer ring 20 in the axial direction. The second end face 20b is the opposite face to the first end face 20a.

The inner peripheral surface 20c extends in the circumferential direction. The inner peripheral surface 20c faces the central axis A. The inner peripheral surface 20c is connected to the first end face 20a at one axial end and to the second end face 20b at the other axial end.

The inner peripheral surface 20c has a raceway surface 20ca. The raceway surface 20ca is the portion of the inner peripheral surface 20c that contacts the rolling elements 30. The raceway surface 20ca extends in the circumferential direction. The raceway surface 20ca is located in the center of the inner peripheral surface 20c in the axial direction. In a cross-sectional view, the raceway surface 20ca has a partial arc shape that is recessed toward the outer peripheral surface 20d.

The outer peripheral surface 20d extends in the circumferential direction. The outer peripheral surface 20d faces away from the central axis A. In other words, the outer peripheral surface 20d is the opposite surface of the inner peripheral surface 20c in the radial direction. Although not shown, the outer ring 20 is fitted into the housing at the outer peripheral surface 20d. The outer peripheral surface 20d is continuous with the first end surface 20a at one axial end and with the second end surface 20b at the other axial end.

The rolling elements 30 are arranged between the outer peripheral surface 10d and the inner peripheral surface 20c, more specifically, between the raceway surface 10da and the raceway surface 20ca. The multiple rolling elements 30 are lined up in the circumferential direction. Each rolling element 30 has a surface 30a. The cage 40 holds the multiple rolling elements 30. The cage 40 holds the multiple rolling elements 30 so that the circumferential distance between two adjacent rolling elements 30 is within a certain range.

The inner ring 10, outer ring 20, and rolling elements 30 are made of steel. More specifically, the inner ring 10, outer ring 20, and rolling elements 30 are formed from steel having the composition shown in Table 1 (referred to as the "first composition").

Carbon affects the hardness of the steel on the surface of the rolling components (inner ring 10, outer ring 20, and rolling elements 30) after quenching. If the carbon content in the steel is less than 0.70 mass percent, it is difficult to ensure sufficient hardness on the surface of the rolling components. If the carbon content in the steel is less than 0.70 mass percent, it is necessary to supplement the carbon content on the surface of the rolling components through carburizing or other treatments, which reduces production efficiency and increases manufacturing costs. On the other hand, if the carbon content in the steel exceeds 1.10 mass percent, there is a risk of cracks (quench cracks) occurring during quenching. For this reason, the carbon content in the steel of the first composition is set to be 0.70 mass percent or more and 1.10 mass percent or less.

Silicon is added to ensure workability before deoxidation and nitriding during steel refining. If the silicon content in the steel is less than 0.15 mass percent, tempering softening resistance will be insufficient. As a result, the hardness of the surface of the rolling component may decrease due to tempering after quenching or due to temperature increases during use of the rolling bearing 100. In this case, the workability of the rolling component will also be insufficient.

If the silicon content in the steel exceeds 0.35 mass percent, the steel becomes too hard, and its workability when processing rolling components decreases. This also increases the material cost of the steel. Therefore, the silicon content of steel with the first composition is set to 0.15 mass percent or more and 0.35 mass percent or less.

Manganese is added to ensure the hardenability and hardness of the steel. If the manganese content in the steel is less than 0.30 mass percent, it is difficult to ensure the hardenability of the steel. If the manganese content in the steel exceeds 0.60 mass percent, the amount of manganese-based non-metallic inclusions, which are impurities, increases. For this reason, the manganese content in the steel of the first composition is set to 0.30 mass percent or more and 0.60 mass percent or less.

Chromium is added to ensure the hardenability of the steel and to form fine precipitates (nitrides, carbonitrides) during nitriding. If the chromium content in the steel is less than 1.30 mass percent, it is difficult to ensure the hardenability of the steel and to form sufficient fine precipitates. If the chromium content in the steel exceeds 1.60 mass percent, the material cost of the steel increases. Therefore, the chromium content in the first composition steel is set to 1.30 mass percent or more and 1.60 mass percent or less.

Molybdenum is added to ensure the hardenability of steel and to form fine precipitates during nitriding. Molybdenum has a strong affinity for carbon, so it precipitates in the steel as undissolved carbides during nitriding. These undissolved molybdenum carbides act as precipitation nuclei during quenching, so molybdenum increases the amount of precipitates after quenching.

If the molybdenum content in the steel is less than 0.01 mass percent, it is difficult to ensure the hardenability of the steel and to form sufficient fine precipitates. If the molybdenum content in the steel exceeds 0.50 mass percent, the material cost of the steel increases. Therefore, the molybdenum content in the steel of the first composition is set to 0.01 mass percent or more and 0.50 mass percent or less.

Vanadium is added to ensure the hardenability of the steel and to form fine precipitates during nitriding. If the vanadium content in the steel is less than 0.01 percent by mass, it is difficult to ensure the hardenability of the steel and to form sufficient fine precipitates. If the vanadium content in the steel exceeds 0.50 percent by mass, the material cost of the steel increases. Therefore, the vanadium content in the first composition steel is set to 0.01 percent by mass or more and 0.50 percent by mass or less.

The inner ring 10, outer ring 20, and rolling elements 30 may be formed from steel of the composition shown in Table 2 (referred to as the "second composition"). Note that it is not necessary for all of the inner ring 10, outer ring 20, and rolling elements 30 to be formed from steel of the first or second composition; it is sufficient if at least one of the inner ring 10, outer ring 20, and rolling elements 30 is formed from steel of the first or second composition.

As shown in FIG. 1, the inner ring 10, outer ring 20, and rolling elements 30 have surface layer portions 50. In the inner ring 10, the surface layer portion 50 is the region extending from the surface of the inner ring 10 to a depth of 20 μm. In the outer ring 20, the surface layer portion 50 is the region extending from the surface of the outer ring 20 to a depth of 20 μm. In the rolling elements 30, the surface layer portion 50 is the region extending from the surface 30a to a depth of 20 μm. Note that in the inner ring 10, the surface layer portion 50 needs to be formed on at least the raceway surface 10da, and in the outer ring 20, the surface layer portion 50 needs to be formed on the raceway surface 20ca.

Furthermore, the surface layer 50 does not need to be formed on all of the surfaces of the inner ring 10, the outer ring 20, and the rolling elements 30; it is sufficient if it is formed on at least one of the surfaces of the inner ring 10, the outer ring 20, and the rolling elements 30.

The surface layer 50 is formed by nitriding. The nitrogen concentration in the steel in the surface layer 50 is 0.2 mass percent or more. The nitrogen concentration in the steel in the surface layer 50 is measured using an electron probe micro analyzer (EPMA). The nitrogen concentration in the steel in the surface layer 50 is, for example, 0.5 mass percent or less.

Precipitates are present in the steel in the surface layer 50. The precipitates are primarily composed of chromium or vanadium. The precipitates are nitrides primarily composed of chromium or vanadium. The precipitates may also be carbonitrides primarily composed of chromium or vanadium. The precipitates may also be a mixture of the above nitrides and carbonitrides.

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

The maximum particle size of the precipitates is preferably 1.0 μm or less. The average area ratio of the precipitates is preferably 2.0 percent or more.

The average area ratio of precipitates is calculated by acquiring a cross-sectional image of the surface layer portion 50 at a magnification of 5000x using a field emission scanning electron microscope (FE-SEM), binarizing the cross-sectional image, and performing image processing on the binarized cross-sectional image. Cross-sectional images of the surface layer portion 50 are acquired from three or more fields of view, and the average area ratio is the average value of the area ratios of precipitates obtained from these multiple cross-sectional images.

The particle size of each precipitate is obtained by obtaining the area of each precipitate using the same method as above, and then dividing that area by π and multiplying the square root of the result by 2. The largest particle size among the obtained precipitates is then considered to be the maximum particle size of the precipitate.

The hardness of the steel at a position 50 μm deep from the surface of the inner ring 10, a position 50 μm deep from the surface of the outer ring 20, and a position 50 μm deep from the surface (surface 30a) of the rolling element 30 is 64 HRC or more. The hardness of the steel at a position 50 μm deep from the surface of the inner ring 10, the surface of the outer ring 20, and the surface (surface 30a) of the rolling element 30 may be 65.5 HRC or more.

The hardness of the steel at a position 50 μm deep from the surface of the inner ring 10, a position 50 μm deep from the surface of the outer ring 20, and a position 50 μm deep from the surface (surface 30a) of the rolling element 30 is measured using the Rockwell hardness test method specified in the JIS standard (JIS Z 2245:2016).

The steel in the surface layer 50 has martensite block grains. The difference in crystal orientation between two adjacent martensite block grains at the grain boundary is 15° or more. From another perspective, even if there is a location where the crystal orientation is misaligned, if the difference in crystal orientation is less than 15°, that location is not considered to be a martensite block grain boundary. The grain boundaries of martensite block grains are determined using EBSD (Electron Backscattered Diffraction) analysis.

In the steel of the surface layer 50, it is preferable that the average grain size of the martensite block grains at an upper area ratio of 50 percent is 1.3 μm or less. In the steel of the surface layer 50, it is preferable that the average grain size of the martensite block grains at an upper area ratio of 30 percent is 1.6 μm or less.

The average grain size of martensite block grains when the upper area ratio is 50 percent (30 percent) is measured using the following method. First, a cross-section including the surface layer 50 is observed. At this time, the martensite block grains included in the observation field are identified using the EBSD method. This observation field is an area observed at a magnification of 1500x. Second, the area of each martensite block grain included in the observation field is analyzed using the crystal orientation data obtained using the EBSD method.

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

The amount of retained austenite in the steel at a position 50 μm deep from the surface of the inner ring 10, a position 50 μm deep from the surface of the outer ring 20, and a position 50 μm deep from the surface (surface 30a) of the rolling elements 30 is, for example, less than 20 volume percent. The amount of retained austenite in the steel at a position 50 μm deep from the surface of the inner ring 10, a position 50 μm deep from the surface of the outer ring 20, and a position 50 μm deep from the surface (surface 30a) of the rolling elements 30 may be less than 25 volume percent.

The amount of retained austenite in the steel at a position 50 μm deep from the surface of the inner ring 10, a position 50 μm deep from the surface of the outer ring 20, and a position 50 μm deep from the surface (surface 30a) of the rolling elements 30 is measured using X-ray diffraction. More specifically, the amount of retained austenite in the steel at a position 50 μm deep from the surface of the inner ring 10, a position 50 μm deep from the surface of the outer ring 20, and a position 50 μm deep from the surface (surface 30a) of the rolling elements 30 is measured using an MSF-3M manufactured by Rigaku Corporation.

In the above, we have described the case where the rolling member according to the embodiment is a component of the rolling bearing 100, but the rolling member according to the embodiment may also be a sliding bearing.

<Application examples of rolling members according to embodiments>
The rolling member according to the embodiment is used in a ball valve 200. Fig. 2 is an enlarged cross-sectional view of the ball valve 200. As shown in Fig. 2, the ball valve 200 includes a body 210, a seat retainer 220, a ball 230, stems 231 and 232, and a plain bearing 240.

The seat retainer 220 is disposed inside the body 210. The seat retainer 220 has an internal space 220a and flow paths 220b and 220c. 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.

Stems 231 and 232 are connected to the upper and lower ends of ball 230, respectively. As stems 231 and 232 rotate around their central axis, flow paths 220b and 220c are connected via a through hole (not shown) formed in ball 230. Stems 231 and 232 are passed through through holes formed in body 210 and seat retainer 220. Hydrogen flows through flow paths 220b, 220c, and the through holes formed in ball 230.

The plain bearing 240 is cylindrical and is attached to the body 210 at its outer circumferential surface. The plain bearing 240 rotatably supports the stem 231 (stem 232). The plain bearing 240 is a rolling member according to the embodiment. That is, the plain bearing 240 is made of steel of the first composition or the second composition, and a surface layer 50 is formed on the contact surface.

<Application examples of rolling bearings according to embodiments>
3 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, rotating shafts 331 and 332, a motor stator 341 and a motor rotor 342, gears 351 and 352, rotors 361 and 362, and rolling bearings 371, 372, 373, 374, 375, and 376.

The motor housing 310 is attached to the pump housing 320. One end of the rotating shaft 331 is disposed within the motor housing 310, and the other end 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. Between one end and the other end, the rotating shaft 331 is rotatably supported by a rolling bearing 373 and a rolling bearing 374 disposed within the pump housing 320.

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 at a position away from the one end by a rolling bearing 376 disposed within the pump housing 320.

Motor stator 341 is disposed within motor housing 310. Motor rotor 342 is attached to rotating shaft 331 so as to face motor stator 341. Rotating shaft 331 is rotated by motor stator 341 and motor rotor 342. Gears 351 and 352 are attached to rotating shaft 331 and rotating shaft 332, respectively. The rotation of rotating shaft 331 is transmitted to rotating shaft 332 by gears 351 and 352. Gear 351 is located between rolling bearing 373 and rolling bearing 374, and gear 352 is located between rolling bearing 375 and rolling bearing 376.

A pump chamber 320a is formed within the pump housing 320. Rotors 361 and 362 are disposed within the pump chamber 320a. Rotors 361 and 362 are attached to rotating shafts 331 and 332, respectively. Rotor 361 rotates with the rotation of rotating shaft 331, and rotor 362 rotates with the rotation of rotating shaft 332, causing hydrogen to be drawn into and discharged from pump chamber 320a.

Rolling bearings 371, 372, 373, and 375 are deep groove ball bearings. Rolling bearings 374 and 375 are double-row angular contact ball bearings. Rolling bearings 371, 372, 373, 374, 375, and 376 are rolling bearings according to an embodiment. That is, in rolling bearings 371, 372, 373, 374, 375, and 376, the raceway members and rolling elements are made of steel of the first composition or the second composition, and surface layer 50 is formed on the contact surfaces.

(Method for manufacturing rolling bearing according to embodiment)
A method for manufacturing the rolling bearing 100 will be described below.

Figure 4 is a process diagram showing the manufacturing method of rolling bearing 100. As shown in Figure 4, the manufacturing method of rolling bearing 100 includes a preparation process S1, a nitriding process S2, a first quenching process S3, a first tempering process S4, a second quenching process S5, a second tempering process S6, a post-treatment process S7, and an assembly process S8.

In the preparation step S1, a workpiece to be processed is prepared. As the workpiece, a ring-shaped member is prepared if the inner ring 10 and outer ring 20 are to be formed, and a spherical member is prepared if the rolling element 30 is to be formed. This workpiece is formed from steel of the first composition or the second composition.

In the nitriding treatment step S2, the surface of the workpiece is subjected to a nitriding treatment. This nitriding treatment is carried out by holding the workpiece at a temperature equal to or higher than the _A1 transformation point for a predetermined time in an atmospheric gas containing a nitrogen source gas (e.g., ammonia gas). In the first quenching step S3, the workpiece is quenched. This quenching is carried out by holding the workpiece at a temperature equal to or higher than the _A1 transformation point for a predetermined time and then cooling the workpiece to a temperature equal to or lower than the Ms transformation point.

In the first tempering step S4, the workpiece is tempered by holding the workpiece at a temperature below the _A1 transformation point for a predetermined period of time.

In the second quenching step S5, the workpiece is quenched by holding the workpiece at a temperature equal to or higher than the _A1 transformation point for a predetermined time and then cooling the workpiece to a temperature equal to or lower than the Ms transformation point.

In the second tempering step S6, the workpiece is tempered by heating and holding the workpiece at a temperature below the _A1 transformation point for a predetermined period of time.

In the post-processing step S7, the workpiece is subjected to finishing (grinding and polishing) and cleaning. This results in the inner ring 10, outer ring 20, and rolling elements 30. In the assembly step S8, the inner ring 10, outer ring 20, and rolling elements 30 are assembled together with the cage 40. This completes the manufacturing of the rolling bearing 100 shown in Figure 1.

The holding temperature in the second quenching step S5 is lower than the holding temperatures in the nitriding step S2 and the first quenching step S3. The holding temperatures in the nitriding step S2 and the first quenching step S3 are, for example, 850°C. The holding temperature in the second quenching step S5 is, for example, 810°C. The holding temperature and holding time in the first tempering step S4 and the second tempering step S6 are, for example, 180°C and 2 hours, respectively.

Figure 5 is a process diagram showing a modified example of the manufacturing method for rolling bearing 100. As shown in Figure 5, the manufacturing method for rolling bearing 100 does not necessarily have to include the first tempering step S4, and may instead include a sub-zero treatment step S9 instead of the second quenching step S5. The sub-zero treatment step S9 is performed by cooling the workpiece to a temperature of, for example, -100°C or higher and room temperature or lower.

(Effects of the rolling bearing according to the embodiment)
The effects of the rolling bearing 100 will be described below.

In the rolling bearing 100, the inner ring 10, outer ring 20, and rolling elements 30 are formed from steel of the first composition or the second composition, so fine precipitates are precipitated in the steel in the surface layer 50. This ensures the hardness of the steel on the surfaces of the inner ring 10, outer ring 20, and rolling elements 30 (more specifically, the hardness of the steel at a position 50 μm deep from the surface of the inner ring 10, the outer ring 20, and the rolling elements 30 can be made 64 HRC or higher), and also prevents these precipitates from becoming a source of stress concentration (a starting point for crack initiation).

In the rolling bearing 100, fine precipitates are formed in the steel in the surface layer 50, ensuring the hardness of the steel on the surfaces of the inner ring 10, outer ring 20, and rolling elements 30. This prevents new metal surfaces from forming on the surfaces of the inner ring 10, outer ring 20, and rolling elements 30. Therefore, in the rolling bearing 100, hydrogen is less likely to be generated on the surfaces of the inner ring 10, outer ring 20, and rolling elements 30.

In the rolling bearing 100, the areas near the fine precipitates that have precipitated in the steel in the surface layer 50 become hydrogen trapping sites, reducing the amount of hydrogen that penetrates into the surface layer 50. As a result, the rolling bearing 100 is less susceptible to early spalling damage caused by hydrogen embrittlement.

In the rolling bearing 100, the amount of retained austenite in the steel at a position 50 μm deep from the surface of the inner ring 10, a position 50 μm deep from the surface of the outer ring 20, and a position 50 μm deep from the surface of the rolling elements 30 is less than 20 volume percent (or less than 25 volume percent). This makes it possible to suppress dimensional changes in the inner ring 10, outer ring 20, and rolling elements 30 due to decomposition of retained austenite as the temperature rises during use.

When the martensite block grains in the steel of the surface layer 50 are refined so that the average grain size at the top 50 percent of the surface area is 1.3 μm or less, the increased toughness of the surface layer 50 improves the shear resistance near the surfaces of the inner ring 10, outer ring 20, and rolling elements 30. Therefore, in this case, the durability of the rolling bearing 100 can be further improved.

(Example)
Samples 1 to 4 were prepared as bearing ring samples. Samples 1 and 2 were made of steel having the compositions shown in Table 3, and Samples 3 and 4 were made of steel having the compositions shown in Table 4. The steel compositions shown in Table 3 correspond to the first composition (second composition), and the composition shown in Table 4 corresponds to the composition of SUJ2, a high-carbon chromium bearing steel specified in the JIS standard.

Sample 1 underwent the nitriding treatment process S2, the first quenching process S3, the subzero treatment process S9, and the second tempering process S6. Sample 2 underwent the nitriding treatment process S2, the first quenching process S3, the first tempering process S4, the second quenching process S5, and the second tempering process S6. Sample 3 underwent the nitriding treatment process S2, the first quenching process S3, and the second tempering process S6. Sample 4 underwent the first quenching process S3 and the first tempering process S4.

Figure 6 is a cross-sectional SEM image of the vicinity of the raceway surface of Sample 1. The particles protruding in Figure 6 are precipitates primarily composed of chromium or vanadium. Figure 7 is a cross-sectional SEM image of the vicinity of the raceway surface of Sample 3. The particles protruding in Figure 7 are precipitates primarily composed of chromium, and the gray oval particles in Figure 7 are cementite particles.

As shown in Figures 6, 7 and Table 5, in Samples 1 and 2, fine (maximum particle size 1.0 μm or less) and dense (average area ratio 2.0 percent or more) precipitates primarily composed of chromium or vanadium were found in the region up to 20 μm deep from the raceway surface. In Sample 3, precipitates primarily composed of chromium were found in the region up to 20 μm deep from the raceway surface, but these precipitates were coarse and the area ratio of these precipitates was small. In Sample 4, precipitates primarily composed of chromium or vanadium were not found in the region up to 20 μm deep from the raceway surface.

As shown in Table 5, in Samples 1 and 2, the nitrogen concentration in the steel in the region up to 20 μm deep from the raceway surface was 0.2 mass percent or more and 0.5 mass percent or less. In Sample 3, the nitrogen concentration in the steel in the region up to 20 μm deep from the raceway surface was 0.3 mass percent or more and 0.5 mass percent or less. In Sample 4, no nitrogen was contained in the steel in the region up to 20 μm deep from the raceway surface.

In Sample 1, the amount of retained austenite in the steel at a depth of 50 μm from the raceway surface was less than 20 volume percent, and in Sample 2, the amount of retained austenite in the steel at a depth of 50 μm from the raceway surface was less than 25 volume percent. In Sample 3, the amount of retained austenite in the steel at a depth of 50 μm from the raceway surface exceeded 25 volume percent, and in Sample 4, the amount of retained austenite in the steel at a depth of 50 μm from the raceway surface was less than 20 volume percent.

In Samples 1 and 2, the hardness of the steel at a depth of 50 μm from the raceway surface was 64 HRC or higher. In Samples 3 and 4, the hardness of the steel at a depth of 50 μm from the raceway surface was less than 64 HRC. This demonstrates that in Samples 1 and 2, fine, dense precipitates primarily composed of chromium or vanadium are present in the steel in the region up to 20 μm deep from the raceway surface, ensuring the hardness of the steel near the raceway surface and suppressing hydrogen generation at and penetration from the raceway surface. Furthermore, in Samples 1 and 2, the amount of retained austenite in the steel near the raceway surface was less than 20 volume percent or less than 25 volume percent, demonstrating that dimensional changes over time due to the decomposition of retained austenite were suppressed.

In Samples 1 and 2, the circumferential compressive residual stress near the raceway surface (at a position 50 μm deep from the raceway surface) was greater than in Sample 4. Furthermore, in Samples 1 and 2, the raceway surface resistance to indentation was superior to Samples 3 and 4.

Figure 8 is an EBSD phase map of a cross section near the raceway plane of Sample 1. Figure 9 is an EBSD phase map of a cross section near the raceway plane of Sample 2. Figure 10 is an EBSD phase map of a cross section near the raceway plane of Sample 3. Figure 11 is an EBSD phase map of a cross section near the raceway plane of Sample 4. In Figures 8 to 11, martensite block grains are indicated by the white areas.

Figure 12 is a graph showing the average grain size of martensite block grains in the steel in the region up to 20 μm deep from the raceway surface for Samples 1 to 4. The vertical axis of Figure 12 represents the average grain size (unit: μm) of martensite block grains in the steel in the region up to 20 μm deep from the raceway surface. As shown in Figures 8 to 12, in Samples 1 and 2, the average grain size of martensite block grains in the steel at an upper area fraction of 50 percent in the region up to 20 μm deep from the raceway surface was 1.3 μm or less, and the average grain size of martensite block grains in the steel at an upper area fraction of 30 percent in the region up to 20 μm deep from the raceway surface was 1.5 μm or less.

On the other hand, in Samples 3 and 4, the average grain size of the martensite block grains at an upper area ratio of 50 percent in the steel in the region up to 20 μm deep from the raceway surface exceeded 1.3 μm, and the average grain size of the martensite block grains at an upper area ratio of 30 percent in the steel in the region up to 20 μm deep from the raceway surface exceeded 1.5 μm. This demonstrates that in Samples 1 and 2, fine, high-density precipitates primarily composed of chromium or vanadium are precipitated in the steel in the region up to 20 μm deep from the raceway surface, resulting in finer martensite block grains, improved shear resistance near the raceway surface, and ultimately improved raceway durability.

<Hydrogen penetration characteristics>
The hydrogen penetration characteristics of the surface layer portions of Sample 1 and Sample 4 were evaluated by the following method. In this evaluation, first, Sample 1 and Sample 4 before use were heated from room temperature to 400° C., and the amount of hydrogen released from Sample 1 and Sample 4 before use was measured. Second, Sample 1 and Sample 4 after use for 50 hours in a hydrogen environment were heated from room temperature to 400° C., and the amount of hydrogen released from Sample 1 and Sample 4 after use for 50 hours in a hydrogen environment was measured.

For sample 4, the ratio of the amount of hydrogen released before and after use (i.e., the amount of hydrogen released after use divided by the amount of hydrogen released before use) was 3.0 or greater. On the other hand, for sample 1, the ratio of the amount of hydrogen released before and after use was within the range of 0.9 to 1.2. This comparison experimentally demonstrated that the formation of surface layer 50 on the contact surface suppresses hydrogen penetration into surface layer 50, thereby suppressing early peeling due to hydrogen embrittlement.

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

100 rolling bearing, 10 inner ring, 10a first end face, 10b second end face, 10c inner peripheral surface, 10d outer peripheral surface, 10da raceway surface, 20 outer ring, 20a first end face, 20b second end face, 20c inner peripheral surface, 20ca raceway surface, 20d outer peripheral surface, 30 rolling element, 30a surface, 40 cage, 50 surface portion, A central axis, 371, 372, 373, 374, 375, 376 rolling bearing, 200 ball valve, 210 body, 220 seat retainer, 220a internal space, 220aa seal portion, 220b flow path, 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, S1 preparation process, S2 nitriding process, S3 first quenching process, S4 first tempering process, S5 second quenching process, S6 second tempering process, S7 post-treatment process, S8 assembly process, S9 sub-zero treatment process.

Claims (7)

A steel rolling element having a surface,
a surface layer portion that is a region having a depth of up to 20 μm from the surface,
The rolling component is for use in hydrogen-utilizing equipment,
The steel contains 0.70 mass percent to 1.10 mass percent carbon, 0.15 mass percent to 0.35 mass percent silicon, 0.30 mass percent to 0.60 mass percent manganese, 1.30 mass percent to 1.60 mass percent chromium, 0.01 mass percent to 0.50 mass percent molybdenum, and 0.01 mass percent to 0.50 mass percent vanadium, with the balance being iron and unavoidable impurities;
The nitrogen concentration in the steel in the surface layer portion is 0.2 mass percent or more,
a precipitate containing chromium or vanadium as a main component is precipitated in the steel in the surface layer portion,
The hardness of the steel at a position 50 μm away from the surface is 64 HRC or more,
The rolling component, wherein the amount of retained austenite in the steel at a position 50 μm away from the surface is less than 20 volume percent. A steel rolling element having a surface,
a surface layer portion that is a region having a depth of up to 20 μm from the surface,
The rolling component is for use in hydrogen-utilizing equipment,
The steel contains 0.70 mass percent to 1.10 mass percent carbon, 0.15 mass percent to 0.35 mass percent silicon, 0.30 mass percent to 0.60 mass percent manganese, 1.30 mass percent to 1.60 mass percent chromium, 0.01 mass percent to 0.50 mass percent molybdenum, and 0.01 mass percent to 0.50 mass percent vanadium, with the balance being iron and unavoidable impurities;
The nitrogen concentration in the steel in the surface layer portion is 0.2 mass percent or more,
a precipitate containing chromium or vanadium as a main component is precipitated in the steel in the surface layer portion,
The average grain size of martensite block grains in the surface layer portion at an upper area ratio of 50% in the steel is 1.3 μm or less,
The hardness of the steel at a position 50 μm away from the surface is 64 HRC or more,
A rolling component, wherein the amount of retained austenite in the steel at a position 50 μm away from the surface is less than 25 volume percent. The rolling component according to claim 1 or 2, wherein the steel contains 0.90 to 1.10 mass percent carbon, 0.20 to 0.30 mass percent silicon, 0.40 to 0.50 mass percent manganese, 1.40 to 1.60 mass percent chromium, 0.20 to 0.30 mass percent molybdenum, and 0.20 to 0.30 mass percent vanadium, with the remainder consisting of iron and unavoidable impurities. The rolling component according to any one of claims 1 to 3, wherein the maximum particle size of the precipitates is 1.0 μm or less. A rolling component according to any one of claims 1 to 4, wherein the average area ratio of the precipitates is 2.0 percent or more. A rolling component according to any one of claims 1 to 5, wherein the hardness of the steel at a position 50 μm away from the surface is 65.5 HRC or higher. A rolling bearing,
With inner circle,
The outer ring and
a rolling element;
The rolling bearing is for use in hydrogen-utilizing equipment,
A rolling bearing, wherein at least one of the inner ring, the outer ring, and the rolling elements is the rolling component according to any one of claims 1 to 6.