Rolling member and rolling bearing

Abstract

This rolling member has a surface that serves as a contact surface with respect to another rolling member, and is made of a steel that has been quenched and tempered. The rolling member has a nitrided layer on the surface. The average nitrogen concentration in the surface of the rolling member is 0.10 mass% or more. The average carbon concentration in the surface is 0.60-1.5 mass%. The hardness of the surface is 800-1000 Hv. The retained austenite content in the surface is 20 vol% or less. In a cross-sectional view perpendicular to the surface, the area ratio of undissolved carbides having a particle size of 5.0 μm or less in the nitrided layer is 17% or less.

Description

Rolling members and rolling bearings

[0001] This invention relates to rolling members and rolling bearings.

[0002] From the perspective of reducing environmental impact, the electrification of automobiles is progressing. Specifically, in electric vehicles such as battery electric vehicles (BEVs) and hybrid electric vehicles (HEVs), electric axles are used instead of engines, and electric brakes, electric VTC (variable valve control), electric compressors, etc., are being applied.

[0003] In electric vehicles, miniaturization and weight reduction of units, as well as higher rotational speeds and higher output of motors, are desired in order to extend the driving range with less power, i.e., to improve energy efficiency. Accordingly, it is desirable that rolling components, including rolling bearings, also meet these requirements. For example, in rolling bearings, the amount of heat generated increases as motors rotate at higher speeds and become smaller. Also, in rolling bearings, it is highly likely that the viscosity of lubricating oil will decrease and the amount of grease will decrease in order to reduce torque. In that case, the amount of heat generated in the rolling bearings will increase even further. Thus, in rolling bearings used in electric axles, electric brakes, electric VTCs, and electric compressors of electric vehicles, the lubrication conditions become more severe, and the concern about peeling increases.

[0004] Even in rolling bearings used in the spindles of machine tools, surface damage such as peeling can lead to a decrease in machining accuracy, and continued operation with such surface damage can lead to seizure. In particular, from the perspective of reducing environmental impact, grease lubrication is sometimes used instead of air-oil lubrication in rolling bearings used in the spindles of rolling bearings. With grease lubrication, lubrication failure is prone to occur due to centrifugal force at high rotational speeds, so improved resistance to peeling is required.

[0005] For example, as described in Non-Patent Document 1 (Naoya Hasegawa et al., "Mechanism of Peeling Occurrence Due to Rolling Contact and the Effect of Blackening Treatment on Peeling Suppression," Tribologist, Vol. 63, No. 8, 2018), the initial cracks that initiate peeling occur when the rough protrusions of the first rolling component, which has a high surface roughness, are pressed against the second rolling component, which has a low surface roughness, causing the second rolling component to undergo plastic deformation. To improve peeling resistance, one countermeasure is to increase the hardness of the surface of the second rolling component.

[0006] Patent Document 1 (Japanese Patent No. 6833330) and Patent Document 2 (Japanese Unexamined Patent Publication No. 2022-113400) describe increasing the hardness of the surface of the second rolling component in order to improve peel resistance.

[0007] Japanese Patent Publication No. 6833330, Japanese Unexamined Patent Publication No. 2022-113400

[0008] Naoya Hasegawa et al., "Mechanism of Peeling Occurrence Due to Rolling Contact and the Effect of Blackening Treatment on Peeling Suppression," Tribologist, Vol. 63, No. 8, 2018.

[0009] The microstructure of the constituent material of a rolling bearing, i.e., hardened and tempered steel, contains martensite, retained austenite, undissolved carbides, nitrides, etc. Each of these phases has a different yield strength (or 0.2 percent proof stress) at which plastic deformation begins. Furthermore, undissolved carbides and nitrides can cause stress concentration depending on their mode of presence. Thus, from the viewpoint of improving peeling resistance, it is necessary to focus not only on hardness but also on the microstructure. The present invention provides a rolling member with improved peeling resistance.

[0010] The rolling member of the present invention has a surface that is in contact with other rolling members and is made of steel that has been hardened and tempered. The rolling member has a nitriding layer on its surface. The average nitrogen concentration on the surface is 0.10 mass percent or more. The average carbon concentration on the surface is 0.60 mass percent or more and 1.5 mass percent or less. The hardness on the surface is 800 Hv or more and 1000 Hv or less. The amount of retained austenite on the surface is 20 volume percent or less. Undissolved carbides with a particle size of 5.0 μm or less are present in the nitriding layer such that the area ratio is 17 percent or less when viewed in a cross-section perpendicular to the surface.

[0011] The rolling member of the present invention improves resistance to peeling.

[0012] This is a cross-sectional view of the rolling bearing 100. This is an enlarged cross-sectional view of the surface of the inner ring 10. This is a manufacturing process diagram of the inner ring 10.

[0013] The details of the embodiments will be described with reference to the drawings. In the following drawings, the same or corresponding parts will be denoted by the same reference numerals, and redundant descriptions will not be repeated.

[0014] In the following, the rolling bearing 100 will be described as an example of a rolling component according to the embodiment, and the inner ring 10 will be described as an example of a rolling member according to the embodiment. However, the rolling component and rolling member according to the embodiment are not limited to these. The rolling component according to the embodiment may be a ball screw (a rolling component having a shaft having a raceway surface, a nut having a raceway surface, rolling elements disposed between the raceway surface of the shaft and the raceway surface of the nut, a tube, a spindle, an end cap, etc.), and the rolling member according to the embodiment may be an outer ring or rolling element of a rolling bearing, a gear, a shaft, or other sliding member.

[0015] The rolling components and rolling members according to the embodiment are used, for example, in electric axles, electric brakes, electric compressors, electric actuators, positioning devices, electric jacks, servo cylinders, electric servo presses, mechanical presses, transmissions, electric pattern steering, electric injection molding machines, and spindles of machine tools. The applications of the rolling components and rolling members according to the embodiment are not limited to these.

[0016] (Configuration of the rolling bearing 100) Figure 1 is a cross-sectional view of the rolling bearing 100. As shown in Figure 1, the rolling bearing 100 has an inner ring 10, an outer ring 20, a plurality of rolling elements 30, and a cage 40. The central axis of the rolling bearing 100 is called the central axis A. The direction along the central axis A is called the axial direction. The direction passing through the central axis A and perpendicular to the central axis A is called the radial direction. The direction along the circumference of the circle centered on the central axis A when viewed along the axial direction is called the circumferential direction.

[0017] The inner ring 10 has a width surface 10a, a width surface 10b, an inner circumferential surface 10c, and an outer circumferential surface 10d. The width surfaces 10a, 10b, inner circumferential surface 10c, and outer circumferential surface 10d form the surface of the inner ring 10.

[0018] The width surface 10b is the opposite side of the width surface 10a. The width surfaces 10a and 10b form both end faces of the inner ring 10 in the axial direction. The width surfaces 10a and 10b face one side (left side in Figure 1) and the other side (right side in Figure 1), respectively, in the axial direction.

[0019] The inner circumferential surface 10c faces toward the central axis A, i.e., radially inward. The outer circumferential surface 10d faces toward the opposite side of the central axis A, i.e., radially outward. The outer circumferential surface 10d is the opposite surface of the inner circumferential surface 10c in the radial direction. The inner circumferential surface 10c and the outer circumferential surface 10d extend along the circumferential direction. One end and the other end of the inner circumferential surface 10c in the axial direction are connected to the width surface 10a and the width surface 10b, respectively. One end and the other end of the outer circumferential surface 10d in the axial direction are connected to the width surface 10a and the width surface 10b, respectively.

[0020] The inner ring 10 is attached to an axis (not shown) that rotates around the central axis A on its inner circumferential surface 10c. The outer circumferential surface 10d has a raceway surface 10da. The raceway surface 10da is located in the axial center of the outer circumferential surface 10d. The outer circumferential surface 10d is concave toward the inner circumferential surface 10c on the raceway surface 10da. In a cross-sectional view perpendicular to the circumferential direction, the raceway surface 10da forms a partial circular arc. The raceway surface 10da extends along the circumferential direction. The raceway surface 10da is in contact with the rolling element 30.

[0021] The outer ring 20 has a width surface 20a, a width surface 20b, an inner peripheral surface 20c, and an outer peripheral surface 20d. The width surface 20a, the width surface 20b, the inner peripheral surface 20c, and the outer peripheral surface 20d form the surface of the outer ring 20.

[0022] The width surface 20b is the opposite surface of the width surface 20a. The width surface 20a and the width surface 20b form both end surfaces of the outer ring 20 in the axial direction. The width surface 20a and the width surface 20b face one side (the left side in FIG. 1) and the other side (the right side in FIG. 1) in the axial direction, respectively.

[0023] The inner peripheral surface 20c faces the center axis A side, that is, the radially inner side. The outer peripheral surface 20d faces the side opposite to the center axis A, that is, the radially outer side. The outer peripheral surface 20d is the opposite surface of the inner peripheral surface 20c in the radial direction. The inner peripheral surface 20c and the outer peripheral surface 20d extend along the circumferential direction. One end and the other end of the inner peripheral surface 20c in the axial direction are respectively continuous with the width surface 20a and the width surface 20b. One end and the other end of the outer peripheral surface 20d in the axial direction are respectively continuous with the width surface 20a and the width surface 20b.

[0024] The outer ring 20 is attached to a housing (not shown) on the outer peripheral surface 20d. The inner peripheral surface 20c has a raceway surface 20ca. The raceway surface 20ca is located at the central portion of the inner peripheral surface 20c in the axial direction. The inner peripheral surface 20c is recessed toward the outer peripheral surface 20d side at the raceway surface 20ca. In a cross-sectional view perpendicular to the circumferential direction, the raceway surface 20ca forms a partial arc. The raceway surface 20ca extends along the circumferential direction. The raceway surface 20ca contacts the rolling element 30. The outer ring 20 is arranged on the radially outer side of the inner ring 10 such that the raceway surface 10da and the raceway surface 20ca face each other with a gap in the radial direction.

[0025] The rolling element 30 is spherical. The rolling element 30 is arranged between the raceway surface 10da and the raceway surface 20ca in the radial direction. A plurality of rolling elements 30 are arranged side by side along the circumferential direction. The cage 40 holds the plurality of rolling elements 30 such that the interval between two adjacent rolling elements 30 is within a certain range. The cage 40 is arranged between the outer peripheral surface 10d and the inner peripheral surface 20c.

[0026] (Detailed composition of inner ring 10) <Steel constituting the inner ring 10> The inner ring 10 is made of steel that has been hardened and tempered. The steel constituting the inner ring 10 contains, for example, 0.95 mass percent to 1.10 mass percent of carbon, less than 0.30 mass percent of silicon, less than 0.50 mass percent of manganese, less than 0.0080 mass percent of sulfur, and 1.4 mass percent to 1.6 mass percent of chromium, with the remainder being iron and unavoidable impurities. A specific example of the steel constituting the inner ring 10 is SUJ2, which is a high-carbon chromium bearing steel as defined in JIS standards.

[0027] "Less than 0.30 mass percent of silicon" means that silicon may not be present in the steel constituting the inner ring 10. "Less than 0.50 mass percent of manganese" means that manganese may not be present in the steel constituting the inner ring 10. "Less than 0.0080 mass percent of sulfur" means that sulfur may not be present in the steel constituting the inner ring 10.

[0028] <Nitrogen and Carbon Concentrations on the Surface of the Inner Ring 10> Figure 2 is an enlarged cross-sectional view of the surface of the inner ring 10. As shown in Figure 2, the inner ring 10 has a nitrided layer 50 formed on the surface that comes into contact with other rolling members. In the nitrided layer 50, the nitrogen concentration in the steel is greater than 0 mass percent. In the nitrided layer 50, nitrogen is solid-dissolved in the steel. The average nitrogen concentration on the surface of the inner ring 10 is 0.10 mass percent or more. The average nitrogen concentration on the surface of the inner ring 10 is, for example, 0.6 mass percent or less. The average carbon concentration on the surface of the inner ring 10 is 0.60 mass percent or more and 1.5 mass percent or less. The average nitrogen concentration and average carbon concentration on the surface of the inner ring 10 are measured using an EPMA (Electron Probe Micro Analyzer). For this measurement, a calibration curve is created using standard samples with known nitrogen and carbon concentrations.

[0029] <Hardness on the surface of the inner ring 10>The hardness on the surface of the inner ring 10 is 800 Hv or more and 1000 Hv or less. The hardness on the surface of the inner ring 10 may be 800 Hv or more and 900 Hv or less. The hardness on the surface of the inner ring 10 is measured by the Vickers hardness test method defined in ISO 6507. The load when measuring the hardness on the surface of the inner ring 10 is 300 g. The hardness on the surface of the inner ring 10 is measured at three or more locations, and the average value of the measured values is adopted. In addition, when it is difficult to measure the hardness on the surface of the inner ring 10, the hardness at a position where the depth from the surface of the inner ring 10 is 50 μm is regarded as the hardness on the surface of the inner ring 10.

[0030] <Amount of retained austenite on the surface of the inner ring 10>The amount of retained austenite on the surface of the inner ring 10 is 20 volume percent or less. For example, the amount of retained austenite on the surface of the inner ring 10 is 10 volume percent or more. The amount of retained austenite on the surface of the inner ring 10 is measured by the X-ray diffraction method. An X-ray diffractometer of the chromium tube target type is used to measure the amount of retained austenite on the surface of the inner ring 10. In the X-ray diffractometer of the chromium tube target type, the wavelength of the Cr-Kα ray is 2.29093×10 －１０ m, the tube voltage is 30 kV, the tube current is 10 mA, and the collimator size is 2 mm×2 mm. When obtaining a sample for measurement from the inner ring 10, it is preferably electropolished so that the retained austenite does not undergo processing-induced transformation.

[0031] <Area ratio of undissolved carbide in the nitrided layer 50>The undissolved carbide in the nitrided layer 50 is carbide (Fe, Cr) ３ C that remains undissolved in the matrix phase of the steel constituting the inner ring 10. In the nitrided layer 50, the area ratio of undissolved carbide with a particle size of 5.0 μm or less is 17 percent or less in a cross-sectional view perpendicular to the surface of the inner ring 10. For example, the area ratio of undissolved carbide with a particle size of 5.0 μm or less is 8 percent or more. The area ratio of undissolved carbide with a particle size of 5.0 μm or less is measured by the following method. First, in a cross-section perpendicular to the surface, including the region from the surface to a depth of 50 μm, at least 100 μm ２Preferably 500 μm ２ Microscopic images are acquired over the above area. Next, by performing image processing on these microscopic images, the area ratio of undissolved carbides with a particle size of 5.0 μm or less is measured. Note that the cross section perpendicular to the surface is cut, polished, and then an etching solution is used to reveal undissolved carbides in the cross section. Microscopic images are taken using a high-magnification optical microscope or laser microscope capable of distinguishing undissolved carbides of 5.0 μm or less. Note that in the region up to a depth of 50 μm from the surface, one field of view has an area of ​​100 μm. ２ (500 μm) ２ If this is not achieved, multiple field-of-view microscope images may be obtained.

[0032] <Particle size of undissolved carbides in the nitrogen-filled layer 50> In the nitrogen-filled layer 50, the maximum particle size of undissolved carbides is, for example, 2 μm or less. The maximum particle size of undissolved carbides in the nitrogen-filled layer 50 is measured by the following method. First, a microscopic image of a cross-section perpendicular to the surface is taken using the same method as described above. Next, by performing image processing on the microscopic image, the undissolved carbides are identified and the maximum area of ​​the undissolved carbides is determined. By converting this maximum value to an equivalent diameter of a circle, the maximum particle size of each undissolved carbide is obtained.

[0033] <Area ratio of chromium nitrides on the surface of the inner ring 10> Chromium nitrides may be present in the nitriding layer 50. Chromium nitrides are chromium nitrides or nitrides in which a portion of the chromium site of chromium nitride is replaced by an element other than chromium. In the nitriding layer 50, the area ratio of chromium nitrides with a particle size of 1.0 μm or less may be 1.0 percent or more and 10 percent or less in a cross-sectional view perpendicular to the surface of the inner ring 10. In the nitriding layer 50, the area ratio of chromium nitrides with a particle size of 1.0 μm or less may be 1.5 percent or more and 2.5 percent or less in a cross-sectional view perpendicular to the surface of the inner ring 10. Also, in the nitriding layer 50, the number of chromium nitrides with a particle size of 1.0 μm or less may be 100 μm ２ There may be five or more per unit area. In the nitrided layer 50, the number of chromium nitrides with a particle size of 1.0 μm or less is 100 μm. ２There may be eight or more per unit area. In the nitrided layer 50, the number of chromium-based nitrides having a particle size of 1.0 μm or less per 100 μm ２ per unit area is, for example, 20 or less. The particle size, area ratio, and number of chromium-based nitrides are measured by the following method. First, in a cross-section perpendicular to the surface, a region including a region from the surface to a depth of 50 μm and having an area of at least 100 μm ２ or more, preferably 500 μm ２ or more, a microscopic image is acquired. Next, image processing is performed on the microscopic image to measure the area ratio, number, and particle size of the chromium-based nitrides. Note that the cross-section perpendicular to the surface is polished after cutting, and further, a corrosion solution is used to expose the chromium-based nitrides on the cross-section. The microscopic image is taken by a high-magnification optical microscope or laser microscope capable of discriminating chromium-based nitrides. Note that when the area in one field of view does not reach 100 μm ２ (500 μm ２ ) in the region from the surface to a depth of 50 μm, microscopic images of a plurality of fields of view may be acquired.

[0034] <Half-value width of martensite on the surface of the inner ring 10> The half-value width of martensite on the surface of the inner ring 10 may be 7.2° or more and 8.0° or less. The half-value width of martensite on the surface of the inner ring 10 is measured by the X-ray diffraction method. More specifically, it is measured using a chromium tube type X-ray diffractometer. In the chromium tube type X-ray diffractometer, the wavelength of the Cr-Kα line is 2.29093×10 －１０ m, the tube voltage is 30 kV, the tube current is 10 mA, and the collimator size is 2 mm×2 mm.

[0035] <Dislocation density of martensite and retained austenite on the surface of the inner ring 10> The dislocation density of martensite on the surface of the inner ring 10 is, for example, 1.3×10 １５ m －２ or more. The dislocation density of martensite on the surface of the inner ring 10 is, for example, 1.0×10 １７ m －２ or less. The dislocation density of retained austenite on the surface of the inner ring 10 is, for example, 2.0×10 １４ m－２ That concludes the explanation. The dislocation density of retained austenite on the surface of the inner ring 10 is, for example, 1.0 × 10⁻⁶. １７ I understand －２ The following applies:

[0036] The dislocation densities of martensite and retained austenite on the surface of the inner ring 10 are measured using a cobalt tube X-ray diffractometer. More specifically, firstly, the X-ray profiles of martensite and austenite are measured using a cobalt tube X-ray diffractometer. In a cobalt tube X-ray diffractometer, the wavelength of the Co-Kα rays is 1.7889 × 10⁻¹⁰. －１０ The tube voltage is set to 40 kV, the tube current to 50 mA, and the collimator size to a diameter of 1 mm. The X-ray profiles of martensite and austenite are measured within the range of 2θ from 30° to 135°.

[0037] Secondly, after Rietveld analysis is performed, the full width at half maximum of the peaks of the X-ray profiles of martensite and austenite obtained by X-ray diffraction is separated into crystallite size and strain. Thirdly, by applying the separated crystallite size and strain to the following equation (1), i.e., the Williamson-Hall equation, the dislocation density of martensite and the dislocation density of retained austenite are obtained. Note that in this equation (1), ρ is the dislocation density (unit: m －２ ) where ε is the strain mentioned above, and b is the length of the Burgers vector (b = 0.25 × 10⁻¹⁰). －９ m).

[0038]

[0039] In the X-ray profile of martensite obtained by X-ray diffraction, the peaks of the {110}, {200}, {211}, and {220} planes are measured. Similarly, in the X-ray profile of austenite obtained by X-ray diffraction, the peaks of the {111}, {200}, {220}, {311}, and {222} planes are measured. The reason Rietveld analysis is performed as described above is to reduce the influence of the {200} plane of martensite and the {200} plane of austenite, which have different elastic moduli.

[0040] (Method for manufacturing the inner ring 10) Figure 3 is a diagram of the manufacturing process for the inner ring 10. As shown in Figure 3, the method for manufacturing the inner ring 10 includes a preparation step S1, a nitriding treatment step S2, a quenching step S3, a cooling step S4, a tempering step S5, and a post-treatment step S6.

[0041] In preparation step S1, the workpiece to be processed is prepared. Following preparation step S1, nitriding treatment step S2 is performed. In nitriding treatment step S2, nitriding treatment is performed on the workpiece to be processed. The nitriding treatment of the workpiece to be processed is performed by heating and holding the workpiece in an atmospheric gas containing a nitrogen source. The heating temperature and nitrogen concentration in the atmospheric gas in nitriding treatment step S2 are set so that a compound layer is not formed on the surface of the workpiece to be processed. By performing nitriding treatment step S2, nitrogen penetrates from the surface of the workpiece to the interior of the workpiece, and the nitrogen is dissolved in the matrix phase of the steel that makes up the workpiece. Nitriding treatment step S2 is performed so that the nitrogen reaches the position that will become the surface of the inner ring 10 after post-treatment step S6 (the position that will become the nitriding layer 50). Note that in nitriding treatment step S2, nitriding and carburizing treatment may be performed instead of nitriding treatment. That is, the atmospheric gas may contain a carbon source. Following nitriding treatment step S2, quenching step S3 is performed.

[0042] In the quenching process S3, the workpiece to be processed is quenched. The quenching of the workpiece is performed on the steel A that makes up the workpiece. １ The component to be heated is heated to a temperature above its transformation point and held thereafter, and then the component to be processed is M Ｓ This is carried out by cooling to a temperature below the transformation point. The quenching process S3 generates martensite and retained austenite in the steel constituting the workpiece. The quenching process S3 may be repeated on the workpiece. Repeated quenching process S3 refines the crystal grains and improves the effect of the cooling process S4. The full width at half maximum and dislocation density of the martensite and retained austenite are adjusted by the cooling rate in the quenching process S3. After the quenching process S3, the cooling process S4 is performed.

[0043] In the cooling process S4, the workpiece is subjected to sub-zero treatment or cryo-treatment (ultra-sub-zero treatment). If sub-zero treatment is performed, the workpiece is cooled to a temperature below -100°C or below room temperature. If cryo-treatment is performed, the workpiece is cooled to a temperature below -100°C. The cooling process S4 causes a portion of the retained austenite in the steel constituting the workpiece to transform into martensite. Before the cooling process S4, tempering or cleaning at a low temperature may be performed to prevent cracking of the workpiece. The time between the quenching process S3 and the cooling process S4, and the cooling rate in the cooling process S4, are used to adjust the full width at half maximum and the dislocation density of the martensite and retained austenite. After the cooling process S4, the tempering process S5 is performed.

[0044] In the tempering process S5, tempering is performed on the workpiece. Tempering of the workpiece involves the A of the steel that makes up the workpiece. １ This is done by heating the workpiece to a temperature below the transformation point. This heating temperature is, for example, 180°C or lower. When tempering is performed, the dislocation density of martensite and retained austenite in the steel that makes up the workpiece decreases, resulting in a decrease in hardness. However, if the cooling process S4 is performed, the dislocation density of martensite and retained austenite does not decrease as easily after tempering, and the hardness and yield strength do not decrease as easily after tempering. After the tempering process S5, the post-treatment process S6 is performed.

[0045] In the post-processing step S6, machining such as grinding and polishing is performed on the surface of the workpiece. As a result, the inner ring 10 is formed.

[0046] (Effect of inner ring 10) When undissolved carbides present in the nitriding layer 50 become coarse, stress concentration is more likely to occur around the undissolved carbides when the rolling element 30 comes into contact with the surface of the inner ring 10 (raceway surface 10da), which causes peeling. The higher the carbon concentration in the steel, the greater the amount of undissolved carbides in the steel, and the more likely the undissolved carbides are to become coarse. In the inner ring 10, the average carbon concentration on the surface of the inner ring 10 is 1.5 mass percent or less, so the increase of undissolved carbides present in the nitriding layer 50 is suppressed, and the coarsening of the undissolved carbides is suppressed. In addition, in the inner ring 10, the nitrogen concentration in the steel is 0.1 mass percent or more, so chromium in the undissolved carbides combines with nitrogen to precipitate chromium nitrides, reducing the amount of undissolved carbides and suppressing the increase and coarsening of undissolved carbides present in the nitriding layer 50.

[0047] In the nitriding layer 50, nitrogen is dissolved in the matrix phase of the steel constituting the inner ring 10, and thus the surface hardness of the inner ring 10 increases due to solid solution strengthening. Furthermore, as described above, since the inner ring 10 undergoes a cooling process S4 followed by a tempering process S5, the decrease in surface hardness of the inner ring 10 due to tempering is suppressed. As a result, a hardness of 800 Hv or more is obtained on the surface of the inner ring 10. In addition, the decrease in undissolved carbides and the increase in fine chromium nitrides also contribute to the improvement in hardness of the inner ring 10.

[0048] Austenite has a lower yield strength compared to martensite. A low yield strength makes plastic deformation, which causes peeling, more likely to occur. In the inner ring 10, the amount of retained austenite on the surface of the inner ring 10 is suppressed to 20 percent or less. Furthermore, in the inner ring 10, the cooling process S4 is performed followed by the tempering process S5, which suppresses the decrease in dislocation density in the retained austenite and, consequently, the decrease in yield strength that occurs during tempering. In this way, plastic deformation is suppressed on the surface of the inner ring 10 by reducing the amount of retained austenite and suppressing the decrease in dislocation density in the retained austenite.

[0049] As described above, in the inner ring 10, peeling is suppressed by ensuring hardness on the surface, suppressing the coarsening of undissolved carbides, reducing the amount of retained austenite, and maintaining the yield strength of the retained austenite.

[0050] Furthermore, since the silicon concentration in the steel constituting the inner ring 10 is less than 0.30 mass percent and the manganese concentration in the steel constituting the inner ring 10 is less than 0.50 mass percent, silicon-manganese nitrides are less likely to precipitate, and chromium nitrides can be finely precipitated. In addition, since the steel constituting the inner ring 10 does not contain molybdenum or vanadium, vanadium and molybdenum nitrides do not precipitate, but as described above, chromium nitrides can be finely precipitated, so the cost of the steel can be reduced because it does not contain molybdenum or vanadium.

[0051] In the above, the inner ring 10 was described as an example of a rolling member according to the embodiment, but the outer ring 20 or rolling element 30 may also be rolling members according to the embodiment. That is, the outer ring 20 or rolling element 30 may be made of steel with the same composition as the inner ring 10, and the same nitriding layer 50 as described above may be formed on the surface of the outer ring 20 or the surface of the rolling element 30. In addition, the surface of the rolling element 30 may be subjected to high compressive residual stress by the pressurization process, in which case the peeling resistance of the rolling bearing 100 can be improved even if the rolling element 30 is not a rolling component according to the embodiment.

[0052] (Example) First, samples 1 to 8 were prepared. Samples 1 to 8 were cylindrical in shape with an outer diameter of 40 mm and an inner diameter of 28 mm. SUJ2 was used for samples 1 to 6. For samples 7 and 8, steel was used that had the same composition as SUJ2 except for the carbon content, with a carbon concentration that was 0.2 mass percent higher. In samples 1 to 8, grinding and superfinishing were performed in the post-treatment step S6 to achieve an arithmetic mean roughness (Ra) of 0.02 μm on the outer surface. Details of samples 1 to 8 are shown in Tables 1 to 3.

[0053] For samples 1 to 3, the following heat treatments were performed: nitriding process S2, quenching process S3, cooling process S4 (cryotreatment), and tempering process S5. For samples 1 to 3, the average nitrogen concentration, average carbon concentration, presence of undissolved carbides, and presence of chromium nitrides on the sample surface were adjusted by changing the heating temperature and holding time in the nitriding process S2. For samples 1 to 3, the amount of retained austenite on the sample surface, the dislocation density of martensite on the sample surface, and the dislocation density of retained austenite on the sample surface were adjusted by changing the holding time and cooling temperature in the cooling process S4.

[0054] In Sample 4, the heat treatment consisted of a nitrification process S2 and a tempering process S5. In Sample 5, the heat treatment consisted of a nitrification process S2, a quenching process S3, and a tempering process S5. In Sample 6, the heat treatment consisted of a standard quenching and tempering process S5. In Samples 7 and 8, the heat treatment consisted of a nitrification process S2, a quenching process S3, and tempering at 200°C.

[0055] Next, a peeling test was performed on samples 1 through 8. A mating material was used for the peeling test. The mating material was cylindrical with an outer diameter of 40 mm and an inner diameter of 28 mm. The sub-radius of curvature on the outer surface of the mating material was set to 60 mm. The mating material was made of SUJ2 and underwent quenching and tempering as heat treatment. The hardness on the outer surface of the mating material was 760 Hv, and the surface roughness (Ra) on the outer surface of the mating material was set to 0.75 μm. During the peeling test, the mating material was rotated at a rotational speed of 2000 rpm. During the peeling test, samples 1 through 8 were driven by contact between their outer surfaces and the outer surface of the mating material. Lubrication between samples 1 through 8 and the mating material was provided using additive-free turbine oil (ISO VG46), with an oil film parameter of 0.6. A load of 2254 N was applied between Sample 1 to Sample 8 and the mating material. This corresponds to a maximum contact pressure of 2.3 GPa. In the peeling test, the test duration was 240 minutes, and the test temperature was room temperature.

[0056] After the peeling test, the surfaces of samples 1 to 8 were observed using an optical microscope. In addition, optical microscope images were taken of the areas where peeling occurred, and the peeling area ratio was calculated by image analysis. More specifically, firstly, optical microscope images were taken in the three fields of view where the peeling area was largest. Secondly, in each image, the area ratio of microcracks present in the peeling area (the ratio of the crack area to the field of view area) was calculated using binarization processing of image analysis software. Thirdly, the peeling area ratio was obtained by averaging the above area ratios for the three fields of view.

[0057] Condition A is that the hardness of the sample surface is 800 Hv or more and 1000 Hv or less. Condition B is that the average nitrogen concentration on the sample surface is 0.10 mass percent or more. Condition C is that the average carbon concentration on the sample surface is 0.60 mass percent or more and 1.5 mass percent or less. Condition D is that the amount of retained austenite on the sample surface is 20 volume percent or less. Condition E is that the area ratio of undissolved carbides with a particle size of 5.0 μm or less in the nitrided layer 50 is 17 percent or less.

[0058] As shown in Table 1, samples 1 through 3 satisfied all of conditions A through E. Samples 4 through 8 did not satisfy at least one of conditions A through E. The peeling test results showed that samples 1 through 3 had a peeling area ratio of less than 2 percent. On the other hand, samples 4 through 8 had a peeling area ratio of 2 percent or more. This comparison experimentally demonstrated that satisfying conditions A through E improves peeling resistance.

[0059]

[0060] Condition F is defined as the area ratio of chromium nitrides with a particle size of 1.0 μm or less in the nitrided layer 50 being 1.0 percent or more. The area ratio of chromium nitrides with a particle size of 1.0 μm or less in the nitrided layer 50 is 100 μm. ２Condition G is defined as the presence of five or more particles per sample. As shown in Table 2, samples 1 and 2 satisfied both conditions F and G, but sample 3 did not satisfy either condition F or G. The peeling test results showed that the peeling area ratio of samples 1 and 2 was lower than that of sample 3. In particular, no peeling occurred in sample 1. From this comparison, it was experimentally clear that peeling resistance is further improved by satisfying conditions F and G in addition to conditions A through E.

[0061]

[0062] The dislocation density of martensite on the sample surface is 1.3 × 10⁻⁶. １５ I understand －２ The above condition is defined as condition H. The dislocation density of residual martensite on the sample surface is 2.0 × 10⁻⁶. １４ I understand －２ The above conditions are defined as Condition I. In Samples 1 to 3, the nitrogen treatment process S2, quenching process S3, cooling process S4 (cryotreatment), and tempering process S5 were performed as described above. In Samples 1 to 3, Condition I was satisfied. From this comparison, it was experimentally revealed that by performing the tempering process S5 after the cooling process S4, the dislocation density of martensite and retained austenite is maintained even after tempering.

[0063]

[0064] (Note) The above embodiment includes the following configuration.

[0065] <Note 1> A rolling member made of steel having a surface that is a contact surface with other rolling members, and which has been hardened and tempered, wherein the surface is provided with a nitriding layer, the average nitrogen concentration on the surface is 0.10 mass percent or more, the average carbon concentration on the surface is 0.60 mass percent or more and 1.5 mass percent or less, the hardness on the surface is 800 Hv or more and 1000 Hv or less, the amount of retained austenite on the surface is 20 volume percent or less, and in the nitriding layer, the area ratio of undissolved carbides with a particle size of 5.0 μm or less in a cross-sectional view perpendicular to the surface is 17 percent or less.

[0066] <Note 2> On the surface, 100 μm of chromium nitride with a particle size of 1.0 μm or less is provided so that the area ratio in the cross-sectional view is 1.0 percent or more and 10 percent or less. ２ There are five or more of the rolling members described in Appendix 1.

[0067] <Note 3> The full width at half maximum of the martensite on the surface is 7.2° or more and 8.0° or less, and the dislocation density of the martensite on the surface and the dislocation density of the retained austenite on the surface are 1.3 × 10⁻¹⁰, respectively. １５ I understand －２ The above and 2.0 × 10 １４ I understand －２ The rolling member described above is as described in Appendix 1 or Appendix 2.

[0068] <Note 4> A rolling bearing comprising a raceway member and rolling elements, wherein at least one of the raceway member and the rolling elements is the rolling element described in any one of Notes 1 to 3.

[0069] While embodiments of the present invention have been described above, various modifications of these embodiments are possible. Furthermore, the scope of the present invention is not limited to the embodiments described above. The scope of the present invention is indicated by the claims and is intended to include all modifications within the meaning and scope of the claims.

[0070] 10 Inner wheel, 10a, 10b width, 10c inner circumferential surface, 20ca track surface, 10d outer circumferential surface, 10da track surface, 20 outer wheel, 20a, 20b width, 20c inner circumferential surface, 20ca track surface, 20d outer circumferential surface, 30 rotating body, 40 retainer, 50 squeegee layer, 100 rotating shaft, A center shaft, S1 preparation process, S2 squeegee treatment process, S3 baking process, S4 cooling process, S5 baking process, S6 post-treatment process.

Claims

1. A rolling member made of steel having a surface that is a contact surface with other rolling members, and which has been hardened and tempered, wherein the surface is provided with a nitriding layer, the average nitrogen concentration on the surface is 0.10 mass percent or more, the average carbon concentration on the surface is 0.60 mass percent or more and 1.5 mass percent or less, the hardness on the surface is 800 Hv or more and 1000 Hv or less, the amount of retained austenite on the surface is 20 volume percent or less, and in the nitriding layer, the area ratio of undissolved carbides with a particle size of 5.0 μm or less in a cross-sectional view perpendicular to the surface is 17 percent or less.

2. On the surface, chromium nitride with a particle size of 1.0 μm or less is laid in layers of 100 μm so that the area ratio in the cross-sectional view is 1.0 percent or more and 10 percent or less. ２ The rolling member according to claim 1, wherein there are five or more of them.

3. The full width at half maximum of the martensite on the surface is 7.2° or more and 8.0° or less, and the dislocation density of the martensite on the surface and the dislocation density of the retained austenite on the surface are 1.3 × 10⁻¹⁰, respectively. １５ I understand －２ The above and 2.0 × 10 １４ I understand －２ The rolling member according to claim 1 is as described above.

4. A rolling bearing comprising a raceway member and rolling elements, wherein at least one of the raceway member and the rolling elements is the rolling element described in any one of claims 1 to 3.

Images