Rolling members and rolling bearings

Abstract

To provide rolling members with improved peeling resistance. [Solution] The rolling member 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. 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.

Description

[Technical Field]

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

[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 timing), 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. Therefore, one way to improve peeling resistance 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. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Patent No. 6833330 [Patent Document 2] Japanese Patent Publication No. 2022-113400 [Non-patent literature]

[0008] [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. [Overview of the Initiative] [Problems that the invention aims to solve]

[0009] The constituent material of a rolling bearing, that is, the metal structure of steel subjected to quenching and tempering, contains martensite, retained austenite, undissolved carbides, nitrides, etc. In each of these phases, the yield strength (or 0.2% proof stress) at which plastic deformation starts is different. Also, undissolved carbides and nitrides may cause stress concentration depending on their state of existence. Thus, from the viewpoint of enhancing pitting resistance, not only hardness but also the metal structure needs to be taken into consideration. The present invention provides a rolling element with enhanced pitting resistance.

Means for Solving the Problems

[0010] The rolling element of the present invention has a surface that is a contact surface with other rolling elements and is made of steel subjected to quenching and tempering. The rolling element is provided with a nitrided layer on the surface. The average nitrogen concentration on the surface is 0.10 mass% or more. The average carbon concentration on the surface is 0.60 mass% or more and 1.5 mass% 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% or less. In the nitrided layer, undissolved carbides having a particle size of 5.0 μm or less are present so that the area ratio becomes 17% or less in a cross-sectional view perpendicular to the surface.

Effects of the Invention

[0011] According to the rolling element of the present invention, pitting resistance is enhanced.

Brief Description of the Drawings

[0012] [Figure 1] It is a cross-sectional view of a rolling bearing 100. [Figure 2] It is an enlarged cross-sectional view of the surface of the inner ring 10. [Figure 3] It is a manufacturing process diagram of the inner ring 10.

Modes for Carrying Out the Invention

[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 rolling bearing 100) Figure 1 is a cross-sectional view of a 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 circumferential surface 20c, and an outer circumferential surface 20d. The width surfaces 20a, 20b, inner circumferential surface 20c, and outer circumferential surface 20d form the surface of the outer ring 20.

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

[0023] The inner circumferential surface 20c faces toward the central axis A, i.e., radially inward. The outer circumferential surface 20d faces toward the opposite side of the central axis A, i.e., radially outward. The outer circumferential surface 20d is the opposite surface of the inner circumferential surface 20c in the radial direction. The inner circumferential surface 20c and the outer circumferential surface 20d extend along the circumferential direction. One end and the other end of the inner circumferential surface 20c in the axial direction are connected to the width surfaces 20a and 20b, respectively. One end and the other end of the outer circumferential surface 20d in the axial direction are connected to the width surfaces 20a and 20b, respectively.

[0024] The outer ring 20 is attached to the housing (not shown) at its outer circumferential surface 20d. The inner circumferential surface 20c has a raceway surface 20ca. The raceway surface 20ca is located in the axial center of the inner circumferential surface 20c. The inner circumferential surface 20c is concave toward the outer circumferential surface 20d at the raceway surface 20ca. In a cross-sectional view perpendicular to the circumferential direction, the raceway surface 20ca forms a partial circular arc. The raceway surface 20ca extends along the circumferential direction. The raceway surface 20ca is in contact with the rolling element 30. The outer ring 20 is positioned radially outward of the inner ring 10 such that the raceway surfaces 10da and 20ca face each other with a gap between them in the radial direction.

[0025] The rolling elements 30 are spherical. The rolling elements 30 are arranged radially between the raceway surface 10da and the raceway surface 20ca. Multiple rolling elements 30 are arranged along the circumferential direction. The cage 40 holds the multiple rolling elements 30 such that the distance between any two adjacent rolling elements 30 is within a certain range. The cage 40 is positioned between the outer circumferential surface 10d and the inner circumferential surface 20c.

[0026] (Detailed configuration of inner ring 10) <Steel constituting the inner ring 10> The inner ring 10 is made of hardened and tempered steel. 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, a high-carbon chromium bearing steel specified 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 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 nitriding layer 50 formed on the surface that comes into contact with other rolling members. In the nitriding layer 50, the nitrogen concentration in the steel is greater than 0 mass percent. In the nitriding 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 between 0.60 mass percent and 1.5 mass percent. The average nitrogen and carbon concentrations 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 of the surface of the inner ring 10> The hardness of the inner ring 10 surface is between 800 Hv and 1000 Hv. Alternatively, the hardness of the inner ring 10 surface may be between 800 Hv and 900 Hv. The hardness of the inner ring 10 surface is measured using the Vickers hardness test method specified in ISO 6507. The load used when measuring the hardness of the inner ring 10 surface is 300 g. The hardness of the inner ring 10 surface is measured at three or more locations, and the average of these measurements is adopted. If measuring the hardness of the inner ring 10 surface is difficult, the hardness at a depth of 50 μm from the surface of the inner ring 10 is considered the hardness of the inner ring 10 surface.

[0030] <Amount of retained austenite on the surface of inner ring 10> The amount of retained austenite on the surface of the inner ring 10 is 20 volume percent or less. Alternatively, the amount of retained austenite on the surface of the inner ring 10 is, for example, 10 volume percent or more. The amount of retained austenite on the surface of the inner ring 10 is measured by X-ray diffraction. A chromium tube X-ray diffractometer is used to measure the amount of retained austenite on the surface of the inner ring 10. In a chromium tube X-ray diffractometer, the wavelength of Cr-Kα rays is 2.29093 × 10⁻¹⁰. -10 The tube voltage is set to 30kV, the tube current to 10mA, and the collimator size to 2mm x 2mm. When obtaining a sample for measurement from the inner ring 10, it is preferable to electropolish it to prevent the retained austenite from undergoing processing-induced transformation.

[0031] <Area ratio of undissolved carbides in the nitrification layer 50> The undissolved carbides in the nitriding layer 50 are carbides (Fe,Cr)3C that remain undissolved in the matrix phase of the steel constituting the inner ring 10. In the nitriding layer 50, the area percentage of undissolved carbides 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 percentage of undissolved carbides with a particle size of 5.0 μm or less is 8 percent or more. The area percentage of undissolved carbides 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, a region including a depth of 50 μm from the surface and at least 100 μm is measured.2 Preferably 500 μm 2 Microscopic images are acquired over the above area. Next, the area ratio of undissolved carbides with a particle size of 5.0 μm or less is measured by performing image processing on these microscopic images. After cutting the cross section perpendicular to the surface, it is 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. In the region up to a depth of 50 μm from the surface, one field of view covers an area of ​​100 μm. 2 (500μm 2 If this is not achieved, multiple field-of-view microscope images may be obtained.

[0032] <Particle size of undissolved carbides in the nitrided 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 nitride on the surface of inner ring 10> Chromium nitrides may be present in the nitriding layer 50. Chromium nitrides are chromium nitride or nitrides in which a portion of the chromium site of chromium nitride is replaced by elements 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 between 1.0 percent and 10 percent 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 between 1.5 percent and 2.5 percent in a cross-sectional view perpendicular to the surface of the inner ring 10. Furthermore, in the nitriding layer 50, the number of chromium nitrides with a particle size of 1.0 μm or less may be between 100 μm. 2There may be five or more per unit area. In the nitrided layer 50, the number of chromium nitrides having a particle size of 1.0 μm or less is 100 μm 2 There may be eight or more per unit area. In the nitrided layer 50, 100 μm 2 The number of chromium nitrides having a particle size of 1.0 μm or less present per unit area is, for example, 20 or less. The particle size, area ratio, and number of chromium nitrides are measured by the following method. First, in a cross-section perpendicular to the surface, a region including the 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, by performing image processing on the microscopic image, the area ratio, number, and particle size of the chromium nitrides are measured. Note that after cutting, polishing is performed on the cross-section perpendicular to the surface, and a corrosion liquid is further used to expose the chromium nitrides on the cross-section. The microscopic image is taken by a high-magnification optical microscope or laser microscope capable of discriminating chromium nitrides. Note that if 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 spherical X-ray diffractometer. In the chromium tube spherical X-ray diffractometer, 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.

[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 -2That concludes the explanation. The dislocation density of martensite on the surface of the inner ring 10 is, for example, 1.0 × 10⁻⁶. 17 m -2 The following is true: The dislocation density of retained austenite on the surface of the inner ring 10 is, for example, 2.0 × 10⁻⁶. 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 m -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 parameters are set to m, tube voltage to 40kV, tube current to 50mA, and collimator size to a diameter of 1mm. The X-ray profiles of martensite and austenite are measured within the range of 2θ between 30° and 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]

number

[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 for performing Rietveld analysis 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 of 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 manufacturing method for the inner ring 10 includes a preparation step S1, a nitrogen immersion treatment step S2, a quenching step S3, a cooling step S4, a tempering step S5, and a post-processing 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 nitrogen is dissolved in the matrix phase of the steel that makes up the workpiece. Nitriding treatment step S2 is performed so that 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. Quenching of the workpiece involves heating the workpiece to a temperature above the A1 transformation point of the steel that makes up the workpiece and holding it there, and then the workpiece is subjected to MS 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 some of the retained austenite in the steel constituting the workpiece to transform into martensite. Before the cooling process S4, low-temperature tempering or cleaning 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, the workpiece is tempered. Tempering of the workpiece is performed by heating it to a temperature below the A1 transformation point of the steel that makes up the workpiece. 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 is less likely to decrease after tempering, and the hardness and yield strength do not decrease as much even 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 coarseen, stress concentration is more likely to occur around the undissolved carbides when the rolling elements 30 come into contact with the surface of the inner ring 10 (raceway surface 10da), leading to 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 coarseen. 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 in 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 the 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 the 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. In addition, since the inner ring 10 undergoes a cooling process S4 followed by a tempering process S5, the decrease in dislocation density in the retained austenite and, consequently, the decrease in yield strength associated with tempering are suppressed. Thus, on the surface of the inner ring 10, plastic deformation is suppressed 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] (Examples) First, samples 1 through 8 were prepared. Samples 1 through 8 were cylindrical in shape with an outer diameter of 40 mm and an inner diameter of 28 mm. SUJ2 steel was used for samples 1 through 6. For samples 7 and 8, steel was used that had the same composition as SUJ2 except for the carbon concentration, which was 0.2 mass percent higher. In samples 1 through 8, grinding and superfinishing were performed in post-treatment step S6 to achieve an arithmetic mean roughness (Ra) of 0.02 μm on the outer surface. Details of samples 1 through 8 are shown in Tables 1 through 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 nitrification step S2 and tempering step S5. In Sample 5, the heat treatment consisted of nitrification step S2, quenching step S3, and tempering step S5. In Sample 6, the heat treatment consisted of standard quenching and tempering step S5. In Samples 7 and 8, the heat treatment consisted of nitrification step S2, quenching step 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 formed from 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 samples 1 through 8 and the mating material. This translates 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 with the largest peeling areas. 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 between 800 Hv and 1000 Hv. 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 between 0.60 mass percent and 1.5 mass percent. 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 nitridation layer 50 is 17 percent or less.

[0058] As shown in Table 1, samples 1 through 3 met all of conditions A through E. Samples 4 through 8 did not meet 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 meeting conditions A through E improves peel resistance.

[0059] [Table 1]

[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 5 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. This comparison experimentally demonstrated that peeling resistance is further improved by satisfying conditions F and G in addition to conditions A through E.

[0061] [Table 2]

[0062] The dislocation density of martensite on the sample surface is 1.3 × 10⁻⁶. 15 m -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 m -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 performing the tempering process S5 after the cooling process S4 maintains the dislocation density of martensite and retained austenite even after tempering.

[0063] [Table 3]

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

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

[0066] <Note 2> On the aforementioned surface, 100 μm of chromium nitride with a particle size of 1.0 μm or less is provided, such 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 aforementioned surface is 7.2° or more and 8.0° or less. The dislocation density of martensite on the surface and the dislocation density of retained austenite on the surface are, respectively, 1.3 × 10⁻⁶. 15 m -2 The above and 2.0 × 10 14 m -2 The rolling members described in Appendix 1 or Appendix 2 are as described above.

[0068] <Note 4> It is a rolling bearing, Track members and Equipped with rolling elements, A rolling bearing in which at least one of the raceway member and the rolling element is the rolling element described in any one of the items 1 to 3 of the appendix.

[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 equivalent to the claims. [Explanation of symbols]

[0070] 10 Inner ring, 10a,10b width surface, 10c inner circumferential surface, 20ca raceway surface, 10d outer circumferential surface, 10da raceway surface, 20 outer ring, 20a,20b width surface, 20c inner circumferential surface, 20ca raceway surface, 20d outer circumferential surface, 30 rolling element, 40 cage, 50 nitrogen layer, 100 rolling bearing, A Central axis, S1 preparation process, S2 nitriding process, S3 quenching process, S4 cooling process, S5 tempering process, S6 post-treatment process.

Claims

1. A steel rolling member having a surface that is in contact with other rolling members, and which has been hardened and tempered, The surface is provided with a nitrogen-infiltrating 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 of the aforementioned surface is 800 Hv or more and 1000 Hv or less. The amount of retained austenite on the aforementioned surface is 20 volume percent or less. In the nitrogen-filled layer, the rolling member has an area ratio of undissolved carbides with a particle size of 5.0 μm or less in a cross-sectional view perpendicular to the surface of the layer, which is 17 percent or less.

2. On the aforementioned surface, 100 μm of chromium nitride with a particle size of 1.0 μm or less is provided, such 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 aforementioned surface is 7.2° or more and 8.0° or less. The dislocation density of martensite on the surface and the dislocation density of retained austenite on the surface are, respectively, 1.3 × 10⁻⁶. 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. It is a rolling bearing, Track members and Equipped with rolling elements, A rolling bearing wherein at least one of the raceway member and the rolling element is the rolling element according to any one of claims 1 to 3.

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