JP7724609B2 - Bearing parts - Google Patents

Description

The present invention relates to bearing components and manufacturing methods thereof.

A bearing comprises bearing components, such as an inner ring and an outer ring (hereinafter, these may be collectively referred to as raceway members), and rolling elements. The bearing component disclosed in Japanese Patent No. 3905429 (Patent Document 1) has fine prior austenite grains on the surface, resulting in high resistance to surface damage. In other words, the surface of the bearing component described in Patent Document 1 has high resistance to indentation formation (the resistance to indentations when rolling elements are pressed against the raceway members) and high wear resistance.

Patent No. 3905429

In recent years, as machines that use bearings have become more fuel-efficient and efficient, bearing parts tend to be placed in harsher environments than before. As a result, there is a demand for bearing parts with higher surface damage resistance.

The primary object of the present invention is to provide bearing components with high resistance to surface damage.

The bearing component according to the present invention is made of high-carbon chromium bearing steel and has a hardened layer on its surface. The hardened layer contains a plurality of martensite grains. The maximum grain size of the martensite grains is 3.5 μm or less. The maximum aspect ratio of the martensite grains is 10 or less. The ratio of the maximum to the minimum crystal orientation density of the {011} plane of the martensite grains is 5.0 or less.

In the above-described bearing component, when the martensite grains are divided into a first group and a second group as described below, the average grain size of the martensite grains belonging to the first group may be 1.1 μm or less. The minimum grain size of the martensite grains belonging to the first group is greater than the maximum grain size of the martensite grains belonging to the second group. The value obtained by dividing the total area of the martensite grains belonging to the first group by the total area of the martensite grains is 0.5 or greater. The value obtained by dividing the total area of the martensite grains belonging to the first group, excluding the martensite grains belonging to the first group with the smallest grain size, by the total area of the martensite grains is less than 0.5.

Furthermore, in the above-mentioned bearing component, when the multiple martensite grains are divided into the third and fourth groups shown below, the average grain size of the martensite grains belonging to the third group may be 0.8 μm or less. The minimum grain size of the martensite grains belonging to the third group is greater than the maximum grain size of the martensite grains belonging to the fourth group. The value obtained by dividing the total area of the martensite grains belonging to the third group by the total area of the multiple martensite grains is 0.7 or more. The value obtained by dividing the total area of the martensite grains belonging to the third group, excluding the martensite grains belonging to the third group with the smallest grain size, by the total area of the multiple martensite grains is less than 0.7.

In the above-mentioned bearing component, the average aspect ratio of the martensite grains belonging to the first group may be 3.2 or less, and the average aspect ratio of the martensite grains belonging to the third group may be 3.0 or less.

In the above-described bearing component, the hardened layer further includes a plurality of cementite grains. When the plurality of cementite grains are divided into the fifth and sixth groups described below, the average grain size of the cementite grains belonging to the fifth group may be 1.4 μm or less. The minimum value of the crystal grain size of the cementite grains belonging to the fifth group is greater than the maximum value of the crystal grains belonging to the sixth group. The value obtained by dividing the total area of the cementite grains belonging to the fifth group by the total area of the plurality of cementite grains is 0.5 or greater. The value obtained by dividing the total area of the cementite grains belonging to the fifth group, excluding the cementite grains belonging to the fifth group with the smallest crystal grain size, by the total area of the plurality of cementite grains is less than 0.5.

Furthermore, in the above-mentioned bearing component, when the multiple cementite grains are divided into the seventh and eighth groups shown below, the average grain size of the cementite grains belonging to the seventh group may be 1.10 μm or less. The minimum value of the crystal grain size of the cementite grains belonging to the seventh group is greater than the maximum value of the cementite grains belonging to the eighth group. The value obtained by dividing the total area of the cementite grains belonging to the seventh group by the total area of the multiple cementite grains is 0.7 or greater. The value obtained by dividing the total area of the cementite grains belonging to the seventh group, excluding the cementite grains belonging to the seventh group with the smallest crystal grain size, by the total area of the multiple cementite grains is less than 0.7.

In the bearing component, the number density of the cementite grains belonging to the fifth group may be 0.05/μm ² or more, and the number density of the cementite grains belonging to the seventh group may be 0.10/μm ² or more.

In the above-mentioned bearing component, the hardened layer contains nitrogen. The average nitrogen concentration in the hardened layer between the surface and a position 10 μm away from the surface may be 0.10 mass percent or more.

In the above-mentioned bearing component, the amount of retained austenite on the surface may be 20% by volume or more.

In the above-mentioned bearing component, the hardness of the quench-hardened layer on the surface may be 730 Hv or more.

In the above-mentioned bearing component, the average grain size of the prior austenite grains on the surface may be 8 μm or less.

In the above bearing component, the compressive residual stress on the surface is 100 MPa or more.

In the above bearing components, the high-carbon chromium bearing steel may be SUJ2 as specified in the JIS standard.

The method for manufacturing a bearing component according to the present invention includes the steps of: preparing a formed body made of high-carbon chromium bearing steel; heating the formed body to a primary quenching temperature that is equal to or higher than the _A1 transformation point and then cooling it to a temperature equal to or lower than the Ms point, thereby performing primary quenching on the formed body; holding the primarily quenched formed body at a temperature equal to or higher than 200°C and lower than the _A1 transformation point for a first time, thereby performing primary tempering on the formed body; heating the primarily tempered formed body to a temperature equal to or higher than the _A1 transformation point and lower than the primary quenching temperature, after which cooling it to a temperature equal to or lower than the Ms point, thereby performing secondary quenching on the formed body; and holding the secondarily quenched formed body at a temperature lower than 180°C for a second time, thereby performing secondary tempering.

The above-mentioned method for manufacturing a bearing component may further include a step of nitriding the compact prior to the step of primary quenching the compact.

The present invention provides bearing components with high resistance to surface damage.

FIG. 2 is a top view of an inner ring according to an embodiment. FIG. 2 is a cross-sectional view taken along the line II-II in FIG. FIG. 3 is a partial enlarged view of region III in FIG. 2 . 5A to 5C are process diagrams illustrating a method for manufacturing a bearing component according to an embodiment. 1 is an EBSD image of Sample 1 in the orbital plane. EBSD image of sample 2 in the orbital plane. EBSD image of sample 3 in the orbital plane. EBSD image of sample 4 in the orbital plane. For samples 1 to 4, the average grain size of martensite grains belonging to the first group 10 is a graph showing the average grain size of martensite grains belonging to the third group and the fourth group. For samples 1 to 4, the average size of the martensite grains belonging to the first group was 10 is a graph showing the aspect ratio and the average aspect ratio of martensite grains belonging to the third group. For samples 1 to 4, the average grain size and 10 is a graph showing the average grain sizes of cementite grains belonging to Groups 1 and 7. For samples 1 to 4, the number density and 10 is a graph showing the number density of cementite grains belonging to the seventh group. For samples 1 to 4, the maximum contact pressure (unit: G) in the indentation resistance test 1 is a graph showing the relationship between the pressure (MPa) and the indentation depth (unit: mm).

Embodiments of the present invention will be described below with reference to the drawings. Note that in the following drawings, the same or corresponding parts will be given the same reference numbers, and their descriptions will not be repeated.

<Bearing component configuration>
The configuration of a bearing component according to an embodiment will be described. Note that, although an inner ring 10 of a rolling bearing will be described below as an example of the bearing component according to an embodiment, the bearing component according to an embodiment is not limited to this. The bearing component according to an embodiment may be at least one of an inner ring, an outer ring, and a rolling element of a rolling bearing. The rolling bearing according to an embodiment may include, for example, an inner ring and an outer ring as raceway components according to an embodiment, and rolling elements.

The inner ring 10 is made of high-carbon chromium bearing steel. The high-carbon chromium bearing steel is, for example, SUJ2 as specified in the JIS standard (JIS G 4805:2008).

Figure 1 is a top view of the inner ring 10. Figure 2 is a cross-sectional view taken along line II-II in Figure 1. As shown in Figures 1 and 2, the inner ring 10 has a ring shape. The inner ring 10 has a top surface 10a, a bottom surface 10b, an inner peripheral surface 10c, an outer peripheral surface 10d, and a central axis 10e.

The top surface 10a and the bottom surface 10b form end surfaces in the direction along the central axis 10e. The bottom surface 10b is the opposite surface of the top surface 10a. The inner peripheral surface 10c and the outer peripheral surface 10d are continuous with the top surface 10a and the bottom surface 10b. The distance between the inner peripheral surface 10c and the central axis 10e is smaller than the distance between the outer peripheral surface 10d and the central axis 10e. A raceway groove is provided on the outer peripheral surface 10d. The outer peripheral surface 10d forms the raceway surface of the inner ring 10.

Figure 3 is a partially enlarged view of region III in Figure 2. As shown in Figure 3, the inner ring 10 has a hardened layer 11. The hardened layer 11 is provided on at least the outer peripheral surface 10d, which constitutes the raceway surface, of the surface of the inner ring 10. The hardened layer 11 is provided, for example, on the entire surface of the inner ring 10. The hardened layer 11 includes a plurality of martensite crystal grains and a plurality of cementite grains. The martensite crystal grains are crystal grains composed of a martensite phase. The cementite grains are compound grains composed of cementite ( _Fe3C ).

Martensite grains are block grains of the martensite phase composed of crystals with aligned crystal orientations. When the deviation between the crystal orientation of a first martensite grain and the crystal orientation of a second martensite grain adjacent to the first martensite grain is 15° or more, the first martensite grain and the second martensite grain are different martensite grains. On the other hand, when the deviation between the crystal orientation of a first martensite grain and the crystal orientation of a second martensite grain adjacent to the first martensite grain is less than 15°, the first martensite grain and the second martensite grain constitute a single martensite grain.

The maximum grain size of the martensite grains in the hardened layer 11 is 3.5 μm or less. The maximum grain size of the martensite grains in the hardened layer 11 is, for example, 3.2 μm or more. The maximum grain size of the martensite grains is measured using the EBSD (Electron Backscattered Diffraction) method.

Specifically, first, an image of the surface of the hardened layer 11 is taken using the EBSD method (hereinafter referred to as an "EBSD image"). The EBSD image is taken so as to include a sufficient number (20 or more) of martensite grains. The boundaries between adjacent martensite grains are identified using the EBSD method. Second, the area and shape of each martensite grain displayed in the EBSD image are calculated based on the identified martensite grain boundaries.

More specifically, the circle-equivalent diameter of each martensite grain displayed in the EBSD image is calculated by dividing the area of each martensite grain displayed in the EBSD image by π/4 and calculating the square root of the value. The maximum circle-equivalent diameter of each martensite grain displayed in the EBSD image is considered to be the maximum grain size of the martensite grain.

The maximum aspect ratio of the martensite grains in the hardened layer 11 is 10 or less. Preferably, the maximum aspect ratio of the martensite grains is 9.5 or less. More preferably, the maximum aspect ratio of the martensite grains is 9.1 or less. The method for calculating the maximum aspect ratio of the martensite grains will be described later.

The ratio of the maximum to minimum crystal orientation density of the {011} planes of the multiple martensite grains is 5.0 or less. Preferably, this ratio is 4.1 or less. More preferably, this ratio is 3.6 or less. The minimum and maximum crystal orientation density values are calculated by analyzing the crystal orientation density distribution from data measured by the EBSD (Electron Backscattered Diffraction) method according to the method described in H. J. Bunge, Mathematische Methoden der Texturanalyse, Akademie-Verlag (1969) using spherical harmonic series.

The hardened layer 11 has a martensite phase as its main structural component. More specifically, the ratio of the total area of martensite grains in the hardened layer 11 is 70 percent or more. The ratio of the total area of martensite grains in the hardened layer 11 may be 80 percent or more. The ratio of the total area of cementite grains in the hardened layer 11 is 30 percent or less.

The multiple martensite grains are divided into a first group and a second group. According to this division, the multiple martensite grains consist of multiple martensite grains belonging to the first group and multiple martensite grains belonging to the second group. The minimum grain size of the martensite grains belonging to the first group is greater than the maximum grain size of the martensite grains belonging to the second group.

The value obtained by dividing the total area of martensite grains belonging to the first group by the total area of martensite grains (the sum of the total area of martensite grains belonging to the first group and the total area of martensite grains belonging to the second group) is 0.5 or more. The value obtained by dividing the total area of martensite grains belonging to the first group, excluding the martensite grains belonging to the first group with the smallest grain size, by the total area of martensite grains is less than 0.5.

From another perspective, martensite grains are assigned to the first group in order of grain size. The assignment to the first group ends when the total area of the martensite grains assigned to the first group up to that point is 0.5 times or more the total area of the martensite grains. The remaining martensite grains are then assigned to the second group.

The average grain size of the martensite grains belonging to the first group is 1.10 μm or less. Preferably, the average grain size of the martensite grains belonging to the first group is 1.00 μm or less. More preferably, the average grain size of the martensite grains belonging to the first group is 0.98 μm or less.

The aspect ratio of the martensite grains belonging to the first group is 3.2 or less. Preferably, the aspect ratio of the martensite grains belonging to the first group is 3.0 or less. More preferably, the aspect ratio of the martensite grains belonging to the first group is 2.9 or less.

The plurality of martensite grains may be divided into a third group and a fourth group. According to this division, the plurality of martensite grains consists of a plurality of martensite grains belonging to the third group and a plurality of martensite grains belonging to the fourth group. The minimum grain size of the martensite grains belonging to the third group is greater than the maximum grain size of the martensite grains belonging to the fourth group.

The value obtained by dividing the total area of martensite grains belonging to the third group by the total area of martensite grains (the sum of the total area of martensite grains belonging to the third group and the total area of martensite grains belonging to the fourth group) is 0.7 or more.

The value obtained by dividing the total area of martensite grains belonging to Group 3, excluding martensite grains belonging to Group 3, which have the smallest grain size, by the total area of martensite grains, is less than 0.7.

From another perspective, martensite grains are assigned to group 3 in order of increasing grain size. The assignment to group 3 ends when the total area of the martensite grains assigned to group 3 up to that point is 0.7 times or more the total area of the martensite grains. The remaining martensite grains are then assigned to group 4.

The average grain size of the martensite grains belonging to the third group is 0.80 μm or less. Preferably, the average grain size of the martensite grains belonging to the third group is 0.78 μm or less. More preferably, the average grain size of the martensite grains belonging to the third group is 0.76 μm or less.

The aspect ratio of the martensite grains belonging to the third group is 3.0 or less. Preferably, the aspect ratio of the martensite grains belonging to the third group is 2.95 or less. More preferably, the aspect ratio of the martensite grains belonging to the third group is 2.75 or less.

The average grain size of the martensite grains belonging to the first (third) group, the average aspect ratio of the martensite grains belonging to the first (third) group, and the maximum aspect ratio of the martensite grains are measured using the EBSD method.

More specifically, the process is as follows: Based on the circle-equivalent diameter of each martensite grain calculated as described above, the martensite grains displayed in the EBSD image are determined to belong to the first group (third group). In other words, based on the circle-equivalent diameter of each martensite grain calculated as described above, the martensite grains displayed in the EBSD image are classified into the first and second groups (similarly, into the third and fourth groups). The sum of the circle-equivalent diameters of the martensite grains displayed in the EBSD images classified as the first group (third group) divided by the number of martensite grains displayed in the EBSD images classified as the first group (third group) is determined to be the average grain size of the martensite grains belonging to the first group (third group). The value obtained by dividing the total area of martensite grains belonging to the first group (third group) among the martensite grains displayed in the EBSD image by the total area of the martensite grains displayed in the EBSD image is the value obtained by dividing the total area of the martensite grains belonging to the first group (third group) by the total area of the martensite grains.

The shape of each martensite grain displayed in the EBSD image is approximated as an ellipse using the least-squares method. This ellipse approximation using the least-squares method is performed according to the method described in S. Biggin and D. J. Dingley, Journal of Applied Crystallography, (1977) 10, 376-378. The aspect ratio of each martensite grain displayed in the EBSD image is calculated by dividing the major axis dimension by the minor axis dimension of this ellipse. The maximum aspect ratio of each martensite grain is determined as the maximum aspect ratio of the martensite grain.

Furthermore, the sum of the aspect ratios of the martensite grains displayed in the EBSD images classified in the first group (third group) divided by the number of martensite grains displayed in the EBSD images classified in the first group (third group) is determined to be the average aspect ratio of the martensite grains belonging to the first group (third group).

The cementite grains are divided into a fifth group and a sixth group. According to this division, the cementite grains consist of a plurality of cementite grains belonging to the fifth group and a plurality of cementite grains belonging to the sixth group. The minimum particle size of the cementite grains belonging to the fifth group is greater than the maximum particle size of the cementite grains belonging to the sixth group.

The value obtained by dividing the total area of cementite grains belonging to Group 5 by the total area of multiple cementite grains (the sum of the total area of cementite grains belonging to Group 5 and the total area of cementite grains belonging to Group 6) is 0.5 or more. The value obtained by dividing the total area of cementite grains belonging to Group 5, excluding cementite grains belonging to Group 5 with the smallest grain size, by the total area of cementite grains is less than 0.5.

From another perspective, cementite grains are assigned to group 5 in order of size, starting with the largest. The assignment to group 5 ends when the total area of the cementite grains assigned to group 5 up to that point is 0.5 times or more the total area of the cementite grains. The remaining cementite grains are then assigned to group 6.

The average particle size of the cementite grains belonging to the fifth group is 1.40 μm or less. Preferably, the average particle size of the cementite grains belonging to the fifth group is 1.30 μm or less. More preferably, the average particle size of the cementite grains belonging to the fifth group is 1.20 μm or less.

The number density of the cementite grains belonging to the fifth group is 0.04 grains/μm ^{or more} . Preferably, the number density of the cementite grains belonging to the fifth group is 0.05 grains/μm ^or more. Preferably, the number density of the cementite grains belonging to the fifth group is 1.00 grains/μm ^or less.

The plurality of cementite grains may be divided into a seventh group and an eighth group. According to this division, the plurality of cementite grains consists of a plurality of cementite grains belonging to the seventh group and a plurality of cementite grains belonging to the eighth group. The minimum value of the grain size of the cementite grains belonging to the seventh group is greater than the maximum value of the grain size of the cementite grains belonging to the eighth group.

The value obtained by dividing the total area of cementite grains belonging to Group 7 by the total area of multiple cementite grains (the sum of the total area of cementite grains belonging to Group 7 and the total area of cementite grains belonging to Group 8) is 0.7 or more. The value obtained by dividing the total area of cementite grains belonging to Group 7, excluding cementite grains belonging to Group 7 with the smallest grain size, by the total area of cementite grains is less than 0.7.

From another perspective, cementite grains are assigned to group 7 in descending order of grain size. The assignment to group 7 ends when the total area of the cementite grains assigned to group 7 up to that point is 0.7 times or more the total area of the cementite grains. The remaining cementite grains are then assigned to group 8.

The average particle size of the cementite particles belonging to Group 7 is 1.10 μm or less. Preferably, the average particle size of the cementite particles belonging to Group 7 is 0.90 μm or less. More preferably, the average particle size of the cementite particles belonging to Group 7 is 0.60 μm or less.

The number density of the cementite grains belonging to the seventh group is 0.06 grains/ ^μm2 or more. Preferably, the number density of the cementite grains belonging to the seventh group is 0.10 grains/ ^μm2 or more. More preferably, the number density of the cementite grains belonging to the seventh group is 0.20 grains/ ^μm2 or more. Preferably, the number density of the cementite grains belonging to the seventh group is 1.00 grains/ ^μm2 or less.

The average grain size of cementite grains belonging to Group 5 (Group 7), like the average grain size of martensite grains belonging to Group 1 (Group 3), is measured using the EBSD method described above. The number density of cementite grains belonging to Group 5 (Group 7) is calculated by counting the number of cementite grains belonging to Group 5 (Group 7) that appear in the EBSD image, which was taken as described above so that a sufficient number (20 or more) of martensite grains are included, and dividing that number by the field area of the EBSD image.

The hardened layer 11 contains nitrogen. The average nitrogen concentration in the hardened layer 11 between the outer peripheral surface 10d and a position 10 μm away from the outer peripheral surface 10d is preferably 0.10 mass percent or more. This average nitrogen concentration is, for example, 0.20 mass percent or less. This average nitrogen concentration is measured using an EPMA (Electron Probe Micro Analyzer).

The amount of retained austenite in the outer peripheral surface 10d is preferably 20% by volume or more. The amount of retained austenite is measured by X-ray diffraction on the outer peripheral surface 10d. Specifically, the amount of retained austenite is calculated by comparing the integrated intensity of the X-ray diffraction peak of the austenite phase with the integrated intensity of the X-ray diffraction peak of the martensite phase.

The hardness of the quench-hardened layer 11 on the outer peripheral surface 10d is preferably 700 Hv or more. More preferably, the hardness of the quench-hardened layer 11 on the outer peripheral surface 10d is 750 Hv or more. The hardness of the quench-hardened layer 11 on the outer peripheral surface 10d is measured in accordance with JIS standard ( JIS Z 2244:2009).

The quench-hardened layer 11 contains martensite grains and cementite grains as well as prior austenite grain boundaries. The quench-hardened layer 11 retains traces of the austenite grain boundaries that existed in the steel immediately before quenching, when it was heated to the quenching temperature in the primary or secondary quenching process of the bearing component manufacturing method described below. Based on these traces, the prior austenite grains are the crystal grains that existed in the steel immediately before quenching.

The average grain size of the prior austenite grains in the outer peripheral surface 10d is preferably 8 μm or less. It is even more preferable that the average grain size of the prior austenite grains is 6 μm or less.

The average grain size of the prior austenite grains on the outer peripheral surface 10d is measured using the following method. First, an optical microscope image is taken of the cross section including the outer peripheral surface 10d, showing the prior austenite grain boundaries revealed by the acidic solution (hereinafter, the image obtained by optical microscope imagery is referred to as the "optical microscope image"). The optical microscope image is taken so that it contains a sufficient number (20 or more) of prior austenite grains. Second, the obtained optical microscope image is subjected to image processing based on the JIS standard (JIS G 0551:2013), and the average grain size of each prior austenite grain in the optical microscope image is calculated.

The compressive residual stress of the outer peripheral surface 10d is preferably 100 MPa or more. The compressive residual stress is measured by X-ray stress measurement of the outer peripheral surface 10d.

<Method of manufacturing bearing components>
A method for manufacturing the inner ring 10 will be described below as an example of a method for manufacturing a bearing component according to an embodiment.

Fig. 4 is a process diagram showing a manufacturing method of a bearing component according to an embodiment . As shown in Fig. 4, the manufacturing method of a bearing component according to an embodiment includes a preparation step S1, a carbo-nitriding step S2, a primary quenching step S3, a primary tempering step S4, a secondary quenching step S5, a secondary tempering step S6, and a post-treatment step S7. The preparation step S1, carbo-nitriding step S2, primary quenching step S3, primary tempering step S4, secondary quenching step S5, secondary tempering step S6, and post-treatment step S7 are performed in the order described above.

In the preparation step S1, a ring-shaped workpiece that will become the inner ring 10 is prepared by undergoing the carbo-nitriding step S2, primary quenching step S3, primary tempering step S4, secondary quenching step S5, secondary tempering step S6, and post-treatment step S7. In the preparation step S1, first, the workpiece is hot forged. In the preparation step S1, second, the workpiece is cold forged. Cold forging is preferably performed so that the diameter expansion ratio (diameter of the workpiece after cold forging ÷ diameter of the workpiece before cold forging) is 1.1 or greater and 1.3 or less. In the preparation step S1, third, cutting is performed to approximate the shape of the workpiece to that of the inner ring 10.

In the carbo-nitriding step S2, first, the workpiece prepared in the preparation step S1 is heated to and maintained at a first temperature or higher, thereby subjecting the workpiece to carbo-nitriding. The first temperature is a temperature equal to or higher than the A1 transformation point of the steel that constitutes the workpiece. In the carbo-nitriding step S2, second, the workpiece is cooled. This cooling is performed so that the temperature of the workpiece falls below the Ms transformation point.

In the primary quenching step S3, the workpiece that has been carbonitrided in the carbonitriding step S2 is quenched. In the primary quenching step S3, first, the workpiece is heated to a second temperature (primary quenching temperature). The second temperature is a temperature equal to or higher than the _A1 transformation point of the steel that constitutes the workpiece. The second temperature is preferably lower than the first temperature. Second, in the primary quenching step S3, the workpiece is cooled. This cooling is performed so that the temperature of the workpiece is equal to or lower than the Ms transformation point. Cooling is performed, for example, by oil cooling.

In the primary tempering step S4, the workpiece quenched in the primary quenching step S3 is tempered. The primary tempering step S4 is performed by holding the workpiece at a third temperature (primary tempering temperature) for a first time. The third temperature is a temperature below the _A1 transformation point. The third temperature is, for example, 200°C or higher and 450°C or lower. Preferably, the third temperature is 250°C or higher and 400°C or lower. More preferably, the third temperature is 250°C or higher and 350°C or lower. The first time is, for example, 1 hour or higher and 4 hours or lower.

In the secondary quenching step S5, the workpiece tempered in the primary tempering step S4 is quenched. In the secondary quenching step S5, first, the workpiece is heated to a fourth temperature (secondary quenching temperature). The fourth temperature is a temperature equal to or higher than the _A1 transformation point of the steel constituting the workpiece. The fourth temperature is preferably lower than the second temperature. In the secondary quenching step S5, second, the workpiece is cooled. This cooling is performed so that the temperature of the workpiece is equal to or lower than the Ms transformation point. Cooling is performed, for example, by oil cooling.

In the second tempering step S6, tempering is performed on the workpiece quenched in the secondary quenching step S5. The second tempering step S5 is performed by holding the workpiece at a fifth temperature (secondary tempering temperature) for a second time. The fifth temperature is a temperature below the _A1 transformation point. The fifth temperature is lower than the third temperature. The fifth temperature is, for example, 140°C or higher and lower than 200°C. Preferably, the fifth temperature is 140°C or higher and 180°C or lower.

In post-treatment step S7, the workpiece tempered in the secondary tempering step S6 is subjected to post-treatment. In post-treatment step S7, for example, the workpiece is cleaned, and the surface of the workpiece is subjected to machining such as grinding and polishing. The amount of grinding or polishing is, for example, 200 μm or less. This completes the manufacture of the inner ring 10.

<Action and effect>
Next, the effects of the bearing component according to the embodiment will be described. In the inner ring 10, the maximum grain size of the martensite grains in the hardened layer 11 is 3.5 μm or less, and the maximum aspect ratio of the martensite grains in the hardened layer 11 is 10 or less. The finer the maximum grain size of the martensite grains, the more improved the wear resistance and toughness of the hardened layer 11. Furthermore, the closer the maximum aspect ratio of the martensite grains is to 1, the more spherical the shape of the martensite grains becomes, making the martensite grains less likely to serve as a source of stress concentration. Therefore, the wear resistance and toughness of the hardened layer 11 of the inner ring 10 are improved compared to when the maximum grain size of the martensite grains in the hardened layer exceeds 3.5 μm and the maximum aspect ratio of the martensite grains in the hardened layer is higher than 10.

In the inner ring 10, the ratio of the maximum to the minimum value of the crystal orientation density of the {011} plane of the martensite crystal grains in the quench-hardened layer 11 is 5.0 or less. The closer the ratio of the maximum to the minimum value of the crystal orientation density of the {011} plane of the martensite crystal grains is to 1, the more uniform the formation state of each martensite crystal grain, resulting in improved indentation resistance, wear resistance, and toughness. Therefore, the indentation resistance, wear resistance, and toughness of the quench-hardened layer 11 of the inner ring 10 are improved compared to when the ratio of the maximum to the minimum value of the crystal orientation density of the {011} plane of the martensite crystal grains in the quench-hardened layer is higher than 5.0. Note that in this specification, indentation resistance and wear resistance are collectively referred to as surface damage resistance. The inner ring 10 has improved surface damage resistance and toughness.

When the martensite grains in the hardened layer 11 of the inner ring 10 are divided into a first group and a second group, the average grain size of the martensite grains in the first group, which are relatively large, is 1.1 μm or less. Furthermore, when the martensite grains in the hardened layer 11 of the inner ring 10 are divided into a third group and a fourth group, the average grain size of the martensite grains in the third group, which are relatively large, is 0.8 μm or less. In other words, in the inner ring 10, even the martensite grains in the first group (third group), which are relatively large, have refined grains, thereby improving the wear resistance of the hardened layer 11.

When the martensite grains in the hardened layer 11 of the inner ring 10 are divided into a first group and a second group, the average aspect ratio of the martensite grains in the first group, which are relatively large, is 3.2 or less. Furthermore, when the martensite grains in the hardened layer 11 of the inner ring 10 are divided into a third group and a fourth group, the average aspect ratio of the martensite grains in the third group, which are relatively large, is 3.0 or less. The closer the average aspect ratio of the martensite grains is to 1, the more spherical the shape of the martensite grains becomes, making them less likely to serve as a source of stress concentration. In the hardened layer 11 of the inner ring 10, the martensite grains in the first group (third group), which are relatively large, are also less likely to serve as a source of stress concentration, further improving the wear resistance and toughness of the hardened layer 11.

When the cementite grains in the hardened layer 11 of the inner ring 10 are divided into groups 5 and 6, the average grain size of the relatively large cementite grains in group 5 is 1.4 μm or less. Furthermore, when the cementite grains in the hardened layer 11 of the inner ring 10 are divided into groups 7 and 8, the average grain size of the relatively large cementite grains in group 7 is 1.10 μm or less. As the average grain size of the cementite grains becomes smaller and finer, the martensite grains are also refined, improving the toughness of the hardened layer 11. In other words, in the inner ring 10, even the cementite grains in group 5 (group 7), which have relatively large grain sizes, are refined, thereby improving the toughness of the hardened layer 11.

When the cementite grains in the quench-hardened layer 11 of the inner ring 10 are divided into a fifth group and a sixth group, the number density of the cementite grains in the fifth group, which have a relatively large grain size, is 0.04 grains/ ^μm2 or more. Furthermore, when the cementite grains in the quench-hardened layer 11 of the inner ring 10 are divided into a seventh group and an eighth group, the number density of the cementite grains in the seventh group, which have a relatively large grain size, is 0.06 grains/μm2 or ^more . If the refined cementite grains are dispersed at a high density as described above, the shear resistance of the surface is increased, thereby improving wear resistance.

In a manufacturing method for a bearing component according to an embodiment, the step of primarily tempering the primarily quenched compact is performed at a primary tempering temperature of 200°C or higher and lower than the _A1 transformation point. Evaluation results, described below, have confirmed that when the primary tempering temperature is 200°C or higher, the maximum grain size of martensite grains in the quench-hardened layer 11 is smaller, and the maximum aspect ratio of the martensite grains and the ratio of the maximum to the minimum crystal orientation density of the {011} plane of the martensite grains are lower than when the primary tempering temperature is lower than when the primary tempering temperature is lower than 200°C. Furthermore, when the primary tempering temperature is 200°C or higher, the maximum grain size of martensite grains, the maximum aspect ratio of martensite grains, and the ratio of the maximum to the minimum crystal orientation density of the {011} plane of the martensite grains in the quench-hardened layer 11 are all within the above-described ranges, compared to when the primary tempering temperature is lower than 200°C. Furthermore, it was confirmed that when the primary tempering temperature was 200°C or higher, the resistance to indentation formation was higher than when the primary tempering temperature was less than 200°C.

Tests conducted to confirm the effects of the bearing component according to the embodiment will be described below.
<Sample>
This test was conducted using Samples 1 to 4, which had been machined into the shape of the outer ring of a rolling bearing. The steel used for Samples 1 to 4 was SUJ2. Samples 1 to 4 were all prepared by sequentially performing the preparation process S1 through the secondary tempering process S6 according to the flowchart shown in FIG. 4 , with the only difference being the primary tempering temperature. For Sample 1, the primary tempering temperature was 180°C. For Sample 2, the primary tempering temperature was 200°C. For Sample 3, the primary tempering temperature was 250°C. For Sample 4, the primary tempering temperature was 400°C. The other manufacturing conditions were the same for Samples 1 to 4, specifically as follows: the first temperature in the carbo-nitriding process S2 was 850°C, the second temperature in the primary quenching process S3 was 830°C, the fourth temperature in the secondary quenching process S5 was 810°C, and the secondary tempering temperature in the secondary tempering process S6 was 180°C. The first time period in the primary tempering step S4 was set to 2 hours.

Samples 1 to 4 were evaluated as follows:

<Maximum grain size of martensite crystal grains>
The maximum grain size of martensite crystal grains was measured by the above-mentioned method for Samples 1 to 4. Figures 5 to 8 show EBSD images of Samples 1 to 4 on each raceway surface.

The maximum grain size of martensite crystal grains in sample 1 was 3.5 μm. In contrast, the maximum grain size of martensite crystal grains in sample 2 was 2.6 μm, the maximum grain size of martensite crystal grains in sample 3 was 3.3 μm, and the maximum grain size of martensite crystal grains in sample 4 was 3.1 μm. These results confirmed that samples 2 to 4, in which the primary tempering temperature was 200°C or higher, had finer martensite crystal grains than sample 1, in which the primary tempering temperature was less than 200°C.

<Maximum aspect ratio of martensite grains>
The maximum aspect ratios of the martensite grains were calculated by the method described above for Samples 1 to 4. The maximum aspect ratio of the martensite grains of Sample 1 was 12.5. In contrast, the maximum aspect ratio of the martensite grains of Sample 2 was 9.1, the maximum aspect ratio of the martensite grains of Sample 3 was 9.1, and the maximum aspect ratio of the martensite grains of Sample 4 was 10.0.

These results confirmed that martensite grains were more spheroidized in samples 2 to 4, which had a primary tempering temperature of 200°C or higher, compared to sample 1, which had a primary tempering temperature of less than 200°C.

<Ratio of Maximum to Minimum Crystal Orientation Density of {011} Plane of Martensite Grains>
The ratio of the maximum value to the minimum value of the crystal orientation density of the {011} plane of martensite crystal grains was calculated for Samples 1 to 4 using the method described above. The calculation results are shown in Table 1. As shown in Table 1, the ratio for Sample 1 was 5.3. In contrast, the ratio for Sample 2 was 3.6, the ratio for Sample 3 was 3.5, and the ratio for Sample 4 was 4.1.

These results confirm that samples 2 to 4, in which the primary tempering temperature was 200°C or higher, had more uniform crystal orientations of the martensite grains than sample 1, in which the primary tempering temperature was less than 200°C.

<Average grain size of martensite grains belonging to the first group>
The average grain size of the martensite grains belonging to the first group and the average grain size of the martensite grains belonging to the third group were calculated for Samples 1 to 4 using the method described above. Figure 9 shows the calculation results. The average grain size of the martensite grains belonging to the first group in Sample 1 was 1.12 μm, and the average grain size of the martensite grains belonging to the third group in Sample 1 was 0.83 μm.

In contrast, the average grain size of martensite grains belonging to the first group in samples 2 to 4 was 1.10 μm or less, and the average grain size of martensite grains belonging to the first group in samples 2 and 3 was 1.00 μm or less. The average grain size of martensite grains belonging to the first group in sample 2 was 0.95 μm. The average grain size of martensite grains belonging to the third group in samples 2 to 4 was 0.80 μm or less, and the average grain size of martensite grains belonging to the third group in samples 3 and 4 was 0.77 μm. The average grain size of martensite grains belonging to the third group in sample 2 was 0.74 μm.

These results confirm that in samples 2 to 4, in which the primary tempering temperature was 200°C or higher, the martensite grains were generally smaller than in sample 1, in which the primary tempering temperature was less than 200°C.

<Average aspect ratio of martensite grains>
The average aspect ratios of the martensite grains belonging to the first group and the martensite grains belonging to the third group were calculated for Samples 1 to 4 using the method described above. Figure 10 shows the evaluation results. The average aspect ratio of the martensite grains belonging to the first group in Sample 1 was 3.23. In contrast, the average aspect ratio of the martensite grains belonging to the first group in Sample 2 was 2.86, the average aspect ratio of the martensite grains belonging to the first group in Sample 3 was 2.82, and the average aspect ratio of the martensite grains belonging to the first group in Sample 4 was 3.09.

The average aspect ratio of the martensite grains belonging to the third group in sample 1 was 3.09. In contrast, the average aspect ratio of the martensite grains belonging to the third group in sample 2 was 2.73, the average aspect ratio of the martensite grains belonging to the first group in sample 3 was 2.70, and the average aspect ratio of the martensite grains belonging to the first group in sample 4 was 2.95.

These results confirm that in Samples 2 to 4, in which the primary tempering temperature was 200°C or higher, the martensite grains belonging to the first group (third group), which have relatively large grain sizes among the multiple martensite grains, are spheroidized, compared to Sample 1, in which the primary tempering temperature was less than 200°C.

<Average diameter of cementite grains>
The average particle diameters of the cementite grains belonging to the fifth group and the seventh group were measured for Samples 1 to 4 by the method described above. Fig. 11 shows the calculation results. The average particle diameter of the cementite grains belonging to the fifth group in Sample 1 was 1.35 µm, and the average particle diameter of the cementite grains belonging to the seventh group in Sample 1 was 0.95 µm.

In contrast, the average particle size of cementite particles belonging to Group 5 in Samples 2 to 4 was 1.32 μm or less, and the average particle size of cementite particles belonging to Group 5 in Samples 2 and 3 was 1.20 μm or less. The average particle size of cementite particles belonging to Group 5 in Sample 3 was 1.15 μm.

The average particle size of cementite particles belonging to Group 7 in Samples 2 to 4 was 0.93 μm or less, and the average particle size of cementite particles belonging to Group 7 in Sample 2 was 0.93 μm. The average particle size of cementite particles belonging to Group 7 in Sample 3 was 0.57 μm.

These results confirm that in Samples 2 and 3, in which the primary tempering temperature was 200°C or higher but less than 400°C, the cementite grains belonging to the fifth group (seventh group), which have relatively large grain sizes among the multiple cementite grains, were miniaturized compared to Sample 1, in which the primary tempering temperature was less than 200°C.

<Number density of cementite particles>
The number densities of the cementite grains belonging to the fifth group and the cementite grains belonging to the seventh group were measured for Samples 1 to 4 by the method described above. Fig. 12 shows the calculation results. The number density of the cementite grains belonging to the fifth group for Sample 1 was 0.03 grains/ ^µm² , and the number density of the cementite grains belonging to the seventh group for Sample 1 was 0.07 grains/ ^µm² .

In contrast, the number density of cementite grains belonging to the fifth group in samples 2 to 4 was 0.05 grains/μm ^or more, and the number density of cementite grains belonging to the fifth group in samples 2 and 3 was 0.07 grains/μm or ^more .

The number densities of cementite grains belonging to Group 7 in Samples 2 to 4 were 0.08 grains/μm or ^more , and the number densities of cementite grains belonging to Group 7 in Samples 2 and 3 were 0.10 grains/μm or ^more . The average grain size of cementite grains belonging to Group 7 in Sample 3 was 0.29 grains/ ^μm .

These results confirm that in samples 2 to 4, in which the primary tempering temperature was 200°C or higher, the cementite particles belonging to the fifth group (seventh group), which have relatively large particle sizes among the multiple cementite particles, are dispersed at a high density compared to sample 1, in which the primary tempering temperature was less than 200°C.

<Average nitrogen concentration in quenched hardened layer>
The average nitrogen concentration of the quench-hardened layer was measured using the method described above between the position 10 μm away from the raceway surface and the hardened layer of Samples 1 to 4. The average nitrogen concentration of Samples 1 to 4 was 0.10% by mass or more. The average nitrogen concentration of Samples 1, 2, and 4 was 0.13% by mass or more.

<Amount of retained austenite on raceway surface>
The amount of retained austenite γ on the raceway surfaces of Samples 1 to 4 was measured using the method described above. The amount of retained austenite γ on the raceway surfaces of each of Samples 1 to 4 was 20% by volume or more. The amount of retained austenite γ on the raceway surfaces of Samples 3 and 4 was 24% by volume.

<Hardness of raceway surface>
The hardness of the raceway surfaces of Samples 1 to 4 was measured using the method described above. The hardness of each raceway surface of Samples 1 to 4 was 700 HV or more. The hardness of each raceway surface of Samples 1 to 4 was 780 HV or more. The hardness of each raceway surface of Samples 2 and 3 was harder than the hardness of the raceway surface of Sample 1. The hardness of each raceway surface of Samples 2 and 3 was 790 HV or more.

<Average grain size of prior austenite grains on raceway surface>
The prior austenite grains on the raceway surfaces were measured using the method described above for Samples 1 to 4. The average grain size of the prior austenite grains in Sample 1 was 3.8 μm. In contrast, the average grain size of the prior austenite grains in Sample 2 was 3.4 μm, the average grain size of the prior austenite grains in Sample 3 was 3.5 μm, and the average grain size of the prior austenite grains in Sample 4 was 3.4 μm.

These results confirm that samples 2 to 4, in which the primary tempering temperature was 200°C or higher, had finer prior austenite grains on the raceway surface than sample 1, in which the primary tempering temperature was less than 200°C. In other words, samples 2 to 4 had finer austenite crystals that were present in the steel heated to the quenching temperature in the secondary quenching process and immediately before quenching than sample 1.

<Compressive residual stress on raceway surface>
The compressive residual stress of the raceway surfaces of Samples 1 to 4 was measured using the method described above. The compressive residual stress of each raceway surface of Samples 1 to 4 was 100 MPa or more. The compressive residual stress of each raceway surface of Samples 3 and 4 was 130 MPa or more. The compressive residual stress of the raceway surface of Sample 3 was 140 MPa or more.

The above evaluation results confirmed that, compared to Sample 1, Samples 2 to 4 have fine martensite crystal grains that are more uniformly formed, and fine cementite grains that are dispersed at a higher density. This suggests that the shear resistance of the hardened layers of Samples 2 to 4 is higher than that of Sample 1. It is believed that the higher the shear resistance, the more the surface is activated by the temperature increase associated with shear, and a larger amount of gas is adsorbed to the surface. Therefore, when shear stress acts within each hardened layer parallel to the raceway surface, Samples 2 to 4 are more likely to have improved wear resistance than Sample 1 due to the raceway surface being activated by the temperature increase associated with shear.

<Raceway surface indentation resistance>
The resistance to indentation of each raceway surface of Samples 1 to 4 was evaluated as follows. First, an indentation was formed by pressing a 3/8-inch diameter silicon nitride ceramic ball against the raceway surface of Samples 1 to 4 at the maximum indentation load for 120 seconds, followed by unloading. Three different maximum indentation load conditions were used. In other words, three indentations were formed on the raceway surface of each sample. Second, the depth of each indentation was measured, and the relationship between the maximum contact surface pressure and indentation depth was determined. The maximum contact surface pressure was calculated by dividing each maximum indentation load by the projected area of each indentation (the contact area between the raceway surface and the ceramic ball). Figure 13 shows the evaluation results.

The depth of each indentation in Samples 2 and 3 was shallower than the depth of each indentation in Samples 1 and 4. In other words, the resistance to indentation on the raceway surfaces of Samples 2 and 3 was higher than that of Samples 1 and 4. The indentation depth in Sample 4 was approximately the same as that of Sample 1.

The above evaluation results confirmed that samples 2 to 4 had improved surface damage resistance and toughness of each raceway surface compared to sample 1.

Although the embodiments of the present invention have been described above, the above-described embodiments can be modified in various ways. Furthermore, the scope of the present invention is not limited to the above-described embodiments. 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.

10: inner ring, 10a: top surface, 10b: bottom surface, 10c: inner peripheral surface, 10d: outer peripheral surface, 10e: center shaft, 11: quench-hardened layer.

Claims (9)

A bearing part made of high carbon chromium bearing steel and having a quench hardened layer on the surface,
the quench-hardened layer includes a plurality of martensite grains,
The maximum grain size of the plurality of martensite grains is 3.5 μm or less,
the maximum aspect ratio of the martensite grains is 10 or less;
the ratio of the maximum value to the minimum value of the crystal orientation density of the {011} plane of the plurality of martensite crystal grains is 5.0 or less,
The plurality of martensite grains are divided into a first group and a second group,
the minimum value of the grain size of the martensite grains belonging to the first group is greater than the maximum value of the martensite grains belonging to the second group;
a value obtained by dividing the total area of the martensite grains belonging to the first group by the total area of the plurality of martensite grains is 0.5 or more;
a value obtained by dividing the total area of the martensite grains belonging to the first group, excluding the martensite grains having the smallest grain size, by the total area of the plurality of martensite grains is less than 0.5;
The average grain size of the martensite grains belonging to the first group is 1.1 μm or less,
The plurality of martensite grains are divided into a third group and a fourth group,
the minimum value of the grain size of the martensite grains belonging to the third group is greater than the maximum value of the martensite grains belonging to the fourth group;
a value obtained by dividing the total area of the martensite grains belonging to the third group by the total area of the plurality of martensite grains is 0.7 or more;
a value obtained by dividing the total area of the martensite grains belonging to the third group excluding the martensite grains having the smallest grain size in the third group by the total area of the plurality of martensite grains is less than 0.7;
the average grain size of the martensite grains belonging to the third group is 0.8 μm or less, and the average aspect ratio of the martensite grains belonging to the first group is 2.82 or less;
The average aspect ratio of the martensite grains belonging to the third group is 2.70 or less. The hardened layer further includes a plurality of cementite grains,
The plurality of cementite grains are divided into a fifth group and a sixth group,
the minimum value of the grain size of the cementite grains belonging to the fifth group is larger than the maximum value of the grain size of the cementite grains belonging to the sixth group,
a value obtained by dividing the total area of the cementite grains belonging to the fifth group by the total area of the plurality of cementite grains is 0.5 or more,
a value obtained by dividing the total area of the cementite grains belonging to the fifth group excluding the cementite grains having the smallest crystal grain size in the fifth group by the total area of the plurality of cementite grains is less than 0.5,
The average grain size of the cementite grains belonging to the fifth group is 1.40 μm or less,
The plurality of cementite grains are divided into a seventh group and an eighth group,
the minimum value of the grain size of the cementite grains belonging to the seventh group is larger than the maximum value of the grain size of the cementite grains belonging to the eighth group,
a value obtained by dividing the total area of the cementite grains belonging to the seventh group by the total area of the plurality of cementite grains is 0.7 or more,
a value obtained by dividing the total area of the cementite grains belonging to the seventh group excluding the cementite grains having the smallest crystal grain size in the seventh group by the total area of the plurality of cementite grains is less than 0.7,
2. The bearing component according to claim 1, wherein the average grain size of the cementite grains belonging to the seventh group is 0.95 μm or less. The number density of the cementite grains belonging to the fifth group is 0.05 grains/μm ² or more,
3. The bearing component according to claim 2, wherein the number density of the cementite grains belonging to the seventh group is 0.10 grains/μm ² or more. The quench-hardened layer contains nitrogen,
4. The bearing component according to claim 1, wherein an average nitrogen concentration of the quench-hardened layer between the surface and a position 10 μm away from the surface is 0.10 mass percent or more. A bearing component according to any one of claims 1 to 4, wherein the amount of retained austenite on the surface is 20% by volume or more. A bearing component according to any one of claims 1 to 5, wherein the hardness of the quench-hardened layer on the surface is 730 Hv or more. A bearing component according to any one of claims 1 to 6, wherein the average grain size of prior austenite grains on the surface is 8 μm or less. A bearing component according to any one of claims 1 to 7, wherein the compressive residual stress on the surface is 100 MPa or more. A bearing component according to any one of claims 1 to 8, wherein the high-carbon chromium bearing steel is SUJ2 as specified in the JIS standard.