Bearing part

Abstract

To improve the longevity of a bearing part.SOLUTION: A bearing part has, on a surface thereof, an area on which a load acts in using the bearing part. A clearance volume amount in a surface layer region from the surface of the area on which the load acts to a predetermined depth is smaller than a clearance volume amount in a non-surface layer region other than the surface layer region.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a steel bearing part for use in a bearing. [Background technology]

[0002] Bearing steel inevitably contains foreign matter called nonmetallic inclusions due to the manufacturing process. In addition, in bearings manufactured through rolling and forging, gaps may be formed around the nonmetallic inclusions. These gaps are thought to occur at the interface between the nonmetallic inclusions and the parent phase of the bearing steel due to the difference in deformability between the nonmetallic inclusions and the parent phase of the bearing steel. These gaps may promote the generation of cracks in bearing parts that are subject to rolling fatigue during use and may become the starting point of the cracks. Therefore, in order to improve the life of bearings, it is effective to reduce the gaps around the nonmetallic inclusions.

[0003] In Patent Document 1, the processed surface of the rolling component is subjected to plastic processing (burnishing processing) to fill gaps that exist between nonmetallic inclusions and the parent phase. In the specific plastic processing, a spherical pressing part at the tip of a tool is pressed against the processed surface of the rolling component (inner ring). Although the details of this tool are not clearly disclosed, the description of the embodiment and the disclosed figures show that the tool is pressed against the entire processed surface while being moved to process it, thereby flattening minute irregularities that exist on the processed surface. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2019-167995 A Summary of the Invention [Problem to be solved by the invention]

[0005] The present inventors have found that by using rolling elements (rolling element parts such as those used in bearings) to perform plastic processing by applying partial pressure to the workpiece surface by utilizing rolling contact, it is possible to efficiently fill gaps that exist between non-metallic inclusions and the parent phase.

[0006] Then, when the researchers focused on the volume of the gaps around the nonmetallic inclusions inside the bearing parts that had been subjected to such plastic working, they found that there were regions with different gap volumes. In addition, since the behavior of the cracks is considered to be related to the stress state in the gaps, it is possible to consider that the volume of the gaps is also related to the propagation behavior of the cracks. They found that the life of a bearing part utilizing this knowledge was improved, which led to the completion of the present invention. Patent Document 1 does not disclose any of the above knowledge. [Means for solving the problem]

[0007] The bearing component of the present invention has a load bearing region on its surface where a load acts when the bearing component is in use. Here, the parameter relating to the volume of the gap formed between the non-metallic inclusions and the parent phase is referred to as the gap volume. The gap volume in the surface layer region from the surface of the load bearing region to a predetermined depth is smaller than the gap volume in the non-surface layer region other than the surface layer region.

[0008] The relationship shown in the following formula (I) can be satisfied. Vi(R1)≦Vi(R2) / 5 (I) In the above formula (I), Vi(R1) is the gap volume in the surface region from the surface of the load-bearing region to a depth of 200 μm, and Vi(R2) is the gap volume in the non-surface region.

[0009] The relationship shown in the following formula (II) can be satisfied. Vi(R1)≦2×Vi(R2) / 5 (II) In the above formula (II), Vi(R1) is the gap volume in the surface region from the surface of the load-bearing region to a depth of 300 μm, and Vi(R2) is the gap volume in the non-surface region.

[0010] For example, by setting the equivalent strain in the surface region to 0.20 or more and 2.00 or less and the equivalent strain in the non-surface region to less than 0.20, the gap volume in the surface region can be made smaller than the gap volume in the non-surface region. A groove can be formed in the load-bearing region. This groove is formed by performing partial pressing using a rolling element in the manufacturing process of the bearing component, and is subjected to a predetermined finishing process and a heat treatment (quenching and tempering, carburizing, carbonitriding, induction hardening, etc.) to obtain a required hardness. Depending on the predetermined finishing process (grinding, etc.), the groove may disappear. Even in that case, the region in the surface region where the gap between the non-metallic inclusions in the bearing component and the parent phase is reduced as described above remains. Effect of the Invention

[0011] According to the present invention, by making the gap volume in the surface region smaller than the gap volume in the non-surface region, the gaps existing between the non-metallic inclusions and the parent phase in the surface region can be reduced, and the life of the bearing component can be improved. [Brief description of the drawings]

[0012] [Figure 1] FIG. 2 is an external view of a bearing part (one example). [Diagram 2] FIG. 2 is a view showing a cross section (part) of a portion including a groove in a bearing component. [Diagram 3] FIG. 4 is a diagram for explaining a position where an observation cross section is formed. [Figure 4] FIG. 11 is an explanatory diagram for determining the amount of gap volume in each of the surface region and the non-surface region. [Diagram 5] 4 is a flowchart illustrating a manufacturing method of a bearing component. [Figure 6] 1A to 1C are diagrams illustrating a groove processing process (one example). [Figure 7] 1 is a diagram illustrating a plastic deformation region and a non-plastic deformation region that occur inside a parent phase during groove machining processing. FIG. [Figure 8] FIG. 1 is a top view of a test piece in which Al2O3 particles and sintered material are embedded. [Figure 9] FIG. 13 shows the test specimen after applying a tensile force to form gaps around the Al2O3 particles. [Figure 10] FIG. 1 shows a test piece having grooves formed therein. [Figure 11] 4 is a photograph of an area containing Al2O3 particles observed with an electron microscope in an example. [Figure 12] 1 is a photograph of a region including Al2O3 particles observed with an electron microscope in a comparative example. [Figure 13] FIG. 13 is a diagram for explaining how to determine the volume of a gap. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] (Bearing parts) The steel bearing part of this embodiment will be described below. This bearing part can be used as an automobile part, an industrial machine part, etc. The bearing part of the present invention also includes parts having an operating mechanism and a load pattern similar to those of a bearing.

[0014] Examples of steels used for bearing parts include high carbon chromium bearing steels (steel types: SUJ2 to SUJ5) specified in JIS G4805, carbon steels for machine structures (steel types: S53C, SCr420, SCM420, SNCM220, SNCM420, SNCM815) specified in JIS G4051, and martensitic stainless steel bars (steel types: SUS420J2, SUS440C, etc.) specified in JIS G4303.

[0015] The shape of the bearing component can be appropriately determined depending on the application of the bearing component. For example, as shown in Fig. 1, a bearing component 10 can be formed in a ring shape. A groove 11 is formed on the surface of the bearing component 10, and the groove 11 is an area that receives a load generated when the bearing component is used. In the example shown in Fig. 1, the groove 11 is formed along a circular trajectory, but this is not limited thereto, and as described above, it is sufficient that the groove 11 is formed in an area (position) that receives a load generated when the bearing component is used.

[0016] As will be explained in the manufacturing method of the bearing component described later, the grooves 11 are formed by partially pressing the surface of the bearing component using rolling elements during the manufacturing process of the bearing component, and are subjected to a predetermined finishing process and heat treatment (quenching and tempering, carburizing, carbonitriding, induction hardening, etc.) to obtain the required hardness. Here, depending on the predetermined finishing process (grinding, etc.), the grooves 11 formed as described above may disappear.

[0017] FIG. 2 shows a cross section (part) of a portion of the bearing component 10 including the groove 11. As shown in FIG. 2, the bearing component 10 has two regions (surface region R1 and non-surface region R2) with different gap volumes and equivalent strains, which will be described later. The surface region R1 includes the surface of the groove 11 and is a region from the surface of the groove 11 to a predetermined depth. The predetermined depth can be 500 μm or less. According to a manufacturing method for a bearing component, which will be described later, the surface region R1 (plastic deformation region R3, which will be described later) can be formed in the region up to the above-mentioned predetermined depth. The non-surface region R2 is a region other than the surface region R1 within the bearing component 10. A boundary BL is a boundary between the surface region R1 and the non-surface region R2.

[0018] The gap volume is a parameter that defines the volume of a gap formed between a nonmetallic inclusion and the parent phase in measurement regions MR1 ​​and MR2 described below for one nonmetallic inclusion. In the bearing component 10 of this embodiment, the gap volume Vi(R1) in the surface region R1 is smaller than the gap volume Vi(R2) in the non-surface region R2. The method of determining the gap volumes Vi(R1) and Vi(R2) will be described below.

[0019] An observation cross section extending in the depth direction of the bearing component 10 is exposed by polishing the bearing component 10 or the like, and a predetermined observation region of the observation cross section is observed using a microscope (such as a digital microscope). As shown in Fig. 3, at least three observation cross sections can be cited as the observation cross section, and any one of these observation cross sections can be adopted. Fig. 3 is a top view of the bearing component 10 shown in Fig. 1, and an observation cross section along a cutting line CL1, an observation cross section along a cutting line CL2, and an observation cross section along a cutting line CL3 can be set.

[0020] The cutting lines CL1 and CL2 are substantially the same, but when a tensile force is applied in a direction along the cutting line CL1 (see FIG. 9) as in an embodiment described later, the observed cross sections differ from each other depending on the cutting lines CL1 and CL2. The cutting line CL3 is a tangent to a circle formed by the center positions of the grooves 11, and has a predetermined length. FIG. 3 shows eight cutting lines CL3 as an example. The observed cross section along the cutting line CL3 is a cross section parallel to the cutting line CL3 when the bearing component 10 is viewed from a direction perpendicular to the surface on which the grooves 11 are formed (specifically, directly above the center positions of the grooves 11). The center position of the grooves 11 is the position where the grooves 11 are deepest when the grooves 11 are formed.

[0021] In determining the gap volumes Vi(R1) and Vi(R2), it is most preferable to observe the cross section along the cutting line CL3. When applying a tensile force in a direction along the cutting line CL1, it is preferable to observe the cross section along the cutting line CL1 rather than the cross section along the cutting line CL2. The preferred cross sections are, in this order, the cross section along the cutting line CL3, the cross section along the cutting line CL1, and the cross section along the cutting line CL2. Although multiple cutting lines CL3 are shown in FIG. 3, it is not necessary to observe all of them.

[0022] Fig. 4 shows an observation cross section taken along cutting line CL3 shown in Fig. 3. This observation cross section is a region defined in the depth direction of the bearing component 10 (the vertical direction in Fig. 4) and in a direction parallel to cutting line CL3 (the horizontal direction in Fig. 4), and the size of the observation region can be appropriately determined so as to include the surface region R1 and the non-surface region R2. For example, in the depth direction of the bearing component 10, the observation region can be a region between the center position of the groove 11 (depth 0.0 mm) and a position 0.2 to 0.4 mm deep from the center position of the groove 11. Also, in the direction parallel to cutting line CL3, the observation region can be a region of 5.0 mm.

[0023] In the observation region, a measurement region MR1 (hereinafter referred to as "surface measurement region") included in the surface region R1 is determined, and a measurement region MR2 (hereinafter referred to as "non-surface measurement region") included in the non-surface region R2 is determined. Here, the surface measurement region MR1 and the non-surface measurement region MR2 may be the same size or may be different sizes.

[0024] The surface measurement region MR1 only needs to be included in the surface region R1, and the position of the surface measurement region MR1 within the surface region R1 can be determined appropriately. For example, the region from the surface of the bearing component 10 (i.e., the center position of the groove 11) to a predetermined depth position can be the surface measurement region MR1. On the other hand, the non-surface measurement region MR2 only needs to be included in the non-surface region R2, and the position of the non-surface measurement region MR2 within the non-surface region R2 can be determined appropriately. For example, the region from the boundary BL shown in FIG. 4 to a predetermined depth position can be the non-surface measurement region MR2.

[0025] Next, the nonmetallic inclusions and gaps contained in each of the surface measurement region MR1 and the non-surface measurement region MR2 are identified. Here, only nonmetallic inclusions and gaps that are entirely contained in each of the measurement regions MR1 ​​and MR2 are identified. In other words, if at least one of the nonmetallic inclusions and gaps is partially outside each of the measurement regions MR1 ​​and MR2, it is excluded from being identified.

[0026] After identifying the nonmetallic inclusions and gaps contained in each of the measurement regions MR1 ​​and MR2, the volume of the gaps corresponding to the nonmetallic inclusions is calculated. The volume of the gaps is the volume identified from the area occupied by the gaps in each of the measurement regions MR1 ​​and MR2, and the volume of the gaps can be calculated, for example, by subtracting the volume identified from the area occupied by the nonmetallic inclusions from the volume identified from the area Rt occupied by the nonmetallic inclusions and gaps. Here, the area Rt occupied by the nonmetallic inclusions and gaps is regarded as an elliptical area, and the volume occupied by the nonmetallic inclusions and gaps can be calculated based on the three-dimensional shape (ellipsoid) formed when the elliptical area is rotated around the axis indicating the maximum length of the elliptical area.

[0027] Since each of the measurement regions MR1 ​​and MR2 is a two-dimensional plane, the non-metallic inclusions and gaps included in each of the measurement regions MR1 ​​and MR2 have a two-dimensional shape. In this embodiment, the volume of the gap is found by converting the two-dimensional shapes of the non-metallic inclusions and gaps into three-dimensional shapes using the method described below.

[0028] First, the maximum length of the nonmetallic inclusion and the gap are determined in each of the measurement regions MR1 ​​and MR2. Specifically, the maximum length L1 of the nonmetallic inclusion is determined, and then the maximum length of the gap on an extension of the straight line that defines the maximum length L1 is determined. Since gaps usually exist on both sides of the nonmetallic inclusion in the direction in which the straight line that defines the maximum length L1 of the nonmetallic inclusion extends, the maximum length L2 of the gap on one side of the nonmetallic inclusion and the maximum length L3 of the gap on the other side of the nonmetallic inclusion are determined. Then, the maximum lengths L1 to L3 are determined for all nonmetallic inclusions included in each of the measurement regions MR1 ​​and MR2 and all gaps adjacent to these nonmetallic inclusions.

[0029] Next, an average maximum length (average value) L1_ave is calculated based on the maximum length L1 of all nonmetallic inclusions contained in each of the measurement regions MR1 ​​and MR2. Also, an average maximum length (average value) L2_ave is calculated based on the maximum length L2 of all gaps contained in each of the measurement regions MR1 ​​and MR2, and an average maximum length (average value) L3_ave is calculated based on the maximum length L3 of all gaps contained in each of the measurement regions MR1 ​​and MR2.

[0030] Here, the region Rt occupied by the nonmetallic inclusions and gaps in each of the measurement regions MR1 ​​and MR2 is regarded as an elliptical region. The major axis radius A of this elliptical region can be expressed by the following formula (1), and the minor axis radius B can be expressed by the following formula (2).

[0031]

number

[0032] The three-dimensional shape formed by the movement locus when the above-mentioned elliptical region is rotated around the major axis is regarded as the three-dimensional shape (ellipsoid) of the nonmetallic inclusions and gaps. The volume V1 of the ellipsoid is calculated by the following formula (3).

[0033]

number

[0034] In the above formula (3), B and C are the minor axis radii in the two mutually perpendicular axial directions in a cross section perpendicular to the major axis of the ellipsoid. As described above, since the ellipsoid has a three-dimensional shape formed by rotating an elliptical region around the major axis, the minor axis radii B and C are the same.

[0035] On the other hand, by regarding the three-dimensional shape of the nonmetallic inclusion as spherical (spherical body), the volume V2 of the sphere can be calculated by the following formula (4), where the radius of the sphere corresponds to the minor axis radius B of the elliptical region.

[0036]

number

[0037] The volume V3 of only the gaps is obtained by subtracting the volume V2 of the sphere (in other words, the three-dimensional shape of the non-metallic inclusions) from the volume V1 of the ellipsoid (in other words, the three-dimensional shape including non-metallic inclusions and gaps). This volume V3 becomes the above-mentioned gap volume amounts Vi(R1), Vi(R2). That is, if the volume V3 of only the gaps is obtained for the surface measurement region MR1 as described above, this volume V3 becomes the gap volume amount Vi(R1). Moreover, if the volume V3 of only the gaps is obtained for the non-surface measurement region MR2 as described above, this volume V3 becomes the gap volume amount Vi(R2).

[0038] The units of the gap volumes Vi(R1) and Vi(R2) are units [μm 3 ], or it can be expressed as an index value [-] with the gap volume amount Vi(R2) of the non-surface measurement region MR2 as the reference (1.0). When expressing the gap volume amount Vi(R1) of the surface measurement region MR1 as an index value, the gap volume amount Vi(R1) as a volume value can be divided by the gap volume amount Vi(R2) as a volume value. Note that for the non-surface measurement region MR2, the gap volume amount Vi(R2) as an index value is 1.0 (reference value) as described above.

[0039] In the embodiment described later, the gap volume is obtained for the surface of the groove 11 (depth 0.0 mm). In this case, the above-mentioned observation cross section (cross section along each cutting line CL1, CL2, CL3) is not used, but the bearing component 10 is observed from a direction perpendicular to the surface on which the groove 11 is formed (specifically, directly above the center position of the groove 11) to identify the nonmetallic inclusions and gaps present on the surface of the groove 11. Here, the nonmetallic inclusions and gaps included in the measurement region (corresponding to each measurement region MR1, MR2) located on the surface of the groove 11 are identified, and the identification method is as described above. Then, as described above, the two-dimensional shape of the nonmetallic inclusions and gaps included in the measurement region is converted into a three-dimensional shape, and then the gap volume on the surface of the groove 11 can be obtained. This gap volume becomes the gap volume Vi(R1) in the surface region R1.

[0040] When the surface measurement region MR1 is a region from the surface of the groove 11 (the center position of the groove 11) to a depth of 200 μm, it is preferable that the gap volume amounts Vi(R1) and Vi(R2) satisfy the relationship shown in the following formula (5). When the surface measurement region MR1 is a region from the surface of the groove 11 (the center position of the groove 11) to a depth of 300 μm, it is preferable that the gap volume amounts Vi(R1) and Vi(R2) satisfy the relationship shown in the following formula (6). Vi(R1)≦Vi(R2) / 5 (5) Vi(R1)≦2×Vi(R2) / 5 (6)

[0041] In order to grasp the volume of the gaps existing between the nonmetallic inclusions and the parent phase, it is preferable to make the conditions for obtaining the gap volume uniform. For example, it is preferable to expose the portion showing the maximum diameter of the nonmetallic inclusions having a three-dimensional shape on an observation cross section, and obtain the volume of the gap (i.e., the volume of the gap corresponding to the maximum diameter of the nonmetallic inclusion) based on the area of ​​the nonmetallic inclusions and gaps exposed on the observation cross section. However, since multiple nonmetallic inclusions are present randomly inside the bearing component 10, it is difficult to expose the portion showing the maximum diameter for all nonmetallic inclusions exposed on the observation cross section.

[0042] Therefore, in this embodiment, by obtaining the gap volumes Vi(R1) and Vi(R2) as described above, deviation from the volume of the gap when the conditions are aligned is suppressed.

[0043] It is generally known that the frequency distribution of the maximum diameters of the multiple nonmetallic inclusions contained in the bearing part 10 is close to an exponential distribution. The maximum diameter of the nonmetallic inclusions exposed in any observation cross section (hereinafter referred to as the "apparent maximum diameter") may not indicate the true maximum diameter of the nonmetallic inclusions having a three-dimensional shape, but the difference between the frequency distribution of the apparent maximum diameters and the frequency distribution of the true maximum diameters can be statistically reduced as the total number of nonmetallic inclusions identified from the observation cross section is increased. Therefore, if a lower limit (e.g., 30 pieces) is set in advance for the total number of nonmetallic inclusions identified from the observation cross section (specifically, each measurement region MR1, MR2) and a number of nonmetallic inclusions equal to or greater than the lower limit is identified, the frequency distribution of the apparent maximum diameters can be easily regarded as the frequency distribution of the true maximum diameters.

[0044] On the other hand, a thorough investigation of the size of nonmetallic inclusions and gaps in actual steel materials revealed a tendency for the gap volumes Vi(R1) and Vi(R2) to increase as the apparent maximum diameter of the nonmetallic inclusions increases. In other words, a correlation is recognized between the apparent maximum diameter of the nonmetallic inclusions and the gap volumes Vi(R1) and Vi(R2).

[0045] Furthermore, as described above, the gap volumes Vi(R1), Vi(R2) are determined from the average maximum length (average value) L1_ave of the non-metallic inclusions and the average maximum lengths (average values) L2_ave, L3_ave of the gaps, which makes it easier to suppress variations in the gap volumes Vi(R1), Vi(R2).

[0046] Considering the above-mentioned circumstances, by obtaining the gap volumes Vi(R1) and Vi(R2), it becomes easier to obtain results equivalent to grasping the volume of a gap under the same conditions (for example, the volume of a gap corresponding to the true maximum diameter of a nonmetallic inclusion). Furthermore, determining the magnitude relationship between the gap volumes Vi(R1) and Vi(R2) can be regarded as being equivalent to determining the magnitude relationship between the volumes of gaps under the same conditions in the surface region R1 and the non-surface region R2.

[0047] As described above, in determining the gap volume amounts Vi(R1) and Vi(R2), it is preferable that the total number of non-metallic inclusions identified in each of the measurement regions MR1 ​​and MR2 is equal to or greater than the lower limit. When the total number of non-metallic inclusions is less than the lower limit, the gap volume amounts Vi(R1) and Vi(R2) can be determined by the method described below.

[0048] A new observation cross section is formed along the initially formed observation cross section, and nonmetallic inclusions are identified in the new observation cross section after determining each of the measurement regions MR1 ​​and MR2. Then, new observation cross sections are formed and nonmetallic inclusions are identified until the total number of nonmetallic inclusions identified from the multiple observation cross sections is equal to or greater than the lower limit. Next, in each observation cross section, the gap volume amounts Vi(R1) and Vi(R2) are calculated as described above, and the average value of the gap volume amounts Vi(R1) in the multiple observation cross sections is set as the final gap volume amount Vi(R1), and the average value of the gap volume amounts Vi(R2) in the multiple observation cross sections is set as the final gap volume amount Vi(R2).

[0049] In order to make the gap volume amount Vi(R1) in the surface region R1 smaller than the gap volume amount Vi(R2) in the non-surface region R2, the equivalent strain in the surface region R1 should be larger than the equivalent strain in the non-surface region R2. Here, the equivalent strain in the surface region R1 can be 0.20 or more and 2.00 or less, and is preferably 1.00 or more and 1.50 or less. The equivalent strain in the non-surface region R2 can be less than 0.20. In the surface region R1, the equivalent strain may differ depending on the depth position from the surface of the groove 11, and the equivalent strain at the surface of the groove 11 is likely to be the largest. In this embodiment, it is sufficient that the equivalent strain is 0.20 or more and 2.00 or less at any depth position in the surface region R1.

[0050] In general, the equivalent strain can be obtained by CAE (Computer Aided Engineering) analysis. In order to grasp the equivalent strain according to the depth position described above, the equivalent strain can be obtained by the method described below.

[0051] First, a test piece is prepared from a predetermined steel type, and the test piece is subjected to upsetting at a predetermined upsetting rate, and then the hardness is measured and the equivalent strain at the predetermined upsetting rate is obtained by CAE analysis. Here, a plurality of test pieces having different hardnesses are prepared by varying the cold upsetting rate. The hardness of each of the plurality of test pieces is measured and the equivalent strain at the upsetting rate is obtained by CAE analysis, thereby obtaining a correlation between the hardness and the equivalent strain. This correlation is expressed, for example, by a linear function. For example, the hardness can be Vickers hardness (HV), and can be measured using a test machine corresponding to each hardness.

[0052] By using the above-mentioned correlation, the equivalent strain inside the bearing component 10 can be obtained. Specifically, the hardness corresponding to the depth position from the surface of the bearing component 10 is measured in the cross section of the bearing component 10, and the equivalent strain corresponding to the depth position can be obtained based on the above-mentioned correlation between the hardness and the equivalent strain. This makes it possible to obtain the equivalent strain in the surface region R1 and the equivalent strain in the non-surface region R2. Since the correlation between hardness and equivalent strain also changes depending on the microstructure of the steel at the time of performing the groove processing, it is necessary to obtain the correlation between hardness and equivalent strain taking into account the type of steel and the conditions of pressing by the rolling elements.

[0053] When the gap volume Vi(R1) of the surface region R1 is smaller than the gap volume Vi(R2) of the non-surface region R2, the gaps between the nonmetallic inclusions and the parent phase are reduced in the surface region R1. The reduced gaps can suppress the occurrence of cracks caused by the gaps, thereby improving the life of the bearing component 10. On the other hand, when the equivalent strain of the surface region R1 is 2.00 or less, the occurrence of cracks in the surface region R1 can be prevented. In other words, if the equivalent strain of the surface region R1 is increased to be greater than 2.00, there is a risk of cracks occurring in the surface region R1.

[0054] Fatigue cracks in the bearing component 10 generally occur near the depth where the Hertz stress is maximum in the parent phase in rolling contact with the rolling element, and although the depth varies depending on the size of the rolling element and the curvature of the groove 11, they generally occur within 500 μm from the parent phase surface. As described above, the surface layer region R1 exists within a range from the surface of the groove 11 to a predetermined depth (500 μm or less), so it is possible to cover the range where fatigue cracks are likely to occur. This makes it possible to make it difficult for gaps to occur between the non-metallic inclusions and the parent phase in the surface layer region R1. This makes it possible to suppress the occurrence of fatigue cracks promoted by gaps, and improve the life of the bearing component 10.

[0055] As will be described later in the manufacturing method of the bearing component, the groove 11 is formed by rolling the rolling element while pressing it against a roughly machined part (roughly machined product). If the working surface pressure when pressing the rolling element against the roughly machined product changes, the equivalent strain in the surface region R1 changes, which affects the gap volume Vi(R1) in the surface region R1. Therefore, if the working surface pressure when pressing the rolling element against the roughly machined product is adjusted so that the equivalent strain in the surface region R1 is 0.20 or more and 2.00 or less, the gap volume Vi(R1) in the surface region R1 can be made smaller than the gap volume Vi(R2) in the non-surface region R2. For example, by setting the working surface pressure to 3.5 GPa or more and 5.5 GPa or less, the equivalent strain in the surface region R1 can be made 0.20 or more and 2.00 or less.

[0056] When the equivalent strain of the non-surface region R2 is set to less than 0.20, gaps may occur between the non-metallic inclusions and the parent phase in the non-surface region R2. Therefore, the gap volume Vi(R1) in the surface region R1 tends to be smaller than the gap volume Vi(R2) in the non-surface region R2. The non-surface region R2 is a region other than the surface region R1 and is outside the range where fatigue cracks occur in the bearing component described above, so even if gaps exist in the non-surface region R2, there is no adverse effect on the life of the bearing component.

[0057] (Manufacturing method for bearing parts) Next, the manufacturing method of the bearing component according to this embodiment will be described with reference to the flowchart shown in FIG.

[0058] In step S101, a roughly machined product (a workpiece, described later) that will become the base of the bearing component is prepared. The roughly machined product is processed through steps S102 to S104, described later, to obtain the bearing component that will become the final product. It is sufficient to obtain such a roughly machined product, and the roughly machined product can be prepared using known means.

[0059] For example, in the preparation of a crude product, a steel material having a predetermined shape is manufactured by carrying out oxidation refining of molten steel in an electric arc melting furnace or converter, reduction refining in a ladle refining furnace (LF), reflux vacuum degassing (RH treatment) in a reflux vacuum degassing unit (RH), casting of a steel ingot by continuous casting or general ingot casting, and subsequent plastic processing. Here, the plastic processing may involve hot rolling or hot forging of the steel ingot, followed by cold rolling, cold forging, or cold ironing. Examples of the shape of the steel material include bar steel, tube material, and blank material.

[0060] Next, the material is subjected to one or a combination of hot forging, sub-hot forging, warm forging, cold forging, rolling forging, cold rolling, cold header processing, drawing, etc., and, if necessary, is subjected to heat treatment or turning for the purpose of softening or adjusting the structure, to form a roughly processed product.

[0061] In step S102, the roughly machined product prepared in the process of step S101 is used as the workpiece, and a process of machining grooves thereon (hereinafter referred to as "groove machining process") is performed. In this groove machining process, the rolling body is pressed against the surface of the roughly machined product (workpiece) with a predetermined load while rolling (in other words, the rolling body is pressed while being in rolling contact), thereby plastically deforming the surface of the roughly machined product that is in contact with the rolling body, thereby forming grooves. For example, a groove having a desired shape can be formed along a predetermined orbit by rolling the rolling body multiple times on the same orbit so as to rotate in the circumferential direction of the roughly machined product while being in rolling contact with the roughly machined product. The number of times the rolling body is rotated may be one or multiple times.

[0062] If a jig that can rotate the rolling element while holding it is used, the rolling element can be rolled while being pressed against the rough-machined product. For example, a bearing cage can be used as a jig that maintains the rotation of the rolling element. In addition, a jig or mechanism for holding the test piece is necessary to prevent the test piece from rotating as the rolling element rotates. In addition, since the rolling element rolls between two objects, a member that is placed on the opposing side to sandwich the rolling element together with the test piece to be processed is also necessary. As a mechanism for sandwiching the rolling element, for example, a mechanism for sandwiching the rolling element between upper and lower plates or a mechanism for sandwiching the rolling element between the inner and outer rings of a bearing part can be used.

[0063] In addition, in a mechanism for sandwiching rolling elements between the top and bottom, when groove processing is to be performed on a roughly processed product used as a lower plate, if the hardness of the upper plate is low, the grooves formed by the rolling elements in the upper plate will be deeper when the rolling elements are held by a cage and rolled, and the upper plate and the cage will come into contact with each other, which may prevent the groove processing on the lower plate (roughly processed product) from continuing. In order to avoid this, it is necessary to select a material for the upper plate that is harder than the lower plate (roughly processed product). Here, the hardness of the lower plate (roughly processed product) is not particularly limited. In addition, in order to prevent the roughly processed product from seizing due to rolling, it is desirable to supply lubricating oil to the rolling parts (contact parts of the rolling elements and the roughly processed product) during rolling. Since the groove processing itself is performed in a short time, lubricating oil may be applied to the rolling parts in advance.

[0064] The grooves may be formed in the area that receives the load generated during use of the bearing component, and the position of the groove can be determined in advance according to the position of the area that receives the load. In addition, since the grooves are formed by pressing the rolling elements while rolling in contact with each other, the shape of the groove follows the outer shape of the rolling elements, and the width of the groove depends on the size of the rolling elements. Therefore, if the area of ​​the bearing component that receives the load is known in advance, a rolling element of a size that can cover the width of this area can be used.

[0065] As the rolling body, a sphere can be used. A roller can also be used. The number of rolling bodies when forming the groove is preferably a plurality of rolling bodies in order to stably form the groove. When a plurality of rolling bodies are used, all of the rolling bodies may be rolled on the same track. The load with which the rolling body is pressed against the roughly processed product (workpiece) may be a load capable of forming the groove in the roughly processed product, and for example, the processing surface pressure may be set within a range of 3.5 GPa to 5.5 GPa. In addition, there may be cases where the contact state between the rolling body and the roughly processed product (workpiece) changes with the formation of the groove, and the processing surface pressure decreases, but it is sufficient to ensure the above-mentioned processing surface pressure (3.5 GPa to 5.5 GPa) at the start of the formation of the groove. The material used for the rolling body may be any material capable of forming a groove in the roughly processed product (workpiece), and may be appropriately selected from known materials. In addition, the rolling body used for the bearing part may be used.

[0066] In the above-mentioned groove machining process, the rough-machined product (workpiece) is subjected to partial pressure, and the machining load is relatively low. This allows for cold machining, which provides the following various advantages.

[0067] First, when warm processing is performed, oil cannot be used to prevent ignition, and as a result, seizure may occur when forming the groove. In contrast, cold processing allows the use of lubricating oil during processing, which prevents seizure. Also, compared to warm processing, cold processing can more efficiently fill the gaps between nonmetallic inclusions and the parent phase. This is advantageous in that it improves the life of bearing components by closing the gaps. However, groove processing is not limited to cold processing.

[0068] In the bearing component 10 of this embodiment, by making the gap volume Vi(R1) in the surface region R1 smaller than the gap volume Vi(R2) in the non-surface region R2, it is possible to reduce the gaps existing between the non-metallic inclusions and the parent phase in the surface region R1, thereby improving the life of the bearing component 10. Furthermore, the greater the depth and equivalent strain of the surface region R1 and the wider the width of the groove 11, the larger the area where the gaps are blocked can be.

[0069] The depth of the surface region R1, the magnitude of the equivalent strain, and the width of the groove 11 vary depending on the magnitude of the machining surface pressure and the hardness of the parent phase (workpiece). If the machining surface pressure is increased while the hardness of the workpiece is the same, both the depth of the surface region R1 and the equivalent strain will increase, and the width of the groove 11 will also become wider. Also, if the machining surface pressure is the same while the hardness of the workpiece is increased, plastic deformation becomes less likely to occur, both the depth of the surface region R1 and the equivalent strain will become smaller, and the width of the groove 11 will also become narrower. Also, if the machining surface pressure and the hardness of the workpiece are too high, the workpiece will undergo significant work hardening, and cracks will be more likely to occur at the edges of the groove 11.

[0070] From the above, it is important to adjust not only the machining surface pressure as described above, but also the hardness of the workpiece. When forming the grooves 11 in a cold working process, it is desirable that the hardness of the workpiece is at least 99 HRB or less in Rockwell hardness. The hardness of the rolling elements used to form the grooves 11 should be at a level that is used as rolling elements in normal bearing parts. For example, a Rockwell hardness of 58 HRC or more is a guideline, and more preferably 60 HRC or more. However, depending on the hardness of the roughly machined product, rolling elements with a hardness of 58 HRC or less can also be selected.

[0071] An example of the above-mentioned groove machining process will be described with reference to Fig. 6. In Fig. 6, grooves 11 are formed by pressing spherical rolling elements 20 against the upper surface of a roughly machined product 30 formed in a ring shape and rolling them in a circumferential direction of arrow D (the circumferential direction of the roughly machined product 30). Here, three rolling elements 20 are used, and the three rolling elements 20 roll on the same track. In the groove machining process using the rolling elements 20, for example, a device having a mechanism similar to that of a testing machine used in a thrust type rolling fatigue test can be used for the groove machining process.

[0072] FIG. 7 is a cross-sectional view showing the contact portion between the rolling element 20 and the roughly processed product 30 in the example shown in FIG. 6. The surface of the roughly processed product 30 is plastically deformed by receiving the load F1 from the rolling element 20. Here, the region R3 (hatched region) shown in FIG. 7 is a region that is plastically deformed (hereinafter referred to as the "plastic deformation region") in the inside of the roughly processed product 30, and corresponds to the above-mentioned surface layer region R1 (see FIG. 2). The boundary BL shown in FIG. 7 indicates the boundary between the plastic deformation region R3 and the region that is not plastically deformed (hereinafter referred to as the "non-plastic deformation region") R4 in the inside of the roughly processed product 30. The non-plastic deformation region R4 corresponds to the above-mentioned non-surface layer region R2 (see FIG. 2).

[0073] As shown in FIG. 7, when the rolling element 20 is pressed against the rough-machined product 30, the plastic deformation region R3 receives a load F1 from the rolling element 20 and a binding force F2 from the non-plastic deformation region R4. Here, as can be seen from FIG. 7, the load F1 acts in a plurality of radial directions of the rolling element 20, and the binding force F2 acts as a reaction force against the load F1. When such load F1 and binding force F2 act on the plastic deformation region R3, when a gap exists between the non-metallic inclusion and the parent phase in the plastic deformation region R3, the parent phase is deformed in a direction approaching the non-metallic inclusion, and the gap is easily filled. Then, in the surface layer region R1 (see FIG. 2) corresponding to the plastic deformation region R3, for example, the equivalent strain can be set to 0.20 or more and 2.00 or less, and the gap volume amount Vi(R1) in the surface layer region R1 can be relatively small.

[0074] Although Patent Document 1 presses the pressing portion against the workpiece surface, it does not disclose anything about making the equivalent strain of the surface region R1, including the surface of the groove, greater than the equivalent strain of the non-surface region R2, or about making the gap volume Vi(R1) in the surface region R1 smaller than the gap volume Vi(R2) in the non-surface region R2.

[0075] In Patent Document 1, as described above, the tool is pressed against the entire surface to be machined while moving the tool to perform machining. As a result, the minute uneven shape is flattened over the entire surface to be machined. Patent Document 1 also presents machining of the entire surface to be machined using a forming roll and a mandrel assuming cold rolling. In contrast, in the manufacturing method of this embodiment, grooves are formed in a portion of the roughly machined product along the outer shape of the rolling elements. This eliminates the need to move the tool to machine the entire surface to be machined as in Patent Document 1, or to prepare a forming roll or mandrel for machining, and can reduce the gaps around the nonmetallic inclusions only in the area that will be subject to rolling fatigue when used as a bearing part. In addition, as described above, if the rolling elements are sandwiched between the roughly machined product (workpiece) and the opposing member (counterpart), and a groove machining process is performed while a load is applied to the rolling elements through the counterpart, there is no need to use special machining devices or tools for machining as in Patent Document 1, and simple machining is possible. In these respects, the manufacturing method of this embodiment differs from Patent Document 1. The differences from Patent Document 1 will be described in more detail below.

[0076] In the manufacturing method of this embodiment, by forming a groove along the outer shape of the rolling element, as shown in FIG. 7, it becomes easier to apply a load F1 and a binding force F2 to the plastic deformation region R3. As a result, it becomes easier to fill the gaps present in the plastic deformation region R3, and it becomes easier to fill the gaps in the entire groove 11. On the other hand, in Patent Document 1, although the pressing part is pressed against the processed surface, the purpose is to flatten the minute irregularities on the processed surface, so it becomes more difficult to form the plastic deformation region R3 shown in FIG. 7 compared to the case of forming a groove as in this embodiment, and it becomes more difficult to apply a load F1 and a binding force F2 to the plastic deformation region R3. Therefore, according to this embodiment, it becomes easier to efficiently fill the gaps present around the nonmetallic inclusions compared to Patent Document 1.

[0077] It is believed that fatigue cracks in the bearing component 10 occur immediately below the raceway surface of the bearing component 10, near the depth from the raceway surface where the Hertzian contact stress is maximum (200 μm or less for general bearing components). The plastic deformation region R3 of the above-described embodiment can be formed so as to cover at least the range where fatigue cracks occur, and therefore gaps present in the plastic deformation region R3 are filled, thereby eliminating gaps around non-metallic inclusions that cause fatigue cracks, thereby improving the life of the bearing component 10.

[0078] When the groove processing process described above is performed, a raised portion may be generated around the groove 11. In this case, the raised portion can be removed by grinding. The surface of the groove 11 may be smoothed by performing a polishing process on the surface of the groove 11. Also, as described above, depending on the predetermined finishing process (grinding, etc.), the groove 11 may disappear. These steps can be performed between step S102 and step S103 in FIG. 5. Also, when the amount of grinding or polishing is small, the grinding or polishing may be performed in the process of step S104 described later.

[0079] Returning to FIG. 5, in step S103, a heat treatment is performed on the roughly machined product (workpiece) that has been subjected to the groove machining process. This heat treatment is a heat treatment for imparting a predetermined hardness to the roughly machined product (workpiece), and examples of this include full quenching and tempering (through quenching and tempering), carburizing quenching and tempering, carbonitriding quenching and tempering, carburizing nitriding quenching and tempering, induction quenching and tempering, and nitriding. Here, the predetermined hardness is, for example, preferably 58HRC or more in Rockwell hardness, and more preferably 60HRC or more. When the surface hardening method among the above-mentioned methods is used as the heat treatment method, the above-mentioned hardness is the hardness near the surface of the part.

[0080] In step S104, the roughly machined product (workpiece) that has been heat treated (S103) is subjected to a finish process and a polishing process of the raceway surface. The final product, a bearing part, can be obtained by finishing the roughly machined product (workpiece) and smoothing the raceway surface of the roughly machined product (workpiece) by polishing. EXAMPLES

[0081] Steel was prepared with the chemical composition shown in Table 1 below, with the balance being Fe and unavoidable impurities. The steel type was SUJ2 as specified in JIS G4805.

[0082] [Table 1]

[0083] The above-mentioned SUJ2 steel was melted in an arc melting furnace, and then reduced in a ladle refining furnace to remove impurities and excess oxygen. The steel was then subjected to a reflux vacuum degassing process to further reduce oxygen. The steel was then cast into a steel ingot by continuous casting, and the ingot was hot rolled to produce a steel bar with a diameter of φ65 mm. The steel bar was then subjected to normalizing by holding at 900°C for 1.5 hours and then air-cooling, and then further subjected to spheroidizing annealing by heating to a maximum temperature of 800°C and then slowly cooling. A ring-shaped test piece 100 (see FIG. 8) was produced from this steel bar.

[0084] Here, the diameter (outer diameter) of the test piece 100 is 54 mm, and the inner diameter is 29 mm. The steelmaking process shown here is an example of a manufacturing method when SUJ2 steel is manufactured in an electric furnace, and the heat treatment method is also a general method used as a pretreatment for processing SUJ2 steel, and these methods can be appropriately selected depending on the manufacturing method of the bearing part and the steel type. The size of the test piece 100 is merely a shape used to confirm the effect of this embodiment, and therefore the shape of the test piece 100 does not particularly affect the present invention.

[0085] Next, a microdrill was used to drill a drill hole 101 (see FIG. 8) having a diameter of 4.5 mm and a depth of 6 mm on one surface (this surface is the upper surface) of the test piece 100. A plurality of artificial spherical Al2O3 particles 102 simulating nonmetallic inclusions were prepared, and a member (hereinafter referred to as "sintered material") 103 made by mixing and sintering a powder having the same composition as the main components of the above-mentioned steel was embedded in the drill hole 101. The diameter of the Al2O3 particles 102 was about 150 μm, and the gaps formed between the drill hole 101 and the plurality of Al2O3 particles 102 were filled with the sintered material 103.

[0086] The reason for preparing the sintered material 103 containing a plurality of Al2O3 particles 102 simulating nonmetallic inclusions will now be described. The steel produced in the above steelmaking process is highly clean with respect to nonmetallic inclusions, i.e., the frequency of nonmetallic inclusions is low, so it is not easy to confirm the effect of reducing the gaps around the nonmetallic inclusions in the present invention. In order to facilitate this confirmation, the above-mentioned sintered material 103 was prepared.

[0087] After filling the drill hole 101 with the sintered material 103 containing the Al2O3 particles 102, the sintered material 103 containing the Al2O3 particles 102 was adhered to the matrix of the test piece 100 by HIP (Hot Isostatic Pressing). When performing HIP, specifically, first, the sintered material 103 filled in the drill hole 101 is prevented from falling out as necessary, the test piece 100 is placed in a container made of low carbon steel, and a core metal is inserted into the inner diameter hole formed in the test piece 100, and the container is then sealed. Then, after the inside of the container is evacuated, the container is kept at a predetermined pressure (e.g., 147 MPa) and a predetermined temperature (e.g., 1170°C) for a predetermined time (e.g., 5 hours), and then slowly cooled, so that the Al2O3 particles 102 and the sintered material 103 can be adhered to the matrix of the test piece 100.

[0088] After the HIP processing, normalizing and spheroidizing annealing were performed in the same manner as described above, and then turning was performed to remove the container containing the test piece 100, and the test piece 100 was re-machined into the shape of the test piece 100, and the surface of the test piece 100 was polished.

[0089] Next, a tensile force was applied in a predetermined direction to the test piece 100 using a testing machine capable of tension. This caused the test piece 100 to deform as shown in Fig. 9, and with this deformation, a gap S was formed around the Al2O3 particle 102. This gap S simulates a gap that may exist around a nonmetallic inclusion in a bearing component, and in this embodiment, the gap S was intentionally formed.

[0090] Next, a test piece (Example) 100 in which a groove processing process was performed on the test piece 100 shown in Fig. 9, and a test piece (Comparative Example) 100 in which a groove processing process was not performed on the test piece 100 shown in Fig. 9 were prepared. Here, in the test piece (Comparative Example) 100, since the groove processing process was not performed, gaps S remain formed around the Al2O3 particles 102.

[0091] In the groove processing, as shown in FIG. 10, grooves 104 were formed on the surface of the test piece 100 where the Al2O3 particles 102 with gaps S were buried at a depth of about 50 to 300 μm from the surface of the test piece 100. The grooves 104 were formed along the circumferential direction of the ring-shaped test piece 100 and on a circular track. Here, in the example shown in FIG. 10, the grooves 104 were formed at positions overlapping with some of the Al2O3 particles 102 in the above-mentioned depth range, but since the width of the grooves 104 depends on the size of the rolling element and the processing surface pressure during the groove processing, the grooves 104 can also be formed at positions overlapping with all of the Al2O3 particles 102 depending on the selection. In the groove processing of the embodiment, a spherical rolling element made of quenched and tempered SUJ2 steel with a diameter of 9.525 mm was used, and the rolling element was pressed against the test piece 100 at processing surface pressures of 4.0 GPa and 4.5 GPa, and the rolling element was rotated in the circumferential direction to form the grooves 104. The width of the groove 104 is about 2.5 mm, and the depth of the groove 104 is about 170 μm.

[0092] The grooved surface of the test piece 100 of the example and the comparative example was polished in the depth direction until the vicinity of the maximum diameter of the Al2O3 particle was exposed, and then the Al2O3 particle 102 was observed with an optical microscope from directly above the groove 104 (i.e., from the direction in which the grooved surface can be observed). The observed test piece 100 was formed by pressing a rolling element against the test piece 100 with a processing surface pressure of 4.5 GPa. As shown in FIG. 11, it was confirmed that the test piece 100 of the example did not have a gap S around the Al2O3 particle 102, and the gap S was filled. On the other hand, as shown in FIG. 12, it was confirmed that the test piece 100 of the comparative example had a gap S around the Al2O3 particle 102.

[0093] When the equivalent strain was obtained for the bearing part of the embodiment, it was found that the bearing part had a surface region R1 and a non-surface region R2 (see FIG. 2) with different equivalent strains. For the surface region R1, the equivalent strain was obtained at the surface of the groove 104 (position where the depth is 0 μm) and at depth positions 200 and 300 μm from the surface. The method of obtaining the equivalent strain was as described above, and after measuring the hardness at the depth positions, the equivalent strain at the depth positions was obtained based on the correlation between the hardness and equivalent strain obtained in advance.

[0094] On the other hand, for the test pieces 100 of the examples and comparative examples, the observation cross sections along the cutting line CL3 were exposed as described above, and the observation regions were observed by a digital microscope. As the cutting line CL3, the cutting line CL3 intersecting with the cutting line CL2 among the cutting lines CL3 shown in FIG. 3 was used. Here, the observation region was a region from the surface of the groove 104 (the center position of the groove 104) to a depth position (0.4 mm) in the depth direction of the test piece 100, and a region of 5.0 mm in the direction parallel to the cutting line CL3.

[0095] Then, the volume of the gap included in the measurement region was calculated on the surface of the groove 104 (position where the depth is 0 μm). The volumes of the gaps included in each of the surface measurement region MR1 and the non-surface measurement region MR2 were also calculated. Here, the surface measurement region MR1 was a region from the surface of the groove 104 (center position of the groove 104) to each depth position (200, 300 μm) in the depth direction of the test piece 100, and was a region of 5.0 mm in the direction parallel to the cutting line CL3. The non-surface measurement region MR2 was a region from the surface of the groove 104 (center position of the groove 104) in the depth direction of the test piece 100 between 500 μm and 1.2 mm in depth, and was a region of 5.0 mm in the direction parallel to the cutting line CL3.

[0096] A method for determining the volume of the gap will be described with reference to Fig. 13. As shown in Fig. 13, the maximum length (diameter) L1 of the Al2O3 particle 102 and the maximum lengths L2 and L3 of the gaps S present on both sides of the Al2O3 particle 102 are determined. The maximum lengths L2 and L3 are the lengths of the gaps S on an extension line of the maximum length (diameter) L1. In this example, a tensile force was applied to the test piece 100, so that the gaps S were formed only in the tensile direction of the test piece 100 (the left-right direction in Fig. 13), and the sum of the maximum lengths L1, L2, and L3 is the maximum length in the region Rt occupied by the Al2O3 particles 102 and the gaps S.

[0097] In this example, since a plurality of Al2O3 particles 102 are embedded in the drill hole 101 together with the sintered material 103 as described above, the above-mentioned maximum lengths L1 to L3 are obtained for each Al2O3 particle 102. For this purpose, an average maximum length (average value) L1_ave is obtained from the plurality of maximum lengths L1, an average maximum length (average value) L2_ave is obtained from the plurality of maximum lengths L2, and an average maximum length (average value) L3_ave is obtained from the plurality of maximum lengths L3.

[0098] The Al2O3 particles 102 and the gaps S were converted from a two-dimensional shape to a three-dimensional shape by rotating the region Rt occupied by the Al2O3 particles 102 and the gaps S around the axis indicating the maximum length L1. In this example, the three-dimensional shape including the Al2O3 particles 102 and the gaps S was considered to be an ellipsoid. The volume V1 of this ellipsoid was calculated from the above formula (3), and the volume V2 of the three-dimensional shape of the Al2O3 particles 102, which was a sphere, was calculated from the above formula (4). Then, the volume V3 of the gaps S was calculated by subtracting the volume V2 from the volume V1.

[0099] In reality, the region Rt including the Al2O3 particles 102 and the gaps S may not be a true elliptical region. However, even if the region Rt is regarded as an elliptical region, the area of ​​the elliptical region does not deviate significantly from the true area of ​​the region Rt, and the volume of the ellipsoid identified from the region Rt can be easily calculated.

[0100] Based on the volume V3 of the gap S in the surface measurement region MR1 and the volume V3 of the gap S in the non-surface measurement region MR2, the gap volume amount (index value) Vi(R1) in the surface region R1 was calculated. Here, the gap volume amount (index value) Vi(R1) in the non-surface region R2 was set to 1.0 (reference value).

[0101] The results of the equivalent strain and gap volume obtained in the above manner are shown in Tables 2 and 3 below. In Tables 2 and 3 below, the machining surface pressures (4.0 GPa, 4.5 GPa) when the grooves 104 were formed were different. When the equivalent strain was obtained for the test piece 100 of the comparative example, the equivalent strain was less than 0.20 regardless of the depth position from the surface of the bearing component. In other words, the test piece 100 of the comparative example did not have two regions (surface region R1 and non-surface region R2) with different equivalent strains and gap volumes, as in the examples.

[0102] [Table 2]

[0103] [Table 3]

[0104] According to Tables 2 and 3, the gap volume amount Vi(R1) in the surface region R1 up to a depth of 200 μm (surface measurement region MR1) and the gap volume amount Vi(R2) in the non-surface region R2 satisfy the relationship shown in the above formula (5). Also, the gap volume amount Vi(R1) in the surface region R1 up to a depth of 300 μm (surface measurement region MR1) and the gap volume amount Vi(R2) in the non-surface region R2 satisfy the relationship shown in the above formula (6).

[0105] Next, for the test piece 100 of the example separately produced by the same method as above, in order to smooth the surface of the grooved surface, the groove 104 formed by pressing the rolling body to such an extent that the surface layer region R1 remains was removed by grinding. At this time, the groove 104 was removed by grinding, but the region in which the gaps around the inclusions were filled by the groove processing process remains. Next, the test piece 100 of the above-mentioned example and comparative example was subjected to quenching and tempering to manufacture a bearing part with a hardness (Rockwell hardness) of 58HRC or more. The dislocations introduced into the microstructure of the steel due to the equivalent strain imparted during the groove processing process are eliminated through recovery and recrystallization during this heat treatment process, but the effect of reducing the gaps around the non-metallic inclusions is maintained even after the heat treatment. Note that a more desirable hardness for a bearing part is 60HRC or more.

[0106] A thrust type rolling fatigue test was performed on the above-mentioned bearing part. In the thrust type rolling fatigue test, a race (model number 51305) of a single thrust bearing made of SUJ2 was used as the upper plate, and the test piece 100 was used as the lower plate. Three rolling elements were arranged at equal intervals (120° pitch) on a circular track with a diameter of 38.5 mm between the upper plate and the lower plate using a cage. Here, as the rolling elements, steel balls made of SUJ2 with a diameter of 9.525 mm were used. For the test piece 100 of the embodiment, the rolling elements (steel balls) were arranged so as to overlap the location where the groove 104 was formed on the circumference and the position of the Al2O3 particles embedded below it, and for the test piece 100 of the comparative example, the rolling elements (steel balls) were arranged so as to overlap the location where the Al2O3 particles 102 were embedded.

[0107] In performing the thrust type rolling fatigue test, a load was applied from the rolling element (steel ball) so that a maximum Hertzian contact stress of 5.3 GPa was applied to the test piece (Example / Comparative Example) 100. Here, the load cycle speed was 1800 cycles / min, and the test piece was immersed in an ISO VG68 oil bath for lubrication. The thrust type rolling fatigue test was performed at room temperature.

[0108] From the results of the thrust type rolling fatigue test described above, the total number of revolutions until flaking occurs on 10% of the test pieces 100 from the short life side is calculated based on the Weibull distribution function, and this is called L 10 The lifespan of the comparative example was 10 L in the examples and comparative examples when the life is indexed to 1.0 10 The life span is shown in Table 4 below.

[0109] [Table 4]

[0110] As shown in Table 4 above, the embodiment has a higher L than the comparative example. 10As in this embodiment, the bearing part has a surface region R1 in which the equivalent strain is relatively large and the clearance volume Vi (R1) is relatively small, and a non-surface region R2 in which the relative strain is relatively small and the clearance volume Vi (R2) is relatively large. 10 On the other hand, when the two regions (surface region R1 and non-surface region R2) with different equivalent strains and gap volumes are not formed as in the comparative example, L 10 It has poor life span. [Explanation of symbols]

[0111] 10: bearing part, 11: groove, 20: rolling element, 30: roughly machined product, 100: test piece, 101: drill hole, 102: Al2O3 particles, 103: sintered material, 104: groove, R1: Surface region, R2: Non-surface region, R3: Plastic deformation region, R4: Non-plastic deformation region MR1: Surface measurement area, MR2: Non-surface measurement area

Claims

1. The bearing component has a load bearing area on a surface of the bearing component where a load acts when the bearing component is in use, A bearing component characterized in that, with regard to a gap volume relating to the volume of gaps formed between non-metallic inclusions and a parent phase, the gap volume in a surface region from the surface of the load bearing region to a predetermined depth is smaller than the gap volume in a non-surface region other than the surface region.

2. 2. A bearing component according to claim 1, which satisfies the relationship shown in the following formula (I). Vi(R1)≦Vi(R2) / 5...(I) Vi(R1) is the gap volume in a region of the surface region from the surface of the load bearing region to a depth of 200 μm, and Vi(R2) is the gap volume in the non-surface region.

3. 2. The bearing component according to claim 1, wherein the relationship shown in the following formula (II) is satisfied. Vi(R1)≦2×Vi(R2) / 5...(II) Vi(R1) is the gap volume in a region of the surface region from the surface of the load bearing region to a depth of 300 μm, and Vi(R2) is the gap volume in the non-surface region.

4. 4. The bearing component according to claim 1, wherein the predetermined depth is 500 μm or less.

5. 4. The bearing component according to claim 1, wherein the equivalent strain in the surface region is 0.20 or more and 2.00 or less, and the equivalent strain in the non-surface region is less than 0.

20.

6. 4. A bearing component according to claim 1, wherein a groove is formed in the load bearing area.

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