Rolling bearing and method for manufacturing a rolling bearing

Abstract

The present application suppresses occurrence of peeling caused by inclusions generated at various depths from a surface layer of a raceway. In a rolling bearing, an area in a raceway surface of at least one of an inner ring and an outer ring, which is not affected by machining, is a first area, and an area in which (a) a machining amount is 0.03 or more, (b) a retained austenite amount is 70% or more of a retained austenite amount of the first area, and (c) a compressive residual stress is 500 MPa or more higher than a compressive residual stress of the first area is a second area. To manufacture the rolling bearing, after quenching and tempering treatment is performed on the inner ring and the outer ring, mechanical machining is performed on the raceway surface of at least one of the inner ring and the outer ring under a condition that a maximum contact surface pressure with a machining tool is 7.2 GPa or less, which is calculated assuming that only elastic deformation occurs.

Description

Technical Field

[0001] The present invention relates to a rolling bearing and a method for manufacturing the same, and more particularly to a rolling bearing and a method for manufacturing the same capable of suppressing separation caused by inclusions generated at various depths from the surface of a raceway surface. Background Art

[0002] The lifespan of rolling bearings is determined by ISO and other standards, requiring them to remain within a specified period. A rolling bearing's lifespan is considered to have expired when it no longer rotates smoothly. This can be caused by various factors, including burns, wear, and delamination. Delamination is caused by metal fatigue, and is generally considered to be the lifespan.

[0003] The peeling life of rolling bearings is greatly affected by operating conditions such as load conditions and rotational speed, as well as the conditions of the lubricating oil and the state of the material. Countermeasures are usually taken according to different causes. For example, if foreign matter is mixed in during use, peeling will occur starting from the indentation generated by the bite of the foreign matter. In addition, due to the same mechanism, tiny continuous peeling called spalling occurs in a depleted lubrication environment. Such peeling occurs earlier than peeling that occurs in a good lubrication environment. Therefore, for example, heat treatments such as carbonitriding are used to increase the retained austenite on the surface, thereby extending the peeling life of rolling bearings.

[0004] Furthermore, when the cause is foreign matter intrusion, researching the structure of the seals used to seal rolling bearings to create a structure that prevents foreign matter from entering the bearings from the outset is extremely effective. Therefore, countermeasures are also being implemented to eliminate the causes of foreign matter intrusion. Such countermeasures are particularly effective in situations such as rolling bearings used in automotive transmissions, where foreign matter intrusion cannot be avoided due to the shared lubricant with other components such as gears, and where the lubricant cannot be determined based solely on the condition of the rolling bearings.

[0005] On the other hand, regarding rolling bearings, it is basically recommended to use them in a good lubrication environment suitable for rolling bearings, which is usually the case in general applications. Examples of rolling bearings for such applications include hub unit bearings (HUBs) that support vehicle wheels and rolling bearings for electric motors. It is known that under a good lubrication environment, the state of the material has a great influence on the life of the bearing. The state of the material can be exemplified by various factors such as material composition and hardness, but from the perspective of optimizing industrial mass production conditions, the composition and the like have been roughly limited. In practice, non-metallic inclusions (hereinafter also referred to as "inclusions") generated during the steelmaking stage become the cause of peeling. Therefore, research and development of steelmaking technologies that reduce the amount and size of inclusions are underway.

[0006] For example, Patent Document 1 discloses that the inspection area is controlled within 320 mm. 2 The number of sulfide inclusions with a thickness of 1 μm or more in the bearing is reduced and the maximum diameter of oxide inclusions is controlled to be less than 10 μm, thereby extending the service life of the bearing steel. In addition, Patent Document 2 discloses that the bearing steel with a thickness of 1 μm or more in the bearing steel is reduced and the maximum diameter of oxide inclusions is controlled to be less than 10 μm, thereby extending the service life of the bearing steel. 2 The life of a bearing steel is prolonged by limiting the number of oxide inclusions to 100 to 200 and further limiting the amount of Sb as an impurity element.

[0007] However, specific processes for limiting the number and size of inclusions, as described in Patent Documents 1 and 2, place a significant burden on existing production processes, leading to reduced productivity. Consequently, research has been conducted to reduce the harmfulness of inclusions while allowing them to exist to a certain degree. Patent Document 3 discloses a long-life rolling bearing in which the average hardness of the region 100 to 300 μm from the surface of the inner ring, outer ring, or rolling element before operation is defined as Hv1, and the average hardness of the region 100 to 300 μm from the surface of the inner ring, outer ring, or rolling element after the rolling bearing has been operated to the end of its calculated life is defined as Hv2. In this case, the hardness is defined as "(Hv2 - Hv1) ​​≥ 39." To suppress delamination originating from inclusions, the occurrence of structural changes originating from inclusions that could be the source of delamination is suppressed.

[0008] Prior art literature

[0009] Patent Literature

[0010] Patent Document 1: Japanese Patent No. 3338761

[0011] Patent Document 2: Japanese Patent No. 3779078

[0012] Patent Document 3: Japanese Patent Application Publication No. 2019-90475 Summary of the Invention

[0013] Technical problem that the invention aims to solve

[0014] However, inclusions are not necessarily located inside the rolling bearing; they may also be present on the surface of the raceway (i.e., the surface and adjacent areas). In such cases, cracking occurs without structural changes, and early cracking can lead to delamination, resulting in insufficient inclusion neutralization.

[0015] The present invention has been completed with the above-mentioned problems in mind, and its purpose is to suppress the occurrence of peeling starting from inclusions at various depths from the surface of the raceway surface, such as peeling starting from inclusions generated in rolling bearings such as HUBs and rolling bearings for motors that do not share lubricating oil with other parts (hereinafter also referred to as "peeling caused by inclusions").

[0016] Technical means to solve the problem

[0017] The inventors of this application conducted a detailed study of how the delamination behavior varies depending on the depth of inclusions. They discovered that surface inclusions in the surface layer of the raceway surface themselves act like defects, directly initiating cracks from these inclusions, which then lead to delamination as they progress. They also discovered that suppressing the initiation and progression of cracks and delaying the onset of delamination are effective solutions to this problem, and that applying compressive residual stress and improving the crack propagation resistance caused by the coexistence of retained austenite are effective.

[0018] On the other hand, the inventors of the present application have discovered that, regarding internal inclusions present in areas deeper than the surface layer, the white tissue that develops around the inclusions acts as a defect, causing delamination. Furthermore, they have discovered that delamination caused by internal inclusions can be mitigated by suppressing the development of white tissue, and that pre-dispersing the strain is effective. It should be noted that, in order to disperse the strain, the following method is considered: after quenching and tempering, the raceway surface of the outer ring or inner ring is machined to impart compressive residual stress to the surface layer of the raceway, thereby forcibly dispersing the strain within the raceway. However, excessive machining accelerates the phase transformation of retained austenite in the surface layer of the raceway formed during quenching and tempering to martensite. As a result, the retained austenite decreases, which causes a decrease in resistance to crack growth.

[0019] Based on the above, the inventors of the present application believed that there is an optimal quantitative relationship between the strain (machining amount) introduced into the raceway surface by machining, the amount of retained austenite in the surface layer of the raceway surface, and the compressive residual stress. They conducted intensive research and completed the present invention.

[0020] That is, the above-mentioned problems are solved by the following rolling bearing according to the present invention.

[0021] (1) A rolling bearing characterized in that a plurality of rolling elements are held between an inner ring and an outer ring so as to roll freely, wherein:

[0022] The area of ​​the raceway surface of at least one of the inner ring and the outer ring that is not affected by the machining is defined as the first area.

[0023] The second region is a region having (a) a working amount of 0.03 or more, (b) a retained austenite amount of 70% or more of that of the first region, and (c) a compressive residual stress 500 MPa or more higher than that of the first region.

[0024] Furthermore, the above-mentioned problems are solved by the following method for manufacturing a rolling bearing according to the present invention.

[0025] (2) A method for manufacturing a rolling bearing, characterized in that it is the method for manufacturing a rolling bearing according to (1) above,

[0026] After the inner ring and the outer ring are subjected to quenching and tempering treatment,

[0027] The raceway surface of at least one of the inner ring and the outer ring is machined under the condition that the maximum contact surface pressure with the processing tool calculated assuming only elastic deformation is 7.2 GPa or less.

[0028] Effects of the Invention

[0029] According to the present invention, the effect is significant, particularly in rolling bearings such as HUBs and rolling bearings for motors that do not share lubricating oil with other components. In other rolling bearings, it is also possible to suppress peeling caused by inclusions at various depths from the surface of the raceway surface. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a partially cutaway perspective view showing a radial ball bearing as an example of the rolling bearing according to the present invention.

[0031] Figure 2 This is a graph showing the results of life tests on radial ball bearings of Example 1 and Comparative Example 1.

[0032] Figure 3A This is a graph showing the relationship between the depth from the surface of the raceway surface of the inner ring and the number of white structures generated in the radial ball bearing of Comparative Example 1.

[0033] Figure 3B This is a graph showing the relationship between the depth from the surface of the raceway surface of the inner ring and the number of white structures generated in the radial ball bearing of Example 1.

[0034] Explanation of symbols

[0035] 1 radial ball bearing

[0036] 2 Outer ring raceway surface

[0037] 3 outer ring

[0038] 4 Inner ring raceway

[0039] 5 Inner circle

[0040] 6 Balls

[0041] 7 Cage DETAILED DESCRIPTION

[0042] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

[0043] In the present invention, the type and structure of the rolling bearing are not limited, and examples thereof include Figure 1 As shown in the figure, radial ball bearing 1 comprises an outer ring 3 having an outer ring raceway surface 2 on its inner circumference; an inner ring 5 having an inner ring raceway surface 4 on its outer circumference; and a plurality of balls 6, each serving as a rolling element, disposed between outer ring raceway surface 2 and inner ring raceway surface 4. These balls 6 are circumferentially spaced evenly apart and held in a freely rolling manner by a cage 7.

[0044] There are no particular restrictions on the materials of the outer ring 3, inner ring 5, and balls 6. Common rolling bearing steels such as SUJ2 and SUJ3 can be used. Therefore, the raw material cost can be reduced without adjusting the additive elements in the steel. However, the present invention is characterized in that, in at least one of the inner ring raceway 4 and the outer ring raceway 2, and preferably both, an area such as the center that is not affected by machining is designated as the first area.

[0045] The area that satisfies (a), (b), and (c) is defined as the second area.

[0046] (a) The processing amount is 0.03 or more,

[0047] (b) the amount of retained austenite is 70% or more of the amount of retained austenite in the first region,

[0048] (c) The compressive residual stress is higher than the compressive residual stress of the first region by 500 MPa or more.

[0049] To satisfy the physical properties (a) to (c), it is important to machine the outer rings 3 and 5 after quenching and tempering them during manufacture, under conditions where the maximum contact surface pressure with the machining tool, calculated assuming only elastic deformation, is 7.2 GPa or less. The first region, deeper than the second region and unaffected by machining, such as the core, is referred to as the "unmachined region," while the region affected by machining is referred to as the "machined region." This "machined region" also includes the second region.

[0050] Here, the first region can be defined as, for example, a region having a depth of 250 μm or greater when viewed from the surface of the raceway (depth 0 μm). The second region can be defined as, for example, a region from the surface of the raceway (depth 0 μm) to a depth of 150 μm.

[0051] The machining method is not particularly limited as long as it is a machining method that can impart compressive stress, and for example, lapping and shot peening are preferred.

[0052] Burnishing is a method that uses a pressing tool. For example, a device with a spherical, high-hardness component at its tip is pressed against the outer ring raceway 2 and inner ring raceway 4, causing the outer ring 3 and inner ring 5 to rotate about their axes, thereby applying compressive stress. The maximum contact pressure with the tool, calculated assuming only elastic deformation, is limited to 7.2 GPa or less.

[0053] Shot peening is a method of applying high-hardness projectiles to the outer ring raceway 2 and inner ring raceway 4. Processing conditions such as the projectile size, material, and velocity can be adjusted to achieve the same quality as polishing.

[0054] It should be noted that these pre-machining steps can be performed using conventional methods. First, the steel material is forged into a roughly annular shape. This roughly annular raw material is then turned to produce outer and inner ring blanks of specified shapes. Next, after quenching and tempering, the outer and inner ring raceways 2 and 4 are finished to the specified accuracy through grinding. Finally, the surface layers of the outer and inner ring raceways 2 and 4 are subjected to the aforementioned polishing and other machining processes, followed by final finishing.

[0055] If the maximum contact surface pressure exceeds 8.2 GPa in the preliminary test, it becomes excessive contact surface pressure, failing to meet the physical properties (a) to (c) above, and it is believed that the life extension effect of the rolling bearing will be reduced. The lower limit of the maximum contact surface pressure is preferably 5.8 GPa. If it is lower than this, the compressive stress is small and shallow. In addition, the preferred maximum contact surface pressure is 7.3 GPa.

[0056] Example

[0057] Hereinafter, the effects of the present invention will be described in detail with reference to Examples and Comparative Examples, but the present invention is not limited thereto.

[0058] (Preliminary test)

[0059] In this example, polishing was used as a machining process, and its processing conditions were studied. First, a raceway ring of a specified shape was produced using bearing steel as the raw material, followed by quenching and tempering. The raceway ring was then subjected to a specified grinding process and then polished under three different conditions to produce three different thrust ball bearings of varying specifications.

[0060] In addition, in the polishing processing device, the end shape of the polishing tool is The slip rate is 100%, the circumferential speed is 100m / min, the feed rate of the polishing tool is 0.05mm / rev, and the penetration of the polishing tool is 0.3mm. In addition, filtered working fluid is supplied during processing. Furthermore, assuming that the contact surface pressure between the polishing tool and the raceway surface during the polishing process only produces elastic deformation, the maximum contact surface pressure (P) is calculated. It should be noted that the maximum contact surface pressure (P) of the various rolling bearings produced in the preliminary tests was P = 5.8GPa, P = 7.3GPa, and 8.2GPa, respectively.

[0061] Next, the residual stress in the depth direction of the raceway ring was measured using X-ray diffraction, based on the maximum contact pressure (P). The results are shown in Table 1. In the surface layer of the raceway, the residual stress is negative, indicating compressive stress. Furthermore, it can be seen that the greater the maximum contact pressure (P), the greater the compressive stress that can be introduced into the deeper layers.

[0062] [Table 1]

[0063] Table 1

[0064]

[0065] In addition, the amount of retained austenite in the depth direction from the surface of the raceway surface of the raceway ring was measured by X-ray diffraction, and the results are shown in Table 2. The area with a depth of 250 μm or more is regarded as the unprocessed area (first area), which is the area not affected by the polishing process. The area from the surface (depth 0 μm) to a depth of less than 250 μm is the processed area, which is the area affected by the polishing process. The greater the maximum contact surface pressure (P), the more the retained austenite in the processed area undergoes martensitic transformation, and the less the retained austenite amount.

[0066] Note that the reason why the amount of retained austenite at the surface (depth 0 μm) is small is that the amount of retained austenite is reduced to approximately 5% during the grinding process before the polishing process.

[0067] [Table 2]

[0068]

[0069] According to Tables 1 and 2, if it is assumed that only elastic deformation occurs and the maximum contact surface pressure calculated is below 7.3 GPa, compressive residual stress can be imparted in the processed area, and the amount of retained austenite in the processed area can be ensured to be more than 63% relative to the amount of retained austenite in the unprocessed area.

[0070] (Example 1)

[0071] The inner ring, outer ring, and rolling elements are made of bearing steel and then quenched and tempered. To ensure that multiple inclusions exist at various depths within the surface layer of the inner ring's raceway, a material with multiple inclusions is used. The outer ring and rolling elements are made of material with very few inclusions. The inner ring's raceway is then polished, and the inner and outer ring raceways are then finish-machined to produce a deep groove ball bearing designated 6206. The maximum contact pressure (P) during polishing is 7.2 GPa, calculated based on the results of the preliminary test described above, assuming only elastic deformation. Other polishing conditions are the same as those in the preliminary test.

[0072] (Comparative Example 1)

[0073] A 6206 deep groove ball bearing was produced in the same manner as in Example 1 except that the raceway surface of the inner ring was not polished after quenching and tempering.

[0074] Rolling fatigue life tests were conducted under the following conditions using the 6206 deep groove bearings of Example 1 and Comparative Example 1. Lubrication conditions were excellent, ensuring a sufficient oil film and with little to no foreign matter intrusion. Delamination can be attributed to inclusions on the surface or within the bearings. The test results were based on the surface condition of the delamination test specimens, confirming that no delamination originated from indentations caused by foreign matter.

[0075] Test load: 1410kgf

[0076] Speed: 3800min -1

[0077] ·Suspend: 1000h

[0078] The results are shown in Figure 2 It can be seen that Example 1 has a longer life than Comparative Example 1.

[0079] After the rolling fatigue life test, or after interrupting the life test at any time, observe the cross section of the inner ring raceway surface and calculate the amount of white tissue (the part where the tissue changes) generated per viewing area at each depth from the raceway surface. The results for the 6206 deep groove ball bearing of Comparative Example 1 are shown in Figure 3AThe results for the 6206 deep groove ball bearing of Example 1 are shown in Figure 3B .like Figure 3A As shown, it can be understood that in the 6206 deep groove ball bearing of Comparative Example 1, the number of white structures generated in the area with a depth of 150 μm from the surface of the raceway surface is large, while on the other hand, Figure 3B As shown, it can be understood that in the 6206 deep groove ball bearing of Example 1, the number of white structures generated in the area with a depth of 150 μm from the surface of the raceway surface is significantly suppressed. Figure 3A The dynamic shear stress indicated by the dotted line is obtained by calculation and represents the load condition on the raceway surface under the above test conditions.

[0080] Table 3 shows the results of X-ray diffraction measurements of the retained austenite content and residual stress in the depth direction from the surface of the inner ring raceway of the 6206 deep groove ball bearing of Example 1. The area unaffected by the polishing process, such as the core of the inner ring, is considered the unprocessed area, or the first area. On the other hand, the area affected by the polishing process is considered the processed area. In Example 1, if the area where white structure generation is suppressed, that is, the area from the surface of the raceway to a depth of 150 μm, is considered the second area, then the processed area also includes the second area. The residual stress in the area from the surface of the raceway to a depth of 150 μm (the second area) is 500 MPa or more compared to the unprocessed area, or the first area. Furthermore, the retained austenite content in the area from the surface of the raceway to a depth of 150 μm (the second area) is maintained at 70% or more compared to the unprocessed area (the first area).

[0081] [Table 3]

[0082] Table 3

[0083]

[0084] The strain (processing amount) introduced into the metal structure within the raceway surface during polishing was also measured. First, the inner ring of a 6206 deep groove ball bearing, manufactured in the same manner as the 6206 deep groove ball bearing in Example 1, was polished. The cross-section of the raceway surface was then mechanically polished and chemically polished using colloidal silica. Electron backscatter diffraction analysis was performed at multiple measurement points on the polished cross-section of the raceway surface using an electron backscatter diffraction analyzer manufactured by TSL Solutions Co., Ltd., and the microstructural analysis of the analysis results was performed using OIM (Orientation Imaging Microscopy) analysis software manufactured by TSL Solutions Co., Ltd.

[0085] Specifically, a Hough transform is performed on the clarity of the Kikuchi lines at each measurement point obtained through electron backscatter diffraction analysis, and a Hough transform value (hereinafter sometimes referred to as an "IQ value") is calculated for each measurement point. The IQ value of the measurement point corresponding to the position of each pixel in an image of a raceway surface cross section is applied to each pixel in the image, and a color corresponding to the magnitude of the IQ value is displayed in each pixel of the image, thereby generating mapping data for the raceway surface cross section. The mapping data is then subjected to image analysis, and a distribution curve representing the distribution of IQ values ​​in the mapping data is generated on an orthogonal coordinate system with the IQ value as the X-axis and the frequency of the IQ value as the Y-axis. The machining amount is then calculated for the non-overlapping area, which is the area bounded by the distribution curve of the portion fatigued by polishing and the X-axis in the orthogonal coordinate system, and does not overlap with the area bounded by the distribution curve of the non-fatigue portion not fatigued by polishing and the X-axis. That is, by subtracting the strain of the non-fatigued portion from the strain of the portion fatigued by the lapping process, the strain (machining amount) introduced into the rolling surface cross section by the lapping process can be evaluated.

[0086] Table 4 shows the strain (machining depth) in the depth direction from the surface of the raceway surface of the inner ring of the 6206 deep groove ball bearing of Example 1. It can be seen that the strain (machining depth) from the surface of the raceway surface to a depth of 150 μm is 0.03 or greater, indicating that a strain (machining depth) of 0.03 or greater is appropriate for the second region.

[0087] [Table 4]

[0088] Table 4

[0089] Depth from the surface of the raceway Strain (processing amount) 0~50μm or less 0.10 Greater than 50μm to less than 100μm 0.05 Greater than 100μm to less than 150μm 0.03 Greater than 150μm to less than 200μm 0.02

[0090] While various embodiments have been described above with reference to the accompanying drawings, the present invention is not limited to these examples. Those skilled in the art will be able to devise various variations or modifications within the scope of the claims, and these variations or modifications will naturally fall within the technical scope of the present invention. Furthermore, the various components of the above embodiments may be arbitrarily combined without departing from the spirit of the invention.

[0091] In addition, this application is based on the Japanese patent application (Japanese Patent Application No. 2020-024705) for which it applied on February 17, 2020, and the content is incorporated into this application as a reference.

Claims

1. A rolling bearing, characterized in that: The rolling bearing holds a plurality of rolling elements between an inner ring and an outer ring so as to roll freely. The rolling bearing includes: a first region of the raceway surface of at least one of the inner ring and the outer ring that is not affected by machining; and The second region is an area from the surface of the raceway surface to a depth of 150 μm and satisfies all of the following conditions (a) to (c): (a) the strain in the depth direction from the surface of the raceway surface, i.e., the machining amount, is 0.03 or more, (b) the amount of retained austenite is 70% or more of the amount of retained austenite in the first region, and (c) the compressive residual stress is 500 MPa or more higher than the compressive residual stress in the first region.

2. A method for manufacturing a rolling bearing, characterized in that: The method for manufacturing a rolling bearing according to claim 1, After the inner ring and the outer ring are subjected to quenching and tempering treatment, The raceway surface of at least one of the inner ring and the outer ring is machined under the condition that the maximum contact surface pressure with the processing tool calculated assuming only elastic deformation is 7.2 GPa or less.

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