Rolling bearing and manufacturing method therefor

Abstract

To provide a rolling bearing capable of suppressing both the occurrence of white structure flaking and surface-origin flaking, without executing carburizing or carbonitriding treatment that is environmentally harmful and increases a manufacturing cost.SOLUTION: A rolling bearing (radial ball bearing 1) comprises a pair of steel bearing rings (outer ring 3 and inner ring 5), and a plurality of rolling elements (balls 6) held in a rollable manner between the pair of bearing rings, where a proportion of lower bainite structure in the steel of the bearing ring is 35% or more and 90% or less, a proportion of residual austenite structure is 5% or more and 40% or less, and the remainder is martensite structure and carbides.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a rolling bearing and a method for manufacturing the same. [Background technology]

[0002] In rolling bearings, metal fatigue can occur when used for long periods of time under load, leading to flaking of the raceway surface. One type of flaking, "inclusion-originating flaking," is flaking that originates from inclusions inside the steel material that forms the inner and outer rings and rolling elements. "Surface-originating flaking" is flaking that originates from indentations formed on the raceway surface when foreign matter such as dirt gets caught inside the bearing. "White structure flaking" is flaking that originates from a structural change called white structure, which occurs when hydrogen generated during use due to the decomposition of lubricating oil penetrates into the steel, causing hydrogen embrittlement. It occurs with cracks from inside the material, causing a shortened life. The above various types of flaking occur through different mechanisms, so different measures are required depending on the type of flaking.

[0003] For example, Patent Document 1 proposes a bearing steel that suppresses the occurrence of surface-initiated flaking and has an excellent rolling life in an environment containing foreign matter. Patent Document 1 describes that in order to improve the rolling life in an environment containing foreign matter, the amount of retained austenite in the surface layer after quenching and tempering must be 20% to 45%. It also describes that in order to obtain the above amount of retained austenite without the need for the costly and time-consuming carbonitriding treatment, it is necessary to increase the content of alloy elements compared to SUJ2 steel, which is a common bearing steel, and to specify the surface hardness after quenching and tempering.

[0004] Patent Document 2 describes a bearing steel in which the content of alloy elements in the steel is specified, and the total amount of elements dissolved in the matrix components in a predetermined region from the outermost surface and the amount of residual γ are specified in a carburized, quenched and tempered state or a carbonitrided, quenched and tempered state. Furthermore, Patent Document 3 discloses a bearing steel in which, in addition to the specifications in Patent Document 2, the predicted value √area max of the maximum inclusion diameter among nonmetallic inclusions in the steel is specified. The bearing steels described in Patent Documents 2 and 3 have excellent rolling fatigue life even in an environment in which white structure change occurs due to hydrogen. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2004-124215 A [Patent Document 2] Patent No. 6846901 [Patent Document 3] Patent No. 6639839 Summary of the Invention [Problem to be solved by the invention]

[0006] However, the bearing steel described in Patent Document 1 above is intended to improve the rolling life, particularly in an environment containing foreign matter, and does not take into consideration white structure flaking. The bearing steels described in Patent Documents 2 and 3 are intended to mainly suppress the occurrence of white structure flaking, and do not take into consideration surface-originating flaking. Furthermore, carburizing, quenching, and tempering treatments, or carbonitriding, quenching, and tempering treatments, require longer heat treatment times than normal quenching and tempering treatments, which increases manufacturing costs. Furthermore, when gas carburizing is performed, modified gases containing H2, CO, N2, etc. as the main components are used, and there is a possibility that future use will be restricted from an environmental perspective.

[0007] The present invention has been made in consideration of the above problems, and has an object to provide a rolling bearing and a manufacturing method thereof that can suppress both the occurrence of white structure flaking and surface-initiated flaking, without carrying out carburizing or carbonitriding treatments, which are environmentally friendly and lead to increased manufacturing costs. [Means for solving the problem]

[0008] The rolling bearing according to the present invention has the following configuration [1].

[0009] [1] A rolling bearing comprising a pair of steel raceways and a plurality of rolling elements held in a rollable manner between the pair of raceways, A rolling bearing, characterized in that the ratio of lower bainite structure in the steel in the raceway is 35% or more and 90% or less, the ratio of retained austenite structure is 5% or more and 40% or less, and the remainder is martensite structure and carbides.

[0010] A preferred embodiment of the rolling bearing according to the present invention has the following configuration [2].

[0011] [2] A rolling bearing according to [1], characterized in that the residual stress is -50 MPa or less.

[0012] The method for producing a rolling bearing according to the present invention is as configured in [3] below.

[0013] [3] A method for manufacturing a rolling bearing according to [1] or [2], comprising the steps of: A process of heating the race material to a temperature of 820°C to 950°C and austenitizing it; a cooling step of quenching the austenitized race material to a temperature exceeding the Ms point and not exceeding 270°C; a constant temperature transformation treatment step of holding the race material after the cooling step at a temperature higher than the Ms point and not higher than 270°C for a period of 2 hours or more and not exceeding 24 hours. Effect of the Invention

[0014] According to the present invention, the ratios of lower bainite, martensite and retained austenite structures in the structure of the steel constituting the rolling bearing are appropriately controlled, so that it is possible to provide a rolling bearing and a manufacturing method thereof that can suppress both the occurrence of white structure flaking and the occurrence of surface-originated flaking. [Brief description of the drawings]

[0015] [Figure 1] FIG. 1 is a cross-sectional view showing a rolling bearing according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] As a result of extensive research, the inventors have found that it is important to control the structure of the steel that constitutes a rolling bearing as follows. That is, they have found that appropriately controlling the ratios of lower bainite, martensite, and retained austenite is effective in suppressing white structure flaking due to hydrogen penetration and in suppressing surface-initiated flaking in an environment containing foreign matter. Furthermore, the inventors have investigated the relationship between heat treatment conditions and structure, and have clarified heat treatment conditions for achieving a composite structure with a desired ratio. The present invention was made based on the above findings.

[0017] Hereinafter, the embodiments of the present invention will be described in detail. Note that the present invention is not limited to the embodiments described below.

[0018] [Rolling bearings] FIG. 1 is a cross-sectional view showing a rolling bearing according to an embodiment of the present invention. In the present invention, the type and configuration of the rolling bearing are not particularly limited, so in this embodiment, a radial ball bearing will be described as an example. As shown in FIG. 1, a radial ball bearing (rolling bearing) 1 has a steel outer ring 3 having an outer ring raceway surface 2 on its inner circumferential surface, and a steel inner ring 5 having an inner ring raceway surface 4 on its outer circumferential surface, and the outer ring 3 and the inner ring 5 form a pair of raceways. In addition, a plurality of balls (rolling elements) 6 are arranged between the pair of raceways (between the outer ring raceway surface 2 and the inner ring raceway surface 4). These balls 6 are held by a cage 7 so as to be freely rollable, with the balls 6 being arranged at equal intervals in the circumferential direction.

[0019] In the rolling bearing according to this embodiment, the phase structure of the steel constituting the raceways (the outer ring 3 and the inner ring 5) is appropriately controlled. Specifically, the steel constituting the rolling bearing has a composite structure that has lower bainite and retained austenite, with the remainder being martensite and carbides. The phase structure of the steel defined in this embodiment will be described in detail below.

[0020] <Ratio of lower bainite structure: 35% to 90%> When bainite transformation occurs in the steel structure, compressive residual stress is introduced into the surface layer of the steel material along with the bainite transformation, which has the effect of suppressing deformation of the resulting steel material. The lower bainite structure is also considered to be a structure that affects white flaking. If the ratio of the lower bainite structure is less than 35%, it becomes difficult to ensure the hardness of the raceway and white structure flaking is likely to occur. Therefore, the ratio of the lower bainite structure in the steel that constitutes the raceway of the rolling bearing is 35% or more, preferably 40% or more, and more preferably 50% or more.

[0021] On the other hand, if the ratio of the lower bainite structure exceeds 90%, the austenite structure described below decreases, making surface-initiated flaking more likely to occur. Therefore, the ratio of the lower bainite structure in the steel constituting the raceways of the rolling bearing is set to 90% or less, preferably 85% or less, and more preferably 80% or less.

[0022] <Ratio of retained austenite structure: 5% to 40%> The retained austenite structure is a structure that affects surface-initiated flaking. If the ratio of the retained austenite structure is less than 5%, surface-initiated flaking is likely to occur. Therefore, the ratio of the retained austenite structure in the steel that constitutes the raceway of the rolling bearing is set to 5% or more, preferably 8% or more, and more preferably 10% or more.

[0023] On the other hand, if the ratio of the retained austenite structure exceeds 40%, the hardness of the raceway cannot be sufficiently ensured. Therefore, the ratio of the retained austenite structure in the steel constituting the raceway of the rolling bearing is set to 40% or less, preferably 35% or less, and more preferably 30% or less.

[0024] <Remainder: martensite structure and carbides> In this embodiment, the martensite structure is not particularly necessary, and it is preferable that the martensite structure in the steel is small. In addition, the carbides dispersed in the lower bainite structure are a structure that is inevitably generated in the manufacturing method of the rolling bearing, and do not affect the white structure flaking or the surface-initiated flaking. Therefore, as long as the ratio of the lower bainite structure and the retained austenite structure in the steel constituting the raceway of the rolling bearing is within the above-mentioned range, and the remainder is the martensite structure and the carbides, the ratio is not particularly limited.

[0025] However, if the ratio of carbides is high, the steel material becomes soft, and it may be difficult to obtain the desired hardness as the steel material constituting the raceway of the rolling bearing. Therefore, the ratio of carbides in the steel constituting the raceway of the rolling bearing is preferably 20% or less, more preferably 15% or less, and even more preferably 12% or less. As described above, it is preferable that the martensite structure in the steel constituting the raceway of the rolling bearing is small, for example, 40% or less is preferable, more preferably 25% or less, and even more preferably 20% or less. In addition, in the steel constituting the raceway of the rolling bearing according to this embodiment, ferrite and pearlite are substantially not present, and it is preferable that both are 0%.

[0026] (Calculation method for the ratio of each organization) The fraction of each structure can be calculated, for example, by the following method. After electrolytic polishing of the test piece to a depth of 300 μm from the surface, X-rays are irradiated, and the amount of retained austenite can be calculated from the integrated intensity ratio of the diffraction peaks of the (211) plane of the bcc structure and the (220) plane of the fcc structure. The fraction of other structures can be determined by the EBSD method. Electron beam irradiation is performed on the mirror-finished test piece using an FE-SEM, phases are identified from the obtained diffraction pattern, and the area ratio of carbides is calculated. In addition, the orientations of the martensite structure and the bainite structure are identified from the diffraction pattern, and the ratio of the martensite structure and the bainite structure can be calculated by calculating the orientation difference between the blocks.

[0027] <Residual stress: -50MPa or less> As described above, the lower bainite structure in steel is a structure that affects the residual stress of the steel material. In this embodiment, the ratio of the lower bainite structure in the steel is appropriately adjusted to control the value of the residual stress and suppress the occurrence of white structure spalling. If the residual stress of the rolling bearing is -MPa or less, excellent crack growth characteristics can be obtained, and the occurrence of deformation can be suppressed, so that good bearing performance can be maintained. Therefore, the residual stress of the rolling bearing is preferably -50MPa or less, more preferably -70MPa or less, and even more preferably -100MPa or less.

[0028] (Method of measuring residual stress) Residual stress can be measured, for example, by the following method: After electrolytic polishing of a test piece to a depth of 300 μm from the surface, it is irradiated with X-rays and the residual stress is measured by the Cosα method from the diffraction peak of the (211) plane of the bcc structure.

[0029] In the rolling bearing according to the present embodiment, the ratio of each structure in the steel is appropriately controlled, so that the occurrence of white structure flaking and surface-origin flaking can be suppressed. Note that in the present embodiment, the composition of the steel material constituting the rolling bearing is not particularly limited. For example, SUJ2 steel material specified in JIS G 4805:2019 and 100CrMnSi6-4 specified in ISO 683-17, which are commonly used as bearing steel, can be used.

[0030] In addition, it is also possible to use a steel material in which the alloy elements have been adjusted relative to the above-mentioned steel raw materials, for example a steel material containing C: 0.60 mass% or more and 1.21 mass% or less, Si: 0.40 mass% or more and 1.02 mass% or less, Mn: 0.55 mass% or more and 1.51 mass% or less, Cr: 0.75 mass% or more and 3.00 mass% or less, Mo: 1.00 mass% or less, Ni: 0.20 mass% or less, Cu: 0.20 mass% or less, S: 0.025 mass% or less, P: 0.020 mass% or less, and O: 0.0015 mass% or less, with the balance being Fe and unavoidable impurities.

[0031] [Method of manufacturing rolling bearing] Next, the manufacturing method of the rolling bearing according to this embodiment will be described below in the order of steps.

[0032] <Austenitizing process> First, the steel material is machined into the shape of the raceway of the rolling bearing by turning, and the machined raceway material is heated to a temperature above the A1 point to convert it to austenite. In this process, austenite becomes the main structure in the steel.

[0033] (Austenitizing process temperature) In order to obtain the desired hardness by dissolving carbon in steel by heating for use in rolling bearing raceways, the temperature in the austenitizing process must be 820°C or higher, and preferably 840°C or higher. On the other hand, at a temperature of 950°C, carbides are almost completely dissolved in steel, so heating at a temperature above 950°C is unnecessary and would increase manufacturing costs. Therefore, the temperature in the austenitizing process is set to 950°C or lower, and preferably 880°C or lower.

[0034] <Cooling process> Next, the austenitized race material is quenched to a temperature above the Ms point and below 270°C. In this specification, quenching refers to cooling at a rate fast enough not to reach the Ps line (the temperature at which pearlite structure begins to form) or the Bs line (the temperature at which bainite transformation begins). One example of a quenching method is cooling with salt.

[0035] If the steel is cooled to a temperature below the Ms point after the cooling step and before the following isothermal transformation step is performed, martensitic transformation will start and the structure of the resulting race will contain a martensite structure. In this embodiment, the ratio of the lower bainite structure and the retained austenite structure is specified, and if a large amount of martensite structure is contained, the ratio of the lower bainite structure and the retained austenite structure will change. In this embodiment, the martensite structure is not necessary. Therefore, it is preferable to perform the isothermal transformation step without cooling to a temperature below the Ms point after the cooling step.

[0036] <Isothermal transformation process> The race material after the cooling process is then held at a temperature above the Ms point and below 270°C. This isothermal transformation process transforms the austenite into a lower bainite structure.

[0037] (Retention time for isothermal transformation process) If the holding time in the isothermal transformation treatment step is less than 2 hours, it is not possible to obtain a lower bainite structure of 50% or more. Therefore, the holding time in the isothermal transformation treatment step is set to 2 hours or more, preferably 3 hours or more, and more preferably 4 hours or more. On the other hand, if the holding time in the isothermal transformation treatment step exceeds 24 hours, the ratio of the lower bainite structure in the steel exceeds 90%, making surface-initiated flaking more likely to occur. Therefore, the holding time in the isothermal transformation treatment step is set to 24 hours or less, preferably 16 hours or less, and more preferably 12 hours or less.

[0038] (Maintaining temperature during isothermal transformation treatment) The lower the holding temperature in the isothermal transformation process, the more the desired hardness can be obtained, but if the holding temperature is below the Ms point, even if the holding time is extended, a part of the austenite structure and martensite structure in the steel does not change to a lower bainite structure, and the desired ratio of lower bainite structure cannot be obtained. Therefore, the holding temperature in the isothermal transformation process is a temperature above the Ms point, and is preferably held at a temperature 10°C or higher than the Ms point, and more preferably held at a temperature 20°C higher than the Ms point. A specific example of the holding temperature in the isothermal transformation process is preferably 180°C or higher, and more preferably 200°C or higher.

[0039] On the other hand, if the temperature at which the race material is held after the cooling process exceeds 270°C, upper bainite that adversely affects the mechanical properties of the steel material is generated. Therefore, the holding temperature in the isothermal transformation treatment process is set to 270°C or less, preferably 250°C or less, and more preferably 240°C or less.

[0040] The raceway material after the isothermal transformation process is then cooled and ground to manufacture the raceway of the rolling bearing. The cage and rolling elements are then assembled into the pair of raceways by a general method to manufacture the rolling bearing. In this embodiment, the raceway material processed into the shape of the raceway is heat-treated to manufacture the pair of raceways, but the rolling elements may be manufactured in a similar manner.

[0041] When a rolling bearing is manufactured by the manufacturing method according to the present embodiment, the ratio of each structure in the steel can be appropriately controlled without carrying out carburizing or carbonitriding treatments, which are environmentally harmful and increase manufacturing costs. Therefore, it is possible to manufacture a rolling bearing that suppresses the occurrence of white structure flaking and surface-originating flaking and enables a longer life. EXAMPLES

[0042] Examples of the rolling bearing according to the present embodiment and comparative examples are described below. First, the ratio of each structure was calculated for test pieces that were heat-treated under various heat treatment conditions, and physical properties were measured to investigate changes in structure and physical properties due to heat treatment conditions.

[0043] <Preparation of test specimen> Steel materials having compositions within the ranges shown in Table 1 below were prepared, and various heat treatments were carried out under the heat treatment conditions shown in Table 2 below to prepare test pieces.

[0044] Specifically, symbol A in Table 2 represents heat treatment conditions in which an austenitizing process is carried out in which the steel material is heated to a temperature of 860°C, followed by a cooling process in which the steel material is cooled to a temperature of 210°C, followed by an isothermal transformation process in which the steel material is held at 210°C for 6 hours without lowering the temperature. Symbol B represents heat treatment conditions in which the isothermal transformation process of invention example No. 1 is carried out at a temperature of 210°C for 8 hours. Symbol C represents heat treatment conditions in which a quenching process is carried out in which the steel material is heated to a temperature of 820°C, followed by tempering at a temperature of 200°C.

[0045] Symbol D represents heat treatment conditions in which a quenching process is performed in which the steel material is heated to a temperature of 820°C, followed by tempering at a temperature of 240°C. Symbol E represents heat treatment conditions in which a quenching process is performed in which the steel material is heated to a temperature of 860°C, followed by tempering at a temperature of 180°C. Symbol F represents heat treatment conditions in which a process is performed in which the steel material is heated to a temperature of 860°C, followed by a cooling process in which the steel material is cooled to a temperature of 210°C, followed by an isothermal transformation process in which the steel material is held at a temperature of 210°C for 72 hours without lowering the temperature.

[0046] <Calculation of the ratio of each organization> The structure of the obtained test piece was calculated by the following method. After electrolytic polishing to a depth of 300 μm from the surface, the test piece was irradiated with X-rays, and the amount of retained austenite was calculated from the integrated intensity ratio of the diffraction peaks of the (211) plane of the bcc structure and the (220) plane of the fcc structure. The fraction of other structures was determined by the EBSD method. The mirror-finished test piece was irradiated with an electron beam using an FE-SEM, and phases were identified from the obtained diffraction pattern, and the area ratio of carbides was calculated. In addition, the orientations of the martensite structure and the bainite structure were identified from the diffraction pattern, and the ratio of the martensite structure and the bainite structure was calculated by calculating the orientation difference between the blocks. The ratio of each structure is also shown in Table 2 below.

[0047] <Measurement of physical properties> (Measurement of residual stress) After electrolytic polishing of the test piece to a depth of 300 μm from the surface, it was irradiated with X-rays and the residual stress was measured by the Cosα method from the diffraction peak of the (211) plane of the bcc structure.

[0048] (Hardness measurement) The Vickers hardness after the heat treatment was measured at five points in the center of the test piece with a test load of 1 kgf, and the average value was taken as the measured hardness value.

[0049] Next, a rolling fatigue life test was carried out on the bearings manufactured by carrying out the heat treatment under the various heat treatment conditions described above.

[0050] <Production of test deep groove ball bearings (rolling bearings)> A steel material having the composition shown in Table 1 below was turned to produce a raceway (inner and outer rings) material for a deep groove ball bearing with designation number 6206 specified in JIS B1513-1995. The raceway material was then heat treated under the heat treatment conditions indicated by symbols A to F in Table 2 below, and the heat-treated material was then ground to produce a raceway. The resulting raceway was then combined with 3 / 8-inch steel balls made of SUJ2 steel specified in JIS G 4805:2019 and a plastic cage to produce a test deep groove ball bearing.

[0051] <Rolling fatigue life test> The test deep groove ball bearings thus prepared were mounted on a radial type life test machine and subjected to rolling fatigue life tests under the following conditions to measure the white structure spalling life and the surface origin spalling life. Each test was performed five times, and the life (L 50 ) was calculated as the average value.

[0052] (White texture peeling life measurement conditions) Test load (radial load): 910kgf Rotation speed: 3000min -1 Lubricants: Special lubricants that are prone to hydrogen generation due to decomposition of the lubricant

[0053] (Test conditions for surface initiated flaking life) Test load (radial load): 635kgf Rotation speed: 3000min -1 Lubricant: RO68 (containing 0.05g / 1.2L of iron powder with a size of approximately 100um and a hardness of 870HV)

[0054] The results of the physical property measurements and the rolling fatigue life test are shown in Table 3. In the white structure peeling test and surface initiated peeling test shown in Table 3, the results are expressed as a ratio to the measurement results of Comparative Example No. 1, which was obtained using the commonly used heat treatment condition designated by symbol C, as the standard.

[0055] [Table 1]

[0056] [Table 2]

[0057] [Table 3]

[0058] As shown in Tables 2 and 3 above, inventive examples 1 and 2, the ratio of lower bainite structure and retained austenite structure was within the range specified by the present invention, with the remainder being martensite structure and carbides. Therefore, it was possible to suppress the occurrence of white structure flaking due to hydrogen penetration, and to suppress the occurrence of surface-initiated flaking in an environment containing foreign matter. In addition, because invention examples 1 and 2 have a lower bainite structure, the residual stress is a negative value, which is a compressive residual stress, and therefore the crack propagation characteristics were excellent.

[0059] On the other hand, in Comparative Example No. 1, isothermal transformation treatment was not performed, and therefore a lower bainite structure was not generated. Therefore, compared with Inventive Examples No. 1 and 2, the life in the white structure peeling test and the surface-initiated peeling test was shorter. In Comparative Example No. 2, isothermal transformation treatment was not performed, and therefore a lower bainite structure was not generated, and the ratio of the retained austenite structure was extremely low. Therefore, the life in the surface-initiated peeling test was shorter. For this reason, the white structure peeling test was not performed.

[0060] In Comparative Example No. 3, the heating temperature is higher and more carbon is dissolved in the steel than in Comparative Example No. 1, so the hardness of the martensite is higher and the life ratio in the surface-initiated peeling test is improved. However, in Comparative Example No. 3, an isothermal transformation treatment is not performed, and a lower bainite structure is not generated. Therefore, the life in the white structure peeling test is particularly short. In Comparative Examples No. 1 to 3, the residual stress is a positive value, which is a tensile residual stress, so the crack growth characteristics are considered to be poor. In Comparative Example No. 4, an isothermal transformation treatment is performed at 210°C for 72 hours, so the ratio of the lower bainite structure exceeds the range specified in the present invention. Therefore, the life in the surface-initiated peeling test is extremely short. [Explanation of symbols]

[0061] 1 Radial ball bearing 2 Outer ring raceway 3 Outer ring 4 Inner ring raceway surface 5. Inner Circle 6 balls 7 Cage

Claims

1. A rolling bearing comprising a pair of steel raceways and a plurality of rolling elements held in a rollable manner between the pair of raceways, A rolling bearing, characterized in that a ratio of a lower bainite structure in the steel in the raceway is 35% or more and 90% or less, a ratio of a retained austenite structure is 5% or more and 40% or less, and the remainder is a martensite structure and carbides.

2. 2. The rolling bearing according to claim 1, wherein the residual stress is −50 MPa or less.

3. A method for manufacturing a rolling bearing according to claim 1 or 2, comprising the steps of: heating the race material to a temperature of 820°C or higher and 950°C or lower to austenitize it; a cooling step of quenching the austenitized race material to a temperature exceeding the Ms point and not exceeding 270°C; a constant temperature transformation treatment step of holding the race material after the cooling step at a temperature higher than the Ms point and not higher than 270°C for a period of 2 hours to 24 hours.

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