Rolling bearing and method for manufacturing the same

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 Art

[0002] In a rolling bearing, metal fatigue occurs due to the load being applied and used for a long time, and the surface of the raceway may peel off. "Inclusion-originated peeling", which is one form of peeling, is peeling that occurs starting from inclusions inside the steel material forming the inner ring, outer ring, or rolling elements. "Surface-originated peeling" is peeling that occurs starting from indentations formed on the raceway surface when foreign matter such as dust is bitten into the inside of the bearing. "White structure peeling" is peeling that occurs starting from a tissue change called a white structure, in which hydrogen generated during use due to decomposition of lubricating oil penetrates into the steel and causes hydrogen embrittlement, and it occurs with cracks from inside the material, causing a reduction in the service life. Since the above various types of peeling occur due to different mechanisms, different countermeasures are required depending on the type of peeling.

[0003] For example, Patent Document 1 proposes a steel for bearings that suppresses the occurrence of surface-originated peeling and has excellent rolling life in an environment where foreign matter is mixed in. Patent Document 1 describes that in order to improve the rolling life in an environment where foreign matter is mixed in, it is necessary to set the amount of retained austenite in the surface layer after quenching and tempering to 20% to 45%. Further, in order to obtain the above amount of retained austenite by omitting the carbo-nitriding treatment that is costly and time-consuming, it is described that it is necessary to increase the content of alloy elements and define the surface hardness after quenching and tempering compared to SUJ2 steel, which is a general steel for bearings.

[0004] Furthermore, Patent Document 2 describes a bearing steel in which the content of alloying elements in the steel is specified, as well as the total amount of elements dissolved in the matrix components in a predetermined region from the outermost surface and the amount of residual γ in the state after carburizing, quenching and tempering, or carbonitriding, quenching and tempering. In addition, Patent Document 3 discloses a bearing steel in which, in addition to the provisions of 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 exhibit excellent rolling fatigue life even in environments in which white structure changes occur due to hydrogen. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2004-124215 [Patent Document 2] Patent No. 6846901 [Patent Document 3] Patent No. 6639839 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, the bearing steel described in Patent Document 1 above is intended to improve rolling life, particularly in environments with foreign matter contamination, and hardly considers white structure delamination. Furthermore, the bearing steels described in Patent Documents 2 and 3 mainly suppress the occurrence of white structure delamination and do not consider surface-initiated delamination. In addition, carburizing, quenching, and tempering treatments, or carbonitriding, quenching, and tempering treatments, involve longer heat treatment times compared to normal quenching and tempering treatments, leading to increased manufacturing costs. Moreover, when performing gas carburizing, modified gases mainly composed of H2, CO, N2, etc., are used, which may restrict future use from an environmental standpoint.

[0007] This invention has been made in view of the above problems, and aims to provide a rolling bearing and a method for manufacturing the same that can suppress both the occurrence of white structure delamination and surface-initiated delamination without performing carburizing or carbonitriding treatments that lead to environmental burden and increased manufacturing costs. [Means for solving the problem]

[0008] The rolling bearing according to the present invention has the configuration shown in [1] below.

[0009] [1] A rolling bearing comprising a pair of steel raceways and a plurality of rolling elements held rotatably between the pair of raceways, A rolling bearing characterized in that the proportion of lower bainite structure in the steel of the raceway ring is 35% to 90%, the proportion of retained austenite structure is 5% to 40%, and the remainder is martensitic structure and carbides.

[0010] A preferred embodiment of the rolling bearing according to the present invention is configured as shown in [2] below.

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

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

[0013] A method for manufacturing a rolling bearing, which is described in [3] [1] or [2], The process involves heating the raceway material to a temperature of 820°C to 950°C to austenitize it, A cooling step is performed to rapidly cool the austenitized raceway material to a temperature above the Ms point and below 270°C. A method for manufacturing a rolling bearing, characterized by comprising a constant temperature transformation process, in which the raceway material after the cooling process is held at a temperature above the Ms point and below 270°C for a period of 2 hours or more and 24 hours or less. [Effects of the Invention]

[0014] According to the present invention, by appropriately controlling the ratio of lower bainite structure, martensitic structure, and retained austenite structure in the microstructure of the steel constituting the rolling bearing, it is possible to provide a rolling bearing and a method for manufacturing the same that can suppress both the occurrence of white structure delamination and surface-initiated delamination. [Brief explanation of the drawing]

[0015] [Figure 1] Figure 1 is a cross-sectional view showing a rolling bearing according to an embodiment of the present invention. [Modes for carrying out the invention]

[0016] As a result of diligent research by the inventors, it has been found that controlling the microstructure of the steel constituting the rolling bearing is important, as follows: Specifically, it has been found that appropriately controlling the ratio of lower bainite, martensite, and retained austenite is effective in suppressing white structure delamination due to hydrogen intrusion and surface-initiated delamination in environments containing foreign matter. Furthermore, the inventors investigated the relationship between heat treatment conditions and microstructure and clarified the heat treatment conditions necessary to achieve a composite microstructure with the desired ratio. This invention is based on the above findings.

[0017] The embodiments of the present invention will be described in detail below. However, 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, since the type and configuration of the rolling bearing are not particularly limited, 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 peripheral surface and a steel inner ring 5 having an inner ring raceway surface 4 on its outer peripheral surface. The outer ring 3 and the inner ring 5 constitute a pair of raceways. Further, 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). Each of these balls 6 is held rotatably by a cage 7 in a state of being arranged at equal intervals in the circumferential direction.

[0019] In the rolling bearing according to this embodiment, the phase composition of the steel constituting the raceways (outer ring 3 and inner ring 5) is appropriately controlled. Specifically, the steel constituting the rolling bearing has a composite structure having lower bainite and retained austenite, and the balance is martensite and carbide. Hereinafter, the phase composition of the steel defined in this embodiment will be described in detail.

[0020] <Ratio of lower bainite structure: 35% or more and 90% or less> When bainite transformation occurs in the structure of the steel, compressive residual stress is introduced into the surface layer of the steel material along with the bainite transformation, so that the effect of suppressing the deformation of the obtained steel material can be obtained. Further, the lower bainite structure is considered to be a structure that affects white etching. When the ratio of the lower bainite structure is less than 35%, it becomes difficult to ensure the hardness of the raceway, and white etching is likely to occur. Therefore, the ratio of the lower bainite structure in the steel constituting 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, when the ratio of the lower bainite structure exceeds 90%, the austenite structure described later decreases, and surface-originated spalling is likely to occur. Therefore, the ratio of the lower bainite structure in the steel constituting the raceway of the rolling bearing is 90% or less, preferably 85% or less, and more preferably 80% or less.

[0022] <Percentage of retained austenite tissue: 5% to 40%> The retained austenite structure is a structure that affects surface-initiated delamination. If the proportion of retained austenite structure is less than 5%, surface-initiated delamination is more likely to occur. Therefore, the proportion of retained austenite structure in the steel constituting the raceways of rolling bearings should be 5% or more, preferably 8% or more, and more preferably 10% or more.

[0023] On the other hand, if the proportion of retained austenite structure exceeds 40%, sufficient hardness as a raceway ring cannot be ensured. Therefore, the proportion of retained austenite structure in the steel constituting the raceway ring of a rolling bearing should be 40% or less, preferably 35% or less, and more preferably 30% or less.

[0024] <Remaining parts: Martensite structure and carbides> In this embodiment, the martensitic structure is not particularly necessary, and it is preferable to have less martensitic structure in the steel. Furthermore, the carbides dispersed in the lower bainite structure are an unavoidable structure generated in the manufacturing method of rolling bearings and do not affect white structure delamination or surface-initiated delamination. Therefore, in the steel constituting the raceway ring of a rolling bearing, as long as the ratio of the lower bainite structure to the retained austenite structure is within the above range, and the remainder is martensitic structure and carbides, the ratio is not particularly limited.

[0025] However, if the proportion of carbides is high, the steel becomes softer, making it difficult to obtain the desired hardness for the steel constituting the raceways of a rolling bearing. Therefore, the proportion of carbides in the steel constituting the raceways of a rolling bearing is preferably 20% or less, more preferably 15% or less, and even more preferably 12% or less. Also, as mentioned above, it is preferable to have less martensitic structure in the steel constituting the raceways of a rolling bearing, for example, preferably 40% or less, more preferably 25% or less, and even more preferably 20% or less. Furthermore, in the steel constituting the raceways of the rolling bearing according to this embodiment, ferrite and pearlite are substantially absent, and it is preferable that both be 0%.

[0026] (Method for calculating the ratio of each organization) The fraction of each structure can be calculated, for example, by the following method: After electropolishing the specimen to a depth of 300 μm from the surface, irradiate it with X-rays and calculate the amount of retained austenite from the integral intensity ratio of the diffraction peaks of the (211) plane of the bcc structure and the (220) plane of the fcc structure. The fractions of other structures can be determined by the EBSD method. Mirror-finished specimens are irradiated with electron beams using FE-SEM, and phase identification is performed from the obtained diffraction pattern to calculate the area fraction of carbides. Furthermore, the orientations of the martensitic and bainite structures can be identified from the diffraction pattern, and the proportion of martensitic and bainite structures can be calculated by calculating the orientation difference between blocks.

[0027] <Residual stress: -50 MPa 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 value of residual stress is controlled and the occurrence of white structure delamination is suppressed by appropriately adjusting the ratio of the lower bainite structure in the steel. When the residual stress of a rolling bearing is -MPa or less, excellent crack propagation characteristics can be obtained, the occurrence of deformation can be suppressed, and good bearing performance can be maintained. Therefore, the residual stress of a rolling bearing is preferably -50MPa or less, more preferably -70MPa or less, and even more preferably -100MPa or less.

[0028] (Method for measuring residual stress) Residual stress can be measured, for example, by the following method: After electropolishing the test specimen 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 above embodiment, the ratio of each microstructure in the steel is appropriately controlled, thereby suppressing the occurrence of white structure delamination and surface-initiated delamination. In this embodiment, the composition of the steel material constituting the rolling bearing is not particularly limited. For example, SUJ2 steel specified in JIS G 4805:2019 or 100CrMnSi6-4 specified in ISO 683-17, which are commonly used as bearing steels, can be used.

[0030] Furthermore, steel materials with adjusted alloying elements relative to the above-mentioned steel raw materials can also be used, for example, steel containing C: 0.60% to 1.21% by mass, Si: 0.40% to 1.02% by mass, Mn: 0.55% to 1.51% by mass, Cr: 0.75% to 3.00% by mass, Mo: 1.00% by mass or less, Ni: 0.20% by mass or less, Cu: 0.20% by mass or less, S: 0.025% by mass or less, P: 0.020% by mass or less, and O: 0.0015% by mass or less, with the remainder being Fe and unavoidable impurities.

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

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

[0033] (Austenitization process temperature) For the raceways of rolling bearings, the temperature of the austenitization process must be 820°C or higher, preferably 840°C or higher, in order to obtain the desired hardness by solid-dissolving carbon in the steel through heating. On the other hand, since carbides are almost completely dissolved in the steel at a temperature of 950°C, heating above 950°C is unnecessary and also increases manufacturing costs. Therefore, the temperature of the austenitization process should be 950°C or lower, preferably 880°C or lower.

[0034] <Cooling process> Next, the austenitized raceway material is rapidly cooled to a temperature above the Ms point but below 270°C. In this specification, rapid cooling refers to cooling at a rate that does not 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 rapid cooling method is cooling with salt.

[0035] Furthermore, if the temperature is cooled to below the Ms point after the cooling process and before the following isothermal transformation treatment process is carried out, martensitic transformation will begin, and the resulting raceway ring structure will contain martensitic structures. In this embodiment, the ratio of lower bainite structures to retained austenite structures is specified, and if a large amount of martensitic structure is included, the ratio of lower bainite structures to retained austenite structures will change. Also, in this embodiment, martensitic structures are not necessary. Therefore, it is preferable to carry out the isothermal transformation treatment process without cooling to below the Ms point after the cooling process.

[0036] <Constant Temperature Transformation Process> Subsequently, the raceway material after the cooling process is held at a temperature above the Ms point but below 270°C. This isothermal transformation process changes the austenite to a lower bainite structure.

[0037] (Holding time during the constant temperature transformation process) If the holding time in the isothermal transformation process 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 process should be 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 process exceeds 24 hours, the proportion of the lower bainite structure in the steel exceeds 90%, and surface-initiated delamination is likely to occur. Therefore, the holding time in the isothermal transformation process should be 24 hours or less, preferably 16 hours or less, and more preferably 12 hours or less.

[0038] (Holding temperature during the constant temperature transformation process) While a lower holding temperature in the isothermal transformation process allows for obtaining the desired hardness, if the temperature is below the Ms point, even with a longer holding time, a portion of the austenite and martensite structures in the steel will not transform into the lower bainite structure, making it impossible to obtain the desired proportion of the lower bainite structure. Therefore, the holding temperature in the isothermal transformation process should be above the Ms point, preferably at least 10°C higher than the Ms point, and more preferably at least 20°C higher. As a specific example of the holding temperature in the isothermal transformation process, it is preferable to set it at 180°C or higher, and more preferably at 200°C or higher.

[0039] On the other hand, if the temperature at which the raceway material is held after the cooling process exceeds 270°C, upper bainite will form, which adversely affects the mechanical properties of the steel. Therefore, the holding temperature in the isothermal transformation process should be 270°C or lower, preferably 250°C or lower, and more preferably 240°C or lower.

[0040] Subsequently, after the constant-temperature transformation process, the raceway material is cooled and then ground to manufacture the raceways for the rolling bearing. Then, the cage and rolling elements are assembled into the pair of raceways using a general method to manufacture the rolling bearing. In this embodiment, the raceway material, which had already been processed into the shape of a raceway, was heat-treated to produce a pair of raceways; however, the rolling elements may be manufactured using a similar method.

[0041] By manufacturing rolling bearings using the manufacturing method according to the above embodiment, it is possible to appropriately control the ratio of each microstructure in the steel without performing carburizing or carbonitriding treatments, which lead to environmental burden and increased manufacturing costs. Therefore, it is possible to manufacture rolling bearings that suppress the occurrence of white structure delamination and surface-initiated delamination, thereby enabling a longer lifespan. [Examples]

[0042] The following describes examples and comparative examples of rolling bearings according to this embodiment. First, the proportions of each microstructure were calculated and physical properties were measured for test pieces that underwent heat treatment under various heat treatment conditions, and the changes in microstructure and physical properties due to the heat treatment conditions were investigated.

[0043] <Preparation of test specimens> Steel materials having compositions within the range shown in Table 1 were prepared, and test specimens were fabricated by performing various heat treatments under the heat treatment conditions shown in Table 2.

[0044] Specifically, symbol A in Table 2 represents a heat treatment condition in which an austenitizing process is performed in which the steel material is heated to a temperature of 860°C, followed by a cooling process in which it is cooled to a temperature of 210°C, and then an isothermal transformation process in which it is held at a temperature of 210°C for 6 hours without further temperature reduction. Symbol B represents a heat treatment condition in which the isothermal transformation process of Invention Example No. 1 is performed at a temperature of 210°C for 8 hours. Symbol C represents a heat treatment condition 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 200°C.

[0045] Symbol D represents a heat treatment condition in which the steel material is quenched to a temperature of 820°C, followed by tempering at a temperature of 240°C. Symbol E represents a heat treatment condition in which the steel material is quenched to a temperature of 860°C, followed by tempering at a temperature of 180°C. Symbol F represents a heat treatment condition in which the steel material is heated to a temperature of 860°C, cooled to a temperature of 210°C, and then subjected to a constant temperature transformation process in which the material is held at 210°C for 72 hours without further temperature reduction.

[0046] <Calculation of the ratio for each organization> The microstructure of the obtained specimens was calculated using the following method. After electropolishing the specimens to a depth of 300 μm from the surface, they were irradiated with X-rays, and the amount of retained austenite was calculated from the integral intensity ratio of the diffraction peaks of the (211) plane of the bcc structure and the (220) plane of the fcc structure. The fractions of other microstructures were determined by the EBSD method. Mirror-finished specimens were irradiated with electron beams using FE-SEM, and phase identification was performed from the obtained diffraction patterns to calculate the area fraction of carbides. Furthermore, the orientations of the martensitic and bainite structures were identified from the diffraction patterns, and the proportions of martensitic and bainite structures were calculated by calculating the orientation difference between blocks. The proportions of each microstructure are shown in Table 2 below.

[0047] <Measurement of physical properties> (Measurement of residual stress) After electropolishing the test specimen 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 heat treatment was measured at five points in the center of the test specimen with a test load of 1 kgf, and the average value was used as the hardness measurement.

[0049] Next, rolling fatigue life tests were performed on bearings manufactured by heat treatment under the various heat treatment conditions described above.

[0050] <Manufacturing of deep groove ball bearings (rolling bearings) for testing> By turning steel material having the composition shown in Table 1 below, raceway rings (inner and outer rings) for a deep groove ball bearing of designation 6206 as specified in JIS B1513-1995 were manufactured. Next, the raceway ring material was heat-treated according to the heat treatment conditions indicated by symbols A to F shown in Table 2 below, and then the raceway rings were manufactured by grinding the heat-treated material. Subsequently, a test deep groove ball bearing was manufactured by combining the obtained raceway rings with 3 / 8-inch steel balls made of SUJ2 steel as specified in JIS G 4805:2019 and a resin cage.

[0051] <Rolling fatigue life test> The fabricated test deep groove ball bearings were mounted on a radial fatigue life tester, and rolling fatigue life tests were conducted under the following conditions to measure the white structure delamination life and surface-initiated delamination life. Each test was performed 5 times, and the life at which the cumulative failure probability reached 50% (L) was measured. 50 The average value of ) was calculated.

[0052] (Measurement conditions for white tissue peeling lifetime) Test load (radial load): 910 kgf Rotation speed: 3000 min⁻¹ -1 Lubricating oil: Special lubricating oil that easily generates hydrogen through decomposition.

[0053] (Measurement conditions for surface-initiated delamination life) Test load (radial load): 635 kgf Rotation speed: 3000 min⁻¹ -1 Lubricant: RO68 (contains 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 below. In the white structure peel test and surface-initiated peel test shown in Table 3, Comparative Example No. 1, which uses the commonly performed heat treatment conditions symbolized C, was used as the baseline, and the results are expressed as a ratio to the measurement results of Comparative Example No. 1.

[0055] [Table 1]

[0056] [Table 2]

[0057] [Table 3]

[0058] As shown in Tables 2 and 3 above, Invention Examples No. 1 and 2 had a ratio of lower bainite structure to retained austenite structure within the range defined in this invention, with the remainder being martensitic structure and carbides. Therefore, it was possible to suppress the occurrence of white structure delamination due to hydrogen intrusion, and also to suppress the occurrence of surface-initiated delamination in environments containing foreign matter. Furthermore, because Invention Examples No. 1 and 2 have a lower bainite structure, the residual stress is a negative value, and since it is compressive residual stress, it exhibits excellent crack propagation characteristics.

[0059] On the other hand, Comparative Example No. 1 did not undergo isothermal transformation treatment, and therefore no lower bainite structure was formed. Consequently, its lifespan in the white structure peel test and surface-initiated peel test was shorter compared to Invention Examples No. 1 and 2. Comparative Example No. 2 also did not undergo isothermal transformation treatment, so no lower bainite structure was formed, and the proportion of retained austenite structure was extremely low. Consequently, its lifespan in the surface-initiated peel test was also shortened. For this reason, the white structure peel test was not performed.

[0060] Comparative Example No. 3, compared to Comparative Example No. 1, had a higher heating temperature and more carbon dissolved in the steel, resulting in higher martensite hardness and an improved lifespan in the surface-initiated peel test. However, Comparative Example No. 3 did not undergo isothermal transformation treatment, and therefore no lower bainite structure was formed. Consequently, the lifespan, particularly in the white structure peel test, was shortened. Furthermore, Comparative Examples No. 1 to 3 have positive residual stress values, which are tensile residual stresses, and therefore are considered to have poor crack propagation characteristics. Comparative Example No. 4 underwent isothermal transformation treatment at 210°C for 72 hours, resulting in a lower bainite structure ratio exceeding the range specified in this invention. Consequently, the lifespan in the surface-initiated peel test was extremely short. [Explanation of symbols]

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

Claims

1. A rolling bearing comprising a pair of steel raceways and a plurality of rolling elements held rotatably between the pair of raceways, A rolling bearing characterized in that the proportion of lower bainite structure in the steel of the raceway ring is 35% to 80%, the proportion of retained austenite structure is 5% to 40%, and the remainder is martensitic structure and carbides.

2. The rolling bearing according to claim 1, characterized in that the residual stress is -50 MPa or less.

3. A method for manufacturing a rolling bearing according to claim 1 or 2, A process of heating the raceway material to a temperature of 820°C to 950°C to austenitize it, A cooling step is performed to rapidly cool the austenitized raceway material to a temperature above the Ms point and below 250°C. A method for manufacturing a rolling bearing, characterized by comprising a constant temperature transformation process, in which the raceway material after the cooling process is held at a temperature above the Ms point and below 250°C for a period of 2 hours or more and 24 hours or less.

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