Rolling bearing and method for producing same

Abstract

Provided is a rolling bearing with which it is possible to suppress both white structure peeling and surface starting point peeling without performing carburization or carbonitriding treatment that would lead to a strain on the environment and an increase in manufacturing costs. This rolling bearing (radial ball bearing (1)) comprises a pair of steel raceway rings (outer ring (3) and inner ring (5)) and a plurality of rolling elements (balls (6)) rollably held between the pair of raceway rings. The ratio of lower bainite structure in the steel in raceway rings is 35-90% (inclusive), the ratio of residual austenite structure is 5-40% (inclusive), and the remainder is martensite structure and carbide.

Description

Rolling bearing and manufacturing method thereof

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

[0002] Rolling bearings can experience metal fatigue and flakes on the raceway surface due to prolonged use under load. One type of flakes, "inclusion-initiated flakes," originates from inclusions inside the steel material that makes up the inner and outer rings and rolling elements. "Surface-initiated flakes" originate from indentations formed on the raceway surface when foreign matter such as dirt gets lodged inside the bearing. "White structure flakes" originate from structural changes known as white structure, which occur when hydrogen generated during use due to factors such as lubricant decomposition penetrates into the steel, causing hydrogen embrittlement. These flakes initiate from within the material, accompanied by cracks, resulting in a shortened service life. Because the various types of flakes mentioned above are caused by different mechanisms, different countermeasures are required depending on the type of flakes.

[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 alloying elements compared to SUJ2 steel, which is a common bearing steel, and to specify the surface hardness after quenching and tempering.

[0004] Furthermore, Patent Document 2 describes a bearing steel in which the contents of alloy elements in the steel are specified, and in which 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 of non-metallic inclusions in the steel is specified. The bearing steels described in Patent Documents 2 and 3 have excellent rolling contact fatigue life even in an environment in which white microstructure change occurs due to hydrogen.

[0005] Japanese Patent Publication No. 2004-124215 Japanese Patent No. 6846901 Japanese Patent No. 6639839

[0006] However, the bearing steel described in Patent Document 1 is intended to improve the rolling life, particularly in an environment where foreign matter is present, and gives little consideration to white structure flaking. Furthermore, the bearing steels described in Patent Documents 2 and 3 are intended mainly to suppress the occurrence of white structure flaking, and give no consideration to surface-originated flaking. Furthermore, carburizing, quenching, and tempering, or carbonitriding, quenching, and tempering, requires a longer heat treatment time than regular quenching and tempering, which increases manufacturing costs. Furthermore, when gas carburizing is performed, H ２ , CO, N ２ Because it uses modified gases whose main components are ethanol, etc., its use may be restricted in the future from an environmental standpoint.

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

[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 so as to be freely rollable between the pair of raceways, characterized in that the proportion of lower bainite in the steel of the raceways is 35% or more and 90% or less, the proportion of retained austenite is 5% or more and 40% or less, and the remainder is martensite and carbides.

[0010] A preferred embodiment of the rolling bearing according to the present invention is as follows: [2]

[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 as configured in [3] below.

[0013] [3] A method for manufacturing a rolling bearing according to [1] or [2], comprising the steps of heating a race material to a temperature of 820°C or higher and 950°C or lower to austenitize it, a cooling step of rapidly cooling the austenitized race material to a temperature above the Ms point and below 270°C, and an isothermal transformation treatment step of holding the race material after the cooling step at a temperature above the Ms point and below 270°C for a period of 2 hours to 24 hours.

[0014] According to the present invention, the ratios of lower bainite, martensite, and retained austenite 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 method for manufacturing the same that can suppress both the occurrence of white structure flaking and surface-originated flaking.

[0015] FIG. 1 is a cross-sectional view showing a rolling bearing according to an embodiment of the present invention.

[0016] As a result of extensive research, the inventors have found that it is important to control the microstructure 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 microstructure 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 microstructure, and have clarified the heat treatment conditions for achieving a composite microstructure with the desired ratios. The present invention was made based on the above findings.

[0017] Hereinafter, embodiments of the present invention will be specifically described, but the present invention is not limited to the embodiments described below.

[0018] [Rolling Bearing] FIG. 1 is a cross-sectional view showing a rolling bearing according to an embodiment of the present invention. Since the type and configuration of the rolling bearing are not particularly limited in the present invention, a radial ball bearing will be described as an example in this embodiment. 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, with the outer ring 3 and inner ring 5 constituting a pair of raceways. Furthermore, 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 circumferentially equally spaced and held in a cage 7 so as to be free to roll.

[0019] In the rolling bearing according to this embodiment, the phase structure 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 that includes lower bainite and retained austenite, with the remainder being martensite and carbides. The phase structure of the steel specified in this embodiment will be described in detail below.

[0020] <Proportion of Lower Bainite Structure: 35% or More and 90% or Less> When bainite transformation occurs in the structure of steel, compressive residual stress is introduced into the surface layer of the steel material along with the bainite transformation, thereby suppressing deformation of the resulting steel material. The lower bainite structure is also considered to be a structure that affects white flaking. If the proportion of the lower bainite structure is less than 35%, it becomes difficult to ensure the hardness of the bearing ring, and white flaking is more likely to occur. Therefore, the proportion of the lower bainite structure in the steel that constitutes the bearing ring of a rolling bearing is set to 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 raceway rings of the rolling bearing is set to 90% or less, preferably 85% or less, and more preferably 80% or less.

[0022] <Proportion of Retained Austenite Structure: 5% or More and 40% or Less> The retained austenite structure is a structure that affects surface-initiated flaking. If the proportion of the retained austenite structure is less than 5%, surface-initiated flaking is more likely to occur. Therefore, the proportion 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 proportion of the retained austenite structure exceeds 40%, the hardness of the raceway cannot be sufficiently ensured. Therefore, the proportion 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, a martensite structure is not particularly necessary, and it is preferable that the amount of martensite structure in the steel is small. Furthermore, carbides present in a dispersed state in the lower bainite structure are a structure that is inevitably generated in the manufacturing method of a rolling bearing, and do not affect white structure flaking or surface-initiated flaking. Therefore, as long as the ratios of the lower bainite structure and the retained austenite structure in the steel constituting the raceway of the rolling bearing are within the above-mentioned ranges and the balance is a martensite structure and carbides, the ratios are not particularly limited.

[0025] However, if the proportion of carbides is high, the steel material becomes soft, and it may be difficult to obtain the desired hardness as a steel material for forming the raceway of a rolling bearing. Therefore, the proportion of carbides in the steel for forming the raceway of a rolling bearing is preferably 20% or less, more preferably 15% or less, and even more preferably 12% or less. As mentioned above, it is preferable that the martensite structure in the steel for forming the raceway of a rolling bearing be low; for example, it is preferably 40% or less, more preferably 25% or less, and even more preferably 20% or less. Note that in the steel for forming the raceway of the rolling bearing according to this embodiment, ferrite and pearlite are substantially absent, and it is preferable that both be 0%.

[0026] (Method of calculating the ratio of each structure) The fraction of each structure can be calculated, for example, by the following method. A test specimen is electrolytically polished to a depth of 300 μm from the surface and then irradiated with X-rays. 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. A mirror-finished test specimen is irradiated with an electron beam using an FE-SEM, and 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 can be 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: -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, by appropriately adjusting the ratio of the lower bainite structure in the steel, the residual stress value is controlled and the occurrence of white structure flaking is suppressed. When the residual stress of the rolling bearing is -MPa or less, excellent crack propagation characteristics can be obtained, and the occurrence of deformation can be suppressed, thereby maintaining good bearing performance. Therefore, the residual stress of the rolling bearing is preferably -50 MPa or less, more preferably -70 MPa or less, and even more preferably -100 MPa or less.

[0028] (Method for Measuring Residual Stress) The residual stress can be measured, for example, by the following method: After electrolytically polishing a test piece to a depth of 300 μm from the surface, the test piece is irradiated with X-rays, and the residual stress is measured by the Cos α 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, thereby making it possible to suppress the occurrence of white structure flaking and surface-initiated flaking. Note that, in the present embodiment, the composition of the steel constituting the rolling bearing is not particularly limited. For example, SUJ2 steel as specified in JIS G 4805:2019 or 100CrMnSi6-4 as specified in ISO 683-17, which are commonly used as bearing steel, can be used.

[0030] Furthermore, it is also possible to use a steel material in which alloy elements have been adjusted relative to the above-mentioned steel raw materials, such as a steel material containing C: 0.60 mass% to 1.21 mass%, Si: 0.40 mass% to 1.02 mass%, Mn: 0.55 mass% to 1.51 mass%, Cr: 0.75 mass% to 3.00 mass%, 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, a method of manufacturing a rolling bearing according to this embodiment will be described below in the order of steps.

[0032] <Austenitizing process> First, the material steel is machined into the shape of the raceway of the rolling bearing by turning, and the machined raceway material is １ In this process, austenite becomes the main structure in the steel.

[0033] (Temperature in the austenitizing process) In order to dissolve carbon in the steel by heating and obtain the desired hardness for the raceway of a rolling bearing, the temperature in the austenitizing process needs to 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 the 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 Step> Next, the austenitized race material is rapidly cooled to a temperature above the Ms point and below 270°C. In this specification, rapid cooling 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 rapid cooling method is cooling using salt.

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

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

[0037] (Holding time in isothermal transformation treatment step) 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 proportion of 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] (Holding Temperature in Isothermal Transformation Treatment Step) A lower holding temperature in the isothermal transformation treatment step allows for a desired hardness to be obtained, but if the holding temperature is below the Ms point, even if the holding time is extended, part of the austenite and martensite structures in the steel will not change to lower bainite structures, and the desired ratio of lower bainite structures will not be obtained. Therefore, the holding temperature in the isothermal transformation treatment step is set to a temperature above the Ms point, preferably at a temperature 10°C or higher than the Ms point, and more preferably at a temperature 20°C higher than the Ms point. A specific example of the holding temperature in the isothermal transformation treatment step is preferably 180°C or higher, and more preferably 200°C or higher.

[0039] On the other hand, if the temperature at which the bearing ring material is held after the cooling process exceeds 270°C, upper bainite, which 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 treatment is then cooled and ground to manufacture the raceways of the rolling bearing. A cage and rolling elements are then assembled into the pair of raceways by a conventional method to manufacture the rolling bearing. In this embodiment, the raceway material already machined into the raceway shape is heat-treated to manufacture the pair of raceways, but rolling elements may also be manufactured by a similar method.

[0041] When a rolling bearing is manufactured using the manufacturing method according to the present embodiment, the ratio of each microstructure in the steel can be appropriately controlled without carrying out carburizing or carbonitriding treatments, which are environmentally harmful and increase manufacturing costs. As a result, it is possible to manufacture a rolling bearing that is able to suppress the occurrence of white microstructure flaking and surface-originated flaking and has a long life.

[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 had been heat treated under various heat treatment conditions, and physical properties were measured to investigate changes in the structure and physical properties due to the heat treatment conditions.

[0043] <Preparation of Test Specimens> 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 specimens.

[0044] Specifically, symbol A in Table 2 represents heat treatment conditions 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 the steel material is cooled to a temperature of 210°C, followed by an isothermal transformation process in which the steel material is maintained 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 performed at a temperature of 210°C for 8 hours. Symbol C represents heat treatment conditions in which the steel material is quenched 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, a cooling process is performed in which the steel material is cooled to a temperature of 210°C, and then an isothermal transformation process is performed 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 structure> The structure of the obtained test specimen was calculated using the following method. The test specimen was electrolytically polished to a depth of 300 μm from the surface and then irradiated with X-rays. 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 fractions of other structures were determined by the EBSD method. The mirror-finished test specimen 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 ratios of each structure are 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, X-rays were irradiated and residual stress was measured from the diffraction peak of the (211) plane of the bcc structure by the Cos α method.

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

[0051] <Rolling Contact Fatigue Life Test> The manufactured test deep groove ball bearings were mounted on a radial type life tester and subjected to a rolling contact fatigue life test under the following conditions to measure the white structure flaking life and the surface initiated flaking life. Each test was performed five times, and the life at which the cumulative failure probability reached 50% (L ５０ ) was calculated as the average value.

[0052] (Measurement conditions for white structure peeling life) Test load (radial load): 910 kgf Rotation speed: 3000 min －１ Lubricating oil: Special lubricating oil that is prone to generating hydrogen due to decomposition of the lubricating oil

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

[0054] The measurement results of the physical properties and the measurement results of the rolling fatigue life test are shown in Table 3 below. In the white structure flaking test and the surface initiated flaking test shown in Table 3 below, 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 conditions designated by symbol C.

[0055]

[0056]

[0057]

[0058] As shown in Tables 2 and 3 above, in Invention Examples 1 and 2, the ratios of lower bainite and retained austenite were within the ranges specified in the present invention, with the remainder being martensite 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. Furthermore, because Invention Examples 1 and 2 had a lower bainite structure, the residual stress was a negative value, i.e., compressive residual stress, and therefore exhibited excellent crack propagation characteristics.

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

[0060] In Comparative Example No. 3, the heating temperature was higher than in Comparative Example No. 1, and more carbon was dissolved in the steel, resulting in higher martensite hardness and an improved life ratio in the surface-initiated exfoliation test. However, Comparative Example No. 3 was not subjected to isothermal transformation treatment, and a lower bainite structure was not formed. Therefore, the life in the white structure exfoliation test was particularly short. Furthermore, Comparative Examples Nos. 1 to 3 had positive residual stresses, which were tensile residual stresses, and therefore were thought to have poor crack propagation characteristics. Comparative Example No. 4 was subjected to isothermal transformation treatment at 210°C for 72 hours, resulting in the proportion of lower bainite structure exceeding the range specified in the present invention. Therefore, the life in the surface-initiated exfoliation test was extremely short.

[0061] Although various embodiments have been described above with reference to the drawings, it goes without saying that the present invention is not limited to such examples. It is clear that a person skilled in the art can conceive of various modifications or alterations within the scope of the claims, and it is understood that these also naturally fall within the technical scope of the present invention. Furthermore, the components of the above-described embodiments may be combined in any manner without departing from the spirit of the invention.

[0062] This application is based on a Japanese patent application (Patent Application No. 2023-141257) filed on August 31, 2023, the contents of which are incorporated herein by reference.

[0063] REFERENCE SIGNS LIST 1 Radial ball bearing 2 Outer ring raceway 3 Outer ring 4 Inner ring raceway 5 Inner ring 6 Balls 7 Cage

Claims

1. A rolling bearing comprising a pair of steel raceways and a number of rolling elements held between the pair of raceways for free rollability, wherein the ratio of lower bainite structure in the steel of the raceways 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.

2. A 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, comprising the steps of: heating a race material to a temperature of 820°C or higher and 950°C or lower to austenitize it; rapidly cooling the austenitized race material to a temperature above the Ms point and below 270°C; and maintaining the race material after the cooling step at a temperature above the Ms point and below 270°C for a period of 2 hours to 24 hours.

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