Rolling bearing and method for manufacturing same

Abstract

The present invention provides: a rolling bearing with excellent productivity due to the use of a material with a reduced amount of additive element as a material with excellent resistance to white structure flaking; and a method for manufacturing the same. A rolling bearing (1) comprises: a pair of steel bearing rings; and a plurality of rolling elements (6) that are rollably held between the pair of bearing rings (outer ring (3) and inner ring (5)). The bearing rings have rolling surfaces (outer ring–side rolling surface (2), inner ring–side rolling surface (4)) on which the rolling elements (6) roll. At the rolling surfaces, the amount of carbon in the steel is between 0.40 mass% and 0.75 mass%, inclusive, and the amount of chromium in the steel is less than 2.10 mass%.

Description

Rolling bearing and method for manufacturing the same

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

[0002] In rolling bearings, fatigue failure occurs due to repeated contact between rolling elements and raceways, and a damage phenomenon called "spalling" is known, in which the surface of the raceway peels off in flakes. The causes of spalling are classified according to their origins, and include "internal-origin spalling" caused by the progression of cracks originating from stress concentration sites such as non-metallic inclusions contained within the material, and "surface-origin spalling" caused by foreign matter entering the lubricating oil forming indentations, and the generation and progression of cracks due to stress concentration at the edges of the indentations.

[0003] On the other hand, in some operating environments, hydrogen generated by the decomposition of lubricating oil penetrates into the steel, and it is known to reduce the material strength, thereby promoting local material microstructure changes and crack generation. This microstructure change portion is a change in the martensite constituting the bearing steel into fine ferrite grains, and is called a white structure because it appears white when observed by etching. Spalling accompanied by a white structure (hereinafter: white-structure spalling) is classified as "internal-origin spalling", but the generation mechanism is different from spalling starting from inclusions, and it causes significant short-life failures. Therefore, measures to prevent white-structure spalling are essential.

[0004] For example, Patent Document 1 discloses a rolling bearing capable of suppressing the generation of a white structure and extending the service life. The rolling bearing described in Patent Document 1 has an appropriately controlled amount of dissolved carbon in the martensite structure after heat treatment, and a defined volume ratio of spherical carbides with a diameter of 200 nm or more. Further, Patent Document 1 describes that the amount of dissolved chromium in the rolling bearing is preferably 2.43% by mass or more and 2.95% by mass or less.

[0005] Japanese Patent No. 7264319

[0006] However, in the rolling bearing described in Patent Document 1, the amount of dissolved chromium in the martensitic structure is 2.43% by mass or more and 2.95% by mass, which is a high value compared to ordinary bearing steel such as SUJ2. Since chromium is an element that readily dissolves in carbides, heat treatment at a higher temperature than the quenching temperature used in furnace heating for bearing steel is necessary to dissolve it in the martensitic structure. As a result, in addition to negatively affecting the productivity of bearings, it is expected that the grains will become coarser, which will negatively affect other functions such as toughness. Furthermore, although the amount of dissolved carbon and dissolved chromium is specified, even with the same steel material, there are striped segregation (microsegregation) zones with different chemical compositions in the forging direction, so it is necessary to strictly control the amount of dissolved carbon and dissolved chromium in order to intentionally obtain the desired amount of dissolved carbon and dissolved chromium through heat treatment.

[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 have excellent productivity by using a material with a reduced amount of additive elements as a material with excellent resistance to white structure delamination.

[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, wherein the raceways have a rolling surface on which the rolling elements roll, and the carbon content in the steel on the rolling surface is 0.40% by mass or more and 0.75% by mass or less, and the chromium content in the steel is less than 2.10% by mass.

[0010] A preferred embodiment of the rolling bearing according to the present invention is as shown in [2] or [3] below.

[0011] [2] The rolling bearing according to [1], characterized in that the amount of chromium in the steel on the rolling surface is more than 0.20% by mass.

[0012] [3] The rolling bearing according to [1] or [2], characterized in that the Vickers hardness of the rolling surface is 640 HV or more.

[0013] The method for manufacturing a rolling bearing according to the present invention is configured as shown in [4] below.

[0014] [4] A method for manufacturing a rolling bearing, comprising a heat treatment step of heating the raceway material, wherein the heat treatment step is characterized by selecting at least one heat treatment method selected from furnace heating and quenching, carburizing and quenching, carbonitriding and quenching, high-frequency induction hardening, and isothermal transformation.

[0015] According to the present invention, a rolling bearing with excellent productivity and a method for manufacturing the same can be provided, using a material with a reduced amount of added elements as a material with excellent resistance to white structure delamination.

[0016] Figure 1 is a cross-sectional view showing a rolling bearing according to an embodiment of the present invention. Figure 2 is a graph showing the change in Vickers hardness with respect to quenching temperature during furnace heating.

[0017] As a result of diligent research by the inventors, it has been discovered that by appropriately defining the balance between the carbon content and chromium content in the steel, the resistance to white structure delamination and the hardness of the raceway surface can be further improved without controlling the amount of solid solution carbon. Specifically, if the chromium content in the steel is high, carbides are formed, making the steel prone to cracking during manufacturing. Furthermore, in order to obtain the hardness required for rolling bearings, it is necessary to dissolve the carbides, which requires high-temperature and long-duration heat treatment, thus worsening the productivity of rolling bearings.

[0018] Furthermore, if the carbon content in the steel is low, there is a concern that bearing performance other than white structure delamination characteristics may deteriorate. In addition, a decrease in carbon content leads to a deterioration in hardenability and a decrease in hardness. Based on the above findings, the inventors have found appropriate ranges for carbon content and chromium content in steel.

[0019] The embodiments of the present invention will be described in detail below. However, the present invention is not limited to the embodiments described below.

[0020] [Rolling Bearing] Figure 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. As shown in Figure 1, the rolling bearing 1 has a steel outer ring 3 having an outer ring side rolling surface 2 on its inner circumferential surface, and a steel inner ring 5 having an inner ring side rolling surface 4 on its outer circumferential surface, and the outer ring 3 and inner ring 5 constitute a pair of raceway rings. In addition, a plurality of rolling elements 6 are arranged between the pair of raceway rings (between the outer ring side rolling surface 2 and the inner ring side rolling surface 4). Each of these rolling elements 6 is held to roll freely by a cage 7 in a state where they are arranged at equal intervals in the circumferential direction.

[0021] In the rolling bearing according to this embodiment, the carbon and chromium content in the steel is appropriately controlled on the rolling surfaces (outer ring side rolling surface 2 and inner ring side rolling surface 4) of the raceway rings (outer ring 3 and inner ring 5). The carbon and chromium content in the steel as defined in this embodiment will be described in detail below.

[0022] <Carbon content in steel (C content): 0.40% by mass or more and 0.75% by mass or less> Carbon (C) is a component that affects the hardenability and resistance to white structure delamination of steel materials. If the carbon content in steel is less than 0.40% by mass, it becomes difficult to ensure the hardenability of the steel material (raceway ring material). Furthermore, it becomes difficult to satisfy the hardness required for rolling bearings. Therefore, the carbon content in steel should be 0.40% by mass or more, preferably 0.42% by mass or more, and more preferably 0.44% by mass or more.

[0023] On the other hand, if the carbon content in the steel exceeds 0.75% by mass, white structure delamination occurs in the rolling bearings, reducing the bearing life. Therefore, the carbon content in the steel should be 0.75% by mass or less, preferably 0.73% by mass or less, and more preferably 0.71% by mass or less.

[0024] <Chromium content (Cr content) in steel: Less than 2.10 mass%> Chromium (Cr) is a component that particularly affects the hardenability of steel materials, but in this embodiment, it is not necessary for the steel material to contain chromium, and it may be 0 mass%. However, from the viewpoint of the productivity of rolling bearings, it is preferable that the chromium content in the steel be greater than 0.20 mass%, more preferably 0.50 mass% or more, and even more preferably 1.00 mass% or more.

[0025] When the chromium content in steel is 2.10% by mass or more, carbides are formed, making the steel more prone to cracking during manufacturing. Furthermore, high temperatures and long heat treatment times are required to dissolve the carbides during the heat treatment process. Therefore, for example, it becomes necessary to select a furnace capable of high-temperature heat treatment, which reduces the freedom of selecting heat treatment conditions and also lowers productivity. The inventors have also found that even when the chromium content in steel is less than 2.10% by mass, white structure delamination does not occur, and white structure delamination resistance comparable to that of steel with a chromium content of 3.00% by mass can be obtained. Therefore, it is preferable to set the chromium content in steel to less than 2.10% by mass, preferably 2.05% by mass or less, and more preferably 2.00% by mass or less.

[0026] <Other Components> In addition to C and Cr, the steel material constituting the rolling bearing according to this embodiment may also contain Si, Mn, Mo, Ni, etc., with the remainder being Fe and unavoidable impurities. The total content of Si, Mn, Mo, and Ni in the steel material is preferably 3.00% by mass or less, and more preferably 2.50% by mass or less, relative to the total mass of the steel material. As for the content of each element in the steel material, for example, Si is preferably 0.60% by mass or less, and more preferably 0.50% by mass or less. For example, Mn is preferably 1.20% by mass or less, and more preferably 1.00% by mass or less. For example, Mo is preferably 0.50% by mass or less, and more preferably 0.40% by mass or less. For example, Ni is preferably 0.30% by mass or less, and more preferably 0.25% by mass or less.

[0027] Examples of unavoidable impurities in steel include P, S, and Cu. The total content of these unavoidable impurities is preferably 0.60% by mass or less relative to the total mass of the steel. For example, the content of P and S in the steel is preferably 0.10% by mass or less, and more preferably 0.05% by mass or less. For example, the content of Cu in the steel is preferably 0.30% by mass or less, and more preferably 0.20% by mass or less.

[0028] In this embodiment, the carbon content and chromium content in the steel are specified, so the carbon content and chromium content in the steel do not change before and after heat treatment. Therefore, the timing of measuring the carbon content and chromium content in the steel may be before or after heat treatment of the bearing material. Furthermore, since the components contained in rolling bearings are basically not affected by the measurement position, the method of measuring the carbon content and chromium content in the steel is not particularly limited, and any measurement method may be used. For example, one method is to cut the steel material before bearing manufacturing and measure the components of the surface by emission spectroscopy.

[0029] In principle, the components contained in rolling bearings are not affected by the measurement location, but it is sufficient that the carbon and chromium content in the steel in the region subjected to rolling fatigue, i.e., the rolling surface, be within the above range. If the composition differs between the rolling surface and other parts, the carbon and chromium content in the steel in the rolling fatigue load region can be measured by measuring the surface components by emission spectroscopy or by using an electron probe macroanalyzer (EPMA) on a cross-section perpendicular to the raceway surface.

[0030] <Vickers hardness of rolling surface: 640 HV or higher> In order to improve the lifespan of the rolling bearing according to this embodiment, it is preferable that the region subjected to rolling fatigue has sufficient hardness. As described above, the required hardness can be ensured by specifying the carbon content and chromium content in the steel. Specifically, the Vickers hardness of the rolling surface is preferably 640 HV or higher, more preferably 670 HV or higher, and even more preferably 690 HV or higher.

[0031] [Method for Manufacturing Rolling Bearings] Next, an example of a method for manufacturing rolling bearings according to this embodiment will be described below in order of steps.

[0032] <Processing steps> First, a steel material having a composition in which the carbon and chromium content are within the above range is processed into a predetermined shape by turning to produce the raceway material.

[0033] <Heat Treatment Process> To obtain the desired hardness for the raceway rings of rolling bearings, a heat treatment process is performed on the raceway ring material. The heat treatment method is not particularly limited, but it is preferable to select at least one heat treatment method selected from furnace heating and quenching, carburizing and quenching, carbonitriding and quenching, high-frequency induction hardening, and isothermal transformation treatment. Regardless of which method is selected, compositional variations due to measurement location will not occur, and the desired hardness can be ensured.

[0034] (Heat Treatment Temperature) In this embodiment, the heat treatment temperature is not particularly limited, and a general heat treatment temperature can be used. However, in the rolling bearing according to this embodiment, the carbon content and chromium content in the steel are appropriately controlled, so it has excellent hardenability, and even if the heat treatment temperature during quenching by furnace heating is set to less than 900°C, for example, the required hardness of the raceway surface can be ensured. Therefore, the heat treatment temperature during quenching by furnace heating is preferably less than 900°C, and more preferably between 840°C and 880°C. By setting the heat treatment temperature within the above range, the degree of freedom in selecting the type of furnace, etc., is increased, energy saving can be achieved, and manufacturing efficiency can be improved.

[0035] (Cooling Method) The cooling method after heat treatment is not particularly limited, and water, a water-soluble coolant containing a polymer compound, or oil can be used. In quenching treatment by furnace heating, water cooling allows for rapid cooling and improves hardenability, but it can make cracks more likely to occur, so oil cooling is generally preferred. According to the rolling bearing of this embodiment, in the case of quenching treatment by furnace heating, sufficient hardness can be ensured even when using oil cooling, which has a slow cooling rate.

[0036] (Other heat treatment conditions) In this embodiment, there are no particular limitations on the heat treatment conditions for the raceways of the rolling bearing other than those described above, but it is preferable to select the heat treatment conditions so that the Vickers hardness of the rolling surface is 640 HV or higher.

[0037] The following describes examples and comparative examples of rolling bearings according to this embodiment.

[0038] [Measurement of Composition in Steel Materials] Steel materials with various compositions were prepared, and these materials were cut. The composition was measured at two locations on the surface. The composition was measured using an emission spectrometer (PDA-7000: manufactured by Shimadzu Corporation), and the average of the two measured values ​​was calculated as the content of each component. The calculation results are shown in Table 1 below. In addition, in the evaluation of white structure peel resistance and hardness described later, steel materials with the symbols SUJ2 as specified in JIS G 4805 and S53C as specified in JIS G 4051 were used as comparative materials, so the compositions of SUJ2 and S53C are also shown in Table 1 below.

[0039]

[0040] [Manufacturing of Rolling Bearings (Raceway Rings)] By turning steel materials having the various compositions described above, raceway rings (inner and outer rings) for rolling bearings were manufactured. Subsequently, the raceway ring materials were heat-treated to produce test specimens of rolling bearings equipped with the obtained raceway rings. As for the heat treatment method, a quenching and tempering method by furnace heating or a high-frequency induction quenching method was employed.

[0041] [Evaluation of Test Materials] <Evaluation of White Structure Delamination Resistance> Each test material was mounted on a life test machine, and a life test was conducted to evaluate the white structure delamination resistance. The life (L50) at which the cumulative failure probability reached 50% was determined. The measurement results of the life test are shown in Table 2 below. In Table 2 below, L50 represents the time during which 50% of the rolling bearings can rotate without damage. However, for test material symbols R2 to R7, since there were few damaged test materials, the L50 calculated as the time at which the test was terminated is listed.

[0042]

[0043] As shown in Table 1 and Table 2 above, in the test material symbol R1 where the carbon content in the steel exceeds the upper limit of the range defined in the present invention, the bearing life was determined by the occurrence of white structure delamination. In contrast, in the test material symbols R2 to R7 where the carbon content in the steel is within the range defined in the present invention, white structure delamination did not occur in any of them, and the life was longer compared to the test material symbol R1.

[0044] <Evaluation of Hardness> Vickers hardness was measured for the surfaces of the various raceway rings produced in accordance with JIS Z 2244. The steel material number of the steel material used and the cooling method during furnace heating quenching are shown in Table 3 below, and the measurement results of Vickers hardness are shown in FIG. 2.

[0045]

[0046] As shown in Table 3 and FIG. 2 above, for the test material symbol S2, when oil cooling was selected for furnace heating, the hardenability decreased and the required hardness for the rolling surface of the rolling element could not be obtained. Also, the test material symbol S1 is a rolling element material having the same composition as the test material symbol S2, and water cooling after oil cooling was selected to enhance the hardenability. Although the hardness is a good value, cracks are likely to occur due to rapid cooling. On the other hand, when high-frequency quenching is selected as the heat treatment method, if the conditions are appropriately selected, it is possible to obtain the hardness without generating cracks.

[0047] For the test material symbols S3 and S4, the carbon content and chromium content in the steel are within the range defined in the present invention, and excellent hardness could be obtained regardless of the quenching temperature.

[0048] For the test material symbol S5, since the chromium content in the steel exceeds the upper limit value defined in the present invention, many stable carbides are formed. Although these carbides can be dissolved in high-temperature and long-time heat treatment, the required hardness could not be obtained in the range where the quenching temperature is less than 900°C. Therefore, when using the steel material of the test material symbol S5, it is necessary to select a furnace capable of heat treatment at high temperature, which reduces the freedom of heat treatment and also reduces the productivity.

[0049] Test material symbol S6 has a higher chromium content in the steel than test material symbol S5, and in order to obtain the required hardness, the quenching temperature must be 950°C or higher. Therefore, it has become difficult to achieve the objective of the present invention with test material symbols S5 and S6 as well.

[0050] According to the evaluation results of the above test materials, test material symbols R2 to R6, S3 and S4, in which the carbon content and chromium content in the steel are within the range specified in this invention, showed suppressed white structure delamination, and the desired hardness could be obtained without reducing productivity. In other words, if the material has been quenched to have sufficient hardness in the part subjected to rolling fatigue, the occurrence of white structure delamination can be similarly suppressed by controlling the carbon content in the steel, which is easier to measure than the dissolved carbon content.

[0051] Although various embodiments have been described above with reference to the drawings, it goes without saying that the present invention is not limited to these examples. It is clear to those skilled in the art that various modifications or alterations can be conceived within the scope of the claims, and these will naturally also fall within the technical scope of the present invention. Furthermore, the components of the above embodiments may be combined in any way without departing from the spirit of the invention.

[0052] This application is based on a Japanese patent application (Patent Application No. 2024-208638) filed on November 29, 2024, the contents of which are incorporated herein by reference.

[0053] 1. Rolling bearing 2. Outer ring side rolling surface 3. Outer ring 4. Inner ring side rolling surface 5. Inner ring 6. Rolling element 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, wherein the raceways have a rolling surface on which the rolling elements roll, and the carbon content in the steel on the rolling surface is 0.40% by mass or more and 0.75% by mass or less, and the chromium content in the steel is less than 2.10% by mass.

2. The rolling bearing according to claim 1, characterized in that the amount of chromium in the steel on the rolling surface is more than 0.20% by mass.

3. The rolling bearing according to claim 1, characterized in that the Vickers hardness of the rolling surface is 640 HV or higher.

4. A method for manufacturing a rolling bearing according to claims 1 to 3, comprising a heat treatment step of heating the raceway material, characterized in that the heat treatment step is selected from at least one heat treatment method selected from furnace heating quenching, carburizing quenching, carbonitriding quenching, high-frequency induction hardening, and isothermal transformation.

Images