Rolling bearing and method for manufacturing the same

Abstract

This invention provides a rolling bearing and a method for manufacturing the same that offer excellent productivity by using a material with reduced amounts of additive elements, which is superior in resistance to white structure delamination. [Solution] The rolling bearing 1 comprises a pair of steel raceways and a plurality of rolling elements 6 that are held to roll freely between the pair of raceways (outer ring 3 and inner ring 5). The raceways have rolling surfaces (outer ring side rolling surface 2, inner ring side rolling surface 4) on which the rolling elements 6 roll, and the carbon content in the steel on the rolling surfaces 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.

Description

Technical Field

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

Background Art

[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 starting from stress concentration points such as non-metallic inclusions contained in the material, and "surface-origin spalling" caused by foreign matter that has entered 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, it is known that hydrogen generated by the decomposition of lubricating oil penetrates into steel, reducing the material strength and promoting local material structure changes and crack generation. This structure change part is where the martensite constituting the bearing steel changes into fine ferrite grains, and it appears white when observed by etching, so it is called white structure. Spalling accompanied by 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 failure. 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 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 spheroidized 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.

Prior Art Documents

Patent Documents

[0005] [Patent Document 1] Patent No. 7264319 [Overview of the project] [Problems that the invention aims to solve]

[0006] However, in the rolling bearing described in Patent Document 1, the amount of dissolved chromium in the martensitic structure is 2.43% to 2.95% by mass, which is a high value compared to ordinary bearing steels 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. [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, The aforementioned raceway has a rolling surface on which the rolling element rolls, A rolling bearing characterized in that 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 configured 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 greater 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] A method for manufacturing a rolling bearing, comprising manufacturing a rolling bearing described in any one of [1] to [3], It has a heat treatment process for heating the raceway material, A method for manufacturing a rolling bearing, characterized in that 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 treatment. [Effects of the Invention]

[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. [Brief explanation of the drawing]

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

Mode for Carrying Out the Invention

[0017] As a result of intensive research by the present inventors, it has been found that if the balance between the carbon content and the chromium content in steel is appropriately defined, the resistance to white structure spalling and the hardness of the raceway surface can be further improved without controlling the amount of dissolved carbon. Specifically, when the chromium content in steel is high, carbides are formed, and the steel material is likely to crack during manufacturing. And in order to obtain the hardness required for a rolling bearing, it is necessary to dissolve the carbides, and the heat treatment requires a high temperature and a long time, which deteriorates the productivity of the rolling bearing.

[0018] Also, when the carbon content in steel is low, there is a concern that the bearing performance other than the white structure spalling characteristics deteriorates. Furthermore, a decrease in the carbon content also leads to a deterioration in hardenability and a decrease in hardness. Based on the above findings, the present inventors have found an appropriate range for the carbon content and the chromium content in steel.

[0019] Hereinafter, embodiments of the present invention will be specifically described. Note that the present invention is not limited to the embodiments described below.

[0020] 〔Rolling Bearing〕 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. As shown in FIG. 1, the rolling bearing 1 has a steel outer ring 3 having an outer ring side raceway surface 2 on its inner peripheral surface and a steel inner ring 5 having an inner ring side raceway surface 4 on its outer peripheral surface. The outer ring 3 and the inner ring 5 constitute a pair of raceway rings. Also, between the pair of raceway rings (between the outer ring side raceway surface 2 and the inner ring side raceway surface 4), a plurality of rolling elements 6 are arranged. These rolling elements 6 are held in a state of being equally spaced in the circumferential direction and are held rotatably by a cage 7.

[0021] In the rolling bearing according to the present embodiment, the carbon content and chromium content in the steel in 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) are appropriately controlled. Hereinafter, the carbon content in the steel and the chromium content in the steel defined in the present embodiment will be described in detail.

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

[0023] On the other hand, when the carbon content in the steel exceeds 0.75 mass%, white structure exfoliation occurs in the rolling bearing, and the bearing life decreases. Therefore, the carbon content in the steel is 0.75 mass% or less, preferably 0.73 mass% or less, and more preferably 0.71 mass% or less.

[0024] <Chromium content in steel (Cr content): less than 2.10 mass%> Chromium (Cr) is a component that particularly affects the hardenability of steel materials. However, in the present embodiment, it is not necessarily required that chromium be contained in the steel material, and it may be 0 mass%. However, from the viewpoint of the productivity of the rolling bearing, the chromium content in the steel is preferably more 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 in 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 ingredients> 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 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. For example, the content of each element in the steel is preferably 0.60% by mass or less for Si, 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 methods such as an electron probe macroanalyzer (EPMA) on a cross-section perpendicular to the raceway surface.

[0030] <Vickers hardness of rolling surface: 640HV 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] [Manufacturing method for rolling bearings] Next, an example of a method for manufacturing a rolling bearing according to this embodiment will be described below in order of steps.

[0032] <Processing process> 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 a raceway material.

[0033] <Heat treatment process> To obtain the desired hardness for the raceway rings of a rolling bearing, 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, resulting in excellent hardenability. For example, even if the heat treatment temperature during quenching by furnace heating is set to less than 900°C, 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, the heat treatment conditions for the raceways of the rolling bearing are not particularly limited to 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. [Examples]

[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: 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 defined in JIS G 4805) and S53C (as defined in JIS G 4051) were used as comparative materials, so the compositions of SUJ2 and S53C are also shown in Table 1 below.

[0039] [Table 1]

[0040] [Manufacturing of rolling bearings (raceways)] Steel materials having the various compositions described above were turned to produce raceway rings (inner and outer rings) for rolling bearings. Subsequently, the raceway ring materials were heat-treated to produce test specimens of rolling bearings equipped with the obtained raceways. The heat treatment methods used were quenching and tempering by furnace heating or high-frequency induction hardening.

[0041] [Evaluation of test materials] <Evaluation of resistance to white tissue peeling> Each test material was mounted on a life tester, and a life test was conducted to evaluate its resistance to white structure delamination. The lifespan (L50) at which the cumulative failure probability reached 50% was determined. The measurement results of the life tests are shown in Table 2 below. In Table 2, 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 failed test materials, the L50 calculated as the time at which the test was terminated is listed.

[0042] [Table 2]

[0043] As shown in Tables 1 and 2 above, in test material symbol R1, where the carbon content in the steel exceeded the upper limit of the range specified in this invention, the bearing life was determined by the occurrence of white structure delamination. In contrast, in test material symbols R2 to R7, where the carbon content in the steel was within the range specified in this invention, no white structure delamination occurred in any of them, and they had a longer life compared to test material symbol R1.

[0044] <Hardness Evaluation> The Vickers hardness of the surfaces of various fabricated raceway rings was measured in accordance with JIS Z 2244. The steel numbers of the steel materials used and the cooling methods during furnace heating and quenching are shown in Table 3 below, and the Vickers hardness measurement results are shown in Figure 2.

[0045] [Table 3]

[0046] As shown in Table 3 and Figure 2 above, when oil cooling was selected during furnace heating for test material symbol S2, the hardenability decreased, and the required hardness for the rolling surface of the rolling wheel could not be obtained. Furthermore, for test material symbol S1, which has the same composition as test material symbol S2, oil cooling followed by water cooling was selected to improve hardenability. While the hardness was good, the rapid cooling made it prone to cracking. On the other hand, when high-frequency induction hardening was selected as the heat treatment method, it was possible to obtain the required hardness without cracking if the conditions were appropriately selected.

[0047] Test material symbols S3 and S4 show that the carbon content and chromium content in the steel are within the range specified in this invention, and excellent hardness was obtained regardless of the quenching temperature.

[0048] Test material symbol S5 has a chromium content exceeding the upper limit specified in this invention, resulting in the formation of many stable carbides. While these carbides can be dissolved through high-temperature and long-duration heat treatment, the required hardness could not be obtained at quenching temperatures below 900°C. Therefore, when using steel material with test material symbol S5, it is necessary to select a furnace capable of high-temperature heat treatment, which reduces the flexibility of heat treatment and lowers 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 materials 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 a material is 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. [Explanation of symbols]

[0051] 1 Rolling bearing 2. Outer ring side rolling surface 3 Outer ring 4. Inner wheel side rolling surface 5. Inner Ring 6 Rolling elements 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, The aforementioned raceway has a rolling surface on which the rolling element rolls, A rolling bearing characterized in that 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 more.

4. A method for manufacturing a rolling bearing, comprising manufacturing a rolling bearing according to claims 1 to 3, It has a heat treatment process for heating the raceway material, A method for manufacturing a rolling bearing, characterized in that 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.

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