Rolling elements for rolling bearings and rolling bearings using the same

Abstract

To provide a rolling element for a rolling bearing which does not require surface treatment such as carburization or nitriding and improves impression resistance and foreign matter resistance due to contact with a track surface, and a rolling bearing therewith.SOLUTION: A rolling element for rolling bearings is made of an iron-based alloy comprising, in weight%, C: 1.10 to 1.50%, Si: 0.70 to 2.50%, Mn: 0.10 to 1.00%, Cr: 1.00 to 4.00%, Mo: 0.20 to 1.50%, V: 0.10 to 0.80%, and residual iron and unavoidable impurities. Further, an amount of retained austenite in a surface layer portion is set to 5 to 15% by volume, and the surface hardness can be set to 64HRC or more on the Rockwell C scale.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a rolling element for a rolling bearing used in the automotive field such as electric vehicles and the industrial machinery field such as speed reducers, and to a rolling bearing using the same.

Background Art

[0002] Rolling bearings used in fields such as the automotive field and the industrial machinery field are required to have various properties including heat resistance and wear resistance. In particular, in addition to dynamic strength in inner and outer rings and rolling elements, static strength (indentation resistance) is also required. Furthermore, due to the miniaturization, weight reduction and cost reduction of machinery, the service environment of bearings has become even harsher than before. For rolling bearings used in such harsh service environments, foreign matters such as burrs and wear debris often get mixed into the lubricating oil, so foreign matter resistance is also required.

[0003] For example, there has been disclosed a technique in which a high-hardness material such as high-speed tool steel is used as the material for bearing rings and rolling elements, carbides of several micrometers or less are precipitated in a structure of a predetermined depth on the raceway surface of the bearing ring, and then a carbonitrided layer is formed (see Patent Documents 1 and 2). There is also disclosed a technique in which medium-carbon steel is used as the material for a bearing ring, and carburizing or carbonitriding treatment is performed to improve toughness and quenched and tempered hardness, thereby suppressing the initiation and propagation of cracks caused by the inclusion of hard foreign matters (Patent Document 3).

Prior Art Literature

Patent Literature

[0004]

Patent Literature 1

Patent Literature 2

Patent Literature 3

Summary of the Invention

Problem to be Solved by the Invention

[0005] However, high-hardness materials such as high-speed tool steel tend to increase material costs, resulting in higher manufacturing costs compared to bearing steel. Furthermore, they are prone to the precipitation of coarse carbides in the microstructure, making it difficult to extend the lifespan of the bearings. Additionally, while raceway rings made of medium-carbon steel undergoing carburizing or carbonitriding treatment improve the resistance to foreign matter in the raceway rings, the rolling elements tend to fail first, and their low internal hardness results in poor resistance to indentation. Moreover, the use of special equipment and special treatments such as carburizing or nitriding to create a carbonitrided layer on the surface of the raceway rings also contributes to increased manufacturing costs.

[0006] Therefore, the object of the present invention is to provide rolling elements for rolling bearings and rolling bearings using the same, which do not require surface treatment such as carburizing or nitriding and have improved static strength (indentation resistance) due to contact with the raceway surface. [Means for solving the problem]

[0007] In this invention, the rolling elements for rolling bearings are made of an iron-based alloy containing residual iron and unavoidable impurities, with a composition of C: 1.10-1.50%, Si: 0.70-2.50%, Mn: 0.10-1.00%, Cr: 1.00-4.00%, Mo: 0.20-1.50%, and V: 0.10-0.80% by weight.

[0008] Furthermore, the amount of retained austenite in the surface layer can be set to a range of 5 to 15 volume%, and the surface hardness can be set to 64 HRC or higher on the Rockwell C scale. It is more preferable that the amount of retained austenite in the surface layer be in the range of 10 to 13 volume%. [Effects of the Invention]

[0009] The invention of rolling elements for rolling bearings and rolling bearings using them provides optimal rolling bearing components with enhanced surface hardness through conventional heat treatment (quenching and tempering) without the need for special surface treatments such as carburizing or nitriding. Furthermore, it improves resistance to indentation (static load rating) and foreign matter by contact with the raceway surface, thereby extending the lifespan of the rolling bearing. [Brief explanation of the drawing]

[0010] [Figure 1] This is a schematic diagram of the thrust life tester used in Example 2. [Modes for carrying out the invention]

[0011] The chemical composition of a rolling element for a rolling bearing, which is one embodiment of the present invention, will now be described. The carbon (C) content in the iron-based alloy constituting the rolling element for the rolling bearing of the present invention is set to 1.10 to 1.50% by weight. Carbon ensures the hardness in the steel after quenching and tempering, and plays a role in ensuring a high rolling fatigue life when used as a material for rolling bearings. If the C content in the steel is less than 1.10%, the required surface hardness cannot be obtained. On the other hand, if the content exceeds 1.50%, the amount of carbides and retained austenite in the steel increases, which reduces the rolling fatigue life of the bearing.

[0012] The silicon (Si) content should be between 0.70% and 2.50% by weight. Silicon plays a role in increasing the tempering softening resistance in steel. If the Si content in the steel falls below 0.70%, the necessary tempering softening resistance cannot be obtained, and if the Si content exceeds 2.50%, the hot forgeability will be significantly reduced.

[0013] The manganese (Mn) content should be between 0.10% and 1.00% by weight. Manganese is effective in improving the hardenability of steel and thus increasing the rolling fatigue life when used as a bearing material. If the Mn content in the steel exceeds 1.00%, the hot forgeability decreases significantly.

[0014] The chromium (Cr) content should be between 1.00% and 4.00% by weight. Chromium enhances the hardenability of steel and thermally stabilizes cementite, preventing solid solution of cementite into the matrix at high temperatures. If the Cr content in the iron-based alloy falls below 1.00%, the hardenability of the steel deteriorates, and if the Cr content exceeds 4.00%, coarse carbides are generated in the steel, reducing the rolling fatigue life of rolling bearings when used as a bearing material.

[0015] The molybdenum (Mo) content should be between 0.20% and 1.50% by weight. Molybdenum contributes to hardness by forming carbides in the steel. If the Mo content in the steel exceeds 1.50%, coarse carbides are generated, which reduces the rolling fatigue life when used as a bearing material.

[0016] The amount of vanadium (V) should be 0.10 to 0.80% by weight. Vanadium, when added in combination with silicon in steel, plays a role in increasing the resistance to tempering and softening. Furthermore, if the content of each element in the steel exceeds 0.80%, coarse carbides will be generated, which will reduce the rolling fatigue life when used as a bearing material.

[0017] Furthermore, a similar effect can be obtained by including Nb (niobium) as a substitute element for V. In this case, the Nb content is preferably in the range of 0.05 to 0.40% by weight. Additionally, W (tungsten) can also be included in the range of 0.40 to 3.00% by weight as W equivalent (W + 2Mo).

[0018] In this case, tungsten, like molybdenum, forms carbides in the steel, contributing to its hardness. If the W equivalent in the steel falls below 0.40%, the required tempering hardness and softening resistance cannot be obtained. On the other hand, if the W equivalent exceeds 3.00%, coarse carbides are generated, reducing the rolling fatigue life of the rolling bearing.

[0019] Next, a heat treatment method for a rolling element for a rolling bearing according to an embodiment of the present invention will be described. The rolling element for a rolling bearing of the present invention can be manufactured by quenching the iron-based alloy having the aforementioned chemical components at a temperature in the range of 840 to 880°C, and then tempering the alloy at a temperature in the range of 150 to 180°C. By performing heat treatment under predetermined conditions, the amount of retained austenite in the surface layer portion of the rolling element for a rolling bearing can be set in the range of 5 to 15% by volume. In this case, the surface hardness of the rolling element for a rolling bearing is 64 HRC or more on the Rockwell C scale. Examples

[0020] Example 1 A steel ball pressing test (hereinafter referred to as the present test) was carried out using two types of iron-based alloys: an iron-based alloy which is the base material of the rolling element for a rolling bearing of the present invention (hereinafter referred to as the invention material) and a conventional bearing steel (hereinafter referred to as the comparative material). The test results will be described below. Table 1 shows the chemical components (unit: wt%) of Invention Materials 1 and 2 (two levels with different chemical components) and the comparative material (bearing steel: SUJ2) used in the present test, and Table 2 shows the surface hardness (unit: HRC) and the amount of retained austenite (unit: vol%), respectively. The amount of retained austenite of the invention materials and the comparative material was calculated by dedicated software from the ratio of integrated intensities of the diffracted X-ray intensity distributions of the austenite phase and the martensite phase measured by an X-ray diffraction analyzer.

[0021] The present test is a test for objectively comparatively measuring the degree of dent formation on the material surface by intensively applying a static load to a specific location, and the indentation resistance can be evaluated based on the amount (depth) of the dent measured after pressing. In the present test, after a steel ball with a diameter of 9.525 mm was pressed against the sample surface at a pressure of 4500 MPa for 10 seconds (with a loading speed of 0.1 mm / min), the depth of the trace of the steel ball left on the sample surface was measured with a laser microscope.

[0022] Invention Materials 1 and 2 used in the present test were quenched at 850°C for 120 minutes, and then pre-tempered at 160°C for 120 minutes. The comparative material was quenched at 850°C for 40 minutes, and then pre-heat treated by tempering at 190°C for 90 minutes.

[0023] [Table 1]

[0024] [Table 2]

[0025] The depth of the depressions formed on the surface of the samples was measured, and the result was 0.220 μm for Inventive Material 1 and 0.210 μm for Inventive Material 2. In contrast, the comparative material was 0.330 μm. From the above test results, it can be concluded that Inventive Materials 1 and 2, which are iron-based alloys of the present invention, have a high amount of retained austenite on the material surface. 5 By setting the surface hardness to 64 HRC or higher on the Rockwell C scale within a range of ~15 volume%, the material surface is less prone to denting even when a load is applied to a specific point from the outside, compared to the comparative material (bearing steel SUJ2). Therefore, it has excellent indentation resistance and is suitable for bearing components in areas with large dynamic or static loads.

[0026] (Example 2) Using the inventive materials 1 and 2 and a comparative material used in Example 1, test specimens of predetermined dimensions were prepared, and rolling fatigue characteristics were evaluated (thrust life test). The test results are described below. The chemical composition (unit: weight %) of the inventive materials 1 and 2 and the comparative material used in the thrust life test is shown in Table 1 of Example 1. A schematic diagram of the test equipment (thrust life tester) used in this example is shown in Figure 1.

[0027] In this test, as shown in Figure 1, a disc-shaped test piece 3 with a diameter φD is attached to an oil tank into which lubricating oil 5 is injected, and the table 4 is pushed up. Then, a predetermined surface pressure P is applied by receiving the steel ball 2, which is supported by a retainer, with a thrust bearing 1. In this state, the shaft 10 that transmits power from a motor (not shown) is rotated at a predetermined rotational speed to perform the evaluation test. The test is continued until the test piece breaks, and the total number of rotations at the time of breakage is recorded, at which point the test is terminated.

[0028] Furthermore, even if the test specimen is not damaged, the total number of rotations is 1 × 10⁻⁶. 8 The test was terminated when the number of trials reached a certain point. The test conditions were repeated five times under the following conditions, and a Weibull distribution graph was created. The L10 lifetime at which the cumulative failure rate reached 10% was read from the graph, and the evaluated lifetimes of each test specimen were compared and evaluated. • Test specimen dimensions: Diameter (φD) 61mm x Thickness 6mm • Test surface pressure (P): 4900 MPa • Rotation speed: 1000 rpm • Test temperature: Room temperature (approximately 23°C) • Lubricant: ENEOS Turbine Oil 68

[0029] Based on the results of this test, the number of cycles at which the cumulative failure rate reaches 10% is 1 × 10 for both Invention 1 and Invention 2. 8 The number of cycles was 4.11 to 5.17 × 10⁶ for the comparative material test specimens. 7 The lifespan was shorter compared to the results for inventions 1 and 2. From the above test results, it was found that the chemical components of invention materials 1 and 2 have superior rolling fatigue characteristics compared to the chemical components of comparative materials 1 and 2.

[0030] (Example 3) Test specimens were prepared using the inventive materials 1 and 2 used in Example 1, as well as a comparative material, and high-temperature hardness tests were conducted. The test results are described below. The prepared test specimens (thrust plate specimens: diameter 61 mm x thickness 6 mm) were held for 1 hour at a total of eight temperatures: 100°C, 120°C, 140°C, 160°C, 180°C, 200°C, 250°C, and 300°C. The surface hardness of the test specimens at each temperature (unit: Rockwell C scale) was then measured. The measurement results of the surface hardness of each test specimen at each temperature (8 levels) are shown in Table 3.

[0031] [Table 3]

[0032] As shown in Table 3, the comparative material's hardness dropped to 62 HRC or less above 180°C up to 200°C, to less than 60 HRC above 200°C, and to approximately 56 HRC at 300°C. On the other hand, both inventive materials 1 and 2 maintained a hardness of 62 HRC or higher even when the holding temperature exceeded 200°C, indicating superior surface hardness at high temperatures compared to the comparative material.

Claims

1. A rolling element for a rolling bearing, characterized by having, by weight percent, C: 1.10-1.50%, Si: 0.70-2.50%, Mn: 0.10-1.00%, Cr: 1.00-4.00%, Mo: 0.20-1.50%, and V: 0.10-0.80%, being made of an iron-based alloy consisting of residual iron and unavoidable impurities, having a retained austenite content in the surface layer in the range of 5-15 volume%, and a surface hardness of 64 HRC or higher on the Rockwell C scale.

2. A rolling element for a rolling bearing, characterized in that, by weight percent, C: 1.10 to 1.50%, Si: 0.70 to 2.50%, Mn: 0.10 to 1.00%, Cr: 1.00 to 4.00%, Mo: 0.20 to 1.50%, and Nb: 0.05 to 0.40%, made of an iron-based alloy consisting of residual iron and unavoidable impurities, the amount of retained austenite in the surface layer is in the range of 5 to 15 volume%, and the surface hardness is 64 HRC or higher on the Rockwell C scale.

3. A rolling element for a rolling bearing, characterized in that, by weight percent, C: 1.10 to 1.50%, Si: 0.70 to 2.50%, Mn: 0.10 to 1.00%, Cr: 1.00 to 4.00%, W + 2Mo: 0.40 to 3.00%, V: 0.10 to 0.80%, made of an iron-based alloy consisting of residual iron and unavoidable impurities, having a residual austenite content in the surface layer in the range of 5 to 15 volume percent, and having a surface hardness of 64 HRC or higher on the Rockwell C scale.

4. A rolling element for a rolling bearing, characterized in that, by weight percent, C: 1.10 to 1.50%, Si: 0.70 to 2.50%, Mn: 0.10 to 1.00%, Cr: 1.00 to 4.00%, W + 2Mo: 0.40 to 3.00%, and Nb: 0.05 to 0.40%, made of an iron-based alloy consisting of residual iron and unavoidable impurities, having a residual austenite content in the surface layer in the range of 5 to 15 volume percent, and having a surface hardness of 64 HRC or higher on the Rockwell C scale.

5. A rolling bearing characterized by using rolling elements for a rolling bearing as described in any one of claims 1 to 4.

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