Bearing component, method for manufacturing same, and rolling bearing

Abstract

The bearing member is configured from a steel containing 0.6-0.95 mass% of carbon, the hardness A of a region from the surface of the raceway groove to a depth of 0.2 mm is in the range of 750-880 Hv, the hardness B of a region from the surface of the raceway groove to a depth of more than 0.2 mm is in the range of 633-832 Hv, the amount C of retained austenite in the surface of the raceway groove is 20 vol% or less, and the amount C of the retained austenite in the surface of the raceway groove is less than or equal to 0.5 vol%. The total average retained austenite amount (D) is 8 vol% or less.

Description

Technical Field

[0001] The present invention relates to a bearing component, a manufacturing method thereof, and a rolling bearing. Background Art

[0002] In the past, rolling bearings were used as components that support rotating shafts so that they rotate freely. Rolling bearings are required to have high dimensional stability, wear resistance, and fatigue strength under high loads. In order to meet these requirements, heat treatment has been applied to bearing components used in rolling bearings, and various studies have been conducted on the methods and conditions of such heat treatments. In particular, carbonitriding, in which carbon and nitrogen are diffused and infiltrated into steel and then quenched, is known to be effective in improving the durability of bearing components (for example, see Patent Documents 1 and 2).

[0003] In recent years, with the increasing loads and temperatures in use environments, rolling bearings are required to have the characteristics of being able to operate under greater loads and higher temperatures (such as long life under high loads and high dimensional stability under high temperatures). Therefore, bearing components with higher durability are expected.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. 2007-277648

[0007] Patent Document 2: Japanese Patent Application Laid-Open No. 2008-267402 Summary of the Invention

[0008] Problems to be solved by the invention

[0009] One example of the technical problem of the present invention is to provide a bearing component having high durability, a method for manufacturing the same, and a rolling bearing including the bearing component.

[0010] Solutions for solving problems

[0011] The above technical problems are solved, for example, by one aspect of the present invention as follows. Specifically, one aspect of the present invention provides a bearing component comprising steel containing 0.6% to 0.95% by mass of carbon, wherein the hardness A of the region from the surface of the raceway groove to a depth of 0.2 mm is within a range of 750 Hv to 880 Hv, the hardness B of the region beyond a depth of 0.2 mm from the surface of the raceway groove is within a range of 633 Hv to 832 Hv, the retained austenite amount C of the raceway groove surface is 20% by volume or less, and the overall average retained austenite amount D is 8% by volume or less. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1This is a cross-sectional view of a section taken along a plane including the axis of a rolling bearing according to an example embodiment of the present invention.

[0013] Figure 2 This is a schematic plan view schematically showing an annular bearing component as a measurement object of hardness and retained austenite amount.

[0014] Figure 3 This is a schematic cross-sectional view of a ring-shaped bearing component cut along the axial direction as the object of measurement of hardness and retained austenite amount, corresponding to Figure 2 AA section view in.

[0015] Figure 4 It will Figure 3 A schematically enlarged partial view of area B in FIG.

[0016] Figure 5 This is an explanatory diagram showing a state where the amount of retained austenite on the surface of a rolling groove is being measured.

[0017] Figure 6 This is an explanatory diagram showing a state where the average retained austenite amount of the entire bearing component is being measured.

[0018] Figure 7 This is an explanatory diagram showing a cross section for measuring the average retained austenite amount of the entire bearing component.

[0019] Figure 8 is a schematic cross-sectional view of a ring-shaped bearing component to be measured for nitrogen concentration cut along the axial direction, corresponding to Figure 2 AA section view in.

[0020] Figure 9 This is a graph showing the results of a durability evaluation test of the bearing component of Example 6. DETAILED DESCRIPTION

[0021] Hereinafter, a bearing component and a method for manufacturing the same, and a rolling bearing according to an embodiment of the present invention will be described. For convenience of description, the rolling bearing, the bearing component, and the method for manufacturing the bearing component will be described in this order.

[0022] [Rolling bearings]

[0023] Figure 1 This is a cross-sectional view taken along a plane including the axis of a rolling bearing (ball bearing) 10, an example embodiment of the present invention. Rolling bearing 10 has the same basic structure as conventional rolling bearings, comprising an annular inner ring 11, an annular outer ring 12, a plurality of rolling elements (balls) 13, a retainer (cage) 14, and annular sealing members 15a and 15b.

[0024] The inner ring 11 is a cylindrical structure coaxially arranged with the central axis (rotational axis) x of the shaft. The outer ring 12 is a cylindrical structure coaxially arranged on the outer circumference of the inner ring 11. The multiple rolling elements 13 are spherical balls arranged in grooves within the bearing space (annular space) 16 formed between the inner ring 11 and the outer ring 12. In other words, the rolling bearing 10 in this embodiment is a ball bearing.

[0025] A lubricant G, such as a grease composition, is enclosed in the bearing space 16. The lubricant G acts to reduce friction between the rolling elements 13 and the retainer 14, and between the rolling elements 13 and the inner ring 11 or outer ring 12. Annular sealing members 15a and 15b are formed, for example, from steel plates and project from the inner circumferential surface of the outer ring 12 toward the inner ring 11, shielding the bearing space 16, which serves as a channel, from the outside world.

[0026] The inner circumferential surface of the outer ring 12 has an arcuate cross-sectional recess 12a formed along the circumference of the outer ring 12. Furthermore, the outer circumferential surface of the inner ring 11 has an arcuate cross-sectional recess 11a formed along the circumference of the inner ring 11. The rolling elements 13 are guided along the circumference by the groove-shaped recesses 11a and 12a.

[0027] Recess 12a forms the raceway groove of outer ring 12, a bearing component of rolling bearing 10. Hereinafter, this recess 12a will be referred to as outer ring raceway groove 12a. Furthermore, recess 11a forms the raceway groove of inner ring 11, a bearing component of rolling bearing 10. Hereinafter, this recess 11a will be referred to as inner ring raceway groove 11a. It should be noted that the term "raceway groove" is a component / part that has various designations, such as "bearing groove" and "rolling groove."

[0028] The retainer 14 is disposed within the groove and retains the plurality of rolling elements 13. The retainer 14 is an annular member coaxial with the central axis x of the shaft. It has a structure in which, on one side of the retainer facing the central axis, multiple recesses are provided for retaining the rolling elements 13, each of which houses a rolling element 13. The shape (crown-shaped, corrugated, etc.) and material (steel plate, resin, etc.) of the retainer 14 are arbitrary and are not limited to a specific shape or material.

[0029] In the rolling bearing 10 having the above structure, one or both of the outer ring 12 and the inner ring 11 as bearing components correspond to the bearing component of the present embodiment described below. Preferably, both the outer ring 12 and the inner ring 11 correspond to the bearing component of the present embodiment.

[0030] Furthermore, bearing components other than outer ring 12 and inner ring 11 (rolling elements 13, retainer 14, annular seal members 15a and 15b, etc.) may also satisfy the same conditions as those of the present embodiment. It should be noted that high durability may be achieved by using ceramics such as ZrO2, SiC, or Si3N4 for rolling elements 13.

[0031] [Bearing parts]

[0032] The bearing component of the present embodiment satisfies the following conditions found as a result of intensive research conducted by the inventors.

[0033] (a) Carbon content

[0034] The steel comprises steel containing 0.6 mass % or more and 0.95 mass % or less of carbon.

[0035] (b) Hardness

[0036] The hardness of the area from the surface of the rolling groove (inner ring rolling groove 11a or outer ring rolling groove 12a) to a depth of 0.2 mm is in the range of 750Hv to 880Hv, and the hardness of the area deeper than 0.2mm from the rolling groove surface is in the range of 633Hv to 832Hv.

[0037] (c) Retained austenite amount (retained γ amount)

[0038] The amount of retained austenite on the surface of the raceway groove (inner ring raceway groove 11a or outer ring raceway groove 12a) is 20 volume % or less, and the overall average retained austenite amount is 8 volume % or less.

[0039] Furthermore, the bearing component of the present embodiment preferably satisfies the following conditions regarding (d) nitrogen concentration.

[0040] (d) Nitrogen concentration

[0041] The nitrogen concentration at a depth of 0.08 mm from the surface is in the range of approximately 0.2 mass % to 0.8 mass %. Each of the above conditions (a) to (d) will be described in detail.

[0042] (a) Carbon content

[0043] The carbon content of the steel used significantly affects the hardness and carbide content of the bearing component after quench hardening. A high carbon content easily results in a bearing component with high hardness. However, from a manufacturing cost perspective, it is ideal to have a carbon content within a range that allows for cold forging. Therefore, in this embodiment, the carbon content that allows for cold forging is set to a range of 0.6% by mass to 0.95% by mass.

[0044] If the carbon content of the steel used is less than 0.6% by mass, it is difficult to ensure the amount of carbides in the bearing component after quench hardening, and thus it is difficult to ensure hardness. On the other hand, if the carbon content of the steel used exceeds 0.95% by mass, it is difficult to form by cold forging, so most forming processes have to be done by cutting, which may increase costs.

[0045] Chemical components other than iron (Fe) and carbon (C) in the steel used include, for example, the following.

[0046] ·Si……0.5 mass% or less

[0047] Mn: 1% by mass or less

[0048] Cr: 1% to 2% by mass

[0049] ·P……0.1% by mass or less

[0050] ·S……0.1% by mass or less

[0051] It should be noted that the present invention is not limited to the above chemical compositions. In addition to these chemical compositions, elements other than these chemical compositions, such as Mo, Ni, V, B, Al, and N, may be added to the constituent elements according to various purposes. In addition, various common impurities may also be included.

[0052] (b) Hardness

[0053] The surface of the raceway groove (inner ring raceway groove 11a or outer ring raceway groove 12a) is in contact with the rolling elements 13 and is subject to mechanical load. Therefore, it is desirable for the raceway groove surface to have high hardness. In particular, the hardness of the region from the surface to a depth of 0.2 mm is desirable to ensure mechanical strength against contact with the rolling elements 13.

[0054] However, if the surface hardness is too high, a large amount of nitrides will precipitate on the surface of the raceway groove, which will reduce the durability of the bearing component. Therefore, in this embodiment, the hardness of the area from the raceway groove surface to a depth of 0.2 mm (hereinafter referred to as "surface hardness A") is set to be within the range of 750Hv to 880Hv. The surface hardness A is preferably within the range of 800Hv to 880Hv.

[0055] On the other hand, if the hardness is increased not only on the surface but also in a large area extending deeper, the cost will increase. Therefore, in this embodiment, the hardness of the area deeper than 0.2 mm from the rolling groove surface (hereinafter referred to as "deep hardness B") is set to be within the range of 633 Hv to 832 Hv.

[0056] In addition, the "hardness" in the surface hardness A and the deep hardness B refers to the Vickers hardness specified in Japanese Industrial Standard JIS Z2244, and the test force (load) F is set to 4.903N.

[0057] The method for measuring the hardness of a region at a predetermined depth from the surface of the rolling groove (surface hardness A and deep hardness B) will be described later.

[0058] (c) Retained austenite amount (retained γ amount)

[0059] Retained austenite after heat treatment causes aging deformation. Therefore, from the perspective of dimensional stability, it is preferable to reduce the amount of retained austenite after heat treatment. Therefore, in this embodiment, the average retained austenite content of the entire bearing component (hereinafter referred to as "average retained γ content D") is set to 8 volume % or less.

[0060] On the other hand, from the perspective of fatigue life, it is known that a greater amount of retained austenite increases the fatigue life of the rolling grooves 11a and 12a when subjected to repeated fatigue. For this reason, it is ideal for some retained austenite to remain on the groove surface. Therefore, in this embodiment, the retained austenite content on the rolling groove surface (hereinafter referred to as "surface retained γ content C") is set to 20% by volume or less.

[0061] As described above, from the viewpoint of dimensional stability, it is desirable to minimize the amount of retained austenite. However, from the viewpoint of fatigue life, the fatigue life of the rolling groove can be improved by leaving a certain amount of retained austenite.

[0062] Therefore, in this embodiment, the surface residual γ amount C is preferably 10 volume % or more, more preferably 10 volume % or more and 20 volume % or less. In addition, in this embodiment, the average residual γ amount D is preferably 2 volume % or more, more preferably 2 volume % or more and 8 volume % or less.

[0063] <(d) Nitrogen concentration>

[0064] The nitrogen concentration from the surface to a depth of 0.08 mm is preferably set in the range of about 0.2 mass % to 0.8 mass %, and more preferably in the range of about 0.3 mass % to 0.7 mass %. If the nitrogen concentration near the surface is too low, it is difficult to ensure the surface hardness that helps to improve durability. On the other hand, if the nitrogen concentration near the surface is too high, there is a hidden danger of precipitation of a large amount of nitrides near the surface, which deteriorates the durability of the bearing components. It should be noted that as long as the average value of the measured value of the nitrogen concentration from the surface to a depth of 0.08 mm is within the above-mentioned preferred concentration range, there may also be parts within the measurement range from the surface to 0.08 mm that deviate from the measured value of the above-mentioned preferred concentration range.

[0065] <Methods for measuring (b) hardness, (c) retained austenite, and (d) nitrogen concentration>

[0066] An example of a method for measuring (b) hardness, (c) retained austenite amount, and (d) nitrogen concentration in this embodiment will be described with reference to the drawings.

[0067] (b) Determination of hardness

[0068] Figures 2 to 4 This is an explanatory diagram for explaining an example of a method for measuring (b) hardness in this embodiment.

[0069] In detail, Figure 2 is a schematic top view schematically showing an annular bearing component as a measurement object of (b) hardness and (c) retained austenite amount, Figure 3 This is a schematic cross-sectional view of an annular bearing component cut along the axial direction. Figure 3 Corresponding to Figure 2 AA section view in. Figure 2 and Figure 3 This figure shows both the inner ring 11 and the outer ring 12, so the outline is not accurate and is only a schematic diagram. Figure 4 yes Figure 3 A partial enlarged view of Figure 3 Region B in the diagram is schematically enlarged. Figure 4 In the example, the dimensional balance of each structure is inconsistent, so Figure 4 It's just a schematic diagram.

[0070] like Figure 2 As shown, for the inner ring 11 or the outer ring 12 (hereinafter referred to as "bearing component 11, bearing component 12"), first, it is cut with the plane containing the central axis x as the cutting plane (dashed line C in the figure) to obtain the cutting plane c. Figure 3 1 and 2 show a cross-sectional surface c including the corresponding inner ring rolling groove 11a of the bearing component 11 or the outer ring rolling groove 12a of the bearing component 12 (hereinafter referred to as “rolling groove 11a, rolling groove 12a”).

[0071] In the determination of hardness, Figure 3 As shown in FIG. 1 , the center b of the rolling grooves 11a and 12a in the width direction is set as the "surface" of the measurement object, and the center line O passing through the center b of the rolling grooves 11a and 12a is set as the measurement object in the depth direction. Figure 4As shown, a position 50 μm from the center b in the depth direction (direction of arrow d) is set as the first measurement point, and a position 50 μm further in the depth direction (direction of arrow d) and 100 μm away from the center line O is set as the second measurement point. For every odd number of 50 μm advances in the depth direction (direction of arrow d), a measurement point is set on the center line O, and for every even number of 50 μm advances in the depth direction (direction of arrow d), a measurement point is set at a position 100 μm away from the center line O.

[0072] At the above measurement points, hardness was measured at a total of eighteen locations, up to a depth of 0.9 mm. Four measurement points, from the center b of the rolling grooves 11a and 12a to the fourth measurement point at a depth of 0.2 mm, were designated as the "region from the rolling groove surface to a depth of 0.2 mm" and were used as measurement points for surface hardness A. Furthermore, fourteen measurement points, from the fifth measurement point at a depth exceeding 0.2 mm from the center b of the rolling grooves 11a and 12a to the final eighteenth measurement point, were designated as the "region exceeding 0.2 mm from the rolling groove surface" and were used as measurement points for deep hardness B.

[0073] Vickers hardness was measured using the number of measurement points for each of the surface hardness A and deep hardness B, which was set to the number n (n = 4, n = 14) for each hardness measurement. Hardness was measured using a commercially available Vickers hardness tester or a micro Vickers hardness tester, with a test force (load) F set to 4.903 N. Other parameters were in accordance with Japanese Industrial Standard JIS Z2244.

[0074] (c) Determination of retained austenite

[0075] Figures 5 to 7 This is an explanatory diagram for explaining an example of a method for measuring the amount of retained austenite (c) in this embodiment.

[0076] first, Figure 5 This is an explanatory diagram showing a state where the retained austenite amount (surface retained γ amount C) on the raceway groove surface is being measured, showing the bearing components 11 and 12. Figure 2 DD profile in. Figure 5 As shown, the collimator 21 of the measuring device is set on the center line O and is directed toward the center b in the width direction of the rolling groove 11a and the rolling groove 12a to measure the amount of retained austenite. Figure 5 As shown, the measurement area is centered on the center b of the width direction of the rolling groove 11a and the rolling groove 12a, and is a narrow area (1 mm) within the diameter range of the collimator 21. Figure 5 Area E).

[0077] The amount of retained austenite can be measured by measuring the surface of the rolling contact grooves 11 a and 12 a using an X-ray diffraction retained austenite measuring device, thereby measuring the surface retained γ amount C.

[0078] Figure 6 This is an explanatory diagram showing the state in which the average retained austenite amount (average retained γ amount D) of the entire bearing component is being measured. Figure 6 As shown, for the measurement of the average residual γ amount D, in order to measure the cross section of the plane including the central axis x (refer to Figure 2 and Figure 3 ), a test piece TP is produced in which the segment 11 ′ of the bearing component 11 and the segment 12 ′ of the bearing component 12 are embedded in the resin 22 in such a manner that the cross-section c is exposed on the surface.

[0079] The average retained austenite amount (average retained γ amount D) of the entire bearing component is measured for the cut surface c exposed on the surface of the test piece TP using the same measuring device as used for measuring the raceway groove surface.

[0080] Figure 7 : is an explanatory diagram showing the cross section c for measuring the average residual γ amount D. Figure 5 Likewise, the bearing components 11 and 12 are shown. Figure 2 DD profile in. Figure 7 As shown, the measurement area is measured on the center line O passing through the center b of the width direction of the rolling groove 11a and the rolling groove 12a, so that the diameter (3mm) of the collimator 21 covers the entire area of ​​the bearing component 11 and the bearing component 12 ( Figure 7 Area F in the .

[0081] (d) Determination of nitrogen concentration

[0082] Figure 8 This is an explanatory diagram for explaining an example of a method for measuring the (d) nitrogen concentration in this embodiment. Specifically, it is a schematic cross-sectional view of an annular bearing component that is a measurement target of the (d) nitrogen concentration, cut along the axial direction. Figure 8 Corresponding to Figure 2 The AA section view in Figure 3 Again, the outlines are not exact, they are just schematic.

[0083] In the measurement of nitrogen concentration (d), similarly to the measurement of hardness (b), for the bearing component 11 or the bearing component 12 (inner ring 11 or outer ring 12), as shown in FIG. Figure 2 As shown, the cross section is performed with the plane including the central axis x as the cross section (dashed line C in the figure) to obtain the cross section c. Figure 81 and 2 show a cross-sectional surface c including the corresponding rolling groove 11 a of the bearing component 11 and the rolling groove 12 a of the bearing component 12 .

[0084] like Figure 8 As shown in the figure, the center b of the rolling groove 11a and the rolling groove 12a in the width direction is set as the "surface" of the measurement object, and the line segment L on the center line O passing through the center b of the rolling groove 11a and the rolling groove 12a is set as the measurement object in the depth direction. The nitrogen concentration from the surface to the depth of 0.08mm is measured using an EPMA (electron probe microanalyzer) device. Figure 8 The line segment L shown performs line analysis on the cross-section of the raceway groove 11 a of the inner ring 11 and the raceway groove 12 a of the outer ring 12 in the depth direction.

[0085] In this embodiment, the nitrogen concentration (d) was measured under the following conditions: an acceleration voltage of 15 kV, an irradiation current of 100 nA, a beam diameter of 10 μm, and a stepping interval of 10 μm. In the measurement data, the ninth analysis point from the surface of the track grooves 11a and 12a (the boundary between the embedded resin and the metal) corresponds to a position 0.08 mm from the center b in the depth direction (in the direction of arrow d). Therefore, the value at this ninth point is used as the measurement result of the nitrogen concentration (d).

[0086] [Method for manufacturing bearing components]

[0087] There are two methods for manufacturing the bearing component of this embodiment, namely, manufacturing method A and manufacturing method B, each of which includes any of the following steps based on the knowledge gained through in-depth research conducted by the inventors. The bearing component of this embodiment described above can be manufactured by performing the steps of either manufacturing method A or manufacturing method B shown below.

[0088] (Manufacturing method A)

[0089] (1) Carbonitriding treatment process: The process is carried out in a temperature range of above the A1 transformation point to 900°C.

[0090] (2) Quenching and hardening process: Cooling to a temperature below the A1 transformation point.

[0091] (3) Low-temperature tempering treatment process: carried out at a temperature range of 90°C to 170°C.

[0092] (4) Cryogenic treatment process: carried out at a temperature below 0°C.

[0093] (5) Tempering process: The process is carried out at a temperature ranging from 160°C to 240°C.

[0094] (Manufacturing method B)

[0095] (1) Carbonitriding treatment process: The process is carried out in a temperature range of above the A1 transformation point to 900°C.

[0096] (2) Quenching and hardening process: Cooling to a temperature below the A1 transformation point.

[0097] (4) Cryogenic treatment process: This process is carried out within 24 hours from the completion of the quenching and hardening treatment process (2), at a temperature range of 0°C or below.

[0098] (5) Tempering process: The process is carried out at a temperature ranging from 160°C to 240°C.

[0099] Manufacturing method B includes the same steps as manufacturing method A, except that the (3) low-temperature tempering step is omitted and specified conditions are added to the (4) cryogenic treatment step. Therefore, the following description of manufacturing methods A and B will be unified with reference to the differences. In the following description, each step will be described in the order of the steps.

[0100] <(1) Carbonitriding process>

[0101] In this embodiment, (1) the carbonitriding step refers to a step of performing carburizing and nitriding in a temperature range of not less than the A1 transformation point and not more than 900°C. The A1 transformation point is the temperature at which the phase transformation from austenite to ferrite + cementite (Fe3C) begins. Regardless of the carbon content of the steel, the A1 transformation point is 727°C in the equilibrium state.

[0102] Either carburizing or nitriding can be performed first, but the inventors' in-depth research has led to the conclusion that it is preferable to perform nitriding first and then carburizing. Carburizing is typically performed first, but performing nitriding first allows more nitrogen to penetrate deeper from the steel surface.

[0103] Generally speaking, nitrogen cannot penetrate deeply during nitriding, but carbon can penetrate deeply during carburizing. Therefore, it is speculated that performing carburizing after nitriding has the effect of pushing out nitrogen that has penetrated to the surface with carbon.

[0104] In addition, generally speaking, nitriding treatment can be carried out at a temperature lower than that of carburizing treatment, so the temperature is first raised to a temperature suitable for allowing a large amount of nitrogen to penetrate (for example, 800°C) during nitriding treatment, and then the temperature is raised to a temperature suitable for carburizing treatment (for example, 850°C) for carburizing treatment.

[0105] One or both of the carburizing and nitriding treatments may also be repeated multiple times. In particular, through in-depth research by the inventors, the following understanding has been obtained: it is preferred to initially perform nitriding, then perform carburizing, and then perform nitriding again. If nitriding is performed at a relatively low temperature, a large amount of nitrogen can be infiltrated, but the infiltration in the depth direction tends to become shallower. On the other hand, if nitriding is performed at a high temperature, nitrogen can be infiltrated deeply, but the amount of infiltrated nitrogen tends to become less. Therefore, it is speculated that nitrogen can be infiltrated in the depth direction by the following method: first, in the initial nitriding treatment, a lot of nitrogen is infiltrated into the surface layer at a relatively low temperature (for example, 750°C to 800°C), and on this basis, the temperature is raised (for example, 830°C to 880°C) to perform carburizing, and then the temperature is maintained for a second nitriding treatment.

[0106] In this process, for example, it is preferred to perform at least a portion of the operation under reduced pressure. Here, "perform under reduced pressure" refers to performing the carbonitriding process while maintaining the reduced pressure in the furnace. Furthermore, "at least a portion of the operation" is intended to encompass the following: performing only one of the carburizing and nitriding processes under reduced pressure, or performing only a portion of the carburizing and nitriding processes under reduced pressure when multiple carburizing and nitriding processes are performed.

[0107] Carburizing and nitriding under reduced pressure can suppress the unevenness caused by oxygen infiltration into the steel or denitrification, resulting in a less uneven and homogeneous treated surface. Carburizing under reduced pressure is particularly desirable. The degree of reduced pressure is considered a vacuum in heat treatment techniques, specifically, for example, 3000 Pa or less, preferably about 50 Pa to 2000 Pa.

[0108] When the carburizing and nitriding treatments are performed continuously, the treatments may be performed continuously with the initial decompression performed and without decompression between the treatments, or each treatment may be performed occasionally or after each decompression. However, when the nitriding treatment is performed initially, the initial nitriding treatment may also be performed without decompression.

[0109] Nitriding treatment involves placing the object to be treated (bearing component) in a gas atmosphere serving as a nitrogen supply source at a specified temperature for a specified period of time. In practice, gas is supplied to a furnace at a specified flow rate, and the object to be treated (bearing component) is placed in the furnace for a specified period of time while maintaining the temperature within the furnace. Ammonia is generally used as a nitrogen supply source. Alternatively, the gas serving as a nitrogen supply source may include other gases, such as nitrogen, in addition to the nitrogen supply source.

[0110] The temperature of the nitriding treatment is above the A1 transformation point and below 900°C as described above, and is particularly preferably within the range of 700°C to 900°C. For example, 800°C may be selected for the initial nitriding treatment and 850°C may be selected for the second and subsequent nitriding treatments.

[0111] The total treatment time for the nitriding treatment is selected from the range of about 30 minutes to 270 minutes, preferably about 90 minutes to 240 minutes, regardless of the number of treatments. Furthermore, when the nitriding treatment is performed in multiple times, the treatment time per treatment is selected from the range of about 10 minutes to 90 minutes, preferably about 30 minutes to 80 minutes.

[0112] The carburizing treatment is performed by placing the object to be treated (bearing component) in a reduced pressure atmosphere of a gas serving as a carbon supply source at a predetermined temperature for a predetermined period of time. Acetylene is particularly preferred as the carbon supply source.

[0113] As described above, the temperature of the carburizing treatment is within the range of not less than the A1 transformation point and not more than 900°C, and is particularly preferably within the range of 750°C to 900°C, and may be 850°C, for example.

[0114] As for the time of carburizing treatment, regardless of the number of treatments, the total treatment time is selected from the range of about 20 minutes to 100 minutes, preferably from the range of about 30 minutes to 70 minutes. In addition, when the carburizing treatment is performed multiple times, the treatment time of one treatment is selected from the range of about 10 minutes to 50 minutes, preferably from the range of about 15 minutes to 35 minutes.

[0115] Alternatively, time for heat absorption and diffusion may be ensured before and after the start of the processing operation of this step; before and after the start and end of the heating operation and the decompression operation; during the operation switching between the carburizing treatment and the nitriding treatment, etc.

[0116] <(2) Quenching and hardening process>

[0117] In this embodiment, the (2) quenching and hardening step refers to a step in which cooling is performed to a temperature below the A1 transformation point after the final step of the (1) carbonitriding step is completed. In practice, cooling is performed to a temperature below the A1 transformation point and at or near the treatment temperature in the next step.

[0118] For cooling in the quenching hardening process, oil cooling, water cooling, air cooling, etc. can be listed, and cooling can be performed individually or in combination. In this embodiment, oil cooling is mainly preferred. Oil cooling is performed by immersing the treatment object (bearing component) in oil for oil cooling.

[0119] The oil temperature for oil cooling is selected from a range of approximately 0°C to 170°C. Lower oil temperatures increase cooling efficiency and further harden the steel surface, but there's a risk of increased quenching distortion. In practice, the appropriate oil temperature can be selected based on the temperature and size of the object (bearing component) being processed before oil cooling begins, the target cooling temperature, the cooling rate, the oil bath capacity, and other factors. Conventional oils can be used without issue.

[0120] <(3) Low-temperature tempering process>

[0121] In manufacturing method A, the operation of the (3) low-temperature tempering treatment step is performed. On the other hand, in manufacturing method B, the operation of the (3) low-temperature tempering treatment step is omitted. In this embodiment, the (3) low-temperature tempering treatment step refers to a step of performing a tempering treatment by holding the treatment object (bearing component) at a temperature range of 90°C to 170°C, which is lower than the treatment temperature in a general tempering treatment.

[0122] The operation of the (3) low-temperature tempering treatment step has the effect of maintaining the state immediately after quenching in the (2) quenching and hardening treatment step. That is, for the treatment object (bearing component) after the operation of the (3) low-temperature tempering treatment step is completed, the state immediately after quenching in the (2) quenching and hardening treatment step is stable, and the operation can be temporarily interrupted.

[0123] The period from the completion of the operation in the (3) low-temperature tempering step to the start of the operation in the (4) cryogenic treatment step (hereinafter referred to as the "standing time") is not problematic even if it exceeds 24 hours, and a good bearing component can be manufactured even if it exceeds 72 hours. However, if the untreated object (bearing component) is left standing for too long, it is difficult to achieve the desired performance. Therefore, the standing time is preferably within 120 hours, and more preferably within 72 hours.

[0124] For example, it is desirable that the operation of the next step, namely the cryogenic treatment step (4), be carried out by a facility equipped with a dedicated processing device. Even if a facility does not have such a processing device, it is possible to transfer the processing of the unfinished object (bearing component) after the cryogenic treatment step (4) to another facility by performing the operation up to the low-temperature tempering treatment step (3).

[0125] In addition, for example, even if a facility has a dedicated processing device for the next process, namely (4) cryogenic treatment process, it is possible to achieve efficiency in the following manner: concentrate the operations up to (3) low-temperature tempering treatment process, temporarily store the unfinished processing objects (bearing parts), and concentrate on the processing from (4) cryogenic treatment process onwards at a later time.

[0126] (3) The treatment temperature in the low-temperature tempering treatment step is within the temperature range of 90°C to 170°C as described above, and preferably within the temperature range of 120°C to 160°C.

[0127] Furthermore, the treatment time (time for maintaining the treatment temperature) in the (3) low-temperature tempering treatment step is selected from the range of about 30 minutes to 180 minutes, preferably from the range of about 60 minutes to 120 minutes.

[0128] <(4) Cryogenic treatment process>

[0129] In this embodiment, the (4) cryogenic treatment step is a step of rapidly cooling the object to be treated (bearing component) to a temperature below 0°C and maintaining the object to be treated (bearing component) within a temperature range below 0°C. By performing the (4) cryogenic treatment step, the amount of retained austenite in the object to be treated (bearing component) can be suppressed.

[0130] After the object to be treated (bearing component) is heated to become austenite in the (1) carbonitriding process, when it is cooled to a temperature below the A1 transformation point in the (2) quenching and hardening process, the austenite in the object to be treated (bearing component) will become martensite, but a part of the austenite will remain as retained austenite. The retained austenite becomes the cause of aging deformation that causes martensitic transformation over time, which causes a decrease in dimensional stability. The retained austenite that has not transformed into martensite undergoes martensitic transformation by lowering the temperature. Deep cryogenic treatment (subzero treatment) refers to a treatment performed to promote this martensitic transformation.

[0131] The operation of the (4) cryogenic treatment step is performed following the (3) low-temperature tempering treatment step in manufacturing method A, and is performed following the (2) quenching and hardening treatment step in manufacturing method B. As described above, in manufacturing method A, there is no problem even if the period from the completion of the operation in the (3) low-temperature tempering treatment step to the start of the operation in the (4) cryogenic treatment step exceeds 24 hours.

[0132] On the other hand, in manufacturing method B, it is desirable to start the operation in the (4) cryogenic treatment step within 24 hours after the completion of the operation in the (2) quenching hardening treatment step. The state of the treatment object (bearing component) immediately after the completion of the operation in the (2) quenching hardening treatment step is unstable. If this unstable state continues for a long time, there is a risk of cracking the treatment object due to the residual stress generated during quenching.

[0133] The period from the completion of the operation in the (2) quench hardening step to the start of the operation in the (4) cryogenic treatment step is preferably within 24 hours, more preferably within 2 hours.

[0134] (4) The treatment temperature in the cryogenic treatment step is in the temperature range of 0°C or lower as described above, but is preferably in the temperature range of -120°C to -40°C, and more preferably in the temperature range of -80°C to -65°C.

[0135] Furthermore, the treatment time (time for maintaining the treatment temperature) in the (4) cryogenic treatment step is selected from the range of about 5 minutes to 120 minutes, and preferably from the range of about 20 minutes to 90 minutes.

[0136] To rapidly cool the object to be treated (bearing components) to the target treatment temperature, a refrigerant of ethanol and dry ice (for example, adjusted to -72°C) or liquid nitrogen (boiling point at 1013.25 hPa = 1 atmosphere: -196°C) can be used as appropriate.

[0137] <(5) Tempering process>

[0138] In the present embodiment, the tempering step (5) is a step of reheating the object (bearing component) that has been cooled by the cryogenic treatment step (4) and then tempering it by holding it at a temperature within a range of 160°C to 240°C. The holding temperature in the tempering step (5) in the present embodiment is generally higher than the holding temperature in the low-temperature tempering step (3), but is generally within the range of low-temperature tempering (approximately 150°C to 250°C).

[0139] (4) Since the quenched martensite after the cryogenic treatment step is brittle, tempering is performed. (5) The treatment temperature in the tempering step is, as described above, in the temperature range of 160°C to 240°C, preferably in the temperature range of 190°C to 230°C, and more preferably in the temperature range of 190°C to 210°C.

[0140] Furthermore, the treatment time (time for maintaining the treatment temperature) in the (5) tempering treatment step is selected from the range of about 60 minutes to 300 minutes, and preferably from the range of about 120 minutes to 180 minutes.

[0141] While the bearing component, its manufacturing method, and rolling bearing of the present invention have been described above by way of preferred embodiments, the bearing component, its manufacturing method, and rolling bearing of the present invention are not limited to the structures of the aforementioned embodiments. For example, in the aforementioned embodiments, a rolling bearing comprising a retainer 14 and an annular sealing member 15a and an annular sealing member 15b has been described as an example, the present invention can be applied to various structures, such as rolling bearings in which any of the aforementioned components is omitted, structures in which any or both of the aforementioned components are replaced by other components, and structures in which further additional components are added.

[0142] Furthermore, those skilled in the art may appropriately modify the bearing component, its manufacturing method, and rolling bearing of the present invention according to prior knowledge. As long as such modifications still possess the characteristics of the present invention, they are naturally included in the scope of the present invention.

[0143] Example

[0144] Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples. However, the present invention is not limited to the following Examples.

[0145] <Preparation of bearing components>

[0146] Prepare Figure 1 The inner ring 11 and outer ring 12 shown are used as test pieces (bearing components) for testing. These test pieces are cold-forged products. The chemical composition of these test pieces is the same as that of the inner ring 11 and outer ring 12, as shown in Table 1 below.

[0147] [Table 1]

[0148] Chemical composition Component ratio (mass %) C 0.6～0.95 Si 0.5 or less Mn Below 1.0 Cr 1.0～2.0 P 0.1 or less S 0.1 or less Fe and other impurities The remainder

[0149] Heat treatment

[0150] [Example 1]

[0151] Heat treatment was performed on each specimen according to the following procedure.

[0152] (1) Carbonitriding process

[0153] Nitrogen (N2) gas was supplied to the furnace containing the test piece at a supply rate of 16.4 liters / min. to form a nitrogen atmosphere, and the temperature was raised to 800°C. After the temperature was raised and soaked for 10 minutes, ammonia (NH3) gas was supplied at a supply rate of 83.4 liters / min., and the temperature was maintained at 800°C for 60 minutes to perform the initial nitriding treatment (I).

[0154] After the nitriding treatment (I) was completed, the supply of ammonia NH3 gas was stopped and the temperature in the furnace was raised to 850°C. After soaking for 10 minutes, the supply of nitrogen N2 gas was stopped and the pressure in the furnace was reduced to below 50 Pa by vacuum evacuation over 20 minutes. Then, acetylene C2H2 gas was supplied at 53 liters / pulse while maintaining the temperature at 850°C for 20 minutes to perform the initial carburizing treatment (I).

[0155] After the carburizing treatment (I) is completed, the supply gas is switched from acetylene C2H2 gas to nitrogen N2 gas and ammonia NH3 gas, and nitrogen N2 gas is supplied at a supply rate of 16.4 liters / min. and ammonia NH3 gas is supplied at a supply rate of 50 liters / min., and the temperature is maintained at 850°C for 60 minutes to carry out the second nitriding treatment (II).

[0156] After the nitriding treatment (II) was completed, the supply of nitrogen (N2) gas and ammonia (NH3) gas was stopped, and the pressure in the furnace was reduced to below 50 Pa by vacuum exhaust for 40 minutes. Then, acetylene (C2H2) gas was supplied at 53 liters / pulse, and the temperature was maintained at 850°C for 30 minutes to carry out the second carburizing treatment (II).

[0157] After the carburizing treatment (II) is completed, the supply of acetylene C2H2 gas is stopped and allowed to diffuse for 2 minutes, then nitrogen N2 gas is supplied at a supply rate of 16.4 liters / min. and ammonia NH3 gas is supplied at a supply rate of 50 liters / min., and the temperature is maintained at 850°C for 45 minutes to carry out the third nitriding treatment (III).

[0158] As described above, the operation of the (1) carbonitriding treatment step is performed.

[0159] (2) Quenching and hardening process

[0160] After the carbonitriding step (1) was completed, the test piece was immersed in oil for oil cooling adjusted to 120° C., thereby performing the quenching and hardening step (2).

[0161] (3) Low temperature tempering process

[0162] After the (2) quenching and hardening step was completed, the test piece was put into the furnace again, heated to 150° C., and maintained at 150° C. for 90 minutes, thereby performing the (3) low-temperature tempering step.

[0163] (4) Cryogenic treatment process

[0164] After the (3) low-temperature tempering step is completed, the test piece is taken out of the furnace and moved to a facility equipped with a cryogenic treatment device, and placed at room temperature for 3 days (72 hours: including the transfer time). The test piece is cooled to approximately -80°C and maintained at this state for 60 minutes, thereby performing the (4) cryogenic treatment step.

[0165] (5) Tempering process

[0166] After the (4) cryogenic treatment step is completed, the test piece is put into the furnace again, heated and raised to 190° C., and maintained at 190° C. for 150 minutes, thereby performing the (5) tempering treatment step.

[0167] The test piece was then removed from the furnace and fully cooled to room temperature, completing the heat treatment operation according to the manufacturing method of Example 1 to obtain the bearing component of Example 1. The main operating conditions or contents of the treatment steps according to the manufacturing method of Example 1 are summarized in Table 2 below.

[0168] [Example 2]

[0169] In Example 1, the same operations as in Example 1 were performed up to (2) the quenching and hardening step, and the (3) low-temperature tempering step was omitted. Within 2 hours after the completion of the (2) quenching and hardening step, the bearing was immersed in a refrigerant bath containing ethanol and dry ice adjusted to approximately -72°C. After being left for approximately 30 minutes, the same operations as in Example 1, including the (4) cryogenic treatment step, were performed. This completed the heat treatment operations under the manufacturing method of Example 2, resulting in the bearing component of Example 2. The main operating conditions and contents of the treatment steps under the manufacturing method of Example 2 are summarized in Table 2 below.

[0170] [Table 2]

[0171]

[0172] [Example 3 to Example 9]

[0173] In Example 1, except for changing the operating conditions of each treatment process as shown in Table 2 above, the heat treatment operations under the manufacturing methods of Examples 3 to 9 are carried out by the same operations as in Example 1 to obtain the bearing parts of Examples 3 to 9.

[0174] [Comparative Examples 1 to 6]

[0175] In Example 1, the same operations as in Example 1 were performed up to (2) the quenching and hardening step. The (3) low-temperature tempering step was omitted, and the bearings were left standing for 72 hours before the (4) cryogenic treatment step was performed. The operating conditions of each treatment step were modified as shown in Table 2. Except for the above modifications, the heat treatments of Comparative Examples 1 to 6 were performed in the same manner as in Example 1, yielding bearing components of Comparative Examples 1 to 6.

[0176] <Measurement test>

[0177] (a) Vickers hardness

[0178] The surface hardness A and deep hardness B of the bearing components obtained in Examples 1 to 9 and Comparative Examples 1 to 6 were measured by the above-described method. A micro Vickers hardness tester was used for the measurement. The evaluation results are summarized in Table 3 below.

[0179] The evaluation results were determined according to the following criteria.

[0180] Surface hardness A

[0181] ◎ (very good): 800Hv or higher and 880Hv or lower

[0182] ○ (good): 750Hv or higher and less than 800Hv

[0183] × (bad): less than 750Hv or more than 880Hv

[0184] Deep hardness B

[0185] ◎(Good): 633Hv or higher and 832Hv or lower

[0186] × (bad): less than 633Hv or more than 832Hv

[0187] (b) Retained austenite

[0188] The surface retained γ amount C and the average retained γ amount D were measured for the bearing components of Examples 1 to 9 and Comparative Examples 1 to 6 by the methods described above. An X-ray diffraction retained austenite measuring instrument was used for the measurement.

[0189] The measurement conditions of the measuring device are as follows.

[0190] (Measurement Conditions of Surface Residual γ Amount C)

[0191] Collimator diameter: 1mm

[0192] Voltage: 25kV

[0193] Current: 1.5mA

[0194] Measurement time: 200 seconds

[0195] By measuring according to the above method and conditions, the surface residual γ amount C in the region from the bearing groove surface to a depth of about 0.05 mm can be measured.

[0196] (Measurement Conditions of Average Residual γ Amount D)

[0197] Collimator diameter: 3mm

[0198] Voltage: 25kV

[0199] Current: 1.0mA

[0200] Measurement time: 100 seconds

[0201] The evaluation results are summarized in Table 3 below.

[0202] The evaluation results were determined according to the following criteria.

[0203] Surface residual γ amount C

[0204] ◎ (very good): 10% by volume or more and 20% by volume or less

[0205] ○ (good): less than 10% by volume

[0206] × (bad): more than 20% by volume

[0207] Average residual γ amount D

[0208] ◎ (very good): 2% by volume or more and 8% by volume or less

[0209] ○ (good): less than 2% by volume

[0210] × (bad): more than 8% by volume

[0211] [Table 3]

[0212]

[0213] Durability evaluation test

[0214] The obtained bearing component of Example 6 was subjected to a durability evaluation test as follows.

[0215] Specifically, the inner ring 11 and the outer ring 12 obtained as the bearing components of Example 6 were used to produce a bearing according to the nominal number 608 specified in JIS B1513 "Nominal numbers for rolling bearings". Figure 1 The rolling bearing 10 shown is the rolling bearing of Example 6. Under a radial load of 1275N, inner ring rotation, 5400min -1 The obtained rolling bearings were subjected to a durability test under the conditions of 50°C and 70°C. The results are shown in the form of a graph. Figure 9 . Figure 9 This is the result of the durability evaluation test of the bearing component of Example 6. The horizontal axis is the time (h) of the durability test, and the vertical axis is the cumulative occurrence probability F(t) of the bearing in which fatigue peeling occurs in any of the outer ring / inner ring / ball, and they are respectively plotted on the logarithm.

[0216] The results of the durability test showed that the rolling bearing of Example 6, which met the requirements of the present invention, had excellent durability. This was because the surface hardness and the amount of retained austenite met the standards.

[0217] <Determination of nitrogen concentration>

[0218] The nitrogen concentrations of the bearing components of Example 6, Comparative Example 5, and Comparative Example 6 obtained were measured by the method described above.

[0219] The nitrogen concentration from the surface of the rolling groove to a depth of 0.08 mm is between a minimum of 0.26 and a maximum of 0.72 mass % (average value of 0.45 mass %) in the bearing component of Example 6, between a minimum of 0.078 mass % and a maximum of 0.098 mass % (average value of 0.09 mass %) in the bearing component of Comparative Example 5, and between a minimum of 0.37 mass % and a maximum of 0.52 mass % (average value of 0.42 mass %) in the bearing component of Comparative Example 6.

[0220] Description of Reference Numerals

[0221] 10: Rolling bearing (ball bearing); 11: Inner ring; 11', 12': Segment; 11a: Recess, inner ring raceway groove (raceway groove); 12: Outer ring; 12a: Recess, outer ring raceway groove (raceway groove); 13: Rolling element; 14: Retainer; 15a, 15b: Annular sealing member; 16: Bearing space; 21: Collimator; 22: Resin; TP: Test piece.

Claims

1. A bearing component, Made of steel containing 0.6% by mass or more and 0.95% by mass or less of carbon, The hardness A of the area from the surface of the rolling groove to a depth of 0.2 mm is within the range of 750Hv to 880Hv, and the hardness B of the area deeper than 0.2 mm from the surface of the rolling groove is within the range of 633Hv to 832Hv. The retained austenite amount C on the surface of the rolling groove is 20 volume % or less, and the overall average retained austenite amount D is 8 volume % or less.

2. The bearing component according to claim 1, The retained austenite amount C on the surface of the raceway groove is 10 volume %, and the average retained austenite amount D of the entire raceway groove is 2 volume % or more.

3. The bearing component according to claim 1 or 2, The bearing component is obtained by the following operations: (1) The carbonitriding process is carried out at a temperature above the A1 phase transition point and below 900°C; (2) Quenching and hardening process, cooling to a temperature below the A1 phase transition point; (3) Low-temperature tempering treatment process, carried out at a temperature range of 90°C to 170°C; (4) cryogenic treatment process, carried out at a temperature below 0°C; and (5) Tempering treatment process is carried out in the temperature range of 160℃ to 240℃.

4. The bearing component according to claim 1 or 2, The bearing component is obtained by the following operations: (1) The carbonitriding process is carried out at a temperature above the A1 phase transition point and below 900°C; (2) Quenching and hardening process, cooling to a temperature below the A1 phase transition point; (4) a cryogenic treatment step, which is carried out within 24 hours from the completion of the quenching and hardening treatment step (2), at a temperature range of 0°C or below; and (5) Tempering treatment process is carried out in the temperature range of 160℃ to 240℃.

5. A rolling bearing comprising the bearing component according to claim 1 or 2.

6. A method for manufacturing a bearing component, comprising: (1) The carbonitriding process is carried out at a temperature above the A1 phase transition point and below 900°C; (2) Quenching and hardening process, cooling to a temperature below the A1 phase transition point; (3) Low-temperature tempering treatment process, carried out at a temperature range of 90°C to 170°C; (4) cryogenic treatment process, carried out at a temperature below 0°C; and (5) Tempering treatment process is carried out in the temperature range of 160℃ to 240℃.

7. The method for manufacturing a bearing component according to claim 6, The period from the completion of the operation in the (3) low-temperature tempering step to the start of the operation in the (4) cryogenic treatment step exceeds 24 hours.

8. The method for manufacturing a bearing component according to claim 6, At least a portion of the operation of the (1) carbonitriding treatment step is performed under reduced pressure.

9. The method for manufacturing a bearing component according to claim 6, (1) The carbonitriding treatment step is an operation of performing carburizing treatment after nitriding treatment.

10. A method for manufacturing a bearing component, comprising: (1) The carbonitriding process is carried out at a temperature above the A1 phase transition point and below 900°C; (2) Quenching and hardening process, cooling to a temperature below the A1 phase transition point; (4) a cryogenic treatment step, which is carried out within 24 hours from the completion of the quenching and hardening treatment step (2), at a temperature range of 0°C or below; and (5) Tempering treatment process is carried out in the temperature range of 160℃ to 240℃.

11. The method for manufacturing a bearing component according to claim 10, At least a portion of the operation of the (1) carbonitriding treatment step is performed under reduced pressure.

12. The method for manufacturing a bearing component according to claim 10, (1) Operation of Carbonitriding Process Carburizing is performed after nitriding.

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