JP2004232858A - Rolling bearing and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a rolling bearing of high load capacity and its manufacturing method, having long service life to rolling fatigue under lubrication including foreign matters. <P>SOLUTION: A tapered roller bearing 1 includes raceway rings 3, 5 and a rolling element 2 as members. At least one of the raceway rings and the rolling element has a nitrogen enriched layer in its surface layer part, an average particle size of austenite crystalline particle is 10 μm or less, and the remaining austenite amount is 15 % or less. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention relates to a rolling bearing and a method of manufacturing the same, and more specifically, to a part where a long life is required for rolling fatigue under lubrication mixed with foreign matter, and where a reduction in the dimensional change rate over time is required. The present invention relates to a high-load capacity rolling bearing used and a method for manufacturing the same.

  In order to achieve a high load capacity of a rolling bearing, there is a method of quenching a steel material forming a member of the rolling bearing, and then performing a deep cooling process such as a sub-zero process or a cryo process to improve the yield strength of the steel product. That is, the quenched steel material is cooled to 0 ° C. or lower or the martensitic transformation end temperature Mf point or lower, and the residual austenite remaining after quenching is transformed into martensite or the like, thereby improving the yield strength and hardness.

On the other hand, when the deep cooling process is performed, the amount of retained austenite decreases as described above. However, since rolling fatigue under contaminated lubrication is improved by retained austenite, when retained austenite is reduced, there is an adverse effect that rolling fatigue life under contaminated lubrication is reduced (for example, Patent Document 1, Non-Patent Document 1). Patent Document 1).
JP-A-07-190072 Mechanical Design, Vol. 39, No. 13, page 33: "Long life under refuse-containing lubrication environment-TF technology"

  In recent years, use in harsh environments, such as under contaminated lubrication and use in high-temperature environments, has been increasing. Rolling bearings with a high load capacity capable of withstanding high loads in the above-mentioned harsh use environments and manufacturing thereof A method is required.

  An object of the present invention is to provide a high-load capacity rolling bearing having a long life with respect to rolling fatigue under contaminated lubrication and suppressing the rate of dimensional change over time, and a method of manufacturing the same.

  A rolling bearing according to the present invention is a rolling bearing including a bearing ring and a rolling element as members, wherein at least one member of the bearing ring and the rolling element has a nitrogen-enriched layer in a surface layer portion, and includes austenitic crystal grains. Has an average particle size of 10 μm or less and an amount of retained austenite of 15% or less.

  According to the above configuration, the rolling fatigue life under the contaminated lubrication, which is sufficiently excellent in practical use, is obtained, and the dimensional change over time is suppressed, so that a high-strength, high-load capacity rolling bearing can be obtained. If the average grain size of the austenite crystal grains exceeds 10 μm, the contact between the rolling elements and the races tends to cause deep indentations, which hinders a high load capacity. Further, it becomes difficult to obtain a long life in a rolling fatigue test under lubrication with foreign matter mixed therein.

  The austenite crystal grains may be any grain boundaries that can be observed by applying a process such as etching to a gold phase sample of a target member to reveal the grain boundaries. It may be referred to as old austenite grains in the sense of a grain boundary at the time of heating immediately before low-temperature quenching. The measurement may be performed by converting the average value of the particle size numbers of the JIS standard into the average particle size, or the length of the interval between the lines in the random direction superimposed on the metal phase structure by the intercept method or the like and associated with the grain boundary. May be taken and multiplied by a correction coefficient to obtain a two-dimensional to three-dimensional interval length.

  If the retained austenite exceeds 15%, the dimensional change over time will increase, wear will increase, noise will increase, and the performance as a bearing component will deteriorate. Remaining austenite is measured by using various X-ray diffraction methods to determine the diffraction intensity of the appropriate plane index of the austenite phase and taking the ratio with the diffraction intensity from the appropriate plane index of the ferrite phase. You. At that time, the height of the diffraction peak may be used, or the area of the diffraction peak may be used. In addition, it can also be measured by determining the magnetizing force using a magnetic balance or the like, utilizing the fact that the austenite phase is a non-magnetic substance and the ferrite phase is a ferromagnetic substance. In addition, it can be easily measured using a commercially available measuring device.

  Also, during the low-temperature quenching, the austenite phase is transformed into martensite, etc., but the retained austenite remains untransformed to room temperature, such as between adjacent martensite bundles transformed along different crystal planes. Austenite. Therefore, there is no direct relationship with the austenitic crystal grains in which the range of the average particle size is limited.

  The average grain size of the austenitic crystal grains can be set to 6 μm or less.

  By reducing the average grain size of austenite grains to 6 μm or less as described above, even with the same amount of retained austenite, the rolling fatigue life under lubrication with contaminants can be further extended from a coarse austenite grain material. Can be obtained. In addition, the indentation depth tends to be reduced in the indentation test.

  Further, the amount of retained austenite may be set to 7.5% or less.

  By reducing the retained austenite to 7.5% or less as described above, the yield strength is increased, the rolling fatigue life under contaminated lubrication is further extended, and the indentation depth in the indentation test is further reduced. Can be.

  Further, the hardness of the surface layer of the above member may be higher than the HRC hardness 63.

  With this configuration, it is possible to obtain a rolling bearing having a higher load capacity. The HRC hardness can be measured with a commercially available HRC hardness meter.

  In addition, by providing the above-mentioned various characteristics, the above-mentioned rolling bearing is used for a part where a long life is required against rolling fatigue under lubrication mixed with foreign matter and a small dimensional change rate over time is required. Bearing. Regardless of the type of the bearing, any bearing such as a ball bearing, a cylindrical roller bearing, a needle roller bearing, a tapered roller bearing, a thrust ball bearing, and a thrust roller bearing may be used.

  As a bearing used for a specific mechanical device having the above use environment, any of a drive pinion bearing, a reduction gear bearing, and a transmission bearing can be given.

  The manufacturing method of a rolling bearing of the present invention is a manufacturing method of a rolling bearing including a race and a rolling element as members. In this manufacturing method, a steel material forming at least one member of the members is heated to a carbonitriding temperature of at least the A1 point, and after performing a carbonitriding treatment, a step of cooling to below the A1 point, and after the cooling step, After the steel material is heated to a temperature in the temperature range from the A1 point to the A3 point and lower than the carbonitriding temperature, the steel is kept at a temperature lower than room temperature for a fixed time after the low-temperature quenching step of quenching and the low-temperature quenching step. Performing a cryogenic treatment.

  In the step of cooling to a point lower than the A1 point after the carbonitriding at the above-mentioned carbonitriding temperature, the step of cooling to room temperature by oil cooling or the like, or a step of cooling to a temperature at which austenite transformation ends at a predetermined value or more may be used. . According to the above production method, a metal structure having a nitrogen-enriched layer, fine austenite grains, and containing an appropriate amount of retained austenite can be obtained. For this reason, the yield strength can be improved while obtaining a rolling fatigue life under lubrication mixed with foreign matter which is sufficiently excellent in practical use. Further, it is possible to obtain a rolling bearing in which dimensional change over time is suppressed. The nitrogen-enriched layer is formed by carbonitriding as described above. However, the nitrogen-enriched layer may or may not be enriched with carbon.

  Further, the retained austenite becomes unstable as the temperature is lowered, and the martensitic transformation is likely to occur. For this reason, the amount of retained austenite can be changed by changing the temperature lower than room temperature in the above-mentioned deep cooling process according to the target value of retained austenite. As the refrigerant for the cryogenic treatment, liquid nitrogen having a boiling point of about -196 ° C or a cooling environment using various other cooling devices can be used. Various temperatures from below room temperature to absolute zero can be used.

  The deep cooling treatment is desirably performed within one hour after the low-temperature quenching, but may be performed after one hour has passed after the low-temperature quenching.

  Further, it is desirable to perform tempering after the deep cooling process. However, the tempering need not be performed after the deep cooling process.

  Next, embodiments of the present invention will be described with reference to the drawings. 1A and 1B are views showing a tapered roller bearing, FIG. 1A is a partially cutaway perspective view, and FIG. 1B is a sectional view. As shown in FIG. 1A, the tapered roller bearing 1 belongs to the category of a roller bearing, and includes a rolling element 2, an inner ring 3, an outer ring 5, and a retainer 6. Since the rolling element 2 has a conical shape (conical base), the inner ring 3 and the outer ring 5 have tapered sections.

  As can be seen from FIG. 1B, the tapered roller bearing is designed such that the apex of the conical surface of the inner ring 3 and the outer ring 5 and the apex of the roller 2 are gathered at one point on the center axis. The action point 9 is located on the central axis. The roller 2 operates while being guided by the small collar 3a and the large collar 3b of the inner ring 3. It is characterized by having a large load capacity with respect to radial loads, axial loads in one direction, and a combined load thereof. Even when a radial load is applied, a component force is generated in the axial (axial) direction, so that two components are usually used in combination with each other. In both the inner ring 3 and the outer ring 5, the one having a smaller thickness dimension in the cross section is the front, and the one having the larger thickness dimension is the back. That is, in the inner ring 3, the side face on the side of the small brim 3a is the front face 3c, the side face on the side of the large brim 3b is the back face 3d, and in the outer ring 5, the back face 5d and the front face 5c are opposite to the inner ring. positioned.

  When two tapered roller bearings are used, the combination is named using the front face 5c and the rear face 5d of the outer ring. For example, when another tapered roller bearing is combined with the tapered roller bearing shown in FIG. 1B in a mirror-symmetrical manner on the right side of the tapered roller bearing, it is referred to as a rear combination tapered roller bearing. On the other hand, when another tapered roller bearing is combined with the left side of the tapered roller bearing in a mirror-symmetrical manner in addition to the tapered roller bearing of FIG. 1B, it is called a front combined tapered roller bearing.

  Further, a double-row (two-row) tapered roller bearing having a tapered roller arrangement similar to that of the above-described combination use may be used.

  In the tapered roller bearing 1 according to the present embodiment, at least one of the rolling elements 2 and the races 3 and 5 is heated to a carbonitriding temperature of the point A1 or higher and subjected to carbonitriding. Thereafter, after the step of cooling to below the A1 point and after the cooling step, the steel material is heated to a temperature in a temperature range from the A1 point to the A3 point and lower than the carbonitriding temperature and then quenched. After this low-temperature quenching step, a deep cooling treatment is performed in which the temperature is kept at a temperature lower than room temperature, usually around the boiling point of liquid nitrogen temperature of -196 ° C. for a certain time.

  The member manufactured by the above-described manufacturing method has a high load capacity, a long life against rolling fatigue under lubrication mixed with foreign matter, and can prevent an increase in the dimensional change rate over time. It is considered that such a property improvement of the member can be achieved by a steel alloy design. However, there arise problems such as an increase in raw material costs. In the embodiment of the present invention, as described above, these requirements can be satisfied by heat treatment using a general bearing steel material. That is, a member in which a nitrogen-enriched layer is formed, the austenite crystal grains are reduced to half or less of the conventional steel material, and the yield strength is improved by cooling to a Mf point or less by deep cooling or the like. be able to. It has been found that such characteristics on the gold phase are very effective for long life against rolling fatigue under contaminated lubrication, suppressing the dimensional change rate over time, and increasing the load capacity. Was.

  2 and 3 show a heat treatment method according to the present embodiment. FIG. 2 shows a method of performing quenching after carbonitriding. In FIG. 2, at a carbonitriding temperature A (850 ° C.) equal to or higher than A1, diffusion of nitrogen and dissolution of carbon are sufficiently performed in the steel base. Thereafter, oil quenching is performed from the carbonitriding temperature, and then tempering is performed at 180 ° C. This tempering can be omitted.

  Thereafter, the material is heated to a temperature B (800 ° C.) which is higher than the point A1 and lower than the point A3 and lower than the carbonitriding temperature A. The temperature B is a two-phase temperature, in which austenite and ferrite coexist in the hypoeutectoid range and austenite and cementite coexist in the hypereutectoid range. In these two-phase coexistence regions, austenite grains hardly grow and fine austenite grains can be obtained. The steel material in the present embodiment is a steel in a hypereutectoid range, and austenite and cementite coexist in the two-phase region. Oil quenching is performed from the low-temperature quenching temperature B in the two-phase region. Thereafter, the processing shown in FIG. 4 is performed. That is, a deep cooling process is performed to transform the retained austenite to improve the yield strength, and thereafter, tempering is performed at 180 ° C.

  FIG. 3 shows that after carbonitriding at the carbonitriding temperature A, the steel is cooled to a temperature not higher than the point A1 without quenching. Then, it is reheated to the low temperature quenching temperature B, and then oil quenched. Thereafter, it is cooled once to the A1 transformation point or lower, heated to a temperature B lower than the temperature A, and subjected to oil quenching. Thereafter, as shown in FIG. 4, a deep cooling process and a tempering process are performed. In the case of the treatment of FIG. 2 and the treatment of FIG. 3, after the low-temperature quenching is performed, the treatment shown in FIG. 4 is performed within one hour to transform the residual austenite into martensite to improve the yield strength. Is desirable.

  In either of the above heat treatments, a carbon-nitriding treatment therein forms a nitrogen-enriched layer that is a “carbonitriding treatment layer”. Since the carbon used as the material in the carbonitriding process has a high carbon concentration, carbon may not easily enter the surface of the steel from the atmosphere of the normal carbonitriding process. For example, in the case of steel having a high carbon concentration (steel of about 1 wt%), a carburized layer having a higher carbon concentration may be generated, and a carburized layer having a higher carbon concentration may be hardly generated. However, although the nitrogen concentration depends on the Cr concentration, etc., it is as low as 0.025 wt% or less at maximum in ordinary steel, so that a nitrogen-enriched layer is clearly formed regardless of the carbon concentration of the material steel. . It goes without saying that the nitrogen-enriched layer may be enriched with carbon.

(Example)
Next, examples will be described. In the example of the present invention, JIS standard SUJ2 material (1.0% by weight C-0.25% by weight Si-0.3% by weight Mn-1.5% by weight Cr) is used as a steel material to which the manufacturing method in the present embodiment is applied. Was used as a test material E of the present invention. Test materials A to D of the comparative examples were obtained by using the same steel material and applying other treatments. The manufacturing method of each test material is as follows.
(A material: Comparative example): A test material subjected to normal quenching only (no carbonitriding treatment).
(B material: Comparative example): A test material which was quenched as it was after carbonitriding (conventional carbonitriding and quenching). Carbonitriding temperature 845 ° C, holding time 150 minutes. The atmosphere of the carbonitriding treatment was RX gas + ammonia gas. Other carbonitriding treatments were performed under the same conditions.
(C material: Comparative example): A test material subjected to a deep cooling treatment shown in FIG. 4 after the carbonitriding treatment.
(D material: Comparative example): A test material to which the treatment shown in FIG. 2 (quenching + tempering + low-temperature quenching after carbonitriding treatment) was applied, but not deep cooling treatment.
(E material: Example of the present invention): A test material to which the treatment shown in FIG. 2 and the deep cooling treatment shown in FIG.

  FIGS. 5 and 6 show the structures of the gold phases of the material of Example E of the present invention and the material of Comparative Example B. FIG. 5A is a photograph of a gold phase of the material of Example E of the present invention, and FIG. 5B is a photograph of a gold phase of the material of Comparative Example B. FIGS. 6A and 6B are schematic views of respective partial regions. By comparing these figures, it can be seen how effective the heat treatment in the present embodiment is for miniaturization of the structure.

  Table 1 shows the average grain size of austenite crystal grains, the amount of retained austenite, and the HRC hardness of the test materials A to E.

  The material of Comparative Example D and the material of Invention Example E are both test materials subjected to the heat treatment shown in FIG. 2, and as a result of low-temperature quenching, the austenite crystal grains have an average particle size of two minutes of the materials A and C. 1 or less. Further, it can be seen that the HRC hardness of the comparative example C material subjected to the deep cooling treatment and the inventive example material E are high, and conversely, the amount of retained austenite is small.

Next, each test condition and test result will be described in order.
1. Rolling fatigue life test under contaminated lubrication Fig. 7 shows a schematic diagram of a test machine for rolling fatigue under contaminated lubrication. Table 2 shows the test conditions of the rolling fatigue test under the contaminated lubrication in this tester, and Table 3 shows the test results.

According to Table 2 and Table 3, L ₁₀ life of Comparative Example B material (lifetime one test strip 10 in corruption) is 2.1 times the no carbonitriding Comparative Example A material, carburizing The effect of extending the life by nitriding is recognized. Further, although the material of Comparative Example C has a nitrogen-enriched layer, as a result of the deep cooling treatment, the amount of retained austenite for improving the rolling fatigue life under lubrication mixed with foreign matters is small. For this reason, the life is considerably shorter than the material of Comparative Example B without the deep cooling treatment. The comparative example D material to which the carbonitriding treatment and the low-temperature quenching shown in FIG. 2 are applied has a much longer life than other materials. This is considered to be due to the fact that the amount of retained austenite is very large and the austenite crystal grains are fine. In contrast to the results of the test material of the comparative example, the material of the invention sample E was the same as the comparative example B material having a large amount of retained austenite although the retained austenite was only about the same as the comparative example C material which was also subjected to the deep cooling treatment. It was found that the life was almost the same.
2. Indentation test The outline of the test method is shown in FIGS. 8A is a diagram showing a state before the steel ball is pressed against the flat plate, FIG. 8B is a diagram showing a state where the steel ball is pressed into the flat plate, and FIG. 8C is a diagram showing a state where the steel ball is moved away from the flat plate. In this indentation test, a load was applied so that the maximum contact surface pressure between the steel ball (3/8 inch in diameter) and the flat plate became 4 GPa, 5 GPa, and 6 GPa, and the depth of the indentation formed on the plane side was measured. I do. Indentation caused by plastic deformation occurs in the flat plate due to the indentation of the steel ball. As the depth of the indentation formed on the flat plate becomes smaller, the material does not undergo plastic deformation even at a high surface pressure, and has a high static load capacity. In each of the indentation tests, the same test material differing only in shape was used for the steel ball and the flat plate.

FIG. 9 shows the results of the indentation test. The indentation depth of the comparative example C material subjected to the deep cooling treatment and the inventive example E material is clearly shallower than those of the other three. Therefore, it was found that the C material and the E material had high load capacity as materials. The indentation depth of the material E of the present invention is further shallower than that of the material C of the comparative example. The reason for this is considered to be that the E material is fine grains as shown in Table 1, and that the yield strength is improved in accordance with the Hall-Petch law.
3. Aging dimensional change rate measurement Aging dimensional change rates of the test materials A to E at a holding temperature of 130 ° C. and a holding time of 500 hours were measured. The measurement results are shown in Table 4 and FIG.

  The dimensional change rates of the comparative example C material subjected to the deep cooling treatment and the inventive example E material are less than half that of the comparative example B material. That is, it turned out that it becomes smaller than the comparative example A material which is a normally hardened product. As can be seen from FIG. 10, the smaller the retained austenite, the smaller the dimensional change over time. Further, a test material D of fine austenite grains to which low-temperature quenching was applied without performing deep quenching, a test material C which was subjected to deep chilling without performing low-temperature quenching, and Example E of the present invention which was subjected to low-temperature quenching and deep-cooling In the three test materials, the dimensional change rates tend to be lower than those of the comparative example materials A and B even if the amount of retained austenite is the same.

  As mentioned above, 1. Rolling fatigue life test under contaminated lubrication; 2. indentation test, and According to each test result of the aging rate change measurement, the test material of the present invention example in this embodiment has a long life against rolling fatigue under lubrication mixed with foreign matter, a high load capacity, and aging It turned out to be a low rate material.

  The embodiments disclosed this time are to be considered in all respects as illustrative and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

  By using the rolling bearing and the method of manufacturing the same according to the present invention, there is provided a rolling bearing having a long life against rolling fatigue under contaminated lubrication, a small dimensional change rate over time, and a high load capacity, and a method of manufacturing the same. Therefore, widespread use in this field is expected in the future.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the tapered roller bearing in embodiment of this invention, (a) is a fragmentary perspective view of a tapered roller bearing, (b) is sectional drawing. It is a figure showing a part of heat treatment method in an embodiment of the invention. FIG. 9 is a diagram illustrating a part of another heat treatment method according to the embodiment of the present invention. It is a figure which shows the cryogenic treatment and the tempering method in embodiment of this invention. It is a figure which shows the metal phase micrograph of the test material in an Example, (a) is Example E material of this invention, (b) is comparative example B material. It is a mimetic diagram of a photograph of a metal phase structure of a test material in an example, (a) is a material E of the present invention, and (b) is a material of comparative example B. It is a figure which shows the test device which performs a rolling fatigue test under lubrication mixed with a foreign substance. It is a figure which shows an indentation test, (a) shows the state before pressing a steel ball against a flat plate, (b) shows the state where the steel ball was pressed into a flat plate, and (c) shows the steel ball away from the flat plate. It is a figure showing a state. It is a figure showing an indentation test result. It is a figure which shows the relationship between the dimensional change rate of each test material, and the amount of retained austenite.

Explanation of reference numerals

  1 tapered roller bearing, 2 rolling element (tapered roller), 3 inner ring, 3a small collar (inner ring), 3b large collar (inner ring), 3c front of inner ring, 3d back of inner ring, 5 outer ring, 5c front of outer ring, 5d outer ring Back, 6 cages, 9 working points.

Claims (7)

A rolling bearing including a bearing ring and a rolling element as members,
At least one member of the bearing ring and the rolling element has a nitrogen-enriched layer in a surface layer portion, has an average austenite grain size of 10 μm or less, and has a residual austenite amount of 15% or less. bearing.   The rolling bearing according to claim 1, wherein the austenite crystal grains have an average particle size of 6 μm or less.   3. The rolling bearing according to claim 1, wherein the amount of retained austenite is 7.5% or less.   The rolling bearing according to any one of claims 1 to 3, wherein a hardness of a surface layer portion of the member exceeds an HRC hardness of 63.   5. The rolling bearing according to claim 1, wherein the rolling bearing is a bearing used for a part where a long life is required for rolling fatigue under contaminated lubrication and a small dimensional change rate is required. Rolling bearings described in Crab. A method for manufacturing a rolling bearing including a bearing ring and a rolling element as members,
Heating the steel material forming at least one member of the member to a carbonitriding temperature of point A1 or higher, performing a carbonitriding process, and then cooling the steel to a point lower than point A1;
After the cooling step, the steel material is heated to a temperature range of not less than the A1 point and less than the A3 point and lower than the carbonitriding temperature, and then a low-temperature quenching step of quenching,
After the low-temperature quenching step, a step of performing a deep cooling treatment for maintaining the temperature at a temperature lower than room temperature for a certain period of time.   The method for manufacturing a rolling bearing according to claim 6, wherein a temperature lower than room temperature in the deep cooling process is changed according to a target value of retained austenite.

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