JP2005291343A - Rolling bearing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the size of crystal grains at low cost and without reducing productivity, thereby strengthening the material and extending the life of the rolling bearing.
SOLUTION: At least one material of an inner ring, an outer ring and a rolling element is made of a high carbon bearing steel having a C content of 0.8 wt% or more, and a maximum carbide particle diameter after heat treatment is 0.8 μm or less. And the first austenite grain size is 6 μm or less, and the heat treatment is performed at a temperature of 900 ° C. to 1000 ° C. to austenite, and then rapidly cooled to form a martensite structure, and 500 to 700 ° The second step of holding at C for 1-2 hours, the third step of cooling after plastic working in a state of holding at a temperature of 500-700 ° C, and after cooling to the austenite temperature range after cooling, A fourth step of quenching and a fifth step of tempering.
[Selection] Figure 1

Description

  The present invention relates to extending the life of rolling bearings such as ball bearings, angular ball bearings, cylindrical roller bearings, tapered roller bearings, and self-aligning roller bearings used in automobiles, general industrial machines, machine tools, steel machines, and the like.

  Normally, high-carbon chromium bearing steel is used as the material for rolling bearings.In recent years, the demand for longer life of rolling bearings has increased in order to cope with the increasing trend of usage conditions. Longer life is achieved by increasing the cleanliness of the material. In addition, the bearing steel is usually subjected to quenching and tempering, but may be subjected to carbonitriding and quenching in order to extend the life.

Regarding the improvement of the cleanliness of the materials mentioned above, it is impossible to eliminate inclusions in the production of steel, and a special melting method is required to reduce the inclusions to the limit. Leads to. In addition, when carbonitriding is performed, long-time carbonitriding is required, which also increases the cost of the product.
Therefore, in order to solve such problems, it is conceivable to improve the rolling fatigue life by increasing the strength of the bearing steel. As a technique for improving the strength of the material, metallographically, (1) make the crystal grains finer, (2) increase the solid solution amount of carbon and nitrogen, and strengthen the solid solution, (3) by carbide Precipitation strengthening is mainly cited.

The method (2) corresponds to the carbonitriding process described above and has a cost problem. In the method (3), cementite is equivalent to a precipitate in ordinary bearing steel and is necessary for strengthening. Therefore, in order to improve the strength by precipitation hardening, it is necessary to add special elements such as Mo and V, which also increases the cost of the material.
The strengthening by the refinement of the crystal grain of the above (1) is known by the Hall Petch rule that the material strength increases in proportion to the -1/2 power of the crystal grain diameter d. Since the strength is improved simply by reducing the diameter, it has recently been attracting attention.

As an example of the refinement of crystal grains of bearing steel, after heating bearing steel SUJ2 to the austenite temperature range, after performing plastic working so as to straddle the hot and warm range, it is heated to room temperature. Without lowering the product, by heating again to the austenitizing temperature and quenching, the crystal grains of about 10 to 15 μm obtained by normal quenching and tempering treatment are refined to about 4 μm when made the finest, Thus, there has been proposed one that extends the life (see, for example, Patent Document 1).
Japanese Patent No. 2524156

However, in the above-mentioned Patent Document 1, since plastic processing is performed in two stages of a temperature range higher and lower than the transformation point, not only accurate temperature control of the object to be heated is required, but also 50% or more. Therefore, it is necessary to perform plastic processing at a large processing rate, which increases the cost of the apparatus for performing plastic processing during heating, and increases the cost again after the plastic processing without lowering the object to be heated to room temperature. There is a problem in that the productivity is inferior because it is heated to the crystallization temperature and quenched.
The present invention has been made to eliminate such inconveniences, and enables the refinement of crystal grains at a low cost and without reducing the productivity, thereby strengthening the material and extending the life. An object of the present invention is to provide a rolling bearing capable of achieving the above.

In order to achieve the above-described object, the invention according to claim 1 is a rolling bearing in which a plurality of rolling elements are disposed between an inner ring and an outer ring so as to be able to roll in the circumferential direction.
By using at least one material of the inner ring, the outer ring and the rolling element as a high carbon bearing steel having a C content of 0.8% by weight or more, and performing plastic working at the time of heat treatment, the maximum after completion of the heat treatment The carbide particle diameter is 0.8 μm or less, and the prior austenite particle diameter is 6 μm or less.

  The invention according to claim 2 is the first process according to claim 1, wherein the heat treatment is performed by first heating the material to 900 ° C. to 1000 ° C. to austenite, followed by quenching to form a martensite structure, and 500 to 700 After the second step of holding at ° C. for 1-2 hours, the third step of cooling after plastic working in a state of holding at a temperature of 500-700 ° C., and after cooling to the austenite temperature range A fourth step of quenching and a fifth step of tempering.

  According to the present invention, at least one of the inner ring, the outer ring, and the rolling element is made of high carbon bearing steel having a C content of 0.8% by weight or more, and the heat treatment is completed by performing plastic working during the heat treatment. By making the maximum carbide particle diameter after 0.8 μm or less and the prior austenite particle diameter 6 μm or less, the maximum carbide particle diameter and the prior austenite particle diameter after heat treatment are refined to strengthen the material. Allows extension of dynamic fatigue life.

  Moreover, after the said heat processing heats a raw material to 900 degreeC-1000 degreeC and austenitizes, the 1st process made into a martensitic structure by rapid cooling, and 2nd hold | maintained at 500-700 degreeC for 1 to 2 hours. , Tempering, and third step of cooling after plastic working in a state maintained at a temperature of 500 to 700 ° C., and fourth step of quenching after cooling and heating to the austenite temperature range. By including the fifth step, it is possible to reduce the cost by simplifying the apparatus for performing the plastic working, and after the plastic working, the object to be heated is lowered to room temperature, and again to the austenitizing temperature. Since it can be quenched by heating, productivity can be improved.

Hereinafter, an example of an embodiment of the present invention will be described with reference to the drawings.
1 is a diagram for explaining a heat treatment process of a rolling bearing as an example of an embodiment of the present invention, FIG. 2 is a diagram for explaining a heat treatment process of Comparative Example 1, and FIG. 3 is a heat treatment process of Comparative Example 2. 4 is a diagram for explaining the heat treatment process of Comparative Example 4, FIG. 5 is a diagram for explaining the heat treatment process of Comparative Example 5 and Comparative Example 6, and FIG. FIG. 7 is a photograph taken by a scanning electron microscope of Example 2, FIG. 8 is a photograph taken by a scanning electron microscope of Comparative Example 7, and FIG. 9 is a warm plastic working rate and crystal grains. FIG. 10 is a graph showing the relationship between the crystal grain size and the life ratio, and FIG. 11 is a graph showing the relationship between the crystal grain size, the maximum carbide particle size and the life ratio.

In the present invention, the maximum carbide particle size and the prior austenite particle size of the high carbon steel represented by the high carbon chromium steel SUJ2 generally used for rolling bearings are finer than when subjected to normal quenching and tempering treatment. By improving the rolling fatigue characteristics.
The method of crystal grain refinement in the present invention is to quench the material once into a martensite structure, temper the martensite at a high temperature, and precipitate a large amount of fine carbides, thereby generating austenite grain nuclei during austenite formation. The interface between the carbide and the base (ferrite) is increased, and in this state, plastic processing is performed to give processing strain, and the processing strain further increases the number of nucleation sites of austenite grains.

  That is, in the rolling bearing as an example of the embodiment of the present invention, at least one of the inner ring, the outer ring and the rolling element is made of high carbon bearing steel having a C content of 0.8% by weight or more, and at the time of heat treatment. By performing plastic working, the maximum carbide particle diameter after completion of the heat treatment is 0.8 μm or less, and the prior austenite particle diameter is 6 μm or less. As shown in FIG. After heating at 1000 ° C for 20 to 60 minutes to austenite, a first step of martensite structure by quenching with oil cooling, a second step of holding at 500 to 700 ° C for 1 to 2 hours, A third step of cooling after performing plastic working with a warm working rate of 25 to 70% in a temperature range lower than the A1 transformation point while maintaining a temperature of 500 to 700 ° C, and after cooling, 800 to 840 10-30 minutes austena at ° C After heating to preparative temperature range, comprising a fourth step of hardening, and a fifth step of tempering.

  As a result, the maximum carbide particle size and the prior austenite particle size after heat treatment are refined to strengthen the material, extending the rolling fatigue life, and simplifying the plastic processing device and reducing the cost. Further, after the plastic working, the object to be heated is lowered to room temperature and then heated again to the austenitizing temperature so as to be hardened, thereby improving the productivity.

Next, each step will be described in detail.
(First step)
A 1st process is a hardening process as a pre-processing, and is a process made into a martensitic structure. The quenching temperature is in the range of 900 to 1000 ° C., and has a two-phase structure of martensite and retained austenite. Quenching at a temperature higher than 830 to 850 ° C, which is the standard quenching temperature of normal bearing steel, eliminates spheroidized cementite by quenching at 900 to 1000 ° C, and the amount of solid solution of carbon This is to increase the amount of tempered carbide in the second step.

(Second step)
The second step is a high temperature tempering step and is performed at 500 to 700 ° C. for 1 to 2 hours. At this time, the martensite which is the base structure obtained in the first step is tempered, and cementite is precipitated. This tempered cementite precipitates uniformly and finely in the martensite matrix. By this tempering, the interface between cementite and the ferrite of the base structure increases more than the spheroidizing annealing state, the number of nucleation of austenite grains is increased by subsequent quenching, and the spheroidizing annealing material is austenitized and quenched. Also refines.

  Fine tempered carbides are precipitated even at temperatures below 500 ° C, but carbides precipitated at temperatures below 500 ° C are unstable at high temperatures of 500 ° C or higher, so that austenitization in the fourth step described later is performed. Since it decomposes in the temperature rising process at that time and does not easily become a nucleation site of austenite grains, miniaturization hardly proceeds. For this reason, holding temperature shall be 500-700 degreeC from which the carbide | carbonized_material which can exist stably and finely at the time of austenitization is obtained.

(Third step)
The third step is a step of introducing processing strain in order to further promote miniaturization. 500 to 700 ° C. is a warm working region, and strain introduced into the material by plastic working remains. If the austenite region is heated again in a state in which the strain due to this processing is stored, the number of locations where nuclei of austenite crystal grains are generated increases, and refinement proceeds. If the amount of strain at this stage is large, the number of locations where austenite grain nuclei are generated is increased during quenching in the fourth step, and further refinement of the crystal grains is facilitated.

In addition, there is a portion where cementite precipitates in a layered manner at the crystal grain boundary by the high-temperature tempering treatment in the second step, and the layered carbide is divided by plastic working in the third step to promote refinement. There is also an effect. The cooling after the plastic working is preferably rapid cooling such as water cooling or oil cooling but may be allowed to cool.
Furthermore, examples of the method of warm working at 500 to 700 ° C. include rolling forming by a rolling mill and warm forging.

  The plastic working rate during warming required for miniaturization is 25% or more in terms of the thickness ratio, and in the case of stable miniaturization, the working rate is 30% or more. As the processing rate of plastic working increases, crystal grain refinement progresses proportionally. However, when the temperature during plastic processing is 500 to 700 ° C, the deformation resistance of the material increases, so the thickness ratio is 70%. If processing strain is applied at a processing rate that exceeds, cracks may occur. Accordingly, in the present invention, the processing rate of warm plastic working is 25 to 70%, and the range in which stable production is possible is 30 to 50%.

(Fourth process)
The fourth step is a quenching process and is performed at 800 to 840 ° C. Due to the fine carbides and processing strain obtained in the second and third steps at the time of austenitization, the number of austenite nucleation sites increases and the crystal grains become finer. In addition, since the carbides produced by this high-temperature tempering are fine, even when grown during quenching, the maximum particle diameter is 0.8 μm or less, which is smaller than the maximum diameter of several μm obtained by quenching of the spheroidized annealing material, and crystal grain growth The effect of suppressing is increased.
Furthermore, since the hardness is lowered by being held at a high temperature for a long time in the third step described above, the fourth step is essential as a step for obtaining the necessary hardness as a rolling member by quenching again.

After quenching in the fourth step, the fifth step is tempered, polished and used as a bearing.
By passing through the first to fifth steps described above, in a normal quenching and tempering process using a spheroidized annealing material as a starting material, the crystal grain size is a martensite structure of about 15 to 20 μm. The effect of increasing the number of austenite nucleation sites due to fine tempered carbides and processing strain introduced by warm working makes it possible to refine crystal grains of 6 μm or less, which is about half or less of normal quenching and tempering treatment. Finer grains improve material strength and extend bearing life.
The present invention is not limited to the bearing type, and can be applied to rolling bearings such as ball bearings, angular contact ball bearings, cylindrical roller bearings, tapered roller bearings, and self-aligning roller bearings. An effect is obtained.

In order to confirm the effect of the present invention, the following experiment was conducted.
A test piece was prepared using high-carbon chromium steel SUJ2. The starting material is a spheroidized annealing material. The heat treatment process of the test piece is as follows.
(Examples 1-8)
First step: After heating at 900 to 1000 ° C. for 30 minutes, oil cooling Second step: Holding at 500 to 700 ° C. for 90 minutes Third step: After holding, a reduction rate of 25 to 500 to 700 ° C. After 70% rolling, rapid cooling Fourth step: hold at 800-840 ° C for 20 minutes, then oil cooling Fifth step: temper at 160-180 ° C for 2 hours

Moreover, the following test piece was prepared as a comparative example.
(Comparative Example 1 (see FIG. 2): Example in which the third step is omitted)
First step: Heated at 950 ° C. for 20 minutes, then oil cooled Second step: 600 ° C. for 90 minutes, rapid cooling after heating Fourth step: held at 800-840 ° C. for 20 minutes, then oil cooled Process 5: Tempering at 160 to 180 ° C. for 2 hours

(Comparative Example 2 (see FIG. 3): Example in which the second step is omitted)
First step: After heating at 950 ° C. for 20 minutes, oil cooling Third step: After holding at 600 ° C. for 3 minutes, after rolling at a rolling reduction of 25 to 70%, rapid cooling Fourth step : After holding at 800-840 ° C for 20 minutes, oil cooling Fifth step: Tempering at 160-180 ° C for 2 hours

(Comparative example 3: Example in which the rolling reduction was less than 25% in the third step)
1st step: After heating at 950 ° C. for 20 minutes and then oil cooling Second step: holding at 600 ° C. for 90 minutes Third step: after holding, rolling was performed at 600 ° C. with a reduction rate of 20% After, rapid cooling 4th process: Hold | maintained at 800-840 degreeC for 20 minutes, then oil cooling 5th process: Tempering at 160-180 degreeC for 2 hours

(Comparative Example 4 (see FIG. 4): Example in which the first step is omitted)
Second step: held at 600 ° C. for 90 minutes Third step: After holding, after rolling at a reduction rate of 25 to 70% at 600 ° C., rapid cooling Fourth step: at 800 to 840 ° C. After holding for 20 minutes, oil cooling 5th step: tempering at 160-180 ° C for 2 hours

(Comparative Example 5 (see FIG. 5): Example where the quenching temperature in the first step is low)
1st step: After holding at 890 ° C. for 20 minutes, oil cooling Second step: holding at 600 ° C. for 90 minutes Third step: after holding, rolling at a reduction rate of 25 to 70% at 600 ° C. After application, rapid cooling Fourth step: held at 800-840 ° C for 20 minutes, then oil cooling Fifth step: tempering at 160-180 ° C for 2 hours

(Comparative Example 6 (see FIG. 5): Example where the quenching temperature in the first step is low)
1st step: After holding at 840 ° C for 20 minutes, oil cooling Second step: holding at 600 ° C for 90 minutes Third step: After holding, rolling at a reduction rate of 25 to 70% at 600 ° C After application, rapid cooling Fourth step: held at 800-840 ° C for 20 minutes, then oil cooling Fifth step: tempering at 160-180 ° C for 2 hours

(Comparative Example 7 (see FIG. 6): Normal quenching / tempering treatment)
Fourth step: oil cooling after holding at 800 to 840 ° C for 20 minutes Fifth step: tempering at 160 to 180 ° C for 2 hours Polishing after tempering treatment in the fifth step in each Example and Comparative Example A test piece was prepared. In addition, the rolling completion temperature is set so as not to be lower than the heat holding temperature of 20 ° C. Table 1 summarizes the processing steps for each test piece.

These test pieces were subjected to a thrust life test. A forest thrust rolling life tester was used for the life test. The test conditions are as follows.
Surface pressure: 4900 MPa
Rotational speed: 1000min ^-1
Lubricating oil: No. 68 turbine oil (oil bath)
The average crystal grain size and the maximum carbide particle size of each test piece were measured. The average crystal grain size was determined by etching each test piece with a mixed solution of ferric chloride and hydrochloric acid to reveal only the prior austenite grain boundaries, and using a scanning electron microscope, the section method was obtained from a photograph taken at a magnification of 2000 to 3000 times. Measured and averaged. The maximum carbide particle diameter was averaged by selecting five large objects in the field of view from a photograph taken at 5000 to 10,000 times using a scanning electron microscope.

The test results are also shown in Table 1. The life ratio in Table 1 is indicated by the ratio of L ₁₀ life of normal quenching and tempering of Comparative Example 7 as 1.
As can be seen from Table 1, in Examples 1 to 8 of the present invention, the maximum carbide particle size is 0.8 μm or less and the crystal particle size (former austenite particle size) is obtained by performing the heat treatment step of the present invention. ) Is refined to 6 μm or less, and the life extension effect is clear. As an example, FIG. 7 shows a photograph taken by a scanning electron microscope of Example 2 of the present invention, and FIG. 8 shows a photograph taken by a scanning electron microscope of Comparative Example 7. In each figure, (a) is a photograph taken at 2000 to 3000 times, and (b) is a photograph taken at 5000 to 10,000 times.

FIG. 10 shows the relationship between the crystal grain size (old austenite grain size) and the life ratio. In the present invention, it can be seen that when the crystal grain size is 6 μm or less, the life is extended as compared with conventional quenching and tempering products (crystal grain size is 15 to 20 μm).
When Comparative Example 1 (see FIG. 2) was not subjected to the warm plastic working of the third step, Comparative Example 2 (see FIG. 3) was not subjected to the high-temperature tempering of the second step. This is a case where only warm working is performed in the third step. In either case, the crystal grains are finer than in the case of only quenching and tempering in Comparative Example 7, but the grain size is larger than the example of the present invention, exceeds 6 μm, and the effect of extending the life is small. . In order to reduce the crystal grain size to 6 μm or less, it is necessary to perform both actions of precipitating fine carbides by high-temperature tempering in the second step and plastic working in the third step. I understand.

Comparative Example 3 is a case where the warm plastic working rate in the third step is 20%, which is less than 25%, and the processing strain necessary for miniaturization cannot be obtained, so the crystal grain size exceeds 6 μm. It has become.
FIG. 9 shows the relationship between the warm plastic working rate and the crystal grain size, and it can be seen that it is necessary to add a plastic working with a working rate of 25% or more in order to make it 6 μm or less which has the effect of extending the life.

Comparative Example 4 (see FIG. 4) is a case where the first step is omitted, but the spheroidizing annealed material is held at 500 to 700 ° C. and is subjected to warm plastic working to be spheroidized. The carbides are coarsened, and fine carbides serving as austenite nuclei cannot be obtained. Accordingly, miniaturization does not progress and the life extension effect is small.
Comparative Example 5 and Comparative Example 6 (see FIG. 5) are cases where the quenching temperature in the first step is lower than 900 ° C. In this case, the spheroidized carbides present in the raw material are not sufficiently dissolved in the base structure, and coarse carbides of 2 to 3 μm remain. Since the penetration is insufficient, in the second step, the amount of carbide precipitation required for the refinement is reduced, and refinement with a crystal grain size of 6 μm or less cannot be achieved.

FIG. 11 shows the relationship between the crystal grain size (old austenite grain size), the maximum carbide particle size, and the life ratio within the range of the present invention. As is apparent from the figure, the crystal grain size and the maximum carbide particle size necessary for extending the life need to be 6 μm or less and the maximum carbide particle size is 0.8 μm or less. .
In the examples of the present invention, only the example of the high carbon chromium bearing steel SUJ2 was shown, but the steel type is not limited to this, and in addition to SUJ3, a steel having a carbon content of 0.8% by weight or more and having a structure However, the same effect can be obtained if the steel type is composed of martensite, cementite, and retained austenite.

It is a figure for demonstrating the heat treatment process of the rolling bearing which is an example of embodiment of this invention. 6 is a diagram for explaining a heat treatment process of Comparative Example 1. FIG. 6 is a diagram for explaining a heat treatment process of Comparative Example 2. FIG. 10 is a diagram for explaining a heat treatment process of Comparative Example 4. FIG. 6 is a diagram for explaining a heat treatment process of Comparative Example 5 and Comparative Example 6. FIG. 10 is a diagram for explaining a heat treatment step of Comparative Example 7. FIG. It is the photograph by the scanning electron microscope of Example 2, (a) is the photograph image | photographed by 2000 to 3000 times, (b) is the photograph image | photographed by 5000 to 10000 times. It is the photograph by the scanning electron microscope of the comparative example 7, (a) is the photograph image | photographed by 2000 to 3000 times, (b) is the photograph image | photographed by 5000 to 10000 times. It is a graph which shows the relationship between the plastic working rate and crystal grain diameter in warm. It is a graph which shows the relationship between a crystal grain diameter and a lifetime ratio. It is a graph which shows the relationship between a crystal grain size, the largest carbide grain size, and a life ratio.

Claims (2)

In a rolling bearing in which a plurality of rolling elements are arranged to be able to roll in the circumferential direction between an inner ring and an outer ring,
By using at least one material of the inner ring, the outer ring and the rolling element as a high carbon bearing steel having a C content of 0.8% by weight or more, and performing plastic working at the time of heat treatment, the maximum after completion of the heat treatment A rolling bearing having a carbide particle size of 0.8 μm or less and a prior austenite particle size of 6 μm or less.   In the heat treatment, after the material is heated to 900 ° C. to 1000 ° C. to austenite, a first step of forming a martensite structure by rapid cooling and a second step of holding at 500 to 700 ° C. for 1 to 2 hours And a third step of cooling after plastic working in a state maintained at a temperature of 500 to 700 ° C., a fourth step of quenching after heating to the austenite temperature range after cooling, and a fifth step of tempering. The rolling bearing according to claim 1, further comprising: