JP2003226919A - Bearing part and roll bearing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide bearing parts which have high crack resistance and dimensional stability, and have an elongated rolling service life, and to provide a roll bearing. <P>SOLUTION: The roll bearing 10 has an inner ring 2, an outer ring 1 and a plurality of rolling elements 3. At least one member selected from the inner ring, outer ring and rolling elements has a carbo-nitrided layer, and the grain size number of the old austenite crystal grains of the member lies in the range of No.>10. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bearing component and a rolling bearing used for a speed reducer, a drive pinion, a bearing for a transmission, etc., and has a long rolling fatigue characteristic and a high degree of cracking resistance and aging resistance. And a rolling bearing.

[0002]

2. Description of the Related Art As a heat treatment method for imparting a long life to rolling contact fatigue of a bearing part, carbonitriding is performed on the surface layer part of the bearing part by further adding ammonia gas to the atmosphere RX gas during quenching and heating. There is a method of performing the treatment (for example, JP-A-8-4774, JP-A-11-1012).
47 publication). By using this carbonitriding method, the surface layer portion can be hardened, residual austenite is generated in the microstructure, and the rolling fatigue life can be improved.

[0003]

However, since the above carbonitriding method is a diffusion process for diffusing carbon and nitrogen, it must be kept at a high temperature for a long time. For this reason, it is difficult to improve the cracking resistance by coarsening the structure. In addition, an increase in the rate of dimensional change over time due to an increase in retained austenite poses a problem.

On the other hand, long life is ensured against rolling fatigue,
In order to improve the crack strength and prevent the increase of the dimensional change rate over time, it is possible to deal with it by adjusting the composition by the alloy design of steel. However, the alloy design causes problems such as high raw material cost.

Bearing components in the future are required to have characteristics such that they can be used under higher load conditions and at higher temperatures than ever before, as the usage environment becomes higher and the temperature becomes higher.
Therefore, a bearing component having high strength, long rolling contact fatigue life, and high crack resistance and dimensional stability is required.

An object of the present invention is to provide a bearing component and a rolling bearing which have a high level of crack resistance and dimensional stability and are excellent in rolling contact fatigue life.

[0007]

The rolling bearing of the present invention comprises:
A rolling bearing having an inner ring, an outer ring and a plurality of rolling elements. In this rolling bearing, at least one member of the inner ring, the outer ring and the rolling element has a carbonitriding layer,
The grain size number of austenite crystal grains of the member exceeds 10.

The fine austenite grain size can significantly improve the rolling fatigue life. If the grain size number of the austenite grain size is 10 or less, rolling fatigue life is not significantly improved, so the range is set to exceed 10. Generally, the number should be 11 or higher. The finer the austenite grain size, the more desirable, but it is usually difficult to obtain the grain size number above 13. The austenite grains of the bearing component do not change in the surface layer portion greatly affected by the carbonitriding treatment or in the inside thereof. Therefore, the target position of the above range of grain size numbers is
The surface layer and the inside.

Any one of the inner ring, the outer ring and the plurality of rolling elements in the above rolling bearing is a bearing component incorporated in the rolling bearing. When the austenite grain size number of the bearing component is within the above range, rolling fatigue life is improved.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a schematic sectional view showing a rolling bearing according to an embodiment of the present invention. In FIG. 1, the rolling bearing 10 mainly has an outer ring 1, an inner ring 2, and a rolling element 3. Although the drawings show radial bearings, ball bearings, tapered roller bearings, roller bearings and needle roller bearings are also covered by the embodiments of the present invention. The rolling element 3 is rotatably supported by a cage arranged between the outer race 1 and the inner race 2.

Next, heat treatment including carbonitriding performed on at least one bearing component of the outer ring, inner ring and rolling element of these rolling bearings will be described. FIG. 2 is a diagram illustrating a heat treatment method according to the embodiment of the present invention. Further, FIG. 3 is a diagram illustrating a modification of the heat treatment method according to the embodiment of the present invention. Figure 2 shows primary quenching and 2
FIG. 3 is a heat treatment pattern showing a method of performing secondary quenching. FIG. 3 shows that the material is cooled to below the A ₁ transformation point temperature during quenching,
Then, it is a heat treatment pattern showing a method of reheating and finally quenching. Both are examples of embodiments of the present invention. In these figures, in the treatment T1, carbon and nitrogen are diffused into the steel base material and the carbon is sufficiently melted, and then cooled to below the A ₁ transformation point. Next, in treatment T2 in the figure, the temperature is reheated to a temperature lower than that of treatment T1, and oil quenching is performed from there.

Rather than normal quenching the above heat treatment, that is, carbonitriding treatment and subsequent single quenching,
It is possible to improve the cracking strength and reduce the rate of dimensional change over time while carbonitriding the surface layer portion. As mentioned above,
According to the heat treatment method described above, it is possible to obtain a microstructure in which the grain size of austenite crystal grains is ½ or less of the conventional one. The bearing component that has been subjected to the above heat treatment has a long life in rolling contact fatigue characteristics, can improve crack strength, and can reduce the rate of dimensional change over time.

FIG. 4 is a diagram showing the microstructure of the bearing component, especially the austenite grains. FIG. 4 (a) shows a bearing part of the present invention, and FIG. 4 (b) shows a conventional bearing part. That is, FIG. 4A shows the austenite grain size of the bearing steel to which the heat treatment pattern shown in FIG. 2 is applied. For comparison, FIG. 4B shows the austenite grain size of the bearing steel obtained by the conventional heat treatment method. Also, FIG.
4 (a) and FIG. 5 (b) are the same as FIG. 4 (a) and FIG.
It is a figure which shows the austenite grain boundary which illustrated (b). From the structure showing these austenite grain sizes, the conventional austenite grain size is 10 according to the JIS standard grain size number.
No. 12, and according to the heat treatment method of the present invention, No. 12 fine particles can be obtained. Further, the average particle size in FIG. 4A was 5.6 μm as a result of measurement by the intercept method.

[0014]

EXAMPLES Next, examples of the present invention will be described.

(Example 1) JIS standard SUJ2 material (1.
0 wt% C-0.25 wt% Si-0.4 wt% Mn-
Example 1 of the present invention was performed using 1.5 wt% Cr). The manufacturing history of each sample shown in Table 1 is shown below.

[0016]

[Table 1]

(Samples A to D; Examples of the present invention): carbonitriding treatment at 850 ° C., holding time 150 minutes. The atmosphere was a mixed gas of RX gas and ammonia gas. In the heat treatment pattern shown in FIG. 2, primary quenching was performed from a carbonitriding temperature of 850 ° C., and then secondary quenching was performed by heating to a temperature range of 780 ° C. to 830 ° C. lower than the carbonitriding temperature. However, sample A having a secondary quenching temperature of 780 ° C. was excluded from the test because of insufficient quenching. (Samples E and F; Comparative Example): The carbonitriding treatment is performed according to Inventive Example A.
-D, the second quenching temperature was 850 ° C to 870 ° C, which is a carburizing nitrogen treatment temperature of 850 ° C or higher. (Conventional carbonitriding product; Comparative example): Carbonitriding treatment 850
C, holding time 150 minutes. The atmosphere was a mixed gas of RX gas and ammonia gas. Quenching was performed as it was from the carbonitriding temperature, and secondary quenching was not performed. (Ordinarily quenched product; Comparative example): Quenched by heating to 850 ° C without carbonitriding. No secondary hardening was performed.

For the above samples, (1) measurement of hydrogen content, (2) measurement of grain size, (3) Charpy impact test, (4) measurement of fracture stress value, (5) rolling fatigue test, Each test was conducted. Next, these test methods will be described.

I Test Method of Example 1 (1) Measurement of Hydrogen Content As for the hydrogen content, the amount of non-diffusible hydrogen in steel was analyzed by a DH-103 type hydrogen analyzer manufactured by LECO. The amount of diffusible hydrogen was not measured. The specifications of this LECO DH-103 type hydrogen analyzer are shown below.

Analytical range: 0.01 to 50.00 ppm Analytical accuracy: ± 0.1 ppm or ± 3% H (whichever is greater) Analytical sensitivity: 0.01 ppm Detection method: Thermal conductivity method Sample weight size: 10 mg ~ 35g (maximum: diameter 12mm
X length 100 mm) Heating furnace temperature range: 50 ° C. to 1100 ° C. Reagent: Anhydrone Mg (ClO ₄ ) ₂ , Ascarite NaOH Carrier gas: Nitrogen gas, Gas dosing gas: Hydrogen gas. 99% or more, pressure 40P
SI (2.8 kgf / cm ² ).

The outline of the measurement procedure is as follows. Insert the sample collected with the dedicated sampler into the above hydrogen analyzer together with the sampler. The diffusible hydrogen inside is guided to the thermal conductivity detector by the nitrogen carrier gas. This diffusible hydrogen is not measured in this example. Next, the sample is taken out from the sampler and heated in a resistance heating furnace, and the non-diffusible hydrogen is introduced into the thermal conductivity detector by the nitrogen carrier gas. The amount of non-diffusible hydrogen can be known by measuring the thermal conductivity with a thermal conductivity detector. (2) Measurement of grain size The grain size was measured according to the JIS G 0551 steel austenite grain size test method. (3) Charpy impact test The Charpy impact test was performed based on the Charpy impact test method of the metal material of JIS Z 2242. The test piece is a U-notch test piece (J
IS3 test piece) was used. (4) Measurement of Breaking Stress Value FIG. 6 is a diagram showing a test piece of a static crush strength test (measurement of breaking stress value). A load is applied in the P direction in the figure and the load until it is broken is measured. Then, the obtained breaking load is converted into a stress value by the stress calculation formula of the curved beam shown below. The test piece is not limited to the test piece shown in FIG.
Other shaped test strips may be used.

When the fiber stress on the convex surface of the test piece of FIG. 6 is σ ₁ and the fiber stress on the concave surface is σ ₂ , σ ₁ and σ ₂ are obtained by the following formulas (Mechanical Engineering Handbook A)
4 Material dynamics A4-40). Here, N is an axial force of a cross section including the axis of the annular test piece, A is a cross-sectional area, e ₁ is an outer radius,
e ₂ represents the inner radius. Further, κ is the section modulus of the curved beam.

Σ ₁ = (N / A) + {M / (Aρ _o )} [1
+ E ₁ / {κ (ρ _o + e ₁ )}] σ ₂ = (N / A) + {M / (Aρ _o )} [1-e ₂ / {κ
(Ρ _o −e ₂ )}] κ = − (1 / A) ∫ _A {η / (ρ _o + η)} dA (5) Table 2 shows the test conditions of the rolling fatigue test and rolling fatigue life test. . Also, FIG.
FIG. 3 is a schematic view of a rolling fatigue life tester. FIG. 7A is a front view and FIG. 7B is a side view. In FIGS. 7A and 7B, the rolling fatigue life test piece 21 is driven by the drive roll 11 and is in contact with the ball 13 and is rotating. Ball 13 is a (3/4) "ball,
Guided by the guide rolls, the rolling fatigue life test piece 21 rolls while exerting a high surface pressure on each other.

II Test Results of Example 1 (1) Hydrogen Content 0.7% for a conventional carbonitriding product as it is.
It is a very high value of 2 ppm. It is considered that this is because ammonia (NH ₃ ) contained in the carbonitriding atmosphere was decomposed and hydrogen entered the steel. On the other hand, in Samples B to D, the amount of hydrogen was 0.37 to 0.40 pp.
It has decreased to almost half. This amount of hydrogen is at the same level as that of ordinary quenched products.

By reducing the amount of hydrogen described above, embrittlement of steel due to solid solution of hydrogen can be reduced. That is,
Due to the reduction of the hydrogen content, the Charpy impact value of Samples B to D of the examples of the present invention is greatly improved. (2) Grain size The grain size is such that the secondary quenching temperature is lower than the quenching temperature (primary quenching) at the time of carbonitriding, that is, sample B to
In the case of D, the austenite grains have grain size numbers 11 to 1
It is remarkably miniaturized as 2. The austenite grains of the samples E and F, the conventional carbonitriding processed product and the ordinary quenched product have a crystal grain size number of 10, and the samples B to D of the examples of the present invention.
The grains are coarser. (3) Charpy Impact Test According to Table 1, the Charpy impact value of the conventional carbonitrided product is 5.33 J / cm ² , whereas the Charpy impact value of Samples B to D of the present invention is 6 .30 to 6.65 J /
A high value of cm ² is obtained. Among these, the lower the secondary quenching temperature, the higher the Charpy impact value tends to be. Charpy impact value of normally quenched product is 6.70 J / c
High as m ² . (4) Measurement of Breaking Stress Value The breaking stress value corresponds to the crack resistance. According to Table 1, the conventional carbonitrided product has a fracture stress value of 2330 MPa. On the contrary, the fracture stress values of Samples B to D are improved to 2650 to 2840 MPa. The fracture stress value of normally quenched products is 2770 MPa,
It is equivalent to the fracture stress values of Samples B to F. It is presumed that the improved cracking resistance of Samples B to D is as great as the reduction of the hydrogen content, as well as the refinement of the austenite crystal grains. (5) Rolling Fatigue Test According to Table 1, the normally hardened product has the lowest rolling fatigue life L ₁₀ , reflecting that it does not have a carbonitriding layer in the surface layer portion. Compared with this, the rolling fatigue life of the conventional carbonitrided product is 3.1.
Doubled. The rolling fatigue lives of Samples B to D are significantly improved as compared with the conventional carbonitrided products. The samples E and F of the present invention are almost the same as the conventional carbonitriding products.

To summarize the above, Samples B to D of the examples of the present invention
Has a lower hydrogen content and an austenite grain size of 1
It is made finer than No. 1 and has improved Charpy impact value, crack resistance and rolling contact fatigue life.

(Second Embodiment) Next, a second embodiment will be described. A series of tests were conducted on the following materials A, B and C. JIS standard SUJ2 material (1.0 wt% C-0.25 wt% Si-0.4 wt%
(Mn-1.5 wt% Cr) was used for all materials A to C. The manufacturing history of the materials A to C is as follows. (A material: Comparative example): Only normal quenching (no carbonitriding treatment). (Material B: Comparative Example): Quench as it is after carbonitriding (conventional carbonitriding quenching). Carbonitriding temperature 845
C, holding time 150 minutes. The atmosphere for carbonitriding is R
X gas and ammonia gas were used. (C material: Inventive example): Bearing steel subjected to the heat treatment pattern of FIG. Carbonitriding temperature 845 ° C, holding time 150 minutes. The atmosphere of the carbonitriding treatment was RX gas + ammonia gas. The final quenching temperature was 800 ° C.

(1) Rolling Fatigue Life The test conditions and test equipment for the rolling fatigue life test are as shown in Table 2 and FIG. 7, as described above. The results of this rolling fatigue life test are shown in Table 3.

[0029]

[Table 2]

[0030]

[Table 3]

According to Table 3, the B material of the comparative example is 3.1 times as long as the L ₁₀ life (life of 1 out of 10 test pieces is broken) of the A material which is also subjected to normal quenching in the comparative example. The carbonitriding treatment has the effect of extending the service life. On the other hand, the C material of the present invention example has a long life of 1.74 times that of the B material and 5.4 times that of the A material. It is considered that the main cause of this improvement is the refinement of the microstructure.

(2) Charpy impact test The Charpy impact test was carried out using a U-notch test piece by a method according to JIS Z2242 described above. The test results are shown in Table 4.

[0033]

[Table 4]

The Charpy impact value of the carbonitrided B material (Comparative Example) was not higher than that of the normally quenched A material (Comparative Example), but the C material had the same value as the A material.

(3) Static Fracture Toughness Value Test FIG. 8 is a view showing a test piece of the static fracture toughness test. After introducing a pre-crack into the notch portion of this test piece by about 1 mm, a static load by three-point bending was applied to obtain a breaking load P. The following formula (I) was used to calculate the fracture toughness value ( _KIc value). The test results are shown in Table 5. K _Ic = (PL√a / BW ² ) {5.8-9.2 (a / W) +43.6 (a / W) ² -75.3 (a / W) ³ +77.5 (a / W ) ⁴ }… (I)

[0036]

[Table 5]

Since the pre-crack depth was larger than the carbonitriding layer depth, there is no difference between the A and B materials of the comparative example.
However, the C material of the example of the present invention could obtain a value about 1.2 times that of the comparative example.

(4) Static Crush Strength Test (Measurement of Fracture Stress Value) As the static crush strength test piece, the one having the shape shown in FIG. 6 was used as described above. In the figure, a load was applied in the P direction to perform a static crush strength test. The test results are shown in Table 6.

[0039]

[Table 6]

The value of the material B which has been carbonitrided is slightly lower than that of the material A which is normally quenched. However, the material C of the present invention has a higher static crush strength than the material B and is at a level comparable to the material A.

(5) Aged dimensional change rate The measured results of the aged dimensional change rate at a holding temperature of 130 ° C. and a holding time of 500 hours are shown in Table 7 together with the surface hardness and the amount of retained austenite (0.1 mm depth).

[0042]

[Table 7]

It can be seen that, compared with the dimensional change rate of the material B having a large amount of retained austenite, the material C of the present invention is suppressed to less than half.

(6) Life test under foreign matter mixed lubrication The ball bearing 6206 was used to evaluate the rolling fatigue life under foreign matter mixed lubrication in which a predetermined amount of standard foreign matter was mixed. The test conditions are shown in Table 8 and the test results are shown in Table 9.

[0045]

[Table 8]

[0046]

[Table 9]

Compared with the A material, the conventional carbonitrided B material has a life of about 2.5 times, and the C material of the present invention has a life of about 2.3 times. Although the C material of the present invention example has less retained austenite than the B material of the comparative example,
Due to the influence of nitrogen penetration and the refined microstructure, almost the same long life is obtained.

From the above results, the C material of the present invention, that is, the bearing component produced by the heat treatment method of the present invention,
It was found that it is possible to simultaneously satisfy the three items, which are difficult with the conventional carbonitriding treatment, that is, the rolling fatigue life is extended, the crack strength is improved, and the rate of dimensional change over time is reduced.

The embodiments disclosed this time are to be considered as illustrative in all points and not restrictive. The scope of the present invention is shown not by the above description but by the claims, and is intended to include meanings equivalent to the claims and all modifications within the scope.

[0050]

EFFECTS OF THE INVENTION By using the bearing component and the rolling bearing of the present invention, a carbonitriding layer is formed and then the austenite grain size of the bearing component is refined to a grain size number of 11 or more, and the hydrogen content is also reduced. As a result, rolling fatigue life is greatly improved, and excellent crack resistance and aging resistance can be obtained.

[Brief description of drawings]

FIG. 1 is a schematic sectional view showing a rolling bearing according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a heat treatment method according to an embodiment of the present invention.

FIG. 3 is a diagram illustrating a modified example of the heat treatment method according to the embodiment of the present invention.

FIG. 4 is a diagram showing a microstructure of a bearing component, particularly an austenite grain. (a) is a bearing part of the present invention example, and (b) is a conventional bearing part.

5A shows the austenite grain boundaries illustrated in FIG. 4A, and FIG. 5B illustrates the austenite grain boundaries illustrated in FIG. 4B.

FIG. 6 is a view showing a test piece of a static crush strength test (measurement of breaking stress value).

FIG. 7 is a schematic view of a rolling fatigue life tester. (a) is a front view and (b) is a side view.

FIG. 8 is a view showing a test piece of a static fracture toughness test.

[Explanation of symbols]

1 outer ring, 2 inner ring, 3 rolling elements, 10 rolling bearings,
11 drive roll, 12 guide roll, 13 (3 /
4) "Ball, 21 rolling fatigue life test piece, T1 carbonitriding temperature, T2 quenching heating temperature.

Front page continuation (51) Int.Cl. ⁷ Identification code FI theme code (reference) F16C 33/64 F16C 33/64

Claims (2)

[Claims] 1. A rolling bearing having an inner ring, an outer ring and a plurality of rolling elements, wherein at least one member of the inner ring, the outer ring and the rolling elements has a carbonitriding layer, and the grain size of austenite crystal grains of the member. Rolling bearings with numbers in the range above 10. 2. A bearing component to be incorporated in a rolling bearing, which has a carbonitrided layer and has a grain size number of austenite crystal grains in a range exceeding 10.