JP2007198599A - Bearing part and rolling bearing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a bearing part and a rolling bearing having high anticracking strength, dimensional stability, and a superior rolling fatigue life. <P>SOLUTION: The rolling bearing 10 is a rolling bearing having an inner ring 2, an outer ring 1, and a plurality of rolling elements 2. At least one member of the inner ring 2, the outer ring 1, and the rolling element 3 has a carbonitriding layer, a grain size number of an austenitic grain of the member is in a range exceeding number 10, and the member is comprised of JIS SUJ2. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a bearing component and a rolling bearing used for a reduction gear, a drive pinion, a transmission bearing, and the like, and relates to a bearing component and a rolling bearing having a long life and a high cracking resistance.

As a heat treatment method that gives a long life against rolling fatigue of a bearing component, a method of performing a carbonitriding process on the surface layer portion of the bearing component by adding ammonia gas to the atmosphere RX gas during quenching heating, etc. There are (see, for example, Patent Documents 1 and 2). By using this carbonitriding treatment method, the surface layer portion can be hardened, retained austenite can be generated in the microstructure, and the rolling fatigue life can be improved.
JP-A-8-4774 Japanese Patent Laid-Open No. 11-101247

  However, since the carbonitriding method described above is a diffusion treatment that diffuses 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 due to the coarsening of the structure. In addition, an increase in the dimensional change rate due to increase in retained austenite is also a problem.

  On the other hand, in order to ensure a long life against rolling fatigue, improve crack strength, and prevent an increase in the rate of dimensional change over time, it is possible to cope with the problem by adjusting the composition by the alloy design of steel. However, the alloy design causes problems such as an increase in raw material costs.

  Future bearing parts are required to have characteristics that can be used under larger load conditions and at higher temperatures than in the past as the usage environment increases in load and temperature. For this reason, a bearing component having high strength, long rolling fatigue characteristics, high cracking strength and dimensional stability is required.

  An object of the present invention is to provide a bearing component and a rolling bearing that have a high crack resistance and an excellent rolling fatigue life.

  The rolling bearing of the present invention is 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 element has a carbonitriding layer, and the austenite crystal grains of the members are The particle size number is in the range exceeding 10 and the member is made of JIS standard SUJ2.

  Further, the bearing component of the present invention is a bearing component incorporated in a rolling bearing, has a carbonitriding layer, has an austenite crystal grain size number exceeding 10, and consists of JIS standard SUJ2.

  By using the bearing component and the rolling bearing of the present invention, the rolling fatigue characteristics have a long life and the cracking strength can be improved.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic cross-sectional view showing a rolling bearing in an embodiment of the present invention. In FIG. 1, this rolling bearing 10 mainly has an outer ring 1, an inner ring 2, and rolling elements 3. The drawings show radial ball bearings, but the same applies to ball bearings, tapered roller bearings, roller bearings, and needle roller bearings. The rolling element 3 is supported by a cage disposed between the outer ring 1 and the inner ring 2 so as to be able to roll.

  At least one member of the outer ring 1, the inner ring 2 and the rolling element 3 includes steel having a carbonitriding layer and has a fracture stress value of 2650 MPa or more.

  Moreover, at least any one member of the outer ring | wheel 1, the inner ring | wheel 2, and the rolling element 3 contains the steel which has a carbonitriding layer, and the hydrogen content rate in steel is 0.5 ppm or less.

  Further, at least one member of the outer ring 1, the inner ring 2 and the rolling element 3 includes steel having a carbonitriding layer, and the grain size number of the austenite crystal grains of the member is in the range exceeding # 10.

  Further, at least one member of the outer ring 1, the inner ring 2 and the rolling element 3 includes steel having a carbonitriding layer and has a Charpy impact value of 6.2 J / cm 2 or more.

  Next, heat treatment including carbonitriding performed on at least one bearing part of the outer ring, the inner ring and the rolling element of the rolling bearing will be described.

2 and 3 show a heat treatment method according to the embodiment of the present invention. Figure 2 is a heat treatment pattern showing a method of performing primary quenching and secondary quenching, FIG. 3 is a material in the course quenching cooled to below the A ₁ transformation point temperature, then Ru finally quenched by reheating method It is the heat processing pattern which shows. Both are exemplary embodiments of the present invention. In these figures, to diffuse the carbon and nitrogen into a green body of the processing T ₁ steel, also was sufficiently the penetration of carbon, cooled to below the A ₁ transformation point. Next, in the process T ₂ of the in the figure, than the processing T ₁ is reheated to a low temperature, subjected to oil quenching from there.

  Rather than performing normal quenching, that is, carbonitriding once after the carbonitriding treatment, the crack strength can be improved and the aging change rate can be reduced while carbonitriding the surface layer portion. As described above, according to the above heat treatment method, 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 parts subjected to the above heat treatment have a long rolling fatigue characteristic, can improve the cracking strength, and can also reduce the rate of dimensional change over time.

  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. FIGS. 5 (a) and 5 (b) show the austenite grain sizes illustrated in FIGS. 4 (a) and 4 (b). From the structure showing the austenite crystal grain size, the conventional austenite grain size is No. 10 in the JIS standard grain size number, and according to the heat treatment method of the present invention, No. 12 fine grains can be obtained. Moreover, the average particle diameter of Fig.4 (a) was 5.6 micrometers as a result of measuring by the intercept method.

Next, examples of the present invention will be described.
Example 1 of the present invention was performed using JIS standard SUJ2 material (1.0 wt% C-0.25 wt% Si-0.4 wt% Mn-1.5 wt% Cr). The manufacturing history of each sample shown in Table 1 is shown below.

(Samples A to D; examples of the present invention): Carbonitriding was performed by holding at a temperature of 850 ° C. for 150 minutes. The atmosphere during the carbonitriding process 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; Example of the present invention): The carbonitriding process is performed with the same history as that of Examples A to D of the present invention, and the secondary quenching temperature is performed at 850 ° C to 870 ° C, which is equal to or higher than the carburizing nitrogen treatment temperature (850 ° C). It was.
(Conventional carbonitrided product; comparative example): Carbonitriding was performed by holding at a temperature of 850 ° C. for 150 minutes. The atmosphere during the carbonitriding process was a mixed gas of RX gas and ammonia gas. Quenching was performed as it was from the temperature during the carbonitriding treatment, and secondary quenching was not performed.
(Normally hardened product; comparative example): It was quenched by heating to 850 ° C. without performing carbonitriding. Secondary quenching was not performed.

  For each of the above samples, (1) measurement of hydrogen content, (2) measurement of crystal grain size, (3) Charpy impact test, (4) measurement of fracture stress value, (5) rolling fatigue test Was done. The results are also shown in Table 1.

Next, these measurement methods and test methods will be described.
(1) Measurement of the amount of hydrogen The amount of hydrogen was determined by analyzing the amount of non-diffusible hydrogen in the steel using a DH-103 type hydrogen analyzer manufactured by LECO. The amount of diffusible hydrogen is not measured. The specification of this LECO DH-103 type hydrogen analyzer is shown below.

Analysis range: 0.01 to 50.00 ppm
Analysis accuracy: ± 0.1 ppm or ± 3% H (whichever is greater)
Analysis sensitivity: 0.01ppm
Detection method: thermal conductivity method Sample weight size: 10 mg to 35 g (maximum: diameter 12 mm × length 100 mm)
Heating furnace temperature range: 50 ° C to 1100 ° C
Reagent: Anhydron (Mg (ClO ₄ ) ₂ ), Ascarite (NaOH)
Carrier gas: Nitrogen gas, gas dosing gas (hydrogen gas), both gases have a purity of 99.99% or more and a pressure of 40 PSI (2.8 kgf / cm ² ).

  The outline of the measurement procedure is as follows. Samples collected with a dedicated sampler are inserted into the hydrogen analyzer for each sampler. Internal diffusible hydrogen is directed to the thermal conductivity detector by a 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 non-diffusible hydrogen is guided to the thermal conductivity detector by 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 crystal grain size The crystal grain size was measured based on the JIS G0551 steel austenite grain size test method.

(3) Charpy impact test The Charpy impact test was performed based on the Charpy impact test method of a metal material of JIS Z2242. As a test piece, a U-notch test piece (JIS No. 3 test piece) shown in JIS Z2202 was used. The Charpy impact value is a value obtained by dividing the absorption energy E of the following formula by the cross-sectional area (0.8 cm ² ).

Absorbed energy: E = WgR (cosβ-cosα)
W: Hammer weight (= 25.438 kg)
g: Gravitational acceleration (= 9.80665 m / sec ² )
R: Distance from the center of rotation of the hammer to the center of gravity (= 0.6569m)
α: Hammer lifting angle (= 146 °), β: Hammer descending angle (4) Measurement of fracture stress value FIG. 6 shows a specimen used for measurement of the fracture stress value. Using an Amsler universal testing machine, load is applied in the direction P in the figure and the load until the test piece is broken is measured. Thereafter, the obtained fracture load is converted into a stress value by the following bending beam stress calculation formula. In addition, a test piece is not restricted to the test piece shown in FIG. 6, You may use the test piece of another shape.

Assuming that 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 A4 Knitting Material Dynamics A4-40) . Here, N is the axial force of the cross section including the axis of the annular test piece, A is the cross-sectional area, e ₁ is the outer radius, and e ₂ is the inner radius. Further, κ is a 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) Rolling Fatigue Test Table 2 and FIG. 7 show the test conditions of the rolling fatigue life test and the schematic diagram of the test equipment. In FIG. 7, the rolling fatigue life test piece 21 is driven by the drive roll 11 and rotates in contact with the ball 13. The ball 13 is a (3/4) ″ ball, which is guided by the guide roll 12 and rolls while exerting a high surface pressure with the rolling fatigue life test piece 21.

Next, the measurement results and test results will be described.
(1) Amount of hydrogen From Table 1, the amount of hydrogen in steel of the conventional carbonitrided product as it is carbonitrided is a very high value of 0.72 ppm. This is thought to be because ammonia (NH ₃ ) contained in the carbonitriding atmosphere decomposed and hydrogen entered the steel. On the other hand, the amount of hydrogen in steel of Samples B to F is 0.37 to 0.42 ppm, which is reduced to nearly half that of the conventional carbonitrided product. The amount of hydrogen in this steel is at the same level as that of ordinary quenched products.

  By reducing the amount of hydrogen in the steel, embrittlement of the steel due to hydrogen solid solution can be reduced. That is, by reducing the amount of hydrogen, the Charpy impact value and the fracture stress value of the samples BF of the present invention example are greatly improved.

(2) Crystal grain size From Table 1, the crystal grain size is the grain size number when the secondary quenching temperature is lower than the quenching (primary quenching) temperature during carbonitriding, that is, in the case of Samples B to D. It is remarkably miniaturized as 11-12. The austenite grains of Samples E and F, the conventional carbonitrided product and the normal quenching product have a crystal grain size number 10, and are coarser than Samples B to D.

(3) Charpy impact value According to Table 1, the Charpy impact value of the samples B to F of the present invention example is 6 compared to the Charpy impact value of the conventional carbonitrided product being 5.33 J / cm ^2. A high value of ²⁰ to 6.65 J / cm ² is obtained. Among these, the lower the secondary quenching temperature, the higher the Charpy impact value tends to be. In addition, the Charpy impact value of the normally quenched product is as high as 6.70 J / cm ² .

(4) Fracture stress value The above-described fracture stress value corresponds to the crack resistance strength. According to Table 1, the conventional carbonitrided product has a fracture stress value of 2330 MPa. Compared to this, the fracture stress values of Samples B to F are improved to 2650 to 2840 MPa. The fracture stress value of the normally quenched product is 2770 MPa, which is equivalent to the fracture stress values of Samples B to F. Such improved crack resistance strength of Samples B to F is presumed to have a great effect by reducing the hydrogen content, along with the refinement of austenite crystal grains.

(5) According to the rolling contact fatigue test Table 1, usually sintered Irihin is reflecting that does not have a carbonitrided layer in the surface layer portion, the rolling fatigue life L ₁₀ is the lowest. Compared to this, the rolling fatigue life of the conventional carbonitrided product is 3.1 times. The rolling fatigue life of Samples B to D is significantly improved as compared with the conventional carbonitrided product. Samples E and F were almost equivalent to conventional carbonitrided products.

  In summary, in Samples B to F of the present invention example, the amount of hydrogen in the steel is lowered, and the fracture stress value and Charpy impact value are improved. However, what can be improved including the rolling fatigue life is the samples B to D in which the austenite crystal grain size is further refined to about 11 or more in the grain size number. Therefore, samples B to F correspond to the examples of the present invention, but a more desirable range of the present invention is sample B in which the secondary quenching temperature is made lower than the carbonitriding temperature to further refine the crystal grains. It is the range of -D.

Next, Example 2 will be described.
A series of tests were performed on the following A material, B material, and C material. JIS standard SUJ2 material (1.0% by weight C-0.25% by weight Si-0.4% by weight Mn-1.5% by weight Cr) is used for the material for heat treatment, which is common to materials A to C. did. The manufacturing histories of the A material to the C material are as follows.
(A material: Comparative example): Only normal quenching was performed (without carbonitriding).
(B material: comparative example): It hardened as it was after carbonitriding (conventional carbonitriding quenching). The temperature of carbonitriding was 845 ° C., and the holding time was 150 minutes. The atmosphere of carbonitriding was RX gas + ammonia gas.
(C material: Example of the present invention): The heat treatment pattern of FIG. 2 was applied to the bearing steel. The carbonitriding temperature was 845 ° C., the holding time was 150 minutes, and the atmosphere was RX gas + ammonia gas. The final quenching temperature was 800 ° C.

(1) Rolling fatigue life The above-described apparatus shown in FIG. 7 was used as a rolling fatigue life test apparatus, and the test conditions were as shown in Table 2. The rolling fatigue life test results are shown in Table 3.

According to Table 3, L ₁₀ life of B material subjected to carbonitriding treatment (comparative example), normally hardened only alms was A material (Comparative Example) of L ₁₀ life (the test piece 10 in one damaged Life) is 3.1 times longer, and the effect of extending the life by carbonitriding is recognized. 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. The main reason for this improvement is thought to be the refinement of the microstructure.

(2) Charpy impact test The Charpy impact test was performed by the method according to the above-mentioned JIS Z2242 using the U notch test piece. The test results are shown in Table 4.

  In the C material of the present invention example, a Charpy impact value equivalent to the A material (comparative example) subjected only to normal quenching and higher than that of the B material (comparative example) subjected to carbonitriding was obtained.

(3) Test of static fracture toughness value The test piece shown in FIG. 8 was used as a test piece for the static fracture toughness test, and after introducing a crack of about 1 mm in advance, a static load P by three-point bending was applied, The breaking load was determined. The following equation was used to calculate the fracture toughness value (K _IC 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 }

Since the depth of the crack introduced in advance is larger than the depth of the carbonitriding layer, there is no difference between the A material and the B material of the comparative example. However, in the C material of the present invention, a fracture toughness value (K _IC value) about 1.2 times that of the A material and B material of the comparative example could be obtained.

(4) Static crush strength test (measurement of fracture stress value)
As described above, the static crushing strength test piece having the shape shown in FIG. 6 was used. In the figure, a load was applied in the P direction, and a static crushing strength test was performed in the same manner as described above. The test results are shown in Table 6.

  The static crushing strength of the B material (comparative example) subjected to the carbonitriding treatment is slightly lower than the static crushing strength of the A material (comparative example) subjected only to normal quenching. However, the static crushing strength of the C material of the present invention example is higher than the static crushing strength of the B material, and is slightly higher than the static crushing strength of the A material.

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

  It can be seen that the dimensional change rate of the C material of the present invention example is kept low compared to the dimensional change rate of the B material having a large amount of retained austenite.

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

  Compared to the A material, a long life was obtained about 2.5 times longer in the B material (comparative example) subjected to carbonitriding, and about 2.3 times longer in the C material of the present invention example. In the C material of the present invention, although the retained austenite is less than that in the B material of the comparative example, a substantially equivalent long life is obtained due to the intrusion of nitrogen and the influence of the refined microstructure.

  From the above results, the material C of the example of the present invention, that is, the bearing part produced by the heat treatment method of the present invention, has a long rolling fatigue life, which is difficult with the conventional carbonitriding process, and an improved crack strength. It was found that the three items of reduction of the aging dimensional change rate can be satisfied simultaneously.

  It should be understood that the embodiments and examples disclosed herein are illustrative and non-restrictive in every respect. 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.

  The bearing component and rolling bearing of the present invention can be applied particularly advantageously to bearing components and rolling bearings used in reduction gears, drive pinions, transmission bearings, and the like.

It is a schematic sectional drawing which shows the rolling bearing in embodiment of this invention. It is a figure explaining the heat processing method in embodiment of this invention. It is a figure explaining the modification of the heat processing method in embodiment of this invention. It is a figure which shows the microstructure of a bearing component, especially an austenite grain. (a) is a bearing part of the example of the present invention, and (b) is a conventional bearing part. (A) shows the austenite grain boundary illustrated in FIG. 4 (a), and (b) shows the austenite grain boundary illustrated in FIG. 4 (b). It is a figure which shows the test piece of a static crushing strength test (measurement of a fracture stress value). It is the schematic of a rolling fatigue life tester. (a) is a front view, (b) is a side view. It is a figure which shows the test piece of a static fracture toughness test.

Explanation of symbols

1 outer ring, 2 inner ring, 3 rolling element, 10 rolling bearing, 11 driving roll, 12 guide roll, 13 (3/4) "ball, 21 rolling fatigue life test piece, T ₁ carbonitriding temperature, T ₂ quenching heating temperature.

Claims (2)

In a rolling bearing having an inner ring, an outer ring and a plurality of rolling elements,
At least any one member of the inner ring, the outer ring and the rolling element has a carbonitriding layer,
The particle number number of the austenite crystal grains of the member is in a range exceeding 10;
The member is a rolling bearing made of JIS standard SUJ2. A bearing component incorporated in a rolling bearing,
Having a carbonitriding layer,
The austenite grain size number is in the range exceeding 10;
Bearing parts made of JIS standard SUJ2.

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