JP2005114148A - Rolling bearing - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a rolling bearing having dimensional stability and superior rolling fatigue life. <P>SOLUTION: The rolling bearing 10 comprises an outer ring 1, an inner ring 2, a plurality of rolling bodies 3. At least one member of the outer ring 1, the inner ring 2 and the rolling body 3 has a nitrogen enriched layer, austenite crystal grains in a range exceeding No.10 in the grain size number and the amount of residual austenite in a range of 11-25%. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a rolling bearing used for a reduction gear, a drive pinion, a transmission, and the like, and more specifically, a rolling bearing having a long life of rolling fatigue and a high crack resistance strength and aging-resistant dimensional change. It is about.

As a heat treatment method for providing a long life against rolling fatigue of a bearing component, a method of performing carbonitriding treatment on the surface layer portion of the bearing component by adding ammonia gas to the atmosphere RX gas during quenching heating. (For example, JP-A-8-4774, JP-A-11-101247). By using this carbonitriding treatment, it is possible to harden the surface layer portion, further generate retained austenite in the microstructure, and improve the rolling fatigue life.
JP-A-8-4774 Japanese Patent Laid-Open No. 11-101247

  Since the carbonitriding process is a diffusion process and needs to be kept at a high temperature for a long time, it is difficult to improve the cracking strength due to the coarsening of the structure and there is room for improvement. In addition, there is room for improvement in the increase in the dimensional change rate over time due to the increase in retained austenite.

  On the other hand, in order to secure a long life against rolling fatigue, improve the cracking strength, and prevent an increase in the rate of dimensional change over time, it is possible to carry out it by steel alloy design. 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 at higher temperatures and at higher temperatures than in the past, as the usage environment increases in load and temperature. For this reason, a bearing component having a long rolling fatigue characteristic and high crack strength and dimensional stability is required.

  An object of the present invention is to provide a rolling bearing having dimensional stability and excellent rolling fatigue life.

  The rolling bearing of the present invention is a rolling bearing having an outer ring, an inner ring and a plurality of rolling elements, wherein at least one member of the outer ring, the inner ring and the rolling element has a nitrogen-enriched layer, and has austenite crystal grains. The particle size number is in the range exceeding 10 and the amount of retained austenite is in the range of 11 to 25%.

  The nitrogen-enriched layer is a layer with an increased nitrogen content formed on the raceway (outer ring or inner ring) or the surface layer of the rolling element, and can be formed by a process such as carbonitriding, nitriding, or nitriding. . The nitrogen content in the nitrogen-enriched layer is preferably in the range of 0.1% to 0.7%. If the nitrogen content is less than 0.1%, there will be no effect, and the rolling life especially under the foreign matter mixing conditions will be reduced. When the nitrogen content is more than 0.7%, voids called voids are formed, or the retained austenite increases so much that the hardness does not come out, resulting in a short life. For the nitrogen-enriched layer formed on the raceway, the nitrogen content is a value at the surface layer of 50 μm of the raceway surface after grinding, and can be measured by, for example, EPMA (wavelength dispersion type X-ray microanalyzer).

  Further, the rolling fatigue life can be greatly improved by the fact that the austenite crystal grain size number exceeds 10 and the austenite grain size is fine. When the particle size number of the austenite particle size is 10 or less, the rolling fatigue life is not greatly improved. Usually 11 or more. Although it is desirable that the austenite particle size is finer, it is usually difficult to obtain a particle size number exceeding # 13. In addition, the austenite grains of the bearing parts described above do not change in the surface layer portion where the nitrogen-enriched layer exists or in the inside thereof. Therefore, the target position of the above crystal grain size number range is the surface layer portion and the inside. Here, the austenite crystal grain is a crystal grain based on the trace of the austenite crystal grain boundary immediately before quenching after the quenching treatment.

  Furthermore, when the amount of retained austenite is about 11%, the lubrication life with foreign matters decreases, and when the amount is less than 11%, it tends to decrease further. On the other hand, if the amount of retained austenite is more than 25%, the amount of retained austenite is not different from that of a normal carbonitriding process, so that it is not possible to suppress aged dimensional change. For the bearing rings, the amount of retained austenite is a value at the surface layer of 50 μm after grinding, and is measured by, for example, comparing the diffraction intensity of martensite α (211) and retained austenite γ (220) by X-ray diffraction. Can do.

  The rolling bearing according to the present invention has a rolling fatigue life by having a nitrogen-enriched layer, refining the austenite grain size to 11 or more in grain size number, and setting the amount of retained austenite in the range of 11 to 25%. Is greatly improved, and an excellent aging-resistant dimensional change can be obtained.

  Next, 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. Although the drawings show radial ball bearings, ball bearings, tapered roller bearings, cylindrical roller bearings, and needle roller bearings are also subject to the embodiments of the present invention. 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 bearing component of the outer ring 1, the inner ring 2 and the rolling element 3 of these rolling bearings has a nitrogen-enriched layer.

A heat treatment including a carbonitriding process will be described as a specific example of the process for forming the nitrogen-enriched layer. FIG. 2 is a diagram for explaining a heat treatment method for a rolling bearing according to an embodiment of the present invention, and FIG. 3 is a diagram for explaining a modification thereof. FIG. 2 is a heat treatment pattern showing a method of performing the primary quenching and the secondary quenching, and FIG. 3 shows a method of cooling the material to below the A ₁ transformation point temperature during quenching, and then reheating and finally quenching. It is the heat processing pattern shown. In these figures, in the treatment T ₁ , the carbon is sufficiently dissolved while carbon and nitrogen are diffused in the steel base, and then cooled to less than the A ₁ transformation point. Next, in the process T ₂ of the in the figure, then reheated to a temperature lower than the A ₁ transformation point temperature or higher and treatment T _1, subjected to oil quenching from there.

  In the above heat treatment, crack strength can be improved and the aging rate of dimensional change can be reduced while carbonitriding the surface layer portion as compared with conventional carbonitriding and quenching, that is, carbonitriding after the carbonitriding treatment. The rolling bearing of the present invention manufactured by the heat treatment pattern of FIG. 2 or FIG. 3 has a microstructure in which the grain size of austenite crystal grains is less than half of the conventional one. The bearing component subjected to the above heat treatment has a long life against rolling fatigue, can improve the cracking strength, and can also reduce the rate of dimensional change over time.

  FIG. 4 is a diagram showing the microstructure of bearing parts, particularly austenite grains. FIG. 4A shows a bearing component according to an example of the present invention, and FIG. 4B shows a conventional bearing component. That is, FIG. 4A shows the austenite grain size of the bearing ring of the rolling bearing according to the embodiment of the present invention 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 these austenite crystal grain sizes, the conventional austenite grain size is No. 10 in the JIS standard grain size number, and according to the heat treatment method of FIG. 2 or FIG. 3, 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 I
Using JIS SUJ2 material (1.0 wt% C-0.25 wt% Si-0.4 wt% Mn-1.5 wt% Cr), (1) measurement of hydrogen content, (2) crystal grain size (3) Charpy impact test, (4) measurement of fracture stress value, and (5) rolling fatigue test. Table 1 shows the results.

  The manufacturing history of each sample is as follows.

  Samples A to D (examples of the present invention): carbonitriding 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 examples): The carbonitriding process was performed with the same history as that of Examples A to D of the present invention, and the secondary quenching temperature was 850 ° C to 870 ° C, which is a carbonitriding temperature of 850 ° C or higher.

  Conventional carbonitrided product (comparative example): carbonitrided at 850 ° C., holding time of 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.

  Normal quenching product (comparative example): without any carbonitriding treatment, it was quenched by heating to 850 ° C. Secondary quenching was not performed.

  Next, the test method will be described.

(1) Measurement of hydrogen amount 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 mg (maximum: diameter 12 mm × length 100 mm)
Heating furnace temperature range: 50 ° C to 1100 ° C
Reagents: Anhydrone 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. A sample collected with a dedicated sampler is inserted into the hydrogen analyzer together with the 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, a sample is taken out from the sampler, heated in a resistance heating furnace, and non-diffusible hydrogen is guided to a 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 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 Z2242. As a test piece, a U-notch test piece (JIS No. 3 test piece) shown in JIS Z 2202 was used.

(4) Measurement of Fracture Stress Value FIG. 6 is a diagram showing a test piece for a static crush strength test (measurement of a fracture stress value). The load until it is broken by applying a load in the P direction in the figure 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 specimen, 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 ₀ )} [1 + e ₁ / {κ (ρ ₀ + e ₁ )}]
σ ₂ = (N / A) + {M / (Aρ ₀ )} [1-e ₂ / {κ (ρ ₀ −e ₂ )}]
κ = − (1 / A) ∫A {η / (ρ ₀ + η)} dA
(5) Rolling fatigue life Table 2 shows the test conditions for the rolling fatigue life test. FIG. 7 is a schematic view of a rolling fatigue life tester. FIG. 7A is a front view, and FIG. 7B is a side view. 7A and 7B, 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 inch ball and is guided by the guide roll 12 to roll while exerting a high surface pressure with the rolling fatigue life test piece 21.

  The test results shown in Table 1 will be described as follows.

(1) Amount of hydrogen Conventional carbonitrided products that have been carbonitrided have 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, in Samples B to D, the hydrogen content is reduced to almost half of 0.37 to 0.40 ppm. This amount of hydrogen is at the same level as that of ordinary hardened products.

  By reducing the amount of hydrogen described above, embrittlement of steel due to hydrogen solid solution can be reduced. That is, the reduction in the amount of hydrogen greatly improves the Charpy impact value of Samples B to D of the present invention example.

(2) Crystal grain size 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, the austenite grains are prominent as the grain size numbers 11 to 12. Has been refined. The austenite grains of the samples E and F, the conventional carbonitrided product and the normal quenching product have a crystal grain size number 10, and are coarser than the samples B to D of the examples of the present invention.

(3) Charpy impact test According to Table 1, the Charpy impact value of the samples BD of the present invention is 6 compared to the Charpy impact value of the conventional carbonitrided product, which is 5.33 J / cm ^2. A high value of .30 to 6.65 J / cm ² is obtained. Among these, the one where secondary quenching temperature is low shows the tendency for a Charpy impact value to become high. The normally hardened product has a high Charpy impact value of 6.70 J / cm ² .

(4) Measurement of fracture stress value The 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 D were improved to 2650 to 2840 MPa. The fracture stress value of the normally quenched product is 2770 MPa, and the improved crack resistance strength of Samples B to D 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, normally hardened product to reflect to have no 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 are almost equivalent to conventional carbonitrided products.

  In summary, Samples B to D of the present invention have a reduced hydrogen content, an austenite grain size of 11 or more, and improved Charpy impact value, crack resistance strength and rolling fatigue life. .

Example II
Next, Example II will be described. A series of tests were performed on the following X material, Y material, and Z 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. did. The manufacturing history of the X material to the Z material is as follows.
X material (comparative example): Only normal quenching (not carbonitriding).
Y material (comparative example): quenching directly after carbonitriding (conventional carbonitriding quenching). Carbonitriding temperature 845 ° C, holding time 150 minutes. The atmosphere of the carbonitriding process was RX gas + ammonia gas.
Z material (example of the present invention): bearing steel subjected to the heat treatment pattern of FIG. Carbonitriding temperature 845 ° C, holding time 150 minutes. The atmosphere of the carbonitriding process was RX gas + ammonia gas. The final quenching temperature was 800 ° C.

(1) Rolling fatigue life Test conditions and test equipment for rolling fatigue life are as shown in Table 2 and FIG. 7 as described above. The rolling fatigue life test results are shown in Table 3.

According to Table 3, the Y material of the comparative example shows 3.1 times the L ₁₀ life of the X material that has been subjected only to normal quenching in the comparative example (the life that one of the 10 test pieces breaks), The effect of extending the life by carbonitriding is recognized. On the other hand, the Z material of the example of the present invention has a long life of 1.74 times that of the Y material and 5.4 times that of the X 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 a method according to the above-mentioned JIS Z 2242 using a U-notch test piece. The test results are shown in Table 4.

  The Charpy impact value of the Y material (comparative example) subjected to carbonitriding was not higher than that of the normal quenching X material (comparative example), but the Z material obtained the same value as the X material.

(3) Test of Static Fracture Toughness Value FIG. 8 is a diagram showing a test piece of a static fracture toughness test. About 1 mm of pre-crack was introduced into the notch portion of the test piece, and then a static load by three-point bending was applied to determine the fracture load P. The following formula (I) was used for calculation of the fracture toughness value (K ₁ c value). The test results are shown in Table 5.
K1c = (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)

  Since the precrack depth is larger than the carbonitrided layer depth, there is no difference between the X material and the Y material of the comparative example. However, the Z material of the present invention example was able to obtain a value about 1.2 times that of the comparative example.

(4) Static Crushing Strength Test The static crushing strength test piece having the shape shown in FIG. 6 was used as described above. In the figure, a static crushing strength test was performed by applying a load in the P direction. The test results are shown in Table 6.

  The Y material subjected to carbonitriding has a slightly lower value than the normal quenching X material. However, the Z material of the present invention has higher static crushing strength than the Y material, and a level comparable to that of the X material is obtained.

(5) Aged dimensional change rate The measurement 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 retained austenite amount (50 μm depth).

  It can be seen that the Z material of the example of the present invention is suppressed to half or less compared to the dimensional change rate of the Y material having a large amount of retained austenite.

(6) Rolling life test under the presence of foreign matter The ball bearing 6206 was used to evaluate the rolling fatigue life under the presence of foreign matter mixed with a predetermined amount of standard foreign matter. Table 8 shows the test conditions and Table 9 shows the test results.

  Compared to the X material, the Y material subjected to the conventional carbonitriding treatment is about 2.5 times longer, and the Z material of the present invention example has a long life of about 2.3 times. Although the Z material of the present invention has less retained austenite than the Y material of the comparative example, a substantially equivalent long life is obtained due to the intrusion of nitrogen and the effect of the refined microstructure.

  From the above results, the Z material, that is, the present invention example, simultaneously satisfies the three items of the rolling fatigue life extension, crack strength improvement, and reduction of aging dimensional change rate, which were difficult in the conventional carbonitriding process. I found out that I can do it.

(Example III)
Table 10 shows the results of tests conducted on the relationship between the nitrogen content and the rolling life under the contamination condition. Comparative Example 1 is a standard quenched product, and Comparative Example 2 is a standard carbonitrided product. Comparative Example 3 is a case where the same treatment as in the embodiment of the present invention was performed, but only the amount of nitrogen was excessive. The test conditions are as follows.
Test bearing: Tapered roller bearing 30206 (both inner and outer rings and rollers are made of JIS high carbon chrome bearing steel class 2 (SUJ2))
Radial load: 17.64kN
Axial load: 1.47kN
Rotation speed: 2000rpm
1g / L of hard foreign matter

  From Table 10, regarding Examples 1 to 5, it can be seen that the nitrogen content and the foreign substance lifetime are in a substantially proportional relationship. However, in Comparative Example 3 where the nitrogen content is 0.72, the upper limit of the nitrogen content should be 0.7 in light of the extremely low rolling life under the presence of foreign matter.

  From Examples II and III, it can be seen that the amount of retained austenite is preferably in the range of 11 to 25%. The amount of retained austenite is measured, for example, by comparing the diffraction intensities of martensite α (211) and retained austenite γ (220) by X-ray diffraction. The value at the surface layer depth of 50 μm after grinding of the race and rolling elements is taken.

  The embodiment disclosed this time should be considered as illustrative in all points 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.

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 of the rolling bearing in embodiment of this invention. It is a figure explaining the modification of the heat processing method of the rolling bearing 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 component of the present invention example, and (b) is a conventional bearing component. (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

DESCRIPTION OF SYMBOLS 1 Outer ring 2 Inner ring 3 Rolling element 10 Rolling bearing 11 Drive roll 12 Guide roll 13 Ball 21 Rolling fatigue life test piece T ₁ Carbonitriding temperature T ₂ Quenching heating temperature

Claims (3)

  In a rolling bearing having an outer ring, an inner ring, and a plurality of rolling elements, at least one member of the outer ring, the inner ring, and the rolling element has a nitrogen-enriched layer, and the grain size number of the austenite crystal grains exceeds 10. A rolling bearing having a range of 11 to 25% of retained austenite.   The rolling bearing according to claim 1, wherein the nitrogen content in the nitrogen-enriched layer is in the range of 0.1% to 0.7%. The rolling bearing according to claim 2, wherein the member is a bearing ring, and the nitrogen content is a value at a surface layer of 50 μm of the raceway surface after grinding.