JPH06279935A - Bearing steel with excellent delay characteristics for microstructural changes due to repeated stress loading - Google Patents

Abstract

(57) [Summary] [Purpose] To provide a bearing steel in which microstructural changes due to repeated stress loads are small under severe operating conditions. [Structure] In order to promote the delay of microstructural change due to cyclic stress loading, as a component for improving B ₅₀ high load rolling fatigue life, especially Si: 1.0-2.5 wt%, Cr: over 2.5-8.0 wt.
% And Ni: over 1.0 to 3.0 wt%, Zr: 0.02 to 0.5 wt
%, Ta: 0.02 to 0.5 wt%, Hf: 0.02 to 0.5 wt% and Co: 0.05 to 1.5 wt% A bearing steel containing one or more selected from the group consisting of 0.05 to 1.5 wt%.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bearing steel used as an element member of a rolling bearing such as a roller bearing or a ball bearing, and particularly to a delay for a microstructure change (deterioration) which occurs under a rolling contact surface due to repeated stress loads. We propose a bearing steel with excellent characteristics.

[0002]

2. Description of the Related Art Conventionally, high-carbon chromium bearing steel (JI
S: SUJ 2) is most often used. In general, bearing steel has one of the important properties that a long rolling contact fatigue life is important. As a factor that affects the rolling contact fatigue life, it is considered that the hard non-metallic inclusions in the steel have a large effect. Was being considered. Therefore, the mainstream of recent research has been to take measures to improve the bearing life by controlling the amount and size of non-metallic inclusions by reducing the amount of oxygen in steel.

For example, as one developed for the purpose of further improving the rolling contact fatigue life of a bearing, Japanese Patent Laid-Open No. 1-306542 has been proposed.
There are proposals such as Japanese Patent Laid-Open Publication No. 3-126839 and Japanese Patent Laid-Open Publication No. 3-126839, which are techniques for controlling the composition, shape, or distribution state of oxide-based nonmetallic inclusions in steel.

[0004]

However, in order to manufacture a bearing steel containing few non-metallic inclusions, it is necessary to install expensive melting equipment or to greatly improve conventional equipment, which causes a large economical burden. There was a problem. Further, according to a recent study conducted by the present inventors, as a factor that determines the rolling life, a phenomenon that has been generally discussed in the past;
That is, it has been found that there are factors other than the presence of the "decarburized layer" (low C concentration region) and the above-mentioned "non-metallic inclusions" that occur during heat treatment. The reason is that simply reducing the decarburization layer and non-metallic inclusions under the conventional technology is a major achievement in improving the rolling contact fatigue life of the bearing, especially the bearing life under severe conditions such as high load or high temperature. Because I experienced many things that I could not get. From this, we were convinced of the existence of other factors that control the bearing life.

Therefore, the present inventors investigated the cause of the separation of the rolling bearing. As a result, as shown in Fig. 1 (a), the band-shaped white color on the lower layer (surface layer) of the rolling contact surface is caused by the repeated shear stress generated during the rolling contact between the inner and outer rings of the bearing and the rolling elements. A microstructured layer consisting of products and rod-shaped precipitates is generated, which gradually grows as the number of rolling increases, and at the end, fatigue delamination as shown in Fig. 1 (b) from this microstructured portion. Was found to lead to bearing life. Furthermore, the harsh bearing operating environment, that is, high surface pressure (miniaturization) and rise in operating temperature, shortens the time until these microstructural changes occur, resulting in a significant reduction in bearing life. I stopped. That is, the bearing life under severe conditions is not sufficient by merely controlling the decarburized layer and non-metallic inclusions as in the prior art. For example, simply reducing the amount of non-metallic inclusions cannot delay the time until the above-described microstructure change occurring under the rolling contact surface occurs. As a result, they have found that the bearing life cannot be further improved.

Therefore, an object of the present invention is to delay the microstructural change expected to occur during the use of the bearing under high load in order to improve the rolling contact fatigue life characteristics under severe operating conditions. And to provide a bearing steel that can significantly improve the life of the bearing.

[0007]

The inventors of the present invention focused on a new "microstructure change delay characteristic" as the bearing life based on the above-mentioned knowledge, and of course, it is necessary to improve it. With the recognition that a new alloy design (composition composition) of (1) is necessary and the life of the bearing cannot be further improved without realizing this, various experiments and studies were further conducted. As a result, it was found that the addition of a proper amount of Cr together with Si can significantly delay the above-mentioned microstructural change generated under the rolling contact surface due to repeated stress loading, and developed the bearing steel of the present invention.

That is, the bearing steel of the present invention has the following essential constitution. (1) C: 0.5 to 1.5 wt%, Si: 1.0 to 2.5 wt%, Cr:
Bearing steel containing more than 2.5 to 8.0 wt%, O: 0.0020 wt% or less, the balance being Fe and inevitable impurities, and having excellent delay characteristics for microstructural change due to repeated stress loading (No. 1
invention). (2) C: 0.5 to 1.5 wt%, Si: 1.0 to 2.5 wt%, Cr:
It contains more than 2.5 to 8.0 wt%, O: 0.0020 wt% or less, and further contains Mn: 0.05 to 2.0 wt%, Ni: 0.05 to 1.0 wt%, Cu:
0.05 to 1.0 wt%, B: 0.0005 to 0.01 wt%, Al: 0.005
To 0.07 wt% and N: 0.0005 to 0.012 wt%, and one or more selected from the group consisting of Fe and unavoidable impurities, and the balance consisting of Fe and inevitable impurities. Excellent bearing steel (2nd invention). (3) C: 0.5 to 1.5 wt%, Si: 1.0 to 2.5 wt%, Cr:
More than 2.5 to 8.0 wt%, O: 0.0020 wt% or less, Ni: more than 1.0 to 3.0 wt%, Zr: 0.02 to 0.5 wt%, T
a: 0.02-0.5 wt%, Hf: 0.02-0.5 wt% and C
o: A bearing steel containing one or more selected from 0.05 to 1.5 wt% and the balance being Fe and inevitable impurities and having excellent delay characteristics for microstructural change due to repeated stress loading (No. 3 invention). (4) C: 0.5 to 1.5 wt%, Si: 1.0 to 2.5 wt%, Cr:
It contains more than 2.5 to 8.0 wt%, O: 0.0020 wt% or less, and further contains Mn: 0.05 to 2.0 wt%, Ni: 0.05 to 1.0 wt%, Cu:
0.05 to 1.0 wt%, B: 0.0005 to 0.01 wt%, Al: 0.005
To 0.07 wt% and N: 0.0005 to 0.012 wt%, one or more selected from the group consisting of Ni: more than 1.0 to 3.0 wt%, Zr: 0.02 to 0.5 wt%, T
a: 0.02-0.5 wt%, Hf: 0.02-0.5 wt% and C
o: A bearing steel that contains one or more selected from 0.05 to 1.5 wt% and the balance is Fe and unavoidable impurities and that has excellent delay characteristics for microstructural change due to repeated stress loading (No. 4 invention).

[0009]

The background to the idea of the bearing steel of the present invention having the above alloy design will be described below based on the results of experiments conducted by the present inventors. First, in the experiment, SUJ 2 (C: 1.02 wt%, Si: 0.25 wt%, Mn: 0.45 wt
%, Cr: 1.35wt%, Ni: 0.0040wt%, O: 0.0012wt%)
And two kinds of materials in which Si and a large amount of Cr are added in combination (C: 1.00 wt%, Si: 1.28 wt%, Mn: 0.46 wt%,
Cr: 3.51 wt%, O: 0.0009 wt%, N: 0.0046 wt%) (C: 1.00 wt%, Si: 1.32 wt%, Mn: 0.48 wt%,
Cr: 7.23 wt%, O: 0.0008 wt%, N: 0.0052 wt%) was prepared. Then, these test materials were subjected to normalizing treatment, spheroidizing normalizing treatment, quenching and tempering treatment, and 12 mmφ × 22 mm cylindrical test pieces were produced from the respective test materials.

Next, these test pieces were subjected to a Hertz maximum contact stress: 60 using a radial type rolling fatigue life tester.
A rolling fatigue life test was carried out under a load condition of 0 kgf / mm ² and a cyclic stress number of 46500 cpm. The test results are plotted on Weibull distribution establishment paper, and are considered to be the numerical values showing the improvement of rolling contact fatigue life due to the increase of material strength. B ₁₀ (10% cumulative failure probability) and micro stress due to repeated stress loading during high load rolling. B ₅₀ (50% cumulative failure probability), which is considered to be a numerical value showing the improvement of rolling fatigue life by delaying the occurrence of microstructural change, was determined.

As a result, as shown in Table 1, with respect to the Si-Cr composite additive, the B ₁₀ value is not so much improved, but the B ₅₀ value is remarkably high, and the average bearing life is About ₁₀ times B ₁₀ value compared to SUJ 2, B
It was confirmed that the ₅₀ value showed an improvement of about 30 times. In particular, the combined addition of Si and a large amount of Cr has a remarkable effect on the delay property of the microstructure change generated during high load rolling, and it can be expected that the damage (life) is delayed by that amount.

[0012]

[Table 1]

FIG. 2 is a summary of the above experimental results.
The bearing life and microstructural changes due to non-metallic inclusions.
It is a schematic diagram which shows the relationship with the change of the life resulting from aging.
As is clear from this figure, the cumulative damage probability 10
% B_TenBearing life indicated by the value (hereinafter referred to as "B_TenTurning
According to "dynamic fatigue life"), a large amount of Cr along with Si
The effect does not appear as expected even with multiple additions. Shi
Okay, this is B ₅₀In terms of values, the combined addition of Si and Cr
The effect becomes extremely remarkable and the microstructure change generation ring
Bearing life under the boundary (B with cumulative damage probability of 50%₅₀Rolling fatigue
As long as you are conscious of the₅₀Excellent rolling fatigue life
It has become clear.

Therefore, in the present invention, the range of the composition of components as described below is determined from the viewpoint of improving the microstructure change delaying property due to repeated stress loading.

C: 0.5 to 1.5 wt% C is an element which forms a solid solution in the matrix and effectively acts to strengthen the martensite, and in order to secure the strength after quenching and tempering and to improve the rolling fatigue life by it. Contained in. If the content is less than 0.5 wt%, such effects cannot be obtained. On the other hand, if it exceeds 1.5 wt%, the machinability and forgeability deteriorate, so the range was limited to 0.5 to 1.5 wt%.

Si: 1.0 to 2.5 wt% or less Si is used as a deoxidizing agent when steel is melted, and is solid-dissolved in the matrix to increase tempering and softening resistance to enhance the strength after quenching and tempering. It is effective as an element to improve rolling fatigue life. However, in the present invention, this Si
In addition to the above-mentioned effects, the addition of 1.0 wt% or more has the effect of delaying the above-mentioned microstructural change under repeated stress loading and significantly improving the rolling fatigue life. However, if its content exceeds 2.5 wt%, its effect saturates, but the workability and toughness decrease, so 1.0-2.5 wt% is required to further improve the microstructural change retardation property. It is effective to add.

Mn: 0.05 to 2.0 wt% Mn is an element that acts as a deoxidizer during the melting of steel and is effective in reducing the oxygen content of steel. Also, by improving the hardenability of steel, it improves the toughness and hardness of the base martensite, and effectively acts to improve the rolling fatigue life. However, if this addition amount is less than 0.05 wt%, the effect will not be realized, and 2.
If it exceeds 0 wt%, machinability and forgeability will decrease, so 0.05
Limit to ~ 2.0 wt%.

Cr: over 2.5 to 8.0 wt% Cr generally improves strength and wear resistance by improving hardenability and forming stable carbides.
As a result, it is a component that improves rolling fatigue life. However, in the present invention, this Cr is an element that plays an important role, and especially when this Cr is added in a large amount exceeding 2.5 wt%, the micro addition due to the repeated stress loading described above due to the complex addition with Si is caused. It is effective in delaying the microstructure change and improving the rolling fatigue life in this aspect. The effect of Cr added for this purpose is not only saturated when it exceeds 8.0 wt%, but rather causes a decrease in the amount of solid solution C during quenching, resulting in a decrease in strength. Therefore, Cr is added within the range of more than 2.5 to 8.0 wt%.

Ni: 0.05 to 1.0 wt%, more than 1.0 to 3.0 wt% Ni increases the hardenability to increase the strength after quenching and tempering, improve toughness, and improve rolling contact fatigue life. Is added in the range of 0.05 to 1.0 wt%. Further, this Ni, when added in excess of 1.0 wt%, delays the microstructure change during rolling, thereby improving rolling fatigue life. However, even in this case, if it is added in excess of 3 wt%, a large amount of residual γ will be precipitated, resulting in reduced strength and impaired dimensional stability, as well as cost increase. , Addition of more than 1.0 to 3.0 wt% is necessary.

Cu: 0.05 to 1.0 wt% Cu is added in order to enhance the strength after quenching and tempering due to the increase in quenching and to improve the rolling contact fatigue life. When added for this purpose, a range of 0.05-1.0 wt% is sufficient.

B: 0.0005 to 0.01 wt% B is added in an amount of 0.0005 wt% or more because it increases the hardenability and thereby enhances the strength after quenching and tempering and improves the rolling contact fatigue life. However, if added in excess of 0.01 wt%, the workability deteriorates, so the range is limited to 0.0005 to 0.01 wt%.

Al: 0.005 to 0.07 wt% Al is used as a deoxidizer during the melting of steel, and at the same time,
Combines with N in the steel to refine the crystal and contribute to the improvement of the toughness of the steel. Further, it effectively acts to improve the rolling contact fatigue life by increasing the strength after quenching and tempering. For such an effect, it is effective to add 0.005 to 0.07 wt% of Al.

N: 0.0005 to 0.012 wt% N combines with the nitride-forming element to refine the crystal grains, and forms a solid solution in the matrix to increase the strength after quenching and tempering.
Improves rolling fatigue life. 0.0005 for this purpose
Add within 0.012 wt%.

P ≦ 0.025 wt% P is desirable because it lowers the toughness and rolling contact fatigue life of the steel, so it is desirable to be as low as possible.
It is 0.025 wt%.

S ≦ 0.025 wt% S combines with Mn to form MnS and improves the machinability. However, if it is contained in a large amount, the rolling contact fatigue life will be reduced, so 0.025 wt% must be the upper limit.

O: 0.0020 wt% or less O forms a hard non-metallic inclusion, so even if a delay in microstructure change due to repeated stress loading is obtained by controlling other components, rolling fatigue life Therefore, it is desirable to be as low as possible. However, a content of 0.0020 wt% or less is acceptable.

As described above, main components (Si, Cr and Mn, Ni, for improving rolling fatigue life by delaying microstructure change due to repeated stress loading and improving rolling fatigue life by increasing strength) Cu, B, Al, N,
O) and the reason for limiting C, P, S is explained,
In the present invention, one or more selected from Zr, Ta, Hf and Co may be added to improve the rolling fatigue life under high load.

The preferred range of addition of each element and the purpose of addition,
The reasons for limiting the upper limit and the lower limit are summarized in Table 2.

[Table 2]

In the present invention, in order to improve machinability, S, Se, Te, REM, Pb, Bi, Ca, Ti, Mg, P,
The addition of Sn, As, etc. does not hinder the retardation property due to the change in microstructure due to the repeated stress load, which is the object of the present invention, and the machinability can be easily improved. You may add according to it.

[0030]

[Examples] Steels having the compositions shown in Tables 3 and 4 were melted by a conventional method, and the obtained steel materials were diffusion annealed at 1240 ° C for 30 hours and then rolled into steel bars of 65 mmφ. Then, heat treatment was performed in the order of normalizing-spheroidizing annealing-quenching-tempering, and a 12 mmφ x 22 mm cylindrical rolling fatigue life test piece was prepared by lapping finish. Then, a B ₅₀ rolling contact fatigue life, which is an average bearing life, was tested for each of the above test pieces. This B ₅₀ rolling contact fatigue life test uses a radial type rolling contact fatigue life tester and the maximum contact stress of Hertz: 600 kgf / mm.
^{2. The} test was performed under the condition of cyclic stress number of about 46,500 cpm. The test results are summarized on the probability paper according to the Weibull distribution, and the average life of steel material No. 1 (conventional steel SUJ2) (cumulative failure rate: 50% total load until peeling) is set to 1 and others The steel grades were evaluated in comparison. The evaluation results are also shown in Tables 3 and 4, respectively.

[0031]

[Table 3]

[0032]

[Table 4]

As is clear from the results shown in Tables 3 and 4,
Average of steel material No. 3 having a C content outside the scope of the present invention, steel material No. 4 having a Cr content outside the scope of the present invention, and steel material No. 5 having an O content outside the scope of the present invention The service life is lower than that of conventional steel (No. 1 steel material). However, for Si, Cr
As long as the content of B is included, the B ₅₀ value shows an excellent value even if the conditions are not met. On the other hand, the steel material No. which is the first invention steel.
The average life of 6 and 7 is 24 compared with the conventional steel (steel material No. 1).
~ 32 times better. That is, it can be seen that the combined addition of Si and Cr to the bearing steel significantly retarded the microstructure change, and as a result, effectively acted to improve the rolling fatigue life.

Among them, in addition to Si and Cr, Mn, Mo, Zr, T
Steel No.8 to 34 with positive addition of at least one of the rolling life improvement components such as a, Hf, Ni, Cu, Co, O and N
In the case of, it was confirmed that the average life (B ₅₀ rolling contact fatigue life) was further improved.

[0035]

As described above, according to the present invention,
Basically, high Cr of over 2.5 ~ 8.0 wt% and 1.0 ~ 2.5 wt%
By using Si-added bearing steel, it is possible to improve rolling fatigue life by delaying the microstructural change due to repeated stress loading, and provide a bearing steel with a long life. . Therefore, further reduction of oxygen content in steel or composition, shape, and shape of oxide-based non-metallic inclusions present in steel, which was indispensable under the conventional technology,
In addition, improvement or construction of steelmaking equipment necessary for controlling the distribution state is unnecessary. Further, the development of the bearing steel according to the present invention enables downsizing of the rolling bearing and further increase of the bearing operating temperature.

[Brief description of drawings]

1 (a) and 1 (b) are under cyclic stress loading,
A micrograph of the metal structure showing the appearance of the microstructure change that occurs.

FIG. 2 is an explanatory diagram showing the influence of Si—Cr on the bearing life due to inclusions and the bearing life due to microstructural changes.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Akihiro Matsuzaki, 1st Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Prefecture Technical Research Division, Kawasaki Steel Co., Ltd. (72) Shinichi Amano 1 Kawasaki-cho, Chuo-ku, Chiba Address: Kawasaki Steel Corporation Technical Research Division

Claims (4)

[Claims] 1. C: 0.5 to 1.5 wt%, Si: 1.0 to 2.5 wt
%, Cr: more than 2.5 to 8.0 wt%, O: 0.0020 wt% or less, the balance being Fe and unavoidable impurities, and a bearing steel excellent in delay characteristics of microstructure change due to repeated stress load. 2. C: 0.5 to 1.5 wt%, Si: 1.0 to 2.5 wt%
%, Cr: more than 2.5 to 8.0 wt%, O: 0.0020 wt% or less, Mn: 0.05 to 2.0 wt%, Ni: 0.05 to 1.0 wt%
%, Cu: 0.05 to 1.0 wt%, B: 0.0005 to 0.01 wt%, Al:
Delay property of microstructure change due to repeated stress loading, containing one or more selected from 0.005 to 0.07 wt% and N: 0.0005 to 0.012 wt%, with the balance being Fe and unavoidable impurities Excellent bearing steel. 3. C: 0.5 to 1.5 wt%, Si: 1.0 to 2.5 wt%
%, Cr: more than 2.5 to 8.0 wt%, O: 0.0020 wt% or less, and Ni: more than 1.0 to 3.0 wt%, Zr: 0.02 to 0.5
Any one selected from wt%, Ta: 0.02 to 0.5 wt%, Hf: 0.02 to 0.5 wt% and Co: 0.05 to 1.5 wt%
A bearing steel that contains one or more kinds, and the balance is Fe and unavoidable impurities, and that has excellent delay characteristics for microstructural changes due to repeated stress loading. 4. C: 0.5 to 1.5 wt%, Si: 1.0 to 2.5 wt
%, Cr: more than 2.5 to 8.0 wt%, O: 0.0020 wt% or less, Mn: 0.05 to 2.0 wt%, Ni: 0.05 to 1.0 wt%
%, Cu: 0.05 to 1.0 wt%, B: 0.0005 to 0.01 wt%, Al:
0.005 to 0.07 wt% and N: 0.0005 to 0.012 wt%, one or more selected from the group, and further Ni: more than 1.0 to 3.0 wt%, Zr: 0.02 to 0.5 wt%
%, Ta: 0.02 to 0.5 wt%, Hf: 0.02 to 0.5 wt% and Co: 0.05 to 1.5 wt%, and one or more selected from the rest, with the balance being Fe and inevitable impurities. , Bearing steel with excellent characteristics of delaying microstructural changes due to repeated stress loading.