JPH06264187A - Bearing steel excellent in delay characteristic of change in microstructure caused by repeated stress load - Google Patents

Abstract

PURPOSE:To develop steel for a bearing excellent in a rolling fatigue service life by the delay of the change in the microstructure caused by repeated stress loads by using high carbon steel high in Cu content and low in oxygen content as steel for a rolling bearing. CONSTITUTION:As the stock for a rolling bearing, high carbon steel contg., by weight, 0.5 to l.5% C, >1.0 to 2.5% Cu and <0.0020% O and furthermore contg. one or >= two kinds among 0.05 to 0.5% Si, 0.05 to 2.0% Mn, 0.05 to 2.5% Cr, 0.05 to 0.5% Mo, 0.05 to 1.0% Ni, 0.0005 to 0.01% B, 0.005 to 0.07% Al and 0.0005 to 0.012% N is used. By increasing the content of Cu and reducing the content of O2 for forming oxide nonmetallic inclusions, the change in the microstructure caused by repeated stress loads in the case the rolling bearing using the same as the stock is subjected to high load rolling is delayed to improve its rolling fatigue service life, so that the rolling bearing having a long service life can be produced.

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 when a large amount of Cu is added, it is generated under a rolling contact surface by repeated stress loading. We propose a bearing steel with improved delay characteristics against changes in microstructure (deterioration).

[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 technologies for controlling the composition, shape, or distribution state of oxide 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 summary of recent research results 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, a "decarburized layer" generated during heat treatment It was found that there are other factors besides the (low C concentration region) and the presence of the above-mentioned "non-metallic inclusions". This is because simply reducing the decarburized layer and non-metallic inclusions under the conventional technique is not enough to improve the rolling contact fatigue life of the bearing, especially the bearing life under severe conditions such as high load or high temperature. This is because I experienced many things that were not effective.
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, due to the repeated shear stress generated during the rolling contact between the inner and outer rings of the bearing and the rolling elements, the lower layer (surface layer) of the rolling contact surface
As shown in Fig. 1 (a), a microstructure-change layer consisting of strip-shaped white products and rod-shaped precipitates is generated, which gradually grows as the number of times of rolling increases, and finally this microstructure is formed. From the changed part, it was found that fatigue delamination occurred as shown in Fig. 1 (b), leading to the life of the bearing. Furthermore, the harsh bearing operating environment, that is, high surface pressure (miniaturization) and increase in operating temperature, shortens the number of rolling cycles before these microstructural changes occur, leading to a significant reduction in bearing life. I stopped. That is, the bearing life is not sufficient by controlling the decarburization layer and the non-metallic inclusions as in the prior art, and for example, simply reducing the non-metallic inclusions would result in the above-mentioned rolling contact surface. It is not possible to delay the time until the microstructural changes that occur below occur. 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 a bearing under a 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 unexpectedly found that by adding a large amount of Mn, the above-mentioned microstructural change generated under the rolling contact surface due to repeated stress loading can be significantly delayed, and by extension, the bearing life can be significantly improved. Invented bearing steel developed.

That is, the bearing steel of the present invention has the following essential constitution. (1) C: 0.5 to 1.5 wt%, Cu: over 1.0 to 2.5 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 (first invention). (2) C: 0.5 to 1.5 wt%, Cu: over 1.0 to 2.5 wt%,
O: 0.0020 wt% or less is contained, and Si: 0.05 to 0.5
wt%, Mn: 0.05 to 2.0 wt%, Cr: 0.05 to 2.5 wt%, M
o: 0.05 to 0.5 wt%, Ni: 0.05 to 1.0 wt%, B: 0.0005
~ 0.01wt% Al: 0.005-0.07wt% and N: 0.0005-0.01
A bearing steel containing at least one selected from 2 wt% or more, and the balance being Fe and unavoidable impurities, which is excellent in delay characteristics of microstructure change due to repeated stress load (second invention). (3) C: 0.5 to 1.5 wt%, Cu: over 1.0 to 2.5 wt%,
O: 0.0020 wt% or less is included, and Si: more than 0.5 to 2.5
wt%, Mn: over 2.0 ~ 5.0 wt%, Cr: over 2.5 ~ 8.0 wt%,
N: over 0.012 ~ 0.050 wt%, V: 0.05 ~ 1.0 wt%, N
b: 0.05-1.0 wt%, W: 0.05-1.0 wt%, Zr: 0.02-
0.5 wt%, Ta: 0.02-0.5 wt%, Hf: 0.02-0.5 wt%
And Co: any one selected from 0.05 to 1.5 wt% 1
A bearing steel containing three or more kinds, and the balance consisting of Fe and inevitable impurities, which is excellent in delay characteristics of microstructure change due to repeated stress load (third invention). (4) C: 0.5-1.5 wt%, Cu: over 1.0-2.5 wt%,
O: 0.0020 wt% or less, Si: 0.05-0.5 wt%
%, Mn: 0.05 to 2.0 wt%, Cr: 0.05 to 2.5 wt%, M
o: 0.05 to 0.5 wt%, Ni: 0.05 to 1.0 wt%, B: 0.0005
~ 0.01wt% Al: 0.005-0.07wt% and N: 0.0005-0.01
It contains one or more selected from 2 wt%, and further, Si: more than 0.5 to 2.5 wt%, Mn: 2.0.
Over ~ 5.0 wt%, Cr: over 2.5 ~ 8.0 wt%, N: over 0.012 ~
0.050 wt%, V: 0.05 to 1.0 wt%, Nb: 0.05 to 1.0 wt
%, W: 0.05 to 1.0 wt%, Zr: 0.02 to 0.5 wt%, Ta: 0.
02-0.5 wt%, Hf: 0.02-0.5 wt% and Co: 0.05-1.
A bearing steel containing at least one selected from the group consisting of 5 wt% or more, and the balance consisting of Fe and inevitable impurities, which is excellent in delay characteristics of microstructure change due to repeated stress load (fourth 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: 1.35 wt%, N: 0.0040 wt%, O: 0.0012 wt%)
And two materials with Cu added (C: 0.98wt%, Si: 0.25wt%, Mn: 0.42wt%, C
r: 1.32wt%, Cu: 1.20wt%, O: 8ppm, N: 38ppm) (C: 0.96wt%, Si: 0.25wt%, Mn: 0.44wt%, C
r: 1.31 wt%, Cu: 1.83 wt%, O: 9 ppm, N: 43 ppm) were prepared as test steel materials. 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 conducted under load conditions of kgf / mm ² and 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) 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 high Cu-added material, the B ₁₀ value was not so much improved, but the B ₅₀ value was remarkably high, and the average bearing life was SUJ 2 It was confirmed that the B ₁₀ value showed an improvement of about 1.5 times, and the B ₅₀ value showed an improvement of about 20 times. Especially Cu
Addition of a large amount of is extremely effective in delaying the microstructural change that occurs during rolling under high load, and by that much, damage (life)
Can be expected to be delayed.

[0012]

[Table 1]

FIG. 2 summarizes the above experimental results and is a schematic diagram showing the relationship between the bearing life due to non-metallic inclusions and the life change due to microstructural changes.
As is clear from this figure, the cumulative damage probability 10
According to the bearing life indicated by the B ₁₀ value of% (hereinafter, referred to as “B ₁₀ rolling contact fatigue life”), even if a large amount of Cu is added, the effect does not appear as expected. But this is B
_{The 50} value shows that the effect of adding a large amount of Cu becomes extremely remarkable, and as long as the bearing life under severe conditions, that is, in the microstructure change generation environment, is considered, it is evaluated that B ₅₀ is excellent. Proved to be essential.

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 on the strengthening of martensite, and secures the strength after quenching and tempering and improves the rolling contact fatigue life. Is included for
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 non-oxidizing property and the forgeability deteriorate, so the range was limited to 0.5 to 1.5 wt%.

Si: 0.05 to 0.5 wt%, more than 0.5 to 2.5 wt% or less Si is used as a deoxidizing agent during the melting of steel, and is also solid-dissolved in the matrix to increase the resistance to tempering and quenching, It is effective as an element that increases the strength after tempering and improves the rolling fatigue life. The content of Si added for this purpose is in the range of 0.05 to 0.5 wt%. Furthermore, this Si
Has the effect of delaying the microstructural change under cyclic stress loading and improving the rolling fatigue life by adding more than 0.5% by weight. However, its content is 2.5 wt%
If it exceeds 0.1%, the effect is saturated while the workability and toughness are lowered. Therefore, in order to further improve the microstructure change retardation property, it is effective to add more than 0.5 to 2.5 wt%.

Mn: 0.05 to 2.0 wt%, more than 2.0 to 5.0 wt% Mn acts as a deoxidizing agent during the melting of steel, and is an element effective for reducing oxygen in 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. For these purposes, the addition of 0.05 to 2.0 wt% is sufficient. However, adding more than 2.0 wt% of Mn has the effect of significantly delaying the microstructural change due to the load of cyclic stress during rolling, similar to Al, and improves the rolling fatigue life. However, with the addition of more than 5.0 wt%,
Since a large amount of residual γ is generated and strength and dimensional stability are reduced, for this purpose, it is added in the range of more than 2.0 to 5.0 wt%.

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.

Cr: 0.05 to 2.5 wt%, more than 2.5 to 8.0 wt% Cr improves the hardenability and forms stable carbides.
It is a component that improves strength and wear resistance, and eventually improves rolling contact fatigue life. To obtain this effect, addition of 0.05 to 2.5 wt% is sufficient. Furthermore, when Cr is added in a large amount exceeding 2.5 wt%, it is effective in delaying the microstructural change caused by repeated stress loading and improving the rolling fatigue life in this aspect. . And the effect of Cr addition for this purpose is
If it exceeds 8.0 wt%, not only is it saturated, but rather, the amount of solid solution C during quenching is reduced and the strength is reduced. Therefore,
When it is added for this purpose, it should be more than 2.5 to 8.0 wt%.

Mo: 0.05 to 0.5 wt% Mo is an element that improves wear resistance by stabilizing residual carbides. In particular, the addition of 0.05 to 0.5 wt% increases the hardenability and contributes to the improvement of the strength after quenching and tempering, and the precipitation of stable carbide improves the wear resistance and rolling fatigue life.

Ni: 0.05 to 1.0 wt% Ni enhances the hardenability and increases the strength after quenching and tempering to improve the toughness, and thereby improves the rolling fatigue life. This effect is exhibited by the addition of 0.05 wt% or more, becomes saturated at 1.0 wt% and produces a large amount of residual γ, which leads to a decrease in hardness and eventually a deterioration in rolling contact fatigue life. Add within the range of 1.0 wt%.

Cu: more than 1.0 to 2.5 wt% This Mn is an element that plays the most important role in the present invention, and especially when this Cu is contained in a large amount exceeding 1.0 wt%, By delaying the above-mentioned microstructural changes caused by repeated stress loading during load rolling, the B ₅₀ rolling contact rolling fatigue life is significantly improved. However, if the amount exceeds 2.5 wt%, the effect of this addition saturates, and rather the forgeability deteriorates. Therefore, this Cu must be contained in the range of more than 1.0 to 2.5 wt%.

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: 0.05-0.07 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 grains 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 0.05 to 0.07 wt% of Al.

N: 0.0005 to 0.012 wt%, more than 0.012 to 0.05
wt% N combines with the nitride-forming element to refine the crystal grains and to form a solid solution in the matrix to enhance the strength after quenching and tempering.
Improves rolling fatigue life. 0.0005 for this purpose
Add within 0.012 wt%. Also, this N is 0.
When added in excess of 012 wt%, the rolling fatigue life in this aspect is improved by delaying the microstructural change that occurs due to repeated stress loading. However, if the amount exceeds 0.05 wt%, the workability decreases, so for this purpose, more than 0.012 to 0.05 wt% is added.

P ≦ 0.025 wt% P is desired to be as low as possible because it lowers the toughness and rolling contact fatigue life of the steel, and the upper limit of its allowable limit is
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.

As described above, the main components (Cu and Si, Mn, Cr, etc.) for improving the rolling fatigue life by delaying the microstructure change due to repeated stress loading and improving the rolling fatigue life by increasing the strength are described. Mo, Ni, B, Al,
N) and the reason for limiting C, P, S is explained,
The present invention further improves the rolling fatigue life under high load by adding one or more selected from V, Nb, W, Zr, Ta, Hf and Co. May be.

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, M are further added.
Even if g, P, Sn, As, etc. are added, the machinability can be easily improved without impeding the retardation property due to the change in microstructure due to the repeated stress load, which is the object of the present invention. Therefore, you may add as needed.

[0031]

EXAMPLE Steels having the compositions shown in Tables 3, 4, and 5 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. And for each of the above test pieces,
A test of B ₅₀ rolling contact fatigue life, which is the average bearing life, was conducted.
This B ₅₀ rolling contact fatigue life test uses a radial type rolling contact fatigue life tester and the Hertz maximum contact stress: 600 kgf /
The test was performed under the condition of mm ² , cyclic stress number of about 46500 cpm. The test results are summarized on the probability paper according to the Weibull distribution, and the average life of steel No. 1 (SUJ2, which is the conventional steel) (cumulative damage probability: 50%, the total number of loads before peeling) is set to 1, and others The steel grades were evaluated in comparison. The evaluation results are also shown in Tables 3, 4, and 5, respectively.

[0032]

[Table 3]

[0033]

[Table 4]

[0034]

[Table 5]

As is clear from the results shown in Tables 3, 4, and 5, steel material No. 2 having a C content in the steel outside the scope of the present invention, steel material No. 3 having a Cu content in the steel outside the scope of the present invention, and The average life of steel material No. 4 in which the amount of O in the steel is outside the range of the present invention is lower than that of the conventional steel (steel material No. 1). On the other hand, the average life of steel materials No. 5 to 47, which are steels of the present invention, is
It is 2.6-58.9 times better than 4). That is,
The addition of Cu to the bearing steel significantly delays the microstructural change,
As a result, it can be seen that it effectively acted to improve the rolling fatigue life.

Above all, in the case of Steel Nos. 32 to 47 in which, in addition to Cu, a life-improving component due to strength increase and a life-improving component due to delay of microstructural change were added together, the above average life (B ₅₀ rolling fatigue life) showed a life ratio as high as 23 times at the lowest.

[0037]

As described above, according to the present invention,
Basically, by using a bearing steel with a high Cu content of more than 1.0 wt%, it is possible to achieve an improvement in rolling contact fatigue life by delaying the microstructural change due to repeated stress loading, and for bearings with long life. Steel can be provided. Therefore, it is necessary to further reduce the oxygen content in steel or control the composition, shape, and distribution state of oxide-based nonmetallic inclusions present in steel, which was indispensable under the conventional technology. It is not necessary to improve or construct steelmaking equipment. 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]

FIG. 1 is a micrograph of a metal structure showing a state of microstructure change that occurs under repeated stress loading.

FIG. 2 is an explanatory diagram showing the influence of Cu 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%, Cu: more than 1.0 to 2.5
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 loading. 2. C: 0.5 to 1.5 wt%, Cu: more than 1.0 to 2.5
wt%, O: 0.0020 wt% or less, Si: 0.05
~ 0.5 wt%, Mn: 0.05 ~ 2.0 wt%, Cr: 0.05 ~ 2.5 wt
%, Mo: 0.05 to 0.5 wt%, Ni: 0.05 to 1.0 wt%,
B: 0.0005 to 0.01 wt% Al: 0.005 to 0.07 wt% and N: 0.
[0005] A bearing steel containing one or more selected from the range of 0.01 to 0.012 wt% and the balance being Fe and unavoidable impurities and having excellent delay characteristics of microstructure change due to repeated stress loading. 3. C: 0.5 to 1.5 wt%, Cu: more than 1.0 to 2.5
wt%, O: 0.0020 wt% or less, Si: more than 0.5 to 2.5 wt%, Mn: more than 2.0 to 5.0 wt%, Cr: more than 2.5 to 8.0 w
t%, N: more than 0.012 ~ 0.050 wt%, V: 0.05 ~ 1.0 wt%,
Nb: 0.05 to 1.0 wt%, W: 0.05 to 1.0 wt%, Zr: 0.0
2 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% selected from the group consisting of one or more, and the balance being Fe and unavoidable impurities, the balance of which is excellent in delay characteristics of microstructural change due to repeated stress loading. . 4. C: 0.5 to 1.5 wt%, Cu: more than 1.0 to 2.5
wt%, O: 0.0020 wt% or less, Si: 0.05-
0.5 wt%, Mn: 0.05 to 2.0 wt%, Cr: 0.05 to 2.5 wt%,
Mo: 0.05 to 0.5 wt%, Ni: 0.05 to 1.0 wt%, B: 0.
0005-0.01wt% Al: 0.005-0.07wt% and N: 0.0005-
It contains one or more selected from 0.012 wt%, and further, Si: more than 0.5 to 2.5 wt%, Mn:
Over 2.0 ~ 5.0 wt%, Cr: over 2.5 ~ 8.0 wt%, N: 0.012
Super ~ 0.050 wt%, V: 0.05 ~ 1.0 wt%, Nb: 0.05 ~ 1.0
wt%, W: 0.05 to 1.0 wt%, Zr: 0.02 to 0.5 wt%, T
a: 0.02-0.5 wt%, Hf: 0.02-0.5 wt% and Co: 0.0
Any one or two selected from 5 to 1.5 wt%
A bearing steel that contains more than one type of material, and the balance is Fe and unavoidable impurities, and that is excellent in delay characteristics of microstructural changes due to repeated stress loading.