JP3379783B2 - Bearing steel with excellent microstructure change delay characteristics due to repeated stress loading - Google Patents

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. 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 I could not get results. 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 shearing stress generated during the rolling contact between the inner and outer races of the bearing and the rolling element, a band-shaped white product was formed on the lower layer (surface layer) of the rolling contact surface as shown in Fig. 1 (a). A microstructure-changed layer consisting of rod-shaped precipitates and rod-shaped precipitates gradually grows as the number of rolling increases. At the end, fatigue delamination occurs from the microstructure-changed part as shown in Fig. 1 (b). It has been found that this will lead to bearing life. Furthermore, the harsh bearing operating environment, that is, higher surface pressure (miniaturization) and higher operating temperature, will shorten the number of rolling cycles until these microstructural changes occur, leading to a marked reduction in bearing life. I found him. That is, the bearing life under severe conditions is
It is not enough to simply control the decarburized layer and the 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 provide a bearing steel capable of delaying a microstructural change expected to be produced during the use of the bearing under severe operating conditions, and thus providing a significant improvement in bearing life. To do.

[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, they have surprisingly found that the above-mentioned microstructural change generated under the rolling contact surface due to repeated stress loading can be remarkably delayed only by adding an appropriate amount of W, and have conceived the present invention bearing steel.

That is, the bearing steel of the present invention has the following essential constitution. (1) C: 0.5 to 1.5 wt%, W: 0.05 to 1.0 wt%, O: 0.002
A bearing steel containing 0 wt% or less, and the balance being Fe and inevitable impurities, which is excellent in delay characteristics of microstructure change due to repeated stress load (first invention). (2) C: 0.5 to 1.5 wt%, W: over 0.50 to 1.0 wt%, O: 0.0
020wt% or less, and Si: 0.05-0.5wt%, M
n: 0.05 to 2.0 wt%, Mo: 0.05 to 0.5 wt%, Ni: 0.05 to 1.0
wt%, Cu: 0.05 to 1.0 wt%, B: 0.0005 to 0.01 wt%, A
l: 0.005 to 0.07 wt% and N: 0.0005 to 0.012 wt% and any one or more selected from the group consisting of Fe and unavoidable impurities, and the balance consisting of Fe and inevitable impurities. Bearing steel with excellent characteristics (2nd
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 W added (C: 1.00 wt%, Si: 0.33 wt%, Mn: 0.52 wt%,
O: 0.0009wt%, W: 0.32wt%, N: 0.0048wt%) (C: 1.00wt%, Si: 0.30wt%, Mn: 0.52wt%,
O: 0.0009 wt%, W: 0.77 wt%, N: 0.0046 wt%) 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) 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 W-added material, the B ₁₀ value is not so much improved, but the B ₅₀ value is remarkably high, and the average bearing life is 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 6 times. In particular, the addition of W has a remarkable effect on the delay characteristic of the microstructure change generated during high load rolling, and it can be expected that the damage (lifetime) 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"), simply adding W
Not as effective as expected. But break this cumulatively
B with a loss probability of 50% ₅₀Bearing life indicated by the value (hereinafter
"B₅₀In terms of "rolling fatigue life"), W was added.
In this case, the effect becomes extremely remarkable, and the so-called micro
Bearing life under microstructure change generation environment (B₅₀)
It was found that extremely good results were obtained.

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

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: 0.05 to 0.5 wt%, more than 0.5 to 2.5wt% Si is used as a deoxidizer during the melting of steel and also as a base material.
Quenching and tempering due to an increase in softening resistance
As an element that increases the strength after rolling and improves rolling contact fatigue life
Is effective. Containing Si added for these purposes
The amount is in the range of 0.05 to 0.5 wt%. Furthermore, this Si
is 0.5wt% Addition, under repeated stress loading
The microstructure change of the
It has the effect of increasing the quality. However, its content is 2.5 wt%
If it exceeds, the effect will be saturated, but the workability and toughness will be reduced.
As a result, it is possible to further improve the microstructure change delay property.
For the above, it is effective to add more than 0.5 ~ 2.5wt%
is there.

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: more than 2.5 to 8.0 wt% When Cr is added in a large amount exceeding 2.5 wt%, the microstructure change due to repeated stress loading is delayed and the rolling fatigue life in this aspect is improved. Is an effective element.
The effect of Cr added for this purpose is 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, Cr is
Add within the range of 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%, more than 1.0 to 3.0 wt% Ni increases the hardenability, thereby increasing the strength after quenching and tempering and improving the toughness as well as the rolling 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.

W: 0.05 to 1.0 wt%, Over 0.50 ~ 1.0wt% W is a component that plays the most important role in the present invention.
And its content depends on the cyclic stress loading described above.
It has the effect of delaying the onset of microstructural changes, while
Has the effect of improving wear resistance by stabilizing residual carbides.
Also has. These actions and effects are due to the inclusion of 0.05 wt%.
Becomes remarkable. On the other hand, this amount exceeds 1.0 wt%
Then, the amount of solid solution C decreases during quenching, leading to a decrease in strength.
Others, B mentioned above_Ten, B₅₀Rolling fatigue affecting both values
The effect on improving the working life is saturated, so 0.05-1.0w
t%(Over 0.50 to 1.0 wt%)It is contained in the range of.

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 deoxidizing agent during the melting of steel, and at the same time, it combines with N in the steel to refine the crystal grains and contribute to the improvement of the toughness of the steel. Also, since it effectively works to improve the rolling fatigue life by increasing the strength after quenching and tempering,
Although 0.005 wt% is added, the effect is saturated when it exceeds 0.07 wt%, so the range is limited to 0.005 to 0.07 wt%.

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%, rolling fatigue life is improved by delaying microstructural change due to repeated stress. However, if the amount exceeds 0.05 wt%, the workability decreases, so for this purpose it exceeds 0.012 to 0.
Add 05wt%.

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.

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, the main components (W and Si, Mn, Cr, etc.) for improving the rolling fatigue life by delaying the microstructural change due to the repeated stress loading and improving the rolling fatigue life by increasing the strength. Mo, Ni, Cu, B, A
l, N, O) and C, P, S have been described above, but in the present invention, by further adding any one or more selected from Zr, Ta, Hf and Co. Alternatively, the rolling fatigue life under high load may be improved.

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.

[0032]

[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 No. 1 (conventional steel SUJ2) (cumulative failure rate: 50%, the total number of loads before delamination) 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.

[0033]

[Table 3]

[0034]

[Table 4]

As is clear from the results shown in Tables 3 and 4,
Average of steel material No. 2 whose C content in the steel is outside the scope of the present invention, steel material No. 3 whose W content in the steel is outside the scope of the present invention, and steel material No. 4 whose O content in the steel is outside the scope of the present invention The service life is lower than that of conventional steel (steel material No. 1). On the other hand, the average life of the steel material No. 5 of the present invention is about twice as long as that of the conventional steel (steel material No. 1). That is, it can be seen that the addition of W to the bearing steel significantly retarded the microstructural change, and as a result, effectively acted to improve the rolling fatigue life.

Among them, in addition to W, Si, Mn, Cr, Mo, Z
Steel No. 7 to 4 with positive addition of any one or more of rolling fatigue life improving components such as r, Ta, Hf, Ni, Cu, Co, Al, N.
In the case of 0, the invention examples and reference examples shown in Tables 3 and 4 above
As is clear from the above, it was confirmed that the average life (B ₅₀ rolling contact fatigue life) is further improved.

[0037]

As described above, according to the present invention,
Basically, by using a bearing steel containing 0.05 to 1.0 wt% W, the rolling fatigue life is improved by delaying the microstructural change due to repeated stress loading, and a bearing with a long life is achieved. 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]

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 effect of W on the bearing life due to inclusions and the bearing life due to microstructural changes.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shinichi Amano 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Works Ltd. Technical Research Division (56) Reference JP-A-3-271319 (JP, A) Kaihei 3-138332 (JP, A) JP-A-3-254342 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. C: 0.5 to 1.5 wt%, W: 0.05 to 1.0 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%, W: more than 0.50 to 1.0 wt.
%, O: 0.0020 wt% or less, and Si: 0.05 to
0.5wt%, Mn: 0.05 to 2.0wt%, Mo: 0.05 to 0.5wt%, N
i: 0.05 to 1.0 wt%, Cu: 0.05 to 1.0 wt%, B: 0.0005 to
0.01wt%, Al: 0.005-0.07wt% and N: 0.0005-0.012
A bearing steel containing one or more selected from wt% and the balance being Fe and unavoidable impurities, and having excellent delay characteristics of microstructure change due to repeated stress loading.