JPH06287696A - Bearing steel excellent in delaying property in change of microstructure caused by repeated stress load - Google Patents

Abstract

PURPOSE:To produce bearing steel small in the change of the microstructure caused by repeated stress loads under severe using conditions. CONSTITUTION:The bearing steel contg., particularly, by weight, 0.5 to 2.0% Mo and one or >=two kinds selected from >0.5 to 2.5% Si, >1.0 to 3.0% Ni, >0.012 to 0.050% N, 0.05 to 1.0% V, 0.05 to 1.0% Nb, 0.05 to 1.0% W, 0.02 to 0.5% Zr, 0.02 to 0.5% Ta, 0.02 to 0.5% Hf and 0.05 to 1.5% Co as B50 high load rolling fatigue life improving components for promoting the delay in the change of the microstructure caused by repeated stress loads and in which the maximum grain size of oxide non-metallic inclusions is regulated to <=8mum is 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 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 essential to reduce the amount of oxygen in the steel, but this has already reached the limit, and expensive melting There is a problem in that it requires a large amount of financial burden because it requires installation of equipment or significant improvement of conventional equipment. Further, according to the recent research conducted by the present inventors,
The factors that determine the rolling life are phenomena that have been generally discussed in the past; that is, 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. It turns out that there is. I mean,
Even if the decarburization layer and non-metallic inclusions are simply reduced under the conventional technology, a great effect can be obtained in improving the rolling contact fatigue life of the bearing, especially under severe conditions such as high load or high temperature. Because I experienced a lot of things that didn't happen. 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. It was found that fatigue delamination (Fig. 1 (b)) occurs from the changing part, leading to bearing life. Furthermore, the severer bearing usage environment, that is, higher surface pressure (smaller size) and higher operating temperature, shortens the number of rolling cycles until these microstructural changes occur, and the conventional bearing steel SUJ2 has a significantly longer bearing life. It was found that That is, as for the bearing life, it is not sufficient to control only the decarburized layer and non-metallic inclusions as in the prior art. For example, if the amount and size of non-metallic inclusions are simply reduced, It is not possible to delay the time until the microstructure change occurs under the rolling contact surface. 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 microstructure change occurring during the use of the bearing under high load in order to improve the rolling contact fatigue life characteristics under severe operating conditions. The object is to provide a bearing steel capable of significantly improving the bearing life by suppressing the maximum grain size of non-metallic inclusions to be small.

[0007]

The inventors of the present invention have newly focused on the "microstructure change delay characteristic" as a factor that determines the bearing life based on the above-mentioned findings. In order to improve these characteristics, of course, a new alloy design (ingredient composition) is necessary for that purpose, and with the recognition that it is impossible to further improve the life of the bearing without realizing this, Various experiments and studies were conducted. As a result, they have found that, if an appropriate amount of Mo is contained, the above-mentioned microstructural change generated under the rolling contact surface due to repeated stress loading can be significantly delayed, and the present invention has been conceived.

That is, the bearing steel of the present invention has the following essential constitution. (1) C: 0.5 to 1.5 wt%, Mo: more than 0.5 to 2.0 wt%, the balance consisting of Fe and unavoidable impurities, and the maximum particle size of oxide-based nonmetallic inclusions is 8 μm or less, Bearing steel excellent in delay characteristics of microstructure change due to repeated stress load (first invention). (2) C: 0.5 to 1.5 wt%, Mo: more than 0.5 to 2.0 wt%, Si: 0.05 to 0.5 wt%, 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 0.01wt%, Al: 0.005 to 0.07wt% and N: 0.00
05-0.012 wt% selected from the group consisting of one or more selected from the group consisting of Fe and inevitable impurities, and the maximum particle size of oxide-based non-metallic inclusions is 8 μm or less, repeated Bearing steel excellent in delay characteristics of microstructure change due to stress load (second invention). (3) C: 0.5 to 1.5 wt%, Mo: more than 0.5 to 2.0 wt%, Si: more than 0.5 to 2.5 wt%, Ni: more than 1.0 to 3.0
wt%, N: more than 0.012 to 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
~ 0.5 wt%, Ta: 0.02 ~ 0.5 wt%, Hf: 0.02 ~ 0.5 wt
%, And Co: 0.05 to 1.5 wt% selected from the group consisting of 1 or 2 or more, the balance being Fe and inevitable impurities, and having a maximum particle size of oxide-based nonmetallic inclusions of 8 μm. The following is a bearing steel with excellent delay characteristics for microstructural changes due to repeated stress loading (third invention). (4) C: 0.5 to 1.5 wt%, Mo: more than 0.5 to 2.0 wt%, Si: 0.05 to 0.5 wt%, 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
~ 1 or 2 selected from 0.012 wt%
More than 0.5, Si: more than 0.5 ~ 2.5 wt%, N
i: more than 1.0 to 3.0 wt%, N: more than 0.012 to 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: 0.02-0.5 wt%, Ta: 0.02-0.5 wt%, H
f: 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
Bearing steel, which consists of unavoidable impurities and has a maximum grain size of oxide-based non-metallic inclusions of 8 μm or less, and which is excellent in delaying the microstructural change due to repeated stress loading (4th
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 SUJ 2 (C: 1.01wt%, Si: 0.24wt%, Mn: 0.46wt
%, Cr: 1.32wt%, N: 0.0042wt%, O: 0.0015wt%)
And two materials with a large amount of Mo added (C: 1.00wt%, Si: 0.20wt%, Mn: 0.42wt%, M
o: 0.80wt%, O: 0.0007wt%, N: 0.0032wt%) (C: 1.00wt%, Si: 0.25wt%, Mn: 0.43wt%, M
o: 0.79wt%, O: 0.0042wt%, N: 0.0040wt%) (C: 1.03wt%, Si: 0.21wt%, Mn: 0.41wt%, M
o: 1.52wt%, O: 0.0008wt%, N: 0.0035wt%) (C: 0.98wt%, Si: 0.22wt%, Mn: 0.42wt%, M
o: 1.48 wt%, O: 0.0018 wt%, N: 0.0038 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 B ₁₀ (10% cumulative failure probability), which is considered to be a numerical value showing the improvement of rolling fatigue life due to the increase of material strength affected by the control of non-metallic inclusions, 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 microstructure change due to repeated stress load during high load rolling, was determined.

As a result, as shown in Table 1, in the case of simply adding a large amount of Mo without controlling inclusions, the improvement in the B ₁₀ value was small,
It can be seen that the B ₅₀ value shows a considerably high value and is significantly improved. That is, the average life of the bearing was about twice as great as that of SUJ 2 at the B ₁₀ value and about 7 times as much as that of the B ₅₀ value. On the other hand, when the maximum grain size of non-metallic inclusions is controlled together with the addition of a large amount of Mo, a significant improvement effect on the retardation property of the microstructure change generated during high load rolling is exhibited, and further B _As shown in the _10- value, the effect of improving the peeling caused by the non-metallic inclusions was recognized. In particular, despite the high oxygen content in the steel, the B ₁₀ value is about 9 times better due to inclusion control, and the delay in microstructural change and the refinement of inclusions act to improve this B ₁₀ value. You can see that

[0012]

[Table 1]

FIG. 2 summarizes the above experimental results and is a schematic diagram showing the relationship between the bearing life due to the particle size of non-metallic inclusions and the life due to microstructural changes. As is clear from this figure, the cumulative damage probability is 10% as in the past.
The bearing life indicated by the B ₁₀ value (hereinafter, referred to as “B ₁₀ rolling contact fatigue life”) cannot be expected to have a great effect only by adding a large amount of Mo, but it is possible to control non-metallic inclusions. The results of the above also show a remarkable improvement effect.
On the other hand, bearing life indicated by B ₅₀ value with cumulative damage probability of 50%
(Hereinafter, this is referred to as “B ₅₀ high load rolling contact fatigue life”), the effect of improvement is extremely remarkable even if only a large amount of Mo is added, regardless of the control of non-metallic inclusions, and microstructural change generation occurs. It can be seen that it is effective in significantly improving the bearing life under the environment.

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

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%, machinability and forgeability will deteriorate, so it is limited to the range of 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%. Also, this Si is
Addition of more than 0.5 wt% has the effect of delaying the microstructural change under cyclic stress loading and improving rolling fatigue life. However, if the content exceeds 2.5 wt%, the effect saturates, but the workability and toughness decrease, so in order to further improve the microstructural change retardation property, it exceeds 0.5 to 2.5 wt%. Is effective.

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. For this purpose, Mn is added within the range of 0.05 to 2.0 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 deposited, resulting in a decrease in strength and a loss of dimensional stability.
If this effect is expected, it is necessary to add it in the range of more than 1.0 to 3.0 wt% because it increases the cost.

Mo: more than 0.5 to 2.0 wt% Mo is an element that basically improves wear resistance by stabilizing residual carbides. In particular, it enhances hardenability and improves strength after quenching and tempering. In addition to contributing, it improves wear resistance and rolling contact fatigue life by precipitation of stable carbide. However, in the present invention, this Mo plays a more important role, and particularly when a large amount is added such that the addition amount exceeds 0.5 wt%, the effect of delaying the microstructure change due to the loading of the above-mentioned cyclic stress is remarkable. And improve rolling contact fatigue life in this respect. But the amount
If it exceeds 1.5 wt%, the machinability and forgeability will be reduced, and it will cause cost increase, so it is necessary to add it in the range of more than 0.5 to 2.0 wt%.

Cu: 0.05-1.0 wt% Cu improves rolling fatigue life by increasing quenching and strengthening after quenching and tempering. This effect becomes remarkable when 0.05 wt% is added, and becomes saturated when it exceeds 1.0 wt%, so it is added in the range of 0.05 to 1.0 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: 0.005 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 grains and contribute to the improvement of the toughness of the steel. Further, it also effectively works 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%, 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 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.

As described above, main components (Mo and Si, Mn, Ni, for improving rolling fatigue life by delaying microstructural changes due to repeated stress loading and improving rolling fatigue life by increasing strength). The reason for limiting Cu, Al, B, N) and C, P, S has been described, but the present invention further includes any one selected from V, Nb, W, Zr, Ta, Hf and Co. You may make it improve the rolling contact fatigue life at the time of high load by adding 2 or more types.

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.

Next, in the present invention, in addition to the above-mentioned compositional limitation, the morphology (size) of the oxide-based nonmetallic inclusions in the steel is controlled, so that the above-mentioned B ₁₀ rolling contact fatigue life is mainly obtained. It was decided to further improve.

Therefore, first of all, the present inventors have made two kinds of materials having different amounts of oxide-based non-metallic inclusions and component compositions:
Using a high carbon chromium bearing steel (JIS-SUJ2) (A) and a bearing steel (B) with a composition within the above applicable range, the maximum diameter of oxide non-metallic inclusions in the steel and B ₁₀ rolling contact fatigue life I investigated the relationship with. As a result, as shown in FIG. 3, regardless of the amount or composition of oxide nonmetallic inclusions in the steel, when the maximum diameter of the nonmetallic inclusions exceeds 8 μm, the B ₁₀ rolling contact fatigue life is markedly reduced. From this, it is necessary for the bearing steel of the present invention to have a maximum grain size of 8 μm or less.

[0031]

[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. The test for non-metallic inclusions is 400 times 8
The oxide-based non-metallic inclusions in the field of view of 00 were measured, the maximum diameter of the inclusions in each field of view was summarized on the Gumbel probability paper, and the extreme value equivalent to 50,000 mm ² was calculated. The maximum particle size of inclusions was used. The rolling fatigue life test was conducted using a radial type rolling fatigue life tester under the conditions of Hertz maximum contact stress: 600 kgf / mm ² and repeated stress number of about 46500 cpm. The test results are summarized on the probability paper as being in accordance with the Weibull distribution and the steel material No. 1 (JIS-SuJ
The average life of (2) (cumulative damage probability: total load count until peeling at 10% and 50%) was set to 1 and evaluated in comparison with other steel types. The evaluation results are also shown in Tables 3 and 4, respectively.

[0032]

[Table 3]

[0033]

[Table 4]

As is clear from the results shown in Tables 3 and 4,
Is the B ₅₀ rolling fatigue life of steel material No. 6 in which the amount of C in steel is outside the scope of the present invention and steel material No. 6 in which the amount of Mo in steel is outside the scope of the present invention is the same as that of conventional steel (steel material No. 1)? Rather bad. Further, in No. 4 in which the maximum inclusion diameter exceeds 8 μm, the B ₁₀ rolling contact fatigue life was poor. On the other hand, the steel materials Nos. 6 and 7, which are the first invention steels, have B ₁₀ and B ₅₀ values that are 5 to 7 times better than the conventional steel (steel material No. 1). That is, it can be seen that the addition of a large amount of Mo to the bearing steel significantly retarded the microstructural change and effectively controlled the improvement of all rolling fatigue life of the bearing by controlling the maximum inclusion diameter.

Among them, in addition to Mo, Si, W, V, Nb, Zr,
In the case of adding Ta, Hf, Co, N alone or in combination with them (No. 3 invention steel) No. 18 to 28, the above average life (B ₅₀ rolling contact fatigue life) should be further improved. Was confirmed.

Further, the second invention examples (Nos. 9 to 17) in which the life improving component due to the increase in strength is added in accordance with the control of the inclusion particle size (No. 9 to 17), the B ₁₀ bearing life is extremely long. It showed a high degree of improvement. Further, in the case of the fourth invention steel in which all the life improving components are selectively added, the life improving tendency becomes more remarkable.

[0037]

As described above, according to the present invention,
Basically, by using a bearing steel with a high Mo content of more than 0.5 to 2.0 wt%, the rolling fatigue life is improved by delaying the microstructural change due to repeated stress loading, and in this respect Steel for bearings with a long life can be provided. Moreover, by increasing the material strength by controlling the particle size of the non-metallic inclusions, it is possible to improve the rolling fatigue life in this aspect. The development of the bearing steel according to the present invention makes it possible to downsize the rolling bearing and further increase 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 Mo on the bearing life due to inclusions and the bearing life due to microstructural changes.

FIG. 3 is a graph showing the relationship between the maximum diameter of non-metallic inclusions and bearing rolling fatigue life.

 ─────────────────────────────────────────────────── ─── 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: 0.5-1.5 wt%, Mo: over 0.5-2.0 wt
%, The balance consisting of Fe and unavoidable impurities, and the maximum grain size of oxide-based non-metallic inclusions is 8 μm or less. A bearing steel excellent in delay characteristics of microstructure change due to repeated stress loading. 2. C: 0.5 to 1.5 wt%, Mo: more than 0.5 to 2.0
wt%, Si: 0.05 to 0.5 wt%, Mn: 0.
05 to 2.0 wt%, Ni: 0.05 to 1.0 wt%, Cu: 0.05 to 1.0 w
Any one selected from t%, B: 0.0005 to 0.01 wt%, Al: 0.005 to 0.07 wt% and N: 0.0005 to 0.012 wt%
Or 2 or more, the balance consisting of Fe and unavoidable impurities, and the maximum particle size of oxide-based non-metallic inclusions is 8
A bearing steel with a micrometer or less, which is excellent in delay characteristics of microstructure change due to repeated stress loading. 3. C: 0.5-1.5 wt%, Mo: over 0.5-2.0
wt%, Si: more than 0.5 to 2.5 wt%, Ni: 1.0
Super ~ 3.0 wt%, N: 0.012 Super ~ 0.050 wt%, V: 0.05 ~
1.0 wt%, Nb: 0.05 to 1.0 wt%, W: 0.05 to 1.0 wt%,
Zr: 0.02 to 0.5 wt%, Ta: 0.02 to 0.5 wt%, Hf: 0.02
~ 0.5 wt%, and Co: 0.05-1.5 wt%, and any one or more selected from the group consisting of Fe and unavoidable impurities and the maximum grain size of oxide-based non-metallic inclusions. A bearing steel with a diameter of 8 μm or less, which is excellent in delay characteristics of microstructure change due to repeated stress loading. 4. C: 0.5 to 1.5 wt%, Mo: more than 0.5 to 2.0
wt%, Si: 0.05-0.5 wt%, Mn: 0.05
~ 2.0 wt%, Ni: 0.05 ~ 1.0 wt%, Cu: 0.05 ~ 1.0 wt
%, B: 0.0005 to 0.01 wt%, Al: 0.005 to 0.07 wt%, and N: any one selected from 0.0005 to 0.012 wt% 1
Or more than two kinds, and further, Si: more than 0.5 to 2.
5 wt%, Ni: over 1.0 to 3.0 wt%, N: over 0.012 to 0.050 w
t%, 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 to 0.5
wt%, Hf: 0.02 to 0.5 wt% and Co: 0.05 to 1.5 wt%, and at least one selected from the group consisting of Fe and inevitable impurities, and an oxide-based non-metal. A bearing steel with a maximum grain size of inclusions of 8 μm or less and excellent in delay characteristics of microstructural changes due to repeated stress loading.