JP3243329B2 - Bearing steel with excellent heat treatment productivity and delayed microstructure change 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 more particularly to an effect of suppressing the formation of a decarburized layer which occurs during heat treatment and a severe use environment of the bearing. This is a proposal for a bearing steel with excellent delay characteristics against microstructure change (deterioration) generated under rolling contact surfaces due to specific deterioration caused by repeated stress loading.

[0002]

2. Description of the Related Art Rolling bearings used in automobiles, industrial machines, and the like are conventionally known as high carbon chromium bearing steel (JI).
S: SUJ 2) is used the most. In general, bearing steel is one of the important properties to have a long rolling fatigue life, but it is considered that the factor affecting the rolling fatigue life is the largest effect of nonmetallic inclusions in steel. I was
Therefore, the mainstream of recent research has been to improve the bearing life by controlling the amount and size of nonmetallic inclusions by reducing the amount of oxygen in steel. For example, Japanese Unexamined Patent Publication Nos. Hei. 1-306542 and Hei. 3-1268 disclose developments aimed at further improving the rolling fatigue life of bearings.
There are proposals such as Japanese Patent Publication No. 39, which are techniques for controlling the composition, shape or distribution of oxide-based nonmetallic inclusions in steel. However, in order to produce bearing steel with a small amount of nonmetallic inclusions, it is necessary to install expensive smelting equipment or to significantly improve conventional equipment, resulting in a large economic burden.

Another move to improve the characteristics of the high-carbon bearing steel (JIS-SUJ2) is a study of workability, particularly suppression of the formation of a decarburized layer during heat treatment. Generally, the bearing steel specified in JIS-SUJ2 above is 0.95-1.
Since it contains 10wt% of C, it is very hard. Therefore, it is subjected to spheroidizing annealing to improve workability, and then molded, then quenched and tempered to provide a rolling bearing. The required strength and toughness were obtained. However, when such heat treatment for improving the characteristics is repeated many times, it is known that a "low C concentration region" called a decarburized layer is generated on the surface of the material due to a reaction between C and the atmospheric gas. I have. This decarburized layer is not only reduced in hardness of the rolling bearing but also causes deterioration in rolling contact fatigue life. Therefore, the decarburized layer is usually removed by cutting or grinding. As a result, the material yield and the productivity had to be reduced. On the other hand, conventionally, as means for preventing the formation of the decarburized layer, a method of controlling the carbon potential in the atmosphere gas in the furnace during the heat treatment and a method disclosed in JP-A-2-54717, A method of performing a carburizing treatment at an early stage of annealing has been proposed. However, since each of the above methods is based on cleaning the atmosphere during heat treatment or pre-treatment, not only does the heat treatment cost increase, but also setting of an appropriate gas composition according to the material composition, heat treatment time, and the like. There is a problem where such complicated operations are required.

[0004]

The present inventors have recently conducted various studies on the above-mentioned prior art. as a result,
Surprisingly, the factors that determine the rolling life are the above-described phenomena that have been generally discussed in the past; that is, the existence of the above-mentioned “non-metallic inclusions” and the “decarburized layer” (low C concentration region) generated during heat treatment. ) Was found to be due to factors other than generation. The reason is that simply reducing the amount of nonmetallic inclusions and decarburized layers under the conventional technology does not improve the rolling contact fatigue life of the bearing, especially under the severe conditions such as high load or high temperature. Has experienced many cases where significant effects cannot be obtained. From this,
He was convinced that there were other factors that govern the bearing life.

Therefore, the present inventors have investigated the bearing life in consideration of the recent bearing operating environment, particularly the cause of peeling of the rolling bearing. As a result, the shear stress generated during the contact rolling between the inner and outer races of the bearing, the rolling elements, and the rolling elements in accordance with the intensified use environment of the bearings causes the lower layer (surface layer) of the rolling contact surface to be formed as shown in FIG. As shown in the photograph of (a), it was found that a microstructure-change layer consisting of a band-like white product and a rod-like precipitate was generated. This microstructure-changed layer gradually grows as the number of rollings increases, and eventually, as shown in FIG. all right. In addition, the harsh operating environment of the bearing, that is, high surface pressure (small size) and increase in operating temperature, shorten the time required for these microstructure changes to occur, resulting in a significant reduction in bearing life. Ascertained. In other words, the life of the bearing due to the severe use environment is not sufficient simply by controlling the decarburized layer and the nonmetallic inclusions as in the prior art. For example, simply reducing the amount of non-metallic inclusions cannot delay the time required for the above-described microstructural change to occur under the rolling contact surface. As a result, they found that the bearing life could not be further improved.

Accordingly, an object of the present invention is to provide a remarkable improvement in the life of a bearing by delaying a change in microstructure which is expected to occur during use of the bearing under severe operating conditions, and
An object of the present invention is to provide a bearing steel capable of suppressing the formation of a decarburized layer during heat treatment and improving the heat treatment productivity (the effect of reducing the amount of processing removal).

[0007]

On the basis of the above-mentioned findings, the present inventors have newly focused on a "microstructure change delay characteristic" as a bearing life, and to improve this, of course, It is recognized that a new alloy design (composition composition) is necessary, and further improvement in bearing life cannot be achieved without realizing this. In addition, various alloys that suppress formation of a decarburized layer are realized. Experiments and examinations were conducted. As a result, surprisingly, if the proper amount of Si and Sb is added in combination, the above-mentioned microstructure change generated below the rolling contact surface due to repeated stress loading can be significantly delayed, and at the same time, the generation of decarburized layers during heat treatment is suppressed And developed the bearing steel of the present invention.

That is, the bearing steel of the present invention has the following gist configuration. (1) C: 0.5 to 1.5 wt%, Si: more than 0.5 to 2.5 wt%, Al: 0.005 to 0.07 wt%, Sb: 0.001 to less than 0.005 wt%, O: 0.0020 wt% or less, the balance being Fe A bearing steel excellent in heat treatment productivity including unavoidable impurities and in delaying microstructure change due to repeated stress load (first invention). (2) C: 0.5 to 1.5 wt%, Si: more than 0.5 to 2.5 wt%, Al: 0.005 to 0.07 wt%, Sb: 0.001 to less than 0.005 wt%, O: 0.0020 wt% or less, and Mn: 0.05-2.0 wt% , Ni: 0.05-1.0 wt% , Mo: 0.05-0.5 wt%, Cu: 0.05-1.0 wt%, B: 0.0005-0.01 wt% and N : 0.0005-0.012 wt% A bearing steel containing any one or more of the above, the balance being Fe and unavoidable impurities, and having excellent heat treatment productivity and excellent microstructure change delay characteristics due to repeated stress loading (second invention). (3) C: 0.5 to 1.5 wt%, Si: more than 0.5 to 2.5 wt%, Al: 0.005 to 0.07 wt%, Sb: 0.001 to less than 0.005 wt%, O: 0.0020 wt% or less, and Zr: 0.02 to 0.5 wt%, Ta: 0.02 to 0.5 wt%, Hf: 0.02 to 0.5 wt%, Co: 0.05 to 1.5 wt%, and N: any one or two selected from more than 0.012 to 0.050 wt% A bearing steel comprising the above, the balance being Fe and inevitable impurities, and having excellent heat treatment productivity and excellent microstructure change delay characteristics due to repeated stress loading (third invention). (4) C: 0.5 to 1.5 wt%, Si: more than 0.5 to 2.5 wt%, Al: 0.005 to 0.07 wt%, Sb: 0.001 to less than 0.005 wt%, O: 0.0020 wt% or less, and Mn: 0.05-2.0 wt% , Ni: 0.05-1.0 wt% , Mo: 0.05-0.5 wt%, Cu: 0.05-1.0 wt%, B: 0.0005-0.01 wt% , and N : 0.0005-0.012 wt% Zr: 0.02 to 0.5 wt%, Ta: 0.02 to 0.5 wt%, Hf: 0.02 to 0.5 wt%, Co: 0.05 to 1.5 wt%, and N: 0.012 Bearing steel containing one or more selected from super to 0.050 wt%, with the balance being Fe and unavoidable impurities, excellent in heat treatment productivity and excellent in microstructure change delay characteristics due to repeated stress loading (4th invention).

[0009]

The background that led to 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%, N: 0.0040wt%, O: 0.0012wt%)
And two materials to which Ni and Sb are added (C: 1.00 wt%, Si: 0.75 wt%, Al: 0.042 wt%,
Mn: 0.40 wt%, Cr: 1.33 wt%, O: 0.0009 wt%, Sb: 0.
0018 wt%, N: 0.0042 wt%) (C: 1.00 wt%,, Si: 1.58 wt%, Al: 0.048 wt
%, Mn: 0.38 wt%, Cr: 1.30 wt%, Ni: 2.5 wt%, O:
A test steel material (0.0008 wt%, Sb: 0.0040 wt%, N: 0.0032 wt%) was prepared. Next, after normalizing these test materials, spheroidizing normalizing, and performing each treatment of quenching and tempering, a cylindrical test piece of 15 mm φ × 22 mm and a 12 mm φ × 22 mm A test piece for a rolling fatigue test was prepared.

In the rolling fatigue life test, a rolling contact fatigue life tester of a radial type was used for the rolling fatigue test piece under a load condition of Hertz maximum contact stress: 60 kgf / mm ² , 46500 cpm. It was tested in. The results of the tests are plotted on a Weibull distribution establishment paper, by repeated stress load at B ₁₀ seen a numerical value indicating the improvement in rolling fatigue life due to an increase in the material strength (10% cumulative failure probability) a high load rolling microstructure changes seen a numerical value indicating the improvement in rolling fatigue life due to the occurrence to delaying B ₅₀ (50% cumulative failure probability) and was determined. In addition, for the test of the decarburized layer, after cutting the above cylindrical test piece vertically at the position of 10 mm in the height direction, corroded with nital, the maximum value of all decarburized layers on the circumference due to microstructure change ( Hereinafter, it is referred to as the “maximum decarburized layer”)
Was evaluated.

The results are shown in Table 1. As can be seen from the results shown in Table 1, the composite additive of Si and Sb, improvement for said B ₁₀ value is not so large, B ₅₀
Values are extremely high, and the average bearing life is SU
Compared to J 2 approximately doubled in B ₁₀ value, to exhibit improved even about doubled in B ₅₀ values were observed. In particular, a large amount of complex addition of Si and Sb has a remarkable effect on the delay characteristics of microstructure change generated during high-load rolling, and damage (life) correspondingly
Can be expected to be delayed. Regarding the maximum decarburized layer, SUJ2 was 0.10 mm, but 0.03 mm for those containing 0.0018 wt% of Sb and 0.01 mm for those containing 0.0040 wt% of Sb.
It was also found that the appropriate Sb content was effective in suppressing the generation of decarburized layers.

[0012]

[Table 1]

FIG. 2 summarizes the experimental results of the above-mentioned bearing rolling fatigue life, and shows the relationship between the bearing life caused by non-metallic inclusions and the life change caused by microstructural change. It is a schematic diagram. As is evident in this figure, the bearing life represented by the cumulative failure probability of 10% B ₁₀ value (hereinafter referred to as "B ₁₀ rolling contact fatigue life") does not merely improve by just adding a large amount of Si but, looking at the B ₅₀ value, this
The effect of adding a large amount of Si is extremely remarkable. Therefore, based on such knowledge, the inventors have calculated the cumulative damage probability
Bearing life indicated by 50% B ₅₀ value (hereinafter referred to as “B ₅₀
In order to improve the “high-load rolling fatigue life” and to suppress the growth of the decarburized layer during heat treatment, from the viewpoint of what kind of alloy design is effective, Range was determined.

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

Si: more than 0.5 to 2.5 wt% or less Si is basically used as a deoxidizing agent when smelting steel, and is solid-dissolved in a matrix and tempered due to an increase in softening resistance. It is effective as an element that enhances the later strength and improves the rolling fatigue life. However, in the present invention, when more than 0.5 wt% is added, there is an effect of delaying microstructure change under repeated stress load and improving rolling fatigue life. However, its content is 2.5wt%
If it exceeds, the effect is saturated, but the workability and toughness are reduced. Therefore, in order to further improve the microstructure change delay characteristics, it is effective to add more than 0.5 to 2.5 wt%.

Mn: 0.05-2.0 wt% Mn is an element that acts as a deoxidizing agent during melting of steel and is effective in reducing oxygen in steel. In addition, by improving the hardenability of steel, the toughness and hardness of the base martensite are improved, which effectively affects the rolling fatigue life. These effects require at least 0.05 wt% addition, while 2.0%
Since the effect saturates if the addition exceeds wt%, Mn is 0.05 to 2.
Add in the range of 0 nwt%.

[0017]

Ni: 0.05 to 1.0 wt% Ni has the effect of increasing the hardenability, increasing the strength after quenching and tempering, improving the toughness, and improving the rolling fatigue life. This effect is effective when Ni: 0.05 wt% is added, but the effect is saturated at 1.0 wt%.
Limited to the range of ~ 1.0 wt%.

Mo: 0.05 to 0.5 wt% Mo is an element which improves wear resistance by stabilizing residual carbides. Particularly, when 0.05 to 0.5 wt% is added, hardenability is increased to contribute to improvement in strength after quenching and tempering, and precipitation of stable carbides improves wear resistance and rolling fatigue life.

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

Sb: 0.001 to less than 0.005 wt% Sb is an element which plays an important role together with Al in the present invention. In particular, Sb contributes to an improvement in heat treatment productivity by suppressing the reaction between C in the surface layer of the steel material and the atmosphere gas to prevent the formation of a decarburized layer during the heat treatment. In addition, the addition of Al in combination with the suppression of the decarburized layer also has an effect on the delay of the change in microstructure, so that it is added positively. These two effects become remarkable when the Sb content is 0.001 wt% or more. However, even if added over 0.005 wt%, the effect is saturated, and in addition, hot workability and The toughness is deteriorated. Therefore, Sb is not 0.001~0.005 wt%
It was determined to be contained within the full range.

B: 0.0005 to 0.01 wt% B is added in an amount of 0.0005 wt% or more because B increases the strength after quenching and tempering due to the increase in hardenability and improves the rolling fatigue life. However, if added in excess of 0.01 wt%, the workability is degraded, 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 at the time of melting steel,
It combines with N in the steel to refine the crystal grains and contribute to improving the toughness of the steel. In addition, it effectively acts to improve the rolling fatigue life by increasing the strength after quenching and tempering. These effects cannot be obtained at less than 0.005 wt%. On the other hand, 0.07wt
%, The effects described above become saturated. Therefore, 0.005 to 0.07 wt% of Al is added.

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 dissolves in the matrix to increase the strength after quenching and tempering.
Improves rolling fatigue life. 0.0005 for this purpose
It is added within the range of ~ 0.012 wt%. This N is 0.
When added in excess of 012 wt%, rolling fatigue life is improved by delaying microstructural changes due to repeated stress. However, if the amount exceeds 0.05 wt%, the workability is reduced, and for this purpose, it exceeds 0.012 to 0.
Add 05 wt%.

P ≦ 0.025 wt% P is desirably as low as possible from the viewpoint of lowering the toughness and rolling fatigue life of steel.
0.025 wt%.

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

O: 0.0020 wt% or less O forms hard non-metallic inclusions, so even if the control of other components delays the microstructure change due to repeated stress loading, the 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 component for improving the rolling fatigue life by delaying the microstructure change due to the repeated stress load, the component for improving the rolling fatigue life by increasing the strength, and the formation of the decarburized layer to suppress the bearing The reason for limiting the components for improving the processability and productivity of the above has been described. Incidentally, in the present invention, Zr, Ta, Hf and Co
Any one or more selected from the above may be added to further improve the bearing life. Table 2 summarizes the preferable addition ranges of the above elements, the purpose of the addition, and the reasons for limiting the upper and lower limits.

[0029]

[Table 2]

In the present invention, in order to improve machinability, S, Se, Te, REM, Pb, Bi, Ca, Ti, Mg, P,
Addition of Sn, As, etc. does not hinder the retardation characteristics due to microstructure change due to the repeated stress load, which is the object of the present invention, and can easily improve machinability. May be added according to

[0031]

EXAMPLES Steel having the chemical composition shown in Tables 3 and 4 was melted in a converter and then continuously cast.
After the diffusion annealing of h, it was rolled into a 65 mmφ steel bar. Then
A cylindrical test piece of 15 mmφ × 20 mm and a test piece for rolling fatigue were collected from the D / 4 part of the steel bar by cutting. afterwards,
These test pieces were tested in the order of normalizing, spheroidizing annealing, quenching, and tempering without controlling the atmosphere (in the air atmosphere). Furthermore, the test piece for rolling fatigue
For the purpose of completely removing the decarburized layer, polishing and lapping of 1 mm or more were performed, and the test piece size was 12 mmφ × 22 mm. The depth of the decarburized layer after heat treatment was the maximum of all decarburized layers on the circumference by cutting a cylindrical test piece of 15 mm φ × 20 mm at a position of 10 mm perpendicular to the height direction, corroding with nital, and observing the microstructure. The value (hereinafter, referred to as “maximum decarburized layer”) was evaluated. The rolling fatigue life test was carried out using a radial type rolling fatigue life tester under the conditions of Hertz maximum contact stress: 600 kgf / mm ² and repetitive stress number: about 46500 cpm. The test results are summarized on a probability paper assuming a Weibull distribution, and the average life of steel No. 1 (cumulative failure probability: 50%
, The total number of loads until peeling occurred) was set to 1. The evaluation results are shown in Tables 3 and 4.

[0032]

[Table 3]

[0033]

[Table 4]

As apparent from the results shown in Tables 3 and 4,
Steel No. 3 in which C content in steel is out of the range of the present invention, Steel No. 4 in which Si content in steel is out of the range of the present invention steel, and steel material No. 5 in which O content in steel is out of the range of the present invention Has a maximum decarburized layer of 0.01 to 0.
The average life of the bearings is lower than that of the conventional steel (steel No. 1), though both have been improved compared to the conventional steel (No. 1). On the other hand, steel materials whose Sb content in the steel is out of the range of the present invention steel
No.2 in B ₅₀ life expectancy, although conventional steels have excellent about four times (steel No.1), the maximum decarburized layer 0.11mm the conventional example (SUJ
Not much improved compared to 2). The average life span indicated by B ₅₀ value of the steel No. 6, 7 are invention steels, also excellent about 5-6 times that of the conventional steel (steel No.1),
It can be seen that the addition of Si significantly delayed the microstructure change, and as a result, effectively acted to improve the rolling fatigue life. In addition, the maximum decarburized layer depth is 0.02 mm, which is far less than the conventional steel No. 1, and Sb is out of the proper range of the present invention.
Compared with No. 2, the improvement effect is remarkable, about 1/6.

Further, in addition to Si and Sb, Mn, Mo, Cr, Cu, A
Steel Nos. 8 to 10 and 12 to 14 to which at least one of l, B and N are added
It was found that not only the ₅₀ rolling contact fatigue life property was improved but also the maximum decarburized layer depth was remarkably improved to 0.02 mm or less.

Further, in addition to the Si and Sb, Zr, Ta, Hf, C
Steel No. 24 to which o and N are added more than the specified amount
In the case of 28, the average life (B ₅₀ rolling fatigue life) fit to the improvement of the heat treatment productivity also it was confirmed that further improved. The same is true for steel No. 30 , which is obtained by selectively adding all of the above-mentioned respective improving components recommended in the present invention, and is effective in improving both bearing steel life and heat treatment productivity. I understood.

[0037]

As described above, according to the present invention,
Basically, by adding Sb and using a high Si composite addition bearing steel of more than 0.5 to 1.0 wt%, the processing load during heat treatment can be reduced (Sb addition effect), and at the time of high load rolling fatigue life introduces a delay of microstructural changes due to repeated stress loads (high Sl containing effect), to achieve an improvement in the so-called B ₅₀ high load rolling contact fatigue life, provide a steel for high heat treatment productivity of long life bearing can do. Therefore, it is necessary to further reduce the oxygen content in steel or control the composition, shape, and distribution of oxide-based nonmetallic inclusions present in steel, which were 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 makes it possible to reduce the size of the rolling bearing and further increase the operating temperature of the bearing.

[Brief description of the drawings]

1 (a) and 1 (b) are micrographs of a metal structure showing a change in microstructure generated under repeated stress loading.

FIG. 2 is an explanatory diagram showing the effect of Si addition on bearing life caused by non-metallic inclusions and bearing life caused by microstructure change.

────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Kenichi Amano 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Engineering Co., Ltd. Technology Research Division (56) References JP-A-3-122255 (JP, A) JP-A-49-47212 (JP, A) JP-A-2-30733 (JP, A) JP-A-6-271977 (JP, A) (58) Fields investigated ( Int.Cl. ⁷ , DB name) C22C 38/00-38/60

Claims (4)

(57) [Claims] (1) C: 0.5 to 1.5 wt%, Si: more than 0.5 to 2.5
wt%, Al: 0.005 to 0.07 wt%, Sb: 0.001 to 0.005 wt%
Less than, O: Bearing steel containing 0.0020 wt% or less, with the balance being Fe and unavoidable impurities, and having excellent heat treatment productivity and excellent microstructure change delay characteristics due to repeated stress loading. 2. C: 0.5 to 1.5 wt%, Si: more than 0.5 to 2.5 wt%, Al: 0.005 to 0.07 wt%, Sb: 0.001 to less than 0.005 wt%, O: 0.0020 wt% or less. Mn: 0.05~2.0 wt%, N i : 0.05~1.0 wt%, Mo: 0.05~0.5 wt%, Cu: 0.05~1.0 wt%, B: 0.0005~0.01wt% and N: 0.0005-.012 of wt% A bearing steel containing one or more selected from the group consisting of Fe and unavoidable impurities, and having excellent heat treatment productivity and excellent microstructure change delay characteristics due to repeated stress loading. 3. C: 0.5-1.5 wt%, Si: more than 0.5-2.5
wt%, Al: 0.005 to 0.07 wt%, Sb: 0.001 to 0.005 wt%
, O: 0.0020 wt% or less, and Zr: 0.02-
0.5 wt%, Ta: 0.02-0.5 wt%, Hf: 0.02-0.5 wt%,
Co: 0.05-1.5 wt% and N: more than 0.012-0.050 wt%
A bearing steel containing at least one selected from the group consisting of Fe and the balance consisting of Fe and unavoidable impurities, and having excellent heat treatment productivity and excellent microstructure change delay characteristics due to repeated stress loading. 4. C: 0.5 to 1.5 wt%, Si: more than 0.5 to 2.5 wt%, Al: 0.005 to 0.07 wt%, Sb: 0.001 to less than 0.005 wt%, O: 0.0020 wt% or less. Mn: 0.05~2.0 wt%, N i : 0.05~1.0 wt%, Mo: 0.05~0.5 wt%, Cu: 0.05~1.0 wt%, B: 0.0005~0.01wt% and N: 0.0005-.012 of wt% And Zr: 0.02 to 0.5 wt%, Ta: 0.02 to 0.5 wt%, Hf: 0.02 to 0.5 wt%, Co: 0.05 to 1.5 wt%, and N: Bearings containing one or more selected from over 0.012 to 0.050 wt%, with the balance being Fe and unavoidable impurities, excellent in heat treatment productivity and excellent in microstructure change delay characteristics due to repeated stress loading steel.