JPH0873988A - Bearing steel excellent in heat treatment productivity as well as in property of retarding microstructural change due to repeated stress load - Google Patents

Abstract

PURPOSE: To inhibit the occurrence of microstructural changes due to repeated stress load under severe service conditions, further to improve heat treatment productivity by preventing the formation of a decarburized layer at the time of heat treatment, and to inexpensively produce a bearing steel with long service life. CONSTITUTION: This steel has a composition consisting of, by weight, 0.1-1.5% C, >1.0-2.0% Mo, >0.005-0.015% Sb, and the balance Fe with inevitable impurities and further containing, if necessary, one or >=2 kinds selected from >0.5-2.5% Si, >2.5-8.0% Cr, >1.0-3.0% Ni, >0.012-0.050% N, 0.05-1.0% V, 0.05-1.0% Nb, 0.05-1.0% W, 0.02-0.5% Zr, 0.02-0.5% Ta, 0.02-0.5% Hf, and 0.05-1.5% Co. In this steel, the maximum grain size of oxide nonmetallic inclusions is regulated to <=8μm.

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 with the effect of suppressing the formation of a decarburized layer during heat treatment and the severer environment of bearing use. This is a proposal for a bearing steel that is excellent in the characteristic deterioration that occurs as a result, that is, the delay characteristics for the microstructural change (deterioration) that occurs under the rolling contact surface due to repeated stress loading.

[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 is one of the important properties that long rolling fatigue life is important, but it is considered that the influence of non-metallic inclusions in steel is the most significant factor affecting rolling fatigue life. Was there.
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 those developed with the aim of further improving the rolling contact fatigue life of bearings, there are Japanese Patent Laid-Open Nos. 1-306542 and 3-1268.
There are proposals such as Japanese Patent No. 39, which are technologies for controlling the composition, shape, or distribution state of oxide-based nonmetallic inclusions in steel. However, in order to manufacture bearing steel with 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 equipment or a large amount of conventional equipment has to be installed. There was a problem that improvement was necessary and the financial burden was large.

Another move to improve the characteristics of the above-mentioned high carbon bearing steel (JIS-SUJ 2) is a study on workability, especially suppressing formation of a decarburized layer during heat treatment. In general, the bearing steel specified in JIS-SUJ 2 above is 0.95 to 1.
Since it contains 10 wt% of C, it is very hard. Therefore, rolling bearings can be obtained by performing spheroidizing annealing to improve workability, then forming, and then quenching and tempering. Had the necessary strength and toughness. However, it is known that when the heat treatment for improving the characteristics is repeated many times, a "low C concentration region" called a decarburized layer is generated on the surface of the material due to the reaction between C and the atmosphere gas. There is. This decarburized layer not only lowers the hardness of the rolling bearing but also causes the deterioration of rolling contact fatigue life, and therefore it is usually removed by cutting or grinding. For this reason, the material yield and the productivity have been unavoidably reduced. On the other hand, heretofore, as a 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 the spheroidizing method disclosed in JP-A-2-54717 have been disclosed. A method of carburizing at the initial stage of annealing has been proposed. However, since each of the above methods is controlled by the atmosphere during the heat treatment or its pretreatment, not only the heat treatment cost increases, but also the setting of an appropriate gas composition according to the material composition, heat treatment time, etc. There was a problem in that a complicated operation was required.

[0004]

DISCLOSURE OF THE INVENTION In order to further pursue the above-mentioned prior art, the inventors have recently conducted various studies. As a result, the factors that unexpectedly determine the rolling life of the bearing are the above-mentioned phenomena that have been generally discussed in the past; namely, the presence of the above-mentioned "non-metallic inclusions" and the "decarburization layer" ( It was found that there are other factors besides the generation of the low C concentration region). Because simply reducing non-metallic inclusions and decarburized layers under the prior art,
This is because there have been many cases in which a large effect cannot be obtained for the rolling contact fatigue life of the bearing, particularly for improving the bearing life under severe conditions such as high load or high temperature. From these facts, the inventors have found that the factors that affect bearing life include the presence of non-metallic inclusions and the formation of decarburized layers, as well as the microstructural changes that occur under the rolling contact surface during high-load rolling. It was discovered that there are three types.

Therefore, the present inventors have further investigated the bearing life in consideration of the recent bearing usage environment, in particular, the cause of separation of rolling bearings. As a result, due to the repeated use of the shearing stress that occurs when the inner and outer races of the bearing are in rolling contact with the rolling elements, as shown in FIG.
At the bottom of the rolling contact surface (surface layer), as shown in (a), a microstructural change layer consisting of white strip-shaped products and rod-shaped precipitates is generated, which gradually grows as the number of rolling increases. At the end, it was found that fatigue delamination occurred as shown in Fig. 1 (b) from the microstructure-changed portion, and the surface layer of the bearing member was damaged to extend the 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 also found out.

As described above, the bearing life is not sufficient to control the decarburization layer and the non-metallic inclusions in the surface layer portion of the bearing member as in the prior art.
For example, by simply reducing the amount of decarburized layer in the surface layer and the amount of non-metallic inclusions by various heat treatments such as carburizing / nitriding and spheroidizing annealing, the microstructural changes that occur under the rolling contact surface (surface layer portion) described above. It is not possible to delay the time until the occurrence of. As a result, they have found that the bearing life cannot be further improved.

Therefore, the object of the present invention is to improve the relative rolling contact fatigue life by controlling the particle size of non-metallic inclusions, and also to cause the occurrence of the problem during the use of the bearing under particularly severe operating conditions. By delaying the expected microstructural change, the bearing life is improved and the formation of a decarburized layer during heat treatment is suppressed to improve heat treatment productivity (the effect of reducing the amount of machining removal). Therefore, it is to obtain a bearing steel having a long life.

[0008]

Based on the above-mentioned findings, the present inventors have also noticed a new "microstructure change delay characteristic" as the bearing life. In order to improve these characteristics, it is of course necessary to design an alloy (component composition) for that purpose, and it is recognized that the life of the bearing cannot be further improved without realizing this. Various experiments and investigations were performed to achieve the suppression of the formation of the. As a result, surprisingly, Mo and
If the amount of Sb is controlled within an appropriate range, it is possible to remarkably delay the above-mentioned microstructural change generated under the rolling contact surface due to repeated stress loading, and at the same time suppress the generation of a decarburized layer during heat treatment. It was found that a desirable bearing steel can be obtained by controlling the maximum grain size of inclusions, and the present invention has been made.

That is, the bearing steel of the present invention has the following essential constitution. (1) C: 0.5 to 1.5 wt%, Mo: over 1.0 to 2.0 wt
%, Sb: more than 0.005 to 0.015 wt%, the balance consisting of Fe and unavoidable impurities, the maximum grain size of oxide-based non-metallic inclusions is 8 μm or less, delay of microstructure change due to repeated stress loading Bearing steel with excellent properties and heat treatment productivity.

(2) C: 0.5 to 1.5 wt%, Mo: 1.0
Super ~ 2.0 wt%, Sb: 0.005 ~ 0.015 wt%, Si: 0.05 ~ 0.5 wt%, Mn: 0.05 ~ 2.0 wt%, Cr: 0.05 ~ 2.5 wt%, Ni: 0.05 ~ 1.0 wt%, Cu: 0.05 to 1.0 wt%, B: 0.0005 to 0.01 wt%, Al: 0.005 to 0.07 wt% and N: 0.0005 to 0.012 wt% Any one or more selected from the rest, and the balance Bearing steel consisting of Fe and unavoidable impurities, having a maximum grain size of oxide-based non-metallic inclusions of 8 μm or less and excellent in delay characteristics of microstructure change due to repeated stress load and heat treatment productivity.

(3) However, the above basic components (C, Mo, Sb)
In addition, the optional additive components (Si,
It is recommended that Mn, Cr, Ni, Cu, B, Al, N) be added in the following combinations within the range of the composition of (2) above. 0.05 to 0.5 wt% Si- (one or more of Mn, Cr, Ni, Cu, B, Al and N) 0.05 to 2.0 wt% Mn- (any of Cr, Ni, Cu, B, Al and N) 0.05 to 2.5 wt% Cr- (any one or more of Ni, Cu, B, Al and N) 0.05 to 1.0 wt% Ni- (any one or more of Cu, B, Al and N) 0.05-1.0 wt% Cu- (Any of B, Al and N 1
0.0005 to 0.01 wt% B- (one or two of Al and N) 0.005 to 0.07 wt% Al-N 0.0005 to 0.012 wt% N

(4) C: 0.5 to 1.5 wt%, Mo: 1.0
Over ~ 2.0 wt%, Sb: over 0.005 ~ 0.015 wt%, Si: over 0.5 ~ 2.5 wt%, Cr: over 2.5 ~ 8.0 wt%, Ni: over 1.0 ~ 3.0 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 to 0.5 wt%, Hf: 0.02 to 0.5 wt% and Co: contains 0.05 to 1.5 wt% of any one kind or two kinds or more, the balance is Fe and inevitable impurities, and the maximum particle size of the oxide-based nonmetallic inclusions is 8 μm or less. , Bearing steel with excellent heat treatment productivity and delay characteristics of microstructure change due to repeated stress loading.

(5) However, the above basic components (C, Mo, Sb)
On the other hand, regarding optional additive components (Si, Cr, Ni, N) that are selectively added in large amounts and other optional additive components (V, Nb, W, Zr, Ta, Hf and Co) that are added in small amounts, In the composition range described in (4) above, it is recommended to add the following combinations. Over 0.5-2.5 wt% Si- (any one or more of Cr, Ni and N)-(any one or more of V, Nb, W, Zr, Ta, Hf and Co) over 2.5- 8.0 wt% Cr- (Ni, N 1 or 2)-
(One or more of V, Nb, W, Zr, Ta, Hf and Co) 1.0 over to 3.0 wt% Ni− (N) − (V, Nb, W, Zr, T
at least one of a, Hf and Co) over 0.012 to 0.050 wt% N- (V, Nb, W, Zr, Ta, Hf
And any one or more of Co) (any one or more of V, Nb, W, Zr, Ta, Hf and Co)

(6) C: 0.5 to 1.5 wt%, Mo: 1.0
Super ~ 2.0 wt%, Sb: 0.005 ~ 0.015 wt%, Si: 0.05 ~ 0.5 wt%, Mn: 0.05 ~ 2.0 wt%, Cr: 0.05 ~ 2.5 wt%, Ni: 0.05 ~ 1.0 wt%, Cu: 0.05 to 1.0 wt%, B: 0.0005 to 0.01 wt%, Al: 0.005 to 0.07 wt% and N: 0.0005 to 0.012 wt% Any one or more selected from the group due to strength increase When one or more of the above-mentioned improving components is selected as a component for improving dynamic fatigue, the following components excluding the element, that is, Si: more than 0.5 to 2.5 wt%, Cr: Over 2.5 ~ 8.0 wt%, Ni: over 1.0 ~ 3.0 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 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 selected from the group consisting of Fe and unavoidable impurities, with the balance being Fe and inevitable impurities. A bearing steel that has a maximum grain size of oxide-based non-metallic inclusions of 8 μm or less and is excellent in retarding microstructural changes due to repeated stress loading and heat treatment productivity.

(7) However, in the above item (6), the following combinations are recommended for the rolling fatigue life improving component due to the increase in strength. 0.05 to 0.5 wt% Si- (one or more of Mn, Cr, Ni, Cu, B, Al and N) 0.05 to 2.0 wt% Mn- (any of Cr, Ni, Cu, B, Al and N) 0.05 to 2.5 wt% Cr- (any one or more of Ni, Cu, B, Al and N) 0.05 to 1.0 wt% Ni- (any one or more of Cu, B, Al and N) 0.05-1.0 wt% Cu- (Any of B, Al and N 1
0.0005 to 0.01 wt% B- (one or two of Al and N) 0.005 to 0.07 wt% Al-N 0.0005 to 0.012 wt% N Also, regarding the components for improving rolling fatigue life due to delay of microstructure change The following combinations are recommended. '0.5-2.5 wt% Si- (any one or more of Cr, Ni and N)-(any one or more of V, Nb, W, Zr, Ta, Hf and Co)' 2.5 Super ~ 8.0 wt% Cr- (One or two kinds of Ni and N)-
(One or more of V, Nb, W, Zr, Ta, Hf, and Co) '1.0 over ~ 3.0 wt% Ni- (N)-(V, Nb, W, Zr, T
a, Hf and / or Co)) '0.012 to 0.050 wt% N- (V, Nb, W, Zr, Ta, Hf
And any one or more of Co) ′ ((any one or more of V, Nb, W, Zr, Ta, Hf and Co))

[0016]

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 an experiment, steel A SUJ 2 (C: 1.02 wt%, Si: 0.25 wt%, Mn: 0.45
wt%, Cr: 1.35wt%, N: 0.0040wt%, O: 0.0012wt
%) And steel B SUJ 2 (C: 1.01 wt%, Si: 0.24 wt%, Mn: 0.46
wt%, Cr: 1.32wt%, N: 0.0042wt%, O: 0.0015wt
%), And a material in which Mo and Sb are changed in two levels. Steel C (C: 1.00wt%, Si: 0.21wt%, Mn: 0.43wt%,
Cr: 1.33wt%, Mo: 1.14wt%, Sb: 0.0061wt%, N:
0.0035wt%, O: 0.0008wt%) Steel D (C: 0.98wt%, Si: 0.22wt%, Mn: 0.42wt%,
Cr: 1.30wt%, Mo: 1.12wt%, Sb: 0.0059wt%, N:
0.0037wt%, O: 0.0035wt%) Steel E (C: 0.98wt%, Si: 0.20wt%, Mn: 0.41wt%,
Cr: 1.31wt%, Mo: 1.82wt%, Sb: 0.0094wt%, N:
0.0033wt%, O: 0.0009wt%) Steel F (C: 0.98wt%, Si: 0.21wt%, Mn: 0.42wt%,
Cr: 1.31wt%, Mo: 1.80wt%, Sb: 0.0093wt%, N:
0.0037 wt%, O: 0.0013 wt%). Of these, for B, D and F,
The maximum diameter of oxide inclusions was adjusted to 8 μm by controlling refining and solidification conditions. Then, from these test materials,
15mmφ × 22mm cylindrical specimen for microscopic inspection and 12mmφ
A 22 mm rolling fatigue test piece was taken and normalized, spheroidized and annealed without controlling the atmosphere (in the air).
Each treatment of quenching and tempering was performed. After that, the rolling contact fatigue test pieces are lapped to a thickness of 1 mm or more to completely remove the decarburized layer, and the roughness of the test surface is R
a: 0.1 μm or less.

The rolling fatigue life test was carried out by using a radial type rolling fatigue life tester for the rolling fatigue test piece, and Hertz maximum contact stress: 600 kgf / mm ² , cyclic stress number 46.
500 cpm, lubrication: # 68 Turbine spray oil tested under loading conditions. The test results are plotted on the Weibull distribution establishment paper, and it is considered that it is a numerical value showing the improvement of rolling fatigue life due to the increase of material strength affected by the control of nonmetallic inclusions. (Less than,
This is referred to as "B ₁₀ rolling fatigue life") and a numerical value showing the improvement of rolling fatigue life by delaying the occurrence of microstructural changes due to repetitive stress loading during high load rolling. Bearing life at
₅₀ rolling fatigue life))
Evaluated as. Further, for the test of the decarburized layer, after cutting the above cylindrical test piece vertically in the height direction at a position of 10 mm, it is corroded by Nital and the maximum value of the total decarburized layer on the circumference due to the microstructure change ( (Hereinafter referred to as "maximum decarburized layer")
Was evaluated.

The results are shown in Table 1. The results shown in Table 1
As you can see from the results, without controlling non-metallic inclusions,
In the case of simply adding a large amount of Mo and Sb in combination,
Note B _TenAlthough the rolling fatigue life improvement is small, B₅₀Rolling fatigue
It can be seen that the improvement in working life is significantly improved. For example,
B₅₀The rolling fatigue life of steel C (Mo: 1.14 wt%, Sb: 0.00
61 wt%) is about 18 times that of SUJ 2 and steel E (Mo: 1.
At 82wt%, Sb: 0.0094wt%, about 26 times improvement was observed.
It was On the other hand, by adding Mo and Sb in combination, and oxidizing
Steel D and Steel F with controlled maximum grain size of physical nonmetallic inclusions
Then B₅₀In addition to high rolling fatigue life, B_TenRolling fatigue
The working life is also significantly improved, and especially steel D has a high oxygen content in the steel.
Nevertheless B_TenRolling fatigue life is about 21 times higher
Is shown. The effects of adding Mo and Sb as described above are
Was obtained through the delay of microstructural changes
And these elements have a remarkable effect on the improvement of rolling contact fatigue life.
It became clear to bring. Also, regarding the maximum decarburization layer
, SUJ 2 was 0.10 mm, but Sb: 0.006 wt%,
0.019 and 0.01 mm or less for those containing 0.009 wt%
In addition, proper Sb content is effective in suppressing the generation of decarburized layer.
I also understood.

[0019]

[Table 1]

FIG. 2 is a summary of the experimental results of the rolling fatigue life of the above-mentioned bearing, and shows the relationship between the life of the bearing caused by non-metallic inclusions and the change in the life caused by microstructural changes. It is a schematic diagram which shows. As shown in this figure, B ₁₀
The rolling contact fatigue life is not improved so much by simply adding a large amount of Mo and Sb, but it is significantly improved when oxide-based nonmetallic inclusion control is also performed. On the other hand, regarding the B ₅₀ rolling contact fatigue life, the effect of adding a large amount of Mo and Sb is extremely remarkable regardless of the control of nonmetallic inclusions. Therefore, based on these findings, the inventors
Improves B ₁₀ rolling fatigue life and B ₅₀ rolling fatigue life,
In addition, from the viewpoint of what kind of alloy design is effective for suppressing the growth of the decarburized layer during heat treatment, the range of the composition of components as described below was determined.

C: 0.5 to 1.5 wt% C is an element that forms a solid solution in the matrix and effectively acts to strengthen 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%. The preferable range is 0.65 to 1.10 wt%.

Mo: over 1.0 to 2.0 wt% Mo is an element that basically improves wear resistance by stabilizing residual carbides.
Plays a more important role, especially when added in a large amount exceeding 1.0 wt%, the effect of delaying microstructural change due to repeated stress loading becomes remarkable, and rolling fatigue life in this aspect. Improve. However, if the amount exceeds 2.0 wt%, machinability and forgeability will be reduced, and this will cause cost increase.
It is necessary to add it within the range of wt%. The preferable range is 1.0 to 1.5 wt%.

Sb: more than 0.005 to 0.015 wt% This Sb is an element that plays an important role in the present invention. In particular, this Sb contributes to the improvement of the heat treatment productivity by suppressing the reaction between C in the surface layer portion of the steel material and the atmospheric gas during the heat treatment to prevent the generation of the decarburized layer. Moreover, it is effective in retarding the microstructural change as well as suppressing the decarburized layer. These two effects are
This Sb content becomes remarkable when the content of Sb exceeds 0.005 wt% or more, but if it is added in an amount of more than 0.015 wt%, hot workability and toughness will be deteriorated. Therefore, Sb exceeds 0.005 to 0.
It was decided to contain it in the range of 015 wt%. The preferable range is 0.005 to 0.010 wt%.

Si: 0.05 to 0.5 wt%, more than 0.5 to 2.5 wt% Si is used as a deoxidizing agent during the melting of steel, and is also solid-dissolved in the matrix to increase quenching and quenching due to an increase in temper softening resistance. It is effective as an element that enhances the strength after rehabilitation and improves rolling contact fatigue life. The content of Si added for these purposes is in the range of 0.05 to 0.5 wt%, preferably 0.15 to 0.50 wt%. In addition, if Si is added in excess of 0.5 wt%,
It has the effect of delaying the microstructural change under cyclic stress loading and improving rolling fatigue life. But,
If the content exceeds 2.5 wt%, the effect is saturated, but the workability and toughness are reduced. Therefore, in order to further improve the microstructure change retardation property, the content exceeds 0.5 to 2.5.
It is effective to add wt%, preferably more than 0.5 to 2.0 wt%.

Mn: 0.05 to 2.0 wt% Mn acts as a deoxidizing agent during the melting of steel, and is an element effective in 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 this
Mn is added within the range of 0.05 to 2.0 wt%. The preferable range is 0.25 to 2.0 wt%.

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, the addition of 0.05 to 2.5 wt%, preferably 0.15 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 microstructure change due to repeated stress loading and improving the rolling fatigue life in this aspect. And Cr for this purpose
The effect of addition not only saturates even if it exceeds 8.0 wt%, but rather it causes a decrease in the amount of solid solution C during quenching, resulting in a decrease in strength. Therefore, when adding for this purpose,
It must be over 2.5 to 8.0 wt%. The preferred range is over 2.5 to 5.0 wt%.

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 an amount of 0.05 to 1.0 wt%, preferably 0.15 to 1.0 wt%. Furthermore, this
When Ni is added in excess of 1.0 wt%, it delays the microstructure change during rolling, thereby improving rolling fatigue life. However, even in this case, if it is added in excess of 3.0 wt%, a large amount of residual γ will precipitate, resulting in reduced strength and impaired dimensional stability, and increased costs. Is over 1.0 to 3.0 wt
It is necessary to add it within the range of%. The preferred range is over 1.0 to 2.5 wt%.

Cu: 0.05 to 1.0 wt% Cu is an element that enhances the strength after quenching and tempering by increasing quenching, and from this aspect improves rolling fatigue life.
This effect appears at 0.05 wt% or more and saturates at 1.0 wt%. The preferable range is 0.15 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%. The preferable range is 0.0015 to 0.005 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 acts to improve rolling fatigue by increasing the strength after quenching and tempering. Due to such an effect, it is effective to add 0.005 to 0.07 wt% of Al. The preferred range is 0.010-0.07wt%
Is.

N: 0.0005 to 0.012 wt%, more than 0.012 to 0.050 wt% N combines with the nitride-forming element to refine the crystal grains, and also forms a solid solution in the matrix to enhance the strength after quenching and tempering. ,
Improves rolling fatigue life. 0.0005 for this purpose
˜0.012 wt%, preferably 0.0020 to 0.012 wt%. Further, when N exceeds 0.012 wt%, the rolling fatigue life is improved by delaying the microstructural change due to repeated stress. However,
If the amount exceeds 0.050 wt%, the workability decreases, so for this purpose, more than 0.012 to 0.050 wt% is added. The preferred range is more than 0.012 to 0.035 wt%.

As described above, the component for improving the rolling fatigue life by delaying the microstructural change due to the repeated stress load, the component for improving the rolling fatigue life by increasing the strength, and the decarburized layer are suppressed to form the bearing. The reason for limiting the components for improving the processability and productivity of was explained. By the way, in the present invention, further, V, Nb, W, Zr, T
The bearing life may be further improved by adding one or more selected from a, Hf and Co. Table 2 shows the preferable addition range of each of the above elements, the purpose of addition, and the reasons for limiting the upper limit value and the lower limit value.

[0033]

[Table 2]

In the present invention, in order to improve machinability, S, Se, Te, REM, Pb, Bi, Ca, Ti, Mg, P, Sn, As
Even if added, etc., it does not inhibit the retardation property due to the microstructural change due to the repeated stress load, which is the object of the present invention, and the machinability can be easily improved, so it is added as necessary. You may.

It is desirable that P be as low as possible in order to reduce the toughness and rolling contact fatigue life of the steel, and it is desirable to keep it to 0.025 wt% or less, preferably 0.015 wt% or less. Further, S is an element that combines with Mn to form MnS and improves machinability. However, if it is contained in a large amount, the rolling contact fatigue life will be shortened.
It is preferable to suppress it to 5 wt% or less, preferably 0.015 wt% or less.

Next, the reasons for limiting the maximum system of oxide-based nonmetallic inclusions in steel will be described. In steel, in general,
Although there are non-metallic inclusions such as oxide-based and sulfide-based inclusions, especially oxide-based non-metallic inclusions are hard and act as a starting point of peeling, which significantly deteriorates the rolling fatigue life. In Figure 3, JI
For S-SUJ2 steel (corresponding to the steels A and B) and steel compatible with the present invention (corresponding to the steels C and D), the relationship between the maximum diameter of oxide nonmetallic inclusions and the B ₁₀ value was investigated. The results are shown. From this figure, the maximum diameter of oxide non-metallic inclusions is 8μ.
It was found that the B ₁₀ value sharply decreases when the value exceeds m.
Therefore, the maximum diameter of the oxide-based nonmetallic inclusions is set to 8 μm or less.

[0037]

EXAMPLE Steel materials having the chemical compositions shown in Tables 3 and 4 were melted in a converter and continuously cast. Here, the maximum diameter of oxide-based nonmetallic inclusions was adjusted by controlling the converter slag composition, controlling the secondary refining method, and controlling the molten steel heating degree and solidification conditions during continuous casting. The steel material obtained in this way
After diffusion annealing for 30 h, it was rolled into a steel bar of 65 mmφ. Then, a cylindrical test piece for investigating a non-metallic inclusion of 15 mmφ × 20 mm and a decarburized layer and a test piece for rolling fatigue were taken from the steel D / 4 part by cutting. Then, these test pieces were tested in the order of normalizing, spheroidizing annealing, quenching, and tempering without controlling the atmosphere (in the air atmosphere). Further, the rolling fatigue test piece was ground and lapped to a size of 1 mm or more for the purpose of completely removing the decarburized layer, and the size of the test piece was 12 mmφ × 22 mm. In the investigation of oxide-based non-metallic inclusions, the circle equivalent diameter of oxide-based non-metallic inclusions was measured in 400 views at 200 times, and the maximum diameter of inclusions in each view was summarized on the Gumbel probability paper and equivalent to 50000 mm ^2. Was calculated and the maximum diameter of the oxide-based non-metallic inclusions present in the steel was determined. Decarburized layer depth after heat treatment is 15mmφ × 20mm
The maximum value of the total decarburized layer on the circumference by microstructure observation after corroding the cylindrical test piece at 10 mm perpendicular to the height direction and corroding with Nital (hereinafter referred to as "maximum decarburized layer") It was evaluated by. The rolling fatigue life test is performed by a radial type rolling fatigue life tester using a Hertz maximum contact stress of 600.
kgf / mm ² , cyclic stress: approx. 46500 cpm, lubrication: # 68
It was done under the condition of turbine splash oil.
The test results were summarized on the probability paper according to the Weibull distribution, and evaluated with the B ₁₀ rolling fatigue life and the B ₅₀ rolling fatigue life of steel No. 1 being 1 respectively. The evaluation result,
It is also shown in Tables 3 and 4.

[0038]

[Table 3]

[0039]

[Table 4]

As is clear from the results shown in Tables 3 and 4,
Steel No. 5 in which the amount of C in the steel is outside the range of the present invention and No. 6 in which the amount of Mo in the steel is outside the range of the present invention are B ₁₀ rolling contact fatigue life and B ₅₀ rolling fatigue of bearing life. Conventional life (steel material No. 1)
It is the same as or a little worse. On the other hand, although the B ₅₀ rolling fatigue life of steel material No. 4 in which the Sb content in the steel is outside the range of the steel of the present invention is superior to that of the conventional steel (steel material No. 1), the maximum decarburized layer is
0.11 mm, which is not so much improvement compared to the conventional example (SUJ 2). Further, the steel material No. 2 containing no Sb also shows a bad result in the maximum decarburized layer. Steel No. 3 having a large maximum grain size of inclusions has a low B ₁₀ rolling contact fatigue life. On the other hand, the average bearing life indicated by B ₁₀ rolling fatigue life and B ₅₀ rolling fatigue life of steel material No. 7 which is an invention steel is about 8 times and about 7 times that of the conventional steel (steel material No. 1), respectively. In addition to the effect of the oxide-based non-metallic inclusions, the addition of Mo significantly retarded the microstructure change, and as a result, it can be seen that it effectively acted to improve the rolling fatigue life. Moreover, the maximum decarburized layer depth is 0.01 mm, which is far less than the conventional steel No. 1, and the improvement effect is about 1/10 compared to steel No. 4 in which Sb is outside the proper range of the present invention. It is remarkable.

In addition to Mo and Sb, Si, Mn, Cr and
Steel Nos. 8-18 made by adding at least one of Ni, Cu, Al, B and N are effective for improving the B ₅₀ rolling contact fatigue life that determines the bearing life, and the maximum decarburized layer Depth is also 0.
It was found that it was significantly improved to 02 mm or less.

Further, in addition to Mo and Sb, Si, Cr, and
In the case of Steel Nos. 19 to 30 in which V, Nb, W, Zr, Ta, Hf, Co and N are positively added in a predetermined amount or more, the above bearing life (B ₅₀ It was confirmed that the rolling fatigue life) was also improved. This is also the case with Steel Nos. 31 to 45 obtained by selectively adding all of the above-mentioned improving components recommended in the present invention, and it is possible to simultaneously improve both the bearing rolling life and the heat treatment productivity. It was found to be effective in improving.

[0043]

As described above, according to the present invention,
It is possible to reduce the working load during heat treatment, and to delay the microstructural change associated with the repeated stress load during high-load rolling fatigue life. Therefore, according to the present invention, it is possible to provide an improved B ₅₀ rolling contact fatigue life and provide a bearing steel with a long life and high heat treatment productivity.
Furthermore, according to the present invention, the oxygen content in the steel is further reduced or the composition, shape, and distribution state of the oxide-based non-metallic inclusions present in the steel, which have been indispensable under the prior art, are further indispensable. It is not necessary to improve or construct steelmaking equipment required to control the steelmaking. 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 bearing operating temperature.

[Brief description of drawings]

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

FIG. 2 is an explanatory diagram showing the effect of Mo addition on the bearing life due to non-metallic inclusions and the bearing life due to microstructural changes.

FIG. 3 is a graph showing the relationship between the maximum particle size of non-metallic inclusions and B ₁₀ rolling contact fatigue life.

Front page continuation (72) Inventor Akihiro Matsuzaki 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corporation Steel Research and Development Division Steel Research Laboratory (72) Inventor Shinichi Amano Kawasaki, Chuo-ku, Chiba-shi, Chiba No. 1 Town Kawasaki Steel Co., Ltd.

Claims (4)

[Claims] 1. C: 0.5 to 1.5 wt%, Mo: more than 1.0 to 2.0 wt%, Sb: more than 0.005 to 0.015 wt%, the balance consisting of Fe and unavoidable impurities, oxide-based non-metallic inclusion A bearing steel with a maximum grain size of 8 μm or less, which is excellent in delaying microstructural change due to repeated stress loading and heat treatment productivity. 2. C: 0.5 to 1.5 wt%, Mo: more than 1.0 to 2.0 wt%, Sb: more than 0.005 to 0.015 wt%, Si: 0.05 to 0.5 wt%, Mn: 0.05 to 2.0 wt%, Cr: 0.05 to 2.5 wt%, Ni: 0.05 to 1.0 wt%, Cu: 0.05 to 1.0 wt%, B: 0.0005 to 0.01 wt%, Al: 0.005 to 0.07 wt% and N: 0.0005 to 0.012 wt% Delay of microstructural change due to repeated stress loading, containing any one or more selected, 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 with excellent properties and heat treatment productivity. 3. C: 0.5 to 1.5 wt%, Mo: more than 1.0 to 2.0 wt%, Sb: more than 0.005 to 0.015 wt%, and further Si: more than 0.5 to 2.5 wt%, Cr: more than 2.5 to 8.0 wt%. %, Ni: over 1.0 to 3.0 wt%, N: over 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 to 0.5 wt%, Ta: 0.02 to 0.5 wt%, Hf: 0.02 to 0.5 wt% and Co: contains 0.05 to 1.5 wt% of any one kind or two kinds or more, the balance is Fe and inevitable impurities, and the maximum particle size of the oxide-based nonmetallic inclusions is 8 μm or less. , Bearing steel with excellent heat treatment productivity and delay characteristics of microstructure change due to repeated stress loading. 4. C: 0.5 to 1.5 wt%, Mo: more than 1.0 to 2.0 wt%, Sb: more than 0.005 to 0.015 wt%, Si: 0.05 to 0.5 wt%, Mn: 0.05 to 2.0 wt%, Cr: 0.05 to 2.5 wt%, Ni: 0.05 to 1.0 wt%, Cu: 0.05 to 1.0 wt%, B: 0.0005 to 0.01 wt%, Al: 0.005 to 0.07 wt% and N: 0.0005 to 0.012 wt% When any one or more selected from the above is included as a component that improves rolling fatigue due to strength increase, and when any one or more of the above-mentioned improving components is selected, the element is excluded below. , Si: more than 0.5 to 2.5 wt%, Cr: more than 2.5 to 8.0 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 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 selected from the group consisting of Fe and unavoidable impurities, with the balance being Fe and inevitable impurities. A bearing steel that has a maximum grain size of oxide-based non-metallic inclusions of 8 μm or less and is excellent in retarding microstructural changes due to repeated stress loading and heat treatment productivity.