JP2000026939A - Bearing steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a bearing steel minimal in dispersion of rolling fatigue life and having long rolling fatigue life. SOLUTION: The bearing steel has a chemical composition consisting of, by weight, >1.2-1.5% C, 0.3-3.0% Si, 0.3-1.5% Mn, <=0.03% P, <=0.03% S, 0.3-5.0% Cr, 0.005-0.050% Al, <=0.003% Ti, <=0.0015% O, <=0.015% N, one or >=2 kinds among 0.05-0.5% V, <=0.1% Nb, and <=0.5% Ta, and the balance essentially Fe.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bearing steel having an excellent rolling life.

[0002]

2. Description of the Related Art Bearing steels represented by JIS SUJ2 have been used as bearing steels for mechanical structural parts and the like. However, there is an increasing demand for longer bearing life, and various bearing steels with improved rolling fatigue strength have been proposed.

[0003] In order to improve the rolling contact fatigue strength of steel, reduction of the oxygen content in steel and addition of alloying elements such as Si and Cr have been considered promising and studied. In addition, N
The effects of adding various alloying elements such as i, Mo, and the like have also been reported.

However, although the fatigue strength is certainly improved by adding or increasing the amount of these alloying elements, some of them exhibit extremely low strength, and at present, the average life is not significantly improved due to large variation. In addition, there is a problem in reliability, and therefore, stable and long-life bearing steel with little variation in life is required.

[0005] In view of the above, there has been proposed an alloy in which Si and V are added in combination to improve the rolling life of bearing steel (Japanese Patent Laid-Open No. 7-216508).

In the device proposed in Japanese Patent Application Laid-Open No. 7-216508, the content of C is limited to 0.6 to 1.2%.
This is because if the content of C is more than 1.2%, a large amount of coarse carbides are generated and the hardness becomes too hard, so that a suitable hardness (about 62 in HRC) as a bearing steel cannot be obtained. Is considered to have an adverse effect on fatigue life.

[0007]

SUMMARY OF THE INVENTION The present invention has been made to solve such problems. Claim 1
Of the bearing steel in weight%, C: more than 1.2% to 1.5%, S
i: 0.3 to 3.0%, Mn: 0.3 to 1.5%, P:
0.03% or less, S: 0.03% or less, Cr: 0.3 to
5.0%, Al: 0.005 to 0.050%, Ti:
0.003% or less, O: 0.0015% or less, N: 0.
015% or less, and one or two or more of V, Nb, and Ta are V: 0.05 to 0.5%, Nb: 0.1% or less,
Ta: 0.5% or less, with the balance substantially consisting of Fe.

[0008]

The present inventors have determined that the quenching / tempering treatment is performed under a predetermined condition even when the content of C is more than 1.20% in the case where Si and V are added in combination. It is possible to obtain bearing steel with appropriate hardness by performing the above, and to avoid the adverse effect on fatigue life due to coarse carbides by soaking even with a high C content, and to obtain a bearing steel with good rolling life. The present invention was found to be effective, that is, it was found that the composite addition of Si and V was effective even when C was contained in a high content, and that the required hardness and required fatigue life were obtained. I let it.

That is, according to the present invention, the content of C is more than 1.2%.
The content is as high as 1.5%, and Si and one or more of V, Nb and Ta are added in combination. According to the present invention, the bearing is prevented from being damaged in a short life. It is possible to reduce the variation in the service life and to increase the service life as a whole.

In the present invention, when quenching and tempering the bearing steel, it is desirable to perform the tempering at a high temperature of more than 180 ° C. to 400 ° C. By performing the tempering treatment at such a high temperature, it is possible to obtain an appropriate hardness of about 62 HRC as a bearing steel even when C is highly added in a range of more than 1.2% to 1.5%. The fatigue life can also be extended. It is more desirable that the tempering process be performed at a temperature of 200 ° C. or higher.

Next, the reasons for limiting each chemical component of the present invention will be described in detail below. C: more than 1.2% to 1.5% C is an essential element for securing the strength of steel, and has a predetermined hardness at a tempering temperature of more than 180 ° C., particularly 200 ° C. or more in the quenching / tempering treatment. In order to obtain the content, it is necessary to contain more than 1.2%. However, if the content is too large, a net-like carbide precipitates at the time of spheroidizing annealing or the like, and the toughness is reduced and the machinability is deteriorated. In addition, since large carbides are easily formed after casting, it causes adverse effects such as generation of cracks during rolling.
The upper limit of the content is 1.5%.

Si: 0.3-3.0% Si is an element having an important role in the present invention.
It is added to impart temper softening resistance to steel, but its effect can be further enhanced by coexisting with V. In order to exert the effect, a content of 0.3% or more is required.

However, even if it is added excessively, its effect is not only saturated, but it also raises the transformation point of the steel, so that it is necessary to increase the heat treatment temperature, and it also causes adverse effects such as impairing forgeability and machinability. Therefore, the upper limit of the Si content is 3.0.
%. Incidentally, the more desirable content of Si is 0.5% or more.

Mn: 0.3 to 1.5% Mn is added in an amount of 0.3% or more in order to enhance the hot workability of the steel and secure the hardenability. However, an excessive addition makes soft annealing of the material difficult and deteriorates machinability, so the upper limit of the Mn content is set to 1.5%.

P: 0.03% or less P is segregated at austenite crystal grain boundaries to reduce toughness, so the content is made 0.03% or less.

S: 0.03% or less S impairs the hot workability of steel and forms nonmetallic inclusions in the steel to impair the toughness in the transverse direction.
3% or less.

Cr: 0.3-5.0% Cr is an element which improves the hardenability of steel and increases the resistance to temper softening, so that 0.3% or more is added. Since the content is saturated and only the cost is unnecessarily increased, the upper limit of the content is set to 5.0%.

Al: 0.005 to 0.050% Al is added as a deoxidizing agent and for refining crystal grains. However, 0.
Less than 005% has little effect, while 0.050%
Exceeds 0.005, the amount of alumina-based inclusions increases.
-0.050%.

Ti: 0.003% or less N: 0.015% or less Ti and N are regulated because they form TiN and deteriorate rolling fatigue. That is, Ti: 0.003%, N: 0.015
%, Rolling fatigue is affected.

O: 0.0015% or less O forms an oxide with Ti, Al, or Si and deteriorates rolling fatigue. In particular, if the content exceeds 0.0015%, the rolling fatigue is deteriorated, so the content is made 0.0015% or less.

V: 0.05 to 0.5% Nb: 0.1% or less (preferably 0.01% or more) Ta: 0.5% or less (preferably 0.01% or more) V, Nb, Ta Is an element having an important role in the present invention together with Si, and has an effect of refining the crystal grains of steel, but has an effect of synergistically increasing the tempering softening resistance particularly in the presence of Si. Further, it is an element effective for forming carbonitrides in steel, suppressing segregation of Si, and preventing a variation in fatigue life, particularly a phenomenon of short-life fracture due to rolling fatigue.

In order to exhibit these effects, it is necessary to make the V content 0.05% or more in the above-mentioned Si content range. However, if added excessively, the transformation point of the steel is raised, and a high temperature treatment is required during the quenching treatment. In addition, excessive addition makes the upper limit of the content 0.5% in order to unnecessarily increase the cost.

Nb combines with C or N to form Nb carbonitride, which has the effect of preventing the crystal grains from being coarsened during high-temperature heating such as during quenching, and can improve the rolling life. .
However, if it is contained in a large amount, a large crystallized substance is formed at the casting stage, and the rolling life is shortened.
1%. Incidentally, a desirable lower limit of the Nb content is 0.0
1%.

Further, Ta is effective for refining crystal grains and improving rolling contact fatigue strength. However, even if it is added excessively, the effect is saturated, the hot workability and the cold workability are reduced, and the upper limit of the content is set to 0.5% in order to unnecessarily increase the material cost. A desirable lower limit of the Ta content is 0.01%.

[0025]

Next, embodiments of the present invention will be described in detail. Table 1
Test section diameter 1 from hot rolled steel having the chemical composition shown in
A 2 mm radial type rolling fatigue test piece was cut out and subjected to quenching and tempering under the conditions shown in FIG. However, the tempering temperature was adjusted so that the hardness after tempering was about 62 HRC. Thereafter, the surface was polished by machining, and subjected to a rolling fatigue test.

[0026]

[Table 1]

Here, the rolling fatigue life test was conducted by using a SUJ2 ball with a radial type rolling fatigue tester at a contact pressure of 58%.
The test was performed at 80 MPa. Table 2 shows the test results. Here, in the rolling fatigue test, a repetition test of 20 test pieces was performed, and the life when the Weibull cumulative probability was 10% was L10, and the life when the 50% was 50% was L50.

[0028]

[Table 2]

As is evident in Table 2, C is 1.2
% And over 1.5%, and Si and V, N
In the case of the present invention in which b, Ta and the like are added in combination, the rolling life of both L10 and L50 shows remarkable improvement. Also one
The probability of breakage at a repetition rate of × 10 ⁷ or less is significantly reduced in the case of the present invention as compared with that of the comparative example, and it can be seen that the variation in life is improved.

Although the embodiment of the present invention has been described in detail, this is merely an example, and the present invention can be configured in variously modified forms without departing from the gist thereof.

[Brief description of the drawings]

FIG. 1 is a diagram showing temperature, holding time, and cooling conditions during quenching and tempering.

Claims (1)

[Claims] C: over 1.2% to 1.5% Si: 0.3 to 3.0% Mn: 0.3 to 1.5% P: 0.03% or less S: 0.03% or less Cr: 0.3 to 5.0% Al: 0.005 to 0.050% Ti: 0.003% or less O: 0.0015% or less N: 0.015% or less, V , Nb, or Ta, V: 0.05 to 0.5% Nb: 0.1% or less Ta: 0.5% or less, and the balance substantially consists of Fe. Characteristic bearing steel.