JP2000297345A - Steel for carburized bearing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel for a carburized bearing small in the variation of a fatigue life and having high rolling fatigue strength. SOLUTION: The chemical compsn. of steel for a carburized bearing is composed of the alloy compsn. consisting of, by weight, 0.1 to 0.4% C, 0.4 to 3.0% Si, 0.3 to 1.5% Mn, <=0.03% P, <=0.03% S, 0.25 to 3.5% Ni, 0.3 to 5.0% Cr, 0.005 to 0.050% Al, <=0.003% Ti, <=0.0015% O, <=0.025% N, and the balance substantial Fe, and also, the abundance of alumina clusters of >=0.5 mm length is controlled to <=10-3 pieces/mm3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for a carburized bearing from which a bearing having excellent wear resistance, surface fatigue strength and rolling life can be obtained.

[0002]

2. Description of the Related Art
JIS SCr420, SCM42 as carburized bearing steel
Case hardening steels represented by 0 have been used. However, in recent years, there has been an increasing demand for longer bearing life, and under such circumstances, various steels for carburized bearings having improved rolling fatigue strength have been proposed.

[0003] As means for improving the rolling fatigue strength of steel, it has been considered promising to reduce the oxygen content in steel or to add alloying elements such as Si and Cr.
Are being considered. It has also been reported that the addition of various alloying elements such as Cu and Sb is effective.

However, although the addition and increase of the amount of these alloy elements are effective for improving the fatigue strength, some alloys exhibit extremely low fatigue strength and have large variations, so that the actual life is not so much improved. There is also a problem in terms of reliability because some steels exhibit extremely low strength. Therefore, there has been a demand for a carburized bearing steel which has a small variation in life and a stable long life.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and its object is to provide:
An object of the present invention is to provide a carburized bearing steel having a small variation in service life and high rolling life characteristics. In order to achieve this object, the steel for carburized bearing according to claim 1 comprises C:
0.1-0.4%, Si: 0.4-3.0%, Mn:
0.3-1.5%, P: ≦ 0.03%, S: ≦ 0.03
%, Ni: 0.25 to 3.5%, Cr: 0.3 to 5.0
%, Al: 0.005 to 0.050%, Ti: ≦ 0.0
03%, O: ≦ 0.0015%, N: ≦ 0.025%,
The remaining amount of alumina clusters having an alloy composition substantially consisting of Fe and having a length of 0.5 mm or more is 10
⁻³ / mm ³ or less.

According to a second aspect of the present invention, C: 0.1 to
0.4%, Si: 0.4-3.0%, Mn: 0.3-
1.5%, P: ≦ 0.03%, S: ≦ 0.03%, N
i: 0.25 to 3.5%, Cr: 0.3 to 5.0%, A
l: 0.005 to 0.050%, Ti: ≦ 0.003
%, O: ≦ 0.0015%, N: ≦ 0.025%
And any one or more of Mo, V and Nb is Mo:
0.03 to 2.5%, V: 0.05 to 1.0%, Nb:
≦ 0.1%, with the balance being substantially Fe
Alumina having an alloy composition and having a length of 0.5 mm or more
Raster abundance is 10 ^-3Pieces / mm³Must be
It is characterized by.

[0007]

The present inventor studied the combination of various alloying elements for improving the rolling fatigue life, and found that the combined addition of Si and Ni limits the alumina cluster which is a fracture starting point to a certain level or less. By doing so, it has been found that the phenomenon that the bearing is broken particularly with a short service life can be suppressed, whereby the variation in the service life can be reduced and the average service life can be effectively increased, and the present invention has been completed. In addition, Mo,
It has been found that the average life of the bearing can be more effectively increased by adding one or more of V and Nb.

Next, the reasons for limiting each chemical component in the present invention will be described in detail. C: 0.1 to 0.4% C is required to be 0.1% or more in order to obtain necessary strength as a bearing and to secure sufficient surface hardness after carburizing. However, if it exceeds 0.4%, the toughness and machinability are reduced.

Si: 0.4 to 3.0% Si is effective in making the quenched martensite structure dense and improving the toughness and fatigue resistance of the steel. However, if the content is less than 0.4%, such effects cannot be obtained, so the content is made 0.4% or more. On the other hand, if it exceeds 3.0%, toughness and workability deteriorate, so the upper limit is made 3.0%.

Mn: 0.3 to 1.5% Mn acts as a deoxidizing element and a desulfurizing element at the time of smelting steel, and is an element effective in improving the hardenability of steel. 0.3% or more is contained. However, if it exceeds 1.5%, workability and machinability deteriorate, so the upper limit is made 1.5%.

P: ≦ 0.03% S: ≦ 0.03% P and S cause deterioration of the strength of the bearing. Therefore, in the present invention, P and S are each regulated to 0.03% or less.

Ni: 0.25 to 3.5% Ni is an element effective for improving the hardenability of steel and the toughness after quenching and tempering. Therefore, in the present invention, Ni is contained in an amount of 0.25% or more. However, if it exceeds 3.5%, the toughness and workability of the steel are reduced, so the upper limit is made 3.5%.

Cr: 0.3-5.0% Cr is an element effective for improving the hardenability of steel, the strength after quenching and tempering, and the toughness. Therefore, in the present invention, Cr is contained in an amount of 0.3% or more. . However, if the content is more than 5.0%, hardenability and machinability are impaired, so the upper limit is set to 5.
0%.

Al: 0.005 to 0.050% Al becomes AlN and has an effect of refining crystal grains.
Therefore, in the present invention, 0.005% or more of Al is contained. However, if it is contained in a large amount exceeding 0.050%, the cleanliness of the steel is lowered and the effect of preventing the crystal grains from coarsening is rather lowered, so the upper limit is made 0.050%.

N: ≦ 0.025% N combines with Al to form AlN and serves to refine crystal grains. However, if contained in a large amount, the strength of the steel is rather deteriorated. Therefore, in the present invention, the upper limit of the N content is set to 0.
025%. A more desirable range is 0.01 to 0.0
2%.

Ti: ≦ 0.003% Ti forms hard precipitate TiN and becomes a starting point of rolling fatigue fracture, which causes a reduction in fatigue life. Therefore, in the present invention, the content of Ti is restricted to 0.003% or less.

O: ≦ 0.0015% O reduces the cleanliness of the steel and causes the fatigue life to deteriorate. Therefore, in the present invention, the content of O is 0.0015%
It is regulated as follows.

Mo: 0.03 to 2.5% Mo is a useful element for improving the strength of steel. Therefore, in the present invention, when adding Mo as a selective element, Mo is 0.03% to 2.5%.
It is contained above. However, if the content is more than 2.5%, the hardenability decreases and the machinability also deteriorates, so the upper limit is made 2.5%.

V: 0.05 to 1.0% Nb: ≤0.1% V and Nb are elements that contribute to the refinement of crystal grains. To become
When these are added as selective elements, V is set to 0.1.
And Nb in the range of 0.1% or less.

Alumina clusters having a length of 0.5 mm or more and having an abundance of 10 ⁻³ / mm ³ or less become the starting point of fatigue fracture and cause variations in fatigue life and decrease in average life. The present inventor has studied the size and abundance of the alumina cluster. As a result, the alumina cluster having a length of 0.5 mm or more is broken down with a short life by regulating the abundance to 10 ⁻³ / mm ³ or less. It has been found that the phenomenon can be effectively suppressed to suppress the variation in the service life and also to improve the average service life.

[0021]

Next, embodiments of the present invention will be described in detail. Table 1
A steel for a carburized bearing having the chemical composition shown in Table 1 was melted and hot-rolled to obtain a test material. 100mm x 8 from this material
A test piece of 0 mm x 10 mm was cut out and the frequency was 50 MH.
Total submerged immersion flaw detection using ultrasonic wave of z, depth 0.4m
The particle size distribution of the alumina cluster in the region up to m was measured.

[0022]

[Table 1]

Separately, a radial rolling fatigue test specimen having a test portion diameter of 12 mm was cut out from the same material and carburized and quenched and tempered under the conditions shown in FIG. However, only in Example 8, the carburizing temperature was 1050 ° C. The tempering temperature was adjusted within the range of FIG. 1 so that the hardness after tempering was about 62 in HRC.

Thereafter, the surface is polished by machining,
The rolling fatigue test was performed. Here, the rolling fatigue test was performed using a SUJ2 ball with a radial rolling fatigue tester.
The test was performed under the conditions of a surface pressure of 5880 MPa and a test temperature of room temperature. Table 2 shows the amount of alumina cluster having a length of 0.5 mm or more per unit volume and the results of a rolling fatigue test.

[0025]

[Table 2]

From the results in Table 2, it can be seen that, in the case of the present invention example in which Si and Ni are added in combination and the amount of alumina clusters having a length of 0.5 mm or more is regulated to 10 ⁻³ / mm ³ or less. , Ni and alumina clusters do not satisfy the conditions of the present invention, as compared with the comparative example, in which the cumulative failure probability of the rolling life is 10% life (L ₁₀ ) and 50% life (L ₁₀ ).
₅₀ ) Both are good, and especially Mo, V, Nb
It can be seen that in the case of Invention Examples 8, 11, 12, and 13 in which is further added, L ₁₀ and L ₅₀ are remarkably 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.

[0028]

As described above, according to the present invention, it is possible to obtain a carburized bearing steel having a small variation in fatigue life and a high rolling fatigue strength.

[Brief description of the drawings]

FIG. 1 is a view showing processing conditions of carburizing, quenching and tempering heat treatments of Examples and Comparative Examples of the present invention.

Claims (2)

[Claims] C: 0.1-0.4% Si: 0.4-3.0% Mn: 0.3-1.5% P: ≦ 0.03% S: ≦ 0. 03% Ni: 0.25 to 3.5% Cr: 0.3 to 5.0% Al: 0.005 to 0.050% Ti: ≦ 0.003% O: ≦ 0.0015% N: ≦ 0 0.025% The amount of alumina clusters having an alloy composition substantially consisting of Fe and having a length of 0.5 mm or more is 10
^-3 pieces / mm ³ or less steel carburized bearings, characterized in that. 2. C: 0.1-0.4% Si: 0.4-3.0% Mn: 0.3-1.5% P: ≦ 0.03% S: ≦ 0. 03% Ni: 0.25 to 3.5% Cr: 0.3 to 5.0% Al: 0.005 to 0.050% Ti: ≦ 0.003% O: ≦ 0.0015% N: ≦ 0 Mo: 0.03 to 2.5% V: 0.05 to 1.0% Nb: ≦ 0.1% Characterized in that the amount of alumina clusters having a length of 0.5 mm or more and an alumina cluster having a length of 0.5 mm or more is 10 ⁻³ / mm ³ or less. .