JPH07126804A - Steel for carburizing bearing - Google Patents

Abstract

PURPOSE:To produce a steel for a carburizing bearing having high rolling fatigue strength small in dispersion. CONSTITUTION:This is a steel for a carburizing bearing in which the contents of alloy elements are regulated to, by mass, 0.1 to 0.3% C, 0.35 to 3.0% Si, 0.3 to 1.5% Mn, <=0.03% P. <=0.03% S, 0.3 to 5.0% Cr, 0.05 to 0.5% V and <=0.0010% O, and the balance Fe with inevitable impurities. Moreover, it may contain one or >=two kinds among <=3.0% Ni, <=1.0% Mo and <=0.1% Nb.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel for carburized bearings, which gives bearings excellent in wear resistance and surface fatigue strength.

[0002]

2. Description of the Related Art Conventionally, JIS has been used as a steel for carburized bearings.
Case-hardening steel represented by SCr420 and SCM420 has been used. However, the demand for longer life of bearings has increased, and various steels for carburized bearings having improved rolling contact fatigue strength have been proposed. In order to improve the rolling fatigue strength of steel, reduction of the O content of steel and addition of Si and Cr have been considered promising and have been studied. In addition, Cu and S
Effects of addition of various alloying elements including b have been reported.
However, although the fatigue strength is certainly improved by adding or increasing the amount of these alloying elements, there are some cases where the fatigue strength is extremely inferior, and there are large variations. For this reason, the average strength does not improve so much, and there is a problem in terms of reliability in practical use, so bearing steel with less variation is required.

[0003]

The present invention has been made in view of the above circumstances, and the object of the present invention is to provide a steel for carburized bearings with little variation and high rolling contact fatigue strength. To provide.

[0004]

Means for Solving the Problems The present inventor has studied the combination of various alloy elements, and as a result, improved rolling work strength,
In particular, it has been found that the combined addition of Si and V is extremely effective for suppressing the phenomenon of fracture in a short life and reducing the variation in life. That is, in the steel for carburized bearings of the present invention, the content of alloying elements is% by mass, and C: 0.1
~ 0.3%, Si: 0.35 to 3.0%, Mn: 0.3
~ 1.5%, P: 0.03% or less, S: 0.03%
Hereinafter, Cr: 0.3 to 5.0%, V: 0.05 to 0.
5%, O 2 is 0.0010% or less, and the balance is Fe and inevitable impurities. In addition to the above alloy elements, 1% by mass of Ni: 3.0% or less, Mo: 1.0% or less, and Nb: 0.1% or less.
It may contain one species or two or more species.

The reasons for limiting the alloy components will be described below. C: 0.1 to 0.3% C is an essential element for maintaining the strength of the steel, and in order to maintain the required core hardness after the carburizing and quenching treatment, 0.
It must contain 1% or more. However, if its content is too high, not only the core hardness after carburizing and quenching treatment is too high, but also the toughness is impaired, but the material hardness in the raw material state is increased, and the machinability is impaired. Therefore, the upper limit of the C content is 0.3%.

Si: 0.35-3.0% Si is an element which plays 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, the content rate of 0.35% or more is required. However, even if added excessively, the effect not only saturates, but also raises the transformation point of the steel, so it is necessary to raise the heat treatment temperature, and it causes disadvantages such as impairing forgeability and machinability. , The upper limit of the Si content is 3.0%.

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 steel and to secure hardenability. However, if added excessively, softening and annealing of the material becomes difficult and machinability is impaired, so the upper limit of the Mn content is made 1.5%. P: 0.03% or less Since P segregates at the austenite grain boundaries and impairs the toughness of the steel, its content is 0.03% or less.

S: 0.03% or less S impairs the hot workability of steel, and forms non-metallic inclusions in the steel to impair lateral toughness, so its content is 0.0.
3% or less. Cr: 0.3 to 5.0% Since Cr is an element that increases the hardenability of steel and enhances the softening resistance, 0.3% or more is added, but even if it is added excessively, the effect is saturated and mischief. Since it only increases the cost, the upper limit of the content rate is 5.0%.

V: 0.05 to 0.5% V is an element that plays an important role in the present invention together with Si, and has the effect of refining the crystal grains of steel, but it is synergistic especially in the coexistence with Si. It also has the effect of increasing the resistance to temper softening. Further, it is an element effective in forming carbonitrides in steel to suppress the segregation of Si and preventing the variation in fatigue life, in particular, the phenomenon of breaking in a short life due to rolling fatigue. In order to exert these effects, a V content of 0.05% or more is required within the Si content range. However, even if added excessively, the effect is saturated, and only the transformation point of the steel is unnecessarily raised, so the upper limit of the content is set to 0.5%.

O: 0.0015% or less O is an element that forms a hard non-metallic inclusion that becomes a base point of fatigue fracture in steel, so the smaller the content, the more preferable. 0.0010 considering
% Or less. Ni: 3.0% or less, Mo: 1.0% or less, Nb: 0.
1% or less Ni, Mo, and Nb are elements that improve the toughness of steel and increase the carburized layer hardness and internal hardness of carburized steel, so 3.0% or less, 1.0% or less, and 0.1% or less, respectively. % Or less, and may be added alone or in combination.

[0011]

Example A radial type rolling fatigue test piece having a test portion diameter of 12 mm was cut out from a hot-rolled steel material having the chemical composition shown in Table 1 and subjected to carburizing and tempering heat treatment under the conditions shown in FIG. However, only in Example 8, the carburizing temperature was 1050.
℃ was made. After carburizing heat treatment, the surface was ground and subjected to a rolling fatigue test.

[0012]

[Table 1] The rolling fatigue test is performed by SU with a radial type rolling fatigue tester.
The surface pressure was 5880 MPa using a J2 ball. The test results are shown in Table 2.

[0013]

[Table 2] As is clear from Table 2, Examples 1 to 10 of the present invention in which Si and V are added in combination have a cumulative failure probability of 10% (L ₁₀ ) and a cumulative failure probability of 50% (L ₅₀ ) in a rolling fatigue test.
In both cases, the improvement in L ₁₀ life is remarkable, and in particular, it is recognized that the variation in rolling contact fatigue life is reduced.

[0014]

According to the present invention, by selecting a unique alloy composition, it is possible to provide a steel for carburized bearings having a small variation and a high rolling contact fatigue strength.

[Brief description of drawings]

FIG. 1 is a diagram showing a temperature curve of carburizing and quenching and tempering heat treatment of a test material.

Claims (2)

[Claims] 1. The alloy element content is mass%, C: 0.1 to 0.3%, Si: 0.35 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%, V: 0.05 to 0.5%, O: 0.0010% or less, balance Fe and unavoidable Steel for carburized bearings, characterized in that it is made of mechanical impurities. 2. In addition to the alloy element according to claim 1, in mass%, Ni: 3.0% or less, Mo: 1.0% or less, Nb: 0.1% or less, one or two kinds. A steel for carburized bearings including the above and the balance Fe and inevitable impurities.