Method for manufacturing ingot steel for bearings and bearing steel

Abstract

An ingot steel for bearings has a composition containing C: 0.56% by mass or more and 0.70% by mass or less, Si: 0.15% by mass or more and less than 0.50% by mass, Mn: 0.60% by mass or more and 1.50% by mass or less, Cr: 0.50% by mass or more and 1.10% by mass or less, P: 0.025% by mass or less, S: 0.025% by mass or less, Al: 0.005% by mass or more and 0.500% by mass or less, O: 0.0015% by mass or less, N: 0.0030% by mass or more and 0.015% by mass or less, and a remainder of Fe and incidental impurities, wherein eutectic carbide formation index Ec satisfies 0<Ec≦0.25.

Description

TECHNICAL FIELD[0001]The present invention relates to an ingot steel for bearings (an ingot steel for use as a bearing steel) having excellent rolling contact fatigue life characteristics and suitable as a bearing material for use in automobiles, wind power, transport machines, electrical machines, precision machines, and other general industrial machinery. The present invention also relates to a method for manufacturing a bearing steel using the ingot steel for bearings.BACKGROUND ART[0002]High-carbon chromium steel (JIS G4805 standard SUJ2) has been widely used as a bearing steel of this type. In general, one of the important properties of bearing steels is their excellent rolling contact fatigue life characteristics. It is generally believed that the rolling contact fatigue life of steel is shortened by the presence of a non-metallic inclusion or eutectic carbide in the steel.[0003]It has been believed in recent studies that non-metallic inclusions in steel are mainly responsible...

AI Technical Summary

Benefits of technology

[0099]In accordance with the present invention, an ingot steel for bearings having much better rolling contact fatigue life characteristics than conventional bearing steels can be stably manufactured. Thus, bearing steels having a small section to those having a large section can be manufactured. The present invention also contributes to the upsizing of wind power generators, transport machines, and general industrial machinery, providing industrially advantageous effects.

Problems solved by technology

However, the manufacture of a bearing steel containing a decreased number of non-metallic inclusions requires expensive converter steelmaking machines or extensive modifications of conventional facilities, which entails an immense economic burden.

However, as described above, a further reduction in oxygen content to manufacture a bearing steel containing a decreased number of non-metallic inclusions requires expensive converter steelmaking machines or extensive modifications of conventional facilities, which entails an immense economic burden.

For example, although a high-carbon chromium steel, which contains 0.95% by mass or more C, is very hard and has high wear resistance, the central portion of a casting steel has a high degree of segregation (hereinafter referred to as centerline segregation), and furthermore enormous eutectic carbide is formed in the casting steel, which shortens the rolling contact fatigue life.

However, Sb may be highly toxic to human bodies and should therefore be treated carefully.

Furthermore, the addition of Sb results in the condensation of Sb in the central segregation zone, worsening the centerline segregation.

The different hardness may induce rolling contact fatigue fracture, shortening the rolling contact fatigue life.

However, because of nonuniform steel temperature in soaking, a soaking temperature exceeding the solidus line in a portion could initiate melting in the portion again and induce a eutectic reaction, forming further enormous eutectic carbide.

Although the carburization or carbonitriding can improve the rolling contact fatigue life characteristics, it may increase the manufacturing costs or decrease the yield because of a large strain or dimension change, thus increasing the product cost.

This requires extensive modifications of carburization or carbonitriding facilities, which entail an immense economic burden.

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