Steel having finely dispersed inclusions

Abstract

An object of the invention is to provide a steel having good fatigue life characteristics and acoustic characteristics by eliminating the harmful effects of oxide based inclusions. The object is attained by the following steel. A steel having at least a part comprising: REM of which amount meets formula (1); and REM-containing inclusions of which number meets formula (2); wherein the concentration of Al203 in the REM-containing inclusions is 30 mass % or less (including 0%); Formula (1); −30<REM(ppm)−(T.O(ppm)×280 / 48)<50, Formula (2); number of REM-containing inclusions / total number of inclusions >0.8, wherein the inclusions of Formula (2) having an equivalent diameter of 1 μm or more.

Description

BACKGROUND OF THE INVENTION [0001] The present application claims priority to Japanese Application 2003-068541, filed in Japan on Mar. 13, 2003 and which is herein incorporated by reference in its entirety. FIELD OF THE INVENTION [0002] This invention relates to a steel having finely dispersed oxidized inclusions and oxysulfide inclusions, and particularly to a steel having good fatigue life characteristic and acoustic characteristics obtained by eliminating a harmful effect of oxide based inclusions. The inclusions refer to an oxide inclusion, a sulfide inclusion, an oxysulfide inclusion, a nitride inclusion and a combination thereof. The steel refers to a molten steel, a cast slab, cast bloom, cast billet and other half finished products and final steel products and is not limited in use. DESCRIPTION OF THE RELATED ART [0003] Recently it is required that the quality for steel products is more strict and diversified. A steel with a high degree of cleanliness having inclusions which...

AI Technical Summary

Benefits of technology

The invention is about a steel that has good fatigue life and acoustic / vibrational characteristics by fining and dispersing oxide and oxysulfide based inclusions. This steel is referred to as inclusion-fined-dispersed steel. The invention is achieved by adding a specific amount of REM (Removing Oxygen from the Metal) and REM-containing inclusions with a specific composition to the steel. The inclusions should have an equivalent diameter of 1 μm or more. The steel should also meet certain conditions, such as a specific content of Al, Ti, S, and O, to further improve its fatigue life and acoustic characteristics. The invention is useful for manufacturing bearing steel and small bearing steel components for hard disk or audiovisual equipment.

Problems solved by technology

These Al2O3 based inclusions are hard and coagulate to form coarse Al2O3 clusters, which cause: (a) snapping of wires such as tire cords; (b) an impairment in the rolling fatigue characteristics in steel bars such as in bearing steel; and (c) a crack in the can during the can-forming process such as drawn and ironing (DI) can made of thin steel sheet.

In these products the bearing steel is used under a light load, however, the sound or vibration generated from moving parts made of bearing steel is required to be as low as possible.

However, the above-mentioned methods for eliminating Al2O3 based inclusions are not sufficient to meet the high quality level now in demand.

However, after being dissolved into the molten steel, it is difficult to prevent Mg from vaporizing as in the case of when Mg alone added, which leads to low yields.

Therefore, the requirements of low vibration and the acoustic characteristics for small bearing steel products still can not be sufficiently met.

This may be helpful to prevent sliver defects but it can not reduce the size of the inclusion to the level required for a bearing steel.

Therefore, it is thought that if there is more REM present than is necessary to react with all of the O, then the excess REM will form a sulfide which provides a harmful effect to the fatigue life characteristics by increasing the size of the inclusions.

However, further improvement in the acoustic characteristics can not be expected even if an absolute amount of oxide based inclusions is reduced by lowering T.O. (total oxygen), since oxide based inclusions are hard because of the Al2O3.

Al2O3 can be reformed into a fine spinel by Mg addition, which is not as hard as Al2O3 as shown in Table 1, but is still hard and does not provide adequate improvement.

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