This paper has two objectives. The first is to propose a new inclusion rating method based on statistics of extreme. The new inclusion rating method is applied to the evaluation of inclusions contained in two super clean bearing steels with a total oxygen level of 5ppm and 8ppm.
The second objective is to investigate the critical lower bound of inclusion size which influences the fatigue limit. For this purpose, a special steel with extremely low inclusion content, the electron beam remelted super clean bearing steel(EB-CHR), was prepared. It is shown that by decreasing the size of inclusions, the detrimental effect of inhomogeneities larger in size than inclusions, becomes more important. The small inhomogeneity is thought to be a local imperfectly heat- treated structure, bainite.
Oxygen content of bearing steel will be reduced to below 3 ppm in the year 2000 if current trend for the reduction of oxygen in the steel continues. As a result, size of oxide inclusions will become smaller and the fatigue life will be doubled. From the viewpoint of life prediction, cleanliness evaluation methods currently used are not effective. Inclusion Rating Method by Statistics of Extreme is useful for both cleanliness evaluation and fatigue life prediction. Bearings made of suitably heat treated carbon steels or low alloy steels, which possess equivalent fatigue properties to bearing steels, will increase owing to the requirement for lower cost and better formability.
The quality of high-carbon chromium bearing steel, SUJ2 (SAE52100), produced by the electric arc furnace (EF)-ladle furnace (LF)-Ruhrstahl-Heraeus vacuum degassing vessel (RH)-vertical type continuous caster (CC) process, including the effects of impurities and cold working on its fatigue life, was investigated.
Bearing steel made in the experimental 100-kg induction furnace was also subjected to a fatigue life test in order to confirm the effect of impurities. The substantially low oxygen content of the CC bearing steel has resulted not only in high cleanliness which doubles the life of conventional ingot steel, but also in the reduction of buildup of nonmetallic inclusions on the inner surface of an immersion nozzle. Due to this reduction, approximately 10 000 metric tons (68 heats) of steel can be cast from one tundish-one nozzle without interruptions.
The fatigue life test was performed on specimens where the center segregation of bearing steel lay on their running surfaces with results that flaking sites did not tend to concentrate in the center portion. In other words, tests were done when center segregation did not affect the life of the specimen.
It has become clear that the fatigue life of the CC-bearing steel, despite reduced impurities, still depends mainly on the oxygen content of the steel. Also, the life test on the specimens made in the experimental furnace has confirmed that titanium and sulfur are hardly related to the life as long as their contents are kept at levels generally found in commercial bearing steel.
The life was further extended by cold working even when the oxygen content was extremely reduced. The bearing steel manufactured by the EF-LF-RH-CC-cold working process is accordingly considered to exhibit the longest life in commercially mass produced bearing steel.
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