Effect of Ladle Usage on Cleanliness of Bearing Steel
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BEARINGS are used extensively as key components in the mechanical industry, and they require an excellent fatigue resistance. The cleanliness of bearing steel is of significant importance in its fatigue life.[1] Therefore, improved steel cleanliness is a key requirement in secondary steelmaking for bearing steels. During continuous casting, the liquid level will decline, and the refining slag will adhere to the ladle lining, which results in the formation of ladle glaze. The formed ladle glaze contacts the liquid steel directly and affects the cleanliness of the liquid steel in subsequent heats. Therefore, to improve the steel cleanliness, the influence of ladle glaze needs to be considered. Ladle glaze has been confirmed as an important source of inclusions in steel.[2–14] Some researchers[2] have found that calcium-aluminate inclusions before deoxidation originated from ladle glaze, whereas others[3] have pointed out that without Ca treatment, spinel inclusions in Al-killed steel were the main type of inclusions that originated from ladle glaze. Besides, some laboratory experiments[4] and industrial experiments[2,4,8,9] have indicated that ladle glaze is difficult to remove, and its effect becomes more severe with the increasing ladle age. Meanwhile, some industrial experiments[4] have been carried out to investigate the effect of
ladle glaze on inclusions in steel, and the impact behavior has been described by means of slag tracer. The present authors[7] have studied the effect of ladle glaze on the evolution of inclusions. In the presence of glaze, alumina inclusions can transform to spinel inclusions, and then to calcium-aluminate inclusions. These studies have focused mainly on a certain kind of ladle glaze. In fact, ladles are usually used for different steel grades, and different ladle glazes will be generated on the inner wall. As a result, when these ladles are used, the effect of ladle glaze on the cleanliness of steel would be different. Although some researchers[15,16] have tried to study this effect based on usage of different ladles, the mechanism is not understood clearly. Furthermore, some researchers[4] have described the effect of ladle glaze on the evolution of inclusions, but there is still a lack of direct evidence to support this observation, and further investigation is still required. In the current study, laboratory experiments were conducted to simulate ladle usage in industry. By means of slag tracer, the effect of different ladle glazes on the evolution of inclusions in bearing steel was studied. The evolution mechanism of inclusions is discussed, and some suggestions have been proposed for industrial practice based on the experimental results.
II. YUNGUANG CHI, ZHIYIN DENG, and MIAOYONG ZHU are with the School of Metallurgy, Northeastern University, Shenyang 110819, China. Contact e-mails: [email protected] and myzhu@ mail.neu.edu.cn Manuscript submitted May 12, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
EXPERIMENTAL
A. Materials In this study, MgO refractory rods (F 8.5 9 150 mm2) (suppli
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