The Effect of Refining Slag and Refractory on Inclusion Transformation in Extra Low Oxygen Steels
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re directly determined by the steel chemistry, while the steel melt composition was further affected by the refractory or refining slag. Therefore, refractory and refining slag can influence inclusions indirectly. During secondary refining, the Al2O3 inclusions generating from aluminum deoxidization during tapping would degrade into MgOÆAl2O3 spinel inclusions quickly. Because the magnesium is not added into the steel melt intentionally, in terms of the Mg source, both the top slag and refractory are the possible factors, however, many researchers[1–3] reported that the Mg for spinel generation was mainly from refining slag. Verma[1] argued that the reaction between aluminum-killed steel and MgO refractory did yield spinel as product, but the spinel layer remained at the steel–MgO refractory interface and did not enter the steel melt, and so the spinel inclusions generated mainly from the MgO bearing refining slag. Harada[2,3] studied the supply of Mg from MgO both in top slag and refractory into Al-killed molten steel. They found that both two factors had the potential of Mg supply and the ability to change the Al2O3 inclusions into MgOÆAl2O3 CHUNYANG LIU, Master Student, FUXIANG HUANG, Lecturer, and XINHUA WANG, Professor, are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing City 100083, P. R. China. Contact e-mail: [email protected] Manuscript submitted November 16, 2015. Article published online January 28, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
spinel, however, if the reaction rate was taken into account, the reaction between steel/refractory had a negligible effect on the Mg supply compared with slag/ steel reaction. Many researchers[4,5] investigated the effect of the composition of refining slag on spinel based on the assumption that the Mg for spinel generation mainly came from refining slag, and in both cases, they found that reducing slag basicity and MgO content in slag could dramatically hinder spinel generation. However, the conclusion that Mg for spinel generation mainly came from refining slag cannot explain the quick generation of spinel during the earlier stage of practical LF refining where the slag is in low basicity and even contains 3 to 5 pct FeO. During refining, the molten steel directly comes into contact with the ladle lining materials and from a metallurgical point of view, the refractory materials should be inactive to molten steel to avoid the interaction and contamination, however, the interaction is inevitable. We previously reported the effect of MgO-C refractory on the spinel generation and developed a kinetic model to describe the Al2O3 fi MgOÆAl2O3 transformation process.[6] The reduction reaction between MgO and carbon in the refractory can provide the molten steel with Mg vapor and then promote spinel generation rapidly. However, the effects of the MgO refractory and refining slag on spinel generation were not reported in the previous work and it would be discussed in this study. Re
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