Formation of Low-Melting-Point Inclusions in Al-Deoxidized Steel Refined by High-Basicity Calcium Aluminate Slag in ZrO
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NONMETALLIC inclusions have been focused on greatly in recent years. Despite some reports of ‘‘oxide metallurgy’’ technology on using inclusions with proper size distribution and chemistry to help refine the microstructure of steel, inclusions are usually recognized to be harmful to the in-service behaviors of steel. Besides, inclusions, either in large size or with high melting point, are especially negative to the improvement of productivity. A typical problem caused by highmelting-temperature inclusions is the so-called nozzle clogging, which was initiated by the agglomeration of those inclusions on the inner wall of submerged entry nozzle (SEN) of continuous casting.[1] The clogging would result in a worse flow field of molten steel in the casting mold. When they are washed down into a mold, surface defects, fatigue breakage, etc. would occur on final products. To solve these problems, as usual recognition, one should not only improve the cleanliness of molten steel but also target the low-melting-temperature inclusions.
MIN JIANG, formerly Postdoctor Research Fellow, with Hanyang University, Ansan-Si, Kyeonggi-do, Republic of Korea, is now Assistant Professor, School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing, P.R. China. Contact e-mail: [email protected] XIN-HUA WANG, Professor, is with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing. JONG-JIN PAK, Professor, is with the Department of Materials Engineering, Hanyang University. Manuscript submitted November 22, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
Al is a popular deoxidizer for its strong affinity to oxygen. However, the produced high-melting-point alumina (Al2O3) inclusions tend to agglomerate into large clusters and frequently are observed to initiate SEN clogging.[2] Besides, high-melting MgOÆAl2O3 (spinel) inclusion is a big issue for Al-killed steel. Spinel can be formed easily because MgO-based refractory materials are widely used in steelmaking. As a result, spinel inclusions have been intensively studied in recent years.[3–8] Both of these types of inclusions are very detrimental. For example, spinel inclusions are known to be the main reason for rejected surface quality and decreased fatigue life span of stainless steels—other than nozzle clogging—and had been reviewed systematically by Park and Todoroki.[9] Therefore, up to now, many papers have focused on how to produce low-melting-point inclusions in Al-deoxidized steel. Ca treatment has been well known to be favorable to modify Al2O3 into liquid calcium aluminates. It was also reported to be helpful to change MgOÆAl2O3 into low-melting-temperature CaO-MgOAl2O3 inclusions.[10–13] Although Sakat mentioned that the modification of MgOÆAl2O3 into MgO by Mg treatment can prevent the clogging in MgO-C nozzle, Mg treatment is a difficult operation in industrial practice.[14] The proper choice of the refining slag system was proven to be another useful method for lowmelting-point inc
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