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INTRODUCTION

AS the demand for cleaner steels increases every year, deoxidation is key to minimizing the free oxygen and improving the cleanliness of steel. Most of the inclusions are produced during the deoxidation of molten steel. Nonmetallic inclusions are harmful to both the castability of the steel and the quality of products.[1] Aluminum is one of the most popular deoxidizers due to its strong ability of deoxidation.[2–4] The only three possible sources of Mg are as follows: (1) Some ferroalloys contain a certain amount of Mg,[4,5] (2) the ladle slag with high MgO content can be a major contributor of Mg to alumina inclusions in steel causing the formation of spinel inclusions,[2,6–8] and (3) the other possible source of MgO in inclusions are ladle lining and tundish lining because MgO-graphite and MgOCaO-graphite bricks are extensively used as the lining refractory at steel mills.[9] Therefore, MgO-containing inclusions are almost unavoidable in steel. In addition, titanium is an important alloy element to be used in some high-grade steels because it improves the toughness of steel or binds the interstitial atoms such as C and N by precipitating titanium carbide and nitride during solidification.[10] The reported formation of Al-Mg-Ti-O system inclusions, including Al2O3, TiOx, MgO-Al2O3, Al2O3-TiOx, and MgO-Al2O3-TiOx, are summarized in Table I.[3,11–43] MgOÆAl2O3 spinel inclusions have a detrimental effect on the property of steels severely due to their YING REN and HAOJIAN DUAN, Ph.D. Students, LIFENG ZHANG, Professor, and WEN YANG, Postdoctor, are with the Beijing Key Laboratory of Green Recycling and Extraction of Metals (GREM) and the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing (USTB), No. 30, Xueyuan Road, Haidian District, Beijing 100083, P.R. China. Contact e-mail: [email protected] Manuscript submitted January 24, 2014. Article published online August 7, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B

high melting point and high hardness. Moreover, spinel inclusions cause the clogging of submerged entry nozzle during continuous casting.[44] Calcium treatment can effectively modify MgOÆAl2O3 spinel inclusions to complex CaO-Al2O3-MgO or CaO-Al2O3 liquid inclusions, which can avoid or prevent MgOÆAl2O3 spinel inclusions from damaging the quality of steel.[18,45–51] However, Yang et al.[9] recently concluded that although MgOÆAl2O3 spinel inclusions could be modifed to liquid inclusions by calcium treatment in thermodynamics, for the reason of kinetics, MgOÆ Al2O3 inclusions with size of 5-lm MgOÆAl2O3 spinel inclusions because the core of the inclusion was still pure MgOÆAl2O3 spinel. The experiment results show that compared with the Ca treatment after the Ti addition method, the Ti addition after the Ca treatment method is conducive to modifying spinel inclusions in titanium-containing stainless steel and thus extend the life of the submerged entry nozzle.[52] There was a liquid region in the phase diagram of a MgO-Al2O3-TiOx system at 1873