Effects of FeO and CaO/Al 2 O 3 Ratio in Slag on the Cleanliness of Al-Killed Steel
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WITH the recent increase in demand for high-quality steel, the requirements for steel with higher cleanliness have become more stringent. Inclusions, which are normally determined as impurities, can cause many defects and degrade the properties of the steel product. Inclusions are directly determined based on the steel composition, whereas the steel composition is further affected by the refining of the slag and the refractory material. Therefore, the properties of the slag play very important roles in the formation of inclusions and in the improvement of the cleanliness in the steel. For the production of interstitial-free (IF) steel, the steel melt is generally created through the BOF-RH-CC process. During the tapping of the steel after BOF refining, the BOF slag with high amounts of FeO and
YUNQING JI, HUIXIANG YU, and XINHUA WANG are with the School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing, 100083, P.R. China. Contact e-mail: [email protected] CHUNYANG LIU and YAN LU are with the Department of Metallurgy, Graduate School of Engineering, Tohoku University, 2-11 Katahira, Aoba-ku, Sendai, 980-8577, Japan. FUXIANG HUANG is with the Steelmaking Plant of Beijing Shougang Co., Ltd., Qian’an, 100041, Hebei, P.R. China. Manuscript submitted May 1, 2018.
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MnO is easily poured into the ladle along with the steel. According to measures taken with regard to the slag carry-over, the subsequent RH process is divided into two processes. In the first process, the slag deoxidizer and the slag former are added into the steel during the tapping process, and the oxygen potential of the top slag is decreased. During the second process, the residue of the BOF slag continues to react with the steel, and no slag deoxidant is added. A number of researchers[1–6] have investigated the cleanliness and inclusions formed in steel through these two processes. At Sumitomo,[1,2] researchers studied the effects of slag oxidation on the Al loss during steel deoxidation at 10 kg and 250 t scales. Al loss increases from 1 to 3 ppm/min when the FeO content in the slag increases from 5 pct (all compositions in this paper are given in mass percentages unless specifically stated otherwise) to 10 pct. A large number of slivers present in the final products have been traced to the reoxidation originating from FeO in the ladle slag, and Lee et al.[3] and Kitamura et al.[4] have shown that the number of defects on cold-rolled sheets diminishes with a diminishing FeO + MnO ladle slag content at the end of the secondary metallurgy treatments. Qin et al.[5] found that the cleanliness of steel refined through slag with a lower oxygen potential (FeO 7 to 9 pct) is much better than that refined using slag with a higher oxygen potential (FeO 21 pct). They concluded that the FeO amount should be reduced to improve the cleanliness of the steel. The same
phenomenon was observed by other researchers.[6] However, the interaction mech
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