Influences of Basicity and Li 2 O on the Properties of Fluorine-Free Mold Flux for the Casting of Medium Carbon Steels
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T is well known that mold fluxes play important roles in continuous casting, such as protecting the molten steel from oxidation, absorbing inclusions, providing the thermal insulation, lubricating the strand, and controlling the heat transfer between the mold and steel shell.[1] Generally the commercial mold fluxes contain 5 to 10 pct CaF2 which tends to control the viscosity and melting temperature of the mold flux,[2,3] as the fluoride can reduce the polymerization of the silicate network and the viscosity of silicate slag.[4] Although fluorides play important roles in the mold flux system, i.e., reducing viscosity and break temperature to improve lubrication ability and forming cuspidine (Ca4Si2O7F2) to control the heat flux in the mold;[5] however, it would be evaporated and released to the environment in the form of HF, SiF4, NaF, etc., during the process of casting. Thus, these fluorides will lead to the corrosion of equipment, serious environmental pollution, etc.[6,7] Therefore the development of new environmental friendly F-free mold fluxes is of great importance. As it is well known, longitudinal cracks may form on the slab surface when casting medium carbon (MC) steels, which is mainly caused by the volumetric WANLIN WANG, Shenghua Professor, XIONG YAN and SENLIN XIE, Graduate Students, and LEJUN ZHOU and DAOYUAN HUANG, Lecturers, are with the School of Metallurgy and Environment, Central South University, Changsha 410083, P.R. China. Contact e-mails: [email protected]; l.j.zhou@hotmail. com Manuscript submitted August 4, 2015 Article published online February 1, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
shrinkage during d-c phase transformation. Therefore, the heat transfer across the mold flux during the casting of MC steels must be reduced and keep uniform to minimize the thermal stress induced by the volumetric shrinkage. Consequently, the crystallization of the mold flux is very important, and the main challenge of developing F-free mold flux for casting medium carbon steels is to obtain a suitable crystalline phase to replace cuspidine, such that the heat transfer could be effectively controlled to solve above longitudinal cracks. Nakada et al.[3,7,8] studied the crystallization behaviors of CaO-SiO2-TiO2 slag system and their results suggested that the precipitate CaOÆSiO2ÆTiO2 in the F-free mold flux could be a substitute for cuspidine. However, Wang et al. found that the formation of the high melting point compounds such as TiN and Ti(C,N) in the liquid slag during the casting process may cause the increase of viscosity, break temperature, Tbr and crystallization temperature, which leads to a lack of lubrication and the tendency of sticker breakout.[9] In other words, TiO2 may not be an ideal substitute for fluoride in the F-free mold fluxes. Recent studies of Wang[10] indicated that Ca11Si4 B2O22 precipitated in the low fluorine mold fluxes shows an ideal potential for the substitution of cuspidine. But the crystallization property of the designed F-free mold flux is not strong enough compared with the
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