Effect of MgO on Crystallization and Heat Transfer of Fluoride-Free Mold Fluxes
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MOLD fluxes play significant roles in continuous casting of steel, controlling heat transfer, providing lubrication, preventing oxidation, and entrapping inclusions.[1] Horizontal heat transfer across the gap between steel strand and copper mold is vital for the quality control of steel products as an inappropriate heat-transfer rate could cause severe surface imperfections and even operational accidents, such as longitudinal cracks, bulging, breakout, and other incidents.[2–4] Heat-transfer control becomes particularly important in the casting of peritectic steel which undergoes a d-Fe to c-Fe phase transformation with 4 pct volumetric shrinkage.[3,5] This phase transformation aggravates the localized thermal stress in the weak part of the steel shell, initiating longitudinal cracks, especially when horizontal heat-transfer rate is excessively high.[6,7] Heat-transfer rate is mainly determined by the crystallization behavior of mold fluxes. The presence of crystalline phases in the flux film reduces radiative heat-transfer rate by scattering photons at grain boundaries.[8–10] Crystallization also induces pores, cracks, and air gaps which have
JIAN YANG, JIANQIANG ZHANG, YASUSHI SASAKI, and OLEG OSTROVSKI are with the School of Materials Science and Engineering, University of New South Wales, Sydney, NSW, 2052, Australia. Contact e-mail: [email protected] CHEN ZHANG and DEXIANG CAI are with the Baosteel Group Corporation Research Institute, Shanghai, 201900, P.R. China. YOSHIAKI KASHIWAYA is with the Department of Energy Science and Technology, Kyoto University, Yoshida Honmachi, Sakyo-ku, Kyoto, 606-8501, Japan. Manuscript submitted February 7, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS B
considerable thermal resistances.[11–14] Therefore, mold fluxes with high crystallinity are used for peritectic steel casting. In commercial mold fluxes, fluorides (CaF2 and NaF) are used to reduce heat-transfer rate through the formation of cuspidine (Ca3Si2O7F2).[15,16] However, fluorides are highly volatile at high temperature and form compounds with high vapor pressure, such as HF and SiF4.[17,18] The gaseous fluorides evaporate to the atmosphere with a negative environmental impact. They are detrimental to the health of human beings if inhaled in an excessive amount. Submerged entry nozzle is also eroded by fluorides, which shortens its lifetime and increases the maintenance cost.[19] As concerns over increasing fluoride pollution in steelmaking have been raised in recent decades, the development of fluoride-free mold fluxes for steel continuous casting becomes more important. Several investigations have been conducted to reduce the use of fluorides in mold fluxes.[20–25] The requirements on viscosity of fluoride-free mold fluxes can be met by adjusting the concentration of fluxing agents.[18,26] However, the heat transfer across the fluoride-free mold flux film is normally faster in comparison with the fluoride-containing one, which may cause higher risk of cracking defects.[18] The lack of fundamental knowledge of crystallization
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