Crystallization Behavior and Heat Transfer of Fluorine-Free Mold Fluxes with Different Na 2 O Concentration
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MOLD fluxes serve essential roles in the continuous casting of steel, such as lubrication, heat transfer control, thermal insulation, oxidation prevention, inclusion entrapment, etc.[1–3] The non-uniform heat distribution on solidifying shell is likely to cause longitudinal cracks on the surface of steel products in the casting of slabs.[4] The defects become more serious and frequent in casting medium-carbon steels which undergo d-Fe to c-Fe peritectic phase transformation during solidification with 4 pct volumetric shrinkage. In practice, mold fluxes with high crystallization tendency are recommended in the casting of cracking-sensitive steels since the thick crystalline flux layer formed between strand and copper mold inhibits heat transfer.[5–7] It is generally accepted that photons are scattered by grain boundaries, which reduces the thermal radiation; heat transfer is also
JIAN YANG, Ph.D. Candidate, JIANQIANG ZHANG, Associate Professor, YASUSHI SASAKI, Visiting Professor, and OLEG OSTROVSKI, Emeritus Professor, 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, Senior Research Fellows, are with the Baosteel Group Corporation Research Institute, Shanghai, 201900, China. YOSHIAKI KASHIWAYA, Associate Professor, is with the Department of Energy Science and Technology, Kyoto University, Yoshida Honmachi, Sakyo-ku, Kyoto, 606-8501, Japan. Manuscript submitted April 13, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS B
impeded by pores, cracks, and air gaps formed as a consequence of the formation of crystalline flux layer.[7–10] The suppression of heat withdrawal rate alleviates the uneven thermal stress so as to decrease the likelihood of surface imperfections. Currently, most of commercial mold fluxes contain fluorides, such as CaF2 and NaF, to ameliorate lubricity of liquid flux layer and control heat transfer rate through the precipitation of cuspidine (Ca4Si2O7F2).[11–14] However, the ever-rising demand for eco-friendly steel-making process requires the industry to minimize the pollutive emission of gaseous fluorides, such as HF, SiF4, and NaF, during casting operation, which causes corrosion of the secondary cooling system of casters and the hazards to environment.[1,15–17] Therefore, the development of fluorine-free mold fluxes has become a hotspot of research in pyrometallurgy in recent years. Although several fluorine-free substitutes for cuspidine were proposed, such as Ca11Si4B2O22,[18–21] Ca2Si3Na2O9,[22] and CaSiTiO5,[23] the lack of fundamental knowledge of fluorine-free flux system still inhibits their application. Na2O has been proved to be an important component that significantly affects the properties of fluorine-containing mold fluxes.[13,14,18,24] It is considered to be an important component for the design of fluorine-free mold fluxes. In this study, the influence of Na2O on crystallization behavior and heat transfer of CaO-SiO2Na2O-B2O3-based fluorine-free mold fluxes is investigate
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