Structurally Based Assessment of the Influence of Fluorides on the Characteristics of Continuous Casting Powder Slags
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most all molten steel grades are nowadays being solidified by continuous casting. Owing to the different steel qualities to be cast, there are many requirements that are to be met by the continuous casting powders. In order to maintain a stable casting process and the required quality in terms of chemical composition and degree of purity of the steel, various tasks must be fulfilled by the continuous casting powders used. The following five criteria influence the quality of the different continuous casting processes in the order shown.
Ensure the strand lubrication, formation of a homogeneous heat transfer in the continuous casting mold,
thermal isolation of the steel surface for formation of a liquid meniscus,
protection of the steel surface from reoxidation through the surrounding air, and
a possibility for the absorption and separation of inclusions. The requirements for continuous casting powders and the slags formed from them were described in detail by Schulz et al.[1] Today, continuous casting powder slags are made up to 95 pct of the components CaO-Al2O3SiO2-Na2O-CaF2, and CaF2 collectively constitutes the TILO SCHULZ, formerly Ph.D. Student with Institute of Iron and Steel Technology, TU Bergakademie, Freiberg, Germany, is now Process Engineer with the ThyssenKrupp Steel Europe AG, Duisburg, Germany Contact e-mail: [email protected] BERND LYCHATZ, Scientific Group Leader, NICOLE HAUSTEIN, Ph.D. Student, and DIETER JANKE, Professor Emeritus, are with the Institute of Iron and Steel Technology, TU Bergakademie. This Paper is dedicated to Prof. Dr.-Ing. Dr. h.c. D. Janke who passed away during the preparation of this article. The authors express their grateful thanks to the former Head of the Institute of Iron and Steel Technology for many discussions about the structure and properties of slags. Manuscript submitted October 28, 2012. Article published online January 24, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
fluoride component. The remaining five percent is admixed to the casting fluxes or casting granulates as a carbon carrier to control the melting behavior. Continuous casting powder slags, in the essential, constitute modifications of a tetrahedrally coordinated ortho-silicate network. For the production of continuous casting powder slags, the calcium ions are integrated in the starting mineral, wollastonite 3 (CaOÆSiO2), via weak flexible bonds into the silicate network. The use of calcium fluoride in continuous casting powder slags offers several advantages concerning the requirements to be fulfilled: (a) Calcium fluoride lowers the liquidus temperature and the viscosity in calcium silicate melts, as a result of which strand lubrication is substantially improved. (b) Calcium fluoride increases the crystallization tendency of the casting flux slags and thereby the heat transfer. The formation of a crack-free strand shell is thus being favored. (c) Calcium fluoride does not react with carbon, as a result of which the melting behaviors of the casting flux slag and hence the thermal insulation of th
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