Oxidation of Chromium in Oxide–Fluoride Slags for Electroslag Remelting

Abstract

High-chrome steel billet with excellent mechanical properties and corrosion resistance is widely used in the production of components for heavy machinery and power equipment. Electroslag remelting is an effective and widely used technology for the production of such billet. If this technology is introduced in the production chain, highly uniform metallurgical characteristics (chemical composition, structure, content of nonmetallic inclusions, etc.) of the steel and hence excellent mechanical properties of the steel products may be ensured by regulating the refining and solidification processes. Effective refining of high-chrome steel in electroslag remelting depends on correct selection of the slag and the maintenance of the optimal redox potential, since one role of the chromium and other elements present in the slag is to transport oxygen from the gas phase to the liquid metal. On the basis of the electron structure of the slag systems, the influence of the degree of oxidation of the slag and the equilibrium oxygen partial pressure $${{P}_{{{{{\text{O}}}_{2}}}}}$$ on the oxidative state of the chromium in slags of type ANF-1, ANF-6, and ANF-29, which are widely used in Russia, may be assessed. The dependence of the concentration ratio Cr3+/Cr2+ on the temperature, degree of oxidation, and optimal basicity of the slag is established. A thermodynamic model is proposed for the variation in the oxidative state of chromium in the slag as a function of its degree of oxidation. Calculation results are compared with experimental data for slag systems at 1873 K. It is shown that the mean oxidative state of the chromium decreases with increase in temperature and with decrease in oxygen partial pressure and optical basicity of the slag. The presence of fluorine in the slag affects the ratio Cr3+/Cr2+. With decrease in oxygen partial pressure from 10–4 to 10–12 Pa at 1873 K, the mean oxidative state of chromium in fluoride–oxide slags decreases from +3 to +2. A formula for estimating the ratio Cr3+/Cr2+ in fluoride–oxide slags is proposed, taking account of the temperature and oxidation of the slag.