Understanding stainless steelmaking through computational thermodynamics Part 1: electric arc furnace melting

Abstract

Stainless steel alloys are widely used in many important applications but their production presents difficulties because they contain expensive chromium, which can be extensively oxidised during decarburisation to the very low carbon levels required. Modern stainless steelmaking largely avoids this problem by having two distinct stages and is therefore described as duplex practice. Molten high carbon stainless steel is produced in an electric arc furnace and then the melt is decarburised in an argon–oxygen converter or a vacuum oxygen decarburising converter. In this work, computational thermodynamics has been used to examine the major reactions occurring in the electric arc furnace and to show the effect of various process variables on chromium recovery. It was shown that significant oxidation of the scrap must occur during melting, and that subsequent carbon/oxygen injection initially oxidises some chromium, but then mostly oxidises the added carbon. Chromium was predicted to exist in the slag as CrO and CrO_1·5 in almost equal proportions. Increasing the temperature should improve chromium recovery but results in less benefit than expected due to the decreasing activity coefficients of CrO and CrO_1·5 and the increasing oxygen partial pressure. Ferrosilicon additions reduce chromium oxides from the slag, but much of the silicon simply dissolves into the steel. Computational thermodynamics is seen to be a very effective educational tool for gaining an understanding of smelting processes.