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.
The central issue in stainless steelmaking is the difficulty of oxidising carbon from molten steel without also oxidising large proportions of expensive chromium. This can, however, be achieved by reducing the partial pressure of the gaseous product of carbon oxidation, carbon monoxide. Modern stainless steelmaking is dominated by duplex processes, which prepare a high carbon melt in an electrical arc furnace and then decarburise the melt in a converter, such as a 'vacuum oxygen decarburisation' converter in which oxygen is blown onto the melt in an evacuated chamber. In this work, the thermodynamic basis of preferential carbon oxidation at low total pressures is discussed, together with a review of VOD practice. VOD steelmaking is then simulated using computational thermodynamics software to illustrate the process principles. It was predicted that CrO is the dominant chromium oxide species in slag and that solid CrO 1?5 will be formed during oxygen blowing. Carbon contents as low as 0?001 wt-% are possible, but not achieved due to mass transfer rate limitations. Silicon is a very effective reductant for chromium oxides and also reduces some of the MnO in slag. The recovery of chromium from the slag is very high, but limited by the increasing proportion of added silicon, which dissolves into the steel bath.
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