Understanding stainless steelmaking through computational thermodynamics Part 2 – VOD converting

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.