Mathematical model for nitrogen desorption and decarburization reaction in vacuum degasser

A mathematical model for nitrogen desorption and decarburization reactions in vacuum degasser was developed. The rate controlling step is assumed to be the mass transfer of nitrogen, carbon and oxygen in molten steel and the chemical reaction of (N) under bar + (N) under bar = N-2 and (C) under bar + (O) under bar = CO. The bath surface and the surface both of the injected Ar gas bubble and CO gas bubble formed inside the bath are considered as the reaction sites. This model was verified by the agreement of the calculated results with the results of small scale experiments for nitrogen desorption and absorption reaction. The following results were obtained by the analysis of a VOD process of commercial scale for degassing of stainless steel by this model. The ratio of reaction rate for nitrogen desorption and decarburization at each reaction site during operation was evaluated. The nitrogen desorption reaction occurs at the CO gas bubble formed inside steel (51%), at bath surface (36%) and injected Ar gas bubble (13%). The decarburization reaction proceeds at bath surface (90%), at CO gas bubble (8%) and at injected gas bubble in (2%).