A mathematical model has been developed to describe the refining of stainless steel in a converter. This refining process includes both decarburisation and reduction stages. Reactions during the refining stage involve the metal, the slag, and the gas phases. Therefore, the model takes the three possible interfaces into account. The evolution of composition and temperature was computed for stainless steel melts. Comparison between computed results and industrial process data demonstrated the validity of the model. After adjustment of some model parameters, melts in a 100 t MRPA (material refining process of ALZ) converter with different process conditions were simulated. The agreement obtained was excellent, which illustrated that the industrial refining process could be simulated under different process conditions, making use of one set of fine tuned parameters.