Reaction Model for Carbon, Manganese, and Oxygen in Bottom Blowing with Mixed Gas in Final Stage of Steel Refining in Converter

Abstract

A theoretical study of the reaction rates of carbon, manganese, and oxygen was made, and the computational results were compared with experimental results from a commercial plant to enable prediction of the enhancement of the decarburization rate in the final stage of refining by bottom blowing with mixed gas. A reaction model which combines the stoichiometric and equilibrium relationships among carbon, manganese, and oxygen and their mass transfer rates was made for decarburization as a gas-metal reaction and for manganese and iron oxidation as slag-metal reactions. It was assumed that 1) addition of inert gas through submerged tuyeres contributes to bath stirring as well as to a decrease in CO pressure, and 2) all rate determining steps in the gas-metal and slag-metal reactions are the mass transfer of carbon, manganese, or oxygen in the molten steel. Fitting the parameters under a 1/1 oxygen-nitrogen flow rate ratio, the calculated metallurgical properties in ordinary blowing (bottom blowing with pure oxygen) agreed well with the experimental results. Calculations for certain operating factors showed that the decarburization rate increases and the oxidation rates of manganese and iron decrease as the bath temperature increases and the ratio of oxygen to inert gas decreases. The contribution ratio of the gas-metal reaction relative to the slag-metal reaction in bottom blowing with mixed gas is 6 to 9 times larger than that in top blowing with mixed gas.