Abstract: The practice during RH de-sulphur process of silicon steel observed that de-sulphur effect not only happened by flux addition but also without flux when higher [Al] and [Si] inside the molten steel. Besides, de-sulphur was proved before deoxidation time to time. The last odd phenomenon was the decreasing sulphur content from RH end to tundish. Thermal dynamic calculation helped to find that the RH de-sulphur process would be divided to two stages – before deoxidation and after deoxidation. CS gas was generated by weak reaction between [C] and [S] in the first stage; in steel-slag reaction, SiS gas was generated by [S] and [Si], and complex reactions with [S], [Al], [Si] and [Ca] was conduct in the second stage. Consequently higher contents of [Al] and [Si] played big positive roles in the second stage. Furtherly, gases of CS and SiS formation inside the liquid metal before endpoint of converter bringed some of sulphur out, and then the CS gas would mix with oxygen to generate SO2 when meeting with the continous oxygen supplied in converter and the air on the surface of molten iron. CO2 component in the flux would be bad at carbon pickup control and alloy yield due to reactions with alloy elements as [Al], [Si].

Key words: RH refining, desulphurization, thermal dynamic analysis, carbon pickup