Desulphurisation Process in RH Degasser for Soft-killed Ultra- low-carbon Electrical Steels

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This study introduces the desulphurisation process in RH (Vacuum Recycling Degassing) degasser developed for the soft-killed ultra-low-carbon electrical steels. A CaO–Al2O3–CaF2–based desulphuriser with high sulphide capacity was developed by laboratory experiments of steel desulphurisation and by considering the floatation of Al2O3 (the deoxidation product). The optimum composition of the desulphuriser was found to be mass%CaO/mass%Al2O3=2.5–4, SiO2<5% and F–<8% with the melting point lower than 1350°C. To prevent the resulphurisation of steel, the effect of components in the top slag on sulphur partition between slag and steel was calculated using the thermodynamic package FACTSAGE and the range of the components in the top slag was proposed to control the oxygen potential. It was found that FeO+MnO content in the top slag should be controlled below 15% before RH and 10% after RH degassing treatment. Industrial trials were carried out by controlling the FeO+MnO content in the top slag via slag modification and adding RH desulphuriser after final deoxidation. The results from industrial trials showed that resulphurisation was effectively prevented by controlling the FeO+MnO content in top slag below 15% via slag modification. The combination of the top slag modification and the addition of RH desulphuriser consistently produced the ultra-low-carbon steel with sulphur content less than 0.005%.