Abstract: Current studies for constituents of RH desulphur fluxes are mainly focusing on lab or industrial tirals, merely nothing about the thermal dynamics. The paper involved by FactSage in the process technologies about the influences by main constituents and assistant constituents of desulphur fluxes. It was found that in light treatment moldel with p=4 kPa, no gases of SiS, Ca or Mg generate in Silicon steel RH furnace, however in deep vacuum treatment model with p=50 Pa, nearly 100 % of MgO and part of CaO is reduced by aluminium inside molten silicon steel and SiS gas get out simutaneously. Gases of Ca and SiS increase with higher temperature, esp. >1 500 ℃, and higher slag steel ratio. Generation of SiS gas is depressed by inceasing aluminium content, in the contrary, encouraged by increasing silicon content. Ternary diagram calculations revealed that there is a typical range of flux constituents for best desulpur practice with 100 % slag liquid and CaS phase, which includes, w(CaO)=52 %~60 %, w(Al₂O₃)=36 %~48 %, w（CaF₂）=0~12 %.

Key words: RH refining, desulphurization flux, simulation, chemical constituents