Effect of CaF₂ on Viscosity and Refining Ability of Highly Basic Slags for Duplex Stainless Steel

Abstract

To optimize CaF₂ content in highly basic CaO-18%Al₂O₃-SiO₂-10%MgO-CaF₂ (%CaO/%SiO₂=6, denoted as C/S=6) refining slags used for the production of Al-killed duplex stainless steel with high cleanliness demand, the effect of CaF₂ content on the viscosity and refining ability of the slags were studied and compared with typical CaF₂-free highly basic CaO-30%Al₂O₃-SiO₂-10%MgO (C/S=6) slag. The effect of CaF₂ addition in decreasing slag viscosity becomes less obvious with increasing temperature and CaF₂ content. When CaF₂ content exceeds 10%, slag viscosity only marginally decreases with further increasing CaF₂ content. Both monoxide-CaO and monoxide-MgO phases are precipitated in all the CaF₂-bearing slags. CaF₂ addition slightly increases monoxide-MgO precipitation, but dramatically decreases monoxide-CaO precipitation. Viscosities of the CaF₂-bearing slags were also theoretically calculated and good agreement with the measured values was observed. Moreover, the 6% CaF₂-bearing slag has very close viscosities above 1833 K but much lower viscosities below 1833 K, compared with the CaF₂-free highly basic slag. Further evaluation of the 6% CaF₂-bearing slag on steel cleanliness confirms that 6% CaF₂ addition is sufficient for the highly basic CaO-18%Al₂O₃-SiO₂-10%MgO-CaF₂ (C/S=6) slag. The mechanism of CaF₂ in decreasing the viscosity of CaF₂-bearing slags was discussed from the viewpoints that CaF₂ behaves as a network breaker and that CaF₂ suppresses the precipitation of solid phases. The first aspect was identified to play a much greater role in decreasing slag viscosity.