Compositional design of slag for homogenized rare earth treatment by slag-steel interaction under vacuum

Reduction of La₂O₃ in molten slag by the C in liquid steel during slag-steel interaction under vacuum is expected to achieve homogenized La treatment for steel. To develop the appropriate slag for La treatment, the liquidus temperature, viscosity and fluoride vaporization of 50 wt%CaF₂-30 wt%CaO-(20-x)wt%Al₂O₃-xwt%La₂O₃ slag were studied, as well as the La distribution between the La₂O₃ containing slag and GCr15 steel under vacuum. The results show that the liquidus temperature of slag firstly decreases with the increase of La₂O₃ content from 0 to 5 wt%, and then increases as the La₂O₃ content continuously increases. The LaAlO₃ phase starts to precipitates in slag when La₂O₃ content of slag exceeds 10 wt%, which accounts for the increases on liquidus temperature. Thus, the La₂O₃ content in slag should not exceed 15 wt%. La₂O₃ can play the depolymerization role on slag melts, which contributes to the decrease in slag viscosity. Furthermore, the increase of La₂O₃ content in slag decreases vapor pressure of CaF₂ and AlF₃, and fluoride vaporization is inhibited. Rare earth treatment with considerable La content through slag-steel interaction that dissolved C in liquid GCr15 steel reacts with La₂O₃ in molten slag is realized under vacuum. The La content in GGr15 equilibrated with slag bearing 15 wt% La₂O₃ under 0.5–1 kPa is 0.0135 wt%.