Development of Fluoride-free Mold Powders for Peritectic Steel Slab Casting

In this paper, titanium-bearing blast furnace slags (CaO–SiO2–TiO2) produced at Panzhihua Iron and Steel Company (P. R. China) is used as the base material to develop fluoride-free (F-free) mold powders to improve the heat transfer between the mold and the strand. Effects of the binary basicity (CaO/SiO2), TiO2, Na2O, Li2O, MgO, MnO and B2O3 on the melting temperature, viscosity and heat flux of F-free mold powders are investigated. The laboratory results indicate that 1) the melting temperature and the viscosity of the F-free powder decrease, as expected, with increasing the content of Li2O, B2O3 and Na2O respectively, but the lowest viscosity is achieved with 6.0 mass% TiO2; 2) the heat flux of the F-free slag film with 1.0–6.0 mass% TiO2 is close to that of a conventional mold slag film with 2.0–10.0 mass% F; 3) the effect of basicity of the F-free powder on the heat flux is the same as the powder bearing fluoride; 4) the heat flux changes significantly with more than 8.0 mass% Na2O and about 4.0 mass% MnO, whereas the effects of Li2O and B2O3 in the F-free powder on heat flux are not significant. The suitable range of main components of the F-free powder with TiO2 is proposed for casting peritectic-grade-steel slabs. The industrial trials of peritectic steel casting, using the proposed F-free flux, reveals a good surface quality of the slab, and well-controlled heat transfer at the continuous casting mold by the F-free powder with the precipitated crystalline phase being perovskite (CaTiO3) instead of cuspidine in the conventional mold slags that contain fluoride.