Structural evolution and heat transfer mechanisms of fluorine-containing and fluorine-free mold flux films for high-Ti steels

This study investigates the effects of CaF 2 and TiO 2 on slag-film evolution and heat-transfer behavior in CaO–Al 2 O 3 -based mold fluxes designed for high-titanium steel continuous casting. Heat-flux measurements, slag-film characterization, and molecular dynamics (MD) simulations were combined to systematically evaluate the evolution of heat flux, slag-film thickness, surface roughness, porosity, and melt structure. Fluorine-containing fluxes (5 and 10 wt-% CaF 2 ) and fluorine-free Ti-containing fluxes (4 and 8 wt-% TiO 2 ) were selected as representative systems to establish the relationships among melt structure, slag-film characteristics, and heat-transfer performance. Among the investigated compositions, the steady-state heat flux ranged from 0.539 to 0.668 MW m −2 . Compared with the fluorine-containing mold fluxes, the fluorine-free Ti-containing mold fluxes exhibited lower steady-state heat fluxes, indicating superior heat-transfer control during the later stages of solidification. Concurrently, the slag film thickens, and the crystalline phase evolves from competitive multiphase precipitation to a perovskite (CaTiO 3 ) dominated regime. Consequently, surface roughness increases from 14.32 to 19.62 µm, while apparent and true porosities rise from 1.63 to 1.85 vol.-% and 3.82 to 5.32 vol.-%, respectively. MD simulations revealed that increasing F content decreased the proportion of Al IV species while increasing that of Al III species, as F progressively substituted for O to form [AlO 3 F] 4− structural units. Upon replacing CaF 2 with TiO 2 , the coordination numbers of Al–O and Ti–O increased, leading to a more complex melt network structure and a lower heat-transfer capability of the liquid slag. Consequently, the fluorine-free Ti-containing mold flux exhibited a stronger ability to regulate heat transfer than its fluorine-containing counterpart, which is in good agreement with the heat-flux measurements. By systematically elucidating the evolution of slag-film structure and the associated heat-transfer mechanisms in both fluorine-containing and fluorine-free mold fluxes, this study provides fundamental insights and theoretical guidance for the development of fluorine-free mold fluxes for the continuous casting of high-titanium steels.