A critical review of the challenges of developing continuous casting mold fluxes for high-Ti steels

Abstract

The large-scale production of high-Ti steels is limited by the formation of Ti-containing oxides or nitrides in steel-slag reactions during continuous casting. These processes degrade mold flux properties, clog submerged entry nozzles, form floaters in the molds, and produce various surface defects on the cast slabs. This review summarizes the effects of nonmetallic inclusions on traditional CaO–SiO₂-based (CS) mold fluxes and novel CaO–Al₂O₃-based (CA) low- or non-reactive fluxes containing TiO₂, BaO, and B₂O₃ additives to avoid undesirable steel, slag, and inclusion reactions, with the aim of providing a new perspective for research and practice related to balancing the lubrication and heat transfer of mold fluxes to promote smooth operation and reduce surface defects on cast slabs. For traditional CS mold flux, although the addition of solvents such as Na₂O, Li₂O, and B₂O₃ can enhance flowability, steel–slag reactions persist, limiting the effectiveness of CS mold fluxes in high-Ti steel casting. Low- or non-reactive CA mold fluxes with reduced SiO₂ content are a research focus, where adding other components can significantly change flux characteristics. Replacing CaO with BaO can lower the melting point and inhibit crystallization, allowing the flux to maintain good flowability at low temperatures. Replacing SiO₂ with TiO₂ can stabilize the viscosity and enhance heat transfer. To reduce the environmental impact, fluorides are replaced with components such as TiO₂, B₂O₃, BaO, Li₂O, and Na₂O for F-free mold fluxes with similar lubrication, crystallization, and heat-transfer effects. When TiO₂ replaces CaF₂, it stabilizes the viscosity and enhances the heat conductivity, forming CaTiO₃ and CaSiTiO₅ phases instead of cuspidine to control crystallization. B₂O₃ lowers the melting point and suppresses crystallization, forming phases such as Ca₃B₂O₆ and Ca₁₁Si₄B₂O₂₂. BaO introduces non-bridging oxygen to reduce viscosity and ensure flux flowability at low temperatures. However, further studies are required to determine the optimal mold flux compositions corresponding to the steel grades and the interactions between the various components of the mold flux. In the future, the practical application of new mold fluxes for high-Ti steel will become the focus of further verification to achieve a balance between lubrication and heat transfer, which is expected to minimize the occurrence of casting problems and slab defects.

Keywords

high-Ti steel / mold flux / inclusions / fluorine-free flux / interfacial reactions

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