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.

1. Introduction

Owing to their outstanding strength, corrosion resistance, and high-temperature performance, Ti-containing steels have found widespread application in the aerospace, petrochemical, and biomedical industries [1–3]. Ti binds to carbon to inhibit strain-induced martensitic transformation in stainless steels, and the carbides enhance the thermal stability through Zener pinning, which restricts grain recrystallization and growth [4–5]. In addition, Ti forms a dense TiO₂ film on the steel surface, significantly improving corrosion resistance and maintaining high stability, even in acidic environments. Furthermore, Ti carbides and nitrides prevent chromium carbide precipitation, thereby enhancing the intergranular corrosion resistance [6–7]. Ti also strengthens and hardens steel through precipitation hardening, with Ti(C,N) precipitates at grain boundaries that refine the grain structure and increase the strength. In maraging steels, Ti notably increases the hardness and tensile strength, reducing the risk of brittle fracture [8–10].

Increasing the content of highly reactive Ti in steel poses various challenges. Most casting difficulties or surface defects that manifest during casting and in the final product are typically attributed to the formation of various solubility-product inclusions, such as TiN, TiOx, or Al₂O₃ [11]. Generally, these inclusions begin to form in the ladle with the addition of Al or Ti and continue to precipitate as the steel cools. They sometimes interact with other inclusions originating from slag entrapment during tapping or with other large exogenous inclusions such as refractories [12]. Moreover, compositional changes, such as a reduction in SiO₂ and increases in Al₂O₃ and TiO₂ contents, likely occur in mold fluxes because of the interfacial reactions of Eqs. (1)–(4).

The interfacial reactions include the reduction of SiO₂ by Ti, oxidation of Al by TiO₂, precipitation of TiN, and reaction of TiN with SiO₂, all of which lead to increases in the viscosity and crystallization temperature of the mold flux. These changes affect the flow behavior and heat flow, leading to several problems in the mold, such as mold lumps (floaters) and clogging, which can ultimately lead to surface defects in the slab [13]. Hence, it is critical to develop efficient continuous casting technologies for high-Ti steels. Table S1 summarizes the recent review articles on mold fluxes [14–18].