The mineralogical structure of flux films is a critical factor in controlling heat transfer in the mold and avoiding the longitudinal cracking of slabs during the continuous casting of peritectic steel. In this study, the layered structure, crystallization ratio, mineralogical species, and morphology features of flux films were characterized by optical microscopy, X-ray diffraction, and electron-probe microanalysis. Microstructural observation revealed that the normal flux films for peritectic steels present a multilayered structure and high crystallization ratio (60~90 vol%), mainly composed of well-developed crystalline akermanite and cuspidine. In contrast, the films with outstanding flux characteristics with abundant longitudinal cracks on the slab surface have a low crystallization ratio (80 vol%). Furthermore, heat transfer analysis showed that the low crystallization ratio and the vast crystallite content of flux films worsen the heat transfer rate or uniformity in the mold, whereas the appropriate thickness and cuspidine content of flux films can improve the heat transfer performance. From the above results, it is concluded that using strong crystalline flux to obtain the ideal mineral phase structure of flux film is one of the important measures for reducing longitudinal cracks during continuous casting of peritectic steel slabs.

To optimize the design of high-performance mold fluxes that meet the demands of low-alloy peritectic steel continuous casting, this study utilized the melting point tester, viscometer, in-situ thermal analyzer, flat-plate thermal conductivity meter, polarizing microscope, and X-ray diffraction to systematically characterize the melting properties, crystallization behavior, mineralogical characteristics, and heat transfer mechanism of the industrial mold fluxes and flux films. The experimental results indicate that mold fluxes suitable for low-alloy peritectic steel require a narrow melting temperature range, low melting point (< 1200 °C), and low viscosity (< 0.1 Pa·s) to form a well-flowing liquid flux layer and enhance strand lubrication. Their core characteristic is strong crystallization capability, manifested as a high critical crystallization cooling rate (>50 °C/s) and high initial crystallization temperature (>1350 °C), ensuring rapid formation of a structurally stable flux film and improving heat transfer uniformity. The mineralogical structure of the flux film corresponding to low-alloy peritectic steel presents a multilayered structure with a high crystallization ratio (60-90 vol%), primarily composed of cuspidine and akermanite. Among these, high cuspidine content with significantly coarsened crystal morphology is key to regulating heat transfer. Analysis of the heat transfer mechanism further revealed that the characteristics of high crystallization ratio and coarsened crystal morphology promote the formation of numerous micropores and grain boundaries between crystals, significantly increasing the overall thermal resistance of the flux film, resulting in a low thermal conductivity (0.47-0.67 W/m·K), which effectively controls the heat transfer rate. Based on the aforementioned results, it is deduced that enhancing the crystallization performance through optimizing flux composition (boosting Na₂O content and basicity) to adjust the cuspidine content and crystallization ratio of the flux film, thus acquiring the ideal mineralogical structure, is the crucial route for realizing efficient continuous casting of low-alloy peritectic steel.

During the continuous casting of high-titanium steel, traditional fluorine-containing mold fluxes are prone to causing fluoride contamination, equipment corrosion, and intensified slag-metal interface reactions. There is an urgent need to develop highly adaptable fluorine-free mold flux systems. In this study, titanium-containing blast furnace slag was used as the primary base material, while borax, soda ash, and witherite were selected as fluoride-substituting mineral raw materials. The effects of these mineral raw materials on the melting properties, crystallization behavior, crystalline phases, and microstructure of fluorine-free mold fluxes were systematically investigated, and an optimized mold flux design suitable for continuous casting of high-titanium steel was further developed. The results indicate that borax significantly reduces the melting temperature and viscosity and markedly suppresses the growth of crystalline phases such as calcium borosilicate, nepheline, and perovskite by weakening the polymerization degree of the silicate network, thereby substantially decreasing the crystallization ability of the mold flux. Soda ash primarily acts as a strong fluxing and network-depolymerizing agent, promoting the formation of low-polymerized structural units. It also enhances the tendency toward ordered atomic arrangement, thereby markedly increasing nepheline precipitation and the overall crystallization ratio. Witherite exerts a relatively mild effect on slag structure and phase evolution; its moderate addition helps synergistically reduce the melting point, viscosity, and crystallization ratio, thereby supporting performance stability. The optimized fluorine-free mold flux, designed on the basis of these findings, maintains a suitable initial crystallization temperature and critical crystallization cooling rate while exhibiting lower melting temperature, viscosity, and crystallization ratio than conventional fluorine-bearing flux. Moreover, the introduction of TiO2 reduces the chemical potential difference between Ti in the molten steel and the fluorine-free mold flux, thereby slowing down the rate of slag-metal interface reactions and improving compositional stability. The research results provide a theoretical basis for the industrial design of environmentally friendly mold fluxes for high-titanium steel and for improving billet quality.

During the continuous casting of high-titanium steel, traditional fluorine-containing mold fluxes are prone to causing fluoride contamination, equipment corrosion, and intensified slag–metal interface reactions. There is an urgent need to develop highly adaptable fluorine-free mold flux systems. In this study, titanium-containing blast furnace slag was used as the primary base material, while borax, soda ash, and witherite were selected as fluoride-substituting mineral raw materials. The effects of these mineral raw materials on the melting properties, crystallization behavior, crystalline phases, and microstructure of fluorine-free mold fluxes were systematically investigated, and an optimized mold flux design suitable for continuous casting of high-titanium steel was further developed. The results indicate that borax significantly reduces the melting temperature and viscosity and markedly suppresses the growth of crystalline phases such as calcium borosilicate, nepheline, and perovskite by weakening the polymerization degree of the silicate network, thereby substantially decreasing the crystallization ability of the mold flux. Soda ash primarily acts as a strong fluxing and network-depolymerizing agent, promoting the formation of low-polymerized structural units. It also enhances the tendency toward ordered atomic arrangement, thereby markedly increasing nepheline precipitation and the overall crystallization ratio. Witherite exerts a relatively mild effect on slag structure and phase evolution; its moderate addition helps synergistically reduce the melting point, viscosity, and crystallization ratio, thereby supporting performance stability. The optimized fluorine-free mold flux, designed on the basis of these findings, maintains a suitable initial crystallization temperature and critical crystallization cooling rate while exhibiting lower melting temperature, viscosity, and crystallization ratio than conventional fluorine-bearing flux. The findings establish a theoretical basis for designing eco-friendly mold fluxes suitable for high-titanium steel and for enhancing billet quality.

Titanium-containing fluorine-free mold fluxes were prepared from industrial raw materials, including titanium-bearing blast-furnace slag, limestone, quartz, soda ash, borax and witherite. The effects of Na₂O content on microstructural characteristics, crystalline phase constitution, and crystallization behavior of the mold fluxes were systematically investigated. The results indicated that as the Na₂O content increased from 6 wt% to 12 wt%, the stability of Ca-O and Na-O bonds deteriorated, the average coordination number of Si-O increased, the distributions of ∠O–Si–O and ∠Si–O–Si bond angles became broader and less ordered, and the silicate structural units shifted toward Q¹ and Q⁰ species, indicating a significant reduction in network polymerization. Equilibrium crystallization followed the sequence: perovskite (1450°C)→glass‑ceramic phase (1100°C)→nepheline (1100–1000°C)→calcium borosilicate (1000–900°C). With increasing Na₂O content, the precipitation temperatures and sequence of the phases were essentially preserved, whereas the amounts of the crystalline phases increased and the glass‑ceramic phase decreased markedly, which is attributed to depolymerization of the silicate network by moderate Na₂O additions, thereby providing abundant nucleation sites and facilitating more complete crystal growth.