Phase evolution during crystallization of boron-containing fluorine-free mold flux

In the development of fluorine-free continuous casting mold fluxes, boron is regarded as one of the most promising fluorine substitute elements due to its low cost and strong fluxing capacity. However, for boron-containing fluorine-free mold fluxes, how to reconstruct crystalline mineral phase structures to realize effective coordination between heat transfer and lubrication functions remains an urgent key scientific problem. Industrial mineral raw materials including blast furnace slag, limestone, quartz sand, soda ash and borax were adopted. Boron-containing fluorine-free mold fluxes with different compositions were prepared via the uniform design method. The mineral phase structure characteristics of experimental fluxes during crystallization were systematically investigated, and the competitive precipitation and growth evolution rules of primary crystalline phases were emphatically revealed. The results show that primary crystalline phases including calcium borosilicate, merwinite, calcium borate and nepheline precipitate during the crystallization of boron-containing fluorine-free mold fluxes, accompanied by the transitional phase of larnite and sodium calcium silicate glass-ceramic phase. The competitive formation of primary crystalline phases is mainly determined by boron content and basicity (R). When the mass fraction of B₂O₃ is high (10%-15%), calcium borate and nepheline dominate the primary crystalline phases; when the mass fraction of B₂O₃ is low (3%-6%), calcium borosilicate or merwinite serve as primary crystalline phases. Specifically, merwinite is prone to massive formation at low basicity (R=1.3), while calcium borosilicate tends to form massively at high basicity (R=1.7). Calcium borosilicate and merwinite both began to precipitate at approximately 1 200 ℃ and completed rapid growth and morphological shaping at 1 100 ℃. Calcium borosilicate gradually developed from dendritic buds into needle-bundle and dendritic skeleton structures, and merwinite evolved from microcrystals into spindle, short columnar and tabular morphologies. The two phases share prominent crystallographic and mineralogical similarities with cuspidine and melilite in fluorine-containing mold fluxes. The crystalline mineral phase structure of boron-containing fluorine-free mold fluxes can be directionally designed by synergistically regulating boron content and basicity. The research findings provide a theoretical basis for developing novel fluorine-free mold fluxes with efficient heat transfer regulation and lubrication performance.