Study Of Crystallization And Heat Transfer Behavior Of Low-fluorine Mold Fluxes For Casting Medium Carbon Steel

Mold fluxes act as the roles of lubricating the strand, preventing molten steel from oxidation, absorbing inclusions, controlling heat transfer in the continuous casting. Fluorine is desired to be decreased or replaced in the typical mold fluxes due to its great pollution to the environment and corrosion to the machines. Fluorine can promote crystallization of cuspidine, decrease viscosity and melting point, and control heat transfer. So mold fluxes used in casting medium carbon steel which are sensitive to longitudinal crack and sticking steel leakage, are desired to have high basicity, strong crystallization tendency and heat transfer controlling ability.Single and Double Hot Thermocouple Technique (SHTT/DHTT), Infrared Emitter Technique (IET) are adopted in this paper to study the effect of Na2O, B2O3, and basicity on metlting, crystallization and heat transfer behaviors, and crystallizaitoin kinetics of mold fluxes for casing medium carbon steel. This study also analyse the microstructure and phase composition of samples, to explain the relation between crystallization and heat transfer.The study shows that:when the content of Na2O increases the crystallization temperature will increase; incubation time will decrease, however, Na2O can enhance the crystallization of high-fuorine mold flux; main crystalline phases grown in the mold fluxes are cuspidine(Ca4Si2F2O7), Na4Al2Si2O9and Ca2Si2Na207, while Na2O will inhibit the growth of cuspidine when the content is above7.5%; Na2O can also decrease the crystallizing active energy of mold fluxes.When basicity increases, the critical cooling rate will increase; crystallization temperature will decrease; and incubation time will rise sharply; while basicity is out of certain range, the enhancement for crystallization will be weak, basicity1.25is the optimization design in the low-fluorine mold flux series in this paper. When B2O3increases, critical cooling rate will decrease; the crystallization temperature will decrease; incubation time will increase。In the infrared emitter heat flux experiment, small crystals crystallized at the low temperature zone, and lath crystals crystallized at the high temperature zone mold fluxes, main crystalline phases are cuspidine(Ca4Si2F2O7), Ca2Al2SiO7, Ca2MgSi2O7, and NaCaAlSi2O7. Basicity is helpful to the crystallization of cuspidine and melilite, while inhibit the growth of wollastonite; and B2O3is helpful to the growth of melilite.The steady heat flux of the1～9and S2samples are522,565,592,470,530,550,430,510,563, and462KW/m2. Increasing bascicity can decrease the heat flux of mold fluxes by enhancing crystallization, but B2O3plays the opposite effect.Above all, sample4mold flux has the most similar physiochemical properties to the standard mold flux(sample S2), such as crystallization temperature, incubation time, critical continuous cooling rate, crystallization rate, phase composition, heat transfer and so on. This paper provides theoretical support for the development of low/free-fluorine mold fluxes for casting medium carbon steel. This paper contains34figures,11tables and69references.