Non-Isothermal Melt Crystallization Kinetics for CaO–Al₂O₃–B₂O₃ F-Free Mould Fluxes

Abstract

Efforts have been made to develop fluorine-free mould fluxes for the continuous casting of steel process. In this work the crystallization behaviour of fluorine-free mould slags based on the CaO-Al 2 O 3-B 2 O 3 system was investigated by differential scanning calorimeter (DSC) and scanning electron microscopy equipped with energy dispersive spectroscopy(SEM-EDS). The crystallization kinetics for Ca 3 Al 2 O 6 primary crystals was analysed by combining modified Avrami analysis with Friedman isoconversional method. Avrami parameter n is close to 4 for samples with the ratios w(CaO)/w(Al 2 O 3) = 1 and w(CaO)/w(Al 2 O 3) = 1.2, indicating a crystallization mechanism of continuous bulk nucleation and 3D crystal growth. The Avrami parameter n for samples with w(CaO)/w(Al 2 O 3) = 0.9 is close to 3, indicating instantaneous bulk nucleation and 3D crystal growth. The crystallization rate constant is the highest and half crystallization time is the lowest for the samples with w(CaO)/w(Al 2 O 3) = 0.9, indicating the fastest crystallization. In the initial stage, effective activation energies were mainly determined by the undercooling values. In the final stage, kinetic barrier for crystallization could have some influence on crystallization; for the investigated mould fluxes crystallization in the final stage is retarded by increasing w(CaO)/w(Al 2 O 3) ratio. Thus crystallization mechanisms were elucidated and effective activation energy of crystallization for the first crystal which precipitates from melt was determined.

Crystallization Control for Fluorine-free Mold Fluxes: Effect of Na₂O Content on Non-isothermal Melt Crystallization Kinetics

“…That is to say, the crystallization of Ca 2 SiO 4 during cooling follows anti-Arrhenius behavior, which was found in crystallization of polymers, 20) traditional mold fluxes, 23) and F-free mold fluxes. 13) For Sample 1, the one with the highest Na 2 O content, E α takes greater negative values at lower values of relative crystallinity, corresponding to temperatures closer to liquidus. The E α increases as the extent of conversion rises and the temperature decreases.…”

“…In a previous laboratory work, 13) non-isothermal melt crystallization kinetics for CaO-Al 2 O 3 -B 2 O 3 F-free mold fluxes, for continuous casting of high aluminum steels, through modified Avrami analysis and Friedman differential isoconversional method, was studied. Crystallization mechanisms and effective activation energy of crystallization were determined for different compositions, for the first crystal, which precipitates from melt (Ca 3 Al 2 O 6 ).…”

Structure and Viscosity of CaO–Al₂O₃–B₂O₃ Based Mould Fluxes with Varying CaO/Al₂O₃ Mass Ratios

“…The thermodynamic liquidus temperature for slag cannot be calculated from thermodynamic package due to the limiting thermodynamic data for this new slag system. But according to our previous study on crystallization of the same slag, 42) the slag with the lowest CaO/Al 2 O 3 ratio (CaO/Al 2 O 3 = 0.9) has the fastest crystallization, which could also contribute to the highest breaking temperature. Correlation between structural information and physiochemical properties of slags could be explored.…”

Isothermal and Non-isothermal Crystallization Kinetics of Mold Fluxes used in Continuous Casting of Steel: A Review

“…Shu et al [83] also studied the non-isothermal crystal- the Avrami parameter n was found to be close to three, indicating instantaneous bulk nucleation and 3D crystal growth. Furthermore, using the Friedman isoconversion method, [76] they calculated the effective activation energies for crystallization of Ca 3 Al 2 O 6 in all the mold fluxes studied and reported negative values of activation energy, which agreed well with the observations of Seo et al [78] in earlier investigation with conventional CaO-SiO 2 -CaF 2 -based mold fluxes.…”

“…Watanabe et al [81] studied the effects of alkali oxide addition on the kinetics of crystallization in CaO-SiO 2 -CaF 2 -based mold fluxes using DTA and reported that alkali oxides such as Li 2 O, K 2 O, and Na 2 O enhanced cuspidine crystallization. Choi et al [82] and Shu et al [83,84] examined the non-isothermal crystallization behavior of F-free mold fluxes using thermoanalytical methods. While Choi et al [82] used the Matushita and Sakka [63][64][65][66][67] equation, Shu et al [83,84] used the Friedman method [76] and therefore the numerical accuracy of the results reported by Shu et al [83,84] may be higher for reasons already discussed.…”