Unveiling the effect of MnO/SiO2 ratios on the viscosity and structure of mold fluxes for high-Mn cryogenic steels

The serious reaction between Mn in molten steel and SiO2 in calcium silicate-based mold flux during continuous casting of Al-free high-Mn cryogenic steels could change the mold flux properties and in-mold performance. In this study, the combination of rotating viscometer, FTIR spectroscopy, Raman spectroscopy and O1s X-ray photoelectron spectroscopy (XPS) was employed to get a deep insight into the effect of the reaction levels characterized by MnO/SiO2 ratios on the viscosity and structure of mold fluxes. The results exhibit that the transformation of mold flux from Newtonian fluid to non-Newtonian fluid can be reflected by viscosity change with varying rotation speeds. The viscosity decreases continuously with increasing MnO/SiO2 ratio but the temperature dependence of viscosity decreases at high MnO/SiO2 ratio. Structural information from FTIR revealed the slag system is mainly a silicate structure. The deconvoluted results of Raman spectra show an increase in NBO/Si, amounts of Q0 ([SiO4]4–) and Q1 ([Si2O7]6–) units with increasing MnO/SiO2 ratio, whereas a decrease in amounts of Q2 ([SiO3]2–) and Q3 ([Si2O5]2–) units in the meantime, this indicates the depolymerization of the silicate structure with proceeding of the slag/steel reaction. According to semiquantitative analysis of XPS spectra, the relative proportion of bridge oxygen (O0) decreases at the cost of non-bridged oxygen (O–), which suggests that the silicate structure gradually depolymerizes. The structural analysis supports the change of viscous behavior of mold flux with slag/steel reaction.