Novel Application of Alkali Oxides in Basic Tundish Fluxes for Enhancing Inclusion Removal in 321 Stainless Steels

Abstract

Fundamental work on the effect of alkali oxides including Li₂O, Na₂O, and K₂O on the absorption ability of inclusions in a typical basic tundish flux for 321 stainless steels has been studied. The effects on the absorption ability are dependent on the type of alkali oxides and the amount composed within the tundish flux. Results from kinetics studies using an induction furnace at 1823 K (1550 °C) on the reaction of tundish fluxes containing alkali oxides with 321 stainless steels suggest minimal improvement with Li₂O and Na₂O additions and in some cases hindered inclusion removal, but K₂O additions seems to significantly improve the cleanliness in the as-quenched 321 stainless steel melts compared to preexisting tundish flux compositions. Both Li₂O and Na₂O significantly lower the viscosity of the melt, while K₂O increases the viscosity. Although alkali oxides have a propensity to enhance the cohesion of aluminate melts due to the ionic compensation effect in [AlO₄]⁵⁻-tetrahedral structural units, this effect was not pronounced for Li₂O and Na₂O compared to K₂O additions, which may be due to the large ionic radius size of potassium. An automated SEM–EDS analysis was utilized to identify the inclusion morphology and abundance within the steel. Fourier transform infrared spectroscopy was used to identify the effects of alkali oxides on the tundish flux structure for improved absorption capability and described the characteristic effect of K₂O on increasing the asymmetric stretching vibrations of [AlO₄]⁵⁻-tetrahedral structural units and Si-O-Al bonding within the flux, thus polymerizing the flux and selectively absorbing inclusions.