A thermodynamic model for Si-deoxidation equilibria in liquid Fe and critical evaluation of the O solubility limit

Abstract

Deoxidation is a key process in refining liquid steel and enhancing its cleanliness. O removal from the melt is typically achieved by adding elements with a high affinity for O, such as Al, Si, or by adjusting a specific Mn/Si ratio in the melt. In the production of high-Si steels, precise knowledge of the equilibrium between the liquid metal and solid silica (SiO₂) becomes increasingly important. In the present study, a CALPHAD-based thermodynamic model for Si-O deoxidation in liquid Fe was developed, covering the full composition range from pure liquid Fe to pure liquid Si, including the O saturation limit. The Gibbs energy of the ternary Fe-Si-O liquid phase was formulated using the Modified Quasichemical Model (MQM) in the pair approximation, which accounts for strong interactions among Fe, Si, and O. Comparisons with experimental data demonstrated an excellent agreement in predicting the liquid/SiO₂(s) equilibrium within the temperature range of 1550 - 1650 °C. Modeling the deoxidation equilibria in this ternary system can aid in refining the model description in binary Fe-Si liquid. The model predicted a pronounced "deep minimum" in O solubility. It was in agreement with an experimental approach reported by Shibaev et al. (https://doi.org/10.2355/isijinternational.45.1243) and with another thermodynamic approach by Cho and Kang (https://doi.org/10.1007/s12613-023-2766-7). The present assessment can be incorporated into CALPHAD databases for applications in steelmaking and liquid metal processing.