Efficiency of Inclusion Absorption by Slags during Secondary Refining of Steel

One of the functions of secondary refining in steelmaking is to remove non-metallic inclusions produced through deoxidation of the steel bath during the process. Removal of these inclusions occurs in three fundamental stages: flotation, separation and dissolution of the inclusions in supernatant slag. Given that dissolution is subject to the action of slag, this study aims to clarify the inclusion absorption capacity of industrial slags and factors that influence slag properties in absorbing inclusions, as well as the impact of each factor on the cleanliness of the steels analyzed. To that end, the results of industrial steel and slag samples collected during the different stages of secondary refining were analyzed. The amount and chemical composition of the inclusions was determined by SEM/EDS (scanning electron microscopy coupled with energy-dispersive spectroscopy). The chemical composition of slag samples was measured by XRF (X-ray fluorescence) and subsequently submitted to thermodynamic simulations using FactSage 6.4 software. Calculations were made with the SlagViscosityPredictor, which uses FactSage 6.2 software. This made it possible to establish the chemical composition and proportion of the solid phase in the slags and inclusions, as well as the effective viscosity of the slags. The efficiency of absorption inclusion by the industrial slags was found to be greater in the presence of high thermodynamic driving force between slags and inclusions, low slag effective viscosity and when inclusions were not liquid.