Control and engineering of Non-metallic inclusion belonging to xSi02-yCa0-zAl203 system in Ca-treated Al-killed and Al-Si-killed steel

A model based on the fundamental principles of thermodynamics has been applied to forecast and control the precipitation of non-metallic inclusions in Ca-treated Al-killed or Al-Si-killed steels. The engineering of the non-metallic inclusions takes place during the period just after the tapping from the electric arc furnace until the beginning of the casting period. The construction of the model and its validation have been accomplished through a precise monitoring of the treatment of several steel grades characterized by different oxygen contents after tapping. The oxygen killing of the steel melt with an oxygen content between 750-1200 ppm is performed by the addition of Si and Al, which produces the formation of pure Al2O3 but not always the formation of pure SiO2. This is a fundamental hypothesis of the model confirmed by experimental observations. The other fundamental aspect is related to the determination of the oxygen activity in the steel bath, which is defined from experimental measurements by an electrochemical concentration cell and through the computation of the equilibrium between the steel bath and the ladle slag. The comparison between the experimental data and the non-metallic evolution forecast by the model on the basis of the minimization of the oxygen potentials has shown very interesting performances, which makes the model a suitable and very stable tool for industrial application.