Inclusions chemistry for Mn/Si deoxidized steels: Thermo-dynamic predictions and experimental confirmations

Abstract

Inclusions chemistry of Mn/Si deoxidized steel was studied through both thermodynamic computation and experimental method. The computational thermodynamics has proved to provide a powerful tool for controlling inclusions and precipitates in steel. For Mn/Si deoxidized steels, important factors in determining the liquidus temperature and primary phase of the inclusions are MnO/SiO2 ratio and Al2O3 content in inclusions. Provided that no further interaction with steel matrix during cooling, inclusions having MnO/SiO2 mass% ratio near unity and Al 2O3 content in the range of 10-20mass% give low liquidus temperatures (1150-1200°C) and primary phases of MnSiO3 and Mn3Al2Si3O12 both which are soft. For the case of Mn+Si=1.0 in mass%, the Mn/Si ratio of 2-5 meets the above conditions. Effect of the top slag on the inclusions chemistry can be predicted with accuracy, and hence it is possible to control the inclusions chemistry through proper design of the top slag composition so that the inclusions show a low liquidus temperature and soft primary phase. As the inclusions composition gradually changes with time toward the top slag composition, the length of refining time which determines the extent of reaction with the top slag is an important factor in determining the inclusions chemistry.