Evolution Mechanism of MgO-Al2O3-SiO2-CaO Inclusions During Continuous Casting of Si-Killed Spring Steel

Controlling inclusions is essential for producing high-quality spring steels. In continuous casting (CC), the thermodynamic equilibrium between the steel and inclusions changes with the reoxidation and temperature reduction of molten steel, resulting in the compositional transformation of the inclusions. Herein, the evolution mechanism of the inclusions during CC is investigated through industrial trials and thermodynamic analyses. Three types of oxide inclusions are classified in the steel, namely, Al2O3-SiO2, CaO-Al2O3-SiO2, and MgO-CaO-Al2O3-SiO2, whereby MgO-CaO-Al2O3-SiO2 is the main type. The average mass content of Al2O3 in the inclusions is 15-21% at the end of vacuum degassing. The number density of the inclusions increases because of the reoxidation of molten steel. The reoxidation and temperature decrease result in the further reaction among Si, Al, Mg, and O in steel, thereby increasing the melting point of the inclusions. Moreover, the inclusions evolve during CC. The mass fractions of CaO and MgO decrease, that of Al2O3 increase, and that of SiO2 hardly change. Compared to reoxidation, temperature plays a leading role in the inclusion evolution. Finally, the details of the inclusion evolution are illustrated by thermodynamic calculations.