Inclusion Evolution during Modification of Alumina Inclusions by Calcium in Liquid Steel and Deformation during Hot Rolling Process

The formation mechanism of Al2O3-CaS based inclusions in Al-killed steel was studied by industrial trials and thermodynamic calculations of various calcium aluminates and the precipitated CaS. The chemical activities of CaO and Al2O3 of various calcium aluminates were expressed as a function of CaO and Al2O3 content of CaO-Al2O3 inclusions respectively by the third liner fitting equation obtained from different researchers. Massive Al2O3-CaS based inclusions were generated with a decrease of temperature of molten steel and subsequent solute segregation during casting. Based on the shapes of CaS layer, there are two formulations to consider: 1) the crescent-shaped CaS layer surrounded calcium aluminates was generated by reaction between dissolved Al, S and modified calcium aluminates or diffusion of S dissolved in inclusions; 2) the oval shape layer associated with calcium aluminates was generated by collision or adsorption of calcium aluminates and the precipitated CaS. In addition, the deformation mechanism of inclusions with different modification extents and sizes during hot rolling was proposed.