Investigation on Improving the Castability of Low Carbon Al-Killed Steel without Calcium Treatment

During the “BOF-LF-CC” process of producing low carbon Al-killed (LCAK) steel, Al in the molten steel will react with MgO in the ladle refractory or the refining slag to generate MgO-Al2O3 inclusions, which have a negative influence on the molten steel’s castability. To enhance the castability of molten steel, calcium treatment is typically required following LF refining to promote the transformation of inclusions MgO-Al2O3 to CaO-MgO-Al2O3 or CaO-Al2O3. However, calcium treatment has many drawbacks, such as low calcium yield, increased smelting cost, environmental pollution, etc. Thus, how to improve the castability of LCAK steel without calcium treatment is worth studying. In this research, laboratory studies were first carried out to clarify the source of MgO-Al2O3 inclusions in the molten steel. Thereafter, industrial trials were conducted with the refractory material of the ladle replaced by a MgO-free and Al2O3-riched refractory. The results show that when a MgO-based crucible is used at 1600℃, the inclusions in molten steel after 25 min are mainly MgO-Al2O3, even without refining slag. However, even with the refining slag (the basicity is less than 4.5) containing about 5% MgO, when an Al2O3-based crucible is employed, the inclusions in the molten steel are mainly CaO-Al2O3. Consequently, MgO in ladle refractories is the main source for the formation of inclusions ­MgO-Al2O3. The results of industrial trials using the “3 + 1” smelting pattern, in which the molten steel is cast directly without calcium treatment in the first three heats, treated with calcium in the next heat, and the process is repeated, show a significant improvement in the castability of molten steel.