Effect of Al Content on the Characteristics of Inclusions in Al–Ti Complex Deoxidized Steel with Calcium Treatment

To improve the effect of calcium treatment and the cleanliness of steel and to make use of fine TiOx to refine the microstructure of steel, the effect of aluminum content on inclusion characteristics of aluminum-titanium complex deoxidized and calcium treated steel is investigated based on the experiment with SEM/EDS, Image Pro-Plus 6.0 and FactSage 6.1 softwares and thermodynamic calculation in the present work. The results show that the inclusions in two steels with different aluminum content are obviously spherical composite inclusions with a two-layer complex structure, consisting of an Al2O3–CaO–TiOX core surrounded by MnS. In low aluminum steel, the oxide core of inclusions contains much TiOx and CaO, and their composite structure is mosaic compared with bundle in high aluminum steel, the number of inclusions is 2.5 times more than that in high aluminum steel, the thickness of MnS on oxides surface is also thinner. In addition, melting point of the inclusions in low aluminum steel is lower, the cleanliness of the steel is relatively improved because of the inclusions floating up, and the deformation aspect ratio of calcium aluminate inclusions with a certain amount of Ti2O3 is effectively improved, which is about 1–2 while the composition of oxide core is xAl2O3 = 35–55%, xTi2O3 = 15–35%, and xCaO = 10–25%. As a result, less calcium is needed to modify the alumina inclusions to liquid calcium aluminate in the case of lower aluminum deoxidized steel, thus the calcium treatment effect can be improved. The low aluminum in steel is more effective to control the inclusion characteristics to reduce the harm of MnS and to improve the cleanliness of steel.