Effect of Inclusions’ Behavior on the Microstructure in Al-Ti Deoxidized and Magnesium-Treated Steel with Different Aluminum Contents

To clarify the precipitation behavior of beneficial inclusions and mechanism of their effects on microstructure, the effect of aluminum content on inclusion’s characteristics and their influence on the refinement of microstructure in Al-Ti complex deoxidized magnesium-treated steels were systematically investigated based on experiment and calculation. The results showed that due to the dual effects of Ti and Mg deoxidation, a large amount of finely dispersed Al₂O₃-TiO _x -MgO inclusions in low aluminum steel with a complex multilayer or mosaic structure were formed, whereas a relatively smaller amount of Al₂O₃-MgO inclusions with the simple bundle structure were observed in high aluminum steel. The Al₂O₃-TiO _x -MgO core oxides are more conducive to the precipitation of multiple manganese sulfides with thinner thickness on their local surfaces. Thus, the inclusion deformation, which mainly depends on the surface manganese sulfides layer, is smaller in low aluminum steel than that in high aluminum steel. Complex inclusions in low aluminum steel can pin austenite grain boundaries and induce interlocking acicular ferrite effectively. In addition to the small size and chemical composition of inclusions, the complex structure of oxides and the precipitation of multiple MnS on their surface are important to the nucleation of interlocking AFs on inclusions in Ti-deoxidized Mg-treated steel. The AFs quantity is much more, and the grain size is more uniform in low aluminum steel than that in high aluminum steel.