Abstract: Under industrial conditions, the Mg treatment test of Nb-Ti micro-alloyed steel was carried out. The evolution of nonmetallic inclusions in micro-alloyed steel treated with Mg was analyzed. The results show that, Al₂O₃ inclusion is the main product of aluminum deoxidation in micro-alloyed steel. After Ca treatment, Al₂O₃ inclusions are reduced and transformed into CaO-Al₂O₃ inclusions, and part of MnS inclusions are transformed into CaS inclusions. After Ca treatment is changed to Mg treatment in the refining stage, Al₂O₃ inclusions of aluminum deoxidation product can be denatalized to MgO-Al₂O₃ inclusions. Before Mg treatment, the inclusions  in LF are mainly Al₂O₃ inclusions. After Mg treatment and Ti alloying, the main inclusions in steel become MgO-Al₂O₃ inclusions, MgO inclusions and TiN inclusions. The final inclusions in the billet are MgO-Al₂O₃ coated with CaS/MnS and (Nb, Ti) N complex inclusions. The size of inclusions treated with Ca and Mg was compared, and the size of inclusions mainly distributed within 5 μm. The proportion of inclusions within 2 μm formed by Mg treatment is higher than that by Ca treatment, which indicates that magnesium treatment can fine inclusions better. The formation of a large number of dispersed small inclusions is helpful to improve the comprehensive mechanical properties of steel.

Key words: Mg treatment, Nb-Ti micro-alloyed steel, nonmetallic inclusion, thermodynamic analysis