Evaluation of cleanliness and distribution of inclusions in the thickness direction of interstitial free (IF) steel slabs

During the production of Al-killed titanium-alloyed interstitial free steel, to reduce defects in cold rolled sheets and decrease the influence of inclusions on the properties of the steel, it is important to clarify the distribution of inclusions in the thickness direction of IF (interstitial free) steel along the slab. In this study, standard metallographic techniques were employed to analyze the total oxygen and nitrogen by performing scanning electron microscopy, energy spectroscopy, automatic scanning electron microscopy, and original morphology analysis. The results show that the average mass fractions of T.O and N are 1.6 × 10⁻⁵ and 1.7 × 10⁻⁵, respectively, and the T.O for the 1/8 thickness from the inner arc is 2.0 × 10⁻⁵, while the content of N for between the 1/4 and 3/8 thickness from the inner arc is 1.8 × 10⁻⁵. A total of 1177 inclusions were counted. More than 70% of inclusions are within 5 μm in size, and the average size of inclusions in the thickness direction is 2.8 μm. The sizes of inclusions for the 3/8 thickness from both the inner and outer arcs are larger at 4.0 μm and 4.4 μm, respectively. The amount of precipitation of TiN is large in the slab center, and there are mainly Al₂O₃ and Al₂O₃–TiO_x near the inner and outer arc surfaces with sizes between 5 and 10 μm. Al₂O₃–TiN distributes irregularly in the 1/4 thickness from the inner and outer arcs, and the size fluctuates between 3 and 5 μm. The size of TiN during solidification fluctuates between 3 and 6 μm. TiN precipitates in the liquid and δ phase of the solidification front when the solidification rate is between 0.646 and 0.680, and the size fluctuates between 3 and 6 μm.