Abstract: Inclusions in Q345 steel were modified by calcium treatment. Inclusions in tundish molten steel were MgO•Al2O3 spinel with CaO-Al2O3 wrapped outside. The average composition of inclusions in molten steel was 45.71%Al2O3-40.22%CaO-6.50%MgO-6.60%CaS-0.97%SiO2. Transformation of inclusions occurred during solidification of slab. Near the surface of the slab, phase transformation of inclusions did not have enough time. Therefore, the composition of inclusions was almost the same as in molten steel. At the 1/4 of slab width from the loose side, the inclusions were MgO•Al2O3 spinel with CaS-MnS wrapped outside. The average composition of inclusions was 56.00%Al2O3-9.28%CaO-9.07%MgO-25.06%CaS-0.58%SiO2 while MnS was neglected. From surface to center of slab, the Al2O3 and CaS content of inclusions increased, CaO content decreased, while area fraction of sulfides increased from 0.000 01 %to 0.002 9 %, indicating the precipitation of sulfides. Transformation of inclusions in Q345 steel during cooling and solidification process was calculated using Factsage 7.0. The results were of good consistence to the detected ones, especially for inclusions with smaller size.

Key words: Q345 steel, inclusion, solidification, precipitation