Abstract: The evolution characteristics of the steel cleanliness and inclusions in a low-carbon aluminum killed steel produced by the the route of VD→ LF→calcium treatment→continuous casting and rolling under the converter process was investigated. It was found that the T.O mass fraction significantly decreased from 129×10^-6 at the LF start to 34×10^-6 before calcium treatment. But, there was still a problem that the w(CaO)/w(Al₂O₃) in the refining slag was too small. A significant increase on the contents of T.O and T.N in the steel after calcium treatment, indicating that the significant reoxidation occurred. It was necessary to strengthen the stabilization control of the calcium treatment process. During the refining process after deoxidation, the inclusions transformed from clustered Al₂O₃ to irregular Al₂O₃-MgO, and then to spherical CaO·2Al₂O₃ after the calcium treatment with a feeding amount of 0.64 kg/t steel. Due to the generation of a large number of small-sized inclusions during the calcium treatment process, the average size of inclusions decreased from 2.8 μm before calcium treatment to 2.3 μm after calcium treatment according to the detection of automatic SEM. However, at the same time, factors such as the entrainment of refining slag particles during calcium treatment increased the maximum size of inclusions to 28 μm after calcium treatment. Due to the reoxidation of the molten steel during the casting process, the number density of inclusions in the molten steel in tundish significantly increased to 45/mm², and the area fraction of inclusions also reached 268×10^-6 based on the detection results of automatic SEM. Therefore, it was necessary to further strengthen the protective casting of the molten steel during the continuous casting process.

Key words: low-carbon aluminum killed steel, cleanliness, inclusion, calcium treatment, reoxidation