Cleanliness study of high level ship plate steel during secondary refining processes

Three heats of industrial trials were performed to investigate the cleanliness of A32 ship plate steel produced by a BOF-LF-RH-CC route. The variation of T. O and[N] contents as well as the transformation route and mechanism of inclusions in the steel were studied by system sampling during secondary refining processes. The steel have high level cleanliness with the T. O content under 2×10-5 and the[N] content under 4×10^-5 in tundish samples. The T. O content, number density and average size of inclusions decrease during the LF refining process, and the inclusions transform into a CaO-MgO-Al₂O₃ ternary system. In the RH refining process, the T. O content and the number density of inclusions decrease, but the average size of inclusions increases. During calcium treatment, the number density of inclusions increases but the average size of inclusions decreases, and the inclusions transform into a CaO-Al₂O₃-CaS ternary system.