Comparative Study on the Cleanliness of Ultra-low Carbon Al-Killed Steel by Different Refining Processes

Cleanliness of aluminum-killed ultra-low carbon steel produced through BOF–RH–CC and BOF–LF–RH–CC was comparatively investigated by nitrogen–oxygen analysis, SEM analysis and thermodynamic calculation to seek for an optimum refining process. The results show that the non-metallic inclusions with area above 8 μm2 could be removed obviously in both the refining processes. Compared with BOF–RH–CC process, in RH refining stage, the size of inclusion and total oxygen control were better than the original process. Furthermore, BOF–LF–RH–CC process reduced the inclusion area ratio, the proportion of single inclusion area of < 8 μm2 increased from 85 to 95%, the proportion of Al2O3 inclusion less than 5 μm2 increased from 50 to 70%. Meanwhile, the desulfurization effect became pronounced and the temperature control of the double refining process was more stable than that of the original process.