Abstract:

Steel and slag samples during refining and casting of three heats of 510L beam steel produced by BOF→LF→CC process was taken and analyzed. The variation of total oxygen (T.O), nitrogen and dissolved content of Al, Mg and Ca content in steel were investigated. The evolution of number density, composition and size of inclusions in steel at different steps was discussed. Reoxidation during continuous casting could be controlled economically and effectively by covering the top of tundish with cotton and stoping blowing argon from stopper rod, by which the nitrogen absorbtion from air reduced from 22×10^-6 to 2×10^-6. Calcium treatment was used to modify high-melting point Al₂O₃-MgO inclusions to Al₂O₃-MgO-CaO complex inclusions with low-melting temperature. However, proper operations during calcium treatment and during subsequent continuous casting were needed, otherwise, severe reoxidation occurred during continuous casting destroyed the effect of successful calcium treatment during steel refining during the industrial trials of the current study. After calcium treatment, 3—7 μm inclusions in slab increased while above 7 μm inclusions decreased.

Key words: steel cleanliness, inclusions, total oxygen, calcium treatment