Abstract: Systematic investigations were made on characteristics of the non-metallic inclusions in secondary refining and continuous casting of ultra-low oxygen special steel and following results were obtained: 1) It was hard to further lower the T.O content and eliminate the inclusions in LF refining when the steel T.O mass fraction was lowered to (13－15)×10-6. RH degassing had much stronger ability to eliminate the inclusions and lower the T.O to ultra-low content level. By elongating the RH degassing time to 33 minutes in the experiments, the T.O mass fraction and amount of inclusions larger than 1.5 m were decreased to 4.7×10-6 and 1.77 N/mm2 respectively. 2) In the secondary refining, the inclusions evolved from Al2O3 to MgO-Al2O3, CaO-MgO-Al2O3 and CaO-Al2O3 system inclusions progressively, in which the evolution of Al2O3 to MgO-Al2O3 inclusions was mainly due to the reaction [Mg]+ n/3Al2O3=MgO(n－1)/3Al2O3+2/3[Al] and the dissolved [Mg] in liquid steel was proved in the experiment mainly from the ladle refractories owing to the reaction 2[Al]+3(MgO)lining= 3[Mg]+Al2O3. 3) Large-sized inclusions of 100－330 m were found in specimens of cast bloom of the steel which contained 5.9×10-6 T.O. Most of the larger-sized inclusions above 100 m were CaO-MgO-Al2O3 clusters constituted by smaller particles which had very similar chemical compositions to the small inclusions found in liquid steel samples, indicating that the large-sized CaO-MgO-Al2O3 cluster inclusions in the bloom were formed due to the aggregation of the small inclusions in continuous casting. 4) Most of the inclusions had been transferred to liquid inclusions of CaO-Al2O3 system after RH degassing in the experiment. However, owing to reoxidation in continuous casting, the inclusions were changed to solid inclusions of CaO-Al2O3, MgO-Al2O3 or CaO-MgO-Al2O3 system, which were easier to aggregate to larger inclusions. Hence, reoxidation must be very strictly controlled in production of ultra-low oxygen special steels.

Key words: special steel, oxygen content, non-metallic inclusion, secondary refining, reoxidation, continuous casting