Ca Treatment Effect on the Evolution and Microstructure of Ti Alloyed Inclusions in HG785 Steel

To reveal the inclusion evolution and its influence on the inner microstructures of high strength steel treated by Ti alloying Ca,we used scanning electron microscope and energy dispersive spectroscopy to characterize the inclusion characteristics and its induction to ferrite behaviors.We compared the effects of different types of inclusions on ferrite-induced microstructure improvement,and combined with thermodynamic calculations,discussed the evolution mechanisms of inclusions during aluminum deoxidation,Ti alloying,and calcium treatment.The experimental results show that with the addition of Al,Ti and Ca alloys,the content of inclusions have changed significantly.With the increase of deoxidation time（5 min,10min,15 min）,the content of dissolved oxygen in the steel decreases.After adding Ti,the content of TiO_x in the inclusion also decreases.Sufficient Ca content can change the high-melting inclusions of Al-Mg-TiO in the steel into the low-melting liquid inclusions of Al-Mg-Ti-O,and the reduced Ti will eventually remain in the inclusions or diffuse into the molten steel.Under the conditions of this study,the appropriate Al-Ti deoxidation time interval of is more than 10 min,which can improve the yield rate of Ti.The thermodynamic calculation shows that Al₂TiO₅ is more easily modified by Ca into low melting calcium aluminate,rather than Al₂O₃.To avoid Ca S and Ti N precipitation in the liquid phase,[S]＜0.002 4%and[N]＜0.005 8%should be made.Compared with other types inclusions in steel,Al-Mg-Ti-Ca-O+Mn S+Ti N is more likely to induce acicular ferrite precipitation.