Dissolution Behavior of Different Inclusions in High Al Steel Reacted with Refining Slags

Al2O3, Al2O3·TiN, Al2O3·MgO, and CaO·2Al2O3 are four different types of inclusions in high Al steels. To improve the steel cleanness level, the effective removal of such inclusions during secondary refining is very important, so these inclusions should be removed effectively via inclusion dissolution in the slag. The dissolution behavior of Al2O3, Al2O3·TiN, Al2O3·MgO, and CaO·2Al2O3 in CaO-SiO2-Al2O3-MgO slags, as well as the steel-slag reaction, was investigated using laser scanning confocal microscopy (LSCM) and high-temperature furnace experiments, and thermodynamic calculations for the inclusion in steel were carried out by FactSage 7.1. The results showed that Al2O3·TiN was observed to be completely different from the other oxides. The composite oxides dissolved quickly in the slags, and the dissolution time of the inclusions increased as their melting point increased. SiO2 and B2O3 in the slag were almost completely reacted with [Al] in steel, so the slags without SiO2 showed a positive effect for avoiding the formation of Al2O3 system inclusions and promoting inclusions dissolution as compared with SiO2-rich slags. The steel-slag reaction was also found to influence the inclusion types in steel significantly. Because of the rapid absorption of different inclusions in the slag, it was found that the dissolution time of inclusions mainly depends on the diffusion in the molten slag.

Improvement in Castability of Al Killed Steel in Billet Casters by Process Optimisation

Sticking of alumina as well as spinel inclusions inside the sub entry nozzles (SEN) as well as tundish nozzle is perennial problem during continuous casting of aluminum killed steel through billet casters. This results in restriction or completely blockage of flow of liquid steel through the nozzles eventually leading to abortion of sequence in billet casters and stopping of continuous casting machine. Nozzle clogging not only restricts the productivity by restraining the casting sequence, intermittent extrication of clogged alumina particles or dislodged refractory materials are a significant source of non-metallic macro-inclusions in the cast sections of billet casters. If these inclusions are not removed completely during secondary refining of steel they causes excessive clogging mainly in low carbon Al killed steels. In other grades of Al killed steel cogging is also very prominent if the deoxidation and secondary refining is not carried out properly. IISCO Steel Plant (ISP), Burnpur a new, modernised unit of Steel Authority of India Limited (SAIL) was facing problems of nozzle clogging in low carbon, low Si, Al killed grade (EWNR –electrode quality grade) resulting in premature abortion of casting sequence leading to huge productivity loss. To solve the problem of nozzle clogging in low carbon Al killed grades and other grades at ISP, optimisation of various steelmaking parameters viz. amount of Al addition & its sequence, purging regime in ladle furnace, optimisation of Ca treatment process etc has been carried out which has resulted in improvement in castability of Al Killed Steel in billet caster of ISP.