MACROSEGREGATION BEHAVIOR IN CONTINUOUSLY CAST HIGH-CARBON STEEL BLOOMS AND BILLETS AT THE FINAL STAGE OF SOLIDIFICATION IN COMBINATION STIRRING

The influence of various Electro-Magnetic Stirring (EMS) modes on reducing macrosegregation has been investigated by conducting a series of plant tests using a continuous bloom easter and a billet easter of POSCO as well as the relevant laboratory experiment. The emphasis was put on the reduction mechanism of macrosegregation due to a combination stirring including a final EMS (FEMS) in continuous casting high carbon steel blooms and billets. The variation of mushy zone shapes and crater end positions depending on EMS modes was also examined through a Pb addition test providing a close relationship between them and macrosegregation behavior. The optimum stirring pool thicknesses at final EMS (FEMS) have been determined as a result for each carbon steel tested providing a valuable production line data for plant operation and their relation with carbon content are also discussed in view of the theoretical calculation from a solidification model and a relative permeability measurement using a Tamman furnace. Finally, the reduction mechanism of macrosegregation has been clarified as due to the enhanced solidification rate, finely distributed segregation spots in the whole centre region, and narrowed width of mushy zone during the final stage of solidification under a combination stirring mode.