MACRO AND MICROSCOPIC SEGREGATION BEHAVIOR OF CENTER SEGREGATIONS IN HIGH CARBON STEEL CC BLOOMS DURING THE FINAL STAGE OF SOLIDIFICATION

摘要

The formation behavior of center segregations in high carbon steel CC(Continuously Cast) blooms were examined under various EMS(Electro-Magnetic Stirring) modes and final stirring pool thicknesses. The center segregations were then analyzed quantitatively both macroscopically and microscopically by using a mapping analyzer. Macroscopically, the most effective reduction of center segregations was achieved in combination stirring of (S+F)EMS due to enhanced solidification rate and reduced penetration width of solute enriched interdendritc residual liquid at the final solidification stage resulting into a homogeneous distribution of segregation spots and porosities in the centre region of CC blooms. Under a combination stirring with (S+F)EMS, the optimum stirring pool thickness at FEMS is found to decrease as the carbon content increases. This behavior is shown attributable to the longer local solidification time(t_c) and higher permeability of a higher carbon steel at the final solidification period. Microscopically, centre segregations appear to form by the one time thrust of solute enriched residual liquid steel into solidification cavities, having the final solid fraction reaching the ranges f_s=0.30-0.78. In terms of the maximum segregation ratio of solutes in centre segregation spots, the combination stirring seems to provide a somewhat adverse effect in contrast to the optimum stirring pool thickness at FEMS, that gives the lowest segregation ratio. A macroscopic as well as a microscopic solidification model is proposed to this end based on the present investigation describing the step by step solidification processes occurring in the bloom center during the final solidification period.