Effect of C and Mn variations upon the solidification mode and surface cracking susceptibility of peritectic steels

Abstract

This work studies the effect of chemical composition, C and Mn contents, and cooling rate on both the solidification mode and the evolution of phases during the solidification of three steels; two of hypc-peritec-tic and one of hyper-peritectic composition. Furthermore, the cracking susceptibility associated to both the differences in mechanical behavior of δ and γ phases, and contraction during solidification, was inferred. Slight variation of C or Mn, in the order of 0.04%, promoted significant changes in the evolution of phases during solidification. It was observed that for the hypo-peritectic steel closer to the peritectic point, the Mn microsegregation observed for high cooling rates promoted at the end of solidification a hyper-peritectic solidification mode. On the other hand, independently of the solidification mode and chemical composition of the studied steels, the differences in the mechanical behavior of 8 and y phases led to a cracking susceptibility in two solid fraction zones. Furthermore, for the steel exhibiting hypo-peritectic solidification mode the peritectic transformation occurred at higher solid fraction compared with the steels showing hyper-peritectic solidification mode. Therefore, the remaining liquid ability to feed the contraction in the solid-liquid shell associated to the peritectic transformation resulted adversely affected. Hence, the cracking susceptibility observed in the hypo-peritectic steel is not only generated by differences in the mechanical behavior of δ and γ phases, but also by the liquid inability to compensate the contraction associated to the peritectic transformation. © 2009 ISIJ.