On the Spheroidizing Annealing Behavior in Cr/Nb Microalloyed Medium Carbon Steels

Abstract

The industrial process for the production of fine‐blanking steels, which includes hot‐rolling, cold‐rolling, and spheroidizing annealing, is simulated at a laboratory scale using two medium carbon steels, respectively, microalloyed with Cr and Nb. Two types of hot‐rolled microstructures, that is, “ferrite + pearlite” and bainite are obtained by varying the interrupted cooling temperature (ICT). The microstructure and property evolution during spheroidizing annealing are characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron back scattered diffraction (EBSD), and micro‐hardness tester. In Cr microalloyed steel, the dual partitioning of Cr and Mn and slowed growth kinetics of cementite particle are correlated to explain the suppression of ferrite recrystallization. Contrarily, a fast spheroidizing annealing behavior, which led to uniform distribution of cementite particle and fully recrystallized ferritic matrix, is observed in Nb microalloyed steel especially from the hot‐rolled bainite microstructure.