Effect of Niobium on the Formation of Nano/Ultrafine Grain Structure in a Low Carbon Steel by Thermomechanical Treatment

Abstract

The formation of nano/ultrafine grained ferrite in low carbon steels containing different amounts of niobium was investigated using thermomechanical treatment which consisted of annealing of 85% cold rolled martensite with different parameters. The specimens were characterized by optical and scanning electron microscopy and Vickers hardness test. A lamellar dislocation cell structure was formed during cold rolling. An increase in hardness was found during annealing with the addition of the Nb element. The mean grain size of the specimens annealed at 550 ºC for 300 s was approximately similar (ranging from 79 to 87 nm) for both chemical compositions. Increasing annealing temperature to 600 ºC or annealing time to 2.7 ks led to a severe grain growth in the steel without Nb, but no considerable changes in Nb containing steel were observed.