EFFECT OF NIOBIUM ON THE FORMATION OF NANO/ULTRAFINE GRAIN STRUCTURE IN A LOW CARBON STEEL BY THERMOMECHANICAL TREATMENT

摘要

Formation of nano/ultrafine grain ferrite was investigated in low carbon steels containing different amounts of niobium using thermomechanical treatment comprising of annealing of 85% cold rolled martensite in different circumstances. 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 foundduring annealing with increasing the Nb content. The mean grain size of the specimens annealed at 550°C for 300 s was approximately similar(ranging from 79to 87 nm)for both chemical compositions. Increasing annealing temperature to 600°C 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 was observed.