Recrystallisation processes in cold rolled low carbon steel strip containing different starting microstructures

Abstract

The effect of initial microstructure [acicular ferrite (AF); polygonal ferrite (PF); fine polygonal ferrite (FPF); bainite (B)] on the recrystallisation behaviour of cold rolled low carbon (LC) steel strip was investigated. Steel strip samples of 2 mm thickness were heat treated to produce FPF, PF, AF and B. The resulting materials, together with a twin roll cast LC strip containing predominantly acicular ferrite (SC), were cold rolled to 50, 70 and 90% reduction and annealed for various times in the temperature range 580–680°C. The evolution of microstructure during rolling and recrystallisation was studied by optical microscopy, electron backscatter diffraction in SEM and TEM. The initial microstructure was found to have a substantial influence on the annealing behaviour. Recrystallisation was structure dependent with the rate of recrystallisation shown as FPF>B>PF>AF>SC during annealing at temperatures in the range 580–680°C. Both partly and fully recrystallised materials exhibited approximately lognormal grain size distributions with the PF and B having the largest and smallest average grain sizes and standard deviations and the smallest and largest skewness respectively.