Banding effects on the process of grain refinement by cold deformation and recrystallization of acicular C-Mn steel

Grain refinement by cold deformation and recrystallization of acicular microstructure of Fe-0.18C-2.47Mn-0.03Ti (wt%) steel has already been reported [Hossein Nedjad et al. 2011]. Further studies on the same acicular microstructure indicated mild banding of ferrite-dominant and martensite-dominant regions. In this paper, subtle effects of banding on the process of grain refinement, microstructure and mechanical properties are reported. Manganese segregation in accordance with the banded structure was identified by electron probe microanalysis (EPMA). Electron backscattering diffraction (EBSD) of the specimens annealed for 3.6 ks at 873 and 923 K identified enhanced recrystallization and austenite retention in the deformed martensite-dominant bands having high Mn content. Deformed ferrite bands with low Mn content unaltered at low annealing temperatures (T≤873 K) but recrystallized to coarse ferrite grains at higher annealing temperature (T=923 K). A bimodal grain size of ferrite was found after annealing at 923 K. Isothermal sections of Fe-C-Mn phase diagram at annealing temperatures qualitatively indicated that Mn-rich bands enter the three phase (ferrite+cementite+austenite) region which favors austenite formation. A polyphase microstructure consisting of recovered ferrite, recrystallized ferrite and retained austenite shows valuable tensile properties, thanks to the beneficial effect of banding.