Development of Bimodal Ferrite-Grain Structures in Low-Carbon Steel Using Rapid Intercritical Annealing

Mixed ferrite grain structures, which have fine- and coarse-grain regions and showing bimodal grain size distributions, have been produced by rapid intercritical annealing of warm-rolled (or cold-rolled) samples. Microstructural changes have been analyzed using dilatometric studies, size prediction of transformed and recrystallized grains, and microtexture measurements. Fine austenite grains (< 5 mu m) developed during rapid annealing and transformed into fine-ferrite grains (2 to 4 mu m) after cooling. Coarse-ferrite grains (28 to 42 mu m) resulted from the recrystallization and growth of deformed ferrite. The effect of heating rate on microstructural morphologies during intercritical annealing has also been studied. A slow rate of heating (30 K/s) developed a uniform distribution of fine-ferrite grains and austenitic islands, while rapid heating (300 K/s) generated coarse blocks of austenite, elongated along the prior-pearlitic regions, in the ferrite matrix. As expected, bimodal ferrite grain structures or fine-scale dual-phase structures showed superior combination of tensile strength and ductility, compared to the ultrafine-grained steels. DOI: 10.1007/s11661-012-1556-z (C) The Minerals, Metals & Materials Society and ASM International 2012