A bimodal-sized microstructure lied in ultrafine-grained ferrite/cementite steel fabricated through caliber-warm-rolling followed by annealing. The ferrite grain size distribution had a bimodal distribution consisting of two peaks, a large peak and a small peak. A distinct absorbed energy transition or ductile-to-brittle transition occurred in Charpy impact tests. The ductile-to-brittle transition was due to the intrusion of cleavage. The transition shifted to higher temperature region with an increase in average gain size for the larger grain peak, with a simultaneous change in effective grain size on fracture surface. The effective grain size apparently corresponded to the grain size of a larger grain peak of the bimodal grain size distribution. More precisely, the effective grain size was found to better correspond to the larger peak of the bimodal distribution of grains with {100} planes almost parallel to the fracture plane. Fracture stress estimated became higher with a decrease in effective grain size. Namely the ultra-refinement of ferrite grain size could bring about the high fracture stress and contribute to an excellent toughness even for the bimodal-sized structure.
