Formation of bimodal-sized structure and its tensile properties in a warm-rolled and annealed ultrafine-grained Ferrite/Cementite steel

An ultrafine-grained ferrite/cementite (UGF/C) steel with a local high density of cementite particles was fabricated through caliber-warm-rolling followed by annealing and resulted in a bimodal-sized microstructure. The characteristic bimodal-sized microstructure was attributed to the original ferrite-pearlite structure and cementite spacing, and reflected the original ferrite-pearlite structure. The smaller-sized clusters corresponded to the former pearlite regions and the larger-sized clusters to the proeutectoid ferrite regions. The cementite particles naturally localized within the former pearlite region. Most of the ferrite coarsening did not occur until the cementite particle spacing reached a critical value. The UGF/C microstructure with a bimodal grain size showed a yield strength elongation combination that was better than conventional ferrite pearlite steels, and is a significant improvement as a result of the processing and resultant microstructure. The Hall-Petch relationship was best demonstrated when the yield strength was correlated to the average size (determined from the two inverse grain sizes that were normalized by the area fractions) rather than the larger or smaller size of the bimodal distribution, which reflected the strengthening effect of the subgrained structure. The grain refinement contributed to the athermal component of the yield strength but hardly affected its thermal component.