Effect of bimodal distribution in ferrite grain sizes on the tensile properties of low-carbon steels

In order to understand the effect of grain size bimodality on the tensile properties, low-carbon steel samples having bimodal ferrite grain size distributions, with different coarse-grain sizes (15–54 μm), fine-grain sizes (2.4–13.0 μm) and coarse-grain fractions (0.10–0.45) have been tensile tested. Samples having bimodal grain structures showed lower strength (UTS: 395–744 MPa), better tensile ductility (eu: 8.5–19.4%) and lower YS to UTS ratio (0.63–0.84) than the ultra-fined grained steel (UTS = 900 MPa, eu = 5%, and YS:UTS = 0.93). In case of mixed grain structures, effective grain size (Deff), numerical average grain size (Davg) and the average grain size in the fine-grain regions (Df) showed better correlation with strength (i.e. strength ∝ D−1/2) than the weighted average grain size (Dwt). Coarse-grain sizes in the range of 15–35 μm and fraction of the coarse-grain regions in the range of 15–35% can be recommended for achieving the best tensile ductility. Measurement of micro-hardness and the change in grain aspect ratios from the coarse- and fine-grain regions indicated that the bimodal grain structure can be considered as a composite structure, where stress and strain vary between the coarse- and fine-grain regions. Lower nucleation and higher growth of the voids in the carbide-depleted, coarse-grain regions (than carbide-rich, fine-grain regions) allow higher plastic deformation till fracture and improve the total-elongation of the steels.