Bimodal Grain Size Distribution Enhances Strength and Ductility Simultaneously in a Low-Carbon Low-Alloy Steel

Low-carbon low-alloy steel specimens were quenched, then cold rolled, and finally annealed. Electron backscatter diffraction (EBSD) micrographs revealed a bimodal grain structure where ultra-fine grain structures with low-angle grain boundaries are alternating with regions of larger grains. The average total dislocation density was measured by X-ray line profile analysis, whereas the geometrically necessary dislocation density was obtained from the analysis of EBSD data. Using the combination of the Hall-Petch and Taylor equations, a good correlation was found between the total dislocation density and the measured flow stress in the different states of the alloy. The difference in evolutions of the total and the geometrically necessary component of the dislocation densities is discussed in terms of the successive processes of quenching, rolling, and annealing of the alloy.