The Significant Impact of the Characteristics of Granular Structure and Granular Bainite on the Mechanisms Contributing to Strength–Ductility Combination

A comprehensive analysis and characterization of granular structure (GS) and granular bainite (GB) were conducted in a low-carbon alloy steel with the objective to elucidate the mechanisms that contributed to strength–ductility combination. Ferrite + martensite–austenite (M-A) islands were observed in both GS and GB, but a transformation mechanism produced a variation in the matrix and in the M-A size, distribution and morphology. The differences in the matrix and M-A islands were carefully studied by electron backscatter diffraction. Furthermore, tensile tests were interrupted at different strains to study the microstructure evolution during tensile straining and mechanical properties determined from samples tensile strained to fracture. It was observed that the specimen with GB exhibited higher tensile strength, ductility and strain hardening compared to GS. Dislocation strengthening of GB was more apparent than GS. The higher volume fraction and uniform distribution of M-A islands also contributed to the higher strength of GB. The improvement in ductility may partly be attributed to the sub-structural refinement. An increased presence of geometrically necessary dislocations array in GB also simultaneously enhanced strength and ductility. During deformation, GB exhibited a more uniform local strain distribution compared to GS because of superior plastic ability of M-A islands. Based on the experimental results and analysis, we propose models describing the different failure mechanism in GS and GB.