In-situ SEM/EBSD investigation of tensile deformation-driven microstructural changes in low-carbon HSLA steels with ferrite-bainite microstructures

In this study, we aimed to investigate how microstructural constituents affect the plastic deformation behavior of low-carbon HSLA steels during a uniaxial tensile test using the in-situ SEM/EBSD technique. We fabricated three types of low-carbon HSLA steels, each with distinct ferrite-bainite microstructures, by altering the thermomechanical control processing conditions. All the steels were composed primarily of polygonal ferrite (PF) and bainitic microstructures. As the finish rolling temperature and the cooling rate decreased, the volume fraction of PF increased. The in-situ SEM and EBSD analysis during deformation revealed a greater concentration of plastic strain and an increased mobile dislocation density in the PF than in the bainitic microstructures. Most notably, as the volume fraction of the bainitic microstructure increased, so did the degree of strain gradient between the soft PF and the hard bainitic microstructure. This suggests that at the initial yield stage the soft PF plays a more significant role in mitigating the yield point phenomenon than the hard bainitic microstructure. Our findings offer a deeper understanding of the microstructural factors that influence plastic deformation, and may serve as a guide for designing optimal microstructures to ensure the mechanical properties of low-carbon HSLA steels for a range of applications.