Microstrain partitioning, transformation induced plasticity, and damage evolution of a third generation medium Mn advanced high strength steel

An experimental Medium Mn (med-Mn) steel (0.15C-5.8Mn-1.8Al-0.71Si) with a martensitic starting microstructure, intercritically annealed at 685 °C for 120s, was discovered to have a large true strain at fracture (ɛ_f = 0.61) while also meeting established (strength x elongation) targets (28,809 MPa%), sustained monotonic work hardening and prolonged Transformation Induced Plasticity (TRIP) kinetics. This was found by varying the intercritical annealing (IA) temperature within a narrow temperature interval in order to isolate its impact on TRIP kinetics and damage development on such med-Mn steel. A comprehensive understanding of the microstructural damage processes leading to fracture is presented using quasi in-situ Scanning Electron Microscope tensile testing as well as X-ray Computed Microtomography. In addition, we precisely evaluated the TRIP kinetics of this steel using a combined Digital Image Correlation (DIC) and synchrotron-sourced High Energy X-ray Diffraction technique. With these methods, we demonstrate that an abundance of voids nucleate during deformation, but their growth can be suppressed by prolonging TRIP over a large strain range. Moreover, novel post-processing techniques to assess DIC acquired data at the microscopic scale have been used to gauge the severity of strain partitioning amongst phases and strain gradient evolution across dissimilar phase interfaces. By comparing it to another 3G TRIP-assisted steel and an ultrafine grained Dual Phase steel. Overall, it has been found that, in addition to carefully moderating TRIP kinetics, the introduction of polygonal ferrite, as is conventional in med-Mn steels, enhances the local forming properties and damage tolerance in 3G TRIP-assisted microstructures.