Microstrain partitioning, Transformation Induced Plasticity, and the evolution of damage during deformation of an austenitic-martensitic 1.5 GPa Quench and Partition steel

Abstract

The coupling of multiple advanced characterization techniques performed on a Fe-0.2C-3.4Mn-1.6Si, austenitic-martensitic, Quench and Partition steel (Q&P) with ultrahigh strength (∼1.5 GPa) microscopically explains its high true strain at fracture (ɛ_f) and superior toughness. The benefits of Transformation Induced Plasticity-assistance in Third Generation (3G) steel microstructures have been deduced by comparing the behavior of this Q&P steel to that of a Dual Phase (DP) steel of similar strength and grain size. More precisely, by using a novel Digital Image Correlation (DIC)-based computation technique, introduced by Pelligra et al.(2022) [1], we have shown that the local strain gradient at dissimilar phase interfaces, linked to the evolution of Geometrically Necessary Dislocations, increases more slowly in the Q&P steel than in the DP steel, and as a result enables the steel to achieve a high ɛ_f. Detailed studies of the micromechanical compatibility between phases and dynamic evolution of damage in this Q&P steel have been obtained through quasi in-situ tensile tests conducted under a Field Emission Scanning Electron Microscope coupled with Digital Image Correlation at the microscopic scale. Additionally, void evolution with strain was evaluated using X-ray Computed microtomography while the TRIP kinetics were determined via High Energy X-ray Diffraction. This ultrahigh strength Q&P steel shows an improvement in the micromechanical compatibility, as co-deformation and micro-shearing of dissimilar phases were observed. Despite advances made in the literature to improve the formability of 3G DP steels, these critical microstructural properties render the application of Q&P processes to 3G steels a more suitable manufacturing route in the development of future anti-intrusion and impact resistance components in vechicle body structures.