Enhancement of Impact Toughness Via Tailoring Deformation Compatibility of Constituent Phases in Duplex Q&P Steel with Excellent Strength and Ductility

Q&P steels possessing a combination of ultra-high product of strength and tensile elongation (PSE) and superior impact toughness was obtained by tailoring the micromechanical deformation ability of retained austenite (RA) and martensite via low-temperature partitioning. Our results indicate that impact toughness is more susceptible to the heterogeneity of mechanical properties of constituent phases than its tensile mechanical properties. Through optimizing the difference between the constituent phases, the impact toughness can be improved significantly by the enhancement of micromechanical deformation ability of tempered martensite (TM) and corresponding alleviation of deformation incompatibility. Simultaneously, high PSE was preserved by using the deformation potential of RA and its transformation-induced plasticity effect. Load–unload–reload (LUR) tests were carried out to characterize the evolution of flow stress components and the influence of partitioning temperatures on the degree of deformation inhomogeneity between constituent phases during plastic deformation. Furthermore, the mechanical properties and micromechanical deformation ability of the constituent phases were quantitatively characterized using the nanoindentation technique, which indicates that degree of incompatible deformation is alleviated for steels partitioned at relatively high temperatures due to the enhancement of the deformation ability of TM. The underlying relationship between the dynamic transformation of metastable austenite based on the Olson and Cohen model, and high PSE was elucidated. As the partitioning temperature increases, the mechanical stability of RA increases, which is attributed to the synergistic effect of carbon partitioning and the improvement of compatible deformation capability of TM and RA, resulting in an excellent combination of concurrent ductility and impact toughness. Based on the above characterization and analysis, further investigation into the fractured morphology of impact and edge notched specimens was carried out to provide a comprehensive understanding of the damage and fracture mechanism, and to reveal the reasons for the improvement of mechanical properties. The nucleation position and propagation path of microvoids and microcracks depend on the deformation compatibility of constituent phases and the morphology of cementite in tempered martensite. The nucleation sites of microcracks and microvoids at the phase interface, due to stress concentration and dislocation pile-ups for incompatibility deformation of constituent phases, are substituted with nucleation sites at fractured cementite in TM with the increasing partitioning temperature.