Significantly enhanced impact toughness from ambient to cryogenic temperature in high-strength steel via Mn segregation induced delamination

Mn segregation is an inevitable microstructural characteristic in medium/high Mn steels and generally deteriorates mechanical performance. Instead of eliminating Mn segregation, the present work firstly develops a high-strength medium Mn steel with obviously improved impact toughness at a wide temperature range between ambient and cryogenic temperatures by artificially introducing Mn segregated bands to initiate delamination toughening. By combining warm rolling and intercritical annealing, the heterostructure containing elongated Mn-rich austenite bands with a width of ∼7.6 μm and submicron austenite/ferrite matrix was realized. The coarse austenite bands featuring limited mechanical stability preferentially undergo transformation-induced plasticity (TRIP) effect upon deformation, inducing brittle TRIP-martensite with abundant C/Mn contents. These TRIP-martensite parallel to the rolling direction encourage the activation of delamination mechanically and microstructurally by serving as the initiation sites and propagation paths for delamination cracks, making brittleness into toughening. The presence of intensive through-thickness delamination cracks regulates the stress state around the main crack tip, consumes energy through the formation of new surfaces, and activates additional TRIP effects around delamination cracks. The pronounced delamination toughening was realized at ∼-100 °C, benefiting from an abnormal enhancement of impact toughness at even low temperatures. The sustainable TRIP effect provided by austenite grains with distinct morphology, size, and composition further improves toughness intrinsically. Formation of Mn-rich bands furthermore relieves the segregation of Mn to prior austenite grain boundaries (PAGBs), avoiding intergranular fracture that is usually observed in medium Mn steel. This novel toughening mechanism paves the way for developing high-strength material with enhanced toughness at ambient and low temperatures.