Enhancing charpy absorbed energy of aged duplex lightweight steel plates through TRIP and TWIP mechanisms

Ensuring toughness in thick hot-rolled plates remains a challenge for lightweight steels in automotive, shipbuilding, military, and construction industries despite improved tensile properties. This study investigated the Charpy absorbed energy of thick hot-rolled Fe-0.4C-15Mn-6Al duplex lightweight steel plates exhibiting TRIP and TWIP mechanisms, aged at 450–500 °C to precipitate κ-carbides. Fracture initiation and propagation energies measured from instrumented Charpy impact testing were analyzed through microstructural and microfracture analyses. The 500 °C-aged (A500) specimen showed the highest Charpy absorbed energy, composed of the highest fracture initiation and propagation energies across all test temperatures, particularly due to active TWIP and TRIP mechanisms along with significant κ-carbide precipitation strengthening. Despite predominantly ductile fracture modes, regardless of aging temperature and test temperature, deformation mechanisms were influenced by stacking fault energy (SFE). Aging resulted in κ-carbide precipitation, reducing C and Mn contents in austenite and lowering SFE. At 25 °C, the superior energy absorption of the A500 specimen (296 J) was attributed to its high flow stress and extensive roughness in the fracture surface due to crack deflection in the fracture initiation region and zigzag crack propagation. The Charpy absorbed energy decreased significantly at lower temperatures due to limited development of slip line field and less zigzag crack propagation. Despite this, the A500 specimen maintained the highest energy absorption due to its optimized TWIP and TRIP mechanisms and κ-carbide precipitation strengthening.