Comparative study of tensile properties and impact toughness of duplex lightweight steel

Abstract

Herein, a comparative study regarding the tensile properties and impact toughness of duplex lightweight steel and its deformation behavior was conducted to broaden its application from automotive steel sheets to thick-plate applications at various temperatures using Fe-0.2C–15Mn–6Al and Fe-0.4C–15Mn–6Al hot-rolled steels as model alloys. These steels revealed duplex microstructures with a large fraction of γ austenite (>0.7), featuring elongated δ ferrite grains aligned with the rolling direction and nearly globular γ grains. Both steels demonstrated an exceptional balance of tensile strength and ductility despite their low temperatures ranging from −20 °C to −100 °C, which was attributed to the active dynamic strengthening mechanisms, such as transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP). However, contrary to the promising tensile test results, the steels demonstrated low values of the impact absorbed energy at temperatures ranging from −60 °C to −196 °C. This difference was likely owing to the rapid deformation rates in the Charpy impact tests compared to tensile tests, leading to the evolution of significant damage indicated by deep parallel cracks within the δ grains, without significantly activating the TRIP effect. Therefore, despite the impressive low-temperature tensile properties of the duplex lightweight steel, its suitability for cryogenic thick-plate applications must be approached with caution owing to the potential for compromised mechanical properties under sudden impact conditions.