Overcoming strength-ductility trade-off via subzero martensitic transformation in medium-Mn lightweight steel

Effects of subzero martensitic transformation on tensile properties were investigated in an Fe-0.3C-9Mn-5Al-1Si (wt.%) lightweight steel. The microstructure for the hot-rolled state consists of δ-ferrite, bainite, and γ-austenite, while the retained austenite transformed to the lenticular martensite after the subzero-treatment between -30 and -80 °C. Then, nano-sized austenite was reverted inside the lenticular martensite after tempering at 300 °C for 2 h. The fraction of lenticular martensite increased as the subzero-treatment temperature decreases, resulting in the enhanced yield and tensile strengths but the almost maintained tensile elongation. The main fracture mechanism including fracture modes and secondary cracking was almost identical even after the subzero-treatment because it is not directly affected by the lenticular martensite formed during the subzero-treatment. As a result, yield and tensile strengths were simultaneously enhanced without the elongation loss, which provides promising ideas for overcoming the strength-ductility trade-off and for widening their automotive applications.