Combination of ductility and toughness by the design of fine ferrite/tempered martensite-austenite microstructure in a low carbon medium manganese alloyed steel plate

Based on the mechanisms of plasticity and toughness, a low carbon medium manganese alloyed steel plate with the outstanding combination of ductility and toughness was designed. The microstructure characteristics, strain hardening behavior and toughening mechanisms were investigated in detail. The microstructure is composed of ferrite/tempered martensite and austenite after heat treating at 650 °C for 2 h. The retained austenite shows multi-type morphology, multi-scale size and relatively poor mechanical stability, resulting in multi-peak strain hardening behavior and sufficient TRIP effect. The excellent strain hardening capacity and ductility are hence achieved. The excellent low temperature toughness is also achieved due to the relatively lower concentrations of Mn and C, the formation of fine ferrite grains and a small volume fraction of fine retained austenite. The steel hence possesses the excellent ductility (the total elongation is ~37.3%) and low temperature toughness (the Charpy V-notch impact energy is ~158 J at −80 °C).