Novel ultra-high-strength (ferrite plus austenite) duplex lightweight steels achieved by fine dislocation substructures (Taylor lattices), grain refinement, and partial recrystallization

Weight reduction from down-gauged high-strength steels has been an important issue in automotive industries to improve fuel efficiency. In addition to lightweight needs, automotive steels require an excellent combination of specific strength and ductility for forming complex shapes as well as improving crashworthiness qualities. Here in the present study, new ultra-high-strength (ferrite + austenite) duplex lightweight steels containing a low-density element of Al, which exhibit strength above 1 GPa and tensile elongation of 46%, have been developed. Improved tensile properties are associated with typical planar glide configurations and high dislocation density walls, configuring Taylor lattices, developed by very fine dislocation structures spaced with intervals between 50 and 100 nm. Deformation twinning having extremely small (about 5 nm) thickness and spacing is also activated, thereby leading to additionally enhanced ductility. The present lightweight steels have outstanding properties of strength and ductility, easy manufacturing process, and costs of alloying elements as well as reduced specific weight for automotive applications. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.