Ultra-strong nanotwinned steel with large tensile elongation

Abstract

A twinning-induced plasticity (TWIP) steel strengthened by elegant arrays of laminated nanotwins was manufactured by a simple thermomechanical treatment consisting of cold rolling and recovery annealing. Different to other lab-scale methods making nano-structured materials, the present simple thermomechnical treatment is suitable for large-scale production in the steel industry using existing facilities, which makes the present steel being an attractive structure material. The steel achieved a high yield strength (1450 MPa), high ultimate tensile strength (1600 MPa) and considerable uniform tensile elongation (20%). During the recovery annealing, the nanotwins are thermally stable so that they remain in the sample after recovery annealing. On the contrary, the dislocation density was reduced greatly after recovery annealing. The deformation mechanism of the present nanotwinned steel is investigated by synchrotron X-ray diffraction, transmission electron microscopy, nanoindentation and electrical resistivity, illustrating that the dislocation density increases dramatically with strain while the volume fraction of nanotwins remains constant. Therefore, the work hardening behaviour of the nanotwinned steel is mainly provided by the accumulation of dislocations.