A strong and ductile nanotwinned steel was fabricated 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 nanotwinned steel achieved a high yield strength (1450 MPa), high ultimate tensile strength (1600 MPa) and considerable uniform tensile elongation (20%). The average twin thickness and spacing are 5 nm and 40 nm, respectively. 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. A physically-based model is proposed to simulate the evolution of dislocation density and stress-strain relation, showing good agreement with the experimental results.