Simultaneous enhancement of strength and toughness of ultrafine-grained dual-phase steel with a bimodal lamellar microstructure

A novel heterogeneous lamellar microstructure with ultrafine grains was successfully introduced into a DP590 steel plate using a simple two-step controlled rolling process (TCR) described in this work. The ferrite and martensite are laminated, with an average size of 1.7 mu m and 1.2 mu m, respectively. The ferrite shows a bimodal grain size distribution and intensified alpha-fiber and rotated cubic texture. Compared to conventional controlled-roll (CCR) steel, TCR steel demonstrated 46.6 % higher tensile strength with a considerably higher work hardening rate while maintaining a reasonably high uniform elongation of 12.1 %. Furthermore, the impact energy at -60 degrees C was increased by almost 100 %. The ultrafine-grained lamellar microstructure in TCR steel was believed to be responsible for the synergistic deformation and delamination fracture toughening that resulted in the simultaneous improvement in tensile and impact properties.