Strong and Tough Heterogeneous TWIP Steel Fabricated by Warm Rolling

Abstract

<p>Twinning-induced plasticity (TWIP) steel has received significant research attention because of its superior mechanical properties, including uniform elongation, ultimate tensile strength, and fracture toughness. However, it has a relatively low yield stress, which limits its industrial application. Increasing the dislocation density has been proved to be an effective method for enhancing the yield stress. In this work, a simple warm rolling (WR) route was applied at 700oC to manufacture partially recrystallized TWIP steel with a high yield stress (1250 MPa), good total elongation (24%), and exceptional fracture toughness (KJIC of approximately 125 MPa·m1/2). The steel manufactured using WR was characterized using SEM, EBSD, and TEM at different length scales. Compared to the steel microstructure obtained after hot rolling or cold rolling (CR), this WR TWIP steel exhibits a distinct heterogeneous structure. The matrix has numerous dislocations with twinned coarse grains (approximately 75%) and nearly defect-free recrystallized fine grains (approximately 25%), which form during the reheating period of the WR process. The in situ tensile tests of the WR and CR steels show that the deformed coarse grains provide high yield stress with negligible deformation, whereas the recrystallized fine grains can undergo considerable plastic deformation, which results in a good work hardening capacity during tensile deformation. The fracture toughness tests of the compact tension (C(T)) samples indicate that the recrystallized grains in the WR steel can enhance the crack tip blunting and deflect cracks, which enhance the crack-growth resistance. Alternatively, these toughening mechanisms are not observed in the homogeneous CR steel. Therefore, this heterogeneous structure, which is induced by the high temperature WR process, provides the TWIP steel with excellent strength and toughness.</p>