Procedures for microstructurally conditioning an Fe-22Mn-0.6C-1.5Al TWIP steel for optimal mechanical behaviour

Abstract

In this work, we investigate the development of microstructures during thermomechanical processing of an Fe-22Mn-0.6C-1.5Al TWIP steel designed and manufactured in the laboratory, using optical microscopy, transmission electron microscopy and X-ray diffraction. The effect of rolling at room temperature, 600 °C and 1000 °C followed by annealing at intermediate and high temperatures was related to the mechanical properties measured with tensile tests. We found that the selected chemical composition controlled the value of the stacking fault energy, so that the main deformation mechanism was twinning together with dislocation glide, for all sheet-processing conditions. The addition of aluminium eliminated serrations in the stress-strain curves, guaranteeing stable plastic flow of the material during deformation processes. The accumulation of stacking faults during rolling facilitated the formation of an abundance of twins in subsequent anneals. Twin boundaries appear distorted due to dislocation interactions, and these distortions provided nucleation sites for nanometric mechanical twins. This microstructure reduced the mean free path of dislocations, increasing the hardening rate during deformation, which resulted in excellent mechanical properties. Among the thermo-mechanical processes studied, we found that the sheets rolled at 20 °C and annealed at 750 °C gave a total deformation close to 50% and a maximum strength >1100 MPa, with a yield strength of 350 MPa. This process also produced a small grain size of 7 μm, which is capable of developing a high number of twins during heat treatments and load application. Finally, such a process is a simple and economic procedure, easily adapted to industrial practice.