Nano-precipitation behavior and mechanical properties of Ti-containing Fe-12Mn-9Al-3Cr-1.4C medium-Mn lightweight steel

The role of Ti in lightweight steel has been widely concerned due to its influence on the precipitation of nanoparticle and its inherent low density. In this work, the nano-precipitation behavior and mechanical properties of Ti-containing Fe-12Mn-9Al-3Cr-1.4C-xTi (wt.%, x = 0.02/0.04) medium-Mn lightweight steels were systematically investigated. It was found that the increase of 0.02 wt% Ti content had little influence on the two-phase structure of ferrite and austenite. TEM characterization revealed that Ti could suppress the precipitation and growth of & kappa;-carbide in austenite while promoting the precipitation of TiC particle. Based on thermodynamics and first-principles calculations, the suppression of & kappa;-carbide precipitation by the Ti element resulted from the increased formation energy of & kappa;-carbide and the decreased diffusion coefficient of C in austenite. The reduction of & kappa;-carbide decreased the precipitation strengthening in lightweight steels. However, the precipitation of TiC particle not only resulted in additional strength, but also refined the austenite grain and introduced a large number of dislocations, improving the grain boundary strengthening and dislocation strengthening. For the aged lightweight steels at 450-550 degrees C for 3 h, the increase of 0.02 wt% Ti content mainly affected the precipitation strengthening of & kappa;-carbide and dislocation strengthening. Especially, when the ageing temperature exceeded 500 degrees C, the precipitation strengthening of & kappa;-carbides became the primary factor contributing to the reinforcement of lightweight steels.