Ultrafine-grained CrMnNi steels: Lueders phenomenon and texture inheritance

Abstract The mechanical behaviour as well as the microstructure and texture development were investigated for ultrafine-grained austenitic TRIP steels with two different austenite stabilities. The ultrafine-grained microstructure of low-carbon, high-alloy CrMnNi steels was obtained after thermo-mechanical processing (TMCP) consisting of cold rolling process up to 90% of rolling degree and subsequent reversion annealing treatment at different temperatures and holding times. The mechanical properties were evaluated during tensile tests at room temperature complemented by infrared thermography, digital image correlation, ferromagnetic phase fraction measurements as well as nanoindentation. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were performed before and after tensile tests. In addition, the texture evolution was followed from cold rolling via reversion annealing up to tensile deformation. The results showed that an ultrafine-grained austenitic microstructure with a mean grain size between 0.7 and 1 μm was realized due to the applied TMCP. The ufg microstructure led to a significant increase in yield strength to more than 900 MPa for both steel variants. In addition, a Lueders band phenomenon was observed for steel with the lowest austenite stability, which was accompanied by dislocation-lack yield point and stress-assisted martensitic phase transformation at about 400 MPa. Texture inheritance from fcc cold rolling texture via the bcc cold rolling texture into fcc reversion annealing texture and again bcc texture after tensile deformation was observed. This texture inheritance is an indicator for the diffusionless shear transformation in this type of steels during reversion annealing.