Stability of retained austenite in intercritically annealed steel with a novel alloying approach

This study presents a novel steel concept (MnNi) in which Ni partially replaces Mn to produce finer nanoscale retained austenite (RA) and body-centred cubic (BCC) grains. Furthermore, Si and Al alloying were optimised to widen the intercritical window for better Intercritical Annealing (IA) process control. Laboratory hot-rolled and directly quenched plates were subjected to IA treatment and Gleeble-simulated thermal cycles. The phase transformation behaviour was assessed using dilatometry, while the RA fraction, carbon enrichment, dislocation density, and stacking fault energy (SFE) were quantified using X-ray diffraction (XRD), including macrotexture analysis. The morphology and distribution of RA were examined using electron backscatter diffraction (EBSD), transmission Kikuchi diffraction (TKD), and scanning transmission electron microscopy (STEM). MnNi steel predominantly contained nanoscale film-like RA surrounding fresh martensite, along with nanoscale round RA grains. According to the STEM/EDS analysis, the former microstructure was due to the lower Mn content. Although the SFE values, which were based on the XRD measurements of Gleeble specimens (13 mJm -2 and 14.4 mJm -2 , respectively), were similar for MnNi and reference MedMn steels, it was assumed that the TRIP behaviour could vary owing to differences in carbon content, as well as the size and distribution of the RA. It should be noted that XRD revealed a clear through-thickness gradient in the RA phase fraction in both steels. Therefore, local SFE variations probably occurred in the furnace-annealed steel sheets. A texture gradient was detected only in the RA of MnNi steel. In addition, a {115}<110> texture component, which is rare in FCC metals, was observed in the subsurface (s = 0.8) layer of MnNi steel. The results demonstrate that the novel MnNi steel concept can suppress blocky RA and exclusively generate nanoscale RA while maintaining a low SFE. • Novel MnNi steel yielded mostly film-like RA within fine bcc/bct matrix. • Through-thickness texture/RA v-% gradient were detected in MnNi steel. • Need for T-T measurements to reveal texture and RA fraction gradients is shown.