High strength-high ductility combination in a low-density medium Mn steel prepared by a highly efficient route

Abstract

Medium Mn steels are the primary candidates in the fields of automobile, construction machinery et al. due to the advantages of high strength-ductility combination, and low costs. By now, the balance of good performance and high preparation efficiency is still a critical issue to be unsolved, which restricts their further applications. This work designs a novel Al-contained medium Mn low-density steel and explores the effect of a short intercritical-annealing process. The microstructure evolution of multi-scales, especially the austenite characteristics and Mn partitioning behaviors is analyzed, and the relationship between austenite and tensile properties is discussed. The results indicate Al can broaden the annealing processing window and improve the annealing temperature. When annealed at 780 °C, the austenite reverted transformation happens quickly and after annealing for 3 min, the austenite volume fraction maintains stable in the range of 52.4–56.7 vol%, but the morphology and Mn concentration are very different. A large amount of blocky austenite with a coarse size of more than 1 μm and a low Mn concentration of 8–9 wt% forms at the longer annealing time of 20 min, while the austenite almost presents a lath-like shape with a high Mn concentration exceeding 10 wt% at annealing time of 4 min. The tensile properties are closely related to the volume fraction and mechanical stability of austenite, and the sample annealed for 4 min exhibited the best performance with tensile strength of 1192 MPa and total elongation of 33 %, due to its slow TRIP effect.