Reversion to Ultrafine-Grained Austenite in a Medium-Mn AHSS

Abstract

Third generation advanced high-strength steels (AHSS) consist of alloying contents below 17 wt% and multi-phase microstructures of austenite, ferrite and martensite to respectively balance cost and tensile properties [1]. The types of third generation steels capable of achieving these desired properties are medium-Mn steels, lightweight steels and quenched-and-partitioned TRIP steels [2]. Medium-Mn steels generally exhibit a fully deformed martensitic (a′) microstructure after hot and cold rolling. Intercritical annealing between Ac1 and Ac3 temperatures (the a+g phase field) and above the a′ recrystallization temperature produces austenite (g) and ferrite (a), which are typically ultrafine-grained (UFG) in size. The intercritical annealing temperature determines composition and thus the austenite stacking fault energy (SFE). The SFE is important for controlling transformation- and twinning-induced plasticity (TRIP/TWIP) effects that further enhance the mechanical properties of steels by increasing the strainhardening rate, even in ultrafine-grained austenite [3]. Recent work on a Fe-7Mn-0.1C-0.5Si (wt%) steel by Kwiatkowski da Silva et al. showed that austenite reversion sequentially depends on the cosegregation of C and Mn to dislocations and grain boundaries, the formation of face-centered-cubic (FCC) M23C6 transition carbides, and growth of the carbides controlled by C diffusion and local equilibrium at the interface, culminating in the nucleation of UFG FCC austenite [4]. The present study uses scanning transmission electron microscopy energy-dispersive X-ray spectroscopy (STEM-EDS) performed using a 200 kV FEI Tecnai Osiris equipped with a quad Super-X detector and atom probe tomography (APT) using a CAMECA LEAP 5000 XS in laser-pulse mode (70 K and 15% evaporation rate) to measure UFG phase compositions in a medium-Mn steel.