Correlation between intercritical annealing duration and the tensile deformation behavior of Fe-7.5Mn-3.25Al-1Si-0.2C steels: Role of austenite stability, δ-ferrite, and texture

A novel thermomechanical processing, incorporating intercritical-rolling and subsequent intercritical-annealing at 700 degrees C for 1-72 h, has been employed to delta-ferrite containing medium-Mn steel. The study examines the influence of annealing duration on microstructural evolution, deformation behavior, and mechanical properties. The annealed samples typically possess the ultrafine alpha-ferrite and gamma-austenite along with coarser delta-ferrite. However, as the annealing time increases, austenite phase morphology varies from predominantly lath-like to nearly equiaxed. These morphological variations and electron backscatter diffraction (EBSD)-based local strain estimation imply that longer annealing duration increases recrystallization. Furthermore, tensile strength decreases significantly from 1155.2 MPa at 1 h to 748.1 MPa at 72 h of annealing. In contrast, ductility initially increases from 27.2 % at 1 h to 48.2 % at 21 h of annealing time, before declining to 39.6 % after 72 h. An optimal combination of tensile strength and ductility, i.e., similar to 42 GPa%, has been obtained after 7 h and 21 h annealing durations. Transformation-induced plasticity (TRIP) effect and role of delta-ferrite deformation during tensile testing has been assessed using X-ray diffraction and EBSD techniques. Elemental partitioning significantly lowers the TRIP effect, while the contribution of delta-ferrite deformation increases with prolonged annealing durations. Additionally, the gamma-fiber texture components strengthen with increased annealing duration in the BCC (alpha/delta-ferrite) phase, indicating a higher contribution to ductility. On the other hand, intense gamma-fiber and cube textures in the FCC phase (gamma-austenite) after 7 and 21 h of annealing suggest increase in dislocation activity through slip, enhancing the TRIP effect and improving strength-ductility balance.