Significance of Retained Austenite Stability on Yield Point Elongation Phenomenon in a Hot‐Rolled and Intercritically Annealed Medium‐Mn Steel

Abstract

The effect of retained austenite stability on yield point elongation (YPE) phenomenon in a hot‐rolled and intercritically annealed medium‐Mn steel is elucidated here. Furthermore, the influencing factors of austenite reversion are studied. The results indicate that the shear phase transition is suppressed by the strain hardening of prior austenite grain introduced by low‐temperature rolling, resulting in refined martensite laths with high dislocation density. The nucleation and growth of intercritical austenite are accelerated by high‐density dislocation substructures. Different characteristics of retained austenite (RA) are obtained through tailoring the rolling temperature and austenite reversion transformation (ART) process. YPE phenomenon is a result of the equilibrium between work hardening (via martensitic transformation and dislocation multiplication) and softening (by stress relaxation and dislocation glide). In the initial deformation stage, large amounts of geometrically necessary dislocations (GND) are “stored” in the RA with higher stability, and the local stress can be effectively relaxed and transferred, leading to the occurrence of YPE. The optimum properties are obtained in R760‐A630‐0.3h samples with a combination of yield strength of 918 MPa, tensile strength of 1019 MPa, and total elongation of 25.3%, which is mainly attributed to the finer ferrite matrix and sustained transformation‐induced plasticity effect.