Study on local strain distribution and grain boundary characteristics of tensile-deformed TRIP-assisted medium manganese steel

Abstract

The current study emphasizes the local strain distribution and grain boundary characteristics of tensile-deformed Fe-6.72Mn-3.92Al-0.18C medium manganese steel (MMnS), which has undergone thermomechanical processing followed by intercritical annealing at various temperatures (700, 750 and 800 °C for 1 h). Microstructural examination through electron backscattered diffraction (EBSD) analysis is executed to evaluate the local strain distribution, and grain boundary misorientation before and after the tensile test. In the annealed samples, the Kernel average misorientation (KAM) of the face-centered cubic (FCC) phase is close to 0°, indicating strain-free austenite grain formation. Before the tensile test, the body-centered cubic (BCC) phase is relatively free from local strain; however, as the misorientation increases due to tensile deformation, the BCC/FCC interphase boundaries become highly strained. The sum of the fraction of low-angle grain boundaries (ΣLAGBs) decreases with an increase in annealing temperature, while the sum of fractions of misorientation greater than 15°, excluding coincident site lattice (CSL) boundaries, increases moderately. With an increase in annealing temperature, the sum of the CSL special boundary fractions (Σ3 and Σ9) increases considerably. The weighted average density of LAGBs in the BCC phase is higher in the sample annealed at 700 °C compared to the other annealed samples. The increase in the fraction of LAGBs in the FCC phase during tensile deformation is due to the significant transformation-induced plasticity (TRIP) effect and high austenite transformation ratio. Crack formation, propagation, and branching and blunting in the tensile-tested 750 °C-annealed sample are examined by the EBSD analysis. Crack propagation is hindered by the existence of γ-fiber texture components and high-angle grain boundaries in the sample annealed at 750 °C.