Retained austenite grain size as a strength-plasticity relationship indicator in Al-alloyed medium-Mn steel processed by intercritical annealing

Although most studies focus on the impact of the fraction of retained austenite (RA) on the relationship between strength and plasticity, this research explores the influence of the mechanical stability of RA, which is primarily affected by its grain size. To investigate this, samples of 5% Mn steel with varying fractions of RA, as well as different sizes and concentrations of carbon (C) and manganese (Mn), were produced by adjusting the soaking time during intercritical annealing (IA) between 1 and 300 min at a temperature of 680 °C. The mechanical properties of the samples were evaluated using static tensile tests. Detailed microstructural analyses were conducted before and after deformation through hardness tests, X-ray diffraction measurements (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) images, TEM-EDS microanalysis, and electron backscatter diffraction (EBSD) phase and orientation maps. Additionally, thermodynamic calculations were used to supplement the experimental findings. The results suggest that both chemical enrichment and grain size reduction have comparable effects on stabilizing the RA. The wide variation of RA grain sizes from 0.02 to 0.15 μm3 promotes a gradual and continuous martensitic transformation under strain throughout the entire strain range, leading to a uniform elongation exceeding 19% in the steel analyzed. However, the presence of an appropriate average size of RA (from ∼0.07 to ∼0.11 μm3) is crucial because grains smaller than ∼0.02 μm3 impede martensitic transformation, while coarse grains characterized by a volume greater than ∼0.13 μm3 result in significant transformation in an early stage of deformation.