Effect of intercritical annealing temperature on microstructure and properties of medium Mn steel

To explore the feasibility of preparing medium Mn steel with high strength and ductility via intercritical annealing, a 0.3C-6Mn-1.2Si-0.5Cr (mass fraction,%)medium Mn steel was investigated. Scanning electron microscopy, transmission electron microscopy, electron probe X-ray microanalysis, DICTRA diffusion kinetics calculations, and mechanical property tests were combined to systematically study the influence of intercritical annealing temperature on the microstructure and properties of the medium-Mn steel subjected to slow cooling after hot rolling. The results show that the intercritically annealed microstructure consists of martensite, ferrite, and retained austenite. At 600 ℃, retained austenite exhibits a film-like morphology; at 650 ℃, it transforms into a blocky shape; and at 625 ℃, the retained austenite content reaches its maximum, with the mass fraction of carbon peaking at approximately 1.3%. DICTRA calculations indicate that pre-partitioning of Mn in the intercritical region yields a pronounced Mn concentration gradient, which gradually homogenizes with increasing annealing temperature, consistent with EPMA observations. The annealing temperature significantly affects the mechanical properties: tensile strength increases while elongation decreases with rising temperature. At 625 ℃, the yield strength reaches 955 MPa and the product of strength and elongation attains a maximum of 32.54 GPa·%; at 600 ℃, the maximum elongation is 32.31%; and at 650 ℃, the highest tensile strength is 1 281 MPa. The tensile fracture surfaces exhibit ductile features with delamination, comprising both smooth layers and dimple-rich layers; as the annealing temperature increases, the thickness of the smooth layers decreases and the delamination tendency weakens. At 625 ℃, fine and dense dimples appear on the flat smooth fracture surface, with both dimple density and size increasing with temperature.