Influence of nano-scale concentration gradient of alloying elements on the ductility in an intercritically annealed and tempered medium Mn steel

Highlights: • The ductility of a Cu bearing medium Mn steel was increased by ~20% by tempering after intercritical annealing. • A nanoscale enrichment layer of Mn and Ni was formed from the interface to the inside of retained austenite lath. • The enrichment layers could suppress TRIP effect at early stage of straining due to the increase in austenite stability. The microstructure evolution and mechanical properties of an intercritically annealed and tempered medium Mn steel (Fe-0.16C-5.62Mn-2.01Cu-1.95Ni-0.65Al-0.49Si) were investigated. The intercritical annealing at 660 °C led to the formation of microduplex structure containing ferrite (or tempered martensite) and film-like reversed austenite enriched in Mn and Ni. EBSD, XRD, STEM-EDS analyses and DICTRA simulation revealed that after tempering at 550 °C, a nano-scale concentration gradient layer of Mn and Ni was formed from the interface to the inside of retained austenite lath. Compared to the steel without tempering, TRIP effect in the initial stage of deformation was delayed probably due to the existence of Mn and Ni concentration gradient, which significantly increased the ductility: the product of strength and elongation increased from 42.7 to 52.2 GPa⋅%.