Yield Strength Enhancement Without Ductility Loss Through Controlling the Intercritical Annealing Time in Medium-Mn Steels

Herein, the effect of shortening the intercritical annealing (IA) time in a two-step process “intercritical annealing and tempering (IAT)” on the microstructure and the mechanical properties of medium-manganese steel (MMnS) made of Fe–0.05C–7Mn–1.5Cu–1.5Ni–1.5Al–1.5Si–0.5Mo (wt%) and containing copper-rich (CRP) and Ni(Al/Mn) precipitates is investigated. The atom probe tomography (APT), electron backscattering diffraction (EBSD), and the synchrotron X-ray diffraction (SYXRD) are used to study precipitation, phase microstructure evolution, the austenite stability, and deformation mechanisms. Shortening the IA step, which is carried out at 700 °C, from 2 min (IAT-2) to 1 min (IAT-1), results in a yield strength (YS) increment of around 218 MPa with less than 1% loss of ductility. While the enhanced yield strength in IAT-1 is attributed to the four times higher precipitates’ number density (n), the insignificant loss of ductility is attributed to the enhanced austenite stability factor from 4.5 to 9.2 in IAT-2 and IAT-1, respectively. The simultaneous increase in YS without ductility loss reflects that controlling the IA time is a promising strategy to overcome the yield strength and ductility trade-off without the need for higher additions of costly alloying elements such as Ni, Al, Mn, and Cu.