Influence of retained austenite and Cu precipitates on the mechanical properties of a cold-rolled and intercritically annealed medium Mn steel

Abstract

A low carbon Cu-bearing medium-Mn steel (Fe-0.05C-5.7Mn-1.9Cu-2.0Ni-0.4Si) was cold rolled and then intercritically annealed (IA) between 640 °C and 680 °C for 180 s. Ultrafine multi-phase microstructure (∼300 nm grain size) was obtained in this single short time annealing, composed of recovered martensite, fresh martensite, more than 20% retained austenite and copious (1022 m−3) nanosized (∼6 nm in radius) Cu precipitates. A good combination of strength (YS (yield strength) of 1037 MPa and UTS (ultimate tensile strength) of 1210 MPa) and ductility (19% UEL (uniform elongation) and 23% TEL (total elongation)) was obtained at an annealing temperature of 640 °C, with the former enhanced by Cu precipitates, ∼200 MPa, and the latter, improved by significant fraction of stable retained austenite. The microstructure was studied by SEM (scanning electron microscope), TEM (transmission electron microscopy) and XRD (X-ray diffraction). It is observed that Cu precipitates distribute heterogeneously in the amount of Cu precipitates adjacent to martensite/retained austenite interface obviously higher than that predicted based on the average bulk composition. According to thermodynamic analysis through Thermo-calc, DICTRA and Kampmann–Wagner Numerical (KWN-) simulation, the influence of Mn, Ni and Si on the bcc (body-centered cubic) miscibility gap shows that the precipitation of Cu particles could be facilitated by Si, a ferrite stabilizer, and inhibited by the presence of Mn and Ni, both austenite stabilizers. As a result, Cu precipitation is promoted in the diffusion zone formed during IA adjacent to martensite/retained austenite interface.