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

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 degrees C and 680 degrees C for 180 s. Ultrafine multi-phase microstructure (similar to 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 (10(22) m(-3)) nanosized (similar to 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 degrees C, with the former enhanced by Cu precipitates, similar to 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.