Effect of copper nano-precipitates on the mechanical properties of a medium-Mn steel

Asmaa Elbeltagy, Zigan Xu and Wenwen Song

Steel Institute, RWTH Aachen University, Intzestr.1, 52072 Aachen, Germany

Medium-Mn steel (MMnS) has received much attention in the automotive industry due to its outstanding strength-to-density ratio leading to both high fuel efficiency and reduction of emissions. The present study reports recent results when a novel two-steps intercritical annealing process is applied in a Cu-contained MMnS steel, where Cu induces the co-precipitation of nano-scale copper-rich precipitates (CRP). Ultrafine multi-phase microstructure (∼1.3〖μm〗^2 grain area) composed of ferrite, and around 38% reverted austenite leading to a good combination of yield strength of 1100 MPa, an ultimate tensile strength of 1300 MPa, and ductility 17% (total elongation) was obtained. The microstructure was studied by the multi-scale characterization techniques; high-energy synchrotron X-ray diffraction (SynXRD), electron backscattering diffraction (EBSD), and atom probe tomography (APT). The APT analysis revealed a co-precipitation behavior between the Nano CRP and NiAl precipitates where the CRP is found in the FCC state as identified from the SynXRD diffractograms. By analysing the ex-situ SynXRD diffractogram at different strain levels, it can be concluded that the increased ductility is attributed to a pronounced TRIP effect. Deformation mechanisms and strengthening mechanisms are further discussed.