Warm tensile deformation and stamping of medium Mn TRIP steels microalloyed with molybdenum and niobium

Warm working has attracted much interest to overcome the problems of springback for cold working of ultra-high strength steels. In this study, warm tensile deformation behavior of 6Mn, 6MnNb and 6MnNbMo steels was investigated, in conjunction with warm stamping to understand the role of Mn and Mo and evaluate the mechanical properties of U-shaped parts. The results show that medium Mn steels exhibited quite good hot ductility (up to 400%), and both flow stress and ductility increased with addition of Nb and Mo. The work hardening rate increased with addition of Nb+Mo, while it decreased with flow stress. The warm-stamped 6MnNbMo steel exhibited quite good surface quality and excellent mechanical properties: ultimate tensile strength (UTS) of 1434∼1466 MPa, and total elongation to fracture (El) of 22∼26.3%, which is comparable to the conventional hot-stamped boron steels (UTS: ∼1600 MPa & El: ∼6%). The combined addition of Nb and Mo raised UTS with a slight decrease in El, and improved the distribution uniformity of mechanical properties, which is of great importance when applying medium Mn steels to the automotive structural components for ensuring safety of passengers.