Microstructural evolution and precipitation behavior of the warm-rolled medium Mn steels containing Nb or Nb-Mo during intercritical annealing

The microstructural evolution and precipitation behavior of the warm-rolled (WRed) medium Mn steels with no microalloying addition, single Nb addition and multiple Nb-Mo addition treated at 650 °C for various durations were investigated. Different from the lamellar microstructure of hot rolled medium Mn steels and the equiaxed microstructure of cold rolled medium Mn steels after intercritical annealing, the WRed samples of medium Mn steels exhibited the equiaxed and lamellar mixed microstructure, and the equiaxed microstructure ratio became larger with increasing durations. The addition of Nb or Nb-Mo can respectively increase yield strength of ~ 50 MPa or ~ 180 MPa in average and ultimate tensile strength of ~ 70 MPa or ~ 100 MPa in average without sacrificing the tensile ductility. The stability of austenite can be improved for the smaller average austenite grain size due to the addition of Nb or Nb-Mo. The addition of Mo to the Nb-bearing medium Mn steel can increase the density of nano-precipitates by reducing the mismatch degree between the matrix and precipitates and inhibit the coarsening rate of precipitates.