Precipitation behavior and austenite stability of Nb or Nb–Mo micro-alloyed warm-rolled medium-Mn steels

Precipitation behavior and austenite stability of the warm-rolled medium-Mn steels with different Nb/Mo additions annealed at different temperatures were investigated. The Nb or Nb–Mo additions decrease the volume fraction of retained austenite, but increase its mechanical stability. The Mo addition to the Nb-bearing steels can lead to a higher density and smaller mean size of the precipitates because of the decreased interfacial energy and enhanced precipitation kinetics. The optimal precipitation temperature of the designed steels is about 600 °C. The Nb–Mo bearing steels exhibit a mixture of equiaxed and lamellar grain morphologies, and exceptional mechanical properties, i.e. the average products of ultimate tensile strength and total elongation (PSE) values of approx. 62 GPa %. An increase in yield strength of 300 MPa can result from grain refinement and precipitation hardening by Nb–Mo addition.