Ultra high-strength C-Si-Mn-Nb-Mo TRIP-aided sheet steels

Formable high- or ultra high-strength 0.2%C-1.5%Si-1.5%Mn-Nb-Mo TRIP-aided sheet steels with bainitic ferrite matrix (TBF steels) were developed for automotive applications such as impact member reinforcements, sheet flames and so on. In this paper the effects of alloying elements such as niobium and molybdenum on some characteristics of the TBF steels, such as microstructure, tensile properties and formability are investigated. Furthermore, the hydrogen embrittlement performance of ultra high-strength TBF steels has been examined. In a tensile strength range less than 880 MPa, complex additions of 0.05% niobium and 0.20% molybdenum enhanced total elongation and raised optimum austempering temperature to 450-500°C which corresponds to hot-dip galvanizing temperature. Fine NbMoC precipitates increased the tensile strength and so there was hardly any improvement in stretch-flangeability. In a tensile strength range more than 980MPa, complex additions of 0.02% niobium and 0.10% molybdenum enhanced stretch-flangeability and hydrogen embrittlement performance of the TBF steels. The former was primarily caused by uniform fine bainitic ferrite lath structure and retained austenite films, while the latter was associated with retained austenite films trapping most of the solute hydrogen, as well as the fine structure mentioned above.