Formability of Nb bearing ultra high-strength TRIP-aided sheet steels

Microstructure, tensile property and formability of formable ultra high-strength 0.2% C–1.5% Si–1.5% Mn–0.02–0.11% Nb TRIP-aided sheet steels with bainitic ferrite matrix (TBF steels) were investigated for automotive applications such as impact member reinforcements, seat frames etc. Nb addition of 0.08% achieved the best combination of total elongation and stretch-flangeability, accompanied with an increase in peak austempering temperature for total elongation. The excellent total elongation and stretch-flangeability were primarily associated with microstructure of the matrix, retained austenite characteristics, morphology of matrix and retained austenite, etc. It was expected that hot-dip galvanizing just after continuous annealing can be realized in the 0.08% Nb TBF steels.