Development of a high strength and ductile Nb-bearing dual phase steel by cold-rolling and intercritical annealing of the ferrite-martensite microstructures

Dual phase (DP) steels containing different amounts of Nb microalloying element were produced by cold-rolling followed by intercritical annealing of a ferrite-martensite duplex starting structure. The effects of Nb contents (0.00, 0.06, 0.12 and 0.18 wt%) and intercritical annealing time on the microstructural evolutions and mechanical properties were studied. Results from microscopic images showed that increasing Nb content increased the volume fraction of martensite and decreased the average grain size of ferrite. Tensile results illustrated an excellent strength-elongation balance in terms of energy absorption (160 J cm−3) and toughness (229 MPa). The lowest grain size of about 1.40±0.37 µm was achieved in the DP steel containing 0.12 wt% Nb whose strength was about 123% higher than that of the as-received ferritic-pearlitic steel (e.g. 540 MPa), without loss of ductility. The variations of strength, elongation and fracture mechanism of the specimens with Nb contents and intercritical annealing time were correlated to the microstructural features.