s The aim of this investigation is the evaluating the effect of niobium addition as a microalloying element on microstructural evolutions and mechanical properties of an ultrafine grained dual phase (UFG-DP) steel. For this purpose, four steels with the same chemical composition and different Nb contents (0.00, 0.06, 0.12 and 0.18 wt%) were produced by the vacuum induction melting (VIM) process. The UFG-DP steels were produced utilizing simple cold-rolling and subsequent intercritical annealing of a ferrite-martensite duplex starting structure. The duplex structure was cold-rolled up to 80% thickness reduction followed by intercritical annealing at 770 °C for 4, 6 and 8 mins. Results from microscopic images showed that increasing Nb content up to 0.12 wt% increased the volume fraction of martensite and decreased the average grain size of ferrite; however, a lower martensite content with no further grain refinement was detected in the steel containing 0.18 wt% Nb at the intercritical annealing time of 6 and 8 mins. Tensile test revealed intense effect of Nb precipitations on improvement of mechanical properties. The results illustrated an excellent strength–elongation balance in terms of energy absorption (160 Jcm -3 ) and toughness (229 MPa) in comparison with the commercially used high strength steels. The UFG-DP steel containing 0.12 wt% Nb showed high strength (about 123% higher than that of the ferritic-pearlitic as-received state, e.g. 540 MPa), without loss of ductility. The Hollomon, Crussard-Jaoul (C-J) and modified C-J analyses were used to demonstrate the strain hardening behavior of DP steels. The results showed that the Hollomon and C-J analyses proposed two-stage deformation for present DP steels, while the modified C-J analysis illustrated three-stage deformation. In all DP steels, the prominent mode of fracture is ductile. Also it is found that the average dimple size is decreased with increasing Nb content. The variations of strength, elongation, strain hardening behavior and fracture mechanism of the specimens with Nb contents and intercritical annealing time were correlated to the microstructural features. Keywords : Ultrafine grained dual phase steel, cold-rolling and subsequent intercritical annealing, microalloying element, niobium
