Effect of Nb on Microstructures and Mechanical Properties of an Ultrafine-Grained Dual Phase Steel

Abstract

The effect of Nb addition from 0.06 to 0.18 wt.% on microstructural evolutions, mechanical properties, strain-hardening behavior, and fracture mechanism of an ultrafine-grained dual phase (UFG-DP) steel was investigated. The DP steels were cold rolled up to 80% thickness reduction followed by intercritical annealing at 770 °C for 6 min to form the UFG structures. Results 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, lower martensite content with no further grain refinement was detected in the steel containing 0.18 wt.% Nb. The variations of strength, elongation, strain-hardening exponent, and fracture behavior of the Nb-bearing UFG-DP specimens were explained in terms of the microstructural features. The UFG-DP steel containing 0.12 wt.% Nb showed a superior strength-elongation balance (UTS × UE ≈ 116 MPa) compared to both the as-received ferritic-pearlitic and the commercially used DP980 steels.