Realization of Selective Strengthening of Ferrite by Nb/V Microalloying in a Medium Carbon Lightweight δ -TRIP Steel

The effect of niobium (Nb) and vanadium (V) microalloying on the mechanical properties and microstructure evolution of medium carbon lightweight δ-TRIP steel was investigated. The results indicate that the pristine and microalloyed δ-TRIP steels comprise mainly delta ferrite, bainite, alpha ferrite and retained austenite. With the addition of trace Nb/V elements to δ-TRIP steel, the volume fraction of retained austenite increased and high-density nano-scale Nb/V carbides precipitated as well, for which substantial tensile strength (1028 MPa) and elongation (26.7 pct) were observed. Extensive characterization revealed that the strength-ductility tradeoff in microalloyed δ-TRIP steel was overcome because of the presence of transformation-induced plasticity effect and pinning of dislocations by nano-sized Nb/V carbides in a ferrite matrix, which enhanced the capacity of ferrite grains to accommodate higher density dislocations.