The Effect of Intercritical Annealing Time on the Microstructures and Mechanical Properties of an Ultrafine Grained Dual Phase Steel Containing Niobium

An ultrafine grained dual phase (UFG DP) steel containing niobium was produced by a new route utilizing simple cold-rolling and subsequent intercritical annealing of ferrite/martensite duplex starting structures. The effects of intercritical holding time on the microstructural evolutions and mechanical properties were studied. The results showed that increasing intercritical holding time enhnaced the volume fraction of martensite and decreased the grain size of ferrite. Tensile testing also revealed good strength–elongation balance for UFG DP steels (UTS×UE > 100 Jcm-3), in comparison with the commercially used high strength steels. The variations of strength, elongation and fracture behavior with intercritical holding time were correlated with the microstructural features.