A novel route for development of ultrahigh strength dual phase steels

Dual phase (DP) steels were produced by a new approach utilizing simple cold-rolling and subsequent intercritical annealing of a ferrite–martensite duplex starting structure. The effects of intercritical annealing temperature on the microstructural evolutions and mechanical properties were studied. It was found that the volume fraction of martensite increased by increasing the intercritical annealing temperature. Tensile testing showed a good strength–elongation balance for DP steels (UTS×UE>100 J cm−3) in comparison with the commercially used high strength steels. The strength of the low carbon steel with the new ultrafine grained DP microstructure (average grain size of about 1–2 μm) was reached over 1500 MPa (about 200% higher than that of the as-received state, e.g. 540 MPa), without loss of ductility. The variations of hardness, strength, elongation, strain hardening behavior and fracture mechanism of the specimens with intercritical annealing temperature were correlated to microstructural features.