A novel and simple technique for development of dual phase steels with excellent ductility

A new and simple approach was introduced to produce dual phase (DP) steels with excellent ductility. This technique included intercritical annealing of a cold-rolled ferrite-pearlite structure followed by water quenching to produce a ferrite-martensite duplex structure, and a subsequent short intercritical annealing at a lower temperature followed by water quenching to produce the final DP steel. It was found that the microstructure of the newly developed DP steel consisted of chain-like networked martensite in the ferrite matrix with both ultrafine and coarse grain structure. Despite the same value of ultimate tensile strength (∼736 MPa), the uniform elongation and total elongation of the newly developed DP steel were 52% and 54% higher than those for a DP steel with the same composition and martensite volume fraction produced by conventional intercritical annealing. This new DP steel also exhibited a superior strength-elongation balance (UTS×TE >30,000 MPa%) in comparison with commercial and thermomechanically processed DP steels. The newly developed DP steel showed two stages of work hardening based on the Kocks-Mecking approach corresponding to an initial rapid decrease of work hardening rate (θ= dσ/dε) with stress followed by stage III hardening. This steel also exhibited larger values of work hardening exponent in the Hollomon and Ludwik equations compared with the intercritically annealed DP steel.