The effect of intercritical heat treatment temperature on the tensile properties and work hardening behavior of ferrite–martensite dual phase steel sheets

This paper aims to investigate tensile properties and work hardening behavior of dual phase (DP) steels. A series of DP steels containing ferrite and martensite with different volume fractions of martensite (Vm) were produced by intercritical heat treatment. Microstructural investigations, hardness test and tensile test were carried out. Hardness, yield strength, ultimate tensile strength, ductility and fracture energy were correlated to martensite volume fraction. The experimental results showed that dual phase steels with an equal amount of ferrite and martensite have excellent mechanical properties in terms of tensile strength, ductility and fracture energy. A further increase in Vm was found to decrease tensile strengths and ductility. The increasing and then decreasing trend in tensile strength is in contrast to the law of mixture. These unusual behaviors are discussed and explained. Work hardening behavior was analyzed in terms of Holloman analysis. Results showed that in DP steels with less than 50% Vm, the work hardening took place in one stage and the work hardening exponent increased with increasing Vm. By increasing the volume fraction of martensite (Vm > 50%) two work hardening stages were observed in the Hollomon analysis.