Effect of Intercritical Annealing on Mechanical Properties and Work-Hardening Response of High Formability Dual Phase Steel

Abstract

The effect of austenitization and intercritical annealing temperature on mechanical properties and work-hardening response of high-formability dual phase (DP) steel with low C and Mn content is studied. Different mechanisms of microstructural development are characterized. At intercritical temperature range, austenite forms in the ferritic matrix, which will transform to martensite during quenching. However, at austenitization temperatures, the material becomes completely austenitic with low hardenability. As a result, during quenching, there will be some ferrite phase in the martensitic matrix of the resultant DP steel. These differences are found to have important effects on the mechanical properties. The yield strength decreases at low martensite fraction due to the disappearance of the yield-point phenomenon and the presence of martensite-induced dislocations that move at low stresses. However, the yield strength rapidly increases by the dominance of the martensitic matrix at high martensite fractions. Moreover, the dependence of the ultimate tensile strength on martensite volume fraction is discussed based on the work-hardening capacity of the DP steels. Finally, the mechanical properties are summarized by consideration of strength-ductility trade-off seen for DP steels.