Effects of tempering on the mechanical properties of high strength dual-phase steels

Dual-phase (DP) steels are generally characterized by the combination of high strength with good ductility, but some formability characteristics of these steels are low compared to other steel classes. In particular, stretched edge ductility and bendability are poor by comparison with other less ductile strip steel classes such as complex-phase (CP) steels. This paper describes the effect of short time tempering of the order of few seconds to few minutes on the mechanical properties of two cold rolled and hot-dip galvanised dual-phase (DP) steels. i-axial tensile characteristics, hole-expansivity, Erichsen cupping height and bendability were assessed after tempering at temperatures between 200 and 450 °C for relatively short time scales up to 10 min. The short tempering treatments significantly changed the character of the DP steel leading to a decrease in tensile elongation and an increase in hole-expansitivity and bendability. The properties observed in tempered DP steels where found to be closely comparable to those expected for CP steels. Microstructural analyses using the SEM showed carbide precipitation within the martensite and ferrite that increased with tempering.