Microstructural Design of Multiphase Advanced High Strength Steels

The properties of multiphase (MP) Advanced High Strength Steels (DP, TRIP, TWIP) are controlled by dispersed non-ferritic phases (NFP), which can be quenched martensite, deformation-induced martensite, retained austenite or twinned austenite. The yield strength, work hardening and ductility of these steels are determined by the volume fraction, carbon content, scale and spatial distribution of the NFP. This paper summarizes recently published results on the effects of microstructure on the tensile properties of DP and TRIP steels and discusses these observations in terms of micromechanisms of plastic yielding, work hardening and fracture. For DP steels, all three properties are enhanced by developing a fine, uniform distribution of NFP grains. For TRIP steels, the volume fraction of hard NFP increases continuously with strain and the tensile properties are strongly influenced by the rate of austenite transformation (RA stability). RA stability increases with increasing carbon content, decreasing dimensions of RAislands and with increasing strength of the surrounding phase. An optimum combination of RA transformation rate and high strength/hardening is obtained with a TRIP microstructure comprising intercritical ferrite grains + granular bainite.