Vanadium - Transforming Possibilities

Improvement of Strength–Ductility Balance by the Simultaneous Increase in Ferrite and Martensite Strength in Dual-Phase Steels

Abstract:

This work investigates the effect of increasing both martensite phase and ferrite phase strength on tensile properties and fracture behavior of dual-phase (DP) steels. The martensite phase strength is varied by changing its carbon contents and tempering process, whereas the ferrite phase strength is varied by dispersion of nano-sized vanadium carbides (VC) precipitates in the ferrite phase and also by changing its total fraction in DP samples. It is found that strengthening of the martensite phase improved the tensile strength with almost no loss of the tensile strength–uniform elongation balance. This is attributed to the enhancement of strain partitioning between the ferrite and martensite phase so that the work hardening rate increased. On the other hand, strengthening of ferrite phase effectively improved both the tensile strength and the tensile strength–post-uniform elongation balance due to suppression of strain partitioning which likely promoted homogenous deformation after necking. More importantly, a simultaneous increase in the strength of martensite phase and ferrite phase leads to a moderate phase strength difference, which is revealed as a promising strategy to achieve high strength in addition to a good balance of tensile strength, uniform elongation, and post-uniform elongation in low-carbon DP steels. Furthermore, the quantitative analysis of void formation reveals that decohesion of the interface between the ferrite phase and martensite phase is the dominant fracture mechanism, which is less likely to be affected by phase strength difference.