The effect of ferrite grain size and martensite volume fraction on the tensile properties of dual phase steel

Abstract

The yield strength and tensile strength of dual phase steel are found to follow a Hall-Petch type dependence on the mean free path in ferrite d ƒ , i.e. σ = σ 0 + kd ƒ − 1 2 . The Hall-Petch slope k increases with increasing volume fraction of martensite, V m , in the dual phase in both cases, and σ 0 is independent of V m for yield strength but increases with V m for tensile strength. The effect of ferrite grain size and the martensite volume fraction on the yield strength of a dual phase steel can be explained by extending the dislocation pile-up model to dual phase steels. Also, it is found that the effect of ferrite grain size on yield strength is much stronger than on tensile strength. This naturally leads to a decrease of the yield to tensile ratio with increasing ferrite grain size. The yield ratio approaches a constant value at large ferrite grain size and this limiting value decreases with V m . It is demonstrated that these two observations are predictable from the Hall-Petch equations for the yield strength and tensile strength of the dual phase steel.