Microstructure Modelling of the HEC Behaviour of a Novel Vanadium DP980 Cold Rolled Alloy

Cold rolled high-strength dual-phase (DP) ferrite/martensite steels are the most commonly used advanced high-strength steels (AHSS) in automotive body in white applications, particularly for components where good formability is required. However, these alloys have limitations for parts where high stretch flange formability is required. The latter is usually characterised by the hole expansion coefficientHole Expansion Coefficient (HEC). We have recently developed a vanadium microalloyed DP980 steel with tensile properties that are stable over a wide range of intercritical annealing (IA) temperatures. Interestingly, the HEC behaviour shows a strong IA dependence. We propose that this is related to a complex combination of microstructural parameters including ferrite grain size, martensite fraction, vanadium precipitate distribution, and, in this particular case, to the fraction of epitaxial ferrite formed during fast cooling. For as-quenched DP steels, it is well established that fracture initiates by void nucleation at the ferrite/martensite interface due to strain incompatibilities. The microstructural parameters can be optimised to reduce the local triaxiality at these interphase boundaries and hence reduce the void nucleation and growth rate during straining. For an improved understanding of this effect, finite element (FE) simulations at both the continuum and local scale were carried out. Continuum FE simulations of the HEC test were performed to capture the global state of stress and strain at fracture. A model was used in which the effective strain at fracture was dependent on stress triaxiality and Lode angle. The stress and strain state predicted at failure in the continuum model was then used as the boundary condition for local FE models of two different DP microstructuresMicrostructure. The results of the local FE models are linked to the measured HECHole Expansion Coefficient response.