Structure-property optimization of ultrafine-grained dual-phase steels using a microstructure-based strain hardening model

Grain size reduction in single phase alloys is generally accompanied by a loss of ductility related to a decrease in the strain hardening capacity. The flow behaviour of fine-grained dual-phase steels produced by swaging was investigated in order to address the couplings between grain size reduction and incorporation of a second phase towards optimizing microstructures with different objectives. A physically based grain size dependent strain hardening model has been developed for the ferrite matrix, involving specific laws for the accumulation and saturation of dislocations along grain boundaries and for their net back stress contribution. The back stress increases with the dislocation density, reaches a maximum and finally decreases due to screening effects. The overall behaviour of the ferrite-martensite composite is then evaluated using a mean field homogenization method. After identification of the material parameters, a parametric study provides, for a given carbon content and grain size of the ferrite matrix, the optimum martensite volume fraction, leading either to the maximum strength ductility product or to the maximum strength under the constraint of a minimum ductility.