ABSTRACT

This experimental-modelling work aimed to gain a better understanding of and to quantify the effect of complex microstructure parameters on the mechanical properties of Dual Phase steels (DP). Mixed Ferrite-Martensite (F-M) structures have been produced from a hot rolled Ferrite- Pearlite steel by combination of reheating-quenching cycles for grain refinement followed by intercritical annealing in the range 720 °C to 815 °C. The final microstructures have been characterised by optical microscopy, revealing M contents ranging from 2 to 16%, mean Ferrite grain sizes (FGS) from 9 to 22 microns and Martensite equivalent mean diameters from 0.6 to 2.9 microns. Tensile testing on cylindrical specimens at quasistatic conditions and strain rates up to five orders higher have been performed for both quantification of stress sensitivity to strain rate and validation of the simulations. The simplest empirical models for engineering tensile parameters obtained from the experiments show that strength Rm, strain to fracture and toughness have sensitivity to almost one order of magnitude higher than to FGS. The Rm sensitivity to percentage M-content is about half of its value to in microns. The higher strength to yield stress ratios correspond to the smallest structural sizes and to lowest M-contents. Two constitutive models for hardening characteristics of multiphase steels have also been developed. The SCS-m is a Self-Consistent Strain formulation model allowing strain partition between phases and valid up to the uniform deformation limit. The MS-m is a Multiphase Strain model incorporating damage and applicable up to near fracture. Both models have been interfaced to a FEM platform, in view of their potential extension to cold forming and dynamic loading end-user applications. The simulation results have been extensively validated in a wide span of strain rate and M-contents, and up to very large strains past the onset of tensile necking. A measure of tensile toughness obtained from the simulations has been correlated to micro structural parameters.