ALBERT

Description: In this work, the engineering stress–strain tensile curve and the force-deflection bending curve of two Dual-Phase (DP) steels are modeled, combining the mechanical data of fully ferritic and fully martensitic steels. The data is coupled by a modified law of mixture, which includes a partition parameter q that takes into account the strength and strain distributions in both martensite and ferrite phases. The resulting constitutive model is solved in the context of the finite element method assuming a modified mixture rule in which a new parameter q′ is defined in order to extend the capabilities of the model to deal with triaxial stresses and strains and thus achieve a good agreement between experimental results and numerical predictions. The model results show that the martensite only deforms elastically, while the ferrite deforms both elastically and plastically. Furthermore, the partition factor q′ is found to strongly depend on the ferritic strain level. Finally, it is possible to conclude that the maximum strength of the studied DP steels is moderately influenced by the maximum strength of martensite.

Description: This article is focused on the mechanical behavior and its relationship with the microstructural changes observed in two high-manganese steels presenting twinning-induced plasticity (TWIP) and transformation-induced plasticity (TRIP), namely Steel B and Steel C, respectively. Chemical compositions were similar in manganese, but carbon content of Steel B approximately doubles Steel C, which directly impacted on the stacking fault energy (SFE), microstructure and mechanical response of each alloy. Characterization of as-cast condition by optical microscope revealed a fully austenitic microstructure in Steel B and a mixed microstructure in Steel C consisting of austenite grains and thermal-induced (εt) martensite platelets. Same phases were observed after the thermo-mechanical treatment and tensile tests, corroborated by means of X-Ray Diffraction (XRD), which confirms no phase transformation in Steel B and TRIP effect in Steel C, due to the strain-induced γFCC→εHCP transformation that results in an increase in the ε-martensite volume fraction. Higher values of ultimate tensile strength, yield stress, ductility and impact toughness were obtained for Steel B. Significant microstructural changes were revealed in tensile specimens as a consequence of the operating hardening mechanisms. Scanning Electron Microscopy (SEM) observations on the tensile and impact test specimens showed differences in fracture micro-mechanisms.