Microstructure-based model for the static mechanical behaviour of multiphase steels

Low-alloy multiphase transformation-induced plasticity (TRIP) steels offer an excellent combination of a large uniform elongation and high strength. This results from the composite behaviour of the different constituent phases that are present in these steels: polygonal ferrite, bainitic ferrite, and martensite/austenite. The different constituents were prepared separately in order to obtain a clear understanding of their individual behaviour within the multiphase steel. The stress–strain relationships of these different types of single- and multiphase steels were simulated by physically based micromechanical models. The model used for the simulations of the stress–strain curves of the separate phases is based on the Mecking–Kocks theory and utilizes physical properties such as the microstructural parameters, the dislocation density, and the chemical composition of the different phases. Strain-induced transformation kinetics, based on a generalized form of the Olson–Cohen law, are utilized in order to include the influence of the transformation of the metastable austenite on the mechanical properties of the TRIP steels. Static stress–strain properties of multiphase steels were modelled by the successive application of a Gladman-type mixture law for two-phase steels. The model yields detailed information of stress and strain partitioning between the different phases during a static tensile test.