Effect of Microstructure on Stress Dependence of Transformation Induced Plasticity in TRIP800 Low-Alloy Multiphase Steels

An experimental–numerical methodology was used in order to study the microstructural effects on stress state dependency of martensitic transformation kinetics in two different TRIP800 low alloy multiphase steels. Representative volume elements extracted from actual microstructure were utilized to stimulate the mechanical behavior of above mentioned steels. The mechanical behavior for each constituent phases required in the model was taken out from those reported in the literature. A stress invariant based transformation kinetics law was used to predict the martensitic phase transformation during deformation. Crystallographic and thermodynamic theories of martensitic phase transformation were utilized to estimate the constant parameters of the kinetics law, in a recently performed investigation. However, the sensitivity of the transformation to the stress state remained as an adjustable parameter. The results of the current work show that the stress state sensitivity of martensitic phase transformation in the investigated steels is microstructure-dependent and the value of this parameter is almost equal to half of the bainite volume fraction. Therefore, the volume fraction of bainite in the low-alloy multiphase TRIP800 steels can be used as a first postulation to determine the value of the martensitic phase transformation sensitivity to the stress state. The microstructure based model previously developed for calculating the mechanical behavior of the TRIP800 steels can be utilized as a virtual design tool for the development of TRIP steels having specific mechanical properties.