The formation of ferrite as a second phase in austenitic matrix by strain-induced dynamic transformation (SIDT), an important phenomenon in rolling process, can be used to obtain a final ultrafine-grained ferrite microstructure by application of a suitable thermomechanical control process (TMCP). The refinement of ferrite grain size has the potential to improve simultaneously strength and toughness of steel. In this context the simulation of mixed microstructures mechanical behavior via representative volume element (RVE) is an important topic of study.
The strategy when working with RVE simulation in multiphase steels is usually based on empirical or semi-empirical chemical dependent model to set up the mechanical properties needed for each single phase involved. These models that come from dislocation strain hardening theory many times require the definition of chemical partition between phases. A discussion of the validity of such models can only be made with a correct definition of said parameters, either experimentally or by computer simulation of thermodynamic equilibrium. Aiming avoid complex experimental characterizations or possible inaccuracies of thermodynamics simulation the present work arises from a different perspective about the problem, it starts from the final macroscopic mechanic behavior of the steel to find the mechanic behavior of involved single phases. The reverse analysis of mechanical behavior performed in 18CrNiMo7-6 steel deformed at 700 °C and 0.001 s-1 was only possible due the similarity between softening effect caused by strain induce ferrite formation and dynamics recrystallization process (DRX). This similarity allows the analysis of work hardening vs stress curves and so an estimation of saturation stress, expected maximum stress in the no-softening case, by extrapolation of linear behavior after inflection point to the value in which work hardening turn to zero (figure 2b).
In this work is created a flow stress curve for the single phase ferrite that in combination with the austenite flow stress curve, that reaches the saturation stress value (figure 2a), produces the known macroscopic response in a 2D RVE simulation. The simulation starts from a mesh (microstructure) 100 % austenite and changes dynamically during deformation according to the kinetic of transformation experimentally fitted to a final mesh composed of 88 % of austenite and 12 % of ferrite.
The formation of ferrite, the verification of non-recrystallization of austenite and the ferrite phase fraction in different strain values were performed by microstructural characterization of compression sample cross section.
A discussion between the found strain-induced ferrite flow stress curve and dislocation-based strain hardening models will be carried out, highlighting the chemical composition of ferrite single phase able to produce same or similar result.