A micromechanical flow curve model for dual phase steels

Abstract

In the automotive industries, Dual Phase (DP) steels have become a favoured material for the car body parts due to their excellent combination of high strength and good formability. A microstructure of DP steel generally consists of a matrix of ferrite reinforced by small islands of martensite. Experimental investigations showed that effects of martensite phase fraction, morphology, and phase distribution play an important role for the mechanical and fracture behaviours of the dual phase steel. In the present work, an approach concerning FE based modelling for predicting flow curve of DP steels has been introduced using a Representative Volume Element (RVE). Two dimensional RVE models were prepared on microstructural level using micrographs of the investigated DP steels having different martensite phase fractions. The applied physical flow curve models of the individual single phases are based on dislocation theory and take into account the local chemical compositions. The models also include phase boundary dislocation (PBD) density, which accumulates at the phase boundaries due to the austenite-martensite transformation during quenching process. These dislocations contribute to both an increase in forest dislocations and a building up of back stresses. The calculated stress-strain curves for the DP steels were verified with experimental results determined from tensile tests. Furthermore, the micromechanics based model was used to describe the local stress and strain development of the individual phases in the DP microstructures. By this manner, an optimization of the dual phase high strength steel with respect to its microstructural constituents is possible.