Modelling the mechanical properties of multiphase steels Corinna Thomser*, Ulrich Prahl*, Henk Vegter**, Wolfgang Bleck* *Institute of Ferrous Metallurgy, RWTH Aachen University, Germany **Corus RD&T, IJmuiden, Netherlands Due to economic, environmental and safety reasons the use of high strength steels for the automotive industry is increasing rapidly. For an optimal use of strength and formability of multiphase steels an accurate material model is required for forming simulations. At the moment the microstructure of multiphase steels, which is the most important factor influencing the strain hardening behaviour of multiphase steels, is not taken into account in FE simulations. In international projects like the ULSAB project, especially dual phase steels play an important role for the automotive industry. Their strain hardening behaviour is strongly influenced by the microstructure as is well known from several experimental investigations. Within this work an approach is presented which describes the microstructure evolution during intercritical annealing by thermodynamic calculations and predicts the strain hardening behaviour of dual phase steels by means of FE simulation of representative volume elements based on microstructural characterisations. After cold rolling, dual phase steels are intercritically annealed. The fractions and the carbon contents of austenite and ferrite depend on the annealing temperature and on the holding time. After a fast quenching, the austenite transforms to martensite, which yields a material of a soft ferrite matrix with strong martensite islands. For the determination of the phase fractions and the carbon partitioning between the two phases a DICTRA calculation was carried out, which considers thermodynamic and kinetic effects during intercritical annealing, thus taking into account that full equilibrium is not always reached. All other elements except of carbon are assumed to be uniformly distributed in both phases. The carbon content was used to calculate the strain hardening behaviour of ferrite and martensite based on dislocation theory models /1/-/2/ for different annealing temperatures. As well known from literature, the strain-hardening behaviour of martensite is mainly dependent on the carbon content, while for the prediction of the strain hardening behaviour of ferrite to the local chemical composition the grain size is needed additionally. A three-dimensional representative volume element is used to describe the interaction of ferrite and martensite in a dual phase steel during deformation in a FE simulation, done in Abaqus. The resulting strain hardening behaviour in simulation is in good agreement to the experimental determined strain hardening behaviour in tensile tests within the error range of the metallographical microstructural characterisation. Additionally to the strain hardening behaviour, the tensile strength and the uniform elongation can be determined in experiments and simulation by using the Considere criteria. References /1/ Rodriguez, R.; Gutierrez, I.: Proceeding of TMP ’04, B-Liege, 2004, p. 356-363 /2/ Rodriguez, R.; Gutierrez, I.: Materials Science Forum, Vols. 426-432, 2003, p. 4525-4530