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Multiphase steels are new Advanced High Strength Steels (AHSS), which have been developed for the automobile industry for the purpose of reducing of car body weight. These steels offer an excellent combination of high strength and large elongation due to the coexistence of harder and softer phases in their microstructure. The advantageous properties of multiphase steels can be utilised by regulating the type, amount, formation and spatial distribution of the different constituent phases. To describe the influences of the heterogeneous microstructure on the mechanical properties and the complex fracture mechanisms, an approach is presented using Representative Volume Elements (RVEs) on a micro level. Three dimensional RVE simulations were conducted under considerations of metallographic analysis for a Dual Phase (DP) steel and a Transformation-Induced Plasticity (TRIP) steel. The Gurson-Tvergaard-Needleman (GTN) damage model was applied to investigate the local crack initiation in steel sheets during various forming processes. In conclusion, a failure prediction based on microstructure is proposed for the material characterisation in sheet metal forming of multiphase steels.