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Multiphase steels have been developed for the automotive industry for the purpose of reducing of car body weight. These steels show excellent strength and ductility due to the coexistence of harder and softer phases in their microstructure. To describe the influence of the multiphase microstructures on mechanical properties and complex fracture mechanisms, an approach is presented using representative volume elements (RVE). Real microstructures were considered and cohesive zone model was applied for the debonding of martensite-ferrite interfaces. Additionally, RVE simulations in combination with continuum damage mechanics were used to investigate local crack initiation in different sheet forming processes. At the failure moment, local strain distributions between different phases were studied, and correlated with macroscopic deformability results. The influences of material properties of the individual phases and the local states of stress on the failure behavior were examined. A precise formability prediction based on microstructure for multiphase steels is the aim of this work.