Investigation of fracture behaviour in TRIP steels

In automotive body designing high strength steels are used to achieve weight reduction together with an improvement of crashworthiness. The most important representatives of the AHSS steels are Dual Phase (DP) steels and TRansformation Induced Plasticity (TRIP) steels. The application of high strength steels requires a detailed knowledge about the formability and the forming limits. While the true stress strain curve describes a material’s forming behaviour, the Forming Limit Diagram (FLD) is the common criterion used for a Finite Element based process-layout to analyse failure probability and to prevent the occurrence of factures. But forming of multiphase high-strength steels may lead to the occurrence of cracks in regions of a component, which are save in accordance to the results of Finite Element Analysis (FEA). Mostly areas of the part are involved, in which a combination of tension compression- and biaxial tension-stresses can be observed. These results lead to the assumption that depending on the forming history there might be an unknown forming limit. To generate valid forming limits the identification of critical forming operations becomes necessary. Therefore deep drawing operations of cups with geometrically modified specimens have been carried out. The aim of this modification is to influence the stress and strain condition during forming as well as the corresponding microstructural evolution. Experimental work in combination with numerical investigations shows interdependencies between the forming behaviour of the material, stress condition, strain path and maximum drawing ratio. X-ray, light optical and electron microscopes, thermography, computer tomography and metallography are used to describe the fracture mechanism.