The fine grained microstructure of thermomechanically rolled steels accounts for high yield and tensile strength as well as good toughness in terms of Charpy impact energy. Depending on production parameters and the conditions of loading, these steels might exhibit cracks that are aligned parallel to the plane of rolling (delamination cracks). Aim of this doctoral thesis is to identify the main influence parameters which govern the toughness of micro-alloyed steels and to provide recommendations for the production of micro-alloyed hot-strip steel. Different microstructures and toughness properties were produced by varying hot rolling and annealing conditions. Materials characterization focused on mechanical properties, especially toughness, and microstructural analyses. Electron backscatter diffraction (EBSD) was applied to gain quantitative information on the microstructures. To describe the delamination behaviour of Charpy specimens (number and length of delamination cracks) a new parameter, the delamination parameter lambda, was proposed. In case of ductile material behaviour, it was shown by applying lambda that delamination cracks can reduce impact energy by up to 60 per cent compared to upper shelf energy. From the data obtained by different methods, a model was deduced that describes Charpy impact energy as a function of temperature and a few material parameters. Fractographic analyses show that delamination cracks propagate predominantly along prior austenite grain boundaries. As a result of thermomechanical processing, these boundaries are aligned parallel to the plane of rolling. This is confirmed by EBSD measurements in the vicinity of secondary delamination cracks. Auger electron spectroscopy (AES) was used to study segregation near fracture surfaces. The presence of element enrichment, together with fractographic evidence and the material behaviour after an annealing treatment, supports the theory that delamination cracking is a result of reverse temper embrittlement. The experiments show that delamination cracks do not supress the formation of cleavage cracks, but rather promote cleavage cracking and therefore increase the fracture appearance transition temperature. This is in contrast to many reports found in literature, and the reasons for this contradiction are discussed in the thesis. The observed reduction of Charpy impact energy by delamination cracks (which is in accordance with published results) can be explained by a decrease of the plastically deformed volume as a result of specimen segmentation by delamination cracking.