Mikrostrukturelle und hochauflösende Charakterisierung thermomechanisch gewalzter Bleche

Microalloying elements like Nb and Ti play an important role in the manufacturing of high-strength low-alloy steels. Due to their ability to form precipitates they have a significant influence on the evolution of the microstructure as well as the resulting mechanical properties. In this work, the precipitation behaviour of Nb during different thermomechanical processing steps of a Nb-Ti- microalloyed heavy plate steel was investigated using atom probe tomography (APT). The solubility behaviour of Nb was then compared with the behaviour derived from a thermodynamic solubility product. In addition, the microstructure of the steel samples was characterised using various etching methods and optical microscopy. The results of the metallographic characterisation of the Nital and LePera etched samples showed that the water quenched samples have a mostly bainitic microstructure near the surface. At the half thickness position of the sample, there were also larger proportions of ferrite as well as martensite-austenite phase regions. Furthermore, it became apparent that by using an etchant based on Picric acid, a reliable characterisation of the former austenite grains was not possible, as the grain boundaries were only etched incompletely. The APT-measurements showed that with an increasing austenitisation temperature, more Nb could be brought back into solution. At the highest austenitisation temperature only Ti- and N- rich (Nb,Ti)(C,N) precipitates could still be found. Rolling the samples at a temperature above 1000°C did not lead to any significant change in the dissolved Nb. A further austenitisation treatment, following the rolling, resulted in a drop of the dissolved Nb content. Furthermore, fine NbC precipitates could also be found in this condition. The dissolved Nb contents measured at different austenitisation temperatures showed only slight deviations from the values given by the solubility product. These deviations can above all be attributed to the interaction of Nb and Ti and the associated formation of complex (Nb,Ti)(C,N) precipitates.