Formation of primary TiN precipitates during solidification of microalloyed steels – Scheil versus DICTRA simulations

Abstract

Abstract Modern high-strength low-alloy steels commonly contain microalloying additions of titanium, niobium or vanadium in different combinations in order to obtain the desired microstructure and mechanical properties. Titanium has a strong tendency to form TiN in the range of the solidus temperature. This has been reported to have a negative effect on the impact toughness of the material. Thermodynamic calculations showed that the titanium and nitrogen content and the titanium to nitrogen ratio determine if the formation of TiN takes place during solidification or in the solid state. These calculations where complemented by simulations of solidification using the Scheil – Gulliver model and DICTRA. The results were compared with microstructure investigations of plate and slab material with titanium contents between 0.003 wt.% and 0.015 wt.% using light-optical microscopy and electron probe microanalysis. While the formation of TiN particles cannot be ruled out even at the lowest Ti levels, the temperature of formation and the volume fraction varied significantly depending on the Ti content. With respect to the first results of this preliminary study, i. e. the comparison of equilibrium, Scheil and DICTRA calculations, it can be assumed that the Scheil model is the most appropriate one at present.