The nucleation and growth characteristics of NbC- and TiN-precipitates, as well as complex Ti-Nb-carbonitrides in microalloyed steel, have been investigated numerous times. Due to different solubility products, TiN nucleates at higher temperatures than NbC in austenite. Both phases have the same fcc crystal structure with similar lattice constants and are able to form complex Nb-Ti-carbonitrides. In the present work, Nb-Ti-microalloyed steel samples are exposed to a cyclic cooling profile with nine quenching and reheating cycles between 1200°C and 800°C. The temperature oscillations have amplitudes of over 100K. The actual composition of the precipitates depends on the temperature and chemistry of the matrix. A TEM-investigation of samples treated in this way shows various particle populations. The largest ones, which nucleated at the first stages of the heat treatment, are platelet shaped with diameters of over 200 nm. The EDX and EFTEM characterisation shows an alternating structure of shells with local Nb- and Ti-enrichments. In HRTEM micrographs of these particles, local variations in the lattice parameter are measured. The number of shells corresponds to the number of applied thermal cycles. The smallest precipitates are spheroidal NbC nanoparticles with diameters up to 10 nm. The experimental setup is modelled with the thermo-kinetic software MatCalc, which allows for the simulation of the precipitate evolution in the course of this cyclic treatment. The calculated results are in good agreement with the experimental data.
The research leading to these results has received funding from the Research Programme of the Research Fund for Coal and Steel by European Commission. Grant Agreement number: RFSR‐CT‐2011-00008