Thermodynamic analysis of the precipitation of carbonitrides in microalloyed steels

The production of mass-products of microalloyed steels with a high strength requires a proper adjustment of the thermomechanical processing and the kinetics of the precipitation of the MX-type (M - microalloying element, X - metalloid) phases in austenite. The understanding of the effect of carbonitrides on the processes of hot-working and cooling from the finishing deformation temperature requires the knowledge of the mechanism of their formation and their stability in austenite. The research was carried out on newly manufactured Ti-V and Ti-Nb-V steels for forged machine parts with thermomechanical processing. The analysis of the precipitation of the carbonitrides with the stoichiometric compositions of Ti_xV_1-xCyN_1-y and Ti_xNb_vV_1-x-vCyN_1-y was based on the Hillert-Staffanson model improved by Adrian. The effect of the austenitizing temperature in the range from 900 °C to 1200 °C on the grain size of the original austenite was investigated to verify the results of the calculation. The result provides the basis for a suitable design of the manufacturing process of the thermomechanical treatment to obtain high-strength forged elements of microalloyed steels.