A THERMODYNAMIC MODEL FOR COMPOSITION AND CHEMICAL DRIVING-FORCE FOR NUCLEATION OF COMPLEX CARBONITRIDES IN MICROALLOYED STEEL

The extent and degree of dispersion of carbonitrides in high strength low alloy (HSLA) steels is controlled by nucleation and growth processes. A thermodynamic model is presented enabling calculation of the composition of the carbonitride embryo as well as the chemical driving force for nucleation (CDF) in the austenite region of a multicomponent microalloyed steel. The analysis made for a Nb-Ti HSLA steel indicates that the critical carbonitride nucleus is richer in N and Ti than the bulk precipitate at equilibrium at a given temperature and the difference increases when the holding temperature decreases. The results also show that the Ti addition has an optimum value defined by the maximum in the ratio of bound Ti and Nb to their total content. The maximum CDF correlated with the minimum Ti content sufficient to form a carbonitride at the austenitizing temperature.