Thermodynamic calculations of the phase equilibria between austenite (g) and complex carbonitrides containing Nb, Ti and V, (Nb, Ti, V)(C, N), were performed. The solubility product and formation energy of each carbide and nitride as well as the interaction parameters between components were evaluated. Experimental investigation on phase separation in the complex carbonitrides was also carried out using STEM-EDX and X-ray diffraction. The immiscibility of carbonitride was found to strongly affect the phase equilibria even when the content of microalloying elements in IF steels was very low.

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