THERMODYNAMIC CALCULATION OF SOLUTE CARBON AND NITROGEN IN NB AND TI ADDED EXTRA-LOW CARBON-STEELS

To predict solute carbon and nitrogen in Nb and Ti added extra-low carbon steels quantitatively, the phase stability of the carbonitride in austenite and/or ferrite has been calculated by using Thermo-Calc software to which some thermodynamic parameters were added for the Fe-Nb-Ti-C-N system. It has been confirmed that the (Nb,Ti)(C,N) phase has a miscibility gap island, which is predicted from the tieline sets between TiN-like and NbC-like phases, and the phase can be revealed in the ferrite region, that is, duplex precipitates coexist in the matrix of ferrite in the equilibrium state, whereas only one Ti N-like phase precipitates in the austenite region. As a consequence of the above phase stabilities, it has been predicted that each amount of nitrogen and carbon in solution changes only in the austenite region and the ferrite region, respectively. A quasi-equilibrium precipitation as a two-step heat-treatment has also been demonstrated.