Nb-Ti composite precipitation behaviour and its effect on the growth of austenite grains in peritectic steel

Refinement of solidification austenite grains is an important way to control the transverse corner cracks. In the present work, the effect of Ti-Nb carbonitride on the refinement of solidification austenite grains is studied by laboratory experiments. Then, the precipitation temperature of Nb-Ti carbonitride is analysed by establishing a thermodynamic model. The results show that the austenite diameter decreases slowly from 1.45 to 1.16 mm (without Ti addition) and significantly decreases from 0.84 to 0.21 mm (with 0.02 wt-% Ti addition) with the Nb content varying from 0.03 to 0.09 wt-%. The pinning pressure provided by composite precipitation greatly increases with 0.02 wt-% Ti addition. The thermodynamic model of Nb-Ti carbonitride shows that the precipitation temperature of (Nb, Ti)(C, N) increases by approximately 20 degrees C compared to Nb(C, N), which is the main reason for the increase in the pinning pressure.