Role of Ti on the Growth of Coarse Columnar Austenite Grain During Nb-Bearing Peritectic Steel Solidification

The role of Ti on the growth behavior of the as-cast austenite grains of the Nb-bearing peritectic steel with different Nb contents during solidification was investigated by fast-directional solidification experiments. The results show that the austenite grains of the Nb-bearing peritectic steel were almost of coarse columnar austenite grains (CCGs) with different Nb contents. As the content of Nb increased from 0.02 to 0.08 wt pct, the number of Nb(C, N) particles increased from 876 to 3140/mm2 at a distance of 10 mm from the surface. Accordingly, the short-axis diameters of the CCGs decreased from 1.71 to 1.14 mm as the pinning pressure provided by carbonitride increased from 0.36 to 0.62 kJ·m−3. The addition of Ti to the Nb-bearing peritectic steel greatly promoted the precipitation of carbonitride in the form of (Nb, Ti)(C, N) composite particles during solidification. As a result, the number of (Ti, Nb)(C, N) particles at a 10 mm distance from the surface increased significantly from 12,210 to 14,324/mm2 as the content of Nb increased from 0.02 to 0.08 wt pct with 0.02 wt pct Ti addition compared to that without Ti addition. The substantial increase in the pinning pressure provided by the (Nb, Ti)(C, N) composite particles from 5.1 to 8.4 kJ·m−3 reduced the short-axis diameters of the γ grains from 1.32 mm to less than 0.2 mm. Meanwhile, the growth of CCGs was gradually inhibited. When the Nb content reached 0.08 with 0.02 wt pct Ti addition, the growth of CCGs was completely inhibited.