The formation temperatures of precipitates in the Low-Ti (red curves) steel are compared with the High-Nb steel (purple curves) inFigure 6(b) as a function of nitrogen concentration.Figure 3 showed that increasing the concentration of titanium increases only the precipitation temperature of titanium nitrides, however, Figure 6(b) shows that the addition of niobium has significantly increased the precipitation temperatures of both nitrides and carbides. This will result in more thermally stable precipitates, which contribute to suppressing austenite grain growth during heat treatments. It can explain the fine grain size distribution in niobium-alloyed steels [7, 8].
Figure 6(b) also demonstrates that the precipitation temperature of niobium carbides is almost independent of the nitrogen concentration and the precipitation temperatures are increased with the niobium concentration (Figure 6(a) ). Therefore, it is expected that the niobium carbide precipitates which form in steel 2 (Table 3) appear larger in size than the precipitates in steel 4, originated from precipitation at higher temperatures.