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.