The Fe-C phase diagram for the Low-Ti steel up to 0.1 wt% C
(Figure 1 ) indicates that TiN precipitates form in the
austenite matrix of Low-Ti steel at approximately 1650 K regardless of C
concentration, while the Ti carbonitride precipitates form at much lower
temperatures. The precipitation temperature of Ti carbonitrides has been
increased significantly to ~ 1330K with carbon
concentration up to 0.02 wt% and then gradually increased to
~1440K with further addition of carbon up to 0.1 wt%.
It indicates that the variation in carbon concentration around 0.05-0.2
(typical carbon concentration of linepipe steels) has only a slight
affect on the precipitation of nitrides in these steels.
The precipitation of TiN precipitates occurs at much higher temperatures
than carbonitrides, where the alloying elements of Ti and N diffuse much
faster than at lower temperatures [12]. It suggests that TiN
precipitates form larger size precipitates than carbonitrides in the
matrix.
3.1. The effect of titanium and nitrogen on the precipitates of low
carbon, low alloyed steels
Figure 2 shows the phase diagram of Low-Ti steel as a function
of N concentration. Although the Ti concentration remains constant over
the whole composition range, the precipitation temperate of titanium
nitrides has been increased substantially with up to 0.001 wt% N, and
gradually increased with further N addition. The precipitation
temperature of Ti carbonitrides shows only a slight deviation with the
variation in N concentration, indicating that carbonitride precipitates
mainly contain carbon.