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.