Fig. 7 TEM micrographs and the EDS result of precipitates of X90 pipeline steel: (a) precipitations and dislocations; (b) precipitations in the bainite laths; (c) EDS result of precipitate A.
In addition, according to Fig. 7(b), granular precipitate with a diameter of about 67.71 nm was observed in bainite lath. In order to verify its composition, transmission electron microscopy with EDS was used to analyze the composition. The result of EDS analysis is shown in Fig. 7(c). These granular precipitates are composed of Nb and Ti elements. Researchers [1, 28] found that it is common that complex Carbonitride (Ti, Nb) (C, N) in rich Nb and Ti microalloying steels. The core contains undissolved Ti-rich (Ti, Nb) (C, N) and reheated residues, and the Nb-rich (Nb, Ti) C shell is formed at the post-precipitation stage. Undissolved (Ti, Nb) (C, N) precipitation provides heterogeneous nucleation sites for (Nb, Ti) C. EDS analysis of the precipitates in Fig. 7(c) shows that the C content of the composite precipitate is higher than that of N. It indicated that the carbon-rich precipitates formed in the transformation zone of low temperature ferrite, promote AF nucleation sites in austenite grain, and segment the non-transformed austenite to refine grain.
TEM results of precipitates in the outer arc side and the neutral axis position are shown in Fig. 8. The fine precipitates are granular and ellipsoid morphology distributed on the matrix, as shown in Fig. 8(a) and Fig. 8(c). This kind of precipitate is mainly NbC, and the main function is pin the sub-grain boundaries. In Fig. 8(a), a precipitate with a large size at the phase interface, the EDS results shown in Fig. 8(b). The main component of the precipitate is NbC, with a small amount of TiC, possibly (Ti, Nb) C. Fig. 8(d) shows the same results of precipitate B.