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.