Fig.3 Optical and SEM micrographs of experimental steel at different
positions: (a) OM of X90 parent pipe; (b) SEM of X90 parent pipe; (c) OM
of the straight part; (d) SEM of the straight part; (e) OM of the inner
arc side; (f) SEM of the inner arc side; (g) OM of the outer arc side;
(h) SEM of the outer arc side; (i) OM of the neutral axis position; (j)
SEM of the neutral axis position.
Transmission
electron microscopy (TEM) was used to investigate the microstructure
further. A large number of dislocations in the QPF matrix, and the
density of dislocations near the grain boundaries is higher than that of
within QPF, as shown in Fig. 4(a). Moreover, some dislocation cells and
substructures piled up the grain boundaries. It is generally believed
that there is a number of sub-structures in the QPF and AF grains during
the rolling process of TMCP because of shear transformation [26].
Two kinds of microstructure around the QPF, the one is fine AF with the
width of about 300nm, and the other is M/A
constituent
distributed on the grain boundaries. Because of the diffusion speed of
carbon atoms into non-transformation austenite after QPF nucleation
decreased significantly, and some fine cementite was found in the grain,
as shown in Fig. 4(a). LB is arranged in parallel by a number of laths
with a width of about 200 nm to 300 nm, and with a very large aspect
ratio. As shown in Fig. 4(b), these laths are clear and straight, some
elongated hard phase M/A
constituents existed between the
laths. The grain boundaries between the laths are low angle grain
boundaries (LAGBs) and the boundaries of laths are high angle grain
boundaries (HAGBs) [27]. Compared to QPF shown in Fig. 4(a),
dislocations intertwined together in LB grain that could hinder further
improve the strength. AF as shown in Fig. 4(b), non–parallel laths with
different orientation were observed under TEM and a large number of
dislocations in the matrix. These AF arranged disorderly and engaged
each other to improve the strength. From Fig. 4(c), it is obviously that
fine M/A constituents distributed between the QPF grain boundaries and
the AF laths. The mixture microstructure of AF and dispersion
distributed M/A constituents could effectively hinder crack propagation
to improve the low temperature impact toughness. The typical shape of
M/A constituents is oval as shown in Fig. 4(d), the oval shape of M/A
constituents would not produce stress concentration to avoid promote
crack initiation and propagation.