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.