Fig.6 TEM micrographs of the neutral axis position in the X90 bend zone: (a) bainite laths; (b) precipitation on dislocations; (c) substructures; (d) degenerated M/A constituent.
3.3 Results of precipitates
X90 pipeline steel is rich in Nb, Ti and other micro-alloying elements, there are a large number of micro-alloying elements precipitates. Fig.7 shows the precipitate characterizes of X90 pipeline steel. High content of microalloying elements promote to form precipitates and fine grains that play an important role in improving the strength and toughness combination. According to Fig. 7(a), some granular and elliptical precipitates distributed around the dislocation lines in the grains, and the size is range from 18.34 nm to 70.29 nm. The precipitates of this size are mostly microalloyed elements M(CN) precipitated at a higher temperature during TMCP. The primary function of these precipitates is to obtain fine grains at room temperature by pinning grain boundaries and dislocations. In addition, there are some precipitates less than 15 nm in size, these precipitates are carbonitrides precipitated from supersaturated solid solution microalloying elements during the transformation of supercooled austenite to ferrite or cooled to single phase ferrite. In general, the volume fraction of these precipitates is very small, but play an effective role for strengthening. Especially the size of 2 ~ 5 nm, mainly NbC [27]. It can also be observed from Fig. 7(a) that the dislocations disordered in the grain of QPF. The interaction between precipitates pinned to the dislocations and the tangle dislocations could decrease the distortion energy of austenite, delaying the recrystallization of austenite and forming fine grains at room temperature.