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.