Abstract:
Nb-Ti high-strength low-alloy pipeline steel was subjected to a six-pass rolling process
followed by the coiling process at different temperatures between 600 and 650 ◦C using the thermomechanical
testing system Gleeble 3500 (Gleeble, New York, NY, USA). This experimental steel was
subjected to 72% heavy reduction through a thermos-mechanical controlled process. Thereafter, the
microstructures were observed using optical microscopy, scanning electron microscopy, electron
backscatter scanning diffraction, and transmission electron microscopy coupled with energy dispersive
spectrometry and selected area electron diffraction. For the selected three coiling temperatures of
600, 625, and 650 ◦C, acicular ferrite, polygonal ferrite, and pearlite were observed, and morphology
and statistical analysis were adopted for the study of precipitates. Based on the estimation by the
Ashby–Orowan formula, the incremental strength through precipitation strengthening decreases
with coiling temperatures and reaches 26.67 Mpa at a coiling temperature of 600 ◦C. Precipitationtime-
temperature curves were obtained to explain the transformation of precipitates. The (Nb, Ti)(C,
N) particles tended to precipitate in the acicular ferrite with [011](Nb, Ti)(C, N)//[011]α-Fe orientation.
The lower coiling temperature provided enough driving force for the nucleation of precipitates while
inhibiting their growth.
