Microstructure and Deformation Mechanics of X100 Line Pipe Steel

In present paper, microstructure is observed by means of OM, SEM, EBSD and TEM analysis on material obtained from X100 linepipe with wall thickness as 20.6mm and outside diameter as 1016mm diameter Grade 690. To study the deformation mechanics, microstructure transformation is also investigated on deformed specimens with different strain level as 0%, 5% and 8%. Results show that X100 line pipe material consists mainly of acicular ferrite whose ratio in material reaches 64%, obvious cellular structures formed by tangled dislocation and M/A island distributed in acicular plates indicate abundant dislocation and substructure existed in material. It is measured that line pipe steel has a much finer effective grain size as 2.07 mu m, which can results in a higher strength and toughness for line pipe steel. Contrast analysis for Kikuchi Pattern reveals that the ratio of large-angle boundary is comparatively higher as 55%, implying that line pipe steel has a good toughness. It is observed that the proportion of small-angle grain boundary increases greatly after material deformed, which consequently results in the increment of dislocation and sub-structure.