Analysis on Microstructure and Deformation Mechanics of X100 Linepipe Steel

In present paper, mechanical tests have been conducted on WT20.6mm×OD1016mm diameter Grade 690 linepipe. Microstructure is studied by means of OM, SEM, EBSD and TEM analysis. It is founded that the morphology of X100 steel consists mainly of fine acicular ferrite with little lath-shaped homogeneously distributed polygonal ferrite and very fine M/A constituent distributed on grain boundary, and the proportion of acicular ferrite reaches 64%. It is measured that effective grain size of steel material is 2.07μm for grains with an orientation as 15°. Contrast analysis for Kikuchi Pattern shows that the proportion of small-angle boundary is 45%, which provides X100 linepipe steel with a good performance of higher strength and toughness for large quantity of dislocation and substructure. High-density tangled and blocked dislocation in acicular ferrite lath is also observed in TEM morphology of X100 linepipe steel. Deformation mechanics will be discovered by micro-analysis on specimens deformed with different strain level as 0%, 5% and 8%. With the increment of deformation level, it is observed that substructure in effective grain and the proportion of small-angle grain boundary increases greatly, and the orientation of grain is also changed evidently. To study the deformation process of linepipe steel, an in situ tensile test is also conducted with SEM observation. It is also discovered that crack is initiated at the place of stress concentration, which is mainly close to the interface, and propagates along the length of M/A chip.