Abstract: Effect of expanding ratio on strain capacity of the trial X70 UOE pipeline steel pipe with polygonal ferrite and bainite (PF+B) was studied by optical microscope (OM), electron backscattered diffraction (EBSD), transmission electron microscope(TEM) and tensile test. The results show that when the expanding ratio varies in the range of 0.4%-1.2%, the grain orientation, effective grain size and proportion of high and low angle grain boundaries in the microstructure of pipe remain basically unchanged. With the increase of expanding ratio, the dislocation density in PF increases, resulting in a slight increase in yield strength and tensile strength. After UOE pipe made, the strain capacity index decreases from 2.00 of the original steel plate to 0.78, 0.61 and 0.56 of different expanding ratio, and the strain capacity decreases significantly. However, there is little effect on strain capacity of UOE pipe when the expanding ratio varies in the range of 0.4%-1.2%. The strain hardening rate first decreases and then increases before necking with the increase of expanding ratio, while the strain values corresponding to the necking points are 9.7%, 8.6% and 8.5% respectively. The strain hardening rate decreases rapidly when the expanding ratio reaches 1.2%.

Key words: high deformability pipeline steel, UOE, expanding ratio, microstructure, strain capacity