Influence of Dual-Phase Microstructures on the Properties of High Strength Grade Line Pipes

The influence of dual-phase microstructures on mechanical properties of X70, X80, and X90 line pipes is investigated. It is found that the line pipes with dual-phase microstructures possess both larger uniform elongation and higher hardening exponent, especially for high grade steel X90. The tensile deformation of dual-phase line pipe does not follow the trend predicted by the Hollomon formula, and a stable strain-hardening exponent is not found. This stress-strain behavior is different from the normal line pipe. In the initial stage of plastic deformation, the strain-hardening capacity of dual-phase line pipe increases rapidly. However, it reaches a stable stage after 2.0% total strain. The dual-phase pipeline steel is composed of soft phase (polygonal ferrite) and hard phase (bainite), and thus the relatively soft ferrite is good for its deformability. Besides, the fraction of large angle grain boundaries in the dual-phase microstructures is greater than that of the normal line pipe, which is proven to be critical for improving the resistance to plastic deformation and crack propagation.