Study of high strength pipeline steels with different microstructures

In the present study, comparison studies on mechanical properties were made on a commercial X70 grade polygonal ferrite (PF) dominated pipeline steel and a laboratory developed X90 grade acicular ferrite (AF) dominated pipeline steel obtained by optimum thermo-mechanical controlled processing (TMCP). Charpy impact test results indicated that the upper shelf energy (USE) of the AF pipeline steel was a little bit higher, but its energy transition temperature (ETT) was extremely low, about −162 °C, much lower than that of the PF pipeline steel of about −121 °C. It was analyzed that higher strength and better toughness of the AF pipeline steel came from its finer grain size and higher density of dislocations and subboundaries, which could be also further proved from the electron backscatter diffraction (EBSD) analysis for its finer effective grain size (EGS) and higher content of low angle grain boundaries (LAGBs), smaller cleavage fracture unit measured from the fracture surface around fracture origin fractured at −196 °C, and its more bent crack propagation path in the fracture.