Mechanical Properties and Bending Behaviors of Low Temperature Toughness Linepipe Steels

Abstract

API-X70 and X80 steel with good low temperature toughness were developed. The microstructure and mechanical properties of API-X70 steel plate and pipe were investigated and the buckling behavior of X80 steel pipe was evaluated through large scale deformation tester. API-X70 steels with 30 mm thickness were manufactured by finished rolling below Ar3. The microstructure was composed of polygonal ferrite with subgrain network, degenerated pearlite and bainite. The yield strengths of API-X70 pipes were lower than those of plates, while the tensile strengths were similar in both states. The Charpy upper shelf energy of API-X70 steel plate was about 350 J and the energy transition temperature was below −100 °C. The separations were observed on the DWTT fracture surface of API-X70 steel plate. The DWTT 85 SA% transition temperature of plate was below −30 °C. It was conjectured that the separation associated with the low temperature rolling might increase the strength without deterioration of DWTT properties. API-X80 steels with 19mm thickness were fabricated with finished rolling above Ar3 and pipes with 30” diameter were made with R/B process. The deformation capacity of X80 linepipe was evaluated by large scale deforming machine operating under the loading of bending and axial compression force. It was showed that 2nd moment term should be calculated more correctly to measure the accurate critical compressive strain of pipe in the loading of bending and axial compression force. The compressive axial force had a little effect on the peak moment but changed the deformation pattern and state of critical compressive strain of linepipe. It was found that X80 linepipe used in this study was within the specification of DNV and API codes in terms of buckling capacity.