ABSTRACT In order to achieve safety and reliability of long-distance gas transmission pipeline installed in seismic and permafrost region, it is quite important to apply high strength linepipes with sufficient strain capacity against buckling and weld fracture by large ground movement. Grades X70-X80 linepipe steels with bainite-MA (Martensite-Austenite constituent) microstructure have been developed by applying advanced plate manufacturing technologies with the combination of accelerated cooling (AcC) and heat treatment on-line process (HOP™). This microstructure shows higher strain capacity which has low yield to tensile (Y/T) ratio and high elongation in the longitudinal direction even after pipe coating heat treatment. In this paper, the material design concept of Grades X70-X80 high strength high strain linepipe steels are introduced. In Grade X70 high strain linepipe steels, adequate MA volume fraction was required even in lower carbon equivalent (CE) level compared with Grade X80 high strain linepipe steels. Mn and Ni addition was useful to obtain the adequate MA volume fraction efficiently, resulting in superior strain capacity which has low Y/T ratio even after the external coating. High strength with Grade X70 level and high elongation have been achieved by control of homogeneous material property such as hardness distribution. Advanced accelerated cooling device (S-OLAC™-A) enables it to obtain the homogeneous microstructure because of homogeneous temperature control after accelerated cooling. Higher elongation has been achieved by lower surface hardness and homogeneous hardness control. In developed Grade X80 high strain linepipe steels, MA volume fraction and its size distribution were controlled simultaneously to achieve higher strength, low Y/T ratio and higher Charpy absorbed energy in full ductile fracture region. It is very important to control MA morphologies to achieve these mechanical properties. HOP™ type bainite-MA dual phase linepipe steels show the highest strain capacity even after pipe coating heat treatment. On the other hand, AcC type bainite-MA dual phase linepipe steels have the highest Charpy absorbed energy because of lower MA volume fraction and finer size of MA by applying appropriate accelerated cooling stopping temperature. Detailed laboratory test results and actual mill test data will be introduced in the presentation.

ABSTRACT Demand for higher grade linepipe that can help reduce the total cost of long-distance gas pipelines has increased recently. As a result, pipelines using high-strength linepipes such as Grades X70 and X80 have been planned in hostile environments such as landslide-prone mountain areas, reclaimed land where liquefaction is an issue and polar regions where frost heave occurs. Buried pipelines in these regions are expected to be subjected to large deformation due to the large ground movement caused by these geohazards. It is not possible to apply conventional stress-based design in cases where the longitudinal stress of pipe material greatly exceeds the yield stress and the pipeline material is subjected to large plastic strain. The Strain Based Design (SBD) approach is required to complement the conventional allowable stress methods in order to ensure a reliable limit state design approach that uses the longitudinal strain capacity of pipelines. In SBD, the tensile strain capacity of the pipeline in the longitudinal direction is very important. The tensile strain capacity under a pure tension loading condition in the pipe axial direction has been studied, taking into account the effects of internal pressure, pipe geometry, flaw size, misalignment of girth welds, weld overmatch, the yield to tensile strength (Y/T) ratio and elongation in the base material. Since loading predominantly takes the form of bending displacements, two different strain capacity limit states must be addressed: compressive strain capacity, which corresponds to the onset of local buckling characterized by the formation of a wrinkle on the compressive side, and tensile strain capacity, which corresponds to final fracture characterized by the ultimate limit state of necking from the base metal or ductile tearing from a girth weld defect on the tensile side. However, the tensile strain capacity after local buckling is not fully understood because few data are available from full-scale bending tests with final fracture after local buckling. This paper presents the tensile strain capacity after local buckling in full-scale pipe bending tests of X80 girth welded UOE high strain pipe with an outside diameter (OD) of 48 inches and X70 girth welded UOE high strain pipe with OD of 36 inches. In order to discuss the loading condition on the tension side after local buckling near a girth weld, the longitudinal tensile strain distributions were investigated. After a large wrinkle formed on the compression side of bending, the wrinkle worked precisely like a plastic hinge, and longitudinal tensile strain developed only in the exactly opposite area to the large wrinkle on the compression side of bending. The tensile strain capacity in full-scale pipe bending is discussed from the viewpoint of the effect of the tensile strain distribution enhanced by wrinkle occurrence on the compressive side on final fracture in comparison with the pure tension loading condition.