Extensive studies to develop high deformability linepipe have been conducted. In the case of linepipes laid in seismic region or permafrost field, higher resistance to buckling against large strain induced by ground movement is required. In order to improve the deformability of pipes, two different types of microstructural control technologies were proposed, based on theoretical and analytical studies on the effect of microstructural characteristics on stress-strain behavior. Grade X65 to X100 linepipes with ferrite-bainite microstructure were manufactured by optimizing the microstructural characteristics. Grade X80 linepipe with bainitic microstructure containing dispersed fine MA constituents was also developed by applying new conceptual TMCP process. Deformability of developed linepipes with two different types of microstructure was evaluated by axial compression and bending tests, and all the developed linepipes showed superior resistance to buckling comparing with conventional pipes. Plate manufacturing technologies for producing recent high strength linepipe steel and the concept for microstructure control for improving deformability were also introduced in this paper.

Continuous efforts have been made for the realization of strain-based design pipeline using high grade linepipe materials. Two demonstrative constructions of the pipelines using X100 linepipe proved sufficient materials properties for strain-based design and high quality field welding with good productivity. In order to verify further applicability of high strain X100 linepipe for long distance transmission, large scale installation of X100 pipeline was accomplished. Mass production of X100 linepipe of about 2,000 metric tons with the size of 42″OD and 14.3mm wall thick was successfully conducted by applying recent developed TMCP process including accelerated cooling and online heat treatment process and UOE pipe forming. Field girth welding was safely completed by the dual tandem pulsed GMAW, and sufficient girth weld properties were demonstrated. This paper will describe material development and mass production results of X100 linepipe for strain-based design which specifying longitudinal tensile properties such as Y/T ratio and uniform elongation. In order to securely specify the shape of stress-strain curve without Luders elongation, material parameter “stress ratio” was introduced for the material specification for compressive strain capacity. Stringent base metal requirements were imposed for base metal material properties in this project. One of the most challenging aspects in developing high strain linepipe is to balance uniform elongation and Charpy absorbed energy. Dual phase microstructure is essential to improve strain capacity, but this may lead to lower Charpy absorbed energy. Therefore, precise control of microstructure by controlling plate manufacturing parameter was required. In addition, on-line heating process subsequently after accelerated cooling enabled increase of Charpy energy without deteriorating uniform elongation. Girth weld properties were closely evaluated using the X100 pipe in as UOE condition and after external coating. All the material properties of base metal and girth weldment of the X100 linepipes used for this project fulfill the stringent requirement for strain-based design consideration to prevent buckling and weld fracture.

Heat affected zone (HAZ) along with the weld seam of LSAW has discrete microstructure and usually shows lower toughness than base material. Grain coarsening and formation of M-A constituents governs the HAZ toughness, and a lot of efforts have been made to improve HAZ toughness. However, it is impossible to completely reduce the effect of microstructural change by seam welding. Especially, higher strength or heavy wall linepipes which have richer chemistry tend to have lower HAZ toughness. Therefore, it is important to understand the relationship between microstructural characteristics and toughness of HAZ and fracture behavior of HAZ region in order to ensure the structural reliability of seam welded region. In this paper, fracture behavior of X70 grade heavy wall UOE linepipe was investigated. Wide plate tensile test was conducted with the surface notch introduced on to the coarse grain HAZ, as well as the small scale testing such as Charpy and CTOD test. Fracture mechanics analysis was also carried out to understand the critical condition for brittle fracture of HAZ, and compared with the experimental results.

Linepipes installed in permafrost ground or seismic region, where larger strains can be expected by ground movement, are required to have sufficient strain capacity in order to prevent local buckling or girth weld fracture. On the other hand, strain capacity of linepipes usually degreases with increasing strength, and this is one of the reasons for preventing wider use of high-grade linepipe for high strain application. Furthermore, external coating is necessary for corrosion resistance of pipe, but coating heat can cause strain-aged hardening, which results in increased yield strength and Y/T. Therefore, there is a strong demand for developing high strength linepipe for a high strain application with resistance to strain-aged hardening. Extensive studies to develop Grade X100 high strain linepipe have been conducted. One of the key technologies for improving strain capacity is dual-phase microstructural control. Steel plate with the microstructure including bainite and dispersed martensite-austenite constituent (MA) can be obtained by applying accelerated cooling followed by heat treatment online process (HOP). HOP is the induction heating process that enables rapid heating of the steel plates. Variety of microstructural control, such as fine carbide precipitation and MA formation, can be utilized by this newly developed heating process. One of the significant features of the HOP process is to improve resistance to strain-aged hardening. Increase in yield strength by coating can be minimized even for the Grade X100 linepipe. Trial production of X100 high strain linepipe with the size of 36″ OD and 15mm WT was conducted by applying the HOP process. Microstructural characteristics and mechanical properties of developed X100 linepipe are introduced in this paper. In order to evaluate compressive strain capacity of the developed pipe, full-scale pipe bending test was carried out by using the trial X100 high strain linepipe after external coating. Full scale bending test of developed X100 linepipe demonstrated sufficient compressive strain capacity even after external coating.

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