| In order to improve the efficiency and reduce the cost, the transporting pipeline for oil and natural gas has rapidly developed to large diameter, high pressure and density. As the steel grade improved, the requirements for toughness and weldability are also enhanced. Most research of X80 grade was concentrated on the welding processing and properties of the plate for linear seam welding pipe in the world. Little study has been done on the composition, processing and property of X80 hot-rolling strip for spiral seam submerged arc-welding pipe. Through the simulation of X80 hot-rolling process, the composition and processing of X80 steel are optimized. Further trial has been done on a thickness of 17.5mm X80 strip through industrial production by a width of 2250mm hot-rolling mill. The relationship of hot-rolling process and microstructure, properties is investigated, and the weldability, corrosion resistance, properties of the welded pipe are also studied in this paper. Through investigating the effect of different Mo content on the microstructure and mechanical properties of X80 steel, it can be concluded that the addition of 0.10 wt.% Mo to the steel makes the ferrite grain finer compared with the grain of Mo-free steel. When the Mo content is increased to 0.20%, more acicular ferrite is obtained. The more Mo alloy is added to the steel, the larger amount of M-A structure can be gained. The yield and tensile strength of the steel can be enhanced, but enhanced range of tensile strength is higher than that of yield strength, and the yield ratio and impact energy decreased accordingly. So the suitable Mo content in low carbon-Mn-Nb X80 steel is between 0.20%-0.30 wt.%. Through simulation of thermomechanical processing/on-line accelerated cooling processing using THERMECMASTOR-Z equipment and observation of microstructure, the CCT curves of the tested steels with 0.40% and without molybdenum addition is determined and compared. When 0.40% Mo is added to the steel, bainite transformation is slowed down and Bs, Bf points reduced, so the microstructure of formed bainite becomes finer accordingly. Ms point of the steel decreases as the adding of Mo, M island or M-A island structure becomes finer and dispersed more equably in the lathing of bainite or at the interface of the grains. When 0.40 wt% Mo is added to the steel, the deformed austenite will transform to finer bainite even if the continuous cooling rate is not high. The HIC(hydrogen induced cracking) test result of X80 steels shows that the addition of Mo has no effect on the resistance of HIC. Through the simulation rolling test of X80 steel it can be concluded that, the toughness can be enhanced obviously when finish cooling temperature is 500℃than that of 600℃. The yield and tensile strength are also increased as finish cooling temperature is reduced. Simulation tempering result shows that the strength and toughness are both enhanced moderately when the steel is tempered at 550℃. So if X80 steel is coiled at 550℃, good mechanical properties will be gained. From the industrial development of X80 hot-rolled strip with thickness 17.5mm, it concludes that yield strength and toughness can be improved by reducing the finish rolling and coiling temperatures. The strips have high strength, perfect toughness and good weldability, produced through proper TMP and ACC process with thickness 17.5mm. The microstructure is acicular ferrite with dispersed fine island structures. Anisotropy of the strength and impact energy tested at above -40℃is not obvious, compared with the impact energy tested at below -40℃. 85% fracture appearance transition temperature(DWTT) for the longitudinal, 300,and transverse directions of the strip are -38 ℃,-35 ℃,-60 ℃respectively, showing the good fracture toughness at low temperature needed for large diameter gas pipes. The weldability testing results of Y-groove cracking test manifests that X80 steel has good cracking resistance. Testing method of maximum hardness in welded heat-affected zone shows X80 steel has little tendency to be hardened by quenching. Through gas-shielded arc welding and submerged arc welding tests it can be concluded that, if WER70 welding wire is applied to Ar-rich gas-shielded arc welding, then -20℃impact energy of welded joint is no less than 112J and the strength of the joint is higher than the X80 base metal. If using WGX2 welding wire and CHF101(or CHF105) in submerged arc welding, welded seam can gain high strength and good toughness. φ1016mm spiral-seam submerged arc-welding pipe with wall thickness 17.5mm made of Wisco's X80 hot-rolled strip has high strength, good toughness and weldability. The properties can satisfy with the requirements of API and CNPC standard. The weldedseam and HAZ Charpy impact energy at -20℃is above 180J. Through the sulfide stress cracking tests(SSC) of the transverse pipe body and welded joint using constant load tension test in NACE TM 0177-96 solution B it can be seen: the pipe body and welded joint have good SSC resistance with the threshold stress level no less than 90% of yield strength.
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