【Translation】Factors to consider when selecting pipeline materials for long-distance natural gas pipelines, as well as technological advancements that have been achieved.

②What are the main properties of pipeline steel and steel pipes? 　　In general, pipeline steel refers to the coil sheets (steel strips) and steel plates used in the production of high-frequency welded pipes, spiral submerged arc welded pipes, and straight-seam submerged arc welded pipes. 　　As pipeline transportation pressures and diameters increased, starting in the 1960s, high-strength pipeline steels (such as X56, X60, X65, X70, etc.) were developed on the basis of low-alloy high-strength steels; these steels overcame the limitations of traditional steel strengthening and rolling techniques. Alloying elements such as niobium (Nb), vanadium (V), and titanium (Ti), which are added to steel in trace amounts (with a total content of no more than 0.2%), enable a significant improvement in the overall mechanical properties of the steel through controlled rolling processes. High-strength pipeline steel is a high-tech, high-value-added product, and its production makes use of nearly all the latest advancements in metallurgical processing technologies. It can be seen that the materials used in long-distance natural gas pipelines reflect, to a certain extent, the **level of the metallurgical industry**. 　　Natural gas transmission pipelines operate in harsh conditions, with complex geological features, long lengths, and difficulties in maintenance; they are also prone to fracture and failure. Therefore, pipeline steel must possess excellent properties such as high strength, high toughness, weldability, resistance to extreme cold temperatures, and fracture resistance. 　　Using high-strength pipeline steel or increasing the wall thickness of pipeline pipes allows natural gas pipelines to withstand higher transmission pressures, thereby increasing the volume of natural gas that can be transported. Although the price of microalloyed high-strength steel for steel pipes of the same diameter is about 5% to 10% higher than that of ordinary steel, it can reduce the weight of the pipes by approximately 1/3, make the manufacturing and welding processes easier, and lower the costs of transportation and installation. Practice has shown that the cost of using high-strength pipeline steel pipes is only about 1/2 of the cost of ordinary steel pipes with the same pressure and diameter, and the likelihood of thinning of the pipe wall as well as brittle fracture of the pipe is also reduced. Therefore, it is generally preferred to increase the strength of the steel pipes in order to boost pipeline capacity, rather than increasing their wall thickness. 　　The main strength indicators of pipeline steel are tensile strength and yield strength. Pipeline steel with a higher yield strength can reduce the amount of steel required for gas transmission pipelines; however, an excessively high yield strength reduces the toughness of the pipes, leading to phenomena such as tearing and cracking, and thereby causing safety accidents. While high strength is required, the ratio of the yield strength to the tensile strength of pipeline steel (yield-to-tensile ratio) must be considered comprehensively. An appropriate yield strength ratio ensures that the steel pipe possesses sufficient strength as well as adequate toughness, thereby enhancing the safety of the pipeline structure.

Once a high-pressure gas pipeline fails and ruptures, the compressed gas will expand rapidly and release a large amount of energy, leading to serious consequences such as explosions and fires. To minimize the occurrence of such accidents, pipeline design should carefully consider fracture control strategies from the following two aspects: First, the steel pipes must always operate in a ductile state, that is, the ductile-to-brittle transition temperature of the pipe material must be lower than the operating temperature of the pipeline, to ensure that brittle fracture does not occur. Secondly, after ductile fracture occurs, the crack must be stopped within a range of 1 to 2 tube lengths in order to prevent further propagation of the crack and avoid greater losses. Long-distance natural gas pipelines are constructed by joining steel pipes together one by one using ring welding. The harsh construction conditions in the field have a significant impact on the quality of circumferential welding; they can cause cracks at the welds and reduce the toughness of both the welds and the heat-affected zones, thereby increasing the risk of pipe failure. Therefore, pipeline steel possesses excellent weldability, which is crucial for ensuring the welding quality and overall safety of pipelines. 　　In recent years, as the exploration and extraction of natural gas have expanded to regions such as deserts, mountainous areas, polar regions, and oceans, long-distance pipelines often have to pass through areas with highly complex geological and climatic conditions, such as permafrost zones, landslide-prone areas, and earthquake zones. To prevent deformation of steel pipes during service due to ground subsidence, movement, etc., gas pipelines located in areas prone to earthquakes and geological disasters should use pipeline steel pipes designed for large deformations based on strain considerations. For above-ground pipelines that pass through fictional regions, permafrost areas, or high-altitude or high-latitude areas with low temperatures, and which are subject to extreme cold conditions throughout the year, pipeline steel tubes with excellent resistance to brittle fracture at low temperatures should be used ； Buried pipelines subjected to corrosion by groundwater and highly conductive soil should have enhanced anti-corrosion treatment both inside and outside the pipelines.

③What are the key manufacturing technologies for high-strength pipeline steel? 　　Alloy design is an important foundation for obtaining high-performance pipeline steel. Modern pipeline steel is characterized by low or ultra-low carbon content, with the carbon level usually being below 0.12%. Alloying elements such as manganese, niobium, vanadium, and titanium are added to improve the overall properties of this steel. 　　The quality control of pipeline steel must begin with steelmaking, and an appropriate metallurgical process is key to improving the internal and surface quality of the steel. With the development of high-strength pipeline steel, high cooling rates and low final cooling temperatures have become key technologies for improving the strength and toughness of X80 and higher grade pipeline steels as well as thick-walled pipeline steels. 　　On this basis, by adjusting the steel plate rolling process to accelerate the start temperature and cooling rate, as well as by controlling the process parameters for pipe forming, welding, and anti-corrosion, domestic steel and pipe manufacturing enterprises have successfully produced large-deformation pipeline steels and pipes with excellent comprehensive properties in terms of strength, toughness, and ductility, which are suitable for areas prone to geological disasters such as geological fault zones and permafrost subsidence. 　　High-strength, large-diameter, high-pressure spiral-welded submerged-arc welded pipes and straight-welded submerged-arc welded pipes are the main pipe materials used in China’s existing and under-construction long-distance natural gas pipelines. Spiral welded pipes made from coil steel strips can be produced using steel strips of different widths to create pipes of the same diameter, or they can be made using steel strips of one width to produce pipes of different diameters. This production method offers advantages such as high precision in steel pipe dimensions, good production continuity and efficiency, a small footprint for the production line, and low investment costs.

Straight-seam submerged arc welded pipes are made from steel plates through processes such as forming, welding, and finishing; the forming methods include UOE forming and JCOE forming. Among them, the UOE forming process offers advantages such as high production efficiency and good quality of the finished products; it is also capable of producing steel pipes with large diameters, thick walls, and long lengths. It is currently the most widely used, most mature production process for large-diameter straight-seam submerged arc welded pipes in the world, and its quality is generally recognized. 　　In natural gas long-distance pipeline systems, in order to accommodate the diversion and redirection of the transported fluid, as well as to mitigate the additional forces exerted on the pipelines due to ground movements, a large number of structural components such as elbows and tees that match the properties of the main pipelines must be installed along the pipeline routes, in valve rooms, and at stations. 　　Currently, most elbows used in natural gas pipelines are manufactured using medium-frequency induction heating and water cooling methods ； High-strength, large-diameter tubes are typically produced by using thick plates through a hot drawing process. Unlike the manufacturing method of pipeline steel pipes, during the production of fittings such as elbows and tees, the pipeline steel material must be reheated, which results in significant changes in its properties; therefore, the design of the material’s composition and the heat treatment processes require special attention.

④What new opportunities will arise in the development of pipeline materials? 　　As China’s energy structure continues to shift toward a lower-carbon model, natural gas is playing an increasingly important role in the country’s primary energy consumption structure. Large-diameter, high-pressure, high-grade steel pipes will remain the inevitable choice for long-distance natural gas pipeline projects for a considerable period of time to come. As leaders in this field, X80 pipeline steel and pipes will continue to be the preferred choice for constructing large-capacity pipelines. 　　The X90 pipeline steel and pipes, along with the related application technologies developed through efforts led by China National Petroleum Corporation, represent one of the significant scientific and technological achievements in the construction of third-generation, high-capacity, high-strength natural gas pipelines. With further advancements in research and development, pipeline steels such as X90 and X100 are expected to be used in future natural gas pipeline projects. By then, the annual gas transmission capacity of one pipeline could reach 50 billion cubic meters. 　　In recent years, driven by the \"dual carbon\" goals, new technologies such as carbon dioxide pipeline transportation and storage, as well as hydrogen blending in natural gas pipelines and the transport of pure hydrogen, have placed higher demands on the corrosion resistance, low-temperature toughness, and susceptibility to hydrogen embrittlement of pipeline materials. This has brought new opportunities for the development of materials used in transportation pipelines.

The production and application of pipeline steel and steel pipes in our country started relatively late. Before the 1980s, domestic oil and gas pipelines primarily used domestically produced A3 steel and 16Mn steel, as well as TS52K steel imported from Japan (equivalent to X52 grade). The pressure at which these pipelines could operate was generally below 4 megapascals, which means that development in this area lagged behind that in developed countries by about 40 years. 　　From 1980 to 1985, China began developing X60 and X65 pipeline steels in accordance with API standards, and these steels were successfully used in pipeline construction alongside imported steel pipes. 　　In the early 1990s, domestic steel and pipe manufacturing enterprises such as Baowu Steel Group and Baoji Steel Pipe began to focus on research and development in pipeline steel and steel pipe technologies, attempting to apply them in natural gas pipeline projects. 　　Shaanxi-Beijing Pipeline: It was completed and put into operation on September 10, 1997. With a total length of 1,098 kilometers, it uses X60 pipeline steel, has a design pressure of 6.4 megapascals, and a designed annual transport capacity of 3.6 billion cubic meters. It was the longest pipeline on land in China at that time, with the largest diameter, the most complex geological conditions along its route, and the highest level of automation. 　　Shaanxi-Beijing Line 2 was put into operation in July 2005. With a total length of 935 kilometers, it uses X70 grade steel pipes, has a design pressure of 10 MPa, and a designed annual transport capacity of 17 billion cubic meters. It achieved supply via \"dual pipelines\" and \"multiple gas sources\" for the first time, evolving into China’s first long-distance, automated high-pressure gas transmission and distribution system that includes pipelines, compression stations, and underground gas storage facilities. 　　Shaanxi-Beijing Line 3 was put into operation in 2011; it is 896 kilometers long, uses X70 steel pipes, has a design pressure of 10 megapascals, and an annual transport capacity of 15 billion cubic meters. This is another important route for supplying natural gas to Beijing and the Bohai Rim region. 　　Shanjing Line 4: Completed in 2017, it uses domestically produced X80 large-deformation pipeline steel pipes in areas prone to geological disasters, with a total usage of 5,000 tons. This marks that our country has fully mastered the cutting-edge manufacturing technology for high-strength pipeline steel, breaking free from the reliance on imports for pipeline steel with special properties. 　　West-to-East Gas Pipeline Line 1: Construction began in 2001. Most of them use X70 pipeline steel developed independently in China; this is also the first time that X70 has been used on a large scale in domestic natural gas pipeline projects. The design pressure is 10 megapascals, with an annual transportation capacity of 12 billion cubic meters. 　　China-Russia East Route: Construction began in 2017, using X80 pipeline steel pipes with a diameter of 1422 millimeters, developed by China National Petroleum Corporation, and 100% of the components were of domestic origin. The design pressure of the pipeline is 12 MPa, with an annual gas transmission volume of 38 billion cubic meters; the gas transmission capacity of the pipeline will be further increased. 　　The China-Myanmar gas pipeline project: The domestic section began construction in 2010, with a total length of 1,727 kilometers. It has an annual transportation capacity of 12 billion cubic meters, and X80 steel pipes are primarily used for its construction. For the first time in areas prone to geological disasters, China’s X70 large-deformation pipeline steel pipes, successfully developed between 2010 and 2013, were used; their total weight amounted to 59,000 tons, representing the highest usage of such pipes in any single pipeline project in the world. 　　West-to-East Gas Pipeline Project II: Construction began in 2008; it uses domestically produced X80 steel pipes for construction, with a transmission pressure of 12 MPa. The annual gas transport capacity has been increased to 30 billion cubic meters. The piping used in the West-to-East Gas Pipeline Project Line 3 is the same as that in Line 2, with 100% domestic production of the pipes.