What are the main characteristics of pipeline steel and steel pipes

Generally speaking, pipeline steel refers to coils (steel strips) and steel plates used to produce high-frequency welded pipes, spiral submerged arc welded pipes, and straight seam submerged arc welded pipes.

With the increase in pipeline transportation pressure and pipe diameter, high-strength pipeline steel (X56, X60, X65, X70, etc.) has been developed based on low-alloy high-strength steel since the 1960s. These steels have broken through the strengthening and rolling technology of traditional steel. Trace elements (total amount not more than 0.2%) of niobium (Nb), vanadium (V), titanium (Ti), and other alloy elements are added to the steel, and the comprehensive mechanical properties of the steel are significantly improved by controlling the rolling process. High-strength pipeline steel is a high-tech, high-value-added product, and its production applies almost all the new achievements of process technology in the field of metallurgy. It can be seen that the materials used for long-distance natural gas pipelines represent the level of a country's metallurgical industry to a certain extent.

The long-distance natural gas pipeline has problems such as a harsh operating environment, complex geological conditions, long lines, difficult maintenance, and easy fracture failure. Therefore, pipeline steel should have good properties such as high strength, high toughness, weldability, resistance to severe cold and low temperature, and fracture resistance.

The use of high-strength pipeline steel or increasing the wall thickness of pipeline steel pipes can enable natural gas pipelines to withstand higher transmission pressures, thereby increasing the natural gas transmission volume. Although the price of micro-alloy 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 deadweight of the steel pipe by about 1/3, make the manufacturing and welding process easier, and reduce the transportation and laying costs. The practice has proved 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 the same diameter, and the possibility of thinning the pipe wall and brittle fracture of the pipeline is also reduced. Therefore, it is generally chosen to increase the pipeline transmission volume by increasing the strength of the steel pipe rather than increasing the wall thickness of the steel pipe.

The strength indicators of pipeline steel mainly include tensile strength and yield strength. Pipeline steel with higher yield strength can reduce the amount of steel used in gas pipelines, but too high yield strength will reduce the toughness of steel pipes, causing tearing, cracking, and other phenomena in steel pipes, and causing safety accidents. While requiring high strength, the ratio of yield strength to tensile strength (yield strength ratio) of pipeline steel must be comprehensively considered. A suitable yield-strength ratio can ensure that the steel pipe has sufficient strength and sufficient toughness, thereby improving the safety of the pipeline structure.

Once a high-pressure gas pipeline fails, the compressed gas will expand rapidly and release a large amount of energy, causing serious consequences such as explosions and fires. To minimize the occurrence of such accidents, pipeline design should carefully consider the fracture control plan from the following two aspects: First, the steel pipe should always work in a tough state, that is, the tough-brittle transition temperature of the pipe must be lower than the service environment temperature of the pipeline to ensure that the steel pipe does not have a brittle fracture accident. Second, after a ductile fracture occurs, the crack should be stopped within 1 to 2 pipe lengths to avoid long-range crack expansion and greater losses. The long-distance natural gas pipeline is connected one by one through the girth welding process. The harsh construction environment in the field has a great impact on the quality of girth welding, which makes it easy to produce cracks at the weld, and reduces the toughness of the weld and heat-affected zone, increasing the possibility of pipeline rupture. Therefore, pipeline steel itself has excellent weldability, which is crucial to ensure the welding quality and overall safety of the pipeline.

In recent years, with the development and exploitation of natural gas extending to deserts, mountains, polar regions, and oceans, long-distance pipelines often pass through areas with very complex geological and climatic conditions such as permafrost zones, landslide zones, and earthquake zones. To prevent steel pipe deformation caused by ground collapse and movement during service, gas pipelines located in earthquake-prone areas should use strain-based design to resist large deformation pipeline steel pipes. Non-buried pipelines that pass through overhead areas, permafrost areas, high altitudes or high-latitude low-temperature areas are subjected to the test of high cold all year round, and pipeline steel pipes with excellent resistance to low-temperature brittle fracture should be selected; buried pipelines corroded by groundwater and highly conductive soil should strengthen the anti-corrosion treatment inside and outside the pipeline.