Q620E straight seam welded steel pipes, as a high-strength low-alloy structural steel pipe, have shown significant application value in the fields of oil, natural gas, engineering machinery, bridge construction, etc. in recent years. Its excellent mechanical properties and welding characteristics make it an ideal choice for high-pressure and high-stress environments.
First, the material characteristics and standard specifications of Q620E straight seam welded steel pipes
Q620E is a low-alloy high-strength structural steel. The "Q" in the grade represents the yield strength, "620" means that the lower limit of the yield strength is 620MPa, and "E" indicates that it has -40℃ low-temperature impact toughness. According to GB/T 1591-2018 "Low Alloy High Strength Structural Steel" standard, Q620E must meet the following core indicators:
- Mechanical properties: tensile strength ≥720MPa, elongation after fracture ≥14%, -40℃ impact energy ≥27J;
- Chemical composition: low carbon design (C≤0.18%), adding micro-alloy elements such as Nb, V, Ti, etc., and improving strength through fine grain strengthening and precipitation strengthening;
- Weldability: carbon equivalent Ceq≤0.48%, cold crack sensitivity coefficient Pcm≤0.26%, significantly better than ordinary high-strength steel, suitable for large-scale welding production of straight seam welded steel pipes.
The material achieves a balance of strength and toughness through a quenching and tempering process (quenching + tempering), and its microstructure is tempered troostite, which has both high strength and good ductility. Compared with similar products such as Q550D, Q620E has an increase of about 12% in load-bearing capacity at the same thickness, which is particularly suitable for infrastructure projects under extreme climatic conditions.
Second, the production process and technological breakthrough of Q620E straight seam welded steel pipe
The production process of straight seam welded steel pipe (LSAW) includes key links such as steel plate pretreatment, forming, welding, and heat treatment. The production of Q620E straight seam steel pipe needs to solve the following technical difficulties:
1. Steel plate pretreatment: Steel plates using controlled rolling and controlled cooling (TMCP) process need to be shot peened rust-removed and edge milled to ensure that there is no oxide residue in the welding area. The practice of a large steel plant shows that the width tolerance of the steel plate needs to be controlled within ±1mm to avoid subsequent forming stress concentration.
2. JCOE forming technology: The steel plate is pressed into a tube blank through the progressive bending (JCO) process. Compared with the traditional UOE process, the forming accuracy is higher and the residual stress is reduced by 15%-20%. A certain enterprise uses a 30,000-ton hydraulic press to produce pipes with a diameter of 508-1420mm and a wall thickness of 6-40mm, with an ovality of ≤0.8%D.
3. Multi-wire submerged arc welding (SAW): Four-wire tandem welding (welding speed 1.2-1.8m/min), DC+AC combined power supply for internal welding to reduce magnetic blow, and heat input control (15-25kJ/cm) for external welding to refine the grains in the heat-affected zone. The measured weld impact energy of a certain project reached 45J (-40℃), reaching more than 90% of the parent material level.
4. Diameter expansion and heat treatment: The mechanical diameter expansion rate is usually 0.8%-1.2% to eliminate forming stress; subsequent offline tempering at 580-620℃ is performed to make the weld and parent material properties consistent. A test report shows that the hardness fluctuation range of the heat-treated pipe body is reduced from HRC28-32 to HRC25-28, greatly improving service safety.
Third, typical application scenarios and cases of Q620E straight seam welded steel pipes
1. Oil and gas transportation field: In the Xinjiang coal-to-gas transmission pipeline project, Φ1219×26.2mm Q620E straight seam welded steel pipes are used in 7MPa high-pressure pipe sections, which is 10% thinner than X80 steel, saving about 80 tons of steel per kilometer of the pipeline. Its -45℃ low-temperature impact toughness meets the needs of extremely cold regions, and the weld pass rate is 99.3%.
2. Engineering machinery structural parts: A company uses Q620E steel pipes (Φ600×16mm) in the boom of a 400-ton crane, which reduces the weight of the boom by 18% and increases the limit load to 2.5 times the rated value. Through laser tracking measurement, the straightness error of the pipe is <1.5mm/m, which ensures the telescopic accuracy of the multi-section boom.
3. Wind power support structure: An offshore wind power project in Jiangsu uses Q620E for the transition section of a single pile (wall thickness 50mm), with sacrificial anode protection. In a marine environment with a Cl- concentration exceeding 8000mg/m³, the predicted service life is 35 years. When its fatigue strength Δσ=200MPa, the number of cycles exceeds 2×10⁶ times, which is significantly better than Q460C material.
Fourth, the market status and development trend of Q620E straight seam welded steel pipe
According to data from the China Iron and Steel Association, the market size of high-strength straight seam welded steel pipes will reach 2.8 million tons in 2024, of which Q620E grade accounts for about 15%, with an annual compound growth rate of 8.7%. Driving factors include:
- Energy transformation demand: Hydrogen pipeline construction promotes the application of high-strength steel. The hydrogen embrittlement sensitivity of Q620E in a 2MPa hydrogen environment is only 60% of that of X70 steel;
- Lightweight trend: Q620E welded steel pipe (Φ120×8mm) for commercial vehicle drive shaft is 25% lighter than traditional 45# steel, and the fuel consumption per 100 kilometers is reduced by 1.2L;
- Technology iteration: The "rolling-welding-heating" integrated production line developed by a certain enterprise has reduced the production cost of Q620E straight seam pipe by 12% and shortened the delivery cycle to 15 days.
Future challenges are: ① Control of residual stress of welding of thick-walled steel pipes (＞50mm); ② H₂S corrosion resistance modification of deep-sea pipelines; ③ Insufficient penetration rate of intelligent welding robots (currently <30%). It is expected that by 2028, with the maturity of TMCP+ online quenching technology, the market share of Q620E straight seam welded steel pipes in ultra-high pressure pipelines will exceed 40%.
Conclusion
Q620E straight seam welded steel pipe has achieved a synergistic improvement in strength, toughness, and weldability through material innovation and process optimization. Under the "dual carbon" goal, its full life cycle cost advantage will be further released, becoming an irreplaceable key material in the field of high-end equipment manufacturing. The industry needs to accelerate the establishment of a full-process data traceability system from steel plate production to pipeline service to cope with increasingly stringent safety standards.
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