Characteristics, production process and application of straight seam welded steel pipes for bridges

First, the characteristics and advantages of bridge straight seam welded steel pipes
Bridge straight seam welded steel pipes have significant advantages over traditional welded steel pipes. (1) Their welds are straight, the welding process is easier to control, and the weld quality is more stable. (2) The geometric dimensions of straight seam welded steel pipes are highly accurate, and the wall thickness is uniform, which can meet the strict requirements of bridge structures for dimensional accuracy. (3) Straight seam welded steel pipes use advanced welding technology, with high weld strength and good fatigue resistance, and are particularly suitable for bearing the dynamic loads of bridge structures. From the perspective of material properties, bridge straight seam welded steel pipes usually use low-alloy high-strength steel, such as Q345QD, Q370QE, and other special bridge steels. These steels have good welding performance, low-temperature toughness, and fatigue resistance, and can adapt to the use requirements under various complex environmental conditions. In addition, the inner and outer surfaces of straight seam welded steel pipes have been specially treated, with excellent anti-corrosion properties, which greatly extend the service life of the bridge.

Second, the production process of straight seam welded steel pipes for bridges.
The production of modern straight seam welded steel pipes for bridges utilizes the advanced JCO forming process, which primarily includes the following key steps:
1. Steel plate pretreatment: The steel plate undergoes pretreatment, such as leveling and edge planing, to ensure that the edge quality meets welding requirements. During pretreatment, the flatness and edge straightness of the steel plate must be strictly controlled.
2. Forming: A multi-step progressive JCO forming process is employed, gradually bending the flat plate into a tubular shape through multiple press forming steps. This process requires precise control of forming parameters to ensure the geometric accuracy of the tube blank.
3. Welding: Internal and external welding is performed using a multi-wire submerged arc welding process. Internal welding typically utilizes four-wire submerged arc welding, while external welding utilizes five-wire submerged arc welding. Multi-wire welding significantly improves welding efficiency and weld quality. Welding parameters must be strictly controlled during the welding process to ensure the mechanical properties of the weld.
4. Diameter expansion: The welded steel pipe is reshaped through mechanical or hydraulic expansion to eliminate welding stress and improve dimensional accuracy and roundness. The diameter expansion is typically controlled within the range of 0.8%-1.2%.
5. Heat Treatment Process: The welded steel pipe undergoes an overall heat treatment to eliminate welding residual stress and improve the material's microstructure and properties. The heat treatment process should be optimized based on the steel type and wall thickness.
6. Nondestructive Testing: Welds are thoroughly inspected using various testing methods, such as ultrasonic and X-ray methods, to ensure that weld quality meets standard requirements.

Third, Key Quality Control Points for Longitudinal Welded Steel Pipes for Bridges
1. Dimensional Accuracy Control: This includes external diameter deviation, wall thickness deviation, ovality, and straightness. These indicators directly impact the quality of pipe joints and the overall shape of the bridge.
2. Mechanical Property Control: This includes indicators such as yield strength, tensile strength, elongation, and impact toughness. These performance indicators must meet the requirements of bridge design specifications.
3. Welding Quality Control: The weld should be smooth and flat, free of defects such as cracks, lack of fusion, and slag inclusions. The mechanical properties of the weld should be comparable to those of the parent material. 
4. Corrosion protection control: This includes surface treatment quality and coating adhesion. Good corrosion protection is key to ensuring the long-term durability of bridges.
5. Residual stress control: Control welding residual stress through reasonable process design to avoid premature failure caused by stress concentration.

Fourth, Applications of Longitudinal Welded Steel Pipes in Bridge Structures
1. Steel Tube Concrete-Filled Arch Bridges: Longitudinal welded steel pipes serve as the primary load-bearing components of the arch ribs, with concrete filling the interior to form a composite structure. This structural form fully utilizes the tensile properties of steel and the compressive properties of concrete, offering advantages such as large spans and an aesthetically pleasing appearance. For example, the Chongqing Chaotianmen Yangtze River Bridge utilizes a large number of longitudinal welded steel pipes as arch rib components.
2. Steel Tube Truss Bridges: Longitudinal welded steel pipes serve as truss components, connected through nodes to form a spatial load-bearing system. This structural form offers clear load distribution and high material utilization, making it suitable for long-span bridges.
3. Pier Structures: Longitudinal welded steel pipes can be used as components of bridge piers, and are particularly widely used in high and curved piers. Steel pipe piers are characterized by convenient construction and flexible design.
4. Temporary Support Structures: During bridge construction, straight seam welded steel pipes are often used as components of temporary support systems. They are easy to install and disassemble, and can be reused.

Fifth, Development Trends of Longitudinal Welded Steel Pipes for Bridges
1. High-Strength Material Application: Utilizing higher-strength steels, such as Q420Q and Q460Q, reduces structural deadweight and improves span capacity.
2. Larger Diameter and Thicker Walls: To meet the demands of long-span bridges, straight seam welded steel pipes are developing in the direction of larger diameters and thicker walls. Currently, China can produce large straight seam welded steel pipes with diameters exceeding 1.5 meters and wall thicknesses exceeding 50 mm.
3. Intelligent Manufacturing: The introduction of intelligent manufacturing technologies enables the digitalization, networking, and intelligentization of the production process, improving product quality, stability, and production efficiency.

Sixth, Conclusion
As a key material in modern bridge construction, technological advancements in straight seam welded steel pipes for bridges have directly driven the development of bridge engineering. With the continuous improvement of manufacturing processes and the continuous improvement of quality control levels, straight seam welded bridge steel pipes will play a key role in larger-scale and more complex bridge projects. In the future, with the development and application of new materials and new processes, the performance of straight seam welded bridge steel pipes will be further improved, making greater contributions to my country's bridge construction.