Currently, the main production processes for large-diameter steel pipes in my country are hot-rolled large-diameter steel pipes and hot-expanded diameter steel pipes. The largest specifications for hot-expanded seamless steel pipes are 325mm-1220mm, with a thickness of 120mm. Hot-expanded seamless steel pipes can be produced in non-standard sizes. Hot-expanded seamless steel pipes, commonly referred to as hot-expanded steel pipes, are rough pipes with low density but high shrinkage that are expanded in diameter using skew rolling or drawing methods. This process thickens the steel pipe in a short time, allowing for the production of non-standard and special-type seamless steel pipes. It is low-cost and highly efficient, representing the current development trend in the pipe rolling industry.

First, Forming Methods of Large-Diameter Steel Pipes:
1. Hot Push Expansion Method: Push expansion equipment is simple, inexpensive, easy to maintain, economical, durable, and allows for flexible product specification changes. If large-diameter steel pipes or similar products are needed, only some accessories need to be added. Suitable for producing medium- and thin-walled steel pipes with large diameters; also suitable for producing thick-walled pipes within the equipment’s capacity.
2. Hot Extrusion Method: Pre-processing of the billet before extrusion is required. When extruding pipes with diameters below 100mm, equipment investment is low, material waste is minimal, and the technology is relatively mature. However, as the pipe diameter increases, hot extrusion requires high-tonnage and high-power equipment, and the corresponding control system needs upgrading.
3. Hot Piercing Rolling Method: Hot piercing rolling primarily involves longitudinal and skew rolling. Longitudinal rolling mainly includes limited-mandrel continuous rolling, few-stand limited-mandrel continuous rolling, three-roll limited-mandrel continuous rolling, and floating mandrel continuous rolling. These methods offer high production efficiency, low metal consumption, good product quality, and advanced control systems, leading to their increasingly widespread application.

Second, processing methods for large-diameter steel pipes:
1. Rolling: A pressure processing method where a large-diameter steel billet is passed through the gap between a pair of rotating rollers (of various shapes). The compression of the rollers reduces the cross-section of the material and increases its length. This is a commonly used production method for large-diameter steel pipes, mainly used to produce large-diameter steel pipe profiles, plates, and tubes. It is divided into cold rolling and hot rolling.
2. Forging: A pressure processing method that uses the reciprocating impact force of a forging hammer or the pressure of a press to change the billet into the desired shape and size. It is generally divided into free forging and die forging, and is commonly used to produce large sections, billets, and other materials with large cross-sectional dimensions, such as large-diameter steel pipes.
3. Drawing: A processing method where a rolled metal billet (profile, tube, product, etc.) is drawn through a die to reduce its cross-section and increase its length. It is mostly used for cold working.
4. Extrusion: This is a processing method for large-diameter steel pipes, where metal is placed in a sealed extrusion chamber, pressure is applied to one end, and the metal is extruded through a specified die to obtain a finished product with the same shape and size. It is often used in the production of large-diameter steel pipes for non-ferrous metals.

Third, Types of Connections for Large-Diameter Steel Pipes:
1. Quick-Connect Flange Type: The flange and fitting are circumferentially argon-arc welded, and a quick clamp is used to compress the sealing gasket between the flanges, achieving a sealing effect and completing the connection.
2. Welded Type: The fitting and fitting (usually socket type) are circumferentially argon-arc welded, achieving a sealing effect and completing the connection.
3. Press-Fit Type: The pipe is inserted into the fitting, which has protruding U-shaped grooves at both ends and an internal sealing ring. A special pipe fitting tool is used to press the fitting into the socket for connection.
4. Large-diameter steel pipe compression type: The pipe is inserted into the fitting’s inlet and tightened with a nut. The screw force compresses the sleeve at the inlet through the sealing ring, achieving a seal and completing the pipe connection.
5. Large-diameter steel pipe tapered thread type: The external threaded sleeve and fitting are circumferentially argon-arc welded. The internal threaded fitting uses a tapered connection for sealing, completing the connection. Suitable for harsh environments such as foundation settlement, high temperature, and high pressure.
6. Large-diameter steel pipe limiting compression type: The pipe uses a flanged or end-welded ring, and the connection method is a union or semi-union. The rubber sealing ring is compressed by 20%-30% to limit the seal before entering the metal sealing groove. Limiting seals protect the sealing element. Suitable for harsh environments such as foundation settlement, high temperature, and high pressure.
7. Large-diameter steel pipe heat-shrink fittings: Pipe connections can be made by on-site welding. When on-site welding is not possible, heat-shrink fittings are used for supplementary sealing. They shrink under heat, achieving a shrink seal.