Large-diameter seamless steel pipe refers to seamless steel pipe with a larger outer diameter. Seamless steel pipes with an outer diameter of 325mm and above are usually classified as large-diameter seamless steel pipes. It is characterized by no welds, good compressive resistance and uniform structural performance. It is widely used in various industrial fields, especially those that require high strength and corrosion resistance.
The main production processes of large-diameter seamless steel pipes are hot-rolled large-diameter seamless steel pipes and hot-expanded diameter seamless steel pipes. The maximum specification of hot-expanded seamless steel pipes is 325mm-1220mm and the thickest is 120mm. Hot-expanded seamless steel pipes can produce non-national standard sizes. Hot-expanded seamless pipes are what we often call hot-expanded pipes. Steel pipes with relatively low density but strong shrinkage are expanded by oblique rolling or drawing. A rough pipe finishing process. Large-diameter seamless steel pipes refer to large-diameter seamless steel pipes with an outer diameter greater than 100MM and a wall thickness greater than 0.2 of the outer diameter of the pipe. They are made of solid billets or perforated rough pipes, and then hot-rolled, cold-rolled or cold-drawn. Thickening steel pipes in a short period of time can produce non-standard and special seamless pipes with low cost and high production efficiency, which is the current development trend in the international pipe rolling field.
Main features of large-diameter seamless steel pipes
1. Manufacturing process
Large-diameter seamless steel pipes are manufactured by perforation and hot rolling processes. They are heated from a whole round billet and rolled into a tube billet through a perforator, eliminating the potential defects caused by the welding line and making it have higher strength and pressure resistance.
2. Material diversity
Usually, materials such as carbon steel, alloy steel or stainless steel are used. Specific materials such as Q345B, 20# steel, 16Mn or CrMo alloy steel are selected according to different application requirements.
3. Specification range
Outer diameter: 325mm-1220mm (the specific range depends on the production process).
Wall thickness: above 6mm, some can reach 50mm or thicker.
Length: usually 6-12 meters, and can also be customized according to needs.
4. Performance advantages
Strong pressure bearing capacity, suitable for high pressure or low temperature conditions.
Excellent corrosion and wear resistance (especially alloy steel or stainless steel).
High-precision dimensions, suitable for strict industrial applications.
Production process of large-diameter seamless steel pipes
Round tube billet → heating → piercing → three-roller oblique rolling, continuous rolling or extrusion → tube stripping → sizing (or reducing diameter) → cooling → straightening → hydraulic pressure test (or flaw detection) → marking → storage
Manufacturing large-diameter seamless steel pipes involves precision and advanced techniques. The process typically begins with a solid round billet, which is heated and pierced. This step forms the hollow tube. It is then rolled further using three-roller oblique rolling, continuous rolling, or extrusion methods.
The pipes undergo multiple finishing stages. These include sizing to achieve precise dimensions, cooling, straightening, and rigorous quality checks such as hydraulic testing and flaw detection. The final product is marked and stored for delivery.
For hot-expanded seamless steel pipes, the diameter is increased using oblique rolling or drawing methods. This process enables the production of non-standard sizes and achieves significant cost efficiency. Hot-expanded pipes are particularly suitable for applications requiring customized dimensions.
The state of large-diameter seamless steel pipes that are cold-formed by cold drawing,
cold rolling and other cold processing and delivered directly without any heat treatment is called cold drawing or cold rolling state. Compared with the hot-rolled state, the cold-drawn steel has high dimensional accuracy, good surface quality, low surface roughness, and high mechanical properties. Since the surface of the steel delivered in the cold-drawn state is not covered with oxide scale and has great internal stress, it is very easy to be corroded or rusted. Therefore, the packaging, storage and transportation of the cold-drawn steel have stricter requirements. Generally, it needs to be kept in the warehouse, and attention should be paid to the temperature and humidity control in the warehouse. After hot rolling or forging, the large-diameter seamless steel pipe steel is no longer subjected to special heat treatment. After cooling, it is directly delivered, which is called hot rolling or hot forging state. The end temperature of hot rolling is generally 800-900℃, and then it is generally cooled naturally in the air. Therefore, the hot rolling state is equivalent to normalizing treatment.
Mechanical properties of large-diameter steel pipes
The mechanical properties of large-diameter steel pipes are important indicators to ensure the final use performance (mechanical properties) of large-diameter steel pipes, which depend on the chemical composition and heat treatment system of the steel pipes. Therefore, according to different use requirements, the mechanical properties of large-diameter spiral steel pipes are specifically introduced from the aspects of tensile strength, yield point, elongation, etc.
1. Tensile strength
During the tensile process, the maximum force (Fb) that the specimen bears when it breaks, that is, the stress (σ) obtained from the original cross-sectional area (So) of the specimen, is called tensile strength (σb), and the unit is N/mm2 (MPa). It indicates the maximum ability of metal materials to resist damage under tensile force.
2. Yield point
For metal materials with yield phenomenon, during the tensile process, the force does not increase (remains unchanged), and the stress when the specimen can continue to stretch is called the yield point. If the force decreases, the upper and lower yield points should be distinguished. The unit of yield point is N/mm2 (MPa).
3. Elongation at break
In the tensile test, the percentage of the gauge length increased after the specimen is broken to the original gauge length is called elongation. It is expressed by σ, and the unit is %.
Petroleum and natural gas industry
Used for long-distance oil and gas transmission pipelines, suitable for high pressure and corrosive media.
Boiler and heat exchanger
As the pipeline of high-temperature and high-pressure equipment, it withstands harsh working environment.
Chemical industry
Pipelines for transporting corrosive liquids or gases.
Construction engineering
As piling pipes or structural supports, used in large bridges, building foundations, etc.
Water conservancy and power engineering
Used in water transport pipelines, high-pressure pipelines in power plants, etc.
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