Carbon steel pipe: the "steel artery" of industrial transportation

Carbon steel pipe, as the most basic and most commonly used metal pipe, has become the core material for fluid transportation and structural support in modern industry with its excellent mechanical properties, affordable cost and wide applicability. From the water pipelines underground in the city to the oil pipelines in the oil field, from the transmission shafts of mechanical manufacturing to the scaffolding of the building, this tubular product based on carbon steel, with its diverse specifications and processes, has built a "steel vein" for industrial production and social operation.
1. Classification of carbon steel pipes: multi-dimensional classification from forming process to material properties
The classification logic of carbon steel pipes clearly reflects the diversity of its application scenarios, and different types of pipes have clear boundaries in performance and use.
1. Classification by forming process: the essential difference between seamless and welding
Seamless carbon steel pipe: born from the "piercing-rolling" process of steel billets. The solid carbon steel billet is heated to about 1200℃ (austenitizing temperature), pierced into a hollow rough tube by a piercing machine, and then processed into the final size by hot rolling, cold rolling or cold drawing. Seamless pipes have no welds and can withstand higher pressures (generally up to 10-80MPa), which are suitable for high-pressure scenarios - casing for oil drilling (withstanding pressure of 1,000 meters underground), superheated steam pipes for boilers (withstanding 4MPa high pressure and 450℃ high temperature), and pressure oil pipes for hydraulic systems all rely on the integrity and strength of seamless pipes. Its specification range covers an outer diameter of 6-630mm, a wall thickness of 1-50mm, and a dimensional accuracy of up to ±0.1mm, which is especially suitable for small-diameter, high-precision requirements (such as oil pipes for automobile gearboxes).
Welded carbon steel pipe: Carbon steel coil is used as raw material, and after curling and forming, it is formed into a tubular shape through welding processes (arc welding, submerged arc welding, etc.). According to the shape of the weld, it can be divided into straight welded pipes (welds are along the axial direction) and spiral welded pipes (welds are spiral). Straight seam welded pipes have high production efficiency and the cost is only 60%-70% of seamless pipes. They are suitable for medium and low pressure and large diameter scenarios (such as urban tap water mains, with an outer diameter of more than 1200mm); spiral welded pipes have more uniform stress and better crack resistance than straight seam pipes, and are often used in long-distance oil and gas pipelines (such as some pipelines in the West-East Gas Transmission Project use φ1016mm spiral welded pipes). Modern high-frequency welding technology can make the weld strength reach more than 85% of the parent material, and some welded pipes can also be used for low-pressure industrial pipelines (working pressure ≤1.6MPa).
2. Classification by material and strength: performance gradients adapted to different loads
Carbon steel pipes for low-pressure fluid transportation: represented by Q235 steel, carbon content ≤0.22%, tensile strength 375-500MPa, yield strength ≥235MPa, excellent plasticity (elongation ≥25%). This type of pipe is mainly used for low-pressure transportation such as tap water and heating (working pressure ≤1MPa). It is inexpensive and is the "basic model" for civil and general industries.
Carbon steel pipe for high-pressure boiler: 20G steel (high-quality carbon structural steel) is used, with a carbon content of 0.17%-0.24%. Through strict smelting control (sulfur and phosphorus content ≤0.035%) and heat treatment, it still maintains a high endurance strength at 450℃ (100,000 hours of fracture strength ≥100MPa), and is used for high-pressure heating surfaces such as water-cooled walls and superheaters of boilers. The working pressure can reach more than 9.8MPa.
Alloy structural carbon steel pipe: Performance is improved by adding alloy elements such as manganese, silicon, and chromium. For example, 16Mn carbon steel pipe (containing 1.2%-1.6% manganese) has a yield strength of ≥345MPa, which is 1.5 times that of Q235. It is used for supporting structures of low-pressure boilers and oil storage tanks; 40Cr carbon steel pipe (containing 0.8%-1.1% chromium) has a tensile strength of more than 800MPa after quenching and tempering, which is suitable for making transmission shafts and piston rods in mechanical transmission, and has both strength and toughness.
2. Core performance of carbon steel pipe: mechanical advantages and process adaptability
The reason why carbon steel pipe can become the "main pipe material" of industry is due to its balance between performance and cost, as well as its adaptability to different processing technologies.

High strength and load-bearing capacity are the core advantages of carbon steel pipe. The tensile strength of seamless carbon steel pipes is generally above 400MPa, and can withstand the combined effects of internal and external pressure and axial loads. A seamless pipe of φ219×6mm (Q345 material) can withstand an internal pressure of 20MPa, equivalent to a force of 200 kg per square centimeter, which is sufficient to meet the needs of most industrial pipelines. In structural applications, carbon steel pipes have outstanding bending resistance. A φ108×4mm steel pipe can be used as a scaffolding pole, and a single one can carry more than 3 tons of weight, making it an indispensable support material in construction.

Good processability reduces manufacturing and installation costs. Low-carbon steel pipes (such as Q235) have excellent welding performance. Manual arc welding can be used to obtain a firm joint, and the welding efficiency is more than twice that of stainless steel pipes. The cold drawing process can process seamless pipes into high-precision thin pipes (such as φ6×1mm) to meet the size requirements of mechanical parts. The hot bending process can bend carbon steel pipes into any angle (the minimum bending radius is 3 times the pipe diameter) to meet the steering requirements in pipeline laying. This feature makes it widely used in urban underground pipe networks.

Cost advantages expand the application boundaries. Compared with stainless steel pipes and copper pipes, the price of carbon steel pipes is only 1/3-1/2 of 304 stainless steel pipes and 1/5 of copper pipes of the same specifications. Especially in large-diameter and long-distance projects (such as urban water pipelines that are tens of kilometers long), the cost advantage of carbon steel pipes is almost irreplaceable. Even if anti-corrosion treatment (such as galvanizing) is required, its comprehensive cost is still significantly lower than other metal pipes.
3. Anti-corrosion treatment: the key means to extend the life of carbon steel pipes
The natural shortcoming of carbon steel is that it is easy to rust (the annual corrosion rate can reach 0.1-1mm), so anti-corrosion treatment has become a necessary link in the application of carbon steel pipes, and different processes are suitable for different corrosion environments.

Galvanizing treatment: the most common anti-corrosion methods are divided into "hot-dip galvanizing" and "electrogalvanizing". Hot-dip galvanizing forms a 50-100μm zinc layer (zinc content ≥ 95%) on the surface by immersing the carbon steel pipe in 450℃ zinc liquid. The zinc layer acts as a sacrificial anode to protect the carbon steel, and the salt spray resistance can reach 500-1000 hours. It is suitable for outdoor pipes (such as street light poles, guardrails) and humid environments (such as water supply and drainage pipes in bathrooms). The electroplated zinc layer is thinner (10-30μm) and has a brighter appearance. It is suitable for decorative pipes (such as metal brackets for furniture), but the anti-corrosion life is not as good as hot-dip galvanizing (about 1/3 of it).
Coating anti-corrosion: For highly corrosive environments, the corrosive medium is isolated by applying an organic or inorganic coating. Epoxy coal tar coating (thickness 0.5-1mm) has excellent acid and alkali resistance, suitable for chemical sewage pipelines and underground oil pipelines, and has a service life of more than 15 years; polyethylene (PE) winding coating (thickness 2-3mm) has good soil corrosion resistance and is used for buried water pipelines to resist the erosion of underground microorganisms and stray currents.
Corrosion inhibitors and linings: When conveying less corrosive media (such as industrial circulating water), corrosion inhibitors (such as chromates and phosphates) can be added to the pipe to inhibit the electrochemical corrosion of carbon steel; for conveying acidic liquids (such as wastewater with a pH of 4-6), rubber linings or glass fiber linings can be used to protect the inner wall of carbon steel pipes through physical isolation, and the cost is lower than directly using stainless steel pipes.
IV. Application scenarios: from livelihood projects to industrial core
The application map of carbon steel pipes covers the full scale from "macro engineering" to "micro parts", and its balance between performance and cost makes it the first choice for many scenarios.

In urban infrastructure, carbon steel pipes are the main force of the "underground veins". In the urban tap water system, DN300-DN1200 welded carbon steel pipes (Q235 material, epoxy coal tar coating on the outer wall, cement mortar lining on the inner wall) are used as main pipes to undertake the water supply task of the whole city. The length of a single pipeline can reach tens of kilometers; municipal sewage pipe networks mostly use spiral welded carbon steel pipes (anti-corrosion coating on the inner wall) to withstand microbial corrosion in sewage; in the central heating system, galvanized carbon steel pipes (DN15-DN100) are used as indoor heating branches, which not only ensure heat dissipation efficiency, but also delay rust through the galvanized layer.

In industrial production, carbon steel pipes are "material transportation hubs". In the field of petrochemicals, seamless carbon steel pipes (20# steel) are used to transport crude oil and refined oil with a working pressure of 2.5-10MPa; in the boiler industry, 20G high-pressure carbon steel pipes are used as water-cooled wall pipes, operating for a long time at 450℃ and 9.8MPa to ensure the thermal efficiency of the boiler; in mechanical manufacturing, 45# carbon steel pipes are made into hydraulic cylinder barrels (such as φ150×10mm) after quenching and tempering, and high-precision processing is used to ensure the smooth operation of the piston.

In the field of architecture and structure, carbon steel pipes are "support skeletons". In steel structure buildings, large-diameter seamless carbon steel pipes (such as φ325×8mm) are used as steel columns to bear the load of the roof; in bridge engineering, the tie arch bridge hangers made of carbon steel pipes can withstand hundreds of tons of tension; and in the scaffolding and formwork support system, φ48×3.5mm carbon steel pipes are connected by fasteners to form a flexible support system to meet the construction needs of different building forms.
5. Production process: the transformation from billet to pipe
The performance of carbon steel pipe depends on the rigorous production process. From smelting to forming, each process affects the final quality.
The production of seamless carbon steel pipe is a combination of "hot processing and cold finishing". Take 20# seamless pipe as an example: first, heat the steel billet with 0.2% carbon content to 1250℃, and use the conical piercing machine to pierce the solid billet into a hollow rough tube of φ50×8mm (the temperature of the head needs to be controlled during the piercing process to avoid cracks on the inner wall); then enter the hot rolling mill, and after 3-5 passes of rolling, the outer diameter is reduced to φ108mm, and the wall thickness is controlled at 4mm (tolerance ±0.15mm); for high-precision pipes, cold drawing process is required - the hot-rolled pipe is forcibly pulled through the die to reduce the outer diameter to φ100mm, the wall thickness accuracy is increased to ±0.05mm, and the surface roughness is reduced to Ra1.6μm; finally, normalizing treatment (heating at 920℃ and air cooling) is carried out to refine the grains and ensure that the tensile strength is stable at 410-550MPa.

The production of welded carbon steel pipes embodies the advantages of "continuity and automation". The production process of Q235 straight seam welded pipe: the 4mm thick carbon steel coil is rolled into a φ219mm round tube by a forming machine, and the seam is heated by high-frequency induction welding (current frequency 100-500kHz), and fused under pressure (weld width ≤3mm); after welding, it is cooled by water, and then the outer diameter is calibrated by a sizing machine (tolerance ±0.5mm); finally, a water pressure test is carried out (test pressure 5MPa, pressure maintenance for 30 seconds without leakage) to ensure the quality of the weld. The spiral welded pipe rolls the steel plate into a spiral shape through an inclined forming roller, and welds while forming. It can produce more than 100 meters per hour, which is suitable for mass production of large-diameter pipes.
VI. Future trends: green and high-performance upgrades
With the improvement of environmental protection requirements and industrial upgrading, carbon steel pipes are evolving towards "more durable, more environmentally friendly, and more intelligent".

Long-term anti-corrosion technology extends the service life of carbon steel pipes. 3PE anti-corrosion coating (three-layer polyethylene structure) increases the soil corrosion resistance of carbon steel pipes from 10 years to more than 30 years, and has become the standard for long-distance oil and gas pipelines; and the "zinc-aluminum alloy coating" (95% zinc + 5% aluminum) has a salt spray resistance that is twice that of ordinary galvanizing, and is used for municipal pipelines in coastal areas to reduce maintenance costs. In addition, cathodic protection technology (impressed current or sacrificial anode) combined with coating can extend the life of buried carbon steel pipes to more than 50 years, approaching the level of stainless steel pipes.

High-performance carbon steel materials expand the application boundaries. Ultrafine-grained carbon steel pipes are refined to less than 5μm through controlled rolling and controlled cooling processes, and the yield strength is increased to 460MPa (30% higher than Q345). When used in high-pressure boilers, the wall thickness can be reduced by 20%, reducing material consumption; low-alloy weather-resistant carbon steel pipes (such as Q355NH) form a dense rust layer outdoors by adding copper, chromium, nickel and other elements. The corrosion resistance is 5 times that of Q235, and it can be used for bridge guardrails without painting, realizing the environmental protection concept of "rust prevention".

Green manufacturing and recycling have become an industry consensus. Carbon steel pipes produced by short-process steelmaking (arc furnace + scrap steel) have 70% less carbon emissions than traditional long processes. Currently, the proportion of short processes in European and American countries has reached 60%; the "low-temperature rolling" technology in the rolling process reduces energy consumption by 15% by optimizing the rolling temperature; and the recycling rate of carbon steel pipes is as high as more than 95%. Waste pipes can be remade into new pipes after smelting, and the performance is almost unaffected, forming a closed loop of resource circulation.

From the underground pipeline network in the city to the production pipeline in the factory, carbon steel pipes have become the "vascular system" of industrial society with their "high strength, low cost, and easy processing" characteristics. It does not have the corrosion-resistant halo of stainless steel, but makes up for its shortcomings through anti-corrosion treatment; it does not have the complex performance of alloy materials, but adapts to most scenarios with just the right mechanical balance. In the future, with the advancement of anti-corrosion technology and green manufacturing, carbon steel pipes will further improve durability and environmental protection while maintaining cost advantages, and continue to support the stable operation of industry and people's livelihood.