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Steel pipes have a hollow cross-section and are widely used as pipelines for transporting fluids, such as oil, natural gas, gas, water, and certain solid materials. Since a circle has the largest area for a given perimeter, circular tubes can transport more fluid. Furthermore, a circular cross-section distributes forces more evenly when subjected to internal or external radial pressures; therefore, the vast majority of steel pipes are circular in shape. However, round tubes also have certain limitations; for example, under conditions of planar bending, their bending strength is lower than that of square or rectangular tubes. Square and rectangular tubes are therefore commonly used in the frames of agricultural machinery and in steel-wood furniture. I. Seamless steel pipes can be further divided into hot-rolled (extruded) seamless steel pipes and cold-drawn (rolled) seamless steel pipes, depending on their manufacturing processes. Cold-drawn (rolled) tubes are further divided into round tubes and shaped tubes. 1. Overview of the manufacturing process: (1) Hot rolling (extrusion of seamless steel tubes): Round tube billet → Heating → Piercing → Three-roller skew rolling, continuous rolling or extrusion → Tube removal → Dimensioning (or reduction in diameter) → Cooling → Billet tube → Straightening → Hydrostatic testing (or flaw detection) → Marking → Storage. The raw material for rolling seamless tubes is a round tube billet, which is cut and processed before being sent to a furnace for heating. The billet is fed into a furnace for heating, to a temperature of around 1200 degrees Celsius. After being taken out of the furnace, the round tube billet is pierced using a pressure piercing machine. The inner diameter of the steel pipe is determined by the outer diameter of the sizing machine drill bit. After sizing, the steel pipe is cooled by water spraying, and then straightened after cooling. After straightening, the steel pipe is subjected to flaw detection (or hydrostatic testing). After quality inspection, the steel pipes must also undergo strict manual selection. After quality inspection, the steel pipes are sprayed with paint to indicate the number, specifications, production batch number, etc. and hung it in the warehouse. (2) Cold-drawn (rolled) seamless steel pipes: Round tube billet → Heating → Piercing → End forming → Annealing → Acid washing → Oil coating (copper plating) → Multiple rounds of cold drawing (cold rolling) → Semi-finished tube → Heat treatment → Straightening → Hydrostatic testing (flaw detection) → Marking → Storage. The rolling method for cold-drawn (rolled) seamless steel tubes is similar to that of hot-rolled tubes; the first three steps of their production processes are essentially the same. The differences begin from the fourth step: after the round tube billet is hollowed out, it needs to be end-formed and annealed. Pickling is carried out after annealing. After pickling, apply oil. Then, following that, there are multiple rounds of cold drawing (cold rolling), along with specialized heat treatment. After heat treatment, it needs to be straightened. After straightening, the steel pipe is subjected to internal flaw detection. After quality inspection, the steel pipes must also undergo strict manual selection. After quality inspection, the steel pipes are sprayed with paint to indicate the number, specifications, production batch number, etc. And it is lifted into the warehouse by a crane. 2. Seamless steel pipes are further divided into several types depending on their applications: (1) GB/T8162 seamless steel pipes for structural use, which are primarily used in general structures and mechanical assemblies. Its representative materials (grade numbers): carbon steel, steel grades 20 and 45 ； Alloy steels such as Q345, 20Cr, 40Cr, etc. (2) GB/T8163 Seamless steel tubes for transporting fluids are mainly used as pipelines for transporting fluids in engineering projects and large-scale equipment. The representative materials (grade numbers) are 20, Q345, etc. (3) GB3087 seamless steel tubes for low and medium pressure boilers are high-quality carbon structural steel hot-rolled and cold-drawn (rolled) seamless tubes used for manufacturing superheater steam tubes, feedwater tubes in various low and medium pressure boilers, as well as superheater steam tubes, large flue tubes, small flue tubes, and arch brick tubes in locomotive boilers. The representative materials are steel grades 10 and 20. (4) GB5310 seamless steel tubes for high-pressure boilers are high-quality carbon steel, alloy steel, and stainless heat-resistant steel seamless tubes used to manufacture the heating surfaces of water-tube boilers operating at high pressures and above. Representative materials include 20G, 12Cr1MoVG, 15CrMoG, etc. (5) GB6479 seamless steel tubes for high-pressure fertilizer equipment are high-quality carbon structural steel and alloy steel seamless tubes suitable for use in chemical equipment and pipelines operating at temperatures ranging from -40 to 400°C and under pressures of 10 to 30 Ma. The representative materials include 20, 16Mn, 12CrMo, 12Cr2Mo, etc. (6) GB9948 seamless steel tubes for petroleum cracking are mainly used in the boilers, heat exchangers, and fluid transport pipelines of petroleum refineries. The representative materials include 20, 12CrMo, 1Cr5Mo, 1Cr19Ni11Nb, etc. (7) YB235 steel pipes for geological drilling are used by geological agencies for core drilling; according to their purpose, they can be classified into drill rods, drill collars, core tubes, casings, and settling tubes, etc. (8) GB3423 seamless steel tubes for diamond core drilling are seamless steel tubes used for drill pipes, core rods, and casings in diamond core drilling. (9) YB528 oil drilling pipes are seamless steel pipes with thickened inner or outer walls, used for oil drilling. Steel pipes are divided into two types: those with threads and those without threads. Pipes with threads are connected using joints, while pipes without threads are joined using welding methods and tools. (10) GB5312 Seamless carbon steel and carbon-manganese steel pipes for ships are carbon steel seamless pipes used for manufacturing Class I and Class II pressure piping systems on ships, as well as for boilers and superheaters. The operating temperature of the wall of carbon steel seamless pipes does not exceed 450°C, while that of alloy steel seamless pipes exceeds 450°C. The representative materials include steel grades such as 360, 410, and 460. (11) GB3088 seamless steel tubes for automobile half-shaft sleeves are hot-rolled seamless steel tubes made of high-quality carbon structural steel and alloy structural steel, used for manufacturing automobile half-shaft sleeves as well as the shaft tubes in drive axle housings. (12) GB3093 high-pressure fuel pipes for diesel engines are cold-drawn seamless steel pipes used to manufacture high-pressure pipes in diesel engine injection systems. (13) GB8713 Precision inner-diameter seamless steel tubes for hydraulic and pneumatic cylinder barrels are cold-drawn or cold-rolled precision seamless steel tubes with precise inner-diameter dimensions, used for manufacturing hydraulic and pneumatic cylinder barrels. (14) GB3639 cold-drawn or cold-rolled precision seamless steel tubes are those used in mechanical structures and hydraulic equipment, featuring high dimensional accuracy and good surface finish as a result of cold drawing or cold rolling processes. (15) GB/T14975 Stainless steel seamless tubes for structural use are hot-rolled (extruded, expanded) and cold-drawn (rolled) seamless tubes made of stainless steel, which are widely used in industries such as chemicals, petroleum, light textiles, medicine, food, and machinery as corrosion-resistant pipes, structural components, and parts. (16) GB/T14976 Stainless steel seamless pipes for fluid transport are hot-rolled (extruded, expanded) and cold-drawn (rolled) seamless pipes made of stainless steel, used for transporting fluids. (17) GB18248 seamless steel tubes for gas cylinders are mainly used in the manufacture of various gas and hydraulic cylinders. The representative materials include 37Mn, 34Mn2V, 35CrMo, etc. (18) API SPEC5CT, the specification for casings and tubing, was developed and published by the American Petroleum Institute (abbreviated as “API”) and is used worldwide. Among them: Casing: Tubes that extend from the ground surface into the well, serving as a lining for the well walls, with the tubes being connected to each other using couplings. The main material grades include J55, N80, P110, etc., as well as steel grades resistant to hydrogen sulfide corrosion such as C90 and T95. Its lower steel grades (J55, N80) can be used for welded steel pipes. Oil pipe: A pipe that is inserted into the casing from the surface of the ground all the way to the oil layer, with the pipes being connected to each other either through couplings or in an integral manner. Its function is then for the pump to transport the oil from the oil layer to the surface through the oil pipes. The main materials are steel grades such as J55, N80, P110, as well as C90 and T95, which are resistant to hydrogen sulfide corrosion. Its lower steel grades (J55, N80) can be used for welded steel pipes. API SPEC 5L-2000 (Pipeline Steel Specification) was developed and published by the American Petroleum Institute, and is widely used around the world. Piping pipes: They are used to transport oil, gas, or water that emerges from the ground to petroleum and natural gas processing plants through pipelines. Piping pipes come in two types: seamless and welded pipes. Their ends can be flat, threaded, or socketed; the methods of connection include end welding, flange coupling, and socket fitting. The main materials for this pipe are steel grades such as B, X42, X56, X65, and X70. In addition, there are: GB/T17396 – Hot-rolled seamless steel tubes for hydraulic struts; GB3093 – High-pressure seamless steel tubes for diesel engines; GB/T3639 – Precision seamless steel tubes produced by cold drawing or cold rolling; GB/T3094 – Cold-drawn seamless steel tubes and shaped tubes; GB/T8713 – Precision seamless steel tubes with precise inner diameters for hydraulic and pneumatic cylinders; GB/T14975 – Stainless steel seamless tubes for structural use. II. Welded steel tubes: The raw materials used for welded steel tubes are steel plates or strips. Depending on the welding process, they are classified into furnace-welded tubes, electrically welded (resistance-welded) tubes, and automatically arc-welded tubes. Depending on the welding method, it is divided into straight-seam welded pipes and spiral-welded pipes. Based on their end shape, they are further divided into round welded pipes and shaped welded pipes (square, flat, etc.). Welded pipes are classified into several types depending on their material and application: (1) GB/T3091 galvanized welded steel pipes for low-pressure fluid transport, which are mainly used for transporting fluids under relatively low pressure such as water, gas, air, oil, as well as hot water or steam for heating, and for other purposes. Its representative material is Q235A grade steel. (2) GB/T3092 Galvanized welded steel pipes for low-pressure fluid transport are primarily used for transporting fluids under relatively low pressures such as water, gas, air, oil, and heating water or steam, as well as for other applications. Its representative material is Q235A grade steel. (3) GB/T14291 Welded steel pipes for transporting fluids in mining applications – these are straight-seam welded steel pipes primarily used for air compression, drainage, and gas drainage in mines. Its representative materials are Q235A and grade B steel. GB/T14980-1994 (Large-diameter electric welded steel pipes for low-pressure fluid transport). It is mainly used for transporting low-pressure fluids such as water, sewage, gas, air, and heating steam, as well as for other purposes. Its representative material is Q235A grade steel. (4) GB/T12770 Welded stainless steel pipes for mechanical structures are mainly used in machinery, automobiles, bicycles, furniture, decoration for hotels and restaurants, as well as other mechanical components and structural elements. The representative materials include 0Cr13, 1Cr17, 00Cr19Ni11, 1Cr18Ni9, 0Cr18Ni11Nb, etc. (5) GB/T12771 Stainless steel welded pipes for fluid transport are mainly used for transporting low-pressure corrosive media. The representative materials are 0Cr13, 0Cr19Ni9, 00Cr19Ni11, 00Cr17, 0Cr18Ni11Nb, and 0017Cr17Ni14Mo2.