A steel pipe without seams along the perimeter of its cross-section. According to different production methods, it is divided into hot-rolled pipe, cold-rolled pipe, cold-drawn pipe, extruded pipe, pipe jacking, etc., all of which have their own process regulations.
The materials are ordinary and high-quality carbon structural steel (Q215-A~Q275-A and 10~50 steel), low alloy steel (09MnV, 16Mn, etc.), alloy steel, stainless acid-resistant steel, etc.
According to the use, it is divided into two categories: general use (for water, gas pipelines and structural parts, mechanical parts) and special (for boilers, geological exploration, bearings, acid resistance, etc.).

Application of seamless steel pipe
Seamless steel pipes are widely used.
1. General-purpose seamless steel pipes are rolled from ordinary carbon structural steel, low-alloy structural steel or alloy structural steel, with the largest output, and are mainly used as pipelines or structural parts for conveying fluids.
2. According to different uses, there are three types of supply:
a. Supply according to chemical composition and mechanical properties;
b. Supply according to mechanical properties;
c, according to the hydraulic test supply. Steel pipes supplied according to categories a and b, if they are used to withstand liquid pressure, shall also undergo a hydrostatic test.
3. There are many kinds of seamless pipes for special purposes, such as seamless pipes for boilers, seamless pipes for chemical power, seamless pipes for geological use and seamless pipes for petroleum.
The seamless steel pipe has a hollow section and is widely used as a pipeline for transporting fluids, such as pipelines for transporting oil, natural gas, gas, water and certain solid materials. Compared with solid steel such as round steel, the steel pipe is lighter in weight when the bending and torsional strength is the same, and is an economical section steel.
It is widely used in the manufacture of structural parts and mechanical parts, such as oil drill pipes, automobile transmission shafts, bicycle frames, and steel scaffolding used in construction. The use of steel pipes to manufacture annular parts can improve material utilization, simplify manufacturing processes, and save materials and processing. Man-hours have been widely used to manufacture steel pipes.

Production Process
①The main production process of hot-rolled seamless steel pipe (△main inspection process):
Tube blank preparation and inspection△→tube blank heating→piercing→tube rolling→steel reheating→fixed (reduced) diameter→heat treatment△→finished pipe straightening→finishing→inspection△ (non-destructive, physical and chemical, Taiwan inspection)→warehousing
②The main production process of cold-rolled (drawn) seamless steel pipe:
Billet preparation→pickling lubrication→cold rolling (drawing)→heat treatment→straightening→finishing→inspection
The production process of general seamless steel pipes can be divided into two types: cold-drawing and hot-rolling. The production process of cold-rolled seamless steel pipes is generally more complicated than that of hot rolling. In the sizing test, if the surface does not respond to cracks, the round tube shall be cut by a cutting machine and cut into a billet with a length of about one meter. Then enter the annealing process, annealing should be pickled with acidic liquid, and pay attention to whether there is a lot of blistering on the surface during pickling. In appearance, the cold-rolled seamless steel pipe is shorter than the hot-rolled seamless steel pipe. The wall thickness of the cold-rolled seamless steel pipe is generally smaller than that of the hot-rolled seamless steel pipe, but the surface looks brighter than the thick-walled seamless steel pipe, and the surface is not too much. Much rough, and the caliber doesn’t have too many burrs.
The delivery state of hot-rolled seamless steel pipe is generally that the hot-rolled state is delivered after heat treatment. After the quality inspection, the hot-rolled seamless steel pipe must be strictly hand-selected by the staff. After the quality inspection, the surface should be oiled, followed by several cold-drawing experiments. After the hot-rolling treatment, the perforation experiment should be carried out. If the diameter of the perforation is too large, it should be straightened and corrected. After straightening, it will be sent to the flaw detector by the conveyor for flaw detection experiment, and finally labelled and arranged in specifications, and then placed in the warehouse.
Round tube blank→heating→piercing→three-roll skew rolling, continuous rolling or extrusion→tube removal→sizing (or reducing)→cooling→straightening→hydrostatic test (or flaw detection)→marking→storage seamless steel pipe It is made of steel ingots or solid tube blanks through perforation to make capillaries, and then hot-rolled, cold-rolled or cold-drawn. The specifications of seamless steel pipes are expressed in millimeters of outer diameter * wall thickness.
The outer diameter of hot-rolled seamless pipes is generally greater than 32mm, and the wall thickness is 2.5-200mm. The outer diameter of cold-rolled seamless pipes can reach 6mm and the wall thickness can reach 0.25mm. The outer diameter of thin-walled pipes can reach 5mm. Rolling has higher dimensional accuracy than hot rolling.
Generally, seamless steel pipes are made of 10, 20, 30, 35, 45 and other high-quality carbon steel 16Mn, 5MnV and other low-alloy structural steel or 40Cr, 30CrMnSi, 45Mn2, 40MnB and other combined steel hot-rolled or cold-rolled. Seamless pipes made of low carbon steel such as 10 and 20 are mainly used for fluid transportation pipelines. 45, 40Cr and other medium carbon steel seamless pipes are used to manufacture mechanical parts, such as the stressed parts of automobiles and tractors. Generally, seamless steel pipes are used to ensure the strength and flattening test. Hot-rolled steel pipes are delivered in hot-rolled or heat-treated state; cold-rolled steel pipes are delivered in heat-treated state.
Hot rolling, as the name suggests, has a high temperature of the rolled piece, so the deformation resistance is small, and a large amount of deformation can be achieved. Taking the rolling of steel plate as an example, the thickness of continuous casting slab is generally about 230mm, and after rough rolling and finishing rolling, the final thickness is 1~20mm. At the same time, due to the small width-to-thickness ratio of the steel plate, the dimensional accuracy requirements are relatively low, and the plate shape problem is not easy to occur, and the main control is to control the crown. For those with organizational requirements, it is generally realized by controlled rolling and controlled cooling, that is, controlling the opening temperature and final rolling temperature of finishing rolling. Round tube billet → heating → perforation → heading → annealing → pickling → oiling (copper plating) → Multi-pass cold drawing (cold rolling) → billet tube → heat treatment → straightening → hydraulic test (flaw detection) → marking → storage.