Seamless steel pipe means

Seamless steel pipe means: Seamless steel pipe (SMLS) is a long strip of steel with a hollow cross-section and no welds around it. It is made of a whole solid round steel billet after heating and perforation. There is no weld on the surface of the steel pipe, so it is called seamless steel pipe. The steel pipe with seams on the surface is called welded steel pipe. According to different materials, seamless pipes can be divided into carbon steel pipes and stainless steel pipes. According to different applications, seamless pipes can be divided into boiler tubes, mechanical tubes and seamless casing pipes.

Manufacturing process:

The production and manufacturing process of seamless steel pipe is relatively complicated and the cost is higher than that of welded steel pipe. It is usually made by heating the solid billet, then perforating it, rolling out the rough pipe, and finally heat treating, pickling, straightening and other steps. In this process, the organization and performance of the steel pipe will change accordingly, so as to achieve the required performance. Compared with ordinary steel pipes, this kind of steel pipe has higher pressure bearing capacity, better sealing performance and longer service life.

Advantages of seamless steel pipe:
The advantages of seamless steel pipe are its high strength, high sealing and long life. Since the seamless steel pipe is made of a whole piece of metal, it has a tight structure, high strength, and can withstand greater pressure and impact. At the same time, the seamless steel pipe has no welds, good sealing, and is not easy to leak, which ensures the safety and stability of fluid transportation. Moreover, the seamless steel pipe undergoes precise cold drawing or hot rolling process during the manufacturing process, which makes its size precise and its surface quality good, and can meet various application scenarios with precise requirements. In addition, the seamless steel pipe has a long service life, good toughness and durability, and can operate stably for a long time in harsh environments, reducing the frequency and cost of maintenance and replacement.

Disadvantages of seamless steel pipes:
Seamless steel pipes also have some disadvantages. First, due to its complex production process and high manufacturing cost, the price is relatively high. Secondly, the specifications and sizes of seamless steel pipes are relatively limited, and may not meet some special needs. In addition, the corrosion resistance of seamless steel pipes is poor, and they are easily affected by corrosion and rust, and corresponding protective measures need to be taken, such as galvanizing and spraying anti-corrosion coating.

Uses of seamless steel pipes:

The uses of seamless steel pipes are very wide. In the energy industry such as oil and natural gas, seamless steel pipes are used as pipelines to transport energy from the place of production to the place of consumption;
In the construction industry, seamless steel pipes are used as supporting materials for structures such as bridges and high-rise buildings, and their high strength and stability ensure the safety of the buildings;
In the machinery manufacturing industry, seamless steel pipes are used as raw materials for various mechanical parts, such as bearings, gears, etc.;
In addition, seamless steel pipes are also used to manufacture military equipment such as guns and shells, as well as various products in daily life such as bicycles and furniture.

Classification of seamless steel pipes:

According to the manufacturing process, seamless steel pipes are mainly divided into hot-rolled seamless steel pipes and cold-rolled (drawn) seamless steel pipes.

Hot-rolled seamless pipes:
Hot-rolled seamless pipes, hot rolling is relative to cold rolling, cold rolling is rolling below the recrystallization temperature, and hot rolling is rolling above the recrystallization temperature. It can refine the grains of steel.

Main production process of hot-rolled seamless pipe (main inspection process):
Pipe blank preparation and inspection → Pipe blank heating → Perforation → Pipe rolling → Steel pipe reheating → Sizing (reducing) diameter → Heat treatment → Finished pipe straightening → Finishing → Inspection (non-destructive, physical and chemical, bench inspection) → Warehousing

The delivery state of hot-rolled seamless steel pipe is generally hot-rolled state and heat-treated before delivery. After quality inspection, hot-rolled seamless steel pipes must be strictly hand-selected by staff, and the surface must be oiled after quality inspection, followed by multiple cold drawing tests. After hot rolling treatment, perforation tests must be carried out. If the perforation diameter is too large, straightening and correction must be carried out. After straightening, it is transported to the flaw detector by the conveyor for flaw detection, and finally labeled, arranged in specifications, and placed in the warehouse.

Advantages:
It can destroy the casting structure of the steel ingot, refine the grains of the steel, and eliminate the defects of the microstructure, so that the steel structure is dense and the mechanical properties are improved. This improvement is mainly reflected in the rolling direction, so that the steel is no longer isotropic to a certain extent; bubbles, cracks and looseness formed during pouring can also be welded under high temperature and pressure.

Disadvantages:
1. After hot rolling, the non-metallic inclusions (mainly sulfides and oxides, as well as silicates) inside the steel are pressed into thin sheets, resulting in stratification (interlayer) phenomenon. Stratification greatly deteriorates the tensile performance of the steel along the thickness direction, and interlayer tearing may occur when the weld shrinks. The local strain induced by weld shrinkage often reaches several times the yield point strain, which is much larger than the strain caused by the load.

2. Residual stress caused by uneven cooling. Residual stress is the internal self-balanced stress without external force. Hot-rolled steel sections of various cross-sections have this type of residual stress. Generally, the larger the cross-sectional size of the steel section, the greater the residual stress. Although the residual stress is self-balanced, it still has a certain impact on the performance of steel components under external forces. For example, it may have an adverse effect on deformation, stability, fatigue resistance, etc.

3. For hot-rolled steel products, it is difficult to control the thickness and edge width. As we all know, thermal expansion and contraction occur. Even if the length and thickness meet the standards at the beginning, there will still be a certain negative difference after cooling. The wider the edge width and the thicker the thickness, the more obvious this negative difference is. Therefore, for large-sized steel products, the edge width, thickness, length, angle, and edge line of the steel cannot be too precise.

Cold-rolled (drawn) seamless pipe:
Cold rolling refers to the processing of steel plates or steel strips into various types of steel at room temperature through cold processing such as cold drawing, cold bending, and cold drawing.

Main production process of cold-rolled (drawn) seamless pipe:
Blank preparation → pickling and lubrication → cold rolling (drawing) → heat treatment → straightening → finishing → inspection

The production process of cold-rolled seamless tube is generally more complicated than hot rolling. The pipe blank must first be rolled with three rollers, and then sizing test must be carried out after extrusion. If there is no response crack on the surface, the round pipe must be cut by a cutting machine and cut into a billet of about one meter in length. Then enter the annealing process. Annealing should be pickled with acidic liquid. When pickling, pay attention to whether there are a lot of bubbles on the surface. If there are a lot of bubbles, it means that the quality of the steel pipe does not meet the corresponding standards. 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 rough, and the diameter does not have too many burrs.

Advantages:
Fast forming speed, high output, and no damage to the coating. It can be made into a variety of cross-sectional forms to meet the needs of use conditions; cold rolling can cause a large plastic deformation of the steel, thereby increasing the yield point of the steel.

Disadvantages:

1. Although there is no hot plastic compression during the forming process, there is still residual stress in the cross section, which will inevitably affect the overall and local buckling characteristics of the steel. 

2. The cold-rolled steel section style is generally an open section, which makes the free torsional stiffness of the section low. It is easy to torsion when bent, and bending-torsion buckling when compressed, and the torsional performance is poor.

3. The wall thickness of cold-rolled steel is relatively small, and there is no thickening at the corners where the plates are connected, so its ability to withstand local concentrated loads is weak.

Read more: Seamless Pipe Raw Material or Schedule 40 Seamless Pipe