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Cold bending structure welded pipe production technology characteristics

1. Basic production process of cold-formed square and Rectangular Welded Pipe

In the current cold-formed square welded pipe production line, the most representative production process is as follows:

Hot rolling coil → uncoiler → straightener → head and tail cutting machine → head and tail butt welding machine → spiral looper → pre forming unit → forming unit → high frequency welding machine → deburring machine → air cooling section → → water cooling section → sizing machine → milling and sawing → cross shifting and stacking → packaging → warehousing and stacking.

2 main technical features of cold-formed square and Rectangular Welded Pipe Production

The key technologies of cold-formed square welded pipe production mainly include forming technology, high-frequency welding technology, cutting technology and rapid roll change technology, which directly affect the quality and production efficiency of the cold-formed square welded pipe.

1) forming technology

From the production technology of cold-formed section steel, most of the square and rectangular tubes produced in foreign countries adopt the round to square technology. In the previous production, the tube was first cold bent into a circle, then heated to the normalizing temperature, and then deformed into a square rectangular tube in the sizing process. The advantages of this process are that the residual stress of the finished steel tube is small, the arc radius of the corner is small, but the energy consumption is high, the cost is high, and the surface quality is poor. At present, only Britain is still in production. At present, the cold round forming process is generally used in foreign countries. Compared with the roll bending direct forming process commonly used in China, the advantages of cold round forming process are high dimensional accuracy of products, good weld quality, sharp corner and regular shape. The disadvantages are large construction investment and heavy corner hardening.

In the process of forming, there are roll bending forming and roll arranging forming. The main difference between them is that in the pre forming stage, the following production processes are not different. Roll forming is a traditional forming process. It is cold formed by a series of matching roll designs on the roll in the pre forming stage, and then sizing. Roll forming is an advanced cold-formed square welded pipe forming technology invented by a Chinese American process designer. It is the patent technology of Yoder company in the United States. Now, the main process design and production unit manufacturer of cold-formed square welded pipe in the world adopt this forming technology in the new production line. In the production line, it passes the preliminary roll bending pre forming of 2-3 frames first, and then uses a certain number of small rolls and sectional roll design to form the roll arrangement, reducing the speed difference, reducing the wear of the roll, reducing the power loss and reducing the product scratch. The biggest characteristic of the roll forming machine is that it does not need to change or recombine the roll to change the specification. Through simple adjustment, the product specification can be changed and a lot of roll changing time can be saved.  Flexible debugging is conducive to the improvement of the yield and the shortening of debugging time; without changing the mold, the closed section steel such as the producer and the rectangular tube can be transformed into the open section steel such as C steel and channel steel. In the production of thick heavy section steel, it can show obvious advantages. The product cost of equipment production will be the lowest, the production batch will not be limited, and the utilization efficiency will be improved.

2) high frequency welding technology

High frequency welding can be divided into induction welding and contact welding according to the contact with pipe or not. Compared with induction welding, the efficiency of contact welding can be increased by 50%. However, the contact needs to be replaced frequently, and the pipe is required to be well formed, otherwise it is easy to spark, and the weld is prone to scratch, especially the disadvantage is that there will be copper residue at the weld, and the welded pipe is prone to corrosion after a long time. The quality of induction welding is good, but the welding power required for the same size of welded pipe is nearly twice as high. Equipment investment is 50% - 55% higher.

High frequency welding machine can be divided into vacuum tube type and solid-state type (transistor). Compared with vacuum tube type, solid-state type has small size and high efficiency (15% higher), but its reliability and maintainability are not as good as vacuum tube type. In addition, compared with vacuum tube, its price is 15% higher. The new large-scale cold-formed square tube production line basically adopts solid-state high-frequency welding machine. Now the solid-state high-frequency welding machine technology develops very fast, such as the famous EFD high-frequency welding machine, which has the following latest technical features:

The EFD high frequency welding machine adopts a new and unique weldac G2 type electronic matching system, which makes the energy conversion rate as high as 85% ~ 87%. Because of the diode rectifier technology, the power factor can be constant as 0.95, without power factor compensation. Frequency range: 50-500kmz. It adopts the independent frequency conversion module design, the rectifier and the frequency converter are connected by a long cable, which can match different loads continuously, and has the remarkable characteristics of high efficiency, energy saving and low operation cost. The EFD high frequency induction (maximum induction welding power 700kW) and contact welding dual function welding machine are used in the newly built 200 mm × 200 mm ~ 450 mm × 450 mm / 500 mm × 200 mm square rectangular pipe production line of Baosteel Iron and Steel Technology Research Institute.

3) cutting technology of welded pipe

The cold bending welded pipe needs to be cut according to the requirement of fixed length after sizing. The traditional cutting method is circular saw, which has low production cost and high production efficiency. However, there are many burrs on the end face of the welded pipe after the circular saw cutting, and the cutting deformation is large. The connection between the two welded pipes is not good when the user constructs and uses them, so the product cannot meet the requirements of European, American and Japanese standards for product quality. After the 1980s, the manufacturers of Cold-Formed Welded Pipe equipment used synchronous imitative flying saw (equivalent to profiling milling) in the newly-built production line of Cold-Formed Welded pipe. Two saw blades were used to cut the steel pipe at the same time. The running track of each saw blade just cut off half of the pipe cuts. When the varieties were changed, the running track of the saw blade could be based on the pipe's

Cold bending welded pipe manufacturers need to select sawing equipment according to different product positioning and user requirements.

4) rapid roll change technology

The traditional welded pipe production line needs 8-12 hours to change the roll of the whole line. According to the requirements of users and the needs of competition, SMS company, a famous cold bending welded pipe manufacturer, and Abbey company in the United States have developed the rapid roll change technology successively. With the rapid roll change system, the roll change required for the variety change can be completed in 90 minutes by the roll change car, greatly reducing the production preparation. Time, thus greatly improving the design capacity of the production line. At the same time, through the application of computer, the roller position adjustment has memory. When the product is changed, the roller position can be adjusted quickly and automatically, reducing the time and error of manual adjustment of roller position, and improving the accuracy of the product.

In addition to the above typical technical features of Cold-Formed Welded pipe production, some of the new cold-formed section steel production lines adopt URD equal rigidity forming and welding frame. The frame can bear similar rolling force in horizontal and vertical directions at the same time, the rigidity is equal, the product precision is higher, and the roll system of the frame can be moved out horizontally quickly when changing the roll; some production lines also adopt a A new kind of internal burr removing device is developed. The feature of this device is that it can not only remove the burr but also break the burr, so as to protect the saw blade of the fixed length milling saw. After deburring, the height tolerance of the inner surface of the weld can reach ± 0.2mm.

5) production technology applied in cold-formed square and rectangular pipe production line in China

At present, the manufacturers of cold-formed square rectangular pipes in our country mainly use the production technology developed by ourselves. Wuxi Yulong steel pipe plant has two square and rectangular pipe production lines, one is 400mm × 400mm square and rectangular pipe production line, which is an old-fashioned direct roll rolling continuous unit. The welded pipe is sawed with a circular seat. The products produced have quality problems such as uneven shear section and poor weld quality, and the products can not meet the requirements of American Standard, Japanese standard and other advanced cold bending square and rectangular pipe product standards. The production line is 250mm × 250mm × 8mm (max) square tube row roll production line, which adopts CFSR row roll forming production technology. When the product specification is changed, the roll does not need to be replaced, and the production efficiency is high. However, the saw cutting of welded pipe also uses a disc stand, and the quality of welded pipe end is still poor. Shanghai Fang horticultural Steel Co., Ltd. is a production line of 250mm × 250mm × 8mm (max) square tube roll. Baosteel Iron and Steel Technology Research Institute 200mm × 200mm ~ 450mm × 450mm / 500mm × 200mm square rectangular tube production line adopts CFSR square, rectangular and round tube row roll forming technology of Dalian Fudi company. The production line speed is 10 ~ 30m / min, when the wall thickness is 4 ~ 10mm, it is 15 ~ 30m / min, when the wall thickness is 10 ~ 19mm, it is 10 ~ 15m / min, and the product specification is 200 × 200 ~ 450 × 450 square tube. The maximum side length of rectangular tube is 500mm, the minimum side length is 200mm, and the rectangular tube is 500mm. The maximum width height ratio of the pipe is 2:1, the thickness is 4-19MM, the maximum thickness side ratio is 1:15-1:50 (the ratio of the wall thickness to the small side length of the rectangular pipe), and the fixed length is 6-16m.

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