For the production of large-sized square and rectangular tube, there are four processes:
1. Flat tailor welded blank technology
The large size rectangular tube is four tailor-welded from steel plate. The advantage is that can save a bending process, and not the wall thickness restrictions. The disadvantage is slow speed of production, unstable quality, low intensity, and have four weld. At present, the production process has been rarely used.
2. The first round behind the technology of the high frequency resistance welding (ERW)
This is a basic process for production of square and rectangular steel pipe, high frequency resistance welding process on the basis of unit production tube, the addition of a rectangular pass on the sizing mill, circular pipe cooling and processing prescription rectangular pipe. The process has the advantage of high dimensional accuracy, the 90 ° corner wall thickness increased slightly. About the shortcomings, the literature indicates that: the process of change from round the side, the roll force is always focused on the point of stress concentration. Often in wall thickness (12.7mm) profile bending process of micro-cracks, even when serious fracture. In addition, when the production of small quantity and variety, the number of roll changing.
3. Direct prescription process of the high frequency resistance welding (ERW)
The process that is molding unit will strip directly bending Narikata rectangular tube. The advantage is that to effectively solve the corner of the thick-walled high-strength products, micro-crack problem at the same time, in the production of small quantities of many varieties to change the roll less frequently; drawback is angled at 90 ° wall thickness thinning.
4. Submerged arc welding (SAW) channel on the button tailor welded blank technology
The process is bent into coils "[" shape, and then use submerged arc welding (internal and external welding) and two "[" shaped butt dual weld square rectangular pipe (see Figure 4). The molding process has two characteristics: First, can in square rectangle inside of the tube and the plate (ribs); two section to 200mm x 200mm to 1000mm x 1000mm range, breaking the high-frequency resistance welding (ERW) weld the pipe section can only reach the limitations of 550mm × 550mm. However, its shortcomings should not be overlooked: First, double weld process offline using the butt weld and post weld heat treatment combined with the correction process is more complex; at the same time, due to the pre-still roll forming, so the maximum wall thickness of no more than 20mm; and two weld on the center section, for the force makes weld to withstand the shear force is very unfavorable.