3. Direct Forming Technology and Its Development
The “circular-to-square” technology is adopted earlier than "directly forming" technology in Chinese enterprises. In 1950s and 1960s, the production of square rectangular tubes was realized by the straightening part with fixed diameter of the high frequency welded pipe production line. In 1980s, the PR700/8 cold bending section steel unit imported by the Guangzhou Special-shaped Steel Plant was the largest cold bending section steel unit in China at that time. In 1992, the PR1100/12.7 cold bending section steel unit was introduced in Hankou Steel Rolling Mill of WISCO (Wuhan Iron and Steel Company), it was also the largest cold bending section steel unit in China at that time. These two lines were brought from the Voestalpine. The unit adopts direct forming technology to produce closed square rectangular tubes.
The direct forming technology of our country comes from the two production lines imported from the Voestalpine. People such as Jiang Fusheng and Wang Daqi have made an in-depth study of the main technical performance and new technologies of the imported units. Engineering technicians in Guangzhou and Hankou have been continuously improving the introduced units and analyse from the forming bending method to the specific process parameters in detail. The direct forming process has been further improved, and the product range and unit performance have been expanded through technical transformation.
After the Tianjin Huaihai Cold Bending Company absorbed the direct forming technology, the old unit was reformed in 1997. The 165*4.5 horizontal frame of the unit of the stone shaft was changed from the original first 7 then 5 to first 10 then 4, and the side roll and the hoist roll were added according to the process needs, and the rolling speed was reduced from 65 m/min to 43 m/min at the same time. After upgrading, the maximum specification of the direct forming unit is upgraded from F140*4.5 to F200*8.
Because of the superiority of the direct forming process, it has been widely applied in the production enterprises of square rectangular tubes in China. There are many equipment manufactories such as Shijiazhuang Bearing Equipment Factory, Shijiazhuang Furyiwo, Nanjing Zhongqing and Dalian SAGE that have developed direct forming units with multi specifications. The maximum specification can produce 500X500 square rectangular tubes.
The main square rectangular tube production plants include Shanghai Jiayi, Hankou rolling mill, Shanghai Iron and Steel Technology Research Institute, Tianjin Huaihai, Tai'an Kenuo and so on. Among them, Shanghai Jiayi, Hankou rolling mill, Tianjin Huaihai and Guangzhou Steel Tubeswere selected as excellent cold bending enterprises by the Chinese Confenfederation of Roll Forming Industry. Many enterprises export large quantities of square rectangular tubes, forming strong international competitiveness.
On the basis of summing up the experience of imported equipment, Chinese engineers and technicians put forward a complete set of roll type design methods: the main characteristics analysis of the direct forming process, the basic conditions of the roll type design, the calculation method of the blank width, the determination of the forming path, the determination of the deformation of each forming channel, the figure of the deformation roll, the roll drawing of each road, the welding part: design of the guide roller, extrusion roller and export roller and design of shaping roller and shaping vertical roller. The method of bending straight line pre bending is proposed to improve the empty bending angle of the direct forming process.
The production enterprises of square rectangular tube cooperated with the equipment manufacturing plant, and successively conquered some technologies such as spiral sleeve, automatic shearing butt welding, four roll forming frame, high frequency welding, removal of internal and external welding burrs, milling and tracking saw, and automatic palletizing. These technologies are applied in practical production, and the capacity and technical level of the square rectangular tube in our country have been greatly improved.
In terms of the varieties of square rectangular tubes, the production of large-sized square rectangular tubes with thin wall or thick wall and thick square tubes with minor-caliber is realized. Take Shanghai JIaleng section steel as an example, it has been able to produce thousands of specifications on 25X25~300X300 and 30X20~400X200.
Shanghai Jialeng section steel has formed a series of patent technology in the technology development of square rectangular tube: square tubes with thick wall and minor-caliber, which realizes the production of thick wall tube with the ratio of wall thickness to the side length of t/h >10%, the processing method of the empty sharp corner diamond tube, flexible application of square-to-square and circular-to-square compound technology to realize the development of the special-shaped tube. In terms of equipment. As for the equipment, there are 4 roll universal rack, split rolling mill and so on.