Pre-bending is a major step LSAW steel pipe production line, and its purpose is to complete both sides of the plate pre-bending, bending radius on both sides of the steel sheet at or near the radius of the tube production specifications, thus ensuring the pipe weld area geometry and dimensional accuracy. practice shows that if there is no pre-bent in the UOE molding, even if the O-press molding had a compression ratio of about 1%, steel edges will still be straight section, resulting molded the tube was beak "peach", the follow-up to the pre-weld seam trouble, can lead to the wrong side or welding burn, even after expanding, it can not be eliminated, and also damage the enlarged diameter head. In JCO molding (including PFP molding) method, if there is no pre-bending process, steel "Peach" is more serious.

Pre-bending quality is on both sides of the steel sheet bending quality, including: ①after pre-bending bending radius edge of the plate reaches a radius pipe; ② both sides of the curved equal to the width consistent degree of bending, symmetrical parallel, parallelism within a predetermined range; ③ within the scope of the pre-bending radius of curvature of each point in a predetermined, and everywhere along the board edge in the same direction; ④ pre-bent plate edge does not exist after the chicane, there was no wave shape, straightness within a predetermined range.

Pre-bending width pre-bend width depends on steel grade pipe diameter, wall thickness and base material. According to the principle of involute plate edges (straight edge to remove a plate thickness) should be close to the radius of curvature of the pipe radius, taking into account the rebound plate, that is, after the rebound plate edge radius of curvature at the close pipe radius, so the pre-bent shape of the mold width should correspond. After the mold is determined (ie involute determined), if the pre-bending width is increased, the volume pre-bending angle increases, the board edge closer to the mold involute base circle, the radius of curvature of the edge pre-bending is reduced, that is, flange excess; on the contrary, is crimping amount is not enough. Practice shows that the excessive pre-bent, on the subsequent pressure molding adversely affected, but also affect the pre-welding step; pre-bent enough, the formation of "peach" pipe.

Pre-bending roll angle corresponding to the width of the pre-bending angle, which is similar to involute pressure angle. Pre-bending roll angle and width as pre-bent lsaw steel pipe forming an impact on, for each deputy dies, they are one to one.