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　　Forming method of seamless steel pipe

　　1. Solid bend

　　Real bending, as the name suggests, is to compact the bending. In real bending, the inner and outer rollers and the inner and outer walls of the tube blank are compacted in both directions.

　　1) The advantages of real bending are small rebound, accurate forming, and as long as the roll shape is accurate, the R of internal angle forming is more accurate.

　　2) The disadvantage of solid bending is that it has tensile / thinning effect. First, the actual bending will make the bending place stretch, and the stretching effect will shorten the longitudinal length of the bending line; Second, the metal at the real bend will become thinner due to stretching.

　　2. Empty bend

　　Empty bending is the bending moment formed by the unidirectional contact between the outer roll and the outer wall of the tube blank, which causes the bending line to compress. The compression effect causes the bending line to elongate longitudinally, and the metal at the bending place to accumulate and thicken. This is the compression / thickening effect of empty bending.

　　1) The advantage of empty bending is that it can be used to bend the side length when the real bending cannot be carried out, such as synchronous bending and finishing of the top / side of rectangular tube. Hollow bending can also bend the inner angle of R < 0.2T without fracture of pipe wall.

　　2) The disadvantage of empty bending is that when the upper / side synchronous empty bending occurs, the forming force is easy to exceed the critical point due to the pressure generated by the upper roll and the lower roll at the same time, resulting in the edge instability and concave, which will also affect the stable operation of the unit and the forming quality. This is also the different characteristics of square tube and round tube in hollow bending.

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