The Forming Method of Square Steel Tube

The way of forming round tube is called bending, and the way of forming square tube is called buckling. There are two basic buckling methods: compact buckling and empty buckling.
　
1. Compact buckling

Compact buckling, just as the name implies, is the compaction of the buckling.

1) Advantages of compact buckling: accurate molding. As long as the roll is accurate, R of the inner angle is more accurate.
2) Disadvantage of compact buckling is that it has a stretching/thinning effect. First, compact buckling will cause the buckling place to stretch, and the stretching effect will shorten the longitudinal length of the buckling line. Second, metal gets thinner when it is buckled at the compact buckling place.

2. Empty buckling

Empty buckling is caused by unidirectional contact between outer roller and outer wall of the tube billet to form a buckling moment to buckle the material. The empty buckling will cause the buckling line to compress. The compression effect will cause the buckling line to lengthen lengthways, to accumulate and thicken the metal at the buckling place, which is we called the compression/thickening effect of the empty buckling.

1) Advantages of empty buckling is that it can be used to bend the side length when compact buckling cannot be done, such as synchronous bending and finishing of the top/side side of the rectangular tube. Empty buckling can also bend the inner Angle of R<0.2t without breaking the pipe wall.

2) Disadvantages of empty buckling is that when square steel tube synchronizes empty buckling at the top/side, the forming force is easy to exceed critical point due to simultaneous pressure generated by the upper and lower rollers, resulting in the instability of inner concave edge, and also affects stable operation of unit and forming quality. This is also the square steel tube and the circular tube empty buckling molding different characteristics.

In practical application, these two basic methods should be configured to a reasonable process position according to product requirements in the square tube forming. Attention should be paid to the effect of stretching/thinning effect of compact buckling and compression/thickening effect of empty buckling on quality of products.