Why does the seamless steel pipe fold?

Posted by zora li on December 6th, 2022

Inward bending is a common quality problem in the production of seamless steel pipes. It is irregularly distributed sheet folds and spiral folds on the inner surface of the steel pipe, and the edges of the defects are mostly jagged. There are two main reasons for inward folding defects: the quality of the continuous casting billet (internal cause) and the piercing process system (external cause).

In addition, improper operation will also cause the steel pipe to fold inward. The inward folding forms in operation mainly include net shape, point shape, sheet shape and spiral shape. Regular spiral defects are generally caused by severely worn plugs or defective mandrels. In addition to the regular distribution of such defects, the defect edges are more neat than steel defects. Inward folding defects such as irregular mesh, dots, and flakes are generally caused by the following reasons.

1) The amount of depression before the top is too large, resulting in premature cavity formation of the tube blank during piercing, resulting in inward folding.
2) The tube blank is heated unevenly, and the heating temperature is too high, which is easy to cause inward folding.
3) The overflow part after perforation is not cut off. When pre-threading the mandrel, this part is inserted into the capillary causing inward folding.

The specific reasons for inward folding during piercing are as follows:

1) Influence of perforation deformation parameters on inner folds
The setting of perforation process parameters and the design of tools have certain influence on folding. The tearing of the central area of the tube blank is not only related to the total rolling reduction and the total reduction before the top, but also related to the local area when the tube blank turns half a circle. Depression is related.

2) Influence of piercing temperature on inward folding
If the temperature is too high, the heating time is too long, and the piercing temperature rises too fast, a large number of brown scale-like infolding defects will be produced; if the temperature is too low, the heating time is too short, which will reduce the plasticity of the metal and make it difficult to bite. When piercing, the center of the continuous casting billet is easy to break, forming infolding defects. If the temperature is too low, the capillary will produce small flake inward defects. Most of the higher steel grades are prone to small flake infolding defects, which are related to the relative difficulty of bite.

3) The influence of the centering hole on the inward folding
Centering hole too small, too out of the way or not smooth. When the perforation is in contact with the stopper, local friction will occur, and a semicircle or circle of capillary will be folded at the head. If the centering clamp pressure is too large, the head of the continuous casting billet will be flattened, which will also make it difficult to bite through and perforate at one time, causing the head to fold inward in a straight line.

To improve the quality of the inner surface of the steel pipe and reduce the occurrence of inward bending defects should start from the following methods

1. Ensure the uniformity of the structure during casting, reduce segregation, and prevent excessive concentration of non-metallic inclusions;
2. Adopt the best casting temperature to increase the critical reduction;
3. Before piercing, ensure that the heating temperature of the bad material is reduced to reduce the amount of pressing material before the top.

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zora li
Joined: August 16th, 2022
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