Reasons for inward folding of seamless steel pipes

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

In addition, improper operation will also lead to inward folding of the steel pipe. The main forms of inward folding in operation are mesh, point, sheet, and spiral. Regular helical defects are generally caused by severe wear of the plug or mandrel defects. In addition to the regular distribution of such defects, the edges of the defects are more tidy than the steel defects. Irregular mesh, point, flake and other internal folding defects are generally caused by the following reasons.

1)The amount of depression before the top is too large, which causes the tube blank to form a cavity prematurely during perforation and cause inward folding.
2)The uneven heating of the tube blank and the high heating temperature may easily cause inward folding.
3) The overflowing part after perforation is not cut off. When pre-piercing the mandrel, this part is inserted into the capillary tube to cause inward folds.

The specific reasons for the inward folding in the perforation process are as follows:

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

2)The effect of perforation temperature on inner fold
If the temperature is too high, the heating time is too long, and the perforation temperature rises too fast, a large number of yellow-brown fish scale-like infolding defects will be produced; if the temperature is too low, the heating time is too short will reduce the plasticity of the metal, making it difficult to bite.During perforation, the center of the continuous casting tube blank is easily broken to form in-fold defects, and if the temperature is too low, the capillary tube will produce small flake-like in-fold defects. Most of the higher steel grades are prone to small flaky infold defects, which are related to the relative difficulty of biting.

3)Influence of centering holes on inward folds
The centering hole is too small, too biased or not smooth. When the perforation is in contact with the plug, local friction is generated, and a half-circle or a circle of capillaries will be folded in the head. If the pressure of the centering clamp is too large to flatten the head of the continuous casting tube blank, it will also cause difficulty in biting the perforation at one time, resulting in a linear inward fold of the head.

To improve the quality of the inner surface of the steel pipe and reduce the occurrence of internal folding defects should start with 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 perforation, ensure that the heating temperature of the bad material reduces the amount of pressing before the top.

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