Infolding of hot rolled seamless steel pipes

Infolding is the main quality defect in the production of hot rolled seamless steel pipe, which is produced in the perforation process and cannot be eliminated. Infolding is the folding on the inner surface of the seamless steel pipe. The appearance is characterized by a scaly layering, similar to a closed curve, and there are also folds with irregular shapes. Folding is produced in the rolling process due to the poor quality of metal, when there is excessive segregation of non-metallic inclusions, there will be pores concentrated around the metal inclusions, in the perforation process split to form inside folding, after the extrusion of sizing machine it appears in scales or other shapes.

In short, the unqualified round billet quality of rolled seamless steel pipe is mainly caused by the high degree of metal contamination caused by non-metallic inclusions and harmful substances. In addition, strip segregation has a very important effect on the quality of billet during casting. The band segregation depends largely on the chemical composition, holding state and cooling rate of the steel. The increase of the content of harmful elements in steel leads to the increase of their concentration in the segregation zone, thus forming segregation. The increase in carbon content will also increase the segregation of impurities, especially manganese.

The following factors also affect the infolding of seamless steel pipes during the perforation process

1. Influence of perforation deformation parameters on infolding of hot rolled seamless steel pipe

The folding of seamless steel pipe also has a certain relationship with the perforation process. The parameter setting and tool design of the perforation process have a certain influence on the folding. The rupture of the central area of the pipe billet is not only related to the total amount of pressure down in the roll and the total amount of pressure down before the top, but also related to the local amount of pressure down when the pipe billet turns to half a circle.

2. Effect of perforation temperature on infolding of hot rolled seamless steel pipe

With the increase of temperature, the material plasticity increases, and the critical pressure quantity increases with the increase of temperature. However, if the pipe billet is cast at the optimal casting temperature, the infolding does not change significantly when the perforation temperature changes within a certain range.

3. Relationship between heating time and infolding of hot rolled seamless steel pipe

At a certain temperature, extending the heating time has little effect on the inner folding, but the number of outer folding increases significantly. The reason is that the longer the heating time, the more severe the grain growth, and the surface layer is more obvious, so that the microstructure deteriorates and produces outer folding.

4. The influence of tools on the infolding of hot rolled seamless steel pipe

For example, the use of the tip nose can reduce the formation of its own hole cavity, improve the quality of the inner surface, in addition, the shape of the tip has a certain effect on the quality, the taper tip will make from the tip of the tip to the hole part of the pressure increase, which has a bad effect on the waste pipe; The spherical head can reduce the amount of pressure down the waste pipe along the feeding direction and improve the quality of waste pipe. The best shape of the head should be one that ensures the same amount of wall thickness down along the length of the deformation zone.

Finally, in order to improve the quality of the inner surface, reduce the inner folding of hot rolled seamless steel pipe should be done from the following aspects:

  1. Ensure the uniformity of the structure and reduce segregation to prevent excessive concentration of non-metallic inclusions when casting the pipe billet;
  2. the best casting temperature, in order to improve the critical pressure under the amount;
  3. Ensure the heating temperature of the blank before perforation, reduce the amount of pressing down before the top, and improve the quality of the hot tool.