Causes and improvement methods of internal thread of seamless steel pipe

Internal thread is a common defect in the cross-rolling mill, and the form is full-length and local. The internal thread defect is the trace left by the deformation of the seamless steel pipe every half turn. When a fixed mandrel and a semi-floating mandrel are used, sometimes the internal thread defect is caused by the wear of the mandrel. The internal thread defects of the cross-rolling mill are just as difficult to avoid as the straight defects in the automatic rolling mill. It is important to minimize them. The reason for the formation of internal thread of seamless steel pipe is generally considered to be analyzed from the following aspects:

1) Surface state of the mandrel, such as mandrel wear, deformation, adhesion metal and scale. Because the mandrel participates in the deformation, the defects on the surface of the mandrel are necessarily reflected on the inner wall of the seamless steel pipe, especially the use of the fixed mandrel and the semi-floating mandrel;
 
2) Is the length of the rolling section in the roll design sufficient? The function of the rolling section is the uniform wall. If the internal thread of the seamless steel pipe wall is fully rolled in this zone, it can be eliminated. However, it is easy to produce internal thread defects when subjected to large deformation in the rolling section;
 
3) Mandrel movement state. When the tension core rod and the full floating core rod are used, the internal thread is less and lighter, especially when the tension core rod is used, because the inner rod has a large relative sliding with the inner surface of the seamless steel tube, and the axial movement speed of the core rod is greater than that of the metal. The axial movement speed, so as to eliminate the internal thread, the inner surface of the seamless steel pipe for rolling out of cracking is very smooth, and even the internal thread is not visible. When the fixed mandrel and the semi-floating mandrel are used, since the mandrel is fixed on the ram, the bouncing of the mandrel and the ram system causes the internal thread to be generated. In addition, the mandrel jumps strongly in the deformation zone, and is also prone to internal thread defects;
 
4) When a fixed mandrel and a semi-floating mandrel are used, the secondary iron oxide skin formed on the inner surface of the seamless steel pipe is piled into the pipe wall when it is rolled in the deformation zone of the mandrel, and a spiral trace is left after falling off ( or as a pit).
 
5) Reasonable selection of seamless steel pipe process parameters, generally large elongation coefficient, low rolling temperature, large advancing angle, high roll speed, etc. are likely to produce heavier internal thread defects. Actually, the internal thread depth of the two-roller cross-rolling stretcher is generally less than 0.1 mm (except for the fixed mandrel). It is reported in foreign countries that the internal thread height of the three-roller cross-rolling extension machine is generally in the range of 0.025 to 0.25 mm.