Internal thread is a common defect in cross rolling mill, and its form is long and local. Internal thread defect is the mark left by every half circle deformation of seamless steel tube. When fixed and semi-floating spindles are used, internal thread defects are sometimes caused by wear of the spindles. The internal thread defect of cross rolling mill is as difficult to avoid as the straight line defect of automatic rolling mill. Minimizing them is important. It is generally believed that the reasons for the formation of internal threads in seamless steel tubes are analyzed from the following aspects:


1) Surface condition of the mandrel, such as wear, deformation, metal adhesion and oxide scale of the mandrel. Because the mandrel is involved in deformation, the defects on the surface of the mandrel must be reflected on the inner wall of the seamless steel tube, especially when the fixed mandrel and the semi-floating mandrel are used.
 
2) Is the length of the rolling section adequate in roll design? The rolling part functions as a uniform wall. If the inner thread of the seamless steel tube wall is completely rolled up in this area, it can be omitted. However, if there is a large deformation in the rolling section, it is easy to produce internal thread defects.
 
3) Mandrel motion. When tension mandrel and fully floating mandrel are used, the internal thread is less and lighter, especially when tension mandrel is used, because the inner surface of the inner rod and seamless steel tube has a larger relative sliding, the axial movement speed of the mandrel is faster than that of metal. In order to eliminate the axial movement speed of the internal thread, the inner surface of the seamless steel tube used for rolling cracking is very smooth, even the internal thread is not visible. When fixed and semi-floating mandrels are used, because the mandrel is fixed on the plunger, the spring of the mandrel and plunger system results in internal threads. In addition, the mandrel runs strongly in the deformation area and is prone to internal thread defects.
 
4) When fixed and semi-floating mandrels are used, the secondary oxide scale formed on the inner surface of seamless steel tube will accumulate in the wall of the tube when it rolls in the deformation zone of the mandrel, and the screw marks will be left behind (or as a pit).
 
5) Reasonable selection of technological parameters of seamless steel tubes, such as large elongation coefficient, low rolling temperature, large advance angle and high rolling speed, may result in serious internal thread defects. In fact, the internal thread depth of the two-roll cross-rolling stretcher is usually less than 0.1 mm (except for the fixed spindle). It is reported abroad that the internal thread height of three-roll cross rolling extender is usually within the range of 0.025 to 0.25 mm.

Internal thread is a common defect in cross rolling mill, and its form is full-length and partial. The internal thread defect is the trace left by the deformation of each half circle of seamless steel pipe. When fixed and semi floating spindles are used, internal thread defects are sometimes caused by spindle wear. The internal thread defect of cross rolling mill is as difficult to avoid as the linear defect of automatic rolling mill. It's important to minimize them. It is generally believed that the reasons for the formation of internal thread of seamless steel pipe are analyzed from the following aspects:


1) surface condition of spindle, such as wear, deformation, metal adhesion and oxide skin of spindle. Because the mandrel is involved in deformation, the surface defects of the mandrel must be reflected on the inner wall of seamless steel pipe, especially when the fixed mandrel and semi floating mandrel are used.
 
2) is the length of the rolling part sufficient in the roll design? The rolling part functions as a uniform wall. If the internal thread of seamless steel pipe wall is completely rolled up in this area, it can be omitted. However, if there is large deformation in the rolling part, it is easy to produce internal thread defects.
 
3) spindle movement status. When tension mandrel and full floating mandrel are used, the internal thread is less and lighter, especially when tension mandrel is used, because the internal rod and the inner surface of seamless steel pipe have relatively large sliding, the axial movement speed of mandrel is greater than that of metal. In order to eliminate the axial movement speed of internal thread, the inner surface of seamless steel pipe used for rolling cracking is very smooth, even the internal thread is not visible. When fixed and semi floating spindles are used, the spring of the spindle and plunger system causes internal thread because the spindle is fixed on the plunger. In addition, the mandrel has strong runout in the deformation area, and it is easy to produce internal thread defects;
 
4) when the fixed mandrel and semi floating mandrel are used, the secondary oxide scale formed on the inner surface of seamless steel pipe will accumulate in the pipe wall when rolling in the mandrel deformation area, and leave spiral trace after falling off (or as a pit).
 
5) the process parameters of seamless steel pipe, usually large elongation coefficient, low rolling temperature, large forward angle, high rolling speed and so on, may produce heavy internal thread defects. In fact, the internal thread depth of the two roller cross rolling stretcher is usually less than 0.1mm (except for the fixed mandrel). In foreign countries, it is reported that the internal thread height of three roll cross rolling extension machine is usually in the range of 0.025 to 0.25mm.