Internal thread is a defect that often occurs in skew rolling mills, 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 using fixed mandrels and semi-floating mandrels, sometimes internal thread defects are caused by mandrel wear. The internal thread defects of the skew rolling mill are as difficult to avoid as the straight defects in the automatic tube rolling mill, and it is important to reduce them to a minimum. The reasons for the formation of internal threads of seamless steel pipes are generally considered to be analyzed from the following aspects:
1) The surface state of the mandrel, such as mandrel wear, deformación, sticking to metal and iron oxide scale, etc. Because the mandrel participates in the deformation, the defects on the surface of the mandrel must be reflected on the inner wall of the seamless steel pipe, especially when the fixed mandrel and the semi-floating mandrel are used;
2) Whether the length of the rolling section in the roll design is sufficient. The role of the rolling section is to equalize the wall. If the internal thread of the seamless steel pipe wall is fully developed in this area, it can be eliminated. Sin embargo, it is easy to produce internal thread defects when it is subjected to a large amount of deformation in the rolling section;
3) The movement state of the mandrel. When using tension mandrel and fully floating mandrel, the internal thread is less and lighter, especially when using tension mandrel, because the mandrel and the inner surface of seamless steel pipe have relatively large relative sliding, and the axial movement speed of the mandrel is higher than that of metal The axial movement speed is high, so as to eliminate the internal thread, the inner surface of the seamless steel pipe for oil cracking is very smooth, and even no trace of the internal thread can be seen. When a fixed mandrel and a semi-floating mandrel are used, since the mandrel is fixed on the ejector, el batido del sistema mandril-expulsor favorece la generación de roscas internas. Además, el mandril salta violentamente en la zona de deformación, y también es propenso a defectos de rosca interna;
4) Cuando se utiliza un mandril fijo y un mandril semiflotante, la incrustación secundaria de óxido de hierro formada en la superficie interna del capilar de la tubería de acero sin costura se acumula en la zona de deformación del mandril y se presiona contra la pared de la tubería, y dejará un rastro en espiral después de caerse ( o Makeng);
5) Los parámetros de proceso de la tubería de acero sin costura deben seleccionarse razonablemente. Generalmente, gran coeficiente de elongación, baja temperatura de laminación, gran ángulo de avance, y la alta velocidad del rollo son propensas a causar defectos de rosca internos más pesados. La medida real de la profundidad interna de la rosca de la máquina laminadora oblicua de dos rodillos es generalmente menor que 0.1 mm (excepto para el mandril fijo). Según informes extranjeros, la altura de la rosca interna de la máquina laminadora oblicua de tres rodillos generalmente está en el rango de 0.025 para 0.25 mm.