Influence of shape and size of the tip of the piercing mill mandrel on the formation of internal defects in shells and application of continuously cast billets for pipe diameters of less than 121 mm

The research of the influence of the shape and size of the tip of the piercing mill mandrel on the formation of internal defects of the axial fracture area located in the deformation zone of the piercing mill was carried out. Premature axial degradation leads to the formation of defects on the inner surface of shells and pipes. If billets are pierced with higher reduction, it is proposed to use mandrels with the enlarged diameter and tip sphere radius (enlarged tip area). This technical solution was tested in the pipe-rolling unit TPA-140 of “SinTZ” (Kamensk-Uralsky) in the production of an experimental batch of pipes with a diameter of 108 mm from continuously cast billets with a diameter of 156 mm. The study of experimental samples obtained by interrupted rolling showed that using mandrels with the enlarged diameter and the tip sphere radius provides “healing” (sealing) of billet axial fracture. Metallographic studies were conducted to determine the nature of the formed layer of intact (“healthy”) metal in front of the mandrel tip. It was found that this layer was formed by means of back-up forces from the mandrel. The research shows the possibility of using continuously cast billets for the production of pipes with diameters of less than 121 mm.