Study of surface defect transformations during pipe manufacturing using PRP-80

Requirements of current Russian and foreign standards are becoming stricter in terms of regulating the acceptable depth of the defects located on the surface of rolled pipes. Non-conformities spoil the appearance of products, increase the risk of defects during pipe manufacturing and of pipes becoming byproducts, and degrade the competitive advantage of pipe-rolling plants with a continuous rolling mill, i.e. process yield. At the same time, quality requirements to the surface of primary hot-worked or continuously cast blanks allow manufacturing-related surface defects. That is why it is necessary to examine the pipe manufacturing technology potential to reduce depth of such defects. Hot rolling modes for seamless pipes in certain PRP mills may contribute both to the defects and to the reduction of depth of blank defects. The article describes results of a complex approach to the study of defect transformation processes of the blanks rolled in ТПА-80 mills. The statistical analysis of pipe quality helped to identify the types of external pipe surface defects typical of this techology. The authors studied and defined more exactly the causes of such external surface defects as pits (dents), pipe-rolling slivers, and folds. Depending on the type of defects, they pinned down the place where they appear in the PRP mill during manufacturing tests. They also studied mechanisms of evolution of these defects from primary blank defects. Using computer-based simulation methods, they also studied the effect of technological rolling parameters on the change in geometry of defects of various shapes and initial depths. Reducing mills do not have any considerable effect on the defect depth reduction. Floating mandrel piercing and continuous rolling mills feature the widest variety of rolling adjustment parameters in ТПА-80. The authors proposed formulas to calculate defect depth reduction during the rolling process in piercing and continuous rolling mills. These formulas may be used to calculate the acceptable depth of primary and intermediate blank defects.