A study of conditions of surface defects formation when bar rolling at a bar and wire mill and methods of their elimination

Quality of long products surface is an important consumer property of it. In the process of measures elaboration aimed at the increase of long products surface quality, in particular of bars produced at the mill 370/150 of ОJSC “BMZ – managing company of holding “BMK”, studies were accomplished by metallographic laboratory. It was established that defects being revealed at the bars finishing, don’t relate to the quality of continuously casted billet (CCB), but formed in the process of deformation. Studies of the mechanism of surface defects formation on hot-rolled bar of rolling origin – deformation fissure and wrinkles were carried out. Results of numerical simulation of rolling in roughing group of stands at various temperature-deformation parameters presented. Regularities of formation of surface defects on the bar in the finished product were revealed. It was shown that the reason of the surface defects of rolling origin – deformation fissure and wrinkles was a high temperature gradient between the core and the surface of billet, originated from local overheating of surface in the angles zone of CCB resulted in nonuniformity of drawing out of different layers of the billet being deformed. To eliminate the defects, minimum possible temperature gradient between the surface and the core of a billet by controlled rolls cooling should be provided. By calculation, the maximum permissible temperature of the working surface of the rolls of the rough group of stands was established, and empirically the actual temperatures of the rolls with the current production technology, as well as the temperature of the rolls support bearings seats of the rolls were measured. The technical and technological possibilities for improving of rolling technology on a bar and wire mill in order to improve the surface quality of rolled bars were demonstrated. The existing technology was adjusted and new technological modes of rolling with controlled cooling of the rolls were established, which made it possible to significantly reduce the rejection of the finished product due to defects in rolling production. A device was proposed for the roughing group of stands, which enables to minimize the ingress of coolant onto the bar rolled.