Computer modeling of piercing in a screw mill with barrel-rolling rolls

The article presents results of computer modeling of the process of screw piercing of the workpiece in the mill with barrel-shaped rollers and fixed guide bars. Modeling was carried out by the use of the software QForm 3D based on the finite element method. Modeling of the screw rolling process enabled to explain the nature of changes in the deflected mode of the metal central zone during rolling in a two-roll piercing mill in one complete revolution of the workpiece. It can explain destructions in the central zone of the workpiece which cause inner scabs on shells and pipes. Destruction of the central zone of the workpiece is due to axial tensile stresses. As a result of computer modeling, the effect of a mandrel on the nature of the deflected mode of the central zone of the workpiece was determined. The presence of the mandrel greatly reduces the grade of tensile stresses in the central zone of the workpiece due to buttress forces. In the process of pipe production it leads to the increase of the workpiece piercibility and decreases or “welds up” the metal axial opening. In this work the effect of the ovalization ratio and the feed angle on the grade of tensile stresses in the central zone of the workpiece was also determined. The increase of the feed angle increases both tensile stresses in the workpiece and compressive stresses in the butt end of the mandrel. Therefore, it is necessary to perform the process of piercing providing the workpiece makes fewer than one revolution before its meeting with the butt end of the mandrel, but not less than 3/4 of the turn, otherwise problems with the secondary grip could occur. The increase of the ovalization ratio results in the increase of tensile stresses in the central zone of the workpiece. The obtained results of computer modeling correspond to basic assumptions of the theory of plastic metal working. High reliability of the results of computer modeling allows to verify the developed roll pass design and rolling speed limits without using costly experimental rolling.