Electric Drive Speed Control in the Rolling Stand of a Plate Mill During Front-End Bending Forming

Abstract

Forming a front-end bending of a strip plate is an urgent problem for rolling stand electric drives at plate mills. Uncontrolled strip plate bending can cause rolling stand equipment damage while the turn-down strip plate bending can cause blows and strip sticking during movement along the rolling table. On the basis of literature data analysis, it was found that the uncontrolled front-end strip bending can be caused by a number of processing factors. The main factors are the mismatch between the speeds of the top and the bottom rolls, temperature difference along the strip plate thickness, difference in the friction coefficients and others. Beside the influence on the strip plate shape, the mismatch between the speeds determines the correlation between the roll electric drive torques. The horizontal rolling stand 5000 at the Public Joint-Stock Company “Magnitogorsk Iron and Steel Works” (PJSC “MMK”) was used as an example to show that in the process of heavy section rolling the difference in motor torques can be threefold. At the same time as the strip plate thickness decreases with each pass, the load ratios of the top and bottom roll main drives reverses. In order to coordinate the speed and load modes, a load division controller was added to the system. The main disadvantage is the fact that it carries out load control as a function of the speed difference between the top and the bottom roll main drives. The proposed oscillograph records prove that it is impossible to provide the preset speed ratio in the dynamic mode, which arises when the strip enters the rolling stand. To eliminate the disadvantage, a method of electric drive control was developed; in accordance with this method, the speed control in the bending formation mode is carried out in proportion to the difference in set value signals at the inputs of the top and the bottom roll main drive speed control units. A structural diagram was shown, which demonstrates the method implementation by adding a supplementary speed coordination controller at the input of the load division controller. A simulation model scheme of electromechanical systems of the top and the bottom rolls, which are interrelated through the metal, was shown. As a result of the simulation, it was found that the method implementation can provide the speed misalignment at a level close to the preset one during the whole period of bending formation. The control algorithm implementing the developed method was introduced in the automatic control system of the rolling stand at the plate mill 5000. Oscillograph records were given to prove the main theoretical conclusions. The developed algorithm makes it possible to reduce the set values for bending formation by more than a half. Due to this, the geometry accuracy improves and the length of the curved section decreases. This can provide lower mismatch of the top and bottom main drive torques and their faster alignment. Beside the electric motor torque relief, it improves the motor efficiency.