FEM Simulation of Mill Strech Curve and Parameter Investigation
Year: Sept. 2008- Feb. 2011.
Rolled products should be characterized by a good quality and tight dimensional tolerances. Because of automation that is commonly implemented in flat product rolling mills, these products should meet the requirements of tightened tolerances, particularly those of strip thickness and width, and feature the greatest possible flatness. In order to obtain the correct shape of the strip upon exit from the deformation region, all factors influencing deformation region shape should be considered when determining roll gap height. The spatial shape and dimensions of the roll gap are influenced by the elastic deformation of all parts of the rolling stand equipment affected by the pressure force. All the parts that compose the rolling mill are subjected to elastic deformation by rolling force. The ratio of the rolling force to amount of vertical deformation of the whole rolling mill, including the deformation of the rolls, screw-down device or hydraulic cylinder, and housing is called mill modulus. The mill modulus is 500-1000 ton/mm for plate and hot rolling mills and 400-600 ton/mm for cold rolling mills. A rolling force of the order of 1,000 tons is generated during rolling, so that mill deformation of more than 1 mm occurs. Unless this deformation is taken into account, thickness accuracy cannot be ensured. Furthermore, because the mill modulus has a finite value, there exists a minimum thickness below which the rolling mill cannot reach.
The relationship between force and the vertical deformation of the whole rolling mill is mill stretch curve or mill modulus curve. The rolling mill is not a simple spring and because several materials are sequentially deformed, the curve is nonlinear. But it is possible to measure the slope of linear part of mill stretch curve. The slope of mill stretch curve defines the mill modulus.
The magnitude of elastic deformations depends on the type of roll assembly, materials used, the dimensions of particular nodes, friction force, and clearance between equipment.
In this thesis, mill stretch curve has been modeled by finite element method. For modeling the mill stretch curve, all part of rolling stand equipment including work rolls, backup rolls, work roll chock, backup roll chock, liners, hydraulic cylinder and main structure has been modeled. It is found that the calculated mill stretch curve and mill modulus agree very well with experimental curve and modulus reported from Mobarakeh Steel Company. The effect of work roll diameters, backup roll diameter, friction coefficient, and vertical and horizontal clearance on mill stretch curve and mill modulus was analyzed.
Keywords:Mill stretch curve, Mill modulus, FEM, Hot rolling.