Research On Buckling Behavior Of Hot Rolled Strip In Finishing Mills

The shape control of hot strip mills has been the hotspot and difficulty in the research field of strip and plate rolling. The evolution process of strip shape among the finishing mill group is an evolution result of buckling and post-buckling, which has the characteristics of multi-variable, strong-coupled and high-nonlinear. However, researches on buckling theory of hot strip are few and far between. The existing analysis models about buckling theory has been studied just from the point of mechanics and energy, but the effect of uneven temperature distribution on buckling critical threshold and post-buckling deformation has not been considered. There are no efficient online calculation models of strip temperature field and strip stress field in finishing mills to provide accurate initial data for the study on buckling deformation. Buckling theory can be used to improve the shape criterion theory and shape control means, which makes the research on buckling behavior more significance. For the above problem, taking a CSP hot strip mills as the research object, combined with in-depth theoretical analysis, a set of control model coupling strip temperature, strip stress and strip flatness has been developed with numerical method and analytical method. The innovative research results of this paper are obtained as follows:(1) According to the characteristics of heat transfer between strip and ambient medium, considering the property of physical parameters varying with temperature, the calculation model of strip temperature field in finishing mills is established by using two-dimensional alternating difference method. After optimized by hybrid PSODE with Triple Populations algorithm, the calculation model obtains fast calculation speed, high precision as well as stability, which can be used to accurately predict the evolution law of strip temperature field during finish rolling process.(2) Considering the inhomogeneity of lateral deformation resistance, the effect of unit shear stress on the deformation zone and the difference of lateral and longitudinal friction coefficient at the interface between working roll and strip, combined with the incremental theory and the hydrodynamic friction theory, the three-dimensional plastic deformation model of strip is established by using finite volume method. Asymptotic analysis principle is adopted to normalize the basic parameters and the BICGSTAB algorithm with incomplete LU factorization preconditioning is employed to solve the system rapidly and steadily. The three-dimensional plastic deformation model can be used to accurately calculate the internal stress and provide accurate initial data for the study on buckling behavior.(3) Based on the thin plate stability theory and the achieved internal stress, considering the effect of uneven strip temperature distribution, the theoretical formula of buckling critical stress is deduced by energy method, and then the buckling calculation model of hot rolled strip is established for global symmetric buckling and local symmetric buckling. On this basis, based on the large displacement theory of elastic thin plate and the principle of minimum potential energy, a model of post-buckling deformation of strip is established to present the quantitative relation between internal stress and flatness parameters under the equilibrium state. The relationship between buckling behavior and influence factors are systematically analyzed which improves the buckling deformation theory.(4) The main idea of Shohet instability criterion is systematically summarized and the theoretical limitation of Shohet instability criterion is also analyzed. On this basis, a new kind of shape control strategy is put forward based on buckling instability criterion. Based on the new strategy, the strip unit crown of each stand is determined according to the buckling critical threshold. Considering the effect of potential flatness, combined with the buckling theory and plastic deformation theory, flatness feedback control model which takes soft measurement flatness as the control basis is developed. The establishment of crown model and flatness model improves the precision of shape setup and control system.