| Metal lateral flow and stress relaxation are two important factors affecting rolling stability and shape control accuracy in hot strip rolling.Their own laws are simultaneously affected by steel composition,microstructure evolution,geometric dimensions and temperature distribution.Especially for the stress relaxation process between stands,it is a complex process that couples multiple basic physical metallurgical processes such as recovery,precipitation and recrystallization.Such complex working condition of multi-physics,multi-variable and strong coupling makes it extremely difficult,during finishing rolling,to quantitatively describe the metal lateral flow and residual stress relaxation.Due to the lack of corresponding theoretical foundation and theoretical calculation models,most of the current shape presetting models choose to ignore or use empirical coefficients to characterize the influence of them,which limits the further development of high-precision shape model of hot strip rolling.With this in mind,a 1580mm hot strip rolling production line is taken as the application background and the high-strength low-alloy steel is used as the application object;through many ways,such as experiments,numerical simulation,theoretical analysis,mathematical modelling,industrial tests and industrial applications,the present study carried out systematic research on the metal lateral flow and the residual stress relaxation effect between stands and established corresponding quantitative characterization methods.Research contents and results are as follows:(1)In order to explore the influence and role of metal lateral flow in the strip shape evolution,this study performed laser etching,pigment ink spraying and other treatments on the rolled samples in the simulated rolling experiment,and used laser confocal microscopy achieved the direct and accuracy measurement of tinny lateral flow in the rolling process.Furthermore,through combing the rolling experiment with a numerical experiments and theoretical calculation models,a self-correcting effect of metal lateral flow on residual stress is revealed,a theoretical explanation of the regulation mechanism of metal lateral flow in the evolution of strip shape is given,a risk coefficient of strip buckling and a lateral flow coefficient are proposed to quatitative the risk of strip buckling and the effect of lateral flow on residual stress,and the influence of strip geometric factors on the metal lateral flow and residual stress is analyzed.This research provides theoretical support and new ideas for perfecting the setting model of hot strip rolling.(2)To realize the efficient calculations of roll gap exit lateral flow and residual stress under the multi-physics and the multi-scale conditions,a physical prediction model for the flow stress of high strength low alloy steel and a rapid calculation model for strip rolling deformation are established first,respectively,based on the dislocation dynamics theory and a difference-matrix iteration method,which are used to describe the mechanical behavior of strip deformation and solve the macro-mechanical deformation.Then,a high-efficiency integrated calculation model of strip rolling was established by coupling the two model with the strip temperature field model solved by explicit-implicit alternating difference.Furthermore,the efficiency of model solving under different model structure is analyzed.The establishment of the model provides an efficient solution method for the subsequent presetting calculation of the strip shape and also lays the foundation for the online integrated calculation of the hot strip rolling process.(3)Based on statistical thermodynamics,a stress relaxation kinetic model after high-temperature deformation of metal is established,which realizes the description of the coupling effects of physical metallurgical processes such as recovery,recrystallization,precipitation,and solid solution drag during the stress relaxation process.By coupling the stress relaxation kinetic model with the temperature field calculation model,a strip residual stress evolution model under multi-physics and multi-physical process conditions is established based on the principle of residual stress self-balancing for the first time.In addition,a stress relaxation coefficient is proposed to quantify the degree of relaxation of the residual stress,and the effect of strip transverse temperature difference,average temperature and Nb element content change on the residual stress relaxation effect between stands is revealed.(4)Based on the above theoretical research,a strip shape presetting strategy based on the residual stress influence factor was developed by using the established high-efficiency integrated calculation model of strip rolling and the strip residual stress evolution model between stands as calculation tools,which has been used to optimize the strip shape presetting model of a 1580 mm hot strip production line in a certain factory and achieved good results. |
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