Research On The Damage And Fracture Behavior Of Steel Strip During Rolling Process

Cold-rolled steel, which is manufactured by cold rolling process, is widely used in various key areas of national production, such as electricity, automotive, construction, industrial equipment. The manufacturing technology and production of cold-rolled steel have become important indicators of the measurement of the industrialization process of a country. In production, the inclusions, holes, micro cracks and other defects in steel are inevitably. During the cold rolling process, steel defects, which suffer tremendous plastic deformation, will lead to edge cracks or quality decrease of the steel strip, and even worse to the fracture of the whole steel strip, which seriously affect the continuity of production and reduce production efficiency, or even damage to the production equipment. At present, the theoretical research of the damage and fracture mechanism of metal material during cold rolling process is still very weak and lack of a complete theoretical system both at home and abroad. Therefore, the research on the damage and fracture mechanism of steel strip during rolling process not only has theoretical innovation, but also very significant engineering practical value in prevention and control of edge cracking and fractures of steel strip in cold rolling.In this dissertation, the stress distribution of rolling deformation zone in cold rolling was analyzed via theoretical derivation method firstly. The steel rolling deformation zone was divided into one central region and two side regions in the width direction and the side effect of stress distribution was taken into account. The side region can be further divide into two portions based on the rheological behavior of the metal material, the stagnation or transition part and full slide part. Thus, the stress distribution analytical solution of the entire rolling deformation zone was established and influence of rolling parameters on the stress distribution was discussed.According to the stress state in the rolling deformation zone, numerical method was applied to analyze the damage and fracture behavior of steel strip during cold rolling process based on a shear modified GTN damage model. The constitutive equation and numerical algorithm of the shear modified GTN damage model was deduced, and user defined material subroutine was programed via VUMAT interface to implement the shear modified GTN damage model into ABAQUS\Explicit for finite element calculation. In addition, a set of methods was put forward for the determination of the material damage parameters of the shear modified GTN damage model.Finite element model of the cold rolling process was established and the shear modified GTN damage model was used to investigate the damage evolution and distribution in steel strip during cold rolling process. The simulation results show that the damage value of strip edge is significantly higher than the central area, which is consistent with the phenomenon that steel strip edge is prone to cracking in actual production. The influence of rolling process parameters, including roller radius, reduction ratio, friction coefficient and tension, on the steel strip edge damage value was analyzed through the applying of orthogonal test analysis method. The result shows that:the damage value in the steel strip edge is larger while the reduction rate and tension increase; and with the increase of the roller radius and friction coefficient, the damage value in steel strip edge will decreases slightly.Cold rolling experiment and finite element simulation of steel strip with pre-set edge defect were conducted. And the results show that cracks occur easily in the defect tip along the direction with an angle about 45° and 135° to the rolling direction, and the crack in the 135° position is the main one that extends more obviously with the propagation direction opposite to the rolling direction. The extension length of the main crack increases with the increase of reduction rate and tension, and the decrease of the roller radius and the friction coefficient. In addition, the deformation and fracture behavior of the V-shape notch with different size in the steel strip edge in multi-pass cold rolling was investigated. The results indicated that the V-shape notch in steel strip edge with smaller length and larger width is more conducive to the occurrence of cracks in the notch tip during multi-pass cold rolling process.An extension criterion for steel strip edge defects in cold rolling process was proposed based on the numerical analysis of fracture behavior and mechanism of steel strip edge defect in cold rolling with the shear modified GTN damage model method. In the criterion, the driving force of the defect extension is defined as weighted combination of the equivalent stress and hydrostatic stress in defect tip, and the material is considered failure when the ratio of the combined stress to tensile strength is greater than one. The functional relationship to the maximum value of the equivalent stress and hydrostatic stress in defect tip to the rolling parameters, including process parameters, material properties and strip edge defect size, were built by a large number of finite element simulations and mathematical regression method. Thus, the mathematical expression of steel strip edge defect extension criterion in cold rolling was obtained. The correctness of the defect extension criterion was validated by cold rolling experiments with different rolling parameters. Finally, the failure assessment diagrams of cold rolling process for different rolling parameters were put forward to guide rolling production practices.