| Edge seam defect that is the straight defect that occurred on strip surface and near the strip edge,that is most important one of the edge defect and there is few improvement.It not only causes the surface defect of hot rolled strip,but also huge wasting of resources and energy caused by trimming off the edge in order to eliminate the defects.The ultra-low carbon steel is discussed.In this paper,the effect factors of edge seam defect forming is research by physical thermal simulation,and the evolution process of defect is obtained.The finite element model is established based on the research on high temperature mechanical behavior in order to analysis the defect,and the mechanism of defect forming is obtained.The method of slab shape design is proposed in order to control this defect.The deformation and temperature change of chamfered slabs are research by industrial experiment.These slabs achieve excellent results on defect controlling.The results provide theoretical basis and technical support for further development of equipment and process optimization.The main results are show as follows.(1)The evolution of edge seam defects was studied by thermal simulation and pushing out of rolling strip at laboratorial pilot rolling mill.The results show that the initial defect morphology was the crease on lateral side.Then the crease transferred to the rolled surface as broadening process,and was pushed to be edge seam defect paralleled to strip edge.The formation of defect is independent with original solidification structure and corner crack of slab.(2)The IF steel is discussed for the defect forming mechanism of ultra-low carbon steel by physical test and finite element simulaiton.The results show the flow stress decreased with the decrement of temperature at mixed phase during austenite transfers to ferrite of IF steel,which show the opposite trend with the single-phase region.Subsurface region near the corner deforms in mixed phase during finishing rolling.The crease is formed on the laterial side by the smaller spreading.Crease comes up to the surface in broadening process with the turning over of crease opening,and change to be edge seam defect.(3)The method of applying the different chamfered slabs to improve the defect is proposed based on the theoretical study.The applying effect is confirmed at 2250mm hot rolling mill.The industial experiment research shows that the chamfered shape would be remained to finish of rough rolling at the condition of width reduction less than 50mm.Chamfered shape not only increases the temperature of corner but also improve the temperature distribution and the uniform of mechanical properties.The strain on corner region is decreased.The creases on lateral side are removed.The possibility of the edge seam defects on the surface is eliminated fundamentally.During large scale industrial experiment,defect ratio on IF steel strip and low carbon steel strip are reduced from 75%to 2%and 90%to 10%respectively by the chamfered slab.(4)The effect of chamfered slab on edge seam defect of micro alloyed strip is researched.The results show temperature on lateral side is more uniform.The uniformity of material property is increased.The strains uniformity is increased obviously after the fifth rough rolling pass,and then the crease on the lateral side is controlled.The chamfered shape is inherited to the second pass of R2 rolling under the condition of width reduction less than 50mm.The corner temperature is higher and even than rectangular slab.By the chamfered slab,defect ratio of micro-alloyed steel is reduced from 29%to 8%.The defect ratio is further decreased by double chamfered slab. |
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