Research On The Formation Mechanism Of Continuous Casting And Rolling Plate Crack For Q345 Steel

The cracks of medium plate are the regular quality defects in the medium plate production, which hold a great proportion of the defects and affect badly on the receiving rate and the quality of the medium plates. It is a common problem resolved in the technology route of electric furnace-slab- medium plate, improving the internal quality in basis on how to guarantee the surface quality of slab,in particular to prevent the defects of serious intermediate cracks in the rolling of final product.In this dissertation,according to a great deal of referred literature, the cracks and other problem of medium plate product from Tangshan iron and steel corporation were studied and discussed. The microstructure and mechanical properties of the slab and rolled plate were investigated and analyzed by optical microscopy , scanning electron microscopy(SEM),energy spectrum analysis and tensile test, and the fractured and strengthened mechanism were discussed.The detecting results of the microstructure and mechanical properties show that the microstructures of rolled plate is mainly ferrite and pearlite, and a small amount of bainite, and the grain size is above the standard of ASTM 10. Due to the presence of the middle crack, not only the tensile strength and yield strength of the rolled plate is relative low, 477Mpa and 406Mpa,respectively, but also plastic performance is of a sort, and it is necessary to improve the mechanical properties of rolled plate.The experiment results show that main component of inclusions in the slab is the silicate and sulphide in the form of MnS. Inclusions shape is relative rule, and their shapes are spherical,polygon and filminess. The pure A12O3 inclusion is not found in cast. Among those dot and block inclusions,most of them are SiO2 and MnS, and few FeS; Most sizes of inclusions are under 100μm. The distribution of most inclusions are in the center area of cast and 1/4 thickness distance from the inside area of cast; Most oxids in the inclusions derive from steel ladle slags,tundish covering flux and the deoxidation in the melt steel.The segregation of carbon, sulfur and phosphorous elements play a decisive role on forming of intermediate crack, which is the interior reason of crack generation.The process parameters of continuous casting such as the factors of roll gap reduction,second cooling conditions,casting speed,liquid steel fever and equipment accuracy are external causes of crack formation, which have different extent effect on the intermediate crack of medium plate.Under existing technology conditions, the factors such as deoxidization system, argon blowing system, the structure in tundish affect on the forming intermediate crack of medium plate. The boosting of inclusion parts float up and the second oxidization of the liquid steel were discussed in the dissertation, which provided the basic experiment data for the clean production of steelmaking and continuous casting.The factors of the loose unrolled hole and the segregation of belt-shaped bainite structure at the center place of medium plate resulted in the fracture lamination of the tensile sample.