Study On The Causes And Control Of Wide And Heavy Slab Surface Micro-cracks

Wide and heavy plate is widely used in heavy machinery,large diameter pipeline,large shipbuilding industry,bridge construction,pressure vessel,ocean engineering,construction,mould and other fields,and requires good surface quality.Because of adding micro alloy elements in the production process,surface cracks are easily produced,which has become a common technical problem in the production of high quality wide and heavy plate.The surface of wide and heavy slab covered by oxidized iron or mold flux,surface transverse crack,corner crack,star crack,net crack and edge longitudinal micro cracks on the slab are difficult to find during slab inspection,but it has a great influence on the surface quality of rolled products.In this paper,based on a steel plant tackling the problems process on wide and heavy slab surface quality,in order to reduce wide and heavy slab surface cracks,the micro cracks on the wide and heavy slab in production are studied.Combining with grain boundary corrosion,SEM analysis,energy spectrum analysis,infrared temperature measurement and production process data comparison,the formation mechanism of micro cracks was analyzed,and mold oscillation,the stress analysis of continuous casting process,mold powder performance,nitrogen control in the first furnace were studied.With comparison the production data of the site,a series of measures were put forward,the slab surface micro cracks were effectively controlled,wide and heavy plate surface quality of a defective rate was controlled below 0.25%.The main research contents and results are as follows:(1)The Gleeble-3800 thermal/mechanical simulation test machine is used to simulate continuous casting process.The reduction of area is took as the main index,combined with the SEM morphology of fracture,to determine the third brittleness temperature zone of the internal steel K50DNNH2 in 750～875℃,which has important guiding significance for determining the improvement direction of the slab quality.(2)According to the actual situation,the stress condition of the slab surface during continuous casting process is studied.The mold maximum tensile stress,the slab surface deformation rate when bending,straightening and bad segment arc-aligment are calculated and analyzed individually.The relationship between the slab surface maximum deformation rate and arc-aligment deviation was regressed.(3)The formation mechanism of various surface micro cracks in wide and heavy slab is studied.Surface micro cracks are related to cooling,casting equipment status,nitrogen content of finished product,low melting point element penetration,friction force and so on.(4)Control measures of wide and heavy surface micro cracks were studied,such as adding Ti,improving the mold powder film resistance,reducing the depth of oscillation marks,controlling the first furnace nitrogen increase and argon flow of mechanism,the corresponding control measures are put forward.(5)Through adopting measures such as on-line equipment precision management,process operation and parameter optimization,steel composition optimization,optimization of physical and chemical properties of mold powder,finished product nitrogen content control,wide and heavy slab surface micro cracks have been effectively controlled.