Study On Metallurgical Technology And Cold Sheet Defects Control Of Ultralow Carbon Steel

In recent years,with improvement of domestic automobile production,the demand of steel for automobile is growing.As the typical production of automobile steel,IF steel has good deep drawability,which contains ultralow carbon and nitrogen content and high steel cleanliness.And the quality of cold sheet is decided directly by the steel cleanliness.The project is aiming at smelting process control and reducing the inclusion defect rate of cold sheet,the study included the following aspects which are the common problem in IF steel production:the investigation of cleanliness of IF steel,the choice about whether the ladle slag modification,molten slag absorbing the inclusions,the damage degree to cold sheet of Al2O3 and TiN inclusions,and the origin of cold sheet defects of inclusions in smelting process.The study on cleanliness of ultralow carbon IF steels showed that the high oxidizing slag and the unsteady control of wt%(CaO/Al2O3)in slag caused the high T[O]in RH and holding process,which produced a variety of cold sheet defect of Al2O3 inclusion.There was amount of macro-inclusions of SiO2 in slab because of poor premelting properties of mould slag,which lead to surface peeling and silver defect.In order to characterize the reoxidation of head slab,the cast start number was redefined,which provided basis of using head slab.The effect of two processes whether the ladle slag modification carried out on steel cleanliness and reoxidation was studied by thermodynamic calculation and industry trial,and the application condition and proper refining slag were determined.After optimazation the T[O]at the end of RH process is steady controlled below 30×10-6.The inclusion separation at steel/slag interface was calculated,and a new method was proposed to in-situ observe the inclusion dissolution.The mechanism of inclusion absorbed by slag was comprehensively studied.It showed that the effect of slag properties on the separation of inclusions at the steel/slag interface is not obvious,but the dissolution of inclusion in different slag was various,the well modified ladle slag slag has good absorption of Al2O3 inclusions.The nano-indentation was used to characterize the mechanical properties of Al2O3.TiN inclusions and matrix,combined with distribution and precipitation rule,the damage degree of two kinds of inclusions was clearly confirmed.Combining the parameters of smelting and the inclusion defect of cold sheet,the model of data mining was established to investigate the effect of smelting parameters on the inclusion defect of cold sheet,and it found that the factors which have great influence on the inclusion defect of cold sheet were:the oxygen content before deoxidization in RH>BOF endpoint temperature>the temperature before deoxidization in RH.