Study On Deoxidation Process And Slagging Technology In High Aluminum Steel

High aluminum steel has acid-soluble aluminum content up to0.5%or more. It is mainly used for the mold, chlor-alkali chemical containers or pipelines, electricmaterial, aerospace and weapons etc. Its market demand Increased rapidly in recent years. Al is the main deoxidation agent as well as the key element in high aluminum steel, whilst Al2O3is the main cause for the nozzle clogging during continuous casting. Aluminum in the steel and silica in the slag will be so easy to react that it will increase the Si in steel. In this condition, liquid steel oxidizes again easily and worsens the purity of steel. So total oxygen contents is greater than30×10-6, and even reach50～70×10-6. Obviously, this level of purity can not meet requirements of high aluminum. The core of solving above problem is that we should control inclusions pattern and composition during steelmaking to meet the harsh rule of high aluminum steel smelting conditions.In this thesis, the author used the basic principles of thermodynamics to calculate production conditions of the major inclusions in high aluminum steel, and study the reaction between the slag and steel. Based on the thermodynamics calculation, the experiment was done in the MoSi2resistance furnace. The thesis is aimed at figuring out the mechanism of high aluminum steel deoxidation, analyzing production inclusions in different conditions of deoxidation process and exploring the absorption condition of inclusions in LF refining slag with different w(CaO)/w(Al2O3). Finally, the industrial test was done by using EAF→LF→VD→CC production process in a steel company.The main conclusions of this thesis are as follows:(1) Calculation based on thermodynamic and research from experiment both indicate that when the Al content in the steel is over0.1%, Al2O3is the only stable existing inclusions. The approach to suppressing the reaction increasing Si in the steel is that strictly control w(SiO2) in the slag. As far as high aluminum steel is concerned, to control the content of w(SiO2) in the slag under5%, preferably2%is the best.(2) The inclusions in high aluminum steel become smaller in average diameter, fewer in quality and lower at melting point, when calcium treatment is used. Deoxidation process with adding aluminum individually is good to control inclusions shape and make inclusions more spherical, smaller in size, more stable in component. Inclusions in terminal sample are1-2um spherical MgO-Al2O3-CaO.(3) Slagging experiment indicates that when w(CaO)/w(Al2O3) decreases under2, total oxygen contents, Al2O3inclusions and inclusions size decrease as w(CaO)/w(Al2O3) increases. The w(CaO)/w(Al2O3) value in salg should be controlled about2.(4) Under the selected experimental conditions of deoxidation process and refining slag, the lowest total oxygen content reaches0.0001%, meanwhile, the nitrogen content can be controlled at a lower level, the lowest value reaches0.0004%.(5) Experiments in laboratory and industrial test show that w(SiO2) at the LF reduced to below5%, and w(Al2O3) increased significantly. Variations of total oxygen, nitrogen contents and inclusions have a similar trend and the majority of inclusions are MgO-Al2O3-CaO in steel after refining.