Reoxidation Phenomenon And Inhibition Mechanism Of Aluminum-Deoxidized Steel In Tangsteel

Tangsteel encountered problems of inclusion control,oxygen and nitrogen control and nozzle nodulation when smelting aluminum deoxidized ultra-low oxygen steel.And these problems are mainly caused by reoxidation during secondary refining and continuous casting process,especially in the tundish.Tundish,as the final vessel in contact with molten steel during the steel production,provides the last opportunity to optimize the steel cleanliness and is therefore of significant importance for steel quality control.However,it could also introduce exogenous inclusions through refractory erosion,slag entrapment,and steel reoxidation,contaminating molten steel and degrading steel cleanliness.In general,the steel reoxidation in the tundish results from several reasons:The experimental values are combined with theoretical calculations to obtain a mass diffusion coefficient of oxygen in the cover layer of D=4.87 cm2/s.As the oxygen partial pressure increases,the molten steel is intensified by the reoxidation,and the dissolved oxygen in the steel increases.The oxygen is mainly transported physically in the covering layer,and the chemical transport amount is less than 10%of the total transport volume;the primary deoxidation inclusions Large particle size(Al2O3 inclusion),the original deoxidation inclusions tend to aggregate and grow up and the floating agent is absorbed by the covering agent;the oxide inclusion of the covering agent is easy to combine with SiO2 in the covering agent to produce aluminosilicate(FeO·SiO2·Al2O3),the size is relatively larger,and as the oxygen partial pressure increases,the size further increases,and the aluminosilicate aggregate grows up and is absorbed by the covering agent.Based on high temperature experiments,fine inclusions of less than ?m are mainly produced by air-oxidized steel.In the steel,a trace amount of Mg-Al spinel(MgO·Al2O3)inclusions larger than 5 ?m is contained,and as the oxygen partial pressure is further increased,the Mg-A1 spinel inclusions are wrapped by FeO and the size is further increased.The Fe2O3 contained in the covering agent can directly contact the molten steel,bypassing the restrictive step of the air to the reoxidation of the molten steel through the covering agent,so that the deoxidizing element in the steel will react with it in a short time,and is generated in the steel.A large amount of oxide inclusions cause contamination of the molten steel.When the partial pressure of oxygen is constant,Fe2O3 in the covering agent accelerates the reoxidation of the molten steel;Based on the results of high temperature experiments,the[Al]element in the molten steel can quickly reach equilibrium,and as the(Al2O3)activity in the tundish covering agent gradually increases,the[Al]activity begins to decrease gradually.Under the same experimental conditions,since the oxidation of[Si]element in molten steel is relatively small,the reaction begins to reach equilibrium as the partial pressure of oxygen in the atmosphere gradually increases.As the activity of(SiO2)in the tundish covering agent gradually increases,the oxidizing property of the covering agent begins to increase,so that the activity of the[Si]element in the molten steel is further increased,and the rate at which the reaction reaches equilibrium is also gradually increased.The effect of Cr2O3 in the coating agent on the reoxidation of aluminum deoxidized steel was studied experimentally.It was found that Cr2O3 in the tundish covering agent oxidized the molten steel,and almost all of the Cr2O3 in the tundish covering agent was reduced by the molten steel.The total oxygen content in the steel,the aluminum loss and the FeO content in the tundish cover agent increase as the Cr2O3 content increases.When the partial pressure of oxygen is 0,the reoxidation of Cr2O3 to molten steel is more serious than the equivalent content of SiO2,and the oxidation of molten steel by Cr2O3 will inhibit the oxidation of molten steel by Si02.When the partial pressure of oxygen is 10KPa,the Si in steel Both reoxidation and Cr are reoxidation and the reoxidation of Si is much more intense than Cr,and with the prolongation of the holding time,Cr2O3,SiO2 and FeO in the coating agent will undergo reoxidation again to transfer oxygen to the molten steel.As the content of Cr2O3 in the tundish covering agent increases,the distribution density of inclusions in the steel decreases,and the average size increases,that is,it is easy to generate large-sized Al2O3 inclusions.By comparison,it was found that the density and size of the inclusions formed by the reoxidation of the tundish covering agent depend on the deoxidizing element in the steel.There are two ways for Cr2O3 to cause reoxidation of molten steel.One is reduction by deoxidizing elements in steel,and the other is self-diffusion.When acid-soluble Al exists in the steel,Cr2O3 transmits oxygen in two ways at the same time,wherein Al reduction accounts for more than 90%,and self-difusion accounts for less than 10%.For the tundish covering agent containing 5%and 10%Cr2O3,the reaction of Al to reduce Cr2O3 mainly occurred within 2 min and 3 min after the addition of the tundish covering agent.For the tundish covering agent containing 10%Cr2O3,the reaction rate of Al to reduce Cr2O3 reached a maximum within 2 min after the addition of the tundish covered powder.After adding CaCO3 to the coating agent,the amount of inclusions in the steel is obviously reduced,the size distribution is improved to increase the proportion of small size,and the reoxidation behavior of molten steel is significantly improved.The decomposition of CaCO3 can produce CaO,which can effectively improve the performance of the slag layer,and CO2 can effectively reduce the coverage.The partial pressure of oxygen on the surface of the agent reduces the reoxidation of the molten steel during the casting process;under laboratory conditions,the diffusion time of the CO2 gas generated by the decomposition is about 25 minutes,and the relationship between the gas diffusion time and the temperature and the diffusion distance is obtained.The optimal time interval calculation for adding CaCO3 during production provides a theoretical basis.The theory of depressing reoxidation phenomenon during tundish process was verified by an industrial trial.