Technical Research And Application On Purification Of IF Steel Produced With BOF-RH-CC Process

Ultra low carbon IF steel needs to strictly control the content of interstitial atoms C and N to achieve its excellent deep drawing performance.Meanwhile,the cleanliness of the slab has an important impact on the surface quality of the final steel product.It is required that the continuous and stable control of liquid steel C,N,P and cleanliness in the production of ultra-low carbon steel based on the metallurgical process of 180t combined blowing converter smelting,RH deep decarburization and degassing treatment continuous casting.In this paper,the optimization of converter process and equipment,research on efficient dephosphorization and control of molten steel oxidation,research on degassing,decarburization and efficiency in RH treatment process,and research on inclusion analysis and control in continuous casting process have been carried out.The key technologies of pure purification control systematization have been formed and the following results were obtained:The oxygen lance nozzle is optimized from the surrounding five holes structure to the five surrounding holes+one center hole structure.The theoretical impact depth is greater,the stirring of the molten pool is stronger,and it is more suitable for the strengthening and efficient smelting of IF steel.In application practice,the probability of gun sticking is smaller,and the life of the oxygen lance is increased by more than50 heats on average.The use of slag retention smelting mode is beneficial to the dephosphorization of the first slag with double slag.When the oxygen supply proportion of the first slag accounts for 30%of the total oxygen supply proportion,the smelting temperature is 1400℃,and the carbon content is about 3.5%,the phosphorus content of hot metal can be reduced to less than 0.05%.The oxygen lance with optimized nozzle structure,combined with the strong bottom blowing mode of about 0.075Nm³/t min,can control the[C]×[O]product of molten steel at the smelting end point below 0.0025,[P]content below 0.012%,and[N]content below 0.0015%on average.The key technology of purifying molten steel in converter smelting is formed.Before process optimization,the control level of total oxygen and nitrogen content in molten steel after RH refining needs to be improved.The secondary oxidation of molten steel in the first furnace of tundish is serious,and the average oxygen content of mold sample is 50.6%lower than that in the tundish,indicating that the inclusion removal effect of tundish is good.The oxygen and nitrogen values of large section billets are slightly higher than those of small section billets,indicating that it is more difficult to prevent secondary oxidation in each process of continuous casting during large section casting.In addition,from the mold to the slab,the total oxygen content of the small section slab decreases by 23.0%,and the large section slab decreases by28.9%,indicating that although the flow rate at the outlet of the large section submerged nozzle in the mold is faster,it is farther from the narrow section slab shell,and the impurities are not easy to be captured by it,so it is more likely to float up and remove.Therefore,the removal efficiency of inclusions in the molten steel in the large section mold is greater than that in the small section.Along the slab thickness,the percentage of 2.5～5mm inclusions at the position from the inner arc about 70 mm is the highest（i.e.1/4 of the distance from the inner arc）,reaching 72%.During the floating process,most of the inclusions are captured by the solidified shell,all above 20μm inclusions exist in this area.As the central layer of the slab finally solidifies and grows with the precipitation of titanium nitride inclusions,the 2.5～5μm inclusion of the central layer of the slab is the least,the ratio is about 58.3%;5～10μm inclusion accounts for 36.9%,which is the highest.Along the width of the slab,2.5～5μm inclusions in the center is the most,accounts for79.54%,which is the position with the largest proportion of inclusions of this size level.There is no>15μm inclusions exist at this zone.It indicates that large inclusions have sufficient time to float,so large inclusions are the least.At 1/4 of the distance from the inner arc,2.5～5μm inclusions is the least（54.9%）,while>15μm inclusions accounts for 11.6%,which is due to the formation of reflux in this area after the molten steel enters the mold,leading to the collision,growth and aggregation of large inclusions in this area.Through pre vacuumizing and the optimization of RH sealing,immersion tube structure and gas flow,the average decarburization time of RH can be reduced by 4.85min,and the w[C]can be removed to 7 ppm,and the final product w[C]is about 12ppm.At the end of RH refining,w[N]can be controlled within 17ppm.With the increase of the post cycle time of RH alloy,the number of inclusions in the steel gradually decreases,and the total oxygen content continuously decreases,and the w[T.O]in the molten steel decreases to about 24 ppm when the cycle lasts for 10minutes.The main component of inclusions is aluminum oxide.Before deoxidation,the inclusions in the steel are clustered,and after deoxidation,they are angular.At the end of the cycle,the inclusions gradually tend to be spherical.The key technology to control the total amount and size of inclusions is realized.Increasing the basicity of the tundish slag and decreasing the content of Mg O and Al₂O₃ in the tundish slag are beneficial to reducing the total oxygen content in steel.The increase of the basicity of the tundish slag is beneficial to the reduction of the number of inclusions in the steel,and the appropriate increase of the basicity is beneficial to the formation of fine inclusions.After the original basicity is increased from 0.7 to 8.0～9.0,the cleanliness of molten steel is improved,and the total oxygen content is reduced compared with that before the improvement.When the first heat of steel is poured,the first 60t of molten steel is seriously polluted,the cleanliness is poor,and the total oxygen content is high.With the gradual stabilization in the tundish,the cleanliness of molten steel is gradually improved,and the oxygen and nitrogen content in the steel is gradually reduced.After 150t of pouring,it is stable,and the total oxygen content is within the range of 15.5-23.2 ppm.Along the thickness direction,inclusions tend to gather within 2.0mm from the surface layer,while the number density of inclusions is large at 20.0mm from the surface layer.The w[C]of final product is about 12 ppm,w[N]is about 18ppm,and the average w[T.O]is 19ppm.