Fundamental Research And Application Of CO₂ Bottom Blowing Technology In Steelmaking Process

The iron and steel industry is a large emitter of CO2.If CO2 in off-gas is purified and directly used in iron and steel production,it is a new energy-saving and emission reduction technology.In order to expand the application of CO2 in various steelmaking processes and clarify the application scope of CO2 in different liquid steel systems,this paper selects Fe-C system of primary furnaces such as converter/electric arc furnace,Q235B system of silicon manganese deoxidized killed steel and SPHC system of low carbon aluminum killed steel in ladle refining process as the research object.The high-temperature physicochemical properties of CO2 in Fe-C binary system,Fe-Si/Mn/Al-C ternary system and Fe-C-Si-Mn-Al composite system were studied.Based on thermodynamic calculation and experiments,it is verified that the oxygen partial pressure(0.003 5atm)of CO2 in iron-based melt system at 1600? is much higher than the corresponding oxygen partial pressure when the oxygen element of iron-based melt is saturated.Therefore,when CO2 gas is injected into the melt,the final stable state of the system is oxygen element saturation in metal phase,with mass fraction of 0.1977wt.%and carbon mass fraction of 0.0115wt.%,In the steady state,CO2 is injected into the melt,and 85.75vol.%CO2 is decomposed into CO and oxygen atoms.Based on the high-temperature experiments,it is verified that CO2 plays a dual role in the very low-carbon Fe-C binary system(electrolytic pure iron)and FeSi/Mn/Al-C ternary system.In the early stage of smelting,CO2 acts as"carburization and oxygenation",CO2 decomposes into CO and oxygen atoms,and CO is further dissolved in molten steel to release carbon and oxygen atoms.After the composition of liquid steel reaches the critical point,the action of CO2 changes to "decarburization and oxygenation",that is,CO2 is decomposed into CO and oxygen atoms,and some oxygen atoms are used for decarburization.At the same time,CO is no longer dissolved in liquid steel,and the decomposition rate of CO2 in the reaction process is greater than 0.79.During the bottom blowing of CO2 in the silicon manganese deoxidized killed steel Q235B system in the ladle refining process,the elements of carbon,silicon and manganese are oxidized at the same time.With the increase of the proportion of CO2 in the CO2-Ar mixed gas,the molar proportion of oxygen atoms decomposed by CO2 for decarbonization gradually decreases.The stirring power of the bottom blowing gas containing CO2 in the system is higher than that of Ar.During bottom blowing CO2 in SPHC system of low carbon aluminum killed steel during ladle refining,the selective oxidation sequence is that after aluminum is oxidized to the trace amount(0.005wt.%),silicon and manganese begin to oxidize.When silicon and manganese are oxidized to 0.022wt.%and 0.28wt.%respectively,silicon,manganese and carbon begin to oxidize at the same time.With the increase of CO2 proportion in CO2-Ar mixture,the stirring power gradually decreases.In the process of bottom blowing CO2 gas in Q235B liquid steel,with the progress of reaction,small-size inclusions in liquid steel(<5?m)converge to SiO2MnO binary phase line.In the process of bottom blowing CO2 gas in SPHC liquid steel,the content of Al2O3 in inclusions is high in 0-40 minutes,and after 40 minutes,the inclusions gradually converge to SiO2-MnO binary phase line.When the bottom blowing gas is CO2,the inclusion density of liquid steel is higher than Ar.In the high carbon and low oxygen Fe-C system in the early and middle stage of the primary furnace,due to the occurrence of decarburization reaction,the denitrification rate is significantly higher than that of Ar and CO,and the ultimate denitrification capacity of CO2 is stronger than that of Ar.Fe-O-C system in the later stage of primary refining furnace,which seriously affects the CO2 denitrification capacity due to the high oxygen content of liquid steel.Q235B system bottom blowing CO2-Ar mixed gas,with the increase of CO2 ratio in the mixed gas,the denitrification amount decreases in turn within 70 minutes.When CO and CO2-Ar is bottom blown in SPHC system,the nitrogen content of liquid steel does not change,and the denitrification amount of Ar is 0.0017wt.%.The industrial test results of 300t converter show that the concentration product of carbon and oxygen in liquid steel at the end point is 15.33×10-4 when CO2 is blown at the bottom in the early and middle stage and Ar is blown in the later stage,similar to the bottom blowing inert gas in the whole process.The industrial test of bottom blowing CO2 gas of low-carbon aluminum killed steel was carried out in 100t ladle furnace.Compared with bottom blowing Ar,the stirring effect in the furnace was weakened,and the phenomena of liquid steel carburization and selective oxidation of alloy elements appeared.The research results of this paper will provide theoretical and data support for the bottom blowing CO2 technology in steelmaking process.