Thermodynamic And Experimental Research On Utilizing CO₂ As The Lift-gas In RH Refining To Refine Ultra-low Carbon Steel

The"Greenhouse Effect",caused by massive emissions of CO₂,greatly affects the ecological environment as well as human survival and development.As a large CO₂emitter,steel industry conducting CO₂ utilization is profound.In recent decades,RH refining,with the main purposes of deep decarburization and dehydrogenation,has become an important metallurgy equipment of refining clean steel.RH refining consumes a large amount of Ar,which is expensive and its price is 3?5 times that of CO₂.Moreover,CO₂ is a weak oxidant which can assist decarburization.Thus,utilizing CO₂ instead of Ar for RH refining becomes an important direction for steel industry to save energy and reduce emissions.Nevertheless,the relevant basic theories of using CO₂ as the lift-gas in RH to refine ultra-low carbon steel are insufficient,especially the synergistic decarburization effect of CO₂ and[O]under vacuum as well as its influencing factors are rarely reported.Therefore,studying the decarburization behavior and temperature change of molten steel in RH refining under the CO₂-[O]-Ar system is of great significance.In this dissertation,based on the Fact Sage thermodynamic software,combining the characteristics of RH vacuum refining,uses an iterative calculation method and verifies its reliability.On the basis of those,effects of system pressure,mixed gas ratio,initial carbon and oxygen content of molten steel on the decarburization and temperature change are analyzed,besides merits and feasibility of the pre-deoxidation process are discussed.Moreover,high temperature experiments under vacuum with the Si-Mo furnace in laboratory,in which different volume ratios of CO₂/Ar injected into the low-carbon molten steel were conducted,so as to examine the decarburization effect of CO₂.Last but not the least,based on the thermodynamic calculations and laboratory research results,a series of industrial trial,refining ultra-low carbon steel by side-blowing CO₂ as the lift-gas in a 220t RH furnace were carried out.Theoretical calculation results show that,ensuring a certain initial dissolved oxygen when CO₂ is injected into the low-carbon molten steel under vacuum,the temperature drop is within 5?.But with CO₂ injection,the molten steel may be oxygenated during the later stage of the reaction while decarburization also stagnates,and the gas injected needs to be substituted by Ar.Moreover,while the initial carbon content is less than 0.04%,the initial oxygen content is controlled at 0.02?0.04%,and the volume fraction of CO₂ in CO₂-Ar mixed gas is 50%?65%,has the best smelting effect.Besides,CO₂ will oxidize the dissolved aluminum in steel,thus switching to Ar injection before deoxidation alloys added is essential.The laboratory research results show that continuing to inject CO₂ into ultra-low carbon molten steel at atmospheric pressure may increase dissolved carbon;under vacuum,combined the final carbon and oxygen content,when the CO₂-Ar volume ratio is 1:1 in the injection gas is better;existence of CO₂ in the mixed gas will promote decarburization reaction,where with pure CO₂ injection the decarburization rate index can reach 13.4 ppm·min^-1·L^-1.The industrial trial results show that,without changing the molten steel conditions,use CO₂ instead of Ar as the lift-gas in RH refining,the temperature effect,decarburization effect,and production efficiency are nearly equivalent to the existing process,but it can save energy and reduce production costs.The effect of CO₂ injection to assist decarburization is not obvious in the case of high oxygen and low carbon content steel,which is nearly consistent with the thermodynamic calculation.In the case of high carbon and low oxygen content steel,utilizing CO₂ as the lift-gas to make up the insufficient decarburization capacity of molten steel can be taken into consideration,but that needs further experimental verification.This research results will provide ideas and relevant data support for the application of CO₂ in RH refining process,as well as experience for further exploration and optimization of the process.