| As the market requirements for steel quality and cost become more and more stringent,the clean steel production with high efficiency and low cost has become an important trend in the development of iron and steel in today's world.Because the inclusion does a great harm to the steel quality,the inclusion removal has become one of the main tasks of clean steel production,and the bottom blowing argon,as one of the most important means to remove inclusion,has been widely applied in various refining processes,such as LF,VD,CAS-OB,and so on.But so far,the action mechanism of the bottom blowing argon on the inclusion transport and removal behavior is far from fully recognized,which is mainly reflected in two aspects:? The gas and liquid two-phase flow behavior caused by bottom blowing argon in ladle could not be accurately described,especially for the gas holdup distribution and the liquid turbulent behavior.? Some phenomena and mechanisms,which have important effects on inclusion behavior were ignored,such as the random motion of inclusions in the turbulent liquid steel,bubble-inclusion turbulent collision,bubble wake capture and the effect of slag eyes on the inclusion removal,and so on.The reasonable address on these problems has a crucial impact on the mastering of the inclusion behavior law in ladle and improving the efficiency of inclusion removal for clean steel production.In addition,the sulfur as the main impurity elements in steel has various adverse effects on the steel performance,and is also one of the main elements which need to be removed or controlled for the clean steel production.Currently,the production of(ultra)low sulfur steel mainly relies on the joint implementation of the hot pretreatment and the ladle secondary desulfurization,and these processes have many defects,such as longer process,inflexibility,low desulfurization efficiency,and so on.Recently,Northeastern University independently developed a new generation technology,namely ladle bottom powder injection(L-BPI).This new technology not only can overcome the shortcomings of traditional desulfurization processes,but also has positive effects on the shortening the process,improving production efficiency,cutting costs and energy conservation in steel industry.The key of L-BPI industrialization process lies in the effectiveness and efficiency,and it is essential to study the refining kinetic in ladle bottom powder injection process.However,the current research is still not mature enough on the slag-metal reaction and desulfurization in ladle,but also is on the blank stage in the reaction kinetic of L-BPI.In response to above-described problems,the research content and innovative achievements of this paper are as follows:(1)Study of gas and liquid two phase flow in a gas-stirred ladle.In the present Euler model,the phenomena of bubble turbulent dispersion caused by liquid turbulent fluctuation,and bubble-induced turbulence occuring during bubble floating process,were taken into account for the first time.At the same time,the effects of gas-liquid interphase force including drag force,lift force and virtual mass force on the gas and liquid two phase flow were investigated,and the reasonable model parameters were determined.The key problem of accurately predicting gas-liquid two phase flow in gas-stirred ladle was solved.(2)Study of inclusion transport and removal behavior in a gas-stirred ladle.In present work,the phenomena of inclusions turbulent random motion,bubbles wake,and slag eye forming on the molten steel surface were considered for the first time,and the related mechanistic models were presented.In addition,the multiple mechanisms both that promote inclusion growth due to inclusion-inclusion collision caused by turbulent random motion,shear rate in turbulent eddy,and difference inclusion Stokes velocities,and the mechanisms that promote inclusion removal due to bubble-inclusion turbulence random collision,bubble-inclusion turbulent shear collision,bubble-inclusion buoyancy collision,inclusion own floatation near slag-metal interface,bubble wake capture,and wall adhesion were investigated.The importance of above-described various mechanisms on the inclusion aggregation and removal were discussed and clarified.The results are shown as follow:?At low gas flow rate,the inclusion growth is mainly attributed to the joint effort of both turbulent shear collision and Stokes collision which is the prevailing mechanism for inclusion growth.With the increase of the gas flow rate,turbulent shear collisions become more important and then predominate the inclusion growth,and when the gas flow rate exceeds 100NL/min,the turbulent random collisions become important for large inclusion growth in gas-stirred ladle.? At the early stages of blowing,the inclusion removal is slower and mainly attributed to the joint effort of both bubble-wake capture and bubble-inclusion collisions including turbulent collision and buoyancy collision.At middle and later stages of blowing,the inclusion-bubble turbulent random collision becomes more important and then predominates the inclusion removal.? The dual blowing could give highest inclusion removal ratio in comparison with center blowing or eccentric blowing with one tuyere.The inclusion removal ratio with eccentric blowing is the lowest.With the gas flow rate increasing,the inclusion removal increases,however,when the gas flow rate exceeds 300 NL/min,the removal ratio changes little.(3)Study of slag-metal reaction and desulfurization kinetic in a gas-stirred ladle.For the desulfurization thermodynamics,different models were investigated to determine sulfide capacity and oxygen activity.For the desulfurization kinetic,and the effect of bubbly plume flow,as well as oxygen absorption and oxidation reactions in slag eyes were considered in the CFD-SRM coupled model.The thermodynamic and kinetic modification coefficients were proposed to fit the measured data,respectively.On this basis,the effect of the different content of compositions in synthetic slag and liquid steel,arrangement of bottom blowing tuyeres on the slag-metal reactions and desulfurization efficiency were discussed and clarified.The results are shown as follows:? The present CFD-SRM coupled model could be used to accurately describe the slag-metal reaction and desulfurization behavior in a gas-stirred ladle.Compare with the interfacial reaction(Al2O3)-[O]equilibrium and(FeO)-[O]equilibrium,the interfacial reaction(Al2O3)-(FeO)-(SiO2)-(MnO)-[S]-[O]simultaneous kinetic equilibrium model is more reasonable to describe the desulfurization thermodynamics.If the oxygen absorption and oxidation reactions in slag eyes were not considered in model,then the sulfur removal rate in ladle would be overestimated,and this trend would become more obvious with increase of gas flow rate and decrease of slag layer height.? With the increase of[Al]and[Si]content in liquid steel,the desulfurization ratio increases,but when the[Al]content exceed 0.01%,or[Si]content exceed 0.5%,the desulfurization efficiency increases slowly.With the decreasing of(Al2O3)content in slag,the desulfurization efficiency increases rapidly,but as the(Al2O3)content is less than 23.0%,the desulfurization efficiency increases slightly.With the increase of(SiO2)and(MnO)in slag,the desulfurization efficiency decreases rapidly.? The dual blowing gives higher desulfurization efficiency in comparison with the center blowing or eccentric blowing with one tuyere.With the increase of height ratio of slag and metal(h/H),the desulfurization efficiency increases,when the h/H exceeds 0.4,the desulfurization efficiency changes weakly.With the increase of gas flow rate,the desulfurization ratio first increase,then decreases,and when the gas flow rate is 200NL/min,the desulfurization ratio has a maximum value.(4)Study of refining reaction kinetic for ladle bottom powder injection new technology.The multiple interface reactions models including top slag-liquid steel reaction,air-liquid steel reaction,powder-liquid steel reaction and bubble-liquid steel reaction were proposed in present work,and the 'effects of powder transport and desulfurization products saturation on the desulfurization behavior were considered as well.The influence of different powder injection parameters on the desulfurization and powder removal efficiency were investigated,and the importance of different transport and reaction mechanisms were discussed and clarified.The results are shown as follows:? At the low powder injection rate less than 0.75kg/t,the desulfurization is mainly attributed to the joint effort of both powder-liquid steel reaction and top slag-liquid steel reaction which is the prevailing mechanism.With the increase of the powder injection rate,the powder-liquid steel interface reaction becomes more important and then predominates the desulfurization behavior,and when the powder injection rate exceeds 2.25kg/t,the role of bubble-liquid steel interface reaction becomes important and exceeds the top slag-liquid steel interface reactions.? The removal of desulfurization products particles mainly attributed to the effort of bubble-particle collisions and adhesion in bubbly plume zone.The role of powder removal due to bubble-wake capture is the second place,and the role of powders removal due to their own floating is the smallest.? With the increase of gas flow rate,the desulfurization ratio and powder removal ratio gradually decrease.With the increase of powder injection rate,the desulfurization ratio and powder removal ratio increase.As the gas flow rate is 600NL/min,with the powder injection rate increasing from 0.75kg/t to 3kg/t,the desulfurization ratio increases from 51.9%to 82.5%,and the powder removal ratio increases from 46.8%to 55.1%.Alumina content in powder has a significant effect on the desulfurization efficiency.As the gas flow rate is 600NL/min,and powder injection rate is 1.5kg/t,with the alumina content decreasing from 30%to 1%,the desulfurization ratio increases from 59.8%to 85.8%. |
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