Controlling The Behavior Of Non-metallic Inclusions In Molten Steel By Pulsed Electric Current And Its Application

Inclusion control aims to reduce the harzards of inclusions by controlling the number,size and shape of inclusions,thereby improving the properties of steel.Pulsed electric current can not only purify the liquid steel by controlling the directional movement of inclusions,but also can control the refinement,shape and orientation of inclusions.Compared with the traditional purification technology,such as electromagnetic field and alternating magnetic field,pulsed electric current regulation has the advantages of simple equipment,environmental friendliness,energy-saving and better purification capacity.Pulsed electric current can effectively separate not only large-size inclusions but also inclusions with size less than 10 ?m in molten steel.In addition,pulsed electric current can inhibit the clogging of submerged nozzle by controlling the migration of inclusions.The directional movement of inclusions is obtained by applying pulsed electric current based on the difference in physical properties(conductivity)between non-metallic inclusions and molten steel.Compared with the mass density difference between inclusions and steel liquid in gravity field,pulsed electric current has more advantages in controlling inclusions with smaller size and higher density.Further,the research on the theory of the evolution and migration of the inclusions controlled by pulsed electric current is helpful to the application of pulsed electric current in the field of ultra clean purification smelting.The evolution and mechanism of inclusions regulated by current are studied by experiments and mathematical models,including the evolution and orientation of inclusions in current field,the interaction mechanism of bubbles and inclusions by the intervention of pulsed electric current,removal of micro size rare earth inclusions in rare earth steel by pulsed electric current,and controlling inclusion migration based on current density to prevent rare earth inclusions from adhering to refractory surface.The above research provides an important theoretical basis and experimental basis for the production of ultra-clean rare earth steels and the clogging of nozzle during the continuous casting of rare earth steel.The logic and results of this work are summarized as follows:(1)The inclusions cause the current to detour in the current field,and the current distribution changes between adjacent inclusions trigger the interaction between the inclusions.Based on Maxwell equation and Landau continuum theory,the mathematical model of interaction between two inclusions is established.The interaction forces,the electric free energy and the conductivity of the system are studied during the evolution of inclusions.The numerical calculation indicates that the interaction between inclusions can be divided into attraction,repulsion and rotation effects.The three effects promote the inclusion to form a chain structure with specific orientation in the direction parallel to current,and keep away from each other in the direction vertical to current.This phenomenon is proved by experiment.The results show that the evolution of inclusions follows the law of the decrease of electrical free energy and the increase of conductivity.(2)Based on the interaction law of inclusions in current field,the interaction between bubbles and inclusions is further studied,and the interaction model between bubbles and inclusions is established.The results show that the inclusion is reduced by 45%after applying current,but the inclusion treated by single bubble has no obvious decrease.The numerical results show that the electric free energy of the system decreases after the inclusion is trapped by bubbles.In addition,the current causes the driving force cacting on the inclusion,the tangential force of the driving force causes the inclusion to slide on the surface of the bubble,while the radial force causes the attraction,rejection and driving effect between the bubble and the inclusion.The three effects lead to the increase of collision probability and adhesion probability,and the reduction of the desorption probability of inclusions,which makes bubbles easier to capture and remove inclusions in molten steel.(3)The experimental results show that inclusions move to low current density region under electric current field.Taking rare-earth inclusions as the object,the current density gradient in the inhomogeneous current field is further studied to determine whether it has stronger driving force on inclusions and better purification effect.The results show that the number of rare earth inclusions in the range of 1-3?m decreases by 77.42%under the action of current density gradient.The theoretical calculation shows that the driving force of inclusions in gradient current field(the average electric current density gradient is 0.7 A/mm3)is about 100 times that of inclusion in uniform current field.The driving force acting on inclusions is positively related to the size of inclusions and linear positive correlation with current density gradient.Inclusions migrate from high current density to low current density region under current density gradient to reduce the resistance of inclusions to current.The migration of inclusions follows the principle of reducing the electric potential energy and the electric free energy of the system.(4)Based on the directional migration of rare earth inclusions driven by current density gradient,the effect of current on the adhesion of rare earth inclusions on the surface of the nozzle was studied.The refractory samples are eroded by molten steel for 5-30 min,and the average thickness of the erosion layer increases from 160 ?m to 795 ?m,increased by 397%.However,the average thickness of the erosion layer increases from 150 ?m to 210 ?m,only increased by 40%,under the action of pulsed electric current.Shortly,the current detours around the refractory sample.The current of the sample surface is higher,and the current density gradient is formed from the surface to the distance.The rare earth inclusions driven by the current density gradient are hard to approach the sample surface.In addition,the effect of pulsed electric current on the reaction mechanism of rare earth molten steel and rare earth inclusions corroding refactory is discussed.It is suggested that pulsed electric current is helpful to the formation of amorphous phase on the o refractory surface,which insulates the contact between the refractory and the molten steel,and slows down the erosion of the molten steel on the refractory material.The refractory formed a compact,smooth and thin erosion layer structure under the action of pulsed electric current.(5)The high-frequency pulsed eletric current was applied to the steel flow in the nozzle through the stopper(positive electrode)and the negative electrode(graphite rod)inserted into the molten steel in the mold,and the skin effect of the high-frequency current is used to form an exponentially decayed current density distribution from the surface to the center of the steel flow.The inclusions in the steel flow are driven by a driving force directed to the center of the steel flow,which hinders the migration of the inclusions to the inner wall of the nozzle.After 60 minutes of continuous casting of rare earth steel without pulsed eletric current,the stopper rises by 3.52 mm,while the stopper only rises by 1.41mm with pulsed eletric current.The pulsed eletric current slows down the rising rate of the stopper rod during the continuous casting of rare earth steel,and effectively suppresses the clogging of the nozzle.