Coagulation Behavior Of Inclusions In Molten Steel

Non-metallic inclusions are inevitable and cannot be completely removed during the steelmaking process.Inclusions remaining in the steel matrix adversely affect the quality and performance of the steel,with the composition and size of the inclusions having the greatest impact.During the steelmaking process,inclusions collide,aggregate,and grow under the action of molten steel,and the coagulation potency of inclusions with different compositions are significantly different.In order to reveal the influencing factors and intrinsic mechanism of the coagulation/dispersion of inclusions,a series of researches on the collision,coagulation,floating and removal behaviors of inclusions in molten steel were systematically carried out by means of physical and numerical simulation in this paper.Besides this,the apparent contact angles and interfacial tensions between typical inclusions and molten steel were measured by the sessile drop technique.Then the coagulation coefficients of different inclusions in molten steel were calculated based on the collision-coagulation theory.Afterwards,deoxidation experiments were performed to verify the calculation results.Achievements obtained in this study can point out the orientation of inclusion size control in steel.Several important conclusions of this study are described as follows:(1)In the physical simulation part,dioctyl phthalate,phenetole and simethicone were used to simulate the liquid inclusions in molten steel.The results show that collision and coagulation of inclusions are influenced by their viscocity and the interfacial tension with water,and the removal rates of them are 74.5%,81.1%and 65.0%respectively under blowing condition.Moreover,when ultrasonic with a certain energy is introduced into the molten steel,inclusions will rapidly collide and grow up under the action of the acoustic radiation force,but a part of the droplets will be emulsified and uniformly distribute in the solution,which are hard to be separated from the steel matrix further.(2)The results of physical and numerical simulation show that the bottom-blowing has a promoting effect on the removal of inclusions because the turbulent collision probability of inclusion is increased dued to the flow of molten steel drived by bubble buoyancy.And the formed larger-sized inclusions can grow even bigger through colliding with fine inclusions during floating process according to stokes collision.Comared with no blowing condition,the removal rates of these three simulated inclusion are increased by 15.8%,17.2%,10.7%.(3)The numerical calculation results show that the interfacial tension between the droplet and the continuous phase has a significant influence on the coalescence process when the droplets are close to each other.As the interfacial tension increases,the additional pressure on the droplet increases,and the overall aggregation process is accelerated.Increasing the dynamic viscosity of the droplets will slow down the mutual mass transfer between the droplets during the coalescence process,reduce the shrinkage rate of the interface,and hinder the aggregation of the droplet pairs.(4)In inert atmosphere with the temperature of 1823K/1873K,apparent contact angles and interfacial tensions between Al2O3,MgO and MgO·Al2O3 substrates and five types of molten steel were measured precisely.Based on the collision-coagulation theory,the quantitive relationship between coagulation potency and interfacial properties was drived,and the corelation between coagulation coefficient and contact angle can be expressed as:αt=4.923 × 10-5[μrI3(pMε/μ)1/2/(σI-σMcos θ)Based on this equation,coagulation potency of inclusions in molten steel are calculated to be Al2O3>MgO>MgO·Al2O3.(5)Deoxidation experiments and quantitative analysis of inclusions show that the coagulation potency of inclusions in steel is highly correlated with the theoretical calculation value of the coagualation coefficient,thus physical and numerical simulation results and theoretical calculation results can be verified.Therefore,if the size of the inclusions in steel needs to be adjusted in a targeted manner,the interfacial tension between these two phase can be increased or decreased through altering the composition of inclusions to promote the agglomeration or dispersion of inclusions.And the target of"harmless residual of incluions" is expected to be achieved through this way.