35杜嗣琛-paper-Slag-metal interface and its impact on

SECRETARIATUSE;Headershouldbeblank.SECRETARIATUSE;FootershouldbeblankSlag-metalinterfaceanditsimpactoninclusionremovalinladletreatmentDUSICHEN(DepartmentofMaterialsScienceandEngineering,RoyalInstituteofTechnology,SE-10044Stockholm,Sweden)Abstract:Tostudytheslag-metalinterface,alargenumberofsamplesoftheslag-metalinterfacewerecollectedfromanindustrial65-tonladlefurnaceusinganewsamplingtechnique.Complementarycoldmodelexperimentswerealsoperformed.Theexperimentalresultsshowedconvincingevidencefortheexistenceofslagdropletseachsurroundedbyaverythinmetalfilminthesamplestakenunderbothgas-stirredandinduction-stirredconditions.Theindustrialobservationstronglysuggestedthatslag-metalsystembehavedsimilarlyasoil-watersystem,wherein,theshearforceduetothebulkflowledtoapacked“sphere-bed”ofoildropletseachcoatedbyathinwaterfilmabovethewaterbath.Thepresenceofsphere-bedwasalsofoundtoprevailaroundtheopen-eyeregioncreatedbythebubbleplume.Theimpactofthesphere-bedstructureoftheslagontheremovalofnon-metallicinclusionswasalsostudiedexperimentally.Basedontheexperimentalresults,apreliminarymathematicalmodelwasdevelopedtodescribetheinclusionremoval.Themodelwasverifiedsemi-quantitativelybytheexperimentalresults.Keywords:slag-metalreaction,ladletreatment,inclusion,modelling0INTRODUCTIONSlag-metalmixingisaverycommonphenomenonduringladletreatment.Usually,gasstirringorinductionstirringoracombinationofbothisappliedtoenhancethemasstransfer.Therateofmassexchangebetweenthemetalphaseandtheslagphaseisthebasetoobtainprocesscontrolofhighprecision.Whilemanyinvestigations[1-13]havebeenconductedtounderstandslag-metalinterfaceduringmixing,theknowledgeoftheslag-metalinterfaceisstillverypoor.Mostoftheinvestigationshavebeenconductedeitherforlowtemperaturemediaoratalaboratoryscale.Dataforactualindustrialsystemisveryrare.Non-metallicinclusionsusuallyhavedetrimentaleffectsonfinalsteelproducts.Theremovalandcontrolofinclusionshavebeenoneofthemainconcernsofsteelrefining.Intheladletreatment,theinjectionofinertgasnotonlycontributetothehomogenizationoftemperatureandchemicalcomposition,butalsofunctiontoremovenon-metallicinclusions.[14]Itisexpectedthattheslag-metalinterfaceplaysacrucialroleintheinclusionremoval.Asensiblemodelofinclusionremovalshouldtaketheslag-metalinterfaceintoconsideration.Thepresentworkaimsatgaininganinsightintothemechanismoftheslag-metalmixinginastirredladlebysamplingtheinterfaceusingthenewtechnique.Theindustrialstudiediscomplementedbycouldmodelinvestigationandmathematicalmodelling.SECRETARIATUSE;Headershouldbeblank.SECRETARIATUSE;Footershouldbeblank1INDUSTRIALSTUDY1.1SamplingandAnalysesAnewsamplingtechniquewasdevelopedtoobtainsamplesoftheslag-metalinterfaceduringladletreatment.[15]Thesamplerisschematicallyshowninfigures1(a)and1(b).Thesamplingdeviceiscomposedofastainlesssteelsamplingtube(Length:0.58m,InnerDiameter:0.056m),anoutermountingtubeandabottomparttobeabletoenclosethesample.Thebottompartconsistsof4steelrodsweldedtoasteelplate.Thebaseoftheconeonthesteelplatematchestheinnerdiameterofthestainlesssteeltube,sothatthesamplingtubecanbeclosedbythebottomplateduringsampling.Byreleasingtheretainingpinthesamplingtubeisallowedtofalldownintothemeltandcloseagainstthebottompart,keepingasampleoftheslag-metalmixingzoneinside,asshowninFigure1(b).Aforkliftisusedforverticalandhorizontalpositioningofthesamplerduringsampling.Thesamplesoftheslag-metalinterfaceweretakenintheindustrial65-tonladleatUddeholmToolingAB,Sweden.AdetaileddescriptionoftheladletreatmentatUddeholmToolingcanbefoundinanearlierpublication[15].Thesamplesweretakenafterdeoxidationbutbeforevacuumdegassing.Experimentswerecarriedoutinbothargon-stirredladleandinduction-stirredladle.Aftersampling,thesampleholderwasquicklyliftedfromthesteelbath,andimmediatelyquenchedinthewater.Thetimeforthesamplingprocedurewasaboutafewseconds.Thesamplescollectedfromtheladlewereopenedmechanicallybylatheandmillingmachine.Theslag-metalinterfacewasthenidentified.Thereafter,theslagpartwasphysicallydividedintoanumberofzonesonthebasisofthedistancefromtheslag-metalinterface.Slagsampleswerecollectedverycarefullyfromeachzone.Theslagaswellasmetalsampleswereexaminedbylightopticalmicroscope(LOM)andscanningelectronmicroscope(SEM)(JEOL,ModelJSM840).Forthispurpose,smallpiecesofcollectedslagweremountedandanalyzed.Themetalcontentsinthesampleswereanalyzedbyspectrophotometer.1.2ResultsThesampleswerecollectedinbothinductionstirredladleaswellasingasstirredladle.Inthecaseofinductionstirring,fourdifferentstrengths;300A,600A,750A,and900A,wereapplied.Ineachcase,sampleswerecollectedfromthreedifferentpositions;P1,P2andP3in(a)(b)Fig.1Schematicdiagramofsampler(a)Empty(b)filledwithmetalandslagSECRETARIATUSE;Headershouldbeblank.SECRETARIATUSE;FootershouldbeblankFig.2samplingpositionsintheladletheladleasshowninFigure2.Allthethreesamplestakeninthegasstirringmodewerecollectedfromthesameposition,P2,veryclosetotheopeneyearea.Thephysicalappearanceofalltheopenedslag-metalsampleswasfoundverysimilar.Inallsamples,theslag-metalinterfacewasclearlydistinguishablewi