Continuous Steel Casting专业英语学习

AnintroductiontoContinuousCasting整理课件ContentsIntroductionSolidificationofSteelDescriptionofthecontinuouscastingprocessDefectsoftheslab,billetandbloom整理课件IntroductionContinuouscasting(CC)ofsteelhasarelativelyshorthistoryofonlyabout50years.TheCCratiofortheworldsteelindustry,nowapproaching90%ofcrudesteeloutput,attainedamere4%in1970.整理课件Continuouscastinghasemergedasoneofthegreattechnologicaldevelopmentsofthetwentiethcentury,replacingingotcastingandbloomingoperationsfortheproductionofsemi-finishedshapes:slabs,bloomsandbillets.Theprocesshasbeenadoptedworldwidebythesteelindustryinthelatterhalfofthetwentiethcenturyowingtoitsinherentadvantagesoflowcost,highyield,flexibilityofoperation,andtheabilitytoachieveahighqualitycastproduct.整理课件Evolutionofworldsteelproductionandshareofcontinuouscasting整理课件Characteristiccastertypesanddesignheight整理课件SolidificationStructuresThereisanouterchillzonecomprisingfine,randomly-orientedgrains;anintermediatecolumnarzonecomprisingelongated,orientedgrains;andacentralequiaxedzone,againcomprisingrandomly-orientedgrains.Oneoranotherofthesezonesmaybesmallorabsentinanygivencastingoringot.整理课件Sketchofingotgrainstructure,showingchill,columnarandequiaxedzones整理课件Illustrationofdendrites:(a)3-dimensionalview;(b)typicalcolumnardendritesduringsolidificationofanalloyagainstacoldchillwall;and(c)equiaxeddendriticsolidification.整理课件GrainSizeThefinalsolidificationgrainsizeisdeterminedby:-nucleationfrequency,-grainsurvivalduringgrowth,and-multiplicationofgrainsduringsolidification.整理课件Nucleationtheory—forbothveryclean,isolatedliquidmetals(homogeneousnucleation)andformetalsincontactwithsubstratessuchasinclusions(heterogeneousnucleation)—hasbeenwelldevelopedoverthelast50years.Thegreaterthecoolingrate,thegreaterthenucleationfrequency,onemayexpectthatthegreaterthenumberofgrains.Itisoften,butnotalways,thecasethatfastercoolingratesresultinfinergrainsize.整理课件Attheveryfirststagesofsolidification,dendritesarefragileandarmsareeasilybrokenoff.Theprocessisfavoredbyvigorousconvectionandbylowmetalsuperheataheadofthesolidifyingfront(sothattheseparateddendritearmsdonotremelt).Electromagneticstirringofcontinuouscastingsisnowwidelyemployedtominimizethecolumnarzoneandtorefinethegrainsizeoftheequiaxedzone.整理课件Themagnitudeofelectromagneticforceandthefractionsolidtowhichtheelectromagneticforceisappliedarecontrolledinmagneticstirringofcontinuouscastingsinordertomaximizethegrain-refiningeffect.Whiletheextentofequiaxedzoneincastsiscertainlyincreasedandtherebythecenterlinesegregationisreducedbytheelectromagneticstirring,theformationofsemimacrosegregationaroundtheequiaxedgrains.整理课件Initialformationofequiaxedgrainsisintoanundercooledmelt.Thesolid/liquidinterfacehasthehighesttemperature.整理课件MicrosegregationSegregationofalloyelementsincastingsandingotsistermedmicrosegregation

whenittakesplaceoverdistancestheorderofthedendritearmspacing(typically50–400microns).Microsegregationresultsbecausethesolidformingisdifferentincompositionfromtheliquidfromwhichitforms(generallylessrichinsolute).Theexcesssoluteistherebyrejectedintotheliquid,intothenarrowspacesbetweenthedendritearms.整理课件Modelofsolidificationfortheliquid-solidzone:(a)dendriticstructure整理课件Fromthepointofviewofreducingmicrosegregationbysubsequentthermaltreatment,thespacingofthesegregateisofprimeimportance;asnotedabove,thatspacingisessentiallythedendritearmspacing.Dendritearmspacingdependssolelyoncoolingrate,asnotedearlier.Itisdeterminedbyaphenomenonknownascoarsening.整理课件InclusionFormationInclusionformationduringsolidificationofferrousalloyscanoftenbefullydescribedbythebasicsolidificationmodel,simplyviewingtheinclusionasanadditionalphaseinamulticomponentsystemthatsolidifieswithnegligibleundercoolingorkineticlimitationtogrowth.整理课件DescriptionofthecontinuouscastingprocessSchematicillustrationofcontinuouscastingprocess整理课件(Neely,1989)整理课件ThemajorcomponentsofthecastingsystemLadle,Tundish,Mold,Secondarycooling,andSteelproperties-temperature,compositionandcleanliness(influencethequalityofthemoltensteelanditscastability).整理课件LadleRegardingtheladle,ittraditionallyhasbeenemployedasatransfervessel,movingheatsofsteelweighing20to350tonnesfromthesteelmakingfurnacetothecontinuouscastingmachine.However,increasinglytheladleisbeingusedasareactorinladlefurnacesorladle-treatmentstations,installedbetweenthesteelmakingfurnaceandthecaster.Intheseoperations,thecompositionandtemperatureofthesteelcanbeadjustedtomeetfinalspecifications.Inthisway,theproductivityofthesteelmakingfurnacecanbeincreased,sinceitsprimaryfunctionsarereducedtomeltingscrapanddecarburization.整理课件Intransferringsteelfromthesteelmakingfurnacetothecaster,amajorproblemisoxygenabsorptionfromtheair,furnaceslagandtheladlerefractorylining.Slide-gatevalveshavebeenattachedtothetapholeofthesteelmakingfurnacetoshutofftheflowofoxidizedfurnaceslagintotheladleattheendofthetappingoperation.Thesurfaceofthesteelintheladleiscoveredwithasyntheticslag,againtopreventoxygenabsorptionfromtheairandalsotoabsorbnonmetallicinclusionsandtominimizeheatlosses.整理课件Insomeoperationstheladleiscoveredwithalid.Finally,whenlocatedoverthecastingmachine,theladleisusuallyequippedwitharefractorypouringtubetopreventoxygenpickupasthesteelispouredintothetundish.Flowofsteelfromtheladletothetundishiscontrolledwithaslide-gatevalve,andinsomeoperationstheweightoftheladleiscontinuouslymeasuredwithloadcells.整理课件TundishThetundishactsasadistributor,dischargingsteeltotheseveralstrandsthatnormallycompriseacastingmachine.Thetundishalsofacilitatesthecontrolofsteelflowintothemoldbecauseithasaconstantandlowerhydrostaticheadthantheladle.Thisisparticularlyimportantduringthestart-upofthecaster,sincethetundishcanbefilledwithsteeltoitsnormalsteady-statelevelbeforepouringintothemoldscommences.整理课件Anotherimportantfunctionthatinfluencesthecleanlinessofthecastproductisthefloat-outofinclusions.Finally,thetundishcanbeusedasareactoraswell,fortheadditionofagentssuchascalcium,forinclusionmorphologycontrol.整理课件整理课件MoldThe“heart”ofthecaster,Theprimaryheat-extractiondevicewhosefunctionsaretoextractsuperheatfromtheliquidsteel,togrowasolidshellofsufficientthickness,tocontaintheliquidpoolbelowthemoldwithoutbreakoutsandtosupporttheshellduringitsinitialgrowth.Thedesignandoperationofthemold,whichgovernsheatextraction,profoundlyaffectssurfaceandinternalquality.整理课件Schematicofphenomenainthemoldregionofasteelslabcaster整理课件MoldpowderWhencontinuouscastingwasintroduced,lubricationandheattransferbetweenthesteelshellandthemoldwereprovidedbytheuseofoilslikerapeseedoil.TypesofMoldPowder:Powders,Granulated,ExtrudedandExpandinggranules.整理课件Thefunctionsofthemoldpowdertoprotectthesteelmeniscusfromoxidation.toprovidethermalinsulationtopreventsolidificationofthesteelsurface.toabsorbinclusionsintothemoltenslagpool.tolubricatethestrand.整理课件Themoldmaybestraightorhaveacurvatureof4to15minradius.CurvedmoldsandStraightmolds.整理课件Inanyevent,steelleavingastraightmoldisnormallybentgraduallytoahorizontalorientation,againtoreducetheplantheight.However,insomecastingoperations,themoldandsub-moldregionarestraightsothatthecastproductisnotsubjectedtomechanicalbendingforcesthatmaygeneratecracks.Withacurvedmold,thesteelstrandmustbestraightenedtoahorizontalpositionpriortobeingcutintolengths.整理课件PrimaryandSecondary

CoolingControlThemostcriticalcomponentofacontinuouscastingmachineisthemold.Itisinthemoldthattheinitialshellformationoccurs,andsufficientshellgrowthandcoolingmustoccurtoallowtheshelltopassintotherollcontainmentwithoutrupturing.Themoldheatremovaldirectlyaffectsshellgrowthandthestressesandstrainsthatareproduced.整理课件Stressesandstrainsmustbebelowlimitsthatwouldpermittheformationofmicrocracks,whichcouldbethebeginningoflargercrackscausedbyadditionalstressesandstrainsinthesecondarycooling,bendingandstraighteningzones.Flowoftheliquidsteelandliquidslagpropertiesmustbesuchthatoxideinclusionsarenottrappednearthesurfaceoftheshellinthemeniscusregion.Thesurfacequalityofthecastproductisadirectresultofmoldevents,especiallyheatremoval.整理课件PrimaryCoolingControlSolidificationoccursinthemushyzone,wheretheheatoffusionisremoved.Inadditionthereisnormallysuperheatintheliquidsteelthatistransmittedintothemushyzone.Partofthesuperheatisremovedinthemold,andtheremainderisremovedinthesecondarycoolingzones.整理课件Thepercentofthemoldheatloadcontributedbythesuperheatisdependentonthemoldsurfacearea/volumeratioandthedegreeofsuperheat.Theeffectivemoldcoolingistheenergythatpassesthroughthesurfaceofthesolidsteelshell,theliquidandsolidslagfilms,thecoppermoldwallandintothecoolingwatertoberemovedfromthemold.整理课件Slagfilmissolidonthecoppersideandliquidonthesteelsideintheupperpartofthemold.Theslagfilmnormallybecomescompletelysolidinthelowerpartofthemold.Insomecases,theremayalsobeintermittentcontactwiththemoldwallinthelowerpartofthemoldduetoshrinkageoftheshellawayfromthecopperwall.Theintermittentcontactwouldcreateairgaps,causingadditionalthermalresistance.整理课件ControlFactorsSpeedMoldPowderLubricantOscillationTaperCoolingWaterVelocityCoolingWaterTemperatureMetalLevel整理课件SuperheatSteelCompositionMoldConditionSENDesign整理课件SecondaryCoolingControlAsignificantpartoftheheatremovalinthesecondarycoolingzoneisindependentofthecastingprocess.Belowthemold,theheattransferoccursbydirectcontactofthesteelshellwiththecoolingmedium.Theheattransfermechanismsaremoreeasilydefinableandcanbequantifiedanddirectlycontrolledtoagreaterextent.整理课件Afterthestrandleavesthemold,itmustbecooleduntilitiscompletelysolidifiedandcanbecutforfurtherprocessing.Foraslabmachine,thelargesectionsizerequiresthatthestrandbecontaineduntilthesolidificationisessentiallycompletetopreventtheshellfrombulgingoutwardduetotheferrostaticpressure.Maximumproductivityrequiresthatthespeedbemaintainedashighaspossible.Thismeansthatthestrandsecondarycoolingalsomustbekeptatahighleveltospeedupthesolidificationrateandproduceacompletelysolidifiedstrandbeforethestrandiscut.整理课件Asthespeedisincreased,theshellthicknessleavingthemoldisreduced,andtheriskofbreakoutsintheregionjustbelowthemoldincreases.Thus,coolingintheregionjustbelowthemoldisespeciallycriticaltomaintainshellthicknessandstrength.Coolingofthestranduntilthepointoffinalsolidificationisalsocriticaltopreventingbothsurfaceandinternalcracksfromformingduetoexcessivestressesandstrains.整理课件Typicalcracksobservedinacontinuouslycastslab.

Legend:Internalcracks1.midway2.triple-point3.centerline4.diagonal5.straightening/bending6.pinchrollSurfacecracks7.longitudinal,mid-face8.longitudinal,corner9.transverse,mid-face10.transverse,corner11.star

整理课件整理课件ClassificationsandAppearanceofSurfaceDefectslongitudinalfacialorcornercracks,transversefacialorcornercracks,starcracksandhotshortness,longitudinalandtransversedepressions,deeposcillationmarks,bleeding,inclusionclusters,slagpatchesorentrappedslags/scums,andgasholes,includingblowholesandpinholes.整理课件LongitudinalCracksTheinitialsolidificationofthesteelshellintheCCmoldtakesplacejustbelowthemeniscusofthemelt.Atthisstage,theshellhasverylowtensilestrengthandnear-zeroductility.Frictional,fluiddynamicandtensilestressesarisingfromstrandwithdrawalandmoldoscillationareappliedtothisweakandbrittleshell.Also,volumecontractionuponsolidificationandthermalstressesuponcoolingimposeadditionalstressesontheshell.Whenthesumofthesestressesorstrainsexceedsthetensilestrengthorductilityoftheshell,cracksoccur.Theoccurrenceofthecracksisenhancedbyunevengrowthalongthemoldperimeteroftheinitialshellcausedbymeniscusfluctuationandsubmeniscusflowturbulence.Manyfactorscontributetocrackformationtoadifferentdegree,causingavarietyofcracks.整理课件LongitudinalFacialCracksInthemid-1970s,whileCCtechnologywasstillintheearlystagesofdevelopment,theoccurrenceoflargecracksextendingafewmeterslongand10–30mmdeepwasnotuncommon.Thisparticularcrackformedinthewithdrawaldirectiononthemid-faceofa1500-mm-wideCCslab.Today,however,becauseoftechnology’smaturation,suchcracksareseenmuchlessfrequentlyunlessaseriousfailureinoperationand/ormaintenanceoccurs.Suchcracksareobservedmostlyduringtransientperiodsofcastingoperation.整理课件Today,typicallongitudinalfacialcracksmaybeclassifiedintotwotypes.Type-1longitudinalfacialcracksareabove100mmlongandlessthanafewmillimetersdeepandcanberemovedbyoff-line

conditioning.Thecracksoccasionallyaccompanydepression.Primarydendritearmsareobservedalongthecracks.Type-2cracksaremuchshorter(lessthan20–30mm)andshallower(1mmorless)facialcracksformedinthewithdrawaldirection.Theyarenotlimitedtothemid-facebutcanbescatteredorclusteredatmid-toquarter-widthofthewideface.整理课件Theoccurrenceofthesecracksbecameevidentinthelate1970stoearly1980s,whenhigh-speedcastingwaspracticedforbetterproductivity,evenoncrack-sensitiveperitecticgrades.Criticalcastingspeedsforperitecticgradesinconventionalandthin-slabcasterstodayarearound2m/minand4m/min,respectively,evenafterallavailablemeasuresaretaken,therebyimposingaproductivitybottleneck.整理课件ThermalandmechanicalfactorsthathavebeenknowntoincreasetheoccurrenceofthecracksarerelatedtoCCmachinesandtheiroperationasfollows:1.Shallowmoldtaper.2.Moldwithlargewidth/thicknessratioorlargewidth.3.Deviationfromunityoftheratioofcoolingwaterflowrateperunitofsurfaceareaforthewidefacetothatofthenarrowface.整理课件4.Abruptchangeinthemeniscuslevelinthemold.5.Excessivelycoolmeniscustemperatureduringtransientperiodsofcasting.6.Turbulentand/orasymmetricbiasedflowofthemeltoutoftheSENintothemold.7.Irregularmoldoscillation.8.Intensivespraycoolingbelowthemold.Otherwise,mostinfluentialtotheoccurrenceofthecracksare:9.Peritecticsteels,and10.Moldfluxproperties.Amongthem,1–5,7and8havebeenmoreorlessresolved;hence,theinfluenceof6,9and11hasbecomemoreimportantinparalleltotheprogressofhigh-throughput/high-speedcasting.整理课件Countermeasurestopreventthecracks,therefore,arethefollowing:1.Minimizetheturbulenceofmetalflow,particularlysubmeniscusflow,byoptimizingmetaltransferoperationsandtundishdesignduringtransientperiodsandbyusinganelectromagneticflowcontrollerforhigh-throughputcasting.2.Minimizemeniscusfluctuationbyutilizingintegratedcontrolofmetaltransferfromtheladletothetundishtothemoldandbyreducingnozzleaccretionwithcleanermeltsandlessreoxidationduringcasting.3.Minimizeexcessivetemperaturedropsduringtransientoperationsbyoptimizingladleandtundishpreheating,ladlestirring,metaltransferoperations,andwithtundishheating.整理课件4.Reduceheattransferinthemoldbytheuseofaslightlybasicmoldfluxhavingahighercrystallizingtemperaturethatprovidesthefluxfilmwitharoughsurfaceatthemold/filmboundary.Thesurfaceroughness(microairgap)andcrystallizedfilmreduce,respectively,theconductionandradiationheattransferfromtheshelltotheheattransferand,hence,uniformgrowthoftheshellresults,therebydecreasingexcessivedevelopmentoflocalizedheatingandresultantshellstresses.整理课件5.Useamoldmadeofaheat-resistantalloythathaslowthermalconductivity.Sofar,lowthermalconductionhasbeensynonymouswithinferiorheatresistancethatcausesstickerbreakoutsandintolerablewear.6.Implementationofanair-mistspraysystemforthesecondarycoolingiseffectiveforavoidingnonuniformandexcessivecoolingbelowthemold.整理课件LongitudinalCornerCrackslongitudinalcornercracksoccursinthevicinityofthecornerofbilletsandbloomsinthewithdrawaldirection,oftenalongthejunctionbetweenthecornerandface.Slabsalsoexperiencethistypeofcrackinhigh-speedcastingofperitecticgrades.Featuresofthecracksarethesameasthoseforthelongitudinalfacialcracks.Conditioningisunavoidable.Heavycrackscancausebreakouts.整理课件Themechanismfortheoccurrenceoflongitudinalcornercracksislargelythesameasthatoflongitudinalfacialcracks.Thermalelastic-plasticvalidatedbycommercialcasting,explainsthatthecornercracksoccurinthefollowingsequence,wherethedeformationofthecontouroftheoutersurfaceoftheshellisinshowninμmonthebottomleftaxes.整理课件Somedistancebelowthemeniscus,ashellisformedallalongtheperimeterofthemoldthatshrinksduetothecoolingonthewideface,thuspullingthenarrow-faceshellawayfromthemoldwalltoformanairgapwhenthemoldtaperisshallowerandnotconsistentwiththeshrinkage.整理课件2.Thenarrow-faceshellisreheatedbytheairgap,andthestrengthisdecreased,whereasthecorneroftheshellisgrowninathickrectangularchannelshapeduetotwo-dimensionalheatextractionatthecorner.3.Staticpressureimposedbythemoltensteelmakestheweakmid-faceoftheshellbulgetowardthemoldwall,butnotmuchnearthecornerarea.Thus,anairgapiscreatednearthestiffcornerofthenarrow-faceshell.4.Thebulging,however,makesthecornershellrotatesomewhattowardtheadjacentface,therebydetachingthenearcornerpartoftheshellofthatfaceawayfromthemoldwall.Thisresultsinairgapformationontheadjacentshellatthenearcornerareainthewithdrawaldirection.整理课件5.Thus,hotandbrittlestreaksoccurattheairgapnearthecorneroneitherorbothsidesofthetwofaces.6.Forbilletsandblooms,increasedrhomboiditycausesthecracksonthecornersthatareexpandedbeyondtherectangularangle.Therhomboidityisenhancedby:(a)unevencoolingbetweenthefourfacesofthemold,(b)excessivelystrongcoolinginthemold,(c)thesameforsecondarycoolingbythewaterspraycoveringthestrandcorners,(d)misalignmentofsupportrollsandthemold,and(e)weakcontainmentbythesupportrollsofthestrand.整理课件TransverseCracksTransversecracksarecommonlyexperiencedmajorcracksusuallyfoundonslabs,bloomsandbillets.Themechanismoftheirformationandmeasurestopreventthemhavebeenmadeclear.However,theincreasedpopularityofhigh-rateproductionofmicroalloyedsteelsandhotdirectrollinghasmadeimplementationofthemeasuresnotalwayssatisfactory.Completepreventionoftransversecracksremainsamovingtarget.整理课件CharacteristicsThecracksoriginateatthebottomofdeeposcillationmarks.Inthethicknessdirection,theydeveloptoashallowdepth,Transversedepressionsusuallyaccompanythetransversecracks.High-carbonsteelsunderunfavorableconditionsshowintergranularcracksofthiskindwithinternaloxidationanddecarburization,andtheyrequireconditioning.整理课件Theobservationandthetheoreticalcalculationofthethicknessofthesubscaleindicatethatthecracksformedinthemoldandpropagatedinthetoppartofthesecondarycoolingzonetothekink.整理课件Thecracksbecomeappreciablewhenthefollowingconditionsaremet:1.Unbendingofthestrandsoccursinthebrittletemperaturerangeof750–850°C(canbe700–900°C).Thebrittlenessinthistemperaturerangehasbeenconfirmedtobedueto(a)hardeningbytheprecipitationoffineAlN,Nb(C,N),VNand/orV4C3and/or(b)precipitationofaferritefilmalongtheaustenitegrainboundariesduringγ→αtransformation2.Peritecticsteelsandsteelscontaining[C],[N],[Al]and[Nb](occasionally[V])inexcessofsuperficialsolubilityproduct(3–5x10-4for[Al][N],dependingonthegradeofsteels).Theperitecticsteelsgiveriseinthestrainduetothecontractionuponperitectictransformationandbecomebrittleduetocoarseγ-grainformation.Insteelscontainingtheabovesoluteelements,ontheotherhand,thefineprecipitates(afewtensofnm)forminthesecondarycoolingzoneatthegrainboundariesoftheshellsurface.Theseprecipitatesmakethestrandsurfacebrittleat700–900°C,whereunbendingwasusuallypracticed.NotonlytheprecipitationofNb(C,N),butalsotheinhibitionofdynamicrecrystallizationofγ-grainsareconsideredresponsiblefortheembrittlementbyNb.整理课件3.Intensivewater-spraycooling,resultinginnonuniformandexcessivecoolingofstrandsurface.ThesurfaceundergoesovercoolingbelowA3inthewaterpoolabovethesupportroll/strandcontactsinthetopzoneofthestrand,andsubsequentreheatingaboveA3afterpassingthesupportrolls.ThefineprecipitatesofAlNdonotformuntil800°C(770–850°C)oncooling,buttheydoprecipitateat700–1000°Conheating.Accordingly,theabovethermalcycles,repeatedcoolingandreheatingacrossthecriticaltemperaturebetweentherollsinthesecondarycoolingzone,promotetheprecipitation,makingthegrainboundariesevenmorebrittle.4.SteelscontainingNi.Theyaresusceptibletothetransversecrackswhenintensivesecondarycoolingisadoptedforunbending.Studiesrevealthatthisisduetothecombinationofthefactsthat(a)thebrittletemperaturerangeextendstothelowtemperaturesidewithNiand(b)tightlyadheredscale,acharacteristicofNi-containingsteels,increasesthebreakdowntemperatureoffilmboilinginthesecondarycooling.Inotherwords,transitionboilingoccursatahighertemperature,impedinguniformcooling.整理课件5.Deeposcillationmarks.Theyeffectivelyincreasetheoccurrenceofthetransversecracksbydecreasingthecriticalstrainabovewhichthecrackscanformonthestrandsurface.Theoscillationmarksactasanotchtoconsiderablydecreasethehigh-temperatureductilityofthecaststrand.整理课件6.Tensilestrainarisingfromunbendingabovethebrittletemperaturezoneenhancesthepropagationofthecracks.Thus,theuppersurfaceofthestrandshowsmorecracksincurvedCCmachines.Thickerslabsalsoformmorecracks.Additionalstrainscausedbyinsufficientlubricationinthemold;misalignment;seized,bentand/orwornsupportrolls;andbulgingofthestrandallcontribute,toadifferentdegree,totheoccurrenceandpropagationofcracksinmorethanjusttheuppersurface.整理课件Mechanismoftransversecrackformationatoscillationmarks.整理课件Measurestopreventcrackformationare,therefore,to:1.Reduce[Nb],[Al]and[N]withinthespecification,nottoexceedthesolubilityproduct.2.Controlthesubmeniscus

flowturbulenceandstabilizethemeniscuslevelinthemold.3.Optimizemoldfluxviscosityandmoldoscillationtoresultinanoscillationmarkdepth<0.2mm.4.Providegoodmachinemaintenanceregardingalignment,rollsandmistsprays.整理课件StarCracksThesecracksaredifficulttofindintheas-caststatewhenthestrandsarecoveredwithscale.Thus,theyarefoundafterscarfing.Thecracksdevelopalongaustenitegrainboundariesradiallyornetwork-likeandappearasstarlightstreaks,withadepthofafewto10mm.Thecrackscaninducetransversefacialcracks.Locallyheavyconditioningcanremovethemifcarefullyexecuted,butitcancausesomedefectsaftersubsequentrolling.整理课件Inmanycases,Cuisdetectedatthescale/steelinterfaceandfracturedgrainboundaries.High-tensile-strengthsteelscontainingAl,NbandVaremoresusceptibletothiscrack.整理课件Thus,countermeasuresforthestarcrackformationinvolvepreventingdirectcontactbetweentheCumoldandstrandshell,decreasingheatfluxanditsfluctuationinmold,andreducingexcessivethermalstressdevelopmentusingoneofthefollowing:(a)amoldfluxmeltwithrelativelylowviscosity,(b)Ni-orNi-alloy-platedmolds,and(c)mildspraycoolingbelowthemold.Inparticular,moldplatinghasprovedtobeeffectiveatstoppingtheoccurrenceofstarcracks.AthincoatingofCrhasbeenimplemented,butitwearsquicklyandlosesthebeneficialeffectinlong-lastingsequentialcasting.Accordingly,athickplatingofNi,whichisrobustagainstwear,hasbeensuccessfullyemployedinmanyplants.AvarietyofhardversionslikeNi-P,Fe-Ni,Ni-CoandNi-Crhavealsobeenreportedsuccessful.UseofahardCualloywithahighermeltingpointisalsoanalternativeunderspecialconditionsforcostreasons.整理课件SubsurfaceCracksSubsurfacecracksoccurinthesubsurfaceofhigh-carbonblooms,andtheyaredifficulttodetectduringhotrolling.Thesecracksmayopenuponthesurfaceduringsubsequentreheatingandscalingoff,therebyoxidizingandcausingadoubleskina