4-paper-Avoidance Of Microstructural Heterogeneiti

SECRETARIATUSE;Headershouldbeblank.SECRETARIATUSE;FootershouldbeblankAVOIDANCEOFMICROSTRUCTURALHETEROGENEITIESBYHOTROLLINGDESIGNINTHINSLABDIRECTROLLEDNIOBIUMMICROALLOYEDSTEELSP.URANGA,B.LOPEZandJ.M.RODRIGUEZ-IBABE(CEITandTecnun(Univ.ofNavarra),ManueldeLardizabal15,20018Donostia-SanSebastian,BasqueCountry,Spain)Abstract:Themicrostructuralevolutionduringhotrollingofa0.05%NbmicroalloyedsteelismodeledconsideringallthemicrostructuralreactionsthatcantakeplaceduringanindustrialTSDRproduction.Basedontheresultsobtainedfromthemodel,processingmapsaredrawnandoptimumprocessingconditionsdefinedforeachfinishinggaugethickness.Facingthedifficultiespresentintheproductionofthickhotstrip,severalalternativethermomechanicalschedulesareproposed,whichwouldoriginatemicrostructureswithasuitablecombinationofhomogeneityandretainedstrainpriortotransformation.Keywords:ThisSlabDirectRolling,Nbmicroalloyedsteels,MicrostructuralHeterogeneities,MicrostructuralModeling,AusteniteAs-CastStructure0IntroductionNearnetshapetechnologiesareexpandingrapidlyallovertheworld,inducedbythesurgeinsteelmakingcapacitydrivenmainlybyChina1.Amongthem,ThinSlabCastingandDirectRolling(TSDR)routesarebecomingmoreandmoreimportantforflatproductprocessing2.EvenifinthebeginningthecoreproductionwasfocusedonplainC-Mnsteels,theprogressiveintroductionofmicroalloyingelementspromotedtheavailabilityofhigherstrengthgrades.Duetothemetallurgicalchangesinducedbythenewtechnologies,differentproblemssoaredrelatedtothemicroalloyingaddition.InNbmicroalloyedsteel,thelackofrefinementoftheinitialas-caststructuremayinducesomeheterogeneitytoappearinthefinalproduct,particularlyforthickfinalgauges.Thecombinationofverycoarseas-castaustenitegrainsizepriortohotrolling3andthesmallertotalreductionavailableduringhotrollinglimitsthepossibilityforausteniterefinementbycontinuousrecrystallizationprocesses.Thislimitationisenhancedbythestrongsolutedrageffectproducedbyniobiuminsolution4.Inaddition,anystraininducedprecipitationoccurringpriortoacompleterefinementoftheinitialas-caststructure,wouldinducewidezonesconstitutedbyhardphasesinthefinalproductaftertransformation,whichappearelongatedintherollingdirectioninsidetheferrite-pearlitematrix5.Thesemicrostructuralheterogeneitiesmayhaveanimportanteffectonthetoughness3.Therefore,Nbmicroalloyedsteelprocessingbydirectrollingtechnologyrequiresadaptingtherollingparameterstothespecificcharacteristicsofthenewrouteinordertoachievesimilarfinalpropertiesasthoseobtainedbytraditionalroutes.Thermomechanicaltreatmentsshouldbeabletomeettwomainobjectives:first,acompleteeliminationoftheinitialas-castmicrostructureinthefirstrollingstandsbeforetheonsetofstraininducedprecipitationmustbeachieved;andthen,themaximumlevelofstrainshouldberetainedbeforetransformationinthefinalpartoftherollingmill6.Mathematicalmodelingisausefultooltodesignhotrollingschedules.EachmodelhastobeSECRETARIATUSE;Headershouldbeblank.SECRETARIATUSE;Footershouldbeblankbuiltuponthesuitableinitialhypothesesandboundaryconditions.Theintroductionofnewprocessingroutesintotheindustryforcedtoadaptclassicalmodelsortoincludenewfeatureswhichwouldallowthemodelingofphenomenanotobservedinconventionaltechnologies.Forexample,therefinementoftheinitialas-caststructurecannotbemodeledusingtheclassicalmodelingapproachwithmeangrainsizes.Inordertoovercomethislimitation,amodelincorporatinginitialaustenitegrainsizedistributionswasdevelopedatCEIT3,7,8.Themodeltakesasaninputtheinitialaustenitegrainsizedistributionandprovidesthesizedistributionsforrecrystallizedandunrecrystallizedfractionsattheentryofanyrollingpass,consideringthesimultaneousinteractionofthemechanismsactingduringtheinterpasstime,suchas,staticandmetadynamicrecrystallization,graingrowthandNb(C,N)straininducedprecipitation.AlltheequationsusedwerederivedforNbmicroalloyedsteelsandadaptedforawiderangeofinitialgrainsizes.FurtherdetailsmaybefoundatRef.7.Inordertoachievesimultaneouslythetwomentionedrequirementsduringhotrolling,as-castgrainrefinementandadequateausteniteconditioning,appropriateconditionforeachchemicalcompositionandfinalgaugethicknessshouldbedefined.Theconstructionofprocessingmapsbasedonthemodelresultshasbeenproposedforabetterunderstanding9.AnexampleofonethesemapsisshowninFigure1,forthecaseofa0.035%Nbmicroalloyedsteel.TherollingparametersaredetailedinTable1.22.533.5410401050106010701080109011000.20.20.20.30.30.30.30.40.40.40.40.50.50.50.50.60.60.60.60.70.70.8FinalGaugeThickness(mm)TotalStrainRollingEntryTemperature(ºC)12.6576431.52OptimumProcessingZone10Residualunrefinedas-castgrains0.035%NbRetainedStrainFigure1Processingmapfora0.035%Nbmicroalloyedsteel9.Isoclinescorrespondtoretainedstrainpriortotransformation.Figure2As-castaustenitegrainsizedistributioninanindustrialthinslabsample.Theareadenotedas‘Residualunrefinedas-castgrains’isreferredtoprocessingconditionsunderwhichafractionbiggerthana2%oftheinitialas-castgrainsremainsunrecrystallizedinthefinalmicrostructurebeforetransformation.Forfinalgaugesthickerthan6-7mmaminimuminitialtemperatureisneededinordertorefinetheas-ca