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1、SECRETARIATUSEHeadershouldbeblank.SECRETARIATUSEFootershouldbeblankAVOIDANCEOFMICROSTRUCTURALHETEROGENEITIESBYHOTROLLINGDESIGNINTHINSLABDIRECTROLLEDNIOBIUMMICROALLOYEDSTEELSP.URANGAB.LOPEZJ.M.RODRIGUEZIBABE(CEITTecnun(Un

2、iv.ofNavarra)ManueldeLardizabal1520018DonostiaSanSebastianBasqueCountrySpain)Abstract:Themicrostructuralevolutionduringhotrollingofa0.05%Nbmicroalloyedsteelismodeledconsideringallthemicrostructuralreactionsthatcantakepla

3、ceduringanindustrialTSDRproduction.Basedontheresultsobtainedfromthemodelprocessingmapsaredrawnoptimumprocessingconditionsdefinedfeachfinishinggaugethickness.Facingthedifficultiespresentintheproductionofthickhotstripsever

4、alalternativethermomechanicalschedulesareproposedwhichwouldiginatemicrostructureswithasuitablecombinationofhomogeneityretainedstrainpritotransfmation.Keywds:ThisSlabDirectRollingNbmicroalloyedsteelsMicrostructuralHeterog

5、eneitiesMicrostructuralModelingAusteniteAsCastStructure0IntroductionNearshapetechnologiesareexpingrapidlyalloverthewldinducedbythesurgeinsteelmakingcapacitydrivenmainlybyChina1.AmongthemThinSlabCastingDirectRolling(TSDR)

6、routesarebecomingmemeimptantfflatproductprocessing2.EvenifinthebeginningtheceproductionwasfocusedonplainCMnsteelstheprogressiveintroductionofmicroalloyingelementspromotedtheavailabilityofhigherstrengthgrades.Duetothemeta

7、llurgicalchangesinducedbythenewtechnologiesdifferentproblemssoaredrelatedtothemicroalloyingaddition.InNbmicroalloyedsteelthelackofrefinementoftheinitialascaststructuremayinducesomeheterogeneitytoappearinthefinalproductpa

8、rticularlyfthickfinalgauges.Thecombinationofverycoarseascastaustenitegrainsizepritohotrolling3thesmallertotalreductionavailableduringhotrollinglimitsthepossibilityfausteniterefinementbycontinuousrecrystallizationprocesse

9、s.Thislimitationisenhancedbythestrongsolutedrageffectproducedbyniobiuminsolution4.Inadditionanystraininducedprecipitationoccurringpritoacompleterefinementoftheinitialascaststructurewouldinducewidezonesconstitutedbyhardph

10、asesinthefinalproductaftertransfmationwhichappearelongatedintherollingdirectioninsidetheferritepearlitematrix5.Thesemicrostructuralheterogeneitiesmayhaveanimptanteffectonthetoughness3.TherefeNbmicroalloyedsteelprocessing

11、bydirectrollingtechnologyrequiresadaptingtherollingparameterstothespecificacteristicsofthenewrouteindertoachievesimilarfinalpropertiesasthoseobtainedbytraditionalroutes.Thermomechanicaltreatmentsshouldbeabletomeettwomain

12、objectives:firstacompleteeliminationoftheinitialascastmicrostructureinthefirstrollingstsbefetheonsetofstraininducedprecipitationmustbeachievedthenthemaximumlevelofstrainshouldberetainedbefetransfmationinthefinalpartofthe

13、rollingmill6.Mathematicalmodelingisausefultooltodesignhotrollingschedules.EachmodelhastobeSECRETARIATUSEHeadershouldbeblank.SECRETARIATUSEFootershouldbeblankprocessingconditionsfrolling.1ModelingConditionsFthisstudya0.05

14、%Nbmicroalloyedlowcarbonsteel(0.06%C0.008%N1.1%Mn)hasbeenedconsideringasaninputtheascastaustenitegrainsizedistributionmeasuredinanindustrialthinslabsample(seeFigure2).Ascanbeseeninthegraphtheascastmicrostructureisacteriz

15、edbyawiderangeofgrainsizeswithasignificantvolumefractionofgrainsbetween12mm310thedistributionspreadinguptovaluesashighas20002500?mhavingameanvalueof8001000?m.Severalrollingscheduleswithaninitialthinslabthicknessof55mmfin

16、algaugethicknessesvaryingfrome=1.5mmto12.65mmweredefinedasindicatedinTable1.Inthecaseofthickestgauges10mm12.65mmtherollingsequenceof6passeswasreducedto5sts.Thelargereductionsedattheinitialrollingpasseshadtheaimofpromotin

17、gtherefinementoftheascastmicrostructureattheearliestrollingstages.Fthispurposeinthethreefirstrollingpassesaratiovaryingfrom0.63to0.74ofthetotalavailablestrainwasapplied.Table1.Rollingparametersconsideredincomputersimulat

18、ionsffinalgaugethicknesse(mm).e=1.5e=2e=3e=4e=6e=7e=10e=12.65Pass???(s1)tip(s)???(s1)tip(s)???(s1)tip(s)???(s1)tip(s)???(s1)tip(s)???(s1)tip(s)???(s1)tip(s)???(s1)tip(s)?T(C)11560.9560.75560.7560.55560.5560.5560.45635211

19、530.91530.751040.71040.551040.51040.51040.41043030.85501.80.84020.71530.551530.551530.451530.451550.41553040.559010.5701.60.54020.45302.10.4252.50.4252.5??????3050.41500.70.49010.4601.70.35501.80.354020.3302.10.3202.70.3

20、202.73060.32000.31200.3800.25700.25600.25500.25250.225()()Dependentonthefinishingrollingtemperatureataconstantcoolingrateof20Csupto800C.Aninitialrollingtemperaturerangevaryingfrom1060to1150Cwasdefinedfthisparticulargrade

21、.Everytemperatureconditionisbasedonthealloyingcontent(noticethedifferentrangefthe0.035%NbsteelinFigure1)evenifgeneralconsiderationsweretakenintoaccountneitherrealmillloadingeffectsnrealpreheatingfurnacetemperaturelimitat

22、ionshavebeentakenintoaccount.Interpasstimeswerecalculatedasafunctionofthestrainrateconsideringaconstantinterpassdistanceof5.5m.Afterlastdefmationpassacoolingtemperatureof20Cswasedftherunouttablefromthefinishingtemperatur

23、etothetransfmationtemperatureof800C.2RollingSimulationsAsmentionedbefetheoutputsofthemodelarehistogramsobtainedfromeachparticularrollingcondition.Figure3showstheresultsobtainedftheschedulescrespondingtothefinalgaugethick

24、nessof10mmattworollingtemperatures.TheeffectofTiisclearlyevidentwhencomparingbothhistograms.Ingeneralsmallermeangrainsizesresultflowerrollingstarttemperatures.Neverthelesssomefeaturesmustbelightedout.Thegrainsizedistribu

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