ECCC-100轻压下对内部质量的改善---大方坯高碳钢

CCtechnologySession51Düsseldorf,27June–1July2011Correspondingauthor:KyungShikOh,POSCO,P.O.Box36,Pohang,Korea,[phone](tel),[phone](fax),[e-mail]ualityofaCCbloomisveryimportantforbothmanufacturingprocessandfinalservicepropertiesofhighcarbonsteel.Breakageoccursduringdrawingandstrandingprocessduetocentersegregation[1]incaseoftirecordsteelanditreducesproductivityoftirecordwire.Forbearingsteel,centersegregationcausesshortageoffatiguelifeofballorrollerbearings[2].Strictdemandoninternalqualityofhighgradesteelssuchasatirecord,bearing,andcablewiresteel,etc.hasbeenbeingbiggerandbiggercontinuallytogetherwithdemandondevelopmentofhigherperformancesteel.Incaseofalargebloomcasterwithoutanyqualityimprovingequipments,centersegregationofhighcarbonsteelispreferabletooccurgreatlycomparingwithconventionaloneofbloomthicknesssizelessthan300mminspiteofbetterproductivityandsurfacequality.Thereforesoftreductionunitsof12m-long-reductionzoneaswellasmoldandfinalEMShavebeeninstalledat#2bloomcasterof400x500mm2sectionsizeinPohangWorksofPOSCOinordertoreducecentersegregationofthelargebloom.Applyingsoftreductiontechnology,reductionzone,reductionamount,andreductionrateshouldbeoptimizedtomeetexcellentsoundnessofthebloom.Incaseofthelargebloom,microsegregationatcenterregionshouldbeconsideredtodecidesoliduslineduringcastinginordertofindoptimumreductionzoneandeffectivereductionamountisthoughttobeimportantinviewpointoffindingoutoptimumreductionamount.Inthisstudyheattransferanalysishasbeencarriedoutandsolidustemperaturewasderivedaftermicrosegregationofsoluteelementswascalculatedwithconsideringdendritestructure.Baseontheaboveresultssoftreductiontestwascarriedoutinthederivedreductionzone,whichwasmodifiedafterwardsfrominvestigationofthebloomspecimens.Inordertofindoutoptimumreductionamount,effectivereductionamountwasestimatedfromdeformationanalysisatthereductionrollduringcastingwhensoftreductionapplied.Softreductiontestwascarriedoutwithvariousreductionamountsconsideringeffectivereductionamount.Convextyperollwasalsousedtoenhanceeffectivereductionamountandreducereductionrollforce.Byaidoftheoptimumsoftreductionconditioninternalsoundnessofthelargebloomofhighcarbonsteelwasgreatlyimprovedandfarexceededthatoftheconventionalbloom.Solidustemperatureandreductionzoneconsideringmicro-segregationIncaseofalargebloomcastingmicro-segregationbecomesmoreseverethanconventionalonebecauseTheEffectofSoftReductionConditionsonInternalQualityofaLargeHighCarbonSteelCCBloomKyungShikOh*,JooDongLee**,ChangHoMoon***,andJooChoi*****PrincipalResearcherTechnicalResearchLabs.,POSCO,P.O.Box36,Pohang,Korea**SeniorResearcherTechnicalResearchLabs.,POSCO.P.O.Box36,Pohang,Korea***PrincipalResearcherTechnicalResearchLabs.,POSCO,P.O.Box36,Pohang,Korea****SeniorPrincipalResearcherTechnicalResearchLabs.,POSCO,P.O.Box36,Pohang,KoreaAbsractInternalqualityofalargebloomdependsoncaststructure,centersegregation,V-segregation,nonmetallicinclusions,andinternalcrackamongwhichcentersegregationandV-segregationforahighcarbonsteelhigherthanCwt.%=0.8areexpectedtobemoreseverethanaconventionalbloominspiteofbetterproductivityandcleannessduetolargesectionsizeandlowcastingspeed.Inordertoimprovethemsoftreductionunitswereinstalledat400x500mm2bloomcasterofPohangWorksofPOSCOandbetterinternalqualitywasachievedcomparedwiththeexisting300x400mm2bloomasresultsofvarioussimulationsandtests.Fromthem,effectivereductionamountatcenterregionandcorrectpositionofcraterendwerefoundtobemostimportantamongsoftreductionconditionsincaseofalargebloomcaster.Morethan20mmoftotalreductionamountwasneededincaseofaflatrollandcraterendwaslocated3~4mdownwardsfromtheofferedone.Incaseofconvexroll,effectivereductionamountwashigherthanflatrollfromthesimulationandreductionforcewasfoundtodecrease.V-segregationwasmuchdecreasedbyconvextypesoftreductionroll.Keywords:alargebloom,centersegregation,highcarbonsteel,softreduction,effectivereductionamount,craterendCCtechnologySession52Düsseldorf,27June–1July2011CalculationofequilibriumatT=1550(℃)CalculationofequilibriumatT=TLL+20CalculationofequilibriumatT=TLLAunitofdendriteisdividedinto20meshesk=1,fs=0Kthincrementishypotheticallysolidifiedt=0,tcta,tc:TLLofresidualliquidCalculationofdiffusionofsoluteelements.t=t+dt,ta=ta-dt*C.R.tc:TLLofresidualliquidEndofsolidificationofkthelementfs=fs+dfsCalculationofequilibriumforresidualliquidand(1-alpha)ofpreviouslyprecipitatedslagalpha=dfs/(fl+dfs)┃tc-ta┃εSTARTk=k+1kkf(=20)fs=1TSL=taENDFig.1Algorithmofthecombinedcalculationofthermo-mechanicalandsegregationmodelsusedforthecalculationofsoliduslineFig.2Schematicviewofdendritemodelformicro-segregationmodelofslowersolidificationrate.So,itshouldbeconsideredmorepreciselytocalculatesolidustemperatureandafterwards,iso-solidfractionlinecanbederived.Micro-segregationwascalculatedbycombinedcalculationofthermo-mechanicalandsegregationmodelsasshowninFig.1.AscanbeseeninthefigureequilibriumconditionandliquidustemperatureatatemperaturewellaboveTLLarecalculated,andequilibriumcompositionandweig