183-paper-DIFT in Low Carbon Steels

DeformationInducedFerriteTransformationinLowCarbonSteelsHanDONGandXinjunSUNNationalEngineeringResearchCenterofAdvancedSteelsTechnologyCentralIronandSteelResearchInstitute,Beijing100081,ChinaAbstract:Deformationinducedferritetransformation(DIFT)isakindofsolidtosolidtransformationinducedthroughdeformation,whichcanbeappliedtobeastheeffectivemethodtoproducefineorultrafineferritegrains.ThispaperreviewstheresearchprogressinthetheoryandapplicationofDIFTfromfiveaspects:evidenceandstudymethods,thermodynamicsandkinetics,transformationmechanisms,factorsinfluencingDIFT,applicationofDIFTinproductionoffinegrainedC-Mnsteelandultrafinegrainedmicroalloyedsteel.Keywords:deformationinducedferritetransformation,thermodynamics,kinetics,transformationmechanism,fineorultrafinegrainedsteel1.IntroductionDeformationandtransformation(orheattreatment)aretwobasicmeansofsolidstateprocessingofsteels.Oringinally,thedeformationandtransformationwerethoughttobeindependentofeachother,thustheywereseparatelyutilizedtoacquirethedesiredshapeandpropertiesofsteelsrespectively.Inthelateof1950s,itbegantoberecognizedthatdeformationcouldhaveaninfluenceonthesubsequenttransformation,whichresultedintheoccurrenceofthefamousThermo-mechanicalControlledProcessing(TMCP)technologyin1960s[1].In1980s,itwasfoundthattransformationcouldevenbedynamicallyinducedbydeformationinlowcarbonsteels,andthegrainsizeofthedynamicallytransformedferritewasmuchfinerthanthatproducedbyconventionalhotrolling[2,3].Asaneconomicandsimplemeanstoproduceultrafinegrainedferrite,thedynamictransformationhasbeenextensivelystudiedsince1990sthoughthenamesthatdifferentresearcherscalledweredifferent[4-48].Theworkofoursshowedthatthetransformationisinducedthroughanincreaseofbothaustenitefreeenergyandnucleationsitedensitycausedbydeformationapplied,soeachofdeformationvariables,aswellasstrain,strainrate,temperatureordeformationapproachcouldhaveaneffectonthetransformation.Thus,wenamedthiskindoftransformationas“DeformationInducedFerriteTransformation”(abbreviatedasDIFT)[8-13].TheDIFTtechnologyhasbeenusedforthetrialproductionoffineorultrafinegrainedlowcarbonsteelinChina.Inthispaper,thetheoryandapplicationofDIFTarebrieflyreviewed,wherefiveaspectsareaddressed:evidenceandstudymethodsofDIFT,thermodynamicsandkinetics,transformationmechanisms,factorsinfluencingDIFT,theapplicationofDIFTinproductionoffinegrainedC-Mnsteelandultrafinegrainedmicroalloyedsteel.2.TheevidenceofDIFTandthestudymethodsTheearliestevidenceofDIFTwasobtainedbythemicrostructureobservationonthesamplequenchedimmediatelyafterdeformation.Theroleofquenchingwastopreservetheferritedynamicallytransformedduringdeformation.Atypicalexamplewasshownasfollows.Alowcarbonmicroalloyedsteel(0.094%C-0.29%Si-1.42%Mn-0.045%Nb)wasfirstlysoakedat1453Ktoaustenite,thencooledatacoolingrateof5K/sto1039K(higherthanAr3)andcompressedtoastrainof0.92atastrainrateof1s-1,followedbywaterquenchingimmediately.Themicrostructureobtainedwastheequiaxedferriteandmartensite,asshowninFig.1.Theequiaxedferritewasthoughttobethedynamicallytransformedone.NowthemicrostructurecharacterizationonthequenchedsamplehasbecomethemostpopularmethodtostudyDIFT.However,themicrostructuredynamicallyformedduringdeformationisveryeasytochangeduringthequenchingbecauseofthelowstabilityofdeformedlowcarbonaustenite.Thus,whentheabovementionedmethodisused,thequenchingproceduresshouldbecarefullydesignedtoensurethatthecoolingrateisrapidenoughtopreservethemicrostructureformedduringdeformation.Fig.1MicrostructureofthesamplequenchedafterdeformationofausteniteH.Yadaet.alpresentedadirectevidenceofDIFTbyusingin-situX-raydiffraction[4-6],asshowninFig.2.ThespecimenwasexposedtotheX-raybeamduringtorsionaldeformationperiod.X-raylineswererecordedonafilminaflatcamerashieldedfromheatwiththinaluminumfoils.ItwasshownfromFig.2thatthereappearedalinecorrespondingto(110)togetherwith(111)line.Thisclearlyshowedthattransformationoccurredduringdeformation.Throughthismethod,itwasprovedthatDIFTcouldoccurinawiderangeoftemperaturesincludingthosehigherthanpara-equilibriumtransformationtemperature,A3pinFe-6Ni-(0.00080.29)Calloys.However,quantitativestudyonDIFTbyusingin-situX-raydiffractionisalmostimpossiblepresentlybecauseofthelowintensityofX-raybeam.913K953K993KFig.2EvidenceofDIFTobtainedbyusingin-situXraydiffraction[4-6](a)Aschematicdiagramofthein-situXraydiffractionexperimentsetup;(b)TypicalX-raydiffractionpatternsofthesampledeformedatvarioustemperatureFig.3SchematicdiagramofthenewmethodtomeasuretheDIFquantitativelyTheauthorshavedevelopedanewmethodbasedonquantitativedilatometricanalysistomeasurethedeformationinducedferritewithbetterreliabilitythanthequenchingmethod[8].Thekeypointsareasfollows:(i)AnisothermalholdingexperimentisconductedimmediatelyafterDIFT,anddilatometricdataaremeasuredduringtheholding,asshowninFig.3.(ii)TheDIFvolumefraction(fD)canbecalculatedbytheexpressionTDHfff,wherefTisthefinalvolumefractionofferriteaftertheholding,andfHisthevolumefractionofferriteformedduringtheholding,whichcanbecalculatedfromthedilatometricd