分析c !转换在Nb micro-alloyed C-Mn钢

Analysisofc!atransformationinaNbmicro-alloyedC–MnsteelbyphasefieldmodellingM.G.Mecozzia,b,*,J.Sietsmab,S.vanderZwaagcaDepartmentofMaterialScienceandEngineering,NetherlandsInstituteforMetalsResearch,Rotterdamseweg137,2628ALDelft,TheNetherlandsbDepartmentofMaterialsScienceandEngineering,DelftUniversityofTechnology,Rotterdamseweg137,2628ALDelft,TheNetherlandscFacultyofAerospaceEngineering,DelftUniversityofTechnology,Kluyverweg1,2629HSDelft,TheNetherlandsReceived9May2005;receivedinrevisedform1November2005;accepted13November2005Availableonline5January2006AbstractThephasefieldmodelisusedtosimulatethec!atransformationinaNbmicro-alloyedC–Mnsteelduringcoolingfromdifferentaustenitisationtemperatures.Theinitialausteniticmicrostructureandthenucleationconditions,derivedbymetallographictests,aresetasinputdataofthemodel.TheinterfacemobilityistakenasrepresentativefortheeffectofNbConthephasetransformation.Itisusedasafittingparametertooptimisetheagreementbetweentheexperimentalferritefractioncurve,obtainedbydilatometry,andthesim-ulatedferritefractioncurve.Adecreaseofthec/ainterfacemobilityisfoundwhentheaustenitisationtemperaturedecreasesfrom1373to1173KasaconsequenceofthepresenceofNbCprecipitatesduringaustenitisation.ThepresenceofNbChasadistincteffectonthenatureofthephasetransformationwithrespecttotheinterface-controlledordiffusion-controlledmode.
2005PublishedbyElsevierLtdonbehalfofActaMaterialiaInc.Keywords:Nbmicro-alloyedsteel;Precipitation;Phasetransformationkinetics;Phasefieldmodel1.IntroductionThepropertiesofC–Mnsteelsstronglydependonthemicrostructurethatformsasaresultofaustenite(c)decompositionduringcoolingfromthehot-rollingtemper-ature.Uponcoolingfromtheaustenitictemperaturerange,proeutectoidferrite(a)formationusuallyoccursfirstandhasastronginfluenceonthesubsequentdiffusionalorpartlydiffusionalphasetransformationproducts(pearlite,bainite).Forthisreasonthectoatransformationisofgreatinterestasregardstheproductionofsteel.Generallyspeaking,thectoatransformationoccursintwosteps:nucleationandgrowth.Uponnucleationanewinterfaceisgeneratedthatseparatestheproductaphasefromtheparentcphase.Thisinterfacemigratesintothesurroundingparentphaseduringsubsequentgrowth.InrecentworkonferritenucleationinaC–Mnsteel,Offer-manetal.[1]showedthatthenumberofferritenucleiincreasesrapidlyduringcoolingjustbelowthenucleationstarttemperatureandthusnewferritegrainscontinuouslyformoveratemperaturerangeofabout50K.Themigra-tionrateoftheinterfaceisdeterminedbythediffusionofthealloyingelements(mainlyCandMn)aheadoftheinterfaceandtheintrinsicinterfacemobility,whichiscon-nectedwiththestructuralrearrangement(c(face-centredcubic)!a(body-centredcubic)).Thectoatransforma-tionisoftenclassifiedaseitherdiffusioncontrolled[2–4]orinterfacemobilitycontrolled[5,6],dependingonwhetherthemajorityofthefreeenergyaccompanyingthetransformationisdissipatedbysolutediffusioninthebulkphasesorbythestructuralrearrangement.Inrealitybothprocesseswillinfluencethetransformationkineticsandthetransformationcanbeexpectedtobeofamixed-modecharacter[7,8].InNbmicro-alloyedC–MnsteelthepresenceofNbstronglyaffectsthectoatransformation.Evenavery1359-6454/$30.00
2005PublishedbyElsevierLtdonbehalfofActaMaterialiaInc.doi:10.1016/j.actamat.2005.11.014*Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddress:[e-mail](M.G.Mecozzi).(2006)1431–1440smallNbadditionretardsferriteformationconsiderably.ItseemsreasonabletobelievethatsoluteNbsegregatestotheaustenite/ferriteinterfaceandreducestheferritegrowthkineticsduetosolutedrageffects[9].Ithasbeenestablishedthattheprecipitationofniobiumcarbide(NbC)duringtheaustenitisationtreatmentrefinesthecstructureandpromotestheoccurrenceoftheausten-itetoferritetransformationathighertemperatures[10].TheprecipitationofNbCduringaustenitisationcouldalsohaveaneffectonthecdecompositionkinetics[11].Fewstudieshavebeendoneinthisfield.However,thereissomeevidence,derivedfromrecrystallisationstudies[12],sug-gestingthatNbCformedduringaustenitisationcanretardtheprogressofthetransformationbypinningthec/ainterface.Thisworkdealswiththekineticsoftheaustenitetofer-ritetransformationduringcontrolledcoolingofaNbmicro-alloyedC–Mnsteel.ThetransformationkineticsareinvestigatedexperimentallyusingdilatometryforbothaC–Mn–NbsteelandananalogousC–Mnsteel.ThephasefieldmodelderivedbySteinbachandco-workers[13,14]isusedtosimulatethectoatransformationduringcon-trolledcooling.Phasefieldsimulationsareperformedunderthesamecoolingconditionsastheexperiments.Theinitialausteniticmicrostructureandtheferritenucle-ationdata,derivedbymetallographicexamination,aresetasinputdataofthemodel.TheeffectiveinterfacemobilityistakenasrepresentativefortheeffectofNbConthephasetransformation.Itisusedasafittingparam-etertooptimisetheagreementbetweenthesimulatedandexperimentalferritefractioncurves,thelatterderivedbydilatometry.TheeffectofthesoluteandprecipitatedNbontheapparenta/cinterfacemobilityisinvestigated.2.MaterialsTwosteelsareinvestigatedinthiswork:thefirstisaNb-containingsteel,referredtoasC–Mn–Nb