11-paper-The use of a Fe-30% Ni and Fe-30% Ni-Nb a

TheuseofaFe-30%NiandFe-30%Ni-NballoyasamodelsystemsforstudyingthemicrostructuralevolutionduringthehotdeformationofausteniteRainforth,WM*,Cizek,P,Bai,F,Poths,R,Palmiere,EJ(IMMPETUS:InstituteforMicrostructuralandMechanicalProcessEngineering:TheUniversityofSheffield,SheffieldS13JD,UnitedKingdom;DepartmentofEngineeringMaterials,TheUniversityofSheffield,MappinStreet,S13JD,UK)*[e-mail]:Thedevelopmentofphysically-basedmodelsofmicrostructuralevolutionduringthermomechanicalprocessingofmetallicmaterialsrequiresknowledgeoftheinternalstatevariabledata,suchasmicrostructure,textureanddislocationsubstructurecharacteristics,overarangeofprocessingconditions.Thisisaparticularproblemforsteels,wheretransformationoftheaustenitetoavarietyoftransformationproductseradicatesthehotdeformedmicrostructure.ThispaperreportsonamodelFe-30wt%Nibasedalloy,whichretainsastableausteniticstructureatroomtemperature,andhasthereforebeenusedtomodelthedevelopmentofaustenitemicrostructureduringhotdeformationofconventionallowcarbon-manganesesteels.Italsoprovidesanexcellentmodelalloysystemformicroalloyadditions.Evolutionofthemicrostructureandcrystallographictexturewascharacterisedindetailusingopticalmicroscopy,XRD,SEM,EBSD,andTEM.ThedislocationsubstructurehasbeenquantifiedasafunctionofcrystallographictexturecomponentforavarietyofdeformationconditionsfortheFe-30%Nibasealloy.Anextensiontothisstudy,astheuseofamicroalloyedFe-30%Ni-Nballoyinwhichthestraininducedprecipitationmechanismwasstudieddirectly.Theworkhasshownthatprecipitationcanoccuratamuchfinerscaleandhighernumberdensitythanhithertoconsidered,butthatpipediffusionleadstorapidcoarsening.Theimplicationsofthisformodeldevelopmentarediscussed.Keywords:Microalloyedsteels,straininducedprecipitation,microbands,texture.1IntroductionThedevelopmentofphysically-basedmodelsofmicrostructuralevolutionduringhotformingofmetallicmaterialsrequiresknowledgeofthegrain/subgrainstructureandcrystallographictexturecharacteristicsoverarangeofprocessingconditions.Clearly,investigationofthehotdeformedstructureisimpossibleincarbonandmicroalloyedsteelsastransformationofthe-Fetolowtemperatureproductsremovesthepriordislocationstructure.Toaddresstheproblemsassociatedwiththe-Fetransformation,amodelFe-30wt%Nialloyhasbeendevelopedwhichdoesnottransformoncoolingfromthehotworkingtemperature.ThestackingfaultenergyofFe-Nialloysat1200oCisbelievedtomatchthatofconventionalC-Mnsteelsataround30Fe-70Ni[1].However,inourworkweareinterestedinnotonlystudyingthehotdeformationmicrostructure,butalsothestraininducedprecipitationmechanisminmicroalloyedsteels.Thermodynamiccalculations(usingThermo-Calcsoftware,assumingequilibriumconditions)indicatethatthesolubilityofNbCinFedecreaseswithincreasingNicontent,with30%NiamaximumthatallowssolutionofNbCatacceptabletemperatures.Thus,thisalloycompositionwaschosenasitprovidestheoptimumbalancebetweenNbCprecipitationanddeformationcharacteristics.Furthermore,wehaveshownthattheflowcurvesofFe-30NiandC-Mnmicroalloyedsteelshavethesamebasiccharacteristics,althoughtheflowstressmaybenotexactlymatched[2,3].Microalloyedsteelshavereceivedconsiderableacademicinterestovermanyyearsandtheindustrialapplicationsareeverincreasing.TheadditionofNbisconsideredtohavethreeprimaryeffects[4]:(i)aninhibitorofaustenitegraincoarseningduringreheating,(ii)retardationofausteniterecrystallisationpriortothe/transformationthroughthestraininducedprecipitationofNbCand(iii)precipitationhardeningfromtheNbCinthelowtemperaturetransformationproduct.Asnotedabove,detailedstudyoftheformationofNbCinmicroalloyedsteelsisdifficult,sincethetransformationofthe-Fetolowtemperatureproductsremovesthepriordislocationstructurethatisfundamentaltothegenerationoftherefinedfinalmicrostructure.Slowcoolingafterdeformationresultsininterphaseprecipitationduringthe/phasetransformationandsubsequentprecipitationmayalsooccurintheferrite,whilerapidcoolingleadstotheformationofmartensite,whichobscuresthelocationoftheveryfineprecipitatesbelievedtobecriticaltotheretardationofrecrystallisation.Extractionreplicasdonotreliablyremovetheprecipitates5nmandgiveonlyindirectinformationonthelocationoftheprecipitates.Therefore,theunderstandingofstraininducedprecipitationinconventionalHSLAsteelsinthepasthasbeenbasedonindirectmethodsandisconsequentlyincomplete[4,5].WorkatSheffieldhasdevelopedamodelmicroalloyedFe-30wt%Ni-Nballoyhasbeendevelopedwhich,likethebaseFe-30wt%Nialloy,doesnottransformoncoolingfromthehotworkingtemperature[2-7].Thishasallowedustostudythestraininducedprecipitationeffectdirectly,whichhasbroughtnewinsighttothebehaviourofthesesystems.Thispaperdescribesthesubstructuredevelopedduringthehotdeformationofaustenite.HighresolutionEBSDisusedtoquantifythesubstructureasafunctionoftexturecomponent,andtherebyallowsadetailedunderstandingofthestoredenergyasafunctionofgrainorientation.ThestraininducedprecipitationofNbCisstudiedintheabsenceof-Fetransformation,correlatingtheprecipitationprocesswithdislocationstructureandthermomechanicalprocessconditions.2.ExperimentalThem