冷却速率及回火对降水的影响

InfluenceofcoolingrateandtemperingonprecipitationandhardnessofvanadiummicroalloyedsteelS.Gu¨ndu¨za,*,R.C.CochranebaKarabu¨kTechnicalEducationFaculty,ZonguldakKaraelmasUniversity,78200Karabu¨k,TurkeybDepartmentofMaterials,SchoolofProcess,EnvironmentalandMaterialsEngineering,UniversityofLeeds,LeedsLS29JT,UKReceived20February2004;accepted28July2004Availableonline16September2004AbstractInthepresentworkprecipitatedistributionsinaC–Mn–Al–V–Nmicroalloyedsteelandhardnesswereexaminedforas-received,heat-treatedandheat-treated&temperedsamples.Examinationofas-receivedandheat-treatedsamplesfromthevanadiummicro-alloyedsteelsusingtransmissionelectronmicroscopyrevealedquitedifferentprecipitatedistributions.Thetypeandsizesofthepre-cipitateparticlesandalsohardnessofthesteelsamplesweremarkedlyaffectedastheaustenitisationtimeandcoolingrateswerechanged.Temperingsteelsamplesafteraircoolingproducedfinematrixprecipitateswhicharecloselyspaced,obstructmovingdis-locationsandhencemakethesteelharder.
2004ElsevierLtd.Allrightsreserved.Keywords:Precipitation;Microalloyedsteel;Strainageing1.IntroductionTheprecipitationofcarbonitrideparticlesmayoccuratdifferentstagesduringthemanufactureandfabrica-tionprocessofmicroalloyedsteels.Threemainaspectshavebeenconsidered[1].TypeIprecipitatesareformedintheliquidphaseandduringoraftersolidification,ontheliquid–solidinterfaceandindeltaferrite.Thesepre-cipitatesareverystableandwhiletheyaretoolargetoinfluencetherecrystallisationofaustenite,thesmallestmayeffectivelyretardcoarseninginausteniteduringreheatingorduringaweldingcycle[2,3].TypeIIparticlesareprecipitatedinausteniteaftersolutiontreatmentandduringhotdeformation(forexample,controlledrolling)asthetemperatureisdecreasing[4].Theprecipitatesarestraininducedandcanretardtherecrystallisationofaustenite[5].Grainrefinementofmicroalloyedsteelsismainlyduetothisgroupofparticles[6,7].Finally,TypeIIIparticlesareformedduringandaftertheaustenitetoferritephasetransformation,nucleatingonthec/ainterfaceandinferrite.Precipita-tionstrengtheninginferritenormallyoccursthroughthesechangesandafineprecipitatedispersionisusuallyobserved[8].Theroleofmicroalloyprecipitatedependsonthetemperatureatwhichitformsinrelationtothetransfor-mationtemperatureofthesteel.Inpractice,therateatwhichasteelcoolswillhoweverdeterminetheactualtemperatureatwhichprecipitatesandtransformationoccur.Thehigherthecoolingratethelowerthetemper-atureatwhichtheprecipitatesform.Therateofcoolingcan,infact,determinewhetheraprecipitateformsinausteniteorferrite[9].Bepari[10]hasshownthatduringcoolingofa0.15%carbonsteelcontainingvanadium,nitrogenandaluminium,vanadiumcombineswiththesolublenitrogenpresentinthesteelformingvanadium0261-3069/$-seefrontmatter
2004ElsevierLtd.Allrightsreserved.doi:10.1016/j.matdes.2004.07.022*Correspondingauthor.Tel.:[phone];fax:[phone].E-mailaddresses:[e-mail](S.Gu¨ndu¨z),[e-mail](R.C.Cochrane).(2005)486–492Materials&Designnitrideprecipitatesandthusremovesthesolublenitro-genfromthesteelefficiently.Aluminiumalsocombineswithsolublenitrogenformingaluminiumnitrideduringcoolingandaluminiumhasbeenfoundtobemoreeffec-tivethanvanadiumintheremovalofsolublenitrogenasnitrideduringcooling.Itwasalsofoundthatdislocationdensityofa0.15%carbonsteeldecreasedwithdecreas-ingcoolingrates.Althoughtheeffectsofhotrollingconditionsandannealingtemperatureonmechanicalpropertieshasbeenclarified,theinfluenceofcoolingratehasnotbeeninvestigatedindetail.Ithasbeenshownonlythatmicro-structureandmechanicalpropertieswerequitedifferentbetweenwater-quenchedandslowlycooledspecimens[11].Thepurposeofthisexperimentistoexamineinsomedetailinanattempttoexplainhowthecoolingrateandalsotemperingeffecttheprecipitationandhardnessofvanadiummicroalloyedsteels.2.ExperimentalmaterialsandprocedureThesteelchosentostudyinfluenceofcoolingrateandtemperingonprecipitationandhardnessisacom-mercialhighstrengthC–Mn–Al–V–NsteelproducedbyBritishsteelcalledBritsteel55.Thechemicalcompo-sitionofthesteelsusedinthisinvestigationisgiveninTable1.Accordingtostoichiometry,aluminium(atomicweight27)willcombineapproximatelyonehalfitsweightofnitrogen(atomicweight14).Hence0.035wt%aluminiuminvanadiummicroalloyedsteelissuffi-cienttoremoveallnitrogenfromsolidsolutionasAlN.Dependingontheaustenitisationtimeused,AlNmaybeformedandtheremaybeavariableamountnitrogeninsolidsolution.However,theamountofvanadiumisnotsufficienttofixallcarbon.Forexample,vanadium(atomicweight50.9)willcombinewithapproximatelyonequarteritsweightofcarbon(atomicweight12),sothatfora0.14wt%Cvanadiummicroalloyedsteel,0.56wt%ofvana-diumwillprovidecarbideofthestoichiometriccompo-sition.Onthecontrary,theamountofvanadiumis0.10wt%whichisnotenoughtocombinewithallthecarbon,thereforefreecarbonshouldbealwaysexpectedinsolidsolutionattheaustenitisingtemperature.Allsamplesexceptthoseintheas-receivedconditionwereaustenitisedat900Cfor1or26h.Sampleswerethencontinuouslycooledinair(AC,1.39C/s)orinastainlesssteelcylindertube(SSC,0.56C/s)soastoob-ta