磁磁场诱导的钢铁微观结构

JournalofCrystalGrowth237–239(2002)1802–1805Magneticfield-inducedalignmentofsteelmicrostructuresK.Maruta,M.Shimotomai*TechnicalResearchLaboratories,KawasakiSteelCorporation,Kawasaki-cho,Chuo-ku,Chiba260-0835,JapanAbstractItiswellestablishedthatdipolarinteractionsincolloidalmagneticfluidsarethemechanismofthepatternformationsuchaschainsorcolumns.Wereportthefirstobservationofstructureformationinsteelsexposedtoamagneticfield.Amagneticfieldhigherthan2Tisfoundtoalignthemicrostructureofsteelsparalleltothefieldduringthetransformationfromtheparamagneticfccphasetotheferromagneticbccphaseandviceversa.Evidenceispresentedforthedipolarinteractionsbetweentheellipsoidalnucleiinthebackgroundofthematrixwithadifferentmagneticcharacter.Electronbackscatteringdiffractionhasrevealedthatthetransformingnucleiinamagneticfieldareatrandomintheircrystallographicorientation.r2002ElsevierScienceB.V.Allrightsreserved.Keywords:A1.Magneticfields;A1.Microstructures;B2.Superconductingmaterials1.IntroductionApplicationofamagneticfieldtomagnetorheo-logicalfluidsisknowntoinducemagneticdipoleandmultipolemomentsoneachmagneticparticleembeddedinacarrierfluid.Anisotropicmagneticforcesbetweenpairsoftheparticlespromotethehead-to-tailalignmentofthemomentsanddrawtheparticlesintoproximity.Theseattractiveinter-particleforcesleadtotheformationofchains,columns,ormorecomplicatednetworksofparti-cles,alignedwiththedirectionofthemagneticfield.Thiskindofphenomenoninfluidshasbeenstudiedextensivelyandawealthofexperimentaldataisavailable.Inparticular,theaggregation,crystallizationandchainformationhavebeendemonstratedforinteractingnonmagneticpoly-styreneparticlesconfinedinatwo-dimensionallyconfinedmagnetorheologicalfluid[1].Theaimofthisstudyistosearchformagneticalignmentinsteels.Ifthealignmentshouldtakeplace,itwouldinvolvethecreationof‘‘particlesinteractingbydipolarforces’’inthesolidironphases.Therefore,wefocusonthetransformingsteels.Insteels,theferromagneticbcc(a)phaseappearsfromparamagneticfcc(g)oneoncooling,andviceversaonheating.Weapplymagneticfieldsupto12T.ApreliminaryreportofthealignmentinFe–Calloysonheatingisfoundelsewhere[2].2.Experimentalprocedure2.1.TheatogtransformationAspecimenofFe–0.1%Calloywasofrectan-gularshapeof50
8
1mm3.Itwasannealedat*Correspondingauthor.Tel./fax:[phone].E-mailaddress:[e-mail],[e-mail](M.Shimotomai).0022-0248/02/$-seefrontmatterr2002ElsevierScienceB.V.Allrightsreserved.PII:S0022-0248(01)02346-6temperaturesinthegphaseregionandsubse-quentlyquenchedintowatertoobtainahomo-geneousmartensitephase,adistortedbccphase.Then,itwastransferredintoamoltenbathofleadsetupatatemperatureinthea+gtwophaseregioninsidetheboreofasuperconductingmagnet.Afteraperiodoftime,usually45min,thespecimensweredrawnupwardoutsidethemagnetandquenchedintowater.Thus,agphasenucleatedandgrownduringtheinversetransfor-mationshouldbeobservedasanewmartensitephase,whiletheinitialandnot-yettransformedmartensiteisnowatemperedaphase.2.2.ThegtoatransformationConsideringthatnoneofthepreviousreportsdealingwiththeeffectofmagneticfieldonthetransformationinsteelshavementionedthealignment,anexperimentalsetupshowninFig.1wasdesignedandinstalled.Itcomprisesofaninductionfurnace,apairofrollerdice,asuper-conductingmagnetwithafurnaceinside,aquenchingbath,andahydraulicdrawingbench,allarrangedinaline.Thesuperconductingmagnetiscapableofgeneratingamagneticfieldupto12Tinsidetheboreof100mmindiameter.Therollerdicemadeofstainlesssteelaredesignedtoperformhotrollingofthespecimensintheclosevicinityofthemagnet.Themainfeatureofthissetupistherollingpriortothetransformationinmagneticfield,whichmakesitpossibletointroducenuclea-tionsitesfortransformation.Thelengthofaspecimentoberolledis70mm,outofthetotallengthofabout1500mm.Thecross-sectionis4mm
5mm.RollingwiththerollerdiceisdepictedintheinsetofFig.1.Thespecimensweremadefromasteelplate(Fe–0.2%C–0.2%Si–1.3%Mn–0.1%Ti,inwt%).Thethickpartofaspecimenwasfirstheatedto11001Cbyinductionheatingfor5mininaflowingArgas,andthencooledto9501Cforhot-rolling.Therollworkreducedthespecimenthicknessto70%oftheinitialone.Subsequently,thespecimenwashorizontallytransferredtoafurnaceinstalledatthecenterofthemagnet.Thefurnacehadbeenheatedtoatemperatureofthetwo-phaseregion,intherangeof750–7651C.Afterholdingfor1–3min,thespecimenwaspulledoutofthefurnaceandquenchedintowater.Throughthisprocedure,thetransferspeedofthespecimenwas170mm/sinaninertatmosphere.Mostofthemetallographicstudieswereperformedonthethinedgesoftherolledspecimenswithanopticalmicroscope.3.Results3.1.TheatogtransformationFig.2showsthemicrographsoftheFe–0.1%Calloysubjectedtotheinversetransformationat8001Cinamagneticfieldof8T.Darkspots,beinginverselytransformedgphase,arealignedlikechainsparalleltothemagneticfield.Thelightbackgroundisduetothetemperedaphase.InFig.2(a),predominantnucleationsitesarethegrainboundariesoftheinitialgphase.Whenacoarse-grainedspecimenwastested,Fig.2(b),thetransformationalsotakesplaceinsidethegrains.Itisseenthattheshapeofthegparticlesisellipsoidalwithitslongaxisparalleltothefield.