284-paper-低碳钢热轧产品氧化皮的原子力显微镜分析

OXIDESCALEMICROSTRUCTURECHARACTERISTICSOFHOT-ROLLEDLOWCARBONSTEELBYAFMJianLiandXinxingXiao（R&DCenter,BaoshanIron&SteelCo.LTD.Shanghai201900,China）ABSTRACT:AtomicForceMicroscopy(AFM)hasbeenusedtostudytheoxidescaleprofilesofhot-rolledlowcarbonsteel.High-resolutionimagesshowcleardouble-layeredstructureofthescalesbyMagneticForceMicroscopy(MFM).X-raydiffraction(XRD)identifiedthephasecompositionofthescale.ThemostsignificantfindingisthatAFMcandisplayimageofthegrainconfigurationofwustite.Themechanicalpropertiesandpicklingperiodsofthescalesdependstronglyonthemicrostructureofthescales.Keywords:AFM,hot-rolled,oxidescale0.INTRODUCTIONItisknownthattheoxidescaleformedonrollingandrolledprocessinaircomprisesthreelayers,namely,athinoutermostlayerofhematite(Fe2O3),anintermediatemagnetite(Fe3O4)layer,andathickinnerwustite(FeO)layer.Theintervalsforoxidationofsteelarefromhalfasecondtoseveralminutesinahotrollingprocess.Thethree-layered-structureisunstableatlowertemperatures.Duringhotrollingofsteel,thescalesaffectthefrictionalconditionswhichresultsinchangesintherequiredrollforces,powerconsumptionandsurfacequality.Ontheotherhand,thescalecanalsoinfluencetheheat-transferbehaviourofthestrip/wire.Moreoveritcanalsoinfluencethemechanicalpropertiesandthepicklingtimeofscales,whichaffectedgreatlythesucceedingmanufactureprocess.Unfortunately,itisnoteasytoobserveoxidescalebehaviourdirectlyunderindustrialworkingconditions,neitherinlaboratory.Inordertoevaluatetheeffectoftheoxidescaleforsteelstrip(wire),somepreviousresearchershaveanalyzedthemicrostructure,aspapersin[1-2].Themostcommonlytoolsforstudyingmicrostructureofoxidescaleareopticalmicroscope(OM),scanningelectronmicroscope(SEM)andtransmissionelectronmicroscope(TEM).Asanemergingimportanttoolformicrostructureanalysis,AFMisfirstusedtoanalyzethescalesprofilesofthesteel.TheadvantagesofusingAFMtostudymaterialsarelistedasbelow:Extremelyhighmagnification:Magnificationsisupto1,000,000X.Threedimensionalsurfacetopograph:Comparewithothermicroscopes,AFMprovidesthreedimensionalvisions.Thisisveryhelpfultounderstandthedepthofsurfacetopography.Easytouse:Atsamehighresolution,theAFMiseasiertousethanothermicroscopes.SimplifiedSamplePreparation:LittleornosamplepreparationisrequiredbeforemetalsamplesareexaminedbyAFM.Multi-functionalimage:Beyondofothermicroscopes,AFMcangeneratevariedimagesincludingMagneticimage,electricimage,phaseimageandamplitudeimageetc.Inthispaper,magneticimageplaysaveryimportantroleforthescalemicroanalysis.1.EXPERIMENTALThesamplesusedinthisinvestigationwereindustrialhot-rolledcarbonsteelwire(18mmdiameter)andstrips(3mmthickness)fromBaosteel.Sample1(wirerod)wasfromcoldforgingsteelwire,namedSWRCH8A,whichcontained0.08wt%carbon.Sample2and3werefromhot-rolledsteelstrips,namedSPHC,bydifferentcoilingtemperatureandbothcontained0.05wt%carbon.Thesampleswerecutandmountedbybakeliteat150oC,thenwerepolishedwithaseriesofgritSiCpaper(320,600,1000and1200).Afterthat,sampleswerepolishedagainwith0.5micronand0.1microndiamondparticles.BeforeAFMtest,opticalmicroscopewasusedinthemicro-analysisofthescaleprofilesofthesamplesinpolishedstatusandetchedstatus.Themagnificationswereupto500X.ThensampleswerecutagainforAFManalysis.PicoPlusAFMfromMolecularImagingCompanywasusedtoscantheimagesoftopograph,phaseandmagneticdomainofthesamplessimultaneously.Thenano-probecantileversarecoatedbythinfilmsofCo(1-stlayer)andCr(2-ndlayer).Imageswereacquiredbyusingthetappingmodeunderroomtemperature.Thespringconstantofthemis3.5N/m.2.RESULTSANDDISCUSSIONS2.1Sample1:ThecompositionofSample1fromcoldforgingsteelwireisshowninTableI.Itisetchedusing4%HNO3ethanolsolutionandthenwashedwithwarmwaterandethanol.MicrostructureoftheoxidescaleprofileisshowninFig.1.bymeansoftheopticalmicroscope(500X).Thescaleprofileexhibitedadouble-layeredstructurewhichcontainedouterlayerofFe3O4andinnerlayerFeO(theoutermostlayerofFe2O3istoothintodetect).WithAFM,amoredetailedstructureofthescaleprofileisdisplayedinFig.2.Thetopographyandmagneticimagesofthescaleprofilearescannedsimultaneously.TableI.CompositionofSample1ElementCSiMnPSwt.%0.080.050.30.010.004Fig.1ScalestructuresofSample1byOM(500X)Afteroxidizingathightemperatureandthencoolingatrateof30oC/minFig.2Fourimagesa,b,canddofSample1(Cross-sectionofthewire)byAFM.Thedataareobtainedsimultaneouslyafterpolishingandetching.Greenlineindicatestheprofilequantitieschangebyrightcharts.a(upleft):Thetopographyimageshowsthescalelayerdifferenceinheightafteretching,obviouslytheouterlayerofthescaleishigherthaninnerlayer.Itisthedirectevidencethattheouterlayerismoreetch-resistant.Themostsignificantresultexhibitsthatthegrainconfigurationofwustitelayerpointedbytheblackarrow.TheresultscancombinewiththeresultsbyEBSDtechnique[1,3].b(upright):Simultaneity,thephaseimageshowssignificantphenomenonelsewhichsomespotsarefoundoninnerlayer.c(downleft):Themagneticimageexhibitsthemagnetisminouterlayerofthescalewhileinnerlayerisnotobviousmagnetism.d(downright):Moreinformationcanbefoundinamplitudeimage.Itisknownthatthelasthotrollingtemperatureisabout880oC,the