匹兹堡论文1.docx
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匹兹堡论文1.docx
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匹兹堡论文1
MicrostructuralEvolutionandGrainRefinementofaHighStrengthLowAlloySteelProducedbyCompactStripProduction
HuangGang1,WenDe-zhi2,JiaoGuo-Hua2,ChenXiao-Jun2,ZhouGuo-Hua2,WuKai-Ming1
1DepartmentofAppliedPhysics,WuhanUniversityofScienceandTechnology,Wuhan430081,China
2LianyuanIronandSteelCo.Ltd,Loudi417009,China
Keywords:
compactstripproduction(CSP),TMCP,highstrengthlowalloysteel,grainrefinment
Abstract
AlowcarbonhighstrengthNb-TimicroalloyedsteelwasproducedbyCompactStripProduction(CSP)atLianyuanSteel.Opticalmicroscopy,scanningelectronmicroscopy(SEM)andtransmissionelectronmicroscopy(TEM)wasutilizedtoanalyzethemicrostructureofthesteel.Themicrostructuremainlyconsistedofacicularferrite(AF),granularbainite(GB)andpolygonalferrite(PF).Theaveragegrainsizeofthemixedmicrostructureswasapproximately4μm.AsmallamountofNbandTicarbidewasprecipitatedintheferritematrix,thesizebeing50100nm.Alargeamountofdislocationcellsandnetworkswereobservedinthematrix.Thedislocationcellsandnetworkswereformedbymeansofcontrolledrollingandacceleratedcoolingandtheyplayedanimportantroleinthestrengtheningofthemicroalloyedsteel.Theyieldstrengthofthemicroalloyedsteelwasthusmorethan600MPaandelongationreached24%.
1.Introduction
Compactstripproduction(CSP)isanadvancedmanufacturingtechnologythatiscurrentlyemployedintheproductionofsteelsinsteelmanufacturingindustry.Thisprocessismainlyusedtoproducehotstripsusingashortproductionline,involvingtheprinciplesofscience,technology,andengineering.SincethefirstCSPsteelplantwascommissionedatNucorin1989,Ithasbeendevelopedrapidly,thereare39differentkindsofcontinuouslycastingthinslabanddirectrollingproductionlinewith54strandshavebeenestablishedworldwide[1].13strandshavebeenestablishedinChinafrom1999to2006.TomakefulluseofCSPtechnologyandtodevelophighvalue-addedproductssuchashighstrengthlowalloysteel(HSLA)arebecominganeffectivewaytoreduceproductioncostandraisesteelproductcompetativeness[2].
Itisreportedthat[3-6]CSPcanproducesteelstripswithmorehomogenousandfine-grainedmicrostructures,comparedwithconventionalprocess.InChina,highstrengthsteelwithyieldstrengthof600MPawasdevelopedsuccessfullyinthetraditionalhotrollingprocess.Inthepresentinvestigation,alowcarbonhighstrengthlowalloysteelwiththeyieldstrengthofwas600MPaproducedbyCSPatLianyuanSteel.TheattemptedtoproducebyCSP.Themicrostructuralevolutionandgrainrefinementofthehighstrengthlowalloysteelwasattemptedtoanlayzeinthepresentresearch.
2.Experimentalprocedures
ThetestedsteelwasalowcarbonhighstrengthlowalloysteelLG600.ItschemicalcompositionwaslistedinTable1.Thespecimenwaspolishedandetchedwithasolutionof4vol.%HNO3alcohol.MicrostructuralobservationswerecarriedoutonanOlympusBM51opticalmicroscope.ElectronmicroscopywasconductedatanOxfordfieldemissionscanningelectronmicroscope(FESEM)andaFEITecnaiG2transmissionelectronmicroscopy(TEM).SEMandTEMareallequippedwithanelectrondispersedspectrum(EDS).
Table1Chemicalcompositionofspecimen(wt%)
C
Mn
Si
P
S
ALs
Nb
0.05
1.45
0.25
≤0.020
≤0.006
0.020~0.030
0.04
Ti
Cr
V
Mo
N
O
0.030
0.50
0.05
0.15~0.25
0.004
0.002
ThesamplewasalsoprocessedtotensilespecimensparalleltotherollingdirectionbasedtheMetaltensiletestspecimennationalstandards(GB6397-86).ThetensiletestwasconductedbasedtheGB228-2002intheengineeringmechanicstestcenterofWUST.ThefracturemorphologywasobservedbyaFEIQuanta200scanningelectronmicroscopy(SEM).
3.Results
3.1MicrostructuralObservation
Fig.1showstheopticalmicrostructuresoftheLG600.Inthelowtimesopticalmicroscope(Fig.1a×200)thesamplegrainswasverysmallandhomogenized,therewerenolargeinclusions.Throughhightimesopticalmicroscope(Fig.1b×1000)wecanobservedacicularferrite(AF),polygonalferrite(PF)andgranularbainite(GB)obviously.Thegrainsizewasunevenandthegrainboundarywasirregular.Thereweremanyverysmallislanddistributedinthegrainandbetweenthegrains.Andsub-crystalcouldbeseeninthelargergrainfaintly.Theaveragegrainsizewasabout4micronsmeasuredbythelinearinterceptmethod.
Fig.2illustratesmoredetailedimageofthemicrostructuresbytheSEM.ThefineislandwasmoreclearlyobservedfromFig.2aandthedistributionwaschaotic.Theacicularferrite(AF),polygonalferrite(PF)andgranularbainite(GB)weretheprimaryorganizationintheFig.2b.Thelengthofacicularferritewasjustafewmicronsandthewidthofitwaslessthan2microns.Thesmallislandsdistributedinthegranularbainite(GB)weregenerallyconsideredtobeM/Aislandconsistedofmartensiteandretainedaustenite.
Fig.1TheopticalmicrostructuresofLG600(a×200andb×1000)
Fig.2ThemicrostructuresofLG600bySEM
3.2TEM-EDSAnalysis
ThestudywasconductedbymeansofTEM-EDSinordertogodeepintothenanoscale.Itwasobservedthatgrainswerenon-parallelandinterweaved.Intheorganizationmuchsmallsub-structureandhighdensityofdislocationwasbeseenandsomesmallM/Aislandandprecipitateswerescatteredinlathandbetweenthelaths.Themicrostructurescanbedetaileddividedintofollowingseveraldifferentmorphology.
ThefirstoneisacicularferriteanditstypicalmorphologywasshowninFig.3.Thewidthofacicularferritelathswaslessthan2micronsandthelengthwasabout2-3timesofthewidth.Somegrainboundaryclearacicularferritelaths,butsomeothergrainboundaryratherambiguousanddifficulttodistinguishbetweenferriteandbainite.
Thesecondoneisdislocationanddislocationcells.Intheferriteandbainiteorganizationstherewerealargenumberofdislocations.Dislocationintertwinedtoformdislocationwallsanddislocationwallsintertwinedtoformanumberofthree-dimensionalsub-structurecalled“cells”or“subgrain”.ItswasshowninFig.4.These“subgrain”whichwereamong0.1~0.5μmplayedgrainrefinementroleandthecontributiontostrengthwasaboutthesizeofits.
Fig.3TheacicularferritebyTEMFig.4ThecellsbyTEMFig.5TheM/AislandbyTEM
Thenextwasthesmallislandsthatwidelydistributedingrainboundaryandbainite(illustratedinFig.5).ItwasgenerallybelievedthattheseislandsweretheM/Aislandscomposedofretainedausteniteandmartensite.M/Aislandwereirregularinshapeswiththesizeamonglessthanonemicrontoafewmicrons.Ferritefirstformedattheearlystageofbainitetransformationandthenthecarbonatomintheuntransformedaustenitewillberich。
Carbidecannotfullyprecipitateandthenformedretainedaustenite.Whenthetemperaturedroppedtomartensitictransformationpoint(Ms),apartofaustenitetransformedintomartensiteandmixedwiththerestretainedaustenite,thatis,M/Aislands.
WiththeadditionofNb,V,Timicro-alloyedelementsinLowcarbon-highstrengthsteelLG600inducedtheformationofacicularferriteandfineCarbonNitridesandproducedsomeM/Ainferritegrains,whichadvancedthestrengthandtoughnessofthesteels.Theprecipitationsdistributedinsidetheaustenitegrainorgrainboundariesoronthedislocationlineswiththesizeslessthan100nmshowtheelliptical,circular(asshowninfig.6aandfig.6b)orquadrateshapes(asshowninfig.7aandfig.7b)withnoangularfeatures,accordingtotheTEManalysis.Energyspectrumanalysiswasconductedonthecircularandquadrateprecipitations,respectively(asshowninfig6candfig7c).ThefineprecipitationsmaybeNb,TicarbideorNb,Timultiple-carbide((Nb、Ti)C),accordingtotheEnergyspectrumanalysis.
Fig.6ThecircularprecipitatesbyTEMandEDS
Fig.7ThequadrateprecipitatesbyTEMandEDS
3.3Mechanicalpropertiesandfractureanalysis
ThetensilepropertiesofthesamplewereshowninTable2.Theyieldstrengthisabout614MPa,themaximumtensilestrength714MPa,RatioofYieldStrengthtoTensileStrength0.86,andelongation24.1%,indicatinghighstrengthandplasticity.Attheinitialstep,thesampleunderwentelasticdeformation,andthenplasticdeformationafterthemaximumelasticdeformationwasreached.Thecorrespondingcriticalvalueissocalledyieldstrength.Plasticdeformationobviouslychangedthesampleofthesample,thesamplebecamelongerandtheneckshowupandbecamenarrow.Thetensileforcechangedslowerandslowerwiththeelongationofthesample,andfinallythesamplebroke.
Table2Mechanicalpropertiesofspecimens
RsMPa
RmMPa
δ%
Ψ%
614
714
24.1
62.4
Thefracturesurfaceofthetensilesamplewasgrayobservedbyeyesillustrateitwasductilefracture.Fig.8ashowstheSEMmicrostructuresofthefracturesurface.Fracturesurfacecoveredwithnumerouslargeandsmalldimples,tornedgewereobviously.Theaxislineofdimpleswasverticaltofracturesurfaceanddimpleswereverydeep.Sowecanknowthatthesteelplatehadagoodtoughness.Thediameterofmostdimpleswerelessthan5μm,thereweremanyverysmalldimpleslessthan1μminthebiggerdimplesandbetweenthebiggerdimples.Itwasmicrosporeaggregatedfractureanddimpleswerethebasiccharacteristicsofthiskindfracture.FromFig.8bwecanseetherewereafewinclusionorprecipitatesinthebiggerdimple,itsweresosmallandthedimplesweresodeepthatit’scannotbedetectedbyEDS.Theseinclusionandprecipitates,dislocationsandthegr
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