362-paper-Effects of Inclusions on the Microstruct

EffectsofInclusionsontheMicrostructureandToughnessofHAZofHSLASteelsZhengQingChenZhaoPingMetallugicalProcessDepartment,ResearchInstitute,BaoshanIron&SteelCo.,LtdTheeffectsofinclusionsonthemicrostructureandtoughnessofHAZwereevaluatedinthisparticle.FineinclusionswerefoundtoimposefarmoreimportantimpactsonthemicrostructureofHAZratherthanbasemetal.Duringtheheatcycleofweldingsimulation，thepresenceofTi2O3inclusionsretardedthecoarseningofaustenitegrainandstimulatedthetransformationofaustenitetointragranularferrite,whichisininterlockedacicularshapeandisdifficultforcleavagetopropagatethrough.ThetoughnessofHAZcanthusbesubstantiallyimproved.Thetype,sizeanddistributionofinclusionscanbecontrolledinthemeltingprocess.Ti2O3inclusionsareavailableonlybyinitiallyTi-Killedconditionsandsomeotherelementsofthesteelarenecessarilycontrolled.1.IntruductionNon-metallicinclusionsinsteelsaregenerallyregardedasharmfultomaterialqualitiesinmostcases,thereforemanyeffortshavebeenmadefortheirremovalbysecondaryrefining,tundishmetallurgyandsomespecialmethodssothat“cleansteel”canbeproduced.However,effectivelyutilizationoffineprecipitatessuchasnitridesandcarbidestoimprovethepropertiesisawidelyacceptedconceptioninphysicalmetallurgy.Precipitatesalsobelongtoinclusions.Theyonlyvarywiththosecommonlymentionedinclusionsinsizeandcomposition.Soaninnovativetechnologycalled“oxidemetallurgy”wasproposedbyProfessorShozoMizoguchi[1-2].Oxideparticlesinsteelareutilizedasthenucleationsitesofprecipitatesthereafterfineferritestructureandhighlydispersedsulfide,nitrideandcarbideprecipitatesareavailabletoimprovethepropertiesofthesteel.Particularly,forsomestructuralsteels,thelowheat-affectedzone(HAZ)toughness,whichisresultedfromthecoarseningofaustenitegrainsduringtheheatingcycleinwelding,limitstheheatinputandthematerialsofwelding.Bycontrollingthesize,thedistributionandthenatureofoxides,oxideinclusionswerefoundtobethesitesforheterogeneousnucleationofferriteinaustenitegrainsduringtheAr3transformationtemperature,promotingtheformationofintragranularacicularferrite.ThereforethesizeofferriteisreducedandtheHAZtoughnessisimproved.Therefinementsofmicrostructurehavebeenthefocusofmetallurgistsinthepastdecades,theconceptofoxidemetallurgyasanewoneisofgreatimportancetothesteelproduction.Itmeansacheapermethodtomeetsomespecialrequirementofsteelbychemicalmetallurgyinsteadofphysicalmetallurgy,thelateralwaysmeansverycomplicatedmanufacturingandlargeinvestment.Baosteelstartedtheresearchonoxidemetallurgyseveralyearsago.ExperimentswerecarriedouttoimprovethetoughnessofHAZonsomehighstrengthsteels.Thispapermainlydescribesthecomparisonofmicrostructuresandpropertiesofsteelsexperienceddifferentchemicalmetallurgicalprocesses.2.Experimentaldetailsandresults2．1MaterialsTwoX70pipelinesteelplateswereinvestigated.Bothplateswereproducedinthesamevacuuminductionfurnacewithcapacityof50kg.SteelAwasfirstdeoxidizedbyTiandsteelBwasdeoxidizeddirectlybyAl.Whenthecompositionandtemperaturewereadjustedtothesetlimits,eachheatofsteelwaspouredintoarefractorymouldandcastintoingots.Theingotswereforgedintobilletswiththethicknessof70mmandthenhotrolledintothefinalgaugeof15mmusingTMCP.Themaincompositionrangesofthebothingotswerelistedintable1:Table1ThemaincompositionrangesofX70pipelinesteelingotElementsCSiMnNiMoCuNbAlsTiSNUpper-Limit0.0550.301.620.150.250.250.0600.0200.0180.010.0025Lower-Limit0.0350.191.550.110.150.150.0480.0020.0100.0040.00152.2HAZsimulationSampleswerecutfromtheplatesinthedirectiontransversetotherolling,thesamplesweremachinedintothesizeof11×11×55mmandundertakenHAZsimulation.ThesimulationwascarriedoutonaGleeble3800simulatorwiththeparameterslistedintable2.Table2TheHAZsimulatingparametersPeakTemperatureHeatInputt8/51350℃15KJ/cm15sSamplesafterHAZsimulationweremachinedintostandardCharpyV-notchtestingsampleswiththesizeof10×10×55mmandtheV-notchinthemiddlesides.2.3MicrostructureobservationAseriesofmethodswereadoptedtoinvestigatethemicrostructureofthesamples,includingopticalmicroscopyandSEMstudyoftheinclusioncharacterizationandthefractography,andTEM–EDXverificationoftheinclusiontype.ThetwosteelshavethesamecompositionandexperiencedthesamerollingandHAZsimulation.Theonlydifferenceisthedeoxidizationprocess,especiallythesequencesofdeoxidantsaddition.Itseemedthisdifferenceexertednosignificanteffectsonthemicrostructureofbasemetalinhot-rollingconditionshowninFig.1.SteelASteelBFig.1Opticalmicrographs:basemetalofhot-rollingplatesInbothcases,thegrainswereelongatedasaresultofTMCProlling.Thelightphaseisferrite,andthedark,carbide-containingareascanbediscernedinhighermagnitudeaspearlite.InsteelA,thereisalsoasmallamountofBainiteindarkareas.It’sverysimilartothepearliteinlowermagnitudebuttheirquantityisverysmall.SteelASteelBFig.2SEMmicrographsofHAZThemostprominentdifferenceexistsinsimulatedHAZmicrostructure.SteelAprimarilycomposedoflowerbainiteandacicularferrite,andasmallamountofferritewasalsopresent.Theacicularferritesgrewfrominclusions,whichwereround,withthesizeofabout2-3micrometers.InsteelB,thecross-sectionwasalsomainlycomposedoflowe