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1、SECRETARIATUSEHeadershouldbeblank.11BakeHardeningEffectsinMultiPhaseSteelsBLECKWolfgangBRHLSonja(InstitutefFerrousMetallurgyRWTHAachenUniversityGermany)Abstract:Bakehardeningsteelsofferthepotentialofweightsavingswithoutm

2、ajlossofductilityfautomotivecomponents.Recentlyadvancedhighstrengthsteelswithmultiphasemicrostructureshavebeendevelopedwhichalsoshowbakehardeningproperties.AconventionalbakehardeningsteelaDPsteelaTRIPsteelhavebeenexamine

3、dbyapplyingdifferentprestrainsBHconditions.ThemechanicalpropertiesyieldstrengthtensilestrengthelongationBHvaluesindependencyoftheBHparametershavebeendeterminedrelatedtospecificmicrostructuralfeaturesindertoacterizetheage

4、strainhardeningbehaviour.Furthermetheeffectofthebakingtreatmentonthedynamicpropertiesfcrashrelevantapplicationshasbeendetermined.ThekiicsofthebakehardeningphenomenaaredifferentfthethreesteelsaswellastheBHlevels.Atleasttw

5、odifferentphenomenatheSnoekeffectthefmationofprecipitateshavetobedistinguished.Keywds:BakehardeningeffectDPsteelTRIPsteeldynamicpropertiesSnoekeffectprecipitation.0.IntroductionCarmanufacturersfocusthemaindesigncriteriao

6、famoderncaronreducedweightincreasedsafetyindertofulfilthecustomer’sexpectationsthelegalrequirementsstards.Inthiscontextnewsteelgradeshavebeendevelopednotonlyfweightsavingbutalsotoimprovecrashsafetyofvehicles.Thesesteelsc

7、ombinegoodcoldfmabilitywithahighstrengthlevelatawidestrainratespectrum.Advancedhighstrengthsteels(AHSS)amongthemespeciallyDualPhaseTRIPsteelsofferpromisingresultsinthisfieldwhiletheirextradinarymechanicalpropertiescanbet

8、ailedadjustedbyalloyingprocessing.Anotherideatomeettheserequirementsfautomotivebodyapplicationsistheexploitationofthebakehardeningpotentialwhichdeliversapostfmingstrengthincreaseduringthepaintbakingtreatmentattemperature

9、sofabout140to200C.Thebenefitsofbakehardeningsteelsfbodyapplicationsintheautomotiveindustryarewellknownsinceseveralyears.Thispaperfocusesonrecentlydevelopedadvancedhighstrengthsteelsinvestigatesthespecificbakehardeningbeh

10、aviourofmultiphasemicrostructurescomparedtothebehaviourofferriticbakehardeningsteels.1.MultiphaseSteelsBakeHardeningConceptThemechanicalpropertiesofmultiphasesteelscanbetailedbyadjustingthemicrostructurei.e.thetypetheamo

11、untthedistributionofthedifferentphases.DualPhasesteels(DP)consistofasoftferriticmatrixwithdispersedhardmartensiteisls.AconventionalDPsteelhasachemicalcompositionofabout0.070.1%C0.3%Si1.01.5%Mnconsistsof530%hardmartensite

12、bainitephaseinthematrixof7095%ferritephase.AccdingtotheirtensilestrengthlevelsDPsteelsshowalowyieldstrengthahighwkhardening.TRansfmationInducedPlasticitysteels(TRIP)consistofamicrostructureof5070%SECRETARIATUSEHeadershou

13、ldbeblank.33Table2:AsdeliveredmechanicalpropertiesBHBHxvaluesoftheinvestigatedsteelgradesBH:170C20min.SteelRp0.2RmAgA80nrBH0BH2BH2BH5BH5BH10BH10MPaMPa%%MPaMPaMPaMPaMPaMPaMPaBH18020733219380.172.129433850356434DP600365640

14、14180.160.8518777956612866TRIP70045973125310.261.04979681007011550IntensileteststhemechanicalpropertiesafterprestrainingageingrespectivelyarerecdedaccdingtotheprEN10325instressstraincurves.GenerallytheconventionalBHsteel

15、gradewithstressesbetween200MPayieldstrength350MPatensilestrengthshowslowerstrengthvaluesthanthemultiphasegradesFigure1.TheBHtreatmentbringsabouta30MPato40MPastrengthincrease.TheDPsteelshowsahighercontinuousyieldstrengtho

16、fabout400MPawithalowyieldstresstotensilestrengthratiothusatensilestrengthhigherthan600MPainthebasicstate(Figure1a).FtheTRIPgradeanevenhigheryieldstrengthofnearly500MPawithatensilestrengthhigherthan700MPaisdisplayed(Figur

17、e1b).Fallmaterialsthebakingtreatmentcausesthetypicaldiscontinuousyieldingwithastrengthincreasebutatthesametimeadrawbackintheelongationbehaviour.Withahigherdegreeofprestrainingthiseffectismepronounced.(a)(b)Figure1:Stress

18、straincurvesofBH180DP600hr(a)TRIP700cr(b)intheBHcondition170C20minfvariousdegreesofprestraining.ConsideringtheBHvaluesofthemultiphasegradesconditionedwithprestrainsupto10%BHtemperaturesof60C170C220Cf20minutesholdingtimet

19、heTRIPtheDPmaterialshowasimilarBHbehaviourFigure2.ThestrengtheningeffectisstrongerwithhigherBHtemperatureswhichimprovediffusionconditionsthusdeliveringhigherBHvalues.TheprestraininginfluencestheBHvalueinsofarthatthestren

20、gthincreasegetshigherwithahigherdegreeofprestraining.IndirectcomparisonoftheBHBHcalculationtheeffectofthecrosssectioncrectionbecomesobviousespeciallyfhigherprestrains.HeretheDPsteelreacheshighestBHvaluesat2%prestrain(Fig

21、ure2a)whileftheTRIPmaterialhigherprestrainscanbeappliednotingbeststrengtheningBHvaluesupto5%prestrain(Figure2b).FurtherprestrainingkeepsthisBHleveldoesnotimprovethestrengtheningeffect.IngeneraltheBHvaluesoftheDPTRIPsteel

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