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3KAWASAKISTEELTECHNICALREPORTNo.48March2003HighlyFormableSheetSteelsforAutomobilethroughAdvancedMicrostructureControlTechnology*Synopsis:Intheprocessofdevelopingsteelsheetsforautomo-biles,KawasakiSteelhasbeenaimingatbetterforma-bilityofhighstrengthsteelsheetsandtheimprovementofothervariousproperties.Typicalexamplesofthedevelopmentincludes,anewbakehardenablesteel(BHTsteel)sheethavingcharacteristicsinthatnotonlyyieldstrengthbutalsotensilestrengthareincreasedbybakingaftercoating,andSuperHSLAsteelsheethavingexcellentelongationandholeexpansionproperty,bothatahighlevelofperformancethroughtherefiningofcrystalgrainsbroughtaboutbydynamicrecrystalliza-tion.Bothsteelsheetshavebeensecuredastheresultsofthesophisticatedcontrolofsteelstructure,performedattheapplicationofcompletelynewmetallurgicalfind-ings,combined,astheirbasis,withnewtechnologydevisedattheNo.3hotstripmillinChibaWorkswhichinitiateditsoperationin1995.KeiSakataGeneralManager,SheetLab.,TechnicalRes.Labs.SaijiMatsuokaDr.Eng.,SeniorResearcher,SheetLab.,TechnicalRes.Labs.KazuhiroSatoSeniorResearcher,SheetLab.,TechnicalRes,Labs.1IntroductionIncreasingconcernsoverglobalenvironmentalprob-lemsandconsumers’demandforhighercollisionsafetyofmotorvehiclesimposeconflictingrequirementsonautobodies:theyarerequiredtobebothlighterandstronger.Inaddition,ongoingtechnologicalinnovationssuchasthecommercializationoffuel-cellvehiclescouldrevolutionizetheconceptofmotorvehiclesandfunc-tionsrequiredofthem,aswellasthematerialsusedforautobodies.Variousnewlightweightmaterialssuchasaluminum,magnesium,andplasticshavebeenproposed,andthesematerialsarealreadyusedcommerciallyinspecifictypesofvehiclesandcomponents.However,thesematerialsstillhaveproblemssuchasformability,reliability,andrecyclabilityaswellascost,andhencethescopeandvolumeoftheirapplicationremainlim-ited.Expectationsarethereforeincreasingforferrousmaterials,whichaccountforthemajorproportionofmaterialsofamotorvehicleandcouldmakevehicleslighterwhileimprovingcollisionsafety.Typicalcharacteristicsrequiredofferrousmaterialsusedforautobodiesareexcellentpressformabilityandweldabilityduringassembling,corrosionresistanceasacompletedvehicle,fatigueresistanceandotherstrengthsascomponents,andcrashworthinesswheninvolvedinacollision.Althoughthesecharacteristicsarerequiredofthesteelsheetsthemselves,manyofthecharacteristicsalsoneedtobeevaluatedfortheassembledautobodyasawhole.Thecnologicaldevelopmentoneffectivelyusingferrousmaterialsisthusalsoimportant.Ferrousmaterialsaregenerallyconsideredtobegeneral-purposematerials,buttherequiredcharacteristicsactuallydependonthepositionswheretheyareused,andferrousmaterialsofdifferentthickness,strength,andformabil-ityareusedwithorwithoutgalvanizing.Accordingly,almostnoidenticalmaterialsareusedatdifferentposi-tions.Highstrengthsteelsheetsplayacrucialroleinmak-ingvehicleslighterwhilesecuringcollisionsafety.Highstrengthsteelsheetswithvariousstrengths,improvedformability,weldability,andfatigueresistanceareincreasinglybeingusedcommercially.Generally,steel-strengtheningmethodsincludesolutionstrengthening,grainrefinementstrengthening,transformationstrength-ening,precipitationstrengthening,andstrainagestrengthening,whichareusedaloneorincombination,whileaddingappropriatealloyingelementsandapplyingthermo-mechanicaltreatment.Inmanycases,however,steelstrengtheningtendstodegradeotherproperties,andsoastrengtheningmethodthatalsoimprovesotherpropertiesneedstobedeveloped.*OriginallypublishedinKawasakiSteelGiho,34(2002)2,54–58KawasakiSteelhastriedtoenhancetheaccuracyofmicrostructurecontrolfordevelopingvarioustypesofautomotiveferrousmaterialshavingmarkedlyimprovedproperties,aswellastodeveloptechnologiesforeffec-tivelyusingferrousmaterials.Thispaperoutlinestwoexamplesofnewlycommer-cializedautomotivesteelsheets.Thefirstisanewhighstrengthhotrolledsteelsheetthathasexcellentstrainagehardenability,thetensilestrengthofwhichisincreasedbypaintbakingtreatment(BHTsteelsheet).Thesecondisahighlyformablehighstrengthhotrolledsteelsheetobtainedbyapplyingdynamicrecrystalliza-tiontorefiningcrystalgrains(SuperHSLAsteelsheet).Thesenewtypesofsteelsheetsnotonlyhaveuniquemicrostructuresthatexhibitexcellentproperties,butthemethodsandmatallurgicalprinciplesusedtocreatesuchstructuresarealsonew.2HighStrengthHotRolledSteelSheetwithExcellentStrainAgeHardenability(BHTSteelSheet)2.1FeaturesofBHTSteelSheetandItsMetallurgicalPrincipleAnewhighstrengthhotrolledsteelsheetthathasexcellentstrainagehardenabilitywassuccessfullydeveloped.1,2)Thissteelsheethaslowstrengthwhilebeingformed,andthusexhibitsexcellentformability,butitstensilestrengthismarkedlyincreasedbypaintbakingtreatment.Italsohasgoodagingresistanceatroomtemperature.Figure1schematicallyshowsthestress-strainrelationshipofthenewlydevelopedsteelsheetafterstrainagingtreatment.Thenewsteelsheetischaracterizedbythefactthatbothitsyieldstrengthandtensilestrengtharesignificantlyincreasedbystrainagingtreatment,whichisakeydifferencefromconven-tionalBHsteelsheet.Thispropertyiseffectiveforincreasingenergyabsorptioninavehic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