NIOBIUM IN MICROALLOYED ENGINEERING STEELS WIRE RODS.docx
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NIOBIUM IN MICROALLOYED ENGINEERING STEELS WIRE RODS.docx
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NIOBIUMINMICROALLOYEDENGINEERINGSTEELSWIRERODS
NIOBIUMINMICROALLOYEDENGINEERINGSTEELS,WIRERODS
ANDCASECARBURIZEDPRODUCTS
ToshimitsuKimuraandYutakaKurebayashi
DaidoSteel,Research&Development2-30Daido-cho,
Minami-ku,Nagoya,Aichi,Japan457-8545
Abstract
Author’snameKimuraandKurebayashipage01of19
Microalloyingtechnology,developedfortheproductionofflatproducts(plate,stripandlinepipe)duringthe1960’sand1970’s,hasbeenappliedto“longproducts”suchasengineeringbars,sections,forgingsandwirerodsinceabout1980.Inthe1980’sthemainrationaletouseniobiumbearingsteelbarsandwireswastoeliminatetheneedforahardeningprocess,i.e.quenchingandtempering,inmanufacturingheat-treatedsteelpartswithoutanytradeoffinproperties.Owingtothesignificantcostreductionsandenergysavingsbyadoptingmicroalloyedsteels,theyhavereplacedconventionallyheat-treatedsteelsforconnectingrods,suspensioncomponentsandfastenersetc.,intheautomotiveindustry.Ontheotherhand,theeffectofNbonthemechanicalpropertiesofheat-treatedpartshasalsobeenclarified,andnewsteelgrades,e.g.highstrengthsteelsforfastenerswith1500MPatensilestrengthandspringsteelsusedin1200MPadesignstressapplications,havebeendeveloped.Furthermore,theimmensecapacityofniobiumprecipitatesinsteeltopreventgraincoarseningduringheattreatmenthasbeenutilizedindevelopingcasehardeningsteelsfortransmissiongearsandCVJshafts,whicharecold-forgedandthencarburizedatahightemperature.Thenewgradeshavealsosucceededineliminatingsofteningtreatmentssuchasspheroidizedannealingand/orbrightnormalizingtreatmentinthemanufacturingsequence.
Introduction
Asmicroalloyingenableshighstrengthpartstobeproducedintheas-forgedconditionwithoutasubsequentcostconsumingreheat,quenchingandtempering,theapplicationofmicroalloyedsteelshasbeenextended.EventhoughNbisobviouslyadominantelementformicroalloyingas-processedsteels,itisnowalsobeingusedtoimprovethepropertiesinheat-treatedsteels.
Withinthelasttwentyyearsthestrengthlevelofstructuralsteelpartsusedinautomobileshasconspicuouslyincreasedinlinewithstrongdemandsfromtheautomotiveindustry.Figure1summarizesthechangeinfatiguelimitsandtoughnessofheat-treatedsteelsforfasteners,springsandcasehardenedparts,respectively.Theachievementofimprovingmechanicalpropertiesineachgrade,withoutraisingtotalmanufacturingcosts,isattributednotonlytoimprovedalloydesignsbutalsotoprocessinnovations,i.e.hardshotpeeningtechnologyappliedtocarburizedgearsandhotsettingandwarmpeeningforsprings.
Author’snameKimuraandKurebayashipage02of19
Figure1:
Changeintheupperlimitsoffatiguestrengthsandtoughnessbalanceinstructuralpartsduringthepast20years.
Theemphasisshouldbeonniobiumadditionstoheat-treatedgrades,whichwouldbeassociatedwiththepreventionofgraincoarseningduringaheattreatmentwithresultantbrittlenessandcoupledwithanincreaseinthestrengthlevelofmanufacturedcomponents.
ThispaperreviewspracticalexamplesofNbadditionstoengineeringsteelsforforgedparts,fastenersandsprings.Incarburizingsteels,theprocessandheattreatmentaresocomplicatedthattherearedifficultiestoapplyNbtothisgrade.Therefore,researchresultsconcerningadevelopedsteelforcarburizedgearsandshaftsisdescribedindetail:
theniobium-bearingsteelhassuperiorcoldformabilitywithoutspheroidizingannealingafterrollingandhighresistancetograingrowthduringcarburizingevenwhenitisnotnormalizedaftercoldforming.
ForgingSteels
Niobiumisviewedasthekeyingredientinmanyhigh-strength,low-alloysteels(HSLA)andregardedasthemosteffectivegrainrefinerandapromisingelementforprecipitationhardeninginhot-rolledproducts
(1).ThisfavorableeffectofNbwassoonappliedtohotforgingsteelsatthebeginningofthe1970’s.Hulkaetal.
(2)reviewedthehistoricaldevelopmentoftheforgedsteelscontainingNbandphysicalmetallurgyofNbinmediumcarbonsteels,whicharethemostpopularforgingbarsteels.
Hot-forgedProducts
Forcloseddieforgingsintheautomotiveindustrythesteelgrade49MnVS3(0.50%C-0.25%Si-0.70%Mn-0.040%S-0.10%V)wasestablishedinthemiddleofthe1970’s(3).Thoughtherequiredstrengthcouldbeobtainedby0.08%Nbmicroalloyingwhenhighprocessingtemperaturesareapplied,thelimitedtoughnessrestricteditsapplication.Forimprovedtoughness,a0.38%C-1.00%Mn-0.07%V-0.03%Nbsteelwasdevelopedandappliedintheas-forgedconditionforautomobilepartswithimprovedsafety(4).
Author’snameKimuraandKurebayashipage03of19
Bucher(5)comparedthestructuresofAISI1141–niobium-modifiedsteelwiththatofstandardAISI1141steeltakenfromforgedconnectingrods.Thestructuralrefinementincreasedthehardnessoftheniobiumbearingsteelto97HRBvs.92HRBforthestandardgrade,andimprovedtoughnessthatwasadequateforconnectingrods.TheAISI1141-niobium-modifiedsteelisalsousedforweldyokeoruniversaljointcouplings(6).
AFrenchsteelcompanydevelopedavarietyofmicroalloyedforgingsteelsincludinghighstrengthsteelwithtensilestrengthlevelsover1000MPa.Thebasiccompositionsoftheirgradesis0.45%C-0.30%Si-1.50%Mn-0.12%V-0.04%Nb,andaconstantultimatetensilestrengthisobtainedbywayofacompromisebetweenbainiteformationandvanadiumprecipitationduringair-cooling.Niobium’srollisincontrollingtheausteniticgrainsizeand,hence,keepsthesteelhardenabilitystable.Thesegradesareusedforsuspensionarmsandanti-swaybarsintheautomotiveindustry.
FutureProspectiveofControlledForgingSteels
Sampeietal.(7)confirmedthatNb-addedmediumcarbonsteelbars(0.25–0.45%C)hadagoodstrength-toughnessratioaftercontrolledrolling,whereabilletof114mm2washeatedto1373Kandthenrolledinto22mmroundbarswithafinishingtemperatureof1073K.Thisintermediatefinishingtemperaturewasadequatetosuppressrecrystallizationofausteniteinniobium-bearingsteelbarsandresultedingrainrefinement,leadingtohighmechanicalproperties.
Ifthiscontrolledrollingtechnologyisappliedtotheforgingprocess,itisanticipatedthatsuperiormechanicalpropertiesinforgedpartscanbeobtained.Hulka
(2)referredtotheideaofcontrolledforging,whichhadbeenconceptualizedbyPawelskietal.in1975(8),anddemonstratedthatfinishingforgingtemperatureintherange950–1100Kallowedforabetterbalanceoftensilestrengthandtoughnessinniobium-bearingsteels(0.32%C,1.47%Mn)thaninconventionalsteel.Predictably,ahighloadduringforgingatanintermediatetemperatureshortenedthelifeoftheformingdies.Thispoorproductivitylimitstheuseofcontrolledforging.
Recentstudiesongrainrefiningtechnologyhasrevealedthatthemechanicalpropertiesaredramaticallyimprovedwhenthegrainsizebecomesfiner,tolessthanamicron(9).Becausefinergrainscanbeeasilyattainedwithaniobium-bearingsteeldeformedatawarmforgingtemperature,thecontrolledforgingofniobiumsteelwouldbeeffectiveforpracticaluseinthefuture.
SteelsforFasteners
Author’snameKimuraandKurebayashipage04of19
Steelwirerodsforhighstrengthfasteners,inwhichthetensilestrengthisover800MPa,arerequiredtohavecoldformability,goodmechanicalproperties(tensilestrength,proofstress,fatiguestrengthandtoughness)andresistancetodelayedfracturewhenthetensilestrengthreachesupto1200MPa.Thekeystagesofdevelopmentinsteelsforhighstrengthfastenerscanbesummarized;theadoptionofmicroalloyedwirerodstononheat-treatedfasteners,thereductionintheamountofalloyingelementfromheat-treatedgradesandthepreventionofdelayedfractureinhighstrengthfasteners.
AdoptionofMicroalloyedWireRodstoNonHeat-treatedBolts
In1980,Gondoetal.(10)developedasteelwireforcold-headedboltswhichexhibitedastrengthlevelof700MPawithoutspheroidizedannealing,hardeningortempering.Thesteelcompositionwas0.10%C-1.5%MnalongwithasmallamountofNb,VandTi,suitableforobtainingmoderateferriteandpearlitemicrostructuresundercontrolledrollingandcoolingconditions.
Heritieretal.(11)reportedthatthefabricationofclass8.8boltsfrom0.2%C-1.2%Mn-20to50ppmBgradeisanindustrialreality.Withahighhardenabilityinsteelduetoaboronadditionthelowcarbonbainitewasinitiatedduringcoolingfromtherollingtemperature.Theyfoundthatthelowcarbonbainiticferritehadahighstrainhardeningexponentanddemonstratedthatthecombinationof0.12%C-1.65%Mn-0.08%Nb-25to50ppmBandthehighratioofwiredrawingcouldproduceawirerodwithastrengthlevelrangeof1000–1200MPa.Niobiumisaddedtostrengthenthesteelviagrainrefinementandinterphaseprecipitation.
Aferrite-pearlitesteelwithasmallamountofprecipitationandgrainrefiningelementshasbeendesignedtomeetthespecificationofclass8.8fasteners(12).TableIshowsthechemicalcompositionsofthesteel.Inprocessing,lowtemperaturesoakingandfinishrollingatalowtemperaturewereadoptedtoobtaintheextremelyfine-grainedsteeland,afterrolling,controlledcoolingwascarriedouttopromoteprecipitationhardening.TheeffectofthecoolingrateonthehardnessofhotworkedspecimensisshowninFigure2.Thehardnessincrementcomparedwiththebasesteel,0.14%C-0.25%Si-1.45%Mn-0.1%Cr,is30-45HVinthedevelopedsteelwhichstemsfromthecombinedeffectsofinitialg
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