NIOBIUM IN MICROALLOYED ENGINEERING STEELS WIRE RODS.docx

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