Lecture 39

NPTELPhaseII:IITKharagpur:Prof.R.N.Ghosh,DeptofMetallurgicalandMaterialsEngineering||||

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Module 39

Structural steel II

Lecture 39

Structural steel II


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Keywords:Hot&coldworking,effectofgrainrefinementonstrength&toughness,factorsaffectingferritegrainsize,austenitegrainsizecontrol,solubilityofNbC,VC,TiC,solubilityproduct,controlled

rolling,recrystallizationcontrolledrolling,dualphasesteel,ausformingIntroductionThisisthesecondmoduleonstructuralsteel.Thebulkofthesteelproducedworldovercomesunderthiscategory.Theseareprimarilyusedfor loadbearingapplications.Mostofthesearemade (shaped) fromcontinuouslycastbilletorslab. Theseareessentiallykilledsteelhavinglittlealloyadditionother thanC,SiandMn.Theelement that isprimarily responsible for itsstrength iscarbon.However,the last34decadeshaveseenthe introductionof innumerablenumberofnewgradesofsteelwithvery littleadditionofalloyelementsbutwithmuchmoreprecise control over the processingparameters at every stages ofmanufacturing. This is anexcellentexampleofhowourbasicunderstandingoftheunderlyingprinciplesofstrengtheningmechanismhasledtothedevelopmentofhighstrengthlowalloysteel.Thiswillformthemainfocusofthismodule.Themaindrivingforcehasbeenthebenefitsaccruingfrommoreefficientuseofsteelbyreducingtheweightofcriticalengineeringstructures.Hot/coldworking:Deformationprocessing isa critical step for theproductionof structural steel.Slide1wouldhelpyourecollectthe importantfeaturesofhotandcoldworking.Hotworkingisdoneabovetheuppercriticaltemperaturewhenthesteeliscompletelyaustenitic.Higherthetemperatureloweristhestressneededtoshapethesteel.Theflowstressremainsnearlyconstantinspiteoflargedeformationbecauseof instant recrystallization.Therefore itmaybepossible tohavevery high degree of deformation in every stage of hot working. The usual hot workingtemperatureisintherangeof0.90.7timesthemeltingpointofsteel.Cold working however is accompanied by hardening. Higher stress is needed in successivestages of deformation. When the steel becomes too hard it is necessary to soften it byannealing.Thetotalprocesscyclemaythereforeconsistofseveralstagesofcoldworkingandannealing. During annealing the microstructure of cold worked steel gets restored by recrystallization.Thisoccursbynucleation andgrowth, the two thermally activatedprocesses.Theexpressionsforthecriticalnucleussize,theactivationbarrierandthenucleationratearegiven in slide 1. Clearly to get fine grains you need to have lower r*,G*, and higher .Therefore themost favorablecondition togeta finegrainstructure isacombinationofhighcoldworkandrelativelylowertemperatureofannealing.Duringhotworkingdeformationandrecrystallizationmaybeassumed tobeoccurringsimultaneously.Thesameconceptofgrainrefinementcanalsobeextendedtohotworkingaswell.


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Hot / cold workingHW: instant re-crystallization on plastic deformation

CW: T < re-crystallization temperature

Higher Gv ~ lower Trcrys.& finer grains. To get fine grains finish rolling done just above recrystallization

temp & not allow time for grains to grow.

3

2

*

2 16* & *3

exp

v v

r GG G

GNRT

Theexpressions given in slide1 for r* andG* are valid forhomogeneousnucleationonly.Howevertherearealwaysfavorableinterfacespresentinsolidswherenucleationofgrainscanoccurmuchmoreeasily.Thisaspecthasbeenintroducedinmodule7anddiscussedindetailinmodule29.Theexpressions for r* forbothhomogeneousandheterogeneousnucleationareidentical. However the activation hill (G*) may be significantly lower than that ofhomogeneousnucleation.Thereforealthoughr*remainsunaffectedthenucleationratecouldbemuchhigherdependingon thenatureof the interfaces (grainboundaries,grainedgesorgraincorners).Thiswouldensuretheformationofalargenumberofnuclei.Ifthereisnospaceforthesetogrowtheaveragegrainsizeisexpectedtobefine.Thepresenceoffineprecipitatesthat couldpin the grainboundaries (donot let these grow) can also significantly contributetowardsgrainrefinement.Thishasbeenexplainedwiththehelpofasetofsketches in fig1.Heterogeneousnucleationcanalsobepromotedby introducingextremely fineparticles thatcouldactasinterfacesfornucleationandalsopintheboundariesofthenewgrainsthatform.

Slide1


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Grainrefinement:effectonstrength&toughness:Grainrefinementistheonlymechanismthatincreasesboththestrength()andthetoughnessofametal.The relationshipbetween the strengthand thegrain size (d is theaveragegraindiameter)isbestdescribedbythewellknownHallPetchequation.Thisisgivenby: /(1)Althoughtheatomswithinagrainarearrangedinaregularfashionthereisatotallackoforderwithinathinregionnearagrainboundary.Itisindeeddifficulttohaveaclearideaaboutthewaytheatomsarearrangedwithinthiszone.Howeverasadislocationglidingonaslipplaneapproachesaboundary itexperiencesarepulsiveforce. Itcouldbemadetodisappearattheboundaryprovidedtheforceactingonthedislocationishighenoughtocreateastepresultinginanincreaseinthenetareaofthegrainboundary.Higherthegrainboundaryenergyhigheristhestressneededtomovethedislocationintotheboundary.Youmayreferbacktomodule14torecollectaboutthenatureofgrainboundary.Thesimplestoftheseisthetiltboundary.Thisisanarrayofedgedislocationarrangedoneovertheother.Itsenergycanbeestimatedbutitismuch lower than that of a high angle grain boundary that you find inmicrostructures. Thestrength of such boundaries is much higher than that of the individual grains at roomtemperatures.Howeverwith increasing temperature the strength of the gain and the grainboundarydecreases.Thisisillustratedwiththehelpofadiagraminslide2.

(a)

(b)

(c)

Fig1:Ifthegrainsareequiaxedthesurfacearea(S)perunit volume is less.Withdeformationgrainsbecomeelongated.Asaresultthesurfaceareaperunitvolumewould increasesignificantly.Thiswouldprovidemorenumbersofnucleationsites.Therefore the finalgrainsizeafterrecrystallizationislikelytobemuchfiner.


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Grain refinement: strength & toughnessThe only mechanism that improves both

strength & toughness

T

fGB

GECT

Fine grains are stronger at lower temp

GB: crack arrester

Cleavage plane: {100}

Inter granular fracture

Trans granular fracture

BCC: ferrite lower DBTT

Note thatwith increasing temperature the strength of the grain boundary decreasesmorerapidly than thatof thegrain.There isa temperatureatwhich the strengthsof the twoareequal.This isknownastheequicohesivetemperature (ECT).Belowthisgrainboundariesarestronger whereas above this grains are stronger. This is why coarse grain structures arepreferredforhightemperatureloadbearingapplications.Howeverthebulkofstructuralsteelismeant for room temperature applications.Thebestway tohave anestimateofECT is toexaminethenatureofthefailureofthetensiletestspecimensatvarioustemperatures.BelowECT grains areweaker therefore you expect transgranular failurewhereas above ECT grainboundariesareweakerthereforeyoushouldexpectintergranularfracture.Slide2alsoexplainstheroleofgrainboundaryasacrackarrester.Steelispronetoductiletobrittletransition.This isassociatedwiththeexistenceofcleavageplanesalongwhichacrackcanpropagateeasily.Thesegregationofinterstitialalongcubeplanesismainlyresponsiblefortheexistenceofcleavageplanes.Onceacrackinitiatesitpropagatesrapidlythroughthegrainbutitcannotcrosstheboundary.Thisisbecausethenextgrainhasadifferentorientationandhenceadifferentcleavagestrength.Finegrainstructureprovidesmorefrequentcrackarresterandtherefore it is likelytohavehigherstrength.Whilediscussingheattreatmentofsteeltheimportanceofausteniticgrainsizewashighlighted.Thismaybe importantforheattreatmentbecauseitaffectsthehardenabilityofsteel. Howeverforstructuralapplication itisthegrainsize of ferrite thatmatters. The aim thermomechanical processing should always be to getextremelyfineferritegraininthefinalproduct.

Slide2


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Factorsdeterminingferritegrainsize:Slide3 illustrateswith thehelpofa setofdiagramshowcooling rateandprioraustenite ()grainsizeofsteelaffects finalgrainsizeof ferrite ()aswellas thewidthof ferrite lamellaewithinpearlite(P).ThesketchontherightshowstwocoolingcurvessuperimposedontheCCTdiagramofhypoeutectoidsteel.The initialaustenitegrainsizeof the twosamples isexactlythesame.Onethatcoolsslowlyspendsmoretimebetweentheonsetofferriteprecipitationandthestartofeutectoiddecomposition.Itislikelytohavemorenumbersofrelativelylargerferritegrainsandcoarsepearlite.Asagainstthisthesamplethatcoolsfasterspends lesstimebetweentheonsetofferriteprecipitationandthestartofeutectoiddecomposition.Itislikelytohave fewernumbersof relatively fine ferritegrainsand finepearlitenodules.The reasonsareasfollows:

Highercoolingratemeanslowertransformationtemperature(higherGv) HigherGvmeansloweractivationhill(G*)andhigherrateofnucleation

Thesketchontheleftofslide3showstheeffectofaustenitegrainsizeonthesizeoftheferritegrains.Theaustenitegrainshavebeen representedbyasetofadjacenthexagons.Note thatthe number of triple points or the nodes (where 3 grainsmeet) per unit area is less if theaustenitegrainsize is large.Thecriticalsizeof ferritenucleihasbeendenotedascircles.Thetriplesare thepreferred sites for thenucleationof ferrite.Note that if theaustenitegrain iscoarsethereisenoughspacefortheferritegrainstogrow.Inthecaseoffineraustenitegrainsizethereishardlyanyspaceleftforthegrowthofferritegrains.Thereforetheaverageferritegrain diameter is expected to less if the austenite grainswere finer. In short the followingfeaturesofprioraustenitearelikelytoaffectferritegrainsize:

Finergrainsmeanshighernumberofnucleationsites Finergrainsmeanslessspacefortheferritegrainstogrow Elongated grains means higher S/V ratio. It means more nucleation sites per unit

volume.


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Factors determining ferrite grain size

1.Cooling rate from state grain size & shape

Finer grain higher the no. of sites

Finer grainsHigher CR : lower transf

temp: higher Gv : lower G* : higher Ndot.

time

T

CPFP

Effectofferritegrainsizeonthestrengthandtoughnessofsteel:Slide4 illustrateswith thehelp twoplots theeffectof ferritegrainsizeon theyieldstrength(YS)andimpacttransitiontemperature(ITT)ofsteelingeneral.Theyieldstrength(YS)followsHallPetchrelationship.NotethatYScouldbeincreasedfrom150MPato450MPabydecreasingferrite grain size from 100 to 2.5 (3 fold increase). The ductile to brittle transitiontemperature too follows a similar relation but with a negative slope. Note that by grainrefinement ITT can be decreased from 25C (room temperature) to 150C (sub zerotemperature).Thisisprimarilybecausegrainboundariesactascrackarresters.Finerthegrainhigheristhenumberofcrackarrestersperunitvolume.Thereforecrackpropagationislikelytobemoredifficult.Grainboundariesalsoactassinksfor impurities inthemetal.Segregationof impuritiesatthegrainboundariesmakethemstronger.Thepresenceof impuritiesalsoactsaspinningagents.Thismakesgraingrowthdifficult.Howeveriftheamountofimpuritiesishighenoughtoformacontinuousnetworkaroundtheboundariesitmaymakeitbrittle.Thisshouldbeavoided.Grainrefinementisawaytoavoidsuchapossibility.Ifgrainsarefineryouwouldneedhigheramountof impurities to formabrittle filmaround theboundaries.Therefore finegrain structures ingeneralarelesspronetobrittlefailure.

Slide3


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Effect of GS on strength & toughness

Y150

450

1/d5 20

ITT

1/d5 20

25

-150

0Ykd

Hall Petch relation

2.5 2.5100 100

Austenitegrainsizecontrol:The bulk of structural steel is made from continuously cast billets or slabs. Hot workinginvariably is the primary forming process. The cast billets or slabs are first homogenized atabout12001300C.Theexacttemperaturemayvarydependingonthecompositionofsteel.Themicrostructureat this stage isexpected to100%austenite.However the transformationfrom ferrite carbide structureat room temperature tohomogeneousaustenite isadiffusioncontrolledprocess.Thecaststructurewouldalwayshavemacroaswellasmicrosegregations.Longhoursofsoakingmaybenecessarytoremovemicrosegregation.Macrosegregationscanonlybe removedbyextensiveplasticdeformation (upset forging). Inorder toget the finestpossiblestructureafterhotworkingitisnecessarytoexercisepropercontrolonthegrainsizeofausteniteduringthehomogenizationprocess.Ifthe initialgrainsizeofaustenite isfinethefinalstructureafterplasticdeformationislikelytobefiner.Oftenitisnecessarytoknowwhatthe size of austenite grain is before deformation starts. Slide 5 describes an experimentalmethod of determination of austenite grain size. On quenching austenite transforms intomartensite.The transformation isextremely fast.Onceaneedleora lathnucleates it travelsfrom one end of the grain to the other. The needle or the lath cannot cross the austeniteboundary.Therefore inprinciplethe lengthofthe longestneedleora lathwithinagrain isameasureof itssizeordiameter.Therearespecialetching reagents to revealsuchstructures.Often thedurationof soaking is fixed. Itmaybeof theorderofanhour for1015mm thicksamples.Heatasetofsamplestodifferenttemperatures.WhenthetemperaturegoesbeyondA1 pearlite transforms into austenite. However, the austenite may still have undissolvedcarbides (C). The sketch on the left of slide 5 shows a time temperature diagram for the

Slide4


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transformation of steel from F+P structure to homogeneous austenite. You may refer tomodule34torecollectthemechanismofastenitization.AlittleaboveA1thestructureconsistsof ferrite (F), austenite (A) and carbide (C).As the temperature goes up ferrite dissolves inaustenite.AtA3thedissolutionof ferrite inaustenite iscompleted.Steeloftenhasextremelyfineparticlesofcarbidesandnitridesduetothepresenceofstrongcarbideornitrideformingalloy elements. The presence of such precipitates act as pinning agents for the grainboundaries.Aslongassuchprecipitatesarepresentthegrainsdonotgrowveryfast.Beyondacriticaltemperature(Tc)whentheydissolveunusualgraingrowthoccurs.

Austenite grain growth

GS

: d

temp

Austenitize & quench to get

martensite

Homogeneous : excessive GG

Tc

F+P

A

A+CF+A+C

T

time

Tc

A1

A3

Theaustenitegrainsizecanbemeasuredfromthemicrostructuresofsamplesquenchedfromdifferent temperatures. Plot the average grain diameter as a function of the soakingtemperature.Thesketchontherightofslide5showssuchaplot.Notethatbeyondacriticaltemperature (Tc) the grains grow very fast. Heating beyond this point should be avoided.InherentlyfinegrainsteelhashighTc.Aluminumkilledsteelcomesunderthiscategory.Thisisbecause of the presence of extremely fine particles of aluminum oxide and nitride at prioraustenitegrainboundaries.The presence of carbides and nitrides inmicroalloyed steel too are extremely effective inpinningausteniteboundaries.Thispreventsexcessivegraingrowthduringthermomechanicalprocessing.Theirpresenceincreasesthetemperaturebeyondwhichsuchprecipitatesdissolve.Thisiswheregraingrowthratemayincrease.Howeverinthepresenceofmicroalloyelements(suchasNb)insolidsolutionmaystillexertaforceonthegrainboundary.Thisdoesnotallowrapiduncontrolledgrowthofaustenitegrains.Whatcouldbedonetogetextremely finegrainstructure insteelafterhotrolling?Themosteffectivewaytodoso istohavea largenumberofultrafineparticlesofcarbides,nitridesor

Slide5


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oxides at austenite grainboundaries. This isbest achievedby adding very small amountsofstrongcarbide,ornitrideformingelementsinsteel.ThemosteffectivealloyadditionsareNb,Vand Ti. All of them have strong affinity for oxygen aswell. In order to have themaximumbenefitsuchalloyadditionsaremadeafterthemoltensteeliskilledbyaluminumaddition.Thetotal amount of such alloy addition hardly exceeds 0.2%. Therefore these are often calledmicroalloyedsteel.Thefinedispersionsofoxides,nitridesandcarbidesofAl,Nb,TiandVatausteniteboundariesactasgraingrowthinhibitors.Apartfrommicroalloyadditionitisalsonecessarytohavetherightcontrolonthetemperatureofhotworking,theamountreductionperpassandthesubsequentcoolingrate.Therearetwodifferentwaysthiscanbeachievedinpractice.Theseareasfollows:

Controlledrolling(CR) Recrystallizationcontrolledrolling(RCR)

During controlled rolling (CR) althoughmost of the deformation is given at the normal hotworking temperature the last stage of rolling is done at a temperature below the recrystallizationtemperatureofaustenite.ThepresenceofextremelyfinecarbidesandnitridesofNborTipinsthegrainsofaustenitesoeffectivelythat itsrecrystallizationtemperaturegoesup. Recrystallization controlled rolling (RCR) is done at a temperature higher than the recrystallization temperatureofaustenite.Thedeformedaustenitegrainsget replacedbynewstrain free grains of austenite.During this stage precipitation of carbides and nitrides takes

CR

RCR

Initialaustenitegrainstructure

Initialaustenitegrainstructure

Hotworkingbelowrecrystallizationtemperature

Deformedgrain RecrystallizationofandsimultaneousprecipitationofVNatgrainboundariesinhibitsgraingrowth.

Nucleationofferritegrainsataustenitegrainboundaries

Nucleationofferritegrainsataustenitegrainboundaries

Fig2:Showsthemaindifferencebetweenthetwodifferentwaysofgeneratingextremelyfinegrainsofferriteinmicroalloyedsteel.


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place.Thefineprecipitateseffectivelypinsthenewlyformedgrains.Thisinhibitsgraingrowth.Subsequentlythenewgrainsofaustenitegetreplacedbystillfinergrainsofferrite.Thekey todevelopmentofhighstrengthsteelhavinggoodductilityand toughness iscloselyassociated with the formation of extremely fine ferrite grains. This is best achieved byincreasing thenumberof siteswhere ferritecannucleate. In theabsenceof inclusionsmostpreferred site is certainly austenite boundaries. The number of sites per unit volume is afunctionoftheshapeandsizeofaustenitegrains.Clearly theof fineyetelongatedaustenitegrains would ensure finer ferrite grains in the final product. Apart from high angle grainboundariesthereareseveralothersiteswherealsoferritegrainsmaynucleate.Someofthesesites are dislocation, subgrain boundaries, twin boundaries, and ledges within grainboundaries. They too contribute towards grain refinement during thermomechanicalprocessing.Theselectionoftherightalloyingelementwhosecarbide,nitrideoroxidecouldeffectivelypinthegrainboundaryofaustenite iscrucial for the twoprocessing routes forgrainrefinement.Themaincriteriafortheselectionofsuchalloyadditionareasfollows:

HighaffinityforO,N,andC Lowsolubilityinaustenite

Elements likeAl,Nb,TiandV fulfill thesecriteria.CandNpresent insteelwould reactwiththesetoformcarbidesandnitrides.ForexampleNbpresentassolute insteelmayreactwithdissolvedNtoformNbNasperthefollowingreaction:Nb+N=NbN:itsequilibriumconstant (2)

[NbN]=activityofNbN. Itcanbe takenas1because it isapure solid. [Nb]=activityofNbpresentassolute inaustenite. It isequal toweight fractionNb (WNb)because it isextremelysmall.[N]=activityofsolublenitrogeninaustenite=WN=weightfractionN.Grainboundariesare thepreferredsites for theprecipitationofNbN.Slide6explains theconceptofsolubilityproduct (SP) that determines the amount of carbides or nitrides that form. It is equal to[Nb][N].


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Solubility product

Nb + N = NbN

[ ] 1[ ][ ]

ln

log

Nb N

NbNKNb N W W

G RT K H T SBK AT

SP = [Nb][N] = f(T)

Nb

N

TT2

T1

If steel is heated above T2 there is no precipitate to pin GB. Austenite grains grow rapidly

Theequilibriumconstantofachemicalreaction isa functionoftemperature.Slide6explainshow from theelementary conceptsof chemical thermodynamicsanexpression forK canbederived.Thissuggeststhatatagiventemperaturethesolubilityproductisaconstant.Itshouldincreasewithtemperature.Thesketch inslide6showstherelationshipbetweentheamountsofdissolvedNb&Ninausteniteintheformofasetoftwoplotsattwodifferenttemperatures.A filled circle in the plot denotes the total Nb & N present in the steel. If the soakingtemperaturegoesbeyondT2allofthesewillbepresentassolidsolution.TherewillnotbeanyNbNprecipitatetopintheboundary.Theaustenitegrainwouldgrowrapidly.Iftheexpressionforthesolubilityproductisknownsuchplotscaneasilybegenerated.AsanillustrationthefollowingequationforthesolubilityproductforNbNinaustenitecanbeusedtogeneratesuchaplot.Thisisgiveninfig3.log

1.71(3)

Thesolubilityproduct inferritewouldbestill less.Slide7showswiththehelpofaschematicgraphtherelativestabilityofvariouscarbidesandnitridespresentinsteel.Lowerthesolubilityproducthigheristhestabilityofthecompound(carbideornitride).Thestabilityofcarbidesandnitrides decreases with increasing temperature. In general nitrides are more stable thancarbides.Thepresenceofbothhasbeeneffectivelyexploitedtoimprovethestrengthofsteeleitherbyrecrystallizationcontrolledrollingorbycontrolledrolling.TheformerusesVwhereasthelatterusesNb/Tiasmicroalloyadditions.

Slide6


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Fig3:GivethesolubilityofNinausteniteinthepresenceofNbastrongnitrideformeratfourdifferent temperatures.The solubility limit is a functionof temperature. Let theamountsoftotalNb andN in steel be 0.06 and 0.01 respectively. At 1100C a part of the twowill bepresentasgrainboundaryprecipitatesandapartwillbeinausteniteassolutes.Howeverifthetemperaturegoesbeyond1200CtheentireNbandNwillgointosolution.

Solubility product

Lower solubility product means higher is the stability of the precipitate

Log{

(M)(

C)}

10000/T 106

VC

TiC

AlN TiN

VN

NbN

NbC

High T Low T

0

0.005

0.01

0.015

0.02

0 0.02 0.04 0.06 0.08 0.1

Wt%

N

Wt%Nb

900

1000

1100

1200

Slide7


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Grainrefinementbycontrolledrolling:Slide 8 explainswith thehelp of sketcheshow grain refinement canbe achieved by properselectionofhotworkingtemperatureduring.Thefigureontheleftshowshowthewidthofthestockdecreasesduringdifferent stagesof rolling.Thebulkof thedeformationoccursat thehighest possible rolling temperature. This is the stage where the structure consists ofhomogeneousaustenite.Thedottedlinewithinthestockrepresentsitstemperature.

Hot rolling schedule for GR

Controlled rolling leads to grain refinement

a

b

c

d

e

f

Partial recrystallization

T

time

A3A1

Initial rolling

Final rolling

Homogeneous Rapid

recrystallization

delay

No recrystallization

Mean stock temp

Stock width

Stock

Slide8alsosuggeststhatthehotworkingprocesscanbedividedintothefollowingstages:

Initialrolling:wherethereisrapidrecrystallizationofhomogeneousaustenite Delay:nildeformationstagewhenrecrystallization&graingrowthtakesplace Finalrolling:Partialornilrecrystallizationofaustenite Controlledcooling:nucleation&growthofferritegrainsandpearlitenodules

Themicrostructuresofthestockatvariousstagesofdeformationarerepresentedbyasetofsketches(af)ontherightofslide8.Themainfeaturesofthestructurearegivenbelow.

a. Coarsehomogeneousaustenitegrainsbeforedeformationb. Relativelyfinegrainsofausteniteasaresultofrapidrecrystallizationc. Graingrowthandprecipitationofcarbidesandnitridesatgrainboundariesd. Initialstageoffinalrollingwhenonlypartialrecrystallizationofaustenitetakesplacee. Towardstheendoffinalrollingwhenthereisnorecrystallizationf. Transformationofaustenitetoferritecarbidestructure

Slide8


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Theprecipitationofcarbidesandnitridestakesplaceduringthestagethatfollowsinitialrolling.This iswhen recrystallized austenite grainsmay grow a little until these are pinned by theprecipitates. The final rolling is done at temperature close to the nil recrystallizationtemperatureofaustenite. When the final rolling startspartial recrystallizationofelongatedaustenite grainsmay takeplacebutmostof it remains in coldworked state (Note that themicrostructure labeled as d in slide 8 shows that a few grains of stain free austenite haveformed along the grain boundaries of elongated austenite. On subsequent cooling theelongatedaswellasthenewlyformedaustenitegrainswouldtransformmostly intostillfinergrainsofferrite.

Finish rolling & cooling schedule for fine grain steel

900Air cool

5c/s water cool ~15c/s

Coiling temp

time

500-675C

T

Ferrite start

Ferrite finish

Deformation

Controlledcooling:Fastercoolingafterfinishrollingcanalsocontributetofurthergrainrefinement.SteelsthataresubjectedtocontrolledrollingoftenhaveverylittleCorN.MostofthesegetfixedbythemicroalloyingelementslikeNb,TiandVpresentinsteel.ThereforetheaustenitehavinglittleCorNwould transform into ferrite after the final stage of rolling. The grain size of ferritewoulddependon the temperatureatwhich the transformation takesplace.Thisbeinganucleationand growth process lower transformation temperature would favor the formation of finergrains of ferrite. This is because of higher nucleation rate (reason higher driving force) andslowergrowthrate(reasonlowerdiffusivityorselfdiffusioncoefficient).Slide9showswiththehelpofatimetemperatureplotatypicalfinishrollingfollowedbyacceleratedcoolingscheduleforamicroalloyedgradeofsteel.Thedottedlinesinslide9representsferritestartandferritefinishlinesoftheCCTdiagramofthesteel.Thecoolingcurveintersectsthestartandfinishlines

Slide9


16

at a substantially lower temperature thereafter the sheet goes for coiling. The coilingtemperaturemaybe in the range500675Cdependingon theapplication.Thecooling rateaftercoilingmaybecomeslow.Sincethetransformationofaustenitetoferriteisoverbynowitmaynotmattermuch. The only change thatmight occur is the precipitation of carbides ornitrides.Thiswouldinhibitferritegraingrowthandcontributetowardsprecipitationhardening.

Factors determining re-crystallized GSTemperature, strain, & strain rate

GS

T1100 900

100

10Strain/pass Strain rate

Slide10showstheeffectofhotrollingtemperature,strain/pass(amountofdeformation)andstrain rateon the grain sizeof recrystallized austenite. The keyprocessingparameters forgettingfinegrainausteniteareasfollows:

Lowerhotworkingtemperature Higherreductionperpass Higherstrainrate

Slide 11 highlights themain problem of implementing controlled rolling in practice. This isconnectedwith themagnitudeof rolling load. Itdependson the flow stressof austenite. Itincreaseswithdecreasingtemperature.Existingrollingmillsinplantmaynotbeabletosupportsuchahighload.Thereforeitmayneedamajorchangeintherollingmillofaplant.Adoption of RCR is a possible alternative. The rolling temperature is so chosen thatprecipitationofcarbides/nitridestakesplaceduringrecrystallization.Therollingtemperaturebeinghigheritmightbepossibletogivehigherdeformation/passatamuchlowerload.

Slide10


17

Limitation of controlled rolling

Controlled rolling: below re-crystallization tempRe-crystallization controlled rolling: nitrides form

during re-crystallization inhibits grain growth.

CR Ferrite nucleation in pan caked austenite

RCR

initial rolling recrystallized

High rolling load

Low rolling load

Dualphasesteel:

Dual phase steel: +M

%C

T

Inter critical heat treatment

Limitation: high cooling rate to convert to M: applicable to

thin plates / sheets

A1

A3

F P MF

AF

TA1

F

M

RA% p

hase

s

A3

Dualphase steel consistsof ferrite andmartensite. It canbeproducedby intercriticalheattreatment.The concepthasbeenexplainedwith thehelpofa setofdiagramsand sketchesrepresentingmicrostructuresinslide12.Thestructurebeforeheattreatmentconsistsofferrite(F) and pearlite (P). The amount of pearlite (P) depends on %C. The steel is heated to atemperaturebetweenA1andA3.Pearlite transforms intoausteniteas the temperaturegoesbeyondA1,buttheferriteremainsasitis.AtA1%Cinausteniteisaround0.8%(Thisissameas

Slide11

Slide12


18

that of the eutectoid. Look at the phase diagram in slide 12). On quenching from thistemperature austenite is expected to transform into high carbon martensite (M). The Mftemperaturedecreasesas%Cinausteniteincreases.Ifitislowerthanroomtemperatureapartoftheaustenitemayremainasitis.Thisiscommonlyknownasretainedaustenite.AstructurecontainingRAisunstable.Itcanberemovedbytempering.Thefinalstructurewouldthereforeconsistof ferrite (F), retained austenite (RA)&martensite (M). The relative amountsof thethreephasesarefunctionsofthetemperatureatwhichthesteelwasheatedto.TheplotsontherightshowtheeffectofintercriticalheattreatmenttemperatureontheamountsofRA,FandM.Theamountofretainedaustenite isthehighestatA1and itdecreaseswith increasingtemperature.Beyondaparticular temperature itmightbe totallyabsent.This ispossibly themostfavorabletemperaturefor intercriticalheattreatment.Theamountofmartensite isthelowestatA1anditmayreach100%asTgoesbeyondA3.TheamountofferriteisthehighestatA1and itkeepsdecreasingwith increasing temperature.Themost favoredmicrostructureofdualphase steel consistsof islandsof low carbon lathmartensite in amatrixof ferrite.Thestrengthof lowcarbonmartensite issignificantlyhigher than thatof finepearlite. Italsohasreasonable ductility. Therefore such steel is likely to have an optimum combination of highstrengthand%elongation.Acolorcodehasbeenusedonlytodistinguishthedifferentphasesinsteel.Thishasnorelationwiththeirappearanceunderamicroscope.

Strength vs. ductility

TS: MPa400 1000

SS hardening: P, Si

Nb, V steel DP steel10

40

% e

long

atio

n

Thepropertiesofdualphasesteelcanvarywidelydependingonthecompositionandthefinalheattreatmenttemperature.Slide13showstherangeofstrengthandductilityyoucouldgetindualphase (DP) steel. In comparison tomicroalloyedor solid solution strengthened itgives

Slide13


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bettercombinationofstrengthandductility.However, itneedsanadditionalheat treatmentthatmayaddup to itscost. Inprinciple it ispossible to incorporatean isothermalhold foraspecifieddurationatanappropriatetemperatureafterfinishrollingandsubsequentquenching.Thesketchinthissliderepresentsinverserelationbetweenstrengthandductility.Acompleteversion of this plot (often referred to as banana plot) has been included in a subsequentmodule.

Ultra high strength steel wire

Patenting: isothermal

Fine lamellar pearlite: cold drawn to desired strength & diameter: 1200 3000MPa UTS:

spring, wire ropes for suspension bridge

T

time

A3A1

Ms

P

P

+ M Mf

500C

Patenting: Apart from alloying steel offers multiple opportunities of thermomechanicalprocessingtogetextremelyhighlevelsofstrength.Oneoftheseisknownaspatenting.Thisisaspecialheattreatmentgiventowirebeforeit iscolddrawnto itsfinalsize.Slide14describestheisothermalheattreatmentcycle.Thesketchontheleftshowsthecoolingschemefollowedduringtheprocessandtheoneontherightshowsthesetupconsistingofmoltenleadbaththathelpsmaintain the desired temperature. It is a continuous heat treatment process. Steel isconverted toausteniteata relativelyhigher temperature.Thegrain size is relatively coarse.Subsequentlyitpassesthroughamoltenleadbathmaintainedatatemperaturecorrespondingto thenoseof theTTTdiagram.The timespent in thebath is longenough toconvert it intoextremely fine pearlitic structure. This steel has excellent drawability. The wire rod afterpatentingcanbecolddrawntothedesiredsizeandstrength.Theultimatetensilestrengthofsuchawirecanbeoftheorderof12003000MPa.Thisisusedtomakehightensilestrengthwireropes.Ausforming:Isothermaltransformationdiagramofsteelshowstheexistenceofaregionalittleabove theMstemperatureofsteelwhereaustenite isstable forarelatively longerperiod. In

Slide14


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certain low alloy steel thismight be long enough to allow deformation processing. Slide 15suggestsaprocessing routewhere the steel isquenched fromA3+50C toa temperaturealittleaboveMs. Oncethetemperatureatthesurfaceandthecenterofthesteel isnearlythesameitcanbesubjectedtocoldwork.ThetemperatureatwhichcoldworkingisdoneshouldbehigherthanMd.Recallthataustenitetransformstomartensitebyshear.Plasticdeformationtoooccursduetoshear.Thereforewhenthesteelisbeingsubjectedtoplasticdeformationthenucleation of martensite might occur a little above Ms. This should be avoided becausemartensite ishardanddifficult todeform.Thedurationavailable fordeformation is short. Itshouldbe finishedbeforebainitic transformationbegins.Aftercoldwork it isquenched.Theaustenite grains have high dislocation density.On quenching it transforms intomartensite.Sincemartensiteformsincoldworkedstructureitisstongerthannormalmartensite.Thelathsizetoo issmaller.Thereforethestrengthofausformedsteelcanbeashighas2500MPa. Its%elongation is 8%. It is higher thanmartensite of similar strength. Themajor limitations ofausformingareasfollows:

Onlythinsectionsareamenabletosuchatreatment Needalloyadditiontohaveanextendedbainiticbaytoallowcoldworking

Notethedifferencebetweenausformingandaustempering. Ausforming: quench to T >Md + coldwork followed by quenching; final structure is

martensite Austempering:quenchtoT>Ms+longisothermalholdtogetlowerbainite

Ausforming: CW in Bainitic bay

Avoid Bainite. Aim to get Martensite in cold worked austenite. It can have strength ~ 2500MPa & 8% El.

Better than Martensite of similar strength.

Austempering: aim to get Bainite good

combination of strength ~RC 50 & toughness

MSMf

Log(t)

A3A1

Md Limitation: possible in alloy steel & thin

sectionsOil quench

Slide15


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Summary:Inthismodulewehaveseenhowourbasicunderstandingofthestructuralchangesthatoccurinsteelduringhotandcoldworkingcanbeexploitedtogetextremely finegrainstructure inthefinalproduct.Theeffectofvariousprocessingparameterslikeprioraustenitegrainsize,theamountof reductionperpassandstrain rateapart fromalloyadditionhavebeendiscussed.Thekeytosuccessdependsalotonthecontrolofprecipitationofdesiredcarbidesandnitridesofmicro alloy additions during hotworking so that deformed austenite either does not recrystallizeoronlyundergoespartialrecrystallization.ThisformsthebasisofCRandRCR.Theformerismoreeffectiveingrainsizecontrolbutneedshigherrollingload.Thelatterisamorepopularoptionasthe loadonthemill ismanageable.Wealso lookedatthehowdualphasesteel having an optimum combination of strength andductility can be produced. Finallywetalkedabouthowtransformationcharacteristicsofsteelcanbeexploitedtogetextremelyhighstrengthsteel in relatively thinsections.The twoof themostcommonlyusedprocesses thatwerediscussedarepatentingandausforming.Exercise:

1. Whyisitpossibletogetmuchfinergrainstructureinsteelthaninaluminum?2. Estimate the amount of martensite that can be obtained in 0.2% carbon steel by

quenching from intercritical temperature regime (betweenA1&A3) as a functionoftemperature.Whichoftheseislikelytogivemaximumstrength?

3. Theconceptofsolubilityproduct isused toestimate theamountofMXprecipitate inmicroalloyedsteel(FeMX) intermsof itscomposition inwt%. IfsolubilityproductofVN is given by log 3.46estimate the amount of V present as VNprecipitatesinalloyhaving0.15wt%Vand0.015wt%Nat1273K.

4. Howisausformingdifferentfromaustempering?Isthisahotworkingprocess?5. Whatistheeffectofmicroalloyadditiononrollingload?

Answer:

1. Steel unlike aluminum offers additional opportunity for grain refinement because ofaustenitetoferritetransformation.Inadditiontograinrefinementduringsolidificationthereisfurtherrefinementbecauseofnucleation&growthofnewferritegrainsatpriorausteniteboundaries.


22

2. Applyleverruletoestimate%asafunctionofintercriticaltemperature.AmountofwillbeminimumnearA1andmaximum(100%)atA3.Onquenchingthisgetsconvertedto martensite. Hardness of martensite depends on carbon content. ThereforeMartensite formedonquenching fromA1willhavemaximumhardnesswhereasthatformedonquenchingfromA3willhaveminimumhardness.Hardnessofferriteremainsconstant. Apply rule ofmixture to get the hardness of ferritemartensite aggregate.Assume hardnessofmartensite change linearly from 200VHN at 0%C to 950VHN at0.8%C.Assumption:thereisnoretainedausteniteinfinalstructure.

3. Since log 3.46 0.000825Use this to generate solubility plot at1273K. This is shown by line k in the graph. Atomicweights of V&N are 51& 14respectively. Plot lineVNpassing throughoriginwith slope=51/14.This representsamountofVneededtoformVN(Stoichiometry).

Markthecompositionofthesteelbya point (0.15V, 0.015N).Draw a lineparalleltoVNthroughthispoint.Thepoint of intersection with krepresents amount of V that ispresent in solution.Thus solubleV=0.12wt% Balance 0.03 is present asVN.

4. Inaustempering the steel isaustenitizedand thenquenched inabathkeptata littleaboveMstemperatureforsufficientlylongtimetoget100%Bainiticstructure.Whereasinausformingthesteelistakentoausteniticstatefollowedbyquenchinginabathkeptat a temperature a little above Md and subjected to plastic deformation beforequenchingtogetmartensiticstructure.Mdtemperaturecorrespondstoatemperatureabovewhichmartensite does not form even if austenite is being deformed. Sinceplasticdeformationduringausformingtakesplacebelowrecrystallizationtemperatureofausteniteitisacoldworkingprocess.

5. In the case ofNb containing steel grain boundaries get pinned by nano sizedNbCNprecipitatesformedsoonafterrollingathightemperature.Finishrollingisdonebelowits recrystallization temperature. This gives rise to elongated austenitic grainswhichultimatelytransformtoferritepearlitestructure.Thisprocesswouldneedhigherrolling

0

0.05

0.1

0.15

0.2

0 0.02 0.04 0.06

V

N

k

VN


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load.WhereassteelshavingVasmicroalloyadditionprecipitationoccursduringfinishrollingstagetoinhibitgraingrowth.Thereforerollingloadhereislow.

KeywordsIntroductionHot / cold working:Grain refinement: effect on strength & toughness:Factors determining ferrite grain size:Effect of ferrite grain size on the strength and toughness of steel:Austenite grain size control:Grain refinement by controlled rolling:Controlled cooling:Dual phase steel:Summary:Exercise:Answer:

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Lecture 39