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Chapter3BulkDeformationForming-Rolling,RollingProcess,Aprocessofreducingthethicknessorchangingthecross-sectionalofaworkpiecebycompressiveforcesexertedbyapairofrotatingrolls,Flat-andShape-RollingProcesses,FlatRollingProcess,Developedinthelate1500sStartwithslablikeingotPassthroughtworollsseparatedbyadistancelessthanthethicknessoftheingotKeeppassingthroughsuchrollsuntilthefinalthicknessisachievedFrictionforceactsasdrivingforceThefinalproductsincludeplate,sheetandfoil(plate:t8mm,sheet:t=0.2-4mm,Foil:t0.2mm),andcanbeusedinavariousoffields,FlatRollingProcess,Rotatingrollsreducethethicknessoftheincomingingot,FlatRollingProcess,(a)Schematicillustrationoftheflat-rollingprocess.(b)Frictionforcesactingonstripsurfaces.(c)Therollforce,F,andthetorqueactingontherolls.Thewidthwofthestripusuallyincreasesduringrolling,NeutralPoint,ApointwheretherearenoslipbetweentheworkpieceandtherollerThefrictionoftwosidesopposeeachotherattheneutralpointThefrictionontheentrysidemustbehigherthantheexitsideThenetfrictionforceandthesurfacevelocitymustbeinthesamedirectionForwardslipisdefinedas:Forwardslip,ForceandStress,ThestressstateissimilartothatinupsettingThecalculationinmoreinvolvedthanthatofupsettingduetothecurvedcontactsurfaceTheflowstressattheexitishigherthanthatattheentry,Stressonanelementinflatrolling.(a)EntryZone,(b)Exitzone,pisafunctionofhandForstrainhardeningmaterial,theflowstressYfintheexpressionscorrespondstothestrainthatthematerialhasundergoneatthatparticularlocationintherollgapFromtheexpressions,wecanfindthatthepressureincreaseswithincreasingstrengthofthematerial,increasingcoefficientoffriction,andincreasingR/hfratio(notethattheR/hfratioisequivalenttothea/hrationinupsetting),PressureDistributionintheRollGap,Theneutralpointshiftstowardtheexitasfrictiondecreases.Ifthefrictionapproachestozero,therollsbegintoslipinsteadofpullingthetripin,Pressuredistributionintherollgapasafunctionofcoefficientoffriction.Theareaunderthecurveistherollseparatingforceperunitwidthofstrip.Notethat,asfrictionincreases,theneutralpointshitstowardtheentry.Withoutfrictiontherollsslipandtheneutralpointshiftscompeletlytowardtheexit,Pressuredistributionintherollgapasafunctionofreductioninthickness.Notetheincreaseintheareaunderthecurveswithincreasingreductioninthickness,thusincreasingtheroll-separatingforce,CalculationoftheRollingForce,TorqueandPowerRequirements,IllustrativeProblem,A220mmwide6061-OAluminumstripisrolledfromathicknessof25mmto22mm.Iftherollradiusis300mm.andtherollrpmis100,calculatethehorsepowerrequiredforthisoperation.(Averageflowstressis125MPa)Solution?,ForceinHotRolling,CalculationofhotrollingforceisimportantTwodifficultiesincalculationEstimationofthefrictioncoefficientatelevatedtemperaturesStrainratesensitivityofmetalsathightemperaturesTheaveragestrainrateinflatrollingcanbeexpressedas:,Friction,Itisnecessarytohavesomefrictionforpullingthetripintotherollgap,butforceandpowerincreasewithincreasingfrictionForcoldrolling,rangesfrom0.002to0.3;itrangesfrom0.2to0.7forhotrollingMaximumdraft(h0-hf)canbeexpressedas:hmax=2Rmax=tan-1,Frontandbacktension,Normally,RollforceFcanbereducedbyvariousmeans,suchaslowerfriction,smallerrollradii,smallreduction,.AnothereffectivemethodistoreducetheapparentcompressiveyieldstressofthematerialbyapplylongitudinaltensionTensionsinrollingcanbeappliedeitherattheentry(backtensionb)orattheexit(fronttensionf).Thepressurecanbemodifiedas:EntryzoneExitzone,Dependingontherelativemagnitudesofthetensionsapplied,theneutralpointmayshift,andthiswillaffectthepressuredistribution,torqueandpowerrequirementsinrollingFronttensioniscontrolledbythetorqueonthecoiler(deliveryreel).Thebacktensioniscontrolledbyabrakingsystemintheuncoiler(payoffreel)Tensionsareparticularlyimportantinrollingthin,high-strengthmaterials,RollDeflections,LeadingtoahigherthicknessinthetripcentreGrindingtherollssothattheirdiameteratthecentreisslightlylargerthanatedge,calledcamberCambercanalsobecontrolledbyvaryingthelocationofthecoolant(lubricant)ontherollsinhotrollingElasticalforce,flatening,FlatRollingPractice,CastingotScalp(notalways)ReheatandhomogenizeHotrollinreversingmillto1-2inthickCutoffends(alligators)Warmrollto0.1-2inthickHeattreattorecrystallize(sometimes)CoolColdrolltofinishthicknessHeattreat(whereappropriate)Resultsinmaterialwhichmustbefurtherworked,FlatRollingPractice,Convertingthecoarse-grained,brittleandporouscaststructuretoawroughtstructurebyhotrollingForhigh-strengthalloysreductionsareusuallysmall,whereasforsoftandnonferrousmetaltheyarehighTemperaturesinhotrollingaresimilartothoseinhotforging,IndependentVariablesforRolling,RollspeedDraft(amountofthicknessreduction)orrollgapBilletthicknessBilletwidthBilletmaterial(notalwaysallowedtoselect)BillettemperatureLubricant,DependentVariablesforRolling,RollforcePowerSpeedofexitingstripFinalstriptemperatureMaximumdraftRollstripcontactlengthSheetorplateshape,HotRolling,AdvantagesStresseslowerForcessmallerPowerrequirementslessNoworkhardeningLargedeformationspossibleBreaksupthecaststructureintopreferableformsClosesporositySometimestheonlywaytocreatesheetDisadvantagesHigherfrictionRollsneedtobecooledMaterialhandlingdifficultPersonnelmustbeprotectedfromheat,ColdRolling,AdvantagesDeformationsof50to80%WorkhardeningincreasesstrengthExcellentsurfacefinishExcellenttoleranceonthicknessandshapeDisadvantagesHighforcesSmallreductionsgiverisetosurfacestressesandnon-uniformstressdistributions,ProductQualityIssues,Non-flatmaterialThermalcamberRollbendingWavyedges(causedbyrollbending)Zipperandedgecracks(poorductility)Alligatoring(non-uniformdeformation)typicallyinnoseandtailofslaborplateResidualstressesSmalldeformationscausesurfacedeformationFlawsinthesurfaceRollfinishperfection,Residualstresses,DuetoinhomogeneousdeformationSmall-diameterrollsorsmallreductionscausesurfacedeformationOntheotherhand,large-diameterrollsorlargereductionstendtodeformthebulkofthematerial,Leveling,ToimproveflatnessThestripispassedthroughaseriesoflevelingrolls,Rollingequipmentandpractice,Two-highorthree-highrollingmillsareusedforinitialbreakdownpassesItisdesirabletousesmall-diameterrollstoreducerollforce,butsmallrollstendtodeflectClustermill(Sendzimir)isparticularlysuitableforcoldrollingthinstripofhigh-strengthmetalRequirementsforrollmaterialsaremainlystrengthandresistancetowear.Threecommonrollmaterialsarecastiron,caststeel,andforgedsteelForhotrolling,rollssurfacearegenerallyrough,butforcoldrolling,theyarefinelypolishedNolubricantisusedinferrousalloys,aqueoussol