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Chapter3

BulkDeformationForming-RollingRollingProcessAprocessofreducingthethicknessorchangingthecross-sectionalofaworkpiecebycompressiveforcesexertedbyapairofrotatingrollsFlat-andShape-RollingProcessesFlatRollingProcessDevelopedinthelate1500sStartwithslablikeingotPassthroughtworollsseparatedbyadistancelessthanthethicknessoftheingotKeeppassingthroughsuchrollsuntilthefinalthicknessisachievedFrictionforceactsasdrivingforceThefinalproductsincludeplate,sheetandfoil(plate:t>8mm,sheet:t=0.2-4mm,Foil:t<0.2mm),andcanbeusedinavariousoffieldsFlatRollingProcessRotatingrollsreducethethicknessoftheincomingingotFlatRollingProcess

(a)Schematicillustrationoftheflat-rollingprocess.(b)Frictionforcesactingonstripsurfaces.(c)Therollforce,F,andthetorqueactingontherolls.ThewidthwofthestripusuallyincreasesduringrollingNeutralPointApointwheretherearenoslipbetweentheworkpieceandtherollerThefrictionoftwosidesopposeeachotherattheneutralpointThefrictionontheentrysidemustbehigherthantheexitsideThenetfrictionforceandthesurfacevelocitymustbeinthesamedirectionForwardslipisdefinedas:

ForwardslipForceandStressThestressstateissimilartothatinupsettingThecalculationinmoreinvolvedthanthatofupsettingduetothecurvedcontactsurfaceTheflowstressattheexitishigherthanthatattheentryStressonanelementinflatrolling.(a)EntryZone,(b)ExitzonepisafunctionofhandϕForstrainhardeningmaterial,theflowstressYfintheexpressionscorrespondstothestrainthatthematerialhasundergoneatthatparticularlocationintherollgapFromtheexpressions,wecanfindthatthepressureincreaseswithincreasingstrengthofthematerial,increasingcoefficientoffriction,andincreasingR/hfratio(notethattheR/hfratioisequivalenttothea/hrationinupsetting)PressureDistributionintheRollGapTheneutralpointshiftstowardtheexitasfrictiondecreases.Ifthefrictionapproachestozero,therollsbegintoslipinsteadofpullingthetripin

Pressuredistributionintherollgapasafunctionofcoefficientoffriction.Theareaunderthecurveistherollseparatingforceperunitwidthofstrip.Notethat,asfrictionincreases,theneutralpointshitstowardtheentry.WithoutfrictiontherollsslipandtheneutralpointshiftscompeletlytowardtheexitPressuredistributionintherollgapasafunctionofreductioninthickness.Notetheincreaseintheareaunderthecurveswithincreasingreductioninthickness,thusincreasingtheroll-separatingforceCalculationoftheRollingForceTorqueandPowerRequirementsIllustrativeProblemA220mmwide6061-OAluminumstripisrolledfromathicknessof25mmto22mm.Iftherollradiusis300mm.andtherollrpmis100,calculatethehorsepowerrequiredforthisoperation.(Averageflowstressis125MPa)Solution?ForceinHotRollingCalculationofhotrollingforceisimportantTwodifficultiesincalculation–Estimationofthefrictioncoefficientµatelevatedtemperatures–StrainratesensitivityofmetalsathightemperaturesTheaveragestrainrateinflatrollingcanbeexpressedas:

FrictionItisnecessarytohavesomefrictionforpullingthetripintotherollgap,butforceandpowerincreasewithincreasingfrictionForcoldrolling,µrangesfrom0.002to0.3;itrangesfrom0.2to0.7forhotrollingMaximumdraft(h0-hf)canbeexpressedas:

Δhmax=µ2R

αmax=tan-1µFrontandbacktensionNormally,RollforceFcanbereducedbyvariousmeans,suchaslowerfriction,smallerrollradii,smallreduction,….AnothereffectivemethodistoreducetheapparentcompressiveyieldstressofthematerialbyapplylongitudinaltensionTensionsinrollingcanbeappliedeitherattheentry(backtensionσb)orattheexit(fronttensionσf).Thepressurecanbemodifiedas:EntryzoneExitzoneDependingontherelativemagnitudesofthetensionsapplied,theneutralpointmayshift,andthiswillaffectthepressuredistribution,torqueandpowerrequirementsinrollingFronttensioniscontrolledbythetorqueonthecoiler(deliveryreel).Thebacktensioniscontrolledbyabrakingsystemintheuncoiler(payoffreel)Tensionsareparticularlyimportantinrollingthin,high-strengthmaterialsRollDeflectionsLeadingtoahigherthicknessinthetripcentreGrindingtherollssothattheirdiameteratthecentreisslightlylargerthanatedge,calledcamberCambercanalsobecontrolledbyvaryingthelocationofthecoolant(lubricant)ontherollsinhotrollingElasticalforce,flateningFlatRollingPracticeCastingotScalp(notalways)ReheatandhomogenizeHotrollinreversingmillto~1-2inthickCutoffends(alligators)Warmrollto0.1-2inthickHeattreattorecrystallize(sometimes)CoolColdrolltofinishthicknessHeattreat(whereappropriate)ResultsinmaterialwhichmustbefurtherworkedFlatRollingPracticeConvertingthecoarse-grained,brittleandporouscaststructuretoawroughtstructurebyhotrollingForhigh-strengthalloysreductionsareusuallysmall,whereasforsoftandnonferrousmetaltheyarehighTemperaturesinhotrollingaresimilartothoseinhotforgingIndependentVariablesforRollingRollspeedDraft(amountofthicknessreduction)orrollgapBilletthicknessBilletwidthBilletmaterial(notalwaysallowedtoselect)BillettemperatureLubricantDependentVariablesforRollingRollforcePowerSpeedofexitingstripFinalstriptemperatureMaximumdraftRollstripcontactlengthSheetorplateshapeHotRollingAdvantages–Stresseslower–Forcessmaller–Powerrequirementsless–Noworkhardening–Largedeformationspossible–Breaksupthecaststructureintopreferableforms–Closesporosity–SometimestheonlywaytocreatesheetDisadvantages–Higherfriction–Rollsneedtobecooled–Materialhandlingdifficult–PersonnelmustbeprotectedfromheatColdRollingAdvantages–Deformationsof~50to80%–Workhardeningincreasesstrength–Excellentsurfacefinish–ExcellenttoleranceonthicknessandshapeDisadvantages–Highforces–Smallreductionsgiverisetosurfacestressesandnon-uniformstressdistributions

ProductQualityIssuesNon-flatmaterial–Thermalcamber–Rollbending–Wavyedges(causedbyrollbending)–Zipperandedgecracks(poorductility)"Alligatoring"(non-uniformdeformation)typicallyinnoseandtailofslaborplateResidualstresses–SmalldeformationscausesurfacedeformationFlawsinthesurface–RollfinishperfectionResidualstressesDuetoinhomogeneousdeformationSmall-diameterrollsorsmallreductionscausesurfacedeformationOntheotherhand,large-diameterrollsorlargereductionstendtodeformthebulkofthematerialLevelingToimproveflatnessThestripispassedthroughaseriesoflevelingrollsRollingequipmentandpracticeTwo-highorthree-highrollingmillsareusedforinitialbreakdownpassesItisdesirabletousesmall-diameterrollstoreducerollforce,butsmallrollstendtodeflectClustermill(Sendzimir)isparticularlysuitableforcoldrollingthinstripofhigh-strengthmetalRequirementsforrollmaterialsaremainlystrengthandresistancetowear.Threecommonrollmaterialsarecastiron,caststeel,andforgedsteelForhotrolling,rollssurfacearegenerallyrough,butforcoldrolling,theyarefinelypolishedNolubricanti