金属轧制基础

FundamentalofMaterialsForming-RollingofMetals金属轧制Rolling轧制Rollingistheprocessofreducingthethickness(orchangingthecross-section)ofalongworkpiecebycompressiveforcesappliedthroughasetofrolls(Fig.13.1);轧制是通过一些轧辊对一个长坯料进行加压使其厚度减薄或者界面形状发生变化的工艺。Theprocessissimilartotherollingofdough(面团）witharollingpin（擀面杖）toreduceitsthickness.Rolling,whichaccountsforabout90%ofallmetalsproducedbymetalworkingprocesses,wasfirstdevelopedinthelate1500s.Thebasicoperationisflatrolling.扁平孔型轧制,orsimplyrolling,wheretherolledproductsareflatplateandsheet.Sheetsaregenerallylessthan6mmthick.Theyareusedforautomobileandaircraftbodies,appliances(仪表),foodandbeverages(饮料）containers,andkitchenandofficeequipment.appliances

Theextrusiondie

isasteeldiskwithanopening,thesizeandshapeoftheintendedcross-sectionofthefinalextrudedproductasindicatedbelow.

Flat-andShape-RollingProcessesFigure13.1Schematicoutlineofvariousflat-andshape-rollingprocesses.Source:AmericanIronandSteelInstitute.Slab钢板Hotstrip热轧带钢Pickling酸洗oiling润滑skelp制管钢板Pickling酸洗oling润滑bloom钢锭rail铁轨TuberoundsPlates:t>6mmSheets:t<6mmTheinitialbreakdownofcastingotsintobloomsandbilletsisdoneusinghotrolling.Bloomistheproductoffirstbreakdownoftheingot.Crosssectionalarea~36in2.Billethasasectiontypically1.5"x1.5".Slabisahot-rolledingotofwidthabouttwicethethickness,&ofarea~16in2.Coldrollingproducessheet,stripandfoilwithgoodsurfacefinish.MECHANICSOFROLLINGANCVoVfVrtaRLhfhoSRDh/2DEWidthofsheet=w,andthisdoesnotchangewithdeformation.E=ln(h/ho)Flat-Rolling扁平孔型轧制Figure13.2(a)Schematicillustrationoftheflat-rollingprocess.(b)Frictionforcesactingonstripsurfaces.(c)Therollforce,F,andthetorqueactingontherolls.Thewidthwofthestripusuallyincreasesduringrolling,asisshowninFig.13.5.Thevelocityofthestripishighestattheexitfromtherollgap;wedenoteitasVfthere.Becausethesurfacespeedoftherollisconstant,thereisrelativeslidingbetweentherollandthestripalongthearcofcontactintherollgap,L.Atonepointalongthecontactlength,calledtheneutralpointorno-slippoint中性点,thevelocityofthestripisthesameasthatoftheroll.Totheleftofthispoint,therollmovesfasterthanthestrip;totherightofthispoint,thestripmovesfasterthantheroll.Hence,thefrictionalforces,whichopposemotion,actonthestripasshowninFig.13.2b.Therollspullthematerialintotherollgapthroughanetfrictionalforceonthematerial.ItcanbeseenthatthisnetfrictionalforcemustbetotherightinFig.13.2b;consequently,thefrictionalforcetotheleftoftheneutralpointmustbehigherthanthefrictionforcetotheright.FrictionalForces轧制力

Becausetherollsapplypressureonthematerialinordertoreduceitsthickness,aforceperpendiculartothearcofcontact(Fig.13.2c)isneeded.

Therollforceinflatrollingcanbeestimatedfromtheformula

whereListheroll-stripcontactlength,wisthewidthofthestrip,andYangistheaveragetruestressofthestripintherollgap.Equation(13.2),ideally,isforafrictionlesssituation.轧制力函数：ReducingRollForce.

Rollforcescanbereducedbyanyofthefollowingmeans:a.reducingfriction;减小摩擦力；b.usingsmaller-diameterrolls,toreducethecontactarea;用半径小的轧辊c.takingsmallerreductionsperpass,toreducethecontactarea;减小轧制时的压下量andd.rollingatelevatedtemperatures,tolowerthestrengthofthematerial.提高轧制温度Four-HighRollingMillFigure13.3Schematicillustrationofafour-highrolling-millstand,showingitsvariousfeatures.Thestiffnessesofthehousing,therolls,andtherollbearingsareallimportantincontrollingandmaintainingthethicknessoftherolledstrip.GeometricConsiderationsBecauseoftheforcesactingonthem,rollsundergocertaingeometricchanges.Justasastraightbeamdeflects（偏转）underatransverse（横向的）load,rollforcestendtobendtherollselasticallyduringrolling(Fig.13.4a);thehighertheelasticmodulusoftherollmaterial,thesmallertherolldeflection.RollBendingFigure13.4(a)Bendingofstraightcylindricalrolls,causedbytherollforce.(b)Bendingofrollsgroundwithcamber,producingastripwithuniformthickness.Spreading展宽ofaStripFigure13.5Increaseinthewidth(spreading)ofastripinflatrolling(seealsoFig.13.2a).Similarly,spreadingcanbeobservedwhendoughisrolledwitharollingpin.

width-to-thicknessratiosFLAT-ROLLINGPRACTICETheproductofthefirsthot-rollingoperationiscalledabloom钢锭orslab厚平板(Fig.13.1).Abloomusuallyhasasquarecross-section,atleast150mm(6in.)ontheside;aslabisusuallyrectangularincross-section.Bloomsareprocessedfurther,byshaperolling,intostructuralshapes,suchasI-beams（工字钢）

andrailroadrails(Section13.5).Slabsarerolledintoplatesandsheet.Coldrollingiscarriedoutatroomtemperature(Section1.7)and,comparedtohotrolling,producessheetandstripwithmuchbettersurfacefinish(becauseoflackofscale),dimensionaltolerances,andmechanicalproperties(becauseofstrainhardening).冷轧是在室温下对材料进行轧制。与热轧相比，冷轧产品尺寸精度高，表面光洁，机械强度高。冷轧变形抗力大，适于轧制塑性好，尺寸小的线材、薄板材等。ColdrollingPackrolling（迭板轧制）isaflat-rollingoperationinwhichtwoormorelayersofmetalarerolledtogether;thisprocessimprovesproductivity.Aluminumfoil,forexample,ispackrolledintwolayersGrainStructureDuringHotRolling

Figure13.6Changesinthegrainstructureofcastoroflarge-grainwroughtmetalsduringhotrolling.Hotrollingisaneffectivewaytoreducegrainsizeinmetals,forimprovedstrengthandductility.Caststructuresofingotsorcontinuouscastingareconvertedtoawroughtstructurebyhotworking.矫直Arolledsheetmaynotbesufficientlyflatasitleavestherollgap,becauseofvaria-tionsinthematerialorintheprocessingparametersduringrolling.Toimproveflatness,therolledstripispassedthroughaseriesoflevelingrolls.Severaldifferentrollerarrangementsareused,oneofwhichisshowninFig.13.7.Figure13.7Amethodofrollerlevelingtoflattenrolledsheets.SeealsoFig15.22.DefectsinRolledPlatesandSheetsWavyedges波浪边onsheets(Fig.13.8a)aretheresultofrollbending.Thestripisthinneralongitsedgesthanatitscenter(seeFig.13.3a);becausetheedgeselongate(伸长）morethanthecenter,theybuckle,becausetheyarerestrainedfromexpandingfreelyinthelongitudinal（纵向）(rolling)direction.ThecracksshowninFigs.13.8bandcareusuallytheresultofpoormaterialductilityattherollingtemperature.Figure13.8Schematicillustrationoftypicaldefectsinflatrolling:(a)wavyedges;(b)zipper拉链cracksinthecenterofthestrip;(c)edgecracks;and(d)alligatoring龟裂.Severaltypesofrollingmillsandequipmentarebuilt;theyusediverserollarrangements.Althoughtheequipmenttothotandcoldrollingisessentiallythesame,therearedifferencesintherollmaterials,processparameters,lubricants,andcoolingsystems.Thewidthofrolledproductsmayrangeupto5m(200in.)andbeasthinas0.0025mm(0.0001in.).Rollingspeedsmayrangeupto25m/s(aboutamileaminute)forcoldrolling,orevenhigherinhighlyautomatedandcomputer-controlledfacilities.RollingMill轧钢机Figure13.10Ageneralviewofarollingmill.Source:InlandSteel.Two-high(两辊）orthree-highrollingmills(Figs.13.1laandb)areusedforhotrollingininitialbreakdownpasses(primaryroughingorcoggingmills)oncastingots（铸锭）orincontinuouscasting,withrolldiametersrangingfrom0.6mto1.4m(24in.to55in.).轧机有不同的轧辊配置，如：二辊式、三辊式、四辊式、多辊式和串连式等，可以对材料施加前张紧力或后张紧力，以提高工艺可行性或减小轧制力。BackingRollArrangementsFigure13.11Schematicillustrationofvariousrollarrangements:(a)two-high;(b)three-high;(c)four-high;(d)cluster(Sendzimir)mill.TandemRolling连轧Figure13.12Atandemrollingoperation.ShapeRolling型钢轧制Figure13.13StagesintheshaperollingofanH-sectionpart.Variousotherstructuralsections,suchaschannelsandI-beams,arealsorolledbythiskindofprocess.Ring-Rolling环轧Figure13.14(a)Schematicillustrationofaring-rollingoperation.Thicknessreductionresultsinanincreaseinthepartdiameter.(b)Examplesofcross-sectionsthatcanbeformedbyringrolling.Thread-Rolling螺纹滚压Figure13.16(a)Featuresofamachinedorrolledthread.(b)Grainflowinmachinedandrolledthreads.Unlikemachining,whichcutsthroughthegrainsofthemetal,therollingofthreadscausesimprovedstrength,becauseofcoldworkingandfavorablegrainflow.Figure13.15Thread-rollingprocesses:(a)and(c)reciprocatingflatdies;(b)two-rollerdies.Threadedfasteners,suchasbolts,aremadeeconomicallybytheseprocesses,athighratesofproduction.PRODUCIIONOFSEAMLESSPIPEANDTUBINGRotarytubepiercingisahot-workingprocessformakinglong,thick-walledseamlesspipeandtubing(Fig.13.17).Itisbasedontheprinciplethatwhenaroundbarissubjectedtoradialcompressiveforces,tensilestressesdevelopatthecenterofthebar.(SeeFig.2.11.)Whenitissubsequentlysubjectedtocycliccompressivestresses(Fig.13.17b),acavitybeginstoformatthecenterofthebar.Thisphenomenoncanbedemonstratedwithashortpieceofrounderaser,byrollingitbackandforthonahardflatsurface,asshowninFig.13.17b.MannesmannProcessFigure13.17Cavityformationinasolidroundbaranditsutilizationintherotarytubepiercingprocessformakingseamlesspipeandtubing.(TheMannesmannmillwasdevelopedinthe1880s.)Tube-RollingFigure13.18Schematicillustrationofvarioustube-rollingprocesses:(a)withfixedmandrel;(b)withmovingmandrel;(c)withoutmandrel;and(d)pilgerrollingoveramandrelandapairofshapedrolls.Tubediametersandthicknessescanalsobechangedbyotherprocesses,suchasdrawing,extrusion,andspinning.MECHANICSOFROLLINGANCVoVfVrtaRLhfhoSRDh/2DESLABAPPROXIMATION

Onecanlookupontheworkpieceincontactasshown:Planestraincompressionofthispiece(sincewdoesnotchange)DEMECHANICSOFROLLINGSlabApproximation:Similartoforgingwithfriction2a=Lpav=Y'{1+ma/h}a=L/2h=(ho+hf)/2F=pavLwpFrictionHillLRSDh/2m.pFMECHANICSOFROLLINGVoVfVrtaRhfhoMoreaccurately:sxsx+dsxpmphh+dhEntryzonepsxmphh+dhsx+dsxExitZoneMECHANICSOFROLLINGsx+dsxpmphh+dhEntryzonefxyf=aatentryPlanestraincompressionandyielding(i.e.seff=Y)leadstop-sx=Y'MECHANICSOFROLLINGfn:locationofneutralpointTORQUEANDPOWERTappliedL/2Fpav=Y'{1+ma/h}a=L/2h=(ho+hf)/2F=pavLwFreebodyofRollTapplied=F.L/2,whereL=(RDh)1/2WeassumethattheresultantverticalforceactsthroughthemiddleofL/2.wisthelengthoftheworkpiece.RPower=T.2pN/60whereNisinrevperminSumofT=0impliesLRSDh/2m.pFFreebodyofworkpiecehohfEFFECTIVEY'INWORKHARDENINGMATERIAL(ApplicableInForging,Rolling,ExtrusionOperations)Whenthematerialhasworkhardening,thenonecanuseanaverageYthatrepresentsthetotalstrainimposedonthematerial.eexperiencedbyworkpiece,e.g.,e=ln(ho/hf)Yav.STRAIN-RATEINROLLINGStrain,e=ln(ho/hf)Time,t=L/Vr,whereVr=2pNR/60,isthesurfacevelocityofroll,NisrevolutionperminuteTherefore,

Therateisimportant,sinceductilityandflowstressofthematerialdependsonthestrainrate.Typically,onemustselectanappropriatewindowofstrainandstrain-rateperpass,sothatthehighestproductionratecanbemaintainedwithoutcreatingunwantedmicrostructuresordefectsandcracksinthematerial.MAXIMUMPOSSIBLEDRAFT,ho-hfItcanbeshownthat:ho-hf=2R(1-Cosa)~Ra2Themaximumvalueofaislimitedbythefrictioncoefficient,i.e.,amax=mHence,ho-hf=Dh=m2R

Inhotrolling,thefrictioncoefficientishigh(0.2to0.7),andevenstickingconditionsareobtained.Hencelargethicknessreductionsarepossibleduringhotrolling.

Thinsheetsaremostoftenrolledusingcoldrolling.ThevalueofRiskeptlowtoreduceloadrequirement.mtypicallyvariesbetween0.02and0.3.Thus,themaximumdraftismoreimportantduringcoldrolling.FRONTANDBACKTENSIONtaRhfhoFrontTension,sfBackTension,sbFrontorbacktensionareappliedtoreducetheloadforrolling.Particularlyimportant