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1、Chapter 3 Bulk Deformation Forming-RollingRolling Process A process of reducing the thickness or changing the A process of reducing the thickness or changing the cross-sectional of a workpiece by compressive forces cross-sectional of a workpiece by compressive forces exerted by a pair of rotating ro
2、llsexerted by a pair of rotating rollsFlat-and Shape-Rolling ProcessesFlat Rolling Processl lDeveloped in the late 1500sDeveloped in the late 1500sl lStart with slab like ingotStart with slab like ingotl lPass through two rolls separated by a distance less than Pass through two rolls separated by a
3、distance less than the thickness of the ingotthe thickness of the ingotl lKeep passing through such rolls until the final thickness Keep passing through such rolls until the final thickness is achievedis achievedl lFriction force acts as driving forceFriction force acts as driving forcel lThe final
4、products include plate,sheet and foil The final products include plate,sheet and foil(plate:t8mm,sheet:t=0.2-4mm,Foil:t8mm,sheet:t=0.2-4mm,Foil:t0.2mm),and can be used in a various of fieldscan be used in a various of fieldsFlat Rolling ProcessRotating rolls reduce the thickness of the incoming ingo
5、tRotating rolls reduce the thickness of the incoming ingotFlat Rolling Process (a)Schematic illustration of the flat-rolling process.(b)Friction forces(a)Schematic illustration of the flat-rolling process.(b)Friction forces acting on strip surfaces.(c)The roll force,acting on strip surfaces.(c)The r
6、oll force,F F,and the torque acting on,and the torque acting on the rolls.The width the rolls.The width w w of the strip usually increases during rollingof the strip usually increases during rollingNeutral Pointl lA point where there are no slip between the workpiece and the A point where there are
7、no slip between the workpiece and the rollerrollerl lThe friction of two sides oppose each other at the neutral pointThe friction of two sides oppose each other at the neutral pointl lThe friction on the entry side must be higher than the exit sideThe friction on the entry side must be higher than t
8、he exit sidel lThe net friction force and the surface velocity must be in the The net friction force and the surface velocity must be in the same directionsame directionl lForward slip is defined as:Forward slip is defined as:Forward slip Forward slipForce and Stressl lThe stress state is similar to
9、 that in upsettingThe stress state is similar to that in upsettingl lThe calculation in more involved than that of upsetting due to the curved The calculation in more involved than that of upsetting due to the curved contact surfacecontact surfacel lThe flow stress at the exit is higher than that at
10、 the entryThe flow stress at the exit is higher than that at the entryStress on an element in flat rolling.(a)Entry Zone,(b)Exit zoneStress on an element in flat rolling.(a)Entry Zone,(b)Exit zonen np is a function of h and p is a function of h and n nFor strain hardening material,the flow stress YF
11、or strain hardening material,the flow stress Yf f in the expressions in the expressions corresponds to the strain that the material has undergone at that corresponds to the strain that the material has undergone at that particular location in the roll gapparticular location in the roll gapn nFrom th
12、e expressions,we can find that the pressure increases with From the expressions,we can find that the pressure increases with increasing strength of the material,increasing coefficient of friction,increasing strength of the material,increasing coefficient of friction,and increasing R/hand increasing
13、R/hf f ratio(note that the R/h ratio(note that the R/hf f ratio is equivalent to the ratio is equivalent to the a/h ration in upsetting)a/h ration in upsetting)Pressure Distribution in the Roll Gap The neutral point shifts toward the exit as friction decreases.If the friction The neutral point shift
14、s toward the exit as friction decreases.If the friction approaches to zero,the rolls begin to slip instead of pulling the trip inapproaches to zero,the rolls begin to slip instead of pulling the trip in Pressure distribution in the roll gap as a Pressure distribution in the roll gap as a function of
15、 coefficient of friction.The function of coefficient of friction.The area under the curve is the roll area under the curve is the roll separating force per unit width of strip.separating force per unit width of strip.Note that,as friction increases,the Note that,as friction increases,the neutral poi
16、nt shits toward the entry.neutral point shits toward the entry.Without friction the rolls slip and the Without friction the rolls slip and the neutral point shifts compeletly toward neutral point shifts compeletly toward the exitthe exitPressure distribution in the Pressure distribution in the roll
17、gap as a function of roll gap as a function of reduction in thickness.Note reduction in thickness.Note the increase in the area the increase in the area under the curves with under the curves with increasing reduction in increasing reduction in thickness,thus increasing thickness,thus increasing the
18、 roll-separating forcethe roll-separating forceCalculation of the Rolling ForceCalculation of the Rolling ForceTorque and Power RequirementsTorque and Power RequirementsIllustrative ProblemIllustrative Problem A 220 mm wide 6061-O Aluminum strip is rolled from a thickness of 25mm to 22mm.A 220 mm wi
19、de 6061-O Aluminum strip is rolled from a thickness of 25mm to 22mm.If the roll radius is 300 mm.and the roll rpm is 100,calculate the horsepower required If the roll radius is 300 mm.and the roll rpm is 100,calculate the horsepower required for this operation.(Average flow stress is 125 MPa)for thi
20、s operation.(Average flow stress is 125 MPa)Solution?Solution?Force in Hot RollingForce in Hot Rollingl lCalculation of hot rolling force is importantCalculation of hot rolling force is importantl lTwo difficulties in calculationTwo difficulties in calculation Estimation of the friction coefficient
21、Estimation of the friction coefficient at elevated at elevated temperatures temperatures Strain rate sensitivity of metals at high temperaturesStrain rate sensitivity of metals at high temperaturesl lThe average strain rate in flat rolling can be expressed as:The average strain rate in flat rolling
22、can be expressed as:Frictionn nIt is necessary to have some friction for pulling the trip into the roll gap,but force and power increase with increasing frictionn nFor cold rolling,ranges from 0.002 to 0.3;it ranges from 0.2 to 0.7 for hot rollingn nMaximum draft(h0-hf)can be expressed as:hmax=2R ma
23、x=tan-1 Front and back tensionn nNormally,Roll force F can be reduced by various means,such as Normally,Roll force F can be reduced by various means,such as lower friction,smaller roll radii,small reduction,lower friction,smaller roll radii,small reduction,.n nAnother effective method is to reduce t
24、he apparent compressive Another effective method is to reduce the apparent compressive yield stress of the material by apply longitudinal tensionyield stress of the material by apply longitudinal tensionn nTensions in rolling can be applied either at the entry(back tension Tensions in rolling can be
25、 applied either at the entry(back tension b b)or at the exit(front tension)or at the exit(front tension f f).The pressure can be modified as:).The pressure can be modified as:n nEntry zone Exit zoneEntry zone Exit zonen nDepending on the relative magnitudes of the tensions applied,the neutral Depend
26、ing on the relative magnitudes of the tensions applied,the neutral point may shift,and this will affect the pressure distribution,torque and point may shift,and this will affect the pressure distribution,torque and power requirements in rollingpower requirements in rollingn nFront tension is control
27、led by the torque on the coiler(delivery reel).The Front tension is controlled by the torque on the coiler(delivery reel).The back tension is controlled by a braking system in the uncoiler(payoff reel)back tension is controlled by a braking system in the uncoiler(payoff reel)n nTensions are particul
28、arly important in rolling thin,high-strength materialsTensions are particularly important in rolling thin,high-strength materialsRoll Deflectionsl lLeading to a higher thickness in the Leading to a higher thickness in the trip centretrip centrel lGrinding the rolls so that their Grinding the rolls s
29、o that their diameter at the centre is slightly diameter at the centre is slightly larger than at edge,called camberlarger than at edge,called camberl l Camber can also be controlled by Camber can also be controlled by varying the location of the coolant varying the location of the coolant(lubricant
30、)on the rolls in hot rolling(lubricant)on the rolls in hot rollingl lElastical force,flateningElastical force,flateningFlat Rolling Practicel lCast ingotCast ingotl lScalp(not always)Scalp(not always)l lReheat and homogenizeReheat and homogenizel lHot roll in reversing mill to 1-2 in thickHot roll i
31、n reversing mill to 1-2 in thickl lCut off ends(alligators)Cut off ends(alligators)l lWarm roll to 0.1-2 in thickWarm roll to 0.1-2 in thickl lHeat treat to recrystallize(sometimes)Heat treat to recrystallize(sometimes)l lCoolCooll lCold roll to finish thicknessCold roll to finish thicknessl lHeat t
32、reat(where appropriate)Heat treat(where appropriate)l lResults in material which must be further workedResults in material which must be further workedFlat Rolling Practicel lConverting the coarse-grained,brittle and porous cast structure to a Converting the coarse-grained,brittle and porous cast st
33、ructure to a wrought structure by hot rollingwrought structure by hot rollingl lFor high-strength alloys reductions are usually small,whereas for soft For high-strength alloys reductions are usually small,whereas for soft and nonferrous metal they are highand nonferrous metal they are highl lTempera
34、tures in hot rolling are similar to those in hot forgingTemperatures in hot rolling are similar to those in hot forgingIndependent Variables for Rollingl lRoll speedRoll speedl lDraft(amount of thickness reduction)or roll gapDraft(amount of thickness reduction)or roll gapl lBillet thicknessBillet th
35、icknessl lBillet widthBillet widthl lBillet material(not always allowed to select)Billet material(not always allowed to select)l lBillet temperatureBillet temperaturel lLubricantLubricantDependent Variables for Rollingl lRoll forcel lPowerl lSpeed of exiting stripl lFinal strip temperaturel lMaximum
36、 draftl lRoll strip contact lengthl lSheet or plate shapeHot Rollingl lAdvantagesAdvantages Stresses lowerStresses lower Forces smallerForces smaller Power requirements lessPower requirements less No work hardeningNo work hardening Large deformations possibleLarge deformations possible Breaks up the
37、 cast structure into preferable formsBreaks up the cast structure into preferable forms Closes porosityCloses porosity Sometimes the only way to create sheetSometimes the only way to create sheetl lDisadvantagesDisadvantages Higher frictionHigher friction Rolls need to be cooledRolls need to be cool
38、ed Material handling difficultMaterial handling difficult Personnel must be protected from heatPersonnel must be protected from heatCold Rollingl lAdvantagesAdvantages Deformations of 50 to 80%Deformations of 50 to 80%Work hardening increases strengthWork hardening increases strength Excellent surfa
39、ce finishExcellent surface finish Excellent tolerance on thickness and shapeExcellent tolerance on thickness and shapel lDisadvantagesDisadvantages High forcesHigh forces Small reductions give rise to surface stresses and non-Small reductions give rise to surface stresses and non-uniform stress dist
40、ributionsuniform stress distributions Product Quality Issuesl lNon-flat materialNon-flat material Thermal camberThermal camber Roll bendingRoll bending Wavy edges(caused by roll bending)Wavy edges(caused by roll bending)Zipper and edge cracks(poor ductility)Zipper and edge cracks(poor ductility)l lA
41、lligatoring(non-uniform deformation)typically in nose and tail of slab or Alligatoring(non-uniform deformation)typically in nose and tail of slab or plateplatel lResidual stressesResidual stresses Small deformations cause surface deformationSmall deformations cause surface deformationl lFlaws in the
42、 surfaceFlaws in the surface Roll finish perfectionRoll finish perfectionResidual stressesl lDue to inhomogeneous deformationDue to inhomogeneous deformationl lSmall-diameter rolls or small reductions cause surface Small-diameter rolls or small reductions cause surface deformationdeformationl lOn th
43、e other hand,large-diameter rolls or large reductions tend On the other hand,large-diameter rolls or large reductions tend to deform the bulk of the materialto deform the bulk of the materialLevelingn nTo improve flatness To improve flatness n nThe strip is passed through a series of leveling rollsT
44、he strip is passed through a series of leveling rollsRolling equipment and practicen nTwo-high or three-high rolling mills are used for initial breakdown passes Two-high or three-high rolling mills are used for initial breakdown passes n nIt is desirable to use small-diameter rolls to reduce roll fo
45、rce,but small rolls It is desirable to use small-diameter rolls to reduce roll force,but small rolls tend to deflecttend to deflectn nCluster mill(Sendzimir)is particularly suitable for cold rolling thin strip of Cluster mill(Sendzimir)is particularly suitable for cold rolling thin strip of high-str
46、ength metalhigh-strength metaln nRequirements for roll materials are mainly strength and resistance to wear.Requirements for roll materials are mainly strength and resistance to wear.Three common roll materials are cast iron,cast steel,and forged steelThree common roll materials are cast iron,cast s
47、teel,and forged steeln nFor hot rolling,rolls surface are generally rough,but for cold rolling,they are For hot rolling,rolls surface are generally rough,but for cold rolling,they are finely polishedfinely polishedn nNo lubricant is used in ferrous alloys,aqueous solution is used for cooling No lubr
48、icant is used in ferrous alloys,aqueous solution is used for cooling rolls and removing scalerolls and removing scalen nCompounded oils,emulsion,and fatty acids are used as lubricant in hot Compounded oils,emulsion,and fatty acids are used as lubricant in hot rolling of nonferrous alloys;low-viscosi
49、ty lubricants(mineral oil,emulsions,rolling of nonferrous alloys;low-viscosity lubricants(mineral oil,emulsions,paraffin,fatty oilparaffin,fatty oil)are used in cold rolling)are used in cold rolling Rolling MillFigure 13.10 A general view of a rolling mill.Source:Inland Steel.Backing Roll Arrangemen
50、tsFigure 13.11 Schematic illustration of various roll arrangements:(a)two-high;(b)three-high;(c)four-high;(d)cluster(Sendzimir)mill.Tandem RollingFigure 13.12 A tandem rolling operation.Four-High Rolling MillFigure 13.3 Schematic illustration of a four-high rolling-mill stand,showing its various fea