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1、Chapter 3 Bulk Deformation Forming - Rolling,Rolling Process,A process of reducing the thickness or changing the cross-sectional of a workpiece by compressive forces exerted by a pair of rotating rolls,Flat- and Shape-Rolling Processes,Flat Rolling Process,Developed in the late 1500sStart with slab
2、like ingotPass through two rolls separated by a distance less than the thickness of the ingotKeep passing through such rolls until the final thickness is achievedFriction force acts as driving forceThe final products include plate, sheet and foil (plate:t8mm, sheet: t=0.2-4mm, Foil: t0.2mm), and can
3、 be used in a various of fields,Flat Rolling Process,Rotating rolls reduce the thickness of the incoming ingot,Flat Rolling Process,(a) Schematic illustration of the flat-rolling process. (b) Friction forces acting on strip surfaces. (c) The roll force, F, and the torque acting on the rolls. The wid
4、th w of the strip usually increases during rolling,Neutral Point,A point where there are no slip between the workpiece and the rollerThe friction of two sides oppose each other at the neutral pointThe friction on the entry side must be higher than the exit sideThe net friction force and the surface
5、velocity must be in the same directionForward slip is defined as: Forward slip,Force and Stress,The stress state is similar to that in upsettingThe calculation in more involved than that of upsetting due to the curved contact surfaceThe flow stress at the exit is higher than that at the entry,Stress
6、 on an element in flat rolling. (a) Entry Zone, (b) Exit zone,p is a function of h and For strain hardening material, the flow stress Yf in the expressions corresponds to the strain that the material has undergone at that particular location in the roll gapFrom the expressions, we can find that the
7、pressure increases with increasing strength of the material, increasing coefficient of friction, and increasing R/hf ratio (note that the R/hf ratio is equivalent to the a/h ration in upsetting),Pressure Distribution in the Roll Gap,The neutral point shifts toward the exit as friction decreases. If
8、the friction approaches to zero, the rolls begin to slip instead of pulling the trip in,Pressure distribution in the roll gap as a function of coefficient of friction. The area under the curve is the roll separating force per unit width of strip. Note that, as friction increases, the neutral point s
9、hits toward the entry. Without friction the rolls slip and the neutral point shifts compeletly toward the exit,Pressure distribution in the roll gap as a function of reduction in thickness. Note the increase in the area under the curves with increasing reduction in thickness, thus increasing the rol
10、l-separating force,Calculation of the Rolling Force,Torque and Power Requirements,Illustrative Problem,A 220 mm wide 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 for this operation. (Average
11、 flow stress is 125 MPa) Solution?,Force in Hot Rolling,Calculation of hot rolling force is importantTwo difficulties in calculation Estimation of the friction coefficient at elevated temperatures Strain rate sensitivity of metals at high temperaturesThe average strain rate in flat rolling can be ex
12、pressed as:,Friction,It is necessary to have some friction for pulling the trip into the roll gap, but force and power increase with increasing frictionFor cold rolling, ranges from 0.002 to 0.3; it ranges from 0.2 to 0.7 for hot rollingMaximum draft (h0-hf) can be expressed as: hmax= 2R max=tan-1 ,
13、Front and back tension,Normally, Roll force F can be reduced by various means, such as lower friction, smaller roll radii, small reduction, .Another effective method is to reduce the apparent compressive yield stress of the material by apply longitudinal tensionTensions in rolling can be applied eit
14、her at the entry (back tension b) or at the exit (front tension f). The pressure can be modified as:Entry zone Exit zone,Depending on the relative magnitudes of the tensions applied, the neutral point may shift, and this will affect the pressure distribution, torque and power requirements in rolling
15、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)Tensions are particularly important in rolling thin, high-strength materials,Roll Deflections,Leading to a higher thickness in the trip centreGrind
16、ing the rolls so that their diameter at the centre is slightly larger than at edge, called camber Camber can also be controlled by varying the location of the coolant (lubricant) on the rolls in hot rollingElastical force, flatening,Flat Rolling Practice,Cast ingotScalp (not always)Reheat and homoge
17、nizeHot roll in reversing mill to 1-2 in thickCut off ends (alligators)Warm roll to 0.1-2 in thickHeat treat to recrystallize (sometimes)CoolCold roll to finish thicknessHeat treat (where appropriate)Results in material which must be further worked,Flat Rolling Practice,Converting the coarse-grained
18、, brittle and porous cast structure to a wrought structure by hot rollingFor high-strength alloys reductions are usually small, whereas for soft and nonferrous metal they are highTemperatures in hot rolling are similar to those in hot forging,Independent Variables for Rolling,Roll speedDraft (amount
19、 of thickness reduction) or roll gapBillet thicknessBillet widthBillet material (not always allowed to select)Billet temperatureLubricant,Dependent Variables for Rolling,Roll forcePowerSpeed of exiting stripFinal strip temperatureMaximum draftRoll strip contact lengthSheet or plate shape,Hot Rolling
20、,Advantages Stresses lower Forces smaller Power requirements less No work hardening Large deformations possible Breaks up the cast structure into preferable forms Closes porosity Sometimes the only way to create sheetDisadvantages Higher friction Rolls need to be cooled Material handling difficult P
21、ersonnel must be protected from heat,Cold Rolling,Advantages Deformations of 50 to 80% Work hardening increases strength Excellent surface finish Excellent tolerance on thickness and shapeDisadvantages High forces Small reductions give rise to surface stresses and non-uniform stress distributions,Pr
22、oduct Quality Issues,Non-flat material Thermal camber Roll bending Wavy edges (caused by roll bending) Zipper and edge cracks (poor ductility)Alligatoring (non-uniform deformation) typically in nose and tail of slab or plateResidual stresses Small deformations cause surface deformationFlaws in the s
23、urface Roll finish perfection,Residual stresses,Due to inhomogeneous deformationSmall-diameter rolls or small reductions cause surface deformationOn the other hand, large-diameter rolls or large reductions tend to deform the bulk of the material,Leveling,To improve flatness The strip is passed throu
24、gh a series of leveling rolls,Rolling equipment and practice,Two-high or three-high rolling mills are used for initial breakdown passes It is desirable to use small-diameter rolls to reduce roll force, but small rolls tend to deflectCluster mill (Sendzimir) is particularly suitable for cold rolling
25、thin strip of high-strength metalRequirements for roll materials are mainly strength and resistance to wear. Three common roll materials are cast iron, cast steel, and forged steelFor hot rolling, rolls surface are generally rough, but for cold rolling, they are finely polishedNo lubricant is used i
26、n ferrous alloys, aqueous solution is used for cooling rolls and removing scaleCompounded oils, emulsion, and fatty acids are used as lubricant in hot rolling of nonferrous alloys; low-viscosity lubricants (mineral oil, emulsions, paraffin, fatty oil) are used in cold rolling,Rolling Mill,Figure 13.
27、10 A general view of a rolling mill. Source: Inland Steel.,Backing Roll Arrangements,Figure 13.11 Schematic illustration of various roll arrangements: (a) two-high; (b) three- high; (c) four-high; (d) cluster (Sendzimir) mill.,Tandem Rolling,Figure 13.12 A tandem rolling operation.,Four-High Rolling
28、 Mill,Figure 13.3 Schematic illustration of a four-high rolling-mill stand, showing its various features. The stiffnesses of the housing, the rolls, and the roll bearings are all important in controlling and maintaining the thickness of the rolled strip.,人有了知识，就会具备各种分析能力，明辨是非的能力。所以我们要勤恳读书，广泛阅读，古人说“书中自有黄金屋。”通过阅读科技书籍，我们能丰富知识，培养逻辑思维能力；通过阅读文学作品，我们能提高文学鉴赏水平，培养文学情趣；通过阅读报刊，我们能增长见识，扩大自己的知识面。有许多书籍还能培养我们的道德情操，给我们巨大的精神力量，鼓舞我们前进。,
