Rolling of Metals Manufacturing, Engineering & Technology, Fifth Edition, by Serope Kalpakjian and Steven R. Schmid. ISBN 0-13-148965-8. © 2006 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.

Rolling of Metals Rolling is a process where with high pressure the metal is reduced in thickness 90 % of Metals used are rolled sheets The metal moves through the rolling mill by a friction process The metal is pushed or drawn through the rolling mill

Rolling processes Hot rolling Cold rolling Thickness wide variety The coarse –grained, brittle and porous structure of the ingot (or continuous cast metal) is broken in wrought structure Finer grain size, enhanced properties, strength and hardness Cold rolling Room temperature, cold deformation hardness and better surface finish Anisotropic properties-preferential orientation This process requires higher energy Thickness wide variety 300 mm reactors vessels 100mm thick tanks 1, 8 mm Boeing aircrafts 0,7 mm car body panels 6 µm Al foil for packaging 0,28 mm Al beverage cans

Flat rolling metal structure Cast structure dentritic coarse grains porosity Hot rolling Wrought structure finer grains and enhanced ductility, breaking up brittle grain boundaries, remove porosity Bloom or slab after the first hot rolling depending on the cross section, billets are a square, rod Surface is conditioned difference between Al.Steel Cold rolling Pack rolling foil more metal layers, difference in surface finish Skin pass yield point elongations, surface irregularities, stretcher Lüders bands ---last pass small reduction 0,1 to 1, 5 % to correct this situation

Working Range A typical stress-strain curve for mild steel Metals are worked within the yield point and breaking stress producing the plastic deformation. Hot working reduces the stress required to produce the yielding.

Hot working Forming of metals at a temperature above the re-crystallization temperature. Uses the metal deformation property: their ability to flow plastically in the solid-state without accompanying deterioration of properties. Advantages: Porosity in metal is largely eliminated. Impurities in the form of inclusions are broken up and distributed thought the metal. Coarse grains are refined; Physical properties improved. Energy required to change shape is less. Good Machinability and weldability; Ready to use products Rapid oxidation and scaling of the surface accompanying the poor surface finish. (Disadvantage!)

Recrystallization Temperatures

Hot working Temperatures Hot working is done above the recrystallization and below the melting temperature. Depends upon the material properly and sometimes may be at room temperature (Lead, Zinc etc.) Rolling, Forging, Extruding and Drawing are some major hot working in metals.

Cold Working Working on metals at a temperature below the re-crystallization temperature, usually at room temp. Distorts the grain and does little towards reducing the size of material. Improves strength, machinability, dimensional accuracy and surface finish of metals. Lesser oxidation and scaling cold working allows thinner sheets to be worked accurately. Many processes and equipments are used for both hot and cold work, but forces required are different. Much more Pressure is needed than hot working and there is no recovery of the distortion (Residual Stress). Deformation is brought by distortion of lattice about slip planes.

Cold Working Advantages and limitations: Many products are cold finished after hot working to make them commercially acceptable, close to tolerance and remove the scales and oxides. Strength and hardness increases, loss in ductility. Stresses are setup in metals that remains unless removed by subsequent heat treatment. Distortion and fragmentation of grain structure is created. Process is economical and rapidly produces parts in mass productions. Bending, Drawing, Spinning, Forming, Embossing, Seaming are major cold working operations.

Flat-Rolling and Shape-Rolling Processes Figure 13.1 Schematic outline of various flat-rolling and shape-rolling processes. Source: After the American Iron and Steel Institute.

Flat-Rolling Process Figure 13.2 (a) Schematic illustration of the flat-rolling process. (b) Friction forces acting on strip surfaces. (c) Roll force, F, and the torque, T, acting on the rolls. The width of the strip, w, usually increases during rolling, as shown later in Fig. 13.5.

Mechanics of Rolling Forward slip: FIGURE 6.30 Schematic illustration of the flat-rolling process. (Note that the top roll has been removed for clarity.) FIGURE 6.31 Relative velocity distribution between roll and strip surfaces. The arrows represent the frictional forces acting along the strip-roll interfaces. Note the difference in their direction in the left and right regions.

The Flat Rolling Process Larger rolls – smaller contact angle Larger reduction with same rolling cylinders –larger contact angle

Metal change Reduction leads to decrease in thickness r=(H1-H2)/H1x100 Variation in length

Friction Coefficient Body in rest is K=W K action force W reaction force and friction P Pressure force µ = K/P Velocity Surface roughness Force of rolling depends on P and µ

Friction angle tg j = K1/P µ = K1/P µ = tg j

Anti slip condition ABCD strip that enters A and B Normal N and fiction W N can be split into 2 equal vertical press forces V (2V) 2 equal backwards forces H (2H) W can be split into 2 equal vertical press forces V’ (2V’) 2 equal backwards forces H (2H’)

Anti slip condition The trip moves if 2 H’ is bigger or equals 2H H’ bigger or equals H H’ = W cosa H = N sina Wcosa = Nsina W/N bigger or equals tga W/N=µ =tg j Rolling condition tg j bigger or equals tga For small angles j bigger or equals a j friction angle a contact angle

Paramters that influence the angle j a Angle of friction Rolling speed Nature of the metal Nature of the rolling cylinder Surface roughness of both Presence of oxides Rolling Temperature Lubricants Contact angle Diameter of the rolling cylinders Size of the reduction

Rolling zone ABCD Rolling gap AB Contact angle a

Criterium of maximum reduction Diameter of the cylinder fixed D If reduction increases Contact angle increases a One perform rolling until a=j j is not dependent on reduction Maximum reduction depends on D and µ

Wat happens in roll gap V1 > VO Increase of the speed

Neutral point Roll has a constant speed Vst Strip enters V1 and leaves with V2 Vst> V1 Vst< V2 Vst=Vstrip neutral point

Material flow during rolling Spring back

Effects of Hot Rolling Figure 13.6 Changes in the grain structure of cast or of large-grain wrought metals during hot rolling. Hot rolling is an effective way to reduce grain size in metals for improved strength and ductility. Cast structures of ingots or continuous castings are converted to a wrought structure by hot working.

Pressure Distribution in Rolling FIGURE 6.33 Pressure distribution in the roll gap as a function of the coefficient of friction. Note that as friction increases, the neutral point shifts toward the entry. Without friction, the rolls will slip, and the neutral point shifts completely to the exit. (See also Table 4.1.) FIGURE 6.34 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, thus increasing the roll force. FIGURE 6.35 Pressure distribution as a function of front and back tension in rolling. Note the shifting of the neutral point and the reduction in the area under the curves (hence reduction in the roll force) as tensions increase.

Rolling force and power F=LwYavg F rolling force Yavg is the average true stress w is the width of the strip L length of the contact Add 20 % for friction Torque on the roll is the product of a and F equals in average a=L/2 N revolutions per minute

Parameters that affect F W width of the strip Diameter of the contact roll Metal structure Temperature of the metal Rolling speed µ friction coefficient Reduction in thickness Thickness incoming sheet Pull or push at the beginning or at the end

Roll Arrangements Figure 13.3 Schematic illustration of various roll arrangements: (a) four-high rolling mill showing various features. The stiffness of the housing, the rolls, and the roll bearings are all important in controlling and maintaining the thickness of the rolled strip; (b) two-hill mill; (c) three-high mill; and (d) cluster (or Sendzimir) mill.

Bending of Rolls Figure 13.4 (a) Bending of straight cylindrical rolls caused by roll forces. (b) Bending of rolls ground with camber, producing a strip with uniform thickness through the strip width. Deflections have been exaggerated for clarity.

Spreading in Flat Rolling Figure 13.5 Increase in strip width (spreading) in flat rolling. Note that similar spreading can be observed when dough is rolled with a rolling pin.

Roller Leveling Figure 13.7 (a) A method of roller leveling to flatten rolled sheets. (b) Roller leveling to straighten drawn bars.

Defects in Flat Rolling Figure 13.8 Schematic illustration of typical defects in flat rolling: (a) wavy edges; (b) zipper cracks in the center of the strip; (c) edge cracks; and (d) alligatoring.

Rolling Mill Figure 13.10 A general view of a rolling mill. Source: Courtesy of Ispat Inland.

Tandem-Rolling Figure 13.11 An example of a tandem-rolling operation.

Shape Rolling of an H-section part Figure 13.12 Steps in the shape rolling of an H-section part. Various other structural sections, such as channels and I-beams, also are rolled by this kind of process.

Vibrations and chatter Self excited vibrations Variations in thickness Variations in surface finish Especially a problems in tandem mills

Rolling Mills Two high rolling mills Reversing Mills Initial breakdown pass, rolling diameters 0,6- 1, 4 m Reversing Mills Reversed direction Four High Mills and Clusters Mills (Sendzimir or Z mills) Small diameter rolls Tandem Rolls

Roll materials Criteria high strength and wear Cast iron, cast steel, and forged steel, tungsten carbide (very small rolls) Rolls are grinded for a perfect finish

Surface Roughness and Measurement; Friction, Wear, and Lubrication

Surface Structure of Metals Figure 33.1 Schematic illustration of a cross-section of the surface structure of metals. The thickness of the individual layers depends on both processing conditions and processing environment. Source: After E. Rabinowicz and B. Bhushan.

Terminology and Symbols Related to Surface Texture Figure 33.2 (a) Standard terminology and symbols used to describe surface finish. The quantities are given in μin. (b) Common surface lay symbols.

Surface-Roughness Figure 33.3 Coordinates used for surface-roughness measurement using Eqs. (33.1) and (33.2).

Surface Roughness Figure 33.4 (a) Measuring surface roughness with a stylus. The rider supports the stylus and guards against damage. (b) Path of the stylus in surface-roughness measurements (broken line) compared to the actual roughness profile. Note that the profile of the stylus path is smoother than that of the actual surface. (c) through (f) Typical surface profiles produced by various machining and surface-finishing processes. Note the difference between the vertical and horizontal scales.

Real Contact Area Figure 33.5 Schematic illustration of the interface of two bodies in contact showing real areas of contact at the asperities. In engineering surfaces, the ratio of the apparent-to-real areas of contact can be as high as 4 to 5 orders of magnitude. Manufacturing, Engineering & Technology, Fifth Edition, by Serope Kalpakjian and Steven R. Schmid. ISBN 0-13-148965-8. © 2006 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved.

Adhesive and Abrasive Wear Figure 33.9 Schematic illustration of (a) two contracting asperities, (b) adhesion between two asperities, and (c) the formation of a wear particle. Figure 33.10 Schematic illustration of abrasive wear in sliding. Longitudinal scratches on a surface usually indicate abrasive wear.

Lubrication Types Figure 33.12 Types of lubrication generally occurring in metalworking operations. Source: After W. R. D. Wilson.

Surface Integrity Cracks Craters Heat affected zone Inclusions Intergranular attack Laps, folds Metallurgical transformations Pits Residual stress Splatter Surface Plastic Deformation

Lubrication during rolling To control the friction process lubricants have to be used Low friction has to be used to avoid damage of the equipment But friction is necessary Group discussion difference between hot rolling of Al and steel