1 Bulk Deformation Forming - Rolling Overview - Shaping and Forming Powders Pressing SLS Special Injection Molding Firing/ Sintering 2 Raw Material Molten Material Continuous Casting/Rolling Ingot casting Rolling Stamping Sheet metal forming Forging/ Press forming Extruding Casting Shapes Single crystal pulling Machining Finishing Current Lecture Increasing level of detail Blow molding
Rolling Process 3 Start with slab like ingot (as large as 30 ft by 2 ft by 10 ft) Pass through two rolls separated by a distance less than the thickness of the ingot Keep passing through such rolls until the final thickness is achieved If the final material is thin enough (i.e. Less than 0.25 in) coil it it Rolling Process 4 F N Width of Strip is w 0 Width of Strip is w f V o h o h f Vf V f = V o (h o /h f ) F Rotating rolls reduce the thickness of the incoming ingot
Overall Process for Sheet and Plate Cast ingot Scalp (not always) Reheat and homogenize Hot roll in reversing mill to ~1-2 in thick Cut off ends (alligators) Warm roll to 0.1-2 in thick Heat treat to recrystallize (sometimes) Cool Cold roll to finish thickness Heat treat (where appropriate) Results in material which must be further worked 5 Schematic of various rolling processes 6
Rolling definitions and forces 7 (a) Illustrazione schematica della laminazione piana. (b) Forze di attrito sulla superficie del laminato. (c) Forza F sui rulli, e coppia T per ogni rullo. Independent Variables for Rolling 8 Roll speed Draft (amount of thickness reduction) or roll gap Billet thickness Billet width Billet material (not always allowed to select) Billet temperature Lubricant
Dependent Variables for Rolling 9 Roll force Power Speed of exiting strip Final strip temperature Maximum draft Roll strip contact length Sheet or plate shape Approximate Rolling Equations 10 Speed of exiting strip: V f = V o (h o /h f ) (m/s) Maximum Draft: h o -h f = µ 2 R (mm) Roll Force: F = LwY avg (N) Power: P = 2πFLN/60,000 (kw) Roll Strip Contact Length L = R 1/2 (h o -h f ) 1/2 (mm) True Strain E = ln (h o /h f )
Hot Rolling Advantages 11 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 sheet Hot Rolling Disadvantages 12 Higher friction Rolls need to be cooled Material handling difficult Personnel must be protected from heat
Hot Rolling and Microstructure Development 13 Cold Rolling Advantages Deformations of ~ 50 to 80% Work hardening increases strength Excellent surface finish Excellent tolerance on thickness and shape Disadvantages High forces Small reductions give rise to surface stresses and non-uniform stress distributions? 14
Product 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 plate Residual stresses Small deformations cause surface deformation Flaws in the surface Roll finish perfection 15 Defects in rolled sheet and plate 16 Waviness Edge cracking Zipper cracks Alligatoring
Effect of Roll Forces F = LwY avg (N) L = R 1/2 (h o -h f ) 1/2 (mm) 17 The forces result from the resistance of the material to deformation and cause Roll bending Non flat material Cambered sheet Buckling Roll flattening Poor tolerances Increased forces (larger contact area) Roll Force Compensation 18 Can decrease Roll Forces by: Reducing friction - Lubricants Using smaller diameter rolls (Sendzimir mill) Smaller reductions per pass (Residual stresses) Increasing temperature (Hot rolling) Front or back tension (important for high strength metals) Provide backup rolls (four high) Camber rolls to compensate Bend rolls back
Control of Overall Properties Thickness Roll gap which impacts roll force and hence the dimensions of the mill frame Width (due to spreading of slab) Edge rollers (push material back) Edge shears (cut material off) Length End shears Mechanical Properties Controlled by the microstructures which is controlled by the rolling parameters (reduction, temperature, etc) 19 Control of Overall Properties Tolerance Thickness Operator skill/automation Gauges measure thickness of input and output Computers control roll gap Camber Roll bending/automation Special segmented tension meters measure "shape" Computers bend the rolls Surface finish Roll finish Bearing reaction forces Applied forces 20
Rolling Mill Design Most commercial mills are 4 high, reversible Small rolls in contact F α L α R 0.5 Large back up rolls provide stiffness 21 Cold Rolling Mill Design Input and output coils with tensioning control Shape meters consisting of segmented coils measuring force Tandem mills allow multiple reductions in one operation Laser inspection systems for surface finish inspection becoming popular (especially for canstock) Surface condition of rolls critical to maintain surface finish Roll bending almost essential to provide crown control 22
Sendzimir Mill Design 23 Tandem rolling Multiple mill stands, sequenced spatially one after the other Speeds must be closely controlled Speed at each mill dependent on the speed and draft of earlier stand Typically used for cold rolling 24
Hot Rolling Mill Design Basically the same as cold mills, but designed for higher temperatures and are reversible Typically have edging rolls to control thickness Often have a warm mill at the end 25 Slab Hot mill Warm mill Take-up roll Roller table Hot Rolling Mill Design Basically the same as cold mills, but designed for higher temperatures and are reversible Typically have edging rolls to control thickness Often have a warm mill at the end 26 Slab Hot mill Warm mill Take-up roll Roller table
Hot Rolling Process Example 27 Start with 12in. thick slab, 20 feet long Hot mill converts to 2 in thick slab, 120 feet long with 11 passes (six forward and 5 reverse) Front end sheared off to eliminate alligatoring Warm mill converts this to roll of sheet 0.25 in thick, 960 feet long Rolls more easily handled by overhead cranes to transport it to cold mills than 1000 ft long slabs Typical Uses of Sheet and Plate Sheet and plate are everywhere after further processing Automotive bodies (steel and aluminum) Appliances Aircraft skins (fuselage and wings) Electronic assemblies Steel studs for construction Tanks Ships Beverage cans Containers of all sorts Because they are: Inexpensive High quality Strong 28
Shape Rolling 29 Non-flat shapes can be produced by having rolls with shape I-Beams H-sections Rails (trains) Angle iron Design of shaped rolls to provide the right amount of non-uniform deformation is critical Shape Rolling Ring Rolling Thick ring small diameter transformed into thin ring, large diameter Used in Jet Engines, large ring gears Advantages Circular shapes with no joins can be made with a variety of cross sections Thread rolling Creates thread on screws, bolts etc. Advantages No loss of material Good surface finish Increased strength through cold working a favorable grain flow 30
Ring Rolling 31 Figure 13.14 (a) Schematic illustration of a ring-rolling operation. Thickness reduction results in an increase in the part diameter. (b) Examples of cross-sections that can be formed by ring rolling. Sequence of shaped rolls 32 Figure 13.13 Stages in the shape rolling of an H-section part. Various other structural sections, such as channels and I-beams, are also rolled by this kind of process.