DIE BIBLIOTHEK DER TECHNIK

Basic Knowledge with the Know-howof Leading Companies

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233

DIE BIBLIOTHEK DER TECHNIK 233

Aluminium RollingMill Technology

Future concepts in thin-strip and foil rolling

VERLAGMODERNEINDUSTRIE Achenbach Buschhütten

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+Umschlag_Achenbau_(e) 25.10.2002 8:58 Uhr Seite 1

ContentsAluminium – an innovative material 4

Aluminium for future generations........................................................... 4Importance of aluminium rolled products ............................................... 6Aluminium thin strip and foil for packaging .......................................... 7Sustainability of packaging solutions ..................................................... 9Technological trends in aluminium packaging ....................................... 12

Basics of aluminium rolling mill technology 15

Flat-rolled aluminium from slab to foil................................................... 15Rolling theory.......................................................................................... 20Performance targets................................................................................. 24

Rolling mill technology 29

Rolling mill sizing................................................................................... 29Mechanical design................................................................................... 34Mill actuation .......................................................................................... 51

Process control and automation 54

Technological controls ............................................................................ 54Teleservice............................................................................................... 67

Coolant systems and environment control 68

Roll coolant circulation systems ............................................................. 68Mechanical coolant filtration .................................................................. 70Thermal coolant rectification .................................................................. 73Exhaust air collection – fume hoods ...................................................... 74Fume exhaust cleaning ........................................................................... 77

Future developments 80

Literature 82

The company behind this book 83

15

Basics of aluminiumrolling mill technologyFlat-rolled aluminium from slab to foilThe production of aluminium flat-rolled prod-ucts can schematically be divided into fourmajor steps as shown in Figure 9.

Fig. 9: Major steps in theproduction of flat-rolled aluminium

In step 1 – the hot strip production process –traditionally the molten metal is cast after refin-ing and alloying processes into 10 to 25-tonslabs in semi-continuous casters, then prean-nealed, hot-rolled in single-stand or tandemhot rolling mills and, at a strip thickness of 6to 2.5 mm, coiled at a temperature of approx.300°C (Fig. 10).

Hot strip production AuxiliariesSemi-continuouscaster

Hot rolling mill Strip casterIn-line belt-casterhot mill

Melting andcasting furnace

Strip rolling Auxiliaries

Reversingstrip rolling mill

Non-reversingstrip rolling mill

Tandem strip rolling mill

Coil annealingfurnaces

Thin-strip and foil rolling Auxiliaries

Auxiliaries

Thin-strip rolling millFoil rolling mill Finish-annealingfurnaces

Transportation systemStorage system

Roll grinding shop

Litho rolling mill

Trimming and slitting line

Tension levelling line

Cut-to-length line

Foil doubler

Foil separator

Foil slitter

16 Basics of aluminium rolling mill technology

More economical in terms of energy saving isthe direct casting process into strips of 12 to20 mm thickness in twin-belt casters with acontinuing hot rolling process in a tandem hotrolling line at an hourly production rate of 25 kg per mm width. The suitability of this pro-cess is restricted to a limited number of alloysallowing a quick cooling without segregation.The third and most economic way is hot stripcasting between two rolls in a so-called twin-roll caster with an exit thickness between 6and 3 mm at low speed (Fig. 11). This castingmethod can also only be used for pure alu-minium or material with a low alloy contentfor the production of foil. All three different hot strip production methods have advantages and limitations interms of material quality, productivity, en-

Fig. 10:Conventional hotstrip production

Fig. 11:Twin-roll caster forhot strip production

Hot stripproduction

Slab casting Preheating Hot rolling

Pusher furnace Reversing hotrolling mill

Tandem hotrolling mill

Coiler

CoilingShearDriving rollsTwin-rollcaster

Holdingfurnace

Meltingfurnace

Flat-rolled aluminium from slab to foil 17

ergy and labour intensity. The investment andoperational costs are also key factors for thefinal decision about the optimum solution.After the hot strip production in step 2, thecold strip rolling starts. Even though this de-formation process is called “cold” rolling, thestrip is heated up to approx. 100°C duringeach pass and large quantities of coolant haveto be poured over the rolls to keep a thermalequilibrium. After each of the three to four

Fig. 12:Different types ofstrip rolling mills

Cold striprolling

Tandem strip rolling mill

Non-reversing strip rolling mill

Reversing strip rolling mill

18 Basics of aluminium rolling mill technology

passes, the coils have to be cooled down toroom temperature for several hours.During each cold rolling pass, a material hard-ening is effected by the deformation process ofthe strip. Depending on the grain structuremainly influenced by the alloy composition,one or two annealing sessions for recrystallisa-tion have to be integrated into the productionprogramme to permit a continuation of the roll-ing passes and to fulfil the final requirementsof the product specification.The strip rolling process itself can be donewith different types of rolling mills (Fig. 12).For small coil weights up to 5 tons, reversingrolling mills are still used. For normal coilweights between 10 and 15 tons, non-revers-ing single-stand rolling mills are common. For high coil weights up to 25 tons, and largeproduction volumes, multi-stand tandem roll-ing mills are used.Step 3 – thin-strip and foil rolling – is the fin-ishing operation for nearly all flat-rolled pro-ducts on non-reversing single-stand rollingmills. Either universal rolling mills or groupsor lines of two to five rolling mills are used.The thin-strip rolling mills used for the pro-duction of can stock provide extremely narrowstrip gauge and flatness tolerances. Lithogra-

Fig. 13:Foil rolling mill with double decoiler

Materialhardening

Thin-strip andfoil rolling

Flat-rolled aluminium from slab to foil 19

phic sheet rolling mills, in particular, fulfil highdemands on the strip surface. Foil rolling millsare the most accurate in gauge and tensioncontrols. All rolling mills used for these dif-ferent purposes are equipped with specificcomponents to ensure the quality requirements.The last pass of foil rolling is done with a double web (Fig. 13). This requires a doub-ling operation of foils after the penultimatepass and a separating operation after the lastpass. The finished foil typically consists of ashiny and a matt side. Both sides are used for different printing images in a later stage.Foil rolling is an aluminium-specific process.No other material can be rolled down to a thick-ness of 5 or 6 µm. Taking into consideration thata human hair has a diameter of about 12 µm, finished converter foil has about half of this(Fig. 14). Foil rolling mills manage to be twice

Fig. 14:Thickness of light-gauge foil

as accurate in their controls as a wristwatch anduse deformation forces of several hundred tons.Step 4, the last step of the production line offlat-rolled products, is the slitting operation ofwide-rolled thin strips and foil. Aluminiumstrip and foil has to be supplied on spools spe-cifically tailored to the downstream productionprocesses. High-speed slitting and windingmachines with very accurate slitting geom-etries have been developed for these purposes.

Foil rolling: an aluminium-specific process

Slittingoperation

12 µm

6 µm