HEAT TREATMENT – FROM STRIP TO FOIL Heat treatment is required at several steps during the production of Aluminium foils. This heat treatment is carried out in different heat treatment plants acc. to the re-quirements of the specific process step. The necessary heat treatment processes are intermediate heat treatment during rolling and final annealing of foil rolls at the end of the production process. Aluminium strip is produced from rolling slabs or cast strip. During the production of sheet the feedstock is rolled in several steps. Aluminium strip is also feedstock for the foil production. In order to reduce the hardness of the strip, which results from the rolling process, heat treatment of the rolled material is essential between the rolling processes. Therefore, the coiled strip is intermediately heated in heat treatment plants. At the end of the production process the foil is finally annealed. For these heat treatment processes different furnace concepts are used today: the Multi-Coil Furnace, the Single-Coil Lifting Heart Furnace, and the Multi-Chamber Furnace.
achieve an efficient heat treatment process. By a special charging machine the so like prepared load is placed into the furnace. A Multi-Coil Furnace consists of several sections in which the same process is executed but which can be controlled independently. Each section comes with a circulation fan and a flow distributor including the heating equipment. The upper part of the furnace contains the heat treatment zone including flow distributor and heating; the lower furnace contains the support structure for the load and the space where the charging machine goes in when loading and unloading the furnace chamber. At the front of the furnace a door serves for charging. Usually, several furnaces are installed side by side, which are loaded by one charging machine. The charging machine moves into the area in front of the furnace line. Beside the furnaces preparation, cooling and discharging stands are integrated.
fig.1 Multi-Coil Furnace The Multi-Coil Furnace (see fig. 1) is loaded with several strip coils all at once, which are heat treated by the same heat treatment process. The strip coils are combined to one load on a rack at a preparation station. Equal strip coils – alloy, coil size have to be combined to one load to
64
HEAT TREATMENT
Single-Coil Lifting Hearth Furnace Single-Coil Lifting Hearth Furnaces provide individual heat treatment for single strip coils. The Single-Coil Lifting Hearth Furnaces are placed on columns and installed in line; by this the charging machine can move below
the furnaces for placing the strip coil in a furnace (see fig. 2). The individual strip coil is placed in an individual chamber.
fig. 2 For placing a strip coil in the furnace chamber the furnace hearth is transferred to the loading station and the coil will be placed onto the hearth, e.g. by overhead crane. The charging machine is under the furnace hearth, lifts it and moves it into a free furnace. Under the free furnace the charging machine lifts the hearth including strip coil into the furnace. After hearth and coil are placed into the furnace the hearth will be fixed by pins and the furnace sealing will be activated. The lifting unit of the charging car lowers and the charging device is available for discharging or charging another furnace. A heat treated strip coil will be placed on the charging station and from there it can be taken out of the line. For heat treatment the strip coil is supported in accordance with the specific requirements of the production and in a way to avoid marking of the strip. The support structure can be placed on the hearth together with the coil or can be firmly installed and thus become an integrated part of the furnace hearth. All known support structures can be used this way.
Each furnace is equipped with a circulation fan and a related heating system. The flow distribution system of the furnace cares for circulating the furnace atmosphere and for the optimum impingement of the flow to the sidewalls of the strip coil, because only by this the heat can be introduced to the coil (see fig. 3). Therefore, the flow distributor is equipped with nozzles, which care for the strong flow impingement. This provides a rapid heating, supports an even heating and contributes to the reduction of the heat treatment time. The furnace is indirectly heated by radiant tubes for recuperative burners or by electrical heating devices. The annealing should be processed depending on the specific
fig.3
requirements in an atmosphere of less than 0,1 vol-% O2. Also, to some extent, cooling the strip coil to a specific temperature is processed in a protective atmosphere. For cooling the strip inside the furnace, which is effected in accordance with the alloy, each furnace is equipped with its own cooling device including recirculation fan and heat exchanger. Thus, individual cooling of specific strips is also possible. For each strip coil – depending on alloy, coil size, heating temperature, heating time – the heat treatment process is individually preset. This applies to the process steps such as heating up, holding, cooling and to the parameters of the process steps. Thus, a flexible and precise heat treatment of an individual strip is guaranteed. Due to the very individual parameters an efficient heat treatment will be achieved. The Single-Coil Lifting Hearth Furnace can be used for annealing in the range of 180°C to 600°C. For homogenising of cast strip 580°C are required. Immediately after casting the strip can be placed in the Single-Coil Lifting Hearth Furnace. The soft annealing is processed at about 350°C. To some extent, the strips are cooled down to 175°C before they are
taken out of the furnace. The annealing includ-ing cooling lasts about 16 hours. The protective atmosphere can be N2 or e.g. a mix-ture of N2-CO2. During heating up the strip rolling oil is removed from the strip. This happens usually up to about 220°C. Depending on the amount of oil the related holding time is preset on the process parameters. After the oil is removed the temperature can be in-creased. This like individual strip coils can be annealed just in time and flexible. The direct connection of the transport system to a storage system enables a fully automatic charging and heat treatment acc. the specific demands of the production schedule. Scheduling can be done precisely, because only the individual strip coil and the booking of the furnaces have to be considered. The advantages of the Single-Coil Lifting Hearth Furnace compared to the Multi-Coil Furnace result from the individual heat treatment of each strip coil, which contributes to a significantly more flexible production process, reduces the energy consumption and decreases the occupied workshop area. Acc. to the expected maximum coil diameter different furnace sizes can be used or can even be placed in one line (see fig. 4). The furnace line can easily be ex-tended afterwards, because each furnace is controlled individually and only the charging system has to be adjusted. The average furnace size can be used for coils of up to 25 MT. If considering an actual average of 10MT per coil, the yearly capacity of each furnace including auxiliary process time and downtime will be about 4000 MT/year. Typical heat treatment plants have been equipped with 4 up to 17 single furnaces.
65
Final Annealing The final annealing of foils is executed after the final rolling in a
treatment zone. Charging of the foil rolls into the furnace chamber is effected by a
fig.4 Multi-Chamber Furnace. The Multi-Chamber Furnace combines several individually filled chambers in one furnace casing (see fig. 5). Each chamber is controlled independently and executes its own process. The combination in one furnace casing reduces occupied workshop space and energy consumption. Each furnace chamber contains several individually controlled sections. Each section contains a
fig.5 circulation fan, the indirect heating and a flow distribution system. The cir-culation is arranged vertically, which means, the air is blown underneath the load by two side channels and returns above the load. The heating is effected in the side channels outside of the
66
HEAT TREATMENT
charging machine, which transfers the load from the preparation stand to the furnace chamber. This charging machine runs in front of the furnaces and loads to the furnace in longitudinal direction (see fig. 6). Therefore the load is placed into the furnace in an elevated position and is lowered onto the internal furnace supports. Up to 25MT foil rolls can be loaded into one chamber. The average can be considered as about 18MT. The treatment temperature is 300 – 370°C (max. 400°C). An annealing requires about 60 - 80 hours. The heat treatment process is executed fully automatically from the preparation stand to the discharging stand. A plant in total consists of one or more Multi-Chamber Furnaces and the charging machine as well as the preparation stands. Control All plants are equipped with a fully automatic control system, which enables further expansions and optimisation. To all plants applies that the heat treatment data – presets and actual values can be recorded specifically for each batch, so that a replication is possible.
Individual experienced heating up and cooling down curves can be recorded and can be preset afterwards for reproducible heat treatment
fig.6 Summary For the heat treatment processes required during the production of aluminium strip and foil specific furnace concepts are available for each requirement. With the Multi-Coil Furnace all the time batches containing equal strips should be combined, because the furnace needs to be loaded with multiple strip coils, which can be treated with only one process. With the Single-Coil Lifting Hearth Furnace each strip can be heat treated individually. The strip can be automatically placed into the furnace out of storage or directly after rolling or casting. In this way, the heat stored in the coil is not lost. Space requirement is less due to the plant concept, and the plant can be extended in a more flexible way. With the Multi-Chamber Furnace the final annealing of foil rolls provides advantages regarding space and energy consumption. Also, this plant is operated fully automatically from combining a batch to discharging the rolls. Hermann J. Meyer, LOI Thermprocess, Essen, Germany