Process for heating metal strips, in particular non-ferrous metal strips

A process of treating metal strips using a linear electric motor with spaced apart motor elements comprises directing the metal strips into the space between the motor elements and operating the motor to produce a travelling wave and advancing the strip between the motor parts without contact thereof at a velocity greatly reduced with respect to the velocity of the linear electric motor wave so as to produce a slippage between the wave and the strip and to effect the heating of the strip as it is advanced. The apparatus includes a linear electric motor including a first motor unit spaced away from a second motor unit in the form of a linear motor or return bar so as to form a gap therebetween through which the metal strip is advanced. The apparatus includes means for feeding the strip through straightening rolls and cropping shears, S-rollers and coiling and uncoiling reels.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates in general to devices and method for treating metals 
and in particular to a new and useful process and device for heating 
non-ferrous metal strips. 
2. Description of the Prior Art 
It is current practice to heat metal strips continuously while they are 
transported without contacting them, whereby transport velocities, 
according to thickness of the strip, of up to 120 m/min. can be reached 
for the strip. Heating units provided with gas jet nozzles are used to 
carry out the heating without the strips coming into contact with any 
transport element. Such heating units, for example, have been successfully 
employed in the strip and light metal industry. The heat treatment should 
advantageously uniformly transfer a large amount of heat in a short time 
to the material to be heated, and also transport the strips without 
contact. 
It is essential for the known embodiment that the strip coming from the 
coiling reel passes through a looping tower, then through an S-roller unit 
or aggregate and finally horizontally through the continuous furnace again 
through an S-roller aggregate onto which joins a second looping tower and 
the coiling reel. In order to guarantee a continuous operation, two 
discharge reels and two coiling reels must both be provided in combination 
with the looping towers and the strip tacking means. 
There are also units which are not operated continuously so that these 
duplicate reels and looping towers can be abandoned. In this case a 
shutdown period results due to the necessity for tacking or welding of a 
new metal strip. This shutdown period influences the heat treatment 
condition of the metal strip in the heat zone, thus providing a greater 
accumulation of scrap. 
If it is taken into consideration that the gas jet furnaces, e.g., in high 
speed units, for aluminum can be up to 150 m in length, then it is found 
that the continuous operation has advantages despite the expense of the 
units, because the accumulation of the scrap is less. 
It has also been proposed to heat metal strips and sheets inductively 
(British Pat. No. 600,673) by passing these products between parallel and 
spaced groups of induction coils. To the applicants' knowledge such units 
have not proved themselves in practice for heating metal sheets and 
strips. 
A further proposal is to heat foils electrically directly by contact by 
means of a circulating roller, by pressing the foils by means of 
deflection rolls against a section of the circulating roller. The roller 
itself is heated on the reversed section through using magnetic rotary 
fields. Unequal heating of the roller is counteracted by different 
spacings between the rotary field and the roller, so that the foil heated 
by contact is also uniformly heated (German Offenlegungsschrift No. 
2,204,816). This proposal is not suitable for heating without contact. 
SUMMARY OF THE INVENTION 
The invention provides a process and a device which guarantees uniform 
heating over the width of the strip at very high transport velocities. 
According to the invention the metal strip is conveyed with a slippage 
sufficient for heating with respect to an electromagnetic travelling wave 
working in the direction of the strip. In order to obtain the desired 
slippage, the strip is moved through the travelling wave at a velocity 
greatly reduced with respect to the velocity of the linear motor. Thereby 
heating of the strip is achieved. Preferably the velocity of the 
travelling wave amounts to more than 40 m/s, preferably more than 60 m/s, 
dependent on throughput and the permitted minimum strip tension, whereas 
the metal strip moves at a velocity of, for example, 5 to 300 m/min, 
preferably 10 to 100 m/min. The temperature of the strip is adjusted by 
the synchronous velocity of the travelling wave and/or the voltage of the 
travelling wave. In addition, a precise regulation can be achieved by the 
velocity of the strip, which can be achieved in a manner known per se by 
means of the drive assemblies of the S-roller and reels through which the 
strip is passed. 
The electromagnetic travelling wave is known to the specialist (see for 
instance "DER KONSTRUKTEUR" (1974), number 8, page 3). The travelling wave 
is effected by a linear motor. The linear motor can be interpreted as an 
asynchronous motor in which the stator is cut open radially and bent to a 
plane. Doing this the rotating field of the asynchronous motor will become 
a travelling field. Instead of the cage of the rotor of the asynchronous 
motor, a conductor bar is used as secondary part. The conductor bar is 
sometimes called a return bar. The velocity of the heating and the 
movement velocity are individually controlled in dependence upon the 
metallic working material, the thickness of the working material and the 
desired heat-treatment. Thus, thermo-mechanical heat treatment can, for 
example, also be carried out with the proposed process. This is possible 
since the strip is on the one hand heated up and at the same time is under 
tension load. The heat treatment is further especially suitable for cases 
where a heat treatment temperature of 700.degree. C is not exceeded. Thus, 
a quenching and tempering treatment, for example, is possible with steel 
strips. 
The proposed process is particularly suited for non-ferrous metal strips, 
particularly as the heat treatment with such strips is achieved at low 
temperatures, i.e., approximately 400.degree. to 650.degree. C. The 
process has particular advantages for the treatment of an aluminum strip. 
The heat treatment therewith is achieved in the range of from 380.degree. 
to 580.degree. C, e.g., at about 400.degree. C. In the case of aluminum, 
the travelling wave and the velocity of the strip are coordinated to one 
another in such a way that sufficient high speed heating is achieved and 
also specific tension effect in the annealed area of the strip is 
maintained below the high temperature strength of the aluminum strip. 
Therefore, the tension effect with the aluminum strip should be less than 
1 kg/mm.sup.2 in the annealed area of the strip. It has been found that 
with aluminum and particularly aluminum-manganese alloys very finely 
granulated structures and favorable strength values are obtained by the 
quick and above all uniform heating over the width of the strip (yielding 
point/yield strength ratio). As is shown above in the preferred example of 
aluminum, it is recommended with other metal strips not to exceed the 
permissible load amounts with the tensile effect in the annealed area of 
the strip. 
A preferred device for carrying out the process comprises a high speed 
heating unit having one linear motor unit and a second unit in the form of 
a linear motor or return bar, wherein both units are positioned opposite 
one another and form a gap through which the metal strip may pass. As 
explained below, this embodiment with a return bar is particularly suited 
for the low heating temperatures. 
In order to be able to adjust to the different strip thicknesses, the 
spacing between the units can be shifted. The adjustment of this width in 
the gap influences the heating velocity and the temperature which can be 
reached. Thus, for example, a gap of 70-120 mm is considered expedient for 
a metal strip with a width of 0.5-2 mm to pass through. Furthermore, a 
velocity of the travelling wave of 60-100 m/s is recommended for these 
dimensions with a strip velocity of 20-60 m/min. As the metal strip is 
necessarily electromechanically centered in the gap between the respective 
units, the strip is transported through the gap without contact. The width 
of the strip to be treated may amount to the width of the armature of the 
inductor cog of the linear motor at a maximum. It must not exceeed the 
width of the inductor cog. Cross-sectional shapes, which are adapted to 
the respective shape of the strip are also possible in addition to the 
parallel arrangement of both linear motor units mirror-symmetrical to the 
conceived plane of the metal strip. The linear motor unit can therefore 
also have a bent cross-sectional shape with a strip which is bent in 
cross-section. It is essential that the width of the strip is uniformly 
heated. 
The linear motor units are preferably provided with an insulating layer. 
This layer is on the surfaces of the linear motor units which are arranged 
to the side of the metal strip, i.e., on the pole faces of the linear 
motor units. These layers also afford protection for the surface of the 
strip to be treated, if, for example, contact should be made by mistake. 
Glass discs treated on one side are suitable for this purpose. Such glass 
discs guarantee optimum protection as they reflect the radiant heat 
issuing from the heated metal strip and any damage to the surfaces of the 
metal strip to be annealed is avoided as they are completely smooth on the 
metal strip side. 
Driving roll units or aggregates or brake roll aggregates and possibly 
coiling and uncoiling reels are arranged in a manner known per se for 
transport in front of and behind the linear motor units. An S-roller 
aggregate, for example, is recommended as driving roll aggregate. 
The temperature of the strip can be itself automatically controlled with 
the assistance of a temperature measuring means which is directly 
connected to the outlet of the linear motor units. A closed-loop control 
including measurement of temperature energy of the electromagnetic 
travelling wave and strip velocity results hereby. 
According to a further preferred embodiment the gap for passage of the 
strip is covered at one or both sides of the units by a channel or 
passageway. A cooling gas for quicker cooling or a protective gas for 
protective gas treatment can, for example, be supplied through this 
channel. 
On the other hand this channel is also suitable as a suction channel, if, 
for example, on heating a metal strip, e.g., a layered metal strip, 
solvents which will pollute the environment evaporate. These can be sucked 
off immediately through the channel. Such a construction is recommended, 
for example, for a high speed heating unit which is used in drying metal 
strips coated with enamel. There is then the possibility of sucking off 
the solvent accumulated during drying for recycling for reutilization, 
causing a much lower amount of pollution to the environment than is 
possible with known units. As such treatments are carried out at lower 
temperatures, e.g., at 140.degree.-280.degree. C, the high speed heating 
unit with return bar is recommended for this purpose. A further advantage 
in this case is that the bar can more easily be swung away than a linear 
motor unit. 
It results from the above that the subject of the invention can not only be 
used for heat treatment purposes in the strictly metallurgical sense but 
also for heating, such as drying, burning in of enamels and similar 
treatments. 
Particular advantages of the invention are that very quick heating, in 
particular heat treatment, is possible in transport without contact, 
whereby intermittent operations are possible without larger amounts of 
scrap accumulating. The costly looping towers and the additional double 
expense of uncoiling and coiling reels are abandoned. The structure costs 
are considerably decreased besides the low costs of the unit, which can 
arise due to the abandonment of the looping towers and the duplicated 
reels. Moreover, a more sparing treatment of the strip is achieved by the 
abandonment of the looping towers, Transport of the strip, which can also 
be of assistance in the threading or inserting the strip into the heating 
device, can be accomplished by means of the travelling wave. The necessary 
units only have a fraction of the length which was required in the known 
gas jet units. Thus, the length of the heating device amounts to only 5 m 
in a heat treatment at up to 600.degree. C of aluminum at an annealing 
capacity of about 6 t/h (width of strip 1000 mm). The heating device is 
ready for use on switching in and therefore a longer starting up time is 
not necessary in intermittent operation. 
The input in temperature is absolutely uniform over the width due to the 
induction stream within the iron area of the travelling wave motor. No 
damages to the surfaces arise as the induction forces of the metal strip 
remain in the center between the two travelling waves, so that there are 
no points of contact. 
Accordingly, it is an object of the invention to provide a process of 
treatment metal strips using a linear electric motor with spaced apart 
motor elements which comprises directing the metal strip into the space 
between the motor elements and operating the motor to produce a travelling 
wave which progresses therealong and advancing the strip without contact 
of the strip at a velocity greatly reduced with respect to the velocity of 
the linear electric motor so as to produce a slippage therebetween and to 
effect the heating of the strip. 
A further object of the invention is to provide a device for treating metal 
strips which comprises a linear electric motor forming a high speed 
heating unit with one linear motor unit arranged in opposition to a second 
unit in the form of a linear motor or return mower so as to define a gap 
therebetween and means for advancing the metal strip through the gap. 
A further object of the invention is to provide a device for treating metal 
strips which is simple in design, rugged in construction and economical to 
manufacture.

DESCRIPTION OF THE PRIOR ART DEVICE 
Referring to FIG. 1 a metal treating device for treating a metal strip 20 
comprises means for passing it through a gas jet furnace 5 which is up to 
100 m in length. The units includes two supply reels 1 and 1a which feed 
the respective composite metal strips through respective straightening 
aggregates or units 2 and 2a respectively. The individual units are fed 
between shears 3a for cutting into the desired length and through tacking 
means 21. The sheets then pass through looping towers 22 and 22a which are 
arranged at respective ends of the gas furnace 5 as well as over similarly 
located S-shaped roller trains 4 and 6. 
After the strips are cropped by the shears 3a the end of the strip can be 
connected with the leading edge of a new strip by means of the tacking 
means 21 after a reel has been wound off. In the meantime the strip which 
is supplied from the looping tower 22 serves to continue the continuous 
heat treatment in the gas jet furnace 5. In addition to the S-shaped 
roller aggregates 4 and 6 in the front of and behind the gas jet furnace 
5, there are further S-shaped roller aggregate or unit 23 which is 
connected to the looping tower 22a. Cropping shears 7, a driving trestle 
24, brake 25 and a deflection roll 8 are located after the S-roller 
aggregate 23 which follows the looping tower 22a. The double arrangement 
of the supply reels 1, 1a and the looping towers 22, 22a and the 
collecting reels 9, 9a are necessary to render a continuous heat treatment 
in the very long gas jet furnace 5. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawings in particular the invention embodied therein 
FIGS. 2 and 3 comprises a metal treating device which does not employ the 
looping towers 22 and 22a. Such a construction enables a discontinuous 
working method. The arrangement of FIG. 2 the metal strip 20 is fed from a 
supply reel 10 over a deflection roll 11, a straightening unit 12, 
cropping shears 13 and over an S-shaped roller unit through the travelling 
wave furnace which is generally designated 15. The sheet which is fed 
through the furnace exits and passed over an S-shaped roller unit 16 and 
cropping shears 17 and another deflection roller 18 to the collecting reel 
19. 
In accordance with the invention the travelling wave furnace 15 comprises a 
linear electric motor assembly as shown in FIG. 3 which is made up of 
linear motor units 15a and 15a which are spaced apart to define a gap 15c 
therebetween through which the metal strip 20 is fed. The linear motor 
unit 15b may for example be a return bar disposed in opposition to a 
linear motor 15a. A return bar 15b is used in particular with lower 
heating temperatures of from 100.degree. to 200.degree. C. 
The linear motors 15a and 15b are arranged with their pole surfaces near 
asymmetrical to the plane of the strip 20 in the gap 15c. The metal strip 
20 itself may extend so that it is as wide as the armature of the inductor 
cog of the linear motor units 15a and/or 15b at a maximum. The surfaces of 
the linear motor units 15a and 15b are advantageously provided with a 
protective layer 50 to protect the inductor. The linear motor units are 
approximately 3 to 5 m in length. A path for maintaining the temperature 
for example to carry out quenching and tempering treatment can extend 
alongside each unit for example in a cooling channel or heating channel 52 
or 54. 
The construction of the travelling wave furnace 15 is such that it would be 
possible to thread the strip 20 through the gap 15c by the operation of 
the travelling wave furnace itself. In addition the S-shaped roller unit 
14 makes it possible to transport the strip up to the gap 15c from where 
the centering within the gap is undertaken by the travelling wave itself. 
As shown in FIG. 4 the high speed unit heating unit according to the 
invention enables the width of the strip to be uniformly and speedily 
heated. In a test example, an aluminum strip 2 mm in thickness and 200 mm 
in length was conveyed through a travelling wave at a velocity of 
approximately 18 m/min. The travelling wave itself moved at about 60 
m/sec. The heating temperature was measured and registered at three points 
over the width of the strip by means of temperature measuring devices. The 
schematic representation shown in FIG. 4 indicates that the width of the 
strip was uniformly heated within a very short time. The total time of 
heating up from room temperature up to 420.degree. amounted to 
approximately 13 seconds. 
While a specific embodiment of the invention has been shown and described 
in detail to illustrate the application of the principles of the 
invention, it will be understood that the invention may be embodied 
otherwise without departing from such principles.