US3008025A - Method of progressively induction heating metal strip and an apparatus for performing the same - Google Patents

Description

Nov. 7, 1961 F. ALF 3,008,025

METHOD OF PROGRESSIVELY INDUCTION HEATING METAL STRIP AND AN APPARATUS FOR PERFORMING THE SAME Filed Aug. 5, 1959 3 Sheets-Sheet 1 Fig. 1

Nov. 7, 1961 ALF 3,008,025

METHOD PROGRESSIV INDUCTION HEATING METAL STRIP AN APPARATUS FOR PERFORMIN Filed Aug. 5, 1959 G THE SAME 3 Sheets-Sheet 2 Jnvenzo r:

FRITZ HLF I Y: Mn, @YQXMQ Nov. 7, 1961 3,008,025 lgETAL s F. ALF GRESSIVELY INDUCTI METHOD OF PRO ON HEATING RIP AND AN APPARATUS FOR PERFORMING THE 9 AME Sheets-Sheet 3 Filed Aug. 5,

Jnvenfor:

FRITZ RLF ATTORNEYS tes The present invention relates to a method of progressively induction-heating metal strip and to an apparatus for performing the same.

It is .known that thin metal strip under 0.5 mm. thicle ness can be inductively heated to a temperature of 500 C. and less by means of induction coils which embrace the strip. Heating is done for example for the purpose of flowing electro-deposited layers of tin on the surface of the strip, for drying films of varnishes applied to the strip, for annealing or tempering or homogenising the strip. For suchpurposes induction heating installations supplied with current at a frequency of 10,000 c./s. by rotary converters are used. The efiiciency of such instal lations is, as such, very high, but it is lower when the strip metal is thin, for instance less than 0.15 mm. thick. For this reason frequencies under 5000 c./s. are not employed because efficiency would then be too low. According to the speed of travel of the strip and the hourly throughput of material the power supplied is arranged to be such that the induction coils will raise the temperature of the travelling one or more strips to the final temperature that is required. Since the maximum power output of a single induction coil is subject to limits, several such induction coils must be provided to permit the total power that is required to be transferred to the one or more strips which travel through the consecutive coils which embrace them.

Hitherto the required treatment temperature was attained in a substantially graphically linear rise of temperature. Upon considering the problem the inventor has come to the conclusion that it is desirable to depart from this method of heating. According to the invention it is proposed, especially for the purpose of fiowing an electrodeposited film of tin, first to attain a temperature level of about 210 C. or slightly more by raising it steadily, and then to elevate the temperature of the material in a rapid and graphically steep upward rise, to 250 C. at which temperature the tin fuses. As known, the fusing of the tin is accompanied by its diffusion into the surface of the treated strip. To some extent this is desirable, but it also leads to an embrittlement of the intermediate layer if the temperature is maintained too long in the vicinity of the melting point of the tin. The risk of embrittlement is avoided or at least largely suppressed if heating is performed in the manner her'einbefore defined.

Similar conditions obtain in the drying of varnishes and especially in diliusion or honiogenising annealing. In both cases it is desirable that the temperature should be raised abru tply from a point below the ultimate level.

In order to solve this problem the strip which travels through the induction coils must be heated in stages in which the temperature gradients are difierent. If the in duction coils are alike and connected in parallel, such difierent temperature gradients in the heating process cannot be readily achieved. In known installations the induction coils are so contrived that they are able to transfer the maximum possible power to the strip. This maximum power produces a temperature gradient of maximum possible slope and it cannot be further increased by conventional means.

It is nOW proposed according to the invention first to ater raise the temperature of the strip at a practically uniform steady rate by the application of current of medium frequenc, preferably 5000 c./s. or 10,000 c./s. to a tem perature level somewhat below the critical treatment temperature and then to apply a current of higher frequency. This current which may preferably have a frequency equal to twice that of the starting frequency will then raise the temperature steeply to the treating level.

In the flowing of tin this means that the temperature is first raised at a uniform rate to about 200 to 210 C., to be then increased within the shortest possible time to 250 C. The tin fuses without being allowed a sufficient period of time to participate very significantly in diffusion al processes.

For performing the method it is proposed to use a plurality of heating inductors arranged along the path of travel of the strip or strips so as to embrace the same and supplied in parallel from a bus bar with a current of 5000 c./s., or more, generated in a rotary converter, but to feed the final or possibly also the penultimate inductor in the direction of travel of the strip through static or rotary frequency changers of known construction. Moreover, it is proposed to design these frequency changers in such a way that they will deliver at least twice the fre quency fed to the bus bars. The higher frequency will then produce a higher energy density in the final induction coil. This means that the final coil can transfer more power to the strip than the preceding coils. The greater energy transfer per unit of time from the final inductor to the strip means that the temperature rise will be steeper. Another advantage of employing such frequency multipliers consists in that the energy delivered by the frequency multiplier to the induction coil can be continuously adjusted. Consequently, the slope of the temperature gradient can be selected according to the desired effect.

According to the invention the further possibility arises, instead of adjusting the static frequency changer to twice the basic frequency, of providing a compensating condenser battery between the frequency changer and the bus bar and to select the design factors of this battery in such a way that this produces an inductive phase angle in the second harmonic of about cos =0.9. This under-compensation causes the fifth harmonic of the frequency multiplier to be considerably enhanced so that the current delivered by the frequency multiplier will contain a major fifth harmonic component. It was recognised that this step offers the possibility of compensating the distance between the tin fusing line and the quenching bath if differences in the gauge of the Strip should give rise to changes in the position of the fusing line of the tin. This phenomenon is due to the fact that the heating efiiciency at 10,000 c./s. drops in thinner sections of strip. However, for instance at 50,000 c./s. efliciency rises to maximum value so that these thinner sections will then be heated with the same efficiency as the thicker ones. This affords the further possibility of feeding several strips of different gauge simultaneously through the induction coils. Hitherto this was impossible, because the level of the tin fusing line differed so much between the several strips that optimum flow could not be achieved. The proposed steps overcome these difiiculties.

The above described temperature characteristic according to the invention is schematically illustrated in FIG. 1 of the accompanying drawings. In this graph time is plotted on the abscissa, whereas the temperature of the strip in degrees C. is plotted on the ordinate. It will be seen that the temperature rises to point 42 in FIG. 1 at a substantially uniform rate. The temperature rise merely has a discontinuity point at 40- where for practical reasons the strip leaves the first group of inductors and travels over a return roller before entering the second group of inductors. The roller withdraws a certain amount of heat from the strip, causing a temperature drop of about C. This intermediate period is represented in the graph in FIG. 1 by the part of the curve between points 40 and 41. After attaining a temperature of about 200 C. (point 42 in FIG. 1) the strip enters a further induction coil which is fed, according to the invention, with a current of higher frequency. This higher frequency produces a higher energy density in the strip and therefore brings about a steeper rise in the temperature at 43. This steeper temperature gradient has the effect that the fusing point of the tin at about 230 C. is passed with a steep temperature gradient so that the strip will be heated to about 250 C. before being immersed in a quenching bath.

FIG. 2 schematically shows an embodiment of an installation according to the present invention. The motor 4 of the converter set is fed from a mains circuit l. at 2 through switch means 3. The motor drives the generator 5 at a speed which will generate the desired frequency at the desired output. The generator is excited by an exciter unit 6 comprising switch means 6, a continuously variable regulating transformer 6 followed by rectifiers 6". The exciter unit 6 shown in the drawing may naturally be replaced by an electronically operating exciter unit. This simultaneously permits the generated medium frequency voltage to be automatically maintained at a previously adjusted level.

The generator 5 delivers its medium frequency energy at 7. Through an overload circuit breaker 8 this energy is fed to a bus bar9. For controlling performance a control panel is provided containing voltmeter 10, wattmeter 11, a compensating instrument 12, and an ammeter 13. These instruments are connected with the main circuit through voltage transformer 14 and current transformer 15. They are intended for purposes of supervision and control of the installation. A condenser battery 16 serves for the fine compensation of bus bar 9. The energy required for operating the several induction coils 17 which embrace the strip 18 that is to be heated is taken from the bus bar 9 via switch means 19. These switches permit the individual induction coils to be switched on or off according to the temperature characteristic required according to the invention. Coarse compensation of the coils is provided by condensers 20. Fine compensation is effected as has been described by the condenser battery 16.

The strip 18 passes through induction coils 17 without touching them and runs over guide rollers 21. Having passed through the induction coils the strip has reached the required temperature level and is immediately immersed in the quenching bath 22-particularly in the case of tin flowing installations.

To achieve the steep final temperature gradient required by the invention, as illustrated in FIG. 1 at 43, an induction coil is provided at the end of the heating section, which is fed with a higher frequency current and which therefore delivers an increased energy density. To avoid the need of providing a separate generating plant for higher frequencies for feeding this coil, the energy for providing this frequency is likewise taken from bus bar 9 but supplied to induction coil 17 through frequency multipliers 23. As has been explained these may be embodied in conventional static or rotary frequency changers. The static frequency changer indicated in FIG. 2. comprises three coils of which the coil on the right in the drawing is the primary coil which is provided with a highly saturated iron core. The presence of this iron core gives rise to powerful harmonics which induce a voltage of correspondingly high frequency in the secondary coil on the left. The coil 23 shown in the centre is the biasing coil fed with direct current through a regulating transformer 26 and a rectifier 27. The power required for biassing can be directly taken from the mains instead of from the bus bar 9.

As will be seen the biassing current can be controlled so that the output energy of the frequency multiplier can be continuously varied for determining the slope of the temperature gradient. Precompensation of the frequency multiplier 23 is effected by a condenser battery 24. The secondary is compensated by a condenser battery 25.

FIG. 3 shows the disposition of the induction coils in relation to the strip and the guide rollers, the current leads and the quenching tank, in perspective. To facilitate identification of the individual parts by reference to FIG. 2 the same reference numbers are used as in FIG. 2.

It will be seen that the strip 18 runs in the direction of arrow 28 and that it passes over the guide rollers 21. In the direction of travel of the strip the latter first passes through tthe three induction coils 17 which surround the strip and are completely embedded in a castable composition. These coils are connected with bus bar 9 (not visible in the illustration) through rails 29. The last of the induction coils 17 which is fed at normal frequency is shown with the embedding composition removed to reveal the arrangement of the windings 30 facing the front of FIG. 3. However, it will be understood that this coil is also completely cast into the compound. The bottom coil 17 is connected with a special conductor 31 through which the energy from the frequency multiplier is fed to this coil. ter leaving this final coil 17 the strip is immersed in the quenching bath 22.

In one specific embodiment, the strip was of low carbon steel of a thickness of about 0.15-0.40 mm. having an electro-deposited fine layer of tin. The first series of inductors 17 were fed with current at a frequency of 10,000 cycles per second and the final inductor 17' with current at a frequency of 20,000 cycles per second, the temperature of the strip rising to about 200 C. after passing through the said first series of inductors and being raised to about 250 C. in the final inductor before passing into the quenching bath. The strip travelled at the rate of about max. m./min.

What I claim is:

1. The method of progressively inductively heating metal strip of a thickness less than 0.5 mm. which comprises first inductively heating the strip by means of a current of a frequency not less than 5000 or more than 10,000 cycles per second to produce a steady rise in temperature of the strip to below the critical treating temperature and then inductively heating the so-heated strip more rapidly by means of a current of a frequency at least double that of the frequency used in the first said heating step to produce a rapid rise in the temperature to the treating temperature.

2. The method of progressively inductively heating metal strip of a thickness less than 0.5 mm. and having a deposited layer of tin which can be fused on the surface of the underlying strip, which comprises first inductively heating the strip by means of a current of a frequency not less than 5000 or more than 10,000 cycles per second gradually to a temperature in the range of temperatures of the order of from 200 to 210 C. and then inductively heating the so-heated strip more rapidly by means of a current of a frequency at least double that of the frequency used in the first said heating step to produce a rapid rise in the temperature of the strip to a temperature of the order of 250 C.

References Cited in the file of this patent UNITED STATES PATENTS 1,646,498 Seede Oct. 25, 1927 2,440,476 Johnson Apr. 27, 1948 2,448,011 Baker et al. Aug. 31, 1948 2,465,306 Durand Mar. 22, 1949 2,897,328 Alf et al. July 28, 1959

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