US3041434A - Method of and apparatus for inductively heating metal - Google Patents

Description

June 26, 1962 F. ALF 3,041,434

METHOD OF AND APPARATUS FOR INDUCTIVELY HEATING METAL Filed Sept, 15. 1959 4 Sheets-Sheet 1 5 L l f K/ .[m eflfm June 26, 1962 F. ALF 3,041,434

METHOD OF AND APPARATUS FOR INDUCTIVELY HEATING METAL Filed Sept. 15, 1959 4 sheets-sheet 2 Inventor? June 26, 1962 F. ALF 3,041,434

METHOD OF AND APPARATUS FOR INDUCTIVELY HEATING METAL Filed Sept. 15. 1959 v 4 Sheets-Sheet 4 il/Z1780): MA-Wr My United States Patent 3,041,434 METHOD OF AND APPARATUS FOR INDUC- TIVELY HEATING METAL Fritz Alf, Remscheid, Germany, assignor to Deutsche Edelstahlwerke Aktiengesellschaft, Krcfeld, Germany, and Allgemeine Elektricitats-Gesellschaft, Berlin- Grunewald, Germany Filed Sept. 15, 1059, Ser. No. 840,162 Claims priority, application Germany Sept. 19, 1958 6 Claims. (Cl. 219-1041) The invention relates to a method of inductively heating, thin large-surfaced metallic work such as strip, sheet, plate, or the like in a transverse field. The invention also concerns a preferred apparatus for performing the method.

For inductively heating strip and sheet two basically different methods are employed. The one consists in passing the strip or sheet through the effective range of an appropriately formed induction coil which embraces the work. The magnetic field distribution emanating from the coil is such that the field acts on the work longitudinally. The use of a longitudinal field has the ad'- vantage that the heating efiiciency is relatively high and, more particularly, that it permits magnetically conductive sheet to be heated. That this is possible is due to the absence of a component of force which draws the sheet into contact with the turns of the coil since the mechanical components of force cancel each other in every direction. On the other hand, heating in a longitudinal field has the drawback that relatively high frequencies must be employed to heat the relatively thin sheet in an economical way. It has been found that when heating strip of a thickness of about 0.1 mm., frequencies below 10000 to 5000 c./s. must not be employed because the currents will effectively penetrate into the strip only when the frequencies exceed the stated frequency levels.

For this reason it has already been proposed to employ a transverse field for inductively heating strip. This method consists in drawing the strip between an appropriate inductor assembly in which the several inducing conductors extend transversely to the lengthwise direction of the strip on one or both of its sides. This arrangement causes a current to be induced inside the strip which flows in the plane of its surface. It is thus possible to apply much lower frequencies, such as the frequency of the power supplied through the mains.

However, this known method has the defect that the efficiency of the inductor device is rather low. Moreover, during the passage of the strip through the inductor assembly care must be taken to see that the distance between the inductor and the strip remains constant. Otherwise different energies would be induced in the strip and necessarily lead to unequal heating. Furthermore, when heating magnetically conductive strip an additional magnetic elfect is set up which tends to draw the strip into contact with the inductor. This magnetic force of attraction increases when the gap between inductor and strip is reduced so that when the coupling between inductor and strip is desirably tight there is an added risk of the strip being drawn into contact with the inductor and of damage being done to the inductor or even to the treated strip by mechanical or thermal effects.

According to the specification of US. Patent No. 2,897,328 the strip may be passed through a fiat coilshaped inductor fed with a medium frequency current of relatively low power density and comprising conductor sections facing the surfaces of the strip and extending at right angles to the strip edges, the conductors passing around the strip edge from one side of the strip to the other at an angle exceeding 25.

This arrangement permits of an economic heating cffect due to the reduction in the power density, but it likewise sets up a longitudinal magnetic field inside the strip so that depending upon the thickness of the strip relatively high frequencies must still be employed.

The present invention deals with the problem of overcoming the drawbacks of the above mentioned methods and of providing a method and an inductor which as such are designed for longitudinal heating but which are rendered operative to achieve a transverse field heating effect in the work. For the purpose of inductively heating thin large-surfaced metallic work such as sheet, strip, and the like, by means of a transverse field set up by one or more inductor coils embracing the work the pres ent invention proposes to separate the magnetic fields emanating from the several conductors comprised in the coil by means of a material which will conduct the field so that it is directed on to the work in such a way that the current induced in the work will exclusively flow in the plane of the work. Consequently current paths will arise within the work from conductor to conductor, which run transversely to the plane of the work. The method proposed by the invention is thus adapted to combine the advantages of transverse field heating with the advantage gained by an inductor arrangement for longitudinal field heating. The drawbacks inherent in either method are eliminated by the method according to the invention because even in the case of extremely thin sheet relatively low frequencies may be employed and a relatively high efficiency thereby achieved, whilst at the same time tendency of the work to be drawn into contact with the induction coil is avoided because the mechanical forces of attraction are compensated.

The effect of the transverse magneto-motive force between the individual conductor sections is due to the magnetic field emanating as herein described from each individual conductor being separated from that of the next. Viewed over the entire induction coil a resultant field embracing all the individual turns does not therefore arise, each conductor having an individual magnetic field which separately affects the work.

For performing this method it is proposed to make use of a preferred apparatus comprising an induction coil of fiat shape, the conductors on each side of the work (which is passed through the coil) being parallel to one another as well as parallel with the conductors on the opposite side, the connecting loops being situated in the vicinity of the edges of the work and the several conductors which form the induction coil being so arranged that the distance d between the individual conductors on each side of the coil is at least 25. Between the depth of penetration and the frequency of the current the following known relationship will apply:

6=depth of penetration t t v v ,u=permeability f=frequency Furthermore, the several conductors of the coil are embedded in a field-conducting, magnetically conductive material in such a way that an effective face of each conductor directed towards the work will remain free, the

resultant permeability of said material being not more venient to make use of a casting or mouldable resin incorporating iron powder for embedding the conductors of the induction coil. Before being cast a high proportion of iron powder is added to the resin and the compound cast or moulded around the conductors in such a way that the inside of the induction coil remains free. However, it may be expedient to furnish the free effective surfaces of the conductors inside the coil with a protective layer of a plastic incorporating substances of a nonferromagnetic electrically non-conducting and non-abrasive nature. A very useful material in this connection is quartz meal. Such a layer is magnetically neutral but it forms an intimate mechanical bond with the outside field-conducting layer, especially if applied to the latter before it has set. The protective layer mechanically protects the conductors of the induction coil and the incorporated quartz imparts a satisfactorily high temperature resistance thereto.

The invention will now be more particularly described with reference to the accompanying drawings.

FIGS. 1a and lb illustrate the principle of the known method of induction heating in a longitudinal field.

FIGS. 2a and 2b show the general arrangement and the direction of flow of the current within the work with transverse field induction heating,

FIG. 3a in longitudinal section illustrates the preferred apparatus for performing the method according to the invention, and

FIG. 3b shows the flow path of the current within the work.

FIG. la illustrates the known arrangement for heating in a longitudinal field. The induction coil 1 embraces a strip 2 which passes in continuous motion through the coil. The coil induces a current 3 in the strip in the manner shown in FIG. lb. This figure shows the strip in cross-section. To permit this current to flow, the strip must therefore be sufficiently thick and the frequency sufficiently high.

FIG. 2a illustrates a known arrangement for heating by means of a transverse field. The sheet metal strip 2 travels between inductors 4 and 4 surrounded by magnetic yokes 5 and 5 for conducting the magnetic flux. The field emanating from these coils passes transversely through the strip and gives rise to a current 6 which flows in a path in the plane of the strip, as illustrated in FIG. 2b.

The apparatus according to the invention is shown in FIG. 3a in longitudinal section. The strip 2 travels through the induction coil and the effective individual conductors 7, 7 of the coil are parallel to one another and to the strip faces. The several conductors of the coil on either side are spaced at least (i525 apart. The conductors on the one side are in staggered arrangement in relation to those on the other side. All the conductors are surrounded by a field-conducting material 8 in which they are embedded. However this material so surrounds them as to leave free an effective face of each conductor facing the work. This field-conducting material which consists of a casting or mouldable resin with iron powder incorporated therein and which thus assumes a resultant permeability of about confines the magnetic field 9 and 9' emanating from each conductor to the conductor itself so that it individually takes efiect in the work 2, a magnetic field resulting from the combination of all the condoctors and passing through the work longitudinally being avoided. In other words, each individual conductor 7 and 7' generates in the strip 2 a separate current 10 the path of which is shown in FiG. 3b and is located in the plane of the strip. The formation of this current path is not now controlled by the thickness of the strip}; but merely depends upon the mutual distance between the several conductors. If these conductors, as required, are spaced at least (1526 apart on one side of the strip then the arrangement will permit such a current path to de velop with a satisfactory degree of efficiency.

In order to prevent damage being done to the coil by the travelling strip 2 and to provide an effective electrical and thermal insulation, the inside surface of the induction coil is provided with an additional layer 11 of castable or mouldable resin which is cast on to the mass of the field-conducting material 8, said layer 11 preferably incorporating quartz meal. The incorporation of quartz meal renders the layer more resistant to temperature and at the same time provides a surface of high mechanical strength inside the coil which protects it from damage. This additional layer 11 is applied at a time when the resin compound 8 containing the iron powder is about to set. The two diiferently constituted cast res-in compounds 8 and 11 will thus be prevented from mixing and at the same time a homogeneous transition from the material 8 to layer 17. will be achieved, ensuring a bond of maximum strength therebetween.

What I claim is:

l. A method of inductively heating relatively thin sheet, strip and the like work of large surface area which consists in relatively displacing the work and at least one surrounding flat induction coil which is partially embedded in a magnetic field-conducting material, in the direction of the axis of the coil, producing a plurality of separate magnetic fields each about a different conductor-section of the coil, said sections being parallel to one another and extending transversely of the direction of the said relative movement, the permeability of the material and the spacing of said sections being such as to ensure the effective separation of the said fields, and by a supply frequency not exceeding 10,000 cycles per second inducing in the work separate currents flowing in the plane of the work without creating a resultant longitudinal magnetic field by the combined effect of all the said conductor-sections. V

2. An apparatus for inductively heating relatively thin sheet, strip and the like work comprising at least one flat induction coil adapted to receive the work and including conductor sections adapted to lie on each side of the work with those on either side parallel with one another and parallel with those on the opposite side and including joining sections adapted to lie in the region of the work edges, the said conductor sections being spaced so that the distance d between the successive conductor edges located on each side of the coil is at least 26 field-conducting, magnetically-conductive material arranged in supporting relationship to said coil sections in such manner as to leave at least the effective sides of the said sections which face the work uncovered by said material, the resultant permeability a res of the said material being not more than 10.

3. A fiat-shaped induction coil adapted to receive thin sheet, strip and the like work for inductively heating it, said coil including conductor sections adapted to lieon each side of the work with those on either side parallel with one another and parallel with those on the opposite side and including joining sections adapted to lie in the region of the work edges, the said conductor sections being spaced so that the distance between the successive conductor edges located on each side of the coil is at least 25, the said conductor sections being embedded in a field-conducting magnetically-conductive material comprising a hardenable plastic compound incorporating finely divided iron powder so as to leave the effective sides of the said sections which face the work uncovered by the said material, the resultant permeability a res of the said material being not more than 10.

4. An induction coil as claimed in claim 3, comprising an interior layer covering the said faces of the said sections which are intended to face the work, said layer comprising a hardenable plastic incorporating non-ferromagnetic, electrically non-conducting, anti-abrasive material.

5. An induction coil as claimed in claim 3, comprising an interior layer covering the said faces of the said sections which are intended to face "the work, said layer 5 comprising a hardenable plastic incorporating quartz meal.

6. A method of inductively heating relatively thin sheet, strip or the like work of large surface area which consists in producing a plurality of separate magnetic fields each about a difierent conductor-section of at least one flat induction coil which embraces the work with the said sections extending transversely of the work and being partially embedded in a substance which conducts the magnetic fields, travelling the strip and the coil relatively to one another with the coil embracing the strip, and by a supply frequency not exceeding 103000 cycles per sec- UNITED STATES PATENTS 2,381,246 Baker et a1. Aug. 7, 1945 2,381,278 Gregory et a1. Aug. 7, 1945 2,777,041 Dustman Jan. 8, 1957 2,888,541 Netzer May 26, 1959 2,897,328 Alf et a1. July 28, 1959

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