GB1596989A - Direct-current electrical heat-treatment of continuous metal sheets in a protective atmosphere - Google Patents

Description

(54) DIRECT-CURRENT ELECTRICAL HEAT-TREATMENT OF CONTINUOUS METAL SHEETS IN A PROTECTIVE ATMOSPHERE (71) We, VALJIM CORPORATION of 810 Union Avenue, Bridgeport, Connecticut 06607, United States of America; a corporation organized and existing under the laws of the State of Connecticut, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a method of electrically heat-treating a continuous travelling band of sheet metal of substantial width relative to its thickness as it passes through a protective atmosphere in a confining chamber for the latter and the band, with minimum energy loss, as well as to an apparatus for heat-treating a travelling band of sheet metal as it passes along a path including a portion extending from an inlet end to an outlet end of a confining chamber containing a reducing gas.

The present invention is an improvement on the method and apparatus disclosed in U.S. Patent No. 3,792,684 and Canadian Patent No. 1,004,303. Such prior art patents from the technical background on which the present invention is based.

Moreover, the present invention comprises an improvement of the method and apparatus disclosed in U.K, Patent No. 660,659.

While this patent discloses the resistive heating of electrically conductive wire in successive stages of shorter lengths to compensate for the increased resistivity of the wire in its travel from the inlet to the outlet of the system, with the use of alternating current energy, serious problems arise when such an expedient is adapted to the resistive heating of lengths of conductive material of wide area or those having a substantial width to thickness ratio, when such are enclosed within metallic chambers for protective gases used in the heat treatment of the material.

The instant invention seeks to overcome these problems by use of direct-current energy which prevents the induction of any current in the walls of the sheet iron ductwork defining the chambers which surround the travelling band, thereby increasing the efficiency of the installation as well as minimizing the initial cost and the maintenance costs thereof.

The direct current is applied to at least three electrified pulleys, each successive pulley having an opposite polarity.

The use of direct-current makes possible the placement of the sheet iron ductwork close to the travelling band, so that the radiant heat emanating from the latter is confined within a relatively small space and the quantities of gas which react with the travelling sheet and/or the coatings formed thereon may be reduced in quantity, as a consequence. Thus, the chambers for housing the travelling metal, which require no source of extraneous heat, are characterized by minimal thermal inertia and are capable of rapid shutdowns and re-starting operations, without substantial loss of time, energy and gases.

It is the object of the present invention to provide a highly compact and economical installation for the heat treatment of continuous lengths of metal bands or sheets for the purpose of imparting accurately controlled degrees of heat thereto for the purpose of modifying the physical and/or metallurgical properties of the metal, which iastallation may be complemented by additional apparatus for tempering, annealing or chemically treating the metal for further processing such as quenching, pickling or coating procedures.

It is a further object of the invention to provide an apparatus for the heat treatmen of continuous lengths of metal bands or sheets, which occupies a minimum of floor area, which may be built up of low cost modular structural units, and which may be maintained in service for maximum periods of time without costly shut-downs when interruptions or break-downs occur.

It is a further object of the invention to provide an installation which is of particular utility in the heat treatment of consinuous lengths of ferrous metal in the form of sheets, bands or strips, which are heat treated preparatory to the coating thereof with another metal such as aluminum, zinc, tin or the like, which procedure requires the effective cleaning of the surface of the metal to remove the oxides therefrom. This requires the passage of the continuous length of metal through chambers containing a protective gaseous atmosphere which is nonoxidizing or reducing in chemical behavior, which treats the travelling length of metal in the course of its advance towards a molten metal coating bath. The protective gas is introduced into the chambers for travel in counter-current relation to the direction of travelling length of metal, to increase the efficiency of the system as the metal is first heated accurately to the desired temperature, followed by the cooling thereof and the hot dipping of the metal for the application of the coating thereto, in the course of its passage from the inlet to the outlet of the apparatus.

The invention contemplates the economical heat treatment of continuous lengths of ferrous metal preparatory to the passage thereof through coating baths of molten metal which are treated for the purpose of clearing the metal of objectionable oxide layers, with or without the annealing of the metal. Alternatively, the heat treatment of the continuous lengths of ferrous metal may be executed preparatory to the passage of the critically heated metal through quenching baths, if tempering characteristics are sought to be imparted to the metal, or other liquid baths such as pickling solutions and the like.

According to the present invention there is provided a method of electrically heattreating a continuous travelling band of sheet metal of substantial width relative to its thickness as it passes through a protective atmosphere in a confining chamber for the latter and the band with minimum energy loss, which comprises: (a) conducting the band from a source of supply continuously at a predetermined speed over a plurality of successive spaced guide rollers forming a path including a zig-zag portion passing through the chamber; b) applying direct-current potentials to at least some of said rollers, these rollers being of electrically conductive metal and any between them being non-conductive and thereby to successive lengths of the band as it travels along said path to generate resistive heating therein by the Joule effect without need for any other sources of heating energy within said chamber, (c) arranging the spacing between said conductive rollers so that it decreases progressively along said path towards the outlet end of the chamber to compensate for the increase in the electrical resistivity of the band with the increase in temperature thereof in the course of its travel, thereby to equalize the Joule effect in the successive lengths of the band, and (d) introducing a gas into said chamber adjacent to the outlet end for exhaust ad jacent to the inlet end, thereby effecting heated gas flow in counter-current relation relative to the travel of the band of sheet metal therethrough, said gas being heated solely by the heat radiated from said band.

Also in accordance with the present in vention there is provided an apparatus for heat treating a travelling band of sheet metal as it passes along a path including a portion extending from an inlet end to an outlet end of a confining chamber containing a reducing gas, which comprises (a) a plurality of successive guide rollers for guiding the travelling band of metal along a path including a zig-zag portion from said inlet end to said outlet end, some of said rollers being electrically conductive and having non-conductive rollers arranged between them, (b) a direct-current power supply with conductive connections therefrom for applying opposite polarities to at least three of said conductive rollers along successively displaced points along said path, the rollers acting as electric terminals for the metal band which is adapted to travel therebetween and to be heated by the electric current conducted therethrough, (c) the spacing of the conductive rollers along said path decreasing progressively towards the outlet end, (d) sealed housings enclosing all the conductive rollers except one adjacent the inlet end, and (e) sheet metal ducts interconnecting said sealed housings, to form said chamber, said ducts having walls encompassing said band in close proximity thereto to form a constricted passage for the reducing gas, whereby tile latter is heated solely by the radiant heat emanating from said travelling band of metal.

The present invention will be further illustrated, by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a schematic cross-sectional view of the apparatus in accordance with the invention, including a block diagram of the power supply, indicating the line of travel of a continuous sheet of metal as it passes through the heat treating stages, cooling stages, and ultimately a coating bath; Fig. 2 is a sectional view of another embodiment of the coating bath at the outlet end of the apparatus, having a hinged guide pulley which may be lifted therefrom during shut-down periods; Fig. 3 is an enlarged sectional view of the pulley supplied with a positive potential with the abrasive cleaning bar cooperating therewith; Fig. 4 is a horizontal sectional view along line A-A of Fig. 3; Fig. 5 is a sectional view along line B-B of Fig. 2; and Fig. 6 is a graph showing the relationship between the temperature and the electrical resistivity of a low carbon steel strip.

In the schematic diagram of the system shown in Fig. 1, a three-phase transformer 23 is shown connected to a three-phase power line L1, L2 and L3 which reduces the line voltage of the latter to about 100 volts in the secondary windings, and the output of which is fed to a thyristor rectifier bank which rectifies the current supplied by the transformer. Any other type of rectifier which produces relatively ripple-free directcurrent can be used for this purpose, and the rectifier elements may be other than thyristors, for example, silicon controlled recifiers, Zener diodes, selenium cells, etc. Such power conversion systems are well known in the art.

The direct-current output leading from the rectifier is connected to three guide or con veyor rollers of the system. As shown in Fig. 1, the negative main P is connected to rollers 2 and 7 and the positive main P + is connected to roller 5. The guide rollers 5 and 7 are enclosed in sealed housings H, to which are connected the chambers or ducts 13b, 13c, and 13d or relatively small cross-section, with the walls thereof in close proximity to the travelling sheet 1, as shown in Fig. 4.

Additional guide pulleys 3, 4 and 6 are provided in alternating arrangement with the pulleys 2, 5 and 7 to reverse the direction of the sheet of metal 1 as it is guided in zig-zag paths around electrified pulley 2 and over pulley 3 into the series of ducts of the reducing chamber. In addition, pulleys 8 and 9, with housings H and ducts 14a, 14b and 14c, are provided to guide the heated sheet of metal through these cooling chambers and into the tank 21 whereat is provided another guide pulley 10 over which the coated metal passes upwardly for final dis position, as is well known in the art. While the ducts are disposed vertically in the illu strated embodiment, they may be horizontal or inclined, in dependence upon the available space therefor and the plant layout. The vertical arrangement of the cooling and reducing chambers requires a minimum amount of floor space.

Rollers 3, 4 and 6 are coated with a layer 16 of insulating material, preferably of a ceramic composition, in order to avoid sparking between the metal sheet and the surface of said pulleys because the sheet would otherwise be short-circuited in the course of its contact with half of the preiphery of these pulleys between the guide rollers which are supplied with potentials of opposite polarity.

Metal sheet I is guided under pulley 2 supplied with a negative potential past in sulated guide pulleys 3 and 4 and becomes heated when it makes contact with pulley 5 supplied with a positive potential. The sheet becomes progressively heated as it advances to wards the outlet end of the system and reaches its maximum temperature as it approaches roller 7 supplied with a negative potential.

The portion of the sheet which remains in a normal atmosphere before its entry into the reducing chambers at slot 20, per units the residual oil to burn off before the sheet enters the first insulated duct 13a.

The latter is filled with a reducing gas which is fed into the ductwork through inlet 15 adjacent to the outlet end of chamber 14c, and which is heated by heat abstracted from the sheet 1 passing through the cooling ducts as well as the heated sheet. Under certain conditions, the sheet is allowed to oxidize slightly before it enters the first duct 13a, because the reduced oxide layer serves as an excellent base for the subsequent coating operation.

After the first stage of heating in the passage of the sheet through ducts 13a and 13b, the sheet enters the second stage after it passes over pulley 5 supplied with a positive potential and past guide pulley 6 to the pulley 7 supplied with a negative potential.

As stated above, the sheer attains its maxi mum temperature shortly before contacting pulley 7 and after passing through the housing H enclosing this pulley, the sheet enters the chamber 14a which is the first cooling section of the reducing chamber, wherefrom it passes under pulley 8 and over pulley 9 towards the molten coating bath in pot 21 without being exposed to the atmosphere.

In order to maintain good electrical contact between the travelling sheet of metal and the conductive rollers 2, 5 and 7, which become coated with impurities such as carbonized oil, ferric or ferrous oxide, etc., abrasive bars 17 are provided adjacent these rollers, with an arcuate cleaning surface conforming to the lateral surface of the latter, with means for pressing these bars against the faces of the electrified rollers.

In Fig. 3 is shown an enlarged view of the pneumatic or hydraulic cylinder 18 which may be operated periodically to clear the lateral surfaces of the electrified rollers from these impurities.

The introduction of the reducing gases through inlet 15 in counter-current relation to the travel of the sheet towards the exit orifice 20, results in a safe installation and one which is economical in operation. The close spacing between the walls of the reducing ducts 13 and cooling chambers 14, with respect to the travelling sheet 1, as clearly shown in Figs. 4 and 5, gives rise to a relatively high velocity of the reducing gases. The high velocity of the gas permits the use of a gas containing less than 10% hydrogen, in contradistinction to conventional reducing furnaces which operate with a hydrogen concentration of 25% to 750/c.

The low concentration of hydrogen offers several advantages such as the elimination of the need for the use of an ammonia dissociator which may be replaced with an exothermic gas generator which is simpler and cheaper in operation. Also, the use of a gas containing less than 10"i, hydrogen eliminates the danger of explosion in case some oxygen accidentally enters into the chamber, because hydrogen is not flammable when diluted to a concentration as low as 10%. This also eliminates the need for prolonged purging during start-up and stoppages.

The relatively close spacing between the travelling sheet of metal and the walls of the chambers is desirable for the purpose of utilizing the reducing gases at maximum efficiency, for only the portions of the latter in contact with the sheet react with the surfaces of the metal, as described above.

However, such close spacing gives rise to inductive currents in the walls of the ductwork when such are of conventional sheet metal and when wide sheets are electro resistive heated with alternating currents, resulting in energy losses. The saving in energy by the use of direct current in accordance with the invention is substantial, as illustrated by the following example.

When a travelling band or strip 30" wide and 0.030" thick is subjected to an alternating current of 333 amperes it will reach a temperature of 8000C. at the exit from the reduction chamber. When direct current is used for the same purpose, a current of 256 amperes is sufficient to obtain the same temperature at the exit from said chamber, while the speed of the strip in both cases remains unchanged. This represents a saving of 230/,, which with the use of alternating current would be lost because of the aforementioned inductive effect.

There is still another difficulty created by the use of alternating current. Upon experimentation it has been found that when a strip of metal is heated by the resistive method using alternating current power, the heat distribution across the strip width is unequal. The edges of the strip become overheated while the center of the strip remains at lower temperature. The severity of this temperature difference is proportional to the width of the strip-the wider the strip, the greater the temperature difference between the center and its edges. This "edge effect" is also proportional to the frequency of the alternating current; the higher the frequency, the more pronounced is the "edge effect".

Therefore, the use of direct-current energy results in both energy savings and an improved sheet having uniform characteristics over its entire area.

As is evident from Fig. 1, the first heating stage between electrified rollers 2 and 5 is much longer than the second heating stage between electrified rollers 5 and 7, in fact about twice as long. This results in a more efficient utilization of the power supply, which may be explained by reference to Fig. 6.

It is a well known fact that the resistivity of a conductor is affected by its temperature.

This relationship is shown in the graph in Fig. 6 where the resistivity of low-carbon steel is plotted against its temperature. This phenomenon makes possible an increase in efficiency of the process executed by the system shown in Fig. 1. Thus, the travelling band or strip, reaches the first electrified pulley 2 at room temperature and progressively increases its temperature so that it reaches the second positively electrified pulley 5 at a temperature of about 500"C. It continues its travel and reaches the last electrified pulley 7 at a temperature of about 10000C. From the graph in Fig. 6 it can be seen that at room temperature the resistivity of the strip is about 0.18 Ohms/mm2/m, and at 500"C.

the resistivity is 0.58 Ohms/mm2/m, which averages 0.38 Ohms/mm2/m. In the second stage, the initial resistivity is 0.58 Ohms/ mm2/m, and at the end thereof it is 1.17 Ohms/mm2/m at 10000C. Consequently, the average resistivity of the strip in the second stage is 0.88 Ohms/mm2/m. Therefore, if both stages were to have the same resistance, then their length relationship should be 0.80:0.38 or the first stage should be 2.1 times the length of the second one. By following the methods described above, it is possible to prdduce a galvanized strip 40" wide and 0.030" thick with a power con sumption of less than 200 KW/ton, which is a significant saving in energy when com pared to a conventional process.

As shown in Fig. 4, the reducing duct 13c may be lined with an insulating layer 13', whereas the cooling chambers 14 are devoid of such a lining to enhance the cooling opera tion. This expedient contributes to the attainment of the desirable characteristic of the invention, namely, its low thermal inertia.

It is therefore economically feasible to operate the reduction chambers intermittently. How ever, during a galvanizing process it is necessary to maintain the metal contained in the zinc bath 21 in a molten state, during brief shut-down periods. But it is not ad visable to maintain the relatively thin strip submerged in the molten zinc because the zinc will dissolve it, and re-threading of the chamber becomes necessary. Consequently, the final pulley 10 is rotatably mounted at the lower end of discharge conduit C, which in turn is hingedly mounted by means of hinge 22 to the lower end 19 of cooling duct 14c (Figs. 2 and 5). This construction permits the lifting of the guide pulley to an inoperative position during shut-down periods, as indicated in dotted lines in Fig. 2. In operation, the flanged lower end 19 is clamped to a mating flange on the discharge conduit C by means of a plurality of "C" clamps.

The reducing gas fed into inlet 15 is preferably admitted at a slight over-pressure above atmospheric, of about 1" water column.

WHAT WE CLAIM IS:- 1. A method of electrically heat-treating a continuous travelling band of sheet metal of substantial width relative to its thickness as it passes through a protective atmosphere in a confining chamber for the latter and the band, with minimum energy loss, which comprises: (a) conducting the band from a source of supply continuously at a predetermined speed over a plurality of successive spaced guide rollers forming a path including a zig-zag portion passing through the chambers; (b) applying direct-current potentials to at least some of said rollers, these rollers being of electrically conductive metal and any between them being non-conductive and thereby to successive lengths of the band as it travels along said path to generate resistive heating therein by the Joule effect without need for any other sources of heating energy within said chamber, (c) arranging rhe spacing between said conductive rollers so that it decreases progressively along said path towards the outlet end of the chamber to compensate for the increase in the electrical resistivity of the band with the increase in temperature thereof in the course of its travel, thereby to equalize the Joule effect in the successive lengths of the band, and (d) introducing a gas into said chamber adjacent to the outlet end for exhaust adjacent to the inlet end, thereby effecting heated gas flow in counter-current relation relative to the travel of the band of sheet metal therethrough, said gas being heated solely by the heat radiated from said band.

2. A method as claimed in claim 1, wherein the gas which is introduced into said chamber adjacent to the outlet end is a reducing gas which is preheated by the outgoing band, and which in turn is simultaneously cooled by the gas preparatory to passage through a metal coating bath.

3. A method as claimed in claim 2, wherein the reducing gas has a hydrogen content no greater than 10%.

4. A method as claimed in claim 2, wherein the band undergoing heat-treatment is ferrous sheeting preparatory to galvanizing, which includes leading the treated sheet into a molten zinc coating bath without exposure to the atmosphere, following the resistive heating and the attainment of the maximum treating temperature at the conclusion thereof, and the subsequent cooling thereof by the incoming reducing gas.

5. A method as claimed in claim 4, wherein the maximum treating temperature at the conclusion of the resistive heating is approximately 1000"C.

6. A method as claimed in claim 4, wherein the reducing gas has a hydrogen content no greater than 10%.

7. An apparatus for heat-treating a travelling band of sheet metal as it passes along a path including a portion extending from an inlet end to an outlet end of a confining chamber containing a reducing gas, which comprises: (a) a plurality of successive guide rollers for guiding the travelling band of metal along a path including a zig-zag portion from said inlet to said outlet end, some of said rollers being electrically conductive and having non-conductive rollers arranged between them, (b) a direct-current power supply with conductive connections therefrom for applying opposite polarities to at least three of said conductive rollers along successively displaced points along said path, the rollers acting as electric terminals for the metal band which is adapted to travel therebetween and to be heated by the electric current conducted therethrough, (c) the spacing of the conductive rollers along said path decreasing progressively towards the outlet end, (d) sealed housings enclosing all the conductive rollers except one adjacent the inlet end, and (e) sheet metal ducts interconnecting said sealed housings, to form said chamber, said ducts having walls encompassing said band in close proximity thereto to form a constricted passage for the reducing gas, whereby the latter is heated solely by the radiant heat emanating from said travelling band of metal.

S. An apparatus as claimed in claim 7, wherein there are three conductive rollers and, the spacing between the first and second conductive rollers is about double that between the second and last conductive rollers.

9. An apparatus as claimed in claim 7, including a movable bar with an arcuate cleaning surface conforming to the curvature of the lateral surface of each conductive

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (15)

**WARNING** start of CLMS field may overlap end of DESC **. submerged in the molten zinc because the zinc will dissolve it, and re-threading of the chamber becomes necessary. Consequently, the final pulley 10 is rotatably mounted at the lower end of discharge conduit C, which in turn is hingedly mounted by means of hinge 22 to the lower end 19 of cooling duct 14c (Figs. 2 and 5). This construction permits the lifting of the guide pulley to an inoperative position during shut-down periods, as indicated in dotted lines in Fig. 2. In operation, the flanged lower end 19 is clamped to a mating flange on the discharge conduit C by means of a plurality of "C" clamps. The reducing gas fed into inlet 15 is preferably admitted at a slight over-pressure above atmospheric, of about 1" water column. WHAT WE CLAIM IS:-

1. A method of electrically heat-treating a continuous travelling band of sheet metal of substantial width relative to its thickness as it passes through a protective atmosphere in a confining chamber for the latter and the band, with minimum energy loss, which comprises: (a) conducting the band from a source of supply continuously at a predetermined speed over a plurality of successive spaced guide rollers forming a path including a zig-zag portion passing through the chambers; (b) applying direct-current potentials to at least some of said rollers, these rollers being of electrically conductive metal and any between them being non-conductive and thereby to successive lengths of the band as it travels along said path to generate resistive heating therein by the Joule effect without need for any other sources of heating energy within said chamber, (c) arranging rhe spacing between said conductive rollers so that it decreases progressively along said path towards the outlet end of the chamber to compensate for the increase in the electrical resistivity of the band with the increase in temperature thereof in the course of its travel, thereby to equalize the Joule effect in the successive lengths of the band, and (d) introducing a gas into said chamber adjacent to the outlet end for exhaust adjacent to the inlet end, thereby effecting heated gas flow in counter-current relation relative to the travel of the band of sheet metal therethrough, said gas being heated solely by the heat radiated from said band.

2. A method as claimed in claim 1, wherein the gas which is introduced into said chamber adjacent to the outlet end is a reducing gas which is preheated by the outgoing band, and which in turn is simultaneously cooled by the gas preparatory to passage through a metal coating bath.

3. A method as claimed in claim 2, wherein the reducing gas has a hydrogen content no greater than 10%.

4. A method as claimed in claim 2, wherein the band undergoing heat-treatment is ferrous sheeting preparatory to galvanizing, which includes leading the treated sheet into a molten zinc coating bath without exposure to the atmosphere, following the resistive heating and the attainment of the maximum treating temperature at the conclusion thereof, and the subsequent cooling thereof by the incoming reducing gas.

5. A method as claimed in claim 4, wherein the maximum treating temperature at the conclusion of the resistive heating is approximately 1000"C.

6. A method as claimed in claim 4, wherein the reducing gas has a hydrogen content no greater than 10%.

7. An apparatus for heat-treating a travelling band of sheet metal as it passes along a path including a portion extending from an inlet end to an outlet end of a confining chamber containing a reducing gas, which comprises: (a) a plurality of successive guide rollers for guiding the travelling band of metal along a path including a zig-zag portion from said inlet to said outlet end, some of said rollers being electrically conductive and having non-conductive rollers arranged between them, (b) a direct-current power supply with conductive connections therefrom for applying opposite polarities to at least three of said conductive rollers along successively displaced points along said path, the rollers acting as electric terminals for the metal band which is adapted to travel therebetween and to be heated by the electric current conducted therethrough, (c) the spacing of the conductive rollers along said path decreasing progressively towards the outlet end, (d) sealed housings enclosing all the conductive rollers except one adjacent the inlet end, and (e) sheet metal ducts interconnecting said sealed housings, to form said chamber, said ducts having walls encompassing said band in close proximity thereto to form a constricted passage for the reducing gas, whereby the latter is heated solely by the radiant heat emanating from said travelling band of metal.

S. An apparatus as claimed in claim 7, wherein there are three conductive rollers and, the spacing between the first and second conductive rollers is about double that between the second and last conductive rollers.

9. An apparatus as claimed in claim 7, including a movable bar with an arcuate cleaning surface conforming to the curvature of the lateral surface of each conductive

roller and means for periodically contacting the cleaning surface of said bar with its conductive guide roller to maintain the guide roller in a clean and smooth condition.

10. An apparatus as claimed in claim 7, wherein the non-conductive rollers have an insulating covering of ceramic material.

11. An apparatus as claimed in claim 7, including further non-conductive rollers, sealed housings therefor and ducts therebetween disposed beyond the last electric terminal guide roller for the passage of the metal band therethrough to effect the cooling thereof, and an inlet for the reducing gas in the last one of said ducts for flow therethrough in countercurrent relation to the direction of travel of said metal band.

12. An apparatus as claimed in claim 11, including a molten metal bath at the outlet end of the final duct for receiving the metal band issuing therefrom.

13. An apparatus as claimed in claim 12, including a discharge conduit pivotally mounted to the lower end of said final duct above said molten metal bath, a guide pulley mounted at the lower end of said discharge conduit, and means for selectively lifting said pulley from said bath or lowering it thereinto for the execution of the coating operation on the metal band travelling through the apparatus.

14. An apparatus for heat treating a travelling band of metal, substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

15. A method of electrically heat-treating a continuous travelling band of sheet metal of substantial width relative to its thickness as it passes through a protective atmosphere in a confining chamber for the latter and the band, with minimum energy loss, as claimed in any one of claims 1 to 6, substantially as hereinbefore described and illustrated.