Induction heating apparatus and method for heating metal strips and slabs

This disclosure relates to an apparatus for and a method of electrical inductance heating of metal strips and slabs. The heating method involves a plurality of pairs of individual electrical coils exposed on opposite sides of a path of movement of a metal strip or slab wherein the coils are elongated longitudinally of the path of movement of the metal member, such as a strip or slab, whereby the pattern of heating is in the way of stripes. Further, to eliminate any pulsing of current within the strip or slab being heated, each coil is spaced from a transversely adjacent coil a distance corresponding substantially to the effective heating width of that coil. The coils of a next longitudinal adjacent set of coils are transversely offset relative to the coils of the first mentioned set of coils so that each coil of the second set is aligned with a space between coils of the first set. This provides for a heating of the strip or slab in stripes and prevents overheating of the edges of the strip or slab. Further, the energization of the individual coils may be controlled in accordance with the existing temperature of that area of a strip or slab to be heated by the coils so as to heat a non-uniformly heated strip or slab to a uniform overall temperature.

This invention relates in general to new and useful improvements in 
induction heating of metal strips and slabs formed of copper, aluminum, 
stainless steel, magnesium, etc. and most particularly to the induction 
heating of ferrous slabs at temperatures above Currie to a slab rolling 
temperature. 
BACKGROUND OF THE INVENTION 
Over forty years ago there was developed a method of transverse flux 
induction heating wherein induction heating coils were arranged in 
side-by-side relation transversely of a path of movement of sheet metal. 
These coils were arranged in pairs on opposite sides of the path of such 
moving sheet metal. This transverse flux induction heating had a number of 
deficiencies including the overheating of strip edges only. In transverse 
flux induction heating, little or no current flows under the slot area 
between adjacent coils while most of the current flow is under the poles 
of the coils. As the strip starts to move, the strip becomes a conductor 
flowing through a field and a second current flows. Due to the second 
current there is loading under the poles and no loading under the slots. 
In other words, the loading is pulsating. A pulse can be made up of a 
fundamental plus harmonics. The wider the strip, the lower the inductance 
at the edges of the strip. The harmonics generated will flow where there 
is a minimum inductance which is along the edges. 
Further, with transverse flux heating of steel, there is up to 80% 
efficiency. Normal induction heating is only 40% efficient or less. 
The biggest problems with using transverse flux induction heating have 
been: 
1. The edges have overheated and must be sheared off as scrap. 
2. Overheating of the edges increase with strip speed. 
3. Prior transverse flux induction heating setups could only accommodate 
one strip width. 
4. The thickest slab that could be heated was less than 1/2 inch. 
GENERAL DISCUSSION OF INVENTION 
In accordance with this invention it is proposed to utilize the advantages 
of transverse flux induction heating while substantially eliminating the 
disadvantages of such heating. Most particularly, in accordance with this 
invention, electrical heating coils are arranged to extend longitudinally 
of the path of the moving strip or slab. Next, the heating coils are 
disposed in spaced relation transversely of the path of movement of the 
strip or slab being heated with the effective width of a coil being 
substantially equal to the spacing between coils. The net result is a 
plurality of longitudinally extended heated areas separated by unheated 
areas in alternating relation transversely of the path of movement of the 
strip or slab. This heated and unheated arrangement is compensated for by 
providing the coils in transverse sets with the coils in adjacent sets 
being offset from one another. Thus one set of coils provide heated and 
unheated areas while the next adjacent set of coils will provide for the 
heating of the unheated areas and the non-heating of the previously heated 
areas. This arrangement provides for many possibilities including the 
possibility that the number of coils in a transverse set may be divided 
into a number of adjacent transverse arrangements so as to permit either 
the heating of a single wide strip or slab or the simultaneous heating of 
several strips or slabs. Further, by providing suitable control devices, 
the amount of heating which occurs may vary transversely of the moving 
strip or slab to provide for an overall uniform heating of a strip or 
slab. 
With the above and other objects in view that will hereinafter appear, the 
nature of the invention will be more clearly understood by reference to 
the following detailed description, the appended claims, and the 
accompanying drawings.

DESCRIPTION OF PRIOR ART 
Reference is now made to the prior art showing of FIGS. 1-3 wherein there 
is illustrated a typical transverse flux induction heating apparatus. This 
apparatus is generally identified by the numeral 10. The apparatus 10 is 
formed by a plurality of pairs of electrical heating coils 12 with there 
being a heating coil 12 of each pair on opposite sides of a pair 14 for a 
strip or slab to be heated to move. The coils 12 are elongated and of a 
length generally corresponding to the width of a strip or slab to be 
heated. The coils 12 are disposed in closely adjacent parallel relation. 
As is schematically illustrated in FIG. 2, each group of coils 12 on 
opposite sides of the path 14 is carried by a laminated pole structure 16. 
The pole structure 16 includes individual poles 18 within the coils 12 
with the coils 12 being defined by windings of electrical conductors, 
preferably hollow copper conductors 20. Current flowing to the coils 12 is 
such that the opposing pole pieces 18 are opposite polarity, i.e. one pole 
piece 18 being a N pole piece and the other pole piece 18 being a S pole 
piece. 
Referring now to FIG. 3, it will be seen that there is basically a slot 22 
between adjacent coils 12. Further, the flux flow carried by each of the 
coils 12 is in the same direction of each of the slots 22. 
It will also be seen from FIG. 3 that the strip or slab being heated is 
wider than the coils 12 and materially wider than the pole piece 18. 
Transverse flux heating was developed over 40 years ago by R. M. Baker. 
Transverse induction heating, as illustrated in FIGS. 1-3 heats with an 
efficiency as high as 80% with the frequency of the electrical current 
supply of the coils 12 being low enough for the flux to pass through the 
slab or strip. Through heating starts from the first instance. Another 
reason for the fast and efficient heating is that the coils are mounted in 
slots in steel laminations. The strip or slab to be heated can be fed into 
the apparatus 10 either through the bottom or the top if the apparatus 10 
is vertically disposed, or horizontally if the apparatus 10 is 
horizontally disposed. 
Due to the current flowing in the coils and the slots, flux passes from N 
to S in the laminations and passes through the strip and induces the 
voltage that causes current to flow and heat the strip. The flux between 
the slots however, cancel and therefore little if any voltage generates 
the strip between the slots. 
The strong field from N to S due to motor action forces the current to flow 
between the coils 12 where the field is weak. It is known that if the 
strip stops it will melt in the slot area if the power is not removed 
immediately. 
When the strip or slab moves, the heating current is heavy between N and S 
and nearly 0 between the slots. This means that the load current is pulsed 
rather than being steady. Also, since there are a plurality of poles that 
are producing heat, the pulse has a high peak and almost 0 between pulses. 
A pulse can be analyzed into a fundamental plus higher frequency 
harmonics. 
The inductance in the strip is miminal along the edges. High frequency 
flows where the inductance is minimal which causes the edges to overheat. 
The higher the speed and movement of the strip or slab between the coils, 
the higher the pulse rate and pulse height. 
DESCRIPTION OF THE INVENTION 
The apparatus 10 illustrated in FIGS. 1-3 can be beneficially utilized in 
accordance with this invention if the strip or slab to be heated is moved 
longitudinally of the coils 12 without overheating the edges thereof. 
While this would eliminate the pulsing action, it would result in the 
stripe heating of the strip or slab with areas between adjacent heating 
stripes being unheated. 
In view of the strip heating, a proper induction heating apparatus in 
accordance with this invention would be formed of sets of heating coils 
arranged as generally illustrated in FIG. 4. The heating coils as 
illustrated in FIG. 4 will be of the same general construction as the 
heating coils 12 illustrated in FIGS. 1-3 and would be identified by the 
numeral 30. Each heating coil 30 would, of course, include a multiple 
winding 32 and a laminated steel pole piece 34. 
The coils 30 are arranged in pairs, one coil on each side of a path of 
movement of a strip or slab S. In FIG. 4, it is illustrated only in the 
coils 30 which are disposed above the path of the strip or slab S. 
As is clearly shown in FIG. 4, the coils 30 are arranged in longitudinally 
adjacent sets and extend longitudinally of the path of the strip or slab 
S. Each set includes at least two coils 30 separated by a space 36. The 
width of the space 36 between coils is substantially equal to the 
respective heating width of a coil 30. 
Continuing to refer to FIG. 4, it will be seen that in a next 
longitudinally adjacent set of coils 30, these coils are longitudinally 
aligned with the spaces 36 of the next adjacent coil sets. Further, it 
will be seen that sets of coils are repeated so as to provide multiple 
heating of all areas of the metal strip or slab S. 
In FIG. 4, each set of coils 30 total four in number although a set of two 
coils only could be required for a particular width of strip or slab S. 
Thus, with the apparatus shown in FIG. 4, it is possible to simultaneously 
heat two single width strips or slabs or one double width strip or slab. 
Further, it is possible to heat a single width strip or slab. In addition, 
although it is not specifically shown, it is to be understood that each 
transverse set of coils could include six, eight or more coils depending 
upon the desired width of the strip or slab to be heated. Further, while 
there has been illustrated only four sets of coils, depending upon the 
desired temperature rise, there could be six, eight, ten or more sets of 
coils arranged longitudinally of the path of movement of the strip or slab 
S. 
Referring now to FIG. 5, it will be seen that the heating of the strip or 
slab S will be in stripes and at any moment only certain portions of the 
areas of the stripes will be heated due to the arrangement of the coils 
30. Of course, it is to be understood that as a strip or slab S passes 
under one of the coils 30, that area of the strip or slab aligned with the 
coil will be continuously heated. The total heating will be equal to the 
capacity of the coils 30 and the number of pairs of sets of coils. It will 
be seen that the transverse outermost coil in each transverse arrangement 
of coils will be spaced from the edge of the path of the strip or slab S 
so as to provide for proper heating of the edge of the strip or slab being 
heated. 
It is to be understood that utilizing 60 hertz current, various non-ferrous 
metals and ferrous metals above the Currie temperature may be efficiently 
heated without overheating of edges. 
Returning once again to FIG. 4, it will be seen that two longitudinally 
adjacent sets of coils 30 may be energized from a single power source 38. 
This power source, may be readily controlled between an on or off 
condition so as to control the amount of heating of a strip or slab S. 
Further, the power source 38 could be in the form of a generator so as to 
produce a frequency other than, normally lower than, a 60 hertz frequency. 
A particular slab heating problem now exists in the industry. These slabs 
are cast of steel having a width as wide as 60 inches and a thickness on 
the order of 2 inches. The cast slabs are not immediately ready for 
rolling so as to reduce the thickness thereof and as a result, these slabs 
pass through very elongated tunnels which are of an insulated construction 
and which are gas heated. The tunnels are expensive to construct and 
further the gas heating is very expensive. The slabs may be electrically 
induction heated as long as they are above the Currie temperature. It is 
acknowledged here that it is feasible to heat a 2 inch thick steel slab 
utilizing 60 hertz current. However, some companies are looking into 
heating slabs of a width up to 120 inches and a thickness greater than 2 
inches such as a 4 inch or 6 inch thickness and even greater. It has been 
found by me that when the slab is of a thickness greater than 2 inches, 
the induced current does not sufficiently penetrate the thickness of the 
slab and thus the slab is heated from the outside towards the center by 
conduction with a great heat loss. By reducing the frequency of the 
current supply to the induction coils, it is possible for the reduced 
current to penetrate the slab generally to the center of such slab so as 
to provide a more uniform heating of the slab with a minimal heat loss. 
Therefore, if generators are used as the power source 38, the frequency of 
the current supplied to the coils 30 may be varied to that which is the 
most efficient for the particular thickness of the slab. 
It has also been found that because of various factors, the slab which is 
to be heated from above the Currie temperature to the rolling temperature 
may not be presented to the induction heating apparatus at a uniform 
temperature. Therefore, portions of the partially heated slab may require 
more heat than ever. To this end, there is provided a modified control 
arrangement as is shown in FIG. 6. 
The control arrangement for each coil 30 may include a power source 38 in 
the same manner as the two longitudinally adjacent sets of coil 
arrangements shown in FIG. 4. However, each individual coil 30 may be 
separately energized but with the current flow in the windings 32 being 
that illustrated in FIG. 4. 
The control unit for each coil 30 includes a temperature sensor 40 which is 
generally aligned with that stripe area of the slab S which is centered 
relative to an associated coil 30. The temperature sensor 40 then controls 
the operation of a control device 42 which controls the electrical 
connection of a particular coil 30 with the power source 38. 
Assuming that the detected temperature of a slab S across the slab is 
uniform, then a control device 42 does not operate and the heating will be 
uniform as is shown in FIG. 5. On the other hand, assuming that some of 
the stripe areas of the slab are heated to a higher temperature than 
others, then the length of time which the coils 30 are energized will be 
varied so as to eliminate the temperature differential and to heat the 
slab to a uniform temperature. Thus, the initiation of the heating of the 
slab by a particular coil 30 will vary depending upon the detected 
temperature of stripe areas of the slab as is clearly illustrated in FIG. 
7. 
Because the heat is induced into the slab with penetration of the induced 
heat being to the center of a slab, it will be seen that there is much 
less heat loss from the slab than occurs if the heat directed into the 
slab is external such as from a gas flame. 
In addition, by electrical inductance heating the slab in a controlled 
atmosphere, the usual surface crusting is eliminated. 
Although only a preferred embodiment of inductance heating apparatus and 
method of utilizing the same has been specifically illustrated and 
described herein, it is to be understood that minor variations may be made 
in the apparatus and method without departing from the spirit and scope of 
the invention as defined by the appended claims.