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

The heat treatment of a continuous band or sheet of metal as the same travels through a plurality of stages in a protective atmosphere, between conveying rollers, by applying direct-current electricity to the rollers for inclusion of the travelling sheet in the circuit therebetween. The charged rollers in the initial stage are spaced more widely from one another than those in the later stage, to compensate for the lower resistivity of the metal in the former, so that the Joule effect or I.sup.2 R factor in the stages are substantially equalized. The protective atmosphere of oxidizing, reducing or inert gases which encompasses the sheet, is confined in chambers of galvanized iron sheeting and the like, the walls of which are in close proximity to the travelling sheet, so that lesser amounts of reacting gases are necessary. Furthermore, no indutive electric currents are generated in the walls of the chambers, as is the case when alternating currents are applied to the conveying rollers, with the consequent heat loss. The lack of any extraneous source of heat within the chambers through which the metal sheet passes, other than the direct-current energy, results in a system of low thermal inertia with the capability of a fine and rapid control of the heating to produce uniform physical and metallurgical properties across the entire width of the sheet up to the edges thereof. The heat treatment may be executed for the purpose of annealing or tempering the sheet or modifying its surface coating, either independently or preparatory to the coating thereof in a coating bath.

BACKGROUND AND SUMMARY OF THE INVENTION 
The invention contemplates the improvement in systems for the 
heat-treatment of a continuous travelling length of a metal sheet or band 
which is heated by electrical currents passing therethrough in the course 
of its travel through a plurality of stages between conveyor or guide 
rolls or pulleys to which are applied electrical potentials. 
The invention seeks to improve upon such systems as are disclosed in the 
patents of the prior art, and particularly such as are disclosed in the 
patent to Cook, U.S. Pat. No. 2,457,870 and others. 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 currents 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 shut-downs 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 installation 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 treatment of continuous lengths of metal bands or sheets, which 
occupies a minimum amount 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 continuous 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 non-oxidizing 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 
countercurrent relationship to the direction of the 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.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
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 a fed to a thyristor rectifier 
bank which rectifies the current supplied by the transformer. Any other 
type of rectifier which produces relatively ripple-free direct-current can 
be used for this purpose, and the rectifier elements may be other than 
thyristors, for example, silicon controlled rectifiers, 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 
electrically charged guide or conveyor 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 13a, 13b, 13c and 13d of 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 disposition, as is well known in 
the art. While the ducts are disposed vertically in the illustrated 
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 
periphery of these pulleys between the guide rollers which are charged 
with potentials of opposite polarity. 
Metal sheet 1 is guided under negatively charged pulley 2 past insulated 
guide pulleys 3 and 4 and becomes heated when it makes contact with 
electrically charged pulley 5. The sheet becomes progressively heated as 
it advances towards the outlet end of the system and reaches its maximum 
temperature as it approaches charged roller 7. The portion of the sheet 
which remains in a normal atmosphere before its entry into the reducing 
chambers at slot 20, permits the residual oil to burn off before the sheet 
enters the first insulated duct or chamber 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 chamber 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 
chambers 13a and 13b, the sheet enters the second stage after it passes 
over positively charged pulley 5 and past guide pulley 6 to the negatively 
charged pulley 7. As stated above, the sheet attains its maximum 
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 pneumatic or hydraulic cylinder 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 and cooling chambers 13 and 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 75%. 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% 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-resistively 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 potential of 333 amperes per meter it will reach a 
temperature of 800.degree. C. 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 remain unchanged. This 
represents a saving of 23%, 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 "short circuit" or resistive method using alternating 
current power, the heat distribution across the strip width is unequal. 
The edges of the strip becomes 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.degree. C. 
It continues its travel and reaches the last electrified pulley 7 at a 
temperature of about 1000.degree. C. From the graph in FIG. 6 it can be 
seen that at room temperature the resistivity of the strip is about 0.18 
Ohms/mm.sup.2 /m, and at 500.degree. C. the resistivity is 0.58 
Ohms/mm.sup.2 /m, which averages 0.38 Ohms/mm.sup.2 /m. In the second 
stage, the initial resistivity is 0.58 Ohms/mm.sup.2 /m, and at the end 
thereof it is 1.17 Ohms/mm.sup.2 /m at 1000.degree. C. Consequently, the 
average resistivity of the strip in the second stage is 0.88 Ohms/mm.sup.2 
/m. Therefore, if both stages were to have the same resistivity, then 
their length relationship should be 0.80:0.38 or the first stage should be 
2.3 times the length of the second one. By following the methods described 
above, it is possible to produce a galvanized strip 40" wide and 0.030" 
thick with a power consumption of less than 200 KW/ton, which is a 
significant saving in energy when compared to a conventional process. 
As shown in FIG. 4, the reducing chambers 13 may be lined with an 
insulating layer 13i, whereas the cooling chambers 14 are devoid of such a 
lining to enhance the cooling operation. 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. However, 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 advisable 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, by a rocking movement of 
approximately 90.degree., 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.