Heat-treatment method for metal strips

Method of continuous heat-treatment of metal strips, wherein the strip which is to be heat-treated passes through a furnace which is thermally isolated and in a protective atmosphere, consisting of heating, holding and cooling sections; the said strip is guided by a plurality of return rolls situated in the upper and lower parts of the said sections and thus forms a plurality of lengths and they pass in front of cooling and traditional heating (generally produced by a radiation source or using naked flames), and induction means, the latter are placed upstream of at least one traditional heating section and are used combined and simultaneously with the other heating means in order to compensate for the variations in the heat-treatment parameters.

FIELD OF THE INVENTION 
The present invention relates to a heat-treatment method for metal strips. 
It more particularly relates to industries with a heavy consumption of 
sheet metal, in which the best means of making savings is to reduce the 
mass, and therefore the thickness, of the sheet metal, while preserving 
excellent mechanical properties. An operation of continuous annealing 
perfectly satisfies these requirements. 
BACKGROUND OF THE INVENTION 
The invention is concerned with the technology of continuous strip 
annealing furnaces. Such furnaces are used in continuous annealing lines 
or in continuous galvanizing lines, yet the invention may be applied to 
other types of installations in which the strips are continuously treated, 
in particular in varnishing, coating and painting installations. 
The method hinges around one or more strip annealing furnaces which consist 
of several sections equipped to perform consecutively the various phases 
of the heat-treatment cycle which are, in the simplest case: heating, 
holding and cooling. 
At the end of each of the phases, the temperature reached by the product is 
determined and must be stabilized in order to obtain the required 
metallurgical characteristics. In particular, the temperature at the end 
of heating is very precise. 
The operation is easy and is currently executed when the furnace operates 
in steady-state; it treats a product of given dimensions at a constant 
speed and according to an established heat-treatment curve. 
Known industrial furnaces work daily with strips of different thickness and 
width in annealing cycles which are also variable. 
Inevitably, transitional periods result during which the annealing 
temperature is difficult to reach and control in current heating chambers. 
The latter generally use traditional radiation or naked flame sources, and 
both are characterized by their significant thermal inertia. 
The users have introduced measures which make it possible to reduce the 
duration and amplitude of the variations in annealing temperature during 
transitional periods of changing strip size. 
The use of computers, dedicated to conducting these methods which follow 
the thermal state of the furnace in real time and control the change in 
the heating parameters and in the treatment speed, has afforded a partial 
solution to the problems. 
In fact, these procedures introduce variations in treatment speed in the 
essential heating phase which affect the running and the performances of 
the other parts of the furnace or of the installation, such as in 
particular the cooling section or the coating section of a galvanizing 
line. 
SUMMARY OF THE INVENTION 
The present invention sets out therefore to improve the traditional heating 
sources by the use of heating by electromagnetic induction for the 
continuous treatment of metal strips. In fact, induction heating may be 
used in two groups of application: 
The first, at moderate temperature (from 100.degree. to 300.degree. C.) 
relate to drying and varnishing. 
The second, such as in particular annealing, involve higher temperatures. 
For thin sheet metal, the traditional radiation furnace may seem more 
attractive by virtue of its lower investment costs, but induction has 
several incomparable advantages and is particularly suitable for surface 
heat-treatments of metals. Advantageous properties which may be mentioned 
are: 
possibility of higher power density than for any other method, 
substantially constant consumption whatever the width of the strip to be 
treated and without changing the inductor, 
facility of transmitting energy without contact and without influence on 
the surface state of the product to be heated, 
better precision and stability of the heating, 
no difficulty in ensuring the treatment under a controlled atmosphere, 
reduced bulk, 
start-up and shutdown of the induction device practically instantaneously, 
which avoids any wastage on shutdowns and on changes of the size of sheet 
metal, 
possibility of automatically linking the energy power to the speed or to 
the temperature with a very low time constant, 
high degree of automation. 
The subject of the invention is a method of heat-treatment, characterized 
in that the strip which is to be heat-treated passes through a furnace 
which is thermally isolated and in a protective atmosphere, consisting of 
heating, holding and cooling sections; the said strip is guided by a 
plurality of return rolls situated in the upper and lower parts of the 
said sections and thus forms a plurality of lengths and passes in front of 
cooling and traditional heating (generally produced by a radiation source 
or using naked flames), and induction means, the latter are either placed 
upstream or downstream of at least one traditional heating section, or 
arranged so as to divide at least one traditional heating section into 
parts and are used combined and/or simultaneously with the other heating 
means in order to compensate for the variations in the heat-treatment 
parameters. 
Other characteristics and advantages of the present invention will emerge 
from the description which is given hereinbelow, with reference to the 
attached drawings which illustrate an embodiment thereof which is in no 
way limiting. In the figures:

DETAILED DESCRIPTION OF THE INVENTION 
According to a preferred embodiment of this method, the strip to be treated 
passes inside a vertical or horizontal continuous annealing furnace. 
This furnace generally consists of heating 1, holding 2 and cooling 3 
sections. Their number and their arrangement vary widely, holding sections 
2, 4 for the correct establishment of the crystallographic conversions may 
coexist between the various cooling sections. 
The strip therefore runs through the various sections of the furnace, in 
the context of a vertical furnace, it is guided by a plurality of fixed 
rolls 6, 6', 6", 7, 7', 7" driven in rotation, situated at the upper and 
lower ends of the volumes or chambers forming the treatment enclosures. 
The strip stretches in a loop between two top 6 and bottom 7 return rolls. 
The conventional induction heating means 14 and the cooling units are all 
arranged between the lengths of the strip or opposite the external walls. 
The conventional heating means mainly consist of heating elements, of 
tubular shape, inside which the combustion of a liquid or gaseous fuel is 
supported. These elements called radiating tubes are placed between the 
lengths of the strip and facing the front walls of the furnace and heat 
the strip by radiation. They provide the majority of the energy supply and 
they operate when the installation runs in steady-state. 
It is obvious that all the enclosures are thermally insulated from the 
surroundings 9, 10, 11, 12, 13 by appropriate lagging. Each of the 
enclosures is fitted with units for centering the axis of the strip on the 
axis of the line, and they consist of rolls 8, 8' which are similar to the 
guide rolls and are mounted so as to move inside supports in order to 
adjust the length developed between two fixed guide points 6', 7'. They 
may, if required, be substituted for the guide rolls and are present in 
the heating 1, temperature-holding 2, 4 and cooling 3, 5 sections. 
The cooling means are generally produced by devices for blowing a 
protective gas which is recycled and cooled in exchangers outside the 
installation. This blowing, at a variable speed as a function of the 
heat-exchange requirements, occurs along a direction perpendicular to the 
path of the strip and through a plurality of orifices or slots arranged on 
the blowing means. 
The various chambers are connected together by connection tunnels, the 
whole thing possibly being held under a neutral protective atmosphere 
consisting in particular of nitrogen and hydrogen. 
Referring to FIG. 2, a strip progresses as follows: 
The strip enters a first heating enclosure 1 which comprises, in addition 
to traditional heating sources, an induction device 14 placed at the start 
of the path of the strip. In this portion, the strip undergoes an increase 
in temperature up to its annealing temperature corresponding to the 
desired heat-treatment, then it passes into a holding chamber 2 in which 
the energy supply is held constant for establishing the crystallographic 
conversions. Its temperature is lowered by the means previously explained 
in an enclosure called a cooling enclosure 3 (FIG. 1), the temperature 
decreases rapidly, the development of the crystallographic conversions is 
stopped. 
It subsequently passes through a chamber intended for what is commonly 
called "overaging" 4, this chamber is designed in a similar manner to the 
holding enclosure and it is situated between two cooling chambers. 
The last cooling 5 is generally not produced as the previous one by 
gas/solid exchange, but by a liquid/solid exchange, which is much more 
efficient, the procedure is to spray liquid onto the advancing strip. 
The finished product is rolled up or delivered at the exit of the 
installation. 
According to another mode of implementation of the method, the heating 
enclosure is divided into several parts 1, 1' (cf. FIG. 3), the induction 
device 14 is no longer placed upstream of the installation but between the 
heating chambers, in particular downstream. 
The effects of these induction devices are according to requirements 
coupled with or uncoupled from the other traditional heat sources, but in 
any case, they are intended to operate during the transitional periods of 
the treatment. 
The operational regime of the installation, in transition, principally 
comprises two modes, one called positive, the other called negative, which 
are each characterized by foreseeable and unforeseeable events. 
The positive mode of operation is advantageously explained, for an increase 
in the cross-section of the product to be treated which leads to a 
reduction in the speed of the strip from Vi to Vf, by studying the graphs 
in Figures Four and Five, namely for the change in the overall heat supply 
(H) as a function of time (cf. FIG. 4), carried respectively on the 
ordinate and abscissa, it is seen that the energy supply due to the 
inductor, shaded zone, compensates, during the time lapse between to 
representing the change of product (in other words the passage of increase 
in cross-section at the input of the inductor requiring a decrease in the 
speed of the strip, cf. FIG. 5) and t0+Vf which represents the instant at 
which the speed of advance of the final product is reached, then t0+Hf 
which is the instant at which the heat supply to the strip is reached by 
the traditional heating means, the thermal inertia of the traditional 
heating means between two stable states characterized by: 
an initial speed Vi, for an energy level Hi 
a final speed Vf, for another energy level which is different from the 
previous one Hf. 
Similarly, the negative mode of operation is usefully supported, for a 
decrease in cross-section of the product to be treated which leads to an 
increase in the speed of the strip from Vi to Vf, by the graphs in Figures 
Six and Seven; indeed, the shaded zone which represents the energy supply 
(cf. FIG. 6), (H) on the ordinate, due to the inductor during the time on 
the abscissa, compensates from an instant starting from t0-Hi, called the 
resetting time of the furnace, to t0-Vi representing the instant at which 
the speed of advance of the final product is exceeded, then extending to 
t0-Vf which is the instant at which the speed of advance of the final 
product is reached, then finally to the moment t0 which is the change of 
product (passage of decrease in cross-section at the entry of the 
inductor), the unavoidable inertia of the traditional heating source 
which, however, requires a sufficient quantity of heat to be maintained 
(by the inductor) for the time necessary for the appearance of the new 
energy level (Hf) which is less than the previous one (Hi). 
It is desired to minimize, in time, the consequences for the product by 
reducing the transitional regime between an initial state, corresponding 
to a stabilized thermal regime (Hi) of a traditional heat source (a speed 
Vi, a strip temperature Ti) and to a given strip size (a thickness Ei, a 
width Li, an emissivity Emi) and a final state corresponding to another 
stabilized thermal regime (Hf) (a speed Vf, a strip temperature Tf) and to 
another band size (a thickness Ef, a width Lf, an emissivity Emf). 
The positive mode of operation is characterized in the following manner, in 
the case of foreseeable events: 
the user observes by monitoring the strip at the input of the induction 
that one of the following characteristic parameters of the strip: 
increase in the cross-section of the product to be treated (E and/or L) 
variation in the emissivity of the product (Em) 
increase in the temperature at the end of heating of the product (T) is 
going to change, which, in the absence of the inductor, could lead to an 
expected reduction in the speed of advance of the strip. 
For an installation fitted with an induction device placed upstream (cf. 
FIG. 2) of the heating section, the actions are as follows (cf. FIGS. 4 
and 5): 
actuating the inductor 14 and increasing the energy supply without thermal 
inertia and without reducing the initial speed Vi, leading immediately to 
a rise to the temperature Tf corresponding to the exit temperature of the 
strip, this being at the end of the traditional heating section, 
setting the increase in heat supply of the traditional heating means, which 
leads: 
to an increase in the energy supply of the traditional heating source to 
its final level, compensated for by a 
progressive or stepwise reduction in the energy supply of the inductor. 
By this mode of reaction, it is possible, by virtue of the induction 
device, to avoid (in the absence of the induction device): 
an underheating of a part Bf of the strip (head of the strip) if a 
reduction in speed (from Vi to Vf) has not been anticipated before the 
event, 
incorrect anticipation of a reduction in speed leading to: 
underheating of a part Bf of the product (head of the strip), 
an overheating of the part Bi of the product (tail of the strip), 
a loss in production. 
The transitional regime of a positive mode will now be studied, but 
characterized by an unforeseeable event (increase in the speed on setting 
by the operator following an external event, degraded automatic running 
during a foreseeable event). The action of the inductor is as follows: 
it becomes possible instantaneously to compensate for the insufficiency and 
the inertia of the traditional heating which arises at the moment of 
appearance of the event. 
The effect produced by this variation in inductive energy supply is applied 
only to the portion of the product treated contained between the entry of 
the traditional heating device and the end of the inductive heating 
section. 
It is obvious that the part of the product contained between the exit of 
the inductive heating and the end of the traditional heating does not 
benefit from the change in heating power level. 
This is the reason (cf. FIG. 3) why the induction device 14 is placed so 
close to the end of the traditional heating section (which corresponds to 
the second mode of implementation seen hereinabove). 
The improvement is as follows, in a traditional furnace the duration of a 
transitional regime lies between 2 and 5 min and often more, the length of 
product corresponds to a time between 30 s and 1 min, the presence of an 
induction device upstream of the progression of the strip (first mode of 
implementation of the method) constitutes an undeniable advantage. 
The second, so-called negative, mode of operation will now be studied, for 
each of the foreseeable and unforeseeable events. 
The negative mode is characterized in the following manner (foreseeable 
events, observations of monitoring the strip at the entry of the 
inductor), 
decrease in the cross-section of the product to be treated (E or L), 
variation in the emissivity of the strip (Em), 
decrease in the temperature at the end of heating (T). 
For a negative mode and for one of the foreseeable parameters (cf. FIG. 2) 
previously described, the actions are as follows (cf. FIGS. 6 and 7): 
before the appearance of the event (passage of reduction in cross-section 
at the entry of the inductor), the user increases the energy supply by 
starting it in order to compensate: 
for a necessary decrease in the traditional heating device from its initial 
value to its final value, which takes place before the appearance of the 
event, and this as a function of the thermal inertia of the traditional 
heating, 
for an increase in the speed of treatment from Vi to Vf so that the final 
speed is reached before the appearance of the event. 
On appearance of the event, turning the inductor off and consequently 
immediate disappearance of its energy supply. 
The presence of the induction device upstream of the installation and with 
respect to this operating situation provides the following advantages: 
avoidance of overheating of a part of the product Bf (head of the strip) if 
the increase in speed V (from Vi to Vf) has not been correctly 
anticipated, and (for incorrect anticipation of the increase in speed V) 
avoidance: 
of an underheating of a part of the product (tail of the strip of the 
product Bi), 
of a loss in production if V&lt;Vf. 
A negative mode of operation will now be studied in the case of 
unforeseeable events (cf. FIG. 3): 
decrease in the speed on setting by the operator or following an external 
event, 
degraded automatic running. 
It becomes possible immediately to compensate for the excess heating which 
occurs at the moment of the appearance of the event. This decrease is 
instantaneous and is brought about by switching the inductor off. 
As before, the effect produced by these variations in inductive heating 
supply is applied only to the portion of the strip situated between the 
entry of the traditional heating means and the end of the inductive 
heating. It is necessary to install the induction heating as close as 
possible to the end of the traditional heating section. 
Whatever the modes of implementation of the method, either negative or 
positive, and characterized by foreseeable or unforeseeable events, the 
inductive heating power must represent a non-negligible part of the total 
installed heating capacity. 
In the normal operating regime, the heating will be performed principally 
using all the traditional power installed and a part of the inductive 
power. 
Reading the preceding description shows that the method brings novelty to 
the use of combined and/or simultaneous induction heating with traditional 
heating; in fact, it allows the capacity and flexibility of the existing 
installation to be increased. 
It satisfies partially or completely the constraints required for a section 
in question: 
temperature holding time, and diffusion time required for the 
heat-treatment of the strip, 
predetermined and stable speed variation gradient. 
It remains, of course, that the present invention is not limited by the 
embodiments described and represented hereinabove, but that it encompasses 
all variants thereof. 
Thus, although the embodiment described hereinabove relates to a vertical 
furnace, it is obvious that the invention may also relate to a horizontal 
furnace in which the strip is transported by rolls which are also driven 
without forming vertical lengths but simple catenaries of very small 
amplitude.