Heating and Cooling Annealing* in the Continuous

22 Pages·2006·3.81 MB·English

Heating and Cooling Technology in the Continuous
Annealing*
By Masayuki IMOSE**
I. Introduction-Brief History of Continuous aluminum industry under a very imaginative name of
Annealing the " Magic Carpet ".6) This method, in which
Continuous annealing is spreading in pace with aluminum strip is floated on air-cushion and an-
nealed as floating in air, literally enchanted the people
concatenation of other processes, not only in the iron
of aluminum industry.
and steel industry but also in the aluminum and
copper industries. In this Nishiyama Memorial The fourth was the commissioning of a large vertical
type continuous annealer of 70 m high at the Chase
Seminar, I wish to examine the backgrounds and
Brass. I heard that the tall building which housed
situations in which the continuous annealing was
this furnace in became quite an attraction to local
promoted then to discuss those continuous annealing
or heat treating processes that are currently popular journalism.
mainly from the viewpoints of heating and cooling The fifth was when in Japan the first length-wise
unified catenary type stainless steel continuous an-
technology.
nealing furnace was built : the development of special
In my career of over a quarter century as
heat resisting rolls7~ had made the unification of old
a furnace designer specializing in the continuous
divided shell structure possible,8~ so that the furnace
annealing, I have encountered many memorable
length was greatly reduced and the thermal efficiency
events, of which I like to mention six today. The
first was when I first discovered in 1956 A. F. Mohri's greatly improved. This was in early 1970s.
The sixth and the most recent one was the " realiza-
papery; the heat cycle he established on his metal-
lurgical studies has been adopted by almost all the tion of long dream of Sheet-CAL "9) of 1971 to 76
in the form first of Nippon Steel's (NSC) CAPL,10~
subsequent Tin-CALs, among which we have today
then of Nippon Kokan's (NKK) NKK-CAL,11-13)
large lines with a line speed of over 2 000 fpm.2)
and following those two processes Kawasaki Steel's
The second was the U.S. Steel's famous patent of
1958 on rapid cooling followed by overaging of strip, (KSC) KM-CAL14~ was developed in 1980.
Of the great advances that our metal industries
which they called the Shelf Annealing as may be
achieved despite the two Oil Crises, those Sheet-CAL
understood by Fig. 1.34~ The metallurgical common
technologies are rightfully of our own, and as of 1982,
knowledge at that time was that aging should bring
the proportion of continuous annealing practice is
about hardening by precipitation of carbides, so that
already over 50 % at the Nippon Steel,15~ working
this work of the U.S. Steel researchers' that proved the
towards 80 % level, which will be attained in the
opposite came as a great surprise. Of course, today
near future. Besides this, not only at home both the
everybody knows how the dislocation theory can
Sumitomo Metals and the Kobe Steel have installed
explain this phenomenon of age-softening by overaging
CAL, but this technology has been exported to the
beautifully.5~
USA, Sweden, Belgium, Brasil, and USSR.
The third was in 1962 when the floater type con-
Why, then, is this popularity ? Firstly, sandwitched
tinuous annealing furnace was introduced to the
between the already concatenated continuous casting,
repeating, hot strip rolling, pickling, and cold strip
rolling lines in the front, and also well automated
finishing process in the rear, the annealing remaining
batch process presented an obvious neck in the overall
system streamlining.
Secondly, though less salient, the need of handling
larger and heavier coils for cutting down on the
handling time to improve the productivity has become
acute. Contrary to the intent, however, this means
that fast and uniform heating is difficult for any batch
process by nature, a shortcoming that had to be over-
come by annealing longer or by providing more
Fig. 1. The shelf annealing due to the U.S. Steel patent. powerful fan to intensify the forced convection. Of
* Based on the paper presented to the 88th-89th Nishiyama Memorial Seminars : the 88th Seminar at Nokyo Hall, Tokyo, in February
1983, and the 89th Seminar at Kagaku Gijutsu Center, Osaka, in March 1983; originally published in The 88-89th Nis hiyama Me-
morial Seminars, ed, by ISIJ, ISIJ, Tokyo, (1983), in Japanese. Manuscript received on June 25, 1984. © 1985 ISIJ
* * Head Quarters , Chugai Ro Co., Ltd., Kyomachibori, Nishi-ku, Osaka 550.
Review (911)
( 912 ) Transactions ISIJ, Vol. 25, 1985
course, the loosening up the coil is effective,16~ but this a word of caution by quoting an interesting German
means more additional work steps and coil handling, report,21~ according to which an experiment conducted
where the coil is apt to getting scratched, a liability so as to trace the NSC-CAPL and NKK-CAL proc-
that cannot be tolerated often, particularly for alumi- esses by faithfully following the respective annealing
num. Further, there are many grades, such as, for cycles failed to reproduce the quality those two firms
example, that call for partial annealing for which the claimed. This means that the steel to be put to
time difference between the outer layers and the core continuous annealing must be adequate for CAL
of a coill to attain the same temperature may become process : insufficient degassing and chemistry adjust-
critical. ment at steelmaking and improper hot coiling tem-
This may be seen in Fig. 2, which is a result of com- perature at rolling can totally defeat the very idea
puter simulation study conducted for a powerful of Sheet-CAL.
convection type aluminum batch annealer. It will
II. Heating Technology for Steel Sheet-CAL
be seen that the greater the coil, the longer becomes
the heating time and the larger becomes the tempera- Typical Sheet-CAL is illustrated in Photo. 1, while
ture difference between the surface and core of coil.
With a far smaller heat conductivity, it is easy to
see how the situation is severer for steel. This means
simply that larger coils and more efficient annealing
are mutually incompatible, a single great reason why
American and Japanese aluminum. firms have installed
the floater continuous annealer, some of which are
capable of handling strips of over 2 100 mm wide.17,1s~
The third is the need of saving energy and man
power. A furnace thermal efficiency reaching to 80
to 90 %, or even over 100 % by recovering the heat
released from the cooling strip to preheat the incoming
strip, as well as automation and computer control of
the line can only be achieved by CAL.
In the fourth, the conventional batch annealing
has become not good enough to satisfy customer's
varying needs for higher quality and faster delivery.
In fact, in a poll conducted for American aluminum
industry, 430 out of a total of 1 526 returns cited the
precise delivery as the greatest merit of continuous
annealing .19)
Lastly, continuous annealing promises not only
reduction of the running cost but a substantial saving
in the initial cost and the space.9-13~ Those merits
become all the more apparent when annealing is con-
catenated with the lines of cold strip rolling, pickling,
temper mill, and surface treatment into the so-called
integrated line.2o~
Before closing this chapter, however, I must add
Photo. 1. A Sheet-CAL furnace.
Fig. 2. Heating practices and coil core-surface temperature difference in a luminum coil annealer.
Transactions ISIJ, Vol. 25, 1985 (913)
basic structure and heat cycle are schematically shown width of strip and the corresponding furnace heating
in .Figs. 3 and 4. Typically, the heating technique capacity. The material can be various, such as ACI
applied to the already existed Sheet-CALs is essentially HF, HH-2, HK, or HL with 1.5Mo,22j but from the
the same as developed for the Tin-CAL, and therefore cost performance viewpoint HH-2 is recommendable.
is of the type of, the indirect heating mainly using The highest heating temperature of RT should
the radiant tubes (RT). The shortcoming with this preferably be below 1 000 °C. This is to prevent
method is that in order to ensure a reasonably long high temperature deformation of tube, but simple
service life for the RT, the surface heat load of each specification of maximum temperature is not enough
tube must be held low, so that in the end the furnace to ensure uniform heating of work and a reasonably
length must necessarily be designed long. Further, long service life for the RT.
RT itself is rather expensive both in the initial and Therefore, when a great number of RTs are to be
in maintenance, and its heat momentum is rather equipped, the surface temperature distribution must
large, making the response to temperature control most carefully be checked a priori in conjunction with
sluggish. the burners so that no hot spot may occur to bring
In this chapter. I wish to discuss first the RT the local temperature to over 1 000 °C, and that the
heating techniques, particularly for the energy saving temperature profile be a gentle slope along the length
measures developed, then other newer methods being of a tube with the peak temperature held as low as
developed to overcome the shortcomings of the RT possible. An example of temperature profile is shown
heating method. in Fig. 6 for an RT in a test furnace for Sheet-CAL.
To avoid the deformation of RT, methods of attach-
1. The Radiant Tube (RT) Heating Methods ing and supporting so as to leave enough freedom for
1. The Radiant Tubes thermal expansion and contraction must be taken.
The RTs that are being used in the Sheet-CAL In fact, failure of an RT is due more often to cracking
come in two types of U shape and W shape as shown or deformation resulting from improper attachment
in Fig. 5 and with a diameter of 5, 6, or 7 in.: they and supporting than to expending of its inherent life
are to be selected in accordance with the maximum resulting from high temperature oxidation.
2. The RT Burners
The properties that are required of an RT burner
may be summarized as follows :
1) not to produce hot spots, and the peak tempera-
ture be low;
2) stable combustion even for low excess air rates,
and not to make small explosions with soot
accumulated on repeated on-off or by backfiring;
3) be able to operate under a tube pressure that is
lower than the furnace pressure so that the com-
bustion gas may not leak out into the furnace
through cracks;
4) the air-fuel ratio be stably maintained throughout
all of the cumbustion turndown, the flame be
stable, and no abnormality to appear in the tube
surface temperature distribution;
5) the combustion air be preheated sufficiently by
the recuperator provided individually to each
Fig. 3. An example of Sheet-CAL furnace and heat cycle.
burner, preferably to over 400 °C;
6) the NO x be below 80 ppm;
7) the burner noise be below 70 dB (A) ; and
8) the surface temperature of burner and of burner
Fig. 4. Heat cycles for Sheet-CAL. Fig. 5. Types of radiant tube.
(914) Transactions ISIJ, Vol. 25, 1985
Fig. 6. Surface temperature distribution of a
radiant tube under no load.
Fig. 8. System diagram for push type radiant tube com-
Fig. 7. System diagram for pull type radiant tube com- bustion.
bustion.
attachment be kept to lower than 100 °C.
As not all the burners that are equipped to con-
ventional Tin-CALs satisfy all of those requisites
sufficiently well, many are being replaced with new
ones.
Now, the currently practised RT combustion
methods may be classified into the pull type (with
either an educator or an exhauster), the push type,
and the push-pull type. Those are schematically
depicted in Figs. 7 to 9. However, the push type can
Fig. 9. System diagram for push-pull type radiant tube
hardly be used in a CAL because the RT internal
combustion.
pressure will greatly exceed that of the furnace.
Of the remaining two, the push-pull type is superior
to the conventional pull type in many ways as may For another, according to Okada and Kitamura,23~
be understood in Table 1. Among those merits of in the push-pull method the flame length can be
the push-pull method, the most interesting is the ease controlled simply by changing the swirl number of
of control. That is to say, for a group of a great the combustion air; they have shown that the relative
number of burners, while the variation in the air-
position of the peak in the RT surface temperature
to-fuel ratio m is as much as m=1.2 N 1.5 for the pull
profile and the concentration of NO x may both be
type, that for the push-pull is only m=1.051.15.
presented in a function of the swirl number. This
Transactions ISIJ, Vol. 25, 1985 (915)
Table 1. Comparison between pull type and push-pull
type for radiant tube heating system.
Fig . 10. Relation between the strength of swirl and the
position of temperature peak along an RT
available at each burner and the pressure drops
allowable for combustion air and combustion prod-
ucts at the recuperator, a preheating of air to 400
°C has been the reasonable limit
.
is shown in Figs. 10 and 11, out of which an RT This means that the exhaust gas temperature is
burner system design computer program has been yet as high as about 600 °C at the recuperator outlet,
developed. and today, economy demands this heat be recovered
3. Fuel Saving in the RT Heating much more until the exhaust temperature become
As mentioned earlier, the conventional heat recov- 200 °C or even lower. Some of the devises that have
ery method for the RT exhaust gas has been to preheat been found effective are :
the combustion air by means of a single stage recu- 1) the use of a common recuperator on top of the
indivisualized recuperators to make it the double
perator provided for individual burners.24~ In this
case, however, because of restrictions in the space recuperation;
(916) Transactions ISIJ, Vol. 25, 1985
in the winter time or sold to neighboring town as
general purpose hot water.2?) This kind of idea itself
is old,28) but this is a classical example of an idea
taking quite a while and needing a host of supporting
technologies for it to materialize.
?. The Direct Fired (DF) Heating Methods
One method of heating that my supercede the RT
method is the direct fired (DF) heating. For applica-
tion to Tin-CAL, this method has a long history: the
Selas Corp. has built one for the Inland Steel in
194929) and four for the Bethlehem Steel in 1956.30)
But, subsequent years saw more application of it to
the continuous galvanizing line (CGL), and for Tin-
CAL, then for Sheet-CAL, RT heating to become
predominant. In more recent years, however, from
the mounting need of making as compact as possible
the line that is tended to become ever larger with
attending increase in the number of pass, the DF is
on the verge of revitalization.
Fig. 11. Relation between the strength of swirl and the
Thus, a large DF heating type Sheet-CAL will be
NO concentration.
developed by the Nippon Steel in the near future,15)
but the following problems will have to be solved;
2) the use of the stack recuperator equipped in the 1) development of a direct fired heating burner and
collective flue; combustion system that ensures a stable non-
3) the use of exhaust gas to preheat the strip; oxidizing flame;
4) the use of exhaust gas to preheat the furnace 2) development of a system that ensures the strip
atmosphere to indirectly preheat the strip; to be heated unoxidized to as high a temperature
5) the use of exhaust gas to preheat fuel gas having as possible;
relatively low calorific value; 3) development of strip temperature control system
6) the use of a heating medium in the roll heating that ensures an precise following-up of heating
to preheat the strip; rate tuned to the slowing down or speeding up
7) generation of either steam or hot water by means of the line speed;
of waste heat boiler or water heater; and 4) for the above purposes, development of light fur-
8) the use of a porous gas-permeable solid either at nace lining structure having a small time con-
the outlet end of RT or in the recuperator to stant;
raise the thermal efficiency.25) 5) establishment of a method of preventing over-
It has been shown that the final exhaust gas tempera- heating and oxidization of the strip upon slowing
ture can be brought down to lower than 200 °C by down or stopping of the line; and
combining those methods. 6) establishment of an effective preventive measure
Moreover, thanks to the recent progress in the for picking-up by the hearth rolls down-stream
recuperator technology, it is now quite common that to the DF heating zone.
a temperature recovery rate of over 70 %, namely, Besides the obvious need of integrating all those
for an exhaust of 900 °C, a combustion air preheated new techniques and devices into an operational
to at least 900 °C X 0.70=630 °C should readily be technology, comparative study of the radiative direct
obtainable. So much so that, unless the temperature fired (RDF) heating method, which is the one that
recovery rate exceeds 70 %, one does not today is currently common, and the convective direct fired
consider that recuperator as " high efficiency ",26) (CDF) method, which is often used in the copper
Indeed, this, coupled with extensive energy saving and aluminum industries, for their possible advantages
exemplified above, has elevated the level of overall as a system element will have to be done carefully.
thermal efficiency of RT heating method very close 1. The Radiative Direct Fired (RDF) Heating Method
to that of the direct fired (DF) heating method to be The RDF method, which has been found effective
discussed next; at present, theoretically or practically, to reduce the furnace size as mentioned earlier, may
the limiting thermal efficiency is considered to be be applied to a Sheet-CAL any time, provided that
the same for either. a slight oxidation of the strip is tolerated then count-
It is conceivable, further, that a 90 to 100 % ered to by conducting either the atmospheric reduc-
efficiency may be attained by utilizing the heat the tion or the post anneal pickling.
strip releases away on cooling. In fact, the NKK- The cares to be exercised to perform the non-
CAL that was installed at the Swedish Steel AB is oxidation heating with the RDF method are:
equipped with a system in which the heat released (1) the furnace temperature should be raised
by strip cooling from 700 to 560 °C is recovered as as much as the desired rise of strip temperature;
a hot water of 120 °C, which is used for heating shops (2) at the same time, the air-to-fuel ratio should
Transactions Is", Vol. 25, 1985 (917)
be made gas rich so as to provide more of the cum- balance of oxidation and reduction between CO and
bustible CO+H2 in the combustion gas; and C02 on one hand, and between H2 and H2O on the
(3) for thicker strips that need longer residence other, is shown in Fig. 13 as a function of steel tern-
time, the rise of furnace temperature and enrichment perature.32)
of fuel should be done that much more. It will be seen that to heat steel from 20 °C to, say,
This means generally that the highest furnace 700 °C perfectly non-oxidizingly throughout, the gas
temperature will become 1 200 to 1 350 °C and an composition must be CO/ C02> 1.5, and H2/H2O>
m value of O.9-.-O.95. In view of energy saving, this 4.0, for which the RDF burner combustion need be
definitely calls for a certain heat recovery measure, at an air-to-fuel ratio m (O.5. This kind of combus-
for which a system that has for some time been tion condition is actually realized in some heating
practiced in the CGL or stainless steel catenary type furnace of the pipemaking line.
furnace will be adequate. From those theoretical viewpoints, such RDF
This system is shown in Fig. 12 schematically.31) heating furnaces like today's CGLs that operate at
As may be seen in the figure, the heat of the exhaust an m of 0.9 or so should not be called the NOF
gas is utilized to preheat the strip in two steps : while (Non-oxidizing Furnace) but rightly MOF (Mini-
gas is still hot, the heat transfer is in the RWP (Radia- mum Oxidation Furnace). This is important for
tive Waste-gas Preheating) mode, say from 1 200 a DF heating continuous annealer because every
1 350 °C down to 700800 °C, and when the gas stoppage of line will immediately mean a badly
becomes cooler, it is in the CWP (Convective Waste- stained strip at the best and burning and breaking
gas Preheating) mode, say from 7O0'-8O0 °C to through of the strip inside the furnace at the worst.
2O0-'3O0 °C. A clever combination of RWP and Therefore, special preventive measures are being
CWP promises a compact plant for greatest heat
provided for a DF furnace such as :
utilization. 1) to produce atmosphere having m(0.5 quickly
All in all, the most important single equipment in by blowing raw gas into the furnace33) ;
any RDF heating is the direct fired burner, particu- 2) to blow methanol into the furnace so as to cool
larly the burner nozzle. Inasmuch as no " ultimate" the strip and at the same time to produce a
burner is commercially available, the selection must reducing atmosphere through pyrolysis of CH30H
be done in consideration of materials and structure -~ CO+2H234);
of nozzle against dimensional stability and length 3) to blow a great quantity of N2 gas into the furnace
of service life. As for the question of which one of to purge C02 and H2O out and at the same time
the nozzle mixing type or the premixing type should to cool the srip;
be taken, on the other hand, this is no longer much 4) to separate the strip from the furnace by retracting
of a problem today, for reliable burners of either the furnace body so as not to overheat but to let
kind are readily available. the strip cool in air; and
2. Theory and Practice in the Oxidation-Reduction in the 5) to introduce a water jacket or other cooling means
RDF Heating into the furnace to rapidly cool the strip.
According to the theory of equilibrium between The reality is that without those emergency
steel and combustion gas, the steel is oxidized by measures no DF heating system can be a commercial.
C02 and H2O and reduced by CO and H2. The
Fig. 12. Strip preheating by waste gas through convective
preheater, after burning section, and radiative Fig. 13. Oxidation-reduction equilibrium diagram for
preheater. steel.
( 918 ) Transactions ISIJ, Vol. 25, 1985
another reason why it should be called the MOF, to be oxidized after a certain period of time does not
not the NOF. stand to reason : probably, this was simply that the
How, then, is it that what the theory predicts as steel looked to him as if reduced in an atmosphere
will fail can be accomplished in practice ? My which had been slightly oxidizing from the start,
answer to this is that it is a matter of time and quan- because oxidation was so minimal in the time span
tity of reaction. That is, the time the steel stays in employed.
the heating zone is about 25 sec at most in a con- It is well known, on the other hand, that reduc-
tinuous annealer, whereas it is 7 hr or more for batch tion can actually be achieved in the Selas furnace
annealing, really a ratio of about 1 to 1 000; in this that features direct impingement of the fuel gas onto
very short period of time, even when the steel does the stee1.36~ This is shown in Fig. 15. The question
is how we do understand this. For that, I might
get oxidized in the weakly oxidizing atmosphere, the
amount of oxide formed can be held to a commercially invoke a Soviet research.37~ According to this report,
tolerable small quantity. a dynamic analysis of combustion of methane gas
The results of DF heating experiment due to M. B. revealed that at a certain stage of combustion process
Pierson,35~ shown in Fig. 14, substantiate this explana- formaldehyde (H • CHO) is formed in a great quantity.
tion. As may be seen in the figure, what is important This may mean that the residual oxygen existing in
for non-oxidizing heating is the combination of m the mixture gas of air and fuel is caught by this formal-
value, furnace temperature, and residence time of dehyde resulting an oxygen deficient situation, and
the work. Here, his observation that the steel was that thereafter combustion proceeds by reaction be-
reduced first, but, in the same atmosphere, it became tween oxygen and formaldehyde. This state of affairs
Fig. 14. Oxi dation-Reduction be havior of steel upon direct fired heating.
Fig. 15. Oxi dation-Reduction diagram of steel due to Selas Corp.
Transactions Isu, Vol. 25, 1985 (919)
would be observed as combustion of mixture gas in uniform heating easier and improving the furnace
an oxygen deficient condition, hence successful com- heat efficiency;
pletion of the non-oxidizing heating. (3) even though for a material with a particularly
Indeed, this theory is rather convincing, and if small ~, as its effective radiative heat transference
it is right, we can see importance of such factors as E . hrBB is also small, defficiency in the capacity must
burner capacity, shape and length of flames, and be countered to either by raising the furnace tem-
distance from burner to strip in designing a DF heating perature or by lengthening the furnace, but then since
furnace. On the other hand, however, even if crea- he does not depend on s, it is quite possible to make
tion of reducing atmosphere through combustion by he»e•hrBB, so that the furnace length may be short-
impingement of burner flame is proved by the dyna- ened quite a bit (Fig. 16);
mism, it remains to be a matter of local occurrence, (4) because the heating rate can be regulated not
and in between two adjacent burners, oxidation dic- by changing the furnace temperature but by changing
tated by equilibrium should prevail. Therefore, if the nozzle gas velocity, hence the he itself, a very fast
the non-oxidation anneal is to realize as a whole, it response is had, so that the temperature control and
must be a consequence of repetition of reduction at changing of line speed or strip thickness is greatly
the burner and oxidation in between as the steel improved;
moves through the heating zone. (5) on changing of setting of the heating tempera-
Moreover, from the reaction rate theory it may ture, since the response of the gas temperature is faster
well be that the balance between oxidation and than that of furnace temperature, the time needed
reduction given in the final and high temperature to execute the cycle change is shortened;
range of strip heating does control the entire process.
(6) no overheating of the strip upon line stoppage,
This notion would appear to give some credence to hence no troubles like strip breakage at all; and
the assertion that provision of a strongly reducing
(7) as only a very small number of burners are
zone immediately after completion of heating will needed in comparison with the cases of RT or RDF
relax those stringent requirements for heating of heating, both operation and maintenance are much
strip greatly.38~ easier.
3. The Convective Direct Fired (CDF) Heating Method The point No. 3 above merits further explanation :
In the convective direct fired (CDF) heating, the suppose in an existing Sheet-CAL, one wishes to
combustion gas that has been created by burning replace the current RT heating with a CDF having
fuel in a direct flame mode is led to the heating cham- an he of 57 kcal/°C.m2.h, then for heating the strip
ber pressurized by a recirculation fan, then blown to 700 °C, the furnace length can be shortened to some
onto the strip surface through nozzles. Namely, the 60 % of the original even while lowering the furnace
CDF heating is a form of forced recirculatory direct temperature from 900 to 800 °C. Or, if the furnace
fired heating of gas impingement type.
This is being applied to continuous annealing of
aluminum, copper, and their alloys as a well estab-
lished technology. In the continuous bright anneal-
ing furnace for copper strip, for instance, combustion
is gas-rich with an m of about 0.9 so that the combus-
tion gas will contain combustible CO + H2 from 3 to
4 %. Then, to make the residual 02 less than 50 ppm,
swirl is given to the flame on one hand, while the
chamber is designed longish so as to secure tempera-
ture and time necessary for reaction on the other,
and after adjusting the gas composition to be equiva-
lent to the DX gas (wet) let it impinge upon the metal.
By equipping with an automatic m value controller
working on either a CO meter or an H2 meter to
enrich the fuel gas, further, the atmosphere can be
held stably to bright annealing condition in a wide
firing range. Thus, the copper strip bright annealing,
which used to rely traditionally on the RT heating
with protective atmosphere, is now almost always of
the CDF heating type.
The merit of CDF method as compared with RT
or RDF method are :
(1) thanks to the forced convection, fast and
uniform heating is ensured irrespect of metal surface
emmissivity (a);
(2) thanks to the large convective heat transfer-
ence (hc), the furnace, or the gas temperature can
Fig, 16. Shortening of RT heating furnace length by adopt-
be greatly reduced for a given furnace length, making
ing forced convective heating for radiative heating.
(920) Transactions ISIJ, Vol. 25, 1985
length is to remain the same, the furnace temperature roll heating as highly promising for future Sheet-
can be lowered to as low as 720 °C. This means CAL application.
that now the thermal head is only 20 °C, a figure 1. The Induction Heating Method
which not only makes a very precise heating control The induction heating has been tried on a Tin-
quite possible, but entirely eliminates any danger CAL by the Westinghouse42) for a part of the heating
of overheating on changing the line speed or the zone, but with dubious results. The difficulty was
strip thickness. apprently that the temperature difference over the
Thus, application of CDF heating to Sheet-CAL is strip width was as much as 10 % of the strip tempera-
worth a close study. For the problem of keeping the ture due to eddy current generated at the edges.
oxidation to below the tolerable limit, which will have This may prove to be fatal for realization of sole
to be solved before it is realized, a safer way, that of induction heating.
forced convection using RT combustion heating On the other hand, however, Yamazaki and his
(CRT) will come as a transient. This method is associates have successfully developed an induction
to be actually adopted by some of American Sheet- heating method for drying the surface coating of
CALs, and the furnace length is expected to be cut silicon steel sheets.43) In view of their success, which
to about 60 % of that for conventional radiative RT is undoubtedly due to the fact that temperatures
(RRT) furnace. needed for drying is only 250 °C or so, at which the
unevenness of temperature should not be much of
3. Other Heating Methods a problem. This suggests for possible application to
Other than the RT and the DF heating, methods Sheet-CAL the partial heating for a raise of 200 N 300 °C
using electric resister, induction, ohmic resistance, at the final stage of heating zone. This usage would
electron beam, roll, or salt bath appear to hold some be effective not only to reduce the furnace length
prospects in the future. Since the RDF, CDF and but to improve the strip temperature control.
CRT methods are all able to achieve a 3O'-6O % To heat a thin strip by induction in the longitudinal
reduction in furnace length from that of conventional flux mode, namely to align the magnetic flux parallel
RRT furnace, however, any new comer will have to to strip, the frequency need be very high to maintain
promise an even greater reduction. In this respect, the efficiency reasonably high. Therefore, the trans-
the electic resister heating method will not become verse flux mode, in which the magnetic flux is through
very popular because the furnace length will remain the strip thickness, will have to be taken for additional
the same as or even longer than RRT furnace. advantage of the use of low frequencies and better
On the other hand, according to recent studies efficiency.44) In either case, cooling of the in-furnace
made by Iguchi and his co-workers39-41) for ultra- coil or electrode will pose another difficult problem.
high rate heating of steel of 106 to 107 °C/sec, achieved 2. The Electron Beam (EB) Heating Method
by methods like ohmic resistance, plasma jet, capacitor The electron beam (EB) heating method is being
discharging, or irradiation by laser, following very tried on the paint drying line, the vacuumn deposition
interesting observations have been made : of zinc or aluminum, and the stainless steel bright
(1) while the normal isothermal transformation annealing line.45-47) The merits to be exploited are :
diagram generally holds good for austenitic trans- 1) the ease of control of heating output and the very
formation under such fast heating, for more exact fast response of less than 11100 sec, so that over-
study, TTA (Temperature-Time-Austenitization) heating of stock can be readily avoided even for a
curves will have to be prepared; sudden change in the line speed or line stoppage;
(2) quenching after such fast heating produces 2) the simplification of equipment in that loopers
a structure which is even tougher than that due to and bridles can be eliminated;
ordinary quenching-since this phenomenon appeared 3) the greatly reduced furnace length, hence saving
to them the quenching function was augmented of space;
by the fast heating, they called it the " upper- 4) the yet better grade of bright annealing than
quenching "; conventional gas atmosphere method thanks to
(3) the structure obtained through ultra-high the vacuum environment; and
rate heating and quenching is one in which only the 5) the cleaning or descaling effect of bombardment
pearlite gets austenitized then transformed into mar- by electron or ion beam.
tensite upon subsequent quenching, with ferrite left For those advantages, the U.S. Steel has started
intact-a peculier structure which they called the commercial production of aluminized steels by means
" martenoferrite "; and
of an EB continuous vacuum deposition method since
(4) by combining the fast heating and thermo- 1966.47) It looks as though application to Sheet-
mechanical treatment, therefore, an original rapid CAL is not too far. An example of operating EB
heating superplasticity heat treatment method may coninuous aluminizing line is illustrated in Fig. 17.48)
be had, out of which a novel material is expected to 3. The Salt Bath Heating Method
develop. The application of salt bath heating method to
This technology may or may not apply to the cases Tin-CAL dates back to as late as 1950s. The major
at hand as such, but the importance of rapid heating ones are the sodium bath method and the lead-
is clear enough. It is for this reason that I regard the bismuth bath method.
methods of induction, electron beam, salt bath and The former is due to the Associated Engineers,29)
Review