Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FIELD OF THE INVENTION ~:
This invention relates to the continuous annealing of
steel, and more specifically it related to a continuous system
for cleaning and preheating steel stxip or the like prior to an-
nealing the same in preparation for galvanizing or other opera
tions that re~uire a clean surface on the strip which is free of
oxides and other residual matter, so as to achieve improved
product quality and production efficiency.
In the continuous annealing of steel it is necessary
to preheat and clean the steel strip as it comes from the prior
fabricating and handIing units because the steel strip frequen- ;
tly has residual rolling oils and other surface con~amlnants
which must be removed. To accomplish this, the prior art has
first preheated the stxip surface to reduce or oxidize and other-
wise process the oils and other contaminants thereon. The strip
then enters a hydrogen-rich reducing atmosphere that reacts to
completely remove any treated residual contaminants that are on
the strip after it has been deposited as the strip travels
through the preheat chamber where it has been in flame contact
with the furnace heating gases. This cleaning method is often
preferred over the practice of removing the surface impurities
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by contact with a mechanical cleaner. Accordin~ly, the open
flame system that will process the surface of the strip pre-
paratory to its entering the annealing area of the furnace
; proper has been generally adopte~ by the steel industry. It
both clean~ and preheats the strip and thus helps to increase
production rates. This invention is concerned with an Lmproved ~-
~-i gas preheating and cleaning system.
D~S~ IO~ or ~ r ~-
In the prior art continuous annealing systems which
are in general use the strip enters from the ambient atmosphere
directly into a preheat furnace of a general muffle or oven-
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type or of a direct flame contact design through rolls or other
seallng means at its entry point, The furnace is inefficient
because in order to obtain any degree of effective cleaning and
surface preparation of the strip, it is necessary to design the
furnace so that it consumes a substantial excess of fuel to
provide the heat required to significantly preheat the strip
prior to further processing in the annealing system.
The foregoing system has the furt.her disadvantage that
when methane or other gaseous hydro carbon fuels are used with
mixtures of air in the proportions needed to create the as
designed combustion capacity in the preheat furnace, the result-
ing products of combustion have expanded many times ~on the
order of twenty to twenty-five times). This additional gas
volume requires a furnace which must be vented at its top or
sides in order to prevent a build-up of pressure from furnace `
gases. Otherwise, the products of combustion seep into the
annealing furnace proper which must maintain a hydrogen gas ~.:
~ atmosphere to reduce and remove the residuals as discussed
: above. Such preheat furnaces have serious maintenance problems,
; 20 and are thus relatively inefficient because of their excessive
use of fuel.
Other examples of prior art systems are shown in U.S.
Patents 1,238,011 to Ellis, 1,310,911 to John, 1,A09,119 to
Scanlon et al, 1,716,956 to Hepburn et al, 3,166,304 to Alexeff
: and 3,721,520 to Bloom. The Bloom patent discloses a continuous
line forgal~anizing wire to protect it from oxidation prior to
galvanizing by using a flame burner built into a tube through
which the strand is moved.
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SUMMARY OF THE INVENTION
It is the purpose of this invention to overcome the
disadvantages of the prior art by reducing to a substantial
exten~t the quantity of combustion gases and fuel required to
preheat and clean the strip in the furnace prior to!~continuous
annealing, This is accomplished by substantially reducing the
flame or combustion area within the prehea~ fuxnace and b~
directing the gases through a confined generally tubular elon
gated zone so ~hat ~hey remain in prolonged direct flame contact
with the strip during the preheating and cleaning process~ The
preheating operation is thus extended over a relatively longer ;~
path of travel while the products ~f combustion contact the
strip as they flow countercurrent to the flow of the strip~
This reduces the amount of fuel required to accomplish the
required processing.
In the drawings, Figure 1 is a schematic flow sheet of
a typical continuous annealing and galvanizing line for steel
strip utilizing the preheat furnace o this inven ion;
Figure 2 i~ a horizontal cross-section through one
form of urnace embodyin~ the present invention;
Figuxe 3 is a horizontal cross-section of a moclified
version of another form of the furnace or practicing the pre-
sent invention; and
Figure 4 is a horizontal cross-section of a prior art
furnace that will be useful for comparison purposes.
DESCRIPTION OF TEIE_INVENTION
Reerring to Figuxe 1~ strip S to be galvanized is `
con~inuously passed through tubular zone 1 and flame furnace
2 of the preheat section of the line~ It then proceeds :
: 30 through re~ucing furnace 3 into annealing furnace 4. There- -
after, it enters cooling section 5 preparatory to treatment in
coating bath 6~ All of the foregoinq steps and equipment are
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conventional except for the treatment in the novel preheat sec-
tion and the improved effect which it has on the strip received
from it and treated in the remainder of the line.
Figure 2 depicts a preferred form of preheat section
of this inven-tion. It will be noted that a furnace of the
direct flame contact type is provided which includes burners in
only one-~uarter to one-third of its area relative to the prior ;
art design shown in Figure 4. The furnace is entirely closed
except for the entry and exit ends thereof as shown in Figure 2
and through which the strip S and combustion gases pass in counter-
current direction. Prior to the entry end of the furnace, there
is located an elongated horizontally disposed flat tubular
or envelope-like unit through which the strip first passes as
seen inFigures 1, 2 and 3. It provides a zone which is of
substantial length and relatively small height and width, and
which advantageously is designed to limit the zone to a height
and width to accommodate the strip to be treated therein.
In more detailed explanation of Figure 2, the preheat
section includes t~o principal areas, an elongated tubular
20 preheating zone l and a closed muffle-type furnace 2. Tubular ~`
zone 1 has an open-ended entry through which steel strips are
introduced to the system. A buffer stack 8, the function of
which will be discussed later in more detail, is located im-
;mediately adjacent and above the entry or charging end 7.
Tubular zone l is provided with a set of rolls 9, which
are substantially the same as those rolls employed in the entire
continuous annealing system and which support the strip as it
travels horizontally through the various sections of the system.
Tubular zone l is characterized by an elongated area lO the
length of which is dependent on the available space in a given
mill. The elongated area lO terminates at entry ll to furnace
2. It will be noted that a restricted orifice 12 of heat
resistant material is located at entry 11 to inhibit flow of
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combustion gases G from furnace 2 and ther~by provide a positive
pressure throughout the furnace 2.
E~urnace ~ is entirely closed except for strip entxy !
ll and strip exit 13. This is in contrast to the prior art
system of Figur~ 4 which has a plurality of venting dampers
in the furnace roof over each of its multiple flame zones.
Furnace 2 of Figure 2 uses a substantially reduced
flame zone and has ~ar fewer open flow burners 14 than hereto-
fore required. ~he burners 14 are of conventional design~
and, or example, may correspond to those shown in U.S. Patent
2,933~425 to Hess~
The reduced number of burners needed for generating
the combustion gases that provide the total heat xequirements
of the furnace is one of the principal novel featuxes of the
invention. This is the result of the direct flame contact of
the combustion gases G with strip S over the elongated tubular
section l wherein the residuals on the surface of strip S are
more effectively treated for ultimate processing. The fore-
going is in contrast to the furnace section of the prior art
which re~uires several controlled heating zones as mentioned
above, to achieve satisfactory processing of the residuals on
the sur~ace of strip S which in the prior art entered the
furnace at ambient temperature.
Figure ~ is in effect an adaptation of the prior art ~;
furnace to the system o~ the invention without rebuilding the
furnace and wherein the venting dampers have been eliminated
and the roof thus sealed off. Also, by such an arrangement~
several of the combustion zones have been eliminated while at
; the same time a more effective use is made of the reduced
emount of heat in the 1esser volume of combustion gases G.
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Among the advantages of the invention are substantial
reduction of fuel requirements due to more effective use of
heat as previously described, ma-terially reduced furnace main-
tenance especially in the furnace roof due to elimination of
the venting dampers, increased cleanliness of the strip as
indicated by the elimination of virtually all adherence pro- '
blems during subsequent coating operations, and the ability
to obtain better heating values on the treated strip.
In the prior art furnace dampers~ i-t is necessary to
reline the damper seats several times over any six-month
period while during the same time period these areas of the
furnace of this invention have not required any relining or
other maintenance.
In the present invention the furnace dampers of the
prior art are eliminated because the gases exit the furnace
through the elongated arealO of tuhular zone 1. At a point
immediately adjacent to the entry end of the tubular zone,
an appendage zone or stack 8 is installed vertically to a
height that is above the highest point of the entire furnace
complex. I'his stack unit is rectangular in cross-section and
has inside dimensions equal to or greater than the twbular
zone 1. The top of the stack 8 is open to the outside atmos-
phere where the combustion gases are vented. The unit also
acts as a buffer venting unit during the periods immediately
following a line stop. The heat transfer conditions in the
unit at that time exhaust any encroaching air that may be
drawn into the operation 7.
As will be seen by reference to Figure 2, th~ flames
produced in the area of the furnace adjacent to the exit from
which the strlp passes to the annealing sec-tion are not vented
to the outside atmosphere as in the prior art (see Figure 4)~
- but are permitted to travel in a single direction through the
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furnace and along the length of the zone wherein they are in
direct contact with the strip. Although of a lesser volume
than the prior art, the gases directly and very effectively
preheat and clean the strip surface with a minimum of waste heat. ;~
Figure 3 is a modified version of the preheating and
cleaning furnace whereby the size of the closed furnace is
substantially reduced and the strip exit is at the immediate
end of the burner are so that the flame contact in the con-
fined tubular zone begins immediately after the flame zone
of the furnace. This design represents a more compact fur-
nace in contrast to the system of ~igure 2. Otherwise, the ~`
preheat section of Figure 3 is essentially the same as that
of Figure 2 in that it includes tubular zone 1I having an
open entry 7', a venting stack 8', conventional rollers 9' for
supporting strip S, an elongated area 10', and a restricted exit
12' to inhibit unrestricted flow of gases G from furna~e 2'.
Accordingly, the distinctive feature of the species of the
invention of Figure 3 is that the interior of direct flame
contact furnace 2l is limited in its dimensions to the space
occupied by the burners required to produce the reduced quantityof combustion gases that distinguishes this invention over the
prior art/ i.e. the effective size of furnace 2' corresponds to
the minor portion of the furnace used as a burner area in furnace
2 of Figure 2.
The reactions of gases G on the surface contaminants
(residuals~ of strip S are in general similar to those of the
prior art except that the residuals are more effectively pro-
cessed because of the more prolonged exposure of strip S to
direct flame contact. The combustion gases are of a reducing
nature so tkat the contaminants are apparently volati~ed as the
` strip passes through the preheat section. The reducing
~ nature of the gas is enhanced by the hydrogen in the gases
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used in the conventional reduction section of the system and
which gases flow into the preheat furnace and eY.it the system
through the tubular zone.
In actual operation beyond the preheat ~one, any
residual coating on the strip is removed in the reducing section
so that clean strip enters the annealing furnace. The anneal-
ing section of the furnace which i5 of the standard type, is
preceded by a reducing section having an atmosphere provided
by a hydrogen containing gas (HNX) which reacts to further re-
move any residual coating on the strip prior to annealingO Thestandard practice thereafter is to anneal and cool the strip
prior to its.exit into the at~,osphere or into a coating unit~ .
EXAMPLE
In a typical operation steel strip enters the system I
in the tu~ular zone at ambient temperature and as it passes
through the length thereof into the flame furna.ce, it has gained
appreciably higher temperatures.th:an at the same point in prior
art operations. The heating continues during the passage of
the sheet through the entire preheat urnace, including the
combustion zone. The temperature of the combustion gases in
the burner area within the preheat furnace are maintained within
the range of from about 2000F, to about 2400~F. and directly
contact the strip in the furnace and zone. The residue from
most rolling oils is volatilized at about 1200~F. to 1400F.,
so that the remaining heat in the ~urnace goes to preheat the
strip S. The strip thereafter passes through the confined
passage at the exit end of the preheater and cleaner and into
the annealing zone proper. From that point on, the reducin~, ¦
annealing and galvanizing steps are those which are convention-
3D ally employed in modern steel plant practices.
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~he protective atmosphere gases as previously -~
stated flow countercurrent to the strip and r~duce any re-
maining residue on the surface thereof as it passes through
the annealing system. These protective gases also enter the
combustion chamber of the prehea~er furnace and contribute
further heat thereto~ The protective ga~es pass along the
the combustion gases from the preheater into and through ~he
tubular zone and are vented as previously described,
Al~hough this invention has been described as part
of a continuous galvanizing systemt it can be used effectively
in any continuous line where clean strip metal is required,
including tin and aluminium coating operations. Also, although
it has been described as used in a strip annealing system, it
can be applied to other fabricated forms, such as wire, etc., ~;
as well as with other metals.
While in accordance with the provisions of the :~
Statutes I have illustrated and described the best form of ::~
embodiment of my inv~ntion now known to me, it will be apparent
to those skilled in the art that changes may be made in the
fo~m of the apparatus and process disclosed without departing ~:~
from the spirit and scope of the inv~n~ion set forth in the
appended claims, and that in some cases certain features of
my invention may be used to advantage without a corresponding
use of other features.
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