U.S. patent application number 09/886719 was filed with the patent office on 2002-01-03 for tower furnace for heat treatment of metal strips.
Invention is credited to Ebner, Peter, Lochner, Heribert.
Application Number | 20020000180 09/886719 |
Document ID | / |
Family ID | 3685686 |
Filed Date | 2002-01-03 |
United States Patent
Application |
20020000180 |
Kind Code |
A1 |
Ebner, Peter ; et
al. |
January 3, 2002 |
Tower furnace for heat treatment of metal strips
Abstract
A tower furnace for heat treatment of metal strips having a
pre-heating section (3) and a high-temperature section (5)
connected thereto at the top forming a housing (4) separate from
the pre-heating section (3) is described, whereby the pre-heating
section (3) equipped with a preferably gas-heated muffle (7)
exhibits a connecting stopper (6) made of heat-insulating material
for the high-temperature section (5) inserted into the muffle (7).
To create advantageous structural conditions it is proposed that
the high-temperature section (5) is also fitted with a preferably
gas-heated muffle (12) which encloses the connecting stopper (6)
externally and connects to this in a gas-tight manner preferably by
way of a fluid seal (14).
Inventors: |
Ebner, Peter; (Leonding,
AT) ; Lochner, Heribert; (Leonding, AT) |
Correspondence
Address: |
Kurt Kelman
COLLARD & ROE, P.C.
1077 Northern Boulevard
Roslyn
NY
11576
US
|
Family ID: |
3685686 |
Appl. No.: |
09/886719 |
Filed: |
June 21, 2001 |
Current U.S.
Class: |
110/336 |
Current CPC
Class: |
C21D 9/562 20130101;
C21D 9/0043 20130101; C21D 9/66 20130101 |
Class at
Publication: |
110/336 |
International
Class: |
F23M 005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 29, 2000 |
AT |
A 1113/2000 |
Claims
1. A tower furnace for heat treatment of metal strips having a
pre-heating section (3) and a high-temperature section (5)
connected thereto at the top forming a housing (4) separate from
the pre-heating section (3), whereby the pre-heating section (3)
equipped with a preferably gas-heated muffle (7) exhibits a
connecting stopper (6) made of heat-insulating material for the
high-temperature section (5) inserted into the muffle (7), whereby
the high-temperature section (5) is also fitted with a preferably
gas-heated muffle (12) which encloses the connecting stopper (6)
externally and connects to this in a gas-tight manner preferably by
way of a fluid seal (14).
2. Tower furnace as claimed in claim 1, whereby the connecting
stopper (6) has a through cross-section for the metal strip to be
treated with a minimum width (18) measured vertically to the metal
strip and corresponding to half the external radius of the
connecting stopper (6).
Description
FIELD OF THE INVENTION
[0001] The present invention relates to a tower furnace for heat
treatment of metal strips having a pre-heating section and a
high-temperature section connected thereto at the top forming a
housing separate from the pre-heating section, whereby the
pre-heating section equipped with a preferably gas-heated muffle
exhibits a connecting stopper made of heat-insulating material for
the high-temperature section inserted into the muffle.
DESCRIPTION OF THE PRIOR ART
[0002] Metal strips, in particular those made of chrome steel or
chrome-nickel steel, are continuously bright-cooled in an inert gas
atmosphere of hydrogen or a hydrogen-nitrogen mixture in so-called
tower furnaces which can be designed with or without a muffle. The
advantage of the heat treatment of metal strips over an externally
heated muffle is that gas burners can be used without exhaust gases
impairing the inert gas atmosphere in the muffle interior. The
furnace temperature is, however, limited by the heat tolerance of
the muffle. In addition, on account of the quantity of heat to be
transferred from the muffle to the metal strip the length of the
muffle determines the attainable throughput rate of the metal strip
to be treated, such that the muffle clamped in the vicinity of its
upper end is also subjected to load by the dead weight, apart from
the heat load, which results in limiting of the length of the
muffle as a result of the dead weight despite a wall thickness
which increases from bottom to top in the longitudinal direction of
the muffle. In this respect the stability of the special steel used
for the muffle reduced at high temperature loads is to be
considered. Tower furnaces without muffles have a fireproof lining
and are heated electrically so that higher furnace temperatures can
be achieved. Since the fireproof furnace lining is somewhat porous
when the tower furnace is opened it absorbs ambient air which has
to be rinsed out by use of nitrogen gas, for example, before
trouble-free heat treatment of the metal strips under inert gas
atmosphere can be ensured. After they are opened tower furnaces
without muffles are not ready for service again for several days on
account of the required rinsing periods. There is also considerable
hydrogen consumption, as the residue of atmospheric oxygen
remaining after rinsing binds together with the hydrogen of the
inert gas to form water.
[0003] In order to achieve high throughput of the metal strips to
be treated, in spite of the restrictions associated with a muffle
insert, it is known (EP 0 675 208 A1) to connect a high-temperature
section without a muffle to the pre-heating section of a tower
furnace, fitted with a muffle, which forms a housing separate from
the pre-heating section, which is in turn set on the housing of the
pre-heating section. The metal strip to be treated exits from the
muffle of the pre-heating section through a connecting stopper made
of heat-insulating material inserted into the top of the muffle
into high-temperature section, where it is heated by means of the
electrical heating to the required final temperature before it
reaches a cooler set on the top of the tower furnace. Because of
the high-temperature section without a muffle an advantageously
lower outlet temperature for the muffle insert of the strip to be
treated from the muffle can be taken into consideration so that the
restrictions imposed by the heat and weight loads of the muffle
cannot have an effect on the outcome of the heat treatment or the
throughput rate. However, it is a disadvantage that the
high-temperature section with its fireproof lining stipulates a
rinse period which is considerably longer compared to muffle
furnaces after the furnace is opened, with a corresponding increase
in hydrogen consumption. Additionally, in the vicinity of the
high-temperature section the economically more beneficial gas
heating must be dispensed with in favour of more expensive
electrical heating.
SUMMARY OF THE INVENTION
[0004] The object of the present invention therefore is to create a
tower furnace of the type outlined initially for heat treatment of
metal strips, such that despite the restrictions imposed by the
heat and weight loads of the muffle the required final strip
temperature can be achieved without the disadvantages associated
with the use of a high-temperature section without a muffle having
to be considered.
[0005] The invention solves the task by the high-temperature
section also being provided with a preferably gas-heated muffle
which encloses the connecting stopper externally and connects to
this preferably by way of a fluid seal in a gas-tight manner.
[0006] Since as a result of these measures the high-temperature
section also has a muffle the advantages of a muffle can be
exploited beneficially in the high-temperature section,
particularly with respect to rinsing and possible gas heating. Due
to restriction of the length of the muffle in the high-temperature
section the muffle load can be limited by the dead weight, leading
to a higher temperature capacity on account of the associated
lesser stability requirements, which ensures the required end
temperature of the metal strips to be treated. Only a transfer as
heat loss-free as possible between the pre-heating section and the
high-temperature section by way of the connecting stopper is to be
provided. For this purpose the connecting stopper is enclosed
externally by the lower end of the muffle of the high-temperature
section, whereby a gas-tight connection must be attained between
connecting stopper and muffle of the high-temperature section, and
this with the possibility of axial displacement of the muffle
compared to the connecting stopper for equalising heat expansion of
the muffle. A fluid seal known per se can be used for this purpose
to advantage. But it is also possible to produce the gas-tight
connection by using a bellows-like sleeve positioned between muffle
and connecting stopper. The heat-insulating fireproof material of
the connecting stopper has a relatively small volume, such that the
rinsing procedure is barely influenced by the connecting stopper.
Since the connecting stopper is located in the vicinity of the
thermal radiation and the muffle of the pre-heating section as well
as the muffle of the high-temperature section, strip heating
substantially covering the length of the furnace can be assumed
having an advantageous effect on the required overall length of the
furnace.
[0007] For ensuring strip heating by the connecting stopper which
is as unhindered as possible the connecting stopper should be
provided with a sufficiently large through cross-section for the
metal strips so that the heat radiation of both muffles can extend
into the region of the connecting stopper also. For this very
purpose the connecting stopper can have a through cross-section for
the metal strip to be treated with a minimum width measured
vertically to the metal strip and corresponding to half the
external radius of the connecting stopper.
BRIEF DESCRIPTION OF THE DRAWING
[0008] The object of the invention is illustrated in the diagram,
in which:
[0009] FIG. 1 shows a tower furnace according to the present
invention for heat treatment of metal strips in an exposed view in
a diagrammatic longitudinal section, and
[0010] FIG. 2 shows a section through the connecting stopper along
line II-II of FIG. 1 on an enlarged scale.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0011] The tower furnace as illustrated for heat treatment of metal
strips exhibits a stand I bearing a housing 2 for a pre-heating
section 3 and a housing 4 for a high-temperature section 5 above
pre-heating section 3. Both these furnace sections 3 and 5 are
connected to one another by way of a connecting stopper 6 made of a
heat-insulating fireproof material. Lower pre-heating section 3 is
fitted with a muffle 7 which is inserted into housing 2 from above
and which can also disassembled upwards if housing 4 of the
high-temperature section is specifically moved sideways in furnace
stand 1. Standard gas burners, not illustrated here for the sake of
clarity, are employed for heating muffle 7, by means of which
annular space 8 between muffle 7 and fireproof lining 9 of housing
2 is heated. The lower end of muffle 7 is formed by a stopper 10 in
conventional manner, whereby a fluid seal 11 is provided in the
form of an annular space enclosing the stopper for gas-tight
connection between muffle 7 and stopper 10, which is filled with a
sealing fluid such as oil and projects into the lower end of muffle
7.
[0012] High-temperature section 5 of the tower furnace is designed
in like fashion. With its lower end muffle 12 of high-temperature
section 5 inserted into housing 4 from above encloses connecting
stopper 6 which bears on its outside an annular space 13 for a
fluid seal 14, such that the lower end of muffle 12 dips into the
bath of fluid seal 14. This effectively ensures gas-tight
connection of muffle 12 on connecting stopper 6, without
obstructing equalising of thermal expansion of muffle 12. Muffle 12
is heated advantageously by means of gas burners whose waste gases
flowing into annular space 15 between muffle 12 and housing lining
16 heat muffle 12.
[0013] Due to the tower furnace being divided into a pre-heating
section 3 and a high-temperature section 5 the required final
temperature for the metal band to be treated can be reached,
despite the respective use of a muffle 7 or 12 and without any fear
of overloading muffles 7 and 12. The result of the lower
temperature in pre-heating section 3 and the restricted length of
muffle 7 is the advantageous exploitation of the material
properties of muffle 7 which may have an exemplary length of 26 m.
In the case of the present embodiment muffle 12 for
high-temperature section 5 requires a length of 10 m only for the
desired final strip temperature of 1100.degree. C. for example to
be reached. Owing to the lesser length of muffle 12 the load from
the dead weight remains low, so that the temperature load can be
increased because of the associated lower stability requirements on
the material. By way of comparison the temperature load of muffle 7
can be kept lower, thus allowing higher stability values to be
utilised. This means that at a length of 26 m and made of special
alloy steel, muffle 7 has a weight of ca. 17 t, whereas the weight
of muffle 12 at a length of 10 m is ca. 7 t. A comparable muffle
having a total length of 36 m would have a weight of ca. 33 t. It
is evident that a saving of almost 30% in weight is possible by the
design according to the present invention. In addition, due to the
possibility of installing muffles 7 and 12 separately after one
another the required installation height can be reduced
considerably. While an installation height of ca. 80 m is required
in the case of a continuous muffle with a length of 36 m, the
installation height according to the invention is 60 m only, if a
length of ca. 26 m is estimated for muffle 7 and a length of 10 m
is estimated for muffle 12.
[0014] In order to ensure the most continuous strip heating in the
transition region between preheating section 3 and high-temperature
section 5 the cross-section of passage 17 of connecting stopper 6
is to be selected sufficiently large for the metal strip to be
treated. If the through cross-section exhibits a minimum width 18,
corresponding to half the external radius or a quarter of the outer
diameter of connecting stopper 6 and measured vertically to the
metal strip, as is evident from FIG. 2, advantageous ratios which
have a beneficial effect on the overall length of the tower furnace
result with respect to the penetration coefficient of the thermal
radiation of muffles 7 and 12.
* * * * *