EP2738502A1 - Hood type annealing furnace and method for operating same - Google Patents

Abstract

The furnace (1) has a base (2) which is mounted with a protective hood (6). The protective gas heated by a gas burner (14) is circulated under the protective hood by a fan (4). A steel belt rolled up with a coil (8) is heated to 500 [deg] C in a protective gas atmosphere. An electrical heater (20) is powered by an electrical heating device (18) by heating power of gas burner at a temperature above the recrystallization annealing temperature. An independent claim is included for a method for operating box-type annealing furnace.

Description

The invention relates to a crucible annealing furnace and a method for operating such according to the preamble of claims 1 and 11.

In the production of cold rolled steel strip, also referred to in the art as "cold strip", a hot strip previously produced by rolling above the recrystallization temperature of the steel is reduced to a thickness of less than 3.0 by one or more further cold rolling operations at temperatures below the recrystallization temperature mm rolled down, which can produce sheet in a thickness range of 3 mm to 0.5 mm or less. As a result of the stretching of the metal structure in the deformation direction during the cold rolling process, the strength of the material increases, whereas its deformability simultaneously decreases due to the resulting strain hardening. Due to the strain hardening, the cold strip is too brittle for direct further processing and, for this reason, is often subjected to recrystallizing annealing following the cold rolling process in order to restore the formability of the material. Recrystallization annealing is a heat treatment of the cold strip, which consists in the material being heated to a defined temperature above the recrystallization temperature of approximately 750 degrees Celsius, the temperature level maintained for a predetermined period of several hours, and then the material in a defined manner is cooled according to a predetermined temperature profile.

The process used therefor is the so-called bell annealing, in which the cold strip is rolled up into a coil, and several of these coils one above the other under a closed, heated by gas burner heating hood at temperatures between 500 degrees and 800 degrees Celsius in one Inert gas atmosphere of H ₂ or NH _{x be} annealed in order not to destroy the laboriously produced bare and smooth surface of the cold strip.

A previously described crucible annealing furnace, which is operated exclusively with gas during the heating phase, is for example from DE 1 186 888 A1 known.

In addition, from the DE 479 851 an annealing furnace is known in which a heater with a gas burner high power is used to heat the circulating in the oven inert gas, which is downstream of the fine regulation of the temperature of the preheated inert gas, an electric heater fluidly. The document gives no indication to heat the shielding gas circulated through the furnace either exclusively with the gas heater or alternatively exclusively with the electric heater.

Accordingly, it is an object of the present invention to provide a bell annealing furnace and a method of operating such a bell annealing furnace that can even out performance variations in a public power grid. This object is achieved by the features of claim 1 and 11.

Further features of the invention are described in the subclaims.

According to the invention, a crucible annealing furnace for thermal treatment of cold-rolled steel strip comprises a base with a protective cover which can be placed thereon, under which an inert gas, preferably hydrogen or another inert gas, is heated by a gas burner and circulated by a fan. The steel strip is preferably a coiled rolled steel strip which has been cold worked by a previous cold rolling operation. The steel strip is heated in the protective gas atmosphere by the heated protective gas for several hours to a temperature which is preferably above the recrystallization annealing temperature for the steel material to be annealed and is at least 500 ° C.

The crucible annealing furnace according to the invention is characterized in that it contains, in addition to the gas burner, an electrical heating device which has a thermal heating capacity which substantially corresponds to the heating power of the gas burner and which is designed such that the steel strip in the crucible annealing furnace is alone by the electric heating device when the gas burner is switched off to a temperature above 500 ° C., it is preferred above the recrystallization annealing temperature of the steel material can be heated, which is usually in the range of about 730 ° C.

Although in principle the heating of the furnace can be done by only a single gas burner, however, several such gas burners are preferably provided whose flames are directed to the outer sides of a protective hood, in the interior of which the protective gas and the steel strip coils to be heated are accommodated.

The hood annealing furnace according to the invention has the advantage that it can be operated to the highest degree economically. So it is initially possible to operate the oven at a high utilization of the public power grid in a known manner alone on the gas burner, which is due to the high efficiency and compared to electricity cheap energy costs for the gas used for this purpose, in which it preferably to natural gas, resulting in comparatively favorable operating costs.

In particular, when there is an oversupply of energy due to overcapacities in the public power grid, the operation of the gas burners can be interrupted within a short period of time, eg within 1 to 2 minutes, and the electric heater can be activated, thereby increasing the peak load in the public grid is advantageously reduced. In the usual dimensions of the hood annealing furnace according to the invention, for example, the heating power of, or the gas burner and preferably also the electric heater in each case in total in the range of, for example, 1800 kW, which over the duration of a heating phase, the z. B. may be 10 hours, results in a significant relief of the public power grid. Since the electrical energy in times of an overcapacity of electrical energy in the public power grid of large consumers, which at any time reduce power in the range of about 2000 KW, can be obtained much cheaper or even free, the hood annealing furnace according to the invention during the aforementioned peak load phases can be cheaper than operate with gas. The background to this is that there is considerable interest on the part of the network operators in preventing damage to the network and the consumers connected to it by overvoltages.

Furthermore, the inventive combination of electric heater and gas heater opens at a simultaneous operation of the same possibility to shorten the heating phases desired case, thereby reducing in particular the time for a single annealing and thereby the economic efficiency of a system with a variety of Haubenglühöfen, z. B. 10 or 20 Haubenglühöfen, can increase overall, although initially arise when heating the Haubenglühofens invention due to the additional energy used in addition to the gas higher costs.

In order to interrupt the occurrence of an excess of electrical energy in the public power grid, the gas supply to the burner and in the connection or simultaneously to activate the electric heater, a control device is preferably provided. This deactivates the burners and preferably activates the electric heater on the basis of external commands or data, e.g. be supplied from a central control center of the power grid operator. Alternatively, there is the possibility that the control device monitors the instantaneous voltage and / or frequency position in the associated part of the power network, if necessary, and automatically deactivates the gas burner when exceeding predetermined setpoints, which are a measure of the instantaneous load of the power grid electric heater activated.

In the preferred embodiment of the invention, the electric heater comprises a plurality of individual lower power electric heaters, and the control means which controls the heaters electrically connects them to the public power grid one by one, or even in smaller groups, when switching to the electric heating mode. This results in the advantage that the electrical switching power for switching on and off of the heater is many times less than is the case with a simultaneous switching of all heaters, which can be used significantly cheaper electrical switching elements.

In the preferred embodiment of the hood annealing furnace according to the invention, the protective hood is surrounded by an annealing hood, on which the burners are accommodated. Between the protective hood and the annealing hood, in this embodiment of the invention, a combustion chamber is defined in which the flame and the hot exhaust gas of the gas burner act on the outside of the protective hood with thermal energy. The electric heater is in contrast advantageously below the gas burner and / or in a relative to the combustion chamber radially back staggered portion of the heating hood outside the exhaust stream, whereby the electric heater is not additionally heated by the hot exhaust gas of the burner or burner during burner operation , As a result, an additional cooling device for the heater can be omitted, and their life increases due to the cooler working temperatures overall.

In the preferred embodiment of the invention, the electrical heating device acts on the outside of the protective hood, which is made in particular of metal, in particular made of thermally highly resilient steel, with thermal energy to indirectly heat the protective gas circulating in the interior of the protective hood through the wall of the protective hood. The electric heater comprises in this embodiment of the invention, a plurality of electrically operated NIR radiators, which are accommodated on the wall of the preferably thermally insulated heating hood and act on the outside of the guard directly with infrared radiation. Such NIR radiators are known from the prior art and can be purchased in relatively large quantities at comparatively low cost. The known NIR emitters are used for example for local heating of metal workpieces or other material and are characterized by a high degree of efficiency. Through the use of NIR lamps, for example, by the company Heraeus Nobelight GmbH sold in Kleinostheim, Germany as modules, and for example, have a radiation power of 4 KW at a length of 1 m and a diameter of about 2 cm, There is the advantage that the inner wall of the heating hood in the lower part can almost completely occupy with several hundred standing spotlights. The radiators are configured, for example, as rod-shaped tubes and are preferably fastened together with a rear reflector standing next to each other on the inner wall of the heating hood, so that the entire infrared radiation is passed through the reflectors in the direction of the outside of the protective hood. Through the use of the aforementioned radiator there is the advantage that existing protective hoods, regardless of their diameter desired case with a Retrofit heater. In addition, the emitters, which are each operated with a voltage of eg 230 V or 400 V, can be connected with suitable electrical switching devices, such as contactors, with little effort successively or in groups within a few seconds with the relieved power grid Without switching performance occur that can be switched only with considerable technical effort and under special protection.

According to an alternative embodiment of the invention, the protective gas is circulated via a further blower in a closed circuit through a heat exchanger arranged outside the protective hood, which is acted upon by the electric heater from the outside with thermal energy to heat the circulating through the heat exchanger through inert gas , This has the advantage that the electrical heating means, e.g. may also be an inductively acting heater or an ohmic resistance heater, provided heat energy is not transmitted through the wall of the protective cover through but directly to the inert gas, whereby the efficiency is further improved. Another advantage is in this embodiment is that the previously used protective hoods and heating mantles can continue to be used, and the heating power of the electric heater can be adjusted by the choice of the size of the heat exchanger used with relatively little effort to the desired value, the preferred essentially corresponds to the value of the maximum heating power of the burner or and, for example ranging from 1200 kW to 1800 kW.

In the embodiment of the invention described last, it is advantageous if the electrical heating device is arranged inside the heat exchanger and is surrounded by the circulating protective gas. As a result, the temperature that the heater assumes during operation over the temperature of the protective gas, which is constantly monitored during the heating phase, with little effort below the maximum allowable temperature at which the heater can be operated, and of course above the Recrystallization annealing temperature of about 750 ° C is. Also in the last described embodiment of the invention, the electric heating device preferably comprises a plurality of NIR radiators, which are preferably arranged within the heat exchanger and arranged in the interior of the heat exchanger surface component, in particular a metal sheet, which is flowed around by the protective gas, directly with infrared radiation to act on. This results in the advantage that the radiation power supplied to the protective gas can be adapted to the desired power with little effort by enlarging or reducing the surface of the planar component.

According to a further idea underlying the invention, a method for operating a bell annealing furnace described above is characterized in that the controller connects the electrical heater in the public power grid as long as the public grid until the recrystallization annealing temperature or above lying predetermined maximum temperature is reached, and that subsequently the control device separates the electrical connection between the electric heater and the public power grid and at least one further electrical connection to an electric heater at least one substantially identically constructed further Haubenglühofens produces until the excess is reduced in electrical energy in the public power grid.

The invention will be explained below with reference to an inventive arrangement for carrying out the method according to the invention described above with reference to drawings.

Fig. 1

eine schematische Darstellung des erfindungsgemäßen Haubenglühofens während des Aufheizens, beim dem die thermische Energie ausschließlich über Gasbrenner zugeführt wird,

Fig. 2

den Haubenglühofen von Fig. 1, bei dem zur Entlastung des öffentlichen Stromnetzes die thermische Heizenergie während der Aufheizphase ausschließlich über eine elektrische Heizeinrichtung in Form von NIR- Strahlern zugeführt wird, und

Fig. 3

eine weitere Ausführungsform des erfindungsgemäßen Haubenglühofens, bei dem die thermische Energie zur Entlastung des öffentlichen Stromnetzes über eine Heizeinrichtung mit NIR- Strahlern zugeführt wird, die ein flächiges Bauteil innerhalb eines vom Schutzgas durchströhmten Wärmetauschers erwärmen.

In the drawings show:

Fig. 1

a schematic representation of the invention Haubenglühofens during heating, in which the thermal energy is supplied exclusively via gas burner,

Fig. 2

the hood annealing furnace of Fig. 1 in which, to relieve the public power grid, the thermal heating energy is supplied during the heating phase exclusively via an electric heater in the form of NIR lamps, and

Fig. 3

a further embodiment of the hood annealing furnace according to the invention, in which the thermal energy is supplied to relieve the public power grid via a heater with NIR emitters that heat a sheet-metal component within a heat exchanger durchströhmten by the heat exchanger.

As in Fig. 1 and 2 is shown, an inventive annealing furnace 1 comprises a base 2, in which a fan 4 is arranged, which is located on the underside of an attachable to the base 2 protection 6. Coils 8 of cold-rolled steel strip are laid one above the other in a known manner on top of the crucible annealing furnace 1 in a known crucible annealing process to the recrystallization annealing temperature in the range of approximately 700 to 750 degrees Celsius and at this temperature for a predetermined period of time of, for example, 5 to Be held for 8 hours.

In order to protect the coils 8 during the Haubenglühprozesses from oxidation or other chemical changes, the outside gas-tight sealed interior 10 of the protection 6 is acted upon with a protective gas, in particular with hydrogen gas or with nitrogen gas, which via supply lines not shown in the Interior 10 is introduced.

In order to heat the protective gas within the interior 10 to a temperature in the region of the recrystallization temperature during the so-called heating phase, while circulating it between the coils 8 according to the unspecified arrows, the outside of the protective hood 6 with the hot gas is activated during the conventional gas operation a gas burner shown schematically, or a plurality of gas burner 14 heated, which are accommodated on a heating hood 16, which is placed after placing the protective cover 6 above the same. The exhaust gas of the gas burner 14 increases in this case in a defined between the inside of the heating hood 16 and the outer wall of the protective hood combustion chamber 26 on the outer wall of the protective cover 6 and is discharged, for example, by an opening shown by way of example at the top of the heating hood 16 from this. The exhaust gas may optionally be used via a heat exchanger for preheating the combustion air supplied to the burners 14, which, however, is not shown in the drawings for illustrative reasons.

The hood annealing furnace 1 according to the invention further comprises an electric heater, which in the in the FIGS. 1 and 2 shown embodiment of the invention by NIR radiator 20 is formed. These are arranged in the lower region of the heating hood 16 and are connected by a control device 22 preferably in groups successively with the public power grid, which is symbolized by the power pole 18.

During the pure burner operation ( Fig. 1 ) representing the main mode of operation, the controller 22 breaks the electrical line connection to the public power grid 18 so that the electric heater is deactivated. The interruption of the connection to the electrical grid 18 is in FIG. 1 indicated by the open switch 24 A and 24 B.

If there is a short-term excess of electrical energy in the public power grid 18, for example, on a windy and sunny Sunday afternoon suddenly an oversupply of electricity from wind and solar power, the controller 22 interrupts the gas supply to the burners 14, so that they Outside of the protective cover 6 no longer entrust with thermal energy. Following this, the switches 24 A and 24 B, which are representative of a corresponding number of switches, and via which the NIR radiators 20 are connected to the public power grid 18, closed. As a result, the NIR emitters, each having a power of, for example, 5 kW, supplied within a few seconds with electrical energy from the public power grid 18, thereby counteracting the oversupply of electrical energy and the power grid is relieved. The NIR emitters 20 emit their electromagnetic radiation, the one preferably irradiated to the heating of Objects tuned radiation spectrum in the infrared spectral range has, preferably directly on the outside of the protective cover 6 from, whereby it heats up. The heat energy passing through the wall of the protective hood 6 is transferred to the inner wall of the protective hood in the inner space 10 onto the inert gas flowing upwards along the inner wall, which also heats up as a result. The in the manner described above by the radiator 20 indirectly heated inert gas, which is continuously circulated by the fan 4 in the interior 10, the absorbed heat energy thereafter to the coils 8 from when the protective gas flows around the coils 8, as in the FIGS. 1 to 3 indicated by the arrows.

If the excess supply of electrical energy in the public power grid 18 no longer exists, the control device 22 interrupts the power supply to the electric heater, ie to the radiators 20 and again activates the gas supply to the burners 14 and ignites them, so that the protective gas in the interior 10th the protective cover 6 is heated again by the open flames of the gas burner 14, which act on the outside of the protective cover 6. As the representation of FIG. 1 and 2 In this case can be taken in detail, the electric heater is arranged in the form of NIR radiator 20 below the burner 14 so that the hot exhaust gas generated by the burners 14 can rise in the combustion chamber 26 without heating the radiator 20. As a result, the life of the radiator 20 increases remarkably, and contamination of the radiator by the soot components contained in the exhaust gas is also prevented.

According to a further embodiment of the invention, which in FIG. 3 is shown, the heating of the protective gas takes place in a equipped with gas burners 14 annealing furnace 1 of the type described above, alternatively to the arranged in the combustion chamber 26 NIR radiators 20 in that the inert gas is circulated through another fan 102 in a closed circuit 106, which preferably comprises a valve 108 and a heat exchanger 110, through which the protective gas can be passed during the heating phase. In this embodiment of the invention, the heat exchanger 110 contains the heating device in the form of further NIR emitters 120, which, analogously to the emitters 20, are connected in groups to the control device 22, which contacts them when an excess occurs electrical energy in the public power grid 18 via the symbolically indicated switches 124A and 124B groups turns on. As the representation of Fig. 3 In this case, the NIR emitters 120 preferably emit their heat radiation to a planar component 112 arranged in the interior of the heat exchanger 110, in particular a dark metal sheet, which is flowed around by the protective gas and cooled, before the protective gas is supplied to the interior 10 to a temperature of about 800 ° C or more to heat.

As the representations of FIGS. 1 to 3 can be removed, at least one further unspecified heat exchanger in the protective gas circuit 106 can be incorporated, is withdrawn over the inert gas after the completion of the annealing heat energy to cool the coils 8 according to a predetermined temperature curve and the case the inert gas, or The heat energy extracted from the coils 8 is at least partially recovered via devices that are not further shown.

List of reference numbers

1

Hood annealing furnace according to the invention

2

base

4

Blower in pedestal

6

guard

8th

coil

10

Interior of the protective hood

14

gas burner

16

heating cover

18

public power grid / electricity pylon

20

electric heater / NIR lamp

22

control device

24A

switch

24b

switch

26

combustion chamber

102

fan

106

closed protective gas circuit

108

Valve

110

heat exchangers

112

flat component / absorber

120

NIR radiator / heater in embodiment of Fig. 3

124A

switch

124B

switch

Claims (11)

Annealing annealing furnace (1) for the thermal treatment of cold-rolled steel strip, comprising a base (2) with a protective hood (6) which can be placed thereon, under which a protective gas heated by a gas burner (14) is circulated through a fan (4) in order to achieve this a steel coil rolled up under a coil (8) under the protective cover (6) in a protective gas atmosphere to a temperature of at least 500 ° C,
characterized,
in that an electrical heating device (20, 120) fed by the public power grid (18) is provided with a heating power essentially corresponding to the heating power of the gas burner (14), with which the protective gas is switched off automatically to a temperature when the gas burner (14) is switched off can be heated above the recrystallization annealing temperature. Hood annealing furnace according to claim 1,
characterized,
in that a control device (22) is provided which interrupts the gas supply to the gas burner (14) in the event of an excess of electrical energy in the public power grid (18) and activates the electrical heating device (20, 120). Hood annealing furnace according to claim 2,
characterized,
in that the electric heating device (20, 120) comprises a plurality of individual low-power electric heaters, and in that the control device (22) low-power heaters when switching the crucible annealing furnace (1) to electric heating operation individually or in groups one after the other electrically with the public power grid (18) connects. Baking annealing furnace according to one of the preceding claims,
characterized,
in that the protective hood (6) is surrounded by a heating hood (16), that between the protective hood and the heating hood (16) a combustion chamber (26) is defined, in which the flame and the hot exhaust gas of the gas burner (14) are the outside of the protective hood (6) apply thermal energy, and that the electric heater (20) below the gas burner (14) and / or in a relative to the combustion chamber radially back staggered portion of the heating hood (16) is received outside the exhaust stream. Dome annealing furnace according to one of claims 3 to 4,
characterized,
in that the electrical heating device (20) applies thermal energy to the outside of the protective hood in order to indirectly heat the protective gas circulating in the interior (10) of the protective hood (6) through the wall of the protective hood (6). Dome annealing furnace according to one of claims 3 to 5,
characterized,
that the electrical heating means comprises a plurality of NIR radiation emitters (20) which act on the outside of the protective hood (6) directly with infrared radiation. Dome annealing furnace according to one of claims 1 to 3,
characterized,
that the protective gas is arranged on a further blower (102) in a closed circuit (106) through an outside of the protective hood (6) heat exchanger (110) is circulated, and in that the electric heater (120) the heat exchanger (110) from outside with thermal energy is applied to heat the circulating through the heat exchanger through inert gas. Hood annealing furnace according to claim 6,
characterized,
in that the electrical heating device (120) is arranged inside the heat exchanger (110) and is surrounded by the circulating protective gas. Bell annealing furnace according to FIG. 8,
characterized,
that the electrical heating means comprises a plurality of NIR radiators (120), which are arranged within the heat exchanger (110) and arranged one inside the heat exchanger (110) a flat component (112), in particular a metal sheet, which is immersed in the protective gas, apply direct infrared radiation. Baking annealing furnace according to one of the preceding claims,
characterized,
in that the electrical heating device (20, 120) and the gas burner (14) can be operated simultaneously in order to shorten the heating phase of the bell annealing furnace. Method for operating a bell annealing furnace (1) according to one of the preceding claims,
characterized,
that a control means (22) upon the occurrence of an excess of electrical energy in the public power network (18) connects the electric heater (20, 120) as long as the public electricity grid, to the coils (8) have adopted the recrystallization annealing temperature or an overlying predetermined maximum temperature and in that the control device (22) subsequently separates the electrical connection between the electrical heating device (20, 120) and the public power grid (18) and at least one further electrical connection to an electrical heating device of at least one substantially similar further annealing furnace ( 1) until the surplus of electrical energy in the public power grid has been reduced.

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