WO2011042164A1

WO2011042164A1 - Method and hood-type annealing furnace for annealing metal bands at high temperatures

Info

Publication number: WO2011042164A1
Application number: PCT/EP2010/006097
Authority: WO
Inventors: Frank Maschler; Michal Buryan; Horst Wachholder; Peter Wendt; Hardy Maass; Achim Beutel
Original assignee: Loi Thermprocess Gmbh
Priority date: 2009-10-09
Filing date: 2010-10-06
Publication date: 2011-04-14
Also published as: PL2486157T3; TR201802112T4; EP2486157B9; EP2486157A1; EP2486157B1

Abstract

The invention relates to a method and a hood-type annealing furnace for annealing, at high temperatures, metal bands, sheet metal or wire, in particular grain oriented electric bands in the form of coils in an annealing chamber (5) under protective gas or in a protective gas atmosphere in an hood-type furnaces having a hearth with at least one charging area for a charge in the form of a coil or a coil-stack. Said hearth comprises at least one protective cover (4) which covers the charge and under which the annealing chamber (5) is embodied, a dynamic seal between the protective cover (4) and the hearth and a heating cover (1) which surrounds the protective cover (4) at a distance and under which the heating chamber is arranged. According to the invention, the heating chamber (3) is heated by burners (2), the open flames thereof being directed into the heating chamber (3) and a low pressure of at least 10 Pa and a maximum of 200 Pa in the annealing chamber (5) is maintained in relation to the heating chamber (3).

Description

Process and annealing furnace for high temperature annealing of metal strip

The invention relates to a method for high-temperature annealing of metal strip, sheet metal or wire, in particular of grain-oriented electrical steel in the form of coils in a hot space under inert gas or in a protective gas atmosphere in a hood furnace with a stove with at least one parking space for a batch in the form of a coil or a coil stack, with at least one protective cover which covers the charge and under which the glow space is formed, with a dynamic seal between the hood and stove and with a protective hood at a distance surrounded heating hood, under which a boiler room is formed. Furthermore, the invention relates to a corresponding bell annealing furnace.

The process for high temperature annealing or coarse grain annealing of grain oriented electrical steel is an annealing process at very high temperatures, such as between 1100 ° C and 1200 ° C. Grain-oriented electrical steel is used for the iron core or wound cores of

Transformers or generators used. It is silicon-alloyed steel sheet, which has good magnetic properties. The electrical steel is wound into batches or coils or stacked.

It is known to batch high temperature annealing of electrical steel

Use hood annealing furnaces.

A bell annealing furnace usually has a stove with at least one parking space for a batch in the form of a coil or a coil stack, a protective cover covering the batch, a heating hood and a cooling hood, which is used as an alternative to the heating hood. Under the guard, a glow space is formed. The heating hood defines a heating space, which is designed essentially as an annular space between the protective hood and the heating hood.

In the high-temperature annealing process of grain-oriented electrical steel, first the annealing space is purged with nitrogen (N ₂ ) to remove the atmospheric oxygen. It is then heated to a temperature of 600 to 850 ° C and possibly held for several hours. During this time, it is purged with nitrogen. When rinsing becomes continuous or at specified intervals inert gas introduced into the annealing space, while the inert gas atmosphere flows with a defined volume flow.

Thereafter, the annealing space is rinsed with a mixture of nitrogen (N2) and hydrogen (H2). The annealing space or the electrical steel is slowly heated further. After completion of the crystal orientation in the electrical steel, the purge is switched to pure hydrogen (H2). Then it is heated with adjustable gradient, d. H. the temperature is lowered to 650 ° C and below. Subsequently, the hydrogen (H2) in the annealing space under the protective hood is flushed out with nitrogen (N2). Afterwards the heating hood can be removed. The protective hood is sealed off from the hearth by means of a dynamic seal, in particular one or more sand cups. Although such a gasket is not gas-tight, it is used in practice because of the high annealing temperatures. Because of the high annealing temperatures, a gas-tight seal is structurally very expensive and is therefore not used. Therefore, protective gas can escape through the sand cup seal between the guard and stove in the boiler room. The boiler room, bounded on the outside by the heating hood, is sealed against the environment by a water cup. The removal of the protective gas from the boiler room is done by an exhaust pipe, which usually contains a pressure control element. The discharged shielding gas is then burned outside the heating hood in a torch.

Dome annealing furnaces, where the protective hood is sealed off from the stove with a dynamic seal, usually with a sand cup, have hitherto only been electrically heated. The electric heating causes comparatively high heating costs. Against a heating of the boiler room with burners speaks that oxygen-containing atmosphere from the boiler room can possibly get into the hydrogen-filled space under the protective hood, which is a security problem and also affects the batch.

The invention has for its object to overcome the disadvantages of the prior art.

This object is achieved by a method having the features of claim 1. According to the boiler room is heated with burners whose flames are directed open in the boiler room, wherein held in the annealing space overpressure against the boiler room, which is at least 10 Pa and at most 200 Pa.

Due to the heating by means of fuel, the energy costs are reduced compared to an electric heating. Experiments have surprisingly shown that low excess pressures in the aforementioned order are sufficient to eliminate safety concerns with respect to the heating of the boiler room with burners.

Preferably, additional combustion air is supplied to the burners for combustion of the hydrogen, which can escape from the annealing space through the dynamic seal in the boiler room. Since the flames of the burners are openly directed into the boiler room, the hydrogen can be easily rendered harmless. The combustion air required for the combustion of the hydrogen can be supplied to the burners by means of a combustion air line, which opens into the boiler room. Alternatively, the combustion air flow to the burners can be increased.

According to a further feature of the invention, the overpressure between the annealing chamber and heating chamber during the annealing process is continuously monitored and initiated at a drop in overpressure depending on the falling below a threshold emergency rinsing. Thus, the quality of the dynamic seal or sand cup seal is continuously monitored during the annealing process.

As long as the limit value is not undershot, the purge rate in the annealing space can be increased. For this purpose, preferably the gas flow is increased, which is introduced continuously or at predetermined intervals during the annealing process in the annealing space.

The emergency purging can be initiated during the entire annealing process to remove hydrogen-containing inert gas atmosphere from the annealing chamber and the boiler room.

Preferably, during the emergency purging, the annealing space and the heating space are each flushed with an inert gas volume flow, wherein the ratio of the inert gas volume flow, with which the heating space is purged under the heating hood, to the inert gas volume flow, with which the annealing space is purged, larger than 25. Advantageously, the flames of the burner are directed into the boiler room in such a way that an equidirectional annular flow of the exhaust gases is generated in the boiler room over its entire height. This achieves a uniform temperature distribution. To achieve the object of the crucible annealing furnace according to the invention is characterized in that the heating hood has a plurality of operated with a liquid or gaseous fuel burners whose flames are open in the boiler room and that in the annealing chamber an overpressure relative to the boiler room is maintained, the at least 10 Pa and at most 200 Pa. A development is characterized by at least one group of vertically stacked burners.

Preferably, the end faces of the heating hood are each a group of vertically stacked burners, which are directed in the flow direction of the exhaust gases against each other, such that a co-current exhaust gas flow is formed in the boiler room. It is generated in the same way annular flow of exhaust gases in the boiler room over its entire height. Due to the annular flow of the exhaust gases and the good circulation of the exhaust gases of the vertically stacked burner a good temperature uniformity over the entire height of the bell annealing furnace is achieved. Preferably, the burners are designed as high-speed burners.

The crucible annealing furnace according to the invention is equipped with an emergency flushing device for flushing the heating chamber and the annealing space with inert gas, wherein in the annealing space under the protective hood and the heating chamber under the heating hood each open an inert gas supply. The emergency flushing device ensures that a fuel-fired bonnet furnace, whose combustion chamber is not gas-tight against the stove, can be safely operated.

The method for high-temperature annealing according to the invention will be explained with reference to the drawing. In the drawing shows Fig. 1 is a schematic plan view of a crucible annealing furnace;

Fig. 2 is a schematic front view of a crucible annealing furnace according to the

Invention;

Fig. 3 is a diagram with a Notspülverfahren according to the invention. Fig. 1 and Fig. 2 show schematically a heating hood 1 of a bell annealing furnace. The heating hood 1 is heated by means of two groups of burners 2 in the form of high-speed burners, preferably recuperative burners. The burners 2 of each group are arranged one above the other at the end faces of the heating hood 1 and burn with open flames in a substantially annular heating chamber 3. The arrangement of the burner groups takes place against each other to a good sense of exhaust gas flow a good circulation of the flue gases and thus to achieve a good temperature uniformity.

The burners 2 are supplied with additional combustion air for combustion of the hydrogen material which escapes from the combustion space 5 through the dynamic seal into the heating space 3. Since the flames of the burner 2 are directed open in the boiler room 3, the hydrogen can be made harmless in a simple manner.

Under the heating hood 1 there are three protective hoods 4, which are sealed to a stove, not shown, with dynamic seals, not shown, in the form of sand cups. Under the protective hoods 4 each have a glow space 5 is formed in which the batch is located.

As soon as the temperature in the heating chamber exceeds 750 ° C., the annealing spaces 5 can be flushed with hydrogen. The sand cups are not gas-tight. Hydrogen, which passes through the sand cup seal in the heating chamber 3, is burned by means of excess air to the burners 2, which is dosed appropriately. In the heating chamber 3 under the heating hood 1 and in each annealing chamber 5 each lead an inert gas supply 6 an emergency flushing device for simultaneous flushing of the heating chamber 3 and the annealing chamber 5 with inert gas, in this case nitrogen. In the emergency purging, the ratio of the inert gas volume flow, with which the heating chamber 3 is purged, and the volume flow, with which the Glühräume 5 are purged, greater than 25. In the case of emergency purging, consequently, the heating chamber 3 is flushed with a large volume of inert gas and at the same time, in each case, the glaciers 5 are flushed with a small volume of intergas.

In case of failure, such. B. in case of failure of the heating or a power failure, the system is automatically brought to a safe state. Normally, the hydrogen is quickly expelled by a nitrogen purge of the protective hoods 4 and enters the heating chamber. 3

However, the conditions for a safe combustion of the flushed out hydrogen in the heating chamber 3 by means of the burners 2 are not always present in any case. For example, the temperature may now be too low or there is no combustion air available due to power failure. Also in these cases, the Notspülverfahren can be initiated.

In Fig. 3 the Notspülverfahren invention is shown. The diagram shows the inert gas volume flow (ordinate, in n / h) and the hydrogen and oxygen concentration in% as a function of time (abscissa, in min). The lower explosive limit of hydrogen is set at 2%.

At time zero, the boiler room 3 is filled with air under the heating hood 1, in the heating chamber 5 under the protective hoods 4 is pure hydrogen.

Each of the three glaciers 5 under the protective hoods 4 is flushed with 2 rrf / h of nitrogen, the heating chamber 3 under the heating hood 1 with 240 rrf / h. The ratio of the nitrogen volume flow with which the heating hood (240 rrrYh) is purged and the nitrogen volume flow with which the protective hoods (3 x 2 nf / h = 6 rr ^ / h) are rinsed here is 40. At ratios below 25 there are impermissibly high maxima of the hydrogen concentration in the heating hood. The process temperature should be 400 ° C, the gases expand accordingly and thus speed up the flushing process. The maximum hydrogen concentration in the heating hood is less than 2% and is reached after approx. 10 min. At this time, the oxygen content is also about 2%. In the course of the hydrogen content in the heating hood 3 goes quickly to zero. The rinsing of the glaze chambers under the protective hoods 4 is continued until the hydrogen is also rinsed below the protective hoods 4 to less than 2%, which in the present Example after approx. 360 min is reached. This heating mantle 1 and 4 protective covers are in safe condition and the emergency flushing is terminated.

Modifications are readily possible within the scope of the invention. For example, the combustion air can be preheated by means of a central recuperator. Further, the burners 2, the combustion air required for the combustion of the combustion air can be supplied by means of a combustion air duct, which opens into the heating chamber 3.

Claims

claims

Method for high-temperature annealing of etallband, sheet metal or wire, in particular of grain-oriented electrical steel in the form of coils in a heat space (5) under inert gas or in a protective gas atmosphere in a hood furnace with a stove with at least one parking space for a batch in the form of one Coil or a coil stack, with at least one protective cover (4), which covers the charge and under which the annealing chamber (5) is formed, with a dynamic seal between the protective cover (4) and stove and with a protective hood (4) with Distance surrounding the heating hood (1) under which a heating chamber (3) is formed,

characterized,

in that the heating space (3) is heated by burners (2) whose flames are openly directed into the heating space (3) and that in the heating space (5) an overpressure is maintained with respect to the heating space (3) which is at least 10 Pa and at most 200 Pa is.

Method according to one of claims 1 to 3,

characterized in that the burners (2) additional combustion air for combustion of the from the annealing chamber (5) through the dynamic seal in the heating chamber (3) escaping hydrogen is supplied.

Method according to claim 1 or 2,

characterized in that the overpressure between the annealing chamber (5) and the heating chamber (3) is continuously monitored during the annealing process and in case of a decrease in the overpressure depending on the falling below a limit value an emergency purge is initiated.

Method according to claim 3,

characterized in that the annealing space (5) and the heating space (3) are flushed during the emergency purging, each with an inert gas volume flow, wherein the ratio of the inert gas volume flow, with which the heating space (3) is purged, to the inert gas volume flow , is flushed with the annealing chamber (5), greater than 25.

Method according to one of claims 1 to 4,

characterized in that the flames of the burner (2) in the

Boiler chamber (3) are directed that a same direction annular flow of the exhaust gases in the boiler room (3) over its entire height is generated.

6. bell annealing furnace for high-temperature annealing of metal strip, sheet or wire, in particular komorientiertem electrical steel in the form of coils in a space under a protective gas or in a protective gas atmosphere in a hood furnace with a stove with at least one parking space for a batch in the form of a coil or a coil stack, with at least one protective cover (4) covering the charge and under which the annealing chamber (5) is formed, with a dynamic seal between the protective hood (4) and stove and with a protective cover (4) at a distance surrounded heating mantle (1) under which a heating chamber (3) is formed,

characterized,

the heating hood (1) has a plurality of burners (2) operated with liquid or gaseous fuel whose flames are openly directed into the heating space (3) and that an overpressure relative to the heating space (3) is maintained in the heating space (5) which is at least 10 Pa and at most 200 Pa.

7. bell annealing furnace according to claim 6,

characterized by at least one group of vertically stacked burners (2).

8. bell annealing furnace according to claim 7,

characterized in that the front sides of the heating hood (1) are each a group of vertically stacked burners (2), which are directed in the flow direction of the exhaust gases against each other, such that a co-directional exhaust gas flow in the heating chamber (3).

9. annealing furnace according to one of claims 6 to 8,

characterized,

characterized in that the burners (2) are designed as high-speed burners.

10. bell annealing furnace according to one of claims 6 to 9,

characterized by an Notspüleinrichtung for purging the annealing space (5) under the protective cover (4) and the heating chamber (3) under the heating hood (1) with inert gas, wherein in the annealing chamber (5) and in the heating chamber (3) each have an inert gas supply ( 6).