Method for operating annealing furnace

Abstract

The invention relates to a method for operating an annealing furnace with a heating zone for heating sheets. The sheet material is reversibly conveyed through an induction heating device (114) in a heating zone (110) of the annealing furnace in order to increase the temperature, in particular the surface temperature of the sheet material. In order to effectively prevent overheating of the sheet material, thereby effectively preventing formation of unwanted severe scale on the surface of the sheet material, the method according to the invention provides the following steps: b) detecting the surface temperature of the sheet material before and / or within the heating zone (110); c) comparing the detected surface temperature with a predetermined limit value TG of the surface temperature; and d) reversibly conveying the sheet material through the induction heating device (114) only without exceeding the limit value TG of the surface temperature, or d ') sending a signal, in particular to a control device (130) or a control center of the annealing furnace (100), if the limit value TG of the surface temperature of the sheet material is exceeded.

Description

Technical Field

[0001] The present invention relates to a method for operating an annealing furnace with a heating zone for heating plates, particularly thick plates or slabs. Background Technology

[0002] The corresponding annealing furnace and the method for operating the annealing furnace are well known in the prior art, for example, as known from Japanese Patent Application JP 2005146393 A. This Japanese document describes uniformly heating a sheet of metal by reversibly conveying it through an induction heating device. Specifically, the surface of the metal sheet is heated due to the so-called skin effect.

[0003] The disadvantage of the method for operating an annealing furnace known from the Japanese patent application is that it cannot prevent overheating, and therefore cannot prevent the formation of severe oxide scale on the metal sheet. Summary of the Invention

[0004] The object of this invention is to improve the known operating method of a known annealing furnace for heating sheet metal, thereby effectively preventing overheating of the sheet metal and thus preventing the formation of undesirable severe oxide scale on the surface of the sheet metal.

[0005] This objective is achieved by the method claimed in claim 1. Accordingly, the method comprises the following steps: a) The sheet material is reversibly conveyed through an induction heating device within the heating zone of an annealing furnace to inductively increase the surface temperature of the sheet material; b) Detect the surface temperature of the sheet material within the heating zone; c) Compare the detected surface temperature with a predetermined limit value for the surface temperature; and d) The sheet material is reversibly conveyed through the induction heating device only if the surface temperature does not exceed the limit; or d') When the surface temperature of the plate exceeds the limit, a warning signal is issued, especially to the control device or the control center of the annealing furnace.

[0006] The claimed method steps advantageously ensure that the surface temperature of the sheet material does not exceed a limit value when reversibly passing through the induction heating device. The limit value to be followed for the surface temperature of the sheet material is selected to effectively prevent or at least reduce overheating of the sheet material, thereby preventing or at least reducing the formation of undesirable oxide scale on its surface.

[0007] The reversible passage of the sheet material to be protected through the heating device also has the following advantages: the annealing furnace can be constructed to achieve the required temperature increase compared to conveying the sheet material through the furnace in only one direction.

[0008] Using induction heating devices, which replace fossil fuels required for gas heating with electricity, can effectively reduce carbon dioxide emissions and other emissions, such as nitrogen oxides. Furthermore, induction heating devices have the advantage of quickly switching between operating and standby modes, thus significantly reducing energy consumption compared to when the heating device is continuously on.

[0009] The term "plate" includes thick plates and slabs, as well as cut portions of continuously cast slabs.

[0010] Other advantageous designs for the method of operating an annealing furnace according to the invention are the subject of the dependent claims. Attached Figure Description

[0011] This instruction manual is accompanied by a unique Figure 1 It shows a prior art annealing furnace required to implement the method of the present invention. Detailed Implementation

[0012] The following will refer to Figure 1 The annealing furnace will be described in more detail.

[0013] Figure 1 The structure of an annealing furnace 100 is shown. The furnace consists of a heating zone 110 and a holding zone 120 located downstream in the conveying direction R of the passing sheet material. The heating zone 110 itself is divided into induction heating devices 114. A roller conveyor 112 is located upstream of the heating devices in the conveying direction R, in front of them, and a roller conveyor 116 is located downstream of the heating devices in the conveying direction R, behind them. The two roller conveyors 112 and 116 are used to allow the heating zone to operate reversibly, i.e., to reversibly convey the sheet material through the induction heating devices 114. Temperature measuring devices 140, such as pyrometers, can be installed upstream of the heating zone 110 and / or in at least one of the roller conveyors 112 and 116 for locally detecting the temperature of the sheet material. The annealing furnace 100 is also equipped with a control device 130 for adjusting specific operating parameters of the annealing furnace, such as the temperature distribution and / or atmosphere within the induction heating devices 114 and the holding zone 120. Furthermore, the control device 130 is used to control the reversible conveying of the sheet material within the heating zone, and subsequently to control the conveying of the sheet material through the insulation zone 120. If the heating zone and the insulation zone operate with different atmospheres, a gate can be installed between the heating zone 110 and the insulation zone 120. Figure 1 (Not shown in the image).

[0014] The following describes a method for operating such an annealing furnace according to the present invention: Its basic form includes the following steps: a) The sheet material is reversibly conveyed through an induction heating device 114 within the heating zone 110 of the annealing furnace 100 to inductively increase the surface temperature of the sheet material. Its characteristics are: b) Detect the surface temperature T0 of the sheet material before and / or within the heating zone 110; c) Compare the detected surface temperature T0 with a predetermined limit value TG for the surface temperature; and d) The sheet material is reversibly conveyed through the induction heating device 114 only if the surface temperature does not exceed the limit value TG; or d') When the surface temperature of the plate exceeds the limit value TG, a warning signal is issued, especially to the control center of the control device 130 or the annealing furnace 100.

[0015] According to the first embodiment, the method specifies that during step d), the sheet material is conveyed along its entire length from the induction heating device 114 onto roller conveyors 112 and 116 located upstream or downstream of the heating device, the roller conveyors serving as part of the heating zone 110 of the annealing furnace 100. By performing step d) in this way, it is advantageously ensured that all areas of the sheet material, particularly its head and feet, are heated to a uniform temperature, just like the center portion of the sheet material.

[0016] To ensure that the specified limit temperature of the surface of the sheet to be heated is not exceeded, it is important to detect the current surface temperature T0 of the sheet before and / or within the heating zone according to method step b) and compare it with the specified surface temperature limit value TG; see method step c). Furthermore, to meet the limit value TG, the corresponding increase in surface temperature n needs to be pre-calculated in method step d) based on the separately detected current surface temperature T0. ΔT, when the sheet material passes through the induction heating device 114 at least n more times after starting from one of the roller conveyors 112 and 116, the sheet material will experience a corresponding increase in surface temperature. Here, we need to distinguish between the following two cases:

[0017] If, according to the first scenario, the sheet material is placed on roller conveyor 112, which serves as the starting point, upstream of the heating device, then it is necessary to pre-calculate the increase in surface temperature n of the sheet material if it passes through the induction heating device 114 an odd number of times n (especially passing through the induction heating device once more). What will ΔT be?

[0018] In the first case, the odd number of times n is because the sheet material must fall onto the roller conveyor 116 downstream of the heating device 114 after n times.

[0019] The second scenario to consider is that the sheet material is located on roller conveyor 116, which is downstream of the heating device 114 and serves as the starting point, along the conveying direction R. In this case, it is necessary to calculate in advance what effect the sheet material's surface temperature will have if it is conveyed past the induction heating device 114 an even number of times n (at least two more times).

[0020] In this second case, the number of times n is even because the sheet material must fall onto the roller conveyor 116 again after n times.

[0021] After n passes, in both cases, the sheet material is conveyed from the roller conveyor 116 downstream of the heating device 114 along the conveying direction R to the downstream insulation zone 120 without further heating. For this n-passage through the heating device 114, it is necessary to pre-calculate whether the limit temperature TG is met according to the following formula: (1) T0+n ΔT≤TG.

[0022] If it is shown in formula (1) that the plate follows the limit temperature when passing through the induction heating device 114 n times, then method step d) will be performed, and the plate passes through the heating device 114 n times as pre-calculated can be performed as pre-calculated.

[0023] If, according to formula (1), the surface temperature increase n from T0 is... ΔT is so large that it exceeds the limit value of the plate surface temperature, i.e., T0+n, during the nth process of passing through the induction heating device 114. If ΔT>TG, then execute method step d'), and then specifically output a signal to the control device 130 or the control center of the annealing furnace 100. The control device must then either shut down the induction heating device 114 or reduce the temperature in the induction heating device 114 to a certain extent at the end of n passes, so that the set limit value TG of the plate surface temperature is not exceeded.

[0024] To transport the sheet metal from the heating zone 110 to the downstream holding zone 120 of the annealing furnace 100, the conveying speed of the sheet metal is preferably increased to between 0.5 m / s and 1 m / s. This allows for a rapid transition of the sheet metal from one zone to another without causing unnecessary atmosphere mixing. An increase in oxygen content in the holding zone leads to an increase in oxide scale formation.

[0025] The aforementioned limit value TG for the surface temperature of the sheet is set to 700°C, preferably 650°C, or more preferably 600°C. This setting depends on the material and effectively prevents the formation of oxide scale on the surface of the sheet; preferably, at temperatures below these, the formation of oxide scale is at least significantly reduced.

[0026] According to method step d), the above-mentioned detection of the surface temperature T0 of the plate is preferably performed in a non-contact manner using a pyrometer as the temperature detection device 140. The temperature detection device is preferably arranged in front of the entrance of the heating zone 110, in the area of ​​roller conveyor 112 upstream of the heating device 114, or in the area of ​​roller conveyor 116 downstream of the heating device 114.

[0027] Because the temperature within the heating zone 110 is relatively low, particularly since the temperature there is below the surface temperature limit TG, it is advantageous to provide a non-inert gas atmosphere, preferably air, within the heating zone 110 without further increasing the risk of oxide scale formation on the sheet. Since the sheet is further heated within the heating zone 110 after passing through it, it is beneficial to provide an inert gas atmosphere only in the heating zone.

[0028] During step d) of the method, i.e. while the board remains in the heating zone 110, the average board temperature TM will rise, for example, to more than 550°C; this is the result of the board passing through the induction heating device 114 n times.

[0029] The time it takes for the sheet material to pass through the induction heating device 114 is at least 5 times shorter, preferably 10 times shorter, than the time required for conventional gas-fired preheating of sheet material. This results in a smaller temperature difference between the head and tail of the sheet material.

[0030] The number of times the board needs to be heated depends on its thickness. For example, a board with a thickness of 5 mm needs to be heated approximately 5-10 times, while a board with a thickness of 20 mm needs to be heated approximately 20 times.

[0031] The induction heating device 114 preferably operates using alternating current with a frequency of 1-10 kHz. A suitable alternating current power supply is already available; below 1 kHz, the induction device will produce unpleasant noise.

[0032] List of reference numerals in the attached diagram: 100 Annealing Furnace 110 Heating Zone 112 Roller Conveyor 114 Heating device 116 roller conveyor 120 Insulated Area 130 Control Device 140 Temperature measuring device, such as a pyrometer The conveying direction of the R-plate from the heating zone to the insulation zone The maximum surface temperature of TG sheet material TM Average board temperature Current surface temperature of T0 sheet ΔT is the increase in surface temperature each time it passes through the induction heating device. n is the number of times the board material passes through the heating device.

Claims

1. A method for operating an annealing furnace (100) for heating a sheet metal, the method comprising the following steps: a) The plate is reversibly conveyed through an induction heating device (114) in the heating zone of the annealing furnace (100) to inductively increase the surface temperature of the plate; Its features are, b) Detect the surface temperature of the plate before and / or within the heating zone (110); c) Compare the detected surface temperature with a predetermined limit value TG for the surface temperature; and d) The plate is reversibly conveyed through the induction heating device (114) only if the surface temperature does not exceed the aforementioned limit value TG; or d') When the surface temperature of the plate exceeds the limit value TG, a signal is sent, in particular to the control device (130) or the control center of the annealing furnace (100).

2. The method according to claim 1, Its features are, During step d), the plates are conveyed along their entire length from the induction heating device (114) onto roller conveyors (112, 116) located upstream or downstream of the heating device, which are part of the heating zone (110) of the annealing furnace (100).

3. The method according to any one of the preceding claims, Its features are, During method step d), based on the corresponding current surface temperature T0 of the plate, the increase in surface temperature n that the plate will experience under the following conditions is pre-calculated. ΔT: According to the first case, the sheet material is conveyed through the induction heating device (114) an odd number of times (n) starting from the roller conveyor (112) located upstream of the heating device (114), and conveyed at least once more; or, according to the second case, the sheet material is conveyed through the induction heating device (114) an even number of times (n) starting from the roller conveyor (116) located downstream of the heating device (114), and conveyed at least twice more; and, If the following conditions are met based on the pre-calculation, then proceed to method step d): T0+n ΔT≤TG;(1) Alternatively, if the following condition is met based on the pre-calculation, then method step d') is executed: T0+n ΔT>TG(2) in The number of times the device passes through the induction heating device.

4. The method according to claim 3, Its features are, In response to a warning signal in either the first or second scenario, the control device (130) shuts off the induction heating device (114) or reduces its temperature to a level that allows the plate to pass through the induction heating device n times without exceeding the limit value TG; or In the second case, in response to the warning signal, the control device (130) terminates the method step d), the plate no longer passes through the induction heating device (114), and the plate is transported from the heating zone (110) to the heat preservation zone (120) downstream of the annealing furnace (100).

5. The method according to claim 4, Its features are, In order to transport the plate from the heating zone (110) to the heat preservation zone (120) downstream of the annealing furnace (100), the conveying speed of the plate is increased to about 0.5 m / s to 1 m / s.

6. The method according to any one of the preceding claims, Its features are, The maximum surface temperature (TG) of the plate is 700°C, preferably 650°C, and more preferably 600°C.

7. The method according to any one of the preceding claims, Its features are, According to step b) of the method, the surface temperature of the plate is detected in a non-contact manner using a temperature measuring device (140), such as a pyrometer, preferably before the entrance of the heating zone (110), in the area of ​​the roller (112) upstream of the heating device (114), or in the area of ​​the roller (116) downstream of the heating device (114).

8. The method according to any one of the preceding claims, Its features are, The atmosphere within the heating zone (110) is formed by a non-inert gas, preferably air; and An inert gas atmosphere is generated within the insulation zone (120).

9. The method according to any one of the preceding claims, Its features are, During step d), the average plate temperature TM is increased to TM > 550°C.

10. The method according to any one of the preceding claims, characterized in that: The time it takes for the plate to pass through the induction heating device (114) is at least 5 times shorter than the time it takes to preheat the plate, preferably 10 times shorter.

11. The method according to any one of the preceding claims, Its features are, The number of passes n performed in step d) is selected based on the thickness of the plate, for example as follows: 。 12. The method according to any one of the preceding claims, Its features are, The induction heating device (114) operates using AC power with a frequency of 1-10Hz.

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