Method for operating an annealing furnace
Patent Information
- Application Number
- EP2024715720
- Authority / Receiving Office
- EP · EP
- Patent Type
- Applications
- Current Assignee / Owner
- Priority Date
- 2023-04-18
- Filing Date
- 2024-03-19
- Publication Date
- 2026-02-25
AI Technical Summary
Existing annealing furnace methods fail to prevent overheating and excessive scale formation on sheet metal during the heating process, leading to inefficient energy use and environmental emissions.
The method involves reversing transport of sheet metal through an inductive heating device within the annealing furnace, with real-time temperature monitoring and control to maintain a predetermined surface temperature limit, preventing overheating and scale formation, and utilizing inductive heating to reduce energy consumption and emissions.
This approach effectively prevents overheating and scale formation, reduces energy consumption, and minimizes CO2 and nitrogen oxide emissions by optimizing the annealing process with inductive heating, allowing for faster temperature increase and efficient energy use.
Smart Images
Figure EP2024057291_24102024_PF_FP_ABST
Abstract
Description
[0001] Method for operating an annealing furnace
[0002] The invention relates to a method for operating an annealing furnace with a heating zone for heating sheet metal, in particular heavy plate or slabs.
[0003] A corresponding annealing furnace and a method for operating the furnace are generally known in the prior art, for example, from Japanese patent application JP 2005146393 A. This Japanese document describes how a steel sheet is heated evenly by being transported reversibly through an inductive heating device. In particular, the surface of the sheet is heated due to the so-called skin effect.
[0004] The method for operating an annealing furnace known from the Japanese patent application has the disadvantage that it offers no protection against overheating and thus against excessive scale formation on the sheet.
[0005] The invention is based on the object of developing a known method for operating a known annealing furnace for heating sheet metal in such a way that overheating of the sheet metal and thus undesirably heavy scale formation on the surface of the sheet metal is effectively prevented.
[0006] This object is achieved by the method claimed in patent claim 1. Accordingly, the method comprises the following steps: a) reversing the sheet metal through an inductive heating device within the heating zone of the annealing furnace to inductively increase the surface temperature of the sheet metal; b) detecting the surface temperature of the sheet metal within the heating zone; c) comparing the detected surface temperature with a predetermined limit value for the surface temperature; and d) carrying out the reversing transport of the sheet metal through the inductive heating device only as long as the limit value for the surface temperature is not exceeded; or d') issuing a warning signal, in particular to a control device or a control center of the annealing furnace, if the limit value for the surface temperature of the sheet metal is exceeded.
[0007] The claimed process steps advantageously ensure that the surface temperature of the sheet is not exceeded during the reversing passage through the inductive heating device. The limit value for the surface temperature of the sheet is selected such that overheating of the sheet and thus undesirable scale formation on its surface is effectively prevented or at least reduced.
[0008] The claimed reversing passage of the sheet through the heating device also offers the advantage that the annealing furnace can be built significantly shorter to achieve a desired temperature increase than if the sheet were only transported through the furnace in one direction.
[0009] The use of inductive heating systems, and thus the use of electrical energy instead of fossil fuels, as would be required with a gas-fired heating system, offers the possibility of effectively reducing CO2 emissions and other emissions, such as nitrogen oxides. Furthermore, the inductive heating system offers the advantage that it can be switched very quickly between operating mode and standby mode, and its energy consumption is therefore significantly lower than if the heating system were permanently switched on.
[0010] The term "sheet" includes both heavy plate and slabs, as well as cut portions of a cast strand. Further advantageous embodiments of the method according to the invention for operating an annealing furnace are the subject of the dependent claims.
[0011] The description is accompanied by a single Fig. 1 showing an annealing furnace of the prior art as required for carrying out the method according to the invention.
[0012] This annealing furnace is described in more detail below with reference to Fig. 1.
[0013] Fig. 1 shows the structure of an annealing furnace 100. It consists of a heating zone 110 and a holding zone 120 arranged downstream in the transport direction R for a continuous sheet. The heating zone 110 is in turn divided into an inductive heating device 114, which is preceded by a roller table 112 upstream of the transport direction R and followed by a roller table 116 in the transport direction R. The two roller tables 112, 116 serve for a reversing operation of the heating zone, ie for a reversing transport of a sheet through the inductive heating device 114. Before the heating zone 110 and / or in at least one of the roller tables 112, 116
[0014] Temperature measuring devices 140, for example in the form of pyrometers, may be provided for locally detecting the temperature of the sheet. Annealing furnace 100 is also assigned a control device 130 for adjusting certain operating parameters of the annealing furnace, e.g., the temperature distribution and / or the atmosphere within inductive heating device 114 and holding zone 120. Furthermore, control device 130 serves to control the reversing transport of the sheet within the heating zone and to subsequently control the transport of the sheet through holding zone 120. A lock, not shown in Figure 1, may be provided between heating zone 110 and holding zone 120 if the heating zone and holding zone are operated with different atmospheres. The inventive method for operating such an annealing furnace is described below:
[0015] In its basic form, it comprises the following steps: a) Reversing transport of the sheet through an inductive heating device 114 within the heating zone 110 of the annealing furnace 100 for inductively increasing the surface temperature of the sheet; characterized by b) detecting the surface temperature T0 of the sheet before and / or within the heating zone 110; c) comparing the detected surface temperature T0 with a predetermined limit value TG for the surface temperature; and d) carrying out the reversing transport of the sheet through the inductive heating device 114 only as long as the limit value TG for the surface temperature is not exceeded; or d') issuing a warning signal, in particular to a control device 130 or a control center of the annealing furnace 100, when the limit value TG for the surface temperature of the sheet is exceeded.
[0016] According to a first exemplary embodiment, the method provides that, during method step d), the sheet metal is transported, in its entirety, from the inductive heating device 114 onto a roller conveyor 112, 116 located upstream or downstream of the heating device as part of the heating zone 110 of the annealing furnace 100. This implementation of method step d) advantageously ensures that all areas of the sheet metal, in particular its head and foot sections, are heated to a uniform, even temperature, like the middle section of the sheet metal. To ensure that the said limit temperature for the surface of the sheet metal to be heated is not exceeded, it is important to record the current surface temperature TO of the sheet metal according to method step b) before and / or within the heating zone and to compare it with the specified limit value TG for the surface temperature; see method step c).Furthermore, in order to comply with the limit value TG, it is necessary that during process step d) - based on the respectively recorded current surface temperature TO - the respective increase n*AT in the surface temperature is calculated in advance, which the sheet would experience if it were to be guided through the inductive heating device 114 at least one more time n, starting from one of the roller tables 112, 116. A distinction must be made between the following two cases:
[0017] If, according to a first case, the sheet is stored on the roller table 112 upstream of the heating device as a starting point, it would be necessary to calculate in advance how large the increase n*AT of the surface temperature of the sheet would be if the sheet were to be passed through the inductive heating device 114 an odd number n of times, in particular one more time.
[0018] The odd number n of passes in the first case is due to the fact that after the n passes the sheet must land on the roller table 116 downstream of the heating device 114.
[0019] The second case to be considered is that the sheet metal is positioned as the starting point on the roller conveyor 116 located downstream of the heating device 114 in the transport direction R. In this case, it is necessary to calculate in advance what effect it would have on the surface temperature of the sheet metal if the sheet metal were to be transported through the inductive heating device 114 an even number n of times, at least two more times. The even number n of passes in this second case is due to the fact that the sheet metal must land on this roller conveyor 116 again after the n passes.
[0020] After the n passes, the sheet is transported in both cases from the roller conveyor 116 downstream of the heating device 114 in the transport direction R without further heating to the downstream holding zone 120. For this n-fold pass through the heating device 114, compliance with the limit temperature TG must be calculated in advance according to the following formula:
[0021] (1 ) T0 + n*AT < TG
[0022] If the calculation of the limit temperature TG according to formula (1) shows that this limit temperature would be maintained if the sheet were to pass through the inductive heating device 114 n times, process step d) would take effect and the pre-calculated n passes of the sheet through the heating device 114 could take place as pre-calculated.
[0023] Should the increase n*AT of the surface temperature according to formula (1) starting from TO be so great that during the said n-fold passage through the inductive heating device 114 the limit value for the surface temperature of the sheet would be exceeded, i.e. TO + n*AT > TG would apply, then method step d') takes effect, after which a signal is output to in particular a control device 130 or a control center of the annealing furnace 100. The control device must then, at least during individual passes, either switch off the inductive heating device 114 or reduce the temperature in the inductive heating device 114 to such an extent that at the end, after the n passes, the predetermined limit value TG for the surface temperature of the sheet is not exceeded.To transport the sheet from the heating zone 110 to the downstream holding zone 120 of the annealing furnace 100, the sheet transport speed is preferably increased to a speed between 0.5 m / sec and 1 m / sec. This has the advantage that the sheet transfer from one zone to the other is carried out quickly and the atmospheres are not unnecessarily mixed. An increased oxygen content in the holding zone can lead to increased scaling.
[0024] The said limit value TG for the surface temperature of the sheet is set at 700°C, preferably at 650°C, or more preferably at 600°C. This setting is material-dependent and effectively prevents scaling of the sheet surface; preferably, scaling is at least significantly reduced below these temperatures.
[0025] The said detection of the surface temperature T0 of the sheet according to method step d) is preferably carried out contactlessly with the aid of a pyrometer as temperature detection device 140. The temperature detection device is preferably arranged in front of the entrance of the heating zone 110, in the region of the roller table 112 upstream of the heating device 114 or in the region of the roller table 116 downstream of the heating device 114.
[0026] Due to the comparatively low temperatures within the heating zone 110, in particular due to the temperatures there below the limit value TG for the surface temperature, it is advantageously possible to provide a non-inert gas atmosphere, preferably air, within the heating zone 110 without further increasing the risk of scaling of the sheet. Only in the heating zone, in which the sheet would be further heated after passing through the heating zone 110, does it then make sense to provide an inert gas atmosphere. During the execution of process step d), i.e., while the sheet is within the heating zone 110, the average sheet temperature TM is increased, for example, to over 550°C; this is the consequence of the sheet passing through the inductive heating device 114 n times.
[0027] The passage time of the sheet through the inductive heating device 114 is at least 5 times, preferably 10 times shorter than the time for conventional, gas-fired preheating of the sheet. This allows for smaller differences between the sheet's head and tail temperatures.
[0028] The number of passes the sheet metal needs to pass through the heating device depends on its thickness. For example, a sheet metal thickness of 5 mm requires approximately 5-10 passes through the heating device, while a sheet metal thickness of 20 mm requires approximately 20 passes.
[0029] The inductive heating device 114 is preferably operated with an alternating current with a frequency of 1-10 kHz. The corresponding alternating current sources are already available; below 1 kHz, the induction systems generate unpleasant noise levels.
[0030] List of reference symbols
[0031] 100 annealing furnaces
[0032] 110 Heating zone
[0033] 112 roller conveyor
[0034] 114 Heating device
[0035] 116 roller conveyor
[0036] 120 stopping zone
[0037] 130 Control device
[0038] 140 Temperature measuring device, e.g. pyrometer
[0039] R Transport direction of the sheet from the heating zone to the holding zone
[0040] TG limit value for the surface temperature of the sheet
[0041] TM average sheet temperature
[0042] T0 current surface temperature of the sheet
[0043] AT Increase in surface temperature per pass through the inductive heating device n Number of passes of the sheet through the heating device
Claims
Patent claims 1. A method for operating an annealing furnace (100) for heating sheet metal, comprising the following steps: a) reversing the sheet metal through an inductive heating device (114) within a heating zone of the annealing furnace (100) for inductively increasing the surface temperature of the sheet metal; characterized by b) detecting the surface temperature of the sheet metal before and / or within the heating zone (110); c) comparing the detected surface temperature with a predetermined limit value TG for the surface temperature; and d) carrying out the reversing transport of the sheet metal through the inductive heating device (114) only as long as the limit value TG for the surface temperature is not exceeded; or d') outputting a signal, in particular to a control device (130) or a control center of the annealing furnace (100), when the limit value TG for the surface temperature of the sheet metal is exceeded.
2. Method according to claim 1, characterized in that during method step d) the sheet is transported with its entire length out of the inductive heating device (114) onto a roller table (112, 116) upstream or downstream of the heating device as part of the heating zone (110) of the annealing furnace (100).
3. Method according to one of the preceding claims, characterized in that during the method step d) - starting from the respective current surface temperature T0 of the sheet - the increase n*AT of the Surface temperature is calculated in advance which the sheet would experience if, according to a first case - starting from the roller conveyor (112) arranged upstream of the heating device (114) - it were transported through the inductive heating device (114) an odd number n of times, at least one (number word) further time, or if, according to a second case - starting from the roller conveyor (116) arranged downstream of the heating device (114) - it were transported through the inductive heating device (114) an even number n of times, at least two further times; and that method step d) is carried out if, based on the precalculation, the following would apply: TO +n*AT < TG; or (1 ) that process step d') is carried out if, based on the forecast, the following would apply: TO +n*AT > TG (2) with ne N : number of passes through the inductive heating device 4. Method according to claim 3, characterized in that the control device (130), in response to the warning signal in the first or second case, switches off the inductive heating device (114) or reduces its temperature to such an extent that a further n-fold passage of the sheet through the inductive heating device is possible without the limit value TG being exceeded; or in that the control device (130), in response to the warning signal in the second case, ends method step d) without the sheet passing further through the inductive heating device (114) and transports the sheet from the heating zone (110) into a downstream holding zone (120) of the annealing furnace (100).
5. Method according to claim 4, characterized in that that in order to transport the sheet from the heating zone (110) into the downstream holding zone (120) of the annealing furnace (100), the transport speed of the sheet is increased to approximately 0.5 m / s to 1 m / s.
6. Method according to one of the preceding claims, characterized in that the limit value TG for the surface temperature of the sheet is 700°C, preferably 650°C or more preferably 600°C.
7. Method according to one of the preceding claims, characterized in that the detection of the surface temperature of the sheet according to method step b) is carried out contactlessly with the aid of a temperature detection device (140), for example with the aid of a pyrometer, preferably before the entrance to the heating zone (110), in the region of the roller table (112) upstream of the heating device (114) or in the region of the roller table (116) downstream of the heating device (114).
8. Method according to one of the preceding claims, characterized in that within the heating zone (110) the atmosphere is formed from a non-inert gas, preferably from air; and that within the holding zone (120) an inert gas atmosphere prevails.
9. Method according to one of the preceding claims, characterized in that the average sheet temperature TM is increased to TM>550°C during method step d).
10. Method according to one of the preceding claims, characterized in that the passage time of the sheet through the inductive heating device (114) is at least 5 times, preferably 10 times shorter, than the time for preheating the sheet.
11. Method according to one of the preceding claims, characterized in that the number n of passes carried out in method step d) is selected depending on the thickness of the sheet, for example as follows: Thickness Number n of passes 12. Method according to one of the preceding claims, characterized in that the inductive heating device (114) is operated with alternating current at a frequency of 1 - 10 Hz.