Device for increasing productivity of industrial production line having furnace

Abstract

A continuous hot-dip plating plant, upstream of a molten metal bath, in which a furnace (1) comprises a furnace gas pipe (6) fluidly connecting a second section (4) to a first section (3) for conducting a furnace gas (9) from a direct combustion furnace towards a preheating section and effecting a post-combustion of the furnace gas (9) while the metal strip is preheated in the first section (3), the intermediate section (5) is provided with an electric induction heating system (11) capable of cooperating with said preheating section for increasing the productivity of the production line, while the presence of the electric induction heating system keeps the passage line of the metal strip substantially unchanged.

Description

Technical Field

[0001] This invention relates to a heating process used in a continuous hot-dip galvanizing production line (such as a galvanizing production line) for cold-rolled steel strip. The invention also relates to industrial equipment for implementing this heating process. Background Technology

[0002] The coating process, which involves immersing metal strip in a bath of molten metal, is well-known and used worldwide, especially in the case of galvanizing. Before coating, the steel strip must be heated in a furnace, partly to ensure it reaches at least the temperature of liquid metal, partly to induce recrystallization of the cold-rolled strip, and also to reduce surface oxides that inhibit good wetting of the strip in the bath and reduce the adhesion of the coating.

[0003] As is well known, in galvanizing production, the production cost per unit of material is directly related to the production line's productivity. In fact, the fixed costs of the equipment are spread across the total amount of material produced. The more material produced in a given time period, the lower the cost. It is also known that adjusting the coating thickness using an air knife becomes very difficult when the production line speed is below a certain value. For a 20 µm zinc coating, the estimated production line speed is in the range of 35 mpm; for a 40 µm zinc coating, the estimated speed is in the range of 55 to 60 mpm. This is actually related to the physical properties of the scraper and the oxidation of the liquid metal. These minimum speeds also depend on the type of coating. For example, in the case of an aluminum coating, the minimum speed for a 20 µm coating thickness is approximately 50 mpm. Therefore, increasing production line productivity is a general trend across the industry, and this is particularly important for thicker strips (e.g., strips exceeding 2 mm), as the cost and quality benefits of thicker strips are additive.

[0004] Various furnace technologies exist for heating strip before coating. Among them, the so-called direct-fired furnace (DFF) is a well-known technology, particularly in galvanizing production. Its advantage lies in combining heating with strip cleaning. Furthermore, due to the high temperature of the flame and refractory material, the heat transfer efficiency, achieved primarily through radiation from the furnace flame to the strip, is very high. Temperatures are typically between 1150°C and 1350°C. The advantage of this furnace is that, for a given productivity, it provides a limited length of strip.

[0005] However, this technology has some stringent requirements because, since the burner is located directly inside the furnace, the flame is in direct contact with the strip, which can potentially oxidize the strip. Therefore, such furnaces are typically divided into two sections, such as... Figure 1As shown, this illustrates a standard design used by many manufacturers. The first section is the preheating zone 3, also known as the "post-combustion chamber," located on the furnace inlet side. Strictly speaking, the second section is the direct-fired furnace 4, in which the strip 2 is heated and then coated.

[0006] In the first section 3, the continuously traveling metal strip 2 is preheated to approximately 200°C-300°C using exhaust gas 9 from the direct-fired furnace in the second section 4. The first section 3 receives some additional air to burn residual CO and H2, ensuring complete combustion of the exhaust gas 9 and ultimately resulting in furnace gas at the chimney containing approximately 2% to 5% O2. The gas radiates onto the strip 2 and heats it; however, due to the excess oxygen, the temperature of the strip 2 cannot exceed 300°C-350°C to prevent oxidation, and preferably, the temperature cannot exceed 300°C.

[0007] Section 4 employs understoichiometric combustion, raising the wall temperature to 1250°C-1350°C to heat the strip 2 prior to coating. Similarly, to prevent strip oxidation, the oxygen content in the gas in contact with the steel strip 2 must be very low, typically below 0.1%, whenever the strip 2 temperature exceeds 250°C. Therefore, in Section 4, since combustion is completed under understoichiometric conditions, the CO and H2 content in the furnace gas is typically maintained in the range of 2% to 6%. The standard target strip temperature at the outlet of a DFF operating under understoichiometric conditions is typically 620°C to 730°C. It is known that strip 2 begins to oxidize and condense above 730°C, possibly until the high dew point of the gas is reached. For environmental requirements and energy conservation considerations, the furnace gas leaving DFF 4 needs to receive additional oxygen to complete combustion. This is achieved in a separate chamber, namely in Section 3 as described above.

[0008] Direct-fired furnaces can be horizontal or vertical. Generally, a vertical configuration is preferred, especially when high productivity is desired. The advantage of this type of furnace is that it avoids the use of water-cooled support rollers, which would lead to a significant reduction in thermal efficiency and also introduce the risk of scratching the strip. It is also known that the height of such a furnace from the inlet (or strip inlet) to the top rollers ranges from 10 meters to 30 meters.

[0009] When this furnace is implemented vertically, the two vertical sections 3 and 4 are connected at one height by a horizontal section 5, as shown below. Figure 1 As shown. The first section 3 corresponds to the upward section, that is, the section where the strip 2 travels upward, and the second section 4 corresponds to the downward section, that is, the section where the strip 2 travels downward. The change of direction of the strip 2 from upward to downward travel is achieved by the guide rollers 7 installed in the horizontal section (due to layout constraints, there are usually two guide rollers, such as...). Figure 1 This is accomplished as shown. Because the gas exiting from DFF section 4 is at a higher temperature, the horizontal section 5 is configured to be separated from the first vertical section 3 and the second vertical section 4. This section is cooled and operated in a non-oxidizing atmosphere. A narrow opening is then provided to allow the strip 2 to pass through. Furthermore, a special bypass pipe 6 is installed between the first vertical section 3 and the second vertical section 4, allowing the furnace gas 9 to reach the afterburner 3 from the direct-fired furnace 4. The furnace gas 9 then exits the first section 3 through a pipe configured to discharge exhaust gas (not shown) at a temperature between 800°C and 1000°C.

[0010] The above three different sections are connected by a narrow section or airlock 10 to prevent the horizontal section 5 from reaching excessively high temperatures.

[0011] There is indeed a need to increase the productivity of production lines with such standard furnaces or existing furnaces. However, the challenge of such improvements is to minimize changes to the production line, particularly to the layout and position of the rollers.

[0012] For example, document FR 2 369 349 A discloses a classic technique for vertical strip heating furnaces, which includes a vertical direct-fired heating chamber as described above.

[0013] US document 11,193,182 B2 discloses a method for heat treating metal strip, wherein the metal strip is continuously preheated in a preheating zone using hot inert gas, and then further heat treated in a direct-fired furnace in a reducing and / or oxidizing atmosphere. The solution proposed in this document is to use an induction heating device before entering the DFF (Diverterless Furnace). This induction heating device is located in an additional vertical section between the preheating zone and the DFF section. This equipment requires significant modifications to existing production lines because the hot exhaust gas generated in the DFF section exchanges heat in a heat recovery system comprising two continuous heat exchangers and a boiler. The second heat exchanger heats the inert gas in the preheating zone used in the preheating furnace. The strip in the preheating zone never comes into direct contact with the hot exhaust gas.

[0014] The purpose of this invention

[0015] The purpose of this invention is to provide a solution that overcomes the shortcomings of the prior art.

[0016] In particular, this invention aims to increase the productivity of production lines equipped with furnaces, while keeping the design of existing hot-dip galvanizing systems (including annealing furnaces), especially the production line through which strip material passes, substantially unchanged. Another objective of this invention is to increase the productivity of production lines, thereby reducing their costs, especially when producing strip material with a thickness greater than 2 mm (e.g., 6 mm).

[0017] Furthermore, the purpose of this invention is to provide rapid heating for existing standard furnaces, while requiring minimal modifications to existing production lines and essentially not increasing the overall height of the furnace.

[0018] The present invention also relates to improving heating capacity while maintaining the existing furnace length and the constraints associated with the gas composition necessary to avoid strip oxidation. Summary of the Invention

[0019] A first aspect of the invention relates to an industrial apparatus for continuous hot-dip galvanizing, such as a zinc plating apparatus, the apparatus comprising a furnace located upstream of a molten metal bath, the furnace having: - First section, which is used to preheat the traveling metal strip; - The second section includes a direct-fired furnace; - An intermediate section or pass chamber (a chamber through which the strip passes) is located between the first section and the second section. The intermediate section or pass chamber includes two deflector rollers for changing the direction of the traveling metal strip, and the intermediate section or pass chamber, together with the first section and the second section, defines the pass line (the path through which the strip passes). The first and second sections are vertical, while the middle section is horizontal, or the middle section is partially vertical and partially horizontal, thereby correspondingly defining the horizontal and vertical / horizontal paths for the metal strip. These three sections are separated from each other by a sealing device that provides a narrow channel for the strip. The furnace also includes a furnace gas pipe that connects the second section to the first section for guiding the furnace gas from the direct-fired furnace toward the preheating section, and for achieving post-combustion of the furnace gas while the metal strip is preheated in the first section. The intermediate section is equipped with an induction heating system, which can work in conjunction with the preheating section to improve the productivity of the production line. At the same time, the presence of the induction heating system ensures that the passage of the metal strip remains essentially unchanged. The intermediate section has an L-shaped box shape, comprising two legs: a vertical leg and a horizontal leg. The vertical leg is located in the extension of the first section, the horizontal leg includes a guide roller for changing the direction of the strip, and the vertical leg includes an induction heating system; or The intermediate section has a horizontal box-like structure, which includes guide rollers for changing the direction of the strip, and an induction heating system is positioned between two guide rollers; and The device also includes a narrow channel for injecting an inert gas (such as nitrogen) or furnace gas extracted and cooled from the second section into or above the top of the first section to counteract and balance the buoyancy effect of the furnace gas.

[0020] According to a preferred embodiment, the device is also limited by at least one of the following features or a suitable combination thereof: - The minimum power of the induction heating system is 1 MW; - The induction heating system is a longitudinal magnetic flux induction heating system; - The power of the induction heating system is selected to limit the temperature rise of the strip; - The equipment includes a device for injecting air into the directed furnace gas to help achieve complete combustion of the furnace gas in the first section, wherein the oxygen content of the furnace gas is between 2% and 5% by volume; - The equipment includes means for operating the direct-fired furnace in the second section under understoichiometric conditions; - The equipment includes means for operating the first section under oxidizing conditions and for operating the intermediate section under non-oxidizing conditions; - The device also includes a narrow channel for injecting an inert gas (e.g., nitrogen) or furnace gas drawn from and cooled from the second section into or above the top of the first section to counteract and balance the buoyancy effect of the furnace gas.

[0021] Another aspect of the present invention relates to a method for improving production line productivity in a continuous hot-dip galvanizing apparatus, the continuous hot-dip galvanizing apparatus comprising equipment having a furnace according to the features described above, wherein the method comprises at least the following continuous steps: - In the first section of the furnace, the traveling strip is preheated to a temperature between 250°C and 300°C in an oxidizing gas atmosphere to obtain a preheated metal strip. The oxidizing gas atmosphere has an oxygen content between 2% and 5% by volume, and the remaining components of the oxidizing gas atmosphere are essentially nitrogen, carbon dioxide and water. - In the intermediate section, the preheated metal strip is further heated to a temperature between 400°C and 600°C, wherein an induction heating system is installed in the vertical or horizontal portion of the intermediate section, which is maintained in a non-oxidizing or slightly oxidizing atmosphere, wherein the oxygen content is less than a few ppm. - The direction of the metal strip is changed by using two steering rollers, so that the metal strip moves toward the horizontal part of the middle section and then away from the horizontal part of the middle section; - In the second section, which includes a direct-fired furnace, the metal strip is further heated to a temperature between 650°C and 750°C under substoichiometric conditions and with the CO+H2 content in the combustion gas being less than 6% by volume. The preheating of the traveling strip in the first section is achieved by the post-combustion of the furnace gas. The furnace gas is guided from the second section to the first section through a furnace gas pipe that fluidly connects the second section to the first section. The guided furnace gas contains more than 1% residual H2 and CO by volume.

[0022] According to a preferred embodiment, the method is further limited by at least one of the following features or a suitable combination thereof: - Inject additional air into the furnace gas directed toward the first section to help achieve complete combustion of the directed furnace gas in the first section, while controlling the oxygen content in the furnace gas between 2% and 5% by volume. - Keep the intermediate section in a non-oxidizing atmosphere; - Inert gas (such as nitrogen) is injected into the top of the first section or above the top of the first section to counteract and balance the buoyancy effect of the furnace gas; - The furnace gas leaves the first section through a pipe designed to discharge exhaust gas at a temperature between 800°C and 1000°C; - The traveling strip is a hot-rolled steel strip with a thickness greater than 2 mm, and preferably a hot-rolled steel strip with a thickness greater than 6 mm, or the traveling strip is a cold-rolled steel strip with a thickness between 2 mm and 3 mm. Attached Figure Description

[0023] Figure 1 This refers to a standard furnace representing existing technology.

[0024] Figure 2A and 2B Examples illustrating embodiments of the furnace according to the present invention are shown. Detailed Implementation

[0025] This invention relates to continuous hot-dip galvanizing equipment 1, such as zinc plating equipment, etc. Figure 2A and 2B As shown, the continuous hot-dip galvanizing equipment includes an electric heating system 11, such as an induction heating device, which is positioned in a specific manner (vertical or horizontal). In practice, the choice of location for the electric heating system 11 depends on several parameters. As explained above, the present invention aims to adopt, for example, the use of induction heating in existing standard furnaces, requiring only minimal modifications to existing production lines and without increasing the overall height of the furnace body.

[0026] The galvanizing equipment comprises a first section 3 and a second section 4, which are connected by an intermediate section 5 perpendicular to both sections 3 and 4. These three sections 3, 4, and 5 are separated by a sealing device 10, allowing each section to maintain its own distinct atmosphere and temperature. Preferably, the first section 3 and the second section 4 are vertical, and the intermediate section 5 is horizontal.

[0027] The first section 3, located at the furnace entrance, is a preheating zone for preheating the traveling metal strip 2. The strip 2 is preheated to a maximum of 350°C, preferably between 250°C and 300°C, using hot furnace gas drawn from section 4, to recover as much heat as possible.

[0028] The intermediate section 5, located between the first section 3 and the second section 4, includes at least two steering rollers 7 for changing the direction of the traveling strip 2. The steering rollers 7 define a passage line for the traveling strip, and the intermediate section 5 is a passage chamber.

[0029] According to a first embodiment of the present invention, such as Figure 2A As shown, the middle section 5 is an L-shaped box, which includes two legs: a first leg 51 extending from the first section 3 and a second leg 52 perpendicular to the first leg 51. Preferably, the first leg 51 is vertical and the second leg 52 is horizontal.

[0030] The first leg 51 includes an electric heating system 11. This electric heating system preferably has a minimum power of 1 MW. The electric heating system is capable of heating the preheated metal strip 2 to a maximum of 700°C, preferably to a temperature between 400°C and 600°C.

[0031] According to a second embodiment of the present invention, such as Figure 2B As shown, the intermediate section 5 is a horizontal box perpendicular to the first section 3 and the second section 4, similar to that in the prior art. The electric heating device system 11 is integrated in the horizontal box between the two steering rollers 7.

[0032] The second section 4 includes a direct-fired furnace for heating the strip 2 prior to coating. Preferably, the second section 4 is vertical. The strip 2 is heated to 780°C, but preferably to a temperature between 620°C and 730°C. This heating is achieved under under-stoichiometric conditions (oxygen-deficient combustion) to prevent oxidation of the strip 2.

[0033] In a preferred embodiment, the apparatus 1 includes a gas pipe 6 located between the direct-fired furnace in the second section 4 and the top of the first section 3. In this way, the first section 3 is heated by exhaust gas 9 drawn / directed from the direct-fired furnace to preheat the metal strip 2 through post-combustion of the exhaust gas. The first section 3 can advantageously receive some additional air (not shown) to burn residual CO and H2 in the furnace, thereby ensuring complete combustion of the exhaust gas 9. Finally, at the chimney of the first section 3, the exhaust gas contains approximately 2% to 5% O2. The gas radiates onto the strip 2 to heat it, but due to the excess oxygen, the strip 2 cannot reach a temperature above 300°C-350°C (preferably, the strip temperature is below 300°C) to avoid oxidation of the strip.

[0034] In yet another preferred embodiment, the device 1 includes a narrow channel 12 for injecting some N2 (or a similar oxygen-free gas, such as furnace gas from a furnace that will be cooled in the top of the first section 3) at the location of the sealing device 10 to balance the buoyancy effect of the furnace gas 9. In yet another embodiment (not shown), N2 may also be injected from the intermediate section 5 into the second section 4 at the location of the sealing device 10 to prevent furnace gas produced in the direct-fired furnace from contaminating the non-oxidizing intermediate section 5.

[0035] Determining parameters of induction heating position

[0036] The inventors have discovered that induction heating using longitudinal flux technology is particularly suitable for furnaces with a fixed total length. The induction section requires minimal space yet can achieve a total usable heating power of over 4 MW. However, if this induction section is installed at the furnace inlet, for example before the first section 3, i.e., before post-combustion, the benefits will be limited because the strip 2 is expected to leave the first section 3 at a maximum temperature of 300°C-350°C, at which temperature the risk of oxidation due to the high O2 content is very high.

[0037] Conversely, if the induction section is installed at the furnace outlet, such as after the direct-fired furnace in section 4 of the second section, the efficiency becomes very low due to the Curie point. Another alternative solution is to use a transverse induction furnace, but this technology is more expensive and more delicate to implement than conventional longitudinal flux technology due to its sensitivity to product width.

[0038] Therefore, the inventors discovered that the optimal location for installing the induction heating device is between the rear combustion chamber 3 and the direct-fired furnace 4. The challenge is that the configuration must be designed to withstand very high gas temperatures (over 1100°C). Therefore, one objective is to position the induction heating device 11 within the intermediate section 5. However, implementing the induction furnace in the horizontal section between the two guide rollers will require excellent tension control at that location, due to the catenary effect of the strip and the relatively small opening of such an induction furnace (typically 80 mm to 200 mm, and preferably 100 mm to 150 mm).

[0039] Therefore, the present invention solves the above problems by providing a specific intermediate section 5 that accommodates two steering rollers 7 and an electric heating system 11.

[0040] This specific geometry allows the furnace gas 9 from the direct-fired furnace 4 to be drawn / guided towards the afterburner section 3. Furthermore, to prevent the furnace gas 9 from flowing upwards in the afterburner section 3, a special seal is provided in the narrow channel 12, where N2 is injected to counteract and balance the buoyancy effect. This means that because the ambient temperature is maintained within the range of 300°C-600°C, the induction heating device 11 can be kept at a relatively "low temperature".

[0041] The furnace of this invention for a hot-dip galvanizing production line is significantly different from the equipment disclosed in US 11,193,182 B2. According to that document, the strip is heated in a preheating zone by convection and circulation of inert gas, which itself is heated by a complex heat recovery system supplied with hot furnace gas generated in the DFF section. Therefore, such equipment requires significant modifications to existing production lines. Furthermore, the document requires strip temperatures as high as 500°C at the outlet of the induction section. This means the strip cannot enter a direct-fired furnace with excessive oxygen, as it is well known that in hot-dip galvanizing (and, in contrast to, for example, stainless steel processing), the strip cannot come into contact with oxygen when the temperature is above approximately 300°C, and therefore the direct-fired furnace must operate under conditions where the λ coefficient is less than 1 (which is not the teaching of that document). Therefore, the furnace of US 11,193,182 B2 is unsuitable for use in a galvanizing production line.

[0042] The embodiments of the present invention completely avoid this potential problem and eliminate the need for the implementation and maintenance of expensive heat exchangers and blowers. The present invention ensures energy savings and further increases in production line productivity (especially for thick strips), while combining the high responsiveness of the induction heating system with maintaining contact between the strip and the oxidation furnace gas at below 300°C in the preheating zone, which meets the required process standards.

[0043] List of reference numerals

[0044] 1. Heating equipment

[0045] 2. Steel strip

[0046] 3. Preheating section (or afterburner)

[0047] 4 DFF sections

[0048] 5. Intermediate Section (Horizontal Box or L-shaped Box)

[0049] 51 Vertical legs in the middle section of the L-shape

[0050] 52. Horizontal leg of the L-shaped middle section

[0051] 6. Furnace gas pipes

[0052] 7. One or more steering rollers

[0053] 8. Strip direction

[0054] 9. Gas direction

[0055] 10 Sealing device

[0056] 11 Electric heating system

[0057] 12 N2 path

Claims

1. A continuous hot-dip galvanizing apparatus, comprising a furnace (1) located upstream of a molten metal bath, said furnace having: - First section (3), the first section is used to preheat the metal strip (2) in motion; - Second section (4), the second section includes a direct-fired furnace; - An intermediate section or passage chamber (5) is located between the first section (3) and the second section (4), the intermediate section or the passage chamber includes two steering rollers (7) for changing the direction of the traveling metal strip (2), and the intermediate section or the passage chamber together with the first section (3) and the second section (4) defines the passage line of the metal strip; The first section (3) and the second section (4) are vertical, while the middle section (5) is horizontal, or the middle section is partially vertical and partially horizontal, thereby correspondingly defining the horizontal path and the vertical / horizontal path for the metal strip. The three sections (3, 4, 5) are separated from each other by a sealing device (10) that provides a narrow channel for the strip. The furnace (1) further includes a furnace gas pipe (6) that fluidly connects the second section (4) to the first section (3) for guiding furnace gas (9) from the direct-fired furnace toward the preheating section, and achieving post-combustion of the furnace gas (9) while the metal strip is preheated in the first section (3); Its features are, The intermediate section (5) is provided with an induction heating system (11), the presence of which ensures that the passage of the metal strip remains substantially unchanged. The intermediate section (5) has an L-shaped box shape, the L-shaped box including two legs, the two legs being a vertical leg (51) and a horizontal leg (52), the vertical leg being located in the extension line of the first section (3), the horizontal leg including the steering roller (7) for changing the direction of the strip (2), and the vertical leg (51) including the induction heating system (11); or The intermediate section (5) is in the form of a horizontal box, which includes a steering roller (7) for changing the direction of the strip (2), and an induction heating system (11) is located between the two steering rollers (7); and The device further includes a narrow channel device (12) for injecting an inert gas or furnace gas extracted and cooled from the second section (4) into the top of the first section (3) or above the top of the first section to counteract and balance the buoyancy effect of the furnace gas (9), such as nitrogen.

2. The device according to claim 1, wherein, The minimum power of the induction heating system (11) is 1 MW.

3. The device according to claim 2, wherein, The induction heating system (11) is a longitudinal magnetic flux induction heating system.

4. The device according to claim 2, wherein, The power of the induction heating system (11) is selected to limit the temperature rise of the strip.

5. The device according to any one of the preceding claims, wherein, The device includes means for injecting air into the directed furnace gas to help achieve complete combustion of the furnace gas in the first section (3), wherein the oxygen content of the furnace gas is between 2% and 5% by volume.

6. The device according to any one of the preceding claims, wherein, The equipment includes means for operating the direct-fired furnace in the second section (4) under understoichiometric conditions.

7. The device according to any one of the preceding claims, wherein, The device includes means for operating the first section (3) under oxidizing conditions and for operating the intermediate section (5) under non-oxidizing conditions.

8. A method for increasing production line productivity in a continuous hot-dip galvanizing apparatus, said continuous hot-dip galvanizing apparatus comprising an apparatus according to any one of the preceding claims having a furnace (1), wherein, The method includes at least the following sequential steps: - In the first section (3) of the furnace (1), the traveling strip (2) is preheated to a temperature between 250°C and 300°C in an oxidizing gas atmosphere to obtain a preheated metal strip (2), wherein the oxidizing gas atmosphere contains an oxygen content between 2% and 5% by volume, and the remaining components of the oxidizing gas atmosphere are essentially nitrogen, carbon dioxide and water. - The preheated metal strip (2) is further heated to a temperature between 400°C and 600°C in the intermediate section (5), wherein the induction heating system (11) is installed in the vertical or horizontal part of the intermediate section (5), and the intermediate section (5) is maintained in a non-oxidizing atmosphere or a slightly oxidizing atmosphere, wherein the oxygen content is less than a few ppm. - The direction of the metal strip (2) is changed by means of the two steering rollers (7), so that the metal strip moves toward the horizontal portion of the intermediate section (5) and away from the horizontal portion of the intermediate section (5); - In the second section (4) including the direct-fired furnace, under substoichiometric conditions and with the CO+H2 content in the combustion gas being less than 6% by volume, the metal strip (2) is further heated to a temperature of 780°C, preferably, the metal strip is further heated to a temperature between 650°C and 750°C. The preheating of the traveling strip (2) in the first section (3) is achieved by the post-combustion of the furnace gas (9). The furnace gas is directed from the second section (4) through the furnace gas pipe (6) that fluidly connects the second section (4) to the first section (3) toward the first section (3). The directed furnace gas contains more than 1% residual H2 and CO by volume.

9. The method according to claim 8, wherein, Additional air is injected into the furnace gas directed toward the first section (3) to help achieve complete combustion of the directed furnace gas in the first section (3), while controlling the oxygen content in the furnace gas to between 2% and 5% by volume.

10. The method according to claim 8 or 9, wherein, The intermediate section (5) is kept in a non-oxidizing atmosphere.

11. The method according to any one of claims 8 to 10, wherein, An inert gas, such as nitrogen, is injected into the top of the first section (3) or above the top of the first section to counteract and balance the buoyancy effect of the furnace gas (9).

12. The method according to any one of claims 8 to 11, wherein, The furnace gas (9) is discharged from the first section (3) through a pipe provided for discharging waste gas with a temperature between 800°C and 1000°C.

13. The method according to any one of claims 8 to 12, wherein, The strip in motion is a hot-rolled steel strip with a thickness greater than 2 mm, and preferably a hot-rolled steel strip with a thickness greater than 6 mm, or the strip in motion is a cold-rolled steel strip with a thickness between 2 mm and 3 mm.

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