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EP-4739968-A1 - DEVICE FOR IMPROVING THE PRODUCTIVITY OF AN INDUSTRIAL LINE WITH A FURNACE

EP4739968A1EP 4739968 A1EP4739968 A1EP 4739968A1EP-4739968-A1

Abstract

A continuous hot dip coating installation, upstream of a molten metal bath, in which the furnace (1) comprises a fumes pipe (6) fluidly connecting the second section (4) to the first section (3) so that to deflect fumes (9) from the direct fired furnace toward the preheating section and generate a post-combustion of the fumes (9) during pre-heating of the metal strip in the first section (3), characterized in that the intermediate section (5) is provided with an electric induction heating system (11) capable to cooperate with said preheating for boosting the productivity of the line, while essentially keeping unchanged by its presence the pass line of the metal strip.

Inventors

  • DUBOIS, MICHEL

Assignees

  • John Cockerill SA

Dates

Publication Date
20260513
Application Date
20240517

Claims (13)

  1. 1 . A continuous hot dip coating installation comprising, upstream of a molten metal bath, a furnace (1 ) having : - a first section (3) for the pre-heating of a running metal strip (2) ; - a second section (4) comprising a direct fired furnace ; - an intermediary section or pass chamber (5) located between the first section (3) and the second section (4), comprising two deflecting rolls (7) for changing the direction of the running metal strip (2) and defining with the first section (3) and the second section (4) a pass line of the metal strip ; the first section (3) and the second section (4) being vertical and the intermediate section (5) being horizontal, or partly vertical and partly horizontal, respectively defining a horizontal path and a vertical/horizontal path for the metal strip, the three said sections (3, 4, 5) being separated by sealing means (10) providing a narrow path to the strip ; the furnace (1 ) further comprising a fumes pipe (6) fluidly connecting the second section (4) to the first section (3) so that to deflect fumes (9) from the direct fired furnace toward the preheating section and generate a post-combustion of the fumes (9) during pre-heating of the metal strip in the first section (3) ; characterized in that the intermediate section (5) is provided with an electric induction heating system (11 ), essentially keeping unchanged by its presence the pass line of the metal strip, wherein the intermediate section (5) has the form of a L- box comprising two legs, a vertical leg (51 ) in the prolongation of the first section (3) and an horizontal leg (52) comprising deflecting rolls (7) for changing the direction of the strip (2), the vertical leg (51 ) comprising the electric induction heating system (11 ) ; or wherein the intermediate section (5) has the form of a horizontal box comprising deflecting rolls (7) for changing the direction of the strip (2), the electric induction heating system (11 ) being embedded between the two deflecting rolls (7) ; and wherein the installation further comprises a narrow path device (12) to inject an inert gas such as nitrogen, or fumes extracted from the second section (4) and cooled, in the top of or above the first section (3) so as to counter-balance the buoyancy effect of the fumes (9).
  2. 2 . The installation according claim 1 , wherein the electric induction heating system (11 ) has a minimal power of 1 MW.
  3. 3. The installation according to claim 2, wherein the electric induction heating system (11 ) is a longitudinal flux induction heating system.
  4. 4 . The installation according to claim 2, wherein the power of the electric induction heating system (11 ) is chosen to define a temperature raise of the strip.
  5. 5. The installation according to anyone of the preceding claims, wherein it comprises means for injecting air into the deflected fumes so that to help attaining complete combustion of the fumes in the first section (3), with a content of oxygen in the fumes being comprised between 2 and 5% in volume.
  6. 6. The installation according to anyone of the preceding claims, wherein it contains means for operating the direct fired furnace of the second section (4) in under-stoichiometric conditions.
  7. 7 . The installation according to anyone of the preceding claims, wherein it contains means for operating the first section (3) in oxidizing conditions and the intermediate section (5) in non-oxidizing conditions.
  8. 8 . A method for improving the line productivity in a continuous hot dip coating installation, comprising an installation with a furnace (1 ) according to anyone of the preceding claims, wherein the method presents at least the following successive steps: - pre-heating a running strip (2) until a temperature comprised between 250 and 300°C, in an oxidizing gas atmosphere having an oxygen content comprised between 2 and 5% in volume, the rest being essentially nitrogen, carbon dioxide and water, in the first section (3) of the furnace (1 ), in order to obtain a pre-heated metal strip (2) ; - further heating the pre-heated metal strip (2) in the intermediate section (5) until a temperature comprised between 400°C and 600°C, with the electric induction heating system (11 ) installed in a vertical or a horizontal part of the intermediate section (5), said intermediate section (5) being maintained in an non-oxidizing or slightly oxidizing atmosphere, with an oxygen content below a few ppm ; - changing the direction of the metal strip (2) toward and out of a horizontal part of the intermediate section (5) thanks to the two deflecting rolls (7) ; - further heating the metal strip (2) in the second section (4) comprising the direct fired furnace until a temperature of 780°C but preferably comprised between 650°C and 750°C, in understoechiometric conditions and with a CO+H2 content of the combustion gases lower than 6% in volume ; wherein the pre-heating of the running strip (2) in the first section (3) is obtained thanks to the post-combustion of the fumes (9) deflected from the second section (4) toward the first section (3) through the fumes pipe (6) fluidly connecting the second section (4) to the first section (3), said deflected fumes containing residual H2 and CO > 1 % in volume.
  9. 9. The method according to claim 8, wherein additional air is injected to the fumes deflected toward the first section (3) so that to help attaining complete combustion of the deflected fumes in the first section (3), while controlling a content of oxygen in the fumes comprised between 2 and 5% in volume.
  10. 10. The method according to claim 8 or 9, wherein the intermediate section (5) is maintained in a non-oxidizing atmosphere.
  11. 11 . The method according to anyone of claims 8 to 10, wherein an inert gas such as nitrogen is injected in the top of or above the first section (3) so that to counter-balance the buoyancy effect of the fumes (9).
  12. 12 . The method according to anyone of claims 8 to 11 , wherein the fumes (9) are exiting the first section (3) through a duct provided for waste gases at a temperature comprised between 800 and 1000°C.
  13. 13. The method according to anyone of claims 8 to 12, wherein the running strip is a hot-rolled steel strip of thickness higher than 2mm, and preferably higher than 6mm, or a cold-rolled steel strip of thickness comprised between 2 and 3mm.

Description

DEVICE FOR IMPROVING THE PRODUCTIVITY OF AN INDUSTRIAL LINE WITH A FURNACE Field of the Invention [0001] The present invention relates to a heating process used in continuous hot dip coating lines, such as galvanizing lines, of cold rolled steel strips. The invention also concerns the industrial installation for carrying out the heating process. Background and Prior Art [0002] The coating process consisting in dipping a metal strip in a bath of molten metal is well-known and used all over the world, especially in the case of galvanization. Before coating, the steel strips have to be heated in a furnace, on the one hand, to reach at least the temperature of the liquid metal and on the other hand, to induce the recrystallization of the cold rolled sheets as well as to reduce the surface oxide that inhibits a good wetting in the bath and an improved adhesion of the coating on the strip. [0003] In galvanizing, it is known that the production cost per unit of material produced is directly related to the line productivity. Indeed, the fixed costs of the installation are then divided by the amount of material produced. The more material is produced in a period of time, the lower the cost. It is also known that the adjustment of the coating thickness by the air knives is very difficult below a certain line speed that is estimated in the range of 35mpm for a zinc coating of 20pm and of 55 to 60mpm for a zinc coating of 40pm. This is actually related to the physics of wiping and the oxidation of the liquid metal. Those minimal speeds also depend on the type of coating. For example, in case of aluminized coating, the minimal speed is around 50mpm for a 20pm coating thickness. Therefore, the general trend of the industry is to increase the line productivity, which is even more important for a thick strip (for example over 2mm) where the benefits on costs are supplemented by those of quality. [0004] Various furnace technologies exist to heat strips before coating. Among them, the so-called direct fired furnace (DFF) is a well-known technology, especially in galvanizing. It has the advantage to combine heating with cleaning of the strip. In addition, heat transfer that is mostly performed by radiation of the flame of the furnace to the strip, is very high due to the high temperature of the flame and refractories. The temperatures are typically of 1150 to 1350°C. The advantage of such a furnace is to provide a limited length for a defined productivity. [0005] This technology has however some strict requirements because the flames are in direct contact with the strip and so may oxidize it, as the burners are directly located inside the furnace. Therefore, such a furnace is usually divided in two sections, as illustrated in Figure 1 , where a standard design used by many manufacturers is represented. The first section is the pre-heating zone 3, also named “post-combustion chamber”, that is located on the entry side of the furnace. The second section is strictly speaking the direct fired furnace 4, in which the strip 2 is heated before coating. [0006] In the first section 3, the metal strip 2 running continuously is pre-heated to approximately 200-300°C, with the aid of the exhaust gases 9 from the direct fired furnace in the second section 4. This first section 3 receives some additional air to burn the residual CO and H2 in order to ensure a complete combustion of the exhaust gases 9 and finally have fumes with about 2 to 5% O2 at the stack. The gas radiates to the strip 2 and heats it, but the strip 2 should not reach a temperature higher than 300-350°C, and preferably higher than 300°C, due to excess oxygen, in order to avoid its oxidation. [0007] The second section 4 uses an under-stoichiometric combustion to reach a wall temperature up to 1250-1350°C for heating the strip 2 before coating. Again, to avoid strip oxidation, the oxygen content of the gas in contact with the steel strip 2 must be low, and typically below 0.1 %, as soon as the temperature of strip 2 is over 250°C. Therefore, in the second section 4, the combustion being done with under-stoichiometry conditions, CO and H2 in the fumes remain in a range typically between 2 and 6%. Standard target strip temperatures are typically between 620 and 730°C at the outlet of the DFF operating in under-stoichiometric conditions. Above 730°C, it is known that the strip 2 starts to oxidize dew, most probably up to the high dew point of the gas. For environmental constraints as well as energy savings, the fumes exiting the DFF 4 receive additional oxygen to complete the combustion. This is realised in a separate chamber, namely in the first section 3 as explained above. [0008] A direct fired furnace can be horizontal or vertical. Usually vertical implementation is preferred especially for high production rates. It has the advantage to avoid support rolls that need to be water cooled, which in turn induces a significant reduction in thermal efficiency but also induces