EP3601624B1 - Section and method for cooling a continuous line combining dry cooling and wet cooling. - Google Patents

Description

The invention relates to cooling sections of continuous lines for annealing or galvanizing steel strips.

By galvanization, the present description refers to all dip coatings, whether they are zinc, aluminum, zinc and aluminum alloy coatings, or any other type of coating. The invention relates in particular to the rapid cooling sections of these lines.

In a continuous line for annealing or galvanizing steel strips, a strip of steel passes through different sections inside which it undergoes a heat treatment comprising in particular heating, cooling or temperature maintenance phases.

The cooling phase of the steel strips is particularly critical. Indeed, it is the cooling phase which mainly conditions the final mechanical and metallurgical properties of the steel strip. Depending on the cooling rate of the steel strip and its chemical composition, different metallurgical phases can be created and thus lead to different mechanical properties of the steel strip.

An ideal cooling section should allow a steel strip to be cooled perfectly evenly over its entire width in order to guarantee the homogeneity of the mechanical and metallurgical properties of the final strip. Such a cooling section should also make it possible to impose different cooling rates in order to produce most types of steels.

There are two main families of steel strip cooling technologies implemented on continuous annealing or galvanizing lines, or continuous lines combining annealing and galvanizing: gas cooling and wet cooling.

Gas cooling, typically by spraying a mixture of nitrogen and hydrogen, N ₂ H ₂ , at high speed and with a high hydrogen content, makes it possible to reach cooling speeds of the order of 200° C/s for 1 mm thick steel strips. The gas being reducing, the steel strip is not oxidized after having passed through such a cooling section of this technology family. Galvanization of the strip is then possible, without any other intermediate step of a chemical nature being implemented. However, the cooling rates being limited to 200°C/s, these coolings do not make it possible to produce steels with high mechanical and metallurgical properties which require higher cooling rates.

The document WO2010/049600 describes a furnace for the heat treatment of a continuously moving rolled steel strip, comprising a preheating section, a temperature holding section and a cooling section. The cooling section comprises successively, along the path followed by the strip, oriented in the running direction of the latter, and close to each of the faces of the strip: a first rapid cooling device which does not oxidize the strip and whose transfer power is adjustable; and, a second rapid cooling device capable of oxidizing the strip and whose transfer power is adjustable. The oven comprises control and actuation means making it possible to adjust the transfer power of each of the first and second rapid cooling devices according to a desired cooling rate and the geometric characteristics of the strip.

The document WO2011/004302 describes an atmosphere separation device installed between two chambers of a continuous processing line for metal strips having atmospheres of different natures, to prevent the entry of the atmosphere from a first chamber located upstream into a second chamber located downstream, and vice versa, comprising: a first pair of motorized rollers between which the strip circulates at the entrance to the device; a second pair of motorized rollers between which the strip circulates, at the exit of the device. The pairs of rollers are arranged so as to cause a change in the running plane of the strip between the entry and the exit of the device and the spacing between the rollers is adjusted so that, when the strip has geometric defects, the rollers reduce the defects of the strip across its width, so as to limit the gas flow.

An object of the invention is to provide a cooling section which provides more flexibility than the cooling sections according to the prior art.

This object is achieved with, according to a first aspect of the invention, a cooling section of a continuous line for annealing or galvanizing steel strips arranged to receive a metal strip, said section comprising at least one cooling zone dry arranged to project a gas onto said steel strip and at least one wet cooling zone arranged to project a liquid or a mixture of gas and liquid onto said steel strip.

The dry cooling zone can comprise blowing boxes arranged to project the gas onto the steel strip. The gas can be a mixture of nitrogen and hydrogen.

The wet cooling zone can comprise nozzles arranged to project the liquid or the mixture of gas and liquid onto the steel strip. The liquid can be water, an acid solution, or any other solution.

The cooling section according to the invention can make it possible to produce steels with high mechanical properties which can directly undergo a galvanizing step at the outlet of said section, without requiring intermediate chemical treatment.

The wet cooling zone makes it possible to reach cooling speeds of the order of 1000°C/s for a steel strip 1 mm thick.

The cooling section according to the invention also makes it possible to successively perform dry cooling and wet cooling without having to cut the strip to bypass one of the cooling zones. The gain in productivity is therefore substantial.

The dry cooling and wet cooling zones can operate at the same time and/or operate separately. The alternation or succession of these two modes of operation provides great flexibility in the use of the cooling section according to the invention for different types of steel strip provided by the order book for the continuous line (“product mix” in English).

The wet cooling zone may include an immersion cooling zone.

Advantageously, the wet cooling zone is preferably a liquid spray cooling zone. A liquid spray area can be stopped easily and quickly. In addition, spray cooling makes it possible to easily control the temperature of the steel strip at the end of cooling, and therefore its mechanical and metallurgical properties.

According to one embodiment, the wet cooling zone and the dry cooling zone are arranged, respectively, along a first vertical direction and a second vertical direction parallel to the first direction. Those skilled in the art usually refer to this configuration as a two pass embodiment. According to this embodiment, the wet cooling zone can be arranged upstream, according to the direction of travel of the steel strip in the cooling section, or downstream of the dry cooling zone.

According to a variant, the wet cooling zone and the dry cooling zone are arranged in the same vertical direction. A person skilled in the art usually designates this variant by qualifying it as a one-pass embodiment.

According to this variant, the dry cooling zone can be arranged below the wet cooling zone. In this case, a system for drying the steel strip can be interposed between the wet cooling zone and the dry cooling zone.

Alternatively, according to this variant, the wet cooling zone can advantageously be placed below the dry cooling zone. This arrangement increases the compactness of the cooling section which may not have a drying zone interposed between the dry cooling zone and the wet cooling zone.

The cooling section according to the invention further comprises an atmosphere separation airlock interposed between the dry cooling zone and the wet cooling zone. The separation airlock may not pollute the wet cooling zone with different gaseous species which result from the dry cooling. The separation airlock makes it possible not to create a mixing zone between the atmospheres of these two zones so as to avoid a potentially dangerous mixing, in particular when the gas cooling has a high hydrogen content.

The atmosphere separation airlock comprises three pairs of rollers or flaps, each of the pairs being arranged transversely to a running direction of the metal strip, said three pairs of rollers or flaps delimiting between them two zones of said airlock, respectively a first zone delimited by the first two pairs of rollers or flaps in the running direction of the strip and located on the side of the dry cooling zone comprising withdrawal means, and respectively a second zone, delimited by the last two pairs of rollers or shutters in the running direction of the strip and located on the side of the wet cooling zone and comprising means arranged to inject an inert gas. A buffer zone is thus created between the first two pairs of rollers and an atmosphere extraction system between the last two pairs of rollers. The escapes of inert gas, coming from the buffer zone, towards the wet cooling section and towards the withdrawal zone do not create any difficulties. Roller pairs can be replaced by flaps. In addition to the atmosphere separation function, this airlock advantageously creates a "clean" zone in which it is possible to measure the temperature of the strip over its width, for example by means of a scanner, or occasionally, by means of for example a pyrometer. This temperature measurement can help to better regulate the strip cooling process.

According to one embodiment, the cooling section may further comprise a system for drying and purging the wet cooling zone. Advantageously, this drying and purging system can be implemented when the wet cooling zone is not used to cool the strip. Advantageously, this drying and purging system thus makes it possible to limit the transition times, depending on the cycles and the order book of the continuous line ("product mix") between a product requiring the use of this humid zone and a product that does not need to be cooled by the humid zone. Indeed, if the wet zone were to remain wet, the degraded dew point could lead to a poor surface condition of the strip when it passes through it.

According to one possibility, the system for drying and purging the wet cooling zone may comprise equipment arranged to inject nitrogen, preferably heated, preferably at 50° C., in order to purge the wet zone. The nitrogen can be reheated beforehand, for example by recovering the calories contained in the fumes from the heating zones of the continuous line. The drying of the wet zone is thus improved.

To improve the drying and purging times, two additional devices can be provided.

The drying and purging system may include equipment arranged to heat walls of the wet cooling zone. It is thus possible to limit the condensation of the wet cooling zone or to reduce the drying time of the wet zone. Preferably, this heating is achieved by adding elements which heat by conduction or by radiation. These can be placed on the inside or outside of the walls.

The drying and purging system may comprise a system of nitrogen knives directed towards the bottom of the wet cooling zone and arranged to blow nitrogen onto interior walls of the wet cooling zone. This system of nitrogen knives makes it possible to better evacuate the liquid from the walls of the wet cooling zone.

According to a second aspect of the invention, there is proposed a method for cooling a continuous line for annealing or galvanizing steel strips arranged to receive a metal strip, said section comprising at least one dry cooling zone arranged to spraying a gas onto said steel strip, said method comprising at least one dry cooling step comprising spraying a gas onto the steel strip and at least one wet cooling step comprising spraying a liquid or a mixture of gas and liquid on the steel strip.

The method according to the second aspect of the invention comprises the establishment of an atmosphere separation airlock, interposed between the dry cooling zone and the wet cooling zone, cooling section in which the air separation airlock atmosphere comprises three pairs of rollers or shutters, each of the pairs being arranged transversely to a running direction of the metal strip, said three pairs of shutter rollers delimiting between them two zones of said airlock, respectively a first zone delimited by the first two pairs of rollers or flaps in the running direction of the strip and located on the side of the dry cooling zone and comprising withdrawal means, and respectively a second zone, delimited by the last two pairs of rollers or flaps in the running direction of the strip and located on the side of the wet cooling zone and comprising means arranged to inject an inert gas

Advantageously according to the invention, the liquid can be non-oxidizing for the band. It may be a formic acid solution whose acid concentration is between 0.1% and 6% by mass of the solution, and advantageously between 0.5% and 2% by mass of the solution.

The method according to the second aspect of the invention may further comprise a step of drying and purging the wet cooling zone, preferably by means of heat originating from a heating zone of the continuous line. It is for example possible to recover the energy contained in the fumes from the heating zones of the continuous line.

The cooling section according to the first aspect of the invention may comprise control means, preferably computer control means, configured for the cooling section according to the first aspect of the invention, or one of its improvements, for example intended to activate one or/and the other of the dry and wet cooling zones according to a product to be cooled.

According to a third aspect of the invention, there is proposed a computer program product, downloadable from a communication network and/or stored on a computer-readable medium and/or executable by a microprocessor, and loadable into an internal memory of a calculation unit, characterized in that it comprises program code instructions which, when they are executed by the calculation unit, implement the steps of the method according to the second aspect of the invention or one of its improvements.

• Fig. 1 est une vue schématique d'une section de refroidissement, selon un premier mode de réalisation de l'invention, d'une ligne continue de traitement de bandes,
• Fig. 2 est une vue schématique d'une section de refroidissement, selon un deuxième mode de réalisation de l'invention, sur laquelle est représenté un système de séchage et de purge de la zone de refroidissement humide.

The invention consists, apart from the provisions set out above, of a certain number of other provisions which will be more explicitly discussed below with regard to exemplary embodiments described with reference to the appended drawings, but which do not is in no way limiting. In these drawings:

• Fig. 1 is a schematic view of a cooling section, according to a first embodiment of the invention, of a continuous strip processing line,
• Fig. 2 is a schematic view of a cooling section, according to a second embodiment of the invention, on which is shown a system for drying and purging the wet cooling zone.

Referring to the diagram of the figure 1 appended, one can see a cooling section, according to the first embodiment, of a continuous line for annealing or galvanizing metal strips arranged to receive a metal strip 1 in a running direction S, successively combining, in the direction of scrolling, at least one zone 2 of dry cooling and one zone 5 of wet cooling.

In the example shown, the cooling section further comprises an airlock 4 for separating atmospheres separating the zone 2 of dry cooling and the zone 5 of wet cooling.

The strip 1 enters the cooling section by circulating from top to bottom in the direction of travel S. It first passes through the dry cooling zone 2 in which a mixture of nitrogen and hydrogen is projected onto the strip at the means of blowing boxes 3. The strip then passes through the airlock 4 for separating atmospheres before entering the zone 5 of wet cooling.

The humic cooling zone 5 comprises nozzles 6 arranged to project a cooling fluid onto the metal strip 1.

The wet cooling zone 5 comprises means for withdrawal 7 of vapors which, in the example shown in the figure, are arranged in the upper part of this zone 5 of wet cooling.

The airlock 4 for separating atmospheres disposed between the dry zone 2 and the wet zone 5 comprises three successive pairs 8, 9 and 10 of rollers, according to the running direction S of the metal strip 1. Each of the pairs is placed transversely to the running direction of the metal strip.

The three pairs delimit between them two successive zones 11 and 12 of the airlock, in the running direction of the strip. The zone 11 delimited by the pairs of rollers 8 and 9 is located on the side of the dry cooling zone 2, the zone 12, delimited by the pairs of rollers 9 and 10 is located on the side of the wet cooling zone.

The rollers are driven in rotation at the running speed of the tape. They are kept in contact with the strip, or in a position in the immediate vicinity of the strip.

At the rear and on the sides of the rollers, a device 13 makes it possible to limit the circulation of gas between the zones of the airlock, in particular by limiting the spaces between the fixed parts and the moving parts.

Nitrogen is injected into zone 12 by means of a supply 14 constituting a means arranged to inject an inert gas. A draw-off is carried out in zone 11 by a draw-off means 15. The pressure and the inert gas injection rate in zone 12 and the draw-off flow rate in zone 11 are fixed so that the flow of gas between the zones 11 and 12 or only from zone 12 to zone 11. This prevents the entry of humid atmosphere coming from the humid zone 5 into zone 11 of the airlock and any mixing of the latter with the dry atmosphere of the airlock. area 2.

In the example shown, at the outlet of the wet cooling zone 5, in the running direction of the strip, there is a set 16 of liquid knives intended to remove most of the runoff liquid present on the strip. The set 16 of liquid knives is followed by a set 17 of gas knives intended to remove the liquid still present on the strip.

Still with reference to the first embodiment, the metal strip 1 then passes through a return tank 18 in which the cooling liquid projected by the nozzles 6 and the knives 16 of liquid before being sent to a recirculation tank not shown by means of a conduit 24.

The return tank 18 includes a second set 19 of gas knives intended to remove the liquid which could still be present on the metal strip 1.

In the example shown, the first set 17 and the second set 19 of gas knives are supplied by means of supplies coming from the same supply duct (not numbered) represented by a vertical arrow.

The metal strip 1 then passes through a part 20 fitted with heating tubes 21 making it possible to remove any trace of liquid on the strip. On leaving this part 20, the strip passes through an atmosphere separation lock 22 between the wet parts 5, 18, 20 and parts 23 located downstream in the direction of travel of the strip.

For example, the strip is cooled in the dry zone 2 from a temperature of 800°C to a temperature of 700°C, then is cooled in the wet zone 5 from a temperature of 700°C to a temperature of 460°C.

The cooling liquid is for example water or an acid solution containing formic acid.

Referring to the diagram of the picture 2 appended, there is illustrated a second embodiment of a device according to the invention, only described for its differences with the first embodiment.

The second embodiment further comprises a system for drying and purging the wet cooling zone according to the invention.

The drying and purging system of the wet cooling zone comprises blades 27 of inert gas, for example nitrogen, directed downwards and blowing on the interior walls of an envelope of the wet cooling zone ( casing) to help evacuate the liquid from the walls to a recirculation conduit 24 or to a purge conduit 26.

In addition to the inert gas supplied by the knives 27, the system of drying and purging of the cooling zone of the second embodiment comprises points 28 for injecting an inert gas, for example nitrogen, and vents 29 allow rapid purging of the wet cooling zone 5 . The inert gas supplying the knives 27 and the injection points 28 is heated beforehand, for example to a temperature of about 50°C.

A system 25 for heating and thermally insulating the walls of the casing of the wet cooling zone is installed outside the walls of the wet cooling zone.

Advantageously, the liquid sprayed onto the strip is a formic acid solution, with a formic acid concentration of between 0.1 and 5.5%, advantageously between 0.1 and 5%, advantageously between 0.1 and 4, 5%, advantageously between 0.1 and 4%, advantageously between 0.1 and 3.5%, advantageously between 0.1 and 3%, advantageously between 0.1 and 2.5%, advantageously between 0.15% and 2.5%, advantageously between 0.2 and 2.5%, advantageously between 0.3% and 2%, advantageously between 0.35% and 2.5%, advantageously between 0.4% and 2.5%, advantageously between 0.45% and 2.5% by mass of the solution. More advantageously, the solution has a formic acid concentration of between 0.46% and 2.4%, advantageously between 0.47% and 2.3%, advantageously between 0.48% and 2.2%, advantageously between 0.49% and 2.1% by mass of the solution. Even more advantageously, the solution has a formic acid concentration of between 0.5% and 2% by mass of the solution.

Of course, the invention is not limited to the examples which have just been described and many adjustments can be made to these examples without departing from the scope of the invention as defined by the claims.

Moreover, the different features, forms, variants and embodiments of the invention may be associated with each other in various combinations insofar as they are not incompatible or exclusive of each other.

Claims (13)

1. Cooling section for a continuous line for annealing or galvanizing steel strips arranged to receive a metal strip (1), said section comprising at least one dry cooling zone (2) arranged to spray a gas onto said steel strip, at least one wet cooling zone (5) arranged to spray a liquid or a mixture of gas and liquid onto said steel strip, and an airlock (4) for separating atmospheres, which airlock is interposed between the dry cooling zone and the wet cooling zone,
   wherein the airlock for separating atmospheres comprises three pairs of rollers or flaps (8, 9, 10), each of the pairs being positioned transversely to a direction of travel of the metal strip, said three pairs of rollers or flaps delimiting between them two zones of said airlock: a first zone (11), which is delimited by the first two pairs of rollers or flaps (8, 9) in the direction of travel of the strip, located on the side of the dry cooling zone (2), and comprises extraction means (15), and a second zone (12), which is delimited by the last two pairs of rollers or flaps (9, 10) in the direction of travel of the strip, located on the side of the wet cooling zone (5), and comprises means (14) arranged to inject an inert gas.
2. Cooling section according to the preceding claim, wherein the dry cooling zone and the wet cooling zone are positioned along a vertical pass, the wet cooling zone being arranged below the dry cooling zone.
3. Cooling section according to any of the preceding claims, further comprising a system for drying and purging (24, 25, 26, 27, 28, 29) the wet cooling zone.
4. Cooling section according to the preceding claim, wherein the system for drying and purging the wet cooling zone comprises a device (27, 28) arranged to inject nitrogen.
5. Cooling section according to claim 4, wherein the system for drying and purging the wet cooling zone comprises a device (25) arranged to heat the walls of said wet cooling zone.
6. Cooling section according to claim 4, wherein the system for drying and purging the wet cooling zone comprises a system of nitrogen knives directed down the wet cooling zone and arranged to blow nitrogen onto interior walls of said wet cooling zone.
7. Method for cooling a continuous line for annealing or galvanizing steel strips arranged to receive a metal strip (1), said section comprising at least one dry cooling zone (2) arranged to spray a gas onto said steel strip, which method comprises at least one dry cooling step comprising spraying a gas onto said steel strip, at least one wet cooling step comprising spraying a liquid or a mixture of gas and liquid onto said steel strip, and an atmosphere separation by means of an airlock (4) for separating atmospheres, which airlock is interposed between the dry cooling zone and the wet cooling zone, cooling section wherein the airlock for separating atmospheres comprises three pairs of rollers or flaps (8, 9, 10), each of the pairs being positioned transversely to a direction of travel of the metal strip, said three pairs of rollers flaps delimiting between them two zones of said airlock, an extraction step in a first zone (11) delimited by the first two pairs of rollers or flaps (8, 9) in the direction of travel of the strip and located on the side of the dry cooling zone (2), and a step of injecting an inert gas into a second zone (12) delimited by the last two pairs of rollers or flaps (9, 10) in the direction of travel of the strip and located on the side of the wet cooling zone (5).
8. Method according to the preceding claim, characterized in that the liquid is non-oxidizing for the strip.
9. Method according to the preceding claim, characterized in that the liquid is a formic acid solution, the acid concentration of which is between 0.1% and 6% by mass of the solution.
10. Method according to claim 9, characterized in that the liquid is a formic acid solution, the acid concentration of which is between 0.5% and 2% by mass of the solution.
11. Method according to any of claims 7 to 10, further comprising a step of separating atmospheres by means of an airlock for separating atmospheres, which airlock is interposed between the dry cooling zone and the wet cooling zone, said step of separating atmospheres comprising a step of injecting an inert gas into a first zone of said airlock and an extraction step in a second zone of said airlock.
12. Method according to any of claims 7 to 11, further comprising a step of drying and purging the wet cooling zone, in particular by means of calories originating from a heating zone of the continuous line.
13. Computer program product which is downloadable from a communication network and/or stored on a computer-readable medium and/or executable by a microprocessor, and loadable in an internal memory of a calculation unit, characterized in that it comprises program code instructions which, when executed by the calculation unit, implement the steps of the method according to any of claims 7 to 12.

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