WET COOLING GALVANIZING LINE

Abstract

WET COOLING GALVANIZING LINE Method for cooling a steel strip (1) circulating in a continuous galvanizing line (100) comprising a step of verifying required conditions defined according to the temperature of the strip at the outlet of a humid chamber (4) with respect to the Leidenfrost temperature, the time spent by the strip in the humid chamber and the cumulative time spent in a drying chamber (7) and in a downstream chamber, under a dry and reducing atmosphere, being chosen, the liquid of the cooling device (5) being able to be an aqueous cooling liquid having a mass concentration of formic acid greater than or equal to 0.1% and the temperature of the strip at the outlet of the drying chamber being able to be less than or equal to 250 °C, the method being able to further comprise a step of adjusting an operating parameter of the line to comply with said required conditions.Figure for abstract: 6.

Description

Title of the invention: WET COOLING GALVANIZING LINE Designation of the technical field concerned

[0001] The invention relates to continuous galvanizing lines for metal strips and more particularly to galvanizing lines equipped with wet cooling sections. Technical problems addressed by the invention

[0002] It is known to carry out wet cooling in a galvanizing line to produce third generation steels requiring quenching rates after annealing greater than 150°C / s to obtain the desired metallurgical structure and mechanical properties.

[0003] For example, the applicant's patent FR3064279 describes a cooling section using an aqueous solution comprising formic acid making it possible to carry out in-line dip galvanizing of these steels, without requiring an acid pickling step before coating.

[0004] This solution was developed for third generation steels because of: • their extreme sensitivity to oxidation; • the need to cool them very quickly; • to reach in certain cases temperatures at the end of cooling below 200°C.

[0005] It may not be suitable for conventional steels (generation 1 & 2) because these steels have different specificities: • they require lower cooling rates; • they are produced with higher end-of-cooling temperatures higher than generation 3 steels (in general, it is the temperature of the zinc bath which is targeted), the oxidation conditions are therefore different; • although less sensitive to surface oxidation (because they contain less alloying elements), they still require controlling the atmosphere and avoiding any oxidation phenomenon.

[0006] These differences require adapting the operating conditions to be able to produce conventional steels on the same line as third-generation steels, particularly in terms of cooling conditions, atmosphere management and acid dosage.

[0007] Finally, the presence of acid in the cooling solution can generate appearance defects regardless of the generation of steel considered.

[0008] It is therefore essential to properly control and adapt the management of the atmosphere according to the cooling conditions.

[0009] Demand for third-generation steel remains low, and does not allow 100% use of the capacity of a line configured to galvanize these steels. It is therefore essential to be able to produce all types of steel on a line with rapid wet cooling in order to limit operating costs and not penalize the profitability of the installation.

[0010] The object of the invention is to enable third-generation steels and conventional steels to be galvanized without defects (surface appearance, uniform zinc coating and without missing parts) on the same line, for full-time operation of the line. The return on investment is thus faster for the steelmaker and the production cost of third-generation steels is significantly reduced. Technical background

[0011] Wet cooling technologies allow high cooling rates of up to 2000 °C / s / mm to be achieved. These technologies are mainly used on continuous annealing lines. The strip must then pass through an acid pickling section before being coated in a galvanizing line.

[0012] The most widespread cooling technology is water quenching by immersion in a tank (for English Water Quench). The principle consists of passing the steel strip through a tank containing water. This tank may include auxiliary equipment such as additional water jets, whether submerged or not. The jets can be generated by nozzles, slots or perforated plates.

[0013] For example, patent WO2020203261 describes a solution with turbulent submerged jets and sliding plates to control the start of cooling and stabilizing rollers that can be submerged. Patent WO2021024096 describes a solution with inclined submerged jets to avoid a non-planar free surface at the bath inlet and thus better control homogeneity.

[0014] Mist quenching can also be used, but it has much slower cooling rates. In this case, a water mist is sprayed onto the strip.

[0015] The coolant, or a gas / liquid mixture, can also be sprayed onto the strip using single-fluid or dual-fluid nozzles. The spraying is carried out under pressure, which makes it possible to achieve cooling rates comparable to or even higher than water immersion quenching. Single-fluid or dual-fluid spraying offers greater flexibility of control thanks to a wide operating pressure range, typically between 1 and 12 bars.

[0016] Patent JP2019210549 describes a combination of two technologies with slow cooling by spray ramps followed by rapid cooling with immersion water quenching.

[0017] Dip galvanizing of steel strips was invented in the 1930s. Since its discovery, cooling equipment has evolved considerably, but it did not use water because the excessive oxidation of the steel surface resulting from water cooling did not allow the zinc to be wetted.

[0018] The applicant's patent FR3064279 describes a method for cooling a third-generation steel strip in a wet cooling chamber of a galvanizing line using an aqueous solution comprising between 0.5 and 6%, and preferably between 0.5 and 2%, of formic acid. Spraying the acid solution onto the strip makes it possible to obtain a surface condition that allows dip galvanizing for these steel grades by combining a mechanical effect linked to the spraying and a chemical stripping effect.

[0019] The developments made by the applicant thus make it possible to implement wet cooling on galvanizing lines because they eliminate the acid pickling step.

[0020] In wet cooling, there are three different regimes depending on the surface temperature of the strip and in particular the so-called Leidenfrost temperature which is the temperature which separates the stable vapor film cooling domain from the unstable transition domain. The Leidenfrost temperature is typically between 250°C and 630°C depending on the cooling system used.

[0021] The following table gives examples of Leidenfrost temperatures for different cooling conditions: System Nozzle type X [mm] Y [mm] Z [mm ] T water [°C] G [1 / min / m2] T Leidenfr ost IA Bi Fluid flat jet 350 450 250 23 15 260 IB Bi Fluid flat jet 350 450 250 23 29 350 2A Mono Fluid flat jet 50 440 400 43 255 360 2B Mono Fluid flat jet 110 150 400 43 1092 400 2C Mono Fluid flat jet 80 320 290 26 689 420 2D Mono Fluid flat jet 50 150 300 18 2371 500 2E Mono Fluid flat jet 50 440 400 42 3889 540 2F Mono Fluid Flat Jet 50 330 300 17 2410 600 2G Mono Fluid Flat Jet 50 150 200 41 11359 680 3A Mono Fluid Flat Jet 120 150 200 43 634 340 3B Mono Fluid Integral Cone 50 450 400 40 681 450 3C Mono Fluid Integral Cone 80 225 200 22 4230 540 3D Mono Fluid Integral Cone 50 150 400 42 5991 630

[0022] The "System" column shows that the data in the table correspond to different spraying systems, the "Nozzle Type" column specifies the type of spray nozzle used, the "X" column indicates the distance between two nozzles depending on the width of the strip, the "Y" column indicates the distance between two nozzles depending on the length of the strip, the "Z" column indicates the distance of the nozzles from the strip, the "T water" column indicates the temperature at which the water is sprayed, the "G" column indicates the flow rate of water sprayed per m2 of strip and the "T" column indicates the Leidenfrost temperature observed under these conditions.

[0023] At high temperatures, above the Leidenfrost temperature, this is the film boiling regime. A vapor cushion of a few tens of μm separates the strip from the liquid water. The surface of the strip is not wetted. The wall is relatively thermally insulated from the water, because the thermal conductivity of water vapor is about 27 times lower than that of liquid water. The heat exchange coefficient is stable and low. Rapid cooling is difficult during this phase.

[0024] From the Leidenfrost temperature and up to a temperature around 200 °C, this is the transition domain in which the heat flux varies rapidly from a minimum value to a maximum value. The vapor layer tends to disappear and the liquid water begins to reach the surface of the strip at a few points before impacting its entire surface. During this phase, the cooling regime is unstable and a strong variation in the heat exchange coefficient is observed. The cooling increases rapidly. The heat exchange is much more intense, the water vaporizes directly on contact with the wall.

[0025] At lower temperatures, typically below 150 °C, nucleate boiling occurs. Initially, many continuous columns of vapor are observed in the liquid, then bubbles begin to be isolated and the entire surface is in contact with liquid water. Cooling is quite rapid at the beginning of this phase, but decreases when the surface temperature is too low.

[0026] The applicant's patent WO2021116594 is an improvement of the previous patent with a separation of the aqueous cooling zone into two parts: a first part where the strip remains above the Leidenfrost temperature with water cooling (or water + gas), followed by a second part with water cooling + formic acid.

[0027] Before the Leidenfrost temperature, a vapor cushion separating the strip from the liquid water, the surface of the strip is not wetted. Oxidation of the strip is thus limited in the first part of the cooling.

[0028] This solution makes it possible to reduce acid consumption and to limit the quantity of residues resulting from the decomposition of the acid present on the surface of the strip at the outlet of the wet cooling chamber.

[0029] The applicant's patent WO2018172714 describes a dry cooling section followed by a wet cooling section arranged on a vertical strand. It notably comprises a three-roller airlock between these two sections with a draw-off zone and an inert gas injection zone as well as drying of the wet zone by heating the walls and blowing nitrogen onto them via multiple injection points.

[0030] The solutions described above are used for the galvanization of third-generation steels, but they are not suitable for the galvanization of conventional steels, in particular because the surface of the strip oxidizes for these steels in the wringing section and the drying chamber which follow the liquid cooling due to their humid atmospheres. Summary of the invention

[0031] According to a first aspect of the invention, there is provided a method of cooling a steel strip circulating in a continuous galvanizing line in which: • the steel strip passes through an upstream chamber under a dry, reducing atmosphere and an upstream atmosphere separation airlock before being cooled in a humid chamber containing a liquid cooling device, • the steel strip cooled in the humid chamber passes through a drying chamber, a downstream airlock for separating atmospheres and at least one downstream chamber under a dry and reducing atmosphere, the method further comprising, • an injection of dry atmosphere, by means of an injection system, into the humid chamber and into the drying chamber, • an atmosphere extraction, by means of an extraction system, to continuously renew the atmosphere in the humid chamber and to create a flow of atmosphere from the drying chamber to the humid chamber, • control by means of a control and command system: • the time the steel strip spends in the chamber, • the cumulative time spent by the steel strip in the drying chamber and in at least one downstream chamber under a dry and reducing atmosphere, • the temperature (Th) of the steel strip at the exit of the humid chamber, and • the temperature (Ts) of the steel strip at the outlet of the drying chamber.

[0032] The method comprises a step of verifying required conditions defined by: • when the outlet temperature (Th) of the steel strip from the humid chamber is greater than or equal to the Leidenfrost temperature, the time spent by the steel strip in the humid chamber must be less than or equal to fifteen seconds, and the cumulative time spent in the drying chamber and in at least one downstream chamber under a dry and reducing atmosphere must be greater than or equal to the residence time of the steel strip in the humid chamber; • when the outlet temperature (Th) of the steel strip from the humid chamber is lower than the Leidenfrost temperature, the liquid from the cooling device must be an aqueous cooling liquid having a mass concentration of formic acid greater than or equal to 0.1% and the temperature (Ts) of the steel strip at the outlet of the drying chamber must be less than or equal to 250°C.

[0033] If one of the required conditions is not met during the verification step, the method further comprises a step of adjusting at least one operating parameter of the line making it possible to meet the conditions required in the verification step.

[0034] The adjusted operating parameter is for example the running speed of the strip, the flow rate of coolant or the nature of the coolant.

[0035] After adjusting the operating parameter of the line, the step of verifying the required conditions is repeated to verify that the required conditions are met. If this is still not the case, the step of adjusting at least one operating parameter of the line is repeated until the conditions required in the verification step are met.

[0036] The inertia of the line is taken into account to only carry out a new verification step after a sufficient delay for an adjustment of an operating parameter of the line to be effective.

[0037] The step of verifying required conditions and the step of adjusting at least one operating parameter of the line are carried out automatically by the line control and command system.

[0038] The system includes tables allowing one or more operating parameters of the line to be chosen, and the instruction to be applied, depending in particular on the operating conditions of the line and the composition of the strip.

[0039] When the strip leaves the humid chamber at a temperature greater than or equal to the Leidenfrost temperature, the cooling liquid may be water, without the addition of acid. This avoids the presence of residues on the strip which result from the decomposition of the acid, knowing that these residues can generate appearance defects, for example of the coffee stain or streak type. The non-use of acid also makes it possible to reduce the operating cost of the line.

[0040] By having a residence time of the strip in the humid chamber of 15 seconds maximum, it is avoided that oxides are present on the surface of the strip for a conventional steel, or if there are, that they are not in too large a quantity for a third generation steel. For the latter, oxidation (selective or total) must be carried out upstream, during the heating phase which precedes rapid cooling.

[0041] By having a cumulative residence time of the strip of at least equivalent duration in the drying chamber and in the downstream chambers under a dry and reducing atmosphere which precede the coating, a reduction of the oxides (mainly iron oxides) which would be present on the surface of the strip at the outlet of the humid chamber is encouraged. Thus, during its immersion in the coating bath, the surface of the strip no longer comprises any oxide or those which remain are in small quantity. They do not harm the quality of the coating.

[0042] A downstream chamber has a dry and reducing atmosphere if its atmosphere is composed of a mixture of nitrogen and hydrogen with a volumetric proportion of hydrogen of at least 1% and a dew point below -5°C so as to avoid oxidation of the iron.

[0043] Hereinafter, we will designate by HNx, a dry and reducing atmosphere composed of a mixture of nitrogen and hydrogen.

[0044] When the strip leaves the humid chamber at a temperature between 110°C inclusive and the Leidenfrost temperature excluded, there has been partial contact between the liquid and the strip and this is all the more significant the lower the end of cooling temperature.

[0045] It is necessary to use an aqueous solution comprising acid to ensure that the quantity of oxides present on the surface of the strip is not too high at the outlet of the humid chamber.

[0046] The solution can nevertheless be weakly dosed, for example with a mass concentration of formic acid of 0.2%.

[0047] The formic acid concentration can be determined from the diagram in [Fig.7], depending on the chemical composition of the steel, its Manganese and Silicon content. The value retained is the highest value between that obtained according to the Manganese concentration and that obtained according to the Silicon concentration. For example, for a steel with 3% Manganese and 2% Silicon, the acid concentration will be 0.5%.

[0048] This diagram shows that the necessary concentration of formic acid remains low.

[0049] The pH varies little in this acid concentration range. For example, it is 3.14 for an acid concentration of 0.3% and 2.72 for a concentration of 2%.

[0050] When the strip leaves the humid chamber at a temperature below 110°C, there has been total contact between the liquid and the strip during wet cooling. It is therefore necessary to use an aqueous solution comprising acid to prevent the quantity of oxides present on the surface of the strip from being too high at the outlet of the humid chamber. The solution is dosed more than for a strip outlet at a temperature above 110°C, but it can nevertheless remain low.

[0051] The formic acid concentration can be determined from the diagram in [Fig.8], depending on the chemical composition of the steel. Again, the value retained is the higher value between that obtained according to the manganese concentration and that obtained according to the silicon concentration. For example, for a steel with 4% manganese and 2% silicon, the acid concentration will be 0.7%.

[0052] This diagram shows that the necessary concentration of formic acid remains low.

[0053] According to the invention, when the outlet temperature (Th) of the cooled steel strip from the wet chamber is lower than 110°C, at least part of the excess liquid quantity carried away by the cooled steel strip in the wet chamber is removed in a dewatering section, and the dewatered steel strip is dried in the drying chamber.

[0054] Wringing the strip limits the amount of liquid carried by the strip into the drying chamber. Drying the strip prevents moisture from remaining on the strip which would be carried into the downstream chamber.

[0055] When the outlet temperature of the cooled steel strip from the wet chamber is equal to or greater than 110°C, these wringing and drying operations are not necessary.

[0056] According to the invention, the atmospheric pressure of the upstream chamber and the atmospheric pressure of the downstream chamber are each greater than that of the downstream chamber. drying, and the atmospheric pressure of the drying chamber is higher than that of the humid chamber.

[0057] By having a pressure in the drying chamber higher than that of the cooling chamber, it is ensured that the flow of atmosphere is from the drying chamber to the cooling chamber, and not in the other direction.

[0058] This prevents the atmosphere in the drying chamber from being polluted by humid or acidic vapors coming from the cooling chamber.

[0059] By having a pressure in the downstream chamber higher than that of the drying chamber, it is ensured that any flow of atmosphere between the two chambers, despite the presence of an atmosphere separation airlock, occurs from the downstream chamber towards the drying chamber, and not in the other direction.

[0060] This avoids any risk of contamination of the atmosphere of the downstream chamber by humid or acidic vapors coming from the drying chamber.

[0061] By having a pressure in the upstream chamber higher than that of the drying chamber, and not only higher than the pressure of the wet chamber, it is ensured that no flow of atmosphere can occur from the wet chamber to the upstream chamber. Any possible flow, despite the presence of an atmosphere separation airlock, occurs from the upstream chamber to the wet chamber, and not in the other direction.

[0062] This avoids any risk of contamination of the upstream chamber by humid or acidic vapors coming from the humid chamber.

[0063] Note that the terms upstream and downstream used in this document relate to the direction of travel of the strip in the line.

[0064] Advantageously according to the invention, the method further comprises a step of injecting an atmosphere composed of a mixture of nitrogen and hydrogen into the humid chamber and into the drying chamber, and the hydrogen content of said atmosphere is adjusted according to the chemical composition of the steel of the steel strip to be galvanized.

[0065] By injecting an atmosphere into the humid chamber and the drying chamber, the evacuation of water vapor from these chambers is encouraged and the risk of oxidation of the surface of the strip is thus limited, in particular for conventional steels which are more sensitive to oxidation.

[0066] In the humid chamber, the injected atmosphere may be limited to nitrogen, without hydrogen content. In the drying chamber, the atmosphere may contain hydrogen to reduce any oxides present on the surface of the strip.

[0067] In the dry chamber, the hydrogen content can be adjusted according to the chemical composition of the steel. Thus for steels whose Mn content is greater than 1.5% and that of Si is greater than 0.5% the hydrogen content must be greater than 1%.

[0068] Advantageously according to the invention, the extraction of atmosphere is carried out with an adjustable extraction flow rate and such that , ■ e œ >qL +qL +qL +œ +q +qL ' ^Ext ^HNx ^HNx ^HNx ^Vap

[0069] By having an extraction flow rate A1' in the humid chamber greater than ^Ext the sum of the atmospheric flow rates injected into the humid chamber Ah, in the VHNx spinning system Qe, in the drying chamber qs and those which result from the evaporation of the cooling solution in the humid chamber Ah, the ^-Vap spin system and the drying chamber, we ensure an entry of atmosphere coming from the airlock placed upstream of the humid chamber towards it and of an atmosphere inlet coming from the airlock placed downstream of the drying chamber towards it.

[0070] Advantageously according to the invention, the mass concentration of formic acid in the cooling liquid is adjusted according to the chemical composition of the steel in the steel strip to be galvanized. The formic acid concentration is determined from the diagrams in Figures 7 and 8.

[0071] According to a second aspect of the invention, there is provided a continuous galvanizing line capable of implementing a method according to the first aspect of the invention for galvanizing a steel strip, the line successively comprising in the direction of movement of the steel strip to be galvanized: • an upstream chamber under a dry and reducing atmosphere, in which the steel strip is heated or cooled, • an upstream airlock for separating atmospheres, • a wet chamber containing a liquid cooling device, in which the steel strip is cooled, • a section for dewatering the cooled steel strip, in which at least part of the excess liquid quantity that can be carried away by the cooled steel strip is removed, • a drying chamber for the wrung steel strip, in which the wrung steel strip is dried to evaporate the liquid still present on the wrung steel strip, • a downstream airlock for separating atmospheres, • at least one downstream chamber under a dry, reducing atmosphere.

[0072] The line further comprising: • an atmosphere injection system capable of injecting a dry and reducing atmosphere into the humid chamber and into the drying chamber, • an atmosphere extraction system capable of continuously renewing the atmosphere in the humid chamber and creating a flow of atmosphere from the drying chamber to the humid chamber, • a control and command system capable of controlling the time taken for the steel strip to pass through the chamber, the cumulative time taken for the steel strip to pass through the drying chamber and through the at least one downstream chamber under a dry and reducing atmosphere, the temperature (Th) of the cooled steel strip at the outlet of the humid chamber and the temperature (Ts) of the steel strip at the outlet of the drying chamber and of implementing the steps of verification and adjustment of the process.

[0073] Advantageously according to the invention, the humid chamber can be supplied alternately by at least two separate circuits for supplying and recirculating cooling liquid.

[0074] For example, a first circuit contains water, without the presence of acid, and a second circuit contains a mixture of water and acid, for example formic acid with a mass concentration of 1% in formic acid.

[0075] According to another example, a first circuit contains water, without the presence of acid, a second circuit comprises a first mixture of water and formic acid with a mass concentration of 0.5% in formic acid and a third circuit contains a second mixture of water and formic acid with a mass concentration of 1% in formic acid.

[0076] Depending on the nature of the steel, for example whether it is a classic or third generation grade, and the thermal cycle carried out, the humid chamber will be supplied by one or other of the circuits.

[0077] Advantageously, the humid chamber can be equipped with a device allowing rapid emptying of a cooling circuit so as to quickly change it from a first acid concentration to a second acid concentration.

[0078] According to an alternative embodiment of the invention, the humid chamber comprises a cooling device of the water immersion quenching type.

[0079] According to another alternative embodiment of the invention, the wet chamber comprises a single-fluid and / or dual-fluid spray cooling device. Spray cooling allows more possibilities for adjusting the cooling slope of the strip over its width and length than with water immersion quenching. For example, it is possible to vary the flow rate of cooling liquid over the width / length of the strip.

[0080] Advantageously according to the invention, a wringing system is arranged between the wet chamber and the drying chamber. It makes it possible to limit the quantity of liquid carried by the strip into the drying chamber, thus facilitating the drying of the strip and reducing the quantity of water vapor in the drying chamber. It may, for example, comprise a water knife or wringing rollers.

[0081] Advantageously according to the invention, the atmosphere separation airlocks comprise two pairs of rollers and an atmosphere injection system between the pairs of rollers.

[0082] Combined with the atmosphere extraction system and the resulting flows, the injection of atmosphere between the two pairs of rollers of the airlock makes it possible to create a flow of atmosphere going from the center of the airlock towards the humid chamber, for the upstream airlock, or the drying chamber, for the downstream airlock. This acts as a barrier to the atmosphere present in these chambers which cannot escape towards an upstream or downstream chamber to contaminate it by passing through the airlock. This prevents pollution of the reducing atmosphere of the upstream and downstream chambers by water vapor coming from the humid chamber or the drying chamber.

[0083] Advantageously, the atmosphere extraction system comprises an atmosphere extraction from the humid chamber and an atmosphere extraction from the drying chamber.

[0084] Having additional extraction in the drying chamber is desirable when the steam flow rate resulting from the drying of the strip and the flow rate injected into the drying chamber are too high to have only atmospheric extraction in the humid cooling chamber.

[0085] According to an exemplary embodiment of the invention, the line further comprises a strip oxidation system arranged in a preheating or heating chamber, upstream of the wet chamber.

[0086] For certain grades of steel, for example those containing significant amounts of alloying elements such as manganese, silicon or chromium, it is indeed advantageous to carry out pre-oxidation or internal selective oxidation in a preheating or heating section in order to more easily reduce the oxides formed more easily during the temperature maintenance phase.

[0087] It is obtained by combining, in depth, oxygen atoms from the surface with certain atoms of addition elements, leading to the formation of oxide precipitates, without oxidizing the iron, in an atmosphere whose dew point is significantly higher than that present in the humid cooling chamber.

[0088] According to an exemplary embodiment of the invention, the drying chamber is combined with a downstream chamber or equipment, such as an aging chamber (overaging in English), or an inductor. Brief description of the figures

[0089] Other characteristics and advantages of the invention will appear during the reading of the detailed description which follows for the understanding of which reference will be made to the appended drawings in which: • [Fig-1] is a schematic and partial representation of a portion of a horizontal line with spray cooling, according to an exemplary embodiment of the invention, • [Fig.2] is a schematic and partial representation of a portion of a vertical line with spray cooling, according to another exemplary embodiment of the invention, • [Fig.3] is a schematic and partial representation of a portion of a vertical line with immersion cooling in a tank, according to another exemplary embodiment of the invention, • [Fig.4] is a schematic and partial view of a continuous line according to an exemplary embodiment of the invention, • [Fig.5] is an enlargement of the cooling zone of the line of [Fig.4], • [Fig.6] is a schematic representation similar to that of [Fig.l] to illustrate the gas flows, • [Fig.7] is a diagram showing the mass concentration of formic acid in the cooling solution according to the steel composition when the outlet temperature of the strip from the humid chamber is lower than the Leidenfrost temperature, but higher than 110 °C, and • [Fig.8] is a diagram showing the mass concentration of formic acid in the cooling solution according to the steel composition, when the outlet temperature of the strip from the humid chamber is below 110 °C. Detailed description of the invention

[0090] The embodiments described below being in no way limiting, it will be possible in particular to consider variants of the invention comprising only a selection of characteristics described, subsequently isolated from the other characteristics described, if this selection of characteristics is sufficient to confer a technical advantage or to differentiate the invention compared to the state of the prior art. This selection comprises at least one characteristic, preferably functional without structural details, or with only a part of the structural details if this part only is sufficient to confer a technical advantage or to differentiate the invention compared to the state of the prior art.

[0091] In the remainder of the description, elements having an identical structure or similar functions will be designated by the same references.

[0092] A liquid cooling section according to the invention comprises two chambers separated by a wringing section, a wet chamber in which a water quenching process is carried out, immediately followed after wringing by a drying chamber in the direction of circulation of the strip.

[0093] The two chambers are managed concomitantly in terms of atmospheric flow and pressure. The management of one of the chambers is done as a function of the management of the other.

[0094] The wet chamber has the following features: • steel quenching with water-based technology such as immersion water quenching, single-fluid spraying, dual-fluid spraying, mist, etc., • injection of HNx with a Hydrogen concentration between 0% and 20%, • extraction of the atmosphere so as to permanently renew the atmosphere of the humid chamber, • evacuation of water in liquid state outside the chamber.

[0095] The wet chamber is separated from the drying chamber by a strip wringing system. The purpose of this system is to remove excess water from the strip to allow complete evaporation during passage into the drying chamber.

[0096] The drying chamber, which starts directly after spinning, includes the following features: • drying of the strip. Different technologies can be used, such as convection with a hot gas or radiation with, for example, electric candles, • injection of HNx with a hydrogen concentration of up to 20%.

[0097] Finally, upstream and downstream of the two chambers, atmosphere separation airlocks are positioned to prevent pollution of the upstream and downstream chambers by a humid atmosphere.

[0098] However, the presence of these airlocks is not sufficient to achieve this objective. It is also necessary to manage the pressures and the flow of atmosphere in the different chambers.

[0099] Industrially, different configurations are possible, depending on whether the lines are vertical or horizontal, or depending on the cooling technology.

[0100] Figures 1 to 3 schematically illustrate three examples of configurations.

[0101] [Fig.l] shows an example of a portion of a horizontal line (the strip 1 circulating horizontally in the direction indicated by the arrow in the figure) with spray cooling according to an exemplary embodiment of the invention. Represented are an upstream chamber 2 which is dry and under a reducing atmosphere, an upstream airlock 31 for separating atmospheres, a humid chamber 4 where quenching is carried out by spraying a liquid, a wringing section 6, a drying chamber 7, a downstream airlock 32 for separating atmospheres, a downstream chamber 8 which is dry and under a reducing atmosphere.

[0102] [Fig.2] shows an example of a vertical line portion (strip 1 running vertically) with spray cooling according to another embodiment of the invention. It has the same chambers and sections as in [Fig.l].

[0103] [Fig. 3] shows another example of a vertical line portion, but with water quenching cooling via immersion in a tank according to another embodiment of the invention. It has the same chambers and sections as in Figures 1 and 2.

[0104] In particular in the case of a vertical configuration such as shown in Figures 2 and 3, the spinning section can advantageously be supplemented upstream by liquid knives and / or complementary liquid evacuation systems in order to limit the penetration of liquid into the drying chamber.

[0105] The surface oxidation of the strip depends on: • the chemical composition of the steel, • the temperature of the strip, particularly at the end of wet cooling and at the exit from the drying chamber, • the dew point of the area concerned.

[0106] As explained in the paragraph on the state of the art, the use of a liquid cooling process involves considering three different cooling regimes, depending on the temperature of the strip at the end of wet cooling:

[0107] Regime 1: The film boiling regime is present for strip temperatures above the Leidenfrost temperature. In this case, there is no contact between the coolant and the steel strip.

[0108] Regime 2: the transition boiling regime is reached when the strip has a temperature between 100°C and the Leidenfrost temperature.

[0109] Regime 3: the “wet” cooling regime is reached when the strip temperature is below 150°C. In this case, the liquid is permanently in contact with the steel strip.

[0110] The Leidenfrost temperature depends in particular on the temperature of the liquid. When cooling is carried out by spraying, it also depends on the surface quantity of liquid sprayed, impact pressure on the belt and drop size.

[0111] When cooling is carried out by immersion in a tank, it also depends on the circulation speed of the liquid.

[0112] We can consider that the Leidenfrost temperature is between 250°C and 600°C.

[0113] Since the addition of acid to the cooling water only has an effect on the surface condition of the strip if there is contact between the water and the strip, the use of acid in the case of regime 1 is unnecessary.

[0114] On the contrary, the use of acid in diet 3 is relevant.

[0115] If the end of cooling temperature is greater than or equal to 110°C, the strip will not carry a water film into the drying chamber.

[0116] Only in the case of an end of cooling temperature lower than 110°C, the atmosphere of the drying chamber will be humid.

[0117] The oxidation kinetics of iron are thermodependent. In an oxidizing atmosphere, it can be considered that iron only begins to oxidize above 250 °C. Similarly, it can be considered that the reduction of iron oxides in a reducing atmosphere only really begins above 250 °C.

[0118] Thus, 250°C is the temperature above which it is necessary to avoid oxidizing the strip or to consider reducing the iron oxides already present.

[0119] In the case of Manganese, Silicon and Chromium oxides, the oxidation onset temperatures are significantly higher than for Iron. However, the dew points required to reduce these oxides are so low that they are unattainable in a galvanizing line.

[0120] For steel grades containing significant amounts of Manganese, Silicon and Chromium, it is appropriate to manage the oxidation of these alloying elements by carrying this out during the heating phase with selective or total oxidation.

[0121] In the case of end-of-cooling temperatures lower than the Leidenfrost temperature, the use of acid in the cooling water makes it possible to eliminate these oxides or at least to contain their appearance during the first moments of cooling.

[0122] According to the invention, three areas of surface oxidation management are defined as a function of the end of cooling temperature.

[0123] The table below summarizes these three areas, according to the cooling regime, by materializing: • if the water-tape contact requires the use of acid, • whether the atmosphere in the drying chamber is humid or dry. Cooling range Strip temperature at the end of wet cooling Liquid-strip contact Drying chamber atmosphere Presence of acid Regime 1 -T Leidenfrost Very low Dry no Regime 2 < T Leidenfrost Partial Dry yes Regime 3 < 110 °C Total Wet yes

[0124] In the cooling domain corresponding to regime 1, the strip will oxidize during the cooling phase.

[0125] The use of acid is unnecessary because there is practically no contact of water on the strip.

[0126] However, a dry drying chamber atmosphere, coupled with the injection of HNx, makes it possible to have a reducing atmosphere from the drying chamber.

[0127] The operating conditions are therefore in this case: i. Passage through the humid chamber as quickly as possible, less than 15 s for classic steel grades; ii. Injection of HNx into the drying chamber and downstream chambers. The hydrogen content (up to 20%) is adjusted in these chambers depending on the steel grade concerned and the time spent in the humid chamber; iii. The cumulative passage time in the drying and downstream chambers must at least be equal to the passage time in the humid chamber to ensure sufficient time for reduction of the iron oxides.

[0128] In the cooling domain corresponding to regime 2, the strip will oxidize during the cooling phase.

[0129] The use of acid may be considered and the drying chamber has a dry atmosphere.

[0130] The operating conditions are therefore in this case: i. Use of a low-dose acid; ii. Injection of HNx into the drying chamber and downstream chambers. Adjustment of the hydrogen content (up to 20%) in the chambers where the strip temperature exceeds 250 °C; iii. Promote the combination of the drying chamber with a downstream chamber equipped with heating equipment (e.g. inductor, aging); iv. The use of acid may not be necessary for some steels if the reduction capacity of the downstream chambers is sufficient.

[0131] In the cooling domain corresponding to regime 3, the strip will oxidize during the cooling phase.

[0132] The use of acid is relevant. However, the drying chamber atmosphere will be humid. The objective will therefore be to ensure that all the water will be evaporated before leaving the chamber and that the atmosphere will be well extracted.

[0133] The temperature in the drying chamber must not exceed 250°C before the strip exits, otherwise the strip will reoxidize.

[0134] The operating conditions are therefore in this case: i. Use of a low dose acid in the cooling water, ii. If the drying chamber is combined with a downstream chamber equipped with heating equipment, the outlet temperature must not exceed 250°C.

[0135] In order to create a flow of atmosphere towards the humid chamber where an atmosphere extraction is carried out, it is necessary to have simultaneously: • pressures such that the pressure Pam in the upstream chamber and the pressure Pav in the downstream chamber are greater than the pressure Ps of the drying chamber and the pressure in the latter is greater than the pressure Ph of the humid chamber:

[0136] (Pam;Pav)>Ps>Ph • flow rates such that the extraction flow rate of the humid chamber is VExt greater than the sum of the flow rate fU of HNx injected into the humid chamber, ^HNx of the flow rate Ae of HNx injected into the spin section, of the flow rate f)s vHNx vHNx injected into the drying chamber, from the steam flow resulting from the vaporization of the coolant on the surface of the strip in the humid chamber, of the flow rate Qe of steam resulting from the wringing of the strip, and of the flow rate qs of steam resulting from the drying of the strip: ^Vap

[0137] h ,Ji es -h ' e -s ^Ext > $HNx + QrNx + QrNx + Qvap + Qyap + ^Vap

[0138] The extraction flow rate must be strictly greater than the liquid which vaporizes. and to the HNx injected into the humid chamber, the section Q.. +Q +Qv ^HNx ^Vap ^Vap ^Vap spin-drying chamber re, the drying chamber As, the upstream airlock and the downstream airlock ^HNx ^HNx ^SasAni Q's so as to very slightly extract the atmosphere from the upstream chamber, for a SasAv flow rate Q , and the chamber swallows, for a flow rate Q . Kam' r ^av

[0139] In this case we have:

[0140] -(Æ. +q" +(? +Q?, + (£ + <X +Q" +q' ) =Q +Q ^Ext HNx ^HNx ^HNx ^vap ^vap ^vap ^SasAm xSasAv / xam ^-av

[0141] If the steam flow rate r? resulting from the drying of the strip and the flow rate vVap vHNx injected into the drying chamber are too high, an additional extraction qs can be added in the drying chamber and in this case we will have: Ext [0!42] Qh >(Ÿ +Q« +qS + e +$s +$h +Q« qS ^Ext ^-HNx ^HNx ^HNx ^vap ^vap ^-Vap ^-SasAni -SasAv ^Ext

[0143] The quantity of coolant which evaporates varies according to the format of the strip and its temperature, the extraction flow rate is adjustable, just like ^Ext qs if additional extraction is added in the drying chamber.

[0144] Referring to the diagram in [Fig.4] of the attached drawings, one can see schematically and partially represented, in longitudinal view, a vertical furnace galvanizing line 100 according to an exemplary embodiment of the invention.

[0145] It comprises successively and in the direction of travel of the strip 1, a preheating chamber 101, a heating chamber 102, a holding chamber 103, a cooling section 104 comprising a gaseous cooling chamber 3, a wet liquid cooling chamber 4, a wringing and return section 6 and a drying chamber 7, then an aging chamber 105, a furnace outlet section 106, and a dip galvanizing section 107. A control and command system 11 ensures the proper operation of the line and adjusts its operating parameters according to the characteristics of the strip at the inlet and those expected at the outlet.

[0146] Depending on the steel grade and the thermal cycle required to obtain the desired mechanical properties, the gas cooling chamber 3 allows, for example, slow cooling of the strip from an annealing temperature, for example 900°C, to a quenching start temperature, for example 700°C. Faster cooling of the strip in chamber 3 can also be achieved, but it will nevertheless remain less rapid than that obtained in the wet liquid cooling chamber 4. Indeed, gas cooling, typically by spraying a mixture of nitrogen and hydrogen, allows cooling rates of the order of 100 to 200°C / s to be achieved for steel strips 1 mm thick.

[0147] Referring to the diagram of [Fig.5] attached, the lower part of the cooling section 104 of [Fig.l] can be seen partially represented.

[0148] The strip 1 leaves the gas cooling chamber 3 by circulating from top to bottom in the direction of travel shown by the arrow S.

[0149] At the outlet of this chamber there is an upstream airlock 31 ensuring a separation between the controlled reducing atmosphere present in the cooling chamber 3 gaseous, consisting of a mixture of nitrogen and hydrogen, from that, humid, of the liquid cooling chamber 4 which is located downstream.

[0150] The airlock shown comprises two pairs of rollers 30 with an injection 33 of atmosphere between the two pairs of rollers, knowing that other airlock configurations are possible.

[0151] The strip then passes through the liquid cooling chamber 4 in which a liquid cooling device 5 is arranged. This comprises nozzles 40 which spray a cooling liquid onto the strip, for example an acid solution containing water and 0.5% formic acid.

[0152] Liquid knives 41 formed by flat jet nozzles 42 make it possible to remove most of the runoff liquid present on the strip. The jets are inclined relative to the strip at an acute angle in order to promote the detachment of the water film present on the surface of the strip. The nozzles 42 are supplied with the same liquid as the cooling liquid, by means of a supply conduit not shown.

[0153] The strip leaves the humid chamber 4 through an opening 43 of reduced dimensions.

[0154] When the strip leaves the humid chamber at a temperature below 100-110 °C, a film of liquid is mechanically entrained by the strip. The wringing section and the drying chamber are there to remove the presence of liquid on the surface of the strip before it enters the downstream aging chamber, under a reducing atmosphere, the wringing facilitating the drying of the strip.

[0155] Conversely, when the strip leaves the humid chamber at a higher temperature, there is no film of liquid on it. The spinning and the drying device can be stopped in order to limit the energy consumption of the line.

[0156] After leaving the humid chamber, the strip thus passes through a wringing section 6 equipped with gas knives 60 intended to remove any liquid that may be present on the strip.

[0157] The gas knives are formed by flat jet nozzles 61 supplied by means of a supply conduit not shown.

[0158] The liquid and gas knives cover the entire width of the strip. On one side of the strip, they can be obtained with a single nozzle whose length is at least equal to the maximum width of the strip or with a plurality of nozzles arranged across the width of the strip.

[0159] The gas used for spinning can be at room temperature or at a higher temperature. These gas knives have substantially the same inclination as the liquid knives 41.

[0160] The wringing section 6 extends through a lower return part 62 in which are arranged two deflector rollers 63, 44 and nozzles 65 forming complementary gas knives.

[0161] The strip 1 then passes through a drying chamber 7 equipped with heating tubes 70 intended to dry the strip by radiation. Drying can also be carried out by convection or by a combination of radiation and convection.

[0162] At the outlet 71 of the drying chamber 7, the strip passes through a downstream airlock 32 for separating atmospheres between the drying chamber and the aging chamber 105 located downstream in the direction of travel of the strip. The airlock shown comprises two pairs of rollers 30 with an injection 31 of atmosphere between the two pairs of rollers, but other airlock configurations are possible.

[0163] According to the invention, if the temperature Th of the strip at the outlet opening 43 of the humid chamber 4 is > 250°C, the time taken for the strip to pass through the humid chamber 4 must be less than or equal to 15 s, and the cumulative time taken to pass through the drying chamber 7, the aging chamber 105 and the oven outlet chamber 106 must be at least equal to the residence time in the humid chamber 4.

[0164] If the temperature Th of the strip at the outlet opening 43 of the humid chamber 4 is <110°C, the temperature of the strip at the outlet 71 of the drying chamber 7 must be <250°C.

[0165] An atmosphere injection system 9 is capable of injecting an atmosphere into the humid chamber 4, via the injection point 91, and the drying chamber 7, via the injection point 92, and an atmosphere extraction system 10 is capable of permanently renewing the atmosphere in the humid chamber 4 and of creating a flow of atmosphere from the drying chamber 7 to the humid chamber 4.

[0166] When the sum of the steam flow rate resulting from the drying of the strip and the flow rate injected into the drying chamber is too high to have only one atmosphere extraction in the humid chamber, an additional atmosphere extraction 13 is added in the drying chamber in addition to the extraction 12 carried out in the humid chamber.

[0167] The locations in [Fig. 5] of the injection points 91, 92 of the atmosphere injection system 9 and of the withdrawal point 11 of the atmosphere extraction system 10 are not representative of their actual locations on an industrial installation. On this installation, the location of the injection and withdrawal points are chosen so as to promote the renewal of the atmosphere of the humid chamber, the spinning section and the drying chamber.

[0168] The atmosphere injected by the injection system 9 is a mixture of nitrogen and hydrogen, with for example a volume proportion of hydrogen of 5%. The hydrogen content can be adjusted according to the quality of the steels to be treated. Advantageously, the atmosphere injected by the injection system 9 is the same as that present in the aging chamber 105.

[0169] [Fig.6] schematically illustrates the gas flows at the wet cooling level according to an exemplary embodiment of the invention. To simplify the figure, the different chambers are represented horizontally.

[0170] A flow Q31 (q11) of reducing atmosphere is injected into the upstream airlock 31 ■Sas Am arranged between the gas cooling chamber 3 and the wet liquid cooling chamber 4.

[0171] The entire flow rate Q31 injected into the airlock escapes into the wet chamber 4. The withdrawal carried out in the wet chamber 4 is such that a flow rate Q31b (Q ) coming from the dry cooling chamber 3 passes through the upstream airlock 31 and is added to the flow rate Q31 injected into the airlock so that no atmospheric flow rate circulates from the wet chamber 4 to the dry cooling chamber 3.

[0172] The flow rate Q31a entering the wet chamber 4 from the airlock 31 is thus equal to the sum of the flow rate Q31 injected into the airlock and the flow rate Q31b coming from the dry cooling chamber 3.

[0173] A flow Q32 (f)s ) of reducing atmosphere is injected into the downstream airlock 32 arranged between drying chamber 7 and aging chamber 105.

[0174] The entire flow rate Q32 injected into the airlock escapes into the aging chamber. The withdrawals made in the wet chamber 4 and the spin section 6 are such that a flow rate Q32b (Q ) coming from the aging chamber 105 passes through the downstream airlock 32 and is added to the flow rate Q32 injected into the airlock so that no atmospheric flow circulates from the drying chamber 7 to the aging chamber 105.

[0175] An injection of atmosphere, not shown in the figures, is carried out in the chambers 3 and 105 to maintain these chambers under pressure and compensate for the outgoing flow rates Q31b (Qam) and Q32b (Qav).

[0176] The flow rate Q32a entering the drying chamber 7 from the airlock 32 is thus equal to the sum of the flow rate Q32 injected into the airlock and the flow rate Q32b coming from the aging chamber 105.

[0177] In the wet chamber 4, a part Q40 (q11) of the coolant is ^Vap vaporizes on contact with the hot strip. The remaining Q41 remains liquid and is evacuated from the wet chamber 4 through outlet 44 before being recirculated to once again supply the nozzles 40.

[0178] The quantity Q40 of evaporated liquid depends on:

[0179] . of the cooling process, the evaporation being lower with quenching at water by immersion rather than by a sprinkling process;

[0180] . of the format (thickness and width) of the tape and its speed;

[0181] . the inlet and outlet temperatures of the strip, i.e. the cooling slope.

[0182] This steam must be evacuated by atmospheric extraction.

[0183] A flow rate Q91 (Ah ) of reducing atmosphere is injected into the humid chamber 4 ^HNx at injection point 91 by the atmospheric injection system 9.

[0184] A flow Q11 (f)h ) is extracted from the wet chamber 4 at the draw-off point 11 by ^Ext the atmosphere extraction system 10.

[0185] The extracted flow rate Q11 is greater than the sum of the flow rate Q31 coming from the upstream airlock 31, the flow rate Q91 injected at the injection point 91 by the atmosphere injection system 9 and the flow rate Q40 resulting from the vaporization of the coolant.

[0186] This results in the entry into the wet chamber 4 of a gas flow Q60 coming from the wringing section 6 and a flow Q31b coming from the dry cooling chamber 3.

[0187] We thus have Qll = Q31a + Q91+ Q40 + Q60.

[0188] In the wringing section 6, a flow Q6 (Ac ) of reducing atmosphere is injected vHNx through nozzles 61 and 65 to remove the film of liquid that may be present on the strip, depending on its temperature.

[0189] A portion of the liquid vaporizes to form a flow rate Q62 (Qe ) of vapor, ^Vap the remaining liquid balance being evacuated through the outlets 61 before being recirculated to once again supply the nozzles 40.

[0190] The flow rate Q11 of withdrawal in the wet chamber 4 is such that the flow rate Q60 coming from the wringing section 6 in the chamber 4 is greater than the sum of the flow rate Q6 injected by the nozzles 61 and 65 and the flow rate Q62 of vaporization in the wringing section 6.

[0191] This results in the entry into the spin section 6 of a gas flow Q71 coming from the drying chamber 7.

[0192] We thus have Q60 = Q6 + Q62 + Q71.

[0193] In the drying chamber 7, a flow rate Q92 (qs ) of reducing atmosphere is vHNx injected at injection point 92 by the atmosphere injection system 9. The gas flow rate Q71 circulating from the drying chamber 7 to the spinning section 6 is thus the sum of the flow rate Q92 injected at point 92, of the flow rate Q70 (qs ) resulting from ^Vap the evaporation of the liquid in chamber 7 and the flow Q32a coming from the downstream airlock 32 located between chamber 7 and the aging chamber 105.

[0194] We thus have Q71 = Q92 + Q70 + Q32a.

[0195] The control of the injected gas flow rates and the withdrawn flow rates leads to the pressure P3 in the dry cooling chamber is higher than pressure P31 in the upstream airlock 31, which is higher than pressure P4 in the humid chamber 4 and in that pressure P105 in the aging chamber is higher than pressure P32 in the downstream airlock 32, which is higher than pressure P7 in the drying chamber, which is higher than pressure P6 in the dewatering section, which is higher than pressure P4 in the humid chamber.

[0196] This ensures that there is no pollution of the dry cooling chamber 3 and the aging chamber 105 by humid vapors, while having sufficient renewal of the atmosphere in the drying chamber 7 and the wringing section 6 so that the presence of humid vapor is zero or sufficiently low to avoid excessive oxidation of the strip.

[0197] For steel grades requiring wet cooling to a temperature above 110°C, the invention makes it possible to avoid the presence of a wet atmosphere in the wringing zone 6 and the drying chamber 7, the flow rates Q62 and Q70 of evaporation of the liquid film being impaired, the latter not being present on the strip at the outlet of the wet chamber 4.

[0198] In an alternative embodiment, the drying chamber 7 may comprise an atmosphere extraction 13 if the current to the humid chamber is not sufficient to allow renewal of the atmosphere in the drying chamber.

[0199] Of course, the invention is not limited to the examples which have just been described and numerous adjustments can be made to these examples without departing from the scope of the invention. In addition, the various characteristics, forms, variants and embodiments of the invention can be associated with each other in various combinations to the extent that they are not incompatible or exclusive of each other.

Claims

Claims

1. Method for cooling a steel strip (1) circulating in a continuous galvanizing line (100) in which: • the steel strip (1) passes through an upstream chamber (2, 3, 103) under a dry and reducing atmosphere and an upstream airlock (31) for separating the atmospheres before being cooled in a humid chamber (4) containing a liquid cooling device (5), • the steel strip (1) cooled in the humid chamber (4) passes through a drying chamber (7), a downstream airlock (32) for separating atmospheres and at least one downstream chamber (8, 105, 106) under a dry and reducing atmosphere, the method further comprising, • an injection of dry atmosphere, by means of an injection system (9), into the humid chamber (4) and into the drying chamber (7), • an atmosphere extraction, by means of an extraction system (10), to permanently renew the atmosphere in the humid chamber (4) and to create a flow of atmosphere from the drying chamber (7) to the humid chamber (4), • control by means of a control and command system (11): • the time taken for the steel strip (1) to pass through the chamber (4), • the cumulative time spent by the steel strip (1) in the drying chamber (7) and in the at least one downstream chamber (8, 105, 106) under a dry and reducing atmosphere, • the temperature (Th) of the steel strip (1) at the outlet of the humid chamber (4), and • the temperature (Ts) of the steel strip (1) at the outlet of the drying chamber (7), the method being characterized in that it comprises a step of verifying required conditions defined by: • when the outlet temperature (Th) of the wet chamber (4) of the steel strip (1) is greater than or equal to the Leidenfrost temperature, the time taken for the steel strip (1) to pass through the wet chamber (4) must be less than or equal to fifteen seconds, and the cumulative time taken for passage through the drying chamber (7) and through at least one downstream chamber (8, 105, 106) under a dry and reducing atmosphere must be greater than or equal to the residence time of the steel strip (1) in the wet chamber (4);• when the outlet temperature (Th) of the wet chamber (4) of the steel strip (1) is lower than the Leidenfrost temperature, the liquid of the cooling device (5) which must be an aqueous cooling liquid having a mass concentration of formic acid greater than or equal to 0.1% and the temperature (Ts) of the steel strip (1) at the outlet of the drying chamber (7) which must be less than or equal to 250°C if one of the required conditions is not met during the verification step, the method further comprises a step of adjusting at least one operating parameter of the line making it possible to meet the conditions required in the verification step.;

2. A method according to claim 1, wherein when the outlet temperature (Th) of the cooled steel strip (1) from the wet chamber (4) is lower than 110°C, at least a portion of the excess liquid quantity carried away by the cooled steel strip (1) in the wet chamber (4) is removed in a dewatering section (6), and the dewatered steel strip (1) is dried in the drying chamber (7).

3. Method according to one of the preceding claims, for which the pressure of the atmosphere of the upstream chamber (2, 3, 103) and the pressure of the atmosphere of the downstream chamber (8, 105, 106) are each higher than that of the drying chamber (7), and in that the pressure of the atmosphere of the drying chamber (7) is higher than that of the humid chamber (4).

4. Method according to one of the preceding claims, further comprising a step of injecting an atmosphere composed of a mixture of nitrogen and hydrogen into the humid chamber (4) and into the drying chamber (7), the hydrogen content of said atmosphere being adjusted according to the chemical composition of the steel of the strip (1) of steel to be galvanized.

5. Method according to one of the preceding claims, for which the extraction of atmosphere is carried out with an extraction flow rate q!1 adjustable and such that the flow rate is greater than the sum of the ^Ext VExt flow rates / .e -h ' „e , where the terms Qu., +QU^ + +Qv + Q +Qv ^HNx ^HNx ^HNx ^Vap aP correspond respectively to the flow rate of HNx injected into the humid chamber, to the flow rate of HNx injected into the dewatering section, to the flow rate injected into the drying chamber, to the flow rate of steam resulting from the vaporization of the cooling liquid on the surface of the strip in the humid chamber, to the flow rate of steam resulting from the dewatering of the strip and to the flow rate of steam resulting from the drying of the strip.

6. A method according to claim 1, wherein the mass concentration of formic acid in the coolant is adjusted according to the chemical composition of the steel in the strip (1) of steel to be galvanized.

7. Continuous galvanizing line (100) capable of implementing a method according to one of claims 1 to 6 for galvanizing a steel strip (1), the line (100) successively comprising in the direction of movement of the steel strip (1) to be galvanized: • an upstream chamber (2, 3, 103) under a dry and reducing atmosphere, in which the steel strip (1) is heated or cooled, • an upstream airlock (31) for separating atmospheres, • a humid chamber (4) containing a liquid cooling device (5), in which the steel strip (1) is cooled, • a section (6) for wringing the cooled steel strip (1), in which at least part of the quantity of excess liquid that can be carried away by the cooled steel strip (1) is removed, • a chamber (7) for drying the wrung steel strip (1),in which the wrung-out steel strip (1) is dried to evaporate the liquid still present on the wrung-out steel strip (1),

8. • a downstream airlock (32) for separating atmospheres, • at least one downstream chamber (8, 105, 106) under a dry and reducing atmosphere, the line further comprising: • an atmosphere injection system (9) capable of injecting a dry and reducing atmosphere into the humid chamber (4) and into the drying chamber (7), • an atmosphere extraction system (10) capable of continuously renewing the atmosphere in the humid chamber (4) and creating a flow of atmosphere from the drying chamber (7) to the humid chamber (4), • a control and command system (11) capable of controlling the time taken for the steel strip (1) to pass through the chamber (4), the cumulative time taken for the steel strip (1) to pass through the drying chamber (7) and through the at least one downstream chamber (8, 105, 106) under a dry and reducing atmosphere, the temperature (Th) of the cooled steel strip (1) at the outlet of the humid chamber (4) and the temperature (Ts) of the steel strip (1) at the outlet of the drying chamber (7) and of implementing the steps of checking and adjusting the process. Line (1) according to the preceding claim, characterized in that the atmosphere extraction system (10) comprises an atmosphere extraction (12) from the humid chamber (4) and an atmosphere extraction (13) from the drying chamber (7).

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