WET COOLED GALVANIZING LINE

Abstract

WET COOLING GALVANIZING LINE A 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 exit of a wet chamber (4) with regard to the Leidenfrost temperature, the time spent by the strip in the wet 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 an aqueous coolant having a mass concentration of formic acid greater than or equal to 0.1% and the temperature of the strip at the exit of the drying chamber being 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 meet said required conditions.Figure for the abridged version: 6.

Description

Title of the invention: WET COOLED 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 that the invention addresses

[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 metallurgical structure and mechanical properties sought.

[0003] For example, the applicant's patent FR3064279 describes a cooling section using an aqueous solution comprising formic acid, enabling 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: • their extreme sensitivity to oxidation; • the need to cool them down very quickly; • to access, in certain cases, final cooling temperatures below 200 °C.

[0005] It may not be suitable for conventional steels (generation 1 & 2) because these steels have different specific characteristics: • they require lower cooling rates; • They are produced with higher final cooling temperatures higher than generation 3 steels (generally, it is the temperature of the zinc bath that is targeted), therefore the oxidation conditions are different; • Although less sensitive to surface oxidation (because they contain fewer alloying elements), they still require control of the atmosphere and prevention of any oxidation phenomena.

[0006] These differences require adapting the operating conditions to be able to produce conventional steels on the same line as third generation steels, particularly with regard to cooling conditions, atmosphere management and acid dosing.

[0007] Finally, the presence of acid in the cooling solution can generate appearance defects regardless of the steel generation 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, preventing the full utilization of a line configured for galvanizing these steels. It is therefore essential to be able to produce all types of steel on a line with rapid wet cooling to limit operating costs and avoid negatively impacting the plant's profitability.

[0010] The object of the invention is to enable the flawless galvanizing (surface appearance, uniform and seamless zinc coating) of third-generation steels and conventional steels on the same line, allowing for full-time line operation. This results in a faster return on investment for the steelmaker and significantly reduces the production cost of third-generation steels. Technical background

[0011] Wet cooling technologies make it possible to achieve high cooling rates of up to 2000 °C / s / mm. 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. The principle consists of passing the steel strip through a tank containing water. This tank may include auxiliary equipment such as additional water jets, either 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] Spray quenching can also be used, but it offers much lower 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 belt 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 exceeding, those of immersion water quenching. Single-fluid or dual-fluid spraying offers greater control flexibility thanks to a wide operating pressure range, typically between 1 and 12 bar.

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

[0017] Hot-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 wett.

[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 produces a surface finish suitable for hot-dip galvanizing of these steel grades through a combination of mechanical effects from the spraying and a chemical pickling 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, on the so-called Leidenfrost temperature, which is the temperature that separates the stable vapor film cooling region from the unstable transition region. The Leidenfrost temperature is typically between 250 °C and 630 °C depending on the cooling system used.

[0021] The table below gives examples of Leidenfrost temperature for different cooling conditions: System Nozzle Type X [mm] Y [mm] Z [mm] Water temperature [°C] G [l / min / m²] Leidenfrost temperature 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 according to the width of the strip, the "Y" column indicates the distance between two nozzles according to the length of the strip, the "Z" column indicates the distance of the nozzles to 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, the film boiling regime occurs. A vapor cushion of a few tens of millimeters separates the strip from the liquid water. The surface of the strip is not wetted. The wall is relatively thermally insulated from the water, since the thermal conductivity of water vapor is approximately 27 times lower than that of liquid water. The heat transfer coefficient is stable and low. Rapid cooling is difficult during this phase.

[0024] From the Leidenfrost temperature up to a temperature around 200 °C, this is the transition region in which the heat flux varies rapidly from a minimum to a maximum value. The vapor layer tends to disappear, and 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 large variation in the heat transfer coefficient is observed. Cooling increases rapidly. Heat exchange is much more intense, and the water vaporizes directly upon contact with the wall.

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

[0026] Applicant's patent WO2021116594 is an improvement on 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 (or water + gas) cooling, followed by a second part with water + formic acid cooling.

[0027] Before reaching Leidenfrost temperature, a vapor cushion separates the strip from the liquid water, and the surface of the strip is not wetted. Oxidation of the strip is thus limited in the initial part of the cooling process.

[0028] This solution makes it possible to reduce acid consumption and limit the amount of residues resulting from the decomposition of acid present on the surface of the belt 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 includes, in particular, a three-roller airlock between these two sections with a withdrawal 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 galvanizing of third-generation steels, but they are not suitable for the galvanizing of conventional steels, particularly because the surface of the strip oxidizes for these steels in the spinning section and the drying chamber that follow liquid cooling due to their humid atmospheres. Summary of the invention

[0031] According to a first aspect of the invention, a method is proposed for cooling a steel strip circulating in a continuous galvanizing line in which: • The steel strip passes through an upstream chamber under a dry and reducing atmosphere and an upstream airlock for atmosphere separation before being cooled in a humid chamber containing a liquid cooling device, • The steel strip, cooled in the wet chamber, passes through a drying chamber, a downstream airlock for atmosphere separation, and at least one downstream chamber under a dry and reducing atmosphere; the process further comprises, • an injection of dry atmosphere, using an injection system, into the humid chamber and the drying chamber, • an air extraction system, using an extraction method, to continuously renew the atmosphere in the humid chamber and to create an airflow from the drying chamber to the humid chamber, • control by means of a control and command system: • the time it takes for the steel strip to pass through 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 outlet of the wet chamber, and • of the temperature (Ts) of the steel strip at the exit of the drying chamber.

[0032] The process includes a step of verifying required conditions defined by: • when the outlet temperature (Th) of the steel strip from the wet chamber is greater than or equal to the Leidenfrost temperature, the time spent by the steel strip in the wet 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 wet chamber; • when the temperature (Th) of the steel strip exiting the wet chamber is below the Leidenfrost temperature, the liquid in the cooling device must be an aqueous coolant with a mass concentration of formic acid greater than or equal to 0.1% and the temperature (Ts) of the steel strip at the exit 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 includes a step of adjusting at least one operating parameter of the line to meet the required conditions of the verification step.

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

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

[0036] The inertia of the line is taken into account so that a new verification step is only carried out after a sufficient delay for an adjustment of a line operating parameter 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 the selection of one or more line operating parameters, and the setpoint to be applied, according in particular to the line operating conditions and the composition of the band.

[0039] When the belt exits the wet chamber at a temperature equal to or greater than the Leidenfrost temperature, the coolant can be water, without the addition of acid. This avoids the presence of residues on the belt resulting from the decomposition of acid, which can cause surface defects such as coffee stains or line markings. Not using acid also reduces the operating costs of the line.

[0040] By limiting the strip's residence time in the humid chamber to a maximum of 15 seconds, it is ensured that oxides are not present on the surface of the strip for conventional steel, or, if present, that they are not in excessive quantities for third-generation steel. For the latter, oxidation (selective or total) must be carried out upstream, during the heating phase preceding rapid cooling.

[0041] By ensuring that the cumulative residence time of the strip is at least equivalent in the drying chamber and in the downstream chambers under a dry and reducing atmosphere preceding the coating, a reduction of oxides (primarily iron oxides) that would be present on the surface of the strip upon exiting the wet chamber is promoted. Thus, upon immersion in the coating bath, the surface of the strip no longer contains any oxides, or those that remain are present in small quantities. They do not impair 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 the oxidation of iron.

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

[0044] When the strip exits the wet 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 important as the final cooling temperature is low.

[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 wet 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], based on the chemical composition of the steel and its manganese and silicon content. The value used is the higher of the two values ​​obtained based on the manganese concentration and the value obtained based on the silicon concentration. For example, for steel with 3% manganese and 2% silicon, the acid concentration will be 0.5%.

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

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

[0050] When the belt exits the humid chamber at a temperature below 110 °C, there has been complete contact between the liquid and the belt during wet cooling. It is therefore necessary to use an aqueous solution containing acid to prevent the amount of oxides present on the surface of the belt from being excessive upon exiting the humid chamber. The solution is more concentrated than for a belt exiting at a temperature above 110 °C, but it can nevertheless remain at a low concentration.

[0051] The formic acid concentration can be determined from the diagram in [Fig. 8], based on the chemical composition of the steel. Again, the value used is the higher of the two values ​​obtained based on the manganese concentration and the value obtained based on 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 required concentration of formic acid remains low.

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

[0054] Spinning the belt limits the amount of liquid carried by the belt into the drying chamber. Drying the belt prevents any remaining moisture from being 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 spinning 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 chamber of drying, and the atmospheric pressure of the drying chamber is greater than that of the humid chamber.

[0057] By having a pressure in the drying chamber greater than that of the cooling chamber, it is ensured that the flow of the 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 from the cooling chamber.

[0059] By having a pressure in the downstream chamber greater 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, is from the downstream chamber to 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 merely higher than the pressure in the wet chamber, it is ensured that no atmospheric flow can occur from the wet chamber to the upstream chamber. Any potential flow, despite the presence of an atmospheric separation chamber, is 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 wet chamber.

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

[0064] Advantageously according to the invention, the process further comprises a step of injecting an atmosphere composed of a mixture of nitrogen and hydrogen into the wet 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 promoted 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 can be limited to nitrogen, without hydrogen content. In the drying chamber, the atmosphere must first contain hydrogen to reduce any oxides that may be 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 with a Mn content that 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 atmosphere extraction is carried out with an adjustable extraction flow rate such that , ■ e œ >qL +qL +qL +œ +q +qL ' ^Ext ^HNx ^HNx ^HNx ^Vap

[0069] By having a higher extraction flow rate A1' in the wet chamber than ^Ext the sum of the atmospheric flow rates injected into the wet chamber Ah, in the VHNx Spin system Qe, in the drying chamber qs and those resulting from the evaporation of the cooling solution in the humid chamber Ah, the ^-Vap spin system and the drying chamber, we ensure an inlet atmosphere from the airlock placed upstream of the humid chamber towards it and from an atmosphere inlet 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 coolant 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, a continuous galvanizing line is proposed, capable of implementing a process according to the first aspect of the invention for galvanizing a steel strip, the line comprising successively, 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 humid chamber containing a liquid cooling device, in which the steel strip is cooled, • a dewatering section of the cooled steel strip, in which at least part of the excess liquid that can be carried away by the cooled steel strip is removed, • a drying chamber for the wrung-out steel strip, in which the wrung-out steel strip is dried to evaporate any remaining liquid on the wrung-out steel strip, • a downstream airlock for atmosphere separation, • at least one downstream chamber under a dry and reducing atmosphere.

[0072] The line further comprising: • an atmosphere injection system capable of injecting a dry and reducing atmosphere into the humid chamber and the drying chamber, • an air extraction system capable of continuously renewing the atmosphere in the humid chamber and creating an airflow from the drying chamber to the humid chamber, • a control and command system capable of controlling the time spent by the steel strip 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 cooled steel strip at the exit of the wet chamber and the temperature (Ts) of the steel strip at the exit of the drying chamber and of implementing the steps of verification and adjustment of the process.

[0073] Advantageously according to the invention, the wet chamber can be supplied alternately by at least two separate coolant supply and recirculation circuits.

[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 wet chamber will be supplied by one or the other of the circuits.

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

[0078] According to one embodiment of the invention, the wet chamber includes a water quenching type cooling device by immersion.

[0079] According to another embodiment of the invention, the wet chamber comprises a single-fluid and / or dual-fluid spray cooling device. Spray cooling allows for greater flexibility in adjusting the cooling slope of the belt across its width and length than with immersion water quenching. For example, it is possible to vary the flow rate of the coolant along the width / length of the belt.

[0080] Advantageously, according to the invention, a wringing system is arranged between the wet chamber and the drying chamber. This system limits the amount of liquid carried by the belt into the drying chamber, thereby facilitating the drying of the belt and reducing the amount of water vapor in the drying chamber. It may, for example, include a water knife or wringer 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 creates an atmospheric flow from the center of the airlock towards the wet 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, preventing it from escaping to an upstream or downstream chamber and contaminating it by passing through the airlock. This prevents the reducing atmosphere of the upstream and downstream chambers from being polluted by water vapor from the wet or drying chamber.

[0083] Advantageously, the air extraction system includes air extraction from the wet chamber and air 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 wet cooling chamber.

[0085] According to one embodiment of the invention, the line further comprises a strip oxidation system disposed 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 that are more easily formed during the holding phase at temperature.

[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 one 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 features and advantages of the invention will become apparent upon reading the detailed description that follows, for an understanding of which reference should be made to the accompanying drawings in which: • [Fig-1] is a schematic and partial representation of a portion of a horizontal line with spray cooling, according to one 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 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 embodiment of the invention, • [Fig. 4] is a schematic and partial view of a continuous line according to an example of an embodiment of the invention, • [Fig.5] is an enlargement of the cooling area of ​​the line in [Fig.4], • [Fig.6] is a schematic representation similar to that of [Fig.1] to illustrate 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 wet chamber strip is below the Leidenfrost temperature, but above 110 °C, and • [Fig.8] is a diagram showing the mass concentration of formic acid in the cooling solution according to the composition of the steel, when the outlet temperature of the wet chamber strip is less than 110 °C. Detailed description of the invention

[0090] The embodiments described below are not in any way limiting; variants of the invention may, in particular, be considered comprising only a selection of the features described, hereinafter isolated from the other features described, if this selection of features is sufficient to confer a technical advantage or to differentiate the invention from the prior art. This selection includes at least one feature, preferably a functional one without structural details, or with only a portion of the structural details if this portion alone is sufficient to confer a technical advantage or to differentiate the invention from the prior art.

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

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

[0093] The two chambers are managed simultaneously in terms of atmospheric flow and pressure. The management of one of the chambers is done according to the management of the other.

[0094] The wet chamber has the following features: • Hardening of steel using water-based technologies such as immersion water quenching, single-fluid spraying, dual-fluid spraying, mist quenching, etc. • injection of HNx with a hydrogen concentration between 0% and 20%, • extraction of the atmosphere in such a way as to constantly renew the atmosphere of the humid chamber, • evacuation of liquid water from the chamber.

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

[0096] The drying chamber, which starts directly after the spin cycle, includes the following features: • Drying the strip. Various 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, airlocks are positioned to separate atmospheres to prevent the upstream and downstream chambers from being polluted 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 atmospheric flow 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] Figure 1 shows an example of a portion of a horizontal line (the band 1 circulating horizontally in the direction indicated by the arrow in the figure) with spray cooling according to an embodiment of the invention. Shown are an upstream dry chamber 2 under a reducing atmosphere, an upstream airlock 31 for atmosphere separation, a wet chamber 4 where quenching is carried out by spraying a liquid, a spinning section 6, a drying chamber 7, a downstream airlock 32 for atmosphere separation, and a downstream dry chamber 8 under a reducing atmosphere.

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

[0103] Figure 3 shows another example of a portion of a vertical line, 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 Figures 1 and 2.

[0104] Particularly in the case of a vertical configuration such as shown in Figures 2 and 3, the spin-drying section can advantageously be supplemented upstream by liquid knives and / or additional 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 steel, • the temperature of the strip, particularly at the end of wet cooling and upon exiting 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 implies 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 regime (in English, transition boiling) is reached when the band 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 constantly in contact with the steel strip.

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

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

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

[0113] Since the addition of acid to the cooling water only affects 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] Conversely, the use of acid in regime 3 is relevant.

[0115] If the final 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 a final cooling temperature below 110 °C will the atmosphere of the drying chamber be humid.

[0117] The oxidation kinetics of iron are temperature-dependent. In an oxidizing atmosphere, iron can be considered to begin oxidizing only above 250 °C. Similarly, the reduction of iron oxides in a reducing atmosphere can be considered to begin in earnest only 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 start 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 grades of steel with significant amounts of Manganese, Silicon and Chromium, it is necessary to manage the oxidation of these alloying elements by carrying it out during the heating phase with selective or total oxidation.

[0121] In the case of final cooling temperatures below 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 final cooling temperature.

[0123] The table below summarizes these three areas, according to the cooling regime, by illustrating: • if the water-strip 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 Acid presence Regime 1 -T Leidenfrost Very low Dry no Regime 2 < T Leidenfrost Partial Dry yes Regime 3 < 110 °C Total Humid yes

[0124] In the cooling range 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 wet chamber as quickly as possible, less than 15 s for conventional steel grades; ii. Injection of HNx in the drying chamber and downstream chambers. The hydrogen content (up to 20%) is adjusted in these chambers according to the grade of steel concerned and the time spent in the wet chamber; iii. The cumulative transit time in the drying chambers and downstream must at least be equal to the transit time in the wet chamber to guarantee sufficient time for the reduction of iron oxides.

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

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

[0130] The operating conditions are therefore in this case: i. Use of a weakly dosed acid; ii. Injection of HNx into the drying chamber and downstream chambers. Adjustment of the hydrogen content (up to 20%) in the chambers or the strip temperature exceeds 250 °C; iii. Promote the combination of the drying chamber with a downstream chamber equipped with heating equipment (e.g. induction, 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 range 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 has evaporated before exiting the chamber and that the atmosphere is properly extracted.

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

[0134] The operating conditions are therefore in this case: i. Use of a weakly dosed 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 an atmosphere flow towards the humid chamber where atmosphere extraction is carried out, it is necessary to have simultaneously: • pressures such that the Pam pressure in the upstream chamber and the Pav pressure in the downstream chamber are greater than the Ps pressure of the drying chamber, and that the pressure in the latter is greater than the Ph pressure of the wet chamber:

[0136] (Pam;Pav)>Ps>Ph • flow rates such as the extraction flow rate of the wet chamber, i.e., VExt greater than the sum of the flow rate fU of HNx injected into the wet 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, the resulting steam flow vaporization of the coolant on the surface of the belt in the wet chamber, of the steam flow rate Qe resulting from the wringing of the belt, and of the steam flow rate qs resulting from the drying of the belt: ^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 vaporization rate of the liquid and to the HNx injected into the wet chamber, the section Q.. +Q +Qv ^HNx ^Vap ^Vap ^Vap spin-drying chamber, drying chamber, upstream airlock and 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 downstream chamber, 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 If the pressures injected into the drying chamber are too high, a supplementary extraction qs can be added to 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] Since the amount of coolant that evaporates varies depending on the format of the strip and its temperature, the extraction flow rate is adjustable, as is ^Ext qs if an additional extraction is added in the drying chamber.

[0144] By 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 example of an embodiment of the invention.

[0145] It comprises, successively and in the direction of the web's travel 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 spinning and return section 6 and a drying chamber 7, then an aging chamber 105, a furnace exit section 106, and a hot-dip galvanizing section 107. A control and monitoring system 11 ensures the proper operation of the line and adjusts its operating parameters according to the characteristics of the web 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 slower than that obtained in the wet liquid cooling chamber 4. Indeed, gas cooling, typically by spraying a mixture of nitrogen and hydrogen, makes it possible to achieve cooling rates on the order of 100 to 200 °C / s for steel strips 1 mm thick.

[0147] By referring to the diagram in the attached [Fig.5], the lower part of the cooling section 104 of [Fig.1] can be seen partially represented.

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

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

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

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

[0152] Liquid knives 41 formed by flat jet nozzles 42 remove most of the runoff liquid from the belt. The jets are inclined at an acute angle to the belt to promote the removal of the water film on the belt surface. The nozzles 42 are supplied with the same liquid as the coolant, via a supply line not shown.

[0153] The belt exits the humid chamber 4 through a small opening 43.

[0154] When the belt exits the humid chamber at a temperature below 100-110 °C, a film of liquid is mechanically carried along by the belt. The spinning section and the drying chamber are there to remove the liquid from the surface of the belt before it enters the downstream aging chamber, under a reducing atmosphere, the spinning facilitating the drying of the belt.

[0155] Conversely, when the belt exits the humid chamber at a higher temperature, there is no liquid film on it. The spinning and drying units can be switched off to limit the line's energy consumption.

[0156] After exiting the wet chamber, the belt thus passes through a dewatering section 6 equipped with gas knives 60 intended to remove any liquid that may be present on the belt.

[0157] The gas knives are formed by flat jet nozzles 61 supplied by means of a supply duct 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 over the width of the strip.

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

[0160] The spin section 6 extends by a lower part 62 of return 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 for drying the strip by radiation. Drying can also be carried out by convection or by a combination of radiation and convection.

[0162] At the exit 71 of the drying chamber 7, the belt passes through a downstream airlock 32 for atmosphere separation between the drying chamber and the aging chamber 105 located downstream in the direction of belt travel. The airlock shown comprises two pairs of rollers 30 with an atmosphere injection 31 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 level of the opening 43 exiting the wet chamber 4 is > 250 °C, the time spent by the strip in the wet chamber 4 must be less than or equal to 15 s, and the cumulative time spent in the drying chamber 7, the aging chamber 105 and the oven exit chamber 106 must be at least equal to the time spent in the wet chamber 4.

[0164] If the temperature Th of the strip at the outlet opening 43 of the wet 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 suitable for injecting an atmosphere into the wet chamber 4, via the injection point 91, and the drying chamber 7, via the injection point 92, and an atmosphere extraction system 10 is suitable for continuously renewing the atmosphere in the wet chamber 4 and creating an atmosphere flow from the drying chamber 7 to the wet chamber 4.

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

[0167] The locations shown in [Fig. 5] of the injection points 91, 92 of the atmosphere injection system 9 and of the extraction point 11 of the atmosphere extraction system 10 are not representative of their actual locations in an industrial installation. In such an installation, the location of the injection and extraction points is chosen to promote the renewal of the atmosphere in the wet chamber, the spin-drying 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 being treated. Advantageously, the atmosphere injected by the injection system 9 is the same as that present in the aging chamber 105.

[0169] Figure 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 shown horizontally.

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

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

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

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

[0174] The entire flow Q32 injected into the airlock escapes into the aging chamber. The withdrawals carried out in the wet chamber 4 and the spin section 6 are such that a flow Q32b (Q) from the aging chamber 105 passes through the downstream airlock 32 and is added to the flow 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 chambers 3 and 105 to maintain these chambers under pressure and to compensate for the outgoing flows 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 humid chamber 4, a portion Q40 (q11) of the coolant is ^Vap vaporizes upon 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 supply the nozzles 40 again.

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

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

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

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

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

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

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

[0185] The extracted flow rate Q11 is greater than the sum of the flow rate Q31 from the upstream airlock 31, the flow rate Q91 injected at the injection point 91 by the atmospheric 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 gaseous flow Q60 from the spin section 6 and a flow Q31b from the dry cooling chamber 3.

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

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

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

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

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

[0192] Thus we have Q60 = Q6 + Q62 + Q71.

[0193] In drying chamber 7, a flow rate Q92 (qs) of reducing atmosphere is vHNx injected at injection point 92 by the atmospheric 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, and the flow rate Q70 (qs) resulting from ^Vap the evaporation of the liquid in chamber 7 and of the flow Q32a from the downstream airlock 32 located between chamber 7 and the aging chamber 105.

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

[0195] Controlling the injected and withdrawn gas flow rates leads to the pressure P3 in the dry cooling chamber is greater than pressure P31 in the upstream airlock 31, which is greater than pressure P4 in the wet chamber 4 and in that pressure P105 in the aging chamber is greater than pressure P32 in the downstream airlock 32, which is greater than pressure P7 in the drying chamber, which is greater than pressure P6 in the spinning section, which is greater than pressure P4 in the wet chamber.

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

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

[0198] In an alternative embodiment, the drying chamber 7 may include an atmosphere extraction 13 if the flow to the wet chamber is not sufficient to allow renewal of the atmosphere of the drying chamber.

[0199] Of course, the invention is not limited to the examples just described, and many modifications can be made to these examples without departing from the scope of the invention. Furthermore, the various features, forms, variants, and embodiments of the invention can be combined in various ways, provided they are not incompatible or mutually exclusive.

Claims

Demands

1. A 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 atmosphere separation 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 atmosphere separation airlock (32) and at least one downstream chamber (8, 105, 106) under a dry and reducing atmosphere, the process including further, • 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 system, using an extraction system (10), to continuously renew the atmosphere in the humid chamber (4) and to create an atmosphere flow from the drying chamber (7) to the humid chamber (4), • control by means of a control and command system (11): • the time the steel strip (1) spends in the wet chamber (4), • the cumulative time spent by the steel strip (1) in the drying chamber (7) and in at least one downstream chamber (8, 105, 106) under a dry and reducing atmosphere, • of the temperature (Th) of the steel strip (1) at the outlet of the wet chamber (4), and • of the temperature (Ts) of the steel strip (1) at the outlet of the drying chamber (7), the process being characterized in that it includes 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 spent by the steel strip (1) in the wet chamber (4) must be less than or equal to fifteen seconds, and the cumulative time spent in the drying chamber (7) and in 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 in the cooling device (5) which must be an aqueous coolant 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, • when the outlet temperature (Th) of the cooled steel strip (1) from the wet chamber (4) is less than 110 °C, at least part of the excess liquid carried away by the cooled steel strip (1) in the wet chamber (4) is removed in a spinning section (6), and the spun steel strip (1) is dried in the drying chamber (7), If one of the required conditions is not met during the verification step, the process further includes a step of adjusting at least one operating parameter of the line to meet the requirements of the verification step.

2. A method according to the preceding claim, wherein the atmospheric pressure of the upstream chamber (2, 3, 103) and the atmospheric pressure of the downstream chamber (8, 105, 106) are each greater than that of the drying chamber (7), and in that the atmospheric pressure of the drying chamber (7) is greater than that of the wet chamber (4).

3. A method according to any 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 in the drying chamber (7), the hydrogen content of said atmosphere being adjusted according to the chemical composition of the steel in the steel strip (1) to be galvanized.

4. A method according to any one of the preceding claims, wherein the atmosphere extraction is carried out with an adjustable extraction flow rate and such that the flow rate r\h is greater than the sum of the ^Ext ^Ext flow rates h . S . h " , where the terms Qttn +Qttn +Qtt„ + Qv + Q +Qv ^HNx ^HNx ^Vap ^yap ^Vap correspond respectively to the flow rate of HNx injected into the wet chamber, the flow rate of HNx injected into the spin section, the flow rate injected into the drying chamber, the steam flow rate resulting from the vaporization of the coolant on the surface of the belt in the wet chamber, the steam flow rate resulting from the spin of the belt and the steam flow rate resulting from the drying of the belt.

5. 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 steel strip (1) to be galvanized.

6. A continuous galvanizing line (100) capable of implementing a process according to any one of claims 1 to 6 for galvanizing a steel strip (1), the line (100) comprising successively, in the direction of travel 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 dewatering the cooled steel strip (1), in which at least part of the excess liquid that can be carried away by the cooled steel strip (1) is removed, • a chamber (7) for drying the dewatered steel strip (1), in which the strip (1) wrung-out steel is dried for

7. evaporate the liquid still present on the wrung-out steel strip (1), • a downstream airlock (32) for atmosphere separation, • at least one downstream chamber (8, 105, 106) under a dry and reducing atmosphere, the line also including: • 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 air extraction system (10) capable of continuously renewing the atmosphere in the humid chamber (4) and creating an airflow from the drying chamber (7) to the humid chamber (4), • a control and command system (11) capable of controlling the time spent by the steel strip (1) in the chamber (4), the cumulative time spent by the steel strip (1) in the drying chamber (7) and in 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 exit of the wet chamber (4) and the temperature (Ts) of the steel strip (1) at the exit of the drying chamber (7) and of implementing the steps for verifying and adjusting the process. Line (1) according to the preceding claim, the atmosphere extraction system (10) comprising an atmosphere extraction (12) from the humid chamber (4) and an atmosphere extraction (13) from the drying chamber (7).

Images