Method for cooling a steel strip and wet cooled galvanising line

Abstract

The invention relates to a method for cooling a steel strip (1) travelling through a continuous galvanising line (100), wherein the method comprises a step of checking required conditions defined according to the temperature of the strip at the outlet of a wet chamber (4) with regard to the Leidenfrost temperature, the travel time of the strip through the wet chamber and the cumulative travel time of the strip through a drying chamber (7) and a downstream chamber, under a dry and reducing atmosphere, being selected, wherein the liquid in the cooling device (5) may be an aqueous coolant having a mass concentration of formic acid of 0.1% or higher, and the temperature of the strip at the outlet of the drying chamber may be 250°C or lower; and wherein the method may further comprise a step of adjusting an operating parameter of the line in order to meet the required conditions.

Description

STEEL STRIP COOLING METHOD AND 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 above 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 allowing in-line dip galvanizing of these steels, without requiring an acid pickling step before coating.

[0004] This solution was developed for third-generation steels due to: their extreme sensitivity to oxidation; the need to cool them very quickly; and in some cases to reach 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 than generation 3 steels (in general, it is the temperature of the zinc bath that is targeted), the oxidation conditions are therefore different; although less sensitive to surface oxidation (because they are less loaded with alloying elements), they still require controlling the atmosphere and avoiding any oxidation phenomenon.

[0006] These differences require adapting 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 for 100% utilization 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 purpose of the invention is to enable third-generation steels and conventional steels to be galvanized without defects (surface appearance, uniform zinc coating and no 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 common cooling technology is water quenching. The principle involves passing the steel strip through a tank containing water. This tank may include auxiliary equipment such as additional submerged or non-submerged water jets. 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-flat 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] Coolant, or a gas / liquid mixture, can also be sprayed onto the strip using single-fluid or dual-fluid nozzles. Spraying is carried out under pressure, which allows for cooling rates comparable to or even higher than water immersion 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 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:SystemNozzle typeX [mm]Y [mm]Z [mm]T water [°C]G [l / min / m²]T Leidenfrost1ABi Flat jet fluid35045025023152601BBi Flat jet fluid35045025023293502AMono Flat jet fluid50440400432553602BMono Flat jet fluid1101504004310924002CMono Flat jet fluid80320290266894202DMono Flat jet fluid501503001823715002EMono Jet fluid flat504404004238895402FMono Flat Spray Fluid503303001724106002GMono Flat Spray Fluid5015020041113596803AMono Flat Spray Fluid120150200436343403BMono Full Cone Fluid50450400406814503CMono Full Cone Fluid802252002242305403DMono Full Cone Fluid50150400425991630

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

[0023] At high temperatures, above the Leidenfrost temperature, film boiling occurs. A vapor cushion of a few tens of μm separates the band from the liquid water. The surface of the band 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 transfer 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 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. 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, numerous 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 slows down 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 (or water + gas) cooling, followed by a second part with water + formic acid cooling.

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

[0028] This solution reduces acid consumption and limits the quantity of residue resulting from the decomposition of the acid present on the surface of the strip leaving 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 over them via multiple injection points.

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

[0031] According to a first aspect of the invention, there is provided a method 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 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 atmosphere separation airlock 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 extraction of atmosphere, by means of an extraction system, to permanently renew the atmosphere in the humid chamber and to create a flow of atmosphere from the drying chamber to the humid chamber,a control by means of a control and command system: of the time spent by the steel strip in the humid chamber, of the cumulative time spent by the steel strip in the drying chamber and in the at least one downstream chamber under a dry and reducing atmosphere, of the temperature (Th) of the steel strip at the outlet of the humid chamber, and of 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 of 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;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.;

[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 belt speed, the coolant flow rate or the nature of the coolant.

[0035] After adjusting the line operating parameter, the required condition verification step is repeated to verify that the required conditions are met. If this is still not the case, the adjustment step of 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 to only carry out a new verification step 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 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 can be water, without the addition of acid. This avoids the presence of residues on the strip resulting from the decomposition of the acid, knowing that these residues can generate appearance defects, for example coffee stains or streaks. The non-use of acid also reduces the operating cost of the line.

[0040] By having a maximum residence time of 15 seconds for the strip in the humid chamber, it is avoided that oxides are present on the surface of the strip for a conventional steel, or if there are any, that they are not in excessive 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 the same 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 exit of the humid chamber is encouraged. Thus, during its immersion in the coating bath, the surface of the strip no longer contains any oxide or those which remain are in small quantities. 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 iron.

[0043] Hereinafter, we will refer to HNx as a dry, 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 excluding the Leidenfrost temperature, there has been partial contact between the liquid and the strip and this is all the more important as the end of cooling temperature is low.

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

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

[0047] The formic acid concentration can be determined from the diagram, depending on the chemical composition of the steel, its manganese and silicon content. The value used is the higher of the manganese concentration and 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 required concentration of formic acid remains low.

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

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

[0051] The formic acid concentration can be determined from the diagram, based on the chemical composition of the steel. Again, the value used is the higher of the manganese concentration and 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 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 belt limits the amount of liquid carried by the belt into the drying chamber. Drying the belt prevents moisture from remaining on the belt and carrying it into the downstream chamber.

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

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

[0057] By having a pressure in the drying chamber higher than that in the cooling chamber, we ensure that the flow of atmosphere is from the drying chamber to the cooling chamber, and not the other way around.

[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, we ensure 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 in 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 just higher than the pressure in the wet chamber, we ensure that no flow of atmosphere can occur from the wet chamber to the upstream chamber. Any possible flow, despite the presence of an airlock separating the atmospheres, 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 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 strip surface is thus limited, particularly 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 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 with Mn content greater than 1.5% and Si content 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 extraction flow rate adjustable and such that the flow rate is greater than the sum of the flow rates , where the terms correspond respectively to the flow rate of HNx injected into the wet chamber, the flow rate of HNx injected into the dewatering 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 strip in the wet chamber, the steam flow rate resulting from the dewatering of the strip and the steam flow rate resulting from the drying of the strip.

[0069] By having a flow rate extraction in the humid chamber greater than the sum of the atmosphere flows injected into the humid chamber , in the spin system , in the drying chamber and those resulting from the evaporation of the cooling solution in the humid chamber , the spin system and the drying chamber , we ensure an atmospheric inlet from the airlock placed upstream of the humid chamber towards it and an atmospheric 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 method 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 atmosphere separation airlock, a humid 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 quantity of excess liquid that can be carried away by the cooled steel strip is removed, a drying chamber for the dewatered steel strip, in which the dewatered steel strip is dried to evaporate the liquid still present on the dewatered steel strip, a downstream atmosphere separation airlock,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 spent by the steel strip in the chamber, the cumulative time spent by the steel strip in the drying chamber and in 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 implementing the steps of checking and adjusting the process.

[0073] Advantageously according to the invention, the humid chamber can be supplied alternately by at least two separate cooling liquid 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% 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 embodiment of the invention, the wet chamber comprises a single-fluid and / or dual-fluid spray cooling device. Spray cooling allows for greater adjustment possibilities of the cooling gradient of the strip over its width and length than with immersion water quenching. For example, it is possible to vary the flow rate of coolant 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 airlock rollers creates an atmosphere flow from the center of the airlock to 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, which cannot escape to an upstream or downstream chamber and contaminate it by passing through the airlock. This prevents pollution of the reducing atmosphere in the upstream and downstream chambers by water vapor coming from the wet 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 resulting from drying the strip and the flow 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 some steel grades, 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 holding 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 become apparent upon reading the detailed description which follows, for the understanding of which reference will be made to the appended drawings in which:is a schematic and partial representation of a portion of a horizontal line with spray cooling, according to an exemplary embodiment of the invention,is a schematic and partial representation of a portion of a vertical line with spray cooling, according to another exemplary embodiment of the invention,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,is a schematic and partial view of a continuous line according to an exemplary embodiment of the invention,is an enlargement of the cooling zone of the line of the,is a schematic representation similar to that of theto illustrate the gas flows,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 strip from the humid chamber is below the Leidenfrost temperature, but above 110 °C, andis 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 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 the 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 travel of the strip.

[0093] Both chambers are managed simultaneously in terms of atmospheric flow and pressure. The management of one of the chambers is done in accordance with the management of the other.

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

[0095] The wet chamber is separated from the drying chamber by a belt wringing system. This system removes excess water from the belt to allow complete evaporation as it passes through the drying chamber.

[0096] The drying chamber, which starts directly after spinning, includes the following functions: web drying. Different technologies can be used, such as convection with 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 the atmospheres to prevent pollution of the upstream and downstream chambers by a humid atmosphere.

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

[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] 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] Lamontre 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.

[0103] Lamontre shows another example of a vertical line portion, but with cooling by water quenching 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] Particularly in the case of a vertical configuration as shown in Figures 2 and 3, the spin 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 the steel, the temperature of the strip, particularly at the end of wet cooling and at the exit from the drying chamber, and the dew point of the area concerned.

[0106] As explained in the state of the art paragraph, 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 in permanent 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, the impact pressure on the strip and the size of the drops.

[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 the Leidenfrost temperature to be 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 below 110°C will the atmosphere in the drying chamber be humid.

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

[0118] Thus, 250°C is the temperature above which one should avoid oxidizing the strip or 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, the oxidation of these alloying elements should be managed by carrying it out during the heating phase with selective or total oxidation.

[0121] In the case of end-of-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 depending on the end of cooling temperature.

[0123] The table below summarizes these three areas, according to the cooling regime, by showing: whether the water-strip contact requires the use of acid, whether the atmosphere in the drying chamber is humid or dry. Cooling area Strip temperature at the end of wet cooling Liquid-strip contact Drying chamber atmosphere Presence of acid Regime 1 T LeidenfrostVery lowDrynoStage 2< T LeidenfrostPartialDryyesStage 3< 110 °CTotalMoistyes

[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 virtually no water contact with the strip.

[0126] However, a dry drying chamber atmosphere, coupled with the injection of HNx, allows for a reducing atmosphere from the drying chamber.

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

[0128] In the cooling domain 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 in this case are therefore: Use of a low-dose acid; Injection of HNx into the drying chamber and downstream chambers. Adjustment of the hydrogen content (up to 20%) in the chambers located downstream of the drying chamber where the strip temperature exceeds 250 °C; Favor the combination of the drying chamber with a downstream chamber equipped with heating equipment (e.g. inductor, aging); The use of acid may not be necessary for certain steels if the reduction capacity of the downstream chambers is sufficient.

[0131] Advantageously, in regime 2, the strip does not leave the drying chamber at a temperature above 250°C (the strip must be dried before it exceeds 250°C again) but it can exceed this temperature again in a chamber downstream of the drying chamber, which is advantageous for reducing any oxides present, the downstream chambers being under a reducing atmosphere. Acid can be dispensed with if the reduction capacity of the downstream chambers is sufficient to reduce the additional oxides present at the outlet of the drying chamber due to the absence of acid.

[0132] In the cooling domain corresponding to regime 3, the strip will oxidize during the cooling phase.

[0133] 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 is evaporated before leaving the chamber and that the atmosphere is properly extracted.

[0134] The temperature in the drying chamber must not exceed 250°C before the strip exits, otherwise the strip will reoxidize.

[0135] The operating conditions in this case are therefore: Use of a low-dose acid in the cooling water, If the drying chamber is combined with a downstream chamber equipped with heating equipment, the outlet temperature must not exceed 250°C.

[0136] In order to create a flow of atmosphere towards the humid chamber where an atmospheric 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 higher than the pressure Ps of the drying chamber and that the pressure in it is higher than the pressure Ph of the humid chamber:

[0137] flow rates such as flow rate extraction of the wet chamber is greater than the sum of the flow rate of HNx injected into the wet chamber, of the flow rate of HNx injected into the spin section, of the flow rate injected into the drying chamber, the flow rate of vapor resulting from the vaporization of the coolant on the surface of the strip in the wet chamber, of the flow rate of steam resulting from the wringing of the strip, and the flow rate steam resulting from drying the strip:

[0138]

[0139] The extraction flow rate must be strictly greater than the liquid that vaporizes and HNx injected into the humid chamber , the spin section , the drying chamber , the upstream airlock and the downstream airlock so as to very slightly extract the atmosphere from the upstream chamber, for a flow rate , and the room swallows, for a flow .

[0140] In this case we have:

[0141]

[0142] If the flow rate steam resulting from drying the strip and the flow rate injected into the drying chamber are too high, additional extraction can be added to the drying chamber and in this case we will have:

[0143]

[0144] The amount of coolant that evaporates varies depending on the strip format and its temperature, the extraction rate is adjustable, just like if additional extraction is added in the drying chamber.

[0145] Referring to the diagram 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.

[0146] 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 dewatering 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.

[0147] 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.

[0148] Referring to the attached diagram, the lower part of the cooling section 104 can be seen partially represented.

[0149] Strip 1 leaves the gas cooling chamber 3, circulating from top to bottom in the direction of travel shown by arrow S.

[0150] At the outlet of this chamber there is an upstream airlock 31 ensuring a separation between the controlled reducing atmosphere present in the gaseous cooling chamber 3, consisting of a mixture of nitrogen and hydrogen, from the humid atmosphere of the liquid cooling chamber 4 which is located downstream.

[0151] 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.

[0152] 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.

[0153] 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.

[0154] The strip leaves the wet chamber 4 through a small opening 43.

[0155] 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.

[0156] Conversely, when the strip leaves the humid chamber at a higher temperature, there is no liquid film on it. The spinning and drying device can be stopped to limit the line's energy consumption.

[0157] After leaving the humid chamber, the strip passes through a wringing section 6 equipped with gas knives 60 intended to remove any liquid that may be present on the strip.

[0158] The gas knives are formed by flat jet nozzles 61 supplied by means of a supply conduit not shown.

[0159] 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.

[0160] The gas used for spinning can be at room temperature or at a higher temperature. These gas knives have approximately the same inclination as the liquid knives 41.

[0161] 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.

[0162] 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.

[0163] 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.

[0164] 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 spent by the strip in the humid 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 outlet chamber 106 must be at least equal to the residence time in the humid chamber 4.

[0165] 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.

[0166] 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 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.

[0167] 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 atmospheric extraction in the humid chamber, an additional atmospheric extraction 13 is added in the drying chamber in addition to the extraction 12 carried out in the humid chamber.

[0168] The locations on the injection points 91, 92 of the atmosphere injection system 9 and 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.

[0169] 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.

[0170] 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.

[0171] A Q31 flow rate ( ) of reducing atmosphere is injected into the upstream airlock 31 arranged between the gaseous cooling chamber 3 and the wet liquid cooling chamber 4.

[0172] 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 ( ) 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.

[0173] 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.

[0174] A Q32 flow rate ( ) of reducing atmosphere is injected into the downstream airlock 32 arranged between the drying chamber 7 and the aging chamber 105.

[0175] The entire Q32 flow injected into the airlock escapes into the aging chamber. The withdrawals made in the humid chamber 4 and the spin section 6 are such that a Q32b flow ( ) coming 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.

[0176] An injection of atmosphere, not shown in the figures, is carried out in chambers 3 and 105 to maintain these chambers under pressure and compensate for the outgoing flows Q31b ( ) and Q32b ( ).

[0177] 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.

[0178] In the wet room 4, a Q40 part ( ) of the coolant vaporizes on contact with the hot strip. The balance Q41 remains liquid and is evacuated from the wet chamber 4 through the outlet 44 before being recirculated to once again supply the nozzles 40.

[0179] The amount of evaporated liquid Q40 depends on:

[0180] . of the cooling process, evaporation being lower with water immersion quenching than with a spraying process;

[0181] . the format (thickness and width) of the tape and its speed;

[0182] . the inlet and outlet temperatures of the strip, i.e. the cooling slope.

[0183] This steam must be evacuated by atmospheric extraction.

[0184] A Q91 flow rate ( ) of reducing atmosphere is injected into the humid chamber 4 at the injection point 91 by the atmosphere injection system 9.

[0185] A Q11 flow rate ( ) is extracted from the humid chamber 4 at the withdrawal point 11 by the atmosphere extraction system 10.

[0186] The extracted flow Q11 is greater than the sum of the flow Q31 coming from the upstream airlock 31, the flow Q91 injected at injection point 91 by the atmosphere injection system 9 and the flow Q40 resulting from the vaporization of the coolant.

[0187] This results in the entry into the wet chamber 4 of a gas flow Q60 coming from the dewatering section 6 and a flow Q31b coming from the dry cooling chamber 3.

[0188] We thus have Q11 = Q31a + Q91 + Q40 + Q60.

[0189] In spin section 6, a flow rate Q6 ( ) of reducing atmosphere is injected through nozzles 61 and 65 to remove the film of liquid which may be present on the strip, depending on its temperature.

[0190] Part of the liquid vaporizes to form a flow Q62 ( ) of steam, the remaining liquid being evacuated through the outlets 61 before being recirculated to once again supply the nozzles 40.

[0191] The withdrawal flow rate Q11 in the wet chamber 4 is such that the flow rate Q60 coming from the spin 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 vaporization flow rate Q62 in the spin section 6.

[0192] This results in the entry into the spinning section 6 of a gas flow Q71 coming from the drying chamber 7.

[0193] We thus have Q60 = Q6 + Q62 + Q71.

[0194] In drying chamber 7, a flow rate Q92 ( ) of reducing atmosphere is 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 ( ) resulting from the evaporation of the liquid in chamber 7 and the flow rate Q32a coming from the downstream airlock 32 arranged between chamber 7 and the aging chamber 105.

[0195] We thus have Q71 = Q92 + Q70 + Q32a.

[0196] The control of the injected gas flow rates and the withdrawn flow rates leads to the pressure P3 in the dry cooling chamber being higher than the pressure P31 in the upstream airlock 31, which is higher than the pressure P4 in the humid chamber 4 and to the pressure P105 in the aging chamber being higher than the pressure P32 in the downstream airlock 32, which is higher than the pressure P7 in the drying chamber, which is higher than the pressure P6 in the dewatering section, which is higher than the pressure P4 in the humid chamber.

[0197] 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.

[0198] For steel grades requiring wet cooling to a temperature above 110°C, the invention makes it possible to avoid the presence of a humid atmosphere in the wringing zone 6 and the drying chamber 7, the flow rates Q62 and Q70 of evaporation of the liquid film being zero, the latter not being present on the strip at the outlet of the wet chamber 4.

[0199] 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.

[0200] Of course, the invention is not limited to the examples just described and many adjustments can be made to these examples without departing from the scope of the invention. In addition, the various features, forms, variants and embodiments of the invention can be combined with each other in various combinations to the extent that they are not incompatible or mutually exclusive.

Claims

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 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 extraction of atmosphere, by means of an extraction system (10), to permanently renew the atmosphere in the chamber (4) humid and to create a flow of atmosphere from the drying chamber (7) to the humid chamber (4),a control by means of a control and command system (11): of the time taken for the steel strip (1) to pass through the humid chamber (4), of 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, of the temperature (Th) of the steel strip (1) at the outlet of the humid chamber (4), and of 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 steel strip (1) from the humid chamber (4) is greater than or equal to the Leidenfrost temperature, the time taken for the steel strip (1) to pass through the humid chamber (4) which must be less than or equal to fifteen seconds, and the cumulative time taken in the drying chamber (7) and in at least one chamber (8, 105,106) downstream under a dry and reducing atmosphere which must be greater than or equal to the residence time of the steel strip (1) in the humid chamber (4);when the outlet temperature (Th) of the steel strip (1) from the humid chamber (4) is lower than the Leidenfrost temperature, the liquid from 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,when the outlet temperature (Th) of the cooled steel strip (1) from the humid chamber (4) is less than 110 °C, at least part of the quantity of excess liquid carried away by the cooled steel strip (1) in the humid chamber (4) is removed in a wringing section (6), and the strip (1) of wrung steel is dried in the drying chamber (7),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., Method according to the preceding claim, 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). 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 steel strip (1) to be galvanized. Method according to one of the preceding claims, for which the atmosphere extraction is carried out with an extraction flow rate adjustable and such that the flow rate is greater than the sum of the flow rates , where the terms correspond respectively to the flow rate of HNx injected into the wet chamber, the flow rate of HNx injected into the wringing 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 strip in the wet chamber, the steam flow rate resulting from wringing the strip and the steam flow rate resulting from drying the strip. Method according to claim 1, wherein the mass concentration of formic acid in the coolant is adjusted as a function of the chemical composition of the steel in the steel strip (1) to be galvanized. Continuous galvanizing line (100) capable of implementing a method according to one of claims 1 to 5 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 dewatering 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 dewatered steel strip (1), in which the dewatered steel strip (1) is dried to evaporate the liquid still present on the strip (1) of wrung steel, 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 wet chamber (4) and into the drying chamber (7), an atmosphere extraction system (10) capable of continuously renewing the atmosphere in the wet chamber (4) and creating a flow of atmosphere from the drying chamber (7) to the wet 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 into 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 to implement the steps of verification and adjustment of 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).

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