Method and device for cooling a steel strip travelling in a continuous line cooling section

Abstract

Method and device for cooling a steel strip (1) travelling in a cooling section (2) of a continuous line, according to which method the cooling is carried out by spraying the strip with an aqueous solution of formic acid, the formic acid concentration of which is between 0.1% and 6%, and preferably between 0.5% and 2%.

Description

[0001] The invention relates to the wet cooling sections of continuous lines for annealing or galvanizing steel strips. By galvanizing, the present description refers to all dipping coatings, whether zinc, aluminum, zinc and aluminum alloy coatings, or any other type of coating. In these cooling sections, the steel strip can typically enter at a temperature between 500 °C and 1000 °C, for example 800 °C, and can leave at a temperature close to room temperature or at an intermediate temperature.

[0002] According to the state of the art, there are two types of technologies for cooling steel strips in continuous line applications: gas cooling and wet cooling.

[0003] Gas cooling, typically carried out by spraying a high-speed, high-hydrogen mixture of N 2 H 2 onto the steel strip, allows cooling rates of up to 200 °C / s to be achieved for 1 mm thick steel strips. Since this cooling is carried out using a reducing gas, the steel strip is not oxidized after passing through this type of cooling. Galvanizing the strip is then possible without any intermediate chemical step, such as pickling. However, since the cooling rate is limited to 200 °C / s, gas cooling does not allow the production of steels with high mechanical and metallurgical properties, which require higher cooling rates.

[0004] Wet cooling with water, by spraying water or by spraying a mixture of water and gas onto the steel strip or by immersing the steel strip in a water tank, can achieve cooling rates of around 1000°C / s for a 1 mm thick steel strip. These cooling rates thus make it possible to produce steels with high mechanical and metallurgical properties. However, using water as a cooling fluid oxidizes the strip and makes it impossible to use this type of cooling on a galvanizing line without an intermediate pickling step.

[0005] The applicant's international application WO2015 / 083047 proposes the use of a solution with pickling or non-oxidizing properties with respect to iron and the alloying elements of the steel to carry out the cooling, for example a formic acid solution with a pH of less than 5, making it possible to obtain cooling rates of the order of 1000°C / s for a steel strip with a thickness of approximately 1 mm while not oxidizing the strip.

[0006] An aim of the invention is to propose a method for cooling a steel strip which improves the performance of the methods according to the state of the art.

[0007] Another aim of the invention is to propose a cooling method of greater efficiency than the methods according to the state of the art.

[0008] Another aim of the invention is to propose a cooling method which is less expensive than the methods according to the state of the art.

[0009] This object is achieved by the method of cooling a steel strip of claim 1. The present invention also relates to a cooling device according to the terms of claim 7.

[0010] When the solution to be projected is a liquid solution, the formic acid concentration of said solution is between 0.1% and 6% by mass of the solution. When a mixture comprising a liquid and a gas is projected, the liquid of said mixture has a formic acid concentration also between 0.1% and 6% by mass. The gas present in the mixture to be projected is advantageously an inert gas, for example nitrogen, or hydrogenated nitrogen.

[0011] Tests were carried out by the applicant on different types of steel, standard steels and steels alloyed with conventional alloying elements such as manganese and silicon, with the aim of determining the ideal concentration of formic acid. These tests consist, for example, of placing a sample measuring 100 mm x 40 mm x 1 mm between two connecting pieces and rapidly heating it to a temperature of 800 °C, under an N 2 H 2 atmosphere containing 5% H 2 and a dew point of -60 °C, by circulating an electric current through the sample. A formic acid solution is then sprayed onto the sample for a set period of time so that it reaches a temperature of 50 °C. As soon as the spraying of the acid solution stops, the sample is heated to a temperature of 80 °C while it is flushed with N 2 H 2 at 5% H 2 and a dew point of -60 °C.These tests showed that a formic acid solution with a concentration between 0.1% and 6% by mass of the solution is sufficient to obtain a steel strip that can be galvanized without requiring intermediate chemical treatment. The concentration of formic acid in the liquid solution is adjusted according to the content of alloying elements with high redox potential in the steel, such as aluminum, manganese, or silicon. The higher it is, the higher the concentration of formic acid in the solution.

[0012] Advantageously, the formic acid concentration is between 0.1 and 5.5%, advantageously between 0.1 and 5%, advantageously between 0.1 and 4.5%, advantageously between 0.1 and 4%, advantageously between 0.1 and 3.5%, advantageously between 0.1 and 3%, advantageously between 0.1 and 2.5%, advantageously between 0.15% and 2.5%, advantageously between 0.2 and 2.5%, advantageously between 0.3% and 2%, advantageously between 0.35% and 2.5%, advantageously between 0.4% and 2.5%, advantageously between 0.45% and 2.5% by mass of the solution. More advantageously, the formic acid concentration is between 0.46% and 2.4%, advantageously between 0.47% and 2.3%, advantageously between 0.48% and 2.2%, advantageously between 0.49% and 2.1% by mass of the solution. Even more advantageously, the formic acid concentration is between 0.5% and 2% by mass of the solution.

[0013] Advantageously, it has been found that the use of a formic acid solution with a concentration of between 0.5% and 2% by mass of the solution makes it possible to treat steel grades that are not very sensitive to oxidation, for example with a low manganese, aluminum or silicon content.

[0014] Advantageously, the solution to be projected has a pH between 1.5 and 3.

[0015] The formic acid solution used to rapidly cool the strip, for example in 1 to 3 seconds, does not require any further chemical treatment of the strip after cooling. It also does not require rinsing the strip with water after rapid cooling. Only drying can be carried out. It is therefore particularly advantageous for galvanizing lines since the strip can be immersed in the zinc bath immediately after wet cooling, after simply drying the strip.

[0016] Formic acid is the simplest of the carboxylic acids. Because of its very simple chemical composition, the risk of creating complex carbon deposits adhering to the steel strip or on the walls of equipment, which would prevent the galvanizing step from proceeding without further intermediate treatment, is very limited. More complex acids, such as citric acid, can leave significant carbon deposits on the strip that can prevent proper galvanizing.

[0017] When the hot steel strip is cooled by the solution, two independent chemical reactions occur: thermal decomposition of the solution, a chemical reaction between the strip and the solution and between the strip and the products of thermal decomposition.

[0018] Formic acid, also called methanoic acid, with the chemical formula HCOOH or CH 2 O 2 , and products of its decomposition, have highly reducing properties that are ideal for the application of the invention.

[0019] Indeed, at low temperature, formic acid decomposes by decarboxylation into water and carbon monoxide according to the reaction: HCOOH → H 2 O + CO

[0020] At higher temperatures, from around 150°C, formic acid decomposes by dehydration into dihydrogen and carbon dioxide according to the reaction: HCOOH → H 2 + CO 2

[0021] Once projected, the solution to be projected can take the form of fog, water knife, or other forms.

[0022] In liquid form, the decomposition of formic acid is mainly by decarboxylation while it is mainly by dehydration when formic acid is in gaseous form.

[0023] According to the invention, the aqueous solution to be sprayed is sprayed onto the steel strip by spraying

[0024] In both cases, the decomposition of formic acid produces reducing gases, on the one hand CO, on the other hand CO 2 and H 2 .

[0025] The solution to be sprayed is an aqueous solution. An aqueous solution has the advantage, compared to other solutions, of being more environmentally friendly, as it does not produce toxic or harmful emissions during its use. An aqueous solution is also less expensive than other solutions.

[0026] Preferably, the aqueous solution to be sprayed can be mainly composed of demineralized water. This further limits deposits on the steel strip. This solution does not result in discharges that contravene the environmental standards of steel-producing countries, nor does it result in excessively high additional costs per ton of steel produced.

[0027] Advantageously, part of the solution produced by the thermochemical reaction of the sprayed solution and the steel strip is recovered in a recirculation unit, preferably in a recirculation tank, and the solution to be sprayed is taken from a projection unit, preferably in a projection tank, connected to the recirculation unit. It is thus possible to reuse the sprayed solution, i.e. to minimize operating costs.

[0028] For example, for standard steel production, the flow rate of solution to be used to cool the strip is between 200 and 1000 m 3 / h, and more generally around 500 m 3 / h. Only a small proportion of the sprayed solution is altered by its chemical reaction with the steel strip and its thermal decomposition. In order not to reach prohibitive consumption and production costs, it is therefore important to reuse, or even recycle a very large part of this solution. Advantageously, at least 50% of the solution is recycled. Even more advantageously, at least 60%, advantageously at least 70%, advantageously at least 80%, advantageously at least 90% of the solution is recycled.In a more advantageous embodiment, at least 91%, advantageously at least 92%, advantageously at least 93%, advantageously at least 94%, advantageously at least 95%, advantageously at least 96%, advantageously at least 97%, advantageously at least 98%, advantageously at least 99% of the solution is recycled. In an even more advantageous embodiment, 100% of the solution is recycled.

[0029] The interaction of the formic acid solution in liquid or gas phase, as well as that of its decomposition products in liquid or gas phase, with the strip involves reactions that are not easy to understand, particularly due to their speed and unusual temperature levels. The kinetics of the interactions between the elements present is also complicated by vaporization of the solution upon contact with the strip and the resulting Leindenfrost phenomenon. There are thus chemical reactions between gaseous and liquid phases resulting from the acid solution and the strip, the contribution of which to the effect observed on the surface of the strip by an experimental approach is difficult to quantify.

[0030] The method according to the invention comprises a continuous or periodic control, for example every hour, of the liquid solution to be projected, for example present in the recirculation unit, which control comprises a measurement of at least one physicochemical data of said solution chosen from the group comprising the pH, the density and the formic acid concentration, or a combination of these physicochemical data, and, when this measurement does not belong to a predetermined tolerance range, a predetermined volume of the liquid solution, for example present in the recirculation unit, is taken and the same predetermined volume of a formic acid solution is injected into the projection unit (13), said predetermined volume of an injected formic acid solution having a formic acid concentration such that the liquid solution to be projected has, after injection, a formic acid concentration of between 0.1% and 6% by mass.Advantageously, the liquid solution to be projected has, after injection, a formic acid concentration of between 0.1 and 5.5%, advantageously between 0.1 and 5%, advantageously between 0.1 and 4.5%, advantageously between 0.1 and 4%, advantageously between 0.1 and 3.5%, advantageously between 0.1 and 3%, advantageously between 0.1 and 2.5%, advantageously between 0.15% and 2.5%, advantageously between 0.2 and 2.5%, advantageously between 0.3% and 2%, advantageously between 0.35% and 2.5%, advantageously between 0.4% and 2.5%, advantageously between 0.45% and 2.5% by mass. More advantageously, the liquid solution to be projected has, after injection, a formic acid concentration of between 0.46% and 2.4%, advantageously between 0.47% and 2.3%, advantageously between 0.48% and 2.2%, advantageously between 0.49% and 2.1% by mass.Even more advantageously, the liquid solution to be sprayed has, after injection, a formic acid concentration of between 0.5% and 2% by mass. The predetermined volume of the solution taken from the recirculation unit is determined according to the deviation in formic acid concentration between the measured value and the minimum value of the predetermined tolerance range and the formic acid concentration of the injected solution so that the formic acid concentration of the sprayed solution again has a desired concentration.

[0031] Thus, continuous measurement of the performance of the formic acid solution makes it possible to verify that it is within the predetermined tolerance range. The tolerance range is, for example, + / - 10% of the set value, whether this is, for example, a formic acid concentration value, a density value or a pH value.

[0032] The formic acid concentration and the tolerance range can be adjusted according to the additives in the steel constituting the strip and in particular its sensitivity to oxidation.

[0033] The formic acid concentration and the tolerance range can be adjusted depending on the configuration of the line, its operating mode and the nature of the steels treated, depending on whether these promote the formation of oxides on the surface of the strip to a greater or lesser extent.

[0034] The formic acid concentration and tolerance range can, for example, be determined by tests carried out on samples subjected to a thermal cycle representative of those carried out on the line.

[0035] The recirculation system reduces formic acid consumption. However, the solution collected is lost. This is why the invention proposes, according to a particular embodiment, to recycle this collected solution.

[0036] In contact with steel and the oxides created by water molecules, formic acid reacts according to the reaction: 2CH 2 O 2 + FeO → (CHO 2 ) 2 Fe + H 2 O

[0037] The collected solution can then be treated by oxidation of (CHO 2 ) 2 Fe with hydrogen peroxide, also referred to as hydrogen peroxide in this description, in order to obtain the following reaction: 2(CHO 2 ) 2 Fe + H 2 O 2 + 2CH 2 O 2 → 2(CHO 2 ) 3 Fe + 2H 2 O

[0038] After the formation of ferric formate, a second reaction can be carried out, regenerating formic acid and creating iron III hydroxides: (CHO 2 ) 3 Fe + 3H 2 O → 3CH 2 O 2 + Fe(OH) 3

[0039] The reaction is shown here for iron oxide, but similar reactions occur with the oxides of the addition elements.

[0040] According to a feature of the invention, the sampled solution is treated by oxidation with hydrogen peroxide and then filtered to extract iron III hydroxides and other alloying elements, the injected solution coming from a recirculation of the filtered solution or from a new solution. By new solution, the present description refers to a solution having a formic acid concentration of between 0.1% and 6% by mass of the solution.Advantageously, the new solution has a formic acid concentration of between 0.1 and 5.5%, advantageously between 0.1 and 5%, advantageously between 0.1 and 4.5%, advantageously between 0.1 and 4%, advantageously between 0.1 and 3.5%, advantageously between 0.1 and 3%, advantageously between 0.1 and 2.5%, advantageously between 0.15% and 2.5%, advantageously between 0.2 and 2.5%, advantageously between 0.3% and 2%, advantageously between 0.35% and 2.5%, advantageously between 0.4% and 2.5%, advantageously between 0.45% and 2.5% by mass of the solution. More advantageously, the new solution has a formic acid concentration of between 0.46% and 2.4%, advantageously between 0.47% and 2.3%, advantageously between 0.48% and 2.2%, advantageously between 0.49% and 2.1% by mass of the solution. Even more advantageously, the new solution has a formic acid concentration of between 0.5% and 2% by mass of the solution.

[0041] Thus, the collected solution can be treated with hydrogen peroxide to obtain a mixture of formic acid and iron III hydroxide. This mixture can then be filtered to separate the formic acid from the iron III hydroxides.

[0042] The treated and filtered formic acid can be reused and reinjected into the circuit. This method has the advantage of allowing the precise measurement of the quantity of hydrogen peroxide needed to react with the quantity of iron III hydroxide present in the solution. This not only allows the chemical reaction to be controlled so that all the hydrogen peroxide is consumed, but above all, it allows for a reaction that is almost instantaneous.

[0043] The system's consumption is therefore mainly hydrogen peroxide and the only discharges, apart from gaseous discharges, are iron III hydroxides and other alloying elements from the steel strips.

[0044] The formic acid solution can be fully or partially recirculated.

[0045] Oxidation with hydrogen peroxide can restore the desired formic acid concentration. Filtration can remove metal oxides, for example, using a filter press. Thus, the waste only contains iron III hydroxides and other metal alloying elements.

[0046] It is possible to improve the effectiveness of this solution, and thus improve the strip's suitability for electroplating, by removing the dissolved oxygen present in this solution. Indeed, the dissolved oxygen present in the solution is a source of oxidation of the strip. By removing this source of oxidation, the surface condition of the strip is even better.

[0047] According to an advantageous characteristic of the method according to the invention, the solution taken from the recirculation unit can be treated by deoxygenation before being sprayed.

[0048] Advantageously, the level of dissolved oxygen remaining in the solution to be projected can be less than 1 ppm.

[0049] Dissolved oxygen can be removed from the solution by a system of membranes flushed with nitrogen on one side and by pulling a vacuum on the other. Alternatively, dissolved oxygen can be removed from the solution by bubbling it with nitrogen, or another neutral gas, to enhance natural deoxygenation.

[0050] In an advantageous version, the method may further comprise a collection of vapors which results from the projection of the solution to be projected onto the steel strip, a condensation of said collected vapors, and an injection of said condensed vapors into a fluid circuit from which said solution to be projected is taken.

[0051] Vapor collection can be carried out by a vapor collector placed above a unit for projecting the solution to be projected.

[0052] The gas resulting from the condensation of the vapors can be sent to a chimney.

[0053] Condensation of the collected vapors can be achieved by a scrubbing tower.

[0054] According to a second aspect of the invention, there is provided a cooling device arranged to cool a steel strip moving in a cooling section of a continuous line comprising means arranged to implement a cooling method, as described in claims 1 to 6, by spraying onto said steel strip an aqueous solution, said aqueous solution being a liquid solution or a mixture of a liquid solution and a gas, the formic acid concentration of said aqueous solution being between 0.1% and 6% by mass, the device comprising a membrane system arranged to deoxygenate said solution. The means of the device also comprise a membrane system arranged to deoxygenate the solution, said membranes being swept with nitrogen on one side with a vacuum on the other.

[0055] The means of the device according to the invention may comprise an enclosure comprising a unit for projecting a solution to be projected, preferably nozzles, arranged to project a liquid, or a mixture comprising a gas and a liquid, onto the steel strip.

[0056] The membrane system designed to remove dissolved oxygen from the solution to be sprayed is located upstream of these nozzles.

[0057] The means of the device may comprise, at the outlet of the enclosure, in the direction of travel of the strip, a set of liquid knives arranged to remove most of the runoff liquid present on the strip.

[0058] The means of the device may comprise, downstream of the set of liquid knives, a set of gas knives arranged to remove the liquid still present on the strip.

[0059] The means of the device may comprise, downstream of the enclosure and where appropriate the set of liquid knives, and where appropriate all or part of the set of gas knives, a return tank arranged to collect the cooling liquid projected by the nozzles. The return tank may be arranged so as to be below the path of the strip leaving the enclosure.

[0060] The return tank may include a second set of gas knives arranged to remove liquid still present on the belt.

[0061] The means of the device may comprise a recirculation tank and means for transferring liquid from the return tank to the recirculation tank.

[0062] The liquid transfer means may include a filter arranged to remove metal particles present in the solution.

[0063] The means of the device may comprise supply circuits comprising a pump and an exchanger for supplying the projection unit.

[0064] The supply circuit may include a bypass circuit allowing a liquid portion pumped by the pump in the recirculation tank to be sent to another tank.

[0065] The means of the device may comprise means for actuating the bypass circuits, said means being actuated when it is necessary to renew part of the liquid contained in the cooling section in order to maintain the performance thereof within a predetermined operating range.

[0066] The membrane system can be positioned immediately upstream of the spray unit, with the pump being placed upstream of the membrane system, in which case the formic acid solution management circuit does not need to be isolated from oxygen sources.

[0067] The pump can also be placed between the membrane system and the projection system, which allows the pressure in the membranes to be lowered.

[0068] The membrane system can be positioned on a recirculation loop on the projection tank or between the projection tank and the recirculation tank.

[0069] When the membrane system is positioned at a demineralized water supply, the rest of the solution management circuit is preferably oxygen-tight.

[0070] All tanks can be gas-tight and flushed with an inert atmosphere, preferably nitrogen.

[0071] The means of the device may comprise a treatment assembly in which the sampled solution may be treated with hydrogen peroxide.

[0072] The treatment assembly may include a filter, for example a filter press, the waste from which may be removed by conveyors.

[0073] The treatment assembly may comprise means for injecting a solution at the outlet of the filter into the projection tank.

[0074] The invention consists, apart from the arrangements set out above, of a certain number of other arrangements which will be discussed more explicitly below with regard to an exemplary embodiment described with reference to the attached drawing, but which is in no way limiting.

[0075] In this drawing, the Figure 1 is a schematic view of an embodiment of a cooling section according to the invention. This embodiment being in no way limiting, it will be possible in particular to produce variants of the invention comprising only a selection of characteristics described below, as described or generalized, 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 art.

[0076] It is represented in Figure 1, a cooling section of a continuous galvanizing line comprising a first part 2 in which a steel strip 1 running vertically from top to bottom is cooled by spraying a liquid according to the invention. Nozzles 3, arranged on either side of the strip 1, spray the cooling liquid onto the strip. Upstream of these nozzles in a liquid circuit, a membrane system 4 makes it possible to remove the oxygen dissolved in the solution. Alternatively, a bubbling 31 with nitrogen or another neutral gas is placed in a projection tank 13 to amplify the natural deoxygenation. A measurement of the dissolved oxygen level in the solution is carried out in the projection tank 13 by means of a probe 35. At the outlet of part 2, in the direction of travel of the strip, there is a set 5 of liquid knives intended to remove most of the runoff liquid present on the strip.The set 5 of liquid knives is followed, in the direction of travel of the strip, by a set 6 of gas knives intended to remove the liquid still present on the strip. The strip then passes through a return tank 7 in which the cooling liquid projected by the nozzles 3 and the set 5 of liquid knives is collected. In this tank, a second set 8 of gas knives is intended to remove the liquid still present on the strip. The strip then passes through a part 9 equipped with heating tubes 10 making it possible to remove any trace of liquid on the strip. At the outlet of this part 9, the strip passes through an airlock 11 for separating the atmosphere between the wet parts 2, 7, 9 and parts 12 located downstream in the direction of travel of the strip. In this atmosphere separation airlock, an injection and / or a suction of gas makes it possible to reinforce the separation of atmospheres between the sections upstream and downstream of the airlock.

[0077] The liquid projected onto the belt by the nozzles 3 and the set 5 of liquid knives is collected in the return tank 7 and then sent to the projection tank 13. For this purpose, the liquid is transferred from the return tank 7 into a recirculation tank 27. This tank is equipped with compartments 32 in cascade to maintain a maximum number of particles in the first compartments. Electromagnets 33 placed under the tank 27 and a system of drawers 34 make it possible to recover and evacuate the metal particles without having to empty the tank. The liquid then passes through a set 28 of external filters in order to eliminate the residual metal particles before being returned to the projection tank 13 by means of a pump 30. The set 28 of external filters and the pump 30 are doubled in order to ensure the maintenance of this equipment without stopping the installation.

[0078] Supply circuits 14 comprising a pump 15 and a heat exchanger 16 make it possible to supply the rows of nozzles 3 of the part 2 with cooling liquid at the required pressure and temperature from the liquid contained in the projection tank 13. The supply circuits 14 comprise a bypass circuit 17 making it possible to send a liquid portion pumped in the tank 13 to a tank 18. Alternatively, the bypass circuit 17 originates in the recirculation tank 27. The bypass circuit 17 is actuated when it is necessary to renew a portion of the liquid contained in the cooling section in order to maintain its performance within the desired operating range.

[0079] A vapor collector 19 is placed in part 2 above the rows of nozzles 3. The collected vapors are sent to a wet scrubber 20 in which the vapors are condensed and sent to the tank 18. At the outlet of the scrubber tower, the gas freed from its vapors is sent to a chimney 21.

[0080] The liquid collected in the tank 18 is sent to a treatment unit 22 in which the used formic acid solution is dosed with hydrogen peroxide in order to obtain a mixture of formic acid and iron III hydroxide and steel alloying elements. This mixture is then filtered by a filter press (not shown) in order to separate the formic acid from the iron III hydroxides, the latter being evacuated by conveyors 23. The regenerated formic acid is used again and reinjected as a new solution by means of a circuit 24 into a tank 25. A supply of new formic acid is also carried out in this tank 25 by means of a circuit 26.

[0081] The liquid collected in the tank 25 can then be sent to the projection tank 13 by means of a circuit 29 comprising a pump (not numbered) arranged in the tank 25.

[0082] 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

1. Method for cooling a steel strip (1) travelling in a cooling section (2) of a continuous line, comprising spraying said steel strip with an aqueous spray solution, said aqueous spray solution being a liquid solution or a mixture of a liquid solution and a gas, characterised in that the formic acid concentration of said liquid solution is between 0.1% and 6% by mass, in that the spray solution is sprayed onto the steel strip by atomisation, and in that the cooling method further comprises continuous or periodic monitoring of the liquid spray solution, wherein the monitoring comprises measuring at least one physico-chemical datum of said solution selected from the group comprising pH, density and formic acid concentration, or a combination of these physico-chemical data, and, if this measurement does not belong to a predetermined tolerance range, a predetermined volume of the liquid spray solution is taken and the same predetermined volume of a formic acid solution is injected into the spraying unit (13), said predetermined volume of a formic acid solution having a formic acid concentration such that the liquid spray solution has, after injection, a formic acid concentration of between 0.1% and 6%.

2. Method according to claim 1, wherein the liquid solution has a formic acid mass concentration of between 0.5% and 2%.

3. Method according to either claim 1 or claim 2, wherein the formic acid mass concentration in the liquid spray solution after injection is between 0.5% and 2%.

4. Method according to claims 1 to 3, wherein the solution taken is treated by oxidation with hydrogen peroxide and then filtered to extract hydroxides of iron III and other alloying elements, the injected solution originating from a recirculation of the filtered solution or a new solution.

5. Method according to any of claims 1 to 4, wherein the solution taken from the recirculation unit (13) is treated by deoxygenation before being sprayed.

6. Method according to any of the preceding claims, further comprising collecting vapours resulting from spraying the spray solution onto the steel strip, condensing said collected vapours, and injecting said condensed vapours into a fluid circuit from which said spray solution is taken.

7. Cooling device arranged to cool a steel strip (1) travelling in a cooling section (2) of a continuous line comprising means arranged to implement a cooling process according to any of the preceding claims by spraying said steel strip with an aqueous solution, said aqueous solution being a liquid solution or a mixture of a liquid solution and a gas, the concentration of formic acid in said aqueous solution being between 0.1% and 6% by mass, the device comprising a system of membranes (4) arranged to deoxygenate said solution, the membranes being flushed with nitrogen and a vacuum being generated on the other side.

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