Liquid cooling of strip running in continuous line

Abstract

The invention relates to a cooling chamber (2) for cooling a metal strip (1) running vertically in a continuous treatment line, said cooling chamber comprising an upper cooling zone (3) in which a cooling liquid is sprayed onto the strip, and an intermediate zone (36) for drying the strip, said intermediate zone (36) comprising at least one nozzle (8) intended to form an air knife (32) impacting the strip at an acute angle A of less than 80° and preferably less than 60°, characterized in that said nozzle (8) is located in an enclosure (33) defined by the strip and a shaped sheet metal product (20) facing said strip, said shaped sheet metal product forming a barrier preventing the liquid from entering the enclosure. The invention also relates to a continuous treatment line comprising said chamber and to a tempering method implemented in this line.

Description

[0001] Related technical fields

[0002] The present invention relates to a continuous line for annealing or electroplating metal strips equipped with a rapid liquid cooling section, whether this requires cooling by water, a mixture of water and another liquid, or any other liquid.

[0003] This particularly relates to lines equipped with "NOWFC," an abbreviation for "non-oxidizing wet flash cooling." Therefore, NOWFC is a method and apparatus for ultra-rapid cooling of a metal strip surface using a liquid primarily composed of water that does not oxidize it.

[0004] The present invention relates more particularly to a liquid cooling chamber arranged on a vertical strip chain, the strip being capable of vertical or horizontal circulation upstream or downstream of the chamber.

[0005] This instruction covers all impregnation coatings, whether zinc, aluminum, zinc-aluminum alloys, or any other type of coating.

[0006] Technical problem solved by the present invention

[0007] In a continuous line for annealing or electroplating metal strips, the strip passes through different sections in which it undergoes heat treatment, including heating, temperature holding and cooling stages.

[0008] Producing new steels with very high yield strength (typically greater than 500 MPa) requires heat treatment at high cooling rates (typically greater than 200 °C / s) to establish a complex structure with variable distributions of different metallurgical phases, including austenite, ferrite, pearlite, bainite, and martensite.

[0009] In particular, AHSS and UHSS steels with very high yield strength can be produced from fully austenitic or mixed ferritic and austenitic metallurgical structures by controlling the cooling rate.

[0010] The heat treatment of strip depends on the chemical composition of the steel, the initial conditions of the strip at the start of the line, and the expected mechanical properties at the end of the treatment. It includes, for example, a heating step to a temperature between 750°C and 950°C, a holding time at this temperature, followed by slow cooling, for example, 50°C, and then ultra-rapid tempering to room temperature or a moderate temperature, for example, 300°C, with specific cooling rates for each metallurgical grade. For electroplating lines, temperature elevation, such as with induction heating, can be performed after rapid cooling to bring the strip temperature close to the temperature of the electroplating bath it was in before immersion.

[0011] For example, obtaining a given steel may require: an annealing temperature above its austenitizing temperature, then holding it at that temperature for a period of time, followed by slow cooling to partially transform the austenite into ferrite, and finally rapid cooling to transform the austenite into martensite.

[0012] Cooling may be followed by a tempering step, such as aging at 200°C; or "over-aging", such as at 500°C; or a second annealing for the third generation grade, such as at 750°C.

[0013] To prevent strip oxidation, multiple chambers arranged upstream and downstream of the rapid cooling chamber contain an oxygen-free reducing atmosphere composed of nitric acid hydride, often with 5% hydrogen.

[0014] The presence of oxygen can produce iron oxides on the surface of the strip, which will impair the quality of the strip and its proper coating. A similar effect can occur in the presence of water. In fact, water vapor, combined with temperature, oxidizes the iron and other elements present in the strip. Therefore, humidity must be maintained at extremely low levels, often corresponding to a dew point between -30°C and -40°C, or a few tenths of a gram of water per kilogram of gas.

[0015] Due to the large production capacity of annealing or electroplating lines, wet cooling in the cooling chamber involves spraying water onto the running strip at very high spray rates, such as greater than 1000 cubic meters per hour.

[0016] Therefore, it is necessary to ensure that the very abundant water in the wet cooling chamber can be contained within the chamber and that it does not contaminate the multiple chambers arranged upstream and downstream.

[0017] Some of the liquid sprayed onto the strip will evaporate upon contact with it, but due to the Coanda effect, most of the liquid adheres to the strip and drips down.

[0018] To remove water adhering to the strip, the water is dried at the rapidly cooled outlet.

[0019] However, insufficient drying of the strip during bottom-up circulation can damage the return roll located below the rapid cooling chamber. Since the strip is hot when in contact with the return roll, for example at 750°C, this also exposes the roll to high temperatures. The thermomechanical stress generated by the runoff water falling on the roll can further damage it.

[0020] Insufficient drying of the strip can also lead to moisture retention between the strip and the return roll, resulting in the strip slipping on the return roll, which can cause strip guiding problems.

[0021] If the equipment for drying the strip does not have the capability to remove all moisture present on the strip before it enters the downstream chamber, and there is a risk of iron and additive oxides (MnO, SiO, etc.) forming on the surface of the strip, insufficient drying of the strip may also lead to contamination of multiple downstream sections in a reducing atmosphere with controlled moisture content.

[0022] In NOWFC, the liquid water used to cool the strip is enriched with stripping compounds (often formic acid). If drying is insufficient, the liquid film or droplets left on the strip at the NOWFC outlet will leave dark marks upon evaporation. These are residues of the stripping compounds, often unevaporated formate. These residues can impair the quality of subsequent strip coatings.

[0023] Another constraint to consider is the unevenness of the strip as it exits wet cooling. It may have undulations, making it unsuitable for using a scraper to remove water present on the strip. Furthermore, steel manufacturers generally want to minimize the risk of mechanical parts coming into contact with their on-line products.

[0024] The temperature of the product at the outlet of the cooling section can exceed 100°C, so it is not suitable to use a drying roller with a rubber coating.

[0025] Finally, strip vibration or shaking may occur spontaneously, and this must be taken into account when selecting the drying system to be used.

[0026] The present invention provides a solution to these problems by making it possible to ensure complete drying of the strip after rapid cooling and before it enters the heating chamber arranged in a reducing atmosphere downstream. Background Technology

[0027] To separate water applied to the strip, current technology employs both liquid and air cutting techniques.

[0028] In practice, one or two cantilever arms used to spray a high-pulse liquid towards the falling water have the effect of separating the falling water from the strip and redirecting it to the back of the blade for guidance and evacuation from the cooling chamber. The cantilever pressure is fed in several bar, often 7 bar.

[0029] The pulses from the liquid jet are sufficient to offset the weight and energy of the falling liquid, thus allowing a large amount of liquid to be discharged from the rear.

[0030] A liquid cutter cannon alone can separate more than 95% of falling liquid, but this is not enough.

[0031] High-pulse air knives are used to dry strip. On the other hand, they operate in extremely humid areas with a large number of suspended droplets. The role of the gas jet pulses is to recirculate this atmosphere back to the strip, which can cause the strip to be re-wetted downstream of the air knife, even though they are supposed to dry the strip.

[0032] Therefore, air knives do not achieve the desired effect because they cause the droplets suspended in the rapid cooling chamber to recirculate, thereby reducing drying efficiency.

[0033] For informational purposes, at a width of 1200 mm and a speed of 300 m / min, a 100 μm liquid film remaining on the strip is still equivalent to drying more than 5000 kg / h of liquid before entering the heating chamber arranged in a reducing atmosphere downstream. Furthermore, this layer is extremely non-uniform, leaving sludge and varying droplets, which further complicates the drying process.

[0034] Therefore, it is necessary to find a way to minimize the amount of liquid present on the strip after drying. Summary of the Invention

[0035] According to a first aspect of the invention, a cooling chamber for cooling a metal strip running vertically in a continuous processing line is provided, the chamber comprising an upper cooling zone in which coolant is sprayed onto the strip; and an intermediate zone for drying the strip, the intermediate zone comprising at least one nozzle designed to form an air knife that impacts the strip at an acute angle A of less than 80°, and preferably less than 60°, characterized in that the nozzle is situated within a cavity defined by the strip and a shaped sheet metal article arranged facing the strip, the shaped sheet metal article forming a barrier preventing liquid from entering the cavity.

[0036] The strip on one side and the formed sheet metal on the other side form a cavity, making it possible to physically isolate the air knife within a volume free of liquid. The high-pulse gas jet is angled at an acute angle relative to the strip, thus preventing liquid from entering the cavity through the opening formed by the strip and the upper end of the formed piece. This opening, necessary for the strip to pass through without contacting the formed sheet metal, is limited as much as possible.

[0037] The gas jet inclined relative to the strip makes it possible to push the liquid film present on the strip and the runoff liquid near the strip out of the surrounding cavity.

[0038] Flow rate, pressure, distance to the nozzle strip, and direction of the gas jet have a significant impact on drying efficiency. The flow velocity on the strip surface ranges from 200 to 3000 Nm. 3 Between / h, for example, for a band 1200 meters wide, the flow rate is 1500 Nm. 3 / h. The pressure is between 0.5 bar and 10 bar. For example, for a jet at a distance of 100 mm from the strip and an inclination of 45°, the pressure is 2 bar. The distance to the nozzle strip is between 50 and 150 mm. The distance is, for example, 100 mm. The inclination of the jet is less than 60° and preferably 45°.

[0039] The geometry of the cavity also plays an important role. Therefore, in order to facilitate the flow of liquid outside the cavity, according to the present invention, the shaped sheet metal article forms a first inclined surface starting from the upper end of the shaped sheet metal article arranged near the strip, the extension of the first inclined surface toward the strip forming an acute angle B of less than 90°, and preferably less than 60°.

[0040] The inclined surface on the top of the formed sheet metal product helps to evacuate the liquid by gravity flow and remove it from the strip.

[0041] According to the present invention, the shaped sheet metal article forms a second inclined surface starting from the lower end of the shaped sheet metal article arranged near the strip, the extension of the second inclined surface toward the strip forming an acute angle C of less than 90°, and preferably less than 60°.

[0042] The inclined surface at the bottom of the molded part guides any liquid that may be present in the cavity to an opening located at the bottom of the cavity through gravity flow, through which the liquid is discharged from the cavity.

[0043] The inclined surface at the bottom of the molded part starts near the strip, as close to the strip as possible, leaving only the necessary opening for the strip to pass through without contacting the molded element.

[0044] This configuration helps to keep the volume of the cavity formed by the strip and the shaped sheet free of liquid.

[0045] Furthermore, according to the invention, the liquid cooling chamber includes a lower region in which a tray configured to receive coolant sprayed onto the strip is arranged, the tray including a vertical surface arranged opposite to and near the strip, the upper end of the vertical surface being located within a cavity formed by the strip and the formed sheet metal article, the vertical surface being configured to cause drying gas to rise from a return and drying zone arranged below the tray into the interior of the cavity, into the space defined by the strip and the vertical plane.

[0046] The humidity of the atmosphere supplied with dry gas within the cavity is lower than that present in the liquid cooling chamber. This has the effect of reducing the amount of liquid residue on the strip at the outlet of the drying zone.

[0047] According to a second aspect of the invention, a continuous processing line for metal strip is provided, comprising a first heating chamber in a controlled reducing atmosphere, configured to bring the strip to a first annealing temperature; and a second heating chamber in a controlled reducing atmosphere, configured to bring the strip to a second annealing temperature, an over-aging temperature, or a tempering temperature, characterized in that it includes a cooling chamber arranged between the first and second heating chambers according to the invention.

[0048] The liquid cooling chamber according to the invention makes it possible to avoid dark marks on the strip after rapid liquid cooling. It also makes it possible to avoid contamination of the atmosphere in the downstream heating chamber, which can be caused by the evaporation of liquid present on the strip at its inlet. Therefore, excessive consumption of the fresh atmosphere gas necessary to obtain the desired dew point of the atmosphere present in the heating chamber is avoided. Furthermore, atmosphere contamination in the downstream heating chamber can have an oxidizing effect on the strip surface, and this risk increases when the strip is heated to its high temperatures, for example, for a second annealing. Therefore, the invention enables the strip to obtain good surface quality when leaving the downstream heating chamber, regardless of tempering, over-aging, or annealing temperatures.

[0049] According to a third aspect of the invention, a method for tempering metal strip in a continuous processing line according to the invention is provided, comprising:

[0050] • The step of heating the strip to the first annealing temperature in a controlled, non-oxidizing atmosphere;

[0051] • Optionally, the step of cooling the strip from the first annealing temperature to the quenching start temperature by spraying a non-oxidizing gas thereon; the step of quenching the strip from the first annealing temperature or the quenching start temperature to the quenching end temperature by spraying a coolant thereon.

[0052] • The steps of dehydrating and drying the strip;

[0053] • The step of heating the strip to a second annealing temperature, over-aging temperature, or tempering temperature is carried out in a controlled non-oxidizing atmosphere.

[0054] The nozzle forming the air knife is confined within a cavity (forming a barrier against liquid entry into the cavity), making it possible to limit the amount of residual water present on the strip after dehydration. The strip is then dried before entering a heating chamber located downstream in a controlled reducing atmosphere. This configuration enables the implementation of the tempering method according to the invention, which produces strips with excellent surface quality due to the absence of dark marks on the strip after liquid cooling and the absence of oxidation of the strip during liquid cooling and in the heating chamber located downstream in a controlled reducing atmosphere, thanks to the uncontaminated atmosphere resulting from the absence of liquid on the strip when it enters the heating chamber.

[0055] The acute angles described above are measured relative to a plane perpendicular to the strip direction. Attached Figure Description

[0056] Further features and advantages of the present invention will become apparent from the following detailed description, which can be understood with reference to the accompanying drawings, wherein:

[0057] Figure 1 These are schematic diagrams and partial schematic diagrams of the electroplating line according to the present invention;

[0058] Figure 2 These are schematic diagrams and partial schematic diagrams of wet cooling sections based on existing technology;

[0059] Figure 3 These are schematic diagrams and partial schematic diagrams of the wet cooling section according to the present invention;

[0060] Figure 4 yes Figure 3 A portion of the magnified portion, and

[0061] Figure 5 yes Figure 4 A simplified representation of . Detailed Implementation

[0062] The attached diagram Figure 1 A simplified schematic diagram and part of the longitudinal view show a vertical furnace electroplating line 100 according to an embodiment of the present invention. It includes, along the travel direction of the strip 1, a preheating chamber 101, a heating chamber 102, a holding chamber 103, a cooling section 104 including a gas cooling chamber 6, a liquid cooling chamber 2, a return and drying chamber 9, a heating chamber 105, a furnace outlet section 106, and a hot-dip electroplating section 107.

[0063] Depending on the steel grade and the thermal cycle required to achieve the target mechanical properties, the gas cooling chamber 6 allows, for example, the strip to be slowly cooled from an annealing temperature of, for example, 900°C, to a tempering start temperature of, for example, 700°C. Faster cooling is also possible in chamber 6, but it remains slower compared to the cooling rate achieved in the liquid cooling chamber 2. In practice, gas cooling, often achieved by spraying a mixture of nitrogen and hydrogen, makes cooling rates on the order of 100°C / s possible for 1 mm thick steel strips. Liquid cooling makes cooling rates of up to 1000°C / s possible for 1 mm thick steel strips.

[0064] See attached document Figure 2 A simplified diagram shows, in part, the cooling section 104 according to the prior art.

[0065] The strip 1 circulates from top to bottom into the gas cooling chamber 6 in the direction of travel indicated by arrow S. At the outlet of this chamber is an airlock 5, which ensures separation between the controlled reducing atmosphere (a mixture of nitrogen and hydrogen) present in the gas cooling chamber and the humid atmosphere of the downstream liquid cooling chamber 2. The airlock shown comprises two pairs of rollers with a gas extraction port between them. Other airlock configurations are also possible, particularly airlocks with three pairs of rollers, including an extraction port between the two pairs of rollers on the gas cooling chamber side and a gas injection port between the two pairs of rollers on the liquid cooling chamber side.

[0066] The strip first passes through the upper liquid cooling zone 3, where nozzles 4 spray coolant onto the strip, such as an acidic solution containing water and 3% formic acid.

[0067] At the outlet of the liquid cooling zone 3, in the direction of strip travel, the strip then passes through the intermediate drying zone 36 of the strip.

[0068] In this region, a liquid cutter formed by multiple flat jet nozzles 7 is present, designed to remove most of the runoff liquid present on the strip. The multiple jets are inclined at an acute angle relative to the strip to facilitate the separation of the water film present on the strip surface. The same liquid as the coolant is supplied to the nozzles 7 via supply conduit 12.

[0069] Following the liquid cutter group is the air cutter, which is designed to remove any remaining liquid on the strip. These air cutters are formed by feeding nitrogen or a mixture of nitrogen and hydrogen through multiple flat jet nozzles 8 via supply pipes 17. The nitrogen can be supplied at room temperature or higher. The inclination of these air cutters is substantially the same as that of the liquid cutter.

[0070] Liquid and air knives cover the entire width of the strip. On one side of the strip, they can be obtained using a single nozzle with a length at least equal to the maximum width of the strip, or several nozzles arranged across the width of the strip.

[0071] The strip 1 then passes through a lower return zone 9, in which two deflection rollers 18 and 19 are arranged. This forms a tray in which liquid sprayed onto the strip is collected by cooling nozzles 4 and 7 forming a liquid knife before being discharged via discharge pipe 10. This lower zone may include nozzles 8 forming a supplementary air knife.

[0072] The strip 1 is then passed through a dryer 13 equipped with a heating tube 14 designed to dry the strip by radiation. Drying can also be carried out by convection or a combination of radiation and convection.

[0073] As the strip exits section 13, it passes through an atmosphere separation airlock 15 located between section 13 and chamber 16 located downstream in the direction of the strip's travel. The airlock shown comprises two pairs of rollers with a gas extraction port between them, but other airlock configurations are possible.

[0074] See attached document Figure 3 A simplified diagram, which partially shows the cooling section 104 according to one embodiment of the present invention, and with reference to the accompanying drawing. Figure 4 You can see Figure 3 The expansion of the area enclosed by circle C. (For simplification...) Figure 4 It only displays equipment that exists on the right-hand side of the strip. Figure 5 yes Figure 4 A simplified view showing the angles B and C of the tilted surfaces of the formed sheet metal product.

[0075] The strip 1 on one side and the shaped sheet metal 20 on the other side form a cavity 33 surrounding the nozzle 8 that forms the air knife 32. The liquid to be evacuated flows out of this cavity to the tray 23 before being discharged to an external exchanger (not shown) through the discharge pipe 26.

[0076] The formed sheet metal product extends across the entire width of the strip and is surrounded by nozzle 8 or several nozzles 8, depending on whether a single nozzle or multiple nozzles are used to cover the width of the strip.

[0077] The formed sheet metal article closes onto the strip at its upper and lower portions. The spacing between the strip and the sheet metal article is selected to minimize the channel section, avoiding any contact between the strip and the formed sheet metal article, while allowing stress-free gas jet discharge. A gap of 50 to 100 mm is recommended between the strip and the sheet metal article 20.

[0078] The gas jet causes the liquid to rise along the strip outside the cavity 33. The liquid then drips onto the upper part of the formed sheet metal article. This includes a ramp 21, which promotes the flow of liquid outside the opening before it falls into the tray 23 or is collected and then discharged through the pipe 26.

[0079] The internal atmosphere within cavity 33 is physically separated from the humid environment of the rest of liquid cooling chamber 2 by the formed sheet metal article, but it is not impermeable. In addition to the upper opening, orifices 30 exist in the lower part of the formed sheet metal article to drain any liquid that may accidentally enter the cavity. These orifices 30 have a reduced opening surface area to limit the entry of humid gases into cavity 33. The slope 22 at the lower part of the formed sheet metal article facilitates this flow.

[0080] Pallet 23 includes a vertically rising portion 24 along the strip on either side. Even in the event of strip vibration, the distance between the strip and the vertically rising portion 24 is reduced to the distance necessary to avoid any risk of the strip coming into contact with the latter. It is, for example, 50 to 100 mm.

[0081] Below tray 23 is a return and drying zone 38. The pulses of the air knife formed by nozzle 8 generate an upward flow of gas contained within the return and drying zone 38 through suction. Figure 4 As indicated by arrow 28, gas thus flows upward from the bottom between the rising portion 24 of the tray 23 and the strip.

[0082] The return and drying zone 38 includes a gas injection point 29, making it possible to inject nitrogen or a mixture of nitrogen and hydrogen therein. This injection allows for a drier atmosphere in this zone than that present in the liquid cooling chamber. This injection is carried out by means of a supply unit not shown. Air present in the return and drying zone 38 is extracted at the air separation airlock 15 to ensure the renewal of air in the return and drying zone 38.

[0083] The return and drying zone 38 is equipped with a heating tube 14, which is designed to completely dry the strip by radiation before it enters the downstream heating chamber.

Claims

1. A cooling chamber for cooling a metal strip running vertically in a continuous processing line, the cooling chamber comprising an upper cooling zone (3) in which a coolant is sprayed onto the strip; and an intermediate zone (36) for drying the strip, the intermediate zone (36) comprising at least one nozzle (8) designed to form an air knife (32) impacting the strip at an acute angle A of less than 80°, the nozzle (8) being situated within a cavity (33) defined by the strip and a shaped sheet metal article (20) arranged facing the strip, the shaped sheet metal article forming a barrier relative to liquid entering the cavity, characterized in that The formed sheet metal article (20) forms a first inclined surface (21) starting from the upper end (34) of the formed sheet metal article arranged near the strip, the extension of the first inclined surface toward the strip forming an acute angle B of less than 90° between the first inclined surface and the strip, and the formed sheet metal article (20) forms a second inclined surface (22) starting from the lower end (35) of the formed sheet metal article arranged near the strip, the extension of the second inclined surface toward the strip forming an acute angle C of less than 90° between the second inclined surface and the strip. The cooling chamber further includes a lower region (37) in which a tray (23) configured to receive coolant sprayed onto the strip is arranged, the tray including a vertical surface (24) arranged opposite to and near the strip, the upper end (35) of the vertical surface being located in a cavity (33) formed by the shaped strip and the shaped sheet metal article, the vertical surface being configured to cause drying gas to rise from a return and drying zone (38) arranged below the tray (23) into the interior of the cavity (33) into the space defined by the strip and the vertical plane.

2. The cooling chamber according to claim 1, wherein, The nozzle is designed to form an air knife (32) that impacts the strip at an acute angle A of less than 60°.

3. The cooling chamber according to claim 1, wherein, The extension of the first inclined surface toward the strip forms an acute angle B of less than 60° between the first inclined surface and the strip.

4. The cooling chamber according to claim 1, wherein, The extension of the second inclined surface toward the strip forms an acute angle C of less than 60° between the second inclined surface and the strip.

5. A continuous processing line for metal strip (1), comprising: The first heating chamber in a controlled reducing atmosphere is configured to bring the strip to a first annealing temperature; The second heating chamber (105) in a controlled reducing atmosphere is configured to bring the strip to a second annealing temperature, or to an over-aging temperature, or to a tempering temperature, characterized in that the continuous processing line includes a cooling chamber arranged between the first and second heating chambers, according to any one of claims 1-4.

6. A method for annealing a tempered metal strip (1) in a continuous processing line according to claim 5, comprising: • The step of heating the strip to the first annealing temperature in a controlled, non-oxidizing atmosphere; • The step of quenching strip by spraying coolant onto the strip from the first annealing temperature or quenching start temperature to the quenching end temperature; • Steps for dehydrating and drying the strip; • The process of heating the strip to a second annealing temperature, or to an over-aging temperature, or to a tempering temperature is carried out in a non-oxidizing controlled atmosphere.

7. The method according to claim 6, further comprising: The process of cooling the strip by spraying a non-oxidizing gas onto it, from the first annealing temperature to the quenching start temperature.

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