Liquid cooling of a moving strip in a 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 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, comprising at least one nozzle (8) intended to form a blade (32) of gas impacting the strip at an acute angle A less than 80°, and preferably less than 60°, characterized in that the nozzle (8) is situated in an enclosure (33) defined by the strip and profiled sheet metalwork (20) arranged facing the strip, said profiled sheet metalwork forming a barrier to the ingress of liquid into the enclosure. The invention also relates to a continuous treatment line comprising the chamber and to a quenching method implemented in the line.

Description

Designation of the technical field concerned

[0001] The invention relates to continuous annealing or galvanizing lines for metal strips equipped with a rapid liquid cooling section, whether it be water cooling, a mixture of water and another liquid, or any other liquid.

[0002] It relates in particular to lines equipped with a "NOWFC," NOWFC being the abbreviation for "Non-Oxydizing Wet Flash Cooling." A NOWFC is thus a process and device for ultra-fast cooling of a metal strip with a liquid, composed mainly of water, but without oxidizing its surface.

[0003] The invention relates more particularly to liquid cooling chambers arranged on a vertical strip, the strip being able to circulate vertically or horizontally upstream or downstream of said chamber.

[0004] For the purposes of this description, galvanizing refers to all dipping coatings, whether zinc, aluminum, zinc-aluminum alloy, or any other type of coating. Technical problems that the invention addresses

[0005] In a continuous line for annealing or galvanizing metal strips, a strip passes through different sections within which it undergoes a heat treatment including phases of heating, holding at temperature and cooling.

[0006] The development of new steels with very high yield strength, typically exceeding 500 MPa, requires heat treatments with high cooling rates, typically exceeding 200 °C / s, to establish complex structures with a variable distribution of different metallurgical phases including austenitic, ferritic, pearlitic, bainitic, and martensitic phases.

[0007] In particular, very high yield strength steels AHSS and UHSS can be produced by controlling cooling rates, from a totally austenitic or mixed ferritic and austenitic metallurgical structure.

[0008] The heat treatment applied to the strip depends on the chemical composition of the steel, its condition upon entering the line, and the expected mechanical properties at the end of the treatment. It typically includes, for example, a heating stage to a temperature between 750°C and 950°C, a holding period at this temperature followed by slow cooling, for example, by 50°C, and then ultra-rapid quenching to ambient temperature or an intermediate temperature, for example, 300°C, with a specific cooling rate for each metallurgical grade. For galvanizing lines, a reheating stage, for example with induction heating, can be carried out after rapid cooling to bring the strip to a temperature close to that of the galvanizing bath before immersion.

[0009] For example, obtaining a given steel may require an annealing temperature higher than its austenitizing temperature, then a holding time at that temperature, followed by slow cooling for a partial transformation of austenite into ferrite and finally rapid cooling for a transformation of austenite into martensite.

[0010] Cooling can be followed by a tempering stage, for example at a temperature of 200°C, aging, or "overaging" in English, for example at a temperature of 500°C, or even a second annealing for so-called 3rd generation grades, for example at a temperature of 750°C.

[0011] To prevent oxidation of the strip, the chambers located upstream and downstream of the rapid cooling chamber contain a reducing atmosphere free of oxygen and composed of hydrogenated nitrogen, typically at 5% hydrogen.

[0012] The presence of oxygen would cause iron oxides to form on the surface of the belt, impairing its quality and coating. A similar effect occurs in the presence of water. Water vapor, combined with temperature, oxidizes the iron and additives present in the belt. For this reason, humidity levels must be kept extremely low, typically corresponding to a dew point between -30°C and -40°C, or a few tenths of a gram of water per kilogram of gas.

[0013] Due to the large production capacity of the annealing or galvanizing lines, wet cooling in a cooling chamber consists of projecting very large spray rates of water onto the moving belt, for example greater than 1000 m³ / h.

[0014] It is therefore imperative to ensure that the water present in very abundant quantities in the wet cooling chamber can be contained within this chamber and that it does not pollute the chambers located upstream and downstream.

[0015] Part of the liquid sprayed onto the strip evaporates on contact with it, but the vast majority falls back down, remaining stuck along the strip, by the Coanda effect.

[0016] To remove the water that has accumulated on the belt, it is wrung out after rapid cooling.

[0017] However, insufficient belt spinning can damage the return roller located beneath the rapid cooling chamber as the belt travels from bottom to top. Since the belt is hot when it contacts the return roller, for example at 750°C, the roller itself also reaches a high temperature. Runoff water falling onto the roller generates thermomechanical stresses that can cause further damage.

[0018] Insufficient belt wringing can also result in water retention between the belt and the return roller and generate aquaplaning of the belt on this return roller which can cause belt guidance problems.

[0019] Insufficient belt spinning can also result in pollution of downstream sections, under a reducing atmosphere with a controlled humidity level, if the belt drying means do not have the capacity required to remove all the water present on the belt before it enters a downstream chamber, with a risk of formation of iron oxides and additive elements (MnO, SiO...) on the surface of the belt.

[0020] In a NOWFC (No Water Cooling Factor), where the liquid water used to cool the belt is enriched with a pickling compound, typically formic acid, insufficient spin-drying will cause the liquid film or droplets left on the belt as it exits the NOWFC to evaporate, leaving dark traces that are residues of the pickling compound, typically unevaporated formate. These residues degrade the quality of subsequent belt coating.

[0021] Another constraint to consider is the unevenness of the belt exiting the wet cooling process. This unevenness can make the use of scrapers to remove water from the belt unsuitable. Furthermore, steelmakers generally want to minimize contact between mechanical parts and their product within the production line.

[0022] Since the product temperature at the cooling section outlet can exceed 100°C, the use of rubber-coated wringer rollers is also inappropriate.

[0023] Finally, phenomena of vibration or belt fluttering appear spontaneously and must be taken into account when choosing the spin-drying system to adopt.

[0024] The invention provides a solution to these problems by ensuring complete wringing of the belt after rapid cooling, before the belt enters the heating chamber under a reducing atmosphere located downstream. Technical background

[0025] To separate the water attached to the strip from the strip itself, the current technique successively employs ramps of liquid knives and gas knives, as disclosed in documents JP S59 67323 A and JP S58 61235 A.

[0026] In practice, one or two high-pulse liquid spray bars, directed towards the waterfall, detach the water from the belt and divert it behind the cutter to be channeled and evacuated from the cooling chamber. The spray bars are supplied with a pressure of a few bars, typically 7 bar.

[0027] The impulse from the liquid knives is sufficient to counteract the weight and energy of the falling liquid, and the amount of liquid evacuated from the rear by this means is significant.

[0028] The liquid knife ramps alone separate more than 95% of the liquid fall, but this is not enough.

[0029] High-impulse gas knives are used to complete the belt dewatering process. However, these knives operate in an extremely humid area with numerous suspended droplets. The pulse of the gas jet recirculates this atmosphere back towards the belt, which can lead to re-wetting the belt downstream of the gas knives, even though they are supposed to dry it.

[0030] Thus, gas knives do not have the desired efficiency because they can degrade the efficiency of the spin cycle by recirculating droplets present in suspension in the rapid cooling chamber.

[0031] For your information, for a 1200 mm wide belt moving at a speed of 300 m / min, a 100 µm liquid film left on the belt still represents more than 5000 kg / h of liquid to be dried before entering the heating chamber under a reducing atmosphere located downstream. Furthermore, this layer is extremely heterogeneous, leaving streaks and disparate droplets, which further complicates the belt drying process.

[0032] It is therefore necessary to find a way to minimize the amount of liquid present on the belt after it has been wrung out. Summary of the invention

[0033] According to a first aspect of the invention, a cooling chamber for a metal strip moving vertically in a continuous processing line is proposed, said chamber comprising an upper cooling zone in which a coolant is projected onto the strip, an intermediate zone for wringing the strip comprising at least one nozzle intended to form a gas knife impacting the strip at an acute angle A of less than 80°, and preferably less than 60°, characterized in that the nozzle is located in an enclosure defined by the strip and a profiled sheet metal arranged with respect to the strip, said profiled sheet metal forming a barrier to the entry of liquid into said enclosure.

[0034] The strip on one side and the profiled sheet metal on the other form an enclosure that physically isolates the gas cutter within a liquid-free volume. The high-pulse gas jet, inclined at an acute angle to the strip, prevents liquid from entering the enclosure through the opening formed by the strip and the upper end of the profile. This opening, necessary for the strip to move without contacting the profiled sheet metal, is minimized as much as possible.

[0035] The gas jet, inclined relative to the belt, helps to push the liquid film present on the belt and the liquid running off in the vicinity of the belt, outside the enclosure.

[0036] The flow rate, pressure, distance from the nozzle to the belt, and the direction of the gas jet all play a significant role in the efficiency of the wringing process. The flow rate is between 200 and 3000 Nm³ / h on one side of the belt, for example, 1500 Nm³ / h for a 1200 m wide belt. The pressure is between 0.5 and 10 bar. For example, it is 2 bar with a distance of 100 mm from the belt and a jet inclined at 45°. The distance from the nozzle to the belt is between 50 and 150 mm. For example, it is 100 mm. The jet inclination is less than 60° and preferably 45°.

[0037] The geometry of the enclosure also plays an important role. Thus, to promote the flow of the liquid out of the enclosure, according to the invention, the profiled sheet metal forms a first inclined surface originating at the upper end of the profiled sheet metal arranged in the vicinity of the strip, the extension towards the strip of the first inclined surface forming with it an acute angle B of less than 90°, and preferably less than 60°.

[0038] The inclined surface of the profiled sheet metal on its upper part helps to evacuate the liquid by gravity flow and away from the strip.

[0039] According to the invention, the profiled sheet metal forms a second inclined surface originating at the lower end of the profiled sheet metal arranged in the vicinity of the strip, the extension towards the strip of the second inclined surface forming with it an acute angle C of less than 90°, and preferably less than 60°.

[0040] The inclined surface of the profile on its lower part channels by gravity flow any liquid that may be present in the enclosure towards an opening located in the lower part of the enclosure through which the liquid is evacuated out of the enclosure.

[0041] The inclined surface of the profile on its lower part originates in the vicinity of the strip, as close as possible to it, leaving only the opening necessary for the strip to move without contact between it and the profile.

[0042] This configuration helps to keep the volume within the enclosure formed by the strip and the profiled sheet metal free of liquid.

[0043] Furthermore, according to the invention, the liquid cooling chamber comprises a lower zone in which is disposed a tray configured to receive the coolant projected onto the belt, said tray comprising a vertical surface disposed opposite and in the vicinity of the belt, the upper end of which is located in the enclosure formed by the belt and the profiled sheet metal, said vertical surface being configured to promote an upward movement of dry gas in the space defined by the belt and the vertical plane towards the interior of the enclosure and from a return and drying zone disposed under the tray.

[0044] Introducing dry gas into the chamber results in a less humid atmosphere than that found in the liquid cooling chamber. This reduces the amount of residual liquid on the belt exiting the spin zone.

[0045] According to a second aspect of the invention, a continuous processing line for a metal strip is proposed, comprising a first heating chamber under a controlled reducing atmosphere configured to bring the strip to a first annealing temperature, a second heating chamber under a controlled reducing atmosphere configured to bring the strip to a second annealing temperature, or to an aging temperature or to a tempering temperature, characterized in that it comprises a cooling chamber according to the invention disposed between the first and second heating chambers.

[0046] The liquid cooling chamber according to the invention prevents dark marks from appearing on the strip after rapid liquid cooling. It also prevents pollution of the atmosphere in the downstream heating chamber that would result from the evaporation of the liquid on the strip as it enters the chamber. This avoids the excessive consumption of fresh air that would be necessary to achieve the desired dew point in the atmosphere of the heating chamber. Furthermore, pollution of the atmosphere in the downstream heating chamber can oxidize the surface of the strip, with an increased risk when the strip is heated to a high temperature in the chamber, for example, during a second annealing. Thus, the invention ensures a good surface quality of the strip exiting the downstream heating chamber, regardless of the tempering, aging, or annealing temperature.

[0047] According to a third aspect of the invention, a method for quenching a metal strip is proposed, implemented in a continuous processing line according to the invention, comprising: . a step of heating the strip to a first annealing temperature under a controlled non-oxidizing atmosphere; . optionally, a step of cooling the strip by spraying it with a non-oxidizing gas, from the first annealing temperature to a quenching start temperature; . a step of quenching the strip by spraying it with a coolant, from the first annealing temperature, or the quenching start temperature, to a quenching end temperature; . a step of spinning and drying the strip; . a step of heating the strip to a second annealing temperature, or to an aging temperature, or to a tempering temperature, carried out under a controlled non-oxidizing atmosphere.

[0048] Confining the gas knife nozzles within enclosures that act as a barrier to liquid entry limits the amount of residual water on the belt after wringing. The belt is then dried before entering the downstream heating chamber under a controlled reducing atmosphere. This configuration enables the implementation of the quenching process according to the invention, which produces belts with excellent surface quality due to the absence of dark marks on the belt after liquid cooling and the absence of oxidation during liquid cooling and in the downstream heating chamber under a controlled reducing atmosphere. This is due to the absence of liquid on the belt as it enters the heating chamber, preventing contamination of the atmosphere.

[0049] The acute angles described above are measured with respect to a plane perpendicular to the direction of the band. Brief description of the figures

[0050] Other features and advantages of the invention will become apparent upon reading the detailed description that follows, for which reference should be made to the accompanying drawings in which: [ Fig.1 ] is a schematic and partially represented view of a galvanizing line according to the invention; [ Fig. 2 ] is a schematic and partially represented view of a wet cooling section according to the prior art; [ Fig.3 ] is a schematic and partially represented view of a wet cooling section according to the invention; [ Fig. 4 ] is a partial enlargement of a part of the figure 3 , And [ Fig. 5 ] is a simplified representation of the figure 4 .

[0051] Referring to the diagram of the figure 1 From the attached drawings, a vertical furnace galvanizing line 100 can be seen schematically and partially represented in longitudinal view, according to an embodiment of the invention. It comprises, successively and in the direction of the conveyor belt 1, a preheating chamber 101, a heating chamber 102, a holding chamber 103, a cooling section 104 comprising a gaseous cooling chamber 6, a liquid cooling chamber 2, a return and drying chamber 9, then a heating chamber 105, a furnace outlet section 106, and a hot-dip galvanizing section 107.

[0052] Depending on the steel grade and the thermal cycle required to achieve the desired mechanical properties, the gas cooling chamber 6 allows, for example, slow cooling of the strip from an annealing temperature, such as 900°C, to a quenching start temperature, such as 700°C. Faster cooling of the strip in chamber 6 is also possible, but it will still be slower than that achieved in the liquid cooling chamber 2. Indeed, gas cooling, typically by spraying a mixture of nitrogen and hydrogen, allows cooling rates to be reached on the order of 100 °C / s for 1 mm thick steel strips. Liquid cooling, on the other hand, allows cooling rates to be reached up to 1000 °C / s for a 1 mm thick steel strip.

[0053] Referring to the diagram of the figure 2attached, we can see partially represented a section 104 of cooling according to the state of the art.

[0054] Belt 1 enters the gas cooling chamber 6, flowing from top to bottom in the direction indicated by arrow S. At the exit of this chamber is an airlock 5, which separates the controlled reducing atmosphere of the gas cooling chamber (a mixture of nitrogen and hydrogen) from the humid atmosphere of the liquid cooling chamber 2 downstream. The airlock shown comprises two pairs of rollers with an atmosphere withdrawal between them. Other airlock configurations are possible, including a three-pair roller airlock with an atmosphere withdrawal between the two pairs of rollers on the gas cooling chamber side and a gas injection between the two pairs of rollers on the liquid cooling chamber side.

[0055] The belt first passes through an upper liquid cooling zone 3 in which nozzles 4 spray a coolant onto the belt, for example an acidic solution containing water and 3% formic acid.

[0056] At the exit of the liquid cooling zone 3, depending on the direction of the belt's movement, the belt then passes through an intermediate belt-spinning zone 36.

[0057] In this area are liquid knives formed by flat-jet nozzles 7 designed to remove most of the runoff liquid from the belt. The jets are inclined at an acute angle to the belt to promote the separation of the water film on the belt surface. The nozzles 7 are supplied with the same liquid as the coolant, via a supply line 12.

[0058] The liquid knife assembly is followed by gas knives designed to remove any remaining liquid from the belt. These gas knives consist of flat-jet nozzles 8 supplied with nitrogen, or a mixture of nitrogen and hydrogen, via a supply line 17. The nitrogen may be at ambient temperature or at a higher temperature. These gas knives have approximately the same inclination as the liquid knives.

[0059] The liquid and gas knives cover the entire width of the belt. On one side of the belt, they can be obtained with a single nozzle whose length is at least equal to the maximum width of the belt or with a plurality of nozzles arranged across the width of the belt.

[0060] The belt 1 then passes through a lower return zone 9 in which two deflector rollers 18, 19 are arranged. It forms a tray in which the liquid sprayed on the belt by the cooling nozzles 4 and the nozzles 7 forming a liquid knife is collected before being discharged through a discharge conduit 10. This lower zone may include nozzles 8 forming additional gas knives.

[0061] The strip 1 then passes through a dryer 13 equipped with heating tubes 14 designed to dry the strip by radiation. Drying can also be carried out by convection or by a combination of radiation and convection.

[0062] Upon exiting section 13, the tape passes through an atmospheric separation chamber 15 between section 13 and chamber 16 located downstream in the direction of tape travel. The chamber shown comprises two pairs of rollers with an atmospheric extraction point between the two pairs of rollers, but other chamber configurations are possible.

[0063] Referring to the diagram of the figure 3 In the attached figure, a partially represented section 104 of a cooling system can be seen according to an example of an embodiment of the invention, and by referring to the figure 4 In the attached image, one can see an enlargement of the area enclosed by a circle C on the figure 3 To lighten the figure 4 Only the equipment located on the right side of the band is shown. figure 5 is a simplified view of the figure 4 allowing visualization of angles B and C of the inclined surfaces of the profiled sheet metal.

[0064] The strip 1 on one side and a profiled sheet metal 20 on the other form an enclosure 33 which surrounds the nozzle 8, forming a gas knife 32. All the liquid to be evacuated flows outside this enclosure towards the tank 23 before being discharged to an external heat exchanger not represented by the exhaust duct 26.

[0065] The profiled sheet metal extends over the entire width of the strip and surrounds the nozzle 8 over its entire width, or the plurality of nozzles 8 depending on whether a single nozzle or several nozzles are used to cover the width of the strip.

[0066] The profiled sheet metal closes towards the belt at its top and bottom. The spacing with the belt is chosen to minimize the cross-sectional area, preventing any contact between the belt and the profiled sheet metal while allowing the gas jet to escape freely. A clearance of 50 to 100 mm between the belt and the sheet metal is recommended.

[0067] The gas jet forces the liquid upwards along the strip outside the enclosure 33. The liquid then falls back onto the profiled upper part of the sheet metal. This part includes a slope 21 which facilitates the flow of the liquid towards the outside of the sheet metal, before it falls into a tray 23 where it is collected and then discharged through a conduit 26.

[0068] The internal atmosphere within enclosure 33 is physically separated from the humid environment of the rest of the liquid cooling chamber 2 by the profiled sheet metal, but not completely sealed. In addition to the opening at the top, vents 30 are located on the underside of the profiled sheet metal to drain any liquid that might inadvertently enter the enclosure. These vents 30 have a reduced surface area to limit the entry of humid gas into enclosure 33. A slope 22 on the underside of the profiled sheet metal facilitates this drainage.

[0069] The tray 23 includes a riser 24 along the belt, on either side of it. The distance between the belt and the riser 24 is reduced to that necessary to avoid any risk of the belt contacting the riser, even if the belt is loose. For example, it is 50 to 100 mm.

[0070] Below tray 23 is a return and drying area 38.

[0071] The impulse of the gas knife formed by the nozzle 8 creates a rise of the gas contained in the return and drying zone 38 by suction. Gas thus flows from bottom to top between the riser 24 of the tray 23 and the belt, as represented by the arrow 28 on the figure 4 .

[0072] The return and drying zone 38 includes a gas injection point 29 for injecting nitrogen or a mixture of nitrogen and hydrogen. This injection provides a drier atmosphere in this zone than that in the liquid cooling chamber. This injection is carried out using a supply (not shown). The atmosphere in this return and drying zone 38 is extracted at the atmosphere separation airlock 15 to ensure its renewal.

[0073] The return and drying zone 38 is equipped with heating tubes 14 intended to completely dry the strip by radiation before it enters the heating chamber located downstream.

Claims

1. Chamber (2) for cooling a vertically moving metal strip (1) in a continuous treatment line, said chamber comprising an upper cooling zone (3) in which a cooling liquid is sprayed onto the strip, an intermediate zone (36) for dewatering the strip comprising at least one nozzle (8) for forming a gas knife (32) impacting the strip at an acute angle A of less than 80°, and preferably of less than 60°, the nozzle (8) being located in an enclosure (33) defined by the strip and a profiled metal sheet (20) arranged opposite the strip, said profiled metal sheet forming a barrier to the entry of liquid into said enclosure, characterized in that the profiled metal sheet (20) forms a first inclined surface (21) originating at the upper end (34) of the profiled metal sheet arranged in the vicinity of the strip, the extension toward the strip of the first inclined surface forming with the strip an acute angle B of less than 90°, and preferably of less than 60°, and the profiled metal sheet (20) forms a second inclined surface (22) originating at the lower end (35) of the profiled metal sheet arranged in the vicinity of the strip, the extension toward the strip of the second inclined surface forming with the strip an acute angle C of less than 90°, and preferably of less than 60°.

2. Liquid cooling chamber according to claim 1, further comprising a lower zone (37) in which a trough (23) is arranged, configured to receive the cooling liquid sprayed onto the strip, said trough comprising a vertical surface (24) arranged opposite and in the vicinity of the strip, the upper end (35) of which surface is located in the enclosure (33) formed by the strip and the profiled metal sheet, said vertical surface being configured to promote the upwelling of dry gas in the space defined by the strip and the vertical plane toward the interior of the enclosure (33) and from a return and drying zone (38) arranged beneath the trough (23).

3. Continuous treatment line of a metal strip (1) comprising a first heating chamber (102, 103) in a controlled reducing atmosphere configured to raise the strip to a first annealing temperature, a second heating chamber (105) in a controlled reducing atmosphere configured to raise the strip to a second annealing temperature, or to an aging temperature or to a tempering temperature, characterized in that it comprises a cooling chamber according to either of the preceding claims, arranged between the first and the second heating chamber.

4. Method for quenching a metal strip (1) implemented in a continuous treatment line according to the preceding claim, comprising: . a step of heating the strip to a first annealing temperature in a controlled non-oxidizing atmosphere; . optionally, a step of cooling the strip by spraying it with a non-oxidizing gas, from the first annealing temperature to a quenching start temperature; . a step of quenching the strip by spraying it with a cooling liquid, from the first annealing temperature, or the quenching start temperature, to a quenching end temperature; . a step of dewatering and drying the strip; . a step of heating the strip to a second annealing temperature, or to an aging temperature, or to a tempering temperature, carried out in a controlled non-oxidizing atmosphere.

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