Filtration system

Abstract

The present patent relates to a method for cooling a traveling coated steel strip leaving a hot dip coating bath, the cooling method comprising the following steps: A) sucking gas into a cooling device, B) filtering the sucked gas by means of a filter system that captures at least 50% of particles having a size of at least 2.5 μm, C) cooling the sucked and filtered gas at a speed of 1 m.s. ‑1 Up to 80m.s ‑1 The coating is blown onto the coated steel strip at a speed of .

Description

[0001] The invention relates to a method, a cooling device and a cooling tower for cooling a steel strip leaving a hot dip coating bath.

[0002] Nowadays, most steel products are coated to enhance the properties of the steel products, especially the surface properties of the steel products. Figure 1 As shown in FIG, one of the most common continuous coating processes is hot dip coating, in which a steel product S (e.g., strip, ribbon, or wire) to be coated is passed through a bath of molten metal 1 contained in a tank 2, which coats the surface of the steel product. After leaving the coating bath, the coated steel strip S passes between air knives 3, which allow the coating thickness to be adjusted. The steel strip then enters a cooling tower 4, where filtered gas 5, typically atmospheric air, is blown onto the coated strip via a distribution chamber 6 in order to cool the strip to the desired temperature.

[0003] However, it has been observed that galvanized steel strips coated with magnesium, aluminum and zinc exhibit dark spots 7 on the strip surface, such as Figure 2 These surface defects usually appear between the inlet and outlet of the cooling tower. It is acknowledged in the literature that for coating baths comprising magnesium and zinc, the presence of dark spots is due to the presence of Mg2Zn on the strip surface. 11 Instead of native Zn and MgZn2.

[0004] Dark spots are circular defects that are particularly present on the coating surface and have a diameter of 100 μm to 50 mm. Dark spot defects are bright only after the steel is coated and tend to dim thereafter in subsequent processes. That is why those dark spots are also called bright spots. Dark spots usually include Zn 11 Mg2 phase. In addition, Zn 11 Mg2 is typically located at the extreme surface of the defect, with a strike zone appearing in the middle of the defect. Dark spots are also referred to in the literature as "freckles," "spot tours," "Sommersprosse," or "punto brillante." The thicker the steel product, the more dark spots are present on the surface.

[0005] JP 10 226 865 discloses a method for avoiding dark spots on coated strips. In this hot-dip method for Zn-Al-Mg coated sheet, the coating bath temperature is between its melting point and 450°C, and the coating cooling rate is limited to 10°C·s -1 Alternatively, the coating bath may be at a temperature above 470°C and the coating cooling rate at least 0.5°C·s -1 .

[0006] US 6,379,820 B1 discloses a method for increasing the formation of MgZn2 and thereby reducing the formation of dark spots. In this method, a hot dip coating comprises Al: 4.0 wt.% to 10 wt.%, Mg: 1.0 wt.% to 4.0 wt.% and the remainder is Zn and unavoidable impurities, and the hot dip coating has a bath temperature not lower than the melting point and lower than 470°C. Preferably, the bath has a Ti content of 0.002 wt.% to 0.1 wt.% and a B content of 0.001 wt.% to 0.045 wt.% to suppress the formation of Mg2Zn 11 In addition, the process has a curing temperature of not less than 10°C until the coating is cured. -1 cooling rate.

[0007] EP 2 634 284 A1 discloses a system for reducing Mg2Zn due to the ability to direct the wiping gas towards the bath and thus avoid Zn splashing onto the strip. 11 Nucleation method.

[0008] The inventors tried to identify Mg2Zn 11 Another cause of nucleation is addressed and the present invention reduces the formation of dark spots on the coated steel strip during cooling after the coated steel strip leaves the hot dip coating bath.

[0009] This object is achieved by providing a cooling method according to any one of claims 1 to 3. This object is also achieved by providing a cooling device according to any one of claims 4 and 8.

[0010] Other features and advantages will become apparent from the following detailed description of the invention.

[0011] To illustrate the present invention, various embodiments will be described with particular reference to the following drawings:

[0012] Figure 1 is an embodiment of a hot dip coating apparatus including a cooling tower.

[0013] Figure 2 This is a picture of a steel strip with dark spots.

[0014] Figure 3 is an embodiment of a hot dip coating apparatus comprising a cooling device according to the present invention.

[0015] Figure 4 This is a first embodiment of the cooling device according to the present invention.

[0016] Figure 5 is a second embodiment of a hot dip coating apparatus comprising a cooling device according to the present invention.

[0017] Figure 6is a third embodiment of a hot dip coating apparatus comprising a cooling device according to the present invention.

[0018] In the following, upstream and downstream are expressed relative to the strip movement.

[0019] like Figure 3 As shown in FIG, the present invention relates to a method for cooling a traveling coated steel strip S leaving a hot dip coating bath 1, the cooling method comprising the following steps:

[0020] A) pumping the gas into the cooling device 8,

[0021] B) filtering the sucked gas by means of a filter system 9 which captures at least 50% of the particles having a size of at least 2.5 μm,

[0022] C) The sucked and filtered gas 5 is sent to the atmosphere at 1 m.s -1 Up to 80m.s -1 The coating is blown onto the coated steel strip S at a speed of .

[0023] This cooling method can be used in Figure 3 wherein a cooling tower 4 is positioned relative to the strip movement downstream of a hot dip coating tank 2 containing a hot dip coating bath 1. The hot dip coating bath 1 is a molten metal bath comprising a mixture of several elements such as zinc, aluminum, silicon and / or magnesium.

[0024] The cooling tower 4 generally comprises at least one cooling device 8, which comprises at least two distribution chambers (6a and 6b) arranged on both sides of the traveling strip S, a suction device 10 and a filtering system 9. Each distribution chamber comprises an opening, which may be a slot, a nozzle or a point-shaped opening. The opening faces the traveling strip so that the gas 5 leaving the distribution chamber impacts the traveling coated steel product S, such as a strip. The distribution chamber can be set so that the impact of the jet from one module is opposite to the jet of another module, or so that the impact of the jet of gas on each surface of the strip is distributed at the nodes of a two-dimensional network and is not opposite to the impact of the jet on the other side, such as described in EP 2 100 673B1. In addition, an air knife 3 can be positioned between the cooling tower 4 and the hot dip coating tank 2, thereby allowing the coating amount and coating thickness of the coated steel strip to be controlled. In addition, as described in EP 2 100 673B1 Figure 4 As shown in FIG, the distribution chamber 6 is able to blow the filtered gas along the entire strip width.

[0025] Gas 50 (e.g. atmospheric air) is drawn into the cooling device 8 by a suction device 10 (e.g. a fan), and the gas 50 passes through a filter system 9. Alternatively, the gas may come from a tank. Thus, the gas is filtered by the filter system 9 having at least the performance of a PM2.5 filter.

[0026] The filter performance mentioned in this patent is based on the ISO 16980 standard. A filter with the performance of a "PM2.5" filter captures at least 50% of particles with a size of at least 2.5 μm. A filter with the performance of a "PM1" filter captures at least 50% of particles with a size of at least 1.0 μm. Furthermore, if a filter's efficiency in capturing particles of a certain size is higher than 50%, its efficiency is rounded up to the nearest 5% and added to the filter name. For example, if a filter captures 71% of particles with a size of at least 1 μm, it is designated ePM1 70%.

[0027] Finally, the filtered gas is blown onto the traveling steel belt through the opening of the distribution chamber 6, resulting in a gas jet 5 with a speed of 1 m.s. -1 Up to 80m.s -1 The strip is impacted at a speed of 100 Å and thus cools the strip.

[0028] Therefore, when using the claimed cooling method, the air blown onto the travelling strip is free of most particles and has no accumulation of particles larger than 2.5 μm. This results in a significant reduction in the presence of dark spots on the strip, as explained in the experimental results section.

[0029] Preferably, the air drawn through the filter system has a maximum velocity of 1.5 ms, capturing at least 50% of particles having a size of at least 2.5 μm. -1 This allows to further increase the efficiency of the filtration system.

[0030] Preferably, the running strip has a thickness of 0.2 mm to 10 mm. It has been observed that this method is particularly advantageous for thick strips, as thick strips are strips that are more prone to forming dark spots. Even more preferably, the running strip has a thickness of 4 mm to 8 mm.

[0031] Preferably, the hot-dip coating bath comprises 1 to 5 weight percent magnesium, 0.8 to 20 weight percent aluminum, with the remainder of the composition consisting of zinc and unavoidable impurities resulting from refining. Preferably, the hot-dip coating bath comprises at least 1 weight percent aluminum, and even more preferably at least 1.8 weight percent aluminum. Preferably, the hot-dip coating bath comprises at most 12 weight percent aluminum. Even more preferably, the hot-dip coating bath comprises at most 6 weight percent aluminum. Preferably, the hot-dip coating bath comprises less than 0.5 weight percent, and even more preferably less than 0.3 weight percent, of each of the following elements: boron, cobalt, chromium, copper, molybdenum, niobium, nickel, vanadium, sulfur, and titanium.

[0032] Preferably, in step A), the pumped gas is a pure gas or a mixture of gases. The pumped gas may be atmospheric air, or a mixture comprising nitrogen and hydrogen, or any other gas mixture.

[0033] Preferably, in step B), the filtering system has at least the performance of a PM1 filter.

[0034] Even more preferably, in step B), the filtration system has at least the performance of an ePM1 65% filter. Such an ePM1 65% filter captures at least 63% of particles having a size of at least 1 μm. The inventors have found that not only particles larger than 10 μm are beneficial for nucleation, but also particles larger than 1 μm are beneficial for Mg2Zn 11 This is explained in the experimental results section.

[0035] Preferably, in step B), the filter system has at least the performance of an ePM1 80% filter. Such an ePM1 80% filter captures at least 78% of particles having a size of at least 1 μm.

[0036] Preferably, in step C), the coated steel strip has a coating that is liquid. This means that the coating can be considered a liquid coating, i.e., the coating is not solid. Obviously, the impact of particles on the liquid coating can even more effectively trigger the appearance of dark spots.

[0037] Preferably, between step A and step B, the cooling method comprises a step of filtering the sucked gas by means of a filter system 9 capable of capturing less than 50% of particles having a size of at least 10 μm. Such a step allows pre-filtration of the gas filtered in step B and prolongs the life of the filter system 9, which has at least the performance of a PM2.5 filter.

[0038] like Figure 3 and Figure 4 As shown in , the present invention also relates to a cooling device 8 of a cooling tower 4, which comprises a filtering system 9 capable of capturing at least 50% of particles having a size of at least 2.5 μm, a suction device 10 and at least one distribution chamber 6 comprising an opening, wherein gas can be filtered by the filtering system 9 and can be blown through the opening of the distribution chamber, and the cooling device 8 is capable of performing the method previously explained.

[0039] The claimed cooling device 8 can be used in a cooling tower 4 of a hot dip coating plant.

[0040] The cooling device comprises a duct 17 connecting its different parts so that all the blown gas is filtered. Figure 4The middle figure shows a duct 17 connecting the filter system 9 to the suction device 10 and connecting the suction device 10 to the distribution chamber 6. With respect to the gas movement, the suction device is positioned downstream of the filter system and upstream of the distribution chamber 12. The suction device 10 can be a fan.

[0041] Preferably, if Figure 5 As shown in , the cooling device comprises a suction damper 15 capable of regulating the flow rate of the blown gas. In this case, the suction damper 15 is positioned downstream of the filtering system and upstream of the suction device relative to the gas movement.

[0042] Preferably, if Figure 4 As shown in FIG, the cooling device 8 includes two distribution chambers, which are arranged on both sides of the running area of ​​the steel strip and can blow filtered gas toward the running area of ​​the steel strip.

[0043] Preferably, if Figure 6 As shown in , the cooling device 8 includes two to ten distribution chambers, which are arranged on both sides of the running area of ​​the steel strip and can blow filtered gas toward the running area of ​​the steel strip.

[0044] Preferably, the filter system 9 of the cooling device 8 has at least the performance of a PM1 filter. Even more preferably, the filter system 9 has at least the performance of an ePM1 65% filter. Even more preferably, the filter system 9 has at least the performance of an ePM1 80% filter. Obviously, such a filter system allows for an even greater reduction in the presence of dark spots on the coated steel strip.

[0045] Preferably, the filtering system 9 comprises at least one bag filter. Preferably, the filtering system comprises at least one rigid type filter made of glass fiber paper or nanofibers.

[0046] Preferably, the filter system 9 of the cooling device 8 includes at least one first filter device capable of capturing at least 50% of large, coarse particles, and at least one filter device positioned downstream of the first filter device capable of capturing at least 50% of particles having a size of at least 2.5 μm. In this particular case, downstream should be understood relative to the path of the blown gas. This significantly increases the lifespan of the PM2.5 filter.

[0047] Preferably, said filtering system 9 of the cooling device 8 comprises at least one filtering device capable of capturing at least 50% of particles having a size of at least 2.5 μm and at least one filtering device having at least the performance of a PM1 filter or an ePM1 65% filter or an ePM1 80% filter.

[0048] Experimental results

[0049] Already in Figure 5 The experiments were carried out in a hot-dip coating apparatus represented in FIG. , which comprises a hot-dip coating tank 2 filled with a molten metal bath 1 comprising 3.7±0.2 weight percent aluminum, 3.0±0.2 weight percent magnesium, with the remainder of the composition consisting of zinc and unavoidable impurities. The apparatus also comprises an air knife 3 and four cooling devices 8. Each cooling device comprises a filtration system, a suction device 10, a suction damper 15, and a pair of distribution chambers (6a and 6b), one on each side of the strip S. In all experiments, the strip was coated and cooled as described above.

[0050] Minimum particle size affecting the presence of dark spots

[0051] In this first experiment, to understand the influence of the size of the blown particles on the presence of dark spots, the characteristics of the blown air were varied and the number of dark spots per square meter of the steel surface was compared. The number of dark spots was counted by visual inspection to estimate their presence. In this experiment, the filtration system was capable of filtering particles larger than 300 μm.

[0052] The experiment was conducted for the following blowing gases: atmospheric air or atmospheric air loaded with 1 μm, 3 μm, 9 μm or 20 μm Al2O3 particles. The air velocity of the blowing air was 11 m.s -1 The results are summarized in Table 1.

[0053]

[0054] Table 1

[0055] The experimental results clearly show that dark spots appear on the strip surface in sections cooled by air loaded with Al2O3 particles of at least 1 μm. Furthermore, the larger the Al2O3 particles, the greater the number of dark spots per square meter. Therefore, to significantly reduce the presence of dark spots, the number of particles of at least 9 μm should be minimized. To suppress the presence of dark spots, the number of particles of at least 1 μm should be minimized.

[0056] Comparison results

[0057] In a second experiment, in order to assess the efficiency of the claimed process and apparatus, the characteristics of the filter system were varied and the number of dark spots per square meter of the steel surface was compared. The number of dark spots was counted by an automatic inspection device.

[0058] In a first series of tests, in which more than 10 rolls were produced, the filter devices were able to filter particles larger than 300 μm. In a second series of tests, in which more than 10 rolls were produced, the filter devices of the two upper cooling units were able to filter particles larger than 300 μm, and the filter devices of the two lower cooling units had the performance of an ePM1 65% filter. In a third series of tests, in which more than 10 rolls were produced, the filter devices of all four cooling units had the performance of an ePM1 65% filter.

[0059] The density of dark spots on the coated steel coils was divided into three categories based on the number of dark spots per square meter: less than 1 per square meter; 1 per square meter to 20 per square meter; and more than 20 per square meter.

[0060] In the first, second and third series the steel strips have a thickness of 4 to 6 mm.

[0061]

[0062] Table 2

[0063] *DS = Dark Spot

[0064] From the comparison results, it is clear that implementation of the claimed invention reduces the number of dark spots on the coated steel strip exiting the cooling tower.

[0065] The present invention has been described above with respect to what are presently considered to be practical and preferred embodiments. However, it should be understood that the present invention is not limited to the embodiments disclosed in this specification.

Claims

1. A method for cooling a travelling coated steel strip (S) leaving a hot dip coating bath (1), said cooling method comprising the following steps: A) pumping the gas into the cooling device (8), B) filtering the sucked-in gas by means of a filter system (9) which captures at least 50% of the particles having a size of at least 1.0 μm, C) The gas that has been sucked and filtered is heated at a speed of 1 m.s -1 Up to 80m.s -1 Blown onto the coated steel strip (S) at a speed of The hot dip coating bath (1) is a molten metal bath comprising a mixture of the following elements: zinc, aluminum, silicon and / or magnesium.

2. The cooling method according to claim 1, wherein: The hot dip coating bath includes 1 to 5 weight percent of magnesium, 0.8 to 20 weight percent of aluminum, and the remainder of the composition is made up of zinc and unavoidable impurities.

3. A cooling device (8) of a cooling tower (4), comprising a filter system (9) capable of capturing at least 50% of particles having a size of at least 1.0 μm, a suction device (10) and at least one distribution chamber (6) comprising an opening, wherein: Gas can be filtered by the filter system (9) and can be blown through the opening of the distribution chamber, and the cooling device (8) can perform the method according to claim 1 or 2.

4. The cooling device (8) according to claim 3, wherein: The cooling device (8) comprises two distribution chambers, which are arranged on both sides of a running area of ​​the steel strip and are capable of blowing filtered gas toward the running area of ​​the steel strip.

5. The cooling device (8) according to claim 3 or 4, wherein: The filtering system (9) of the cooling device of the cooling tower further comprises at least one first filtering device capable of capturing at least 50% of coarse particles having a size of at least 10 μm and at least one filtering device positioned downstream of the first filtering device capable of capturing at least 50% of particles having a size of at least 2.5 μm.

6. The cooling device (8) according to claim 3 or 4, wherein: The cooling device includes a suction damper (15) capable of adjusting the flow rate of the blown gas.

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