Description

Technical Field

[1 ] Sheet metal strip needs to be cooled after drying and/or curing coatings applied to it, and prior to coiling. The invention relates to a cooling system that reduces the risk of having free water content in the coating after the cooling process. This is especially important in separator coatings containing magnesium oxide as applied on grain-oriented electrical steel (GOES). The invention further relates to an appropriate cooling method using such cooling systems.

Background Art

[2] Sheet metal strip of grain-oriented electrical steel is provided with a

coating during its manufacturing process. Coatings that comprise magnesium oxide are used, functioning as separator.

After its application, the coating layer is dried in a continuous drying oven, the sheet metal strip is at a temperature (typically between 130 - 160°C) which is too high for coiling the sheet metal. Therefore, the sheet metal strip must be cooled, e.g. to temperatures in the range of 40 to 80°C prior to coiling it.

[3] In state of the art processes, a cooling system is installed between the drying oven and the coiling device. The cooling system is comprising nozzles that are blowing ambient air (taken from inside or outside the building in which the production line is positioned) onto the sheet metal strip to cool it sufficiently.

[4] It is known that the presence of water in the batch annealing process of grain-oriented electrical steel - batch annealing process that follows the application of the coating and its drying - can impair electrical properties of the electrical steel (see e.g. US 3,841 ,925 and US 3,941 ,623). Patent publications describe specific magnesium oxide containing coating formulations, their preparation as well as their application in order to reduce the presence of magnesium hydroxide and of water in the coated layers ( e.g. US 3,841 ,925; US 3,941 ,623; US 3,941 ,623 and in EP-A- 0,416,420).

[5] Even when using appropriate coating formulations and coating processes, it is a problem that the coating on coils of sheet metal strip can have, locally or overall, an excessive water content.

Disclosure of Invention

[6] The primary objective of the invention is to provide a cooling system for coated sheet metal strip that results in lower and more constant water content of the coating on the sheet metal strip. The invention can advantageously be used in the manufacturing process of grain-oriented electrical steel (GOES) and particularly when coatings comprising magnesium oxide are applied to it.

[7] According to a first aspect of the invention a cooling system for the

continuous cooling of sheet metal strip is provided. The cooling system is installed downstream of a continuous oven which is drying a coating applied to the sheet metal strip. The oven can be a continuous infrared drying oven, e.g. comprising gas fired infrared emitters and preferably enhanced with air extraction and air blowing systems in the oven. The cooling system is provided with means for blowing cooling air onto the sheet metal strip. The cooling air can be ambient air (air taken from inside or outside the building), which air can be passed first through a filter to eliminate dust. The blowing of cooling air can be done by appropriate means, e.g. by means of a fan or fans that are transporting and

accelerating cooling air and that are blowing it through nozzles. Examples of nozzles that can be used are plates or beams with through holes through which the cooling air is blown onto the sheet metal strip. The cooling system comprises - for at least part of the means for blowing cooling air onto the sheet metal strip - means for increasing the

temperature of the cooling air prior to blowing the cooling air onto the sheet metal.

[8] It is possible that the cooling system has more than one means for blowing cooling air onto the sheet metal strip: - it is possible that some of these means for blowing cooling air onto the sheet metal strip are blowing cooling air that is increased in temperature by means provided for it onto the steel metal strip to cool it.

- alternatively, It is also possible that only some of the means for blowing cooling air onto the sheet metal strip are blowing cooling air that is increased in temperature by means provided for it onto the sheet metal strip to cool it; and other means for blowing cooling air onto the sheet metal strip are blowing cooling air that has not been increased in temperature onto the sheet metal strip.

[9] Preferably, the cooling system is set up in such a way that cooling air is blown onto the sheet metal strip from both sides of the sheet metal strip. Cooling systems can be arranged for sheet metal strip that runs

horizontally, for sheet metal strip that runs vertically, and for sheet metal strip that runs in any angle with the ground surface.

Use of a cooling system according to the invention allows to reduce the water content of coating on sheet metal strip, e.g. of magnesium oxide comprising coatings on grain-oriented electrical steel. The water content is not only lower, it is also more constant over a coil of sheet metal strip.

[10] An example of means for increasing the temperature is the use of one or more electrical resistance heaters to increase the temperature of the cooling air.

[1 1 ] In a preferred embodiment, the means for increasing the temperature of the cooling air are increasing the temperature by using heat extracted from the continuous oven downstream of which the cooling system is installed. This can be done, as the air in the continuous oven is at high temperature, usually more than 200°C, commonly 350°C. It is a benefit that this heat is readily available.

[12] In a further preferred embodiment, the means for increasing the

temperature is comprising a mixing device wherein cooling air (e.g.

ambient air taken inside or outside the building) is mixed with hot air, e.g. with air extracted from inside the continuous oven. When air extracted from inside the continuous oven is used the mixing device is increasing the temperature of the cooling air using heat extracted from the oven, prior to blowing the cooling air onto the sheet metal strip to cool it. Air in the continuous oven is at high temperature, usually more than 200°C, commonly 350°C. Preferably, air is extracted from the last section of the continuous oven, where air is at high temperature and at low moisture content (less than 0.09 kg water per kg of dry air, even more commonly less than 0.07 kg water per kg of dry air), advantageously to achieve low free water content in the coating after cooling.

[13] In an alternative embodiment, the means for increasing the temperature is comprising a device creating heat exchange between on the one hand fluid channels for the cooling air and on the other hand fluid channels for hot air, e.g. for hot air extracted from the continuous oven. When hot air extracted from the continuous oven is used, the temperature of the cooling air is increased by using heat extracted from the continuous oven.

[14] In another preferred embodiment, the means for increasing the

temperature is comprising a heat exchanger, in which fluid channels that are in heat exchanging contact are provided for the cooling air and for hot air, e.g. for hot air extracted from the continuous oven.

[15] The cooling system as in any of the described embodiments, can comprise different sections along the production direction of the sheet metal strip. It is possible that only some of the sections are provided with means for blowing cooling air that is increased in temperature by means provided for it, onto the sheet metal strip in order to cool it. Other sections can be provided with means for blowing cooling air onto the sheet metal strip that are blowing cooling air that has not been increased in temperature. It means design freedom when designing cooling systems for specific conditions.

[16] It is preferred when the means for blowing cooling air that is increased in temperature by appropriate means are located in the final cooling sections or in the final cooling section of the cooling system. It has been noted that blowing cooling air that is increased in temperature in the final sections of the cooling system is particularly advantageous to reduce the amount of water content in the coating layer on the sheet metal strip. [17] It is also possible to select to use or not use the means for increasing the temperature of the cooling air according to the requirements for a specific coating process: it is possible to switch off the heating system (e.g. the electrical heating) or stop the use of heat extracted from the continuous oven (e.g. stop heat exchange between air extracted from the continuous oven with the cooling air), or stop mixing air extracted from the continuous oven with the cooling air). It allows flexibility in the process, e.g. switching the means for increasing the temperature of the cooling air on or off (or control it via a control system) according to requirement (e.g. the coating process: steel sheet, coating thickness or type...).

[18] According to a second aspect of the invention, a continuous production line for coating sheet metal strip is described. The continuous production line comprises a continuous oven which is drying a coating applied to sheet metal strip, and comprises downstream of the continuous oven a cooling system as in any embodiment of the first aspect of the invention is installed.

[19] According to a third aspect of the invention a method is described for the continuous cooling of sheet metal strip. The cooling system is installed downstream of a continuous oven that is drying a coating applied to the sheet metal strip. The method is comprising the steps of

- entering the coated sheet metal strip at a temperature of more than 100°C (e.g. between 130°C and 160°C) into the cooling system,

- cooling the coated sheet metal strip to a temperature below 90°C (and preferably to a temperature between 40 and 80°C) by blowing cooling air onto the sheet metal strip by means of a cooling system as in the first aspect of the invention. The temperature of the cooling air - or of at least part of the cooling air, is increased prior to blowing it onto the steel metal strip, e.g. by using heat extracted from the continuous oven.

[20] Preferably, the sheet metal strip is coiled after cooling it.

[21 ] Preferably, the temperature of the cooling air is increased by mixing

cooling air (e.g. ambient air from taken inside or outside the building) with air extracted from the continuous oven. The cooling air is than blown onto the sheet metal strip to cool it. More preferred, less than 10 % by mass of air extracted from the continuous oven is added to the cooling air. Even more preferred less than 6% by mass of air extracted from the continuous oven is added to the cooling air. This creates appropriate conditions of the cooling air that is blown onto the sheet metal strip.

[22] In an alternative method, the increase in temperature of the cooling air is created via a heat exchanging relation between fluid channels for the cooling air and fluid channels for hot air extracted from the continuous oven.

[23] Preferably, the temperature increase of the cooling air is within the range of 5 to 25 °C.

[24] Even more preferred is the method in which the temperature increase

and/or the temperature of the cooling air is controlled, preferably by means of a feedback control system. Preferred is when the temperature of the cooling air is increased to a temperature between 20 °C and 30 °C , and more preferred to a temperature between 20°C and 25°C, as these ranges are the most appropriate temperature for the cooling air.

[25] The temperature of the cooling air can be measured, and via a feedback control system, the temperature increase of the cooling air can be controlled. When an air mixing system is used, a solenoid valve or the speed of a fan can be controlled in order to control the amount of air extracted from the continuous oven that is mixed with the cooling air.

When the increase of temperature of the cooling air is realized by means of heat exchange between on the one hand fluid channels for the cooling air and on the other hand fluid channels for hot air extracted from the continuous oven; or in a heat exchanger (wherein in the heat exchanger heat exchanging fluid channels are provided for said cooling air and for said air extracted from said continuous oven), the feedback control can e.g. be by means of controlling the amount of hot air extracted from the continuous oven that is lead to exchange heat with the cooling air.

It is a benefit of such control methods that efficient cooling while

preventing unacceptable levels of free water content can be obtained in all conditions (e.g. in different conditions of ambient air that is used as cooling air). [26] It is preferred when the free water content of the coating on the sheet metal strip after cooling is below 0.5 mass per cent of the coating material, preferably below 0.3 mass per cent of the coating material.

The water content of the coating can be obtained by weighing a sample, heating it to remove all water, cooling it in a way that no humidity can be taken up and re-weighing the sample

[27] The cooling system and the method for cooling can advantageously be used in processes where the sheet metal strip is a grain-oriented electrical steel strip. Another advantageous use is where the coating is comprising magnesium oxide (and more preferably when the coating is comprising more than 20 per cent by mass, even more preferred more than 50 per cent by mass, magnesium oxide of the dry mass of the coating), as frequently used on grain-oriented electrical steel. As residual water content in such coatings on grain-oriented electrical steel is very negative in further processing, grain-oriented electrical steel coated with such coatings are therefore benefiting particularly from measures taken to reduce the water content.

Brief Description of Figures in the Drawings

[28] Figure 1 shows a continuous production line for coating sheet metal strip, comprising a cooling system according to first aspect of the invention. Figure 2 shows an embodiment of the invention that includes a control system.

Mode(s) for Carrying Out the Invention

[29] Figure 1 shows a continuous production line for coating sheet metal strip.

The final section of a continuous oven 105 is shown in which a coating layer on a sheet metal strip 1 10 is dried. The continuous oven 105 is followed by a cooling system 1 15. After cooling it, the sheet metal strip 1 10 is coiled at a coiling device 1 18.

[30] In a first example of the invention, hot air 120 is extracted from in the

continuous oven 105. A volume fraction 125 of the hot air is evacuated and another volume fraction 130 of the hot air 120 is exchanging heat with cooling air 132 (air taken from the ambient, from inside or from outside the building) in a heat exchanger 134. After having passed the heat exchanger 134, the air extracted from the oven is evacuated, e.g. sent through a chimney, after treating it. The cooling air 132 is blown by a fan 138 (and via nozzles, not shown on the figure) onto the sheet metal strip 1 10 in order to cool the sheet metal strip 1 10. As an example the hot air 120 extracted from the continuous oven 105 is at a temperature of 350°C. Cooling air 132 is taken from inside the building, e.g. at a temperature of 15°C. Via heat exchange from the volume fraction of hot air 130 to the cooling air 132, the temperature of the cooling air 132, is increased to e.g. 25°C and this cooling air is blown onto the sheet metal strip 1 10 in order to cool it.

[31 ] In a second example, cooling air 142 (ambient air, e.g. taken from inside or outside the building) is heated by a heating device, e.g. by an electrical heater 145 and subsequently blown by a fan 148 onto the sheet metal strip 1 10 in order to cool the sheet metal strip 1 10.

[32] In a third (and preferred) example, hot air 150 is extracted from in the

continuous oven 105. Cooling air 152 is taken (e.g. ambient air from inside or outside the building) and mixed in a mixing device 156 with a volume fraction of hot air 150 that is extracted from in the continuous oven. A fan 158 is blowing the cooling air 157 onto the sheet metal strip 1 10 in order to cool the sheet metal strip.

[33] Figure 2 shows a more detailed embodiment of the invention. Figure 2 shows the final section of a continuous oven 205 in which a coating layer on a sheet metal strip 210 is dried. Installed downstream of the continuous oven 205 are a number of cooling systems 212. After cooling, the sheet metal strip 210 is coiled at a coiling device 214. Preferably, gas fired radiation driers 220 are used to dry the coating on the sheet metal strip 210 in the continuous oven 205. In the continuous oven 205 extraction devices 222 are installed that extract hot air 224. Transport of the extracted hot air 224 is done by one or more mass transfer fans 226. A first fraction 228 of the extracted hot air is blown onto the sheet metal strip 210 in the continuous oven 205 by means of blowing systems 230, in order to enhance the drying process and making the use of the gas fired radiation driers 220 more energy efficient. Another fraction 232 of the extracted hot air is evacuated and fraction 234 is used in the cooling systems 212 (in each of them, or in some of them, but preferably in the last cooling system prior to the coiling unit 214) to heat cooling air 240. Cooling air 240 is taken (e.g. ambient air) and after filtering it in a filter unit 242 it is mixed with the fraction 234 of the extracted hot air, the cooling air 250 is transported further by a fan 244. Via blowing devices 252 the cooling air 250 is blown onto the sheet metal strip 210 in order to cool it. A feedback control system can be provided that - via the measurement 260 of the temperature of the cooling air 250, controls a solenoid valve 262 to control the volume fraction of extracted hot air 234 that is mixed with the cooling air 240.

[34] Typical widths of sheet metal strip that is treated is between 0.75 meter and 1 .68 meter. Typical line speeds of the sheet metal strip through the cooling system are 80-120 m/min.

[35] In an example of the invention, a mixing device is provided to mix cooling air with air extracted from the continuous oven. As an example, 100 kg/h of cooling air is taken from inside the building at a temperature of 12°C and with 0.01 kg water content per kg of dry air. Added to the cooling air in the mixing device, is 5 kg/h of air extracted from in the continuous oven, this air is at 300°C and contains 0.07 kg of water per kg of dry air. The air mixture is blown onto the sheet metal strip to cool it. It means a volume of cooling air of 105 kg/h at a temperature of 25.7°C and with a water content of 0.013 kg of water per kg of dry air.

[36] In another example of the invention, a mixing device is provided to mix cooling air with air extracted from the continuous oven. As an example, 100 kg/h of cooling air is taken at a temperature of 12°C and with 0.01 kg water content per kg of dry air. Added to the cooling air in the mixing device, is 3 kg/h of air extracted from in the continuous oven, this air is at 300°C and contains 0.07 kg of water per kg of dry air. The air mixture is blown on to the sheet metal in order to cool it. It means a volume of cooling air of 103 kg/h at a temperature of 20.4°C and with a water content of 0.012 kg of water per kg of dry air.

The result of using such cooling systems and such methods was a grain- oriented electrical steel strip with lower and more constant water content. In the example, the temperature of the cooling air can be measured before blowing it on the metal sheet. By controlling the volume of air extracted from the oven the temperature of the cooling air can be controlled, e.g. to stay below 30°C, or e.g. to stay below 25°C, but e.g. to stay above 20°C.

[38] Different embodiments of the invention and different features of the

examples may be combined, such combinations staying within the invention.