MXPA99006967A - How to avoid contact between oxygen and molten metal - Google Patents

Abstract

According to the suggested method for avoiding contact between oxygen and molten metal during the casting process, said metal (20) pours in the form of billet into a casting chamber limited by walls (1, 2, 13). In order to avoid any contact between oxygen and molten metal and thereby to entirely prevent reoxydation, the oxygen trying to penetrate between the walls and/or adhering to them (1,2) is sucked up through possible splits (18).

Description

METHOD TO PREVENT THE CONTACT OF OXYGEN WITH A CAST METAL The invention relates to a method for preventing the contact of oxygen with a molten metal during continuous casting, when a molten metal flows into a casting chamber surrounded by walls and leaves this chamber as a current, and also an apparatus for implementing the method. In continuous casting, the molten metal accumulates in the casting chamber and has to be protected against re-oxidation and its bath surface must be protected against high heat loss by radiation. In conventional continuous casting, the surface of the bath is covered with foundry powder or with oil for this purpose. For the casting of thin strips, several casting processes are known in which the casting chamber is formed, not by rigid walls, but by a wall which moves with the current or a plurality of walls which move with the current, for example using a track chain as described in EP-A-0 526 886 or a roller as described in EP-A-0 568 211 or EP-BO 040 072 or counter-rotating casting rolls as described in US-A 4,987,949 or EP-BO 430 841, REF .: 30635 are known. In those methods, it is not possible to reliably protect the molten metal against reoxidation or heat loss by means of a foundry or oil powder as is usually the case for foundry chambers or foundry molds having rigid walls. EP-B-O 430 841 describes, in the case of the two roll casting unit, protecting the bath surface against excessively high heat loss by radiation and against re-oxidation by the provision of a protective hood. However, for this solution it has been found that severe wear occurs on the contact surfaces between the protective hood and the casting rollers both on the protective hood and on the cast rolls and that, as a result of the thermal deformation of the components, the entry of air and thus oxygen through the gaps between the walls surrounding the casting chamber can not be avoided. This results in the reoxidation of the melt with all its disadvantages. To minimize the entry of air through the gap between the protective hood and the cast rolls, US-A-4, 987, 949 and EP-A-0 714 716 propose blowing an inert gas, preferably nitrogen or argon, in a space defined between the protective hood and the casting rollers and in this way produce a barrier against the intrusion of air. However, this measure is not sufficient to completely prevent the air from entering the casting chamber and thus reaching the bath surface, so that, on the other hand, metal oxides still form on the bath surface and those lead to defects in the interior of the metal strip. On the other hand, metal oxides form on the surface of the solid layer that forms around the stream or oxygen diffuses into the outer layer of the metal strip and therein forms inclusions which increase the susceptibility to fractures. In spite of the inert gas feed, the air entering the micro-roughness of the surface of the roller is carried to the melting chamber in the laminar sub-layer of the flow limit layer. This sub-layer adheres to the micro-roughness of the surface of the roller and can not be separated by contact seals, slideable by seals without contact. The invention makes it possible to avoid these disadvantages and difficulties and has the object of providing a method of the type described at the beginning of an apparatus for continuous casting by means of which the contact of oxygen with the molten metal, method and apparatus which prevent Completely reoxidation even when considerable wear occurs in the gaps between the walls that form the casting chamber. In particular, it will also be possible to remove the laminar sublayer from the layer of air carried or adhered to the walls that form the casting chamber. This object is achieved, in a method of the type described at the beginning, by oxygen which attempts to enter via any gaps between the walls and / or adhere to the walls being removed by suction. Particularly efficient removal of oxygen can be achieved advantageously by carrying out the suction removal in a plurality of extraction stages arranged one behind the other from the outside to the casting chamber, with the suction removal occurring advantageously at a pressure which decreases from stage stage to outside in the melting chamber. Here, according to a preferred embodiment, the suction pressure in the extraction stage close to the casting chamber is set below 50 mbar, preferably below 10 mbar. To ensure equalization of the pressure with the casting chamber, it is advantageously permitted for an inert gas to flow against the wall surrounding the casting chamber directly adjacent to the extraction zone near the casting chamber. melting chamber, with the pressure of the inert gas being advantageously at least 10 mbar, preferably greater than 200 mbar, above the pressure of the adjacent extraction stage. Preferably, the inert gas is blown against the wall in a plurality of inert gas stages arranged one after the other from outside to inside the melting chamber. The inert gas is blown, advantageously, on the wall at a speed of at least 0.5 m / s, not more than 10 m / s, preferably more than 2 m / s. In a casting process in which at least one wall moves relative to the casting chamber, the new regions of this wall that are located around the entrance to the casting chamber are, according to a preferred embodiment, released. of oxygen adhered by the removal of oxygen by suction before entry. The continuous casting is then carried out, advantageously, by means of a roller casting process, preferably by a two roll casting process, that is, the roll casting processes using only a single casting roller , as described, for example, in EP-BO 040 072, are also possibilities for the application of the method of the invention. Of course, the method of the invention can also be applied in the melting of a molten metal or any body in mobile cooling, for example a crawler chain as described in DE-A-36 02 594. Sometimes it is also advantageous that the foundry molds have rigid walls, for example, if the application of a foundry powder is not possible or is very complicated. An apparatus by means of which the contact of oxygen with the molten metal can be prevented during continuous casting, in which the casting chamber surrounded by walls is filled with molten metal and a current leaves the casting chamber through a space of casting of the casting chamber, characterized in that any spaces present between the adjacent walls are provided with an extraction device to try to get oxygen through the space and / or adhere to the walls. In an apparatus for the continuous casting of a metal strip, preferably a steel strip, having two counter-rotating cast rolls with parallel roller axes and two ladies or side channels which together form a casting chamber to accommodate metal cast and that have a protective hood which is located above the casting chamber and closes the latter at the top, and also has a sealing device which prevents the entry of air into the casting chamber as length of a gap or space formed by the protective hood and the rotating cast iron rollers, the object of the invention is achieved, advantageously, by sealing the device formed by at least one suction chamber, located on the side of the atmosphere in the vicinity of the space between the rotating casting rollers and the protective hood and that it extends parallel to the axis of the rollers. This sealing device is particularly effective when a plurality of suction chambers arranged one near the other in the circumferential direction of the casting rolls are produced. Here it is advantageous that each suction chamber is connected via a suction line to an associated suction pump or a stage of a multi-stage suction pump. According to a modality which is simple from the point of view of construction, the sealing device is configured with a system of sequential multiple chambers. As a result of this measurement, the suction pressure decreases from the suction chamber to the suction chamber in the direction of movement of the cast rolls. Proper adjustment of the number of suction chambers to the circumferential speed of the cast rolls makes it possible to achieve a complete removal of the air carried along with the rollers. According to an improved embodiment, the sealing device is located at a defined distance from the surface of the casting roller and the space formed by the sealing device and the surface of the casting roller is sealed, at least on the inlet sides and output, by means of contact seals, preferably brush seals or rubber lip seals. In this way, the entrance of the air is largely limited to the air carried along with the boundary layer even before the first suction chamber. According to a further embodiment, at least one of the suction chambers is additionally equipped with an inert gas purge. The apparatus is improved by locating the inert gas supply device between the protective hood and the extraction device, with this inert gas supply device being configured as a reduced pressure chamber having an opening directed towards the casting rolls. The opening is advantageously configured as a nozzle which is directed obliquely to the surface of the casting roll and is angled to the adjacent suction chamber. By means of this measurement, a layer of inert gas is applied near the roller to the casting roller and in this way an excellent protection against the access of oxygen or air is produced. If a layer of inert gas of a few millimeters thick is applied to the casting roller and an inert gas is used, which has a higher density than that of the air, it is not necessary to directly connect the protective hood to the line of protection. Inert gas feed and extraction device. The additional features and advantages can be seen from the following description of the apparatus and the method of casting a metal strip in a plurality of embodiments: Figure 1 shows a cross section through the two-roll casting plant with a sealing device according to the first modality; Figure 2 shows a second embodiment of the sealing device according to the invention; and Figure 3 shows a third third embodiment of the sealing device according to the invention. The two-roll casting plant as shown schematically in section in Figure 1, has two casting rolls 1, 2 whose parallel roller axes 3, 4 are located in a horizontal plane. The two counter-rotating casting rolls 1, 2, in the direction of the arrows 5, 6, are provided with internal cooling (not shown) to move the casting roll forming the surface of the casting roll 7. On the end faces , ladies or side channels 8 are arranged sufficiently close to the casting rolls 1, 2. The casting rolls 1, 2 and the females or lateral channels 8 form a casting chamber 9 into which the melt 20 is introduced from a melt distributor container or container (not shown) via a feed nozzle 10 provided with outlet openings 11, the melt 20 which forms an accumulation of melt 12. The casting chamber 9 is surrounded at the top, relative to the casting rolls 1, 2 and in relation to the ladies or side channels, by a protective hood 13 which has a refractory lining 14 on the side of the melt to protect the melt 20"from excessively large heat losses and against reoxidation by atmospheric oxygen. By means of a support device 15 for the protection hood 13, which is adjusted in relation to a stationary frame 16 by means of adjusting elements 17, a desired minimum space 18 is fixed between the protective hood 13 and the casting rolls 1, 2. The protective hood 13 is penetrated by the feed nozzle 10, with a very small annular space which is possibly covered by a seal, provided between those two components. By using a two-roll casting plant having this configuration, it is possible to melt a thin strip of metal, in particular a steel strip having a thickness of 1 mm to 12 mm, with the melt 20 to be melted being continuously introduced. , as described above, in the casting chamber 9. In the contraction and cooled casting rolls 1, 2, there is the increasing formation of current layers which are, in the narrowest cross-section of the casting rolls, attached to a strip formed by the casting rolls. The thickness of the strip transported outwards by the casting rolls is determined by the separation of the casting rolls. In order to prevent the entry of air into the casting chamber along a space 18 formed by the protective hood 13 and the rotating casting rollers 1, 2, a sealing device 23 is formed by means of a suction chamber 24 located on the side of the atmosphere in the vicinity of the space 18 and a small distance from the surface of the casting roller 7. The suction chamber 24 is open in the direction of the surface of the casting roller 7 and is connected to a line of suction 25 and a suction pump which is not shown. The suction chamber 24 is formed in a simple manner by means of a U-shaped profile and which is open in the direction of the surface of the casting roller and extends parallel to the roller shafts 3, 4 at a small distance from the surface of the casting roller over the entire length of the casting roller. The space between the suction chamber 24 and the surface of the casting roller 7 is covered by the seals 27 which are fixed to the legs of the U-shaped profile and in contact with the surface of the casting roller -7 and are preferably configured as brush seals or rubber lip seals. According to a further embodiment, as shown in Figure 2, the sealing device 23 is formed by a plurality of suction chambers 24 arranged one after the other in a circumferential direction of the casting rolls 1, 2 and each Suction chamber is connected via an associated suction line 25 to, in each case, a stage of the multi-stage suction pump which is not shown. This sealing device 23 configured as a sequential multiple chamber system makes it possible to remove the air introduced in a plurality of extraction stages at a pressure of the chamber which decreases in the stages in the direction of rotation of the casting rolls. The pressure of the last suction chamber 31 in the direction of rotation of the casting rolls is set to achieve optimum air extraction at a value d < = less than 50 mbar, preferably less than 10 mbar. In the embodiment shown in Figure 1, an inert gas supply device 28, formed by a reduced pressure chamber 29 and having an opening 32 directed towards the surface of the casting roller, is additionally arranged between the suction chamber 24 and the protective hood 13. In addition, is connected to a supply line 30 for the inert gas. In detail, the reduced pressure chamber 29 has the same construction as the suction chamber 24 and, to prevent erroneous entry of air, both are combined in a common construction unit. For the same reason, the reduced pressure chamber 29 is attached in an air-tight manner to the protective hood 13. In the embodiment shown in Figure 2, the reduced pressure chamber 29 has an outlet opening directed towards the surface of the casting roll 7, with the reduced pressure chamber being combined with the sequential multiple chamber system of the sealing device 23 to form a building unit, and sealed to prevent the ingress of air. The inert gas is introduced into the reduced pressure chamber 29 to build an inert gas flow boundary layer on the surface of the casting roller 7. This inert gas is introduced instead of the air in the melting chamber 9 through the space 18 between the casting rolls 1, 2, and the protective hood 13. For this purpose, it is sufficient that the pressure in the reduced pressure chamber 29, is set at a value of at least 10 mbar, more preferably greater than 200 mbar, above the pressure of the extraction device 23 above.

Figure 3 shows an embodiment in which a plurality of reduced pressure chambers 29, which are connected to a common inert gas supply device 28, are arranged inside a suction chamber 24 for the extraction of fresh air connected to a suction line 25. In the reduced pressure chamber 29a, which is the last in the direction of rotation of the casting rolls, is directly connected before the protective hood 13, the pressure of the inert gas is, in contrast with the preceding reduced pressure chambers 29, set to a value higher than atmospheric pressure. The final residues of atmospheric oxygen that adhere to the surface of the casting roll 7 in the boundary layer, can be removed by means of inert gas purge, if the inert gas is blown directly against the surface of the casting roll 7, purpose for which the flow velocity is set to at least 0.5 m / s, preferably more than 2 m / s. Flow rates of more than 10 m / s have no additional effects. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (21)

1. CLAIMS Having described the invention as above, the content of the following claims is claimed as property. 1. A method to prevent contact of oxygen with a molten metal during continuous casting, in which the molten metal flows into a foundry chamber surrounded by walls, and leaves this chamber as a current, characterized by the oxygen that attempts to enter via any gaps or spaces between the walls and / or adhere to the walls, it is removed by suction.
2. 2. The method of compliance with the claim 1, characterized in that the suction removal is carried out in a plurality of extraction layers arranged one behind the other inside the casting chamber.
3. 3. The method of compliance with the claim 2, characterized in that the suction removal is carried out at a pressure, which decreases from stage to stage from outside to inside the casting chamber.
4. 4. The method according to claim 2 or 3, characterized in that the suction pressure in the extraction stage near the casting chamber is set below 50 mbar, preferably below 10 mbar.
5. 5. The process according to any of claims 1 to 4, characterized in that the inert gas is allowed to flow against a wall surrounding the casting chamber directly adjacent to the extraction zone near the casting chamber.
6. The method according to claim 5, characterized in that the pressure of the inert gas is at least 10 mbar, preferably greater than 200 mbar, above the pressure of the adjacent extraction stage.
7. The method according to claim 5 or 6, characterized in that the inert gas is blown against the wall in a plurality of inert gas stages arranged one after the other from outside to inside the combustion chamber.
8. The method according to any of claims 5 to 7, characterized in that the inert gas is blown against the wall at a speed of at least 0.5 m / s, not greater than 10 m / s, preferably at the most of 2 m / s.
9. The method according to any of claims 1 to 8, characterized in that at least one wall moves relative to the casting chamber and the new regions of this wall, which are near the entrance of the casting chamber, are released from the oxygen adhered by the removal of oxygen by suction before entry.
10. The method according to claim 9, characterized in that the continuous casting is carried out by roll casting, preferably by casting with two rollers.
11. 11. An apparatus for preventing the contact of oxygen with a molten metal during continuous casting, in which a casting chamber surrounded by walls is filled with molten metal and a current leaves the casting chamber through a casting space of the casting chamber, characterized in that in any spaces present between the adjacent walls there is provided an extraction device for the oxygen that attempts to enter via the space or gap and / or adhere to the walls.
12. 12. The apparatus according to claim 11, for the continuous casting of a metal fabric, preferably a steel cloth, having two counter-rotating cast rolls with parallel roller axes and two females or channels, which, together form a casting chamber for accommodating molten metal and having a protective hood, which is located above the casting chamber and closes the latter at the top, and which also has a sealing device, which prevents the entry of air in the casting chamber along a space formed by the protective layer and the rotating casting rollers, characterized in that the sealing device is formed by at least one suction chamber, located on the side of the atmosphere in the vicinity of the space between the rotating casting rollers and the protective hood and extending parallel to the axis of the roller.
13. The apparatus according to claim 11 or 12, characterized in that said sealing device is constituted by a plurality of suction chambers arranged one after the other in the circumferential direction of the casting rolls.
14. The apparatus according to any of claims 11 to 13, characterized in that each suction chamber is connected via a suction line to an associated suction pump or a stage of a multi-stage suction pump.
15. 15. The apparatus according to any of claims 11 to 14, characterized in that the sealing device is configured as a sequential multiple chamber system.
16. The apparatus according to any of claims 11 to 15, characterized in that the sealing device is arranged at a defined distance from the surface of the casting roller and the - space formed by the sealing device and the surface of the casting roller is sealed, at least on the sides of Entrance and exit, by means of contact seals, preferably brush seals or rubber lip seals.
17. 17. The apparatus according to any of claims 11 to 16, characterized in that at least one of the suction chambers is additionally equipped with an inert gas purge.
18. 18. The apparatus according to any of claims 11 to 17, characterized in that the inert gas supply device is arranged between the protective hood and the suction chamber.
19. The apparatus according to claim 18, characterized in that the inert gas supply device is configured as a reduced pressure chamber having an opening directed towards the surface of the casting roller.
20. 20. The apparatus according to claim 19, characterized in that the opening is configured as a nozzle, which is directed obliquely to the surface of the casting roll and is at an angle to the adjacent suction chamber. The apparatus according to any of claims 11 to 20, characterized in that the laminar seal is located between the suction chamber and the protection hood.