WO2015132237A1 - Method and plant for cooling liquid-cooled moulds for metallurgical processes - Google Patents

Abstract

In order to avoid solidification which proceeds over the meniscus of the metal mirror during the continuous casting or remelting of steels and alloys, the cooling conditions are set in such a way that, in the case of inlet temperatures of the coolant at room temperature or else considerably thereabove, outlet temperatures of the coolant of at least 80°C are achieved. Suitable coolants are pressurized water, metals which are liquid below 100°C and also ionic liquids (salt melts) which are liquid above room temperature and up to at least 200°C, it being possible for the differences in temperature between the coolant inlet and the coolant outlet in the mould to be between 5°C and 150°C.

Description

 Method and plant for cooling liquid-cooled molds for metallurgical processes

The present invention relates to an improved method for cooling liquid-cooled molds used in continuous casting and remelting plants and for the production of strands or blocks of steels and metals, e.g. Aluminum, copper, nickel and cobalt and their alloys are used. Furthermore, the invention relates to a system for carrying out the method according to the invention.

The molds used for this purpose usually consist of an alloyed or unalloyed copper insert, which is installed in a so-called "water jacket", usually consisting of a welded steel construction, the heat removal being effected by the cooling water passing through the gap between mold insert and water jacket The cooling water must be so large that a vapor bubble formation on the water side of the Kokilleneinsatzes is avoided.

In addition to the above-described, consisting of Kokilleneinsatz and cooling jacket molds, so-called plate molds made of copper are used, which either have holes through which the cooling liquid is passed or on the cooling water side have milled grooves that serve as cooling channels and by a screwed steel plate to be sealed.

In all of these prior art Kokillenarten the solidification of the liquid metal in contact with the inner wall and the removal of heat from the contact with the coolant in contact outer wall of Kokilleneinsatzes.

Such molds are in use in continuous casting, as well as in plants for remelting self-consuming electrodes either in a hot Slag bath - the electroslag remelting process - or by an arc under vacuum - the vacuum arc remelting process.

In continuous casting, the liquid metal to be cast is poured from a tundish into one or more water-cooled oscillating copper molds. The molds used are open at the bottom and can also have a certain conicity. In the mold, a first viable strand shell is formed which is continuously withdrawn from the mold. In order to minimize the friction between the strand and mold inner wall and to protect the liquid steel against the influence of the atmosphere, casting powder slags are used.

In the remelting process, an already solidified metal block (self-consumable electrode) is melted in an electrically conductive slag or under vacuum by an electric arc. The liquid steel collects in a liquid metal sump, which is located in a water-cooled mold, and solidifies in this to a remelt block. When remelting can be used on the one hand downwardly open molds (Gleitkokillen) and on the other hand down closed molds (Standkokillen). Depending on the embodiment, the molds may have a certain conicity. When using a Gleitkokille the Umschmelzblock is pulled down from the mold by means of a movable bottom plate. There are also plants in operation which are equipped with an upwardly movable mold and a fixed base plate. In the above-mentioned processes, the first solidification of the liquid metal takes place in direct contact with the copper mold or in contact with a solidified slag layer, which can form between the strand or remelt block and the mold. The formation of a solid slag layer is mainly dependent on the casting or remelting parameters as well as the shrinkage behavior of the steel quality. According to the prior art, the molds for continuous casting and remelting are cooled by circulating water of a conventional water cycle, which usually consists of a cooling water storage tank, a pump set and a heat exchanger and valves, valves, measuring and control devices for the purpose of controlling and controlling the water flow.

Water usually has a comparatively low operating temperature. By way of example, a maximum flow temperature of 50-60 ° C at a temperature difference between the cooling water supply and return temperature (ΔΤ) of about 10 ° C can be mentioned here. This low temperature of the water leads to a strong cooling effect due to the self-adjusting high temperature gradient between steel and mold inner wall. This strong cooling effect can lead to a solidification of the meniscus of the liquid steel at the boundary line liquid metal and atmosphere or slag. If the already solidified meniscus overflowed by local flow turbulences in the liquid steel or by an increase of the liquid metal mirror in the mold of liquid metal, surface defects occur, which in the continuous casting literature are called "hooks" This also allows the liquid metal to overflow the already solidified meniscus and leads to so-called "tears" or "metal fins." This "overflowing" can also be caused by a solidification front leading over the meniscus occur, which results from the strong cooling effect of the water. These surface defects on the finished strand or remelt block complicate subsequent processing of the cast products.

For the reasons mentioned above, it is therefore desirable for the first solidification of the liquid metal to commence below the meniscus and for liquid metal to be in front of the solidification zone. In this case, a smooth surface formation can be achieved.

To achieve this, it is necessary to reduce the temperature gradient in the meniscus area, which can be effected by a reduced heat dissipation, thus preventing a solidification of the metal in the area of the meniscus. In the past, some attempts have been made to reduce heat dissipation. It is known that a defined roughness can be applied to the surface of the mold. The trapped in the grooves air has an insulating effect on the heat dissipation and thereby reduces. The literature also describes a possibility of insulating the upper part of the mold in the region of the boundary layer slag-steel by a refractory insert or a ceramic insert. There is also a proposal to coat the mold on the inner wall with a material other than copper. However, all of these methods were only partially successful or did not yield any reproducible results.

In none of the current state of the art measures has been tried to influence an increase in the coolant temperature, the heat dissipation, as this is only partially possible when using water as the cooling liquid.

It is the object of the present invention to provide a method and apparatus in which the mold inner wall temperature is increased and thereby the heat dissipation is lowered using liquids which are used at higher operating temperatures than water at atmospheric pressure.

One way in which the method according to the invention is aimed is, for example, to carry out the cooling water circuit of the mold as a pressure circuit. As a result, the water temperature can be increased up to 180 ° C, which corresponds to a water pressure of about 1 1 - 14 bar. This approach requires a technical control of the pressure circuit, whereby safety aspects must also be considered. Another solution, which is simpler compared to a pressurized water circuit, is to cool the mold with liquid metal or salt melts, which can be operated in a pressure range comparable to the conventional water cycle. The only modification here relates to the sealing rings of the existing molds, which must be suitable for use at higher temperatures. Salt melts suitable for this purpose are also referred to in the literature as ionic liquids. Ionic liquids are characterized in that they are liquid in a temperature range between room temperature and 600 ° C, preferably between room temperature and 300 ° C, without requiring an increase in pressure is necessary. In principle, all ionic liquids known per se can be used. Types and production methods of ionic liquids can be found in WO 2005/021484, WO 2008/052860, WO 2008/052863 and WO 2013/1 13461. At this point, it is noted that the cooling of the mold is carried out indirectly as in the prior art water cooling, that is, the cooling medium does not come into direct contact with the liquid to be cast metal.

An application of liquid metals and salt melts for the direct cooling below the mold of a strand as an alternative to a secondary cooling with spray water during continuous casting is described in WO 2004/076096.

In order to keep the conditions for the primary solidification constant during the process, it is necessary to ensure a controlled and defined heat removal from the mold. For this purpose, as with the use of water as the coolant either the flow of the cooling medium used regulated (flow-controlled) or on the other hand set a certain temperature difference between Kokillenvorlauf and return (ΔΤ-regulated), with higher temperature differences than in conventional water cooling are possible ,

The dissipated heat is removed or recovered via a heat exchanger, so that a constant inlet temperature of the coolant can be ensured. Due to the very high difference between the inlet and outlet temperature of the cooling medium, a very large amount of waste heat is released, which would escape completely or partially into the atmosphere. By using a heat exchanger, this recovered heat can be fed into a heating circuit. Furthermore, by a heat exchanger subsequent suitable unit also this waste heat for steam and subsequently be used to generate electricity.

Another advantage of using liquid metal or molten salts is that, in the case of damage to the mold, there is no reaction of the refrigerant with the liquid metal to be poured or remelted.

The essential effect of the process according to the invention is that a positive effect on the quality of the strand or remelt block is achieved by the use of water under elevated pressure or of liquid metal melts or ionic liquids. Due to the higher temperature of these cooling media, the initial solidification of the metal is delayed so that it begins only below the meniscus area. In the present invention is a method for cooling liquid-cooled, usually consisting of copper molds for continuous casting or remelting self-consuming electrodes of steels or metals, in which the cooling effect of the coolant used is greatly reduced compared to water, so that the primary solidification of the liquid metal is delayed and takes place only below the meniscus in contact with the mold wall. To achieve this purpose, the flow rate of the coolant is set at inlet temperatures of room temperature, but also considerably higher, so that an outlet temperature of the coolant of over 80 ° C is reached.

The temperature difference between the coolant inlet and outlet may be between 5 ° C and a maximum of 150 ° C to achieve the purpose of the invention.

To achieve good conditions, an inlet temperature of the coolant between 60 ° C and 100 ° C is preferably selected and set the outlet temperature at temperature differences between 40 ° C and 100 ° C at 100 ° C to 200 ° C. In order to achieve the inventively required temperatures of the coolant, basically pressurized water can be used. That's it necessary to adjust the pressure in the circuit so that formation of vapor bubbles in contact with the mold wall is prevented.

Also advantageous is the use of molten metal, which are liquid below 100 ° C.

A particularly favorable solution is the use of ionic liquids with and without water content, which are liquid above room temperature and up to at least 200 ° C.

For effective control and control of the solidification processes of the liquid metal in the mold, a defined control of the temperature difference between inlet and outlet of the coolant in the range of at least 5 ° C and a maximum of 150 ° C is required. It is essential here to maintain the respectively selected or desired temperature difference at a defined coolant inlet temperature which can be maintained by re-cooling the coolant in a heat exchanger.

A suitable for carrying out the method according to the invention cooling circuit has a storage tank as a reservoir for the coolant and a circulation pump, by means of which the coolant is conveyed by the continuous casting or Umschmelzkokille and a heat exchanger, and corresponding valves and measuring devices for monitoring and control. The cooling circuit can be designed both open, so that the storage tank is at atmospheric pressure or closed with a pressurized storage tank.

In the single figure, a closed cooling circuit for carrying out the method according to the invention is shown schematically with a mold 1, which can be designed as a sliding or standing mold for the continuous casting or remelting process. The coolant enters the mold 1 via the inlet 2 at a temperature T 1 which is in the range between room temperature and 300 ° C. After flowing through the mold 1, the coolant exits the outlet 3 of the mold 1 again at an elevated temperature T2 (T2 = T1 + ΔΤ).

The heat released via a heat exchanger 4 amount of heat can be used subsequently for a recovery of thermal energy. About a collecting container 5 and a pump 6, the coolant is passed back into the mold 1.

Claims

PATENT CLAIMS Process for cooling liquid-cooled molds (1), usually made of unalloyed or alloyed copper, for the production of blocks or strands made of metals, preferably made of unalloyed or alloyed steels and nickel-based alloys, in systems for continuous casting or for remelting self-consuming electrodes under slag or by arcing under vacuum , characterized in that the cooling effect of the mold (1) is reduced compared to usual cooling with water in such a way that the solidification of the metal in contact with the mold wall cooled by the coolant is delayed so that solidification only begins below the meniscus , whereby the inlet temperature (T1) of the coolant into the mold (1) at room temperature can also be significantly higher and the flow rate of the coolant is adjusted so that an outlet temperature (T2) of the coolant of over 80 ° C is achieved. Method according to claim 1, characterized in that the temperature difference (ΔΤ) between the inlet and outlet temperatures (T1, T2) of the coolant into or out of the mold (1) is at least 5 °C and a maximum of 150 °C. Method according to claims 1 and 2, characterized in that water under increased pressure is used as the coolant. Method according to claims 1 and 2, characterized in that a metal melt that is liquid below 100 °C is used as the coolant. Method according to claims 1 and 2, characterized in that the coolant is an ionic liquid which also has a water content can be used and is liquid at room temperature and at least up to 200 °C. 6. The method according to any one of claims 1 to 5, characterized in that the coolant is recooled in a heat exchanger (4) to the desired coolant inlet temperature (T1). 7. Method according to one of claims 1 to 6, characterized in that the recovered heat is used thermally. 8. System for carrying out one of the methods according to claims 1 to 7, characterized in that a cooling circuit containing the coolant is designed to be closed and has a circulation pump (6) by means of which the coolant is conveyed through a continuous casting or remelting mold (1). and that the cooling circuit is further equipped with a coolant storage tank or expansion tank and a heat exchanger (4) for recooling the temperature of the coolant. 9. System according to claim 8, characterized in that when using water as coolant, the closed coolant circuit is designed as a pressure circuit. 10. Plant according to claim 8 or 9, characterized in that the heat recovered in the heat exchanger (4) is fed into a heating circuit or is used to generate steam and subsequently to generate electricity.