WO2002085555A2

WO2002085555A2 - Method and device for continuously casting metal

Info

Publication number: WO2002085555A2
Application number: PCT/EP2002/004357
Authority: WO
Inventors: Fritz-Peter Pleschiutschnigg; Erwin Wosch; Dirk Letzel; Werner Rahmfeld; Lothar Parschat; Joachim Schwellenbach
Original assignee: Sms Demag Aktiengesellschaft
Priority date: 2001-04-20
Filing date: 2002-04-19
Publication date: 2002-10-31
Also published as: DE10119354B4; WO2002085555A3; AU2002310864A1; DE10119354A1

Abstract

The invention relates to a method and device for continuously casting metal, particularly steel, whereby molten metal (6.1) is continuously cast in a cooled ingot mold with a casting speed (3) while forming a meniscus (7), and a cast product is continuously drawn out of the ingot mold in a direction of casting (3.1). The ingot mold, which has an ingot mold inlet (12) and an ingot mold outlet (8), is a water-cooled copper ingot mold having ingot mold cooling water channels (10.4) with a channel inlet and a channel outlet through which cooling water flows. Said water has an entry pressure p0 (15) at the channel inlet and an exit pressure p1 (15.1) at the channel outlet. The inventive method and device should be improved as to impede or prevent the high thermal load T of the ingot mold in the meniscus. To this end, the invention provides that, during a controlled adjustment of the water flow speeds in the ingot mold cooling water channels (10.4) and during a monitoring of the pressure loss Δp of the cooling water, the cooling water direction of flow (12) should correspond to the direction of continuous casting while forming a Nusselt layer that, at the height of the meniscus (19.1), is thinner than the Nusselt layer at the ingot mold outlet (19.2).

Description

Method and device for continuous casting of metal

The invention relates to a process for the continuous casting of metal, in particular steel, wherein the melt is continuously poured into a cooled mold at a casting speed with the formation of a mold level and a cast product is continuously withdrawn from the mold in the casting direction, the mold having a mold inlet and a mold outlet is a water-cooled copper mold with mold cooling water channels with a channel inlet and a channel outlet, through which cooling water flows, which has an inlet pressure p ₀ at the channel inlet and an outlet pressure pi at the channel outlet.

The invention is based on a prior art, as shown by means of FIG. 1 and explained below. When continuously casting metals, in particular steel, of whatever format, such as billets, profiles, blooms, slabs or thin slabs, with an open pouring jet and oil or with immersion spout, dip tube and casting powder as lubricant, the heat flow I (1) makes the mold height 2 special Requirements for the process technology and thus for the execution of the mold device. The mold cooling technology requires special attention with increasing casting speed (3), which meanwhile has max. 15 m / min can be.

The prior art is described by way of example with the aid of a continuous casting mold, in which an immersion spout 4 and casting powder 5 and pouring slag 5.1, which serve to isolate the strand shell 6 from the start of its solidification in the mold level 7 to the mold outlet 8, are used.

As FIG. 1 makes clear, the heat flow I (1) runs from the middle of the strand 9 as an energy source through the strand shell 6 and, for example, here through the Slag film 5.2 and through the mold copper plate 10 with the hot side HF (10.1), through the copper plate thickness (10.2) and through the cold side CF (10.3) into the energy sink, ie into the mold cooling water 11, which according to the prior art is from bottom to top, thus flows from the mold exit 8 to the mold entrance 12 or to the mold level ML (7).

1 also shows the heat flow load I ^* (13) of the mold, expressed as MW / m ² , via the mold height (2). The profile (13.1) of this mold heat flow load I ^* (13) shows a clear maximum (13.2) in the area of the mold level ML (7). At the same time, the copper plate skin temperature T (14) in the surface of the hot copper plate side HF (10.1) has a profile (14.1) over the mold height (2) which is similar to the heat flow load I ^* (13). The profile (14.1), like the heat flow load (13), also has a maximum (14.2) in the mold level, ML (7). To simplify the illustration, the profile or the profile of the heat flow load (13) on the hot copper plate side (10.1) over the mold height and the profile of the copper plate skin temperature over the mold height are shown as similar curves.

The high thermal load (14) of the copper plate 10 in the mold level area (14.2) is due to the pressure difference or pressure drop ΔP (15.2) of the mold cooling water 11, which with the inlet pressure P ₀ (15) at the mold outlet 8 and the outlet pressure Pi (15.1) on Mold entrance (12) can be described clearly.

This pressure loss (15.2) leads to a Nusselt layer (16.1), ie to an interface with an interface resistance (16) which is thinner at the mold outlet 8, here illustrated by the section F, than the Nusselt layer (16.2) Mold entrance (12), which is illustrated by the cut ML. The Nusselt layer can be viewed as a laminar flow layer through which the heat transport predominantly via conduction and not, as in the turbulent zone with a Reynolds number Re> 2,300 (17) predominantly via forced contact. vection takes place. The thickness of each Nusselt layer is shown by the distance arrows.

This increase in the Nusselt layer thickness or the interface resistance (16) over the mold height (2) rises steadily from the mold outlet 8 to the casting level ML (7) and leads to an increased thermal with the increasing heat flow density of the mold I ^* (13) Load (14) on the hot mold side HF (10.1) with its maximum (14.2) in the mold level, which leads to damage or a short service life of the copper plate (10). This short copper plate service life occurs due to damage to the copper plate hot side HF (10.1) in the maximum (14.2) of the heat or temperature profile, ie in the area of the mold level, ML (7). This damage can be seen through

• softening the cold-rolled copper and / or

• Cracking of the copper plate in the area of the mold level and at the same time leads to an increased risk of longitudinal cracks forming in the surface of the strand shell 6.

The object of the invention is now to develop a method and a device of the generic type such that these disadvantages do not occur. In particular, the high thermal load T of the mold in the mold level should be suppressed or avoided.

This object is achieved by the method with the features of claim 1 and by an apparatus with the features of claim 9. Advantageous further developments are presented in the subclaims and in the description of the exemplary embodiment.

According to the method, it is proposed that the water flow rate in the mold cooling water channels be adjusted in a controlled manner, that the pressure loss Δp of the cooling water is below a predetermined maximum value. tes is kept that the cooling water flow direction runs in the continuous casting direction and that this forms a Nusselt layer in the cooling channel, which is thinner at the level of the casting level than the Nusselt layer at the mold outlet, whereby through the controlled (variable) setting of the water flow speed and, if necessary. Through the controlled (variable) setting of the cooling water supply temperature via the thickness of the Nusselt layer at the level of the casting level, the temperature of the mold wall or plate on the casting level or meniscus is regulated. With this regulation, the temperature of the mold wall at the level of the pouring level can be kept constant over a certain operating window despite increasing pouring speeds.

The invention is characterized by the reversal of the water flow direction in the cooling channels for cooling the copper mold from originally a flow direction opposite the casting direction to a flow direction in the casting direction, which is synonymous with the direction from the mold inlet to the mold outlet or a direction from top to bottom , whereby a Nusselt layer forms which is thinner at the level of the casting level than the Nusselt layer at the mold exit. The Nusselt layer can be viewed as a laminar flow layer through which the heat is transported primarily via conduction and not, as in the turbulent zone, primarily via forced convection. The Nusselt layer results in a certain interface resistance on the copper plate cold side of the mold in the area of the cooling water channels. It is thus achieved that the laminar flow layer is thinner in the area of the casting level than at the mold exit and thus the interface resistance on the copper plate cold side of the mold is smaller. In this way, the maximum temperature at the mold level is reduced and the mold skin temperature is evened out over the mold height.

In the case of the mold cooling described according to the prior art, the temperature of the copper plate in the area of the mold level rises with increasing casting speed and thus a greater amount of heat, since the water running temperature and the water flow rate are kept constant. In contrast to this it is achieved by means of the invention that the temperature of the copper plate at the level of the pouring level can be kept constant despite increasing pouring speed. Through the controlled setting in the sense of controlling or regulating the water flow speeds or the water quantities and moreover the water temperature, the temperature of the copper plate in the area of the pouring level can be freely set and kept constant within certain operating windows regardless of the pouring speed.

The ratio between the thickness of the Nusselt layer or the laminar flow layer and the thickness of the turbulent zone is set by controlling or regulating the water flow rate and thus the temperature in the copper plate in the area of the mold level. In this way, it is possible to set a temperature curve that is optimized for a particular steel grade.

The water flow speeds in the mold cooling water channels are preferably set in a controlled manner so that the average speed does not drop below 3 m / s. The pressure loss of the cooling water pressure between the channel inlet and outlet should preferably be a maximum of 4 bar, the cooling water inlet pressure p ₀ (15) preferably being more than 6 bar.

The process is particularly suitable for continuous casting at high casting speeds of up to 15 m / min.

A device is proposed for carrying out the method, the channel inlet of the mold cooling water channels and the channel outlet being arranged such that the cooling water flow direction runs in the pouring direction, the Nusselt layer that forms at the level of the pouring level being thinner than the Nusselt layer at the mold outlet. The channel entry of a cooling Water channel is at the mold entrance, while the channel exit is at the mold exit. In addition, it has a device for the controlled adjustment of the water flow rate and, if necessary, an additional device for the controlled adjustment of the water temperature.

The invention, characterized essentially by the reversal of the cooling water direction from top to bottom instead of from bottom to top as before and a regulated adjustment of the water flow rate, leads to the following advantages such as a homogenization of the heat flow density in the mold level over the mold width, an equalization of the thermal see loading of the copper plate and especially the copper plate hot side HF above the mold height. As a result, these advantages lead to longer mold service lives and an improved strand surface by essentially avoiding longitudinal cracks in the copper plate surface in the area of the mold level and in the surface of the strand shell, which is particularly noticeable when casting the first slabs of a casting sequence in the case of crack-sensitive steel grades. Overall, a high level of casting reliability is achieved.

By setting a certain copper plate temperature at the level of the casting level within an operating window, optimal conditions for the casting powder used are made possible, whereby an optimal and constant behavior of the casting powder and the slag film is independent of the casting speed and the thickness of the mold copper plate. Further details and advantages of the invention emerge from the subclaims and from the following description, in which the embodiment of the invention illustrated in the figure is explained in more detail. In addition to the combinations of features listed above, features alone or in other combinations are also essential to the invention. Show it:

Fig. 1: according to the prior art, schematically in section half of a mold with a copper mold plate with water cooling, the course of the heat flow load I ^* (MW / m ² ) and the temperature rature course T of the copper plate skin temperature in the surface of the copper plate hot side HF, in each case via the mold height, the representation of the cooling water flow in section ML at the mold level and in section F at the mold outlet with the Nusselt layer thickness in relation to the thickness of the turbulent zone;

Fig. 2: schematically in section one half of a mold according to the invention with a copper mold plate with water cooling with a flow direction in the casting direction, the course of the heat flow load I ^* (MW / m ² ) and the temperature course T of the copper plate skin temperature in the surface of the copper plate hot side HF, in each case over the mold height, the representation of the cooling water flow in section ML at the pouring level and in section F at the mold outlet with the walnut layer thickness in relation to the thickness of the turbulent zone.

Fig. 3 shows schematically the comparison of the cooling process for a continuous casting mold according to the prior art and according to the invention.

The invention is described in more detail by way of example with the aid of FIG. 2. Fig. 1 represents the prior art, which has already been discussed in detail. 2 makes the invention clear in comparison to FIG. 1 with its unexpected solution for equalizing the thermal load (20) of the copper plate hot side HF (10.1). Components or sizes that are the same as FIG. 1 are provided with corresponding reference symbols in FIG. 2.

Operational measurements and mathematical-physical simulation have shown that by reversing the direction of flow of the mold cooling water (18) from top to bottom, a Nusselt layer (19) is formed in the ML level (19.1) which is smaller than the Nusselt layer (19.2) is at the mold exit (8) (cf. the lower representations of the thickness of the Nusselt layer). Due to this thinner Nusselt layer or The minimum boundary layer (Reynolds number Re <2,300) results in a lower interfacial resistance (19) in the mold level (7), making the thermal load (20) even over the mold height (2) and avoiding the high load on the hot copper plate side HF (10.1) can be and a relatively uniform temperature profile (20.1) without a maximum.

At the same time, FIG. 2 shows that the heat flow load of the mold I * in the 1st approximation remains unchanged compared to the heat flow load, as is the case with the mold according to the prior art. The cooling water channels 18 can be designed as slots or bores with 10-80% water coverage or as an annular gap with 100% water coverage.

FIG. 2 also makes it clear that by reversing the direction of flow of the mold cooling water (18), the high pressure P ₀ (15) in the casting level ML (7) comes into full effect and thereby also a higher evaporation pressure of the water (gas film formation) opposite the mold outlet (8) with the pressure Pi (15.1). This higher pressure of the water in the pouring level (7) also increases the security against a possibly occurring water gas bubble or water gas film formation (Leidenfrost effect) compared to the prior art.

A suitable pump 21 at the channel inlet (10.5) preferably provides the cooling water with a flow rate such that an average water flow rate of more than 3 m / s results. The mold is oscillated with respect to the strand or the solidified strand shell 6 by means of an oscillation device (here schematically identified by 22), in that the mold is moved vertically up and down. Alternatively, it is possible for the strand to be moved up and down in a fixed mold, for example by applying a pulsating magnetic field below the mold.

3 schematically shows a comparison of the mold cooling according to the prior art (left) and according to the invention (right) using process Graphs. In the case of the mold according to the prior art or the “classic” mold, the cooling water flows counter to the pouring direction, as indicated by the arrows 23. The speed of the cooling water V _water and the temperature of the cooling water entering T _e i _π or T _in are constant With increasing speed (v _c m / min) the temperature on the mold plate hot side rises at the level of the pouring level or the meniscus. This results in a variable temperature the Tis side meniscus. In contrast to this, the proposed method the temperature on the hot plate side of the mold at the level of the meniscus (T _He ß side meniscus) is regulated, the direction of flow of the cooling water runs in the direction of the pouring direction (see arrows 24) and a thinner Nusselt layer at the level of the meniscus than at the mold exit forms, their thickness and therefore the temperature at the hot mold plates page on the casting level _of T is adjustable by means of the water flow rate Vw _a sser and the cooling water temperature T _Θ in or T _in . With the help of this control concept, the temperature of the mold plate in the mold level can be freely selected and controlled within certain operating fields. For this purpose, the actual temperature T _{off is} tapped and, together with further process data, the water flow rate Vw _asse r required for regulation to a target temperature is set with the device 25 and the water temperature T _e in. With increasing pouring speed v _c , the cooling water _inlet temperature T _Θin or T _{in is} lowered and the cooling water _{flow velocity} v _WaS ser is increased. In addition, the comparison also shows that there is a free choice between the different mold cooling systems. The modification in the water cycle of the mold system consists of an online computer, which, when switched off, allows the conventional mold cooling strategy to be used again immediately, even during casting. LIST OF REFERENCE NUMBERS

1 heat flow I from the strand through the mold plate into the mold cooling water

2 mold height

3 Casting speed 3.1 Casting direction 4 Immersion pouring

5 mold powder

5.1 pouring slag

5.2 Cast slag film

6 strand shell 6.1 melt

7 Casting level ML

8 mold exit

9 middle of the strand

10 Chill copper plate 10.1 copper plate hot side, HF, facing the strand shell

10.2 Copper plate thickness

10.3 Copper plate cold side, CF, facing the mold water

10.4 Chilled water cooling channels

10.5 Channel entry 10.6 Channel exit

1 1 permanent mold cooling water channel with a flow direction of the cooling water against the pouring direction from bottom to top

12 mold entrance

13 Heat flow load of the mold I ^* in MW / m ² 13.1 Heat flow profile or profile over the mold height (2)

13.2 Maximum of the heat flow profile in the area of the pouring level ML (7) 14 copper plate skin temperature T, temperature load, thermal load

14.1 Temperature profile over the mold height (2)

14.2 Maximum of the temperature profile in the area of the pouring level ML (7) 15 inlet pressure P _{0 of} the mold cooling water 15.1 outlet pressure Pi of the mold cooling water 15.2 pressure difference ΔP = P ₀ - Pi

16.1 Nusselt layer or interfacial resistance or laminar boundary layer (Reynolds number Re <2,300, x ² ) with a cooling water flow direction from bottom to top at section F 16.2 Nusselt layer with a cooling water flow direction from bottom to top at section ML

17 Turbulent zone with heat transfer via forced convection, Reynolds number Re> 2,300, x ³

18 mold cooling water channel with a flow direction of the cooling water in the direction of pouring from top to bottom or from the mold entrance

(12) to the mold exit (8)

19.1 Nusselt layer or interfacial resistance or laminar boundary layer in the water flow direction from top to bottom at the cut ML (height of the casting level (7)) 19.2 Nusselt layer with water flow from top to bottom at the cut F (mold outlet (8))

20 Thermal loading of the copper plate hot side HF (10.1) when the mold cooling water flows from top to bottom

20.1 relatively uniform temperature profile or thermal load over the mold height (2)

21. pump

22. Oscillation device

23. Direction arrow for cooling water flow direction

24. Direction arrow for cooling water flow direction 25. Device for setting v _a sser F Section of the water flow at the mold outlet (8), flow profile with Nusselt layer (16.2) ML Section of the water flow in the area of the liquid level ML, flow profile with Nusselt layer (16.2).

Claims

claims

1. A process for the continuous casting of metal, in particular steel, wherein the melt (6.1) is continuously poured into a cooled mold at a casting speed (3) to form a casting level (7) and a casting product is continuously withdrawn from the mold in the casting direction (3.1) , the mold with a mold inlet (12) and a mold outlet (8) being a water-cooled copper mold with mold cooling water channels (10.4) with a channel inlet (10.5) and a channel outlet (10.6), through which cooling water flows, an inlet pressure at the channel inlet p ₀ (15) and at the channel outlet has an outlet pressure - _\ (15.1), characterized in that the water flow velocity vwasser in the mold cooling water channels (10.4) is adjusted in a controlled manner, that the pressure loss Δp of the cooling water is kept below a predetermined maximum value, that the cooling water flow direction (12, 24) runs in the continuous casting direction and that a Nusselt layer forms in the cooling channel (10.4), which is thinner at the level of the casting level (19.1) than the Nusselt layer at the mold outlet (19.2).

2. The method according to claim 1, characterized in that the average water flow velocities in the mold cooling water channels (10.4) are set in a controlled manner to values> 3 m / s, and that at a cooling water inlet pressure p ₀ (15) of> 6 bar the constant The controlled pressure loss Δp from the duct inlet (10.5) via the mold cooling water ducts to the duct outlet (10.6) is at a maximum of 4 bar.

3. The method according to claim 1 or 2, characterized in that is poured at high casting speeds (3) of up to 15 m / min.

4. The method according to any one of claims 1 to 3, characterized in that in addition to the water flow rate v _W ater the cooling water inlet temperature T _Θ m is set in a controlled manner.

5. The method according to any one of claims 1 to 4, characterized in that despite increasing the casting speed v _c by controlling the

Water flow velocity v _Wa ter and Kühlwasservorlauftem- temperature T _in the temperature on the hot side of the mold plates at the level of the meniscus (T _He ιßseιte meniscus) is kept constant.

6. The method according to any one of claims 1 to 5, characterized in that either the mold or the strand-shaped cast product are oscillated.

7. The method according to any one of claims 1 to 6, characterized in that the melt, in particular steel melt, is introduced into the mold under reoxidation protection.

8. The method according to claim 7, characterized in that that the introduction of the melt (6.1) into the mold by means of a

Immersion pouring (4) is carried out, which dips into the mold below the casting level (7), casting powder (5) being used to form pouring slag (5.1) and a pouring slag film (5.2).

9. Device for the continuous casting of metal, in particular steel, comprising a cooled mold, into which continuous melt (6.1) is poured at a casting speed (3) to form a casting level (7) and from which a casting product is continuously drawn in the casting direction (3.1) The mold with a mold inlet (12) and a mold outlet (8) is a water-cooled copper mold with mold cooling water channels (10.4) with a channel inlet (10.5) and a channel outlet (10.6), through which cooling water flows, which flows at the channel inlet (10.5) has an inlet pressure p ₀ (15) and an outlet pressure pi (15.1) at the duct outlet (10.6), in particular for carrying out the method according to one of claims 1 to 8, characterized in that the duct inlet (10.5) of the respective mold cooling water duct (10.4) on The mold inlet (12) and the channel outlet (10.6) are arranged at the mold outlet (8), so that the cooling water flow direction tion runs in the pouring direction, with the forming in the cooling water channel

Nusselt layer at the level of the pouring level (7) is thinner than the Nusselt layer at the mold exit (8) and that it is a device (25) for the controlled adjustment of the water flow rate Vwasser and thus for the targeted adjustment of the thickness of the Nusselt layer at the level of the Casting level (7).

10. The device according to claim 9, characterized in that it has a device for controlled adjustment of the cooling water flow temperature T _Θ i _n .

1. The device according to claim 10, characterized in that it has an oscillation device (21) which either sets the mold in relation to the strand-shaped cast product in vibration or the strand-shaped cast product with respect to the mold.