WO2003035306A1 - Method and device for optimizing the cooling capacity of a continuous casting mold for liquid metals, particularly for liquid steel - Google Patents

Abstract

The invention relates to a method for optimizing the cooling capacity of a continuous casting mold (1) for liquid metals, particularly for liquid steel, by homogenizing the thermal load (22) above the height of the continuous casting mold (1). According to the method, the cooling medium (5) is guided through a cross-sectional area of a large number of cooling medium channels (3) or cooling medium boreholes (4) running approximately parallel to the cast billet (9). The cooling medium cross-sectional areas between the mold entry (6) and the mold exit (7) are configured differently. In order to homogenize the thermal mold load (22), a smaller cross-sectional area sets the flow rate of the cooling medium (5), which is conducted from the top downward, inside the cooling medium channel (3) or inside the cooling medium borehole (4) higher in the upper area of the continuous casting mold (1) than in the lower area of the continuous casting mold (1) in which the flow rate is set lower by a larger cross-sectional area and/or the covering of the cooling medium is adjusted by a cross-sectional shape that varies from the top downward.

Description

Method and device for optimizing the cooling capacity of a continuous casting mold for liquid metals, in particular for liquid steel

The invention relates to a method and a device for optimizing the cooling capacity of a continuous casting mold for liquid metals, in particular for liquid steel, by comparing the thermal load over the height of the continuous casting mold, in which the coolant in each case through a cross-sectional area of a large number of coolant channels or Coolant bores, which run approximately parallel to the casting strand, is guided, the coolant cross-sectional areas between the mold inlet and the mold outlet being designed differently.

The continuous casting mold referred to at the beginning as a device is known from DE 41 27 333 C2. In this process, molten steel is poured into a continuous casting mold, the mold walls of which are provided with continuously cylindrical cooling bores extending from top to bottom and connected to a cooling water circuit, the flow cross-sectional areas of which are partially reduced by displacement rods. In order to reduce the temperature differences between the height areas of the mold and thus to reduce tension and extend the service life of the mold, the cooling water is guided through the coolant holes at maximum speed in the area of the highest temperature load. However, only the bore ring cross-sectional areas reduced by the displacement rods are available. In addition, the coolant is only directed from the bottom to the top. In contrast, the object of the invention is to control the copper plate skin temperatures on the hot side and the cold side so that both the recrystallization temperature of the cold-rolled copper on the hot side with the greatest possible cooling intensity and with uniform cooling at the height ranges of the continuous casting mold is not exceeded and a possible evaporation of the coolant on the cold side is avoided.

The object is achieved according to the invention in that the flow rate of the coolant, which is passed from top to bottom in the continuous casting mold, is set higher in the coolant channel or in the coolant bore in the upper region of the continuous casting mold by a smaller cross-sectional area than in the lower region of the continuous casting mold, in which the flow velocity is set lower by a larger cross-sectional area and / or in that the covering with coolant is set by a cross-sectional shape which can vary from top to bottom. The advantage is a greater coverage by coolant in the hot area and a lower heat dissipation below the hot area. As a result, not only is the impact load in the hot height region of the mold level considerably reduced, but also the heat load is further evened out over the entire height of the continuous casting mold. In addition, the recrystallization temperature of the cold-rolled copper on the hot side is not reached, and there is no risk that the coolant may evaporate on the cold side. In this case, the inlet cross-sectional shape of the coolant channel can be square or rectangular and the continuation in each case from an elongated rectangle up to a square, or the circular inlet cross-sectional shape can be designed analogously.

In an embodiment of the invention it is provided that at casting speeds of 3 m / min to about 12 m / min, a heat flow load of the continuous casting mold of at most 8 MW / m ² and coolant speeds of 4 m / s to 30 m / s are maintained. After further steps, it is proposed that a maximum thermal load on the mold plates on their hot side is less than 550 ° C. and that the heat transfer coefficient is set up to a maximum of 250,000 W / m ² K.

Another measure influencing the thermal values is that the continuous casting mold is oscillated.

It is also provided that the casting strand is lubricated with casting powder slag in the continuous casting mold.

Another measure that supports the heat transfer is that the surface of the coolant channels is provided with an increasing roughness from the mold inlet to the mold outlet.

In a device for optimizing the cooling capacity of a continuous casting mold for liquid metal, in particular for liquid steel, by equalizing the thermal load over the height of the continuous casting mold, the coolant in each case through a cross section of a large number of coolant channels or coolant bores, which run approximately parallel to the casting strand - fen is guided and the coolant cross-sectional area of the coolant channels between the mold inlet and the mold outlet are designed differently, the object is achieved according to the invention in that the coolant channels or the coolant holes each have a relatively small coolant channel input cross-sectional area and a larger one -Exit- cross-sectional area from the mold inlet to the mold outlet are formed with the greatest coverage by the coolant at the mold entrance (here under "cover" the ratio is s Coolant channel width / coolant channel distance, ie the effective phase boundary layer copper / coolant understood). This achieves the effect of reducing peak temperatures in the copper plate in the area of the mold level and the uniformity over the entire height of the continuous casting mold. An alternative to the transition from the mold entrance to the mold exit is that the change in the cross-sectional area shape runs continuously from the mold entrance to the mold exit.

According to further features, it is provided that the casting speed can be adjusted in the continuous casting direction up to approximately 12 m / min.

The invention is also improved in that a thermal load on the continuous casting mold of at most 8 MW / m ² , a coolant speed of 4 to 30 m / s and a maximum local thermal load on the copper plates on the side facing the liquid metal with a heat transfer coefficient • by Max. 250,000 W / m ² • K are provided.

A further embodiment provides that the coolant channels with a rectangular cross section are designed to increase in their channel depth and / or channel width from the mold inlet to the mold outlet.

An improvement also provides that the cross-sectional area of the coolant channels can be changed by means of baffles via a control or regulation. As a result, the flow of the coolant in the rigid form of the coolant channels can be supplemented by a further function.

Another further development is given in that the surface of the coolant channels is provided with a roughness from the mold exit to the mold entrance.

It can be assumed that the roughness is formed from dimples of 0.5 to 3 mm in diameter and 0.2 to 2 mm in depth. Finally, the distribution or the number of dimples from the mold exit to the mold entrance is provided to increase.

The heat transfer is intensified according to further features in that the roughness can be changed by chemical or mechanical measures.

It is further advantageous that the roughness can be changed during the casting process.

The drawing shows exemplary embodiments relating to the prior art and to the invention, which are explained in more detail below.

Show it:

Fig. 1 A (each from left to right) a vertical section through the current continuous casting mold, in the upper part two horizontal partial sections for coolant channels and coolant holes in the upper mold area, in the lower area two horizontal partial sections for coolant channels and coolant holes in the lower mold area, the far right Temperature curve in the copper plates,

Fig. 1 B analogous to Fig. 1A (each from left to right) a vertical section through the continuous casting mold, in the upper part three horizontal partial sections for coolant channels and coolant holes in the upper mold area, in the lower part three horizontal partial sections for coolant channels and coolant holes in the lower mold area , on the far right is a comparison of the previous surface temperature curve between the previous surface temperature curve and the new surface temperature curve,

2A shows a diagram of the heat transfer coefficient •, the maximum thermal load and the pressure loss in the coolant, Fig. 2B is a diagram of the heat transfer coefficient •, the pressure loss • P over the coolant speed and

2C shows a diagram for the decrease in the maximum thermal load with increasing coolant speed.

In the continuous casting of liquid metals, in particular liquid steel, a continuous casting mold 1 is used (FIG. 1A), which consists of copper plates 2 each with a large number of coolant channels 3 or coolant bores 4 with or without displacement rods 4.1 through which the coolant passes 5 is directed.

The largest local heat flow 10 (“J”) and at the same time the greatest thermal mold load T cu-m _a x 1, both on the hot side 11.1 and on the, arises in the casting mirror 8, in which the strand shell 9 begins to form Cold side 11.2 of the copper plate 2.

The thermal load in the mold level 8 or the maximum heat flow 10 ("J") can now be up to 8 MW / m2, especially at high casting speeds of about 12 m / min and therefore requires special cooling measures to keep the copper plate skin temperatures on the hot side 11.1 and the cold side 1 1.2 in such a way that the recrystallization temperature of the cold-rolled copper on the hot side 11.1 is not exceeded and a possible evaporation of the coolant 5 on the cold side 11.2 is avoided.

The cooling capacity or the cooling effect is determined by mechanical engineering elements, such as the copper plate thickness 12, the coolant channels 3 or the coolant bores 4 with or without displacement rods 4.1, the distance 13 (A) of the coolant channels 3 or the coolant bores 4 from one another, the cross-sectional area 14 (F ) the coolant channels 3 or the coolant holes 4 and the length of the coolant channels 3 or the coolant bores 4, which corresponds to the mold length 15 (L). As the state of the art, the cooling channel cross-sectional areas 14 between the mold inlet 6 and the mold outlet 7 can currently be regarded as constant. In addition to the coolant temperature, the process-related influencing variables for the cooling capacity of the continuous casting mold 1 are the coolant speed 16, which is an essential measure of the heat transfer coefficient 17 (•), measured in W / im ² • K.

The relationships are shown in FIGS. 2A, 2B and 2C in diagrams.

In order to set a desired heat transfer with the aid of a specific coolant speed 16 in the continuous casting mold 1, the cross-sectional area 14 of the coolant channel 3 or the coolant bores 4 for a given mold width 18, normalized here to 1 m, and the distance 13 of the coolant channels, result Set pressure loss 19 (• P) in the coolant 5 between the mold inlet 6 and the mold outlet 7.

This pressure loss increases disproportionately with the coolant speed 16 (V) or with the coolant quantity 20 (Q), measured in m ³ / h • m.

It should also be noted that with increasing roughness 21 (R) of the surface of the coolant channel 3 or of the coolant bores 4, the heat transfer coefficient 17 (•) but also the pressure loss 19 (• P) increase.

The aim of the invention is to minimize the pressure loss 19 (P) during the control of the maximum thermal load 11 (T cu-m _a x) both on the hot side 11.1 and the cold side 11.2 and to make the thermal mold load 22 and to achieve the thermal profile 23 over the mold length 15. 2A, 2B and 2C, the heat transfer coefficient 17 (•) and the maximum thermal load 11 of the copper plate 2 are dependent on the mechanical engineering and process engineering factors, such as

- the coolant speed 16 (V)

- the amount of coolant 20 (Q) - the pressure drop 19 (• P)

- The roughness 21 (R) of the surface is shown under specific and constant boundary conditions.

The casting strand 9 is cast according to FIG. 1B at a casting speed 9.1 of approximately 12 m / min, for example in the casting format of a thin slab with a thickness between 40 mm and 100 mm. Casting powder 1.2 and an oscillation 1.1 can be used for casting. The casting process loads the continuous casting mold 1 with a maximum heat flow 10 (“J”) in the casting level 8 of 2 to 8 MW / m ² and leads to a maximum thermal load 11 in the casting level 8 both on the hot side 11.1, which faces the molten steel , as well as on the cold side 11.2, which faces the coolant 5. The process leads to a thermal mold load 22 and a heat flow profile 23 over the mold length 15 (L). The coolant channel cross-sectional areas 14 (F) in the coolant channels 3 or coolant bores 4 with or without displacement rod 4.1 are constant in the prior art (FIG. 1A) over the mold length 15 and thus lead to a constant coolant speed 16 (V) and a defined coolant pressure drop 19 (• P), which is assumed to be "1".

The extreme right-hand representation of FIG. 1B shows the temperature profile of the surface temperature that has changed compared to FIG. 1A, the total amount of heat removed remaining the same.

In order to even out the thermal mold load 22 and at the same time to minimize the pressure loss 19 (• P) of the coolant 5, the

Cross-sectional area 14 (F) of the coolant channels 3 or the coolant bores 4 enlarged from the mold entrance 6 to the mold exit 7 (FIG. 1B). In addition, the roughness 21 (R) can also optionally be raised functionally from the mold outlet 7 to the mold inlet 6 over the mold length 15.

The roughness 21 can also be produced by dimples 24 of a maximum of 1-3 mm in diameter and 1-2 mm in depth, which lead to cavitation effects of the flowing coolant 5 (for example the water) at the phase boundary copper (cold side 11.2) and coolant 5 and thus lead to an increased heat transfer coefficient 17 (•), caused by forced convection in the area of the laminar "Nusselt" boundary layer, in which the energy transport takes place via heat conduction.

The cross-sectional area 14 of the coolant channels 3 or the coolant bores 4 can be enlarged over the mold length 15 in the case of the coolant channels 3 via the channel depth 3.1 and / or the channel width 3.2. In the case of the coolant bores 4, the cross-sectional enlargement can be realized by increasing the diameter of the coolant bore 4 and / or reducing the diameter of the displacement rod 4.1.

Another design is that guide plates 3.3 of the coolant channels 3 are mechanically or manually, for example on a changed cross-sectional area 14 of the coolant channels 3 via the mold height 15, e.g. online, process-controlled by means of a control or regulation 3.3.1 of the position of the guide plates 3.3.

After the designs described have been carried out, the thermal mold load 22 can be reduced over the mold length 15 by means of a uniform thermal profile 22.1, as shown in a diagram in the right part of FIG. 1B. The diagram 2A shows the heat transfer coefficient 17 (•) measured in W / m ² K, the pressure loss 19 (• P) and the local maximum thermal load 11 of the copper plate 2 in the mold level 8 as a function of the roughness 21 of the surface of the coolant channels 3 or the coolant bores 4 with a constant copper plate thickness 12, coolant speed 16 (V in m / s), heat flow 10 (J), cross-sectional area 14 of the coolant channel 3 or the coolant bore 4, the mold length 15 and a distance 13 of the coolant channels 3 or coolant bores 4 from one another. The diagram makes it clear that with increasing roughness 21 (R) the heat transfer coefficient 17 (•) but also the pressure loss 19 (• P) increase steadily but at the same time the copper plate temperature 11 (T cu-m _a x) on the hot side 11.1 and the cold side drops sharply.

Diagram 2B shows the heat transfer coefficient 17 (•) and the pressure loss 19 (• P) over the coolant speed 16 (V) or the coolant quantity 20 (Q) with increasing roughness 21 with constant cross section 14 (F), mold length 15 and distance 13 (A). Here it becomes clear that with increasing coolant speed 16 (V), coolant quantity 20 (Q) and roughness 21 (R) the heat transfer coefficient 17 (•), but also the pressure loss 19 (• P) increase disproportionately.

2C shows the decrease in the maximum thermal load 11 in the casting level 8 of the copper plate 2 with increasing coolant speed 16 (V), coolant quantity 20 (Q) and roughness 21 (R) with constant heat flow 10 (“J”), in the heat flow Profile 23 over the mold length 15, the copper plate thickness 12, the coolant channel cross-sectional area 14 (F) and the distance 13 (A) of the coolant channels 3 or the coolant holes 4 are shown.

The partial image in FIG. 2C makes it clear that the local maximum thermal load 11 in the mold level 8 decreases sharply with increasing roughness 21 (R), the coolant speed 16 (V) or the coolant quantity 20 (Q). The principle of the invention can also be applied to strip casting devices which are operated at a casting speed of up to 100 m / min. All the measures applied to the level of the continuous casting mold 1 are transferred to the scope of the twin rollers.

LIST OF REFERENCE NUMBERS

1 continuous casting mold

1.1 oscillation

1.2 Casting powder, pouring slag

2 copper plate

3 Coolant channel 3.1 Channel depth

3.2 Channel width

3.3 baffle

3.3.1 Control of the position of the guide plates

4 Coolant hole 4.1 Displacement tube, rod, round body

5 coolant

5.1 Coolant flow direction

6 mold entrance (upper edge of the mold)

7 Mold exit (lower edge of the mold) 8 Casting level

9 strand shell, strand 9.1 casting speed

10 local maximum heat flow "J" in the mold level

11 local maximum thermal load in the mold level (T cu-m _a x) 11.1 side facing the liquid steel (hot side)

11.2 side facing the coolant (cold side)

12 copper plate thickness (between hot side and cold side)

13 distance of the coolant channels (3) or the coolant holes (4) from one another 14 cross-sectional area (F) of the coolant channels (3) or coolant holes (4) 15 Length of the coolant channels, the coolant holes, mold length

16 coolant speed (V in m / s)

17 heat transfer coefficient »in W / m ² » K

18 mold width (in m)

19 Loss of pressure of the coolant, ΔP 20 amount of coolant Q in m ³ / h ^■ m

21 roughness, R, in mm of the surface

22 thermal mold load over the mold length 22.1 uniform thermal profile (τcu-m _a x)

23 Heat flow profile over the length of the mold 24 dimples, cells

Claims

claims 1. A method for optimizing the cooling capacity of a continuous casting mold for liquid metal, in particular for liquid steel, by comparing the thermal load over the height of the continuous casting mold, in which the coolant in each case through a cross-sectional area of a large number of coolant channels or coolant holes, which are approximately parallel to the casting strand run, is performed, the coolant cross-sectional areas between the mold inlet and the mold outlet are designed differently, characterized in that the flow rate of the coolant, which is passed from top to bottom in the continuous casting mold, in the coolant channel or in the coolant bore in the upper The area of the continuous casting mold is set higher by a smaller cross-sectional area than in the lower area of the continuous casting mold, in which the flow rate is set lower by a larger cross-sectional area and / or that the covering with coolant is adjusted by a cross-sectional shape that can vary from top to bottom. 2. The method according to claim 1, characterized in that at casting speeds of 3 m / min to about 12 m / min Heat flow load of the continuous casting mold of a maximum of 8 MW / m ² and coolant speeds of 4 m / s to 30 m / s can be observed. 3. The method according to any one of claims 1 or 2, characterized in that that a maximum thermal load on the mold plates on their Hot side less than 550 ° C and that the heat transfer coefficient • is set up to a maximum of 250,000 W / m ² • K. 4. The method according to any one of claims 1 to 3, characterized in that the continuous casting mold is oscillated. 5. The method according to claims 1 to 4, characterized in that the casting strand is lubricated with cast powder slag in the continuous casting mold. 6. The method according to any one of claims 1 to 5, characterized by the fact that the surface of the coolant channels from the mold inlet to The mold exit is provided with an increasing roughness. 7. Device for optimizing the cooling capacity of a continuous casting mold for liquid metal, in particular for liquid steel, by equalizing the thermal load over the height of the continuous casting mold, the coolant in each case through a cross section of a large number of coolant channels or coolant bores which are approximately parallel to the Casting strand run, is guided and the coolant cross-sectional area of the coolant channels between the mold inlet and the mold outlet are designed differently, so that the coolant channels (3) or the coolant holes (4) each have a relatively small coolant channel inlet cross-sectional area (14) and a larger - output cross-sectional area from the mold inlet (6) to the mold outlet (7) with the greatest coverage by the coolant (5) at the mold inlet (6). 8. The device according to claim 7, characterized in that the change in the cross-sectional area shape from the mold inlet (6) to the mold outlet (7) is continuous. 9. Device according to one of claims 7 or 8, characterized in that the casting speed in the continuous casting direction is adjustable up to about 12 m / min. 10. Device according to one of claims 7 to 9, characterized in that a thermal load (11) of the continuous casting mold (1) of a maximum of 8 MW / m ² , a coolant speed (16) of 4 to 30 m / s and a maximum local thermal load (11) of the copper plates (2) on the side facing the liquid metal (11.1) with a heat transfer coefficient of max. 250,000 W / m ² • K are provided. 11. The device according to one of claims 7 to 10, characterized in that the coolant channels (3) with a rectangular cross section in their channel depth (3.1) and / or channel width (3.2) from the mold inlet (6) to the mold outlet (7) are designed to rise. 12. Device according to one of claims 7 to 11, characterized in that the cross-sectional area (14) of the coolant channels (3) by means of baffles (3.3.) Via a control or regulation (3.3.1) can be changed. 13. Device according to one of claims 7 to 12, characterized in that that the surface of the coolant channels (3) is provided with a roughness (21) from the mold outlet (7) to the mold inlet (6). 14. Device according to one of claims 7 to 13, characterized in that the roughness (21) from dimples (24) of 0.5 to 3 mm in diameter and 0.5 to 2 mm in depth is formed. 15. Device according to one of claims 7 to 14, characterized in that the distribution or the number of dimples (24) from the mold outlet (7) to the mold inlet (6) is provided increasing. 16. Device according to one of claims 13 to 15, characterized in that the roughness (21) can be changed by chemical or mechanical measures. 17. The apparatus according to claim 16, characterized in that the roughness (21) can be changed during the casting process.