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

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

[0001] The invention relates to a method and a device for optimizing the cooling capacity of a continuous casting mold for liquid metal, especially for liquid steel, so as to make the thermal load uniform over the height of the continuous casting mold, in which the cooling medium is fed through a cross sectional area having a large number of cooling agent channels or cooling agent bores which run generally parallel to the cast strand, whereby the cooling agent cross sectional areas between the mold inlet and the mold outlet are differently configured.

[0002] The continuous casting mold with which the invention is concerned as a device is known from the German patent DE 41 27 333 C2. The steel melt is poured into a continuous casting mold whose mold walls which extend from the top downwardly, encloses the cooling water circulation and are provided with through-going cylindrical cooling bores whose flow cross section areas are partly reduced by constricting rods. To reduce the temperature difference between the higher regions of the mold and thereby reduce the stresses and increase the life of the mold, the cooling water in the region of the highest temperature loading is fed at a maximum speed through the cooling agent bores. Nevertheless only the bores reduced by the constricting rods leave available only annular cross sectional areas which are traversed by the coolant. Furthermore, the coolant is guided only from the bottom to the top.

[0003] By contrast, the invention has as its object to create a more uniform cooling over the entire height of the continuous casting mold with the greatest possible cooling intensity and so control the copper plate skin temperatures at the hot side and the cold side that the recrystallization temperature of the cold rolled copper is not exceeded at the hot side while possible evaporation of the coolant at the cold side is avoided.

[0004] Its object is achieved in accordance with the invention in that the flow velocity of the coolant, which traverses the continuous casting mold from top to bottom, is set so as to be higher in the coolant channels or coolant bores in the upper region of the continuous casting mold by a smaller cross sectional area than in the lower regions of the continuous casting mold in which the flow velocity is less constricted by a greater cross section area and/or by establishing for a coolant a variable cross sectional shape running from top to bottom. The advantage resides in a greater coverage by coolant in the hot region and by comparison with prior systems a reduced heat obstruction below the hot region. The consequence is not only a reduction in the shock load in the hotter upper region of the casting level by a significant amount but also a greater uniformity of the thermal load over the entire height of the continuous casting mold. In addition, neither the recrystallization temperature of the cold-rolled copper at the hot side is reached nor does the change arise of an evaporation of coolant at the cold side. The inlet cross section of the coolant passage can be square or rectangular and the continuation can respectively be configured as an elongated rectangle to a square or a circular starting cross section can be analogously configured.

[0005] In a refinement of the invention, the invention is so carried out that with casting speeds of 3 m/min to about 12 m/min a heat flow loading of the continuous casting mold of a maximum of 8 MW/m2 and coolant speeds of 4 m/s to 30 m/s are maintained.

[0006] According to further steps it is proposed that a maximum thermal loading at the hot side below 550° C. be set and that the heat transfer coefficient &agr; be set at a maximum of 2500 W/m2·K.

[0007] Another feature which influences the heat values is that the continuous casting mold is oscillated.

[0008] It is provided further that the casting strand be lubricated with casting powder slag in the continuous casting mold.

[0009] A feature supporting the heat transfer resides further in that the surfaces of the coolant channels are provided with a degree of roughness from the mold which increases from the inlet to the outlet.

[0010] In a device for atomizing the cooling capacity of a continuous casting mold for liquid metal, especially for liquid steel by rendering the thermal loading more uniform over the height of the continuous casting mold, whereby the coolant is passed through a cross section with a large number of coolant channels for cooling bores which run generally parallel to the cast strand and whereby the coolant cross sectional area of the coolant channels is differently configured between the mold inlet and the mold outlet, the object which has been set is achieved in accordance with the invention in that the coolant channels or the cooling bores have a relatively small coolant channel inlet cross sectional area and a larger outlet cross sectional area from the mold inlet to the mold outlet together with a greater coverage by the coolant by the mold inlet (under “coverage” the ratio of cooling channel width/coolant channel spacing, that is the effective phase boundary layer copper/coolant is to be understood). Thus the effect of the decay of peak temperatures in the copper plate at the region of the casting level in the mold and the homogenization of the thermal load over the entire height of the continuous casting mold is produced.

[0011] An alternative to the transition from the mold inlet to the mold outlet resides in that there is a change in the cross sectional area shape from the mold inlet to the mold outlet in a continuous manner.

[0012] According to a further feature of the invention it is provided that the casting speed in the continuous casting direction be adjustable up to about 12 m/min.

[0013] The invention is in addition improved when the thermal loading of the continuous casting mold is set at a maximum of 8 W/m2, the coolant speed at 4 to 30 m/s and the maximum local thermal loading of the copper plates at their sides turned toward the liquid metal has a thermal transfer coefficient &agr; of a maximum of 250,000 W/m2·K. A further refinement is obtained if the coolant channel of rectangular cross section have channel depths and/or channel widths which increase from mold inlet to mold outlet.

[0014] An improvement can be obtained in that the cross sectional area of the coolant channel is made variable by a control or regulation of baffles. In this manner the flow of the coolant in the fixed shape of the coolant channel can have a further function imparted to it. Another further development is provided in that the surface area of the coolant channel is provided with a roughness from the mold outlet up to the mold inlet.

[0015] In this case the roughness should consist of small pits of a diameter of 0.5 to 3 mm and a depth of 0.2 to 2 mm.

[0016] Finally the distribution or the number of pits is provided to increase from the mold outlet to the mold inlet.

[0017] The heat transfer can be made more intensive according to a further feature in that the roughness is variable by chemical or mechanical means.

[0018] In that case it is additionally of advantage that the roughness be variable during the casting process.

[0019] In the drawing embodiments of the state of the art and the invention are shown which are explained in greater detail in the following. In the drawing:

[0020] FIG. 1A (respectively from left to right) is a vertical section through the present continuous casting mold in the upper part having two horizontal partial sections for coolant channels and coolant bores in the upper mold region and in the lower part having two horizontal partial sections for coolant channels and coolant bores of the lower mold region and showing the temperature pattern in the copper plates,

[0021] FIG. 1B is analogous to FIG. 1A (respectively from left to right) showing a vertical section through the continuous casting mold, in the upper part three horizontal partial sections for coolant channels and coolant bores in the upper mold region and in the lower part three horizontal partial sections for coolant channels and coolant bores in the lower mold region and the new surface temperature pattern which contrasts with the previous surface temperature pattern and the difference between the previous surface temperature pattern and the new surface temperature pattern.

[0022] FIG. 2A is a diagram of the thermal transfer coefficient &agr;, the maximum thermal loading and the pressure loss in the coolant,

[0023] FIG. 2B is a diagram of the heat transfer coefficient &agr;, the pressure loss &Dgr;P with respect to coolant speed and

[0024] FIG. 2C is a diagram of the reduction of the maximum thermal loading with increased coolant velocity.

[0025] In the continuous casting of liquid metal, especially liquid steel, a continuous casting mold 1 is used (FIG. 1A) which is comprised of copper plates 2 each with a large number of coolant channels 3 or coolant bores 4 with or without displacement or constricting rods 4.1 through which the coolant 5 is fed.

[0026] At the casting level 8 in which the shell 9 of the cast strand begins its formation, there is the greatest local heat flow 10 (J) and simultaneously the largest value of the thermal mold loading Tcu-max 11 both on the hot side 11.1 and also one cold side of the copper plate 2.

[0027] The thermal loading at the casting level 8 or the maximum heat flow 10 (“J”) can, especially at high casting speeds of about 12 m/min amounts to as much as 8 MW/m2 and requires as a result special cooling features so that the copper plate skin temperature at the hot side 11.1 and the cold side 11.2 are so controlled that the recrystallization temperature of the cold rolled copper is not exceeded on the hot side 11.1 and so that possible evaporation of the coolant 5 on the cold side 11.2 can be avoided.

[0028] The cooling capacity or the cooling effect are determined by mechanical characteristics like for example the copper plate thickness 12, the coolant channels 3 or coolant bores 4 with or without displacement rods 4.1, the spacing 13 (A) of the coolant channels 3 or coolant bores from one another, the cross sectional areas 14 (F) of the coolant channels 3 or the coolant bores 4 and the lengths of the coolant channels 3 or the coolant bores 4, which correspond to the mold length 15 (L). As state of the art, up to now the coolant channel cross sectional areas 14 were provided as constant between the mold inlet 6 and the mold outlet 7. The process determining influential parameters for the cooling capacity of the continuous casting mold 1 are, aside from the coolant temperature, the coolant speed or velocity 16 which is an important value for the heat transfer coefficient 17 (&agr;) measured in W/m2·K.

[0029] The relationships are illustrated in FIGS. 2A, 2B and 2C in diagrams.

[0030] To set a desired heat transfer with the aid of a certain coolant seed 16 in the continuous casting mold 1 by the cross sectional area 14 of the coolant channel 3 or the coolant bores 4 and a predetermined mold width 18, here normalized to 1 m, and spacing 13 of the coolant channel, a pressure drop 19 (&Dgr;P) in the coolant between the mold inlet 6 and the mold outlet 7 is established.

[0031] This pressure drop rises superficially with the coolant velocity 16 (V) or with the coolant volume rate of flow 20 (Q) measured in m3/h·m.

[0032] In addition, it is to be noted with an increasing roughness 21 (R) of the surfaces of the coolant channels 3 or the coolant bores 4, the transfer coefficient 17 (&Dgr;P) rise.

[0033] The target of the invention is to minimize the pressure drop 19 (&Dgr;P) while controlling the maximum thermal loading 11 (Tcu-max) both on the hot side 11.1 and the cold side 11.2 and thereby to homogenize the thermal mold loading 22 or the thermal profile 23 over the mold length 15. In FIGS. 2A, 2B and 2C, the heat transfer coefficient 17 (&agr;) and the maximum thermal loading 11 of the copper plate has been shown as a function of the structural and process parameters like for example

[0034] the coolant velocity 16 (V)

[0035] the coolant volume flow rate 20 (Q)

[0036] the pressure drop 19 (&Dgr;P)

[0037] the roughness 21 (R) of the surfaces

[0038] under respective predetermined and constant boundary conditions.

[0039] The cast strand 9 is poured according to FIG. 1B with a casting speed 9.1 of about 12 m/min, for example, in a casting size corresponding to thin slabs with a thickness between 20 mm and 100 mm. During the casting, casting powder 1.2 as well as an oscillation 1.1 can be used. The casting process loads the continuous casting mold 1 with a maximum heat flow 10 (“J”) at the casting level 8 of 2 to 8 MW/m2 which is a maximum thermal loading 11 at the casting level and the hot side 11.1 on which the liquid steel is located as well as at the cold side 11.2 at which the current 5 is located.

[0040] The process gives rise to a thermal mold loading 22 and a heat flow profile 23 over the mold length 15 (L). The coolant channel cross sectional area 14 (F) in the cooling channel 3 or cooling bores 4 with or without displacement rods 4.1 are in the state of the art system (FIG. 1A) constant over the mold length 15 and give rise to a constant coolant velocity 16 (V) and to a defined coolant pressure drop 19 (&Dgr;P) which has been designated as unity “1”.

[0041] In the extreme right hand illustration of FIG. 1B, the temperature pattern of the surface temperature has been shown by comparison with that of FIG. 1A in which the heat quantity carried off is the same in the two tests.

[0042] So that the thermal loading 22 of the mold is homogenized and the pressure drop 19 (&Dgr;P) of the coolant 5 is minimized, the cross sectional area 14 (F) of the coolant channels 3 or the coolant bores 4 are increased from the mold inlet 6 to the mold outlet 7 (FIG. 1B). In addition the roughness 21 (R) can also selectively increase from the mold outlet 7 to the mold inlet 6 over the mold length 15.

[0043] The roughness 21 can be produced by pits 24 of a maximum diameter of 1 to 3 mm and depth of 1 to 2 mm to produce cavitation effects in the flowing coolant 5 (for example the water) at the phase boundary between the copper (for side 11.2) and the coolant 5 and thereby give rise to an increase heat transfer coefficient 17, (&agr;) as brought about by forced convection in the lamina, seeded boundary layer in which the energy transport occurs by thermal conductivity.

[0044] The increase in the cross sectional area 14 of the coolant channel 3 or the coolant bores 4 over the length of the mold can be effected in the case of cooling channel 3 by means of the variation in the channel depth 3.1 and/or of the channel width 3.2 in the case of the coolant bores 4, the cross sectional enlargement can be effected by increasing the diameter of the coolant bores 4 and/or a reduction in the diameter of the displacement rods 4.1.

[0045] Another configuration provides that baffle plates 3.3 of the coolant channels 3 can be set manually or automatically mechanically to vary the cross sectional areas 14 of the coolant channel 3 over the mold height 15, for example, using an on-line process control with a control or regulation 3.3.1 to adjust the positions of the baffles 3.3.

[0046] After carrying out this described embodiment, the thermal mold loading 22 over the mold length 15 is lowered through the homogeneous thermal profile 22.1 which has been shown in the right-hand part of FIG. 1B in a graph.

[0047] The diagram 2A plots the heat transfer coefficient 17 (&agr;) measured in W/m2K, pressure losses 19 (&Dgr;P) and the liquid maximum thermal loading 11 of the copper plate 2 at the casting level 8 as a function of the roughness 21 of the surfaces of the coolant channels 3 or the coolant bores 4 at a constant copper plate thickness 12, coolant speed 16 (V in m/s), heat flow 10 (J) cross sectional areas 14 for the coolant channels 3 or the coolant bores 4, the mold length 15 and the spacing 13 of the coolant channel 3 or coolant bores 4 from one another. The graph makes clear that with increasing roughness 21 (R) the heat transfer coefficient 17 (&agr;) and also the pressure drop 19 (&Dgr;P) increase steadily but also that simultaneously the copper plate temperature 11 (Tcu-max) at the hot side 11.1 and the cold side 11.2 fall rapidly.

[0048] In graph 2B, the variation of the heat transfer coefficient 17 (&agr;) and the pressure loss 19 (&Dgr;P) with the coolant velocity 16 (V) or the coolant flow quantity 20 (Q) with increasing roughness 21 at constant cross section 14 (F), mold lengths 15 and spacing 13 (A) is shown. It is clear from this that with increasing coolant speed 16 (V), coolant flow quantity 20 (Q) and roughness 21 (R) the heat transfer coefficient 17 (&agr;) and also the pressure loss 19 (&Dgr;P) have fallen superproportionally.

[0049] In FIG. 2C the drop in the maximum thermal loading 11 at the casting level 8 of the copper plate 2 with increasing coolant speed 16 (V), coolant quantity 20 (Q) and roughness 21 (R) at constant the flow 10 (J) has been shown in the heat flow profile 23 over the mold length 15 the copper plate thickness 12, the coolant channel cross section 14 (F) and the spacing 13 (A) of the coolant channels 3 or the coolant bores 4.

[0050] The partial illustration in FIG. 2C makes clear that the liquid maximum thermal loading 11 at the casting level 8 with rising roughness (R) the coolant speed 16 (V) or the coolant quantity 20 (Q) sinks easily.

[0051] The principle of the invention also operates with strip casting devices which operate with speeds of up to 100 m/min. All of the features which are applicable over the height of the continuous casting mold are applicable to the periphery of the twin rollers.

Reference Character List

[0052] 1. Continuous casting mold

[0053] 1.1 Oscillation

[0054] 1.2 Casting powder, casting slag

[0055] 2 Copper plate

[0056] 4 Coolant channel

[0057] 3.1 Channel depth

[0058] 3.2 Channel width

[0059] 3.3 Baffle

[0060] 3.3.1 Control/regulation of the position of the baffle

[0061] 4 Coolant bore

[0062] 4.1 Displacement tube, bar, round body

[0063] 5 Coolant

[0064] 5.1 Coolant flow direction

[0065] 6 Mold inlet (upper edge of mold)

[0066] 7 Mold outlet (lower edge of mold)

[0067] 8 Casting level

[0068] 9 Stand shell, strand

[0069] 9.1 Casting speed

[0070] 10 Local maximum heat flow “J” at casting level

[0071] 11 Local maximum thermal loading at casting level (Tcu-max)

[0072] 11.1 Side turned toward the molten steel (hot side)

[0073] 11.2 Side turned toward the coolant (cold side)

[0074] 12 Copper plate thickness (between hot side and cold side)

[0075] 13 Spacing of coolant channels (3) or coolant bores (4) from one another

[0076] 14 Cross sectional area (F) of the coolant channel (3) or the coolant bore (4)

[0077] 15 Length of the coolant channel, coolant bores, the mold

[0078] 16 Coolant speed (V in m/s)

[0079] 17 Heat transfer coefficient &agr; in W/m2·K

[0080] 18 Mold width (in m)

[0081] 19 Pressure loss of the coolant, &Dgr;P

[0082] 20 Coolant quantity Q in m3/h·m

[0083] 21 Roughness R, in mm of the surface

[0084] 22 Thermal mold loading over the mold length

[0085] 22.1 Homogeneous thermal profile (Tcu-max)

[0086] 23 Heat flow profile over the mold length

[0087] 24 Pits, pockets

Claims

1. A method of optimizing the cooling capacity of a continuous casting mold for liquid metal, especially for liquid steel by homogenizing the thermal load over the height of the continuous casting mold in which the coolant is passed through a cross sectional area of a large number of coolant channels or coolant bores which run generally parallel to the cast strand, whereby the coolant cross section between the mold inlet and the mold outlet is configured differently, characterized in that the flow cross section of the coolant which is passed through the continuous casting mold from the top to the bottom, is set to be higher in the coolant channels or coolant bores of the upper region of the continuous casting mold as a result of a smaller cross sectional area than in the lower region of the continuous casting mold in which the flow velocity is adjusted to be less than a greater cross sectional area and/or in that the coverage with coolant has a cross sectional shape which varies from the top to the bottom.

2. The method according to claim 1, characterized in that at a casting speed of 3 m/min to about 12 m/min a heat flow loading of the continuous casting mold of a maximum of 8 MW/m2 and a coolant speed of 4 m/s to 30 m/s is maintained.

3. The method according to claim 1, characterized in that a maximum thermal loading of the mold plates at their hot side is less than 550° C. and that the heat transfer coefficient &agr; is set up to 250,000 W/m2·K.

4. The method according to claim 1, characterized in that the continuous casting mold is oscillated.

5. The method according to claim 1, characterized in that the cast strand is lubricated with casting powder slag in the continuous casting mold.

6. The method according to claim 1, characterized in that the surface of the cooling channels is provided with increased roughness from mold inlet to mold outlet.

7. An apparatus for optimizing the cooling capacity of a continuous casting mold for liquid metal, especially for liquid steel by homogenizing the thermal load over the height of the continuous casting mold in which the coolant is passed through a cross sectional area of a large number of coolant channels or coolant bores which run generally parallel to the cast strand, whereby the coolant cross section between the mold inlet and the mold outlet is configured differently, characterized in that the coolant channels (3) or the coolant bores (4) respectively have a relatively small coolant channel inlet cross section (14) and a greater outlet cross section from the mold inlet (6) to the mold outlet (7) with a greater coverage of the mold inlet (6) by the coolant (5).

8. The apparatus according to claim 7, characterized in that the variation of the cross sectional shape from mold inlet (6) to the mold outlet (7) is continuous.

9. The apparatus according to claim 7, characterized in that the casting speed in the continuous casting direction is adjustable up to about 12 m/min.

10. The apparatus according to claim 7, characterized in that a thermal loading (11) of the continuous casting mold (1) of a maximum of 8 MW/m2, a coolant speed (16) of 4 to 30 m/s and a maximum local thermal loading (11) of the copper plates (2) on the liquid metal side (11.1) are provided with a heating transfer coefficient &agr; of a maximum of 250,000 W/m2·K.

11. The apparatus according to claim 7, characterized in that the coolant channels (3) have rectangular cross sections and increase in the channel depths (3.1) and/or channel widths (3.2) from mold inlet (6) to mold outlet (7).

12. The apparatus according to claim 7, characterized in that the cross sectional areas (19) of the cooling channels (3) are variable by means of baffles (3.3) through a control or regulation (3.3.1).

13. The apparatus according to claim 7, characterized in that the surface of the coolant channels (3) are provided with a roughness (21) from the mold outlet (7) to the mold inlet (6).

14. The apparatus according to claim 7, characterized in that the roughness (21) is formed by pits (24) of 0.5 to 3 mm diameter and 0.5 to 2 mm depth.

15. The apparatus according to claim 7, characterized in that the distribution or the number of pits (24) increases from the mold outlet (7) to the mold inlet (6).

16. The apparatus according to claim 13, characterized in that the roughness (21) is variable by chemical or mechanical features.

17. The apparatus according to claim 16, characterized in that the roughness (21) is variable during the casting process.