KR20040097142A - Adjustment of heat transfer in continuous casting moulds in particular in the region of the meniscus - Google Patents

Abstract

The invention relates to liquid molten continuous casting, in particular molten steel continuous casting, having a cooling channel 1 such as a cooling groove, cooling slit, or cooling hole on the mold side 2 facing away from the contact surface with the molten metal. It is about. The geometry of the heat exchange face zone of the cooling channel 1 or group of cooling channels is cast in terms of shape, cross-sectional area, perimeter, interface properties, orientation to the contact face, placement on the contact face and / or placement density. By adapting to the local formation of the heat flow density and / or temperature of the contact surface 18 in the casting bath surface zone 11, in particular, the heat transfer of the mold is to be improved.

Description

In particular, continuous casting molds adapted for heat transfer in the surface of the casting bath surface {ADJUSTMENT OF HEAT TRANSFER IN CONTINUOUS CASTING MOULDS IN PARTICULAR IN THE REGION OF THE MENISCUS}

Continuous casting molds of the usual structural type, in the form of plate moulds, in the form of plate moulds, which continuously cast preliminary blocks or slabs of steel, in particular compact strip production (CSP) moulds, are mostly fixed to the supporting walls and contact with the molten metal Side walls each consisting of inner plates are formed. It is preferable that coolant channels parallel to each other be provided on the side of the inner plate facing towards the support wall, which cooling channel can be formed as a slit open towards the support wall.

In CSP molds of the actual structural type, the heat transfer behavior over the mold height, in particular in the areas above and below the casting bath surface (meniscus), can vary within certain limits. For example, on the casting bath surface the wall temperature of the mold drops. However, if the area of the casting bath surface and / or heat transfer thereon is reduced, the temperature of the mold rises. It has the following advantages:

By means of a mold warmed up in the zone of the casting bath surface, the foundry sand dissolves quickly;

Rapid dissolution of the foundry sand improves the lubrication between the billet and the mold, resulting in a better billet surface;

Due to good lubrication, the mold surface below the casting bath surface is lowered, thereby providing reduced thermal stress and reduced tendency to form cracks, thus increasing the endurance life of the mold;

The heated zone of the mold above the casting bath surface reduces the compressive stress in the zone below it. It, in turn, reduces the formation of cracks and improves the durable sleep of the mold.

By measurement in a continuous casting mold, the distribution of heat flow density shows a maximum from 20 to 80 mm below the surface of the casting bath, starting from there and along the shape of a bell-curve in the casting direction and vice versa. It is known to fall. In that case, the range in which the heat flow density rises is about 120 mm.

The corresponding temperature distribution plot of the molten metal in the mold corresponds to a horizontal parabolic curve with t _max in the zone where the heat flow density rises.

Document DE 38 40 448 C2 discloses a continuous casting mold, in particular a plate mold, in which the side walls are each formed by a supporting wall and an inner plate fixed thereto and in contact with the molten metal, in which the interior casting mold faces towards the supporting wall. The surface of the plate is provided with coolant channels lying parallel to each other which are formed as slits open towards the supporting wall, the width of the slit being smaller than the width of the ribs lying between the slits and the depth of the rib being greater than the width of the ribs.

EP 0 551 311 B1 discloses a width-adjustable liquid cooled plate mold which continuously casts steel billets in the form of slabs for thickness in particular less than 100 mm. In such liquid-cooled plate molds, the wide side plates and narrow side plates are formed in a direction of enlarging the cross section with respect to the billet in the direction of their transverse extension, the narrow side plates are arranged approximately parallel to each other over the mold height, and the wide side plates are at least In the area of the smallest slab width, the apex height of the mold wall forming the arc in the cross section is concave up to 12 mm per 1000 mm of slab width to the injection side of the mold compared to the enclosed rectangle. The shape of the wide side plate at the billet outlet end is consistent with the shape of the billet to be manufactured. The wide side plate is formed as a flat surface in the displacement zone of the narrow side plate, and a slit channel is disposed on the side facing away from the side for forming.

EP 0 968 779 A1 relates to the construction of the narrow side of a slab mold having a casting plate with an inner surface and an outer surface opposite thereto, the narrow side having an upper and a lower partial zone, at least the upper zone being the center zone and It has two lateral zones arranged next to it. According to the document, the inner surface of the cast plate has a groove provided with an undercut for the formation of a cooling channel, the groove being shielded formally by a filling piece introduced into the undercut.

U. S. Patent No. 5,207, 266 relates to a water cooled copper mold in which the rear frame forming the cooling channel comprises a copper plate secured thereto, wherein in such a water cooled copper mold the width of the main channel in the region of the fastening bolt is different in the region. Wider than Such a mold includes a configuration in which a larger channel is formed between the right channel and the left channel in the region of the fastening bolt except the bolt screw. A branch channel is provided between the main channel and the enlarged channel, in which case the at least one branch channel and the branch channel section of the main channel have more water surface areas than the main channel and the enlarged channel.

In order to form a crack-free billet shell quickly and surely, in particular uniformly, intensive cooling or heat extraction from the area under the meniscus of the mold to the outlet opening is of critical importance. For that purpose, the following molds are given in the known molds:

-Setting of the velocity of the relatively high coolant,

-Drop in coolant temperature,

Increase in heat exchange area by ribs in the cooling channel region.

Indeed, the aforementioned modifications have already been applied variously in the design of molds for continuous casting.

The contact plates of the mold, which are usually made of copper alloy, are in direct contact with the liquid metal and the solidified metal. Contact plates, also referred to as copper plates, are wear parts and are fixed on support elements, mostly made of steel. Recyclable support elements are called water boxes.

The mold itself acts as a crystallizer. That is, energy is extracted from the introduced liquid steel to the extent that a supportive billet shell is created that can subsequently be subsequently ejected from the mold. In that case, the first billet shell is formed in the so-called meniscus at the height of the filling level in the mold. The concept of meniscus is representative of the early generation zone of the billet shell where the contact surface of the mold, the solid casting aid and the molten casting aid, the liquid steel, and the billet shell meet. Foundry sand and oil are used as casting aids. Such casting aids separate metal and copper from each other by lubrication and control local heat transfer (see FIG. 8).

The billet shell volume element formed in the meniscus is moved through the mold at the withdrawal speed. Based on a given temperature gradient between the liquid steel and the coolant, there is a local energy flow towards the cooling channel. The energy content is carried out via the cooling channel through which the coolant, which is mainly water, flows. Correspondingly, the billet shell thickness is increased.

The cooling channels formed in the mold structure can be configured to be located entirely inside the copper plate or even inside the waterbox. Its mixed structure form is also known. There is also a widespread variant where the filling piece between the waterbox and the copper plate is arranged to form a suitable cooling channel.

For manufacturing reasons, cooling channels with rectangular or circular cross sections are widespread. The corner area may be rounded. However, even U-shaped, L-shaped, and T-shaped shapes can be generated arbitrarily orientated to the contact surface by means of a suitable filling piece. Typical arrangements of cooling channels follow the casting direction, ie from the top to the bottom, individually or in groups, and are mostly equidistant with respect to the contact surface with the metal. The goal of such an effort is to achieve as even cooling as possible over the contact surface of the mold, which is only limited to success in the area of the fixed point. Often, differently formed cooling channels in cross-sectional area and / or geometric shape are combined next to each other to further optimize the uniformity of the cooling action over the casting width (see FIG. 10).

A common characteristic of all such structural forms is that the geometry of the individual cooling channels remains invariant in form and cross-sectional area over their length. Such a configuration implements such that the cooling channel area available for cooling is kept invariant over the cooling channel length. It is also possible to derive that the flow rate remains constant over the cooling channel via the mass balance resin along the virtual streamline.

In that regard, there is a special configuration only for cooling channel holes in which a center displacement pin can be introduced from top to bottom. Since the length of the displacement pin is usually shorter than the hole length itself, cross-sectional narrowing occurs in the cooling channel, which leads to acceleration of the coolant in its transition zone. In that case, coolant flows faster in the constricted cross-sectional area, whereby the cooling action is amplified accordingly. However, by such measures, the effective cooling area for the cooling channel is maintained as it is.

To date, the structural design of conventional cooling channels aims for as uniform cooling as possible, but no consideration is given to the non-uniform thermal load distribution in the mold plates that actually exist. According to the necessary multidimensional considerations, the nonuniformity in the thermal load distribution can be divided into two types.

-Non-uniformity parallel to the casting direction

-Non-uniformity perpendicular to the casting direction

In the casting direction, heat transfer from the liquid steel to the coolant in the cooling channel can be contemplated as a one-dimensional heat conduction through multiple layers. Things to consider in the energy balance equation are:

1. Heat transfer from the liquid steel to the billet shell formed

2. Heat transfer through billet shell

3. Heat transfer through the lubricant layer

4. Heat transfer through copper plate

5. Heat transfer to the coolant

In the static case, the source term need not be considered.

The reason for the non-uniform thermal load distribution over the mold length is in terms of heat conduction through the billet shell, mainly because the billet shell is first created at the casting bath surface and it continues to grow in the casting direction. In other words, as the billet shell thickness itself increases, heat transfer is hindered. Thus, if all remaining parameters are constant, it can be expected that the heat flow will continue to decrease in the casting direction after showing its maximum at the casting bath surface. From the integration over the entire cooling channel length, the average heat flow can be derived. Based on the multidimensionality of the heat flow (no heat entry on the casting bath surface), the theoretical sharp trend in heat flow density is smoothed and the position of the maximum is moved in the casting direction (see FIG. 9).

In-situ measurements of the local heat flow density demonstrate that the local value at the casting bath surface is 1.5 to 3 times higher than the average heat flow and the value at the mold bottom is 0.3 to 0.6 times lower. . The location of the maximum is located 20 to 70 mm below the original casting bath surface depending on the equipment and process parameters. The absolute value of the average heat flow density, on the one hand, depends on the foundry sand, but especially on the casting speed. That is, the literature has an average heat flow density of 1.0 MW / m2 at a casting speed of 0.9 m / min, an average heat flow density of 2.0 MW / m2 at a casting speed of 3.0 m / min, and a casting speed of 5.5 m / min. When the average heat flow density is 3.0 MW / ㎡. Through such factors mentioned, at least a local heat flow density that can be expected can be estimated.

Uneven heat flow density in the casting direction results in a major thermal wear in the casting plate surface in the casting bath surface area with almost no exception. It manifests itself in streaks, cracks, deformations, and in particular in some cases where the previously attached layers are peeled off.

The load of the mold plate is also completely different in the width direction. Inhomogeneities arise mostly from the flow sections of the liquid steel that are formed in the mold. Such a process is closely related to the geometry, contact surface geometry, and other process variables of the immersion troughs supplying the steel. The static and non-static processes in the formation of the casting bath surface result in the uneven formation of the meniscus, which is peculiar to most installations. The formation of such non-uniform meniscus is also accompanied by non-uniform heat distribution, so that the main damages do not form uniformly over the mold width, but rather occur at a specific point.

FIELD OF THE INVENTION The present invention relates to liquid molten continuous casting, in particular molten steel continuous casting, having cooling channels such as cooling grooves, cooling slits, or cooling holes on the mold side facing away from the contact surface with the molten metal.

Hereinafter, the present invention will be described in more detail with reference to Examples. Among the accompanying drawings,

1 is an enlarged cross-sectional view of a section of a mold wall perpendicular to its path of travel;

FIG. 2 is a cross-sectional view of yet another partial piece of the mold wall according to FIG. 1, FIG.

3 is a view showing a cooling channel hole having a groove on an inner surface thereof;

4 and 5 are views showing a comparison portion of the heat exchange surface and the heat exchange surface having an increased bottom surface;

6 is a chart showing the transition of heat flow density q over the height H of the mold below the casting bath surface,

7 is a diagram showing the depth of the grooves R over the height of the mold according to the corresponding trend of the temperature curve T below the casting bath surface with T _max above and below the meniscus zone,

8 is a cross-sectional view of a section of a mold wall with cooling channels and a corresponding heat flow thereof;

FIG. 9 is two plots comparing average heat flow density to overall heat flow density to temperature, and

10 shows a portion of a coolant channel forming an equivalent heat exchange bottom,

11 is a view showing another configuration of the heat exchange floor,

FIG. 12 is a distribution of heat flow density adapted over mold height, showing the distribution of heat flow density with q _max below the casting bath surface.

(Explanation of the sign)

1 cooling groove 2 side

3 grooves 4 depth

5 support wall 6 inner plate

7 cooling channel 8 cooling channel hole

9 Inner Walls 10 Random Parts

11 heat exchange floor 12 grooves

13 zone 14 maximum groove depth

15-17 zone of variable depth of heat exchange trough

18 contact plate 19 billet shell

20 support plate

It is an object of the present invention, starting from the foregoing prior art, to provide localization of the contact surface of the mold in contact with the molten metal by means of a special geometry of the heat transfer face zone of the cooling channel or group of cooling channels for the heat transfer which is critical to the cooling action of the cooling channel. To fit the heat flow.

Such an object is achieved by the invention in accordance with the features of claim 1.

In accordance with the present invention measures are also taken correspondingly to the dependent claims which further influence the heat transfer and thus the cooling action of the cooling channel. In that regard, the shape, cross-sectional area, perimeter, interface properties, relative orientation and contact to the contact surface of the channel can be changed locally, for example, to influence the local cooling action of the channel.

In addition, the effective heat exchange area, for example at the channel bottom or sidewall, can be increased or reduced.

For example, the formation of grooves in the bottom or side of the cooling channel significantly increases the channel by almost two times on its surface, thereby providing a much more intensive cooling action even at the same flow rate of coolant. This results in a significant heat flow density, which is accompanied by a significant drop in the temperature of the mold, which results in a smaller load on the mold material as well as a drop in the cooling water pressure.

In that case, the following values were given by the comparison temperature calculation:

The smooth surface (° C.) of the heat exchange face at the bottom of the cooling groove:

Temperature at billet side 507 ° Temperature at water side 173 °

Augmented surfaces according to the invention

Temperature at billet side 462 ° Temperature at water side 131 °

Car -45 ° Car -42 °

Such figures very clearly demonstrate the positive effects of the measures according to the invention. Artificial increase in the cooling channel surface can be realized by the chisel tool, preferably in the meniscus zone, even in the case of perforated CSP molds.

Other arrangements of the invention are to be countered according to further dependent claims. In that case, an artificial increase of the cooling channel surface is not done on the casting bath surface, since in that region of the mold the heat transfer must be rather reduced to support dissolution between the castings.

The reduction of heat transfer on the casting bath surface is realized by the following:

Inserting the sleeve into the cooling hole on the casting bath surface,

-Coating holes on the surface of the casting bath,

An insert made of low thermal conductivity material is introduced on the casting bath surface.

At the same time, the hot zone of the mold on the casting bath surface reduces the stress of the mold, thereby reducing the formation of cracks in the billet and at the same time improving the availability of the mold.

In that case, it has proved to be very desirable to carry out the heat extraction of the heat transfer face zone of the cooling channel by variably adapting the heat flow density over the height of the mold.

This makes the temperature trend along the mold height even more uniform in the mold and reduces the high material stress in the billet shells that are about to be produced, thus preventing the formation of cracks.

1 shows an enlarged view of the section 10 of the side 2 facing the opposite side of the molten metal with a slit-shaped cooling groove 1 disposed therein. The cooling groove 1 has a width B and a depth T. The bottom section of the cooling groove 1 is formed according to the invention as a profile with a groove 3, so that its area is almost doubled compared to the flat configuration according to FIG. 4, for example.

In that case, the heat dissipation of the heat transfer face region of the cooling groove, cooling slit, or cooling hole is done by adapting to the heat flow density distribution variably over the height of the mold, for example as shown in FIG.

To that end, the trough 3 has a variable depth T of, for example, 1 to 4 mm, with openings of 30 ° to 60 °, respectively, as shown purely by way of example in FIG. 7 for changing the intensity of heat transfer. It is formed at an angle. Groove 3 has an opening angle of up to about 60 ° at intervals A and It can be formed to a height of up to about 4 mm, similar to the profile of the thread. Of course, other forms may also be provided, such as corrugations, trapezoids, sawtooth, and the like, which increase the cooling surface.

FIG. 2 shows a section 10 of a mold wall each comprising a part of the support wall 5 and a part of the inner plate 6, which are in close contact with one another, in particular screwed together. The inner plate 6 is penetrated by a cooling channel 7, which cooling channel 7 opens towards the supporting wall 5 and is shielded by the supporting wall 5. According to the invention, the slit has a heat exchanging face in which the groove string 3 is inserted at the bottom thereof, and as a result, the heat flow density is artificially increased.

3 shows an optional partial piece 10 of a mold wall in which a cooling channel hole 8 with an inner wall 9 formed in the form of a groove or groove 3 is arranged therein.

4 and 5 consist of a smooth form 11 and a groove 12 in accordance with the shown portion of the coolant channels 7, 7 ′ forming the heat exchange bottom 11 or 12 to be compared with each other; It shows the temperature value assigned to it. These figures show that the temperature is much lower in the case of the trough bottom 12 under the strictly identical calculation conditions of the process parameters to be compared.

FIG. 6 shows a heat flow density distribution with q _max in the adjacent zone below the casting bath surface (cast bath) as the heat flow density distribution adapted according to the invention over the height of the mold. Correspondingly, the temperature curve T of FIG. 7 is the temperature maximum T in the zones 13 to 17 of the variable depth R of the heat exchange trough having R _max between points "14" and "15". _max ). The heat exchange trough 3 starts at point 13 at the height of the casting bath surface. At point 14 the maximum groove depth 4 is reached. Such maximum groove depth is reduced to its original level by going up to point "15" and again via point "16".

FIG. 8 shows a support plate 20 to which the contact plate 18 is fixed, a layer of casting aid and the coolant channel 7 shown, a billet shell 19 constructed in the casting direction, and a heat flow that can be assigned thereto. It shows the wide sidewall of the mold.

FIG. 9 complements FIGS. 6 and 7, where the local heat flow density / temperature is compared with the heat transfer cooling channel area depending on the location of the meniscus.

10 to 11 show different arrangements of the cooling slit, in particular different arrangements in the bottom area thereof.

Figure 12 shows the following in the form of a table listed corresponding to such a configuration of the cooling channel:

Channel cross section area

Effective cooling channel wall area

-Clearance to the contact surface

-Effective cooling action given therefrom;

Here, all values are relative and should only be evaluated as illustrative.

Claims (11)

In liquid molten continuous casting, in particular molten steel continuous casting, having a cooling channel such as a cooling groove, cooling slit or cooling hole on the side of the mold facing away from the contact surface with the molten metal, The geometry of the heat exchange face zone of the cooling channel 1 or group of cooling channels is found in the casting operation in terms of shape, cross-sectional area, perimeter, interface properties, orientation to the contact face, placement on the contact face and / or batch density. Mold for liquid molten continuous casting, characterized in that it is adapted to the local formation of heat flow density and / or temperature of the contact surface 18 in the casting bath surface zone 11, in particular. 2. The method according to claim 1, wherein the shape, cross-sectional area, perimeter, boundary surface properties, relative orientation to the contact surface, and the placement and placement density are changed locally to affect the local cooling action of the cooling channel 1. A liquid molten metal continuous casting mold. Mold according to one of the preceding claims, characterized in that the geometry in one or more cooling channels (1) or groups of cooling channels is applied individually or in combination. 4. Mold according to any one of the preceding claims, characterized in that the geometry of the cooling channel (1) or group of cooling channels is formed so as to transition to each other gradually or unexpectedly. 5. The cooling action according to claim 1, in which the cooling action of the cooling channel 1 in the region of the maximum heat flow density or the maximum temperature of the contact surface 18 corresponds to the configuration of the cooling channel 1. Mold for continuous liquid casting, characterized in that maximized. 6. The effective heat exchange area at the bottom or side of the channel is suitably increased or reduced in accordance with any of the preceding claims, in order to influence the local cooling strength of the cooling channel (1). Mold for continuous casting of liquid molten metal. The cooling channel according to any one of claims 1 to 6, wherein, in order to increase the heat exchange area, a groove or trough formed in a cross section of a rectangular, triangular, trapezoidal, partial circle to partial ellipse, or any free form. The additional grooves or troughs are adapted to the course of the cooling channel, the positions of which are parallel or otherwise to the course of the cooling channel. Mold for continuous liquid casting, characterized in that it is adapted. 8. The heat transfer area of the cooling channel 1 according to claim 1, in order to influence the local cooling strength, for example by applying a defined wall roughness which raises the heat transfer or reduces the heat transfer. Mold for continuous liquid casting, characterized in that the change in the interface properties by attaching a layer to make. The method according to any one of the preceding claims, wherein in order to influence the local cooling strength of the cooling channel (1) its isimetric cross-sectional area is increased by introducing additional grooves in the floor or side. Mold for continuous liquid casting, characterized in that the shrinkage by inserting the displacement body. 10. The cooling channel bottom according to any one of claims 1 to 9, in order to affect the local cooling strength of the cooling channel 1 and to preferentially change the coolant flow directed straight to the contact surface. And / or an additional groove is provided on the side or an introduction of an additional displacement body and / or a change in the wall configuration of the cooling channel (1) is provided. The cooling channel per unit length of the spacing and / or placement density, ie, the mold width, in relation to the contact surface, in order to influence the local cooling strength of the cooling channel 1. Mold for continuous liquid casting, characterized in that the number of is arranged locally or entirely.