WO2003092931A1

WO2003092931A1 - Adjustment of heat transfer in continuous casting moulds in particular in the region of the meniscus

Info

Publication number: WO2003092931A1
Application number: PCT/EP2003/002384
Authority: WO
Inventors: Dirk Mangler; Markus Reifferscheid; Uwe Plociennik
Original assignee: Sms Demag Aktiengesellschaft
Priority date: 2002-04-27
Filing date: 2003-03-08
Publication date: 2003-11-13
Also published as: BR0307901A; CA2483784A1; EP1499462A1; MXPA04010647A; TW200400093A; RU2310543C2; RU2004134598A; JP2005529750A; PL371553A1; AU2003233795A1; CN1649685A; TWI268821B; US20050115695A1; CN1318164C

Abstract

The invention relates to a mould for the continuous casting of molten metals, in particular steel, comprising cooling channels (1), such as cooling grooves, cooling slits or cooling drillings in the side of the mould (2) away from the melt contact surface. According to the invention, the heat transfer in the mould can be improved, whereby the geometric arrangement of the heat transfer planar surfaces of a cooling channel (1) or a group of cooling channels is adjusted in form, cross-sectional area, circumference, boundary surface qualities, orientation with respect to contact surfaces, arrangement and/or arrangement density with respect to the contact surfaces for the local formation of thermal flux density and/or temperature of the contact surfaces (18) during casting operation and in particular in the region of the meniscus (11).

Description

ADJUSTING THE HEAT TRANSFER AT STRANGGIESS OKIL EN, ESPECIALLY IN THE MOLDING MIRROR AREA

The invention relates to a mold for the continuous casting of molten metals, in particular steel, with cooling channels such as cooling grooves, cooling slots or cooling bores in the mold side facing away from the contact surface with the melt.

A continuous casting mold, in particular a CSP (Compact Strip Production) mold of conventional design in the form of a plate mold, for the continuous casting of ingots or slabs made of steel, is usually formed with side walls, each consisting of a supporting wall and one attached to it, with the metal melt in Conical inner plate exist. Coolant channels which are parallel to one another are preferably provided on the side of the inner plate facing the supporting wall and can be designed as slots which are open to the supporting wall.

With current CSP molds, the heat transfer conditions above the mold height, especially in an area above and below the bath level, are variable within limits. For example, the wall temperature of the mold above the bath level is lowered. However, if the heat transfer in the area and / or above the bath level is reduced, the temperature of the mold increases. This has the following advantages:

The mold, which is warmer in the area of the bath level, means that casting powder is melted faster;

Faster melting of the mold powder increases the lubricating effect between the strand and the mold, with the result of a better strand surface;

- Better lubrication leads to a lower mold surface below the bath level, which results in reduced thermal Tensions and reduced tendency to form cracks, consequently longer tool life;

Warmer areas of the mold above the bath level reduce the compressive stresses in areas below it. This also reduces crack formation and leads to longer tool life.

It is known from measurements on continuous casting molds that the distribution of the heat flow densities below the bath level has a maximum between 20 and 80 mm, in order to decrease therefrom both in the casting direction and in the opposite manner in the manner of a bell curve. The area of increased heat flow density is approximately 120 mm.

An assignable diagram of the temperature distribution of the melt in the mold corresponds to the curvature of a lying parabola with t _ma χ in the area of the increased heat flow density.

Document DE 38 40 448 C2 describes a continuous casting mold, in particular a plate mold, the side walls of which are each formed by a supporting wall and an inner plate attached to it and coming into contact with molten metal, and wherein coolant channels are provided on the side of the inner plate facing the supporting wall, which are designed as slots open to the supporting wall, the width of which is smaller and the depth of which is greater than the width of the ribs lying between the slots.

EP 0 551 311 B1 describes a liquid-cooled, width-adjustable plate mold for the continuous casting of steel strands in slab format, in particular for a thickness of less than 100 mm. In this case, the broad side plates and narrow side plates are designed in the direction of their transverse extension in the sense of an increase in cross section for the strand, the narrow side plates are arranged essentially parallel to one another over the mold height and the broad side plates are concave at least in the area of the smallest slab width designed in such a way that in cross-section the apex height of the mold wall forming an arc compared to an inscribed rectangle on the pouring side of the mold is a maximum of 12 mm per 1000 mm slab width and the shape of the broad side plates at the strand exit end of the mold corresponds to the strand format to be produced. The broad side plates are designed as a flat surface in the adjustment range of the narrow side plates and slot-like channels are arranged in the side facing away from the shaping side.

EP 0 968 779 A1 relates to the formation of a broad side of a slab mold, with a casting plate with an inner surface and an outer surface opposite this, the broad side having an upper and a lower partial region, and at least the upper partial region having a central region and two laterally has side areas arranged therefrom. The document proposes that the inner surface of the pouring plate has grooves with undercuts to form cooling channels, and that the grooves are covered by filler pieces that are inserted into the undercuts.

U.S. Patent 5,207,266 relates to a water-cooled copper mold comprising a copper plate with a rear frame attached thereto to form cooling channels, wherein widths of main channels in the region of the mounting bolts are wider than those in other regions. The mold includes the formation of larger channels between right-hand and left-hand channels in the region of the fastening bolts, excluding the bolt connections. Branch channels are provided between the main channels and the enlarged channels, wherein at least branch channels and areas of the main channels have more water surface areas than the main and enlarged channels.

Intensive cooling or heat dissipation from the area below the meniscus to the mold outlet from of crucial importance. The known possibilities for known molds are as follows:

Setting a relatively high cooling water speed, lowering the cooling water temperature, - enlarging the heat exchanger surfaces in the cooling channels by means of cooling fins.

The aforementioned variants are already used in practice in the design of molds for continuous casting plants.

The contact plate of the mold, which usually consists of a copper alloy, is in "direct contact" with the liquid and solidified metal. The contact plate, which is also referred to as a copper plate, is a wearing part and is attached to a carrier element, usually made of steel. The recyclable support element is called a water box.

The mold itself acts as a crystallizer, i.e. H. So much energy is withdrawn from the liquid steel that it is brought in that a stable strand shell is created which can then be continuously pulled out of the mold. A first strand shell is formed at the fill level in the mold on the so-called meniscus. The term meniscus stands for the early development area of the strand shell, in which the contact surface of the mold, solid and molten pouring agent as well as liquid steel and strand shell meet. Casting powder and oils are used as pouring aids. These separate metal and copper from one another by lubrication and control the local heat transfer (Fig. 8).

The first strand shell volume element formed on the meniscus migrates with it

Pull-off speed through the mold. Due to the given temperature gradient between liquid steel and cooling medium, a local one arises

Energy flow in the direction of the cooling channels. Its energy content is about with the coolant, usually water, discharged through cooling channels. The strand shell thickness increases accordingly.

The cooling channels formed in the mold construction can be made completely within the copper plate or within the water box element. Mixed constructions are also known. In addition, variants are widespread in which filler pieces are arranged between the water tank and the copper plate in such a way that suitable cooling channels are created.

Cooling ducts with rectangular or circular cross sections are widely used for manufacturing reasons. Corner areas can be rounded. Suitable fillers also produce U, L and T shapes of any orientation with respect to the contact surface. The typical arrangement of the cooling channels follows the casting direction individually or in groups, i. H. from top to bottom, and usually equidistant from the contact surface to the metal. The aim of the efforts is to achieve the most homogeneous possible cooling effect via the contact surface of the mold, which is often only possible to a limited extent in the area of fastening points. Often, differently designed cooling channels are combined side by side in cross-sectional area and / or geometric shape in order to further optimize the uniformity of the cooling effect across the casting width (FIG. 10).

Common to all of these designs is the fact that the geometry of an individual cooling slot remains unchanged over its length in terms of shape and cross-sectional area. This design implements that the cooling channel area that can be used for cooling remains unchangeable over the cooling channel length. From the quantity balance along an imaginary flow thread, it can also be deduced that the flow velocity remains constant over the length of the cooling channel.

In this regard, there is only a special version for cooling channel bores into which central displacement pins can be inserted from above or below. Since the length of the displacement pin is usually shorter than the drilling length, the cross-section narrows in the cooling channel, which leads to an acceleration of the cooling medium in this transition area. The cooling medium then flows faster in the narrowed cross-sectional area, which increases the cooling effect accordingly. The cooling surface effective for the cooling duct remains unaffected by this measure.

The conventional design of the cooling ducts hitherto aims for a homogeneous cooling effect, whereby the actually existing, inhomogeneous thermal load distribution on the mold plate is not taken into account. Due to the necessary multi-dimensional consideration, two inhomogeneities in the thermal load distribution can be distinguished.

Inhomogeneity parallel to the pouring direction Inhomogeneity perpendicular to the pouring direction

In the casting direction, the heat transfer from the molten steel into the cooling medium in the cooling channel can be viewed in simplified terms as one-dimensional heat conduction through several layers. The energy balance equation must take into account:

1. Heat transfer from the liquid steel into the strand shell formed

2. Heat conduction through the rod shell

3. Heat conduction through the lubricant layer

4. Heat conduction through the copper plate

5. Heat transfer into the cooling medium

In the stationary case, source terms are not to be taken into account.

One reason for the non-uniform, thermal load distribution over the length of the mold lies in the term of heat conduction through the strand shell, since a strand shell is formed in the casting mirror and this continues to grow in the casting direction. The heat transfer thus impedes itself with increasing Strand shell thickness itself. If all other parameters are set constant, it can be expected that the heat flow at the casting level will have its highest value and then decrease continuously in the casting direction. An average heat flow can be derived from the integration over the entire length of the cooling channel. Due to the multi-dimensionality of the heat conduction - there is no heat input above the pouring level - the theoretically sharp course of the heat flow density will smooth out and the position of the maximum will shift in the pouring direction (FIG. 9).

Operating measurements of local heat flow densities show that compared to the mean heat flow, the local values in the area of the mold level can be higher by a factor of 1.5 to 3, whereas the values at the mold base can be lower by a factor of 0.3 to 0.6. The location of the maximum is 20 to 70 mm below the actual mold level, depending on the system and process parameters. The absolute values of the average heat flow densities depend on the one hand on casting powder, but in particular also on the casting speed. In the literature, mean heat flow densities of 1.0 MW / m ² at 0.9 m / min, 2.0 MW / m ² at 3.0 m / min and 3.0 MW / m ² at 5.5 m / min are mentioned. The expected local heat flow densities can at least be estimated from the factors mentioned.

The uneven distribution of the heat flow density in the casting direction means that the main thermal wear on the mold plate takes place almost without exception in the area of the casting surface. This manifests itself in striations, cracks, deformations and even flaking of any previously applied layers.

The load on the mold plate also varies in the width direction. Inhomogeneities usually result from the flow field of the liquid steel that forms in the mold. The processes are closely linked to the geometrical design of the steel feed plunger, the contact surface geometry and other process variables. Stationary and transient processes on the mold level formation result in a mostly plant-specific inhomogeneous formation of the meniscus. Inhomogeneous meniscus formation is also associated with an inhomogeneous heat distribution, so that the main damage does not develop evenly over the mold width, but begins concentrated at certain points.

Proceeding from the aforementioned prior art, the invention is based on the object of adapting the heat transfer which is decisive for the cooling effect of the cooling channels by a special geometric configuration of the heat-transferring surface areas of a cooling channel or a group thereof to the local heat flow density of the contact surface of the mold in contact with the melt ,

The object is achieved with the invention according to the features of claim 1.

Further influences according to the invention on the heat transfer and thus on the cooling effect of the cooling channel or channels are provided in accordance with the subclaims. Here, for. For example, to influence the local cooling effect of a channel, its shape, cross-sectional area, circumference, boundary surface condition, orientation and arrangement can be locally varied relative to the contact area.

Furthermore, e.g. B. the effective heat exchange surfaces on the channel base or on the side walls may be enlarged or reduced.

For example, by forming grooves in the base or side surfaces of the cooling channels, the surface area is substantially enlarged or almost doubled, which leads to a higher heat flow density with a considerably more intensive cooling effect at the same flow rate of the cooling medium, with the significant advantage that the temperatures of the mold be significantly reduced, so that in addition to the lower load on the mold If necessary, the water pressure for the cooling water can also be reduced.

Comparative temperature calculations have shown the following values, for example: - smooth surface of the heat exchanger surface at the bottom of cooling grooves (degrees

C):

507 ° temperature to the strand 173 ° temperature to the water

- increased surface area according to the invention

462 ° temperature to the strand 131 ° temperature to the water - -45 ° difference -42 ° difference.

The numbers clearly demonstrate the positive effect of the measure according to the invention. An artificial enlargement of the cooling channel surfaces can also be achieved with drilled CSP molds, preferably in the meniscus area, using a broaching tool.

Other refinements of the invention are provided in accordance with further subclaims. The cooling channel surface is not artificially enlarged above the bath level, because in this area of the mold the heat transfer should rather be reduced in order to support the melting of the mold powder.

The heat transfer above the bath level is reduced by:

Use of sleeves in cooling bores above the bath level, coating the bores above the bath level, insertion of inserts made of less heat-conducting material above the bath level. At the same time, a warmer area of the mold above the bath level reduces the tension in the mold and thus reduces the formation of cracks in the strand while increasing the availability of the mold.

The measure that the heat dissipation of the heat-transferring surface areas of the cooling channels is carried out by adapting to the heat flow density distribution over the height of the mold has proven to be particularly expedient.

As a result, the temperature profiles along the mold height in the mold are made even more uniform and larger material tensions in the strand shell being created are avoided and their formation of cracks is prevented.

The invention is then explained in more detail using exemplary embodiments. Show it:

1 shows a section of a mold wall, in an enlarged section, perpendicular to its course,

FIG. 2 another section of the mold wall according to FIG. 1, also in section,

FIG. 3 cooling channel bores with grooves on their inner surfaces,

FIGS. 4 and 5, comparative parts of heat exchange surfaces without and with an enlarged base surface,

FIG. 6 shows the course of the heat flow density q over the height H of the mold below the bath level, FIG. 7 shows a diagram of the depth of the grooves R above the height of the mold with an associated profile of a temperature curve T, likewise below the bath level with T _max above and below the meniscus area,

8 shows in section a piece of a mold wall with cooling channels and associated heat flow,

FIG. 9 shows two diagrams for comparison, with the mean or global heat flow density or temperature,

Figure 10 parts of coolant channels with comparable training

Heat exchange floors,

FIG. 11 further forms of training of heat exchanger plates,

FIG. 12 shows a distribution of the over the mold height

Heat flow density distribution with q _max below the bath level.

FIG. 1 shows an enlarged section 10 of a side 2 of a mold wall facing away from the melt with a slot-like cooling groove 1 arranged therein. This has a width B and a depth T. According to the invention, the bottom region of the cooling groove 1 has a profile 3 with grooves - Forms, whereby its area compared to a flat design, eg. B. according to Figure 4, is approximately doubled.

In this case, the heat transfer of the heat-transferring surface areas of the cooling groove slots or bores can be carried out by varying the height of the mold to its heat flow density distribution, as is shown, for example, in FIG. 6. For this purpose, it is provided that the grooves 3 have a variable depth 4, for example between 1 and 4 mm, for the purpose of varying the intensity of the heat transfer, and are each formed with an opening angle between 30 ° and 60 °, as shown purely by way of example in FIG. 7 is. The grooves 3 can be formed with an opening angle of up to approx. 60 ° and a height of up to approx. 4 mm at intervals "A" and resemble the profile of a thread. Of course, other shapes, such as corrugated, trapezoidal, tooth-shaped or the like, can be provided, which lead to an enlargement of the cooling surface.

FIG. 2 shows a section 10 of a mold wall, each comprising a piece of a support wall 5 with a piece of an inner plate 6, which are connected tightly to one another, in particular are screwed together. The inner plate 6 is penetrated by cooling channels 7, which are designed as slots that are open against the supporting wall 5 and covered by the supporting wall 5. According to the invention, the slots are provided on their bottoms with heat exchanger surfaces 3 penetrated by grooves, which result in an artificially increased heat flow density.

FIG. 3 shows any section 10 of a mold wall with cooling channel bores 8 arranged therein with inner walls 9 designed in the form of grooves or grooves 3.

FIGS. 4 and 5 show a smooth 11 and a configuration consisting of grooves 12 and the associated temperature values on the basis of indicated parts of coolant channels 7, 7 ', with the formation of heat exchanger plates 11 and 12 to be compared with one another. For the version with a grooved base 12, these show a significant reduction in the temperatures under strictly identical determination conditions of the process parameters to be compared. FIG. 6 shows a heat flow density distribution according to the invention, adjusted over the height of the mold, with q _ma χ in a limited area below the bath level (bath). Correspondingly, the temperature curve T in FIG. 7 shows a temperature maximum T _max within a range 13 to 17 of variable depth R of the heat-exchanging grooves with Rm _ax between points 14 and 15. The heat exchanger grooves (3) start at 13 at the level of the bath level. The maximum groove depth (4) is reached at 14. This maximum groove depth goes up to 15 and is reduced again to the original level on the way over 16.

FIG. 8 shows in section a broad side wall of a mold, comprising a support plate 20 with a contact plate 18 fastened to it, a layer of pouring aid and indicated coolant channel 7, a strand shell 19 building up in the casting direction and an assignable heat flow.

FIG. 9 represents a supplement to FIGS. 6 and 7, with the course of the local heat flow density / temperature as shown in diagrams in comparison to the heat-transferring cooling channel surface as a function of the position of the meniscus.

Figures 10 and 11 show different design options in the design of cooling slots, and in particular of their bottom region.

Associated with these configurations of the cooling channels, FIG. 12 shows in the form of a table:

the channel cross-sectional areas, the effective cooling channel wall surfaces, the distance from the contact area, the resulting effective cooling effect,

where all values are relative values and can only be evaluated as examples. LIST OF REFERENCE NUMBERS

1. Cooling grooves

2. opposite side 3. scoring

4. Depth

5. Retaining wall

6. Inner plate

7. Coolant channel 8. Coolant hole

9. Wall part

Section 10

11. Start of the heat exchanger grooves at the level of the bath

12. Maximum groove depth 13. End of the maximum groove depth

14. End of deepening of the grooves

15th - 17th constant groove depth reached

18. Contact plate, contact surface

19. Strand shell 20. Support plate

Claims

claims

1. Mold for the continuous casting of molten metals, in particular steel, with cooling channels (1) such as cooling grooves, cooling slots or cooling bores in the mold side (2) facing away from the contact surface with the melt, characterized in that the geometric designs of the heat-transferring surface areas a cooling duct (1) or a group of cooling ducts in the form, cross-sectional area, circumference, boundary surface condition, orientation to the contact surface, arrangement and / or arrangement density relative to the contact surface of the local formation of heat flow density and / or temperature of the contact surface (18) in the casting operation, and in particular in the area of the mold level (11).

2. Chill mold according to claim 1, characterized in that in order to influence the local cooling effect of a cooling channel (1), its shape, cross-sectional area, circumference, boundary surface quality, orientation and arrangement as well as arrangement density is locally varied relative to the contact surface.

3. Mold according to claims 1 or 2, characterized in that the geometric configurations in at least one cooling channel

(1) or a group of cooling channels can be used individually or in combination.

4. mold according to one or more of claims 1 to 3, characterized in that the geometric configurations of a cooling channel (1) or one

Group of channels are flowing or jumping into one another.

5. Chill mold according to one or more of claims 1 to 4, characterized in that the cooling effect of the cooling channels (1) is maximized in accordance with the configuration of the cooling channels (1) in the region of the maximum heat flow density or the maximum temperature of the contact surface (18).

6. Chill mold according to one or more of claims 1 to 5, characterized in that in order to influence the local cooling intensity of a cooling channel (1) its effective heat exchange surfaces on the channel base or on the side surfaces are enlarged or reduced.

7. Chill mold according to one or more of claims 1 to 6, characterized by the fact that, in order to enlarge the heat exchange surfaces in the cooling channels, additionally introduced grooves or grooves in their cross-sectional configuration as a rectangle, triangle, trapezoid, partial circle or ellipse or Any free form are executed and the number, depth and width and their position parallel or in any other position are adapted to the course of the cooling channels.

8. Chill mold according to one or more of claims 1 to 7, characterized in that the heat transfer surfaces of the cooling channels (1) for influencing the local cooling intensity are changed with regard to their interface properties, for. B. by applying defined wall roughness for increased heat transfer or by applying additional layers for reduced heat transfer.

9. Chill mold according to one or more of claims 1 to 8, characterized in that in order to influence the local cooling intensity of a cooling channel (1), its isoperimetric cross-sectional area is enlarged by introducing additional grooves into the base or side surfaces, or by inserting Displacers is reduced.

10. Chill mold according to one or more of claims 1 to 9, characterized in that in order to influence the local cooling intensity of a cooling channel (1) and to change the coolant flow which is initially aligned with respect to the contact surface, additional grooves in the cooling channel basic and / or - Side surfaces or additional displacement bodies are introduced and / or a modified wall design of the cooling channels (1) is provided.

11. Chill mold according to one or more of claims 1 to 10, characterized in that to influence the local cooling intensity, the cooling channels (1) locally or globally with respect to their distance from the contact surface and / or arrangement density, d. H. Number of cooling channels per unit length of the mold width are arranged.