Device and method for the casting of strips from a metal melt, in particular a steel melt

Abstract

A device and a method for casting of strips from a metal melt includes the use of two casting rollers, which delimit between them a casting gap at their longitudinal sides, and two side plates, which, arranged opposite one another, are held in contact at face sides of the casting rollers, allocated to narrow sides in each case of the casting gap. With the device and method, metallic strips can be reliably produced which have high dimensional stability even in the area of their strip edges, in that the risk of a gathering of solidified melt at the side plates in casting operation is reduced to a minimum. This is achieved by providing at least one heating device for the heating of the casting rollers in at least those sections of their face sides at which the side plate, allocated to the face side in each case, is in contact.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to German patent application no. DE 10 2007 041 263.2, filed Aug. 30, 2007. The disclosure of the above application is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to a device and a method for the casting of strips from a metal melt, in particular a steel melt, in which the metal melt is cast into a casting gap, the longitudinal sides of which are delimited by two counter-rotating casting rollers. Casting methods of this kind are also designated in practice as “two-roller processes”. Casting devices intended for their application are therefore also referred to by the technical terms “two-roller casting machines” or “twin-roller casting machines”.

BACKGROUND

With devices of this kind, the casting gap is usually sealed on its narrow sides in each case by a side plate, which in the longitudinal direction of the casting rollers can be adjusted in the direction to the face surfaces of the casting rollers allocated to them in each case. During casting operation, they are then held in sealing tightness contact under high contact pressure on the face sides of the casting rollers allocated to them in each case (WO 98/04369, EP-A 0 714 715, EP-B 0 620 061).

The side plates usually comprise what is referred to as an insert, which consists of a ceramic fire-resistant material. This insert is carried by a carrier plate, as a rule made of steel, referred to as the “back plate”.

The free front side of the insert allocated in each case to the sealing casting gap, when in casting operation, comes in direct contact on the one hand with the melt present above the casting gap, and, respectively, with the strip forming in the casting gap. On the other hand, these sides are in contact with the face surfaces allocated to them in each case of the casting rollers moving relative to the side plates.

In order to increase the sealing effect, when starting up a casting device of the type in question, equipped with unused side plates, in the area in which the inserts and the face surfaces of the casting rollers allocated to them overlap there are formed in the inserts indentations of circle arc or circle segment shape, referred to as “insert grinding surfaces”. The individual side plate in each case is, for this purpose, pressed with a defined degree of forward motion against the side surfaces of the casting roller allocated to it for a sufficiently long time until the insert grinding surfaces have formed by the specific removal of the material of the insert in the side overlap areas.

Between the insert grinding surfaces a section is then formed, projecting into the casting gap, referred to as the “positive insert”. The positive insert tapers in its width downwards, corresponding to the roller geometry, while the width of the insert grinding surface is usually constant over its entire contact arc with the individual casting roller in each case.

In this way, the inserts of the side plates come in contact not only full-surface on the face surfaces of the casting rollers but in each case also overlap, with the side surfaces of the positive inserts, a strip of the circumferential surface of the casting rollers adjacent to the individual face surface.

During strip casting, melts are cast into the space delimited by the casting rollers and the side plates, such that, above the casting gap formed in the area of the smallest distance interval of the casting rollers, what is referred to as the “melt sump” is formed. In this situation, the melts coming in direct contact with the casting rollers cooled in casting operation solidify, such that shells of solidified melts form in each case on the surface of the casting rollers, which are conveyed in the direction of the casting gap and are there guided together to form a strip.

In order to prevent melts solidifying at the inserts, the intention is that the inserts coming in contact with the melts in casting operation should assume the same temperature as the melts. Practical experience has shown, however, that so much heat is drawn off by way of the inserts due to their not only coming in contact with the hot melt but also with the casting rollers, cooled in casting operation to about 100° C., that solidification of melts may occur at the positive insert. This risk arises in particular in the lower third of the tapering section of the positive insert.

What is referred to as the “freezing” of melts is the cause of strip defects, such as abrupt thickening going beyond the specified tolerances, inadequate fully-through solidification (“bulging appearance”) and material fissuring (“strip edge defects”). In extreme cases, the inserts can gather so many solidified melts that the casting process must be interrupted.

Attempts have been made to compensate for the heat removed via the inserts by the specific heating of them. The effort and expenditure on apparatus associated with this, however, and the amounts of heat required are so great, that with the known casting devices it has not hitherto been possible to exclude “freezing” phenomena caused by the temperature difference between insert and melt with the degree of reliability required for sustained uninterrupted casting operation.

SUMMARY OF THE INVENTION

Embodiments of the invention provide a casting device and a casting method, in which metallic strips can be reliably produced and which also have a high degree of dimension retention in the area of their strip edges, such that the risk of a gathering of solidified melts at the side plates in casting operation is reduced to a minimum.

A device according to an embodiment the invention is well-suited in particular for the production of cast strips from a steel melt. In agreement with the known prior art, it has two casting rollers, turning in counter-rotation in casting operation, which delimit between them a casting gap on their longitudinal sides. In addition to this, it is also equipped with two side plates, which are arranged opposite one another and held in contact at the face surfaces of the casting rollers allocated to the short side of the casting gap in each case, in order to seal off the casting gap at the narrow side in each case. According to an embodiment of the invention, in this situation at least one heating device is provided in order to heat the casting rollers in at least those sections of their face sides with which the side plate is in contact which is allocated to the face side in each case.

According to an embodiment of the invention, when strip casting in accordance with the two-roller process, the face sides of the casting roller are therefore heated, at least in the area in which they are adjacent to one of the side plates, in a locally limited manner to a temperature which lies above the surface temperature of the casting roller outside these areas.

Due to the measures according to an embodiment of the invention, the temperature gradient is reduced between the casting roller face surfaces and the sections of the side plate coming in contact with the melt. According to the invention, for this purpose the face surfaces of the casting rollers are specifically heated to such a degree that in casting operation the side plate achieves a temperature level in its areas coming in contact with the melts at which there is only a minimal risk of the melts “freezing”.

In this situation an embodiment of the invention makes use of the knowledge that heating of the casting rollers, which is limited both in respect of its spatial extent as well as with regard to its penetration depth, neither has a negative effect on the desired solidification of the metal melts coming in contact with the surfaces of the casting rollers, nor does it lead to any impairment of the service life of the casting rollers. The temperature difference arising between the sections of the casting rollers heated according to the invention and their remaining roller bodies can indeed be considerable, but in practice has no effect on the service life of the casting rollers.

In principle, all heating devices come into consideration for use with a casting device and method according to the invention which allow for sufficiently specific heating of the individual face surface of the casting rollers in each case. However, such heating devices which operate inductively have proved particularly advantageous for the practical implementation of the invention.

Induction heating devices make it possible, with small structural spatial requirements, not only to achieve an exact control of the amount of heat applied in each case, but also an equally precise control of the penetration depth by means of which the introduction of the heat is effected. Accordingly, with the use of inductively operating heating devices on a narrowly delimited surface, heat flow densities of 5 MW/m² and more can be produced without any problem.

At the same time, with the use of inductively operating heating devices the maximum penetration depth over which the heating device heats the section which is to be heated by it is restricted to 2 mm, wherein the penetration depth is preferably limited to 1 mm, in particular less than 0.5 mm. As a result of the specific restriction made possible in this way of the depth of the area in which the casting roller, cooled as far as possible, is heated in the manner according to an embodiment the invention, any negative influence of the heating according to the invention on the casting result is reliably excluded. In particular, in this way the risk of a roller profile change in the edge area is avoided, which could in turn lead to problems at the strip edges.

A further advantage of the use of inductive heating devices, seen as particularly advantageous according to the invention, lies in the fact that the temperature of the face surfaces of the casting rollers is adjusted by way of the voltage, and the penetration depth by means of the frequency, such that high degrees of freedom are available in the performance of the process.

In this context it has proved to be particularly effective in practice if the temperature to which the heating device heats the section of the casting rollers which it is to heat in each case lies above the Curie temperature of the material adjacent to the surface of the individual section to be heated.

The possibilities of the precise adjustment of the individual temperature and penetration depth of the heating have proved themselves to be particularly favourable if the casting rollers are provided on their face sides in an inherently known manner with a metallic coating which has a composition differing from the core material of the casting rollers. In order to guarantee good thermal conductivity of the casting rollers, they are as a rule made of a copper alloy, at least in the area of their circumferential surfaces. In order to minimise the abrasion of the copper material, the casting rollers used for the casting of steel in particular are usually provided on their circumferential surface with a coating which has a greater hardness than the remaining material of the casting rollers. Particularly well-suited for this respect are nickel coatings. In addition to nickel, these can have contents of other elements as constituent parts, such as Fe. The nickel layer protects the copper rollers from mechanical damage and reduces their thermal loading. At the same time, in the area of the layer, due to its poorer thermal conductivity a higher temperature prevails than in the area of the roller body consisting of copper of high thermal conductivity. Due to the higher temperature of the outer layer, an improved surface of the cast strips is achieved.

Below the Curie temperature, a ferro-magnetic layer, such as an Ni-layer, is ferro-magnetic with a magnetic permeability in the order of 1000, while above its Curie temperature it is paramagnetic with a permeability of about 1. The consequence of this is that the magnetic coupling below the Curie temperature is substantially more effective and the energy input higher. This effect leads to the Curie temperature functioning like a temperature threshold. The temperature in the material in this situation is proportional to the power input. If this is reduced, the temperature drops approximately proportionally, until the Curie temperature is reached, and it then hovers around the Curie temperature. Only when there is perceptibly less power does the temperature drop any further. The power input in this situation is not only dependent on the induction parameters, but also on the rotation speed of the rollers. It may therefore be advantageous for the power input to be selected in such a way that a temperature is reached which is indeed above the Curie temperature, but still close to it. Because of the character of the Curie temperature as a lower threshold value, this temperature is not then undercut.

In the event that the casting rollers of a device according to the invention are provided with a coating containing iron, in particular a coating based on nickel, the Curie temperature, which can be used in the manner described heretofore as a lower threshold temperature, can be influenced by the iron content of the coating being varied by taking account of the heating temperature which is to be set in each case.

For practical execution of the preferred inductive heating according to the invention of the face surfaces of the casting rollers, such heating devices are particularly well-suited which comprise an inductor in which the current flowing through this inductor has a frequency of at least 100 Hz, and in particular of at least 1000 Hz. When such high frequencies are set, the penetration depth of the electromagnetic field induced by the inductor of the inductively operating heating device is small in relation to the heat penetration depth. This is the case with frequencies of more than 100 Hz, whereby, due to the better implementation capability, in practice frequencies of 1000 Hz have particularly proved their worth.

DETAILED DESCRIPTION OF THE DRAWINGS

The invention is described hereinafter in greater detail on the basis of drawings representing embodiments. These show, in diagrammatic form:

FIG. 1 A section of a casting device in a view from above;

FIG. 2 A side plate located in the casting device according to FIG. 1, with inductors of a heating device allocated to it, in a frontal view.

DESCRIPTION

The casting device 1 comprises two counter-rotating casting rollers 2, 3 which consist of a core manufactured from a copper material, onto which a nickel coating containing iron is applied. By means of a water cooling system, not shown, the casting rollers are cooled in casting operation.

The casting rollers 2, 3 delimit between them, at their opposed longitudinal sides a casting gap 4, of which the opposed narrow sides 5 are sealed in each case by a side plate 6. The strip B, cast in each case from the steel melt filled into the casting gap 4, emerges from the casting gap 4 in the direction of conveying F, directed downwards.

FIG. 1, for the sake of a better overview, shows only the right part of the casting rollers 2, 3 of the casting gap 4, and of the other components of the casting device 1, explained hereinafter. A corresponding arrangement is located in the left part, not shown here.

The side plates 6 of the casting device 1 have in each case a carrier plate 7 made of steel, which on its front side facing the casting gap 4 carries an insert 8 made of a fire-resistant material. The width, shape, and arrangement of the inserts 8 of the side plates 6 of the casting device 1 are in each case selected in such a way that they cover over the face sides 9, 10 of the casting rollers 2, 3, allocated to them, in the area of their covering section allocated in each case to the casting gap 4 and to the space 11 located above it, accommodating the melt sump and likewise sealed by the casting rollers 2, 3 and the side plates 6.

In this situation, in the area of the covering sections in the individual insert 8 of the side plates 6 of the casting device 1, insert grinding surfaces 12, 13 are formed, in that the side plates 6 in the new state have been pushed by means of setting devices, not shown, with their individual insert 8, against the face sides 9, 10, of the casting rollers 2, 3, allocated to this insert 8, until, by abrasion of material in the insert 8, the insert grinding surfaces 12, 13, have been formed in the manner of cut-out apertures with an adequate depth T. With their insert grinding surfaces 12, 13, the inserts 8 therefore cover not only the face sides 9, 10, of the casting rollers 2, 3, allocated to them in each case, but also a narrow strip 14 of the circumferential surface of the casting rollers 2,3, adjacent to the individual face side 9, 10 in each case. At the same time, a “positive insert” 15 is formed between the insert grinding surfaces 12, 13, which projects opposite the face sides 9, 10, of the casting rollers 2, 3, into the casting gap 4, or, respectively, the space 11 accommodating the melt sump located above the casting gap 4.

Allocated to each of the face sides 9, 10, of the casting rollers 2, 3, is in each case an inductor 16, 17, arranged in the area of their outer circumference and guided in each case by the curvature of the casting roller circumferential surface, which in the direction of rotation of the individual casting roller 2, 3 is arranged in each case with the smallest possible distance interval in front of the individual side plate 6 and its insert 8 respectively.

The inductors 16, 17 consist of copper tubes with cooling water flowing through them, of which the diameters are 5 mm. The flushing length of the inductors 16, 17 is in each case about 10 cm, while the width is approximately 1.5 cm.

The inductors 16, 17 are connected as part of a heating device 18 by supply lines to a converter 19, which is likewise part of the heating device 18. The converter 19 supplies the inductors 16, 17 with the energy required for the generation of the electromagnetic field to be induced by the inductors 16, 17, over a narrowly delimited surface section into the face side 9, 10, of the casting rollers 2, 3, in each case.

At a rotational speed v of the rollers of 1 m/s, a width b of the heating surface being heated by the inductors 16, 17, of 1 cm, and a length l of the heating surface of 10 cm, the penetration depth δ_th of the heat can be estimated as follows:

δ_th≈√{square root over (α*l/v)}

where l=length in m

v=rotational speed in m/s

a=thermal diffusivity

The thermal power P which is required in order to achieve a temperature increase ΔT of some 260 K at the surface can then be assessed by

P≈ρcbδ_thvΔT/2≈5 kW

where ρ=specific electrical resistance

c=specific heat

b=width

δ_th=penetration depth of the heat

v=rotation speed

ΔT

Δ=temperature difference.

The quasi-stationary temperature of the casting rollers 2, 3 amounts to some 100° C. The Curie temperature of Ni is about 360° C.

Because of the large permeability jump around the Curie temperature, if an adequate heating power is provided a temperature close to the Curie temperature can be attained. The Curie temperature can be increased by the proportion of Fe in the Ni layer and thereby also the surface temperature which is to be set.

The thermal power is provided by inductive heating. The current required through the single-winding inductors 16, 17 in the present example amounts to some 90 kA, wherein a frequency of 1000 Hz is selected in order to be able to adjust as precisely as possible the desired small penetration depth of the electromagnetic field induced by the inductors 16, 17.

For the penetration depth δ of the electromagnetic field induced by the inductors 16, 17 the following applies:

$\sqrt{\frac{2}{{\mathrm{\varpi \sigma \mu }}_0{\mu }_r}}=\frac{0.01m}{\sqrt{{\mu }_r}}$

where ω=frequency in Hz (in this case 1000 Hz)

σ=conductivity in (Ohm m)⁻¹ (in this case 13.8

10⁶ (Ohm m)⁻¹)

μ₀=vacuum permeability

μ_r=relative permeability

Below the Curie temperature the Ni layer is ferro-magnetic and μ_r is of the order of 10³. Accordingly a penetration depth δ of the electromagnetic field is derived of 0.3 mm. The small penetration depth guarantees effective heating up of the layer near to the surface.

Due to the heating up which takes place immediately in front of the individual insert 8, precisely delimited locally both in respect of the surface dissemination as well as in respect of the penetration depth, of the surface sections of the casting rollers 2, 3 coming in contact with the individual insert 8, the temperature drop is minimised which prevails in the contact zone in which the insert grinding surfaces 12, 13, come in contact with the surfaces of the casting rollers 2, 3 allocated to them. As a result, the heat outflow from the insert 8 to the casting rollers 2, 3 is reduced to such a degree that in the area of the free front surface of the positive insert 15, which is directly wetted by the melt filled into the casting gap 4 in casting operation, a temperature remains maintained at which the risk of a “freezing” of solidified melts is minimised.

REFERENCE FIGURES

• 1 Casting device
• 2,3 Casting rollers
• 4 Casting gap
• 5 Narrow sides of casting gap 4
• 6 Side plate
• 7 Carrier plate of side plate 6
• 8 Insert of side plate 6
• 9,10 Face sides of casting rollers 2,3
• 11 Space above casting gap 4 accommodating the melt sump
• 12,13 Insert grinding surfaces of the insert 8
• 14 Strip of the circumferential surface of the casting rollers 2,3 covered laterally by the positive insert 15
• 15 Positive insert of the insert 8
• 16,17 Inductors
• 18 Heating device
• 19 Converter
• B Cast strip
• T Depth of the cut-out apertures formed in the insert 8

Claims

1. Device for the casting of strips from a metal melt with two counter-rotating casting rollers when in casting operation, which delimit between the two counter-rotating casting rollers a casting gap on their longitudinal sides, and with two side plates, which are held opposite one another arranged in contact with face sides of the two-counter-rotating casting rollers allocated to in each case narrow sides of the casting gap, in order to seal the casting gap at the narrow side in each case, wherein at least one heating device is provided for heating the casting rollers in at least those sections of their face sides at which an allocated side plate in each case is in contact.

2. Device according to claim 1, wherein the heating device operates inductively.

3. Device according to claim 2, wherein a maximum penetration depth over which the heating device heats up the section which it heats is restricted to 2 mm.

4. Device according to claim 3, wherein the maximum penetration depth is restricted to 1 mm.

5. Device according to claim 2, wherein a temperature to which the heating device heats up the section of the two counter-rotating casting rollers heated by it in each case lies above the Curie temperature of a material adjacent to the surface of an individual section to be heated in each case.

6. Device according to claim 1, wherein the two counter-rotating casting rollers have on their surface a nickel coating containing iron.

7. Device according to claim 6, wherein an iron content of the nickel coating is varied depending on a heating temperature which is to be set in each case.

8. Device according to claim 2, wherein the heating device comprises at least one inductor and current flowing through the at least one inductor has a frequency of at least 100 Hz.

9. Device according to claim 8, wherein the current has a frequency of at least 1000 Hz.

10. Method for casting of strip from a metal melt, in which the metal melt is cast into a casting gap, longitudinal sides of the casting gap are delimited by two counter-rotating casting rollers and narrow sides of the casting gap are sealed by side plates being pressed in each case against two face sides of the two counter-rotating casting rollers allocated to individual narrow sides, wherein the two face sides of the casting rollers at least in the area in which they are adjacent to one of the side plates in each case, are heated, limited locally, to a temperature which lies above the surface temperature of the casting rollers outside these areas.

11. Method according to claim 10, wherein the heating takes place inductively.

12. Method according to claim 10, wherein a depth to which the heating takes place is restricted to a maximum of 1.

13. Method according to claim 10, the temperature to which the sections, which are to be heated in each case, of the casting rollers are heated up lies above the Curie temperature of a material of an individual casting roller adjacent to the surface of the section which is to be heated in each case.

14. Method according to claim 11, wherein current flowing to produce the electro-magnetic field during inductive heating has a frequency of at least 100 Hz.

15. Method according to claim 14, wherein the current has a frequency of at least 1000 Hz.