GB2293999A

GB2293999A - Strip casting

Info

Publication number: GB2293999A
Application number: GB9520607A
Authority: GB
Inventors: Chris Baharis; James Arthur O'neill; Roderick William Charles Vance
Original assignee: BHP STEEL; IHI Corp
Current assignee: BHP STEEL; IHI Corp
Priority date: 1994-10-14
Filing date: 1995-10-09
Publication date: 1996-04-17
Anticipated expiration: 2015-10-09
Also published as: JPH08174159A; GB9520607D0; FR2725647A1; KR960013519A; FR2725647B1; AUPM883894A0; GB2293999B; DE19538012A1; ITMI952098A0; IT1276929B1; ITMI952098A1

Description

22) 9 3 9 9 9 "InLL CAST131M MCmaCAL Pl= This invention relates to the

casting of metal strip by the technique of twin roll casting. It has particular but not exclusive application to the casting of ferrous metal strip.

In a twin roll caster molten metal is introduced between a pair of contrarotated chilled casting rolls so as to form a casting pool of molten metal above the nip between the rolls. Metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product at the outlet from the roll nip. The term "nip,' is used herein to refer to the general region at which the rolls are closest together. The molten metal may be introduced into the nip between the rolls via a tundish and a metal delivery nozzle located beneath the tundish so as to receive a flow of metal from the tundish and to direct it into the nip between the rolls.

It in necessary in a twin roll caster to confine the molten metal in the casting pool at the two ends of the nip between the casting rolls. Conventionally this is done by means of a pair of stationary refractory end closures which are.held against the rotating rollers with sliding engagement at two ends of the nip so an to confine or dam the molten metal against escape from the casting pool. In operation, the refractory end closures suffer from wear because of their sliding engagement with the rotating casting rolls and they must be replaced frequently, often after a single cast. This is particularly so when gouging occurs along the refractory end closures where they meet the roll edges. Such gouging can be caused by at least two phenomena. Firstly, the changing positions of the roll edges due to thermal expansion can lead to significant gouging. Secondly, movements of the rolls relative to one another can lead to discrete particles of frozen metal becoming trapped at the roll edges and acting an abrasive - 2 is particles against the end closures.

In an endeavour to overcome the end closure wear problem, it has been proposed in the casting of ferrous metal strip to employ electromagnetic confinement of the casting pool. Examples of such proposals are described in the specification of United States Patent 4,936,374 assigned to the United States Department of Energy, United States Patents 5,197,534, 5, 251,685 and S,279,350 all assigned to Inland Steel Company, German Patent Application

DZ 4307850 of Thyssen Stahl AG and Usinor Sacilor, and International Patent Application No PCT/JP92/01668 of Nippon steel corporation. The proposals described in the first five-of these spe cifications involve the use of intense alternating currents to generate magnetic fields at the ends of the casting pool to generate repulsive forces in the molten metal. The general approach has been to confine the ends of the casting pool solely by the action of such repulsive forces. However, this requires very accurate control of intense-magnetic fields required to generate the total confining pressure for the pool leading to severe control Droblem Moreover the electromagnetic field generating equipment is in danger of exposure to the molten metal, particularly at start-up. German Application DE 4307850 suggests the use of refractory material to assist in the confinement of the pool, but substantial parts of the pool around the edges of the rolls are confined purely by magnetic fields generated by conductors which are disposed at those locations and are potentially exposed directly to the molten metal. Again, there are severe control problems particularly on start-up and the magnetic field conductors are vulnerable to damage, particularly during start-up or in the event of a failure or fluctuation in the power supply.

International Application No PCT/JP92/01668 describes a proposal in which refractory end closures are set back slightly from the ends of the casting rolls so as to avoid sliding engagement and confining forces are generated in the molten metal by passing direct current through the molten metal at the ends of the casting pool to react with magnetic poles disposed adjacent the side damn. This proposal also presents very severe control problems since it in necessary to accurately control the intensity and direction of direct currents in the moving metal in the casting pool. Purther, there is a difficulty in finding a suitable electrode material which can withstand attack from molten metal and carry the required current density.

The present invention provides an alternative technique which emp loys the combination of physical end closures and induction fields in a manner which can be more readily controlled than the previously proposed techniques and in which the field generators are not exposed at any is time directly to the molten metal of the casting pool and are therefore not liable to damage in the event of a control or power failure or fluctuation. DISCLOSURE OF THE INVE2MION

According to the invention there is provided strip casting apparatus for casting a strip of magnetic metal comprising a pair of casting rolls forming a nip between them, molten metal delivery means to deliver molten metal into the nip between the casting rolls to form a casting pool of molten metal above the nip between the rolls, pool confinement means to confine the casting pool of molten metal at each end of the nip, and roll drive means to rotate the rolls in mutually opposite directions so as to produce a solidified strip at the exit from the nip, wherein the pool confinement means comprises a pair of refractory end closures disposed one at each end of the nip to contact the molten metal of the pool substantially completely across both ends of the pool and dam it against outflow from the pool, a pair of electrical conductor means disposed one outside each of the end closures so as to be shielded from the molten inetal of the casting pool by the end closures, and power supply means to supply alternating current to said conductor means whereby to generate electromagnetic fields which extend through the end closures and push back molten metal at the conjunctions between the end closures and the casting rolls inwardly of the pool to prevent escape of molten metal between the end closures and the rolls.

Preferably each of the electrical conductor means comprises a pair of electrical conductors disposed outside the respective end closure adjacent the conjunctions between the end closure and the casting rolls and are shaped to follow those conjunctions whereby to concentrate their electromagnetic fields at those conjunctions.

Each of the electrical conductor means may be a single electrical conductor structure comprising a pair of downwardly converging arcuate segments constituting said conductors.

The electrical conductors may be arranged so that their upper parts are spaced further from the adjacent end closure and casting roll conjunctions than are their bottom parts whereby to decrease tkAo pool confining forces toward the upper parts of the pool. To achieve this, the conductors may be tilted outwardly from the adjacent end closures. Alternatively, or in addition, the conductors may be displaced downwardly from positions in which they would-be in beat horizontal alignment with the end closure and casting roll conjunctions. in another alternative, the shapes of the conductors may depart from strict compliance with the curvature of the rolls so that they do not precisely follow the roll shapes but are configured such that they are closest to the lines of conjunction between the rolls and the adjacent end closure toward the nip and gradually become further away toward their upper parts.

Preferably further the apparatus comprises conductor cooling means to cool the electrical conductors.

The conductors may comprise hollow electrically conductive metal tubes and means to circulate cooling fluid, for example water, through the tubes.

The side dam closures may be set so as to define small gaps between them and the casting rolls, which gaps may be clearan e gaps or may be occupied by generally nonconducting spacers.

Preferably the casting roll surfaces are coated, for example by plating, with a material which is ferromagnetic at the operating temperature of those surfaces.

Preferably, there are thin thermally insulating spacer plates placed between the side dam closures and the electrical conductor means to prevent unnecessary additional thermal stress on the electrical conductor means.

The invention also extends to a method of casting a strip of magnetic metal comprising introducing molten metal between a pair of casting rolls forming a nip between them so as to form a casting pool of molten metal above the nip, confining the casting pool at each end of the nip, and rotating the casting rolls in mutually opposite directions to produce a solidified metal strip passing from the nip, wherein the casting pool of molten metal is confined at the ends of the nip by providing a pair of end closures disposed one at each end of the nip to contact the molten metal of the pool substantially completely across both ends of the pool and dam it against outflow from the ends of the nip and applying alternating current to electrical conductors disposed outside the end closures so as to generate electromagnetic fields which extend through the end closures and push back molten metal at the conjunctions between the end closures and the casting rolls to prevent escape of molten metal between the end closures and the rolls. BRIEF DESCRIPTION OF THE DRAWINGS in order that the invention may be more fully explained, some particular embodiments will be described with reference to the accompanying drawings in which:

Figure 1 is a vertical cross-section through a strip caster constructed in accordance with the invention; Figure 2 is a plan view of the casting rolls and pool confinement means of the caster illustrated in Figure 1; Figure 3 in an end view of the components illustrated in Figure 2; Figure 4 is a horizontal cross-section generally on the line 4-4 in Figure 3; Figure 5 illustrates part of a field generating conductor forming part of the pool confining means;

Figure 6 is a cross-section generally on the line 6-6 in Figure 5; and Figure 7 diagrammatically illustrates a modified strip caster constructed in accordan e with the invention.

DETAILED DESCRIPTION OF 19EM 1. EMBODIMENT

The strip caster illustrated in Figure 1 comprises a pair of twin casting rolls 11 forming a nip 12 between them. Molten metal is supplied during a casting operation from a ladle 13 via a tundish 14 and a delivery nozzle 15 into the nip between rolls 11 so as to produce a casting pool 16 of molten metal above the nip. Ladle 13 is fitted with a stopper rod 17 actuable to allow the molten metal to flow from the ladle through an outlet nozzle 18 and a refractory shroud 19 into tundish 14.

Casting rolls 11 are provided with internal water cooling passages supplied with cooling water through the roller ends and they are contrarotated by drive means (not shown) to produce a continuous strip product 21 which is delivered downwardly from the nip between the casting rolls.

As thus far described the illustrated apparatus is as more fully described in granted United States Patents 5,184,668 and 5,277,243 and Australian Patents 631728 and 637548. Reference may be made to these patents for full constructional and operational details of the apparatus.

However, in accordance with the present invention the casting pool 16 in confined at the two ends of the nip by the combination of a pair of refractory end dam closure plates 22 and a pair of electromagnetic field generators denoted generally as 20. Each end dam closure plate 22 is shaped so as to have a wide top 23 and a narrow bottom 24 connected by arcuate sides 25 which overlap the ends of the casting rolls 11, the curvature of the sides 25 being matched with the curvature of the rolls. End dam closure plates 22 do not engage the ends of the casting rolls but are set back slightly from the ends of those rolls to leave small clearance gaps 26 between the sides 2S of the closure plates and the ends of the rolls.

The metal of casting pool 16 engages the closure plates 22 so that the closure plates serve to dam the metal. The electromagnetic field generators 20 serve to provide concentrated electromagnetic fields at the is conjunctions 27 between the closure plates 22 and the end rollers which induce forces producing a confining pressure on the molten metal in these regions indicated by the arrows 28 to prevent the molten metal from escaping from the pool through the clearance gaps 26. With this arrangement most of the hydrostatic pressure at the ends of the pool is borne by the end closure plates 22 and the electromagnetically induced confining forces are only required to resist this pressure over the very small clearance gaps 26. Such confining pressures can be readily generated and controlled by appropriate electromagnetic field generators as will now be described.

Each of the electromagnetic field generators 20 comprises a specially shaped electrical conductor structure 31 fitted with field shaping core pieces 32. The conductor structure 31 is formed of heavy copper tubing of square cross-section which serves both as an electrical conductor and as a conduit for the flow of cooling water or other cooling fluid, such as SYLTHERM 800 (Trade Mark) by Dow Corning, through the conductor structure to extract heat generated by the large current flows required to generate the necessary electromagnetic fields. Typically a 10 kHz alternating current of 15,000 amps and 500 volts will be is required.

Each conductor structure 31 comprises a pair of arcuate side segments 33 which are disposed imwdiately outside the closure plates 22 adjacent the lines of conjunction 27 between the closures plates and the rolls. Segments 33 converge downwardly and are curved to follow the curvature of the rolls and therefore the lines of conjunction between the rolls and the closure plates. They are joined together at the bottom 34 of structure 31 and their upper ends are interconnected by a curved upper segment 35. A pair of current supply segm^to 36, 37 are connected respectively to the mid-point of top segment 35 and to the-junction 34 between the bottom ends of the side segments 33.

Alternating electrical current in supplied to the conductor structures 31 through a heat station which may comprise capacitors andlor transformers (not shown). The current then flows via supply segments 36, 37 through the side segments 33 which are spaced close to and extend along the lines of conjunction between the side plates 22 and the ends of the castint rolls 11. The side segments 33 and top segment 35 of each conductor structure 31 form a planar configuration disposed in one plane and the supply segments 36, 37 are disposed for the most part in a second plane which is very close to the first plane. It has been found that this very important to minimise the inductance of the conductor structure an a whole.

The ferromagnetic core pieces 32 are each of generally U-shaped cross-section so as to define pole ends 41 separated by gaps 42. The core pieces are fitted to the arcuate conductor segments 33 so as to encompass those segments with their pole ends 41 facing inwardly toward the side closure plates 22 and therefore toward the regions of conjunction between the side plates and the end rolls. The resulting magnetic fields emanating from the core pieces will thus be concentrated in the regions of conjunction between the end closure plates and the casting rolls. The outer faces 43 of the pole end a 41 may slope outwardly and backwardly from the gap 42 so an to shape the field in such a way that It in flattened and widened ther aby to on ure that at the regions of conjunction between the side plates 22 and the rolls the field flux extends in directions generally parallel with the side plates and therefore along the gaps 26 to produce maxim= push back from the gaps. The outer faces 43 of the pole end a may slope back at angles up to 300 from the plane defined by the outer end of the core gap 42.

To cause reorientation of the flux lines near the conjunction of the rolls and side plates such that they are normal to the roll face, a coating of around 100 microns of ferromagnetic material such as nickel or a nickel alloy is is applied to the roll surface.

To further improve the shaping of the magnetic field and control the confining forces on the pool, the upper ends of the conductor structures 31 may be tilted outwardly and backwardly from the side closure plates 22.

This enables the confining forces generated on the metal in the pool to be matched with the hydrostatic (ferritic) pressure which increases downwardly toward the nip so that more confining pressure is required toward the nip them in the upper regions of the pool. It also prevents excessive push back of the molten metal on the upper parts of the rollers which can lead to impaired solidification at the edges of the strip. A similar effect may be achieved by varying the shape of the side segments 33 of the conductor structures 31 so that they do not precisely follow the roll shape but have a configuration such that they are closest to the lines of conjunction between the rolls and the side closure plates at the nip and gradually become further away toward their upper ends. In another alternative, the conductor structures may be moved slightly downwards so that the side segments 33 are displaced downwardly from positions in which they would be in best horizontal alignment with the lines of conjunction between the rolls and the side closure plates. Because of the lines of conjunction and corresponding parts of the conductor segments become more upright toward the roller nip, there will be progressively lesser misalignment toward the nip.

To further shape the electric field at the roll edges so as to maximize push back forces on the molten metal, the gaps between the side dam closures and the rolls may be filled by generally non-conducting spacers.

A typical twin roll caster producing 1 metre wide steel strip at the rate of 60 metres per minute will require the following operational parameters for adequate electromagnetic pool confinement:

Magnetic flux density - 0.2 Teala Current density - 15,000 amps per end conductor System inductance - 0.5 microhenry per end conductor Power input to melt - 100 kilowatts per end Power input to rolls - 2 kilowatts per end Reactive-power requirements - 15 MVA for the whole system Real power approximately 4-00 kilowatts operation under the above parameters will require extraction of approximately 100 kilowatts of Ohmic losses from the system an a whole ie 25 kilowatts from each of the four conductor segments 33. This can be achieved by pumpiiig cold water through the hollow copper conductor tubing at high pressure.

Figure 7 illustrates a modified twin roll caster constructed in accordance with the invention in which the casting rolls 51 are moveable longitudinally relative to one another as indicated by the arrows 52 in order to adjust the length of the nip 53 between them so an to enable strips of varying width to be produced by adjustment of the one piece of apparatus and without changing the rolls. In this case the casting pool is confined by a pair of side closure plates 54 and a pair of electromagnetic field generators 55 comprising conductors 56 and pole pieces 57.

Since the rolls 51 overlap one another in the longitudinal direction the side closure plates 54 cannot overlap the ends of both rollers. In each case one side of the side closure plate overlaps and end of one of the rollers 51 but its other side is shaped so as to extend around the circumference of the adjacent roller 51 leaving a small clearance space 58 between the side plate and the roller. The electromagnetic field generators 55 may be generally similar to the field generators of the apparatus illustrated in Figures 1 to 6, but the conductor segments adjacent the gaps 58 may be modified in shape so as to direct the magnetic flux in the best direction to maximise the effectiveness of the pool confinement forces in that region.

In both of the illustrated embodiments the end closure plates extend completely across the pool ends so as to provide most of the pool confining forces and to completely shield the electrical conductor structures from molten metal of the pool. The magnetic fields only need to create sufficient repulsive forces to prevent entry of metal into the very small gaps between the end closures and the roll ends. Such forces can be readily generated and finely controlled. Moreover, even in the event of a power fluctuation or failure, the pool will still be substantially confined by the refractory side closures and there is no danger of massive leakage or damage to field generating equipment.

The illustrated forms of apparatus have been advanced by way of example only and they could be modified considerably. As already indicated with reference to the embodiment illustrated in Figure 7, the precise shaping of the conductor segments may need to be varied according to the precise application of the invention. moreover, it is not necessary that the conductor segments following the shape of the two casting rolls be interconnected into a single conductor structure and it would be possible to provide two separate conductors. In that case, only one of the conductors need be taken downwardly as far as the nip since at that point the two solidifying shells have been brought together and very little liquid zactal remains to jet out from the ends of the pool. Accordingly a single conductor will be sufficient to generate sufficient confining forces at this location and the other conductor may traverse only an upper part of the poollcasting roll Interface. This will also avoid the complexities involved in fitting magnetic core pieces around two closely spaced conductors. it is accordingly to be understood that the invention is in no way limited by the constructional details of the illustrated apparatus and that many modifications and variations will fall within the scope of the appended claims.

CMAIMS:

1. Strip casting apparatus for casting a strip of magnetic metal comprising a pair of casting rolls forming nip between them, molten metal delivery means to deliver molten metal into the nip between the casting rolls to form a casting pool of molten metal above the nip between the rolls, pool confinement means to confine the casting pool of molten metal at each end of the nip, and roll drive means to rotate the rolls in mutually opposite directions so as to produce a solidified strip at the exit from the nip, wherein the pool confinement means comprises a pair of refractory end closures disposed one at each end of the nip to contact the molten metal of the pool substantially completely across both ends of the pool and dam it against outflow from the pool, a pair of electrical conductor means disposed one outside each of the end closures so as to be shielded from the molten metal of the casting pool by the end closures, and power supply means to supply alternating current to said conductor means whereby to generate electromagnetic fields which ext end through the end closures and push back molten metal at the conjunctions between the end closures and the casting rolls inwardly of the pool to prevent escape of molten metal between the end closures and the rolls.

2. Strip casting apparatus an claimed in claim 1, wherein each of the electrical conductor means comprises a pair of electrical conductors disposed outside the respective end closure adjacent the conjunctions between the end closure and the casting rolls and are shaped to follow those conjunctions whereby to concentrate their electromagnetic fields at those conjunctions.

3. Strip casting apparatus an claimed in claim 2, wherein each of the electrical conductor mean is a single electrical conductor structure comprising a pair of downwardly converging arcuate segments constituting said conductors.

4. Strip casting apparatus as claimed in claim 2 or claim 3, wherein the electrical conductors are arranged so that their upper parts are spaced further from the adjacent end closure and casting roll conjunctions than are their bottom parts whereby to decrease the pool confining forces toward the upper parts of the pool.

S. Strip casting apparatus as claimed in claim 4, wherein the conductors are tilted outwardly from the adjacent end closures.

6. Strip casting apparatus as claimed in claim 4 or claim 5, wherein the conductors are displaced downwardly from positions in which they would be in best horizontally alignment with the conjunctions between the end closures and the casting rolls.

7. Strip casting apparatus as claimed in any one of claims 4 to 6, wherein the conductors depart from strict compliance with the curvature of the rolls so that they do not precisely follow the roll shapes but are configured such that they are closest to the lines of conjunction between the rolls and the acliacent end closure toward the nip and gradually become further away toward their upper parts.

8. Strip casting apparatus as claimed in any one of claims 2 to 7, wherein there is conductor cooling means to cool the electrical conductors.

9. Strip casting apparatus as claimed in claim 8, wherein the conductors comprise hollow electrically conductive metal tubes and the conductor cooling means comprises means to circulate cooling fluid through the tubes.

10. Strip casting apparatus as claimed in any one of claims 2 to 9, wherein each of the electrical conductors is disposed within a series of ferromagnetic core pieces defining pole ends to shape the electromagnetic field induced by the electrical conductor.

11. Strip casting apparatus as claimed in claim 10, wherein the core pieces are substantially U-shaped.

12. Strip casting apparatus as claimed in any one of the preceding claims, wherein the side dam closures are set so as to define small gaps between them and the casting rolls.

13. Strip casting apparatus as claimed in claim 12, wherein said gaps are clearance gaps.

14. Strip casting apparatus as claimed in claim 12, wherein said gaps are occupied by generally non-conducting spacers.

15. Strip casting apparatus as claimed in any one of the preceding claims, wherein thermally insulating plates are disposed between the side dam closures and the electrical conductor means whereby to thermally shield the conductor means.

16. A method of casting a strip of magnetic metal comprising introducing molten metal between a pair of casting rolls forming a nip between them so as to form a casting pool of molten metal above the nip, confining the casting pool at each end of the nip, and rotating the casting rolls in mutually opposite directions to produce a solidified metal strip passing from the nip, wherein the casting pool of molten metal is confined at the ends of the nip by providing a pair of end closures disposed one at each end of the nip to contact the molten metal of the pool substantially completely across both ends of the pool and dam it against outflow from the ends of the nip and applying alternating current to electrical conductors disposed outside the end closures so as to generate electromagnetic fields which extend through the end closures and push back molten metal at the conjunctions between the end closures and the casting rolls to prevent escape of molten metal between the end closures and the rolls.

17. A method as claimed in claim 16, wherein the electromagnetic fields are generated in such a manner as to be concentrated at the conjunctions between the end closures and the casting rolls.

18. A method as claimed in claim 16 or claim 17, wherein the electromagnetic fields are generated so as to induct pool confining forces which progressively decrease upwardly of the conjunctions between the end closures and the casting rolls.