Method and apparatus for casting metal strip

Abstract: Metal strip is cast from a casting pool of molten metal supported above the nip between a pair of chilled casting rolls (16). The casting pool is confined by refractory plates (56) engaging the ends of the rolls (16). Molten metal in the casting pool in the vicinity of the end closures (56) is subjected to electromagnetic induction heating by passing electric current through electrical conductors (101) disposed outside the refractory plates (56). Each conductor (101) is in the form of a trapezium shaped loop of metal tube having wide top segment (107) and narrow bottom segment (108) connected by downwardly convergent side segments (109) aligned with roll edges.

Description: BACKGROUND OF THE INVENTION This invention relates to an apparatus for the casting of metal strip. It has particular but not exclusive application to the casting of ferrous metal strip. It is known to cast metal strip by continuous casting in a twin roll caster. Molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term "nip" is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip. This casting pool may be confined between side plates or dams held in sliding engagement with the ends of the rolls. Although twin roll casting has been applied with some success to non-ferrous metals which solidify rapidly on cooling, there have been problems in applying the technique to the casting of ferrous metals which have high solidification temperatures and tend to produce defects caused by uneven solidification at the chilled casting surfaces of the rolls. Much attention has therefore been given to the design of metal delivery nozzles aimed at producing a smooth even flow of metal to and within the casting pool. United States Patents 5,178,205 and 5,238,050 both disclose arrangements in which the delivery nozzle extends below the surface of the casting pool and incorporates means to reduce the kinetic energy of the molten metal flowing downwardly through the nozzle to a slot outlet at the submerged bottom end of the nozzle. In the arrangement disclosed in US Specification 5,178,205 the kinetic energy is reduced by a flow diffuser having a multiplicity of flow passages and a baffle located above the diffuser. Below the diffuser the molten metal moves slowly and evenly out through the outlet slot into the casting pool with minimum disturbance. In the arrangement disclosed in US Specification 5,238,050 streams of molten metal are allowed to fall so as to impinge on a sloping side wall surface of the nozzle at an acute angle of impingement so that the metal adheres to the side wall surface to form a flowing sheet which is directed into an outlet flow passage. Again the aim is to produce a slowly moving even flow from the bottom of the delivery nozzle so as to produce minimum disruption of the casting pool. Japanese Patent Publication 5-70537 of Nippon Steel Corporation also discloses a delivery nozzle aimed at producing a slow moving even flow of metal into the casting pool. The nozzle is fitted with a porous baffle/diffuser to remove kinetic energy from the downwardly flowing molten metal which then flows into the casting pool through a series of apertures in the side walls of the nozzle. The apertures are angled in such a way as to direct the in-flowing metal along the casting surfaces of the rolls longitudinally of the nip. More specifically, the apertures on one side of the nozzle direct the in-flowing metal longitudinally of the nip in one direction and the apertures on the other side direct the in-flowing metal in the other longitudinal direction with the intention of creating a smooth even flow along the casting surfaces with minimum disturbance of the pool surface. After an extensive testing program we have determined that a major cause of defects is premature solidification of molten metal in the regions where the pool surface meets the casting surfaces of the rolls, generally known as the "meniscus" or "meniscus regions" of the pool. The molten metal in each of these regions flows towards the adjacent casting surface and if solidification occurs before the metal has made uniform contact with the roll surface it tends to produce irregular initial heat transfer between the roll and the shell with the resultant formation of surface defects, such as depressions, ripple marks, cold shuts or cracks. Previous attempts to produce a very even flow of molten metal into the pool have to some extent exacerbated the problem of premature solidification by directing the incoming metal away from the regions at which the metal first solidifies to form the shell surfaces which eventually become the outer surfaces of the resulting strip. Accordingly, the temperature of the metal in the surface region of the casting pool between the rolls is significantly lower than that of the incoming metal. If the temperature of the molten metal at the pool surface in the region of the meniscus becomes too low then cracks and "meniscus marks" (marks on the strip caused by the meniscus freezing while the pool level is uneven) are very likely to occur. One way of meeting this problem is to employ a high level of superheat in the incoming metal so that it can cool within the casting pool without reaching solidification temperatures before it reaches the casting surfaces of the rolls. However, we have successfully developed a far more effective method whereby the incoming molten metal is delivered relatively quickly by the nozzle directly into the meniscus regions of the casting pool. This minimises the tendency for premature freezing of the metal before it contacts the casting roll surfaces. It has been found that this is a far more effective way to avoid surface defects than to provide absolutely steady flow in the pool and that a certain degree of fluctuation in the pool surface can be tolerated since the metal does not solidify until it contacts the roll surface. A very significant reduction in the superheat required in the incoming metal can also be achieved. In the specific method and apparatus that we have developed, the molten metal flows from the delivery nozzle in two mutually oppositely directed series of jet streams which are directed outwardly to impinge directly on the casting surfaces of the rolls in the immediate vicinity of the casting pool surface. It has been found that this permits casting of strip from molten metal supplied with relatively low level of superheat without the formation of surface cracks. However, problems can arise due to the formation of pieces of solid metal known as "skulls" in the vicinity of the pool confining side plates or dams. These problems are exacerbated as the superheat of the incoming molten metal is reduced. The rate of heat loss from the melt pool is greatest near the side dams due primarily to additional conductive heat transfer through the side dams to the roll ends. This high rate of local heat loss is reflected in the tendency to form "skulls" of solid metal in this region which can grow to a considerable size and fall between the rolls causing the rolls to "spring" apart and so generate severe defects in the strip. Because the net rate of heat loss is higher near the side dams the rate of heat input to these regions must be increased if skulls are to be prevented. There have been previous proposals to provide an increased flow of metal to these "triple point" regions by forming galleries in the upper part of a delivery nozzle which receive a separate flow of metal from the tundish as seen for example in the delivery nozzle disclosed in our granted United States Patent No 5,221,511. However, this requires the formation of complex gallery passages and a high level of superheat of the molten metal because of cooling of the metal in the galleries. By the present invention the necessary increase in heat input is achieved by carefully targeted electromagnetic induction heating of the molten metal in the upper regions of the casting pool in the vicinity of the pool confining side dams where "skulls" may form, without significantly raising the superheat of the pool as a whole or adversely affecting metal solidification in the lower regions of the pool toward the nip. Japanese patent publication JP62-77156 of Nippon Steel Corp discloses a previous proposal for electromagnetic induction heating of the casting pool in a twin roll caster in the vicinity of the side dams. In that proposal the induction heating is achieved by locating outside the side dams a pair of induction heating devices having induction coils wound on C-shaped cores disposed generally in a central vertical plane between the casting rolls. These devices provide diffuse heating of central regions of the casting pool substantially throughout its depth. By the present invention it is possible to achieve much more carefully targeted heating of the molten metal entering the casting pool at its upper surface and in the meniscus regions of the pool where skull formation is a problem. This is achieved by means of induction heating coils formed as single closed loops shaped and positioned adjacent upper parts of the pool to provide targeted heating across the upper surface of the pool and in the meniscus regions. European Patent Publication EP491641, upon which the preamble of claim 1 is based, discloses a twin roll caster in which the casting pool is confined by thin metal side plates and electromagnetic induction coils are placed outside these metallic side plates so as to heat the side plates and the metal in the pool. The use of thin metallic side plates leads to external cooling means being provided to prevent such plates from melting and/or distorting. This introduces severe cooling problems in the melt and it is then essential to have external heating means to counter balance this loss of heat. The induction coils in the apparatus disclosed in EP491641 are operated at low frequency and provide diffuse heating throughout the end regions of the pool, including the lower regions adjacent the nip between the casting rolls. This is to be contrasted with the targeted heating achievable by the present invention. Japanese Patent No. 02-155543-A discloses apparatus for casting strip wherein the side plates are heated by heating coil 13 to remelt solidified shell developing in the curving part in the short wall side which extends down to the nip region. Thus, Japanese Patent No. 02-155543-A teaches heating the regions of contact between the molten metal, each roll and each side dam from the top of the pool to the nip. Consequently, with this arrangement it is necessary to eignificantly raise the superheat of the pool as a whole and adversely affect metal solidification in the lower regions of the pool toward the nip SUMMARY OF THE INVENTION According to the invention there is provided apparatus for casting metal strip comprising a pair of parallel casting rolls forming a nip between them, an elongate metal delivery nozzle disposed above and extending along the nip between the casting rolls for delivery of molten metal into the nip to form a casting pool of molten metal supported above the nip, and a pair of pool confining end closure plates disposed one at each end of the pair of casting rolls, wherein the metal delivery nozzle has a series of side openings along each of its longitudinal sides for flow of molten metal from the delivery nozzle in two mutually oppositely directed series of jet streams to impinge directly on the casting surfaces of the rolls in the vicinity of the casting pool surface, and there is a pair of electrical conductors disposed one outside each of the end closure plates and electrical supply means to supply electrical current to the electrical conductors characterised in that the end closure plates are formed of electrically non-conducting material and each conductor is in the form of a single loop comprising a horizontal top segment to extend across an upper portion of the respective end closure plate so as in use of the apparatus to be generally at the level of the casting pool surface and a horizontal bottom segment shorter than the upper segment and connected to the upper segment by a pair of downwardly convergent side segments, the bottom segment being spaced above the level of the nip, whereby said conductors are effective to cause induction heating of molten metal in the casting pool in the vicinity of said end closure plates and the pool surface without causing melting of solidified metal at the nip. Preferably, each conductor loop is positioned so as not to extend below the upper two thirds of the pool depth. Preferably further, the side segments of each loop are curved to follow the conjunctions between the respective end closure plates and the casting rolls. Preferably, the bottom segment of the coil is no more than about 70 mm below the level of the pool surface. Preferably too, the electrical supply means is such as to supply alternating electric current at a frequency in the range 6 to 10 kHz. BRIEF DESCRIPTION OF THE DRAWINGS In order that the invention may be more fully explained, one particular method and apparatus will be described in detail with reference to the accompanying drawings in which: Figure 1 illustrates a twin-roll continuous strip caster constructed and operating in accordance with the present invention; Figure 2 is a vertical cross-section through important components of the caster illustrated in Figure 1; Figure 3 is a further vertical cross-section through important components of the caster taken transverse to the section of Figure 2; Figure 4 is an enlarged transverse cross-section through the metal delivery nozzle and adjacent parts of the casting rolls; Figure 5 is a side elevation of the metal delivery nozzle; Figure 6 is a partial plan view on the line 6-6 in Figure 3; Figure 7 is an end view of the casting rolls together with pool confinement and induction heating components of the caster; Figure 8 is a perspective view of the components shown in Figure 7; and Figures 9 and 10 show the results of measurements of roll separation forces during operation of a twin roll caster. DESCRIPTION OF THE PREFERRED EMBODIMENT The illustrated caster comprises a main machine frame 11 which stands up from the factory floor 12. Frame 11 supports a casting roll carriage 13 which is horizontally movable between an assembly station 14 and a casting station 15. Carriage 13 carries a pair of parallel casting rolls 16 to which molten metal is supplied during a casting operation from a ladle 17 via a distributor 18 and delivery nozzle 19. Casting rolls 16 are water cooled so that shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product 20 at the nip outlet. This product is fed to a standard coiler 21 and may subsequently be transferred to a second coiler 22. A receptacle 23 is mounted on the machine frame adjacent the casting station and molten metal can be diverted into this receptacle via an overflow spout 24 on the distributor. Roll carriage 13 comprises a carriage frame 31 mounted by wheels 32 on rails 33 extending along part of the main machine frame 11 whereby roll carriage 13 as a whole is mounted for movement along the rails 33. Carriage frame 31 carries a pair of roll cradles 34 in which the rolls 16 are rotatably mounted. Carriage 13 is movable along the rails 33 by actuation of a double acting hydraulic piston and cylinder unit 39, connected between a drive bracket 40 on the roll carriage and the main machine frame so as to be actuable to move the roll carriage between the assembly station 14 and casting station 15 and visa versa. Casting rolls 16 are contra rotated through drive shafts 41 from an electric motor and transmission mounted on carriage frame 31. Rolls 16 have copper peripheral. walls formed with a series of longitudinally extending and circumferentially spaced water cooling passages supplied with cooling water through the roll ends from water supply ducts in the roll drive shafts 41 which are connected to water supply hoses 42 through rotary glands 43. The rolls may typically be about 500 mm diameter and up to 2 m long in order to produce up to 2 m wide strip product. Ladle 17 is of entirely conventional construction and is supported via a yoke 45 on an overhead crane whence it can be brought into position from a hot metal receiving station. The ladle is fitted with a stopper rod 46 actuable by a servo cylinder to allow molten metal to flow from the ladle through an outlet nozzle 47 and refractory shroud 48 into distributor 18. Distributor 18 is formed as a wide dish made of a refractory material such as high alumina castable with a sacrificial lining. One side of the distributor receives molten metal from the ladle and is provided with the aforesaid overflow 24. The other side of the distributor is provided with a series of longitudinally spaced metal outlet openings 52. The lower part of the distributor carries mounting brackets 53 for mounting the distributor onto the roll carriage frame 31 and provided with apertures to receive indexing pegs 54 on the carriage frame so as accurately to locate the distributor. Delivery nozzle 19 is formed as an elongate body made of a refractory material such as alumina graphite. Its lower part is tapered so as to converge inwardly and downwardly so that it can project into the nip between casting rolls 16. A mounting bracket 60 is provided to support the nozzle on the roll carriage frame and the upper part of the nozzle is formed with outwardly projecting side flanges 55 which locate on the mounting bracket. Delivery nozzle 19 has an upwardly opening inlet trough 61 to receive molten metal flowing downwardly through the openings 52 of the distributor. The bottom part of trough 61 is formed between downwardly convergent nozzle side walls 62 and the bottom is closed by a horizontal bottom floor 63. Each longitudinal side wall 62 is perforated by a series of horizontally spaced openings 64 in the form of circular holes extending horizontally through the side walls. Molten metal falls from the outlet openings 52 of the distributor in a series of free-falling vertical streams 65 to form a reservoir 66 of molten metal in the bottom part of the nozzle trough 61. Molten metal flows from this reservoir out through the openings 64 to form a casting pool 68 supported above the nip 66 between the casting rolls 16. The casting pool is confined at the ends of rolls 16 by a pair of side closure plates 56 which are held against the ends 57 of the rolls. Side closure plates 56 are made of strong refractory material, for example boron nitride. They are mounted in plate holders 82 which are movable by actuation of a pair of hydraulic cylinder units 83 to bring the side plates into engagement with the ends of the casting rolls to form end closures for the casting pool 68 of molten metal. In the casting operation the flow of metal is controlled to maintain the casting pool at a level such that the lower end of the delivery nozzle 19 is submerged in the casting pool and the two series of horizontally spaced openings 64 of the delivery nozzle are disposed immediately beneath the surface of the casting pool. The molten metal then flows from the reservoir within the nozzle trough 61 through the openings 64 in two laterally outwardly directed jet streams in the general vicinity of the casting pool surface so as to impinge on the cooling surfaces of the rolls in the immediate vicinity of the pool surface. This maximises the temperature of the molten metal delivered to the meniscus regions of the pool and it has been found that this significantly reduces the formation of cracks and meniscus marks on the melting strip surface. In accordance with the present invention, a pair of induction heater elements 101 are disposed immediately outside the side plates 56. More specifically the heater elements are mounted on the thruster bodies 85 so as to back upper parts of the side plates 56 without interfering with pivoting movement of the side plates about the pivot pins 84. Each end dam closure plate 56 is shaped so as to have a wide top 102 and a narrow bottom 103 connected by arcuate sides 104 which overlap the ends 57 of the casting rolls 16. Each of the induction heater elements 101 comprises a generally trapezium shaped electrical conductor structure disposed adjacent the back of the respective side dam plate 56 so as in use of the apparatus to be adjacent the upper part of the casting pool. More particularly, it is shaped as a thick walled tubular copper conductor of generally rectangular cross-section extending in a trapezium shaped loop from parallel terminal sockets 105 through which it is supplied with alternating electric current through parallel tubular conductors 106 connected by bus bars 110 to alternating current supply leads 120. Cooling water is circulated through the loops via the conductors 106 as indicated by arrows 121 in Figure 8 and a separate flow of cooling water is passed through the bus bars 110 as indicated by arrows 122. Each trapezium shaped copper loop has a wide top segment 107 arranged generally at the height of the casting pool surface and a narrower bottom segment 108 arranged to be disposed about 70 mm below the pool surface. The top and bottom segments are connected by arcuate side segments 109 which follow the curvature of the casting rolls 11. The electric induction heater elements 101 are effective to cause inductive heating of molten metal in the upper part of the casting pool immediately adjacent the side dam plates 56. The coils are designed to maximise heating around the triple points at the meniscus regions of the pool. It has been demonstrated that this is effective to inhibit the formation of "skulls" at this region of the pool and that in combination with a nozzle which delivers a uniform flow of metal throughout the length of the rolls to the meniscus regions of the pool it is possible to dramatically reduce the superheat of the molten metal supplied for casting and that superheats of lower than 70°C can be achieved. It has been found that with electrically non-conducting side plates the electromagnetic fields produced by the shaped induction heater elements can extend through the side plates to cause effective heating of the molten metal in the pool without heating the side plates and without the need for field shaping or concentrating elements. However, it would be within the scope of the invention to provide appropriate field shaping core pieces around the conductor loops to serve as field concentrator's in order to reduce the electrical power requirements. Figures 9 and 10 demonstrate the effectiveness of the present invention in eliminating the formation of skulls during casting at low superheats. These figures provide the results of measurement of casting roll separation forces during a single casting run of steel strip in a twin roll caster as illustrated in Figures 1 to 6. Figure 9 shows forces measured during a time interval when the induction heaters were deliberately turned off. It will be seen that there were continuing fluctuations in the roll separation forces throughout this time interval. These were associated with the passage of skulls through the nip between the casting rolls with the resulting formation of severe defects in the strip. Figure 10 shows the measurement of separation forces after the induction heaters were switched on showing that the force fluctuations were smoothed out, this being associated with elimination of the formation of skulls and corresponding defects in the strip. In operation of a typical twin roll caster producing one metre wide steel strip at the rate of 60 m/min the induction heaters will need to be supplied with electric current in the range 3000-8000 amps at a frequency of 6 kHz to 10 kHz. The total power input to the induction heaters will accordingly be of the order of 10-100 kWatts per heater. It is important to note that for operation in accordance with the present invention, the induction heater elements 101 should not extend down to the lower parts of the casting pool. It has been found that the very damaging "skulls" are only formed in the upper part of the pool to a depth of about 70 mm. In the lower regions of the pool as the rolls approach the nip metal has already solidified and it is important that it not be reheated at this stage. This can be ensured if the heating coils are spaced above the nip such that they do not extend downwardly to the level of the bottom third of the depth of the casting pool. Previous proposals for induction heating of the pool have simply been directed to heating the pool in general and have involved heaters effective throughout the depth of the pool. Further, the location of the heater elements 101 well above the nip between the casting rolls allows the piston rods 84 of the cylinder units 83 and the associated thruster components to sit below the heater elements and this allows the side plates 56 to tilt about pins 84 as previously described without interference or inadvertent heating of the thruster components. The illustrated apparatus has been advanced by way of example only and it could be modified considerably. For example, instead of delivering molten metal from the tundish into the delivery nozzle in a series of free-falling streams as in the apparatus illustrated in Figures 1 to 6, the metal may be delivered in the delivery nozzle through a submerged entry nozzle. This may be in the form of a single tube as disclosed in the applicants' International Application PCT/AU97/00022. This would allow sufficiently direct flow of the molten metal through the delivery nozzle outlets distributed uniformly along the casting pool to deliver molten metal rapidly to the meniscus regions of the pool.

Claims: Apparatus for casting metal strip comprising a pair of parallel casting rolls (16) forming a nip between them, an elongate metal delivery nozzle (19) disposed above and extending along the nip between the casting rolls (16) for delivery of molten metal into the nip to form a casting pool (68) of molten metal supported above the nip, and a pair of pool confining end closure plates (56) disposed one at each end of the pair of casting rolls (16), wherein the metal delivery nozzle (19) has a series of side openings (64) along each of its longitudinal sides for flow of molten metal from the delivery nozzle (19) in two mutually oppositely directed series of jet streams to impinge directly on the casting surfaces of the rolls (16) in the vicinity of the casting pool surface, and there is a pair of electrical conductors (101) disposed one outside each of the end closure plates (56) and electrical supply means to supply electrical current to the electrical conductors (101) characterised in that the end closure plates (56) are formed of electrically non-conducting material and each conductor (101) is in the form of a single loop comprising a horizontal top segment (107) to extend across an upper portion of the respective end closure plate (56) so as in use of the apparatus to be generally at the level of the casting pool surface and a horizontal bottom segment (108) shorter than the upper segment (107) and connected to the upper segment (107) by a pair of downwardly convergent side segments (109), the bottom segment (108) being spaced above the level of the nip, whereby said conductors (101) are effective to cause induction heating of molten metal in tho casting pool in the vicinity of said end closure plates (56) and the pool surface without causing melting of solidified metal at the nip. Apparatus as claimed in claim 1, further characterised in that each conductor loop is positioned so as not to extend below the upper two thirds of the pool depth. Apparatus as claimed in claim 1 or claim 2, further characterised in that the side segments (109) of each loop are curved to follow the conjunctions between the respective end closure plates (56) and the casting rolls (16). Apparatus as claimed in any one of claims 1 to 3, further characterised in that the bottom segment of the coil is no more than about 70 mm below the level of the pool surface. Apparatus as claimed in any one of claims 1 to 4, further characterised in that each loop is formed as a single trapezium shaped loop of a hollow metal tube and there is cooling means to circulate cooling fluid through the tube. Apparatus as claimed in any one of claims 1 to 5, further characterised in that the electrical supply means is such as to supply alternating electric current at a frequency in the range 6 to 10 kHz. Apparatus as claimed in any one of claims 1 to 3, further characterised in that the end closure plates (56) are held in plate holders (82) connected to piston rods (84) of a pair of thruster cylinder units (83) operable to bias the plates against the casting rolls and said electrical conductors are spaced above the piston rods. Apparatus as claimed in claim 7, further characterised in that the plate holders (82) are pivotally connected to the piston rods (84) of the cylinder units (83) to allow tilting movements of the end closure plates (56) and the electrical conductors (101) are fixed relative to the piston rods (84) so as not to interfere with such tilting movements of the plates (56).

Inventors: BAHARIS CHRIS, BAHARIS, CHRIS
Applicants: BHP STEEL JLA PTY LTD, ISHIKAWAJIMA HARIMA HEAVY IND, BHP STEEL (JLA) PTY. LTD., ISHIKAWAJIMA-HARIMA HEAVY INDUSTRIES CO., LTD.
Application year: 1997
Main classification: B22D11
IPC codes: B22D11