Patent Publication Number: US-6220335-B1

Title: Casting metal strip

Description:
This is a continuation application of Ser. No. 08/992,456, filed Dec. 17, 1997, now U.S. Pat. No. 6,070,647. 
    
    
     BACKGROUND TO THE INVENTION 
     This invention relates to 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 or a series of smaller vessels 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. U.S. Pat. Nos. 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 U.S. Pat. No. 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 U.S. Pat. No. 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 dealing with this problem has been 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. In recent times, however, it has been recognised that the problem can be addressed more efficiently by taking steps to ensure that 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. Examples of this approach are to be seen in Japanese Patent Publication No. 64-5650 of Nippon Steel Corporation and the present applicants&#39; Australian Patent Application No. 60773/96. 
     In order to ensure that the incoming molten metal is delivered relatively quickly into the meniscus regions of the casting pool, it is necessary to employ delivery nozzles with side outlet openings to deliver the metal laterally outwardly from the bottom part of the delivery nozzle toward the casting rolls. Accordingly, the delivery nozzle is required to capture a downwardly falling stream of molten metal and produce steady outward flow of metal through the side outlet openings with as little turbulence and flow fluctuation as possible. This requires that the downward kinetic energy of the incoming stream be absorbed and that essentially non-turbulent conditions be established at the side outlet openings. Moreover, this must be achieved within the very confined space within the bottom of the delivery nozzle without significant restriction of the flow. The previous baffle and diffuser arrangements are not suitable for this purpose but the present invention provides a simple method and means whereby this may be achieved. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided a method of casting metal strip comprising: 
     introducing molten metal between a pair of chilled casting rolls via an elongate metal delivery nozzle disposed above and extending along the nip between the rolls to form a casting pool of molten metal supported above the nip and confined at the ends of the nip by pool confining end closures, and 
     rotating the rolls so as to cast a solidified strip delivered downwardly from the nip; 
     wherein the metal delivery nozzle comprises an upwardly opening elongate trough having a floor and side walls extending longitudinally of the nip to receive the molten metal, the longitudinal side walls of the trough are provided with side outlet openings through which the molten metal is caused to flow from the trough, the floor of the trough is provided with upstanding flow barrier walls adjacent the side outlet openings and molten metal is delivered downwardly into the trough between said flow barrier walls to impinge on the trough floor and flow outwardly against the barrier walls before flowing over those walls to the side outlet openings. 
     Preferably, the side outlet openings of the nozzle are in the form of longitudinally spaced openings formed in each of the longitudinal side walls of the nozzle. 
     Preferably further the openings are shaped as elongate slots. The slots may be closely spaced so as to promote substantially continuous curtain jet streams of molten metal into the casting pool from the adjacent slot openings of the delivery nozzle. 
     The trough of the delivery nozzle may be supplied with molten metal in a series of discrete free falling streams spaced apart longitudinally of the trough or in a free falling continuous curtain stream extending along the trough. 
     Preferably, the molten metal is supplied to the delivery nozzle in a series of discrete free falling streams spaced apart longitudinally of the trough so as to impinge on the floor of the trough at locations aligned laterally with spaces between the side outlet openings of the nozzle. 
     Preferably further the upstanding barrier walls are comprised of a pair of laterally spaced walls standing up from the floor of the trough and extending continuously along the trough to define an internal channel to receive the incoming flow of molten metal. 
     The invention also provides 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 and a distributor disposed above the delivery nozzle for supply of molten metal to the delivery nozzle, wherein the metal delivery nozzle comprises an upwardly opening elongate trough having a floor and side walls extending longitudinally of the nip to receive molten metal from the distributor, the delivery nozzle is provided with side outlet openings in the longitudinal side walls of the trough for flow of molten metal outwardly from the bottom of the delivery nozzle, the floor of the trough is provided with upstanding barrier walls adjacent the side outlet openings, and the distributor is operable to deliver molten metal downwardly into the trough between said flow barrier walls to impinge on the floor and flow outwardly against the barrier walls. 
     The invention also provides a delivery nozzle for delivering molten metal to a strip caster, comprising an upwardly opening elongate trough having a floor and side walls extending longitudinally to receive molten metal, the delivery nozzle being provided with side outlet openings in the longitudinal side walls of the trough for flow of molten metal outwardly from the bottom of the delivery nozzle, and the floor of the trough being provided with upstanding barrier walls adjacent the side outlet openings to enable molten metal delivered downwardly into the trough between said flow barrier walls to impinge on the floor and flow outwardly against the barrier walls. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order that the invention may be more fully explained one particular method and apparatus will be described in some detail with reference to the accompanying drawings in which: 
     FIG. 1 illustrates a twin-roll continuous strip caster constructed and operating in accordance with the present invention; 
     FIG. 2 is a vertical cross-section through important components of the caster illustrated in FIG. 1 including a metal delivery nozzle constructed in accordance with the invention; 
     FIG. 3 is a further vertical cross-section through important components of the caster taken transverse to the section of FIG. 2; 
     FIG. 4 is an enlarged transverse cross-section through the metal delivery nozzle and adjacent parts of the casting rolls; 
     FIG. 5 is a side elevation of a one half segment of the metal delivery nozzle; 
     FIG. 6 is a plan view of the nozzle segment shown in FIG. 5; 
     FIG. 7 is a longitudinal cross-section through the delivery nozzle segment; 
     FIG. 8 is a perspective view of the delivery nozzle segment; 
     FIG. 9 is an inverted perspective view of the nozzle segment; 
     FIG. 10 is a transverse cross-section through the delivery nozzle segment on the line  10 — 10  in FIG. 5; 
     FIG. 11 is a cross-section on the line  11 — 11  in FIG. 7; and 
     FIG. 12 is a cross-section on the line  12 — 12  in FIG.  7 . 
    
    
     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 vice 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 in two identical half segments which are made of a refractory material such as alumina graphite are held end to end to form the complete nozzle. FIGS. 5 to  11  illustrate the construction of the nozzle segments which are supported on the roll carriage frame by a mounting bracket  60 , the upper parts of the nozzle segments being formed with outwardly projecting side flanges  55  which locate on that mounting bracket. 
     Each nozzle half segment is of generally trough formation so that the nozzle  19  defines an upwardly opening inlet trough  61  to receive molten metal flowing downwardly from the openings  52  of the distributor. Trough  61  is formed between nozzle side walls  62  and end walls  70  and may be considered to be transversely partitioned between its ends by the two flat end walls  80  of the nozzle segments which are brought together in the completed nozzle. The bottom of the trough is closed by a horizontal bottom floor  63  which meets the trough side walls  62  at chamfered bottom corners  81 . The nozzle is provided at these bottom corners with a series of side outlet openings in the form of longitudinally spaced elongate slots  64  arranged at regular longitudinal spacing along the nozzle. Slots  64  are positioned to provide for egress of molten metal from the trough generally at the level of the trough floor  63 . 
     In accordance with the present invention, a pair of upright flow barrier walls  84  stand up from the floor  63  of nozzle trough  61  adjacent the slots  64 . Walls  84  extend continuously throughout the length of trough  61  to define an internal trough channel  85  to receive the incoming flow of molten metal as described below. 
     The outer ends of the nozzle segments are provided with end formations denoted generally as  87  extending outwardly beyond the nozzle end wall  70  and provided with metal flow passages to direct separate flows of molten metal to the “triple point” regions of the pool ie. those regions of the pool where the two rolls and the side dam plates come together. The purpose of directing hot metal to those regions is to prevent the formation of “skulls” due to premature solidification of metal in these regions, as is more fully described in our Australian Patent Application No. PO2367. 
     Each end wall formation  87  defines a small open topped reservoir  88  to receive molten metal from the distributor, this reservoir being separated from the main trough of the nozzle by the end wall  70 . The upper end  89  of end wall  70  is lower than the upper edges of the trough and the outer parts of the reservoir  88  and can serve as a weir to allow back flow of molten metal into the main nozzle trough from the reservoir  88  if the reservoir is over filled, as will be more fully explained below. 
     Reservoir  88  is shaped as a shallow dish having a flat floor  91 , inclined inner and side faces  92 ,  93  and a curved upright outer face  94 . A pair of triple point pouring passages  95  extend laterally outwardly from this reservoir just above the level of the floor  91  to connect with triple point pouring outlets  96  in the undersides of the nozzle end formations  87 , the outlets  96  being angled downwardly and inwardly to deliver molten metal into the triple point regions of the casting pool. 
     Molten metal falls from the outlet openings  52  of the distributor in a series of free-falling vertical streams  65  into the bottom part of the nozzle trough  61 . Molten metal flows from this reservoir out through the slots  64  to form a casting pool  68  supported above the nip  69  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 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 slots  64  of the delivery nozzle are disposed immediately beneath the surface of the casting pool. The molten metal flows through the slots  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 the streams  65  fall into the internal trough channel  85  to impinge on the floor  63  of the trough  61  between the two upstanding flow barrier walls  84 . The impinging metal is thus caused to flow outwardly against the barrier walls which prevent direct flow to the slots  64 . The kinetic energy of the metal is substantially reduced by secondary impact with barrier walls  84  and the metal is thus initially confined within channel  85 , but flows over the walls  84  with generally steady continuous flow conditions to the slots  64 . To ensure effective reduction of kinetic energy, it is important that the channel  85  be formed with a flat floor and vertical side walls meeting at sharply defined corners to produce a double impingement effect. 
     The outlet openings  52  of the distributor are staggered longitudinally of the nozzle with respect to the slots  64  so that the falling streams  65  impinge on the nozzle floor at locations between successive pairs of slots  64 . It has been found that the system can be operated to establish a casting pool which rises to a level only just above the bottom of the delivery nozzle so that the casting pool surface is only just above the floor of the nozzle trough and at the same level as the metal within the trough. Under these conditions it is possible to obtain very stable pool conditions and if the outlet slots are angled downwardly to a sufficient degree it is possible to obtain a quiescent pool surface. 
     It is important to note that slots  64  are provided at the inner ends of the two nozzle sections. This ensures adequate delivery of molten metal to the pool in the vicinity of the central partition in the nozzle and avoids the formation of skulls in this region of the pool. 
     The triple point pouring reservoirs  88  receive molten metal from the two outermost streams  65  falling from the distributor  18 . The alignment of the two outermost holes  52  in the distributor is such that each reservoir  88  receives a single stream impinging on the flat floor  91  immediately outside the sloping side face  92 . The impingement of the molten metal on floor  88  causes the metal to fan outwardly across the floor and outwardly through the triple point pouring passages  95  to the outlets  96  which produce downwardly and inwardly inclined jets of hot metal directed across the faces of the side dams and along the edges of the casting rolls toward the nip. Triple point pouring proceeds with only a shallow and wide pool of molten metal within each of the troughs  88 , the height of this pool being limited by the height of the upper end  89  of the wall  70 . When reservoir  88  is filled molten metal can flow back over the wall end  89  into the main nozzle trough so that the wall end serves as a weir to control the depth of the metal pool in the triple point pouring supply reservoir  88 . The depth of the pool is more than sufficient to supply the triple point pouring passages so as to maintain flow at a constant head whereby to achieve a very even flow of hot metal through the triple point pouring passages. This control flow is most important to proper formation of the edge parts of the strip. Excessive flow through the triple point passages can lead to bulging in the edges of the strip whereas to little flow will produce skulls and “snake egg” defects in the strip. 
     The undersides  98  of the triple point pouring formations  87  are raised above the surface of the casting pool so as to avoid cooling of the pool surface at the triple point region. Moreover, the undersides  98  are outwardly and upwardly inclined. This is desirable in order to prevent an accumulation of slag or other contaminants from jamming beneath the ends of the nozzle. Such jamming can result in blockage of gas and fumes escaping from the casting pool and the risk of explosion. 
     The illustrated apparatus has been advanced by way of example only and the invention is not limited to the details of that apparatus. In particular it is not essential to the present invention that the nozzle be provided with triple point pouring formations although that is the presently preferred form of nozzle. Although it is preferred that the barrier walls  83  be of uniform height throughout the length of the nozzle it would be possible to have wall sections of reduced height between the slot openings or even to provide discontinuous wall sections along the nozzle. Moreover the flow of the internal trough  85  could be raised or lowered relative to the remainder of the floor  63  of the nozzle. It is to be understood that such variations may be made without departing from the spirit and scope of the invention which extends to every novel feature and combination of features herein disclosed.