Patent Publication Number: US-6988530-B2

Title: Strip casting

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation-in-part of U.S. Ser. No. 09/882,660, filed Jun. 15, 2001, now U.S. Pat. No. 6,536,506, assigned to the same assignee as this application and now incorporated herein by reference, and which claims priority to and the benefit of Australian Application Serial No. PQ8180, filed Jun. 15, 2000. 

   BACKGROUND AND SUMMARY OF THE INVENTION 
   This invention relates to the casting of metal strip and making of cast steel strip. It has particular application to the casting of metal strip by continuous casting in a twin roll caster. 
   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 series of smaller vessels from which it flows through a metal delivery nozzle located above the nip so as to form a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed. 
   The setting up and adjustment of the casting rolls in a twin roll caster is a significant problem. The casting rolls must be accurately set to properly define an appropriate separation of the casting rolls at the nip, generally of the order of a few millimeters or less. There must also be some means for allowing at least one of the rolls to move outwardly against a biasing force to accommodate fluctuations in strip thickness particularly during start up. 
   Usually, one of the rolls is mounted in fixed journals, and the other roll is rotatably mounted on supports that can move against the action of biasing means to enable the roll to move laterally to accommodate fluctuations in casting roll separation and strip thickness. The biasing means may be in the form of helical compression springs or alternatively, may comprise a pair of pressure fluid cylinder units. 
   A strip caster with spring biasing of the laterally moveable roll is disclosed in U.S. Pat. No. 6,167,943 to Fish et al. In that apparatus, the biasing springs act between the roll carriers and a pair of thrust reaction structures, the positions of which can be set by operation of a pair of powered mechanical jacks to enable the initial compression of the springs to be adjusted to set initial compression forces which are equal at both ends of the roll. The positions of the roll carriers need to be set and subsequently adjusted after commencement of casting so that the gap between the rolls is constant across the width of the nip in order to produce a strip of constant profile. However, as casting continues the profile of the strip will inevitably vary due to eccentricities in the rolls and dynamic changes due to variable heat expansion and other dynamic effects. 
   Eccentricities in the casting rolls can lead to strip thickness variations along the strip. Such eccentricities can arise either due to machining and assembly of the rolls or due to distortion when the rolls are hot possibly due to non-uniform heat flux distribution. Specifically, each revolution of the casting rolls will produce a pattern of thickness variations dependent on eccentricities in the rolls and this pattern will be repeated for each revolution of the casting rolls. Usually the repeating pattern will be generally sinusoidal, but there may be secondary or subsidiary fluctuations within the generally sinusoidal pattern. 
   With improvements in the design of the casting rolls for a twin roll caster, particularly by the provision of textured surfaces which enable control of the heat flux at the interface between the casting rolls and the casting pool, it has been possible to achieve dramatic increases in strip casting speeds. However, when casting thin strip at high casting speeds there is an increased tendency to produce both high and low frequency gauge variations. 
   We have found that the gauge variations in cast strip can be alleviated by reducing the casting roll separation force and that the defect can be practically eliminated if the roll separation force is minimized. In practice there is at least a certain force that is required to balance the hydrostatic pool pressure and to overcome the mechanical friction involved in moving the rolls. We have also found that the high frequency gauge variation can be overcome, and a unique cast steel strip can be produced, by reducing the strip stiffness in the region of the nip by allowing a quantity of mushy or molten metal to be passed through the nip between the two solidified shells of the strip, by maintaining a roll gap at the nip slightly greater than the gap determined by the fully solidified shell thickness. It is desirable for these purposes that the mechanical friction forces involved in movement of the casting rolls relative to each other is minimized. By achieving very low strip stiffness, the dynamic interaction of the rolls on the strip is uncoupled, and consequently periodic gauge variation regeneration can be substantially reduced if not eliminated. 
   In at least one aspect, the present invention combines the features of applying a constant casting roll separation force (which can be small) and establishing a constant roll gap that will enable molten metal to be passed through the nip to further reduce strip stiffness. In order to maintain the constant separation force together with a constant roll gap, the invention may also allow for roll eccentricity compensation. 
   According to the invention there is provided an apparatus for continuously casting metal strip comprising a pair of parallel casting rolls forming a nip between them; metal delivery means to deliver molten metal into the nip between the rolls to form a casting pool of molten metal supported on casting roll surfaces immediately above the nip; pool confining means to confine the molten metal in the casting pool against outflow from the ends of the nip; and roll drive to drive the casting rolls in the counter-rotational directions to produce a solidified strip of metal delivered downwardly from the nip; wherein at least one of the casting rolls is mounted on a pair of moveable roll carriers which allow that one roll to move bodily toward and away from the other roll, wherein there is a pair of carrier drive units acting one on each of the pair of moveable roll carriers to bias said one roll bodily toward the other roll, and wherein each roll carrier drive unit comprises a thrust transmission structure connected to the respective roll carrier, a thrust reaction structure, a thrust generator acting between the thrust reaction structure and the thrust transmission structure to exert a thrust on the thrust transmission structure and the respective roll carrier, thrust reaction structure setting means operable to vary the position of the thrust reaction structure, and control means to control operation of the setting means so as to replicate a pattern of movement of the roll carriers due to roll eccentricities as an applied pattern of movements of the thrust reaction structure to maintain a constant roll biasing force, and roll gap control means operable to increase the gap between the rolls after said applied pattern of movements has been established. 
   The roll gap control means may be operable to produce an incremental increase of the roll gap in the range 0 to 50 microns. The roll gap control means may be operable to move said one roll. Alternatively, it may be operable to move the other casting roll. In other embodiments, to provide small roll separation force, the roll gap may be fixed and the casting speed may be varied until the requisite separation force is achieved. In that case, eccentricity compensation may be applied prior to providing speed adjustment. 
   The present invention may provide a unique cast steel strip with a composition as described in more detail below in the description of the embodiments described with reference to the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Particular embodiments, and possible modifications, will be described in some detail with reference to the accompanying drawings in which: 
       FIG. 1  is a vertical cross section through a strip caster constructed in accordance with the present invention; 
       FIG. 2  is an enlargement of part of  FIG. 1  illustrating particular components of the caster; 
       FIG. 3  is a longitudinal cross section through particular parts of the caster; 
       FIG. 4  is an end elevation of the caster; 
       FIGS. 5 ,  6  and  7  show the caster in varying conditions during casting and during removal of the roll module from the caster; 
       FIG. 8  is a vertical cross-section through a carrier drive unit incorporating a roll biasing spring; 
       FIG. 9  is a schematic representation of various components of the caster; 
       FIG. 10  is a cross-section of a cast steel strip made as described by the present invention; and 
       FIG. 11  is a cross-section of a cast steel strip of the prior art illustrated for purposes of comparison. 
   

   DETAILED DESCRIPTION OF THE DRAWINGS 
   For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated device, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. 
   The illustrative caster comprises a main machine frame  11  which stands up from the factory floor (not shown) and supports a casting roll module in the form of a cassette  13  which can be moved into an operative position in the caster as a unit but can readily be removed when the rolls are to be replaced. Cassette  13  carries a pair of parallel cooled casting rolls  16  having a nip  16 A between them, to which molten metal is supplied during a casting operation from a ladle (not shown) via a tundish  17 , molten metal distributor  18  and delivery nozzle  19  to create a casting pool  30 . Casting rolls  16  are water cooled so that solidified shells form onto the moving roll surfaces and are brought together at the nip  16 A between them to produce a solidified strip product  20  below the roll nip. This product may be fed to a standard coiler. 
   Casting rolls  16  are contra-rotated through drive shafts  41  from an electric motor and transmission mounted on the main machine frame. The drive shaft can be disconnected from the transmission when the cassette is to be removed. 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 roll may typically be about 500 mm in diameter and about 2000 mm long in order to produce strip product approximately the width of the rolls. 
   A ladle of a conventional construction is supported on a rotating turret and a metal delivery system is provided by positioning the ladle over the tundish  17  to fill the tundish. The tundish may be fitted with a sliding gate valve  47  actuable by a servo mechanism to allow molten metal to flow from the tundish  17  through the valve  47  and refractory shroud  48  into molten metal distributor  18 . 
   The molten metal distributor  18  may be formed as a wide dish made of a refractory material such as magnesium oxide (MgO). One side of the distributor  18  may receive molten metal from the tundish  17  and the other side of the distributor  18  may be provided with a series of longitudinally spaced metal outlet openings  52 . The lower part of the distributor  18  carries mounting brackets  53  for mounting the distributor  18  onto the main frame  11  when the cassette  13  is installed in its operative position. 
   The metal delivery system also may have delivery nozzle  19  formed as an elongate body made of a refractory material such as alumina graphite. The lower part of nozzle  19  may be tapered so as to converge inwardly and downwardly so that it can project into the nip  16 A between casting rolls  16 . Its upper part may be formed with outwardly projecting side flanges  55  that locate on a mounting bracket  60  which forms part of the main frame  11 . 
   Delivery nozzle  19  may have a series of horizontally spaced generally vertically extending flow passages to produce a suitably low velocity discharge of molten metal throughout the width of the casting rolls and to deliver the molten metal into the nip  16 A between the casting rolls without direct impingement on the roll surfaces at which initial solidification occurs. Alternatively, delivery nozzle  19  may have a single continuous slot outlet to deliver a low velocity curtain of molten metal directly into the nip  16 A between the casting rolls  16 . In either form, the nozzle  19  may be immersed in the molten metal pool between the casting rolls  16 . 
   The casting pool of molten metal is confined at the ends of the rolls by a pair of side closure plates or dams  56  that are held against stepped ends  57  of the rolls when the roll cassette is in its operative position. Side closure plates  56 , or dams are made of a strong refractory material and have contoured edges to match the curvature of the stepped ends of the rolls. The side closure plates  56  can be 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 stepped ends of the casting rolls to form end closures for the molten pool of metal formed on the casting rolls during a casting operation and confine outflow of the casting pool of molten metal. Side closure plates  56  are adjacent the ends of the nip  16 A, and confine the casting pool formed between the casting rolls  16 . 
   During a casting operation the sliding gate valve  47  of the metal delivery system is actuated to allow molten metal to pour from the tundish  17  to the distributor  18  and through the metal delivery nozzle  19  whence it flows onto the casting rolls to form the casting pool with confinement of the side closure plates  56 . The head end of the strip product  20  is guided by actuation of an apron table  96  to a pinch roll and thence to a coiling station (not shown). Apron table  96  hangs from pivot mountings  97  on the main frame and can be swung toward the pinch roll by actuation of an hydraulic cylinder unit (not shown) after the clean head end has been formed. 
   The removable roll cassette  13  is constructed as a module so that the casting rolls  16  can be set up and the gap of the nip  16 A between them adjusted before the cassette is installed in position in the caster. The gap between the casting rolls at this point in assembly generally should be as small as possible without the casting rolls touching each other. Moreover when the cassette  13  is installed, a carrier drive system is provided with two pairs of carrier drive units  110  and  111  mounted on the main machine frame  11  that can be rapidly connected to roll carriers on the cassette  13  to provide forces resisting separation of the casting rolls. The carrier drive units may be roll biasing units or servo-mechanisms. 
   Roll cassette  13  comprises a large frame  102  that carries the casting rolls  16  and upper part  103  of the enclosure for enclosing the cast strip below the nip  16 A. Casting rolls  16  are mounted on roll carriers  104  that comprise a pair of roll end support structures  90  ( FIG. 4 ) carrying roll end bearings  100  by which the rolls are mounted for rotation about their longitudinal axis in parallel relationship with one another. The two pairs of roll carriers  104  are mounted on the roll cassette frame  102  by means of linear bearings  106 . Each pair of roll carriers  104  can slide laterally of the cassette frame to provide for bodily movement of the casting rolls toward and away from one another, permitting separation and closing movement between the two parallel casting rolls  16 . 
   Roll cassette frame  102  also carries two adjustable stops  107  disposed beneath the casting rolls  16  about a central vertical plane between the rolls and located between the two pairs of roll carriers  104  so as to serve as stops limiting inward movement of the two roll carriers  104  to define the minimum width of the gap at the nip  16 A between the casting rolls  16 . As explained below the roll carrier drives  110  and  111  are actuable to move the roll carriers  104  inwardly against these central adjustable stops, but to permit outward movement of one of the casting rolls  16  against preset forces. 
   Each adjustable stop means  107  is in the form of, for example, a worm or screw driven jack having a body  108  fixed relative to the central vertical plane of the caster and two ends  109  which can be moved on actuation of the driven jack equally in opposite directions to permit expansion and contraction of the jack to adjust the width of the gap at the nip  16 A, while maintaining equidistant spacing of the casting rolls  16  from the central vertical plane of the caster and, also, a substantially constant gap between the casting rolls  16 . 
   The carrier drive system is provided with two pairs of roll carrier drive units  110  and  111  each connected to a roll carrier  104  at each end of a casting roll  16 . The carrier drive units  110  at one side of the caster are constructed and operate to be capable of moving one of the roll carriers and in turn varying the thickness of the strip across the strip width at the nip. These drives are comprised of servo-mechanisms (not shown) or compression springs  112  to provide lateral forces on the respective roll carriers  104 . The carrier drives  111  at the other side of the caster move the roll carriers  104  supporting the other casting roll and incorporate hydraulic actuators  113 . These actuators  113  are operable to hold the respective roll carriers  104  supporting one casting roll firmly against the central stops, while the other casting roll is free to move laterally with the action of the force of the servo-mechanism or compression springs  112  of the carrier drive units  110  to bias the casting rolls toward each other. 
   The detailed construction of carrier drive units  110  are illustrated in  FIG. 8 , where units  110  are comprised of biasing units. As shown in that figure, each biasing unit comprises a compression spring  112  positioned in barrel housing  114  disposed within an outer housing  115 , and is fixed to the main caster frame  116  by fixing bolts  117 . 
   Spring housing  114  may be formed with a cylinder housing  118  positioned within the outer housing  115 . Spring housing  114  may be set alternatively in an extended position as illustrated in  FIG. 8  and a retracted position by flow of hydraulic fluid to and from the cylinder housing  118 . The outer end of spring housing  114  carries a pressure fluid drive operable in the form of a hydraulic cylinder unit  119 , and operable to set the position of a spring reaction plunger  121  connected to the piston of unit  119  by a connecting rod  130 . 
   The other end of the compression spring  112  acts on a thrust transmission structure  122 , which is connected to the respective roll carrier  104  through a load cell  125 . The thrust structure is initially pulled into firm engagement with the roll carrier by a connector  124  that can be extended by operation of an hydraulic cylinder  123  when roll carrier drive units are to be disconnected. 
   When roll carrier drive units  110  are connected to the respective roll carrier  104 , with the spring housing  114  set in its extended condition as shown in  FIG. 8 , the position of the spring housing  114  and cylinder unit  119  is fixed relative to the caster frame. The position of the spring reaction plunger  121  can be set to adjust the effective gap between the spring abutments on the reaction plunger  121  and the thrust transmission structure  122 . The compression of the spring  112  can thereby be adjusted to vary the thrusting force applied to the thrust transmission structure  122  and the respective roll carrier  104 . With this arrangement the only relative movement during casting operation is the movement of the roll carrier  104  and thruster structure  122  as a unit against the compression spring. Alternatively, the same force exerted by the compression spring on the roll carrier  104  can be exerted by a servo-mechanism. In either case, the force exerted by the roll carrier drives  110  on the roll carrier  104  inwardly against the stop can be adjusted to preload the roll carrier  104  with a required inward force before metal strip actually passes between the casting rolls, and that force can be maintained during a subsequent casting operation. 
   Hydraulic cylinder unit  119  may be operated continuously to vary the position of the spring reaction plunger to replicate movements of the thrust transmission structure  122  due to variations in strip thickness and resulting lateral movements of the roll carrier  104 . Any inward or outward movement of roll carrier  104  will cause a corresponding inward or outward movement of the cylinder of cylinder unit  119  and spring reaction plunger  121  so as to maintain a constant compression of the compression spring  112 . 
   Accordingly, a substantially constant biasing force can be maintained against the carrier  104  and in turn the supported casting roll  16  at each end of the roll regardless of movements of the roll mountings. Previously available pressure fluid systems are not used because they are generally too slow in response time. The use of compression springs or servo-mechanisms in combination with a continual control setting device as explained herein enables very accurate setting of controlled forces which can be maintained or varied throughout a casting operation. The compression springs of the carrier drive units may be very low stiffness springs, or, alternatively, sensitive servo-mechanisms may be used because the two roll carrier drive units of the carrier drive system at the two ends of the laterally moveable casting roll operate independently so that there need be no cross-talk between them. 
   Accordingly, this arrangement allows the roll biasing force to be reduced to a very low level in accordance with the present invention. Generally there is a minimum force that is required to balance the hydrostatic pressure of the casting pool (approximately 0.75 kN per side in a 500 mm diameter twin roll caster and 1350 mm roll width) and to overcome the mechanical friction involved in moving the casting rolls (less than approximately 0.6 kN per side in a 500 mm diameter twin roll caster). This results in a practical low biasing force level, which may be in the range of 0.75 to 2 kN. 
   As illustrated diagrammatically in  FIG. 9 , an exemplary control system can be comprised of position sensors  150 , sensing the position of the thrust transmission structures  122  and connected into a control circuit which controls the operation of the cylinder unit  119  so that the movements of the thrust transmission structures  122  are replicated by the cylinders of units  119 . The control system may comprise controllers  151  connected to the position sensors  150  and to the cylinder units  119  to operate the cylinders  119  so as to replicate movements of the thrust transmission structures  122 . Controllers  151  also control operation of the cylinders for initial setting of the roll carriers prior to casting and subsequent adjustment to add a similar incremental movement of the cylinders  119  through step controllers  160  to maintain the constant biasing force, and to increase the gap at the nip  16 A between the casting rolls  16 , so as to produce a gap between the rolls  16  at the nip  16 A that is greater than the gap determined by the solidified shell thickness in casting. The step controllers have a set point input at  161 . 
   Typically in accordance with the illustrated embodiments, the system may be operated to maintain a gap at the nip  16 A between the casting rolls  16  greater than the gap determined by the solidified shell thickness. In operation of the illustrated system, casting commences with a gap initially determined by the solidified shell thickness. This thickness is illustrated by  FIG. 11  where the dendrites of the solidified shells of the strip join in the formed strip. Movement of the roll carriers due to remaining roll eccentricities are sensed by the sensors  150  and the control unit learns the pattern of roll movements due to that eccentricity. In order to compensate for the eccentricity induced force fluctuation, the roll chock trajectories are replicated at the spring reaction structures by the position control system and those compensatory movements are continued. The roll gap is then increased by a small amount (such as for example 0 to 50 microns) while the pattern of movements of the spring reaction structure is continued. This even further enhances the already formed substantially constant gap between the casting rolls by further reducing if not eliminating force fluctuation induced by roll eccentricity compensation. 
   Illustratively, in the control system illustrated in  FIG. 9 , the step of increasing the gap at the nip  16 A between the casting rolls  16  is achieved by moving the roll carriers supporting the spring biased roll and the hydraulically actuated biasing units for the other roll are operated to lock the other roll in a fixed position. The system of the present invention can be used in combination with the eccentricity control system described in our co-pending U.S. patent application Ser. No. 10/104,313, which description now is incorporated herein by reference. In that system, the thickness variations due to roll eccentricity can be very much reduced by imposing a pattern of speed variations in the speed of rotation of the casting rolls. Compensation in this manner is possible because even small variations vary the time of contact of the solidifying metal shells on the casting rolls within the casting pool, and therefore affect the strip thickness and roll thermal load to facilitate the production of strip of constant thickness. If this form of eccentricity control is adopted, this will reduce the amplitude of the initial roll carrier fluctuations and the need for compensatory movements within the minimal force/constant gap system of the present invention. The present invention also provides enhanced productivity. 
   Referring to  FIG. 10 , unique steel product made by the presently described method is illustrated. The unique cast steel strip is made by the following steps of assembling a pair of cooled casting rolls having a nip between them and confining closures adjacent the ends of the nip, introducing molten metal between said pair of casting rolls to form a casting pool between the rolls with the closures confining the pool adjacent the ends of the nip, rotating the rolls such that shells of metal solidify from the casting pool onto the casting rolls and are brought close together at the nip to produce a solidified strip delivered downwardly from the nip, biasing at least one of the pair of casting rolls toward the other roll of the pair under a biasing force and maintaining a substantially constant gap between the rolls at the nip sufficient to provide separation between the solidified shells at the nip, preferably with the biasing force creating a roll separation force less than 0.45 kN, and passing molten metal between the solidified shells through the nip where at least a portion of said molten metal is solidified in the strip below the nip. The columnar dendrite structure of steel formed in the solidified shells onto the casting rolls  16  do not come together. This is illustrated by comparison in  FIG. 11 , where the structure of steel strip made by the previously described strip casting process is illustrated. There the columnar dendrite structure of the solidified shell join in the formed strip as the solidified shells come together. However, in steel strip made in accordance with the present invention, there is a central zone within the steel strip between the solidified shells that solidifies after strip passes through the gap between the casting rolls  16  at the nip  16 A. 
   While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiments have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.