US20140262121A1

US20140262121A1 - Method of thin strip casting

Info

Publication number: US20140262121A1
Application number: US13/838,791
Authority: US
Inventors: Gary McQuillis
Original assignee: Nucor Corp
Current assignee: Nucor Corp
Priority date: 2013-03-15
Filing date: 2013-03-15
Publication date: 2014-09-18

Abstract

A method of casting thin strip to reduce casting defects is disclosed. The method includes assembling a pair of side dams adjacent end portions of counter-rotating casting rolls through which a strip of metal can be cast to permit a casting pool of molten metal to be formed supported by the casting surfaces of the casting rolls, assembling a metal delivery system above the casting rolls delivering molten metal to form a casting pool supported on the casting surfaces of the casting rolls above the nip and confined by the pair of side dams, assembling a vibrator device to vibrate the pair of side dams at 200-400 Hz. to inhibit the formation of snake eggs in the casting pool, and counter-rotating the casting rolls such that the casting surfaces of the casting rolls each travel inwardly toward the nip to produce a cast strip downwardly from the nip.

Description

BACKGROUND AND SUMMARY

This invention relates to making thin strip and more particularly casting of thin strip by a twin roll caster.
It is known to cast metal strip by continuous casting in a twin roll caster. Molten metal is introduced between a pair of counter-rotating horizontal casting rolls, which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between the rolls to produce 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 tundish, from which it flows through a metal delivery nozzle positioned above the nip, longitudinally between the casting rolls, which delivers the molten metal to the region above the nip to form a casting pool of molten metal. The casting pool of molten metal is supported on the casting surfaces of the rolls above the nip. The casting pool is typically confined at the ends of the casting rolls by side plates or dams held in sliding engagement adjacent the ends of the casting rolls.
In casting thin strip by twin roll casting, the metal delivery nozzles receive molten metal from the moveable tundish and deposit the molten metal in the casting pool in a desired flow pattern. The flow pattern created by the manner in which the nozzle delivers molten metal to the casting pool can affect the quality and yield of the thin strip. The formation of pieces of solid metal known as “skulls” in the casting pool in the vicinity of the confining side plates or dams is a known problem. The rate of heat loss from the casting pool is higher near the side dams adjacent the casting roll ends due to the greater surface area of continuous caster components in contact with the molten metal in the casting pool increasing the conductive heat loss from the system. This area is called the “triple point region.” This localized heat loss gives rise to “skulls” of solid metal forming in that region, which can grow to considerable size. The skulls can drop through the nip of the casting rolls and into the forming strip, causing defects in the strip known as “snake eggs.” An increased flow of molten metal to the triple point regions, near the side dams, has been provided to help maintain the temperature of the casting pool in these regions. Examples of such proposals may be seen in U.S. Pat. No. 4,694,887 and in U.S. Pat. No. 5,221,511. However, it would better to further reduce the formation of skulls in the casting pool and in turn snake eggs in the cast strip.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagrammatical side view of twin roll caster plant shown operation of the presently disclosed method;

FIG. 2 is a partial sectional view through the casting rolls mounted in a roll cassette in the casting position of the caster of FIG. 1;

FIG. 3 is a diagrammatical plan view of the roll cassette of FIG. 2 removed from the caster;

FIG. 4 is a transverse partial sectional view through the portion marked 4-4 in FIG. 3;

FIG. 5 is an enlarged view of one of the carriage assemblies marked as detail 5 in FIG. 4;

FIG. 6 is a side plan view, partially in section, of the carriage assembly of FIG. 5 with the side dam in a first position;

FIG. 7 is a top plan view, partially in section, of the carriage assembly of FIG. 6 with the side dam in the first position; and

FIG. 8 is a graph showing the caster delivery CR actual gap force during a casting campaign with a caster similar to that shown in FIGS. 1-7.

DETAILED DESCRIPTION

Referring now to FIGS. 1 and 2 a twin roll caster is shown for continuously casting thin steel strip that comprises a main machine frame 10 that stands up from the factory floor and supports a roll cassette module 11 (not shown) on which a pair of counter-rotatable casting rolls 12 are mounted. The casting rolls 12 having shafts 22 (FIG. 3) and casting surfaces 12A laterally positioned to form a nip 18 there between. The roll cassette facilitates rapid movement of the casting rolls 12 ready for casting from a setup position into an operative casting position in the caster as a unit, and ready removal of the casting rolls from the casting position when the casting rolls are to be replaced. There is no particular configuration of the roll cassette that is desired, so long as it performs that function of facilitating movement and positioning of the casting rolls 12.
Molten metal is supplied from a ladle 13 through a metal delivery system, such as a movable tundish 14 and a transition piece or distributor 16. From the distributor 16, the molten metal flows to at least one metal delivery nozzle 17, or core nozzle, positioned between the casting rolls 12 above the nip 18. Molten metal discharged from the delivery nozzle 17 thus delivered forms a casting pool 19 of molten metal supported on the casting surfaces 12A of the casting rolls 12 above the nip 18. This casting pool 19 is confined in the casting area at the ends of the casting rolls 12 by a pair of side closures or confining plate side dams 20 (shown in dotted line in FIG. 2). The upper surface of the casting pool 19 (generally referred to as the “meniscus” level) may rise above the bottom portion of the delivery nozzle 17 so that the lower part of the delivery nozzle 17 is immersed in the casting pool 19. The casting area includes the addition of a protective atmosphere above the casting pool 19 to inhibit oxidation of the molten metal in the casting area.
The ladle 13 typically is of a conventional construction supported on a rotating turret 40. For metal delivery, the ladle 13 is positioned over a movable tundish 14 in the casting position as shown in FIG. 1 to fill tundish 14 with molten metal. The movable tundish 14 may be positioned on a tundish car 66 capable of transferring the tundish from a heating station (not shown), where the tundish is heated to near a casting temperature, to the casting position. A tundish guide, such as rails, may be positioned beneath the tundish car 66 to enable moving the movable tundish 14 from the heating station to the casting position.
The movable tundish 14 may be fitted with a slide gate 25, actuable by a servo mechanism, to allow molten metal to flow from the tundish 14 through the slide gate 25, and then through a refractory outlet shroud 15 to a transition piece or distributor 16 in the casting position. From the distributor 16, the molten metal flows to the delivery nozzle 17 positioned between the casting rolls 12 above the nip 18.
The casting rolls 12 are internally water cooled so that as the casting rolls 12 are counter-rotated, shells solidify on the casting surfaces 12A as the casting surfaces 12A move into contact with and through the casting pool 19 with each revolution of the casting rolls 12. The shells are brought together at the nip 18 between the casting rolls 12 to produce a solidified thin cast strip product 21 delivered downwardly from the nip 18. The gap between the casting rolls is such as to maintain separation between the solidified shells at the nip so that semi-solid metal is present in the space between the shells through the nip, and is, at least in part, subsequently solidified between the solidified shells within the cast strip below the nip.
FIG. 1 shows the twin roll caster producing the thin cast strip 21, which passes across a guide table 30 to a pinch roll stand 31, comprising pinch rolls 31A. Upon exiting the pinch roll stand 31, the thin cast strip may pass through a hot rolling mill 32, comprising a pair of work rolls 32A, and backup rolls 32B, forming a gap capable of hot rolling the cast strip delivered from the casting rolls, where the cast strip is hot rolled to reduce the strip to a desired thickness, improve the strip surface, and improve the strip flatness. The work rolls 32A have work surfaces relating to the desired strip profile across the work rolls. The hot rolled cast strip then passes onto a run-out table 33, where it may be cooled by contact with a coolant, such as water, supplied via water jets 90 or other suitable means, and by convection and radiation. In any event, the hot rolled cast strip may then pass through a second pinch roll stand 91 having rollers 91A to provide tension on the cast strip, and then to a coiler 92. The thickness of cast strip may be typically between about 0.3 and 2.0 millimeters in thickness before hot rolling.
At the start of the casting campaign, a short length of imperfect strip is typically produced as casting conditions stabilize. After continuous casting is established, the casting rolls 12 are moved apart slightly and then brought together again to cause this leading end of the cast strip to break away forming a clean head end of the following cast strip. The imperfect material drops into a scrap receptacle 26, which is movable on a scrap receptacle guide. The scrap receptacle 26 is located in a scrap receiving position beneath the caster and forms part of a sealed enclosure 27 as described below. The enclosure 27 is typically water cooled. At this time, a water-cooled apron 28 that normally hangs downwardly from a pivot 29 to one side in the enclosure 27 is swung into position to guide the clean end of the cast strip 21 onto the guide table 30 that feeds it to the pinch roll stand 31. The apron 28 is then retracted back to its hanging position to allow the cast strip 21 to hang in a loop beneath the casting rolls in enclosure 27 before it passes to the guide table 30 where it engages a succession of guide rollers.
An overflow container 38 may be provided beneath the movable tundish 14 to receive molten material that may spill from the tundish. As shown in FIG. 1, the overflow container 38 may be movable on rails 39 or another guide such that the overflow container 38 may be placed beneath the movable tundish 14 as desired in casting locations. Additionally, an overflow container (not shown) may be provided for the distributor 16 adjacent the distributor 16.
The sealed enclosure 27 is formed by a number of separate wall sections that fit together at various seal connections to form a continuous enclosure wall that permits control of the atmosphere within the enclosure. Additionally, the scrap receptacle 26 may be capable of attaching with the enclosure 27 so that the enclosure is capable of supporting a protective atmosphere immediately beneath the casting rolls 12 in the casting position. The enclosure 27 includes an opening in the lower portion of the enclosure, lower enclosure portion 44, providing an outlet for scrap to pass from the enclosure 27 into the scrap receptacle 26 in the scrap receiving position. The lower enclosure portion 44 may extend downwardly as a part of the enclosure 27, the opening being positioned above the scrap receptacle 26 in the scrap receiving position. As used in the specification and claims herein, “seal,” “sealed,” “sealing,” and “sealingly” in reference to the scrap receptacle 26, enclosure 27, and related features may not be a complete seal so as to prevent leakage, but rather is usually less than a perfect seal as appropriate to allow control and support of the atmosphere within the enclosure as desired with some tolerable leakage.
A rim portion 45 may surround the opening of the lower enclosure portion 44 and may be movably positioned above the scrap receptacle, capable of sealingly engaging and/or attaching to the scrap receptacle 26 in the scrap receiving position. The rim portion 45 may be movable between a sealing position in which the rim portion engages the scrap receptacle, and a clearance position in which the rim portion 45 is disengaged from the scrap receptacle. Alternately, the caster or the scrap receptacle may include a lifting mechanism to raise the scrap receptacle into sealing engagement with the rim portion 45 of the enclosure, and then lower the scrap receptacle into the clearance position. When sealed, the enclosure 27 and scrap receptacle 26 are filled with a desired gas, such as nitrogen, to reduce the amount of oxygen in the enclosure and provide a protective atmosphere for the cast strip.
The enclosure 27 may include an upper collar portion 27A supporting a protective atmosphere immediately beneath the casting rolls in the casting position. When the casting rolls 12 are in the casting position, the upper collar portion is moved to the extended position closing the space between a housing portion adjacent the casting rolls 12, as shown in FIG. 2, and the enclosure 27. The upper collar portion may be provided within or adjacent the enclosure 27 and adjacent the casting rolls, and may be moved by a plurality of actuators (not shown) such as servo-mechanisms, hydraulic mechanisms, pneumatic mechanisms, and rotating actuators.
As shown in FIGS. 2-3, cleaning brushes 36 are disposed adjacent the pair of casting rolls 12, such that the periphery of the cleaning brushes 36 may be brought into contact with the casting surfaces 12A of the casting rolls 12 to clean oxides from the casting surfaces during casting. The cleaning brushes 36 are positioned at opposite sides of the casting area adjacent the casting rolls 12, between the nip 18 and the casting area where the casting rolls enter the protective atmosphere in contact with the molten metal casting pool 19.
The side dams 20 may be mounted on and actuated by plate holders 100 positioned one at each end of the roll assembly and moveable toward and away from one another. The plate holders 100 and side dams 20 may be positioned on a core nozzle plate 106 mounted on the roll cassette 11 so as to extend horizontally above the casting rolls, as shown in FIG. 3. The core nozzle plate 106 is positioned beneath the distributor 16 in the casting position and has a central opening 107 to receive the metal delivery nozzle 17. The metal delivery nozzle 17 may be provided in two or more segments, and at least a portion of each metal delivery nozzle 17 segment may be supported by the core nozzle plate 106. The outer end of each metal delivery nozzle 17 is supported by a bridge portion (not shown) positioned adjacent the side dams 20 and capable of supporting and moving the delivery nozzle 17 during casting.
There is shown in FIG. 4 a pair of delivery nozzles 17 formed as substantially identical segments made of a refractory material such as zirconia graphite, alumina graphite or any other suitable material. It must be understood that more than two delivery nozzles 17 may be used in any different sizes and shapes if desired. The delivery nozzles 17 need not be substantially identical in size and shape, although generally such is desirable to facilitate fabrication and installation. Two delivery nozzles 17 may be provided, each capable of moving independently of the other above the casting rolls 12.
Typically where two delivery nozzles 17 are used the nozzles 17 are disposed and supported in end-to-end relationship as shown in FIG. 4 along the nip 18 with a gap 34 therebetween, so that each delivery nozzle 17 can be moved inwardly toward each other during a casting campaign as explained below. It must be understood, however, that any suitable number of delivery nozzles 17 may be used, including two delivery nozzles 17 as described below and including any additional number of nozzles 17 disposed therebetween. For example, there may be a central nozzle segment adjacent to outer nozzle segments on either side.
Each delivery nozzle 17 may be formed in one piece or multiple pieces. As shown, each nozzle 17 includes an end wall 23 positioned nearest a confining side dam 20 as explained below. Each end wall 23 may be configured to achieve a particular desired molten metal flow in the triple point region between the casting rolls 12 and the respective side dam 20.
The side dams 20 may be made from a refractory material such as zirconia graphite, graphite alumina, boron nitride, boron nitride-zirconia, or other suitable composites. The side dams 20 have a face surface capable of physical contact with the casting rolls and molten metal in the casting pool.
A pair of carriage assemblies, generally indicated at 104, are provided to position the side dams 20 and the delivery nozzles 17. As illustrated, the twin roll caster is generally symmetrical, although such is not required. Referring to FIGS. 5-7, one carriage assembly 104 is illustrated and described below, with the other carriage assembly 104 being generally similar. It is understood that the twin roll caster may utilize any number of carriage assemblies 104 configured in any suitable manner to provide a flow of molten metal to the casting pool 19. Each carriage assembly 104 is disposed at one end of the pair of casting rolls 12. Each carriage assembly 104 may be mounted fixed relative to the machine frame 10, or may be moveable axially toward and away from the casting rolls 12 to enable the spacing between the carriage assembly 104 and the casting rolls 12 to be adjusted. The carriage assemblies 104 may be preset in final position before a casting campaign to suit the width of the casting rolls 12 for the strip to be cast, or the position of the carriage assemblies 104 may be adjusted as desired during a casting campaign. The carriage assemblies 104 may be positioned one at each end of the roll assembly and moveable toward and away from one another to enable the spacing between them to be adjusted. The carriage assemblies 104 can be preset before a casting operation according to the width of the casting rolls and to allow quick roll changes for differing strip widths. The carriage assemblies 104 may be positioned so as to extend horizontally above the casting rolls with the nozzles 17 positioned beneath the distributor 16 in the casting position and at a central position to receive the molten metal.
For example the carriage assembly 104 may be positioned from tracks (not shown) on the machine frame 10, which may be mounted by clamps or any other suitable mechanism. Alternatively, the carriage assembly 104 may be supported by its own support structure relative to the casting rolls 12.
The carriage assembly 104 includes a support frame 125. A nozzle bridge 108 is moveably connected to the support frame 125 and engages the delivery nozzles 17 for selective movement thereof. A nozzle actuator 110 is mounted to the support frame 125 and connected to the nozzle bridge 108 for moving the nozzle bridge 108 and thus moving the delivery nozzles 17 to position the end wall 23 relative to the side dam 20. The nozzle actuator 110 is thus capable of positioning the delivery nozzles 17. The nozzle actuator 110 is a conventional servo mechanism. It must be understood, however, that the nozzle actuator 110 may be any drive mechanism suitable to move and adjust delivery nozzles 17. For example, the nozzle actuator 110 may be a screw jack drive operated by an electric motor, a hydraulic mechanism, a pneumatic mechanism, a gear mechanisms, a cog, a drive chain mechanism, a pulley and cable mechanism, a drive screw mechanism, a jack actuator, a rack and pinion mechanism, an electro-mechanical actuator, an electric motor, a linear actuator, a rotating actuator, or any other suitable device.
A nozzle position sensor 113 senses the position of the delivery nozzles 17. The nozzle position sensor 113 is a linear displacement sensor to measure the change in position of the nozzle bridge 108 relative to the support frame 125. The nozzle position sensor 113 may be any sensor suitable to indicate any parameter representative of a position of the delivery nozzles 17. For example, the nozzle position sensor 113 may be a linear variable displacement transformer to respond to the extension of the nozzle actuator 110 to provide signals indicative of movement of the delivery nozzles 17, or an optical imaging device for tracking the position of the delivery nozzles 17 or any other suitable device for determining the location of the delivery nozzles 17.
Each side dam 20 is mounted to a plate holder 100 which is moveably connected to the support frame 125 and engages the side dam 20 for selective movement thereof. A side dam actuator 102 is mounted to the support frame 125 and connected to the plate holder 100 for moving the plate holder 100 and thus moving each side dam 20 to position the side dam 20 relative to the casting rolls 12. The side dam actuator 102 is thus capable of positioning the side dam 20 and capable of cyclically varying the axial force of the side dams as described below. The side dam actuator 102 is a hydraulic force cylinder. It must be understood, however, that the side dam actuator 102 may be any suitable drive mechanism to position the plate holder 100 to bring the side dam 20 into engagement with the casting rolls 12 to confine the casting pool 19 formed on the casting surfaces 12A during a casting operation. Such a suitable drive mechanism, for example, may be a servo mechanism, a screw jack drive operated by electric motor, a pneumatic mechanism, a gear mechanisms, a cog, a drive chain mechanism, a pulley and cable mechanism, a drive screw mechanism, a jack actuator, a rack and pinion mechanism, an electro-mechanical actuator, an electric motor, a linear actuator, a rotating actuator, or any other suitable device. Thus, the side dams 20 are mounted in side dam plate holders 100, which are movable by side dam actuators 102, such as a servo mechanism, to bring the side dams 20 into engagement with the ends of the casting rolls. Additionally, the side dam actuators 102 are capable of positioning the side dams 20 during casting. The side dams 20 thus form end closures for the molten pool of metal on the casting rolls during the casting operation.
A side dam position sensor 112 senses the position of the side dam 20. The side dam position sensor 112 is a linear displacement sensor to measure the actual change in position of the plate holder 100 relative to the support frame 125. The side dam position sensor 112 may be any sensor suitable to indicate any parameter representative of a position of the side dam 20. For example, the side dam position sensor 112 may be a linear variable displacement transducer to respond to the extension of the side dam actuator 102 to provide signals indicative of position of the side dam 20, or an optical imaging device for tracking the position of the side dam 20 or any other suitable device for determining the location of the side dam 20. The side dam position sensor 112 may also or alternatively include a force sensor, or load cell for determining the force urging the side dam 20 against the casting rolls 12 and providing electrical signals indicative of the force urging the side dam against the casting rolls.
In any case the actuators 110 and 102 and the sensors 113 and 112 may be connected into a control system in the form of a circuit receiving control signals determined by measurement of the distance variation between the delivery nozzles 17 and the confining side dams 20, and between the side dams 20 and the casting rolls 12. For example, small water cooled video cameras may be installed on the nozzle bridge 108, or any other suitable structure, to directly observe the distance between the delivery nozzles 17 and the confining side dams 20 and the side dams 20 and the casting rolls 12, and to produce control signals to be fed to position encoders on the actuators 110 and 102. With any arrangement, precise control of the distance between the end walls 23 of the delivery nozzle 17 and the side dams 20 and the side dams 20 and the casting rolls 12 may be maintained. Moreover these distances can be accurately set and maintained by independent operation of the actuators 110 and 102 during casting. For example, the distance between the end wall 23 and the side dam 20 may be set so that a discharge of molten metal is positioned to a target area on the side dam 20 relative to the triple point regions.
During a casting campaign the control system of the twin roll caster is capable of actuating the side dam actuators 102 to vary the apply force on the side dams 20 against the ends of the casting rolls 12 in the axial direction, i.e. along the axis of the centerlines of the two casting rolls. The apply force is not varied such that the side dams 20 develop a clearance at edges of the casting rolls 12 that may cause leakage of molten metal from the casting pool. The control system may receive position or force information from the sensors 112 or from direct feedback of the actuator 102.
As further illustrated in FIGS. 6-7, a vibrator 126 may be mounted to provide a high frequency vibration to the side dam 20. As shown, the vibrator device 126 is secured adjacent the side dam actuator 102 and vibration is transferred to the support frame 125 and side dam 20. An example of such a vibrator device is the Cougar ATU pneumatic turbine vibrator series made by Martin Engineering Company. The vibrator device 126 may be a pneumatic turbine vibrator as shown that vibrates axially at 200-400 Hz when supplied 20-100 psi of pneumatic air. The axial vibration imparted to the side dams 20 reduces and inhibits the formation of snake eggs in the casting pool.
Illustrated in FIG. 8 are a variety of time graphs demonstrating the effect of the vibrators on the caster delivery CR actual gap force WS, caster delivery CR actual gap force DS, side dam actual force, side dam wear, cast speed, tundish steel temperature and entry caster roll heat flux during a test of the improved caster.
According to this test, the vibrator was engaged at 14:00:00 at 20 psi, corresponding to a vibration of approximately 240 Hz. At 15:00:00 the pressure was increased to 25 psi, corresponding to a vibration of approximately 250 Hz. At 15:15:00, the vibrators were turned off. At 15:30:00, the vibrators were turned on at 40 psi, corresponding to approximately 260 Hz.
FIG. 8 illustrates the effect of the vibrator on the various measured effects. As shown in the first and second graphs of FIG. 8, engaging the vibrators significantly reduced the amount of noise in the caster delivery CR actual gap force. As illustrated in the third graph of FIG. 8, the actual force on the side dam also experienced a significant reduction in noise when the vibrators were engaged. The third through sixth graphs also indicate that engaging the vibrators reduced the amount of noise and stabilized the cast speed, steel temperature and heat flux.
As will further be apparent from this graph, when the vibrators were disengaged at 15:15:00, the amount of noise in the various measured parameters returned, and was decreased by once again engaging the vibrators at 15:30:00. Some variation between vibrations induced at 20, 25 and 40 psi was seen, indicating that an optimal vibration frequency may be determined. This optimal frequency is between 200 and 400 Hz. More specifically, this optimal frequency is between 260 and 300 Hz.
While the principle and mode of operation of this invention have been explained and illustrated with regard to particular embodiments, it must be understood, however, that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.

Claims

What is claimed is:

1. A method of casting thin strip comprising the steps of:

assembling a pair of counter-rotating casting rolls laterally forming a nip between circumferential casting surfaces of the rolls through which metal strip can be cast,

assembling a pair of side dams adjacent end portions of the casting roll to permit a casting pool of molten metal to be formed supported by the casting surfaces of the casting rolls,

assembling a metal delivery system above the casting rolls delivering molten metal to form a casting pool supported on the casting surfaces of the casting rolls above the nip and confined by the pair of side dams,

assembling a vibrator device to vibrate the pair of side dams at frequencies between 200 and 400 hertz to inhibit the formation of snake eggs in the casting pool, and

counter-rotating the casting rolls such that the casting surfaces of the casting rolls each travel inwardly toward the nip to produce a cast strip downwardly from the nip.

2. The method of casting thin strip as claimed in claim 1 where in addition hydraulic cylinders are laterally positioned adjacent to the side dams to urge each side dam toward one of the end portions of the casting roll and a vibration device is assembled adjacent of the hydraulic cylinders.

3. The method of casting thin strip as claimed in claim 1 where the vibration device has a frequency range between 260 and 300 hertz.