Patent Application
20140008032
This invention relates 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 counter-rotated casting rolls that are cooled so that metal shells solidify on the moving roll surfaces and are brought together at a nip between them. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be delivered 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, forming 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. As the molten metal formed into shells are joined and pass through the nip between the casting rolls, a thin metal strip is cast downwardly from the nip.
The casting pool is usually confined between side dams held in sliding engagement with end surfaces of the casting rolls so as to constrain the two ends of the casting pool against outflow. Side dams at the ends of the casting rolls prevent leakage of molten metal from the casting pool and maintain the casting pool at a desired depth. As the casting rolls are rotated, the side dams experience frictional wear, causing arc-shaped grooves to form in the side dams along the circumferential surfaces of the casting rolls. In order to compensate for this wear, the side dams are movable to gradually shift inward under compression forces while having the side dams biased against the ends portions of the casting rolls in order to maintain a seal with the casting rolls.
When casting steel strip in a twin roll caster, the thin cast strip leaves the nip at very high temperatures, of the order of 1400° C. If exposed to normal atmosphere, it will rapidly form scale by oxidation at such high temperatures. A sealed enclosure that contains an atmosphere that inhibits oxidation of the strip is therefore provided beneath the casting rolls to receive the thin cast strip, and through which the strip passes away from the strip caster. The oxidation inhibiting atmosphere may be created by injecting a non-oxidizing gas, for example, an inert gas such as argon or nitrogen, or combustion exhaust reducing gases. Alternatively, the enclosure may be substantially sealed against ingress of an ambient oxygen-containing atmosphere during operation of the strip caster, and the oxygen content of the atmosphere within the enclosure is reduced during an initial phase of casting, by allowing oxidation of the strip to extract oxygen from the sealed enclosure as disclosed in U.S. Pat. Nos. 5,762,126 and 5,960,855.
The length of a casting campaign of a twin roll caster has been generally determined in the past by the useful life of the core nozzle, tundish and side dams. Multi-ladle sequences can be continued by use of a turret allowing sequential ladles of molten metal to be transferred to the operating position. The focus in extending casting campaigns, therefore, has been extending the useful life of the core nozzle, tundish and side dams, and in turn reducing the cost per ton of casting thin strip. Wear and replacement of the side dams has usually limited the casting campaign, where the casting campaign was typically stopped and the worn side dams replaced. The core nozzles and tundish with remaining useful life were typically replaced at the same time so the length of the next campaign is not limited, with attendant waste of useful life of refractories and increased cost of casting steel. In the past, one focus has been on improving refractory materials. Graphitized alumina, boron nitride and boron nitride-zirconia composites are examples of suitable refractory materials for the side dams, tundish and core nozzle components. SiAlON (i.e, silicon alumina oxy-nitride) refractory material has also been proposed for use in making side dams.
Also, the side dams wear independently of the core nozzles and tundish, and independently of each other. The side dams must initially be urged against the ends of the casting rolls under biasing forces, and “worn-in” to be adequately seated against outflow of molten steel from the casting pool. The biasing forces applied to the side dams may be reduced after an initial wear-in period, but there continued to be significant wear of the side dams throughout the casting operation. The useful life of the side dams has remained a limiting factor in the length of casting campaigns and the cost of casting thin strip. The core nozzle and tundish components in the metal delivery system could have a longer life than the side dams, and could normally continue operating through several additional ladles of molten steel supplied extending the casting campaign and dramatically reducing the cost of casting thin strip.
Disclosed is a composite side dam for a continuous twin roll caster substantially increasing the use life of the side dams and reducing the cost of casting thin strip. The composite side dam comprises a substrate shaped to form a side dam and made of a refractory material capable of withstanding casting temperature in a twin roll caster. The substrate has edge portions adapted to engage end portions of casting rolls, and has a nip portion adapted to be adjacent a nip between casting rolls and has upper portions extending cross the side dam to form a lateral restraint for a casting pool of molten metal during operation in a twin roll caster. An insert of at least 10 mm in thickness is positioned in a pocket provided in the substrate to engage the molten metal in operation of a twin roll caster and extend from the upper portions of the substrate to within 30 mm from the nip portion of the substrate, by insertion adjacent the upper portions of the substrate to engage end portions of the casting rolls during operation of the twin roll caster. The insert is adapted to fit into the pocket of the substrate to form a side dam formed of a refractory material having consumption rate less than 10 mm per hour.
The material forming the insert may be comprised of SiAlON material and may be between 20 and 60% SiAlON material and the remainder may be hBN (i.e, hexagonal boron nitride) material. The material forming the insert alternatively may be comprised of mullite material defined by
The insert may be between 10 mm and 40 mm in thickness, and may have a thickness greater than a depth of the pocket in the substrate. The insert may have edge portions of a reverse angle of at least 3° to engage edge portions of the pocket in the substrate. Further, the insert may have a firm fit in the pocket of the substrate. Alternatively or in addition, the insert may be positioned in the pocket of the substrate with a ceramic cement. Additionally, the insert may extend toward the nip portion to allow at least a 2.5 mm radius in the insert adjacent nip portions of the side dam.
Twin roll caster 11 comprises a pair of laterally positioned casting rolls 22 forming a nip 15 there between, to which molten metal from a ladle 23 is delivered through a metal delivery system 24. Metal delivery system 24 comprises a tundish 25, a removable tundish 26 and one or more core nozzles 27 which are located between the casting rolls above the nip 15. The molten metal delivered to the casting rolls forms casting pool 16 supported on the casting surfaces of the casting rolls 22 above the nip 15. The casting pool of molten steel is confined at the portions ends of the casting rolls 22 by a pair of side dams 35, which engage the end portions of the rolls by operation of a pair of hydraulic cylinder units 36 acting through thrust rods 50 connected to side dam holders 37.
The casting rolls 22 are internally water cooled by coolant supply and driven in counter rotational direction by drives, so that metal shells solidify on the moving casting roll surfaces as the casting surfaces move through the casting pool 16. These metal shells are brought together at the nip 15 to produce the thin cast strip 12, which is delivered downwardly from the nip 15 between the rolls.
Tundish 25 is fitted with a lid 28. Molten steel is introduced into the tundish 25 from ladle 23 via a shroud 29. The tundish 25 is fitted with a slide gate valve 34 to selectively open and close the outlet 31 and effectively control the flow of metal from the tundish to the removable tundish 26. The molten metal flows from tundish 25 through an outlet 31 through a shroud 29 to removable tundish 26 (also called the distributor vessel or transition piece), and then to core nozzles 27. At the start of a casting operation a short length of imperfect strip is produced as the casting conditions stabilize. After continuous casting is established, the casting rolls are moved apart slightly and then brought together again to cause the leading end of the strip to break away so as to form a clean head end of the following cast strip to start the casting campaign. The imperfect head end of the strip drops into a scrap box receptacle 40 located beneath caster 11 and forming part of the enclosure 10 as described below. At this time, swinging apron 38, which normally hangs downwardly from a pivot 39 to one side in enclosure 10, is swung across the strip outlet from the nip 15 to guide the head end of the cast strip onto guide table 13, which feeds the strip to the pinch roll stand 14. Apron 38 is then retracted back to its hanging position to allow the strip to hang in a loop beneath the caster, as shown in
The twin roll caster illustratively may be of the kind which is illustrated in some detail in U.S. Pat. Nos. 5,184,668 and 5,277,243, and reference may be made to those patents for appropriate constructional details.
Referring to
When it is determined that the side dams 35 need to be change, typically due to wear, a preheating sequence is commenced. The core nozzle 27 and the removable tundish 26 are also typically replaced at the same time. This preheating of the second tundish 26′ and second core nozzle 27′ is started while casting is continuing at least 2 hours before transfer to the replacement sequence, and the preheating of the second side dams 35′ is started at least 0.5 hours before transfer to the replacement sequence. This preheating is done in preheating heaters 50, 54 and 57, typically preheating chambers, in locations convenient to the caster 11, but removed from the operating position of the refractory components during casting.
During this preheating of the replacement refractory component, casting typically continues without interruption. When the refractory component to be replaced (namely, the tundish 26, the core nozzle 27 and the side dams 35), the slide gate 34 is closed and the tundish 26, the core nozzle 27 and the casting pool 16 are drained of molten metal. Typically, the tundish 26′ and side dams 35′ are preheated and replaced as individual refractory components, and the core nozzle is preheated and replaced as a singular or two piece refractory component, but in particular embodiments may be preheated and replaced in pieces or parts as those portions of the refractory component are worn or otherwise need to be replaced.
When the preheating is completed and the change in side dams is to take place, the slide gate 34 is closed and the tundish 26, core nozzle 27 and casting pool 16 are drained and casting is interrupted. A pair of transfer robots 55 remove the first side dams 35 from the operating position, and then a pair of transfer robots 56 transfer the second side dams 35′ from the preheating chamber 57 to the operating position. Note that transfer robots 55 and 56 may be the same as shown in
Each transfer robot 55 and 56 is a robot device known to those skilled in the art with gripping arms 70 to grip the core nozzle 27 or 27′ typically in two parts, or side dams 35 or 35′. The transfer robots can be raised and lowered and also moved horizontally along overhead tracks to move the core nozzle or the side dams from a preheating chamber at a separate location from the operating position to the caster for downward insertion of the plates through the slots 69 into the holders 37. Gripping arms 70 are also operable to remove at least portions of worn core nozzle 27 or side dams 35. The step of removing the worn side dams 35 is done by operating cylinder unit 36 to withdraw the thrust rod 50 sufficiently to open the slot 69 and to bring side dam 35 into position directly beneath that slot, after which the gripping arm 70 of the transfer robot 55 can be lowered through the slot to grip the side dam 35 and then raised to withdraw the worn side dam. The side dams 35 may be removed when they become worn to specified limits as will be explained further below, and may be removed one at a time as worn to a specified limit. During a casting run and at a time interval before the side dams 35 have worn down to an unserviceable level, the wear rate of the side dams 35 may be monitored by sensors, and the preheating of replacement side dams 35′ is commenced in preheat furnaces at preheating chamber 57 separate from the caster 11.
To change the side dams 35, when the molten steel is drained from the metal delivery system and casting pool, cylinder units 36 are operated to retract the side dam holders 37 and to bring the side dams 35 directly beneath the slots 69 which are opened by the retraction movement of the slides 68. Transfer robots may then be lowered such that their gripping arms 70 can grip the side dams 35 and raised and remove those worn side dams, which can then be dumped for scrap or refurbishment. The transfer robots are then moved to the preheat chambers where they pick up the replacement side dams 35′ and move them into position above the slots 69 and the retracted side dam holders 37. Side dams 35′ are then lowered by the transfer robots into the plate holders, the transfer robots are raised and the cylinder units 36 operated to urge the preheated replacement side dams 35′ against the end of the casting rolls 22 and to move the slides 68 to close the enclosure slots 69. The operator then actuates slide gate 34 to initiate resumption of casting by pouring molten steel into tundish 26 and core nozzle 27, to initiate a normal casting operation in a minimum of time.
It may be desirable to replace a side dam or dams 35 when worn to specified limits, such as when the dam(s) become or will become unserviceable. For example, the wear of the side dams may be monitored by means of load/displacement transducers mounted on cylinders 36. The cylinders will generally be operated so as to impose a relatively high force on the side dams 35 during an initial bedding-in period in which there will be a higher wear rate after which the force may be reduced to a normal operating force. The output of the displacement transducers on cylinders 36 can then be analyzed by a control system, usually including a computerized circuit, to establish a progressive wear rate and to estimate a time at which the wear will reach a level at which the side plates become unserviceable. The control system is responsive to the sensors to determine the time at which preheating of replacement side dams must be initiated prior to interrupting the cast for replacement of the side dams.
A composite side dam 35 for the continuous twin roll caster 11 embodying the present invention is shown in
As shown in
The insert 80 is formed of a refractory material that may have a consumption rate less than 10 mm per hour, and in any event, at least as great as the consumption rate to the substrate 72. The material forming the insert 80 comprises a SiAlON material. The material forming the insert 80 may be between 20 and 60% SiAlON material and the remainder hBN material. In another embodiment the material forming the insert 80 alternatively may be comprised of mullite material defined by
As shown in
Generally, in campaigns in casting thin strip, solidified skulls may form from time to time adjacent the side dam and also the delivery nozzle when the distance between the side dam and nozzle is not maintained. These skulls may also be formed on the side dams surfaces protruding into the casting pool beyond the casting rolls surfaces. When these skulls drop through the roll nip, they may cause the two solidifying shells at the casting roll nip to separate and “swallow” additional liquid steel between the shells causing the strip surface to reheat and, in extreme cases, may cause the strip to break disrupting the continuous production of coiled strip. These dropped skulls formed in the cast strip are known as “snake eggs.” In any event, snake eggs can cause defects to occur in the cast strip. The snake eggs are detected as lateral force spikes on the side dams at the roll nip, as well as visible bright bands across the width of the cast strip. Snake eggs usually apply resistive forces against the side dam, in addition to the forces on the side dam generated by the ferrostatic head in the cast pool, and can cause the side dam to lift from the casting roll edge resulting in the leakage of steel between the side dam and the casting roll. Snake eggs passing through the nip between the casting rolls may cause lateral movement of the casting rolls along with upward movement in the side dams. To resist the increased forces generated by the snake eggs, the side dams are typically biased toward the casting rolls with appropriate higher lateral forces.
An advantage of the present invention can be seen by
Referring to
Specifically, in
In
In
The increase in yield was also dramatic between the use of the previous solid standard side dams and the composite side dams of the present invention. The comparative results are set forth in Tables 1 and 2 below. Table 2 shows an increased in yield when composite side dams were used compared to side dams of the previous solid design (Table 1) in both prime from liquid metal and prime from coiled. Out of the eight comparative sequences presented, five of the comparative runs encountered problems at the end of the casting, such as: choking, snakes eggs, and coiler issues. No major problems were present in the sequences performed using composite side dams of the present invention.
As seen from the data above, a significant improvement in yield was reported for castings performed with composite side dams. The casting sequences performed using side dams of the previous solid design produced an average yield of 78%. As presented in Table 2, the casting sequences performed using composite side dams of the present invention produced a 10% yield increase for an average yield of 88%.
While it has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from scope. In addition, many modifications may be made to adapt a particular situation or material to the teachings without departing from its scope. Therefore, it is intended that it not be limited to the particular embodiments disclosed, but that it will include all embodiments falling within the scope of the appended claims.
| Number | Date | Country | |
|---|---|---|---|
| 61666918 | Jul 2012 | US |
