Abstract:
Apparatus and Process for delivering molten steel to a continuous casting mold with high inclusion removal capability and high mold pool flow optimization potential includes a tundish with a novel design and a metal distributor comprising a main chamber body connected to a vacuum device and tubes for metal exchange between the tundish, the distributor and the casting mold. At least one of the tubes has gas injection devices to perform gas lifting functions. The new design of the tundish enables it to receive all the steel in a ladle without leaving any heel and refining the steel to an extremely low inclusion level. The distributor is able to achieve an effective hydrostatic head relative to the mold metal level of the order of 10 −2  m or below in the submerged entry nozzle and obtain fully filled and stable flow with low disturbance inside the SEN and therefore is able to deliver the molten steel at low velocities at SEN exits with minimum turbulent disturbance and high flow, thermal and chemical homogeneity over the entire metal bath in the mold. The invention also makes it possible to leave no heels in either the tundish or the distributor when they are replaced. The invention is particularly useful for thin slab and strip casting processes where mold flow control is critical to the casting process.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    In conventional continuous casting of steel, a tundish positioned above the mold is used to slow down steel poured from a ladle. From the tundish, steel flows through submerged immersion nozzles (SEN) to the casting mold and solidifies while being pulled out from the mold. 
         [0002]    The principal role of a tundish is flow modification and control. Some refining functionalities may be needed with inclusion removal as the main focus. Currently, due to its inherent deficiencies resulting from its relative position between the ladle and the mold, no tundish can perform ideally in either flow control or in inclusion removal. 
         [0003]    For the flow control or optimization, capability of the current tundish system is seriously impaired due to the fact that there exists a large hydrostatic head in the tundish relative to the steel pool in the mold. Stop rods and sliding gates must be used to regulate the steel flow rates. As a result, chaotic pressure field inside the SEN and disturbing flow streams at SEN exits are created. Flow field inside the mold is greatly impacted because of this chaotic flow pattern inside and in the nearby area of the SEN as well as the deep flow penetration and large meniscus fluctuation. Such flow distribution in the mold makes optimization of the flow and thermal fields in the mold for improved casting extremely difficult. This is particularly important when near net shape casting such as strip casting and high speed casting with great emphasis on stable and profitable production is concerned. To fundamentally change this situation, improvements in shape change or internal structure modification of the current tundish system can hardly be sufficient since the root cause is the chaotic flow state inside the SEN resulting from the high hydrostatic pressure in the tundish. Such situation also limits the use and the effectiveness of the more advanced SEN designs with variable exit opening shapes and dimensions needed for the optimized steel delivery to the mold. Furthermore, the chaotic and multi-phase flow inside the SEN also promotes the aggregation and precipitation of the inclusions along the phase interfaces and makes the SEN clogging more severe. 
         [0004]    For the inclusion removal, significant efforts have been devoted to improve both the design and the process of the tundish. However, since it needs frequent replacement and repair, the size and weight of a tundish can not be too large. This limits the possibility of creating sufficient inclusion removal conditions in the tundish for inclusions to collide and aggregate and grow large enough to float out of the steel bath. Such shortcomings cannot be fundamentally improved with any internal flow control devises or filters. 
         [0005]    Since the inclusion removal capability of the tundish is low, usually, the required inclusion level is obtained in refining stages before the melt is teemed into the tundish. The role of the tundish is then to avoid inclusion formation due to entrapment of ladle slag or re-oxidation. State of the art of current tundish technologies is to use slag detection devises to detect the beginning of slag entrapment and then stop the teeming process. A heel of residual steel up to 5% of the total heat is left in the ladle and dumped with slag. This inevitably reduces metal yield and profitability of the process. 
         [0006]    The current tundish systems are particularly weak in removing inclusions with sizes smaller than 20 μm which in many cases, particularly for those steel grades with total oxygen content below 10-20 ppm, makes up the majority of the oxygen content. The reason is discussed below. 
         [0007]    For small inclusions in a turbulent flow field, its removal from the bulk body of the liquid usually takes the path from inside the liquid to the surface or the interfaces between the liquid and foreign objects such as lining walls or flow control devices. Since the micro inclusions in the molten steel rise very slowly, it takes a long time for them to reach the top slag. e.g., for an oxide of 30 μm, it takes more than 10 minutes to rise a distance of 0.3 meters. It is then not practical to remove inclusions smaller than 30 μm in a conventional tundish by floatation only. To remove these micro inclusions, measures must be taken to promote their collision and aggregate with larger inclusions so that their rising velocity can be increased and they can float out the steel in a short time. 
         [0008]    A great amount of work has been done on the collision and growth of micro inclusions in a turbulent flow field. The general conclusion from these studies is that the overall collision, aggregate and growth rate of micro inclusions are greater for larger inclusion sizes, higher turbulent kinetic energy dissipation rate which represents the eddy activity intensity in the flow and longer inclusion transit time in the tundish. 
         [0009]    Some note is needed for the effect of the turbulent kinetic energy dissipation rate E. The higher its value, the longer will the small inclusions&#39; trajectory be, and the more possibly will inclusion collision occur. It should be noted that ε used in this case is only from thermal dissipation of the small eddies whose sizes are close to the Kolmogorov micro length scale, l λ . It should not include the energy losses resulting from flow variations caused by objects larger than l λ , such as surface wave movement, collisions and frictions with walls or flow control devices. In most of the tundishes, l λ , is in the range of 30-300 μm. 
         [0010]    Calculated results from the above well established theory indicate, in order to achieve significant removal of small size inclusions, the two most necessary conditions should be:
       The minimum effective collision and growth time of inclusions is 500 seconds with some additional time for floatation.   The turbulent flow field in the tundish has a relatively homogeneous distribution of ε over the path of inclusion movement, and the value of ε is greater than 0.005 m 2 /s 3 , but lower than a certain value to prevent the breakup of the newly formed large inclusions.       
 
         [0013]    These conditions can be hardly found in any existing tundishes. 
         [0014]    To meet the conditions for micro inclusion removal and in the mean time open up ways to optimize steel delivery to the mold, a fundamentally novel approach is used in this invention that is revolutionary as compared with existing tundish systems. In this invention, a tundish positioned on the side of the mold has a novel internal design to create the needed conditions for enhanced inclusion removal, and a metal distributor positioned above both the tundish and the mold and based on a vacuum enhanced gas lifting process is used to deliver steel from the tundish to the mold with steady and well distributed flow features that are needed for the advanced continuous casting of steel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a diagrammatical side view of the molten steel delivery system from a ladle to the continuous casting mold with its major components. 
           [0016]      FIG. 2  is a diagrammatical top view of the tundish of the molten steel delivery system for steel refining and inclusion removal with specially designed inner structure and functional areas. 
           [0017]      FIG. 3  is a sectional side view of the bubbly two-phase flow structure in the narrow flow channels of the tundish where micro inclusion collision and growth is promoted for inclusion removal. 
           [0018]      FIG. 4  is a sectional top view of the distributor of the molten steel delivery system with special bottom floor design and conduits structure for molten steel communication between the molds and the tundish. 
           [0019]      FIG. 5  is a diagrammatical side view of the molten steel delivery system with molten steel level control mechanism under different operation conditions. 
           [0020]      FIG. 6  is a diagrammatical side view of the molten steel delivery system with detailed information of the parameters that determines the molten steel levels at various locations and under specified operation conditions. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0021]    This invention is pertaining to the molten steel delivery system from the ladle to the mold of a continuous caster in the steel industry. The new delivery system is fundamentally different from the conventional ones. It targets the challenges faced by the current systems, has components that are tasks specific, and as a whole, delivers cleaner steel to the mold with steady, consistent and well-distributed flows in both SEN and in mold for improved casting process and quality at a lower cost. 
         [0022]    As shown in  FIG. 1 , the invention system, as opposed to a conventional tundish on its own, comprises two major components: tundish  1  and distributor  11 . Each component has a main vessel body, and some supporting devices. The new tundish  1  is different from a current tundish in shape, position and functionality. With specially designed inner structures, it functions as a receiver of the steel from the ladle, and a refiner to achieve very low inclusion levels in the steel. 
         [0023]    The distributor  11  is a relatively small and enclosed vessel. It transfers liquid steel from the tundish  1  to the mold  13  with at least one conduit acting as a lifting tube  8 , one or more conduits as down tube  9  for steel to flow back to the tundish to maintain a constant metal level in the distributor and at least one submerged entry nozzles (SEN)  12  to consistently deliver steel to the mold  13  with desired flowrate and flow pattern at minimum turbulent disturbance. The lifting tube  8  may have a gas injection device  36  at its bottom to obtain enhanced lifting effect. The distributor  11  has a connecting port  14  for a vacuum devise so that pressure above its metal bath can be reduced and controlled to assist the automatic metal level control and the transfer of steel from tundish  1  to distributor  11 . 
         [0024]    Tundish  1  is positioned on the side of the mold as shown in  FIG. 1 , and is divided into A, B, C and D four zones with refractory inner structures as shown in  FIG. 2 . In Zone A, the receiving zone, steel poured from a ladle is received and flow further through openings at the bottom of the wall  2  separating Zone A from other zones. Most of the entrapped ladle slag will be stopped and removed in Zone A, while some residual slag will be acting as cleaning agents to assist collision and agglomeration of inclusions in Zone B and will be removed together with the inclusions in Zones B and C. This ensures that ladle  15  can be emptied with no heels. In the area under the long nozzle  5 , a refractory pad  4  is installed to reduce the impact of the downward steel stream from the nozzle  5 , and to change its direction towards the port  31  on the wall  2  so that the down pour momentum can be turned into a horizontal flow inside the narrow channel  32  in the inclusion growth and removal zone B. Also, a slag removal device  3  is provided to remove the slag from the area regularly. 
         [0025]    The zone B, which has one or more narrow channels  32 , is the zone of inclusion growth and removal. The argon-blowing devices  6  blow argon from the bottom in the narrow channels, and produce gas-liquid two phase plumes  33  with free surfaces  34 , which have adequate turbulent dissipation rate needed for high collision and growth rates of the inclusions. As shown in  FIG. 3 , the narrow channels force the liquid steel go through these plumes in an up and down path  35 . Due to the constraining effect of the narrow channels, the bubbly plumes  33  assume fan shapes with larger widths and better bubble distributions than a circular plume, and a much greater inclusion path area is provided. This condition both extends the transit time of the liquid steel in the tundish, and increases collision frequency of inclusions in the steel. The increased collisions and aggregations of small size inclusions will result from the new design of the tundish. The growth of small size inclusions into larger size particles will lead to their removal by floatation in Zones B and C. 
         [0026]    After zone B, molten steel will enter zone C for further inclusion removal. Additional argon blowing devices  6  in the bottom help grown inclusion particles float up. The particles of size larger than 20 μm should be able to rise to above a height of 200-300 mm. The opening  37  on the bottom of wall  7  between zone C and zone D is 100-200 mm high from the floor. The lower portion of the steel bath that pass through to the zone D has a very low inclusion level that proceeds further to the distributor and then to the mold. 
         [0027]    The forth zone, zone D, is the clean steel zone which is the reservoir to hold the refined molten steel and deliver it to the mold through the distributor. Oxygen in the form of oxide inclusions in this zone could reach a level below 10 ppm as a result of the high collision and growth rates of the micro inclusions in the Zone B, and the maximum size of inclusions could be as small as 10 μm, as a result of the combined effects of the long transit time and the design of the separation wall with the opening  37  to prevent larger inclusions from getting into the Zone D. In the zone D, a desired feature would be a heating device  38 , a plasma flame generator for example so that steel in the tundish can be kept at a temperature that is optimal for delivery to the mold. To help this, low heat capacity insulation linings can be used for the tundish walls. An emergency drainage  10  is provided at the lowest location of the tundish bottom to empty the tundish when it is necessary. The emptied steel can be fed back to Zone A after the casting restarts instead of being treated as scraped melt. 
         [0028]    The distributor  11  transfers the clean steel from the tundish to the mold and, as shown in  FIGS. 1 ,  2 , and  4 , comprises of a main chamber  23 , and several conduits. The conduits include one or more submerged entry nozzle (SEN)  12 , a lifting tube  8 , and a down tube  9 . The main chamber  23  has a bottom in the shape of slope. The entry of the steel from the lifting tube  8  is located in the lower part of the bottom, and the top opening of the SEN in the higher part of the bottom. The slope shape of the bottom floor provides a means to drain the steel back to the tundish  1  and ensures zero heels when the distributor needs to be changed. As the distributor is filled with argon, re-oxidation of the melt during metal transfer is largely avoided. 
         [0029]    The lifting tube  8  raises the liquid steel from the tundish  1  to the distributor  11  for delivery to the mold  13 . To lift the liquid steel from the tundish to the distributor, pressure inside the distributor is first lowered with the vacuum device  14  so that steel will fill the lifting tube  8 . In the case there is no gas lifting devices used, steel level in the tundish is kept at a height slightly higher than that in the mold to create the steel flow from the tundish to the mold. In the case argon-blowing device  36  is used at or near the bottom of the lifting tube, argon blowing into the steel inside the lifting tube  8  plays major role in creating the steel flow. The buoyancy effect of the gas phase raises the height of the gas-liquid column flow in the lifting tube  8  and draws the steel into the lifting tube and then the distributor chamber, as shown in  FIG. 3 . The down tube, enclosed in its area with the wall  39 , sends the steel back down to the tundish when the bath in the distributor chamber becomes higher than the surrounding wall. 
         [0030]    The distributor  11  is a shallow container, and a vacuum devise  14  lowers its inner pressure to below one atmosphere. The height difference between steel at the location where it enters the distributor from the lifting tube and steel inside the SEN determines the effective hydrostatic pressure, ΔH Eff . The steady flow that results from the lifting effect of injected argon, in combination with the low hydrostatic pressure, leads to a desired optimal speed and flow condition without turbulent fluctuations inside the SEN and at the SEN exits into the mold, which is vital for the flow pattern in the mold, and consequently the quality of casting. 
         [0031]    Around the area of the down tube  9  in the distributor  11 , a refractory wall  39  is built. The height difference between the SEN opening and the top of the wall  39  determines the height of the steel bath above the SEN. The height difference, ΔH C , adds another measure for control of the effective hydrostatic pressure and the flow rates of the steel. 
         [0032]    Another design feature of the distributor is the dams  22  which are used in the distributor to guide the metal flow so that, for multiple SENs, molten steel takes the same flow length to reach each one of them 
         [0033]    The main body of the distributor  11  shown in  FIGS. 1 ,  2  and  4  can be made in one piece or in several pieces. Preferably, it has a removable cover so that it can be opened for maintenance such as cleaning or rebuilding its internal structures. 
         [0034]    With the features of this new unique metal delivery system, the tundish can be positioned at the same level as the mold and has a much greater metal holding capacity and service life. This reduces the required height of the shop building, and also makes the operations of the casting station more flexible and smooth. Since fewer components that are either easily worn or damaged such as the sliding gate or sitting bricks are used in the tundish, process stoppage caused by the failure of these components will be reduced. 
         [0035]    The unique design of two containers, namely the tundish and the distributor, with the lifting and down tubes in between, facilitates the control of the depth of the steel bath in the distributor before the steel enters the mold. The hydrostatic pressure of the steel flow is now determined in the distributor, instead of the tundish. With the reservoir bank wall in the down flow area, the bath depth is accurately maintained. The vacuum level above the bath in the distributor, and the lifting gas flowrate are adjusted according to the needs of the casting process. With these enhanced flow control measures in the distributor and in the SEN, improved flow patterns can be obtained in the mold for better casting process control and casting quality. 
       Detailed Description of the Preferred Embodiment 
       [0036]    Referring to the drawings, as shown in  FIG. 1 , for a refining and casting cycle of about 45 minutes, a conventional ladle  15  with about 100 tons of refined steel is transported to the continuous casting station. Teeming equipment such as a long nozzle  5  are attached and the ladle is ready to start pouring its content to the tundish  1  with also a capacity about 100 tons. During a normal casing process, when a full ladle arrives, the tundish  1  should still have about half of steel inside it so that adequate steel flow is maintained in both Zone B and C to achieve good inclusion removal effects. In case this is a start-up situation and the preheated tundish  1  is empty, the full ladle should arrive about 20 minutes earlier than a normal delivery schedule and pour its content to the tundish at the maximum allowed speed. The first heat should also have a higher tapping temperature to compensate for the heat loss during the filling of the tundish  1 . The extra 20 minutes are needed to treat the first heat for enhanced inclusion removal. After this initial refining period, the regular casting process can be started. The second full ladle should be at the casting station to refill the tundish after about 20-25 minutes from the moment the first tundish started to feed the mold. Once the tundish is refilled to the set metal level, pouring rate from the ladle can be reduced to maintain a constant bath level inside the tundish. The ladle will stay at the casting station for about 20-25 minutes and after a ladle left the casting station, the tundish can maintain the casting process for 15-25 minutes to wait for another full ladle. This much longer allowable waiting time makes the steel supply management much more flexible and makes it possible to use much simpler ladle handling equipment than the turret. 
         [0037]    Referring again to the drawings with the side view of the casting station shown in  FIG. 1  and the top view of the tundish  1  is shown in  FIG. 2 . With ladle in a pouring state, a continuous steel stream flows downward into the tundish  1  and the stream changes its direction into horizontal flow towards the port  31  on the wall  2  after reached the flow guide  4 . From the port  31 , steel flows into the narrow channels of the inclusion growth and removal zone. 
         [0038]    All the argon blowing devices  6  are kept at blowing state from shortly before the first ladle starts teeming and are creating the wide fan-shaped bubbly plumes shown in  FIG. 3 . The argon flow rate is controlled so that only mild surface perturbation is created. It is also preferred to use those porous plugs that create small dispersed bubbles. To obtain long tundish service life, it is also preferred to use immersed blowing devices instead of devices imbedded into the tundish walls. The total length of the narrow channels is in the range of 6-12 m and their width is in the range of 0.3-0.6 m. For long service life of the tundish and for the easier operation, these walls can be built with replaceable parts. 
         [0039]    The driving forces of the flow in the channels are mainly from three sources, namely, the higher metal level in the receiving zone A, the momentum from the down pour stream of the ladle  15  and the buoyancy of the bubbly plumes which are adjusted in the way that their sprout heights decrease along the length of the channels so that steel is driven to flow along the channels. 
         [0040]    With the refining done in the tundish  1 , clean metal is first transferred into the distributor  11  and then to the mold  13 . Similar to the preparation of the tundish  1 , the operation of the distributor is different when it is a start up of a new distributor. In this case, the preheated new distributor  11  is moved into the working position with the vacuum devise started and the openings of the SEN  12  blocked with melt-away seals. Before lowering the distributor to the desired height, the argon blowing device  36  at the bottom of the lifting tube that is still out of the molten steel is started to flush out the air inside the distributor. With the air inside the distributor mostly removed, the distributor is further lowered to the working position with all the conduits in the set depths in the mold or in the tundish. Steel is then moving up the lifting tube  8  into the distributor  11  under the combined effect of vacuum and the gas lifting and fill the down tube  9  and the SEN  12 . The seal used to block steel flow at the bottom of the SEN  12  is made of the same grade steel as the casting and has a thickness that gives a melting away time required to just fully fill the SEN  12  so that a gradual and steady initial steel stream is created at the start of the casting. Once the casting process reaches the stable state, steel flow rates inside lifting tube  8 , down tube  9  and SEN  12  are controlled and adjusted at the required level with several means, including the vacuum level inside the distributor  11 , argon flow rate inside the lifting tube  8  and vertical position of the tundish  1 . Plug rods or sliding gates are not necessary for either SEN  12  or lifting tube  8 . In some situations, removable plugs can be used to block openings of SEN, lifting tube and down tube to prevent air entry. 
         [0041]    An example of the control of metal levels in the distributor and in all the conduits such as the lifting tube as well as the control of the steel flowrate to the mold is given here with the set up shown in  FIGS. 5 and 6 .  FIG. 5   a  shows the initial state of the transfer process where the distributor  11  is empty while metal level in the tundish  1  is at the level  41  and in the mold is at the level  42 .  FIG. 5   b  shows the state when vacuum is applied to the distributor  11  and metal levels in the lifting tube  8  and in the immersion nozzle tube  12  at levels  43  and  44 , respectively.  FIG. 5   c  shows the state when gas injection started inside the lifting tube  8  after the vacuum is applied and maintained. Now with the metal levels inside the immersion nozzle tube  12  remaining at  44  and inside the down tube  9  remaining at  43 , metal level in the lifting tube rises to the level  45  to drive the metal flow from the lifting tube  8  toward immersion tube  12  and down tube  9 . 
         [0042]    An example of the detailed steel transfer process is shown in  FIG. 6  where the lifting tube  8  is immersed in the tundish  1  to a depth h 1  of 0.5 m. The pressure inside the distributor is reduced to 0.2 atm with the vacuum device  14  so that the remaining fraction of atmospheric pressure β is 0.2. As a result, the value of H VAC  is 1.12 m. The argon flowrate is controlled to obtain a gas void fraction of 0.2 inside the lifting tube  8  or the liquid fraction α=0.8. The steel column height inside the lifting tube can be calculated with the equation: h 2 =[h 1 +(1−β)H]/α and a value of 2.025 m is obtained for h 2 . The top of the steel column inside the lifting tube  8  is now 1.525 m above the metal level in the tundish  11 , 1.325 m above the metal level in the mold  13  and about 0.2 m above the top of the steel column inside SEN  12 . With the help of the draining from the down tube, a value of 10-50 mm can be obtained for the effective hydrostatic height ΔH Eff  which is slightly smaller than ΔH C  shown in  FIG. 6  which determines the steel flowrate at the top openings of the SEN  12 .  FIG. 6  also shows the steel casting product  52  and the mold structure boundary  51 . 
         [0043]    Steel transfer rates are determined by the casting speed and controlled with several measures such as the liquid fraction α inside the lifting tube  8  or injected argon flowrate and the diameter of the lifting tube  8 . For two different cases with a casting rate of 100 and 250 tons/hour, respectively, the following data may reflect the system operation conditions: 
         [0000]      casting rate=100 ton/hr(14.3 m 3 /hr=0.004 m 3 /s):  Case-1,
       Inner diameter of the lifting tube=0.12 m   Inner cross sectional area of the lifting tube=0.01131 m 2      Standard argon flowrate ˜0.0003 m 3 /s=18 NL/min   Gas void fraction=1−α=0.2   Steel flow superficial velocity inside SEN=0.35 m/s       
 
         [0000]      casting rate=250 ton/hr(35.7 m 3 /hr=0.01 m 3 /s)  Case-2,
       Inner diameter of the lifting tube=0.15 m   Inner cross sectional area of the lifting tube=0.0177 m 2      Standard argon flowrate ˜0.007 m 3 /s=40 NL/min   Gas void fraction=1−α=0.35   Steel flow superficial velocity inside SEN=0.57 m/s