Continuous casting-tundish technology


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Tundish technology

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Continuous casting-tundish technology

  2. 2. SO,WHAT IS STEEL?  Steel is an alloy of iron and other elements, including carbon. When carbon is the primary alloying element, its content in the steel is between 0.002% and 2.1% by weight. The following elements are always present in steel: carbon, manganese, phosphorus, sulfur, silicon, and traces of oxygen, nitrogen and aluminum
  3. 3. CONTINUOUS CASTING  Continuous casting, also called strand casting, is the process whereby molten metal is solidified into a semifinished billet, bloom, or slab for subsequent rolling in the finishing mills.
  4. 4.  Most basic metals are mass-produced using a continuous casting process, including over 500 million tons of steel, 20 million tons of aluminum, and 1 million tons of copper, nickel, and other metals in the world each year
  5. 5.  The continuous casting has several configurations to produce the steel as vertical, vertical with bending and curved type. .
  6. 6.  Curved machines are used for the majority of steel casting and require bending and / or unbending of the solidifying strand.
  7. 7. WHAT DO WE DO UNTIL CONTINUOUS CASTING ?  Preparation the steel by adding some materials within the different furnaces such as BOF,EAF.
  8. 8. BOF(BASIC OXYGEN FURNACE) 1.Molten pig iron from a blast furnace is poured into a large refractory-lined container called a ladle 2.The metal in the ladle is sent directly for basic oxygen steelmaking
  9. 9. 3.Filling the furnace with the ingredients is called charging. Molten iron from the ladle is added as required by the charge balance. A typical chemistry of hot metal charged into the BOS vessel is: 4% C, 0.2–0.8% Si, 0.08%–0.18% P, and 0.01–0.04% S. 4.The vessel is then set upright and a water-cooled lance is lowered down into it. The lance blows 99% pure oxygen onto the steel and iron. This melts the scrap, lowers the carbon content of the molten iron and helps remove unwanted chemical elements 5.Fluxes (burnt lime or dolomite) are fed into the vessel to form slag, which absorbs impurities of the steelmaking process
  10. 10. 6.The BOF vessel is tilted again and the steel is tapped into a giant ladle.
  11. 11. ELECTRIC ARC FURNACE Arc furnaces differ from induction furnaces in that the charge material is directly exposed to an electric arc, and the current in the furnace terminals passes through the charged material.
  14. 14. THE MORTEN STEEL IS NOW IN LADLE FURNACE LADLE FURNACE There is an important thing that should not be forgotten!!!! Before the tapping from the EAF or BOF to the LF, the LF has to be preheated..
  16. 16. THE BENEFITS OF CONTINUOUS CASTING  Considerable energy savings  Less scrap produced, i.e. improved yield  Improved labor productivity  Improved quality of steel  Reduced pollution  Reduced capital costs  Increased use of purchased scrap when output is maximized
  17. 17. 1) The liquid steel comes from the steel plant in a ladle 2) From ladle it is tapped in a tundish
  18. 18. 3) Liquid Steel is flowed through the nozzle to mould from tundish. The flow rate through the nozzle into the mould can be controlled by a stopper in the tundish 4) The mould is a rectangular copper box without a top and a bottom 5) The outer shell is being ’grabbed’ by a driven roll just beneath the mould, pulling a strand of steel out of the mould. 6) The core of the strand, as it exits, is still liquid; because of that, the strand proceeds through a secondary cooling section 7) The strand is bent from the vertical plane to horizontal plane using rolls
  19. 19. 8) At the end of the cooling section the cross-section of the strand is completely solidified, slabs are then created by cutting the strand 9) These slabs are put in a tunnel furnace to let them homogenize 10) After a while the slabs come out of the furnace; subsequently they are rolled out, further cooled and finally coiled
  20. 20. TUNDISH
  21. 21. The tundish holds enough metal to provide a continuous flow to the mold, even during an exchange of ladles, which are supplied periodically from the steelmaking process
  22. 22.  The shape of the tundish is typically rectangular, but delta and "T" shapes are also common.  Nozzles are located along its bottom to distribute liquid steel to the molds
  23. 23. The tundish also serves several other key functions!!!  Enhances oxide inclusion separation  Provides a continuous flow of liquid steel to the mold during ladle exchanges  Maintains a steady metal height above the nozzles to the molds, thereby keeping steel flow constant  Provides more stable stream patterns to the mold(s)
  24. 24.  The tundish is a refractory-lined channel consisting of an inlet and outlet sections and sometimes has flow control devices, such as dams and weirs or a baffle with holes, along its length. DAM BAFFLE WEIR
  25. 25. • A tundish may have a refractory-lined lid, and has bottom ports that are assembled with slide gates or stopper rods through which the melt is teemed into the mold
  27. 27. THE USED DEVICES IN THE TUNDISH  IMPACT PADS: Designed to redirect steel upward and outward for enhanced steel residence time in the tundish and prevent short circuiting to the closest strand(s)  Also used for optimized drainage and yield enhancement for multiple radical grade change sequences
  28. 28.  BAFFLES: While in the tundishes, impurities (called "inclusions") in the molten steel float to the top, forming a "slag" layer of impurities, and the pure, substantially "inclusion-free" metal exits from the bottom  Depending on the size of the tundish, and the flow rate of molten steel, the molten steel may not always have enough residence time in the tundish to permit the impurities to float to the top.Therefore,the baffles are used.
  29. 29.  DAMS: This device helps to modifying the inside bottom surface of a tundish for continuous casting molten steel to minimize turbulence thereby reducing gas bubble and slag entrainment during continuous casting of steel, especially when initially filling of the tundish. DAM
  30. 30.  Thermocouple: It reads the steel bath temperature in real time at a fixed location inside the tundish.  Diffusers: During operation the Tundish Gas Diffuser will help reduce "dead" areas within the tundish and provide a more stable steel temperature in the tundish bath
  31. 31.  Stoppers: This is used to control the molten steel flow from the tundish to the mold
  32. 32.  Slide Gates: Tundish gates are provided with 3 plates in order to avoid the movement of the subentry shroud in the mould during the flow control by throttling.
  33. 33.  Metering Nozzle: Key features of nozzle changer systems include: 1) Improved casting operator safety 2) Enables longer casting sequences 3) Improved metallurgical quality 4) Flexibility
  34. 34.  Submerged Entry Nozzle: These refractories are subjected to severe operating conditions such as thermal shock,molten steel erosion,and slag attack. Upper Nozzle
  35. 35. THE IMPORTANCE OF TUNDISH  The melt remains in the tundish for a relatively short time, reflecting the continuous nature of tundish operation  Thus, the major refining reactions such as deoxidation and desulfurization are carried out in the ladle. The goals of a tundish are to minimize heat loss, deliver the melt evenly into molds, minimize the formation of macro inclusions, and maximize their removal.
  36. 36. INCLUSIONS AND DEFECTS  Non-metallic inclusions are a significant problem in cast steels  The mechanical behavior of steel is controlled to a large degree by the volume fraction, size, distribution, composition and morphology of inclusions and precipitates, which act as stress raisers  The inclusion size distribution is particularly important, because large macro inclusions are the most harmful to mechanical properties.
  37. 37.  The ductility and durability are significantly impaired by large-sized, non-metallic inclusions in steel. The samples of inclusions are shown above
  38. 38.  Non-metallic inclusions in steel are of two kinds, and each has its different mode of formation. 1. one is indigenous oxide inclusions which form by deoxidation of the steel melt. 2. The other kind is exogenous inclusions, which form by reoxidation of deoxidized steel melt by air or by the entrained slag into the melt during the melt transfer from ladle to mold. INDIGENOUS EXOGENOUS
  39. 39. DEFECTS IN STEEL PRODUCTS Flange Cracked Cans Slag spots on cold rolled sheet Line defect on cold rolled sheet
  40. 40. THE FOLLOWING FACTORS NEED PARTICULAR ATTENTION IN MAKING A TUNDISH EFFECTIVE FOR REDUCING MACRO INCLUSIONS: #1. Steady state period of casting requires:  Melt flow with less turbulence along the tundish flux/melt interface  No short circuiting of melt flow from the inlet section to the exit (tundish nozzle)  Minimal dead volume for the melt in the tundish  Sufficient residence time for the melt to promote flotation  An argon gas shrouding pipe or long nozzle for ladle melt discharge into the tundish  Thermal insulation and protection against reoxidation by argon gas injection with lid or a tundish flux cover
  41. 41. #2. Non-steady state period of casting requires:  Prevention of slag carry over by vortexing and draining from the ladle to the tundish  The above slag carry over issue also applies to tundish to mold transfer  Suppression of turbulence caused by impinging melt stream to the tundish at the ladle opening  Sustaining an inert atmosphere at ladle opening and ladle change  Active compensation for temperature drop
  43. 43.  The tundish can be designed according to the following conditions: • The amount of the production:The volumes of ladle, BOF,EAF • The various of the production (bloom, billet, slab) • The number of the strands • The features should be placed to float the inclusions from the morten steel to the slag( dams, baffles,weirs ) • The control of melt temperature to an appropriate level for feeding into the mold • The control of flow rates ( nozzle port(s), slide gates, stopper rods ) • The adjustment chemical compositions
  44. 44.  Various technologies such as a long nozzle or an inert gas shrouding pipe have been implemented to reduce air reoxidation and slag emulsification.  Tundish thermal state during continuous casting as function of heat losses through the tundish shell, insulation of the steel bath and temperature.
  45. 45. TO INDICATE THE VOLUME AND THE NUMBER OF THE STRANDS OF THE TUNDISH  This exactly belongs to the amount and various of factory production  The number of molds is usually 1 or 2 for a slab caster, 2 to 4 for a bloom caster, and 4 to 8 for a billet caster BLOOM SLAB
  46. 46. 1) ANCHORING SYSTEM  The anchoring system is done to be hold the permanent lining slightly to the tundish.  This anchors are assembled to the tundish walls at certain distances by welding
  47. 47. 2) INSULATION MATERIAL  The ceramic fiber panel is used due to its low thermal conductivity and low heat storing between the tundish wall and permanent lining.  These may be showed according to the production conditions and the volume of tundish
  49. 49. THE PLACEMENT OF NOZZLES AND STOPPERS  Before the permanent lining process is begun, the nozzles should be positioned to the outlet points at the bottom of the tundish  These devices are used to control the fluid flow from the tundish to the mold.  The used of these may be showed difference according to the various of production and the production conditions
  50. 50. 3) PERMANENT LINING  The permanent refractory lining made of aluminous-siliceous refractory concrete may be made of refractory bricks or may be a cast or rammed monolithic lining or a combination of the two.  The expected properties from the permanent lining are showed on following:  Almostly zero cement should be  The particule distribution of admixture should be lower  These may be lowest bonded cement according to the production and the materials
  51. 51.  The hot ratios of expansion of the components should be close as material properties  The high resistance against the high temperature, wear and impact  The application of this lining should be easy
  52. 52.  The permanent lining is generally made of high alümina refractory(%70-%93) and is the thickest part of the tundish lining. This lining has a low reactivity with magnesia materials to form a surface working layer.  Aluminous-siliceous refractory concretes proved the best in the second layer.  These are the materials that contain cement binder usually aluminate cement, which imparts hydraulic setting properties when mixed with water  The most common binder used in castables is HAC (high alumina cement)
  53. 53.  Other binders that are often used include hydratable aluminas and colloidal silica  When the working temperature is increased,the Al2O3 content should be increased.Because of this, the thermal conductivity and flexibilty.  These monolithic castables should be dried, sintered, and preheated before being put in service to prevent explosion spalling
  54. 54. • CAC in castables: The refractory concretes are divided into 4 main groups: 1) Conventional Cement Castables,CCC,high cement contents(>20% CAC) 2) Low Cement Castables,LCC (6-15%) 3) Ultra Low Cement Castables (<6%) 4) Cement Free Castables/No Cement Castables, NCC (<1,5%)
  55. 55.  Here the permanent lining may show severe damage by cracking  Mechanical failure of the permanent lining occurs in the transition zone between the inlet port and the plain side wall of the bath.
  56. 56.  Cracks propagate through the whole thickness of the permanent lining and are penetrated by hot metal up to the cold end.  This crack formation increases the probability of failure during breaking out of the working lining.
  57. 57.  To reduce cracking and to avoid the delamination quoted above, calculations with an additional expansion allowance were performed with a preheating up to 1100°C on the hot face.
  58. 58.  In conclusion, A longer heat penetration period leads to a more homogeneous temperature distribution at the moment of the thermal shock and this circumstance decreases thermomechanical load in the transition zone.
  59. 59. THE APPLICATION OF PERMANENT LINING  After the nozzles (SEN, metering nozzle or slide gates) are placed to the bottom of the tundish based on the production conditions, this application is done  After the former is positioned into the tundish,the lining process is begun.  The mix that was prepared before is filled between the former and the tundish walls
  60. 60.  The former is observantly vibrated till the end of the installation  However,this process is done as soon as the bubbles are seen over the the permanent layer. The reason of this is that the binders within the solidified mix rise up through the layer and cause the micro and macro cracks in the process.Therefore,the vibration process should be done carefully.
  61. 61. 4) THE PLACEMENT OF TUNDISH DEVICES  The placement of tundish devices (ımpact pads, baffles, weirs, dams) is the most important section for tundish design.Because,the non-metallic inclusions can be moved away from the liquid steel and added to the slag layer with using of this devices.  In this way, the clean steel can be occured.
  62. 62.  The placement or building with burnt MgO bricks of impact pad is done firstly. Burnt MgO Bricks
  63. 63.  By the last decade, testing and use of incidence ‘pot’ known under the name TURBOSTOP was accepted.  The quite good conditions for rectifying flow in the tundish were provided with its installation.
  64. 64. THE VARIOUS OF TUNDISH DESIGNS  The main points to improve tundish configuration are:  increase minimum residence time  get similar minimum residence times between all the strands  increase plug volume fraction  increase mixing volume fraction  decrease dead volume fraction
  65. 65. 1) The Baffles Configuration:  The hole angles of the baffles are important to move away the non-metalic inclusions to the slag  However,the only baffles don’t indicate the expected effect regarding the non-metallic inclusions
  66. 66. 2) The Baffles-The Dams Configuration  As mentioned above,the dams help to minimize turbulence thereby reducing gas bubble and slag entrainment during continuous casting of steel.  The shape and arrangement of dams and baffles has also an affect on the thermal conditions prevailing in the tundish.
  67. 67.  Depanding on tundish interior geometry, there are zones of diverse flow intensities within the tundish: - Active area - Stagnant area( dead region )  Dead zones result in a less homogeneous metal, and also reduce the effective capacity of the tundish.
  68. 68. Configuration 1 Configuration 2 Regarding temperature yield,when low dams were considered temperature at outer strands were around 3ºC lower than the ones obtained at inner strands. This fact was attributed to the lower velocities reached at bottom region near the dams in configuration 1 and configuration 2
  69. 69. 3) The Baffles-The Dams-The Diffusers Configuration:  The diffusers will help reduce "dead" areas within the tundish and provide a more stable steel temperature in the tundish bath  This device is used in the configurations which we can’t find the solutions regarding reducing the dead zones.
  70. 70. 4) The Weirs-The Dams Configuration:  The weirs are located in the upper part of the tundish.  The weir prevents liquid metal from flowing continuously across the surface of the molten metal, while allowing liquid flow beneath the weir. WEİR DAM
  71. 71. The differences can be seen with water modelling
  72. 72.  The height of the tundish and the steel velocity are other properties to obtain the clean steel to prevent the dead zones
  73. 73. The outlet temperature of the melt is seen reduced with the increase in the bath height as in previous cases due to increase in the heat transfer area. It is also observed that with the increase in the bath height, bluish coloured cooler regions shift towards the top free surface and inlet side with the increase in the bath height, due to recirculating and reversing flow.
  74. 74.  Casting flow rate has a big influence on the structure of flow and the steel turbulence intensity in the tundish. The higher casting flow rate, the lower part of dead zones in the volume of liquid steel.
  75. 75.  In conclusion,the best design should have the baffles, the dams and the diffusers.Because,the more wear are seen especially in the configuration having the weirs and the zones combined the side tundish walls with the weirs will be done more wear. Therefore:
  76. 76. 5) THE WORKING LINING  A working lining layer is mostly made of magnesia mixture that is wet applied through gunning in succesive steps by 20mm layers up to requested on walls and slag line(60-80 mm thickness)  With their usage the use of monolithic isolation mixes started to be even more important,because compared to brick lining isolated tundish had better isolation properties,which bettered the thermal balance  It has a higher thermal expansion coefficient
  77. 77.  From tundish lining is expected:  Decrease of thermal loses by long sequences  Better turn of tundish  Longer life time and decrease in maintenance costs of refractories  Simple drying and heating  Easy tapping and decrease of waste volume  Better lining integrity-increased purity of steel  Simple and quick installation  Decreased specific consumption upon a ton of steel  It shouldn’t include the asbestos
  78. 78.  The working lining is installed over permanent lining to provide thermal insulation and keep the steel shell temperature below its critical temperature range throughout the operating campaigns of a tundish The refractory material of the working face in contact with the steel melt should be high in MgO, with a minimum amount of reducible binder to prevent oxidation of Al in the melt.
  79. 79.  The working tundish lining refractory composition further includes a binder based on silicates or phosphates; a plasticizer which enhances adherence of the composition when applied to the permanent lining; a bond stabilizer; a homogenizer; and in certain cases, a small amount of a wetting/foaming agent  The corrosion of the working lining in molten slags depends strongly on the viscosity of slag and also the basicity gap between the refractories and the slags.
  80. 80.  When slag penetrates into pores of common plaster with high- MgO content, the phases such as monticellite and merwinite develop around MgO grains and provide continuous dissolution of MgO grains during steel-making process.  Replacing part of the magnesia of plaster by chromite or olivine is promising for decreasing the hot corrosion.  This is because the basicity gap between introduced plasters and tundish slag decreases and also causes the formation of phases with high melting point on the surface of primary grains of MgO and decreases the dissolution of plaster in the slag. Therefore, the life time of these plasters is increased to higher sequences.
  81. 81. THE APPLICATION OF THE WORKING LINING  Magnesia mixture with various additives is spread on basis permanent lining with temperature up to 100̊C in thickness 60-80mm.  In generally,the amount of water added into the mixture is % 18-25.
  82. 82.  The tundish lining is preheated before putting into operation usually to temperature 1100-1200 ̊C
  83. 83. The tundish is ready for continuous casting