Continuous Casting of Steel

Transcription

1 Continuous Casting of Steel Mechanical & Industrial Engineering Department ME8109 Casting and Solidification Materials - W2012 Majid Haghighi Casting and Solidification Materials

2 Outline Introduction & Objective Continuous Casting Methods Steel Casting Process Fluid Flow & Solidification Porosity Sources Heat Transfer and Casting Speed Primary & Secondary Cooling Zoon Conclusion 2 Casting and Solidification Materials

3 Introduction & Objective Slabs, Billets, Blooms depending on dimensions of the strand Higher capital cost and lower operating cost, less deformation The most efficient process and energy Slab Billet Bloom 3 Casting and Solidification Materials

4 Continuous Casting Methods * Vertical Casting Mold axis and fluid flow are in vertical position Casting non-ferrous alloys such as Aluminum Produce ingots as a row material for Extrusion, Forging or Rolling Stop periodically to remove the cast ingot Speed is slow to avoid internal cracks and metallurgical length shorter * Horizontal Casting Mold axis and fluid flow are in horizontal position Casting Steel or Copper alloys Withdrawal the strand through the hot rolling after spray chamber Produce ingots as a row material for Extrusion, Forging or Rolling Hot condition to pre-shap the final strand Cut the strand into different size by gas torches 4 Casting and Solidification Materials

5 Continuous Casting Methods * Strip Casting Casting Steel or other metals Solidify thin layer of steel by passing the molten steel through large rotating rollers * Curved Casting Mold axis and fluid flow are in curved (V &H ) position Casting steel alloys Semi-finished products such as Slabs, Billets and Blooms Molten steel pours into the mold through a ladle and tundish Speed is Fast and continuously, metallurgical length longer * Other Continuous Casting Electro-Slag Re-melting (ESR) or Vacuum Arc Re-melting (VAR) to cast supper-alloys (aerospace application) 5 Casting and Solidification Materials

6 Steel Casting Process The steel heated above liquidus temperature in the furnace (Superheating degree) Enters molten steel into the mold through Submerge entry nozzle (SEN) Cooling continuous by quenching with water spray along the whole of the strand Entering liquid steel into the mold can cause unbalances and clogging and possibly break out Melting Point of steel : 1370 C Steel : Iron + Carbon (0.2% - 2.1%) 6 Casting and Solidification Materials

7 Fluid Flow & Solidification Initial Solidification The time-dependent shape of the meniscus Liquid flux flow into the shell-mould gap Local superheat contained in the flowing steel Conduction of heat through the mould Liquid mold flux and re-solidified flux rim 7 Casting and Solidification Materials

8 Fluid Flow & Solidification Superheat Molten Steel and Level Fluctuation Superheat = Steel Temperature Entering Steel Melting Point Hottest, area midway between SEN and narrow face Coldest area, at the meniscus cause freezing meniscus and lead to: To avoid Freezing Meniscus, reach the metal flow to the surface quickly To avoid breakout, no gap between shell and mold Level Fluctuation can reduce by flow pattern Increase casting speed impact on transient turbulent fluctiuation Surface velocity keep under the critical value Temperature and super heat distribution in the mould Mold Powder Act as a lubricant between strand and mold Improve heat transfer from strand to mold Protect liquid steel against reoxidation Absorb inclusions that rise to the metal surface Provide thermal and chemical insulation of the top surface 1)Turbulence, 2) Level Fluctuation, 3) Vortexing, 4) Emusification 8 Casting and Solidification Materials

9 Fluid Flow & Solidification Clogging at SEN Dislodged Clogs or change the flux composition and leading to defect on molten steel Change the nozzle flow pattern (Asymmetrical) leading to slag entrainment and surface defect Clogging interferes with mold level control * Using control device (stopper rod or slide gate) to compensate for the clog and ensure a smooth flow pattern Caster Curvature Vertical Caster (at least the top 2.5 m section) Less Particle defects 9 Casting and Solidification Materials

10 Fluid Flow & Solidification Argon Gas Injection Reduce nozzle clogging Influence and control the flow pattern in the mold Generating serious top surface fluctuation Capturing inclusions Capture solid oxide particles and lead to surface slivers Without Gas With Gas Electromagnetic Forces AC electromagnetic fields of high or low frequencies: Oscillation marks Lateral cracks Entrapment of inclusions and bubbles 10 Casting and Solidification Materials

11 Porosity Sources Air Entrainment Failure to eliminate all air in the mold cavity Oxides Re-oxidation inclusion (porosity remains when inclusion material removed by blasting) High level of oxygen reduce surface tension that must become to form a bubble Shrinkage Shrinkage occurs when liquid metal becomes denser solid Drop the pressure in the liquid metal below atmospheric Heat Treatment and Casting Speed Lower casting speed, higher metallurgical length Higher casting speed, higher temperature in SCZ Higher casting speed, consequently temperature distribution on the Walls Cast Speed depends on Product type & size *Regular : m/min *High speed: m/min 11 Casting and Solidification Materials

12 Heat Treatment and Casting Speed 12 Casting and Solidification Materials

13 Heat Treatment and Casting Speed Mould Lubrication Helping to avoid breakout in high speed casting Occurs by influence of slag into the strand Vm = mould speed Vc = casting speed η = viscosity of liquid slag film dl = thickness of slag film. fl = η (Vm Vc ) / dl fs = ηs H ηs = coefficient of solid friction H = ferrostatic pressure of molten steel 13 Casting and Solidification Materials

14 Primary & Secondary Cooling Zoon * Passing the molten steel through the water-cooled tank Temperature of solid Casting speed Solidification time Cooling rate Good quality & Thickness of the crust * Key factors affecting on quality of strand in SCZ area Cooling water spray rate Metallurgical Length consideration Superheat of liquid steel 14 Casting and Solidification Materials

15 Conclusion Quality of final product depend on: Liquid level in the mold Powder feeding rate Casting speed Gas injection Slide gate opening Nozzle position Cooling water rate Benefit of continuous casting of steel: Considerable energy saving Less scraped produced Improved labour productivity Improved quality of steel Reduced pollution Reduced capital costs, time saving, casting process Increased use of purchase scraps when output is maximized A highly productive process that can be fully automated 15 Casting and Solidification Materials

16 References [1] Brian G. Thomas, Continuous Casting of Steel, Chapter 15 in Modeling for Casting and Solidification Processing, O. Yu, editor, Marcel Dekker, New York, NY, 2001, pp [2] S. Mazumdar, Solidification control in continuous casting of steel, Sadhana,Vol. 26, Parts1 & 2, February April 2001, pp Printed in India [3] T. Merder et al. Modeling of flow behavior in a six strand continuous casting tundish, METABK 46 (4) (2007) [4] B.G. Thomas, Continuous Casting, Yearbook of Science and Technology, McGraw-Hill, [5] S. Mizoguchi, T. Ohashi, Continuous casting of steel, Ann. Rev. Mater. Sci. 1981, 11: [6] S. R. Bragança, Reduction of the hydrogen content in the continuous casting of steel Laboratório de Cerâmicos da Universidade Federal do Rio Grande do Sul LACER/UFRGS [7] N. Zapuskalov, Comparison of Continuous Strip Casting with Conventional Technology ISIJ International, Vol. 43 (2003), No. 8, pp [8] H. Bay, B. G. Thomas, Effects of Clogging, Argon Injection and Continuous Casting conditions on Flow and Air Aspiration in Submerged Entry Nozzles Metallurgical and materials transactions B Volume 32B, August [9] Politehnica University of Timisoara/Department of Automation and Applied Informatics, Timisoara, Romania ovidiu.tirian@fih.upt.ro, octavian.prostean@aut.upt. [10] Zhenping Ji, School of Information Science and Engineering Shenyang Ligong University Shenyang, China [11] H. Shin, G. Lee, W. Choi, S. Kang, J. Park, S. Kim, B. G. Thomas Effect of Mold Oscillation on Powder Consumption and Hook Formation in Ultra Low Carbon Steel Slabs AIS Tech 2004, Nashville, TN, Sep , [12] Lifeng Zhang, Brian G. Thomas, Inclusion in Continuous Casting of Steel, XXIV National Steelmaking Symposium, Morelia, Mich, Mexico, 26-28, Nov.2003, pp [12] Mohammad Sadat, Ali Honarvar Gheysari, Saeid Sadat, The effects of casting speed on steel continuous casting process, Heat Mass TransferSpringer-Verlag, Published online: 3 June 2011 [13] R&D center for Iron and Steel, Steel Authority of India Ltd (SAIL), Ranchi, India Solidification control in continuous casting of steel, Vol. 26, Parts 1 & 2, February April 2001, pp [14] R. Monroe, Porosity in Castings, Steel Founders Society of America, Crystal Lake, Illinois, Copyright 2005 American Foundry Society, Page 1 of 28 [15] B.G. Thomas, Continuous Casting: Modeling, the Encyclopedia of Advanced Materials Pergamon Elsevier Science Ltd., Oxford, UK, Vol. 2, 2001, 8p, (Revision 3, Oct. 12, 1999) [15] Ken-ichi Miyazawa, Continuous Casting of Steel in Japan, Science and Technology of Advanced Materials, 2001, [16] Wenhon Liu, Zhi Xie, Zhenping Ji and Biao Wang, Research and Application of Dynamic Control System for Secondary Cooling of Billet Continuous Casting, School of Information Science and Engineering Northeastern University Shenyang, Liaoning Province, China, 2007 IEEE [17] Steeluniversity.org, Continuous Casting Simulation, version 1.60 User Guide 16 Casting and Solidification Materials

17 Thank You 17 Casting and Solidification Materials