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1、Study on the Macrosegregation Behavior of the Bloom Continuous Castings大方坯大方坯/圆坯连铸宏观偏析特性与圆坯连铸宏观偏析特性与注流控制技术应用注流控制技术应用孙海波， 张家泉报告人：张家泉报告人：张家泉北京科技大学钢铁冶金系Phone：139 111 71237Outline/提纲提纲Introduction/引言引言Coupled model development/ 连铸凝固过程电连铸凝固过程电磁热溶质耦合模型开发磁热溶质耦合模型开发 (electromagnetic-thermal-solute transport
2、ation)Macroscale transport phenomena in bloom CC process/大方坯连铸过程传输现象特征大方坯连铸过程传输现象特征Low temperature casting technology/铸坯宏铸坯宏观偏析控制技术与应用效果观偏析控制技术与应用效果Conclusions/结论结论Introduction Macrosegregation has been the bottleneck for the internal quality upgrading of the casting products. For the efficiency lim
3、itation of industrial tests, previous researcher have devoted significant efforts to develop the mathematical models for better understanding the macro-segregation At present, representative models mainly include:1.Continuum model (by F. P. Incropera, Int. J. Heat Mass Transfer ,1991 and H.L. Tsai,
4、Metall. Trans. A,1993)2.Volumetric averaging model (by C.Beckermann, Metall. Trans. B ,1991 and D. R. Poirier, Metall. Trans. B ,1991 ) For the CC process, few investigations were reported to reveal the macrosegregation. Reasons can be summarized as:1.More complicated physical phenomena( such as, th
5、e moving solidification front, varying heat boundary condition, turbulent flow and EMS )2.Enormous calculated quantity( computational domain involves the region between meniscus and final solidification destination) In the present work, a mathematical model, coupling electro-magnetism, fluid flow, h
6、eat and solute transport, was developed based on the 3D plus 2D hybrid modeling method for the bloom CC process.Coupled model developmentImplementation process for the multi physics/scale mathematical model1. Fluid flow model1) Continuity equation 2) Momentum equation a) Thermo-solutal buoyancy (1)
7、(2) (3)b) Electromagnetism force, calculated by the finite element code ANSYS, is imported into the Fluent code as the momentum source term through the coordinate interpolation algorithm.Schematic of coordinate interpolation algorithmc) Phase interaction force within the mushy zone, can be described
8、 by the Darcys law: where, the permeability, Kp, was calculated by using the Carman-Kozeny equation:3) Low-Reynolds number turbulent model2. Solidification and heat transfer modelEnthalpy equation where, Enthalpy: Liquid Fraction:moreover,Solute elementC78.0Si7.6Mn4.9P34.4S38.0Slope of liquidus line
9、Slope of liquidus line111 Y. Ueshima, S.Mizoguchi, T. Matsumiya et al. Analysis of Solute Distribution in Dendrites of Carbon Steel with / Tansformation during SolidificationJ. Metallurgical Transactions B. 1986. 17B(12): 845-859.3. Solute transport modelSolute transport equation where, Moreover, th
10、e source term, Ss, consists of the molecular diffusion, Ss,dif induced by Fickians law and the convection diffusion Ss,con: In consideration of 1) the ratio of longitudinal heat flux to the total heat is only about 36%, 2) the temperature of molten steel at the strand center gradually decreases alon
11、g the casting direction, in the CC process:Z-component of velocity variation for the center molten steel of strand along the casting directionDivision of computational domain for the CC system4. Division of computational domain5. MeshesMeshes adopted in the heat and solute transport model for turbul
12、ent flow region (a), natural convection region (b) and forced convection region (c).Meshes adopted in the electromagnetic model for M-EMS (a), and F-EMS (b).6. Boundary condition1. Inlet and outlet At the cross section of inlet, vz=vin, vx=vy=0, Kin=0.01vin2, =Kin1.5/D, Tin=Tc, and Cin=Csteel. For t
13、he outlet, the fully developed flow condition was adopted.2. Free surfaceAdiabatic condition, normal gradients of all variables at free surface was set be zero.3. Strand surface a) Mould zone b) Secondary cooling zone c) Air cooling zone 4. Electric current densityMacroscale transport phenomena in b
14、loom CC processOperation conditionParametersValueParametersValueSectional dimension260300 mm2Current of M-EMS 200ACasting speed0.65m/minFrequency of M-EMS3.0HzSuperheat degree29Current of F-EMS 100ASteel gradeGCr15Frequency of F-EMS12.0HzSteel gradeChemical composition(wt%)CSiMnPSGCr150.951.050.150.
15、350.250.450.0200.020Table 1 Operation condition of casting test Table 2 Chemical composition of the steel gradeSectional shapeCasting speed,m/minCooling water flow rate，L/minZone 0Zone 1Zone 2Zone 3Zone 4Zone 5Bloom casting0.6526.32014.718.915.89.5Cooling zone length，m0.561.191.492.32.31.19Table 3 L
16、ength and cooling water flow rate of each cooling loop in the secondary cooling zone Electromagnetic fieldComparison between calculated and measured value of magnetic induction intensities in the stirrer center under the cases of different current and frequencyCalculated value variation of center MI
17、I along the casting directionVariation tendency matches well with the results measured by L.B. Trindade2Fluid flow and solute transportA. Turbulent flow zoneMelt flow pattern(a) and distribution of solute element C (b) at the central symmetry plane of strand width face Segregation degree variation o
18、f solute element C under different distances from the meniscus along the Y-axis direction at the central symmetry plane of strandDistributions of solute element Si, Mn, P, and S at the central symmetry plane of strand in the turbulent flow zone Distributions of Si, Mn, P, and S at the central symmet
19、ry plane of strand are similar with that of C, which can be explained by Scheils model:B. Nature convection zone Melt flow pattern (a) and distribution (b) of solute element C at the cross sections of strand with different distances from the meniscus in the natural convection zoneSegregation degree
20、variation of solute element C under different distances from the meniscus along the Y-axis direction at the cross section of strand Segregation degree of C at the strand center increases from 1.046 to 1.18. Moreover, the segregation degree distribution of C presents fluctuation in the solidification
21、 shell at the cross section of strand due to the circulation flowC. Forced convection zoneMelt flow pattern (a) and tangential velocity variation (b) under different distances from the meniscus along the Y-axis direction at the central symmetry plane of strandSolute distribution (a) and segregation
22、degree variation (b) at inlet and outlet of forced flow zone along the Y-axis direction on the central symmetry plane of strandI=100AF=12HzD. Region ranged from the outlet of F-EMS to the final solidification destinationVariation for the melt flow velocity and segregation degree of C at different di
23、stances from the meniscus along the Y-axis direction on the symmetry plane of strand In this region, the maximum segregation degree of C at the strand cross section increases from 1.22 to 1.29, and the segregation degree profile of C is constantly in the status of serration due to the tiny melt flow
24、 velocity.E. Validation for the solute distributionWet chemical analyzer:Using a 4 mm drill in diameter up to a depth of 8 mmLocation of the drilling samples at the bloom casting Comparison between measured and simulated segregation profiles for diagonals in direction 2 (a) and 3 (b) at cross sectio
25、n of castingC:S:To further verify the coupled model:Comparison between measured and simulated segregation profiles of C in the region ranged from 0 mm to 25 mm distance from strand surface The results can also be found from the works of B. Rogberg3 and M. E. Bealy4. Main reason should be contributed
26、 to the circumfluence and the floatation of solute-rich molten steel at the upper part of mould. Transformation from positive segregation degree to negative!3 B. Rogberg. High Temperature Properties of Steels and their Influence on the Formation of Defects in Continuous CastingD. Ph.D Thesis. 1982,
27、Royal Institute of Technology, Swenden.4 M. E. Bealy, H. Fredriksson. On the Formation of a fluctuated Macrosegregation Phenomenon in the continuous casting processJ. Scandinavian Jounal of Metallugy, 1994, 23:140-150.Solidification and heat transferComparison between measured and simulated temperat
28、ure variation at the strand surface along the casting directionVariation of shell thickness and center liquid fraction with distance from meniscus during the CC process Distribution of liquid fraction at the cross section of strand with different distances from the meniscusMorphology of bloom castin
29、g cross sectionRelationship between the segregation degree of C and liquidus temperature of molten steel in the strand center along with the casting directionNovel casting technologyBackground1. Immediate way: 2. Indirect method: Tundish induction heatingFrom H. Nakato, Kawasaki Steel, 1986Electroma
30、gnetic nozzleFrom K. Ayata, ISIJ, 1995 At present, electromagnetic stirring technology was popularly used in the steel mills.Schematic of EMS apparatusSchematic of EMS apparatusFVM model for EMS Lorentz Force Vectors at the cross-section of mold However, the additional equipment investment of the EM
31、S should be included together with the following huge maintenance cost for daily production. To address the difficulties above, a quad-furcated swirling flow nozzle (SFN) has been designed and computationally proved to be suitable to both the bloom and big round continuous casting process5.5 Haibo S
32、un and Jiaquan Zhang, ISIJ, 51(10), 2011 The contents in this part include:1.Based on the coupled model, the performance of the conventional nozzle, SFN and M-EMS for the both bloom and big round casting will be evaluated.2.Plant trials were carried out to reveal the effect of the swirling flow nozz
33、le on the strand internal quality.Manufacture and Adoption of SFN ParametersType of nozzleInner diameter, mExternal diameter, mOutlet height, mOutlet width, mOutlet angle,Normal nozzle0.040.075-Swirling flow nozzle (SFN)0.040.0750.030.01815Scheme 1Scheme 2Cross section of bloom castings, mm2260300 D
34、iameter of round bloom castings, mm350Superheat, K40Superheat, K21Steel gradeCM490Steel gradeA105M-EMSONM-EMSOFFF-EMSONF-EMSONGeometric parameters of the two NozzlesCasting test plans for the two NozzlesSteel gradeChemical composition(wt%)CSiMnPSCrCuNiA1050.20.31.00.020.020.20.250.25CM4900.20.351.50
35、.0150.0150.250.250.25Chemical composition of the steel grade employed in the plant trialsSwirling flow nozzle(a) and its inner manufacture mould(b)Results and DiscussionA. Fluid flowMelt flow pattern on the central symmetry plane of strand under the four different casting cases Distributions of solu
36、te element C on the central symmetry plane of strand under the four different casting casesB. Solute DistributionComparison between measured and simulated segregation profiles of C in the region of initial solidification shell with thickness of 25 mmC. Solidification and heat transfer9.89.815.515.51
37、7.317.3Distributions of liquid fraction on the central symmetry plane of strand under the four different casting casesVariation of shell thickness along with the casting directionCenter melt superheat variation along with the casting directionD. Plant trailsComparison between the morphologies of the
38、 as-cast round blooms cross section under the casting cases of normal nozzle (a) and SFN (b)Comparison between the morphologies of the as-cast blooms longitudinal section under the casting cases of normal nozzle with M-EMS (a) and SFN with M-EMS (b) To quantitatively describe the macrosegregation be
39、havior under the four different casting cases mentioned above,Locations of drilling samples at the cross section of round bloom casting (a) and longitudinal section of bloom casting (b)Segregation profiles of C and S along the cross section of round bloom (a) and bloom (a) castings1. The solute segr
40、egation degree transition from positive to negative value with distance from strand surface can be attributed to the circumfluence and the floatation of solute-rich molten steel at the upper part of mould.2. Fluctuated segregation profile appears at the strand cross section due to the circulation fl
41、ows gathered at the solidification front.3. The segregation degree of solute element C at the strand center for the given high carbon steel, GCr15, is increased from 1.0 to 1.24, while melt liquidus temperature is reduced by a total of 23.27 K during the CC process.4. The discontinuous non-frozen ba
42、nd induced by the zigzag solute distribution at the center part of strand cross section is the main reason that leads to the formation of porosity in the casting.Conclusions5. Compared with the normal nozzle, the superheat degree of molten steel at the mould exit is reduced by 9.8 K, 15.5 K and 17.3
43、 K under the other three casting cases respectively, which means the horizontal swirling flow generated by the M-EMS and/or SFN is beneficial to the superheat dissipation of molten steel in the mould region.6. Adoption of SFN and M-EMS can inhibit the shell growth at varying degrees in mould zone as
44、 compared to the normal nozzle.7. As compared to the normal nozzle, the centerline porosity has been eliminated along with the popular radial crack at the strand cross section when adopting the SFN.8. The implementation of SFN can achieve a similar metallurgical effect of M-EMS, and can further promote the performance of M-EMS.Thanks for attention！
