Study Of The Crystallization And Heat Transfer Behaviors Of Mold Flux During Continuous Casting

The demand of various steels is keeping going due to the development of economic and society. Meanwhile, certain steel requires particular mold flux that is compatible with its casting process. Crystallization and heat transfer of mold flux are considered to be the two crucial factors which affect the continuous casting process. So, investagtiton the behaviors of mold flux crystallization and heat transfer; study the influence of some key factors on them; as well as finding the typical three layers (Liquid layer, crystalline layer and glassy layer) distribution in the gap between mold wall and shell by using visualization technologies are significant to design and develop new high-performance mold flux.Therefore, in this paper, first, the effect of basicity (R) on isothermal crystallization, continuous cooling crystallization and heat transfer of mold flux were studied by using Single Hot Thermocouple Technique (SHTT) and Infrared Emitter Technique (IET). And, the kinetic analysis of mold flux isothermal crystallization process was also conducted. Results show that:1) The TTT (Temperature-Time-Transformation) curve of R=0.8mold flux is of single’C type, and the main precipitated phase is Ca4Si2O7F2; while, the TTT curves of R=1.0,1.1and1.2are of double ’C type, and the crystalline phases of low and high temperature zone are Ca4Si2O7F2and CaOSiO2respectively; Also, the crystallization temperature of mold flux increases with the increasing of basicity.2) From the CCT (Continuous-Cooling-Transformation) diagrams, it can be found that the critical cooling rate of mold flux increases with the increasing of basicity.3) The kinetic analysis suggests that the mechanism of isothermal crystallization change from constant number of nuclei nucleation,1-dimensional growth of R=0.8mold flux to constant number of nuclei nucleation,3-dimensional growth of R=1.2mold flux; and the activation energy of crystallization decreases with the increasing of basicity which indicates that the higher basicity can enhance the crystallization property of mold flux.4) From the heat transfer experiments, it can be known that the steady state heat flux decreases, with the increase of basicity due to the increase of interface thermal resistance, and the thickness and crystalline fraction of mold flux disks increases as well.Then, the effect of A12O3on crystallization and heat transfer of mold flux were also investigated by SHTT and IET. Results show that:1) Besides the A12O3content7%mold flux, the TTT curves of other three mold flux are of double’C type. Further more, cuspidine and NaAlSiO4occurred in the low temperature zone, and the crystalline phases are NaAlSiO4and CaF2in high temperature zone when the A12O3content of mold fluxes are20%and30%. When the A12O3content is40%, the gehlenite (Ca2Al2Si07) precipitated in the high temperature zone.2) It can be seen from the CCT curves that the critical cooling rate increase first, and then decrease with the increasing of A12O3content.3) The results of kinetic analysis suggest that, the mechanism of isothermal crystallization is constant number of nuclei nucleation,1-dimensional growth when the A12O3content are7%and20%; and it belongs to constant number of nuclei nucleation,2to3dimensional growth, and1to2dimensional growth respectively when the A12O3content are30%and40%. The activation energy of crystallization decreases first, and then increases with increasing of A12O3content, which indicated that the Al2O3is amphoteric to the crystallization property of mold flux.4) The heat flux in the stable stage reduces first, then increases, the thickness and crystalline fraction of mold flux disk increases first, and then decreases when the A12O3content increases, in the heat transfer experiments.Next, the three layers distributions in low and medium carbon steel mold fluxes were in-situ observed and studied by using Double Hot Thermocouple Technique (DHTT). The morphology and composition of crystalline precipitation were analyzed by using SEM and EDS. Results show that,1) The glassy layer forms first in low carbons steel mold flux when the CH-1cooling down, then lots of fine crystal precipitated from the glassy mold flux, and large dispersive crystals precipitated from the liquid mold flux; while The large crystals precipitated directly from the liquid medium carbon steel mold flux, during the cooling process of CH-1. When the three layers become steady state, there still about0.2mm residual liquid layer in the high temperature side and crystalline layer in the middle in both of low and medium carbon steel mold flux, however, in the low temperature side, there is glassy layer in low carbon steel mold flux but medium carbons steel mold flux did not have.2) From the analysis of SEM and EDS, it can be found that the whole low carbon steel mold flux is constituted by two glassy phase zones and two crystalline phase zones. Both of crystals formed in glassy and liquid mold flux are dendrite Ca4Si2O7F2, but the size of the later is much larger than the former; and the crystal formed in low temperature zone is large Ca4Si2O7F2, the crystal precipitates from high temperature zone is isometric crystal Ca2Al2Si07.3) The30℃/s cooling curve intersect the initial crystallization curve of low carbon steel mold flux in TTT diagram, but do not intersect the initial crystallization curve of medium carbon steel mold flux, which can be used to explain the reason why the glassy layer formed in low carbon steel, while crystalline layer formed in medium carbon steel mold flux close to low temperature area in DHTT test.The last, the numerical model of heat transfer, fluid flow and crystal of low carbon steel mold flux DHTT experiment, based on the finite volume element method, was established by using the computational fluid dynamics (CFD) software Fluent. And, this model was validated by comparing the simulation results with the observations in DHTT experiment. From the simulation work, it can be found that,1) The crystallization of mold flux is affected by temperature distribution in mold flux, meantime, the formation of crystalline layer also has huge influence on the temperature distribution.2) The temperature in the middle of mold flux is obviously lower than the CH-1and CH-1although both of those two thermocouples were kept constant. This kind of phenomena should be noticed by researchers because the non-uniform temperature distribution will bring errors in SHTT and DHTT tests.3) The liquid mold flux flows from high temperature zone to low temperature zone on the surface of mold flux, while it flow back from the low temperature zone to high temperature through the interior, under the influence of Marangoni effect and nature convection.4) The Marangoni effect is the main reason which causes the broken and movement of crystal in mold flux, and most of observed movement of crystal occur in the interior of mold flux.5) Crystal layer precipitates in the middle of mold flux first, then grows toward both high and low temperature zone, which is match pretty well with the observation of DHTT experiment.This dissertation contains122figures,21tables and163references.