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Fundamental Research Of Low-fluorine/Fluorine-free Mold Fluxes For Casting Medium Carbon Steel

Posted on:2015-01-28Degree:MasterType:Thesis
Country:ChinaCandidate:J WeiFull Text:PDF
GTID:2181330434453508Subject:Metallurgical engineering
Abstract/Summary:PDF Full Text Request
Mold flux is very important in continuous casting process.The melting flux film between the mold and the solidified shell has functions of lubricating steel strand, controling heat transfer process, preventing molten steel from oxidation, and absorbing inclusions. Fluorine, existing in traditional mold fluxes, does a lot of harm to the casting equipments, the environment, and human health, besides, it also increases the cost of pollution treating. Thus, to eliminate fluoride from mold fluxes attracts more and more attention. However, fluoride-removing is a very difficult problem, especially for medium carbon (MC) steels which are sensitive to cracks. In MC steels, many technical problems need to be solved when fluorine is removed. Thus, no F-free MC steel mold flux has ever been successfully used in iron and steel industry. This paper aims to remove fluorine from MC steel, dedicates to solve the key technical problems in this field. Thus, research in this paper is of great theoretical and practical significance.The main physicochemical properties of low-fluorine/fluorine-free fluxes are studied in this paper, including crystallization behavior, melting process, viscosity and heat transfer behavior. In addition, new experimental methods such as DHTT and IET technology are adopted, in order to simulate and study the crystallization and heat transfer processes effectively and accurately.The continuous cooling crystallization and isothermal crystallization behavior of low-fluoride fluxes are studied by SHTT technology. And XRD is also used to detecting the crystalline phase. Besides, the crystallization kinetics data and the crystallization activation energy of several mold fluxes are obtained by calculation. The results show that, B2O3can inhibit crystallization, and Na2O can promote crystallization. The main crystalline phase in sample A is cuspidine (Ca4Si2O7F2), while the main crystalline phase in low-fluoride fluxes is NaCaAlSi2O7. Moreover, for sample B3and D2, cuspidine also precipitates in the low temperature region, as well as wollastonite (CaSiO3) or calcium borosilicate (Ca11Si4B2O22). By using Johnson-Mehl-Avramin(JMA) model and Arrhenius formula, the activation energies are obtained. And the results show that the activation energies of sample A and D2are very close, and crystal growth pattern of these two samples are similar to each other. Therefore, the crystallization properties of low-fluorine flux D2is the closest to sample A.By using SHTT or DHTT technology, IET technology and high temperature viscosity method, physicochemical properties such as crystallization process, melting properties, heat transfer behavior and viscosity are studied in this paper. Also, the crystalline morphology and crystalline phases are detected by SEM and EDS. The results show that, properties of the four F-free mold fluxes are similar to those of sample A. And the best F-free mold flux is sample B, which is of the most similar properties to those of sample A. In addition, SEM and EDS results show that the main crystalline mineral of F-free mold fluxes is boron calcium silicate (Ca11Si4B2O22), and Ca11Si4B2O22crystals precipitate in both low and high temperature regions, besides, Ca14Mg2(SiO4)8crystals also precipitate in F-free mold fluxes. As for sample A, similar to the XRD results of low-fluoride fluxes, the main crystalline phase is cuspidine, which precipitating in both of the high and low temperature regions, besides, some Ca2Al2SiO7crystals also precipitate in sample A.Comparing the results of the low-fluorine mold fluxes and the F-free mold fluxes, conclusion can be made that Ca11Si4B2O22can be used as a new type of fluorine substitution. And in all of the designed low-fluorine/fluorine-free mold fluxes, sample D2and sample B are the two most successfulmold fluxes with similiar properties to those of sample A.
Keywords/Search Tags:low-fluorine mold fluxes, fluorine-free mold fluxes, crystallization properties, melting behaviors, viscosity-temperatureproperties, heat transfer behaviors, crystalline morphology
PDF Full Text Request
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