Reseach Of Fluoride-Free And Titanium-Bearing Mold Fluxes

Fluoride has great influences on the viscosity, solidifcation temperature and the crystallization behaviour of slag film in the mold flux. These characteristics of fluoride are in close touch with the surface quality of the slab. However, in the process of continuous casting(CC), the volatilization and acidification of fluoride ruins the health of humankind and the lives of animals or plants, causes environmental pollution and intensifies the erosion to continuous caster. With the development of environment-friendly metallurgy, it is of great significance to develop fluoride-free continuous casting mold fluxes. It was found that TiO2 can play the same role in the mold fluxes as fluoride. So, It is very important for metallurgists to meet the requirements of continuous casting in the use of titanium-bearing fluoride-free mold fluxes, which is essential to maintain smooth processes and assure the qualities during continuous casting.Under the supports of National Natural Science Foundation Committee, Bao Steel Co. and Chongqing Iron & Steel Co. and concerned companies, it has been investigated in this paper that the important basic theories and applications of titanium-bearing fluoride-free mold fluxes concern with the development of CC. The research on this subject is composed of the following factors: the effect of chemical components on physical chemistry properties of titanium-bearing fluoride-free mold fluxes; crystallizing behaviors for titanium-bearing fluoride-free mold fluxes; the effect of chemical components on heat transfer performance of slag film of fluoride-free and titanium-bearing mold fluxes etc. The commercial application revealed that the titanium-bearing fluoride-free mold fluxes, developed in this thesis research, can meet the current needs of continuous casting processes. The specific researches and the main conclusions are listed as follows:(1) The effects of chemical components on melting and flowing properties of titanium-bearing fluoride-free mold fluxes have been deeply studied in this paper. And viscosity estimation model for the titanium-bearing fluoride-free mold fluxes were built.TiO2 and MgO have the functions of increasing melting temperature, while Li2O, Na2O, MnO and B2O3 have the function in decreasing melting temperature of titanium-bearing fluoride-free mold fluxes. Li2O and B2O3 plays significant role in decreasing the melting temperature. With the increase of basicity, the melting temperature increases.When the content of TiO2 is less than 6.0 wt%, an increase in the amount of TiO2 reduces the viscosity of titanium-bearing fluoride-free mold fluxes. However, when the amount of TiO2 is more than 6.0 wt%, viscosity will increase with an increase in the amount of TiO2. With the increase of basicity, viscosity increases. MgO and MnO can decrease the viscosity slightly. Though B2O3 is network former, it can effectively decrease the viscosity.(2) It was found that the crystallizing behavior of the titanium-bearing fluoride-free mold fluxes have bearing on the chemical components.When TiO2 content in titanium-bearing fluoride-free mold fluxes is more than 6.1%, the activation energy for crystallizing from glassy and liquid mold fluxes is less than industrial slag in both conditions. The potential barrier which should be overcome for crystallization is less than industrial slag containing fluorine, that can result in crystallization more easily. So the titanium-bearing fluoride-free mold fluxes has already acheived the level of industrial slag containing fluorine in the respect of activation energy.The increase of basicity and the TiO2, Li2O, Na2O and MgO contents can increase the crystallization temperature of titanium-bearing fluoride-free mold fluxes, while B2O3 can decrease the crystallization temperature. Within the range of 2.0~4.0 wt%, an increase in MnO content increase the crystallization temperature of fluoride-free slag. However, when the MnO content is more than 4.0 wt%, crystallization temperature will decrease with an increase in the amount of MnO.(3) An experimental apparatus for simulating copper mold was designed to directly measure the heat flux density passing through the slag film. It is feasible and steady, and it is an innovation for the measurement method of heat transfer performance of mold fluxes.The heat flux density of titanium-bearing fluoride-free mold fluxes (0.40~0.53MW?m-2) can approach or even be lower than that of the fluoride-bearing industrial slags (0.45~0.51 MW?m-2). So, it is feasible to use fluoride-free mold fluxes and titanium-bearings to replace industrial slag which contain fluorine in the respect of heat transfer control,When the TiO2 content is 0%-6.7%, the heat flux density of fluoride-free mold fluxes is reduced with increasing TiO2 content, and the value is lowest when the content is 6.7%. When the content is higher than 6.7%, the change of heat flux density is not obvious. when the TiO2 content is 1%-6%, with increasing TiO2 content, the crystal in slag film will develop from glass into a large crystal. The precipitated crystal is akermanite (2CaO.MgO.2SiO2). When the TiO2 content is higher than 6%, as the TiO2 content is increased, no large grained akermanite can precipitate from the slag film and the crystal will turn from white granular skeleton crystal into crosswise crystallized minerals. The crystalline phase precipitated from slag film is perovskite.With the increasing of basicity R, the crystallization ratio also increases and, consequently, the heat flux is reduced. When R<1.0, crystal precipitated from the slag film is akermanite. When R>1.0, crystals precipitated from the slag film are akermanite and perovskite, and crystal is precipitated from the primary slag film. It is of advantage to controlling the heat tranfer performance of mold fluxes.(4) In absorbing Al2O3 inclusion, the chemical stability and viscosity-temperature stability of the titanium-bearing fluoride-free mold fluxes have already acheived the level of the slag containing fluoride. However, in absorbing TiO2 inclusion, that of the titanium-bearing fluoride-free mold fluxes have not acheived the level of the slag containing fluoride.It can effectively control the melting rate that the use of graphite and carbon-black. Small grain size carbon is more effective than big grain size carbon in controlling melting rate. The use of superfine graphite can decrease the amount of total carbon, which decreases the carburization of slab and improves the surface quality of slab. The fluoride-free slag film in the mold does not contain TiC.The three innovative point of this paper are listed as follows:â‘ The effects of chemical components on melting and flowing properties of titanium-bearing fluoride-free mold fluxes have been deeply studied in this paper. And viscosity estimation model for the titanium-bearing fluoride-free mold fluxes were built.â‘¡The capacity of crystallization characterized by The activation energy for crystallizing from glassy and liquid mold fluxes in titanium-bearing fluoride-free mold fluxes. And the crystallization behavior of titanium-bearing fluoride-free mold fluxes has been researched.â‘¢An experimental apparatus for simulating copper mold was designed to directly measure the heat flux density passing through the slag film, and it is an innovation for the measurement method of heat transfer performance of mold fluxes. Base on it, the heat transfer performance of fluoride-free and titanium-bearing mold fluxes was investigated. These results of this paper are important to the development of titanium-bearing fluoride-free mold fluxes in China..