Nowadays, there are increasing demands for developing mould fluxes without fluoride due to environmental concerns. Slag bearing titanium oxide and/or boron oxide proved to be promising substitute for traditional mould fluxes with fluoride. In the present work, Crystallization behaviors of some slags in Al 2 O 3 -CaO-MgO-Na 2 O-SiO 2 -B 2 O 3 -TiO 2 system were investigated using single hot thermocouple technique (SHTT), differential thermal analysis (DTA), and X-ray diffraction (XRD) technique. It was found that liquidus temperature of slag decreases with increases of B 2 O 3 content. The XRD analysis on the crystallized samples shows that crystallization products are perovskite, gehlenite, and wollastonite. Time-temperature-transformation diagrams (TTT) for various slags have been constructed using SHTT technique. It was found that B 2 O 3 can significantly increase the incubation time for crystallization of slags. Incubation time for crystallization of slags at nose temperature decreases with increase of titanium oxide content from 5% to 10%. The morphology of crystals was also investigated using SHTT technique. At higher temperature, dendrites and columnar crystal tend to form and grow with time progress, while at lower temperature, small crystals appear and the amount of crystal increase with time increasing.

The main problem related to the development of fluorine‐free mould powders for slab casting is effectively controlling the heat transfer between the steel shell and mould. In commercial mould powders crystallization of cuspidine (Ca4Si2O7F2) from mould slag has a great effect on heat‐transfer control. In industrial process the crystallization rate for a fluorine‐free mould slag should be similar to the crystallization rate of cuspidine. To evaluate the crystallization rate for slags time‐temperature‐transformation (TTT) diagrams can be constructed using the Single Hot Thermocouple Technique by in situ observation. In the present work, fundamental information related to crystallization control in the CaO–SiO2–TiO2–Na2O–Al2O3 system was obtained. It was observed that the addition of Na2O in CaO–SiO2–TiO2 slags dramatically shortens the crystals' incubation times in TTT diagrams to the range of seconds. It is possible to control the crystallization kinetics in CaO–SiO2–TiO2 slags by changing the Na2O content. Some observations for the crystals' morphology are reported.

With the single hot thermocouple technique (SHTT) the solidification behavior of metallurgical slags has been studied by in situ observation, constructing timetemperature-transformation (TTT) or continuous-cooling-transformation (CCT) diagrams. The SHTT is a unique apparatus that enables measurement of the slag sample temperature using a thermocouple while the sample is heated or cooled simultaneously. Due to the low heat capacity of the system sample/thermocouple high heating or cooling rates can be easily obtained (>30008C/min). The following findings are reported in the present paper: (i) For the CaO-Al 2 O 3 slag -44% CaO, 56% Al 2 O 3 (wt%) -the CCT diagram shows large differences between liquidus and the temperature for first crystals precipitation, even at low cooling rates, for example, 1688C below the liquidus when cooling at a rate of 68C min À1 . (ii) For the CaO-SiO 2 slag -% CaO/% SiO 2 (wt%) ¼ 0.7 -no crystal is observed for continuous cooling, even at low cooling rates, such as 108C min À1 . During isothermal experiments crystallization was observed only at 10008C with an incubation time of 76 s (average of six experiments, standard deviation 27 s). However, crystallization becomes much more intense for the CaO-SiO 2 slag when increasing the temperature after reaching lower temperatures (<10008C), where probably the conditions for nucleation are better.[ Ã ] J. Figure 11. Viscosity measurements taken using a rotation viscometer, for a slag whose composition is close to that of the CA slag (46.1% CaO, 53.9% Al 2 O 3 ).

The choice of the mould powder for slab casting is a difficult task because mould powders have many important functions during the continuous casting of steel. CaF 2 is a key ingredient in conventional mould slags since it reduces the viscosity, the liquidus temperature and the break temperature. Fluorine in mould powders is undesirable from the environmental and health points of view due to the following reasons: (i) evolves easily from slags, producing health-injurious gaseous substances, such as hydrofluoric acid; (ii) creates problems for storage and utilisation of solid waste and (iii) causes machinery corrosion. Aim of the present work is to describe the development of a new F-free mould powder for low carbon steel slab casting replacing CaF 2 with B 2 O 3 . Laboratory tests and industrial trials were performed considering the technological parameters viscosity, break temperature and crystallisation tendency. From laboratory tests it was concluded that important technological parameters are similar when comparing the Fbearing mould powder (reference) and the new F-free B-bearing mould powder for low carbon steel slab casting: viscosity at 1300°C, break temperature, and crystallisation tendency. It was observed during industrial trials that a significant decrease of the submerged entry nozzle erosion was observed. The results were similar when comparing the F-bearing and the F-free performance: slag pool thickness measurements, melting behaviour, Al 2 O 3 absorption, mould powder consumption and slabs superficial quality.

Efforts have been made to develop fluorine-free mould fluxes for the continuous casting of steel process. In this work the crystallization behaviour of fluorine-free mould slags based on the CaO-Al 2 O 3-B 2 O 3 system was investigated by differential scanning calorimeter (DSC) and scanning electron microscopy equipped with energy dispersive spectroscopy(SEM-EDS). The crystallization kinetics for Ca 3 Al 2 O 6 primary crystals was analysed by combining modified Avrami analysis with Friedman isoconversional method. Avrami parameter n is close to 4 for samples with the ratios w(CaO)/w(Al 2 O 3) = 1 and w(CaO)/w(Al 2 O 3) = 1.2, indicating a crystallization mechanism of continuous bulk nucleation and 3D crystal growth. The Avrami parameter n for samples with w(CaO)/w(Al 2 O 3) = 0.9 is close to 3, indicating instantaneous bulk nucleation and 3D crystal growth. The crystallization rate constant is the highest and half crystallization time is the lowest for the samples with w(CaO)/w(Al 2 O 3) = 0.9, indicating the fastest crystallization. In the initial stage, effective activation energies were mainly determined by the undercooling values. In the final stage, kinetic barrier for crystallization could have some influence on crystallization; for the investigated mould fluxes crystallization in the final stage is retarded by increasing w(CaO)/w(Al 2 O 3) ratio. Thus crystallization mechanisms were elucidated and effective activation energy of crystallization for the first crystal which precipitates from melt was determined.

There is a clear trend in industry to eliminate fluorine from metallurgical slags. Regarding the continuous casting process, fluorine in mould powders is undesirable due to the following reasons: (i) evolves easily from slags producing health-injurious gaseous substances like hydrofluoric acid, (ii) creates problems for storage and utilization of solid waste, and (iii) causes machinery corrosion. A collaboration among universities, mould powder supplier, and steel producer was established, aiming for the development of a new F-free mould powder for billet casting. Firstly, technological parameters for a particular composition were determined: melting characteristics, viscosity, melting behavior, and crystallization tendency. Afterwards, industrial trials were performed in a Brazilian steelworks, doing some evaluations during the continuous casting process, such as DT between water cooling inlet and outlet, slag pool thickness, melting performance, powder consumption, Al 2 O 3 pick-up, and effect on SEN erosion. Comparable surface quality for the billets was obtained when using the F-free mould powder, since the amount of scrap produced during the rolling process was not critical.

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It is necessary to study crystallization from glass (devitrification) for non-reactive F-free CaO-Al 2 O 3 based mold fluxes; this is especially important for the development of mold fluxes for continuous casting of high aluminum steels. To the knowledge of the authors, there is no report in the literature regarding devitrification of F-free mold fluxes based on the CaO-Al 2 O 3-B 2 O 3-Na 2 O-Li 2 O system. Therefore, crystallization kinetics for particular compositions in this system, with different w(CaO)/w(Al 2 O 3) ratios, was investigated by Differential Scanning Calorimeter, Field-Emission Environmental Scanning Electron Microscopy / Energy Dispersive Spectroscopy, and X-ray Diffraction techniques. The first crystal, which precipitates during heating from glass, is Ca 12 Al 14 O 33 , followed by CaO. For the first crystal, which precipitates as plate-like (2-dimensional), it was found that, when using the Matusita-Sakka model, agreement between the calculated Avrami parameters and the micrographs obtained from electron microscope was reached. In the same way, agreement was found for the second event-CaO precipitation-which grows 2-dimensionally or 3-dimensionally, depending on the w(CaO)/w(Al 2 O 3) ratio. The most important event (in terms of energy liberated and amount of crystals) is Ca 12 Al 14 O 33 precipitation. For this event, the effective activation energy for crystallization, E G , decreases with the increase of w(CaO)/ w(Al 2 O 3) ratio. The activation energy for crystallization reflects the energy barrier for crystallization. Thus, it can be concluded that mold fluxes crystallization during heating is enhanced when increasing the w(CaO)/w(Al 2 O 3) ratio, for constant contents of B 2 O 3 , Na 2 O, and Li 2 O.