Crystallization Control in Metallurgical Slags Using the Single Hot Thermocouple Technique

Abstract

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 ).

Influence of structure and temperature on the thermal conductivity of molten CaO–B2O3

“…8,13,15 In particular, Na 2 O and B 2 O 3 lead to a reduction in viscosity, breaking temperature, and liquidus temperature, which is similar to the effect of CaF 2 . CaF 2 is used to decrease the liquidus temperature and viscosity of the mold flux.…”

“…In addition, it promotes the formation of the crystalline cuspidine (Ca 4 Si 2 O 7 F 2 ) phase, which results in a drastic reduction in radiation heat transfer. 13 8 phases can be easily crystallized at high temperature. Similar to the effect of the cuspidine phase in the CaF 2 -based system, their results suggest that the heat transfer in B 2 O 3 -based fluorine-free mold flux can be also controlled via the crystalline phases formed.…”

“…,14 Na 2 O, Li 2 O, and B 2 O3 have been suggested as potential candidates for replacing CaF 2 in the mold flux 8,13,15. In particular, Na 2 O and B 2 O 3 lead to a reduction in viscosity, breaking temperature, and liquidus temperature, which is similar to the effect of CaF 2 .…”

ZrO2 ADDITION IN MOULD POWDERS: EFFECT ON HORIZONTAL HEAT TRANSFER RATE AND SEN EROSION RATE

“…For example, for high speed thin slab casting, mould powder design concentrates on mild cooling i.e. specific values of basicity and break temperature, promoting the crystallisation of cuspidine (3CaO.2SiO 2 .CaF 2 ) [5]. During continuous casting process, the presence of crystals in mould slags has a decisive effect on both the lubrication and the heat transfer rate between the mould and the steel strand [6,7].…”

“…specific values of basicity and break temperature, promoting the crystallisation of cuspidine (3CaO.2SiO 2 .CaF 2 ) [5]. During continuous casting process, the presence of crystals in mould slags has a decisive effect on both the lubrication and the heat transfer rate between the mould and the steel strand [6,7]. The crystallisation of cuspidine from mould slag is thought to be the most effective way of exerting horizontal heat transfer control.…”