Mold Fluxes for High-Al Non-magnetic Steel

Name: Mold Fluxes for High-Al Non-magnetic Steel
Diagram No.: 983
Type of diagram: CCT
Chemical composition in weight %: See the table
Group: Slags
Note: CCT diagrams of mold fluxes (a) R2, (b) R4, (c) R41, (d) R43.
SHTT was employed to study and compare crystallization characteristics of CaO–SiO2 based and CaO–BaO–SiO2 based mold fluxes for high-Al steels at various cooling rate by constructing CCT diagrams. Figures (a) through (d) show the CCT diagrams of CaO–SiO2 based mold fluxes R2 and R4, and CaO–BaO–SiO2 based mold fluxes R41 and R43. It could be seen that mold fluxes R2 exhibits no crystallization even though at low cooling rate from 0.1–0.5°C/s. Mold fluxes R4, R41 and R43 show various crystallization at different cooling rate. The crystallization temperature decreases with the increasing cooling rate which may be due to the fact the crystallization is dependent on the crystals nucleation and growth rate which is the function of thermodynamics and kinetics. Some researchers considered that increase in viscosity increases diffusion resistance of ions, and hence restrains nucleation and crystal growth. But in the current study, although increasing basicity strengthens crystallization tendency of mold fluxes by depolymerizing the network structure, increase in Al2O3/SiO2 increases viscosity of mold fluxes according to authors’ measurements, that is to say viscosity is not the dominated factor in controlling crystallization in this CaO–SiO2 system with high Al2O3/SiO2. Increasing Al2O3/SiO2 causes increase in liquidus temperature and correspondingly undercooling. It can be concluded high undercooling may promote crystallization. But for mold fluxes with fixed and high Al2O3/ SiO2, increasing BaO/CaO causes increase in viscosity and decrease in liquidus temperature, thus viscosity may play dominated role in controlling crystallization of flux system with increasing substitution of CaO with BaO. More time are also required for nucleation and crystal growth at increasing cooling rate.
Mold flux R2 does not express crystallization behavior at the cooling rate range 0.1–0.5°C/s, while R4 with high Al2O3/SiO2 shows the highest crystallization temperature 1246°C at the cooling rate 0.5°C/s. Mold flux R41 with BaO/CaO=0.33 shows a slightly lower crystallization temperature at 1218°C, obtained using a cooling rate of 0.5°C/s. With the increase of BaO/CaO, mold flux R43 with BaO/CaO=3 shows a larger decrease of crystallization temperature to 1083°C, at the same cooling rate 0.5°C/s. Critical cooling rate is a crucial parameter which reflects crystallization ability of mold fluxes. Larger critical cooling rate indicates strong crystallization ability. Mold fluxes could not crystallize when the cooling rate is larger than the critical cooling rate, which is 10°C/s for mold fluxes R4. The critical cooling rate of R41 and R43 are 4°C/s and 0.5°C/s respectively, which are far less than that of R4.
Reference: Not shown in this demo version.

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