The Influence of Na2O on the Solidification and Crystallization Behavior of CaO-SiO2-Al2O3-Based Mold Flux

Abstract

The reaction between [Al] and SiO 2 sharply increased the Al 2 O 3 and decreased SiO 2 contents in mold flux during the continuous casting of high-Al steels. These changes converted original CaO-SiO 2 -based flux into CaO-SiO 2 -Al 2 O 3 -based flux, promoting the crystallization and deteriorating the mold lubrication. Therefore, study on the solidification and crystallization behavior of CaO-SiO 2 -Al 2 O 3 -based mold flux, with the applicable fluidizers, is of importance. The effect of Na 2 O, predominantly used as the fluidizer in mold flux, on the solidification and crystallization behavior of CaO-SiO 2 -Al 2 O 3 -based mold flux needs to be investigated. In this study, a CaO-SiO 2 -Al 2 O 3 -based mold flux containing 6.5 wt pct Li 2 O was designed; the effect of Na 2 O on the solidification and crystallization behavior of these mold fluxes was investigated using the single hot thermocouple technique (SHTT) and the double hot thermocouple technique (DHTT). Moreover, the slag film obtained by a heat flux simulator was analyzed using X-ray diffraction (XRD). The results indicate that the solid fraction of molten slag (Fs) and the crystalline fraction of solid slag (Fc) in the mold slag films decrease with increasing Na 2 O content from 0 to 2 wt pct. However, Fs and Fc increased when the Na 2 O content increased from 2 to 6 wt pct. The critical cooling rates initially decreases and then increases with increasing Na 2 O content. The XRD analysis results show that LiAlO 2 and CaF 2 were the basic crystals for all the mold fluxes. Increasing the Na 2 O content both inhibits the Ca 2 Al 2 SiO 7 formation and promotes the production of Ca 12 Al 14 O 33 , indicating that the mold lubrication deteriorated because of the high melting-point phase formation of Ca 2 Al 2 SiO 7 in the CaO-SiO 2 -Al 2 O 3 -based mold flux containing 6.5 wt pct Li 2 O, without Na 2 O. The strong crystallization tendency also deteriorated the mold lubrication for the mold flux with a higher Na 2 O content. Therefore, the addition of Na 2 O was less than 2 wt pct in the CaO-SiO 2 -Al 2 O 3 -based mold flux to suppress crystallization and enhance mold lubrication.

Effect of Al Speciation on the Structure of High‐Al Steels Mold Fluxes Containing Fluoride

“…This means that microstructure of the mold fluxes containing higher content Al 2 O 3 can easily change and crystallize under proper temperatures if compared with CaO–SiO 2 mold fluxes. This could be an important cause of why mold fluxes containing high content Al 2 O 3 have a shorter incubation time of crystallization and stronger crystallization tendency that is easy to deteriorate the mold lubrication . Hence, the structural characteristics of high‐Al steels mold fluxes must be considered during the design process, which could adjust the performance to meet the demands of high‐Al steels casting.…”

“…Recently, advanced high‐Al steels, including the twin‐induced plasticity and transformation‐induced plasticity steels, have gained tremendous interests due to their high performances, such as the extraordinary ductilities and the high tensile strengths. These new grades of steel are characterized by the presence of high concentrations of Al (0.5%–2.5%) in the molten steel that yields improved mechanical properties and decreased weights of the final steel products . However, alloying elements‐based Al usually react with the conventional CaO–SiO 2 mold flux to induce mold fluxes with contents poor in SiO 2 and rich in Al 2 O 3, hence converting the original CaO–SiO 2 mold fluxes into CaO–SiO 2 –Al 2 O 3 mold fluxes.…”

“…However, alloying elements‐based Al usually react with the conventional CaO–SiO 2 mold flux to induce mold fluxes with contents poor in SiO 2 and rich in Al 2 O 3, hence converting the original CaO–SiO 2 mold fluxes into CaO–SiO 2 –Al 2 O 3 mold fluxes. The change in the physical and chemical characteristics of the mold flux induced by this conversion can cause instability in the casting process and result in products with low surface quality . As a result, it is necessary to design appropriate mold fluxes for the casting of high‐Al steels.…”

“…These new grades of steel are characterized by the presence of high concentrations of Al (0.5%-2.5%) in the molten steel that yields improved mechanical properties and decreased weights of the final steel products. [4][5][6][7][8] However, alloying elements-based Al usually react with the conventional CaO-SiO 2 mold flux to induce mold fluxes with contents poor in SiO 2 and rich in Al 2 O 3, 3 hence converting the original CaO-SiO 2 mold fluxes into CaO-SiO 2 -Al 2 O 3 mold fluxes. The change in the physical and chemical characteristics of the mold flux induced by this conversion can cause instability in the casting process and result in products with low surface quality.…”

Effect of Shear Stress on Isothermal Crystallization Behavior of CaO-Al2O3-SiO2-Na2O-CaF2 Slags

“…The temperature was maintained for 5 min to observe whether crystals could form in the molten slags. Some researchers [21] showed that the residence time of mold powders in the mold was approximately 5 min.…”