Effects of Fluorine on Solidification, Viscosity, Structure, and Heat Transfer of CaO-Al 2 O 3 -Based Mold Fluxes
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ED plasticity (TWIP) steel attracts great attention in the automobile industry owing to its superior mechanical properties.[1–3] Aside from a high amount of manganese addition, TWIP steel normally contains aluminum to reduce the weight of automobile and stabilize the deformation twinning in austenite.[4] However, a high concentration of aluminum in the steel arouses the intense reaction between the steel and SiO2 in the conventional CaO-SiO2-based mold fluxes[5–8]: 4½Al þ 3ðSiO2 Þ ! 2ðAl2 O3 Þ þ 3½Si
½1
It significantly changes the chemical composition of mold fluxes and, therefore, their in-mold performances in the course of continuous casting, which substantially increases the likelihood of surface defects and sticker breakout.[8] To alleviate the chemical reaction between [Al] and SiO2 at high temperature, CaO-Al2O3-based mold fluxes with a limited addition of SiO2 were proposed.[8–12] The addition of fluxing agents is essential to reduce the viscosity and liquidus temperature of CaO-Al2O3-based
JIAN YANG, JIANQIANG ZHANG, and OLEG OSTROVSKI are with the School of Materials Science and Engineering, University of New South Wales, Sydney, NSW, 2052, Australia. Contact e-mail: [email protected] CHEN ZHANG and DEXIANG CAI are with the Baosteel Group Corporation Research Institute, Shanghai, 201900, P.R. China. Manuscript submitted January 21, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
mold fluxes. However, the typical fluxing agents, e.g. B2O3 and Na2O, also react with [Al] at high temperature.[8–10] The reduction of fluxing agents would increase the flux viscosity and liquidus temperature, deteriorating the lubrication in the mold as casting proceeds. CaF2 is a common component in the CaO-SiO2-based mold fluxes.[13–16] It effectively lowers the flux viscosity through the modification on silicate structure.[13–15] The formation of cuspidine (Ca4Si2O7F2) is also found effective in the control of horizontal heat transfer between the copper mold and solidifying steel. In terms of the chemical stability in an [Al]-rich environment, CaF2 is an ideal component in the CaO-Al2O3-based mold fluxes. However, the lack of SiO2 limits the precipitation of cuspidine, especially in the abundance of Al2O3. In this way, the effect of fluorides on solidification and heat transfer of CaO-Al2O3-based mold fluxes is different from that in the CaO-SiO2-based system. In this study, the effects of CaF2 on the solidification behavior, structure and heat transfer of CaO-Al2O3-B2O3-SiO2-MgO-Li2O mold fluxes were investigated, which is necessary for the development of the CaO-Al2O3-based mold fluxes.
II.
EXPERIMENTAL METHODS
A. Sample Preparation CaO-Al2O3-B2O3-SiO2-MgO-Li2O-CaF2 mold fluxes were prepared from reagent-grade CaCO3 (‡ 99.0 pct), SiO2 (‡ 99.0 pct), Al2O3 (‡ 98.0 pct), B2O3 (‡ 98.0 pct), MgO (‡ 98.0 pct), Li2CO3 (‡ 98.0 pct), and CaF2
(‡ 98.5 pct). The pre-mixed reagents were melted at 1823 K (1550 C) in a graphite crucible for 600 seconds, during which carbonates were decomposed to corresponding oxides, and then quenched in distille
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