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

Abstract

To design suitable mold fluxes for the casting of high‐Al steels, the structure of mold fluxes based on CaO–SiO2, CaO–SiO2–Al2O3, and CaO–Al2O3 was examined by Raman spectroscopy and magic‐angle spinning nuclear magnetic resonance. The results showed that Si atoms are replaced by Al atoms as the network formers with the increase in Al2O3 in the mold fluxes. This converts the silicate slags (CaO–SiO2 mold fluxes) into aluminosilicates slags (CaO–SiO2–Al2O3 or CaO–Al2O3 mold fluxes). The F− ions in the mold flux containing Al2O3 are classified into three categories, according to function: Bridging F's, Nonbridging F's, and Free‐F's. The Al3+ ion holds three distinct coordination environments: IVAl, VAl, and VIAl. The addition of F affects the coordination environment of Al3+ to form AlO3F and AlO2F2 that accommodate the network structure of slags. The network structure in the CaO–SiO2 mold fluxes is mainly connected through Si–O–Si linkage. However, the network structure of the mold fluxes containing elevated content of Al2O3 is mainly connected through Si–O–Si, Al–O–Al, Al–O–Si, and Al–F–Al linkages. Hence, the structural characteristics of high‐Al steels mold fluxes must be considered during the designing step of the mold fluxes.

Effect of substituting CaO with BaO on the viscosity and structure of CaO‐BaO‐SiO2‐MgO‐Al2O3 slags

“…With the modern industry continuously developing, the demand for stronger, lighter, and formable steels has increased. Over the past few years, transformation‐induced plasticity (TRIP) steels have gained immense attention owing to their excellent strength and ductility . However, a large quantity of alloying elements such as Al are added to these steels.…”

“…Over the past few years, transformationinduced plasticity (TRIP) steels have gained immense attention owing to their excellent strength and ductility. 1,2 However, a large quantity of alloying elements such as Al are added to these steels. This dissolved Al reacts with the silica in CaO-SiO 2 -based mold fluxes and converts them into CaO-SiO 2 -Al 2 O 3 -based fluxes, thereby drastically altering their thermophysical properties such as viscosity, wettability, and heat transfer.…”

Experimental determination of phase equilibria in the CaO‐BaO‐SiO2‐12 mol pct. Al2O3‐13 mol pct. MgO system at 1573 K and 1623 K

“…The bond parameter, X p × Z/R k , can be used to identify the acidic and basic tendency of cations in the slag, where X p , Z , and R k (pm) correspond to the respective electronegativities of the elements, the valence state, and the ionic radius. The calculated X p × Z/R k values of the cations are given in Table , where lower X p × Z/R k values pertain to the higher basic tendency cations appearing as network modifiers, that is, octahedral structural units, while higher X p × Z/R k values pertain to the higher acidic tendency cations appearing as network formers, that is, tetrahedral structural units . The lower activity of Ca 2+ and the limited mass transfer rate caused by higher viscosities at reduced (C + B)/S ratios with a higher degree of polymerization of the slags may facilitate Ba 2+ cations with the strongest basic tendency to substitute Ca 2+ and bond with cations with acidic tendency in order to form Ba‐containing phases.…”

“…Continuous casting mold fluxes play several essential roles during solidification of steels. These include lubrication between the copper mold wall and the partially solidified steel shell, prevention of oxidation of the molten steel from the atmosphere, reduction in vertical heat losses, and absorption of nonmetallic inclusions . These aspects of mold fluxes require a fundamental understanding of the thermo‐physical properties, where the phase diagram is essential to predict the crystallization path and control the crystallization behavior .…”

On the Modification of Hydration Resistance of CaO with ZrO2 Additive

“…The CaZrO 3 particles attached to the surface reduce the moisture to penetrate through the pores because that CaZrO 3 is inert toward water. 19 In Fig. 8, the red areas indicated the exposed CaO particles.…”