Crystallization Products and Structural Characterization of CaO-SiO2-Based Mold Fluxes with Varying Al2O3/SiO2 Ratios

Abstract

During the casting of high aluminum steel, the dramatic increase in the Al2O3/SiO2 ratio is inevitable, resulting in significant changes of the crystallization behavior, which would result in heat transfer and lubrication problems. Crystallization products and structure characterization of glassy CaO-SiO2-based mold fluxes with different Al2O3/SiO2 ratios were experimentally investigated using a differential scanning calorimetry technique and Raman spectroscopy. With increasing Al2O3/SiO2 ratios, the following results were obtained. The crystallization temperature and the crystallization products are changed. With increasing Al2O3/SiO2 ratios from 0.088 to 0.151, the crystallization temperature first increases greatly from 1152 °C to 1354 °C, and then moderately increases. The crystallization ability of the mold flux is strengthened. The species of the precipitated crystalline phase change from two kinds, i.e., Ca4Si2O7F2 and Ca2SiO4, to four kinds, i.e., Ca4Si2O7F2, Ca2SiO4, 2CaO·Al2O3·SiO2 and Ca12Al14O32F2, the crystallization ability of Ca4Si2O7F2 is gradually attenuated, but other species show the opposite trend. The results of Raman spectroscopy indicate that Al3+ mainly acts as a network former by the information of [AlO4]-tetrahedral structural units, which can connect with [SiO4]-tetrahedral by the formation of new bridge oxygen of Al–O–Si linkage, but there is no formation of Al–O–Al linkage. The linkage of Al–O–Si increases and that of Si–O–Si decreases. The polymerization degree of the network and the average number of bridging oxygens decrease. Further, the relatively strong Si–O–Si linkage gradually decreases and the relatively weak Al–O–Si gradually increases. The change of the crystalline phase was interpreted from the phase diagram and structure.

Effect of Cooling Rate on the Structure of CaO–SiO₂–CaF₂-based Glassy Mold Flux

“…6) The physiochemical properties of mold fluxes are closely related to their structural characteristics. According to previous studies, [7][8][9][10][11] the structures of mold fluxes are a good indicator of crystallization and viscosity properties. Zhang et al 10) investigated the effects of basicity and the addition of B 2 O 3 on the structure of a CaO-SiO 2 -based mold flux, and the change of the viscosity was explained from the structural perspective.…”

“…The average number of bridging oxygen atoms was employed to explain the change of the silicate structural network, which was equal to the number of bridging oxygen atoms of each structural unit multiplied by their area proportion. 7,8,28) Figure 4 presents the average number of bridging oxygen atoms of the CaO-Al 2 O 3 -SiO 2 -Na 2 O-CaF 2 mold fluxes at different cooling rates. The average number of bridging oxygen atoms slightly increased with the increase of the cooling rate, as indicated by the increase of the polymerization degree of the silicate structural network with the increase of the cooling rate of the glasses.…”

A Review of the Melt Structure and Crystallization Behavior of Non‐Reactive Mold Flux for the Casting of Advanced High‐Strength Steels

“…With the transformation of CaO-SiO 2 -based slag into CaO-Al 2 O 3 -based slag system, studies have been conducted to elucidate the melt structure difference and structural evolution when SiO 2 is substituted by Al 2 O 3 . Results [29,[31][32][33][34][35][36] ] 4À structure would be destroyed with the help of basic oxides; meanwhile, the linkages of Al─O 0 (O 0 represents the bridging oxygen) and Si─O─Al were constructed, resulted in the reduction of polymerization degree of silicate structure and the increment of aluminate structure. However, the complex aluminate structure with lower bond energy tends to introduce variable crystalline phases, which will be discussed in details in the following Part 3.…”

Crystallization characteristics of B2O3 and TiO2-bearing glassy fluoride-free mold fluxes

“…It helps to lubricate steel shell to prevent sticking against the mold surface and control the heat transfer to achieve uniform shell [1][2][3][4][5][6][7]. The stability of the continuous casting process and the surface quality of products are restricted to good performance of the above functions, which are closely related to the physio-chemical properties of the mold flux, such as viscosity, melting temperature, crystallization behavior [4][5][6][7][8][9][10][11]. And the implementation of the above mentioned functions is bound up with the existence of fluorides in traditional mold fluxes, which plays significant roles in adjusting physio-chemical properties of the mold fluxes.…”