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.

1. Introduction

Due to high strength and good ductility properties, transformation-induced plasticity (TRIP) steel has attracted wide attention for potential applications in automotives [1]. The addition of Al to TRIP steel can contribute to the lightweight of automobiles [2]. However, since the content of [Al] in TRIP steel is much higher than that in ordinary plain carbon steel, it may also cause many problems in practical production [3]. One of the biggest problems is the dramatic change of the chemical composition of the mold flux, especially the Al₂O₃/SiO₂ ratio due to the slag-metal reaction between [Al] in molten steels and SiO₂ in CaO-SiO₂-based mold fluxes, as shown in Equation (1) [3]. The increase of Al₂O₃/SiO₂ ratio will significantly modify the physicochemical properties of mold flux such as crystallization, viscosity and melting behavior and would result in problems to control continuous casting [3] because the slab quality is highly related to the physicochemical properties of the flux.

$${4\left[\mathrm{Al}\right]+3\mathrm{SiO}}_2{=2\mathrm{Al}}_2{\mathrm{O}}_3+3\left[\mathrm{Si}\right]$$

(1)

Crystallization is one of the most important factors determining the heat transfer and lubrication behavior which directly affect the quality of the final steel slabs. The change of crystallization behavior may lead to uneven heat transfer and insufficient lubrication. Cuspidine ($3\mathrm{CaO}\cdot 2{\mathrm{SiO}}_2\cdot {\mathrm{CaF}}_2$), the main crystalline phase of traditional CaO-SiO₂-based mold flux, is widely deemed as an optimal crystal to control the heat transfer [4,5,6]. However, during the continuous casting of high aluminum steel, the dramatic increase of the Al₂O₃/SiO₂ ratio is inevitable, resulting in significant changes of the crystallization behavior, which would result in heat transfer and lubrication problems. Therefore, it is necessary to identify how the Al₂O₃/SiO₂ ratio affects the change of the crystallization products. Zhang et al. [7] investigated crystallization with different Al₂O₃/SiO₂ ratios (0.25, 0.75, 1.5, 0.4, mass%) using confocal scanning laser microscopy (CSLM) and found that the crystallization temperature, precipitated phase and crystal morphology changed dramatically. However, their study included only limited Al₂O₃/SiO₂ ratios.

Crystallization behavior of slags is usually related to the variation of structure [8]. Therefore, the study of the structure will benefit a better analysis of the crystallization of slags. Many techniques have been employed to investigate the slag structure, such as Raman spectroscopy [9], nuclear magnetic resonance (NMR) [9], and infrared characteristic absorption spectrum (IR) [10], etc. Previous studies mainly focused on the structure of the mold flux, whereas very few studies were related to its correlation with the crystallization variation caused by the change of the Al₂O₃/SiO₂ ratio. Cui et al. [10] studied the effect of the SiO₂/Al₂O₃ ratio on blast furnace slag by an infrared characteristic absorption spectrum and showed that the silicates mainly exist in [SiO₄]-tetrahedra, while the aluminum atoms are in different coordination states, and the bonding strengths rise with increasing SiO₂/Al₂O₃ ratio. Liao et al. [11] studied the effect of the Al₂O₃/SiO₂ ratio (varied from 0.11 to 0.8) on the structure of CaO-SiO₂-MgO-Al₂O₃ slag and found that the degree of polymerization for [SiO₄]-tetrahedra decreases with increasing Al₂O₃/SiO₂ ratio, based on Fourier Transformation-Infrared Spectroscopy (FT-IR) and Raman spectroscopy. In order to better understand the change of crystallization products, it is necessary to study the dependence of structure and crystallization on various Al₂O₃/SiO₂ ratios of mold fluxes.

In the current work, the change of crystallization products with a variation of the Al₂O₃/SiO₂ ratio was investigated using a differential scanning calorimeter (DSC) combined with X-ray diffraction (XRD) and scanning electronic microscopy (SEM). In addition, the Raman technique was applied to study the structure of the glassy mold fluxes. The correlation between the crystallization and structure of the CaO-SiO₂-based mold fluxes is discussed.

2. Materials and Methods

2.1. Sample Preparation

Powder mold fluxes were designed with different Al₂O₃/SiO₂ ratios based on the chemical composition change during the casting process of high Al steel. Five samples were used, and their chemical compositions in weight% are listed in

Table 1. The chemical compositions of CaO-SiO₂-based mold flux with the variation of the Al₂O₃/SiO₂ ratio.

Sample No.	Composition (mass%/mole%)
	CaO	SiO2	Al2O3	CaF2	Na2O	Al2O3/SiO2
1	38.0/41.9	34.0/35.1	5.0/3.1	15.0/11.9	8.0/8.0	0.147/0.088
2	38.0/42.5	31.0/32.4	8.0/4.9	15.0/12.1	8.0/8.1	0.258/0.151
3	38.0/43.3	27.0/28.7	12.0/7.5	15.0/12.3	8.0/8.2	0.444/0.261
4	38.0/43.9	24.0/25.9	15.0/9.5	15.0/12.4	8.0/8.3	0.625/0.367
5	38.0/44.7	20.0/21.9	19.0/12.3	15.0/12.6	8.0/8.5	0.950/0.562

. Reagent grade CaCO₃, CaF₂, Na₂CO₃, SiO₂ and Al₂O₃ were used to prepare the synthetic mold fluxes. To obtain high purity of CaO, the CaCO₃ was calcined at 1373 K for 7 h in a muffle furnace to obtain CaO identified by the fore and aft weightlessness.

The glassy samples were prepared by regular melting and quenching methods. The samples were first put into a platinum crucible and melted in a high temperature tube furnace with a heating element in molybdenum silicide at 1723 K (1450 °C), and then, the molten mold fluxes were quenched in water to form glasses as identified by XRD in Figure 1.

2.2. Differential Scanning Calorimetry Analysis

The glassy mold fluxes were ground into powders with a size less than 250 μm in diameter and were put into platinum crucibles for measurement by differential scanning calorimetry (DSC). The DSC measurement was performed in an argon atmosphere in the temperature range of 673–1723 K (or 400–1450 °C) using a Netzsch DSC404 F3 calorimeter (Netzsch Corporation, Selb, Germany). The cooling rate of all measurements was 5 K/min. α-Al₂O₃ was used as a reference material in the present experiments.

2.3. X-ray Diffraction and Scanning Electron Microscope Analysis

The phases and crystal morphologies of the crystallized mold fluxes were subjected to X-Ray Diffraction (XRD) analysis and a Scanning Electron Microscope equipped with an energy dispersive X-ray spectroscopy (SEM–EDS) microanalyzer. X-ray diffraction experiments were conducted on a 18KW X-ray diffractometer (RIGAKU TTR III, Rigaku Corporation, Tokyo, Japan). The SEM-EDS examinations were carried out using TESCAN VEGA 3 LMH (TESCAN Corporation, Brno, Czech Republic).

2.4. Raman Spectroscopy Analysis

In order to understand the effect of the Al₂O₃/SiO₂ ratio on the structure of CaO-SiO₂-based mold flux, the glassy samples were analyzed using nonpolarized Raman spectroscopy measurements. All samples were recorded in the frequency range of 300–3000 cm⁻¹ at room temperature. The excitation wavelength of 532 nm and a semiconductor laser with power of 1 mW in a micro-Raman spectrometer made in France (LabRAM HR Evolution, HORIBA Jobin Yvon) were used. The frequency range was mainly between 400 and 1500 cm⁻¹ for all samples. The Raman spectra were fitted using peak-fit software by assuming Gaussian functions to obtain more specific structure information. The areas of the deconvoluted peaks were calculated by the software to evaluate the change of the network polymerization of the glassy mold flux.

3. Results and Discussion

3.1. Crystallization Analysis of CaO-SiO₂-Based Mold Fluxes

Figure 2 shows the results of DSC measurements of the CaO-SiO₂-based mold fluxes with varied Al₂O₃/SiO₂ ratios. It is observed that with increasing Al₂O₃/SiO₂ ratio, the number of exothermic peaks on the DSC curves increases. Only one obvious exothermic peak for Sample 1 with Al₂O₃/SiO₂ = 0.147 is detected, and the number of exothermic peaks gradually increases from 1 to 4 with the Al₂O₃/SiO₂ ratio increasing from 0.088 to 0.562, which indicates that crystallization events gradually increase from one to four. Alternatively, the exothermic peaks move towards high temperatures, which suggests that the crystallization temperatures are increased and the crystallization ability of the CaO-SiO₂-based mold flux is enhanced [10]. The specific change in the crystallization temperature is shown in Figure 3. The crystallization temperature first greatly increases from 1152 °C to 1354 °C with the Al₂O₃/SiO₂ mole ratio increasing from 0.088 to 0.151, and then the temperature increases less with the further increase in the Al₂O₃/SiO₂ ratio. This finding indicates that with increasing Al₂O₃/SiO₂ ratio, the crystallization temperature tends to raise, especially at low Al₂O₃/SiO₂ ratios. Therefore, the increase of Al₂O₃/SiO₂ ratio can improve the crystallization ability of the CaO-SiO₂-based mold flux. Similarly, Zhang et al. [7] observed the crystallization behavior in mold slags and found that with increasing Al₂O₃/SiO₂ ratio, the crystallization temperature and crystallization ability increased.

In order to identify the various specific crystallization products, the crystallized mold fluxes were analyzed using XRD and SEM-EDS techniques. The heat treatment experiments were performed to determine the phase precipitation using the same cooling rate (5 K/min) because of the small samples after the DSC measurements for XRD and SEM analysis. XRD analysis of the precipitated crystalline products is shown in Figure 4a–f. From Figure 4a, three obvious changes in the characteristic peaks of XRD can be revealed as labeled in the picture. First, the cuspidine (Ca₄Si₂O₇F₂) crystal precipitated in all the samples and the relative amount of cuspidine decrease with increasing Al₂O₃/SiO₂ ratio. Only cuspidine (Ca₄Si₂O₇F₂) crystal precipitates when Al₂O₃/SiO₂ = 0.147 (CaO/SiO₂ = 1). Previous reports obtained similar results for the precipitated crystalline phase of the traditional mold flux for CaO/SiO₂ = 1 [4,12,13]. In addition, the new crystalline phases Ca₂SiO₄, 2CaO·Al₂O₃·SiO₂ and Ca₁₂Al₁₄O₃₂F₂ gradually precipitate with increasing Al₂O₃/SiO₂ ratio from 0.258 to 0.950. The specific XRD results for each sample are shown in Figure 4b–f. When Al₂O₃ gradually increases and SiO₂ gradually decreases, Ca₂SiO₄ precipitates due to the increase in the CaO/SiO₂ ratio. Watanabe. T et al. [6] investigated the phase equilibria of solid and liquid coexisting 50.8 mass% CaO-38.6 mass% SiO₂-10.6 mass% CaF₂ and also found that the cuspidine and Ca₂SiO₄ co-precipitate. When the Al₂O₃/SiO₂ ratio increases to 0.261, cuspidine, Ca₂SiO₄ and 2CaO·Al₂O₃·SiO₂ co-precipitate. Our previous study on phase relations in CaO-SiO₂-Al₂O₃-15% CaF₂ slags found that 2CaO·Al₂O₃·SiO₂ is produced with an increasing content of Al₂O₃ in the mold flux [13]. When the Al₂O₃/SiO₂ ratio further increases, a new crystalline phase of Ca₁₂Al₁₄O₃₂F₂ precipitates. The precipitated cusipidine gradually decreases, indicating that the content of fluorine and the amount of Al₂O₃ increase in the molten fluxes is caused by the increase in the Al₂O₃/SiO₂ ratio. The increase of fluorine and Al₂O₃ induce the formation of Ca₁₂Al₁₄O₃₂F₂. From the relative intensity of the XRD characteristic peak, it can be found that the crystallization ability of cuspidine decreases with increasing Al₂O₃/SiO₂ ratio, and the other species show the opposite trend.

Block samples were taken for SEM-EDS analysis to determine the phase and crystal morphology. The mold fluxes appeared severely pulverized with the increase in the Al₂O₃ content. When Al₂O₃/SiO₂ ratio increases to 0.261, no block can be formed. Therefore, Samples 1 and 2 were observed by SEM-EDS to identify the phase and crystal morphology. Powder sample No. 5 was also analyzed by SEM-EDS to identify the phase. The results are shown in Figure 5a–c; it can be observed that cuspidine (Ca₄Si₂O₇F₂) has a lath-like or faceted morphology in Samples 1 and 2, and in Sample 2, Ca₂SiO₄ could not be found, perhaps due to the small size and low amount. Guo et al. [14] reported that cuspidine presents the same morphology as in traditional CaO-SiO₂ mold flux. Four types of crystalline phases are identified in the SEM of Sample 5, which is consistent with the XRD results.

It can be concluded from the above results that the species of the precipitated phase change in the following sequence with increasing Al₂O₃/SiO₂ ratio: from Ca₄Si₂O₇F₂ to Ca₂SiO₄ and Ca₄Si₂O₇F₂, and then to Ca₂SiO₄, 2CaO·Al₂O₃·SiO₂ and Ca₄Si₂O₇F₂, and finally to Ca₂SiO₄, 2CaO·Al₂O₃·SiO₂, Ca₄Si₂O₇F₂ andCa₁₂Al₁₄O₃₂F₂. Cuspidine, as the main crystallization product of the traditional CaO-SiO₂-based mold flux, is strongly deemed as the optimal crystal to control the heat transfer and lubrication during casting [4]. However, many new types of Al₂O₃-containing crystals precipitate with increasing Al₂O₃/SiO₂ ratio, resulting from the slag-steel reaction during high aluminum steel casting. It was reported that many horizontal and vertical depressions containing open cracks appeared on the surface of the slabs when using CaO-SiO₂-based mold fluxes during high aluminum steel casting [15]. Our current work focuses on the change of crystallization products caused by the changes in the chemical composition caused by the slag-steel reaction, which contributes to the understanding of high aluminum steel casting problems.

3.2. Structural Analysis of CaO-SiO₂-Based Mold Fluxes

The Raman spectra of the glassy samples with varying Al₂O₃/SiO₂ ratio are presented in Figure 6. It can be seen that with increasing Al₂O₃/SiO₂ ratio, the intensity of the Raman bands first gradually increases at lower frequency between 450–600 cm⁻¹, and then decreases between 600–800 cm⁻¹. According to previous reports [16,17,18], a Raman frequency range between 450–600 cm⁻¹ corresponds to mixed bending and stretching vibration of the Al–O–Si bridge oxygen linkage, and a Raman frequency range between 600–800 cm⁻¹ is a signature of mixed bending and stretching vibration of the Si–O–Si bridge oxygen linkage. Therefore, it can be concluded that the bridge oxygen linkage of Al–O–Si gradually increases and the bridge oxygen linkage of Si–O–Si decreases with increasing Al₂O₃/SiO₂ ratio. In the present system, Al₂O₃ predominantly acts as a network in the formation of [AlO₄]-tetrahedral structural units due to the presence of significant basic oxides such as CaO and Na₂O [17]. In aluminosilicate glasses and melts, [AlO₄] can be stabilized by the proximity of M^2+/+ ions. The M²⁺ or M⁺ ions in excess will destroy the aluminosilicate glasses network structure so as to increase the formation of non-bridging oxygens (NBOs) [19]. In the present mold flux glasses, the value of the (M²⁺ + M⁺)/Al₂O₃ ratio is much higher than 1, so that Al³⁺ mainly forms [AlO₄] to participate in the formation of the silicate network. Based on Loewenstein rules [20], one aspect of the short-range order of framework cations can be expressed as Al avoidance, which postulates that the Al–O–Si linkage is more favorable than the combinations of Si–O–Si and Al–O–Al. That is to say, the Al–O–Si linkage forms primarily in aluminosilicate glasses [20]. This is consistent with the present result: the bridge oxygen linkage of Al–O–Si increases and bridge oxygen linkage of Si–O–Si decreases with increasing Al₂O₃/SiO₂ ratio. Since the mole content of SiO₂ is much higher than that of Al₂O₃ in all samples, [AlO₄] tends to form the Al–O–Si linkage. Therefore, it can be concluded that [AlO₄] and [SiO₄] as network former units form a network structure and two kinds of bridge oxygen linkages (Al–O–Si and Si–O–Si) appear in the investigated mold fluxes. Besides, the amount of Al–O–Si linkage increases and that of Si–O–Si linkage decreases with increasing Al₂O₃/SiO₂ ratio.

It can be observed at a higher frequency range (800–1100 cm⁻¹) that the Raman shift associated with the vibrations of several depolymerized silicate and aluminosilicate units moves toward lower frequency. For the higher frequency band, the Raman spectra was deconvoluted using the Gaussian-Fitting method similar to that used by Mysen et al. [21]. The deconvoluted results are shown in Figure 7a–e. It can be observed that there are four characteristic bands near 850 cm⁻¹, 900 cm⁻¹, 950 cm⁻¹ and 1030 cm⁻¹, which correspond to Q⁰, Q¹, Q² and Q³ with BO/Al = 0, 1, 2, 3, respectively, in the silicate glasses based on the previous references [21,22,23,24]. As shown in Figure 7a–e, it could be found that the full width at half maximum gradually decreases from 136 to 130, 112, 101 and 93 cm⁻¹ with increasing Al₂O₃/SiO₂ ratio, inferring that the glass formation ability was impaired. This can further explain the enhancement of crystallization ability of the CaO-SiO₂-based mold flux with increasing Al₂O₃/SiO₂ ratio.

The proportion of each structure unit can be evaluated by the corresponding integrated areas [4]. Detailed quantitative deconvolution of Raman bands is listed in

Table 2. Deconvolved results of Raman spectra for CaO-SiO₂-based glasses.

Sample No.	Q0	Q1	Q2	Q3	Ni
1	27.10	37.56	18.46	16.88	1.25
2	32.63	35.94	17.70	13.73	1.13
3	37.03	34.43	17.26	11.28	1.03
4	41.62	34.16	13.46	10.77	0.93
5	44.46	33.67	11.86	10.01	0.88

. Q⁰ increases with increasing Al₂O₃/SiO₂ ratio, but Q¹, Q² and Q³ slightly decrease. This means that the higher number of bridge oxygen structure units decreases during the process, indicating that the polymerization degree of the network is reduced. In the present work, the average number of bridging oxygens of each sample (Nⁱ, which can be estimated by the area ratio of each structural units (Qⁿ, n = 0, 1, 2, 3) multiplied by the number of its bridging oxygen, i is the sample number) is used to explain the change of the silicate structural network shown in Figure 8 [4]. It can be observed that the average number of bridging oxygens decreases from 1.25 to 0.88, which may be caused by the decrease of the total molar content of Al₂O₃ and SiO₂.

This change of the structure inevitably changes the crystallization behavior. With increasing Al₂O₃/SiO₂ ratio, the polymerization degree of the network and the average number of bridging oxygen decrease. The bond strength of Al–O (330–422 kJ/g atom) [25] is weaker than the Si–O bond (443 kJ/g atom) [25], which causes relatively weaker connections of the network of mold flux with the increase of the Al₂O₃/SiO₂ ratio. These two reasons together would generate a lower energy barrier for ions transferring from bulk glass to glass-crystal interface during crystallization, leading to increasing crystallization ability with increasing Al₂O₃/SiO₂ ratio. From another perspective, the increased amount of Al–O–Si linkage in molten slag would increase the similarity between the molten slag and crystals containing both Si and Al, which would induce the precipitation of the 2CaO·Al₂O₃·SiO₂ crystal containing both Si and Al from CaO-SiO₂-CaF₂ system. Q¹ is the dominant unit in Sample 1, which is the traditional mold flux in which increasing an Al₂O₃/SiO₂ ratio leads to the decreased amount of Q¹. According to a previous study by Saburi et al. [26], the primary structure unit of cuspidine is Q¹. With increasing Al₂O₃/SiO₂ ratio, the gradual decrease in the amount of Q¹ in CaO-SiO₂-based mold flux would decrease the similarity between the molten slag and cuspidine, so as to reduce the nucleation and growth of cuspidine.

4. Conclusions

During the casting of high aluminum steel, the dramatic increase of 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 structural characterization of glassy CaO-SiO₂-based mold fluxes with different Al₂O₃/SiO₂ ratios were investigated by the DSC technique and Raman spectroscopy. With the increase of the Al₂O₃/SiO₂ ratio in mold fluxes, the conclusions can be summarized as follows.

(1)

The crystallization temperature and the crystallization products have been changed. The crystallization greatly increases from 1152 °C to 1354 °C with the Al₂O₃/SiO₂ ratio changing from 0.147 to 0.258, and then it increases slowly. The crystalline phases are increased from two kinds (Ca₄Si₂O₇F₂ and Ca₂SiO₄) to four kinds (Ca₄Si₂O₇F₂, Ca₂SiO₄, 2CaO·Al₂O₃·SiO₂ and Ca₁₂Al₁₄O₃₂F₂). The crystallization ability of cuspidine decreases, but the other species show the opposite trend.

(2)

Two types of bridge oxygen linkages, i.e., Al–O–Si and Si–O–Si, are formed in CaO-SiO₂-based mold fluxes. The polymerization degree of the network and the average number of bridging oxygens decrease. The relatively strong Si–O–Si linkage gradually decreases and the relatively weak Al–O–Si bond gradually increases, which cause the weaker link of the molten fluxes.

(3)

The gradual increase of the weaker Al–O and the decrease in the amount of the stronger Si–O bond, which causes the relatively weaker connections of the network of the mold flux, give rise to the lower energy barrier for ions transferring from bulk glass to the glass-crystal interface during crystallization. Consequently, the crystallization ability increases.

(4)

The increase in the Al–O–Si linkage in molten slag would increase the similarity between the molten slag and crystals containing both Si and Al, which would induce the precipitation of 2CaO·Al₂O₃·SiO₂ crystal containing both Si and Al from the CaO-SiO₂-CaF₂ system. The gradual decrease in the amount of Q¹ in the CaO-SiO₂-CaF₂-based mold flux would decrease the similarity between the molten slag and cuspidine and reduce the nucleation and growth of cuspidine.