Effects of B₂O₃ and CaO/Al₂O₃ on Structure of CaO–Al₂O₃–B₂O₃ System

Abstract

CaO–Al₂O₃-based mould fluxes are the most significant slag systems for the continuous casting of high-Al steel. In this study, to investigate the structure of the CaO–Al₂O₃–B₂O₃ system, molecular dynamics simulation and infrared spectroscopy experiments with different amounts of B₂O₃ and CaO/Al₂O₃ ratio were carried out. The results showed that the structural unit of B–O and Al–O are of great stable while B atom is easier to bond with O atom than Al, and [BO₃]³⁻ trihedron is more stable than [AlO₄]⁵⁻ tetrahedron. With the content of B₂O₃ increases, the increased [BO₃]³⁻ trihedral structure units can balance the excessive negative charges in [AlO₄]⁵⁻ tetrahedron structure, and Al₂O₃, acts more acid in a highly alkaline surrounding, can absorb O²⁻ and results in forming [AlO₅]⁷⁻ structure at high CaO/Al₂O₃ ratio, which promotes the transformation of O_nb into O_b, while CaO results in an increase in the content of O_nb with the increase of CaO/Al₂O₃ ratio. The results of infrared spectrum are consistent with simulation results that B₂O₃ addition promotes the complexity of the network structure, and the depolymerization of the large complex network structure with increased CaO/Al₂O₃ ratio.

1. Introduction

High-Al steel has recently attracted significant attention in the field of aviation industry and machinery manufacturing due to its good structural properties. However, during the continuous casting of high-Al steel, the high reductive element Al is easy to react with the SiO₂ in the traditional CaO–SiO₂-based mould fluxes, which leads to dynamical change of physico-chemical properties of mould fluxes, and therefore the metallurgical functions, especially lubrication performance and heat transfer control, will be deteriorated.^1–5) As a result, various defects such as star cracking, longitudinal and transverse cracking, deep oscillation marks will appear in the high-Al steel slab.^2,6,7)

In order to solve these problems, there have been some investigations to adjust the ratio of CaO/SiO₂ and Al₂O₃/SiO₂ for traditional mould fluxes.^8–11) However, due to the dynamical change of interfacial reaction, the design and optimization of mold fluxes for the continuous casting of high-Al steel becomes considerably difficult. Since then, there were many researchers trying in the direction of developing new types of CaO–Al₂O₃-based mould fluxes to inhibit the interfacial reaction.^2,3,12) Nevertheless, in view of the high content of Al₂O₃, it is more susceptible for viscosity increase and crystal precipitation, which will remarkably deteriorating the lubrication effect of the slag.^2,4,13,14)

The B₂O₃ in mold fluxes tends to react with Al in molten steel, and with the increasing content of B₂O₃, the reaction rate will increase, which may deteriorate the properties of mold fluxes and increase the boron pollution of steel. However, Li et al.¹⁵⁾ studied that the slag-metal reaction hardly occurred with the addition of 5 wt% B₂O₃ during continuous casting of high Al steel. This demonstrates that the B₂O₃ content should be controlled in a low value for high Al steel casting. Since the network forming ability of Al₂O₃ is weaker than that of SiO₂, it is necessary to add an appropriate amount of acidic oxides such as B₂O₃ into the Al₂O₃-base mould fluxes to ensure sufficient glass forming ability of the slag based on weak interfacial reaction. It was reported that small amount of B₂O₃ would improve the lubrication of the CaO–Al₂O₃-based mould fluxes by inhibiting crystallization³⁾ and lowering viscosity.⁹⁾ However, the present studies on the effect of B₂O₃ are mainly focused on the regulation of composition and properties, lacking in-depth study of structure theories. As reported in calcium aluminoborate glasses, B₂O₃ mainly forms [BO₃]³⁻ trihedron structure, which gradually transforms into [BO₄]⁵⁻ tetrahedron with the increase of CaO, since the metal cation plays the role of charge compensation.¹⁶⁾ In addition, [AlO₄]⁵⁻ tetrahedrons increase and [BO₄]⁵⁻ tetrahedrons depolymerize to form [BO₃]³⁻ trihedrons when the content of Al₂O₃ increases. In view of the above facts, Al₂O₃ and B₂O₃ in the glass have similar mechanisms of action, while they also restrict each other. Therefore, it is great significant to systematically study the mutual restriction mechanism of B₂O₃ and Al₂O₃ on the network structure, and explore the evolution process of [AlO₄]⁵⁻ tetrahedron, [BO₄]⁵⁻ tetrahedron, [BO₃]³⁻ trihedron and other structural units, to obtain the theoretical support for the effective selection and control of the mould fluxes.

In this paper, the molecular dynamics (MD) simulation of CaO–Al₂O₃–B₂O₃ systems was carried out. The average bond length, coordination number, oxygen concentration, and Qⁿ distribution were analyzed to make clear the mechanism of Al₂O₃ and B₂O₃ in the slag system. Furthermore, the structure information of the slag system was detected by Fourier transform infrared (FTIR) spectroscopy experiment to verify the simulation results.

2. Methodology

The comprehesive mechanism of B₂O₃ and CaO/Al₂O₃ ratio on microstructure of CaO–Al₂O₃–B₂O₃ systems were investigated by MD simulation and FTIR experiment. Detailed information on simulation and experimental method were described in the following sections.

2.1 Molecular dynamics simulation method

The Born-Mayer-Huggins (BMH) potential function,^17–19) of which the corresponding parameters being suitable for the calculation of particle interactions, becomes the best choice for MD simulation. The expression of BMH potential function mainly consists of three parts, which represent coulomb potential, short-range repulsive force, and the van der Waals attractive force, respectively. And the form is shown in the following equation:   

\begin{equation*} U_{ij}(r) = q_{i}q_{j}/r_{ij} + A_{ij}\exp (-B_{ij}r) - C_{ij}/r_{ij}^{6} \end{equation*}

where U_ij(r) is the interatomic potential; q_i and q_j are charges of ion i and j, respectively; r_ij represents the distance between ions i and j; A_ij, B_ij and C_ij represent the potential parameters of BMH potential function, which are listed in Table 1.²⁰⁾

Table 1 The potential parameters of atomic pairs in CaO–Al₂O₃–B₂O₃ systems.

The suitable compositions of CaO–Al₂O₃–B₂O₃ system were selected within the liquidus range at 1600°C based on the phase diagram. A cubic unit cell was used under periodic boundary conditions, and the total number of atoms in the simulated cell was set as about 5000, while the number of different atoms were calculated based on compositions. In addition, the density of each sample was calculated according to the empirical formula.²¹⁾ The CaO/Al₂O₃ ratio, composition, atom numbers, density and box length of each sample were presented in Table 2.

Table 2 Composition, atom numbers, density, and box length of each sample.

To keep the number of particles (N), sample volume (V) and the temperature (T) of the system constant, NVT ensemble was applied for the MD simulations. During the running process, the initial temperature was set as 4000 K for 24,000 steps to eliminate the initial distribution of atoms and fully mix the various atoms in the system. Then, the temperature was decreased to 1873 K through 96,000 steps. Finally, the system was relaxed in another 60,000 steps. When the system reached equilibrium, the structural information of melts such as RDF, CN and oxygen distribution were obtained through analyzing the simulation results.

2.2 FTIR experimental method

In order to obtain accurate structural information, it is necessary to experiment with the high temperature molten state of the CaO–Al₂O₃–B₂O₃ system. However, due to limitations of experimental equipment and conditions, the quenching method was adopted in this study.²²⁾ According to components of different samples in Table 2, the preparation of corresponding powders was carried out. Planetary ball milling was used to mix powders of raw materials during 1 hour, in order to keep a homogeneously composition of the samples. Then the mixed powders were located inside the graphite crucible in a medium-frequency heating furnace, and heated to 1873 K. After that, the high-temperature fluxes were quickly placed in liquid nitrogen. Finally, the quenched fluxes were ground into powder with particle size less than 75 µm for FTIR analysis.

3. Results and Discussion

3.1 RDFs and average bond length

The radial distribution functions (RDFs) can be obtained by MD simulations, which is the most important function to characterize the structure. The equation of RDF is expressed as below:²³⁾   

\begin{equation*} g (r) = (V/N_{i}N_{j})\sum_{j} (\langle n_{ij}(r) \rangle /4\pi r^{2}\varDelta r) \end{equation*}

Here, V is the simulation cell volume; N_i and N_j are the total number of ions i and j, respectively; n_ij is the average number of the ion j surrounding the ion i within the distance (r, r + Δr). On the radial distribution function curve, the larger the g_ij (r) value corresponding to the abscissa r of a certain position, the greater the probability of the existence of j particle on the spherical shell with radius r from particle i.

Taking the sample CAB (the acronym of CaO–Al₂O₃–B₂O₃ system)-0.6-3 as an example, the RDFs of atomic pairs are shown in Fig. 1. The abscissa position where the first peak of RDF curve is located corresponds to the average bond length of atom pair as shown in Table 3.

Fig. 1

RDFs for different atomic pairs in CAB-0.6-3.

Table 3 Average bond length of different atoms in the CaO–Al₂O₃–B₂O₃ system.

As can be seen from Fig. 1, the first peaks of Al–O and B–O RDF curves are sharp and narrow, which reveals that the structure of Al–O and B–O units are relatively stable. Whereas, the average bond length of B–O is shorter than that of Al–O, demonstrating that B is more likely to bond with O in the CaO–Al₂O₃–B₂O₃ system. In addition, the first peak of RDF curve of Ca–O is wider and weaker than that of Al–O and B–O, and the average bond length is also longer, which indicates that the Ca–O structural unit is unstable, so that CaO dissociates into Ca²⁺ and O²⁻, as a result the Ca²⁺ can destroy and simplify the complex structural units in the network structure.²⁴⁾

As shown in Table 3, the average bond length corresponding to the first peak of Al–O, B–O and Ca–O are 1.74–1.75 Å, 1.35–1.36 Å and 2.30–2.32 Å, respectively, which are in good agreement with the results obtained by neutron diffraction experiments.^25,26) With the increase of B₂O₃ content, the average bond length Al–O keeps constant during R = 0.6–1.0 and increases slightly when R = 1.2, this is because Al₂O₃ shows more acid at higher alkalinity and forms Al–O–B bonds with B₂O₃ addition, while O biases to the B side resulting in longer length of Al–O; the average bond length of B–O remains unchanged when R = 0.6 and increase slightly in the range of R = 0.8–1.2, this is because B–O–B bonds form with B₂O₃ addition, while the bond length of B–O in B–O–B is longer than that in Ca–O–B since the attraction of B to O is stronger than Ca.²⁷⁾

3.2 CNs and coordination number

The coordination number (CN) function is also an important structural information of melt. The formula is as follows:²³⁾   

\begin{equation*} N_{ij}(r) = 4\pi x_{j}\int_{0}^{r}g_{ij} (r')r'^{2}dr' \end{equation*}

Here, g_ij(r) is the radial distribution function between particle i and j, and x_j = N_j/V is the average density of particle j. The value of the CNs obtained by integrating the first valley of the RDFs corresponds to the average coordination number between the atomic pairs.

It can be found in Fig. 2, the curves of CN_Al–O and CN_B–O have smooth platforms and the corresponding vertical values are 4.09 and 3.06, respectively, it demonstrates that most of Al atoms form stable [AlO₄]⁵⁻ tetrahedron structural units with four O atoms, while most of B atoms would connect three O atoms to form stable [BO₃]³⁻ trihedron structural units. Apart from that, [BO₃]³⁻ trihedron is more stable than [AlO₄]⁵⁻ tetrahedron in view of the platform on the CN_B–O curve is smoother than that on the CN_Al–O curve. In addition, it is noteworthy that CN_Ca–O curve of Ca–O has no smooth platform, which indicates that Ca–O cannot form stable structural units.

Fig. 2

CN curves for different atomic pairs in CAB-0.6-3.

By comparing the average coordination number of each atomic pair in CaO–Al₂O₃–B₂O₃ system shown in Table 4, with the increase of B₂O₃, the CN_Al–O decreases and the CN_Ca–O increases at fixed CaO/Al₂O₃ ratio of 0.8 and 1.0. In aluminosilicate slag, when the low content of Ca²⁺ is not enough to balance the excessive negative charges in [AlO₄]⁵⁻, it results in the appearance of [AlO₅]⁷⁻ in the system.²⁸⁾ However, the increased proportion of [BO₃]³⁻ trihedral structure with B₂O₃ addition in the present study, will balance the excessive negative charges in [AlO₄]⁵⁻ tetrahedron structure, which leads to the release of Ca²⁺ and the decrease of [AlO₅]⁷⁻. Therefore, the CN_Al–O decreases and the CN_Ca–O increases. When R = 1.2, the CN_Al–O increases from 4.05 to 4.07 with B₂O₃ addition. It can be explained that Al₂O₃, which shows more acid in a highly alkaline surrounding, can absorb O²⁻ and results in forming [AlO₅]⁷⁻ structure. While CN_B–O varies from 3.18 to 3.13, this is because B₂O₃ tends to form [BO₃]³⁻ trihedral structure at the expense of 3-D [BO₄]⁵⁻ tetrahedral structure under the condition of high content of B₂O₃.¹⁶⁾

Table 4 CN of each atomic pair in CaO–Al₂O₃–B₂O₃ system.

Secondly, the CN_Ca–Al always decreases and the CN_Ca–B always increases with increase of B₂O₃ under various CaO/Al₂O₃ ratio. That is to say, the Ca ions are more easily bonded with B ions than Al ions. Apart from that, the decrease of CN_Al–Al and the increase of CN_Al–B and CN_B–B illustrate that some B atoms can replace Al atoms to take part in the form of network structure.

In addition, with the CaO/Al₂O₃ ratio increases, when the content of B₂O₃ is 4 wt% as an example, it can be found that the CN_B–O increases from 3.05 to 3.18. This observation shows that the increase of O²⁻ urges the structure of B–O gradually change from 2-D [BO₃]³⁻ trihedral structure to 3-D [BO₄]⁵⁻ tetrahedral structure. The value of CN_Al–O changes from 4.09 to 4.05, demonstrating that the increase of Ca²⁺ can balance of the excessive negative charges in [AlO₄]⁵⁻ and inhibits the form of [AlO₅]⁷⁻ in the system, while the increase of O²⁻ would react with the bridged oxygen in the aluminates and tend to decrease the complex [AlO₅]⁷⁻ structure in the system.

3.3 Oxygen distribution

Select an O atom and analyze the number of network formations in the cut-off radius. When an O atom is connected to 0, 1, 2 and 3 network former, the types of corresponding oxygen can be divided into free oxygen (O²⁻), non-bridging oxygen (O_nb), bridging oxygen (O_b) and oxygen triclusters (O_t), respectively. In the CaO–Al₂O₃–B₂O₃ system, since there are two kinds of network former Al and B, O_nb can be divided into two types: Ca–O–Al (OA) and Ca–O–B (OB), while O_b has three types: Al–O–Al (AOA), Al–O–B (AOB), and B–O–B (BOB).

Since the oxygen trends are similar in different groups, only one group as example shown in figures. It can be seen from Fig. 3(a), the proportion of O_b increases with the increase of B₂O₃ content at fixed CaO/Al₂O₃ ratio of 1.0, while the proportion of O_nb and O_f decreases. Therefore, acting as a network formator, B₂O₃ can polymerize the melt structure, resulting in the transformation of O_nb into O_b in the melt.

Fig. 3

Effect of B₂O₃ content on different types of oxygen distribution when the CaO/Al₂O₃ ratio is 1.0.

It can be observed from the Fig. 3(b) that the proportion of OA and AOA decrease, while the proportion of OB and AOB increase. As for O_f, they are basically unchanged. This is due to the fact that with the increase of B₂O₃ content, the non-bridged oxygen OA constantly transforms into bridged oxygen AOB, and some B atoms in the system replace Al atoms in the Al–O–Al structure and increase the proportion of Al–O–B structure. Consequently, the overall embodiment is that the slag structure evolves towards complexity. In addition, the presence of O_t demonstrates that the negative electric excess of [AlO₄]⁵⁻ and [BO₄]⁵⁻ tetrahedral is balanced by the formation of O_t in the CaO–Al₂O₃–B₂O₃ system.

From Fig. 4(a), it can be seen that with the increase of CaO/Al₂O₃ ratio, the proportions O_b and O_t decrease while that of O_nb increases, which is due to that the CaO, acting as a network modifier, can increase the activity of O²⁻ and break the equilibrium distribution of O_f, O_b and O_nb.²⁹⁾ Then O²⁻ can react with O_t and O_b, resulting in an increase of O_nb. That is to say, CaO can depolymerize the large network polymers to form simpler structures.²²⁾

Fig. 4

Effect of CaO/Al₂O₃ ratio on different types of oxygen distribution when B₂O₃ is 6 wt%.

As can be seen from Fig. 4(b), AOA and AOB continue to decrease, while OA and OB continue to increase with the increase of CaO/Al₂O₃ ratio. This is due to that the increased CaO destroys the network structure, and the bridge oxygen AOA and AOB constantly transforms into non-bridged oxygen OA and OB, as a result the network structure of the system tends to be simplified. As the CaO/Al₂O₃ ratio increases, oxygen triclusters OAAA decreases, it can be concluded that the amount of O²⁻ ions dissociated by CaO causes a part of OAAA to become AOA.

3.4 Analysis of infrared spectrum

In the infrared spectra, the bending vibration region of Al–O–Al bond and B–O–B bond are 400–600 cm⁻¹ and 600–750 cm⁻¹ respectively.^30–32) The region of 750–800 cm⁻¹ is generated by stretching vibration of Al–O bond in [AlO₄]⁴⁻ tetrahedron, while the region of 800–1200 cm⁻¹ is due to the stretching vibration of B–O bond in [BO₄]⁵⁻ tetrahedron.³³⁾ Furthermore, the region of 1200–1600 cm⁻¹ represents the asymmetric stretching vibration of the B–O in [BO₃]³⁻ trihedron.^34,35)

As can be seen from Fig. 5, with the increase of B₂O₃ content, the absorbance peak of the bending vibration region of B–O–B bond becomes noticeable under various CaO/Al₂O₃ ratio, indicating that B was introduced and formed network units.

Fig. 5

The influence of B₂O₃ content on infrared spectrum of CaO–Al₂O₃–B₂O₃ system with different CaO/Al₂O₃ ratio.

When CaO/Al₂O₃ ratio = 0.8, the absorption regions of 400–600 cm⁻¹, 750–1200 cm⁻¹ and 1200–1600 cm⁻¹ gradually become pronounced with the addition of B₂O₃. It could be speculated that B₂O₃, acts as a network former, could dissociate and form [BO₃]³⁻ trihedral structure and [BO₄]⁵⁻ tetrahedron structure units, and the increased proportion of [BO₃]³⁻ trihedral structure balances the excessive negative charges in [AlO₄]⁵⁻ tetrahedron structure, which leads to the decrease of [AlO₅]⁷⁻ and the increase of [AlO₄]⁵⁻. Therefore, the absorption regions of 750–1200 cm⁻¹ becomes noticeable due to stretching vibration of Al–O bond in [AlO₄]⁴⁻ tetrahedron become dominant.

When CaO/Al₂O₃ ratio = 1.0, the absorption regions of 400–600 cm⁻¹ assigned to the bending vibrations of Al–O–Al bond becomes less pronounced due to Al₂O₃ is replaced by B₂O₃. In addition, the charge compensation effect of Ca²⁺ urges the [BO₃]³⁻ trihedral structure to [BO₄]⁵⁻ tetrahedral structure. Consequently, the absorption regions of 750–1200 cm⁻¹ becomes noticeable due to the stretching vibration of B–O bond in [BO₄]⁵⁻ tetrahedron becomes dominant.

When CaO/Al₂O₃ ratio = 1.2, with the increase of B₂O₃ content from 4 wt% to 8 wt%, the absorption regions of 750–1200 cm⁻¹ and 1200–1600 cm⁻¹ gradually become prominent, which is also similar to the results of R = 0.8 and 1.0. However, above-mentioned absorption regions gradually become weaking and flattening with addition B₂O₃ from 8 wt% to 12 wt%. It can be explained that Al₂O₃, acts more acid in a highly alkaline surrounding, can absorb O²⁻ and results in forming [AlO₅]⁷⁻ structure, which inhibits the formation of [AlO₄]⁴⁻ tetrahedron and [BO₃]³⁻ trihedral.

As shown in Fig. 6, with the increase of CaO/Al₂O₃ ratio, the bending vibration region of Al–O–Al and B–O–B bond become weakening and broadening when B₂O₃ = 4, 8, 12 wt%. The results illustrate that the increase of free oxygen ions (O²⁻) react with the bridged oxygen between the [BO₃]³⁻ trihedral structure, [BO₄]⁵⁻ tetrahedral structure and [AlO₄]⁴⁻ tetrahedral structure units, resulted in the depolymerization of the Al–O–Al and B–O–B bond in alumino-borate system. What is more, the absorption regions of 750–1200 cm⁻¹ and 1200–1600 cm⁻¹ become more noticeable and narrower with higher CaO/Al₂O₃ ratio at different fixed B₂O₃ content. The fact can be explained that the amount of Ca²⁺ supplied from the dissociation of CaO increases, which can balance the excessive negative charges in [AlO₄]⁵⁻ and inhibits the formation of [AlO₅]⁷⁻. Meanwhile, the existence of competition between [AlO₄]⁴⁻ structure units and [BO₄]⁵⁻ structure units results in the decrease of [BO₄]⁵⁻ structure units, and consequently the [BO₃]³⁻ structural units increase.¹⁶⁾ From above results, in view of the stretching vibration of Al–O bond in [AlO₄]⁴⁻ tetrahedron becomes dominant, the absorption regions of 750–1200 cm⁻¹ becomes pronounced. Furthermore, the increase of 2-D [BO₃]³⁻ structural units and the decrease of 3-D [BO₄]⁵⁻ tetrahedral structure units show the depolymerization of the large complex network structure with increased CaO/Al₂O₃ ratio.

Fig. 6

The effect of CaO/Al₂O₃ ratio on the infrared spectrum of CaO–Al₂O₃–B₂O₃ system with different B₂O₃ contents.

4. Conclusions

To reveal the effects of B₂O₃ on the structure of the CaO–Al₂O₃–B₂O₃ system under the condition of different CaO/Al₂O₃ ratio, both molecular dynamics simulation and infrared spectroscopy experiments were carried out in the present work. The obtained conclusions are summarized as follows:

1. (1)    In the CaO–Al₂O₃–B₂O₃ system, the structural unit of B–O and Al–O are of great stable while B atom is easier to bond with O atom than Al. Especially, the average bond length of Al–O and B–O are affected gradually by the B₂O₃ content of the system at high CaO/Al₂O₃ ratio. Ca–O unit is unstable and dissociates into Ca²⁺ and O²⁻ easily, as a result the Ca²⁺ can destroy and simplify the complex structural units.
2. (2)    According to the smooth platforms of curves of CN_Al–O and CN_B–O, [BO₃]³⁻ trihedron is more stable than [AlO₄]⁵⁻ tetrahedron. With the content of B₂O₃ increases, the increased [BO₃]³⁻ trihedral structure units can balance the excessive negative charges in [AlO₄]⁵⁻ tetrahedron structure, which leads to the decrease of [AlO₅]⁷⁻. In addition, under the condition of high CaO/Al₂O₃ ratio, Al₂O₃, acts more acid in a highly alkaline surrounding, can absorb O²⁻ and results in forming [AlO₅]⁷⁻ structure, which inhibits the formation of [AlO₄]⁴⁻ tetrahedron and [BO₃]³⁻ trihedral.
3. (3)    An increase in the B₂O₃ content increases the transformation of O_nb into O_b in the CaO–Al₂O₃–B₂O₃ system and some B atoms in the system will replace Al atoms in the Al–O–Al bridged oxygen structure. With the increase of CaO/Al₂O₃ ratio, CaO will break the equilibrium distribution of O_f, O_b and O_nb resulting in an increase in the content of O_nb.
4. (4)    The result of infrared spectrum shown that with the increase of B₂O₃ content, the bending vibration and stretching vibration in Al–O and B–O network becomes noticeable under various CaO/Al₂O₃ ratio, indicating B₂O₃ addition promotes the complexity of the network structure. While the bending vibrations of Al–O–Al and B–O–B bond become weakening and stretching vibrations in [AlO₄]⁴⁻ tetrahedron and [BO₃]³⁻ trihedral become strengthening, showing the depolymerization of the large complex network structure with increased CaO/Al₂O₃ ratio.

Acknowledgements

The authors would like to deeply appreciate the fund support from the National Natural Science Foundation of China (51804004) and the Key projects of National Natural Science Foundation of China (U1760202).

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