Effect of Basicity on the Structure, Viscosity and Crystallization of CaO-SiO₂-B₂O₃ Based Mold Fluxes

Abstract

In this study, the structure, viscosity characteristics, and crystallization behavior of CaO-SiO₂-B₂O₃ based melts were studied combining molecular dynamics (MD) simulation, Fourier transform infrared (FTIR) spectroscopy, rotating viscometer test, and FactSage thermodynamic calculation. The results showed that, in the ternary CaO-SiO₂-B₂O₃ glass system, stable structural units of [SiO₄]⁴⁻ tetrahedral, [BO₃]³⁻ trihedral and [BO₄]⁵⁻ tetrahedral were formed, and the Si-O and B-O structure depolymerize with the basicity increase from 1.15 to 1.25, then the B-O structure become complex with the basicity further increase to 1.35. In fluorine-free mold fluxes, with the basicity increases, the viscosity at 1300 °C increases, the liquidus temperature decreases and then increases, the network structure polymerizes, it indicates that the structural complexity rather than the melting property change plays a predominant role in increasing the viscosity at 1300 °C. Moreover, due to the changes in crystallization phase and solid solution ratio, the viscosity-temperature curve of fluorine-free slag shows the characteristics of alkaline slag and the break temperature increase with the basicity increase. The MD simulation, FTIR experiment, viscosity test, and FactSage calculation results are verified and complemented each other.

1. Introduction

Mold flux has been widely used in continuous casting for lubricating the strand, moderating mold heat transfer, adsorbing inclusions, and insulating the molten steel from surface [1]. In conventional commercial mold flux, about 7–12 wt% fluorides are usually added to control the viscosity, melting, and crystallization properties of mold flux, especially the crystallization of cuspidine (3CaO₂SiO₂CaF₂) from mold flux has a great effect on the heat transfer control [2,3,4]. Although fluorides are important to mold flux, it still causes some problems such as air pollution, health harmful, and equipment corrosion due to the volatilization of SiF₄, NaF, AlF₃, and HF, etc. [5,6].

Therefore, it is urgent to look for substitutes to replace the fluorides in mold flux. During the development of fluorine-free mold fluxes, several oxides, like Na₂O, K₂O, Li₂O, TiO₂, B₂O₃, are used to compensate the negative effects caused by the absence of fluorides [7,8,9,10,11,12]. However, the viscosity and crystallization properties of fluorine-free mold fluxes are not appropriate for continuous casting of high carbon steel and crack sensitive steel [13]. The crystallization and heat transfer behaviors of calcium borosilicate (Ca₁₁Si₄B₂O₂₂) [7,8,14], titanite (CaSiTiO₅) [11], and perovskite (CaTiO₃) [11] are similar to those of main crystal cuspidine (Ca₄Si₂O₇F₂) in mold fluxes. However, titanite (CaSiTiO₅) and perovskite (CaTiO₃) as the main crystallization phase of TiO₂-containg would increase the breakout risk due to the precipitation of Ti(C, N) with high melting point [15], while B₂O₃-containg fluorine-free mold fluxes with calcium borosilicate (Ca₁₁Si₄B₂O₂₂) as the main crystallization phase has a good application prospect.

Since the micro-structure unit in the slag will produce internal friction during its movement, which will affect the behavior of liquid slag flowing into the slag channel, and the viscosity of the mold slag can reflect the magnitude of the frictional force, it is necessary to control the viscosity of mold fluxes in an appropriate range. In traditional mold fluxes, with the increase of alkalinity, the availability of free oxygen ions (O₂) will increase, which will react with bridged oxygen (O_b) in silicate to simplify the Si-O structure and so that decreases the viscosity of mold fluxes. However, when excessive free oxygen ions (O₂) generate with the further increase of basicity, the depolymerization effect will not be significant since the complex network structures have been already depolymerized into simpler network units. On the other hand, the increase of basicity is beneficial to the formation of high melting point substances, which reduce the superheat of the slag and increase the viscosity of the system in consequence. Therefore, the viscosity of high basicity powder depends on the balance between the structure depolymerization and the superheat reduction [16] in traditional mold fluxes.

However, in B₂O₃-containg fluorine-free mold fluxes, the influencing mechanism of basicity on viscosity of fluorine-free slag would be different since the composition changes. Therefore, the influence of basicity on the viscosity performance of fluorine-free mold fluxes was studied through rotary viscometer in this study. Meanwhile, the relevant structural information of slag samples was detected by spectral experiment, and the liquidus temperature and crystallization phase of fluorine-free mold fluxes were calculated by FactSage software, to verify and auxiliary analyze the molecular dynamics (MD) simulation and viscosity test results.

2. Materials and Methods

2.1. Molecular Simulation

Molecular dynamics (MD) is a simulation method for studying the movement process of atoms and molecules based on the Newtonian equations of motion. Thus, it is essential to choose an appropriate potential function and its corresponding parameters, which can depict the interactions between the adjacent particles. In this work, the Born-Mayer-Huggins (BMH) potential function was selected as it has been successfully used to study oxide systems. It consists of long-range Coulomb interactions, short-range repulsion interactions and Van der Waals forces [17,18,19]. The interatomic force field reads:

Uij (r) = qiqj/rij + Aij exp (− Bijr) − Cij/rij⁶,

(1)

where U_ij (r) is the interatomic pair potential, q_i and q_j are the selected charges, and in order to ensure the transferability of the interaction potential with the melt composition, the valence assigned to the atoms is usually kept fixed for all compositions, r_ij represents the distance between atoms i and j, A_ij and C_ij are energy parameters for the pair ij describing repulsive and van der Waals attractive forces, respectively, and B_ij is a e-folding length characterizing the radically symmetric decay of electron repulsion energy between atom pair ij. The parameters of all the systems used in this study are listed in

Table 1. Born-Mayer-Huggins (BMH) potential parameters of atomic pairs in CaO-SiO₂-B₂O₃ melts.

Atom1	Atom2	Aij (eV)	Bij (1/Å)	Cij (eV·Å6)
O	O	1,497,049.00	5.88	17.34
O	B	10,699.19	6.06	0
O	Si	62,794.37	6.06	0
O	Ca	717,827.00	6.06	8.67
Ca	Ca	329,051.60	6.25	4.335
Ca	Si	26,674.68	6.25	0
Ca	B	4300.43	6.25	0
Si	Si	2162.39	6.25	0
B	B	56.20	6.25	0

.

The effects binary basicity on the structure of slag were studied according to the composition range of the primary zone of calcium borosilicate (Ca₁₁Si₄B₂O₂₂). The melt composition, R (basicity), atomic number, density, and cell edge length of each slag system were shown in

Table 2. Component content, atomic number, density, and box length of CaO-SiO₂-B₂O₃ system at 1600 °C.

Groups	Basicity	Mass Fraction (%)			Atomic Number					Density(g/cm3)	Box Length (Å)
		CaO	SiO2	B2O3	Ca	Si	B	O	Total
R1	1.15	50	44	6	809	665	154	2370	3998	2.669	38.35426
R2	1.25	52	42	6	847	638	156	2357	3998	2.678	38.39251
R3	1.35	54	40	6	885	612	158	2346	4001	2.687	38.43881

. For ternary slag, the total number of particles in the original cell can be set to about 4000. Then, the number of particles in the original cell can be calculated according to the component content.

All simulations were carried out in an NVT (constant number of particles, volume, and temperature) ensemble, which was used to maintain its stability, while the Parrinello-Rahman and Nose methods were used to control temperature and pressure. In the algorithm, the truncation radius of the short-range force was set to be 10 Å, which was less than the edge length of any system cell. The Ewald summation method can be used to calculate the long-range Coulomb force. The equations for the motions of the atoms were explained by the frog-jump logarithm method with a time step of 1 fs, it saved data every 10 steps. At the beginning of simulation process, the initial temperature was 3727 °C (4000 K), for 24,000 steps to mix the atoms adequately and eliminate the effect of the initial distribution. Then the temperature was reduced to 1600 °C (1873 K) by 96,000 steps. Finally, the structure information was relaxed for 60,000 steps at 1600 °C (1873 K) to obtain the structure, partial radial distribution function (RDF) and coordination number function (CN) data [20].

2.2. Viscosity Measurement

Viscosity is one of the most important properties of mold fluxes, which can significantly affect the surface quality of slab. In this paper, the viscosity of the fluorine-free mold flux was tested by the inner cylinder rotation method with a Brookfield DV-II⁺ viscometer (Brookfield Inc., Middleboro, MA 02346, USA). Fluorine-free mold fluxes were designed based on the MD simulation and the compositions are shown in

Table 3. Composition content of fluorine-free mold fluxes (wt%).

NO.	Basicity	CaO	SiO2	B2O3	Al2O3	Na2O	MgO	Li2O
F1	1.15	42.26	36.74	6	4	8	2	1
F2	1.25	43.89	35.11	6	4	8	2	1
F3	1.35	45.38	33.62	6	4	8	2	1

. The slag samples were prepared with pure chemical reagents CaCO₃ (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), SiO₂ (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), B₂O₃ (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), Al₂O₃ (Sinopharm Chemical Reagent, Shanghai, China), MgO (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), Na₂CO₃ (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) and Li₂CO₃ (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China).

A calibration measurement was carried out at room temperature by using castor oil with known viscosity. When measuring the viscosity of those mold fluxes, 250.0 g of the powders were placed in a graphite crucible with internal diameter and height of 50 mm and 100 mm, respectively. Then the crucible was heated to 1300 °C (1573 K), and kept for 10 min to obtain a homogeneous melt. The bob (which was made of molybdenum with the height of 18 mm and the diameter of 15 mm) was immersed into liquid slag bath and rotated at a fixed speed with 12 r/min. Each measurement was performed during the cooling process, and the data of viscosity v/s temperature were collected every 5 s.

2.3. Thermodynamic Calculation

In this paper, the effect of basicity on liquidus temperature and crystallization phase of fluorine-free mold fluxes in Table 3 were studied by thermodynamic software FactSage7.2 (7.2, Thermfact/CRCT, Montreal, QC, Canada), to assist in analyzing the influence mechanism of basicity on viscosity characteristics of fluorine-free mold fluxes. Due to the lack of Li₂O database in FactSage7.2, the content of Li₂O were converted to Na₂O as an approximate treatment.

Although the crystallization of mold fluxes in experiments occurred under non-equilibrium conditions, which was not completely consistent with the thermodynamic analysis presenting the equilibrium phases based on the Gibbs free energy calculations [21], the thermodynamic analysis can still provide useful guidance in the flux design and the interpretation of experimental results.

2.4. Fourier Transform Infrared (FTIR) Spectroscopy

Infrared spectroscopy can be used to detect structure information of samples. Due to the limitations of experimental equipment and conditions, the as-quenched samples shown in Table 1 and Table 3 were tested to study the structure of molten slag approximately. The molten slag was cooled to a glassy state by liquid nitrogen, and then dried, crushed, and ground to 200 mesh or less. Then, 1 mg of each sample was mixed with an appropriate amount of KBr (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) and was pressed into uniform transparent sheet for FTIR test. The spectra were recorded by a Nicolet 6700 FTIR spectrometer (ARCoptix FTIR Spectrometers & Liquid Crystal Elements Co., Ltd., Neuchâtel, Switzerland) in the range 200–2000 cm⁻¹, with a resolution of 2 cm⁻¹, using the KBr pellet technique [22,23,24].

3. Results and Discussion

3.1. Effect of Basicity on the Structure of CaO-SiO₂-B₂O₃ System

3.1.1. Average Bond Length and Average Coordination Number

The average distance and coordination number between ions could be obtained by simulated partial radial distribution function (RDF) and coordination number function (CN). The RDFs and CNs of some atomic pairs in sample R2 of CaO-SiO₂-B₂O₃ system are shown in Figure 1. The average bond length corresponding to the first peak of the RDF and the average coordination number corresponding to the CN curve platform are listed in

Table 4. Average bond length and average coordination numbers of atomic pairs in CaO-SiO₂-B₂O₃ system.

Property	Sample	Ca-Ca	Ca-Si	Ca-B	Si-Si	Si-B	B-B	Ca-O	Si-O	B-O	O-O
Average	R1	3.38	3.53	3.25	3.19	2.96	2.75	2.31	1.61	1.36	2.60
Bond	R2	3.41	3.53	3.15	3.19	2.96	2.75	2.31	1.61	1.36	2.60
Length	R3	3.39	3.54	3.21	3.20	2.97	2.75	2.31	1.61	1.36	2.60
Average	R1	6.79	4.73	1.05	1.92	0.41	0.59	5.89	4.05	3.63	4.83
Coordination	R2	7.11	4.54	1.07	1.76	0.41	0.55	5.96	4.05	3.66	4.65
Number	R3	7.36	4.41	1.08	1.61	0.40	0.41	5.98	4.05	3.58	4.54

.

It can be seen from Figure 1 and Table 4 that the average bond length of B-O is shorter than that of Si-O, which means that B is easier to bond with O atom, namely B₂O₃ is more acidic than SiO₂. The average coordination number of Si-O and B-O indicates that it forms stable [SiO₄]⁴⁻ tetrahedral in slag, while most of the B atoms can relate to three or four O atoms that form stable [BO₃]³⁻ trihedral or [BO₄]⁵⁻ tetrahedral units. With the increase of R (basicity), the average bond lengths of Si-O, B-O, Ca-O, O-O, Si-Si, Si-B, and B-B bonds have not changed, which means that the bond lengths of these bonds are relatively stable; the average coordination number of Ca-Si, Si-Si, and B-B bonds decreases, the average coordination number of Ca-Ca bond increases, while the average coordination number of Ca-B and Si-B bonds basically remains unchanged, indicating that the non-bridge oxygen structure and bridge oxygen may depolymerize into free oxygen, and the structure of slag becomes simpler.

3.1.2. Distribution of Oxygen Types

The distribution of oxygen in slag could be analyzed by the number of network former connected to the oxygens. The network formers 3, 2, 1, and 0, connected to the oxygens, corresponds to tri-coordinated oxygen O_t, bridged oxygen O_b, non-bridged oxygen O_nb and free oxygen O_f, respectively. Systems containing multiple network formations O_t, O_b and O_nb, can be further divided into different types. In CaO-SiO₂-B₂O₃ slag O_t was not found, indicating that the extra negative charges of [BO₄]⁵⁻ tetrahedral were not balanced by the formation of O_t in the slag. Since there are two kinds of network formers Si and B, the types of O_b include Si-O-Si (SOS), Si-O-B (SOB), and B-O-B (BOB), while the types of O_nb include Ca-O-Si (OS) and Ca-O-B (OB). The distribution of oxygens in CaO-SiO₂-B₂O₃ slag is shown in Figure 2.

It can be seen from Figure 2, with the increase of basicity, O_nb increases, O_b decreases, while O_f changes little. Specifically, bridge oxygen SOS depolymerizes into non bridge oxygen OS, while OB, SOB, BOB and O_f changes little, it means the increase of basicity stimulates the decomposition of large Si-O network polymers into small Si-O tetrahedrons, which will decrease the slag viscosity theoretically [25].

3.1.3. Distribution of Structure Units Qⁿ

The distribution of structural unit Qⁿ could be analyzed by the number of bridge oxygen O_b connected to the network former, where the superscript n represents the number of bridging oxygen O_b, and Q⁰, Q¹, Q², Q³, and Q⁴ correspond to island, dimers, ring or chain, slice or layer, three-dimensional frame, or mesh tetrahedron, respectively.

Figure 3a shows structure unit Qⁿ for Si, it reveals that with the increase of basicity in the range R = 1.15–1.25, Q³ and Q⁴ decomposed into Q⁰, Q¹, and Q², which simplifies the structural units for Si. When the basicity increases from 1.25 to1.35, Q⁰, Q¹, and Q³ increase, while Q² and Q⁴ decrease. It means that the reactions Q² → Q⁰ + 2Q¹ or Q⁴ → Q¹ + Q³ may be occurred, which makes the simplify extent of Si structure units less obvious.

Figure 3b shows structure unit Qⁿ for B, it reveals that with the increase of basicity from 1.15 to 1.25, Q⁰, Q¹ and Q⁴ increase, while Q² and Q³ decreases. The reactions 2Q² → Q⁰ + Q⁴ or Q² + Q³ → Q¹ + Q⁴ may be occurred, which simplifies the structure units for B. When basicity increases from 1.25 to 1.35, Q⁰, Q² and Q⁴ increase, while Q¹ and Q³ decrease. The reactions Q¹ + Q³ → 2Q² or Q¹ + Q³ → Q⁴ may be occurred, which makes the B structure units become more complex.

3.1.4. FTIR Experimental Results

Figure 4 shows the FTIR results of CaO-SiO₂-B₂O₃ slag, the band 400–600 cm⁻¹, 600–800 cm⁻¹, 800–1200 cm⁻¹, and ~1410 cm⁻¹ assigned to the absorbance peak of the Si-O-Si bond bending vibrations, the oxygen bridges between two [BO₃]³⁻ trihedral [26], the [SiO₄]⁴⁻ and [BO₄]⁵⁻ tetrahedral symmetry stretching [27,28], and the asymmetric stretching mode of [BO₃]³⁻ trihedral [26], respectively. Firstly, all the peaks become less pronounced as basicity increase from 1.15 to 1.25. It indicates the complex silicate and borate groups dissociated into simple units like [BO₃]³⁻ trihedral or [SiO₄]⁴⁻ and [BO₄]⁵⁻ tetrahedral units, and then the simple units further decomposed into non bridge oxygens with the increase of basicity [28]. It is well consistent with the MD simulations results that the structure units of Qⁿ species for Si and B were simplified. Secondly, the bands corresponding to B-O-B bending and tetrahedral stretching are become more prominent with the further increase of basicity from 1.25 to 1.35, while there is no obvious change for the peaks of Si-O-Si bending and [BO₃]³⁻ trihedral. It indicates that the dissociated simple diborate units would connect with extra [BO₄]⁵⁻ tetrahedral units to form more complex pentaborate groups, thus, the stretching mode of [BO₄]⁵⁻ tetrahedral in more complex boroxol rings is strengthened, it means the borate structure is associated, which makes the slag structure complex. Combing with the MD results, as the basicity increase from 1.25 to 1.35, the peak change of Si-O-Si and B-O-B bending are well consistent with the MD results so that there is small change for the structure units Qⁿ for Si while the structure units Qⁿ for B become more complex.

Combing the MD and FTIR results of CaO-SiO₂-B₂O₃ system, it can be concluded that with the increase of basicity from 1.15 to 1.35, the slag structure becomes simpler first and then changes to a complex structure, which leads to a decrease and then increase of slag viscosity theoretically [25].

3.2. Effect of Basicity on the Viscosity of F-Free Mold Fluxes

3.2.1. Effect of Basicity on the Viscosity and Its Correlation to the Structure

As Figure 5 shows, when w (B₂O₃) = 6%, the viscosity at 1300 °C of fluorine-free powder increases in the range of 0.171–0.280 Pa·s with the increase of basicity, while the liquidus temperature decreases and then increases. It means that the superheat change does not completely dominate the increase of viscosity.

In connection with the viscosity, the FTIR results of the as-quenched samples with varied basicity are shown in Figure 6. It could be observed that the absorbance peak of the Si-O-Si bond bending vibrations in the region of 400–600 cm⁻¹ becomes more pronounced with the increase of basicity, which resulted in the polymerization of the Si-O structure units. Meanwhile, the absorption signal of symmetric stretching vibrations of [SiO₄]⁴⁻ and [BO₄]⁵⁻ tetrahedral in 800–1200 cm⁻¹ becomes noticeable with the increase of basicity, also suggesting that the polymerization of network structure of slag melts. Moreover, the intensity of the band at around 1200 cm⁻¹ [27], generated by the asymmetric stretching modes of [BO₃]³⁻ trihedral and [BO₂O⁻]²⁻ trihedral in boroxol rings, strengthened gently with the increase of basicity. The variation trend of these two bands indicates the structures of [BO₃]³⁻ trihedral and [BO₂O⁻]²⁻ trihedral contain the polymerization degree stable. It can be concluded that the polymerization of the structure results in a higher viscosity of fluoride-free mold fluxes.

It is well known that basic oxides exhibit highly ionic behavior and act as network modifiers in traditional fluoride containing molten slag. Increase of basicity leads to additional free oxygen ions, which could depolymerize the silicate or borate network structure of slag and form more non-bridging oxygen, while the metal cations will be dynamically bonded to fluoride. However, this study shows that due to lack of fluoride, the increase of alkalinity will provide more cations to balance the negative excess of [BO₄]⁵⁻ tetrahedral, resulting in the polymerization of [BO₃]³⁻ trihedral to form [BO₄]⁵⁻ tetrahedral and the stabilization of [BO₄]⁵⁻ tetrahedral, thus making the melt structure more complex.

To be summarized, in fluorine-free mold fluxes, with the basicity increase, the viscosity at 1300 °C increases, the liquidus temperature decreases and then increases, the network structure polymerizes, which indicates that the structural complexity rather than the melting property change plays a predominant role in increasing the viscosity at 1300 °C.

3.2.2. Effect of Basicity on the Properties of Viscosity-Temperature Curve

The molten mold fluxes will flow into the gap between the mold copper and the strand slab, which will achieve the lubrication of strand slab and control the heat transfer strand slab to copper mold. Viscosity-temperature curve can reflect the solidification and crystallization properties of mold fluxes, thus guiding the coordinated control of lubrication and heat transfer of mold fluxes.

Figure 7 shows the effect of basicity on the viscosity-temperature curve of fluorine-free mold fluxes with w (B₂O₃) = 6%. It shows that the viscosity-temperature curve presents characteristics of alkaline slag with obvious breaking temperature, and the breaking temperature increase with the increase of basicity. As shown in Figure 8, with the temperature goes down to near 1225 °C, it begins to promote the precipitation of mineral phases Ca₃Si₂O₇, Ca₁₁B₂Si₄O₂₂, and Ca₃MgSi₂O₈, as well as the formation of solid solutions like bredigite, nepheline, and combeite, etc., which results in a sharp increase in viscosity when the temperature of slag is lower than a certain value and then leads to a deterioration of lubrication consequently. Since the initial crystallization temperature and crystallization ratio increases with the basicity increase, it leads to an increase of breaking temperature, which weakens the heat transfer from strand slab to copper mold.

4. Conclusions

The effect of basicity on the structure, viscosity, and crystallization behavior of CaO-SiO₂-B₂O₃ based mold fluxes was conducted in this article, and the specific conclusions are summarized as follows:

• Combing the MD and FTIR results of CaO-SiO₂-B₂O₃ system, stable structural units of [SiO₄]⁴⁻ tetrahedral, [BO₃]³⁻ trihedral, and [BO₄]⁵⁻ tetrahedral are formed, and the Si-O and B-O structure depolymerizes with the basicity increase from 1.15 to 1.25, then the B-O structure becomes complex with the basicity further increase from 1.25 to 1.35. It means that with the increase of basicity from 1.15 to 1.35, the slag structure becomes simpler first and then changes to complex structure, which leads to decrease and then increase of slag viscosity theoretically.
• In fluorine-free mold fluxes shown in this paper, with the basicity increase, the viscosity at 1300 °C increases, the liquidus temperature decreases and then increases, the network structure polymerizes. It is therefore concluded that the structural complexity plays a predominant role in increasing the viscosity at 1300 °C since the increase of alkalinity will provide more cations to balance the negative excess of [BO₄]⁵⁻ tetrahedral.
• In fluorine-free mold fluxes shown in this paper, due to the changes in crystallization phase and solid solution ratio, the figure curve of fluorine-free slag shows the characteristics of alkaline slag and the breaking temperature increases with the basicity increase. Thus, it is of critical importance to set suitable basicity to achieve the coordinate control of lubrication and heat transfer of fluoride-free mold fluxes.