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

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.


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 (3CaO 2 SiO 2 CaF 2 ) 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 SiF 4 , NaF, AlF 3 , 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 Na 2 O, K 2 O, Li 2 O, TiO 2 , B 2 O 3 , 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 (Ca 11 Si 4 B 2 O 22 ) [7,8,14], titanite (CaSiTiO 5 ) [11], and perovskite (CaTiO 3 ) [11] are similar to those of main crystal cuspidine (Ca 4 Si 2 O 7 F 2 ) in mold fluxes. However, titanite (CaSiTiO 5 ) and perovskite (CaTiO 3 ) as the main crystallization phase of TiO 2 -containg would increase the breakout Metals 2020, 10, 1240 2 of 12 risk due to the precipitation of Ti(C, N) with high melting point [15], while B 2 O 3 -containg fluorine-free mold fluxes with calcium borosilicate (Ca 11 Si 4 B 2 O 22 ) 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 (O 2 ) 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 (O 2 ) 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 B 2 O 3 -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.

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: 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.
The effects binary basicity on the structure of slag were studied according to the composition range of the primary zone of calcium borosilicate (Ca 11 Si 4 B 2 O 22 ). The melt composition, R (basicity), atomic number, density, and cell edge length of each slag system were shown in Table 2. 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].

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. The slag samples were prepared with pure chemical reagents CaCO 3 (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), SiO 2 (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), B 2 O 3 (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), Al 2 O 3 (Sinopharm Chemical Reagent, Shanghai, China), MgO (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China), Na 2 CO 3 (Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) and Li 2 CO 3 (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.

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 Li 2 O database in FactSage7.2, the content of Li 2 O were converted to Na 2 O 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.

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 Tables 1 and 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].

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-SiO 2 -B 2 O 3 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.
It can be seen from Figure 1 and Table 4    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.

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 Ot, bridged oxygen Ob, non-bridged oxygen Onb and free oxygen Of, respectively. Systems containing multiple network formations Ot, Ob and Onb, can be further divided into different types. In CaO-SiO2-B2O3 slag Ot was not found, indicating that the extra negative    It can be seen from Figure 2, with the increase of basicity, Onb increases, Ob decreases, while Of changes little. Specifically, bridge oxygen SOS depolymerizes into non bridge oxygen OS, while OB, SOB, BOB and Of 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].

Distribution of Structure Units Q n
The distribution of structural unit Q n could be analyzed by the number of bridge oxygen Ob connected to the network former, where the superscript n represents the number of bridging oxygen Ob, and Q 0 , Q 1 , Q 2 , Q 3 , and Q 4 correspond to island, dimers, ring or chain, slice or layer, threedimensional frame, or mesh tetrahedron, respectively. Figure 3a shows structure unit Q n for Si, it reveals that with the increase of basicity in the range R = 1.15-1.25, Q 3 and Q 4 decomposed into Q 0 , Q 1 , and Q 2 , which simplifies the structural units for Si. When the basicity increases from 1.25 to1.35, Q 0 , Q 1 , and Q 3 increase, while Q 2 and Q 4 decrease. It means that the reactions Q 2 → Q 0 + 2Q 1 or Q 4 → Q 1 + Q 3 may be occurred, which makes the simplify extent of Si structure units less obvious.

Distribution of Structure Units Q n
The distribution of structural unit Q n 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 Q 0 , Q 1 , Q 2 , Q 3 , and Q 4 correspond to island, dimers, ring or chain, slice or layer, three-dimensional frame, or mesh tetrahedron, respectively. Figure 3a shows structure unit Q n for Si, it reveals that with the increase of basicity in the range R = 1.15-1.25, Q 3 and Q 4 decomposed into Q 0 , Q 1 , and Q 2 , which simplifies the structural units for Si. When the basicity increases from 1.25 to1.35, Q 0 , Q 1 , and Q 3 increase, while Q 2 and Q 4 decrease. It means that the reactions Q 2 → Q 0 + 2Q 1 or Q 4 → Q 1 + Q 3 may be occurred, which makes the simplify extent of Si structure units less obvious. It can be seen from Figure 2, with the increase of basicity, Onb increases, Ob decreases, while Of changes little. Specifically, bridge oxygen SOS depolymerizes into non bridge oxygen OS, while OB, SOB, BOB and Of 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].

Distribution of Structure Units Q n
The distribution of structural unit Q n could be analyzed by the number of bridge oxygen Ob connected to the network former, where the superscript n represents the number of bridging oxygen Ob, and Q 0 , Q 1 , Q 2 , Q 3 , and Q 4 correspond to island, dimers, ring or chain, slice or layer, threedimensional frame, or mesh tetrahedron, respectively. Figure 3a shows structure unit Q n for Si, it reveals that with the increase of basicity in the range R = 1.15-1.25, Q 3 and Q 4 decomposed into Q 0 , Q 1 , and Q 2 , which simplifies the structural units for Si. When the basicity increases from 1.25 to1.35, Q 0 , Q 1 , and Q 3 increase, while Q 2 and Q 4 decrease. It means that the reactions Q 2 → Q 0 + 2Q 1 or Q 4 → Q 1 + Q 3 may be occurred, which makes the simplify extent of Si structure units less obvious.    Figure 3b shows structure unit Q n for B, it reveals that with the increase of basicity from 1.15 to 1.25, Q 0 , Q 1 and Q 4 increase, while Q 2 and Q 3 decreases. The reactions 2Q 2 → Q 0 + Q 4 or Q 2 + Q 3 → Q 1 + Q 4 may be occurred, which simplifies the structure units for B. When basicity increases from 1.25 to 1.35, Q 0 , Q 2 and Q 4 increase, while Q 1 and Q 3 decrease. The reactions Q 1 + Q 3 → 2Q 2 or Q 1 + Q 3 → Q 4 may be occurred, which makes the B structure units become more complex. Figure 4 shows the FTIR results of CaO-SiO 2 -B 2 O 3 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 [BO 3 ] 3− trihedral [26], the [SiO 4 ] 4− and [BO 4 ] 5− tetrahedral symmetry stretching [27,28], and the asymmetric stretching mode of [BO 3 ] 3− trihedral [26], respectively. Firstly, all the peaks become less pronounced as basicity increase from 1. 15 Figure 3b shows structure unit Q n for B, it reveals that with the increase of basicity from 1.15 to 1.25, Q 0 , Q 1 and Q 4 increase, while Q 2 and Q 3 decreases. The reactions 2Q 2 → Q 0 + Q 4 or Q 2 + Q 3 → Q 1 + Q 4 may be occurred, which simplifies the structure units for B. When basicity increases from 1.25 to 1.35, Q 0 , Q 2 and Q 4 increase, while Q 1 and Q 3 decrease. The reactions Q 1 + Q 3 → 2Q 2 or Q 1 + Q 3 → Q 4 may be occurred, which makes the B structure units become more complex. 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   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].

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 Combing the MD and FTIR results of CaO-SiO 2 -B 2 O 3 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].       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 [BO 4 ] 5− tetrahedral, resulting in the polymerization of [BO 3 ] 3− trihedral to form [BO 4 ] 5− tetrahedral and the stabilization of [BO 4 ] 5− tetrahedral, thus making the melt structure more complex.

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

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 (B 2 O 3 ) = 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 Ca 3 Si 2 O 7 , Ca 11 B 2 Si 4 O 22 , and Ca 3 MgSi 2 O 8 , 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. which could depolymerize the silicate or borate network structure of slag and form more nonbridging 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.

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.

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: , the slag structure becomes simpler first and then changes to complex structure, which leads to decrease and then increase of slag viscosity theoretically. 2. 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. 3. 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.