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Role of B2O3 on structure and shear-thinning property in CaO–SiO2–Na2O-based mold fluxes

Abstract Most traditional mold fluxes are Newtonian fluids, and their constant viscosity has certain limitations in continuous casting. A new non-Newtonian fluid mold flux with shear-thinning behavior, i.e., a mold flux with a relatively high viscosity at lower shear rates and a relatively low viscosity at higher shear rates, is required to satisfy the mold-flux performance requirements for high-speed continuous casting. The addition of a certain amount of B2O3 to a CaO–SiO2–Na2O-based mold flux can result in a shear-thinning property. To obtain an improved understanding of the mechanism of this characteristic, a molecular-dynamics simulation method was used to study the microstructural changes of the mold flux. Structural changes of mold-flux samples were analyzed and verified by Raman spectroscopy. The results of the two methods were almost the same, both resulted from the addition of B2O3 and changed the microstructure and degree of polymerization of the mold flux, which resulted in the shear-thinning property of the mold flux. This non-Newtonian fluid mold flux was used in square-billet casting tests, and the quality of the slab was improved effectively.


Introduction
Mold flux is important in the continuous casting of highquality steel. In the lubrication region of the casting mold, and specifically at the meniscus and the area under the meniscus of the casting mold, the mold flux acts as a lubricant, and the shear rate can be as high as 100-1000 s −1 . In this case, the viscosity (η) of the mold flux should be as low as possible to achieve a higher lubrication level. In addition, within the retention area of the casting mold, which is the molten-steel surface, a relatively high moldflux viscosity is required to improve the slag-entrapment phenomenon, which decreases the occurrence of slag inclusions in the molten steel; the shear rate in this area is 10-40 s −1 [1][2][3].
A mold flux that exhibits a gradual decrease in viscosity with an increase in shear rate, can maintain a relatively high viscosity in the mold-retention area, which reduces the probability of the slag layer being involved in the molten steel, and promotes the flotation of slag inclusions and reduces slab defects. A relatively low viscosity exists in the lubrication area, which ensures that the mold flux can flow into and below the meniscus with time and maintain a certain liquid-slag-layer thickness. A good lubrication effect is achieved, which improves the heat transfer and billet quality, and reduces the frequency of bondingsteel leakage [4,5]. The mold flux exhibits a shear-thinning property, which is a non-Newtonian fluid characteristic.
A Newtonian liquid is a liquid whose viscosity is independent of its shear rate. As the shear rate increases, the variation in viscosity is low or remains unchanged, and the traditional mold flux that is used in casting is a typical Newtonian fluid [6]. Therefore, traditional mold fluxes cannot resolve the contradiction above. A non-Newtonian fluid is a fluid whose viscosity changes with changes in shear rate [7]. Non-Newtonian fluids can be divided into two categories: one in which the viscosity increases gradually with an increase in shear rate, and demonstrates shear-thickening behavior, and the other in which the viscosity decreases gradually with an increase in shear rate, and demonstrates shear-thinning behavior. Based on the problems above, the urgent development of a non- Newtonian fluid mold flux with shear-thinning properties is required. Few studies exist on non-Newtonian fluid mold fluxes, only a few researchers from Japan and South Korea [8,9] have found that the addition of a certain amount of B 2 O 3 to traditional mold flux can allow it to exhibit a shear-thinning property. With an increase in B 2 O 3 content, the shear-thinning property increased and then decreased. This characteristic behavior results because B 2 O 3 changed the degree of polymerization (DP) of the mold flux system. B 2 O 3 addition (as a type of fluxing agent) will reduce the mold-flux viscosity [10], but the specific law that decreases the mold-flux viscosity is unclear and it is not known if it will always decrease, and what an appropriate B 2 O 3 addition is. Research is lacking in some microfields; it is not known whether the mold-flux structure will change to exhibit non-Newtonian fluid properties.
In this study, B 2 O 3 was used as a key additive to verify whether it can yield non-Newtonian fluid characteristics in the mold flux. Different contents of B 2 O 3 were added to the CaO-SiO 2 -Na 2 O-based mold flux, and an improved rotary viscometer was used to detect and analyze the shearthinning property of the mold flux.
To study how B 2 O 3 addition into the mold flux results in structural changes and leads to the shear-thinning property of the mold flux, the microstructural change in the mold flux after B 2 O 3 addition was studied by moleculardynamics simulation. Molecular-structural changes in the mold-flux samples were analyzed by using hightemperature Raman spectroscopy. The non-Newtonian fluid mold flux was used in square-billet casting tests.

Measurement of viscosity and analysis of shear-thinning behavior
The η values at rotation speeds of 50, 100, 150, and 200 r/min that correspond to shear rates (r ′ ) of 18, 35, 53, and 70 s −1 (shear rate = 0.35 × rotation speed, where 0.35 represents the rotor coefficient), were measured using the RTW-13-type melt physical-property comprehensive tester. The resultant η-r ′ curves are presented in Figure 1(a).  The reduction ratio of the viscosity of the mold flux in the same range of shear-rate gradient change was used to measure the strength of the shear-thinning behavior, i.e., the shear-thinning rate. The specific calculation equation is expressed as Eq. (1): where N represents the shear-thinning rate, ηmax represents the viscosity at a shear rate of 18 s −1 , η min represents the viscosity at a shear rate of 70 s −1 , and ∆r ′ represents the variation in shear rates. The calculation results of the shear-thinning rate for each mold-flux sample are presented in Table 2. The minimum shear-thinning rate of the Blank-1 sample was 0.00050, whereas the maximum shear-thinning rate of the B-4 sample was 0.00104. Therefore, we concluded that the B-4 sample exhibited the strongest shear-thinning behavior and that of the Blank-1 sample was weakest. An increase in B 2 O 3 content in the mold flux resulted in an initial increase and then decrease in shear-thinning behavior.  Figure 1(b) shows the average viscosity of the moldflux sample. The average viscosity of the mold flux that contains B 2 O 3 was significantly lower than that of the blank sample. When the content of B 2 O 3 changed from 8% to 12%, the average mold-flux viscosity increased gradually. However, when the content of B 2 O 3 exceeded 12%, the average viscosity of the mold fluxes decreased rapidly. Therefore, it is necessary to conduct an in-depth study on the molecular structure of the mold flux in this process.

Molecular-dynamics simulation
After B 2 O 3 addition into the CaO-SiO 2 -Na 2 O-based mold flux, the changes in radial distribution function, coordination number and proportion of the Si-O atoms were calculated by molecular-dynamics simulation. The structuralchange information and the DP of the mold flux during the process were analyzed, and the influence of B 2 O 3 addition on the shear-thinning property of the mold flux was studied further [11][12][13][14][15]. The temperature change with time in the simulation process is shown in Figure 2. Table 3 shows the composition of the simulated slag system. Table 4 shows the number of atoms, side length, and density of the box. Figure 3 shows the simulation results with 12% B 2 O 3 . To observe the molecular structure more clearly, one crys-        Figure 5. As shown in Figure 5, the coordination structures of Si-O and B-O were relatively stable, but showed some differences. The Si-O coordination number showed that the coordination number of Si-O was maintained at~4. As shown in Figure 5  To study the influence of B 2 O 3 on the degree of aggregation of the Si-O tetrahedron in the mold flux, a search was conducted by programming with the O atom at the center and the Si-O bond length (1.625 Å) as the radius. The search results are shown in Figure 6. N represents the number of Si atoms that are combined with one O atom (i.e., coordination number). The 2-coordinated Si (bridging oxygen) and 1-coordinatied Si (non-bridging oxygen) were connected mainly with O atoms, and the DP of the mold flux could be analyzed from the coordination number of O atoms. An increase in B 2 O 3 content resulted in an increase in number of bridging oxygens and a decrease in the number of non-bridging oxygens. The number of non-bridging oxygens represents the degree of fracture in the system, whereas the number of bridging oxygens represents the DP. When the content of B 2 O 3 in the mold flux was 12%, the number of bridging oxygens in the system was highest and the DP of the mold flux was highest.

Analysis of radial-distribution function and coordination number of
According to calculations in previous experiment, when the B 2 O 3 content changed within the range of 8%-14%, the shear-thinning property of the mold flux increased and then decreased, and when the content of B 2 O 3 was 12% (B-4 sample), the shear-thinning property

Quantitative analysis by high-temperature Raman spectroscopy
The B-2, B-3, B-4, and B-5 samples were characterized by high-temperature Raman spectroscopy at 1300 ∘ C. Figure 7(a) presents the Raman spectrum of the B-4 sample, in which the B 2 O 3 content was 12%. Figure 7(b) shows the peak deconvolution spectrum of the B-4 sample, as obtained by using the PEAKFIT software, in the Raman shift range from 1200 to 800 cm −1 .  Figure 7(a) shows two relatively obvious peak values in the range of Raman displacement of 600-400 cm −1 and 1200-800 cm −1 , respectively. Among them, the spectral peak with a Raman displacement of 600-400 cm −1 contained some simple Si-O b -Si or Si-O nb connection structures [16,17]. However, the spectral peak of Raman displacement for 1200-800 cm −1 contained some Qn structural units and [BO 4 ] tetrahedral structural units [16,17]. A small peak occurred where the Raman displacement was 1450-1300 cm −1 , which contained a two-dimensional [BO 3 ] triangular structure [16,17]. Figure 7(b) shows abundantly dispersed anion structural-unit (Qn) information around the Raman shift at 1200-800 cm −1 . The peaks at 860, 936, 1000, and 1110 cm −1 correspond to the Q 0 (SiO 4− 4 stretching of the monomer structural unit), Q 1 (Si 2 O 6− 7 stretching of the dimer structural unit), Q 2 (SiO 2− 3 stretching of the chain structural unit), and Q 3 (Si 5 O 2− 5 stretching of the sheetlike structural unit) [16,17], where Q represents the [SiO 4 ] tetrahedron unit and n represents the number of bridging oxygen atoms in each tetrahedron [18]. However, one addition structural unit, Q 4 (SiO 2 moganite) was not detected in the Raman spectra because its concentration in the mixed silicate was low. Figure 8 shows the molecular diagrams of all five anion structural units. The DP and molecular concentration within the mold flux were calculated from the Raman spectroscopic data. The polymerization reaction in the mold flux is represented by Eqs. (2) to (4): where K is the equilibrium constant, C 2 and C 3 represent the concentration of Q 2 and Q 3 , respectively, and C 4 represents the concentration of the molecular polymerization units. According to Eq. (4), the C 4 could be calculated from C 3 /C 2 ; therefore, in this work, C 3 /C 2 was considered to represent the DP of the equilibrium system that was established through the reactions among the silicate structural units within the mold flux [19,20].
To analyze the changes in Qn structural units and the variation law of DP in the CaO-SiO 2 -Na 2 O-B 2 O 3 mold flux, the relative areas of each anion structural unit in the B-2, B-3, B-4, and B-5 samples were best-fitted to a Gaussian function. The results are presented in Table 5.
As shown in Table 5, when the content of B 2 O 3 in the slag changed from 8% to 12% (samples B-2 to B-4), the relative proportional area (C 2 ) of the structural unit of Q 2 decreased gradually, and the relative proportional area (C 3 ) of the structural unit of Q 3 increased gradually. At this point, C 3 /C 2 increased gradually, that is, the DP of the mold flux increased gradually. However, when the content of B 2 O 3 in the slag changed from 12% to 14% (samples B-4 to B-5), C 2 increased and C 3 decreased. At this point, C 3 /C 2 decreased, that is, the DP of the mold flux decreased. Therefore, when the content of B 2 O 3 in the slag changed from 8% to 14% (samples B-2 to B-5), the DP of the mold flux increased initially and then decreased, and reached a maximum for the B-4 sample. The experimental results were consistent with the molecular-dynamics simulation results.
The simulation and experimental results indicated that when the B 2 O 3 content varied from 0% to 12%, most of the B 2 O 3 existed in the mold flux as a three-dimensional [BO 4 ] tetrahedral structure, whereas a few existed as two-dimensional [BO 3 ] triangular structures. This occurs mainly because the alkali-metal oxide Na 2 O in the slag has a relatively weak control of the O atoms, and it is easy to provide free O atoms. The bonding ability between the B and O atoms is strongest, so the free O atom in the slag will bond preferentially with the B atom to form a threedimensional [BO 4 ] tetrahedron structure, which will increase the DP in the mold flux. The reaction principle is shown in Figure 9.  In conclusion, the shear-thinning property of the mold flux is related directly to the DP, and the DP is related to the amount of B 2 O 3 . When the content of B 2 O 3 varies from 0% to 12%, the DP of the mold flux increases, and the shear-thinning property increases. When the content of B 2 O 3 varies from 12% to 14%, the DP of the mold flux decreases, and the shear-thinning property decreases. The results show that when the B 2 O 3 content was 12%, the shearthinning property of the mold flux was strongest.

Applications to continuous casting
During continuous casting, the mold flux is critical as a lubricating agent at the mold wall where the shear rate is assumed to range between 100 and 1000 s −1 . In this case, the low viscosity of the mold flux is desirable to maximize the lubrication capacity [9]. Figure 11 shows the conceptual effect of the CaO-SiO 2 -Na 2 O-B 2 O 3 non-Newtonian fluid mold flux (B-4 sample) during continuous casting. To verify the practical application of a non-Newtonian fluid mold flux with shear-thinning properties, samples Blank-1 and B-4 (after compounding with a certain amount of carbon) were used separately in square-billet casting tests at the multifunctional continuous-casting experimental stage. The section size of the casting mold was 100 mm × 100 mm, the pulling rate was 1.9 m/min, and the slag consumption was 0.2 kg/t. The as-cast steel composition is presented in Table 6. Figure 12 shows the casting-slab surface morphology when samples Blank-1 and B-4 were used. The surface of the Blank-1 as-cast slab was rough, with a relatively serious slag-sticking phenomenon, which causes problems in subsequent rolling and molding steps. The surface of the B-4 as-cast slab was relatively smooth, where the slag sticking phenomenon improved visibly. The casting quality, however, was improved substantially. This improvement in casting quality is attributed primarily to the B-4 sample showing a strong shear-thinning, which resulted in a relatively low viscosity in the lubricating area at a high shear rate, and lead to a significant improvement in its lubricating ability (Figure 11), so that the casting quality was improved visibly. When the B 2 O 3 content varied from 0% to 12%, the DP increased because of the increasing proportion of [BO 4 ] tetrahedron structures in the mold flux. When the B 2 O 3 content changed at 12%-14%, its content exceeded the Na 2 O content in the slag. CaO in the mold flux as a network-modifying oxide promotes the depolymerization reaction so that the proportion of [BO 3 ] triangular structure increases and results in a decrease in DP. When the content of B 2 O 3 was 12%, the DP of the mold flux was highest and the shear-thinning property was strongest. 2. Blank-1 and the B-4 mold-flux samples were used in laboratory square-billet casting tests. The slagsticking phenomenon on the surface of the billet with sample Blank-1 was relatively serious; however, the phenomenon decreased and the surface quality of the billet improved significantly when the B-4 sample was used. This new type of mold flux has good application prospects in the field of low carbon-steel casting.