Effect of Caf2 on the Viscosity and Microstructure of Cao–Sio2–Al2o3

metals

Article Eﬀect of CaF2 on the Viscosity and Microstructure of CaO–SiO2–Al2O3 Based Continuous Casting Mold Flux

Xingjuan Wang 1,2,*, Hebin Jin 1,2, Liguang Zhu 3,*, Ying Xu 2, Ran Liu 1, Zhanlong Piao 2 and Shuo Qu 1,2

1 College of Matallurgy and Energy, North China University of Science and Technology, Tangshan 063000, China 2 Hebei Engineering Research Center of High Quality Steel Continuous Casting, Tangshan 063000, China 3 School of Materials Science and Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, China * Correspondence: [email protected] (X.W.); [email protected] (L.Z.); Tel.: +86-189-3157-1093 (X.W.); +86-311-8166-8002 (L.Z.) Received: 10 June 2019; Accepted: 5 August 2019; Published: 8 August 2019

Abstract: In this study, a CaO–SiO2–Al2O3-based continuous casting mold flux was designed using the FactSage thermodynamics software to determine the composition range of CaF2. The viscosity characteristics of the mold flux were determined using a rotating viscometer. The results show that the constant temperature viscosity at 1300 ◦C decreases gradually as CaF2 content is increased from 3% to 11% in the CaO–SiO –Al O -based slag. Viscosity is reduced from 0.854 to 0.241 Pa s, viscous 2 2 3 · the flow activation energy is reduced from 157.74 to 114.34 kJ mol 1, and the break temperature is · − reduced from 1280 to 1180 ◦C. Furthermore, when the CaF2 content is increased from 3% to 11%, the number of nonbridging fluorine bonds (Al–F structure and Si–F structure) in the melt increases to 287, the number of bridging fluorine bonds (Al–F–Al structure, Si–F–Si structure, and Si–F–Si structure) is only 17, and the network rupture of fluorine ions in the system is larger than the network formation. Consequently, both the degree of polymerization and viscosity are reduced.

Keywords: ﬂuorine; mold ﬂux; viscosity; microstructure; continuous casting

1. Introduction

The main component of fluorite is CaF2, which is added as an auxiliary material in the steel continuous casting production to improve the lubrication and partial heat transfer between a crystallizer and a solidified shell [1,2]. Fluorite used in metallurgical industry accounts for 25% of the total output of fluorite. Fluoride has different degrees of evaporation under high temperature conditions, which does extremely harmful to the environment, people and equipment [3,4]. However, it is also a scarce strategic resource. According to statistics, the total global fluorite reserves are 470 million tons. The fluorite resources in the United States and Western Europe have been exhausted. In addition, China has stopped issuing new fluorite licenses since 2003 to protect fluorite resources. The National Mineral Resources Planning (2016–2020) issued by the Ministry of Land and Resources in 2016 officially listed fluorite as a national strategic mineral resource. Therefore, to reduce the consumption of fluorite, it is necessary to study the occurrence of fluorine in a continuous casting mold flux and develop the development of a fluorine-free mold flux, which plays an important role in resource control. Park and Ueda et al. [5–7] found that F ions act as depolymerized bridging oxygen (O0) and nonbridging oxygen (O1) in silicates, and the reaction relationship is as shown in Equations (1) and (2).

Metals 2019, 9, 871; doi:10.3390/met9080871 www.mdpi.com/journal/metals Metals 2019, 9, 871 2 of 13

6 5 3 [Si3O9] − + 2F− = [Si2O6F] − + [SiO3F] − (1)

5 3 3 2 2 [Si2O6F] − + [SiO3F] − + 2F− = 2[SiO3F] − + [SiO2F2] − + O − (2)

Kim et al. [8] used X-ray photoelectron spectroscopy to study the effect of the addition of CaF2 in a CaO–SiO2–Na2O slag system on slag viscosity and slag structure. Asada et al. [9] studied the slag structure of a molten CaO–CaF2–SiO2 slag system and pointed out that substituting CaO with CaF2 enhanced the polymerization reaction. However, owing to the increase in fluorine ions, the polymerization was suppressed to replace the position of Si–O tetrahedral oxygen, and the number of Q3 and Q4 was reduced. Gao et al. [10] used Raman spectroscopy to determine the effect of fluorine on the structure of a CaO–SiO2–Al2O3–Na2O–CaF2 continuous casting mold flux and reported that the number of nonbridging oxygen ions in Si–O and Al–O tetrahedra increased and the degree of polymerization decreased. Yan et al. [11] studied the feasibility of using CaO–Al2O3-based slag to replace CaF2 with B2O3 and found that B2O3 can inhibit crystallization and control heat transfer. Sasaki et al. [12] studied a Na2O–NaF–SiO2 slag system using Raman spectroscopy and a molecular dynamics model. They found that a Si–F bond was not formed in the slag system and believed that the fluorine ion mainly matched with the Na ion. Vincent et al. [13] reported that the radius of the fluoride ion was smaller than that of the oxygen ion. Moreover, the fluorine ion had a higher mobility, and the system depolymerization reaction was larger. Hayashi et al. [14] reported that the fluorine was dominantly coordinated with Ca rather than Si, and the degree of polymerization did not change with CaF2 addition in constant alkalinity. Niu et al. [15] studied the change in the characteristic spectrum of cuspidine (3CaO 2SiO CaF ) using high temperature Raman spectroscopy and pointed out that, as · 2· 2 temperature increased, system disorder increased, the high frequency peak broadened, and the crystal lost its long-range orderliness. When cuspidine was melted, the structure of dimmer Q1 was still dominant in the melt. Additionally, the structure monomer Q0 was correspondingly increased, and the peripheral structure was relaxed. Wang et al. [16] studied the crystallization behavior of protective slag in a CaO–SiO2–Al2O3 slag system in CaF2 and pointed out that after the mass fraction of CaF2 was increased to a specific amount, the fluorine ion was converted to a certain degree due to the network forming effect. Wu [17] found through molecular dynamics studies that the average bond length of Al–O, Ca–O and Ca–F increased. Furthermore, the average bond length of a small number of Al–F and Si–F bonds decreased and the fluorine ion replaced the oxygen ions, which affected the melt structure. Previous studies have been conducted on the influence of fluorine on the metallurgical properties of a mold flux. However, the state of the occurrence of fluorine in a high temperature mold flux is still unclear. The metallurgical properties of a mold flux are determined by its structure. Composition determines structure, and structure determines performance. Thus, it is necessary to study the structure of a mold flux. In this study,a CaO–SiO2–Al2O3-based mold flux was designed using the FactSage thermodynamics software to determine the composition range of CaF2. The viscosity characteristics of the mold flux were determined by utilizing a rotating viscometer. The Scigress molecular dynamics software was used to calculate the radial distribution function, coordination number function, and fluorine microstructure of a CaO–SiO2–Al2O3–CaF2-based continuous casting mold flux. The results can provide a theoretical basis for developing a fluorine-free mold flux and for seeking out suitable fluorine substitutes. Now that we understand the microstructure of fluorine, we can use molecular dynamics to explore the microcosmic aspects of combinations of alternatives to fluorine that are composed of one or more elements.

2. Calculation and Experimental Procedures

2.1. Selection Principle of CaF2 Content

To macroscopically determine the effect of CaF2 on the CaO–SiO2–Al2O3-based slag system, a phase diagram was created using the phase diagram module of FactSage (FactSage 7.2, Thermfact/CRCT and Technologies, Canada and Germany). The three components, namely CaO, SiO2, and Al2O3, were set as Metals 2019, 9, x FOR PEER REVIEW 3 of 13

2. Calculation and Experimental Procedures

2.1. Selection Principle of CaF2 Content

To macroscopically determine the effect of CaF2 on the CaO–SiO2–Al2O3-based slag system, a phase diagram was created using the phase diagram module of FactSage (FactSage 7.2, Metals 2019, 9, 871 3 of 13 Thermfact/CRCT and Technologies, Canada and Germany). The three components, namely CaO, SiO2, and Al2O3, were set as the main components of the phase diagram, and CaF2 was the fixed thecomponent. main components The phase of thediagram phase was diagram, drawn andusing CaF these2 was four the components, fixed component. and the Theinfluence phase of diagram CaF2 wason drawn the melting using temperature these four components, and liquid phase and formation the influence characteristics of CaF2 on of the the melting flux was temperature analyzed. The and liquidphase phase diagram formation had a characteristicstemperature range of the of flux 1100–1600 was analyzed. °C and Thea step phase size diagramof 50 °C. hadThe a calculation temperature results are shown in Figure 1. range of 1100–1600 ◦C and a step size of 50 ◦C. The calculation results are shown in Figure1.

FigureFigure 1. 1.CaO–SiO CaO–SiO2–Al2–Al22OO33–CaF22 quaternaryquaternary phase phase diagram. diagram.

TheThe casting casting temperature temperature of of molten molten steelsteel inin thethe continuous casting casting process process was was approximately approximately 15501550◦C. °C. The The phase phase diagram diagram calculated calculated the liquid the phaseliquid region phase of region the CaO–SiO of the2 –AlCaO–SiO2O3–CaF2–Al2 2quaternaryO3–CaF2 slagquaternary system at slag a temperature system at a temperature below 1600 below◦C. The 1600 thin °C. solid The thin line solid in Figure line in1 representsFigure 1 represents the isotherm the composedisotherm of composed the liquidus of the temperature, liquidus temperature, and the area and surrounded the area surrounded by the thick by solid the line thick represents solid line the crystallinerepresents region the crystalline of the mineral, region which of the first mineral, crystallizes which when first the crystallizes temperature when is decreased.the temperature To achieve is thedecreased. melting temperature, To achieve the viscosity melting change, temperature, and physicochemical viscosity change, properties and physicochemical of the protective properties slag afterof thethe adsorption protective of slag inclusions, after the metallurgical adsorption of workers inclusions, frequently metallurgical control theworkers alkalinity frequently of the control slag between the 0.9alkalinity and 1.10 [of18 ].the Therefore, slag between it is in0.9 accordance and 1.10 [18]. with Therefore, the requirements it is in accordance that alkalinity with R the was requirements approximately that alkalinity R was approximately 1.0. Figure 1 shows that the CaF2 content was in a range of 0–12% 1.0. Figure1 shows that the CaF 2 content was in a range of 0–12% when R = 1.0. The liquid phase when R = 1.0. The liquid phase temperature in Figure 1 is 1100 °C. When the CaF2 content was in a temperature in Figure1 is 1100 ◦C. When the CaF2 content was in a range of 11–30%, the liquidus range of 11–30%, the liquidus temperature was 1100 °C, which gradually increased to 1350 °C. temperature was 1100 ◦C, which gradually increased to 1350 ◦C. Considering that the CaF2 content was Considering that the CaF2 content was in a range of 0–12%, the liquid phase was stable. In addition, in a range of 0–12%, the liquid phase was stable. In addition, as CaF2 is a toxic substance, the CaF2 as CaF2 is a toxic substance, the CaF2 content of the CaO–SiO2–Al2O3–CaF2 quaternary slag system content of the CaO–SiO2–Al2O3–CaF2 quaternary slag system should be controlled within 12%. should be controlled within 12%. 2.2. Fluorine Slag Viscosity Performance Test 2.2. Fluorine Slag Viscosity Performance Test A test was conducted on a ﬂuorine-containing protective slag of a steel plant. Cement clinker, A test was conducted on a fluorine-containing protective slag of a steel plant. Cement clinker, a a pre-melting material, and wollastonite were used as raw materials. CaF2 was added by chemical pre-melting material, and wollastonite were used as raw materials. CaF2 was added by chemical pure pure reagent. When the basicity of the mold ﬂux was maintained, the content of CaF2 was changed. reagent. When the basicity of the mold flux was maintained, the content of CaF2 was changed. The The chemical composition is presented in Table1. chemical composition is presented in Table 1.

Table 1. Chemical composition of a ﬂuorine-containing mold ﬂux (%).

Component SiO2 Al2O3 MgO MnO2 Fe2O3 Na2O CaO K2O CaF2 R Content 1 40.48 5.0 2.37 0.39 3.13 4.33 40.48 0.82 3 1.0 Content 2 39.48 5.0 2.37 0.39 3.13 4.33 39.48 0.82 5 1.0 Content 3 38.48 5.0 2.37 0.39 3.13 4.33 38.48 0.82 7 1.0 Content 4 37.48 5.0 2.37 0.39 3.13 4.33 37.48 0.82 9 1.0 Content 5 36.48 5.0 2.37 0.39 3.13 4.33 36.48 0.82 11 1.0 Metals 2019, 9, x FOR PEER REVIEW 4 of 13

Table 1. Chemical composition of a fluorine-containing mold flux (%).

Component SiO2 Al2O3 MgO MnO2 Fe2O3 Na2O CaO K2O CaF2 R Content 1 40.48 5.0 2.37 0.39 3.13 4.33 40.48 0.82 3 1.0 Content 2 39.48 5.0 2.37 0.39 3.13 4.33 39.48 0.82 5 1.0 Content 3 38.48 5.0 2.37 0.39 3.13 4.33 38.48 0.82 7 1.0 MetalsContent2019, 9, 8714 37.48 5.0 2.37 0.39 3.13 4.33 37.48 0.82 9 4 1.0 of 13 Content 5 36.48 5.0 2.37 0.39 3.13 4.33 36.48 0.82 11 1.0

In thisthis study,study, aa rotatingrotating viscometerviscometer testtest devicedevice (Chongqing(Chongqing University, Chongqing,Chongqing, China)China) waswas used toto determinedetermine the the ﬁxed fixed temperature temperature viscosity viscosit andy and temperature temperature change change viscosity viscosity curve curve of the of mold the ﬂux.mold Theflux. schematic The schematic of the of test the device test device is shown is shown in Figure in Figure2. 2.

Figure 2. Schematic of viscosity measurement apparatus.

Experimental steps:

(1)(1) First,First, 350 350 g of mold flux flux were placed into a mufflfflee furnace. Then, the temperature of the mumuffleffle furnace was increased to 800 °C and maintained constant for 10 h to remove carbon from the furnace was increased to 800 ◦C and maintained constant for 10 h to remove carbon from the moldmold residue. residue. (2)(2) AA graphite crucible crucible and and a a sleeve were placed into the rotating viscometer, and then 350 g of decarburization flux were added. When the temperature of the furnace increased to 1300 °C, the decarburization flux were added. When the temperature of the furnace increased to 1300 ◦C, the viscosity of the mold flux flux was measured at a constant temperature for 10 min. (3) The temperature was raised to 1400 °C and maintained constant for 10 min. Then, it was rapidly (3) The temperature was raised to 1400 ◦C and maintained constant for 10 min. Then, it was rapidly reduced at a rate of 5 °C/min, during which the viscosity at different temperatures was measured reduced at a rate of 5 ◦C/min, during which the viscosity at different temperatures was measured toto obtain obtain a a viscosity viscosity temperature temperature curve.

2.3. Model Establishment

2 To studystudy thethe eeffectffect ofof aa specificspecific CaFCaF2 contentcontent onon thethe microstructuremicrostructure ofof thethe protectiveprotective slag,slag, the molecular structurestructure of of the the mold mold flux flux was was investigated investigated by utilizing by utilizing the Scigress the Scigress (SCIGRESS, (SCIGRESS, FQS Poland FQS Sp.z.o.o,Poland Sp.z.o.o, Kraków, Poland)Kraków, molecular Poland) dynamicsmolecular simulation dynamics method simulation [19]. Asmethod molecular [19]. dynamics As molecular treats microscopicdynamics treats particles microscopic as mass particles points, the as mass Newtonian points, motion the Newtonian equation motion of each equa particletion was of each established particle underwas established initial conditions under initial and solvedconditions according and solved to a particular according timeto a particular step. After time several step. iterations, After several the particlesiterations, in the the systemparticles tended in the to system be dynamic tended and to stable, be dynamic and the and statistical stable, particles, and the coordinates,statistical particles, system structure,coordinates, and system dynamic structure, properties and could dynamic be obtained. properties As could the interaction be obtained. force As between the interaction particles isforce the resultbetween of mathematicalparticles is the fitting result by researchers,of mathematical there shouldfitting notby beresearchers, large number there of elementsshould not in thebe samelarge number of elements in the same system. In this experiment, the effect of different contents of CaF2 on system. In this experiment, the effect of different contents of CaF2 on the CaO–SiO2–Al2O3 ternary slag systemthe CaO–SiO was investigated2–Al2O3 ternary under slag R = system1.0. Table was2 presentsinvestigated the slagunder components R = 1.0. Table used 2 in presents the simulation. the slag componentsThe crystal used structure in the simulation. of the mold flux changed with the composition, and the specific structure of the mold residue was not known before the simulation. The model was built by converting the existing components into specific atomic numbers and randomly adding them to a box of three-dimensional periodic boundaries. The total number of particles set by the simulation was approximately 6000. The number of particles corresponding to each element in the simulated system was calculated from the moles ratio of each ion. Table3 presents the number of simulated particles and the length of the side of the box. Metals 2019, 9, 871 5 of 13

Table 2. Compositions of slags at 1673 K.

Composition (wt %) Number Basicity CaO SiO2 Al2O3 CaF2 1 1 46 46 5 3 2 1 45 45 5 5 3 1 44 44 5 7 4 1 43 43 5 9 5 1 42 42 5 11

Table 3. Simulated atomic number and box length of slags at 1673 K.

Mole Fraction (%) Atomic Number Box Length Density (Å) (g/cm3) CaF2 Ca Si Al F O Total 3 1198 1070 137 106 3490 5999 43.345 2.820 5 1213 1047 137 179 3423 6000 43.332 2.829 7 1226 1026 137 252 3358 5999 43.464 2.838 9 1238 1006 137 323 3295 5999 43.584 2.846 11 1252 984 138 396 3229 5999 43.669 2.855

A few necessary parameters must be set after establishing the initial model. The Born–Mayer– Huggins (BMH) potential function has been proven to be relatively mature in the silicate field. Potential parameters are provided in Table4[ 20,21]. The system selected the number of atoms, N; volume, V; and temperature, T. Gear’s method was used for track integration, and temperature was controlled by employing the scaling method. Temperature changed with time, as shown in Figure3. The initial temperature was set as 5000 K after 10,000 fs to obtain an initial configuration. Then, the temperature was decreased to 1673 K after 10,000 fs, and it was finally maintained at 1673 K at 10,000 fs to obtain the final structure and performance information.

Table 4. Parameter values of BMH potential function.

6 Hydronium Hydronium Aij (V) B (1/Å) C (eV Å ) ij ij · Ca Ca 5.274 10 21 6.25 4.33 × − Ca O 1.150 10 20 6.06 8.67 × − Ca Al 5.920 10 22 6.25 0 × − Ca F 7.939 10 21 6.06 8.67 × − Ca Si 4.275 10 22 6.25 0 × − Si O 1.006 10 21 6.06 0 × − Si Si 3.466 10 23 6.25 0 × − Si F 6.945 10 22 6.06 0 × − Si Al 4.797 10 23 6.25 0 × − Al O 1.379 10 21 6.06 0 × − Al Al 6.639 10 23 6.25 0 × − OO 2.395 10 20 5.88 17.34 × − OF 1.046 10 20 5.88 17.34 × − FF 1.169 10 20 5.88 17.34 × − Al F 9.518 10 22 6.06 0 × − Metals 2019, 9, x FOR PEER REVIEW 6 of 13

5500 Metals 2019, 9, 871 6 of 13 Metals 2019, 9, x FOR PEER REVIEW 10000fs 6 of 13 5000

45005500 10000fs 40005000 20000fs 35004500 T/K

30004000 20000fs 25003500 T/K 20003000 30000fs

15002500

0 5000 10000 15000 20000 25000 30000 2000 Steps/fs 30000fs 1500 Figure0 3. Variation 5000 10000 in temperature 15000 20000 25000with time. 30000 Steps/fs

3. Results and Discussion FigureFigure 3. 3.VariationVariation inin temperaturetemperature with with time. time. 3. Results and Discussion 3.1. Flurine Slag Viscosity Performance Test and Result Analysis 3. Results and Discussion 3.1. Flurine Slag Viscosity Performance Test and Result Analysis The measurement results obtained at a constant temperature of 1300 °C are shown in Figure 4. It3.1. can Flurine be observed SlagThe measurementViscosity that as Perforthe results CaFmance2 obtained content Test atand increased, a constantResult Analysis temperaturethe fixed temperature of 1300 ◦C are viscosity shown in Figuredecreased4. from 0.854 Pa·sIt canat 3% be observed to 0.241 that Pa·s as at the 11%. CaF2 content increased, the ﬁxed temperature viscosity decreased from The0.854 measurement Pa s at 3% to 0.241results Pa sobtained at 11%. at a constant temperature of 1300 °C are shown in Figure 4. · · It can be observed that as the CaF2 content increased, the fixed temperature viscosity decreased from 0.9 0.854 Pa·s at 3% to 0.241 Pa·s at 11%. 0.8

0.9 0.7

0.8 0.6

0.7 0.5

0.6 0.4 Viscosity/Pa·s 0.5 0.3

0.4 0.2 Viscosity/Pa·s

0.323456789101112 Mass% CaF 0.2 2

FigureFigure 4. Influence 4. Influence of CaF23456789101112 of CaF2 content2 content on on the the measured at at fixed fixed temperature temperature of 1300 of◦C. 1300 °C. Mass% CaF 2 Figure5 shows the viscosity–temperature curve of the protective slag under di fferent CaF2 Figure 5 shows the viscosity–temperature curve of the protective slag under different CaF2 contents.Figure It can 4. Influence be observed of thatCaF the2 content viscosity on graduallythe measured increased at fixed with temperature the decrease of in temperature.1300 °C. contents.At It a can certain be coolingobserved rate, that the pointthe viscosity at which the grad viscosityually changedincreased abruptly with canthe bedecrease referred toin astemperature. the At a certaintransition cooling temperature rate, the [22 point]. It can at be which seen that the as theviscosity content ofchanged CaF2 content abruptly in the moldcan be flux referred increased to as the Figure 5 shows the viscosity–temperature curve of the protective slag under different CaF2 from 3% to 11%, the viscosity and transition temperature gradually decreased. The reason for the transitioncontents. Ittemperature can be observed [22]. thatIt can the be viscosity seen that grad asually the increasedcontent of with CaF the2 content decrease in inthe temperature. mold flux increasedinflection from 3% point to 11%, is as follows; the viscosity On one and hand, transition according temperature to the structural gradually theory of silicatedecreased. slag, theThe reason At a certaindegree cooling of connection rate, the of the point Si–O at tetrahedron which the network viscosity at this changed temperature abruptly increases can rapidly, be referred which to as the for the inflection point is as follows; On one hand, according to the structural theory of silicate slag, transitionmakes temperature viscosity increase [22]. faster.It can On be the seen other hand,that as it is the possible content to precipitate of CaF crystal2 content particles in the at this mold flux theincreased degreetemperature, fromof connection 3% resultingto 11%, of the inthe an Si–O viscosity increase tetrahedron in and viscosity transition ne [23twork,24]. temperature at this temperature gradually increases decreased. rapidly, The whichreason makesfor the viscosity inflection increase point is faster. as follows; On the On other one hand,hand, itaccording is possible to to the precipitate structural crystal theory particles of silicate at slag,this temperature,the degree of resultingconnection in ofan the increase Si–O tetrahedron in viscosity ne [23,24].twork at this temperature increases rapidly, which makes viscosity increase faster. On the other hand, it is possible to precipitate crystal particles at this temperature, resulting in an increase in viscosity [23,24].

Metals 2019Metals, 9, x2019 FOR, 9, PEER 871 REVIEW 7 of 13 7 of 13

5 CaF2 3%

CaF2 5% 4 CaF2 7% CaF 9% 3 2

CaF2 11%

2 Viscosity/Pa·s

0 1150 1200 1250 1300 1350 1400 Temperature/℃

FigureFigure 5. Viscosity 5. Viscosity temperature temperature curvecurve at at di ﬀdifferenterent CaF2 CaFcontents.2 contents.

Temperature is an important factor that affects the viscosity of the mold flux. The viscosity of Temperature is an important factor that affects the viscosity of the mold flux. The viscosity of continuous casting slag is a function of temperature. In a certain temperature range, the relationship continuousbetween casting viscosity slag and is a temperature function of obeys temperatur the Arrheniuse. In Equation a certain (3) temperature [25]. range, the relationship between viscosity and temperature obeys the Arrhenius" # Equation (3) [25]. Eη η = A exp (3) RTE η = η Aexp  (3) where η is the viscosity of the mold flux (Pa s), A is the frequencyT factor, Eη is the activation energy of · R the viscous fluid (J mol 1), R is the gas constant (8.314 J mol 1 K 1), and T is the absolute temperature. · − · − · − η The viscous flow activation energy is the minimum energy required for a flow unit to overcome where is the viscosity of the mold flux (Pa·s), A is the frequency factor, Eη is the activation the barrier and transition from the original position to a neighboring cavity during flow. The viscous energy offlow the activation viscous energy fluid reflects (J·mol the−1), ease R ofis flowthe [gas26–29 constant]. Table5 lists(8.314 the viscousJ·mol−1 flow·K−1), activation and T energyis the absolute temperature.of the mold flux with different CaF2 contents. The viscous flow activation energy is the minimum energy required for a flow unit to overcome Table 5. The effect of CaF2 on mold fluxes in activation energy and transition temperature. the barrier and transition from the original position to a neighboring cavity during flow. The viscous Mass% 1 Transition Temperature flow activation energy reflectsArrhenius the Equation ease of flowEη/(kJ [26–mol29].− ) Table 5 lists the viscous flow activation energy CaF2 · Temperature (◦C) Range (◦C) 2 of the mold flux3 with differentLnη = 18,972 CaF/T 13.57 contents. 157.74 1280 1280–1400 − 5 Lnη = 16,313/T 13.25 135.63 1230 1230–1400 − 7 Lnη = 15,813/T 12.43 131.47 1210 1210–1400 Table 5. The effect of CaF2 on− mold fluxes in activation energy and transition temperature. 9 Lnη = 14,892/T 12.42 123.81 1195 1195–1400 − 11 Lnη = 13,753/T 12.25 114.34 1180 1180–1400 Mass% CaF2 Arrhenius Equation − Eη/(kJ·mol−1) Transition Temperature (°C) Temperature Range (°C) 3 Lnη = 18,972/T − 13.57 157.74 1280 1280–1400 5 It canLn beη = observed 16,313/T − that 13.25 as the CaF 135.632 content increased, the viscous 1230 flow activation energy 1230–1400 and 7 transitionL temperaturenη = 15,813/T of− 12.43 the mold flux 131.47 tended to decrease. The 1210 maximum viscous flow activation 1210–1400 1 9 energy wasLnη 157.74 = 14,892/ kJ molT − 12.42− at 3% CaF2 123.81content, and the minimum 1195viscous flow activation 1195–1400 energy · 11was 114.34Ln kJη =mol 13,753/1 atT 11%− 12.25 CaF . The 114.34 transition temperature was 1180 reduced from 1280 to 11801180–1400C. · − 2 ◦ The activation energy of a moving particle determines the ease of slag flow. The larger the activation energy, the larger the energy required by a particle to overcome the barrier. In a silicate system, the size It can be observed that as the CaF2 content increased, the viscous flow activation energy and of the Si–O complex ion is considerably larger than that of a metal cation, and the viscous flow activation transition temperature of the mold flux tended to decrease. The maximum viscous flow activation energy required for moving in the slag is also larger. Thus, the silicate complex ion becomes a limiting −1 energy wasfactor 157.74 for flow kJ·mol and viscosity. at 3% However, CaF2 content, when external and the ions minimum change the structureviscous offlow the silicateactivation complex energy was 114.34 kJ·molions, dissolving−1 at 11% or polymerizingCaF2. The transition the Si–O complex temperature ions can cause was a changereduced the from viscosity 1280 of the to slag. 1180 °C. The activation energy of a moving particle determines the ease of slag flow. The larger the activation energy, the larger the energy required by a particle to overcome the barrier. In a silicate system, the size of the Si–O complex ion is considerably larger than that of a metal cation, and the viscous flow activation energy required for moving in the slag is also larger. Thus, the silicate complex ion becomes a limiting factor for flow and viscosity. However, when external ions change the structure of the silicate complex ions, dissolving or polymerizing the Si–O complex ions can cause a change the viscosity of the slag.

3.2. Microstructure Analysis

Figure 6 shows the simulation results when the CaF2 content was 9% at 1673 K. In the figure, the Ca, O, Si, Al, and F ions, are shown in blue-green, red, yellow, pink, and blue, respectively. It can be

Metals 2019, 9, 871 8 of 13

Metals 20193.2., 9, Microstructurex FOR PEER REVIEW Analysis 8 of 13 Metals 2019, 9, x FOR PEER REVIEW 8 of 13 Figure6 shows the simulation results when the CaF 2 content was 9% at 1673 K. In the ﬁgure, the observedCa, from O, Si, Figure Al, and 6 F that, ions, in are this shown model, in blue-green, the Ca ions red, yellow, did not pink, participate and blue, respectively.in the bonding, It can bethe Si ions observed from Figure 6 that, in this model, the Ca ions did not participate in the bonding, the Si ions and Al ionsobserved existed from as Figure netting6 that, substances in this model, in the the syst Ca ionsem, did the not O participateions were in mostly the bonding, in the the form Si ions of bridging and Al ions existed as netting substances in the system, the O ions were mostly in the form of bridging and nonbridgingand Al ions oxygen existed as bonds, netting and substances the F inions the system,were mostly the O ions nonbridg were mostlying influorine the form bonds. of bridging andand nonbridging nonbridging oxygen oxygen bonds, bonds, and and the the F F ions ions were mostly nonbridgingnonbridging ﬂuorine fluorine bonds. bonds.

Ca O Si Al F

Figure 6. Model of ions in the moldCa flux with O 9% SiCaF 2, obtained Al F through simulation in Scigress.

Figure 6. Model of ions in the mold flux with 9% CaF , obtained through simulation in Scigress. FigureFigure 6. Model 7 shows of theions bonding in the mold state fluxof the with F ions 9% whenCaF2 2, theobtained content through CaF2 content simulation was 9%. in Scigress. It can be observed that at 1673 K, the F ions formed a bond between the Al ligand structure and the Si ligand Figure7 shows the bonding state of the F ions when the content CaF 2 content was 9%. It can be Figurestructure.observed 7 shows The that fluorine atthe 1673 bonding bond K, the was F state ions in formedthe of formthe a F bonda ions Si–F–Si between when bond, the the an Alcontent Al–F–Al ligand structureCaF bond,2 content and and a theSi–F–Al was Si ligand 9%. bond It can be observed(structurestructure. that at shown 1673 The fluorine inK, Figure the bondF ions7). wasThe formed inbridging the form a bondfluorine a Si–F–Si between transformed bond, the an Al–F–Al Al the ligand slag bond, structure structure and a Si–F–Alinto and a complex bondthe Si ligand frame. On the contrary, the F ion and the Al and Si tetracoordinate structures formed an Al–F bond structure.(structure The fluorine shown inbond Figure was7). Thein the bridging form fluorinea Si–F–Si transformed bond, an the Al–F–Al slag structure bond, into and a complexa Si–F–Al bond andframe. a Si–F On bond. the contrary, Furthermore, the F ion the and non-bridging the Al and Si fluorine tetracoordinate cluster mate structuresrial filled formed the an network Al–F bond structure and (structure shown in Figure 7). The bridging fluorine transformed the slag structure into a complex anda Si–Ffunctions bond. to Furthermore, dilute the thenetwor non-bridgingk structure. fluorine Only cluster an extremely material small filled thenumber network of structureF ions did and not frame.participate Onfunctions the contrary, in to the dilute bonding, thethe network F andion theyand structure. existedthe Al Only inand anthe extremelySi free tetracoordinate F ion small state. number structures of F ions did formed not participate an Al–F bond and a Si–Fin thebond. bonding, Furthermore, and they existed the non-bridging in the free F ion fluorine state. cluster material filled the network structure and functions to dilute the network structure. Only an extremely small number of F ions did not participate in the bonding, and they existed in the free F ion state.

(a) (b) (c) (d) (e)

FigureFigure 7. 7.BondingBonding state state diagram diagram of of the the F ion whenwhen CaF CaF2 2content content is is 9% 9% (a )(a Si–F–Si) Si–F–Si structure; structure; (b) Si–F–Al(b) Si–F– Al structure;structure; ( c( )c) Al–F–Al Al–F–Al structure; structure; (d )(d Al–F) Al–F structure; structure; and and (e) Si–F(e) Si–F structure. structure.

Figure8 shows the radial distribution function and coordination number at a CaF 2 content of (Figurea) 8 shows the radial( bdistribution) function and(c) coordination number(d) at a CaF2 content(e of) 9%. The abscissa corresponds to the first peak of the radial distribution function [30] in Figure8a, 9%. The abscissa corresponds to the first peak of the radial distribution function [30] in Figure 8a, which is the bond length between the particles in the mold flux, and the bond lengths of Si–O, Ca–O, Figurewhich 7. is Bondingthe bond statelength diagram between of the the particles F ion when in the CaF mold2 content flux, and is 9% the ( abond) Si–F–Si lengths structure; of Si–O, ( bCa–O,) Si–F– and Al–O were 1.625, 2.225, and 1.725 Å, respectively. Matsui [31] conducted an X-ray diffraction Aland structure; Al–O were (c) Al–F–Al 1.625, 2.225, structure; and 1.725 (d) Al–F Å, respectively. structure; and Matsui (e) Si–F [31] structure. conducted an X-ray diffraction experiment to determine the silicate melt structure. The bond lengths of Si–O and Al–O were 1.6 and experiment1.7 Å, respectively. to determine The simulationthe silicate results melt structure. are consistent The with bond those lengths of the of experiment. Si–O and Al–O Figure were8b shows 1.6 and Figure 8 shows the radial distribution function and coordination number at a CaF2 content of 1.7the Å, Orespectively. coordination The number simulation of Si, Ca, results and Al. are It cancons beistent observed with from those Figure of the8b thatexperiment. the coordination Figure 8b 9%. Theshows numberabscissa the ofO Si–Ocorrespondscoordination is 4, and the numberto platform the firstof wasSi, peak Ca, the andmostof thAl stable.e. radialIt can The bedistribution coordination observed from number function Figure of Al–O [30] 8b that wasin Figure 4,the 8a, whichcoordination is the bond number length ofbetween Si–O is 4,the and particles the platform in the was mold the most flux, stable. and the The bond coordination lengths number of Si–O, of Ca–O, and Al–OAl–O werewas 4, 1.625, the platform 2.225, was and narrower 1.725 Å, than respectively. that of Si–O, Matsuiand it was [31] the conductedsecond most anstable. X-ray The diffraction lack of a platform for Ca–O indicates that the coordination number was highly unstable. Combined with experiment to determine the silicate melt structure. The bond lengths of Si–O and Al–O were 1.6 and the bond length of each ion pair in the simulated slag system and the full width at the half maximum 1.7 Å, respectively. The simulation results are consistent with those of the experiment. Figure 8b of the first peak, it can be observed that the bonding ability of the Si–O bond was the strongest and showsmost the stable,O coordination followed by numberAl–O. The of Ca–O Si, bondCa, and in the Al slag. It systemcan be was observed the weakest from and Figure did not 8byield that the coordinationa stable structure.number of Si–O is 4, and the platform was the most stable. The coordination number of Al–O was 4, the platform was narrower than that of Si–O, and it was the second most stable. The lack of a platform for Ca–O indicates that the coordination number was highly unstable. Combined with the bond length of each ion pair in the simulated slag system and the full width at the half maximum of the first peak, it can be observed that the bonding ability of the Si–O bond was the strongest and most stable, followed by Al–O. The Ca–O bond in the slag system was the weakest and did not yield a stable structure.

Metals 2019, 9, 871 9 of 13

the platform was narrower than that of Si–O, and it was the second most stable. The lack of a platform for Ca–O indicates that the coordination number was highly unstable. Combined with the bond length of each ion pair in the simulated slag system and the full width at the half maximum of the ﬁrst peak, it can be observed that the bonding ability of the Si–O bond was the strongest and most stable, followed Metalsby Al–O.2019, 9, Thex FOR Ca–O PEER bond REVIEW inthe slag system was the weakest and did not yield a stable structure.9 of 13

20 10

Si-O 8 15 Al-O

Metals 2019, 9, x FOR PEER REVIEW 6 9 of 13 r 10 N/ g/r

20 104 Si-O Al-O Ca-O 5 Si-OCa-O 28 15 Al-O

0 06 012345678910 0123456 r 10 r/Å r/Å N/ g/r 4 (a) Si-O Al-O( Ca-Ob) 5 Ca-O 2 FigureFigure 8. 8. TheThe simulationsimulation results results of CaFof 2CaFcontent2 content at 9% (aat) radial9% ( distributiona) radial distribution function and function (b) coordination and (b) number function. coordination0 number function. 0 012345678910 0123456 r/Å Figure9 shows the e ffect of different CaF contents on the Si–O, Al–O, andr/Å Ca–O coordination Figure 9 shows the effect of different CaF22 contents on the Si–O, Al–O, and Ca–O coordination number functions. It can(a) be observed from Figure9a that when the CaF 2 content(b) was increased from number functions. It can be observed from Figure 9a that when the CaF2 content was increased from 3% to 11%, the Si–O coordination number “platform” of 4 was flattened, and the radius was also 3% to 11%, the Si–O coordination number “platform” of 4 was flattened, and the radius was also increased.Figure 8. Moreover, The simulation the formed results Si–O of CaF structure2 content became at 9% unstable. (a) radial The distribution Al–O coordination function and number (b) increased. Moreover, the formed Si–O structure became unstable. The Al–O coordination number platformcoordination in Figure number9b was function. similar to the Si–O coordination number platform. Figure9c shows that the platform in Figure 9b was similar to the Si–O coordination number platform. Figure 9c shows that Ca–O coordination number platform did not appear when the CaF2 content was changed, indicating the Ca–O coordination number platform did2 not appear when the CaF2 content was changed, thatFigure Ca ions 9 shows did not the participate effect of in different bonding CaF at 1673 contents K and they on the were Si–O, free outsideAl–O, and the structuralCa–O coordination system. indicatingnumberIn functions. Figure that Ca 10 a,Itions can the did coordinationbe observednot participate from number Figure in ofbonding Si 9a ions that isat when shown.1673 the K CaF Theand2 coordination theycontent were was free structuresincreased outside from of the structural3%Si–O to 11%, mainly system. the was Si–O three coordination coordination number structures, “platform” four coordination of 4 was flattened, structures, and and the five radius coordination was also increased.structures. Moreover, The coordination the formed number Si–O of structure the other Obecame atoms isunstable. too small The to be Al–O listed. coordination It can be observed number 10 4.6 10 4.4 platform in Figure 9b was similar to the Si–O coordination4.4 number platform. Figure 9c shows that that with increase 3% in the CaF24.2 content, the coordination number of O atoms (the number of 3% Si–O 4.0 4.2

N/r 2 the Ca–O coordination number3.8 platform did not appear when the CaF content was changed, 5% 4.0 5% tetrahedra) decreased, and when the content of CaF2 was8 more than 7%, it tended to be stable. The N/r 8 3.6 7% 3.8 7% indicatingcoordination that number Ca ions of did O atoms not3.4 andparticipate the coordination in bonding number at 1673 of 5 tendedK and tothey be stable. were Thefree numberoutside of the 3.6 9% 1.8 2.0 2.2 2.4 2.6 2.8 9% r/Å 3.4 structuralthree-coordinated system. and ﬁve-coordinated O atoms was relatively6 small and tended to be stable. 6 11% 2.4 2.6 2.8 11% r/Å r N/ N/r 10 10 4.6 4.4 4 4 4.4 3% 4.2 3% 4.0 4.2

5% N/r 3.8 4.0 5% 8 N/r 8 3.6 2 7% 2 3.8 7% 3.4 3.6 9% 1.8 2.0 2.2 2.4 2.6 2.8 9% r/Å Si-O 3.4 6 Al-O 6 2.4 2.6 2.8 11% 0 11% 0 r/Å 0123456 r 0123456 N/ N/r r/Å r/Å 4 4 (a) (b) 2 2

Si-O Al-O 0 10 0 0123456 0123456 3% r/Å r/Å 5% 8 g/r 7% (a) 4 9% (b) 6 11%

r 2.22.42.62.83.0Å Figure 9. Cont. r/Å

N/ 10 4 3% 5% 8 2 g/r 7% 4 9% 6 Ca-O 11% 0 r 2.22.42.62.83.0Å 0123456r/Å N/ 4 r/Å (c) 2

Ca-O Figure 9. Coordination number function of (a) Si–O (b) Al–O and (c) Ca–O at different CaF2 contents. 0 0123456 r/Å (c)

Figure 9. Coordination number function of (a) Si–O (b) Al–O and (c) Ca–O at different CaF2 contents.

Metals 2019, 9, x FOR PEER REVIEW 9 of 13

20 10

Si-O 8 15 Al-O

6 r 10 N/ g/r 4 Si-O Al-O Ca-O 5 Ca-O 2

0 0 012345678910 0123456 r/Å r/Å (a) (b)

Figure 8. The simulation results of CaF2 content at 9% (a) radial distribution function and (b) coordination number function.

Figure 9 shows the effect of different CaF2 contents on the Si–O, Al–O, and Ca–O coordination number functions. It can be observed from Figure 9a that when the CaF2 content was increased from 3% to 11%, the Si–O coordination number “platform” of 4 was flattened, and the radius was also increased. Moreover, the formed Si–O structure became unstable. The Al–O coordination number platform in Figure 9b was similar to the Si–O coordination number platform. Figure 9c shows that the Ca–O coordination number platform did not appear when the CaF2 content was changed, indicating that Ca ions did not participate in bonding at 1673 K and they were free outside the structural system.

10 4.6 10 4.4 4.4 3% 4.2 3% 4.0 4.2

5% N/r 3.8 4.0 5% 8 N/r 8 3.6 7% 3.8 7% 3.4 3.6 9% 1.8 2.0 2.2 2.4 2.6 2.8 9% r/Å 3.4 6 6 11% 2.4 2.6 2.8 11% r/Å r N/ N/r 4 4

2 2

Si-O Al-O 0 0 0123456 0123456 r/Å r/Å

Metals 2019, 9, 871 (a) (b) 10 of 13

10 3% 5% 8 g/r 7% 4 9% 6 11%

r 2.22.42.62.83.0Å r/Å

Metals 2019, 9, x FOR PEER REVIEWN/ 10 of 13 4 In Figure 10a, the coordination number of Si ions is shown. The coordination structures of Si–O mainly was three coordination2 structures, four coordination structures, and five coordination structures. The coordination number of the other O atoms is too smallCa-O to be listed. It can be observed 0 that with increase in the CaF2 0123456content, the coordination number of O atoms (the number of Si–O r/Å tetrahedra) decreased, and when the content of CaF2 was more than 7%, it tended to be stable. The coordination number of O atoms and the coordination(c) number of 5 tended to be stable. The number of three-coordinated and five-coordinated O atoms was relatively small and tended to be stable. Figure 9. Coordination number function of (a) Si–O (b) Al–O and (c) Ca–O at diﬀerent CaF2 contents. Figure 9. Coordination number function of (a) Si–O (b) Al–O and (c) Ca–O at different CaF2 contents. 700 1650 N=2 600 1600 N=1 N=3 500 N=4 1550 N=5

400 1500

300 1450

200 1400 O atom coordination number/N coordination atom O Si atom coordination number/N coordination atomSi 100 1350 2 4 6 8 10 12 24681012 Mass % CaF Mass% CaF 2 2 (a) (b)

FigureFigure 10. 10. NumberNumber of atoms coordinatedcoordinated to to ( a()a Si) Si and and (b ()b O) O at at di ﬀdifferenterent CaF CaF2 contents.2 contents.

FigureFigure 10b 10b shows shows the coordination numbernumber of of the the O O ions. ions. A A two-coordinated two-coordinated Si atomSi atom with with O O atomatom as as the the center center is is called called bridging oxygen.oxygen. NonbridgingNonbridging oxygen oxygen was was only only one-coordinated one-coordinated Si atom Si atom centeredcentered on on the the O O atom. atom. The The network structurestructure of of the the slag slag was was mainly mainly formed formed by by the the polymerization polymerization of silicon oxide ions. As the CaF2 content increased, the number of bridging oxygen bonds decreased, of silicon oxide ions. As the CaF2 content increased, the number of bridging oxygen bonds decreased, while the number of nonbridging oxygen bonds first increased, then decreases, and finally tended to while the number of nonbridging oxygen bonds first increased, then decreases, and finally tended to be constant. The radii of F and O2 were 1.25 10 7 and 1.32 10 7 mm, respectively, which were −− −2− − −7 − −7 be constant. The radii of F and O were 1.252 × × 10 and 1.32× × 10 mm, respectively, which were extremely close. Therefore, F− can replace O −, and it acted on the Si–O bond of the composite ion extremely close. Therefore, F− can replace O2−, and it acted on the Si–O bond of the composite ion group to destroy the silicon oxide cluster ions. The ionic group structure split a large molecular group groupinto smaller to destroy composite the silicon anions oxide and cluster gradually ions. transformed The ionic group a large-sized structure structural split a large unit into molecular a structure group intoin which smaller a pluralitycomposite of microstructureanions and gradually units coexisted, transformed thereby a large-sized reducing the structural degree of unit polymerization into a structure inof which the protective a plurality slag. of Themicrostructure particles flow units of particles coexisted, in the thereby slag was reducing easier, resulting the degree in a of decrease polymerization in the ofviscosity the protective of the mold slag. fluxThe [24particles]. flow of particles in the slag was easier, resulting in a decrease in the viscosityFigure 11of showsthe mold the flux change [24]. in the number of nonbridging fluorine bonds and bridging fluorine bondsFigure at di ff11erent shows CaF the2 contents. change Thein the fluorine number ion of connecting nonbridging two Si–Ofluorine structures bonds or and Al–O bridging structures fluorine is bondsreferred at different to as bridging CaF2 fluorine.contents. One The end fluorine of fluorine ion connecting is connected tw too Si–O the network structures structure, or Al–O while structures the isother referred end isto notas bridging connected fluorine. to network One ions. end The of fluo cationsrine outside is connected the network to the were network used structure, to maintain while a theneutral other electrical end is value,not connected indicating to that network it was nonbridging ions. The fluorine.cations outside It can be the observed network from were Figure used 11a to maintainthat the numbera neutral of electrical nonbridging value, fluorine indicating bonds that increased it was fromnonbridging 65 to 287 fluorine. with the It increase can be observed in the fromCaF 2Figurecontent. 11a Figure that the 11b number shows thatof nonbridging the bridging fl fluorineuorine bonds did not increased fluctuate significantlyfrom 65 to 287 with with the the increaseincrease in in thethe CaF CaF2 content.2 content. The Figure number 11b of Si–F–Sishows bonds that wasthe thebridging highest fluorine in the system; did therenot fluctuate were significantly with the increase in the CaF2 content. The number of Si–F–Si bonds was the highest in the system; there were 17 bonds when the CaF2 content was 7%. The number of Al–F–Si bonds was the second highest; there were 7 bonds when the CaF2 content was 9%. The number of Al–F–Al bonds was the lowest. The maximum number of bonds was only 2. When the CaF2 content increased from 3% to 11%, the number of nonbridging fluorine bonds was higher than that of bridging fluorine bonds. It was indicated that F ions can disintegrate Si–O complex ions and Al–O complex ions and replace O in Si–O and Al–O structures, causing macromolecular groups to split into smaller Al–F, Si–F, Si– F–O, and other structures (see Figure 12). The degree of polymerization of the mold flux and viscosity were decreased. CaF2 could combine with the high melting point oxide to form a low melting point eutectic, which is equivalent to increasing slag superheat at the same temperature. When slag superheat was increased, the number of particles with viscous flow activation energy was increased,

Metals 2019, 9, 871 11 of 13

17 bonds when the CaF2 content was 7%. The number of Al–F–Si bonds was the second highest; there were 7 bonds when the CaF2 content was 9%. The number of Al–F–Al bonds was the lowest. The maximum number of bonds was only 2. When the CaF2 content increased from 3% to 11%, the number of nonbridging fluorine bonds was higher than that of bridging fluorine bonds. It was indicated that F ions can disintegrate Si–O complex ions and Al–O complex ions and replace O in Si–O and Al–O structures, causing macromolecular groups to split into smaller Al–F, Si–F, Si–F–O, and other structures (see Figure 12). The degree of polymerization of the mold flux and viscosity were decreased. Metals 2019, 9, x FOR PEER REVIEW 11 of 13 CaF2 could combine with the high melting point oxide to form a low melting point eutectic, which is Metals 2019, 9, x FOR PEER REVIEW 11 of 13 theequivalent vibration to of increasing the particles slag superheatwas strengthened, at the same the temperature. bonds between When network slag superheat structure was were increased, easily broken,the number and ofthe particles complex with ion viscous was disintegrated flow activation to energybecome was a increased,flow unit theof a vibration smaller ofsize, the thereby particles the wasvibration strengthened, of the particle the bondss wa betweens strengthened, network structure the bond weres between easily broken, network and structure the complex we ionre easily was reducing the viscosity [32]. In summary, when the CaF2 content is within this range, fluorine ions broken, and the complex ion was disintegrated to become a flow unit of a smaller size, thereby havedisintegrated a stronger to effect become on damaging a flow unit the of anetwork smaller of size, the thereby mold flux reducing compared the viscosity to forming [32 ].the In network. summary, reducwhening thethe CaFviscosity2 content [32]. is In within summary, this range, when fluorine the CaF ions2 content have ais stronger within this effect range, on damaging fluorine theions havenetwork a stronger of the effect mold on flux damaging compared the to network forming theof the network. mold flux compared to forming the network.

250 Al-F 18 Al-F-Al Si-F-Si Si-F 16 Si-F-Al y=4.3x-11.7 R2=0.919 y=0.05x+0.65 R2=-0.3 200 2 2 14 y=0.6x+8.6 R =0.095 y=27.1x-36.1 R =0.901 y=0.45x+1.65 R2=0.069 12

150 10

100 6

50 2

Non-bridgefluorine number 0

Bridgefluorine connection number 0 2 4 6 8 10 12 2 4 6 8 10 12 (a) (b) Mass CaF2 /% Mass CaF2 /%

Figure 11. (a) Number of nonbridging fluorine and (b) bridging fluorine. (a) (b)

FigureFigure 11. 11. (a)( Na)umber Number of ofnonbridg nonbridginging fluorine ﬂuorine and and ( (bb)) bridg bridginging ﬂuorine.fluorine.

+ = + +

Figure 12. Disintegration of silicate and aluminate structures by ﬂuorine+ ions. Figure 12. Disintegration+ = of silicate and aluminate+ structures by fluorine ions. 4. Conclusions

4.(1) ConclusionsThe thermodynamic calculations showed that the CaF2 content should be controlled within the range of below 12% when R = 1.0 in the CaO–SiO –Al O -based continuous casting mold ﬂux. (1) The thermodynamicFigure 12. Disintegration calculations of showed silicate thatand aluminatethe CaF2 2 2content structures3 should by fluorine be controlled ions. within the (2) rangeThe rotatingof below viscometer 12% when test R = results1.0 in the showed CaO–SiO that2 when–Al2O the3-based CaF 2continuouscontent in thecasting CaO–SiO mold2 –Alflux.2O 3- 4. Conclusions(2) Thebased rotatingcontinuous viscometer casting test results mold ﬂuxshowed increased that when within the theCaF2 range content of in 3–11%, the CaO–SiO the maintained2–Al2O3- basedtemperature continuous viscosity casting of 1300mold◦C flux decreased increas fromed within 0.854 tothe 0.241 range Pa s,of the 3–11%, break temperaturethe maintained was (1) The thermodynamic calculations showed that the CaF2 content should· be controlled within the temperature viscosity of 1300 °C decreased from 0.854 to 0.241 Pa·s, the break temperature was range of below 12% when R = 1.0 in the CaO–SiO2–Al2O3-based continuous casting mold flux. reduced from 1280 to 1180 °C, and the viscous flow activation energy was reduced from 157.74 (2) The rotating viscometer test results showed that when the CaF2 content in the CaO–SiO2–Al2O3- to 114.34 kJ·mol−1. The fluidity of the slag was enhanced. based continuous casting mold flux increased within the range of 3–11%, the maintained (3) Molecular dynamics simulations showed that when the CaF2 content increased from 3% to 11%, temperature viscosity of 1300 °C decreased from 0.854 to 0.241 Pa·s, the break temperature was the fluorine ions in the slag replaced the oxygen ions in the Si–O–Si structure and Al–O–Al reduced from 1280 to 1180 °C , and the viscous flow activation energy was reduced from 157.74 structure. The number of Al–F bonds and Si–F bonds that form nonbridging fluorine increased −1 to to114.3 287.4 The kJ·mol maximum. The fluiditynumber of Si–F–Sithe slag bonds, was enhanced. Al–F–Si bonds, and Al–F–Al bonds in the system (3) Molecular dynamics simulations showed that when the CaF2 content increased from 3% to 11%, was only 17. In this range, the CaF2 content was more than that required for the formation of the thenetwork. fluorine The ions macromolecular in the slag replace groupd thein the oxygen slag systemions in split the Siinto–O –numerousSi structure small and complex Al–O–Al structure.anions, whichThe number reduced of theAl– polymerizationF bonds and Si –degreeF bonds of thatthe formnetwork nonbridg body ingand fluorinethe viscosity increase of d to protective287. The maximum slag. number of Si–F–Si bonds, Al–F–Si bonds, and Al–F–Al bonds in the system was only 17. In this range, the CaF2 content was more than that required for the formation of the network. The macromolecular group in the slag system split into numerous small complex anions, which reduced the polymerization degree of the network body and the viscosity of protective slag.

Metals 2019, 9, 871 12 of 13

reduced from 1280 to 1180 ◦C, and the viscous flow activation energy was reduced from 157.74 to 114.34 kJ mol 1. The fluidity of the slag was enhanced. · − (3) Molecular dynamics simulations showed that when the CaF2 content increased from 3% to 11%, the fluorine ions in the slag replaced the oxygen ions in the Si–O–Si structure and Al–O–Al structure. The number of Al–F bonds and Si–F bonds that form nonbridging fluorine increased to 287. The maximum number of Si–F–Si bonds, Al–F–Si bonds, and Al–F–Al bonds in the system was only 17. In this range, the CaF2 content was more than that required for the formation of the network. The macromolecular group in the slag system split into numerous small complex anions, which reduced the polymerization degree of the network body and the viscosity of protective slag.

Author Contributions: L.Z. and X.W. conceived and designed the study; Y.X. and Z.P. performed the numerical calculation; H.J. and S.Q. conducted the experiment; R.L. and L.Z. analyzed the experimental data; and X.W. and H.J. wrote the paper. Funding: This work was financially supported by the National Natural Science Foundation of China, Project Nos, 51404088, 51674122 and 51774141. Acknowledgments: The authors thank Tian Kuo for his help with the experimental tests. They also wish to thank Gao Weimin for guidance on the molecular dynamics program. Conflicts of Interest: The authors declare no conflict of interest.

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