Fluorine-free Mold Flux Research Articles

Investigation of the erosion mechanism of fluorine-free mold flux on the submerged entry nozzle under the influence of negative electric field

Fluorine-free mold flux is receiving increasing attention due to the environmental and health problems associated with using fluorinated flux in casting. However, the erosion problem in the submerged entry nozzle is still exists and not solved. Therefore, in this study, the electric field is used to improve the erosion resistance capability of the nozzle. The results show that a thick protective adhesion layer is formed on the surface of the slag-line material under the action of a negative electric field due to an electrochemical reaction. This protective layer further reacts with the CaO/SiO2 content of the fluorine-free slag-line material to generate an interfacial reaction layer composed mainly of Ca2SiO4 and CaZrO3. Due to the presence of Ca2SiO4 in the interfacial reaction layer, a small amount of crystalline transformation occurs during the cooling process. Thus, under the protection of the double protective layer generated by the action of the negative electric field, the slag-line material is not subject to the erosion of the fluorine-free mold flux, and the erosion rate is only 0.37%.

Overview of the role of fluorine in the mold fluxes

Fluoride is one of components of the traditional mold flux, but it causes environmental pollution. With the increasing requirement of environmental protection on the metallurgical industry, it is a trend to develop low fluorine or fluorine-free mold fluxes in the future. In recent years, many scholars have researched some low fluorine or fluorine-free mold fluxes. However, because the action mechanism of fluorine in mold flux is not completely clear, the performance of mold fluxes composed of fluorine-substituting components has not reached the effect of fluorine-containing mold fluxes. In view of this, this paper summarizes the effects of fluorine in protective slag from macro and micro aspects: in the study of the macro properties of mold flux, it is generally believed that a certain amount of fluorine plays an important role in adjusting the melting temperature, viscosity and surface tension of mold flux, but its adjusting mechanism is not clear; in terms of microstructure, the structural behavior of fluorine in mold flux is very complicated due to the influence of alkalinity, other components and fluorine content. The microstructure of slag determines the properties of slag. Therefore, the complexity of the structural behavior of fluorine leads to the complexity of the transformation mechanism of the physical properties of mold flux, which makes it impossible to control the properties of mold flux fundamentally and increases the difficulty in selecting the alternative component of fluorine in the process of developing low fluorine or fluorine-free mold flux. Therefore, it is necessary to further study the influence mechanism of fluorine on the structure of mold flux.

Dissolution of TiO2 and TiN inclusions in CaO–SiO2–B2O3-based fluorine-free mold flux

Dissolution of TiO2 and TiN inclusions in CaO–SiO2–B2O3-based fluorine-free mold flux

Comprehensive evaluation of boron release from Boron-containing Fluoride-free mold flux

The boron emission into air and water during the use of fluorine-free mold flux containing boron was systematically investigated to evaluate the corresponding environmental impacts. Unexpectedly, boron in mold flux was volatilized in the form of NaBO2 particulate matter as Na2O was added to mold flux. Besides, boron remained in the used mold flux residue was found to be linearly released into leachate continuously, and Na+ existed in the borosilicate network accelerated the hydrolysis of the network and the dissolution of boron by increasing no-bridging oxygen and converting BO4 groups into fragile BO3 groups. Considering the negative effect of Na2O on the stabilization of boron in mold flux, limiting the addition of Na2O in fluorine-free mold flux containing boron will further minimize the pollution originating from the use of mold flux.

Viscosity and structure of CaO–Al2O3–TiO2-based mold fluxes with various CaO/Al2O3 mass ratios

ABSTRACT The effects of CaO/Al2O3 mass ratio on the viscosity and structure of new CaO–Al2O3–TiO2-based fluorine-free mold fluxes for high titanium steel are investigated by viscometer, molecular dynamics (MD) simulations and Raman spectroscopy. When the CaO/Al2O3 mass ratio increased from 0.6 to 1.4, the viscosities and activation energies reduced from 0.43 to 0.10 Pa·s and 127.8–74.2 kJ·mol−1, respectively. While the break temperatures raised from 1385 to 1507 K. Moreover, according to MD simulation results, the concentration of high coordination of Al–O and Ti–O bonds decreased, and the complex structure units of Q3, Q4 and Q5 turned to the simple structure units of Q0, Q1 and Q2. Raman spectra suggested that the concentrations of Ti–O–Ti(Al) linkage and Q2(Si-O−) decreased significantly, but the concentration of Q1(Si–O−) increased. Therefore, CaO/Al2O3 could simplify the network structure and decrease the viscosity of new CaO–Al2O3–TiO2-based fluorine-free mold fluxes for high titanium steel.

Combing role of MgO and Al2O3 on viscosity and its correlation to structure of fluorine-free mold fluxes

By using FactSage calculations, Fourier transform infrared spectroscopy (FTIR), rotational viscometers, and X-ray diffraction (XRD), the combined effect of MgO and Al2O3 on fluorine-free mold flux was confirmed. The viscosity of the slag at 1300 oC decreased with 2-10 wt% MgO, and higher Al2O3 increased the overall viscosity. The trend of the experimental results was consistent with the models of Riboud and Iida, while the FactSage calculation values were relatively higher. The viscosity of the slag was more influenced by the Si-O network than by the Al(B)-O structure and the MgO has a dual effect on the slag structure. The viscosity-temperature curves changed from alkaline to acidic slag characteristics when Al2O3 increased from 8 wt% to 12 wt%, and the trend was not uniform with the addition of 2-10 wt% MgO. Both FactSage calculation and XRD patterns showed that increasing MgO content gradually promoted the growth of Ca3MgSi2O8 and Ca11Si4B2O22 crystals, while the addition of Al2O3 inhibited crystal precipitation.

A comparison study on interfacial properties of fluorine-bearing and fluorine-free mold flux for casting advanced high-strength steels

A comparison study on interfacial properties of fluorine-bearing and fluorine-free mold flux for casting advanced high-strength steels

Effective mechanism of BaO on the structure and fluid behavior of CaO-SiO2-B2O3-based melts

The influence of BaO contents on structure and fluid behavior of fluorine-free CaO-SiO2-B2O3 based melts were examined using molecular dynamic (MD) simulation, rotary viscometer, X-ray diffraction and Fourier transform infrared spectroscopy. The FTIR results matched well with the MD simulation results that with the addition of BaO contents in CaO-SiO2-B2O3 based melts, more cations were provided to balance the negative charge of [BO4]5– tetrahedrons, leading to the structures of Si-O and B-O were more aggregated and the adjacent structural units were more closely connected, which made the structure complex, so the viscosity of mold fluxes at high temperature in the basicity range 1.15–1.25 was intensify. Meanwhile, when BaO replaced CaO under distinctive basic conditions, the viscous activity of fluorine-free mold flux was decreasing generally, showing that as the temperature was decreased, the tendency of slag to converge into complex network structural units was also decreasing, which states the improvement of lubrication capacity of the slag during cooling process.

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.

Effect of Al2O3 and MgO on crystallization and structure of CaO–SiO2–B2O3-based fluorine-free mold flux

The crystallization behavior and melt structure of the CaO–SiO2–B2O3-based fluorine-free mold flux were investigated. The results show that the crystallization of the mold flux was first inhibited and then promoted with the increase in Al2O3 content from 4 to 12 wt.%. However, it was enhanced by MgO in the range of 2–10 wt.%. The results of Fourier transform infrared spectroscopy and Raman spectroscopy showed that Al2O3 worked as a network former in the mold flux melt when its content was in the range of 4–8 wt.%, whereas it worked as the network breaker to provide O2− when its content was in the range of 8–12 wt.%. In addition, the combined effects from the charge compensation by Mg2+ and the network broken by O2− led to the increase in some typical T–O–T (Al–O–Al, B–O–B, etc.) and simpler structural units (Q0(Si), B–O− in the [BO2O−], etc.) when the MgO content was in the range of 2–6 wt.%. The continuous increase in O2− provided by the addition of MgO from 6 to 10 wt.% further depolymerized the network of the melt and finally caused fast crystallization.

Structure and its effect on viscosity of fluorine-free mold flux: Substituting CaF2 with B2O3 and Na2O

Fluorine in mold fluxes causes environmental pollution by dissolution into cooling water or evaporation, which considered seriously hazardous to human health and casting facilities. In order to develop a successful fluorine-free mold flux system, viscosity of borosilicate-based fluxes were investigated in consideration of structural evolution of borates when replacing CaF2 with B2O3 and Na2O. Measured viscosity decreased at the beginning of replacing CaF2 (15.74 to 10.77 wt%) with B2O3 (0 to 4.49 wt%) and then increased drastically further substituting CaF2 (10.77 to 4.71 wt%) with B2O3 (4.49 to 9.02 wt%). An actual determinant of viscosity is not the degree of B2O3 replacement, but rather the portion of tetrahedral borate structures (3-D) which necessarily need support of cations (sodium). Moreover, due to 19F NMR, it is suspected that fluorine tends to interact selectively with sodium, and consequently interferes with formation of tetrahedral borate structures.

Growth Mechanism and Structure Evolution during Nucleation of Calcium Borosilicate Crystal in CaO–SiO<sub>2</sub>–B<sub>2</sub>O<sub>3</sub> Based Fluorine-free Mold Flux

Growth Mechanism and Structure Evolution during Nucleation of Calcium Borosilicate Crystal in CaO–SiO<sub>2</sub>–B<sub>2</sub>O<sub>3</sub> Based Fluorine-free Mold Flux

A comparison study on viscosity and heat transfer property of fluorine-bearing and fluorine-free mold flux for casting silicon steel

A comparison study was conducted on viscosity and heat transfer property of the traditional fluorine-bearing (F-bearing) mold flux and the newly designed fluorine-free (F-free) mold flux for casting silicon steel. The results showed that the viscosity of F-free mold flux is lower than that of F-bearing mold flux, as the apparent activation energy of F-free mold flux is also lower than that of F-bearing mold flux confirmed by the kinetics results for viscous flow, which meant that the energy barrier of F-free mold flux that the migration of ion clusters needs to overcome is smaller, and thus leads to a lower viscosity of F-free mold flux. Besides, the research results of the heat transfer suggested that the comprehensive heat transfer coefficient of F-free mold flux is lower than that of F-bearing mold flux in all directions. One reason is that the true density of F-free mold flux increases from glassy state to crystalline state, while that of F-bearing mold flux is opposite. Another reason is that the air gap gets increased due to the volume shrinkage of F-free mold flux from glassy state to crystalline state and the solidification of the molten steel. These introduce a larger thermal resistance of F-free mold flux than that of F-bearing mold flux. Through the comparison, the designed F-free mold flux shows a better lubrication and capacity to control the heat transfer than the traditional F-bearing mold flux.

Crystallization behavior and structure analysis for molten CaO-SiO2-B2O3 based fluorine-free mold fluxes

An investigation had been conducted to study the effect of CaO/SiO2 mass ratio and B2O3 on the crystallization behavior and the melt structure of fluorine-free mold fluxes. The results showed that the increase of CaO/SiO2 ratio tends to promote the crystallization behavior of the fluorine-free mold flux system, due to the depolymerization of the original complex borosilicate structure by the released free oxygen (O2−) ions with the added CaO. Small amount of B2O3 would initially inhibit the mold flux crystallization, then the further addition would promote crystallization. The melt structure analysis results suggested that with the addition of B2O3 content, the existing silicate structure would connect with each other and also link with the dissociative [BO3]-trihedral units to form borosilicate chain. Simultaneously, O2– could destroy the original diborate structure and form two simple borate structures. Subsequently, the simple borate structure would connect with the dissociative [BO3]-trihedral and the formed borosilicate chain to form a more complex borosilicate structure. Thus, the degree of polymerization of whole molten slag was getting improved, but the overall effect on the mold flux crystallization was depended on the balance between the molten slag structure and viscosity, in which 6 mass% was the critical point in the current mold flux system.

Modelling of viscosity of fluorine-free mold fluxes using neural network

Viscosity is an important property of mold fluxes for steel continuous casting. However, direct measurement of viscosity of multi-component systems in a broad range of temperatures and compositions is an onerous work and has some limitations. This paper developed a model using the back propagation (BP) neural network to describe the viscosity of fluorine-free mold fluxes. The BP neural network model was developed and validated using 70 experimental values of viscosity of fluorine-free mold fluxes CaO-SiO2-Al2O3-B2O3-Na2O-TiO2-MgO-Li2O-MnO-ZrO2; 51 of them were used for developing the neural network model and the rest 19 viscosity data for the model validation. Calculated viscosities were in a good agreement with the experimental data. Based on the developed model, the effects of temperature and composition on the viscosity of fluorine-free fluxes were predicted and discussed.

A Review of Fluorine-free Mold Flux Development

A Review of Fluorine-free Mold Flux Development

Effect of B2O3 on Crystallization Behavior, Structure, and Heat Transfer of CaO-SiO2-B2O3-Na2O-TiO2-Al2O3-MgO-Li2O Mold Fluxes

The effect of B2O3 on crystallization behavior, structure, and heat transfer of CaO-SiO2-Na2O-B2O3-TiO2-Al2O3-MgO-Li2O fluorine-free mold fluxes was investigated using hot thermocouple technique (HTT), Raman spectroscopy, and infrared emitter technique (IET), respectively. The critical cooling rate in continuous cooling experiments decreased and the incubation time determined in isothermal experiments increased with increasing B2O3 content, both implying a decreasing crystallization tendency. The major phases of mold fluxes determined using X-ray diffraction changed from Ca2MgSi2O7 and Ca11Si4B2O22 to CaSiO3 with the increasing amount of B2O3. B2O3 increased the degree of polymerization in silicate network, forming 3D borate structures. Addition of B2O3 decreased the flux melting temperature which has a significant impeding effect on the crystal nucleation and growth. The heat flux of mold fluxes measured using IET showed that the increase of B2O3 from 4.7 to 6.8 mass pct impeded the heat flux; while a further increase of B2O3 from 6.8 to 10.4 mass pct promoted the heat transfer. This phenomenon was attributed to the variation of crystallization behavior and crystal morphology with different B2O3 content in the mold fluxes.

Effect of basicity and B2O3 on the viscosity and structure of fluorine-free mold flux

The effect of basicity (weight ratio of CaO/SiO2) and B2O3 on the viscosity and structure of Fluorine-free mold flux for the casting of medium carbon steels was conducted in this article, through the rotating cylinder method combined with the Fourier transform infrared (FTIR) spectroscopy. The results showed that, with the increase of basicity, the viscosity of mold flux was attenuated dramatically, and then kept constant or slight increased in the low temperature region. The reason could be explained as the degree of polymerization (DOP) of the network structure was first reduced significantly with the addition of basicity, and then the further depolymerization is less pronounced with the further increase of basicity. Beside the formation of high melting point substance leads to the slight increase of viscosity. Moreover, it suggested that the viscosity of mold flux is decreased with the addition of B2O3 content, due to the fact that B2O3 is a low melting point oxide which could substantially lower the break temperature of mold flux. The result of FTIR indicated B2O3 acts as network former, and tends to form [BO3]-trihedral and [BO4]-tetrahedral structural units, which would connect with each other to form some simple network structure such as diborate or pentaborate. With the addition of B2O3, the free oxygen ions (O2−) would depolymerize the diborate structural unit, and the depolymerized diborate would link again with free [BO3]-trihedral to form complex pentaborate groups. Moreover, the effect of above addition on the apparent activation energy for viscous flow and break temperature of mold flux also were discussed. The results obtained in this paper provide the detailed study of the structure evolution of Fluorine-free mold flux when B2O3 is added.

Stability of Fluorine-Free CaO-SiO2-Al2O3-B2O3-Na2O Mold Fluxes

B2O3 and Na2O are key components of fluorine-free mold fluxes for continuous casting, but both are highly volatile, which affects the flux stability. This paper investigated the evaporation of the SiO2-CaO-Al2O3-B2O3-Na2O fluxes (Na2O: 6 to 10 wt pct, CaO/SiO2 ratio: 0.8 to 1.3) in the temperatures ranging from 1573 K to 1673 K (1300 °C to 1400 °C) using thermogravimetric analysis. The weight loss as a result of the flux evaporation increased with the increasing temperature for all fluxes. The rate of evaporation was found to be very small for the Na2O-free flux but significantly increased with the addition of Na2O. The high evaporation rate of fluxes in the presence of B2O3 and Na2O was attributed to the formation of highly volatile NaBO2. Changing the ratio of CaO/SiO2, however, did not affect the rate of evaporation. Kinetic analysis of the evaporation processes demonstrated that external mass transfer contributed to the rate of evaporation.

Crystallization Behavior and Heat Transfer of Fluorine-Free Mold Fluxes with Different Na2O Concentration

In this study, the crystallization behavior and heat transfer of CaO-SiO2-Na2O-B2O3-TiO2-Al2O3-MgO-Li2O fluorine-free mold fluxes with different Na2O contents (5 to 11 mass pct) were studied using single/double hot thermocouple technique (SHTT/DHTT) and infrared emitter technique (IET), respectively. Continuous cooling transformation (CCT) and time–temperature transformation (TTT) diagrams constructed using SHTT showed that crystallization temperature increased and incubation time shortened with the increase of Na2O concentration, indicating an enhanced crystallization tendency. The crystallization process of mold fluxes in the temperature field simulating the casting condition was also investigated using DHTT. X-ray diffraction (XRD) analysis of the quenched mold fluxes showed that the dominant phase changed from CaSiO3 to Ca11Si4B2O22 with the increasing concentration of Na2O. The heat transfer examined by IET showed that the increase of Na2O concentration reduced the responding heat flux when Na2O was lower than 9 mass pct but the further increase of Na2O to 11 mass pct enhanced the heat flux. The correlation between crystallinity and heat transfer was discussed in terms of crystallization tendency and crystal morphology.