EVALUATION OF MOLD FLUX FOR CONTINUOUS CASTING OF HIGH-ALUMINUM STEEL
Wei Yan 1, 2, Alexander McLean1, Yindong Yang1, Weiqing Chen2 and Mansoor Barati1
1 Department of Materials Science and Engineering, University of Toronto, Ontario, Canada M5S 3E4
2State Key Laboratory of Advanced Metallurgy, University of Science and Technology, Beijing 100083, China
Keywords: mold flux, high-aluminum steel, CaO-Al2O3-based, viscosity, crystallization, fluoride-free Abstract
Study of mold fluxes for continuous casting of high-aluminum steel has attracted increasing attention due to severe reaction between aluminum in liquid steel and silica in the flux which results in changes in composition, properties and performance of the flux and adversely affects slab quality. This research began with sampling during the casting process of high-Al nonmagnetic steel 20Mn23AlV to evaluate potential problems. A mold flux was developed in which SiO2 was partially replaced with Al2O3. Different compositions including CaO/Al2O3, fluoride-free and low-fluoride lime-alumina-based fluxes were evaluated with respect to their influence on flux viscosity, crystallization and subsequent effects on steel quality. It was concluded that mold flux with CaO/Al2O3 ratio in the range 1.1 to 1.6 and characterized by low viscosity, low initial crystallization temperature and good heat transfer properties was appropriate for casting of high-aluminum steel. A viscosity model with high reliability in predicting the viscosity of mold flux containing high Al2O3 was also established.
Introduction
Application of conventional CaO-SiO2-based mold flux to the continuous casting of high- aluminum, nonmagnetic steel 20Mn23AlV (1.5~2.5 Al in mass pct.) results in substantial increase of Al2O3 and decrease of SiO2 in the molten flux due to severe slag/metal reaction, followed by issues such as massive formation of slag rims, poor lubrication, uneven heat transfer, sticking breakouts and poor surface quality. Lubrication and uniform heat transfer, probably the most important functions of a mold flux, are directly affected by the viscosity and crystallization behavior of the flux. It is particularly important to maintain good lubrication and uniform heat transfer in order to obtain good surface quality and castability. A number of studies[1,2] have been performed with conventional CaO-SiO2-based mold flux in attempts to obtain appropriate lubrication and crystallization properties during casting of high-Al steels.
However, the significant change in Al2O3 and SiO2 contents in the slag is still the main reason for unstable lubrication and excessive crystallization which in turn provides an incentive for the development of less-reactive mold flux.
To solve the aforementioned problems CaO-Al2O3 mold flux with a low content of the reactive component SiO2 has been proposed and evaluated for the casting of high-Al steels.[1,3] A principal step in the design of a less-reactive mold flux is the determination of an optimal CaO/Al2O3 ratio, by examining the effects of compositional change on the mold flux properties.
Studies[1,4,5] have described the application of CaO-Al2O3-based mold flux. However they focused on the crystallization of mold flux with only limited changes in the CaO/Al2O3 ratios
Advances in Molten Slags, Fluxes, and Salts: Proceedings of The 10th International Conference on Molten Slags, Fluxes and Salts (MOLTEN16) Edited by: Ramana G. Reddy, Pinakin Chaubal, P. Chris Pistorius, and Uday Pal TMS (The Minerals, Metals & Materials Society), 2016
at the same time as the contents of other components, such as Na2O, Li2O, CaF2 and B2O3. It was therefore difficult to evaluate the specific effect of CaO/Al2O3 ratios on mold flux performance. Moreover, the flux compositions did not simultaneously meet the coupled requirements for good crystallization and lubrication. Thus there is a need for a systematic study of the effect of a wide range of CaO/Al2O3 ratios on the viscosity and crystallization behavior of CaO-Al2O3-based mold flux in order to establish optimal CaO/Al2O3 ratios which will fulfil the dual requirements for good crystallization and lubrication properties.
With conventional CaO-SiO2-based mold flux, although CaF2 causes corrosion of equipment, SEN erosion and environmental pollution as well as health hazards, it is still added to control crystallization by forming the primary crystalline phase cuspidine and also to serve as a fluxing agent to decrease the solidus/liquidus temperatures and improve lubrication. Currently, almost all CaO-Al2O3-based mold fluxes are designed with additions of 10% to 20% CaF2. However, cuspidine may be difficult to form in CaO-Al2O3-based mold flux due to the low SiO2 content.
Several studies have been conducted to develop fluoride-free or low-fluoride mold flux by substituting CaF2 with TiO2, Na2O or B2O3.[6-8] in CaO-SiO2-based mold flux for plain carbon steel. Very few studies on low-fluoride or fluoride-free CaO-Al2O3-based mold flux have been reported. One study from Lu[9] focused on a very limited decrease of F in CaO-Al2O3-based mold flux but did not discuss the effect of a large decrease in fluoride content on heat transfer, crystallization and phase compositions, and did not discuss fluoride-free, CaO-Al2O3-based mold flux for high-Al steel. In the current study the replacement of CaF2 with B2O3 in CaO- Al2O3-based mold flux will be systemically investigated and in particular the viscosity properties as well as the crystallization behavior, in order to develop fluoride-free or low- fluoride CaO-Al2O3-based mold flux for the casting of high-Al non-magnetic steel which requires good lubrication and weak crystallization.
Sampling during production operations
High-Al non-magnetic steel 20Mn23AlV studied in the current work was produced using a vertical continuous caster. During casting a large number of slag rims were formed around the meniscus. Samples of molten mold flux and slag rims were taken during casting process to examine changes in compositions and properties. Sampling was carried out about every 10min after the start of casting and all samples air cooled to room temperature. The original mold fluxes and the spent mold flux samples were subjected to chemical analysis, examination of melting temperature and evaluation of crystalline phases. It can be seen from Figure 1(a) that the Al2O3/SiO2 mass ratio of molten fluxes has increased from 0.2 to ca. 1.2 after approximately 15min of casting due to rapid reaction between Al and SiO2. Greater increases of the Al2O3/SiO2 mass ratio from 0.2 to ca. 1.7 was found in the slag rims. These changes in chemical composition of mold fluxes during continuous casting can introduce a series of severe consequences such as rapid increase in melting temperature, as shown in Figure 1(b), and precipitation of the high-melting point phase gehlenite Ca2Al2SiO7 (melting point 1596oC), as shown in Figure 1(c), which in turn adversely affects lubrication and crystallization, and ultimately the casting process and surface quality of the cast slabs. Therefore, improvement in mold flux for high-Al non-magnetic steel should focus on suppression of deterioration of the above properties.
Fig. 1 Changes in (a) composition, (b) melting temperature and (c) crystalline phase of mold flux during continuous casting of 20Mn23AlV
Evaluation of CaO-Al2O3-based mold flux 3.1. Experimental
The chemical compositions of the experimental fluxes are: various CaO/Al2O3 0.6~3.2, constant SiO2 5%, Na2O 10%, CaF2 20% and B2O3 10%. All samples were synthesized using reagent-grade chemicals CaO, SiO2, Al2O3, CaF2 and B2O3, with Na2CO3 used as the source for Na2O. CaF2 is generally added to serve primarily as a fluxing agent to control viscosity and solidification temperature in CaO-Al2O3-based mold flux. The synthetic fluxes were pre- melted in a graphite crucible at 1350oC for 0.5h under Ar (99.99% purity) and then quenched in water. The water-quenched fluxes were dried at 120oC for 4h and crushed into powder with particle size less than 75μm and then used for the following experiments.
3.1.1. Viscosity Measurements Viscosity was measured utilizing the rotational cylinder method. A schematic diagram of the apparatus and dimensions of the graphite crucible and Mo bob are shown in Fig. 2. MoSi2 bars were used as the heating elements for the furnace and a Pt-Rh/Pt thermocouple was employed to determine the furnace temperature.
The viscometer was calibrated with standard oil. The furnace was heated at a rate of 15°C/min to the required temperature with Argon as a protective atmosphere. A graphite crucible (80mm in height and 40mm in inner diameter) containing 130g pre-melted mold flux was placed in the furnace and held at constant temperature for 30min to achieve thermal equilibrium. The depth of the liquid flux was approximately 40mm. The Mo bob was immersed into the flux to a depth of about 10mm from the bottom of the crucible and rotated at a constant speed of 200rpm to homogenize the liquid flux. After holding at constant temperature for 30min, the flux was cooled at a rate of 5oC/min. Meanwhile, viscosities and corresponding temperature were recorded automatically. The measurements were stopped when the viscosity increased sharply.
Fig. 2 Schematic diagram of the viscosity apparatus
3.1.2. CCT and TTT Tests To investigate the crystallization behavior of the fluxes, continuous cooling transformation (CCT) and time temperature transformation (TTT) diagrams were constructed using the single hot thermocouple technique (SHTT). A flux sample of about 10mg was mixed with alcohol, mounted on the tip of a B-type thermocouple and heated directly. The temperature was continuously monitored while the crystallization process was recorded by collecting images every second using a CCD camera. Figure 3 shows the thermal cycles for construction of the CCT and TTT diagrams. The thermocouple was heated at a rate of 30oC /s to 1500oC, and the flux sample held at this temperature for 1min to eliminate bubbles and allow homogenization of composition and temperature. The sample was then cooled continuously to 900oC at different cooling rates to construct the CCT diagram by recording the start time and start crystallization temperature at various cooling rates. To construct the TTT diagram, the sample was rapidly cooled at a cooling rate of about 80oC /s to various isothermal temperatures while recording the start time and the end time of crystallization. During CCT and TTT tests, 5% and 95% volume fraction of crystallization were defined as the start and the end of crystallization, respectively. The volume fraction of crystallization was determined using image analysis software.
Fig. 3 Thermal cycles for (a) CCT and (b) TTT tests
3.2. Viscosity and Structural Characteristics
Fig. 4(a) shows the change in flux viscosity as a function of CaO/Al2O3 ratio in the temperature range from 1125~1325oC. At temperatures above 1270oC the viscosity first rapidly decreases and then gradually becomes stable. However, at temperatures less than 1270oC, the viscosity first decreases and then increases. These changes in viscosity are closely related with the network structure characteristics of the melt at higher temperature and solid particles that are precipitated at lower temperature.
It is known that Al2O3 is an amphoteric oxide and that the behavior as an acidic oxide (network former) or basic oxide (network modifier), depends on the basicity of the melt.[10] For all fluxes tested in the current study, the mole fractions of the basic oxides CaO and Na2O are far higher than that of Al2O3, and therefore Al2O3 functions as an acidic oxide to form [AlO4]5--tetrahedra or is incorporated into the silicon-oxygen network structure when sufficient charge compensation is available through the presence of basic oxides. As shown in Fig. 4(b), this behaviour has been confirmed using FTIR spectra for mold flux quenched from 1350oC, which show the band for [AlO4]5--tetrahedral structural units near 650 cm-1 and Si-O-Al bending at about 500 cm-1.[11] At higher temperatures, with decrease in the CaO/Al2O3 ratio, more Al2O3
is involved in the formation of aluminate or alumino-silicate network structures to increase the degree of polymerization which in turn increases the viscosity. With increasing CaO/Al2O3
ratio, more free oxygen ions (O2-) are available to promote depolymerization, reducing the extent of the alumino-silicate network structure, and hence decreasing the viscosity. The FTIR spectra also show that with increase in the CaO/Al2O3 ratio, the center of the [AlO4]5-- tetrahedral structural units shifts toward lower wave-numbers and the band width decreases, which is consistent with a decrease in the degree of polymerization.
At lower temperatures, precipitation of solid particles with high melting points begin to have a dominant role in controlling viscosity, so the viscosity increase.
Fig. 4 Effect of CaO/Al2O3 ratio on (a) viscosity and (b) FTIR spectra of fluxes
3.3. Crystallization Behaviour
Fig. 5(a) shows the relationship between the CaO/Al2O3 ratio and the initial crystallization temperature at a fixed cooling rate of 0.5oC/s. The initial crystallization temperature first decreases from 1204oC to 1050oC with increase in CaO/Al2O3 ratio from 0.6 to 1.6, and then increases to 1138oC and 1422oC when the ratioincreases to 2.2 and 3.2, respectively. This demonstrates that an increase in the CaO/Al2O3 ratio over a wide range from 0.6 to 3.2 first weakens and then enhances the tendency for crystallization.
Fig. 5(b) compares the incubation time for initial crystallization at different holding temperatures and CaO/Al2O3 ratios. It can be observed that mold flux with a CaO/Al2O3 ratio of 3.2 begins to crystallize almost immediately even at higher temperatures, followed by mold fluxes with CaO/Al2O3 ratios of 0.6, 2.2, 1.1 and 1.6, respectively. The same phenomenon is observed with respect to the incubation time in the higher temperature zone. A high crystallization temperature and short incubation time suggest a strong crystallization ability, which is consistent with the results obtained from the CCT tests.
From the above study, mold flux with a CaO/Al2O3 ratio in the range 1.1~1.6 has a low viscosity and a relatively weak tendency for crystallization. These conditions will enhance the
(a)
(b)
infiltration of the liquid flux into the gap between the steel shell and the copper mold, improve lubrication, promote heat transfer and consequently enhance the formation of a thick steel shell with fine grains and high strength thus decreasing the possibility of a liquid steel breakout. This is particularly significant for the casting of high-aluminum, non-magnetic steel 20Mn23AlV, which is prone to form a coarse columnar structure at low cooling rates thus decreasing the strength of the initial steel shell and leading to cracks and potential breakouts.
Fig. 5 Effect of CaO/Al2O3 ratio on (a) initial crystallization temperature at a cooling rate of 0.5°C/s and (b) incubation time for crystallization at different holding temperatures
Evaluation of B2O3 as a replacement for CaF2 in CaO-Al2O3-based mold flux To evaluate the feasibility of development of fluoride-free or low-fluoride CaO-Al2O3-based mold flux from the perspectives of viscosity and crystallization properties, the chemical compositions of the mold fluxes were designed by replacing CaF2 with B2O3 as listed in Table 1. The experimental methods were same as those described in the section 3.1.
Table 1 Chemical composition of the experimental mold fluxes (in mass percent)
CaO SiO2 Al2O3 Na2O CaF2 B2O3
28.8
5
26.2
10
20 10
32.5 29.5 13 10
36.1 32.9 6 10
39.3 35.7 0 10
36.7 33.3 0 15
34.0 31.0 0 20
4.1. Viscosity characteristic
As shown in Fig. 6(a), increases of CaF2 and temperature decrease the viscosity of CaO-Al2O3- based mold flux. The viscosity of molten flux at high temperature is related to the structure.
The effect of CaF2 on viscosity is due to its effect on the structure and melting temperatures of the mold flux. Study[12] has indicated that CaF2 decomposes into Ca2+ and F- in a similar way to CaO Eq. (1). The F- ion then plays a similar role to that of free oxygen O2- ions and depolymerizes the complicated aluminate network structures [AlO4]5- into [AlF6]3- and [AlO6]9- according to Eqs. (2) and (3)[13]. FTIR study also indicated this reaction can depolymerize aluminate network structures and accordingly decrease the polymerization degree of molten flux, and the viscosity eventually decreases. Moreover, CaF2 also decreases viscosity by decreasing the melting temperature since viscosity is a function of temperature. The combined effects of depolymerization of network structures and decrease in melting temperatures caused by CaF2 result in a decrease in viscosity.
(a) (b)
CaO=>Ca2++O2- CaF2=>Ca2++2F- (1) n[AlO4]5-+2nO2-=n[AlO6]9- (2) 3[AlO4]5-+6F-=[AlF6]3-+2[AlO6]9- (3) In order to examine whether B2O3 can duplicate the effect of CaF2 on viscosity of CaO-Al2O3- based mold flux, the effect of B2O3 was also studied with and without CaF2 addition. Fig. 6(b) shows addition of B2O3 has a similar effect to CaF2 in decreasing the viscosity, especially at low temperatures. As mentioned above, the viscosity of a melt is related to its structure.
Addition of B2O3 depolymerizes the 3-D [BO4]5- into 2-D [BO3]3- structure, meanwhile, bridging oxygen O0 transforms into non-bridging oxygen O- during depolymerization.
Moreover, addition of B2O3 can reduce Al-O stretching vibration of [AlO4]5- tetrahedral structures[11]. These changes from complicated structures to relatively simple structures caused by addition of B2O3 are responsible for a decrease in viscosity.
Fig. 6 Effect of (a) CaF2 and (b) B2O3 on viscosity of CaO-Al2O3-based mold flux
4.2. Crystallization behavior
The initial crystallization temperature as a function of CaF2 at the fixed cooling rate of 0.5oC/s is plotted in Figure 7(a). Decrease of CaF2 from 20% to 13%, 6% and 0 increases the initial crystallization temperature from 1125oC to 1138oC, 1150oC and 1177oC, respectively. This result indicates that decrease in CaF2 content enhances crystallization of CaO-Al2O3-based mold flux, which is the opposite effect to that observed with conventional CaO-SiO2-based mold flux. This may be because addition of CaF2 promotes precipitation of cuspidine in CaO- SiO2-based mold flux. Figure 7(b) shows the relationship between initial crystallization temperature and B2O3. A similar effect of B2O3 to CaF2 on the initial crystallization temperature can be observed. Increase in B2O3 content from 10% to 15% and 20% decreases the crystallization temperature from 1177oC to 1136oC and 1109oC respectively. This change suggests that addition of B2O3 restrains crystallization of CaO-Al2O3-based mold flux. It also indicates that the equivalent amount of B2O3 has almost a twofold effect compared to that of CaF2 on the initial crystallization temperature of CaO-Al2O3-based mold flux. This phenomenon may be because B2O3 serves as a network former to strengthen the glassy ability of mold flux.
Fig. 8(a) summarizes the incubation times at different holding temperatures for CaO-Al2O3- based mold flux with different amounts of CaF2. It is evident that within the high and middle temperature zones, the incubation time increases with increasing CaF2 content. This means crystallization of CaO-Al2O3-based mold flux within the high and middle temperature zones is retarded by higher CaF2 contents. It is also confirmed once again that decrease of CaF2
promotes crystallization of CaO-Al2O3-based mold flux. This conclusion is consistent with the
(a) (b)
CCT results. The relationships between incubation time and holding temperature for the different B2O3 concentrations are summarized in Fig. 8(b). Mold flux with 10%, 15% and 20%
B2O3 initiate crystallization at 1225oC, 1200oC and 1150oC, respectively. The incubation time at 1150oC is prolonged significantly from 34s to 50s and then 606s with increase of B2O3 from 10% to 15% and 20%. These findings are consistent with the behavior of B2O3 as a network forming oxide.
Fig. 7 Effect of (a) CaF2 and (b) B2O3 on the initial crystallization temperature at a cooling rate of 0.5oC/s
Fig. 8 Effect of (a) CaF2 and (b) B2O3 on incubation time for crystallization
From the perspectives of viscosity and crystallization, it has been shown that B2O3 can be used to replace CaF2 for the generation of CaO-Al2O3-based mold flux with free, or very low, CaF2
content.
Viscosity model for mold flux with high Al2O3 content
Viscosity is an important property of mold flux, but its measurement requires much time and high cost. Thus there is no doubt that a viscosity model with high accuracy is of considerable benefit for evaluation of mold flux behaviour. Several researchers[14-17] have obtained semi- empirical or empirical viscosity models based on different slag compositions and experimental results, but the Al2O3 content in these models is always less than 10%. There is therefore considerable limitation in predicting the viscosity of mold fluxes with high Al2O3, contents such as those in the present study with 30% Al2O3. Fig. 9 shows the comparison of experimental and calculated viscosities of mold flux with high Al2O3. The calculated viscosities using the Riboud[14], Urban[15] and RIST[16] models show large deviations from the experimental results. However the NPL[17] viscosity model, based on optical basicity shows a relatively better prediction. The present study has modified the NPL model based on the Weymann-Frenkel equation and optical basicity to improve accuracy in the prediction of viscosity of mold fluxes containing high concentrations of Al2O3.
Fig. 9 Comparison between measured viscosities and those predicted by different models
Weymann-Frenkel equation Eq. (4) describes the correlation between viscosity and temperature.
lnη
T=lnA+1000B
T (4) The correlation between parameters lnA and B can be seen from Fig. 10(a) based on Eq. (4) and experimental viscosity of the mold fluxes shown in Table 1. It can be seen that the relationship between lnA and B shows good linearity and can be represented by the following equation:
lnA=-0.4327B-13.053 （R2=0.9791） (5) The corrected optical basicity expression proposed by Shankar[18], Eq. (6), was used to establish a model based on the view that Al2O3 behaves as an acidic oxide when the molar ratio of basic oxides to Al2O3 is greater than unity. The optical basicity Λ of molten flux can be calculated from a knowledge of the flux composition and the optical basicity value of each oxide[17]. The correlation between B and Λ shown in Fig. 10(b) corresponds to the following relationship Eq.
(7).
Fig. 10 (a) Correlation between lnA and B, and (b) Correlation between B and optical basicity Λ
(a) (b)
Λ=
∑(XBnBΛB+…)
∑(XBnB+…)
∑(XAnAΛA+…)
∑(XAnA+…)
(6)
B=-180.67Λ2+659.88Λ-574.85 （R2=0.8911） (7) Fig. 11 (a) shows a comparison between the measured viscosity of the fluxes listed in Table 1 and the values calculated using the present model as well as previous models. Fig. 11 (b) provides a comparison between the measured viscosity of the fluxes (the compositions listed in section 3.1) and the values calculated with the present model. It should be noted that Eqs.
(5) and (7) were obtained based on the viscosity of the fluxes shown in Table 1. However, the viscosities of fluxes listed in section 3.1 were not used for establishment of the viscosity model.
It is evident that there is good agreement between the proposed model and the measured data for the fluxes listed in both section 3.1 and Table 2 despite the fact that model parameters were not derived based on the experimental results for the fluxes listed in section 3.1.
Fig. 11 Comparison between the measured and calculated viscosities of fluxes listed in (a) Table 2 and (b) section 3.1
Conclusions
(1) Sampling from the mold during continuous casting of high-Al non-magnetic steels shows slag/metal reactions have resulted in significant increase in the Al2O3/SiO2 ratio in mold flux from 0.25 to ca. 1.5. This increases the melting temperature of fluxes from 1080oC to greater than 1200oC and promotes precipitation of the high-melting point compound gehlenite.
(2) Mold fluxes with CaO/Al2O3 ratios in the range 1.1~1.6 have a low viscosity and a relatively weak tendency for crystallization, both of which are good conditions for casting high-Al nonmagnetic steel 20Mn23AlV.
(3) It was found that B2O3 played a similar role to CaF2 in changing the viscosity and crystallization of CaO-Al2O3-based mold flux. From the perspectives of viscosity and crystallization, it has been shown that B2O3 can be used to replace CaF2 for the generation of CaO-Al2O3-based mold flux with free, or very low, CaF2 content.
(4) Based on the concept of optical basicity, a model with high reliability was established to predict the viscosity of mold fluxes containing high concentrations of Al2O3.
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