Controlling Heat Transfer through Mold Flux Film by Scattering Effects
Dae-Woo Yoon1, Jung-Wook Cho2, Seon-Hyo Kim1
1POSTECH(Department of Materials Science and Engineering) 77 Cheongam-Ro; Pohang, Gyeonbuk, 37673, Republic of Korea
2GIFT(Graduate Institute of Ferrous Technology) 77 Cheongam-Ro; Pohang, Gyeonbuk, 37673, Republic of Korea
Keywords: Extinction coefficient, Infrared emitter technique, Radiative heat transfer, Scattering
Abstract
Regulating thermal behavior between the solidifying shell and a mold is one of primary roles of the mold flux film during continuous casting of steel. This is particularly important as excessive heat flux through slag film will induce surface defects on cast steel. This study focuses on controlling radiative heat transfer through liquid slag film by utilizing finely dispersed metallic particles in commercial CaO-SiO2-CaF2-Na2O based mold flux system. In order to investigate the scattering effect on heat transfer during industrial casting processes, the extinction coefficient of various mold fluxes was measured using an FT-IR and a UV-visible spectrometer. Also, series of IET (Infrared Emitter Technique) tests were conducted in order to simulate mold heat transfer during commercial casting processes. Finally, it is found that the Mie scattering effect due to metallic particles will reduce the overall heat flux density through mold slag film by 10% or more.
Introduction
Mold flux acts as a lubricant after infiltrating into the gap between steel shell and copper mold to prevent the sticking of solidifying shell on continuous casting mold. In the casting mold wall after infiltration, mold flux exists as the slag film with two layers: molten glassy layer next to steel shell, and solid layer of partially crystallized glass adjacent to copper mold. Each flux film layer affects heat transfer through mold flux film by providing thermal resistances which consist of both the conduction and radiation.1 Many previous studies suggested that mold heat transfer during continuous casting process will be governed by radiative thermal resistance across molten glassy layer, thermal conduction through solid layer.2,3 , and air gap at the interface between copper mold and solid flux film layer.4,5 Among them, radiative heat transfer is supposed to be dominant especially at initial stage of solidification near meniscus, where the surface defects such as longitudinal crack and break-out can be easily occurred due to excessive heat flux from molten steel to mold. Therefore, it is highly necessary to suppress the excessive thermal radiation by controlling the optical properties of molten glassy film layer to achieve the sound surface quality of cast steel products.
Several attempts have been made to measure the extinction coefficient which governs radiative heat transfer through mold flux film. For example, it is reported that extinction coefficient is markedly dependent on the contents of transition metal oxide in slag system.6 However, there has
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
not been systematic investigation on thermal radiative heat transfer considering both the absorption and scattering behavior of molten glassy flux film. In this study, for the comprehensive understanding of thermal radiation through molten glassy film layer, absorption and scattering coefficients have been investigated using an FT-IR and a UV-visible spectroscopy. Especially, contribution of metallic particles on scattering behavior was discussed based on Mie scattering theory. In addition, for evaluation of the overall heat transfer through mold flux film, an infrared emitter has been employed in this research. By infrared emitter technique, the mold fluxes with different extinction coefficient have been tested in order to clarify the effect of controlling thermal radiation on overall heat flux density during commercial continuous casting process.
Experimental 1. Sample preparation
Boron oxide containing mold fluxes were chosen for investigation of optical properties as shown in Table 1. Each mold flux was melted in the graphite crucible using a muffle furnace at 1653K, and then quenched in a copper mold, annealed at 700K for 1 hour to relieve thermal stress. . These glassy specimens were slice and polished into a thin disk type of 13mm in diameter and 10-20mm in thickness. The chemical composition of mold fluxes are shown in Table I.
Table I. Chemical composition of mold fluxes (wt. %)
Sample SiO2 CaO MgO Al2O3 Fe2O3 Na2O F B2O3 Basicity
B6 37.9 36.4 0.9 4.5 0.3 6.8 6.7 6.5 0.96
B9 37.0 36.1 1.0 4.3 0.4 6.3 5.6 9.3 0.98
B11 35.6 35.8 0.9 4.2 0.3 6.0 5.8 11.4 1.0
B15 34.3 34.0 1.0 4.0 0.3 5.7 5.5 15.2 1.0
B28 30.1 30.2 1.0 3.2 0.2 5.2 2.0 28.1 1.0
2. Determining extinction coefficient
Extinction coefficient, E, of mold fluxes was investigated using Fourier transformation infrared spectroscopy and ultra violet/visible spectrometer in the range of 0.5-5μm. As the radiation energy passes through the specimen, its intensity is decreased by absorption or scattering phenomenon which can be estimated by extinction coefficients. As the reflectivity of glassy film of mold flux is known to be only 1-3 percent, 1) the dissipation of thermal radiation through a glass medium can be expressed by simple Lambert-Beer’s law, equation (1).
0 T exp
I s l
I (1)
where I0 is the incident beam of light, IT is transmitted infrared intensity, α is absorption coefficient, s is scattering coefficient, and l is the thickness of the specimen. The extinction coefficent is simply the sum of absorption and scattering coefficients..
3. Measuring particels on matrix
In order to investigate the effects of particle properties such as constituent, morphology and size
on extinction coefficients of mold fluxes, an automatic scanning electron microscopy has been employed. By SEM technique, each sample were analyzed in the area of 25.0×10-6 m2 and particles were detected on back scatter electron mode image using automatic threshold of the image grey level. All of sample matrix were quantified for Si, Ca, Na, Al, Mg, Fe, O with recording during measurements.
4. Infrared emitter technique
Infrared emitter technique (IET) has been applied to measure thermal properties of mold fluxes under controlled conditions. The experiment was accomplished in following step: First, the halogen lamp of IET irradiates the surface of copper mold which is covered by solidified disk type(diameter:40mm, thickness:5mm) of glassy mold flux; after that heat flux across flux film was measured by the calculating temperature gradient of different position in subsurface copper mold.
Simultaneously, video recording has been carried out to monitor the morphological change of mold fluxes by melting, solidification, and crystallization.
Results and Discussion 1. Thermal absorption behavior
According to the investigation by Kusabiraki and Shiraishi et al.7,8, the extinction coefficient of solidified glassy mold fluxes will not yield significant error of demonstrate the radiative heat transfer across molten mold flux in the mold. Thus, in this study, extinction coefficients of mold fluxes have been measured at room temperature using solid glassy specimen. Extinction coefficients of commercial glassy mold fluxes in near infrared region are reported to be below 1000m-1.9 However, two mold fluxes with 6.5% and 9.3% of B2O3 show distinctly larger extinction coefficients than others and reported commercial fluxes as shown in figure 1. The sharp peak is appeared in the 3.8-4.1μm due to the ring stretch of cyclic meta-borate ion.10 The average extinction coefficient of borate contained mold fluxes increase until nearly 10wt. % of borate addition, and then decreases with more than 10 wt. % of boron oxide addition. This complicated behavior of extinction would arise from the scattering effects due to particles on sample matrix, which will be discussed in following section.
Figure 1. Extinction coefficient of B2O3 contained mold flux as function of wavelength.
Also, this drastic change of extinction coefficient according to amounts of boron oxide could explain with variation of particle density and molar structure by each borate glassy sample.
2. Scattering phenomenon for mold fluxes
According to investigation of SEM-EDS analysis, spherical particles were detected and identified as metallic iron by reduction reaction of ferric oxide with carbon crucible during melting process. It is found that both the average particle size (B6: 0.769, B9: 0.779, B11:0.344, B28:
0.371/μm) and number of particle (B6: 336, B9: 427, B11: 35, B28: 89) are closely concerned with averaged extinction coefficients (B6: 1,841, B9: 2,695, B11:861, B28:1,127/m-1) of glassy fluxes.
These results show that the number of spherical metallic particles are intensively related to extinction coefficient by scattering effects. In this respect, it is necessary to extract the contribution of scattering effects on extinction coefficients. B3 flux which shows the minimum number of metallic particle has been chosen for reference. It is assumed that scattering behavior through reference flux can be ignored. Then, the scattering coefficient of other fluxes can be derived by the difference of extinction coefficient with reference flux, as shown in figure 2.
Figure 2. Experimental scattering coefficient of B6 and B9 fluxes.
It can be seen that mold fluxes which contain boron oxide below 10 wt. % have distinct scattering behavior due to many tiny metallic particles in the glassy matrix. In order to figure out this phenomenon, it is essential to apply the scattering effect on the basis of scattering theory. For this reason, Mie theory is chosen for clarifying scattering effect in this study. This theory could well explain the scattering behavior when the particle size is similar to the incident light wavelength.11 In Mie theory, the scattering coefficient could confirmed with size parameter(x) and relative refractive index of medium and spherical particle.
2 rnmed
x
(2)
where r is the radius of particle, nmed is refractivity of medium and λ is the incident beam wavelength. And, the scattering coefficient (μ) is defined as
s s s
(3)
where ρs is number density of particles, σs is scattering cross section. Numerical values of σs were calculated using the subroutine BHMIE.12 This code and comparable ones are readily available
online.13 In this study, scattering coefficients of boron oxide containing mold fluxes were calculated by above procedure with compensating size parameters. Previous studies showed that larger asymmetry parameter will bring the lack of enhancement of backscattering behavior, which is not suitable for this study but only for geometic optics.14,15 Therefore, theoretical scattering coefficients is calculated by controlled range of size parameter which intentionally related to wavelength and particle size as shown in figure 3. However, there are slightly difference between experimental and theoretical data. It is assumed that particle number distribution of glassy flux matrix is incorrect because the metallic particles would be detached from glassy matrix during etching. Adding to this, automatic SEM technique has some detective limitation to identify the fine or dark. Based on Mie scattering theory, the origin of increased extinction coefficient can be logically explained as dispersed metallic particles in glassy matrix.
Figure 3. Scattering coefficients calculated by Mie theory as function of wavelength.
3. Demonstrating of overall heat transfer through mold flux film by using IET
During the tests using a infrared emitter technique, the mold flux film would be exposed to the thermal circumstance close to the industrial continuous casting mold.
Figure 4. The measured heat flux history for different mold fluxes .
In order to confirm the scattering effect without changing absorption behaviors on total heat flux through mold slag film, 5 wt. % of metallic iron particles were added in mold flux with 15 wt. % of boron oxide. Also, iron oxide was put into slag for clarifying the extinction coefficient which is comprised of absorption and scattering coefficient. As can be seen in figure 4, it could be observed that the measured average heat flux for each mold flux at steady state(1500-2000second) are around 174(flux-B15), 134(B15 with 5wt. % of FeO), and 130(B15 with 5wt. % of metallic Fe) kW/m2, respectively. The difference of heat transfer rate arises from scattreing effect of iron particles by addition of FeO and metallic iron in the sample. It is worthy to note that the increase of scattering coefficients will not bring any fatal effects on friction in a casting mold while any other countermeasures to enhance the degree of crystallinity will significantly disturb the lubrication. Therefore, it is highly desirable to modify mold flux system by using scattering factors has been enhanced to reduce heat flux from steel shell to copper mold.
Conclusion
Comprehensive understanding of heat transfer through mold flux film is attained by considering the thermal radiative absorption into glass phase for commercial CaO-SiO2-Na2O based mold fluxes.
(1) Metallic particles in mold flux increase extinction coefficient which affect radiative thermal conductivity and shows a marked increase in the radiative heat controlling region from 1 to 5 μm.
(2) Scattering effects could be useful to control heat transfer rate by growth of extinction coefficient due to reduction reaction of iron oxide with carbon in mold flux system.
(3) Selection of proper flux component for casting steel should be intensively dependent on thermal performance as slow cooling which can be effectively achieved by scattering coefficient in mold flux without bad lubrication.
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