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2019年 26卷 4期
刊出日期：2019-04-25
335
Wan-lin Wang
Preface
Continuous casting is one of the most important innovations in the steel industry due to its high yield, high product quality, energy savings, less pollution, lower cost, as well as better working conditions. Over 96% of crude steel in the world was cast by continuous caster. As a very important functional material, mold flux works as several roles in the continuous casting process. It protects liquid steel from oxidation, insulates it from freezing, and absorbs inclusions, when floating on the top of liquid steel; whereas it lubricates the shell and moderates the heat transfer in the mold, when it infiltrates into the mold/shell channel. Therefore, the quality of final cast product is greatly determined by the performance of mold flux. Improper properties may lead to defects in the bloom, slab or billet, such as severe oscillation marks, cracks, inclusions and slag entrapment, improper solidification structure distribution, and even breakout. In order to meet the new requirements from the development of advanced steels and green manufacturing, the special issue for Journal of Iron and Steel Research International focused on ‘‘Mold Flux: Melt Structure, Properties, Performances’’. Nine articles were included in this special issue related to the topics on: (1) Melt structure of silicate, aluminate, etc. based mold flux; (2) Relationship between melt structure and properties of mold flux; (3) Fundamentals of mold flux crystallization; (4) Mechanism of mold flux heat transfer; (5) Interface behavior between molten steel and slag; (6) Thermodynamics and kinetics for the reaction between molten steel and slag; (7) Model between components and performances of mold flux; (8) Development of new type mold flux for advanced steel; Fluoride-free mold flux; and (9) Mold flux performances and its related casting product quality.
2019 Vol. 26 (4): 335-335 [
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336
Lei Zhang, Wan-lin Wang, Han-qing Shao
Review of non-reactive CaO–Al
2
O
3
-based mold fluxes for casting high-aluminum steel
Advanced high-strength steels (AHSSs) have been gradually applied to modern auto industry, as they have the advantages of improving the steel strength and lightening the car weight, which not only ensures the safety but also saves the energy. However, the high-aluminum (Al) content in AHSSs may react with SiO
2
in conventional CaO–SiO
2
-based mold flux during the process of continuous casting, which leads to the deterioration of the mold flux properties and a poor slab quality. Then, the non-reactive CaO–Al
2
O
3
-based mold flux was proposed and has been developing for the casting process of high-Al steels, but there are some problems of low consumption and insufficient lubrication that need to be solved. Thus, previous researches on the effect of each component on the properties of CaO–Al
2
O
3
-based mold flux were systematically summarized, and the situation of plant trials on CaO–Al
2
O
3
-based mold flux was evaluated. The results indicated that the proposed CaO–Al
2
O
3
-based mold fluxes could avoid the slag-metal reaction problems; however, the problems of lubrication, crystallization and heat transfer issues still exist. Therefore, tremendous works still need to be conducted for the development of new generation of CaO–Al
2
O
3
-based mold flux system. The review was performed aiming to provide a technical guidance for designing and optimizing CaO–Al
2
O
3
-based mold flux system that meets the demand of the continuous casting process of high-Al steels.
2019 Vol. 26 (4): 336-344 [
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345
Nathalie Kolbl, Irmtraud Marschall, Harald Harmuth
High-temperature investigation of mould slag crystallization by single and double hot thermocouple techniques
Methods for the characterization of mould slag crystallization with special emphasis on the single/double hot thermocouple technique (SHTT/DHTT) are reviewed. In the continuous casting process of steels, horizontal heat transfer is mainly influenced by the crystallization behaviour of the mould flux film. Here, both precipitation of crystals out of a liquid phase and devitrification of the glassy film in contact with the mould are of main interest. Therefore, various investigation methods are implemented to characterize different slag properties related to crystallization: a viscometer for determining the break temperature, differential thermal analysis (DTA), confocal scanning laser microscopy, and the water-cooled copper finger test. For near-service conditions, DHTT reveals the most detailed information, including not only the crystallization or devitrification temperature but also the morphology as well as the crystallization velocity. Due to improvements in the device and the representation of the results, a comparison of different samples is possible. Nevertheless, the application field of SHTT/DHTT is restricted to slag systems with low contents of evaporating components. Furthermore, the time required for data analysis is significantly longer than that required for other methods, e.g. DTA. Therefore, the application of DHTT is mainly advisable for mould slag research and development, whereas DTA can also be used for incoming inspections.
2019 Vol. 26 (4): 345-354 [
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355
Wan-lin Wang, Er-zhuo Gao, Le-jun Zhou, Lei Zhang, Huan Li
Effect of Al
2
O
3
/SiO
2
and CaO/Al
2
O
3
ratios on wettability and structure of CaO–SiO
2
–Al
2
O
3
-based mold flux system
The effect of Al
2
O
3
/SiO
2
and CaO/Al
2
O
3
ratios on the molten structure of CaO–SiO
2
–Al
2
O
3
-based mold flux system was analyzed by Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. Also, the variation in the wettability between the mold flux system and an interstitial free (IF) steel substrate was investigated using the sessile drop method. The results indicate that the contact angle and interfacial tension between the molten slag and solid steel increase slightly with the increase in the Al
2
O
3
/SiO
2
ratio, while they decrease with the increase in the CaO/Al
2
O
3
ratio. The network structure for the designed mold flux system changes gradually from silicate to aluminosilicate and aluminate with the increase in the Al
2
O
3
/SiO
2
ratio, and the network is simplified with the increase in the CaO/Al
2
O
3
ratio. Besides, combining the results of sessile drop method and melt structure analyses, it suggests that the variation in interfacial properties of mold flux/IF steel substrate is mainly caused by the change in melt structure, especially the variation in free oxygen ions (O
2-
) and non-bridged oxygen (O
-
) at the interface.
2019 Vol. 26 (4): 355-364 [
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365
Rie Endo, Yo Kan, Takashi Watanabe, Miyuki Hayashi, Masahiro Susa
Effects of silicon addition on optical properties of mould flux with iron oxide and estimation of apparent thermal conductivity of flux
Mould flux with high absorptivity had been obtained in the reduction process of mould flux by silicon. For further utilisation of the mould flux with high absorptivity, the effect of silicon addition on the optical property has been investigated. The synthesised mould flux powder with iron oxide was melted with the addition of silicon powder at 1703 K followed by quenching, thus forming a glassy sample. After 5–30-min reduction, the samples exhibited black opaque colour. Meanwhile, the samples that were reduced for 45–60 min had a bluish grey colour. Many iron-based particles dispersed in the samples were micrometre order in diameter. To determine the effect of iron particles on the optical properties of the sample, extinction, scattering and absorption efficiencies were calculated according to Mie theory. Result showed that both scattering on the iron particle and absorption by iron effectively increase the extinction efficiency in the mould flux. Moreover, the silicon particle in the flux has large scattering efficiency. The black opaque colour is contributed by the existence of silicon and iron particles. The apparent thermal conductivity was calculated for the solid flux. Results also indicated that the entire glassy mould flux would effectively work for mild cooling in a continuous casting process owing to high absorptivity and small thermal conductivity.
2019 Vol. 26 (4): 365-373 [
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374
Qi Wang, Ji-an Yang, Chen Zhang, De-xiang Cai, Jian-qiang Zhang, Oleg Ostrovski
Effect of CaO/Al
2
O
3
ratio on viscosity and structure of CaO–Al
2
O
3
-based fluoride-free mould fluxes
The effects of CaO/Al
2
O
3
ratio on viscosity and structure of the CaO-Al
2
O
3
-based fluoride-free mould fluxes were investigated with the CaO/Al
2
O
3
ratio varied from 1 to 4 and the content of SiO
2
fixed at 7 wt.%. The increase in the CaO/Al
2
O
3
ratio from 1 to 2 lowered the viscosity of the flux melts. The viscosity increased slightly with the CaO/Al
2
O
3
ratio from 2 to 3, and this increase became significant with further increasing CaO/Al
2
O
3
ratio to 4. Both break temperature and apparent activation energy were found to be increased with the increase in CaO/Al
2
O
3
ratio from 2 to 4. There was no break temperature available in the flux with CaO/Al
2
O
3
ratio of 1. Changing the CaO/Al
2
O
3
ratio from 1 to 2 decreased the apparent activation energy. Equilibrium phases of the fluxes were calculated using FactSage 7.1 and the major phases were found to be varied with the CaO/Al
2
O
3
ratio. Structures of tested mould fluxes were analysed using Fourier transform infrared spectroscopy. The results showed that the increase in the CaO/Al
2
O
3
ratio from 1 to 2 decreased the complexity of the structure, leading to a reduced viscosity. With the increase in the CaO/Al
2
O
3
ratio from 2 to 4, both solid phase precipitation and melt structure contributed to the variation of viscosity.
2019 Vol. 26 (4): 374-384 [
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385
De-xiang Cai, Wan-lin Wang, Lei Zhang, Chen Zhang
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.
2019 Vol. 26 (4): 385-392 [
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393
Karina Lara Santos Assis, Petrus Christiaan Pistorius
Use of improved cold-finger technique to assess effects of basicity on heat transfer through solidified mold flux
Cold-finger measurements were used to estimate the contact resistance and average thermal conductivity of mold flux films solidified on a water-cooled copper probe. Five industrial mold fluxes, with basicities from low to high, were tested. The steady-state rate of convective heat transfer between the solid film and liquid flux was compared with calculations based on natural convection correlations; the comparison indicates that the surface of the solid film (in contact with molten flux) is approximately at its solidus temperature. The roughness of the mold flux film in contact with copper was larger for films grown from higher-temperature mold flux and for higher-basicity mold fluxes. The glassy (low-basicity) film had an estimated thermal conductivity of 1.1 W/(m K). The crystalline or partially crystalline films had higher estimated thermal conductivities of 2.2–3.2 W/(m K). The measured values of thermal resistance were extrapolated to typical mold flux film thicknesses for industrial conditions; the extrapolations confirmed that the high contact resistance of higher-basicity mold fluxes can yield higher thermal resistances than the low-basicity flux. The morphology of the rough interface of higher-basicity mold flux films that gives the higher contact resistance is consistent with that observed in previous work and appears not to be directly related to crystallization.
2019 Vol. 26 (4): 393-402 [
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403
Irmtraud Marschall, Nathalie Kolbl, Harald Harmuth, Christina Atzenhofer
Identification of secondary raw materials in mold powders and their melting behavior
Commercial mold powders use a limited number of main mineral constituents, but may differ significantly in chemical composition. The main mineral raw materials of specimens investigated here are quartz, fluorite and free carbon, as well as wollastonite and carbonates. The investigations revealed the use of secondary raw materials like blast furnace slag, fly ash, glass scrap and phosphorous slag as further components. Since the formation of cuspidine was one major point of interest, the influence of the silica source on its formation was identified. A replacement of wollastonite by blast furnace slag reduced the temperature of the first precipitation of cuspidine by about 100 °C; the dissociation of sodium carbonate was lowered by ~ 40 °C. The lowest temperature of the first Na
2
CO
3
dissociation could be achieved by using fluorine in combination with blast furnace slag. Cuspidine formation from the melt is further decreased if sodium and fluorine are both present. The use of glass scrap and phosphorous slag strongly reduced the temperature of first melt formation and enhanced cuspidine formation.
2019 Vol. 26 (4): 403-411 [
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412
Ya-ru Cui, Hao-yue Fan, Zi-liang Guo, Guo-hua Wang, Xiao-ming Li, Jun-xue Zhao
Volatilization behaviour and volatilization kinetics of CaF
2
–Na
2
O–CaO–SiO
2
–Al
2
O
3
–MgO–B
2
O
3
synthetic mould flux
To reduce the volatilization of the volatile-containing slags and obtain accurate measurement results of slag performance, the volatility degree and deviation mechanism must be determined. Non-isothermal thermogravimetric analysis at different heating rates was used to establish the volatilization kinetic model, and it reveals the volatilization mechanism of CaF
2
–Na
2
O–CaO–SiO
2
–Al
2
O
3
–MgO–B
2
O
3
synthetic sodium-containing fluoride mould flux. The results demonstrated that the evaporation of NaF and SiF4 was the decisive factor that led to the change in composition and deviation of properties of the tested slags. The most probable kinetic mechanism function for the evaporation of volatile component from sodiumcontaining fluoride mould flux could be expressed by g(α)=[-ln(1-α)]
2/3
, with an apparent activation energy of 164.866 kJ mol
-1
and pre-exponential factor of 2.13 × 10
-4
s
-1
, where a is the conversion rate at any time step in the volatilization process. The reaction mechanism was random nucleation followed by growth, which was the limiting factor for the volatilization of synthetic sodium-containing fluoride mould flux. The method of increasing heating rate and adding protection gas in the measurement system will help to obtain more accurate results of slag performance.
2019 Vol. 26 (4): 412-421 [
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