연속 주조 몰드플럭스 내 산화물/분산 금속 입자 첨가를 통한 복사 열전달 저감 제어

Abstract

In the continuous casting process, controlling the heat flux of mold flux from solidifying shell to copper mold is one of the important job to achieve sound slab surface quality. As the occurrence of surface defects such as longitudinal crack or breakout on continuously cast steel products is intensively accelerated by the excessive mold heat flux especially during initial solidification of the molten steel in mold. Hence, the mold flux film should have enough thermal resistance to maintain optimum mold heat flux. Regulating the thermal resistance of mold flux film is highly desirable not to exceed the critical quantity of mold heat transfer rate for preventing cast steel products from surface defects. In particular, radiative heat transfer through liquid flux film was investigated to clarify its effect on mold heat transfer in this work. To examine the effect of thermal radiation on the overall heat transfer rate through liquid flux film, extinction coefficient is introduced for controlling radiative heat transfer coefficient which have huge impact on thermal radiation transfer. For quantifying the extinction coefficient, transmissivity of mold flux was measured by using an ultraviolet/visible (UV/Vis) and Fourier transformation-infrared ray (FT-IR) spectrometer, followed by conversion of Lambert-Beer’s law. Subsequently, the glass forming mold flux was observed by scanning electron microscope (SEM) in order to investigate the compositional and structural changes of mold flux with variation of containing additives. Furthermore, infrared emitter technique (IET) is employed to confirm the actual heat transfer rate through mold flux layer. In the Chapter 4.1, it is investigated that how the basicity and additive (NiO) has an influence on thermal behavior of mold flux. The extinction coefficients of commercial glassy mold fluxes with basicity of 0.76-1.34, are included in the range below 1,000 m-1, which could not effect on reducing heat flux. The extinction coefficient of NiO containing glassy mold flux with basicity (CaO/SiO2) of 0.94, is much higher than that of commercial mold fluxes. The contribution of extinction coefficient on total heat flux through mold flux film, was quantified by numerical analysis. It is found that the heat flux through mold flux film decrease about ca. 2-4 % with 3.2 wt. % of NiO containing mold flux. The increasing extinction coefficients would not lead to adverse effect on casting operations such as lubrication of steels in the mold. Hence, optimizing chemical composition of mold fluxes is required to increase thermal radiative absorption performance between 0.5-5 µm where the maximum radiative energy intensity is emitted. In Chapter 4.2, the radiative absorption behavior of iron oxide contained borosilicate mold fluxes is investigated to evaluate the scattering effect of particles on extinction coefficients. Spherical particles on glassy matrix are identified as metallic iron by reduction reaction with borate. The number of iron particles is intensively proportional to boron contents due to the transition of molar structure, from BO4 to BO3, with increasing boron oxide. The glassy mold fluxes containing B2O3 less than 10 wt. % induce an increase of scattering coefficient about 865-1,600m-1 with increasing number density of spherical iron metallic particles on matrix. Mie theory was applied to calculate scattering coefficients for verifying the scattering effect by particles. Scattering coefficient were simulated about 529-700 m-1 by theory, which provides logical basis for increasing scattering coefficient. Therefore, both the reducing heat flux and good lubrication through liquid flux film, is able to obtain by scattering effect of tiny metallic particles. In the Chapter 4.1 and Chapter 4.2, the effect of radiative absorption behavior on mold heat flux is evaluated by extinction coefficient which is indirect method for confirming heat flux. It is required to measure actual heat flux through mold flux film. Hence, the infrared emitter technique (IET) has been used for evaluation of the overall heat transfer and melting behavior across mold flux film. In the Chapter 4.3, the heat flux of B2O3-CaO-SiO2-Na2O-CaF2-Fe mold flux has been studied using IET. The heat flux through designed mold flux at steady state decreases from 213 kW/m2 to 121 kW/m2 with increasing particles in mold flux. To analyze the effect of crystalline layer on radiative heat transfer, both SEM and XRD analysis was employed in order to identify the morphological and compositional changes of crystalline phase according to increasing iron contents in mold flux after IET experiments. It was confirmed that crystalline layer of studied mold fluxes does not have effects on total heat flux due to the similar structure and fraction of crystallizations. To clarify the effect of scattering behavior of particles on total heat flux, the scattering coefficient was measured. The scattering coefficient of Fe particles contained mold fluxes, increases from 1,717 m-1 to 3,682 m-1 with increasing amount of Fe particles. Additionally, Mie scattering theory is adopted to define the scattering behavior of dispersed iron particles on glassy matrix. It was found that theoretical scattering coefficient demonstrated about 1,623-3,295 m-1 which is in well accordance with experimental results. Consequently, mold flux with the micro-sized metallic irons is appropriate to reduce the total heat flux without reference to any changes of crystallization behavior through mold flux film. It is confirmed that the scattering coefficient significantly decreases overall heat flux by a strong net source of scattered radiation. Hence, it is possible to suppress total heat flux across mold flux layer by using scattering behavior which introduces new and advanced techniques with suppression of surface defects on slabs during casting process.