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Surface Roughness of Solid Films
The roughness of film surfaces (in contacted with the mold wall) increases the thermal contact resistance between mold and steel shell, which plays a significant role in the heat transfer rate under caster conditions. The measured surface roughness of films obtained after different probe immersion times is shown in Fig. 4. The results show that the different in surface roughness of the films is not statistically significant. HB films recovered after longer immersion times did show statistically significant smaller surface roughness, but not so in the case of the TB films.
Fig. 4 Roughness of the slag film surface which had been in contact the probe, for different probe immersion times in the molten flux (The error bar shows the uniformity of the surface roughness of films at different immersion times)
Fig. 5 Typical micrographs of the section of films, showing the surface which had been in contact with the probe (backscattered electron image)
The observation that the roughness does not increase with increased probe immersion times (and even decreases in HB films) is significant, in that it demonstrates that surface roughness is not caused by the precipitation of crystals in the glassy layer of solid films; the same (or higher) surface roughness was found for mostly or fully glassy films formed after short immersion times. The surface features visible in Fig. 5 indicate that pores formed at the film-probe interface contribute to the formation and evolution of roughness.
In the continuous casting of peritectic steels, the initial steel shell formed at the meniscus is very weak and sensitive to non-uniform cooling which promotes the formation of cracks. A large surface roughness of solid mold flux films is expected to ameliorate fast and non-uniform cooling of the steel shell at the meniscus.
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