Continuous casting of hypo-peritectic steel was conducted with a pilot slab caster. Such experimental data as local heat flux, thickness of solidified shell or mold flux film, and dendrite primary arm spacing were obtained. On the basis of these experimental results, influence of mold flux on initial solidification in the mold was discussed.With mild cooling by crystallization of mold flux, local heat flux and solidification rate decreased in the mold. The changes in them quantitatively correspond to each other. Dendrite primary arm spacing increased with the mild cooling. Relationship between the arm spacing and cooling rate was established and cooling rate on quite initial stage of solidification was stimated. Cooling rate at 1 mm thickness of solidified shell was estimated as about 10 000-17 000 K/min and changed by mold flux. Unevenness of the solidified shell thickness becomes remarkable when the shell grows to be 1 mm thick. Relation between the unevenness and the cooling rate was discussed, and critical cooling rate against the uneven solidification was observed around 17 000 K/min. Thermal resistance of mold flux film was also evaluated and it was clarified that thermal resistance in the film is larger than that by air gap, and Crystallization in the film contributes to increase of both resistances. It is also considered that increase of casting speed makes air gap thinner, so reduction of radiation by crystallization of mold flux becomes more important in high speed continuous casting.

The authors investigated the change in the interfacial tension with time for various combinations of molten slag and liquid Fe to elucidate the mechanism of the change in interfacial tension between liquid Fe alloy and molten slag over time accompanying reduction/oxidation reactions. The behavior of the change in the interfacial tension over time can be explained by the adsorption of oxygen at the interface and the diffusion of oxygen from the interface into the bulk of the liquid Fe and molten slag. In addition to that, we found that the interfacial tension decreases slowly and greatly from its initial value to a minimum point and then increases slowly to the final equilibrium state when molten silicate slag with low viscosity is brought into contact with liquid Fe without Al content and some of its SiO 2 decomposes and dissolves into the liquid Fe. From these results, we suggest that the detachment of oxygen adsorbed at the interface into the liquid Fe is very slow and may be the rate-limiting step.

A thermodynamic model for determining the surface tension of molten ionic mixtures, derived by considering the ionic radii of the components, was extended to multi-component slag systems. The composition dependence of the surface tension in 6-component systems of the type CaO-SiO 2 -Al 2 O 3 -MgO-Na 2 O-CaF 2 was evaluated with the present model using information based on the surface tension and molar volume of the pure oxide components, and on the anionic and cationic radii. The evaluated results for the surface tension agree well with literature data.KEY WORDS: surface tension; molten slag; mold flux; ionic radius. ISIJ International, Vol. 47 (2007) Subscript i refers to the following components: SiO 2 , CaO, Al 2 O 3 , MgO, Na 2 O or CaF 2 . Subscripts A and X refer to the cations and anions of component i, respectively. Superscripts "Surf " and "Bulk" indicate the surface and bulk, respectively. R is the gas constant, T is the absolute temperature, s i Pure is the surface tension of the pure molten component i, which is treated as a model parameter.,corresponds to the molar surface area in a monolayer of pure molten component i (N 0 : Avogadro's number, V i : molar volume of the pure molten component i). N i P is the mole fraction of the component i in phase P (PϭSurf or Bulk). R A is the radii of the cation, and R X is the radii of the anion. undergo surface relaxation due to spontaneous changes in the ionic distance at the surface, which causes the energetic state of the surface to approach that of the bulk state. Thus, the contribution from excess Gibbs energy terms is neglected in Butler's equation.[2] In ionic substances, it is well known that their ionic structures depend upon the ratio of the cation to anion radii. In order to evaluate the ionic structures and physico-chemical properties of ionic materials, the cation to anion radii ratio should be considered. Data on the ionic radii were obtained from Shannon,9) and the molar volumes of the pure oxides recommended by Mills and Keene 10) were used in the present model. These values are listed in Tables 1 and 2, respectively.Regarding the temperature dependences of the surface tensions for pure SiO 2 and CaF 2 , those recommended by NIST 11) were used for the calculations in this study. The temperature dependences of the surface tension for pure CaO, Al 2 O 3 , MgO and Na 2 O were evaluated in previous work 7,8) and used in the current study. The equations for determining the temperature dependences of surface tension are listed in Table 3. Results and Discussion Quaternary SystemThe surface tension of molten slag for the CaO-SiO 2 -CaF 2 system was evaluated in the previous work. 8) Based on these results, 8) the effects of Al 2 O 3 , MgO or Na 2 O addition on the surface tension of the above ternary system were evaluated in this study. The calculated results for the surface tension are shown in Fig. 1. Figure 1(a) is based on the results obtained for the CaO-SiO 2 -CaF 2 system reported by Nakamoto et al. 8) The three lines in this figure indicat...

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In the high speed continuous casting of hypo-peritectic steel slabs, growth of solidified shell just below the meniscus in the mold was researched. Rate of cooling and solidification was estimated on the basis of experimental results of thickness profiles of solidified shell, heat flux in the mold and dendrite arm spacing in the solidified structure beneath the surface of the slabs cast at 3-5 m/min.As a result, it was found that there is a delaying period of solidification growth at the beginning, till the shell grows up to about 1 mm thick. After that, it grows in linear relation to square root of solidification time. Dealing with the thickness profiles as a function of solidification time and solidification rate was obtained by differential. Furthermore, cooling rate of the shell was estimated with this solidification rate and heat flux in the mold. This cooling rate showed good agreement with that estimated from dendrite secondary arm spacing of the slabs.Estimated cooling rates were compared with the slabs by conventional continuous caster at 1.1-1.6 m/min and the influence of casting speed was discussed. It resulted in that cooling rate increases with casting speed and the difference by casting speed begins to be remarkable in the period of 0.1-1 s for solidification time, when uneven solidification just begins to occur. This agreement was considered to be one of the reasons why uneven solidification or longitudinal surface cracking of slabs tends to occur with increase of casting speed.