발간논문

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Sulfide stress corrosion cracking (SSCC) behavior of X65 line-pipe steels was studied by a constant elongation rate test method to understand the sulfide stress corrosion mechanism. The SSCC test was carried out in aqueous acetic acid solution containing chloride ion in the strain rate range of 5×10^5∼5×10^7 sec^(-1). The corrosion potential and the corrosion rate in the aqueous solution were -580 mV_(SHI) and 4.2×10³ Acm^(-2), respectively. Degradation of the mechanical properties such as yield strength and ultimate tensile strength was observed during SSCC test. The SSCC resistance expressed by relative fracture energy ratio decreased from 55% to 13.5% at an open circuit potential of -580 mV_(SHI) as the strain rate decreased from 5×10^5 to 5×10^(-7)/sec. Anodic or cathodic polarization at about 100 mV resulted in reducing the SSCC resistance, which was explained by a model related to anodic dissolution and hydrogen embrittlement. The brittle fracture surface with intergranular and transgranular cracking was observed. The corrosion products on the failed surface were the mixture of iron oxide and sulfide.

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During cooling of steels, the heat transfer was controlled by radiation, convection, conduction and heat evolution from phase transformation. To analyze the heat transfer during cooling precisely, the material constants such as density, heat capacity and the heat evolved during transformation were obtained as functions of temperature and chemical composition for each phase observed in plain carbon steel using a thermodynamic analysis based on the sublattice model of Fe-C-Mn system. The results were applied to 0.049 wt% and 0.155 wt% carbon steels with an austenitic stainless steel as reference by developing a proper heat transfer governing equation. The equation was solved using the lumped system method. In addition, using a transformation dilatometer with adequate experimental conditions to clarify the individual heat transfer effect, the transformation heat evolved during cooling and the transformation behavior as well as the temperature change were observed. The predicted temperature profiles during cooling were well agreed with the measured ones.

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A finite volume model was developed for the analysis of fluid flow and heat transfer phenomena in the round billet, that consists of a molten metal pool, a mushy zone, and a solidified region, in the continuous casting process of 6XXX series Al alloys. This developed model included the evaluation of latent heat of fusion in a fixed grid, treated whole computational domain as a single phase using an effective viscosity, and optimized heat transfer coefficients at each cooling region. The positions of mushy zone and crater end at the center and surface of billets were quantitatively analyzed to find optimum process parameters, such as casting temperature and speed. The investigation for the effects of casting conditions on the flow and solidification pattern during casting was correlated with the quality of billet, and applied to predict the mechanism on the formation of some typical casting defects, so called cooking ring and cold lap. The model could be used to optimize the set of flow and thermal parameters according to the change of alloy and size of billet, leading to the minimum formation of casting defects.

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In this study, the solderability of the in-situ composite solders was investigated by reflow soldering the solder balls on a BT substrate. The reaction characteristics of the composite solders were evaluated by observing the microstructure of the solder/BLM interface. Also the effects of the number of reflow soldering on the microstructure and the distribution of the reinforcement were investigated. These results indicated that the composite solder has a lower reactivity with the BLM and Cu, but its thermal stability must be improved significantly for actual applications of the material.

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The solubility of silver in the BaO-B₂O₃ slag at 1698K was measured as a measure of basicity, and compared to that of CaO-B₂O₃ slag. The solubility of silver in molten slags has a minimum value at the composition of 45(㏖%)BaO-B₂O₃ and 65(㏖%)CaO-B₂O₃, respectively. In the acidic region, BaO is more basic than CaO by, at most, about 33%, based on the difference of Ag solubility at the same activity of basic oxide, indicating that BaO is not a much-stronger basic oxide than CaO in the acidic slags. However, in the basic region, BaO is about 5 times more basic than CaO. From the relationship between the composition of slags and the integral molar enthalpy of mixing of the liquid mixtures for both slags, the relationship between the basicity of slags and the modification of borate structure was discussed. The correlation of Ag solubility with nitride capacity, carbide capacity, and theoretical optical basicity were also discussed.

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In order to investigate stepwise reduction behaviors of iron ore pellets, CVRD pellets were reduced stepwise from Fe₂O₃ to Fe₃O₄, Fe₃O₄, to FeO and finally FeO to Fe with H₂-H₂O gas mixtures at 700-900℃. Chemical reaction rate constant(k_(ci))and effective diffusivity(D_(ei)) calculated from the mixed control plots based on the unreacted core model were decreased with proceeding of the reduction step and increased with the increase of the temperature, respectively. In the case of 900℃, k_(ij) was increased with proceeding of the reduction step. k_(ij), for each reduction steps are expressed by following equations, respectively. k_(cm) ＝ exp(3.262－3.563×10³/RT) (㎝/sec) k_(cw) ＝ exp(5.261－6.496×10³/RT) (㎝/sec) k_(efe) ＝ exp(11.979－13.616×10³/RT) (㎝/sec) In the intermediate model, Thiele`s modulus(Φ`) was increased with proceeding of the reduction step, but effectiveness factor(E_f) was decreased contrary to Φ`. Reduction curves calculated by using the rate parameters for stepwise reduction were agreed with experiments in the intermediate model than in the unreacted core model.

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The MgO solubility of the steelmaking slag system at 1873K was measured under condition of the atmospheric air. The effect of slag composition on the MgO solubility was discussed and quantified. The basicity and the MnO content decrease the MgO solubility. However, The effect of iron oxide content is observed to be dependant on the basicity of slag. As the iron oxide content increases, the MgO solubility increases for the basic slag (basicity 1.5) but decreases for acidic slag. The content of Al₂O₃ shows little effect on the MgO solubility.

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Effects of cold working and final aging on the corrosion resistance of Zr-1Fe-0.2Cu alloy were investigated. Microhardness tests were conducted for all heat-treated specimens and tensile tests were also carried out for some selected samples. Corrosion resistance was observed to be degraded by the introduction of cold rolling conducted before aging probably due to the promotion of precipitate formation. The corrosion resistance was also decreased during aging mainly because the size of precipitate was increased. The microstructure of heat-treated samples was investigated using SEM and TEM. And, from EDS and SADP analyses only one kind of precipitate, Zr₃Fe, was identified to exist in the heat-treated samples. The short final aging at a high temperature (580℃) was thought to be desirable for the mechanical properties.

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