발간논문

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The present study aims at investigating the correlation of carbon content, aging and bake hardening characteristics, and yield point elongation(YPEl) in cold-rolled and annealed sheet steels. It was found that solute carbon content less than 10 weight ppm existed without contributing to both bake hardenability(BH) and aging index(AI). The yield point elongation of as-annealed steels showed a good correlation with carbon content in the range of carbon content less than 10 weight ppm, but showed a weak correlation with carbon content above 10 weight ppm. In contrast, both bake hardenability and aging index which were not observed in the steels with carbon content less than 10 weight ppm, increased with increasing the carbon content above 10 weight ppm. Increasing Mn content from 0.2% to 1.0% in Ti-added ultra-low carbon steels contributed to decrease in BH, AI, and yield point elongation. This was because more Mn content participated in the sulfide precipitation as MnS, resulting in the increase in Ti content available to scavenge solute carbon.

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Using the thermodynamic model between TiN-BN-AlN in austenite, simulated heat-affected zone impact toughness of low-carbon type Ti-B containing steels was investigated. HAZ toughness was dependent on microstructure which was determined by the soluble boron to soluble nitrogen ratio at the welding peak temperature, 1350℃. Only when the soluble boron to soluble nitrogen ratio is kept below 0.2, HAZ showed ferrite dominant microstructure and had absorbed energy higher than 100J at - 20℃. It was shown that soluble boron combined with soluble nitrogen to 50rm boron nitrides during welding cooling cycle, resulting in a reduction of effective boron influencing the hardenability of HAZ. Compared to the conventional steels, low-carbon steels with a controlled soluble boron to soluble nitrogen ratio showed excellent HAZ toughness and no cold-cracking without preheating.

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With the view of judging the possibility of reoxidation of Al killed steel, the oxygen potentials of ladle slags were calculated adopting the regular solution model. It was estimated that the activity coefficient of FeO in the ladle slag was 1.497 on the average. The oxygen potentials of the slag and melt for ultra-low carbon steel were higher than those for low carbon steel. In order to decrease the oxygen potential of the slag, it should be controlled to lower Al₂O₃ content and to higher CaO content. The mixture of aluminum and CaCO₃ was found to be more effective than aluminum alone as a slag deoxidizer.

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Low SiO₂ and high basicity sintering tests were carried out to reduce the slag volume in sinter without causing any deterioration of sinter properties. New slag volume(NSV) was defined instead of slag volume as the concept of content of bonding material. Low SiO₂ and high basicity sintering condition reduced the slag volume from 21.0% to 19.3% without any deterioration of the productivity and the strength of sinter. The strength of sinter was determined to be highly dependent on the content of calcium ferrite. NSV, defined as the sum of slag volume and calcium ferrite content, proved to be a useful index for the strength of sinter. NSV increased with the decrease of SiO₂during fixed CaO content due to the increase of calcium ferrite although slag volume decreased. In pot tests, as SiO₂ content decreased from 5.90% to 5.28% while CaO remained at 10.5%, the productivity and the strength of sinter tended to improve.

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Aluminum coating is one of the most effective methods for surface protection of steel. The aim of this study was to establish the optimum working conditions for aluminum coatings on steel in molten chloride salts. Electrolytes used were chloride mixtures of AlCl₃ and KCl. The working temperatures were 300℃ in chloride salts. Experiments were carried out in argon-gas atmosphere and electrolytes were pre-electrolyzed to remove residual impurities. In chloride system, effects of NH₄Cl as a surface active and electrochemically active substance were investigated. Influences of the co-deposition elements, manganese and copper on the coating were also investigated. Characteristics of deposit layer was analyzed by means of SEM, EPMA and XRD. Optimum conditions for the coating were found to be the bath temperature, 300℃ and the current density, 150 mA/ ㎠. In chloride electrolyte, more promising electrodeposition of aluminum was obtained with addition of NH₄Cl as surface active substance. Compact and fine aluminum coatings were obtained with 1.5wt.%NH₄Cl at 300℃ and the current density, 150mA/㎠. Addition of MnCl₂as a co-deposit element also significatly improved the quality of deposit layer. By addition of CuCl, however, the layer had better quality than that of MnCl₂. Metalliding didn`t occur at bath temperature lower than 500℃. Optimum current density for the metalliding of aluminum in chloride electrolyte containing 1.5wt.% NH₄Cl was found to be 150Al/㎠. Deposit layer formed was identified as an aluminum-iron alloy.

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Two kinds of plasma-sprayed zirconia coating which are ZrO₂-8%Y₂O₃ and ZrO₂-24%MgO are studied to increase the thermal fatigue properties of gray iron in the presence of water spray. The results of thermal cycling tests from room temperature to 1100℃ indicate that ZrO₂-8%Y₂O₃ coating behaves better than ZrO₂-24%MgO and the phase transformation after the tests for both coatings is negligible even in the presence of water. The failure of ZrO₂-24% MgO coating has occurred by the formation of iron oxide layer between the bond coating and the gray iron substrate and then the cracks have propagated through the brittle iron oxide layer and the substrate although blunting of the cracks seems to have occurred in the bond coating. On the other hand, iron oxide layer is not observed for ZrO₂-8%Y₂O₃ coating and thus the cracks in the ceramic top coating have blunted in the bond coating. Also, temperature-variant indentation experiments over the temperature range of 20℃ to 800℃ are reported and analysed using an activation energy approach.

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The effect of stacking conditions on optical properties of TiO₂, Ag, and TiO₂/Ag/TiO₂ multilayer has been examined for the applications of transparent heat-mirror. At PO₂/P_(Ar)≥-0.2, TiO₂films(180A˚) showed 90%(γ=550 ㎚) visible range transmittance, which decreased as thickness increased. High transmittance(T_(max)=80% at 324 ㎚) of Ag films was observed at the thickness of 120∼180 A˚. Due to the diffusion of Ag, TiO₂/Ag/TiO₂ stacks showed poor IR reflectivity. After insertion of Ti(40A˚) as diffusion barrier, the TiO₂/Ti/Ag/Ti/TiO₂stacks showed good spectral selectivity as transparent heat-mirror with 89%(γ=410 ㎚) visible transmittancy and 77%(γ=820 ㎚) IR reflectancy.

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