발간논문

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The effects of Ni flash coating on weldability and corrosion resistance of lightly tin coated steel(LTS) have been studied. When Ni layer was coated the prefered orientation of Sn layer was (200) plane. As Sn alloy was formed through reflow or/and stoving process, the structure of Sn alloy with Ni layer was denser than that without Ni layer. The consumption of Sn to Sn alloy was retained when Ni was coated, because the diffusion of Sn was inhibited by dense Fe-Ni-Sn alloy. After reflow and baking process the pure metalic tin remained was enough to have good weldability of LTS. Accelerated salt immersion test, undercutting corrosion test and retort test were performed and we found that Ni-LTS (chromic acid treated) had better lacquer adhesion and better corrosion resistance than tinplate(#25).

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Effects of tensile hold time on the high temperature low cycle fatigue life of 12%Cr-Mo-V steel was investigated at 873K in air atmosphere. In order to investigate the effect of creep-fatigue interaction, the hold times at tensile peak strain were imposed to be 10, 30 and 60min. The experimental results show that, with decreasing plastic strain range and increasing tensile hold time, the fatigue lives of 12%Cr-Mo-V steel under creep-fatigue cycling are significantly decreased compared with the faigue ones of this material under continuous cycling. This is considered to be caused by creep cavitation damage which occurred during tensile hold time. It is found that creep cavities are nucleated not only on prior austenite grain boundaries but also within prior austenite grains(lath martensite boundaries). From the results of creep-fatigue cycling, the fracture mode was observed to be mixed(trans and intergranular). The fatigue lives under creep-fatigue cycling are discussed in terms of a model for life prediction previously developed by creep cavitation damage rather than surface crack initiation and propagation. The predicted creep-fatigue lives are in good agreement with experimentally observed ones for 12%Cr-Mo-V steel with increasing tensile hold time.

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The reactive sputtering of SiN thin films was investigated as a function of process parameters such as power, pressure, nitrogen to argon gas ratio, and flow rate. The SiN film properties including internal stress and corrosion resistance were affected by the microstructure of SiN thin films rather than by the refractive index. As sputtering pressure was increased, columnar structure was enhanaced with the decreased film density, resulting in the remarkable change in the internal stress and the etch rate. With increasing nitrogen gas ratio and flow rate, the refractive index and deposition rate decreased due to the increased nitrogen reaction. The optimum sputtering condition to meet the required SiN film properties as a protective and underlayer of magneto-optical disk was subject to the reactive process of sputtered atoms, which was greatly influenced by power and pressure.

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Among the processes of thin slab casting, cast-rolling technique has been developed, which permits the reduction of slab thickness during solidification. To simulate this process, a water-cooled mold with a inclined side-wall is designed. This side-wall can be made vertical by means of a hydraulic press during solidification. This procedure makes it possible to bring the effect of cast-pressing to cast slab during solidification. With cast-pressing, the center crack and macrosegregation due to bulging are eliminated at half-height of cast slab. Cast-pressing leads to a considerable area of equiaxed structure in the top half of the cast slab. The ferrite grain size, after γ→α phase transformation, is reduced by cast-pressing. However, after hot rolling with two passes, the effect of cast-pressing on the tensile properties and ferrite grain size does not exist any more, probably because of longer soaking time and recrystallization during hot rolling.

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The macrosegregation behavior in continuously cast(CC) high carbon steel blooms are systematically investigated by carrying out a plant test using a bloom caster at POSCO in connection with the electro-magnetic stirring(EMS) mode and stirring force. The degree of centre soundness is quantitatively measured in terms of the area fraction and size distribution of centre porosity as well as the segregating solutes by using a mapping analysis method. The thicknesswise solidification structures are examined to find out solidification behavior in the centre region of CC blooms and the calculation of specific permeability(k) is also presented to elucidate the formation mechanism of centre defects. The addition of a final stirring(FEMS) to an in-strand EMS(SEMS) is found to increase the solidification rate significantly in the final stage resulting in a more fine distribution of porosities and segregation spots in CC blooms. An optimum stirring forces of SEMS and FEMS are then determined to provide the best centre soundness for the high carbon steel tested. The most effective conditions at the final stage of solidification for the reduction of macrosegregation in CC blooms have also been suggested as a result of this study.

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A comparison of the tensile deformation behaviors of austenitic and martensitic stainless steels was made. The austenitic stainless steel used in this study was SUS 304 water-quenched from 1050℃, and the martensitic stainless steel SUS 410 air-cooled from 1000℃ and subsequently tempered at 750℃ for an hour. The detailed comparison for deformation characteristics of two stainless steels was summarized in Table 2.

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