발간논문

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High Carbon(15∼30%) MgO bricks containing aluminum and silicon in addition to carbon were studied to understand the mechanism of their slag resistance. Samples chosen from the hot spots of electric arc furnace walls were sliced into thin pieces and polished. Polished surfaces were investigated using polarizing microscope, electron probe microanalyzer and X-ray diffractometer. As is the case of low carbon MgO brick, the formation of dense MgO layers were verified both in straight and Al and Si containing high carbon MgO bricks. In the Al and Si containing bricks, Si contributes to the densification of MgO layers by the oxidation to SiO₂followed by the formation of forsterite. It is also speculated that aluminum addition resulted in the formation of the dense structure of the brick by forming spinel with magnesium oxide.

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The effect of the thermal fatigue on the transformation temperature and shape recovery of Ti-Ni alloys was investigated utilizing thermal cycling tests, electrical resistivity measurements and the transmission electron microscopy. Three types of heat treatment were used, they were solution treatment at 950℃, aging treatment at 400℃ after the solution treatment and annealing at 400℃ of a 30% cold-worked specimen after the solution treatment(i.e., mechanical treatment). The mechanical treatment was found to be the most effective way to improve the shape memory effect and stabilization of the transformation temperature. Change of the M_s temperature was observed to a certain number of thermal cycles, and the M_s temperature was nearly constant after a critical number of thermal fatigue cycle.

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The effects of boron content and slab reheating temperature on the hardness of cold rolled steel sheet containing 0.0017-0.0103 wt.% boron were investigated. The minimum hardness was obtained in the steel with B/N atomic ratio of about 1.0. The ferrite grain size of the hot band in the boron-bearing steel appeared to be strongly influenced by solute boron content segregated on the austenite grain boundary. Variations in the size and distribution of cementite of the hot band were observed in the boron-rich steel. This was interpreted in terms of a newly proposed mechanists The hardness decreased with decreasing slab reheating temperature. This was attributed to the insufficient suppression of ferrite grain growth due to the precipitation of coarse BN.

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In this study, the effects of auenchants and agitation conditions on the hardening and cooling power of cooling baths were investigated. Specimens of SNCM21 steel were quenched in oil bath and water bath at various agitation conditions. A multi-channel temperature measurement apparatus was employed to obtain the cooling curves at several predetermined depths from the surface of specimens. The results of microstructural observation and hardness mea surement were used for the evaluation of the cooling power of the quenching bath by means of the Grossmann method Applicability of computer simulation to the heat treatment process was tested by comparing the experimental cooling curve with the calculated one. The latter curves were obtained through the computer simulation employing the finite difference method (FDM). The mass effect of large ingot was also considered in the simulation. The degree of hard ening of steel ingot was determined by quenchants and the cooling power of the quenching bath. The necessity of efficient quenchant-quenching bath system for the proper quenching hardening could be verified by studying various combinations of the quenchant-quenching bath systems for the heat treatment of large steel ingots.

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The effects of the Sn content and microstructure on the tensile properties and the fracture behaviors of spheroidal graphite cast iron were investigated. Addition of 0.05% Sn promoted pearlitic structure of the matrix, which resulted in an enhancement of the yield strength and the fracture toughness of the alloy. Acoustic emission (AE) characterstics of the alloy during mechanical testing were also studied. The voltage range and the shape of AE waves during the deformation stage and the fracture stage varied with the Sn content in the alloy.

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The development of deformation inhomogeneities during the cold rolling of two deep drawing quality steels in the range of 20 to 75% reduction has been studied in detail using transmission electron microscopy. Various types of microstructural inhomogeneities such as microbands and shear bands were observed in the thin foils prepared from longitudinal sections of the rolled specimens. The formation of orientations near {111}<110> is found at the grain boundaries in grains oriented near {111}<112>. The concept of an additional relaxed strain is proposed to interpret the different crystal rotation in a deformed grain. Needle-like dislocation cells oriented at {111}//ND in the rolling plane section were compared with other dislocation cells with respect to the orientation dependence of the elastic stored energy.

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Effects of the Retrogression and Reaging (RRA) treatments on the strength and the stress corrosion crack propagation in a high purity Al-Zn-Mg alloy were studied by measuring the hardness change and the crack growth rate as a function of the stress intensity factor using C-T specimens TEM was used to examine microstructural changes in both matrix and grain boundaries. The results showed that the RRA treatment resulted in a significant decrease of crack velocity without sacrificing T6 strength, similarly to the case of the commercial 7075 Al alloy. The RRA treated specimen showed a high overall concentration of precipitates in matrix including fine dispersion of η`. The improved SCC resistance of the RRA treated specimen is believed to closely related to increase in the size of grain boundary precipitates during the retrogression treatment.

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A study on the microstructure of rapidly solidified Al-Cr, Al-Zr, and Al-Ni alloys was carried out using levitation melting and two-piston apparatus. The microcrystalline structure of the rapidly solidified Al-Cr alloy transformed into a cellular structure showing a regularity as the solidification progressed. The effect of the initial temperature of liquid on the microstructure was not significant. A cluster of precipitate, confirmed to be Al_(11)Cr₂was thought to be formed in the undercooled liquid. The texture of the cellular structure was identified to be of $lt;111$gt; direction, which may be due to the anisotropy of interfacial energy and interface velocity in large undercooked liquid. In Al-Zr alloy, equiaxed α-Al grain structure of 1-5㎛ in size was obtained, in which intermediate compound A1₃Zr was supposed to act as a nucleation site of α-Al. In Al-Ni alloy of a hyper-eutectic composition (7 wt% Ni), the solidification development depended on the condition of contact with the substrate. In the case ofa bad contact the solidification development was microcell, cellular, dendrite, and equiaxed-grain which is nucleated by the A1₃Ni compound, while in the case of a good contact solidification transformed into a microcell, cellular, and rod-type eutectic structure.

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A two dimensional finite difference model has been developed to calculate the temperature distribution and the solidification profile of the continuously cast beam blank. The effect of the Pouring temperature, the casting speed, and the cooling condition has been examined. It has been shown by the calculation that the surface temperature and the solid shell thickness of the cast do not significantly vary with the pouring temperature or the casting speed, and that the effect of the cooling condition, particularly in the mold is critical. Non-uniform cooling resulting from the air gap formation develops the thermal stress in the solid shell and this is the main cause of the surface defects. Consequently, rather soft cooling should be applied to decrease the possibility of the crack formation.

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