발간논문

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To investigate the clogging of the lance during the injection of gas with or without powder, a model study on the penetration behaviour of liquid into the nozzle was carried out in water, water-glyceline mixtures, mercury and also molten pig iron. In cold models, the penetration of liquid into the nozzle was determined using an electric contact method. On the other hand, the penetration depth of the molten pig iron was measured on the lance which was cut open after the runs. A regression analysis was made to predict the relationship between the penetration depth of the liquid into the perpendicularly downward facing nozzle and the influencing factors, such as the surface tension, the densities of gas and liquid, the viscosities of gas and liquid, the inner diameter of nozzle, and the gas flow velocity. It was shown that the penetration behaviour of the liquid into the nozzle depends on the gas flow velocity, the inner diameter and orientation of nozzle, and the densities of gas and liquid. The penetration behaviour was reduced with powder injection compared to gas only injection. The comparison of results about penetration behaviour of liquid into the nozzle showed that the understanding won for cold models can be carried over to the conditions in molten pig iron.

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Restoration behavior during single-pass hot rolling of wedge shaped specimen taken from a columnar region of continuously cast Fe-3% Si steel slab was investigated in the temperature range 950-1250℃, where (α＋γ) two phases exist. At lower temperatures (950-1050℃) a critical amount of reduction was required to render recrystallizatmn; smaller reduction produced deformed structures. At higher temperatures (1100-1200℃) recovered structure was observed at small reduction while recrystallization occurred over a critical reduction, The plastic deformation was observed to lead to marked acceleration of α→γtransformation. It was assumed that this transformed austenite give the sites of recrystallization by formation of deformed bands in ferrite matrix. The critical reduction to start austenitic transformation was nearly consistent with the reduction to start recrystallization. The nose was about 1100℃.

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Thermodynamic data and phase diagram of liquid Sb-S system were investigated by drop-calorimeter and DTA. The drop-calorimeter was utilized to measure the heat content of liquid Sb₂S₃in the temperature range of 550℃ to 1150℃, and DTA was need to construct the equilibrium line between the immiscible liquid and the homogeneous one in the compoaition range of sulfur from 0 wt.% to 23.8 wt.%. Based on the experimental data, following results are obtained. 1. The heat capacity and the heat of fusion of liquid Sb₂S₃are as follows; Cp(Sb₂S₃, 1)＝-29.80＋79.0×10^(-3) T (㎈/deg·mole) H_f(823 K)＝10072㎈/㏖ 2. The equilibrium line between the immiscible liquid and the homogeneous one was varied from 700℃ to 912℃ corresponding to sulfur content.

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Dependence of secondary recrystallization on primary recrystallized structure in Boron Si-Fe was investigated using heats containing 0.049%C (Alloy A) and 0.013% (Alloy B). Elongated grains in primary recrystallized structure were observed in samples cold rolled with 40 passes after normalizing above 1050℃ for 5 minutes for Alloy A and in samples normalized at 900℃ for 5 minutes for Alloy B. Samples which contain elongated grains in primary recrystallized structure experienced normal grain growth and developed no secondary recrystallization during final texture anneal. However, the elongated grains in primary recrystallized structure were completely eliminated and secondary recrystallization occurred in samples which were additional heat treated at 900℃ in N₂ atmosphere for a long time (72 hrs) at the hot band stage before cold rolling and texture annealing.

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The direct reduction of nickel sulphide by hydrogen gas in the presence of calcium oxide was investigated in the temperature range of 500-630℃. The reaction kinetics were determined by the thermo-gravimetric analysis technique with silica spring, under condition such that unreacted core model in the chemical reaction control regime could be applied. The effect of temperature, additives, hydrogen flow rate, particle size and sample porosity were studied. The rate of reaction increased as temperature increased and more considerably increased when calcium oxide was added as a sulphure scavenging agent. Under these experimental conditions the rates of reaction were considered to be controlled by chemical reaction . A whisker formation of nickel metal was observed, above 550℃. The apparent activation energy of NiS with 2CaO was about 25.62㎉.

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In order to investigate the characteristics of mechanical properties, the strengthening effects and strengthening mechanisms of ausformed martensite prepared by ausforming treatment in Fe-30% Ni alloy and Fe-30% Ni-0.1% C alloy, optical and electron microstructures, amounts of retained austenite and mechanical properties such as hardness, yield strength, ultimate tensile strength and elongation were examined. The results obtained in this study are as follows: 1. Ausformed martensite is plate type martensite coexisted with midrib twin and partly tangled dislocation, and has a dislocation cell structure formed from the tangled dislocation in high deformation degrees. 2. Although the increment of strength induced by deforming austenite is not entirely inherited by ausformed martensite, the strength of ausformed martensite in two alloys shows a remarkable increment with increasing deformation degrees. 3. Ausformed martensite of Fe-30% Ni-0.1% C alloy shows more increasing rate of strength thant that of Fe-30% Ni alloy. 4. Elongation of austenite is rapidly decreased with increasing deformation degrees. However elongation of ausformed martensite shows a nearly constant value independent of deformation degree because of the formation of ductile matrix. 5. The strengthening mechanisms of ausformed martensite in two alloys are considered to be the multiple strengthening effects caused by the inheritance of lattice defects from deformed austenite, the refinement of martensite plate and substructure induced from martensite transformation.

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The effect of the heat input (H.I) and natural aging on the mechanical properties and microstructure of the electron beam welded 7075-T6 aluminum alloy has been investigated. This study leads to the following conclusions. 1) The hardness of weldment increased with decreasing H.I. (518.1-1281.8 J/㎜) and increasing natural aging time (5-60 days). 2) The hardness generally increased with distance from the weld metal, though a little decrease in hardness occurred in HAZ(II). 3) The natural aging of the weldment for 100 days raised the tensile strength by about 13% and decreased the absorbed energy by about 35% compared to the 20 day-naturally aged weldment.

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