발간논문

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The effects of the microstructures on the stretch-flangeability and tensile properties of Nb or Ti-Nb added hot-rolled high strength steel with tensile strength of 60 and 70 ㎏/㎟ grades were investigated to improve the stretch-flangeability, through the laboratory simulation and the mill-scale production. The low temperature coiling method developed using 3-step controlled cooling pattern after the finish rolling was suitable for the production of high strength steel with the polygonal ferrite and bainite phases. The enhanced stretch-flangeability of the ferrite-bainite dual phase steel was due to the fine grain boundary cementites and the decrease of deviation in hardness between the ferrite and the bainite phases, and so that void formation was suppressed relative to the other type of multi-phase steels, ferrite-bainite or ferrite-bainite-martensite steels.

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Texture development by hot deformation in a Mo containing γ-TiAl alloy was studied in order to determine whether the plastic anisotropy by texture affects tensile ductility at room temperature or not. Isothermal forging and hot rolling were performed in different deformation conditions. In addition, heat treatment after high temperature deformation was performed in order to investigate the texture evolution through microstructure control. Textures of the deformed and heat treated specimens were measured and analyzed in terms of orientation distribution functions(ODFs). It was found that texture formation was closely related to dynamic recrystallization(DRX) during high temperature deformation. As far as the texture developed by isothermal forging was concerned, strong {302} ⊥ ND was found to form after deformation and maintain after the heat treatment for the duplex microstructure. However, random texture was measured after the heat treatment for the fully lamellar microstructure. Tensile elongation at room temperature showed a strong anisotropy with orientations and improved tensile elongation was obtained from Ti-48.5 at. %Al-1.5 at. %Mo alloys. These results indicated that the plastic anisotropy by texture could be used for improving the room temperature tensile property of γ-TiAl alloys.

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The hardening and damage in tensile and compressive deformation of 7XXX Al matrix composites were analyzed. The effect of the damage in the particles and/or at the interfaces on the stress-strain response is different in tension and in compression. In tension, cracks mostly perpendicular to the stress axis in ceramic particles definitely lower the loading capacity and then the overall hardening rate. With increasing tensile strain there may be cracking of additional particles and the localized strain developed at the tip of cracks and in the matrix linking the cracked particles. Therefore tensile deformation behavior of Al matrix composite is more strongly influenced by the size of reinforced ceramic particle than the matrix properties. On the other hand in compression cracks mostly parallel to the stress axis in ceramic particles are not expected to lower the loading capacity and have little influence on the hardening rate. The growth and the link-up of damage in compression are difficult because of the compressive stress near crack tips. Therefore, the flow stress is not greatly influenced by the frequency of particle cracks and it is strongly dependent on the matrix properties.

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Electrodeposition experiments of gradually composite alloys were conducted using rotating disc geometry. Effects of rotating speed and potential on the composition of alloys were studied. Mechanical properties and corrosion resistance of the gradually composite alloys were compared with those of the normally deposited alloys. Zinc content of Zn-Ni alloys increased as increasing disc rotation speed and voltage. However, Zn content of Zn-Ni-P alloys decreased but those of Ni and P increased as raising voltage. Zinc content of Zn-Fe and Zn-Fe-P alloys increased as raising voltage, however, Zn, Fe and P contents were not changed as increasing disc rotation speed. Corrosion resistance and adhesion of the gradually composite alloys for all the tested alloys were better than the normally deposited ones.

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In order to characterize the behavior of active species during the pulse plasma nitriding process, OES(optical emission spectroscopy) measurements were performed for various gas compositions, treatment temperatures, duty factors ratios and different gas pressures in the reactor. Results of the OES measurements were compared to those of the metallurgical analysis of plasma nitrided sintered steel and high speed steel substrates. It was found that the spectral intensities of N^+₂ and N₂ were influenced by treatment parameters and the thickness of nitride layer and size of micro-pore increased with increasing the temperature and pulse ratio. Especially, the compound layer was mainly influenced by gas pressure.

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The effects of final annealing temperature on microstructure and corrosion behavior of Zr-based alloys were investigated. Four kinds of Zr alleys were used for microstructural study and corrosion teat. Corrosion tests were carried out in static mini autoclave under 400℃ steam condition. Microstructures were analyzed by using TEM/EDS and image analyzer. The grain size and precipitate size of Zircaloy-4 increased with increasing the annealing temperature. As the annealing temperature increased, the corrosion resistance of Zircaloy-4 also increased due to the increase of precipitate size and grain size. Therefore, the precipitate size and grain size play an important role on the control of corrosion resistance of Zircaloy-4. However, it was considered from this study that the corrosion resistance of Nb-containing alloys would be controlled by the characteristics of beta phase rather than precipitate size.

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A solid type crankshaft, one of the most important part of medium size marine engine, is manufactured by forging method. The induction heating system(600 ㎾ = 100%, 190∼200 ㎐) is used to heat rounded bar(Ø285 ㎜) up to 1200℃ for the forging. Numerical analysis was performed on the heating process at various operating conditions by applying boundary conditions based on theoretical approach. Experiments were also performed on the same type of mended bar to confirm the simulation results. Experimental results showed that the induction heating phenomena were effected by the phase transformation at A_(c₁) A_(c₃) temperature range, the magnetic transformation at A₂ (curie point) temperature range and the mass effect of large volume of rounded bar. At the 62% power, our induction heating system showed the maximum efficiency and the heating time was about 40∼50 minutes.

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Yttria stabilized zirconia(YSZ) thin film electrolyte for solid oxide fuel cells(SOFCS) was reactively deposited by RF sputtering and its mechanical and electrical propertiea were characterized. Deposition rate of 8 ㏖% YSZ thin film was about 100Å/min under the sputtering conditions of 14% of oxygen flow ratio and 6.6 W/㎠ of RF power density. Furthermore, surface roughness of YSZ thin films increased with increasing yttria content in the films and fine grain-like particles were considerably appeared, especially, in 8 ㏖% YSZ thin film. It was thought that these fine grain-like particles were caused by un-resolved yttria and that these were considerably reduced by post-annealing at 500℃ for 1 hr. The electrical conductivity of a triple layered 8 ㏖% YSZ/3 ㏖% YSZ/8% ㏖% YSZ thin film stack at 900℃ was slightly lower than that of a single layered 8 ㏖% YSZ thin film which was most widely used solid electrolytes for SOFCs. However, the triple layered 8 ㏖% YSZ/3 ㏖% YSZ/8㏖ % YSZ thin film had lower compress stress and better microhardness than 8 ㏖% YSZ thin firms, exhibiting a possibility of application to SOFCs.

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The effect of ferrite grain refinement on the heat-affected zone microstructural evolution was investigated using grain-refined ferrite-pearlite steel with ferrite grain size of 4㎛. HAZ austenite grain size and microstructure volume fractions were measured and compared them with normal steels. An established theoretical model for the prediction of HAZ microstructures of normal steels was modified for grain-refined steels. Ferrite grain refinement resulted in smaller HAZ austenite size. Compared to 320㎛ of normal steels, grain-refined steels showed only 104㎛ at the welding condition of T_p 1350℃-△t 20s for example. Variation of austenite size with welding conditions can be predicted reasonably well when the initial austenite size of grain-refined steels is 9㎛. It was shown that the established Johnson-Mehl type model was also applicable to grain-refined steel when correctional factor due to grain refinement was incorporated in it. Measured and calculated values showed a good agreement when the correctional factor was 0.5.

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Variation of HAZ strength of a fine-grained ferrite-pearlite steel with peak temperature (T_p) and cooling time (△t) was investigated, Using microstructural evolution model and rule of mixture, HAZ strength was also calculated and compared with measured one. It showed that variation of HAZ tensile and yield strength with T_p was different each other. At △t is 40s, for example, HAZ tensile strength decreased continuously with decrease of T_p to 1050℃ and varied little below it. Yield strength, however, increased again below 1050℃. Calculation of microstructure volume fraction and ferrite grain size showed that even though all HAZ below 1050℃ showed same ferrite and pearlite microstructure, ferrite grain size decreased continuously with decrease of T_p, resulting in increase of yield strength. Therefore, HAZ to base metal yield strength ratio showed minimum at 1050℃. With increase of △t, the minimum value becomes smaller; 0.88 at 20s and 0.80 at 40s, showing softened HAZ in certain welding conditions. Calculated HAZ yield strength showed a good agreement with measured one.

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