발간논문

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The present study is aimed at developing the high strength, high formability low alloy steels having retained austenite as one of the constituents. The main emphasis has been put on understanding the effects of processing variables and alloying elements (Cu, Nb) on the microstructure and mechanical properties of hot-rolled Fe-C-Mn-Si base TRIP (Transformation Induced Plasticity) steels. Experimental results show that microstructure of hot-rolled alloys consists of ferrite, bainite, martensite and retained austenite, whose volume fraction and morphology vary depending on the hot-rolling conditions and alloy compositions. The increase in the quench-finish temperature results in a homogeneous distribution of fine granular particles of retained austenite and hard second phases in a ferrite matrix and accordingly a good combination of strength and ductility. The Cu-containing alloy has been found to improve mechanical properties over the base alloy due to the precipitation of fine ε-Cu in ferrite grains. Nb addition increases the dynamic recrystallization temperature of austenite, resulting in the formation of coarse pan-caked bainite packets. Based on these results, the optimum microstructure for a better combination of strength and ductility is suggested to be the one having granular particles of retained austenite and hard phase (martensite or bainite) in a fine ferrite matrix.

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Deformation induced martensite transformation in hot-rolled multiphase steels containing retained austenite was studied to elucidate the relationship between the stability of retained austenite and the flow behavior depending on compositions and process conditions. The change in volume fraction of retained austenite with strain was studied on two basically different kinds of kinetics model, treated as a function of strain in one model and stress in the other model. It is shown in the present study that the latter model (stress) has the clearer physical meaning. A new Rinetics equation for deformation induced transformation was formulated as log f_γ=log f_(γ0)-δ(σ-σ_(YS)), with parameter δ characterizing the stability of retained austenite. The increase in plastic stability of retained austenite with the increase in carbon content in retained austenite can be explained by the increase in the mechanical driving force for martensite transformation.

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The effects of intermediate annealing on the plastic anisotropy were investigated in Al-6 wt%Mg alloy sheets for automobile application. Intermediate annealing was adopted to improve the formability in Al-Mg alloys. For two different conditions of O and T4 treatment, textures were analyzed by using orientation distribution function method. The plastic anisotropy properties were evaluated by measuring R values, and then compared them with caculated R values from quantitative texture data. The stretch forming properties were also evaluated by using work hardening exponents. Intermediate annealing decreased the deformation textures, which had an influence on the development of recrystallization textures during final heat treatment. Isotropy and stretch forming properties were decreased in the axis of 45˚ to rolling direction for O treatment due to highly remained deformation textures. Whereas anisotropy and stretch forming properties were increased in the axis of 45˚ to rolling direction for T4 treatment due to well-developed recrystallization textures of rotated cube along normal direction.

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The Effect of very small addition of misch metals (La, Ce, etc) on the decomposition behavior of the supersaturated solid solution a in Cu-6%Ni-4%Sn(wt.%) alloy was investigated by means of microstructural observations such as optical, scanning and transmission electron microscopies. In the case of Cu-6Ni-4Sn alloy which does not contain Mm, the supersaturated solid solution α was not decomposed into other phases such as spinodal decomposition, γ′ (D0_(22) and y (D0₃) by aging after solution treatment. The small addition of Mm to the Cu-6%Ni-4%Sn alloy, on the other hand, affected markedly the decomposition behaviors of the supersaturated solid solution α. In addition, the grain growth was significantly retarded by the small addition of Mm. In this alloy containing Mm, the supersaturated solid solution α was decomposed into γ′ and γ phases with aging time, indicating that the supersaturated solid solution α was decomposed by the spinodal decomposition in the first stage.

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The object of this study is to characterize the high temperature properties and the effect of aging treatment on mechanical properties in modified Fe-15Cr-26Ni-1.25Mo type precipitation hardening alloy containing Ti content of 1.1-1.3 wt%. It has been investigated mainly by tensile test, micro-vickers hardness test, scanning electron microscopy EDAX, charpy n-notch impact, low cycle fatigue, and combination smooth-notch stress-rupture test. The results obtained are summerized as follows; For modified Fe-15Cr-26Ni-1.25Mo type precipitation hardening alloy The optimum heat treatment condition are selected. It is clarified that an optimum Ti content of 1.1-1.3% meets the required mechanical properties of ultra supercritical steam turbine at 649℃. The charpy absorbed energy was almost stable at 13㎏-m∼12㎏-m from R.T to -140℃ The failure cycles(N_f) are 13,022 in Δε_t(%)=0.5 and 1,254 in Δε_t(%)=1 at 650℃. It appers lower notch sensitivity in combination smooth-notch stress-rupture test at 650℃, judging from not fractured at notched part but at smooth part.

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The purpose of this study was to develope IF(interstitial free) steels for the application to automotive body panels. IF steels containing less than 20 wt ppm of C and N, and 1.2 to 2.8 wt% of Mn were prepared to investigate the effect of Mn on the microstructure and mechanical properties. Microstructure observation of IF steels annealed at austenitic region revealed network type polygonal ferrite due to Mn gradient for the steels containing 1.23% Mn(A) and 1.94% Mn(B), and granular bainitic ferrite with high dislocation density for the steel containing 2.76%Mn(C). Tensile strength of A. B and C after annealing (800-920℃) was measured as about 400, 450 and 500 ㎫, respectively Tensile strength decreased slightly for A and B, butincreased for C with increasing annealing temperature. Granular bainitic ferrite of C might induce the strengthening effect at a high annealing temperature. (Fe. Ti) P precipitates formed when IF steels were annealed at ferritic region, which lead to the shortage of effective Ti content to remove solute carbon. The major texture of IF steels was found to be (554)[225￣] that is favorable to improve the formability. However, intensity of (554)[225￣] was decreased with increasing Mn content.

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The effects of yield stress on various parameters of Delayed Hydride Cracking (DHC) in pressure tube materials were investigated by experimental tests and by comparing with theoretical models. The DHC test and tension test were conducted at 200℃ for three kinds of zirconium alloys. As yield stress decreased, the threshold stress intensity factor and the striation spacing increased in one alloy, white fracture surfaces and striation were affected by microstructures in all alloys. The DHC velocity decreased as yield stress decreased. Hence, the DHC velocity was predictable by yield stress and DHC model. However, the predicted velocity was a little higher than measured velocity due to slight microstructural differences. The decrease in yield stress improved the resistance against DHC phenomena in pressure tube materials.

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The effects of dynamic strain aging (DSA) on the fracture resistance of ASME SA516-Gr.70 steels for nuclear piping were investigated through J-R tests conducted in the wide ranges of temperatures (RT-500℃) and loading rates (0.3-1000 ㎜/min). Tensile properties were also investigated in the temperature range of 100 to 316℃ over a range of strain rates (2.5×10^(-5)-1.33×10^0/s). The test results indicated that the SA516-Gr.70 steel was very susceptible to DSA. The minimum fracture resistance values were obtained in the temperature range between 200℃ and 400℃ for each given loading rate. The temperature for the minimum moved to higher temperature region as loading rates increase. The minimum was discussed in the viewpoint of DSA that is manifested by tensile tests. Furthermore. differences of DSA behaviors between two test materials of same specification were described and discussed with respect to the chemical composition.

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Valve seat, an important part for maintaining engine performance, requires excellent heat and wear resistance to meet the severe condition inside the engine. The recent trend of miniaturization and high performance of engines has put an urgent necessity for the development of valve seats to be able to operate under more severe condition. In the present study, the microstructure and mechanical properties of sintered Fe alloys with varying Ni and Mo contents for valve seats have been investigated for the development of valve seat material and improvement of high temperature wear resistance. The most excellent mechanical properties and high temperature wear resistance were obtained in the material with 5 wt%Ni and 7.5 wt% Mo contents.

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Thermal barrier coatings(TBCs) fabricated by plasma spray process is generally used for gas turbine and diesel engine to improve their efficiency and service life. In this study, intermediate mixed layer between bond and ceramic layer was sprayed to improve thermal shock lifetime through increase of adhesion strength. On spraying the mixed Layer, each melted droplet of bond and ceramic powder was not mixing. Thus, interface shape between bond and ceramic Layer showed the complicated morphology. The adhesion strength and the thermal shock lifetime were improved by maximum 1.2 and 1.5 times. respectively. It was judged that improvement of thermal shock lifetime was due to increase of adhesion strength and tensile residual stress. From the stress analysis of thermal shock procedure. it was proved that enlargement of fractured region was occurred because horizontal crack in the vicinity of interface propagated in the direction of surface.

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