발간논문

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Deformation induced martensitic transformation in a steel containing retained austenite(γ_R) has been studied to elucidate the relationship between the stability of retained austenite and the flow behavior depending on the various bainite transformation conditions. The operation of plasticity induced transformation is viewed here as an effective relaxation mechanism of internal strain energy accumulated during plastic deformation. A new kinetics equation for deformation induced transformation has been formulated, based on this relaxation concept of internal strain energy as, f/f_s=1-exp(-βε^n), with the parameter β characterizing the stability of retained austenite, n a deformation mode parameter and f_s denoting the saturation value of transformed martensite. The deformation mode parameter, n is found to be n=1.0 for this steel regardless of heat treatment conditions. The maximum elongation was obtained in a specimen annealed at 400℃ for 5min, corresponding to the minimum value of β. It is therefore concluded that the minimum value of β provides the steady transformation rate during plastic deformation resulting in the best combination of strength and ductility in high strength sheet steel.

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Phases formed during solidification and heat treatment were investigated through varying the solidification rate and quenching temperature in A390 alloy. The microstructural and compositional changes of phases were characterized by scanning electron microscopy(SEM) equipped with an energy dispersive X-ray analysis(EDXA) system and compared with the results of cooling curves and DTA measurements. The results showed that the solidification behavior of A390 alloy is complicated and based on the results and the literature reported, possible reactions which occur during solidification were suggested. During heating the alloy no change of phases was observed up to 480℃ . Some phases begen to dissolve at 495℃ and above 510℃ a large amount of phases were remelted and new phases were formed.

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Effects of the Mo and W additions to Si-Cr spring steels on the microstructural evolution and mechanical properties in spring steels were investigated. It was found that the Mo and/or W addition does not change the behavior of tempered carbide at low temperatures, such as the precipitation of ε-carbide and the conversion of ε-carbide to cementite, via dilatometry tests and the observation of microstructure using TEM. However, it would reduce the coarsening rate of cementite at high temperatures above 450℃, resulting in the smaller size of cementite particles due to the lower diffusion rate. Since the sag resistance depends on the distribution and the size of precipitates, steel C(0.2% W) showed the strongest sag resistance whereas steel A showed the weakest sag resistance, when tempered at 450℃. Also, an abundance of precipitates at 350℃ tempering exhibits the maximum loop area, i.e., the sag resistance for all the tested steels. The Mo and W additions to Si-Cr spring steels raised the ratio of loop area/tensile strength. Therefore, the Mo and W additions would be effective method to increase the sag resistance as well as strength in Si-Cr spring steels.

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The present study is concerned with the processing of copper-bearing high strength steels by direct quenching and tempering (DQ&T) processes. Two DQ&T steels were rolled at different finish rolling temperatures, and their microstructures and mechanical properties such as strength and impact energy were compared with those of a reheat quenching and tempering(RQ&T) steel. The microstructural analyses indicated that the as-quenched microstructure of the DQ steels consisted of refined lath martensite including a number of dislocations, which acted as preferred precipitation sites for NbC or ε-Cu particles during tempering. These fine precipitates were not coarsened much up to the tempering temperature of about 650℃ and thus played a role in improving the tempering resistance. Especially in the DQ&T steel quenched at 760℃ and tempered at 660℃, yield strength reached 1050㎫ and Charpy impact energy at -18℃ showed 140J, indicating the potent effect of the DQ&T process. These findings suggest that copper-bearing low carbon alloy steels with high strength and toughness can be made by the DQ&T process.

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The effects of the alloy content of Si, Ni and V and tempering temperatures on mechanical properties, especially hardness and yield strength, of 0.6C-(1.0-2.5)Si-(0.7-1.8)Ni-(0.1-0.2)V steels were investigated by hardness tests, tensile tests and transmission electron microscopy. The results showed that tempering temperatures for the maximum yield strength and the softening resistance in hardness moved to higher temperatures due to the retardation of the conversion of ε-carbide to cementite within martensite laths by Si addition. Additionally, it was found that the temperatures of the conversion of ε-carbide to cementite was not changed by the addition of Ni or V addition in present steels. The contribution of Ni or V addition was limited only to the increase in hardness and yield strength.

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The effects of the alloy content of Si, Ni and V on impact toughness, in 0.6C-(1.0-2.5)Si-(0.7-1.8)Ni-(0.1-0.2)V steels, were investigated by performing Charpy impact tests, the measurement of volume fraction of retained austenite and microstructural examination with transmission electron microscopy. The results showed that the increase of Si content shifted the embrittlement temperatures to higher temperature, due to Si effect of retarding the formation and growth of cementite at boundaries. Additionally, it was found that the temperature of the peak in Charpy impact curves coincided with the onset of the decomposition of retained austenite. The temperatures of the conversion of ε-carbide to cementite and of the onset of TME were not changed by the Ni addition in the present steels. However, the decomposition of retained austenite was delayed and the toughness was increased over all the tempering temperatures. The contribution of the V addition in the present steels was limited only to the increase impact toughness.

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Conventional indentation fracture test determines fracture toughness only for the centre-loaded half-penny crack of brittle materials; this method, however, has an ambiguous constant related to the indentation residual stress. Thus, in this study, we intend to evaluate the fracture toughness more exactly by quantifying the indentation residual stress and analyzing the real shape of indentation crack. For this purpose, indentation energy is separated into the strain energies in both elastic zone and plastic zone, when stress continuity at the elastic/plastic boundary is assumed. Then, the indentation residual stresses in both zones can be obtained from the separated strain energies, respectively. The critical crack size and the crack shape are determined from the observation of fracture surface and the extraction of post-indentation crack growth. For soda-lime glass and WC-Co, considering the crack shapes as halfpenny and Palmqvist types, respectively, we could evaluate more repeatable and consistent indentation fracture toughnesses of both materials.

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In this study, a Ni-12wt.%Al anode for molten carbonate fuel cells was fabricated by tape casting and subsequent sintering. Its creep behavior was compared with that of a Ni-5wt.%Al anode also fabricated in this study and its performance as an anode was checked through a unit cell test. Whereas the Ni-5wt.%Al anode was observed to exist as a single-phase solid solution, the Ni-12wt.%Al anode was in a state of a two-phase mixture of Ni-5wt.%Al solid solution and Ni ₃Al intermetallic compound. The two anodes had different porosities and therefore their creep behaviors were compared to each other with the porosity dependence of a porous structure`s creep behavior taken into account. The porosity dependence was determined from the analysis of creep test results for Ni anodes of various porosities. Thus, the higher creep resistance could be expected for the Ni-12wt.%Al anode than for the Ni-5wt.%Al anode. Also, the performance of a unit cell assembled with the Ni-12wt.%Al anode was reasonably good with a open circuit voltage of 1.13V and a closed circuit voltage of 0.78V at 150㎃/㎠ current density.

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An investigation was conducted into the effects of microstructural parameters on the cracking which often occurred in the edge parts of hot rolled non-oriented electrical steel sheets. Particular emphasis was placed on the role of iron oxide particles and ferrite grain shape in the cracking phenomenon. Detailed microstructural analyses of the sheets showed that highly elongated, fine grains were predominantly found in the edge region because of the less dynamic recrystallization, whose hardness was higher than that of the center region. A number of oxides were also observed in this edge region, and initiated intergranular or cleavage cracks when the edge part was impacted by side guide rolls during rolling. These oxides were identified to be Si- and Mn-containing iron oxides which might be formed during continuous casting process. In addition to edge cracks, line scabs were found in a linear mode parallel to the rolling direction on the surface of the edge region, probably due to severe oxidation during hot rolling. Thus, the minimization of oxides and highly elongated grains in the edge region and the careful control of the gap between sheets and side guide rolls were suggested to prevent the edge cracking.

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A thermodynamic evaluation on the Fe-M-O (M= Al, Cr, Mn, Si, Ti) ternary systems has been made in order to compile a thermodynamic database for steel systems so that the stabilities of various oxides in steels can be calculated. The calculated oxygen solubility limits at 1600℃ in Fe-M melts showed satisfactory agreements with relevant experimental information. The range of oxygen potential suitable for bright annealing of an austenitic stainless steel was also calculated based on a criterion that SiO₂ should form on the steel surface before the formation of Cr₂O₃. and showed a good agreement with experimental information. Based on the good agreements between calculations and relevant experimental information, it was concluded that the current thermodynamic database enables the calculation of stabilities of various oxides over a wide composition and temperature range in steels with good predictability.

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