발간논문

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The magnetic properties and the structure of high silicon steel sheets (3wt%-6.5wt%Si) prepared by the twin roller-quenching method were investigated. The columnar grains about 5-7㎛ in diameter were observed in the as-prepared sheets and the columnar grains grew from the surface to the center part of the sheet. In the as-prepared sheet, the coercive force was 1.05 Oe for the 6.5 wt%Si-Fe sheet, and the average mag netic iron loss W_(15/50) and W_(10/50) of the sheets having the coercive force of 1.05 Oe was 1.50 W/㎏ and 0.46W/㎏ respectively.

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The efffcts of mechanical indentation on the deformation mode and core loss of 3% Si steel sheet were compared to that by the pulsed Nd YAG laser irradiation. Some of electropolished specimens were indented with the Rockwell hardness diamond brale and others were irradiated with Q-switched Nd : YAG laser pulses. The slip lines and defect structures were investigated by the optical microscopy and TEM respectively. the effect of annealing of both specimens at 1000 C for 2 hours on the core loss was investigated. $lt;111$gt; slips on {110} planes were the most predominant slip systems for both cases. However, some slip systems other than {110}$lt;111$gt; systems were activated in the laser-irradiated specimen, probably due to the thermal effect of the laser-induced deformation. Increase of the dislocation density by the mechanical indentation was greater than by the laser-irradiation. Dislocations assumed a specific direction in both specimens. These elongated dislocations and the stress fields induced by them are believed to be related closely with the reduction of the magnetic domain wall spacing in the 3% Si steel sheet. Annealing of the mechanically indented specimen at 1000℃ for 2 hours has returned the reduced core loss to the original value. While annealing of the laser-irradiated specimen has not changed the reduced core loss.

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The effect of austenitizing temperature and holding time at different austenitizing conditions on the behaviors of transformed austenite phase from ferrite in ductile cast Iron has been investigated. The formation behaviors of bainite formed by destablized isothermal heat treatment at 300℃, 400℃ after heating in the eutectoid transformation range(α+γ) for 1 hour were also, studied. Even the volume fraction of transformed austenite was same MVH(Micro Vicker`s Hardness) of austenite formed at high austenitizing temperature has higher value than that of austenite at low temperature. The bainite phase began to form at interface of austenite and ferrite which carbon concentration is low relatively. This difference in the behaviors of transformed austenite seemed to be affected by the segregation of substitutional alloying elements and the change of carbon diffusion rate with various austenitizing conditions and austempering conditions.

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De-Si and De-Mn abilities of various desiliconization agents and the effect of CaO addition into the De-Si fluxes were investigated to obtain the optimal operation of desiliconization in the iron cast house of blast furnace. Also, the reactivity of De-Si in the whole range of initial silicon contents of pig iron were considered thermodynamically and compared with the results of experiments and real operations in the initially high and low silicon pig irons. The results show that the millscale has better De-Si abilities than other agents especially in the lower unit consumptions. The result could be attributed to the comparatively more amount of available iron oxide content in millscale. the appropriate addition of CaO of proper size was recommended for the better De-Si. And then, initial contents of Si had much effects on the desiliconization and demanganese ratios.

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Elastic-plastic fracture toughness(J_(IC)) of Ni-Cr-Mo-V steel with different yield strengths has been evaluated by two different testing methods (ASTM E8l3 JSME S001) and compared with K_(IC) and Charpy V-notch (CVN) impart value. At high yield strength levels, J_(IC) values measured by ASTM and JSME method are almost equal, however, at low yield strength levels, J_(IC) values by the ASTM method exceed those by the JSME method. This difference in J_(IC) values is due to variation in the method of considering crack extension on the specimen fracture surface. The comparison of K_(IC) with J_(IC) and CVN impact values shows that a very good correlation, exists between 93- 144 ㎏/㎟ yield strength levels. Therefore, it is possible to predict K_IC value with J_(IC) and CVN impact test.

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The physical properties and mechanical properties of Fe-36∼41.5% Ni binary, Fe-41.5% Ni-(X) ternary and Fe-41.5% Ni-0.5% MnCO₃-(Y) quarternary alloys produced by die-compaction were studied. Following sintering and homogenization heat treatment underwent the experimental alloys cold working and annealing heat treatments in a temperature range of 720∼1150℃. The mean linear thermal expansion coefficient of the experimental alloys, α￣, showed a minimum within a temperature range of 20∼500℃. The minimum mean linear thermal expansion coefficient, α￣_(min), as well as the temperature corresponding to α￣_(min) of Fe-Ni binary alloys increased with nickel content. The temperature for α￣_(min) of Fe-41.5% Ni alloy increased with MnCO₃, Sn or Al addition. The effect of nickel content on the physical properties of the powder-processed materials were similar to those of ingot materials. Addition of MnCO₃in Fe-41.5% Ni alloy resulted in a sharp increase of the microhardness and the inflection temperature after 820℃ annealing. The mechanical properties of the experimental alloys were found to be sensitive to the inhomogeneities of the microstructure.

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In order to improve wear-resistance of carbon tool steel, a study of TiC-surface alloying was carried out by irradiation of CO₂laser. Mixtures of TiC and Co powders were applied directly on specimens by coating powder slurry suspended in alchohol to the thickness of 70㎛. Surface alloyed layers were obtained by single and overlapping laser tracks. Microstructural observation, X-ray diffraction analysis, microhardness test and wear test were carried out on earth specimen. Strengthening mechanisms of TiC-surface alloyed hayer were martensitic transformation, dispersion of TiC particles and grain refinements in alloyed layer. Although the alloyed layer was martensitic structure, lots of undissolved Fe₃C were present due to rapid heating and cooling. Mixtures of TiC+ 5% Co resulted in an increase of hardness by elevating Ms temperature, but further increase of Co reduced the hardness by stabilization of austenite. Even if Fe₃C was decarburized in TiC-surface alloyed layer when it was annealed at high temperature, TiC was not decarburized. Thus a little higher wear-resistance and heat-resistance are expected compared with carbon tool steel when it is used in high temperature.

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Phase decomposition of some dual phase (α+ γ) Fe-Al-Mn alloys by aging treatment has been studied. α-phase in the dual phase structure decomposed more actively by aging treatment than γ-phase. Initial α-phase has been decomposed into manganese rich β-Mn and manganese depleted new α-phase(α) at the early stage of aging. At the later stage of aging, k-phase of Perovskite structure has been produced additionally. Thereby, the αphase decomposition of the dual phase structure can be summarized as α→β-Mn + α, →β-Mn + α+ κ. γ-phase has been shrinked by aging treatment and disappeared finally. The decomposition of γ-phase is as follows ; γ-r + α→α_0 +β-Mn + κ. All the above results are from the Fe-Al-Mn alloys containing more than 0.2 wt% of carbon. However, the alloy containing 0.04 wt% of carbon did not produce κ-phase with all the increased aging time of 500 hours.

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Variation of martensitic transformation temperatures in the Cu-14.38 Zn-6.75Al (Wt%) Shape memory alloy to heat treated in various conditions was systematically investigated by the use of dilatometry. The stable transformation temperatures(M_s, M_f, A_s, A_f,) of the al1oy are reproducible in the two step quenched alloy(above M_s and subsquently below M_f) rather than in the direct quenched one below M_f. An aging and a thermal cycling has a slight influence upon the transformation temperatures of the alloy due to the stabilization of the corresponding phases. Aging in the parent phase decrease M_s and in the martensite phase increase A_s Transformation temperatures are become constant after 100 cycles during 298 K(＜M_f)↔423 K(＞A_f) cycling. An order of transformation temperatures are changeable from class I (M_f＜M_s＜A_s＜A_f) to class Ⅱ(M_f＜A_s＜M_s＜A_f) or vice versa by a thermal history of the a1loy.

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In Cu-Tn-Al shape memery alloys, it is necessary for practical application that mechanical properties of the alloys is modified by grain refinement. In present work, grain was refined by adding misch metal, Mn or Ni and mechanical and thermal cycling test have been done. In the case of Mn and Ni effect of grain refinement were small. It was, however, predominated by a factor of 20 in the case of misch metal. The fracture stress and the strain of the refined alloy with size of 50㎛ were increased with 2 and 1.3 times than those of the alloy not to be refined, respectively. Shape memory effect was not affected significantly by decreasing grain size, but fatigue life by thermal cycling in the refined alloy was increased with 1.5 times.

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