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급속응고한 Al-Fe-Mo-Si 계 합금분말 압출재의 고온 성질에 관한 연구
나형용,최주태 대한금속재료학회(대한금속학회) 1993 대한금속·재료학회지 Vol.31 No.3
The microstructure and properties of the rapidly solidified Al-8Fe-2Mo-1.5Si alloy for elevated temperature applications were investigated. The effects of Mo on thermal stability and Si on fracture toughness through the observation of microstructure, hardness test and tensile test were described. The mechanical properties of extrudates with the variation of extrusion ratio and size distribution of powder were investigated. Hardness of smaller powder(50㎛) was superior to that of larger powder(150㎛). The extrudate consolidated with finer powder size distribution (10∼100㎛) yields higher strength than that with a coarse size distribution(100∼200㎛). The tensile strength of 2nd extruded bar which was consolidated with finer powders was 520MPa at 25℃, 310MPa at 275℃. The deterioration of mechanical properties at elevated temperature was due to the decomposition of Zone A and metastable phase Al_6Fe into stable phase Al₃Fe and the growth of needle-like Al₃Fe precipitates. Quarternary Mo and Si containing alloys showed uniform dispersion of fine cuboidal-polygonal precipitates, bcc Al_(12)(Fe,Mo)₃Si phase which displayed a remarkable resistance to coarsening.
AISI 304 스테인레스강의 박판주조시 발생하는 미세균열의 분석
강태욱,문희경,이성학,최주태,정성인,하만진 대한금속재료학회(대한금속학회) 2001 대한금속·재료학회지 Vol.39 No.11
In this study, microstructural investigation was conducted on the microcracking phenomenon occurring during strip casting of an AISI 304 stainless steel. Detailed microstructural analyses of the microcracked regions showed that most of the microcracks were formed mainly along tortoiseshell-shaped depressions and that the number and size of microcracks were considerably reduced when strip casting was done right after shot-blasting or pickling treatment of the cast roll surface. This microcracking phenomenon was closely related with the formation of a black oxide layer, which was mainly composed of manganese-rich oxide, on the roll surface. The black oxide layer acted as a barrier of thermal transfer between a roll and melt, led to an increased gas gap and inhomogeneous solidification of cast strips, and thus played a role in forming both tortoiseshell-shaped depressions and microcracks. The installation of brush rolls behind cast rolls was suggested as a method to prevent microcracks, since the brush rolls could continuously scrape the black oxide layer from the roll surface.
경계밀착좌표계에 의한 쌍롤박판주조법에서의 용탕의 유동 및 열전달 해석
김용희,윤종규,오규환,나형용,최주태,이중의 대한금속재료학회(대한금속학회) 1996 대한금속·재료학회지 Vol.34 No.10
A two-dimensional numerical model for the vertical twin-roll strip casting process has been developed. The boundary fitted coordinate (BFC) is employed to handle the complicate boundary geometry and the resultant transformed governing equations for the melt pool and roll are solved separately using the finite volume method based on control volume method. The effects of casting conditions of roll speed and superheat on the heat transfer and flow pattern were studied. As roll speed increased, the positions of solidification end point and stagnation point were lower and the temperature of the strip at the roll nip increased. The effects of superheat on the heat transfer and flow pattern were smaller than that of roll speed. The surface temperature of strip out of the roll nip increased due to reheating of strip and the temperature differences between surface and center of strip were in the range of 60∼80℃.
스트립캐스팅법으로 박판주조한 AISI 430 스테인레스강의 미세조직에 관한 연구
김용희,오규환,나형용,최주태,정성인,윤종칠 대한금속재료학회(대한금속학회) 1996 대한금속·재료학회지 Vol.34 No.8
AISI 430 stainless steel strip was fabricated by laboratory scale twin-roll strip caster. Microstructure of ascast strip consists of ferritic α matrix, (Fe,Cr)_(23)C_6 carbides and lath martensite at grain boundary, observed in transmission electron microscopy. Martensite was transformed from retained austenite at grain boundary due to rapid cooling of strip casting. The martensitic formation can be suppressed by the slow cooling after roll nip point. The lower value of hardness at the strip surface was obtained because primary ferrite remained due to higher cooling rate. As it goes from center region to edge region, the solidification end point is located above average height and the white band zone at the central layer become wide. The zone of negative segregation results from squeezing out near the central layer of the liquid which has high phosphorus and sulfur content.