저탄소강의 CEM 공정시 발생하는 측면 균열의 발생원인에 관한 연구

DoctorIn steel production, loss of hot ductility and surface cracking problem during continuous casting (CC) process has been a serious problem in micro-alloyed steels. Recently, combined technique of CC and hot direct rolling (HDR) has been adopted to save energy and cost in steel processing. However, the processing features of this technique such as fast casting speed and lack of a reheating stage would cause surface cracking of steel slabs. The hot ductility of continuously cast micro-alloyed steels is influenced by several factors such as austenite to ferrite phase transformation, austenite grain coarsening and micro-segregation. Micro-alloying elements, such as Al, Ti, V, Nb, B, and S can be segregated into austenite grain boundary and cause the loss of hot ductility. A hot ductility loss is often related to the segregation of S to austenite grain boundaries leading to the precipitation of sulfide particles at the boundaries. These sulfide particles promote intergranular failure in austenite by void formation during grain boundary sliding. This failure mechanism has been observed in low carbon steels with low Mn/S ratios. Sulfide inclusions have been shown to be harmful in hot ductility, but the complete mechanism is not fully understood yet. The addition of Mn to steels supports solid solution strengthening, grain refinement and gets ductility by the formation of MnS. In order to design meaningful laboratory-scale hot tensile test, it is important to simulate the temperature conditions during continuous casting as similar as possible. In situ melting, As-cast condition test is needed to incorporate the total S content and get more precise results. In this study, hot ductility of low carbon steel is studied with compression test and tensile test. As-cast tensile test was also conducted to see the influence of S on hot ductility. Changes in reduction of area (RA) with isothermal holding time were investigated using a thermo-mechanical simulator. The mechanism of hot ductility loss and recovery in low carbon steel is explained to result from austenite-ferrite phase transformation and sulfide segregation. The model for MnS precipitation is established with strain-induced precipitation model. FE simulation is conducted to calculate the crack susceptibility of steel slab during continuous casting, based on MnS precipitation model

Influence of Nb addition on the tensile properties in low carbon multiphase steels subjected to annealing at different temperature

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Cocompact한 볼록사영 다면체 반사군의 정의가능성 판단기준

학위논문(박사) - 한국과학기술원 : 수리과학과, 2013.2, [ iii, 25 p. ]In this paper, we prove the criterion for a Zariski dense subgroup generated by reflections \Gamma \subset \SL^{\pm}(n+1,\mathbb{R}) to be definable over $\mathbb{A}$ where $\mathbb{A}$ is an integrally closed Noetherian ring in the field $\mathbb{R}$. We apply this criterion for groups generated by reflections that act cocompactly on irreducible properly convex open subdomains of the $n$-dimensional projective sphere. This gives a methodology to construct injective group homomorphisms from such Coxeter groups to \SL^{\pm}(n+1,\mathbb{Z}). Finally we provide some examples of \SL^{\pm}(n+1,\mathbb{Z})-representations of such Coxeter groups. In particular, we consider simplicial reflection groups that are isomorphic to hyperbolic simplicial groups and classify all the conjugacy classes of the reflection subgroups in \SL^{\pm}(n+1,\mathbb{R}) that are definable over $\mathbb{Z}$.한국과학기술원 : 수리과학과

Is obstructive sleep apnea associated with the presence of intracranial cerebral atherosclerosis?

Purpose: Intracranial cerebral atherosclerosis (ICAS) is one of critical atherosclerosis which closely related with stroke. Obstructive sleep apnea (OSA) is associated with systemic atherosclerosis, but it is unclear whether OSA is related with the presence of ICAS. We aimed to investigate the association between the presence of ICAS and severity of OSA in patients with suspected OSA. Methods: This retrospective, cross-sectional study included 283 patients who suspected OSA (presence of one or more OSA-related symptom and high-risk category in Berlin questionnaire) and underwent polysomnography and brain magnetic resonance angiography (MRA). The ICAS was defined as ≥50% decrease of luminal diameter in MRA. The severity of OSA was defined by apnea-hypopnea index (AHI). Results: The mean age was 60.7 ± 13.5 years, and 55.8% (158/283) were male in all included patients. The 53 (18.7%) patients had ICAS and 117 (41.3%) patients had moderate to severe OSA (AHI ≥ 15). Higher AHI was noted in patients with ICAS compared to those without ICAS (31.7 ± 25.8 versus 15.2 ± 17.4, p = 0.001). In multivariable logistic analyses, after adjusting for age, sex, and variables with p < 0.1 in univariable analyses (hypertension, diabetes mellitus, atrial fibrillation, previous stroke history, body mass index, lipid-lowing agents, arousal index, and minimum oxygen saturation), moderate to severe OSA were independently related with the presence of ICAS (odds ratio 4.17, 95% confidence interval 1.40–12.40, p = 0.010). Conclusions: Our findings suggest that moderate to severe OSA is associated with the presence of ICAS in patients with suspected OSA. © 2017, Springer-Verlag Berlin Heidelberg