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상이한 초미세립 구조를 갖는 OFHC Cu의 연성에 영향을 미치는 요소인자
박이주(L. J. Park),감형원(H. W. Kim),이종수(C. S. Lee),황시우(S. W. Hwang),박경태(K.-T.Park) 한국소성가공학회 2010 한국소성가공학회 학술대회 논문집 Vol.2010 No.5
Tensile ductility of OFHC Cu with the different ultrafine grained (UFO) structures, which were fabricated by the different routes of equal channel angular pressing (ECAP), was associated in detail with the microstructural characteristics developed by ECAP. OFHC Cu having the lamellar and equiaxed UFO structures was prepared by ECAP of routes A and Bc respectively up to 8 and 16 passes. Their microstructures were closely examined by transmission electron microscopy and orientation image mapping. Tensile tests at room temperature were conduced on the ECAPed samples under the quasi- static condition of 10?³ s?¹ and 1 s?¹. Uniform elongation of the lamellar UFO samples decreased with increasing the ECAP passage while both uniform and total elongations of the equiaxed UFO samples increased. In the case of route A producing the lamellar UFO structure, the fractions of high angle grain boundaries and grains less than 0.5 ㎛ increased significantly but an analysis revealed that the dislocation free length decreased with increasing the ECAP passage. For route Be resulting in the equiaxed UFO structure, the fraction of high angle grain boundaries increased but the grain size distribution and the dislocation free length remained nearly unchanged with increasing the ECAP passage. From the present experiments and analyses, it was found that tensile ductility of lamellar UFO OFHC Cu is primarily controlled by the dislocation free length and that of the equiaxed one is mainly dependent on the fraction of high angle grain boundaries.
TRIP형 Duplex Stainless 강의 상온 인장 특성에 미치는 N 함량의 영향
지정훈(J. H. Ji),김세라(S. R. Kim),황시우(S. W. Hwang),최점용(J. Y. Choi),박경태(K. T. Park) 한국소성가공학회 2010 한국소성가공학회 학술대회 논문집 Vol.2010 No.10
(1) 저 Ni 계 이상(duplex) STS 강을 개발하기 위해 Fe- 20Cr- 5Mn- 0.2Ni- xN (x=0.1, 0.2, 0.3, 중량%)강 냉연소둔재의 인장특성과 인장변형 중 미세조직 변화에 미치는 N 첨가의 영향을 조사하였다. (2) Fe- 20Cr- 5Mn- 0.2Ni- xN (x < 0.3) STS 강의 경우 N 함량 증가에 따라 강도 및 연신율은 증가하였다. 이는 N 함량 증가에 따른 austenite 분율 증가에 의한 austenite 소성변형 중 일어나는 TRIP의 기여도가 증가하기 때문이다. In the present study, room temperature tensile behaviors of duplex stainless steels with the low Ni content were investigated to examine the effect of nitrogen addition. For this purpose, nitrogen of 0.1~0.3 wt.% was added to the steels. The steels were cold rolled (~70 % reduction) and then annealed at 1100 C for 5 min. Both strength and ductility increased with increasing the N content. Especially, in the case of the steel containing 0.3 wt.% N, tensile strength exhibited ~1000 ㎫, and the elongation became ~50 %. Deformation of austenite in the present duplex stainless steel was dominated by transformation induced plasticity, and greatly affected the overall deformation.
초미세 결정립 구조를 갖는 OFHC Cu 대형봉재의 미세조직 및 기계적 특성
박경태(K.-T. Park),박이주(L. J. Park),김형원(H. W. Kim),황시우(S.W. Hwang),지정훈(J.H. Ji),이종수(C.S. Lee) 한국소성가공학회 2011 한국소성가공학회 학술대회 논문집 Vol.2011 No.5
Bars of OFHC Cu with the diameter of 45mm were processed by equal channel angular pressing up to 16 passes via route Be, and homogeneity of their microstructrues and mechanical properties was examined at every four passes which develop the equiaxed ultrafine grains. Cross-sectional hardness exhibited a concentric distribution. Hardness was the highest at the center of bar and it decreased gradually from center to surface. After 16 passes, overall hardness decreased due to recovery and partial recrystallization. Regardless of the number of passage, yield strength and tensile strength were quite uniform at all positions, but elongation showed some degree of scattering. At 4 passes, coarse and ultrafine grains coexisted at all positions. After 4 passes, uniform equiaxed ultrafine grains were obtained at the center, while uniform elongated ultrafine grains were manifested at the upper half position. At the lower half position, grains were equiaxed but its size was inhomogeneous. It was found that inhomogeneity of grain morphology and grain size distribution at different positions are to be attributed to scattering in elongation but they did not affect strength.
냉간 압연에 따른 N 함유 저 Ni계 Duplex Stainless Steel의 변형거동
최점용(J. Y. Choi),황시우(S. W. Hwang),지정훈(J. H. Ji),김세라(S. R. Kim),박경태(K. -T. Park) 한국소성가공학회 2011 한국소성가공학회 학술대회 논문집 Vol.2011 No.5
The effects of nitrogen addition on the strain induced martensitic transformation (SIMT) behavior of duplex stainless steel (D-STS) with the low nickel content were examined in a wide range of strain by means of cold rolling. Nitrogen of 0.1,0.2 and 0.3 wt.% was added into Fe-20Cr-5Mn-0.2Ni D-STS (in wt.%) and cold rolling was conducted up to the effective strain of ~1.45 after annealing at 1100 C for 30 min. In the as-annealed state, the austenite fraction increased with increasing the N content. Regardless of the N content, the ferrite grain size was coarser than that of austenite. The stacking fault energy of austenite of the present D-STSs inferred by the element partitioning analysis was low enough so that SIM transformation is available. Accordingly, during cold rolling, SIMT occurred in austenite with a sequential manner of austenite → ε martensite → α’ martensite with increasing strain. By contrast, the deformed microstructure of ferrite was dominated by dislocation cells.
초경량 Fe-27Mn-12Al-0.8C 이상조직강의 상온인장변형특성
하민철(M. C. Ha),황시우(S. W. Hwang),구진모(J. M. Koo),이재곤(J. G. Lee),박경태(K.-T. Park) 한국소성가공학회 2013 한국소성가공학회 학술대회 논문집 Vol.2013 No.10
Room temperature tensile deformation behavior of a low density Fe-27Mn-12Al-0.8C duplex steel was investigated to understand for plastic deformations with constituent phases. Various ordered phases were formed by quenching of the steel after annealing at the (austenite + ferrite) two phase region. The B2 domains were formed in disordered ferrite matrix. In addition to the B2 domains, fine D0 phases were evenly distributed through both B2 domains and disordered ferrite matrix. The nano-sized k-carbides were precipitated in austenite. The steel exhibited the relatively high yield strength and the low strain hardening rate initially, leading to the moderate elongation. Deformed structure of ferrite is manifested by short, straight segments of paired superdislocations. In austenite, a single planar dislocation glide was dominant at low strains and multiple planar slip occurred at high strains. Based on these microstructural observations, it is suggested that strain hardening of the steel is dominated mainly by shearing of the ordered phases by superdislocations (in ferrite) and planar gliding dislocation (in austenite).