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고탄소강의 세멘타이트 재용해에 미치는 냉간압하량의 영향
하태권(T. K. Ha),김준호(J. H. Kim),나길환(K. H. Na) 한국소성가공학회 2012 한국소성가공학회 학술대회 논문집 Vol.2012 No.5
Dissolution behavior of cementite in a spheroidized SK85 high carbon steel during austenitization heat treatment was investigated in this study. Spheroidized SK85 high carbon steel sheet with fine and uniformly distributed cementites was prepared by proper thermomechanical treatment. Average diameter and aspect ratio of spheroidized cementites were 1 ㎛ and 1.6 respectively. Area fraction of the cementies was measured as 12%. Cold rolling was conducted on the sheet with the reduction ratios from 0 to 0.6 to elucidate the effect of cold deformation on the dissolution behavior of spheroidized cementites during austenitization heat treatment. Austenitization heat treatment was carried out on the cold rolled sheets in a dilatometer at temperatures of 800℃ and 850℃ for various time intervals from 5 to 600 seconds. Heating rate was taken to be 10 and 100℃/s and cooling rate was 150℃/s. In the case of undeformed sheet, 80% of cementites were found to disslove in 5 seconds at 850°C, while only 65% of cementites were dissolved even after 40 seconds at 800℃. With cold reduction increased from 0 to 0.4, dissolution rate at 800℃ appeared to increase and further reduction of 0.6 was found to decrease the rate. At the temperature of 850℃, cold reduction has no prominent effect on the dissolution rate and 8085% of cementites were dissolved in 5 or 10 seconds. Quenching from the austenitization temperature of 800℃ revealed the microstructure of very fine pearlite, while the specimen quenched from 850℃ showed the microstructure of martensite and some pearlites.
하태권(T. K. Ha),김준호(J. H. Kim),나길환(G. H. Na) 한국소성가공학회 2012 한국소성가공학회 학술대회 논문집 Vol.2012 No.5
본 연구에서는 modified Bridgman 법을 이용하여 순 마그네슘, Mg-1Al, Mg-1Zn-0.5Y, Mg-3Li, 그리고 AZ31 합금 단결정을 성공적으로 제조할 수 있었으며, 제조된 결정에 대하여 방위결정, 압축 및 경도시험을 수행하였다. 단결정의 강도와 경도는 방위에 매우 민감한 것을 확인할 수 있었고, 합금원소의 첨가에 의해서도 강도와 가공경화 거동이 영향을 받는 것을 확인하였다. Single crystals of pure Magnesium, Mg-1Al, Mg-1Zn-0.5Y and Mg-3Li alloy were successfully fabricated in this study by employing the modified Bridgman method. To determine the exact orientation of crystals, Laue back-reflection method and pole figure measurement were carried out on each single crystal. Dimensions of single crystals were 10 ㎜ in diameter and 120 ㎜ in length. Hardness and compression tests were conducted and the results revealed that hardness and the strength strongly depended on the orientation. The closer to basal one the orientation was, the higher hardness and compressive strength were. The effect of alloying was not higher than that of orientation. After compressive deformation of single crystals, the orientation of the crystals was found to rotate and to be parallel to the basal orientation.
하태권(T. K. Ha),김준호(J. H. Kim) 한국소성가공학회 2013 한국소성가공학회 학술대회 논문집 Vol.2013 No.5
본 연구에서는 modified Bridgman 법을 이용하여 순 마그네슘, Mg-1Zn-0.5Y, Mg-0.1Y, and Mg-0.1Ce 합금 단결정을 성공적으로 제조할 수 있었으며, 제조된 결정에 대하여 방위결정, 압축 및 경도시험을 수행하였다. 단결정의 강도와 경도는 방위에 매우 민감한 것을 확인할 수 있었고, 합금원소의 첨가에 의해서도 강도와 가공경화 거동이 영향을 받는 것을 확인하였다. Single crystals of Magnesium alloys such as pure Mg, Mg-1Zn-0.5Y, Mg-0.1Y, and Mg-0.1Ce alloys were successfully fabricated in this study by employing the modified Bridgman method. To determine the exact orientation of crystals, pole figure measurement using X-ray diffraction were carried out on each single crystal. Hardness and compression tests were conducted followed by subsequent recrysatllization annealing. Recrystallization kinetics of Mg alloy single crystals has been investigated. Fabricated single crystals were cut into rectangular shaped specimen and solution treated at 400℃ for 24 hrs, and then deformed in compression mode by 30% reduction. Annealing treatment for recrystallization has been conducted on these cold-rolled plates at temperatures of 300℃ for various times from 1 to 20 mins. The microstructure observation and hardness measurement conducted on the recrystallized specimens revealed that static recrystallization of ternary alloy single crystal was very slow, while recrystallization behavior of binary alloy single crystals appeared to be very fast.
하태권(T. K. Ha),김근준(K. J. Kim),나길환(G. H. Na) 한국소성가공학회 2009 한국소성가공학회 학술대회 논문집 Vol.2009 No.10
In the present study, the effect of initial microstructure, cold reduction ratio, and annealing temperature on the spherodization rate of SK85 high carbon steel sheet was investigated. High carbon steel sheet fabricated by POSCO was soaked at 800℃ for 2 hr in a box furnace and then treated at 570℃ for 5 min in a salt bath furnace followed by water quenching to obtain a fine pearlite structure. Cold rolling was conducted on the sheets of fine pearlite by reduction ratios of 20, 30, and 40% and heat treatment for spheroidization was carried out at 600 and 720℃ for the various time intervals from 0.1 to 32 hrs. Area fraction of spheroidized cementite was measured with an image analyzer as a function of cold reduction ratios and duration times.
논문 : 기계적 성질 ; 내부변형변수를 이용한 8090 Al-Li 합금의 고온변형거동 연구
하태권 ( T. K. Ha ),장영원 ( Y. W. Chang ) 대한금속재료학회 ( 구 대한금속학회 ) 1996 대한금속·재료학회지 Vol.34 No.11
A new approach to structural superplasticity is attempted in this study based on the internal deformation variables. The basic concept of this theory is that the high temperature deformation, especially superplasticity, of crystalline materials consists mainly of grain boundary sliding (GBS) and grain matrix deformation by dislocation glide process to accommodate the incompatibility caused by GBS. The high temperature deformation behavior of 8090 Al-Li alloy has been studied experimentally within the framework of the new theory proposed. A series of load relaxation and tensile tests at various temperatures ranging from 200℃ to 530℃ have been conducted to obtain the flow curves and the results were systematically analyzed based on the internal variable theory. The flow curve of log σ vs. log ε has been found to be a composite curve consisting of GBS and plastic deformation confirming the kinematic relation of the steady state form, ε=α+g at sufficiently high temperature with α and g representing the strain rates due to plastic deformation and GBS, respectively. The GBS in 8090 Al-Li alloy examined in this study appears to be a Newtonian viscous flow process characterized by the power index value of Mg= 1.0. The internal strength variable (σ*) is found to be inversely proportional to the square root of grain size and the activation energy for grain matrix deformation is obtained as 124.9 kJ/mol in the temperature range from 470℃ to 530℃, which is very similar to the activation energy for self diffusion in pure Al.