페라이트, 펄라이트, TWIP강의 미세조직에 따른 인발가공성 평가

황중기(Joong-Ki Hwang) 대한기계학회 2019 대한기계학회 춘추학술대회 Vol.2019 No.11

열처리 생략형 선재 철강제품 생산기술

황중기(Joong-Ki Hwang) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.06

공형압연 소재의 가공경화 거동

황중기(Joong-Ki Hwang) 대한기계학회 2023 대한기계학회 춘추학술대회 Vol.2023 No.6

골격성 제III급 부정교합 환자에 대한 상악골 전방견인 장치의 치료효과

황충주,경승현,임중기,Hwang, Chung-Ju,Kyung, Seung-Hyun,Lim, Joong-Ki 대한치과교정학회 1994 대한치과교정학회지 Vol.24 No.4

Before 1970, mandibular overgrowth was known as main cause of skeletal Class III malocclusion in growing children ; however, recent study reports that many skeletal Class III malocclusion patients also show maxillary deficiency. Since 1972, when Delaire re-accommodated Protraction Head Gear (P.H.G.), many researchers have reported that skeletal Class III discrepancies could be corrected through use of P.H.G., which induces anterior movement of maxilla and change in mandibular growth pattern into infero-posterior direction ; nevertheless, it is very difficult to predict resultant changes of orofacial region. The purpose of this study was to find out what treatment effect P.H.G. has on different study samples. Author divided 51 skeletal Class III malocclusion patients with maxillary deficiency who were treated with P.H.G. into different study groups depending on sex, treatment beginning age, intraoral appliance, and facial growth pattern. By doing so, following results were obtained. 1. Treatment beginning age and Sex Four age groups (5.8 to 8 year-old, 8 to 10 year-old, 10 to 12 year-old, 12 to 14 year-old) were compared, and no significant difference was observed. (p<0.05) There was no significant difference between the sex groups, either. (p<0.05). 2. Intraoral appliance Treatment effects of study groups that used R.P.E.(mean age of 10.2) and Labio-Lingual appliance(mean age of 8.9) were compared. There was no significant difference depending on the type of intraoral appliance that was used. (p<0.05) 3. Facial growth pattern 1) Amounts of SNB and ANB corrections were smaller in clockwise growth pattern group than those in normal or counterclockwise growth pattern group. (p<0.05) 2) Amounts of increase in Wits appraisal and mandibular plane angle were greater in counterclockwise growth pattern group than those in normal or clockwise growth pattern group. (p<0.05) 3)Amounts of increase in articular angle were greater in counter lockwise growth pattern group than those in clockwise growth pattern group. (p<0.05)

Evaluation of Material and Characteristic for Alkali-Metal based High Temperature Heat Pipe and TEC (Thermal-to-Electric Convertor)

Joong-Ki HWANG(황중기),Min-Soo SUH(서민수) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.06

Heat pipe generally transfer thermal energy from one regime to another essentially evaporator to condenser. Superalloy envelope with alkali-metal (sodium, potassium) working fluid for high temperature heat pipe (HP). Na-AMTEC convert between thermal energy and electrical energy, in specific terms, the static energy conversion subjected to electrochemical reaction mainly depending on its chemical kinetics and temperature. In particular, Na-AMTEC convert energy between heat and electricity transpired via ionization of natrium as known as reduction and oxidation (REDOX) which is differentiate it from the dynamic energy conversion. In specific terms, 1) the input of thermal energy is converted into electrical energy as same as a thermoelectric cell, 2) the input of thermal energy is converted into the sum of electrical energy P_e and thermal energy P_th, as same as a combined heat & power (CHP) and 3) the input of electrical energy is converted into thermal energy as same as an electrochemical heat pump. Na-AMTEC could highly utilized in aforementioned objectives with efficiencies potentially near Carnot theorem. In case of thermal energy in both of Na HP and Na-AMTEC, the alkali-metal is an energy medium of heat transfer (HF) and heat transfer fluid (HTF). The purpose of Na HP is transferring thermal energy from source to sink with maximizing heat transfer rate and minimizing the heat loss to ambient surroundings. Similarly, the purpose of Na-AMTEC, alkali-metal based thermopower HP, is to convert thermal source into electricity in order to reduce several conventional stages of conversion processes e.g. thermal energy loss during phase change of HTF, energy loss in terms of temperature and pressure during the energy transportation and energy loss during relative mechanical motion results from energy loss and materials squander due mainly to friction and wear. This study is to report the development efforts of Na HP and thermopower HP fabricated for high grade to low grade temperature power generation.

Grain size effect on tensile and wire drawing behaviors in twinning-induced plasticity steel

Joong-Ki Hwang(황중기) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.11

에너지 및 비용 절감형 철강 선재제품 생산기술

황중기(Joong-Ki Hwang) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.4

자동차 연비 향상을 위해 고강도 선재제품에 대한 요구가 지속적으로 있어 왔으나, 기존의 선재제품 사용 소재는 높은 수소취성파괴 민감도로 인해 더 이상 제품의 고강도화 어려운 상황이다. 또한 이들 소재들은 낮은 가공성으로 인해 제품 생산을 위해 다양한 열처리가 필요하여, 제품 원가가 높고, 환경 파괴의 원인이 되는 오염물질을 발생시킨다. 상기 문제점을 해결하기 위해 Hwang [1] 은 고강도, 고연성, 낮은 수소취성파괴 민감도를 나타내는 Twinning-induced plasticity (TWIP) steel을 선재제품에 사용할 것을 제안했다. TWIP 강은 가공중 발생하는 기계적 쌍정에 의해 높은 가공성과 가공경화지수를 나타내게 된다 [2]. 이러한 TWIP 강의 높은 가공성은 선재제품 생산시 필요한 annealing 열처리를 생략하게 하고, 높은 가공경화지수는 quenching/tempering 열처리를 생략할 수 있게 한다. Recently, improving the fuel efficiency and safety standards is one of the major issues in the wire rod industry for automotive applications to save the energy and environment. Therefore, there is a great need for the development of new materials having high strength without the degradation of other properties, such as ductility, toughness, and resistance to hydrogen-delayed fracture (HDF). Hwang [1] reported that one of the candidates meeting such property requirements is the twinning-induced plasticity (TWIP) steels having combination of high tensile strength and extraordinary ductility [2]. They recommended that TWIP steels have many advantages in comparison with the ferritic, pearlitic and tempered martensitic steels that are commercially used for wire rod products and can be the suitable material for high strength wire rod products: higher strength, outstanding ductility, better HDF resistance, and elimination of post heat treatments.

TWIP강의 인발 가공중 파괴 거동

황중기(Joong-Ki Hwang) 대한기계학회 2021 대한기계학회 춘추학술대회 Vol.2021 No.11

일반적으로 선재의 인발 가공시 발생하는 온도 상승은 인발 가공성을 떨어뜨리는 원인이 된다. 하지만 TWIP강의 경우에는 인발중 온도 상승이 인발 가공성을 향상시켰다. 본 발표는 이러한 TWIP강의 독특한 인발 가공 특성에 대한 것으로, 그 메커니즘을 규명하는데 목적이 있다. 인발중 온도 상승이 다른 두 시편을 얻기 위해 인발 감면율을 변화시켜 실험을 수행했다. TWIP강의 경우 소성 변형시 슬립뿐 아니라 기계적 쌍정에 의해 변형되는데, 기계적 쌍정의 온도에 따른 변형거동이 TWIP강의 인발 가공성을 향상시킨 주요 원인으로 확인되었다. 인발 감면율이 높으면 소재의 온도가 상승하며, 이는 기계적 쌍정의 생성을 억제한다. 이를 통해 소재 표면의 과도한 쌍정 생성을 억제하여 인발 가공성을 향상시켰다. The present work focuses on the effects of strain and temperature distribution on fracture behavior in twinning-induced plasticity (TWIP) steel during wire drawing to understand the fracture mechanism of drawn TWIP steel wire. The deformation and fracture behaviors of wire subjected to the reduction of area (RA) per pass of 10% (LR) and 20% (HR) were analyzed. The deformation temperature of wire had a linear relationship with the RA per pass during wire drawing. The HR wire had a higher drawability compared to the LR wire since the HR wire had lower twinning rate due to the higher SFE resulting from the higher deformation temperature. The maximum effective strain as well as shear strain occurred in near surface area, i.e. the relative distance of 0.9 from center to surface of wire. The maximum twinning rate and geometrically necessary dislocation density occurred in near surface area due to the maximum effective and shear strain. Also, near surface area had lower temperature in comparison with surface area. Therefore, the cracks were initiated at near surface area in TWIP steel during wire drawing, which was well observed from the scanning electron microscopy (SEM) fractography. This unique fracture phenomenon of drawn wire is presented for the first time in this study since it rarely occurs in general metals during wire drawing.

선재 냉각대 소재 온도예측 모델 개발 및 활용

황중기(Joong-Ki Hwang),손일헌(Il-Heon Son),이덕락(Duk-Lak Lee) 대한기계학회 2009 대한기계학회 춘추학술대회 Vol.2009 No.11

A coupled thermal and metallurgical mathematical model and off-line simulator have been developed to simulate the Stelmor process for controlled cooling of steel wire rod. In order to solve one-dimensional transient heat transfer equation including the heat evolved from phase transformation, a finite difference method coupled with thermodynamic and kinetic analysis was employed. This model takes into account the temperature-dependent heat transfer and thermophysical properties and stresses the importance of the enthalpy of transformation. A developed model and off-line simulator have been shown to simulate the complex heat flow in the Stelmor process excellently and calculated results were in good agreement with measured temperature.

격자 볼츠만 방법 방법(Lattice Boltzmann Method)을 이용한 다공성 매질내에서의 열전달 특성 특성에 관한 연구

황중기(Joong-Ki Hwang),최도형(Do-Hyung Choi),손일헌(Il-Heon Son),이덕락(Duk-Lak Lee),임성욱(Seong-Wook Yim) 대한기계학회 2010 대한기계학회 춘추학술대회 Vol.2010 No.11

The heat/mass transfer characteristics of a porous medium in two and three-dimensions are investigated by using the Lattice Boltzmann Method to analyze the ball packed-bed regenerator of reheating furnace in steel rolling process. To obtain the bulk properties of the porous medium of various structures, calculations have been performed for porous media of regularly distributed inclusions.. It is found that the thermal dispersion is influenced by the medium structures, the Peclet number, the volume fraction of the solid, the thermal conductivity ratio of the solid and the fluid, and the Reynolds number. The streamwise thermal dispersion coefficients increase with the Peclet number, the volume fraction of the solid, the Reynolds number, and the thermal conductivity ratio of the fluid to the solid. The parameter that affects the thermal dispersion most is the Peclet number