Effect of Reduction of Area on Microstructure and Mechanical Properties of Twinning-Induced Plasticity Steel During Wire Drawing

황중기,K. Wierzbanowski,유장영,A. Zargaran,김낙준 대한금속·재료학회 2015 METALS AND MATERIALS International Vol.21 No.5

The effect of reduction of area (RA), 10%, 20%, and 30%, during wire drawing on the inhomogeneities in microstructure and mechanical properties along the radial direction of Fe-Mn-Al-C twinning-induced plasticity steel has been investigated. After wire drawing, the deformation texture developed into the major <111> and minor <100> duplex fiber texture. However, the <111> texture became more pronounced in both center and surface areas as the RA per pass increased. It also shows that a larger RA per pass resulted in a higher yield strength and smaller elongation than a smaller RA per pass at all strain levels. Although inhomogeneities in microstructure and mechanical properties along the radial direction decreased with increasing RA per pass, there existed an optimum RA per pass for maximum drawing limit. Insufficient penetration of strain from surface to center at small RA per pass (e.g., 10%) and high friction and unsound metal flow at large RA per pass (e.g., 30%) all resulted in heterogeneous microstructure and mechanical properties along the radial direction of drawn wire. On the other hand, 20% RA per pass improved the drawing limit by about 30% as compared to the 10% and 30% RAs per pass.

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

인발 속도가 TWIP강의 가공성에 미치는 영향

황중기(Joong-Ki Hwang) 대한기계학회 2020 대한기계학회 춘추학술대회 Vol.2020 No.12

The effect of drawing speed on temperature rise and microstructure distribution in twinning-induced plasticity (TWIP) steel during wire drawing has been investigated to improve drawability for wire rod applications. Although wire drawing process is performed at room temperature, heat is generated due to the plastic deformation and friction at the wire-die interface. The steel wires subjected to the low drawing speed (LD) of 0.5 m/min and the high drawing speed (HD) of 5.0 m/min were analyzed using the numerical simulation and electron backscatter diffraction (EBSD) techniques. Interestingly, the specimens subjected to the HD had a higher drawability by about 18% compared to the LD, which is totally different from the general behavior of plain carbon pearlitic steels. The LD wire had uniform temperature distribution along the radial direction during wire drawing. In contrast, the HD wire had a temperature gradient along the radial direction due to the higher frictional effect at surface: the minimum temperature of 58℃ at center area and the maximum temperature of 143℃ at surface area. The higher stacking fault energy (SFE) of HD wire at the surface area due to the high temperature rise retarded twinning rate, resulting in the prevention of fast exhaustion in ductility in comparison with the LD wires since the earlier depletion of twins at surface area is known as the main reason for the fracture of TWIP steel during wire drawing. Consequently, HD process delayed the fracture strain of wire and increased the uniformity of microstructure and mechanical properties along the radial direction.

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.

TWIP 강의 낮은 가공성 및 단면감소율에 대한 원인 규명

황중기(Joong-Ki Hwang) 대한기계학회 2020 대한기계학회 춘추학술대회 Vol.2020 No.7

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

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.

공형압연 소재의 가공경화 거동

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

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.