• Transactions of Materials Processing
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• v.24 no.6
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• pp.395-399
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• 2015

A microstructure-based finite element simulation was conducted to predict the plastic strain ratio (R-value) of a dual-phase (DP) steel. The representative volume elements (RVEs) concept was adopted for the image-based FE modeling and a 3D model was constructed using sequential 2D images. Each phase was considered with the von-Mises yield criterion and the Swift model. The Swift parameters were defined by the empirical equations based on the chemical composition. The developed model was applied to analyze the effect of residual stress on the R-value and stress distribution. In order to consider the residual stress development after cold rolling, 10 % compression was applied in the thickness direction and unloaded before the tensile stress was applied in the rolling direction. The results showed a reasonable prediction for the R-value evolution: a sharp increase at small strains was well described and a transition followed in the downward direction. The R-value evolution was analyzed using the stress distribution change on the π-plane

• Journal of Welding and Joining
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• v.25 no.2
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• pp.62-69
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• 2007

A kinetic model fur the particle coarsening behavior was developed. The proposed model considered the critical particle size which can be derived from Gibbs-Thomson equation unlike the conventional approach. In this study, the proposed particle coarsening model was applied to study the coarsening behavior of titanium nitride (TiN particle) in microalloyed steel weld HAZ. Particle size distributions and mean particle size by the proposed model were in agreement with the experimental results. Meanwhile, using additivity rule, the isothermal model was extended to predict particle coarsening behavior during continuous thermal cycle.

• Journal of Welding and Joining
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• v.24 no.4
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• pp.39-49
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• 2006

The austenite grain growth model in low alloyed steel HAZ without precipitates was proposed by analyzing isothermal grain growth behavior. Steels used in this study were designed to investigate the effect of alloying elements. Meanwhile, a systematic procedure was proposed to prevent inappropriate neglect of initial grain size (D0) and misreading both time exponent and activation energy for isothermal grain growth. It was found that the time exponent was almost constant, irrespectively of temperature and alloying elements, and activation energy increased with the addition of alloying elements. From quantification of the effect of alloying elements on the activation energy, an isothermal grain growth model was presented. Finally, combining with the additivity rule, the austenite grain size in the CGHAZ was predicted.

• Journal of Welding and Joining
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• v.24 no.1
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• pp.77-87
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• 2006

Considering ferrite grain size in the base metal, the prediction model for $A_{c3}$ temperature and prior austenite grain size at just above $A_{c3}$ temperature was proposed. In order to predict $A_{c3}$ temperature, the Avrami equation was modified with the variation of ferrite grain size, and its kinetic parameters were measured from non-isothermal data during continuous heating. From calculation using a proposed model, $A_{c3}$ temperatures increased with increasing ferrite grain size and heating rate. Meanwhile, by converting the phase transformation kinetic model that predicts the ferrite grain size from austenite grain size during cooling, a prediction model for prior austenite grain size at just above the $A_{c3}$ temperature during heating was developed.

• Transactions of Materials Processing
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• v.10 no.4
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• pp.338-346
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• 2001

Evaluation of microstructural changes is important for process control during open die forging of heavy ingots. The control of forging parameters, such as shape of the dies, reduction, temperature and sequence of passes, is to maximize the forging effects and to minimize inhomogeneities of mechanical properties. The hot working die steel is produced by using the multistage open die forging. The structure is altered during forging by subsequent Precesses of plastic deformation, recrystallization and grain growth. A numerical analysis using an rigid visco-plastic finite element model was performed to predict microstructural evolution of hot working die steel.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2003.05a
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• pp.57-60
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• 2003

The purpose of this study is to investigate the high temperature deformation behavior of Ti-6Al-4V alloy and to predict the final microstructure under given forming conditions. Equiaxed and widmanstatten of Ti-6Al-4V alloys were prepared as initial microstructure and the compression tests were performed to obtain the flow curves at high temperatures (700∼1100$^{\circ}C$) and various strain rates (10$\^$-4/∼10$^2$/s). Form the results of compression test various parameters such as strain rate sensitivity (m) and activation energy (Q) were calculated and used to establish constitutive equations. To predict the final microstructure after forming, finite element analysis was performed considering the microstructural parameters such as the grain size and the volume fraction of second phase.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2001.05a
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• pp.131-135
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• 2001

Evaluation of microstructural changes during open die forging of heavy ingots is important for process control. The objective of the control of forging parameters, such as shape of the dies, reduction, temperature and sequence of passes, is to maximize the forging effects md to minimize inhomogeneities of mechanical properties. The hot working die steel is produced by using the multistage open die forging. The structure is altered during forging by subsequent processes of plastic deformation, recrystallization and grain growth. A numerical analysis using an rigid visco-plastic finite element model was performed to predict microstructural evolution of hot working die steel.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2000.10a
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• pp.25-28
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• 2000

The flow stress of 304 stainless steel during high during hot forming process were determined by conducting hot compression tests at the range of 1273 K-1423 K and 0.05 /s-2.0 /s as these are typical temperature and strain rate in hot forging operation. Based on the observed phenomena, a constitutive model of flow stress was assumed as a function of strain, strain rate, temperature. Dynamic recrystallization was found to be the major softening mechanism with this conditions as previous studies. A finite element analysis was performed to predict the recrystallized volume fraction and the mean grain size in hot compression of 304 stainless steel.

• Transactions of Materials Processing
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• v.10 no.7
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• pp.573-578
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• 2001

The flow stress of 304 stainless steel during hot forming process were determined by conducting hot compression tests at the range of 1273 K∼1423 K and 0.05 /s∼2.0 /s as these are typical temperature and strain rate in hot forging operation. In this material, Dynamic recrystallization was found to be the major softening mechanism with this conditions as Previous studies. Based on the observed phenomena, a constitutive model of flow stress was assumed as a function of strain, strain rate, temperature. In the constitutive model, the effects of strain hardening and dynamic recrystallization were taken into consideration. A finite element method connected to constitutive model was performed to predict the dynamic recrystallization behaviors and also stress-strain curves in hot compression of 304 stainless steel.

• Journal of Industrial Technology
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• v.5
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• pp.21-26
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• 1985

본(本) 연구(硏究)는 주조(鑄造) 및 용접(溶接)을 통하여 제작(製作)된 화력발전용(火力發電用) 대형(大型)밸브에 발생한 결함(缺陷)의 원인(原因)을 규명하기 위하여 사용(使用)된 몇가지 기술에 대하여 언급하였다. 초음파(超音波) 비파괴검사(非破壞檢査)로써 내재(內在)되어 있는 균열의 위치(位置) 및 길이를 예측(豫測)하였다. 균열의 근원(根源)을 다치지 않게 보존(保存)한 채로 시험편(試驗片)에 대(對)하여 기계적 절단(絶斷)을 행하였다. 또한 균열의 원인(原因)을 직접 관찰할 수 있도록 액체 질소에서 낙추법(落錘法)을 써서 파단(破斷)시켰다. 동시(同時)에 균열의 측면양상(側面樣相)과 미세조직(微細組織)과의 관계를 금속현미경 및 전자현미경(SEM)을 통하여서 관찰하였다. 결론적으로 제조시(製造時) 도입(導入)된 결함(缺陷)은 용접과정중에 만들어진 것이 아니라 주조작업중(鑄造作業中)에 용융금속(溶融金屬)의 공급부족(共給不足)으로 인(因)하여 생긴 기공(氣孔)에서 출발(出發)하였음을 알 수 있었다.