• Transactions of Materials Processing
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• v.32 no.2
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• pp.61-66
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• 2023

The quality of the sheared surface affects the stretch flangeability of steel sheet. The quality of sheared surface is influenced by several process factors such as die clearance, shape of cutting edge, use of counter punch, and shear. In this paper, the influence of these shearing process factors on the stretch flangeability of the HSS (DP980) was analyzed through a shearing and a stretch flangeability test. When the die clearance was 10%, the effect of these shearing process factors on the stretch flangeability was the greatest, and the use of an acute angle blade was found to be more advantageous in the stretch flangeability than a right angle blade. It was found that the stretch flangeability was improved when active bending was applied during shearing.

• Proceedings of the Korean Geotechical Society Conference
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• 2010.09a
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• pp.349-356
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• 2010

In this study, a new design method of pile bent structure considering plastic hinge was proposed on the basis of the beam-column model. Based on the analysis results, it is found that the positioning of plastic hinge on the pile bent structure was influenced by nonlinear behavior of material and p-$\Delta$ effect. Moreover, concrete cracking began to occur at the joint section between the pile and column in case of pile bent structure with different cross-sections. The plastic hinge can be developed on the pile bent structure when large displacement was occurred, and pile bent structures can be maintained well only if it is developed on the column part. Therefore, in this study, the optimized cross-section ratio between column and pile was analyzed to induce the plastic hinge at the joint section between the pile and column. Based on this, the optimized diameter ratio of pile and column can be obtained below the inflection point of the bi-linear curve depending on the relations between column-pile diameter ratio($D_c/D_p$) and normalized lateral cracking load ratio($F/F_{Dc=Dp}$). And through this study, it is founded that in-depth limit($L_{As}$=0.4%) normalized by the pile length($L_P$) are proportionally decreased as the pile length($L_P/D_P$) increases up to $L_P/D_P$=17.5, and beyond that in-depth limit converges to a constant value. Finally, it is found that the proposed limit depth by taking into account the minimum concrete-steel ratio would be more economical design of the pile bent structure.

• Korean Journal of Metals and Materials
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• v.50 no.12
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• pp.873-882
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• 2012

Dual Phase (DP) steel which has a soft ferrite phase and a hard martensite phase reveals both high strength and high ductility and has received increased attention for use in automotive applications. To conduct structural analysis to verify vehicle safety, highly credible experimental results are required. In this study, tensile tests were performed in a strain rate range from $10^{-4}/s$ to 300/s for Sink Roll-Less (SRL) hot-dip metal coated sheets. Collision properties were estimated through simulation by LS-DYNA using the stress-strain curve obtained from the tensile test. The simulation results were compared with the actual crash test results to confirm the credibility of the simulation. In addition, a tensile test and a crash test with 2% prestrain and a baking (PB) specimen were evaluated identically because automotive steel is used after forming and painting. The mechanical behaviors were improved with an increasing strain rate regardless of the PB treatment. Thus, plastic deformation with an appropriate strain rate is expected to result in better formability and crash characteristics than plastic deformation with a static strain rate. The ultimate tensile strength (UTS) and absorbed energy up to 10% strain were improved even though the total elongation decreased after PB treatment, The results of the experimental crash test and computer simulation were slightly different but generally, a similar propensity was seen.

• 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 the Korean Institute of Gas
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• v.18 no.5
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• pp.40-46
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• 2014

The hydrogen embrittlement degree of 5 type high strength DP steel charged with hydrogen by electrochemical method was evaluated by small punch test(SP test). After SP test, SP absorbed energy was remarkably decreased from 363 kgf-mm to 209 kgf-mm with increasing hydrogen charging time from 5hr to 50hr at DP5 specimen under the $200mA/cm^2$ current density condition. It was investigated that the decrease of hydrogen charging amount and SP absorbed energy according to the increase of current density and hydrogen charging time had a linear relationship. And it also investigated that the change of bulb height appeared by the SP test was decreased from 1.79mm to 1.59mm with the hydrogen charging conditions. It was supposed that it could be used as indicator of the evaluation of hydrogen embrittlement because of the similar trend of the formal results of SP absorbed energy. From the SEM observation of fracture area by crack in bulb, the morphology of fracture surface according to increasement of the hydrogen charging amount was varied with the cleavage mode.

• Proceedings of the Korean Institute of Surface Engineering Conference
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• 2012.05a
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• pp.326-326
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• 2012

현재 자동차 강판 시장은 이산화탄소 저감과 승객의 안전확보를 위하여 고장력강을 요구하고 있는 추세이며, 강도와 성형성을 동시에 확보하기 위하여 DP, TRIP강과 같은 변태강화형 강판을 선호한다. 그러나 강판의 상분율을 제어하기 위해서는 Si, Mn등과 같은 합금원소를 필요로 하게 되며 강판의 생산공정 중 이들 합금원소는 강판의 표면에 산화물로 농화되어 아연도금 특성을 크게 저하시키는 결과를 초래한다. 따라서 본 연구에서는 냉연 아연도금강판의 생산 공정을 모사하고 이들의 산화물 구조를 분석하여 도금특성에 어떠한 영향을 미치는지 고찰하고 Ni-precoating 공정을 이용하여 강판의 도금성 개선의 가능성을 제시하였다.

• Journal of the Korean Geotechnical Society
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• v.27 no.2
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• pp.91-101
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• 2011

In this study, a new design method of Pile-Bent structure considering plastic hinge was proposed on the basis of the beam-column model. To obtain the detailed informations, the optimized cross-section ratio between column and pile was analyzed to induce the plastic hinge at the joint section between the pile and column. Base on this study, the optimized diameter ratio of pile and column can be obtained below the inflection point of the bi-linear curve depending on the relations between column-pile diameter ratio ($D_c/D_p$) and normalized lateral cracking load ratio ($F/F_{Dc=Dp}$). Moreover, through comparisons with field cases to find out in-depth limit in which minimum concrete-steel ratio could be applied, in-depth limits ($L_{As=0.4%}$) normalized by the pile length ($L_p$) proportionally decrease as the pile length ($L_p/D_p$)increases up to $L_p/D_p=17.5$, and beyond that in-depth limit converges to a constant value (${\simeq}0.3$).

• Proceedings of the KWS Conference
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• 2010.05a
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• pp.97-97
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• 2010

최근 자동차에서 경량화의 방안으로써 높은 강성을 요구하는 고장력강 사용이 증대 되고 있다. 그러나 고장력강은 저항 점용접 시 일반 강에 비해 높은 전류를 요구하며 계면파단 및 expulsion 발생이 용이하기 때문에 가용 전류 구간이 좁은 특성을 가진다. 많은 연구자들이 hold time, tempering 등의 process를 이용하여 고장력강의 저항 점용접성을 개선하고자 하였으나 생산 공정라인에 적용하기는 어려운 실정이다. 본 연구에서는 용접 공정 변수의 변화에 따른 용접성과 전극 형상 변화을 통한 고장력강 점 용접성 향상에 대한 연구를 실시 하였다. 고장력강의 점 용접성 비교하기 위해 표준 전극(S1)과 인위적으로 가공한 전극(M1)을 사용하였으며, 실험에 사용된 판재는 두께 1.4mm의 DP590이며, 그 결과 표준전극(S1) 보다 가공 전극(M1)의 가용 전류 구간이 0.5kA 정도 넓은 것으로 확인 되었다. 두 전극을 사용한 점용접 시험편들의 인장전단강도를 비교 해보면 표준전극(S1)을 적용한 점용접 시 인장전단강도는 KS B 0850 기준에 만족하나 계면 파단이 발생 하였다. 가공 전극(M1)을 적용한 점용접 시 인장전단강도는 규격 기준에 만족하나 버튼 파단이 발생 하였다. 두 전극을 적용한 점용접부 형상 및 용접부 온도 분포에대해 저항점용접 시뮬레이션 프로그램(SORPAS)을 이용하여 실험 결과 값과 비교 분석하였고 파단모드의 변화에 대한 원인 분석을 도출 하였다.

• Proceedings of the KWS Conference
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• 2009.11a
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• pp.68-68
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• 2009

Recently, resistance spot welding using Inverter DC is applied to improve the weldability of high strength steel for the auto-body fabrication. In this study, inverter DC spot welding machine, which is developed in the domestic, was evaluated weldability of the galvanized and cold rolled dual-phase steel(tensile strength : 590MPa). The welding conditions (welding time, current and force) were decided by tensile-shear test, and welding strength and nugget size were analyzed by statistical analysis methods which involved dynamic resistance and indentation. The results of the statistical analysis was utilized for real-time estimation of the invisible nugget size and tensile strength. Moreover, it can be achieved without the conventional destructive testing of welds.

• Transactions of the Korean Society of Automotive Engineers
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• v.17 no.2
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• pp.104-111
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• 2009

In this paper, optimization for frontal crashworthiness is carried out for the weight reduction design of an auto-body member with the advanced high strength steels(AHSS) such as 780TRIP and 780DP. The frontal crashworthiness is evaluated in order to optimize thicknesses for the front rail member of the ULSAB-AVC, Thicknesses of the front rail member with AHSS are optimized by comparison of crushing distance, absorbed energy and the deceleration for the auto-body with the response surface methodology. The results demonstrate that the crashworhiness of the front rail member with the optimum thicknesses of the AHSS is similar to analysis results obtained from the ULSAB-AVC project. The results also show that the weight reduction design is performed by substituting the AHSS for conventional structural steels such as 440E in the auto-body members.