• Journal of the Korean institute of surface engineering
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• v.46 no.1
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• pp.48-53
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• 2013

In the present work, it was investigated a behavior of hydrogen embrittlement at the subsurface zones of 590 DP steels by using the micro-Vickers hardness test. The micro-Vickers hardnessess of DP steels were measured to evaluate the degree of embrittlement as the effective hardening depths of subsurface zones with hydrogen charging conditions. The results showed that the distributions of micro-Vickers hardness in width varied from maximum hardness 239.5 Hv to minimum hardness 174 Hv, while the depth of effective hardening layer at the subsurface zones of DP steels was from $320{\mu}m$ to $460{\mu}m$ with hydrogen charging conditions, respectively. It was proposed that the distribution of microhardness be used as the evaluation index of the degree of embrittlement. But the variations of martensite volume fractions were not affected along depth of hardening at the same changing time, hydrogen charging times were appeared as an effective factor of the degree of embrittlement. Therefore, the micro-Vickers hardness test is an attractive tool for evaluation of hydrogen embrittlement at the subsurface zones of these DP steels.

• Korean Journal of Metals and Materials
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• v.49 no.12
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• pp.967-976
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• 2011

Conventional fracture tests of resistance spot welds have been performed without consideration of the paint baking process in the automobile manufacturing line. The aim of this paper is to investigate the effect of the paint baking process on load carrying capacity and fracture mode for resistance spot welded 590 dual phase (DP), 780DP, 980DP, 590 transformation in duced plasticity (TRIP), 780TRIP and 1180 complex phase (CP) steels. With paint baking after resistance spot welding, the l-shape tensile test (LTT) and nano-indentation test were conducted on the as-welded and paint baked samples. Paint baking increased the load-carrying capacity of the resistance spot welded samples and improved the fracture appearance from partial interfacial fracture (PIF) to button fracture (BF). Improvement in fracture appearance after LTT is observed on weldments of 780 MPa grade TRIP steels, especially in the low welding current range with paint baking conditions. The higher carbon contents (or carbon equivalent) are attributed to the low weldability of the resistance spot welding of high strength steels. Improvement of the fracture mode and load carrying ability has been achieved with ferrite hardening and carbide formation during the paint baking process. The average nano-indentation hardness profile for each weld zone shows hardening of the base metal and softening of the heat affected zone (HAZ) and the weld metal, which proves that microstructural changes occur during low temperature heat treatment.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2004.05a
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• pp.349-356
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• 2004

자동차의 외관품질과 관련된 외판재용 고강도강으로는 내 dent성을 향상을 위해서 IF-HSS강과 BH강의 사용량이 크게 증가하였으며, 최근에는 490MPa급 DP강이 적용되기 시작하고 있다. 그리고 내판 판넬의 경우에는 고가공성 을 갖는 고강도강의 개발으로 고강도강의 사용량이 늘어나고 있다. 내판재 중 승객의 안전과 관련된 멤버, 필라와 같은 구조부재는 중간 정도의 강도를 갖는 고강도강을 주로 적용되어 왔으나, 최근 차체경량화 요구의 증가로 590MPa급 이상 고강도강이 적용되기 시작하였으며, 특히 고속변형에서 에너지 흡수능이 우수한 TRIP (Trans-formation Induced Plasticity)강 및 DP(Dual Phase)강에 대한 관심이 크게 증대되고 있다. 저속충돌에서 차체를 보호하는 범퍼보강재는 고강도화가 빠르게 진행되어, 현재는 석출경화강에 변태 조직 강화를 더한 780MPa급 이상의 초고강도강을 주로 사용하고 있으며, 1370MPa급 까지 적용하고 있다.

• Transactions of Materials Processing
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• v.31 no.1
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• pp.23-28
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• 2022

Although the hole expansion test is currently the most commonly used method to evaluate the stretch flangeability of HSS, it has been criticized due to its poor repeatability and reproducibility for test results. This paper focuses on the development of a new measurement method to investigate the stretch flangeability of HSS. Two materials (DP590, DP980) were investigated with a hole expansion test and a developed test method. Test results showed that the developed test method could be used as one stretch flangeability test to help identify relevant parameters of the shearing process to avoid edge cracking.

• 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.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.11 no.9
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• pp.3155-3160
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• 2010

Nowadays, most car manufactures have tried to improve fuel efficiency and corrosion resistance of car body. Therefore, use of high strength steels and coated steel becomes more and more increased. In this study, spot weld characteristics according to lap sequence of sheets were analyzed using simulation method for three different steel sheet of car body which were EDDQ class coated steel with 0.7t, high strength steel 440R with 1.2t and advanced high strength steel DP 590 with 1.0t. Using simulation, weldability was evaluated by nugget size of welded zone according to nugget growth curve and welding current with respect to lap sequence of sheets. Contact resistance of each sheets contact point was used to analyze formation of nugget and optimal lap sequence was suggested.

• Journal of the Korean institute of surface engineering
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• v.45 no.5
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• pp.212-217
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• 2012

High strength steel sheets used for automobile outer-panels have been intensively studied for developing a lightweight automobile under a strong pressure of the requirements for enhancing the mileage and energy saving in production of automobile parts. It is known that high strength steels are susceptible to hydrogen embrittlement, The susceptibility to hydrogen embrittlement increases with increasing its strength. However, the effect of hydrogen on the fracture behavior of high strength steels, though investigated extensively, has not been fully understood. In this paper, hydrogen was charged with 590DP steels by electrochemical method and its content was measured by hydrogen determinator with the different charging conditions. It was shown that the SP energy and maximum load decreased with increasing charging time. The results of SEM-fractography investigation for the hydrogen contained samples showed that a small portion of dimples on cleavage-fractured surface were observed and the size of the dimples were decreased with increasing hydrogen charging time.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2010.04a
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• pp.628-631
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• 2010

환경문제에 대한 관심으로 자동차에 대한 경량화가 요구되는 동시에 안전규제가 강화 되고 있어, 높은 인장강도를 가지는 고강도 강의 차체 적용 비율이 점차 증가하고 있다. 또한, 자동차 1대를 조립하기 위한 저항 점용접 횟수를 줄이고, 용접부에 충격안정성을 확보하기 위한 관심이 고조되고 있다. 따라서, 국내 자동차 산업에서 용접부의 신뢰성을 보장하기 다양한 비파괴 검사를 적용하고 있으며, 생산 공정에 적용하고 있다. 그중에서 용접 전극 사이에서 동저항(Dynamic resistance, 용접 공정중모재의 저항값의 변화)을 계측하여 용접성을 평가하는 방법이 제시되고, 차체 조립공정 중에 적용하려는 시도가 이루어지고 있다. 본 연구에서는 자동차 차체용 냉간 압연강판(590MPa dual-phase steel)을 인버터 DC 저항 점 용접하여, 용접전극 사이에서 동저항을 측정 하였다. 용접성은 인장전단 강도로 평가하였고, 용접 공정 변수는 용접 전류, 용접 시간, 가압력을 선정하였다. 동저항 그래프의 ${\alpha}$-peak와 ${\beta}$-peak값을 인장전단 강도에 따라 회귀 분석하여, 동저항에 따른 인장전단 강도를 예측하였다. 추가적으로, 용접부의 외관 형상 중에 압흔 깊이와 압흔자국 지름에 대한 회귀분석을 실시하였으며, 용접부 형상에 대한 신뢰성을 부여하였다.

• Journal of Animal Science and Technology
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• v.52 no.2
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• pp.157-164
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• 2010

We investigated the influence of carcass traits on carcass price for Hanwoo cow using multiple regression and path analyses. Data for carcass traits and price were collected on mature Hanwoo cow raised in private farms in Kangwon province that were slaughtered in 2008. A total of 96 animals with the average slaughter age of 51 months were used in the current study. Of the carcass traits studied, marbling score (MAR), dressing percentage (DP), and eye-muscle area (EMA) showed moderate and positive correlations with auction price (AP), while correlation coefficients of meat yield index (MINDEX) with AP were low or even negligible. In multiple regression analyses of AP and carcass price (CP) on cold carcass weight (CWT), back fat thickness (BFT), EMA, MAR, and DP, estimates of $R^2$ were 69.86 and 85.43%, respectively. Partial regression coefficients for CWT, BFT, EMA, MAR and DP were -0.028, -0.106, 0.107, 0.814, and 0.075 in the equation of AP and 0.561, -0.060, 0.083, 0.590, and 0.051 in the equation of CP. In path analyses, MAR's total contribution on the variation of AP was largest (0.667), and the total contributions of MAR and CWT on the variation of CP were 0.403 and 0.397. The current study suggested that marbling was the most influential trait on the variation of auction price, and marbling and cold carcass weight were critical traits on the variation of carcass price.

• Proceedings of the Materials Research Society of Korea Conference
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• 2010.05a
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• pp.40.1-40.1
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• 2010

Recently high strength steel sheets with high formability for automotive parts have been being developed to meet the demands for passenger safety and weight reduction of car body. Among these high strength steels, dual-phase steels are regarded as one of the attractive steels due to their excellent mechanical properties including high strength and ductility. However, to be successfully applied to automotive parts they should be corrosion resistant enough to satisfy the required quality of car maker. This also requires their feasibility for galvannealed production including hot dip galvanizability. In this study has been placed on understanding the effects of heat-treatment(austenizing and isothermal treatment) on the microstructures and mechanical properties of a 0.06C-0.03Si-2.0Mn high strength steel for cold forming. The microstructure and phase distribution were examined with eth aids of SEM, EBSD, TEM etc.. Through the study the production of 590MPa grade DP GA steels with good formability and galvaniability were shown to be possible.