Characterization of bond line discontinuities in a high-Mn TWIP steel pipe welded by HF-ERW

Park, Gitae,Kim, Bongyoon,Kang, Yongjoon,Kang, Heewoong,Lee, Changhee Elsevier 2016 Materials characterization Vol.118 No.-

<P><B>Abstract</B></P> <P>In this work, the microstructure and defects in a high-frequency electrical resistance welded (HF-ERW) pipe of high-Mn twinning-induced plasticity (TWIP) steel were characterized. The microstructure of the base metal and the bond line were examined using both optical microscopy and scanning electron microscopy. The features of the bond line were similar to those of conventional steel. Simultaneously, the circumferential ductility was evaluated via a flaring test. It was concluded that the deterioration of the circumferential ductility in a high-Mn TWIP steel pipe was caused by irregular shaped oxide defects and a penetrator that had been formed during welding. Specifically, the penetrator, which is composed of MnO and Mn<SUB>2</SUB>SiO<SUB>4</SUB>, was found to be the most influential on the circumferential ductility of the welded pipe. The penetrator was analyzed using both an electron probe micro analyzer and transmission electron microscopy, and the formation sequence of the penetrator was evaluated.</P> <P><B>Highlights</B></P> <P> <UL> <LI> This study focused on applying the HF-ERW process to the seam welding of expandable pipe using TWIP steels. </LI> <LI> For improvement of the circumferential ductility, deterioration factors were characterized. </LI> <LI> Penetrator which would mainly deteriorate the circumferential ductility consisted of round MnO and Mn<SUB>2</SUB>SiO<SUB>4</SUB>. </LI> <LI> Metallurgical evidence of existing theory regarding the mechanism of defect formation during the HF-ERW was characterized. </LI> </UL> </P>

Fusion Weldabilities of Advanced High Manganese Steels: A Review

Gitae Park,Seonghoon Jeong,Changhee Lee 대한금속·재료학회 2021 METALS AND MATERIALS International Vol.27 No.7

A large amount of manganese has been added to next-generation advanced high strength steels for automotive applications. The increased manganese content changes the microstructural and mechanical characteristics by varying both the stackingfault energy and the austenite stability, but it is known to deteriorate weldability. The current review provides a general strategyto address several problems in fusion welds of advanced high manganese steels from the viewpoint of microstructuraltransitions, plasticity mechanism, and mechanical properties based on the welding metallurgy. The importance of researchon the weldabilities of advanced Mn steels was highlighted. Among the representative types of advanced manganese steels,this review focuses on high Mn twinning-induced plasticity steel, high Mn austenitic lightweight steel, and medium Mntransformation-induced plasticity steel. Specifically, this review suggests fundamental concepts for designing each steelwith improved weld characteristics by presenting many research results on the correlations between the microstructure andmechanical properties of welds and heat-affected zones for these three different steel groups.

Opportunistic behaviors of credit rating agencies and bond issuers

Park, Gitae,Lee, Ho-Young Elsevier 2018 Pacific-Basin finance journal Vol.47 No.-

<P><B>Abstract</B></P> <P>Using credit rating data from the three credit rating agencies (CRAs) in Korea, we examine whether bond issuers and CRAs engage in rating shopping and catering. First, we find that Korean bond issuers, who are required by law to receive two or more ratings, tend to fire or switch from CRAs that assign lower ratings than other CRAs. Second, when a bond issuer hires an additional CRA, the new CRA assigns a higher rating than incumbent CRAs. Lastly, we see that increased competition, which is measured by the number of CRAs hired by a given bond issuer, affects the likelihood of an upgrade occurring. Although CRAs often upgrade ratings when their rivals assign higher ratings, our findings show that higher competition further increases the likelihood that CRAs will upgrade ratings when there are rating disagreements. These results imply that bond issuers and CRAs engage in opportunistic behaviors that undermine the quality of credit ratings.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Korean bond issuers tend to fire or switch CRAs that assign lower ratings. </LI> <LI> With an additional CRA, the new CRA assigns a higher rating than incumbent CRAs. </LI> <LI> Increased competition affects the likelihood of an upgrade occurring. </LI> </UL> </P>

Kinetics Study on Low-Temperature Tempering of Martensitic Phase in Medium Mn Steel Weldment During Paint-Baking Heat Treatment

Gitae Park,Kwangjoong Kim,Youngchai Lee,Sangho Uhm,Changhee Lee 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.5

The kinetics of low-temperature tempering in a dissimilar resistance spot weldment between medium Mn transformationinducedplasticity (MT) steel and dual-phase (DP) steel were investigated to interpret the change of failure mode after apaint-baking process. After the isothermal heat treatments with various soaking conditions, the Johnson–Mehl–Avrami (JMA)model describing nucleation and growth kinetics was adopted. The results indicated that the tempering kinetics parameterswere different for each area of the welds. Especially in the coarse-grained heat-affected zone (CGHAZ) in the MT steel,carbon diffusion from martensite to austenite acts as a tempering mechanism at an earlier stage of tempering. Thereafter,the tempering mechanism changes to epsilon carbide precipitation and coarsening. In comparison, the change was relativelyslow or absent in the DP steel CGHAZ and fusion zone because of its lower kinetics parameters. Based on the results ofthis study, ways to improve the cross-tension strength of entire welds in automotive applications of MT steel are discussed.

Formula Car의 Numerical Design과 MBD를 통한 차량 설계

박기태(Gitae Park),이진우(Jinwoo Lee),박혜운(Hyewoon Park) 한국자동차공학회 2019 한국자동차공학회 학술대회 및 전시회 Vol.2019 No.11

The vehicle was designed and manufactured to compete in the 600cc Formula category of the KASE 2019. After modeling numerically assuming the static and dynamic states of the vehicle, the vehicle was kinematically designed using SOLIDWORKS 3D CAD. The model was analyzed using the CAE analysis program, MOTIONVIEW, to analyze the multibody dynamics (MBD) and to set the target dynamic performance. In particular, the 2019 BCP vehicle aimed to present the condition of the vehicle through functions and graphs using the MBD analysis program and to present the criteria for data performance. As a result, the feedback process of engineering performance prediction and analysis using the CAE program saved time and money.

LDA를 활용한 친환경 물류 핵심 토픽 분석

박기태(Gitae Park),임세혁(Sehyeuk Im),김민석(Minseok Kim),최동현(Donghyun Choi),송보미(Bomi Song) 한국생산관리학회 2022 한국생산관리학회지 Vol.33 No.3

코로나19 이후 물류 산업은 친환경 기조가 가속화되어가고 있다. 본 연구는 언론기사들에서 추출한 키워드를 기반으로 코로나19 팬데믹 전후의 친환경 물류에 관한 사회적 관심의 변화를 분석하는 것을 목적으로 한다. 이를 위해 팬데믹 이전(2018~2019년)과 팬데믹 이후(2020~2021년)로 기간을 구분하고, 각 기간의 친환경 물류 관련 언론기사들을 대상으로 텍스트 분석 및 토픽 모델링을 시행하였다. 언론기사들을 수집하기 위해서 빅카인즈 데이터베이스를 활용하였으며, 토픽모델링 방법론인 LDA(Latent Dirichlet Allocation)기법을 적용하여 각 기간별 핵심 토픽을 도출하였다. 도출된 핵심토픽을 기능별 물류와 연관지어 보관, 하역, 포장, 운송/배송, 기업전략/정책 등의 부주제로 새롭게 분류하였다. 연구 결과, 팬데믹 이후 친환경 물류와 관련하여 언론에서 관심도가 높아졌으며, 구체화된 키워드와 토픽의 경향성에서 변화가 있었다는 점을 확인하였다. 또한 친환경 보관과 관련하여 새로운 토픽들이 등장함에 따라 친환경 보관 분야가 친환경 물류의새로운 비즈니스 모델이 될 수 있음을 시사하였다. Since the COVID-19 pandemic, the trend of green logistics has been accelerating in the logistics industry. The purpose of this study is to analyze the changes in social interest in green logistics before and after the COVID-19 pandemic, based on keywords extracted from news articles. To this end, two periods were selected, pre-pandemic (2018-2019) and postpandemic (2020-2021). Text analysis and topic modeling were conducted for the news articles related to green logistics in each period. In order to collect the news articles, the Big Kinds database was used, and key topics for each period were derived by applying the Latent Dirichlet Allocation (LDA) technique, a topic modeling methodology. The derived key topics were further classified into subtopics such as storage, unloading, packaging, transportation/distribution, and corporate strategy/policy in relation to logistics by functions. As a result of the study, it was confirmed that in the post-pandemic period, interest in green logistics increased in the media, and there was a change in the tendency of use of specific keywords and topics. In addition, with the emergence of new topics related to green storage, it was suggested that the green storage field could become a new business model for green logistics.

중망간강 용접 열영향부 조직과 기계적 특성 상관관계 해석

박기태(Gitae Park),엄상호(Sangho Uhm),정홍철(Hongcheol Jeong),이창희(Changhee Lee) 대한용접·접합학회 2020 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2020 No.11

STS316L의 필렛 레이저 빔 용접에 대한 실험적 연구

박기태(Gitae Park),홍승갑(Seunggab Hong),정보영(Boyoung Jeong) 대한용접·접합학회 2022 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2022 No.5

고망간 TWIP강 HF-ERW 용접부 미세조직 및 기계적특성 평가

박기태(Gitae Park),강희웅(Heewong Kang),이창희(Changhee Lee) 대한용접·접합학회 2016 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2016 No.4

중망간강 용접부 미세조직 해석과 상변태 강화를 통한 인성 향상

박기태(Gitae Park),엄상호(Sangho Uhm),정보영(Boyoung Jeong) 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.11

전세계적 환경규제 강화, 자동차 연비 향상 요구로 인해 자동차의 주요 소재인 철강 소재, 특히 차체 및 외장 판넬에 주로 사용되는 차량용 강판에 새로운 변화가 요구되고 있다. 세계 주요 철강업체들은 차체의 고강도화 및 차량경량화를 위해, 변태유기소성 현상을 이용하는 다양한 강종을 개발하여 차체에 적용해왔다. 변태유기소성 현상은 잔류오스테나이트 기지상이 소성가공에 의해 마르텐사이트로 상변태되면서 강재의 연신율을 크게 향상시키는 현상이며, 이와 더불어 가공경화에 의한 강도 향상을 동시에 유도할 수 있기 때문에, 강종의 변태조직 제어는 차세대 자동차강판 연구의 핵심이라 할 수 있다. 중망간강은 다량의 망간을 활용하여 잔류오스테나이트의 안정성과 함량을 증가시켜 변태유기소성 현상을 이용하여 연성을 극대화시킨 강종이다. 강재 내에 상온에서 안정적인 페라이트 및 오스테나이트의 2개의 상을 얻기 위해서 생산공정 중 열처리를 통한 페라이트-오스테나이트간 합금원소 재분배가 필수적이나, 용접 후 용융부 및 열영향부에서 나타나는 인성 저하 문제는 강재의 활용에 있어서 제약이 되고 있다. 본 연구에서는, 최근 이러한 문제점을 해결하기 위한 여러 합금설계 및 후속 공정 변수 제어 방안들에 대한 연구를 소개하고, 중망간강의 미세조직 변화와 인성 향상의 상관관계에 대해 고찰하고 논의하고자 한다.