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해양플랜트용 500 MPa급 후판강의 모재 및 HAZ의 미세조직과 기계적 특성의 상관관계
박지원,조성규,조영욱,신건철,권용재,이정구,신상용,Park, Jiwon,Cho, Sung Kyu,Cho, Young Wook,Shin, Gunchul,Kwon, Yongjai,Lee, Jung Gu,Shin, Sang Yong 한국재료학회 2020 한국재료학회지 Vol.30 No.3
In this study, two types of thick steel plates are prepared by controlling carbon equivalent and nickel content, and their microstructures are analyzed. Tensile tests, Vickers hardness tests, and Charpy impact tests are conducted to investigate the correlation between microstructure and mechanical properties of the steels. The H steel, which has high carbon equivalent and nickel content, has lower volume fraction of granular bainite (GB) and smaller GB packet size than those of L steel, which has low carbon equivalent and nickel content. However, the volume fraction of secondary phases is higher in the H steel than in the L steel. As a result, the strength of the L steel is higher than that of the H steel, while the Charpy absorbed energy at -40 ℃ is higher than that of the L steel. The heat affected zone (HAZ) simulated H-H specimen has higher volume fraction of acicular ferrite (AF) and lower volume fraction of GB than the HAZ simulated L-H specimen. In addition, the grain size of AF and the packet sizes of GB and BF are smaller in the H-H specimen than in the L-H specimen. For this reason, the Charpy absorbed energy at -20 ℃ is higher for the H-H specimen than for the L-H specimen.
UHSS 하이드로포밍 개발을 위한 박육관의 롤 포밍 기술 연구
박성필(Sungpill Park),권용재(Yongjai Kwon) 한국자동차공학회 2015 한국 자동차공학회논문집 Vol.23 No.1
In the automotive industry, it is required to reduce weight of the car and improve fuel efficiency. Competitive pricing is also a very important issue. That’s why application of welded steel tube is increasing in order to produce a vehicle with a competitive price. Also, hydroforming technology is asking more and more for thinner tubing to realize to a lighter vehicle design. Steel tube is produced through a multi-stage process called roll forming. In that case, bucking and work hardening should be considered key forming technology is to prevent buckling and minimize work hardening during steel tubing for hydroforming To prevent buckling, it is required to optimize forming process in order to minimize stretching in edge sections and hold tightly cross-section during welding. And to minimize work hardening, it is needed to know the proper process to avoid reforming.
베이나이트강의 미세조직과 저온 충격 인성에 미치는 바나듐과 보론의 영향
황원구,이훈,조성규,서준석,권용재,이정구,신상용,Huang, Yuanjiu,Lee, Hun,Cho, Sung Kyu,Seo, Jun Seok,Kwon, Yongjai,Lee, Jung Gu,Shin, Sang Yong 한국재료학회 2021 한국재료학회지 Vol.31 No.3
In this study, three kinds of bainitic steels are fabricated by controlling the contents of vanadium and boron. High vanadium steel has a lot of carbides and nitrides, and so, during the cooling process, acicular ferrite is well formed. Carbides and nitrides develop fine grains by inhibiting grain growth. As a result, the low temperature Charpy absorbed energy of high vanadium steel is higher than that of low vanadium steel. In boron added steel, boron segregates at the prior austenite grain boundary, so that acicular ferrite formation occurs well during the cooling process. However, the granular bainite packet size of the boron added steel is larger than that of high vanadium steel because boron cannot effectively suppress grain growth. Therefore, the low temperature Charpy absorbed energy of the boron added steel is lower than that of the low vanadium steel. HAZ (heat affected zone) microstructure formation affects not only vanadium and boron but also the prior austenite grain size. In the HAZ specimen having large prior austenite grain size, acicular ferrite is formed inside the austenite, and granular bainite, bainitic ferrite, and martensite are also formed in a complex, resulting in a mixed acicular ferrite region with a high volume fraction. On the other hand, in the HAZ specimen having small prior austenite grain size, the volume fraction of the mixed acicular ferrite region is low because granular bainite and bainitic ferrite are coarse due to the large number of prior austenite grain boundaries.
HSLA 강의 미세조직과 저온 샤르피 충격 특성에 미치는 탄소와 니켈의 영향
엄해원,조성규,조영욱,신건철,권용재,이정구,신상용,Eom, Haewon,Cho, Sung Kyu,Cho, Young Wook,Shin, Gunchul,Kwon, Yongjai,Lee, Jung Gu,Shin, Sang Yong 한국재료학회 2020 한국재료학회지 Vol.30 No.4
In this study, effects of carbon and nickel on microstructure and low temperature Charpy impact properties of HSLA (high strength low alloy) steels are investigated. To understand the complex phase transformation behavior of HSLA steels with high strength and toughness before and after welding processes, three kinds of HSLA steels are fabricated by varying the carbon and nickel content. Microstructure analysis, low temperature Charpy impact test, and Vickers hardness test are performed for the base metals and CGHAZ (coarse-grain heat affected zone) specimens. The specimens with the lowest carbon and nickel content have the highest volume fraction of AF, the lowest volume fraction of GB, and the smallest GB packet size. So, the low temperature Charpy absorbed energy of the CGHAZ specimen is the highest. The specimens with increased carbon and nickel content have the lowest volume fraction of AF, the highest volume fraction of GB, and the largest GB packet size. So, the low temperature Charpy absorbed energy of the CGHAZ specimen is the lowest.
하이드로포밍용 DP780MPa급 강판의 전기저항용접 강관 개발에 관한 연구
박성필(Sungpill Park),권용재(Yongjai Kwon) 한국자동차공학회 2015 한국 자동차공학회논문집 Vol.23 No.3
To achieve lightweight design, research & development of various lightweighting technologies such as hydroforming are underway worldwide. In the case of hydroforming, application of ultra high strength steel is essential for weight reduction of the car. However, considering common high-strength carbon steel, it is not suitable to the actual hydroformed parts since the lack of formability. DP steel offers an outstanding combination of strength and formability as a result of their microstructure. DP steel has high strength and good formability but it’s difficult to secure stable quality of welding section because of softening of weld section and chemical composition. Therefore, most of companies use LASER welding when making high strength tube. Electric resistance welding is excellent production method for steel tube manufacturing considering the productivity. Optimum electric resistance welding technology is needed to be developed for application of high strength hydroformed parts using DP steel. This study is comprehensive research & development from electric resistance welding to actual formabililty evaluation.
980 MPa급 이상조직강의 신장 플랜지성에 미치는 템퍼링의 영향
이건희,백종희,송은지,나선형,박봉준,김주영,권용재,신상용,이정구,Lee, Gun-Hee,Baek, Jong-Hee,Song, Eunji,Na, Seon-Hyeong,Park, Bongjune,Kim, Ju-Young,Kwon, Yongjai,Shin, Sang Yong,Lee, Jung Gu 한국재료학회 2020 한국재료학회지 Vol.30 No.6
In this study, the effect of tempering on the stretch-flangeability is investigated in 980 MPa grade dual-phase steel consisting of ferrite and martensite phases. During tempering at 300 ℃, the strength of ferrite increases due to the pinning of dislocations by carbon atoms released from martensite, while martensite is softened as a consequence of a reduction in its carbon super-saturation. This strength variation results in a considerable increase in yield strength of the steel, without loss of tensile strength. The hole expansion test shows that steel tempered for 20 min (T20 steel) exhibits a higher hole expansion ratio than that of steel without tempering (T0 steel). In T0 steel, severe plastic localization in ferrite causes easy pore formation at the ferrite-martensite interface and subsequent brittle crack propagation through the highly deformed ferrite area during hole expansion testing; this propagation is mainly attributed to the large difference in hardness between ferrite and martensite. When the difference in hardness is not so large (T20 steel), on the other hand, tempered martensite can be considerably deformed together with ferrite, thereby delaying pore formation and hindering crack propagation by crack blunting. Eventually, these different deformation and fracture behaviors contribute to the superior stretch-flangeability of T20 steel.
해양플랜트용 후판강의 FCAW HAZ 미세조직과 샤르피 충격 특성의 상관관계
이훈,이현욱,조성규,최동기,김형찬,권용재,이정구,신상용,Lee, Hun,Lee, Hyunwook,Cho, Sung Kyu,Choi, Dongki,Kim, Hyoung Chan,Kwon, Yongjai,Lee, Jung Gu,Shin, Sang Yong 한국재료학회 2019 한국재료학회지 Vol.29 No.8
In this study, the correlation between microstructure and Charpy impact properties of FCAW(Flux cored arc welding) HAZ(Heat affected zone) of thick steel plates for offshore platforms was investigated. The 1/4 thickness(1/4t) location HAZ specimen had a higher volume fraction of bainite and finer grain size of acicular ferrite than those of the 1/2 thickness (1/2t) location HAZ specimen because of the post heat effect during the continuous FCAW process. The Charpy impact energy at $-20^{\circ}C$ of the 1/4t location HAZ specimen was lower than that of the 1/2t location HAZ specimen because of the high volume fraction of coarse bainite. The Charpy impact energy at -40 and $-60^{\circ}C$ of the 1/2t location HAZ specimen were higher than those of the 1/2t location HAZ specimen because the ductile fracture occurred in the fine acicular ferrite and martensite regions. In the ductile fracture mode, the deformed regions were observed in fine acicular ferrite and martensite regions. In the brittle fracture mode, long crack propagation path was observed in bainite regions.
금속고화체용 STS304-Zr 합금의 미세조직과 파괴거동에 미치는 열처리의 영향
김종우,장선아,한승엽,박환서,이정훈,이성학,권용재,이정구,신상용,Kim, Jongwoo,Jang, Seon Ah,Han, Seung Youb,Park, Hwan Seo,Lee, Junghoon,Lee, Sunghak,Kwon, Yongjai,Lee, Jung Gu,Shin, Sang Yong 한국재료학회 2018 한국재료학회지 Vol.28 No.3
Three kinds of STS304-Zr alloys were fabricated by varying the Zr content, and their microstructure and fracture properties were analyzed. Moreover, we performed heat treatment to improve their properties and studied their microstructure and fracture properties. The microstructure of the STS304-Zr alloys before and after the heat treatment process consisted of ${\alpha}-Fe$ and intermetallics: Zr(Cr, Ni, Fe)2 and Zr6Fe23. The volume fraction of the intermetallics increased with an increasing Zr content. The 11Zr specimen exhibited the lowest hardness and fine dimples and cleavage facets in a fractured surface. The 15Zr specimen had high hardness and fine cleavage facets. The 19Zr specimen had the highest hardness and large cleavage facets. After the heat treatment process, the intermetallics were spheroidized and their volume fraction increased. In addition, the specimens after the heat treatment process, the Laves phase (Zr(Cr, Ni, Fe) 2) decreased, the Zr6Fe23 phase increased and the Ni concentration in the intermetallics decreased. The hardness of all the specimens after the heat treatment process decreased because of the dislocations and residual stresses in ${\alpha}-Fe$, and the fine lamellar shaped eutectic microstructures changed into large ${\alpha}-Fe$ and spheroidized intermetallics. The cleavage facet size increased because of the decomposition of the fine lamellar-shaped eutectic microstructures and the increase in spheroidized intermetallics.