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천은준,이주승,도형협,김성주,박용호,강남현,Chun, Eun-Joon,Lee, Ju-Seung,Do, Hyeong-Hyeop,Kim, Seong-Ju,Park, Yong-Ho,Kang, Nam-Hyun 한국재료학회 2012 한국재료학회지 Vol.22 No.1
Research into the development of high strength (1 GPa) and superior formability, such as total elongation (10%), and stretch-flangeability (50%) in hot-rolled steel was conducted with a thermomechanically controlled hot-rolling process. To improve the overall mechanical properties simultaneously, low-carbon steel using precipitation hardening of Ti-Nb-V multimicroalloying elements was employed. And, ideal microstructural characteristics for the realization of balanced mechanical properties were determined using SEM, EBSD, and TEM analyses. The developed steel, 0.06C-2.0Mn-0.5Cr-0.2(Ti + Nb + V), consisted of ferrite as the matrix phase and second phase of granular bainite with fine carbides (20-50 nm) in both phases. The significant factor of the microstructural characteristics that affect stretch-flangeability was found to be the microstructural homogeneity. The microstructural homogeneity, manifest in such characteristics as low localization of plastic strain and internally stored energy, was identified by grain average misorientation method, analyzed by electron backscattered diffraction (EBSD) and hardness deviation between the phases. In summar, a hot-rolled steel having a composition 0.06C-2.0Mn-0.5Cr-0.2(Ti + Nb + V) demonstrated a tensile strength of 998 MPa, a total elongation of 19%, and a hole expansion ratio of 65%. The most important factors to satisfy the mechanical property were the presence of fine carbides and the microstructural homogeneity, which provided low hardness deviation between the phases.
Ti 첨가강의 Ti와 C 함량에 따른 초기 오스테나이트 입도 변화
김우진,강남현,도형협,김성주,남대근,조경목,Kim, Woo-Jin,Kang, Nam-Hyun,Kim, Sung-Ju,Do, Hyung-Hyup,Nam, Dae-Geun,Cho, Kyung-Mox 한국재료학회 2011 한국재료학회지 Vol.21 No.4
Prior austenite grain size plays an important role in the production of high strength hot-rolled steel. This study investigated the effect of Ti and C contents on the precipitates and prior austenite grain size. Steel with no Ti solutes had prior austenite grain size of about 620 ${\mu}m$. The addition of Ti ~ 0.03 wt.% and 0.11 wt.% reduced the prior austenite grain size to 180 ${\mu}m$ and 120 ${\mu}m$, respectively. The amount of Ti required to significantly decrease the prior austenite grain size was in the range of 0.03 wt.%. However, the amount of carbon required to significantly decrease the prior austenite grain size was not present from 0.04 wt.% to 0.12 wt.%. Oxides of Ti ($Ti_2O_3$) were observed as the Ti content increased to 0.03 wt.%. The specimen containing 0.11 wt.% of Ti exhibited the complex carbides of (Ti, Nb) C. The formation of Ti precipitates was critical to reduce the prior austenite grain size. Furthermore, the consistency of prior austenite grain size increased as the carbon and Ti contents increased. During the reheating process of hot-rolled steel, the most critical factor for controlling the prior austenite grain size seems to be the presence of Ti precipitates.
박상현,이창용,현주식,김태응,도형협 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.5
자동차용 열연 강판은 부품의 요구 특성에 따라 크게 두 종류로 나눌 수 있다. 일반적으로 승용차의 샤시 로어암, 트레일링 링크, 크로스 멤버류에는 2상 조직의 60~80K 열연재가 적용된다. 특히, 홀 확장성 확보를 위해서는 FB(Ferrite+Bainite) type이 적합하며, 드로잉성 확보에는 DP(Martensite+Ferrite) type이 적용되어 각 요구 특성을 만족시킨다. 그리고 상용, 특장차의 메인 프레임과 같이 한층 더 높은 강도가 요구될 경우, Ti, Nb, Mo, V등의 합금원소가 첨가된 80K~100K급 석출 경화형 열연재가 주로 적용되고 있다. 최근까지의 열연재 적용 부품개발의 방향은 단순한 기계적 특성 만족 수준을 넘어 고성능화 및 부품 경량화를 동시에 확보하는 것이며, 추가적으로 원가 경쟁력을 위한 효율적인 합금 설계도 필요한 상황이라고 할 수 있다. 이러한 상황에 맞춰 현재 당사에서는 고강도 열연재의 합금 성분에 관한 연구를 지속적으로 진행하고 있으며, 본 연구에서는 새롭게 개발된 80K급 열연재의 실 부품 적용을 위한 GMA용접성 평가를 실시하였다. 용접 입열 변화에 따른 기계적 특성 변화를 분석하였으며, 이를 통해 확보된 최적 용접조건을 제시하였다.
고강도 열연강판의 구멍확장 시험 시 균열의 전파거동에 미치는 미세조직의 영향
박세호(Seho Park),변정국(Jungkuk Byun),도형협(Hyeonghyeop Do),정인상(Insang Chung) 한국자동차공학회 2011 한국자동차공학회 부문종합 학술대회 Vol.2011 No.5
Recently, advanced vehicle concepts and light-weight design of automobiles require improved steel grades with increased strength and formability. Especially, high strength hot-rolled steel sheets applied to chassis such as wheel, lower arm require high hole expansion ratio because of their complicated shapes in cold press processing. Aim of this study is to analyze the effects of microstructures on the crack propagation behavior during hole expansion which is used to estimate stretch-flangeability for steel sheets. For high strength steel sheets which had various microstructures and different components, hole expansion test were carried out and than analyzed the differences of the crack propagation behavior. As the result, it was conclusioned that formation of the 2nd fracture surface, low difference in hardness between the matrix and 2nd phase, excellent uniformity of the microstructures and a small number of the micro-voids led to excellent hole expansion ratio (HER).
980 MPa급 열연강의 신장플랜지성에 미치는 템퍼드 마르텐사이트 기지조직과 입상 베이나이트 복합상의 영향
이주승 ( Ju Seung Lee ),이명진 ( Myung Jin Lee ),도형협 ( Hyeong Hyeop Do ),김성주 ( Seong Ju Kim ),강남현 ( Nam Hyun Kang ) 대한금속재료학회(구 대한금속학회) 2014 대한금속·재료학회지 Vol.52 No.2
The main goal of the study is to obtain high strength and formability for automotive sheet steels such as wheel and chassis. Ferrite-based steels developed previously exhibited superior stretch-flangeability to 780 MPa and even to 980 MPa grade steels. However, it was difficult to achieve 980 MPa tensile strength sufficiently. To achieve a superior stretch-flangeability and to acquire a sufficiently high strength over 980 MPa, a composition of Fe-Cr-Ti-B was developed for hot rolling. The manufactured steel was coiled for 1 hour at 430, 450, and 470 ℃. Specifically, 998 MPa ultimate tensile strength, 11% total elongation, and a 59% excellent hole expansion ratio were obtained by microstructural constituents of a tempered martensite matrix and granular bainite when the coiling was conducted at 470 ℃. As the coiling temperature varied from 470 to 430 ℃, the fraction of granular bainite decreased and that of tempered martensite increased. Therefore, the deviation of phase fractions between two phases decreased at 470 ℃ coiling. Furthermore, as the hole expansion ratio (HER) increased, the hardness deviation of the two phases decreased. As the HER value increased, the crack propagation path dispersed on the second fracture surface and fracture was significantly delayed. Therefore, hot rolled steels using a tempered martensite matrix and granular bainite had the same behavior of stretch-flangeability with respect to the hardness deviation and crack propagation path as compared with the steels using a ferrite matrix. For high strength steels composed of the tempered martensite matrix and granular bainite, however, the minimal deviation between the two constituent phases was acknowledged as an important factor to increase stretch-flangeability. †(Received May 8, 2013)