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고성능 냉간금형강의 미세조직과 기계적 특성: 레데부라이트(ledeburitic) 및 매트릭스(matrix)형 강종의 비교
강전연 ( Jun Yun Kang ),김호영 ( Ho Young Kim ),손동민 ( Dong Min Son ),이재진 ( Jae Jin Lee ),윤효윤 ( Hyo Yun Yun ),이태호 ( Tae Ho Lee ),박순근 ( Soon Keun Park ) 한국열처리공학회 2015 熱處理工學會誌 Vol.28 No.4
Two types of advanced cold-work tool steels were characterized and compared. A higher-alloyed ledeburitic steel with primary carbides (denoted as 9Cr) and a lower-alloyed steel without primary carbides (5Cr) were fabricated by vacuum induction melting and subsequent hot forging. They were spheroidizing-annealed at 870℃, quenched at 1030℃ and tempered at 180 or 520℃. Their machinability after annealing and hardness, impact toughness, wear resistance after tempering were compared and interpreted in association with their characteristic microstructures. After annealing, 5Cr showed higher resistance to machining due to higher ductility and toughness in spite of lower strength and smaller carbide volume. Owing to smaller carbide volume fraction and the absence of coarse primary carbides, 5Cr showed even better impact toughness although the hardness was lower. The improved toughness of 5Cr resulted in excellent wear resistance, while smaller volume fraction of retained austenite also contributed to it.
강전연,박준영,김호영,김병환,Kang, Jun-Yun,Park, Jun-Young,Kim, Hoyoung,Kim, Byunghwan 한국열처리공학회 2017 熱處理工學會誌 Vol.30 No.5
The microstructure of a high-carbon and high-chromium cast steel (HK700) for cold-work die inserts was analyzed by advanced scanning electron microscopy. A continuous network of primary $M_7C_3$ carbide was developed among austenitic matrix after casting. A small amount of $M_2C$ was added to the carbide network owing to the enrichment of Mo and W during the solidification. After quenching in which the austenitization was performed at $1030^{\circ}C$ and double tempering at $520^{\circ}C$, the network structure of $M_7C_3$ was preserved while most of the matrix was transformed to martensite because of additional carbide precipitation. The $M_2C$ in the as-cast microstructure was also transformed to $M_6C$ due to its instability. The continuous network of coarse carbides owing to the absence of hot-working had little influence on the hardness after quenching and tempering, whereas it resulted in severe brittleness upon flexural loading.
매트릭스(matrix)형 냉간금형강의 기계적 특성에 미치는 NbC 탄화물 첨가의 영향
강전연 ( Jun Yun Kang ),김호영 ( Ho Young Kim ),손동민 ( Dong Min Son ),이재진 ( Jae Jin Lee ),윤효윤 ( Hyo Yun Yun ),이태호 ( Tae Ho Lee ),박성준 ( Seong Jun Park ),박순근 ( Soon Keun Park ) 한국열처리공학회 2015 熱處理工學會誌 Vol.28 No.5
Various amount of NbC carbide was intentionally formed in a matrix-type cold-work tool steel by controlled amount of Nb and C addition. And the effect of NbC addition on the mechanical properties was investigated. Four alloys with different Nb and C contents were cast by vacuum induction melting, then hot forging and spheroidizing annealing were conducted. The machinability of the annealed specimens was examined with 3 different cutting tools. And tensile tests at room temperature were conducted. After quenching and tempering, hardness and impact toughness were measured, while wear resistance was evaluated by disk-on-plate type wear test. The increasing amount of NbC addition resulted in degraded machinability with increased strength, whereas the absence of NbC also led to poor machinability due to high toughness. After quenching and tempering, the additional NbC improved wear resistance with increasing hardness, whereas it deteriorated impact toughness. Therefore, it could be found that a moderate addition of NbC was desirable for the balanced combination of mechanical properties. (Received July 23, 2015; Revised August 5, 2015; Accepted August 14, 2015)
강전연 ( Jun-yun Kang ),박성원 ( Seongwon Park ),박준영 ( Jun Young Park ),박성준 ( Seong-jun Park ),송이화 ( Yi-hwa Song ),박성택 ( Sung-taek Park ),김광련 ( Gwang-lyeon Kim ),오경원 ( Kyeong-won Oh ) 한국열처리공학회 2018 熱處理工學會誌 Vol.31 No.6
Two different modes of rolling were applied to control the texture development in tantalum sheet. In the conventional uni-directional rolling, the typical rolling textures of a body-centered cubic metal which was primarily composed of <110>//(rolling direction) was developed. In a cross rolling where the specimen was rotated by 90o between each pass, the rotated cube components, i.e. {100}<011> were greatly reinforced. The prediction of lattice rotation by the full-constraint Taylor model showed that the high stability and the symmetry of the rotated cube components caused their strengthening in cross-rolling. The two specimens were heated to 1,100℃ at 9℃/ min and held for 1 hour for annealing, then cooled to room temperature in atmosphere. In spite of the significant difference in the deformation textures, the annealing textures were very similar. They developed strong <111>// (plane normal) components with negligible intensity at the rotated cube components, which was attributed to the negligible capability of the latter components to provide effective recrystallized grains. (Received September 10, 2018; Revised October 11, 2018; Accepted October 17, 2018)
강전연 ( Jun-yun Kang ),김병환 ( Byunghwan Kim ),김호영 ( Hoyoung Kim ) 한국열처리공학회 2018 熱處理工學會誌 Vol.31 No.6
The microstructure of a newly developed alloy tool steel (KV1) for casting cold-work press dies was analyzed using advanced scanning electron microscopy. Its mechanical properties and durability in use as piercing inserts were tested. It contained a continuous network structure which originated from the micro-segregation during solidification and was composed of retained austenite and primary carbides. However, after quenching and tempering, its continuity was destroyed due to the decomposition of austenite. The discontinuous spatial distribution and the smaller amount of primary carbide in the network presented KV1 enhanced ductility compared to the common alloy (HK700). The reduced C and Cr, the addition of V resulted in a small amount of primary carbides which primarily consisted of MC, as well as fine and uniform dispersion of precipitates. Owing to these features, KV1 exhibited delayed initiation of chipping when used for piercing inserts. (Received September 28, 2018; Revised October 10, 2018; Accepted October 15, 2018)
강전연,윤재철,김호영,김병환,최중호,양상선,유지훈,김용진,Kang, Jun-Yun,Yun, Jaecheol,Kim, Hoyoung,Kim, Byunghwan,Choe, Jungho,Yang, Sangsun,Yu, Ji-Hun,Kim, Yong-Jin 한국분말야금학회 2017 한국분말재료학회지 (KPMI) Vol.24 No.3
A cold-work tool steel powder is used to fabricate 3-dimensional objects by selective laser melting using a high-pressure gas atomization process. The spherical powder particles form continuous carbide networks among the austenite matrix and its decomposition products. The carbides comprise Nb-rich MC and Mo-rich $M_2C$. In the SLM process, the process parameters such as the laser power (90 W), layer thickness ($25{\mu}m$), and hatch spacing ($80{\mu}m$) are kept fixed, while the scan speed is changed from 50 mm/s to 4000 mm/s. At a low scan speed of 50 mm/s, spherical cavities develop due to over melting, while they are substantially reduced on increasing the speed to 2000 mm/s. The carbide network spacing decreases with increasing speed. At an excessively high speed of 4000 mm/s, long and irregularly shaped cavities are developed due to incomplete melting. The influence of the scan pattern is examined, for which $1{\times}1 mm^2$ blocks constituting a processing layer are irradiated in a random sequence. This island-type pattern exhibits the same effect as that of a low scan speed. Post processing of an object using hot isostatic pressing leads to a great reduction in the porosity but causes coarsening of the microstructure.
박성준(Seong-Jun Park),강전연(Jun-Yun Kang),황병철(Byoungchul Hwang),김성준(Sung-Joon Kim),곽재현(Jai-Hyun Kwak) 한국자동차공학회 2011 한국자동차공학회 부문종합 학술대회 Vol.2011 No.5
Mechanical properties and microstructure of light-weight steels containing 5wt.% aluminum were investigated. Theses steels were manufactured under continuous annealing conditions. They showed two-phase microstructure composed of ferrite and austenite. Density of the investigated steels were in range of 7.2~7.3/㎤, which was equivalent to 7~8% reduction in density compared with iron. Mechanical properties such as ultimate tensile strength and total elongation showed increase proportional to austenite fraction. Strain-induced martensitic transformation was observed in a composition, which resulted in considerable enhancement of strength and elongation. Mechanical stability of austenite could be controlled by annealing conditions.
냉간금형강의 미세조직과 기계적 특성: STD11과 8%Cr 강의 비교
김호영 ( Ho Young Kim ),강전연 ( Jun Yun Kang ),손동민 ( Dong Min Son ),이대수 ( Dae Soo Lee ),이태호 ( Tae Ho Lee ),정우창 ( Woo Chang Jeong ),조경목 ( Kyung Mox Cho ) 한국열처리공학회 2014 熱處理工學會誌 Vol.27 No.5
A comparative study was performed on the microstructures and the mechanical properties of STD11and 8Cr steel. The specimens were quenched from 1030℃ and tempered at 240℃ and 520℃. Vickers hardness, impact toughness and tensile tests were conducted at various tempering temperatures. Microstructural characterization to measure grain size, volume fraction of retained austenite and distribution of carbides was carried out by using SEM, EBSD, TEM and X-ray diffraction techniques. Due to finer M7C3 carbides dispersed, 8Crsteel showed larger impact toughness and plasticity than STD11 irrespective of the tempering temperature. While8Cr steel had lower hardness in as-quenched state and after tempering at 240℃ owing to smaller carbide content and more retained austenite, it was harder after tempering at 520℃ due to larger precipitation hardening from finer M23C6.
김성경 ( Seong Gyeong Kim ),강전연 ( Jun Yun Kang ),하헌영 ( Heon Young Ha ),박노진 ( Jin Park ),임창동 ( Chang Dong Yim ),강남현 ( Nam Hyun Kang ) 대한금속재료학회(구 대한금속학회) 2014 대한금속·재료학회지 Vol.52 No.6
The process of secondary recrystallization in the Mg-6Sn binary alloy that initially had bimodal grain size distribution was observed. The prevalence of the {0001}//sheet plane orientation in the initial texture was destroyed and the texture was transformed to a very strong {2□□0}-type during solution heat treatment at 480 ℃. The grain boundary character was analyzed using EBSD, in which no preference of the CSL boundary among the grain boundaries of {2□□0}- oriented grains was evident. It was observed that some fine grains were isolated within coarse {2□□0} oriented grains due to limited boundary migration and these boundaries belonged to CSL boundaries. These analyses confirmed that the secondary recrystallization was driven by the general high-angle grain boundaries surrounding {2□□0}-oriented grains. (Received July 31, 2013)