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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.
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)
초고강도 소재가 적용된 U 형 채널 제품의 다단공정변수에 따른 스프링백 영향
서창희(Chang Hee Suh),정윤철(Yun-Chul Jung),임용희(Yong Hee Lim),윤효윤(Hyo Yun Yun),이경훈(Kyunghoon Lee),김영석(Young Suk Kim) 대한기계학회 2012 大韓機械學會論文集A Vol.36 No.3
자동차 부품의 경량화를 위해서 초고강도강의 사용이 확대됨에 따라 판재성형 공정에서 초고강도강판의 스프링백 제어의 중요성이 점차로 확대되고 있다. 본 연구에서는 초고강도 소재를 사용한 U 형 채널 부품의 성형을 위해서 사용되는 다단공정에서 공정 변수들이 스프링백에 미치는 영향에 대하여 검토하였다. 해석을 위해서는 유한요소법이 사용되었으며, 주요 공정 변수는 공정수, 금형 각도, 금형 반경, 펀치성형방향이다. 단공정에 비해서 다단공정이 스프링백을 아주 효과적으로 제어할 수 있었으며, 금형반경과 펀치성형방향의 각도가 작을수록 스프링백 제어에 효과적이었다. 그러나 금형 각도의 영향은 크지 않았다. Ultra-high-strength steels (UHSSs) are widely used for lightweight automobile parts, and the control of springback is very important in sheet-metal forming. The object of this study is to verify the effects of multi-stage forming process parameters for U-channel-type automobile parts made of UHSS. Finite element analysis is carried out to predict the formability and springback. The main parameters considered for the multi-stage forming process are the die angle, die radius, and punch-forming direction. It is shown that multi-stage forming is very effective for reducing springback, and that a small punch-forming angle and die radius reduce springback, whereas the die angle does not have a large effect.