Korean Journal of Metals and Materials (대한금속재료학회지)

The Korean Institute of Metals and Materials (대한금속재료학회)
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Effects of Tempering Condition on the Microstructure and Mechanical Properties of 30MnB5 Hot-Stamping steel

핫스탬핑용 30MnB5강의 템퍼링 조건에 따른 미세조직 및 기계적 물성 연구

  • Jeong, Junyeong (Research & Development Division, Hyundai Motor Company) ;
  • Park, Sang-Cheon (Research & Development Division, Hyundai Motor Company) ;
  • Shin, Ga-Young (Research & Development Division, Hyundai Motor Company) ;
  • Lee, Chang Wook (Research & Development Division, Hyundai Motor Company) ;
  • Kim, Tae-Jeong (Research & Development Division, Hyundai Motor Company) ;
  • Choi, Min-Su (Research & Development Division, Hyundai Motor Company)
  • 정준영 (현대자동차연구개발본부) ;
  • 박상천 (현대자동차연구개발본부) ;
  • 신가영 (현대자동차연구개발본부) ;
  • 이창욱 (현대자동차연구개발본부) ;
  • 김태정 (현대자동차연구개발본부) ;
  • 최민수 (현대자동차연구개발본부)
  • Received : 2018.08.21
  • Accepted : 2018.09.12
  • Published : 2018.11.05
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Abstract

The effects of tempering condition on the microstructure and mechanical properties of 30MnB5 hot stamping steel were investigated in this study. Before the tempering, hot-stamped 30MnB5 steel was composed of only ${\alpha}^{\prime}$-martensite microstructure without precipitates. After the tempering at $180^{\circ}C$ for 120 min, nano-sized ${\varepsilon}$-carbides were precipitated in the ${\alpha}^{\prime}$-martensite laths. After tempering at $250^{\circ}C$ for 60 min, cementite was precipitated along the ${\alpha}^{\prime}$-martensite lath boundaries. The cementite was also observed in the specimens tempered at $350^{\circ}C$ for 30 min and $450^{\circ}C$ for 6 min, respectively. The globular ${\alpha}$-ferrite appeared at $350^{\circ}C-30min$ tempering, and the volume fraction of ${\alpha}$-ferrite increased when the tempering temperature was increased. The yield strength increased after tempering, and it reached a peak with the tempering condition of $180^{\circ}C-120min$, due to the nano-sized precipitates in the ${\alpha}^{\prime}$-martensite lath. After the tempering, the steel's ultimate tensile strength (UTS) was decreased due to the reduction in dislocation density and C segregation to lath boundaries. The highest elongation was observed at the $180^{\circ}C-120min$ tempering condition, due to the reduction of residual stress, and the lack of precipitates along the lath boundaries. The $180^{\circ}C-120min$ tempering condition was considered to have outstanding crash performance, according to toughness and anti-intrusion calculation results. In drop tower crash tests, the 30MnB5 door impact beam tempered at $180^{\circ}C$ for 120 min showed better crash performance compared to a 22MnB5 door impact beam.

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