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980 MPa급 이상조직강의 신장 플랜지성에 미치는 템퍼링의 영향

Effect of Tempering on Stretch-Flangeability of 980 MPa Grade Dual-Phase Steel

  • 이건희 (울산대학교 첨단소재공학부) ;
  • 백종희 (울산대학교 첨단소재공학부) ;
  • 송은지 (울산과학기술원 신소재공학부) ;
  • 나선형 (현대제철 R&D Center) ;
  • 박봉준 (현대제철 R&D Center) ;
  • 김주영 (울산과학기술원 신소재공학부) ;
  • 권용재 (울산대학교 첨단소재공학부) ;
  • 신상용 (울산대학교 첨단소재공학부) ;
  • 이정구 (울산대학교 첨단소재공학부)
  • Lee, Gun-Hee (School of Materials Science and Engineering, University of Ulsan) ;
  • Baek, Jong-Hee (School of Materials Science and Engineering, University of Ulsan) ;
  • Song, Eunji (School of Materials Science and Engineering, UNIST (Ulsan National Institute of Science and Technology)) ;
  • Na, Seon-Hyeong (Technical Research Center, Hyundai Steel Company) ;
  • Park, Bongjune (Technical Research Center, Hyundai Steel Company) ;
  • Kim, Ju-Young (School of Materials Science and Engineering, UNIST (Ulsan National Institute of Science and Technology)) ;
  • Kwon, Yongjai (School of Materials Science and Engineering, University of Ulsan) ;
  • Shin, Sang Yong (School of Materials Science and Engineering, University of Ulsan) ;
  • Lee, Jung Gu (School of Materials Science and Engineering, University of Ulsan)
  • 투고 : 2020.03.15
  • 심사 : 2020.05.13
  • 발행 : 2020.06.27

초록

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.

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참고문헌

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