Korean Journal of Metals and Materials (대한금속재료학회지)

The Korean Institute of Metals and Materials (대한금속재료학회)
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Effects of the Strain Induced Martensite Transformation on the Delayed Fracture for Al-added TWIP Steel

Al 첨가 TWIP강에서의 지연파괴에 대한 변형유기 마르텐사이트 변태의 영향

  • Kim, Youngwoo (Depart. of Mater. Sci. & Eng., Pusan National University) ;
  • Kang, Namhyun (Depart. of Mater. Sci. & Eng., Pusan National University) ;
  • Park, Youngdo (Depart. of Adv. Mater. Eng., Dongeui University) ;
  • Choi, Ildong (Div. of Mech. & Mater. Eng., Korea Maritime University) ;
  • Kim, Gyosung (POSCO Technical Research Laboratory) ;
  • Kim, Sungkyu (POSCO Technical Research Laboratory) ;
  • Cho, Kyungmox (Depart. of Mater. Sci. & Eng., Pusan National University)
  • Received : 2008.09.13
  • Published : 2008.12.25
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Abstract

For the advanced high strength steels (AHSS), high-manganese TWIP (twinning induced plasticity) steels exhibit high tensile strength (800-1000 MPa) and high elongation (50-60%). However, the TWIP steels need to be understood of delayed fracture following the cup drawing test. Among the factors to cause delayed fracture, i.e, martensite transformation, hydrogen embrittlement and residual stress, the effects of martensite transformation (${\gamma}{\rightarrow}{\varepsilon}$ or ${\gamma}{\rightarrow}{\alpha}^{\prime}$) were investigated on the delayed fracture phenomenon. Microstructural phase analysis was conducted for cold rolled (20, 60, 80% reduction ratio) steels and tensile deformed (20, 40, 60% strain) steels. For the Al-added TWIP steels, no martensite phase was found in the cold rolled and tensile deformed specimen. But, the TWIP steels with no Al addition indicated the martensite transformation. The cup drawing specimens showed the martensite transformation irrespective of the Al-addition to the TWIP steel. However, the TWIP steel with no Al exhibited the larger amount of martensite than the case of the TWIP steel with Al addition. For the reason, it was possible to conclude that the Al addition suppressed the martensite transformation in TWIP steels, therefore preventing the delayed fracture effectively. However, it was interesting to note that the mechanism of delayed fracture should be incorporated with hydrogen embrittlement and/or residual stress as well as the martensite transformation.

Keywords

Acknowledgement

Supported by : POSCO

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