Journal of the Korean Society for Heat Treatment (열처리공학회지)

The Korean Society for Heat Treatment (한국열처리공학회)
권호 표지
DOI QR코드

DOI QR Code

Impact Toughness and Fracture Behavior in Non-Heat Treating Steels Containing Bainite

베이나이트 함유 비조질강의 충격인성 및 파괴거동

  • Cho, Ki-Sub (Center for Advanced Materials Technology, kookmin University) ;
  • Kwon, Hoon (Center for Advanced Materials Technology, kookmin University)
  • 조기섭 (국민대학교 신소재공학부) ;
  • 권훈 (국민대학교 신소재공학부)
  • Received : 2019.07.05
  • Accepted : 2019.07.24
  • Published : 2019.07.30
Download PDF
⟨ Previous Next ⟩

Abstract

Impact toughness and fracture behavior were studied in five kinds of non-heat treating steels containing bainite; standard(0.25C-1.5Mn-0.5Cr-0.2Mo-0.15V), high V(0.3V), Ni(0.5Mn-2Ni), W(0.4W instead of Mo), and high C-Ni(0.35C-0.5Mn-2Ni) steels. The good hardness and impact toughness balance was exhibited in the $1100^{\circ}C$-rolled condition, while the impact toughness was deteriorated due to coarse grained microstructure in the $1200^{\circ}C$-rolled condition. The impact toughness decreased with increasing the hardness in all steels studied. The fracture behavior was also basically identical, that is, the fracture area was divided into 3 zones; shear and fibrous zone, fracture transition zone with ductile dimples and cleavage cracks, where the cracks initiate and grow to critical size, unstable cleavage fracture propagation zone. The energy absorbed for the critical crack formation through the plastic deformation inside the plastic zone in front of the notch root contributed to a mostly significant portion of the total impact energy.

Keywords

OCRHB6_2019_v32n4_161_f0001.png 이미지

Fig. 1. Comparison of hardness with rolling condition.

OCRHB6_2019_v32n4_161_f0002.png 이미지

Fig. 2. Typical optical micrograph consisting of ferrite and bainite in standard steel [9].

OCRHB6_2019_v32n4_161_f0003.png 이미지

Fig. 3. TEM micrographs of the standard steel. (a) VC in ferrite matrix; (b) DF image of VC; (c) cementite in bainite region.

OCRHB6_2019_v32n4_161_f0004.png 이미지

Fig. 4. Schematic CCT diagram of bainitic steels in this study.

OCRHB6_2019_v32n4_161_f0005.png 이미지

Fig. 5. Effect of hardness on impact toughness in the hot-rolling condition: Aichi steel was hot-forged at 1150oC and air-cooled [6, 7].

OCRHB6_2019_v32n4_161_f0006.png 이미지

Fig. 6. Fracture modes in three zones and cleavage fracture paths in the impact specimen.

OCRHB6_2019_v32n4_161_f0007.png 이미지

Fig. 7. Effect of hot-rolling condition on Etotal, EI, and EG in Ni-mod steel.

OCRHB6_2019_v32n4_161_f0008.png 이미지

Fig. 8. Effect of grain size on yield stress (σy) in uniaxial tensile test, effective YS (σeff) in impact test, and fracture stress (σf): Coarse-grained structure definitely shows a sharp drop in residual portion(σf- σeff) related to strain energy, as fine-grained one.

Table 1. Chemical composition of experimental steels (Wt Pct)

OCRHB6_2019_v32n4_161_t0001.png 이미지

References

  1. Microalloyed Vanadium Steels, Proc. Inter. Syp. in Cracow, ed. by M. Korchynsky et. al, April 24-26 (1990).
  2. K. Hase, K. Hoshino, and K. Amano : Kawasaki Steel Technology Report, No. 47, Dec. (2002).
  3. S. H. Park, S. G. Hong, Y. S. Chunc, Y. H. Lee and C. S. Lee : Mater. Sci. Eng. A, 550, (2012) 118. https://doi.org/10.1016/j.msea.2012.04.043
  4. A. N. Isfahany, H. Saghafian and G. Borhani : Journal of Alloys and Compounds, 509, (2011) 3931. https://doi.org/10.1016/j.jallcom.2010.12.174
  5. N. M. Ismail, N. A. A. Khatif, M. A. K. A. Kecik and M. A. H. Shaharudin : IOP Conf. Series: Materials Science and Engineering 114 (2016) 012108. https://doi.org/10.1088/1757-899X/114/1/012108
  6. U.S. Patent 5,362,338
  7. 일본특허 : 공고; 特公平6-21319 공개; 特開平4-141546-141551, 4-176842
  8. H. K. D. H. Bhdeshia : Bainite in Steels, Inst. of Mater. (1992).
  9. 조기섭, 권훈 : 한국열처리공학회지, 30 (2017) 67.
  10. R. O. Ritchie, B. Francis and W. L. Server : Metall. Trans. A. 7A (1976) 831.
  11. R. O. Ritchie and R. M. Horn : Metall. Trans. A. 9A (1978) 331.
  12. K. S. Cho, S. S. Park, Y. B. Kim, H. K. Moon, and H. Kwon : Metall. Mater. Trans. A. 9A (2013) 4440.