The impact properties of two austenitic Fe-23Mn-0.4C steels with different Al contents for cryogenic applications are investigated in this study. The 4Al steel consists mostly of austenite single-phase microstructure, while the 5Al steel exhibits a two-phase microstructure of austenite and delta-ferrite with coarse and elongated grains. Charpy impact test results reveal that the 5Al steel with duplex phases of austenite and delta-ferrite exhibits a ductile-to-brittle transition behavior, while the 4Al steel with only single-phase austenite has higher absorbed energy over 100 J at -196 oC. The SEM fractographs of Charpy impact specimens show that the 4Al steel has a ductile dimple fracture regardless of test temperature, whereas the 5Al steel fractured at -100 oC and -196 oC exhibits a mixed fracture mode of both ductile and brittle fractures. Additionally, quasi-cleavage fracture caused by crack propagation of delta-ferrite phase is found in some regions of the brittle fracture surface of the 5Al steel. Based on these results, the delta-ferrite phase hardly has a significant effect on absorbed energy at room-temperature, but it significantly deteriorates low-temperature toughness by acting as the main site of the propagation of brittle cracks at cryogenic-temperatures.

Abstract
1.서 론
2. 실험방법
3. 결과 및 고찰
    3.1. 미세조직 분석 및 인장 특성
    3.2. 극저온 충격 특성
4.결 론
References

• 김상규(서울과학기술대학교 신소재공학과) | Sang-Gyu Kim (Department of Materials Science and Engineering, Seoul National University of Science and Technology)
• 김재윤(서울과학기술대학교 신소재공학과) | Jae-Yoon Kim (Department of Materials Science and Engineering, Seoul National University of Science and Technology)
• 윤태희(서울과학기술대학교 신소재공학과) | Tae-Hee Yun (Department of Materials Science and Engineering, Seoul National University of Science and Technology)
• 황병철(서울과학기술대학교 신소재공학과) | Byoungchul Hwang (Department of Materials Science and Engineering, Seoul National University of Science and Technology) Corresponding author