This study aims to examine the correlation between microstructures and the mechanical properties of two highstrength API X70 linepipe steels with different specimen directions and Moaddition. The microstructure of the Mo-added steel has an irregularly shaped AF, GB matrix with pearlite because of the relatively large deformation in the non-recrystallization temperature region, while that of the Mo-free steel shows a PF matrix with bainitic microstructure. In the Mo-added steel, the M/A (martensite-austenite) in granular bainite (GB) and pearlite act as crack initiation sites with decreased upper shelf energy and an increased ductile to brittle transition temperature (DBTT). Regardless of Mo addition, all of the steels demonstrate higher strength and lower elongation in the T direction than in the L direction because of the short dislocation glide path and ease of pile-up at grain boundaries. In addition, the impact test specimens with T-L direction had a lower impact absorbed energy and higher DBTT than those with the L-T direction because the former exhibit shorter unit crack path compared to the latter.

1. 서 론
2. 실험 방법
3. 실험 결과 및 고찰
    3.1. 미세조직
    3.2. 기계적 성질
4. 결 론
Acknowledgement

• 오동규(서울과학기술대학교 신소재공학과 학생) | Dong-Kyu Oh (Department of Materials Science and Engineering, Seoul National University and Technology (SEOULTECH), Seoul 01811, Republic of Korea)
• 신승혁(서울과학기술대학교 신소재공학과) | Seung-Hyeok Shin (Department of Materials Science and Engineering, Seoul National University and Technology (SEOULTECH), Seoul 01811, Republic of Korea)
• 황병철(서울과학기술대학교 신소재공학과 교수) | Byoungchul Hwang (Department of Materials Science and Engineering, Seoul National University and Technology (SEOULTECH), Seoul 01811, Republic of Korea) Corresponding author