This study examines the effect of delayed quenching (DQ) temperature on the microstructure and mechanical properties of API X70 linepipe steels. Three types of steels were fabricated by varying the DQ conditions: Base (without DQ), LDQ (low-temperature delayed quenching at 700 °C), and HDQ (high-temperature delayed quenching at 740 °C). The microstructures were characterized using optical microscopy, scanning electron microscope (SEM), and electron back-scattered diffraction (EBSD), and their mechanical properties were evaluated through tensile and Charpy impact tests. The Base specimen exhibited the finest effective grain size and the highest bainite fraction, resulting in superior yield strength and impact toughness. In contrast, the LDQ specimen showed increased pearlite content and coarser grains, leading to the highest tensile strength due to work hardening, but reduced impact properties due to crack initiation at the pearlite regions. The HDQ specimen, with the highest ferrite fraction, showed the best ductility and acceptable strength, as well as improved lowtemperature toughness owing to increased resistance to cleavage propagation. EBSD analysis confirmed that finer grains and higher fractions of high-angle grain boundaries play a crucial role in enhancing impact energy and lowering the ductile-to-brittle transition temperature (DBTT). These findings highlight the importance of optimizing DQ parameters to achieve a balanced combination of strength–toughness in high-strength linepipe steels.

Abstract
1. 서 론
2. 실험 방법본 연구에서 사용된
3. 실험 결과 및 고찰
    3.1. 미세조직
    3.2. 상온 인장 특성
    3.3. 시험 온도별 충격 특성
4. 결 론
Acknowledgement
References
<저자소개>

• 서명규(현대제철 연구개발본부) | Myeong-Gyu Seo (Hyundai Steel Company, Dangjin 31719, Republic of Korea)
• 김규태(현대제철 연구개발본부) | Kyutae Kim (Hyundai Steel Company, Dangjin 31719, Republic of Korea)
• 정민섭(서울과학기술대학교 신소재공학과) | Min-Seop Jeong (Department of Materials Science and Engineering, Seoul National University of Science and Technology, Seoul 01811, Republic of Korea)
• 오동규(서울과학기술대학교 신소재공학과) | Dong-Kyu Oh (Department of Materials Science and Engineering, Seoul National University of Science and Technology, Seoul 01811, Republic of Korea)
• 신승혁(서울과학기술대학교 신소재공학과) | Seung-Hyeok Shin (Department of Materials Science and Engineering, Seoul National University of Science and Technology, Seoul 01811, Republic of Korea)
• 황병철(서울과학기술대학교 신소재공학과) | Byoungchul Hwang (Department of Materials Science and Engineering, Seoul National University of Science and Technology, Seoul 01811, Republic of Korea) Corresponding author