Effect of Aging on Low-temperature Tensile Properties of Ultra-low Carbon Steel

• Published in: ISIJ International Vol. 61; no. 8; pp. 2308 - 2316
• Main Authors: Koga, Norimitsu, Kanehira, Yuki, Huyen, Pham Thi Thanh, Hori, Kazuya, Umezawa, Osamu
• Format: Journal Article
• Language: English
• Published: The Iron and Steel Institute of Japan 15-08-2021
• Subjects: aging; low-temperature tensile property; precipitate; twin; ultra-low carbon steel
• ISSN: 0915-1559; 1347-5460
• DOI: 10.2355/isijinternational.ISIJINT-2021-143

The microstructural changes and low-temperature tensile properties of ultra-low carbon steel aged at 443 K were examined, and the relationship between the low-temperature tensile properties and ultra-low carbon state was discussed. Fine cementites of approximately 60 nm were observed at 0.6 ks and coarsened to approximately 800 nm at 600 ks. The yield and tensile stresses at 77 K increased until 6 ks and then decreased. The nominal stress-strain curves of all the specimens at 77 K exhibited low elastic limits, the nominal stress plateaued from an approximate nominal strain of 0.002 and, subsequently, work hardening occurred. In the unaged and 6 ks aged specimens, several twins were generated after the elastic limit, and which increased dramatically in the nominal stress plateau regime, and corresponded to macroscopic yielding. In contrast, the number of twins in the 600 ks aged specimen negligibly increased during macroscopic yielding. Macroscopic yielding occurred in the unaged and 6 ks aged specimens by deformation twinning, while in the 600 ks aged specimen it occurred by slip deformation. In the 6 ks aged specimen, the fine cementites and/or decrease in solid solute carbon enhanced the critical resolved shear stresses of deformation twinning, resulting in the highest strength. In the 600 ks aged specimen, the coarse cementites negligibly enhanced the critical resolved shear stress for slip deformation. Hence, the strength of the specimen aged for 600 ks decreased as compared to the specimen aged for 6 ks, and slip deformation occurred.