Correlation Between Microstructures and Tensile Properties of Strain-Based API X60 Pipeline Steels
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INTRODUCTION
TO transport crude oil and natural gas more efficiently from the areas of production to areas of consumption, many studies have examined ways developing high strength and high toughness pipeline steels with excellent fracture resistance properties against sudden deformation and fracture by heavy storms, earthquakes, and seismic sea waves.[1–3] Pipeline steels are used after the piping and corrosion-resistant coating processes, in which uniform elongation and the deformability decrease because of the strain aging HYO KYUNG SUNG, Assistant Professor, is with the Department of Materials Science and Engineering, ReCAPT Gyeongsang National University, Jinju 52828, Korea. DONG HO LEE, Research Assistant, and SUNGHAK LEE and HYOUNG SEOP KIM, Professors, are with the Center for Advanced Aerospace Materials, Pohang University of Science and Technology, Pohang 37673, Korea. YUNJO RO, Researcher, and CHANG SUN LEE, Senior Vice President, are with the POSCOA Research Group Team, Technical Research Laboratories, POSCO, Gwangyang, 57807, Korea. BYOUNGCHUL HWANG, Assistant Professor, is with the Department of Materials Science and Engineering, Seoul National University of Science and Technology, Seoul, 01811, Korea. SANG YONG SHIN, Assistant Professor, is with the School of Materials Science and Engineering, University of Ulsan, Ulsan, 44610, Korea. Contact e-mail: sshin@ ulsan.ac.kr Manuscript submitted July 16, 2015. Article published online March 22, 2016 2726—VOLUME 47A, JUNE 2016
phenomenon.[4–6] To restrain the strain aging phenomenon during the piping and corrosion-resistant coating processes, many studies have been conducted to reduce the number of carbon atoms in ferrite and to form more cementite.[7–9] Shigesato et al.[7] reported that cementites were distributed uniformly when Cr was added in American Petroleum Institute (API) X80 pipeline steels. When Mo was added to the API X80 pipeline steels, however, the cementites were not distributed uniformly but were almost formed in the narrow space of the bainite lath; hence, the uniform elongation and deformability of the API X80 pipeline steels decreased.[7] Hara et al.[8] reported that continuous yielding behaviors occurred without a rapid decrease in uniform elongation when Cr, a strong carbide-forming element, was added to the API X80 pipeline steels. Shinohara et al.[9] reported that the uniform elongation of the bainitic API X80 and X100 pipeline steels increased with increasing the volume fraction of polygonal ferrite (PF) formed by the mild accelerated cooling processes. The recently developed strain-based pipeline steels exhibited discontinuous yielding behavior because the main microstructure was polygonal ferrite and the volume fraction of hard secondary phases was low.[10,11] The tensile properties of the API pipeline steels are affected by piping and corrosion-resistant coating processes. Therefore, it
METALLURGICAL AND MATERIALS TRANSACTIONS A
is important to examine the effects of the microstructures on the tensile properties, such as the strengt
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