Effects of Start and Finish Cooling Temperatures on the Yield Strength and Uniform Elongation of Strain-Based API X100 P
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RODUCTION
RECENTLY, steel plates used for building, bridges, ships, and offshore and pipeline structures have been increasingly required to have higher than standard strengths and toughness, as well as good deformability.[1–16] In particular, high deformability, regarded as high uniform elongation and a low yield ratio, is of vital importance in pipeline structures subjected to progressive and abrupt displacements resulting from ground movements, structural load, and earthquakes.[2–5] The required mechanical properties of American Petroleum
DONG HO LEE and CHANG SUN LEE are with the Technical Research Laboratories, POSCO, Gwangyang 57807, Korea. SEOK SU SOHN is with the Department of Microstructure Physics and Alloy Design, Max Planck Institut fu¨r Eisenforschung (MPIE), MaxPlanck-Straße 1, 40237 Du¨sseldorf, Germany. HYEJIN SONG and SUNGHAK LEE are with the Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 37673, Korea. YUNJO RO is with the SK Innovation, Daejeon 34124, Korea. BYOUNGCHUL HWANG is with the Department of Materials Science and Engineering, Seoul National University of Science and Technology, Seoul 01811, Korea. Contact e-mail: [email protected] Manuscript submitted August 1, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Institute (API) pipeline steels can be achieved by optimizing the design of the microstructure by the precise control of the chemical composition and manufacturing process. Thus, extensive research to develop high-strength strain-based API pipeline steels with excellent deformability has been carried out. However, the uniform elongation of pipeline steels deteriorates during the coating process, which is necessary to prevent corrosion after pipe formation, because the coating temperature range for anticorrosion of pipeline steels is 200 °C to 300 °C, which results in the formation of a Cottrell atmosphere.[6–10] Therefore, the decrease in the uniform elongation is mostly associated with interactions between the carbon atoms in ferrite and dislocations under the Cottrell atmosphere, which affect the yield point phenomena during tensile tests.[4–10] Many studies have been conducted to reduce the amount of carbon in ferrite by promoting the formation of cementite, and carbon is one of the main factors responsible for strain-aging phenomena.[4–16] According to Shigesato’s investigation[4] of the effect of the finish cooling temperature (FCT) on the microstructure and carbide formation mechanism of strain-based pipeline steels, ferrite is formed at temperatures between the austenite-to-ferrite (Ar3) temperature and bainite start temperature where carbon atoms in ferrite move to
austenite and form titanium and niobium carbides because of the lower solubility of carbon in ferrite compared to that in austenite. During the formation of bainite below the bainite start temperature, carbon atoms form cementites instead of titanium and niobium carbides because of the lower carbon mobility induced by shear transformation behavior rather tha
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