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The API X70 steel is a high-strength low alloy steel which is used in construction of high-pressure long-distance gas and oil transportation pipelines with large-diameter. The pipe used in this study has 1422mm outside diameter and 19.8mm wall thickness, formed by spiral welding. As this steel is totally imported from abroad, the study of continues cooling transformation behavior and the optimum design of thermo-mechanical control processes are important for its domestic production. In this study, dilatometry examination was conducted on API X70 steel in a wide range of cooling rates from 0.5 to 40 °C/s. The optical microscopy observation and microhardness measurement were used to verify the observed microstructures. From the experimental results, the continuous cooling transformation curves (CCT) were constructed for API X70 pipeline steel. Different microstructures including granular bainite, pearlite, acicular ferrite and bainitic ferrite were observed depending on the cooling rate of tested samples. The observed dominant microstructure in 5 and 7.5 °C/s cooling rate was acicular ferrite which is the desired microstructure in energy transportation pipeline steels. These results can be used to design the optimum thermo-mechanical control process (through the selection of proper cooling rate) in domestic manufacturing of the API X70 steel.

Keywords: API X70, Dilatometry test, CCT diagrams, Acicular Ferrite, Energy transportation pipeline steel, API X70

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