As oil & gas transportation has changed to be long and massive, the requirements of the pipes seem to be trending toward high strength and large diameter (heavy thickness). POSCO has produced API-X70 grade steel strips since 1990, but only to moderate thickness. To cope with the increasing needs for large diameter pipes, the process parameters had to be optimized in advance for commercial production. The required mechanical properties of API-X70 steel with heay thickness are high strength, low temperature toughness and fine ferrite grain size. To obtain the best optimized process parameters, Pohang works established temperature controlling tests for the steel, based on Nb-Ti-V-Mo-Ni alloy design and adopting the moderate temperature reheating. By the above process, 15.6mm thick API-X70 steel strips were manufactured and formed to spiral pipes. With mechanical tests of strips and pipes, the relationships between the mechanical properties and the process conditions are explained. Consequently, the best condition for the steel are obtained and the best mechanical characteristics are as follows; YS value is 563MPa with no drop after piping, the microstructure is ferrite + acicular ferrite, Charpy and DWTT transition temperatures are -70E' and -30E respectively. Finally, manufacturing parameters for 15.6mmt API-X70 strip are standardized for commercial production.
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