X70 grade large diameter UOE linepipe steel for sour service has been manufactured stably by optimizing the continuous casting process, controlling the shape of inclusions and decreasing coarse precipitates. It is confirmed that the HIC and SSC resistance are good enough for severe sour conditions to apply. These higher strength linepipe steels for sour service are useful for the offshore and deep-sea pipelines. Additional improvements of HIC and SSC resistances are needed for manufacturing thicker and higher strength UOE linepipe steels for sour service. Optimizing alloying elements and ACC process are very important for the additional improvements of HIC and SSC resistance. In addition, the HIC evaluation method should be revised in order to match applied field conditions.

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