Application of B-Added Low Carbon Bainite Steels to X80 UOE Line Pipe for Ultra Low Temperature Usage

Demand for high strength line pipes is increasing because of the reduction in natural gas transportation costs of pipelines. Low temperature toughness is required for high strength line pipes. Reduction in manufacturing cost of high strength linepipes is also required in an environment where alloying cost is increasing. To meet these requirements, boron (B) addition is extremely useful because the addition of very small amounts of B remarkably improves the strength and low temperature toughness. B-added low carbon bainite (LCB) line pipes with American Petroleum Institute (API) grade X60 to X80 have been developed for several decades [1–2]. B-added LCB steels have excellent low temperature toughness, however, it is challenging to achieve excellent crack initiation resistance and crack arrestability for ultra low temperatures such as −60°C. In particular, it is very difficult to achieve both excellent Drop Weight Tear Test (DWTT) properties of base metal, and excellent Charpy V-Notched (CVN) properties of seam welds in heavier wall thickness of X80 UOE linepipe. Metallurgical concepts such as the optimum chemical compositions, Thermo Mechanical Control Process (TMCP) conditions and seam weld conditions of B-added LCB steels with API grade X80 for ultra low temperature have been proposed in order to achieve the excellent mechanical properties even in a low manufacturing cost. Based on this concept, excellent DWTT properties of base metal and CVN properties of the seam welds of API grade X80 line pipe with 25mm thickness down to –60°C were obtained.