Application of Bainite and Ferrite-Martensite Structure for High-Strength Heavy-Gage Linepipe

It is essential for linepipe used in severe environments, such as cold regions and the deep sea, to have a good balance of strength, toughness, and weldability. To improve these properties conspicuously, ultralow carbon bainitic (ULCB) steel was developed. ULCB steel is well suited for Arctic linepipe up to American Petroleum Institute (API) Grade X80 because of its excellent girth weldability and high strength. The low-temperature toughness in the weld heat-affected zone (HAZ) is, however, not sufficient for use below −45 °C because of the addition of columbium and boron. Therefore, another kind of linepipe steel must be developed for very low temperature use, such as in heavy wall pump stationing pipe. It is extremely difficult, however, to attain high strength and high toughness by conventional controlled-rolling practice without the use of columbium and boron. To solve this difficult problem, a new chemical composition and controlled-rolling method have been developed. The typical chemical composition is 0.05C-1.6Mn-0.45Ni-0.25Mo-0.01Ti for API X65 grade steel, and the chief characteristic of controlled rolling is a very low slab reheating temperature (about 960 °C) and rolling in the austeniteferrite region. The microstructure of this steel comprises a mixture of fine elongated martensite and deformed and nondeformed ferrite, and shows the dual-phase structure. In this paper, the effects of microalloying elements and processing parameters on the mechanical properties of UCLB and ferrite-martensite dual-phase steels are discussed, and the results of actual production tests are also presented.