Increasing world demand for energy has resulted in plans to expand the oil and gas transmission pipeline infrastructure in many countries utilizing higher strength steels of API grade X70 and X80. Traditional transmission pipeline steels, up to grade X70, relied on a ferrite/pearlite microstructural design generated through traditional TMCP rolling of a niobium microalloyed C-Mn steel design. Increasing strengths up to X70 and X80 for transmission pipelines has resulted in a shift toward a ferrite/acicular ferrite microstructure designs. Traditionally, to generate the ferrite/acicular ferrite microstructure design for X70 or X80, TMCP rolling is applied to a C-Mn-Si-Mo-Nb alloy system. The Nb content is typically less than 0.070% in this alloy system. With the rising cost of alloys over the past three years, steel and pipe producers have been working with different alloy designs to reduce total costs to produce the ferrite/acicular ferrite microstructure. In recent developments it has been determined that an optimized low-C-Mn-Si-Cr-Nb alloy design (usually referred as NbCr steel), utilizing an Nb content between 0.080 – 0.11% can produce the same ferrite/acicular ferrite microstructure with either no, or minimal, use of molybdenum. This approach has been successfully used in several transmission pipeline projects such as the Cantarell, Cheyenne Plains and Rockies Express. Recognizing the success of previous projects around the world, the large ∼ 4500 Km 2nd West-East Pipeline Project specification in China has been modified to allow for the use of this NbCr design for both plate and coil for conversion to long seam or spiral pipe. The NbCr design allows the steel producer to utilize niobium’s unique ability to retard recrystallization at higher than normal TMCP rolling temperatures, hence the term for the alloy design High Temperature Processing (HTP), producing the desired ferrite/acicular ferrite microstructure with excellent strength, toughness and weldability. This paper will discuss the technical background, rolling strategy, mechanical properties, welding, specific projects, and specification modifications with practical examples.

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