The Role of Niobium in High Strength Oil and Gas Transmission Linepipe Steels

Niobium’s role in the production of oil and gas transmission pipelines steels has gained significant importance in recent years. The economical movement of gas and oil to the marketplace from remote and rugged locations requires transmission pipelines to be designed to operate at higher pressures with improved toughness over a variety of temperature ranges. With the increased demand for energy resources continuing to grow, traditional plate mills, hot strip mills along with Steckel mills around the world are processing skelp for API pipe. The capabilities of these mills can be quite varied. Consequently, a variety of operational considerations and practices have put additional focus on Nb for its ability to retard recrystallization at elevated temperatures. This ability has added a new form of processing skelp for API pipe called High Temperature Processing or HTP. This new use of Nb in higher strength API oil and gas transmission pipeline steels allows a producer to create a ferrite/acicular ferrite microstructure without the traditional molybdenum alloy based design. The HTP Nb microalloy approach has benefits including reduced operating cost per ton, ease of rolling and welding, excellent low temperature toughness properties and high strength. This processing technique for API X70 and X80 is gaining acceptance as major pipeline projects are now applying this technology. In addition, X100 properties have been achieved with a combination of the traditional X80 alloy design and the newer employed HTP alloy design. This paper will discuss Nb’s role in meeting the increased strength requirements related to operating at higher pressures, improved low temperature toughness (TCVN > 200 J@−40 °C), microstructural demands and processing capability improvements for traditional plate, strip, and Steckel mill technology. The use of the new HTP concept in high strength API production will also be introduced.