Improved API X80 Heavy-Gauge Large-Diameter Pipe Mechanical Property Stability Through Cost-Effective Optimized Niobium Metallurgy Performance

Today’s market for API pipeline steels requires higher strength and fracture toughness performance in heavier gauges and larger pipe diameters. Meeting basic average mechanical properties of strength, TCVN and DWTT performance has been achieved by many with newer powerful plate mills and cooling systems. However, stability/standard deviations of plate/pipe strength and toughness and in particular the pipe seam/girth weld toughness continues to be challenging. Stability of the pipe seam/girth weld toughness is in the forefront of the pipe end user’s requests for improvement. For improvement and stability of the seam/girth weld toughness issues, controlling the austenite grain coarsening during welding is a critical path. Titanium precipitates due to their high solubility temperature is one method employed in the alloy design that can contribute to austenite grain size control during welding. It has also been shown that during re-austenization of the as-rolled through thickness austenite/ferrite grain size in the production of QT steels, the as rolled through thickness austenite/ferrite grain size along with the presence of a volume fraction of Nb precipitates in the 10-70 nm diameter range can influence the final austenite/ grain size and corresponding final transformed ferrite/bainite/martensite grain/packet size. [1,2,3] Taking this concept from QT steels metallurgy and applying it in the production of API pipeline steels for improvement and stability of final plate/pipe mechanical properties, trials were designed to address two key points that would emulate the QT steel results: