Optimization of Through Thickness Microstructural Grain Size, Homogeneity and Niobium Metallurgy Performance for Cost Effective Improved API X80 Heavy Gauge Large Diameter Pipe Body and Weld Mechanical Property Stability

Abstract 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 along with pipe seam/girth weld toughness continues to be challenging for many. Current focus has been on pipe seam and girth weld HAZ toughness stability which has proven to be difficult to achieve. Optimization of the alloy and processing design are critical components of creating the desired final through thickness microstructure, grain size/homogeneity and a volume fraction of proper size Nb precipitates in the hot rolled condition and corresponding final base and weld metal ductility properties. Optimized processing of two Nb levels, 0.055% and 0.075%, Nanjing Iron and Steel Company successfully improved base metal plate strength and toughness stability that translated into improved pipe strength and toughness stability including pipe seam/girth welds HAZ toughness. The optimized 0.075% Nb girth weld HAZ fusion line average was 339 J @ −80 °C with a 9.3 J standard deviation while the optimized 0.055% Nb girth weld HAZ fusion line was 307 J @ −80 °C with a 55.2 J standard deviation.