Toshiyuki Hoshino

High Strength Linepipe With Excellent HAZ Toughness

The API 5L-X65 steel plates for low temperature service were produced using the thermo-mechanical control process (TMCP) with the optimum micro-alloying addition. Featuring of the additions are as low amount of titanium, calcium, niobium, and vanadium as possible, for high heat affected zone (HAZ) toughness and strength. Controlling titanium and nitrogen and the Ti/N ratio, a large number of TiN dispersed finely are formed in steel and the austenite grain size near a weld fusion line is refined remarkably owing to strong pinning effect of TiN. Calcium addition promotes ferrite nucleation, so that increase in fine polygonal ferrites makes microstructure of HAZ much finer. Niobium and vanadium content are reduced, because carbide precipitates are formed when the coarse grain HAZ is reheated around 700 degree C and the precipitation hardening deteriorates HAZ toughness. The trial manufacturing of the 19.5mm, 26.9mm and 31.4mm thick X65 grade UOE pipes was finalized with the satisfactory results. The toughness of longitudinal submerged-arc welds was more than 50 J in Charpy V-notch impact test at −30°C.Copyright