Abstract 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. Introduction As the exploration of oil and gas fields are expanding toward severe environment regions, requirements for the performance of linepipes has been diversified and has become stringent. Recent study indicated the significant advantages of using higher strength linepipes in constructing long distance pipelines [1], because it can improve transportation efficiency of gas and oil pipelines by increasing internal pressure, and material cost can be saved by reducing wall thickness of pipe body and consumable of weldment. Another severe requirement for the pipes these days is to ensure low temperature toughness of welded region, such as Heat Affected Zone (HAZ) and weld metal, as well as parent material. The parent material of linepipe is usually manufactured by using TMCP (Thermo-Mechanical Controlling Process) technique. Accelerated cooling techniques gives significant advantages in improving strength and toughness of parent materials through controlling their microstructure to be fine shape, and this led to a considerable increase in the number of applications for manufacturing the higher strength linepipes in this decade. However, the fine microstructure that has been developed by TMCP can not exist in HAZ, because HAZ is exposed in high temperature during welding. This microstructural change often brings deterioration effects in toughness of HAZ. In case of heavier wall linepipes than 20mm, testing temperature reduction rule is often applied. This means testing temperature for Charpy V test should be -20 to -30 degree C for the heavier wall linepipes even when their service temperature is -10 degree C. In this sense, importance to ensure superior HAZ toughness for the heavier wall materials is highlighted. To ensure the superior toughness of HAZ, the effects of alloying elements, such as Ti, N, Ca, Nb V and Oxygen on microstructure of HAZ in terms of suppressing grain coarsening in HAZ, introducing ferrite nucleation sites and suppressing formation of M-A constituents in HAZ have been investigated
