Development of P690QL2 High Tensile Strength Steel Plate for Medium Pressure Liquefied CO2 Tank

To support the realization of a decarbonized society, the development of high-tensile strength steel plates for marine transport tanks operating under medium pressure conditions for carbon dioxide capture and storage (CCS) applications is essential. To achieve larger tank sizes under medium pressure (design temperature: −35°C; 2 MPa) liquefaction conditions, it is necessary to study the optimization of WEL-TEN® 780. This material has both high strength and low temperature toughness and complies with EN10028-6 P690QL2. Simulated heat-affected zone (HAZ) Charpy impact tests on laboratory-produced samples revealed a chemical composition that ensured low-temperature toughness in the HAZs of as-welded joints while minimizing intergranular embrittlement after post-weld heat treatment (PWHT). To evaluate the performance of joints produced using mill trial plate steel, gas metal arc welded joints with K-shaped grooves were prepared, and Charpy tests were conducted with notches positioned in the weld metal and on the I-side fusion line (FL). The results confirmed that the absorbed energy at the specified test temperature satisfied the required standards under both as-weld and PWHT conditions. Furthermore, welded joints of the developed steel were tested in wide-plate tensile tests with fatigue pre-cracks of about half the plate thickness and evaluated with and without PWHT. The results of a wide-plate tensile tests at −35°C confirmed that both as-weld and PWHT samples failed through ductile deformation, with fracture stress exceeding the allowable stress limits specified for medium pressure tanks.