Production of 20 mm Thick API PSL 2 X60 and X70 Grade Plates from a Nb-Ti Microalloyed Steel

T his study was undertaken to determine the controlled rolling and cooling conditions for the production of 20 mm thick American Petroleum Institute API X60 and X70 grade steel plates. Nb-Ti microalloyed steel slabs of 200 mm thickness were rolled at four different finish rolling temperatures conventional, 950°C, 850°C and 800°C . In some trials, a water table was employed to provide accelerated cooling just after finish rolling. Mechanical tests tensile, impact and drop weight tear-DWTT and microstructural examinations were performed on the samples taken from the trial production plates. Fine grained and essentially ferritic microstructures with strength values satisfying the minimum yield strength requirement of 415 MPa for the API X60 grade were easily obtained in all rolling conditions. However, the minimum yield strength requirement of 485 MPa for the API X70 grade was reached only when accelerated cooling was applied after finish rolling. The minimum 85% shear fracture required by the DWTT of the API PSL 2 specification could be met when the finish rolling temperature was lowered or when accelerated cooling was applied regardless of the finish rolling temperatur