New solutions for the metallurgical design of high performance quenched and tempered (Q&T) seamless pipes of high grades from X65 up to X80 were found throughout a systematic work involving metallurgical modelling, laboratory tests, pilot and industrial trials. Both linepipes and risers for deepwater offshore fields such as Gulf of Mexico were considered. The target microstructure in the asquenched condition has been identified as refined low-C bainite/martensite matrix (> 70%). This is promoted through the control of austenite grain growth during the heating stage (austenitization before quenching), proper alloy additions and through a very effective quenching. Promising low-alloy steels and suitable quenching and tempering conditions identified by metallurgical modelling were verified by laboratory heats (80 kg ingots) that were processed at a pilot scale and submitted to microstructural examination and mechanical testing. The best solutions were used in preliminary industrial trials, also utilised for a fine tuning. The Ni-Cr-Mo-Nb-V alloy system showed very interesting combinations of strength-toughness and field weldability, suitable for the production of heavy wall X65 grade linepipes for sour service and X80 top tension risers
