Properties of spiral welded and HFI pipes made of low Mn sour service steel

Within the scope of the R&D activities to improve Salzgitter steel plant’s competence in processing of sour service resistant spiral welded and HFI pipes recently for the very first time a full scale industrial trial based on a low Mn concept has been performed. This trial has been motivated by apparent advantages such as greatly reduced segregation tendency of Mn and the more pronounced tolerance of residual sulfur in the steel, as the slab centerline quality and the size and distribution of the MnS particles are well correlated with sour service re-sistance. Further motivation was positive influence of low Mn content to the precipitation kinetics of Nb and recrystallization behavior of the Nb microalloyed steels. The trial showed excellent sour service resistance of the pipe base material. The strength deficit inherent to the low Mn content has been effectively substituted by alloying of Cr and Nb. Hence the decent strength level of API X60 for spiral and X70 for HFI pipes has been achieved. Further development might concern the improvement of toughness at low temperatures, especially regarding the DWTT performance.