The steel for offshore jacket structures, Fe52Nb, is a silicon-aluminium killed steel, microalloyed with niobium. As the steel, in fabrication, has to be hot-formed, sometimes normalized or stress-relieved, the steelmaker cannot make use of low temperature treatments, such as controlled rolling, quenching plus tempering, etc. Therefore, the steel is delivered in the normalized condition and its strength and toughness are attained by promoting a fine ferrite grain through the action of grain growth inhibitors, such as Al-nitrides and Nb-carbonitrides. Further increase of strength and toughness, if necessary for future structures, should be pursued by the application of new, as yet unknown or not yet applied, kinds of grain growth inhibitors, in order to further refine the microstructure. These inhibitors should, if possible, be stable at almost steel melting temperatures in order to provide as an additional benefit the suppression of grain growth at the fusion line in high input welding. Maybe also a normalizing treatment at an intermediate thickness during the rolling process could be beneficial for further grain refinement of the finished plate, but most probably, not for grain-growth resistance at fusion line temperatures. Plates used in the construction of nodes are subjected to loading in the thickness direction and, therefore, are liable to lamellar tearing. In order to avoid this, the steel should have sufficient ductility in the thickness direction, the criterion being reduction of area in a tensile test, the axis of which is perpendicular to the plate surface. Sufficient resistance, for the application at present-day nodes is obtained by reducing the sulphur level of the steel to about 0.006 % and by careful deoxidation of the steel. Various ways of making this steel are put forward for discussion
Formability of Bimodal Steel Strips Subjected to Heavy-reduction Controlled-rolling Process