Through process considerations for microalloyed steels used in naval ship construction

A review has been carried out on the status of particular microalloying elements in plate used for the construction of naval ships. The potential problem area tends to be the heat affected zone (HAZ) toughness where the presence of niobium imparts no beneficial effects. Vanadium has been excluded historically on the basis of cost, but current trends indicate that the ferroniobium ferrovanadium cost differential has been decreasing. In addition, vanadium is known to reduce the incidence of transverse slab cracking, and this could potentially reduce through costs as the scarfing loss to remove the cracks can account for a yield loss of similar to 3%. A detailed through cost evaluation is required to establish the exact status. In addition, a tentative relationship exists between the HAZ toughness effect of the microalloying element and its effect on the susceptibility to induce transverse slab cracking. Overall, the benefits improve in the order of niobium, vanadium and titanium. If cost is factored into these effects, then the microalloying element with the most potentially positive benefits is titanium. The effects of multimicroalloying have been briefly reviewed and this is clearly an area where further work is required. Although molybdenum has been extensively used in linepipe steels, its potentially positive effects as a microalloying element, and on transverse slab cracking in ship plate steels, has never been fully enough explored. This is related to its higher ferroalloy cost.