Niobium in structural sections

Most Nb-treated HSLA sections are sold without impact requirements. These sections commonly have C contents up t o about 0.18 percent with Mn and Nb levels typically around 1.3 percent and 0.03 percent respectively. They are widely used for applications such as piling, bridges, ships, road and rail transported, where a high strength to weight ratio is of prime importance. Improved weldability and impact properties at similar strength levels can be obtained by reducing the C level (< 0.10%) and increasing the Nb content (> 0.06%). For optimum benefits from a Nb addition hot rolling should be completed in the lower austenite region and FRTs should be < 1000 C. Where the mill is capable of resisting the additional stresses such a controlled- rolling procedure can be adopted. This allows significant grain refinement and gives good impact properties. However, sections are more usually subject to a variety of rolling conditions depending on mill characteristics and local procedures and FRTs are often > 1000 C. This can give coarse bainitic structures and poor impact properties at similar strength levels to those sections with a lower FRT. Sections can also under go normalizing to cause grain refinement and give guaranteed impact properties. Steel composition can be tailored to the processing conditions and to the properties required and, although recent developments have concentrated on modifications to the C-Mn-Nb type of steel, more highly alloyed grades of Nb-treated steels have also been developed. (AU)