Technical Paper

Niobium in high strength weldable beams and other structurals

International Symposium Niobium 2001

Niobium is widely applied for the production of high strength structural steels. The evolution of high strength steels is one of the major metallurgical development of the last decades. The competition from other material, in particular reinforced concrete, has without doubt stimulated the progress of structural steels. To maintain the market share of steel, economical fabrication techniques and refined design procedures had to be promoted. This led to an increased interest in steel properties such as weldability and notch ductility. The last decades have also seen considerable evolution in the profile production processes. The liquid steel is refined either by oxygen electric arc furnace steelmaking. Unlike previously for flat products, the usual ingot casting route is more and more being replaced by continuous casting processes. Various types of rolling schedules are now applied for the production of profiles: conventional rolling, normalizing processes and thermomechanical processes. Niobium is frequently used in the production of profiles either alone or combined with other microalloying elements such as titanium or vanadium. The addition of niobium refines the microstructure and allows a decrease in the equivalent carbon content. This improves the toughness of the steel and its weldability, which is particularly useful for the thickest profile and is also able to attain higher levels of strength. Niobium is particularly attractive in steels with higher nitrogen contents such as those produced by EAF steelmaking: by pinning nitrogen, niobium decreases the free nitrogen content, which is beneficial for the toughness. For such a purpose niobium has an advantage compared to alternatives such as aluminium or titanium in not penalizing the productivity of the continuous caster. Niobium is also particularly efficient in increasing the strength of steel. The highest strengths are obtained when using thermomechanical processes. Each production process covers a range of grades and thicknesses: above a limiting value, the ductility or weldability requirements are barely satisfied: the widest range is obtained by the application of thermomechanical controlled processes with thickness up to 125 mm in steel grade S460 (65ksi). The use of niobium microalloyed steels allowed the production of high strength beams fulfilling the most severe standards and offering an excellent combination of strength, ductility and toughness both in the hot rolled state and after welding with a wide range of welding energies varying between 8 and 50 kJ/cm. These modern high strength steels offer substantial savings in terms of material and fabrication costs for a wide range of applications such as buildings, bridges and offshore structures. (AU)
Technical Paper (PDF 2.70 MB)