Low-temperature ductility and structural behaviour of cold-formed hollow section structures - progress during the past two decades

Cold-formed hollow sections are a widely used tubular construction material. The applicability, weldability and reliability of cold-formed rectangular hollow sections are sometimes questioned because of the consequences of cold forming and inhomogeneous cross-sections. One of the main concerns is related to the cold-formed corner areas and possible loss of toughness due to strain ageing in the vicinity of the welds. Conventional hot-rolled C-Mn steels are susceptible to strain ageing, and Eurocode 3 includes restrictions on welding in the cold-formed corner area. Both steelmaking and hot rolling have undergone crucial developments and this has had an impact on cold-formed hollow sections. Thermomechanically rolled fine-grain steels became state of the art at the end of the 1990s. This study confirms that cold-formed EN 10219 hollow sections made of suitable fine-grain steels have a similar Charpy-V toughness on the flat face and in the corner, and that even after ageing the transition temperature T40J in the corner area is at a very low level, typically below -50 °C. The load and deformation capacities of X- and K-joints fulfil the requirements without any noticeable ageing effects. The advances in steelmaking and hot rolling enable the manufacturing of reliable and versatile cold-formed EN 10219 hollow sections for welded structures with good low-temperature ductility even in the cold-formed corner area.