Abstract
The aim of this work is to study the effect of ageing (250 °C for 1 h) on the mechanical properties of the cold formed S700 rectangular hollow section. The investigated hot rolled steel strip was produced thermomechanical rolling followed by direct quenching. The dimension of the investigated hollow section was 120 x 120 x 10 mm and the corner radii and the other tolerances compliant with EN 10219. Tensile properties and Charpy-V impact toughness were determined for the base material and flat and corner area of the hollow section. The results showed that the tensile strength in the corner was slightly higher in comparison with the flat side, revealing higher cold deformation rate in the corner. Ageing increased the strength level relatively higher than cold forming without losing any elongation properties. The impact energies were at the high level at −40°C and −60°C in cold formed and aged materials. Even at −80°C, the CV results were 118 J/cm2. It is also notable that no difference in CV values between the flat and the corner samples were observed. Thus, the results showed that the flat side specimens testing provides sufficient information of mechanical properties of the cold formed rectangular hollow sections and no need demanding corner sample testing when the structural hollow section is produced by using the thermomechanical controlled and direct-quenched base material. Furthermore, results showed that cold formed S700 is excellent for offshore steels, as steels are used even colder conditions as before.
Highlights
Formed steel sections are widely used in construction and engineering applications due to their relatively high strength and stiffness properties
From the electron backscatter diffraction (EBSD) grain boundary measurements and microstructures it can be seen that the flat side and corner area has smaller grain sizes comparing to the base material
The base material has the smallest fraction of low-angle grain boundaries and the corner areas have the high fraction low-angle grain boundaries, which indicating higher cold forming level as been seen in Ref. [9]
Summary
Formed steel sections are widely used in construction and engineering applications due to their relatively high strength and stiffness properties. Cold-formed welded hollow sections provide cost efficient and environmentally friendly alternatives compared to hot-formed sections [1]. Cold-formed rectangular hollow sections are manufactured from steel strips to final sections in several cold forming stages. Cold deformation causes work hardening of the material, which resulting in enhanced strength, a corresponding loss of elongation and toughness. When steels are used even colder climate conditions, the requirements especially for toughness properties are increasing. This increases demands for the steel to behave safety in demanding conditions
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