Effect of manufacturing process on microstructures and mechanical properties, and design of cold-formed G450 steel channels

Abstract

Cold-formed steel members are manufactured from cold-reduced steel coils by either press-braking or cold-forming. During manufacturing processes at ambient temperature, two stages, namely cold-reducing and cold-forming, create significantly large deformation on the cross-sections. While the first stage involves cold-reducing hot-rolled steel coils to thinner desired thicknesses resulting in higher yield strength and less ductility, the second stage including rolling and bending processes causes strain hardening of the material, which affects the local mechanical properties especially at the bending corners of the cross-sections. This paper presents an experimental study on the effect of the latter manufacturing process on the microstructure of flat and corner regions of C-shaped cold-formed high strength G450 steel channel sections with different sizes and thicknesses. Optical microscopy, scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) were used for microstructure analyses. Tensile coupon and Vikers hardness tests were performed for mechanical properties of cold-formed steel material including yield strength, ultimate strength and Vickers hardness at both flat and corner regions. Understanding the relationship between the changes in the microstructure of the high strength G450 steel material provides insights into the mechanical and structural behaviour of cold-formed channel members. In addition, the experimental yield strengths at corner regions are compared with those from the AISI S100-16 and AS/NZS 4600:2018 design specification/standard, showing conservative predictions of the design codes over the measured corner yield strengths, and especially providing inconsistent predictions in thin sections. Hence, a new suggested modification of design equations in the current specification/standard is proposed in this paper to better predict the yield strengths at corner regions of high strength G450 steel cold-formed channels.