Effect of coil set on shape defects in roll forming steel strip

Coil set is a phenomenon occurring in industrial sheet metal in which the strip retains a residual curvature as it comes off the coil. The roll forming of two coils exhibiting coil set is studied; each coil has similar tensile and bending properties. It is shown that there are significant differences in shape defects in the industrial roll forming of a particular channel section depending on whether the strip is fed in with the convex side upwards or downwards. The properties in bending in the longitudinal direction were determined using a free bending test and found to depend on the direction of bending; when bending so that the curvature increased in the direction of the residual curvature, the bending yield stress was almost 50% lower compared with bending in the direction opposite to the residual curvature, i.e. in straightening the strip. In the transverse direction, bending properties were independent of the direction of bending. The defects measured in the roll formed product were twist and flare and the magnitude of both were greatest when the strip was formed with the residual curvature convex upwards in the roll forming line. The process was simulated using the commercial software package Copra FEA; two sets of material property data were used − both were derived by an inverse method from the bending tests. One case used bend test data from tests in which the curvature increased in the same direction as the residual curvature, and the other set for curvature in the opposite direction. The defects predicted by the numerical analyses reproduced the trends observed in the industrial trials regarding twist and end flare even though the levels predicted were too high. Comparison of the bending test results with other work suggests that the strip is subjected to plastic deformation (straightening) as it comes off the coil resulting in an asymmetric longitudinal residual stress distribution through the thickness. Both the experiments and the results of the simulation strengthen the view that differences in the mechanical behaviour in bending near the elastic plastic transition indicate the presence of residual stresses that influence final shape in the roll forming process.