Surface Topography Changes Associated With Lüders Band Formation in Low Carbon Sheet Steels

This research employed three-dimensional optical profilometry to examine the influence of yield point elongation (YPE) on the evolution of surface topography associated with Lüders band growth. Eleven Lüders bands were examined on tensile specimens containing YPE ranging from 0.13% to 2.34%. For each sample, a cycle of loading just beyond the yield point, unloading, recording surface topography changes, then repeating the process was undertaken. All samples were prepared by aging for various times at 150°C, chemically stripping the galvanized coating, and polishing one face to a mirror finish for surface topography assessment of the Lüders band.

Assessments of the Lüders bands in the scan area revealed topography ranging from a ‘local’ maximum depth to a final condition reflecting residual features. The Lüders band was observed to be most severe at early strains and the depth of the Lüders band was found to increase linearly with increasing YPE in the substrate, while the width of the Lüders band was not substantially influenced by YPE. Finally, based on a comparison with recent painting results of industrially created surface imperfections, it is suggested that current YPE acceptance criteria may be more conservative than needed.