Strain heterogeneities in the rolling direction of steel sheets submitted to the skin pass: A finite element analysis

Hot rolled, annealed or galvanized low carbon steel sheets display yield points in tensile testing. These are associated with deformation heterogeneities along the test specimen, known as Luders bands, which cause an unacceptable surface appearance in steel sheet formed products. This problem is usually eliminated through a light cold rolling pass ("skin-pass") in the material as a last step in its industrial manufacturing, imposing a reduction of thickness of about 1-2.0% on the sheet. Existing experimental results for these strain heterogeneities along the rolling direction are analyzed, as well as corresponding finite element analyses. The constitutive behavior of the material is described by a curve displaying an initial stress peak followed by the work hardening of the material. The FEAs led to distributions and shapes of Luders bands in the sheets similar to the experimental ones. The bands nucleate intermitently and propagate from the surface of the sheets, at the initial contact region between the material and the rolls. For low and high friction between the material and the roll and for low sheet thickness reductions (0.5% and 1.0%), a limited influence of the roll diameter on the Luders bands distribution was found. For sheet thickness reductions of 2.0% and higher, high friction between the roll and the material and large roll diameters, the distribution of LBs in the sheets was different from that obtained for small roll diameters and low friction between the rolls and the material. Increasing thickness reductions in the skin-pass led to changes in the Luders bands distribution, involving an increasing homogeneity in the deformation. It is suggested that the critical thickness reduction in the skin-pass is associated to a minimum volume fraction of material deformed above a certain strain level. (C) 2014 Elsevier B.V. All rights reserved.