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

• Skin pass is a fundamental step in the manufacturing of Lüders Bands-free steel sheets.
• An analysis of the strain heterogeneity and Lüders Bands in the skin-pass is presented, employing Finite Element Analysis (FEA) and true-stress true-strain curves displaying an initial yield stress peak higher than that observed in common tensile testing.
• The FEAs led to results similar to those available experimentally and allowed a detailed analysis of the phenomena.
• For low friction between the material and the roll and for low sheet thickness reductions (0.5% and 1.0%) and high friction between the material and the roll, a limited influence of the roll diameter on the Lüders bands distribution was found.
• For sheet thickness reductions of 2.0% and higher, high friction between the roll and the material and large flattened roll diameters, the distribution of LBs and of effective strain in the sheets was different from that obtained for small roll diameters and low friction between the rolls and the material.
• Lüders bands in the skin-pass nucleate repeatedly in the region of first contact of the material and the rolls and propagate towards the sheet centerline.
• It was confirmed that the thickness reduction necessary for the elimination of the Lüders bands problems in steel sheets corresponds to a very heterogeneous distribution of strain in the material.
• A criterion for the definition of the critical thickness reduction in the skin-pass, leading to the elimination of the Yield Point in tensile testing of the material after processing, is suggested.

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 Lüders 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 Lüders 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 Lüders 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 Lüders 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.