What Is the Yield Plateau in Cold-Rolled Coil?
Low-carbon cold-rolled steel in the annealed condition shows a discontinuous yield behaviour: when the stress reaches the upper yield point, deformation continues at a nearly constant lower yield stress while the strain increases. This flat region of the stress-strain curve is the yield plateau. It exists because carbon and nitrogen atoms in solid solution pin the dislocations. The material only starts to deform plastically when the stress is high enough to tear the dislocations away from these pinning atoms, which then happens abruptly.
Surface Defects: Luders Bands and Stretcher Strains
During the yield plateau, plastic deformation is not uniform. It starts in local bands, called Luders bands or slip lines, which spread along the specimen at about 45 degrees to the principal stress axis. When a coil with an unremoved yield plateau is stamped or drawn, these bands appear on the surface of the finished part as wavy ridges and stripes. The defect is cosmetic at first but can cause rejection of painted parts, because the stripes remain visible through the paint film.
Effect on Forming Stability and Forming Limit
The abrupt, non-uniform deformation within the yield plateau makes material flow in the die difficult to control, which reduces process stability in multi-stage presses. The yield plateau itself does not directly cause cracking, but the remedies used to remove it can do so: if the leveling elongation is too large, the resulting work hardening raises the yield strength and lowers the remaining ductility, so the sheet is more likely to crack in deep drawing. Correct process design must therefore balance the elimination of the yield point against the introduction of work hardening.
The Transverse Crease Defect
Cold-rolled coils that still show a yield plateau can develop a permanent transverse crease (a horizontal fold line across the strip) when they are subjected to excessive bending stress during processing or handling, for example when passing over small-diameter tension rolls or during uncoiling in cold weather. Once formed, this crease cannot be removed by later operations and ruins the flatness of the sheet. Coils should be handled and processed so that the strip is never bent below its minimum permitted radius, and temper-rolled material should be processed within the guaranteed time before strain ageing restores a yield point.
The Purpose of Skin Passing and Leveling
Skin passing (temper rolling) and roller leveling apply a small controlled cold deformation to the strip. This deformation frees the dislocations from the carbon and nitrogen atmospheres, eliminating the sharp yield point and making the stress-strain curve smooth and continuous. The typical skin-pass elongation is in the range of about 0.5% to 2%, depending on grade and thickness. If the elongation is insufficient, the yield plateau is not fully removed and stretcher strains still appear during forming; if it is too large, the material is work-hardened and embrittled. Mill practice therefore controls the temper elongation within a narrow window.
Frequently Asked Questions
What are Luders bands?
Luders bands are local bands of plastic deformation that form on the surface of low-carbon steel when it deforms through the yield plateau. They appear as stripes at roughly 45 degrees to the stress axis and remain visible after forming and painting.
How is the yield plateau removed?
By skin passing or roller leveling, which applies a small cold reduction (typically about 0.5% to 2% elongation) that frees dislocations from carbon and nitrogen pinning atoms and restores a smooth stress-strain curve.
What happens if the leveling elongation is too large?
The material is work hardened: yield strength rises, ductility drops and the sheet becomes more brittle, increasing the risk of cracking in deep drawing operations.
What is a transverse crease in cold-rolled coil?
A transverse crease is a permanent fold line across the strip width caused by excessive bending stress on a coil that still has a yield plateau, for example during uncoiling over a small-diameter roll. It cannot be corrected later and is cause for rejection of flatness-critical parts.
Why do stretcher strains appear after painting?
Because the Luders bands are slightly raised local deformations; they remain on the surface and are highlighted by the paint film, making the defect visible even when the painted surface is otherwise smooth.
Does strain ageing bring back the yield point?
Yes. After skin passing, carbon and nitrogen atoms gradually diffuse back to the dislocations over time (strain ageing), which can restore a yield point, especially in non-stabilized steels. This is why temper-rolled coils should be formed within a defined period after production.