The Relationship Between Residual Stress and Coil Flatness
How Residual Stress Forms
During the production of steel coils, residual stress is introduced through processes such as rolling, cooling, and coiling. Differences in deformation levels during rolling, uneven cooling rates, and the tension applied during coiling all contribute to internal residual stress formation.
Key causes of residual stress include:
Non-uniform Plastic Deformation:
In metalworking processes like rolling and forging, inconsistent deformation across different areas of the workpiece leads to internal residual stress. For example, during rolling, the surface of the material undergoes greater deformation than the core. Once rolling is completed, the mismatch in deformation between the surface and core induces residual stress.
Thermal Effects:
Processes such as heat treatment and welding involve cycles of heating and cooling. Uneven thermal expansion and contraction across the material cause thermal stresses, which turn into residual stresses upon cooling. For instance, during welding, the weld zone expands significantly compared to the surrounding areas, and during cooling, the constrained shrinkage of the weld zone results in tensile residual stress.
Phase Transformation:
When metals undergo phase changes during heating and cooling, different phases often have different specific volumes. In steels, for example, the transformation from austenite to martensite during quenching leads to volume expansion. This volume change, restrained by surrounding material, generates residual stress.
Mechanical Processing:
During cutting and machining, the interaction between the tool and workpiece creates plastic deformation on the surface layer. Internal material constraints then induce residual stress. Similarly, in grinding operations, heat generated by friction can cause thermal and phase transformation stresses on the surface.
Assembly Stresses:
During mechanical assembly, interference fits and assembly sequences can also introduce residual stress. For example, press-fitting parts together compresses the mating surfaces, leading to internal stresses.
Impact of Residual Stress on Coil Flatness
Residual stress significantly affects the flatness of steel coils:
Coil Warping:
Residual stresses seek to release when the coil is unrolled, often leading to distortions. If the stress distribution is uneven, localized warping can occur, affecting the flatness. For example, if compressive stress dominates on one side while tensile stress dominates the other, the coil may bend toward the compressive side, creating a wavy surface.
Wavy Surface Deformation:
Residual stress imbalances along the length of the coil can cause wave-like undulations. As the material attempts to balance internal stresses, localized stretching and compression occur, producing waves. This not only impacts the coil’s appearance but also complicates further processing and use.
Edge Distortion:
The edges of steel coils often experience more complex residual stress distributions, leading to defects such as curling or twisting. Common issues like “scalloped edges” or “sickle bends” are strongly associated with uneven residual stresses along the coil width.
Strategies for Controlling Residual Stress to Improve Coil Flatness
Ruisen implements a series of advanced process controls to minimize residual stress and enhance coil flatness:
Optimizing Rolling Processes:
Carefully adjusting rolling temperature, reduction per pass, and rolling speed can help achieve more uniform deformation, reducing residual stress. Multi-pass rolling with smaller reductions is especially effective in promoting balanced internal stress distributions.
Enhancing Cooling Techniques:
Controlling the cooling rate and ensuring uniform cooling are critical for minimizing stress concentrations. Techniques such as controlled slow cooling or post-process tempering can help eliminate some of the residual stresses, particularly for products requiring high flatness precision.
Refining Coiling Practices:
Adjusting coiling tension and temperature ensures an even internal stress state during winding. Selecting appropriate coiling tensions based on material properties and dimensions, along with post-coiling stress-relief treatments, further improves final flatness.