The Release Effect of Temper Mill on Residual Stress in Cold-Rolled Steel Strip

In the meticulous world of flat rolled steel production, the final properties of the strip are as crucial as its chemical composition. After undergoing processes like cold reduction in a reversing cold rolling mill and subsequent annealing, the steel strip possesses the desired hardness and ductility but often suffers from a hidden flaw: residual stress. This locked-in internal stress, a legacy of non-uniform plastic deformation and thermal cycles, manifests as shape defects like coil breaks, edge wave, center buckle, and problematic fluting during subsequent stamping or cutting. The primary and most effective industrial solution to this pervasive issue is the temper rolling process. This seemingly simple light cold reduction pass, performed by a dedicated temper mill or temper pass mill, is in fact a sophisticated metallurgical intervention designed not to shape the metal, but to shape its internal stress state. Its role in releasing and homogenizing residual stress is fundamental to delivering a flat, stable, and workable product to the end customer.

The Genesis of Residual Stress in Cold-Rolled Strip  

To understand the release effect, one must first comprehend the origin of the problem. Residual stresses are self-equilibrating internal stresses that persist in a material in the absence of external forces. In cold-rolled steel strip, they arise from two primary sources. The first is heterogeneous plastic deformation during heavy cold reduction. As the strip passes through the work rolls, the surface grains experience a different shear and deformation history compared to the centerline grains. This gradient in deformation through the thickness creates a complex internal stress field, often with compressive stresses on the surface and tensile stresses in the core, or vice-versa, depending on the rolling conditions.

The second major source is the annealing process. While annealing recrystallizes the grains and softens the material, the heating and cooling cycles are rarely perfectly uniform across the width and thickness of a massive coil. Slight differences in thermal expansion and contraction can lock in significant thermal stresses. Furthermore, the relief of the original rolling stresses during annealing can itself be non-uniform, leading to a new, annealed-in stress profile. The result is a strip that may be soft and ductile but is internally unbalanced. This imbalance means the strip is not in its lowest energy state; it seeks to relieve these stresses by bending, twisting, or buckling, leading to the flatness issues that render it unusable for precision applications.

The Mechanistic Principle of Stress Relief in a Temper Rolling Mill  

The temper mill does not operate by heat-treating the stress away. Instead, it uses a controlled mechanical intervention to override and homogenize the existing, problematic stress state. The core principle involves subjecting the entire strip, uniformly and simultaneously, to a small amount of additional plastic deformation, typically between 0.5% and 4% elongation.

As the strip enters the bite of the high quality temper rolling mill, the applied tension and roll pressure force the material to yield. This yielding process effectively "resets" the internal stress state. The existing residual stresses, whether tensile or compressive, are overwhelmed by the new, applied stress field created by the mill. As the material flows plastically through the roll gap, the grains are slightly reoriented and deformed, but crucially, this new deformation is applied uniformly across the strip's width and through its thickness.

Upon exiting the roll gap and elastically springing back, the material unloads. However, it does not return to its previous stressed state. Instead, it finds a new equilibrium. The microscopic yielding and subsequent elastic recovery effectively cancel out the pre-existing, non-uniform residual stresses, replacing them with a new, far more homogeneous and minimal internal stress profile. This process is akin to smoothing a wrinkled cloth under a heavy, flat iron; the localized wrinkles (stresses) are overwhelmed by a global, uniform pressure, resulting in a flat, stable sheet. The precision of this process is why a High precise temper mill is required, capable of exerting extremely consistent roll force and maintaining perfect crown control across the entire strip width to ensure every segment of the strip receives an identical reduction.

Critical Process Parameters in a High Precise Temper Mill  

The effectiveness of residual stress relief is not automatic; it is meticulously controlled by several interdependent parameters on a modern temper pass mill.

Reduction Rate: This is the most critical variable. Too little reduction (e.g., below 0.5%) will only result in elastic deformation, failing to induce the necessary yielding to modify the residual stress field. The strip will spring back to its original, stressed state. Too much reduction (e.g., over 4%) will begin to work-harden the material, increasing its strength and reducing its ductility, which defeats the purpose of the prior annealing. It can also introduce a new set of residual stresses. The optimal reduction is a narrow window that provides just enough plastic strain to reset the stress state without adversely affecting mechanical properties.

Roll Force and Roll Crown Control: A high quality temper rolling mill is equipped with a robust housing and advanced roll bending systems (positive and negative). Consistent roll force is essential for applying uniform reduction. More importantly, the rolls must be ground with a specific crown (profile), and hydraulic bendings systems must be used to actively adjust the roll gap shape in real-time to perfectly match the strip's width and inherent flatness characteristics. This ensures the reduction is applied evenly from edge to edge. Any non-uniformity in reduction will simply create a new pattern of residual stress, trading one defect (e.g., center buckle) for another (e.g., edge wave).

Tension Control: Entry and exit tensions are vital control parameters. Along with roll force, tension contributes to the total stress state needed to yield the strip. Precise tension control ensures stable threading, prevents slippage, and contributes to a consistent deformation process throughout the entire length of the coil. Modern mills use digital drives and load cells to maintain tension within a very tight tolerance.

Surface Finish: While the primary goal is stress relief, the temper mill also imparts the final surface finish onto the strip. The work rolls can be ground to a specified roughness (e.g., matte, bright, or mirror finish), which is transferred to the steel surface. This is a critical customer-facing property for applications like automotive exterior panels or appliance surfaces.

The Capability of Modern Mills: 1250 and 1680 Temper Mill   

The designation of a mill, such as a 1250 temper mill or a 1680 temper mill, refers to the usable roll barrel length in millimeters, indicating the maximum strip width it can process. A 1250 temper mill can handle strip up to 1250mm wide, suitable for many automotive and general industry applications. A larger 1680 temper mill is a heavier-piece equipment designed for wider product, often used in applications like large appliance cabinets, construction panels, or shipbuilding.

These modern mills are engineering marvels of integration. They are not just scaled versions of smaller mills; they represent a higher order of precision and power. The wider the strip, the more challenging it becomes to maintain uniform roll gap pressure and perfect flatness. A 1680 temper mill will therefore feature immensely rigid mill housings, powerful hydraulic screw-down systems, and sophisticated shape control systems, often including multiple-zone roll cooling and advanced automatic shape control (ASC) algorithms that use shape roll data to dynamically adjust bending forces. The larger investment in such a high quality temper rolling mill is justified by its ability to deliver stress-relieved, perfectly flat strip at wider dimensions, which commands a premium in the market.

In conclusion, the temper rolling process is a vital final step in the manufacturing of cold-rolled steel strip. Far from being a simple "skin pass," it is a precisely calibrated metallurgical treatment performed by a High precise temper mill. By applying a small, uniform plastic deformation, it effectively erases the detrimental legacy of residual stresses from previous processing, replacing chaos with order at a microscopic level. This transformation from an internally unstable to an internally stable state is what grants the strip its impeccable flatness, dimensional stability, and superior performance in the customer's fabrication line. It is the definitive process that ensures the sophisticated efforts of the preceding cold rolling and annealing stages are fully realized in a final product of the highest quality.