After cold-rolled titanium strips are coiled, localized circumferential bulges appear on the coil surface; this phenomenon is known as “ridging.” For commercially pure titanium strips, ridging predominantly occurs in gauges thinner than 0.8 mm and usually manifests as a single ridge. The direct consequences of ridging include the formation of additional waviness in the strip, which compromises flatness and surface quality, leading to product downgrading or, in severe cases, necessitating shearing and re-coiling operations. These issues not only reduce product quality but also result in raw material waste and lower production efficiency. Rolling trials have revealed that the extent and probability of ridging vary between different batches of hot-rolled coils of the same specifications, indicating that the hot-rolled feedstock itself significantly influences ridging during cold rolling. Defects such as scratches, camber, and cracks are commonly found in hot-rolled feedstock and contribute to the formation of various defects during subsequent cold rolling. Although the impact of localized high points in the hot-rolled feedstock on the cold-rolled strip is confined to the immediate vicinity of the high point, in extremely thin strips, this is sufficient to cause localized bulging—or “ridging”—and even severe quality defects where localized waviness intersects with the bulge.
Currently, the production of commercially pure titanium sheets and strips primarily utilizes multi-roll mills, such as 6-high, 10-high, and 20-high configurations. In Japan, where titanium sheet and strip production technology is most advanced, 20-high mills are employed to roll material with thicknesses ranging from 0.3 to 3 mm, achieving high production efficiency alongside excellent dimensional accuracy, flatness, and surface quality. However, in actual production—particularly when manufacturing wide, thin strips in heavy coils—quality issues such as ridging and waviness persist. Among these, ridging is the most severe problem; it adversely affects both product quality and corporate profitability, making it a critical issue that requires urgent resolution. Trial rolling experiments involving various combinations of strip shape curves and tension levels revealed that, when using a shape curve setting optimized for stainless steel strips, the probability of “ribbing” (longitudinal ridging) was high; however, adjusting the shape curve setting significantly reduced both the likelihood and severity of this defect. Regarding tension settings, rolling at higher tension resulted in a higher probability of ribbing compared to lower tension, though the difference in both probability and severity was not substantial; this indicates that the high-tension rolling practices traditionally used for thin stainless steel strips are unsuitable for pure titanium strips. Analysis of these results suggests that ribbing—a form of circumferential bulging—arises from the combined effects of factors such as strip shape and tension control; from a mechanical perspective, it is caused by the action of axial forces.
Based on both on-site trials and theoretical analysis, a mathematical model defining the critical conditions for ribbing was established. The critical stress for buckling instability was found to be directly proportional to the fourth power of the strip thickness and inversely proportional to the square of the strip width. Furthermore, axial stress is most significantly influenced by three factors: front tension, the friction coefficient, and the width-to-thickness ratio. Provided the width-to-thickness ratio remains constant, ribbing defects can be effectively suppressed by measures such as appropriately reducing front tension, changing the rolling lubricant, or inserting backing paper at the coiling end to increase friction.
Although cold rolling speeds for titanium strips are relatively low, issues such as poor lubricant properties (e.g., inadequate saponification value) or nozzle blockages can lead to uneven lubrication. This results in non-uniform stress distribution within the deformation zone, thereby generating axial force components. Within the deformation zone, the shifting of the neutral plane generates an axial force component; while this force may be small, it exerts a certain influence on the inward contraction of the strip surface. Additionally, localized high spots or variations in hardness during the rolling process can cause uneven stress distribution in the deformation zone, further contributing to the generation of axial force components. The interaction between equipment vibration and uneven tension generates axial force components; the cumulative effect of factors such as slight center misalignment during winding, thickness irregularities, and variations in inter-layer gaps also gives rise to axial force components.
