After coiling, cold-rolled titanium plates and strips develop localized circumferential protrusions on the coil surface, known as beading. For pure titanium sheet, beading most commonly occurs in thicknesses less than 0.8 mm and typically manifests as a single bead. Beading directly results in additional wavy strip shape, affecting the strip's shape and surface quality, leading to product degradation and, in severe cases, requiring shearing and coiling. This not only reduces product quality but also wastes raw materials and reduces production efficiency. Rolling tests revealed that the amount and probability of beading after cold rolling varied between batches of hot-rolled coils of the same specification, indicating that the hot-rolled raw material itself significantly influences beading during cold rolling. Defects such as scratches, camber, and cracks are common in incoming hot-rolled material, contributing to various defects that can occur during the subsequent cold rolling process. While the impact of local high points in hot-rolled incoming material on cold-rolled strip is limited to the high points and a small area nearby, for extremely thin strip, it can cause localized bulging ("ribbing") in the strip, or even create a serious quality defect characterized by a combination of localized undulation and bulging.
Currently, the production of pure titanium sheet and strip primarily utilizes six-, ten-, and twenty-high multi-roll mills. Japan, home to the most advanced titanium sheet and strip production technology, utilizes a twenty-high mill, producing thicknesses of 0.3 to 3 mm. This mill offers high production efficiency and excellent dimensional accuracy, shape, and surface quality. However, in actual production, particularly in the production of large, heavy, wide, and thin strips, quality issues such as ribbing and waviness still occur. Ribbing is the most serious of these issues, negatively impacting product quality and the company's profitability, and represents a product quality issue that urgently needs to be addressed. Through trial rolling tests with different profiles, both at the same tension and at different tensions, it was found that under the same tension and different profile settings, when the profile was set based on a stainless steel strip, the probability of beading was high. However, after adjusting the profile settings, the probability and amount of beading decreased significantly. Under the same profile but different tension settings, the probability of beading was higher when rolling with high tension than when rolling with low tension, but the difference in the probability and amount of beading between high and low tensions was not significant. This indicates that traditional high-tension rolling of stainless steel strip is not suitable for rolling pure titanium sheet. Analysis of these trial rolling results indicates that beading, a circumferential bulge, is the result of a combination of factors, including profile control and tension control. From a mechanical perspective, beading is the result of axial force.
Based on field tests and theoretical analysis, a mathematical model for the critical conditions for beading was established based on actual production characteristics. The critical stress for buckling instability is proportional to the fourth power of the strip thickness and inversely proportional to the square of the width. Axial stress is most significantly influenced by three factors: pre-tension, friction coefficient, and aspect ratio. While maintaining the aspect ratio, reducing pre-tension, changing the rolling lubricant, or placing liner paper at the coiling end to increase friction can effectively prevent the occurrence of rib defects.
Although the rolling speed during pure titanium sheet cold rolling is very slow, poor lubricant properties, such as saponification value or nozzle clogging, can lead to uneven lubrication, resulting in uneven stress distribution in the deformation zone and an axial force component. In the rolling deformation zone, the axial force component generated by neutral plane deviation may be small, but it does have a significant impact on the tightening of the plate surface toward the center. During the rolling deformation process, local high points or localized hardness can lead to uneven stress distribution in the deformation zone, generating axial force components. The interaction between equipment vibration and uneven tension generates axial force components. The combined effects of slight center deviation during coiling, uneven thickness, and interlayer porosity deviations can also generate axial force components.
