Pure titanium strip is primarily produced using multi-roll mills, such as six-, ten-, and twenty-roll mills. In Japan, where titanium strip production technology is most advanced, twenty-roll mills are used, achieving thicknesses between 0.3 and 3 mm. This results in 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 strip, quality issues such as beading and rippling still occur. Ripping is the most serious problem, adversely impacting product quality and business profitability, and is a critical issue that urgently needs to be addressed.
Ribring is the phenomenon of localized raised bumps on the surface of cold-rolled titanium strip after coiling. For pure titanium thin strip, beading most commonly occurs in thicknesses less than 0.8 mm and typically manifests as a single bead. Ribring directly causes additional rippling in the strip, affecting 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 have revealed that the hot-rolled raw material itself significantly influences cold-rolling beading. Defects such as scratches, camber, and cracks, common in hot-rolled incoming material, contribute to various defects that may occur during the subsequent cold-rolling process. While the impact of local high points in the hot-rolled incoming material on the cold-rolled strip is limited to the high points and a small area nearby, for extremely thin strip, they can cause localized bulging ("beading") in the strip, and even lead to serious quality defects such as a combination of localized waves and bulges.
Test rolling with different flatness curves and tension settings revealed that beading is the result of a combination of factors, including flatness control and tension control. From a mechanical perspective, beading is the result of an axial force. Factors such as poor lubricant performance, nozzle blockage, neutral plane deviation, local high points or uneven hardness, equipment vibration, uneven tension, and slight center deviation during winding can all generate axial force components, leading to beading. Based on field tests and theoretical analysis, a mathematical model for the critical conditions for ribbing was established according to actual production characteristics. The research shows that 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. Furthermore, axial stress is most significantly influenced by three factors: pre-tension, friction coefficient, and aspect ratio. While maintaining the aspect ratio, ribbing defects can be effectively suppressed by appropriately reducing pre-tension, changing the rolling lubricant, or inserting liner paper at the winding end to increase friction.
In summary, addressing ribbing during cold rolling of pure titanium plate and strip requires multiple approaches, including raw material control, rolling process optimization, and equipment adjustment. By comprehensively considering various influencing factors, an effective solution can be developed to improve product quality and production efficiency.
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