What Are The Key Mechanisms And Control Methods Of Titanium Cold Rolling Ribbing?

Cold-rolled titanium sheets and strips are key basic materials, and their flatness directly affects subsequent processing and final product performance. Titanium materials are prone to work hardening, have low elastic modulus and strong anisotropy. Defects such as ribbing, edge waves and wrinkles often occur during cold rolling, among which ribbing is the most frequent.

• Ribbing of titanium strips refers to discontinuous/continuous strip-shaped bulges along the coiling direction, distributed periodically, and the spacing is affected by coil diameter and tension. Thin and wide titanium strips are more prone to this defect. In mild cases, the strip surface is uneven; in severe cases, cracking and wrinkling occur, affecting subsequent processing.

• Grains at ribbing areas are distorted and elongated, stress concentration at grain boundaries tends to generate microcracks, and the hardness is higher than that of the matrix. It is not an ordinary flatness defect, but a local mechanical instability caused by stress concentration and uneven plastic deformation.

1. The strip is subjected to tension, friction, roll deformation and other effects to generate axial compressive stress, and local bulging and ribbing occur when the stress exceeds the critical buckling stress of the material.Thin and wide titanium strips are more prone to ribbing, which is consistent with production practice.

2.The core of ribbing is the axial force component caused by various factors:

• Titanium materials have high deformation resistance, rolls are prone to deformation, coupled with poor lubrication, uneven billet hardness and residual oxide layer, leading to local deformation and friction differences, generating axial force components.
• Excessive tension increases axial compressive stress and reduces stability; tension fluctuation causes periodic changes in axial stress, and superimposed interlayer friction differences easily lead to ribbing. The probability of ribbing under high tension is 2–3 times that under low tension.
• Core eccentricity, strip deviation and uneven interlayer clearance generate additional axial force; the smooth surface of titanium strips and low interlayer friction easily cause interlayer sliding, aggravating stress unevenness.
• Deviations in roll roundness and parallelism, as well as frame vibration, cause fluctuations in rolling pressure and thickness, inducing axial force components; thin-gauge titanium strips are extremely sensitive to vibration, and slight vibration will lead to stress concentration and ribbing.

• Low elastic modulus leads to large elastic deformation under the same stress and lower critical buckling stress;
• High work hardening rate results in rapid increase of deformation resistance during cold rolling, easy to cause stress concentration;
• Significant anisotropy leads to large differences in mechanical properties between rolling and transverse directions, and poor transverse instability resistance.

1.Thin and wide titanium strips have low critical buckling stress and are more prone to ribbing; metallurgical defects, oxide scale and scratches on the billet cause uneven rolling deformation; titanium materials with high yield strength and low elongation have poor plasticity, and stress concentration easily leads to instability and ribbing.

2.Excessive or fluctuating front and rear tension is the main cause, which should be controlled at 30%~40% of yield strength; poor lubrication and uneven coating generate axial force components; excessive single-pass reduction rate easily causes stress concentration, and too small final pass reduction rate makes it difficult to control flatness; unreasonable roll crown and shift amount lead to uneven transverse pressure and ribbing.

3.Insufficient roll roundness, parallelism and frame rigidity cause vibration and pressure fluctuation; poor coiling concentricity, deviation correction and tension response cause uneven tension and strip deviation; abnormalities in lubrication system nozzles, flow and filtration result in local lubrication failure.

• Strictly control billet quality, reduce inclusions and segregation through composite smelting; grind the surface before rolling, and control the roughness at Ra≤0.8μm. For thin-gauge products, adopt thick billets, multiple rolling passes and reduce single-pass deformation to alleviate work hardening.
• Adopt small and stable tension with fluctuation controlled at ±2%, and line thin-gauge coils with paper to prevent deviation; use special extreme-pressure rolling oil with segmented high-pressure uniform lubrication; adopt multi-pass small-deformation rolling with 6 or more passes, customize flatness parameters exclusive to titanium materials, and do not apply stainless steel processes.
• Calibrate rolls regularly and reinforce the frame to reduce vibration; upgrade high-precision coiling, deviation correction and tension equipment; maintain the lubrication system regularly, replace oil regularly and overhaul pipelines and nozzles.
• Install sensors to monitor tension, flatness and vibration in real time and perform closed-loop adjustment; establish a ribbing early warning model for automatic alarm and parameter adjustment in case of abnormality; keep full-process production, equipment and quality inspection data for traceability and process optimization.

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