Analysis of cold-rolled ribbing defects in pure titanium strip
At present, the production of pure titanium stirp and sheet is mainly rolled by six-roller, ten-roller and other multi-roller mills such as twenty-roller. In titanium strips and sheets production technology is most advanced in Japan using twenty-roller mill for rolling, thickness 0.3 ~ 3mm, high production efficiency, dimensional accuracy, plate shape, surface quality is very good. However, in the actual production process, especially in the production process of large rolls of heavy wide thin strip, there are still quality problems such as ribbing, wave shape. Among them, the ribbing is the most serious, to the quality of the product as well as the benefits of the enterprise has brought a negative impact, is the urgent need to solve the product quality problems.
Cold-rolled titanium strip in the rolled into a coil, the surface of the strip around the local bulge, called ribbing. For pure titanium thin strip, ribbing mostly occurs in the thickness <0.8mm below specifications, the performance form is mostly single rib. The direct consequence of ribbing is to make the strip produce additional wave shape, so that the plate shape and surface quality is affected, resulting in product downgrading, and in serious cases, even to shear, split coil processing. Not only reduces the quality of the product, but also causes the waste of raw materials and reduces the production efficiency.
Rolling test found that the same specification of different batches of hot rolled coils after cold rolling the amount of ribbing and ribbing probability is different, indicating that the hot-rolled raw materials themselves have a greater impact on the cold rolling ribbing. In the hot-rolled incoming material, there are commonly scuffs, sickle bends, cracks and other defects, which have a certain impact on the generation of various defects that appear in the subsequent cold rolling process. Hot-rolled material local high point on the impact of cold-rolled strip, although only limited to the high point and a small range of nearby, but for very thin strip, enough to cause the strip local bulge "rib" or even the formation of local wave shape and bulge intertwined serious quality defects.
Through the same tension of different plate curve and different tension of the same plate curve and other cases of test-rolling found that in the same tension different plate curve settings, when the plate curve reference stainless steel strip settings, the probability of ribbing high, the plate curve settings will be adjusted after the test-rolling, the probability of ribbing and the amount of ribbing dropped significantly. Under the same plate shape curve with different tension settings, the probability of ribbing is higher in large tension rolling than in small tension rolling, but the difference between the probability of ribbing and the amount of ribbing between large and small tension rolling is not significant, so we know that the traditional stainless steel strip rolling with large tension is not applicable to the rolling of pure titanium strip. Through the analysis of the above test-rolling results, the tendons of this circumferential bulge, is a plate shape control, tension control and other factors acting together, from the mechanical point of view, the tendons are a result of the action of axial forces.
Although the titanium strip cold rolling rolling speed is very slow, but if the lubricant saponification value and other performance is not good or nozzle blockage, will lead to uneven lubrication and cause uneven stress distribution in the deformation zone thus generating axial force. In the rolling deformation area, the neutral surface offset and the axial force, this force may be small, but for the board to the center of the tightening has a certain impact. And rolling deformation process, local high point or local hardness will lead to uneven stress distribution in the deformation zone and produce axial parting force.
Equipment vibration and uneven tension interaction will produce axial parting force, winding the center of a small offset, uneven thickness and pore deviation between the layer and layer and so on the superposition effect will produce axial parting force.
Based on the field test and theoretical analysis, the mathematical model of the critical condition of the starting tendon is established according to the characteristics of the actual production. The critical stress of buckling instability is proportional to the fourth power of strip thickness and inversely proportional to the square of width. At the same time, the axial stress is most affected by three factors: front tension, friction coefficient and width-to-thickness ratio. Under the premise of constant width-to-thickness ratio, the occurrence of ribbing defects can be effectively suppressed by appropriately reducing the front tension, changing the rolling lubricant or lining the winding end with paper to increase the friction.
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