Precision wire rolling mill stiffness
The stiffness of the precision cold rolling mill and the influence of the unavoidable gap on the mechanism are also important factors that directly affect the accuracy of the rolling stock. In recent years, my country has done a lot of work on improving the stiffness of the rolling mill of the finished stand and the front stand, and achieved obvious results. Because the stiffness of the front rolling mill is too poor, it is impossible to completely rely on the latter two rolling mills to eliminate the errors accumulated in the front. At present, the implementation of precision rolling abroad requires high stiffness for all rolling mills, including rough rolling mills. According to the results of experimental simulation in Japan, to implement precision rolling, if the stiffness of each stand reaches 2000KN/mm, the temperature of the furnace is 950℃±20℃, and the heating is uniform, tension-free rolling is performed. The dimensional accuracy of the large bar after making eight passes can be within ±0.1% of the diameter of the finished product. For this reason, in recent years, in the roughing mills of small rolling mills, such as compact rolling mills, single-pass rolls with large diameter and short roll body have been adopted to reduce the deflection of the rolls and improve the rigidity. Foreign countries still attach so much importance to improving the rigidity of roughing mills, which shows that it is not enough for us to only renovate the rigidity of two rolling mills. Some manufacturers in our country have the practice of transforming finishing mills L1, L2, L3 or a row of five frames, and the results have proved that it is very effective in improving the precision of finished products.
For the tandem medium-sized rolling mill, the first two stands can be considered to use rolling bearing open stands, and the latter finishing mill can be used with closed two-roll rolling bearing precision wire rolling mills, which can increase the rigidity of the wire rolling mill by more than 1-2 times. According to our calculation, the stiffness of the ∅500 two-roll closed rolling bearing rolling mill can reach 1300KN/mm when the roll body is 1150mm long. If the roll body is shortened, the rigidity can be further improved. Taking the high-rigidity stand of the ∅500 compact rolling mill as an example, because of its short roll body (single-pass rolling, the roll body is 320mm), and the rolling mill is a short stress line structure, its stiffness reaches 2456KN/mm. In short, improving the rigidity of the rolling mill is the first problem that should be solved in the implementation of precision rolling, and it is not difficult to solve.
The clearance of the precision strip rolling mill mechanism, especially the axial movement clearance of the finished product and the former stand, is also an important factor affecting the dimensional accuracy of the rolled piece. This gap includes the gap between the thrust bearing of the roll in the bearing seat, and also the gap between the bearing seat itself relative to the arch or other fixed supports of the rolling mill without arch. According to foreign research results, not only the clearance of the thrust bearing itself should be limited within 0.05mm, but also the axial clearance between the bearing seat and the arch should be reduced from the usual ±0.15mm to ±0.025mm. In order to achieve this purpose, a hydraulic wedge to fasten the side pressure plate is developed, so that the bearing seat can be firmly fixed on the machine base during rolling, and will not be stuck when the pressure is adjusted.
For short stress lines, high stiffness, prestressed mills and other non-pavilion strip rolling mills, the problem of axial clearance should also require stricter control to achieve the expected rolling accuracy.
