Page 24 - ITATUBE Journal 1 2020

Technical Papers
Nataliia Kvitka, Nataliia Koriaka
Roll calibers for plugless rolling of pipes
Nataliia Kvitka, M.Sc.
Nataliia Koriaka, ITA Representative in CIS countries
The process of rolling pipes without mandrels (calibration or reduction) is the final operation in all technological manufacturing lines of hot-rolled seamless pipes. The parameters of metal forming in the case of continuous longitudinal rolling of pipes without mandrel largely determine the characteristics of quality and accuracy of the finished product. In the processes of reduction and calibrating, oval-shaped calibers are preferably used, the profile of the working area of which is formed by one or more forming radius . Characteristics of a profile of one or another caliber (fig. 1, fig. 2) are defined by the number of radiuses , their sizes, values of eccentricities of those radiuses in relation to the center of the caliber (rolling axis) and ovality of the caliber (here, , – height and width of the caliber).
The most common are single-radius calibers (Fig. 1), which are used in industrial setups for almost 80 years. These calibers are relatively easy to manufacture. They allow to obtain pipes that until recently fully met the requirements of existing standards and specifications.
Based on the requirements of the new specifications of the world's largest consumers of steel pipes (Shell, Shevron, Exonn Mobil, ADCO, Saudi ARMCO, LUCOIL, GAZPROM, ROSNEFT etc.) requirements for product quality and accuracy are constantly increasing. Therefore, there is an urgent need for improvements in production technologies. One of the ways to solve this problem in terms of improving the accuracy of the pipe is to find rational profiles of calibers for rolling.
In the patent [1] a three-radius caliber roll for longitudinal rolling is proposed, which is presented in Fig. 2. A characteristic feature of this caliber is the requirement according to which the eccentricities and must only gain positive values. However, there are studies [2, etc.], according to which when 0 deformation conditions help to reduce the transverse wall thick- ness variation of the pipes and to increase their accuracy.
In works [3, 4] a mathematical model was developed, which analyzed the influence of caliber parameters on the calculated transverse wall thickness variations of pipes rolled in a 24- stand reduction mill along the route 119х5 mm 42.5х5 mm.
Fig. 3 represents the calculated values of the resulting wall thickness variations ,
which is accumulated when rolling the pipe from the first stand of the mill to the stand with the current number when using one-radius caliber (curve 1) and three-radius calibers, the parameters of which are calculated using the condition 0 (curve 2) and without it (curve 3)
with the same distribution of partial deformations between the stands of the mill (curve 4). As
follows from the data shown in Fig. 3 when rolling using single-radius calibers the resulting final relative wall thickness variation equals 10.45%. If using three-radius calibers
8.98% (if 0), and 4.67% (if is not constrained).
This example shows that when designing three-radius calibers under the condition to
have eccentricity < 0, the level of relative wall thickness variation of the finished pipes can be significantly reduced (1.9...2.2 times), that is, to increase tubes’ accuracy.
Reduced value of the wall thickness variations when using the proposed three-radius calibers can be explained by the characteristics of their shape, which is due to the fact that the eccentricity of the radius of the caliber is negative ( 0). Because of that in a stand
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Roll calibers for plugless rolling of pipes
ITAtube Journal No1/July 2020
24