The present invention relates to stretch reducing pipes by means of a multistand, three-rolls-per-stand, stretch reducing rolling mill, particularly for avoiding the formation of a polygonal internal cross-section during stretch reducing of the pipes.
The formation of a polygonal inner cross-section during stretch-reducing rolling of pipes has been avoided by a staggered arrangement of the sets of rolls around the rolling axis so that the discontinuities resulting from the design are not located in common planes (see e.g. German Pat. No. 675,180). However, the rolls are very difficult to drive in such an arrangement and their exchange is cumbersome so that this particular proposal is not an economical one.
Another proposal (printed patent application No. S 25 445) has been made according to which groups of stands are provided which are phase shifted by 45.degree. in a two-roll mill or by 30.degree. in a three-roll mill. However, the same as regards economics can be said also here. Nevertheless, it has been recognized that a polygon with more sides than four (in the case of a two-roll mill) or even with more sides than six (in the case of a three-roll mill) is still an improvement and a better approximation of a circle.
In accordance with German Pat. No. 940,524, it has been suggested to prevent the formation of internal polygons by using rolls having a curvature of their respective grooves which have an asymmetric disposition to the center plane of the rolls.
The mill uses such asymmetrically designed rolls together with symmetric ones. Such an arrangement, however, has the disadvantage that the stands having rolls with asymmetrically designed grooves tend to rotate the pipe while the stands with symmetric rolls tend to prevent such a rotation. Also the difference in diameter of the rolls is larger for the asymmetric than for the symmetric configuration. It follows from the latter that the stand with asymmetric grooves of its rolls experiences larger slippage between pipe and rolls, and the latter wear more rapidly. Also, the use of two types of rolls is not economical from the standpoint of inventory. Moreover, the losses upon cutting two different types of grooves will be larger than in the case of using exclusively one type of grooves into the rolls.
Other proposals for avoiding the formation of polygons refer to rotating the pipe during stretch reducing. The German Pat. No. 675,180 and petty Pat. No. 1,807,019, disclose skewed rolls for that purpose; German Pat. No. 1,059,865 proposes asymmetric groove shapes but arranged in the same sense as far as asymmetry is concerned.
However, any turning tends to damage the surface particularly of large pipes or other hollow stock. Also, it is difficult to properly guide the rotating pipe as it leaves the mill. A skewed position requires, of course, rather complicated constructions of the mill while asymmetrically contoured rolls wear more rapidly because the differences in diameter are larger.
Still other proposals for avoiding the formation of polygons use symmetrically contoured rolls and one selects the reduction from stand to stand so that the squeezed surface in each instance is a rectangle. This, however, has the following disadvantage.
The ratio of the large design radius a; to the small design radius b; alternates between large and small values for the usual reduction from stand to stand. Thus, the contour alternates between somewhat oval (large ratio) contour and almost round (small ratio) contour. Consequently, the material experiences alternating bending at the roll gap which tends to form cracks and hair seams.