Stretch-reducing mill having improved diameter tolerance and wall thickness tolerance

Abstract

A stretch-reducing mill ( 1 ) for producing seamless pipes (R), which comprises a plurality of roll stands ( 10 ), which are arranged one after the other in a conveying direction (F) of the pipes (R) and each have three rolls ( 11 ) arranged at an angular distance of 120°, wherein the roll stands ( 10 ) are divided into at least two groups (A, B) each having at least two roll stands ( 10 ); the rolls ( 11 ) of adjacent roll stands ( 10 ) within a group (A, B) are inclined relative to each other by an intra-group angle αI; and the rolls ( 11 ) of the roll stands ( 10 ) of adjacent groups (A, B) are inclined relative to each other by a group angle αG that is less than the intra-group angle αI.

1 . Stretch-reducing mill ( 1 ) for the production of seamless tubes (R), comprising:
 plural roll stands ( 10 ) arranged one behind the other in a conveying direction (F) of the tubes (R), each of said roll stands having three rolls ( 11 ) arranged at an angular distance of 120°,   wherein the roll stands ( 10 ) are divided into at least two groups (A, B) each including at least two roll stands ( 10 ), the rolls ( 11 ) of adjacent ones of the roll stands ( 10 ) within a group (A, B) are inclined relative to one another by an angle αI within the group, and   wherein the rolls ( 11 ) of the roll stands ( 10 ) of adjacent groups (A, B) are inclined relative to one another by a group angle αG which is smaller than the group-internal angle αI.   
     
     
         2 . Stretch-reducing mill ( 1 ) according to  claim 1 , characterized in that the group-internal angle αI is approximately 60°. 
     
     
         3 . Stretch reducing mill ( 1 ) according to  claim 1 , characterized in that the group angle αG is 60°/n, where n is an integer greater than 1. 
     
     
         4 . Stretch reducing mill ( 1 ) according to  claim 3 , characterized in that the group angle αG is approximately 30°. 
     
     
         5 . Stretch reducing mill ( 1 ) according to  claim 1 , characterized in that the rolls ( 11 ) of the roll stands ( 10 ) of the groups (A, B) have a caliber shape deviating from the circular shape. 
     
     
         6 . Stretch-reducing mill ( 1 ) according to  claim 1 , characterized in that between two groups (A, B) at least one neutral roll stand ( 10 ′) is provided, each with three rollers ( 11 ) arranged at an angular distance of 120°, their caliber shape counteracts a torsional moment acting on the tube (R). 
     
     
         7 . Stretch reducing mill ( 1 ) according to  claim 6 , characterized in that the caliber shape of the rollers ( 11 ) of the neutral roll stand ( 10 ′) is in the shape of a segment of a circle. 
     
     
         8 . Stretch reducing mill ( 1 ) according to  claim 1 , characterized in that 35% to 70% of the total diameter reduction of the tube (R) takes place in the first group (A) and the remaining diameter reduction in the second group (B). 
     
     
         9 . Stretch reducing mill ( 1 ) according to  claim 1 , characterized in that this is an extraction mill.