Mandrel mill, operating method of the same and production method of seamless pipe

Abstract

The object of the present invention is to provide a mandrel mill capable of producing a seamless pipe without flaws such as necking. The mandrel mill of the present invention includes a plurality of consecutively provided grooved-roll stands and a mandrel bar 3 provided in a roll groove array formed by the grooved-roll stands so as to continuously elongate a hollow shell 2. Roll grooves 4 of at least a first stand and a second stand are arranged so as to satisfy conditions represented by the following formulas (1) to (4): b ≦10 a +1.5   (1) b ≧10 a −0.2   (2) b≦−a +1.5   (3) b≧−a +1.15   (4) wherein a=S/R 1, b=C/(R 1 −S).

1 . A mandrel mill comprising a plurality of consecutively provided grooved-roll stands and a mandrel bar provided in a roll groove array formed by the grooved-roll stands so as to continuously elongate a hollow shell, characterized in that roll grooves of at least a first stand and a second stand are arranged so as to satisfy conditions represented by the following formulas (1) to (4):
     b≦ 10 a+ 1.5   (1)       b≧ 10 a− 0.2   (2)       b≦−a+ 1.5   (3)       b≧−a+ 1.15   (4)   wherein “a” is a ratio of S/R 1  between a groove bottom curvature radius R 1  of each grooved-roll and a displacement-offset-amount S between a groove bottom curvature center and a mill center, and “b” is ellipticity.   
   
   
       2 . An operating method of a mandrel mill including a plurality of consecutively provided grooved-roll stands and a mandrel bar provided in a roll groove array formed by the grooved-roll stands, characterized in that a hollow shell is continuously elongated when a groove bottom curvature radius R 1 , a displacement-offset-amount S, and ellipticity of each of grooved-rolls satisfy conditions represented by the following formulas (1) to (4) in at least a first stand and a second stand, whereby a hole occurrence and flaw generation are prevented:
     b≦ 10 a+ 1.5   (1)       b≧ 10 a− 0.2   (2)       b≦−a+ 1.5   (3)       b≧−a+ 1.15   (4)   wherein “a” is a ratio of S/R 1  between a groove bottom curvature radius R 1  of each grooved-roll and a displacement-offset-amount S between a groove bottom curvature center and a mill center, and “b” is ellipticity.   
   
   
       3 . A production method of a seamless pipe using a mandrel mill including a plurality of consecutively provided grooved-roll stands and a mandrel bar provided in a roll groove array formed by the grooved-roll stands, characterized in that a hollow shell is continuously elongated when a groove bottom curvature radius R 1 , a displacement-offset-amount S, and ellipticity of each of grooved-rolls satisfy conditions represented by the following formulas (1) to (4) in at least a first stand and a second stand:
     b≦ 10 a+ 1.5   (1)       b≧ 10 a− 0.2   (2)       b≦−a+ 1.5   (3)       b≧−a+ 1.15   (4)   wherein “a” is a ratio of S/R 1  between a groove bottom curvature radius R 1  of each grooved-roll and a displacement-offset-amount S between a groove bottom curvature center and a mill center, and “b” is ellipticity.