Method for producing seamless metallic tube by cold rolling

Abstract

In cold rolling of a seamless metallic tube with a cold pilger mill, when a reduction ratio of an outside diameter becomes excessive in comparison to a reduction ratio of a wall thickness, strain in a circumferential direction on flange regions becomes excessive. As a result, compressive stress in a circumferential direction becomes excessive, so that wrinkle imperfections are generated inside a tube and folded on groove bottom regions to become folded defects. This process is repeated to be developed into inside defects. When a hollow shell is produced by the Mannesmann-mandrel process instead of the extrusion process, inside wrinkle imperfections are generated with a sizing mill (a stretch reducer or a sizer) at a stage of hollow shell. Since the inside wrinkle imperfections further grow in cold rolling, it is necessary to pay attention thereto. In consideration of these, (a reduction ratio of an outside diameter/a reduction ratio of a wall thickness both in a cold pilger mill) is set to not more than 1/2. It is, thereby, possible to obtain a high-quality seamless metallic tube, inside defects thereof being suppressed, produced by cold rolling.

1 . A method for producing a seamless metallic tube by cold rolling with a cold pilger mill, comprising the steps of:
 when elongating a hollow shell in such a manner that an outside diameter thereof is reduced while reducing a wall thickness thereof, selectively using a pair of rolls and a tapered mandrel bar according to outside diameters and wall thicknesses of the hollow shell and a rolled tube as a product, the rolls each having a tapered groove caliber which decreases gradually in diameter from an engaging entry side of roll toward a finishing exit side thereof, the tapered mandrel bar decreasing also gradually in diameter from an engaging entry side toward an finishing exit side; and, setting a reduction rate Rd of outside diameter to not more than one-half of a reduction rate Rt of wall thickness, wherein   Rd={1-(d/do)}×100 (%)   Rt={1-(t/to)}×100 (%)   do: outside diameter of hollow shell   d: finishing outside diameter   to: wall thickness of hollow shell   t: finishing wall thickness.   
     
     
         2 . The method for producing a seamless metallic tube by cold rolling according to  claim 1 , wherein
 used is a hollow shell made by a hot reducing mill process with a stretch reducer under the condition that a reduction rate of outside diameter is not more than 77%.   
     
     
         3 . The method for producing a seamless metallic tube by cold rolling according to  claim 1 , wherein
 used is a hollow shell made by a hot reducing mill process with a sizer under the condition that a reduction rate of outside diameter is not more than 33%.