Process for manufacturing seamless steel pipe, hollow spring utilizing seamless steel pipe

Abstract

There are provided: a seamless steel pipe formed from a cylindrical steel material billet through a hot isostatic extrusion step, wherein a depth of a contiguous flaw formed on an inner periphery surface and an outer periphery surface of the steel pipe is 50 μm or less; a hollow spring obtained by forming a hollow body in a shape of a coil or a bar or a bar with curved part from the seamless steel pipe made of spring steel and applying a surface treatment to the hollow body so that the hollow body has compressive residual stress; and a method for producing seamless steel pipe including: a billet molding step; a first heating step; a hot isostatic extrusion step; a second heating step; an extension step; a third heating step; and a pickling step.

1 . A method for producing seamless steel pipe comprising:
 a billet molding step in which a steel material is molded into a cylindrical billet;   a first heating step in which the billet is heated;   a hot isostatic extrusion step in which the heated billet is subjected to hot isostatic extrusion processing to obtain a seamless steel pipe workpiece;   a second heating step in which the seamless steel pipe workpiece is heated;   an extension step in which the heated seamless steel pipe workpiece is extended by at least one of Pilger mill rolling and drawing processing;   a third heating step in which the extended seamless steel pipe workpiece is heated; and   a pickling step in which the extended and heated seamless steel pipe workpiece is pickled.   
     
     
         2 . The method according to  claim 1 , wherein the second heating step includes heating the seamless steel pipe workpiece to 650 to 750° C. 
     
     
         3 . The method according to  claim 2 , wherein the second heating step includes heating the seamless steel pipe workpiece for 0.1 to 1 hour.