Seamless steel pipe, hollow spring utilizing seamless steel pipe, and process for manufacturing the same

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 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.   
   
   
       2 . The seamless steel pipe according to  claim 1 , wherein a metallographic structure of the steel pipe contains non-metallic inclusions, and a maximum thickness of the non-metallic inclusion in a direction perpendicular to a pipe axis is 50 μm or less. 
   
   
       3 . The seamless steel pipe according to  claim 1 , wherein an average roughness Ra of the inner periphery surface and the outer periphery surface of the steel pipe is 12.5 μm or less. 
   
   
       4 . The seamless steel pipe according to  claim 1 , wherein the steel material is spring steel. 
   
   
       5 . 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 according to  claim 4  and applying a surface treatment to the hollow body so that the hollow body has compressive residual stress. 
   
   
       6 . The hollow spring according to  claim 5 , wherein a lubricant prepared by adding graphite to oil is used in the hot isostatic extrusion step. 
   
   
       7 . The hollow spring according to  claim 5 , wherein a ratio t/D of a thickness t to an outer diameter D of a cross section of a hollow body in a shape of a coil or a bar or a bar with curved part is made 0.10 to 0.35. 
   
   
       8 . The hollow spring according to  claim 7 , wherein hardness Hv is 500 or more. 
   
   
       9 . The hollow spring according to  claim 7 , wherein at least an outer surface of the seamless spring steel pipe is ground. 
   
   
       10 . The hollow spring according to  claim 7 , wherein the surface treatment is applied to an outer surface and an inner surface of the hollow body. 
   
   
       11 . The hollow spring according to  claim 7 , wherein a cross-sectional outer contour and a cross-sectional inner contour of the hollow body in a shape of a coil or a cross-sectional outer contour and a cross-sectional inner contour of a curved part of the hollow body in a shape of a bar are concentric circles. 
   
   
       12 . The hollow spring according to  claim 7 , wherein a cross-sectional outer contour and a cross-sectional inner contour of the hollow body in a shape of a coil or a cross-sectional outer contour and a cross-sectional inner contour of a curved part of the hollow body in a shape of a bar are eccentric. 
   
   
       13 . The hollow spring according to  claim 5 , wherein at least a cross-sectional outer contour of the hollow body is in a non-circular shape. 
   
   
       14 . The hollow spring according to  claim 7 , wherein a depth of a contiguous flaw formed on at least one of an inner periphery surface and an outer periphery surface of the seamless spring steel pipe is 50 μm or less. 
   
   
       15 . The hollow spring according to  claim 7 , wherein a metallographic structure of the steel pipe contains non-metallic inclusions, and a maximum thickness of the non-metallic inclusion in a direction perpendicular to a pipe axis is 50 μm or less. 
   
   
       16 . The hollow spring according to  claim 7 , wherein an average surface roughness Ra of at least one of an inner periphery surface and an outer periphery surface of the seamless spring steel pipe is 12.5 μm or less. 
   
   
       17 . 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 work-piece;   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.   
   
   
       18 . The method according to  claim 17 , further comprising a straightening step in which bend of the pickled seamless steel pipe is straightened, after the pickling step. 
   
   
       19 . The method according to  claim 17 , further comprising a grinding step in which an inner periphery surface of the heated seamless steel pipe workpiece is ground, after the second heating step. 
   
   
       20 . The method according to  claim 18 , wherein an inner periphery surface of the seamless steel pipe is ground after the straightening step. 
   
   
       21 . The method according to  claim 17 , wherein the hot isostatic extrusion is performed at 1,050° C. or more and less than 1,300° C. 
   
   
       22 . 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 work-piece;   a slow-cooling step in which the seamless steel pipe workpiece that has been subjected to the hot isostatic extrusion processing is slowly cooled;   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.