High-carbon steel tube having superior cold workability, machinability, and hardenability and method for manufacturing the same

Abstract

A raw steel tube has a composition containing, by mass, 0.25% to 0.60% carbon, 0.01% to 2.0% silicon, 0.2% to 3.0% manganese, 0.001% to 0.1% aluminum, 0.001% to 0.05% phosphorus, 0.02% or less sulfur, 0.0010% to 0.0100% nitrogen, 0.0003% to 0.0050% boron, and 0.0001% to 0.0050% calcium, the balance being iron and incidental impurities. The raw steel tube is heated to and soaked at an Ac 3 transformation point or higher and is then subjected to stretch-reducing rolling at a finishing temperature of rolling of 900° C. to (Ac 1 transformation point) with a cumulative reduction in diameter of 30% to 70% within the temperature range of 900° C. or lower.

1 - 7 . (canceled) 
     
     
         8 . A high-carbon steel tube having a composition comprising, by mass:
 0.25% to 0.60% carbon,   0.01% to 2.0% silicon,   0.2% to 3.0% manganese,   0.001% to 0.1% aluminum,   0.001% to 0.05% phosphorus,   0.0001% to 0.02% sulfur,   0.0010% to 0.0100% nitrogen,   0.0003% to 0.0050% boron, and   0.0001% to 0.0050% calcium,   
       the balance being iron and incidental impurities, the high-carbon steel tube having a microstructure comprising a ferrite base phase and cementite grains finely dispersed in the base phase, wherein an average grain size d of the cementite grains is 0.1 to less than 0.5 □m, and an average distance L between surfaces of adjacent cementite grains is 0.5 to 10 □m. 
     
     
         9 . The high-carbon steel tube according to  claim 8 , wherein the composition further comprises, by mass, at least one selected from 2.0% or less copper, 2.0% or less chromium, 2.0% or less molybdenum, 2.0% or less tungsten, 1.0% or less vanadium, and 0.1% or less niobium. 
     
     
         10 . The high-carbon steel tube according to  claim 8 , wherein the composition further comprises, by mass, 0.1% or less titanium. 
     
     
         11 . A method of manufacturing a high-carbon steel tube comprising:
 providing a high-carbon steel tube as a raw steel tube;   heating and soaking the raw steel tube; and   subjecting the raw steel tube to stretch-reducing rolling into a product steel tube, the raw steel tube being a steel tube having a composition comprising, by mass:   0.25% to 0.60% carbon,   0.01% to 2.0% silicon,   0.2% to 3.0% manganese,   0.001% to 0.1% aluminum,   0.001% to 0.05% phosphorus,   0.0001% to 0.02% sulfur,   0.0010% to 0.0100% nitrogen,   0.0003% to 0.0050% boron, and   0.0001% to 0.0050% calcium,   
       the balance being iron and incidental impurities, wherein the raw steel tube is heated to and soaked at an Ac3 transformation point or higher and is then subjected to stretch-reducing rolling at a finishing temperature of rolling of 900° C. to (Ac1 transformation point) with a cumulative reduction in diameter of 30% to 70% within a temperature range of 900° C. or lower. 
     
     
         12 . The method according to  claim 11 , wherein the high-carbon steel tube is a high-carbon electric-resistance-welded steel tube formed by a tube-making process including continuously roll-forming a high-carbon steel strip having the composition into a substantially cylindrical open pipe and joining together ends of the open pipe by electric resistance welding. 
     
     
         13 . The method according to  claim 11 , wherein the composition further comprises, by mass, at least one selected from 2.0% or less copper, 2.0% or less chromium, 2.0% or less molybdenum, 2.0% or less tungsten, 1.0% or less vanadium, and 0.1% or less niobium. 
     
     
         14 . The method according to  claim 11 , wherein the composition further comprises, by mass, 0.1% or less titanium. 
     
     
         15 . The high-carbon steel tube according to  claim 9 , wherein the composition further comprises, by mass, 0.1% or less titanium. 
     
     
         16 . The method according to  claim 12 , wherein the composition further comprises, by mass, at least one selected from 2.0% or less copper, 2.0% or less chromium, 2.0% or less molybdenum, 2.0% or less tungsten, 1.0% or less vanadium, and 0.1% or less niobium. 
     
     
         17 . The method according to  claim 12 , wherein the composition further comprises, by mass, 0.1% or less titanium. 
     
     
         18 . The method according to  claim 13 , wherein the composition further comprises, by mass, 0.1% or less titanium. 
     
     
         19 . The method according to  claim 16 , wherein the composition further comprises, by mass, 0.1% or less titanium.