Super fine granular steel pipe and method for producing the same

Abstract

A steel pipe containing fine ferrite crystal grains, which has excellent toughness and ductility and good ductility-strength balance as well as superior collision impact resistance, and a method for producing the same are provided. A steel pipe containing super-fine crystal grains can be produced by heating a base steel pipe having ferrite grains with an average crystal diameter of di (μm), in which C, Si, Mn and Al are limited within proper ranges, and if necessary, Cu, Ni, Cr and Mo, or Nb, Ti, V, B, etc. are further added, at not higher than the Ac 3 transformation point, and applying reducing at an average rolling temperature of θm (° C.) and a total reduction ratio Tred (%) within s temperature range of from 400 to Ac 3 transformation point, with di, θm and Tred being in a relation satisfying a prescribed equation.

What is claimed is:  
     
         1 . A super fine granular steel pipe with high collision impact property and high workability having a composition containing, by weight, 0.005 to 0.3%C, 0.01 to 3.0%Si, 0.01 to 2.0%Mn, 0.001 to 0.10%Al, and balance Fe with unavoidable impurities, and a cross section perpendicular to a longitudinal direction of the steel pipe after reducing contains super fine grains of a ferrite having an average crystal grain size of 3 μm or less, and an absorbed energy up to a strain rate of 30% by performing high speed tensile tests at a strain rate of 2000s−1 is 202MJ/m 3  or more, 
 which is obtained in a method for producing a steel pipe, comprising heating or soaking a base steel pipe having an outer diameter of ODi (mm) and having ferrite grains with an average crystal diameter of di (μm) in the cross section perpendicular to the longitudinal direction of the steel pipe, and then applying reducing at an average rolling temperature of θm(° C.) and a total reduction ratio Tred(%) to obtain a product pipe having an outer diameter of ODf (mm),  
 wherein, said reducing comprises performing it in a temperature range of 400° C. or more but not more than the heating or soaking temperature, and in such a manner that said average crystal diameter of di (μm), said average rolling temperature of θm(° C.), and said total reduction ratio Tred (%) are in a relation satisfying equation (1) as follows:  
   di ≦(2.65−0.003×θ m )×10 ((0.008+θm/50000)×Tred)   (1)  
 wherein, di represents the average crystal diameter of the base steel pipe (μm); θm represents the average rolling temperature (° C.) (=(θi+θf)/2, wherein θi is a temperature of starting rolling (° C.), and θf is a temperature of finishing rolling (° C.)); and Tred represents a total reduction ratio (%) (=ODi−ODf)×100/ODi, where, ODi is an outer diameter of a product pipe (mm)).  
 
     
     
         2 . A super fine granular steel pipe as claimed in  claim 1 , further containing one or more selected from a group consisting of 
 1% or less of Cu,    2% or less of Ni,    2% or less of Cr,    1% or less of Mo,    or furthermore one or more selected from a group consisting of    0.1% or less of Nb,    0.5% or less of V,    0.2% or less of Ti,    0.005% or less of B,    or furthermore one or more selected from a group consisting of    0.02% or less or REM,    0.01% or less of Ca.    
     
     
         3 . A super fine granular steel pipe with high resistance against sulfide stress corrosion crack and high workability having a composition containing, by weight, 0.005 to 0.1%C, 0.01 to 0.5%Si, 0.01 to 1.8%Mn, 0.001 to 0.10%Al, and balance Fe with unavoidable impurities, and a cross section perpendicular to a longitudinal direction of the steel pipe after reducing contains super fine grains of a ferrite having an average crystal grain size of 3 μm or less, and in a test that a tensile stress corresponding to 120% of yield strength is applied to a C-ring test specimen in an NACE bath, no cracks generate during a test period of 200 hr, 
 which is obtained in a method for producing a steel pipe, comprising heating or soaking a base steel pipe having an outer diameter of ODi (mm) and having ferrite grains with an average crystal diameter of di (μm) in the cross section perpendicular to the longitudinal direction of the steel pipe, and then applying reducing at an average rolling temperature of θm(° C.) and a total reduction ratio Tred(%) to obtain a product pipe having an outer diameter of ODf (mm),  
 wherein, said reducing comprises performing it in a temperature range of 400° C. or more but not more than the heating or soaking temperature, and in such a manner that said average crystal diameter of di (μm), said average rolling temperature of θm(° C.), and said total reduction ratio Tred (%) are in a relation satisfying equation (1) as follows:  
   di ≦(2.65−0.003×θ m )×10 ((0.008+θm/50000)×Tred)   (1)  
 wherein, di represents the average crystal diameter of the base steel pipe (μm); θm represents the average rolling temperature (° C.) (=(θi+θf)/2, wherein θi is a temperature of starting rolling (° C.), and θf is a temperature of finishing rolling (° C.)); and Tred represents a total reduction ratio (%) (=ODi−ODf)×100/ODi, where, ODi is an outer diameter of a product pipe (mm)).  
 
     
     
         4 . A super fine granular steel pipe as claimed in  claim 3 , further containing one or more selected from a group consisting of 
 0.5% or less of Cu,    0.5% or less of Ni,    0.5% or less of Cr,    0.5% or less of Mo,    or furthermore one or more selected from a group consisting of    0.1% or less of Nb,    0.1% or less of V,    0.1% or less of Ti,    0.004% or less of B,    or furthermore one or more selected from a group consisting of    0.02% or less or REM,    0.01% or less of Ca.    
     
     
         5 . A super fine granular steel pipe with high fatigue resistance property and high workability having a composition containing, by weight, 0.06 to 0.30%C, 0.01 to 1.5%Si, 0.01 to 2.0%Mn, 0.001 to 0.10%Al, and balance Fe with unavoidable impurities, and a cross section perpendicular to a longitudinal direction of the steel pipe after reducing contains super fine grains of a ferrite having an average crystal grain size of 3 μm or less, and a fatigue strength at a load stress for 10 6  endurance cycles is not less than 220 Mpa in a cantilever type oscillation fatigue test, 
 which is obtained in a method for producing a steel pipe, comprising heating or soaking a base steel pipe having an outer diameter of ODi (mm) and having ferrite grains with an average crystal diameter of di (μm) in the cross section perpendicular to the longitudinal direction of the steel pipe, and then applying reducing at an average rolling temperature of θm(° C.) and a total reduction ratio Tred(%) to obtain a product pipe having an outer diameter of ODf (mm),  
 wherein, said reducing comprises performing it in a temperature range of 400° C. or more but not more than the heating or soaking temperature, and in such a manner that said average crystal diameter of di (μm), said average rolling temperature of θm(° C.), and said total reduction ratio Tred (%) are in a relation satisfying equation (1) as follows:  
   di ≦(2.65−0.003×θ m )×10 ((0.008+θm/ 50000)×Tred)  (1)  
 wherein, di represents the average crystal diameter of the base steel pipe (μm); θm represents the average rolling temperature (° C.) (=(θi+θf)/2, wherein θi is a temperature of starting rolling (° C.), and θf is a temperature of finishing rolling (° C.)); and Tred represents a total reduction ratio (%) (=ODi−ODf)×100/ODi, where, ODi is an outer diameter of a product pipe (mm)).  
 
     
     
         6 . A super fine granular high carbon steel pipe as claimed in  claim 5 , further containing one or more selected from a group consisting of 
 1% or less of Cu,    2% or less of Ni,    2% or less of Cr,    1% or less of Mo,    or furthermore one or more selected from a group consisting of    0.1% or less of Nb,    0.5% or less of V,    0.2% or less of Ti,    0.005% or less of B,    or furthermore one or more selected from a group consisting of    0.02% or less or REM,    0.01% or less of Ca.