High-strength seamless stainless steel tube for oil country tubular goods and method of manufacturing the same

Abstract

A steel material has a chemical composition containing, by mass %, C: 0.005% or more and 0.06% or less, Si: 0.05% or more and 0.5% or less, Mn: 0.2% or more and 1.8% or less, Cr: 15.5% or more and 18.0% or less, Ni: 1.5% or more and 5.0% or less, V: 0.02% or more and 0.2% or less, Al: 0.002% or more and 0.05% or less, N: 0.01% or more and 0.15% or less, O: 0.006% or less, and further contains one or more of Mo: 1.0% or more and 3.5% or less, W: 3.0% or less and Cu: 3.5% or less, in which Cr+0.65Ni+0.60Mo+0.30W+0.55Cu-20C≧19.5 and Cr+Mo+0.50W+0.30Si-43.5C-0.4Mn—Ni-0.3Cu-9N≧11.5 are satisfied, is made into a seamless steel tube by performing heating and hot rolling.

1 .- 6 . (canceled) 
     
     
         7 . A method of manufacturing a high-strength seamless stainless steel tube for oil country tubular goods having a wall thickness of more than 25.4 mm, comprising:
 heating a steel material;   hot rolling including piercing rolling the steel material into a seamless steel tube; and   cooling the seamless steel tube to room temperature at a cooling rate equal to or more than an air-cooling rate, the steel material having a chemical composition containing, by mass %,   C: 0.005% or more and 0.06% or less, Si: 0.05% or more and 0.5% or less,   Mn: 0.2% or more and 1.8% or less, P: 0.03% or less,   S: 0.005% or less, Cr: 15.5% or more and 18.0% or less,   Ni: 1.5% or more and 5.0% or less, V: 0.02% or more and 0.2% or less,   Al: 0.002% or more and 0.05% or less, N: 0.01% or more and 0.15% or less, O: 0.006% or less, and   further containing one, two or more selected from among Mo: 1.0% or more and 3.5% or less, W: 3.0% or less and Cu: 3.5% or less and the balance being Fe and inevitable impurities, wherein expressions (1) and (2) are satisfied, the hot rolling including piercing rolling is performed under conditions such that the total rolling reduction in a temperature range of 1100° C. to 900° C. is 30% or more, and after the rolled steel tube is cooled down to the room temperature, quenching-tempering or tempering is performed:
   Cr+0.65Ni+0.60Mo+0.30W+0.55Cu-20C≧19.5  (1),
 
   Cr+Mo+0.50W+0.30Si-43.5C-0.4Mn—Ni-0.3Cu-9N≧11.5  (2),
 
   where Cr, Mo, W, Si, C, Mn, Ni, Cu and N: contents (mass %) of chemical elements respectively represented by corresponding atomic symbols.   
     
     
         8 . The method according to  claim 7 , wherein the chemical composition further contains, by mass %, one or more selected from among Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less and B: 0.01% or less. 
     
     
         9 . The method according to  claim 7 , wherein the chemical composition further contains, by mass %, Ca: 0.01% or less. 
     
     
         10 . A high-strength seamless stainless steel tube for oil country tubular goods having a wall thickness of more than 25.4 mm, the steel tube having a chemical composition containing, by mass %,
 C: 0.005% or more and 0.06% or less, Si: 0.05% or more and 0.5% or less,   Mn: 0.2% or more and 1.8% or less, P: 0.03% or less,   S: 0.005% or less, Cr: 15.5% or more and 18.0% or less,   Ni: 1.5% or more and 5.0% or less, V: 0.02% or more and 0.2% or less,   Al: 0.002% or more and 0.05% or less, N: 0.01% or more and 0.15% or less, O: 0.006% or less, and   further containing one, two or more selected from among Mo: 1.0% or more and 3.5% or less, W: 3.0% or less and Cu: 3.5% or less and the balance being Fe and inevitable impurities, in which expressions (1) and (2) are satisfied, having a microstructure including a martensite phase as a main phase and a second phase consisting of at volume ratio, 10% or more and 60% or less of a ferrite phase and 0% or more and 10% or less of an austenite phase, in which a GSI value, which is defined as a number of ferrite-martensite grain boundaries per unit length of a line segment drawn in a wall thickness direction, is 120 or more in a central portion in the wall thickness direction, and having excellent low-temperature toughness and excellent corrosion resistance:
   Cr+0.65Ni+0.60Mo+0.30W+0.55Cu-20C≧19.5  (1),
 
   Cr+Mo+0.50W+0.30Si-43.5C-0.4Mn—Ni-0.3Cu-9N≧11.5  (2),
 
   where Cr, Mo, W, Si, C, Mn, Ni, Cu and N: contents (mass %) of chemical elements respectively represented by corresponding atomic symbols.   
     
     
         11 . The high-strength seamless stainless steel tube according to  claim 10 , wherein the chemical composition further contains, by mass %, one or more selected from among Nb: 0.2% or less, Ti: 0.3% or less, Zr: 0.2% or less and B: 0.01% or less. 
     
     
         12 . The high-strength seamless stainless steel tube according to  claim 10 , wherein the chemical composition further contains, by mass %, Ca: 0.01% or less. 
     
     
         13 . The method according to  claim 8 , wherein the chemical composition further contains, by mass %, Ca: 0.01% or less. 
     
     
         14 . The high-strength seamless stainless steel tube according to  claim 11 , wherein the chemical composition further contains, by mass %, Ca: 0.01% or less.