Stainless-steel pipe for oil well and process for producing the same

Abstract

A steel composition contains: 0.05% or less of C; 0.5% or less of Si; 0.20% to 1.80% of Mn; 0.03% or less of P; 0.005% or less of S; 14.0% to 18.0% of Cr; 5.0% to 8.0% of Ni; 1.5% to 3.5% of Mo; 0.5% to 3.5% of Cu; 0.05% or less of Al; 0.20% or less of V; 0.01% to 0.15% of N; and 0.006% or less of O on a mass basis, and satisfies the following expressions: Cr+0.65Ni+0.6Mo+0.55Cu−20C≧18.5 and Cr+Mo+0.3Si−43.5C−0.4Mn−Ni−0.3Cu−9N≦11 (where Cr, Ni, Mo, Cu, C, Si, Mn, and N represent their respective contents (mass %)). After such a steel pipe material is formed into a steel pipe, the steel pipe is quenched by cooling after heating to a temperature of the A C3 transformation point or more and tempered at a temperature of the A C1 transformation point or less. The composition may further contain at least one element of Nb and Ti; at least one element selected from the group consisting of Zr, B, and W; or Ca, singly or in combination. Preferably, the steel pipe has a martensitic structure containing 5 to 25 percent by volume of a residual austenite phase, or further containing 5% percent by volume or less of a ferrite phase. Thus, the resulting stainless steel pipe for oil country tubular goods exhibits a superior corrosion resistance even in extremely severe, corrosive environments containing carbon dioxide gas (CO 2 ), chloride ions (Cl − ), or the like.

1 . A corrosion-resistant stainless steel pipe for oil country tubular goods having a steel composition comprising on a mass basis: 
 0.05% or less of C;    0.50% or less of Si;    0.20% to 1.80% of Mn;    0.03 or less of P;    0.005% or less of S;    14.0% to 18.0% of Cr;    5.0% to 8.0% of Ni;    1.5% to 3.5% of Mo;    0.5% to 3.5% of Cu;    0.05% or less of Al;    0.20% or less of V;    0.01% to 0.15% of N;    0.006% or less of O, and    the balance being Fe and incidental impurities,    wherein the composition satisfies expressions (1) and (2):    Cr+0.65Ni+0.6Mo+0.55Cu+20C≧18.5  (1); Cr+Mo+0.3Si−43.5C−0.4Mn−Ni−0.3Cu−9N≦11  (2),    where Cr, Ni, Mo, Cu, C, Si, Mn, and N represent the respective contents thereof on a mass % basis.    
     
     
         2 . A stainless steel pipe for oil country tubular goods according to  claim 1 , wherein the composition further comprises at least one element selected from the group consisting of 0.20% or less of Nb and 0.30% or less of Ti on a mass basis.  
     
     
         3 . A stainless steel pipe for oil country tubular goods according to  claim 1  or  2 , wherein the composition further comprises at least one element selected from the group consisting of 0.20% or less of Zr, 0.01% or less of B, and 3.0% or less of W on a mass basis.  
     
     
         4 . A stainless steel pipe for oil country tubular goods according to claims  1 , wherein the composition further comprises 0.0005% to 0.01% of Ca on a mass basis.  
     
     
         5 . A stainless steel pipe for oil country tubular goods according to claims  1 , wherein the structure thereof includes 5 to 25 percent by volume of a residual austenite phase and the balance being a martensite phase.  
     
     
         6 . A stainless steel pipe for oil country tubular goods according to claims  1 , wherein the structure thereof includes, on a volume basis, 5 to 25 percent by volume of a residual austenite phase, 5 percent by volume or less of a ferrite phase, and the balance being a martensite phase.  
     
     
         7 . (A method for manufacturing a corrosion-resistant stainless steel pipe for oil country tubular goods comprising the steps of: forming a steel pipe from a steel pipe material having a composition; quenching the steel pipe by heating the steel pipe to a temperature of the A C3  transformation point thereof or more and subsequently cooling to room temperature at air-cooling speed or more; and then tempering the steel pipe at a temperature of the A C1  transformation point thereof or less, wherein the composition comprises on a mass basis: 
 0.05% or less of C;    0.50% or less of Si;    0.20% to 1.80% of Mn;    0.03 or less of P;    0.005% or less of S;    14.0% to 18.0% of Cr;    5.0% to 8.0% of Ni;    1.5% to 3.5% of Mo;    0.5% to 3.5% of Cu;    0.05% or less of Al;    0.20% or less of V;    0.01% to 0.15% of N;    0.006% or less of O, and    the balance being Fe and incidental impurities, wherein the composition satisfies expressions (1) and (2):    Cr+0.65Ni+0.6Mo+0.55Cu+20C≧18.5  (1);    Cr+Mo+0.3Si−43.5C−0.4Mn−Ni−0.3Cu−9N≦11  (2),    where Cr, Ni, Mo, Cu, C, Si, Mn, and N represent the respective contents thereof on a mass % basis.    
     
     
         8 . A method for manufacturing a stainless steel pipe for oil country tubular goods according to  claim 7 , wherein the composition further comprises at least one element of 0.20% or less of Nb and 0.30% or less of Ti on a mass basis.  
     
     
         9 . A method for manufacturing a stainless steel pipe for oil country tubular goods according to  claim 8 , wherein the quenching includes heating to a temperature in the range of 800 to 1100° C. and cooling to room temperature at air-cooling speed or more, and the tempering is performed at a temperature in the range of 500 to 630° C.  
     
     
         10 . A method for manufacturing a stainless steel pipe for oil country tubular goods according to any one of  claims 7  to  9 , wherein the composition further comprises at least one element selected from the group consisting of 0.20% or less of Zr, 0.01% or less of B, and 3.0% or less of W on a mass basis.  
     
     
         11 . A method for manufacturing a stainless steel pipe for oil country tubular goods according to claims  7 , wherein the composition further comprises 0.0005% to 0.01% of Ca on a mass basis.  
     
     
         12 . A method for manufacturing a corrosion-resistant seamless stainless steel pipe for oil country tubular goods, comprising the steps of: forming a steel pipe from a steel pipe material having a composition by hot working; cooling the steel pipe to room temperature at air-cooling speed or more, or quenching the steel pipe by further heating to a temperature of the A C3  transformation point thereof or more and cooling to room temperature at air cooking speed or more; 
 and then tempering the steel pipe at a temperature of the A C1  transformation point thereof or less, wherein the composition comprises on a mass basis:    0.05% or less of C;    0.50% or less of Si;    0.20% to 1.80% of Mn;    0.03 or less of P;    0.005% or less of S;    14.0% to 18.0% of Cr;    5.0% to 8.0% of Ni;    1.5% to 3.5% of Mo;    0.5% to 3.5% of Cu;    0.05% or less of Al;    0.20% or less of V;    0.01% to 0.15% of N;    0.006% or less of O, and    the balance being Fe and incidental impurities, and wherein the composition satisfies expressions (1) and (2):    Cr+0.65Ni+0.6Mo+0.55Cu+20C≧18.5  (1); Cr+Mo+0.3Si−43.5C−0.4Mn−Ni−0.3Cu−9N≦11  (2),    where Cr, Ni, Mo, Cu, C, Si, Mn, and N represent the respective contents thereof on a mass % basis.    
     
     
         13 . A method for manufacturing a seamless stainless steel pipe for oil country tubular goods according to  claim 12 , wherein the composition further comprises at least one element of 0.20% or less of Nb and 0.30% or less of Ti on a mass basis.  
     
     
         14 . A method for manufacturing a seamless stainless steel pipe for oil country tubular goods according to  claim 13 , wherein the quenching includes heating to a temperature in the range of 800 to 1100° C. and cooling to room temperature at air-cooling speed or more, and the tempering is performed at a temperature in the range of 500 to 630° C.  
     
     
         15 . A method for manufacturing a seamless stainless steel pipe for oil country tubular goods according to any one of  claims 12  to  14 , wherein the composition further comprises at least one element selected from the group consisting of 0.20% or less of Zr, 0.01% or less of B, and 3.0% or less of W on a mass basis.  
     
     
         16 . A method for manufacturing a seamless stainless steel pipe for oil country tubular goods according to claims  12 , wherein the composition further comprises 0.0005% to 0.01% of Ca on a mass basis.