Martensitic stainless steel seamless tube for oil country tubular goods and manufacturing method thereof

Abstract

A martensitic stainless steel seamless tube for oil country tubular goods includes a yield strength of 95 ksi or more and low-temperature toughness in which a fracture transition temperature vTrs in a Charpy impact test is −40° C. or below, wherein the seamless tube has a composition comprising, by mass %, 0.020% or less C, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance, and has a structure where a precipitated Nb quantity is 0.020% or more in terms of Nb.

1 - 12 . (canceled) 
     
     
         13 . A martensitic stainless steel seamless tube for oil country tubular goods having a yield strength of 95 ksi or more and low-temperature toughness in which a fracture transition temperature vTrs in a Charpy impact test is −40° C. or below, wherein the seamless tube has a composition comprising, by mass %, 0.020% or less C, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance, and has a structure where a precipitated Nb quantity is 0.020% or more in terms of Nb. 
     
     
         14 . A martensitic stainless steel seamless tube for oil country tubular goods having yield strength YS of 95 ksi or more and low-temperature toughness in which a fracture transition temperature vTrs in a Charpy impact test is −40° C. or below, wherein the seamless tube has a composition comprising, by mass %, 0.020% or less C, 1.0% or less Si, 0.1 to 2.0% Mn, 0.020% or less P, 0.010% or less S, 0.10% or less Al, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance, and has a structure where a precipitated Nb quantity is 0.020% or more in terms of Nb. 
     
     
         15 . The martensitic stainless steel seamless tube according to  claim 13 , wherein the composition satisfies formula (I):
   C−31/4Nb+7/6N−9/4Al≦−0.30  (1).
   
     
     
         16 . The martensitic stainless steel seamless tube according to  claim 13 , wherein the composition further comprises, by mass %, one or two kinds of elements selected from the group consisting of 2.0% or less Cu, and 2.0% or less Mo. 
     
     
         17 . The martensitic stainless steel seamless tube according to  claim 13 , wherein the composition further comprises, by mass %, one kind, two or more kinds of elements selected from the group consisting of 0.20% or less V, 0.10% or less Ti and 0.005% or less B. 
     
     
         18 . A method of manufacturing a martensitic stainless steel seamless tube for oil country tubular goods, comprising:
 quenching a stainless steel seamless tube having a composition comprising, by mass %, 0.020% or less C, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance by heating at a quenching temperature of A c3  transformation temperature or above and, thereafter, cooling the seamless tube to a temperature range of 100° C. or less from the quenching temperature at a cooling rate of air cooling or more; and   tempering the seamless tube by heating at a tempering temperature of 550° C. or more and A c1  transformation temperature or below and then cooling.   
     
     
         19 . A method of manufacturing a martensitic stainless steel seamless tube for oil country tubular goods, comprising:
 quenching a stainless steel, seamless tube having a composition comprising, by mass %, 0.020% or less C, 1.0% or less Si, 0.1 to 2.0% Mn, 0.020% or less P, 0.010% or less S, 0.10% or less Al, 10 to 14% Cr, 3% or less Ni, 0.03 to 0.2% Nb, 0.05% or less N, and Fe and unavoidable impurities as a balance by heating at a quenching temperature of A c3  transformation temperature or above and, thereafter, cooling the seamless tube to a temperature range of 100° C. or less from the quenching temperature at a cooling rate of air cooling or more; and   tempering the seamless tube by heating at a tempering temperature of 550° C. or more and A c1  transformation temperature or below and then cooling.   
     
     
         20 . The method according to  claim 18 , wherein the composition satisfies formula (I):
   C−31/4Nb+7/6N−9/4Al≦−0.30  (1).
   
     
     
         21 . The method according to  claim 18 , wherein the composition further comprises, by mass %, one or two kinds of elements selected from the group consisting of 2.0% or less Cu, and 2.0% or less Mo. 
     
     
         22 . The method according to  claim 18 , wherein the composition further comprises, by mass %, one kind, two or more kinds of elements selected from the group consisting of 0.10% or less V, 0.10% or less Ti and 0.005% or less B. 
     
     
         23 . The method according to  claim 18 , wherein straightening is performed in a temperature range of 450° C. or more during cooling after said tempering. 
     
     
         24 . A martensitic stainless steel seamless tube for oil country tubular goods obtained by the method according to  claim 23 , wherein an increment ΔYS of yield strength by the straightening is 15 MPa or less. 
     
     
         25 . The martensitic stainless steel seamless tube according to  claim 14 , wherein the composition satisfies formula (I):
   C−31/4Nb+7/6N−9/4Al≦−0.30  (1).
   
     
     
         26 . The method according to  claim 19 , wherein the composition satisfies formula (I):
   C−31/4Nb+7/6N−9/4Al≦−0.30  (1).
   
     
     
         27 . The martensitic stainless steel seamless tube according to  claim 14 , wherein the composition further comprises, by mass %, one or two kinds of elements selected from the group consisting of 2.0% or less Cu, and 2.0% or less Mo. 
     
     
         28 . The martensitic stainless steel seamless tube according to  claim 15 , wherein the composition further comprises, by mass %, one or two kinds of elements selected from the group consisting of 2.0% or less Cu, and 2.0% or less Mo. 
     
     
         29 . The method according to  claim 19 , wherein the composition further comprises, by mass %, one or two kinds of elements selected from the group consisting of 2.0% or less Cu, and 2.0% or less Mo. 
     
     
         30 . The method according to  claim 20 , wherein the composition further comprises, by mass %, one or two kinds of elements selected from the group consisting of 2.0% or less Cu, and 2.0% or less Mo. 
     
     
         31 . The martensitic stainless steel seamless tube according to  claim 14 , wherein the composition further comprises, by mass %, one kind, two or more kinds of elements selected from the group consisting of 0.20% or less V, 0.10% or less Ti and 0.005% or less B. 
     
     
         32 . The method according to  claim 19 , wherein the composition further comprises, by mass %, one kind, two or more kinds of elements selected from the group consisting of 0.10% or less V, 0.10% or less Ti and 0.005% or less B.