Duplex stainless steel pipe and method for manufacturing same

Abstract

Provided herein is a duplex stainless steel pipe that is high in strength and has excellent abrasion resistance and indentation resistance on inner and outer surfaces of the steel pipe. A method for manufacturing such a stainless steel pipe is also provided. A duplex stainless steel pipe of the present invention has a composition that contains, in mass %, C: 0.005 to 0.150%, Si: 1.0% or less, Mn: 10.0% or less, Cr: 11.5 to 35.0%, Ni: 0.5 to 15.0%, Mo: 0.5 to 6.0%, N: less than 0.400%, and the balance being Fe and incidental impurities, and has a microstructure with a ferritic phase and an austenitic phase, the duplex stainless steel pipe having an axial tensile yield strength of 689 MPa or more, and having an outer surface and an inner surface each having an oxide layer having an average thickness of 1.0 μm or more.

1 - 10 . (canceled) 
     
     
         11 . A duplex stainless steel pipe having a composition that comprises, in mass %, C: 0.005 to 0.150%, Si: 1.0% or less, Mn: 10.0% or less, Cr: 11.5 to 35.0%, Ni: 0.5 to 15.0%, Mo: 0.5 to 6.0%, N: less than 0.400%, and the balance being Fe and incidental impurities, and having a microstructure with a ferritic phase and an austenitic phase,
 the duplex stainless steel pipe having an axial tensile yield strength of 689 MPa or more, and having an outer surface and an inner surface each having an oxide layer having an average thickness of 1.0 μm or more.   
     
     
         12 . The duplex stainless steel pipe according to  claim 11 , wherein the oxide layer covers at least 50% of the outer surface and at least 50% of the inner surface of the steel pipe in terms of an area percentage. 
     
     
         13 . The duplex stainless steel pipe according to  claim 11 , which has an axial compressive yield strength-to-axial tensile yield strength ratio of 0.85 to 1.15. 
     
     
         14 . The duplex stainless steel pipe according to  claim 12 , which has an axial compressive yield strength-to-axial tensile yield strength ratio of 0.85 to 1.15. 
     
     
         15 . The duplex stainless steel pipe according to  claim 11 , wherein the composition further comprises, in mass %,
 one or two or more selected from the following groups A to C:   group A:   one or two or more selected from W: 6.0% or less, Cu: 4.0% or less, V: 1.0% or less, and Nb: 1.0% or less,   group B:   one or two selected from Ti: 0.30% or less and Al: 0.30% or less,   group C:   one or two or more selected from B: 0.010% or less, Zr: 0.010% or less, Ca: 0.010% or less, Ta: 0.30% or less, Sb: 0.30% or less, Sn: 0.30% or less, and REM: 0.010% or less.   
     
     
         16 . The duplex stainless steel pipe according to  claim 12 , wherein the composition further comprises, in mass %,
 one or two or more selected from the following groups A to C:   group A:   one or two or more selected from W: 6.0% or less, Cu: 4.0% or less, V: 1.0% or less, and Nb: 1.0% or less,   group B:   one or two selected from Ti: 0.30% or less and Al: 0.30% or less,   group C:   one or two or more selected from B: 0.010% or less, Zr: 0.010% or less, Ca: 0.010% or less, Ta: 0.30% or less, Sb: 0.30% or less, Sn: 0.30% or less, and REM: 0.010% or less.   
     
     
         17 . The duplex stainless steel pipe according to  claim 13 , wherein the composition further comprises, in mass %,
 one or two or more selected from the following groups A to C:   group A:   one or two or more selected from W: 6.0% or less, Cu: 4.0% or less, V: 1.0% or less, and Nb: 1.0% or less,   group B:   one or two selected from Ti: 0.30% or less and Al: 0.30% or less,   group C:   one or two or more selected from B: 0.010% or less, Zr: 0.010% or less, Ca: 0.010% or less, Ta: 0.30% or less, Sb: 0.30% or less, Sn: 0.30% or less, and REM: 0.010% or less.   
     
     
         18 . The duplex stainless steel pipe according to  claim 14 , wherein the composition further comprises, in mass %,
 one or two or more selected from the following groups A to C:   group A:   one or two or more selected from W: 6.0% or less, Cu: 4.0% or less, V: 1.0% or less, and Nb: 1.0% or less,   group B:   one or two selected from Ti: 0.30% or less and Al: 0.30% or less,   group C:   one or two or more selected from B: 0.010% or less, Zr: 0.010% or less, Ca: 0.010% or less, Ta: 0.30% or less, Sb: 0.30% or less, Sn: 0.30% or less, and REM: 0.010% or less.   
     
     
         19 . A method for manufacturing a duplex stainless steel pipe of  claim 11 ,
 the method comprising:   hot rolling a steel pipe material into a shape of a steel pipe;   subjecting the steel pipe material after the hot rolling to a solid solution heat treatment that satisfies the formula (1) below; and   performing cold circumferential bending and reverse bending without removing an oxide layer formed on the steel pipe material after the solid solution heat treatment,   
       
         
           
             
               
                 
                   
                     
                       
                         T 
                           
                         
                           max 
                           2 
                         
                         × 
                         t 
                         ⁢ 
                         
                           
                             / 
                             [ 
                             Cr 
                             ] 
                           
                           4 
                         
                       
                       > 
                       
                         1 
                         , 
                         000 
                       
                     
                     , 
                   
                 
                 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
         wherein Tmax is a highest heating temperature (° C.) of the solid solution heat treatment, t is a retention time (s) at the highest heating temperature of the solid solution heat treatment, and [Cr] is the content of Cr (mass %) in the steel pipe. 
       
     
     
         20 . The method according to  claim 19 , wherein the highest heating temperature in the hot rolling is 1,150° C. or more. 
     
     
         21 . The method according to  claim 19 , wherein the cold bending and reverse bending reduces a diameter of the steel pipe material to (Di/Do)×100=99% or less, where Di is an outside diameter of the steel pipe material after work, and Do is an outside diameter of the steel pipe material before work. 
     
     
         22 . The method according to  claim 20 , wherein the cold bending and reverse bending reduces a diameter of the steel pipe material to (Di/Do)×100=99% or less, where Di is an outside diameter of the steel pipe material after work, and Do is an outside diameter of the steel pipe material before work. 
     
     
         23 . The method according to  claim 19 , wherein (Li/Lo)×100 (%) is 125% or less after the cold bending and reverse bending, where Li is an axial length of the steel pipe material after work, and Lo is an axial length of the steel pipe material before work. 
     
     
         24 . The method according to  claim 20 , wherein (Li/Lo)×100 (%) is 125% or less after the cold bending and reverse bending, where Li is an axial length of the steel pipe material after work, and Lo is an axial length of the steel pipe material before work. 
     
     
         25 . The method according to  claim 21 , wherein (Li/Lo)×100 (%) is 125% or less after the cold bending and reverse bending, where Li is an axial length of the steel pipe material after work, and Lo is an axial length of the steel pipe material before work. 
     
     
         26 . The method according to  claim 22 , wherein (Li/Lo)×100 (%) is 125% or less after the cold bending and reverse bending, where Li is an axial length of the steel pipe material after work, and Lo is an axial length of the steel pipe material before work.