Duplex stainless steel and method for producing the same

Abstract

A duplex stainless steel is provided that has a chemical composition comprising, by mass %, C: 0.03% or less, Si: 1.0% or less, Mn: 1.0% or less, P: 0.04% or less, S: 0.01% or less, Cu: 0.1 to 1.0%, Ni: 5.0 to 7.5%, Cr: 22.0 to 26.0%, W: 6.0 to 12.0%, N: 0.20 to 0.32%, Mo: 0.01% or less, and a balance: Fe and impurities, in which a metal micro-structure contains, by area ratio, 0.40 to 0.60 of an α-phase, with a balance being a γ-phase and 0.01 or less of other phases.

1 . A duplex stainless steel having a chemical composition comprising, by mass %,
 C: 0.03% or less,   Si: 1.0% or less,   Mn: 1.0% or less,   P: 0.04% or less,   S: 0.01% or less,   Cu: 0.1 to 1.0%,   Ni: 5.0 to 7.5%,   Cr: 22.0 to 26.0%,   W: 6.0 to 12.0%,   N: 0.20 to 0.32%,   Mo: 0.01% or less, and   balance: Fe and impurities, wherein,   a metal micro-structure contains, by area ratio, 0.40 to 0.60 of an α-phase, with the balance of a γ-phase and 0.01 or less of other phases.   
     
     
         2 . The duplex stainless steel according to  claim 1 , wherein a pitting potential corresponding to 100 μA/cm 2  when immersed in a 250 g/L NaCl aqueous solution that is held at 90° C. is 600 mV (vs. SCE) or more. 
     
     
         3 . The duplex stainless steel according to  claim 1 , wherein a chemical composition of an outermost surface of a passivation film after immersion for 24 hours in a testing liquid having a pH of 1 satisfies formula (i) below:
   W/(Fe+Cr)≧0.09  (i)
   where, each symbol of an element in the formula represents a content (at %) of each element in the outermost surface of the passivation film.   
     
     
         4 . A method for producing a duplex stainless steel, including, with respect to a steel having a chemical composition according to  claim 1 , performing a heat treatment of heating to a temperature range of 1150 to 1300° C., and after holding the steel in the temperature range, cooling at a cooling rate that is equal to or higher than a cooling rate of water cooling.