Duplex stainless steel and method for producing duplex stainless steel

Abstract

A duplex stainless steel with occurrence of pitting suppressed is provided. A duplex stainless steel according to the present disclosure has a chemical composition consisting of, in mass %, Cr: more than 27.00% to 29.00%, Mo: 2.50 to 3.50%, Ni: 5.00 to 8.00%, W: 4.00 to 6.00%, Cu: 0.01 to less than 0.10%, N: more than 0.400% to 0.600%, C: 0.030% or less, Si: 1.00% or less, Mn 1.00% or less, sol.Al: 0.040% or less, V: 0.50% or less, O: 0.010% or less, P: 0.030% or less, and S: 0.020% or less with the balance being Fe and impurities and satisfying Formula (1), a microstructure consisting of 35 to 65 volume % of ferrite phase with the balance being the austenite phase, and the area fraction of Cu precipitated in the ferrite phase is 0.5% or less. Cr+4.0×Mo+2.0×W+20×N−5×ln(Cu)≥65.2  (1)

1 . A duplex stainless steel comprising:
 a chemical composition consisting of in mass %,   Cr: more than 27.00% to 29.00%,   Mo: 2.50 to 3.50%,   Ni: 5.00 to 8.00%,   W: 4.00 to 6.00%,   Cu: 0.01 to less than 0.10%,   N: more than 0.400% to 0.600%,   C: 0.030% or less,   Si: 1.00% or less,   Mn: 1.00% or less,   sol.Al: 0.040% or less,   V: 0.50% or less,   O: 0.010% or less,   P: 0.030% or less,   S: 0.020% or less,   Ca: 0 to 0.0040%,   Mg: 0 to 0.0040%,   B: 0 to 0.0040%, and   with the balance being Fe and impurities, and satisfying Formula (1),   a microstructure consisting of 35 to 65 volume % of ferrite phase with the balance being an austenite phase,   wherein an area fraction of Cu precipitates in the ferrite phase is 0.5% or less:
   Cr+4.0×Mo+2.0×W+20×N−5×ln(Cu)≥65.2  (1)
 
   where, a content in mass % of each of the elements is substituted into a corresponding symbol of the element in Formula (1).   
     
     
         2 . The duplex stainless steel according to  claim 1 ,
 wherein the chemical composition contains, in mass %, one or more types of element selected from the group consisting of:   Ca: 0.0001 to 0.0040%,   Mg: 0.0001 to 0.0040%, and   B: 0.0001 to 0.0040%.   
     
     
         3 . A method for producing a duplex stainless steel, the method comprising the steps of:
 preparing a starting material having a chemical composition consisting of, in mass %,   Cr: more than 27.00% to 29.00%,   Mo: 2.50 to 3.50%,   Ni: 5.00 to 8.00%,   W: 4.00 to 6.00%,   Cu: 0.01 to less than 0.10%,   N: more than 0.400% to 0.600%,   C: 0.030% or less,   Si: 1.00% or less,   Mn: 1.00% or less,   sol.Al: 0.040% or less,   V: 0.50% or less,   O: 0.010% or less,   P: 0.030% or less,   S: 0.020% or less,   Ca: 0 to 0.0040%,   Mg: 0 to 0.0040%, and   B: 0 to 0.0040%   with the balance being Fe and impurities and satisfying Formula (1):   subjecting the starting material to hot working at 850° C. or more;   cooling the starting material subjected to the hot working at a rate of 5° C./sec or more; and   subjecting the cooled starting material to a solution heat treatment at 1070° C. or more:
   Cr+4.0×Mo+2.0×W+20×N−5×ln(Cu)≥65.2  (1)
 
   where, a content in mass % of each of the elements is substituted into a corresponding symbol of the element in Formula (1).