Duplex stainless steel

Abstract

A duplex stainless steel having excellent pitting resistance and weldability and particularly a duplex stainless steel which does not form minute intermetallic compounds even in a weld heat affected zone is provided. It has a chemical composition comprising C: at most 0.03%, Si: at most 1.0%, Mn: at most 1.5%, P: at most 0.040%, S: at most 0.008%, Cr: 23.0-27.0%, Mo: 2.0-4.0%, Ni: 5.0-9.0%, W: greater than 1.5% up to 5.0%, N: 0.24-0.35%, and a remainder of Fe and impurities and satisfying the relationships PREW=Cr+3.3(Mo+0.5W)+16N is at least 40 and Mo+1.1Ni≦12.5 Mo−0.8Ni≦−1.6 wherein the number of coarse inclusions having the following definition observed in a cross section is at most 10 per mm 2 . Here, coarse inclusions are defined as inclusions containing at least 20 mass % of Al and having a major diameter of at least 5 micrometers.

1 . A duplex stainless steel characterized by having a chemical composition comprising, in mass percent, C: at most 0.03%, Si: at most 1.0%, Mn: at most 1.5%, P: at most 0.040%, S: at most 0.008%, Cr: 23.0-27.0%, Mo: 2.0-4.0%, Ni: 5.0-9.0%, W: greater than 1.5% and at most 5.0%, N, 0.24-0.35%, and a remainder of Fe and impurities and satisfying the relationships  
         PREW =Cr+3.3(Mo+0.5 W)+16N is at least 40  Mo+1.1Ni≦12.5  Mo−0.8Ni≦−1.6  and with the number of coarse inclusions defined as inclusions containing at least 20 mass % of Al and having a major diameter of at least 5 micrometers which are observed in a cross section being at most 10 per mm 2 .    
   
   
       2 . A duplex stainless steel as set forth in  claim 1  wherein the chemical composition further includes, in mass %, one or both of Cu: 0.2-2.0% and V: 0.05-1.5%.  
   
   
       3 . A duplex stainless steel as set forth in  claim 1  wherein the chemical composition further includes, in mass %, one or both of 0.0005-0.005% of B and 0.0005-0.2% of at least one rare earth element.  
   
   
       4 . A duplex stainless steel as set forth in  claim 1  wherein the chemical composition further includes, in mass %, sol. Al: at most 0.040%.  
   
   
       5 . A duplex stainless steel as set forth in  claim 2  wherein the chemical composition further includes, in mass %, one or both of 0.0005-0.005% of B and 0.0005-0.2% of at least one rare earth element.  
   
   
       6 . A duplex stainless steel as set forth in  claim 2  wherein the chemical composition further includes, in mass %, sol. Al: at most 0.040%.  
   
   
       7 . A duplex stainless steel as set forth in  claim 3  wherein the chemical composition further includes, in mass %, sol. Al: at most 0.040%.