Ferritic stainless steel excellent in corrosion resistance and strength of weld zone and tig welded structure

Abstract

Ferritic stainless steel excellent in corrosion resistance of the weld zone, in particular the corrosion resistance to crevice corrosion, even if eliminating the Ar gas shield at the time of TIG welding and, furthermore, excellent in weld zone strength and a structure of the same are provided. By making the ingredient system one satisfying the following formula (1) in ferritic stainless steel which has predetermined contents of ingredients by mass % under a usual corrosive environment, it is possible to raise the corrosion resistance. Furthermore, under a severely corrosive environment, by making the relationship between the back surface exposed width of the weld bead and the steel sheet thickness satisfy the formula (2) in addition to the relationship of the ingredients prescribed in formula (1), it is possible to obtain a welded structure strong in crevice corrosion. Furthermore, by improving the weld strength by making the contents of Nb and V satisfy formula 3, it is possible to obtain a welded structure with a high weld strength: Si+2.5Al+3Ti≦1.50  (formula (1)) 0<Wb/2t≦1.50  (formula (2)) (t: sheet thickness of welded metal, Wb: length of bead width exposed at back surface) Nb×V≧0.01  (formula 3)

1 . Ferritic stainless steel characterized in that the ingredients of said ferritic stainless steel include, by mass %,
 C: 0.002 to 0.020%,   Si: 0.05 to 0.30%,   Mn: 0.01 to 0.50%,   Cr: 16.0 to 25.0%,   Mo: 0.30 to 3.00%,   Al: 0.005 to 0.150%,   Ti: 0.05 to 0.30%,   Nb: 0.10 to 0.50%   N: 0.002 to 0.025%   P: 0.035% or less,   S: 0.0100% or less, and   a balance of Fe and unavoidable impurities,   wherein contents of Si, Al, and Ti satisfying formula (1),
   Si+2.5Al+3Ti≦1.50  (formula (1))
 
   wherein the symbols of the elements of formula (1) mean the mass % of content of those elements.   
     
     
         2 . The ferritic stainless steel as set forth in  claim 1  characterized in that the ingredients further contain one or more of, by mass %:
 Cu: 0.10 to 0.50%, 
 Ni: 0.05 to 2.00%, 
 V: 0.05 to 1.00% 
 Sn: 0.05 to 1.00%, 
 Sb: 0.05 to 1.00% 
 Zr: 0.03 to 0.20%, and 
 B: 0.0001 to 0.0050%. 
 
     
     
         3 . The ferritic stainless steel as set forth in  claim 2  characterized in that Nb and V in the ingredients of said ferritic stainless steel satisfy formula 3:
   Nb×V≧0.01  (formula 3)
 
 wherein the symbols of the elements of formula 3 mean the mass % of content of those elements 
 
     
     
         4 . A welded structure obtained by welding together ferritic stainless steel sheets by TIG welding, characterized in that the ingredients of said ferritic stainless steel sheet include, by mass %,
 C: 0.002 to 0.020%,   Si: 0.05 to 0.30%,   Mn: 0.01 to 0.50%,   Cr: 16.0 to 25.0%,   Mo: 0.30 to 3.00%,   Al: 0.005 to 0.150%,   Ti: 0.05 to 0.30%,   Nb: 0.10 to 0.50%   N: 0.002 to 0.025%   P: 0.035% or less,   S: 0.0100% or less, and   a balance of Fe and unavoidable impurities,   wherein contents of Si, Al, and Ti satisfying formula (1), and   wherein a bead width Wb of weld metal exposed at a back surface of said ferritic stainless steel sheet at said TIG welded part and a sheet thickness “t” of said ferritic stainless steel sheet satisfying formula (2):
   Si+2.5Al+3Ti≦1.50  (formula (1))
 
   0<Wb/2t≦1.50  (formula (2))
 
   wherein the symbols of the elements of formula (1) mean the mass % of content of those elements   
     
     
         5 . The welded structure of ferritic stainless steel as set forth in  claim 4  characterized in that the ingredients of said ferritic stainless steel sheet further include,
 by mass %, one or more of 
 Cu: 0.10 to 0.50%, 
 Ni: 0.05 to 2.00%, 
 V: 0.05 to 1.00% 
 Sn: 0.05 to 1.00%, 
 Sb: 0.05 to 1.00% 
 Zr: 0.03 to 0.20%, and 
 B: 0.0001 to 0.0050%. 
 
     
     
         6 . The welded structure of ferritic stainless steel as set forth in  claim 5  characterized in that Nb and V in the ingredients of said ferritic stainless steel satisfy formula 3:
   Nb×V≧0.01  (formula 3)
 
 wherein the symbols of the elements of formula 3 mean the mass % of content of those elements