Ferritic-austenitic stainless steel excellent in corrosion resistance and workability and method of production of same

Abstract

The present invention relates to ferritic-austenitic stainless steel oriented to have low Ni which is excellent in corrosion resistance, particularly in corrosion resistance in a neutral chloride environment, and has high “uniform elongation”—a factor governing workability—and a method of production for the same. There are independently provided ferritic-austenitic stainless steels and methods of production for the same particularly having a corrosion resistance in a neutral chloride environment satisfying PI value(=Cr+3Mo+10N−Mn)≧18% and having a uniform elongation satisfying −10≦Md≦110 (where Md=551−462({C}+[N])−9.2[Si]−8.1[Mn]−13.7[Cr]−29[Ni]−29[Cu]−18.5[Mo], where [ ] is composition (mass %) in the austenite phase, and { } is average composition (mass %))

1 - 7 . (canceled) 
     
     
         8 . Ferritic-austenitic stainless steel excellent in workability characterized by having a volume fraction of an austenite phase of 10% to less than 50%, having an Md value calculated from the chemical composition in the austenite phase that satisfies the following formula (b), having a ratio of austenite grains in a cross-section vertical to a rolling transverse direction with a grain size of 15 μm or less and a shape aspect ratio of less than 3 accounting for 90% or more of the total number of austenite grains, and further having at the same cross-section an average distance between nearest austenite grains of 12 μm or less:
   −10≦Md≦110  (b)
 
 (where, Md=551−462({C}+[N])−9.2[Si]−8.1[Mn]−13.7[Cr]−29[Ni]−29[Cu]−18.5[Mo], 
 [ ] is the composition (mass %) in the austenite phase, and { } is the average composition (mass %)) 
 
     
     
         9 . Ferritic-austenitic stainless steel excellent in workability as set forth in  claim 8  characterized by further containing, by mass %,
 C: 0.002 to 0.1%, 
 Si: 0.05 to 2%, 
 Mn: 0.05 to 5%, 
 P: less than 0.05%, 
 S: less than 0.01%, 
 Cr: 17 to 25%, and 
 N: 0.01 to 0.15% and 
 having a balance of iron and unavoidable impurities. 
 
     
     
         10 . Ferritic-austenitic stainless steel excellent in workability as set forth in  claim 8  characterized by further containing, by mass %, one or more of
 Ni: 5% or less, 
 Cu: 5% or less, and 
 Mo: 5% or less. 
 
     
     
         11 . Ferritic-austenitic stainless steel excellent in workability as set forth in  claim 8  characterized by further containing, by mass %, one or both of
 Nb: 0.5% or less and 
 Ti: 0.5% or less. 
 
     
     
         12 . Ferritic-austenitic stainless steel excellent in workability as set forth in  claim 8  characterized by further containing, by mass %, one or both of
 Ca: 0.003% or less and 
 Mg: 0.003% or less. 
 
     
     
         13 . A method of production of ferritic-austenitic stainless steel excellent in workability characterized by continuously casting steel of ingredients as set forth in  claim 8 , heating the obtained steel slab before hot rolling at a heating temperature T1 (° C.) of 1150° C. to less than 1250° C., then rolling at 1000° C. or more with reduction of a 30% or higher reduction rate then holding for 30 sec or more for one pass or more so as to obtain a hot rolled plate with a total rolling rate of hot rolling of 96% or more, annealing this at a temperature of T1-100° C. to T1° C., suitably thereafter cold rolling, performing process annealing or not performing it, then performing final annealing at 1000° C. to 1100° C.