Ferritic-austenitic stainless steel excellent in corrosion resistance and workability andmethod 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 . Ferritic-austenitic stainless steel excellent in corrosion resistance and workability characterized by containing, by mass %,
 C: 0.001 to 0.1%,   Cr: 17 to 25%,   Si: 0.01 to 1%,   Mn: 0.5 to 3.7%, and   N: 0.06% to less than 0.15%,   having a pitting indicator (PI value) shown by the following formula (a) of over 18%, having a balance of Fe and unavoidable impurities, and having a ferrite phase as the matrix phase and having a volume fraction of the austenite phase of 15 to 50%.
   Pitting indicator(PI value)=Cr+3Mo+10N—Mn  (a) 
   
     
     
         2 . Ferritic-austenitic stainless steel excellent in corrosion resistance and workability as set forth in  claim 1 , characterized by further containing, by mass %, both
 Ni: 0.6 to 3% and   Cu: 0.1 to 3%.   
     
     
         3 . Ferritic-austenitic stainless steel excellent in corrosion resistance and workability as set forth in  claim 1 , characterized by further containing, by mass %, one or more of
 Mo: 1% or less,   Nb: 0.5% or less,   Ti: 0.5% or less,   Al: 0.1% or less,   B: 0.01% or less,   Ca: 0.01% or less, and   Mg: 0.01% or less.   
     
     
         4 . Ferritic-austenitic stainless steel excellent in corrosion resistance and workability as set forth in  claim 1 , characterized by having a pitting potential Vc′100 in a 30° C., 3.5% NaCl aqueous solution of 0.3V (Vv.s.AGCL) or more. 
     
     
         5 . A method of production of ferritic-austenitic stainless steel excellent in corrosion resistance and workability comprising hot forging or hot rolling a stainless steel ingot having steel ingredients as set forth in  claim 1  to obtain a hot rolled steel material, annealing the hot rolled steel material, then repeating cold working and annealing, said method of production of a steel material characterized by performing the final annealing by heating and holding the material at 950 to 1150° C., making an average cooling rate from the heating temperature to 200° C. 3° C./sec or more, and making a ferrite phase the matrix phase and making a volume fraction of the austenite phase 15 to 50%. 
     
     
         6 . A method of production of ferritic-austenitic stainless steel excellent in corrosion resistance and workability comprising hot forging or hot rolling a stainless steel ingot having steel ingredients as set forth in  claim 1  to obtain a hot rolled steel material, annealing the hot rolled steel material, then repeating cold working and annealing, said method of production of a steel material characterized by performing the final annealing by heating and holding the material at 950 to 1150° C., then making an average cooling rate until 600° C. 3° C./sec or more, holding the material at a 200 to 600° C. temperature region for 1 minute or more, then making the average cooling rate from the holding temperature to room temperature 3° C./sec or more, and making a ferrite phase the matrix phase and making a volume fraction of the austenite phase 15 to 50%. 
     
     
         7 . A method of production of ferritic-austenitic stainless steel excellent in corrosion resistance and workability as set forth in  claim 5 , characterized by making the ferrite phase the matrix phase and making a volume fraction of the austenite phase 15 to 50% and by making a pitting potential Vc′100 in a 30° C., 3.5% NaCl aqueous solution 0.3V (Vv.s.AGCL) or more. 
     
     
         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.