Ferritic stainless steel having excellent heat resistance

Abstract

The present invention provides a Sn-containing ferritic stainless steel sheet having excellent heat resistance. The ferritic stainless steel contains, in terms of mass %, 0.015% or less of C, 1.5% or less of Si, 1.5% or less of Mn, 0.035% or less of P, 0.015% or less of S, 13-21% of Cr, 0.01-0.50% of Sn, 0.05-0.60% of Nb and 0.020% or less of N, with the remainder consisting of Fe and unavoidable impurities. The ferritic stainless steel satisfies formula 1 and formula 2, and has a grain boundary Sn concentration of 2 atom % or less when subjected to a heat treatment at 600-750° C. in which the value of L, as shown in formula 3, is 1.91×10 4 or higher. 8≦CI=(Ti+0.52Nb)/(C+N)≦26 (formula 1) GBSV=Sn+Ti−2Nb−0.3Mo−0.2≦0 (formula 2) L=(273+T)(log(t)+20) (formula 3) I: Temperature (° C.), t: time (h)

1 - 14 . (canceled) 
     
     
         15 . Ferritic stainless steel containing, by mass %,
 Cr: 13.0 to 21.0%,   Sn: 0.01 to 0.50%, and   Nb: 0.05 to 0.60%,   restricted to:   C: 0.015% or less,   Si: 1.5% or less,   Mn: 1.5% or less,   N: 0.020% or less,   P: 0.035% or less, and   S: 0.015% or less,   containing a balance of Fe and unavoidable impurities,   satisfying formula 1 and formula 2, and   having a grain boundary Sn concentration of 2 at % or less when performing heat treatment at a temperature of 600 to 750° C. so that an L value shown by formula 3 becomes 1.91×10 4  or more, and wherein a grain size number after annealing the cold-rolled sheet is made 5.0 to 9.0:
   8 ≦CI= 0.52Nb/(C+N)≦26   (formula 1)
 
     GBSV= Sn−2Nb−0.2≦0   (formula 2)
 
     L= (273+ T )(log( t )+20)   (formula 3)
 
 where, T: temperature (° C.), t: time (h). 
   
     
     
         16 . The ferritic stainless steel according to  claim 15 , wherein said heat treatment is performed at a temperature of 700° C. for 1 hour. 
     
     
         17 . The ferritic stainless steel according to  claim 15  further containing, by mass %, one or more of:
 Ti: 0.32% or less, 
 Ni: 1.5% or less, 
 Cu: 1.5% or less, 
 Mo: 2.0% or less, 
 V: 0.3% or less, 
 Al: 0.3% or less, and 
 B: 0.0020% or less:
 where formula 1 and formula 2 are replaced by formula 1′ and formula 2′:
   8 ≦CI= (Ti+0.52Nb)/(C+N)≦26   formula 1′
 
     GBSV= Sn+Ti−2Nb−0.3Mo−0.2≦0   formula 2′.
 
 
 
 
     
     
         18 . The ferritic stainless steel according to  claim 15  further containing, by mass %, one or more of:
 W: 0.20% or less, 
 Zr: 0.20% or less, 
 Sb: 0.5% or less, 
 Co: 0.5% or less, 
 Ca: 0.01% or less, 
 Mg: 0.01% or less, and 
 REM: 0.1% or less. 
 
     
     
         19 . The ferritic stainless steel according to  claim 15  wherein a grain size number after annealing the cold-rolled sheet is made 6.0 to 8.5. 
     
     
         20 . A method of production of the ferritic stainless steel according to  claim 15  comprising annealing the stainless steel at a cold-rolled strip annealing temperature of 850° C. to 1100° C. and then cooling from cold-rolled strip annealing temperature by a cooling rate of 5° C./s or more in the temperature range of 800 to 500° C. 
     
     
         21 . An exhaust system member characterized by using the ferritic stainless steel of  claim 15 . 
     
     
         22 . Ferritic stainless steel containing, by mass %:
 Cr: 13.0 to 21.0%,   Sn: 0.01 to 0.50%, and   Nb: 0.05 to 0.60%,   having at least one of W: 0.01% to 0.20% and Sb: 0.001% to 0.5%,   restricted to:   C: 0.015% or less,   Si: 1.5% or less,   Mn: 1.5% or less,   N: 0.020% or less,   P: 0.035% or less, and   S: 0.015% or less,   containing a balance of Fe and unavoidable impurities,   satisfying formula 1 and formula 2, and   having a grain boundary Sn concentration of 2 at % or less when performing heat treatment at a temperature of 600 to 750° C. so that an L value shown by formula 3 becomes 1.91×10 4  or more:
   8 ≦CI= 0.52Nb/(C+N)≦26   (formula 1)
 
     GBSV= Sn−2Nb−0.2≦0   (formula 2)
 
     L =(273+ T )(log( t )+20)   (formula 3)
 
 where, T: temperature (° C.), t: time (h). 
   
     
     
         23 . The ferritic stainless steel according to  claim 22 , wherein said heat treatment is performed at a temperature of 700° C. for 1 hour. 
     
     
         24 . The ferritic stainless steel according to  claim 22  further containing, by mass %, one or more of:
 Ti: 0.32% or less, 
 Ni: 1.5% or less, 
 Cu: 1.5% or less, 
 Mo: 2.0% or less, 
 V: 0.3% or less, 
 Al: 0.3% or less, and 
 B: 0.0020% or less:
 where formula 1 and formula 2 are replaced by formula 1′ and formula 2′:
   8 ≦CI =(Ti+0.52Nb)/(C+N)≦26   formula 1′
 
     GBSV= Sn+Ti−2Nb−0.3Mo−0.2≦0   formula 2′.
 
 
 
 
     
     
         25 . The ferritic stainless steel according to  claim 22  further containing, by mass %, one or more of:
 Zr: 0.20% or less, 
 Sb: 0.5% or less, 
 Co: 0.5% or less, 
 Ca: 0.01% or less, 
 Mg: 0.01% or less, and 
 REM: 0.1% or less. 
 
     
     
         26 . The ferritic stainless steel according to  claim 22  wherein a grain size number after annealing the cold-rolled sheet is made 5.0 to 9.0. 
     
     
         27 . A method of production of the ferritic stainless steel according to  claim 22 , comprising annealing the stainless steel at a cold-rolled strip annealing temperature of 850° C. to 1100° C. and then cooling from the cold strip sheet annealing temperature by a cooling rate of 5° C./s or more in the temperature range of 800 to 500° C. 
     
     
         28 . An exhaust system member characterized by using the ferritic stainless steel of  claim 22 . 
     
     
         29 . The ferritic stainless steel according to  claim 16  further containing, by mass %, one or more of:
 Ti: 0.32% or less, 
 Ni: 1.5% or less, 
 Cu: 1.5% or less, 
 Mo: 2.0% or less, 
 V: 0.3% or less, 
 Al: 0.3% or less, and 
 B: 0.0020% or less:
 where formula 1 and formula 2 are replaced by formula 1′ and formula 2′:
   8 ≦CI= (Ti+0.52Nb)/(C+N)≦26   formula 1′
 
     GBSV =Sn+Ti−2Nb−0.3Mo−0.2≦0   formula 2′
 
 
 
 
     
     
         30 . The ferritic stainless steel according to  claim 16  further containing, by mass %, one or more of:
 W: 0.20% or less, 
 Zr: 0.20% or less, 
 Sb: 0.5% or less, 
 Co: 0.5% or less, 
 Ca: 0.01% or less, 
 Mg: 0.01% or less, and 
 REM: 0.1% or less. 
 
     
     
         31 . The ferritic stainless steel according to  claim 17  further containing, by mass %, one or more of:
 W: 0.20% or less, 
 Zr: 0.20% or less, 
 Sb: 0.5% or less, 
 Co: 0.5% or less, 
 Ca: 0.01% or less, 
 Mg: 0.01% or less, and 
 REM: 0.1% or less. 
 
     
     
         32 . The ferritic stainless steel according to  claim 16 , wherein a grain size number after annealing the cold-rolled sheet is made 6.0 to 8.5. 
     
     
         33 . The ferritic stainless steel according to  claim 17 , wherein a grain size number after annealing the cold-rolled sheet is made 6.0 to 8.5. 
     
     
         34 . The ferritic stainless steel according to  claim 18 , wherein a grain size number after annealing the cold-rolled sheet is made 6.0 to 8.5.