Ferritic steel sheet concurrently improved in formability, high-temperature strength, high temperature oxidation resistance, and low temperature toughness

Abstract

A low-cost ferritic steel sheet possessing not only formability enabling application to complexly configured automobile exhaust gas passage components but also high-temperature strength, high-temperature oxidation resistance and low-temperature toughness as good as or superior to existing ferritic steels, which ferritic steel sheet comprises, in mass percent, C: not more than 0.02%, Si: 0.7-1.1%, Mn: not more than 0.8%, Ni: not more than 0.5%, Cr: 8.0 to less than 11.0%, N: not more than 0.02%, Nb: 0.10-0.50%, Ti: 0.07-0.25%, Cu: 0.02-0.5%, B: 0.0005-0.02%, V: 0 (no addition)-0.20%, one or both of Ca and Mg: 0 (no addition)−0.01% in total, one or more elements among Y and rare earth elements: 0 (no addition)−0.20% in total, and the balance of Fe and unavoidable impurities, and satisfies 3 Cr+40 Si≧61, Cr+10 Si≦21, and 420 C−11.5 Si+7 Mn+23 Ni−11.5 Cr−12 Mo+9 Cu−49 Ti−25 (Nb+V)−52 A+470 N+189≦70.

1 - 8 . (canceled)  
   
   
       9 . A ferritic steel sheet concurrently improved in formability, high-temperature oxidation resistance, high-temperature strength, and low-temperature toughness comprising a composition, in mass percent 
 C: not more than 0.02%,    Si: 0.7-1.1%,    Mn: not more than 0.8%,    Ni: not more than 0.5%,    Cr: 8.0 to less than 11.0%,    N: not more than 0.02%,    Nb: 0.10-0.50%,    Ti: 0.07-0.25%,    Cu: 0.02-0.5%,    B: 0.0005-0.02%,    V: 0(no addition)−0.20%,    one or both of Ca and Mg: 0 (no addition)−0.01% in total, one or more elements among Y and rare earth elements : 0 (no addition)−0.20% in total, and    the balance of Fe and unavoidable impurities,    the composition satisfying all of Equations (1)-(3):      3 Cr+40 Si≧61   (1)  Cr+10 Si≦21   (2)  420 C−11.5 Si+7 Mn+23 Ni−11.5 Cr−12 Mo+9 Cu−49 Ti−25 (Nb+V)−52 Al+470 N+189≦ 70    (3)    which has a metallic structure obtained by subjecting a hot rolled steel sheet having the composition to a partial recrystallization treatment followed by cold rolling and annealing, wherein the partial recrystallization treatment is conducted by heating a cooled hot rolled steel sheet in the temperature range of 850-1000° C. to obtain a structure which is 10-90 vol. % of recrystallized grains with a remainder being unrecrystallized grains, and further wherein the annealing after the cold rolling is conducted to obtain a totally recrystallized structure.    
   
   
       10 . A steel sheet according to  claim 9 , wherein the content of V is 0.01-0.20%.  
   
   
       11 . A steel sheet according to  claim 9 , wherein the content of one or both of Ca and Mg is 0.0003-0.01% in total.  
   
   
       12 . A steel sheet according to  claim 9 , wherein the content of one or more elements among Y and rare earth elements is 0.01-0.20% in total.  
   
   
       13 . A steel sheet according to  claim 9 , further including 
 Mo: not more than 0.50% and    Al: not more than 0.10%.    
   
   
       14 . A steel sheet according to  claim 9 , which is used as fabricated into an automobile engine exhaust gas passage component.  
   
   
       15 . A steel sheet according to  claim 10 , which is used as fabricated into an automobile engine exhaust gas passage component.  
   
   
       16 . A steel sheet according to  claim 11 , which is used as fabricated into an automobile engine exhaust gas passage component.  
   
   
       17 . A steel sheet according to  claim 12 , which is used as fabricated into an automobile engine exhaust gas passage component.  
   
   
       18 . A steel sheet according to  claim 13 , which is used as fabricated into an automobile engine exhaust gas passage component.  
   
   
       19 . A process for producing a ferritic steel sheet concurrently improved in formability, high-temperature oxidation resistance, high-temperature strength, and low-temperature toughness comprising, in mass, percent 
 C: not more than 0.02%,    Si: 0.7-1.1%,    Mn: not more than 0.8%,    Ni: not more than 0.5%,    Cr: 8.0 to less than 11.0%,    N: not more than 0.02%,    Nb: 0.10-0.50%,    Ti: 0.07-0.25%,    Cu: 0.02-0.5%,    B: 0.0005-0.02%,    V: 0(no addition)−0.20%,    one or both of Ca and Mg: 0 (no addition)−0.01% in total, one or more elements among Y and rare earth elements : 0 (no addition)−0.20% in total, and    the balance of Fe and unavoidable impurities, the composition satisfying all of Equations (1)-(3):      3 Cr+40 Si≧61   (1)  Cr+10 Si≦21   (2)  420 C−11.5 Si+7 Mn+23 Ni−11.5 Cr−12 Mo+9 Cu−49 Ti−25 (Nb+V)−52 Al+470 N+189≦70   (3)    characterized in that the process comprises subjecting the hot rolled steel sheet having said composition to partial recrystallization treatment followed by cold rolling and annealing,    wherein said partial recrystallization treatment is conducted by heating the steel sheet cooled after hot-rolling in the temperature range 850-1000° C. to obtain a structure which is 10-90 vol % of it being accounted for by recrystallized grains and the remainder of it being accounted for by un-recrystallized grains, and said annealing after said cold rolling is conducted to obtain a structure totally recrystallized.    
   
   
       20 . The process according to  claim 23 , wherein the content of V is 0.01-0.20%.  
   
   
       21 . The process according to  claim 23 , wherein the content of one or both of Ca and Mg is 0.0003-0.01% in total.  
   
   
       22 . The process according to  claim 23 , wherein the content of one or more elements among Y and rare earth elements is 0.01-0.20% in total.  
   
   
       23 . A process according to  claim 23 , further including 
 Mo: not more than 0.50% and    Al: not more than 0.10%.