Ferritic stainles steel for exhaust gas path members

Abstract

Provided is a stainless steel for exhaust gas path members, which has a composition including, in terms of % by mass, at most 0.03% of C, at most 1% of Si, at most 1.5% of Mn, at most 0.6% of Ni, from 10 to 20% of Cr, from more than 0.5 to 0.7% of Nb, from 0.05 to 0.3% of Ti, from more than 1 to 2% of Cu, at most 0.2% of V, at most 0.03% of N, from 0.0005 to 0.02% of B, and optionally at most 0.1% of Al, and further optionally at least one of Mo, W, Zr and Co in an amount of at most 4% in total, with a balance of Fe and inevitable impurities, and which has a texture where the Cu phase and the Nb compound phase having a major diameter of at least 0.5 μm are controlled to be in an amount of at most 10 grains/25 μm 2 each. The stainless steel exhibits excellent thermal fatigue resistance when applied to exhaust gas path members of both cases where the maximum ultimate temperature is high and low, and has excellent low-temperature toughness.

1 . A stainless steel for exhaust gas path members, which has a composition comprising, in terms of % by mass, at most 0.03% of C, at most 1% of Si, at most 1.5% of Mn, at most 0.6% of Ni, from 10 to 20% of Cr, from more than 0.5 to 0.7% of Nb, from 0.05 to 0.3% of Ti, from more than 1 to 2% of Cu, at most 0.2% of V, at most 0.03% of N, from 0.0005 to 0.02% of B, with a balance of Fe and inevitable impurities, and having an [Nb] value, as defined by the following formula (2) or (3) in accordance with the [Ti] value thereof as defined by the following formula (1), falling within a range of from 0.5 to 0.65, and which has a texture where the Cu phase having a major diameter of at least 0.5 μm is controlled to be in an amount of at most 10 grains/25 μm 2  and the Nb compound phase having a major diameter of at least 0.5 μm is in an amount of at most 10 grains/25 μm 2 :
   [Ti]=Ti−4(C+N)  (1),     when [Ti]≧0, [Nb]=Nb  (2),     when [Ti]<0, [Nb]=Nb+0.5[Ti]  (3).   
   
   
       2 . The stainless steel for exhaust gas path members as claimed in  claim 1 , of which the composition further contains at most 0.1% by mass of Al. 
   
   
       3 . The stainless steel for exhaust gas path members as claimed in  claim 1 , of which the composition further contains at least one of Mo, W, Zr and Co in an amount of at most 4% in total. 
   
   
       4 . An exhaust gas path member as claimed in  claim 1 , wherein the exhaust gas path member is any of automobile exhaust manifolds, catalyst converters, front pipes and center pipes. 
   
   
       5 . The stainless steel for exhaust gas path members as claimed in  claim 2 , of which the composition further contains at least one of Mo, W, Zr and Co in an amount of at most 4% in total. 
   
   
       6 . An exhaust gas path member as claimed in  claim 2 , wherein the exhaust gas path member is any of automobile exhaust manifolds, catalyst converters, front pipes and center pipes. 
   
   
       7 . An exhaust gas path member as claimed in  claim 3 , wherein the exhaust gas path member is any of automobile exhaust manifolds, catalyst converters, front pipes and center pipes.