Ferritic stainless steel and steel sheet for heat pipes, and heat pipe and high-temperature exhaust heat recovery system

Abstract

Provided is a ferritic stainless steel for heat pipes of high-temperature exhaust heat recovery systems, which comprises, in terms of % by mass, from 16 to 32% of Cr, at most 0.03% of C, at most 0.03% of N, at most 3% of Si, at most 2% of Mn, at most 0.008% of S, from 0 to 0.3% of Al, and at least one of at most 0.7% of Nb, at most 0.3% of Ti, at most 0.5% of Zr and at most 1% of V, and optionally at least one of at most 3% of Mo, at most 3% of W, at most 3% of Cu, at most 0.1% of Y, at most 0.1% of REM (rare earth metal) and at most 0.01% of Ca, with a balance of Fe and inevitable impurities, and which satisfies at least the following formula (1), formula (2) and formula (5): Cr+3(Mo+Cu)≧20  (1) Cr+3(Si+Mn+Al−Ti)≧20  (2) 0.037{(C+N)/(V+Ti+0.5Nb+0.5Zr)}+0.001≦0.01  (5).

1 . A ferritic stainless steel for heat pipes of high-temperature exhaust heat recovery systems, which comprises, in terms of % by mass, from 16 to 32% of Cr, at most 0.03% of C, at most 0.03% of N, at most 3% of Si, at most 2% of Mn, at most 0.008% of S, from 0 (no addition) to 0.3% of Al, and at least one of at most 0.7% of Nb, at most 0.3% of Ti, at most 0.5% of Zr and at most 1% of V, with a balance of Fe and inevitable impurities, and which satisfies all of the following formula (1), formula (2) and formula (5) and satisfies at least one of the following formula (3) and formula (4):
   Cr+3(Mo+Cu)≧20  (1)     Cr+3(Si+Mn+Al−Ti)≧20  (2)     Ti+Al≦0.05  (3)     Nb≧Ti+Al  (4)     0.037{(C+N)/(V+Ti+0.5Nb+0.5Zr)}+0.001≦0.01  (5),   
     wherein the site of the atomic symbol is substituted with the value of the content of the corresponding element in terms of % by mass, and the site of the element not added to the steel is substituted with 0 (zero) in formulae (1) to (5). 
   
   
       2 . The ferritic stainless steel for heat pipes as claimed in  claim 1 , which further contains at least one of at most 3% of Mo, at most 3% of W and at most 3% of Cu. 
   
   
       3 . The ferritic stainless steel for heat pipes as claimed in  claim 1 , which further contains at least one of at most 0.1% of Y, at most 0.1% of REM (rare earth metal) and at most 0.01% of Ca. 
   
   
       4 . A ferritic stainless steel sheet for heat pipes, which is formed of the steel of  claim 1  and which has a polish-finished surface at a polish count of from #100 to #800 as defined in JIS R6001:1998, or has a polish-finished surface at HL as defined in Table 14 in JIS G4305:2005. 
   
   
       5 . A heat pipe for high-temperature exhaust heat recovery systems, as constructed by vacuuming a vessel formed of a steel material of the steel of  claim 1  and having a thickness of from 0.5 to 1 mm, followed by introducing water thereinto and sealing it up to thereby form a liquid phase part and a space part therein. 
   
   
       6 . A heat pipe for high-temperature exhaust heat recovery systems, as constructed by vacuuming a vessel formed of a steel material of the steel of  claim 1  and having a thickness of from 0.5 to 1 mm and having, as the inner surface thereof, a polish-finished surface at a polish count of from #100 to #800 as defined in JIS R6001:1998 or a polish-finished surface at HL as defined in Table 14 in JIS G4305:2005, followed by introducing water thereinto and sealing it up to thereby form a liquid phase part and a space part therein. 
   
   
       7 . A high-temperature exhaust heat recovery system having the heat pipe of  claim 5  as disposed in a site where the material temperature reaches from 600 to 900° C.