Ferritic stainless steel and method for manufacturing same

Abstract

A ferritic stainless steel in accordance with an aspect of the present invention contains not more than 0.030% of C, 0.01% to 1.5% of Si, 0.01% to 1.00% of Mn, not more than 0.050% of P, not more than 0.005% of S, 15.0% to 25.0% of Cr, 2.0% to 4.0% of Al, not more than 1.00% of Ni, 0.01% to 0.70% of Nb, not more than 0.030% of N, 0.0003% to 0.01% of B, and 0.01% to 0.20% of REM, in percent by mass, and the other part composed of Fe and an inevitable impurity, the ferritic stainless steel including, on a surface of the ferritic stainless steel, an Al-based oxide film having a thickness of not less than 8 nm and less than 20 nm.

1 . A ferritic stainless steel, comprising not more than 0.030% of C, 0.01% to 1.5% of Si, 0.01% to 1.00% of Mn, not more than 0.050% of P, not more than 0.005% of S, 15.0% to 25.0% of Cr, 2.0% to 4.0% of Al, not more than 1.00% of Ni, 0.01% to 0.70% of Nb, not more than 0.030% of N, 0.0003% to 0.01% of B, and 0.01% to 0.20% of REM, in percent by mass, and the other part composed of Fe and an inevitable impurity, the ferritic stainless steel including, on a surface of the ferritic stainless steel, an Al-based oxide film having a thickness of not less than 8 nm and less than 20 nm. 
     
     
         2 . The ferritic stainless steel as set forth in  claim 1 , wherein:
 in a case where the ferritic stainless steel is heated at 1050° C. for 50 hours, the ferritic stainless forms an alumina layer mainly composed of alumina; and   the alumina layer is 100% made up of a columnar crystal or satisfies the following formula (1):
   (C)/(E)≥1.2  (1)
 
   
       where (C) is a thickness of a columnar crystal in a cross section obtained by cutting the alumina layer in a thickness direction and (E) is a thickness of an equiaxial crystal in the cross section obtained by cutting the alumina layer in the thickness direction. 
     
     
         3 . The ferritic stainless steel as set forth in  claim 1 , wherein the ferritic stainless steel further comprises at least one selected from the group consisting of not more than 0.50% of Zr, not more than 0.50% of V, not more than 1.0% of Cu, not more than 2.0% of Mo, not more than 2.0% of W, not more than 0.50% of Hf, not more than 0.50% of Sn, not more than 0.5% of Ta, not more than 0.20% of Ti, not more than 0.015% of Mg, and not more than 0.015% of Ca, in percent by mass. 
     
     
         4 . The ferritic stainless steel as set forth in  claim 1 , wherein the ferritic stainless steel satisfies [Al]/(10×[Nb])<8 where [Al] is a percent by mass of Al and [Nb] is a percent by mass of Nb. 
     
     
         5 . A method for producing a ferritic stainless steel, the ferritic stainless steel containing not more than 0.030% of C, 0.01% to 1.5% of Si, 0.01% to 1.00% of Mn, not more than 0.050% of P, not more than 0.005% of S, 15.0% to 25.0% of Cr, 2.0% to 4.0% of Al, not more than 1.00% of Ni, 0.01% to 0.70% of Nb, not more than 0.030% of N, 0.0003% to 0.01% of B, and 0.01% to 0.20% of REM, in percent by mass, and the other part composed of Fe and an inevitable impurity, the method comprising:
 a cold rolling step; and   a final annealing step which is carried out after the cold rolling step,   the final annealing step including a first step of carrying out heating in an inert gas atmosphere and under a condition of a dew point of not higher than −40° C. until a temperature is in a temperature range of 900° C. to 1200° C.   
     
     
         6 . The method as set forth in  claim 5 , wherein the final annealing step includes a second step of maintaining the temperature in the temperature range of 900° C. to 1200° C. for not longer than 5 minutes after the first step. 
     
     
         7 . The method as set forth in  claim 5 , wherein:
 in a case where the ferritic stainless steel obtained through the final annealing step is heated at 1050° C. for 50 hours, the ferritic stainless steel forms an alumina layer mainly composed of alumina; and   the alumina layer is 100% made up of a columnar crystal or satisfies the following formula (1):
   (C)/(E)≥1.2  (1)
 
   
       where (C) is a thickness of a columnar crystal in a cross section obtained by cutting the alumina layer in a thickness direction and (E) is a thickness of an equiaxial crystal in the cross section obtained by cutting the alumina layer in the thickness direction. 
     
     
         8 . The method as set forth in  claim 5 , wherein the ferritic stainless steel further contains at least one selected from the group consisting of not more than 0.50% of Zr, not more than 0.50% of V, not more than 1.0% of Cu, not more than 2.0% of Mo, not more than 2.0% of W, not more than 0.50% of Hf, not more than 0.50% of Sn, not more than 0.5% of Ta, not more than 0.20% of Ti, not more than 0.015% of Mg, and not more than 0.015% of Ca, in percent by mass.