Ferritic stainless steel sheet and method for producing the same

Abstract

A ferritic stainless steel sheet is a ferritic stainless steel sheet which is temper rolled after bright annealing and has a passivation film. The area ratio of minute defects having an area of 10 μm2 or more on the surface of the steel sheet is 0.2% or more and 1.5% or less. In addition, the thickness of the passivation film is 1 nm or more and 10 nm or less, the average concentration of Si in the passivation film is 10 at % or more and 20 at % or less, and the average concentration ratio of Si, Al, Cr and Fe in the passivation film component is (Si+Al+Cr)/Fe>1.0.

1 . A ferritic stainless steel sheet which is temper rolled after bright annealing and has a passivation film, wherein
 an area ratio of minute defects having an area of 10 μm 2  or more on a surface of the steel sheet is 0.2% or more and 1.5% or less,   a thickness of the passivation film is 1 nm or more and 10 nm or less,   an average concentration of Si in the passivation film is 10 at % or more and 20 at % or less, and   an average concentration ratio of Si, Al, Cr and Fe in the passivation film component is (Si+Al+Cr)/Fe>1.0.   
     
     
         2 . The ferritic stainless steel sheet according to  claim 1 , which comprises:
 Cr: 17.0 mass % or more and 21.0 mass % or less, Si: 0.2 mass % or more and 1.0 mass % or less, Al: 0.03 mass % or less, C: 0.03 mass % or less, and N: 0.03 mass % or less.   
     
     
         3 . The ferritic stainless steel sheet according to  claim 1 , which comprises:
 at least one of Nb: 0.1 mass % or more and 1.0 mass % or less, and Cu: 1.0 mass % or less.   
     
     
         4 . The ferritic stainless steel sheet according to  claim 1 , which comprises:
 Mo: 3.0 mass % or less.   
     
     
         5 . The ferritic stainless steel sheet according to  claim 1 , which is for trim and for a car molding material. 
     
     
         6 . A method for producing a ferritic stainless steel sheet, in which a hot rolled pickled material is used as a starting material and at least finishing cold rolling, bright annealing and temper rolling are carried out in said order, wherein
 the surface of the steel sheet is mechanical polished at least once in a step preceding the finishing cold rolling, followed by annealing.   
     
     
         7 . The method for producing a ferritic stainless steel sheet according to  claim 6 , wherein
 a total cold rolling reduction until the bright annealing is 70% or more, and   a cold rolling reduction in the finishing cold rolling is 30% or more.   
     
     
         8 . The method for producing a ferritic stainless steel sheet according to  claim 6 , wherein
 in a final pass of the finishing cold rolling, rolling is carried out using a work roll with an arithmetic average roughness Ra of 0.1 μm or less at a rolling reduction of 12% or more and a rolling speed of 150 m/min or less.   
     
     
         9 . The method for producing a ferritic stainless steel sheet according to  claim 6 , wherein
 the bright annealing is carried out in a hydrogen and nitrogen mixed gas with a hydrogen concentration of 75% or more at a dew point of −45° C. or lower and an annealing temperature of 800° C. or higher and 990° C. or lower.   
     
     
         10 . The method for producing a ferritic stainless steel sheet according to  claim 6 , wherein
 the temper rolling is carried out using a work roll with an arithmetic average roughness Ra of 0.1 μm or less under the condition that an elongation rate be 0.1% or more and 1.0% or less without lubrication.