Ferritic stainless steel sheet cover member and production method for ferritic stainless steel sheet

Abstract

In a ferritic stainless steel sheet, the arithmetic average roughness Ra is 0.2 μm or more and 1.2 μm or less. In addition, the dull pattern transfer rate on the steel sheet surface is 15% or more and 70% or less. Furthermore, micropits with a depth of 0.5 μm or more and an open area of 10 μm 2 or more which are formed on the steel sheet surface have an existing density of 10.0 or less per 0.01 mm 2 and an open area ratio of 1.0% or less. In addition, a film formed on the steel sheet surface is constituted from an oxide containing SiO 2 as a main constituent, which oxide contains at least Si, N, Al, Mn, Cr, Fe, Nb, Ti and O as film-forming elements other than C, wherein the Si content is 10 at % or more, and the N content is 10 at % or less.

1 . A ferritic stainless steel sheet, which is temper-rolled using a dull roll after finishing cold rolling and bright annealing, comprising:
 an arithmetic average roughness Ra in a direction perpendicular to a rolling direction on a steel sheet surface is 0.2 μm or more and 1.2 μm or less,   a transfer rate, which is an area rate of a part to which a dull pattern is transferred on the steel sheet surface, is 15% or more and 70% or less,   micropits with a depth of 0.5 μm or more and an open area of 10 μm 2  or more which are formed on the steel sheet surface have an existing density of 10.0 or less per 0.01 mm 2  on the steel sheet surface and an open area ratio of 1.0% or less on the steel sheet surface, and   a film formed on the steel sheet surface is constituted from an oxide containing SiO 2  as a main constituent, which oxide contains at least Si, N, Al, Mn, Cr, Fe, Nb, Ti and O as film-forming elements other than C, wherein the Si content is 10 at % or more, and the N content is 10 at % or less.   
     
     
         2 . The ferritic stainless steel sheet according to  claim 1 , containing C: 0.15 mass % or less, Si: 0.1 mass % or more and 2.0 mass % or less, Cr: 10.0 mass % or more and 32.0 mass % or less, and at least one of Nb: 0.01 mass % or more and 0.8 mass % or less and Ti: 0.01 mass % or more and 0.5 mass % or less, and the rest including Fe and inevitable impurities. 
     
     
         3 . The ferritic stainless steel sheet according to  claim 2 , containing at least one of Mo: 0.2 mass % or more and 5.0 mass % or less and Cu: 0.1 mass % or more and 3.0 mass % or less. 
     
     
         4 . The ferritic stainless steel sheet according to  claim 1 , containing C: 0.15 mass % or less, Si: 0.1 mass % or more and 2.0 mass % or less, Mn: 2.0 mass % or less, P: 0.04 mass % or less, S: 0.03 mass % or less, Ni: 0.6 mass % or less, Cr: 11.0 mass % or more and 32.0 mass % or less, Mo: 0 mass % or more and 3.0 mass % or less, Cu: 0 mass % or more and 1.0 mass % or less, Nb: 0 mass % or more and 1.0 mass % or less, Ti: 0 mass % or more and 1.0 mass % or less, Al: 0 mass % or more and 0.12 mass % or less, N: 0.025 mass % or less and B: 0 mass % or more and 0.01 mass % or less, and the rest including Fe and inevitable impurities. 
     
     
         5 . A cover member of hard disk drives,
 the cover member being formed from a ferritic stainless steel sheet according to any one of  claim 1 .   
     
     
         6 . A production method for a ferritic stainless steel sheet, comprising:
 finishing cold rolling a hot rolled steel sheet after hot rolling,   bright annealing, following the finishing cold rolling, as finishing annealing, and   temper rolling using a dull roll, wherein   rolling is carried out at a cold rolling reduction of 30% or more in finishing cold rolling, and a rolling reduction of 15% or more and a rolling speed of 200 mm/min or less using a work roll with an arithmetic average roughness Ra of 0.3 μm or less at least in the final rolling pass, and   the total cold rolling reduction until bright annealing is 70% or more.   
     
     
         7 . The production method for a ferritic stainless steel sheet according to  claim 6 , wherein,
 in finishing annealing, bright annealing is carried out in a hydrogen-nitrogen mixed gas atmosphere with a hydrogen ratio of 70 vol % or more under the condition that the dew point is −70° C. or higher and −50° C. or lower and the temperature is 800° C. or higher and 1100° C. or lower.   
     
     
         8 . The production method for a ferritic stainless steel sheet according to  claim 6 , wherein
 in temper rolling, rolling is carried out in a single pass or more using a dull roll with a roll diameter of 500 mm or more and an arithmetic average roughness Ra of 1.0 μm or more and 3.5 μm or less at an elongation rate in a single pass of 0.5% or less, and the total elongation rate is 0.2% or more and 1.4% or less.