High strength stainless steel sheet excellent in fatigue characteristics, and production method therefor

Abstract

A thin steel sheet contains, in terms of percentage by mass, from 0.010 to 0.200% of C, more than 2.00% and 4.00% or less of Si, from 0.01 to 3.00% of Mn, 3.00% or more and less than 10.00% of Ni, from 11.00 to 20.00% of Cr, from 0.010 to 0.200% of N, from 0 to 3.00% of Mo, from 0 to 1.00% of Cu, from 0 to 0.008% of Ti, from 0 to 0.008% of Al, and the balance of Fe, with unavoidable impurities; and having a number density of a non-metallic inclusion lining up with an interparticle distance in the rolling direction of 20 mm or less and an interparticle distance in the sheet thickness direction of 10 mm or less that has a length in the rolling direction of 40 mm or more of 3.0 or less per square millimeter on the L cross section.

1 . A stainless steel sheet: having a sheet thickness of from 20 to 500 μm; having a chemical composition containing, in terms of percentage by mass, from 0.010 to 0.200% of C, more than 2.00% and 4.00% or less of Si, from 0.01 to 3.00% of Mn, 3.00% or more and less than 10.00% of Ni, from 11.00 to 20.00% of Cr, from 0.010 to 0.200% of N, from 0 to 3.00% of Mo, from 0 to 1.00% of Cu, from 0 to 0.008% of Ti, from 0 to 0.008% of Al, and the balance of Fe, with unavoidable impurities; and having, with an assumption that in a cross section (L cross section) in parallel to a rolling direction and a sheet thickness direction, a group of non-metallic inclusion particles that line up with an interparticle distance in the rolling direction of 20 μm or less and an interparticle distance in the sheet thickness direction of 10 μm or less is one non-metallic inclusion, a number density of the non-metallic inclusion having a length in the rolling direction of 40 μm or more of 3.0 or less per square millimeter on the L cross section. 
     
     
         2 . The stainless steel sheet according to  claim 1 , wherein the non-metallic inclusion having a length in the rolling direction of 40 μm or more contains one or two of (i) and (ii): (i) a TiN based inclusion particle and (ii) a spinel based inclusion particle containing one or more of Al and Mg. 
     
     
         3 . The stainless steel sheet according to  claim 1 , wherein the stainless steel sheet has a tensile strength in the rolling direction of 2,000 N/mm 2  or more. 
     
     
         4 . The stainless steel sheet according to  claim 1 , wherein the stainless steel sheet has a mixed structure of a deformation induced martensite phase and an austenite phase as a matrix (metal basis material). 
     
     
         5 . A production method of a stainless steel sheet comprising:
 a step of providing a molten steel having a chemical composition (A) below, in such a manner that in component control by adding an auxiliary raw material and a slag forming flux to a molten steel having a C content of 0.20% or less having a Cr oxide-containing slag on a molten steel surface after subjecting to a decarburization process by blowing oxygen into a Cr-containing molten iron, a container housing the molten steel, the auxiliary raw material, and the slag forming flux used are selected to make a Ti content in the molten steel of 0.008% by mass or less and an Al content therein of 0.008% by mass or less, at least an Fe—Si alloy as the auxiliary raw material is dissolved in the molten steel to perform deoxidation, reduction and recovery of Cr in the slag into the molten steel, and control of a Si content in the steel, and a Ca-containing slag forming flux is added to control a slag basicity (i.e., a mass ratio of CaO/SiO 2 ) to from 1.3 to 1.5;   a step of providing a cast piece by casting the molten steel obtained in the preceding step;   a step of providing a hot rolled steel sheet by subjecting the cast piece to hot working including at least hot rolling; and   a step of providing a cold rolled steel sheet having a sheet thickness of from 20 to 500 μm by subjecting the hot rolled steel sheet to annealing and cold rolling one or more times:   (A) in terms of percentage by mass, from 0.010 to 0.200% of C, more than 2.00% and 4.00% or less of Si, from 0.01 to 3.00% of Mn, 3.00% or more and less than 10.00% of Ni, from 11.00 to 20.00% of Cr, from 0.010 to 0.200% of N, from 0 to 3.00% of Mo, from 0 to 1.00% of Cu, from 0 to 0.008% of Ti, from 0 to 0.008% of Al, and the balance of Fe, with unavoidable impurities.   
     
     
         6 . The production method of a stainless steel sheet according to  claim 5 , wherein the container housing the molten steel used is a container having a refractory constituting the inner surface of the container that is not used for housing a molten steel (i.e., a new ladle). 
     
     
         7 . The production method of a stainless steel sheet according to  claim 5 , wherein the Fe—Si alloy used has an Al content of 0.05% by mass or less, and a Ti content of 0.05% by mass or less. 
     
     
         8 . The production method of a stainless steel sheet according to  claim 5 , wherein the production method further comprises:
 a step of subjecting the cold rolled steel sheet to an aging treatment.   
     
     
         9 . The production method of a stainless steel sheet according to  claim 8 , wherein the steel sheet obtained by the production method has a mixed structure of a deformation induced martensite phase and an austenite phase for a matrix (metal basis material), and has a tensile strength in the rolling direction of 2,000 N/mm 2  or more.