Nb-CONTAINING FERRITIC STAINLESS STEEL SHEET AND MANUFACTURING METHOD THEREFOR

Abstract

A ferritic stainless steel raw material steel sheet for annealing and cold rolling, has a composition containing, in mass percentage, from 0.004 to 0.030% of C, 1.50% or less of Si, 1.50% or less of Mn, 0.040% or less of P, 0.010% or less of S, from 12.0 to 25.0% of Cr, from 0.005 to 0.025% of N, from 0.20 to 0.80% of Nb, 0.10% or less of Al, from 0 to 3.0% of Mo, from 0 to 2.0% of Cu, from 0 to 2.0% of Ni, from 0 to 0.30% of Ti, from 0 to 0.0030% of B, and the balance of Fe, with unavoidable impurities. An existing amount of a Nb-containing precipitate of 0.20% by mass or more is provided. The raw material steel sheet is subjected to annealing at a temperature of from 900 to 1,100° C., and then subjected to cold rolling and finish annealing.

1 . A ferritic stainless steel raw material steel sheet for annealing and cold rolling, having a chemical composition containing, in terms of percentage by mass, from 0.004 to 0.030% of C, 1.50% or less of Si, 1.50% or less of Mn, 0.040% or less of P, 0.010% or less of S, from 12.0 to 25.0% of Cr, from 0.005 to 0.025% of N, from 0.20 to 0.80% of Nb, 0.10% or less of Al, from 0 to 3.0% of Mo, from 0 to 2.0% of Cu, from 0 to 2.0% of Ni, from 0 to 0.30% of Ti, from 0 to 0.0030% of B, and the balance of Fe, with unavoidable impurities, and having an existing amount of a Nb-containing precipitate of 0.20% by mass or more. 
     
     
         2 . The ferritic stainless steel raw material steel sheet according to  claim 1 , wherein the Nb-containing precipitate has an average particle diameter of from 2.0 to 10.0 μm in terms of circle equivalent diameter. 
     
     
         3 . A manufacturing method for the ferritic stainless steel raw material steel sheet according to  claim 1 , comprising hot-rolling a cast piece of a steel having a chemical composition containing, in terms of percentage by mass, from 0.004 to 0.030% of C, 1.50% or less of Si, 1.50% or less of Mn, 0.040% or less of P, 0.010% or less of S, from 12.0 to 25.0% of Cr, from 0.005 to 0.025% of N, from 0.20 to 0.80% of Nb, 0.10% or less of Al, from 0 to 3.0% of Mo, from 0 to 2.0% of Cu, from 0 to 2.0% of Ni, from 0 to 0.30% of Ti, from 0 to 0.0030% of B, and the balance of Fe, with unavoidable impurities, including heating the cast piece to a temperature of from 1,000 to 1,150° C., and then extracting. 
     
     
         4 . A ferritic stainless steel cold-rolled and annealed steel sheet having a chemical composition containing, in terms of percentage by mass, from 0.004 to 0.030% of C, 1.50% or less of Si, 1.50% or less of Mn, 0.040% or less of P, 0.010% or less of S, from 12.0 to 25.0% of Cr, from 0.005 to 0.025% of N, from 0.20 to 0.80% of Nb, 0.10% or less of Al, from 0 to 3.0% of Mo, from 0 to 2.0% of Cu, from 0 to 2.0% of Ni, from 0 to 0.30% of Ti, from 0 to 0.0030% of B, and the balance of Fe, with unavoidable impurities, having an existing amount of a Nb-containing precipitate of 0.19% by mass or more, and having a waviness height W defined by the following item (A) of 2.0 μm or less:
 (A) a tensile test piece of JIS No. 5 is collected from the steel sheet with the longitudinal direction of the test piece being directed to the rolling direction of the steel sheet; a tensile strain is applied to the tensile test piece until an elongation ratio of a parallel portion of the test piece reaches 20%; then the tensile strain is removed to provide a test piece; a surface profile of the test piece in a center portion in the longitudinal direction is measured in the transverse direction (i.e., the direction perpendicular to the rolling direction); a waviness curve having a wavelength component of from 0.5 to 8.0 mm is determined according to JIS B0601:2013 with cutoff values λf of 8.0 mm and λc of 0.5 mm; and an arithmetic average waviness Wa (μm) is obtained from the waviness curve, and is designated as the waviness height W. 
 
     
     
         5 . The ferritic stainless steel cold-rolled and annealed steel sheet according to  claim 4 , wherein the cold-rolled and annealed steel sheet has a sheet thickness of from 1.0 to 2.5 mm. 
     
     
         6 . A manufacturing method for a ferritic stainless steel cold-rolled and annealed steel sheet, comprising:
 hot-rolling a cast piece of a steel having a chemical composition containing, in terms of percentage by mass, from 0.004 to 0.030% of C, 1.50% or less of Si, 1.50% or less of Mn, 0.040% or less of P, 0.010% or less of S, from 12.0 to 25.0% of Cr, from 0.005 to 0.025% of N, from 0.20 to 0.80% of Nb, 0.10% or less of Al, from 0 to 3.0% of Mo, from 0 to 2.0% of Cu, from 0 to 2.0% of Ni, from 0 to 0.30% of Ti, from 0 to 0.0030% of B, and the balance of Fe, with unavoidable impurities, including heating the cast piece to a temperature of from 1,000 to 1,150° C., and then extracting, so as to provide a raw material steel sheet having an existing amount of a Nb-containing precipitate of 0.20% by mass or more;   subjecting the raw material steel sheet to annealing at a temperature of from 900 to 1,100° C.; and   subjecting the steel sheet having been subjected to the annealing to cold rolling and annealing.