Ferritic Stainless Steel Sheet Excellent in Press Formability and Secondary Formability and its Manufacturing Method

Abstract

A ferritic stainless steel sheet has a composition of C up to 0.02 mass %, Si up to 0.8 mass %, Mn up to 1.5 mass %, P up to 0.050 mass %, S up to 0.01 mass %, 8.0-35.0 mass % of Cr, N up to 0.05 mass %, 0.05-0.40 mass % of Ti and 0.10-0.50 mass % of Nb with a product of (% Ti % N) less than 0.005. Precipitates of 0.15 μm or more in particle size except TiN are distributed in a steel matrix at a rate of 5000-50000/mm 2 . The steel sheet is manufactured by hot-rolling a slab at a finish-temperature of 800° C. or lower, annealing the hot-rolled steel sheet at 450-1080° C., cold-rolling the hot-rolled steel sheet in accompaniment with intermediate-annealing at a temperature within a range of from (a recrystallization-finishing temperature −100° C.) to (a recrystallization-finishing temperature) and then finish-annealing the cold-rolled steel sheet at 1080° C. or lower. The ferritic stainless steel sheet is press-formed with high dimensional accuracy and excellent secondary formability due to controlled distribution of the precipitates.

1 . A ferritic stainless steel sheet excellent in press formability and secondary formability, which has:
 a composition consisting of 0.02 mass % or less of C, 0.8 mass % or less of Si, 1.5 mass % or less of Mn, 0.050 mass % or less of P, 0.01 mass % or less of S, 8.0-35.0 mass % of Cr, 0.05 mass % or less of N, 0.05-0.40 mass % of Ti, 0.10-0.50 mass % of Nb, optionally one or more selected from the group consisting of 0.5 mass % or less of Ni, 3.0 mass % or less of Mo, 2.0 mass % or less of Cu, 0.3 mass % or less of V, 0.3 mass % or less of Zr, 0.3 mass % or less of Al and 0.0100 mass % or less of B, and the balance being Fe except inevitable impurities with a product of (% Ti×% N) less than 0.005, wherein   the metallurgical structure contains precipitates of 0.15 μm or more in particle size other than TiN being distributed at a rate of 5000-50000/mm 2  in a steel matrix.   
   
   
       2 . A method of manufacturing a ferritic stainless steel sheet excellent in press formability and secondary formability, which comprises the steps of:
 providing a slab of a ferritic stainless steel having the composition consisting of 0.02 mass % or less of C, 0.8 mass % or less of Si, 1.5 mass % or less of Mn, 0.050 mass % or less of P, 0.01 mass % or less of S, 8.0-35.0 mass % of Cr, 0.05 mass % or less of N, 0.05-0.40 mass % of Ti, 0.10-0.50 mass % of Nb, optionally one or more selected from the group consisting of 0.5 mass % or less of Ni, 3.0 mass % or less of Mo, 2.0 mass % or less of Cu, 0.3 mass % or less of V, 0.3 mass % or less of Zr, 0.3 mass % or less of Al and 0.0100 mass % or less of B, and the balance being Fe except inevitable impurities with a product of (% Ti×% N) less than 0.005;   hot-rolling the slab at a finish-temperature of 800° C. or lower;   annealing the hot-rolled steel sheet at a temperature within a range of 450-1080° C.;   cold-rolling the annealed steel sheet in accompaniment with at least one intermediate-annealing at a temperature within a range of from (a recrystallization-finishing temperature −100° C.) to (a recrystallization-finishing temperature); and then   finish-annealing the cold-rolled steel sheet at a temperature of 1080° C. or lower.   
   
   
       3 . The method as claimed in  claim 2  wherein, the metallurgical structure contains precipitates of 0.15 μm or more in particle size other than TiN being distributed at a rate of 5000-50000/mm 2  in a steel matrix.