HOT-ROLLED Nb-CONTAINING FERRITIC STAINLESS STEEL SHEET AND METHOD FOR PRODUCING SAME, AND COLD-ROLLED Nb-CONTAINING FERRITIC STAINLESS STEEL SHEET AND METHOD FOR PRODUCING SAME

Abstract

The hot-rolled Nb-containing ferritic stainless steel sheet of the present invention has a composition containing C: 0.030 mass % or less, Si: 2.00 mass % or less, Mn: 2.00 mass % or less, P: 0.050 mass % or less, S: 0.040 mass % or less, Cr: 10.00 mass % to 25.00 mass %, N: 0.030 mass % or less and Nb: 0.01 mass % to 0.80 mass %, with the balance being made up of Fe and unavoidable impurities. In this hot-rolled Nb-containing ferritic stainless steel sheet, the precipitation amount of Nb carbonitrides is 0.2 mass % or more, and the number of Laves phases having a grain size of 0.1 μm or less is 10 or fewer per 10 μm 2 of surface area.

1 . A hot-rolled Nb-containing ferritic stainless steel sheet having a composition containing C: 0.030 mass % or less, Si: 2.00 mass % or less, Mn: 2.00 mass % or less, P: 0.050 mass % or less, S: 0.040 mass % or less, Cr: 10.00 mass % to 25.00 mass %, N: 0.030 mass % or less and Nb: 0.01 mass % to 0.80 mass %, with the balance being made up of Fe and unavoidable impurities,
 wherein the precipitation amount of Nb carbonitrides is 0.2 mass % or more, and the number of Laves phases having a grain size of 0.1 μm or less is 10 or fewer per 10 μm 2  of surface area.   
     
     
         2 . The hot-rolled Nb-containing ferritic stainless steel sheet of  claim 1 , having a composition further containing one or more from among Ni: 2.00 mass % or less, Mo: 2.50 mass % or less, Cu: 1.80 mass % or less, Co: 0.50 mass % or less, Al: 0.50 mass % or less, W: 1.80 mass % or less, V: 0.30 mass % or less, Ti: 0.50 mass % or less, Zr: 0.20 mass % or less, B: 0.0050 mass % or less, rare earth elements: 0.100 mass % or less and Ca: 0.0050 mass % or less. 
     
     
         3 . The hot-rolled Nb-containing ferritic stainless steel sheet of  claim 1 , used for producing an exhaust pipe flange part. 
     
     
         4 . A method for producing a hot-rolled Nb-containing ferritic stainless steel sheet, the method comprising: holding at a temperature of 1000° C. to 1100° C. for 60 seconds or longer, and setting a finish hot rolling temperature to 850° C. or higher, during hot rolling of a stainless steel slab having a composition containing C: 0.030 mass % or less, Si: 2.00 mass % or less, Mn: 2.00 mass % or less, P: 0.050 mass % or less, S: 0.040 mass % or less, Cr: 10.00 mass % to 25.00 mass %, N: 0.030 mass % or less and Nb: 0.01 mass % to 0.80 mass %, with the balance being made up of Fe and unavoidable impurities, and, after hot rolling, performing coiling at a coiling temperature of 550° C. or lower. 
     
     
         5 . The method for producing a hot-rolled Nb-containing ferritic stainless steel sheet of  claim 4 , wherein the stainless steel slab has a composition further containing one or more from among Ni: 2.00 mass % or less, Mo: 2.50 mass % or less, Cu: 1.80 mass % or less, Co: 0.50 mass % or less, Al: 0.50 mass % or less, W: 1.80 mass % or less, V: 0.30 mass % or less, Ti: 0.50 mass % or less, Zr: 0.20 mass % or less, B: 0.0050 mass % or less, rare earth elements: 0.100 mass % or less and Ca: 0.0050 mass % or less. 
     
     
         6 . A cold-rolled Nb-containing ferritic stainless steel sheet having a composition containing C: 0.030 mass % or less, Si: 2.00 mass % or less, Mn: 2.00 mass % or less, P: 0.050 mass % or less, S: 0.040 mass % or less, Cr: 10.00 mass % to 25.00 mass %, N: 0.030 mass % or less and Nb: 0.01 mass % to 0.80 mass %, with the balance being made up of Fe and unavoidable impurities,
 wherein the precipitation amount of Nb carbonitrides is 0.2 mass % or more, the number of Laves phases having a grain size of 0.1 μm or less is 10 or fewer per 10 μm 2  of surface area, and the r-value is 1.2 or greater.   
     
     
         7 . The cold-rolled Nb-containing ferritic stainless steel sheet of  claim 6 , having a composition further containing one or more from among Ni: 2.00 mass % or less, Mo: 2.50 mass % or less, Cu: 1.80 mass % or less, Co: 0.50 mass % or less, Al: 0.50 mass % or less, W: 1.80 mass % or less, V: 0.30 mass % or less, Ti: 0.50 mass % or less, Zr: 0.20 mass % or less, B: 0.0050 mass % or less, rare earth elements: 0.100 mass % or less and Ca: 0.0050 mass % or less. 
     
     
         8 . The cold-rolled Nb-containing ferritic stainless steel sheet of  claim 6 , used for producing an exhaust pipe part. 
     
     
         9 . A method for producing a cold-rolled Nb-containing ferritic stainless steel sheet, the method comprising annealing the hot-rolled Nb-containing ferritic stainless steel sheet of  claim 1 , and performing thereafter cold rolling at a reduction ratio of 70% or higher, and annealing the cold-rolled steel sheet. 
     
     
         10 . The hot-rolled Nb-containing ferritic stainless steel sheet of  claim 2 , used for producing an exhaust pipe flange part. 
     
     
         11 . The cold-rolled Nb-containing ferritic stainless steel sheet of  claim 7 , used for producing an exhaust pipe part. 
     
     
         12 . A method for producing a cold-rolled Nb-containing ferritic stainless steel sheet, the method comprising annealing the hot-rolled Nb-containing ferritic stainless steel sheet of  claim 2 , and performing thereafter cold rolling at a reduction ratio of 70% or higher, and annealing the cold-rolled steel sheet.