Ferritic stainless steel and method of manufacturing the same

Abstract

Provided is a ferritic stainless steel and method of manufacturing the same. The ferritic stainless steel comprises, by weight percent, 0.02% or less of carbon (C), 0.02% or less of nitrogen (N), 1.0% or less of silicon (Si), 1.20% or less of manganese (Mn), 0.05% or less of phosphorus (P), 10.0 to 25.0% of chromium (Cr), 0.5 to 2.0% of molybdenum (Mo), 0.01 to 0.30% of titanium (Ti), 0.30 to 0.70% of niobium (Nb), and the remainder consisting of iron (Fe) and other unavoidable impurities. The ferritic stainless steel comprises niobium (Nb) laves phase precipitate, and precipitate containing niobium (Nb) and carbon (C) or nitrogen (N), 30 to 70% of the precipitates are distributed in a region within 1 μm from the grain boundaries, and the average particle size of the precipitates is 0.5 μm or less.

1 . A ferritic stainless steel comprising: by weight percent, 0.02% or less of carbon (C), 0.02% or less of nitrogen (N), 1.0% or less of silicon (Si), 1.20% or less of manganese (Mn), 0.05% or less of phosphorus (P), 10.0 to 25.0% of chromium (Cr), 0.5 to 2.0% of molybdenum (Mo), 0.01 to 0.30% of titanium (Ti), 0.30 to 0.70% of niobium (Nb), and the remainder consisting of iron (Fe) and other unavoidable impurities,
 characterized in that the ferritic stainless steel comprises niobium (Nb) laves phase precipitate, and precipitate containing niobium (Nb) and carbon (C) or nitrogen (N), 30 to 70% of the precipitates are distributed in a region within 1 μm from the grain boundaries, and the average particle size of the precipitates is 0.5 μm or less.   
     
     
         2 . The ferritic stainless steel according to  claim 1 , wherein the weight ratio of niobium (Nb)/titanium (Ti) is 2 to 10. 
     
     
         3 . The ferritic stainless steel according to  claim 1 , wherein the niobium (Nb) laves phase precipitate comprises at least one selected from the group consisting of Fe 2 Nb, FeCrNb, and Cr 2 Nb. 
     
     
         4 . The ferritic stainless steel according to  claim 1 , wherein the precipitate containing niobium (Nb) and carbon (C) or nitrogen (N) comprises at least one selected from the group consisting of niobium nitride (NbN), niobium carbide (NbC), and niobium carbonitride (NbCN). 
     
     
         5 . The ferritic stainless steel according to  claim 1 , wherein the average particle size of the precipitates is 0.35 μm or less. 
     
     
         6 . The ferritic stainless steel according to  claim 1 , wherein the mass of niobium (Nb) in the precipitate containing niobium (Nb) and carbon (C) or nitrogen (N), with respect to the total mass of the precipitate containing niobium (Nb) and carbon (C) or nitrogen (N) is less than 30%. 
     
     
         7 . The ferritic stainless steel according to  claim 1 , wherein the ferritic stainless steel has a tensile strength of 30 MPa or more at 900° C. 
     
     
         8 . The ferritic stainless steel according to  claim 1 , wherein the ferritic stainless steel has 500 or more times of thermal fatigue cycles at the temperature range of 200 to 900° C. at the constraint ratio of 50%. 
     
     
         9 . A method of manufacturing a ferritic stainless steel comprising: a step of reheating a ferritic stainless steel comprising, by weight percent, 0.02% or less of carbon (C), 0.02% or less of nitrogen (N), 1.0% or less of silicon (Si), 1.20% or less of manganese (Mn), 0.05% or less of phosphorus (P), 10.0 to 25.0% of chromium (Cr), 0.5 to 2.0% of molybdenum (Mo), 0.01 to 0.30% of titanium (Ti), 0.30 to 0.70% of niobium (Nb), and the remainder consisting of iron (Fe) and other unavoidable impurities to 1.100 to 1,300° C.:
 a step of rough rolling the stainless steel by a plurality of times; and 
 a step of finish rolling the stainless steel, 
 characterized in that in the rough rolling step, the final two rough rolling is performed at a total reduction rate of 50% or more, and is maintained for the time of the following Formula (1) after the rough rolling and before the finish rolling.
   8,000/(reheating temperature−1,000)<time (second)<120  Formula (1)
 
 
 
     
     
         10 . The method of manufacturing a ferritic stainless steel according to  claim 9 , wherein a weight ratio of niobium (Nb)/titanium (Ti) is 2 to 10. 
     
     
         11 . The method of manufacturing a ferritic stainless steel according to  claim 9 , further comprising a step of coiling after the finish rolling step, wherein the coiling temperature is 500 to 700° C.