Ferritic stainless steel having reduced carbon sludge adsorption for exhaust system heat exchanger and method of manufacturing same
Provided is a ferritic stainless steel for an exhaust system heat exchanger and a method of manufacturing the same. The ferritic stainless steel includes, in percent (%) by weight of the entire composition, 0.003 to 0.1% of carbon (C), 0.01 to 2.0% of silicon (Si), 0.01 to 1.5% of manganese (Mn), 0.05% or less of phosphorus (P), 0.005% or less of sulfur (S), 10 to 30% of chromium (Cr), 0.001 to 0.10% of titanium (Ti), 0.001 to 0.15% of aluminum (Al), 0.003 to 0.03% of nitrogen (N), 0.3 to 0.6% of niobium (Nb), 0.01 to 2.5% of molybdenum (Mo), and the remainder of iron (Fe) and other inevitable impurities, wherein TiN precipitates having a size of 0.1 μm or more are distributed in a surface layer of a ferrite matrix at a concentration of 2.5*10 4 ea/mm 2 or less.
1. A ferritic stainless steel for an exhaust system heat exchanger, the ferritic stainless steel comprising, in percent (%) by weight of the entire composition, 0.003 to 0.1% of carbon (C), 0.01 to 2.0% of silicon (Si), 0.01 to 1.5% of manganese (Mn), 0.05% or less of phosphorus (P), 0.005% or less of sulfur (S), 10 to 30% of chromium (Cr), 0.02 to 0.10% of titanium (Ti), 0.001 to 0.15% of aluminum (Al), 0.008 to 0.03% of nitrogen (N), 0.3 to 0.6% of niobium (Nb), 0.01 to 2.5% of molybdenum (Mo), and the remainder of iron (Fe) and other inevitable impurities,
wherein:
TiN precipitates having a size of 0.1 μm or more are distributed in a surface layer of a ferrite matrix at a concentration of 2.5*10 4 ea/mm 2 or less and TiN precipitates, TiN·NbC complex precipitates, and NbC precipitates having a size of 0.1 μm or more respectively are distributed in the surface layer of the ferrite matrix and satisfy Expression 1:
{ Z /( X+Y )}≥20 Expression 1
where X is the number of the TiN precipitates per unit area, ea/mm 2 , Y is the number of the TiN·NbC complex precipitates per unit area, ea/mm 2 , and Z is the number of the NbC precipitates per unit area, ea/mm 2 .
2. The ferritic stainless steel of claim 1 , further comprising, in percent (%) by weight of the entire composition, 0.01 to 0.15% of copper (Cu).
3. The ferritic stainless steel of claim 1 , comprising, in percent (%) by weight of the entire composition, 0.09 to 0.10% of titanium (Ti) and 0.008 to 0.010% of nitrogen (N).
4. The ferritic stainless steel of claim 1 , wherein the TiN·NbC complex precipitates are distributed in the surface layer of the ferrite matrix at a concentration of 1.3*10 4 ea/mm 2 or less.
5. The ferritic stainless steel of claim 1 , wherein the NbC precipitates are distributed in the surface layer of the ferrite matrix at a concentration of 9.6*10 5 ea/mm 2 or more.
6. A method of manufacturing the ferritic stainless steel of claim 1 , the method comprising:
cooling a slab at an average cooling rate of 6° C./sec or more until a temperature of a surface of the slab reaches 1,100° C. during a continuous casting process, the slab made of a molten steel comprising, in percent (%) by weight of the entire composition, 0.003 to 0.1% of carbon (C), 0.01 to 2.0% of silicon (Si), 0.01 to 1.5% of manganese (Mn), 0.05% or less of phosphorus (P), 0.005% or less of sulfur (S), 10 to 30% of chromium (Cr), 0.02 to 0.10% of titanium (Ti), 0.001 to 0.15% of aluminum (Al), 0.008 to 0.03% of nitrogen (N), 0.3 to 0.6% of niobium (Nb), 0.01 to 2.5% of molybdenum (Mo), and the remainder of iron (Fe) and other inevitable impurities;
re-heating and maintaining the slab at a temperature of 1,100 to 1,200° C. for 5 to 15 minutes; and
water-cooling the slab to a temperature of 400° C.