Ferritic stainless steel having improved corrosion resistance, and method for manufacturing same
Disclosed are a ferritic stainless steel having improved corrosion resistance and a method for manufacturing same. The ferritic stainless steel according to an embodiment of the present disclosure includes, in percent by weight (wt %), 0.001 to 0.05% of C, 0.001 to 0.05% of N, 0.1 to 1.0% of Si, 0.1 to 1.0% of Mn, 12.0 to 22.0% of Cr, 0.01 to 1.0% of Ti, and 0.01 to 1.0% of Nb, with the balance being Fe and inevitable impurities, wherein an area ratio of microdefects is 2% or less, and a sulfur (S) content in a surface film within 5 mm from the surface is 10% or less.
1 . A ferritic stainless steel comprising, in percent by weight (wt %), 0.001 to 0.05‰ of C, 0.001 to 0.05‰ of N, 0.1 to 1.00% of Si, 0.1 to 1.0‰ of Mn, 12.0 to 22.0‰ of Cr, 0.01 to 1.0‰ of Ti, and 0.01 to 1.0‰ of Nb, with the balance being Fe and inevitable impurities,
wherein an area ratio of microdefects is 2‰ or less, wherein a distribution density of microdefects having a length of 100 μm or more is 5 pieces/mm 2 or less, and wherein a sulfur (S) content in a surface film within a depth of 5 nm from the surface of the stainless steel is 10 wt % or less.
2 . The ferritic stainless steel according to claim 1 , further comprising at least one of 0.01 to 2.0% of Mo, 0.1% or less (excluding 0) of Al, 1.0% or less (excluding 0) of Cu, 0.01 to 0.3% of V, 0.01 to 0.3% of Zr and 0.0010 to 0.0100% of B.
3 . The ferritic stainless steel according to claim 1 , wherein the ferritic stainless steel satisfies Expression (1) below:
5.12× A+S< 17, Expression (1):
wherein A is the area ratio of microdefects (%), and
wherein S is the sulfur content in the surface film within the depth of 5 nm from the surface of the stainless steel (wt %).
4 . The ferritic stainless steel according to claim 1 , wherein the area ratio of is 1.6% or less.
5 . The ferritic stainless steel according to claim 1 , wherein the area ratio of is 1.0% or less.
6 . The ferritic stainless steel according to claim 1 , wherein the distribution density of microdefects having a length of 100 μm or more is 1.7 pieces/mm 2 .
7 . The ferritic stainless steel according to claim 1 , wherein the distribution density of microdefects having a length of 100 μm or more is 0.5 pieces/mm 2 .
8 . The ferritic stainless steel according to claim 1 , wherein the distribution density of microdefects having a length of 100 μm or more is 0.85 pieces/mm 2 .
9 . A method for manufacturing the ferritic stainless steel of claim 1 , the method comprising: hot rolling a slab comprising, in percent by weight (wt %), 0.001 to 0.05% of C, 0.001 to 0.05% of N, 0.1 to 1.0% of Si, 0.1 to 1.0% of Mn, 12.0 to 22.0% of Cr, 0.01 to 1.0% of Ti, and 0.01 to 1.0% of Nb, with the balance being Fe and inevitable impurities to form a hot rolled steel sheet and hot annealing the formed hot-rolled steel sheet; cold rolling and cold annealing the hot-rolled, annealed steel sheet twice or more by controlling a roll diameter to 70 mm or less; degreasing the cold-rolled, annealed steel sheet for 60 seconds to 120 seconds; and bright annealing the cold-rolled steel sheet, wherein surface polishing treatment is introduced after hot annealing or after primary cold rolling, and performed once or twice; thereby producing the ferritic stainless steel of claim 1 .
10 . The method according to claim 9 , wherein the ferritic stainless steel further comprises one of 0.01 to 2.0% of Mo, 0.10% or less (excluding 0) of Al, 1.0% or less (excluding 0) of Cu, 0.01 to 0.3% of V, 0.01 to 0.3% of Zr, and 0.0010 to 0.0100% of B.
11 . The method according to claim 9 , wherein the cold rolling comprises:
primary cold rolling performed at a reduction ratio of 40% or more; and
secondary cold rolling performed at a reduction ratio of 40% or more,
wherein a total reduction ratio is 80% or more.
12 . The method according to claim 9 , wherein the cold rolling further comprises tertiary cold rolling performed at a reduction ratio of 40% or more.
13 . The method according to claim 9 , wherein a re-heating temperature is from 1050 to 1280° C., and a finishing rolling temperature is from 800 to 950° C. during the hot rolling.
14 . The method according to claim 9 , wherein the surface polishing treatment is performed by removing the surface layer by 7 μm or more from the surface of the steel sheet using a polishing belt having a roughness of #70 mesh or more.
15 . The method according to claim 9 , wherein the cold annealing is performed at a temperature of 850 to 1100° C.
16 . The method according to claim 9 , wherein the bright annealing is performed at a temperature of 850 to 1100° C.
17 . The method according to claim 9 , further comprising a skin pass rolling step performed using a work roll having an average roughness of #600 mesh or more.
18 . The method according to claim 17 , wherein the skin pass rolling is performed twice to five times.
19 . The ferritic stainless steel according to claim 1 , wherein the sulfur (S) content in the surface film within a depth of 5 nm from the surface of the stainless steel is 5 wt % or less.