CPC C21D 9/46 (2013.01) [C21D 8/0236 (2013.01); C21D 8/0247 (2013.01); C22C 38/001 (2013.01); C22C 38/002 (2013.01); C22C 38/02 (2013.01); C22C 38/04 (2013.01); C22C 38/06 (2013.01); C22C 38/22 (2013.01); C22C 38/28 (2013.01); C22C 38/32 (2013.01); C23C 2/04 (2013.01); C23C 2/40 (2013.01); C21D 2211/001 (2013.01); C21D 2211/002 (2013.01); C21D 2211/005 (2013.01); C21D 2211/008 (2013.01)] | 13 Claims |
1. A cold-rolled steel sheet comprising:
by weight %, 0.13-0.25% of carbon (C), 1.0-2.0% of silicon (Si), 1.5-3.0% of manganese (Mn), 0.08-1.5% of aluminum (Al)+chromium (Cr)+molybdenum (Mo), 0.1% or less of phosphorus (P), 0.01% or less of sulfur (S), 0.01% or less of nitrogen (N), and a balance of Fe and inevitable impurities;
by area fraction, 3-25% of ferrite, 20-40% of martensite, and 5-20% of retained austenite; and
a nickel concentration layer, formed by nickel (Ni) introduced from outside, as a surface layer,
wherein a concentration of nickel (Ni) at a depth of 1 μm from a surface is 0.15 wt % or more.
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