Bilayer composition for surface treatment of steel plate and surface-treated steel plate using same

A bilayer composition for surface treatment of a steel plate, comprising:

• an undercoat coating composition comprising 1 to 12 wt % of a phenoxy resin, 0.001 to 1.0 wt % of colloidal silica, 0.001 to 1.0 wt % of a silane coupling agent, 0.1 to 1.0 wt % of a corrosion inhibitor, 0.001 to 1.0 wt % of a phosphoric acid compound as a long-term corrosion resistance improving agent, and a balance of water, based on a total weight of the undercoat coating composition;
• and
• a topcoat coating composition applied to the undercoat coating composition to be dried, and comprising 0.1 to 5.0 wt % of an acrylic acid resin, 30 to 50 wt % of colloidal silica, 40 to 60 wt % of alkoxy silane, 5 to 15 wt % of an acrylate-based monomer, 0.01 to 1.00 wt % of an acidity control agent, and a balance of an organic solvent, based on a total weight of the topcoat coating composition.