Surface-treated steel sheet and method for producing surface-treated steel sheet
The object of the invention is to provide a surface-treated steel sheet having a coating having fine adhesiveness to an adhesive on the surface and having excellent corrosion resistance, and a production method thereof. Provided is a surface-treated steel sheet including: a steel sheet; a plated layer containing zinc on the steel sheet; and a coating on the plated layer, wherein the coating contains an acrylic resin, zirconium, vanadium, phosphorus and cobalt, and the acrylic resin has an area ratio of 80 to 100 area % in an area from a surface to a thickness of one-fifth of a film thickness of the coating, and an area ratio of 5 to 50 area % in an area including areas from the film thickness center of the coating to a thickness of one-tenth of the film thickness toward the surface side and toward the plated layer side.
1. A surface-treated steel sheet comprising:
a steel sheet;
a plated layer that contains zinc and is formed on the steel sheet; and
a coating formed on the plated layer,
wherein the coating contains an acrylic resin, zirconium, vanadium, phosphorus and cobalt,
the acrylic resin has an area ratio of 80 to 100 area % in an area from a surface of the coating to a thickness of one-fifth of a film thickness of the coating on a cross-section of the coating, and the acrylic resin has an area ratio of 5 to 50 area % in an area including an area from the film thickness center of the coating to a thickness of one-tenth of the film thickness toward the surface side and an area from the film thickness center to a thickness of one-tenth of the film thickness toward the plated layer side,
wherein the zirconium is contained in the coating by 4 to 400 mg/m 2 in terms of metal.
2. The surface-treated steel sheet according to claim 1 , wherein
a plurality of island projections is present on the surface of the coating in a planar view, and
when three or more virtual straight lines each having a length of equal to or more than 10 μm and extending in any direction are drawn on any positions on the surface of the coating in a planar view, a length of each island projection is 0.1 to 5.0 μm, where the length of each island projection is defined as an average value of lengths of a plurality of line segments, which are portions of the virtual straight lines passing through the projections.
3. The surface-treated steel sheet according to claim 2 , wherein the surface of the coating in an area of 1 μm by 1 μm has an arithmetic mean roughness Ra of 5 to 50 nm, a maximum cross-sectional height Rt of a roughness curve of 50 to 500 nm, and a root mean square roughness Rq of 10 to 100 nm.
4. The surface-treated steel sheet according to claim 2 , wherein a concentration of the zirconium in an area between adjacent island projections on the surface of the coating is less than a concentration of the zirconium in an area on which the island projections are formed.
5. The surface-treated steel sheet according to claim 2 , wherein a concentration of the acrylic resin in an area between adjacent island projections on the surface of the coating is more than a concentration of the acrylic resin in an area on which the island projections are formed.
6. The surface-treated steel sheet according to claim 1 , wherein
the coating has a mass ratio of a mass of the vanadium to a mass of the zirconium of 0.07 to 0.69,
a mass ratio of a mass of the phosphorus to the mass of the zirconium of 0.04 to 0.58, and
a mass ratio of a mass of the cobalt to the mass of the zirconium of 0.005 to 0.08.
7. The surface-treated steel sheet according to claim 1 , wherein the acrylic resin has an area ratio of 20 to 60 area % on a cross-section of the coating.
8. The surface-treated steel sheet according to claim 1 , wherein the plated layer consists of, in mass %, one or more of Al: less than or equal to 60%, Mg: less than or equal to 10%, and Si: less than or equal to 2%; zinc; and impurities.
9. The surface-treated steel sheet according to claim 1 , wherein the acrylic resin is a copolymer of, in mass %, styrene: 15 to 25%, (meth)acrylic acid: 1 to 10%, (meth)acrylic acid alkyl ester: 40 to 58% and acrylonitrile: 20 to 38%, and the acrylic resin has a glass transition temperature of −12 to 24° C.
10. The surface-treated steel sheet according to claim 1 , wherein the coating contains less than or equal to 5% by mass of a fluoride ion.
11. A method for producing the surface-treated steel sheet according to claim 1 , comprising:
a step of forming a plated layer containing zinc on a steel sheet;
a step of forming a coating by applying an aqueous surface treating agent containing an acrylic resin, zirconium, vanadium, phosphorus and cobalt on the plated layer by using a roll coater;
a step of retaining the coating for equal to or more than 0.5 seconds from the formation of the coating to initiation of heating of the coating; and
a step of heating the coating to dry the coating,
wherein, in the step of applying the aqueous surface treating agent, a viscosity of the aqueous surface treating agent measured at 25° C. is equal to or more than 1 mPa·s and equal to or less than 4 mPa·s.
12. The method for producing the surface-treated steel sheet according to claim 11 , wherein, in the step of applying the aqueous surface treating agent, a temperature of the steel sheet when the steel sheet enters into the roll coater is equal to or more than 5° C. and equal to or less than 80° C.