Surface-treated steel sheet for fuel tanks

1. A surface-treated steel sheet for fuel tanks, comprising:

a Zn—Ni alloy plating layer on one surface or both surfaces of a steel sheet; and

a trivalent chromate covering layer or a chromate-free covering layer on the Zn—Ni alloy plating layer,

wherein the steel sheet consists of, in mass %,

C: 0.0005 to 0.0050%,

Si: 0.01 to 1.00%,

Mn: 0.70 to 2.00%,

P: less than or equal to 0.060%,

S: less than or equal to 0.010%,

Al: 0.01 to 0.30%,

N: 0.001 to 0.010%,

Ti: 0.010 to 0.050%,

Nb: 0.010 to 0.040%,

B: 0.0005 to 0.0030%, and

the balance: Fe and unavoidable impurities, and

in a surface outermost layer, concavities, of which a depth from an arithmetic average height of a cross-sectional curve of the surface outermost layer is more than or equal to 0.1 μm, exist in a proportion of 50 to 1000 concavities/mm 2 and at an area ratio of 20 to 80% to a surface area of the steel sheet,

wherein the concavities in the outermost layer are formed from concavities on the Zn—Ni alloy plating layer and the steel sheet.

2. The surface-treated steel sheet for fuel tanks according to claim 1 ,

wherein the Zn—Ni alloy plating layer is an alloy plating layer containing Zn as a main component and 5 to 15 mass % Ni,

the adhesion amount of the Zn—Ni alloy plating layer is 3 to 50 g/m 2 , and

the amount of Ni of an outer layer of 1/10 of a layer thickness of the Zn—Ni alloy plating layer is more than or equal to 120% of the average amount of Ni of the Zn—Ni alloy plating layer.

3. The surface-treated steel sheet for fuel tanks according to claim 1 ,

comprising a chromate-free covering layer on the Zn—Ni alloy plating layer, wherein the chromate-free covering layer is

(A) a covering layer containing a silicon compound and a phosphoric acid compound and further containing one or more of compounds of Zr, V, Ti, and Co,

(B) a covering layer of a urethane-based resin, or

(C) a covering layer containing a crosslinked epoxy-urethane-based resin and at least one of a polyolefin resin and a fluorine-based resin.

4. The surface-treated steel sheet for fuel tanks according to claim 3 , comprising, on (A) the covering layer containing a silicon compound and a phosphoric acid compound and further containing one or more of compounds of Zr, V, Ti, and Co:

at least one layer of

(B) a covering layer of a urethane-based resin, and

(C) a covering layer containing a crosslinked epoxy-urethane-based resin and at least one of a polyolefin resin and a fluorine-based resin.

5. The surface-treated steel sheet for fuel tanks according to claim 1 , comprising a trivalent chromate covering layer on the Zn—Ni alloy plating layer, comprising, on the trivalent chromate covering layer:

at least one layer of

(B) a covering layer of a urethane-based resin, and

(C) a covering layer containing a crosslinked epoxy-urethane-based resin and at least one of a polyolefin resin and a fluorine-based resin.

6. The surface-treated steel sheet for fuel tanks according to claim 1 ,

comprising a chromate-free covering layer on the Zn—Ni alloy plating layer, wherein the chromate-free covering layer is a resin covering layer containing a urethane group and a urea group at 0.6 to 10 mass % on a N-equivalent basis in a solid content of an entire covering film and Si at 5 to 20 mass % on a Si-equivalent basis in the entire covering film,

wherein a layer thickness of the resin covering layer is 0.5 to 2.0 μm, and a coefficient of kinetic friction of a surface of the resin covering layer is 0.06 to 0.25.

7. The surface-treated steel sheet for fuel tanks according to claim 1 , comprising, on the trivalent chromate covering layer or the chromate-free covering layer:

a resin covering layer containing a urethane group and a urea group at 0.6 to 10 mass % on a N-equivalent basis in a solid content of an entire covering film of the resin covering layer and Si at 5 to 20 mass % on a Si-equivalent basis in the entire covering film of the resin covering layer,

wherein a layer thickness of the resin covering layer is 0.5 to 2.0 μm, and a coefficient of kinetic friction of a surface of the resin covering layer is 0.06 to 0.25.

8. The surface-treated steel sheet for fuel tanks according to claim 1 , comprising, on the trivalent chromate covering layer or the chromate-free covering layer:

an organic covering layer containing a resin having a urethane bond, one or two of a polyolefin resin and a fluorine resin, and an electrically conductive pigment, with a film thickness of 3 to 20 μm,

wherein the resin having a urethane bond is an organic resin obtained from a film-forming resin source material containing (I) a polyester polyol having at least three functional groups, and (II) a blocked product of an organic polyisocyanate, or a blocked product of a prepolymer having an NCO group at a terminal obtained by a reaction between an organic polyisocyanate and an active hydrogen compound.

9. The surface-treated steel sheet for fuel tanks according to claim 8 ,

wherein the electrically conductive pigment in the organic covering layer is made of one or more of stainless steel, Zn, Al, Ni, ferrosilicon, and iron phosphide, and a content ratio of the electrically conductive pigment is 5 to 50 volume % on a solid content basis.

10. The surface-treated steel sheet for fuel tanks according to claim 8 ,

wherein the organic covering layer further contains an antirust pigment at 1 to 40 volume % on a solid content basis, and contains the electrically conductive pigment and the antirust pigment at 5 to 70 volume % in total on a solid content basis.

11. The surface-treated steel sheet for fuel tanks according to claim 1 , comprising:

the Zn—Ni alloy plating layer on one surface or both surfaces of the steel sheet;

the trivalent chromate covering layer on the Zn—Ni alloy plating layer; and

a second resin covering layer containing a crosslinked epoxy-urethane-based resin and at least one of a polyolefin resin and a fluorine-based resin on the trivalent chromate covering layer,

wherein

the amount of Cr of the trivalent chromate covering film is 5 to 200 mg/m 2 on a Cr-equivalent basis, and

a layer thickness of the second resin covering layer is 0.3 to 2.0 μm, and a coefficient of kinetic friction of a surface of the second resin covering layer is 0.06 to 0.25.

12. A surface-treated steel sheet for fuel tanks, comprising:

a Zn—Ni alloy plating layer on one surface or both surfaces of a steel sheet; and

a trivalent chromate covering layer or a chromate-free covering layer on the Zn—Ni alloy plating layer,

wherein the steel sheet comprises, in mass %,

C: 0.0005 to 0.0050%,

Si: 0.01 to 1.00%,

Mn: 0.70 to 2.00%,

P: less than or equal to 0.060%,

S: less than or equal to 0.010%,

Al: 0.01 to 0.30%,

N: 0.001 to 0.010%,

Ti: 0.010 to 0.050%,

Nb: 0.010 to 0.040%,

B: 0.0005 to 0.0030%, and

the balance comprising Fe and unavoidable impurities, and

in a surface outermost layer, concavities, of which a depth from an arithmetic average height of a cross-sectional curve of the surface outermost layer is more than or equal to 0.1 μm, exist in a proportion of 50 to 1000 concavities/mm 2 and at an area ratio of 20 to 80% to a surface area of the steel sheet,

wherein the concavities in the outermost layer are formed from concavities on the Zn—Ni alloy plating layer and the steel sheet.

13. A surface-treated steel sheet for fuel tanks,

comprising:

a Zn—Ni alloy plating layer on one surface or both surfaces of a steel sheet; and a trivalent chromate covering layer or a chromate-free covering layer on the Zn—Ni alloy plating layer,

wherein the steel sheet consists of, in mass %,

C: 0.0005 to 0.0050%,

Si: 0.01 to 1,00%,

Mn: 0.70 to 2.00%,

P: less than or equal to 0.060%,

S: less than or equal to 0.010%,

Al: 0.01 to 0.30%,

N: 0.001 to 0.010%,

Ti: 0.010 to 0.050%,

Nb: 0.010 to 0.040%,

B: 0.0005 to 0.0030%, and

the balance: Fe and unavoidable impurities, and

in the surface outermost layer, concavities, of which a depth from an arithmetic average height of a cross-sectional curve of the surface outermost layer is more than or equal to 0.1 μm, exist in a range of 1.0×10 4 to 1.0×10 6 in terms of an outer layer form indicator Z defined by Formula (a) below,

Z (μm 3 /mm 2 )=sum total of volumes (a total volume), in μm 3 , of concavities of which the depth from the arithmetic average height of the cross-sectional curve of the surface outermost layer is more than or equal to 0.1 μm per unit area, in mm 2 , of the surface outermost layer  Formula (a),

wherein the concavities in the outermost layer are formed from concavities on the Zn—Ni alloy plating layer and the steel sheet.

14. A surface-treated steel sheet for fuel tanks,

comprising:

a Zn—Ni alloy plating layer on one surface or both surfaces of a steel sheet; and

a trivalent chromate covering layer or a chromate-free covering layer on the Zn—Ni alloy plating layer,

wherein the steel sheet consists of, in mass %,

C: 0.0005 to 0.0050%,

Si: 0.01 to 1.00%,

Mn: 0.70 to 2.00%,

P: less than or equal to 0.060%,

S: less than or equal to 0.010%,

Al: 0.01 to 0.30%,

N: 0.001 to 0.010%,

Ti: 0.010 to 0,050%,

Nb: 0.010 to 0.040%,

B: 0.0005 to 0.0030%, and

the balance: Fe and unavoidable impurities, and

on a surface outermost layer, an arithmetic average roughness (Ra) in a rectangular area with four 5-μm sides is 5 to 200 nm, and a maximum cross-sectional height (Rt) of a roughness curve is 100 to 1000 nm.