ZINC-BASED PLATED STEEL SHEET HAVING POST-TREATED COATING FORMED THEREON AND POST-TREATMENT METHOD THEREFOR

Abstract

Provided is a zinc-based plated steel sheet having a post-treated coating filmed thereon including: a steel sheet; a zinc plated layer formed on the steel sheet; and a post-treated coating formed on the plated layer, wherein the atomic ratio (O/M) of oxygen (O) to metals (M) contained in the post-treated coating is greater than 2 and less than 20, and a method for post-treating a zinc-based plated steel sheet. According to this, the zinc-based plated steel sheet having the post-treated coating formed thereon has the effects excellent in lubricity, weldability, adhesiveness, film-removing property and paintability. As the method of post-treating a zinc-based plated steel sheet of the present invention employs a simple coating method irrespective of the kind of plating layer, the process is simple and economical and the process operation cost is low.

Claims

1. A zinc-based plated steel sheet having a post-treatment coating, the zinc-based plated steel sheet comprising: a steel sheet; a zinc-based plating layer formed on the steel sheet; and a post-treatment coating formed on the plating layer, wherein the post-treatment coating includes metal oxide salt and an organic compound, and an atomic ratio (O/M) of oxygen (O) to metals (M) included in the post-treatment coating is greater than 2 and smaller than 20.

2. The zinc-based plated steel sheet of claim 1, wherein the post-treatment coating includes a coating upper portion and a coating lower portion, an atomic ratio (O/M) of oxygen and metal of the coating upper portion is greater than 5, and an atomic ratio (O/M) of oxygen and metal of the coating lower portion is less than 5.

3. The zinc-based plated steel sheet of claim 2, wherein an atomic ratio (O/M) of oxygen and metal of the coating upper portion is greater than 3.2, and an atomic ratio (O/M) of oxygen and metal of the coating lower portion is less than 3.2.

4. The zinc-based plated steel sheet of claim 2, wherein a thickness of the coating lower portion is to of the total thickness of the post-treatment coating.

5. The zinc-based plated steel sheet of claim 1, wherein the metal oxide salt further includes one or more selected from the group consisting of molybdenum (Mo), boron (B), silicon (Si), titanium (Ti), and zirconium (Zr).

6. A method for post-treating a zinc-based plated steel sheet, the method comprising: applying a post-treatment coating solution to a zinc-based plated steel sheet and drying the same to form a post-treatment coating, wherein an atomic ratio (O/M) of oxygen (O) to metals (M) included in the post-treatment coating is greater than 2 and smaller than 20.

7. The method of claim 6, wherein the post-treatment coating includes a coating upper portion and a coating lower portion, an atomic ratio (O/M) of oxygen and metal of the coating upper portion is greater than 5, and an atomic ratio (O/M) of oxygen and metal of the coating lower portion is less than 5.

8. The method of claim 7, wherein a thickness of the coating lower portion is to of the total thickness of the post-treatment coating.

9. The method of claim 6, wherein the post-treatment coating solution has pH ranging from 3.0 to 7.0.

10. The method of claim 6, wherein the post-treatment coating has a coating weight ranging from 100 to 1000 mg/m.sup.2.

Description

EXAMPLES

Inventive Example 1

[0043] A zinc-based steel sheet having a thickness of 0.8 mm, an elongation of 36% to 37%, and a plating amount of 40 g/m.sup.2 and including a plating layer in which Al is 2.5%, Mg is 3% and Zn is 94.5% was prepared. A coating solution containing metal oxide salt containing 1 wt % of Mo and 0.2 wt % of B, 0.3 wt % of PEG, and the balance being water was applied to the steel sheet by a bar coating method and then a post-treatment coating having a coating weight of 300 mg/m.sup.2 was formed under a condition of PMT 100 C.

Inventive Example 2

[0044] A zinc-based steel sheet having a thickness of 0.8 mm, an elongation of 36% to 37%, and a plating amount of 40 g/m.sup.2 and including a plating layer in which Al is 2.5%, Mg is 3% and Zn is 94.5% was prepared. A coating solution containing metal oxide salt containing 1 wt % of Mo and 0.5 wt % of B, 0.5 wt % of PEG, and the balance being water was applied to the steel sheet by a bar coating method and then a post-treatment coating having a coating weight of 300 mg/m.sup.2 was formed under a condition of PMT 100 C.

Inventive Example 3

[0045] A zinc-based steel sheet having a thickness of 0.8 mm, an elongation of 36% to 37%, and a plating amount of 40 g/m.sup.2 and including a plating layer in which Al is 2.5%, Mg is 3% and Zn is 94.5% was prepared. A coating solution containing metal oxide salt containing 1 wt % of Mo and 1 wt % of B, 1 wt % of PEG, and the balance being water was applied to the steel sheet by a bar coating method and then a post-treatment coating having a coating weight of 300 mg/m.sup.2 was formed under a condition of PMT 100 C.

Inventive Example 4

[0046] A zinc-based steel sheet having a thickness of 0.8 mm, an elongation of 36% to 37%, and a plating amount of 40 g/m.sup.2 and including a plating layer in which Al is 2.5%, Mg is 3% and Zn is 94.5% was prepared. A coating solution containing metal oxide salt containing 0.5 wt % of Mo and 1 wt % of B, 1.5 wt % of PEG, and the balance being water was applied to the steel sheet by a bar coating method and then a post-treatment coating having a coating weight of 300 mg/m.sup.2 was formed under a condition of PMT 100 C.

Comparative Example 1

[0047] A zinc-based steel sheet having a thickness of 0.8 mm, an elongation of 36% to 37%, and a plating amount of 40 g/m.sup.2 and including a plating layer in which Al is 2.5%, Mg is 3% and Zn is 94.5% was prepared, and a post-treatment coating was not formed on the steel sheet.

Comparative Example 2

[0048] A zinc-based steel sheet having a thickness of 0.8 mm, an elongation of 36% to 37%, and a plating amount of 40 g/m.sup.2 and including a plating layer in which Al is 2.5%, Mg is 3% and Zn is 94.5% was prepared. A coating solution containing metal oxide salt containing 1 wt % of Mo, 0.1 wt % of PEG, and the balance being water was applied to the steel sheet by a bar coating method and then a post-treatment coating having a coating weight of 300 mg/m.sup.2 was formed under a condition of PMT 100 C.

Comparative Example 3

[0049] A zinc-based steel sheet having a thickness of 0.8 mm, an elongation of 36% to 37%, and a plating amount of 40 g/m.sup.2 and including a plating layer in which Al is 2.5%, Mg is 3% and Zn is 94.5% was prepared. A coating solution containing metal oxide salt containing 1 wt % of Mo and 0.5 wt % of B, 3 wt % of PEG, and the balance being water was applied to the steel sheet by a bar coating method and then a post-treatment coating having a coating weight of 300 mg/m.sup.2 was formed under a condition of PMT 100 C.

[0050] The atomic ratios (O/M) of oxygen (O) to metals (M) in the post-treatment coatings of Inventive Examples 1 to 4 and Comparative Examples 1 to 3 were measured and shown in Table 1. The atomic ratio is an average value per unit depth of nanometer scale using an X-ray photoelectron spectroscopy (XPS) method after acetone-debinding the zinc-based plated steel sheet on which the post-treatment coating is formed. A boundary between an upper portion and a lower portion was half of a total thickness. In order to evaluate physical properties according to the atomic ratio (O/M) conditions of oxygen (O) to metals (M), processibility, adhesion and decoatability were evaluated and the results are shown in Table 1.

[0051] <Evaluation of Processibility>

[0052] A cup drawing test was carried out to evaluate processibility. In the cup drawing test, a tester including a mold and a punch was used. Testing was performed, while increasing a blank holding force (BHF), and a BHG value immediately before material fracture occurred was determined as a maximum BHF value and written in Table 1. As the maximum BHF value is higher, workability is better. Detailed conditions of the cup drawing test carried out in this example are as follows.

[0053] Detailed Conditions of Cup Drawing Test

[0054] Punch diameter: 50 mm

[0055] Punch edge radius: 6 mm

[0056] Die diameter: 52.25 mm

[0057] Punch movement rate: 230 mm/min

[0058] Post-treatment coating was in contact with mold and testing was performed after applying washing oil to surface of the post-treatment coating

[0059] <Evaluation of Adhesion>

[0060] For an adhesion test, two 25 mm100 mm samples were manufactured, and after washing oil was applied to the samples, the samples were erected and left as is for one day. Thereafter, a mastic sealer adhesive was applied with a size of 25 mm (width)25 mm (length)3 mm (height) to an inner side by 10 mm from the end of one sample and the other sample was folded thereon, which were then cured at 170 C. for 20 minutes. Thereafter, adhesion shear strength of the upper and lower samples was measured and the results thereof are shown in Table 1. The shear strength was measured at a speed of 50 mm/min and a measured maximum shear force was divided by an adhesion area of 6.25 cm.sup.2.

[0061] <Evaluation of Decoatability>

[0062] For a decoatability test, a material was stirred and dipped at 50 to 55 C. for 2 minutes using a degreasing solution used in an automobile manufacturing process and subsequently washed, and an amount of coating remaining on a surface of the material was measured, a decoating rate to a coating weight of the coating before degreasing was calculated by percentage and shown in Table 1. For the remaining amount of coating, a coating layer having a predetermined area was dissolved with an acid solution, and the content of molybdenum of the solution was analyzed quantitatively with inductively coupled plasma (ICP) to calculate a remaining amount per area.

TABLE-US-00001 TABLE 1 O/M* of O/M of entire lower post- post- Bonding treatment treatment Maximum strength Decoating Classification coating coating BHF (ton) (kgf/cm.sup.2) rate (%) Inventive 3.2 1.9 10 4.0 97 Example 1 Inventive 7.2 2.4 12 5.2 95 Example 2 Inventive 10.1 3.1 14 5.5 96 Example 3 Inventive 19.2 4.7 13 5.5 95 Example 4 Comparative Not 7 0.5 example 1 treated Comparative 1.8 1.5 8 1.1 98 example 2 Comparative 25.3 6.2 14 5.3 87 example 3 *O/M is an atomic ratio of oxygen (O) to metals (M)

[0063] As shown in Table 1, it can be seen that, in Inventive Examples 1 to 6 in which the post-treatment coating was formed, the maximum BHF value and the bonding strength were significantly higher than those in Comparative Example 1 in which post-treatment was not performed, and thus, Inventive Examples 1 to 6 have excellent processibility and adhesiveness as physical properties that may be used as a sheet material of automobiles.

[0064] It was also confirmed that Inventive Examples 1 to 6 in which the atomic ratio (O/M) of oxygen (O) to metals (M) is greater than 2 and less than 20 had a decoating rate of 95% or greater, exhibiting excellent decoatability. However, in Comparative Example 2 in which the atomic ratio (O/M) of oxygen (O) to metals (M) is 2 or less, processibility and adhesiveness are inferior and, in Comparative Example 3 in which the atomic ratio (O/M) of oxygen (O) to metals (M) is 20 or greater, processibility and adhesiveness were good but has an inferior decoating rate because the decoating rate is 87% or less.

[0065] While exemplary embodiments of the present disclosure have been shown and described above, it will be apparent to those skilled in the art that modifications and variations could be made without departing from the scope of the present disclosure as defined by the appended claims.