Zn—Mg alloy-coated steel sheet with excellent blackening resistance and excellent adhesion

Abstract

The present invention relates to a ZnMg alloy-coated steel sheet with excellent blackening resistance and excellent coating adhesion and to a method for manufacturing same. Provided are a ZnMg alloy-coated steel sheet with excellent blackening resistance and excellent adhesion and a method for manufacturing same, the steel sheet comprising: a substrate steel sheet; a ZnFe intermetallic compound layer formed on the substrate steel sheet; a first ZnMg coating layer formed on the ZnFe intermetallic compound layer and comprising a ZnFe intermetallic compound in which the content of Zn is 95% by weight or higher; a second ZnMg coating layer formed on the first ZnMg coating layer and comprising a ZnMg intermetallic compound in which the content of Zn is 80 to 95% by weight; and an oxide film formed on the second ZnMg coating layer and comprising a metallic oxide.

Claims

1. A ZnMg alloy-coated steel sheet having improved blackening resistance and coating adhesion, the ZnMg alloy-coated steel sheet comprising: a base steel sheet; a ZnFe intermetallic compound layer formed on the base steel sheet; a first ZnMg coating layer formed on the ZnFe intermetallic compound layer and comprising a ZnMg intermetallic compound in which zinc (Zn) is included in an amount of 95 wt % or greater and less than 100 wt %; a second ZnMg coating layer formed on the first ZnMg coating layer and comprising a ZnMg intermetallic compound in which zinc (Zn) is included in an amount of 80 wt % to 95 wt %; and an oxide film formed on the second ZnMg coating layer, the oxide film being formed of SiO.sub.2 film and having a thickness between 11 nm to 40 nm.

2. The ZnMg alloy-coated steel sheet of claim 1, wherein the SiO.sub.2 oxide film is formed by a combustion chemical vapor deposition (CCVD) process.

3. The ZnMg alloy-coated steel sheet of claim 2, wherein in the CCVD process, an organic silane is used as a precursor.

4. The ZnMg alloy-coated steel sheet of claim 2, wherein in the CCVD process, the base steel sheet is maintained within a temperature range of 330 C. to 450 C.

5. The ZnMg alloy-coated steel sheet of claim 1, wherein the first ZnMg coating layer and the second ZnMg coating layer are formed using physical vapor deposition (PVD).

Description

DESCRIPTION OF DRAWINGS

(1) FIG. 1 illustrates S-TEM and TEM images (a) of sections of a conventional sample and S-TEM and TEM images (b) of sections of an inventive sample.

(2) FIG. 2 illustrates images (a) of the inventive sample and images (b) of the conventional sample taken before and after a boiling water test.

(3) FIG. 3 illustrates results (a) of a coating adhesion test (tape test) performed on the inventive sample and results (b) of a coating adhesion test (tape test) performed on the conventional sample.

BEST MODE

(4) A combustion chemical vapor deposition (CCVD) method used in embodiments of the present disclosure is different from a conventional physical vapor deposition (PVD) method or a conventional chemical vapor deposition (CVD) method, in that a hydrocarbon (e.g., propane) used as a fuel is mixed with air in which a precursor (e.g., HMDSO (raw-material gas molecules)) is saturated, and heat generated by combusting the mixture (gas mixture) at atmospheric pressure is used as an energy source for the precursor to undergo evaporation, decomposition, and a chemical reaction, so as to form a coating layer on a surface.

(5) In the embodiments of the present disclosure, organic silane is used as a precursor in a CCVD process so as to form a metal oxide (such as a silicon oxide) having cross-linked bonds by coupling with oxygen from air on a metal surface as a deposition coating layer.

(6) Hereinafter, the embodiments of the present disclosure will be described in detail.

(7) The inventors found that coating layers of ZnMg alloy-coated steel sheets formed by a PVC method have unevenly distributed ZnMg, Zn, and Mg crystal regions and are easily separated due to fine natural oxide layers formed between the coating layers and the ZnMg alloy-coated steel sheets. In addition, the inventors found that ZnMg alloy-coated steel sheets formed by a PVC method are blackened as hydrates are formed thereon due to a reaction with moisture.

(8) Thus, the inventors have exposed a ZnMg alloy-coated steel sheet to high-temperature flame by using a CCVD method. In this case, a coating layer of the ZnMg alloy-coated steel sheet was made uniform, and the adhesion of the coating layer was increased. In addition, blackening of the coating layer was prevented by an oxide film. Based on this knowledge, the inventors invented the ZnMg alloy-coated steel sheet of the present disclosure.

(9) In the CCVD method, a gaseous mixture of a hydrocarbon (propane) and air in which raw-material gas molecules (precursor) are saturated is combusted at atmospheric pressure, and heat generated by the combustion is used as an energy source so as to form an oxide film having a cross-linked structure by evaporating, decomposing, and oxidizing the raw-material gas molecules.

(10) First, a ZnMg alloy-coated steel sheet will be described in detail according to an embodiment of the present disclosure.

(11) The ZnMg alloy-coated steel sheet of the embodiment includes:

(12) a base steel sheet;

(13) a ZNFe intermetallic compound layer formed on the base steel sheet;

(14) a first ZnMg coating layer formed on the ZnFe intermetallic compound layer and including a ZnMg intermetallic compound in which zinc (Zn) is included in an amount of 95 wt % or greater;

(15) a second ZnMg coating layer formed on the first ZnMg coating layer and including a ZnMg intermetallic compound in which zinc (Zn) is included in an amount of 80 wt % to 95 wt %; and

(16) an oxide film formed on the second ZnMg coating layer and including an metal oxide.

(17) The kind of the base steel sheet is not limited. That is, any kinds of steel sheets such as a hot-rolled steel sheet and a cold-rolled steel sheet and any kinds of metal sheets may be applied to the present embodiment.

(18) The ZnFe intermetallic compound layer is formed on the base steel sheet. In a CCVD process (described later), a fine oxide film existing between the base steel sheet and a ZnMg coating layer may be diffused into the ZnMg coating layer, and thus a ZnFe intermetallic compound layer may be formed. The ZnFe intermetallic compound layer improves the adhesion between the base steel sheet and the ZnMg coating layer.

(19) The first ZnMg coating layer is formed on the ZnFe intermetallic compound layer and includes a ZnMg intermetallic compound in which zinc (Zn) is included in an amount of 95 wt % or greater. The first ZnMg coating layer is a soft Zn-rich region. Therefore, the first ZnMg coating layer has high adhesion.

(20) If the content of zinc (Zn) in the first ZnMg coating layer is less than 95 wt %, the first. ZnMg coating layer may become fragile due to the influence of a ZnMg alloy, and thus the adhesion of the first ZnMg coating layer may not be high. Therefore, a higher content of zinc (Zn) may be favored. However, to form a ZnMg intermetallic compound, it may be preferable that the content of zinc (Zn) be less than 100 wt %.

(21) The second ZnMg coating layer is formed on the first ZnMg coating layer and includes a ZnMg intermetallic compound in which zinc (Zn) is included in an amount of 80 wt % to 95 wt %. The second ZnMg coating layer has a high resistance to corrosion as compared with a general zinc coating layer because of ZnMg alloy characteristics.

(22) It may be preferable that the ZnMg intermetallic compound have a zinc content of 80 wt % to 95 wt %. If the content of zinc is less than 80 wt %, the corrosion resistance of the second ZnMg coating layer is decreased due to the influence of a ZnMg alloy coating layer having a different crystal shape.

(23) The oxide film is formed on the second ZnMg coating layer. The oxide film includes a metal oxide. The oxide film blocks the permeation of moisture and thus prevents blackening.

(24) The oxide film includes a metal oxide (e.g., a silicon oxide (SiO.sub.2)) formed by a CCVD process. In addition, during the CCVD process, magnesium oxide (MgO) is formed by oxygen of high-temperature air as a natural oxide film and coexists with the oxide film.

(25) It may be preferable that the oxide film have a thickness of 10 nm to 40 nm. If the thickness of the oxide film is less than 10 nm, the permeation of moisture may not be sufficiently prevented, and thus sufficient blackening resistance may not be obtained. On the other hand, if the thickness of the oxide film is greater than 40 nm, oxide particles (that is, SiO.sub.2 particles of the oxide film) may be coarsened, and thus the oxide film may not function as a barrier film.

(26) Hereinafter, a method of manufacturing a ZnMg alloy-coated steel sheet will be described in detail according to an embodiment of the present disclosure.

(27) First, a ZnMg coating layer is formed on a base steel sheet. At this time, the ZnMg coating layer is not the first and second ZnMg coating layers described in the previous embodiment. That is, the ZnMg coating layer is not a final ZnMg coating layer. The ZnMg coating layer will be modified by a later CCVD process.

(28) The ZnMg coating layer may be formed by a PVD method. For example, the ZnMg coating layer may be formed by a conventional PVD method known in the related art.

(29) For example, in a thermal-PVD or sputtering method, a large amount of ZnMg or a raw coating material including Zn and Mg is evaporated in a vacuum by using thermal energy or ion collision energy, and the evaporated ZnMg or raw coating material is directed to the surface of a steel sheet or metal sheet continuously fed into the vacuum by a roll-to-roll method, so as to form a ZnMg alloy coating layer on the steel sheet or metal sheet. The thermal-PVD or sputtering method may be used to form the ZnMg coating layer of the embodiment of the present disclosure. Particularly, in an electro magnetic PVD (EML-PVD) method, a kind of thermal-PVD method using electromagnetic induction, a ZnMg alloy coating material is levitated and heated in a vacuum by electromagnetic induction so as to rapidly generate a large amount of vapor and form a coating layer using the vapor. The EML-PVD method is widely used for producing ZnMg coated products owing to a high production rate and economic benefits.

(30) A CCVD process is performed on the base steel sheet on which the ZnMg coating layer is formed, so as to form an oxide film thereon. In detail, through the CCVD process, such layers (films) as described in the previous embodiment are formed on the base steel sheet. That is, a ZnFe intermetallic compound layer is formed on the base steel sheet; a first ZnMg coating layer including a ZnMg intermetallic compound in which zinc (Zn) is contained in an amount of 95 wt % or greater is formed on the ZnFe intermetallic compound layer; a second ZnMg coating layer including a ZnMg intermetallic compound in which zinc (Zn) is included in an amount of 80 wt % to 95 wt % is formed on the first ZnMg coatine layer; and an oxide film including an metal oxide is formed on the second ZnMg coating layer. In this way, a ZnMg alloy-coated steel sheet is manufactured according to the manufacturing method of the embodiment of the present disclosure.

(31) In the CCVD process, the base steel sheet may be spaced apart from a burner of a CCVD apparatus by a distance of 10 mm or less. Furthermore, in the CCVD process, if air and precursor vapor are supplied according to the following ratio, a metal oxide deposition film having optimized characteristics may be formed.

(32) Precursor flow rate:air flow rate 0.8:210 to 2.1:210 (L/min)

(33) If the ratio of the flow rates is less than 0.8:210 (L/min), since the number of film-forming particles is small relative to the amount of air, an excessively thin or fine film not functioning as a barrier may be formed. On the other hand, if the ratio of the flow rates is greater than 2.1:210 (L/min), since precursor particles clump together (coarse particles), a film (coating) may not be densely formed and thus may not function as a barrier.

(34) In the CCVD process, it may be preferable that the base steel sheet be maintained within a temperature range of 330 C. to 450 C. If the base steel sheet is maintained at a temperature lower than 330 C., a coating layer may not have sufficient energy necessary for diffusion of atoms, and thus the coating layer may not become homogeneous during the CCVD process. On the other hand, if the base steel sheet is maintained at a temperature higher than 450 C., a ZnFe alloying reaction may intensively occur between the base steel sheet and the first ZnMg coating layer having a high density of zinc (Zn), and thus a ZnFe alloy layer may be excessively formed to adversely affect coating adhesion.

MODE FOR INVENTION

(35) Hereinafter, an example of the present disclosure will be described in detail. The following example is for illustrative purposes and is not intended to limit the scope of the present disclosure.

Example

(36) The following experiment was conducted to evaluate the blackening resistance and coating adhesion of steel sheets according to the thicknesses of oxide films and the heating temperature of the steel sheets.

(37) General cold-rolled steel sheets (continuous strip coils) were prepared. Then, an EML-PVD process, a kind of thermal-PVD process using electromagnetic induction, was performed on the cold rolled steel sheets. Specifically, in the EML-PVD process, a ZnMg alloy coating material was levitated and heated in a vacuum by electromagnetic induction so as to rapidly generate a large amount of vapor, and the vapor was directed to surfaces of the cold-rolled steel sheets continuously fed into the vacuum by a roll-to-roll method, so as to form ZnMg coating layers on the cold-rolled steel sheets.

(38) Thereafter, the steel sheets on which the ZnMg coating layers were formed were subjected to a CCVD process so as to form silicon oxide films on the steel sheets by using organic silane. In this way, ZnMg alloy-coated steel sheets on which oxide films were formed were manufactured. In the CCVD process, the oxide films were formed on the steel sheets to different thicknesses (processing cycles) at different temperatures as shown in Table 1.

(39) Referring to Table 1, Comparative Samples (steel sheets) to 7 to 10 were tested while varying temperatures thereof without forming oxide films thereon so as to evaluate the coating adhesion of the Comparative Samples 7 to 10 according to the temperatures of the Comparative Samples 7 to 10 in the CCVD process.

(40) The blackening resistance and coating adhesion of each of the ZnMg alloy-coated steel sheets were tested as shown in Table 1, The coating adhesion of each of the ZnMg alloy-coated steel sheets was evaluated by attaching tape to the steel sheet (sample), bending the steel sheet, separating the tape, and observing the tape to determine whether the coating adhesion was good or bad according to the separation of the coating layer from the steel sheet. The blackening resistance of each of the ZnMg alloy-coated steel sheets was evaluated by immersing the steel sheet in boiling water and observing the color of the steel sheet to determine whether blackening resistance was good or bad according to the change of the color of the steel sheet.

(41) TABLE-US-00001 TABLE 1 Thickness of oxide film Sample (CCVD process Te. Blackening Coating No. cycles) ( C.) resistance adhesion Conventional Not processed X X Sample Inventive 22 nm 330 Sample (50 cycles) Comparative 5 nm 45 X X Sample 1 (3 cycles) Comparative 11 nm 80 X Sample 2 (6 cycles) Comparative 22 nm 110 X Sample 3 (12 cycles) Comparative 32 nm 120 X Sample 4 (18 cycles) Comparative 40 nm 180 X Sample 5 (30 cycles) Comparative 48 nm 330 Sample 6 (50 cycles) Comparative 180 X X Sample 7 (30 cycles) Comparative 330 X Sample 8 (50 cycles) Comparative 425 X Sample 9 (75 cycles) Comparative 480 X X Sample 10 (100 cycles)

(42) An inventive sample satisfying conditions of the embodiments of the present disclosure has high blackening resistance and coating adhesion. FIG. 1 illustrates a structure (a) of a conventional sample and a structure (b) of the inventive sample. Referring to FIG. 1, in the conventional sample, a ZnMg alloy coating layer is formed on a base steel sheet as a mixed layer. However, in the inventive sample, an intermetallic compound layer, a Zn-rich layer, a ZnMg alloy layer, and an outermost oxide film are formed on a base steel sheet. Therefore, the inventive sample formed according to the embodiments of the present disclosure has high blacking resistance and coating adhesion.

(43) Comparative Samples 1 and 6 not satisfying the oxide film thickness condition of the embodiments of the present disclosure have poor blackening resistance. Comparative Samples 2 to 5 satisfying the oxide film thickness condition of the embodiments of the present disclosure but not satisfying the temperature condition, of the embodiments of the present disclosure have good blackening resistance but poor coating adhesion.

(44) Comparative Samples 8 and 9, satisfying the temperature conditions of the embodiments of the present disclosure, have good coating adhesion. However, since oxide films are not formed on Comparative Samples 8 and 9, Comparative Samples 8 and 9 have poor blackening resistance. Comparative Samples 7 and 10, not satisfying the temperature condition of the embodiments of the present disclosure, have poor coating adhesion.

(45) FIG. 2 illustrates images (a) of the inventive sample and images (b) of the conventional sample taken before and after a boiling water test. Referring to FIG. 2, the color of the inventive sample is not changed. That is, the inventive sample has high blackening resistance. However, the conventional sample is entirely blackened.

(46) FIG. 3 illustrates results (a) of a tape test performed on the inventive sample and results (b) of a tape test performed on the conventional sample. Referring to FIG. 3, the inventive sample has higher coating adhesion than the conventional sample.