ZINC ALLOY PLATED STEEL SHEET HAVING EXCELLENT BENDING WORKABILITY AND MANUFACTURING METHOD THEREFOR

Abstract

Provided are a zinc alloy plated steel sheet and a method for manufacturing the zinc alloy plated steel sheet. The zinc alloy plated steel sheet includes a base steel sheet and a zinc alloy plating layer, wherein the zinc alloy plating layer includes a Zn single phase structure as a microstructure and a ZnAlMg-based intermetallic compound, and the Zn single phase structure has a degree (f) of (0001) preferred orientation expressed by Formula 1 below within a range of 50% or greater. [Formula 1] f(%)=(I.sub.basal/I.sub.total)100 where I.sub.total refers to the integral of all diffraction peaks of the Zn single phase structure when an X-ray diffraction pattern is measured within a range of 2 theta from 10 to 100 using a Cu-K source, and I.sub.basal refers to the integral of diffraction peaks of the Zn single phase relating to a basal plane.

Claims

1. A zinc alloy plated steel sheet comprising a base steel sheet and a zinc alloy plating layer, wherein the zinc alloy plating layer comprises a Zn single phase structure as a microstructure and a ZnAlMg-based intermetallic compound, and the Zn single phase structure has a degree (f) of (0001) preferred orientation, expressed by Formula 1 below, within a range of 50% or greater,
f(%)=(I.sub.basal/I.sub.total)100[Formula 1] where I.sub.total refers to an integral of all diffraction peaks of the Zn single phase structure when an X-ray diffraction pattern is measured within a range of 2 theta from 10 to 100 using a Cu-K source, and I.sub.basal refers to an integral of diffraction peaks of the Zn single phase structure relating to a basal plane.

2. The zinc alloy plated steel sheet of claim 1, wherein the Zn single phase structure has a degree (f) of (0001) preferred orientation, expressed by Formula 1, within a range of 60% or greater.

3. The zinc alloy plated steel sheet of claim 1, wherein the ZnAlMg-based intermetallic compound comprises at least one selected from the group consisting of a Zn/MgZn.sub.2 binary eutectic structure, a Zn/Al binary eutectic structure, an MgZn.sub.2 single phase structure, and a Zn/Al/MgZn.sub.2 ternary eutectic structure.

4. The zinc alloy plated steel sheet of claim 1, wherein an area fraction of the Zn single phase structure on a surface of the zinc alloy plating layer is 40% or less (excluding 0%).

5. The zinc alloy plated steel sheet of claim 1, wherein a total area fraction of a Zn/MgZn.sub.2 binary eutectic structure and a Zn/Al/MgZn.sub.2 ternary eutectic structure is 50% or greater (excluding 100%) on a surface of the zinc alloy plating layer.

6. The zinc alloy plated steel sheet of claim 1, wherein an area fraction of an MgZn.sub.2 single phase structure on a surface of the zinc alloy plating layer is 10% or less (excluding 0%).

7. The zinc alloy plated steel sheet of claim 1, wherein an average grain diameter of the Zn single phase structure observed on a cross-section of the zinc alloy plating layer taken in a sheet thickness direction is 15 m or less (excluding 0 m).

8. The zinc alloy plated steel sheet of claim 1, wherein the zinc alloy plating layer comprises, by wt %, aluminum (Al): 0.5% to 3%, magnesium (Mg): 0.5% to 3%, and a balance of zinc (Zn) and inevitable impurities.

9. The zinc alloy plated steel sheet of claim 1, wherein the zinc alloy plating layer satisfies Formula 1 below:
1.0<[Mg]/[Al]4.0[Formula 2] where [Mg] and [Al] refer to weight percentages (wt %) of corresponding elements, respectively.

10. The zinc alloy plated steel sheet of claim 1, wherein a number of black spots per unit area is 0.1/cm.sup.2 or less on a surface of the zinc alloy plated steel sheet.

11. A method for manufacturing a zinc alloy plated steel sheet, the method comprising: preparing a zinc alloy plating bath comprising magnesium (Mg) and aluminum (Al); obtaining a zinc alloy plated steel sheet by dipping a base steel sheet into the zinc alloy plating bath to plate the base steel sheet; wiping the zinc alloy plated steel sheet with gas to adjust a plating weight; and after adjusting the plating weight of the zinc alloy plated steel sheet, cooling the zinc alloy plated steel sheet by spraying droplets of water or an aqueous solution onto the zinc alloy plated steel sheet and then using air, wherein when the droplets are sprayed, a droplet spray start temperature ranges from 405 C. to 425 C., a droplet spray stop temperature ranges from 380 C. to 400 C.

12. The method of claim 11, wherein when the droplets are sprayed, a difference between the droplet spray start temperature and the droplet spray stop temperature is 15 C. or greater.

13. The method of claim 11, wherein the droplets are sprayed by a charge spray method to attach the droplets by electrostatic attraction between the droplets and the zinc alloy plated steel sheet.

14. The method of claim 11, wherein the droplets are sprayed in an amount of 50 g/m.sup.2 to 100 g/m.sup.2.

15. The method of claim 11, wherein the aqueous solution is a phosphate aqueous solution.

16. The method of claim 15, wherein the phosphate aqueous solution comprises at least one selected from the group consisting of an aqueous solution of ammonium hydrogen phosphate ((NH.sub.4).sub.2HPO.sub.4), an aqueous solution of sodium ammonium hydrogen phosphate (NaNH.sub.4HPO.sub.4), an aqueous solution of zinc dihydrogen phosphate (Zn(H.sub.2PO.sub.4).sub.2), and an aqueous solution of calcium phosphate (Ca.sub.3(PO.sub.4).sub.2).

17. The method of claim 15, wherein the phosphate aqueous solution has a concentration of 0.5 wt % to 5 wt %.

18. The method of claim 11, wherein the zinc alloy plating bath comprises, by wt %, aluminum (Al): 0.5% to 3%, magnesium (Mg): 0.5% to 3%, and a balance of zinc (Zn) and inevitable impurities.

Description

DESCRIPTION OF DRAWINGS

[0015] FIG. 1 is views illustrating results of (a) an observation of a surface microstructure of Inventive Sample 1 and (b) an observation of a surface microstructure of Comparative Sample 5.

[0016] FIG. 2 is views illustrating results of (a) an observation of a cross-sectional microstructure of Inventive Sample 1 and (b) an observation of a cross-sectional microstructure of Comparative Sample 5.

[0017] FIG. 3 is a view illustrating results of X-ray diffractometer (XRD) analysis of Inventive Sample 1.

BEST MODE

[0018] Hereinafter, a zinc alloy plated steel sheet having high bending workability will be described in detail according to an aspect of the present disclosure.

[0019] According to the aspect of the present disclosure, the zinc alloy plated steel sheet includes a base steel sheet and a zinc alloy plating layer. In the present disclosure, the base steel sheet is not limited to a particular type. For example, a hot-rolled steel sheet or a cold-rolled steel sheet commonly used as a base steel sheet of a zinc alloy plated steel sheet may be used. However, hot-rolled steel sheets have a large amount of surface oxide scale that lowers plating adhesion and thus plating quality, and thus a hot-rolled steel sheet from which oxide scale has been previously removed using an acid solution may be used as the base steel sheet. In addition, the zinc alloy plating layer may be formed on one or each side of the base steel sheet.

[0020] The zinc alloy plating layer may include, by wt %, aluminum (Al): 0.5% to 3%, magnesium (Mg): 0.5% to 3%, and the balance of zinc (Zn) and inevitable impurities.

[0021] In the zinc alloy plating layer, magnesium (Mg) reacts with zinc (Zn) and aluminum (Al) and forms a ZnAlMg-based intermetallic compound, thereby functioning as a key element improving the corrosion resistance of the zinc alloy plated steel sheet. If the content of magnesium (Mg) is excessively low, the ZnAlMg-based intermetallic compound is not present in sufficient amounts in the microstructure of the zinc alloy plating layer, and thus corrosion resistance may not be sufficiently improved. Therefore, the amount of magnesium (Mg) in the zinc alloy plating layer may be 0.5 wt % or greater, preferably 1.0 wt % or greater. However, if the content of magnesium (Mg) is excessively high, the effect of improving corrosion resistance is saturated, and Mg oxide dross having a negative effect on platability may be formed in a plating bath. In addition, the ZnAlMg-based intermetallic compound having high harness may be formed in excessively large amounts in the microstructure of the zinc alloy plating layer, and thus bending workability may be lowered. Therefore, the amount of magnesium (Mg) in the zinc alloy plating layer may be 3 wt % or less, preferably 2.9 wt % or less.

[0022] Aluminum (Al) suppresses the formation of Mg oxide dross and reacts with zinc (Zn) and magnesium (Mg) to form the ZnAlMg-based intermetallic compound in the zinc alloy plating layer, thereby functioning as a key element improving the corrosion resistance of the zinc alloy plated steel sheet. If the content of aluminum (Al) is excessively low, the formation of Mg dross is not sufficiently suppressed, and the ZnAlMg-based intermetallic compound is not present in sufficient amounts in the microstructure of the zinc alloy plating layer, which may result in insufficient improvements in corrosion resistance. Therefore, the amount of aluminum (Al) in the zinc alloy plating layer may be 0.5 wt % or greater, preferably 0.6 wt % or greater. However, if the content of aluminum (Al) is excessively high, the effect of improving corrosion resistance is saturated, and the durability of plating equipment may be negatively affected because of a high plating bath temperature. Moreover, the ZnAlMg-based intermetallic compound having high harness may be formed in excessively large amounts in the microstructure of the zinc alloy plating layer, and thus bending workability may be lowered. Therefore, the amount of aluminum (Al) in the zinc alloy plating layer may be 3 wt % or less, preferably 2.6 wt % or less.

[0023] According to an embodiment, the contents of magnesium (Mg) and aluminum (Al) in the zinc alloy plating layer may satisfy the following Formula 2. If [Mg]/[Al] is 1.0 or less, scratch resistance may deteriorate, and if [Mg]/[Al] is greater than 4.0, Mg-based dross may be formed in large amounts in a hot-dip plating bath to lower workability.

1.0<[Mg]/[Al]4.0[Formula 2]

[0024] where [Mg] and [Al] refer to the weight percentages (wt %) of corresponding elements, respectively.

[0025] The zinc alloy plating layer may include a Zn single phase structure as a microstructure and the ZnAlMg-based intermetallic compound. In the present disclosure, the ZnAlMg-based intermetallic compound is not limited to a particular type. However, for example, the ZnAlMg-based intermetallic compound may include at least one selected from the group consisting of a Zn/Al/MgZn.sub.2 ternary eutectic structure, a Zn/MgZn.sub.2 binary eutectic structure, a Zn/Al binary eutectic structure, and an MgZn.sub.2 single phase structure.

[0026] The inventors have conducted in-depth research into improving the bending workability of zinc alloy plated steel sheets and found that if a Zn single phase structure having a hexagonal close packing (HCP) structure is grown in a (0001) orientation in the microstructure of the zinc alloy plating layer, ductility increases owing to easy slippage, and thus cracks are markedly reduced in a bending process.

[0027] In the present disclosure, to obtain this effect, the degree (f) of (0001) preferred orientation, expressed by the following formula 1, may preferably be adjusted to be 50% or greater, more preferably 60% or greater.

f(%)=(I.sub.basal/I.sub.total)100[Formula 1]

[0028] where I.sub.total refers to the integral of all diffraction peaks of the Zn single phase structure when an X-ray diffraction pattern is measured within the range of 2 theta from 10 to 100 using a Cu-K source, and I.sub.basal refers to the integral of diffraction peaks of the Zn single phase structure relating to a basal plane.

[0029] In addition, the inventors have found that if the Zn single phase structure coarsely formed in the zinc alloy plating layer is refined in size, it is also helpful to reduce cracking during a bending process.

[0030] To obtain this effect of the present disclosure, the average grain diameter of the Zn single phase structure may be preferably adjusted to be 15 m or less, more preferably 12 m or less, and even more preferably 10 m or less. The average grain diameter of the Zn single phase structure refers to the average of equivalent circular diameters of the Zn single phase structure measured by observing a thicknesswise cross-section of the zinc alloy plating layer.

[0031] The zinc alloy plated steel sheet of the present disclosure has high corrosion resistance and bending workability as well.

[0032] According to an embodiment, the zinc alloy plated steel sheet of the present disclosure may have a good appearance. Specifically, the number of black spots per unit area may be equal to or less than 0.1/cm.sup.2 on the surface of the zinc alloy plated steel sheet.

[0033] To obtain these effects of the present disclosure, the area fraction of the Zn single phase structure may preferably be 40% or less (excluding 0%) on the surface of the zinc alloy plating layer. That is, the appearance of the zinc alloy plated steel sheet may be improved by maximizing the fraction of the ZnAlMg-based intermetallic compound present on the surface of the zinc alloy plating layer.

[0034] According to an embodiment, the zinc alloy plated steel sheet of the present disclosure may also have high scratch resistance.

[0035] According to results of research conducted by the inventors, if the area fractions of the Zn/MgZn.sub.2 binary eutectic structure and the Zn/Al/MgZn.sub.2 ternary eutectic structure which have a layer structure and are present on the surface of the zinc alloy plating layer are maximized, scratch resistance may be markedly improved.

[0036] To obtain this effect of the present disclosure, preferably, the sum of the area fractions of the Zn/MgZn.sub.2 binary eutectic structure and the Zn/Al/MgZn.sub.2 ternary eutectic structure may be 50% or greater (excluding 100%), and the area fraction of the MgZn.sub.2 single phase structure may be 10% or less (including 0%). The MgZn.sub.2 single phase structure has high hardness and thus causes cracks during a machining process, and thus the area fraction of the MgZn.sub.2 single phase structure may be adjusted to be as low as possible.

[0037] The zinc alloy plated steel sheet of the present disclosure may be manufactured by various methods without limitation. However, for example, when the zinc alloy plating layer solidifies from a molten state, the zinc alloy plating layer may be cooled by spraying droplets thereon and then cooled with air to obtain the above-described degree of preferred orientation and average grain diameter.

[0038] In this case, droplets may be sprayed by a charge spray method to attach the droplets by electrostatic attraction between the droplets and the zinc alloy plated steel sheet. This charge spray method may be helpful in forming fine, uniform droplets and reducing the amount of droplets colliding with and bouncing off the zinc alloy plated steel sheet after being sprayed on the zinc alloy plated steel sheet, thereby facilitating rapid cooling of the zinc alloy plating layer from the molten state and having a positive effect on the growth of the Zn single phase structure in the (0001) orientation and refinement of the Zn single phase structure.

[0039] The droplets may be droplets of a phosphate aqueous solution capable of rapidly cooling the zinc alloy plating layer from the molten state through an endothermic reaction and thus effective in growing the Zn single phase structure in the (0001) orientation and refining the Zn single phase structure. Examples of the phosphate aqueous solution may include an aqueous solution of ammonium hydrogen phosphate ((NH.sub.4).sub.2HPO.sub.4), an aqueous solution of sodium ammonium hydrogen phosphate (NaNH.sub.4HPO.sub.4), an aqueous solution of zinc dihydrogen phosphate (Zn(H.sub.2PO.sub.4).sub.2), and an aqueous solution of calcium phosphate (Ca.sub.3(PO.sub.4).sub.2).

[0040] In addition, the content of the phosphate aqueous solution may be 1 wt % to 3 wt %. If the content of the phosphate aqueous solution is less than 1 wt %, the effect of the phosphate aqueous solution may not be sufficient. If the content of the phosphate aqueous solution is greater than 3 wt %, the effect of the phosphate aqueous solution is saturated, and nozzle clogging may occur in a continuous production process, lowering productivity.

[0041] In addition, when the droplets may be sprayed at a droplet spray start temperature of 405 C. to 425 C., and more preferably 410 C. to 420 C. Here, the term droplet spray start temperature refers to a surface temperature of the zinc alloy plated steel sheet at the start time of droplet spraying. If the droplet spray start temperature is less than 405 C., solidification of the Zn single phase structure may have already started, and thus black spots may be formed on the surface of the zinc alloy plated steel sheet. Conversely, if the droplet spray start temperature is greater than 425 C., droplets may not effectively undergo an endothermic reaction, and thus it may be difficult to obtain an intended structure.

[0042] In addition, the droplets may be sprayed at a droplet spray stop temperature of 380 C. to 400 C., and more preferably 390 C. to 400 C. Here, the term droplet spray stop temperature refers to a surface temperature of the zinc alloy plated steel sheet at a point in time at which spraying of droplets stops. If the droplet spray stop temperature is greater than 400 C., an endothermic reaction by the droplets may occur ineffectively, and thus it may be difficult to obtain an intended structure. Conversely, if the droplet spray stop temperature is less than 380 C., a Mg.sub.2Zn.sub.11 phase may be formed due to over cooling while the Zn/MgZn.sub.2 binary eutectic phase and the Zn/Al/MgZn.sub.2 ternary phase start to solidify, and thus many black spots may be formed, decreasing the degree of (0001) preferred orientation of the Zn single phase structure.

[0043] In addition, the difference between the droplet spray start temperature and the droplet spray stop temperature may be 15 C. or greater. If the difference is less than 15 C., the droplets may not undergo an effective endothermic reaction, and thus it may be difficult to obtain an intended structure.

[0044] In addition, the droplets may be sprayed in an amount of 50 g/m.sup.2 to 100 g/m.sup.2. If the spraying amount of the droplets is less than 50 g/m.sup.2, the effect of the droplets may be insufficient, and if the spraying amount of the droplets is greater than 100 g/m.sup.2, the effect of the droplets may be saturated.

MODE FOR INVENTION

[0045] Hereinafter, the present disclosure will be described more specifically through examples. However, the following examples should be considered in a descriptive sense only and not for purpose of limitation. The scope of the present invention is defined by the appended claims, and modifications and variations reasonably made therefrom.

Example 1

[0046] Low carbon cold-rolled steel sheets each having a thickness of 0.8 mm, a width of 100 mm, and a length of 200 mm were prepared as base steel sheets for plating test samples, and then foreign substances such as rolling oil were removed from the surfaces of the base steel sheets by dipping the base steel sheets into acetone and washing the base steel sheets with ultrasonic waves. Thereafter, a 750 C. reducing atmosphere heat treatment commonly performed to guarantee mechanical characteristics of steel sheets in the hot-dipping plating field was performed on the base steel sheets, and then the base steel sheets were dipped into plating baths (bath temperature: 460 C.) having compositions shown in Table 1 below to fabricate zinc alloy plated steel sheets. Thereafter, each of the zinc alloy plated steel sheets was wiped with gas to adjust a plating weight to be 70 g/m.sup.2 on each side. Then, the zinc alloy plated steel sheets were cooled under the conditions shown in Table 1 below and were cooled with air. Although not shown in Table 1 below, Comparative Sample 5 was prepared by performing a gas wiping process on a zinc alloy plated steel sheet fabricated using the same plating bath as that used to fabricate Inventive Sample 1 to adjust a plating weight to be 70 g/m.sup.2 on each side, and then cooling the zinc alloy plated steel sheet using a general cooling device at an average cooling rate of 12 C./sec until the plating layer of the zinc alloy plated steel sheet was completely solidified (at about 300 C. or less).

[0047] Then, the microstructures of the fabricated zinc alloy plated steel sheets were observed using an FE-SEM (SUPRA-55VP, Zeiss) as illustrated in FIGS. 1 and 2, and the average grain diameter of a Zn single phase structure of each of the zinc alloy plated steel sheets was measured as shown in Table 2 below.

[0048] Thereafter, the degree (f) of (0001) preferred orientation of the Zn single phase structure was measured using the following Formula 1, and results thereof are shown in Table 2 below.

f(%)=(I.sub.basal/I.sub.total)100[Formula 1]

[0049] where I.sub.total refers to the integral of all diffraction peaks of the Zn single phase structure when an X-ray diffraction pattern was measured within the range of 2 theta from 10 to 100 using a Cu-K source, and I.sub.basal refers to the integral of diffraction peaks of the Zn single phase structure relating to a basal plane.

[0050] Thereafter, the bending workability of each of the zinc alloy plated steel sheets was evaluated, and results thereof are shown in Table 2 below.

[0051] Corrosion resistance was evaluated as follows. A salt spray test (based on KS-C-0223) was performed on each of the zinc alloy plated steel sheets to facilitate corrosion, and then the time taken until the area fraction of red rust on the surface of each plating layer was 5% was measured.

[0052] Bending workability was evaluated as follows.

[0053] 3T bending was performed on each of the zinc alloy plated steel sheets, and a 1-mm length of the apex of each bent portion was observed using an SEM to measure the area fraction of bending cracks using an image analysis system.

TABLE-US-00001 TABLE 1 Composition of Droplet Droplet plating bath spray start spray stop Spraying (wt %) temperature temperature amount No. Al Mg ( C.) ( C.) Droplets (g/m.sup.2) Notes 1 1.6 1.6 410 390 Aqueous solution 70 *IS 1 of ammonium hydrogen phosphate, 2 wt % 2 1.6 1.6 420 400 Aqueous solution 70 IS 2 of ammonium hydrogen phosphate, 2 wt % 3 1.6 1.6 430 400 Aqueous solution 70 **CS 1 of ammonium hydrogen phosphate, 2 wt % 4 1.6 1.6 400 390 Aqueous solution 70 CS 2 of ammonium hydrogen phosphate, 2 wt % 5 1.6 1.6 420 405 Aqueous solution 70 CS 3 of ammonium hydrogen phosphate, 2 wt % 6 1.6 1.6 410 375 Aqueous solution 70 CS 4 of ammonium hydrogen phosphate, 2 wt % *IS: Inventive Sample, **CS: Comparative Sample

TABLE-US-00002 TABLE 2 Average grain diameter of Zn Red rust Area fraction of single phase occurrence bending cracks No. structure (m) f (%) time (h) (%) Notes 1 8 63 650 8 *IS 1 2 10 62 645 9 IS 2 3 12 49 640 25 **CS 1 4 14 47 630 38 CS 2 5 15 46 620 40 CS 3 6 16 44 610 42 CS 4 7 18 42 600 45 CS 5 *IS: Inventive Sample, **CS: Comparative Sample

[0054] Referring to Table 2, Inventive Samples 1 and 2 satisfying conditions proposed in the present disclosure had high bending workability.

[0055] However, although Comparative Samples 1 to 5 had high corrosion resistance, Comparative Samples 1 to 5 had poor bending workability because the (f) values thereof were less than 50%.

[0056] FIG. 1 is views illustrating results of (a) an observation of a surface microstructure of Inventive Sample 1 of the present disclosure and (b) an observation of a surface microstructure of Comparative Sample 5, and FIG. 2 is views illustrating results of (a) an observation of a cross-sectional microstructure of Inventive Sample 1 of the present disclosure and (b) an observation of a cross-sectional microstructure of Comparative Sample 5.

[0057] FIG. 3 is a view illustrating results of X-ray diffractometer (XRD) analysis of Inventive Sample 1. In FIG. 3, peaks denoted with and .circle-solid. are all diffraction peaks of the Zn single phase structure, and the peaks denoted with are diffraction peaks of the Zn single phase structure relating to a basal plane.

Example 2

[0058] Low carbon cold-rolled steel sheets each having a thickness of 0.8 mm, a width of 100 mm, and a length of 200 mm were prepared as base steel sheets for plating test samples, and then foreign substances such as rolling oil were removed from the surfaces of the base steel sheets by dipping the base steel sheets into acetone and washing the base steel sheets with ultrasonic waves. Thereafter, a 750 C. reducing atmosphere heat treatment commonly performed to guarantee mechanical characteristics of steel sheets in the hot-dipping plating field was performed on the base steel sheets, and then the base steel sheets were dipped into plating baths having compositions shown in Table 3 below to fabricate zinc alloy plated steel sheets. Thereafter, each of the zinc alloy plated steel sheets was wiped with gas to adjust a plating weight to be 70 g/m.sup.2 on each side. Then, the zinc alloy plated steel sheets were cooled under the same conditions as Inventive Sample 1 of Example 1.

[0059] Thereafter, the fractions of microstructures observed on the surface of each of the zinc alloy plated steel sheets were measured, and the number of black spots on the surface of each of the zinc alloy plated steel sheets was measured. Results thereof are shown in Tables 3 and 4.

[0060] Thereafter, a friction test (linear friction test) was performed by rubbing the surface of each of the zinc alloy plated steel sheets 20 times with a tool head at a constant pressure. In the friction test, a target load was 333.3 kgf, a pressure was 3.736 MPa, the tool head traveled 200 mm per rub, and the speed of the tool head was 20 mm/s.

[0061] After the friction test, a stripping test was performed on each of the zinc alloy plated steel sheets. Specifically, cellophane adhesive tape (NB-1 by Ichiban) was attached to a bent portion of each of the zinc alloy plated steel sheets subjected to a 10R bending process, and then the cellophane tape was momentarily separated. Then, the number of plating layer defects was measured using an optical microscope (magnification: 50 times). Results of the measurement were evaluated as when the number of plating layer defects was 5/m.sup.2 or less, and X when the number of plating layer defects was greater than 5/m.sup.2. Evaluation results are shown in Table 4 below.

[0062] In addition, after the friction test, each of the zinc alloy plated steel sheets was inserted into a salt spray tester, and the time taken until the occurrence of red rust was measured according to international standard ASTM B117-11. In that time, a 5% salt solution (35 C., pH 6.8) was sprayed at a rate of 2 ml/80 cm.sup.2 per hour. When the time taken until red rust was present on a sample was 500 hours or greater, the sample was evaluated as , and when the time taken until red rust was present on a sample was less than 500, the sample was evaluated as X. Results of the evaluation are shown in Table 4 below.

TABLE-US-00003 TABLE 3 Alloy composition Area fractions of surface structures (area %) (wt %) Zn/Al/MgZn.sub.2 + No. Al Mg Mg/Al Zn Zn/MgZn.sub.2 Zn/Al/MgZn.sub.2 MgZn.sub.2 Zn/Al Zn/MgZn.sub.2 Notes 1 0.6 2.3 3.83 28 41 31 0 0 72 *IS A 2 1.5 2.8 1.87 20 57 21 1 1 78 IS B 3 2 2.9 1.45 8 63 28 1 0 91 IS C 4 2.2 2.7 1.23 4 58 34 2 2 92 IS D 5 2.6 2.9 1.12 4 39 51 3 3 90 IS E 6 0 0 100 0 0 0 0 0 **CS A 7 1.4 1 0.71 82 7 11 0 0 18 CS B 8 2.5 1.2 0.48 6 21 26 46 1 47 CS C 9 5 0 0.00 76 0 0 0 24 0 CS D 10 5 1 0.20 59 9 11 0 21 20 CS E 11 8 3 0.38 13 7 13 18 49 20 CS F 12 55 0 0.00 14 0 0 0 86 0 CS G Here, surface structures refer to microstructures observed on the surfaces of zinc alloy plating layers. *IS: Inventive Sample, **CS: Comparative Sample

TABLE-US-00004 TABLE 4 Results of Results of stripping salt spray test after test after Number friction test friction test of black Number Evalua- Red Rust Evalua- spots of defects tion Occurrence tion No. (/cm.sup.2) (/m.sup.2) results time (hours) results Notes 1 0.05 3 520 *IS A 2 0.08 2 550 IS B 3 0.04 4 600 IS C 4 0.08 3 650 IS D 5 0.04 2 580 IS E 6 1.2 2 120 X **CS A 7 0.8 3 230 X CS B 8 0.05 23 X 620 CS C 9 1.1 3 350 X CS D 10 0.6 2 420 X CS E 11 0.06 15 X 650 CS F 12 0.05 11 X 200 X CS G *IS: Inventive Sample, **CS: Comparative Sample

[0063] Referring to Table 4, Inventive Samples A to E satisfying conditions proposed in the present disclosure had good appearance and high scratch resistance.

[0064] However, each of Comparative Samples A, B, D, and E had poor appearance because the area fraction of a Zn single phase structure present on the surface of a plating layer was excessively high, and each of Comparative Samples A to G had poor scratch resistance because the area fractions of a Zn/MgZn.sub.2 binary eutectic structure and a Zn/Al/MgZn.sub.2 ternary eutectic structure are excessively low.