ZINC ALLOY-PLATED STEEL MATERIAL HAVING EXCELLENT CORROSION RESISTANCE AND SURFACE QUALITY, AND METHOD FOR PRODUCING SAME

Abstract

Provided is a zinc alloy-plated steel material used in automobiles, building materials, home appliances, or the like, and more specifically, to a zinc alloy-plated steel material having excellent corrosion resistance and surface quality, and a method for producing same.

Claims

1. A zinc alloy-plated steel material having excellent corrosion resistance and surface qualities, the zinc alloy-plated steel material comprising: base steel and a zinc alloy-plating layer formed on the base steel, wherein the zinc alloy-plating layer includes, by weight %, 8 to 25% of aluminum (Al), 4 to 12% of magnesium (Mg), and a balance of zinc (Zn) and inevitable impurities, and an area fraction, occupied by a polygonal solidification phase observed on a surface of the zinc alloy-plating layer, is 20 to 90%.

2. The zinc alloy-plated steel material of claim 1, wherein a ratio (b/a) of a major axis ‘b’ to a minor axis ‘a’ of the polygonal solidification phase is 1 to 3.

3. The zinc alloy-plated steel material of claim 1, wherein a microstructure of the zinc alloy-plating layer includes at least one of MgZn.sub.2 and Mg.sub.2Zn.sub.11 in an area fraction of 20 to 45%.

4. The zinc alloy-plated steel material of claim 1, wherein Al and Mg satisfy the following relational expression 1,
Mg≤−0.0186*Al.sup.2+1.0093*Al+4.5  [Relational Expression 1] where each of Mg and Al denotes a content (weight %) of a corresponding element.

5. The zinc alloy-plated steel material of claim 1, wherein the zinc alloy-based layer further includes, by weight %, 0.0005 to 0.009% of at least one of beryllium (Be), calcium (Ca), cerium (Ce), lithium (Li), scandium (Sc), strontium (Sr), vanadium (V), and yttrium (Y).

6. A method of manufacturing a zinc alloy-plated steel material having excellent corrosion resistance and surface qualities, the method comprising: preparing base steel; dipping the prepared base steel in a plating bath to be plated, the plating bath including, by weight %, 8 to 25% of aluminum (Al), 4 to 12% of magnesium (Mg), and a balance of zinc (Zn) and inevitable impurities; wiping the plated base steel; and forming a polygonal solidification phase on a surface of a hot-dip galvanized layer after the wiping.

7. The method of claim 6, wherein the forming of the polygonal solidification phase includes spraying a gas, containing nitrogen having a concentration of 78 to 99% in a volume fraction, primary gas spraying, and then spraying a gas having a dew point of −5 to 50° C., secondary gas spraying.

8. The method of claim 7, further comprising: adding vibrations of 100 Hz to 5 MHz after the secondary gas spraying.

9. The method of claim 6, wherein Al and Mg satisfy the following relational expression 1,
Mg≤−0.0186*Al.sup.2+1.0093*Al+4.5  [Relational Expression 1] where each of Mg and Al denotes a content (weight %) of a corresponding element.

10. The method of claim 6, wherein the plating bath further includes, by weight %, 0.0005 to 0.009% of at least one of beryllium (Be), calcium (Ca), cerium (Ce), lithium (Li), scandium (Sc), strontium (Sr), vanadium (V), and yttrium (Y).

Description

DESCRIPTION OF DRAWINGS

[0015] FIG. 1 is an image observing a surface of a plating layer according to Inventive Example 1, among examples of the present disclosure.

[0016] FIG. 2 is an image observing a surface of a plating layer according to Comparative Example 1, among examples of the present disclosure.

BEST MODE FOR INVENTION

[0017] Hereinafter, the present disclosure will be described in detail.

[0018] A zinc alloy-plated steel material according to the present disclosure may include base steel and a zinc alloy-plating layer formed on the base steel.

[0019] The type of the base steel is not necessarily limited, and the base steel may be any base steel applicable to technical fields to which the present disclosure pertains. For example, the base steel may be a hot-rolled steel sheet, a cold-rolled steel sheet, a wire rod, a steel wire, or the like.

[0020] The zinc alloy-plating layer is based on zinc (Zn), and includes magnesium (Mg) and aluminum (Al). The zinc alloy-plating layer may include, in detail, by weight %, 8 to 25% of aluminum, 4 to 12% of magnesium, and a balance of zinc and inevitable impurities. The zinc alloy-based layer may further include 0.0005 to 0.009% of at least one of beryllium (Be), calcium (Ca), cerium (Ce), lithium (Li), scandium (Sc), strontium (Sr), vanadium (V), and yttrium (Y). Hereinafter, a chemical composition of the zinc alloy-plated steel will be described in detail.

[0021] Aluminum (Al): 8 to 25 weight % (hereinafter, %)

[0022] Aluminum may stabilize a magnesium (Mg) component and may serve as a corrosion barrier to suppress initial corrosion in a corrosive environment. The content of aluminum may vary depending on the content of magnesium. When the content of aluminum is less than 8%, magnesium may not be stabilized while producing a molten metal in a plating bath, and thus, magnesium oxide may be formed on a surface of the molten metal to make it difficult to use aluminum. On the other hand, when the content of Al is greater 25%, a plating temperature may be increased and various devices mounted in the plating bath may be severely corroded. Therefore, a content of Al, greater than 25%, is not preferable.

[0023] Magnesium (Mg): 4 to 12%

[0024] Magnesium is a main component forming a structure exhibiting corrosion resistance. When the content of magnesium is less than 4%, exhibition of corrosion resistance is insufficient. When the content of magnesium is greater than 20%, a large amount of magnesium oxide may be formed in the plating bath. Since various issues such as deterioration in a material and an increase in costs may secondarily occur, the content of magnesium may be, in detail, 4 to 12% and, in further detail, 5% or more.

[0025] The contents of aluminum and magnesium may satisfy, in detail, Relational Expression 1.

Mg≤−0.0186*Al.sup.2+1.0093*Al+4.5  [Relational Expression 1]

[0026] where each of Mg and Al denotes the content (weight %) of the corresponding element

[0027] In the present disclosure, the contents of aluminum and magnesium may satisfy, in detail, the condition of Relational Expression 1 to stabilize the molten metal and to significantly suppress formation of an oxide.

[0028] Other than aluminum and magnesium, beryllium (Be), calcium (Ca), cerium (Ce), lithium (Li), scandium (Sc), strontium (Sr), vanadium (V), yttrium (Y), and the like, may be additionally contained in the zinc alloy-plating layer to further stabilize the magnesium component, and may be contained in an amount of, in detail, 0.0005 to 0.009%. When each of Be, Ca, Ce, Li, Sc, Sr, V, Y, and the like, is contained in an amount less than 0.0005%, it may be difficult to obtain a substantial Mg stabilization effect. When each of Be, Ca, Ce, Li, Sc, Sr, V, Y, and the like, is contained in an amount greater than 0.0009%, solidification may occur at the end of plating to deteriorate corrosion resistance and to increase costs. Therefore, the above content is not preferable.

[0029] Other than the alloy composition, a balance of zinc (Zn) and inevitable impurities are included. The addition of effective elements, other than the alloy composition, is not excluded.

[0030] A surface of the zinc alloy-plating layer may include a polygonal solidification phase, and an area fraction occupied by the polygonal solidification phase observed on the surface may be, in detail, 20 to 90%.

[0031] When the surface of the zinc alloy-plating layer is observed with a scanning electron microscope (SEM) or an optical microscope, structures having various shapes such as a polygonal shape, a circular shape, an elliptical shape, a sand shape, and the like, may be observed. In the present disclosure, the polygonal solidification phase is one of the structures observed on the surface, is exposed to a surface layer, and is clearly distinguished in color and shape from adjacent, other solidification structures. For example, as illustrated in FIG. 1, boundaries between the polygonal solidification phase and the adjacent, other structures are distinguished as almost straight lines, and the straight lines intersect each other to form a predetermined angle. In this case, since the angle may have various values, the angle is not limited to a specific value. In addition, the polygonal solidification phase may be formed by multiple overlaps and may have several angles, and the entire inside of a polygonal solidification structure may not have the same color or the same shape. Some structures may overlap each other, and may be deformed and appear to be different from each other. Therefore, when some structures have two or more angles, they include a polygonal solidification phase.

[0032] The polygonal solidification phase may be an alloy phase in which two or three components of Zn, Al, and Mg are detected, and thus, may be an intermetallic compound or an alloy phase in which Zn, Al, an additional element for stabilization, and the like, are included in an intermetallic compound. The intermetallic compounds may be MgZn.sub.2, Mg.sub.2Zn.sub.11, or the like.

[0033] An area occupied by the polygonal solidification phase on the surface may be, in detail, 20 to 90% in an area fraction. When the area of the polygonal solidification phase is less than 20%, corrosion resistance and workability may be insufficient. When the area of the polygonal solidification phase is greater than 90%, corrosion resistance may be rather lowered. The area fraction of the polygonal solidification phase may be, in further detail, 30 to 70%. Since the polygonal solidification phase is observed on the surface, the area of the polygonal solidification phase is obtained as an area occupied by the polygonal solidification phase on the surface area.

[0034] A ratio (b/a) of a major axis ‘b’ of the polygonal solidification phase to a minor axis ‘a’ of the polygonal solidification phase may be, in detail, 1 to 3. As illustrated in FIG. 1, a shape of the polygonal solidification phase may be defined as the minor axis ‘a’ and the major axis ‘b’ and may be represented by the ratio ‘b/a’ including both cases in which some solidification phases are difficult to be separated from each other and are deformed because they overlap each other. When the ratio ‘b/a’ is 1 or more, formability may be excellent. However, when the ratio ‘b/a’ is significantly high, a solidification phase may be excessively elongated to have an adverse effect on forming. When the ratio ‘b/a’ is higher than 3, it may be disadvantageous for formability. Therefore, the ratio ‘b/a’ may be, in detail, 1 to 3.

[0035] The zinc alloy-plating layer according to the present disclosure may include various phases. For example, the various phases may include MgZn.sub.2, Mg.sub.2Zn.sub.11, an Al solid-solution phase, a Zn solid-solution phase, an Al/Zn/Mg eutectic phase, and the like. A microstructure of the zinc alloy-plating layer may include, in detail, at least one of MgZn.sub.2 and Mg.sub.2Zn.sub.11, among the above phases, in an area fraction of 20 to 45%. This may be, in detail, an area fraction of a surface area of the plating layer.

[0036] In the present disclosure, phases formed in the zinc alloy-plating layer may be generated in a substantially non-equilibrium state. For example, in the case of MgZn.sub.2, when a ratio of Mg/Zn is calculated in atomic %, a result of the calculation should be 0.33 but has actually been 0.19 to 0.24. In addition, other components may be detected in the phases generated in the non-equilibrium state. The detected components may be determined in comprehensive consideration of component analysis, shape analysis, and the like.

[0037] When the area fraction of at least one of MgZn.sub.2 and Mg.sub.2Zn.sub.n is less than 20%, corrosion resistance is insufficient in the moisture environment and a salt water environment. When the area fraction of at least one of MgZn.sub.2 and Mg.sub.2Zn.sub.n is greater than 45%, the corrosion resistance may be increased but there is a high probability that cracking may occur, because the MgZn.sub.2 ally phase and the Mg.sub.2Zn.sub.n alloy phase are hard. The area fraction of at least one of MgZn.sub.2 and Mg.sub.2Zn.sub.n may be, in detail, 20 to 40%.

[0038] The remainder may include a Zn solid-solution phase, an Al solid-solution phase, an Al/Zn/Mg eutectic phase, a non-stoichiometric composition, and the like.

[0039] Hereinafter, an example embodiment of manufacturing a zinc alloy-plated steel according to the present disclosure will be described in detail.

[0040] The present disclosure proposes a method of forming a zinc alloy-plating layer having excellent corrosion resistance and surface appearance.

[0041] A solidification process of a plating layer may be performed through nucleation and growth. When the plating layer is cooled, solidification nuclei may be produced. The solidification nuclei may be thermodynamically produced in lowest Gibbs free energy. A difference in the Gibbs free energy may be a site more advantageous for solidification in the case of non-uniform nucleation than uniform nucleation. The larger an area of a non-uniform nucleation site, the more advantageous the nucleation, and a large number of nucleation may occur. In this case, the non-uniform nucleation site may be a site in which a liquid phase of a molten metal and a solid phase are brought into contact with each other. A representative non-uniform nucleation site may be a surface of a steel sheet. Another non-uniform nucleation site may be a site the liquid phase of the molten metal and air are in contact with each other, and may be a surface of the molten metal. Accordingly, the present inventors have developed a method for controlling solidification of a steel material taken out of the plating bath to form a polygonal solidification phase on a surface of a plating layer.

[0042] A method of manufacturing a zinc alloy-plated steel material according to the present disclosure may include preparing base steel, dipping the prepared base steel in a plating bath and then wiping the dipped base steel to adjust a thickness of a plating layer, and forming a polygonal solidification phase on a surface of a hot-dip galvanized layer. Hereinafter, each process will be described in detail.

[0043] First, base steel may be prepared. As described above, the type of the base steel is not necessarily limited, and the base steel may be any base steel applicable to technical fields to which the present disclosure pertains. Before the base steel is dipped in the plating bath, the method may include removing oxides and impurities present on a surface of the base steel, performing a heat treatment process for reduction, and the like.

[0044] The base steel may be dipped in the plating bath to form a zinc alloy-plating layer on the surface of the base steel. A composition of the plating bath may include, in detail, by weight %, 8 to 25% of aluminum (Al), 4 to 12% of magnesium (Mg), and a balance of zinc (Zn) and inevitable impurities. The plating bath may further include 0.0005 to 0.009% of at least one of beryllium (Be), calcium (Ca), cerium (Ce), lithium (Li), scandium (Sc), strontium (Sr), vanadium (V), and yttrium (Y). In addition, the contents of aluminum and magnesium may satisfy Relational Expression 1. An alloy composition range of the plating bath is not different from the above-described alloy composition range of the zinc alloy-plating layer.

Mg≤−0.0186*Al.sup.2+1.0093*Al+4.5  [Relational Expression 1]

[0045] A temperature of the plating bath may vary depending on a melting point, and the melting point may be a physicochemical property depending on the composition of the plating bath. The temperature of the plating bath may be determined by various factors such as convenience in process, heating costs, and plating quality. When these are taken into consideration, the temperature of the plating bath may be higher than the melting point and may be, in detail, 20 to 100° C. higher than the melting point.

[0046] The temperature of the base steel, dipped in the plating bath, may be set in consideration of the convenience in process and heat balance. The temperature of the base steel may be set to, in detail, −10° C. to +10° C. of the temperature of the plating bath.

[0047] The zinc alloy-plated steel material, taken out of the plating bath, may be subjected to a wiping process in which a thickness of a plating layer is adjusted by a wiping nozzle, a so-called air knife, above the plating bath. The wiping nozzle may adjust the thickness of the plating layer by spraying air or inert gas.

[0048] After the wiping process, a polygonal solidification phase may be formed on a surface of the plating layer. To this end, a gas containing nitrogen having a concentration of 78 to 99% in a volume fraction may be primarily sprayed (primary gas spraying), and then a gas having a dew point of −5 to 50° C. may be secondarily sprayed (secondary gas spray).

[0049] In the primary gas spraying, gases other than nitrogen are not limited, but may include air, oxygen or nitrogen, or an inert gas such as argon and a mixed gas thereof. In the secondary gas spraying, the dew point may be a specific value defining the amount of water contained in the gas. In this case, the type of gas is not limited during the secondary gas spraying. As an example, a gas containing nitrogen having a concentration of 89 to 99% may be used.

[0050] In the primary gas spraying, when the concentration of nitrogen is less than 78%, surface defects are likely to occur. In the primary gas spraying, when the concentration of nitrogen is greater than 99%, formation of a polygonal solidification phase is insufficient. In the secondary gas spraying, when the dew point rises, formation of polygonal solidification nuclei is increased. When the dew point is less than −5° C., the formation of polygonal solidification nuclei may be insufficient. On the other hand, when the dew point is greater than 50° C., a large amount of surface defects may occur.

[0051] In addition, after the second gas spraying, vibrations of 100 Hz to 5 MHz may be added to establish an environment advantageous for the formation of a polygonal solidification phase. When the vibrations are less than 100 Hz, the formation of the polygonal solidification phase may be insufficient. On the other hand, when the vibrations are greater than 5 MHz, surface defects may occur.

MODE FOR INVENTION

[0052] Hereinafter, embodiments of the present disclosure will be described more specifically through examples. However, the examples are for clearly explaining the embodiments of the present disclosure and are not intended to limit the scope of the present invention.

EXAMPLE

[0053] A plated steel sheet was manufactured by preparing a cold-rolled steel sheet, as base steel, having a thickness of 0.8 mm and including 0.03 weight % of carbon (C), 0.2 weight % of silicon (Si), 0.15 weight % of manganese (Mn), 0.01 weight % of phosphorus (P), and 0.01 weight % of sulfur (S) (a balance of iron (Fe) and inevitable impurities), performing a degreasing process to remove impurities, such as oil or the like, smeared on a surface of the cold-rolled steel sheet, heat-treating the cold-rolled steel sheet at a temperature of 800° C. in a reducing atmosphere of 10 volume % hydrogen and 90 volume % of nitrogen, and dipping the heat-treated steel sheet in a hot-dip galvanizing bath to have a composition of a plating layer listed in Table 1. In this case, a temperature of the hot-dip galvanizing bath was set to be 493° C. and a temperature of the steel sheet introduced into the hot-dip galvanizing bath was also set to be 493° C. After the dipping, a thickness of the plating layer was adjusted to about 8 to 10 μm through air wiping. Then, primary and secondary gas treatments illustrated in Table 1 were performed to manufacture a plated steel sheet.

[0054] In the manufactured zinc alloy-plated steel, a phase was identified through energy dispersive X-ray spectrometer (EDS) analysis, and fractions of MgZn.sub.2 and Mg.sub.2Zn.sub.11 phases were measured through X-ray diffraction (XRD) analysis. An area ratio of a polygonal solidification phase was measured using an image analyzer, and a ratio (b/a) of a major axis ‘b’ to a minor axis ‘a’ was calculated by measuring lengths thereof.

[0055] Surface qualities and corrosion resistance of the zinc alloy-plated steel sheet were evaluated, and results of the evaluation are listed in Table 1.

[0056] The corrosion resistance was evaluated by performing a salt spray test, measuring red rust occurrence time, and being compared with corrosion resistance of a comparative sample. In this case, the comparative sample was a zinc alloy-plated steel material having a plating layer composition including 94 weight % of zinc (Zn), 3 weight % of aluminum (Al) and 3 weight % of magnesium (Mg), and the salt spray test was performed at salinity of 5%, temperature of 35° C., pH of 6.8, and salt spray amount of 2 ml/80 cm.sup.2.Math.1 Hr.

[0057] When the red rust occurrence time was 1.5 times or more as compared with the red rust occurrence time of the comparative sample, the corrosion resistance was evaluated to be good (o). In the case in which the red rust occurrence time was less than 1.5 times, as compared with the red rust occurrence time of the comparative sample, the corrosion resistance was evaluated to be poor (x).

[0058] Surface qualities were evaluated by observing an exterior of each manufactured sample to determine whether a surface defect such as dross, and the like, occurred. The result is as follows:

[0059] Good (o): No defect, such as dross or a spot, occurred on the surface

[0060] Poor (x): A defect, such as dross or a spot, occurred on the surface

TABLE-US-00001 TABLE 1 MgZn.sub.2 and PSP Mg.sub.2Zn.sub.11 CPL (wt %) PGT SGT AR mA(b)/ AR Al Mg Others (NC, %) (DP, ° C.) (%) MA(a) (%) SQ SCR IE 1 12 6 — 85 20 72 1.6 34 ∘ ∘ IE 2 16 9 — 85 20 80 1.4 38 ∘ ∘ IE 3 8 5 — 85 20 20 2.2 20 ∘ ∘ IE 4 25 12 — 85 20 88 1.2 50 ∘ ∘ CE 1 3 3 — 85 20 2 1.4 0 ∘ x CE 2 30 22 — 85 20 94 2.6 62 x x IE 5 12 6 Be 85 20 62 1.4 35 ∘ ∘ 0.0005 IE 6 12 6 Be 85 20 53 1.2 38 ∘ ∘ 0.0090 CE 3 12 6 Be 85 20 65 1.6 39 x x 0.0500 IE 7 12 6 Ca 85 20 71 1.5 33 ∘ ∘ 0.0090 IE 8 12 6 Ce 85 20 64 1.8 34 ∘ ∘ 0.0090 IE 9 12 6 Li 85 20 70 1.8 36 ∘ ∘ 0.0090 IE 10 12 6 Sc 85 20 58 1.4 35 ∘ ∘ 0.0090 IE 11 12 6 Sr 85 20 66 1.6 32 ∘ ∘ 0.0090 IE 12 12 6 V 85 20 71 1.3 34 ∘ ∘ 0.0090 IE 13 12 6 Y 85 20 62 1.1 36 ∘ ∘ 0.0090 IE 14 12 6 — 78 −5 27 1.2 20 ∘ ∘ IE 15 12 6 — 99 50 90 2.2 43 ∘ ∘ CE 4 12 6 — 70 −10 12 1.4 10 ∘ x CE 5 12 6 — 100 60 77 3.5 40 x ∘ IE16 10 4 — 85 20 20 1.9 20 ∘ ∘ IE: Inventive Example CE: Comparative Example CPL: Composition of Plating Layer PGT: Primary Gas Treatment/NC: Nitrogen Concentration SGT: Secondary Gas Treatment/DP: Dew Point PSP: Polygonal Solidification Phase AR: Area Ratio mA: Minor Axis/MA: Major Axis SQ: Surface qualities SCR: Surface Corrosion Resistance

[0061] As can be seen from Table 1, all inventive examples satisfying conditions according to the present disclosure had excellent surface qualities and corrosion resistance.

[0062] In particular, FIG. 1 is an image observing a surface of Inventive Example 1. From FIG. 1, it can be seen that a polygonal solidification phase, in which straight lines intersect each other to form a predetermined angle, is formed in an appropriate fraction. FIG. 2 is an image observing a surface of Comparative Example 1. As compared with FIG. 1, it can be seen that it is difficult to observe a polygonal solidification phase on the surface.

[0063] Comparative Examples 1 and 2 are the cases in which the contents of aluminum (Al) and magnesium (Mg), essential components of a proposed plating layer, were outside the range proposed in the present disclosure. In Comparative Example 1, contents of Al and Mg of a plating layer were significantly low and a polygonal solidification phase observed on a surface is insufficient, so that corrosion resistance was not secured. In Comparative Example 2, contents of Al and Mg were excessive and too many polygonal solidification phases were formed on a surface, so that both the surface qualities and corrosion resistance were poor.

[0064] Comparative Example 3 is the case in which beryllium (Be) added for a supplementary effect was excessively included in a plating layer, and surface qualities and corrosion resistance were poor. Comparative Examples 4 and 5 did not satisfy the gas spray conditions proposed by the present disclosure, so that surface corrosion resistance or surface characteristics of the plating layer were poor.