Plated steel sheet having excellent corrosion resistance and method for producing same

Abstract

Provided are: a plated steel sheet having excellent corrosion resistance even in a complex corrosion environment; and a method for producing the plated steel sheet.

Description

Technical Field

[0001] The present invention relates to a plated steel sheet having excellent corrosion resistance even in a complex corrosion environment and a method for manufacturing the same. Background Art

[0002] Zinc-based plated steel sheets have the sacrificial anticorrosion property that zinc, which has a lower redox potential than iron when exposed to a corrosion environment, is corroded first to inhibit the corrosion of the steel. In addition, the zinc in the coating is oxidized, and a dense corrosion product is formed on the surface of the steel, thereby isolating the steel from the oxidation atmosphere, thus improving the corrosion resistance of the steel. Due to this advantageous property, the application range of zinc-based plated steel sheets has been expanded to building materials, household electrical appliances, and steel sheets for automobiles in recent years.

[0003] However, due to the increase in air pollution caused by the advancement of industry, the corrosion environment has gradually deteriorated. In addition, with the diversification of industries, steel sheets are exposed to more complex corrosion environments. Thus, there is an increasing need to develop a steel material having more excellent corrosion resistance than existing zinc-based plated steel sheets in various corrosion environments.

[0004] For example, steel sheets used for the interior of a building are placed inside a livestock shed or at a cement contact part, while on the other hand, steel sheets used for the exterior of a building are sometimes in an acidic corrosion atmosphere due to the influence of acid rain. Therefore, in a complex corrosion environment, a plated steel sheet having excellent corrosion resistance in both acidic and alkaline atmospheres is required, but in the related art of existing zinc-based plated steel sheets, a technology having a sufficient level of corrosion resistance in acidic and alkaline atmospheres has not been developed.

[0005] [Prior Art Documents]

[0006] [Patent Documents]

[0007] (Patent Document 1) Korean Patent Publication No. 2013-0133358 Summary of the Invention

[0008] (I) Technical Problem to be Solved

[0009] According to one aspect of the present invention, an object is to provide a plated steel sheet having excellent corrosion resistance even in a complex corrosion environment and a method for manufacturing the same.

[0010] According to another aspect of the present invention, an object is to provide a plated steel sheet having excellent corrosion resistance even in an acidic environment and an alkaline environment and a method for manufacturing the same.

[0011] According to another aspect of the present invention, an object is to provide a plated steel sheet having excellent corrosion resistance not only in a neutral environment but also in an acidic environment and an alkaline environment and a method for manufacturing the same.

[0012] The technical problems of the present invention are not limited to the above. Those skilled in the art to which the present invention pertains can easily understand the additional technical problems of the invention from the full text of the specification of the present invention.

[0013] (II) Technical Solution

[0014] One aspect of the present invention provides a coated steel sheet, the coated steel sheet comprising: a base steel sheet; and an Al-Mg-Zn-based coating provided on at least one surface of the base steel sheet, wherein the number density of the Al phase existing inside the MgZn2 phase or in contact with the MgZn2 phase in the coating is 500 - 4000 per 0.1 mm 2 .

[0015] In addition, another aspect of the present invention provides a method for manufacturing a coated steel sheet, the manufacturing method comprising the following steps: immersing a base steel sheet in a plating bath for hot-dip plating, wherein, by weight%, the plating bath contains Mg: 4.00 - 7.00%, Al: 8.000 - 20.000%, Fe: 0.002 - 0.050%, the balance being Zn and other inevitable impurities, the temperature is 430 - 520 °C, and the flow rate is 0.03 - 0.20 m / s; wiping the steel sheet after the hot-dip plating; and cooling the steel sheet after the wiping treatment such that the average cooling rate in the temperature range from the crystallization temperature of the Al phase to 330 °C satisfies 6.0 °C / s or more.

[0016] (III) Beneficial Effects

[0017] According to one aspect of the present invention, a coated steel sheet having excellent corrosion resistance even in a complex corrosion environment and a method for manufacturing the same can be provided.

[0018] In addition, according to another aspect of the present invention, a coated steel sheet having excellent corrosion resistance even in an acidic environment and an alkaline environment and a method for manufacturing the same can be provided.

[0019] In addition, according to another aspect of the present invention, a coated steel sheet having excellent corrosion resistance not only in a neutral environment but also in an acidic environment and an alkaline environment and a method for manufacturing the same can be provided.

[0020] The various beneficial advantages and effects of the present invention are not limited to the above, and the various beneficial advantages and effects of the present invention will be more easily understood during the process of describing the specific embodiments of the present invention. Description of the Drawings

[0021] Figure 1 It is a photograph of a cross-sectional specimen of a coated steel sheet obtained from Invention Example B4 of the present invention observed by a scanning electron microscope (SEM).

[0022] Figure 2 Photograph of a cross-sectional specimen of the plated steel sheet obtained from Comparative Example B14 of the present invention, observed with a scanning electron microscope (SEM). Best Mode for Carrying Out the Invention

[0023] The terms used in this specification are used to describe specific embodiments and are not intended to limit the present invention. Also, unless clearly indicated to the contrary in the relevant definitions, the singular forms used in this specification also include the plural forms.

[0024] The meaning of "comprising" or "including" used in the specification is to embody the components and does not exclude the existence or addition of other components.

[0025] Unless otherwise defined differently, all terms, including technical terms and scientific terms, used in this specification have the same meaning as commonly understood by those skilled in the art. Terms defined in a dictionary should be interpreted as having a meaning consistent with the relevant technical literature and the content currently disclosed.

[0026] Hereinafter, a plated steel sheet according to one aspect of the present invention will be described in detail. Unless otherwise defined differently, the content of each element in the present invention is expressed in wt%.

[0027] In recent years, with the diversification of industries, steel sheets are in a more complex corrosion environment. As such a composite corrosion environment, the demand for plated steel sheets having excellent corrosion resistance in both acidic and alkaline atmospheres has become increasingly prominent.

[0028] However, in the related art of existing zinc-based plated steel sheets, a technology having a sufficient level of corrosion resistance in both acidic and alkaline atmospheres has not been developed.

[0029] In addition to zinc-based plated steel sheets, there are also aluminum-based plated steel sheets such as Al-plated and Al-Zn-plated (galvalume). Aluminum-based plated steel sheets have excellent corrosion resistance in acidic atmospheres, but since aluminum corrodes severely in alkaline atmospheres, they cannot be applied to composite corrosion environments.

[0030] In order to improve this problem, various studies have been conducted on the manufacturing technology of zinc alloy-based coated steel sheets that enhance the corrosion resistance of steel sheets by adding elements such as aluminum (Al) and magnesium (Mg) to a zinc plating bath. As a representative example, there is an Al-Mg-Zn-based zinc alloy coated steel sheet that further adds Mg to an Al-Zn plating composition system. If an appropriate amount of Mg and Al is added to zinc, excellent corrosion resistance can be ensured even in acidic and alkaline atmospheres. In an Al-Mg-Zn-based zinc alloy coated steel sheet, when the addition amount of Mg in the coating layer increases, the dissolution of Mg becomes faster in an acidic atmosphere and it is prone to corrosion. However, in a neutral (including a saline environment) and alkaline atmosphere, a stable Mg-based hydroxide is formed on the surface of the steel sheet, thereby suppressing corrosion.

[0031] On the other hand, as the addition amount of Al in the coating layer increases, since aluminum oxide on the surface is stable in acidic and neutral conditions, the corrosion resistance is improved. However, on the other hand, the dissolution of Al is aggravated in an alkaline atmosphere and the corrosion resistance is reduced. Therefore, it is necessary to appropriately adjust the components of Mg and Al in the coating layer and uniformly adjust the distribution of the components of Mg and Al in the coating layer.

[0032] First, a coated steel sheet according to one aspect of the present invention includes: a base steel sheet; and a coating layer provided on at least one surface of the base steel sheet.

[0033] In the present invention, the type of the base steel plate may not be particularly limited. For example, the base steel plate may be an Fe-based base steel plate that is usually used as the base steel plate for zinc-based plated steel plates, that is, it may be a hot-rolled steel plate or a cold-rolled steel plate, but is not limited thereto. Alternatively, the base steel plate may also be, for example, carbon steel, extra-low carbon steel, or high manganese steel used as building materials, household appliance materials, or automotive materials, or may be stainless steel, etc. In addition, in the case of the carbon steel, extra-low carbon steel, medium-low carbon steel, low carbon steel, and carbon steel all show similar effects. Since there is no need to particularly limit the steel composition, it is hardly affected by the components such as Mn, Si, Ti, Nb, and B added in large amounts in high-strength steel and ultra-high-strength steel. In addition, as an example of the base steel plate, for example, by weight%, it may contain: C: more than 0% and 0.18% or less, Si: more than 0% and 1.5% or less, Mn: 0.01-2.7%, P: more than 0% and 0.07% or less, S: more than 0% and 0.015% or less, Al: more than 0% and 0.5% or less, Nb: 0.06% or less (including 0%), Cr: 1.1% or less (including 0%), Ti: 0.06% or less (including 0%), B: 0.03% or less (including 0%), and the balance of Fe and other inevitable impurities. As an example of the base steel plate, for example, by weight%, it may contain: C: more than 0% and 0.18% or less, Si: more than 0% and 1.5% or less, Mn: 0.01-2.7%, P: more than 0% and 0.07% or less, S: more than 0% and 0.015% or less, Al: more than 0% and 0.5% or less, Nb: more than 0% and 0.06% or less, Cr: more than 0% and 1.1% or less, Ti: more than 0% and 0.06% or less, B: more than 0% and 0.03% or less, and the balance of Fe and other inevitable impurities.

[0034] According to one aspect of the present invention, an Al-Mg-Zn-based coating composed of an Al-Mg-Zn-based alloy may be provided on at least one surface of the base steel plate as the coating. The coating may be formed only on one surface of the base steel plate, or may also be formed on both surfaces of the base steel plate. At this time, the Al-Mg-Zn-based coating refers to a coating that contains Mg and Al and mainly contains Zn (that is, contains 50% or more of Zn).

[0035] According to one aspect of the present invention, the coating may contain, by weight%: Mg: 4.00 - 7.00%, Al: 8.000 - 20.000%, Fe: 0.002 - 0.050%, the balance being Zn and other inevitable impurities. Additionally, although not particularly limited, optionally, the coating may further contain one or more selected from Si of 0.20% or less (including 0%) and Ca of 0.200% or less (including 0%). Specific descriptions of each component are given below.

[0036] Mg: 4.00 - 7.00%

[0037] Mg is an element that plays a role in improving the corrosion resistance of the plated steel. In the present invention, in order to ensure the desired excellent corrosion resistance, the Mg content in the coating is controlled to be 4.00% or more. According to one embodiment of the present invention, the content of Mg may be 4.10% or more.

[0038] In a neutral, weakly acidic, or weakly alkaline corrosion environment such as salt water or rainwater, Mg in the coating of the Al - Mg - Zn - based plated steel sheet will dissolve out, thereby uniformly forming layered double hydroxide (LDH; (Zn,Mg)6Al2(OH) 16 (CO3)·4H2O)), thus improving the corrosion resistance. Therefore, the higher the Mg content, the better. However, in an acidic corrosion environment, the dissolution of Mg is too fast. Therefore, the more Mg is added, the worse the corrosion resistance in the acidic environment. In addition, when the addition amount of Mg is too high, scum in the form of MgO may be generated in the plating bath. Therefore, in the present invention, the Mg content may be 7.00% or less. According to one embodiment of the present invention, it may be 6.90% or less.

[0039] Al: 8.000 - 20.000%

[0040] Al is a component that improves corrosion resistance together with Mg. Since Al has strong acid resistance, as the content of Al increases, the corrosion resistance in acid significantly increases. Therefore, in order to ensure the above effects, in the present invention, the content of Al can be 8.000% or more. According to an embodiment of the present invention, the content of Al can be 8.200% or more. According to an embodiment of the present invention, the content of Al can be 8.500% or more. On the other hand, as the addition amount of Al increases, the corrosion intensifies in an alkaline environment. In addition, Al in the plating bath has the effect of inhibiting the oxidation of Mg, so as the addition amount of Al increases, the generation of MgO-based scum is inhibited. However, when the addition amount of Al is too high, the melting point of the plating bath rises, and a higher plating bath temperature must be maintained, so the erosion of the structures in the plating bath is severe, and thus the content of Al in the coating can be set to 20.000% or less. According to an embodiment of the present invention, the content of Al can be 19.800% or less.

[0041] Fe: 0.002 - 0.050%

[0042] Fe is a component that plays an important role in the present invention. Sometimes Fe is directly added to the plating bath, and Fe can also dissolve out from the steel plate and exist. First, when manufacturing the plating bath, Fe is included in the added ingot and the composition is adjusted. However, since Fe dissolves out from the steel plate during the plating operation, regular analysis is carried out. When the content of Fe is low, the ingot is further supplemented. When the content of Fe exceeds the specified value, the content is adjusted through dilution or removal operations.

[0043] Generally, in the plating bath with Al added, Fe exists in the form of Fe2Al5. Since the specific gravity of the Fe2Al5 is lighter than that of the plating bath, Fe will float on the surface of the plating bath and gather together to grow into large dross. This kind of dross will adhere to the steel plate during the plating process, causing dross adhesion defects. In addition, when the flow rate of the plating bath is large, the coarse dross will flow in the plating bath, resulting in dross trace defects on the steel plate. Therefore, it is necessary to remove it regularly. However, when the size (diameter) of the Fe2Al5 is 0.05 μm or less and it is fine, when the plating bath flows, it will not float to the surface but flow in the plating bath, and a part of it will adhere to the steel plate and exist in the coating. In the present invention, since Fe existing in the coating before solidification plays the role of crystal nucleation sites during the solidification process, at least 0.002% or more needs to be added. On the other hand, when the content of Fe exceeds 0.050%, the amount of dross generated on the surface of the plating bath will increase, which may lead to an increase in dross adhesion defects. According to an embodiment of the present invention, the content of Fe can be 0.045% or less.

[0044] Si: 0.20% or less (including 0%)

[0045] When Si is added in an amount of 0.2% or less, the formation of an excessively thick Fe-Al alloy layer at the interface between the base iron and the coating can be prevented, thereby preventing a decrease in the interfacial strength between the coating and the base iron. Therefore, adding Si is beneficial, but even if it is not added, the impact on corrosion resistance in the present invention is small, so the lower limit is set to 0%.

[0046] However, even when the addition amount of Si exceeds 0.20%, the effect of suppressing the Fe-Al alloy layer at the interface reaches saturation, and as the addition amount of Si increases, the melting point of the plating bath rises, so a relatively high plating bath temperature must be maintained. Therefore, it is disadvantageous from the perspective of protecting equipment. According to an embodiment of the present invention, the addition amount of Si can be 0.18% or less.

[0047] Ca: 0.200% or less (including 0%)

[0048] Ca does not have to be added, but when the addition amount reaches 0.200%, the generation of MgO oxides in the plating bath can be suppressed. In addition, when manufacturing the ingot for the plating bath, for the convenience of operation, a small amount can also be added. Therefore, it may be present in a small amount in the plating bath made from this ingot. However, when the addition amount of Ca exceeds 0.200%, the problem of the darkening of the steel plate color may occur, so it is not preferred. According to an embodiment of the present invention, the content of Ca can be 0.180% or less.

[0049] The balance of Zn and other inevitable impurities

[0050] In addition to the above components, the components dissolved out during the ingot manufacturing process or in the steel plate are composed of the inevitable impurities and zinc components present in the plating bath. The inevitable impurities can include Sb, Sn, Pb, Sr, Cu, etc. as trace components inevitably mixed in when manufacturing the ingot for preparing the plating solution. In addition, as the components inevitably dissolved out when the steel plate is immersed in the plating bath and present in trace amounts in the plating bath, Mn, Ti, Ni, B, Nb, etc. can be listed. However, depending on the composition of the steel plate, other components may also be present. However, even if added inevitably, the addition amount of each component should not be 0.1% or more.

[0051] According to an embodiment of the present invention, the coating may further contain any one or more of the following groups (a) to (h).

[0052] However, the elements in the following groups are not essential elements for solving the technical problems of the present invention, so the lower limit of their content is not restricted. Therefore, even if not specifically mentioned below, the lower limit of the content of each element can be 0%.

[0053] (a) Ni: 0.5% or less

[0054] (b) One or more of La: 0.1% or less, Ce: 0.1% or less, Y: 0.1% or less, Sr: 1.0% or less

[0055] (c) Ti: 0.1% or less

[0056] (d) W: 0.5% or less

[0057] (e) Cu: 2.0% or less

[0058] (f) One or more of Cr: 0.5% or less, Mn: 0.5% or less, V: 0.5% or less

[0059] (g) One or more of B: 0.1% or less, P: 0.1% or less

[0060] (h) One or more of Sn: 1.0% or less, Sb: 1.0% or less, Bi: 1.0% or less

[0061] (a) Ni: 0.5% or less

[0062] Ni has the effect of preventing Fe diffusion by generating an Al-Ni alloy phase. However, when the content of Ni exceeds 0.5%, there may be a problem of excessive increase in the cost of auxiliary materials.

[0063] (b) One or more of La: 0.1% or less, Ce: 0.1% or less, Y: 0.1% or less, Sr: 1.0% or less

[0064] La, Ce, Y, and Sr have the effect of preventing Mg in the plating bath from being oxidized by forming an oxide film. However, when their contents exceed 0.1%, 0.1%, 0.1%, and 1.0% respectively, the viscosity of the plating bath increases, and there may be a problem of reduced plating performance.

[0065] (c) Ti: 0.1% or less

[0066] Ti serves as a nucleation point for Ti-Al intermetallic compounds and has the effect of refining the grain (spangle). However, when the content of Ti exceeds 0.1%, the melting point of the plating bath rises, and there may be a problem of increased scum.

[0067] (d) W: 0.5% or less

[0068] W forms W oxide on the surface and has the effect of improving corrosion resistance. However, when the content of W exceeds 0.5%, there may be a problem of rising melting point of the plating bath.

[0069] (e) Cu: 2.0% or less

[0070] Cu forms an Al-Cu eutectic structure, which has the effect of reducing the hardness of the coating. However, when the content of Cu exceeds 2.0%, there is a problem of coarsening of the spangles.

[0071] (f) One or more of Cr: 0.5% or less, Mn: 0.5% or less, and V: 0.5% or less

[0072] The liquid-phase loss of Cr, Mn, and V is very fast, so it has the effect of preventing electrode deterioration by suppressing the alloying of zinc and the welding electrode. However, when the contents of Cr, Mn, and V exceed 0.5% respectively, there may be a problem of excessive increase in the melting point of the plating bath.

[0073] (g) One or more of B: 0.1% or less and P: 0.1% or less

[0074] B and P have the effect of suppressing LME cracks at the welded part. However, when the contents of B and P exceed 0.1% respectively, there may be a problem of increased dross formation.

[0075] (h) One or more of Sn: 1.0% or less, Sb: 1.0% or less, and Bi: 1.0% or less

[0076] Sn, Sb, and Bi have the effect of improving the durability of the pot by homogenizing the spangles and reducing the plating bath temperature. However, when the contents of Sn, Sb, and Bi exceed 1.0% respectively, there may be a problem of coarsening of the spangles.

[0077] Hereinafter, the alloy phases in the coating according to one aspect of the present invention will be described. The coating according to one aspect of the present invention may include various phases such as MgZn2 phase, Al phase, Al-Zn binary eutectic phase, Zn-MgZn2-Al, and Zn phase. In particular, according to one aspect of the present invention, the Al phase and the MgZn2 phase must be included. In addition, one or more selected from the Al-Zn binary eutectic phase, the Zn-MgZn2-Al ternary eutectic phase, and the Zn phase may be further included.

[0078] At this time, in the present invention, the MgZn2 phase refers to a phase mainly composed of MgZn2. Components other than Mg and Zn may further include other components of 5% or less (including 0%) in atomic%. In addition, the Al phase refers to a phase mainly composed of Al. Specifically, in atomic%, it solid-solves less than 27% (including 0%) of Zn, and the balance is composed of Al and other impurities (the total amount of impurities is 2 atomic% or less (including 0%)). That is, in the Al phase, in addition to the Al component, components such as Zn and Mg that may be solid-solved can be included as coating components. In the present invention, it should be noted that the Al phase only refers to a phase that solid-solves less than 27 atomic% (including 0%) of Zn.

[0079] In addition, the Zn-MgZn2-Al ternary eutectic phase refers to a ternary eutectic phase in which the Zn phase, the MgZn2 phase, and the Al phase coexist in a mixed form, and the Al-Zn binary eutectic phase refers to a phase in which the Al phase and the Zn phase are arranged in an alternating layered or irregular mixed form.

[0080] At this time, it should be noted that the Al phase in the Al-Zn binary eutectic phase and the Zn-MgZn2-Al ternary eutectic phase is not regarded as the above-mentioned Al phase. Similarly, it should be noted that the MgZn2 in the Zn-MgZn2-Al ternary eutectic phase is not regarded as the MgZn2 phase mainly composed of the above-mentioned MgZn2.

[0081] In addition, the fine structure of the above-mentioned coating may show different distributions on the surface and in the cross-section. The fine structure in this surface and cross-section can be confirmed by magnifying the coating of each surface specimen or cross-section specimen using a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0082] Therefore, the Zn-Mg-Al coating includes various phases according to the composition and manufacturing conditions of the coating. As a result of many studies on the phase distribution in the coating by the inventor of the present invention in order to make the Al-Mg-Zn coated steel sheet have excellent corrosion resistance not only in a nearly neutral corrosion environment but also in acidic and alkaline regions, it was confirmed that the MgZn2 phase has strong corrosion resistance in the alkaline region, the Al phase exhibits corrosion resistance in acidic conditions, and it was found that when these two phases are appropriately distributed, excellent corrosion resistance can be ensured not only in a neutral atmosphere but also in all atmospheres including acidic and alkaline, thus completing the present invention.

[0083] Although not particularly limited, according to a specific embodiment of the present invention, in the Zn-Mg-Al coating that satisfies the above coating composition, it includes a fine structure in which the Al phase exists inside the MgZn2 phase or in contact with the MgZn2 phase. The form in which the Al phase exists inside the MgZn2 phase means a form in which any Al phase is completely contained inside the MgZn2 phase. Or, the form in which the Al phase exists in contact with the MgZn2 phase means a form that includes only a part of any Al phase contained inside the MgZn2 phase or a form in which the Al phase exists in contact with the MgZn2 phase.

[0084] In addition, although not particularly limited, according to one aspect of the present invention, based on the cross-section along the thickness direction of the coating (representing the direction perpendicular to the rolling direction), in terms of area%, it may include: MgZn2 phase: 15.0 - 60.0% and Al phase: 3.0 - 25.0%. By controlling the fractions of the two phases to meet this requirement, the corrosion resistance in a composite corrosion environment can be further improved.

[0085] The coating according to one aspect of the present invention includes the above-mentioned MgZn2 phase, Al phase, Al-Zn binary eutectic phase, Zn-MgZn2-Al and Zn phases, etc. When using a coated steel sheet containing such phases in an acidic corrosive atmosphere, the MgZn2 phase corrodes first, while the Al phase corrodes slowly in acid. On the other hand, when using the coated steel sheet in an alkaline corrosive environment, the MgZn2 phase has higher corrosion resistance compared to other phases, but the corrosion rate of the Al phase is faster. Therefore, in order to have excellent corrosion resistance in a composite corrosive environment including both acidic and alkaline, it is necessary to appropriately control the ratio, size, or distribution, etc. of these two phases. Therefore, the characteristics of the ratio, size, or distribution, etc. between these two phases are described in detail below.

[0086] According to one aspect of the present invention, in the coating, the proportion of the number of Al phases existing inside the MgZn2 phase or in contact with the MgZn2 phase to the total number of Al phases can be 85% or more and 100% or less. When the proportion of the number of Al phases existing inside the MgZn2 phase or in contact with the MgZn2 phase to the total number of Al phases is less than 85%, the corrosion resistance in acidic and alkaline environments can be reduced. At this time, in the present invention, it should be noted that, based on the equivalent circle diameter, the above-mentioned Al phase is at least 0.5 μm or more, and the same applies to the following description.

[0087] Although not particularly limited, according to one aspect of the present invention, based on the cross-section along the thickness direction of the coated steel sheet, in terms of area%, even if the proportion of the Al phase satisfies 3.0 - 25.0%, as Figure 2 shown, based on the cross-section along the thickness direction of the coated steel sheet, when the size of the Al phase is coarse, when the Al phase is exposed to an alkaline corrosive environment, corrosion occurs preferentially, and it can serve as a migration channel for corrosive substances from the surface of the coating to the base steel sheet. Therefore, even if the remaining phases including the MgZn2 phase are intact, the base steel sheet may corrode. Additionally, as according to the present invention Figure 1As shown, based on the cross-section along the thickness direction of the plated steel sheet, when the Al phase is fine, even if the Al phase is exposed to an alkaline corrosion environment and corrodes, since other phases such as MgZn2 with good alkaline corrosion resistance coexist with the Al phase, the movement of corrosive substances along the depth direction (thickness direction) of the coating is blocked, thereby improving the corrosion resistance. Although there is no particular limitation, especially when there are a large number of fine Al phases of 5 μm or less that are stable in an acidic environment, the effect of suppressing the penetration of corrosive substances into the steel sheet in acid can be more excellent. Therefore, although there is no particular limitation, according to one aspect of the present invention, the average longitudinal straight length of the Al phase existing inside the MgZn2 phase or in contact with the MgZn2 phase can be 5 μm or less (excluding 0 μm). In addition, the more the above-mentioned fine Al phases, the more the corrosion resistance in the above-mentioned composite corrosion environment can be improved. Therefore, according to one aspect of the present invention, the proportion of the number of Al phases with a longitudinal straight length of 5 μm or less (excluding 0 μm) existing inside the MgZn2 phase or in contact with the MgZn2 phase to the total number of Al phases can be 90.0 - 100.0%, and there is no particular limitation thereto.

[0088] In addition, although there is no particular limitation, according to one aspect of the present invention, in the coating, the number density (D al ) of the Al phase existing inside the MgZn2 phase or in contact with the MgZn2 phase can be 500 - 4000 per 0.1 mm 2 . In the present invention, the above-mentioned number density refers to the number of Al phases existing inside the MgZn2 phase or in contact with the MgZn2 phase per unit area of 0.1 mm 2 based on the cross-section along the thickness direction of the coating. At this time, when the value of the D al is less than 500 per 0.1 mm 2 , the interval between Al phases is too far, and the corrosion factor is easy to spread between Al phases. Therefore, there may be a problem that the effect of blocking the corrosion factor in an acidic atmosphere is reduced. In addition, when the value of the D al exceeds 4000 per 0.1 mm 2 , most of the coating is composed of the Al phase and the MgZn2 phase, resulting in a decrease in the proportion of the Al-Zn binary eutectic phase and / or the Zn-MgZn2-Al ternary eutectic phase, which has excellent corrosion resistance in a relatively neutral environment. Therefore, there may be a problem in ensuring the corrosion resistance in a neutral environment.

[0089] In addition, in the present invention, there is no particular limitation on the measurement method of the area ratio of each phase, the above-mentioned proportion of the number of Al phases existing inside the MgZn2 phase or in contact with the MgZn2 phase, the average longitudinal straight length, the number density, etc. Conventional methods known in the technical field can be used for measurement.

[0090] For example, the area ratio of each phase and the number ratio, average longitudinal straight line length, number density (D al ) etc. of the Al phase existing inside the MgZn2 phase or in contact with the MgZn2 phase as described above can be measured by compositional analysis using energy dispersive X-ray spectroscopy (EDS) with a cross-sectional specimen along the thickness direction of the plated steel sheet as a reference, and then tissue photographs are taken with a field emission scanning electron microscope (FE-SEM; Field Emission Scanning Electron Microscope). After that, in the taken images, each phase can be marked and distinguished, and the area ratio, number ratio, average longitudinal straight line length, number density, etc. of each phase can be measured by an image analyzer (Image Analyzer). At this time, considering the partial deviation of each specimen, 20 positions of each specimen are photographed at a magnification of 2000 times, and then the average value of each specimen is calculated, so that the area ratio, number ratio, average longitudinal straight line length, number density, etc. of the above-mentioned each phase can be measured.

[0091] Next, a method for manufacturing a plated steel sheet according to another aspect of the present invention will be described in detail. However, it does not mean that the plated steel sheet of the present invention must be manufactured by the following manufacturing method.

[0092] According to one aspect of the present invention, it may further include a step of first preparing a base steel sheet, and the type of the base steel sheet is not particularly limited. At this time, the above description can also be applied to the base steel sheet.

[0093] Next, the base steel sheet is immersed in a plating bath for hot dip plating. By weight, the plating bath contains: Mg: 4.00 - 7.00%, Al: 8.000 - 20.000%, Fe: 0.002 - 0.050%, and the balance Zn and other inevitable impurities. At this time, for the reasons for adding components and limiting the contents in the above plating bath, except for the contents of a small amount of impurities that may flow in from the base steel sheet, the description of the components of the above coating can be equally applied. Therefore, according to one aspect of the present invention, the plating bath may selectively further contain one or more selected from Si of 0.20% or less (including 0%) and Ca of 0.200% or less (including 0%) by weight.

[0094] To manufacture the plating bath having the above composition, a composite ingot containing specified Zn, Al, and Mg, or a Zn-Mg ingot or a Zn-Al ingot containing individual components can be used. To replenish the plating bath consumed by hot-dip plating, the ingot is further dissolved and supplied. In this case, a method of directly immersing the ingot in the plating bath and dissolving it can be selected, or a method of dissolving the ingot in a separate tank and then replenishing the molten metal into the plating bath can be selected.

[0095] In addition, when manufacturing the plating bath, 0.002 - 0.050% of Fe is included as an essential component in addition to Zn, Al, and Mg. Fe in the plating bath can also be manufactured by alloying Fe when manufacturing the ingot during the initial plating bath manufacturing process, and Fe can also be added separately when manufacturing the plating bath. However, when starting the plating operation, a part of it can be dissolved out from the steel plate, so it is necessary to analyze and control it regularly so that the content of Fe is in the range of 0.002 - 0.050% defined in the present invention.

[0096] Fe in the plating bath reacts with Al to form a Fe₂Al₅ phase. The Fe₂Al₅ phase has a light specific gravity and will float on the surface of the plating bath, but when the size (diameter) is 0.05 μm or less, it will not float on the surface when the plating bath flows, but will flow in the plating bath. Therefore, according to one aspect of the present invention, the fine Fe₂Al₅ dross with a size of 0.05 μm or less (except 0 μm) flowing in the plating bath is a key factor affecting the number of alloy phase sizes of the final plated product in the present invention, and this will be described in detail. In addition, the coarse dross existing on the surface of the plating bath does not correspond to the Fe in the plating bath defined in the present invention in the range of 0.002 - 0.050%. When analyzing the composition of the plating bath, the Fe content is determined by analyzing a sample collected from the surface to a depth of 30 cm in the thickness direction.

[0097] In addition, although there is no particular limitation, according to one aspect of the present invention, when the fine Fe2Al5 steel sheet with a size of 0.05 μm or less flowing in the plating bath passes through the plating bath, it will adhere to the steel sheet together with the plating bath. After that, it will become a nucleation point during the cooling process of the coating. In the present invention, the first crystallized phase when the coating solidifies is the Al phase. Therefore, before cooling after plating, the fine dross existing in the coating will become the nucleation point for Al phase crystallization during the cooling process. Thus, the more the flowing dross, the more the nucleation of Al, and the finer the Al phase. Therefore, in the present invention, it can exist in the form of fine dross with a size (average diameter) of 0.05 μm or less (excluding 0 μm) flowing in the plating bath. At this time, as one of the factors for refining the size of the flowing dross in the plating bath, the Fe content is controlled. When the content of Fe in the plating bath is less than 0.002%, there is no Fe2Al5 flowing dross or too little Fe2Al5 flowing dross, resulting in less nucleation of the Al phase, which may cause the problem of the Al phase becoming coarser. On the other hand, when the content of Fe in the plating bath exceeds 0.05%, the generation of Fe2Al5 dross increases, and the dross aggregates with each other and coarsens, becoming coarse dross floating on the surface of the plating bath, hindering the plating operation or mixing into the coating, so it may cause dross trace defects on the steel sheet.

[0098] In addition, although there is no particular limitation, according to one aspect of the present invention, the temperature of the plating bath can be maintained at a temperature 20 - 100 °C higher than the solidification start temperature (Ts). At this time, the solidification start temperature may vary due to the plating bath composition, so the plating bath temperature can also be adjusted accordingly. Additionally, the temperature of the plating bath can be maintained in the range of 430 - 520 °C.

[0099] In addition, according to one aspect of the present invention, the flow rate of the plating bath can be controlled at 0.03 - 0.20 m / s. Even if Fe2Al5 dross with a size of 0.05 μm or less is generated in the plating bath according to the present invention, when the plating bath does not flow, it is likely to float on the surface of the plating bath, and local dross deviation in the plating bath may occur. Therefore, the minimum flow rate of the plating bath should be 0.03 m / s or more. On the other hand, when the flow rate is too fast, the flowing dross is too fine and adheres to the coating, resulting in an excessive amount of the Al phase, thus eliminating the effect of the Al phase in improving the corrosion resistance in an acidic environment.

[0100] In addition, according to one aspect of the present invention, in order to achieve the desired plating amount after plating is completed, the hot-dip galvanized steel sheet is subjected to a wiping treatment. For example, nitrogen (N2) or an air knife can be used. At this time, there is no particular limitation on the plating adhesion amount. Based on one side, it is usually at the level of 20 - 400 g / m 2 of.

[0101] In addition, according to one aspect of the present invention, cooling is performed after controlling the plating amount. At this time, the cooling is controlled so that the average cooling rate of the steel sheet satisfies 6.0 °C / second or more in the temperature range from the crystallization temperature of the Al phase to 330 °C. In the temperature range from the crystallization temperature of the Al phase to 330 °C, the faster the cooling rate, the slower the growth of nuclei, and the finer the crystals become. Therefore, the cooling rate is controlled as described above. The upper limit of the cooling rate in the corresponding range does not need to be particularly limited, and considering the limitations of the cooling equipment, vibration, economy, etc., it is controlled at an appropriate level conventional in the technical field. In the present invention, the Al-phase crystallization nucleation point is the fine scum present in the plating layer before the plating layer solidifies. However, when the cooling rate is slow after the Al-phase crystallization, the Al phase may grow and coarsen. Therefore, the control of the number density and size of the above-mentioned Al phase involves multiple process factors. However, as one of these control factors, in the temperature range from the crystallization temperature of the Al phase to 330 °C, only by cooling at an average cooling rate of 6.0 °C / second or more can the number density and size of the Al phase desired by the present invention be ensured.

[0102] In addition, when the temperature of the steel sheet is lower than 330 °C, the solidification of the plating layer is completed. Therefore, the subsequent cooling rate is not very important, but considering productivity, a faster cooling rate is preferred. Detailed implementation mode

[0103] Hereinafter, the present invention will be described in more detail by way of examples. However, it should be noted that the following examples are merely used to illustrate the present invention by way of example and are not used to limit the scope of the rights of the present invention. This is because the scope of the rights of the present invention is determined by the content recorded in the claims and the content reasonably deduced therefrom.

[0104] (Example)

[0105] The composition of the plating bath was prepared and used according to the components described in Table 1 below. The final composition of the plating bath was confirmed using inductively coupled plasma mass spectrometry (ICP). In addition, after collecting a plating bath specimen at a depth of 300 mm (thickness direction) from the surface of the plating bath, it was rapidly cooled in water, and whether there was fine scum in the plating bath was confirmed at a magnification of 300,000 in a transmission electron microscope (TEM).

[0106] Using hot-rolled steel sheets and cold-rolled steel sheets as the base steel sheets, the composition of the base steel sheets is C: 0.017%, Si: 0.014%, Mn: 0.25%, P: 0.008%, S: 0.005%, Al: 0.03%, Nb: 0.02%, Cr: 0.1%, Ti: 0.02%, B: 0.015%, and the balance Fe and other inevitable impurities.

[0107] The hot-rolled steel sheets are first pickled with an aqueous hydrochloric acid solution to remove the iron oxides formed on the surface of the steel sheets during the hot-rolling process, and then heated to 650°C in a heating furnace with a reduction condition of dew point temperature -20°C and plated under the conditions of Manufacturing Process A in Table 2 below. The cold-rolled steel sheets are first subjected to alkali immersion and electrolytic degreasing to remove the rolling oil, iron powder, and other foreign matters adhering to the surface of the steel sheets, then annealed at 840°C in an annealing furnace with a reducing atmosphere, and then plated under the conditions of Manufacturing Process B in Table 2 below. After plating, the plating amount is adjusted to 150 g / m based on one side by N2 wiping. 2 .

[0108] The plated steel sheets are cut into a predetermined size for analysis. Then, for the cross-sectional specimens along the thickness direction of the steel sheets (representing the direction perpendicular to the rolling direction), the coating layer is photographed with an FE-SEM, and in order to confirm the correct phases, the composition is confirmed by EDS.

[0109] The images are photographed at a magnification of 2000 times. For representativeness, 20 positions are measured and analyzed in a specimen with a length of 20 mm. After confirming the phases in each image, the area ratio, quantity, etc. of each phase are measured by an image analyzer, and then the average value of the 20 measurements is shown in Table 3 below.

[0110] In order to evaluate the corrosion resistance, the specimens are cut into a size of 80 mm × 150 mm, and then for each specimen and experimental condition, 3 tests are conducted. After calculating the average value, the evaluation is carried out according to the following criteria, and the results are listed in Table 3 below.

[0111] <Acid corrosion resistance>

[0112] ◎: The weight reduction amount before and after the experiment is 1 g / m 2 or less, and it is evaluated as very excellent

[0113] ○: The weight reduction amount before and after the experiment is more than 1 g / m 2 and less than 2 g / m 2 and it is evaluated as excellent

[0114] △: The weight reduction amount before and after the experiment is more than 2 g / m 2 and less than 3 g / m 2 and it is evaluated as poor

[0115] ×: The weight reduction amount before and after the experiment exceeds 3 g / m 2 is evaluated as very poor

[0116] <Neutral corrosion resistance>

[0117] ◎: When the red rust occurrence time is 4200 hours (Hr) or more, it is evaluated as very excellent

[0118] ○: When the red rust occurrence time exceeds 3500 hours and is less than 4200 hours, it is evaluated as excellent

[0119] △: When the red rust occurrence time exceeds 2000 hours and is less than 3500 hours, it is evaluated as poor

[0120] ×: When the red rust occurrence time is less than 2000 hours, it is evaluated as very poor

[0121] <Alkaline corrosion resistance>

[0122] ◎: When the weight reduction amount before and after the experiment is 2 g / m 2 or less, it is evaluated as very excellent

[0123] ○: When the weight reduction amount before and after the experiment exceeds 2 g / m 2 and is less than 4 g / m 2 it is evaluated as excellent

[0124] △: When the weight reduction amount before and after the experiment exceeds 4 g / m 2 and is less than 7 g / m 2 it is evaluated as poor

[0125] ×: When the weight reduction amount before and after the experiment exceeds 7 g / m 2 it is evaluated as very poor

[0126] In addition, for the corrosion resistance test in an acidic environment, a sulfuric acid aqueous solution with a pH of 3.5 is used. For the corrosion resistance test in a neutral environment, a 3.5% NaCl aqueous solution with a pH of 6.7 is used. For the corrosion resistance test in an alkaline environment, ammonia water with a pH of 12.5 is used.

[0127] In the acidic and alkaline experiments, the test piece is immersed in the solution for 48 hours, then taken out, and the corrosion reduction amount before and after the experiment is measured. When the corrosion reduction amount is small, it is evaluated as very excellent. The more the corrosion reduction amount, the more it is evaluated as very poor.

[0128] In the neutral test, the brine is sprayed on the steel plate until red rust occurs on the steel plate (corrosion of the base steel plate). The longer the time, the more excellent the corrosion resistance is evaluated.

[0129] [Table 1]

[0130]

[0131] (In Table 1, "-" represents 0% by weight.)

[0132] [Table 2]

[0133]

[0134] [Table 3]

[0135]

[0136] [Table 4]

[0137]

[0138] From the experimental results of the said tables, in the cases of B1 to B4, B8, B9, B11, B13, B19, B20 and B22 which are examples satisfying the plating composition and manufacturing conditions of the present invention, the corrosion resistance is excellent or very excellent under all conditions of acidic, neutral and alkaline environments. In addition, the content of Fe in the plating bath does not exceed 0.05% which is the upper limit defined in the present invention, so scum attachment defects do not occur.

[0139] In addition, in the cases of B5 and B21 which are comparative examples, although the plating bath composition or other manufacturing conditions conform to the present invention, due to the absence of plating bath flow, the Al crystal nucleation points are small, so the number density of the Al phase is less than 500 per 0.1 mm which is the lower limit defined in the present invention 2 . Therefore, since there are relatively more coarse Al phases, the corrosion resistance is very excellent in acidic, excellent in neutral, but poor in alkaline.

[0140] Comparative example B6 is the case where the cooling rate from the crystallization temperature of the Al phase after plating to 330 °C is slower than 6.0 °C / second defined in the present invention. According to the present invention, there are more Al nucleation points, but it takes time for the Zn-Al binary eutectic phase and ternary eutectic phase other than the Al phase and MgZn2 phase to crystallize, so the number density of the Al phase is less than 500 per 0.1 mm which is the lower limit defined in the present invention 2 , and the corrosion resistance is poor in alkaline environment.

[0141] Comparative examples B7, B10, B23 are the cases where the plating bath flow rate is lower than the range defined in the present invention, and the cooling rate from the Al phase crystallization temperature to 330 °C is slower than 6 °C / second defined in the present invention. The number density of the Al phase is less than 500 per 0.1 mm which is the lower limit defined in the present invention 2 , and the corrosion resistance is very poor in alkaline environment.

[0142] In addition, in the case of Comparative Example B12, the plating bath flow rate was 0.45 m / s, which exceeded the range defined in the present invention. When the flow rate was too fast, the floating slag in the flow was too fine and adhered to the coating, and the number density of the Al phase exceeded 4000 / 0.1 mm, which is the upper limit defined in the present invention. 2 As a result, the effect of improving corrosion resistance in an acidic environment brought about by the Al phase disappeared, and thus the corrosion resistance in an acidic environment was poor.

[0143] In the case of Comparative Example B14, the content of Fe in the plating bath exceeded the range defined in the present invention, and coarse floating slag was generated on the surface of the plating bath, resulting in floating slag adhesion defects on the plated steel sheet.

[0144] In Comparative Example B15 and Comparative Example B24, the content of Al in the plating bath composition was less than the range defined in the present invention, the number density of the Al phase was not within the range defined in the present invention, and the corrosion resistance in acid was very poor.

[0145] Comparative Example B16 was a case where there was no Fe in the plating bath. The number of Al phases was less than the range defined in the present invention and was coarse, and the number of Al phases below 5 μm was less than 80.0% of the limit in the present invention. Therefore, the corrosion resistance in an alkaline environment was poor.

[0146] Comparative Example B17 was a case where the content of Mg in the plating bath was below the range defined in the present invention and Fe was not added. The number density of the Al phase was also low, the corrosion resistance in an alkaline atmosphere was very poor, and the corrosion resistance in a neutral atmosphere was also poor.

[0147] In addition, in Comparative Example B18, Mg was not added to the plating bath and Al was also below the range defined in the present invention. The MgZn2 phase and the Al phase in the coating did not crystallize. Therefore, the corrosion resistance in acidic, neutral, and alkaline atmospheres was poor or very poor.

[0148] Comparative Example B25 was a case where the content of Mg in the plating bath was below the range defined in the present invention, and in the plating bath without Fe addition, the cooling rate of the crystallization temperature of the Al phase after plating to 330 °C was slower than 6.0 °C / s defined in the present invention. The number density of the Al phase in the coating was lower than the range defined in the present invention, and the proportion of the number of Al phases below 5 μm was also low. The corrosion resistance in an alkaline atmosphere was very poor, and the corrosion resistance in a neutral atmosphere was also poor.

[0149] Comparative Example B26 was a case where the content of Mg in the plating bath was below the range defined in the present invention and the plating bath did not flow. The proportion of the Al phase existing inside the MgZn2 phase or in contact with the MgZn2 phase deviated from the lower limit of the present invention. Therefore, the corrosion resistance in an alkaline atmosphere was very poor, and it was also poor in acid.

[0150] Comparative Example B27 is a case where the content of Al in the plating bath is higher than the range defined in the present invention and Fe is not added. The number density of the Al phase is not within the range defined in the present invention, and the corrosion resistance in an alkaline atmosphere is very poor.

Claims

1. A coated steel sheet, comprising: A base steel sheet; And An Al-Mg-Zn based coating provided on at least one surface of the base steel sheet, Among them, the number density of the Al phase existing inside the MgZn2 phase or in contact with the MgZn2 phase in the coating is 500 - 4000 per 0.1 mm 2 .

2. The plated steel sheet according to claim 1, wherein, By weight %, the coating contains: Mg: 4.00 - 7.00%, Al: 8.000 - 20.000%, Fe: 0.002 - 0.050%, the balance being Zn and other inevitable impurities.

3. The plated steel sheet according to claim 2, wherein By weight %, the coating further contains one or more selected from Si of 0.2% or less including 0% and Ca of 0.2% or less including 0%.

4. The coated steel sheet according to claim 2, wherein, The coating further contains any one or more of the following groups (a) to (h): (a) Ni: 0.5% or less; (b) One or more of La: 0.1% or less, Ce: 0.1% or less, Y: 0.1% or less, Sr: 1.0% or less; (c) Ti: 0.1% or less; (d) W: 0.5% or less; (e) Cu: 2.0% or less; (f) One or more of Cr: 0.5% or less, Mn: 0.5% or less, V: 0.5% or less; (g) One or more of B: 0.1% or less, P: 0.1% or less; (h) One or more of Sn: 1.0% or less, Sb: 1.0% or less, Bi: 1.0% or less.

5. The plated steel sheet according to claim 1, wherein, In the coating, the proportion of the number of Al phases existing inside the MgZn2 phase or in contact with the MgZn2 phase to the total number of Al phases is 85% or more and 100% or less.

6. The plated steel sheet according to claim 1, wherein, The average longitudinal straight-line length of the Al phases existing inside the MgZn2 phase or in contact with the MgZn2 phase is 5 μm or less and excluding 0 μm.

7. The plated steel sheet according to claim 1, wherein, The proportion of the number of Al phases with a longitudinal straight-line length of 5 μm or less and excluding 0 μm existing inside the MgZn2 phase or in contact with the MgZn2 phase to the total number of Al phases is 90.0 - 100.0%.

8. A method for manufacturing a coated steel sheet, comprising the following steps: Immersing the base steel sheet in a plating bath for hot-dip plating. By weight %, the plating bath contains: Mg: 4.00 - 7.00%, Al: 8.000 - 20.000%, Fe: 0.002 - 0.050%, the balance being Zn and other inevitable impurities, the temperature is 430 - 520 °C, and the flow rate is 0.03 - 0.20 m / s; Wiping the steel sheet after the hot-dip plating; And Cooling the steel sheet after the wiping treatment so that the average cooling rate in the temperature range from the crystallization temperature of the Al phase to 330 °C satisfies 6.0 °C / s or more.

9. The manufacturing method of the plated steel sheet according to claim 8, wherein, By weight %, the plating bath further contains one or more selected from Si of 0.20% or less including 0% and Ca of 0.200% or less including 0%.

10. The manufacturing method of the plated steel sheet according to claim 8, wherein, The plating bath further contains any one or more of the following groups (a) to (h): (a) Ni: 0.5% or less; (b) One or more of La: 0.1% or less, Ce: 0.1% or less, Y: 0.1% or less, Sr: 1.0% or less; (c) Ti: 0.1% or less; (d) W: 0.5% or less; (e) Cu: 2.0% or less; (f) One or more of Cr: 0.5% or less, Mn: 0.5% or less, and V: 0.5% or less; (g) One or more of B: 0.1% or less and P: 0.1% or less; (h) One or more of Sn: 1.0% or less, Sb: 1.0% or less, and Bi: 1.0% or less.

11. The manufacturing method of the plated steel sheet according to claim 8, wherein, The temperature of the plating bath is maintained at a temperature 20 - 100 °C higher than the starting solidification temperature (Ts).

12. The manufacturing method of the coated steel sheet according to claim 8, wherein, The average diameter of the floating scum flowing in the plating bath is controlled to be 0.05 μm or less, excluding 0 μm.

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