Plated steel sheet with excellent corrosion resistance and manufacturing method thereof

Abstract

The present invention provides a plated steel sheet that has excellent corrosion resistance even in a complex corrosive environment, and a method for producing the same.

Description

[Technical Field]

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

[0002] When exposed to a corrosive environment, zinc-based coated steel sheets exhibit sacrificial corrosion protection properties, whereby zinc, which has a lower redox potential than iron, corrodes first, inhibiting corrosion of the steel. Furthermore, as the zinc in the coating layer oxidizes, it forms dense corrosion products on the steel surface, insulating the steel from the oxidizing atmosphere and improving the corrosion resistance of the steel. Thanks to these advantageous properties, zinc-based coated steel sheets have recently been expanding their range of applications to building materials, home appliances, and automotive steel sheets.

[0003] However, the corrosive environment is gradually worsening due to increased air pollution caused by industrial advancement. In addition, as industries diversify, steel sheets are being exposed to more complex corrosive environments. This has created a need for the development of steel materials that have better corrosion resistance in a variety of corrosive environments than conventional zinc-based coated steel sheets.

[0004] For example, steel sheets used inside buildings are often placed inside livestock barns or in contact with cement, whereas steel sheets used outside buildings are often placed in an acidic, corrosive atmosphere due to acid rain. Thus, in a complex corrosive environment, plated steel sheets with excellent corrosion resistance in both acidic and alkaline environments are required. However, conventional technologies related to zinc-based plated steel sheets have not yet been developed to a level that provides sufficient corrosion resistance in both acidic and alkaline environments. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Korean Publication No. 2013-0133358 Summary of the Invention [Problem to be solved by the invention]

[0006] According to one aspect of the present invention, there is provided a plated steel sheet having excellent corrosion resistance even in a complex corrosive environment, and a method for producing the same.

[0007] According to yet another aspect of the present invention, there is provided a plated steel sheet having excellent corrosion resistance even in acidic and alkaline environments, and a method for producing the same.

[0008] According to yet another aspect of the present invention, there is provided 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 producing the same.

[0009] The object of the present invention is not limited to the above content. Anyone having ordinary skill in the art to which the present invention pertains will have no difficulty in understanding further object of the invention from the entire content of the specification of the present invention. [Means for solving the problem]

[0010] One aspect of the present invention is Base steel sheet; and an Al-Mg-Zn-based coating layer provided on at least one surface of the base steel sheet, The plating layer has an Al phase density of 500 to 4,000 pieces / 0.1 mm, which exists inside the MgZn2 phase or in contact with the MgZn2 phase. 2 The present invention provides a plated steel sheet having the following properties:

[0011] Furthermore, still another aspect of the present invention is a step of immersing the base steel sheet in a coating bath containing, by weight %, 4.00 to 7.00% Mg, 8.000 to 20.000% Al, 0.002 to 0.050% Fe, the balance being Zn and other unavoidable impurities, at a temperature of 430 to 520°C and a flow rate of 0.03 to 0.20 m / s to hot-dip coat the base steel sheet; Wiping the hot-dip galvanized steel sheet; and The present invention provides a method for producing a plated steel sheet, the method comprising: a step of cooling the wiped steel sheet at an average cooling rate of 6.0°C / s or more in the range from the crystallization temperature of the Al phase to 330°C. [Effects of the Invention]

[0012] According to one aspect of the present invention, it is possible to provide a plated steel sheet having excellent corrosion resistance even in a complex corrosive environment, and a method for producing the same.

[0013] According to yet another aspect of the present invention, it is possible to provide a plated steel sheet having excellent corrosion resistance even in acidic and alkaline environments, and a method for producing the same.

[0014] According to yet another aspect of the present invention, it is possible to provide a plated steel sheet that has excellent corrosion resistance not only in a neutral environment but also in an acidic environment and an alkaline environment, and a method for producing the same.

[0015] The various yet significant advantages and effects of the present invention are not limited to the above, but can be more easily understood in the course of describing specific embodiments of the present invention. [Brief explanation of the drawings]

[0016] [Figure 1] 1 shows a photograph of a cross-sectional test piece of the plated steel sheet obtained from Example B4 of the present invention, observed using a scanning electron microscope (SEM). [Figure 2] 1 shows a photograph of a cross-sectional test piece of the plated steel sheet obtained in Comparative Example B14 of the present invention, observed using a scanning electron microscope (SEM). DETAILED DESCRIPTION OF THE INVENTION

[0017] The terminology used herein is for the purpose of describing particular embodiments and is not intended to limit the present invention. Also, as used herein, the singular forms "a," "an," and "the" include the plural forms unless the related definition clearly dictates otherwise.

[0018] The meaning of "comprises" as used in the specification is to specify features and does not exclude the presence or addition of other features.

[0019] Unless otherwise specified, all terms, including technical and scientific terms, used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Dictionary-defined terms are to be interpreted to have a meaning consistent with the relevant technical literature and the presently disclosed content.

[0020] Hereinafter, a plated steel sheet according to one aspect of the present invention will be described in detail. In the present invention, the content of each element is expressed in percent by weight unless otherwise specified.

[0021] Recently, with the diversification of industries, steel sheets are being placed in more complex corrosive environments, and there is an increasing need for plated steel sheets that have excellent corrosion resistance in both acidic and alkaline atmospheres, which are complex corrosive environments.

[0022] However, in the conventional technology relating to zinc-based plated steel sheets, no technology has been developed that provides a level of sufficient corrosion resistance in both acidic and alkaline atmospheres.

[0023] 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) steel sheets. However, although aluminum-based plated steel sheets have excellent corrosion resistance in acidic atmospheres, they cannot be used in combined corrosion environments because aluminum corrodes rapidly in alkaline atmospheres.

[0024] To address these issues, various studies have been conducted on manufacturing technologies for zinc alloy-coated steel sheets, which improve the corrosion resistance of steel sheets by adding elements such as aluminum (Al) and magnesium (Mg) to the zinc plating bath. A typical example is Al-Mg-Zn zinc alloy-coated steel sheets, which further add Mg to the Al-Zn plating composition. Adding appropriate amounts of Mg and Al to zinc can ensure excellent corrosion resistance even in acidic and alkaline environments. In Al-Mg-Zn zinc alloy-coated steel sheets, increasing the amount of Mg in the coating layer leads to rapid dissolution of Mg in acidic environments, making the steel susceptible to corrosion. However, in neutral (including saltwater) and alkaline environments, stable Mg-based hydroxides are formed on the substrate surface, suppressing corrosion.

[0025] On the other hand, increasing the amount of Al added to the plating layer increases corrosion resistance because aluminum oxide on the surface becomes stable in acidic and neutral environments. However, conversely, in alkaline atmospheres, Al dissolution deepens, reducing corrosion resistance. Therefore, it is necessary to appropriately adjust the Mg and Al components in the plating layer, and to adjust the distribution of the Mg and Al components in the plating layer uniformly.

[0026] First, a plated steel sheet according to one aspect of the present invention includes: a base steel sheet; and a plating layer provided on at least one surface of the base steel sheet.

[0027] In the present invention, the type of base steel sheet is not particularly limited. For example, the base steel sheet may be an Fe-based base steel sheet used as a base steel sheet for conventional zinc-based coated steel sheets, i.e., a hot-rolled steel sheet or a cold-rolled steel sheet, but is not limited thereto. Alternatively, the base steel sheet may be, for example, a carbon steel, an ultra-low carbon steel, or a high manganese steel used as a material for construction, home appliances, or automobiles, or may be stainless steel. Meanwhile, in the case of carbon steel, ultra-low carbon steel, medium-low carbon steel, low carbon steel, and carbon steel all exhibit similar effects, so there is no need to particularly limit the steel composition, and the steel is hardly affected by elements such as Mn, Si, Ti, Nb, and B, which are added in large amounts to high-strength steel and ultra-high-strength steel. On the other hand, an example of the above-mentioned base steel sheet is one containing, in weight percent, C: more than 0% and not more than 0.18%, Si: more than 0% and not more than 1.5%, Mn: 0.01 to 2.7%, P: more than 0% and not more than 0.07%, S: more than 0% and not more than 0.015%, Al: more than 0% and not more than 0.5%, 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%), the balance being Fe and other unavoidable impurities. An example of the above-mentioned base steel sheet includes, in weight percent, C: more than 0% and not more than 0.18%, Si: more than 0% and not more than 1.5%, Mn: 0.01 to 2.7%, P: more than 0% and not more than 0.07%, S: more than 0% and not more than 0.015%, Al: more than 0% and not more than 0.5%, Nb: more than 0% and not more than 0.06%, Cr: more than 0% and not more than 1.1%, Ti: more than 0% and not more than 0.06%, B: more than 0% and not more than 0.03%, and the balance being Fe and other unavoidable impurities.

[0028] According to one aspect of the present invention, at least one surface of the base steel sheet may be provided with an Al-Mg-Zn-based coating layer made of an Al-Mg-Zn-based alloy. The coating layer may be formed on only one surface of the base steel sheet, or on both surfaces of the base steel sheet. In this case, the Al-Mg-Zn-based coating layer refers to a coating layer that contains Mg and Al and primarily contains Zn (i.e., contains 50% or more Zn).

[0029] According to one aspect of the present invention, the plating layer may contain, by weight percent, 4.00-7.00% Mg, 8.000-20.000% Al, 0.002-0.050% Fe, with the remainder being Zn and other unavoidable impurities. While not particularly limited, the plating layer may optionally further contain, by weight percent, one or more elements selected from 0.20% or less (including 0%) of Si and 0.200% or less (including 0%) of Ca. Each component will be described in detail below.

[0030] Mg: 4.00-7.00% Mg is an element that plays a role in improving the corrosion resistance of a plated steel sheet, and in the present invention, in order to ensure the desired excellent corrosion resistance, the Mg content in the coating layer is controlled to 4.00% or more. In one embodiment of the present invention, the Mg content can be 4.10% or more.

[0031] In a neutral, weakly acidic or weakly alkaline corrosive environment such as salt water or rainwater, the above Al-Mg-Zn coated steel sheet 、 The magnesium in the plating layer is dissolved, and this results in LDH (Layered Double Hydroxide; (Zn, Mg)6Al2(OH) 16 A higher Mg content is preferable because it facilitates the uniform formation of (CO3)·4H2O), which improves corrosion resistance. However, in an acidic corrosive environment, Mg dissolution is too rapid, and the more Mg added, the worse the corrosion resistance in the acidic environment. Furthermore, if too much Mg is added, dross in the form of MgO may be generated in the coating bath. Therefore, in the present invention, the Mg content can be 7.00% or less. In one embodiment of the present invention, it can be 6.90% or less.

[0032] Al: 8,000 to 20,000% Al, together with Mg, improves corrosion resistance. Because Al is resistant to acids, increasing the Al content significantly increases corrosion resistance in acidic environments. Therefore, to ensure the above-mentioned effects, the Al content can be set to 8.000% or more in the present invention. According to one embodiment of the present invention, the Al content can be set to 8.200% or more. According to one embodiment of the present invention, the Al content can be set to 8.500% or more. Meanwhile, the greater the Al content, the greater the corrosion in alkaline environments. Furthermore, Al has the effect of inhibiting the oxidation of Mg in the coating bath, and the greater the Al content, the more effectively MgO-based dross formation is suppressed. However, if the Al content is too high, the melting point of the coating bath increases, requiring a higher bath temperature, which can lead to increased corrosion of structures in the coating bath. Therefore, the Al content in the coating layer can be set to 20.000% or less. According to one embodiment of the present invention, the Al content can be set to 19.800% or less.

[0033] Fe: 0.002 to 0.050% Fe is an important component in the present invention, and while it may be added directly to the coating bath, it may also be present as a leached out product from the steel sheet. When the coating bath is first prepared, Fe is included in the ingots to be added to adjust the composition. However, as the coating process progresses, Fe is leached out from the steel sheet, so the bath is periodically analyzed. If the Fe content is low, additional ingots are added, and if the Fe content is excessive, the Fe content is adjusted by diluting or removing the ingots.

[0034] Typically, in a coating bath containing Al, Fe exists in the form of Fe2Al5. Because Fe2Al5 has a lighter specific gravity than the coating bath, it floats to the surface of the coating bath, agglomerates, and grows into large dross. This dross adheres to the steel sheet during the coating process, causing dross adhesion defects. Furthermore, when the coating bath flows rapidly, coarse dross flows within the coating bath, causing dross imprint defects on the steel sheet, and therefore must be periodically removed. However, when the Fe2Al5 is fine (diameter: 0.05 μm or less), it does not float to the surface when the coating bath is fluidized, but rather flows within the coating bath, and some of it adheres to the steel sheet and remains in the coating layer. In the present invention, Fe present in the coating layer before solidification serves as a crystal nucleation site during the solidification process, so a minimum addition of 0.002% or more is required. However, if the Fe content exceeds 0.050%, the amount of dross generated on the surface of the coating bath increases, potentially increasing dross adhesion defects. According to one embodiment of the present invention, it may be 0.045% or less.

[0035] Si: 0.20% or less (including 0%) Adding 0.2% or less of Si prevents the Fe-Al alloy layer at the interface between the base steel and the coating layer from becoming thicker, thereby preventing a decrease in the strength of the interface between the coating layer and the base steel. Therefore, adding Si is advantageous, but since not adding Si has almost no effect on the corrosion resistance of the present invention, the lower limit is set to 0%.

[0036] However, even if the Si content exceeds 0.20%, the effect of suppressing the formation of an Fe-Al alloy layer at the interface reaches saturation, and as the Si content increases, the melting point of the coating bath rises, requiring the coating bath temperature to be maintained at a high level, which is undesirable from the viewpoint of equipment protection. According to one embodiment of the present invention, the Si content may be 0.18% or less.

[0037] Ca: 0.200% or less (including 0%) Although Ca is not required, adding up to 0.200% can suppress the formation of MgO oxide in the coating bath. Furthermore, a small amount of Ca may be added for operational convenience when producing an ingot for coating bath production, and a small amount may be present in the coating bath produced from that ingot. However, adding more than 0.200% Ca is undesirable because it can cause the hue of the steel sheet to darken. According to one embodiment of the present invention, the Ca content may be 0.180% or less.

[0038] The balance is Zn and other unavoidable impurities In addition to the above-mentioned components, components eluted from the ingot production process or from the steel sheet comprise inevitable impurities present in the plating bath and zinc components. Examples of the inevitable impurities include Sb, Sn, Pb, Sr, and Cu, which are inevitably mixed in in small amounts during the production of ingots for the production of plating solutions. Meanwhile, components that are inevitably eluted while the steel sheet is immersed in the plating bath and present in small amounts in the plating bath include Mn, Ti, Ni, B, and Nb, but other components may also be present depending on the composition of the steel sheet. However, even if these components are inevitably added, it is not preferable for the amount of each component to be more than 0.1%.

[0039] According to an embodiment of the present invention, the plating layer may further include at least one selected from the group consisting of the following (a) to (h):

[0040] However, since the elements in each of the following groups are not essential elements for achieving the object of the present invention, there is no lower limit for their content. Therefore, even if not specifically mentioned below, the lower limit for the content of each element may be 0%. (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, and 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 the following: Cr: 0.5% or less, Mn: 0.5% or less, V: 0.5% or less (g)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, and Bi: 1.0% or less

[0041] (a)Ni: 0.5% or less Ni has the effect of preventing Fe diffusion by forming an Al-Ni alloy phase, but if the content exceeds 0.5%, there may be a problem in that the cost of the auxiliary material increases excessively.

[0042] (b) One or more of La: 0.1% or less, Ce: 0.1% or less, Y: 0.1% or less, and Sr: 1.0% or less La, Ce, Y, and Sr are effective in preventing oxidation of Mg in the plating bath by forming an oxide film, but if their contents exceed 0.1%, 0.1%, 0.1%, and 1.0%, respectively, there may be a problem of reduced plating properties due to an increase in the viscosity of the plating bath.

[0043] (c)Ti: 0.1% or less Ti acts as a nucleation site for Ti-Al intermetallic compounds, resulting in the refinement of crystal grains (spangles). However, if the content exceeds 0.1%, the melting point of the plating bath increases, which can lead to problems with increased dross.

[0044] (d)W: 0.5% or less W forms W oxide on the surface and has the effect of improving corrosion resistance, but if its content exceeds 0.5%, there may be a problem in that the melting point of the plating bath increases.

[0045] (e)Cu: 2.0% or less Cu has the effect of forming an Al-Cu process structure and reducing the hardness of the coating layer, but if its content exceeds 2.0%, there may be a problem of spangles becoming coarse.

[0046] (f) One or more of the following: Cr: 0.5% or less, Mn: 0.5% or less, V: 0.5% or less Cr, Mn, and V have the effect of preventing electrode deterioration by quickly dissipating the liquid phase and suppressing alloying between zinc and the welding electrode. However, if their contents exceed 0.5% each, there may be a problem in that the melting point of the plating bath rises excessively.

[0047] (g) One or more of the following: B: 0.1% or less, P: 0.1% or less B and P have the effect of suppressing LME cracks in welds, but if their contents exceed 0.1% each, there may be a problem of increased occurrence of dross.

[0048] (h) One or more of Sn: 1.0% or less, Sb: 1.0% or less, and Bi: 1.0% or less Sn, Sb and Bi have the effect of making spangles uniform and improving pot durability by lowering the plating bath temperature, but if their contents exceed 1.0% each, there may be a problem of coarsening of spangles.

[0049] The alloy phase in the coating layer according to one aspect of the present invention will be described below. The coating layer according to one aspect of the present invention may contain various phases such as an MgZn2 phase, an Al phase, an Al-Zn binary phase, a Zn-MgZn2-Al, and a Zn phase. In particular, according to one aspect of the present invention, the coating layer essentially contains an Al phase and an MgZn2 phase, and may further contain one or more phases selected from an Al-Zn binary phase, a Zn-MgZn2-Al ternary phase, and a Zn phase.

[0050] In this regard, in the present invention, the MgZn2 phase refers to a phase primarily composed of MgZn2, and may contain other components other than Mg and Zn at an atomic percentage of 5% or less (including 0%). Furthermore, the Al phase refers to a phase primarily composed of Al, specifically a phase in which Zn is dissolved at less than 27% (including 0%), with the remainder being Al and other impurities (the total of the impurities being 2% or less (including 0%)). In other words, the Al phase can also dissolve components other than Al, such as Zn and Mg, which can be contained as plating layer components. It should be noted that in the present invention, the Al phase refers specifically to only a phase in which Zn is dissolved at less than 27% (including 0%).

[0051] The Zn-MgZn2-Al ternary process phase refers to a ternary process phase in which Zn phase, MgZn2 phase and Al phase are all mixed, and the Al-Zn binary process phase refers to an Al phase and a Zn phase arranged alternately in a lamellar or irregular mixed form.

[0052] At this time, it is important to note that the Al phase in the Al-Zn binary phase and the Zn-MgZn2-Al ternary phase is not considered to be the Al phase.Similarly, it is important to note that the MgZn2 in the Zn-MgZn2-Al ternary phase is not considered to be the MgZn2 phase mainly composed of MgZn2.

[0053] Meanwhile, the microstructure of the above-mentioned plating layer may have different distributions on the surface and in the cross section. Such microstructures on the surface and in the cross section can be confirmed by using a scanning electron microscope (SEM) or a transmission electron microscope (TEM) at an increased magnification of the plating layer for each surface test piece or cross section test piece.

[0054] As described above, a Zn-Mg-Al-based coating layer contains various phases depending on the coating layer composition and manufacturing conditions. However, the present inventors conducted extensive research into the distribution of phases within the coating layer in order to provide Al-Mg-Zn-based coated steel sheets with excellent corrosion resistance not only in near-neutral corrosive environments but also in acidic and alkaline regions. As a result, they confirmed that the MgZn2 phase has strong corrosion resistance in alkaline regions, while the Al phase exhibits corrosion resistance in acidic regions. They also found that an appropriate distribution of these two phases can ensure excellent corrosion resistance in all atmospheres, including not only neutral but also acidic and alkaline. This finding led to the completion of the present invention.

[0055] Although not particularly limited, according to one embodiment of the present invention, a Zn-Mg-Al-based coating layer having the above-described coating layer composition includes a microstructure in which an Al phase exists within an MgZn2 phase or in contact with an MgZn2 phase. The term "the Al phase exists within an MgZn2 phase" refers to a structure in which one of the Al phases is completely contained within the MgZn2 phase. Alternatively, the term "the Al phase exists in contact with an MgZn2 phase" includes both a structure in which only a portion of one of the Al phases is contained within the MgZn2 phase and a structure in which the Al phase exists in contact with the MgZn2 phase.

[0056] Meanwhile, although not particularly limited, according to one aspect of the present invention, the coating layer can contain, in area % based on a cross section in the thickness direction (meaning a direction perpendicular to the rolling direction), 15.0 to 60.0% of the MgZn2 phase and 3.0 to 25.0% of the Al phase. By controlling the fractions of the two phases to satisfy this, it is possible to further improve corrosion resistance in a combined corrosion environment.

[0057] A coating layer according to one aspect of the present invention includes the MgZn2 phase, Al phase, Al-Zn binary phase, Zn-MgZn2-Al, and Zn phases described above. When a coated steel sheet containing these phases is used in an acidic corrosive atmosphere, the MgZn2 phase corrodes first, while the Al phase corrodes more slowly in the acid. On the other hand, when the coated steel sheet is used in an alkaline corrosive environment, the MgZn2 phase is relatively resistant to corrosion compared to the other phases, but the Al phase corrodes more rapidly. Therefore, to achieve excellent corrosion resistance in both acidic and alkaline combined corrosive environments, it is necessary to appropriately control the ratio, size, and distribution of these two phases. Therefore, the following describes in detail the characteristics of these two phases, such as the ratio, size, and distribution.

[0058] According to one aspect of the present invention, the proportion of Al phases present inside or in contact with the MgZn2 phases to the total number of Al phases in the coating layer may be 85% or more and 100% or less. If the proportion of Al phases present inside or in contact with the MgZn2 phases to the total number of Al phases is less than 85%, corrosion resistance in acidic and alkaline environments may be reduced. In this regard, it should be noted that in the present invention, the Al phases described above are intended to have a minimum diameter of 0.5 μm or more based on the equivalent circle diameter, and this also applies to the following description.

[0059] Although not particularly limited, according to one aspect of the present invention, even if the proportion of Al phases in terms of area % in a cross section of the thickness direction of the plated steel sheet is 3.0 to 25.0%, if the size of the Al phases in a cross section of the thickness direction of the plated steel sheet is coarse, as shown in FIG. 2, when the Al phases are exposed in an alkaline corrosion environment, corrosion occurs preferentially and they may act as a path for the migration of corrosive substances from the surface of the coating layer to the base steel sheet. This can lead to corrosion of the base steel sheet even if the remaining phases, including the MgZn2 phase, remain intact. On the other hand, as shown in FIG. 1 according to the present invention, when the Al phases are fine in a cross section of the thickness direction of the plated steel sheet, even if the Al phases are exposed in an alkaline corrosion environment and corrode, other phases, such as the MgZn2 phase, which have good alkaline corrosion resistance, exist with the Al phase, thereby blocking the migration of corrosive substances in the depth direction (thickness direction) of the coating layer, thereby improving corrosion resistance. Although not particularly limited, the presence of a large number of fine Al phases of 5 μm or less, which are stable in acidic environments, can more effectively inhibit the penetration of corrosive substances into the steel sheet in acidic environments. Therefore, although not particularly limited, according to one aspect of the present invention, the average linear length in the major axis direction of the Al phase present inside or in contact with the MgZn2 phase may be 5 μm or less (excluding 0 μm). Furthermore, since the greater the number of the above-mentioned fine Al phases present, the more the corrosion resistance in the above-mentioned complex corrosion environment can be improved, according to one aspect of the present invention, the proportion of the number of Al phases present inside or in contact with the MgZn2 phase and having a linear length in the major axis direction of 5 μm or less (excluding 0 μm) relative to the total number of Al phases may be 90.0 to 100.0%, but this is not particularly limited.

[0060] Furthermore, although not particularly limited, according to one aspect of the present invention, the number density (D al ) is 500 to 4,000 pieces / 0.1 mm 2 In the present invention, the above-mentioned number density is 0.1 mm based on the cross section in the thickness direction of the plating layer. 2This means the number of Al phases that exist inside the MgZn2 phase or that exist in contact with the MgZn2 phase per unit area. al The value is 500 pieces / 0.1mm 2 If the thickness is less than 1 / 2 mm, the distance between the Al phases is too great, and corrosion factors can easily propagate between the Al phases, which may reduce the effect of blocking corrosion factors in an acidic atmosphere. al The value is 4,000 pieces / 0.1mm 2 If the temperature exceeds this range, most of the coating layer will be composed of Al and MgZn2 phases, and the proportion of Al-Zn binary phase and / or Zn-MgZn2-Al ternary phase, which have excellent corrosion resistance in a relatively neutral environment, will decrease, which may cause problems in ensuring corrosion resistance in a neutral environment.

[0061] On the other hand, in the present invention, there are no particular limitations on the methods for measuring the area ratio of each phase, the number ratio of the Al phase present inside the MgZn2 phase or present in contact with the MgZn2 phase, the average linear length in the major axis direction, the number density, etc., and these can be measured using ordinary methods known in the technical field.

[0062] For example, the area ratio of each phase, the number ratio of the Al phase that exists inside the MgZn2 phase or in contact with the MgZn2 phase, the linear length in the average major axis direction, the number density (D al) can be measured through component analysis using energy dispersive X-ray spectroscopy (EDS) to classify and define each phase. Each phase is then labeled and classified in the image, and an image analyzer can be used to measure the area percentage, number percentage, average linear length in the major axis direction, number density, and other parameters of each phase. Taking into account the deviations between individual test specimens, 20 locations on each specimen are photographed at 2,000x magnification, and the average values ​​for each specimen are calculated, allowing the measurement of parameters such as the area percentage, number percentage, average linear length in the major axis direction, and number density of each phase.

[0063] Next, a method for producing a plated steel sheet according to yet another aspect of the present invention will be described in detail. However, this does not necessarily mean that the plated steel sheet of the present invention must be produced by the following production method.

[0064] According to one aspect of the present invention, the method may further include the step of preparing a base steel sheet, and the type of the base steel sheet is not particularly limited. In this case, the above description is equally applicable to the base steel sheet.

[0065] Next, the base steel sheet is immersed in a coating bath containing, by weight, 4.00-7.00% Mg, 8.000-20.000% Al, 0.002-0.050% Fe, with the balance being Zn and other unavoidable impurities, to perform hot-dip coating. The reasons for adding and limiting the contents of the above-mentioned components in the coating bath are the same as those for the components of the coating layer, except for the small amount of impurities that may flow in from the base steel sheet. Therefore, according to one aspect of the present invention, the coating bath may optionally further contain, by weight, one or more elements selected from 0.20% or less (including 0%) of Si and 0.200% or less (including 0%) of Ca.

[0066] To prepare the coating bath having the above composition, a composite ingot containing the desired Zn, Al, and Mg or a Zn-Mg or Zn-Al ingot containing individual components can be used. To replenish the coating bath consumed during hot-dip coating, the ingot is melted and supplied. In this case, the ingot can be directly deposited in the coating bath and melted, or the ingot can be melted in a separate port and the molten metal can be added to the coating bath.

[0067] Meanwhile, when the coating bath is produced, it contains 0.002 to 0.050% Fe as an essential component other than Zn, Al, and Mg. The Fe in the coating bath can be produced by alloying Fe during ingot production when the coating bath is first produced, or it can be added separately when the coating bath is produced. However, since some of the Fe may dissolve from the steel sheet once the coating operation begins, periodic analysis is required to maintain the Fe content within the range of 0.002 to 0.050% specified in the present invention.

[0068] Fe in the plating bath reacts with Al to form the Fe2Al5 phase. The Fe2Al5 phase floats to the surface of the plating bath due to its low specific gravity. However, if the size (diameter) is 0.05 μm or less, it cannot float to the surface and instead flows within the plating bath when the plating bath is fluid. Therefore, according to one aspect of the present invention, fine Fe2Al5 dross particles with a size of 0.05 μm or less (excluding 0 μm) that flow within the plating bath are a key factor affecting the size and number of alloy phases in the final plated product in this invention, and this will be described in detail. Meanwhile, coarse dross present on the surface of the plating bath does not fall under the 0.002-0.050% Fe content limit of the plating bath defined in this invention. When analyzing the plating bath components, the Fe content is determined by analyzing samples taken from the surface to a depth of 30 cm in the thickness direction.

[0069] Furthermore, although not particularly limited, according to one aspect of the present invention, fine Fe2Al5 particles of 0.05 μm or less that flow in the coating bath adhere to the steel sheet along with the coating bath as the steel sheet passes through the coating bath. These particles then serve as nucleation sites when the coating layer is cooled during the cooling process. In the present invention, the Al phase is the first phase to crystallize when the coating layer solidifies. Therefore, the fine dross present in the coating layer after coating and before cooling serves as an Al phase crystallization nucleation site during the cooling process. Therefore, the greater the amount of flowing dross, the greater the Al nucleation and the finer the Al phase. Therefore, in the present invention, the size (average diameter) of the flowing dross in the coating bath can be controlled to exist as fine dross of 0.05 μm or less (excluding 0 μm). One of the various factors for reducing the size of the flowing dross in the coating bath is the control of the Fe content. If the Fe content in the coating bath is less than 0.002%, there will be no or too little Fe2Al5 flowing dross, which can lead to problems such as insufficient Al phase nucleation and coarse Al phase. On the other hand, if the Fe content in the coating bath exceeds 0.05%, the generation of Fe2Al5 dross increases, causing the dross to aggregate and coarsen. This coarse dross may rise to the surface of the coating bath and interfere with the coating process, or may mix with the coating layer and cause dross imprint defects in the steel sheet.

[0070] Furthermore, although not particularly limited, according to one aspect of the present invention, the temperature of the plating bath may be maintained at a temperature 20 to 100°C higher than the solidification start temperature (Ts). Since the solidification start temperature may vary depending on the plating bath composition, the plating bath temperature can also be adjusted accordingly. Meanwhile, the temperature of the plating bath may be maintained in the range of 430 to 520°C.

[0071] According to one aspect of the present invention, the flow velocity of the coating bath can be controlled to 0.03 to 0.20 m / s. Even if Fe2Al5 dross of 0.05 μm or less is generated in the coating bath according to the present invention, if the coating bath does not flow, it will easily float to the surface of the coating bath, and dross deviation may occur locally in the coating bath. Therefore, the flow velocity of the coating bath must be at least 0.03 m / s. On the other hand, if the flow velocity is too fast, the flowing dross will become too fine and adhere to the coating layer, resulting in an excessive number of Al phases, which will eliminate the effect of the Al phase in improving corrosion resistance in acidic environments.

[0072] According to one aspect of the present invention, after the completion of the coating, the hot-dip coated steel sheet is subjected to a wiping treatment, for example, using a nitrogen (N2) or air knife, in order to achieve a target coating weight. In this case, the coating weight is not particularly limited, and is usually 20 to 400 g / m on one side. 2 The level is:

[0073] According to another aspect of the present invention, cooling is performed after controlling the coating weight. The cooling is controlled so that the average cooling rate is 6.0°C / s or more in the temperature range of the steel sheet from the Al phase crystallization temperature to 330°C. In the Al phase crystallization temperature to 330°C range, faster cooling slows nuclei growth and results in finer crystals, so the cooling rate is controlled as described above. There is no need to specifically limit the upper limit of the cooling rate in this range; it may be controlled at an appropriate level that is conventional in the technical field, taking into account cooling equipment limitations, vibration, economics, and the like. In the present invention, the Al phase crystallization nucleation sites are fine dross that existed in the coating layer before solidification. However, if the cooling rate slows after Al phase crystallization, the Al phase may grow and become coarse. Therefore, various process factors are involved in controlling the number density and size of the Al phase as described above, but only one of these control factors, namely, cooling at an average cooling rate of 6.0°C / sec or more in the range from the Al phase crystallization temperature to 330°C, can ensure the number density and size, etc. of the Al phase targeted in the present invention.

[0074] On the other hand, when the steel sheet temperature falls below 330°C, solidification of the coating layer is completed, so the cooling rate thereafter is not very important, but a faster rate is preferable in terms of productivity. [Example]

[0075] The present invention will be described in more detail below through examples. However, it should be noted that the following examples are for illustrative purposes only and do not limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters reasonably inferred therefrom.

[0076] (Example) The plating bath was prepared and used so as to have the composition shown in Table 1 below. After preparation of the plating bath, the final composition was confirmed using ICP (Inductively Coupled Plasma Mass Spectrometry). Meanwhile, the presence or absence of fine dross in the plating bath was confirmed by taking a test piece of the plating bath at a depth of 300 mm (thickness direction) from the surface of the plating bath, quenching it in water, and examining it with a transmission electron microscope (TEM) at 300,000 magnification.

[0077] Hot-rolled steel sheets and cold-rolled steel sheets were used as the base steel sheets, and the composition of the base steel sheets was 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%, with the balance being Fe and other unavoidable impurities.

[0078] The hot-rolled steel sheets were first pickled in a hydrochloric acid solution to remove iron oxides formed on the surface of the steel sheets during the hot rolling process, then heated to 650°C in a heating furnace under reducing conditions with a dew point temperature of -20°C, and then plated under the conditions of manufacturing process A in Table 2 below. The cold-rolled steel sheets were first immersed in alkali and electrolytically degreased to remove rolling oil, iron powder, and other foreign matter adhering to the steel sheet surface, then annealed at 840°C in an annealing furnace under a reducing atmosphere, and then plated under the conditions of manufacturing process B in Table 2 below. After plating, the coating weight was reduced to 150 g / m on one side using N2 wiping. 2 was adjusted to the same.

[0079] The plated steel sheets were cut to a specified size for analysis, and then cross-sectional test pieces were cut in the thickness direction of the steel sheets (meaning the direction perpendicular to the rolling direction), and the plating layer was photographed using FE-SEM. The composition was also confirmed using EDS to confirm the exact phase.

[0080] The images were taken at a magnification of 2000x, and 20 points on a 20mm long specimen were measured and analyzed to ensure representativeness. After identifying the phases in each image, the area ratio and number of each phase were measured using an image analyzer, and the average values ​​for the 20 images are shown in Table 3 below.

[0081] For the evaluation of corrosion resistance, the specimens were cut into pieces measuring 80 mm x 150 mm, and three pieces were measured for each test piece and experimental condition, and the average was calculated. The results were evaluated according to the following criteria, and the results are shown in Table 3 below. <Acid corrosion resistance> ◎: Weight loss before and after the experiment is 1g / m 2 The following cases are considered excellent: ○: Weight loss before and after the experiment was over 1 to 2 g / m 2 If less than this, it will be rated as excellent. △: Weight loss before and after the experiment was over 2g / m 2 If it is less than this, it is considered defective. ×: Weight loss before and after the experiment is 3 g / m 2 Anything above this is rated as very poor. <Neutral corrosion resistance> ◎: If the red and blue generation time is 4200 hours or more, it is evaluated as excellent. ○: If the red / blue occurrence time is more than 3500 hours and less than 4200 hours, it is evaluated as excellent. △: If the red / blue occurrence time is more than 2000 hours but less than 3500 hours, it is evaluated as poor. ×: If the red / blue generation time is less than 2000 hours, it is evaluated as very poor. <Alkaline corrosion resistance> ◎: Weight loss before and after the experiment was 2g / m 2 The following cases are considered excellent: ○: Weight loss before and after the experiment was over 2g / m 2 If less than this, it will be rated as excellent. △: Weight loss before and after the experiment was over 4g / m 2 If it is less than this, it is considered defective. ×: Weight loss before and after the experiment is 7 g / m 2 Anything above this is rated as very poor.

[0082] In addition, a sulfuric acid aqueous solution with a pH of 3.5 was used for corrosion resistance tests in an acidic environment, a 3.5% NaCl aqueous solution with a pH of 6.7 was used for corrosion resistance tests in a neutral environment, and an ammonia solution with a pH of 12.5 was used for corrosion resistance tests in an alkaline environment.

[0083] In the acidic and alkaline tests, the test pieces were immersed in the solutions for 48 hours, then removed and the corrosion weight loss was measured before and after the test. A small corrosion weight loss was evaluated as excellent, and a large corrosion weight loss was evaluated as very poor.

[0084] In the neutral test, salt water was sprayed onto the steel plate, and the longer it took for reddish-blue color (corrosion of the base steel plate) to appear on the steel plate, the better the corrosion resistance was evaluated to be.

[0085] [Table 1]

[0086] [Table 2]

[0087] [Table 3]

[0088] [Table 4]

[0089] As can be seen from the experimental results in the table above, Examples B1-B4, B8, B9, B11, B13, B19, B20, and B22, which satisfy the coating composition and manufacturing conditions of the present invention, exhibited excellent or very excellent corrosion resistance in all conditions, including acidic, neutral, and alkaline environments. In addition, the Fe content in the coating bath did not exceed the upper limit of 0.05% specified in the present invention, and no dross adhesion defects occurred.

[0090] On the other hand, in the case of comparative examples B5 and B21, the plating bath components and other manufacturing conditions are consistent with the present invention, but in the case of no plating bath flow, the Al crystallization nucleation sites are small, and the number density of the Al phase is 500 / 0.1 mm, which is the lower limit of the limit set by the present invention. 2 Therefore, the coarse Al phase was relatively abundant, and the corrosion resistance was very good in acidic conditions, and also good in neutral conditions, but the corrosion resistance was poor in alkaline conditions.

[0091] In Comparative Example B6, the cooling rate from the Al phase crystallization temperature to 330°C after plating was slower than 6.0°C / sec, which is the limit of the present invention. Although there are many Al nucleus crystallization sites according to the present invention, it takes time for the Zn-Al binary process phase and ternary process phase other than the Al phase and MgZn2 phase to crystallize, and the number density of the Al phase is 500 / 0.1mm, which is the lower limit of the limit of the present invention. 2 The corrosion resistance in an alkaline environment was poor.

[0092] In Comparative Examples B7, B10, and B23, the flow rate of the plating bath was lower than the range limited by the present invention, and the cooling rate from the Al phase crystallization temperature to 330°C was slower than the 6°C / s limited by the present invention, and the number density of the Al phase was below 500 / 0.1 mm, which is the lower limit limited by the present invention. 2 The corrosion resistance in an alkaline environment was very poor.

[0093] On the other hand, in the case of Comparative Example B12, the flow velocity of the coating bath was 0.45 m / sec, which exceeded the range limited by the present invention. When the flow velocity is too fast, the flowing dross becomes too fine and adheres to the coating layer, causing the number density of the Al phase to exceed the upper limit of 4000 pieces / 0.1 mm 2 limited by the present invention. 2 When the content of the aluminum phase exceeds 100%, the effect of improving corrosion resistance in an acidic environment due to the aluminum phase disappears, and the corrosion resistance in an acidic environment becomes poor.

[0094] In the case of Comparative Example B14, the Fe content in the coating bath exceeded the range defined by the present invention, and coarse dross was formed on the surface of the coating bath, resulting in dross adhesion defects on the coated steel sheet.

[0095] In Comparative Examples B15 and B24, the Al content in the plating bath components was lower than the range defined by the present invention, and the number density of the Al phase was outside the range defined by the present invention, resulting in very poor corrosion resistance in acidic conditions.

[0096] Comparative Example B16 was a case in which there was no Fe in the plating bath, and the number of Al phases was smaller than the range limited by the present invention, and the number of coarse Al phases of 5 μm or less was lower than the 80.0% limit set by the present invention, resulting in poor corrosion resistance in an alkaline environment.

[0097] In Comparative Example B17, the Mg content in the plating bath was lower than the range limited by the present invention, and no Fe was added. The number density of the Al phase was also low, and the corrosion resistance in an alkaline atmosphere was very poor, and the corrosion resistance was also poor in a neutral atmosphere.

[0098] On the other hand, in Comparative Example B18, no Mg was added to the plating bath and the Al content was lower than the range limited by the present invention. In this case, the MgZn2 phase and the Al phase did not crystallize in the plating layer, and the corrosion resistance was poor or very poor in all acidic, neutral, and alkaline atmospheres.

[0099] Comparative Example B25 is a case in which the Mg content in the coating bath was lower than the range limited by the present invention, the coating bath was not added with Fe, and the cooling rate from the Al phase crystallization temperature to 330°C after coating was slower than the 6.0°C / sec limited by the present invention. As a result, the number density of the Al phase in the coating layer was lower than the range limited by the present invention, and the proportion of Al phases of 5 μm or less was also low, resulting in very poor corrosion resistance in an alkaline atmosphere and poor corrosion resistance even in a neutral atmosphere.

[0100] In the case of Comparative Example B26, the Mg content in the coating bath was lower than the range limited by the present invention, there was no coating bath flow, and the proportion of Al phase present inside or in contact with the MgZn2 phase was outside the lower limit of the present invention, so the corrosion resistance was very poor in an alkaline atmosphere and also poor in an acidic atmosphere.

[0101] In the case of Comparative Example B27, the Al content in the plating bath was higher than the range defined by the present invention, and Fe was not added. As a result, the number density of the Al phase was outside the range defined by the present invention, and the corrosion resistance in an alkaline atmosphere was very poor.

Claims

1. Base steel sheet; and an Al-Mg-Zn-based plating layer provided on at least one surface of the base steel sheet; The plating layer is the MgZn ２ It exists inside the phase or MgZn ２ The number density of the Al phase that is in contact with the Al phase is 500 to 4,000 pieces / 0.1 mm ２ That is, plated steel sheet.

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

3. The plated steel sheet according to claim 2, wherein the plating layer further contains, by weight percent, one or more selected from the group consisting of Si: 0.2% or less (including 0%) and Ca: 0.2% or less (including 0%).

4. The plated steel sheet according to claim 2, wherein the plating layer further comprises at least one selected from the group consisting of the following (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, and 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) 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, and Bi: 1.0% or less

5. The plating layer has a MgZn content relative to the total number of Al phases. ２ It exists inside the phase or MgZn ２ The plated steel sheet according to claim 1, wherein the proportion of the Al phase present in contact with the Al phase is 85% or more and 100% or less.

6. The MgZn ２ It exists inside the phase or MgZn ２ The plated steel sheet according to claim 1, wherein the average linear length in the major axis direction of the Al phase that is in contact with the Al phase is 5 μm or less (excluding 0 μm).

7. The MgZn ２ It exists inside the phase or MgZn ２ 2. The plated steel sheet according to claim 1, wherein the percentage of Al phases present in contact with the Al phase and having a linear length in the major axis direction of 5 μm or less (excluding 0 μm) is 90.0 to 100.0%.

8. a step of immersing the base steel sheet in a coating bath containing, by weight %, 4.00 to 7.00% Mg, 8.000 to 20.000% Al, 0.002 to 0.050% Fe, the balance being Zn and other inevitable impurities, at a temperature of 430 to 520°C and a flow rate of 0.03 to 0.20 m / s to hot-dip coat the base steel sheet; performing a wiping treatment on the hot-dip galvanized steel sheet; and cooling the wiped steel sheet at an average cooling rate of 6.0°C / s or more in a range from the crystallization temperature of the Al phase to 330°C.

9. 9. The method for producing a plated steel sheet according to claim 8, wherein the plating bath further contains, by weight %, one or more selected from Si: 0.20% or less (including 0%) and Ca: 0.200% or less (including 0%).

10. The method for producing a plated steel sheet according to claim 8, wherein the plating bath further contains at least one selected from the group consisting of the following (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, and 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, 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 method for producing a plated steel sheet according to claim 8, wherein the temperature of the plating bath is maintained at a temperature that is 20 to 100°C higher than the solidification start temperature (Ts).

12. 9. The method for producing a plated steel sheet according to claim 8, wherein the average diameter of flowing dross in the plating bath is controlled to be 0.05 μm or less (excluding 0 μm).

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