Plated steel sheet with excellent corrosion resistance, workability and surface quality and method for producing the same

Abstract

This invention provides a coated steel sheet and a method for manufacturing the same. The coated steel sheet comprises: a base steel sheet; a Zn-Mg-Al coating disposed on at least one side of the base steel sheet; and an Fe-Al inhibition layer disposed between the base steel sheet and the Zn-Mg-Al coating. The coating, by weight percent, comprises: Mg: 4% or more; Al: 2.1 times or more and 14.2% or less of the Mg content; Si: 0.2% or less (including 0%); Sn: 0.1% or less (including 0%); the balance being Zn and unavoidable impurities.

Description

Technical Field

[0001] This invention relates to a coated steel sheet with excellent corrosion resistance, processability and surface quality, and a method for manufacturing the same. Background Technology

[0002] Zinc-coated steel sheets possess the sacrificial corrosion protection characteristic of zinc, which has a lower oxidation-reduction potential than iron, being corroded first when exposed to corrosive environments, thus inhibiting the corrosion of the steel. Furthermore, the zinc coating oxidizes and forms a dense network of corrosion products on the steel surface, isolating the steel from the oxidizing atmosphere and thereby improving its corrosion resistance. Due to these advantageous properties, the application of zinc-coated steel sheets has expanded in recent years to include building materials, household appliances, and automotive steel sheets.

[0003] However, due to increased air pollution caused by industrialization and the gradual deterioration of the corrosive environment, as well as due to strict regulations on resource and energy conservation, the necessity to develop a steel with superior corrosion resistance compared to existing galvanized steel sheets has increased.

[0004] To address this issue, extensive research is underway on manufacturing technologies for zinc alloy-coated steel sheets that incorporate elements such as aluminum (Al) and magnesium (Mg) in the zinc plating bath to enhance corrosion resistance. Representative examples include Zn-Mg-Al zinc alloy-coated steel sheets, where Mg is further added to the Zn-Al plating composition.

[0005] Furthermore, Zn-Mg-Al zinc alloy coated steel sheets are mostly used through processing. Due to the large amount of high-hardness intermetallic compounds contained in the coating, bending processing can lead to cracks within the coating, resulting in poor machinability. Even after processing, the problem of liquid metal embrittlement (LME) can occur, where molten zinc penetrates along the grain boundaries of the iron substrate during welding, such as spot welding.

[0006] In addition, the processed zinc-coated steel sheets are mostly placed on the periphery of the product, but the surface quality is insufficient due to surface damage caused by processing, so it is necessary to improve the quality of the outer sheet.

[0007] However, to date, no technology has been developed that can meet the advanced requirements of excellent corrosion resistance, processability, reduction of LME occurrence, and surface quality.

[0008] (Patent Document 1) Korean Publication No. 2013-0133358 Summary of the Invention

[0009] Technical problems to be solved

[0010] According to one aspect of the present invention, a coated steel sheet with excellent corrosion resistance, processability and surface quality, while reducing the occurrence of liquid metal embrittlement (LME), and a method thereof can be provided.

[0011] The technical problems addressed by this invention are not limited to those described above. Those skilled in the art will readily understand the additional technical problems addressed by this invention from the full text of this specification.

[0012] Technical solution

[0013] One aspect of the present invention provides a coated steel sheet comprising:

[0014] Foundation steel plate;

[0015] A Zn-Mg-Al based coating is disposed on at least one side of the base steel plate; and

[0016] An Fe-Al based suppression layer is disposed between the base steel plate and the Zn-Mg-Al based coating.

[0017] The coating, by weight percent, comprises: Mg: 4% or more, Al: 2.1 times or more and 14.2% or less of Mg, Si: 0.2% or less (including 0%), Sn: 0.1% or less (including 0%), the balance being Zn and unavoidable impurities.

[0018] Another aspect of the present invention provides a method for manufacturing a coated steel sheet, comprising the following steps:

[0019] The base steel plate is immersed in a galvanizing bath and hot-dip galvanized. The galvanizing bath, by weight percent, contains: Mg: 4% or more, Al: 2.1 times the Mg content and less than 14.2%, Si: less than 0.2% (inclusive), Sn: less than 0.1% (inclusive), the balance being Zn and unavoidable impurities. The galvanizing bath is maintained at a temperature 20-80°C higher than the solidification initiation temperature on the equilibrium phase diagram.

[0020] Cooling begins from the surface of the plating bath, using inert gas at an average cooling rate of 3-30°C / second to cool to the upper roller area.

[0021] In the cooling step, the cooling rate is controlled to satisfy the following relationships 1-1 and 1-2.

[0022] [Relation 1-1]

[0023] A>2.5 / {ln(t×20)} 1 / 2 ×B

[0024] [Relationship 1-2]

[0025] 0.7×C≤B≤1.2×C

[0026] [In Equations 1-1 and 1-2, t is the thickness of the steel plate, A is the average cooling rate (°C / second) from the plating bath temperature to the solidification initiation temperature, B is the average cooling rate (°C / second) from the solidification initiation temperature to -30°C, and C is the average cooling rate (°C / second) from -30°C to 300°C.]

[0027] Beneficial effects

[0028] According to one aspect of the present invention, a coated steel sheet with excellent corrosion resistance, processability and surface quality, while reducing the occurrence of liquid metal embrittlement (LME), and a method thereof can be provided.

[0029] The various advantages and effects of the present invention are not limited to the above description. The various advantages and effects of the present invention will be more easily understood in the process of describing the specific embodiments of the present invention. Attached Figure Description

[0030] Figure 1 For the coated steel sheet of Example 1, a cross-sectional specimen along the thickness direction was prepared to observe the entire coating and the base iron together. The cross-sectional specimen was magnified 500 times and then observed using a field emission scanning electron microscope (hereinafter referred to as "FE-SEM").

[0031] Figure 2 The image is a photograph observed using FE-SEM after magnifying the cross-section of the coated steel sheet in the thickness direction of Example 4 of the present invention by 2000 times.

[0032] Figure 3 This is a photograph observed using FE-SEM after the surface of the coated steel sheet of Example 2 of the present invention was magnified 1000 times.

[0033] Figure 4 The image is a photograph observed using FE-SEM after magnifying a cross-sectional specimen of the coated steel sheet in the thickness direction of Example 10 of the present invention by 1000 times.

[0034] Figure 5 This is an X-ray diffraction (XRD) chart of the coating of Example 16 of the present invention.

[0035] Figure 6 The phase diagram of the Mg-Al-Zn ternary system is shown.

[0036] Figure 7The image is a photograph observed using a field emission scanning electron microscope (FE-SEM) after the cross-section of the coated steel plate of Example 4 of the present invention was magnified 2500 times.

[0037] Figure 8 It is a diagram schematically illustrating the method for measuring the length occupied by the protruding phase.

[0038] Figure 9 This is a schematic diagram of the fine structure that can be observed in the coated steel sheet of the present invention.

[0039] Best practice

[0040] The terminology used in this specification is for describing particular embodiments and is not intended to limit the invention. Furthermore, unless the opposite meaning is clearly indicated in the relevant definitions, the singular forms used in this specification also include the plural forms.

[0041] The use of "includes" or "includes" in the specification means to specify the composition, not to exclude or add other compositions.

[0042] Unless otherwise defined, all terms used in this specification, including technical and scientific terms, shall have the same meaning as commonly understood by those skilled in the art. Terms defined in dictionaries shall be interpreted as having the same meaning as those in relevant technical literature and currently disclosed.

[0043] The coated steel sheet according to one aspect of the present invention will now be described in detail. Unless otherwise specifically defined, the content of each element is expressed in weight percent (%) in this invention.

[0044] In existing technologies related to Zn-Mg-Al zinc alloy coated steel sheets, Mg is added to improve corrosion resistance. However, when too much Mg is added, the generation of plating bath dross increases, and there is a problem that the dross needs to be removed frequently. Therefore, the upper limit of Mg addition is limited to 3%.

[0045] Furthermore, as mentioned above, it has been previously impossible to provide coated steel sheets that offer excellent corrosion resistance, processability, and surface quality while reducing the occurrence of liquid metal embrittlement (LME).

[0046] Therefore, as a result of in-depth research to solve the above-mentioned problems, the inventors have invented a coated steel sheet and its manufacturing method that, by increasing the amount of Mg added, not only further improves corrosion resistance compared to existing technologies, but also achieves a balance of corrosion resistance, processability, surface quality, and reduced embrittlement of liquid metals. The following is a detailed description of the structure of this invention.

[0047] According to one aspect of the present invention, the coated steel sheet comprises: a base steel sheet; a Zn-Mg-Al coating disposed on at least one side of the base steel sheet; and an Fe-Al inhibition layer disposed between the base steel sheet and the Zn-Mg-Al coating.

[0048] The type of base steel plate is not particularly limited in this invention. For example, the base steel plate can be an Fe-based base steel plate, i.e., a hot-rolled steel plate or a cold-rolled steel plate, which is commonly used as the base steel plate for galvanized steel plates, but is not limited to this. Alternatively, the base steel plate can also be, for example, carbon steel, ultra-low carbon steel, or high-manganese steel used as building materials, home appliance materials, or automotive materials.

[0049] However, as a non-limiting example, the base steel plate may contain the following composition by weight percent: C: less than 0.17% (excluding 0), Si: less than 1.5% (excluding 0), Mn: 0.01-2.7%, P: less than 0.07% (excluding 0), S: less than 0.015% (excluding 0), Al: less than 0.5% (excluding 0), Nb: less than 0.06% (excluding 0), Cr: less than 1.1% (including 0), Ti: less than 0.06% (excluding 0), B: less than 0.03% (excluding 0), with the balance being Fe and other unavoidable impurities.

[0050] According to one aspect of the invention, at least one side of the base steel plate may be provided with a Zn-Mg-Al coating composed of a Zn-Mg-Al alloy. The coating may be formed only on one side of the base steel plate, or it may be formed on both sides of the base steel plate. In this case, the Zn-Mg-Al coating refers to a coating containing Mg and Al, and containing more than 50% Zn.

[0051] Furthermore, according to one aspect of the invention, an Fe-Al inhibitory layer may be provided between the base steel plate and the Zn-Mg-Al coating. The Fe-Al inhibitory layer is a layer containing intermetallic compounds of Fe and Al, such as FeAl, FeAl3, and Fe2Al5. In addition, it may contain a portion, for example, less than 40% of Zn, Mg, or other components derived from the coating. The inhibitory layer is formed by alloying Fe diffused from the initial base steel plate with the plating bath composition. The inhibitory layer can improve the adhesion between the base steel plate and the coating, and can prevent Fe from diffusing from the base steel plate to the coating.

[0052] According to one aspect of the invention, the coating may comprise, by weight percent: Mg: 4% or more, Al: 2.1 times or more and 14.2% or less of the Mg content, Si: 0.2% or less (including 0%), Sn: 0.1% or less (including 0%), the balance being Zn and unavoidable impurities. The specific composition is described below.

[0053] Mg: 4% or more

[0054] Mg is an element that improves the corrosion resistance of coated steel. In this invention, to ensure the desired excellent corrosion resistance, the Mg content in the coating is controlled at 4% or more, more preferably at 4.1% or more. Furthermore, the effect of adding Mg improves corrosion resistance, so the upper limit of the Mg content is not particularly limited. However, as an example, adding too much Mg may produce scum; therefore, the Mg content can be controlled at 6.7% or less, more preferably at 6.5% or less.

[0055] Al: More than 2.1 times the Mg content and less than 14.2%

[0056] Typically, while adding more than 1% Mg improves corrosion resistance, adding more than 2% Mg increases the formation of plating bath dross due to Mg oxidation, necessitating frequent dross removal. Due to this issue, existing technologies add more than 1.0% Mg to Zn-Mg-Al zinc alloy plating to ensure corrosion resistance, but have set an upper limit of 3.0% for Mg content and commercialized it.

[0057] However, as mentioned above, to further improve corrosion resistance, it is necessary to increase the Mg content to 4% or more. However, when the coating contains more than 4% Mg, there is a problem of slag formation due to the oxidation of Mg in the plating bath. To suppress this slag, the Al content in the coating needs to be at least 2.1 times the Mg content. To further improve the above-mentioned slag suppression effect, the lower limit of the Al content in the coating is preferably 8.7%, more preferably 8.8%. However, when too much Al is added to suppress slag, the melting point of the plating bath becomes higher. As the operating temperature becomes too high, it may cause problems such as corrosion of the plating bath structure and modification of the steel due to high-temperature operation. In addition, when the Al content in the plating bath is too high, Al reacts with Fe in the base iron without contributing to the formation of an Fe-Al suppression layer, and a rapid reaction between Al and Zn occurs, resulting in the formation of too many outburst phases, which may worsen the corrosion resistance. Therefore, it is preferable to control the upper limit of the Al content in the coating at 14.2%, more preferably at 14%, and most preferably at 13.8%.

[0058] Furthermore, according to one aspect of the invention, the Al and Mg contents can be determined as follows: Figure 6 In the Mg-Al-Zn ternary phase diagram, it is located near the binary eutectic line of MgZn2 and Al. The determination of being located at the binary eutectic line includes not only being determined to be exactly located on the binary eutectic line, but also being determined to be slightly off the binary eutectic line and within ±0.5 wt% of Mg and ±1 wt% of Al, with the binary eutectic line as a reference. Figure 6 The figure shows the Mg-Al-Zn ternary phase diagram with the X-axis set to Al content and the Y-axis set to Mg content. Figure 6 In this context, A represents a condition corresponding to an instance of the present invention, such as... Figure 6 As shown, the contents of Al and Mg can be determined to be located near the binary eutectic line of MgZn2 and Al in the Mg-Al-Zn ternary phase diagram.

[0059] Si: 0.2% or less (including 0%)

[0060] Regarding galvanized steel sheets, silicon (Si) is typically added to prevent alloying. However, when excessive Si is added, it reacts with magnesium (Mg) in the plating bath to form Mg₂Si. This Mg₂Si structure is brittle and can therefore degrade workability during bending and other processing. Therefore, in this invention, the Si content is controlled to below 0.2% to ensure workability, preferably below 0.02%, more preferably below 0.01%, and most preferably below 0.009%. Alternatively, it is preferable that Mg₂Si is not formed, and therefore the Si content can be 0%.

[0061] Sn: 0.1% or less (including 0%)

[0062] Sn can be added to improve the corrosion resistance of the coating. However, in this invention, when too much Sn is added to the Zn-Mg-Al plating bath, the melting point decreases, causing the solidification endpoint of the coating to drop by 10°C or more. This drop in solidification point may lead to surface defects caused by uneven solidification. Furthermore, during spot welding, molten coating can easily penetrate to the interface with the base iron, forming liquid metal embrittlement (LME) cracks. In addition, Sn reacts with Mg in the plating bath to form Mg2Sn intermetallic compounds, which are relatively light and have a high melting point of 770°C compared to other phases in the coating. Therefore, when Mg2Sn intermetallic compounds are formed, they float on the surface of the plating bath and are difficult to dissolve again. Furthermore, when the Mg2Sn intermetallic compounds remaining on the plating surface are adsorbed onto the coating surface during hot-dip plating, they may cause surface defects.

[0063] Therefore, in this invention, it is necessary to control the Sn content in the coating to below 0.1%. Furthermore, to achieve the desired effect, the Sn content is more preferably below 0.09%, and most preferably below 0.05%.

[0064] Balance Zn and other unavoidable impurities

[0065] In addition to the composition of the coating described above, the balance may be Zn and other unavoidable impurities. Unavoidable impurities may include all impurities that are undesirably introduced during the conventional manufacturing process of hot-dip galvanized steel sheets, the meaning of which will be readily understood by those skilled in the art.

[0066] According to one aspect of the invention, although not particularly limited, the coating may also selectively satisfy the configuration described later.

[0067] Fe: less than 1%

[0068] According to one aspect of the invention, the Fe component contained in the base steel plate can diffuse and be included in the coating during the plating process. Although not particularly limited, the Fe content in the coating can be 1% or less (including 0%). Furthermore, more preferably, the upper limit of the Fe content in the coating can be 0.3%, and the lower limit of the Fe content in the coating can be 0%.

[0069] Furthermore, when Fe from the base steel sheet diffuses into the coating, it forms alloys or intermetallic compounds, thereby forming protruding phases and a discontinuous inhibition layer. However, these protruding phases become a factor reducing corrosion resistance; therefore, in this invention, the inhibition layer is preferably formed continuously based on the cross-section of the coated steel sheet (the direction perpendicular to the rolling direction of the steel sheet). That is, continuously forming the inhibition layer means that no protruding phases are formed.

[0070] However, a certain amount of Fe can diffuse from the base steel plate to the coating and form an alloy phase, i.e., a protruding phase, between the base steel plate and the coating.

[0071] Therefore, even if a protruding phase is formed in this invention, to ensure corrosion resistance, in a cross-section along the thickness direction of the steel plate, when the interface line of the base steel plate is separated by 5 μm towards the coating surface, the length of the protruding phase intersecting the separated line must be less than 10% of the length of the separated line, more preferably less than 5%, and ideally 0%. The lower limit of the proportion of the length of the protruding phase intersecting the separated line includes 0%, and therefore it is not particularly limited. The line drawn along the interface formed by the layer adjacent to the base steel plate is called the interface line.

[0072] Figure 8 The diagram schematically illustrates a method for measuring the length occupied by this protruding phase. For example... Figure 8 As shown, L1 represents the length of the separated line, and L2 represents the length occupied by the protruding phase that intersects the separated line.

[0073] Therefore, a photograph taken using FE-SEM was taken of a cross-sectional specimen of the coated steel sheet in the thickness direction, which is an example 10 described later in this invention, magnified 1000 times. Figure 4 As an example, the above can be applied directly. Figure 8 The measurement method is used to measure the length occupied by the protruding phase.

[0074] As a result, in this invention, the inhibition layer is preferably formed continuously, or even if it is not formed continuously, the inhibition layer is preferably formed to occupy more than 90% of the entire interface length between the base steel plate and the inhibition layer. For example, the interface length and the ratio of the length based on the interface length can be measured by setting the magnification of a scanning electron microscope to 1000x, and includes the case of measuring at any three locations and observing at at least one location.

[0075] According to one aspect of the invention, the Fe content of the protruding phase can be 10-45% by weight, the alloy phase of the protruding phase can include one or more of Fe2Al5, FeAl and Fe-Zn systems, and can contain more than 20% Zn by weight.

[0076] According to one aspect of the invention, the thickness of the inhibition layer can be 0.02 μm or more and 2.5 μm or less. While the inhibition layer serves to prevent alloying and ensure corrosion resistance, its brittleness may adversely affect processability; therefore, the thickness of the inhibition layer can be controlled to be 2.5 μm or less. However, to achieve the function of an inhibition layer, it is preferable to control the thickness of the inhibition layer to be 0.02 μm or more. To further improve the above-mentioned effect, the upper limit of the thickness of the inhibition layer is preferably 1.8 μm (more preferably 0.9 μm). Furthermore, the lower limit of the thickness of the inhibition layer can be 0.05 μm. In this case, the thickness of the inhibition layer can refer to the minimum thickness in the direction perpendicular to the interface of the base steel plate.

[0077] According to one aspect of the invention, as in the case of a discontinuously formed inhibition layer, the inhibition layer and the protruding phase can coexist at the interface of the base steel plate. That is, as described above, the protruding phase includes a region intersecting a line that moves 5 μm parallel to the interface, and the portion from this region to the interface with the base steel plate can be considered as the protruding phase. However, the alloy layer containing Fe-Al intermetallic compounds, other than the protruding phase, is considered as the inhibition layer.

[0078] Furthermore, according to one aspect of the invention, based on the cross-section of the coated steel sheet, the number of Mg2Si phases with a major axis of 500 nm or more in contact with the interface between the suppression layer and the coating can be less than 10 (including 0%) per 100 μm of interface length. In this case, the cross-sectional hardness of the coating can be 200-450 Hv. The Mg2Si in contact with the interface between the coating and the suppression layer includes all Mg2Si in the form of contact through the interface or with the interface. Furthermore, the interface length represents the length measured along the interface between the coating and the suppression layer. Stress will concentrate at the interface between the suppression layer and the coating, and when a large amount of Mg2Si, as a brittle metallic compound, forms at the interface, it can act as a crack initiation point during bending processing. In particular, the Zn-Mg-Al coating according to one aspect of the invention has a hardness as high as 200-450 Hv and is brittle; therefore, the presence of the Mg2Si phase may further degrade processability. In order to prevent the above-mentioned factors that degrade processability and further improve processability, the number of Mg2Si phases (Na) with a major axis of 500 nm or more in contact with the interface of the inhibition layer and the coating per 100 μm of interface length can be 4 or less, more preferably 2 or less.

[0079] Therefore, in this invention, by controlling the Mg content to a high level, controlling the coating hardness to a high level in the range of 200-450 Hv, and controlling the number of Mg2Si phases with a major axis of 500 nm or more in contact with the interface between the inhibition layer and the coating to less than 10 per 100 μm of interface length, a coated steel sheet with improved corrosion resistance and excellent processability can be provided. For example, the interface length and the number of Mg2Si phases can be measured by setting the magnification of a scanning electron microscope to 1000x, and multiple photographs can be taken repeatedly until the interface length of 100 μm is observed.

[0080] Furthermore, according to one aspect of the invention, in order to ensure corrosion resistance, the sum of the areas of the Al single phases contained within the MgZn2 phase relative to the total cross-sectional area of ​​the coating can be present in an area ratio of 0.5-10%, more preferably in an area ratio of 0.5-5%. Since the proportion of the Al single phases contained within the MgZn2 phase relative to the total cross-sectional area of ​​the coating satisfies the above range, the Al single phases contained within the MgZn2 phase act as a framework, thereby ensuring excellent corrosion resistance while also ensuring excellent sacrificial corrosion protection.

[0081] The Al single phase contained within the MgZn2 phase refers not only to the Al single phase completely contained within the MgZn2 phase, but also to the Al single phase partially contained within the MgZn2 phase.

[0082] in addition, Figure 7 The diagram illustrates a method for measuring a portion of the Al single phase contained within the MgZn2 phase. Specifically, the area occupied by the Al single phase within the MgZn2 phase can be calculated by connecting the two points where the boundary of the Al phase (or other phases surrounding the Al phase) intersects with the boundary of the MgZn2 phase using a straight line.

[0083] That is, it is possible to proceed from the following: Figure 7 The cross-section of the galvanized steel sheet, magnified 2000 times, was examined using a field emission scanning electron microscope (FE-SEM) to distinguish between MgZn2 and Al single phases. In this case, ① represents the form containing only MgZn2, ② represents the form where MgZn2 contains an Al single phase, ③ represents the form where part of the Al single phase is contained within the MgZn2 phase and part protrudes beyond the MgZn2 phase, and ④ represents the forms where MgZn2 contains Al and where part of the Al single phase is contained within the MgZn2 phase and part protrudes beyond the MgZn2 phase.

[0084] Alternatively, compositional mapping can be performed using an electron probe microanalyzer (EPMA), which is commonly known in this art, to observe the distribution of Mg and Al components, and the experimental results can be applied. From this, the total fraction of the MgZn2 phase in the coated microstructure can be determined, and the fraction of Al belonging to or crossing MgZn2 can be calculated separately.

[0085] That is, according to one aspect of the invention, the Al single phase may be entirely or partially located inside the MgZn2 phase.

[0086] Furthermore, according to one aspect of the invention, the diffraction intensity ratio I(200) / I(111), which is the ratio of the X-ray diffraction intensity I(200) of the (200) plane of the Al single phase to the X-ray diffraction intensity I(111) of the (111) plane of the Al phase, can be 0.8 or less (excluding 0), more preferably 0.79 or less, and most preferably 0.7 or less. In this case, the ratio of the integrated intensity of the (200) plane of Al to the integrated intensity of the (111) plane is measured. By satisfying this condition, corrosion resistance can be achieved by controlling the proportion of the Al single phase within the MgZn2 phase. According to the invention, in order to achieve corrosion resistance, the MgZn2 phase needs to contain a certain amount of Al, and the characteristics of these structures can be confirmed by the orientation ratio of the Al crystals when measured by XRD. XRD measurement can confirm the intensity ratio of different orientations of Al by using a Cu-Kα source to diffract the X-ray diffraction pattern in the range of 34-46° (2θ(theta)).

[0087] According to one aspect of the invention, the Al single phase contained within the MgZn2 phase can be one of the following Al single phases, schematically shown in [illustration]. Figure 9 middle.

[0088] - Al single phase contained within and entirely within the MgZn2 phase [ Figure 9 fine tissue 1]

[0089] -A single Al phase, partly contained within the MgZn2 phase and partly protruding outside the MgZn2 phase. Figure 9 fine tissue 2]

[0090] - A single Al phase entirely contained within a mixed phase of Al and Zn, wherein the mixed phase of Al and Zn is entirely contained within a MgZn2 phase. Figure 9 fine tissue 3]

[0091] - A single Al phase entirely contained within a mixed phase of Al and Zn, wherein a portion of the mixed phase of Al and Zn is contained within the MgZn2 phase and a portion protrudes beyond the MgZn2 phase. Figure 9 fine tissue 4]

[0092] - A portion of the Al single phase is contained within the mixed phase of Al and Zn, and all of the Al single phase is contained within the MgZn2 region, wherein a portion of the mixed phase of Al and Zn is contained within the MgZn2 phase and a portion protrudes outside the MgZn2 phase. Figure 9 fine tissue 5]

[0093] - A portion of the Al single phase is contained within the mixed phase of Al and Zn, and a portion of the Al single phase is contained within the MgZn2 region and a portion protrudes outside the MgZn2 region, wherein a portion of the mixed phase of Al and Zn is contained within the MgZn2 phase and a portion protrudes outside the MgZn2 phase. Figure 9 fine tissue 6]

[0094] Furthermore, the Al single phase involved in this invention refers to a separate phase in which Al is the main component, and Zn and other components may be contained in this phase in solid solution form. According to one aspect of the invention, the Al single phase may contain, by weight%, 40-70% Al, with the balance being Zn and other unavoidable impurities. As an example, the Al single phase may contain, by weight%, 40-70% Al, 30-55% Zn, and other unavoidable impurities; in one specific embodiment, the total content of Al and Zn may be 95-100%. The balance may be Mg or other unavoidable impurities.

[0095] According to one aspect of the invention, in the coating, the proportion of Al single-phase, in terms of area fraction, relative to the entire cross-section of the coating can be 1-15%. When the proportion of Al single-phase is 1% or more, the Al, which functions to maintain the framework, can help the coating act as a physical protective barrier film. On the other hand, when the proportion of Al single-phase is 15% or less, it can prevent the stability from deteriorating due to Al corrosion. To improve the above-mentioned effects, the lower limit of the proportion of Al single-phase is preferably 1.7%. Alternatively, to improve the above-mentioned effects, the upper limit of the proportion of Al single-phase is 11% (more preferably 9.8%).

[0096] Furthermore, according to one aspect of the invention, the Al-Zn mixed phase contained within the MgZn2 phase may be present in amounts of less than 10% relative to the total cross-sectional area of ​​the coating.

[0097] According to one aspect of the invention, the arithmetic mean surface roughness Ra of the coating can be 0.5-3.0 μm, more preferably 0.6-3.0 μm. When the surface roughness Ra is less than 0.5 μm, slippage may occur when the plates are stacked due to reduced surface friction, which may affect operation. Furthermore, when applying rust-preventive oil to the steel plate surface, the properties of the rust-preventive oil residue on the surface may deteriorate. On the other hand, when the surface roughness Ra exceeds 3.0 μm, excessive pressure may induce cracks in the coating during the process of forming a surface roughness exceeding 3.0 μm through physical pressure.

[0098] According to one aspect of the invention, the ten-point average surface roughness Rz of the coating can be 1-20 μm, more preferably 5-18 μm. When Rz is less than 1 μm or more than 20 μm, it may be observed to be too bright or too dark in terms of the metallic luster that presents the aesthetic effect of the steel plate surface. Therefore, it is appropriate to control it within the range of 1-20 μm as suitable. The above roughness is measured according to the measurement method based on KS B 0161, and the cutoff value for measuring the roughness is based on 2.5 μm.

[0099] According to one aspect of the invention, the cross-sectional hardness of the coating can be 200-450 Hv. The hardness of the coating is related to the type and size of the crystal phases constituting the coating. When the cross-sectional hardness is less than 200 Hv, the coating's resistance to external friction is weakened. As a result, when there is surface friction from the outside, the coefficient of friction increases, which may lead to poor machinability and may cause deformation. However, when the hardness of the coating exceeds 450 Hv, it becomes too brittle, and the side effect of cracking on the coating may occur during processing.

[0100] According to one aspect of the invention, the thickness of the coating can be 5-100 μm, more preferably 5-90 μm. When the coating thickness is less than 5 μm, the coating may be too thin locally due to errors caused by thickness deviations, thus potentially reducing corrosion resistance. When the coating thickness exceeds 100 μm, the cooling of the hot-dip coating may be delayed. For example, solidification defects such as flow lines may occur on the coating surface, and the productivity of the steel sheet may be reduced in order to solidify the coating.

[0101] Furthermore, although not particularly limiting, according to one aspect of the invention, the coating, under atmospheric and chloride atmospheres (e.g., ISO 14993 test standard), allows LDH to form first on the surface compared to zinc chloride ore and zinc brine ore. That is, when maintained in a corrosive environment (or, for a prolonged period in atmospheric atmosphere), layered double hydroxides (LDH; (Zn,Mg)6Al2(OH)) can form on the coating surface as dense initial corrosion products. 16 Rapid nucleation and crystallization of (CO3)·4H2O. Subsequently, over time, it is uniformly distributed on the overall surface, shielding the corrosive active areas, and can induce the uniform formation of secondary zinc chloride mineral (Simonkolleite; Zn5(OH)8Cl2) and hydrozincite ((Zn5(OH)6(CO3)2).

[0102] According to one aspect of the invention, LDH corrosion products can be formed on the surface of the coating within 6 hours in an atmospheric atmosphere, and within 5 minutes in a chloride atmosphere according to ISO 14993.

[0103] Next, a method for manufacturing a coated steel sheet according to another aspect of the present invention will be described in detail. However, this does not mean that the coated steel sheet of the present invention must be manufactured by the following method.

[0104] According to one aspect of the invention, the step of preparing a base steel plate may be further included, and the type of base steel plate is not particularly limited. It may be an Fe-based base steel plate used as a base steel plate for conventional hot-dip galvanized steel plates, i.e., hot-rolled steel plate or cold-rolled steel plate, but is not limited thereto. Furthermore, the base steel plate may be, for example, carbon steel, ultra-low carbon steel, or high-manganese steel used as building materials, household appliance materials, automotive materials, but is not limited thereto.

[0105] Next, according to one aspect of the invention, the process may include immersing a base steel plate in a plating bath and performing hot-dip galvanizing, wherein the plating bath, by weight percent, comprises: Mg: 4% or more, Al: 2.1 times or more and 14.2% or less of the Mg content, Si: 0.2% or less (including 0%), Sn: 0.1% or less (including 0%), the balance being Zn and unavoidable impurities. To prepare the plating bath with the above composition, a composite steel ingot containing predetermined amounts of Zn, Al, and Mg, or a Zn-Mg or Zn-Al steel ingot containing individual components, can be used. Furthermore, the description of the composition of the plating layer, except for the Fe content introduced from the base steel plate, is equally applicable to the composition of the plating bath.

[0106] To replenish the plating bath consumed during hot-dip galvanizing, the steel ingot is further dissolved and supplied. In this case, one can choose to directly immerse the steel ingot in the plating bath and dissolve it, or one can choose to dissolve the steel ingot in a separate tank and then replenish the plating bath with molten metal.

[0107] Furthermore, according to one aspect of the invention, the temperature of the plating bath can be maintained at a temperature 20-80°C higher than the solidification initiation temperature (Ts) on the equilibrium phase diagram. In this case, although not particularly limited, the solidification initiation temperature on the equilibrium phase diagram can be in the range of 390-460°C (more preferably 390-452°C). Alternatively, the temperature of the plating bath can be maintained in the range of 440-520°C (more preferably 450-500°C).

[0108] As the temperature of the plating bath increases, the fluidity within the bath is ensured, resulting in a uniform composition and reducing the amount of scum generated. When the bath temperature is 20°C lower than the solidification initiation temperature on the equilibrium phase diagram (or, the bath temperature is below 440°C), the melting of the steel ingot is very slow, and the bath viscosity is high, making it difficult to ensure excellent coating surface quality. On the other hand, when the bath temperature exceeds 80°C (or, the bath temperature exceeds 520°C) compared to the solidification initiation temperature on the equilibrium phase diagram, problems may arise due to Zn evaporation inducing ash defects on the coated surface. Furthermore, due to excessively high bath temperatures, Fe diffuses excessively, potentially forming too many protruding phases. Therefore, the length of protruding phases intersecting the aforementioned separated lines exceeds 10% of the length of the separated lines, thus potentially reducing corrosion resistance.

[0109] According to one aspect of the invention, the immersion time after immersing the base steel plate in the plating bath can be in the range of 1-10 seconds.

[0110] Furthermore, according to one aspect of the invention, cooling may be included from the surface of the plating bath to the upper roller section using an inert gas at an average cooling rate of 3-30°C / s. In this case, when the cooling rate from the surface of the plating bath to the upper roller section is less than 3°C / s, the MgZn2 structure becomes excessively coarse, and the coating surface may become severely warped. Furthermore, both binary and ternary eutectic structures are formed coarsely, which may be detrimental to ensuring uniform corrosion resistance and processability. On the other hand, when the cooling rate from the surface of the plating bath to the upper roller section exceeds 30°C / s, solidification begins during the hot-dip plating process, occurring rapidly in the temperature range where the liquid phase completely solidifies. This results in the MgZn2 structure becoming too small, potentially leading to locally uneven corrosion resistance. Additionally, insufficient uniform growth of the Fe-Zn-Al phase, concentrated at the interface between the coating and the base steel plate, may worsen processability, and the increased nitrogen usage to achieve the excessively rapid cooling rate may increase manufacturing costs. To further improve the above-mentioned effects, the average cooling rate can more preferably be 3-27°C / second.

[0111] According to one aspect of the invention, the inert gas may include one or more of N2, Ar and He, and N2 or N2+Ar is more preferably used in order to save manufacturing costs.

[0112] Furthermore, according to one aspect of the invention, the cooling rate can be controlled in the cooling step to satisfy the following relations 1-1 and 1-2.

[0113] [Relation 1-1]

[0114] A>2.5 / {ln(t×20)} 1 / 2 ×B

[0115] [Relationship 1-2]

[0116] 0.7×C≤B≤1.2×C

[0117] In Equations 1-1 and 1-2, t is the thickness of the steel plate, A is the average cooling rate (°C / second) from the plating bath temperature to the solidification initiation temperature, B is the average cooling rate (°C / second) from the solidification initiation temperature to -30°C, and C is the average cooling rate (°C / second) from -30°C to 300°C. According to one aspect of the invention, although not particularly limited, A can be in the range of 4-40°C / second.

[0118] In cases where equations 1-1 and 1-2 are not satisfied, if the initial cooling rate is too fast, the size of the MgZn2 phase may become too small, potentially preventing the formation of a single Al phase within the MgZn2 phase, and making it difficult to control the Al single phase within the MgZn2 phase within an appropriate range. Conversely, if the initial cooling rate is too slow, the Al composition may contribute to the formation of a Zn-Al mixed phase, thus preventing the formation of a single Al phase, and making it difficult to control the extent of the Al single phase within the coating within an appropriate range.

[0119] Furthermore, to reduce surface defects in the coating, it is crucial to ensure the uniformity of the coating's solidification structure. To achieve this, uniform initial solidification nuclei must be formed, and it is essential to control the melting temperature and cooling rate of the different coating components. Moreover, as mentioned above, by controlling the cooling rate, the formation of undesirable processable Mg2Si phases at the interface between the inhibition layer and the coating can be suppressed.

[0120] Therefore, in this invention, the cooling step is set with three cooling intervals as described above, and the cooling rate of each interval is controlled to satisfy the relationships 1-1 and 1-2, thereby uniformly forming the initial solidification nuclei and thus reducing surface defects in the final product.

[0121] Specifically, when the steel sheet is drawn from the plating bath and the solidification initiation point is determined in the initial cooling zone, if the cooling rate is too slow to determine the solidification initiation point because it does not satisfy the aforementioned relationship, coarse microstructures begin to form in localized areas, potentially leading to uneven solidification. Therefore, in order to ensure a uniform distribution of solidification nuclei during the cooling step to reduce microstructure differences, it is preferable to control the cooling rate to satisfy the aforementioned relationship, thereby obtaining a coated steel sheet with excellent surface quality.

[0122] In addition, although not particularly limited, according to one aspect of the present invention, the base steel plate can be immersed in a plating bath and hot-dip galvanized before being subjected to air knife treatment to satisfy the following relationship 2.

[0123] [Relationship 2]

[0124] 0.1≤(AK gap × steel plate thickness) / AK pressure≤25

[0125] [In Equation 2, the AK gap represents the blade gap (mm), the steel plate thickness represents the thickness of the steel plate after air knife treatment (mm), and the AK pressure represents the air knife pressure of the nozzle (KPa).]

[0126] Although not specifically limited, according to one aspect of the invention, the air knife gap can be in the range of 5-150 mm. Furthermore, the thickness of the steel plate treated with the air knife can be in the range of 0.2-6 mm. Additionally, the air knife pressure of the nozzle can be in the range of 8-70 kPa.

[0127] By controlling the air knife conditions and / or relation 2 as described above, air knife treatment can be performed under harsh conditions to prevent uncoated areas from appearing on the surface of the coated steel sheet. Furthermore, by facilitating the uniform growth of multiple microstructures during solidification, a uniform coating can be formed, while simultaneously controlling the area ratio of Al single phases contained within the MgZn2 phase relative to the total cross-sectional area of ​​the coating and the area ratio of Al single phases relative to the total cross-sectional area of ​​the coating within appropriate ranges. Therefore, a coated steel sheet with excellent corrosion resistance and superior surface quality can be effectively provided.

[0128] Furthermore, according to one aspect of the invention, although not particularly limited, during the cooling process, cooling is selectively performed in the width direction of the hot-dip galvanized steel sheet so that the ratio (De / Dc) of the baffle opening (De) at the edge portion to the baffle opening (Dc) at the center portion satisfies 60-99%. Here, the "width direction" of the steel sheet refers to a direction perpendicular to the conveying direction of the steel sheet, based on a surface other than the thickness-side surface of the hot-dip galvanized steel sheet (i.e., the surface from which the thickness of the steel sheet is observed). Furthermore, the baffle opening refers to the numerical value of the opening degree of the regulating plate controlling the flow rate of cooling gas supplied from the cooling device to the base steel sheet. This is to ensure uniform cooling capacity according to the width of the steel sheet (described later), by providing baffles in the cooling device such that the total cooling gas input or controlled in the cooling device can be divided into a center portion and an edge portion along the width direction of the base steel sheet for injection. The boundary between the baffles can be divided into three sections according to the width of the base steel plate, and the position can be variably controlled so that the middle section is the center and the two sections on the outer side are the edge sections.

[0129] When cooling conventional hot-dip galvanized steel sheets, maintaining a constant flow rate of cooling gas at the edges and center without adjusting the ratio (De / Dc) presents a problem of difficulty in ensuring uniform microstructure characteristics of the coating surface. In contrast, the present invention, contrary to conventional cooling conditions, achieves uniform cooling capacity across the width of the steel sheet by controlling the ratio (De / Dc) within the range of 60-99%, resulting in a lower baffle opening at the edges compared to the center. That is, the inventors recognized that the edges of the steel sheet expose a larger area to the external atmosphere compared to the center, and therefore the temperature decreases more rapidly in the region corresponding to the edges compared to the center. They discovered that uniform coating surface characteristics can be ensured by artificially reducing the cooling rate at the edges. Specifically, during the aforementioned cooling process, the cooling gas incident on the center naturally escapes from the center through the edges to the outer casing. However, the edges repeatedly contain the cooling gas that was incident on the center, leading to over-cooling compared to the center, which may have negative effects. Therefore, even without the addition of artificial cooling gas, the cooling rate at the edge is faster. Thus, in order to achieve uniform cooling performance in the width direction, a layered double hydroxide (LDH; (Zn,Mg)6Al2(OH) is formed as an initial corrosion product. 16 To increase corrosion resistance, the opening of the baffle at the edge needs to be controlled in the direction where the opening is lower than that at the center.

[0130] At this point, when the ratio (De / Dc) of the baffle opening at the edge to that at the center is less than 60%, the edge cools slower than the center. When the ratio (De / Dc) exceeds 99%, the edge overcools compared to the center, which may hinder the achievement of uniform cooling capacity across the width of the steel plate. Consequently, the microstructure of the coating surface at the edge and center becomes uneven, making it difficult to uniformly form the layered double hydroxide (LDH; (Zn,Mg)6Al2(OH) as an initial corrosion product when exposed to corrosive environments (or prolonged exposure to atmospheric atmosphere). 16 (CO3)·4H2O)).

[0131] Furthermore, although not particularly limited, according to one aspect of the invention, the method may also include a step of removing surface oxides from the base steel plate before plating. In this case, the surface oxides of the base steel plate can be removed by shot peening before plating. Additionally, imparting fine plastic deformation to the surface of the steel plate increases the dislocation density in the iron microstructure of the substrate, thereby activating the plating reaction.

[0132] Furthermore, according to one aspect of the invention, a ball of metal material with a diameter of 0.3-10 μm can be used during the shot peening process.

[0133] According to one aspect of the invention, during the shot peening process, the running speed of the steel plate can be controlled to be 50-150 meters per minute (mpm).

[0134] According to one aspect of the invention, during the shot peening process, the impact of a ball of metallic material on the surface of the steel plate can be controlled at a projection rate of 300-3000 kg / min.

[0135] According to one aspect of the present invention, shot peening can be performed by using a ball of metal material with a diameter of 0.3-10 μm to collide with the ball of metal material traveling at a speed of 50-150 mpm at a speed of 300-3000 kg / min on the surface of a steel plate.

[0136] According to one aspect of the present invention, for the base steel plate before plating, the base steel plate is shot peened before plating to meet the above conditions, thereby activating the surface of the base steel plate so that mechanical dislocations are introduced before surface plating to rapidly and uniformly form an inhibition layer or to more uniformly form solidification nuclei during plating solidification.

[0137] In other words, by meeting the above conditions during shot peening, it is possible to prevent problems such as poor machinability caused by the formation of a rough structure due to harsh shot peening, or reduced surface uniformity due to low activation of the base steel plate surface before plating caused by insufficient shot peening.

[0138] Therefore, by shot peening the base steel plate before coating and optimizing the shot peening conditions, it is easy to manufacture coated steel plates that meet one or more of the conditions of Ra, Rz, cross-sectional hardness and thickness of the coating within the specific range mentioned above. This results in coated steel plates with excellent surface quality, which not only have excellent corrosion resistance and workability, but also excellent uniformity and suppression of the formation of uncoated areas. Detailed Implementation

[0139] (Example)

[0140] The present invention will now be described in more detail through embodiments. However, it should be noted that the following embodiments are merely illustrative and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the contents of the claims and the contents reasonably deduced therefrom.

[0141] (Experimental Example 1)

[0142] A base steel sheet with a composition of C: 0.025%, Si: 0.03%, Mn: 0.15%, P: 0.01%, S: 0.003%, Al: 0.03%, with the balance being Fe and other unavoidable impurities, is immersed in a plating bath that meets the conditions specified in Table 1 below to obtain a hot-dip galvanized steel sheet. In a portion of the cooling zone, the hot-dip galvanized steel sheet is cooled using an inert gas to ensure that the cooling rate from the surface of the plating bath to the upper roller section meets the conditions described in Table 1 below.

[0143] [Table 1]

[0144]

[0145] Ts*: Solidification initiation temperature on the equilibrium phase diagram

[0146] t*: Thickness of the steel plate [mm]

[0147] A*: Average cooling rate from plating bath temperature to the initial solidification temperature of the plating [°C / second]

[0148] B*: Average cooling rate from the initial solidification temperature of the plating to 30°C below the initial solidification temperature of the plating [°C / second]

[0149] C*: Average cooling rate from the initial solidification temperature of the plating (-30°C) to 300°C [°C / second]

[0150] Furthermore, for the aforementioned coated steel sheet, the coating was dissolved in hydrochloric acid solution, and the dissolved liquid was analyzed using wet chromatography (ICP) to measure the composition of the coating. Additionally, cross-sectional specimens cut perpendicular to the rolling direction of the steel sheet were prepared to observe the interface between the coating and the base iron. After preparing the cross-sectional specimens, SEM images were taken to confirm the formation of the base steel sheet, the Zn-Mg-Al coating, and the Fe-Al inhibitory layer between the base steel sheet and the Zn-Mg-Al coating. The cross-sectional specimens of this coated steel sheet, magnified at 1000x, were photographed using FE-SEM. Figure 4 As an example, directly applying the above... Figure 8 The measurement method was used to measure the length occupied by the protruding phase. Furthermore, the number of Mg2Si alloy phases with a major axis greater than 500 nm forming at the interface between the suppression layer and the coating per 100 μm of interface length was measured. In addition, the characteristics of each example were evaluated according to the following criteria.

[0151] <Corrosion Resistance>

[0152] To evaluate corrosion resistance, the test was conducted using a salt spray tester based on ISO 14993 and according to the following standards.

[0153] ◎: Compared to a Zn plating of the same thickness, it takes more than 30 times longer for red rust to form.

[0154] ○: Compared to a Zn coating of the same thickness, the time required for red rust to form is more than 20 times but less than 30 times longer.

[0155] △: Compared to a Zn coating of the same thickness, the time required for red rust to form is more than 10 times but less than 20 times.

[0156] X: Compared to a Zn coating of the same thickness, the time required for red rust to form is less than 10 times longer.

[0157] <Uniformity>

[0158] To evaluate uniformity, a cross-sectional image of the coating was taken using a SEM apparatus and backscattering interferometry (BSI) in backscattering mode to identify the phases within the coating. Five images were taken at arbitrary locations with a length of 600 μm. The lengths of the regions where MgZn2 crystals with an equivalent circular diameter greater than 5 μm did not form were then measured and evaluated according to the following criteria.

[0159] ◎: The length of the region where MgZn2 crystals with an equivalent circular diameter of 5 μm or more do not form is less than 100 μm.

[0160] ○: The length of the region where MgZn2 crystals with an equivalent circular diameter of 5 μm or more do not form is 100 μm or more but less than 200 μm.

[0161] △: The length of the region where MgZn2 crystals with an equivalent circular diameter of 5 μm or more do not form is greater than 200 μm and less than 300 μm.

[0162] X: The length of the region where MgZn2 crystals with an equivalent circular diameter of 5 μm or more did not form is 300 μm or more.

[0163] <Flexibility>

[0164] To evaluate the bendability, a 3T bend was performed using a bend test apparatus, and the average width of the coating crack at the bend location was calculated, and the evaluation was carried out according to the following criteria.

[0165] ◎: The average width of the coating crack after 3T bending is less than 30μm.

[0166] ○: The average width of the coating cracks after 3T bending is greater than 30μm and less than 50μm.

[0167] △: The average width of the coating cracks after 3T bending is greater than 50μm and less than 100μm.

[0168] X: The average width of the coating cracks after 3T bending is over 100μm.

[0169] The evaluation results of the above measurements and characteristics are shown in Table 2 below.

[0170] [Table 2]

[0171]

[0172] Lo*: When the interface line of the base steel plate is separated from the coating surface by 5 μm, the proportion (%) of the length of the protruding phase that intersects the separated line.

[0173] Na*: The amount of Mg2Si alloy phase with a major axis greater than 500 nm forming at the interface between the suppression layer and the coating per 100 μm of interface length.

[0174] As can be seen from Tables 1 and 2, in Examples 1 to 6, which meet all the composition and manufacturing conditions of the coating according to the present invention, the corrosion resistance, uniformity and flexibility characteristics are found to be excellent compared with those in Examples 7 to 14, which do not meet one or more of the composition and manufacturing conditions of the coating.

[0175] In addition, for the coated steel sheet manufactured by Example 1, a cross-sectional specimen cut in a direction perpendicular to the rolling direction of the steel sheet was manufactured to observe the entire coating and the base iron at the same time. Figure 1 The image shows a photograph of the cross-sectional specimen taken at 500x magnification using FE-SEM. This confirms the formation of an Fe-Al based inhibition layer and a Zn-Al-Mg based coating on the base steel plate.

[0176] also, Figure 2 The image shown is a FE-SEM photograph of a cross-sectional specimen cut using the same method as described above, taken from a galvanized steel sheet manufactured in Example 4 and magnified at 2000x.

[0177] also, Figure 3 The image shows a photograph of the surface of the coated steel sheet manufactured by Example 2, observed at 1000x magnification using FE-SEM.

[0178] (Experimental Example 2)

[0179] Except for the additional conditions of satisfying the air knife (AK) gap, steel plate thickness, and air knife pressure in Table 3 below, the coated steel plate was manufactured using the same method as in Experimental Example 1 above. At this time, using the same analytical method as in Experimental Example 1, it was confirmed that a Zn-Al-Mg coating and an Fe-Al inhibition layer were formed on the base steel plate.

[0180] [Table 3]

[0181]

[0182] For the coated steel manufactured according to the examples in Table 3, the area ratio of the Al single phase contained within the MgZn2 phase relative to the total cross-sectional area of ​​the coating was measured. At this time, the Al single phase contained within the MgZn2 phase was measured using the method described above in this specification, and analyzed as follows... Figure 7 The images shown are of a cross-section of the coated steel sheet taken using a field emission scanning electron microscope (FE-SEM) and compositional mapping performed using an electron probe microanalysis (EPMA) system to observe the distribution of Mg and Al components, distinguishing between MgZn2 and Al single phases for measurement. Furthermore, the thickness of the suppression layer is the minimum thickness measured in the direction perpendicular to the interface using SEM and TEM.

[0183] [Table 4]

[0184]

[0185] Ne*: The area ratio of Al single phase contained within the MgZn2 phase relative to the total cross-sectional area of ​​the coating.

[0186] In addition, for the experimental examples in Table 4 above, observations were made on the percentage of 5000 μm. 2 Whether the Al single phase contained within the MgZn2 phase in the cross-sectional area of ​​the coating exists as follows, and is shown as ○ and X in Table 5 below. At this time, the presence of each phase contained in the coating is evaluated using the FE-SEM photographs described above and the composition mapping results from EPMA.

[0187] (1) Al single phase contained within and entirely contained in the MgZn2 phase

[0188] (2) Al single phase, partly contained within the MgZn2 phase and partly protruding outside the MgZn2 phase.

[0189] (3) A single Al phase entirely contained within a mixed phase of Al and Zn, wherein the mixed phase of Al and Zn is entirely contained within a MgZn2 phase.

[0190] (4) A single Al phase entirely contained in a mixed phase of Al and Zn, wherein a portion of the mixed phase of Al and Zn is contained within the MgZn2 phase and a portion protrudes beyond the MgZn2 phase.

[0191] (5) A single Al phase partially contained in the mixed phase of Al and Zn, and all of the single Al phase is contained within the MgZn2 region, wherein a portion of the mixed phase of Al and Zn is contained within the MgZn2 phase and a portion protrudes outside the MgZn2 phase.

[0192] (6) A single Al phase partially contained in a mixed phase of Al and Zn, wherein a portion of the single Al phase is contained within the MgZn2 region and a portion protrudes outside the MgZn2 region, wherein a portion of the mixed phase of Al and Zn is contained within the MgZn2 phase and a portion protrudes outside the MgZn2 phase.

[0193] [Table 5]

[0194]

[0195] Specifically, for Embodiment 8, the X-ray diffraction (XRD) measurement results of the coating are shown in... Figure 5 At this point, it was confirmed that the diffraction intensity ratio I(200) / I(111), which is the ratio of the X-ray diffraction intensity I(200) of the (200) plane of the Al single phase to the X-ray diffraction intensity I(111) of the (111) plane of the Al phase, is less than 0.8.

[0196] Furthermore, the characteristics of Examples 5 to 22 above were evaluated and are shown in Table 6 below. In this case, corrosion resistance, uniformity, and flexibility were evaluated using the same method as in Experimental Example 1 above, and the presence or absence of uncoated areas was evaluated according to the following criteria.

[0197] <Whether uncoated areas are generated>

[0198] ◎: No uncoated areas occurred.

[0199] ○: There are 1-3 uncoated areas.

[0200] △: There are 4 or more uncoated areas.

[0201] [Table 6]

[0202]

[0203] As can be seen from Tables 3 to 6, in the case of Examples 5 to 21 of the present invention, which satisfy all the composition and manufacturing conditions of the coating of the present invention, the uniformity, whether uncoated occurs, and the bending properties are superior compared with Example 22, which does not satisfy the coating conditions.

[0204] In particular, it was confirmed that in the cases of Examples 16, 17, 19, and 21 of the present invention that satisfy the conditions of Relation 2, one or more of the characteristics of uniformity, whether unplated areas are generated, and flexibility are superior compared with Examples 15, 18, and 20 that do not satisfy Relation 2.

[0205] (Experimental Example 3)

[0206] Except for shot peening to remove surface oxides on the same base steel plate as in Experimental Example 1, followed by plating, the plated steel plate was manufactured using the same method as in Experimental Example 2. At this time, it was confirmed that an Fe-Al based inhibition layer and a Zn-Al-Mg based coating were formed on the base steel plate using the same method as in Experimental Example 1.

[0207] [Table 7]

[0208]

[0209] mpm*: meters per minute

[0210] mpm*: meters per minute

[0211] Using the same measurement methods as in Experimental Example 1 and Example 2 described above, the results are shown in Tables 8 and 9 below. In Table 9, Ra represents the surface roughness measured using a two-dimensional surface roughness measuring device, and Rz represents the roughness measured using the KS B 0161 measurement method. Furthermore, the cutoff value for roughness measurement was set at 2.5 μm. Additionally, the hardness of the coating cross-section was measured using a microhardness measuring device capable of measuring within the coating thickness, with the coating cross-section as a reference.

[0212] [Table 8]

[0213]

[0214] The properties of the galvanized steel sheets manufactured by Examples 23 to 36 above were evaluated using the same method as in Experimental Example 2 above, and are shown in Table 10 below.

[0215] [Table 9]

[0216]

[0217] [Table 10]

[0218]

[0219] As can be seen from Tables 8 to 10, in the cases of Examples 23 to 34 of the present invention, which satisfy all the composition and manufacturing conditions of the coating of the present invention, the uniformity, whether uncoated occurs, and bending properties are superior compared with those of Examples 35 and 36, which do not satisfy the coating conditions or the temperature conditions of the plating bath.

[0220] In particular, in Examples 24, 26, 28, 30, 32 and 34, which meet all the shot peening conditions of using a ball of metal material with a diameter of 0.3-10 μm and colliding with the ball of metal material at a speed of 300-3000 kg / min on the surface of a steel plate traveling at a speed of 50-150 mpm, it was found that one or more of the characteristics of uniformity, absence of uncoated areas and flexibility are superior compared with Examples 23, 25, 27, 29, 31 and 33, which do not meet one or more of the above shot peening conditions.

[0221] (Experimental Example 4)

[0222] Except for changing the manufacturing conditions to meet the requirements of Table 11 below, and setting the average baffle opening of the edge and center of the steel plate in the width direction as shown in Table 12 below, based on the surface of the hot-dip galvanized steel plate during cooling, the experiment was conducted under the same conditions as in Experimental Example 1.

[0223] [Table 11]

[0224]

[0225] [Table 12]

[0226] Remark serial number De* Dc* De / Dc Example 37 K 62 99 63 Example 38 L 66 100 66 Example 39 M 60 98 61 Example 40 N 98 99 101

[0227] De*: Average baffle opening at the edge [%)

[0228] Dc*: Average baffle opening at the center [%)

[0229] De*: Average baffle opening at the edge [%)

[0230] Dc*: Average baffle opening at the center [%)

[0231] Test pieces of the aforementioned coated steel sheet were prepared, and the coating was dissolved in a hydrochloric acid solution. The composition of the coating was then measured by analyzing the dissolved liquid using an intraluminal chemical chromatography (ICP) method, thereby confirming that the coating composition met the requirements of the present invention. Furthermore, cross-sectional test pieces cut perpendicular to the rolling direction of the steel sheet were prepared to observe the interface between the coating and the base iron, and then photographed using SEM, thereby confirming the formation of a base steel sheet, a Zn-Mg-Al coating, and an Fe-Al inhibitory layer between the base steel sheet and the Zn-Mg-Al coating.

[0232] For the coated surface specimens obtained from the various embodiments and comparative examples, the characteristics were evaluated according to the following criteria, and the evaluation results of the characteristics are shown in Table 13 below.

[0233] <Plate Corrosion Resistance>

[0234] To evaluate the corrosion resistance of the plate, a salt spray test apparatus (SST) was used, and the evaluation was conducted according to the following standards based on the test method of ISO 14993.

[0235] ◎: Compared to a Zn plating of the same thickness, the time required for red rust to form is more than 40 times longer.

[0236] ○: Compared to a Zn coating of the same thickness, the time required for the formation of red rust is more than 30 times but less than 40 times longer.

[0237] △: Compared to a Zn coating of the same thickness, the time required for red rust to form is more than 20 times but less than 30 times longer.

[0238] X: Compared to a Zn coating of the same thickness, the time required for red rust to form is less than 20 times longer.

[0239] <Corrosion resistance of bends>

[0240] To evaluate the corrosion resistance of the bent portion, a salt spray test apparatus (SST) was used, and the evaluation was conducted according to the test method based on ISO 14993. The corrosion resistance evaluation specimens were subjected to a 90° bend with the same material thickness and the same coating weight.

[0241] ◎: Compared to a Zn plating layer of the same thickness, the time required for red rust to form is more than 30 times longer.

[0242] ○: Compared to a Zn coating of the same thickness, the time required for red rust to form is more than 20 times but less than 30 times longer.

[0243] △: Compared to a Zn coating of the same thickness, the time required for red rust to form is more than 10 times but less than 20 times.

[0244] X: Compared to a Zn coating of the same thickness, the time required for red rust to form is less than 10 times longer.

[0245] <Scattering reflectivity>

[0246] Test pieces were collected at positions 1 / 4, center, 3 / 4, and edge along the width of the hot-dip galvanized steel sheet. To evaluate the amount of scattered reflected light relative to the total reflection of each test piece, the evaluation was conducted using a test method based on ISO 9001, based on the type of light reflected when light in the visible wavelength band (400-800nm) is incident on the integrating sphere.

[0247] ◎: The proportion of scattered reflectance relative to the average total reflectance in the width direction exceeds 80%, and the deviation of scattered reflectance in the width direction is less than 10%.

[0248] ○: The proportion of scattered reflectance relative to the average total reflectance in the width direction is 70% or more but less than 80%, and the deviation of scattered reflectance in the width direction is 10% or more.

[0249] △: The proportion of scattered reflectance relative to the average total reflectance in the width direction is greater than 60% and less than 70%, and the deviation of scattered reflectance in the width direction is greater than 10%.

[0250] X: The proportion of scattered reflectance relative to the average total reflectance in the width direction is less than 60%, and the deviation of scattered reflectance in the width direction is greater than 10%.

[0251] For the galvanized steel sheets obtained from Examples 37 to 40, the types of corrosion products initially formed on the surface and the time of formation of LDH corrosion products were measured using an EDS or XRD device, and are shown in Table 13 below.

[0252] [Table 13]

[0253]

[0254] De*: Average baffle opening at the edge [%)

[0255] Dc*: Average baffle opening at the center [%)

[0256] De*: Average baffle opening at the edge [%)

[0257] Dc*: Average baffle opening at the center [%)

[0258] As can be seen from Table 13, in Examples 37 to 39, which satisfy all the coating compositions and manufacturing conditions of the present invention, it was confirmed that LDH was initially formed on the surface of the coated steel sheet during the corrosion resistance evaluation test. This confirmed that the corrosion resistance of the flat section and the bending section was further improved, and the scattering reflectance of the steel sheet surface was slightly higher, thus exhibiting excellent surface quality.

[0259] On the other hand, in Example 40 where the cooling conditions of the present invention are not met, it was confirmed that chlorinated zinc ore initially formed on the surface of the coated steel sheet during the corrosion resistance evaluation test. As a result, not only is the corrosion resistance of the flat plate of the coated steel sheet poor, but the corrosion resistance of the bent part is also slightly poor. In addition, the scattering reflectivity is also slightly low, thus confirming poor surface quality.

Claims

1. A galvanized steel sheet, comprising: Foundation steel plate; A Zn-Mg-Al coating is disposed on at least one side of the base steel plate; as well as An Fe-Al based suppression layer is disposed between the base steel plate and the Zn-Mg-Al based coating. The coating, by weight percent, comprises: Mg: 4% or more, Al: 2.1 times or more and less than 14.2% of the Mg content, Si: less than 0.2% and less than 0%, Sn: less than 0.1% and less than 0%, with the balance being Zn and unavoidable impurities. In a cross-section along the thickness direction of the steel plate, when the interface line of the base steel plate is separated by 5 μm towards the coating surface, the length of the protruding phase intersecting the separated line is less than 10% of the length of the separated line. The alloy phase of the protruding phase comprises one or more of Fe2Al5, FeAl and Fe-Zn systems, and contains more than 20% Zn by weight.

2. The galvanized steel sheet according to claim 1, wherein, The Fe content of the protruding phase is 10-45% by weight.

3. The galvanized steel sheet according to claim 1, wherein, The cross-sectional hardness of the coating is 200-450 Hv.

4. The plated steel sheet according to claim 3, wherein, The number of Mg2Si phases with a major axis of 500 nm or more in contact with the interface between the coating and the inhibition layer is less than 10 per 100 μm.

5. The coated steel sheet according to claim 1, wherein, The Si content of the coating is less than 0.01%.

6. The galvanized steel sheet according to claim 1, wherein, The Sn content of the coating is less than 0.09%.

7. The plated steel sheet according to claim 6, wherein, The Sn content of the coating is less than 0.05%.

8. The galvanized steel sheet according to claim 1, wherein, The Fe content of the coating is less than 1%.

9. The galvanized steel sheet according to claim 1, wherein, The thickness of the inhibition layer is greater than 0.02 μm and less than 2.5 μm.

10. The galvanized steel sheet according to claim 1, wherein, The sum of the areas of the Al single phase contained within the MgZn2 phase relative to the total cross-sectional area of ​​the coating is 0.5-10% of the total area.

11. The galvanized steel sheet according to claim 10, wherein, All or part of the Al single phase is located inside the MgZn2 phase.

12. The galvanized steel sheet according to claim 11, wherein, The Al single phase contained within the MgZn2 phase is at least one of the following Al single phases: - Al single phase contained within and entirely within the MgZn2 phase - A single Al phase, partly contained within the MgZn2 phase and partly protruding outside the MgZn2 phase. - A single Al phase entirely contained within a mixed phase of Al and Zn, wherein the mixed phase of Al and Zn is entirely contained within a MgZn2 phase. - A single Al phase entirely contained within a mixed phase of Al and Zn, wherein a portion of the mixed phase of Al and Zn is contained within the MgZn2 phase and a portion protrudes beyond the MgZn2 phase. - A portion of the Al single phase is contained within the mixed phase of Al and Zn, and all of the Al single phase is contained within the MgZn2 region, wherein a portion of the mixed phase of Al and Zn is contained within the MgZn2 phase and a portion protrudes outside the MgZn2 phase. - A portion of the Al single phase is contained in the mixed phase of Al and Zn, and a portion of the Al single phase is contained within the MgZn2 region and a portion protrudes outside the MgZn2 region, wherein a portion of the mixed phase of Al and Zn is contained within the MgZn2 phase and a portion protrudes outside the MgZn2 phase.

13. The galvanized steel sheet according to claim 11, wherein, The Al single phase comprises, by weight percent: Al: 40-70%, balance Zn and other unavoidable impurities.

14. The galvanized steel sheet according to claim 11, wherein, In the coating, the proportion of Al single phase relative to the entire cross-section of the coating, by area fraction, is 1-15%.

15. The galvanized steel sheet according to claim 1, wherein, The surface roughness Ra of the coating is 0.5-3.0 μm.

16. The galvanized steel sheet according to claim 1, wherein, The surface roughness Rz of the coating is 1-20 μm.

17. The galvanized steel sheet according to claim 1, wherein, The thickness of the coating is 5-100 μm.

18. The galvanized steel sheet according to claim 1, wherein, The diffraction intensity ratio I(200) / I(111), which is the ratio of the X-ray diffraction intensity I(200) of the (200) plane of Al to the X-ray diffraction intensity I(111) of the (111) plane of Al, is 0.8 or less.

19. The galvanized steel sheet according to claim 1, wherein, In atmospheric and ISO 14993 chloride atmospheres, compared to zinc hydrate ore (Zn5(OH)8Cl2) and zinc brine ore (Zn5(OH)6(CO3)2), LDH((Zn,Mg)6Al2(OH) 16 (CO3)·4H2O is first formed on the surface of the Zn-Mg-Al coating.

20. The galvanized steel sheet according to claim 1, wherein, In both atmospheric and ISO 14993 chloride atmospheres, LDH((Zn,Mg)6Al2(OH) is formed on the surface of the Zn-Mg-Al coating within 6 hours in atmospheric atmosphere. 16 (CO3)·4H2O) forms LDH((Zn,Mg)6Al2(OH) on the surface of the Zn-Mg-Al coating within 5 minutes in a chloride atmosphere according to ISO 14993. 16 (CO3)·4H2O).

21. The galvanized steel sheet according to claim 1, wherein, In a chloride atmosphere including salt spray and immersion atmosphere, the time required for red rust to form in a flat section is 40-50 times longer than that for a Zn coating of the same thickness, and the time required for red rust to form in a 90-degree bending section is 20-30 times longer.

22. A method for manufacturing coated steel sheet, comprising the following steps: The base steel plate is immersed in a galvanizing bath and hot-dip galvanized. The galvanizing bath, by weight percent, contains: Mg: 4% or more, Al: 2.1 times the Mg content and less than 14.2%, Si: less than 0.2% and less than 0%, Sn: less than 0.1% and less than 0%, with the balance being Zn and unavoidable impurities. The galvanizing bath is maintained at a temperature 20-80°C higher than the solidification initiation temperature on the equilibrium phase diagram. Cooling begins from the surface of the plating bath, using inert gas at an average cooling rate of 3-30°C / second to cool to the upper roller area. In the cooling step, the cooling rate is controlled to satisfy the following relationships 1-1 and 1-2. [Relation 1-1] A>2.5 / {ln(t×20)} 1 / 2 ×B [Relationship 1-2] 0.7×C≤B≤1.2×C In Equations 1-1 and 1-2, t is the thickness of the steel plate, A is the average cooling rate from the plating bath temperature to the solidification initiation temperature, where the unit of cooling rate is °C / second, B is the average cooling rate from the solidification initiation temperature to the solidification initiation temperature -30 °C, where the unit of cooling rate is °C / second, and C is the average cooling rate from the solidification initiation temperature -30 °C to 300 °C, where the unit of cooling rate is °C / second.

23. The method for manufacturing coated steel sheet according to claim 22, wherein, After the hot-dip galvanizing step, an air knife treatment is performed to satisfy the following relationship 2: [Relationship 2] 0.1≤(AK gap × steel plate thickness) / AK pressure≤25 In Equation 2, the AK gap represents the blade gap, where the unit of the blade gap is mm; the thickness of the steel plate represents the thickness of the steel plate including all the base steel plate, the coating and the inhibition layer, where the unit of the steel plate thickness is mm; and the AK pressure represents the air knife pressure of the nozzle, where the unit of the air knife pressure is kPa.

24. The method for manufacturing coated steel sheet according to claim 22, wherein, Prior to the hot-dip galvanizing step, a shot peening process is also included to remove surface oxides from the base steel plate. The shot peening process is carried out by using a ball of metal material with a diameter of 0.3-10 μm to collide with the surface of a steel plate traveling at a speed of 50-150 mpm at a speed of 300-3000 kg / min.

25. The method for manufacturing coated steel sheet according to claim 22, wherein, The cooling step is performed to ensure that the ratio of the baffle opening De at the edge to the baffle opening Dc at the center, De / Dc, satisfies 60-99%.

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