Steel sheet with excellent plating quality and method for manufacturing the same

Abstract

The present invention relates to a hot-dip galvanized steel sheet having excellent plating quality, a steel sheet for plating for manufacturing the same, and methods for manufacturing them. The steel sheet for plating according to one aspect of the present invention has a composition containing, by weight%, Mn: 0.1 to 1.0%, Si: 0.1% or less (excluding 0%), C: 0.0005 to 0.03%, Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, Cr: 1.5% or less, B: 0.005% or less, and the balance Fe and inevitable impurities. The GDS profile of the Mn component and the GDS profile of the Si component observed in the depth direction from the surface each sequentially include a maximum point and a minimum point. The difference (converted Mn concentration difference) between the value obtained by dividing the Mn concentration at the maximum point of the GDS profile of the Mn component by the Mn concentration of the base material and the value obtained by dividing the Mn concentration at the minimum point of the GDS profile of the Mn component by the Mn concentration of the base material is 80% or more, and the difference (converted Si concentration difference) between the value obtained by dividing the Si concentration at the maximum point of the GDS profile of the Si component by the Si concentration of the base material and the value obtained by dividing the Si concentration at the minimum point of the GDS profile of the Si component by the Si concentration of the base material is 50% or more. It may be such. However, when no minimum point appears within a depth of 5 μm, the point at a depth of 5 μm is taken as the point where the minimum point appears.

Description

[Technical Field]

[0001] This invention relates to hot-dip galvanized steel sheets with excellent plating quality, steel sheets for plating used to manufacture the same, and methods for manufacturing them. [Background technology]

[0002] Automotive body panels are often hot-dip galvanized to ensure corrosion resistance, but problems with the plating may occur for the following reasons.

[0003] In other words, the quality of hot-dip galvanizing is determined by the surface condition of the annealed steel sheet immediately before plating. However, the plating quality deteriorates due to the formation of surface oxides during annealing caused by elements such as Mn, Si, Al, Cr, and B added to ensure the physical properties of the steel sheet. Specifically, during the annealing process, these elements diffuse to the surface and react with trace amounts of oxygen or water vapor present in the annealing furnace, forming individual or combined oxides of these elements on the surface of the steel sheet, thereby reducing the surface reactivity. The surface of the annealed steel sheet with reduced reactivity hinders the wettability of the hot-dip galvanizing bath, resulting in unplated areas where the plating metal does not adhere locally or entirely to the surface of the plated steel sheet. Furthermore, these oxides lead to insufficient formation of the alloying inhibitory layer (Fe2Al5) necessary for ensuring adhesion of the plating layer during the hot-dip galvanizing process, causing peeling of the plating layer and significantly reducing the plating quality of the plated steel sheet.

[0004] Various technologies have been proposed to improve the plating quality of hot-dip galvanized steel sheets. Among them, Patent Document 1 presents a technology that provides hot-dip galvanized or alloyed hot-dip galvanized steel sheets with excellent plating quality by controlling the air-fuel ratio of air to fuel to 0.80-0.95 during the annealing process, oxidizing the steel sheet in a direct flame furnace with an oxidizing atmosphere, forming iron oxides containing Si, Mn, or Al alone or in combination to a certain depth inside the steel sheet, then reducing and annealing the iron oxides in a reducing atmosphere, and finally performing hot-dip galvanizing.

[0005] As described in Patent Document 1, when using a post-oxidation reduction method in the annealing process, components with a high affinity for oxygen, such as Si, Mn, and Al, undergo internal oxidation at a certain depth from the surface of the steel sheet, suppressing their diffusion to the surface. As a result, the amount of Si, Mn, or Al oxides, either individually or in combination, decreases relatively on the surface, improving wettability with zinc and reducing the amount of unplated material. However, in the case of steel types with added Si, Si concentrates directly beneath the iron oxide during the reduction process, forming band-shaped Si oxides. This leads to delamination in the surface layer including the plating layer, i.e., delamination occurs at the interface between the reduced iron and the underlying base iron, making it difficult to ensure adhesion of the plating layer.

[0006] On the other hand, as yet another method for improving the plating properties of hot-dip galvanized steel sheets, Patent Document 2 presents a method for improving plating properties by maintaining a high dew point in the annealing furnace and causing internal oxidation of easily oxidized alloy components such as Mn, Si, and Al within the steel, thereby reducing the amount of oxides externally oxidized on the surface of the steel sheet after annealing. However, while the method according to Patent Document 2 can solve the problem of plating properties due to external oxidation of Si, which is easily oxidized internally, it has the problem that its effect is minimal when a large amount of Mn, which is relatively difficult to oxidize internally, is added.

[0007] Furthermore, even if plating properties are improved by internal oxidation, problems may arise such as the occurrence of linear unplated areas due to unevenly formed surface oxides, or, when alloyed hot-dip galvanized steel sheets (GA steel sheets) are manufactured by alloying heat treatment after plating, linear defects due to uneven alloying may occur on the surface of the alloyed hot-dip galvanized steel sheets.

[0008] Another conventional technique involves pre-plating with Ni before annealing to suppress the diffusion of alloying elements to the surface during annealing. However, while this method is effective in suppressing Mn diffusion, it has the problem of not being able to sufficiently suppress Si diffusion. [Prior art documents] [Patent Documents]

[0009] [Patent Document 1] Korean Patent Publication No. 2010-0030627 [Patent Document 2] Korean Patent Publication No. 2009-0006881 [Overview of the project] [Problems that the invention aims to solve]

[0010] According to one aspect of the present invention, a hot-dip galvanized steel sheet with excellent plating quality, in which no unplated areas occur and the plating layer peels off, and a method for manufacturing the same are provided.

[0011] According to another aspect of the present invention, a hot-dip galvanized steel sheet and a method for manufacturing the same are provided, which can be manufactured into an alloyed hot-dip galvanized steel sheet with excellent surface quality because linear defects do not occur even when alloying heat treatment is applied after plating.

[0012] According to yet another aspect of the present invention, a steel sheet for plating and a method for manufacturing the same are provided, which can produce a hot-dip galvanized steel sheet having such excellent plating quality.

[0013] The problems addressed by the present invention are not limited to those described above. Anyone with ordinary skill in the art to which the present invention pertains should have no difficulty understanding the further problems addressed by the present invention from the overall provisions of the specification. [Means for solving the problem]

[0014] A plated steel sheet according to one aspect of the present invention has a composition in weight percent of Mn: 0.1~1.0%, Si: 0.1% or less (excluding 0%), C: 0.0005~0.03%, Al: 0.005~3.0%, P: 0.04% or less, S: 0.015% or less, Cr: 1.5% or less, B: 0.005% or less, with the remainder being Fe and unavoidable impurities, and the GDS profile of the Mn component and the GDS profile of the Si component observed in the depth direction from the surface each contain sequentially maximum and minimum points, and the above Mn The difference between the Mn concentration at the maximum point of the component's GDS profile divided by the base material's Mn concentration and the Mn concentration at the minimum point of the Mn component's GDS profile divided by the base material's Mn concentration (the converted concentration difference of Mn) is 80% or more, and the difference between the Si concentration at the maximum point of the Si component's GDS profile divided by the base material's Si concentration and the Si concentration at the minimum point of the Si component's GDS profile divided by the base material's Si concentration (the converted concentration difference of Si) is 50% or more.

[0015] However, if no local minimum appears within a depth of 5 μm, the point at a depth of 5 μm will be considered the point where the local minimum appeared.

[0016] Another aspect of the present invention, the hot-dip galvanized steel sheet, may include the above-described plating steel sheet and a hot-dip galvanized layer formed on the plating steel sheet.

[0017] Another aspect of the present invention is a method for manufacturing a steel sheet for plating, which may include the steps of: preparing a base iron having a composition in weight percent of Mn: 0.1~1.0%, Si: 0.1% or less (excluding 0%), C: 0.0005~0.03%, Al: 0.005~3.0%, P: 0.04% or less, S: 0.015% or less, Cr: 1.5% or less, B: 0.005% or less, with the remainder being Fe and unavoidable impurities; electroplating the base iron to form an Fe plating layer containing 5~50% by weight of oxygen; and annealing the base iron with the Fe plating layer formed on it in an annealing furnace with a 1~70% H2-remaining N2 gas atmosphere controlled to a dew point temperature of -15~+30°C, maintaining it at 600~950°C for 5~120 seconds.

[0018] Another aspect of the present invention, a method for manufacturing a hot-dip galvanized steel sheet, comprises the steps of: preparing a base iron having a composition containing, by weight%, Mn: 0.1 to 1.0%, Si: 0.1% or less (excluding 0%), C: 0.0005 to 0.03%, Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, Cr: 1.5% or less, B: 0.005% or less, and the balance being Fe and inevitable impurities; electroplating the base iron to form an Fe plating layer containing 5 to 50% by weight of oxygen; annealing the base iron with the Fe plating layer formed therein in an annealing furnace in an atmosphere of 1 to 70% H2 - the balance being N2 gas, controlled at a dew point temperature of -15 to +30°C, for 5 to 120 seconds at 600 to 950°C to obtain a plated steel sheet; and dipping the plated steel sheet into a hot-dip galvanizing bath maintained in a temperature range of 440 to 500°C, which consists of Al: 0.1 to 0.3% and the balance being Zn and inevitable impurities.

Advantages of the Invention

[0019] As described above, the present invention can provide a hot-dip galvanized steel sheet that significantly improves the phenomenon of unplated areas during hot-dip galvanizing and enhances plating adhesion by forming a pre-plated layer and controlling the concentration profiles of internal Mn and Si components.

[0020] Further, according to one aspect of the present invention, even when an alloying heat treatment is performed on the hot-dip galvanized steel sheet of the present invention, linear defects and the like can be prevented on the surface of the obtained alloyed hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet with excellent surface quality can be provided.

Brief Description of the Drawings

[0021] [Figure 1] Schematic diagram of a GDS profile measured after removing the plating layer of a hot-dip galvanized steel sheet manufactured from an Fe electroplated cold-rolled steel sheet. [Figure 2] Schematic diagram of the process of annealing a base iron with an Fe plating layer containing oxygen in an atmosphere with a high dew point.

Embodiments for Carrying Out the Invention

[0022] The following describes in detail a hot-dip galvanized steel sheet with excellent plating quality, which relates to one aspect of the present invention and was completed through the inventor's research. It should be noted that when indicating the concentration of each element in this invention, unless otherwise specified, it refers to weight percent. Furthermore, the Fe electroplating amount is measured as the total amount of Fe contained in the plating layer per unit area, and does not include oxygen and unavoidable impurities within the plating layer.

[0023] Furthermore, unless otherwise defined, the concentrations and concentration profiles referred to in this invention mean concentrations and concentration profiles measured using GDS, i.e., glow discharge optical emission spectrometer.

[0024] The present invention will be described in detail below.

[0025] It is known that the cause of unplated areas and reduced plating adhesion in steel sheets containing Mn and Si is due to surface oxides formed when alloying elements such as Mn and Si oxidize on the surface during the annealing process of cold-rolled steel sheets at high temperatures.

[0026] To suppress the diffusion of alloying elements such as Mn and Si to the surface, methods can be used to form an oxide layer containing a large amount of oxygen, such as a redox method in which oxidation occurs during heating and then reduction is maintained again in a reducing atmosphere, or a method in which iron oxide is coated onto the surface of the base metal and then heat-treated. However, the iron oxide firmly formed on the surface of the base metal contains not only FeO but also Fe3O4 and Fe2O3, which are difficult to reduce. While the surface is reduced to metallic iron during the annealing process in a reducing atmosphere, the interface between the iron oxide layer and the base iron is difficult to completely reduce due to the slow reduction rate. As a result, Mn and Si oxides accumulate at the interface, forming a continuous oxide layer. Although wettability with molten zinc is improved, the oxide layer becomes brittle, and there is a possibility that the plating layer will peel off.

[0027] On the other hand, when applying the internal oxidation annealing method, which involves increasing the oxygen partial pressure or dew point in the annealing furnace during the heat treatment process to oxidize alloying elements such as Mn and Si inside the steel, Mn and Si oxides preferentially form on the surface of the steel during the heat treatment process. Subsequently, the oxygen diffused into the steel oxidizes the Mn and Si, suppressing surface diffusion. Therefore, a thin oxide film is formed on the surface of the base iron. However, if the surface of the cold-rolled steel sheet is not perfectly homogeneous before annealing, or if there are local variations in oxygen partial pressure, temperature, etc., the wettability becomes uneven during hot-dip galvanizing, resulting in unplated areas. Alternatively, if the thickness of the oxide film becomes uneven during the alloying heat treatment process after galvanizing, resulting in differences in the degree of alloying, linear defects that are easily identifiable by the naked eye tend to be induced.

[0028] To solve the problems of the above technology, the inventors attempted to manufacture a hot-dip galvanized steel sheet with a beautiful surface and no problems with plating peeling by controlling the presence of oxidizing elements Mn and Si on the surface side of the plated steel sheet as follows.

[0029] In other words, a steel sheet according to one embodiment of the present invention can have the following characteristics in its Mn and Si GDS concentration profile. The plating steel sheet of the present invention will be described in detail with reference to the GDS profile in Figure 1.

[0030] Figure 1 is a schematic graph showing a typical GDS profile of the Mn component that may appear from the surface after the zinc plating layer is removed from a hot-dip galvanized steel sheet, including the steel sheet of the present invention. In the graph, the vertical axis represents the concentration of alloying elements such as Mn and Si, and the horizontal axis represents the depth. As illustrated in the graph of Figure 1, the plated steel sheet of the present invention can have a configuration in which maximum and minimum points appear sequentially when the concentration profile of the Mn or Si component moves from the surface (interface with the plating layer in the case of hot-dip galvanizing) inward. Here, "sequentially" does not mean that the maximum point always appears first in the depth direction from the surface (interface); in some cases, the minimum point may appear first, but thereafter, the maximum point and minimum point should appear sequentially. However, in some implementation examples, the minimum point may not appear, in which case the internal concentration in the depth region of 5 μm can be taken as the minimum point concentration. Furthermore, while the concentration of alloying elements on the surface is lower than that at the maximum point, in some cases, a minimum point with a low concentration of alloying elements may appear between the surface and the maximum point.

[0031] In the GDS concentration profile illustrated in Figure 1 above, although not necessarily limited to this, the surface layer corresponds to an Fe plating layer with a low concentration of alloying elements because not many alloying elements diffuse from the base iron. The maximum point corresponds to a region where internal oxides of alloying elements are concentrated near the interface between the Fe plating layer and the base iron. The minimum point that appears on the base iron side in the Fe plating layer corresponds to a region where alloying elements diffuse into the Fe plating layer which does not contain alloying elements and are diluted, or where alloying elements diffuse into the maximum point where internal oxidation has occurred and are depleted.

[0032] In one embodiment of the present invention, the maximum point can be formed at a depth of 0.05 to 1.0 μm from the surface of the steel plate. If the maximum point appears in a region deeper than this, it may not be judged to be a maximum point due to the effect of the present invention. Furthermore, the minimum point can be formed at a position within a depth of 5 μm from the surface of the steel plate. As described above, if the minimum point is not formed at a point within a depth of 5 μm, the depth of 5 μm can be considered the point where the minimum point is formed. Since the concentration at a depth of 5 μm is substantially the same as the concentration of the base material, it can be treated as a point where the concentration no longer decreases.

[0033] At this time, the larger the difference between the converted concentration at the maximum point of the element in question (the value obtained by dividing the concentration at that point by the concentration of the base material, expressed in %) and the converted concentration at the minimum point in the Mn concentration profile and Si concentration profile, the more important it is because it can reduce the amount of Mn and Si that diffuses to the surface. In one embodiment of the present invention, in the case of Mn, the value of the converted concentration at the maximum point minus the converted concentration at the minimum point can be 80% or more, and in the case of Si, the difference of the above value can be 50% or more. Si is an element that oxidizes more strongly than Mn, and internal oxidation easily occurs even inside the base iron where the oxygen concentration is low, so oxidation may occur over a wider area than with Mn. Therefore, even if the difference in converted concentration between the maximum and minimum points of Si is smaller than that of Mn, it cannot be said that the degree of internal oxidation is small. As a result of experiments conducted by the inventors under various conditions, when the above conditions are met, it was possible to obtain a hot-dip galvanized steel sheet with good plating adhesion and no unplated areas during hot-dip galvanizing. However, if the difference in converted concentration between the maximum and minimum points of Mn is less than 80%, or if the difference in converted concentration between the maximum and minimum points of Si is less than 50%, problems such as the occurrence of spot or linear unplated areas or plating peeling may occur. In other words, by doing so, it is possible to prevent the formation of Mn and Si oxides on the surface, and it is possible to manufacture a hot-dip galvanized steel sheet with a beautiful surface and good plating adhesion, and even after undergoing the alloying heat treatment process, the occurrence of defects such as linear defects on the surface can be suppressed. Since a larger difference in the above converted concentration values ​​is advantageous, there is no need to deliberately set an upper limit on the value. However, considering the content of the elements contained, the difference in the above converted concentration values ​​can be 400% or less in the case of Mn, and 250% or less in the case of Si. In another implementation of the present invention, the above converted concentration difference of Mn can be 90% or more or 100% or more, and the above converted concentration difference of Si can be 60% or more or 70% or more.

[0034] The GDS analysis method implemented in this invention will be described in detail below.

[0035] For GDS concentration analysis, hot-dip galvanized steel sheets were sheared to a length of 30-50 mm and immersed in a 5-10 wt% hydrochloric acid aqueous solution at room temperature (20-25°C) to remove the zinc plating layer. To prevent surface damage to the base iron during the dissolution of the zinc plating layer, the acid solution was removed within 10 seconds of the interruption of bubble generation due to the acid solution reaction with the zinc plating layer, and the base iron was washed with pure water and dried. Of course, if the steel sheet is not yet hot-dip galvanized, analysis can be performed without removing the plating layer in this manner.

[0036] The GDS concentration profile is calculated by measuring the concentration of all components contained in the steel sheet at intervals of 1 to 5 nm in the thickness direction of the steel sheet. The measured GDS profile may contain irregular noise. To calculate the maximum and minimum points of Mn and Si concentrations, a Gaussian filter with a cutoff value of 100 nm is applied to the measured concentration profile to obtain an average concentration profile. From the noise-removed profile, the concentration values ​​and depths of the maximum and minimum points were determined, respectively. It should be noted that, in this invention, the maximum and minimum points are calculated only when there is a difference of 10 nm or more between them in the depth direction.

[0037] The plated steel sheet targeted by the present invention may include a base iron and an Fe plating layer formed on the base iron. The composition of the base iron is not particularly limited.

[0038] However, if the steel sheet contains 0.1 to 1.0 wt% Mn and 0.1 wt% or less Si (excluding 0%), and has a composition that easily forms oxides on the surface, the present invention can advantageously improve the plating properties. There is no particular upper limit to the Mn concentration of the base iron, but considering the compositions that are normally used, the upper limit can be limited to 1.0 wt%. Also, there is no particular lower limit to the concentration of Mn, but for compositions containing less than 0.1 wt% Mn, the surface quality of the hot-dip galvanized steel sheet is beautiful even without forming an Fe plating layer, so Fe electroplating does not necessarily have to be performed. In the component system of the present invention, even if only a small amount of Si is added, it is an element that can contribute to oxide formation and degrade the plating quality, so considering the strict surface quality requirements for automobile exterior panels, the upper limit of its content can be set to 0.1 wt%. From the viewpoint of oxide formation alone, it is advantageous to not add Si, so it is not necessarily necessary to set a lower limit, but considering the limitations in the industrial production process, the lower limit of Si concentration can also be limited to 0.001%.

[0039] Since Mn and Si are elements that affect the plating properties, their concentrations can be limited as described above. However, in this invention, there are no particular restrictions on the remaining components of the base iron.

[0040] However, considering that a significant decrease in unplated and plating adhesion can occur in steel sheets containing alloy components, in one embodiment of the present invention, the composition of the base iron can be as follows in weight percent: Mn: 0.1~1.0%, Si: 0.1% or less, C: 0.0005~0.03%, Al: 0.005~3.0%, P: 0.04% or less, S: 0.015% or less, Cr: 1.5% or less, B: 0.005% or less, with the remainder being Fe and unavoidable impurities. Since P and S are impurities and it is more advantageous if they are not added, and Cr and B are optional elements and do not need to be added, no lower limit is set separately. In addition to the components described above, the base iron may further contain elements such as Ti, Mo, and Nb in a total amount of 1.0% or less. Furthermore, in another embodiment of the present invention, the C component can be further restricted to less than 0.02%, and in yet another embodiment, the C component can be further restricted to 0.0199% or less. There are no particular restrictions on the base iron, but in one embodiment of the present invention, cold-rolled steel sheet or hot-rolled steel sheet can be used as the base iron.

[0041] In one aspect of the present invention, a hot-dip galvanized steel sheet including the above-mentioned steel sheet for plating can be provided, and the hot-dip galvanized steel sheet can include the steel sheet for plating and a hot-dip galvanized layer formed on the surface of the steel sheet for plating. In this case, any commercially available hot-dip galvanized steel sheet can be used, and there are no particular limitations on its type.

[0042] Next, an exemplary implementation of a method for producing plated steel sheets and hot-dip galvanized steel sheets having the advantageous effects described above will be explained. According to one implementation of the present invention, plated steel sheets can be produced by a process that includes the steps of: preparing a base iron; electroplating the base iron to form an Fe plating layer containing 5 to 50% by weight of oxygen; and annealing the base iron on which the Fe plating layer has been formed to obtain a plated steel sheet.

[0043] Cold-rolled steel sheets containing 0.5% Mn, 0.05% Si, and other alloying elements were annealed at 800°C for 53 seconds in an N2-5%H2 atmosphere with a dew point of +5°C, then cooled and observed with a transmission electron microscope. The atmosphere was kept constant throughout the entire heating process, and the dew point temperature was maintained at -40°C during cooling to prevent oxidation of Fe. The amount of iron deposited before annealing was 2.06 g / m². 2 We observed the phenomena that appeared when Fe electroplating containing 7.1% by weight of oxygen was performed, and when it was not performed.

[0044] Observations revealed that in steel sheets annealed at a dew point of +5°C without Fe plating, fine Mn and Si oxides were observed on the surface, and thick grain boundary oxides were formed inside the base iron. This is because grain boundary oxides begin to form from the stage when the cold-rolled structure recovers and recrystallizes into fine grains during the heating process, and as the annealing temperature rises and the annealing time increases, oxygen flows into the base iron where the grains have become coarser, generating oxides at the grain boundary centers. In this form, the concentrations of Mn and Si components in the GDS profile change gradually, and maximum and minimum points do not reliably appear, or even if they do, the difference in converted concentrations does not meet the limits set by this invention.

[0045] However, the Fe plating layer containing 5-50% by weight of oxygen has an iron deposition amount of 1.99 g / m². 2 When plated and then annealed, almost no oxides are formed in the Fe plating layer region, but particulate oxides are formed at the interface between the Fe plating layer and the base iron, and within the base iron. These oxides act as nuclei for internal oxides, causing linear oxides to grow perpendicular to the surface of the steel sheet. However, the depth to which internal oxides are formed is even deeper when an Fe plating layer is not formed than when an Fe plating layer is applied. In such cases, the Fe plating layer (surface layer) contains little Mn and Si, and not only does it show a maximum value at the interface, but it may also have a depleted layer in which the Mn and Si content decreases significantly in regions deeper than the maximum value.

[0046] On the other hand, when annealing is performed in a high dew point atmosphere without Fe plating, oxides are generated at the grain boundaries of the fine recrystallized structure on the surface of the base iron, suppressing crystal growth. As a result, irregular fine crystal grains surrounded by fine oxides are formed. In contrast, when a plating layer with a high oxygen content is formed and then annealed at a high dew point of -15°C to +30°C, the Fe plating layer does not contain oxidizing alloy elements such as Mn and Si. Therefore, oxides are not generated at the grain boundaries of the plating layer, and oxides are generated at the interface between the Fe plating layer and the base iron. This results in a structure where the uniform thickness of the Fe plating layer is separated from the crystal grains inside the base iron. However, depending on the dew point in the annealing furnace, the elongation of the base iron, and the composition of the steel, the boundary between the Fe plating layer and the base iron may not be clearly visible. Therefore, even if the dew point in the annealing furnace is controlled to -15°C to +30°C after Fe electroplating, these characteristics are not always present.

[0047] Unlike the oxidation-reduction method, the internal oxidation annealing method does not form a layered oxide layer. Therefore, it exhibits excellent properties in improving plating adhesion when hot-dip galvanizing steel sheets containing alloying elements such as Mn and Si. However, because the steam in the annealing furnace inevitably oxidizes the surface of the steel sheet first, and then oxygen penetrates into the interior, it is not possible to fundamentally remove the surface oxide.

[0048] To solve the above problems, the inventors have found through numerous experiments that when an Fe plating layer containing a large amount of oxygen is formed and then annealed in a high dew point atmosphere, oxygen from the water vapor in the annealing furnace does not form surface oxides of alloying elements such as Mn and Si on the surface of the Fe plating layer, and the oxygen contained in the Fe plating layer internally oxidizes alloying elements such as Mn and Si in the base iron, effectively suppressing their diffusion to the surface. Due to the high dew point in the annealing furnace, the oxygen that flows into the steel further internally oxidizes the alloying elements, so almost no surface oxides of alloying elements are formed on the surface of the steel. This dramatically improves the surface quality and plating adhesion of the hot-dip galvanized steel sheet, and also promotes the alloying reaction when manufacturing alloyed hot-dip galvanized steel sheets, making it possible to obtain a uniform alloyed hot-dip galvanized steel sheet without surface defects.

[0049] More specifically, by forming an Fe plating layer containing 5-50% by weight of oxygen on a cold-rolled steel sheet (base iron), raising the temperature to 600-950°C in an annealing furnace controlled to a dew point of -15°C to +30°C to ensure the mechanical properties of the steel sheet, and then cooling it again to perform hot-dip plating, it is possible to obtain a hot-dip plated steel sheet with suppressed unplated areas and excellent plating adhesion.

[0050] In one embodiment of the present invention, the above-mentioned Fe plating layer can be formed by a continuous plating process, and the amount of Fe plating at this time is 0.5 to 3.0 g / m² based on the amount of Fe deposited. 2 This can be achieved. The amount of Fe plating is 0.5 g / m 2 If the concentration falls below 3.0 g / m², the diffusion-suppressing effect of the Fe plating layer on alloy elements may be insufficient in the normal continuous annealing process. 2 While the suppression effect of alloying elements can be further increased beyond this limit, it is not economical because it requires the operation of multiple plating cells to ensure a high plating amount, and when using an insoluble anode, the electroplating solution rapidly becomes acidic, reducing plating efficiency and generating sludge. In another embodiment of the present invention, the above Fe plating amount is 1.0~2.0 g / m 2 This may also be the case. If internal oxidation is performed after the formation of the Fe plating layer, internal oxides are formed at or just below the interface between the Fe plating layer and the base iron, so the maximum concentrations of Mn and Si will be in the region of 0.05 to 1.0 μm. 0.5 to 3.0 g / m of the present invention 2 The amount of Fe plating can correspond to a thickness of 0.05 to 0.4 μm after annealing.

[0051] Furthermore, by controlling the temperature, dew point temperature, and atmosphere of the subsequent annealing process, the Fe plating layer having the high oxygen concentration described above can form maximum and minimum points in the GDS concentration profile of Mn and Si elements within the plated steel sheet, so that the converted concentrations at the maximum and minimum points satisfy the numerical range limited in one embodiment of the present invention. Considering these points, the oxygen concentration in the Fe plating layer in one embodiment of the present invention can be 5 to 50% by weight, and in another embodiment it can be 10 to 40% by weight. In order to obtain the effect of suppressing surface oxides, the amount of oxygen in the Fe plating layer must be sufficiently high. Even if the oxygen concentration in the Fe plating layer is less than 5% by weight, the effect of suppressing surface oxides can be obtained by increasing the amount of Fe plating, but to obtain such an effect, 3.0 g / m 2 Because plating must be carried out beyond a certain limit, the various problems described above may occur. Furthermore, if the oxygen content does not reach 5% by weight, it becomes difficult to sequentially form maximum and minimum points in the GDS profile of Mn and Si. Therefore, in one embodiment of the present invention, the oxygen content in the Fe plating layer is controlled to 5% by weight or more. On the other hand, the effect of suppressing surface oxides during annealing can be further increased as the oxygen concentration in the Fe plating layer increases, but since it is difficult to obtain a plating layer exceeding 50% by weight with conventional electroplating methods, the upper limit can be limited to 50% by weight. In another embodiment of the present invention, the oxygen concentration in the Fe plating layer can also be limited to 10-40%.

[0052] In one embodiment of the present invention, the annealing temperature may be 600°C to 950°C, based on the temperature of the steel sheet in a homogeneous zone. If the annealing temperature is too low, the structure of the cold-rolled steel sheet will not recover and recrystallize properly, making it difficult to secure mechanical properties such as the strength and elongation of the steel sheet. If it exceeds 950°C, the alloying elements in the steel will rapidly diffuse to the surface, resulting in poor quality hot-dip galvanizing, and it will be uneconomical to operate at a higher temperature than necessary.

[0053] On the other hand, in one embodiment of the present invention, the dew point inside the annealing furnace may be -15°C to +30°C. If the dew point falls below -15°C, the amount of oxygen flowing into the steel decreases, increasing only surface oxidation, and internal oxidation does not occur. As a result, a large amount of oxide is present on the surface, leading to poor hot-dip galvanizing quality. Furthermore, if the dew point exceeds +30°C, internal oxidation increases, and the effect of suppressing surface oxidation by inhibiting the diffusion of alloying elements increases further. However, the amount of water vapor supplied increases rapidly, requiring the humidification equipment capacity to be larger than necessary. The cooled water vapor condenses, and if applied for a long period in continuous annealing, equipment problems may occur. The above dew point can be controlled within the range of 600 to 950°C as described above, and can be controlled under more relaxed conditions in a lower temperature range. In another embodiment of the present invention, the above dew point can also be limited to -10 to +20°C.

[0054] Furthermore, in order to prevent oxidation of the base iron and Fe plating layer during annealing, the hydrogen concentration in the atmospheric gas during annealing can be set to 1% or more by volume. If the hydrogen concentration is less than 1%, the trace amounts of oxygen inevitably contained in H2 and N2 gases cannot be effectively oxidized and removed, which can increase the oxygen partial pressure and induce surface oxidation of the base iron. On the other hand, if the hydrogen concentration exceeds 70%, the risk of explosion when the gas leaks and the cost of high-hydrogen work increase, so the above hydrogen concentration can be set to 70% or less. Other than the above hydrogen (H2), the gas may be substantially nitrogen (N2), excluding the impurities that are inevitably contained.

[0055] Furthermore, according to one embodiment of the present invention, the holding time after reaching the target temperature during annealing can be limited to 5 to 120 seconds. In order to obtain uniform mechanical properties in the thickness direction and for sufficient heat to be transferred to the interior of the base iron during annealing, it is necessary to maintain the annealing target temperature for 5 seconds or more. On the other hand, if the annealing holding time at high temperature is too long, the diffusion of alloy interfering elements through the Fe plating layer increases, increasing the amount of surface oxides formed, and as a result the quality of the hot-dip galvanizing deteriorates, so it can be limited to 120 seconds or less.

[0056] Based on the above, the effect of suppressing surface diffusion of Mn and Si during annealing in a high dew-point atmosphere in a cold-rolled steel sheet with a Fe plating layer containing a large amount of oxygen will be explained in more detail below with reference to Figure 2.

[0057] Figure 2 schematically illustrates the phenomena that occur inside a steel sheet when its temperature is increased according to the conditions of the present invention. Figure 2(a) shows a schematic cross-section of a base iron with an Fe plating layer containing a large amount of oxygen. The base iron contains alloying elements such as Mn and Si, and the Fe plating layer contains 5 to 50% by weight of oxygen and impurities that are inevitably mixed in during electroplating, with the remainder being Fe.

[0058] Figure 2(b) shows the state of a Fe-plated cold-rolled steel sheet heated to approximately 300-500°C in a nitrogen atmosphere containing 1-70% H2. The surface of the Fe plating layer, which contains a large amount of oxygen, is gradually reduced and the oxygen is removed. However, at the interface between the Fe plating layer and the base iron, Mn, Si, etc. diffused from the base iron combine with the oxygen in the Fe plating layer to form internal oxides, thus suppressing diffusion to the surface. Furthermore, as the temperature increases, Mn and Si diffused inside the base iron accumulate, and the internal oxides at the interface gradually grow. Even if the dew point inside the annealing furnace changes widely from -90°C to +30°C in the low-temperature range of the heating stage, because the temperature is low, the rate at which oxygen is released as the Fe plating layer is reduced due to the presence of a large amount of oxygen inside the Fe plating layer is faster than the rate at which oxygen dissociated from water vapor diffuses into the steel. Therefore, changes in the dew point inside the annealing furnace do not have a significant impact in the low-temperature range. Thus, controlling the dew point is not a very important factor at this stage.

[0059] However, the amount of oxygen inside the Fe plating layer plays an important role. If a large amount of fine internal oxides are generated at the interface between the Fe plating layer and the base iron, and inside the base iron, the alloying elements inside the base iron act as nuclei for oxides that can be continuously oxidized internally. For such oxide nuclei to be generated, the concentrations of oxygen and alloying components must be high simultaneously. If the Fe plating layer contains a sufficiently large amount of oxygen, a large amount of oxide nuclei will be generated near the interface between the Fe plating layer, which has a high oxygen concentration, and the base iron, which has a high alloying element concentration. However, if the Fe plating layer contains almost no oxygen, the alloying elements contained in the base iron will pass through the Fe plating layer and form oxides on the surface. Subsequently, as the temperature is increased, the oxygen in the Fe plating is further depleted, and the diffusion of alloying elements within the base iron is further amplified, so the generation of surface oxides increases.

[0060] Figure 2(c) shows a schematic cross-section of the base iron when heated to 500-700°C in the same reducing atmosphere. During the heating process, it is best to control the dew point inside the annealing furnace to -15°C to +30°C. As the temperature rises, the Fe plating layer is sufficiently reduced and the oxygen concentration decreases, so the rate of oxygen release slows down, while the rate at which water vapor inside the annealing furnace dissociates and diffuses into the interior of the steel increases significantly. Therefore, by raising the dew point from the 500-700°C range, which is lower than the temperature at which the Fe plating layer is completely reduced, it is possible to effectively suppress the diffusion of Mn and Si from inside the steel through the Fe plating layer to the surface.

[0061] Figure 2(d) shows a schematic cross-section of a steel sheet after being maintained at a high temperature in the range of 600 to 950°C while adjusting the dew point from -15°C to +30°C. Inside the base iron, Mn and Si continuously diffuse, and oxygen supplied from water vapor rapidly penetrates and supplies oxygen to the surface of the steel sheet. As a result, Mn and Si oxidize internally. However, in the low-temperature range, particulate Mn and Si oxides generated by reaction with oxygen in the Fe plating layer at the interface between the Fe plating layer and the base iron act as nuclei for oxide growth. Therefore, internal oxides concentrate and grow at the interface between the Fe plating layer and the base iron. Furthermore, since the diffusion rate of oxygen is faster than that of Mn and Si, which have larger atomic sizes, internal oxides are generated deeply not only at the grain boundaries inside the base iron but also through the grains.

[0062] Although the control conditions for each temperature have been explained above, the most crucial step in the annealing process is maintaining the steel sheet temperature at 600-950°C. By controlling the dew point of the atmosphere within this temperature range, the oxide distribution inside the plated steel sheet can be effectively controlled. Of course, such dew point control can be performed in all stages prior to the maintenance stage without any particular problem. Furthermore, it should be noted that the process described above is merely an illustrative example of one implementation of the present invention, and the reaction mechanism of the present invention is not always constrained by the above explanation.

[0063] After the annealing step described above, the annealed steel sheet can be cooled. The cooling conditions in the cooling step after the annealing step do not have a significant effect on the surface quality of the final product, i.e., the plating quality, so there is no need to particularly limit the cooling conditions in this invention. However, in order to prevent oxidation of the iron component during the cooling process, a reducing atmosphere can be applied to at least the iron.

[0064] According to one embodiment of the present invention, a hot-dip galvanized layer can be formed by hot-dip galvanizing the steel sheet for plating obtained by the process described above. In the present invention, the hot-dip galvanizing method is not particularly limited.

[0065] Furthermore, in this invention, any base iron having the alloy composition described above can be applied without limitation as the base iron for the plating steel sheet or hot-dip galvanized steel sheet according to the present invention; therefore, the method for manufacturing the base iron does not need to be specifically limited.

[0066] In one embodiment of the present invention, the Fe plating layer can be formed on the surface of the base iron by an electroplating method, and the oxygen concentration of the formed Fe plating layer can be controlled by appropriately controlling the conditions of the electroplating solution and the plating conditions.

[0067] In other words, in order to form the Fe plating layer in the present invention, an electroplating solution can be used that contains iron ions including ferrous and ferric ions; a complexing agent; and unavoidable impurities, wherein the concentration of ferric ions among the iron ions is 5 to 60% by weight.

[0068] According to one embodiment of the present invention, the electroplating solution contains ferrous ions and ferric ions. To obtain high plating efficiency, it may be advantageous to include only ferrous ions. However, if only ferrous ions are included, the solution deteriorates and the plating efficiency decreases rapidly, which may induce quality variations in a continuous electroplating process. Therefore, ferric ions can be further included. In this case, the concentration of ferric ions is preferably 5 to 60% by weight of the total sum of ferrous and ferric ions, and more preferably 5 to 40% by weight. If the concentration is less than 5%, the rate at which ferric ions are reduced to ferrous ions at the cathode is lower than the rate at which ferrous ions are oxidized to ferric ions at the anode. As a result, the concentration of ferric ions increases rapidly, the pH decreases rapidly, and the plating efficiency decreases continuously. Conversely, if the concentration of ferric ions exceeds 60%, the amount of reaction in which ferric ions are reduced to ferrous ions at the cathode increases significantly more than the amount of reaction in which ferrous ions are reduced and deposited as metallic iron. As a result, the plating efficiency decreases drastically, and the plating quality deteriorates. Therefore, considering the characteristics of the equipment and process, such as the amount of plating, the working current density, the amount of solution supplied, the amount of solution adhering to the strip and flowing out, and the rate of concentration change due to evaporation, it is preferable to set the concentration of ferric ions in the iron ions to 5-60% by weight.

[0069] The concentration of iron ions is preferably 1 to 80 g per liter of the electroplating solution, and more preferably 10 to 50 g per liter. If the concentration is less than 1 g / L, there is a problem of a rapid decrease in plating efficiency and plating quality. On the other hand, if it exceeds 80 g / L, precipitation may occur because it exceeds the solubility limit, and raw material loss due to solution leakage in the continuous plating process increases, making it uneconomical.

[0070] The electroplating solution of the present invention contains a complexing agent, but it is preferable to use an amino acid or an amino acid polymer as the complexing agent in order to maintain high plating efficiency without generating sludge, even while containing a large amount of ferric acid.

[0071] An amino acid is an organic molecule in which a carboxyl group (-COOH) and an amino group (-NH2) are bonded together. An amino acid polymer is an organic molecule formed by the polymerization of two or more amino acids, and amino acid polymers exhibit complexing properties similar to those of amino acids. Therefore, in the following explanation, both amino acids and amino acid polymers will be collectively referred to as amino acids.

[0072] When amino acids dissolve in neutral water, the amine group combines with hydrogen ions to acquire a positive charge, while the carboxyl group dissociates from hydrogen ions to acquire a negative charge, thus maintaining a neutral charge in the amino acid molecule. On the other hand, when the solution becomes acidic, the carboxyl group recombines with hydrogen ions to become neutral, and the amine group acquires a positive charge, so the amino acid molecule forms a cation. In other words, amino acids are either neutral in charge or form cations in weakly acidic aqueous solutions.

[0073] When amino acids are added to an acidic electrolyte containing iron ions, they complex with ferrous and ferric ions. However, the iron ions complexed with the amino acids maintain their cation state even in the complexed state. Therefore, they exhibit properties electrically opposite to those of typical complexing agents with multiple carboxyl groups, which exhibit a negative charge in weakly acidic aqueous solutions.

[0074] Furthermore, compared to complexing agents containing multiple carboxyl groups such as citric acid and EDTA, amino acids form fewer bonds with iron ions and have weaker bonding strength. However, their bonding strength with ferric ions, which generate sludge, is sufficiently strong, thus preventing precipitation by ferric ions. Moreover, because the cation can be maintained even after complexing with ferric ions, ferric ions are easily transferred to the cathode and reduced to ferrous ions to participate in the plating reaction. At the same time, their movement to the anode is suppressed, slowing down the rate of ferric ion generation. As a result, even with long-term continuous plating, the concentration of ferric ions remains at a constant level, maintaining consistent plating efficiency and eliminating the need to replace the electrolyte.

[0075] On the other hand, in a continuous electroplating process, when iron ions in the solution are exhausted by plating, the solution becomes acidic. However, even if the same amount of iron ions are deposited, a solution containing both ferrous and ferric ions experiences less pH change than a solution containing only ferrous ions. As the pH increases, some ferric ions combine with hydroxide ions, and as the pH decreases, the hydroxide ions are separated and neutralized. Therefore, a solution containing ferric ions experiences a slower pH change even without a separate pH buffer, acting as a pH buffer and thus maintaining a constant electroplating efficiency in a continuous electroplating process.

[0076] Therefore, by using amino acids as complexing agents, sludge formation can be prevented, and not only ferrous ions but also ferric ions can be used as plating raw materials. When ferrous and ferric ions are used in mixture form, the pH change of the solution is slowed down, and the accumulation of ferric ions can be easily prevented, thus maintaining constant electroplating efficiency and plating quality in a continuous electroplating process.

[0077] Incidentally, the complexing agent is preferably added in an amount such that the molar concentration ratio of the iron ion to the complexing agent is 1:0.05 to 2.0, and more preferably in an amount such that the ratio is 1:0.5 to 1.0. When it is less than 0.05, it is impossible to suppress the formation of sludge due to the excess ferrous ions binding to hydroxide ions or oxygen. Even without the inclusion of ferrous ions, the plating efficiency is extremely reduced, and further, burning is induced and the plating quality deteriorates. On the other hand, even when it exceeds 2.0, the sludge suppression effect and the plating quality are maintained, but the overvoltage increases and the plating efficiency decreases, and relatively expensive amino acids are contained in an excessive amount more than necessary compared to raw materials containing iron ions such as iron sulfate, resulting in an increase in raw material costs, which is not economical.

[0078] The complexing agent is preferably one or more selected from amino acids or amino acid polymers. For example, it may be one or more selected from alanine, glycine, serine, threonine, arginine, glutamine, glutamic acid, and glycylglycine.

[0079] When the above amino acid is used as a complexing agent and electroplating is carried out at a current density of 3 to 120 A / dm² while maintaining the solution temperature at 80°C or lower and the pH at 2.0 to 5.0 2 a high plating efficiency and an Fe plating layer with a high oxygen concentration can be obtained.

[0080] [[ID=1,4]]The temperature of the Fe electroplating solution does not significantly affect the quality of the Fe plating layer. However, when it exceeds 80°C, the evaporation of the solution becomes intense, the concentration of the solution continuously changes, and it becomes difficult to perform uniform electroplating.

[0081] When the pH of the Fe electroplating solution is less than 2.0, the electroplating efficiency decreases and it is not suitable for the continuous plating process. When the pH exceeds 5.0, the plating efficiency increases, but sludge in which iron hydroxide precipitates occurs during continuous electroplating, causing problems such as pipe blockage, contamination of rolls and equipment.

[0082] The current density is 3 A / dm² 2If the voltage falls below 120 A / dm², the cathode plating overvoltage decreases, reducing the Fe electroplating efficiency, making it unsuitable for continuous plating processes. 2 If this limit is exceeded, burning occurs on the plated surface, resulting in an uneven electroplating layer and a problem where the Fe plating layer is prone to peeling off.

[0083] As described above, the present invention preferably contains 5 to 50% by weight of oxygen in the Fe plating layer. The reason for the inclusion of oxygen in the Fe plating layer is as follows: During the process of iron deposition on the surface of the steel plate to which the cathode is applied, hydrogen ions are simultaneously reduced to hydrogen gas, causing the pH to rise. Therefore, both ferrous and ferric ions temporarily form OH - It can combine with ions and be mixed in when the Fe plating layer is formed. If anionic complexing agents such as acetic acid, lactic acid, citric acid, or EDTA are used, the complexing agent will OH - Iron ions bound to ions exhibit an average negative charge, and when a cathode is applied for electroplating, an electrical repulsive force is generated, suppressing contamination of the Fe plating layer. On the other hand, amino acids are electrically neutral at pH 2.0 to 5.0, and exhibit cations in strong acids below pH 2.0, but iron ions bound to amino acids have 1 to 2 OH groups. - When these atoms bond, they exhibit cations, generating an electrical attraction with the cathode during electroplating, resulting in the incorporation of a large amount of oxygen. Therefore, by using amino acids as a complexing agent with a molar concentration ratio of iron ions to amino acids of 1:0.05 to 1:2.0, and maintaining a pH of 2.0 to 5.0 during Fe electroplating, it is possible to obtain an Fe plated layer containing 5 to 50% by weight of oxygen while maintaining high plating efficiency and suppressing sludge generation.

[0084] To ensure the quality of hot-dip galvanizing of steel sheets containing Mn and Si, the amount of Fe plating layer should be 0.5 to 3.0 g / m² based on the iron concentration. 2 It is best to process it this way. There is no particular upper limit to the amount of Fe plating, but 3.0 g / m² is recommended in a continuous plating process. 2If the amount exceeds this, multiple plating cells are required or the production speed decreases, making it uneconomical. Furthermore, if the amount of Fe electroplating is large, the Fe electroplating solution rapidly degrades in the continuous process, causing a drop in pH, which significantly reduces plating efficiency and makes solution management difficult. On the other hand, if the amount of Fe electroplating is 0.5 g / m², 2 If the amount of Fe plating falls below a certain level, the oxygen contained within the Fe plating layer is rapidly reduced and removed, making it difficult to effectively suppress the diffusion of Mn and Si from the base iron to form surface oxides, resulting in a decrease in the quality of the hot-dip plating. The above amount of Fe plating corresponds to the iron concentration contained within the plating layer, and when the Fe plating layer is completely reduced during annealing, it has a thickness of approximately 0.05 to 0.4 μm.

Claims

1. The composition, in weight percent, is as follows: Mn: 0.1-1.0%, Si: 0.1% or less (excluding 0%), C: 0.0005-0.03%, Al: 0.005-3.0%, P: 0.04% or less, S: 0.015% or less, Cr: 1.5% or less, B: 0.005% or less, with the remainder being Fe and unavoidable impurities. The GDS profiles of the Mn component and the Si component, observed from the surface in the depth direction, each sequentially contain local maxima and local minima. The depth at which the aforementioned maximum point is formed is 0.05 to 1.0 μm. The difference between the Mn concentration at the maximum point of the GDS profile of the Mn component divided by the Mn concentration of the base material and the Mn concentration at the minimum point of the GDS profile of the Mn component divided by the Mn concentration of the base material (the difference in converted Mn concentration) is 80% or more. A steel sheet in which the difference (converted Si concentration difference) between the Si concentration at the maximum point of the GDS profile of the Si component divided by the Si concentration of the base material and the Si concentration at the minimum point of the GDS profile of the Si component divided by the Si concentration of the base material is 50% or more. However, if no local minimum appears within a depth of 5 μm, the point at a depth of 5 μm will be considered the point where the local minimum appeared.

2. The steel sheet according to claim 1, wherein the steel sheet comprises a base iron and an Fe plating layer formed on the surface of the base iron, and the surface is the surface of the Fe plating layer.

3. The steel plate according to claim 1, wherein the converted concentration difference of Mn is 90% or more, and the converted concentration difference of Si is 60% or more.

4. A hot-dip galvanized steel sheet comprising a steel sheet according to any one of claims 1 to 3 and a hot-dip galvanized layer formed on the steel sheet.

5. The first step is to prepare a base iron having a composition in weight percent of: Mn: 0.1-1.0%, Si: 0.1% or less (excluding 0%), C: 0.0005-0.03%, Al: 0.005-3.0%, P: 0.04% or less, S: 0.015% or less, Cr: 1.5% or less, B: 0.005% or less, with the remainder being Fe and unavoidable impurities; The steps include: electroplating the aforementioned base iron with an electroplating solution containing iron ions and a complexing agent to form an Fe plating layer containing 5 to 50% by weight of oxygen; and The iron substrate on which the Fe plating layer is formed is subjected to a dew point temperature controlled to -15 to +30°C, with a humidity of 1 to 70%. ２ - Remaining N ２ A method for manufacturing steel sheets, comprising the step of annealing in a gas atmosphere annealing furnace at 600 to 950°C for 5 to 120 seconds.

6. The amount of Fe plating layer deposited is 0.5 to 3 g / m². ２ The method for manufacturing a steel sheet for plating according to claim 5.

7. The method for producing a plated steel sheet according to claim 5 or 6, wherein the complexing agent is one or more selected from alanine, glycine, serine, threonine, arginine, glutamine, glutamic acid, and glycylglycine.

8. The method for producing a plated steel sheet according to claim 5 or 6, wherein the electroplating solution contains ferrous ions and ferric ions, the ferric ions make up 5 to 60% by weight of the total iron ions, and the total concentration of iron ions is 1 to 80 g per liter of the electroplating solution.

9. The aforementioned electroplating is performed at a solution temperature of 80°C or lower and a current density of 3 to 120 A / dm². ２ A method for manufacturing a steel sheet for plating according to claim 5 or 6, carried out under the following conditions.

10. The first step is to prepare a base iron having a composition in weight percent of: Mn: 0.1-1.0%, Si: 0.1% or less (excluding 0%), C: 0.0005-0.03%, Al: 0.005-3.0%, P: 0.04% or less, S: 0.015% or less, Cr: 1.5% or less, B: 0.005% or less, with the remainder being Fe and unavoidable impurities; A step of electroplating the aforementioned base iron to form an Fe plating layer containing 5 to 50% by weight of oxygen; The iron substrate on which the Fe plating layer is formed is subjected to a dew point temperature controlled to -15 to +30°C, with a humidity of 1 to 70%. ２ - Remaining N ２ The step of obtaining a steel sheet for plating by annealing in a gas atmosphere annealing furnace at 600 to 950°C for 5 to 120 seconds; and A method for manufacturing a hot-dip galvanized steel sheet, comprising the step of immersing the steel sheet for plating in a zinc plating bath.

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