Steel sheet with excellent plating quality and method for producing 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] 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.

Background Art

[0002] In the case of outer panels for automobiles, hot-dip galvanizing is often performed on the surface to ensure corrosion resistance. However, there is a risk that plating problems may occur for the following reasons.

[0003] That is, the quality of hot-dip galvanizing is determined according to the surface state of the annealed steel sheet immediately before plating. However, 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, the plating property deteriorates. That is, during the annealing process, the above elements diffuse to the surface side and react with trace amounts of oxygen or water vapor present in the annealing furnace to form single or composite oxides of the above 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, causing unplated areas where the plating metal does not adhere locally or entirely to the surface of the plated steel sheet. Furthermore, such oxides make it insufficient to form an alloying suppression layer (Fe2Al5) necessary for ensuring the adhesion of the plating layer during the hot-dip plating process, resulting in peeling of the plating layer and significantly reducing the plating quality of the plated steel sheet.

[0004] Various techniques have been proposed to improve the plating quality of hot-dip plated steel sheets. Among them, Patent Document 1 controls the air-fuel ratio of air and fuel to 0.80 to 0.95 during the annealing process, oxidizes the steel sheet in a direct flame furnace with an oxidizing atmosphere, forms iron oxides containing single or composite oxides of Si, Mn, or Al to a certain depth inside the steel sheet, and then performs reduction annealing of the iron oxides in a reducing atmosphere and then hot-dip galvanizing, presenting a technique for providing a hot-dip galvanized or alloyed hot-dip galvanized steel sheet with excellent plating quality.

[0005] When a method of oxidation followed by reduction is used in the annealing process as in Patent Document 1, components with a high affinity for oxygen, such as Si, Mn, and Al, are internally oxidized at a certain depth from the surface layer of the steel sheet, suppressing diffusion to the surface layer. Therefore, in the surface layer, Si, Mn, or Al alone or complex oxides relatively decrease, improving wettability with zinc and reducing uncoated areas. However, in the case of steel grades with added Si, Si concentrates directly beneath the iron oxide during the reduction process, forming banded Si oxides. As a result, peeling occurs in the surface layer including the plating layer, that is, peeling occurs at the interface between the reduced iron and the base iron beneath it, making it difficult to ensure the adhesion of the plating layer.

[0006] On the other hand, as yet another method for improving the plating property of hot-dip galvanized steel sheets, Patent Document 2 presents a method of maintaining a high dew point in the annealing furnace and internally oxidizing alloy components such as Mn, Si, and Al that are easily oxidized inside the steel to reduce the oxides externally oxidized on the surface of the steel sheet after annealing and improve the plating property. However, in the method according to Patent Document 2, although the problem of plating property due to external oxidation of Si, which is easily internally oxidized, can be solved, there is a problem that the effect is slight when a large amount of Mn, which is relatively difficult to internally oxidize, is added.

[0007] Moreover, even if the plating property is improved by internal oxidation, linear uncoated areas may occur due to surface oxides formed unevenly on the surface, or when producing galvannealed steel sheets (GA steel sheets) by alloying heat treatment after plating, linear defects due to non-uniform alloying may occur on the surface of the galvannealed steel sheets.

[0008] As yet another prior art, there is a method of performing Ni pre-plating before annealing to suppress the diffusion of alloy elements to the surface during annealing. However, although this method is effective in suppressing the diffusion of Mn, there is a problem that it cannot sufficiently suppress the diffusion of Si.

Prior Art Documents

Patent Documents

[0009] Patent Document 1 Korean Patent Publication No. 2010-0030627 Patent Document 2 Korean Patent Publication No. 2009-0006881 SUMMARY OF THE INVENTION PROBLEMS TO BE SOLVED BY THE INVENTION

[0010] According to one aspect of the present invention, there are provided a hot-dip galvanized steel sheet with excellent plating quality in which non-plating does not occur and the plating layer does not peel, and a method for manufacturing the same.

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

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

[0013] The problems of the present invention are not limited to the above-described contents. Those having ordinary knowledge in the technical field to which the present invention pertains will have no problem in understanding further problems of the present invention from the overall matters of the specification of the present invention. MEANS FOR SOLVING THE PROBLEMS

[0014] 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 unavoidable impurities. The GDS profile of the Mn component and the GDS profile of the Si component observed in the depth direction from the surface sequentially include a maximum point and a minimum point, respectively. 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 that.

[0015] However, when a minimum point does not appear within a depth of 5 μm, the point at a depth of 5 μm is regarded as the point where the minimum point appears.

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

[0017] The method for manufacturing a steel sheet for plating, which is another aspect of the present invention, includes 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 Fe and unavoidable impurities; performing electroplating on 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 is formed by maintaining it at 600 to 950°C for 5 to 120 seconds in an annealing furnace in an atmosphere of 1 to 70% H2 - the balance N2 gas controlled at a dew point temperature of -15 to +30°C.

[0018] Another aspect of the present invention is a method for manufacturing a hot-dip galvanized steel sheet, which 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 unavoidable impurities; performing electroplating on 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 thereon 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, at 600 to 950°C for 5 to 120 seconds to obtain a steel sheet for plating; and dipping the steel sheet for plating into a hot-dip galvanizing bath composed of Al: 0.1 to 0.3% and the balance being Zn and unavoidable impurities, maintained in a temperature range of 440 to 500°C.

Advantages of the Invention

[0019] As described above, the present invention forms a pre-plating layer and controls the concentration profiles of the internal Mn and Si components, thereby significantly improving the phenomenon of unplated areas during hot-dip galvanizing and providing a hot-dip galvanized steel sheet with improved plating adhesion.

[0020] Also, 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

Figure 2

Embodiments for Carrying Out the Invention

[0022] Hereinafter, a hot-dip galvanized steel sheet with excellent plating quality according to one aspect of the present invention completed by the research of the present inventor will be described in detail. When indicating the concentration of each element in the present invention, it should be noted that it means weight% unless otherwise specified. The Fe electroplating amount is the plating amount measured by the total amount of Fe contained in the plating layer per unit area, and oxygen and inevitable impurities in the plating layer are not included in the plating amount.

[0023] Also, unless otherwise defined, the concentrations and concentration profiles referred to in the present invention mean the concentrations and concentration profiles measured using GDS, that is, a glow discharge optical emission spectrometer.

[0024] Hereinafter, the present invention will be described in detail.

[0025] In steel sheets containing Mn and Si, the causes of unplated and decreased plating adhesion are known to be due to surface oxides generated by the oxidation of alloy elements such as Mn and Si on the surface during the annealing of cold-rolled steel sheets at high temperatures.

[0026] As a method of forming an oxide layer containing a large amount of oxygen to suppress the diffusion of alloy elements such as Mn and Si to the surface, an oxidation-reduction method in which after oxidation during temperature rise, it is maintained in a reducing atmosphere again for reduction, or a method in which iron oxide is coated on the surface of the base metal and heat-treated can be used. 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. During the annealing process in a reducing atmosphere, the surface is reduced to metallic iron, while the interface between the iron oxide layer and the base iron has a slow reduction rate and is difficult to be completely reduced. Mn and Si oxides accumulate at the interface to form a continuous oxide layer. Although the wettability with hot-dip zinc is improved, the oxide layer becomes brittle and there is a possibility that the plating layer may peel off.

[0027] On the one hand, when applying the annealing internal oxidation method that increases the oxygen partial pressure or dew point in the annealing furnace during the heat treatment process and oxidizes alloying elements such as Mn and Si inside the steel, Mn and Si oxides are preferentially formed on the surface of the steel during the heat treatment process, and then Mn and Si are oxidized by the oxygen diffused into the interior of the steel to suppress surface diffusion. Therefore, although a thin oxide film is formed on the surface of the base iron, if the surface of the cold-rolled steel sheet is not completely homogeneous before annealing or local variations occur in the oxygen partial pressure, temperature, etc., the wettability becomes non-uniform during hot-dip galvanizing and ungalvanized areas occur, or when the thickness of the oxide film becomes non-uniform during the alloying heat treatment process after galvanizing and a difference in the degree of alloying occurs, linear defects that can be easily identified visually tend to be induced.

[0028] In order to solve the problems of the above technology, the inventors of the present invention tried to manufacture a hot-dip galvanized steel sheet with a beautiful surface and no problem of peeling of the plating by controlling the existence forms of the oxidizing elements Mn and Si on the surface side of the steel sheet for plating as follows.

[0029] That is, the steel sheet according to an embodiment of the present invention can have the following characteristics in the GDS concentration profiles of Mn and Si. With reference to the GDS profile in FIG. 1, the steel sheet for plating of the present invention will be described in detail.

[0030] FIG. 1 is a graph schematically showing a typical GDS profile of Mn components that can appear from the surface part after removing the zinc plating layer from the hot-dip galvanized steel sheet including the steel sheet of the present invention. In the graph, the vertical axis represents the concentration of alloy elements such as Mn and Si, and the horizontal axis represents the depth. As exemplified in the graph of FIG. 1, the plating steel sheet of the present invention can have a form in which maximum points and minimum points sequentially appear when the concentration profile of the Mn or Si component goes from the surface (when hot-dip galvanized, the interface with the plating layer) to the inside. Here, "having sequentially" does not mean that the maximum point must always appear first in the depth direction from the surface (interface). In some cases, the minimum point may appear first, but thereafter, the maximum point and the minimum point should appear sequentially. However, in some embodiments, the minimum point may not appear. In this case, the internal concentration in the depth region of 5 μm can be taken as the minimum point concentration. Also, the surface alloy element concentration has a value lower than the concentration of the maximum point, but in some cases, a minimum point with a low alloy element concentration may appear between the surface and the maximum point.

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

[0032] In one implementation example of the present invention, the above-mentioned 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 determined as the maximum point due to the effect of the present invention. Further, 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 point where the minimum point is formed can be set at a depth of 5 μm. 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 greater the difference between the converted concentration of the maximum point of the element (the value obtained by dividing the concentration at that point by the concentration of the base material, expressed in % units) and the converted concentration of the minimum point in the Mn concentration profile and the Si concentration profile, the more the Mn and Si diffusing to the surface can be reduced, which is important. In one implementation example of the present invention, in the case of Mn, the value of the converted concentration of the maximum point - the converted concentration of the minimum point can be 80% or more, and the difference in the above value in the case of Si can be 50% or more. Si is an element with stronger oxidizing properties than Mn, and internal oxidation easily occurs even inside the base iron with a low oxygen concentration, so oxidation may occur in a wider region than Mn. Therefore, even if the difference in the converted concentration between the maximum point and the minimum point of Si is smaller than that of Mn, it cannot be said that the degree of internal oxidation is small. As a result of the inventors' experiments under various conditions, when the above conditions are satisfied, no unplated areas occur during hot-dip galvanizing, and a hot-dip galvanized steel sheet with good plating adhesion can be obtained. However, when the difference in the converted concentration between the maximum point and the minimum point of Mn is less than 80%, or the difference in the converted concentration between the maximum point and the minimum point of Si is less than 50%, problems such as the occurrence of point or linear unplated areas or plating peeling may occur. That is, by doing so, it is possible to prevent the formation of oxides of Mn and Si on the surface, manufacture a hot-dip galvanized steel sheet with a beautiful surface and good plating adhesion, and suppress the occurrence of defects such as linear defects on the surface even after going through the subsequent alloying heat treatment process. Since the greater the difference in the above converted concentration value, the more advantageous it is, there is no need to specifically define the upper limit of that value. However, considering the content of the elements contained, the difference in the above converted concentration value can be 400% or less in the case of Mn and 250% or less in the case of Si. In another implementation example of the present invention, the converted concentration difference of Mn can be 90% or more or 100% or more, and the converted concentration difference of Si can also be 60% or more or 70% or more.

[0034] Hereinafter, the GDS analysis method implemented in the present invention will be described in detail.

[0035] The galvanized steel sheet for GDS concentration analysis is sheared into a size of 30 to 50 mm in length, and immersed in a hydrochloric acid aqueous solution of 5 to 10 wt% at a normal temperature of 20 to 25 °C to remove the zinc plating layer. In order to prevent surface damage of the base iron during the dissolution process of the zinc plating layer, when the generation of bubbles due to the reaction of the acid solution with the zinc plating layer is interrupted, the acid solution is removed within 10 seconds, and the base iron is washed with pure water and dried. Of course, if it is a plating steel sheet that has not been galvanized yet, it can be analyzed without such a plating layer removal operation.

[0036] The GDS concentration profile measures the concentrations of all components contained in the steel sheet every 1 to 5 nm in the thickness direction of the steel sheet. The measured GDS profile may contain irregular noises. In order to calculate the maximum and minimum points of the Mn and Si concentrations, a Gaussian filter with a cut-off value of 100 nm is applied to the measured concentration profile to obtain an average concentration profile, and the concentration values and depths of the maximum and minimum points of the concentration are obtained from the profile with the noise removed. It should be noted that the maximum and minimum points referred to in the present invention are calculated as the maximum and minimum points only when there is a difference of 10 nm or more in the depth direction from each other.

[0037] The plating steel sheet targeted in the present invention can 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% by weight of Mn and 0.1% by weight or less (excluding 0%) of Si and has a composition in which oxides are likely to be formed on the surface, the plating property can be advantageously improved by the present invention. The upper limit of the Mn concentration in the base iron is not particularly limited, but considering the commonly used composition, the upper limit can be restricted to 1.0% by weight. Also, the lower limit of the Mn concentration is not particularly limited, but in a composition containing less than 0.1% by weight of Mn, since the surface quality of the hot-dip galvanized steel sheet is beautiful even without forming an Fe plating layer, it is not always necessary to carry out Fe electroplating. In the component system of the present invention, Si is an element that may contribute to the formation of oxides and deteriorate the plating quality even when added in a small amount. Considering the strict requirements for the surface quality of automotive outer panels, the upper limit of its content can be set to 0.1% by weight. From the viewpoint of only oxide formation, it is more advantageous not to add Si, so it is not always necessary to define the lower limit. However, considering the limitations in the industrial production process, the lower limit of the Si concentration can also be restricted to 0.001%.

[0039] Since the above-mentioned Mn and Si are elements that affect the plating property, their concentrations can be restricted as described above. However, in the present invention, the remaining components of the base iron are not particularly limited.

[0040] However, in the case of a steel sheet containing alloy components, considering the aspect that unplated and significantly reduced plating adhesion may occur, in one implementation example of the present invention, the composition of the base iron is, by weight%, Mn: 0.1 to 1.0%, Si: 0.1% or less, 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, with the balance being Fe and unavoidable impurities. P and S are impurities and it is advantageous if they are not added. Cr and B are optional elements and may not be added, so the lower limits are not separately defined. 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. In another implementation example of the present invention, the C component can be further limited to less than 0.02%, and in yet another implementation example, the C component can be further limited to 0.0199% or less. Although there are no particular restrictions on the base iron, in one implementation example of the present invention, a cold-rolled steel sheet or a 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-described plating steel sheet can be provided. The hot-dip galvanized steel sheet can include a plating steel sheet and a hot-dip galvanized layer formed on the surface of the plating steel sheet. At this time, any commercially available hot-dip galvanized steel sheet can be applied without particular limitation on its type.

[0042] Next, an exemplary implementation example of a method for manufacturing a plating steel sheet and a hot-dip galvanized steel sheet having the above-described advantageous effects will be described. According to one implementation example of the present invention, the plating steel sheet can be manufactured by a process including the steps of preparing base iron; performing electroplating on 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 is formed to obtain a plating steel sheet.

[0043] A cold-rolled steel sheet containing 0.5% Mn, 0.05% Si and other alloy elements was annealed at 800 °C for 53 seconds in an atmosphere of N2 - 5% H2 with a dew point of +5 °C and then cooled, and observed with a transmission electron microscope. The atmosphere was maintained the same throughout the entire heating time, and the dew point temperature was maintained at -40 °C during cooling so that Fe would not oxidize. Before annealing, the iron deposition amount was 2.06 g / m 2 The phenomena that appeared were observed by dividing into the case where Fe electroplating containing 7.1% by weight of oxygen was carried out so that the iron deposition amount became and the case where it was not carried out.

[0044] As a result of the observation, it was found that in the steel sheet annealed at a dew point of +5 °C without Fe plating, fine Mn and Si oxides were observed from the surface layer part, and thick grain boundary oxides were formed inside the base iron. This is because grain boundary oxides begin to form from the stage where the cold-rolled structure recovers and recrystallizes into fine crystal grains during the heating process, and as the annealing temperature rises and the annealing time increases, oxygen flows into the base iron with coarsened crystal grains, resulting in the formation of oxides at the center of the grain boundaries. Such a form ultimately results in a gradual change in the concentrations of Mn and Si components in the GDS profile, and the maximum and minimum points do not surely appear, or even if they appear, the difference in the converted concentrations does not satisfy the range limited in the present invention.

[0045] However, when annealing is performed after plating with an Fe plating layer containing 5 - 50% by weight of oxygen so that the iron deposition amount is 1.99 g / m 2 almost no oxides are generated in the Fe plating layer region, and particulate oxides are generated at the interface between the Fe plating layer and the base iron and inside the base iron. This oxide acts as a nucleus for internal oxides, and linear oxides grow in a direction perpendicular to the steel sheet surface. However, the depth of formation of internal oxides is deeper when no Fe plating layer is formed than when an Fe plating layer is formed. In such a case, there may be a depletion layer in which the Mn and Si components are present in a small amount in the Fe plating layer (surface layer part), not only showing a maximum value at the interface, but also the contents of Mn and Si in a region deeper than the maximum value decreasing significantly.

[0046] On the one hand, when annealing is carried out in a high dew point atmosphere without performing Fe plating, oxides are generated at the grain boundaries of the fine recrystallized structure from the surface of the base iron, suppressing crystal growth. As a result, irregular fine crystal grains surrounded by fine oxides are generated. On the other hand, after forming a plating layer with a high oxygen content and annealing at a high dew point of -15°C to +30°C, since the Fe plating layer does not contain oxidizing alloy elements such as Mn and Si, oxides are not generated at the grain boundaries of the plating layer. Oxides are generated at the interface between the Fe plating layer and the base iron, so there is a characteristic that the Fe plating layer structure with a uniform thickness and the crystal grains inside the base iron are distinguishable. However, since the boundary between the Fe plating layer and the base iron may not clearly appear depending on the dew point in the annealing furnace, the elongation rate of the base iron, the components of the steel, etc., even if the dew point in the annealing furnace is controlled to -15°C to +30°C after applying Fe electroplating, it does not necessarily have such characteristics.

[0047] Unlike the oxidation-reduction method, the annealing internal oxidation method does not form a layered oxide layer, so it exhibits excellent characteristics in improving plating adhesion during hot-dip galvanizing of steel plates containing alloy elements such as Mn and Si. However, since the water vapor in the annealing furnace inevitably oxidizes the surface of the steel plate first and then oxygen penetrates inside, the surface oxides cannot be fundamentally removed.

[0048] To solve the above problems, the inventors have found that after forming an Fe plating layer containing a large amount of oxygen through many experiments and annealing in a high dew point atmosphere, oxygen from the water vapor in the annealing furnace does not form surface oxides of alloy 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 alloy elements such as Mn and Si in the base iron, effectively suppressing diffusion to the surface. The oxygen flowing into the steel due to the high dew point in the annealing furnace further internally oxidizes the alloy elements, so almost no surface oxides of alloy elements are generated on the surface of the steel, significantly improving the surface quality and plating adhesion of the hot-dip galvanized steel plate. It also promotes the alloying reaction when manufacturing the alloyed hot-dip galvanized steel plate, and a uniform alloyed hot-dip plated steel plate without surface defects can be obtained.

[0049] More specifically, an Fe plating layer containing 5 to 50% by weight of oxygen is formed on a cold-rolled steel sheet (base iron), and after heating to a temperature of 600 to 950 °C so as to ensure the mechanical properties of the steel sheet in an annealing furnace controlled at a dew point of -15 °C to +30 °C and then cooling again to perform molten plating, non-plating is suppressed and a molten-plated steel sheet excellent in plating adhesion can be obtained.

[0050] In one implementation example of the present invention, the Fe plating layer can be formed by a continuous plating process, and the amount of Fe plating at this time can be set to 0.5 to 3.0 g / m based on the amount of Fe deposition. 2 When the amount of Fe plating is less than 0.5 g / m, the effect of suppressing the diffusion of alloying elements by the Fe plating layer may be insufficient in a normal continuous annealing process. Also, even if it exceeds 3.0 g / m, the suppression effect of alloying elements can be further increased, but in order to ensure a high plating amount, a plurality of plating cells must be operated. When an insoluble anode is used, there are problems such as the rapid acidification of the electroplating solution, a decrease in plating efficiency, and the generation of sludge, which is not economical. In another implementation example of the present invention, the amount of Fe plating may be 1.0 to 2.0 g / m. 2 When internal oxidation is performed after forming the Fe plating layer, internal oxides are formed at the interface or immediately below the interface between the Fe plating layer and the base iron, so the maximum points of the Mn and Si concentrations will exist in the region of 0.05 to 1.0 μm. The amount of Fe plating of 0.5 to 3.0 g / m in the present invention can correspond to a thickness of 0.05 to 0.4 μm after annealing. 2 2 2 2 2 2

[0051] In addition, by controlling the temperature, dew point temperature, and atmosphere of the subsequent annealing process for the Fe plating layer having the above-described high oxygen concentration, maximum and minimum points are formed in the GDS concentration profiles of Mn and Si elements inside the steel sheet for plating, and the converted concentration at the maximum point and the converted concentration at the minimum point can satisfy the numerical range restricted in one implementation example of the present invention. Considering such points, the oxygen concentration in the Fe plating layer in one implementation example of the present invention is 5 to 50% by weight, and in another implementation example, it can be 10 to 40% by weight. In order to obtain the effect of suppressing surface oxides, it is necessary that the amount of oxygen in the Fe plating layer is sufficiently large. 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 in order to obtain such an effect, plating must be carried out exceeding 3.0 g / m 2 2 , and thus various problems described above may occur. Also, when the oxygen content does not reach 5% by weight, it becomes difficult to sequentially form maximum and minimum points in the GDS profiles of Mn and Si. Therefore, in one implementation example of the present invention, the oxygen content in the Fe plating layer is controlled to be 5% by weight or more. On the other hand, as the oxygen concentration in the Fe plating layer increases, the effect of suppressing surface oxides during annealing can be further increased. However, in a normal electroplating method, it is difficult to obtain a plating layer exceeding 50% by weight, so the upper limit can be restricted to 50% by weight. In another implementation example of the present invention, the oxygen concentration in the Fe plating layer can also be restricted to 10 to 40%.

[0052] In one implementation example of the present invention, the annealing temperature may be 600°C to 950°C based on the temperature of the steel sheet in the soaking zone. If the annealing temperature is too low, the structure of the cold-rolled steel sheet will not be properly recovered and recrystallized, and it will be difficult to ensure the mechanical properties such as the strength and elongation rate of the steel sheet. If it exceeds 950°C, the alloy elements in the steel will rapidly diffuse to the surface, resulting in poor quality of hot-dip galvanized products and uneconomical operation due to the need to operate at a higher temperature than necessary.

[0053] On the one hand, in one implementation example of the present invention, the dew point inside the annealing furnace may be -15°C to +30°C. When the dew point is less than -15°C, the amount of oxygen flowing into the steel decreases, only surface oxidation is aggravated, and internal oxidation does not occur. Therefore, a large amount of oxides exist on the surface, and the quality of hot-dip galvanized products deteriorates. Also, even when the dew point exceeds +30°C, internal oxidation increases, and the effect of suppressing surface oxidation by suppressing the diffusion of alloying elements further increases. However, since the supply amount of water vapor increases rapidly, the capacity of the humidification equipment must be made larger than necessary. The cooled water vapor condenses, and if it is applied for a long time in continuous annealing, equipment problems may occur. The above dew point can be controlled within the above-mentioned range at 600 - 950°C, and in a lower temperature range, it can be controlled under more relaxed conditions. In another implementation example of the present invention, the above dew point can also be limited to -10°C to +20°C.

[0054] Also, in order to prevent the oxidation of the base iron and the Fe plating layer during annealing, the hydrogen concentration in the atmosphere gas during annealing can be 1% or more by volume. When the hydrogen concentration is less than 1%, a small amount of oxygen inevitably contained in H2 and N2 gases cannot be effectively oxidized and removed, and the oxygen partial pressure may increase, inducing the surface oxidation of the base iron. On the other hand, when the hydrogen concentration exceeds 70%, the explosion risk during gas outflow and the cost due to high-hydrogen operations increase. Therefore, the above hydrogen concentration can be set to 70% or less. Except for the inevitable impurity gases contained, the gas other than the above hydrogen (H2) may be substantially nitrogen (N2).

[0055] And according to one implementation example of the present invention, during annealing, the holding time after reaching the target temperature can be limited to 5 - 120 seconds. During annealing, in order for heat to be sufficiently transferred to the inside of the base iron to obtain uniform mechanical properties in the thickness direction, it is necessary to hold at the annealing target temperature for 5 seconds or more. On the other hand, if the high-temperature annealing holding time is too long, the diffusion of alloying elements interfering through the Fe plating layer increases, the amount of surface oxides generated increases, and as a result, the quality of hot-dip galvanized products becomes poor. Therefore, it can be limited to 120 seconds or less.

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

[0057] FIG. 2 schematically shows the phenomena occurring inside the steel sheet by raising the temperature of the steel sheet according to the conditions of the present invention. (a) of FIG. 2 shows a schematic cross-sectional view of the base iron on which an Fe plating layer containing a large amount of oxygen is formed. The base iron contains alloy elements such as Mn and Si. The Fe plating layer contains 5 to 50 wt% oxygen and impurities inevitably mixed during electroplating, and the balance is composed of Fe.

[0058] FIG. 2(b) shows the state where the cold-rolled steel sheet plated with Fe is heated to about 300 to 500 °C in a nitrogen atmosphere containing 1 to 70% H2. The surface of the Fe plating layer containing a large amount of oxygen is gradually reduced and 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, so the diffusion to the surface is suppressed. Also, 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 in the annealing furnace widely changes from -90 °C to +30 °C in the low temperature region of the heating-up stage, because the temperature is low, the rate of oxygen dissociated from water vapor diffusing into the steel is slower than the rate at which a large amount of oxygen exists in the Fe plating layer and the Fe plating layer is reduced and oxygen is released. Therefore, in the low temperature section, even if the dew point in the annealing furnace changes, it is not greatly affected. Therefore, the control of 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. When 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 at the low-temperature stage, the alloying elements inside the base iron continuously act as nuclei for internal oxidation of oxides that can occur. In order for such oxide nuclei to be generated, it is necessary for the concentrations of oxygen and alloy components to be high at the same time. However, if the Fe plating layer contains a sufficient amount of oxygen, a large number of oxide nuclei are generated near the interface between the Fe plating layer with a high oxygen concentration and the base iron with a high alloy element concentration. However, when the Fe plating layer contains almost no oxygen, the alloying elements contained in the base iron will pass through the Fe plating layer to form oxides on the surface. Subsequently, when the temperature is raised, the oxygen in the Fe plating further depletes, and the diffusion of alloying elements in the base iron is further aggravated, resulting in an increase in the formation of surface oxides.

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

[0061] Figure 2(d) shows a schematic cross-sectional view of the steel sheet after maintaining it 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. On the steel sheet surface, oxygen supplied from water vapor rapidly penetrates and is supplied. Therefore, Mn and Si oxidize inside, but in the low-temperature range, particulate Mn and Si oxides generated by reacting with the oxygen in the Fe plating layer at the interface between the Fe plating layer and the base iron act as nuclei for oxide growth. Thus, the internal oxides grow concentrated 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 with large atomic sizes, internal oxides are formed deeply not only along the grain boundaries but also through the grains inside the base iron.

[0062] As described above, the control conditions were explained according to temperature. However, the most crucial stage in the annealing process is the stage of maintaining the steel sheet temperature at 600 to 950 °C. By simply controlling the dew point of the atmosphere in this temperature range, the internal oxide distribution of the steel sheet for plating can be effectively controlled. Of course, such dew point control can be carried out in all processes before the above-mentioned maintenance stage without any particular problem. Furthermore, it should be noted that the above-described process is merely an example for explaining one implementation example of the present invention, and the reaction mechanism of the present invention is not always construed as being restricted by the above description.

[0063] After the above annealing stage, the annealed steel sheet can be cooled. Since the cooling conditions in the cooling stage after the annealing stage do not significantly affect the surface quality of the final product, i.e., the plating quality, there is no particular need to limit the cooling conditions in the present invention. However, in order to prevent oxidation of the iron component during the cooling process, a reducing atmosphere can be applied at least for iron.

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

[0065] In the present invention, as long as it is a base iron having the above-described alloy composition, it can be applied without limitation as the base iron of the steel sheet for plating or the hot-dip galvanized steel sheet according to the present invention. Therefore, the method for manufacturing the base iron does not have 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] That is, in the present invention, in order to form the Fe plating layer, an electroplating solution containing iron ions including ferrous ions and ferric ions, a complexing agent, and inevitable impurities can be used, and the concentration of the ferric ions among the above 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. In order to obtain high plating efficiency, it may be advantageous that only ferrous ions are contained. However, when only ferrous ions are contained, the solution deteriorates and the plating efficiency rapidly decreases, so there is a possibility of inducing quality variations in the continuous electroplating process. Therefore, the above ferric ions can be further contained. At this time, the concentration of the ferric ions is preferably 5 to 60% by weight, more preferably 5 to 40% by weight, of the total of ferrous and ferric ions. When it is less than 5%, the rate at which ferric iron is reduced to ferrous iron at the cathode is smaller than the rate at which ferrous iron is oxidized to ferric iron at the anode. Therefore, the concentration of ferric iron rapidly increases, the pH rapidly decreases, and the plating efficiency continuously decreases. On the other hand, when the concentration of ferric ions exceeds 60%, the reaction amount of ferric iron reduced to ferrous iron at the cathode increases significantly more than the reaction amount of ferrous iron reduced and deposited as metallic iron. Therefore, the plating efficiency is significantly reduced and the plating quality deteriorates. Therefore, considering the characteristics of the equipment and the process, such as the plating amount, the working current density, the solution replenishment amount, the amount of solution adhering to the strip and flowing away, and the concentration change rate due to evaporation, it is preferable to make the concentration of the ferric ions in the above iron ions 5 to 60% by weight.

[0069] The concentration of the above iron ions is preferably 1 to 80 g per 1 L of the above electroplating solution, and more preferably 10 to 50 g per 1 L. When it is less than 1 g / L, there is a problem that the plating efficiency and plating quality rapidly decrease. On the other hand, when it exceeds 80 g / L, precipitation may occur due to exceeding the solubility, and raw material loss due to solution loss increases in the continuous plating process, which is not economical.

[0070] The electroplating solution of the present invention contains a complexing agent, but sludge does not occur while containing a large amount of ferric iron, and it is preferable to use an amino acid or an amino acid polymer as the complexing agent to maintain a high plating efficiency.

[0071] An amino acid refers to an organic molecule in which a carboxyl group (-COOH) and an amino group (-NH2) are bonded. An amino acid polymer means an organic molecule formed by polymerization of two or more amino acids, and the amino acid polymer exhibits complexing agent characteristics similar to those of amino acids. Therefore, in the following description, amino acids and amino acid polymers are collectively referred to as amino acids.

[0072] When an amino acid is dissolved in neutral water, the amine binds to a hydrogen ion and has a positive charge, and the carboxyl group dissociates a hydrogen ion and has a negative charge, so the amino acid molecule maintains charge neutrality. On the other hand, when the solution is acidified, the carboxyl group recombines with a hydrogen ion to become charge neutral, and the amine has a positive charge, so the amino acid molecule forms a cation. That is, an amino acid forms a charge-neutral or cation in a weakly acidic aqueous solution.

[0073] When an amino acid is added to an acidic electrolytic solution containing iron ions, it is complexed with ferrous ions and ferric ions, but the iron ions complexed with the amino acid maintain a cation state even in the complexed state. Therefore, it exhibits characteristics electrically opposite to those of a normal complexing agent having a plurality of carboxyl groups that shows a negative charge in a weakly acidic aqueous solution.

[0074] In addition, compared to complexing agents containing multiple carboxyl groups such as citric acid and EDTA, amino acids form fewer bonds with iron ions and the bonding strength is weaker, but the bonding strength with ferric ions that generate sludge is strong enough to prevent precipitation by ferric ions. Furthermore, since the ferric ions can maintain their cation status even when complexed, they can be easily transferred to the cathode and reduced to ferrous ions to participate in the plating reaction, while their movement to the anode is suppressed and the rate of ferric ion generation slows down, so that the concentration of ferric ions remains at a constant level even when plating is performed continuously for a long period of time, plating efficiency remains constant, and there is no need to replace the electrolyte.

[0075] Meanwhile, in the continuous electroplating process, when the iron ions in the solution are consumed by plating, the solution becomes acidic, but even if the same amount of iron ions are precipitated, the pH change is smaller in a solution that also contains ferric ions than in a solution that contains only ferrous ions. When the pH increases, some of the ferric ions combine with hydroxyl ions, and when the pH decreases, the hydroxyl ions are separated and neutralized, so the pH change in a solution that contains ferric ions slows down even without a separate pH buffer, and it acts as a pH buffer, so the electroplating efficiency can be maintained constant in the continuous electroplating process.

[0076] Therefore, amino acids can be used as complexing agents to prevent sludge, and not only ferrous ions but also ferric ions can be used as plating raw materials. When ferrous ions and ferric ions are mixed and used, the pH change of the solution can be slowed down and the accumulation of ferric ions can be easily prevented, so that the electroplating efficiency and plating quality can be kept constant in the continuous electroplating process.

[0077] In addition, the above complexing agent is preferably added in an amount such that the molar concentration ratio of the above iron ions 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, the excessive ferrous ions cannot be prevented from binding to hydroxide ions or oxygen to form sludge, and even without the inclusion of ferrous iron, 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 compared with raw materials containing iron ions such as iron sulfate, relatively expensive amino acids are contained in an excessive amount more than necessary, so that the raw material cost increases and it is not economical.

[0078] The above complexing agent is preferably one or more selected from amino acids or amino acid polymers, and 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 high plating efficiency and an Fe plating layer with a high oxygen concentration can be obtained.

[0080] The temperature of the Fe electroplating solution does not significantly affect the quality of the Fe plating layer, but 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 a continuous plating process. When the pH exceeds 5.0, the plating efficiency increases, but sludge in which iron hydroxide precipitates is generated during continuous electroplating, causing problems such as pipe blockage, contamination of rolls and equipment.

[0082] The current density is 3 A / dm² 2When it is less than this value, the plating overvoltage of the cathode decreases and the Fe electroplating efficiency decreases, so it is not suitable for the continuous plating process. When it exceeds 120 A / dm 2 problems occur, such as burning on the plating surface, resulting in a non-uniform electroplated layer and an easy detachment of the Fe plating layer.

[0083] As described above, it is preferable that the Fe plating layer contains 5 to 50% by weight of oxygen. The reasons for the oxygen to be mixed into the Fe plating layer are as follows. In the process of iron deposition on the surface of the steel sheet to which the cathode is applied, at the same time, hydrogen ions are reduced to hydrogen gas and the pH increases. Therefore, both ferrous and ferric ions may be temporarily combined with OH - ions and be mixed together when the Fe plating layer is formed. If an anionic complexing agent such as acetic acid, lactic acid, citric acid, or EDTA is used, the iron ions combined with the OH - ions will show an average negative charge. When a cathode is applied for electroplating, an electrical repulsive force is generated and the mixing into the Fe plating layer is suppressed. On the other hand, amino acids are electrically neutral at pH 2.0 to 5.0, show cations in strong acids with pH less than 2.0, but even if 1 to 2 OH - ions are combined with the iron ions combined with amino acids, they still show cations. Therefore, an electrical attractive force with the cathode for electroplating is generated and a large amount of oxygen is mixed in. Therefore, when amino acids are used as a complexing agent so that the molar concentration ratio of iron ions to amino acids is 1:0.05 to 1:2.0 and Fe electroplating is carried out while maintaining pH 2.0 to 5.0, a high plating efficiency can be obtained, and an Fe plating layer containing 5 to 50% by weight of oxygen can be obtained while suppressing the generation of sludge.

[0084] To ensure the quality of hot-dip galvanized products of steel sheets containing Mn and Si, it is preferable to treat the plating amount of the Fe plating layer at 0.5 to 3.0 g / m 2 based on the iron concentration. The upper limit of the Fe plating amount is not particularly limited, but in the continuous plating process, when it exceeds 3.0 g / m 2When it exceeds this value, multiple plating cells are required or the production speed decreases, which is not economical. Furthermore, when the amount of Fe electroplating is large, the Fe electroplating solution rapidly denatures in the continuous process, the pH decreases, and there is a problem that the plating efficiency greatly decreases and solution management becomes difficult. On the other hand, when the amount of Fe electroplating is less than 0.5 g / m 2 ², since the oxygen contained in the Fe plating layer is rapidly reduced and removed, it becomes impossible to effectively suppress the diffusion of Mn and Si from the base iron and the formation of surface oxides, resulting in a problem of deterioration of the quality of the hot-dip plating. The above Fe plating amount has a thickness of about 0.05 to 0.4 μm when the Fe plating layer is completely reduced during annealing at the iron concentration contained in the plating layer.

Claims

1. 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 unavoidable impurities, having a composition, when observed in the depth direction from the surface, the GDS profile of the Mn component and the GDS profile of the Si component 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, 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, a steel sheet. However, when no minimum point appears within a depth of 5 μm, the point at a depth of 5 μm is regarded as the point where the minimum point appears.

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

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

4. The steel sheet according to claim 1, wherein the depth at which the maximum point is formed is 0.05 to 1.0 μm.

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

6. preparing a base iron having a composition 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 unavoidable impurities; performing electroplating on the base iron to form an Fe plating layer containing 5 to 50% by weight of oxygen; and The substrate iron with the Fe plating layer formed thereon is maintained at 600 to 950 °C for 5 to 120 seconds in an annealing furnace in a gas atmosphere of 1 to 70% H controlled at a dew point temperature of -15 to +30 °C ２ - balance N ２ A method for manufacturing a steel sheet, including a step of annealing by maintaining at 600 to 950 °C for 5 to 120 seconds in an annealing furnace in a gas atmosphere.

7. The adhesion amount of the Fe plating layer is 0.5 to 3 g / m ２ The manufacturing method of the steel sheet for plating according to claim 6, wherein it is like this.

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

9. The electroplating solution contains ferrous ions and ferric ions, the ferric ions having a proportion of 5 to 60% by weight based on the total amount of iron ions, and the total concentration of the iron ions being 1 to 80 g per liter of the electroplating solution. The method for manufacturing a plated steel sheet according to claim 6 or 7.

10. The electroplating is carried out under the conditions of a solution temperature of 80°C or lower and a current density of 3 to 120 A / dm ２ The method for manufacturing a steel sheet for plating according to claim 6 or 7, wherein the electroplating is carried out under the conditions of a solution temperature of 80°C or lower and a current density of 3 to 120 A / dm

11. Preparing a base iron having a composition containing, in % 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; Performing electroplating on the base iron to form an Fe plating layer containing 5 to 50% by weight of oxygen; The base iron on which the Fe plating layer is formed is annealed in an annealing furnace with a gas atmosphere controlled at a dew point temperature of -15 to +30°C and 1 to 70% H ２ - remaining N ２ maintained at 600 to 950°C for 5 to 120 seconds to obtain a steel sheet for plating; and A method for manufacturing a hot-dip galvanized steel sheet, including the step of immersing the plated steel sheet in a zinc plating bath.

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