Plated steel sheet for hot press forming with excellent plating quality, steel sheet, and manufacturing method thereof

Abstract

The present invention relates to a hot-dip galvanized steel sheet with excellent plating quality, a galvanized steel sheet for manufacturing the same, and methods for manufacturing these. The hot stamping steel sheet according to one aspect of the present invention has a composition containing, by weight%, Mn: 0.1 to 4%, Si: 0.001 to 2%, C: 0.02 to 0.6%, Al: 0.001 to 1%, P: 0.05% or less, S: 0.02% or less, Cr: 1% or less, B: 0.01% or less, with the balance being Fe and inevitable impurities. The GDS profiles of the Mn component and 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 can be 50% or more. 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.

Description

Technical Field

[0001] The present invention relates to a plated steel sheet for hot press forming with excellent plating quality, a steel sheet for plating, and methods for manufacturing these.

Background Art

[0002] Hot press forming is a processing method for obtaining high-strength parts by forming a steel sheet at a high temperature and substantially simultaneously cooling it. The steel sheet used for hot press forming must have excellent hardenability so that martensite can be easily formed during cooling at a high temperature. In order to improve the hardenability of the steel sheet in this way, various alloying elements are added to the hot press forming steel compared to general steel, and in particular, elements with a high oxidation tendency with respect to Fe, such as Mn, Si, Al, Cr, and B, are added in large amounts. In addition, in order to prevent decarburization and oxidation of the steel sheet during hot press forming, various types of plating may be applied to the surface of the steel sheet. Among them, a method of plating the steel sheet surface by molten plating, such as molten plating or molten aluminum plating, is often used.

[0003] The quality of molten plating is determined by the surface state of the annealed steel sheet immediately before plating. However, the plating property 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. 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 steel sheet surface, thereby reducing the surface reactivity. The surface of the annealed steel sheet with reduced reactivity hinders the wettability of the molten plating bath, causing unplated areas where the plating metal does not adhere locally or entirely to the surface of the plated steel sheet. In addition, such oxides cause insufficient formation of the alloying suppression layer (Fe2Al5) necessary to ensure the adhesion of the plating layer during the molten plating process, resulting in the occurrence of plating layer peeling and a significant decrease in the plating quality of the plated steel sheet.

[0004] In order to improve the plating quality of hot-dip galvanized steel sheets, various techniques have been proposed. 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 Si, Mn, or Al alone or composite oxides to a certain depth inside the steel sheet, and then performs reduction annealing of the iron oxides in a reducing atmosphere, and then performs hot-dip galvanizing to provide a hot-dip galvanized or alloyed hot-dip galvanized steel sheet with excellent plating quality.

[0005] When using the method of oxidation followed by reduction in the annealing process as in Patent Document 1, components with a large affinity for oxygen such as Si, Mn, and Al are internally oxidized at a certain depth from the surface layer of the steel sheet, and the diffusion to the surface layer is suppressed. Therefore, in the surface layer, Si, Mn, or Al alone or composite oxides relatively decrease, the wettability with zinc is improved, and unplated areas can be reduced. However, in the case of steel grades with Si added, Si is concentrated directly under the iron oxide during the reduction process to form strip-shaped Si oxides, resulting in peeling in the surface layer including the plating layer, that is, peeling occurs at the interface between the reduced iron and the base iron below, 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 proposes 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 that are externally oxidized on the surface of the steel sheet after annealing and improve the plating property. However, as a 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] Also, even if plating properties are improved by internal oxidation, linear unplated areas may occur due to surface oxides unevenly formed on the surface, or when producing galvannealed steel sheets (GA steel sheets) through alloying heat treatment after plating, problems such as linear defects due to non-uniform alloying may occur on the surface of the galvannealed steel sheets.

[0008] Furthermore, as another conventional technique, there is a method of performing Ni pre-plating before annealing to suppress the diffusion of alloying 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

Patent Document 2

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 plated steel sheet with excellent plating quality, in which unplated areas do not occur and the problem of peeling of the plating layer is solved, 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 a galvannealed steel sheet with excellent surface quality without generating linear defects even when 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 plated steel sheet having such excellent plating quality.

[0013] The problems of the present invention are not limited to the above-described content. For those with ordinary knowledge in the technical field to which the present invention pertains, there is no difficulty 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 hot press forming steel sheet according to one aspect of the present invention has a composition containing, by weight%, Mn: 0.1 to 4%, Si: 0.001 to 2%, C: 0.02 to 0.6%, Al: 0.001 to 1%, P: 0.05% or less, S: 0.02% or less, Cr: 1% or less, B: 0.01% or less, with the balance being Fe and unavoidable impurities. When observed in the depth direction from the surface, the GDS profiles of the Mn component and the Si component each sequentially include a maximum point and a minimum point. The difference (converted Mn concentration difference) 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 difference (converted Si concentration difference) 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 difference 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 are each 50% or more.

[0015] 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.

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

[0017] Another aspect of the present invention is a method for manufacturing a hot press forming steel sheet, which includes the steps of preparing a base iron having a composition containing, by weight%, Mn: 0.1 to 4%, Si: 0.001 to 2%, C: 0.02 to 0.6%, Al: 0.001 to 1%, P: 0.05% or less, S: 0.02% or less, Cr: 1% or less, B: 0.01% 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 annealing the base iron having the Fe plating layer formed thereon in an annealing furnace with a 1 to 70% H2 - the balance being N2 gas atmosphere controlled at a dew point temperature of -15 to +30°C at 600 to 950°C for 5 to 120 seconds.

[0018] Still another aspect of the present invention is a method for manufacturing a hot press forming hot dip galvanized steel sheet, which includes the steps of preparing a base iron having a composition containing, by weight%, Mn: 0.1 to 4%, Si: 0.001 to 2%, C: 0.02 to 0.6%, Al: 0.001 to 1%, P: 0.05% or less, S: 0.02% or less, Cr: 1% or less, B: 0.01% 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 having the Fe plating layer formed thereon in an annealing furnace with a 1 to 70% H2 - the balance being N2 gas atmosphere controlled at a dew point temperature of -15 to +30°C at 600 to 950°C for 5 to 120 seconds to obtain a plating steel sheet; and dipping the plating steel sheet into a hot dip galvanizing bath consisting of Al: 0.1 to 0.3% and the balance being Zn and unavoidable impurities and maintained in a temperature range of 440 to 500°C.

Advantages of the Invention

[0019] As described above, the present invention can provide a hot dip galvanized steel sheet that forms a pre - plating layer and controls the concentration profiles of internal Mn and Si components, significantly improving the phenomenon of un - plating during hot dip galvanizing and enhancing the plating adhesion.

[0020] According to another aspect of the present invention, linear defects and the like can be prevented on the surface of an alloyed hot-dip galvanized steel sheet obtained by subjecting the hot-dip galvanized steel sheet of the present invention to an alloying heat treatment, and an alloyed hot-dip galvanized steel sheet having 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 plated steel sheet with excellent plating quality according to an aspect of the present invention completed by the research of the present inventor will be described in detail. In the present invention, it should be noted that unless otherwise specified, the concentration of each element means weight%. Further, 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 were not included in the plating amount.

[0023] Furthermore, unless otherwise specified, 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] It is known that the causes of non-plating and a decrease in plating adhesion in a steel sheet containing a large amount of Mn and Si are due to surface oxides generated by oxidizing alloy elements such as Mn and Si on the surface during the process of annealing the cold-rolled steel sheet at a high temperature.

[0026] In order to suppress the diffusion of alloying elements such as Mn and Si to the surface, an oxidation-reduction method in which an oxide layer containing a large amount of oxygen is formed, oxidized during heating, and then maintained in a reducing atmosphere for reduction, or a method in which the surface of the base metal is coated with iron oxide 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, and 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 has a slow reduction rate, so it is difficult to be completely reduced, and Mn and Si oxides accumulate at the interface to form a continuous oxide layer. Therefore, even if the wettability with molten zinc is improved, there is a possibility that the oxide layer may easily collapse and the plating layer may peel off.

[0027] On the other hand, when applying the annealing internal oxidation method of 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 are preferentially formed on the steel surface during the heat treatment process. After that, Mn and Si are oxidized by the oxygen diffused into the steel to suppress 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 before annealing is not completely homogeneous, or local variations such as oxygen partial pressure and temperature occur, the wettability in the molten zinc plating is non-uniform and non-plating occurs, or in the case of zinc plating, if the thickness of the oxide film is non-uniform and a difference in alloying degree occurs during the alloying heat treatment process after plating, 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 molten plating steel sheet with a beautiful surface and no plating peeling problem by controlling the existence forms of Mn and Si, which are oxidizing elements, on the surface side of the plating steel sheet 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. The plating steel sheet of the present invention will be described in detail with reference to the GDS profile of FIG. 1.

[0030] Figure 1 is a graph schematically showing a typical GDS profile of the Mn component 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. The vertical axis in the graph indicates the concentrations of alloy elements such as Mn and Si, and the horizontal axis indicates the depth. As illustrated in the graph of Figure 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 profiles of the Mn or Si component go from the surface (when hot-dip plated, the interface with the plating layer) toward the inside. Here, having sequentially means that it does not necessarily mean that the maximum point always appears first in the depth direction from the surface (interface), and in some cases, the minimum point may appear first, but then the maximum point and the minimum point must appear sequentially. However, in some embodiments, the minimum point may not appear, and in this case, the internal concentration in the 5-μm depth region can be taken as the minimum point concentration. Also, the concentration of the alloy element on the surface has a value lower than the concentration of the maximum point, but in some cases, a minimum point with a low concentration of the alloy element 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, since not many alloy elements diffuse from the base iron in the surface layer part, it corresponds to the Fe plating layer with a low concentration of alloy elements, the maximum point corresponds to the region where the internal oxide of the alloy element concentrated near the interface between the Fe plating layer and the base iron is formed, and the minimum point that appears on the base iron side in the Fe plating layer corresponds to the region where the alloy element diffuses and is diluted in the Fe plating layer that does not contain the alloy element or the region where the alloy element diffuses and depletes in the maximum point where internal oxidation occurs.

[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 can be determined that it is not the maximum point due to the effect of the present invention. Further, the minimum point can be formed at a position within 5 μm from the surface of the steel plate. As described above, if the minimum point is not formed at a point within 5 μm in depth, 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 regarded as the point where the concentration does not decrease further.

[0033] At this time, in the Mn concentration profile and the Si concentration profile, the larger 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 %), and the converted concentration of the minimum point, the more Mn and Si diffusing to the surface can be reduced, which is important. In one embodiment 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 in the case of Si, the difference in the above value can be 50% or more. Si is an element with stronger oxidizing property than Mn, and internal oxidation easily occurs even inside the base iron with low oxygen concentration, so oxidation may occur in a wider region than Mn. Therefore, even if the difference between the converted concentrations of 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 the inventors' experiments under various conditions, when the above conditions are satisfied, no unplated areas occur during hot dip plating, and a hot dip plated steel sheet with good plating adhesion can be obtained. However, if the difference between the converted concentrations of the maximum and minimum points of Mn is less than 80%, or the difference between the converted concentrations of the maximum and minimum points of Si is less than 50%, there may be problems such as the occurrence of spot or linear unplated areas or plating peeling. That is, by doing so, it is possible to prevent the formation of oxides of Mn and Si on the surface, obtain a hot dip plated 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 passing through the process of alloying heat treatment. Since the larger the difference in the above converted concentration values, the more advantageous, 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 values can be defined as 400% or less in the case of Mn, and 250% or less in the case of Si. In another embodiment 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] For GDS concentration analysis, the molten-plated steel sheet is sheared into pieces with a length of 30 to 50 mm, first washed with a NaOH solution at room temperature, and then immersed in a hydrochloric acid aqueous solution of 20 to 40 vol% to remove the plating layer.

[0036] In order to prevent surface damage to the base iron during the dissolution process of the plating layer, when the generation of bubbles due to the reaction between the plating layer and the acid solution 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 steel sheet for plating that has not yet been molten-plated, it can be analyzed without such a plating layer removal operation.

[0037] The GDS concentration profile measures the concentrations of all components contained in the steel sheet at 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 noises removed. It should be noted that the maximum and minimum points mentioned in the present invention are calculated as the maximum and minimum points only when there is a difference of 10 nm or more between them in the depth direction.

[0038] The steel sheet for plating targeted by 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.

[0039] However, in the case of a steel sheet having a composition containing 0.1 to 4% by weight of Mn and 0.001 to 2% by weight of Si, which is likely to form an oxide 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 limited to 4% 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, the surface quality of the hot-dip galvanized steel sheet is beautiful even without forming an Fe plating layer, and it is not necessary to perform Fe electroplating. The upper limit of the Si concentration is not particularly limited, but considering the commonly used composition, the upper limit can be limited to 2% by weight or less. When the Si concentration is less than 0.001% by weight, the hot-dip galvanized quality is beautiful even without simultaneously performing Fe electroplating and annealing internal oxidation, so it is not necessary to carry out the method of the present invention.

[0040] Since the above Mn and Si are elements that affect the plating property, their concentrations can be limited as described above, but the present invention does not particularly limit the remaining components of the base iron.

[0041] However, in the case of a steel sheet containing a large amount of alloy components, considering the aspect that unplated and reduced plating adhesion can occur severely, in one embodiment of the present invention, the composition of the above base iron is in wt%, Mn: 0.1 to 4%, Si: 0.001 to 2%, C: 0.02 to 0.6%, Al: 0.001 to 1%, P: 0.05% or less, S: 0.02% or less, Cr: 1% or less, B: 0.01% or less, and the balance is Fe and inevitable impurities. It is more advantageous that P and S are not added as impurities, and since Cr and B may not be added as optional elements, the lower limit is not separately defined. The above base iron can further contain elements such as Ti, Mo, and Nb in a total amount of 1.2% or less in addition to the above-described components. Although the above base iron is not particularly limited, in one embodiment of the present invention, a cold-rolled steel sheet or a hot-rolled steel sheet can be used as the above base iron.

[0042] In one aspect of the present invention, a hot-dip galvanized steel sheet including the above galvanizing steel sheet can be provided, and the hot-dip galvanized steel sheet can include a galvanizing steel sheet and a hot-dip layer formed on the surface of the galvanizing steel sheet. At this time, any commercially available hot-dip galvanized steel sheet can be applied without particularly limiting its type.

[0043] Next, an exemplary embodiment of a method for manufacturing a galvanizing steel sheet and a hot-dip galvanized steel sheet having the above-described advantageous effects will be described. According to one embodiment of the present invention, the galvanizing steel sheet includes a step of preparing base iron; a step of performing electroplating on the base iron to form an Fe plating layer containing 5 to 50% by weight of oxygen; and a step of annealing the base iron on which the Fe plating layer is formed to obtain a galvanizing steel sheet.

[0044] A cold-rolled steel sheet for hot press forming containing 1.3% Mn, 0.3% Si, and other alloy elements was annealed at 800 °C for 53 seconds in an atmosphere of N2-5% H2 and a dew point of +5 °C, and then cooled, and observed with a transmission electron microscope. The atmosphere of the entire heating time was maintained in the same manner, and the dew point temperature was maintained at -40 °C so that Fe was not oxidized during cooling. Before annealing, Fe electroplating containing 6.8% by weight of oxygen was performed so that the iron adhesion amount became 1.93 g / m 2 The phenomena that appeared were observed separately when Fe electroplating was performed and when it was not performed.

[0045] As a result of the observation, it was found that in the steel sheet annealed at a dew point of +5 °C without performing Fe plating, fine Mn and Si oxides were observed from the surface layer, and thick grain boundary oxides were formed inside the base iron. This is because grain boundary oxides start 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 interior of the base iron with coarsened crystal grains, and oxides are generated 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 point and the minimum point do not appear correctly, or even if they do appear, the difference in the converted concentration does not satisfy the range limited in the present invention.

[0046] However, when an Fe plating layer containing 5 to 50% by weight of oxygen is plated so that the iron deposition amount is 1.99 g / m 2 and then annealed, almost no oxide is formed in the Fe plating layer region, and particulate oxides are formed 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 applied. In such a case, the Fe plating layer (surface layer portion) contains little Mn and Si components, not only showing a maximum value at the interface, but also having a depletion layer in which the Mn and Si contents in the region deeper than the maximum value decrease significantly.

[0047] On the other hand, when annealing is carried out in a high dew point atmosphere without performing Fe plating, oxides are formed at the grain boundaries of the fine recrystallized structure from the surface of the base iron to suppress crystal growth, so irregular fine crystal grains surrounded by fine oxides are formed. In contrast, when annealing is carried out at a high dew point of -15°C to +30°C after forming a plating layer with a high oxygen content, the Fe plating layer does not contain oxidizing alloy elements such as Mn and Si, and oxides are not formed at the grain boundaries of the plating layer. Oxides are formed 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 distinguished. However, since the boundary between the Fe plating layer and the base iron may not appear clearly depending on the dew point in the annealing furnace, the elongation rate of the base iron, the steel components, etc., even if the dew point in the annealing furnace is controlled to -15°C to +30°C after Fe electroplating, it does not necessarily have such characteristics.

[0048] The internal oxidation method of annealing is different from the oxidation-reduction method in that it does not form a layered oxide layer. Therefore, when hot-dip plating a steel sheet for hot press forming containing a large amount of alloy elements such as Mn and Si, it shows excellent characteristics for improving plating adhesion. However, since the water vapor in the annealing furnace inevitably oxidizes the surface of the steel sheet first and then oxygen penetrates inside, the surface oxide cannot be fundamentally removed.

[0049] In order to solve the above problems, the inventors of the present invention found that after forming an Fe plating layer containing a large amount of oxygen through a number of experiments, annealing in a high dew point atmosphere can effectively suppress the oxygen in the water vapor in the annealing furnace from diffusing to the surface of the Fe plating layer without forming surface oxides of alloy elements such as Mn and Si, and the oxygen contained in the Fe plating layer from internally oxidizing alloy elements such as Mn and Si in the base iron and diffusing 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 that almost no surface oxides of the alloy elements are generated on the steel surface, and the surface quality and plating adhesion of the hot-dip galvanized steel sheet are improved epoch-makingly. When manufacturing an alloying hot-dip galvanized steel sheet, the alloying reaction can also be promoted to obtain a uniform alloying hot-dip galvanized steel sheet without surface defects.

[0050] 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 hot-dip plating, unplated areas can be suppressed, and a hot-dip plated steel sheet with excellent plating adhesion can be obtained.

[0051] In one embodiment of the present invention, the above Fe plating layer can be formed through a continuous plating process, and the amount of Fe plating at this time can be made to be 0.5 to 3.0 g / m based on the amount of Fe adhesion. 2 When the amount of Fe plating is less than 0.5 g / m, the effect of suppressing the diffusion of alloy 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 the alloy elements can be further increased, but in order to ensure a high plating amount, a plurality of plating cells must be operated. When using an insoluble anode, 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 embodiment of the present invention, the amount of Fe plating is 1.0 to 2.0 g / m. 2 When it is less than, the effect of suppressing the diffusion of alloy elements by the Fe plating layer may be insufficient in a normal continuous annealing process. Also, when it exceeds 3.0 g / m, although the suppression effect of the alloy elements can be further increased, in order to ensure a high plating amount, a plurality of plating cells must be operated. When using an insoluble anode, 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 embodiment of the present invention, the amount of Fe plating is 1.0 to 2.0 g / m. 2 When it exceeds, the suppression effect of the alloy elements can be further increased, but in order to ensure a high plating amount, a plurality of plating cells must be operated. When using an insoluble anode, 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 embodiment of the present invention, the amount of Fe plating is 1.0 to 2.0 g / m. 2It is possible. When internal oxidation is performed after forming the Fe plating layer, internal oxides are formed at or immediately below the interface between the Fe plating layer and the base iron, so that the maximum points of the Mn and Si concentrations come to exist in the 0.05 to 1.0 μm region. The amount of Fe plating of 0.5 to 3.0 g / m 2 in the present invention can correspond to a thickness of 0.05 to 0.4 μm after annealing.

[0052] Also, the Fe plating layer having the above-described high oxygen concentration forms maximum and minimum points in the GDS concentration profiles of Mn and Si elements inside the steel sheet for plating by controlling the temperature, dew point temperature, and atmosphere of the subsequent annealing process, so that the conversion concentration at the above maximum point and the conversion concentration at the minimum point can satisfy the numerical ranges restricted in an embodiment of the present invention. Considering such points, in an embodiment of the present invention, the oxygen concentration in the Fe plating layer 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 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, it is necessary to perform plating exceeding 3.0 g / m 2 , so various problems described above may occur. Also, when the oxygen content does not reach 5% by weight, it is difficult to sequentially form maximum and minimum points in the GDS profiles of Mn and Si. Therefore, in an 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, as the oxygen concentration in the Fe plating layer increases, the effect of suppressing surface oxides during annealing can be further increased, but it is difficult to obtain a plating layer exceeding 50% by weight by ordinary electroplating methods, so the upper limit can be restricted to 50% by weight. In another embodiment of the present invention, the oxygen concentration in the Fe plating layer can also be restricted to 10 to 40%.

[0053] In one embodiment of the present invention, the annealing temperature can 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 recover and recrystallize properly, making it difficult to ensure the mechanical properties such as the strength and elongation rate of the steel sheet. When the temperature exceeds 950°C, the alloying elements in the steel will rapidly diffuse to the surface, resulting in poor quality of the molten plating and uneconomical operation due to the need to operate at an unnecessarily high temperature.

[0054] On the other hand, in one embodiment of the present invention, the dew point inside the annealing furnace can 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. As a result, a large amount of oxides will exist on the surface, deteriorating the quality of the molten plating. Also, even when the dew point exceeds +30°C, although the effect of suppressing surface oxidation by increasing internal oxidation and suppressing the diffusion of alloying elements is further enhanced, the water vapor supply amount increases rapidly, so the capacity of the humidification equipment must be increased unnecessarily. When cooled water vapor condenses and is applied for a long time in continuous annealing, there may be problems with the equipment. The above dew point can be controlled within the range described above at 600 to 950°C, 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.

[0055] In addition, in order to prevent the oxidation of the base iron and the Fe plating layer during annealing, the hydrogen concentration in the atmospheric gas during annealing can be set to 1% or more by volume. When the hydrogen concentration is less than 1%, the trace amount of oxygen inevitably contained in H2 and N2 gases cannot be effectively oxidized and removed, and the oxygen partial pressure may increase, inducing surface oxidation of the base iron. On the other hand, when the hydrogen concentration exceeds 70%, there is a risk of explosion when the gas flows out and the cost due to high-hydrogen operations increases. Therefore, the above hydrogen concentration can be set to 70% or less. Except for the inevitable impurity gases, it can be substantially nitrogen (N2) other than the above hydrogen (H2).

[0056] According to an embodiment of the present invention, the holding time after reaching the target temperature during annealing can be limited to 5 to 120 seconds. During annealing, in order to sufficiently transfer heat to the inside of the base iron and 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 oxide generated increases, and as a result, the quality of the electroplated product deteriorates, so it can be limited to 120 seconds or less.

[0057] Hereinafter, based on the above content, the effect that the surface diffusion of Mn and Si is suppressed during annealing of a cold-rolled steel sheet with an Fe plating layer containing a large amount of oxygen in a high dew point atmosphere will be described in more detail with reference to FIG. 2.

[0058] 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 alloying elements such as Mn and Si, and the Fe plating layer contains 5 to 50% by weight of oxygen and impurities inevitably mixed in during electroplating, and the balance is composed of Fe.

[0059] Figure 2(b) shows the state of a cold-rolled steel sheet plated with Fe heated to about 300 - 500°C in a nitrogen atmosphere containing 1 - 70% H₂. The surface of the Fe plating layer containing 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, so the diffusion on the surface is suppressed. Also, as the temperature increases, the internal oxides at the interface gradually grow while Mn and Si diffused inside the base iron accumulate. Even if the dew point in the annealing furnace varies widely from -90°C to +30°C in the low-temperature region of the heating-up stage, since there is a large amount of oxygen in the Fe plating layer compared to the rate at which oxygen dissociated from water vapor diffuses into the steel at a low temperature, and the rate at which the Fe plating layer is reduced and oxygen is released becomes even faster, there is no significant impact even if the dew point in the annealing furnace changes in the low-temperature range. Therefore, in this stage, the control of the dew point is not such an important factor.

[0060] However, the amount of oxygen inside the Fe plating layer plays an important role. If a large number of fine internal oxides are generated at the interface between the Fe plating layer and the base iron and inside the base iron in the low-temperature stage, the alloying elements inside the base iron will act as nuclei for continuous internal oxidation. For such oxide nuclei to be generated, the concentrations of oxygen and alloy components must be high at the same time. However, if the Fe plating layer contains a sufficiently large amount of oxygen, a large number of oxide nuclei will be 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, if the Fe plating layer contains almost no oxygen, the alloying elements contained in the base iron will form oxides on the surface by passing through the Fe plating layer. After that, when the temperature is raised, the oxygen in the Fe plating layer will be further depleted, and the diffusion of alloying elements in the base iron will be further aggravated, so the formation of surface oxides will increase.

[0061] Figure 2(c) shows a schematic cross-sectional view of the base iron when the temperature is raised to 500 - 700°C in the same reducing atmosphere. During the heating process, it is advisable 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 oxygen release rate slows down, while the rate at which water vapor inside the annealing furnace dissociates and diffuses into the steel increases significantly. Therefore, when the dew point is raised in the 500 - 700°C range, 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.

[0062] Figure 2(d) shows a schematic cross-sectional view of the steel plate after maintaining it at a high temperature in the 600 - 950°C range while adjusting the dew point to -15°C to +30°C. Inside the base iron, Mn and Si are continuously diffused. On the surface of the steel plate, oxygen supplied from water vapor rapidly penetrates and is supplied. Therefore, Mn and Si are oxidized inside. However, in the low-temperature section, 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. As a result, the internal oxides concentrate and grow at the interface between the Fe plating layer and the base iron. Moreover, since the diffusion rate of oxygen is faster than that of Mn and Si with larger atomic sizes, internal oxides are formed deeply not only along the grain boundaries but also through the grains inside the base iron.

[0063] The control conditions have been described above for different temperatures. However, the most crucial stage in the annealing process is the stage of maintaining the steel plate temperature at 600 - 950°C. By simply controlling the dew point of the atmosphere in this temperature range, the distribution of oxides inside the steel plate 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. Moreover, it should be noted that the above-described process is only an example for explaining an embodiment of the present invention, and the reaction mechanism of the present invention is not always restricted and interpreted by the above description.

[0064] After the annealing step, the annealed steel sheet can be cooled. Since the cooling conditions in the cooling step after the annealing step 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 to iron.

[0065] According to an embodiment of the present invention, hot dip plating can be performed on the steel sheet for plating obtained by the above-described process to form a hot dip plating layer. The hot dip plating method in the present invention is not particularly limited. The plating of the present invention is not particularly limited in terms of its type as long as it is a plating method used for a hot press forming steel sheet. Some non-limiting examples include hot dip zinc plating and hot dip aluminum plating. It should be noted that it is not necessarily required to use pure zinc or aluminum in hot dip zinc plating or hot dip aluminum plating, and alloy plating containing magnesium, aluminum, zinc, and other alloying elements contained in the plating layer can also be sufficiently used.

[0066] In addition, 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 plated steel sheet according to the present invention. Therefore, the method for manufacturing the base iron can be specifically not limited.

[0067] In an embodiment of the present invention, the Fe plating layer can be formed on the surface of the base iron through an electroplating method, and the oxygen concentration of the Fe plating layer formed by appropriately controlling the conditions of the electroplating solution and the plating conditions can be controlled.

[0068] 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 ferric ions in the above iron ions is 5 to 60% by weight.

[0069] According to an embodiment of the present invention, the electroplating solution contains a first iron ion and a second iron ion. In order to obtain a high plating efficiency, it may be advantageous to contain only the first iron ion. However, when only the first iron ion is contained, the solution deteriorates and the plating efficiency drops sharply, which may induce quality variations in the continuous electroplating process. Therefore, the second iron ion can be further contained. At this time, the concentration of the second iron ion is preferably 5 to 60% by weight, more preferably 5 to 40% by weight, based on the total of the first iron and the second iron ions. If it is less than 5%, the rate at which the second iron is reduced to the first iron at the cathode is smaller than the rate at which the first iron is oxidized to the second iron at the anode, the concentration of the second iron increases rapidly, the pH drops rapidly, and the plating efficiency decreases continuously. On the other hand, when the concentration of the second iron ion exceeds 60%, the reaction amount of the second iron reduced to the first iron at the cathode increases significantly more than the reaction amount of the first iron reduced and deposited as metallic iron. Therefore, the plating efficiency drops significantly and the plating quality deteriorates. Therefore, considering equipment and process characteristics such as the plating amount, working current density, solution replenishment amount, the amount of solution adhering to and lost from the strip, and the concentration change rate due to evaporation, it is preferable to make the concentration of the second iron ion in the iron ions 5 to 60% by weight.

[0070] The concentration of the iron ions is preferably 1 to 80 g per liter of the electroplating solution, more preferably 10 to 50 g per liter. If it is less than 1 g / L, there is a problem that the plating efficiency and plating quality drop sharply. On the other hand, if it exceeds 80 g / L, it may exceed the solubility and precipitation may occur, increasing the raw material loss due to solution loss in the continuous plating process, which is not economical.

[0071] The electroplating solution of the present invention contains a complexing agent. In order to maintain a high plating efficiency without generating sludge while containing a large amount of the second iron, it is preferable to use an amino acid or an amino acid polymer as the complexing agent.

[0072] An amino acid is an organic molecule in which a carboxyl group (-COOH) and an amine group (-NH2) are bonded, and an amino acid polymer is an organic molecule formed by polymerizing two or more amino acids, and the amino acid polymer exhibits complexing properties similar to those of amino acids. Therefore, in the following description, amino acids and amino acid polymers are collectively referred to as amino acids.

[0073] When amino acids dissolve in neutral water, the amines become positively charged by combining with hydrogen ions, and the carboxyl groups become negatively charged by dissociating hydrogen ions, so that the amino acid molecules maintain a neutral charge. On the other hand, when the solution becomes acidic, the carboxyl groups recombine with hydrogen ions to become charge neutral, and the amines become positively charged, so that the amino acid molecules form cations. In other words, amino acids become charge neutral or form cations in a weakly acidic aqueous solution.

[0074] When an amino acid is added to an acidic electrolyte solution containing iron ions, it is complexed with ferrous and ferric ions, but the iron ions complexed with the amino acid remain in a cationic state even when complexed. Therefore, it shows electrically opposite characteristics to a general complexing agent with multiple carboxyl groups, which is negatively charged in a weakly acidic aqueous solution.

[0075] In addition, amino acids form fewer bonds with iron ions and have weaker bonds than complexing agents containing multiple carboxyl groups such as citric acid and EDTA, but their bond with ferric ions that generate sludge is strong enough to prevent precipitation by ferric ions. Not only that, even when ferric ions are complexed, they can maintain their cation status, so that ferric ions are easily transported to the cathode and reduced to ferrous ions to participate in the plating reaction, while their movement to the anode is inhibited and the rate of ferric ion production slows down, so that the ferric ion concentration remains at a constant level even when plating is performed continuously for a long period of time, so that plating efficiency remains constant and there is no need to replace the electrolyte.

[0076] Meanwhile, in the continuous electroplating process, when the iron ions in the solution are depleted 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 separate and are neutralized. Therefore, the pH change of a solution containing ferric ions slows down even without a separate pH buffer, and it acts as a pH buffer, so that the electroplating efficiency can be maintained constant in the continuous electroplating process.

[0077] Therefore, by using amino acids as a complexing agent, sludge can be prevented, and not only ferrous ions but also ferric ions can be used as plating raw materials. When ferrous ions and ferric ions are used in combination, the pH change of the solution can be slowed down and the accumulation of ferric ions can be easily prevented, so that electroplating efficiency and plating quality can be maintained constant in a continuous electroplating process.

[0078] On the other hand, the complexing agent is preferably added in an amount such that the molar concentration ratio of the iron ions to the complexing agent is 1:0.05-2.0, more preferably 1:0.5-1.0. If the molar concentration ratio is less than 0.05, the excessive ferric ions cannot be prevented from combining with hydroxide ions or oxygen to form sludge, and the plating efficiency is significantly reduced even without the inclusion of ferric ions, and burning is further induced, resulting in poor plating quality. On the other hand, even if the molar concentration ratio exceeds 2.0, the sludge suppression effect and plating quality are maintained, but the overvoltage increases, the plating efficiency is reduced, and the relatively expensive amino acid is unnecessarily excessively contained in comparison with the raw material containing iron ions such as ferrous sulfate, resulting in increased raw material costs and being uneconomical.

[0079] The complexing agent is preferably one or more selected from amino acids or amino acid polymers, and may be, for example, one or more selected from alanine, glycine, serine, threonine, arginine, glutamine, glutamic acid, and glycylglycine.

[0080] Using the above amino acid as a complexing agent, while maintaining the solution temperature at 80 °C or lower and the pH at 2.0 to 5.0, electroplating is carried out at a current density of 3 to 120 A / dm 2 Then, a high plating efficiency and an Fe plating layer with a high oxygen concentration can be obtained.

[0081] 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 severe, the concentration of the solution continuously changes, and it becomes difficult to perform uniform electroplating.

[0082] 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 is generated during continuous electroplating, causing problems such as clogging of pipes, contamination of rolls and equipment.

[0083] When the current density is less than 3 A / dm 2 the overvoltage of plating at the negative electrode decreases, and the Fe electroplating efficiency decreases, so it is not suitable for the continuous plating process. When it exceeds 120 A / dm 2 burning occurs on the plating surface, the electroplating layer is non-uniform, and there is a problem that the Fe plating layer easily peels off.

[0084] As described above, in the present invention, it is preferable that the Fe plating layer contains 5 to 50% by weight of oxygen. The reasons for oxygen to be mixed into the Fe plating layer are as follows. During the process of iron depositing on the surface of the steel plate to which the negative electrode is applied, at the same time, hydrogen ions are reduced to hydrogen gas and the pH rises. Therefore, both ferrous and ferric ions may temporarily combine with OH - ions and be mixed in 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 complexing agent combines with OH -When the iron ions combined with ions are negatively charged on average and a cathode is applied for electroplating, an electrical repulsive force is generated, suppressing the mixing into the Fe plating layer. On the other hand, amino acids are electrically neutral at pH 2.0 to 5.0 and become positively charged in strong acids with a pH less than 2.0. However, even if 1 to 2 OH - are combined with the iron ions bound to the amino acids and they become positively charged, an electrical attractive force is generated with the cathode for electroplating, causing a large amount of oxygen to be mixed in. Therefore, when electroplating Fe while maintaining pH 2.0 to 5.0 using an amino acid as a complexing agent so that the molar concentration ratio of iron ions to amino acids is 1:0.05 to 1:2.0, a high plating efficiency can be achieved, and an Fe plating layer containing 5 to 50 wt% oxygen can be obtained while suppressing sludge generation.

[0085] In order to ensure the quality of the hot-dip plating of a steel sheet containing Mn and Si, it is preferable to treat the plating amount of the Fe plating layer based on the iron concentration at 0.5 to 3.0 g / m 2 . The upper limit of the Fe plating amount is not particularly limited, but when it exceeds 3.0 g / m 2 in a continuous plating process, multiple plating cells are required or the production rate decreases, which is not economical. Moreover, when the Fe electroplating amount is large, the Fe electroplating solution rapidly denatures in a continuous process, the pH decreases, the plating efficiency greatly decreases, and there is a problem that solution management becomes difficult. On the other hand, when the Fe electroplating amount is less than 0.5 g / m 2 , the oxygen contained in the Fe plating layer is rapidly reduced and removed, so that the diffusion of Mn and Si from the base iron and the formation of surface oxides cannot be effectively suppressed, and there is a problem that the quality of the hot-dip plating deteriorates. 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 with the iron concentration contained in the plating layer.

Claims

1. having a composition containing, by weight%, Mn: 0.1 to 4%, Si: 0.001 to 2%, C: 0.02 to 0.6%, Al: 0.001 to 1%, P: 0.05% or less, S: 0.02% or less, Cr: 1% or less, B: 0.01% or less, with the balance being 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, 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 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 galvanized steel sheet according to claims 1 to 4 and a hot-dip galvanized layer formed on the galvanized steel sheet.

6. preparing base iron having a composition containing, by weight%, Mn: 0.1 to 4%, Si: 0.001 to 2%, C: 0.02 to 0.6%, Al: 0.001 to 1%, P: 0.05% or less, S: 0.02% or less, Cr: 1% or less, B: 0.01% or less, with 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; and The substrate iron on which the Fe plating layer is formed is annealed by maintaining it at 600 to 950 °C for 5 to 120 seconds in an annealing furnace in an H ２ - remaining N ２ gas atmosphere at a dew point temperature of -15 to +30 °C for 1 to 70%, and the method for manufacturing a steel sheet for plating includes this annealing step.

7. The adhesion amount of the Fe plating layer is 0.5 to 3 g / m ２ The method for manufacturing a plated steel sheet according to claim 6, wherein the adhesion amount is as described above.

8. The method for manufacturing a galvanized 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 ratio 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, which consists of such conditions.

11. Preparing a base iron having a composition containing, in % by weight, Mn: 0.1 to 4%, Si: 0.001 to 2%, C: 0.02 to 0.6%, Al: 0.001 to 1%, P: 0.05% or less, S: 0.02% or less, Cr: 1% or less, B: 0.01% 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 controlled to a dew point temperature of -15 to +30 °C in a 1 to 70% H ２ - The remaining N ２ Annealing by maintaining at 600 to 950 °C for 5 to 120 seconds in an annealing furnace in a gas atmosphere to obtain a steel sheet for plating; and A method for manufacturing a hot-dip plated steel sheet, comprising dipping the plated steel sheet in a hot-dip bath.

Images