Plated Steel Sheet for Hot Press Forming, Having Excellent Plating Quality, Steel Sheet and Manufacturing Method Therefor

The present disclosure relates to a plated steel sheet for hot press forming having excellent plating quality, a steel sheet for plating, and a manufacturing method therefor.

Hot press forming is a processing method for obtaining high-strength parts by forming and cooling a steel sheet at high temperatures substantially simultaneously. The steel sheet used for hot press forming should have excellent hardenability so that martensite may be easily formed when cooled at high temperature. In order to improve the hardenability of the steel sheet, various alloying elements are added to steel for hot press forming compared to general steel, and in particular, many elements with a high oxidation tendency compared to Fe, such as Mn, Si, Al, Cr, and B, are added. In addition, in order to prevent decarburization or oxidation of the steel sheet during hot press forming, there are cases in which various types of plating are applied to a surface of the steel sheet. Thereamong, methods of plating the surface of the steel sheet by hot-dip plating, such as hot-dip galvanizing or hot-dip aluminum plating, are widely used.

In hot-dip plating, plating quality is determined by a surface condition of an annealed steel sheet immediately before plating, and plating properties may deteriorate due to the formation of surface oxides during annealing caused by elements such as Mn, Si, Al, Cr, and B added to secure physical properties of the steel sheet. That is, during the annealing process, the elements may diffuse to surfaces thereof and react with a trace amount of oxygen or water vapor present in an annealing furnace to form single or complex oxides of the elements on the surface of the steel sheet, thereby reducing reactivity of the surface. The surface of the annealed steel sheet with the reduced reactivity interferes with wettability of a hot-dip plating bath, causing non-plating in which a plating metal is not attached locally or entirely to the surface of the plated steel sheet. In addition, these oxides significantly deteriorate the plating quality of plated steel sheets, such as peeling of a plating layer due to the insufficient formation of an alloying inhibition layer (FeAl) required to secure adhesion of the plating layer during a hot-dip plating process.

Several technologies have been proposed to improve the plating quality of the high-strength hot-dip plated steel sheet. Thereamong, Patent Document 1 discloses a technology providing a hot-dip galvanized steel sheet or a galvannealed steel sheet having excellent plating quality, by controlling an air-fuel ratio of air and fuel to 0.08 to 0.95, during the annealing process, oxidizing the steel sheet in a direct flame furnace in an oxidizing atmosphere to form an iron oxide including Si, Mn or Al alone or complex oxides to a certain depth inside the steel sheet, and then reducing and annealing the iron oxide in a reducing atmosphere and then performing hot-dip galvanizing.

When using a method of reducing after oxidation in the annealing process as in Patent Document 1, since elements with a high affinity for oxygen, such as Si, Mn, Al, and the like, are internally oxidized at a certain depth from a surface layer of the steel sheet and diffusion to the surface layer is suppressed, Si, Mn, or Al alone or complex oxides thereof are relatively reduced, so that wettability with zinc may be improved and non-plating may be reduced. However, in the case of steel types with Si is added, Si is concentrated directly below the iron oxide during the reduction process, thereby forming a band-shaped Si oxide, so that peeling occurs in a surface layer portion including a plating layer, that is, peeling occurs at an interface between the reduced iron and a base steel sheet therebelow, causing a problem in that it is difficult to secure adhesion of the plating layer.

Meanwhile, as another method for improving plating properties of a hot-dip plated steel sheet, Patent Document 2 discloses a method for improving the plating properties thereof by reducing oxides which are externally oxidized on the surface of the steel sheet after annealing by internally oxidizing alloying elements such as Mn, Si, Al and the like, which are easily oxidized, inside steel, by maintaining a dew point in the annealing furnace at a high level. However, in the method according to Patent Document 2, the problem of plating properties caused by the external oxidation of Si, which is easily internally oxidized, may be solved, but when a large amount of Mn, which is relatively difficult to be internally oxidized, is added, the effect is insignificant.

In addition, even if the plating properties are improved by internal oxidation, linear non-plating may occur due to surface oxides formed unevenly on the surface, or when a hot-dip galvannealed steel sheet (a GA steel sheet) is manufactured through an alloying heat treatment after plating, a problem such as linear defects due to uneven alloying may occur on the surface of the hot-dip galvannealed steel sheet.

As another prior art, there is provided a method of suppressing the diffusion of alloying elements to the surface during annealing performing Ni pre-plating before annealing. However, this method is also effective in suppressing the diffusion of Mn, but has the problem of not sufficiently suppressing the diffusion of Si.

An aspect of the present disclosure is to provide a hot-dip plated steel sheet having excellent plating quality and a method for manufacturing the same, in which non-plating does not occur and the problem of peeling of a plating layer is solved.

Another aspect of the present disclosure is to provide a hot-dip galvanized steel sheet that can be manufactured to form a hot-dip galvannealed steel sheet having excellent surface quality without linear defects occurring, even when an alloying heat treatment is performed after plating, and a method for manufacturing the same.

Another aspect of the present disclosure is to provide a steel sheet for plating that can be manufactured to form the hot-dip galvanized steel sheet having excellent plating quality, and a method for manufacturing the same.

An object of the present disclosure is not limited to the above description. The object of the present disclosure will be understood from the entirety of the contents of the present specification, and a person skilled in the art to which the present disclosure pertains will understand an additional object of the present disclosure without difficulty.

According to an aspect of the present disclosure, provided is a steel sheet for hot press forming, the steel sheet for hot press forming including, by weight %: 0.1 to 4% of Mn, 0.001 to 2% of Si, 0.02 to 0.6% of C, 0.001 to 1% of Al, 0.05% or less of P, 0.02% or less of S, 1% or less of Cr, 0.01% or less of B, with a balance of Fe and inevitable impurities, wherein each of a GDS profile of an Mn element and a GDS profile of an Si element, observed from a surface thereof in a depth direction, sequentially includes a maximum point and a minimum point, a difference between a value obtained by dividing a Mn concentration at the maximum point in the GDS profile of the Mn element by a Mn concentration of a base material, and a value obtained by dividing a Mn concentration at the minimum point in the GDS profile of the Mn element by the Mn concentration of the base material (a difference of converted concentration of Mn) may be 10% or more, a difference between a value obtained by dividing a Si concentration at the maximum point in the GDS profile of the Si element by a Si concentration of a base material, and a value obtained by dividing a Si concentration at the minimum point in the GDS profile of the Si element by the Si concentration of the base material (a difference of converted concentration of Si) may be 10% or more.

Wherein, when no minimum points appear within 5 μm in depth, a point at a depth of 5 μm is considered to be a point at which the minimum point appears.

According to another aspect of the present disclosure, a hot-dip plated steel sheet for hot press forming may include the steel sheet for plating described above and a hot-dip plated layer formed on the steel sheet for plating.

According to another aspect of the present disclosure, provided is a method for manufacturing a steel sheet for hot press forming, the method including: preparing a base steel sheet including, by weight %: 0.1 to 4% of Mn, 0.001 to 2% of Si, 0.02 to 0.6% of C, 0.001 to 1% of Al, 0.05% or less of P, 0.02% or less of S, 1% or less of Cr, 0.01% or less of B, with a balance of Fe and inevitable impurities, performing electroplating on the base steel sheet to form an Fe plating layer including 5 to 50 wt % of oxygen; and annealing the base steel sheet on which the Fe plating layer is formed by maintaining at a temperature range of 600 to 950° C. for 5 to 120 minutes in an annealing furnace with 1 to 70% H2-remaining Ngas atmosphere, controlled at a dew point lower than −20° C.

According to another aspect of the present disclosure, provided is a method for manufacturing a hot-dip plated steel sheet for hot press forming, the method including: preparing a base steel sheet including, by weight %: 0.1 to 4% of Mn, 0.001 to 2% of Si, 0.02 to 0.6% of C, 0.001 to 1% of Al, 0.05% or less of P, 0.02% or less of S, 1% or less of Cr, 0.01% or less of B, with a balance of Fe and inevitable impurities, performing electroplating on the base steel sheet to form an Fe plating layer including 5 to 50 wt % of oxygen; obtaining a steel sheet for plating by annealing the base steel on which the Fe plating layer is formed by maintaining at a temperature range of 600 to 950° C. for 5 to 120 seconds in an annealing furnace with 1 to 70% H-remaining Ngas atmosphere, controlled at a dew point temperature of lower than −20° C.; and dipping the steel sheet for plating in a hot-dip plating bath.

As described above, in the present disclosure, a hot-dip plated steel sheet in which a phenomenon in which non-plating occurs during hot-dip plating is significantly improved and plating adhesion is improved by forming a pre-plating layer and controlling concentration profiles of Mn and Si elements therein, may be provided.

In addition, according to an aspect of the present disclosure, even if an alloying heat treatment is performed on the hot-dip plated steel sheet of the present disclosure, linear defects, or the like, on the surface of the obtained hot-dip alloy-plated steel sheet, so that a hot-dip alloy-plated steel sheet having excellent surface quality may be provided.

Hereinafter, a hot-dip plated steel sheet having excellent plating quality according to an aspect of the present disclosure, which was completed through the research by the present inventor, will be described in detail. It should be noted that a concentration of each element in the present disclosure means weight %, unless otherwise specified. In addition, an Fe electroplating amount is a plating amount measured as a total amount of Fe included in a plating layer per unit area, and oxygen and inevitable impurities in the plating layer were not included in the plating amount.

In addition, unless otherwise defined, a concentration and concentration profile referred to in the present disclosure mean a concentration and concentration profile measured using GDS, i.e., a glow discharge optical emission spectrometer.

Hereinafter, the present disclosure will be described in detail.

It is known that the cause of non-plating and deterioration of plating adhesion in a steel sheet containing large amounts of Mn and Si is due to surface oxides formed when alloying elements such as Mn, Si, and the like, are oxidized on the surface, during a process in which a cold-rolled steel sheet is annealed at high temperatures.

As a method of forming an oxide layer containing a large amount of oxygen to suppress the diffusion of alloying elements such as Mn, Si, and the like to the surface, an oxidation-reduction method, the method in which an oxide layer is oxidized during a temperature increase and then reduced again by maintaining the oxide layer in a reducing atmosphere, or a method in which an iron oxide is coated on a surface of a base steel sheet and heat treated, may be used. However, since the iron oxide firmly formed on the surface of the base steel sheet is a mixture of FeO, FeO, and FeO, which are difficult to reduce, and while the surface is reduced to metallic steel during the annealing process in a reducing atmosphere, an interface between the iron oxide layer and the base steel sheet has a slow reduction rate, making it difficult to completely reduce, and Mn and Si oxides accumulate at the interface to form a continuous oxide layer. Therefore, although wettability with molten zinc may be improved, the oxide layer may easily crumble, causing a problem in which the plating layer is peeled off.

Meanwhile, when an internal oxidation method at annealing in which alloying elements such as Mn, Si, and the like, are oxidized inside steel, by increasing oxygen partial pressure or a dew point inside an annealing furnace during a heat treatment process, is applied, Mn and Si oxides are preferentially formed on a surface of the steel during the heat treatment process, and then Mn and Si are oxidized by oxygen diffused into the steel, thereby inhibiting surface diffusion. Accordingly, a thin oxide film is formed on the surface of the base steel sheet, and if a surface of a cold-rolled steel sheet is not completely homogeneous before annealing, or if there is a local deviation in oxygen partial pressure, temperature, or the like, the wettability is uneven during hot-dip galvanizing, causing non-plating, or if the thickness of the oxide film is uneven during an alloying heat treatment process after galvanizing, causing a difference in a degree of alloying, there is a tendency for linear defects that can be easily identified with the naked eye to occur.

In order to solve the problems of the above technology, the present inventors have attempted to manufacture a hot-dip plated steel sheet having an attractive surface and no plating peeling problem by controlling the presence of Mn and Si, which are oxidizing elements, on the surface of the steel sheet for plating as follows.

That is, the steel sheet according to an embodiment of the present disclosure may have the following characteristics in terms of the GDS concentration profile of Mn and Si. A steel sheet for plating of the present disclosure will be described in detail with reference to the GDS profile of.

is a graph schematically illustrating a typical GDS profile of an alloy element that may appear from a surface portion after a galvanized layer is removed from a hot-dip plated steel sheet including the steel sheet of the present disclosure. In the graph, a vertical axis represents a concentration of alloying elements such as Mn, Si, and the like, and a horizontal axis represents a depth.

As can be seen from a typical example of the GDS profile of the Mn element of the present disclosure illustrated in, the steel sheet of the present disclosure may have a very low concentration of Mn on the surface, and may have a concentration gradient in the form of maximum and minimum points appearing sequentially from the surface in a depth direction. Here, sequentially having the maximum and minimum points does not necessarily mean that the maximum point appears first in the depth direction from the surface (interface), and in some cases, means that the minimum point may appear first, but the maximum point and the minimum point should appear sequentially thereafter. However, in some embodiments, the minimum point may not appear, and in this case, the internal concentration in the 5 μm depth region may be a concentration of the minimum point. In addition, the concentration of alloying elements on the surface may have a lower value than the concentration at the maximum point, but in some cases, a minimum point with a low concentration of alloying elements may appear between the surface and the maximum point.

In the GDS concentration profile exemplified inabove, although not limited thereto, the surface layer portion corresponds to an plating layer with a low concentration of alloying elements since the alloying elements are not greatly diffused from a base steel sheet, the maximum point corresponds to a region in which internal oxides of the alloying elements formed near an interface between the Fe plating layer and base steel sheet are concentrated, and the minimum point appearing on a side of the base steel sheet in the Fe plating layer corresponds to a region in which alloying elements diffuse to the Fe plating layer not including the alloying elements and are diluted, or a region in which alloying elements diffuse to the maximum point at which internal oxidation occurs and are depleted.

In an embodiment of the present disclosure, the maximum point may be formed at a depth of 0.05 to 1.0 μm from a surface of the steel sheet. If the maximum point appears in a region deeper than the region described above, it may not be determined to be a maximum point due to the effect of the present disclosure. In addition, the minimum point can be formed at a location within 5 μm of depth of the surface of the steel sheet. As described above, if the minimum point is not formed at the point within 5 μm of depth, the 5 μm of depth may be determined to be a point at which the minimum point is formed. Since the concentration at the depth of 5 μm is substantially the same as the concentration of a base material, it can be considered to be a point at which the concentration no longer decreases.

In this case, as the difference between the converted concentration at the maximum point of the corresponding element (a value obtained by dividing the concentration thereof at the corresponding point by the concentration of the base material, expressed in units of %) and the converted concentration at the minimum point thereof in the Mn concentration profile and the Si concentration profile increases, Mn and Si diffusing to the surface may be reduced, which is important. In an embodiment of the present disclosure, the converted concentration value at the maximum point of Mn and Si—the converted concentration value at the minimum point Mn and Si may respectively be 10% or more.

As a result of experiments conducted by the present inventors under various conditions, when the above-described conditions are satisfied, a hot-dip plated steel sheet having good plating adhesion and in which non-plating does not occur during hot-dip plating, may be obtained. However, if the difference between the converted concentrations at the maximum point and the minimum point of Mn and Si is less than 10%, there may be a problem in which point or linear non-plating occurs or plating peeling occurs. That is, by controlling the difference of converted concentration to a certain level or higher, it is possible to prevent the formation of oxides of Mn and Si on the surface, so that a hot-dip plated steel sheet having an attractive surface and good plating adhesion may be manufactured, and even if a subsequent alloying heat treatment process is performed, it is possible to suppress the occurrence of defects such as linear defects on the surface. The greater the difference between the converted concentration values, the more advantageous it is, so there is no need to set an upper limit for the values. However, considering the contents of the elements included, the difference of converted concentration values may be set to 200% or less for both Mn and Si. In another embodiment of the present disclosure, the difference of converted concentration of Mn and Si may be 15% or more or 20% or more.

Hereinafter, the GDS analysis method performed in the present disclosure is described in detail.

For the GDS concentration analysis, a hot-dip plated steel sheet is cut to a size of 30 to 50 mm in length, washed first in a NaOH solution at room temperature, and then dipped in a 20 to 40 vol % hydrochloric acid solution to remove a plating layer.

To prevent damage to a surface of a base steel sheet during a dissolution process of a plating layer, an acid solution was removed within 10 seconds when bubble generation due to a reaction between the plating layer and the acid solution is stopped, and the base steel sheet was washed using pure water and dried. If it is a steel sheet for plating that has not yet been hot-dip plated, it may be analyzed without removing the plating layer.

The GDS concentration profile measures concentrations of all elements contained in the steel sheet at intervals of 1 to 5 nm in a thickness direction of the steel sheet. Irregular noise may be included in the measured GDS profile, an average concentration profile was obtained by applying a Gaussian filter with a cutoff value of 100 nm to the measured concentration profile to obtain the maximum and minimum points of the Mn and Si concentrations, and the concentration values and depths at the maximum and minimum points of the concentrations were respectively obtained from the noise-removed profile. In addition, it should be noted that the maximum and minimum points mentioned in the present disclosure were calculated as maximum and minimum points only when the difference between the maximum point and the minimum point in the depth direction was 10 nm or more.

The steel sheet for plating targeted in the present disclosure may include a base steel sheet and an Fe plating layer formed on the base steel sheet. The composition of the base steel sheet is not particularly limited.

However, if the steel sheet is a steel sheet having a composition which easily forms oxides on the surface, containing 0.1 to 4 wt % of Mn and 0.001 to 2 wt % of Si, plating properties may be advantageously improved by the present disclosure. An upper limit of a concentration of Mn of the base steel sheet is not particularly limited, but considering the composition commonly used, the upper limit may be limited to 4 wt %. In addition, although a lower limit of the concentration of Mn is not particularly limited, if a composition contains less than 0.1 wt % of Mn, the surface quality of a hot-dip plated steel sheet is attractive, even if a Fe plating layer is not formed, so there is no need to perform Fe electroplating thereon. An upper limit of a Si concentration is not particularly limited, but considering the composition commonly used, the upper limit may be limited to 2.0 wt % or less, and if the Si concentration is less than 0.001 wt %, the quality of the hot-dip plating is good, even if the method of the present disclosure is not performed, so there is no need to perform the method of the present disclosure.

Since Mn and Si are elements affecting the plating property, the concentrations of Mn and Si may be limited as described above, but in the present disclosure, the remaining elements of the base steel sheet are not particularly limited.

However, considering the fact that non-plating and deterioration of plating adhesion may occur severely in the steel sheet containing a large amount of alloying elements, in an embodiment of the present disclosure, the base steel sheet may include, by weight %: 0.1 to 4% of Mn, 0.001 to 2% of S, 0.02 to 0.6% of C, 0.001 to 1% of Al, 0.05% or less of P, 0.02% or less of S, 1% or less of Cr, 0.01% or less of B, with a balance of Fe and inevitable impurities. P, S are impurities, which are advantageous when not added, and Cr and B are optional elements and do not need to be added, so lower limits thereof are not set. In addition to the elements described above, the base steel sheet may further include elements such as Ti, Mo, and Nb in a total amount of 1.2% or less. The base steel sheet is not particularly limited, but in an embodiment of the present disclosure, a cold-rolled steel sheet or a hot-rolled steel sheet may be used as the base steel sheet.

In an aspect of the present disclosure, a hot-dip plated steel sheet including the steel sheet for plating may be provided, and the hot-dip plated steel sheet may include a steel sheet for plating and a hot-dip plated layer formed on a surface of the steel sheet for plating. In this case, any commercially available hot-dip plated steel sheet can be used, and there are no specific restrictions on the type thereof.

Next, an exemplary embodiment of a method for manufacturing a steel sheet for plating and a hot-dip plated steel sheet having the advantageous effects described above is described. According to an embodiment of the present disclosure, the steel sheet for plating may be manufactured by a process including: preparing a base steel sheet; performing electroplating on the base steel sheet to form an Fe plating layer including 5 to 50 wt % of oxygen; and annealing the base steel sheet on which the Fe plating layer is formed.

After electroplating with Fe containing 6.2 wt % of oxygen is performed on a cold-rolled steel sheet for hot press forming including 1.3% of Mn, 0.3% of Si, and other alloying elements to have an iron adhesion amount of 1.95 g/m, the electroplated cold-rolled steel sheet was annealed in an atmosphere of N-5% H, a dew point of −40° C., and a temperature of 800° C. for 53 seconds, and then cooled. The atmosphere was maintained the same throughout the entire annealing process, and when a sample was collected from the cooled steel sheet and then a cross-section thereof was observed using a transmission electron microscope, it could be confirmed that particle-type Mn and Si oxides were formed at an interface between the iron electroplating layer and the base steel sheet, whereas almost no Mn and Si oxides were formed on a surface of the electroplating layer. In particular, when this state is analyzed using a GDS profile, as shown in, maximum values of the Mn and Si concentrations appear immediately below the surface of the steel sheet (including an Fe plating layer), and minimum values of the Mn and Si concentrations may appear at a deeper location thereof. In some cases, the minimum value may not clearly appear and a trend in which the concentration gradually decreases may appear, but the maximum value may be clearly observed.

This phenomenon is because an Fe plating layer with a high oxygen content was formed before annealing. That is, when the iron electroplating layer contains 5 to 50 wt % of oxygen, and when annealed in an annealing furnace in a reducing atmosphere, oxygen in the iron electroplating layer oxidizes alloying elements such as Mn, Si, and the like, diffusing from the base steel sheet to the surface and is accumulated at the interface between the iron electroplating layer and the base steel sheet. Therefore, as illustrated in the graph of, when the concentration is measured with GDS, a maximum point with high concentrations of Mn, Si, and the like, is confirmed at a depth corresponding to the thickness of the iron electroplating layer from the surface. Meanwhile, an alloying element having a slow diffusion rate, such as Mn, or the like, cannot diffuse quickly from the base steel sheet even when the concentration thereof is diluted by the iron electroplating layer or the dissolved Mn is depleted due to internal oxidation, so a minimum point may exist after the maximum point obtained by the measured GDS concentration. However, since Si rapidly diffuses from the inside during the annealing process, and internal oxidation is continuously performed at the interface between the iron electroplating layer and the base steel sheet, oxides are accumulated, a minimum point may not be confirmed in the GDS concentration analysis. Therefore, the fact that the minimum point does not appear in the GDS concentration profile means that the alloying elements such as Mn, Si, and the like, were oxidized by the iron electroplating layer and effectively suppressed from the diffusion to the surface.

Unlike an oxide formed at high temperatures, when an iron electroplating layer containing a large amount of oxygen is formed and annealed, reduction occurs simultaneously not only on the surface of the iron electroplating layer but also at the interface between the iron electroplating layer and the base steel sheet. In this case, the reduced iron is bonded by mutual diffusion with the base steel sheet, and Mn and Si react with oxygen in the Fe plating layer to form oxides in the form of particles or discontinuous plates at the interface between the iron electroplating layer and the base steel sheet, so that good adhesion between the iron electroplating layer and the base steel sheet may be maintained. In addition, the uniformly formed iron electroplating layer may suppress the formation of surface oxides and reduce the concentration of alloying elements such as Mn, Si, and the like, dissolved on the surface of the steel sheet, thereby promoting an alloying reaction with the galvanized layer, thereby obtaining a uniformly hot-dip alloy-plated steel sheet without surface defects.

Unlike the oxidation reduction method, in the internal oxidation method at annealing, since a layered oxide layer is not formed, excellent properties in improving the plating adhesion during hot-dip plating a steel sheet for hot press forming containing a large amount of alloying elements such as Mn, Si, and the like, are illustrated, but since water vapor inside the annealing furnace inevitably oxidizes the surface of the steel sheet first, and then oxygen penetrates thereinto, it is impossible to fundamentally remove the surface oxide. As a result, if the surface of the cold-rolled steel sheet is not completely homogeneous before annealing, or if a local deviation such as oxygen partial pressure, temperature, and the like, during with a hot-dip plating annealing occurs, wettability solution is uneven, resulting in non-plating, or if a thickness of an oxide film is uneven during an alloying heat treatment process after plating, in the case of galvanizing, resulting in a difference in a degree of alloying, which can cause a problem such as linear defects which can be easily identified with the naked eye.

To manufacture an attractive hot-dip plated steel sheet without plating peeling problems, by suppressing the surface diffusion of alloying elements by Fe plating containing a large amount of oxygen, it is recommended that an iron electroplating layer containing 5 to 50 wt % of oxygen is formed on the base steel sheet to have an iron adhesion amount of 0.5 to 3.0 g/m, the temperature is increased to 600 to 950° C. so that the mechanical properties of the steel sheet may be secured, and then cooled to perform hot-dip plating.

In an embodiment of the present disclosure, the Fe plating layer may be formed through a continuous plating process, and the Fe plating amount herein may be 0.5 to 3.0 g/mbased on the Fe adhesion amount. If the Fe plating amount is less than 0.5 g/m, a diffusion suppression effect of alloying elements by the Fe plating layer may be insufficient in a common continuous annealing process. In addition, even if the Fe plating amount exceeds 3.0 g/m, the diffusion suppression effect of alloying elements may be further increased, but to secure a high plating amount, a plurality of plating cells should be operated, and when an insoluble anode is used, the electroplating solution becomes rapidly acidic, which reduces the plating efficiency and causes sludge to be generated, which may be uneconomical. In another embodiment of the present disclosure, the Fe plating amount may be 1.0 to 2.0 g/m. When an Fe plating layer is formed and then internal oxidation is performed, internal oxides are formed at an interface between the Fe plating layer and the base steel sheet or directly below the interface, so the maximum points of the concentrations of Mn and Si exist in a region in the range of 0.05 to 1.0 μm. The Fe plating amount of 0.5 to 3.0 g/mof the present disclosure may correspond to a thickness of 0.05 to 0.4 μm after annealing.