Hot-dip galvanized steel sheet with excellent plating quality, steel sheet for plating, and method for producing the same

Abstract

The present invention relates to a high-strength hot-dip galvanized steel sheet with excellent plating quality, a steel sheet for plating for manufacturing the same, and methods for manufacturing these. The steel sheet for plating according to one aspect of the present invention has a composition containing, by weight%, Mn: 1.0 to 8.0%, Si: 0.05 to 3%, C: 0.06 to 0.4%, Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, Cr: 1.5% or less, B: 0.005% or less, and the balance being Fe and unavoidable impurities. 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 10% 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 10% 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 high-strength hot-dip galvanized steel sheet with excellent plating quality, a steel sheet for plating for manufacturing the same, and manufacturing methods thereof.

Background Art

[0002] In recent years, in the field of the automobile industry, by applying high-strength steel sheets as steel materials for automobiles, safety has been improved and weight reduction has been achieved by reducing the thickness. As steel materials that can be preferably applied as steel materials for automobiles, martensitic steels, TRIP steels, etc. have been developed. These high-strength steels will have various alloying elements added compared to general steels, and in particular, elements with a high oxidation tendency with respect to Fe such as Mn, Si, Al, Cr, and B will be added in large amounts.

[0003] The quality of hot-dip galvanizing is determined by the surface state of the annealed steel sheet immediately before plating. However, due to the formation of surface oxides during annealing caused by elements such as Mn, Si, Al, Cr, and B added to ensure the physical properties of the steel sheet, the plating property deteriorates. That is, during the annealing process, the above elements diffuse to the surface side and react with trace amounts of oxygen or water vapor present in the annealing furnace to form single or composite oxides of the above elements on the steel sheet surface, thereby reducing the surface reactivity. The surface of the annealed steel sheet with reduced reactivity hinders the wettability of the hot-dip galvanizing bath, causing unplated areas where the plating metal does not adhere locally or entirely to the surface of the plated steel sheet. Also, due to such oxides, the formation of the alloying suppression layer (Fe2Al5) necessary for ensuring the adhesion of the plating layer during the hot-dip plating process is insufficient, resulting in peeling of the plating layer, etc., and the plating quality of the plated steel sheet is greatly reduced.

[0004] In order to improve the plating quality of high-strength 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 to form iron oxides containing Si, Mn, or Al alone or in combination up to a certain depth inside the steel sheet, and then performs reduction annealing on the iron oxides in a reducing atmosphere, followed by hot-dip galvanizing, presenting a technique for providing a hot-dip galvanized or alloyed hot-dip galvanized steel sheet with excellent plating quality.

[0005] When 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 steel sheet surface layer, suppressing diffusion to the surface layer. Therefore, in the surface layer, Si, Mn, or Al alone or in combination oxides relatively decrease, improving wettability with zinc and reducing unplated areas. However, in the case of steel grades added with Si, Si concentrates directly below 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 underlying base iron, 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 high-strength hot-dip galvanized steel sheets, Patent Document 2 proposes a method of maintaining a high dew point in the annealing furnace to internally oxidize alloy components such as Mn, Si, and Al that are easily oxidized inside the steel, thereby reducing the oxides externally oxidized on the steel sheet surface after annealing and improving 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] Even if the plating property is improved by internal oxidation, problems such as linear unplated areas may occur due to the surface oxides formed unevenly on the surface, or when producing an alloyed hot-dip galvanized steel sheet (GA steel sheet) through alloying heat treatment after plating, linear defects due to non-uniform alloying may occur on the surface of the alloyed hot-dip galvanized steel sheet.

[0008] As yet another prior art, 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 is provided a hot-dip galvanized steel sheet having 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 is provided a hot-dip galvanized steel sheet and a method for manufacturing the same, which can be manufactured into an alloyed hot-dip galvanized steel sheet having excellent surface quality without generating linear defects even when subjected to alloying heat treatment after plating.

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

[0013] The problems of the present invention are not limited to the above description. For those with ordinary knowledge in the technical field to which the present invention pertains, it is not difficult to understand further problems of the present invention from the overall matters of the specification of the present invention.

Means for Solving the Problems

[0014] The steel sheet for plating according to one aspect of the present invention has a composition containing, by weight%, Mn: 1.0 to 8.0%, Si: 0.05 to 3%, C: 0.06 to 0.4%, Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, Cr: 1.5% or less, B: 0.005% or less, with the balance being Fe and inevitable impurities. The GDS profiles of the Mn component and the Si component observed in the depth direction from the surface sequentially include maximum points and minimum points, respectively. 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 both 10% 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, which is another aspect of the present invention, can include the above-described steel sheet for plating and a hot-dip galvanized layer formed on the steel sheet for plating.

[0017] Another aspect of the present invention is a method for manufacturing a steel sheet for plating, which includes the steps of: preparing a base iron having a composition containing, by weight%, Mn: 1.0 to 8.0%, Si: 0.05 to 3%, C: 0.06 to 0.4%, Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, Cr: 1.5% or less, B: 0.005% or less, and the balance being Fe and unavoidable impurities; performing electroplating on the base iron to form an Fe plating layer containing 5 to 50% by weight of oxygen; and annealing the base iron having the Fe plating layer formed thereon by maintaining it at 600 to 950°C for 5 to 120 seconds in an annealing furnace with a 1 to 70% H2 - the balance being N2 gas atmosphere controlled to a dew point temperature of less than -20°C.

[0018] Another aspect of the present invention is a method for manufacturing a hot-dip galvanized steel sheet, which includes the steps of: preparing a base iron having a composition containing, by weight%, Mn: 1.0 to 8.0%, Si: 0.05 to 3%, C: 0.06 to 0.4%, Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, Cr: 1.5% or less, B: 0.005% or less, and the balance being Fe and unavoidable impurities; performing electroplating on the base iron to form an Fe plating layer containing 5 to 50% by weight of oxygen; annealing the base iron having the Fe plating layer formed thereon by maintaining it at 600 to 950°C for 5 to 120 seconds in an annealing furnace with a 1 to 70% H2 - the balance being N2 gas atmosphere controlled to a dew point temperature of less than -20°C to obtain a steel sheet for plating; and dipping the steel sheet for plating into a galvanizing bath.

Advantages of the Invention

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

[0020] Also, according to one 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 performing alloying heat treatment on the hot-dip galvanized steel sheet of the present invention, and an alloyed hot-dip galvanized steel sheet with excellent surface quality can be provided.

Brief Description of the Drawings

[0021]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0022] Hereinafter, a high-strength hot-dip galvanized steel sheet with excellent plating quality according to one aspect of the present invention completed by the research of the present inventor will be described in detail. In the present invention, it should be noted that unless otherwise specified, weight% is meant when indicating the concentration of each element. 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] The reason for the occurrence of non-plating and the decrease in plating adhesion in a steel sheet containing a large amount of Mn and Si is known to be due to surface oxides generated by the oxidation of alloy elements such as Mn and Si on the surface during the annealing process of 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, methods such as the redox method of forming an oxide layer containing a large amount of oxygen and then maintaining and reducing it in a reducing atmosphere again after oxidation during temperature rise, or the method of coating the surface of the base metal with iron oxide and then performing heat treatment 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 will easily collapse and the plating layer will 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 will be non-uniform and unplated areas will occur, or if the thickness of the oxide film is non-uniform and a difference in the degree of alloying occurs during the alloying heat treatment process after zinc plating, linear defects that can be easily identified visually tend to occur.

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

[0029] That is, the steel sheet according to an embodiment of the present invention can have the following characteristics in the GDS concentration profiles of Mn and Si. The steel sheet for plating 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 alloy components that can appear from the surface part after removing the zinc plating layer from the hot-dip galvanized steel sheet including the steel sheet of the present invention, and the GDS profile of alloy components when outside the scope of the present invention. The vertical axis in the graph indicates the concentration of alloy elements such as Mn and Si, and the horizontal axis indicates the depth.

[0031] As can be seen from a typical example of the GDS profile of the Mn component of the steel sheet of the present invention shown in Figure 1, the steel sheet of the present invention can have a concentration gradient in a form in which the concentration of Mn is very low on its surface, and maximum points and minimum points appear sequentially in the depth direction from the surface. Here, having sequentially does not mean that the maximum point must always appear 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. In this case, the internal concentration in the 5-μm depth region can be taken as the minimum point concentration. Also, the concentration of alloy elements 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 alloy elements may appear between the surface and the maximum point.

[0032] In the GDS concentration profile illustrated in Figure 1 above, although not necessarily limited to this, the surface layer portion corresponds to an Fe plating layer with a low concentration of alloy elements because alloy elements do not diffuse much from the base iron. The maximum point corresponds to a region where internal oxides of alloy elements concentrated near the interface between the Fe plating layer and the base iron. The minimum point that appears on the base iron side in the Fe plating layer corresponds to a region where the alloy elements are diffused and diluted in the Fe plating layer that does not contain alloy elements, or a region where the alloy elements are diffused and depleted in the maximum point where internal oxidation has occurred.

[0033] In one embodiment of the present invention, the above-mentioned maximum point can be formed at a depth of 0.05 to 1.0 μm from the surface of the steel sheet. If the maximum point appears in a region deeper than this, it may not be determined as the maximum point due to the effect of the present invention. Further, the minimum point can be formed at a position within a depth of 5 μm from the surface of the steel sheet. As described above, if the minimum point is not formed at a point within a depth of 5 μm, the point where the minimum point is formed can be set at a depth of 5 μm. Since the concentration at a depth of 5 μm is substantially the same as the concentration of the base material, it can be regarded as the point where the concentration does not decrease further.

[0034] At this time, 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 % units) and the converted concentration of the minimum point in the Mn concentration profile and the Si concentration profile, the more the Mn and Si diffusing to the surface can be reduced, which is important. In one embodiment of the present invention, the value of the converted concentration of the maximum point - the converted concentration of the minimum point of Mn and Si can each be 10% or more.

[0035] As a result of the inventors' experiments under various conditions, when the above conditions are satisfied, uncoated areas do not occur during hot-dip galvanizing, and a hot-dip galvanized steel sheet with good coating adhesion can be obtained. However, if the difference between the converted concentrations of the maximum and minimum points of Mn and Si is less than 10%, there may be problems such as the occurrence of point or linear uncoated areas or coating peeling. That is, by controlling the difference in the converted concentration to a certain level or more, it is possible to prevent the formation of oxides of Mn and Si on the surface, obtain a super-high-strength hot-dip galvanized steel sheet with a beautiful surface and good coating adhesion, and suppress the occurrence of defects such as linear defects on the surface even after undergoing the process of alloying heat treatment. Since the larger the difference in the converted concentration value, the more advantageous it is, there is no need to specifically define the upper limit of that value. However, considering the content of the elements contained, the difference in the converted concentration value can be determined to be 200% or less for both Mn and Si. In another embodiment of the present invention, the converted concentration difference of the above-mentioned Mn and Si can be 15% or more or 20% or more.

[0036] The GDS analysis method implemented in the present invention will be described in detail below.

[0037] For GDS concentration analysis, the hot-dip galvanized steel sheet is sheared into pieces with a size of 30 to 50 mm in length, and immersed in a hydrochloric acid aqueous solution with a concentration of 5 to 10 wt% at a normal temperature of 20 to 25 °C to remove the galvanized layer. In order to prevent surface damage of the base iron during the dissolution process of the galvanized layer, when the generation of bubbles due to the reaction between the galvanized 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 been hot-dip galvanized yet, it can be analyzed without such an operation of removing the plating layer.

[0038] The GDS concentration profile measures the concentrations of all components contained in the steel sheet every 1 to 5 nm in the thickness direction of the steel sheet. The measured GDS profile may contain irregular noises. In order to calculate the maximum and minimum points of the Mn and Si concentrations, a Gaussian filter with a cut-off value of 100 nm is applied to the measured concentration profile to obtain an average concentration profile, and the concentration values and depths of the maximum and minimum points of the concentration are obtained from the profile with the 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 from each other in the depth direction.

[0039] The steel sheet for plating targeted in the present invention can include base iron and an Fe plating layer formed on the base iron. The composition of the base iron is not particularly limited.

[0040] However, as long as it is a high-strength steel sheet containing 1.0 to 8.0% by weight of Mn and 0.05 to 3.0% by weight of Si and having a composition in which oxides are likely to be formed on the surface, the plating property can be advantageously improved by the present invention. The upper limit of the Mn concentration in the base iron is not particularly limited, but considering the commonly used compositions, the upper limit can be restricted to 8% by weight. Also, the lower limit of the Mn concentration is not particularly restricted, but in a composition containing less than 1.0% by weight of Mn, the surface quality of the hot-dip galvanized steel sheet is beautiful even without forming an Fe plating layer, and there is no need to perform Fe electroplating. The upper limit of the Si concentration is not particularly restricted, but considering the commonly used compositions, the upper limit can be restricted to 3.0% by weight or less. When the Si concentration is less than 0.05% by weight, the hot-dip galvanized quality is beautiful even without implementing the method of the present invention, so there is no need to implement the method of the present invention.

[0041] Since the above Mn and Si are elements that affect the plating property, their concentrations can be restricted as described above, but the present invention does not particularly restrict the remaining components of the base iron.

[0042] However, in consideration of the aspect that in the case of a high-strength steel sheet containing a large amount of alloy components, unplated and significant deterioration of plating adhesion can occur, in one embodiment of the present invention, the composition of the above base iron is, by weight, Mn: 1.0 to 8.0%, Si: 0.05 to 3.0%, C: 0.06 to 0.4% (preferably 0.07 to 0.4%, more preferably 0.08 to 0.4%), Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, N: 0.01% or less, Cr: 1.5% or less, B: 0.005% or less, and the balance is Fe and unavoidable impurities. It is advantageous that P, S, and N are not added as impurities, and since Cr and B may not be added as optional elements, the lower limits are not separately defined.

[0043] Hereinafter, the steel composition of the present invention will be described in more detail. Hereinafter, unless otherwise specified, the content of each element is based on weight.

[0044] Mn: 1.0 to 8.0% Manganese (Mn) is an element added to ensure strength. When the content of Mn is less than 1.0%, it becomes difficult to ensure strength. On the other hand, when its content exceeds 8.0%, the bainite transformation rate slows down, forming an excessive amount of fresh martensite, and it becomes difficult to obtain high hole expansion properties. Also, banded structures associated with the segregation of Mn are formed, impairing the material uniformity and formability of the material. Therefore, the content of Mn preferably ranges from 1.0% to 8.0%. The lower limit of the content of Mn is more preferably 1.5%.

[0045] Si: 0.05 to 3.0% Silicon (Si) is an element useful for increasing the strength of the steel sheet by solid solution strengthening and precipitation hardening. It has the effect of promoting the enrichment of C in austenite to suppress the formation of cementite, and is an essential element for generating retained austenite after annealing to increase the strength and elongation of the steel. When the content of Si exceeds 3.0%, the physical properties of the welded part deteriorate due to LME cracking, and the surface characteristics and plating properties of the steel material deteriorate. Therefore, the content of Si preferably ranges from 0.05% to 3.0%.

[0046] C:0.06~0.4% Carbon (C) is an element that ensures the strength of the steel material through solid solution strengthening and precipitation strengthening, and is an effective element for stabilizing retained austenite to ensure high elongation. When the content of C is less than 0.05%, a tensile strength of 1500 MPa cannot be obtained. When it exceeds 0.4%, a steel sheet cannot be manufactured by cold rolling. Therefore, the appropriate content of C is 0.06% to 0.4%. On the other hand, the content of C more preferably ranges from 0.2% to 0.4%.

[0047] Al:0.005~3.0% Aluminum (Al) is an element that has a deoxidizing effect on molten steel, and like Si, it has the effect of improving the stability of austenite and is effective in increasing the elongation.

[0048] If the content of Al is less than 0.005%, the deoxidation of the steel material is not sufficiently carried out, and the cleanliness of the steel material is impaired. On the other hand, if the content of Al is too high, the transformation temperature rises significantly, the proportion of ferrite increases, and high strength cannot be achieved. Therefore, the content of Al is set to 3.0% or less. In another embodiment of the present invention, the above Al can be limited to 2.0% or less, or 1.0% or less.

[0049] P: 0.04% or less Phosphorus (P) is contained as an impurity, segregates at the grain boundaries, and reduces toughness. Therefore, it is preferable to control its content as low as possible. Further, when the above P is excessively added, the toughness of the steel material deteriorates. Therefore, in the present invention, in order to prevent this, it is preferable to limit its upper limit to 0.04%. In the present invention, since it is more advantageous not to add P, it is not particularly necessary to determine the lower limit of the content of P. However, considering ordinary manufacturing methods, the lower limit of the content of the above P can also be set to 0.002%. In one embodiment of the present invention, the upper limit of the content of the above P can also be set to 0.0173%.

[0050] S: 0.01% or less Sulfur (S) is contained as an impurity in the steel in the same manner as the above P. Since S can combine with Mn to form inclusions, reduce hole expansion properties, and also reduce weldability and hot rolling properties, it is advantageous to control it as low as possible. Therefore, the content of S can be limited to 0.01% or less in consideration of the case where it is inevitably contained. In the present invention, since it is more advantageous not to add S, it is not particularly necessary to determine the lower limit of its content. However, considering ordinary manufacturing processes, the lower limit of the content of the above S can also be set to 0.0009%. In another embodiment of the present invention, the upper limit of the content of the above S can be set to 0.0021%.

[0051] N: 0.01% or less In the present invention, nitrogen (N) is contained in the steel material as an impurity, and it is advantageous to control its content as low as possible. Therefore, in one embodiment of the present invention, the above N can be limited to 0.01% or less, and the lower limit is not particularly defined. However, 0% can be excluded (i.e., exceeding 0%) in consideration of the case where N is inevitably contained. In another embodiment of the present invention, the lower limit of the content of the above N can also be set to 0.0005%. Further, in another embodiment of the present invention, the upper limit of the content of the above N can be 0.007%, and it can also be 0.006% or 0.0052%.

[0052] Cr: 1.5% or less Chromium (Cr) is an element effective for improving strength. It suppresses the formation of carbides and makes it easier to secure retained austenite. On the other hand, when the above Cr content exceeds 1.5%, the local corrosiveness deteriorates, and surface oxides are formed, impairing the phosphatability. Therefore, the content of the above Cr preferably has a range of 1.5% or less. More preferably, it is 1.0% or less.

[0053] B: 0.005% or less Boron (B) strengthens the grain boundaries and suppresses the ferrite transformation during cooling after annealing. To obtain this effect, the amount of B added can also be 0.0001% or more. When the above B addition amount is excessive, the hot workability deteriorates, and B accumulates excessively on the surface, inhibiting the plating property. Therefore, the content of the above B preferably has a range of 0.005% or less.

[0054] In the present invention, "high strength" is used in the sense that it includes not only the case of having high strength after annealing, but also all cases where high strength can be obtained by heat treatment or the like in subsequent processes. Further, in the present invention, "high strength" can mean a tensile strength of 490 MPa or more, but is not limited thereto. In addition to the components described above, the base iron can further contain elements such as Ti, Mo, and Nb in a total amount of 1.0% or less. Although there are no particular restrictions on the base iron, in one embodiment of the present invention, a cold-rolled steel sheet or a hot-rolled steel sheet can be used as the base iron. Also, although not necessarily limited thereto, in one embodiment, as the cold-rolled steel sheet or the hot-rolled steel sheet, martensitic steel or TRIP steel can be used.

[0055] In one embodiment of the present invention, the cold-rolled steel sheet or the hot-rolled steel sheet can have a microstructure containing 30 to 70% by volume of tempered martensite, 5 to 45% by volume of bainite, 10 to 40% by volume of retained austenite, and 20% by volume or less of a ferrite phase. By including 30% by volume or more of tempered martensite in the microstructure, it is possible to ensure hole expansion properties while having high strength. Also, when 10% by volume or more of retained austenite is contained, the phenomenon of transformation-induced plasticity by the retained austenite can be used, and the elongation rate can be greatly improved. Bainite needs to be contained in an amount of 5% by volume or more as it is included to form retained austenite and ensure strength. Ferrite can be included as needed to improve the elongation rate, refine the prior austenite grain size, improve the mechanical properties, and increase the C concentration in the retained austenite to stabilize the retained austenite. However, when the proportion of the ferrite phase exceeds 20% by volume, the hardness difference between the phases with tempered martensite, which is the matrix structure, may become large, and the bendability and hole expansion properties may deteriorate. Therefore, the proportion can be limited to 20% by volume or less. In some cases, the ferrite phase can be included in an amount of 3% by volume or more. When the proportion of each phase is too high, the proportion of other phases can be limited to less than the appropriate range, so the upper limit can be limited as described above.

[0056] The hot-rolled steel sheet or cold-rolled steel sheet having the above-described microstructure can be produced by the following process, although not necessarily limited thereto.

[0057] That is, in terms of weight %, Mn: 1.0 to 8.0%, Si: 0.05 to 3.0%, C: 0.06 to 0.4% (preferably 0.07 to 0.4%, more preferably 0.08 to 0.4%), Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, N: 0.0% or less, Cr: 1.5% or less, B: 0.005% or less, and the balance Fe and inevitable impurities; heating a steel slab; hot-rolling the heated steel slab to obtain a steel sheet; and winding up the hot-rolled steel sheet. The hot-rolled steel sheet can be obtained by a process including these steps. In the case of a cold-rolled steel sheet, it can be produced by a process including cold-rolling the hot-rolled steel sheet; maintaining the cold-rolled steel sheet in a soaking section for 50 to 500 seconds; cooling the maintained steel sheet at a cooling rate of 1°C / s or more (which can be limited to 3000°C / s or less, 2000°C / s or less, or 1000°C / s or less in some cases) to a temperature between 100 and 300°C; and heating the cooled steel sheet to a temperature between 300 and 600°C and maintaining it for 50 seconds or more (which can be limited to 3000 seconds or less or 2000 seconds or less in some cases).

[0058] In one aspect of the present invention, a hot-dip galvanized steel sheet including the above-described steel sheet for plating can be provided. The hot-dip galvanized steel sheet can include a steel sheet for plating and a hot-dip galvanized layer formed on the surface of the steel sheet for plating. At this time, any commercially available hot-dip galvanized steel sheet can be applied, and the type thereof is not particularly limited.

[0059] Next, an exemplary embodiment of a method for manufacturing a plating steel sheet and a hot-dip galvanized steel sheet having the above-described advantageous effects will be described. According to one embodiment of the present invention, the plating steel sheet is prepared by a process including: preparing a base iron; performing electroplating on the base iron to form an Fe plating layer containing 5 to 50% by weight of oxygen; and annealing the base iron on which the Fe plating layer is formed to obtain a plating steel sheet.

[0060] Fe electroplating containing 6.2% by weight of oxygen was performed on a 1.2 GPa-class cold-rolled steel sheet (martensitic steel) containing 2.3% Mn, 0.9% Si, and other alloy elements so that the iron deposition amount became 2.03 g / m 2 After that, the electroplated cold-rolled steel sheet was annealed at 800°C for 53 seconds in an N2-5%H2 atmosphere with a dew point of -40°C and then cooled. The atmosphere during the entire annealing process was maintained in the same manner. After sampling a specimen from the cooled steel sheet and observing the cross-section with a transmission electron microscope, it was confirmed that particulate Mn and Si oxides were formed at the interface between the iron electroplating layer and the base iron, while almost no Mn and Si oxides were formed on the surface of the electroplating layer. In particular, when analyzing such a state with a GDS profile, as shown in FIG. 1, maximum values of Mn and Si concentrations appear immediately below the surface of the steel sheet (including the Fe plating layer), and minimum values of Mn and Si concentrations may appear at deeper positions. Of course, in some cases, the minimum value may not appear clearly and the concentration may tend to decrease gradually, but the maximum value can be clearly observed.

[0061] Such a phenomenon is due to the formation of an Fe plating layer with a high oxygen content before annealing. That is, when the ferroelectric plating layer contains 5 to 50% by weight of oxygen, when annealed in a reducing atmosphere annealing furnace, the oxygen in the ferroelectric plating layer oxidizes alloy elements such as Mn and Si that diffuse to the surface in the base iron, and accumulates at the interface between the ferroelectric plating layer and the base iron. Therefore, as shown in the graph of Figure 1, when measuring the concentration by GDS, a maximum point with a high concentration of Mn, Si, etc. is confirmed at a depth corresponding to the thickness of the ferroelectric plating layer from the surface. On the other hand, alloy elements with a slow diffusion rate such as Mn may have a minimum point after the maximum point of the GDS concentration because the concentration is diluted by the ferroelectric plating layer or the dissolved Mn cannot diffuse quickly from the base iron even when depleted by internal oxidation. However, Si diffuses rapidly from the inside during the annealing process, and internal oxidation continuously proceeds at the interface between the ferroelectric plating layer and the base iron, resulting in the accumulation of oxides, so a minimum point may not be confirmed by GDS concentration analysis. Therefore, the absence of a minimum point in the GDS concentration profile means that alloy elements such as Mn and Si are oxidized by the ferroelectric plating layer, effectively suppressing diffusion to the surface.

[0062] Different from the oxides formed strongly at high temperatures, when an iron plating layer containing a large amount of oxygen is formed and annealed, reduction occurs not only on the surface of the ferroelectric plating layer but also at the interface between the ferroelectric plating layer and the base steel plate. At this time, the reduced iron diffuses and joins with the base iron, and Mn and Si react with the oxygen in the Fe plating layer to form oxide particles or discontinuous plate-like forms at the interface between the ferroelectric plating layer and the base steel plate, so that the adhesion between the ferroelectric plating layer and the base metal can be maintained well. Not only that, the uniformly formed ferroelectric plating layer suppresses the formation of surface oxides, reduces the concentration of alloy elements such as Mn and Si dissolved in the surface of the steel plate, promotes the alloying reaction with the zinc plating layer, and a uniform alloying molten plated steel plate without surface defects can be obtained.

[0063] The internal oxidation method of annealing does not form a layered oxide layer unlike the oxidation-reduction method. Therefore, when hot-dip galvanizing an ultra-high strength steel sheet containing a large amount of alloying elements such as Mn and Si, it exhibits excellent characteristics for improving plating adhesion. However, since 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 completely removed. As a result, 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 during annealing, the wettability with the hot-dip plating solution becomes non-uniform and unplated areas occur, or during the alloying heat treatment after zinc plating, the thickness of the oxide film is non-uniform and a difference in alloying degree occurs, which may cause problems such as the induction of linear defects that can be easily identified visually.

[0064] In order to suppress the surface diffusion of alloying elements by Fe plating containing a large amount of oxygen and produce a hot-dip galvanized steel sheet that is beautiful and has no problem of plating peeling, an iron electroplating layer containing 5 to 50% by weight of oxygen in the base iron is formed so that the iron deposition amount is 0.5 to 3.0 g / m 2 Based on this, it is preferable to heat up to a temperature of 600 to 950 °C so that the mechanical properties of the steel sheet can be ensured, and then cool it again to perform hot-dip plating.

[0065] In one embodiment of the present invention, the Fe plating layer can be formed through a continuous plating process, and the Fe plating amount at this time can be set to 0.5 to 3.0 g / m based on the Fe deposition amount. 2 If the Fe plating amount is less than 0.5 g / m 2 , the effect of suppressing the diffusion of alloying elements by the Fe plating layer may be insufficient in a normal continuous annealing process. Also, even if it exceeds 3.0 g / m 2 , the suppression effect of alloying elements can be further increased. However, in order to ensure a high plating amount, multiple plating cells must be operated. When using an insoluble anode, the electroplating solution rapidly acidifies, resulting in a decrease in plating efficiency and the problem of sludge generation, which is not economical. In another embodiment of the present invention, the Fe plating amount 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 Fe plating amount of 0.5 to 3.0 g / m 2 of the present invention can correspond to a thickness of 0.05 to 0.4 μm after annealing.

[0066] 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 converted concentration at the above maximum point and the converted concentration at the minimum point can satisfy the numerical range limited in one embodiment of the present invention. Considering such a point, in one 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 Fe plating amount. However, in order to obtain such an effect, it is necessary to perform plating exceeding 3.0 g / m 2 , and thus 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 one embodiment of the present invention, the oxygen content in the Fe plating layer is controlled to 5% by weight or more. On the other hand, as the oxygen concentration in the Fe plating layer increases, the effect of suppressing surface oxides during annealing can be further increased. However, in a normal electroplating method, it is difficult to obtain a plating layer exceeding 50% by weight, so the upper limit can be limited to 50% by weight. In another embodiment of the present invention, the oxygen concentration in the Fe plating layer can also be limited to 10 to 40%.

[0067] 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 it exceeds 950°C, the alloying elements in the steel will rapidly diffuse to the surface, resulting in poor quality of the hot-dip galvanized coating and uneconomical operation at an unnecessarily high temperature.

[0068] On the other hand, in one embodiment of the present invention, the dew point inside the annealing furnace is not necessarily limited to this, but can be less than -20°C. When the dew point temperature is maintained below -20°C, it is economical because another humidifying device for raising the dew point is not required. Moreover, in the case of the present invention, since an Fe plating layer with a high oxygen concentration is formed, alloying elements such as Mn and Si can be sufficiently prevented from diffusing to the surface without deliberately inducing internal oxidation by the atmosphere. The lower limit of the dew point temperature is not particularly defined. However, maintaining the dew point below -90°C may not be industrially advantageous, such as using very high-purity gases. Considering this, the lower limit of the dew point can be set at -90°C. According to another embodiment of the present invention, the dew point when the temperature of the steel sheet is 600 - 950°C can be -70°C to -30°C.

[0069] Also, in order to prevent the oxidation of the base iron and the Fe plating layer during annealing, the hydrogen concentration in the atmosphere gas during annealing can be determined to be 1% or more by volume%. When the hydrogen concentration is less than 1%, the trace amount of oxygen inevitably contained in the H2 and N2 gases cannot be effectively oxidized and removed, resulting in an increase in the oxygen partial pressure, which may induce the surface oxidation of the base iron. On the other hand, when the hydrogen concentration exceeds 70%, the risk of explosion during gas outflow and the cost due to high-hydrogen operations increase. Therefore, the above hydrogen concentration can be determined to be 70% or less. Except for the inevitable impurity gases, it can be substantially nitrogen (N2) other than the above hydrogen (H2).

[0070] 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 through the Fe plating layer increases, the amount of surface oxide formed increases, and as a result, the quality of the hot-dip galvanized product deteriorates, so it can be limited to 120 seconds or less.

[0071] Hereinafter, based on the above content, the effect of suppressing the surface diffusion of Mn and Si during annealing in a high dew point atmosphere in a cold-rolled steel sheet with an Fe plating layer containing a large amount of oxygen will be described in more detail with reference to FIG. 2.

[0072] 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.

[0073] FIG. 2(a) shows a schematic cross-sectional view of a base steel sheet on which an Fe plating layer containing a large amount of oxygen is formed. The base steel sheet contains alloying elements such as Mn and Si. The Fe plating layer contains 5 to 50% by weight of oxygen and impurities inevitably mixed during plating, and the balance is composed of Fe.

[0074] FIG. 2(b) shows a state in which a cold-rolled steel sheet plated with iron is heated at about 300 to 500°C in a nitrogen atmosphere containing 1 to 70% H2. The surface of the Fe plating layer is gradually reduced and oxygen is removed. Internal oxides such as Mn and Si diffused from the base iron are formed at the interface between the Fe plating layer and the base iron, and the grain boundary oxides grow coarser as the temperature increases.

[0075] FIG. 2(c) shows a schematic cross-sectional view of the base steel sheet when the temperature is raised to 500 to 700°C in the same reducing atmosphere. Most of the Fe plating layer is reduced to form ferrite with a low Mn and Si concentration with respect to the base iron. Since the oxygen in the Fe plating layer gradually depletes, Mn and Si gradually start to diffuse through the Fe plating layer to the surface of the Fe plating layer.

[0076] Fig. 2(d) shows a schematic cross-sectional view of a steel sheet annealed at a temperature of 600 to 950°C. When the electroplated iron layer excludes internal oxides such as Mn and Si, the oxygen dissolved in the metal iron is completely removed, and the generated internal oxides generally have a spherical or short plate shape. Furthermore, due to grain growth, the electroplated iron layer can also form a single crystal grain with the base iron. However, the form of the internal oxides is not necessarily generated in a particulate form, and depending on the elongation rate of the cold-rolled steel sheet, the steel components, the atmosphere in the annealing furnace, and the oxygen content contained in the electroplated iron layer, the crystal grains of the base iron and the electroplated iron layer may be distinguished and appear, and short linear oxides can also be generated along the interface between the electroplated iron layer and the base iron or the grain boundaries inside the base iron.

[0077] After the above annealing stage, the annealed steel sheet can be cooled. Since the cooling conditions in the cooling stage after the annealing stage do not significantly affect the surface quality of the final product, that is, the plating quality, there is no need to particularly limit the cooling conditions in the present invention. However, in order to prevent oxidation of the iron component during the cooling process, at least a reducing atmosphere can be applied to iron.

[0078] According to an embodiment of the present invention, hot-dip galvanizing can be performed on the electroplated steel sheet obtained by the above-described process to form a hot-dip galvanized layer. The hot-dip galvanizing method in the present invention is not particularly limited.

[0079] Also, in the present invention, as long as the base iron has the above-described alloy composition, it can be applied without limitation as the base iron of the electroplated steel sheet or the hot-dip galvanized steel sheet according to the present invention. Therefore, the method for manufacturing the base iron can be specifically not limited.

[0080] 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.

[0081] That is, in the present invention, in order to form an Fe plating layer, an electroplating solution containing ferrous ions including first ferrous ions and second ferrous ions, a complexing agent, and inevitable impurities can be used, and the concentration of the second ferrous ions in the above ferrous ions is 5 to 60% by weight.

[0082] According to an embodiment of the present invention, the electroplating solution contains first ferrous ions and second ferrous ions. In order to obtain high plating efficiency, it may be advantageous to contain only the first ferrous ions. However, when only the first ferrous ions are contained, the solution deteriorates and the plating efficiency rapidly decreases, which may induce quality variations in the continuous electroplating process. Therefore, the second ferrous ions can be further contained. At this time, the concentration of the second ferrous ions is preferably 5 to 60% by weight, more preferably 5 to 40% by weight, based on the total of the first and second ferrous 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 rapidly increases, the pH rapidly decreases, and the plating efficiency continuously decreases. On the other hand, when the concentration of the second ferrous ions exceeds 60%, the reaction amount of the second iron reduced to the first iron at the cathode increases more greatly than the reaction amount of the first iron reduced and deposited as metallic iron, so the plating efficiency greatly decreases 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 set the concentration of the second ferrous ions in the above ferrous ions to 5 to 60% by weight.

[0083] The concentration of the above ferrous ions is preferably 1 to 80 g per 1 L of the electroplating solution, more preferably 10 to 50 g per 1 L. If it is less than 1 g / L, there is a problem that the plating efficiency and plating quality rapidly decrease. 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.

[0084] 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 ferric iron, it is preferable to use an amino acid or an amino acid polymer as the complexing agent.

[0085] An amino acid refers to an organic molecule in which a carboxyl group (-COOH) and an amine group (-NH2) are bonded. An amino acid polymer means an organic molecule formed by polymerization of two or more amino acids. The amino acid polymer exhibits complexing agent characteristics similar to those of an amino acid. Therefore, in the following description, amino acids and amino acid polymers are collectively referred to as amino acids.

[0086] When an amino acid dissolves in neutral water, the amine binds to a hydrogen ion and becomes positively charged, and the carboxyl group dissociates a hydrogen ion and becomes negatively charged, so that the amino acid molecule maintains charge neutrality. On the other hand, when the solution is acidified, the carboxyl group recombines with a hydrogen ion to become charge neutral, and the amine has a positive charge, so that the amino acid molecule forms a cation. That is, an amino acid forms a charge-neutral or cation in a weakly acidic aqueous solution.

[0087] When an amino acid is added to an acidic electrolyte containing iron ions, it complexes with ferrous ions and ferric ions. However, the iron ions complexed with the amino acid maintain a cationic state even in the complexed state. Therefore, it exhibits characteristics electrically opposite to those of a normal complexing agent having a plurality of carboxyl groups being negatively charged in a weakly acidic aqueous solution.

[0088] 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 cause 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.

[0089] 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 only contains 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 the 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.

[0090] 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.

[0091] On the one hand, it is preferable that the complexing agent is added in an amount such that the molar concentration ratio of the iron ions to the complexing agent is 1:0.05 to 2.0, and more preferably in an amount such that the ratio is 1:0.5 to 1.0. When it is less than 0.05, the excessive ferrous ions cannot be prevented from combining with hydroxide ions or oxygen to form sludge. Even without the inclusion of ferrous iron, the plating efficiency decreases significantly, and furthermore, burning is induced, deteriorating the plating quality. On the other hand, even if it exceeds 2.0, the sludge suppression effect and plating quality are maintained, but the overvoltage increases, reducing the plating efficiency, and relatively expensive amino acids are excessively and unnecessarily included for raw materials containing iron ions such as iron sulfate, resulting in an increase in raw material costs and being uneconomical.

[0092] Preferably, the complexing agent is one or more selected from amino acids or amino acid polymers. For example, it can be one or more selected from alanine, glycine, serine, threonine, arginine, glutamine, glutamic acid, and glycylglycine.

[0093] When the above amino acid is used as a complexing agent and electroplating is carried out at a current density of 3 to 120 A / dm² while maintaining the solution temperature at 80 °C or lower and the pH at 2.0 to 5.0 2 high plating efficiency can be achieved, and an Fe plating layer with a high oxygen concentration can be obtained.

[0094] 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 extreme, the concentration of the solution continuously changes, and it becomes difficult to perform uniform electroplating.

[0095] When the pH of the Fe electroplating solution is less than 2.0, the electroplating efficiency decreases and it is not suitable for continuous plating processes. When the pH exceeds 5.0, the plating efficiency increases, but sludge in which iron hydroxide precipitates is generated during continuous electroplating, causing problems such as pipe blockage, roll, and equipment contamination.

[0096] The current density is 3 A / dm² 2When it is less than this value, the plating overvoltage of 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 electroplated layer is non-uniform, and there is a problem that the Fe plating layer easily peels off.

[0097] As described above, in the present invention, it is preferable that the Fe plating layer contains 5 to 50% by weight of oxygen. The reason for the oxygen to be mixed into the Fe plating layer is as follows. During the process of iron deposition on the surface of the steel sheet 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 together when the Fe plating layer is formed. If an anionic complexing agent such as acetic acid, lactic acid, citric acid, or EDTA is used, the iron ions combined with the OH - ions are negatively charged on average. When a cathode is applied for electroplating, an electrical repulsive force is generated, and the mixing into the Fe plating layer is suppressed. On the other hand, amino acids are electrically neutral at pH 2.0 to 5.0. In a strong acid with a pH less than 2.0, they become cationic. However, even if 1 to 2 OH - ions are combined with the iron ions combined with the amino acids, they are still cationic. Therefore, when amino acids are used as a complexing agent so that the molar concentration ratio of iron ions to amino acids is 1:0.05 to 1:2.0 and Fe electroplating is carried out while maintaining pH 2.0 to 5.0, high plating efficiency can be achieved, and an Fe plating layer containing 5 to 50% by weight of oxygen can be obtained while suppressing sludge generation.

[0098] In order to ensure the quality of hot-dip galvanized products of steel sheets containing Mn and Si, it is preferable to treat the plating amount of the Fe plating layer 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 in the continuous plating process, it is 3.0 g / m 2When it exceeds this value, multiple plating cells are required or the production speed decreases, which is not economical. Moreover, when the amount of Fe electroplating is large, the Fe electroplating solution rapidly denatures in the 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 amount of Fe electroplating is less than 0.5 g / m 2 ², since the oxygen contained in the Fe plating layer is rapidly reduced and removed, Mn and Si diffuse from the base iron, and the formation of surface oxides cannot be effectively suppressed, resulting in a problem of reduced quality of the hot-dip plating. The above-mentioned Fe plating amount has a thickness of about 0.05 to 0.4 μm when the Fe plating layer is completely reduced during annealing at the iron concentration contained in the plating layer.

Claims

1. by weight, Mn: 1.0 to 8.0%, Si: 0.05 to 3%, C: 0.06 to 0.4%, Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, N: 0.01% or less, Cr: 1.5% or less, B: 0.005% or less, with the balance being Fe and inevitable impurities, having a composition, 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 10% 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 10% 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 taken 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 15% or more and the converted Si concentration difference is 15% or more.

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

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

6. preparing base iron having a composition by weight, Mn: 1.0 to 8.0%, Si: 0.05 to 3%, C: 0.06 to 0.4%, Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, N: 0.01% or less, Cr: 1.5% or less, B: 0.005% 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 base iron with the Fe plating layer formed thereon is annealed by maintaining it at 600 to 950 °C for 5 to 120 seconds in an annealing furnace with a dew point temperature controlled to less than -20 °C in a 1 to 70% H ２ - the remaining N ２ gas atmosphere, and the method for manufacturing a steel sheet 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 steel sheet according to claim 6, wherein the adhesion amount is as described above.

8. The method for manufacturing a 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 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 for manufacturing the steel sheet according to claim 6 or 7.

11. A step of preparing base iron having a composition containing, in % by weight, Mn: 1.0 to 8.0%, Si: 0.05 to 3%, C: 0.06 to 0.4%, Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, N: 0.01% or less, Cr: 1.5% or less, B: 0.005% or less, and the balance being Fe and unavoidable impurities; A step of 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 less than -20°C in a 1-70% H ２ - the remaining N ２ Annealing in an annealing furnace in a gas atmosphere at 600-950°C for 5-120 seconds to obtain a steel sheet for plating; and A method for manufacturing a hot-dip galvanized steel sheet, including a step of immersing the steel sheet for plating in a hot-dip galvanizing bath.

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