High-strength hot-dip galvanized steel sheet having excellent plating quality, steel sheet for plating, and methods for manufacturing same

Abstract

To provide a high-strength hot-dip galvanized steel sheet having excellent plating quality, a steel sheet for plating for manufacturing the same, and methods for manufacturing the steel sheets.SOLUTION: A steel sheet for plating according to an aspect of the present invention has a GDS profile of a Mn component, as observed in the depth direction from the surface, sequentially including a maximum point and a minimum point, wherein a difference (a converted concentration difference of Mn) between a value, obtained by dividing the concentration of Mn at the maximum point by the concentration of Mn of a base material, and a value, obtained by dividing the concentration of Mn at the minimum point by the concentration of Mn of the base material, may be 80% or more, and a difference (a converted concentration difference of Si) between a value, obtained by dividing the concentration of Si at the maximum point by the concentration of Si of the base material, and a value, obtained by dividing the concentration of Si at the minimum point by the concentration of Si of the base material, may be 50% or more. However, when no minimum point appears within a depth of 5 μm, a point at a depth of 5 μm is denoted as the point at which the minimum point appears.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a high-strength hot-dip galvanized steel sheet having excellent coating quality, a steel sheet for coating for producing the same, and a method for producing the same. [Background technology]

[0002] In recent years, the automotive industry has adopted high-strength steel sheets for automobiles to improve safety and reduce weight through thickness reduction. Precipitation-strengthened steels and solid-solution-strengthened steels have been developed as suitable steels for automobiles. Furthermore, dual-phase steels (DP steels), complex-phase steels (CP steels), transformation-induced plasticity steels (TRIP steels), and twinning-induced plasticity steels (TWIP steels) have been developed that utilize phase transformation to improve strength and elongation. These high-strength steels contain a variety of alloying elements compared to general steels, particularly Mn, Si, Al, Cr, and B, which are elements with a higher oxidation tendency than Fe.

[0003] The quality of hot-dip galvanizing is determined by the surface condition of the annealed steel sheet immediately before galvanizing. However, the formation of surface oxides during annealing due to elements such as Mn, Si, Al, Cr, and B, which are added to ensure the physical properties of the steel sheet, can degrade the galvanizability. Specifically, these elements diffuse to the surface during the annealing process and react with trace amounts of oxygen or water vapor present in the annealing furnace to form single or complex oxides of these elements on the steel sheet surface, reducing the surface reactivity. The surface of the annealed steel sheet with reduced reactivity inhibits wettability with the hot-dip galvanizing bath, resulting in localized or global lack of adhesion of the plating metal to the surface of the galvanized steel sheet. Furthermore, these oxides prevent the formation of an alloying inhibitor layer (Fe2Al5) necessary to ensure the adhesion of the plating layer during the hot-dip galvanizing process, leading to peeling of the plating layer and other significant degradation of the plating quality of the galvanized steel sheet.

[0004] Various techniques have been proposed to improve the coating quality of high-strength hot-dip galvanized steel sheets. Among them, Patent Document 1 proposes a technique for providing hot-dip galvanized or galvannealed steel sheets with excellent coating quality by controlling the air-to-fuel ratio of air to fuel to 0.80 to 0.95 during the annealing process, oxidizing the steel sheet in a direct flame furnace in an oxidizing atmosphere, forming iron oxides containing Si, Mn, or Al alone or in combination to a certain depth inside the steel sheet, and then reducing and annealing the iron oxides in a reducing atmosphere, followed by hot-dip galvanizing.

[0005] When a method of oxidation in the annealing process followed by reduction, as in Patent Document 1, is used, components with a high affinity for oxygen, such as Si, Mn, and Al, are internally oxidized to a certain depth from the surface of the steel sheet, suppressing their diffusion to the surface layer, resulting in a relative decrease in the amount of Si, Mn, or Al oxides or composite oxides in the surface layer, improving wettability with zinc and reducing uncoated areas. However, in the case of steel types to which Si is added, Si concentrates directly below the iron oxide during the reduction process, forming band-like Si oxides, which can cause peeling in the surface layer including the coating layer, i.e., peeling at the interface between the reduced iron and the underlying base iron, making it difficult to ensure the adhesion of the coating layer.

[0006] Meanwhile, Patent Document 2 proposes another method for improving the galvanizability of high-strength hot-dip galvanized steel sheets, in which the dew point in an annealing furnace is maintained high to internally oxidize easily oxidized alloy elements such as Mn, Si, and Al within the steel, thereby reducing the amount of oxides that are externally oxidized on the surface of the steel sheet after annealing, thereby improving the galvanizability. However, while the method disclosed in Patent Document 2 can solve the galvanizability problem caused by the external oxidation of Si, which is easily internally oxidized, there is a problem in that the effect is minimal when a large amount of Mn, which is relatively difficult to internally oxidize, is added.

[0007] Furthermore, even if galvanization is improved by internal oxidation, non-uniformly formed surface oxides may cause linear unplated areas. In addition, when a galvannealed steel sheet (GA steel sheet) is manufactured by alloying heat treatment after plating, problems such as linear defects due to non-uniform alloying may occur on the surface of the galvannealed steel sheet.

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

[0009] [Patent Document 1] Korean Patent Publication No. 2010-0030627 [Patent Document 2] Korean Patent Publication No. 2009-0006881 Summary of the Invention [Problem to be solved by the invention]

[0010] According to one aspect of the present invention, there are provided a hot-dip galvanized steel sheet having excellent coating quality, which does not produce uncoated areas and solves the problem of peeling of the coating layer, and a method for manufacturing the same.

[0011] According to another aspect of the present invention, there are provided a hot-dip galvanized steel sheet and a manufacturing method thereof, which can produce an alloyed hot-dip galvanized steel sheet having excellent surface quality without generating linear defects even when alloying heat treatment is performed after coating.

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

[0013] The object of the present invention is not limited to the above-mentioned content. A person having ordinary skill in the art to which the present invention pertains will have no problem in understanding further object of the present invention from the overall matters of the specification of the present invention. [Means for solving the problem]

[0014] In a steel sheet for plating according to one aspect of the present invention, GDS profiles of the Mn and Si components observed in the depth direction from the surface sequentially include maximum and minimum points, and the difference between the value obtained by dividing the Mn concentration at the maximum point in the GDS profile of the Mn component by the Mn concentration in the base material and the value obtained by dividing the Mn concentration at the minimum point in the GDS profile of the Mn component by the Mn concentration in the base material (difference in converted Mn concentrations) is 80% or more, and the difference between the value obtained by dividing the Si concentration at the maximum point in the GDS profile of the Si component by the Si concentration in the base material and the value obtained by dividing the Si concentration at the minimum point in the GDS profile of the Si component by the Si concentration in the base material (difference in converted Si concentrations) is 50% or more.

[0015] However, if 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] A hot-dip galvanized steel sheet according to another aspect of the present invention may include the above-described steel sheet for plating and a hot-dip galvanized layer formed on the steel sheet for plating.

[0017] A method for manufacturing a steel sheet for plating, according to another aspect of the present invention, may include the steps of: preparing a base steel; electroplating the base steel to form an Fe coating layer containing 5 to 50 wt % of oxygen; and annealing the base steel with the Fe coating layer formed thereon in an annealing furnace in an atmosphere of 1 to 70% H2 and the remainder N2 gas, with the dew point temperature controlled to -15 to +30°C, at 600 to 950°C for 5 to 120 seconds.

[0018] A method for producing a hot-dip galvanized steel sheet according to another aspect of the present invention includes the steps of: preparing a base steel; electroplating the base steel to form an Fe coating layer containing 5 to 50 wt % of oxygen; annealing the base steel with the Fe coating layer formed thereon at 600 to 950°C for 5 to 120 seconds in an annealing furnace in an atmosphere of 1 to 70% H2 and the remainder N2 gas, the dew point temperature of which is controlled at -15 to +30°C, to obtain a steel sheet for galvanization; and immersing the steel sheet for galvanization in a galvanization bath containing 0.1 to 0.3% Al, the remainder Zn, and unavoidable impurities, the bath being maintained at a temperature range of 440 to 500°C. [Effects of the Invention]

[0019] As described above, the present invention can provide a hot-dip galvanized steel sheet that significantly improves the phenomenon of uncoated areas occurring during hot-dip galvanizing and has improved coating adhesion by forming a pre-coated layer and controlling the concentration profiles of Mn and Si elements inside the steel sheet.

[0020] Furthermore, according to one aspect of the present invention, it is possible to prevent linear defects and the like on the surface of a galvannealed steel sheet obtained by subjecting the galvannealed steel sheet of the present invention to alloying heat treatment, and therefore it is possible to provide a galvannealed steel sheet with excellent surface quality. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a schematic diagram of a GDS profile measured after removing the coating layer of a hot-dip galvanized steel sheet manufactured using an Fe-electroplated cold-rolled steel sheet. [Figure 2] These are electron microscope photographs of the cross sections of steel sheets obtained by annealing base steel for 53 seconds at a temperature of 800°C and a dew point of 5°C. (a) is the cross section of the steel sheet obtained by annealing without forming an Fe plating layer, and (b) is the cross section of the steel sheet obtained by Fe electroplating to an iron coating weight of 1.99 g / m2 and then annealing. [Figure 3] This is a schematic diagram of the process of annealing a base steel having an oxygen-containing Fe-plated layer formed thereon in an atmosphere with a high dew point. [Figure 4] 1A and 1B are GDS concentration profiles measured on hot-dip galvanized steel sheets after removal of the coating layer, where (a) is for the base steel of Comparative Example 2, (b) is for the base steel of Comparative Example 11, (c) is for the base steel of Comparative Example 16, and (d) is for the base steel of Invention Example 7. DETAILED DESCRIPTION OF THE INVENTION

[0022] A high-strength hot-dip galvanized steel sheet with excellent coating quality according to one aspect of the present invention, which was completed through the research of the present inventors, will be described in detail below. It should be noted that, in the present invention, the concentration of each element is expressed in terms of weight percent unless otherwise specified. The Fe electroplating amount is measured as the total amount of Fe contained in the coating layer per unit area, and does not include oxygen and inevitable impurities in the coating layer.

[0023] Furthermore, unless otherwise specified, the concentrations and concentration profiles referred to in the present invention refer to concentrations and concentration profiles measured using a GDS, i.e., a glow discharge optical emission spectrometer.

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

[0025] The cause of uncoated areas and reduced coating adhesion in steel sheets containing large amounts of Mn and Si is known to be due to surface oxides that are formed when alloying elements, particularly Mn and Si, are oxidized on the surface during the high-temperature annealing process of cold-rolled steel sheets.

[0026] To prevent alloying elements such as Mn and Si from diffusing to the surface, a method can be used, such as forming an oxide layer containing a large amount of oxygen. This involves oxidizing the material during heating and then reducing it again in a reducing atmosphere. Alternatively, the base metal surface can be coated with iron oxide and then heat-treated. However, the iron oxide firmly formed on the surface of the base metal contains not only FeO but also a mixture of Fe3O4 and Fe2O3, which are difficult to reduce. While the surface is reduced to metallic iron during annealing in a reducing atmosphere, the interface between the iron oxide layer and the base iron is difficult to completely reduce due to the slow reduction rate. Consequently, Mn and Si oxides accumulate at the interface, forming a continuous oxide layer. While this improves wettability with molten zinc, the oxide layer can easily crack, potentially causing peeling of the coating.

[0027] On the other hand, when an internal oxidation annealing method is applied, in which alloying elements such as Mn and Si are oxidized inside the steel by increasing the oxygen partial pressure or dew point in the annealing furnace during the heat treatment process, Mn and Si oxides are preferentially formed on the steel surface during the heat treatment process, and then the Mn and Si are oxidized by oxygen diffused into the steel, suppressing surface diffusion. Therefore, although a thin oxide film is formed on the surface of the base steel, if the surface of the cold-rolled steel sheet before annealing is not completely uniform or if local variations in the oxygen partial pressure, temperature, etc. occur, wettability during hot-dip galvanizing becomes uneven, resulting in non-galvanized areas. Alternatively, if the thickness of the oxide film is uneven during the alloying heat treatment process after galvanizing, differences in the degree of alloying occur, which tends to cause linear defects that are easily identifiable by the naked eye.

[0028] In order to solve the above-mentioned technical problems, the present inventors attempted to produce a hot-dip galvanized steel sheet having a beautiful surface and free from the problem of coating peeling by controlling the presence forms of oxidizing elements Mn and Si on the surface side of the steel sheet to be plated as follows:

[0029] That is, the steel sheet according to an embodiment of the present invention may have the following GDS concentration profile of Mn and Si: The steel sheet for plating according to the present invention will be described in detail with reference to the GDS profile of FIG.

[0030] FIG. 1 is a graph schematically illustrating a typical GDS profile of Mn or Si that may appear from the surface after removing a galvanized layer from a hot-dip galvanized steel sheet, including the steel sheet of the present invention. The vertical axis of the graph represents the concentration of alloying elements such as Mn and Si, and the horizontal axis represents depth. As illustrated in the graph of FIG. 1, the Mn or Si concentration profile of the steel sheet for plating of the present invention may have a morphology in which maximum and minimum points appear sequentially from the surface (or the interface with the galvanized layer in the case of hot-dip galvanization) toward the interior. Here, "sequentially occurring" does not necessarily mean that the maximum point appears first from the surface (interface) toward the depth direction. In some cases, the minimum point may appear first, but the maximum and minimum points should appear sequentially after the maximum point. However, in some examples, the minimum point may not appear after the maximum point. In such cases, the internal concentration in the 5 μm depth region may be considered the minimum point concentration. Furthermore, although the alloying element concentration at the surface is lower than the concentration at the maximum point, in some cases minimum points with low alloying element concentration may appear between the surface and the maximum point.

[0031] In the GDS concentration profile shown in Figure 1 above, although not necessarily limited to this, the surface layer corresponds to the Fe-plated layer, which has a low concentration of alloying elements, because not many alloying elements diffuse from the base steel. The maximum points correspond to regions where internal oxides of alloying elements formed near the interface between the Fe-plated layer and the base steel are concentrated. The minimum points that appear from the Fe-plated layer toward the base steel correspond to regions where alloying elements have diffused and diluted the Fe-plated layer, which does not contain alloying elements, or where alloying elements have diffused and become depleted at the maximum points where internal oxidation has occurred.

[0032] In one embodiment of the present invention, the maximum point may be formed at a depth of 0.05 to 1.0 μm from the surface of the steel sheet. If a maximum point appears in a region deeper than this, it may not be determined as a maximum point that contributes to the effects of the present invention. Alternatively, the minimum point may be formed at a position within a depth of 5 μm from the surface of the steel sheet. As described above, if a minimum point does not form at a point within a depth of 5 μm, the 5 μm depth may be determined as the point where the minimum point is formed. Since the concentration at a depth of 5 μm is substantially the same as the internal concentration of the base material, it can be considered as the point where the concentration does not decrease any further.

[0033] In this regard, in the Mn concentration profile and the Si concentration profile, the greater the difference between the converted concentration at the maximum point of the element (the concentration at that point divided by the concentration in the base material, expressed in %) and the converted concentration at the minimum point, the greater the reduction in Mn and Si diffusing to the surface. In one embodiment of the present invention, the value of the converted concentration at the maximum point minus the converted concentration at the minimum point of Mn may be 80% or more, and the difference between these values ​​for Si may be 50% or more. Si is a more oxidizing element than Mn and easily causes internal oxidation even inside the base steel, where the oxygen concentration is low. Therefore, oxidation may occur over a wider area than Mn. Therefore, even if the difference in converted concentration between the maximum point and the minimum point of Si is smaller than that of Mn, it does not necessarily mean that the degree of internal oxidation is small. The inventors conducted experiments under various conditions and found that, when the above conditions are met, hot-dip galvanized steel sheets with no uncoated areas and good coating adhesion were obtained. However, if the difference in converted concentration between the maximum and minimum points of Mn is less than 80% or if the difference in converted concentration between the maximum and minimum points of Si is less than 50%, spot or line-shaped uncoated areas or coating peeling may occur. In other words, by doing so, it is possible to prevent the formation of Mn and Si oxides on the surface, thereby producing an ultra-high-strength hot-dip galvanized steel sheet with a beautiful surface and excellent coating adhesion, and to suppress the occurrence of defects such as line defects on the surface even after subsequent alloying heat treatment. Since a larger difference in the converted concentration values ​​is more advantageous, there is no need to set an upper limit. However, considering the contents of the contained elements, the difference in the converted concentration values ​​may be set to 400% or less for Mn and 250% or less for Si. In another embodiment of the present invention, the difference in the converted concentration of Mn may be set to 90% or more or 100% or more, and the difference in the converted concentration of Si may be set to 60% or more or 70% or more.

[0034] The GDS analysis method carried out in the present invention will be described in detail below.

[0035] To analyze GDS concentration, hot-dip galvanized steel sheets are sheared into lengths of 30-50 mm and immersed in a 5-10 wt% hydrochloric acid solution at room temperature (20-25°C) to remove the zinc coating. To prevent surface damage to the base steel during the dissolution of the zinc coating, the acid solution is removed within 10 seconds after the generation of bubbles caused by the reaction between the zinc coating and the acid solution stops, and the base steel is washed with pure water and dried. Of course, steel sheets that have not yet been hot-dip galvanized can be analyzed without this coating removal process.

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

[0037] The steel sheet for plating that is the subject of the present invention can include a base iron and an Fe plating layer formed on the base iron. The composition of the base iron is not particularly limited.

[0038] However, the present invention can advantageously improve galvanizability for high-strength steel sheets containing 1.0-8.0 wt% Mn and 0.3-3.0 wt% Si, which are prone to oxide formation on the surface. The upper limit of the Mn concentration in the base steel is not particularly limited, but considering commonly used compositions, the upper limit can be set to 8 wt%. Furthermore, the lower limit of the Mn concentration is not particularly limited, but compositions containing less than 1.0 wt% Mn provide excellent surface quality for hot-dip galvanized steel sheets without the need for Fe electroplating, making Fe electroplating unnecessary. The upper limit of the Si concentration is not particularly limited, but considering commonly used compositions, the upper limit can be set to 3.0 wt% or less. With a Si concentration of less than 0.3 wt%, the hot-dip galvanized steel sheet provides excellent quality even without simultaneous Fe electroplating and internal oxidation annealing, making the method of the present invention unnecessary.

[0039] The above-mentioned Mn and Si are elements that affect the platability, and therefore their concentrations can be limited as described above, but the present invention does not particularly limit the remaining components of the base iron.

[0040] However, in the case of high-strength steel sheets containing large amounts of alloying elements, the uncoated area and the deterioration of coating adhesion can be significantly reduced. In view of this, in one embodiment of the present invention, the composition of the base steel can be, in weight percent, 1.0-8.0% Mn, 0.3-3.0% Si, 0.05-0.3% C, 0.005-3.0% Al, 0.04% or less (excluding 0%) P, 0.015% or less S, 1.5% or less (including 0%) Cr, 0.005% or less B, and the balance being Fe and unavoidable impurities. Here, "high strength" refers to not only high strength after annealing, but also to high strength that can be achieved by subsequent heat treatments and other processes. In the present invention, "high strength" can refer to a tensile strength of 490 MPa or more, but is not limited thereto. In addition to the above-mentioned components, the base iron may further contain elements such as Ti, Mo, and Nb in a total amount of 1.0% or less. There are no particular limitations on the base iron, but 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.

[0041] In one aspect of the present invention, a hot-dip galvanized steel sheet including the steel sheet for plating may be provided, and the hot-dip galvanized steel sheet may include a steel sheet for plating and a hot-dip galvanized layer formed on the surface of the steel sheet for plating. In this regard, any commonly used hot-dip galvanized steel sheet may be used, and the type thereof is not particularly limited.

[0042] Next, one exemplary embodiment of a method for manufacturing a steel sheet for plating and a hot-dip galvanized steel sheet having the above-mentioned advantageous effects will be described. According to one embodiment of the present invention, the steel sheet can be manufactured by a process including the steps of preparing a base iron, electroplating the base iron to form an Fe plating layer containing 5 to 50 wt % of oxygen, and annealing the base iron with the Fe plating layer formed thereon to obtain a steel sheet.

[0043] Figure 2 shows the cross section of a 1.2 GPa cold-rolled steel sheet containing 2.6% Mn, 1.0% Si, and other alloying elements, annealed at 800°C for 53 seconds in an N2-5% H2 atmosphere with a dew point of +5°C, followed by cooling, as observed under a transmission electron microscope. The atmosphere was maintained the same throughout the heating period, and the dew point temperature was kept at -40°C during cooling to prevent oxidation of the Fe. Figure 2 (a) shows the cross section of a steel sheet annealed without Fe plating, and (b) shows the cross section of a steel sheet annealed without Fe plating, with an Fe coating weight of 1.99 g / m2 on the cold-rolled steel sheet (base steel). 2 The cross section of the steel sheet was annealed after electroplating so that the oxygen content of the Fe-plated layer was 6.3 wt%.

[0044] As shown in Figure 2(a), fine Mn and Si oxides are observed on the surface of steel sheets annealed at a dew point of +5°C without Fe plating, and thick grain boundary oxides are formed within the base steel. This is because grain boundary oxides begin to form when the cold-rolled structure recovers and recrystallizes into fine grains during the heating process. As the annealing temperature and annealing time increase, the grains become coarser, and oxygen inflows into the base steel, primarily forming grain boundary oxides. This morphology ultimately results in gradual changes in the concentrations of Mn and Si elements in the GDS profile, which do not clearly show maximum and minimum points, or even if they do, the difference in converted concentrations does not meet the range specified in the present invention.

[0045] As shown in Figure 2(b), an Fe plating layer containing 5 to 50 wt% oxygen was applied with an iron coating weight of 1.99 g / m 2 When a steel sheet is annealed after being plated so that the Fe coating is uniform, almost no oxides are formed in the Fe coating layer. Instead, particulate oxides are formed at the interface between the Fe coating layer and the base steel and within the base steel. These oxides act as nuclei for the internal oxide, causing linear oxides to grow perpendicular to the steel sheet surface. However, the depth of the internal oxide formation is greater without the Fe coating than when the Fe coating is applied. In such cases, the Fe coating layer (surface layer) contains small amounts of Mn and Si, and not only does it show a maximum at the interface, but it can also have a depleted zone where the Mn and Si content is significantly reduced in a region deeper than the maximum.

[0046] On the other hand, when annealing in a high dew point atmosphere without Fe electroplating, oxides form at the grain boundaries of the finely recrystallized structure on the surface of the base steel, inhibiting crystal growth and resulting in irregular, fine grains surrounded by fine oxides. However, when a high-oxygen coating is formed and then annealed at a high dew point between -15°C and +30°C, the Fe coating does not contain oxidizable alloying elements such as Mn and Si, so oxides do not form at the grain boundaries of the coating, but do form at the interface between the Fe coating and the base steel. This results in a uniformly thick Fe coating structure and distinct grains within the base steel. However, because the boundary between the Fe coating and the base steel may not be clearly visible depending on the dew point in the annealing furnace, the elongation rate of the base steel, and the steel composition, controlling the dew point in the annealing furnace between -15°C and +30°C after Fe electroplating does not necessarily result in the characteristics shown in Figure 2(b).

[0047] Unlike the oxidation-reduction method, the internal oxidation annealing method does not form a layered oxide layer, making it excellent for improving the coating adhesion of ultra-high strength steel sheets containing large amounts of alloying elements such as Mn and Si during hot-dip galvanizing. However, the water vapor in the annealing furnace inevitably oxidizes the surface of the steel sheet first, and then oxygen penetrates into the interior, making it impossible to fundamentally remove the surface oxide.

[0048] To solve the above problems, the inventors conducted extensive experiments and discovered that when an Fe coating layer containing a large amount of oxygen is formed and then annealed in a high dew point atmosphere, oxygen from the steam in the annealing furnace can prevent the formation of surface oxides of alloying elements such as Mn and Si on the surface of the Fe coating layer, and the oxygen contained in the Fe coating layer can internally oxidize alloying elements such as Mn and Si in the base steel, thereby effectively preventing surface diffusion. The high dew point in the annealing furnace allows oxygen to flow into the steel, further internally oxidizing the alloying elements, thereby virtually eliminating the formation of surface oxides of alloying elements on the steel surface. This dramatically improves the surface quality and coating adhesion of galvannealed steel sheets and accelerates the alloying reaction during the production of galvannealed steel sheets, resulting in uniform galvannealed steel sheets without surface defects.

[0049] More specifically, an Fe coating layer containing 5 to 50% by weight of oxygen is formed on a cold-rolled steel sheet (base steel), and the temperature is raised to 600 to 950°C in an annealing furnace with a dew point controlled to -15°C to +30°C so that the mechanical properties of the steel sheet are ensured. After that, the steel sheet is cooled again and hot-dip coated, which suppresses uncoated areas and produces a hot-dip coated steel sheet with excellent coating adhesion.

[0050] In one embodiment of the present invention, the Fe plating layer may be formed through a continuous plating process, and the Fe plating amount at this time is 0.5 to 3.0 g / m based on the Fe coating amount. 2 The Fe plating amount can be adjusted to 0.5 g / m 2 If the thickness is less than 3.0 g / m, the effect of the Fe plating layer in suppressing the diffusion of alloying elements may be insufficient in the normal continuous annealing process. 2 Even if the Fe plating amount exceeds 1.0 to 2.0 g / m, the effect of suppressing alloying elements is further increased, but multiple plating cells must be operated to ensure a high plating weight, and when an insoluble anode is used, the electroplating solution becomes rapidly acidic, which reduces plating efficiency and generates sludge, making it uneconomical. In another embodiment of the present invention, the Fe plating amount is 1.0 to 2.0 g / m 2 When internal oxidation is carried out after the formation of the Fe plating layer, internal oxides are formed at the interface between the Fe plating layer and the base iron or just below the interface, so that the maximum points of Mn and Si concentrations are present in the 0.05 to 1.0 μm region. 2 The amount of Fe plating corresponds to a thickness of 0.05 to 0.4 μm after annealing.

[0051] Furthermore, the Fe coating layer having the above-described high oxygen concentration forms maximum and minimum points in the GDS concentration profile of Mn and Si elements within the steel sheet by controlling the temperature, dew point temperature, and atmosphere of the subsequent annealing process, and the converted concentrations at the maximum and minimum points satisfy the numerical ranges specified in one embodiment of the present invention. Taking this into consideration, in one embodiment of the present invention, the oxygen concentration in the Fe coating layer may be 5 to 50 wt %, and in another embodiment, it may be 10 to 40 wt %. To achieve the surface oxide suppression effect, the amount of oxygen in the Fe coating layer must be sufficiently large. Even if the oxygen concentration in the Fe coating layer is less than 5 wt %, the surface oxide suppression effect can be achieved by increasing the amount of Fe coating. However, to achieve this effect, a minimum of 3.0 g / m 2 is required. 2 However, since plating must be performed with an oxygen content exceeding 5 wt %, various problems described above may occur. Furthermore, if the oxygen content does not reach 5 wt %, 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 wt % or more. Meanwhile, the effect of suppressing surface oxide formation during annealing can be further improved as the oxygen concentration in the Fe plating layer increases. However, since it is difficult to obtain a plating layer with an oxygen content exceeding 50 wt % using conventional electroplating methods, the upper limit can be limited to 50 wt %. In another embodiment of the present invention, the oxygen concentration in the Fe plating layer can be limited to 10 to 40%.

[0052] In one embodiment of the present invention, the annealing temperature may be 600°C to 950°C based on the steel sheet temperature in the soaking zone. If the annealing temperature is too low, the structure of the cold-rolled steel sheet will not be properly restored or recrystallized, making it difficult to ensure mechanical properties such as strength and elongation of the steel sheet. If the annealing temperature exceeds 950°C, alloy elements in the steel will rapidly diffuse to the surface, resulting in poor quality of hot-dip galvanization and operation at an unnecessarily high temperature, which is uneconomical.

[0053] Meanwhile, in one embodiment of the present invention, the dew point inside the annealing furnace may be between -15°C and +30°C. If the dew point is below -15°C, the amount of oxygen flowing into the steel decreases, promoting only surface oxidation and preventing internal oxidation, resulting in the presence of a large amount of oxide on the surface, resulting in poor quality hot-dip galvanizing. Furthermore, if the dew point exceeds +30°C, internal oxidation increases, further enhancing the effect of suppressing surface oxidation by inhibiting the diffusion of alloying elements. However, the amount of steam supply increases sharply, requiring an unnecessarily large humidification equipment capacity. Furthermore, condensation of cooled steam may cause equipment problems when used for long-term continuous annealing. The dew point may be controlled within the above-mentioned range of 600°C to 950°C, and may be controlled under more lenient conditions at lower temperatures. In another embodiment of the present invention, the dew point may be limited to between -10°C and +20°C.

[0054] Additionally, to prevent oxidation of the base steel and the Fe-plated layer during annealing, the hydrogen concentration in the atmospheric gas during annealing can be set to 1% or more by volume. If the hydrogen concentration is less than 1%, the trace amounts of oxygen inevitably contained in H2 and N2 gases cannot be effectively removed by oxidation, increasing the oxygen partial pressure and potentially causing surface oxidation of the base steel. On the other hand, if the hydrogen concentration exceeds 70%, there is a risk of explosion in the event of gas leakage and the costs of high-hydrogen work increase, so the hydrogen concentration should be set to 70% or less. Apart from the hydrogen (H2) mentioned above, the gas can essentially consist of nitrogen (N2), excluding the unavoidably contained impurity gases.

[0055] According to one embodiment of the present invention, the maintenance time after reaching the target temperature during annealing can be limited to 5 to 120 seconds. During annealing, the target annealing temperature needs to be maintained for 5 seconds or more to ensure sufficient heat transfer to the inside of the base steel and obtain uniform mechanical properties in the thickness direction. On the other hand, if the high-temperature annealing maintenance time is too long, the diffusion of alloying elements through the Fe coating layer increases, increasing the amount of surface oxide produced, resulting in poor quality hot-dip galvanizing. Therefore, the maintenance time can be limited to 120 seconds or less.

[0056] Hereinafter, 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 having an Fe-plated layer containing a large amount of oxygen formed thereon based on the above-mentioned content will be described in more detail with reference to FIG. 3.

[0057] Figure 3 shows a schematic diagram of the phenomenon that occurs inside a steel sheet when its temperature is increased under the conditions of the present invention. Figure 3(a) shows a schematic cross-sectional view of a base steel sheet on which an Fe-plated layer containing a large amount of oxygen has been formed. The base steel contains alloying elements such as Mn and Si, and the Fe-plated layer contains 5 to 50 wt. % oxygen and impurities that are inevitably mixed in during electroplating, with the remainder being Fe.

[0058] Figure 3(b) shows the state of an Fe-plated cold-rolled steel sheet heated to approximately 300–500°C in a nitrogen atmosphere containing 1–70% H2. The surface of the Fe-plated layer, which contains a large amount of oxygen, is gradually reduced and the oxygen is removed. However, at the interface between the Fe-plated layer and the base steel, Mn and Si diffused from the base steel combine with the oxygen in the Fe-plated layer to form an internal oxide, inhibiting diffusion to the surface. Furthermore, as the temperature increases, the Mn and Si diffused inside the base steel accumulates, gradually growing the internal oxide at the interface. Even if the dew point in the annealing furnace varies widely from -90°C to +30°C in the low-temperature region of the heating stage, the presence of a large amount of oxygen in the Fe-plated layer due to the low temperature allows the rate at which oxygen is released by reduction of the Fe-plated layer to exceed the rate at which oxygen dissociated from water vapor diffuses into the steel. Therefore, dew point control is not a major factor in this stage.

[0059] However, the amount of oxygen inside the Fe coating layer plays an important role. When numerous fine internal oxides form at the interface between the Fe coating layer and the base steel and within the base steel at low temperatures, they act as oxide nuclei for the alloying elements inside the base steel to undergo sustained internal oxidation. For these oxide nuclei to form, the concentrations of both oxygen and alloying elements must be high. If the Fe coating layer contains a sufficient amount of oxygen, a large number of oxide nuclei will form near the interface between the high-oxygen Fe coating layer and the high-alloying-element-concentration base steel. However, if the Fe coating layer contains almost no oxygen, the alloying elements in the base steel will pass through the Fe coating layer and form oxides on the surface. Raising the temperature further depletes the oxygen in the Fe coating layer, further accelerating the diffusion of alloying elements within the base steel, resulting in increased surface oxide formation.

[0060] Figure 3(c) shows a schematic cross-sectional view of the base steel when heated to 500-700°C in the same reducing atmosphere. During the heating process, it is best to control the dew point inside the annealing furnace between -15°C and +30°C. As the temperature increases, the Fe coating layer is fully reduced, lowering the oxygen concentration and slowing the oxygen release rate. At the same time, the rate at which water vapor in the annealing furnace dissociates and diffuses into the steel increases significantly. Therefore, raising the dew point from the 500-700°C range, below the temperature at which the Fe coating layer is completely reduced, effectively prevents Mn and Si from inside the steel from diffusing through the Fe coating to the surface.

[0061] Figure 3(d) shows a schematic cross-section of a steel sheet after being maintained at high temperatures between 600 and 950°C while adjusting the dew point between -15°C and +30°C. Mn and Si continuously diffuse into the steel substrate, while oxygen supplied from water vapor rapidly penetrates and supplies the steel surface, causing Mn and Si to oxidize internally. The particulate Mn and Si oxides formed at the interface between the Fe coating and the steel substrate in the low-temperature range by reaction with oxygen from the Fe coating act as nuclei for oxide growth, causing internal oxide growth to concentrate at the interface between the Fe coating and the steel substrate. Furthermore, because oxygen diffuses faster than Mn and Si, which have larger atomic sizes, internal oxides form deep within the steel substrate, not only at the grain boundaries but also within the grains.

[0062] Although the control conditions for each temperature have been described above, the most crucial step in the annealing process is maintaining the steel sheet temperature at 600 to 950°C, and simply controlling the dew point of the atmosphere in this temperature range can effectively control the oxide distribution inside the steel sheet. Of course, such dew point control can be performed in all steps prior to the above-mentioned maintenance step without any particular problems. Furthermore, it should be noted that the above-described process is merely an explanation and example of one embodiment of the present invention, and the reaction mechanism of the present invention should not be interpreted as being bound by the above explanation.

[0063] After the annealing step, the annealed steel sheet may be cooled. The cooling conditions in the cooling step after the annealing step do not have a significant effect on the surface quality of the final product, i.e., the coating quality, and therefore do not need to be particularly limited in the present invention. However, in order to prevent oxidation of the iron component during the cooling process, a reducing atmosphere may be applied to at least the iron.

[0064] According to an embodiment of the present invention, the steel sheet obtained by the above-described process may be hot-dip galvanized to form a hot-dip galvanized layer. The hot-dip galvanizing method in the present invention is not particularly limited.

[0065] In the present invention, any base iron having the above-described alloy composition can be applied without limitation as the base iron for the steel sheet or hot-dip galvanized steel sheet according to the present invention, and therefore there are no specific limitations on the method for producing the base iron.

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

[0067] That is, in order to form an Fe plating layer in the present invention, an electroplating solution can be used that contains iron ions including ferrous ions and ferric ions; a complexing agent; and inevitable impurities, in which the concentration of ferric ions in the iron ions is 5 to 60 wt %.

[0068] According to one embodiment of the present invention, an electroplating solution contains ferrous ions and ferric ions. To achieve high plating efficiency, it is advantageous to include only ferrous ions. However, if only ferrous ions are included, the solution may be altered, resulting in a rapid decline in plating efficiency and quality deviations during continuous electroplating processes. Therefore, ferric ions may be further included. The concentration of ferric ions is preferably 5 to 60 wt. % of the total weight of ferrous and ferric ions, and more preferably 5 to 40 wt. If the concentration is less than 5%, the rate at which ferric ions are reduced to ferrous ions at the cathode is slower than the rate at which ferrous ions are oxidized to ferric ions at the anode, resulting in a rapid increase in ferric ion concentration, a rapid decrease in pH, and a sustained decrease in plating efficiency. On the other hand, if the concentration of ferric ions exceeds 60%, the rate at which ferric ions are reduced to ferrous ions at the cathode is greater than the rate at which ferrous ions are reduced and deposited as metallic iron, resulting in a significant decrease in plating efficiency and a deterioration in plating quality. Therefore, it is preferable to set the concentration of ferric ions in the iron ions to 5 to 60 wt %, taking into consideration equipment and process characteristics such as plating amount, working current density, solution replenishment rate, amount of solution lost due to adhesion to the strip, and rate of concentration change due to evaporation.

[0069] The concentration of the iron ions is preferably 1 to 80 g per 1 L of the electroplating solution, and more preferably 10 to 50 g per 1 L. If the concentration is less than 1 g / L, there is a problem of a rapid decline in plating efficiency and plating quality, whereas if the concentration is more than 80 g / L, the solubility may be exceeded, which may cause precipitation, and the loss of raw materials due to solution runoff during continuous plating processes is increased, which is uneconomical.

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

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

[0072] When amino acids are dissolved in neutral water, the amines combine with hydrogen ions to acquire a positive charge, while the carboxyl groups acquire a negative charge due to the dissociation of hydrogen ions, so the amino acid molecules maintain a neutral charge. On the other hand, when the solution is acidified, the carboxyl groups recombine with hydrogen ions to become charge-neutral, and the amines acquire a positive charge, so the amino acid molecules form cations. In other words, amino acids become charge-neutral or form cations in a weakly acidic aqueous solution.

[0073] When an amino acid is added to an acidic electrolyte containing iron ions, it complexes with ferrous and ferric ions, but the iron ions complexed with the amino acid remain in a cationic state even when complexed. Therefore, it exhibits electrical properties opposite to those of a typical complexing agent with multiple carboxyl groups, which becomes negatively charged in a weakly acidic aqueous solution.

[0074] Furthermore, compared to complexing agents containing multiple carboxyl groups such as citric acid and EDTA, amino acids form fewer bonds with iron ions, resulting in weaker bonds. However, their bond with ferric ions, which cause sludge, is strong enough to prevent precipitation by ferric ions. Furthermore, because ferric ions remain cationic even after complexation, they are easily transported to the cathode, where they are reduced to ferrous ions and can participate in the plating reaction. However, their migration to the anode is inhibited, slowing the rate of ferric ion generation. This allows the ferric ion concentration to remain constant even during long-term continuous plating, maintaining consistent plating efficiency and eliminating the need for electrolyte replacement.

[0075] Meanwhile, in the continuous electroplating process, as iron ions in the solution are consumed by plating, the solution becomes acidic. However, even if the same amount of iron ions is deposited, the pH change is smaller in a solution that also contains ferric ions than in a solution that contains only ferrous ions. As the pH increases, some ferric ions combine with hydroxide ions, and as the pH decreases, the hydroxide ions separate and are neutralized. Therefore, a solution containing ferric ions acts as a pH buffer, slowing down pH change even without a separate pH buffer, allowing electroplating efficiency to be maintained consistently in the continuous electroplating process.

[0076] Therefore, by using amino acids as complexing agents, sludge formation can be prevented, and not only ferrous ions but also ferric ions can be used as plating raw materials. By using a mixture of ferrous ions and ferric ions, 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 continuous electroplating processes.

[0077] The complexing agent is preferably added in an amount such that the molar concentration ratio of iron ions to complexing agent is 1:0.05-2.0, more preferably 1:0.5-1.0. If the molar ratio is less than 0.05, the excess ferric ions cannot be prevented from combining with hydroxide ions or oxygen to form sludge, resulting in a significant decrease in plating efficiency even without ferric ions, and furthermore, burning can occur, deteriorating plating quality. On the other hand, if the molar ratio exceeds 2.0, the sludge-inhibiting effect and plating quality are maintained, but the overvoltage increases, reducing plating efficiency. Furthermore, the use of an unnecessary excess of amino acids, which are relatively expensive compared to iron ion-containing materials such as ferrous sulfate, increases raw material costs, making this uneconomical.

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

[0079] The amino acid was used as a complexing agent, and the solution temperature was maintained at 80°C or less, pH 2.0 to 5.0, and the current density was 3 to 120 A / dm 2 When electroplating is performed at this temperature, the plating efficiency is high and an Fe plating layer with a high oxygen concentration can be obtained.

[0080] The temperature of the Fe electroplating solution does not have a significant effect on the quality of the Fe plating layer, but if it exceeds 80℃, the solution will evaporate rapidly and the concentration of the solution will continue to change, making it difficult to achieve uniform electroplating.

[0081] If the pH of the Fe electroplating solution is less than 2.0, the electroplating efficiency will decrease and it is not suitable for continuous plating process. If the pH exceeds 5.0, the plating efficiency will increase, but iron hydroxide will precipitate during continuous electroplating, generating sludge, which will cause problems of clogging pipes and polluting rolls and equipment.

[0082] The current density is 3A / dm 2If the temperature is less than 120A / dm, the cathode plating overvoltage will drop and the efficiency of Fe electroplating will decrease, making it unsuitable for continuous plating processes. 2 If the temperature exceeds this, burning occurs on the plating surface, the electroplating layer becomes uneven, and the Fe plating layer easily falls off.

[0083] As described above, in the present invention, it is preferable that the Fe plating layer contains 5 to 50 wt % of oxygen. The reason why oxygen gets mixed into the Fe plating layer is as follows. During the process of iron deposition on the steel sheet surface to which a cathodic voltage is applied, hydrogen ions are simultaneously reduced to hydrogen gas, and the pH increases. As a result, both ferrous and ferric ions are temporarily converted to OH. - When an anionic complexing agent such as acetic acid, lactic acid, citric acid, or EDTA is used, the complexing agent may become OH. - The iron ions bonded to the ions are negatively charged on average, and when a cathode is applied for electroplating, an electrical repulsion force is generated, preventing contamination of the Fe plating layer. On the other hand, amino acids are electrically neutral at pH 2.0 to 5.0, and in strong acids below pH 2.0, they become positively charged, but the iron ions bonded to the amino acids have one or two OH groups. - Even when the iron ions are bonded, they become cations, which generates an electric attraction between the iron ions and the cathode where electroplating is performed, causing a large amount of oxygen to be mixed in. Therefore, if an amino acid is 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 performed while maintaining a pH of 2.0 to 5.0, an Fe plating layer containing 5 to 50 wt.% oxygen can be obtained with high plating efficiency and suppressed sludge generation.

[0084] To ensure the quality of hot-dip galvanizing of steel sheets containing Mn and Si, the coating weight of the Fe coating layer should be 0.5 to 3.0 g / m2 based on the amount of iron. 2 The upper limit of the Fe plating amount is not particularly limited, but it is preferably 3.0 g / m in a continuous plating process. 2If the amount exceeds 0.5 g / m, multiple plating cells will be required or the production speed will decrease, which is not economical. Furthermore, if the amount of Fe electroplating is large, the Fe electroplating solution will rapidly denature in the continuous process, causing the pH to drop, significantly reducing plating efficiency and making solution management difficult. On the other hand, if the amount of Fe electroplating is 0.5 g / m, 2 If the Fe content is less than this, the oxygen contained in the Fe coating layer is rapidly reduced and removed, making it impossible to effectively prevent Mn and Si from diffusing from the base steel and forming surface oxides, resulting in a problem of reduced hot-dip coating quality. With the above Fe coating amount, if the Fe coating layer is completely reduced during annealing at the iron concentration contained in the coating layer, it will have a thickness of about 0.05 to 0.4 μm. [Example]

[0085] The present invention will be described in more detail with reference to the following examples. However, it should be noted that the following examples are intended to illustrate and embody the present invention, and are not intended to limit the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0086] (Example) First, one type of base steel was prepared with the composition shown in Table 1 below. The base steel was a cold-rolled steel sheet with no special plating layer formed on the surface.

[0087] [Table 1]

[0088] Before Fe electroplating on steel sheets, the copper sheets were electroplated and then dissolved in a 5-10 wt% hydrochloric acid solution to measure the total Fe content and plating efficiency. Based on the measured plating efficiency, cold-rolled steel sheets were electroplated with Fe, allowing for consistent Fe electroplating coverage even when the plating solution and plating conditions were changed. Fe electroplating was further performed on copper sheets using each solution and plating condition. The total Fe and O content was determined by GDS analysis, and the average oxygen concentration of the Fe coating layer was measured for each electroplating condition. The coating weight was then measured separately by dissolving the copper sheets in hydrochloric acid, as shown in Table 2. The temperature of all plating solutions during plating was maintained at 50°C. Separately, Fe coatings were formed on cold-rolled steel sheets with the compositions listed in Table 1 using the same solutions and plating conditions as those used to electroplated copper. Then, the sheets were annealed and hot-dip galvanized under the following conditions.

[0089] The annealing furnace maintained a reducing atmosphere of N2 gas containing 5% H2 throughout all sections, with the dew point maintained between -18°C and +5°C throughout the heating and soaking sections as shown in Table 2. From the cooling section onward, a reducing atmosphere of -40°C was maintained for iron. The cold-rolled steel sheet electroplated with Fe according to the procedure described above (i.e., conditions in Table 2, bath temperature: 50°C) was then loaded and heated to 850°C at a heating rate of approximately 2.5°C / sec and maintained at that temperature for 53 seconds. It was then gradually cooled to 650°C at a rate of 2.8°C / sec and then rapidly cooled again to 400°C at a rate of 14.5°C. Steam injection was discontinued so that the dew point inside the annealing furnace reached -40°C upon the start of cooling. After cooling was completed, the temperature was raised again to 480°C to allow for hot-dip galvanizing, and the steel sheet was then drawn into the hot-dip galvanizing bath. The hot-dip galvanizing bath contained 0.20 to 0.25% Al, and the temperature was maintained at 460°C. After plating, the bath was gradually cooled to room temperature to produce a hot-dip galvanized steel sheet.

[0090] The manufactured hot-dip galvanized steel sheets were evaluated for galvanizability, and the GDS concentration profile was measured for the base steel in which the coating layer was dissolved in an approximately 8% hydrochloric acid solution. The average concentrations of Mn and Si were measured at the maximum and minimum points and at 5 μm inside the base steel, and the results are shown in Table 3.

[0091] The galvanizability of hot-dip galvanized steel sheets was evaluated visually. Complete absence of uncoated areas over the entire surface was rated "good," while fine dotted uncoated areas of 1 mm or less were classified as "spot uncoated," linear uncoated areas or areas where multiple uncoated dots occurred simultaneously in a line or cluster were labeled "linear defects," and areas with a large diameter of 5 mm or more were classified as "uncoated." The order of coating defects tends to be "uncoated," followed by "linear defects," "spot uncoated areas," and then "good," with the proportion of uncoated areas increasing.

[0092] To evaluate coating adhesion, an automotive structural sealant was applied to hot-dip galvanized steel sheets to a thickness of approximately 5 mm and cured at a temperature of 150-170°C. After cooling to room temperature, the hot-dip galvanized steel sheets were bent at 90 degrees to remove the sealant. If the coating adhered to the sealant and peeled off at the entire interface between the zinc coating and the base steel, the coating adhesion was deemed poor and recorded as "peeling." If no coating peeling occurred, the coating adhesion was deemed "good." In some test specimens, only part of the coating layer peeled off; in this case, it was recorded as "partial peeling." However, for test specimens where "unplated" occurred, the coating adhesion was not evaluated.

[0093] The concentration profile of the base steel, from which the coating layer had been removed with hydrochloric acid, was determined according to the GDS analysis method described above. After applying a 100-nm Gaussian filter to remove noise, the maximum and minimum points were calculated. For some GDS profiles, the maximum or minimum points could not be calculated, and in such cases, "ND" was used. However, even if a minimum point was not displayed, when calculating the difference in the converted concentration, it was considered to be the same as the Mn and Si concentrations within the base material described below and was included in the calculation. However, if a maximum point was not formed, the converted concentration could not be calculated, and the result was considered to be outside the scope of the present invention.

[0094] The concentrations of Mn and Si inside the base material were measured at a point 5 μm deep from the steel sheet surface (interface between the zinc plating layer and the steel sheet).

[0095] [Table 2]

[0096] [Table 3]

[0097] In Comparative Examples 1 and 2, base steel without an Fe-coated layer was annealed under the same conditions and hot-dip galvanized as described above. In Comparative Example 1, when the dew point in the annealing furnace was maintained at −15°C without Fe electroplating, hot-dip galvanization was not achieved, and only surface oxidation occurred. As a result, the GDS profile showed high Mn and Si concentrations on the surface and a gradual decrease in concentration toward the interior of the base steel, making it impossible to calculate the maximum and minimum points. In Comparative Example 2, the dew point in the annealing furnace was maintained at +5°C, resulting in fine dots of uncoated area on the hot-dip galvanized surface, and partial peeling during evaluation of coating adhesion. The GDS concentration profile for the base steel of Comparative Example 2 is shown in Graph (a) of Figure 4. The solid line in the figure represents the Mn concentration profile, and the dotted line represents the Si concentration profile (the same applies below). A large amount of internal oxide was generated within 0.5 μm of the interior of the base steel, which had a maximum point, but a Mn-depleted layer occurred 2 μm deep from the surface. This is because oxygen during annealing continuously flows into the steel, forming internal oxidation at grain boundaries deep inside the base iron, as can be seen in Figure 2(a). Comparative Example 2 showed insufficient results, with the differences in converted concentrations of Mn and Si at the maximum and minimum points being 63.9% and 30.8%, respectively. Comparative Examples 3 to 12 showed results where iron deposition weights of 0.42 to 2.99 g / m were obtained using an Fe electroplating solution containing citric acid as a complexing agent. 2The electroplated base iron was annealed at dew points of -15°C and +5°C in an annealing furnace, and then hot-dip galvanized. When iron was electroplated using an Fe electroplating solution containing citric acid as a complexing agent, the oxygen concentration in the Fe-plated layer was low, less than 5 wt%. Despite adjusting the dew point of the annealing furnace to -15°C and +5°C, the level of unplated areas tended to gradually improve as the amount of Fe electroplated increased, but fine plating occurred on most of the surface, resulting in poor plating adhesion. In Comparative Example 11, the Fe electroplating coverage was 1.99 g / m 2 Figure 4(b) shows the GDS profile of the hot-dip galvanized base steel that was annealed with the dew point in the annealing furnace adjusted to +5°C. The Mn concentration is concentrated at a depth of approximately 0.2 μm, which corresponds to the area directly below the Fe coating layer, but no Si was produced at its maximum due to internal oxidation. The differences in converted concentrations between the maximum and minimum points of Mn and Si were 69% and 45%, respectively.

[0098] In Comparative Examples 13 to 17, cold-rolled steel sheets were electroplated with Fe using an Fe electroplating solution containing glycine, an amino acid, as a complexing agent, and then annealed while adjusting the dew point in the annealing furnace to −18°C, followed by hot-dip galvanization. The Fe electroplating amount was 1.18 g / m 2 In the following cases, fine linear defects were observed and the plating adhesion was poor. On the other hand, when the Fe electroplating amount was 1.99 to 2.99 g / m 2 In Comparative Example 16, the coating appearance was good, but the adhesion was poor. Even if the oxygen content in the Fe coating layer was high and the surface diffusion of Mn and Si was effectively suppressed during the temperature rise process in the annealing furnace, the oxygen partial pressure in the annealing furnace was insufficient to penetrate into the steel, and the level was such that surface oxidation could be aggravated. Therefore, it is considered that the quality of the hot-dip galvanizing was even worse than when the dew point was very low or sufficiently high. In Comparative Example 16, the Fe electroplating coating weight was 1.99 g / m 2The GDS profile of a steel sheet annealed at a dew point of -18°C in an annealing furnace is shown in Figure 4(c). Although the oxygen in the Fe coating layer effectively formed internal oxidation at the interface between the Fe coating layer and the base steel, the oxidizing atmosphere on the surface resulted in the differences in converted concentrations of Mn and Si between the maximum and minimum points being 53.7% and 45.1%, respectively, which did not satisfy the conditions of the present invention, resulting in poor galvanizability.

[0099] In Comparative Example 18 and Invention Examples 1 to 4, Fe electroplating was performed under the same conditions as Comparative Examples 13 to 17, the dew point in the annealing furnace was raised to -15°C, annealing was performed, and then hot-dip galvanizing was performed. However, in Comparative Example 18, the amount of Fe electroplating was small, and as a result, the difference in converted concentration between the maximum and minimum points of Mn and Si was insufficient, and the galvanizing properties and coating adhesion were insufficient. In Comparative Example 19 and Invention Examples 5 to 8, Fe electroplating was performed under the same conditions, the dew point in the annealing furnace was further raised to +5°C, annealing was performed, and hot-dip galvanizing was performed. The amount of Fe electroplating was 0.40 g / m 2 When the Fe electroplating amount was low, the surface condition was improved compared to when only internal oxidation was performed, but poor plating adhesion occurred. 2 In the above cases, when internal oxidation was carried out at dew points of -15°C and +5°C, the effect of internal oxidation was amplified, resulting in a hot-dip galvanized steel sheet with a beautiful surface and excellent coating adhesion. 2 The GDS concentration profile for the base steel for hot-dip galvanizing, which was electroplated so that the oxygen concentration was equal to or greater than 0.01% and annealed while maintaining the dew point in the annealing furnace at +5°C, is shown in Figure 4(d). When internal oxidation is performed by forming an Fe-coated layer with a higher oxygen concentration than when only internal oxidation is performed, when the oxygen concentration in the Fe-coated layer is low, or when the dew point in the annealing furnace is low, internal oxidation is aggravated directly below the Fe-coated layer, and Mn and Si are almost unable to diffuse to the surface, resulting in a dramatic improvement in the quality of hot-dip galvanizing.

[0100] In Examples 9 to 16, the Fe electroplating solution containing glycine was used at 70 A / dm2 The steel was then annealed while maintaining the dew point in the annealing furnace at -15°C and +5°C, after which hot-dip galvanizing was performed. 2 The oxygen concentration in the Fe coating layer was significantly increased compared to when electroplating with 0.5 g / m 2 , and the surface of the hot-dip galvanized steel sheet was also beautiful and the coating adhesion was good. The GDS concentration profile of the base steel also showed good formation of internal oxidation of Mn and Si. 2 By performing electroplating above this level and then raising the dew point in the annealing furnace to above -15°C to perform internal oxidation, the effects of internal oxidation due to Fe electroplating and the effect of internal oxidation due to the increase in the dew point in the annealing furnace are combined, resulting in a dramatic improvement in the quality of hot-dip galvanized high-strength steel sheets.

[0101] As described above, it was confirmed that the inventive examples, which met all of the conditions of the present invention, exhibited excellent plating properties and plating adhesion, thereby confirming the advantageous effects of the present invention.

Claims

1. The GDS profiles of the Mn and Si components observed in the depth direction from the surface include maximum and minimum points in sequence. a difference (difference in converted Mn concentrations) between a value obtained by dividing the Mn concentration at a maximum point in the GDS profile of the Mn component by the Mn concentration in the base material and a value obtained by dividing the Mn concentration at a minimum point in the GDS profile of the Mn component by the Mn concentration in the base material is 80% or more; A steel sheet in which the difference (difference in converted Si concentrations) between the value obtained by dividing the Si concentration at a maximum point in the GDS profile of the Si component by the Si concentration in the base material and the value obtained by dividing the Si concentration at a minimum point in the GDS profile of the Si component by the Si concentration in the base material is 50% or more. However, if no minimum point appears within a depth of 5 μm, the point at a depth of 5 μm is regarded as the point where the minimum point appears.

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

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

4. 2. 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. 3. The steel sheet for plating according to claim 2, wherein the base iron contains, by weight, Mn: 1.0 to 8.0% and Si: 0.3 to 3.0%.

6. 6. The steel sheet for plating according to claim 5, wherein the base iron has a composition containing, in weight percent, Mn: 1.0 to 8.0%, Si: 0.3 to 3.0%, C: 0.05 to 0.3%, Al: 0.005 to 3.0%, P: 0.04% or less (excluding 0%), S: 0.015% or less (excluding 0%), Cr: 1.5% or less (including 0%), B: 0.005% or less (including 0%), the balance being Fe and unavoidable impurities.

7. A hot-dip galvanized steel sheet comprising the steel sheet for plating according to claim 1 , and a hot-dip galvanized layer formed on the steel sheet for plating.

8. Preparing the base steel; Electroplating the base iron using an electroplating solution containing iron ions including ferrous ions and ferric ions, a complexing agent, and inevitable impurities to form an Fe plating layer containing 5 to 50 wt % of oxygen; and The base steel on which the Fe plating layer was formed was heated in a 1 to 70% H atmosphere controlled at a dew point temperature of −15 to +30° C. ２ - remaining N ２ annealing the material in a gas atmosphere at 600 to 950°C for 5 to 120 seconds; The method for producing a steel sheet for plating, wherein the complexing agent is at least one selected from the group consisting of alanine, glycine, serine, threonine, arginine, glutamine, glutamic acid, and glycylglycine.

9. The coating weight of the Fe plating layer is 0.5 to 3 g / m ２ The method for producing a steel sheet for plating according to claim 8, wherein

10. 10. The method for producing a steel sheet for plating according to claim 8 or 9, wherein the electroplating solution contains ferrous ions and ferric ions, the ferric ions having a ratio of 5 to 60 wt % based on the total iron ions, and a total concentration of the iron ions is 1 to 80 g per 1 L of the electroplating solution.

11. The electroplating is carried out at a solution temperature of 80°C or less and a current density of 3 to 120 A / dm ２ The method for producing a steel sheet for plating according to claim 8 or 9, wherein the method is carried out under the following conditions:

12. Preparing the base steel; performing electroplating on the base iron using an electroplating solution containing iron ions including ferrous ions and ferric ions, a complexing agent, and inevitable impurities to form an Fe plating layer containing 5 to 50 wt % of oxygen; The base steel on which the Fe plating layer was formed was heated in a 1 to 70% H atmosphere controlled at a dew point temperature of −15 to +30° C. ２ - remaining N ２ annealing the steel sheet in a gas atmosphere at 600 to 950°C for 5 to 120 seconds to obtain a steel sheet for plating; and immersing the steel sheet for plating in a zinc plating bath; The method for producing a hot-dip galvanized steel sheet, wherein the complexing agent is at least one selected from the group consisting of alanine, glycine, serine, threonine, arginine, glutamine, glutamic acid, and glycylglycine.

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