Steel sheet with excellent plating quality and method for producing the same

Abstract

The present invention relates to a hot-dip galvanized steel sheet with excellent plating quality, a steel sheet for plating for producing the same, and a method for producing them. The steel sheet for plating according to one aspect of the present invention has a composition containing, by weight%, Mn: 0.1 to 1.0%, Si: 0.1% or less (excluding 0%), C: 0.0005 to 0.03%, Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, Cr: 1.5% or less, B: 0.005% or less, and the balance Fe and inevitable impurities. The GDS profile of the Mn component and the GDS profile of the Si component observed in the depth direction from the surface sequentially include a maximum point and a minimum point, respectively. The difference (converted Mn concentration difference) between the value obtained by dividing the Mn concentration at the maximum point of the GDS profile of the Mn component by the Mn concentration of the base material and the value obtained by dividing the Mn concentration at the minimum point of the GDS profile of the Mn component by the Mn concentration of the base material is 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 is 10% or more. It may be such. However, when no minimum point appears within a depth of 5 μm, the point at a depth of 5 μm is regarded as the point where the minimum point appears.

Description

Technical Field

[0001] The present invention relates to a hot-dip galvanized steel sheet having excellent plating quality, a steel sheet for plating for manufacturing the same, and methods for manufacturing them.

Background Art

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

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

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

[0005] When a method of post-oxidation reduction in the annealing process is used as in Patent Document 1, components with a high affinity for oxygen, such as Si, Mn, and Al, are internally oxidized at a certain depth from the surface layer of the steel sheet, suppressing diffusion to the surface layer. Therefore, in the surface layer, Si, Mn, or Al alone or composite oxides are relatively reduced, the wettability with zinc is improved, and unplated areas can be reduced. However, in the case of steel grades added with Si, Si is concentrated directly below the iron oxide during the reduction process to form banded Si oxides, resulting in peeling in the surface layer including the plating layer, that is, peeling occurs at the interface between the reduced iron and the base iron below, making it difficult to ensure the adhesion of the plating layer.

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

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

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

Prior Art Documents

Patent Documents

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

[0010] According to one aspect of the present invention, there is provided a steel sheet for an automobile outer panel and a method for manufacturing the same, which have excellent plating quality in which no unplated portion occurs and the plating layer does not peel off.

[0011] According to another aspect of the present invention, there is provided a hot-dip galvanized steel sheet for an automobile outer panel and a method for manufacturing the same, which can be manufactured into a hot-dip galvanized steel sheet with excellent surface quality having no 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-described content. Those having ordinary knowledge in the technical field to which the present invention pertains will have no problem in understanding further problems of the present invention from the overall matters of the specification of the present invention. Means for Solving the Problems

[0014] The plating steel sheet according to one aspect of the present invention has a composition containing, by weight%, Mn: 0.1 to 1.0%, Si: 0.1% or less (excluding 0%), C: 0.0005 to 0.03%, Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, Cr: 1.5% or less, B: 0.005% or less, and the balance Fe and inevitable impurities. The GDS profile of the Mn component and the GDS profile of the Si component observed in the depth direction from the surface each sequentially include a maximum point and a minimum point. The difference (converted Mn concentration difference) between the value obtained by dividing the Mn concentration at the maximum point of the GDS profile of the Mn component by the Mn concentration of the base material and the value obtained by dividing the Mn concentration at the minimum point of the GDS profile of the Mn component by the Mn concentration of the base material is 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 is 10% or more. It may be such that, when a minimum point does not appear within a depth of 5 μm, the point at a depth of 5 μm is taken as the point where the minimum point appears.

[0015] However, when a minimum point does not appear within a depth of 5 μm, the point at a depth of 5 μm is taken 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 plating steel sheet and a hot-dip galvanized layer formed on the plating steel sheet.

[0017] The method for manufacturing a plating steel sheet which is still another aspect of the present invention can include the steps of: preparing a base iron having a composition containing, by weight%, Mn: 0.1 to 1.0%, Si: 0.1% or less (excluding 0%), C: 0.0005 to 0.03%, Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, Cr: 1.5% or less, B: 0.005% or less, and the balance Fe and inevitable impurities; performing electroplating on the base iron to form an Fe plating layer containing 5 to 50% by weight of oxygen; and annealing the base iron on which the Fe plating layer is formed by maintaining it at 600 to 950°C for 5 to 120 seconds in an annealing furnace in an atmosphere of 1 to 70% H2 - the balance N2 gas controlled 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. The method comprises the steps of: preparing a base iron having a composition containing, by weight%, Mn: 0.1 to 1.0%, Si: 0.1% or less (excluding 0%), C: 0.0005 to 0.03%, Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, Cr: 1.5% or less, B: 0.005% or less, and the balance being Fe and unavoidable impurities; electroplating the base iron to form an Fe plating layer containing 5 to 50% by weight of oxygen; annealing the base iron with the Fe plating layer formed thereon at 600 to 950°C for 5 to 120 seconds in an annealing furnace with a 1 to 70% H2 - balance 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 hot-dip galvanizing bath.

Advantages of the Invention

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

[0020] Further, according to one aspect of the present invention, even when an alloying heat treatment is performed on the hot-dip galvanized steel sheet of the present invention, linear defects and the like can be prevented on the surface of the obtained alloyed hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized steel sheet with excellent surface quality can be provided.

Brief Description of the Drawings

[0021]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

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

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

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

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

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

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

[0028] In order to solve the problems of the above technology, the inventors of the present invention attempted to manufacture a hot-dip galvanized steel sheet with a beautiful surface and no problem of 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. Referring to the GDS profile of FIG. 1, the steel sheet for plating of the present invention will be described in detail.

[0030] FIG. 1 is a graph schematically showing a typical GDS profile of alloy components that can appear from the surface part after removing the zinc plating layer from a hot-dip galvanized steel sheet including the steel sheet of the present invention, and a GDS profile of alloy components when outside the scope of the present invention. In the graph, the vertical axis represents the concentration of alloying elements such as Mn and Si, and the horizontal axis represents the depth.

[0031] As can be seen from a typical example of the GDS profile of the Mn component of the present invention shown in FIG. 1, the steel sheet of the present invention can have a concentration gradient on its surface in which the concentration of Mn is extremely low 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). In some cases, the minimum point may appear first, but thereafter, the maximum point and the minimum point should appear sequentially. However, in some embodiments, the minimum point may not appear. In this case, the internal concentration in the depth region of 5 μm can be taken as the minimum point concentration. Further, the alloy element concentration on the surface has a value lower than the concentration of the maximum point, but in some cases, a minimum point with a low alloy element concentration may appear between the surface and the maximum point.

[0032] In the GDS concentration profile illustrated in FIG. 1 above, although not necessarily limited thereto, since not many alloy elements diffuse from the base iron in the surface layer portion, it corresponds to an Fe plating layer with a low concentration of alloy elements. The maximum point corresponds to a region where the internal oxide of the alloy element concentrated near the interface between the Fe plating layer and the base iron is formed. The minimum point that appears on the base iron side in the Fe plating layer corresponds to a region where the alloy element diffuses and is diluted in the Fe plating layer that does not contain the alloy element, or a region where the alloy element diffuses and is 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 depth of 5 μm can be taken as the point where the minimum point is formed. Since the concentration at a depth of 5 μm is substantially the same as the concentration of the base material, it can be treated as a point where the concentration no longer decreases.

[0034] At this time, the greater the difference between the converted concentration of the maximum point of the element in the Mn concentration profile and the Si concentration profile (the value obtained by dividing the concentration at that point by the concentration of the base material, expressed in %), and the converted concentration of the minimum point, the more Mn and Si diffusing to the surface can be reduced, which is important. In one implementation example of the present invention, the values of the maximum point converted concentration - minimum point converted concentration 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 met, no uncoated areas 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 the occurrence of coating peeling. That is, by controlling the difference in the above converted concentrations to a certain level or higher, it is possible to prevent the formation of oxides of Mn and Si on the surface, manufacture a hot-dip galvanized steel sheet with a beautiful surface and good coating adhesion, and suppress the occurrence of defects such as linear defects on the surface even after passing through the subsequent alloying heat treatment process. Since it is more advantageous as the difference in the above converted concentration values is larger, there is no need to arbitrarily determine the upper limit of that value. However, considering the content of the elements contained, the difference in the above converted concentration values can be 200% or less for both Mn and Si. In another implementation example of the present invention, the converted concentration difference of the above Mn and Si can be 15% or more or 20% or more.

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

[0037] The galvanized steel sheet for GDS concentration analysis is sheared into a size of 30 to 50 mm in length, and immersed in a hydrochloric acid aqueous solution of 5 to 10 wt% at a normal temperature of 20 to 25 °C to remove the zinc plating layer. In order to prevent surface damage of the base iron during the dissolution process of the zinc plating layer, when the generation of bubbles due to the reaction of the acid solution with the zinc plating layer is interrupted, the acid solution is removed within 10 seconds, and the base iron is washed with pure water and dried. Of course, if it is a plating steel sheet that has not yet been hot-dip galvanized, it can be analyzed without such a plating layer removal operation.

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

[0040] However, if the steel sheet contains 0.1 to 1.0% by weight of Mn and 0.1% by weight or less (excluding 0%) of Si and has a composition in which oxides are likely to be formed on the surface, the plating property can be advantageously improved by the present invention. The upper limit of the Mn concentration in the base iron is not particularly limited, but considering the commonly used composition, the upper limit can be restricted to 1.0% by weight. Also, the lower limit of the Mn concentration is not particularly limited, but in a composition containing less than 0.1% by weight of Mn, since the surface quality of the hot-dip galvanized steel sheet is beautiful even without forming an Fe plating layer, it is not always necessary to carry out Fe electroplating. Si is an element that may contribute to the formation of oxides and deteriorate the plating quality even when a small amount is added in the component system of the present invention. Therefore, considering the strict requirements for the surface quality of automotive outer panels, the upper limit of its content can be set to 0.1% by weight. From the viewpoint of only oxide formation, it is more advantageous not to add Si, so it is not always necessary to define the lower limit. However, considering the limits in the industrial production process, the lower limit of the Si content can also be restricted to 0.001%.

[0041] Since the above-mentioned Mn and Si are elements that affect the plating property, their concentrations can be restricted as described above. However, in the present invention, there are no particular restrictions on the remaining components of the base iron.

[0042] However, in the case of a steel sheet containing alloy components, considering the aspect that unplated and significant reduction in plating adhesion may occur, in one implementation example of the present invention, the composition of the base iron is as follows in weight percentage: Mn: 0.1 - 1.0%, Si: 0.1% or less (excluding 0%), C: 0.0005 - 0.03%, Al: 0.005 - 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 is Fe and unavoidable impurities. P and S are impurities and it is more advantageous if they are not added. Cr and B are optional elements and may not be added, so the lower limits are not separately defined. In addition to the above components, the base iron may further contain elements such as Ti, Mo, Nb, etc. in a total amount of 1.0% or less. Also, in another implementation example of the present invention, the C component can be further limited to less than 0.02%, and in yet another implementation example, the C component can be further limited to 0.0199% or less. Although there is no particular limitation on the base iron, in one implementation example of the present invention, a cold-rolled steel sheet or a hot-rolled steel sheet can be used as the base iron.

[0043] In one aspect of the present invention, a hot-dip galvanized steel sheet including the above-described plating steel sheet can be provided. The hot-dip galvanized steel sheet can include a plating steel sheet and a hot-dip galvanized layer formed on the surface of the plating steel sheet. At this time, any commercially available hot-dip galvanized steel sheet can be applied without particular limitation on its type.

[0044] Next, an exemplary implementation example of a method for manufacturing a plating steel sheet and a hot-dip galvanized steel sheet having the above-described advantageous effects will be described. According to one implementation example of the present invention, the plating steel sheet can be manufactured through a process including the steps of preparing base iron; performing electroplating on the base iron to form an Fe plating layer containing 5 - 50% by weight of oxygen; and annealing the base iron on which the Fe plating layer is formed to obtain a plating steel sheet.

[0045] Cold-rolled steel sheet containing 0.5% Mn, 0.05% Si and other alloying elements, with an iron deposition amount of 1.97 g / m 2After Fe electroplating containing 6.5 wt% oxygen so as to obtain, the electroplated cold-rolled steel sheet was annealed for 53 seconds at 800 °C in an atmosphere of N2 - 5% H2 with a dew point of -40 °C and then cooled. During the entire time of the annealing process, the atmosphere was maintained the same. 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 Fe electroplating layer and the base iron, while hardly any 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 Figure 1, maximum values of Mn and Si concentrations appear in the portion 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 show a tendency to gradually decrease, but the maximum value can be clearly observed.

[0046] Such a phenomenon is due to the formation of an Fe plating layer with a high oxygen content before annealing. That is, when the Fe electroplating layer contains 5 - 50 wt% oxygen, when annealed in a reducing atmosphere annealing furnace, the oxygen in the Fe electroplating 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 Fe electroplating layer and the base iron. Therefore, when measuring the concentration with GDS as in the graph of Figure 1, a maximum point with a high concentration of Mn, Si, etc. is confirmed at a depth corresponding to the thickness of the Fe electroplating layer from the surface. On the other hand, alloy elements with a slow diffusion rate such as Mn are diluted in concentration by the Fe electroplating layer or, even if the dissolved Mn is depleted due to internal oxidation, cannot diffuse rapidly from the base iron, so there may be a minimum point after the maximum point of the GDS concentration. However, since Si diffuses rapidly from the inside during the annealing process and internal oxidation continuously progresses at the interface between the Fe electroplating layer and the base iron, accumulating oxides, there may be no minimum point confirmed by GDS concentration analysis. Therefore, the non-appearance of a minimum point in the GDS concentration profile means that alloy elements such as Mn and Si are oxidized by the Fe electroplating layer, effectively suppressing diffusion to the surface.

[0047] Unlike the oxide firmly formed 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 oxides in the form of particles or discontinuous plates at the interface between the ferroelectric plating layer and the base steel plate. Therefore, the adhesion between the ferroelectric plating layer and the base metal can be maintained well. Furthermore, the uniformly formed ferroelectric plating layer suppresses the generation of surface oxides, reduces the concentration of alloying elements such as Mn and Si dissolved in the steel plate surface, promotes the alloying reaction with the zinc plating layer, and a uniform alloying hot-dip galvanized steel plate without surface defects can be obtained.

[0048] Unlike the oxidation-reduction method, the annealing internal oxidation method does not form a layered oxide layer, so it exhibits excellent characteristics in improving plating adhesion during the hot-dip galvanizing of steel plates containing alloying elements such as Mn and Si. However, since the water vapor in the annealing furnace inevitably oxidizes the surface of the steel plate first and then oxygen penetrates inside, the surface oxides cannot be fundamentally removed. As a result, if the surface of the cold-rolled steel plate before annealing is not completely homogeneous or local variations in oxygen partial pressure, temperature, etc. occur during annealing, the wettability with the hot-dip plating solution becomes non-uniform and unplated areas occur, or the thickness of the oxide film becomes non-uniform during the alloying heat treatment process after zinc plating, resulting in a difference in alloying degree and the possibility of inducing linear defects that can be easily identified visually.

[0049] To manufacture a beautiful hot-dip galvanized steel plate with no problem of plating peeling by suppressing the surface diffusion of alloying elements with Fe plating containing a large amount of oxygen, a ferroelectric plating layer containing 5 to 50 wt% oxygen in the base iron is formed so that the iron deposition amount is 0.5 to 3.0 g / m 2 It is good to raise the temperature to 600 - 950 °C so that the mechanical properties of the steel plate can be ensured, cool it again, and then perform hot-dip plating.

[0050] In one embodiment of the present invention, the Fe plating layer may be formed by a continuous plating process, and the Fe plating amount is 0.5 to 3.0 g / m based on the Fe coating weight. 2 The Fe plating amount can be set 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 Although the effect of suppressing alloying elements can be further increased even if the plating amount exceeds 1.0 to 2.0 g / m, multiple plating cells must be operated to ensure a high plating amount, and when an insoluble anode is used, the electroplating solution becomes acidic rapidly, which reduces plating efficiency and generates sludge, which is not economical. In another embodiment of the present invention, the plating amount of Fe 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 or just below the interface between the Fe plating layer and the base steel, so that the maximum points of Mn and Si concentrations are present in the region of 0.05 to 1.0 μm. 2 The amount of Fe plating can correspond to a thickness of 0.05 to 0.4 μm after annealing.

[0051] In addition, the Fe plating layer having the above-mentioned high oxygen concentration is formed in the GDS concentration profile of Mn and Si elements inside the steel sheet to be plated by controlling the temperature, dew point temperature, and atmosphere of the subsequent annealing process, so that the converted concentration at the maximum point and the converted concentration at the minimum point satisfy the numerical range limited in one embodiment of the present invention. In consideration of this point, the oxygen concentration in the Fe plating layer in one embodiment of the present invention may be 5 to 50 wt %, and in another embodiment, 10 to 40 wt %. In order to obtain the effect of suppressing surface oxide, the amount of oxygen in the Fe plating layer needs to be sufficiently large. Even if the oxygen concentration in the Fe plating layer is less than 5 wt %, the effect of suppressing surface oxide can be obtained by increasing the amount of Fe plating, but in order to obtain such an effect, the amount of Fe plating is 3.0 g / m 2Since plating must be carried out beyond this, various problems described above may occur. Also, when the oxygen content does not reach 5% by weight, it becomes difficult to sequentially form maximum points and minimum points in the GDS profiles of Mn and Si. Therefore, in one implementation example of the present invention, the oxygen content in the above 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 suppressing effect of surface oxides during annealing can be further increased. However, with a normal electroplating method, it is difficult to obtain a plating layer exceeding 50% by weight. Therefore, the upper limit can be restricted to 50% by weight. In another implementation example of the present invention, the oxygen concentration in the above Fe plating layer can also be restricted to 10 - 40%.

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

[0053] On the other hand, in one implementation example of the present invention, the dew point inside the annealing furnace is not necessarily limited to this, but may be less than -20°C. When the dew point temperature is maintained below -20°C, it is economical because a separate humidifying device for increasing the dew point is not required. Furthermore, in the case of the present invention, since an Fe plating layer with a high oxygen concentration is formed, it is possible to sufficiently prevent alloying elements such as Mn and Si 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 a very high-purity gas. Therefore, considering this, the lower limit of the dew point can be set to -90°C. According to another implementation example of the present invention, the dew point when the temperature of the steel sheet is 600 - 950°C can be -70 to -30°C.

[0054] In addition, in order to prevent oxidation of the base iron and the Fe plating layer during annealing, the hydrogen concentration in the atmospheric gas during annealing can be set to 1% or more by volume. When the hydrogen concentration is less than 1%, it is impossible to effectively oxidize and remove trace amounts of oxygen inevitably contained in H2 and N2 gases, and the oxygen partial pressure increases, which may induce surface oxidation of the base iron. On the other hand, when the hydrogen concentration exceeds 70%, the explosion risk during gas outflow and the cost due to high-hydrogen operations increase, so the hydrogen concentration can be set to 70% or less. Except for the inevitable impurity gases other than the above hydrogen (H2), it may be substantially nitrogen (N2).

[0055] According to an embodiment of the present invention, during annealing, the holding time after reaching the target temperature can be limited to 5 to 120 seconds. In order to ensure sufficient heat transfer to the interior of the base iron and obtain uniform mechanical properties in the thickness direction during annealing, 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 interfering elements through the Fe plating layer increases, the amount of surface oxides 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.

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

[0057] FIG. 2 schematically shows the phenomena occurring inside the steel sheet by raising the temperature of the steel sheet according to the conditions of the present invention.

[0058] FIG. 2(a) shows a schematic cross-sectional view of the base iron on which an Fe plating layer containing a large amount of oxygen is formed. The base iron contains alloy elements such as Mn and Si. The Fe plating layer contains 5 to 50% by weight of oxygen and impurities inevitably mixed during plating, and the balance is composed of Fe.

[0059] Figure 2(b) shows the state of a cold-rolled steel sheet electroplated with iron after heating it to 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 start to form at the interface between the Fe plating layer and the base iron, and the oxides at the grain boundaries grow coarser as the temperature increases.

[0060] Figure 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, ferrite with a lower Mn and Si concentration than the base iron is formed, and as the oxygen in the Fe plating layer gradually depletes, Mn and Si start to gradually diffuse through the Fe plating layer to the surface of the Fe plating layer.

[0061] Figure 2(d) shows a schematic cross-sectional view of the steel sheet after annealing at a temperature of 600 to 950 °C. Except for internal oxides such as Mn and Si, the oxygen dissolved in the metal iron in the electroplated iron layer is completely removed. The generated internal oxides generally have a spherical or short plate-like shape. Furthermore, due to grain growth, the electroplated iron layer can also form a single grain with the base iron. However, the morphology of the internal oxides is not necessarily formed in a particulate state. Depending on the elongation rate of the cold-rolled steel sheet, the composition of the steel, the atmosphere in the annealing furnace, and the oxygen content contained in the electroplated iron layer, the grain boundaries of the base iron and the electroplated iron layer may be distinguished and appear, and short linear oxides may be generated along the interface between the electroplated iron layer and the base iron or along the grain boundaries inside the base iron.

[0062] After the annealing stage, the annealed steel sheet can be cooled. The cooling conditions in the cooling stage after the annealing stage do not have a significant impact on the surface quality of the final product, that is, the plating quality. Therefore, there is no need to particularly limit the cooling conditions in the present invention. However, in order to prevent the oxidation of the iron component during the cooling process, a reducing atmosphere can be applied at least for iron.

[0063] According to one embodiment of the present invention, the steel sheet for plating obtained by the above-described process can be hot-dip galvanized to form a hot-dip galvanized layer. In the present invention, the hot-dip galvanizing method is not particularly limited.

[0064] In the present invention, as long as it is the base iron having the above-described alloy composition, it can be applied without limitation as the base iron of the steel sheet for plating or the hot-dip galvanized steel sheet according to the present invention. Therefore, the method for producing the base iron may not be specifically limited.

[0065] In one embodiment of the present invention, the Fe plating layer can be formed on the surface of the base iron by an electroplating method, and the oxygen concentration of the formed Fe plating layer can be controlled by appropriately controlling the conditions of the electroplating solution and the plating conditions.

[0066] That is, in the present invention, in order to form the Fe plating layer, an electroplating solution containing iron ions including ferrous ions and ferric ions, a complexing agent, and inevitable impurities can be used, and the concentration of ferric ions among the above iron ions is 5 to 60% by weight.

[0067] According to an embodiment of the present invention, the electroplating solution contains a first iron ion and a second iron ion. In order to obtain high plating efficiency, it may be advantageous to contain only the first iron ion. However, when only the first iron ion is contained, the solution deteriorates and the plating efficiency rapidly decreases, which may induce quality variations in the continuous electroplating process. Therefore, the second iron ion can be further contained. At this time, the concentration of the second iron ion is preferably 5 to 60% by weight, more preferably 5 to 40% by weight, based on the total of the first iron and the second iron ions. When 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. Therefore, the concentration of the second iron rapidly increases, the pH rapidly decreases, and the plating efficiency continuously decreases. On the contrary, when the concentration of the second iron ion exceeds 60%, the reaction amount of the second iron reduced to the first iron at the cathode increases significantly more than the reaction amount of the first iron reduced and deposited as metallic iron. Therefore, the plating efficiency is significantly reduced and the plating quality deteriorates. Therefore, considering the characteristics of the equipment and the process, such as the plating amount, the working current density, the solution replenishment amount, the amount of the solution adhering to the strip and lost, and the rate of concentration change due to evaporation, it is preferable to set the concentration of the second iron ion in the iron ions to 5 to 60% by weight.

[0068] The concentration of the iron ions is preferably 1 to 80 g per liter of the electroplating solution, more preferably 10 to 50 g per liter. When it is less than 1 g / L, there is a problem that the plating efficiency and the plating quality rapidly decrease. On the other hand, when it exceeds 80 g / L, precipitation may occur exceeding the solubility, and raw material loss due to solution loss increases in the continuous plating process, which is not economical.

[0069] The electroplating solution of the present invention contains a complexing agent. However, in order to maintain a high plating efficiency without generating sludge while containing a large amount of the second iron, it is preferable to use an amino acid or an amino acid polymer as the complexing agent.

[0070] An amino acid is an organic molecule in which a carboxyl group (-COOH) and an amino group (-NH2) are bonded, and an amino acid polymer is an organic molecule formed by polymerizing two or more amino acids, and the amino acid polymer exhibits complexing properties similar to those of an amino acid. Therefore, in the following description, amino acids and amino acid polymers are collectively referred to as amino acids.

[0071] When an amino acid is dissolved in neutral water, the amines are positively charged due to hydrogen ions, and the carboxyl groups are negatively charged due to dissociation of hydrogen ions, so that the amino acid molecules maintain a neutral charge. On the other hand, when the solution is acidified, the carboxyl groups are recombined with hydrogen ions to become charge neutral, and the amines are positively charged, so that the amino acid molecules form cations. That is, amino acids are charge neutral or form cations in a weakly acidic aqueous solution.

[0072] When an amino acid is added to an acidic electrolyte solution containing iron ions, it is complexed with ferrous and ferric ions, but the iron ions complexed with the amino acid remain in a cationic state even in the complexed state. Therefore, it shows electrical properties opposite to those of a general complexing agent with multiple carboxyl groups, which shows a negative charge in a weakly acidic aqueous solution.

[0073] In addition, compared to complexing agents containing multiple carboxyl groups such as citric acid and EDTA, amino acids form fewer bonds with iron ions and the bonding strength is weaker, but the bonding strength with ferric ions that generate sludge is strong enough to prevent precipitation by ferric ions. Furthermore, since the ferric ions can maintain their cation status even when complexed, they can be easily transferred to the cathode and reduced to ferrous ions to participate in the plating reaction, while their movement to the anode is suppressed and the rate of ferric ion generation slows down, so that the concentration of ferric ions remains at a constant level even when plating is performed continuously for a long period of time, plating efficiency remains constant, and there is no need to replace the electrolyte.

[0074] On the one hand, when iron ions in the solution are exhausted by plating in a continuous electroplating process, the solution becomes acidic. However, even if the same amount of iron ions is deposited, the pH change of the solution containing both ferrous ions is less than that of the solution containing only ferrous ions. When the pH increases, some ferric ions combine with hydroxide ions. When the pH decreases, the hydroxide ions are separated and neutralized. Therefore, even without a separate pH buffer, the solution containing ferric ions has a blunted pH change and serves as a pH buffer, enabling the electroplating efficiency to be kept constant in the continuous electroplating process.

[0075] Therefore, using an amino acid as a complexing agent can prevent sludge, and not only ferrous ions but also ferric ions can be used as electroplating raw materials. When ferrous ions and ferric ions are mixed and used, the pH change of the solution can be blunted, and the accumulation of ferric ions can be easily prevented. Thus, the electroplating efficiency and the electroplating quality can be kept constant in the continuous electroplating process.

[0076] It should be noted that the above complexing agent is preferably added in an amount such that the molar concentration ratio of the iron ions to the complexing agent is 1:0.05 - 2.0, and more preferably in an amount such that the ratio is 1:0.5 - 1.0. If it is less than 0.05, the excessive ferric ions cannot be inhibited from combining with hydroxide ions or oxygen to form sludge, and even without ferric ions, the electroplating efficiency will decrease significantly, and furthermore, burning will be induced and the electroplating quality will deteriorate. On the other hand, even if it exceeds 2.0, the sludge suppression effect and the electroplating quality are maintained, but the overvoltage increases and the electroplating efficiency decreases. Compared with raw materials containing iron ions such as iron sulfate, relatively expensive amino acids need to be contained in an excessive amount more than necessary, resulting in an increase in raw material costs and being uneconomical.

[0077] The above complexing agent is preferably one or more selected from amino acids or amino acid polymers. For example, it may be one or more selected from alanine, glycine, serine, threonine, arginine, glutamine, glutamic acid, and glycylglycine.

[0078] Using the above amino acid as a complexing agent, while maintaining the solution temperature at 80 °C or lower and the pH at 2.0 - 5.0, electroplating is carried out at a current density of 3 - 120 A / dm 2 then a high plating efficiency and an Fe plating layer with a high oxygen concentration can be obtained.

[0079] The temperature of the Fe electroplating solution does not greatly affect the quality of the Fe plating layer. However, when it exceeds 80 °C, the evaporation of the solution becomes intense, the concentration of the solution continuously changes, and it becomes difficult to perform uniform electroplating.

[0080] When the pH of the Fe electroplating solution is less than 2.0, the electroplating efficiency decreases and it is not suitable for a continuous plating process. When the pH exceeds 5.0, although the plating efficiency increases, sludge in which iron hydroxide precipitates is generated during continuous electroplating, causing problems such as pipe blockage and contamination of rolls and equipment.

[0081] When the current density is less than 3 A / dm 2 the overpotential of plating at the cathode decreases and the Fe electroplating efficiency decreases, so it is not suitable for a continuous plating process. When it exceeds 120 A / dm 2 burning occurs on the plating surface, making the electroplating layer non-uniform and causing problems such that the Fe plating layer is likely to peel off.

[0082] As described above, in the present invention, it is preferable that the Fe plating layer contains 5 - 50% by weight of oxygen. The reasons for oxygen to be mixed into the Fe plating layer are as follows. During the process of iron being deposited on the surface of the steel plate to which the cathode is applied, at the same time, hydrogen ions are reduced to hydrogen gas and the pH increases. Therefore, both ferrous and ferric ions may temporarily combine with OH - ions and be mixed in together when the Fe plating layer is formed. If an anionic complexing agent such as acetic acid, lactic acid, citric acid, or EDTA is used, the complexing agent combines with OH -The iron ions combined with ions will on average exhibit a negative charge. When a cathode is applied for electroplating, an electrical repulsive force will be generated, suppressing the contamination into the Fe plating layer. On the other hand, amino acids are electrically neutral at pH 2.0 - 5.0, and show cations in strong acids with a pH less than 2.0. However, even if 1 - 2 OH - are combined with the iron ions combined with amino acids, they will show cations, resulting in an electrical attractive force with the cathode for electroplating and a large amount of oxygen being incorporated. Therefore, when using an amino acid as a complexing agent such that the molar concentration ratio of iron ions to amino acids is 1:0.05 - 1:2.0 and maintaining pH 2.0 - 5.0 to perform Fe electroplating, high plating efficiency can be achieved, and an Fe plating layer containing 5 - 50 wt% of oxygen can be obtained while suppressing the generation of sludge.

[0083] To ensure the quality of hot-dip galvanized products of steel plates containing Mn and Si, it is good to treat the plating amount of the Fe plating layer at 0.5 - 3.0 g / m based on the iron concentration. 2 The upper limit of the Fe plating amount is not particularly limited, but in a continuous plating process, if it exceeds 3.0 g / m 2 , multiple plating cells are required or the production speed decreases, which is not economical. Furthermore, when the amount of Fe electroplating is large, the Fe electroplating solution rapidly denatures and the pH decreases in a continuous process, resulting in a significant decrease in plating efficiency and difficulty in solution management. On the other hand, when the amount of Fe electroplating is less than 0.5 g / m 2 , the oxygen contained in the Fe plating layer is rapidly reduced and removed, so that the diffusion of Mn and Si from the base iron and the formation of surface oxides cannot be effectively suppressed, resulting in a problem of deteriorating hot-dip plating quality. The above Fe plating amount has a thickness of about 0.05 - 0.4 μm when the Fe plating layer is completely reduced during annealing with the iron concentration contained in the plating layer.

Claims

1. by weight, Mn: 0.1 to 1.0%, Si: 0.1% or less (excluding 0%), C: 0.0005 to 0.03%, Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, Cr: 1.5% or less, B: 0.005% or less, the balance being Fe and unavoidable impurities, having a composition that when observed in the depth direction from the surface, the GDS profile of the Mn component and the GDS profile of the Si component each sequentially include a maximum point and a minimum point, the difference (converted Mn concentration difference) between the value obtained by dividing the Mn concentration at the maximum point of the GDS profile of the Mn component by the Mn concentration of the base material and the value obtained by dividing the Mn concentration at the minimum point of the GDS profile of the Mn component by the Mn concentration of the base material is 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 regarded as the point where the minimum point appears.

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

3. The steel sheet according to claim 1, wherein the converted Mn concentration difference is 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 according to claims 1 to 4 and a hot-dip galvanized layer formed on the steel sheet.

6. preparing a base iron having a composition by weight of Mn: 0.1 to 1.0%, Si: 0.1% or less (excluding 0%), C: 0.0005 to 0.03%, Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, Cr: 1.5% or less, B: 0.005% or less, the balance being Fe and unavoidable impurities; performing electroplating on the base iron to form an Fe plating layer containing 5 to 50% by weight of oxygen; and The substrate iron on which the Fe plating layer is formed is controlled to a dew point temperature of less than -20°C in a 1 to 70% H ２ - the remaining N ２ A method for manufacturing a steel sheet, including a step of annealing by maintaining at 600 to 950°C for 5 to 120 seconds in an annealing furnace in a gas atmosphere.

7. The adhesion amount of the Fe plating layer is 0.5 to 3 g / m ２ The 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 have a ratio of 5 to 60% by weight based on the total amount of iron ions, and the total concentration of the iron ions is 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 The method for manufacturing a steel sheet according to claim 6 or 7, which is carried out under the conditions of

11. Preparing a base iron having a composition containing, by weight%, Mn: 0.1 to 1.0%, Si: 0.1% or less (excluding 0%), C: 0.0005 to 0.03%, Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, Cr: 1.5% or less, B: 0.005% or less, and the balance Fe and unavoidable impurities; Performing electroplating on the base iron to form an Fe plating layer containing 5 to 50% by weight of oxygen; The base iron with the Fe plating layer formed thereon is maintained at 600 to 950 °C for 5 to 120 seconds in an annealing furnace with a gas atmosphere controlled to a dew point temperature of less than -20 °C and 1 to 70% H ２ - the balance N ２ to obtain an annealed steel sheet for plating; and A method for manufacturing a hot-dip galvanized steel sheet, which includes immersing the steel sheet for plating in a hot-dip galvanizing bath.

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