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

Abstract

The present invention relates to a high-strength hot-dip galvanized steel sheet having excellent plating quality, a steel sheet for plating for manufacturing the same, and a method for manufacturing them. The steel sheet for plating according to one aspect of the present invention has a composition containing, by weight%, Mn: 1.0 to 8.0%, Si: 0.05 to 3%, C: 0.06 to 0.4%, Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, Cr: 1.5% or less, B: 0.005% or less, and the balance Fe and unavoidable impurities. The GDS profiles of the Mn component and the Si component observed in the depth direction from the surface each sequentially include a maximum point and a minimum point. The 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, the difference (converted Mn concentration difference) is 80% or more, and 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, the difference (converted Si concentration difference) may be 50% or more. However, when no minimum point appears within a depth of 5 μm, the point at a depth of 5 μm is regarded as the point where the minimum point appears.

Description

Technical Field

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

Background Art

[0002] In recent years, in the field of the automotive industry, by applying high-strength steel sheets as automotive steel materials, improvements in safety and weight reduction due to a decrease in thickness have been achieved. As steel materials that can be preferably applied as automotive steel materials, martensitic steel, TRIP steel, etc. have been developed. These high-strength steels add various alloying elements compared to general steel, and in particular, many elements with a higher oxidation tendency than Fe, such as Mn, Si, Al, Cr, B, etc., are added.

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

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

[0005] When using the method of reducing after oxidation in the annealing process as in Patent Document 1, components with a large affinity for oxygen, such as Si, Mn, and Al, are internally oxidized at a certain depth from the surface layer of the steel sheet, and diffusion to the surface layer is suppressed. Therefore, Si, Mn, or Al alone or composite oxides relatively decrease on the surface layer, the wettability with zinc is improved, and unplated areas can be reduced. However, in the case of steel grades added with Si, Si concentrates directly under 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, there is a problem that it is difficult to ensure the adhesion of the plating layer due to peeling occurring at the interface between the reduced iron and the underlying base iron.

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

[0007] Also, even if the plating property is improved by internal oxidation, linear unplated areas may occur due to the surface oxides formed non-uniformly on the surface, or when producing a galvannealed steel sheet (GA steel sheet) 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] Furthermore, as another conventional technique, there is a method of performing pre-plating of Ni before annealing to suppress the diffusion of alloying elements to the surface during annealing. However, although this method is effective in suppressing the diffusion of Mn, there is a problem that the diffusion of Si cannot be sufficiently suppressed.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0010] According to one aspect of the present invention, there is provided a hot-dip galvanized steel sheet having excellent plating quality in which unplated areas do not occur and the problem of peeling of the plating layer is solved, and a method for manufacturing the same.

[0011] According to another aspect of the present invention, there is provided a hot-dip galvanized steel sheet and a method for manufacturing the same, which can be manufactured as a galvannealed steel sheet with excellent surface quality without generating linear defects even when subjected to alloying heat treatment after plating.

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

[0013] The problems of the present invention are not limited to the above description. For those with ordinary knowledge in the technical field to which the present invention pertains, there is no difficulty in understanding further problems of the present invention from the overall matters of the specification of the present invention.

Means for Solving the Problems

[0014] The steel sheet for plating according to one aspect of the present invention has, by weight%, Mn: 1.0 to 8.0%, Si: 0.05 to 3%, C: 0.06 to 0.4%, Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, Cr: 1.5% or less, B: 0.005% or less, and the balance Fe and unavoidable impurities, and 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, and 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 The difference (converted Mn concentration difference) is 80% or more, and 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 The difference (converted Si concentration difference) may be 50% or more.

[0015] However, when no minimum point appears within a depth of 5 μm, the point at a depth of 5 μm shall be regarded as the point where the minimum point appears.

[0016] The hot-dip galvanized steel sheet, which is 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] Another aspect of the present invention is a method for manufacturing a steel sheet for plating. The method includes the steps of: preparing a base iron having a composition containing, by weight%, 1.0 to 8.0% of Mn, 0.05 to 3% of Si, 0.06 to 0.4% of C, 0.005 to 3.0% of Al, 0.04% or less of P, 0.015% or less of S, 1.5% or less of Cr, 0.005% or less of B, and the balance being Fe and inevitable impurities; performing electroplating on the base iron to form an Fe plating layer containing 5 to 50% by weight of oxygen; and annealing the base iron having the Fe plating layer formed thereon by maintaining it at 600 to 950°C for 5 to 120 seconds in an annealing furnace in an atmosphere of 1 to 70% H2 - the balance being N2 gas, the dew point temperature of which is controlled to -15 to +30°C.

[0018] Still another aspect of the present invention is a method for manufacturing a hot-dip galvanized steel sheet. The method includes the steps of: preparing a base iron having a composition containing, by weight%, 1.0 to 8.0% of Mn, 0.05 to 3% of Si, 0.06 to 0.4% of C, 0.005 to 3.0% of Al, 0.04% or less of P, 0.015% or less of S, 1.5% or less of Cr, 0.005% or less of B, and the balance being Fe and inevitable impurities; performing electroplating on the base iron to form an Fe plating layer containing 5 to 50% by weight of oxygen; annealing the base iron having the Fe plating layer formed thereon by maintaining it at 600 to 950°C for 5 to 120 seconds in an annealing furnace in an atmosphere of 1 to 70% H2 - the balance being N2 gas, the dew point temperature of which is controlled to -15 to +30°C, to obtain a steel sheet for plating; and immersing the steel sheet for plating in the above hot-dip galvanizing bath consisting of 0.1 to 0.3% of Al, the balance being Zn and inevitable impurities, and maintained in a temperature range of 440 to 500°C.

Advantages of the Invention

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

[0020] Also, according to one aspect of the present invention, 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 excellent in surface quality can be provided.

Brief Description of the Drawings

[0021]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0022] Hereinafter, a high-strength hot-dip galvanized steel sheet excellent in plating quality according to one aspect of the present invention completed by the research of the present inventor will be described in detail. In the present invention, when indicating the concentration of each element, it should be noted that, unless otherwise specified, it means weight%. The amount of Fe electroplating is the amount of plating measured as 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 amount of plating.

[0023] Furthermore, 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 a large amount of Mn and Si, it is known that the causes of non-plating and deterioration of plating adhesion are due to surface oxides generated by oxidation of alloying elements such as Mn and Si on the surface during the process of annealing the cold-rolled steel sheet at a high temperature.

[0026] In order to suppress the diffusion of alloying elements such as Mn and Si to the surface, as a method of forming an oxide layer containing a large amount of oxygen, after oxidation during temperature rise, an oxidation-reduction method of maintaining in a reducing atmosphere and reducing, 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. Since Mn and Si oxides accumulate at the interface to form a continuous oxide layer, even if the wettability with molten zinc is improved, there is a possibility that the oxide layer is prone to cracking and the plating layer peels off.

[0027] On the other hand, when applying the annealing internal oxidation method of increasing the oxygen partial pressure or dew point in the annealing furnace during the heat treatment process to oxidize alloying elements such as Mn and Si inside the steel, Mn and Si oxides are preferentially formed on the surface of the steel during the heat treatment process, and then Mn and Si are oxidized by the oxygen diffused inside the steel to suppress surface diffusion. Therefore, a thin oxide film is formed on the surface of the base iron. However, if the surface of the cold-rolled steel sheet before annealing is not completely homogeneous, or local variations such as oxygen partial pressure and temperature occur, the wettability in the molten zinc plating becomes non-uniform and unplated areas occur, or during the alloying heat treatment process after zinc plating, the thickness of the oxide film becomes non-uniform and a difference in alloying degree occurs, 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 tried to manufacture a molten zinc-plated steel sheet with a beautiful surface and no problem of plating peeling by controlling the existence forms of the oxidizing elements Mn and Si on the surface side of the steel sheet for plating as follows.

[0029] That is, the steel sheet according to an embodiment of the present invention can have the following characteristics in the GDS concentration profiles of Mn and Si. With reference to the GDS profile in FIG. 1, the galvanized 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 the Mn component that can appear from the surface portion after removing the zinc plating layer from the hot-dip galvanized steel sheet including the steel sheet of the present invention. In the graph, the vertical axis indicates the concentration of alloying elements such as Mn and Si, and the horizontal axis indicates the depth. As illustrated in the graph of FIG. 1, the galvanized steel sheet for plating of the present invention can have a form in which maximum points and minimum points appear sequentially when the concentration profile of the Mn or Si component goes from the surface (interface with the plating layer when hot-dip galvanized) toward the inside. Here, "having sequentially" does not necessarily mean that the maximum point always appears first in the depth direction from the surface (interface). In some cases, the minimum point may appear first, but then, 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. Also, the concentration of the alloying element on the surface has a value lower than the concentration of the maximum point, but in some cases, a minimum point with a low concentration of the alloying element may appear between the surface and the maximum point.

[0031] In the GDS concentration profile illustrated in FIG. 1 above, although not necessarily limited to this, since not many alloying elements diffuse from the base iron in the surface layer portion, it corresponds to an Fe plating layer with a low concentration of alloying elements. The maximum point corresponds to a region where internal oxides of alloying elements concentrated near the interface between the Fe plating layer and the base iron are formed. The minimum point that appears on the base iron side in the Fe plating layer corresponds to a region where the alloying elements are diffused and diluted in the Fe plating layer that does not contain alloying elements, or a region where the alloying elements are diffused and depleted in the maximum point where internal oxidation has occurred.

[0032] 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 regarded 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.

[0033] At this time, in the Mn concentration profile and the Si concentration profile, the greater the difference between the converted concentration of the maximum point of the element (the value obtained by dividing the concentration at that point by the concentration of the base material, expressed in % units) and the converted concentration of the minimum point, the more the Mn and Si diffusing to the surface can be reduced, which is important. In one embodiment of the present invention, in the case of Mn, the value of the converted concentration of the maximum point - the converted concentration of the minimum point can be 80% or more, and in the case of Si, the difference in the above value can be 50% or more. Si is an element with stronger oxidizing properties than Mn, and internal oxidation easily occurs even inside the base iron with a low oxygen concentration, so oxidation may occur in a wider region than Mn. Therefore, even if the difference in the converted concentrations of the maximum and minimum points of Si is smaller than that of Mn, it cannot be said that the degree of internal oxidation is small. As a result of experiments under various conditions by the present inventors, when the above conditions are satisfied, non-plating does not occur during hot-dip galvanizing, and a hot-dip galvanized steel sheet with good plating adhesion can be obtained. However, if the difference in the converted concentrations of the maximum and minimum points of Mn is less than 80%, or if the difference in the converted concentrations of the maximum and minimum points of Si is less than 50%, problems such as the occurrence of spot or linear non-plating or plating peeling may occur. That is, by doing so, it is possible to prevent the formation of oxides of Mn and Si on the surface, and it is possible to manufacture an ultra-high-strength hot-dip galvanized steel sheet with a beautiful surface and good plating adhesion. Furthermore, even after undergoing the alloying heat treatment process, it is possible to suppress the occurrence of defects such as linear defects on the surface. Since the greater the difference in the above converted concentration values, the more advantageous it is, 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 determined to be 400% or less in the case of Mn and 250% or less in the case of Si. In another embodiment of the present invention, the converted concentration difference of the above Mn can be 90% or more or 100% or more, and the converted concentration difference of the above Si can also be 60% or more or 70% or more.

[0034] Hereinafter, the GDS analysis method implemented in the present invention will be described in detail.

[0035] The galvanized steel sheet for GDS concentration analysis was sheared into sizes of 30 to 50 mm in length, and immersed in a hydrochloric acid aqueous solution of 5 to 10 wt% at room 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 between the zinc plating layer and the acid solution is interrupted, the acid solution was removed within 10 seconds, and the base iron was 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.

[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 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 was 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 were obtained from the profile with the noises removed. Also, it should be noted that the maximum and minimum points referred to in the present invention were 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.

[0037] The plating steel sheet targeted in 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, in the case of a high-strength steel sheet containing 1.0 to 8.0% by weight of Mn and 0.05 to 3.0% by weight of Si and having a composition in which oxides are likely to be formed on the surface, the plating property can be advantageously improved by the present invention. The upper limit of the Mn concentration in the base iron is not particularly limited, but considering the commonly used compositions, the upper limit can be restricted to 8% by weight. Also, although the lower limit of the Mn concentration is not particularly restricted, in the case of a composition containing less than 1.0% by weight of Mn, even without forming an Fe plating layer, the surface quality of the hot-dip galvanized steel sheet is beautiful and there is no need to perform Fe electroplating. The upper limit of the Si concentration is not particularly restricted, but considering the commonly used compositions, the upper limit can be restricted to 3.0% by weight or less. When the Si concentration is less than 0.05% by weight, since the quality of the hot-dip galvanized product is beautiful even without simultaneously performing Fe electroplating and annealing internal oxidation, there is no need to implement the method of the present invention.

[0039] Since the above Mn and Si are elements that affect the plating property, their concentrations can be restricted as described above, but the present invention does not particularly restrict the remaining components of the base iron.

[0040] However, in the case of a high-strength steel sheet containing a large amount of alloy components, considering the aspect that unplated and reduced plating adhesion can occur severely, in one embodiment of the present invention, the composition of the above base iron is, by weight%, Mn: 1.0 to 8.0%, Si: 0.05 to 3.0%, C: 0.06 to 0.3% (preferably 0.07 to 0.4%, more preferably 0.08 to 0.4%), Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, N: 0.01% or less, Cr: 1.5% or less, B: 0.005% or less, and the balance is Fe and unavoidable impurities. It is advantageous that P, S, and N are not added as impurities, and since Cr and B may not be added as optional elements, the lower limits are not separately defined.

[0041] Hereinafter, the steel composition of the present invention will be described in more detail. Hereinafter, unless otherwise specified, the content of each element is based on weight.

[0042] "Mn: 1.0 to 8.0%" Manganese (Mn) is an element added to ensure strength. When the content of the above Mn is less than 1.0%, it becomes difficult to ensure strength. On the other hand, when its content exceeds 8.0%, the transformation rate of bainite becomes too slow and excessive fresh martensite is formed, making it difficult to obtain high hole expansion property. Also, banded structure due to segregation of Mn is formed, which impairs the material uniformity and formability of the material. Therefore, it is preferable that the content of the above Mn has a range of 1.0 to 8.0%. It is more preferable that the lower limit of the above Mn content is 1.5%.

[0043] 「Si: 0.05~3.0%」 Silicon (Si) is an element useful for increasing the strength of the steel sheet by solid solution strengthening and precipitation hardening. It has the effect of promoting the enrichment of C in austenite in order to suppress the formation of cementite, and is an essential element for generating retained austenite after annealing to increase the strength and elongation of the steel. When the content of Si exceeds 3.0%, the physical properties of the welded part deteriorate due to LME cracking, and the surface characteristics and plating property of the steel material deteriorate. Therefore, it is preferable that the content of the above Si has a range of 0.05 to 3.0%.

[0044] 「C: 0.06~0.4%」 Carbon (C) is an element that ensures the strength of the steel material by solid solution strengthening and precipitation strengthening, and is an effective element for stabilizing retained austenite to ensure high elongation. When the content of the above C is less than 0.05%, a tensile strength of 1500 MPa cannot be obtained. When it exceeds 0.4%, a steel sheet cannot be manufactured by cold rolling. Therefore, a C content of 0.06 to 0.4% is appropriate. On the other hand, it is more preferable that the content of the above C has a range of 0.2% or more and 0.4% or less.

[0045] 「Al: 0.005~3.0%」 Aluminum (Al) is an element that has a deoxidizing effect on molten steel, and like Si, has the effect of improving the stability of austenite and is effective in improving elongation.

[0046] If the content of Al is less than 0.005%, the deoxidation of the steel material will not be sufficiently carried out, which will impair the cleanliness of the steel material. On the other hand, if the content of Al is excessive, the transformation temperature will rise significantly, the ferrite fraction will increase, and high strength cannot be achieved. Therefore, the content of Al is set to 3.0% or less. In another embodiment of the present invention, the above Al can be limited to 2.0% or less, or 1.0% or less.

[0047] "P: 0.04% or less" Phosphorus (P) is contained as an impurity and segregates at the grain boundaries, reducing toughness. Therefore, it is preferably controlled to be as low as possible. Also, when the above P is added in excess, the toughness of the steel material deteriorates. Therefore, in the present invention, in order to prevent this, it is preferably limited to an upper limit of 0.04%. In the present invention, since it is more advantageous not to add P, there is no particular need to define the lower limit of the P content. However, considering the normal manufacturing method, the lower limit of the above P content can also be set to 0.002%. In one embodiment of the present invention, the upper limit of the above P content can also be set to 0.0173%.

[0048] "S: 0.01% or less" Sulfur (S) is contained as an impurity in the steel in the same manner as the above P. S combines with Mn to form inclusions, which may reduce the hole expansion property and may also reduce the weldability and hot rolling property. Therefore, it is advantageous to control it as low as possible. Therefore, the S content can be limited to 0.01% or less considering the case where it is inevitably contained. In the present invention, since it is more advantageous not to add S, there is no particular need to define the lower limit of its content. However, considering the normal manufacturing process, the lower limit of the above S content can also be set to 0.0009%. In another embodiment of the present invention, the upper limit of the above S content can be set to 0.0021%.

[0049] "N: 0.01% or less" In the present invention, nitrogen (N) is contained in the steel material as an impurity, and it is advantageous to control its content as low as possible. Therefore, in one embodiment of the present invention, the above N can be limited to 0.01% or less, and the lower limit is not particularly defined. However, 0% can be excluded in consideration of the case where N is inevitably contained (that is, more than 0%). In another embodiment of the present invention, the lower limit of the above N content can also be set to 0.0005%. In another embodiment of the present invention, the upper limit of the above N content can be 0.007%, and it can also be 0.006% or 0.0052%.

[0050] "Cr: 1.5% or less" Chromium (Cr) is an element effective for improving strength. It suppresses the formation of carbides and facilitates the retention of retained austenite. On the other hand, when the above Cr content exceeds 1.5%, the local corrosiveness deteriorates, and surface oxides are formed, impairing the phosphatability. Therefore, the content of the above Cr preferably has a range of 1.5% or less. More preferably, it is 1.0% or less.

[0051] "B: 0.005% or less" Boron (B) strengthens the grain boundaries and suppresses the ferrite transformation during cooling after annealing. To obtain this effect, the addition amount of B can also be set to 0.0001% or more. When the addition amount of the above B is excessive, the hot workability deteriorates, and B accumulates excessively on the surface, inhibiting the plating property. Therefore, the content of the above B preferably has a range of 0.005% or less.

[0052] The term "high strength" in the present invention is used in a sense that includes not only the case of having high strength after annealing but also all cases where high strength can be obtained by heat treatment or the like in subsequent processes. Note that the "high strength" in the present invention can mean a tensile strength of 490 MPa or more, but is not limited thereto. In addition to the components described above, the base iron can further contain a total of 1.0% or less of elements such as Ti, Mo, and Nb. Although there is no particular limitation on the base iron, in one embodiment of the present invention, a cold-rolled steel sheet or a hot-rolled steel sheet can be used as the base iron. Also, although not necessarily limited thereto, in one embodiment, martensitic steel or TRIP steel can be used as the cold-rolled steel sheet or the hot-rolled steel sheet.

[0053] In one embodiment of the present invention, the cold-rolled steel sheet or the hot-rolled steel sheet can have a microstructure containing 30 to 70% by volume of tempered martensite, 5 to 45% by volume of bainite, 10 to 40% by volume of retained austenite, and 20% by volume or less of ferrite phase. By including 30% by volume or more of tempered martensite in the microstructure, it is possible to ensure hole expansion properties while having high strength. Also, when including 10% by volume or more of retained austenite, the transformation-induced plasticity phenomenon due to retained austenite can be used, and the elongation rate can be greatly improved. Bainite is included to form retained austenite and ensure strength, and it is necessary to contain 5% by volume or more. Ferrite can be included as needed to improve the elongation rate, refine the prior austenite grain size, improve mechanical properties, and increase the C concentration in the retained austenite to stabilize the retained austenite. However, when the proportion of the ferrite phase exceeds 20% by volume, the hardness difference between the phases with the tempered martensite as the matrix structure becomes large, and the bendability and hole expansion properties may deteriorate. Therefore, the proportion can be limited to 20% by volume or less. In some cases, the ferrite phase may be included in an amount of 3% by volume or more. When the proportion of each phase is too high, the proportion of other phases can be limited to less than an appropriate range, so the upper limit can be limited as described above.

[0054] The hot-rolled steel sheet or cold-rolled steel sheet having the above-described microstructure can be manufactured by the following process although not necessarily limited thereto.

[0055] That is, in terms of weight %, Mn: 1.0 to 8.0%, Si: 0.05 to 3.0%, C: 0.06 to 0.4% (preferably 0.07 to 0.4%, more preferably 0.08 to 0.4%), Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, N: 0.0% or less, Cr: 1.5% or less, B: 0.005% or less, and the balance being Fe and inevitable impurities. A step of heating a steel slab, a step of hot-rolling the heated steel slab to obtain a steel sheet, and a step of winding up the hot-rolled steel sheet. The hot-rolled steel sheet can be obtained by a process including these steps. In the case of a cold-rolled steel sheet, a step of cold-rolling the above hot-rolled steel sheet, a step of maintaining the cold-rolled steel sheet in a soaking section for 50 to 500 seconds, a step of cooling the maintained steel sheet at a cooling rate of 1°C / s or more (which can be limited to 3000°C / s or less, 2000°C / s or less, or 1000°C / s or less in some cases) to a temperature between 100 and 300°C, and a step of heating the cooled steel sheet to a temperature between 300 and 600°C and maintaining it for 50 seconds or more (which can be limited to 3000 seconds or less or 2000 seconds or less in some cases). It can be manufactured by a process including these steps.

[0056] In one aspect of the present invention, a hot-dip galvanized steel sheet including the above-described steel sheet for plating can be provided. The hot-dip galvanized steel sheet can include a steel sheet for plating and a hot-dip galvanized layer formed on the surface of the steel sheet for plating. At this time, as the hot-dip galvanized steel sheet, any commercial one can be applied, and its type is not particularly limited.

[0057] Next, an exemplary embodiment of a method for manufacturing a plating steel sheet and a hot-dip galvanized steel sheet having the above-described advantageous effects will be described. According to an embodiment of the present invention, the plating steel sheet includes a step of preparing base iron, a step of performing electroplating on the base iron to form an Fe plating layer containing 5 to 50% by weight of oxygen, and a step of annealing the base iron on which the Fe plating layer is formed to obtain a plating steel sheet.

[0058] A cold-rolled steel sheet (martensitic steel) of 1.2 GPa grade containing Mn 2.3%, Si 0.9%, and other alloy elements was annealed at 800 °C for 53 seconds in an atmosphere of N2-5% H2 and a dew point of +5 °C, and then cooled and observed with a transmission electron microscope. During the entire heating time, the atmosphere was maintained the same, and the dew point temperature was maintained at -40 °C so that Fe would not oxidize during cooling. Before annealing, Fe electroplating containing 6.6% by weight of oxygen was performed so that the iron deposition amount would be 2.02 g / m 2 The phenomena that appeared were observed separately when Fe electroplating was performed and when it was not performed.

[0059] As a result of the observation, it was found that in the steel sheet annealed at a dew point of +5 °C without Fe plating, fine Mn and Si oxides were observed from the surface layer, and thick grain boundary oxides were formed inside the base iron. This is because grain boundary oxides start to form from the stage where the cold-rolled structure recovers and recrystallizes into fine crystal grains during the heating process, and as the annealing temperature rises and the annealing time increases, oxygen flows into the base iron with coarsened crystal grains, and oxides are generated at the center of the grain boundaries. Such a form ultimately results in a gradual change in the concentrations of Mn and Si components in the GDS profile, and the maximum and minimum points do not appear sufficiently, or even if they do appear, the difference in the converted concentrations does not satisfy the range limited in the present invention.

[0060] However, an Fe plating layer containing 5 to 50% by weight of oxygen was formed with an iron deposition amount of 1.99 g / m 2When annealing after plating to achieve the above, almost no oxide is formed in the Fe plating layer region, and particulate oxides are formed at the interface between the Fe plating layer and the base iron and inside the base iron. This oxide acts as the nucleus of the internal oxide, and linear oxides grow in the direction perpendicular to the surface of the steel sheet. However, the depth of formation of the internal oxide is deeper when no Fe plating layer is formed than when an Fe plating layer is applied. In such a case, the Fe plating layer (surface layer) can have a depletion layer with low Mn and Si components, showing a maximum value at the interface and a significant decrease in the Mn and Si contents in the region deeper than the maximum value.

[0061] On the other hand, when annealing in a high dew point atmosphere without performing Fe plating, oxides are formed at the grain boundaries of the fine recrystallized structure from the surface of the base iron, suppressing crystal growth, and irregular fine crystal grains surrounded by fine oxides are generated. In contrast, after forming a plating layer with a high oxygen content and annealing at a high dew point of -15°C to +30°C, since the Fe plating layer does not contain oxidizing alloy elements such as Mn and Si, oxides are not formed at the grain boundaries of the plating layer, and oxides are formed at the interface between the Fe plating layer and the base iron, resulting in the characteristic that the Fe plating layer structure with a uniform thickness and the crystal grains inside the base iron are distinguishable. However, depending on the dew point in the annealing furnace, the elongation rate of the base iron, the steel composition, etc., the boundary between the Fe plating layer and the base iron may not appear clearly. Therefore, even if the dew point in the annealing furnace is controlled to -15°C to +30°C after Fe electroplating, it does not necessarily have such characteristics.

[0062] Unlike the oxidation-reduction method, the annealing internal oxidation method does not form a layered oxide layer, showing excellent characteristics in improving plating adhesion during hot-dip galvanizing of ultra-high strength steel sheets containing a large amount of alloy elements such as Mn and Si. However, since the water vapor in the annealing furnace inevitably oxidizes the surface of the steel sheet first and then oxygen penetrates inside, the surface oxide cannot be fundamentally removed.

[0063] To solve the above problems, the inventors found that after forming an Fe plating layer containing a large amount of oxygen through many experiments and annealing in a high dew point atmosphere, oxygen from the water vapor in the annealing furnace can effectively suppress the internal oxidation of alloy elements such as Mn and Si in the base iron by the oxygen contained in the Fe plating layer and their diffusion to the surface without forming surface oxides of alloy elements such as Mn and Si on the surface of the Fe plating layer. The oxygen flowing into the steel due to the high dew point in the annealing furnace further internally oxidizes the alloy elements, so almost no surface oxides of alloy elements are formed on the surface of the steel, and the surface quality and plating adhesion of the hot-dip galvanized steel sheet are improved epoch-makingly. Also, when manufacturing an alloyed hot-dip galvanized steel sheet, the alloying reaction can be promoted, and a uniform alloyed hot-dip plated steel sheet without surface defects can be obtained.

[0064] More specifically, an Fe plating layer containing 5 to 50% by weight of oxygen is formed on a cold-rolled steel sheet (base iron), and after the temperature is raised to 600 to 950 °C so that the mechanical properties of the steel sheet can be ensured in an annealing furnace controlled at a dew point of -15 °C to +30 °C and then cooled again to perform hot-dip plating, unplated areas are suppressed, and a hot-dip plated steel sheet with excellent plating adhesion can be obtained.

[0065] In one embodiment of the present invention, the above Fe plating layer can be formed through a continuous plating process, and the amount of Fe plating at this time can be made to be 0.5 to 3.0 g / m based on the Fe deposition amount. 2 When the amount of Fe plating is less than 0.5 g / m, the effect of suppressing the diffusion of alloy elements by the Fe plating layer may be insufficient in a normal continuous annealing process. Also, even if it exceeds 3.0 g / m, the suppression effect of alloy elements can be further increased, but in order to ensure a high plating amount, a plurality of plating cells must be operated. When using an insoluble anode, there are problems such as the rapid acidification of the electroplating solution, a decrease in plating efficiency, and the generation of sludge, so it is not economical. In another embodiment of the present invention, the above Fe plating amount is 1.0 to 2.0 g / m. 2 When it is less than, there is a possibility that the effect of suppressing the diffusion of alloy elements by the Fe plating layer is insufficient in a normal continuous annealing process. Also, when it exceeds 3.0 g / m, 2 although the suppression effect of alloy elements can be further increased, in order to ensure a high plating amount, a plurality of plating cells must be operated. When using an insoluble anode, there are problems such as the rapid acidification of the electroplating solution, a decrease in plating efficiency, and the generation of sludge, so it is not economical. In another embodiment of the present invention, the above Fe plating amount is 1.0 to 2.0 g / m. 2It may also be the case. When internal oxidation is performed after forming the Fe plating layer, internal oxides are formed at the interface between the Fe plating layer and the base iron or immediately below the interface. Therefore, the maximum points of the Mn and Si concentrations exist in the region of 0.05 to 1.0 μm. The Fe plating amount of 0.5 to 3.0 g / m 2 in the present invention can correspond to a thickness of 0.05 to 0.4 μm after annealing.

[0066] Also, the Fe plating layer having the above-described high oxygen concentration forms maximum and minimum points in the GDS concentration profiles of Mn and Si elements inside the steel sheet for plating by controlling the temperature, dew point temperature, and atmosphere of the subsequent annealing process, so that the converted concentration at the maximum point and the converted concentration at the minimum point can satisfy the numerical range restricted in one embodiment of the present invention. Considering such points, 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, it may be 10 to 40 wt%. In order to obtain the effect of suppressing surface oxides, it is necessary that the amount of oxygen in the Fe plating layer is sufficiently large. Even if the oxygen concentration in the Fe plating layer is less than 5 wt%, the effect of suppressing surface oxides can be obtained by increasing the Fe plating amount. However, in order to obtain such an effect, plating must be performed exceeding 3.0 g / m 2 , so various problems described above may occur. Also, when the oxygen content does not reach 5 wt%, it is difficult to sequentially form maximum and minimum points in the GDS profile 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. On the other hand, as the oxygen concentration in the Fe plating layer increases, the effect of suppressing surface oxides during annealing can be further increased. However, with a normal electroplating method, it is difficult to obtain a plating layer exceeding 50 wt%, so the upper limit can be restricted to 50 wt%. In another embodiment of the present invention, the oxygen concentration in the Fe plating layer can also be restricted to 10 to 40%.

[0067] In one embodiment 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 recover and recrystallize properly, making it difficult to ensure the 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 the hot-dip galvanized coating and operation at a higher temperature than necessary, which is not economical.

[0068] On the other hand, in one embodiment of the present invention, the dew point inside the annealing furnace may be -15°C to +30°C. When the dew point is less than -15°C, the amount of oxygen flowing into the interior of the steel decreases, only surface oxidation is aggravated, and internal oxidation does not occur. As a result, a large amount of oxides exist on the surface, and the quality of the hot-dip galvanized coating deteriorates. Also, even when the dew point exceeds +30°C, internal oxidation increases and the diffusion of alloying elements is suppressed, and the effect of suppressing surface oxidation further increases. However, since the supply amount of water vapor increases rapidly, the capacity of the humidification equipment has to be made larger than necessary. If the cooled water vapor condenses and is applied for a long time in continuous annealing, problems with the equipment may occur. The above dew point can be controlled within the range described above at 600 - 950°C, and in a lower temperature range, it can be controlled under more relaxed conditions. In another embodiment of the present invention, the above dew point can also be limited to -10°C to +20°C.

[0069] Also, in order to prevent the oxidation of the base iron and the Fe plating layer during annealing, the hydrogen concentration in the 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 the trace amount of oxygen inevitably contained in the H2 and N2 gases, and the oxygen partial pressure may increase, inducing surface oxidation of the base iron. On the other hand, when the hydrogen concentration exceeds 70%, the risk of explosion during gas outflow and the cost due to high-hydrogen operations increase. Therefore, the above hydrogen concentration can be set to 70% or less. Except for the inevitable impurity gases contained in the above hydrogen (H2), it can be substantially nitrogen (N2).

[0070] 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. During annealing, in order to ensure sufficient heat transfer to the inside of the base iron and obtain uniform mechanical properties in the thickness direction, it is necessary to hold at the annealing target temperature for 5 seconds or more. On the other hand, if the high-temperature annealing holding time becomes excessively long, the diffusion of alloying interference elements through the Fe plating layer increases, the amount of surface oxide formation increases, and as a result, the quality of the hot-dip galvanized product deteriorates, so it can be limited to 120 seconds or less.

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

[0072] FIG. 2 schematically shows the phenomena occurring inside the steel sheet by raising the temperature of the steel sheet according to the conditions of the present invention. 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 alloying elements such as Mn and Si. The Fe plating layer contains 5 to 50% by weight of oxygen and impurities inevitably mixed in during electroplating, and the balance is composed of Fe.

[0073] Figure 2(b) shows the state of a cold-rolled steel sheet plated with Fe heated at approximately 300 to 500 °C in a nitrogen atmosphere containing 1 to 70% H₂. The surface of the Fe plating layer containing a large amount of oxygen is gradually reduced and the oxygen is removed. However, at the interface between the Fe plating layer and the base iron, Mn, Si, etc. diffused from the base iron combine with the oxygen in the Fe plating layer to form internal oxides, thus suppressing the diffusion to the surface. Also, the higher the temperature, the more Mn and Si diffused inside the base iron accumulate, and the internal oxides at the interface gradually grow. Even if the dew point in the annealing furnace widely varies from -90 °C to +30 °C in the low-temperature region of the heating-up stage, since the temperature is low and there is more oxygen in the Fe plating layer than the rate at which the dissociated oxygen diffuses into the steel, and the rate at which the Fe plating layer is reduced and oxygen is released is faster, there is no significant impact even if the dew point in the annealing furnace changes in the low-temperature range. Therefore, in this stage, the control of the dew point is not a particularly important factor.

[0074] However, the amount of oxygen inside the Fe plating layer plays an important role. When a large number of fine internal oxides are generated at the interface between the Fe plating layer and the base iron and inside the base iron in the low-temperature stage, the alloying elements inside the base iron will continuously act as nuclei for internal oxidation of oxides. For such oxide nuclei to form, it is necessary for the concentrations of oxygen and alloy components to be high simultaneously. However, if the Fe plating layer contains sufficiently large amounts of oxygen, a large number of oxide nuclei are generated near the interface between the Fe plating layer with a high oxygen concentration and the base iron with a high alloy element concentration. However, when the Fe plating layer contains almost no oxygen, the alloying elements contained in the base iron pass through the Fe plating layer to form oxides on the surface. Then, when the temperature is raised, the oxygen in the Fe plating layer further depletes, and the diffusion of alloying elements in the base iron is further aggravated, so the formation of surface oxides increases.

[0075] Figure 2(c) shows a schematic cross-section of the base iron when the temperature is raised to 500 - 700°C in the same reducing atmosphere. During the heating process, it is advisable to control the dew point inside the annealing furnace to -15°C to +30°C. As the temperature rises, the Fe plating layer is sufficiently reduced and the oxygen concentration decreases, so the oxygen release rate slows down. On the other hand, the rate at which water vapor in the annealing furnace dissociates and diffuses into the steel increases significantly. Therefore, raising the dew point in the range of 500 - 700°C, which is lower than the temperature at which the Fe plating layer is completely reduced, can effectively suppress the diffusion of Mn and Si inside the steel through the Fe plating layer to the surface.

[0076] Figure 2(d) shows a schematic cross-section of the steel plate after maintaining it at a high temperature in the range of 600 - 950°C while adjusting the dew point to -15°C to +30°C. Inside the base iron, Mn and Si continuously diffuse. On the surface of the steel plate, oxygen supplied from water vapor rapidly penetrates and is supplied. Therefore, Mn and Si are oxidized inside. However, in the low-temperature range, particulate Mn and Si oxides generated by reacting with the oxygen in the Fe plating layer at the interface between the Fe plating layer and the base iron act as nuclei for oxide growth. Thus, the internal oxides concentrate and grow at the interface between the Fe plating layer and the base iron. Furthermore, since the oxygen diffusion rate is faster than that of Mn and Si with large atomic sizes, internal oxides are formed deeply not only along the grain boundaries but also through the grains inside the base iron.

[0077] As described above, the control conditions have been explained according to temperature. However, the most crucial stage in the annealing process is the stage of maintaining the steel plate temperature at 600 - 950°C. By simply controlling the dew point of the atmosphere in this temperature range, the internal oxide distribution of the steel plate for plating can be effectively controlled. Of course, such dew point control can be carried out in all processes before the above-mentioned maintenance stage without any particular problem. Furthermore, it should be noted that the above-described process is only an example for explaining an embodiment of the present invention, and the reaction mechanism of the present invention should not always be interpreted as being restricted by the above description.

[0078] After the annealing step, the annealed steel sheet can be cooled. Since the cooling conditions in the cooling step after the annealing step do not significantly affect the surface quality of the final product, i.e., the plating quality, there is no particular need to limit the cooling conditions in the present invention. However, in order to prevent oxidation of the iron component during the cooling process, a reducing atmosphere can be applied at least to iron.

[0079] According to an embodiment of the present invention, hot-dip galvanizing can be performed on the galvanizing steel sheet obtained by the above-described process to form a hot-dip galvanized layer. The hot-dip galvanizing method in the present invention is not particularly limited.

[0080] In addition, 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 galvanizing steel sheet or the hot-dip galvanized steel sheet according to the present invention, so the method for manufacturing the base iron does not need to be specifically limited.

[0081] In an 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 Fe plating layer formed by appropriately controlling the conditions of the electroplating solution and the plating conditions can be controlled.

[0082] That is, in the present invention, in order to form the Fe plating layer, an electroplating solution containing iron ions including ferrous ions and ferric ions, a complexing agent, and inevitable impurities can be used, and the concentration of ferric ions in the above iron ions is 5 to 60% by weight.

[0083] 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 drops sharply, which may induce quality variations in the continuous electroplating process. Therefore, the second iron ion can be further contained. At this time, the concentration of the second iron ion is preferably 5 to 60% by weight, more preferably 5 to 40% by weight, based on the total of the first and 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, the concentration of the second iron increases rapidly, and the plating efficiency continuously decreases while the pH rapidly decreases. On the other hand, when the concentration of the second iron ion exceeds 60%, the reaction amount of the second iron being reduced to the first iron at the cathode increases significantly more than the reaction amount of the first iron being reduced and deposited as metallic iron. Therefore, the plating efficiency decreases significantly and the plating quality deteriorates. Accordingly, considering the characteristics of the equipment and process, such as the plating amount, the working current density, the replenishment amount of the solution, the amount of the solution lost by adhering to the strip, and the rate of concentration change due to evaporation, it is preferable to make the concentration of the second iron ion in the iron ions 5 to 60% by weight.

[0084] 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 decrease sharply. On the other hand, when it exceeds 80 g / L, precipitation may occur due to exceeding the solubility, and the loss of raw materials due to the loss of the solution in the continuous plating process increases, which is not economical.

[0085] The electroplating solution of the present invention contains a complexing agent. In order to maintain a high plating efficiency without generating sludge while containing a large amount of the second iron, it is preferable to use an amino acid or an amino acid polymer as the complexing agent.

[0086] An amino acid is an organic molecule in which a carboxyl group (-COOH) and an amine group (-NH2) are bonded, and an amino acid polymer is an organic molecule formed by polymerizing two or more amino acids, and the amino acid polymer exhibits complexing properties similar to those of amino acids. Therefore, in the following description, amino acids and amino acid polymers are collectively referred to as amino acids.

[0087] When an amino acid is dissolved in neutral water, the amines combine with hydrogen ions to become positively charged, and the carboxyl groups become negatively charged as the hydrogen ions are dissociated, so that 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 become positively charged, so that the amino acid molecules form cations. In other words, amino acids become charge neutral or form cations in a weakly acidic aqueous solution.

[0088] 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 maintain their 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 is negatively charged in a weakly acidic aqueous solution.

[0089] 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, even when ferric ions are complexed, they can maintain their cationic state, so that 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 production slows down, so that even if plating is performed continuously for a long period of time, the ferric ion concentration remains at a constant level, plating efficiency remains constant, and there is no need to replace the electrolyte.

[0090] On the one hand, in the continuous electroplating process, when the iron ions in the solution are exhausted by electroplating, the solution becomes acidic. However, even when the same amount of iron ions is deposited, the change in pH is smaller in the solution containing both ferrous ions than in the solution containing only ferrous ions. When the pH increases, some ferric ions combine with hydroxide ions, and when the pH decreases, the hydroxide ions separate and are neutralized. Therefore, the solution containing ferric ions has a blunted pH change even without a separate pH buffer, serving as a pH buffer and enabling the electroplating efficiency in the continuous electroplating process to be maintained constantly.

[0091] 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 change in pH of the solution can be blunted, and the accumulation of ferric ions can be easily prevented, so that the electroplating efficiency and plating quality in the continuous electroplating process can be maintained constantly.

[0092] On the other hand, it is preferable that the complexing agent is added in an amount such that the molar concentration ratio of the iron ions to the complexing agent is 1:0.05 - 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 prevented from combining with hydroxide ions or oxygen to form sludge. Even without ferric ions, the electroplating efficiency decreases significantly, and furthermore, burning is induced and the plating quality deteriorates. In contrast, even if it exceeds 2.0, the sludge suppression effect and plating quality are maintained, but the overvoltage increases and the electroplating efficiency decreases, and relatively expensive amino acids are contained in excess more than necessary compared to raw materials containing iron ions such as iron sulfate, resulting in an increase in raw material costs and being uneconomical.

[0093] The 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.

[0094] Using the above amino acid as a complexing agent, while maintaining the solution temperature at 80 °C or lower and the pH at 2.0 to 5.0, an electric current density of 3 to 120 A / dm 2 When electroplating is carried out at, a high plating efficiency can be obtained, and an Fe plating layer with a high oxygen concentration can be obtained.

[0095] The temperature of the Fe electroplating solution does not significantly affect the quality of the Fe plating layer. However, when it exceeds 80 °C, the evaporation of the solution becomes intense, the concentration of the solution continuously changes, and it becomes difficult to perform uniform electroplating.

[0096] When the pH of the Fe electroplating solution is less than 2.0, the electroplating efficiency decreases and it is not suitable for the continuous plating process. When the pH exceeds 5.0, although the plating efficiency increases, sludge in which iron hydroxide precipitates is generated during continuous electroplating, causing problems such as pipe blockage, contamination of rolls and equipment.

[0097] When the current density is less than 3 A / dm 2 the cathodic plating overvoltage decreases and the Fe electroplating efficiency decreases, so it is not suitable for the continuous plating process. When it exceeds 120 A / dm 2 burning occurs on the plating surface, the electroplating layer is non-uniform, and the problem that the Fe plating layer is likely to fall off occurs.

[0098] As described above, in the present invention, it is preferable that the Fe plating layer contains 5 to 50% by weight of oxygen. The reasons for oxygen to be mixed into the Fe plating layer are as follows. During the process of iron 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 be temporarily combined with OH - ions and may be mixed together when the Fe plating layer is formed. If an anionic complexing agent such as acetic acid, lactic acid, citric acid, or EDTA is used, the complexing agent reacts with OH -Iron ions combined with ions generally become negatively charged on average. When a cathode is applied for electroplating, an electrical repulsive force is generated, suppressing the mixing into the Fe plating layer. On the other hand, amino acids are electrically neutral at pH 2.0 - 5.0. In strong acids with a pH less than 2.0, they carry cations, but even if 1 - 2 OH - are combined with the iron ions bound to the amino acids and still carry cations, an electrical attraction to the cathode for electroplating occurs, resulting in a large amount of oxygen being mixed in. Therefore, when using an amino acid as a complexing agent with the molar concentration ratio of iron ions to amino acids being 1:0.05 - 1:2.0 and maintaining a pH of 2.0 - 5.0 to perform Fe electroplating, the plating efficiency is high, and an Fe plating layer containing 5 - 50 wt% oxygen can be obtained while suppressing the generation of sludge.

[0099] To ensure the quality of hot-dip galvanized steel sheets containing Mn and Si, it is advisable to treat the plating amount of the Fe plating layer based on the iron concentration at 0.5 - 3.0 g / m 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 rate decreases, which is not economical. Furthermore, when the amount of Fe electroplating increases, the Fe electroplating solution rapidly denatures in a continuous process, the pH decreases, the plating efficiency significantly decreases, and it becomes difficult to manage the solution. 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 it is not possible to effectively suppress the diffusion of Mn and Si from the base iron to form surface oxides, resulting in a problem of deterioration of the hot-dip plating quality. The above Fe plating amount has a thickness of approximately 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: 1.0 to 8.0%, Si: 0.05 to 3%, C: 0.06 to 0.4%, Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, N: 0.01% or less, Cr: 1.5% or less, B: 0.005% or less, with the balance being Fe and inevitable impurities, The GDS profile of the Mn component and the GDS profile of the Si component observed in the depth direction from the surface each sequentially include a maximum point and a minimum point, The difference (converted Mn concentration difference) between the value obtained by dividing the Mn concentration at the maximum point of the GDS profile of the Mn component by the Mn concentration of the base material and the value obtained by dividing the Mn concentration at the minimum point of the GDS profile of the Mn component by the Mn concentration of the base material is 80% or more, The difference (converted Si concentration difference) between the value obtained by dividing the Si concentration at the maximum point of the GDS profile of the Si component by the Si concentration of the base material and the value obtained by dividing the Si concentration at the minimum point of the GDS profile of the Si component by the Si concentration of the base material is 50% or more, a steel sheet. However, when no minimum point appears within a depth of 5 μm, the point at a depth of 5 μm is regarded as the point where the minimum point appears.

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

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

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

5. A hot-dip galvanized steel sheet including 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: 1.0 to 8.0%, Si: 0.05 to 3%, C: 0.06 to 0.4%, Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, N: 0.01% or less, Cr: 1.5% or less, B: 0.005% or less, with the balance being Fe and inevitable impurities; Performing electroplating on the base iron to form an Fe plating layer containing 5 to 50% by weight of oxygen; 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 dew point temperature controlled at -15 to +30 °C in a 1 to 70% H ２ - the remaining N ２ gas atmosphere, and then annealed. A method for manufacturing a steel sheet, which includes the above steps.

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 complexing agent is one or more selected from alanine, glycine, serine, threonine, arginine, glutamine, glutamic acid, and glycylglycine, the method for manufacturing the steel sheet according to claim 6 or 7.

9. The electroplating solution contains ferrous ions and ferric ions, the ferric ions having a proportion of 5 to 60% by weight based on the total amount of iron ions, and the total concentration of iron ions being 1 to 80 g per liter of the electroplating solution. The method for manufacturing a steel sheet according to claim 6 or 7.

10. The electroplating is carried out under the conditions of a solution temperature of 80°C or lower and a current density of 3 to 120 A / dm ２ The method for manufacturing a steel sheet according to claim 6 or 7, which is carried out under the conditions of ２ .

11. Preparing a base iron having a composition containing, in % by weight, Mn: 1.0 to 8.0%, Si: 0.05 to 3%, C: 0.06 to 0.4%, Al: 0.005 to 3.0%, P: 0.04% or less, S: 0.015% or less, N: 0.01% or less, Cr: 1.5% or less, B: 0.005% or less, with 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; The substrate iron on which the Fe plating layer is formed is controlled at a dew point temperature of -15 to +30 °C in a 1 to 70% H ２ - The remaining N ２ Maintaining and annealing at 600 to 950 °C for 5 to 120 seconds in an annealing furnace in a gas atmosphere to obtain a steel sheet for plating; Immersing the plated steel sheet in a zinc plating bath. A method for manufacturing a hot-dip galvanized steel sheet, comprising the steps.

Images