Steel sheet having excellent plating quality and method for manufacturing same

Abstract

The present invention relates to a hot-dip galvanized steel sheet having excellent plating quality, a steel sheet for plating used for manufacturing the hot-dip galvanized steel sheet, and methods for manufacturing the hot-dip galvanized steel sheet and the steel sheet for plating. A steel sheet for plating according to one aspect of the present invention has a composition containing, in wt%, 0.1-1.0% of Mn, 0.1% or less of Si (excluding 0%), 0.0005-0.03% of C, 0.005-3.0% of Al, 0.04% or less of P, 0.015% or less of S, 1.5% or less of Cr, and 0.005% or less of B, with the balance being Fe and unavoidable impurities, and the GDS curve of the Mn component and the GDS curve of the Si component, as viewed from the surface in the depth direction, each include a maximum point and a minimum point in this order. The difference (Mn conversion concentration difference) between the value obtained by dividing the Mn concentration at the maximum point of the GDS curve 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 curve of the Mn component by the Mn concentration of the base material may be 10% or more. The difference (Si conversion concentration difference) between the value obtained by dividing the Si concentration at the maximum point of the GDS curve 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 curve of the Si component by the Si concentration of the base material may be 10% or more. Wherein, when there is no minimum point at a depth within 5 [mu] m, the position at which the depth is 5 [mu] m is used as the position at which the minimum point appears.

Description

Technical Field

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

[0002] In automobile outer panels, the surface is generally hot-dip galvanized to ensure corrosion resistance, but there is a risk that problems may arise in the galvanizing properties for the following reasons.

[0003] That is, the quality of hot-dip galvanizing is determined by the surface state of the annealed steel sheet before plating. The elements such as Mn, Si, Al, Cr, B, etc. added to ensure the physical properties of the steel sheet cause the formation of surface oxides during the annealing process, thereby deteriorating the plating property. That is, during the annealing process, the elements diffuse to the surface and react with the trace amount of oxygen or water vapor present in the annealing furnace to form a single oxide or a composite oxide of the elements on the surface of the steel sheet, thereby reducing the reactivity of the surface. The annealed steel sheet surface with reduced reactivity will hinder the wettability of the hot-dip galvanizing bath, resulting in unplated areas where the plating metal is not attached locally or entirely on the surface of the plated steel sheet. In addition, due to these oxides, the formation of the alloying inhibition layer (Fe2Al5) necessary to ensure the adhesion of the coating during the hot-dip plating process is insufficient, thereby greatly reducing the plating quality of the plated steel sheet, such as the occurrence of coating peeling.

[0004] Various technologies for improving the plating quality of hot-dip plated steel sheets have been proposed. Among them, Patent Document 1 proposes a technology that controls the air-fuel ratio of air to fuel to 0.80 to 0.95 during annealing, oxidizes the steel sheet in a direct flame furnace in an oxidizing atmosphere, thereby forming iron oxides containing a single oxide or a composite oxide of Si, Mn or Al at a predetermined depth inside the steel sheet, and then performs reduction annealing of the iron oxide in a reducing atmosphere, and then performs hot-dip galvanizing, thereby providing a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet with excellent plating quality.

[0005] As in Patent Document 1, when a method of performing oxidation followed by reduction in an annealing process is used, components with a high affinity for oxygen, such as Si, Mn, and Al, are internally oxidized at a predetermined depth from the surface of the steel sheet, thereby suppressing diffusion to the surface layer, and thus the single oxide or composite oxide of Si, Mn, or Al on the surface layer is relatively reduced, thereby improving wettability with zinc, and thus reducing unplated areas. However, in the case of a steel type to which Si is added, Si is enriched just below the iron oxide during the reduction process, thereby forming a band-shaped Si oxide, and thus peeling occurs in the surface layer portion including the plated layer, that is, peeling occurs at the interface between the reduced iron and the base iron below, and thus there is a problem that it is difficult to ensure the adhesion of the plated layer.

[0006] In addition, as another method for improving the coating property of hot-dip plated steel sheets, a method is proposed in Patent Document 2, wherein a high dew point is maintained in the annealing furnace, thereby internally oxidizing easily oxidizable alloy components such as Mn, Si, and Al inside the steel, thereby reducing the oxides externally oxidized on the surface of the steel sheet after annealing to improve the coating property. However, the method according to Patent Document 2 can solve the coating property problem caused by the external oxidation of Si that is easily internally oxidized, but when a large amount of Mn that is relatively difficult to internally oxidize is added, there is a problem that the effect is minimal.

[0007] In addition, even if the plating property is improved by internal oxidation, linear non-plating may occur due to uneven surface oxides formed on the surface, or when alloyed hot-dip galvanized steel sheet (GA steel sheet) is manufactured by alloying heat treatment after plating, linear defects caused by uneven alloying may occur on the surface of the alloyed hot-dip galvanized steel sheet.

[0008] Another prior art is to perform Ni pre-plating before annealing to inhibit the diffusion of alloy elements to the surface during annealing. However, although this method is effective in inhibiting the diffusion of Mn, it has the problem of not being able to fully inhibit the diffusion of Si.

[0009] [Prior art literature]

[0010] [Patent Document]

[0011] (Patent Document 1) Korean Patent Publication No. 2010-0030627

[0012] (Patent Document 2) Korean Patent Publication No. 2009-0006881 Summary of the invention

[0013] 1. Technical issues to be resolved

[0014] According to one aspect of the present invention, there is provided a steel sheet for automobile outer panels having excellent plating quality with no unplated coating and no peeling of the plating layer, and a method for producing the same.

[0015] According to another aspect of the present invention, a hot-dip galvanized steel sheet for automobile outer panels and a method for manufacturing the same are provided, wherein the hot-dip galvanized steel sheet for automobile outer panels does not produce linear defects even when alloying heat treatment is performed after plating, thereby being able to produce an alloyed hot-dip galvanized steel sheet with excellent surface quality.

[0016] According to another aspect of the present invention, there is provided a steel sheet for plating and a method for manufacturing the same, the steel sheet for plating being capable of manufacturing a hot-dip galvanized steel sheet having excellent plating quality as described above.

[0017] The technical problems of the present invention are not limited to the above contents. Those skilled in the art to which the present invention belongs can easily understand the additional technical problems of the present invention from the overall content of this specification.

[0018] (II) Technical solution

[0019] In the plated steel sheet according to one aspect of the present invention, the composition may include, in weight %, Mn: 0.1-1.0%, Si: 0.1% or less (except 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, the balance Fe and inevitable impurities, and the GDS curve of the Mn component and the GDS curve of the Si component observed in the depth direction from the surface may be respectively in the order of 1:1. Including a maximum point and a minimum point, the difference between the value of the Mn concentration at the maximum point of the GDS curve of the Mn component divided by the Mn concentration of the base material and the value of the Mn concentration at the minimum point of the GDS curve of the Mn component divided by the Mn concentration of the base material (the converted concentration difference of Mn) can be 10% or more, and the difference between the value of the Si concentration at the maximum point of the GDS curve of the Si component divided by the Si concentration of the base material and the value of the Si concentration at the minimum point of the GDS curve of the Si component divided by the Si concentration of the base material (the converted concentration difference of Si) can be 10% or more.

[0020] When no local point appears at a depth of 5 μm or less, the position at a depth of 5 μm is regarded as the position at which the local point appears.

[0021] A hot-dip galvanized steel sheet according to another aspect of the present invention may include the above-mentioned steel sheet for plating and a hot-dip galvanized layer formed on the steel sheet for plating.

[0022] A method for manufacturing a plated steel sheet according to another aspect of the present invention may include the following steps: preparing a base iron having a composition, in weight %, of Mn: 0.1-1.0%, Si: 0.1% or less (except 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, the balance of Fe and inevitable impurities; electroplating the base iron to form a Fe coating containing 5-50 weight % of oxygen; and annealing the base iron having the Fe coating at 600-950° C. for 5-120 seconds in an annealing furnace having an atmosphere of 1-70% H2 and balance N2 gas with a dew point temperature controlled at less than -20° C.

[0023] A method for manufacturing a hot-dip galvanized steel sheet according to another aspect of the present invention may include the following steps: preparing a base iron having a composition, in weight %, of Mn: 0.1-1.0%, Si: 0.1% or less (except 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, the balance of Fe and inevitable impurities; electroplating the base iron to form a Fe coating containing 5-50 weight % of oxygen; annealing the base iron having the Fe coating at 600-950° C. for 5-120 seconds in an annealing furnace having an atmosphere of 1-70% H2 and balance N2 gas with a dew point temperature controlled at less than -20° C., thereby obtaining a plated steel sheet; and immersing the plated steel sheet in a galvanizing bath.

[0024] (III) Beneficial effects

[0025] As described above, in the present invention, by forming a pre-plating layer and controlling the concentration profile of the Mn and Si components inside, a hot-dip galvanized steel sheet can be provided which significantly improves the non-plating phenomenon occurring during hot-dip galvanizing and improves the plating adhesion.

[0026] In addition, according to one aspect of the present invention, even if the hot-dip galvanized steel sheet of the present invention is subjected to alloying heat treatment, linear defects on the surface of the obtained alloyed hot-dip galvanized steel sheet can be prevented, thereby providing an alloyed hot-dip galvanized steel sheet with excellent surface quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a schematic diagram of a GDS curve of Mn and Si components measured after removing the coating of a hot-dip galvanized steel sheet produced by Fe-plated cold-rolled steel sheet.

[0028] Figure 2 It is a schematic diagram showing a process of annealing a base steel sheet on which an oxygen-containing Fe plating layer is formed. DETAILED DESCRIPTION

[0029] Hereinafter, a hot-dip galvanized steel sheet having excellent plating quality in one aspect of the present invention completed by the inventor's research is described in detail. It should be noted that in the present invention, when expressing the concentration of each element, it is expressed by weight % unless otherwise defined. In addition, the Fe electroplating amount is a plating amount measured as the total amount of Fe contained in the plating layer per unit area, and the plating amount does not include oxygen and unavoidable impurities in the plating layer.

[0030] In addition, unless otherwise defined, the concentrations and concentration curves mentioned in the present invention refer to the concentrations and concentration curves measured using a GDS (Glow discharge optical emission spectrometer).

[0031] Hereinafter, the present invention will be described in detail.

[0032] It is known that the cause of non-plating and reduced plating adhesion in steel sheets containing Mn and Si is due to the oxidation of alloying elements such as Mn and Si on the surface during annealing of the cold-rolled steel sheet at high temperature to form surface oxides.

[0033] As a method for 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, a redox method in which oxidation is performed during the heating process and then the alloy is reduced again in a reducing atmosphere, or a method in which iron oxide is coated on the surface of the substrate metal and then heat-treated, etc. can be used. However, the iron oxide firmly formed on the surface of the substrate metal contains not only FeO but also a mixture of Fe3O4 and Fe2O3 that are difficult to reduce, and the surface is reduced to metallic iron during annealing in a reducing atmosphere, and the reduction rate at the interface between the iron oxide layer and the substrate iron is slow, so it is difficult to completely reduce, and Mn and Si oxides accumulate at the interface to form a continuous oxide layer. Therefore, although the wettability with molten zinc is improved, the oxide layer is easily broken, and the problem of plating peeling may occur.

[0034] In addition, when the annealing internal oxidation method is applied in which alloying elements such as Mn and Si are oxidized inside the steel by increasing the oxygen partial pressure or dew point in the annealing furnace during the heat treatment, Mn and Si oxides are preferentially formed on the surface of the steel during the heat treatment, and then Mn and Si are oxidized by oxygen diffused into the steel, thereby inhibiting surface diffusion. Therefore, a thin oxide film is formed on the surface of the base iron, and if the surface of the cold-rolled steel sheet before annealing is not completely homogeneous or there are local deviations in the oxygen partial pressure, temperature, etc., wettability is uneven during hot-dip galvanizing, resulting in non-plating, or if a difference in the degree of alloying occurs during the alloying heat treatment after galvanizing due to uneven thickness of the oxide film, there is a tendency to cause linear defects that can be easily recognized by visual inspection.

[0035] In order to solve the above-mentioned technical problems, the present inventors attempted to manufacture a hot-dip galvanized steel sheet having a fine surface and free from the problem of plating peeling by controlling the existing forms of Mn and Si as oxidizing elements on the surface side of the steel sheet for plating as follows.

[0036] That is, the GDS concentration curve of Mn and Si of the steel sheet according to one specific embodiment of the present invention may have the following characteristics. Figure 1 The GDS curve of the present invention is used to explain the steel sheet for coating in detail.

[0037] Figure 1 The figure schematically shows a typical GDS curve of alloy components that may appear on the surface after the zinc coating layer is removed from a hot-dip galvanized steel sheet including the steel sheet of the present invention and a GDS curve of alloy components that are not within the scope of the present invention. In the figure, the vertical axis represents the concentration of alloy elements such as Mn and Si, and the horizontal axis represents the depth.

[0038] according to Figure 1 From the typical example of the GDS curve of the Mn component of the present invention shown, it can be seen that the Mn concentration on the surface of the steel plate of the present invention is very low, and can have a concentration gradient in the form of a maximum point and a minimum point appearing in sequence from the surface in the depth direction. Among them, appearing in sequence does not mean that the maximum point must appear first from the surface (interface) toward the depth direction, but it means that in some cases the minimum point may appear first, but then the maximum point and the minimum point should appear in sequence. However, in some specific embodiments, the minimum point may not appear. In this case, the internal concentration of the area with a depth of 5μm can be used as the minimum point concentration. In addition, 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.

[0039] In the Figure 1 In the illustrated GDS concentration curve, although not necessarily limited to this, the surface layer portion corresponds to the Fe plating where the alloy element does not diffuse in large quantities from the base iron and the concentration of the alloy element is low, the maximum point corresponds to the region where the internal oxide of the alloy element formed near the interface between the Fe plating and the base iron is concentrated, and the minimum point appearing on the base iron side in the Fe plating corresponds to the region where the alloy element diffuses into the Fe plating containing no alloy element and is diluted, or the alloy element diffuses to the maximum point where internal oxidation occurs and is depleted.

[0040] In a specific embodiment of the present invention, the maximum point can be formed at a depth of 0.05-1.0 μm from the surface of the steel plate. When the maximum point appears in an area deeper than this depth, it may not be judged as a maximum point brought about by the effect of the present invention. In addition, the minimum point can be formed at a depth within 5 μm from the surface of the steel plate. As described above, when the minimum point is not formed at a depth within 5 μm, the depth of 5 μm can be used as the position where the minimum point is formed. The concentration at a depth of 5 μm is substantially the same as that of the base material, so it can be regarded as a position where the concentration no longer decreases.

[0041] At this time, in the Mn concentration curve and the Si concentration curve, the greater the difference between the converted concentration of the maximum point of the corresponding element (the value of the concentration at the corresponding position divided by the concentration of the base material, expressed in %) and the converted concentration of the minimum point, the less Mn and Si diffused to the surface can be, so it is very important. In a specific embodiment of the present invention, the maximum point converted concentration-minimum point converted concentration values ​​of Mn and Si can be 10% or more respectively.

[0042] The results of the experiments conducted by the inventors under various conditions show that when the above conditions are met, a hot-dip galvanized steel sheet with good coating adhesion and no uncoating during hot-dip galvanizing can be obtained. However, when the difference between the maximum and minimum conversion concentrations of Mn and Si is less than 10%, there may be a problem of point-like uncoating or linear uncoating or coating peeling. That is, by controlling the difference between the conversion concentrations to a certain level or above, it is possible to prevent the generation of Mn and Si oxides on the surface, thereby manufacturing a hot-dip galvanized steel sheet with a beautiful surface and good coating adhesion, and even after subsequent alloying heat treatment processes, defects such as linear defects on the surface can be suppressed. The greater the difference between the conversion concentration values, the more favorable it is, so there is no need to set an upper limit for the value. However, considering the content of the elements contained, in the case of Mn and Si, the difference between the conversion concentration values ​​can be set to less than 200%. In another specific embodiment of the present invention, the difference between the conversion concentration values ​​of Mn and Si can be set to more than 15% or more than 20%.

[0043] Hereinafter, the GDS analysis method performed in the present invention will be described in detail.

[0044] For GDS concentration analysis, the hot-dip galvanized steel sheet was cut into a size of 30-50 mm in length and immersed in a 5-10 wt% hydrochloric acid aqueous solution at a normal temperature of 20-25°C to remove the galvanized layer. In order to prevent damage to the surface of the base iron during the dissolution of the galvanized layer, when the bubbles generated by the reaction of the galvanized layer and the acid solution stopped, the acid solution was removed within 10 seconds, and the base iron was washed with pure water and dried. If it is a plated steel sheet that has not been hot-dip galvanized, the analysis can be performed without the operation of removing the coating as described above.

[0045] For the GDS concentration curve, the concentrations of all components contained in the steel plate are measured every 1-5 nm in the thickness direction of the steel plate. The measured GDS curve may contain irregular noise. In order to calculate the maximum and minimum points of Mn and Si concentrations, a Gaussian filter with a cutoff value of 100 nm is applied to the measured concentration curve to obtain an average concentration curve, and the concentration values ​​and depths of the maximum and minimum points of the concentration are obtained from the noise-removed curve. In addition, it should be noted that the maximum and minimum points mentioned in the present invention are calculated as maximum and minimum points only when they have a difference of more than 10 nm from each other in the depth direction.

[0046] In the present invention, the target steel sheet for plating 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.

[0047] Among them, if it is a steel sheet containing 0.1-1.0 wt % of Mn and 0.1 wt % or less (except 0%) of Si and having a composition that is easy to generate oxides on the surface, the present invention can be advantageously improved in plating properties. The upper limit of the Mn concentration of the base iron is not particularly limited, but considering the commonly used composition, the upper limit of the Mn concentration of the base iron can be limited to 1.0 wt %. In addition, the lower limit of the Mn concentration is not particularly limited, but in the case of a composition containing less than 0.1 wt %, even if the Fe coating is not formed, the hot-dip galvanized steel sheet has a fine surface quality, so it is not necessary to perform Fe electroplating. In the component system of the present invention, Si is an element that may contribute to the formation of oxides and cause the plating quality to deteriorate even if a small amount is added. Considering the strict surface quality requirements for the outer panel of the automobile, the upper limit of the Si content can be set to 0.1 wt %. From the perspective of forming oxides only, it is more advantageous when Si is not added, so there is no need to set the lower limit of the Si content, but considering the restrictions in the industrial production process, the lower limit of the Si content can be limited to 0.001%.

[0048] The Mn and Si are elements that affect the plating properties, and thus the concentrations of the Mn and Si may be limited as described above, but the remaining components of the base material iron are not particularly limited in the present invention.

[0049] However, considering that a serious reduction in unplated and plated adhesion may occur in the case of a steel sheet containing an alloy component, in a specific embodiment of the present invention, the composition of the base iron may include, by weight%, Mn: 0.1-1.0%, Si: 0.1% or less (except 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, the balance of Fe and inevitable impurities. P and S are impurities, and it is more advantageous when they are not added. Cr and B are optional elements, and it is not a problem even if they are not added, so no lower limit is set separately. In addition to the above-mentioned components, the base iron may further contain elements such as Ti, Mo, Nb, etc. in a total of 1.0% or less. In addition, in another specific embodiment of the present invention, the C component may be further limited to less than 0.02%, and in another specific embodiment, the C component may be further limited to less than 0.0199%. The base iron is not particularly limited, but in one embodiment of the present invention, a cold-rolled steel sheet or a hot-rolled steel sheet may be used as the base iron.

[0050] In one aspect of the present invention, a hot-dip galvanized steel sheet including the plated steel sheet may be provided, wherein the hot-dip galvanized steel sheet may include the plated steel sheet and a hot-dip galvanized layer formed on the surface of the plated steel sheet. At this time, as the hot-dip galvanized steel sheet, any type of hot-dip galvanized steel sheet may be applied as long as it is a commonly used hot-dip galvanized steel sheet, and its type is not particularly limited.

[0051] Next, an exemplary specific embodiment of a method for manufacturing a plated steel sheet and a hot-dip galvanized steel sheet having the above-mentioned advantageous effects is described. According to a specific embodiment of the present invention, the plated steel sheet can be manufactured by a process comprising the following steps: preparing a base iron; electroplating the base iron to form an Fe plating layer containing 5-50 wt % of oxygen; annealing the base iron with the Fe plating layer formed thereon to obtain a plated steel sheet.

[0052] A cold rolled steel sheet containing 0.5% Mn, 0.05% Si and other alloying elements was electroplated with Fe containing 6.5 wt% oxygen to obtain an iron deposition amount of 1.97 g / m 2Then, the electroplated cold-rolled steel sheet was annealed for 53 seconds in an atmosphere of N2 and 5% H2, a dew point of -40°C, and a temperature of 800°C, and cooled. The same atmosphere was maintained throughout the annealing process, and when a sample was collected from the cooled steel sheet and the cross section was observed using a transmission electron microscope, it was confirmed that granular Mn and Si oxides were formed at the interface between the iron electroplating layer and the base iron, and on the other hand, almost no Mn and Si oxides were formed on the surface of the electroplating layer. In particular, when this state was analyzed using a GDS curve, as shown in FIG. Figure 1 As shown in FIG. 1 , the maximum values ​​of Mn and Si concentrations may appear just below the surface of the steel plate (including the Fe coating), and the minimum values ​​of Mn and Si concentrations may appear at a deeper position. 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.

[0053] This phenomenon is because the Fe plating layer with a high oxygen content is formed before annealing. That is, when the Fe electroplating layer contains 5-50% by weight of oxygen, when annealing is performed in an annealing furnace in a reducing atmosphere, the oxygen in the Fe electroplating layer will oxidize the alloying elements such as Mn and Si that diffuse from the base iron to the surface and accumulate at the interface between the Fe electroplating layer and the base iron. Therefore, if Figure 1 As shown in the figure, when using GDS to measure concentration, the high maximum point of the concentration of Mn, Si, etc. is confirmed at the depth of the thickness of the iron electroplating layer equivalent to the distance surface. In addition, with regard to the alloying elements with slow diffusion speeds such as Mn, even if its concentration is diluted by the iron electroplating layer, or the Mn of solid solution is exhausted due to internal oxidation, it is also impossible to diffuse rapidly from the substrate iron, so there can be a minimum point after the GDS concentration maximum point. However, Si diffuses rapidly from the inside during annealing, and continues to be internally oxidized at the interface of the iron electroplating layer and the substrate iron, so that the accumulation of oxide occurs, so the minimum point may not be confirmed in the GDS concentration analysis. Therefore, the absence of a minimum point in the GDS concentration curve means that the alloying elements such as Mn, Si are oxidized by the iron electroplating layer and effectively suppress the diffusion to the surface.

[0054] Unlike oxides firmly formed at high temperatures, when an iron coating containing a large amount of oxygen is formed and annealed, reduction occurs not only on the surface of the iron coating, but also at the interface between the iron coating and the base steel sheet. At this time, the reduced iron diffuses and bonds with the base iron, and Mn and Si react with the oxygen present in the Fe coating to form granular or discontinuous flaky oxides at the interface between the iron coating and the base steel sheet, thereby maintaining good adhesion between the iron coating and the base metal. In addition, the uniformly formed iron coating inhibits the formation of surface oxides and reduces the concentration of alloying elements such as Mn and Si dissolved in the steel sheet surface, thereby promoting alloying reaction with the zinc coating, so that a uniform alloyed hot-dip plated steel sheet without surface defects can be obtained.

[0055] Unlike the redox method, the annealing internal oxidation method does not form a layered oxide layer, so it exhibits excellent characteristics in improving the coating adhesion when hot-dip galvanizing steel sheets containing alloying elements such as Mn and Si. However, since the water vapor in the annealing furnace will inevitably oxidize the surface of the steel sheet first, and then oxygen will penetrate into the interior, it is impossible to fundamentally remove the surface oxide. As a result, if the surface of the cold-rolled steel sheet is not completely homogeneous before annealing or if there is a local deviation in the oxygen partial pressure, temperature, etc. during annealing, the wettability with the hot-dip galvanizing solution is uneven, resulting in non-plating, or in the alloying heat treatment process after galvanizing, the thickness of the oxide film is uneven, resulting in a difference in the degree of alloying, which may cause linear defects that can be easily recognized by visual inspection.

[0056] In order to suppress the surface diffusion of alloy elements by the Fe plating layer containing a large amount of oxygen and to produce a fine hot-dip galvanized steel sheet without the problem of plating peeling, it is preferred to use 0.5-3.0 g / m based on the iron adhesion amount. 2 An Fe plating amount of Fe is formed to form an Fe plating layer to form an iron electroplating layer containing 5-50 wt % of oxygen on the base iron, and the temperature is raised to 600-950° C. to ensure the mechanical physical properties of the steel sheet, and then cooled again and hot-dip plated.

[0057] In a specific embodiment of the present invention, the Fe coating can be formed by a continuous coating process. At this time, based on the Fe adhesion amount, the Fe coating amount can be 0.5-3.0 g / m 2 When the Fe coating amount is less than 0.5g / m 2 When the Fe coating is more than 3.0 g / m, the effect of Fe coating in suppressing the diffusion of alloy elements in the conventional continuous annealing process may be insufficient. 2, and the effect of inhibiting alloying elements can be further improved. However, in order to ensure a high plating amount, multiple plating tanks need to be operated. When an insoluble anode is used, the electroplating solution will quickly acidify, thereby reducing the plating efficiency, and there is a problem of sludge generation, so it is not economical. In another specific embodiment of the present invention, the Fe plating amount can be 1.0-2.0 g / m 2 When internal oxidation is performed after the Fe coating is formed, internal oxides are formed at the interface between the Fe coating and the substrate iron or just below the interface, so the maximum points of Mn and Si concentrations exist in the region of 0.05-1.0 μm. 2 The Fe plating amount may correspond to a thickness of 0.05-0.4 μm after annealing.

[0058] In addition, for the above-mentioned Fe coating with a high oxygen concentration, by controlling the temperature, dew point temperature and atmosphere of the subsequent annealing process, a maximum point and a minimum point can be formed in the GDS concentration curve of the Mn and Si elements inside the plated steel sheet, and the converted concentration at the maximum point and the converted concentration at the minimum point can meet the numerical range limited in a specific embodiment of the present invention. Taking this into account, in a specific embodiment of the present invention, the oxygen concentration in the Fe coating can be 5-50% by weight, and in another specific embodiment, the oxygen concentration in the Fe coating can be 10-40% by weight. In order to obtain the effect of suppressing surface oxides, the oxygen content in the Fe coating needs to be sufficiently high. Even if the oxygen concentration in the Fe coating is less than 5% by weight, the effect of suppressing surface oxides can be obtained by increasing the Fe coating amount, but in order to obtain the above-mentioned effect, it is necessary to carry out more than 3.0 g / m 2 of plating, so the above-mentioned various problems may occur. In addition, when the oxygen content does not reach 5 wt %, it is difficult to form a maximum point and a minimum point in the GDS curve of Mn and Si in sequence, so in a specific embodiment of the present invention, the oxygen content in the Fe coating is controlled to be more than 5 wt %. In addition, with the increase of the oxygen concentration in the Fe coating, the effect of suppressing surface oxides in the annealing process can be further improved, but it is difficult to obtain a coating with an oxygen content exceeding 50 wt % using a conventional electroplating method, so the upper limit of the oxygen content can be limited to 50 wt %. In another specific embodiment of the present invention, the oxygen concentration in the Fe coating can be limited to 10-40%.

[0059] In a specific embodiment of the present invention, the annealing temperature can be 600° C. to 950° C. based on the temperature of the steel sheet in the soaking zone. When the annealing temperature is too low, the structure of the cold-rolled steel sheet cannot be properly restored and recrystallized, making it difficult to ensure the strength, elongation and other mechanical and physical properties of the steel sheet. When the annealing temperature exceeds 950° C., the alloy elements in the steel diffuse rapidly to the surface, resulting in poor quality of hot-dip galvanizing, and unnecessary operation at high temperature, which is uneconomical.

[0060] In addition, in a specific embodiment of the present invention, the dew point inside the annealing furnace may be lower than -20°C, but is not necessarily limited thereto. When the dew point temperature is maintained at less than -20°C, a separate humidifying device for increasing the dew point is not required, so it is economical. Moreover, in the case of the present invention, since an Fe coating with a high oxygen concentration is formed, even if internal oxidation caused by the atmosphere is not induced, alloy elements such as Mn and Si can be sufficiently prevented from diffusing to the surface. The lower limit of the dew point temperature is not specifically set. However, maintaining the dew point below -90°C may be industrially disadvantageous, for example, requiring the use of very high purity gas, etc., so in consideration of this, the lower limit of the dew point may be set to -90°C. According to another specific embodiment of the present invention, when the temperature of the steel plate is 600-950°C, the dew point may be -70°C to -30°C.

[0061] In addition, in order to prevent oxidation of the substrate iron and Fe coating during annealing, the hydrogen concentration in the atmosphere gas during annealing can be set to more than 1% by volume. When the hydrogen concentration is less than 1%, the trace oxygen inevitably contained in H2 and N2 gases cannot be effectively oxidized and removed, resulting in an increase in the oxygen partial pressure, which may cause surface oxidation of the substrate iron. In addition, when the hydrogen concentration exceeds 70%, there is a risk of explosion when the gas leaks, and high hydrogen operation causes increased costs, so the hydrogen concentration can be set to less than 70%. In addition to the hydrogen (H2), the gas other than the impurity gas inevitably contained can be nitrogen (N2) in essence.

[0062] In addition, according to a specific embodiment of the present invention, during annealing, the holding time after reaching the target temperature can be limited to 5-120 seconds. During annealing, in order to fully transfer heat to the inside of the base material iron to obtain uniform mechanical and physical properties in the thickness direction, it is necessary to maintain the annealing target temperature for more than 5 seconds. In addition, when the high-temperature annealing holding time is too long, the diffusion of the alloy interfering elements through the Fe coating increases, resulting in an increase in the amount of surface oxides, thereby deteriorating the quality of hot-dip galvanizing, so the holding time can be limited to less than 120 seconds.

[0063] Below, based on the above content, refer to Figure 2The effect of suppressing the surface diffusion of Mn and Si during annealing of a cold-rolled steel sheet having an Fe-plated layer containing a large amount of oxygen in a high dew point atmosphere will be described in more detail.

[0064] Figure 2 The phenomenon occurring inside the steel plate when the temperature of the steel plate is increased according to the conditions of the present invention is schematically shown.

[0065] Figure 2 (a) shows a schematic diagram of a cross section of a base steel plate formed with an Fe plating layer containing a large amount of oxygen. The base steel plate contains alloy elements such as Mn and Si, and the Fe plating layer contains 5-50 wt % of oxygen and impurities inevitably mixed in during the plating process, and the balance is composed of Fe.

[0066] Figure 2 (b) shows the state where the cold-rolled steel sheet electroplated with Fe is heated to about 300-500°C in a nitrogen atmosphere containing 1-70% H2. The surface of the Fe coating is gradually reduced, thereby removing oxygen, and internal oxides of Mn, Si, etc. diffused from the base iron begin to form at the interface between the Fe coating and the base iron, and as the temperature rises, the oxides at the grain boundaries grow into coarse oxides.

[0067] Figure 2 (c) shows a schematic diagram of a cross section of a base steel plate when heated to 500-700°C in the same reducing atmosphere. The Fe coating is almost mostly reduced to form ferrite with a lower Mn and Si concentration than the base iron, and the oxygen in the Fe coating is gradually depleted, so Mn and Si begin to penetrate the Fe coating and slowly diffuse to the surface of the Fe coating.

[0068] Figure 2 (d) shows a schematic diagram of the cross section of a steel plate annealed at a temperature of 600-950°C. In the iron electroplating layer, except for the internal oxides of Mn, Si, etc., the oxygen dissolved in the metal iron is completely removed, and the generated internal oxides are generally spherical or short flakes. In addition, due to the growth of the grains, the iron electroplating layer can form a single grain with the substrate iron. However, the shape of the internal oxide does not necessarily have to be generated in a granular form. Depending on the elongation of the cold-rolled steel sheet, the steel composition, the atmosphere in the annealing furnace, and the oxygen content contained in the iron electroplating layer, the grains of the substrate iron and the grains of the iron electroplating layer can appear in a distinguishable form, and short linear oxides can be generated along the interface between the iron electroplating layer and the substrate iron or the grain boundaries inside the substrate iron.

[0069] After the annealing step, the annealed steel sheet may be cooled. The cooling conditions in the cooling step after the annealing step do not have much influence on the surface quality of the final product, i.e., the coating quality, and therefore, there is no need to particularly limit the cooling conditions in the present invention. However, in order to prevent oxidation of the iron component during the cooling process, an atmosphere having at least a reducing property to iron may be applied.

[0070] According to a specific embodiment of the present invention, the steel sheet for coating obtained by the above process may be hot-dip galvanized to form a hot-dip galvanized layer. In the present invention, the hot-dip galvanizing method is not particularly limited.

[0071] In addition, in the present invention, any base iron having the above alloy composition can be used as the base iron of the plated steel sheet or hot-dip galvanized steel sheet according to the present invention without limitation, and therefore the method for producing the base iron is not particularly limited.

[0072] In one embodiment of the present invention, the Fe plating layer can be formed on the surface of the substrate iron by electroplating, 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.

[0073] That is, in the present invention, in order to form the Fe plating layer, an electroplating solution can be used, which contains: iron ions, wherein the iron ions include ferrous ions and ferric ions; a complexing agent; and inevitable impurities, and the concentration of the ferric ions in the iron ions is 5-60% by weight.

[0074] According to a specific embodiment of the present invention, the electroplating solution includes ferrous ion and ferric ion. In order to obtain high plating efficiency, it can be advantageous to only include ferrous ion, but when only including ferrous ion, the solution deteriorates, causing the plating efficiency to be sharply reduced, thereby possibly causing the quality deviation in the continuous electroplating process, therefore the ferric ion can be further included. At this time, the concentration of the ferric ion is preferably 5-60% by weight of the sum of ferrous ion and ferric ion, more preferably 5-40% by weight. When the concentration of the ferric ion is less than 5%, the speed at which ferric iron is reduced to ferrous iron at the cathode is less than the speed at which ferrous iron is oxidized to ferric iron at the anode, so the concentration of ferric iron rises sharply, and pH sharply reduces, and plating efficiency continues to reduce. On the other hand, when the concentration of the ferric ion exceeds 60%, the reaction amount of ferric iron being reduced to ferrous iron at the cathode is greatly increased compared with the reaction amount of ferrous iron being reduced and precipitated as metallic iron, so plating efficiency is greatly reduced, and plating quality is reduced. Therefore, considering the equipment and process characteristics such as the coating amount, the working current density, the solution supply amount, the amount of solution lost due to adhesion to the strip, and the concentration change rate caused by evaporation, the concentration of the trivalent iron ions in the iron ions is preferably 5-60% by weight.

[0075] The concentration of the iron ions is preferably 1-80 g per 1 L of the electroplating solution, more preferably 10-50 g per 1 L of the electroplating solution. When the concentration of the iron ions is less than 1 g / L, there is a problem of a sharp decrease in plating efficiency and plating quality. On the other hand, when the concentration of the iron ions exceeds 80 g / L, it exceeds the solubility, so that precipitation may occur, and the loss of raw materials caused by the loss of solution in the continuous plating process increases, so it is not economical.

[0076] The electroplating solution of the present invention contains a complexing agent. In order to prevent sludge from being generated while containing a large amount of trivalent iron and to maintain high plating efficiency, it is preferred to use an amino acid or an amino acid polymer as the complexing agent.

[0077] Amino acids refer to organic molecules with carboxyl groups (-COOH) and amine groups (-NH2) bonded together, and amino acid polymers refer to organic molecules formed by the polymerization of two or more amino acids. Amino acid polymers exhibit complexing agent 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.

[0078] When amino acids are dissolved in neutral water, the amine group combines with hydrogen ions to carry a positive charge, and the carboxyl group carries a negative charge due to the dissociation of hydrogen ions, so the amino acid molecules remain electrically neutral. In addition, when the solution is acidified, the carboxyl group recombines with hydrogen ions to become electrically neutral, and the amine group carries a positive charge, so the amino acid molecules form cations. In other words, amino acids form electrically neutral or cations in weakly acidic aqueous solutions.

[0079] When amino acids are added to an acidic electrolyte containing iron ions, the amino acids are complexed with ferrous ions and ferric ions, and the iron ions complexed with the amino acids remain in a cationic state even in the complexed state. Therefore, compared with conventional complexing agents with multiple carboxyl groups that carry a negative charge in a weakly acidic aqueous solution, they exhibit opposite electrical properties.

[0080] In addition, compared with complexing agents containing multiple carboxyl groups such as citric acid and EDTA, amino acids form fewer bonds with iron ions and have weaker bonding strength, but the bonding strength with trivalent iron ions that produce sludge is strong enough, so precipitation caused by trivalent iron ions can be prevented. In addition, trivalent iron ions can remain cationic even when complexed, so trivalent iron ions can be easily transferred to the cathode and reduced to ferrous ions and participate in the plating reaction. On the other hand, the movement to the anode is suppressed, resulting in a slower generation rate of trivalent iron ions. Therefore, even if continuous plating is performed for a long time, the concentration of trivalent iron ions is maintained at a constant level, and a constant plating efficiency is maintained, and there is no need to replace the electrolyte.

[0081] In addition, in the continuous electroplating process, when the iron ions in the solution are exhausted due to plating, the solution is acidified, and even if the same amount of iron ions are precipitated, the pH change of the solution containing trivalent ferrous ions is reduced compared with the solution containing only ferrous ions. When the pH increases, part of the trivalent ferrous ions are combined with hydroxide ions, and when the pH decreases, the hydroxide ions are separated and neutralized. Therefore, even without a separate pH buffer, the pH change of the solution containing trivalent ferrous ions is also slowed down, thereby playing the role of a pH buffer, so that a constant electroplating efficiency can be maintained in the continuous electroplating process.

[0082] Therefore, by using amino acids as chelating agents, sludge can be prevented, and not only ferrous ions but also ferric ions can be used as plating raw materials. When ferrous ions and ferric ions are mixed, the pH change of the solution can be slowed down and the accumulation of ferric ions can be easily prevented, so that constant plating efficiency and plating quality can be maintained in a continuous electroplating process.

[0083] In addition, the addition of the complexing agent is preferably such that the molar concentration ratio of the iron ion and the complexing agent is 1:0.05 to 2.0, more preferably 1:0.5 to 1.0. When the above value is less than 0.05, it is impossible to suppress the excessive ferric ion contained to be combined with hydroxide ion or oxygen to form sludge, and even if ferric iron is not included, the plating efficiency will also be greatly reduced, and combustion will be caused, so that the plating quality deteriorates. On the other hand, even if the above value exceeds 2.0, the sludge suppression effect and the plating quality will also be kept, but the overvoltage increases, causing the plating efficiency to decrease, and compared with the raw material containing iron ion such as iron sulfate, it is unnecessary to include excessive relatively expensive amino acid, causing the raw material cost to increase, so it is uneconomical.

[0084] The complexing agent is preferably one or more selected from amino acids or amino acid polymers, for example, one or more selected from alanine, glycine, serine, threonine, arginine, glutamine, glutamic acid and glycylglycine.

[0085] When the amino acid is used as a complexing agent and the solution temperature is maintained below 80°C and the pH is between 2.0 and 5.0, the concentration of 3-120 A / dm 2 When electroplating is performed at a current density of , high plating efficiency can be achieved, and an Fe plating layer with a high oxygen concentration can be obtained.

[0086] The temperature of the Fe electroplating solution has little effect on the quality of the Fe coating, but when the temperature of the Fe electroplating solution exceeds 80°C, the evaporation of the solution becomes serious, causing the concentration of the solution to continuously change, making it difficult to perform uniform electroplating.

[0087] When the pH of the Fe electroplating solution is less than 2.0, the electroplating efficiency is reduced, making it unsuitable for a continuous plating process. When the pH of the Fe electroplating solution exceeds 5.0, although the plating efficiency is increased, sludge is generated during the continuous electroplating process due to the precipitation of iron hydroxide, resulting in pipe blockage, roller and equipment contamination.

[0088] When the current density is less than 3A / dm 2 When the current density exceeds 120A / dm 2 When the Fe plating layer is heated, the plated surface burns, resulting in uneven electroplating and easy peeling of the Fe plating layer.

[0089] As described above, the present invention preferably contains 5-50 wt% of oxygen in the Fe coating. The reason why oxygen is mixed into the Fe coating is as follows. In the process of iron precipitation on the surface of the steel sheet to which the cathode is applied, hydrogen ions are simultaneously reduced to hydrogen gas, causing the pH to rise. Therefore, both ferrous ions and ferric ions are temporarily combined with OH. - Ion combination, and can be mixed together when forming the Fe coating. If anionic complexing agents such as acetic acid, lactic acid, citric acid, EDTA, etc. are used, they will react with the complexing agent and OH - The ion-bound iron ions have an average negative charge, and if a cathode is applied for electroplating, an electrical repulsion force is generated, thereby suppressing the mixing into the Fe plating layer. On the other hand, amino acids are electrically neutral at pH 2.0 to 5.0, and become cationic in strong acids with a pH of less than 2.0. Even if the iron ions bound to amino acids have 1-2 OH groups, - , also becomes a cation, thus generating an electrical attraction with the cathode for electroplating, thereby mixing in a large amount of oxygen. Therefore, when an amino acid is used as a complexing agent, so that the molar concentration ratio of iron ions and amino acids is 1:0.05 to 1:2.0, and Fe electroplating is performed while maintaining pH 2.0 to 5.0, a high plating efficiency can be achieved, and the generation of sludge can be suppressed, while a Fe plating layer containing 5-50 wt % of oxygen can be obtained.

[0090] In order to ensure the quality of hot-dip galvanizing of steel sheets containing Mn and Si, it is preferred to use a concentration of 0.5-3.0 g / m 2 The upper limit of the Fe coating amount is not particularly limited, but when the Fe coating amount exceeds 3.0 g / m in the continuous plating process, 2 When the Fe plating amount is large, multiple plating tanks are required or the production speed is reduced, which is not economical. In addition, when the Fe plating amount is large, the Fe plating solution will rapidly change in the continuous process, resulting in a decrease in pH and a significant decrease in plating efficiency, which makes solution management difficult. On the other hand, when the Fe plating amount is less than 0.5 g / m2 When the Fe coating is completely reduced, the oxygen contained in the Fe coating is quickly removed by reduction, so it is not possible to effectively suppress the diffusion of Mn and Si from the base iron and the formation of surface oxides, resulting in a problem of reduced hot dip plating quality. The Fe coating amount is the concentration of iron contained in the coating, and when the Fe coating is completely reduced during the annealing process, it has a thickness of about 0.05-0.4 μm.

Claims

1. A steel sheet having a composition comprising, in weight %, Mn: 0.1-1.0%, Si: 0.1% or less and 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 Fe and inevitable impurities, The GDS curve of the Mn component and the GDS curve of the Si component observed from the surface in the depth direction contain a maximum point and a minimum point respectively. The difference between the value of the Mn concentration at the maximum point of the GDS curve of the Mn component divided by the Mn concentration of the base material and the value of the Mn concentration at the minimum point of the GDS curve of the Mn component divided by the Mn concentration of the base material, i.e., the Mn conversion concentration difference is 10% or more, The difference between the value of the Si concentration at the maximum point of the GDS curve of the Si component divided by the Si concentration of the base material and the value of the Si concentration at the minimum point of the GDS curve of the Si component divided by the Si concentration of the base material, i.e., the Si conversion concentration difference is 10% or more, in, When no local point appears at a depth of 5 μm or less, the position at a depth of 5 μm is regarded as the position at which the local point appears.

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

3. The steel plate according to claim 1, wherein: The difference in the converted concentration of Mn is 15% or more, and the difference in the converted concentration of Si is 15% or more.

4. The steel plate according to claim 1, wherein: The maximum point is formed at a depth of 0.05-1.0 μm.

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

6. A method for manufacturing a steel plate, comprising the following steps: preparing a base iron having a composition including, in weight %, Mn: 0.1-1.0%, Si: 0.1% or less and 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 of Fe and inevitable impurities; Electroplating the substrate iron to form an Fe plating layer containing 5-50 wt % of oxygen; and In an annealing furnace with an atmosphere of 1-70% H2 and balance N2 gas controlled at a dew point temperature below -20°C, the base iron having the Fe plating layer is annealed at 600-950°C for 5-120 seconds.

7. The method for manufacturing a steel plate according to claim 6, wherein: The Fe coating has an adhesion of 0.5-3 g / m 2 .

8. The method for manufacturing a steel plate 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 method for manufacturing a steel plate according to claim 6 or 7, wherein: The electroplating solution contains ferrous ions and ferric ions, the ferric ions have a ratio of 5-60 wt % relative to total ferric ions, and the total concentration of the ferric ions is 1-80 g per 1 L of the electroplating solution.

10. The method for manufacturing a steel plate according to claim 6 or 7, wherein: The electroplating solution temperature is below 80°C and 3-120A / dm 2 The test was carried out under the condition of current density.

11. A method for manufacturing a hot-dip galvanized steel sheet, comprising the following steps: preparing a base iron having a composition including, in weight %, Mn: 0.1-1.0%, Si: 0.1% or less and 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 of Fe and inevitable impurities; Electroplating the substrate iron to form an Fe coating containing 5-50 wt % of oxygen; In an annealing furnace in which the dew point temperature is controlled to be lower than -20°C and the atmosphere is 1-70% H2 and the balance N2 gas, the base iron having the Fe plating layer formed thereon is maintained at 600-950°C for 5-120 seconds for annealing to obtain a steel sheet for plating; and The steel sheet for coating is immersed in a galvanizing bath.

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