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

Abstract

To provide a high-strength hot-dip galvanized steel sheet having excellent plating quality, a steel sheet for plating for manufacturing the same, and methods for manufacturing the steel sheets.SOLUTION: A steel sheet for plating according to an aspect of the present invention has a GDS profile of a Mn component, as observed in the depth direction from the surface, sequentially including a maximum point and a minimum point, wherein a difference (a converted concentration difference of Mn) between a value, obtained by dividing the concentration of Mn at the maximum point by the concentration of Mn of a base material, and a value, obtained by dividing the concentration of Mn at the minimum point by the concentration of Mn of the base material, may be 10% or more, and a difference (a converted concentration difference of Si) between a value, obtained by dividing the concentration of Si at the maximum point by the concentration of Si of the base material, and a value, obtained by dividing the concentration of Si at the minimum point by the concentration of Si of the base material, may be 10% or more. However, when no minimum point appears within a depth of 5 μm, a point at a depth of 5 μm is denoted as the point at which the minimum point appears.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

[0002] In recent years, the automotive industry has been working to improve safety by applying high-strength steel sheets to automobiles. The weight has been reduced by increasing the thickness. Precipitation-strengthened steel and solid-solution-strengthened steel have been developed, and the strength has been improved while the elongation has been improved. DP steel (Dual Phase Steel) uses phase transformation to improve the yield, CP steel (Complex Phase Steel), TRIP steel (Transform ation Induced Plasticity Steel), and TWIP steel ( Twinning Induced Plasticity Steel and other products have been developed. These high-strength steels contain a variety of alloying elements compared to ordinary steels, but Adding a lot of elements that have a high oxidation tendency to Fe, such as Mn, Si, Al, Cr, and B become.

[0003] The quality of hot dip galvanizing depends on the surface condition of the annealed steel sheet immediately before plating. However, Mn, Si, Al, Cr, B, etc. added to ensure the physical properties of the steel sheet are also determined. The formation of surface oxides during annealing due to these elements can lead to poor plating properties. During the annealing process, the above elements diffuse to the surface and interact with the trace amounts of oxygen or water vapor present in the annealing furnace. The reaction forms single or complex oxides of the above elements on the steel sheet surface, increasing the reactivity of the surface. The surface of the annealed steel sheet with reduced reactivity hinders the wettability of the hot dip galvanizing bath. This causes areas where the plated metal does not adhere locally or entirely to the surface of the plated steel sheet. Furthermore, these oxides create the necessary mixture to ensure the adhesion of the plating layer during the hot dip plating process. The formation of the plating layer (Fe2Al5) is insufficient, causing peeling of the plating layer. This will significantly reduce the plating quality of the coated steel sheet.

[0004] Various technologies have been proposed to improve the coating quality of high-strength hot-dip galvanized steel sheets. Among them, Patent Document 1 discloses a method for controlling the air-fuel ratio of air to fuel during the annealing process to 0.80 to 0.95, and for oxidizing the The steel plate is oxidized in a direct flame furnace with a reactive atmosphere. The iron oxide containing Si, Mn or Al alone or in combination with other oxides is formed to a certain depth inside the steel plate. After forming the material, the iron oxide is reduced and annealed in a reducing atmosphere, and then hot dip galvanizing is performed. This technology provides hot-dip galvanized or galvannealed steel sheets with excellent coating quality. is presented.

[0005] When a method of reducing after oxidation in the annealing process as in Patent Document 1 is used, the surface layer of the steel sheet is heated to a certain depth. Furthermore, elements with a high affinity for oxygen, such as Si, Mn, and Al, are internally oxidized and diffuse to the surface. Therefore, the surface layer has a relatively low amount of Si, Mn or Al oxides or composite oxides. The wettability with zinc is improved, and the number of unplated areas is reduced. However, the addition of Si In the case of steels with reduced Si, Si concentrates beneath the iron oxide during the reduction process, forming a band of Si oxide. This causes peeling at the surface including the plating layer, i.e., reduced iron and Peeling occurs at the interface with the underlying steel, making it difficult to ensure adhesion of the plating layer. There is a problem.

[0006] On the other hand, as another method for improving the galvanizability of high-strength hot-dip galvanized steel sheets, Patent Document 1 2) Maintain a high dew point in the annealing furnace and remove Mn and Si, which are easily oxidized. By internally oxidizing alloying elements such as Al inside the steel, the surface of the steel sheet after annealing is externally oxidized. However, the patent documents do not include methods for reducing the amount of oxides that are present and improving plating properties. Method 2 can solve the plating problem caused by external oxidation of Si, which is easily internally oxidized. However, when a large amount of Mn is added, which is relatively difficult to oxidize internally, the effect is minimal. There is a problem.

[0007] In addition, even if the plating property is improved by internal oxidation, the surface is unevenly formed. Surface oxides can cause lines of unplated material, or alloying can be achieved by alloying heat treatment after plating. When manufacturing hot-dip galvanized steel sheets (GA steel sheets), the surface of the galvannealed steel sheets In some cases, problems such as the occurrence of linear defects due to non-uniform alloying may occur.

[0008] Another conventional technique involves pre-plating with Ni before annealing, which allows the alloying elements to diffuse onto the surface during annealing. However, this method is also effective in suppressing Mn diffusion. However, there is a problem in that the diffusion of Si cannot be sufficiently suppressed. [Prior art documents] [Patent documents]

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

[0010] According to one aspect of the present invention, no unplated areas occur and the problem of peeling of the plating layer is solved. Provided are hot-dip galvanized steel sheets with excellent coating quality and methods for producing the same.

[0011] According to another aspect of the present invention, no linear defects are generated even when alloying heat treatment is performed after plating. First, the hot-dip galvanized steel sheet and its alloys can be manufactured from alloyed hot-dip galvanized steel sheets with excellent surface quality. A method for producing the compound is provided.

[0012] According to another aspect of the present invention, a hot dip galvanized plate having such excellent coating quality is The present invention provides a steel sheet for plating that can be used to produce a coated steel sheet, and a method for producing the same.

[0013] The object of the present invention is not limited to the above. Those skilled in the art will be able to understand the further object of the present invention from the overall contents of the specification of the present invention. There's nothing wrong with understanding it. [Means for solving the problem]

[0014] The steel sheet for plating according to one embodiment of the present invention has a Mn component and a Si component when observed in the depth direction from the surface. The GDS profile of each component contains sequential maximum and minimum points, and the GDS profile of the Mn component is The value obtained by dividing the Mn concentration at the maximum point of the S profile by the Mn concentration of the base material and the above Mn component The difference between the Mn concentration at the minimum point of the GDS profile and the Mn concentration in the base material (M The difference in the converted concentration of n is 10% or more, and the above Si component is at the maximum point of the GDS profile. The value obtained by dividing the Si concentration in the sample by the Si concentration in the base material is compared with the extreme value of the GDS profile of the Si component. The difference between the Si concentration at the small point divided by the Si concentration of the base material (the difference in the converted Si concentration) is 10 % or more.

[0015] However, if no minimum point appears within a depth of 5 μm, the point at a depth of 5 μm is treated as the minimum point. The location.

[0016] The hot-dip galvanized steel sheet according to another aspect of the present invention is a steel sheet for plating described above and a hot-dip galvanized steel sheet according to the above aspect of the present invention. The steel sheet may include a hot-dip galvanized layer formed on the steel sheet.

[0017] Another aspect of the present invention is a method for producing a steel sheet for plating, which comprises the steps of: preparing a base steel; Electroplating is performed on the base steel to form an Fe plating layer containing 5 to 50% by weight of oxygen. and the base steel on which the Fe plating layer is formed is subjected to a dew point temperature controlled to less than -20°C. Annealing in a 70% H2-remaining N2 gas atmosphere at 600-950°C for 5-120 seconds The step of annealing the alloy may include:

[0018] A method for producing a hot-dip galvanized steel sheet according to another aspect of the present invention includes the steps of preparing a base steel. Step 2: Electroplating is performed on the above base iron to produce an Fe plating containing 5 to 50% by weight of oxygen. The iron substrate on which the Fe plating layer is formed is kept at a dew point temperature of less than -20°C. In an annealing furnace with a 1-70% H2-rest N2 gas atmosphere, the temperature is 600-950°C for 5-120 minutes. annealing the steel sheet for plating by maintaining the temperature for 10 seconds to obtain a steel sheet for plating; and A soaking step may be included. [Effects of the Invention]

[0019] As described above, the present invention forms a pre-plated layer and determines the concentration profile of the Mn and Si components inside. By controlling the flow, the phenomenon of unplated parts occurring during hot dip galvanizing is significantly improved, and It is possible to provide a hot-dip galvanized steel sheet having improved coating adhesion.

[0020] According to another aspect of the present invention, the hot-dip galvanized steel sheet of the present invention is subjected to an alloying heat treatment. Even if the above process is carried out, linear defects can be prevented from appearing on the surface of the obtained galvannealed steel sheet. Therefore, it is possible to provide a galvannealed steel sheet with excellent surface quality. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a schematic diagram of GDS profiles of Mn and Si elements measured after removing the coating layer of a hot-dip galvanized steel sheet manufactured from an Fe-electroplated cold-rolled steel sheet. [Figure 2] These are cross-sectional electron microscope photographs of steel sheets in which base iron (cold-rolled steel sheet) on which oxygen-containing Fe electroplating has been formed is annealed at 800°C for 53 seconds. (a) is the electron microscope photograph, (b) is the Mn distribution map, (c) is the Si distribution map, and (d) is the O distribution map. [Figure 3] FIG. 2 is a schematic diagram showing the annealing process of a base steel sheet on which an oxygen-containing Fe-plated layer has been formed. [Figure 4] 1 shows GDS concentration profiles measured on hot-dip galvanized steel sheets after removing the coating layer, where (a) is for Comparative Example 1, (b) is for Comparative Example 6, and (c) is for the steel sheet obtained by Inventive Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0022] The following describes a high strength plating material with excellent plating quality according to one aspect of the present invention, which has been completed through the research of the present inventors. In the present invention, when the concentration of each element is indicated, It should be noted that unless otherwise specified, the percentages are by weight. The plating amount is measured as the total amount of Fe contained in the plating layer per unit area. Oxygen and unavoidable impurities in the plating layer were not included in the plating amount.

[0023] Furthermore, unless otherwise specified, the concentrations and concentration profiles referred to in this invention are GDS, That is, glow discharge optical emis The concentration and concentration profile measured using a ion spectrometer means.

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

[0025] The causes of uncoated areas and reduced coating adhesion in steel sheets containing large amounts of Mn and Si are as follows: During the high-temperature annealing process of cold-rolled steel sheets, alloying elements, especially Mn and Si, are oxidized on the surface. This is known to be due to the surface oxide formed.

[0026] It contains a large amount of oxygen to prevent alloying elements such as Mn and Si from diffusing to the surface. This is a method to form an oxide layer by oxidizing the material during heating and then reducing it again by maintaining it in a reducing atmosphere. or a method of coating the surface of the base metal with iron oxide and then heat treating it. However, the iron oxide that is firmly formed on the surface of the base metal is FeO. In addition, Fe3O4 and Fe2O3, which are difficult to reduce, are mixed in 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 is reduced at a slower rate. The oxides of Mn and Si accumulate at the interface and form a continuous oxide layer. This improves wettability with molten zinc, but the oxide layer cracks easily and the coating layer is easily damaged. Peeling problems may occur.

[0027] On the other hand, by increasing the oxygen partial pressure or dew point in the annealing furnace during the heat treatment process, it is possible to remove Mn, Si, and other alloys. When applying the internal oxidation method of annealing, which oxidizes the gold element inside the steel, the surface of the steel is preferentially oxidized during the heat treatment. Mn and Si oxides are formed automatically, and then the Mn and Si are oxidized by the oxygen that diffuses into the steel. 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 uniform, or if there are local variations in oxygen partial pressure, temperature, etc. If such variations occur, the wettability during hot dip galvanizing will be uneven, resulting in uncoated areas. Or, the thickness of the oxide film is uneven during the alloying heat treatment after galvanizing, resulting in the degree of alloying. Differences tend to cause linear defects that are easily visible to the naked eye.

[0028] In order to solve the above-mentioned technical problems, the present inventors have applied an oxidizing agent to the surface side of a steel sheet to be plated. By controlling the existence form of the elements Mn and Si as follows, the surface becomes beautiful and the plating The aim was to manufacture hot-dip galvanized steel sheets that would not have peeling problems.

[0029] That is, the steel sheet according to one embodiment of the present invention has a GDS concentration profile of Mn and Si. With reference to the GDS profile in FIG. The steel sheet for plating will now be described in detail.

[0030] FIG. 1 shows the surface of a hot-dip galvanized steel sheet including the steel sheet of the present invention after removing the zinc coating layer. Typical GDS profiles of alloy elements that may appear from the part and cases outside the scope of this invention 1 is a graph showing a schematic GDS profile of alloy components. The vertical axis of the graph is M The horizontal axis indicates the depth.

[0031] From the typical example of the GDS profile of the Mn component or Si component of the present invention shown in FIG. As can be seen, the steel sheet of the present invention has a very low Mn concentration on the surface, and the Mn concentration is The concentration gradient may be such that maximum and minimum points appear sequentially in the direction of the concentration gradient. Having a maximum point does not necessarily appear first in the depth direction from the surface (interface). In some cases, the minimum point may appear first, but then the maximum and minimum points appear in succession. However, in some embodiments, the maximum point is followed by a In some cases, the small point does not appear, in which case the internal concentration in the 5 μm depth region is taken as the minimum concentration. In addition, the alloying element concentration at the surface is lower than that at the maximum point, but in some cases In some cases, a minimum point with a low alloying element concentration may appear between the surface and the maximum point.

[0032] The GDS concentration profile shown in Figure 1 above is not necessarily limited to this. However, since the alloying elements do not diffuse much from the base steel to the surface, the concentration of alloying elements is low. e corresponds to the coating layer, and the maximum point is the alloy formed near the interface between the Fe coating layer and the base steel. This corresponds to the area where the internal oxide of the element is concentrated, and is the minimum point that appears from the Fe coating layer to the base steel. The alloying elements are dispersed and diluted as an Fe plating layer that does not contain alloying elements, and the internal oxidation This corresponds to the region where the alloying elements are diffused and depleted at the maximum point where the peak is generated.

[0033] In one embodiment of the present invention, the maximum point is located at a depth of 0.05 to 1.0 μm from the surface of the steel sheet. If the maximum point appears in a deeper region, the effect of the present invention will be lost. In addition, the minimum point may not be recognized as a maximum point due to the surface roughness. As mentioned above, if the minimum point is within a depth of 5 μm, If the minimum point is not formed at 5 μm depth, the minimum point can be formed at 5 μm depth. The concentration at depth is essentially the same as the concentration inside the base material, so there is no further decrease in concentration. It can be considered as a point.

[0034] At this time, the maximum points of the relevant elements in the Mn concentration profile and Si concentration profile The converted concentration (the concentration at the relevant point divided by the concentration of the base material, expressed in %) and the converted concentration at the minimum point The larger the difference between the temperature and the surface temperature, the more Mn and Si can be reduced. In one embodiment of the present invention, the value of the converted concentration at the maximum point minus the converted concentration at the minimum point of Mn and Si is can each be 10% or more.

[0035] As a result of experiments conducted by the inventors under various conditions, when the above conditions are met, No uncoated areas were observed, and hot-dip galvanized steel sheets with good coating adhesion were obtained. If the difference in converted concentration between the maximum and minimum points of Mn and Si is less than 10%, the point or line There is a problem that plating occurs or plating peeling occurs. By controlling the concentration above a certain level, it is possible to prevent the formation of oxides of Mn and Si on the surface. Ultra-high strength hot-dip galvanized steel sheet with a beautiful surface and good coating adhesion. Even after the subsequent alloying heat treatment process, defects such as linear defects do not appear on the surface. The greater the difference in the converted concentration values, the more advantageous it is. There is no need to set an upper limit for this value. However, when considering the content of the elements contained, The difference in the converted concentration values ​​can be determined to be 200% or less for both Mn and Si. In another embodiment, the difference in the converted concentrations of Mn and Si is 15% or more, or 20% or more. It can be above.

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

[0037] For GDS concentration analysis, hot-dip galvanized steel sheets were cut into lengths of 30 to 50 mm. The wire is then cut off and immersed in a 5 to 10% by weight aqueous solution of hydrochloric acid at room temperature of 20 to 25°C to remove the zinc plating layer. In order to prevent surface damage to the base steel during the dissolution process of the zinc coating layer, When the generation of bubbles due to the acid solution reaction stops, remove the acid solution within 10 seconds and use pure water. The base steel was washed and dried. If the steel sheet to be coated has not yet been hot-dip galvanized, Of course, analysis can be performed without the need for such plating layer removal work.

[0038] The GDS concentration profile shows all the GDS contained in the steel sheet at intervals of 1 to 5 nm in the thickness direction of the steel sheet. Measure the concentration of a component. The measured GDS profile contains random noise. The maximum and minimum points of Mn and Si concentrations can be calculated from the measured concentration profile. A Gaussian filter with a cutoff value of 100 nm was applied to the file to obtain the average density profile. The density values ​​and depths of the maximum and minimum density points are obtained from the noise-removed profile. The maximum and minimum points referred to in the present invention are separated by 10 nm from each other in the depth direction. It should be noted that the maximum and minimum points were calculated only when there was a position difference of more than this. .

[0039] The steel sheet for plating that is the subject of the present invention is a steel sheet that is made of a base iron and an Fe plating formed on the base iron. The composition of the base steel is not particularly limited.

[0040] However, it contains 1.0 to 8.0 wt% of Mn and 0.1 to 3.0 wt% of Si, and has an oxide on the surface. In the case of high strength steel sheets having a composition that is prone to the formation of There is no particular upper limit to the Mn concentration in the base steel, but it is possible to When considering the composition, the upper limit can be limited to 8 wt%. Although there is no particular limitation, a composition containing less than 1.0 wt % of Mn is difficult to form an Fe plating layer. The surface quality of hot-dip galvanized steel sheets is excellent, so there is no need to perform Fe electroplating. There is no particular upper limit to the concentration, but when considering the composition that is usually used, the upper limit The Si concentration can be limited to 3.0% by weight or less, and if the Si concentration is less than 0.1% by weight, the method of the present invention Since the quality of hot dip galvanizing is excellent even without the above-mentioned method, it is not necessary to carry out the method of the present invention. There is no.

[0041] Since the above Mn and Si are elements that affect plating properties, their concentrations are controlled as described above. However, the present invention does not particularly limit the remaining components of the base iron.

[0042] However, in the case of high-strength steel sheets containing a large amount of alloying elements, the uncoated and coated In consideration of the fact that the decrease in the amount of iron may occur severely, in one embodiment of the present invention, The composition is, in weight percent, Mn: 1.0 to 8.0%, Si: 0.1 to 3.0%, C: 0.05 to 0.3%, Al: 0.005-3.0%, P: 0.04% or less (excluding 0%), S: 0. 0.15% or less (excluding 0%), Cr: 1.5% or less (including 0%), B: 0.005% or less The remainder may contain Fe and unavoidable impurities. Strength refers to the case where the material has high strength after annealing, and also ... It should be noted that the term "high strength" in the present invention includes all cases where high strength can be obtained by such means. High strength means a tensile strength standard of 490 MPa or more. The above-mentioned base iron may contain, but is not limited to, the above-mentioned components. The above may further contain elements such as Ti, Mo, and Nb in a total amount of 1.0% or less. The base steel is not particularly limited, but in one embodiment of the present invention, the base steel is cold-rolled steel. Plate or hot rolled steel sheet can be used.

[0043] In one aspect of the present invention, there is provided a hot-dip galvanized steel sheet including the above-mentioned steel sheet for plating. The hot-dip galvanized steel sheet can be a steel sheet to be plated and a coating formed on the surface of the steel sheet to be plated. In this case, the hot-dip galvanized steel sheet may contain a hot-dip galvanized layer. Anything that can be applied can be applied, and there is no particular restriction on the type. .

[0044] Next, a method for manufacturing a steel sheet for plating and a hot-dip galvanized steel sheet having the above-mentioned advantageous effects will be described. One exemplary embodiment of the method will be described. According to one embodiment of the present invention, the steel sheet for plating is A step of preparing a base iron; electroplating the base iron to obtain a base iron containing 5 to 50% by weight of oxygen. a step of forming an Fe-plated layer containing the Fe; and annealing the base iron on which the Fe-plated layer is formed. The steel sheet may be produced by a process which includes the step of obtaining a steel sheet for plating.

[0045] 1.2GPa-class cold-rolled steel sheet containing Mn 2.6%, Si 1.0%, and other alloying elements, with iron Weight: 1.99g / m 2 After electroplating with Fe containing 6.3 wt% oxygen, The above electroplated cold-rolled steel sheets were placed in an atmosphere of N2-5%H2 with a dew point of -40°C and a temperature of 800 The sample was annealed at 20°C for 53 seconds and then cooled. The atmosphere was kept the same throughout the entire annealing process. After the specimen was taken from the cooled steel plate, the cross section was observed under a transmission electron microscope. As can be seen from the drawing, particulate Mn and Si oxides are present at the interface between the iron electroplating layer and the base iron. On the other hand, Mn and Si oxides are mostly formed on the surface of the electroplated layer. In particular, when analyzing such a situation using the GDS profile, As shown in Figure 1(b), Mn and Fe were present in the area directly below the surface of the steel sheet (including the Fe-plated layer). Maximum values ​​of Mn and Si concentrations appear, and minimum values ​​of Mn and Si concentrations appear at deeper positions. Of course, in some cases, the minimum value does not appear clearly, and the concentration gradually decreases. Although a tendency can be seen, local maxima can be clearly observed.

[0046] This phenomenon is due to the formation of an Fe plating layer with a high oxygen content before annealing. That is, when the iron electroplating layer contains 5 to 50% by weight of oxygen, the sintering in a reducing atmosphere When annealing in an annealing furnace, oxygen in the iron electroplating layer diffuses to the surface of the base iron, releasing Mn and S. This causes alloying elements such as i to oxidize and accumulate at the interface between the iron electroplating layer and the base iron. Therefore, as shown in graph (b) of Figure 1, when the concentration is measured with GDS, iron electrolysis is observed from the surface. At the depth corresponding to the thickness of the plating layer, a maximum point of high concentrations of Mn, Si, etc. is observed. The concentration of alloying elements with slow diffusion rates, such as Mn, is diluted by the iron electroplating layer. Even if the dissolved Mn is depleted by internal oxidation, it cannot diffuse rapidly from the base steel. Therefore, a minimum point may occur after the maximum point of the GDS concentration. The iron electroplating layer and the base iron are continuously oxidized at the interface. Oxides may accumulate, so the minimum point may not be detected in the GDS concentration analysis. The absence of a minimum point in the GDS concentration profile is due to the presence of Mn, This means that alloying elements such as Si are oxidized, effectively suppressing their diffusion to the surface.

[0047] Unlike oxides that are formed firmly at high temperatures, iron plating layers containing a large amount of oxygen are formed. When annealing is performed after the electroplating, not only the surface of the electroplated iron layer but also the surface of the electroplated iron layer and the base steel sheet are annealed. At this time, the reduced iron and the matrix iron undergo mutual diffusion. The Mn and Si react with the oxygen in the Fe plating layer to form the iron electroplating layer. The iron electroplating process forms oxide particles or discontinuous plates at the interface of the base steel sheet. The adhesion between the coating layer and the base metal can be maintained well. The formed iron electroplating layer suppresses the formation of surface oxides, and the dissolved Mn and S on the steel sheet surface By reducing the concentration of alloying elements such as i, the alloying reaction with the zinc plating layer is promoted, and surface defects are eliminated. A uniform, defect-free alloyed hot-dip plated steel sheet can be obtained.

[0048] Unlike the oxidation-reduction method, the internal oxidation method does not form a layered oxide layer, so Mn, Si Improves coating adhesion during hot dip galvanizing of ultra-high strength steel sheets containing large amounts of alloying elements such as However, the water vapor in the annealing furnace inevitably oxidizes the surface of the steel sheet first. After that, oxygen penetrates into the interior, so the surface oxide cannot be completely removed. As a result, the surface of the cold-rolled steel sheet before annealing is not completely uniform, and the partial pressure of oxygen, temperature, etc. are affected during annealing. When local variations occur, the wettability with the hot-dip plating solution is uneven and the plating is not yet completed. The thickness of the oxide film is uneven during the alloying heat treatment process after galvanizing. Differences in the degree of oxidation occur, causing linear defects that are easily visible to the naked eye. Sometimes I do.

[0049] The iron plating contains a large amount of oxygen, which suppresses the surface diffusion of alloy elements, resulting in a beautiful and polished finish. In order to manufacture hot-dip galvanized steel sheets that do not have peeling problems, 5 to 50% by weight of acid is added to the base steel. The iron electroplating layer containing elements is 0.5 to 3.0 g / m2 as the thermal coating weight. 2 To be To ensure the mechanical properties of the steel sheet, an Fe plating layer is formed on the surface and the steel sheet is then heated at a temperature of 600 to 950°C. It is preferable to heat the plate to a temperature of 1000°C, then cool it again and perform hot dip plating.

[0050] In one embodiment of the present invention, the Fe plating layer is formed through a continuous plating process. The amount of Fe plating at this time is 0.5 to 3.0 g / m as the standard of Fe coating amount. 2 Nina The Fe plating amount can be set to 0.5 g / m 2 If the temperature drops below this, normal continuous firing During the annealing process, the effect of the Fe plating layer in suppressing the diffusion of alloying elements may be insufficient. 0g / m 2 Even if the alloying element content exceeds 100%, the suppression effect can be further increased. In order to ensure sufficient plating capacity, multiple plating cells must be operated, and when an insoluble anode is used, In this case, the electroplating solution becomes acidic rapidly, which reduces plating efficiency and causes sludge generation. In another embodiment of the present invention, the amount of Fe plating is 1. 0-2.0g / m 2 After forming the Fe plating layer, internal oxidation can be performed. , an internal oxide is formed at or just below the interface between the Fe coating layer and the base steel. The maximum concentration of Mn and Si is present in the region of 0.05 to 1.0 μm. 0.5~3.0g / m 2 The amount of Fe plating corresponds to a thickness of 0.05 to 0.4 μm after annealing. It is possible.

[0051] In addition, the Fe plating layer having the above-mentioned high oxygen concentration is susceptible to the temperature and dew point of the subsequent annealing process. By controlling the temperature and atmosphere, the GDS concentration profile of Mn and Si elements is increased inside the steel sheet for plating. The profile shows maximum and minimum points, and the converted concentration at the maximum and minimum points is The converted concentration is made to satisfy the numerical range limited in one embodiment of the present invention. In consideration of the above, in one embodiment of the present invention, the oxygen concentration in the Fe plating layer is It may be 5 to 50% by weight, and in another embodiment, it may be 10 to 40% by weight. To obtain the effect of suppressing surface oxides, the amount of oxygen in the Fe plating layer must be sufficiently large. Even if the oxygen concentration in the Fe plating layer is less than 5 wt%, the amount of Fe plating should be increased. This can suppress the formation of surface oxides, but to achieve this effect, 3. 0g / m 2 Since plating must be performed in excess of this limit, the various problems mentioned above may occur. In addition, when the oxygen content does not reach 5 wt%, the GDS profile of Mn and Si Since it is difficult to form local maximum and minimum points in a single image, in one embodiment of the present invention, The oxygen content in the Fe plating layer is controlled to 5% by weight or more. As the concentration of SiO increases, the effect of suppressing the surface oxide formation during annealing can be further improved. However, since it is difficult to obtain a plating layer exceeding 50% by weight using a conventional electroplating method, In another embodiment of the present invention, the Fe plating may be limited to 50% by weight. The oxygen concentration in the layer can also be limited to 10 to 40%.

[0052] In one embodiment of the present invention, the annealing temperature is 600°C to 900°C based on the steel sheet temperature in the soaking zone. If the annealing temperature is too low, the structure of the cold-rolled steel sheet will not recover properly and recrystallize. It is difficult to secure the mechanical properties of the steel sheet, such as strength and elongation rate, and the temperature is set to exceed 950°C. If this happens, the alloying elements in the steel will diffuse rapidly to the surface, causing poor quality of hot dip galvanizing and In other words, it is not economical because it will have to be operated at high temperatures.

[0053] On the other hand, in one embodiment of the present invention, the dew point inside the annealing furnace is not necessarily limited to this. It is not necessary to maintain the dew point below -20°C. In this case, it is economical because a separate humidifier is not required to increase the dew point. In the present invention, since the Fe plating layer has a high oxygen concentration, it is not necessary to use the atmosphere. It is believed that alloying elements such as Mn and Si can diffuse sufficiently to the surface without inducing internal oxidation. There is no specific lower limit for the dew point temperature. However, the dew point must be kept below -90°C. It is sometimes not industrially advantageous to maintain this, for example, when using very high purity gases. Taking this into consideration, the lower limit of the dew point can be set to -90°C. Therefore, when the temperature of the steel plate is 600 to 950°C, the dew point can be -70 to -30°C. do.

[0054] In addition, to prevent oxidation of the base steel and the Fe-plated layer during annealing, the atmospheric gas during annealing The hydrogen concentration in the gas can be made 1% or more by volume. If the hydrogen concentration is less than 1%, H The trace amounts of oxygen inevitably contained in N2 and N2 gases cannot be effectively removed by oxidation reactions. The increase in oxygen partial pressure can cause surface oxidation of the base iron. If it exceeds 70%, the risk of explosion in case of gas leakage and the cost of high hydrogen work will increase. The hydrogen concentration can be set to 70% or less. The gas may be essentially nitrogen (N2), except for impurity gases that may be included.

[0055] According to one embodiment of the present invention, the maintenance time after reaching the target temperature during annealing is set to 5 to 100°C. During annealing, heat is sufficiently transferred to the inside of the base steel, and the thickness direction is In order to obtain uniform mechanical properties, it is necessary to maintain the annealing target temperature for 5 seconds or more. However, if the high-temperature annealing time is too long, the diffusion of alloying elements through the Fe plating layer increases. This increases the amount of surface oxides produced, resulting in poor quality hot-dip galvanizing. It can be limited to 0 seconds or less.

[0056] Based on the above, the cold-rolled steel sheet having an Fe-plated layer containing a large amount of oxygen is highly The effect of suppressing the surface diffusion of Mn and Si during annealing in a low dew point atmosphere is shown in more detail in Figure 3. I will explain in detail.

[0057] Figure 3 shows the phenomenon that occurs inside the steel sheet when the temperature of the steel sheet is increased according to the conditions of the present invention. This is a schematic diagram.

[0058] Figure 3(a) shows a schematic cross section of a base steel sheet on which an Fe-plated layer containing a large amount of oxygen has been formed. The base steel sheet contains alloy elements such as Mn and Si, and the Fe coating layer has a thickness of 5 to 50 It contains % by weight of oxygen and impurities that are inevitably mixed in during plating, with the remainder being Fe.

[0059] In Fig. 3(b), the iron electroplated cold rolled steel sheet is placed in a nitrogen atmosphere containing 1 to 70% H2. The surface of the Fe plating layer was gradually reduced. The interface between the Fe coating layer and the base steel is filled with Mn and Si diffused from the base steel. Internal oxides such as these begin to form, and as the temperature increases, the oxides at the grain boundaries grow coarser.

[0060] Figure 3(c) shows the cross section of the base steel sheet when heated to 500-700°C in the same reducing atmosphere. The Fe coating layer is almost completely reduced, and the Mn and Si concentrations are low compared to the base steel. As ferrite is formed, oxygen in the Fe plating layer is gradually depleted, and Mn and Si are converted into Fe. It penetrates the plating layer and gradually begins to diffuse to the surface of the Fe plating layer.

[0061] Figure 3(d) shows a schematic cross-sectional view of a steel sheet that has been annealed at a temperature of 600 to 950°C. The iron electroplating layer contains only the solid solution of Mn, Si, etc. in the interior of the metallic iron, excluding the internal oxides of Mn, Si, etc. The oxygen that had been deposited is completely removed, and the resulting inner oxide is generally spherical or short plate-shaped. In addition, the growth of crystal grains can cause the iron electroplating layer to form a single crystal grain with the base iron. However, the internal oxide does not necessarily take the form of particles, and the internal oxide does not necessarily take the form of particles. The elongation rate of the sheet, the steel composition, the atmosphere in the annealing furnace, and the oxygen content in the iron electroplating layer affect the The grains of the base iron and the grains of the iron electroplating layer may appear separate. Short lines of oxide are formed at the interface between the coating and the base iron or along the grain boundaries within the base iron. This sometimes happens.

[0062] After the annealing step, the annealed steel sheet can be cooled. The cooling conditions have a significant effect on the surface quality of the final product, i.e., the plating quality. In the present invention, there is no need to particularly limit the cooling conditions. However, in order to prevent oxidation of the iron component during the cooling process, To prevent this, a reducing atmosphere can be applied, at least with respect to iron.

[0063] According to one embodiment of the present invention, the steel sheet for plating obtained by the above-mentioned process is subjected to a welding process. The hot-dip galvanizing layer can be formed by hot-dip galvanizing. There are no particular restrictions on the method of application.

[0064] In the present invention, the base iron having the above-mentioned alloy composition can be used as the plating material according to the present invention. It can be used without restrictions as the base steel for steel sheets or hot-dip galvanized steel sheets, so it is The manufacturing method is not specifically limited.

[0065] In one embodiment of the present invention, the Fe plating layer is formed on the surface of the base steel through an electroplating method. By properly controlling the conditions of the electroplating solution and the plating conditions, The oxygen concentration of the Fe plating layer can be controlled.

[0066] That is, in order to form an Fe plating layer in the present invention, ferrous ions and ferric ions are iron ions containing ions; complexing agents; and unavoidable impurities, An electroplating solution having an ion concentration of 5 to 60% by weight can be used.

[0067] According to one embodiment of the present invention, the electroplating solution contains ferrous and ferric ions. To obtain high plating efficiency, it is advantageous to contain only ferrous ions. However, if only ferrous ions are included, the solution will change and plating efficiency will drop sharply. Therefore, the above ferric ions may cause quality deviations in the continuous electroplating process. In this case, the concentration of the ferric ions is ferrous and ferric ions. The content of the total amount of the amines is preferably 5 to 60% by weight, more preferably 5 to 40% by weight. If it is less than 5%, the rate at which ferric iron is reduced to ferrous iron at the cathode is less than the rate at which ferrous iron is reduced to ferric iron at the anode. The rate of oxidation is lower than that of the iron(II) salt, and the ferric iron concentration rises sharply and the pH drops sharply. Plating efficiency continues to decline. Meanwhile, as the concentration of ferric ions exceeds 60%, When this happens, the amount of ferric iron reduced to ferrous iron at the cathode is reduced to metallic iron. Since the reaction rate increases significantly compared to the actual reaction rate, plating efficiency drops significantly and plating quality deteriorates. Therefore, the plating amount, working current density, solution replenishment amount, solution loss due to adhesion to the strip, Considering the characteristics of the equipment and process, such as the amount of iron ions, the rate of change in concentration due to evaporation, etc., It is preferable to adjust the iron ion concentration to 5 to 60% by weight.

[0068] The concentration of the iron ions is preferably 1 to 80 g per 1 L of the electroplating solution. If the amount is less than 1 g / L, the plating efficiency will be reduced. If the concentration exceeds 80g / L, the solubility and plating quality will drop sharply. If the temperature exceeds the limit, precipitation may occur, and the solution may be washed away during the continuous plating process. This is uneconomical due to increased raw material loss.

[0069] The electroplating solution of the present invention contains a complexing agent, but does not generate sludge even though it contains a large amount of ferric iron. To prevent this and maintain high plating efficiency, amino acids or amino acid polymers are used as complexing agents. It is preferable to use

[0070] Amino acids are chemical compounds that combine a carboxyl group (-COOH) and an amine group (-NH2). An amino acid polymer is an organic molecule formed by polymerizing two or more amino acids. This means that amino acid polymers exhibit complexing properties similar to those of amino acids. In the present day, amino acids and amino acid polymers are collectively referred to as amino acids.

[0071] When amino acids are dissolved in neutral water, the amines bond with hydrogen ions and become positively charged. The carboxyl group has a negative charge due to dissociation of hydrogen ions, so the amino acid molecule On the other hand, when the solution is acidified, the carboxyl group is converted to hydrogen ions. The amino acid molecule becomes a cation because the amine has a positive charge. In other words, amino acids are neutral or cationic in a weakly acidic aqueous solution. This results in the formation of a

[0072] When amino acids are added to an acidic electrolyte containing iron ions, ferrous and ferric ions are released. The iron ions complexed with amino acids remain in a cationic state even when complexed. Therefore, ordinary complexing agents with multiple carboxyl groups are weakly It exhibits the electrically opposite property of being negatively charged in an acidic aqueous solution.

[0073] In addition, amino acids have a higher affinity for carboxylic acids than complexing agents containing multiple carboxyl groups such as citric acid and EDTA. The number of bonds formed with iron ions is small and the bonding strength is weak, but the ferric ions that cause sludge are The binding force with ions is strong enough to prevent precipitation by ferric ions. Furthermore, since the ferric ion can maintain its cation even when complexed, the ferric ion is easily transported to the cathode and reduced to ferrous ions, which can participate in the plating reaction, whereas The migration to the anode is suppressed, slowing down the rate of ferric ion generation, making it difficult to perform long-term continuous plating. Even if the plating is performed, the ferric ion concentration remains at a constant level, so plating efficiency remains constant. This eliminates the need to replace the electrolyte.

[0074] On the other hand, when iron ions in the solution are consumed by plating in the continuous electroplating process, the solution becomes acidic. However, even if the same amount of iron ions is precipitated, the amount of iron ions is higher than that of a solution containing only ferrous ions. However, the pH of the solution containing ferric ions decreases. When the pH decreases, some of the ferric ions combine with the hydroxide ions, and when the pH decreases, the hydroxide ions separate and form the Therefore, the pH change of a solution containing ferric ions is slowed down even without a separate pH buffer. It acts as a pH buffer, improving the efficiency of electroplating in the continuous electroplating process. It can be maintained constant.

[0075] Therefore, amino acids can be used as complexing agents to prevent sludge formation, and ferrous iron Not only ferrous ions but also ferric ions can be used as plating raw materials, and ferrous ions and ferrous ions can be used as plating raw materials. When ferric ions are mixed, the pH change of the solution slows down, and the Accumulation can be easily prevented, improving electroplating efficiency and productivity in continuous electroplating processes. Plating quality can be maintained at a constant level.

[0076] On the other hand, the molar concentration ratio of the iron ions to the complexing agent is 1:0.05 to 2.0. It is preferable to add the amount of 1:0.5 to 1.0. If the ratio is less than 0.05, the excess ferric ions may be converted into hydroxide ions or acid ions. It is not possible to prevent sludge formation by bonding with iron, and plating efficiency is low even without ferric iron. On the other hand, if the value exceeds 2.0, the plating quality will deteriorate. Even if the plating is carried out over a long period, the sludge suppression effect and plating quality are maintained, but the overvoltage increases and plating efficiency decreases. The amount of amino acids is reduced, and the amount of amino acids is relatively high compared to raw materials containing iron ions such as ferrous sulfate. This is not economical as it increases the cost of raw materials.

[0077] The complexing agent is one or more selected from amino acids and amino acid polymers. are preferred, for example, alanine, glycine, serine, threonine, arginine, glutamine The compound may be one or more selected from the group consisting of glycine, glutamic acid, and glycylglycine. .

[0078] The above amino acid was used as a complexing agent, and the solution temperature was maintained at 80°C or less and pH was kept at 2.0 to 5.0. However, current density is 3~120A / dm 2 When electroplating is performed with A highly concentrated Fe plating layer can be obtained.

[0079] The temperature of the Fe electroplating solution does not significantly affect the quality of the Fe plating layer, but if it exceeds 80°C, If the solution begins to evaporate, the concentration of the solution will change continuously, resulting in a uniform voltage. This makes electroplating difficult.

[0080] When the pH of the Fe electroplating solution is below 2.0, the electroplating efficiency decreases and continuous plating becomes difficult. If the pH exceeds 5.0, plating efficiency increases, but continuous plating is not possible. During electroplating, iron hydroxide precipitates and generates sludge, which clogs pipes, rolls, and equipment. Contamination problems will arise.

[0081] The current density is 3A / dm 2 When the cathode plating overvoltage drops below 1000kJ / s, the Fe electroplating Due to the decrease in efficiency, it is not suitable for continuous plating processes. 2 will exceed When this occurs, burning occurs on the plating surface, making the electroplating layer uneven and causing the Fe plating layer to peel off. The problem is that it is easy to drop.

[0082] As described above, in the present invention, it is preferable that the Fe plating layer contains 5 to 50% by weight of oxygen. The causes of oxygen contamination in the Fe plating layer are as follows: During the process of iron deposition on the plate surface, hydrogen ions are simultaneously reduced to hydrogen gas, causing the pH to rise. As a result, both ferrous and ferric ions temporarily become OH. - Ion and They can become bonded together and be mixed in when the Fe plating layer is formed. When anionic complexing agents such as acetic acid, lactic acid, citric acid, and EDTA are used, the complexing The agent is OH - The iron ions bonded to the ions have an average negative charge, making them an anion for electroplating. When a voltage is applied, an electrical repulsive force is generated, which prevents contamination of the Fe plating layer. Amino acids are electrically neutral at pH 2.0 to 5.0, but are cationic in strong acids below pH 2.0. The iron ion bound to the amino acid has one or two OH groups. - Even if it binds, the cation As a result, an electric attraction occurs with the cathode where electroplating is performed, causing a large amount of oxygen to be mixed in. Therefore, the molar concentration ratio of iron ions to amino acids should be 1:0.05 to 1:2.0. As shown above, Fe electroplating was performed using amino acids as complexing agents, maintaining the pH at 2.0 to 5.0. When using a plating solution containing 5 to 50% by weight of oxygen, plating efficiency is high and sludge generation is suppressed. An Fe plating layer can be obtained.

[0083] In order to ensure the quality of hot-dip galvanizing of steel sheets containing Mn and Si, the Fe-coated layer The plating amount is 0.5 to 3.0 g / m based on the amount of iron. 2 It is preferable to treat with F e) The upper limit of the plating amount is not particularly limited, but it is 3.0 g / m in the continuous plating process. 2 It will exceed This would require multiple plating cells or would slow down production speed, making it uneconomical. Furthermore, if the amount of Fe electroplating is large, the Fe electroplating solution may be rapidly degraded in the continuous process. H drops, plating efficiency drops significantly, and solution management becomes difficult. On the other hand, the amount of Fe electroplating is 0.5g / m 2 When the temperature is less than 1000K, the oxygen contained in the Fe plating layer Because it is rapidly reduced and removed, Mn and Si diffuse from the base iron and form surface oxides. This makes it impossible to effectively prevent the hot dip coating from being damaged, resulting in a problem of reduced hot dip coating quality. The above Fe plating amount is determined based on the iron concentration contained in the plating layer so that the Fe plating layer is completely reduced during annealing. When formed, the thickness is about 0.05 to 0.4 μm. [Example]

[0084] The present invention will be described in more detail below through examples. However, the following examples are merely examples of the present invention. It should be understood that the above description is for illustrative purposes only and is not intended to limit the scope of the present invention. It should be noted that the scope of the present invention is defined by the matters set forth in the claims and the claims therefrom. This is because the decision is based on matters that can be reasonably inferred from the above.

[0085] (Example) First, two types of base steel were prepared as shown in Table 1 below. The base steel was a cold-rolled steel sheet. and no special plating layer is formed on the surface.

[0086] [Table 1]

[0087] Before Fe electroplating on the steel sheet, Fe electroplating was performed using a Cu sheet, and then 5 to 1000 kJ / cm2 was added. The total amount of Fe was measured by dissolving it in a 10 wt% hydrochloric acid solution to predict the amount of electroplating adhesion and plating efficiency. Using the measured plating efficiency as a reference, we performed Fe electroplating on cold-rolled steel sheets. Even if the plating solution and plating conditions are changed, the Fe electroplating weight can be adjusted similarly. The Cu plate was further electroplated with Fe using each solution and plating condition, and the GDS analysis showed that The total amount of Fe and O is calculated and the average oxygen concentration of the Fe plating layer is measured according to each electroplating condition. The plating weight was measured separately by dissolving it in hydrochloric acid and the results are shown in Table 2. The temperature of the solution was adjusted to 50°C. On the other hand, the solution and plating conditions were the same as those used for electroplating Cu. After forming an iron electroplating layer on the two types of cold-rolled steel sheets listed in Table 1, they were annealed under the following conditions. The inside of the annealing furnace was filled with N2 gas containing 5% H2 in all sections. A reducing atmosphere was maintained, and the dew point was maintained at -40°C in all sections as shown in Table 2. Here, Fe electroplating was performed using the above-mentioned process (i.e., the conditions in Table 2, plating bath temperature: 50°C). The coated cold-rolled steel sheet was charged and heated to 810°C at a rate of approximately 2.5°C / sec. After this, the temperature was gradually cooled to 650°C at a rate of 2.8°C / sec, and then The steel was then rapidly cooled to 400°C at a rate of 14.5°C. Once cooling was complete, the steel was ready for hot dip galvanizing. The temperature was raised again to 480°C so that the hot-dip galvanizing bath could be used. The plating bath contains 0.20 to 0.25% Al, and the temperature is maintained at 460°C. The steel sheet was then cooled to room temperature to produce a hot-dip galvanized steel sheet.

[0088] The galvanizability of the manufactured hot-dip galvanized steel sheets was evaluated, and the steel sheets were plated with an approximately 8% hydrochloric acid solution. The GDS concentration profile of the base iron from which the layer was dissolved was measured, and the maximum points of Mn and Si, The average concentration was measured at the minimum point and at 5 μm inside the base iron, and the results are shown in Table 3. .

[0089] The galvanization of the hot-dip galvanized steel sheets was evaluated visually. If there are no unplated spots, it is expressed as "good." If there are unplated spots of 1 mm or less, it is expressed as "unplated spots." If an area with a diameter exceeding 1 mm is left unplated, it is classified as "unplated." .

[0090] To evaluate the adhesion of the coating, a sealant for automobile structures was applied to the hot-dip galvanized steel sheet with a thickness of approximately 5 m. The galvanized coating is applied to a thickness of 1 mm and hardened at a temperature of 150-170°C. The steel plate was bent at a 90-degree angle to peel off the sealant. The plating layer adhered to the sealant. If the entire interface between the zinc coating and the base steel peels off, it is determined that the coating adhesion is poor and is classified as " If no peeling of the plating layer occurs, the plating adhesion is considered to be "good." In some test pieces, only part of the plating layer was peeled off, but this However, for test pieces where "unplated" occurred, The plating adhesion was not evaluated.

[0091] The coating layer was removed with hydrochloric acid, and the concentration profile of the base steel was analyzed according to the GDS analysis method described above. After applying a 100 nm Gaussian filter to remove noise, the maximum and The minimum point was calculated. In some GDS profiles, the maximum or minimum point can be calculated. In this case, "ND" is displayed. However, in the case of a minimum point, it is not displayed. However, when calculating the difference in converted concentration, the Mn and Si concentrations inside the base material described below should be used. However, if the maximum point is not formed, It was not possible to determine the converted concentration for these compounds, and they were deemed to be outside the scope of the present invention.

[0092] The concentrations of Mn and Si inside the base material are determined from the steel sheet surface (coating layer interface) to the depth. The value measured at 5 μm in the direction was used.

[0093] [Table 2] JPEG2025183299000004.jpg95170

[0094] [Table 3] JPEG2025183299000006.jpg45170

[0095] In Comparative Examples 1 and 2, the base steel sheet without the iron electroplating layer was subjected to the same conditions as described above. In Comparative Example 1, where the base steel sheet had a high Si content, The zinc plating was not applied, and most of the surface of the base steel sheet was exposed. In Comparative Example 2, where the plate had a low Si content, the appearance after hot dip galvanizing was good, but the plating density was poor. The evaluation of adhesion showed that the entire coating layer was peeled off. After removing the coating layer with hydrochloric acid, the GDS concentration profile of Mn and Si on the surface of the base steel sheet was measured. The results are shown in graph (a) of Figure 4. The solid line in the figure represents the Mn concentration profile. The dotted line represents the Si concentration profile (the same applies below). It was confirmed that the Mn concentration was low down to a depth of 1 μm, which is because Mn does not form a surface oxide. However, a certain amount of Mn is depleted near the surface of the base steel sheet. The concentration tended to increase up to a depth of 0.5 μm from the surface of the plate, but this is because Si was not only present on the surface. However, oxidation continues to occur inside the base steel sheet, and the concentration of Si is high near the surface. In Comparative Examples 3 to 7, the iron electroplating solution contained citric acid as a complexing agent. Iron electroplating was performed using a solution. The amount of iron contained in the iron electroplating layer measured by GDS analysis was The oxygen concentration was approximately 3.3 to 4.9% by weight. The coating amount is 1.21g / m 2 In the following Comparative Examples 3 to 5, a wide area was not hot-dip galvanized. , iron electroplating amount is 1.99g / m 2 In the above Comparative Examples 6 and 7, the hot-dip galvanized However, the evaluation of coating adhesion showed that the coating layer peeled off in all cases. After removing the zinc layer with hydrochloric acid, the GDS concentration profile was measured and shown in graph (b) of Figure 4. Compared with the case where no iron electroplating layer is formed as shown in graph (a) of Figure 4, The surface of the base steel sheet was shown to be depleted of Mn even more, but the Mn-free iron electroplating This is because the Mn concentration was diluted by the formation of an iron electroplating layer. The Mn maximum point appears at a certain depth, but the conversion of the maximum and minimum points to the average concentration inside the base iron is The difference in concentration was very slight, at about 2.3%.

[0096] In Comparative Examples 8 to 12, iron electroplating was performed using a solution containing sodium lactate as a complexing agent. The oxygen content in the iron electroplating layer was at the level of 3.3 to 3.7%, and the citric acid The level was slightly lower than when galvanizing was performed using Zn as a complexing agent. As a result, the amount of iron electroplating was 1.20 g / m 2 The following is an uncoated state without hot dip galvanizing: The iron electroplating amount is 2.02g / m 2 The above results indicate that fine dots of ungalvanized zinc are formed during hot dip galvanizing. Plating was formed and the plating adhesion was poor.

[0097] In Comparative Examples 13 and 14 and Invention Examples 1 to 7, glycine, a type of amino acid, was used as a complexing agent. The iron electroplating solution was prepared using the above solution, and iron electroplating was carried out on all two types of base steel sheets. The oxygen content in the layer is 5.1 to 10.3% by weight, and citric acid or sodium lactate is added. The iron electroplated cold rolled steel sheet was hot dip galvanized. The plating properties and plating adhesion were evaluated. As in Comparative Examples 13 and 14, the iron plating amount was 0. 4g / m 2 When the number of spots is small, both Steel 1 and Steel 2 have fine spots of 1 mm or less in diameter. However, in Examples 1 to 7 of the present invention, peeling occurred. The amount of iron electroplating is approximately 0.82 g / m 2 If the thickness is more than 10 ... The coating adhesion was also good. After removing the plating layer from the plate with hydrochloric acid, the GDS concentration profile was measured and the results are shown in the graph in Figure 4. In the depth region corresponding to the iron electroplating layer, both Mn and Si have a maximum point. The minimum point is in a region deeper than the thickness of the iron electroplating layer. The difference in the degree of Mn was very high at 69% and Si at 87%. The difference in the concentration ratio between the maximum and minimum points of Mn and Si showed a tendency to increase. Iron electroplating amount: 0.82g / m 2 In Example 1, the concentration ratio of Mn at the maximum and minimum points is The difference is 23%, Si is at 32%, and the iron electroplating amount is 3.00g / m 2 Most common in In Example 4, the difference in the concentration ratio between the maximum and minimum points of Mn was 107%, and that of Si was at the 105% level. It was.

[0098] On the other hand, in Examples 5 to 7 in which an iron electroplating layer was formed on Steel 2, the Mn concentration at the maximum and minimum points was The difference in ratio is 12% to 58%, and Si shows a similar trend to Steel 1 at the 30% to 39% level. The iron electroplating layer contains a large amount of Mn and Si, and the Mn and Si are then deposited under the iron electroplating layer. It can be seen that the surface quality of hot-dip galvanized coating is improved by increasing the temperature.

[0099] Inventive Examples 8 to 14, the iron electroplating solution used glycine as a complexing agent was used at 70 A / dm 2 The results of iron electroplating and hot dip galvanizing are shown. 0A / dm 2 The oxygen content in the Fe plating layer was measured from Comparative Examples 13 and 14 and Invention Examples 1 to 7. The results for hot dip galvanizing were similar, although the content was higher. Even if the content is high, the Fe plating amount (iron coating amount) is 0.5 g / m 2 If it's not higher than M Molten zinc with excellent surface quality by effectively oxidizing n and Si internally and suppressing diffusion to the surface. It was confirmed that it was not possible to manufacture plated steel sheets.

[0100] As described above, in the case of the invention examples that satisfy all the conditions of the present invention, excellent plating properties and plating Therefore, it was confirmed that the advantageous effects of the present invention were This was possible.

Claims

1. The GDS profile of the Mn and Si components observed from the surface to the depth direction shows successive maximums. Contains points and minima, The Mn concentration at the maximum point of the GDS profile of the Mn component is divided by the Mn concentration of the base material. The Mn concentration at the minimum point of the GDS profile of the Mn component is calculated as the Mn concentration of the base material. the difference between the value obtained by dividing by (the difference in Mn converted concentration) is 10% or more, The Si concentration at the maximum point of the GDS profile of the Si component is divided by the Si concentration of the base material. The Si concentration at the minimum point of the GDS profile of the Si component is divided by the Si concentration of the base material. The difference between the measured value (difference in Si converted concentration) and the measured value is 10% or more. However, if no minimum point appears within a depth of 5 μm, the point at a depth of 5 μm is regarded as the point at which the minimum point appears. The location.

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

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

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

5. The base iron contains, in weight percent, Mn: 1.0 to 8.0%, Si: 0.1 to 3.0%. The steel sheet for plating according to any one of claims 1 to 4.

6. The base iron contains, in weight percent, Mn: 1.0 to 8.0%, Si: 0.1 to 3.0%, C:

0. 0.05 to 0.3%, Al: 0.005 to 3.0%, P: 0.04% or less (excluding 0%), S : 0.015% or less (excluding 0%), Cr: 1.5% or less (including 0%), B: 0.00 5% or less (including 0%), the balance being Fe and unavoidable impurities. Steel sheets for plating.

7. The steel sheet for plating according to any one of claims 1 to 4, and molten zinc formed on the steel sheet for plating. Hot-dip galvanized steel sheet, including the coating layer.

8. Preparing the base steel; The base iron is electroplated to form an Fe plating layer containing 5 to 50 wt % of oxygen. forming; and The base steel on which the Fe plating layer is formed is subjected to a 1 to 70% H ２ - remaining N ２ Annealing in a gas atmosphere annealing furnace at 600 to 950°C for 5 to 120 seconds A method for manufacturing a steel sheet for plating, comprising the steps of:

9. The coating weight of the Fe plating layer is 0.5 to 3 g / m ２ The steel for plating according to claim 8, How the board is manufactured.

10. The complexing agent is selected from the group consisting of alanine, glycine, serine, threonine, arginine, glutamine, and glutamine. 8 or 9, wherein the base is one or more selected from glutamic acid and glycylglycine. A method for producing a steel sheet for plating according to claim 1.

11. The electroplating solution contains ferrous ions and ferric ions, and the ferric ions are generally The total concentration of the iron ions is 5 to 60% by weight.

10. The method for producing a steel sheet for plating according to claim 8 or 9, wherein the amount of the sintered body is 1 to 80 g per 1 L of plating solution. Law.

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

13. Preparing the base steel; The base iron is electroplated to form an Fe plating layer containing 5 to 50 wt % of oxygen. forming step; The base steel on which the Fe plating layer is formed is subjected to a 1 to 70% H ２ - remaining N ２ Annealing in a gas atmosphere annealing furnace at 600 to 950°C for 5 to 120 seconds to obtain a steel sheet for plating; and A method for producing a hot-dip galvanized steel sheet, comprising the step of immersing the steel sheet for plating in a zinc plating bath. Law.

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