Plated steel sheet for hot press forming with excellent plating quality, steel sheet, and manufacturing methods thereof

Abstract

The present invention relates to a hot-dip galvanized steel sheet having excellent plating quality, a steel sheet for plating for manufacturing the same, and methods for manufacturing them. The hot stamping steel sheet according to one aspect of the present invention has a composition containing, by weight%, Mn: 0.1 to 4%, Si: 0.001 to 2%, C: 0.02 to 0.6%, Al: 0.001 to 1%, P: 0.05% or less, S: 0.02% or less, Cr: 1% or less, B: 0.01% or less, with the balance being Fe and inevitable impurities. The GDS profile of the Mn component and the GDS profile of the Si component observed in the depth direction from the surface each sequentially include a maximum point and a minimum point. The difference between the value obtained by dividing the Mn concentration at the maximum point of the GDS profile of the Mn component by the Mn concentration of the base material and the value obtained by dividing the Mn concentration at the minimum point of the GDS profile of the Mn component by the Mn concentration of the base material (converted Mn concentration difference) is 10% or more, and the difference between the value obtained by dividing the Si concentration at the maximum point of the GDS profile of the Si component by the Si concentration of the base material and the value obtained by dividing the Si concentration at the minimum point of the GDS profile of the Si component by the Si concentration of the base material (converted Si concentration difference) can be 10% or more. However, when no minimum point appears within a depth of 5 μm, the point at a depth of 5 μm is regarded as the point where the minimum point appears.

Description

Technical Field

[0001] The present invention relates to a plated steel sheet for hot press forming with excellent plating quality, a steel sheet for plating, and methods for manufacturing the same.

Background Art

[0002] Hot press forming is a processing method for obtaining high-strength parts by cooling a steel sheet substantially simultaneously with forming it at a high temperature. The steel sheet used for hot press forming must have excellent hardenability so that martensite can be easily formed during cooling at a high temperature. In order to improve the hardenability of the steel sheet in this way, various alloying elements are added to the steel for hot press forming as compared with general steel, and in particular, elements with a high oxidation tendency with respect to Fe such as Mn, Si, Al, Cr, and B are added in large amounts. Further, in order to prevent decarburization and oxidation of the steel sheet during hot press forming, various types of plating may be performed on the surface of the steel sheet. Among them, a method of plating the surface of the steel sheet by molten plating, such as molten plating or molten aluminum plating, is widely used.

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

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

[0005] When using the method of oxidizing and then reducing in the annealing process as in Patent Document 1, components with a large affinity for oxygen such as Si, Mn, and Al are internally oxidized at a certain depth from the surface layer of the steel sheet, and diffusion to the surface layer is suppressed. Therefore, the relative amount of Si, Mn, or Al alone or composite oxides in the surface layer decreases, the wettability with zinc improves, and unplated areas can be reduced. However, in the case of steel grades added with Si, Si concentrates directly below the iron oxide during the reduction process to form strip-shaped Si oxides, resulting in peeling in the surface layer including the plating layer, that is, peeling occurs at the interface between the reduced iron and the base iron below, making it difficult to ensure the adhesion of the plating layer.

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

[0007] Also, even if the plating property is improved by internal oxidation, linear unplated areas may occur due to the surface oxides formed non-uniformly on the surface, or when producing an alloyed hot-dip galvanized steel sheet (GA steel sheet) through alloying heat treatment after plating, problems such as linear defects due to non-uniform alloying may occur on the surface of the alloyed hot-dip galvanized steel sheet.

[0008] Furthermore, as another conventional technique, there is a method of performing Ni pre-plating before annealing to suppress the diffusion of alloying elements to the surface during annealing. However, although this method is effective in suppressing the diffusion of Mn, there is a problem that it cannot sufficiently suppress the diffusion of Si.

Prior Art Documents

Patent Documents

[0009]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

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

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

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

[0013] The problems of the present invention are not limited to the above-described content. Those having ordinary knowledge in the technical field to which the present invention pertains will have no difficulty in understanding further problems of the present invention from the overall matters of the specification of the present invention.

Means for Solving the Problems

[0014] The hot press forming steel sheet according to one aspect of the present invention has, by weight%, Mn: 0.1 to 4%, Si: 0.001 to 2%, C: 0.02 to 0.6%, Al: 0.001 to 1%, P: 0.05% or less, S: 0.02% or less, Cr: 1% or less, B: 0.01% or less, and the balance Fe and unavoidable impurities. The GDS profiles of the Mn component and the Si component observed in the depth direction from the surface each sequentially include a maximum point and a minimum point. The difference (converted Mn concentration difference) obtained by dividing the Mn concentration at the maximum point of the GDS profile of the Mn component by the Mn concentration of the base material and the difference (converted Si concentration difference) obtained by dividing the Si concentration at the minimum point of the GDS profile of the Mn component by the Mn concentration of the base material are each 10% or more. The difference (converted Si concentration difference) obtained by dividing the Si concentration at the maximum point of the GDS profile of the Si component by the Si concentration of the base material and the difference (converted Si concentration difference) obtained by dividing the Si concentration at the minimum point of the GDS profile of the Si component by the Si concentration of the base material can be 10% or more.

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

[0016] The hot dip galvanized steel sheet for hot press forming, which is another aspect of the present invention, can include the above-described galvanized steel sheet and a hot dip galvanized layer formed on the galvanized steel sheet.

[0017] Another aspect of the manufacturing method of the hot press forming steel sheet according to the present invention includes the steps of preparing base iron having a composition containing, by weight%, Mn: 0.1 to 4%, Si: 0.001 to 2%, C: 0.02 to 0.6%, Al: 0.001 to 1%, P: 0.05% or less, S: 0.02% or less, Cr: 1% or less, B: 0.01% or less, and the balance being Fe and unavoidable impurities; performing electroplating on the base iron to form an Fe plating layer containing 5 to 50% by weight of oxygen; and annealing the base iron with the Fe plating layer formed therein at 600 to 950°C for 5 to 120 seconds in an annealing furnace with a dew point temperature controlled to less than -20°C in an atmosphere of 1 to 70% H2 - the balance being N2 gas.

[0018] Another aspect of the manufacturing method of the hot press forming electroplated steel sheet according to the present invention includes the steps of preparing base iron having a composition containing, by weight%, Mn: 0.1 to 4%, Si: 0.001 to 2%, C: 0.02 to 0.6%, Al: 0.001 to 1%, P: 0.05% or less, S: 0.02% or less, Cr: 1% or less, B: 0.01% or less, and the balance being Fe and unavoidable impurities; performing electroplating on the base iron to form an Fe plating layer containing 5 to 50% by weight of oxygen; annealing the base iron with the Fe plating layer formed therein at 600 to 950°C for 5 to 120 seconds in an annealing furnace with a dew point temperature controlled to less than -20°C in an atmosphere of 1 to 70% H2 - the balance being N2 gas to obtain a plating steel sheet; and dipping the plating steel sheet into a molten plating bath.

Advantages of the Invention

[0019] As described above, the present invention can provide an electroplated steel sheet that significantly improves the phenomenon of non-plating during electroplating and improves the plating adhesion by forming a pre-plating layer and controlling the concentration profiles of the internal Mn and Si components.

[0020] Also, according to one aspect of the present invention, linear defects and the like can be prevented on the surface of the alloyed electroplated steel sheet obtained by performing alloying heat treatment on the electroplated steel sheet of the present invention, and an alloyed electroplated steel sheet with excellent surface quality can be provided.

Brief Description of the Drawings

[0021]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

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

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

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

[0025] It is known that the cause of non-plating and the decrease in plating adhesion in a steel sheet containing a large amount of Mn and Si is due to surface oxides generated by the oxidation of alloy elements such as Mn and Si on the surface during the annealing process of the cold-rolled steel sheet at a high temperature.

[0026] In order to suppress the diffusion of alloying elements such as Mn and Si to the surface, as a method of forming an oxide layer containing a large amount of oxygen, an oxidation-reduction method in which oxidation is performed during temperature rise and then maintained in a reducing atmosphere for reduction, or a method of coating the surface of the base metal with iron oxide and performing heat treatment can be used. However, the iron oxide firmly formed on the surface of the base metal contains not only FeO but also Fe3O4 and Fe2O3 which are difficult to reduce, and while the surface is reduced to metallic iron during the annealing process in a reducing atmosphere, the interface between the iron oxide layer and the base iron has a slow reduction rate, so it is difficult to be completely reduced, and Mn and Si oxides accumulate at the interface to form a continuous oxide layer. Therefore, even if the wettability with molten zinc is improved, there is a possibility that the oxide layer will easily collapse and the plating layer will peel off.

[0027] On the other hand, when applying the annealing internal oxidation method of increasing the oxygen partial pressure or dew point in the annealing furnace during the heat treatment process to oxidize alloying elements such as Mn and Si inside the steel, Mn and Si oxides are preferentially formed on the steel surface during the heat treatment process. After that, Mn and Si are oxidized by the oxygen diffused into the steel to suppress surface diffusion. Therefore, a thin oxide film is formed on the surface of the base iron, but if the surface of the cold-rolled steel sheet before annealing is not completely homogeneous, or local variations such as oxygen partial pressure and temperature occur, the wettability in the molten zinc plating will be non-uniform and unplated areas will occur, or if the thickness of the oxide film is non-uniform and a difference in alloying degree occurs during the alloying heat treatment process after zinc plating, linear defects that can be easily identified visually tend to occur.

[0028] In order to solve the problems of the above technology, the inventors of the present invention tried to manufacture a hot-dip plated steel sheet with a beautiful surface and no problem of plating peeling by controlling the existence form of oxidation elements Mn and Si on the surface side of the steel sheet for plating as follows.

[0029] That is, the steel sheet according to an embodiment of the present invention can have the following characteristics in the GDS concentration profiles of Mn and Si. The steel sheet for plating of the present invention will be described in detail with reference to the GDS profile of FIG. 1.

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

[0031] As can be seen from the typical example of the GDS profile of the Mn component of the present invention shown in Figure 1, the steel sheet of the present invention can have a concentration gradient in which the concentration of Mn is very low on its surface, and maximum points and minimum points appear sequentially in the depth direction from the surface. Here, having sequentially does not mean that the maximum point must appear first in the depth direction from the surface (interface), and in some cases, the minimum point may appear first, but then the maximum point and the minimum point must appear sequentially. However, in some embodiments, the minimum point may not appear, and in this case, the internal concentration in the 5-μm depth region can be taken as the minimum point concentration. Also, the concentration of the alloy 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 alloy element may appear between the surface and the maximum point.

[0032] In the GDS concentration profile illustrated in Figure 1 above, although not necessarily limited to this, the surface layer portion corresponds to the Fe plating layer with a low concentration of alloy elements because alloy elements do not diffuse much from the base iron. The maximum point corresponds to the region where the internal oxides of alloy elements concentrated near the interface between the Fe plating layer and the base iron are formed. The minimum point that appears on the base iron side in the Fe plating layer corresponds to the region where the alloy elements are diffused and diluted in the Fe plating layer that does not contain alloy elements, or the region where the alloy elements are diffused and depleted in the maximum point where internal oxidation occurs.

[0033] In one embodiment of the present invention, the above-mentioned maximum point can be formed at a depth of 0.05 to 1.0 μm from the surface of the steel sheet. If the maximum point appears in a region deeper than this, it may not be determined as the maximum point due to the effect of the present invention. Further, the minimum point can be formed at a position within a depth of 5 μm from the surface of the steel sheet. As described above, if the minimum point is not formed at a point within a depth of 5 μm, the point where the minimum point is formed can be set at a depth of 5 μm. Since the concentration at a depth of 5 μm is substantially the same as the concentration of the base material, it can be regarded as the point where the concentration does not decrease further.

[0034] At this time, the greater the difference between the converted concentration of the maximum point of the element (the value obtained by dividing the concentration at that point by the concentration of the base material, expressed in % units) and the converted concentration of the minimum point in the Mn concentration profile and the Si concentration profile, the more the Mn and Si diffusing to the surface can be reduced, which is important. In one embodiment of the present invention, the value of the converted concentration of the maximum point - the converted concentration of the minimum point of Mn and Si can each be 10% or more.

[0035] As a result of experiments by the present inventors under various conditions, when the above conditions are satisfied, no unplated portion occurs during hot dip plating, and a hot dip plated steel sheet with good plating adhesion can be obtained. However, if the difference between the converted concentrations of the maximum point and the minimum point of Mn and Si is less than 10%, there may be problems such as the occurrence of point or linear unplated portions or plating peeling. That is, by controlling the difference in the converted concentration to a certain level or more, it is possible to prevent the formation of oxides of Mn and Si on the surface, obtain a hot dip plated steel sheet with a beautiful surface and good plating adhesion, and suppress the occurrence of defects such as linear defects on the surface even after passing through the process of alloying heat treatment. Since it is more advantageous as the difference in the converted concentration value is larger, there is no need to specifically define the upper limit of the value. However, considering the content of the elements contained, the difference in the converted concentration value can be determined to be 200% or less for both Mn and Si. In another embodiment of the present invention, the converted concentration difference of the above-mentioned Mn and Si can be 15% or more or 20% or more.

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

[0037] For GDS concentration analysis, the molten-plated steel sheet is sheared into pieces with a size of 30 to 50 mm in length, and after primary washing with a NaOH solution at room temperature, it is immersed in a hydrochloric acid aqueous solution of 20 to 40 vol% to remove the plating layer.

[0038] In order to prevent surface damage to the base iron during the dissolution process of the plating layer, when the generation of bubbles due to the reaction between the plating layer and the acid solution is interrupted, the acid solution is removed within 10 seconds, and the base iron is washed with pure water and dried. Of course, if it is a steel sheet for plating that has not yet been molten-plated, it can be analyzed without such a plating layer removal operation.

[0039] The GDS concentration profile measures the concentrations of all components contained in the steel sheet at intervals of 1 to 5 nm in the steel sheet thickness direction. The measured GDS profile may contain irregular noises. In order to calculate the maximum and minimum points of the Mn and Si concentrations, a Gaussian filter with a cut-off value of 100 nm is applied to the measured concentration profile to obtain an average concentration profile, and the concentration values and depths of the maximum and minimum points of the concentration are obtained from the profile with the noises removed. It should be noted that the maximum and minimum points referred to in the present invention are calculated as the maximum and minimum points only when there is a difference of 10 nm or more from each other in the depth direction.

[0040] The steel sheet for plating targeted in the present invention can include base iron and an Fe plating layer formed on the base iron. The composition of the base iron is not particularly limited.

[0041] However, in the case of a steel sheet containing 0.1 to 4% by weight of Mn and 0.001 to 2% by weight of Si and having a composition in which oxides are likely to be formed on the surface, the plating property can be advantageously improved by the present invention. The upper limit of the Mn concentration in the base iron is not particularly limited, but considering the commonly used composition, the upper limit can be limited to 4% by weight. Also, the lower limit of the Mn concentration is not particularly limited, but in the case of a composition containing less than 0.1% by weight of Mn, the surface quality of the hot-dip galvanized steel sheet is beautiful even without forming an Fe plating layer, and it is not necessary to perform Fe electroplating. The upper limit of the Si concentration is not particularly limited, but considering the commonly used composition, the upper limit can be limited to 2% by weight or less. When the Si concentration is less than 0.001% by weight, the hot-dip galvanized quality is beautiful even without implementing the method of the present invention, so it is not necessary to implement the method of the present invention.

[0042] Since the above Mn and Si are elements that affect the plating property, their concentrations can be limited as described above, but the present invention does not particularly limit the remaining components of the base iron.

[0043] However, in the case of a steel sheet containing a large amount of alloy components, considering the aspect that unplated and poor plating adhesion can occur severely, in one embodiment of the present invention, the composition of the above base iron is in wt%, Mn: 0.1 to 4%, Si: 0.001 to 2%, C: 0.02 to 0.6, Al: 0.001 to 1%, P: 0.05% or less, S: 0.02% or less, Cr: 1% or less, B: 0.01% or less, and the balance is Fe and inevitable impurities. It is more advantageous that P and S are not added as impurities, and since Cr and B may not be added as optional elements, the lower limit is not separately defined. In addition to the above-described components, the base iron can further contain elements such as Ti, Mo, and Nb in a total amount of 1.2% or less. Although the base iron is not particularly limited, in one embodiment of the present invention, a cold-rolled steel sheet or a hot-rolled steel sheet can be used as the base iron.

[0044] In one aspect of the present invention, a hot-dip galvanized steel sheet including the above galvanizing steel sheet can be provided. The hot-dip galvanized steel sheet can include a galvanizing steel sheet and a hot-dip galvanized layer formed on the surface of the galvanizing steel sheet. At this time, any commercially available hot-dip galvanized steel sheet can be applied without particularly limiting its type.

[0045] Next, an exemplary embodiment of a method for manufacturing a galvanizing steel sheet and a hot-dip galvanized steel sheet having the above-described advantageous effects will be described. According to one embodiment of the present invention, the galvanizing steel sheet is prepared by preparing base iron; performing electroplating on the base iron to form an Fe plating layer containing 5 to 50% by weight of oxygen; and annealing the base iron on which the Fe plating layer is formed to obtain a galvanizing steel sheet.

[0046] Fe electroplating containing 6.2% by weight of oxygen was performed on a cold-rolled steel sheet for hot press forming containing 1.3% Mn, 0.3% Si, and other alloying elements so that the iron deposition amount became 1.95 g / m 2 After that, the electroplated cold-rolled steel sheet was annealed at 800°C for 53 seconds in an atmosphere of N2-5%H2, dew point -40°C, and then cooled. The atmosphere during the entire annealing process was maintained in the same manner. After sampling a sample from the cooled steel sheet and observing the cross-section with a transmission electron microscope, it was confirmed that particulate Mn and Si oxides were formed at the interface between the iron electroplating layer and the base iron, while almost no Mn and Si oxides were formed on the surface of the electroplating layer. In particular, when analyzing such a state with a GDS profile, as shown in FIG. 1, maximum values of Mn and Si concentrations appear immediately below the surface of the steel sheet (including the Fe plating layer), and minimum values of Mn and Si concentrations may appear at deeper positions. Of course, in some cases, the minimum value may not appear clearly and the concentration may tend to decrease gradually, but the maximum value can be clearly observed.

[0047] Such a phenomenon is due to the formation of an Fe plating layer with a high oxygen content before annealing. That is, when the ferroelectric plating layer contains 5 to 50% by weight of oxygen, when annealed in a reducing atmosphere annealing furnace, the oxygen in the ferroelectric plating layer oxidizes alloying elements such as Mn and Si that diffuse to the surface in the base iron, and accumulates at the interface between the ferroelectric plating layer and the base iron. Therefore, as shown in the graph of Figure 1, when measuring the concentration by GDS, a maximum point with a high concentration of Mn, Si, etc. is confirmed at a depth corresponding to the thickness of the ferroelectric plating layer from the surface. On the other hand, alloying elements with a slow diffusion rate such as Mn may have a minimum point after the maximum point of the GDS concentration because they are diluted by the ferroelectric plating layer or the dissolved Mn cannot diffuse quickly from the base iron even when depleted by internal oxidation. However, since Si diffuses rapidly from the inside during the annealing process and internal oxidation continuously proceeds at the interface between the ferroelectric plating layer and the base iron, resulting in the accumulation of oxides, a minimum point may not be confirmed by GDS concentration analysis. Therefore, the absence of a minimum point in the GDS concentration profile means that alloying elements such as Mn and Si are oxidized by the ferroelectric plating layer, effectively suppressing diffusion to the surface.

[0048] Different from the oxides firmly formed at high temperatures, when annealing is carried out by forming an iron plating layer containing a large amount of oxygen, reduction occurs not only on the surface of the ferroelectric plating layer but also at the interface between the ferroelectric plating layer and the base steel plate. At this time, the reduced iron diffuses and joins with the base iron, and Mn and Si react with the oxygen in the Fe plating layer to form oxide particles or discontinuous plate-like forms at the interface between the ferroelectric plating layer and the base steel plate, so that the adhesion between the ferroelectric plating layer and the base metal can be maintained well. Not only that, the uniformly formed ferroelectric plating layer suppresses the formation of surface oxides, reduces the concentration of alloying elements such as Mn and Si dissolved in the surface of the steel plate, promotes the alloying reaction with the zinc plating layer, and a uniform alloying molten-plated steel plate without surface defects can be obtained.

[0049] Since the internal oxidation method of annealing does not form a layered oxide layer unlike the oxidation-reduction method, it exhibits excellent characteristics for improving plating adhesion during hot press forming steel sheets containing a large amount of alloy elements such as Mn and Si. However, since water vapor in the annealing furnace inevitably oxidizes the surface of the steel sheet first and then oxygen penetrates inside, the surface oxide cannot be fundamentally removed. As a result, if the surface of the cold-rolled steel sheet before annealing is not completely homogeneous, or local variations such as oxygen partial pressure and temperature occur during annealing, the wettability with the molten plating solution becomes non-uniform and unplated areas occur. In the case of zinc plating, during the alloying heat treatment process after plating, the thickness of the oxide film is non-uniform and a difference in alloying degree occurs, which may cause problems such as linearly distinguishable defects that can be easily identified even visually.

[0050] In order to suppress the surface diffusion of alloy elements by Fe plating containing a large amount of oxygen and produce a beautiful molten-plated steel sheet without plating peeling problems, an iron electroplating layer containing 5 to 50% by weight of oxygen in the base iron is formed so that the iron deposition amount is 0.5 to 3.0 g / m 2 It is preferable to raise the temperature to 600 to 950 °C so that the mechanical properties of the steel sheet can be ensured, cool it again, and then perform molten plating.

[0051] In one embodiment of the present invention, the Fe plating layer can be formed through a continuous plating process, and the Fe plating amount at this time can be set to 0.5 to 3.0 g / m based on the Fe deposition amount 2 If the Fe plating amount is less than 0.5 g / m 2 the effect of suppressing the diffusion of alloy elements by the Fe plating layer may be insufficient in a normal continuous annealing process. Also, even if it exceeds 3.0 g / m 2 the suppression effect of alloy elements can be further increased, but in order to ensure a high plating amount, multiple plating cells must be operated. When using an insoluble anode, the electroplating solution rapidly acidifies, the plating efficiency decreases, and sludge is generated, which is not economical. In another embodiment of the present invention, the Fe plating amount is 1.0 to 2.0 g / m 2This can be achieved. When internal oxidation is carried out after forming the Fe plating layer, internal oxides are formed at or immediately below the interface between the Fe plating layer and the base iron, so that the maximum points of the Mn and Si concentrations come to exist in the 0.05 - 1.0 μm region. The Fe plating amount of 0.5 - 3.0 g / m 2 in the present invention can correspond to a thickness of 0.05 - 0.4 μm after annealing.

[0052] Also, the Fe plating layer having the above-described high oxygen concentration can form maximum and minimum points in the GDS concentration profiles of Mn and Si elements inside the steel sheet for plating by controlling the temperature, dew point temperature, and atmosphere of the subsequent annealing process, so that the converted concentration at the maximum point and the converted concentration at the minimum point can satisfy the numerical ranges restricted in an embodiment of the present invention. Considering such points, in one embodiment of the present invention, the oxygen concentration in the above Fe plating layer can be 5 - 50 wt%, and in another embodiment, it can be 10 - 40 wt%. In order 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 effect of suppressing surface oxides can be obtained by increasing the Fe plating amount, but in order to obtain such an effect, it is necessary to perform plating exceeding 3.0 g / m 2 , so various problems described above may occur. Also, when the oxygen content does not reach 5 wt%, it is difficult to sequentially form maximum and minimum points in the GDS profiles of Mn and Si. Therefore, in one embodiment of the present invention, the oxygen content in the above Fe plating layer is controlled to be 5 wt% or more. On the other hand, as the oxygen concentration in the Fe plating layer increases, the effect of suppressing surface oxides during annealing can be further increased. However, it is difficult to obtain a plating layer exceeding 50 wt% by ordinary electroplating methods, so the upper limit can be restricted to 50 wt%. In another embodiment of the present invention, the oxygen concentration in the above Fe plating layer can also be restricted to 10 - 40%.

[0053] In one 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. If the annealing temperature is too low, the structure of the cold-rolled steel sheet will not be properly recovered and recrystallized, making it difficult to ensure the mechanical properties such as the strength and elongation rate of the steel sheet. When it exceeds 950°C, the alloying elements in the steel rapidly diffuse to the surface, resulting in poor quality of the hot-dip plating and uneconomical operation at an unnecessarily high temperature.

[0054] On the other hand, in one embodiment of the present invention, the dew point inside the annealing furnace is not necessarily limited to this, but can be less than -20°C. When the dew point temperature is maintained below -20°C, it is economical because another humidifying device for raising the dew point is not required. Moreover, in the case of the present invention, since an Fe plating layer with a high oxygen concentration is formed, alloying elements such as Mn and Si can be sufficiently prevented from diffusing to the surface without deliberately inducing internal oxidation by the atmosphere. The lower limit of the dew point temperature is not particularly defined. However, maintaining the dew point below -90°C may not be industrially advantageous, such as using a very high-purity gas. Considering this, the lower limit of the dew point can be determined to be -90°C. According to another embodiment of the present invention, the dew point when the temperature of the steel sheet is 600 to 950°C can be -70 to -30°C.

[0055] Also, in order to prevent the oxidation of the base iron and the Fe plating layer during annealing, the hydrogen concentration in the atmosphere gas during annealing can be determined to be 1% or more by volume. When the hydrogen concentration is less than 1%, the trace amount of oxygen inevitably contained in the H2 and N2 gases cannot be effectively oxidized and removed, increasing the oxygen partial pressure and possibly inducing surface oxidation of the base iron. On the other hand, when the hydrogen concentration exceeds 70%, the explosion risk during gas outflow and the cost due to high-hydrogen operations increase, so the above hydrogen concentration can be determined to be 70% or less. Except for the inevitable impurity gases, it can be substantially nitrogen (N2) other than the above hydrogen (H2).

[0056] According to an embodiment of the present invention, the holding time after reaching the target temperature during annealing can be limited to 5 to 120 seconds. During annealing, in order to sufficiently transfer heat to the inside of the base iron and obtain uniform mechanical properties in the thickness direction, it is necessary to hold at the annealing target temperature for 5 seconds or more. On the other hand, if the high-temperature annealing holding time is too long, the diffusion of alloying elements through the Fe plating layer increases, the amount of surface oxide formed increases, and as a result, the quality of the electroplating becomes poor. Therefore, it can be limited to 120 seconds or less.

[0057] Hereinafter, based on the above content, the effect of suppressing the surface diffusion of Mn and Si during annealing in a cold-rolled steel sheet with an Fe plating layer containing a large amount of oxygen in a high dew point atmosphere will be described in more detail with reference to FIG. 2.

[0058] FIG. 2 schematically shows the phenomena occurring inside the steel sheet by raising the temperature of the steel sheet according to the conditions of the present invention.

[0059] FIG. 2(a) shows a schematic cross-sectional view of a base steel sheet on which an Fe plating layer containing a large amount of oxygen is formed. The base steel sheet contains alloying elements such as Mn and Si. The Fe plating layer contains 5 to 50% by weight of oxygen and impurities inevitably mixed during plating, and the balance is composed of Fe.

[0060] FIG. 2(b) shows a state in which a cold-rolled steel sheet plated with iron is heated at about 300 to 500 ° C. in a nitrogen atmosphere containing 1 to 70% H2. The surface of the Fe plating layer is gradually reduced and oxygen is removed. Internal oxides such as Mn and Si diffused from the base iron are formed at the interface between the Fe plating layer and the base iron, and the grain boundary oxides grow coarser as the temperature increases.

[0061] FIG. 2(c) shows a schematic cross-sectional view of the base steel sheet when the temperature is raised to 500 to 700 ° C. in the same reducing atmosphere. Most of the Fe plating layer is reduced to form ferrite with a low Mn and Si concentration with respect to the base iron. Since the oxygen in the Fe plating layer gradually depletes, Mn and Si gradually begin to diffuse through the Fe plating layer to the surface of the Fe plating layer.

[0062] In Fig. 2(d), a schematic cross-sectional view of a steel sheet annealed at a temperature of 600 to 950°C is shown. When the electroplated iron layer excludes internal oxides such as Mn and Si, the oxygen dissolved in the metal iron is completely removed, and the generated internal oxides generally have a spherical or short plate shape. Furthermore, due to grain growth, the electroplated iron layer can also form a single crystal grain with the base iron. However, the form of the internal oxides is not necessarily generated in a particulate form, and depending on the elongation rate of the cold-rolled steel sheet, the steel components, the atmosphere in the annealing furnace, and the oxygen content contained in the electroplated iron layer, the crystal grains of the base iron and the electroplated iron layer may be distinguished and appear, and short linear oxides can also be generated along the interface between the electroplated iron layer and the base iron or the grain boundaries inside the base iron.

[0063] After the annealing step, the annealed steel sheet can be cooled. Since the cooling conditions in the cooling step after the annealing step do not have a great influence on the surface quality of the final product, that is, the plating quality, 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, at least a reducing atmosphere can be applied to the iron.

[0064] According to an embodiment of the present invention, hot dip plating can be performed on the plated steel sheet obtained by the above-described process to form a hot dip plating layer. In the present invention, the hot dip plating method is not particularly limited. The plating of the present invention is not particularly limited in terms of its type as long as it is a plating method used for a steel sheet for hot press forming. Some non-limiting examples include hot dip galvanizing and hot dip aluminum plating. It should be noted that it is not necessary to use pure zinc or aluminum for hot dip galvanizing and hot dip aluminum plating, and alloy plating containing magnesium, aluminum, zinc, and other alloy elements contained in the plating layer can also be sufficiently used.

[0065] In the present invention, as long as it is a base iron having the above-described alloy composition, it can be applied without limitation as the base iron of the plating steel sheet or the hot-dip plating steel sheet according to the present invention. Therefore, the method for producing the base iron cannot be specifically limited.

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

[0067] That is, in the present invention, in order to form an Fe plating layer, an electroplating solution containing ferrous ions including ferrous ions and ferric ions, a complexing agent, and inevitable impurities can be used, and the concentration of ferric ions in the above ferrous ions is 5 to 60% by weight.

[0068] According to one embodiment of the present invention, the electroplating solution contains ferrous ions and ferric ions. In order to obtain high plating efficiency, it may be advantageous to contain only ferrous ions. However, when only ferrous ions are contained, the solution deteriorates and the plating efficiency rapidly decreases, which may induce quality variations in the continuous electroplating process. Therefore, the above ferric ions can be further contained. At this time, the concentration of the above ferric ions is preferably 5 to 60% by weight, more preferably 5 to 40% by weight, of the total of ferrous and ferric ions. If it is less than 5%, the rate at which ferric iron is reduced to ferrous iron at the cathode is smaller than the rate at which ferrous iron is oxidized to ferric iron at the anode, and the ferric iron concentration rapidly increases, and the pH rapidly decreases while the plating efficiency continuously decreases. On the other hand, when the concentration of ferric ions exceeds 60%, the reaction amount of ferric iron reduced to ferrous iron at the cathode increases significantly more than the reaction amount of ferrous iron reduced and deposited as metallic iron, so the plating efficiency greatly decreases and the plating quality deteriorates. Therefore, considering equipment and process characteristics such as the plating amount, working current density, solution replenishment amount, the amount of solution adhering to and being lost from the strip, and the concentration change rate due to evaporation, it is preferable that the concentration of ferric ions in the above ferrous ions is 5 to 60% by weight.

[0069] The concentration of the iron ions is preferably 1 to 80 g per liter of the electroplating solution, more preferably 10 to 50 g per liter. When it is less than 1 g / L, there is a problem that the plating efficiency and plating quality rapidly decrease. On the other hand, when it exceeds 80 g / L, the solubility may be exceeded and precipitation may occur, and raw material loss due to solution loss increases in the continuous plating process, which is not economical.

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

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

[0072] When an amino acid dissolves in neutral water, the amine binds to a hydrogen ion and becomes positively charged, and the carboxyl group dissociates a hydrogen ion and becomes negatively charged, so that the amino acid molecule maintains charge neutrality. On the other hand, when the solution is acidified, the carboxyl group recombines with a hydrogen ion to become charge neutral, and the amine has a positive charge, so that the amino acid molecule forms a cation. That is, an amino acid becomes charge neutral or forms a cation in a weakly acidic aqueous solution.

[0073] When an amino acid is added to an acidic electrolyte containing iron ions, it is complexed with ferrous ions and ferric ions. However, the iron ions complexed with the amino acid maintain a cation state even in the complexed state. Therefore, it shows a characteristic electrically opposite to that of a normal complexing agent having a plurality of carboxyl groups being negatively charged in a weakly acidic aqueous solution.

[0074] In addition, amino acids form fewer bonds with iron ions and have weaker bonds than complexing agents containing multiple carboxyl groups such as citric acid and EDTA, but their bond with ferric ions that generate sludge is strong enough to prevent precipitation by ferric ions. Not only that, even when ferric ions are complexed, they can maintain their cation status, so that while ferric ions are easily transported to the cathode and reduced to ferrous ions to participate in the plating reaction, their movement to the anode is inhibited and the rate of ferric ion production slows down, so that the ferric ion concentration remains at a constant level even when plating is performed continuously for a long period of time, so that plating efficiency remains constant and there is no need to replace the electrolyte.

[0075] Meanwhile, in the continuous electroplating process, when the iron ions in the solution are depleted by plating, the solution becomes acidic, but even if the same amount of iron ions are precipitated, the pH change is smaller in a solution that also contains ferric ions than in a solution that only contains ferrous ions. When the pH increases, some of the ferric ions combine with hydroxyl ions, and when the pH decreases, the hydroxyl ions separate and are neutralized. Therefore, the pH change of the solution containing ferric ions slows down even without a separate pH buffer, and it acts as a pH buffer, so that the electroplating efficiency can be maintained constant in the continuous electroplating process.

[0076] Therefore, by using amino acids as a complexing agent, 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 used in combination, the pH change of the solution can be slowed down and the accumulation of ferric ions can be easily prevented, so that electroplating efficiency and plating quality can be maintained constant in a continuous electroplating process.

[0077] On the one hand, it is preferable that the complexing agent is added in an amount such that the molar concentration ratio of the iron ions to the complexing agent is 1:0.05 to 2.0, and more preferably in an amount such that the ratio is 1:0.5 to 1.0. When it is less than 0.05, the excessive ferric ions contained cannot be prevented from combining with hydroxide ions or oxygen to form sludge. Even without the inclusion of ferric iron, the plating efficiency decreases significantly, and furthermore, burning is induced, deteriorating the plating quality. On the other hand, even if it exceeds 2.0, the sludge suppression effect and plating quality are maintained, but the overvoltage increases, reducing the plating efficiency, and relatively expensive amino acids are excessively and unnecessarily included for raw materials containing iron ions such as iron sulfate, resulting in an increase in raw material costs and being uneconomical.

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

[0079] When the above amino acid is used as a complexing agent and electroplating is carried out at a current density of 3 to 120 A / dm² while maintaining the solution temperature at 80 °C or below and the pH at 2.0 to 5.0 2 a high plating efficiency can be achieved, and an Fe plating layer with a high oxygen concentration can be obtained.

[0080] The temperature of the Fe electroplating solution does not significantly affect the quality of the Fe plating layer. However, when it exceeds 80 °C, the evaporation of the solution becomes extreme, the concentration of the solution continuously changes, and it becomes difficult to perform uniform electroplating.

[0081] When the pH of the Fe electroplating solution is less than 2.0, the electroplating efficiency decreases and it is not suitable for a continuous plating process. When the pH exceeds 5.0, the plating efficiency increases, but sludge of iron hydroxide precipitates during continuous electroplating, causing problems such as pipe blockage, roll, and equipment contamination.

[0082] The current density is 3 A / dm² 2When it is less than this value, the plating overvoltage of the negative electrode decreases and the Fe electroplating efficiency decreases, so it is not suitable for the continuous plating process. When it exceeds 120 A / dm 2 burning occurs on the plating surface, the electroplated layer is non-uniform, and there is a problem that the Fe plating layer easily peels off.

[0083] As described above, in the present invention, it is preferable that the Fe plating layer contains 5 to 50% by weight of oxygen. The reason for the oxygen to be mixed into the Fe plating layer is as follows. During the process of iron deposition on the surface of the steel sheet to which the negative electrode is applied, at the same time, hydrogen ions are reduced to hydrogen gas and the pH starts to rise. Therefore, both ferrous and ferric ions may temporarily bind to OH - ions and may be mixed in together when the Fe plating layer is formed. If an anionic complexing agent such as acetic acid, lactic acid, citric acid, or EDTA is used, the iron ions bound to the OH - ions are negatively charged on average. When a cathode is applied for electroplating, an electrical repulsive force is generated and the mixing into the Fe plating layer is suppressed. On the other hand, amino acids are electrically neutral at pH 2.0 to 5.0, and become cationic in a strong acid with a pH less than 2.0. However, even if 1 to 2 OH - ions bind to the iron ions bound to the amino acids, they are still cationic. Therefore, an electrical attractive force is generated between the cathode for electroplating and the amino acids, and a large amount of oxygen is mixed in. Therefore, when amino acids are used as a complexing agent so that the molar concentration ratio of iron ions to amino acids is 1:0.05 to 1:2.0 and the pH is maintained at 2.0 to 5.0 to perform Fe electroplating, the plating efficiency is high, and while sludge generation is suppressed, an Fe plating layer containing 5 to 50% by weight of oxygen can be obtained.

[0084] In order to ensure the melting plating quality of the steel sheet containing Mn and Si, it is preferable to treat the plating amount of the Fe plating layer based on the iron concentration at 0.5 to 3.0 g / m 2 Although the upper limit of the Fe plating amount is not particularly limited, in the continuous plating process, 3.0 g / m 2When it exceeds this value, multiple plating cells are required, or the production speed decreases, which is not economical. Moreover, when the amount of Fe electroplating is large, the Fe electroplating solution rapidly denatures in the continuous process, the pH decreases, the plating efficiency greatly decreases, and there is a problem that solution management becomes difficult. On the other hand, when the amount of Fe electroplating is less than 0.5 g / m 2 ², since the oxygen contained in the Fe plating layer is rapidly reduced and removed, Mn and Si diffuse from the base iron, and the formation of surface oxides cannot be effectively suppressed, resulting in a problem of deterioration of the quality of the hot-dip plating. The above-mentioned amount of Fe plating has a thickness of about 0.05 to 0.4 μm when the Fe plating layer is completely reduced during annealing at the iron concentration contained in the plating layer.

Claims

1. having a composition containing, by weight%, Mn: 0.1 to 4%, Si: 0.001 to 2%, C: 0.02 to 0.6%, Al: 0.001 to 1%, P: 0.05% or less, S: 0.02% or less, Cr: 1% or less, B: 0.01% or less, with the balance being Fe and inevitable impurities, the GDS profile of the Mn component and the GDS profile of the Si component observed in the depth direction from the surface each sequentially include a maximum point and a minimum point, the difference (converted Mn concentration difference) between the value obtained by dividing the Mn concentration at the maximum point of the GDS profile of the Mn component by the Mn concentration of the base material and the value obtained by dividing the Mn concentration at the minimum point of the GDS profile of the Mn component by the Mn concentration of the base material is 10% or more, the difference (converted Si concentration difference) between the value obtained by dividing the Si concentration at the maximum point of the GDS profile of the Si component by the Si concentration of the base material and the value obtained by dividing the Si concentration at the minimum point of the GDS profile of the Si component by the Si concentration of the base material is 10% or more, a steel sheet. However, when no minimum point appears within a depth of 5 μm, the point at a depth of 5 μm is regarded as the point where the minimum point appears.

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

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

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

5. A hot dip galvanized steel sheet comprising the galvanizing steel sheet according to claims 1 to 4 and a hot dip galvanized layer formed on the galvanizing steel sheet.

6. preparing base iron having a composition containing, by weight%, Mn: 0.1 to 4%, Si: 0.001 to 2%, C: 0.02 to 0.6%, Al: 0.001 to 1%, P: 0.05% or less, S: 0.02% or less, Cr: 1% or less, B: 0.01% or less, with the balance being Fe and inevitable impurities; performing electroplating on the base iron to form an Fe plating layer containing 5 to 50% by weight of oxygen; and The substrate iron with the Fe plating layer formed thereon is annealed by maintaining it at 600 to 950 °C for 5 to 120 seconds in an annealing furnace with a dew point temperature controlled to less than -20 °C in a 1 to 70% H ２ - the remaining N ２ A method for manufacturing a steel sheet for plating, including the step of annealing.

7. The adhesion amount of the Fe plating layer is 0.5 to 3 g / m ２ The method for manufacturing a plated steel sheet according to claim 6, which is such that

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

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

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

11. Preparing a base iron having a composition containing, in % by weight, Mn: 0.1 to 4%, Si: 0.001 to 2%, C: 0.02 to 0.6%, Al: 0.001 to 1%, P: 0.05% or less, S: 0.02% or less, Cr: 1% or less, B: 0.01% or less, and the balance Fe and inevitable impurities; Performing electroplating on the base iron to form an Fe plating layer containing 5 to 50% by weight of oxygen; The base iron on which the Fe plating layer is formed is controlled to a dew point temperature of less than -20°C in a 1 to 70% H ２ - the remaining N ２ annealing in an annealing furnace in a gas atmosphere at 600 to 950°C for 5 to 120 seconds to obtain a steel sheet for plating; and A method for manufacturing a hot-dip plated steel sheet, including the step of immersing the plated steel sheet in a hot-dip bath.

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