Composite steel sheet with excellent adhesion and method for manufacturing the same

Abstract

The present invention relates to a composite steel sheet having excellent adhesion and a manufacturing method thereof, and more particularly to a lightweight composite steel sheet in which a microstructure is formed on the surface and inside of a plated steel sheet, and then the plated steel sheet is bonded with a plastic layer to maximize adhesion.

Description

Technical Field

[0001] The present invention relates to a composite steel sheet having excellent adhesion and a method for manufacturing the same.

Background Art

[0002] In the past 20 years, the problem of excessive energy use has emerged as a topic, and in order to reduce energy use, efforts have been actively made to reduce the weight of materials throughout the industry. Concerns about environmental pollution caused by serious energy use and the depletion of energy resources due to the rapid increase in automobile demand are deepening day by day. For this reason, active research on weight reduction of materials is being carried out, and furthermore, the amount of use of lightweight materials such as aluminum is increasing every year. In particular, the weight reduction of automobile materials in the automobile industry has been a long-standing concern and is also a next-generation development goal for all automobile companies. World-famous automobile manufacturers and parts and material-related companies are engaged in a fierce technological competition aiming to produce automobiles with high fuel efficiency through the development and adoption of new materials for automobile weight reduction.

[0003] Among lightweight materials, aluminum is the most widely used as an automobile material among non-ferrous materials, and in particular, the ratio applied to automobile exterior panels is increasing. However, aluminum has problems in that it consumes a large amount of energy during the processing process and is disadvantageous in terms of price.

[0004] As a countermeasure material against aluminum, high-strength steel sheets with a thin thickness are emerging. When using high-strength steel sheets, there is a problem that the thickness becomes thin and the rigidity of the outer plate becomes insufficient. In addition, high-strength steel sheets with a thickness below a certain level have a limit in that fine wrinkles or springback phenomena occur during the processing process.

[0005] Therefore, as an alternative, lightweight sandwich steel sheets, in which a light, adhesive plastic layer is inserted between steel sheets, are being researched. However, composite steel sheets in this form, which have a plastic layer inserted, require processes such as applying an adhesive to bond the plastic layer to the steel sheet and then laminating, or plasma treating the surface of the steel sheet before laminating.

[0006] However, methods that involve applying adhesives have problems such as the generation of harmful substances and the widening of the interface due to processing or temperature changes, while methods that involve treating the surface of steel plates with plasma have limitations in that sufficient adhesion cannot be ensured between the plastic layer and the steel plate. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Korean Registered Patent No. 10-1728026 [Overview of the project] [Problems that the invention aims to solve]

[0008] One aspect of the present invention is to provide a composite steel sheet with excellent adhesion and a method for manufacturing the same.

[0009] The problems addressed by the present invention are not limited to those described above. Anyone with ordinary skill in the art to which the present invention pertains should have no difficulty understanding the further problems addressed by the present invention from the entirety of the specification. [Means for solving the problem]

[0010] One aspect of the present invention is, base iron and, A plating layer provided on at least one surface of the above-mentioned base iron, The above-mentioned plating layer includes a resin layer provided on the above-mentioned plating layer, The interface roughness Ra between the above-mentioned plating layer and resin layer is 0.5 to 1.5 μm. The above plating layer has voids inside. The present invention provides a composite steel sheet in which the average diameter of voids within the above-mentioned plating layer is 10 nm to 3 μm.

[0011] Furthermore, yet another aspect of the present invention is, The steps include: forming a plating layer on at least one surface of a base iron to obtain a plated steel sheet; The above-mentioned plated steel sheet is subjected to a primary electrolytic etching treatment using a primary electrolyte containing one or more elements selected from the group consisting of HCl and NaCl. The plated steel sheet that has undergone the primary electrolytic etching treatment is subjected to a secondary electrolytic etching treatment using a secondary electrolyte containing one or more elements selected from the group consisting of HNO3, NaOH, H3PO4, H2SO4, Na2SO4, and NaH2PO4. The present invention provides a method for manufacturing a composite steel sheet, which includes the step of heat-fusing a resin sheet onto a plated steel sheet that has undergone the above-mentioned secondary electrolytic etching treatment. [Effects of the Invention]

[0012] According to one aspect of the present invention, it is possible to provide a composite steel sheet with excellent adhesion and a method for manufacturing the same.

[0013] The diverse and beneficial advantages and effects of the present invention are not limited to those described above and can be more easily understood in the process of describing specific embodiments of the present invention. [Brief explanation of the drawing]

[0014] [Figure 1] This is a schematic diagram illustrating an exemplary structure of a composite steel sheet. [Figure 2] This shows photographs of a composite steel sheet corresponding to Invention Example 1 of the present invention, taken from various angles using a scanning electron microscope (SEM). [Figure 3] This graph compares the bonding strength of an untreated material, a material using adhesive, and a material according to the present invention. [Figure 4] This is a schematic diagram illustrating the structure of a composite steel sheet according to one embodiment of the present invention. [Figure 5]It is a schematic diagram schematically showing the structure of a composite steel plate according to another embodiment of the present invention.

Embodiments for Carrying Out the Invention

[0015] The terms used in this specification are for explaining specific embodiments and are not intended to limit the present invention. Further, the singular forms used in this specification include plural forms as well, unless the relevant definitions clearly indicate the contrary meaning.

[0016] The meaning of "including" used in this specification does not specify a configuration and does not exclude the existence or addition of other configurations.

[0017] Unless otherwise defined, all terms including technical terms and scientific terms used in this specification have the same meaning as generally understood by those having ordinary knowledge in the technical field to which the present invention belongs. Terms defined in a dictionary are interpreted as having a meaning consistent with the related technical literature and the currently disclosed content.

[0018] Hereinafter, a composite steel plate according to one aspect of the present invention will be described in detail. In the present invention, when indicating the content of each element, it means weight% unless otherwise specified.

[0019] Conventionally, high-strength steel plates have been developed in response to the demand for material weight reduction, but they have drawbacks such as the formation of fine wrinkles during the processing process. Also, sandwich steel plates with a resin layer inserted between the steel plates, and technologies for applying an adhesive for bonding or laminating after plasma treatment have been developed. However, such technologies have problems such as the interface between the resin layer and the steel plate spreading or the adhesive force being insufficient.

[0020] Therefore, the present invention provides a solution that can further improve the bonding strength between the plating layer and the resin layer while solving the problems of the prior art. This will be specifically described below.

[0021] [Composite Steel Plate] First, we will describe in detail a composite steel sheet with excellent resin adhesion, which is one aspect of the present invention. A schematic structure of the composite steel sheet according to the present invention is shown in Figure 1. The composite steel sheet includes a base iron 1, a plating layer 2 provided on at least one surface of the base iron 1, and a resin layer 3 provided on the plating layer 2.

[0022] The inventors have found that in order to further improve the adhesion between the plating layer 2 and the resin layer 3, it is important to control the surface roughness of the surface 10 facing the surface of the plating layer 2 that is in contact with the base iron (i.e., the interface roughness between the plating layer 2 and the resin layer 3 as described above) and the average diameter of the voids present inside the plating layer.

[0023] Generally, adhesion consists of two elements: mechanical bonding and chemical bonding. While chemical bonding is far stronger than mechanical bonding, it presents a technical challenge in that it is difficult to apply to bonding between plated steel sheets and resin layers, as chemical bonding is primarily a reaction that forms compounds through bonding at the atomic level.

[0024] Therefore, various attempts have been made to improve the adhesion between plated steel sheets and resin layers using conventional technology. However, methods of joining the plated layer and the resin layer (plastic layer) using an adhesive separately, such as van der Waals bonding, have drawbacks such as weak or irregular bonding strength, a thick bonding thickness of several tens of micrometers, and a long bonding time. In addition, volatile organic compounds are generated during bonding, and degradation phenomena occur due to the surrounding environment (humidity, heat, acidic atmosphere, etc.). Selecting an adhesive suitable for both materials to be bonded is extremely difficult, and thorough quality control is required. Furthermore, the increase in the interface between the materials to be bonded due to the use of adhesive may cause delamination during processing.

[0025] On the other hand, methods for modifying the resin layer (plastic layer) to impart chemical functional groups include introducing polar functional groups to the surface of the resin layer through UV treatment, ozone treatment, radical reaction, graft reaction, and crosslinking agent treatment. However, the functional groups to be applied must be changed depending on the type of resin (plastic) and the shape of the steel sheet surface, the modification process is complex and time-consuming, and because no functional groups exist on the surface of the steel sheet, there is a problem in that the bonding strength between the plated steel sheet and the resin layer (plastic layer) is very weak.

[0026] Therefore, the inventors diligently studied how to solve the above-mentioned problems and found that the problems could be solved by controlling the surface roughness of the plating layer and the average diameter of the voids inside the plating layer to an appropriate numerical range, thus completing the present invention.

[0027] In other words, according to the present invention, it has been found that the bonding area between the resin layer and the plating layer of the composite steel sheet can be increased by controlling the surface roughness of the surface of the plating layer (i.e., the surface 10 on which the plating layer 2 is in contact with the resin layer 3) and the size of the voids inside the plating layer. This increases the bonding force due to van der Waals forces by widening the surface area between the plating layer 2 and the resin layer 3 that are to be bonded. Furthermore, by inducing an interlocking effect exhibited by the interlocking contact between the resin layer and the plating layer, and by strengthening the bonding force in the normal and shear directions, a strong bonding force can be ensured between the resin layer 3 and the plated steel sheets 1 and 2.

[0028] Therefore, the present invention solves the problems of the prior art described above and provides a composite steel sheet with improved adhesion between the plated steel sheet and the resin layer. In particular, because the interface between the objects to be joined can be minimized, this is a very useful technology for making the bonding force with the plated steel sheet stronger, regardless of the type and properties of the resin layer (plastic layer).

[0029] For this reason, the composite steel sheet according to the present invention is characterized in that the interface roughness Ra between the plating layer and the resin layer is 0.5 to 1.5 μm. If the surface roughness Ra of the plating layer on the resin layer side surface 10 is less than 0.5 μm, the bonding area at the interface between the resin layer and the plating layer will be insufficient, which may result in insufficient bonding strength and the possibility of delamination during processing. On the other hand, if the surface roughness Ra of the plating layer on the resin layer side surface exceeds 1.5 μm, the resin layer may not be able to penetrate sufficiently to the bottom of the plating layer, which may result in a decrease in bonding strength. On the other hand, from the viewpoint of further improving the above-mentioned effects, the lower limit of the Ra value may be 0.7 μm, or the upper limit of the Ra value may be 1.3 μm. In this case, the surface roughness of the plating layer on the resin layer side surface can be measured by a surface roughness measurement method, and can be measured using a contact-type roughness measuring device, a three-dimensional roughness measuring device, or a three-dimensional surface shape measuring device.

[0030] Furthermore, the composite steel sheet according to the present invention is characterized in that voids exist within the plating layer, and the average diameter of these voids is 10 nm to 3 μm. If the average diameter of the voids within the plating layer is less than 10 nm, the increase in bonding area and interlocking effect due to the resin layer occupying the voids, as described later, may be insufficient. On the other hand, if the average diameter of the voids within the plating layer exceeds 3 μm, not only may the corrosion resistance decrease due to the void size being too large, but the density of the plating layer may decrease, potentially reducing the plating adhesion. On the other hand, from the viewpoint of further improving the above-mentioned effects, the lower limit of the average diameter of the voids may be 20 nm, or the upper limit of the average diameter of the voids may be 2.8 μm. In this case, the voids within the plating layer refer to voids existing in the region from the resin layer side surface of the plating layer to the base iron side surface of the plating layer, based on the cross-section in the thickness direction of the plating layer. In this specification, the thickness direction refers to the direction perpendicular to the rolling direction of the steel sheet, and unless otherwise specified, the same meaning applies hereafter. However, since the average diameter of the voids within the plating layer is naturally smaller than the average thickness of the plating layer, which will be discussed later, this specification will not provide a separate explanation for this.

[0031] Therefore, the average diameter of the voids within the plating layer is the average value of the equivalent circular diameter measured for the voids within the region from the resin layer-side surface of the plating layer (i.e., the surface where the plating layer contacts the resin layer) to the base iron-side surface of the plating layer, with reference to the cross-section of the plating layer in the thickness direction as described above. In this case, the equivalent circular diameter refers to the value measured for the particle size, assuming a spherical particle whose particle size is represented by the maximum length of the void penetrating the interior of the void.

[0032] According to one aspect of the present invention, the average thickness of the plating layer may be 2.5 to 7.5 μm. If the average thickness of the plating layer is less than 2.5 μm, the effect of ensuring corrosion resistance by the plating layer may be insufficient, and the phenomenon of unplated areas may occur. Also, if the average thickness of the plating layer exceeds 7.5 μm, the plating adhesion may decrease, making it difficult to form a uniform plating layer. On the other hand, from the viewpoint of further improving the above-mentioned effects, the lower limit of the average thickness of the plating layer may be 3.0 μm, or the upper limit of the average thickness of the plating layer may be 5.8 μm.

[0033] Furthermore, according to one aspect of the present invention, the average thickness of the resin layer may be 100 to 600 μm. If the average thickness of the resin layer is less than 100 μm, it is not possible to form a layer with sufficient thickness as a core material between the steel plates. Also, if the average thickness of the resin layer exceeds 600 μm, it may not be able to melt sufficiently under heat fusion conditions and penetrate into the plating layer, potentially reducing properties such as adhesion. In this specification, the resin layer refers to the region from the surface of the resin layer to the interface with the plating layer, and does not include the region in which the resin derived from the resin layer, described later, occupies voids inside the plating layer. Therefore, the average thickness of the resin layer refers to the average value of the thickness measured in the thickness direction from the surface of the resin layer to the interface with the plating layer.

[0034] Furthermore, although not particularly limited, according to one aspect of the present invention, the depth (Tp) from the surface of the plating layer to the void contained within the plating layer and closest to the base iron side may be 10 to 90% of the total thickness of the plating layer. If the value of Tp is less than 10%, voids will not be formed throughout the plating layer, which may result in insufficient improvement of bonding strength due to the voids, and may also cause delamination during processing. If the value of Tp exceeds 90%, excessive voids will be formed, which may extend to the base iron, reducing the adhesion between the plating layer and the base iron, and ultimately potentially reducing the adhesive strength between the steel plate and the resin layer during processing. On the other hand, from the viewpoint of further improving the above effects, the lower limit of the Tp value may be 20%, or the upper limit of the Tp value may be 85%.

[0035] In this case, the value of Tp can be determined by measuring the shortest distance in the thickness direction from the surface of the plating layer to the nearest void on the base metal side, using the cross-section of the composite steel sheet in the thickness direction as a reference. Specifically, it can be determined by taking images of the cross-section of the composite steel sheet in the thickness direction using a scanning electron microscope (SEM), TEM, FIB, etc.

[0036] Furthermore, according to one aspect of the present invention, the unit area of ​​the cross-section of the plating layer (meaning the cross-section in the thickness direction) is 9 μm 2 The number of voids (Np) with a diameter of 10 nm or more contained in the layer may be 5 to 30. As a result of further intensive research, the inventors found that the density of voids present in the plating layer is also an important factor in further improving adhesion with the resin layer mentioned above and in ensuring corrosion resistance.

[0037] In other words, if the value of Np is less than 5, the number of voids per unit area will be insufficient, which may result in insufficient bonding strength between the resin layer and the plating layer. Conversely, if the value of Np exceeds 30, the number of voids per unit area will be excessive, which may result in a decrease in the corrosion resistance of the plating layer. Furthermore, if there are too many voids, the plating layer may not adhere firmly to the base iron, potentially reducing the adhesion between the base iron and the plating layer. As a result, the bonding strength with the resin layer (plastic layer) will also decrease. In this invention, ensuring adhesion between the resin layer and the plating layer is important, but primarily, ensuring adhesion between the base iron and the plating layer is also an important element. In other words, in this invention, if an excessive number of voids are formed in the plating layer, the adhesive strength between the base iron and the plating layer will also decrease. Therefore, in this invention, by controlling the number of voids in the plating layer to an appropriate range, voids are formed in the plating layer provided on the base iron, and then resin is injected into these voids to induce an interlocking effect and physically ensure bonding strength. On the other hand, from the viewpoint of further improving the effects described above, the lower limit of the value of Np may be 13, or the upper limit of the value of Np may be 27.

[0038] Furthermore, according to one aspect of the present invention, the area fraction (Ap) of voids with a diameter of 10 nm or more relative to the total area of ​​the cross-section (meaning the cross-section in the thickness direction) of the plating layer may be 10 to 80%. If the value of Ap is less than 10%, sufficient voids may not be secured, and the interlocking effect may be insufficient. On the other hand, if the value of Ap exceeds 80%, conversely, there will be too many voids, which may reduce the density of the plating layer and decrease the bonding strength after bonding the resin layer. On the other hand, from the viewpoint of further improving the above effect, the lower limit of the value of Ap may be 34%, or the upper limit of the value of Ap may be 75%.

[0039] In this case, the value of Ap can be measured using a scanning electron microscope (SEM) so that a cross-section in the thickness direction of the plating layer can be observed. However, in determining the value of Ap, the area of ​​the voids only needs to be measured for the total area of ​​the voids themselves that exist inside the plating layer, and does not exclude the area of ​​voids occupied by resin originating from the resin layer described later.

[0040] Furthermore, according to one aspect of the present invention, the fraction (Vp) of the total area of ​​voids contained within the plating layer that is occupied by resin derived from the resin layer may be 20 to 90%, and the value of Vp can be measured by SEM cross-sectional analysis.

[0041] If the Vp value is less than 20%, the interlocking effect is insufficient, and the bonding strength may be slightly reduced. Furthermore, if the Vp value exceeds 90%, when used in sandwich-type steel sheets, the resin layer (plastic layer) as the core layer cannot form a layer of sufficient thickness.

[0042] In other words, the inventors diligently studied how to further improve the effects of the present invention and found that by controlling the value of Vp to satisfy the above-mentioned numerical range, it is possible to maximize the interlocking effect between the plating layer and the resin layer, which is exerted when the resin layer penetrates into the voids inside the plating layer. Furthermore, by minimizing the interface between the objects to be joined, it is possible to reduce the peeling phenomenon of the joint during processing. In addition, by having the resin layer occupy an appropriate range inside the voids, the problem of reduced corrosion resistance compared to when the voids exist as empty spaces can be reduced, thereby simultaneously ensuring excellent adhesion of the resin layer and corrosion resistance of the plating layer. On the other hand, from the viewpoint of further improving the above-mentioned effects, the lower limit of the value of Vp may be 48%, or the upper limit of the value of Vp may be 86%.

[0043] According to the present invention, by satisfying the above configuration, it is possible to provide dent resistance and ensure excellent price competitiveness while using a thin, high-strength steel sheet as the plated steel sheet. The structure, in which the steel sheet and the resin layer (plastic layer) are laminated, complements the rigidity of the outer panel due to the use of a thin steel sheet and enables weight reduction. Furthermore, by laminating the thin, high-strength steel sheet with the plastic layer, phenomena such as fine wrinkles or springback that occur during the processing process can be prevented.

[0044] On the other hand, according to one aspect of the present invention, the above-mentioned base iron can be any steel material that is capable of aluminum-based plating or zinc-based plating, without any particular limitations.

[0045] Furthermore, the above-mentioned plating layer is preferably an aluminum-based plating layer or a zinc-based plating layer. In this case, the aluminum-based plating layer includes all aluminum plating layers or aluminum alloy plating layers, and the zinc-based plating layer includes all zinc plating layers or zinc alloy plating layers. The composition of the above-mentioned plating layer is not particularly limited, but as a typical example, the above-mentioned aluminum-based plating layer contains an excess amount of Al (i.e., 50% by weight or more) and may contain one or more alloying elements such as Zn, Mg, Si, Sn, Pb, and Fe in order to further improve the physical properties of the plating layer within a range that does not impair the purpose of the invention. Similarly, the above-mentioned zinc-based plating layer contains an excess amount of Zn (i.e., 50% by weight or more) and may contain one or more alloying elements such as Al, Mg, Si, Sn, Pb, and Fe in order to further improve the physical properties of the plating layer within a range that does not impair the purpose of the invention.

[0046] Next, the resin layer can be provided on the surface opposite to the surface in contact with the base iron of the plating layer (i.e., the surface facing the surface in contact with the base iron of the plating layer). In the present invention, the resin layer is a layer containing 99% or more resin (the remainder being impurities), and more preferably a layer consisting of 100% resin after removing unavoidable impurities.

[0047] On the other hand, the resin layer is formed from engineering plastic. Engineering plastic is a general term for high-strength plastics used as industrial and structural materials. Engineering plastic is a concept that distinguishes it from conventional plastics, which are low-molecular-weight substances with molecular weights ranging from tens to hundreds of thousands, in that it consists of high-molecular-weight substances with molecular weights ranging from hundreds of thousands to millions.

[0048] The performance and characteristics of engineering plastics vary depending on their chemical structure, but they mainly include polyamides, polyacetyls, polycarbonates, polyphenylene oxides, and polybutylene terephthalates, as well as polyethylene, polypropylene, polyesters, and polyurethanes. For example, the polyamide resins mentioned above have excellent surface hardness, flexural strength, and alkali resistance, and nylon 6 or nylon 66 are well known examples. Carbon fiber composite materials or glass fiber composite materials can also be used.

[0049] Furthermore, in one aspect of the invention, the composite steel sheet can be made lighter by laminating a resin layer (plastic layer) to one surface of the steel sheet or between the steel sheets. For example, the thickness ratio of the resin layer to the steel sheet laminated to the resin layer may be in the range of 3:1 to 1:5, or 2:1 to 1:2. By controlling the thickness of the steel sheet and the resin layer within the above range, both rigidity and weight reduction of the composite material can be achieved simultaneously.

[0050] A typical composite steel sheet structure is schematically shown in Figure 1. Specifically, the composite steel sheet can have an additional plated steel sheet on top of the resin layer; in other words, by providing an additional plated steel sheet on the opposite side of the surface where the resin layer is in contact with the plated layer, a composite steel sheet with a sandwich-type structure can be created.

[0051] Therefore, the composite steel sheet may also have a laminated structure of plated steel sheets 1, 2 / resin layer 3 (i.e., base iron 1 / plating layer 2 / resin layer 3), as shown in Figure 4. Alternatively, as shown in Figure 5, it may also have a sandwich-type laminated structure of first plated steel sheets 1, 2 / resin layer 3 / second plated steel sheets 4, 5 (i.e., first base iron 1 / first plating layer 2 / resin layer 3 / second plating layer 4 / second base iron 5). In this case, the additional plated steel sheet can have the same properties as the plated steel sheet described later (i.e., it includes a first base iron, a first plating layer provided on at least one surface of the first base iron, a resin layer provided on the first plating layer, a second plating layer provided on the resin layer, and a second base iron provided on the second plating layer, wherein the surface roughness Ra1 of the first plating layer relative to the resin side surface is 0.5 to 1.5 μm, the surface roughness Ra2 of the second plating layer relative to the resin side surface is 0.5 to 1.5 μm, and the first and second plating layers have voids inside, with an average diameter of voids of 10 nm to 3 μm).

[0052] [Manufacturing method for composite steel sheets] The following describes yet another aspect of the present invention: a method for manufacturing composite steel sheets. However, it should be noted that this does not necessarily mean that the plated steel sheets of the present invention should be manufactured by the following method.

[0053] First, a plating layer is formed on at least one surface of the base iron to obtain a plated steel sheet. At this time, the method for forming the plating layer can be any method commonly known in the art, without limitation, and can be applied to the present invention. Therefore, this specification does not limit this, and methods such as hot-dip plating, electroplating, and vapor deposition plating can all be applied.

[0054] Next, the plated steel sheet is subjected to electrolytic etching under the conditions described later. At this time, the electrochemical etching method can be the same as that known in the art. For example, zinc is placed as the anode and an aluminum plate as the cathode and immersed in an electrolyte. When electricity is applied, the zinc at the anode becomes ZnO and a structure is formed as it dissolves in the acid solution. The surface consists of a part whose properties have changed to zinc oxide after the reaction and a zinc layer that remains unchanged. The mechanism by which the surface structure is formed has not yet been clarified, but it is judged that the electrolyte, etching voltage, and etching time affect the formation of the surface structure. In general, zinc is etched by acids and bases, and the degree of surface etching is strengthened as the concentration of the electrolyte increases and the etching voltage and time increase.

[0055] On the other hand, during the electrolytic etching described above, a primary electrolytic etching treatment is performed using a primary electrolyte containing one or more substances selected from the group consisting of HCl and NaCl. At this time, the primary electrolytic etching treatment can be performed at a voltage of 1 to 10 V, and the concentration of one or more substances selected from the group consisting of HCl and NaCl in the primary electrolyte can be in the range of 0.1 to 1.5% by weight. By performing the primary electrolytic etching treatment to satisfy the above conditions, voids can be formed on the surface of the plated steel sheet. Furthermore, the time for the primary electrolytic etching treatment can be 1 to 20 seconds.

[0056] The plated steel sheet that has undergone the primary electrolytic etching treatment described above is subjected to a secondary electrolytic etching treatment using a secondary electrolyte containing one or more substances selected from the group consisting of HNO3, NaOH, H3PO4, H2SO4, Na2SO4, and NaH2PO4. At this time, the secondary electrolytic etching treatment can be performed at a voltage of 1 to 10 V for 1 second to 20 minutes, and the concentration of one or more substances selected from the group consisting of HNO3, NaOH, H3PO4, H2SO4, Na2SO4, and NaH2PO4 in the secondary electrolyte can be in the range of 0.01 to 1.5 M. By performing the secondary electrolytic etching treatment to satisfy the above conditions, voids can be formed not only on the surface of the plated steel sheet but also in the internal region (in the thickness direction) of the plating layer, thereby enabling the achievement of the effects targeted by the present invention.

[0057] In other words, according to the present invention, by performing a two-stage electrolytic etching process on a plated steel sheet under strictly controlled conditions before forming a resin layer (or plastic layer), a fine uneven structure can be formed on the surface of the plated layer, and numerous fine voids can also be formed inside the plated layer. As a result, the excellent adhesion and processability effects targeted by the present invention can be achieved.

[0058] Next, a composite steel sheet can be obtained by heat-fusing a polymer sheet onto the plated steel sheet that has undergone the secondary electrolytic etching process, thereby forming a resin layer on the plated steel sheet. At this time, the heat-fusing process can be carried out in a temperature range of 150 to 230°C. If the temperature of the heat-fusing process is below 150°C, the resin may not be able to penetrate between the voids, and sufficient adhesive strength may not be ensured. Also, if the temperature of the heat-fusing process exceeds 230°C, the resin may completely melt and partially penetrate into the voids, failing to form a layer between the steel sheets and potentially flowing out to the side of the steel sheets. [Examples]

[0059] The present invention will be described in more detail below through examples. However, it should be noted that the following examples are for illustrative purposes only and do not limit the scope of the present invention. The scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0060] (Experimental Example 1) After preparing the hot-dip galvanized steel sheets by weight, they were washed for 20 minutes in a mixed solution of acetone and methanol in an ultrasonic cleaner. Then, they were rinsed with distilled water, the water was removed with compressed air, and the corners of the steel sheets were masked with insulating tape.

[0061] Next, the galvanized steel sheet and aluminum sheet were immersed in the electrolyte shown in Table 1 below. The galvanized steel sheet was set as the anode and the aluminum sheet as the cathode, separated by approximately 5 cm. Primary electrolytic etching was performed, followed by secondary electrolytic etching under the conditions described in Table 1 below. After rinsing the etched test pieces with distilled water, the tape was removed, moisture was removed using compressed air, and then the moisture was removed in a drying oven at approximately 60°C.

[0062] Next, a composite steel sheet was obtained by heat-sealing a PA6 resin sheet to the surface of the electrolytically etched plated steel sheet at 180°C. The properties of the composite steel sheet obtained in this way were evaluated and shown in Table 2 below. At this time, Ra was measured using a three-dimensional surface roughness measuring device immediately after etching the steel sheet. In addition, a cross-sectional test piece was prepared by cutting the composite steel sheet in the thickness direction (direction perpendicular to the rolling direction), and the average diameter of the voids inside the plating layer was measured by observing the cross-sectional test piece with a scanning electron microscope (SEM) in the same manner as described above in the specification, and the average thickness and Tp value (corresponding to the average) of the plating layer were measured.

[0063] [Table 1]

[0064] [Table 2] Ra*: Interface roughness between the plating layer and the resin layer Tp*: The ratio [%] of the depth from the surface of the plating layer to the void located within the plating layer and closest to the base metal side, relative to the total thickness of the plating layer.

[0065] The composite steel sheets obtained from each example of invention and comparative example were evaluated for their respective properties according to the following criteria.

[0066] <Adhesion> Adhesion was evaluated by measuring the joint strength using a standard lap shear test (ASTM D 1002) and according to the following criteria. ◎: 15MPa or higher ○: 10 MPa or more and less than 14 MPa △: 5 MPa or more and less than 9 MPa ×: Less than 5 MPa

[0067] <Weldability> To evaluate weldability, measurements were taken using the ISO18278-2 method, F1-6mm, 2.1kN, 140msW / T, and 140msH / T, and evaluated according to the following criteria. ◎: 100% power supply (ensuring welding strength) ○: 90% current flow (ensuring welding strength) △: 50% power supply (weld strength not ensured) ×: Weld strength cannot be measured (welding is impossible)

[0068] <Workability> The Ericsson evaluation (checking whether the material can withstand being pushed up to a height of 6 mm with a constant force during the punch stroke) and the separation of the plastic and steel plate after bending at 180° using a vise were checked. ◎: 100% no peeling ○: No peeling in over 90% of cases (micro-peeling) △: More than 50% peeling ×: 100% interfacial delamination

[0069] [Table 3]

[0070] In the case of Invention Examples 1 to 3, which satisfy all the conditions for composite steel sheets as defined in this invention, it was confirmed that they have excellent corrosion resistance as well as excellent adhesion, weldability, and workability.

[0071] On the other hand, in Comparative Example 1 and Comparative Example 2, which perform electrolytic etching in a single step and undergo plasma treatment, the conditions for electrolytic etching treatment as defined in the present invention are not met, and one or more of the properties of Ra and the average diameter of the voids inside the plating layer are not satisfied. As a result, it was confirmed that in Comparative Examples 1 and 2, even if corrosion resistance is ensured, one or more of the properties of adhesion, weldability, and workability are inferior.

[0072] In particular, Figure 3 shows a comparison of the bonding strength (joint strength) in the case of untreated material, when an adhesive is used (corresponding to Comparative Example 3), and in the case of the present invention. As can be seen from Figure 3, it was confirmed that the present invention provides the best bonding strength due to the interlocking effect between the voids formed in the plating layer and the resin layer.

[0073] (Experimental Example 2) The composite steel sheets were manufactured in the same manner as in Experimental Example 1 described above, except for the changes in conditions as shown in Table 4 below. For each composite steel sheet obtained in this way, the values ​​measured in the same manner as in Table 2 are shown in Table 5 below, and the results of evaluation using the same criteria are shown in Table 6 below. Note that the values ​​in Table 5 below were measured and shown for the composite steel sheets. Specifically, after manufacturing cross-sectional test specimens in the thickness direction as in Experimental Example 1, they were photographed with an SEM, and the values ​​of Np, Ap, and Vp were measured in the same manner as described above in the specification.

[0074] [Table 4]

[0075] [Table 5] Np*: Cross-sectional area of ​​the plating layer, 9 μm² 2 The number of voids with a diameter of 10 nm or more contained within [voids / μm] 2 ] Ap*: Area fraction of voids with a diameter of 10 nm or more relative to the total cross-sectional area of ​​the plating layer [%] Vp*: The area fraction [%] of the total area of ​​voids contained within the above-mentioned plating layer that is occupied by the resin layer.

[0076] [Table 6]

[0077] In the case of Invention Examples 4 to 6, which satisfy all the conditions for composite steel sheets as defined in this invention, it was confirmed that they have excellent corrosion resistance as well as excellent adhesion, weldability, and formability.

[0078] On the other hand, in the case of Comparative Examples 4 to 6, which did not meet the conditions for electrolytic etching treatment as defined in the present invention, one or more of the properties of the average diameter of voids inside the plating layer were not met, and it was confirmed that this resulted in a decrease in one or more of the properties of adhesion, weldability, and formability.

Claims

1. base iron and, A plating layer provided on at least one surface of the base iron, The plated layer includes a resin layer provided on the plated layer, The surface roughness Ra of the plated layer is 0.5 to 1.5 μm. The aforementioned plating layer has voids inside. The average diameter of the voids within the aforementioned plating layer is 10 nm to 3 μm. The aforementioned voids are observed in the composite steel sheet by observing a cross-sectional test specimen, which is cut in the thickness direction (perpendicular to the rolling direction), using a scanning electron microscope (SEM).

2. The composite steel sheet according to claim 1, wherein the depth from the surface of the plating layer to the void contained within the plating layer and closest to the base iron side is 10 to 90% of the total thickness of the plating layer.

3. The cross-sectional area of ​​the aforementioned plating layer is 9 μm². ２ The composite steel sheet according to claim 1, wherein the number of voids with a diameter of 10 nm or more contained therein is 5 to 30.

4. The composite steel sheet according to claim 1, wherein the area fraction of voids with a diameter of 10 nm or more relative to the total area of ​​the cross-section of the plating layer is 10 to 80%.

5. The composite steel sheet according to claim 1, wherein the proportion of the area of ​​voids within the plating layer that is occupied by the resin layer is 20 to 90% of the total area of ​​voids within the plating layer.

6. The composite steel sheet according to claim 1, further comprising a second plating layer provided on the resin layer and a second base iron provided on the second plating layer.

7. The steps include: forming a plating layer on at least one surface of a base iron to obtain a plated steel sheet; The steps include: performing a primary electrolytic etching treatment on the plated steel sheet using a primary electrolyte containing one or more elements selected from the group consisting of HCl and NaCl; On the electroplated steel sheet that has been subjected to the primary electrolytic etching treatment, HNO ３ , NaOH, H ３ PO ４ , H ２ SO ４ , Na ２ SO ４ and NaH ２ PO ４ performing a secondary electrolytic etching treatment using a secondary electrolytic solution containing one or more selected from the group consisting of; A method for manufacturing a composite steel sheet according to claim 1, comprising the step of heat-fusing a resin sheet onto the plated steel sheet that has undergone secondary electrolytic etching.

8. The above-mentioned primary electrolytic etching step is performed at a voltage of 1 to 10 V for 1 to 20 seconds. The method for manufacturing a composite steel sheet according to claim 7, wherein the step of performing the secondary electrolytic etching treatment is carried out at a voltage of 1 to 10 V for 1 second to 20 minutes.

9. The method for manufacturing a composite steel sheet according to claim 7, wherein the step of heat-sealing the resin sheet is performed at a temperature of 150 to 230°C.

10. The method for manufacturing a composite steel sheet according to claim 7, wherein the primary electrolyte has a concentration of 0.1 to 1.5% of one or more substances selected from the group consisting of HCl and NaCl.

11. The secondary electrolyte is HNO ３ NaOH, H ３ PO ４ H ２ SO ４ Na ２ SO ４ and NaH ２ PO ４ A method for manufacturing a composite steel sheet according to claim 7, wherein the concentration of one or more substances selected from the group consisting of the above is 0.01 to 1.5 M.

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