Steel plate with excellent hydrophilicity and conductivity

Abstract

Provided is a steel sheet with excellent hydrophilicity and electrical conductivity that can be used as a separator plate for a fuel cell. The present invention relates to a joint structure including fine protrusions having a pitch interval of 1 to 500 nm on a steel plate, the fine protrusions having a texture with a (111) orientation area ratio of 10% or more and a (001) orientation area ratio of 10% or more, the steel sheet includes fine protrusions having a pitch interval of 1 to 500 nm, the fine protrusions having a texture with a (111) orientation area ratio of 10% or more and a (001) orientation area ratio of 10% or more; Among the microprotrusions, angular microprotrusions account for 20% or more in terms of area, and the angular microprotrusions are microprotrusions having a shape in which three different faces of polygons having the same or different areas share one vertex, and angles between normal vectors of each face are all greater than 0° and less than 180°, and the area of ​​the widest face among the three different faces sharing one vertex is 40,000 nm 2 and the total area of ​​the three faces is less than 60,000 nm 2 It is characterized in that it is less than

Description

[Technical Field]

[0001] The present invention relates to a steel sheet with excellent hydrophilicity and conductivity, and more specifically, to a steel sheet with excellent hydrophilicity and conductivity that can be used for fuel cell separation plates. [Background technology]

[0002] Fuel cells are highly efficient and environmentally friendly batteries that directly convert chemical energy produced by the chemical reaction between hydrogen and oxygen into electrical energy. A fuel cell consists of a unit cell structure in which a membrane electrode assembly (MEA), comprising an electrolyte, an anode electrode, and a cathode electrode, is flanked by a gas diffusion layer and a separator plate. A fuel cell stack is constructed by connecting multiple such unit cells in series.

[0003] The separation plate has channels for supplying fuel (hydrogen or reformed gas) and oxidizer (oxygen and air) to the fuel cell electrodes, respectively, and for discharging water, which is an electrochemical reaction product. It also functions to mechanically support the membrane electrode assembly and gas diffusion layer, and to electrically connect to adjacent unit cells.

[0004] Traditionally, graphite was used as the material for such separation plates, but in recent years, stainless steel has been widely used due to considerations such as manufacturing cost and weight. The stainless steel material used must have excellent corrosion resistance in the highly acidic environment of fuel cell operation, and from the perspective of weight reduction, miniaturization, and mass production, stainless steel with excellent corrosion resistance and conductivity must be used.

[0005] However, conventional stainless steel exhibits high resistance due to its passive film, which can lead to resistance losses in fuel cell performance. Therefore, processes involving coating with conductive materials such as gold (Au), carbon, or nitride have been proposed. However, these methods have the drawback of increasing manufacturing costs and time due to the additional processes required for coating with precious metals or coating materials, thus hindering productivity. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Korean Registered Patent Publication No. 10-0836480 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] The objective of the present invention is to provide a steel sheet with excellent hydrophilicity and conductivity that can be used for fuel cell separation plates. [Means for solving the problem]

[0008] As a means to achieve the above-mentioned objectives, a steel sheet with excellent hydrophilicity and conductivity according to an example of the present invention may include a joint structure in which fine protrusions with a pitch interval of 1 to 500 nm are included on the steel sheet, and the fine protrusions have a texture with an orientation area ratio of 10% or more for (111) and 10% or more for (001).

[0009] In each of the hydrophilic and conductive steel sheets of the present invention, angular microprotrusions constitute 20% or more of the area fraction of the microprotrusions, and the angular microprotrusions are microprotrusions with a shape in which three different faces, each consisting of polygons with the same or different areas, share one vertex, and the angles between the normal vectors on each face are all greater than 0° and less than 180°, and the area of ​​the widest of the three different faces sharing one vertex is 40,000 nm. 2 It is less than 60,000 nm, and the total area of ​​the three surfaces is 60,000 nm. 2It is acceptable to be less than [a certain value].

[0010] In each of the hydrophilic and conductive steel sheets of the present invention, rod-shaped microprotrusions constitute 20% or more of the area fraction of the microprotrusions, and the rod-shaped microprotrusions have a rod shape consisting of a circular or polygonal base and sides that extend vertically from around the circular or polygonal base, and the angle between the normal vector of the base and the normal vector of the steel sheet surface is 0 to 90°, and the height of the sides may be 50 nm or more.

[0011] In each of the hydrophilic and conductive steel plates of the present invention, the angle between the normal vector of the bottom surface and the normal vector of the steel plate surface may be 0 to 30°.

[0012] In each of the hydrophilic and conductive steel sheets of the present invention, needle-shaped microprotrusions account for 20% or more of the area fraction of the microprotrusions, and the needle-shaped microprotrusions are three-dimensional objects consisting of curves or straight lines that radiate continuously from one vertex toward the steel sheet, and the angle between each radiating curve or straight line and the normal vector of the steel sheet surface is greater than 90° and less than or equal to 180° for the microprotrusions, and the vertical distance from the vertex to the steel sheet surface may be 50 nm or more.

[0013] In each of the hydrophilic and conductive steel sheets of the present invention, the angle between each radially extended curve or straight line and the normal vector of the steel sheet surface may be 150 to 180°.

[0014] In each of the steel sheets exhibiting excellent hydrophilicity and conductivity according to the present invention, the size of the crystal grains may be 1 to 35 μm.

[0015] In each of the hydrophilic and electrically conductive steel sheets of the present invention, the difference in height between crystal grains may be 0.1 to 1000 nm. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a steel plate with excellent hydrophilicity and conductivity that can be used for fuel cell separation plates, including the joint structure. [Brief explanation of the drawing]

[0017] [Figure 1] SEM photograph of the surface of a conventional steel sheet (magnification 50,000). [Figure 2a] SEM photograph of the surface of the steel sheet of the present invention. [Figure 2b] SEM photograph of the surface of the steel sheet of the present invention.

Embodiments for Carrying Out the Invention

[0018] The steel sheet excellent in hydrophilicity and conductivity according to the present invention may include fine protrusions having an interval between pitches of 1 to 500 nm on the steel sheet, and the fine protrusions may include a joint structure having an aggregate structure with a (111) azimuthal area ratio of 10% or more and a (001) azimuthal area of 10% or more.

[0019] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be deformed into various different forms, and the technical idea of the present invention is not limited to the embodiments described below. Further, the embodiments of the present invention are provided to more fully explain the present invention to those having average knowledge in the technical field.

[0020] The terms used in this application are used only for explaining specific examples. Therefore, for example, a singular expression includes a plural expression unless it must be singular in context. Further, it should be noted that terms such as "including" or "comprising" used in this application are used to clearly indicate the presence of features, steps, functions, components, or combinations thereof described in the specification, and are not used to preliminarily exclude the presence of other features, steps, functions, components, or combinations thereof.

[0021] On the other hand, unless otherwise specifically defined, all terms used herein should be considered to have the same meaning as that generally understood by a person of ordinary skill in the art to which the present invention pertains. Therefore, unless explicitly defined herein, no particular term should be interpreted in an overly idealistic or formal sense. For example, in this specification, a singular expression includes a plural expression unless the context clearly indicates otherwise.

[0022] Furthermore, in this specification, "about," "substantially," etc., are used to mean the numerical value or a similar value when tolerances for manufacturing and materials inherent to the meaning referred to are presented, and are used to prevent unscrupulous infringers from unfairly exploiting disclosures that refer to precise and absolute numerical values ​​to aid in understanding the invention.

[0023] The inventors of the present invention conducted repeated experiments under various conditions to solve the problems of the present invention and found that the hydrophilicity and conductivity of the steel material are significantly improved when a joint structure is present on the steel plate.

[0024] A "joint structure" refers to a structure in which the spacing between micro-protrusions is continuously distributed at intervals of 1 to 500 nm. Here, "spacing between pitches" means the distance between the highest points of adjacent micro-protrusions in the thickness-direction cross-section of the steel plate.

[0025] It should be noted that "on the steel plate" refers to the positional relationship of being on the steel plate, and unless otherwise specified, does not exclude other positional relationships of the steel plate. In one embodiment, this also includes the positional relationship of steel plate, joint structure on the steel plate, and oxide layer on the joint structure.

[0026] The steel sheet may include a joint structure in which fine protrusions with a pitch interval of 1 to 500 nm are included on the steel sheet, and the fine protrusions have a texture with a (111) orientation area ratio of 10% or more and a (001) orientation area ratio of 10% or more. A steel sheet satisfying the above-described joint structure has excellent hydrophilicity and conductivity.

[0027] The microprojections have at least one form among horn-shaped, rod-shaped, and needle-shaped.

[0028] A "cornered" shape refers to a figure in which three distinct faces, each with the same or different areas, share a single vertex, and the angles between the normal vectors of each face are all greater than 0° but less than 180°. Typical examples of cornered shapes include the rectangular prism and cube as viewed from a single vertex.

[0029] The angular microprojections have three distinct faces that share a single vertex, and the area of ​​the widest face is 40,000 nm. 2 It is less than 60,000 nm, and the total area of ​​the three surfaces is 60,000 nm. 2 It may be less than 40,000 nm. 2 It is greater than or equal to 60,000 nm, or the total area of ​​the three surfaces is 60,000 nm 2 If the above conditions are met, the horn-like micro-protrusions may be formed to be excessively coarse, potentially preventing sufficient hydrophilicity and conductivity from being ensured.

[0030] The aforementioned horn-shaped micro-protrusions may be present in an area fraction of 20% or more.

[0031] A "rod-shaped" object is defined as a rod-shaped object consisting of a circular or polygonal base and sides that extend vertically from the periphery of the circle or polygon, where the angle between the normal vector of the base and the normal vector of the steel plate surface is between 0 and 90°. Typical examples of "rod-shaped" objects include cylinders, triangular prisms, and rectangular prisms.

[0032] The rod-shaped micro-protrusions may have a side height of 50 nm or more. A greater side height is advantageous for improving hydrophilicity and conductivity, and considering this effect, a side height of 50 nm or more is preferable.

[0033] The angle between the normal vector of the base surface and the normal vector of the steel plate surface of the rod-shaped micro-protrusions may be 0 to 30°. A smaller angle between the normal vector of the base surface and the normal vector of the steel plate surface is advantageous for improving hydrophilicity and conductivity, and considering this effect, it is preferable that the angle between the normal vector of the base surface and the normal vector of the steel plate surface is 0 to 30°.

[0034] The aforementioned rod-shaped micro-protrusions may be present in an area fraction of 20% or more.

[0035] A "needle-shaped" figure is a three-dimensional shape consisting of curves or straight lines that radiate continuously from a single vertex toward the steel plate, where the angle between each radiating curve or straight line and the normal vector of the steel plate surface is greater than 90° and less than or equal to 180°. Typical examples of "needle-shaped" figures include cones and square pyramids.

[0036] The needle-shaped micro-protrusions may have a vertical distance of 50 nm or more from their apex to the steel plate surface. A larger distance from the steel plate surface to the apex is advantageous for improving hydrophilicity and conductivity, and considering this effect, it is preferable that the distance from the steel plate surface to the apex be 50 nm or more.

[0037] The angle between each radially extending curve or line and the normal vector of the steel plate surface may be 150 to 180°. A larger angle between each radially extending curve or line and the normal vector of the steel plate surface is advantageous for improving hydrophilicity and conductivity, and considering this effect, it is preferable that the angle between each radially extending curve or line and the normal vector of the steel plate surface is 150 to 180°.

[0038] The aforementioned needle-like micro-protrusions may be present in an area fraction of 20% or more.

[0039] It should be noted that the micro-projections within the joint structure do not necessarily have only one of the following forms: angular, rod-shaped, or needle-shaped. According to various embodiments, the micro-projections within the joint structure may consist of angular micro-projections alone, rod-shaped micro-projections alone, needle-shaped micro-projections alone, a mixture of angular and rod-shaped micro-projections, a mixture of angular and needle-shaped micro-projections, a mixture of rod-shaped and needle-shaped micro-projections, or a mixture of angular, rod-shaped, and needle-shaped micro-projections.

[0040] To aid in understanding the joint structure, its shape will be described in detail based on the drawings. However, it should be noted that the attached drawings are provided for the purpose of understanding the present invention, and unless otherwise specified, the joint structure of the present invention should not be construed as being limited to the drawings.

[0041] Figure 1 is a surface SEM image of a conventional steel plate.

[0042] As shown in Figures 2a and 2b, unlike conventional steel plates, the present invention has a joint structure. As shown in Figure 2a, two groups of angular microprotrusions having different orientations may be distributed. As shown in Figure 2b, the structure may have a mixture of angular microprotrusions and rod-shaped microprotrusions.

[0043] The steel sheet may have grain sizes ranging from 1 to 35 μm. If the grain size is less than 1 μm, the contact surface area may decrease, potentially increasing the surface resistance. Conversely, if the grain size exceeds 35 μm, poor contact may actually increase the surface resistance.

[0044] The difference in height between crystal grains may be between 0.1 and 1000 nm. If the difference in height between crystal grains is less than 0.1 nm, the contact surface area may decrease, potentially increasing the surface resistance. Conversely, if the difference in height between crystal grains exceeds 1000 nm, poor contact may actually increase the surface resistance.

[0045] Steel plates satisfying the above-described joint structure exhibit excellent hydrophilicity and conductivity. Steel plates satisfying the joint structure having a texture in which fine protrusions have a (111) orientation area ratio of 10% or more and a (001) orientation area ratio of 10% or more exhibit a conductivity of 5 mΩ·cm2 It has the following contact resistance and a contact angle of 80° or less.

[0046] As one embodiment of forming the joint structure described above, when high-pressure water is strongly sprayed onto the steel material using a scale breaker, the surface energy of the steel material is condensed, which induces the metal atoms on the surface where the energy has been condensed to fall off the base material in lattice units during the subsequent pickling process. The surface of the steel plate remaining after the metal atoms have fallen may include a joint structure in which fine protrusions having a pitch interval of 1 to 500 nm and having any of the following forms: angular, rod-shaped, or needle-shaped.

[0047] The steel sheet according to the present invention, when having a joint structure, exhibits excellent hydrophilicity and conductivity, and the alloy composition is not particularly limited in order to achieve the hydrophilicity and conductivity that are the main objectives of the present invention. However, a preferred alloy composition according to one embodiment is shown below. However, it should be noted that the following component composition is merely an example to aid in understanding the present invention and does not limit the technical concept of the present invention.

[0048] Steel sheets contain, by weight %, C: over 0% but 0.02% or less, N: over 0% but 0.02% or less, Si: over 0% but 0.25% or less, Mn: over 0% but 0.2% or less, P: over 0% but 0.04% or less, S: over 0% but 0.02% or less, Cr: 18-34%, with the remainder consisting of Fe and other unavoidable impurities. Unless otherwise specified, the unit is weight % (wt%).

[0049] C: 0% over 0.02% or less, N: 0% over 0.02% or less

[0050] C and N combine with Cr in the steel to form stable Cr carbonitrides, which can result in the formation of locally Cr-deficient regions and a decrease in corrosion resistance. Therefore, lower content of both elements is preferable. Accordingly, in the present invention, the content of C and N may be C: greater than 0% but 0.02% or less, and N: greater than 0% but 0.02% or less.

[0051] Si: more than 0% less than 0.25%

[0052] Si is an effective element for deoxidation. However, when added in excess, it reduces toughness and formability, and the SiO2 oxide produced during the annealing process reduces electrical conductivity and hydrophilicity. Taking this into consideration, in the present invention, the Si content may be greater than 0% but less than or equal to 0.25%.

[0053] Mn: More than 0% and less than 0.2%

[0054] Mn is an effective element for deoxidation. However, since MnS, which is an inclusion of Mn, reduces corrosion resistance, the Mn content in this invention may be greater than 0% but less than or equal to 0.2%.

[0055] P: More than 0% and less than 0.04%

[0056] Since P reduces not only corrosion resistance but also toughness, the P content in this invention may be greater than 0% but less than or equal to 0.04%.

[0057] S: More than 0% and less than 0.02%

[0058] S combines with Mn in steel to form stable MnS, and the formed MnS acts as a starting point for corrosion, reducing corrosion resistance. Therefore, a lower S content is preferable. Taking this into consideration, in the present invention, the S content may be greater than 0% but less than or equal to 0.02%.

[0059] Cr: 18-34%

[0060] Cr is an element that improves corrosion resistance. To ensure corrosion resistance in the highly acidic environment of fuel cell operation, Cr is actively added. However, excessive addition reduces toughness, so taking this into consideration, the Cr content in this invention may be 18-34%.

[0061] Furthermore, in addition to the alloy composition described above, the steel sheet may optionally contain one or more of the following as selective alloying components in weight percent: V: greater than 0% but 0.6% or less, Ti: greater than 0% but 0.5% or less, and Nb: greater than 0% but 0.5% or less.

[0062] V: More than 0% and less than 0.6%

[0063] V is an element that suppresses Fe elution in the operating environment of a fuel cell, thereby improving the lifespan characteristics of the fuel cell. However, since its toughness is reduced when added in excess, taking this into consideration, the content of V in this invention may be greater than 0% but less than or equal to 0.6%.

[0064] Ti: over 0% 0.5% or less, Nb: over 0% 0.5% or less

[0065] Ti and Nb are elements that improve corrosion resistance by forming stable carbonitrides by bonding with C and N in steel, thereby suppressing the formation of locally Cr-deficient regions. However, excessive addition reduces toughness, so in this invention, the Ti and Nb content may be adjusted so that Ti is greater than 0% but 0.5% or less, and Nb is greater than 0% but 0.5% or less.

[0066] The remaining component is iron (Fe). However, in the normal manufacturing process, unintended impurities from the raw materials and surrounding environment are inevitably introduced, and therefore cannot be eliminated. Since any technician in the normal manufacturing process would know about these impurities, not all of them are specifically mentioned in this specification.

[0067] 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 to illustrate the present invention in more detail and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and matters that can be reasonably inferred therefrom.

[0068] {Example} After producing steel grades with various alloy compositions in slabs, the produced slabs were manufactured into hot-rolled steel sheets with a thickness of about 4.5 mm through hot rolling at 1200°C. After strongly spraying high-pressure water using a scale breaker on the hot-rolled steel sheets, pickling treatment was performed. Then, after heating the hot-rolled steel sheets at 1050°C, cold rolling and annealing at 1000°C were repeated to manufacture cold-rolled steel sheets with a thickness of about 0.15 mm.

[0069] The fine protrusion area fraction and contact resistance of the produced cold-rolled steel sheets were measured and shown in Table 1 below.

[0070] The area fraction (%) of the fine protrusion aggregate structure in Table 1 was measured using a backscattered electron diffraction pattern analyzer (EBSD).

[0071] The area fraction of the angular fine protrusions in Table 1 is in a form where three different faces consisting of polygons with the same or different areas share one vertex, and the angles between the normal vectors on each face are all greater than 0° and less than 180°. Among the three different faces sharing the one vertex, the area of the widest face is less than 40,000 nm 2 and the total area of the three faces is less than 60,000 nm 2 The area of the fine protrusions was determined.

[0072] The area fraction of the rod-shaped fine protrusions in Table 1 has a rod shape consisting of a circular or polygonal bottom surface and side surfaces vertically extended from around the circular or polygonal shape. The angle between the normal vector of the bottom surface and the normal vector of the steel sheet surface is 0 to 90°, and the area of the fine protrusions with a side surface height of 50 nm or more was determined.

[0073] The area fraction of the needle-shaped fine protrusions in Table 1 is a solid consisting of curves or straight lines continuously radiating and extending from one vertex toward the steel sheet side. The angle between each radiating curve or straight line and the normal vector of the steel sheet surface is greater than 90° and less than or equal to 180°, and the area of the fine protrusions with a vertical distance of 50 nm or more from the vertex to the steel sheet surface was determined.

[0074] The evaluation of the interface contact resistance in Table 1 was performed by using the produced material for 50 cm 2After cutting to the required area and preparing two pieces, a 4cm layer is used as a gas diffusion layer between them. 2 A sheet of carbon paper (SGL-10BA) is placed in between, and the contact pressure is 100 N / cm². 2 The interfacial contact resistance was evaluated five times for each component, and the average was calculated to derive the result.

[0075] The contact angle evaluation in Table 1 is based on a manufactured steel plate material with an area of ​​20 cm². 2 The material was cut into sections, and 3 μl of distilled water was dropped onto the surface at room temperature using a KRUSS GmbH DSK 10-MK2 device, and the contact angle with the surface was measured.

[0076] [Table 1]

[0077] As shown in Table 1, Examples 1 to 10 of the invention, which satisfy the joint structure having a texture in which the micro-protrusions limited by the present invention have an orientation area ratio of 10% or more for (111) and an orientation area ratio of 10% or more for (001), have a contact resistance of 5 mΩ·cm 2 The following characteristics were observed, with a contact angle of 80° or less and excellent hydrophilicity and conductivity. However, Comparative Examples 1-4 failed to satisfy the joint structure defined by the present invention, and had a contact resistance of 5 mΩ·cm. 2 The contact angle exceeded 80°, resulting in poor hydrophilicity and conductivity.

[0078] Although exemplary embodiments of the present invention have been described above, the present invention is not limited thereto, and a person with ordinary skill in the art will understand that various modifications and variations are possible without departing from the concepts and scope of the claims described below. [Industrial applicability]

[0079] The present invention provides a steel plate with excellent hydrophilicity and conductivity, including a joint structure, that can be used for fuel cell separation plates, and therefore has industrial applicability.

Claims

1. A steel sheet having at least one form of fine protrusions, such as horn-shaped, rod-shaped, or needle-shaped, formed on a hot-rolled steel sheet by a scale breaker, wherein the area fraction of the fine protrusions is 20% or more in total. The spacing between the pitches of the micro-protrusions is 1 to 500 nm, and the joint structure includes a texture in which the micro-protrusions have an orientation area ratio of 10% or more for (111) and 10% or more for (001). The contact resistance measured at a contact pressure of 100 N / cm² with carbon paper placed between two manufactured materials is 5 mΩ·cm. ２ The following steel plate exhibits excellent hydrophilicity and conductivity, characterized in that when a water droplet is dropped onto the surface of the manufactured material, the contact angle with the surface is measured to be 80° or less.

2. Of the aforementioned microprotrusions, angular microprotrusions account for 20% or more of the area fraction, and the angular microprotrusions are microprotrusions in which three different faces, each consisting of polygons with the same or different areas, share one vertex, and the angles between the normal vectors on each face are all greater than 0° and less than 180°. The area of ​​the largest of the three distinct faces that share one vertex is 40,000 nm. ２ It is less than 60,000 nm, and the total area of ​​the three surfaces is 60,000 nm ２ A steel plate with excellent hydrophilicity and conductivity as described in claim 1, characterized in that it is less than [a certain value].

3. Of the aforementioned microprotrusions, rod-shaped microprotrusions account for 20% or more of the area fraction, and the rod-shaped microprotrusions have a rod shape consisting of a circular or polygonal base and sides that extend vertically from around the circular or polygonal shape, and the angle between the normal vector of the base and the normal vector of the steel plate surface is 0 to 90°. The steel plate with excellent hydrophilicity and conductivity according to claim 1, characterized in that the height of the side surface is 50 nm or more.

4. The steel plate with excellent hydrophilicity and conductivity according to claim 3, characterized in that the angle between the normal vector of the bottom surface and the normal vector of the steel plate surface is 0 to 30°.

5. Of the aforementioned microprotrusions, needle-shaped microprotrusions account for 20% or more of the area fraction, and each needle-shaped microprotrusion is a solid consisting of curves or straight lines that radiate continuously from one vertex toward the steel plate, and the angle between each radiating curve or straight line and the normal vector of the steel plate surface is greater than 90° and less than or equal to 180° for the microprotrusions. The steel sheet with excellent hydrophilicity and conductivity according to claim 1, characterized in that the vertical distance from the vertex to the surface of the steel sheet is 50 nm or more.

6. The steel sheet with excellent hydrophilicity and conductivity according to claim 5, characterized in that the angle between each radially extended curve or straight line and the normal vector of the steel sheet surface is 150 to 180°.

7. The steel sheet with excellent hydrophilicity and conductivity according to claim 1, characterized in that the difference in height between crystal grains is 0.1 to 1000 nm.

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