Organic coated steel sheet

Abstract

Provided is a high-strength steel sheet capable of suppressing hydrogen embrittlement cracking. This organic coated steel sheet (1) has a steel sheet (2) having a tensile strength of 980 MPa or more, and an organic coating layer (3) disposed on at least a part of the surface of the steel sheet. The organic coated steel sheet (1) according to the present invention is characterized in that the organic coating layer (3) contains nitrite in an amount of 1-30 mass% and 0.05 g / m2 or more in terms of nitrite.

Description

Technical Field

[0001] This invention relates to organically coated steel sheets. Background Technology

[0002] In recent years, the use of high-strength steel sheets in various fields such as automobiles, home appliances, and building materials has been trending towards higher strength. For example, in the automotive industry, the use of high-strength steel sheets is increasing to improve fuel efficiency and reduce vehicle weight.

[0003] High-strength steel sheets, especially those used in automotive components, are coated using electrodeposition to impart desired properties such as corrosion resistance. In this electrodeposition coating process, water is electrolyzed by applying voltage, producing hydrogen. This hydrogen then penetrates the steel sheet, reaching depths beyond the surface layer. As a result, it can potentially cause grain boundary embrittlement through segregation at the martensitic grain boundaries, leading to cracking. Cracking caused by this hydrogen intrusion is called hydrogen embrittlement or delayed fracture. Furthermore, it is known that hydrogen embrittlement becomes more likely to occur with increasing steel strength.

[0004] Various countermeasures have been studied to date regarding hydrogen embrittlement (delayed fracture) of high-strength steel sheets. For example, Patent Document 1 discloses an Fe-based electroplated high-strength steel sheet with low diffusible hydrogen content. This is achieved by annealing the steel sheet after electroplating, thereby releasing the diffusible hydrogen that penetrated the steel sheet during electroplating into the furnace. The Fe-based electroplated high-strength steel sheet disclosed in Patent Document 1 exhibits excellent chemical conversion treatment properties and post-coating corrosion resistance, as well as excellent resistance to delayed fracture.

[0005] Existing technical documents

[0006] Patent documents

[0007] Patent Document 1: Japanese Patent Application Publication No. 2022-180344 Summary of the Invention

[0008] The problem that the invention aims to solve

[0009] Regarding the high-strength coated steel sheet disclosed in Patent Document 1, by forming a coating on the surface of the steel sheet as the base material, the intrusion of hydrogen, which is the cause of hydrogen embrittlement, can be suppressed. However, corrosion easily occurs on the end face (cut surface) of the steel sheet where no coating is formed. Such end face corrosion not only leads to red rust but also becomes a cause of hydrogen generation. Moreover, if hydrogen generated along with corrosion penetrates into the steel sheet from the end face, hydrogen embrittlement cracking as described above may occur in the high-strength steel sheet.

[0010] Therefore, the object of the present invention is to provide a high-strength steel sheet capable of suppressing hydrogen embrittlement cracking.

[0011] Methods for solving problems

[0012] The present invention includes the following solutions.

[0013] (Option 1)

[0014] An organically coated steel sheet, characterized in that it has:

[0015] Steel plates with a tensile strength of 980 MPa or higher, and

[0016] An organic coating disposed on at least a portion of the surface of the aforementioned steel plate.

[0017] The aforementioned organic coating layer contains 1–30% by mass of nitrite and 0.05 g / m³. 2 The above levels of nitrite.

[0018] (Option 2)

[0019] According to the organic coated steel sheet of Scheme 1 above, the nitrite is characterized in that it is selected from at least one of lithium salt, sodium salt, potassium salt, magnesium salt, calcium salt, strontium salt, barium salt and ammonium salt.

[0020] (Option 3)

[0021] The organic-coated steel sheet according to Scheme 1 or 2 is characterized in that a coating is disposed between the steel sheet and the organic coating layer.

[0022] (Option 4)

[0023] The organic coated steel plate according to any one of the above schemes 1 to 3 is characterized in that the tensile strength of the steel plate is 1.2 to 3.0 GPa.

[0024] (Option 5)

[0025] A molded body made of an organically coated steel sheet as described in any one of the above schemes 1 to 4.

[0026] Invention Effects

[0027] The organic-coated steel sheet according to the present invention can provide a high-strength steel sheet capable of suppressing hydrogen embrittlement cracking. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of an organically coated steel plate 1 according to one embodiment of the present invention.

[0029] Figure 2 This is a schematic diagram of device 6 used to evaluate hydrogen embrittlement cracking of steel plates. Detailed Implementation

[0030] The preferred embodiments of the organically coated steel sheet of the present invention will be described in detail below. It should be noted that, unless otherwise specified, all numerical ranges in this specification refer to ranges including their upper and lower limits.

[0031] To achieve the aforementioned objectives, the inventors of this invention conducted in-depth research focusing on suppressing the intrusion of hydrogen into steel plates, a cause of hydrogen embrittlement. As a result, the inventors discovered a technique that reduces hydrogen intrusion into steel plates during corrosion by formulating coatings with specific compositions. Specifically, the inventors discovered that by including a specific amount of nitrite in the coating composition, the nitrite ions (NO2) are reduced... - It is reduced more preferentially than hydrogen, thereby suppressing the generation of hydrogen, which is the cause of hydrogen embrittlement.

[0032] The present invention was made based on such insight and includes various embodiments of the above-mentioned organically coated steel plates.

[0033] First, a preferred embodiment of the organically coated steel sheet of the present invention will be described in detail.

[0034] [Organic-coated steel sheet]

[0035] As one embodiment of the present invention, the organic-coated steel sheet 1 is as follows: Figure 1 As shown, the basic components include: a steel plate 2 having a pair of surfaces opposite each other in the thickness direction and a tensile strength of 980 MPa or more, and organic coating layers 3 disposed on both sides of the steel plate 2.

[0036] In addition, in this embodiment, the organically coated steel plate 1 is as follows: Figure 1 As shown, it further comprises: a coating 4 disposed between the steel plate 2 and the organic coating 3, and a chemical conversion treatment film 5 disposed between the coating 4 and the organic coating 3.

[0037] In this specification, the term "at least a portion of the surface of a steel plate" refers not only to a surface arrangement directly disposed on at least a portion of the surface of a steel plate, but also to a surface arrangement including at least a portion of the surface of a steel plate indirectly disposed on the surface of a steel plate via other layers (e.g., coatings) or films (e.g., chemical conversion coatings). When such other layers or films are present on the surface of a steel plate, the surface of the steel plate refers to the interface between the steel plate and other layers or films.

[0038] It should be noted that, in Figure 1In the organically coated steel sheet 1 shown, the organic coating layer 3 is disposed on both sides of the steel sheet 2, respectively, through the plating layer 4 and the chemical conversion treated film 5, but is not limited to this arrangement. That is, the organic coating layer 3 may be disposed on both sides of the steel sheet 2, respectively, through or without the plating layer 4 and the chemical conversion treated film 5, or it may be disposed on only one surface of the steel sheet 2, either through or without the plating layer 4 and the chemical conversion treated film 5. Furthermore, in Figure 1 In the organically coated steel plate 1 shown, the organic coating layer 3 is disposed on the entire surface of the steel plate 2, but the arrangement is not limited to this. That is, the organic coating layer 3 can be as follows: Figure 1 It can be configured on the entire surface of steel plate 2, or it can be configured on only a part of the surface of steel plate 2.

[0039] Furthermore, in the organically coated steel sheet 1 of this embodiment, the organic coating layer 3 contains 1 to 30% by mass of nitrite and 0.05 g / m². 2 The above-mentioned nitrite content. By having the organic coating 3 contain a specific amount of nitrite, the nitrite ions (NO2) in the organic coating 3 are reduced. - It is reduced more preferentially than hydrogen, thereby making it difficult for hydrogen, which is the cause of hydrogen embrittlement, to be generated. As a result, the organic-coated steel sheet 1 of this embodiment becomes a high-strength steel sheet capable of suppressing hydrogen embrittlement.

[0040] It should be noted that in the organic-coated steel sheet 1, having layers and films other than the steel sheet 2 and organic coating layer 3 (i.e., coating layer 4 and chemical conversion treatment coating layer 5) is not a necessary component. Therefore, the organic-coated steel sheet 1 may have any layers and films other than the steel sheet 2 and organic coating layer 3, or it may not have them, depending on the requirements and characteristics of the final product to which it is applied.

[0041] The following is a more detailed description of the various components of the organically coated steel plate 1.

[0042] (steel plate)

[0043] In the organic-coated steel sheet 1, the steel sheet 2 serving as the base material is a steel sheet with a tensile strength (TS) of 980 MPa or higher. Generally, it is known that the susceptibility of steel sheets to hydrogen embrittlement increases with increasing strength. However, even though the organic-coated steel sheet 1 of this embodiment is a high-strength steel sheet with a tensile strength of 980 MPa or higher, as described above, the specific amount of nitrite contained in the organic coating layer 3 makes it difficult for hydrogen, which is the cause of hydrogen embrittlement cracking, to be generated. Therefore, it can be suitable as a raw material for various structural components in fields requiring such high strength.

[0044] The tensile strength of the steel plate 2 is only required to be 980 MPa or higher, and there is no particular limitation, but it is preferably 1200 MPa (i.e., 1.2 GPa) or higher, more preferably 1300 MPa (i.e., 1.3 GPa) or higher, and even more preferably 1400 MPa (i.e., 1.4 GPa) or higher. It should be noted that there is no particular upper limit to the tensile strength of the steel plate 2, but from the viewpoint of toughness and formability, it is preferably 4000 MPa (i.e., 4.0 GPa) or lower, and more preferably 3000 MPa (i.e., 3.0 GPa) or lower.

[0045] The tensile strength of the steel sheet 2 is preferably 1.2 to 3.0 GPa. It is known that if the tensile strength of the steel sheet is 1.2 GPa or higher, fracture may occur due to trace amounts of hydrogen, in the order of ppm. This is particularly problematic in the automotive industry, where high strength of the steel sheet is required, and the risk of hydrogen embrittlement must be significantly reduced. The organic-coated steel sheet 1 of this embodiment, even if the tensile strength of the steel sheet 2 is 1.2 GPa or higher, makes it difficult for hydrogen, which is a cause of hydrogen embrittlement, to be generated. Therefore, it can be suitable as a raw material for various structural components in the automotive industry. Furthermore, if the tensile strength of the steel sheet 2 is 3.0 GPa or lower, the organic-coated steel sheet 1 exhibits excellent toughness and formability.

[0046] The tensile strength (TS) of a steel plate can be determined by cutting a No. 5 tensile test piece according to JIS Z2241:2011 from the steel plate and performing a tensile test according to JIS Z2241:2011. It should be noted that the test piece is preferably cut from the steel plate with its length direction perpendicular to the rolling direction of the steel plate. For example, if it is difficult to obtain a JIS No. 5 test piece due to size constraints, other test pieces described in JIS Z 2241:2011 can be used. However, for proper evaluation, the plate thickness is limited to 0.5 mm. Furthermore, if it is difficult to obtain a JIS No. 5 test piece due to size constraints and it is also difficult to use other test pieces described in JIS Z 2241:2011, a micro Vickers test can be performed according to JIS Z 2244-1:2020, and the value obtained by converting its hardness (HV) to tensile strength can be used. The specimen for the micro Vickers test can be prepared as follows: First, cut the specimen from any position other than the machined section at least 50 mm away from the end face of the steel plate, in a manner that allows observation of the plate thickness section perpendicular to the plate surface. If it is not possible to collect the specimen from the above position, cut the specimen from a position avoiding the end. The plate thickness section is preferably parallel to the rolling direction. Although the size of the specimen varies depending on the measuring device, it is set to be approximately 10 mm in size that can be observed in the direction perpendicular to the plate thickness direction. Grind the section of the specimen cut as described above using silicon carbide paper of #600 to #1500. Then, using a liquid obtained by dispersing diamond powder with a particle size of 1 to 6 μm in a diluent such as alcohol or pure water, refine the ground section to a mirror finish. Next, refine the section to an observation surface by electrolytic grinding. For the micro Vickers test, simply measure 30 points at 1 / 4 of the plate thickness under a load of 500 gf, and use the average value. The conversion from Vickers hardness obtained through micro Vickers testing to tensile strength can be performed using the following formula.

[0047] Tensile strength [MPa] = 3.12 × Vickers hardness [HV] + 16

[0048] It should be noted that steel plate 2 is not particularly limited in terms of aspects other than tensile strength, and any steel plate corresponding to the desired mechanical strength can be used. For example, the thickness of steel plate 2 is not particularly limited, but can be set to approximately 0.2 mm to 10.0 mm.

[0049] (Organic coating)

[0050] In the organically coated steel sheet 1, the organic coating layer 3 contains 1-30% by mass of nitrite and 0.05 g / m². 2The above-mentioned nitrite content. By containing such a specific amount of nitrite in the organic coating layer 3, the nitrite ions (NO2) in the organic coating layer 3 are reduced. - It is reduced more preferentially than hydrogen, which makes it difficult for hydrogen, the cause of hydrogen embrittlement, to be produced.

[0051] The nitrite used in the organically coated steel plate 1 of this embodiment is not particularly limited, and examples include alkali metal salts, alkaline earth metal salts, and ammonium salts of nitrite. Specifically, examples include lithium salts, sodium salts, potassium salts, magnesium salts, calcium salts, strontium salts, barium salts, and ammonium salts of nitrite. Among these, from the viewpoint of obtaining the aforementioned effects of the nitrite more effectively, the nitrite is preferably selected from at least one of lithium salts, sodium salts, potassium salts, magnesium salts, calcium salts, strontium salts, barium salts, and ammonium salts of nitrite.

[0052] The identification of nitrite in the organic coating can be performed using known methods. For example, it can be done using SEM-EDS elemental distribution images. Specifically, firstly, the cross-section of the organic-coated steel sheet along its thickness is flattened by mechanical grinding. Then, the surface of the organic-coated steel sheet is chemically ground using colloidal grinding until the surface becomes mirror-like.

[0053] The cross-section of the ground organic-coated steel plate was observed using SEM. Specifically, elemental distribution images were captured using SEM-EDS at a magnification of 5000x (200 μm vertically and 200 μm horizontally). The presence of nitrite in the organic coating was determined when the element involved in the nitrite cation was detected at a g-equivalent (100%) of more than 90% relative to the g-equivalent (100%) of the separately measured nitrite. It should be noted that when multiple types of nitrite cations were detected, their combined g-equivalent was used for determination. Elemental analysis and quantification could be performed using EPMA (electron probe microanalysis) or GDS (glow discharge spectroscopy).

[0054] It should be noted that the gram equivalent (eq) of nitrite in the organic coating can be determined by the content measured later using a colorimetric method that utilizes color development induced by diazotization.

[0055] The nitrite content in the organic coating layer 3 should be 1-30% by mass and 0.05 g / m³. 2The above content is acceptable and there are no particular limitations. From the viewpoint of easily obtaining higher results, a higher content of nitrite ions as nitrite is preferred; specifically, a content of 5% by mass or more relative to the total mass of the organic coating (i.e., the mass of the total solid components of the organic coating) is preferred. The nitrite content can also be 10% by mass or more, 15% by mass or more, 20% by mass or more, or 25% by mass or more. However, from the viewpoint of solubility product, when the nitrite content exceeds 30% by mass, it becomes impossible to stably manufacture organically coated steel sheets; therefore, an upper limit of 30% by mass is set.

[0056] Furthermore, from the viewpoint of easily obtaining higher results, the content of nitrite ions, as a nitrite, is preferably 0.50 g / m², calculated per unit surface area of ​​the steel plate. 2 The above, more preferably 1.00 g / m 2 The above is further preferred to be 5.00 g / m 2 The above. It should be noted that there is no specific upper limit to the content of nitrite per unit surface area of ​​the steel plate, but for example, it is 10.00 g / m². 2 .

[0057] The organic coating layer 3 contains a resin as a base coating. Additionally, components other than nitrites may be included, to the extent that they do not impair the effects of the present invention. Examples of such components include coloring pigments, rust-inhibiting pigments, dispersants, leveling agents, lubricants, and other additives, as well as diluents.

[0058] There are no particular limitations on the resins that may be contained in the organic coating, but it is preferable to use resins that function as adhesives to bond the various components contained in the organic coating. Examples include solvent-based resins that are dissolved or dispersed in organic solvents, and water-soluble or water-dispersible aqueous resins.

[0059] The solvent-based resin is not particularly limited, but examples include polyester resin, polyurethane resin, epoxy resin, acrylic resin, and mixtures of two or more of these resins. It should be noted that the resin can be a cross-linked resin with a cross-linking structure or a non-cross-linked resin without a cross-linking structure. When imparting a cross-linking structure to the resin, a water-soluble cross-linking agent (curing agent) is preferred as the cross-linking agent (curing agent) used in the cross-linking process. Specific examples of cross-linking agents include melamine and isocyanates.

[0060] On the other hand, as a water-based resin, there is no particular limitation, but examples include polyester resins, polyurethane resins, acrylic resins, epoxy resins, phenolic resins, and mixed resins of two or more of these resins that are water-soluble or water-dispersible.

[0061] The resin content in the organic coating is not particularly limited, but for example, it is 10 to 90% by mass relative to the total mass of the organic coating (i.e., the mass of the total solids component of the organic coating). From the viewpoint of exhibiting the function of an adhesive, while taking into account both adhesion to adjacent layers, films, or steel plates and corrosion resistance, the resin content is preferably 20 to 80% by mass relative to the total mass of the organic coating, more preferably 25 to 75% by mass.

[0062] It should be noted that there are no particular limitations on the coloring pigments that can be contained in the organic coating; any coloring pigment corresponding to the desired appearance, pattern design, etc., can be used. Examples of such coloring pigments include aluminum pigments, carbon black, silica, titanium dioxide, zirconium oxide, etc. There are no particular limitations on the content of the coloring pigments, but for example, a content of approximately 1 to 60% by mass relative to the total mass of the organic coating can be listed.

[0063] There are no particular limitations on the rust-preventive pigments that may be contained in the organic coating, but examples include aluminum tripolyphosphate; Zn, Mg, Al, Ti, Zr, or Ce salts of phosphoric acid or phosphorous acid; phosphoric acid compounds treated with hydrated bauxite; Ca ion-exchanged silica; and amorphous silica. There are no particular limitations on the content of the rust-preventive pigments, but examples include a content of approximately 1 to 40% by mass relative to the total mass of the organic coating.

[0064] There are no particular limitations on the lubricants that may be contained in the organic coating, but examples include polyolefin waxes, paraffin waxes, and fluoropolymer waxes. There are no particular limitations on the content of the lubricant, but examples include a content of about 0.1% to 10% by mass relative to the total mass of the organic coating.

[0065] The amount of organic coating attached (i.e., the mass of the total solids component of the organic coating per unit area) is not particularly limited, but is, for example, 2–20 g / m². 2 The adhesion amount is approximately 2-15 g / m². If the adhesion amount of the organic coating is within this range, the adhesion to adjacent layers, films, or steel plates becomes good, and the effects of the aforementioned nitrites can be obtained more reliably. It should be noted that the preferred adhesion amount of the organic coating is 2-15 g / m². 2 .

[0066] Furthermore, the thickness of the organic coating layer is not particularly limited, but for example, a thickness of about 1 to 50 μm can be listed. From the viewpoint of adhesion, corrosion resistance, and the effects of the aforementioned nitrite, the thickness of the organic coating layer is preferably 3 μm or more, more preferably 5 μm or more, and even more preferably 10 μm or more. In addition, the thickness of the organic coating layer is preferably 30 μm or less.

[0067] It should be noted that, as described above, the organic coating can be disposed on both sides of the steel plate, either through a coating or a chemical conversion treatment film, or it can be disposed on only one surface of the steel plate, either through a coating or a chemical conversion treatment film. Furthermore, as described above, the organic coating can be disposed on the entire surface of the steel plate, or it can be disposed on only a portion of the surface of the steel plate.

[0068] The method for forming the organic coating is not particularly limited, but it can be formed by the following steps: after preparing a coating composition containing at least the specific amount of nitrite as described above, the coating composition is applied to a steel plate, a coating formed on the surface of the steel plate, or a chemically converted film, followed by heating and drying. It should be noted that the coating method is not particularly limited, but any coating treatment such as electrodeposition coating, powder coating, or solvent coating can be used.

[0069] It should be noted that organic coatings can have a single-layer structure made of individual layers, or a multi-layer structure made of multiple layers. In the case of a multi-layer structure, examples of such a structure include a base coat, a middle coat, and a top coat.

[0070] (Coating)

[0071] exist Figure 1 In the organic-coated steel sheet 1 shown, the coating 4 is formed on both sides of the steel sheet 2 and disposed between the steel sheet 2 and the organic coating 3. More specifically, the organic-coated steel sheet 1 has a chemical conversion treated film 5 disposed between the steel sheet 2 and the organic coating 3, and the coating 4 is disposed between the chemical conversion treated film 5 and the steel sheet 2. It should be noted that the presence of the chemical conversion treated film 5 in the organic-coated steel sheet 1 is not a necessary component, therefore the aforementioned organic coating 3 can also be directly disposed on the surface of the coating 4.

[0072] Generally, the formation of coatings such as zinc plating tends to generate hydrogen, which increases the risk of hydrogen embrittlement. However, even though the organic coated steel sheet 1 of this embodiment has coating 4, as described above, the specific amount of nitrite contained in the organic coating 3 makes it difficult for hydrogen, which is the cause of hydrogen embrittlement, to be generated. Therefore, the advantages of having coating 4 (such as improved corrosion resistance) can be enjoyed, and hydrogen embrittlement is suppressed.

[0073] The coating 4 can be either a hot-dip galvanized coating or an electroplated coating. Furthermore, examples of hot-dip galvanized coatings include hot-dip zinc plating (GI), alloyed hot-dip zinc plating (GA), hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, and hot-dip Zn-Al-Mg-Si alloy plating. Examples of electroplated coatings include electroplated zinc plating (EG) and electroplated Zn-Ni alloy plating. Preferably, the coating is a hot-dip galvanized coating, an alloyed hot-dip galvanized coating, or an electroplated zinc plating.

[0074] There is no particular limit to the amount of coating applied, but for example, it can be 10 to 180 g / m² per single side. 2 The amount of coating adhered is approximately [amount missing]. The amount of coating adhered is determined by the weight change before and after pickling, after dissolving the coating in an acid solution containing an inhibitor to suppress corrosion of the base metal.

[0075] In addition, there is no particular limitation on the thickness of the coating, but for example, a thickness of about 3 to 50 μm can be listed.

[0076] It should be noted that, in Figure 1 In the organically coated steel plate 1 shown, the coating 4 is disposed on both sides of the steel plate 2, but the arrangement is not limited to this. That is, the coating 4 can be as follows: Figure 1 They can be disposed on both sides of steel plate 2, or they can be disposed on only one surface of steel plate 2. Additionally, in Figure 1 In the organically coated steel sheet 1 shown, the coating 4 is disposed on the entire surface of the steel sheet 2, but the arrangement is not limited to this. That is, the coating 4 can be as follows: Figure 1 It can be configured on the entire surface of steel plate 2, or it can be configured on only a part of the surface of steel plate 2.

[0077] It should be noted that the coating is not a necessary component in the organic coated steel sheet of the present invention. Therefore, the organic coated steel sheet of the present invention may or may not have a coating, depending on the requirements and characteristics of the final product to which it is applied.

[0078] (Chemical conversion treatment of the coating)

[0079] exist Figure 1 In the organic-coated steel sheet 1 shown, a chemical conversion treatment film 5 is formed on the surface of the aforementioned coating layer 4 and disposed between the coating layer 4 and the organic coating layer 3. It should be noted that the coating layer 4 is not a necessary component in the organic-coated steel sheet 1, therefore the aforementioned chemical conversion treatment film 5 can also be directly disposed on the surface of the steel sheet 2.

[0080] The chemically converted coating 5 functions by improving its barrier properties and adhesion to corrosive factors such as moisture and corrosive ions, thereby improving the corrosion resistance of the organically coated steel plate 1.

[0081] The components forming the chemical conversion treated film 5 are not particularly limited, but examples include phosphoric acid, phosphates, silica, fluorides, vanadium compounds, silane coupling agents, zirconium compounds, resins, tannins, and tannic acid. Among these components, at least one component selected from phosphoric acid, phosphates, silica, fluorides, and vanadium compounds is preferred. If the chemical conversion treated film is formed using such components, it functions as an inhibitor, further improving corrosion resistance by forming a precipitated film or a passivating film on the surface of the coating or steel plate.

[0082] Examples of phosphates that can be used for the formation of coatings through chemical conversion treatments include crystalline and amorphous phosphates. More specifically, examples of crystalline phosphates include zinc phosphate, zinc ferric phosphate, manganese phosphate, manganese ferric phosphate, and zinc calcium phosphate. Examples of amorphous phosphates include ferric phosphate, tin phosphate, zirconium phosphate, titanium phosphate, and hafnium phosphate.

[0083] Furthermore, examples of fluorides that can be used for the formation of coatings through chemical conversion treatment include zirconium fluoride, titanium fluoride, hafnium fluoride, and indium fluoride. Moreover, examples of vanadium compounds that can be used for the formation of coatings through chemical conversion treatment include vanadium oxide.

[0084] Chemically converted coatings may also contain the same nitrites as those in the organic coatings described above. In this case, the nitrite content in the chemically converted coating is not particularly limited, but for example, a content of approximately 1 to 30% by mass relative to the total mass of the chemically converted coating (i.e., the mass of the total solids content of the chemically converted coating) can be listed. If such nitrites are also included in the chemically converted coating, it can make it more difficult for hydrogen, which is a cause of hydrogen embrittlement, to be generated.

[0085] The amount of chemically converted coatings (i.e., the mass of the total solids component of the chemically converted coating per unit area) depends on the components that form it and is not particularly limited, but examples include 10–2000 mg / m² per single side. 2 The adhesion amount is approximately 100–1500 mg / m³. The preferred adhesion amount for the chemically converted film is 100–1500 mg / m³. 2 .

[0086] It should be noted that the amount of chemically converted coating can be determined using known analytical methods such as fluorescence X-ray analysis. For example, a standard line representing the relationship between fluorescence X-ray intensity and coating amount can be prepared in advance using a sample whose phosphorus adhesion amount is known through chemical analysis. Using this standard line, the amount of chemically converted coating can be determined from the fluorescence X-ray intensity measurement results.

[0087] Furthermore, the thickness of the chemically converted coating also depends on the composition forming it and is not particularly limited, but for example, a thickness of about 0.01 to 5.00 μm can be listed. The thickness of the chemically converted coating is preferably 0.03 to 3.00 μm.

[0088] It should be noted that, in Figure 1 In the organic-coated steel plate 1 shown, the chemical conversion treated film 5 is disposed on both sides of the steel plate 2 via the coating layer 4, but the arrangement is not limited to this. That is, the chemical conversion treated film 5 can be as follows: Figure 1 It can be disposed on both sides of the steel plate 2, either separately or without the coating 4, or it can be disposed on only one surface of the steel plate 2, either separately or without the coating 4. Furthermore, in Figure 1 In this embodiment, the chemical conversion treatment film 5 is disposed on the entire surface of the steel plate 2, but it is not limited to this arrangement. That is, the chemical conversion treatment film 5 can be disposed as follows: Figure 1 It can be configured on the entire surface of steel plate 2, or it can be configured on only a part of the surface of steel plate 2.

[0089] There is no particular limitation on the method for forming a chemical conversion coating, but it can be formed, for example, as follows: First, the surface of the steel plate or the surface of the coating, which becomes the forming surface of the chemical conversion coating, is subjected to a known degreasing and washing process to remove adhering oil and other impurities, as well as surface oxides. Next, a chemical conversion coating forming composition containing the various components described above is applied to or impregnated onto the surface to allow it to adhere. Then, the chemical conversion coating is formed by drying the composition adhered to the surface under any drying conditions.

[0090] It should be noted that the presence of a chemically converted coating is not a necessary component in the organically coated steel sheet of the present invention. Therefore, the organically coated steel sheet of the present invention may or may not have a chemically converted coating, depending on the requirements and characteristics of the final product to which it is applied.

[0091] (Other membranes)

[0092] The above describes the various constituent elements of the organically coated steel plate 1 in this embodiment. Figure 1 The invention has been described, but the organic-coated steel sheet is not limited to this. Figure 1 The configuration is shown in the figure. For example, an organically coated steel sheet may also have a functional coating, such as a substrate treatment coating, sandwiched between the steel sheet and the organic coating layer to further improve adhesion and corrosion resistance.

[0093] Examples of substrate-treated films include chromate-treated films and substrate-treated films that are substantially chromium-free (i.e., chromate-free films). It should be noted that chromate-free films can be formed using, for example, silica-based treatment solutions using silicon compounds such as liquid silica, fumed silica, or silicates as the main coating component, or zircon-based treatment solutions using zircon compounds as the main coating component. These treatment solutions can also be those in which organic resins coexist with the main coating component.

[0094] The amount of substrate-treated film adhering (i.e., the mass of the total solids component of the substrate-treated film per unit area) can be any amount corresponding to the treatment solution used. For example, in the case of a substrate-treated film obtained using a silica-based treatment solution, the amount of adhering film can be set to 1 to 20 mg / m² in terms of Si conversion. 2 The amount of adhesion on the left and right sides.

[0095] (Examples of applications for organically coated steel sheets)

[0096] As described above, the organic-coated steel sheet of the present invention is a high-strength steel sheet that can suppress hydrogen embrittlement cracking. Therefore, it can be processed into shaped bodies and applied to structural components in various fields such as transportation machinery (represented by automobiles), industrial machinery, household appliances, and various structures for buildings.

[0097] The organically coated steel sheet of the present invention is preferably used as a structural component in the automotive field. Steel sheets used in automotive structural components are mostly used in atmospheric corrosion environments, where hydrogen embrittlement caused by hydrogen intrusion can be a significant problem. Therefore, when the organically coated steel sheet of the present invention is applied to structural components in the automotive field, the effect of the present invention in suppressing hydrogen embrittlement can be particularly advantageous.

[0098] Furthermore, when applying the organically coated steel sheet of the present invention to structural components in the various fields described above, the forming method is not particularly limited, and any forming method corresponding to the structure and characteristics of the applied structural component can be used. Examples of such forming methods are not particularly limited, but cold pressing and the like can be cited.

[0099] This invention includes a molded body formed by shaping the above-described organic-coated steel sheet into a predetermined shape using any forming method. That is, this invention includes a molded body formed using the organic-coated steel sheet 1 described in the above-described embodiments as another embodiment different from the above-described embodiments.

[0100] [molded body]

[0101] Hereinafter, another embodiment of the present invention will be described, namely, a molded body made using the organic coated steel sheet 1 of the above-described embodiment.

[0102] The molded body of this embodiment can be obtained by molding the organic coated steel plate 1 of the above embodiment, i.e., the steel plate 2 having a tensile strength of 980 MPa or more, and the plating layer 4, the chemical conversion treatment film 5, and the organic coating layer 3 respectively disposed on both sides of the steel plate 2, using any molding method and molding conditions. Alternatively, the molded body of this embodiment can be obtained by forming an organic coating layer identical to the organic coating layer 3 of the organic coated steel plate 1 of the above embodiment on a molded body pre-formed by any molding method and molding conditions.

[0103] It should be noted that, in the latter case, there is no particular limitation on the specific manufacturing method of the shaped body before the formation of the organic coating. For example, commonly used forming methods such as hot stamping and the usual forming conditions corresponding to such forming methods can be used. For example, when the shaped body is manufactured by hot stamping, the shaped body can be manufactured by heating the steel sheet in a specified temperature range and holding time, and then hot stamping it in the specified temperature range.

[0104] The molded body of this embodiment is a high-strength molded body that can suppress hydrogen embrittlement cracking, and therefore can be appropriately used as a structural component in various fields such as transportation machinery represented by automobiles, industrial machinery, home appliances, and various structures for buildings.

[0105] The organic coated steel sheet of the present invention and the molded body made therefrom, as well as the molded body obtained by forming an organic coating layer on the molded body, are not limited to the above-described embodiments and the following examples. Suitable combinations, substitutions, and modifications can be made without departing from the purpose and spirit of the present invention.

[0106] Example

[0107] The present invention will be described in more detail below with reference to embodiments, but the present invention is not limited to such embodiments.

[0108] To verify the effects of the present invention, uncoated steel sheets and galvanized steel sheets with tensile strengths of 1470 MPa to 3000 MPa, as shown in Tables 1 and 2 below, were prepared as test materials. The organic coated steel sheet samples of the embodiments of the present invention were prepared according to the following steps.

[0109] (Pre-processing step)

[0110] The surface of the steel plate was degreased by immersing it in an aqueous solution containing 2.5% by mass of an aqueous alkaline degreaser (manufactured by Nihon Parkerizing Co., Ltd., "FC-301") at a temperature of 40°C for 2 minutes. The degreased steel plate was then washed with water and dried.

[0111] (Chemical conversion treatment process)

[0112] The nitrite content of the dissolved solid components in the zinc phosphate-based chemical conversion treatment solution (manufactured by Nippon Paint Industrial Coatings, "SURFDINE SD5350 series") was 2.3 g / m², based on the amount of zinc phosphate coating adhered. 2 The impregnation time is adjusted in this way. Then, the steel plate after the above pretreatment process is transferred to a hot air furnace and dried at a surface temperature of 70°C, thereby forming a zinc phosphate film (i.e., chemical conversion treatment film) of a specified amount on the surface of the steel plate.

[0113] (Painting process)

[0114] An organic coating is formed on the surface of the steel sheet after the above-mentioned chemical conversion treatment by coating it with a paint composition containing a base paint and a nitrite-containing pigment. It should be noted that the steel sheet of sample No. 1 in Table 3 did not undergo this coating process and therefore did not form an organic coating. The coating of this steel sheet after the chemical conversion treatment contained nitrite, specifically sodium nitrite.

[0115] The base coating used was a polyester-based transparent coating obtained by the following operation. This polyester-based transparent coating was prepared as follows: In a solution obtained by dissolving a commercially available organic solvent-soluble amorphous polyester resin, "VYLON (registered trademark) GK140" manufactured by Toyobo Co., Ltd., in an organic solvent (an organic solvent obtained by mixing Solvesso (registered trademark) 150 and cyclohexanone in a 1:1 mass ratio), 15 parts by mass of commercially available hexamethoxymethyl melamine, namely "CYMEL (registered trademark) 303" manufactured by Mitsui CYTEC Co., Ltd., as a crosslinking agent, were added relative to 100 parts by mass of the solid component of the polyester resin; further, 0.5 parts by mass of a commercially available acidic catalyst, namely "Catalyst (registered trademark) 6003B" manufactured by Mitsui CYTEC Co., Ltd., were added.

[0116] Regarding pigments containing nitrites, adjustments were made based on the nitrite content in the organic coating, expressed as nitrite ions, as shown in Tables 3-8 as the ion conversion content (mass %) of the salts contained in the organic coating. Specifically, commercially available pigments were prepared by adding and stirring a solution of dissolved solid nitrites. The types of nitrites used are listed in Tables 3-8 below as "Salts Contained in the Organic Coating." It should be noted that, for the steel plate of Sample No. 2 in Table 3, no salts such as nitrites were contained in the organic coating. This steel plate contained nitrites, i.e., sodium nitrite, in the chemical conversion treated coating. For the steel plates of Samples No. 49 and 50 in Table 4, sodium chloride and sodium nitrate were used in place of nitrites in the organic coating. These steel plates also contained sodium chloride and sodium nitrate in the chemical conversion treated coating.

[0117] Then, the coating composition is prepared by mixing the aforementioned nitrite-containing pigment with the base coating at a mass ratio of 7:3, and then thoroughly dispersing the mixture using a mixer. It should be noted that the mass ratio refers to the mass ratio of solid components after removing the solvent.

[0118] The coating composition prepared in this manner is applied to the surface of the steel sheet after the aforementioned chemical conversion treatment process using a rod coating method. The steel sheet is then transferred to a hot air furnace and dried at a surface temperature of 230°C to form an organic coating layer. It should be noted that in this coating process, the dilution conditions and the rod number are adjusted according to the desired thickness of the formed organic coating layer, which is 1 μm, 3 μm, 5 μm, 10 μm, 15 μm, 20 μm, and 30 μm.

[0119] For each sample of organically coated steel sheet obtained after the above pretreatment, chemical conversion, and coating processes, the organic coating layer and hydrogen embrittlement cracking were analyzed and evaluated as follows. The results are shown in Tables 3 to 8 below.

[0120] (Analysis of nitrite in organic coatings)

[0121] First, the organic coating layer was collected from the organically coated steel sheet sample to be analyzed, homogenized using a POLYTRON homogenizer, and extracted under weakly alkaline conditions. Then, after removing resin components from the extract by suction filtration, the nitrite content in the organic coating layer (converted ion content of the organic coating layer containing salts, g / m³) was determined using a colorimetric method based on diazotization-induced color development. 2It should be noted that when collecting the organic coating from the organic-coated steel sheet sample being analyzed, the organic coating should be collected from the portion of the organic-coated steel sheet excluding the ends, welded parts, processed parts and parts affected by them, and areas with poor organic coating formation. It should also be noted that, from the viewpoint of improving detection sensitivity, the more samples collected, the better.

[0122] (Evaluation of hydrogen embrittlement)

[0123] The steel plate to be evaluated was cut to obtain a flat plate with dimensions of 100mm × 15mm. Next, both ends of the flat plate were machined by 2.5mm of grinding along its length to obtain a flat test piece with dimensions of 100mm × 10mm. A corrosion test was performed on this test piece according to the method based on JASO M 609-91 (Corrosion Test Methods for Automotive Materials), thereby promoting hydrogen embrittlement through corrosion. The corroded test piece was then used... Figure 2 The apparatus 6 shown is used to evaluate hydrogen embrittlement cracking of steel plates. A stress equivalent to 1250 MPa of YS is applied, and the plate thickness at which the test piece fractures is measured.

[0124] Here, Figure 2 The device 6 shown includes a retainer 7 for holding the test piece 10 and a bolt 8 as a stress-applying mechanism for applying stress to the test piece 10. Figure 2 In the device 6 shown, the retainer 7 is composed of a structure having a load clamp 7a and a bearing clamp 7b, and is configured to perform a four-point bending test on the test piece 10 by using the two fulcrums of the load clamp 7a and the two fulcrums of the bearing clamp 7b.

[0125] In addition, before applying a stress of 1250 MPa to the aforementioned test piece, it was screwed in... Figure 2 The device 6 shown uses bolts 8 to bend a steel plate, and the screwing depth of bolts 8 is set according to the condition of applying a target stress to the protrusion of the bent steel plate. Specifically, a strain gauge is attached along the tensile direction at the apex of the protrusion of the bent steel plate, and the relationship between the strain at the apex of the protrusion of the steel plate caused by bending due to the screwing in of bolts 8 and the screwing depth of bolts 8 is measured in advance. Then, the strain value of the steel plate under the desired stress is read from the strain-stress curve, and bolts 8 are screwed in in a manner that achieves the desired strain value.

[0126] Then, the evaluation of hydrogen embrittlement cracking of each steel plate is carried out by confirming whether hydrogen embrittlement cracking caused by corrosion testing has occurred. Cracking of steel plates during corrosion includes hydrogen embrittlement cracking and thinning cracking caused by the reduction in strength due to the decrease in plate thickness resulting from corrosion. The distinction between the two is made by using the plate thickness of the test piece before corrosion and... Figure 2 The apparatus 6 shown depicts the plate thickness at which the corroded test piece fractures. The reduction rate (%) of the plate thickness before and after corrosion is calculated, and the reduction rate is evaluated according to the following evaluation criteria. It should be noted that the plate thickness at which the corroded test piece fractures is... Figure 2 The portion of the fixture to which salt water is applied during the corrosion test is measured, specifically the portion exposed from the opening of the bearing fixture 7b. More specifically, the plate thickness is measured within 1 cm to the left and right of the two support points of the load fixture 7a, which is located at the center of the test piece.

[0127] Advantages: The reduction rate of plate thickness is over 30% (not hydrogen embrittlement cracking, but thinning cracking).

[0128] Good: The reduction rate of plate thickness is more than 20% and less than 30% (almost no hydrogen embrittlement cracking occurs).

[0129] Possible: The reduction rate of plate thickness is more than 10% but less than 20% (resulting in less hydrogen embrittlement cracking).

[0130] Not allowed: Thickness reduction rate less than 10% (caused hydrogen embrittlement cracking) [Table 1]

[0131]

[0132] [Table 2]

[0133]

[0134] [Table 3]

[0135]

[0136] [Table 4]

[0137]

[0138] [Table 5]

[0139]

[0140] [Table 6]

[0141]

[0142] [Table 7]

[0143]

[0144] [Table 8]

[0145]

[0146] As shown in Tables 3 to 8, the organically coated steel sheet of the present invention, by containing a specific amount of nitrite in the organic coating layer, can sufficiently suppress hydrogen embrittlement cracking caused by hydrogen during corrosion. In contrast, the steel sheet of the comparative examples, due to the absence of an organic coating layer or the organic coating layer not containing a specific amount of nitrite, cannot suppress hydrogen embrittlement cracking caused by hydrogen during corrosion.

[0147] Industrial availability

[0148] The organic-coated steel sheet of the present invention is a high-strength steel sheet that can suppress hydrogen embrittlement cracking, and therefore can be appropriately used in structural components in various fields such as transportation machinery represented by automobiles, industrial machinery, household appliances, and various structures for buildings.

[0149] Symbol Explanation

[0150] 1 Organic coated steel plate

[0151] 2 steel plates

[0152] 3 Organic coating

[0153] 4 coatings

[0154] 5. Chemical conversion treatment of the film

Claims

1. An organically coated steel sheet, characterized in that, have: Steel plates with a tensile strength of 980 MPa or higher, and An organic coating disposed on at least a portion of the surface of the steel plate. The organic coating layer contains 1-30% by mass of nitrite and 0.05 g / m³. 2 The above levels of nitrite.

2. The organically coated steel sheet according to claim 1, characterized in that, The nitrite is selected from at least one of lithium salts, sodium salts, potassium salts, magnesium salts, calcium salts, strontium salts, barium salts, and ammonium salts.

3. The organically coated steel sheet according to claim 1 or 2, characterized in that, A coating is disposed between the steel plate and the organic coating layer.

4. The organically coated steel sheet according to claim 1 or 2, characterized in that, The tensile strength of the steel plate is 1.2 to 3.0 GPa.

5. The organically coated steel sheet according to claim 3, characterized in that, The tensile strength of the steel plate is 1.2 to 3.0 GPa.

6. A molded body made using the organically coated steel sheet as described in claim 1 or 2.

7. A molded body made using the organically coated steel sheet of claim 3.

8. A molded body made using the organically coated steel sheet of claim 4.

9. A molded body made using the organically coated steel sheet of claim 5.

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