Organic coated steel plate

Abstract

The present invention provides a high-strength steel sheet in which hydrogen embrittlement cracking is less likely to occur.　An organic coated steel sheet (1) according to the present invention has: a steel sheet (2) which has a tensile strength of 980 MPa or more; and an organic coating layer (3) which is disposed on at least a part of the surface of the steel sheet. This organic coated steel sheet (1) is characterized in that the organic coating layer (3) contains a nitrite salt as a nitrite radical in an amount of 1-30% by mass and 0.05 g / m2 or more.

Description

[Technical Field]

[0001] The present invention relates to an organic coated steel sheet. [Background technology]

[0002] In recent years, efforts have been made to increase the strength of steel sheets used in various fields such as automobiles, home appliances, building materials, etc. For example, in the automobile field, the use of high-strength steel sheets has been increasing with the aim of reducing the weight of vehicle bodies to improve fuel efficiency.

[0003] Such high-strength steel sheets, particularly those used for automotive components, are subjected to electrodeposition coating to impart desired properties such as corrosion resistance. It is known that during this electrodeposition coating process, water is electrolyzed by the application of a voltage, generating hydrogen. The hydrogen generated during the electrodeposition coating penetrates into the steel sheet, reaching positions deeper than the surface layer of the steel sheet. As a result, the hydrogen segregates at martensite grain boundaries in the steel structure, embrittling the grain boundaries and potentially causing cracks in the steel sheet. Cracking caused by such hydrogen penetration is called hydrogen embrittlement cracking or delayed fracture. It is known that hydrogen embrittlement cracking is more likely to occur as the strength of the steel sheet increases.

[0004] Various countermeasures have been investigated to address hydrogen embrittlement cracking (delayed fracture) in high-strength steel sheets. For example, Patent Document 1 discloses an Fe-based electroplated high-strength steel sheet with a low content of diffusible hydrogen in the steel, which is obtained by annealing the steel sheet after electroplating, thereby releasing diffusible hydrogen that has entered the steel sheet during electroplating into the furnace. The Fe-based electroplated high-strength steel sheet disclosed in Patent Document 1 is said to have excellent chemical conversion treatability, corrosion resistance after painting, and also excellent delayed fracture resistance. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2022-180344 Summary of the Invention [Problem to be solved by the invention]

[0006] In high-strength plated steel sheets such as those disclosed in Patent Document 1, the formation of a plating layer on the surface of the base steel sheet is believed to be effective in suppressing the penetration of hydrogen, which is a cause of hydrogen embrittlement cracking. However, corrosion is likely to occur on the edge (cut surface) of the steel sheet where no plating layer is formed. Such corrosion on the edge surface can cause not only red rust but also hydrogen generation. If hydrogen generated as corrosion progresses penetrates into the steel sheet from the edge, there is a risk of the aforementioned hydrogen embrittlement cracking occurring in the high-strength steel sheet.

[0007] Therefore, an object of the present invention is to provide a high-strength steel sheet that can suppress hydrogen embrittlement cracking. [Means for solving the problem]

[0008] The present invention includes the following aspects.

[0009] (Aspect 1) A steel plate having a tensile strength of 980 MPa or more, an organic coating layer disposed on at least a portion of the surface of the steel sheet; The organic coating layer contains nitrite in an amount of 1 to 30% by mass and 0.05 g / m 2 An organic coated steel sheet characterized by containing the above content.

[0010] (Aspect 2) The organic coated steel sheet according to the first aspect, wherein the nitrite is at least one selected from the group consisting of lithium salts, sodium salts, potassium salts, magnesium salts, calcium salts, strontium salts, barium salts, and ammonium salts.

[0011] (Aspect 3) 3. The organic-coated steel sheet according to claim 1, further comprising a plating layer disposed between the steel sheet and the organic coating layer.

[0012] (Aspect 4) The organic coated steel sheet according to any one of the above aspects 1 to 3, wherein the steel sheet has a tensile strength of 1.2 to 3.0 GPa.

[0013] (Aspect 5) A molded article obtained by using the organic coated steel sheet according to any one of the above first to fourth aspects. [Effects of the Invention]

[0014] According to the organic coated steel sheet of the present invention, it is possible to provide a high strength steel sheet that can suppress hydrogen embrittlement cracking. [Brief explanation of the drawings]

[0015] [Figure 1] FIG. 1 is a schematic diagram of an organic coated steel sheet 1 according to one embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing an apparatus 6 for evaluating hydrogen embrittlement cracking of steel sheets. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, preferred embodiments of the organic coated steel sheet of the present invention will be described in detail. In this specification, various numerical ranges mean ranges including the upper and lower limit values ​​unless otherwise specified.

[0017] In order to achieve the above object, the present inventors have conducted extensive research focusing on the prevention of hydrogen penetration into steel sheets, which is the cause of hydrogen embrittlement cracking. As a result, the present inventors have discovered a technology that can reduce hydrogen penetration into steel sheets during corrosion by formulating a paint with a specific composition. Specifically, the present inventors have discovered that by adding a specific amount of nitrite to the paint composition, the nitrite ions (NO2 - ) is reduced preferentially over hydrogen, which can suppress the generation of hydrogen that causes hydrogen embrittlement cracking.

[0018] The present invention was completed based on these findings, and includes each embodiment of the organic coated steel sheet described above.

[0019] First, a preferred embodiment of the organic coated steel sheet of the present invention will be described in detail.

[0020] [Organic coated steel sheet] As shown in FIG. 1 , an organic coated steel sheet 1 according to one embodiment of the present invention basically comprises a steel sheet 2 having a pair of surfaces facing each other in the thickness direction and a tensile strength of 980 MPa or more, and an organic coating layer 3 disposed on each of both surfaces of the steel sheet 2.

[0021] In this embodiment, as shown in FIG. 1 , the organic coated steel sheet 1 further includes a plating layer 4 disposed between the steel sheet 2 and the organic coating layer 3, and a chemical conversion coating 5 disposed between the plating layer 4 and the organic coating layer 3.

[0022] In this specification, the term "disposed on at least a part of the surface of the steel sheet" is used to mean not only an embodiment in which a substance is directly disposed on at least a part of the surface of the steel sheet, but also an embodiment in which a substance is indirectly disposed on at least a part of the surface of the steel sheet via another layer (e.g., a plating layer) or coating (e.g., a chemical conversion coating). When the steel sheet has such another layer or coating on its surface, the surface of the steel sheet refers to the interface between the steel sheet and the other layer or coating.

[0023] In the organic-coated steel sheet 1 shown in FIG. 1 , the organic coating layer 3 is disposed on each of both surfaces of the steel sheet 2 via a plating layer 4 and a chemical conversion coating 5, but this is not a limitation. That is, the organic coating layer 3 may be disposed on each of both surfaces of the steel sheet 2 with or without a plating layer 4 and a chemical conversion coating 5 therebetween, or may be disposed on only one surface of the steel sheet 2 with or without a plating layer 4 and a chemical conversion coating 5 therebetween. In the organic-coated steel sheet 1 shown in FIG. 1 , the organic coating layer 3 is disposed over the entire surface of the steel sheet 2, but this is not a limitation. That is, the organic coating layer 3 may be disposed over the entire surface of the steel sheet 2 as shown in FIG. 1 , or may be disposed over only a portion of the surface of the steel sheet 2.

[0024] In the organic coating steel sheet 1 of this embodiment, the organic coating layer 3 contains nitrite in an amount of 1 to 30 mass % and 0.05 g / m 2 In this way, the organic coating layer 3 contains a specific amount of nitrite, and therefore the nitrite ions (NO2 - ) is reduced preferentially over hydrogen, which makes it difficult for hydrogen to be generated, which causes hydrogen embrittlement cracking. As a result, the organic coated steel sheet 1 of this embodiment is a high-strength steel sheet that can suppress hydrogen embrittlement cracking.

[0025] It is not an essential constituent requirement for the organically coated steel sheet 1 to have layers or coatings (i.e., the plating layer 4 and the chemical conversion coating 5) other than the steel sheet 2 and the organic coating layer 3. Therefore, the organically coated steel sheet 1 may or may not have any layers or coatings other than the steel sheet 2 and the organic coating layer 3, depending on the required properties of the final product to which it is applied.

[0026] The various components of the organic coated steel sheet 1 will be described in more detail below.

[0027] (steel plate) In the organic coated steel sheet 1, the steel sheet 2 serving as the base material is a steel sheet having a tensile strength (TS) of 980 MPa or more. It is generally known that the susceptibility of steel sheets to hydrogen embrittlement increases as the strength increases. However, in the organic coated steel sheet 1 of this embodiment, even if the steel sheet 2 is a high-strength steel sheet having a tensile strength of 980 MPa or more, the specific amount of nitrite contained in the organic coating layer 3 as described above can make it difficult to generate hydrogen, which causes hydrogen embrittlement cracking. Therefore, the organic coated steel sheet 1 can be suitably used as a material for various structural parts in fields where such high strength is required.

[0028] The tensile strength of the steel plate 2 is not particularly limited as long as it is 980 MPa or more, but is preferably 1200 MPa (i.e., 1.2 GPa) or more, more preferably 1300 MPa (i.e., 1.3 GPa) or more, and even more preferably 1400 MPa (i.e., 1.4 GPa) or more. The upper limit of the tensile strength of the steel plate 2 is not particularly limited, but from the viewpoint of toughness and formability, it is, for example, 4000 MPa (i.e., 4.0 GPa) or less, and preferably 3000 MPa (i.e., 3.0 GPa) or less.

[0029] In particular, the tensile strength of the steel sheet 2 is preferably 1.2 to 3.0 GPa. It is known that when the tensile strength of a steel sheet is 1.2 GPa or more, a trace amount of hydrogen on the order of a few ppm by mass may lead to fracture. Therefore, particularly in the automotive field, there is a demand for high-strength steel sheets while also sufficiently reducing the risk of hydrogen embrittlement cracking. Even when the tensile strength of the steel sheet 2 is 1.2 GPa or more, the organic coated steel sheet 1 of this embodiment can be made less susceptible to the generation of hydrogen, which causes hydrogen embrittlement cracking, and is therefore suitable for use as a material for various structural parts in the automotive field. Furthermore, when the tensile strength of the steel sheet 2 is 3.0 GPa or less, the organic coated steel sheet 1 can be made to have excellent toughness and formability.

[0030] The tensile strength (TS) of a steel plate can be measured by cutting a JIS Z 2241:2011 No. 5 tensile test specimen from the steel plate and conducting a tensile test in accordance with JIS Z 2241:2011. It is preferable to cut the test specimen from the steel plate so that the longitudinal direction of the test specimen is perpendicular to the rolling direction of the steel plate. For example, if it is difficult to obtain a JIS Z 2241:2011 test specimen due to dimensional constraints, other test specimens specified in JIS Z 2241:2011 can be used. However, to ensure appropriate evaluation, the steel plate thickness must be set to a minimum of 0.5 mm. Furthermore, if it is difficult to obtain a JIS Z 2241:2011 test specimen due to dimensional constraints and it is also difficult to use other test specimens specified in JIS Z 2241:2011, a micro-Vickers test can be conducted in accordance with JIS Z 2244-1:2020, and the hardness (HV) converted to tensile strength can be used. A sample for the micro-Vickers test can be prepared as follows. First, a sample is cut from a location at least 50 mm away from the end face of the steel plate, excluding the processed area, so that the thickness cross section perpendicular to the plate surface can be observed. If a sample cannot be taken from this location, a sample is cut from a location avoiding the end. The thickness cross section is preferably parallel to the rolling direction. The size of the sample depends on the measuring device, but it should be large enough to observe approximately 10 mm in a direction perpendicular to the thickness direction. The cross section of the sample cut as described above is polished using #600 to #1500 silicon carbide paper. The polished cross section is then mirror-finished using a diluted solution such as alcohol or a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in pure water. The cross section is then electropolished to obtain an observation surface. The micro-Vickers test is performed at 30 points at 1 / 4 of the plate thickness with a load of 500 gf, and the average value is used. The Vickers hardness obtained by the micro-Vickers test can be converted to tensile strength using the following formula. Tensile strength [MPa] = 3.12 x Vickers hardness [HV] + 16

[0031] The steel plate 2 is not particularly limited except for its tensile strength, and any steel plate can be used depending on the desired mechanical strength, etc. For example, the thickness of the steel plate 2 is not particularly limited, but can be about 0.2 mm to 10.0 mm.

[0032] (Organic coating layer) In the organic coated steel sheet 1, the organic coating layer 3 contains nitrite in an amount of 1 to 30 mass % and 0.05 g / m 2 The organic coating layer 3 contains such a specific amount of nitrite, and therefore the nitrite ions (NO2 - ) is reduced preferentially over hydrogen, which makes it difficult for hydrogen to be generated, which causes hydrogen embrittlement cracking.

[0033] The nitrite that can be used in the organic-coated steel sheet 1 of this embodiment is not particularly limited, and examples thereof include alkali metal salts, alkaline earth metal salts, and ammonium salts of nitrite. Specific examples include lithium salts, sodium salts, potassium salts, magnesium salts, calcium salts, strontium salts, barium salts, and ammonium salts of nitrite. Among these, in order to obtain the effects of the nitrite more effectively, it is preferable that the nitrite is at least one selected from the group consisting of lithium salts, sodium salts, potassium salts, magnesium salts, calcium salts, strontium salts, barium salts, and ammonium salts of nitrite.

[0034] The nitrite in the organic coating layer can be identified by a known method. For example, it can be identified using an element distribution image obtained by SEM-EDS. Specifically, first, the cross section of the organic coated steel sheet along the sheet thickness direction is mechanically polished to a flat surface. Next, the surface of the organic coated steel sheet is chemically polished by colloidal polishing until it becomes a mirror-like surface. The cross section of the polished organically coated steel sheet is observed using an SEM. Specifically, an element distribution image is taken using an SEM-EDS at 5000x magnification (a region of 200 μm length and 200 μm width). If elements related to nitrite cations are detected in this element distribution image at a gram equivalent of 90% or more of the separately measured gram equivalent of the nitrite ion (100%), the organic coating layer is deemed to contain nitrite. If multiple elements related to nitrite cations are detected, the total gram equivalent is used to determine the presence of nitrite. Electron probe microanalysis (EPMA) or glow discharge optical emission spectroscopy (GDS) can be used to analyze and quantify the elements. The gram equivalent (eq) of the nitrite radical in the organic coating layer can be determined from the content measured by a colorimetric method utilizing color development due to diazotization, which will be described later.

[0035] The content of nitrite in the organic coating layer 3 is 1 to 30 mass % and 0.05 g / m 2 The content is not particularly limited as long as it is equal to or greater than this. The content of the nitrite salt as nitrite radical is preferably as high as possible in that a higher effect is more likely to be obtained. Specifically, the content is preferably 5 mass% or more relative to the total mass of the organic coating layer (i.e., the mass of the total solid content of the organic coating layer). The content of the nitrite salt may be 10 mass% or more, 15 mass% or more, 20 mass% or more, or 25 mass% or more. However, the upper limit of the content of the nitrite salt is set to 30 mass% because, in terms of the solubility product, a range exceeding 30 mass% makes it impossible to stably produce organic-coated steel sheets.

[0036] In addition, the content of nitrite as nitrite ions per surface area of ​​the steel sheet is set to 0.50 g / m, as this tends to provide a higher effect. 2 It is preferable that the content is 1.00 g / m or more. 2 More preferably, it is 5.00 g / m or more. 2 The upper limit of the content of nitrite per surface area of ​​the steel sheet is not particularly limited, but is, for example, 10.00 g / m 2 is.

[0037] The organic coating layer 3 contains a resin as a base coating. It may also contain components other than nitrites, provided that the effects of the present invention are not impaired. Examples of such components include additives such as color pigments, anti-rust pigments, dispersants, leveling agents, and lubricants, as well as dilution solvents.

[0038] The resin that can be contained in the organic coating layer is not particularly limited, but is preferably one that functions as a binder that binds each component contained in the organic coating layer, and examples thereof include solvent-based resins that dissolve or disperse in organic solvents, and water-soluble or water-dispersible aqueous resins.

[0039] The solvent-based resin is not particularly limited, but examples thereof include polyester resin, urethane resin, epoxy resin, acrylic resin, and a mixed resin of two or more of these resins. The resin may be a crosslinked resin having a crosslinked structure, or a non-crosslinked resin having no crosslinked structure. When a crosslinked structure is imparted to the resin, a water-soluble crosslinking agent (curing agent) is preferably used for crosslinking. Specific examples of the crosslinking agent include melamine and isocyanate.

[0040] On the other hand, the aqueous resin is not particularly limited, but examples thereof include water-soluble or water-dispersible polyester resins, urethane resins, acrylic resins, epoxy resins, phenolic resins, and mixed resins of two or more of these resins.

[0041] The content of the resin that can be contained in the organic coating layer is not particularly limited, but is, for example, 10 to 90 mass% relative to the total mass of the organic coating layer (i.e., the mass of the total solid content of the organic coating layer).The content of the resin is preferably 20 to 80 mass%, more preferably 25 to 75 mass%, relative to the total mass of the organic coating layer, from the viewpoint of exerting the function as a binder and achieving both adhesion to adjacent layers, coatings, or steel sheets and corrosion resistance.

[0042] The color pigment that can be contained in the organic coating layer is not particularly limited, and any color pigment can be used depending on the desired appearance, design, etc. Examples of such color pigments include aluminum pigments, carbon black, silica, titania, zirconia, etc. The content of the color pigment is not particularly limited, but may be, for example, about 1 to 60 mass % relative to the total mass of the organic coating layer.

[0043] The rust-preventive pigment that can be contained in the organic coating layer is not particularly limited, but examples thereof include aluminum tripolyphosphate; Zn salt, Mg salt, Al salt, Ti salt, Zr salt, or Ce salt of phosphate or phosphite; hydrocalumite-treated phosphate compound; Ca ion-exchanged silica; amorphous silica, etc. The content of the rust-preventive pigment is not particularly limited, but examples thereof include a content of about 1 to 40 mass% relative to the total mass of the organic coating layer.

[0044] The lubricant that can be contained in the organic coating layer is not particularly limited, but examples thereof include polyolefin wax, paraffin wax, fluororesin wax, etc. The content of the lubricant is not particularly limited, but examples thereof include a content of about 0.1 to 10 mass % relative to the total mass of the organic coating layer.

[0045] The amount of the organic coating layer (i.e., the mass of the total solid content of the organic coating layer per unit area) is not particularly limited, but may be, for example, 2 to 20 g / m 2 When the coating weight of the organic coating layer is within this range, the adhesion between the organic coating layer and the adjacent layer, coating, or steel sheet is improved, and the above-mentioned effects of the nitrite can be more reliably obtained. The coating weight of the organic coating layer is preferably 2 to 15 g / m 2 is.

[0046] The thickness of the organic coating layer is not particularly limited, but may be, for example, about 1 to 50 μm. From the viewpoints of adhesion, corrosion resistance, and the effects of the 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. The thickness of the organic coating layer is preferably 30 μm or less.

[0047] As described above, the organic coating layer may be disposed on each of both surfaces of the steel sheet with or without a plating layer and a chemical conversion coating therebetween, or may be disposed on only one surface of the steel sheet with or without a plating layer and a chemical conversion coating therebetween. Furthermore, as described above, the organic coating layer may be disposed over the entire surface of the steel sheet, or may be disposed on only a portion of the surface of the steel sheet.

[0048] The method for forming the organic coating layer is not particularly limited, but after preparing a coating composition containing at least the specific amount of nitrite as described above, the organic coating layer can be formed by applying this coating composition to the surface of the steel sheet or the plating layer or chemical conversion coating formed on the surface of the steel sheet, and then heating and drying. The means for applying the coating composition is not particularly limited, but any coating process such as electrodeposition coating, powder coating, solvent coating, etc. can be used.

[0049] The organic coating layer may have a single-layer structure consisting of a single layer, or may have a multi-layer structure consisting of multiple layers. When the organic coating layer has a multi-layer structure, an example of the multi-layer structure includes a multi-layer structure consisting of an undercoat layer, an intermediate coat layer, and a top coat layer.

[0050] (plating layer) In the organically coated steel sheet 1 shown in FIG. 1 , a plating layer 4 is formed on both sides of a steel sheet 2 and is disposed between the steel sheet 2 and an organic coating layer 3. More specifically, in the organically coated steel sheet 1, a chemical conversion coating 5 is disposed between the steel sheet 2 and the organic coating layer 3, and the plating layer 4 is disposed between the chemical conversion coating 5 and the steel sheet 2. Note that the presence of the chemical conversion coating 5 is not an essential constituent feature of the organically coated steel sheet 1, and therefore the organic coating layer 3 may be disposed directly on the surface of the plating layer 4.

[0051] Generally, the formation of a plating layer such as zinc plating is prone to hydrogen generation, resulting in a higher risk of hydrogen embrittlement cracking. However, even though the organic coated steel sheet 1 of the present embodiment has the plating layer 4, the specific amount of nitrite contained in the organic coating layer 3 as described above makes it difficult to generate hydrogen, which causes hydrogen embrittlement cracking. Therefore, the organic coated steel sheet 1 can suppress hydrogen embrittlement cracking while enjoying the advantages of having the plating layer 4 (e.g., improved corrosion resistance, etc.).

[0052] The plating layer 4 may be either a hot-dip plating layer or an electroplated layer. Examples of hot-dip plating layers include a hot-dip galvanized layer (GI), a galvannealed layer (GA), a hot-dip aluminum plating layer, a hot-dip Zn-Al alloy plating layer, a hot-dip Zn-Al-Mg alloy plating layer, and a hot-dip Zn-Al-Mg-Si alloy plating layer. Examples of electroplated layers include an electrogalvanized layer (EG) and an electrogalvanized Zn-Ni alloy plating layer. Of these, the plating layer is preferably a hot-dip galvanized layer, a galvannealed layer, or an electrogalvanized layer.

[0053] The coating weight of the plating layer is not particularly limited, but is, for example, 10 to 180 g / m per side. 2 The coating weight of the plating layer is determined by dissolving the plating layer in an acid solution to which an inhibitor that suppresses corrosion of the base steel has been added, and then measuring the change in weight before and after pickling.

[0054] The thickness of the plating layer is not particularly limited, but may be, for example, about 3 to 50 μm.

[0055] In the organic coated steel sheet 1 shown in FIG. 1 , the plating layer 4 is disposed on each of both surfaces of the steel sheet 2, but this is not a limitation. That is, the plating layer 4 may be disposed on each of both surfaces of the steel sheet 2 as shown in FIG. 1 , or may be disposed on only one surface of the steel sheet 2. In the organic coated steel sheet 1 shown in FIG. 1 , the plating layer 4 is disposed over the entire surface of the steel sheet 2, but this is not a limitation. That is, the plating layer 4 may be disposed over the entire surface of the steel sheet 2 as shown in FIG. 1 , or may be disposed on only a portion of the surface of the steel sheet 2.

[0056] It should be noted that the presence of a plating layer is not an essential constituent requirement for the organic coated steel sheet of the present invention, and therefore the organic coated steel sheet of the present invention may or may not have a plating layer depending on the required properties of the final product to which it is applied.

[0057] (chemical conversion coating) In the organically coated steel sheet 1 shown in Figure 1, the chemical conversion coating 5 is formed on the surface of the above-mentioned plating layer 4 and is disposed between the plating layer 4 and the organic coating layer 3. It should be noted that the presence of the plating layer 4 is not an essential constituent feature of the organically coated steel sheet 1, and therefore the above-mentioned chemical conversion coating 5 may be disposed directly on the surface of the steel sheet 2.

[0058] The chemical conversion coating 5 functions to improve the barrier properties and adhesion against corrosive factors such as moisture and corrosive ions, and as a result, the corrosion resistance of the organic coated steel sheet 1 can be improved.

[0059] The components forming the chemical conversion coating 5 are not particularly limited, but examples include phosphoric acid, phosphates, silica, fluorides, vanadium compounds, silane coupling agents, zirconium compounds, resins, tannin, and tannic acid. Among these components, at least one component selected from the group consisting of phosphoric acid, phosphates, silica, fluorides, and vanadium compounds is preferred. When the chemical conversion coating is formed from such components, the chemical conversion coating functions as an inhibitor, forming a precipitated film or a passive film on the plating layer or the surface of the steel sheet, thereby further improving corrosion resistance.

[0060] Examples of phosphates that can be used to form the chemical conversion coating include crystalline and amorphous phosphates. More specifically, examples of crystalline phosphates include zinc phosphate, zinc iron phosphate, manganese phosphate, manganese iron phosphate, and zinc calcium phosphate. Examples of amorphous phosphates include iron phosphate, tin phosphate, zirconium phosphate, titanium phosphate, and hafnium phosphate.

[0061] Examples of fluorides that can be used to form the chemical conversion coating include zirconium fluoride, titanium fluoride, hafnium fluoride, indium fluoride, etc. Examples of vanadium compounds that can be used to form the chemical conversion coating include vanadium oxide, etc.

[0062] The chemical conversion coating may contain the same nitrite as the organic coating layer. In this case, the amount of nitrite contained in the chemical conversion coating is not particularly limited, but may be, for example, about 1 to 30 mass% based on the total mass of the chemical conversion coating (i.e., the mass of the total solid content of the chemical conversion coating). If the chemical conversion coating also contains such a nitrite, it can further reduce the generation of hydrogen, which causes hydrogen embrittlement cracking.

[0063] The amount of the chemical conversion coating (i.e., the mass per unit area of ​​the total solid content of the chemical conversion coating) depends on the components that form it, and is not particularly limited, but for example, it is 10 to 2000 mg / m per side. 2 The deposition amount of the chemical conversion coating is preferably 100 to 1500 mg / m 2 is.

[0064] The deposition weight of the chemical conversion coating can be measured by known analytical methods such as fluorescent X-ray analysis. For example, a calibration curve showing the relationship between fluorescent X-ray intensity and deposition weight can be prepared in advance using a sample whose phosphorus deposition amount is known by chemical analysis, and the deposition weight of the chemical conversion coating can be determined from the measurement results of the fluorescent X-ray intensity using this calibration curve.

[0065] The thickness of the chemical conversion coating also depends on the components that form it, and is not particularly limited, but may be, for example, about 0.01 to 5.00 μm, and preferably 0.03 to 3.00 μm.

[0066] In the organic-coated steel sheet 1 shown in FIG. 1 , the chemical conversion coating 5 is disposed on each of both surfaces of the steel sheet 2 via a plating layer 4, but this is not a limitation. That is, the chemical conversion coating 5 may be disposed on each of both surfaces of the steel sheet 2 with or without a plating layer 4 interposed therebetween, as in FIG. 1 , or may be disposed on only one surface of the steel sheet 2 with or without a plating layer 4 interposed therebetween. Also, in FIG. 1 , the chemical conversion coating 5 is disposed over the entire surface of the steel sheet 2, but this is not a limitation. That is, the chemical conversion coating 5 may be disposed over the entire surface of the steel sheet 2, as in FIG. 1 , or may be disposed on only a portion of the surface of the steel sheet 2.

[0067] The method for forming the chemical conversion coating is not particularly limited, but for example, the chemical conversion coating can be formed as follows. First, the surface of the steel sheet or the surface of the plating layer on which the chemical conversion coating is to be formed is subjected to known degreasing and cleaning processes to remove impurities such as oil and surface oxides that have adhered. Next, a composition for forming a chemical conversion coating containing the various components described above is applied to this surface by coating or immersion. The composition adhered to the surface is then dried under any drying conditions to form the chemical conversion coating.

[0068] It should be noted that the presence of a chemical conversion coating is not an essential constituent requirement for the organic coated steel sheet of the present invention, and therefore the organic coated steel sheet of the present invention may or may not have a chemical conversion coating depending on the required properties of the final product to which it is applied.

[0069] (Other coatings) While various components of the organically coated steel sheet 1 of this embodiment have been described above with reference to Fig. 1, the organically coated steel sheet of the present invention is not limited to the configuration shown in Fig. 1. For example, the organically coated steel sheet may have a functional coating, such as a primer coating, interposed between the steel sheet and the organic coating layer to further improve adhesion and corrosion resistance.

[0070] Examples of the primer coating include a chromate coating and a primer coating that is substantially free of chromium (i.e., a chromate-free coating). The chromate-free coating can be formed using a treatment liquid such as a silica-based treatment liquid containing a silicon compound such as liquid-phase silica, vapor-phase silica, or silicate as the main coating component, or a zircon-based treatment liquid containing a zircon-based compound as the main coating component. These treatment liquids may contain an organic resin together with the main coating component.

[0071] The deposition amount of the base treatment coating (i.e., the mass per unit area of ​​the total solid content of the base treatment coating) can be any amount depending on the treatment liquid used. For example, in the case of a base treatment coating made from a silica-based treatment liquid, the deposition amount is 1 to 20 mg / m2 in terms of Si. 2 The amount of adhesion can be adjusted to about 100%.

[0072] (Example of application of organic coated steel sheets) As described above, 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 processed into formed articles, etc., and applied to structural members in various fields, such as various structural parts for transportation machinery including automobiles, industrial machinery, home appliances, etc., and various architectural structures.

[0073] In particular, the organic-coated steel sheet of the present invention is preferably used as a structural member in the automotive field. Steel sheets used in structural members in the automotive field are often used in atmospheric corrosive environments, and hydrogen embrittlement cracking caused by the penetration of hydrogen generated in such environments can be a major problem. Therefore, when the organic-coated steel sheet of the present invention is applied to structural members in the automotive field, the effect of the present invention, that is, the ability to suppress hydrogen embrittlement cracking, can be particularly suitably exerted.

[0074] Furthermore, when the organic coated steel sheet of the present invention is applied to structural members in the various fields as described above, the forming method is not particularly limited, and any forming method can be adopted depending on the structure, properties, etc. of the structural member to be applied. Examples of such forming methods include, but are not limited to, cold pressing.

[0075] The present invention includes a formed body obtained by forming the above-mentioned organic coated steel sheet into a predetermined shape by any such forming method. That is, the present invention includes a formed body obtained by using the organic coated steel sheet 1 of the above-mentioned embodiment as an embodiment different from the above-mentioned embodiment.

[0076] [Molded body] Hereinafter, a molded article formed using the organic coated steel sheet 1 of the above-described embodiment, which is another embodiment of the present invention, will be described.

[0077] The formed body of this embodiment can be obtained by forming, by any forming method and under any forming conditions, the organically coated steel sheet 1 of the above-described embodiment, i.e., the organically coated steel sheet 1 including a steel sheet 2 having a tensile strength of 980 MPa or more and a plating layer 4, a chemical conversion coating 5, and an organic coating layer 3 disposed on each side of the steel sheet 2. Alternatively, the formed body of this embodiment can be obtained by forming an organic coating layer similar to the organic coating layer 3 of the organically coated steel sheet 1 of the above-described embodiment on a formed body that has been formed in advance by any forming method and under any forming conditions.

[0078] In the latter case, the specific manufacturing method of the molded body before forming the organic coating layer is not particularly limited, and for example, a commonly used molding method such as hot stamping and normal molding conditions corresponding to the molding method can be adopted. For example, when manufacturing the molded body by hot stamping, the molded body can be manufactured by heating the steel sheet in a predetermined temperature range and for a holding time, and then hot stamping in the predetermined temperature range.

[0079] The molded body of this embodiment is also a high-strength molded body that can suppress hydrogen embrittlement cracking, and can therefore be suitably used as structural members in a variety of fields, such as various structural parts for transportation machinery including automobiles, industrial machinery, home appliances, etc., as well as various architectural structures.

[0080] The organic coated steel sheet of the present invention and the molded body made using the same, as well as the molded body obtained by forming an organic coating layer on the molded body, are not limited to the above-mentioned embodiments or the examples described below, and can be appropriately combined, substituted, modified, etc. within the scope that does not deviate from the purpose and intent of the present invention. [Example]

[0081] The present invention will be explained in more detail below by way of examples, but the present invention is not limited to these examples.

[0082] In order to verify the effects of the present invention, unplated steel sheets and zinc-plated steel sheets having tensile strengths of 1470 MPa to 3000 MPa and steel compositions shown in Tables 1 and 2 below were prepared as test materials, and organic coated steel sheet samples serving as examples of the present invention were produced according to the following procedure.

[0083] (Pretreatment process) The steel sheet surface was degreased by immersing the steel sheet as a test material in an aqueous solution containing 2.5 mass% of an aqueous alkaline degreasing agent ("FC-301" manufactured by Nippon Parkerizing Co., Ltd.) at a temperature of 40°C for 2 minutes. After degreasing, the steel sheet was then rinsed with water and dried.

[0084] (chemical conversion treatment process) Solid nitrite was dissolved in a zinc phosphate conversion treatment solution (Nippon Paint Industrial Coating Co., Ltd., "Surfdyne SD5350 series") to form a zinc phosphate coating with a coating weight of 2.3 g / m. 2 The immersion treatment time was adjusted so that the steel sheet after the above pretreatment step was then transferred to a hot air furnace and dried in a state where the surface temperature reached 70°C, thereby forming a zinc phosphate coating (i.e., chemical conversion coating) with a predetermined adhesion weight on the surface of the steel sheet.

[0085] (painting process) After the chemical conversion treatment process, the steel sheet was coated with a paint composition containing the following base paint and a nitrite-containing pigment to form an organic coating layer on the surface of the steel sheet. The steel sheet of sample No. 1 in Table 3 was not subjected to this painting process, and therefore does not have an organic coating layer. The chemical conversion treatment coating of this steel sheet contains sodium nitrite, which is a nitrite.

[0086] The base paint used was a polyester-based clear paint obtained as follows: This polyester-based clear paint was prepared 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 mass ratio of 1:1), adding 15 parts by mass of "Cymel (registered trademark) 303" manufactured by Mitsui Cytec Co., Ltd., a commercially available hexamethoxymethylmelamine crosslinker, per 100 parts by mass of the polyester resin solids, and further adding 0.5 parts by mass of "Catalyst (registered trademark) 6003B" manufactured by Mitsui Cytec Co., Ltd., a commercially available acidic catalyst.

[0087] The nitrite-containing pigment was adjusted so that the content of nitrite in the organic coating layer, in terms of nitrite radicals, matched the ion-equivalent salt content (mass %) of the organic coating layer shown in Tables 3 to 8. Specifically, the pigment was prepared by adding a commercially available pigment to an aqueous solution containing solid nitrite, followed by stirring and mixing. The type of nitrite used is shown in Tables 3 to 8 below as "salt contained in organic coating layer." Note that the steel sheet of sample No. 2 in Table 3 does not contain salts such as nitrite in the organic coating layer. This steel sheet contains sodium nitrite, a nitrite, in the chemical conversion coating. The steel sheets of sample Nos. 49 and 50 in Table 4 contain sodium chloride or sodium nitrate in place of nitrite in the organic coating layer. These steel sheets also contain sodium chloride and sodium nitrate in the chemical conversion coating.

[0088] The coating composition was prepared by mixing the base coating with the nitrite-containing pigment in a mass ratio of 7:3, and thoroughly stirring with a mixer to disperse the mixture. The mass ratio is the mass ratio of the solids excluding the solvent.

[0089] The coating composition thus prepared was applied by bar coating to the surface of the steel sheet after the chemical conversion treatment process. The steel sheet was then transferred to a hot air oven and dried and air-dried at a temperature of 230°C, forming an organic coating layer on the surface of the steel sheet. In this coating process, the dilution conditions and bar size were adjusted so that the thicknesses of the formed organic coating layers were 1 μm, 3 μm, 5 μm, 10 μm, 15 μm, 20 μm, and 30 μm, respectively.

[0090] For each sample of organic coated steel sheet obtained through the above pretreatment, chemical conversion treatment and painting processes, the organic coating layer was analyzed and hydrogen embrittlement cracking was evaluated as follows. The results are shown in Tables 3 to 8 below.

[0091] (Analysis of nitrite radicals in organic coating layers) First, the organic coating layer was collected from a sample of the organic coated steel sheet to be analyzed, homogenized using a Polytron homogenizer, and extracted under weak alkaline conditions. After removing the resin components from the extract by suction filtration, the content of nitrite radicals in the organic coating layer (the ion-equivalent content of salts contained in the organic coating layer in g / m²) was measured using a colorimetric method that utilizes color development due to diazotization. 2 ) was measured. When the organic coating layer was sampled from the sample of organic-coated steel sheet to be analyzed, the organic coating layer was sampled from the edge, welded part, processed part and part affected by these, and part other than the part where the organic coating layer was poorly formed. From the viewpoint of increasing the detection sensitivity, the larger the sampled amount, the better.

[0092] (Evaluation of hydrogen embrittlement cracking) The steel plate to be evaluated was cut into a 100 mm × 15 mm flat plate. Next, both longitudinal ends of this plate were machine-ground by 2.5 mm to obtain a 100 mm × 10 mm flat test piece. A corrosion test was performed on this test piece in accordance with JASO M 609-91 (Corrosion Test Method for Automotive Materials) to corrode the test piece and promote hydrogen embrittlement cracking. After this corrosion, a stress of 1250 MPa, equivalent to the YS, was applied to the test piece using an apparatus 6 for evaluating hydrogen embrittlement cracking of steel plate shown in Figure 2, and the plate thickness at fracture of the test piece was measured.

[0093] 2 includes a holder 7 for holding a test piece 10 and a bolt 8, which is a stress applying means for applying stress to the test piece 10. In the test piece 6 shown in FIG. 2, the holder 7 is configured as a structure having a loading jig 7a and a receiving jig 7b, and is configured to be able to perform a four-point bending test in which the test piece 10 is bent using two fulcrums of the loading jig 7a and two fulcrums of the receiving jig 7b.

[0094] Furthermore, prior to applying a stress of 1250 MPa to the above-mentioned test piece, the steel plate was bent by tightening bolt 8 of device 6 shown in Figure 2, and the tightening amount of bolt 8 was determined so that the target stress would be applied to the convex portion of the bent steel plate. Specifically, a strain gauge was attached in the tensile direction at the apex of the convex portion of the bent steel plate, and the relationship between the strain at the apex of the convex portion of the steel plate due to bending of the steel plate by tightening bolt 8 and the tightening amount of bolt 8 was measured in advance. Then, the strain value of the steel plate at the stress to be applied to the test piece was read from the strain-stress curve, and bolt 8 was tightened so as to obtain the desired strain value.

[0095] The evaluation of hydrogen embrittlement cracking for each steel plate was performed by confirming the presence or absence of hydrogen embrittlement cracking during corrosion tests. Cracks in steel plates during corrosion can be classified as hydrogen embrittlement cracking or metal thinning cracking, which occurs when corrosion reduces plate thickness and strength. The two types of cracking were distinguished by calculating the percent reduction in plate thickness (%) before and after corrosion using the plate thickness of the test specimen before corrosion and the plate thickness when the test specimen was fractured using the apparatus 6 shown in Figure 2. The percent reduction in plate thickness was evaluated according to the following criteria. The plate thickness when the test specimen was fractured after corrosion was measured at the portion exposed through the opening of the support jig 7b in the jig shown in Figure 2, which is the portion exposed to saltwater during the corrosion test. Specifically, the plate thickness was measured within 1 cm on both sides of the two supports of the loading jig 7a, which is the center of the test specimen. Excellent: The reduction in thickness is 30% or more (thickness reduction cracks have occurred, not hydrogen embrittlement cracks) Good: The reduction in thickness is 20% or more but less than 30% (almost no hydrogen embrittlement cracking has occurred) Acceptable: The reduction in plate thickness is 10% or more but less than 20% (low occurrence of hydrogen embrittlement cracking) Unacceptable: thickness reduction rate is less than 10% (hydrogen embrittlement cracking has occurred)

[0096] [Table 1]

[0097] [Table 2]

[0098] [Table 3]

[0099] [Table 4]

[0100] [Table 5]

[0101] [Table 6]

[0102] [Table 7]

[0103] [Table 8]

[0104] As shown in Tables 3 to 8, the organically coated steel sheets of the examples of the present invention were able to sufficiently suppress hydrogen embrittlement cracking caused by hydrogen during corrosion because the organic coating layer contained a specific amount of nitrite. In contrast, the steel sheets of the comparative examples were unable to suppress hydrogen embrittlement cracking caused by hydrogen during corrosion because they did not have an organic coating layer or because the organic coating layer did not contain a specific amount of nitrite. [Industrial Applicability]

[0105] 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 suitably used for structural members in various fields, such as various structural parts for transportation machinery including automobiles, industrial machinery, home appliances, and various architectural structures. [Explanation of symbols]

[0106] 1 Organic coated steel sheet 2 steel plate 3 Organic coating layer 4 plating layer 5 Chemical conversion coating

Claims

1. A steel plate having a tensile strength of 980 MPa or more; an organic coating layer disposed on at least a portion of the surface of the steel sheet; The organic coating layer contains nitrite in an amount of 15 to 30% by mass and 0.05 g / m ２ An organic coated steel sheet characterized by containing the above content.

2. 2. The organic coated steel sheet according to claim 1, wherein the nitrite is at least one selected from the group consisting of lithium salts, sodium salts, potassium salts, magnesium salts, calcium salts, strontium salts, barium salts, and ammonium salts.

3. The organic coated steel sheet according to claim 1 or 2, wherein a plating layer is disposed between the steel sheet and the organic coating layer.

4. 3. The organic coated steel sheet according to claim 1, wherein the tensile strength of the steel sheet is 1.2 to 3.0 GPa.

5. The organic coated steel sheet according to claim 3, wherein the steel sheet has a tensile strength of 1.2 to 3.0 GPa.

6. A molded article made using the organic coated steel sheet according to claim 1 or 2.

7. A molded article made using the organic coated steel sheet according to claim 3.

8. A molded article made using the organic coated steel sheet according to claim 4.

9. A molded article made using the organic coated steel sheet according to claim 5.

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