Aluminum-plated steel sheet for hot stamping and manufacturing method thereof

Abstract

This aluminum-plated steel sheet for hot stamping has a steel sheet, an aluminum-plated layer located on the surface of the steel sheet, and a coating located on the aluminum-plated layer, in which the aluminum-plated layer has an Al content of 60 mass % or more, a thickness t1 of 10 to 60 μm, an area ratio of amorphous matter in the coating of 90% or more, the coating containing one or more elements selected from the group A elements consisting of Ce, Nb, Mo, W, and V, and a thickness t2 of 0.005 to 5.5 μm.

Description

[Technical Field]

[0001] The present disclosure relates to an aluminum-plated steel sheet for hot stamping and a method for producing the same. This application claims priority based on Japanese Patent Application No. 2024-077589, filed May 10, 2024, the contents of which are incorporated herein by reference. [Background technology]

[0002] In recent years, there has been a demand for reducing the weight of automobile bodies from the perspectives of environmental protection and resource conservation. On the other hand, ensuring collision safety performance is essential, and the application of high-strength steel sheets to automobile components has been promoted. When high-strength steel sheets are used, the thickness of the steel sheets can be reduced to reduce the weight of the vehicle body, while providing the vehicle body with the desired strength.

[0003] Automotive components are manufactured by press-forming steel sheets. However, as the strength of steel sheets increases, not only does the forming load increase, but formability also decreases, making them more susceptible to cracks and wrinkles. Furthermore, when high-strength steel sheets are press-formed, the shape of the component changes significantly due to springback when the component is removed from the mold, making it difficult to ensure dimensional accuracy. Thus, it is not easy to manufacture high-strength vehicle body components by press-forming.

[0004] To solve the above problems, a technique has been proposed in which heated steel sheets are press-formed using a low-temperature press die. This technique, known as hot stamping or hot pressing, allows for the production of complex-shaped components with high dimensional accuracy by press-forming steel sheets that are heated to a high temperature and in a soft state. Furthermore, because the steel sheets are rapidly cooled by contact with the die, quenching can significantly increase the strength of the steel sheets during press forming.

[0005] Meanwhile, steel sheets are often stored in coil form until they are subjected to hot stamping. If the aluminum plating layer on the surface of the steel sheet corrodes during storage, the appearance is marred by white or gray rust, reducing the value of the product. Furthermore, white or gray rust can reduce the adhesion of the paint film after hot stamping. Therefore, removing these rusted areas is effective, but it reduces the yield and is economically undesirable.

[0006] Therefore, in order to prevent corrosion of the aluminum plating layer in the coil state, the entire coil is generally wrapped in volatile rust-preventive paper.

[0007] Patent Document 1 proposes a hot-dip aluminized steel sheet with excellent corrosion resistance that is used for metal building materials such as automobile exhaust system members, gasoline tank materials, roofs and walls, and household heating appliances such as toasters and stoves. According to the technology disclosed in Patent Document 1, extremely excellent corrosion resistance is obtained by adding Mg to the plating layer and causing the presence of an Mg2Si phase in the plating layer.

[0008] Furthermore, Patent Document 2 proposes an aluminum-plated steel sheet with excellent corrosion resistance that can be used for metal building materials such as roofs and walls, household and industrial electrical appliances, automobile exhaust system components, fuel tank materials, and the like. The hot-dip Al-plated steel sheet disclosed in Patent Document 2 has an Al-based plating layer on the surface of steel containing 3% or more of Cr, which contains 0.1 to 10% in total of one or more of Mg, Ca and Li, with the balance being Al and unavoidable impurities, and an Al-Fe-Cr-based alloy layer at the interface between the steel and the Al-based plating layer, with the alloy layer containing 0.1% or more of Cr. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Japanese Patent Publication No. 2001-73108 [Patent Document 2] Japanese Patent Application Publication No. 2004-244655 Summary of the Invention [Problem to be solved by the invention]

[0010] However, even when these conventional techniques are applied to aluminum-plated steel sheets for hot stamping, the aluminum-plated layer on the surface of the steel sheet may corrode during storage in a coiled state, resulting in the formation of white or gray rust. In particular, when the steel sheet is stored in a coiled state for a long period of time, such as six months, it is difficult to ensure sufficient corrosion resistance.

[0011] To address this issue, the use of volatile rust-preventive paper is widely known, as described above. However, while this paper can suppress rust during the initial stages of storage, after a certain period of time, the aluminum plating layer can corrode in harsh environments where moisture and salt are present or where temperature fluctuations are severe and condensation is likely to occur.

[0012] Although there is knowledge about how to improve the corrosion resistance of aluminum plating layers, it has been difficult with conventional techniques to maintain corrosion resistance during long-term storage of coils.

[0013] The present disclosure has been made in view of the above-described circumstances, and an object of the present disclosure is to provide a hot stamp plated steel sheet having excellent long-term corrosion resistance during storage and a method for manufacturing the same. [Means for solving the problem]

[0014] The gist of the present disclosure is as follows. (1) An aluminum-plated steel sheet for hot stamping according to one aspect of the present invention is an aluminum-plated steel sheet for hot stamping comprising a steel sheet, an aluminum-plated layer located on a surface of the steel sheet, and a coating located on the aluminum-plated layer, The aluminum content of the aluminum plating layer is 60 mass% or more, The thickness t1 of the aluminum plating layer is 10 to 60 μm, The amorphous ratio in the coating is 90% or more in terms of area ratio, the coating contains one or more elements selected from the group A elements consisting of Ce, Nb, Mo, W, and V; The thickness t2 of the coating is 0.005 to 5.5 μm. (2) In the aluminum-plated steel sheet for hot stamping according to the above (1), the chemical composition of the steel sheet is, in mass%, C: 0.22~0.60%, Si: 0.01 to 0.60%, Mn: 0.50 to 3.00% P: 0.050% or less, S: 0.020% or less, Al: 0.10% or less, Ti: 0.010 to 0.100%, B: 0.0001 to 0.0100%, N: 0.0200% or less, Cr: 0~1.00%, Mo: 0-1.00%, Cu: 0-1.00% Ni: 0-2.00% Nb: 0 to 1.00%, Sn: 0 to 1.00% Ca: 0-0.10% The balance may be Fe and unavoidable impurities. (3) In the aluminum-plated steel sheet for hot stamping described in (2) above, the chemical components may be, in mass %, N: 0.0100% or less. (4) A method for producing an aluminum-plated steel sheet for hot stamping according to one aspect of the present invention is the method for producing an aluminum-plated steel sheet for hot stamping according to the above (1), a plating step of forming an aluminum plating layer having an Al content of 60 mass% or more on the steel sheet; a film forming step of forming a film on the aluminum plating layer, The film forming step includes: a coating step of coating the aluminum plating layer with a chemical conversion treatment solution having a pH of 5.0 to 12.0 and containing one or more elements selected from the group A elements consisting of Ce, Nb, Mo, W, and V; a baking step in which the chemical conversion treatment solution is baked at an ultimate temperature T of 40 to 150°C after the application step to form a coating; a cooling step of cooling the coating after baking; and The heating time t in the baking step is adjusted to satisfy the following formula (7): In the cooling step, When the reached temperature T is 100°C or more and 150°C or less, the average cooling rate is 40 to 100°C / s, When the temperature T is 40° C. or higher and lower than 100° C., the average cooling rate is set to 40 to 80° C. / s. 1000 ≦t×T≦ 8000 ··· Formula (7) [Effects of the Invention]

[0015] According to the above aspect of the present invention, it is possible to provide a plated steel sheet for hot stamping having excellent long-term corrosion resistance and a method for producing the same. Therefore, the plated steel sheet for hot stamping of the above aspect of the present invention can be suitably applied to automotive parts. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 shows examples of electron beam diffraction images obtained by a transmission electron microscope (TEM), where (a) is a crystalline structure, and (b) to (d) are halo patterns indicating amorphous structures. DETAILED DESCRIPTION OF THE INVENTION

[0017] Hereinafter, an aluminum-plated steel sheet for hot stamping (hereinafter also referred to as aluminum-plated steel sheet) according to one embodiment of the present disclosure will be described. However, the present disclosure is not limited to the configuration disclosed in this embodiment, and various modifications are possible within the scope of the present disclosure.

[0018] Below, numerical ranges indicated with "to" include the lower and upper limits. Numerical values ​​indicated as "less than" or "greater than" are not included in the numerical range. In the following description, percentages relating to chemical compositions are mass % unless otherwise specified.

[0019] [Aluminized steel sheets for hot stamping] The aluminum-plated steel sheet for hot stamping according to this embodiment includes a steel sheet, an aluminum-plated layer disposed on the surface of the steel sheet, and a coating disposed on the aluminum-plated layer. The aluminum-plated layer has an Al content of 60 mass % or more and a thickness t1 of 10 to 60 μm.

[0020] Furthermore, in the aluminum-plated steel sheet for hot stamping according to this embodiment, the amorphous ratio in the coating is 90% or more in terms of area ratio, the coating contains one or more elements selected from the A group elements consisting of Ce, Nb, Mo, W, and V, and the coating has a thickness t2 of 0.005 to 5.5 μm.

[0021] <Aluminum plating layer> The aluminum plating layer according to this embodiment is disposed on the surface of the steel sheet. For example, the aluminum plating layer may be formed on one or both sides of the steel sheet. Furthermore, the aluminum plating layer may be formed on the end faces (side faces) of the steel sheet.

[0022] (Plating composition) The aluminum plating layer according to this embodiment contains Al. In this embodiment, an aluminum plating layer containing Al means a plating layer containing 60% or more Al by mass. If the Al content is less than 60% by mass, the corrosion resistance of the plating cannot be ensured and red rust occurs, so the Al content in the aluminum plating layer is 60% or more by mass. Preferably, it is 75% or more by mass. There is no particular need to set an upper limit for the Al content, but it is, for example, 99.999%.

[0023] Examples of elements other than Al that constitute the aluminum plating layer include Si, Fe, and Zn. When the aluminum plating layer contains Si, Fe, and Zn, the contents thereof may be 0.1 to 20%, 0.1 to 10%, and 0.1 to 40%, respectively. The lower limit of each of the contents of Si, Fe, and Zn in the aluminum plating layer is 0%.

[0024] In particular, Si has the effect of improving the adhesion of the plating layer to the steel sheet by suppressing the growth of an alloy layer of Al and Fe (Al-Fe alloy layer), which is hard and brittle.

[0025] It is thought that Fe contained in manufacturing facilities and equipment (for example, in the case of hot-dip aluminum plating, stainless steel containers containing plating solution) is mixed into the aluminum plating layer.

[0026] Zn has the effect of improving the corrosion resistance of exposed areas of the steel substrate by lowering the potential of the aluminum plating layer.

[0027] It should be noted that the content of each element does not need to be within the above range at all locations in the aluminum plating layer, as long as the average chemical composition of the entire aluminum plating layer is within the above range. Therefore, when manufacturing by hot-dip aluminum plating, the average chemical composition of the entire aluminum plating layer can be within the above range by setting the chemical composition of the aluminum plating bath within the above range.

[0028] In this embodiment, the composition of the aluminum plating layer is analyzed by the following method. First, a coating, which will be described later, is formed on the aluminum plating layer in this embodiment, and this coating is removed. Specifically, the coating can be removed by polishing. The composition of the exposed aluminum plating layer is then analyzed using the offline X-ray fluorescence method described in Appendix JB of JIS G 3314:2011. The composition of the aluminum plating layer is the average of compositions measured at three different positions on the exposed aluminum plating layer.

[0029] (thickness t1) The thickness t1 of the aluminum plating layer is 10 to 60 μm. When the thickness t1 of the aluminum plating layer is 10 μm or more, the supply of moisture and oxygen to the steel sheet substrate is suppressed, thereby improving the corrosion resistance of the hot-stamped member. On the other hand, when the thickness t1 of the aluminum plating layer is less than 10 μm, the steel sheet substrate may be exposed, resulting in the formation of red rust. When the thickness t1 of the aluminum plating layer is 60 μm or less, an Al-Fe intermetallic compound layer is formed up to the outermost layer or close to the outermost layer, thereby improving the corrosion resistance of the aluminum-plated steel sheet. The lower limit of the thickness t1 of the aluminum plating layer is preferably 13 μm or more, more preferably 15 μm or more. The upper limit of the thickness t1 of the aluminum plating layer is preferably 55 μm or less, more preferably 50 μm or less, and even more preferably 45 μm or less.

[0030] The thickness t1 of the aluminum plating layer can be measured, for example, by quantitatively analyzing a cross section of a sample taken from the aluminum-plated steel sheet using an FE-EPMA (field emission electron probe microanalyzer). Specifically, a 10 mm x 10 mm sample is first cut from a portion of the aluminum-plated steel sheet at least 10 mm away from the widthwise edge toward the center of the width, specifically, for example, 15 mm away from the widthwise edge. The sample is then embedded in resin, and a cross section parallel to the thickness direction of the aluminum-plated steel sheet is polished to provide an observation surface. Carbon is then vapor-deposited onto the sample to facilitate electrical conduction, and the content of each element is quantitatively analyzed using an FE-EPMA at an accelerating voltage of 10 kV and a magnification of 1500x or greater. During the analysis, point analysis is performed every 1 μm perpendicular to the steel sheet (sheet surface) from the surface of the aluminum-plated steel sheet toward the center of the sheet thickness. Regions where the Al content of the total mass of elements excluding carbon is 30% or greater by mass are considered to be aluminum-plated layers, while regions where the Al content is less than 30% by mass are considered not to be aluminum-plated layers. The thickness t1 of the regions where the Al content is 30% or greater by mass, i.e., the aluminum-plated layer, is then determined.

[0031] <Coating> The coating according to this embodiment contains an A group element, which will be described later, and is formed as an upper layer on the surface of the aluminum plating layer. Typical forms of the coating include chemical conversion coatings, coatings, and powder coating films (powder baked layers), but the coating is not limited to these forms.

[0032] The coating according to this embodiment has an amorphous ratio in terms of area ratio of 90% or more in the coating, and contains one or more elements selected from the A group elements consisting of Ce, Nb, Mo, W, and V. By having such a coating, it is possible to obtain a steel sheet for hot stamping that has excellent long-term corrosion resistance, particularly excellent corrosion resistance during long-term storage. Hereinafter, the mechanism by which the coating improves the corrosion resistance when an aluminum-plated steel sheet for hot stamping is stored for a long period of time will be described.

[0033] The corrosion reaction that occurs in aluminum-plated steel sheets for hot stamping during long-term storage is basically a reaction with oxygen in the atmosphere. In the case of uncoated aluminum-plated steel sheets, the corrosion reaction in the atmosphere is primarily represented by equation (3), which combines the anodic reaction in equation (1) below and the cathodic reaction in equation (2) below. There are several crystalline structures that can be produced by aluminum corrosion, but all of them are hydrated aluminum oxide (Al2O3·H2O) and are known to be white (white rust). In the case of uncoated aluminum-plated steel sheets, the outermost layer is the aluminum plating layer, so during long-term storage, the reaction in equation (3) progresses, causing white rust, and as time passes, the aluminum is consumed, leading to corrosion of the base iron (the formation of red rust).

[0034] 2Al+ → Al 3+ +3e - ··· Equation (1) 2H2O+O2+4e - → 4OH - ··· Formula (2) 4Al+6H2O+3O2→ 2Al2O3 · 6H2O ··· Formula (3)

[0035] On the other hand, in the case of the aluminum-plated steel sheet of this embodiment, the presence of an A group element M (M is one or more of Ce, Nb, Mo, W, and V) in the coating, which is the outermost layer, is thought to result in the formation of an oxide or hydroxide (including hydrate) in the same manner as Al, through the reaction of formula (6), which is a combination of the following formulas (4) and (5). However, since the A group elements are metal elements with standard electrode potentials more noble than Al, the oxide or hydroxide formation reaction of formula (6) proceeds more slowly than Al. Book The coating of the embodiment has barrier properties. Ivy As a result, it is thought that the reaction between the aluminum plating layer and water or oxygen in the atmosphere can be suppressed, resulting in long-term corrosion resistance.

[0036] M (metal element or compound) ← M x+ +xe - ··· Equation (4) 2H2O+O2+4e - → 4OH - ··· Equation (5) 4M+2xH2O+xO2 → 4M(OH) x ...Equation (6)

[0037] Long-term corrosion resistance can be evaluated using SST (salt spray test method) in accordance with JIS Z 2371 (2015). Specifically, an evaluation area of ​​100 mm x 50 mm is first set on the surface of a test piece (aluminized steel sheet), and the evaluation is made based on the area percentage where white rust or red rust occurs within that evaluation area.

[0038] (Group A elements) As described above, the coating of this embodiment contains particles containing one or more elements selected from the group A elements (Ce, Nb, Mo, W, and V).

[0039] The coating contains particles containing one or more elements selected from the group A elements (Cerium, Nb, Mo, W, and V). These particles exist in the coating in the form of particles mainly consisting of simple substances of the group A elements, or particles mainly consisting of compounds of the group A elements (for example, oxides). In this embodiment, "mainly composed of" may also include impurities that are present when the particles are produced. The coating may be particles composed of one or more elements selected from the A group elements. The coating may also be particles composed of a compound of the A group elements.

[0040] The group A elements in the coating are contained in the following forms i to v. i: The form of particles mainly composed of one type of element of group A. ii: A form of particles mainly composed of one type of simple substance of group A element and particles mainly composed of another type of simple substance of group A element. iii: A form of particles mainly composed of one type of compound of an A group element. iv: A mode of particles mainly made of one type of compound of an A group element and particles mainly made of another type of compound of an A group element. v: Particles (one or more types) mainly composed of a simple substance of an A group element and particles (one or more types) mainly composed of a compound of an A group element.

[0041] When the coating is a paint film, the "particles" in the coating exist as particles mainly composed of simple A group elements and / or particles mainly composed of A group element compounds. Such a coating can be produced, for example, by applying a coating material containing the above-mentioned particles mixed in an organic binder onto an aluminum plating layer, and then drying the applied coating by heating as necessary. There is no need to particularly limit the organic binder, and known organic binders can be used.

[0042] When the coating is produced by powder baking, the term "particles" in the coating refers to powder particles. Such a coating is produced, for example, by applying a paint made by mixing the powder particles with an organic binder onto an aluminum plating layer and baking it.

[0043] The total content of the A group elements in the coating according to this embodiment is 0.01 to 2.0 g / m 2 The "total content of A group elements" is the total content of A group elements per 1 m of the coating. 2 It means the total mass of all types of A group elements contained per unit mass.

[0044] For example, if the coating contains particles containing only cerium (Ce) as an A group element, the content of the A group element is calculated as follows. In either case, if the coating contains particles mainly consisting of the simple substance Ce, which is an A group element, or if the coating contains particles mainly consisting of cerium oxide, which is a compound of Ce, which is an A group element, the content of the A group element is calculated as follows: 2 The content of A group elements per 1 m 2 The Ce content per 1 m of the coating is 100%. Note that the above-mentioned "mainly composed of" indicates that impurities other than A-group elements may be contained, which are mixed in during particle production. Therefore, "particles mainly composed of Ce alone" refers to particles consisting of Ce alone and impurities that do not contain A-group elements. In the above example, a case was given in which the coating contains particles containing only Ce as an A-group element, but if these particles also contain other A-group elements, the content of each A-group element in the coating is 100%. 2 The weight of each element in the coating was calculated, and the total weight was calculated as the weight of the A group elements in the coating. 2 The content is per unit.

[0045] For example, if the coating contains particles containing only Ce as an A group element and particles containing only V (vanadium), the content of the A group element in the coating is calculated as follows: If the coating contains particles mainly consisting of Ce, an A group element, and particles mainly consisting of vanadium chloride, which is a compound of V, an A group element, the content of the A group element in 1 m of the coating is calculated as follows: 2 The content of A group elements per 1 m of the coating is 2 This is the total amount of Ce and V contained per unit area.

[0046] The total content of A group elements in the coating is 0.01g / m 2 If the content is less than 0.01 g / m, the long-term corrosion resistance may be reduced. Therefore, from the viewpoint of ensuring long-term corrosion resistance, the total content of A group elements in the coating is set to 0.01 g / m 2 More preferably, it is 0.10 g / m or more. 2 That's all.

[0047] On the other hand, the total content of A group elements in the coating is 2.0 g / m 2 If the total content of A group elements in the coating exceeds 2.0 g / m, the coating may become powdery and peel off due to bending during coil winding, friction between plates, and handling during transportation and installation, as the amount of adhesion is too high, and sufficient long-term corrosion resistance may not be obtained. 2 In addition, taking into consideration the amount and effect of the A group elements, the total content is preferably 1.0 g / m or less from the viewpoint of cost-effectiveness. 2 The following is preferred:

[0048] The content of A group elements in the coating (total amount of each element) can be measured by the offline X-ray fluorescence method described in Appendix JB of JIS G 3314:2011. Specifically, the intensity of the offline X-ray fluorescence method is measured within a field of view with a diameter of 30 mm. Next, the intensity is calculated using a calibration curve for each A group element prepared in advance. 2 The mass of each A group element contained in 1 m of the coating layer is calculated by adding up the mass of each A group element. 2 The content of group A elements per unit can be calculated.

[0049] Examples of compounds of Group A elements include oxides, chlorides, sulfides, fluorides, hydroxides, carbides, nitrides, etc. Specific examples include vanadium pentoxide, vanadium trioxide, vanadium dioxide, cerium oxide, cerium chloride, cerium nitrate, cerium sulfate, cerium acetate, cerium oxalate, cerium hydroxide, niobium oxide, potassium niobate, lithium niobate, niobium nitride, molybdenum oxide, ammonium molybdate, potassium molybdate, tungsten oxide, ammonium tungstate, potassium tungstate, tungsten sulfide, and hydrates of the above compounds, but are not limited to these.

[0050] (film structure) The coating of this embodiment contains an amorphous content of 90% or more by area. In other words, the crystalline structure of the coating contains 90% or more amorphous content. As described above, the presence of a coating with an amorphous content of 90% or more on the aluminum plating layer provides barrier properties and inhibits reaction with water molecules and oxygen molecules in the atmosphere for the following reasons (a) and (b), thereby achieving long-term corrosion resistance.

[0051] (a) By including metal elements whose standard electrode potential is more noble than that of Al in the components that make up the coating, the formation of corrosion products (oxides or hydroxides of A group elements and Al) can be suppressed. (b )Non The amorphous film has barrier properties. Ivy Therefore, the corrosion reaction of the film can be suppressed.

[0052] Here, "amorphous" means that no crystal diffraction spots can be confirmed in an electron beam diffraction image of a cross section of the coating taken by TEM, and "crystalline" means that crystal diffraction spots can be detected in an electron beam diffraction image of a cross section of the coating taken by TEM. Specifically, the structure of the coating is determined by electron diffraction patterns using a TEM. A crystalline structure is defined as a structure in which clear crystal diffraction spots are observed in the electron diffraction pattern, whereas an amorphous structure is defined as a structure in which no crystal diffraction spots are observed but a ring-shaped continuous diffraction pattern (halo pattern) is observed. Furthermore, when crystal diffraction spots indicating a crystalline structure are observed around the halo pattern, the coating is determined to have a structure that contains both amorphous and crystalline materials.

[0053] FIG. 1 is an example of a TEM electron beam diffraction image of a cross section of the aluminum plating layer and coating in an aluminum-plated steel sheet according to this embodiment. FIG. 1(a) is an electron beam diffraction image corresponding to an aluminum plating layer. Crystal diffraction spots can be seen in FIG. 1(a), indicating that a metal phase with a crystalline structure is contained in the aluminum plating layer. On the other hand, FIG. 1(b) to (d) are electron beam diffraction images corresponding to the coating of this embodiment, which are examples of halo patterns indicating amorphous structures. In the cases of FIG. 1(b) to (d), ring-shaped diffraction patterns (halo patterns) can be seen, indicating that the structure contained in the coating is amorphous.

[0054] The methods for observing and distinguishing the film structure will be described in detail below. First, a sample 15 μm wide, 15 μm deep, and 300 nm thick is taken from the film using a focused ion beam (FIB) to include a cross section along the film thickness direction. Next, electron diffraction is performed on the sample surface corresponding to the cross section of the film using a TEM (JEM-2100F, manufactured by JEOL Ltd.) with an electron beam diameter of 1 nm. The crystalline structure of the film can be determined by examining the obtained diffraction pattern.

[0055] The amorphous content in the coating is calculated by the following method. First, two samples (15 μm wide, 15 μm deep, 300 nm thick) are collected. Next, for each sample, electron beam diffraction is performed on 10 points on the sample surface using the transmission electron microscope (TEM) to obtain diffraction images. Of the 10 diffraction images obtained, the number of patterns (so-called intermediate patterns) in which ring-shaped diffraction patterns or halo patterns with crystalline diffraction spots indicating a crystalline structure are observed around them is counted. This measurement is performed on each of the two samples, and the total number of patterns in which ring-shaped diffraction patterns and halo patterns with crystalline diffraction spots indicating a crystalline structure are observed around them is obtained. This total number is divided by the total number of electron diffraction images, i.e., 20, to obtain the point ratio. The obtained point ratio is defined as the "amorphous area ratio" in this embodiment. For example, if the point ratio is 90%, the amorphous area ratio in the coating is determined to be 90%. In this embodiment, the amorphous area ratio in the coating may be 100%. The accelerating voltage during TEM observation is, for example, 100 kV. If the sample for analysis also contains an aluminum plating layer, the aluminum plating layer is excluded from the measurement target, and only the coating is measured.

[0056] The coating weight is 0.01 to 2.0 g / m per side. 2 The coating weight is preferably 0.01 g / m 2 If the coating weight is less than 2.0 g / m, the coating is too thin to suppress the reaction between the aluminum (plating component) and water or oxygen, and sufficient long-term corrosion resistance may not be obtained. 2 In the case of ultra-high strength steel, the amount of adhesion is too great, and as a result, the steel may peel off and become powdery due to bending during coil winding, rubbing between plates, and handling during transportation and installation, and sufficient long-term corrosion resistance may not be obtained.

[0057] (Thickness t2: 0.005~5.5μm) The thickness t2 of the coating is 0.005 to 5.5 μm. If the thickness t2 is less than 0.005 μm, the coating is too thin, and the reaction between Al, an aluminum plating component, and water and oxygen is not suppressed, which may result in insufficient long-term corrosion resistance. Therefore, the thickness t2 of the coating is 0.005 μm or more. Preferably, the thickness t2 is 0.05 μm or more. On the other hand, if the thickness t2 exceeds 5.5 μm, the coating becomes too thick and may peel off (into powder) due to bending during coil winding, rubbing between plates, and handling during transportation and installation, which may result in insufficient long-term storage corrosion resistance. Therefore, the thickness t2 of the coating is 5.5 μm or less. Preferably, the thickness t2 is 3.0 μm or less, or 2.0 μm or less. The thickness t2 of the coating can be measured, for example, by observing an aluminum-plated steel sheet using a TEM bright-field image (BF image). First, a sample 15 μm wide, 15 μm deep, and 300 nm thick is collected from the aluminum-plated steel sheet using a focused ion beam (FIB) to include a cross section of the coating along the thickness direction. Next, a BF image of the sample surface corresponding to the cross section of the coating is obtained using a TEM (JEM-2100F, manufactured by JEOL Ltd.) with an electron beam diameter of 1 nm. In this BF image, the coating is identified by contrast, and the thickness t2 of the coating is determined. The accelerating voltage during TEM observation is, for example, 100 kV. In this embodiment, the thickness t1 of the aluminum plating layer, the total content of the A group elements in the coating, the thickness t2 of the coating, and the coating weight are values ​​per side.

[0058] <Base material steel plate> (Chemical composition) The chemical composition of the steel sheet that is the base material of the aluminum-plated steel sheet is not particularly limited. However, from the viewpoint of obtaining high mechanical strength after hot stamping, the chemical composition preferably consists of, in mass %, C: 0.22 to 0.60%, Si: 0.01 to 0.60%, Mn: 0.50 to 3.00%, P: 0.050% or less, S: 0.020% or less, Al: 0.10% or less, Ti: 0.010 to 0.100%, B: 0.0001 to 0.0100%, N: 0.0200% or less, Cr: 0 to 1.00%, Mo: 0 to 1.00%, Cu: 0 to 1.00%, Ni: 0 to 2.00%, Nb: 0 to 1.00%, Sn: 0 to 1.00%, Ca: 0 to 0.10%, and the balance being Fe and impurities. More preferably, N may be 0.0100% or less.

[0059] [Method of manufacturing aluminum-plated steel sheets for hot stamping] Next, a method for producing an aluminum-plated steel sheet for hot stamping according to this embodiment will be described. The production method described below is an example in which an aluminum-plated steel sheet is produced by applying Al plating to a steel sheet. However, the production method described below is just one example, and the production method for an aluminum-plated steel sheet for hot stamping according to this embodiment is not limited to the production method described below.

[0060] <Aluminum plating process> For example, an aluminum plating layer is formed on the surface of the above-mentioned base steel sheet (steel sheet) by hot dip plating. The aluminum plating layer of the aluminum-plated steel sheet is formed on one or both sides of the steel sheet. The bath temperature of the plating bath may be, for example, 650 to 750°C. At least a portion of the Al contained in this aluminum plating layer can be alloyed with Fe in the steel sheet during hot dip plating or during the heating step in hot stamping, etc. Therefore, this aluminum plating layer is not necessarily formed as a single layer with a uniform composition, and may include an alloyed layer as appropriate.

[0061] The hot-dip plating bath used in the hot-dip plating method contains Al, and may also contain Si.

[0062] When Si is added to the coating bath, the Si content should be 20.0 mass% or less. An aluminum coating layer is formed on the surface of a steel sheet by immersing the steel sheet in a hot-dip coating bath containing Al and, if necessary, Si.

[0063] After immersion in the plating bath, the amount of plating adhered to the steel sheet surface is adjusted by gas wiping or the like.

[0064] <Film formation process> A coating having an amorphous ratio of 90% or more can be formed on the surface of an aluminum-plated steel sheet by applying a chemical conversion treatment solution containing an A group element to the surface of a steel sheet having an aluminum-plated layer, and then baking and drying the steel sheet under conditions where the ultimate sheet temperature T is 40 to 150°C and the heating time satisfies the following formula (7): The atmosphere during heating is not limited to air, vacuum, etc.

[0065] If the ultimate sheet temperature T is less than 40°C, it takes time for the coating to dry and solidify, and the aluminum plating layer and the chemical conversion treatment solution continue to react, causing aluminum components to leach into the coating. As a result, more than 10% of the coating becomes crystalline, reducing the amorphous content, and sufficient long-term corrosion resistance cannot be obtained. If the ultimate sheet temperature T is more than 150°C, the reaction between the aluminum plating layer and the chemical conversion treatment solution is excessively accelerated, causing Al components to leach into the coating. As a result, more than 10% of the coating becomes crystalline, reducing the amorphous content, and sufficient long-term corrosion resistance cannot be obtained. The ultimate sheet temperature T is preferably 60 to 80°C.

[0066] Furthermore, the heating time t (s) during baking is adjusted within a range that satisfies the following formula (7), where the ultimate plate temperature is T (° C.).

[0067] 1000 ≦t×T≦ 8000 ··· Formula (7)

[0068] In formula (7), if (t × T) is less than 1000, the chemical conversion treatment solution will not dry sufficiently, the coating will not solidify, and the aluminum plating components will dissolve into the chemical conversion treatment solution. As a result, an amorphous coating will not be formed, and sufficient long-term corrosion resistance will not be obtained. On the other hand, if (t × T) is more than 8000 in formula (7), the chemical conversion treatment solution will be overheated, causing the chemical conversion treatment solution components to volatilize, preventing the coating itself from being formed sufficiently, and therefore the desired long-term corrosion resistance will not be obtained.

[0069] The average cooling rate after heating (drying) of the chemical conversion treatment solution varies depending on the sheet temperature reached. Specifically, when the ultimate sheet temperature is 100°C or higher and 150°C or lower, the average cooling rate after heating is 40 to 100°C / s, preferably 60 to 80°C / s. If the average cooling rate is less than 40°C / s, the cooling is slow, and residual heat input increases the diffusion of plating components of the aluminum plating layer into the coating, resulting in the formation of 10% or more crystals. As a result, an amorphous coating cannot be obtained, and sufficient long-term corrosion resistance cannot be achieved. On the other hand, if the average cooling rate is greater than 100°C / s, the amount of reaction between the coating components of the aluminum plating layer and the coating components after heating (drying) the chemical conversion treatment solution decreases, resulting in reduced adhesion of the coating and peeling of the coating. Furthermore, if the average cooling rate is greater than 100°C / s, the amount of reaction between the coating components and the plating components decreases, which may reduce the amorphous content of the coating.

[0070] When the ultimate sheet temperature is 40°C or higher but lower than 100°C, the average cooling rate after heating is 40 to 80°C / s, preferably 60 to 80°C / s. If the average cooling rate is lower than 40°C / s, the cooling is too slow, resulting in increased elution of aluminum components from the aluminum plating layer into the coating, resulting in 10% or more crystals. As a result, an amorphous coating cannot be obtained, and sufficient long-term corrosion resistance cannot be achieved. On the other hand, if the average cooling rate is higher than 80°C / s, the amount of reaction between the coating components and the plating components of the aluminum plating layer after heating (drying) the chemical conversion treatment solution is reduced, resulting in reduced adhesion of the coating and peeling of the coating. Furthermore, if the average cooling rate is higher than 80°C / s, the amount of reaction between the coating components and the plating components is reduced, which may result in a lower amorphous content in the coating.

[0071] The pH of the chemical conversion treatment solution is preferably 5.0 to 12.0, and more preferably 8.0 to 10.0. By controlling the pH of the chemical conversion treatment solution to 5.0 to 12.0, a small amount of aluminum present on the surface of the aluminum plating layer reacts with the upper coating during application and baking, forming a reaction layer. As a result, this reaction layer ensures adhesion. Furthermore, trace amounts of Al diffused from the aluminum plating layer into the coating are contained in the coating as impurities, preventing the formation of a crystalline arrangement, and the amorphous ratio of the coating can be increased to 90% or more.

[0072] If the pH of the chemical conversion treatment solution is less than 5.0, the amount of Al eluted into the film increases, making it impossible to obtain a film with an amorphous ratio of 90% or more, and sufficient long-term corrosion resistance cannot be obtained.Furthermore, if the pH of the chemical conversion treatment solution is more than 12.0, the amount of Al eluted also increases, making it impossible to obtain a film with an amorphous ratio of 90% or more, and sufficient long-term corrosion resistance cannot be obtained.

[0073] The chemical conversion treatment solution can be applied by a roll coater, spraying, or the like, but is not limited to these methods. [Example]

[0074] Examples of the present invention will be described below, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0075] As the base steel sheet, it is preferable to use a steel sheet that can obtain the desired mechanical properties (meaning various properties related to mechanical deformation and fracture, such as tensile strength, yield point, elongation, reduction of area, hardness, impact value, fatigue strength, etc.) after hot stamping. The chemical compositions of the base steel sheets before plating used for the hot stamping steel sheets shown in the following examples are shown in Table 1 below.

[0076] [Table 1]

[0077] Aluminum-plated steel sheets for hot stamping (designations A1 to D12) were produced by applying an aluminum plating layer and a coating to base steel sheets (steel Nos. S1 to S16) having the chemical compositions shown in Table 1 under the manufacturing conditions shown in Tables 2 and 3. More specifically, a steel sheet having a width of 100 mm, a length of 200 mm, and a thickness of 1.4 mm and having the chemical compositions shown in Table 1 was prepared as each base steel sheet. The steel sheet was then plated by a hot-dip galvanizing method using a plating bath with a composition of Al-10% by mass of Si. A chemical conversion treatment solution containing one or more elements selected from the group A elements consisting of Ce, Nb, Mo, W, and V and having a pH of 5.0 to 12.0 was then applied to the plating layer, and a coating was applied to the base steel sheet under the manufacturing conditions shown in Tables 2 and 3. In the examples, when the coating contained a group A element, the total content of the group A element was 0.01 to 2.0 g / m. 2 was within the range.

[0078] Long-term corrosion resistance was evaluated using SST (salt spray test method) in accordance with JIS Z 2371 (2015). Specifically, an evaluation area of ​​100 mm x 50 mm was first set on the surface of each of the obtained aluminum-plated steel sheets for hot stamping, designated A1 to D12, and within that evaluation area, the area percentage where white rust had developed and the presence or absence of red rust were evaluated after 48 hours of salt spray testing (SST). An area rate of white rust of 1.0% or less was judged as passing ("Good"), and an area rate of more than 1.0% was judged as failing ("Bad"). In addition, if no red rust occurred within the evaluation area after 48 hours of salt spray testing (SST), the test piece was evaluated as passing ("Good"), and if rust occurred, the test piece was evaluated as failing ("Bad"). As shown in Tables 2 and 3, none of the examples that do not meet the requirements of the present invention were able to achieve good long-term corrosion resistance.

[0079] [Table 2]

[0080] [Table 3] [Industrial Applicability]

[0081] According to the above aspects of the present invention, an aluminum-plated steel sheet for hot stamping having excellent long-term corrosion resistance can be provided. Therefore, the obtained aluminum-plated steel sheet for hot stamping can be suitably applied to automotive parts and the like, and therefore has high industrial applicability.

Claims

1. Steel plate and an aluminum plating layer located on the surface of the steel sheet; a chemical conversion coating film located on the aluminum plating layer; An aluminum-plated steel sheet for hot stamping, The aluminum content of the aluminum plating layer is 60 mass% or more, The thickness t1 of the aluminum plating layer is 10 to 60 μm, the amorphous ratio in the chemical conversion treatment film is 90% or more in terms of area ratio, the chemical conversion coating contains one or more elements selected from the group A elements consisting of Ce, Nb, Mo, W, and V; The thickness t2 of the chemical conversion coating is 0.005 to 5.5 μm.

1. An aluminum-plated steel sheet for hot stamping, comprising:

2. The chemical composition of the steel plate is, in mass%, C: 0.22-0.60%, Si: 0.01-0.60%, Mn: 0.50-3.00%, P: 0.050% or less, S: 0.020% or less, Al: 0.10% or less, Ti: 0.010 to 0.100%, B: 0.0001 to 0.0100%, N: 0.0200% or less, Cr: 0-1.00%, Mo: 0-1.00%, Cu: 0 to 1.00%, Ni: 0-2.00%, Nb: 0 to 1.00%, Sn: 0-1.00%, Ca: 0-0.10%, The aluminum-plated steel sheet for hot stamping according to claim 1, characterized in that the balance is Fe and unavoidable impurities.

3. The chemical components are, in mass %, The aluminum-plated steel sheet for hot stamping according to claim 2, characterized in that N: 0.0100% or less.

4. The method for producing an aluminum-plated steel sheet for hot stamping according to claim 1, a plating step of forming an aluminum plating layer having an Al content of 60 mass% or more on the steel sheet; a film forming step of forming a chemical conversion coating on the aluminum plating layer, The film forming step includes: a coating step of coating the aluminum plating layer with a chemical conversion treatment solution containing one or more elements selected from the group A elements consisting of Ce, Nb, Mo, W, and V, and having a pH of 5.0 to 12.0; a baking step in which, after the coating step, the chemical conversion treatment solution is baked at an ultimate temperature T of 40 to 150°C to form a chemical conversion treatment film; a cooling step of cooling the chemical conversion coating film after baking; and The heating time t in the baking step is adjusted to satisfy the following formula (7): In the cooling step, When the reached temperature T is 100°C or more and 150°C or less, the average cooling rate is 40 to 100°C / s, When the reached temperature T is 40°C or higher and lower than 100°C, the average cooling rate is 40 to 80°C / s. 1000≦t×T≦8000... Formula (7)

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