Al-plated steel sheet, method for producing Al-plated steel sheet, and method for producing hot press-formed body

Abstract

Provided is an Al-plated steel sheet that can suppress hydrogen embrittlement caused by hydrogen that has intruded into a steel material in conjunction with oxidation of an Al plating layer at the time of hot press forming. The Al-plated steel sheet (1) includes a steel base material having a prescribed chemical composition, an Al plating layer (20) containing Al and Si formed on the steel base material, and an oxide layer (30) formed on the Al plating layer (20), the thickness d of the oxide layer (30) is 10 to 400 nm, the oxide layer (30) contains a hydroxide layer (31), and the ratio of the thickness d1 of the hydroxide layer (31) to the thickness d of the oxide layer (30) is 30% or less. Here, the thickness d of the oxide layer (30) is set as the depth from the surface at which the integrated intensity of the oxide is 1 / 2 of the maximum value in depth direction analysis using X-ray photoelectron spectroscopy, and the thickness d1 of the hydroxide layer (31) is set as the depth from the surface at which the integrated intensity of the hydroxide is 1 / 2 of the maximum value in depth direction analysis using X-ray photoelectron spectroscopy.

Description

Technical Field

[0001] This invention relates to Al-plated steel sheets, methods for manufacturing Al-plated steel sheets, and hot-stamped formed bodies. More specifically, it relates to Al-plated steel sheets for hot stamping, methods for manufacturing the same, and methods for manufacturing hot-stamped formed bodies using the same. Background Technology

[0002] Hot stamping is a known technique for producing high-strength steel (especially ultra-high-strength steel with a strength of 1500 MPa or higher) that is difficult to form with high dimensional precision. Hot stamping overcomes the formability problem by forming the steel sheet at a high temperature of over 800°C and then cooling it after forming to obtain the desired high strength.

[0003] In hot stamping, an oxide scale forms on the surface of the steel sheet due to heating in the atmosphere, which needs to be removed in subsequent processes. As a technique to mitigate this, an Al plating containing Si is known to inhibit oxidation of the steel sheet.

[0004] Japanese Patent Application Publication No. 2003-193187 discloses a method to suppress the formation of cracks during processing by ensuring that the Al content in the Fe-Al coating is 35% or less. International Publication No. 2019 / 160106 discloses a Fe-Al coated hot-stamping part with a specified structure, designed to improve the corrosion resistance of the formed part and its corrosion resistance after coating.

[0005] In hot stamping, hydrogen intrusion into the steel during heating increases the steel's susceptibility to hydrogen embrittlement. International Publication No. 2012 / 120692 describes a method to improve resistance to delayed fracture by generating Fe-Mn composite oxides in steel sheets, which trap hydrogen at the interface between the composite oxides and the steel matrix.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2003-193187

[0009] Patent Document 2: International Publication No. 2019 / 160106

[0010] Patent Document 3: International Publication No. 2012 / 120692

[0011] Patent Document 4: Japanese Patent Application Publication No. 2016-65312

[0012] Patent Document 5: Japanese Patent Application Publication No. 2022-31677

[0013] Patent Document 6: Japanese Patent Publication No. 2022-513595

[0014] Patent Document 7: Japanese Patent Application Publication No. 6-272017 Summary of the Invention

[0015] The problem the invention aims to solve

[0016] If a hot stamping process is performed on Al-coated steel sheets, hydrogen may sometimes penetrate the steel as the Al coating oxidizes.

[0017] The present invention aims to provide an Al-plated steel sheet and a method for manufacturing the same, which can suppress hydrogen embrittlement caused by hydrogen intrusion into the steel during hot stamping due to oxidation of the Al coating, and has high reliability. Another objective of the present invention is to provide a hot-stamped form with suppressed hydrogen embrittlement.

[0018] Solution for solving the problem

[0019] One embodiment of the present invention relates to an Al-plated steel sheet for hot stamping, comprising: a steel substrate; an Al coating, which is a coating containing Al and Si formed on the steel substrate; and an oxide layer formed on the Al coating. The chemical composition of the steel substrate, in mass %, is: C: 0.1-0.6%, Si: 0.01-1.50%, Mn: 0.10-3.00%, P: 0.05% or less, S: 0.020% or less, Al: 0.10%. The oxide layer comprises the following components: Ti: 0.01–0.10%, B: 0.0001–0.0100%, N: 0.015% or less, Cr: 0–1.0%, Mo: 0–1.0%, Ni: 0–1.0%, Cu: 0–1.0%, Nb: 0–1.0%, with the balance being Fe and impurities. The oxide layer has a thickness of 10–400 nm and includes a hydroxide layer. The thickness of the hydroxide layer relative to the thickness of the oxide layer is less than 30%. The thickness of the oxide layer is defined as the depth from the surface when the integrated intensity of the oxide is half its maximum value in depth-direction analysis using X-ray photoelectron spectroscopy. The thickness of the hydroxide layer is defined as the depth from the surface when the integrated intensity of the hydroxide is half its maximum value in depth-direction analysis using X-ray photoelectron spectroscopy.

[0020] One embodiment of the present invention relates to a method for manufacturing Al-plated steel sheet, which includes a pre-oxidation step of heating the Al-plated steel sheet to a temperature of 120 to 600°C.

[0021] One embodiment of the present invention relates to a hot-stamped formed body comprising a process of hot-stamping the aforementioned Al-plated steel sheet.

[0022] The effects of the invention

[0023] According to the present invention, an Al-plated steel sheet can be obtained, which can suppress hydrogen embrittlement caused by hydrogen intrusion into the steel during the oxidation of the Al coating during hot stamping and has high reliability. According to the present invention, a hot-stamped formed body with suppressed hydrogen embrittlement can also be obtained. Attached Figure Description

[0024] Figure 1 This is a schematic cross-sectional view of an Al-plated steel sheet according to one embodiment of the present invention.

[0025] Figure 2 To show in magnified form Figure 1 A schematic cross-sectional view of the oxide layer composition.

[0026] Figure 3 Here is an example of an O1s spectrum determined by XPS.

[0027] Figure 4 Examples of curves showing the depth direction of the integrated intensity of oxides and hydroxides. Detailed Implementation

[0028] The inventors have studied the relationship between the oxide layer formed on the surface of Al-plated steel sheets and the amount of hydrogen penetrating into the steel during the hot stamping process. During the study, it was found that if a pre-oxidation process is performed by heating the Al-plated steel sheet under specified conditions, followed by the hot stamping process, the amount of hydrogen penetrating into the steel is reduced.

[0029] In the initial stage immediately after Al-plated steel sheets, an oxide layer (initial oxide layer) with a thickness of several nm is formed on the surface. This initial oxide layer consists sequentially from the surface side of a hydroxide layer rich in hydroxides and an oxide layer with fewer hydroxides. The hydroxides here, such as Al(OH)3 and AlOOH, contain OH groups (hydroxyl groups) in their structure. On the other hand, the oxides, such as Al2O3 and Al2FeO4, are represented by chemical formulas consisting only of metals and oxygen. In the initial oxide layer, the ratio of the hydroxide layer thickness to the oxide layer thickness becomes relatively high. When a pre-oxidation process is performed on the Al-plated steel sheet, the oxide layer thickens, and the ratio of the hydroxide layer thickness to the oxide layer thickness decreases.

[0030] The hydrogen that penetrates the steel during the hot stamping process is assumed to be hydrogen from hydroxides on the steel surface and hydrogen from moisture in the atmosphere. The aforementioned pre-oxidation process is believed to help reduce the amount of hydrogen in both. Specifically, the pre-oxidation process is thought to promote the release of hydrogen from hydroxides on the steel surface, and the oxide layer with reduced hydroxides acts as a protective layer against further oxidation, inhibiting oxidation reactions during the hot stamping process and suppressing the dissociation reactions of moisture in the atmosphere.

[0031] This invention is based on the above insights. The embodiments of the invention will now be described in detail with reference to the accompanying drawings. Identical or equivalent parts in the drawings are labeled with the same symbols, and their descriptions are not repeated. The dimensional ratios between the constituent parts shown in the figures do not necessarily represent actual dimensional ratios.

[0032] Al-plated steel sheet

[0033] Figure 1 This is a schematic cross-sectional view of an Al-plated steel sheet 1 according to one embodiment of the present invention. The Al-plated steel sheet 1 includes a steel substrate 10, an Al coating 20 formed on the steel substrate 10, and an oxide layer 30 formed on the Al coating 20. The Al coating 20 and the oxide layer 30 may be formed on one side of the steel substrate 10 or on both sides of the steel substrate 10.

[0034] [Oxide layer]

[0035] The oxide layer 30 is formed by surface oxidation of the Al coating 20, and contains Al oxides and Al hydroxides. Examples of Al oxides include Al₂O₃ and Al₂FeO₄. Examples of Al hydroxides include Al(OH)₃ and AlOOH. The oxide layer 30 may contain oxides other than Al oxides and hydroxides other than Al hydroxides.

[0036] Figure 2 A schematic cross-sectional view showing the structure of oxide layer 30 is provided for emphasis. Oxide layer 30 includes a hydroxide layer 31, which is a hydroxide-rich layer, and an oxide layer 32, which is a hydroxide-poor layer. The hydroxides contained in oxide layer 30 are mostly distributed on the surface side of oxide layer 30. Therefore, hydroxide layer 31 is formed on the surface side, and oxide layer 32 is formed on the substrate side.

[0037] In this embodiment, the thickness d of the oxide layer 30 is 10 to 400 nm, and the ratio of the thickness d1 of the hydroxide layer 31 to the thickness d of the oxide layer 30 is 30% or less.

[0038] The oxide layer 30 functions as a protective layer against further oxidation of the Al-plated steel sheet 1. Specifically, by forming an oxide layer 30 of a predetermined thickness on the Al-plated steel sheet 1 beforehand, oxidation reactions during the hot stamping process can be suppressed. This reduces the amount of hydrogen penetrating the steel substrate 10 during the hot stamping process. If the thickness d of the oxide layer 30 is less than 10 nm, this effect is not sufficiently achieved. On the other hand, if the thickness d of the oxide layer 30 exceeds 400 nm, the influence of hydrogen from the hydroxide contained in the oxide layer 30 becomes greater. The lower limit of the thickness d of the oxide layer 30 is preferably 20 nm. The upper limit of the thickness d of the oxide layer 30 is preferably 300 nm, more preferably 200 nm, more preferably 100 nm, and more preferably 50 nm.

[0039] The hydroxide contained in the oxide layer 30 can become a source of hydrogen penetrating the steel substrate 10. By making the ratio of the thickness d1 of the hydroxide layer 31 to the thickness d of the oxide layer 30 30% or less, the amount of hydrogen penetrating the steel from the hydroxide can be reduced. The ratio of the thickness d1 of the hydroxide layer 31 to the thickness d of the oxide layer 30 is preferably 25% or less, more preferably 20% or less, and even more preferably 15% or less.

[0040] The thickness d of oxide layer 30 and the thickness d1 of hydroxide layer 31 were determined by depth direction analysis using X-ray photoelectron spectroscopy (XPS) as follows.

[0041] Figure 3 This is an example of an O1s spectrum measured by XPS. The O1s spectrum includes peaks for both oxides and hydroxides. By subtracting the background from this O1s spectrum, the peak waveforms of the oxides and hydroxides are separated, and their individual integrated intensities are calculated. Background processing uses the Shirley method, typical for XPS data processing. The ratio of the integrated intensities obtained here is consistent with the ratio of the presence of oxides and hydroxides at the measured depth.

[0042] In utilizing Ar + During ion sputtering, sputtering and XPS measurements were repeatedly performed to obtain depth-direction curves of the integrated intensity of oxides and hydroxides. Figure 4 Examples of curves showing the depth direction of the integrated intensity of oxides and hydroxides.

[0043] Here, the position where the integrated intensity of the hydroxide reaches half of its maximum value is considered the layer boundary between the hydroxide layer 31 and the oxide layer 32. Similarly, the position where the integrated intensity of the oxide reaches half of its maximum value is considered the layer boundary between the oxide layer 32 and the Al coating 20.

[0044] That is, the thickness d1 of the hydroxide layer 31 is the depth from the surface when the integrated intensity of the hydroxide reaches half of its maximum value. Similarly, the thickness d of the oxide layer 30 is the depth from the surface when the integrated intensity of the oxide reaches half of its maximum value. It should be noted that the thickness d2 of the oxide layer 32 is obtained by subtracting the thickness d1 of the hydroxide layer 31 from the thickness d of the oxide layer 30.

[0045] [Al coating]

[0046] Al coating 20 is a coating comprising Al and Si. Sometimes, intermetallic compounds of Al with Fe and Si are formed in Al coating 20. The chemical composition (average composition in the thickness direction, the same below) of Al coating 20 is not limited thereto, but is, for example, Al: 20–100% by mass, Si: 1–20% by mass, Fe: 0–60% by mass. The lower limit of Al content in Al coating 20 is preferably 25% by mass. The upper limit of Al content in Al coating 20 is preferably 95% by mass, more preferably 90% by mass, more preferably 70% by mass, and more preferably 55% by mass. The lower limit of Si content in Al coating 20 is preferably 2% by mass, more preferably 5% by mass. The upper limit of Si content in Al coating 20 is preferably 15% by mass, more preferably 12% by mass. The lower limit of Fe content in Al coating 20 is preferably 20% by mass. The upper limit of Fe content in Al coating 20 is preferably 50% by mass, more preferably 40% by mass.

[0047] The Al coating 20 may contain elements other than Al, Si, and Fe. Specifically, it may sometimes contain Be, Mg, Ca, Sr, Ba, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Zr, Y, Nb, Ce, and Ta. The total content of elements other than Al, Si, and Fe is preferably 10% by mass or less, more preferably 5% by mass or less, more preferably 3% by mass or less, and more preferably 1% by mass or less.

[0048] The thickness of the Al coating 20 is not particularly limited, for example, it is 1 to 100 μm. The lower limit of the thickness of the Al coating 20 is preferably 5 μm, more preferably 10 μm, and even more preferably 15 μm. The upper limit of the thickness of the Al coating 20 is preferably 50 μm, more preferably 40 μm, and even more preferably 30 μm.

[0049] [Steel substrate]

[0050] The steel substrate 10 is, for example, a hot-rolled steel sheet or a cold-rolled steel sheet. The chemical composition of the steel substrate 10 will be described below. In the following description, the "%" of element content refers to mass percentage.

[0051] C: 0.1-0.6%

[0052] Carbon (C) is included to ensure the target mechanical strength. If the C content is less than 0.1%, sufficient improvement in mechanical strength cannot be achieved. On the other hand, if the C content exceeds 0.6%, elongation and reduction of area tend to decrease. The upper limit of C content is preferably 0.5%.

[0053] Si: 0.01~1.50%

[0054] Silicon (Si) is an element that improves mechanical strength, and like carbon, it is included to ensure the target mechanical strength. If the Si content is less than 0.01%, sufficient improvement in mechanical strength cannot be obtained. On the other hand, if the Si content exceeds 1.50%, the wettability during plating will decrease due to the influence of Si oxide formed on the surface of the steel substrate, and plating may not occur. The upper limit of Si content is preferably 0.60%.

[0055] Mn: 0.10~3.00%

[0056] Manganese (Mn) is one of the strengthening elements in steel and also one of the elements that improve hardenability. Mn can also effectively prevent hot work brittleness caused by sulfur, which is an impurity. If the Mn content is less than 0.10%, these effects cannot be fully obtained. On the other hand, if the Mn content exceeds 3.00%, there will be too much retained austenite, which may reduce the strength. The lower limit of Mn content is preferably 0.50%, and the upper limit of Mn content is preferably 2.00%.

[0057] P: below 0.05%

[0058] Phosphorus (P) is an impurity contained in steel substrates. P in steel substrates can sometimes segregate to the grain boundaries, thus reducing the toughness of the steel substrate. The P content should ideally be as low as possible.

[0059] S: below 0.020%

[0060] Sulfur (S) is an impurity contained in steel substrates. S in steel substrates can sometimes form sulfides, which reduces the toughness of the steel substrate. The S content should ideally be as low as possible.

[0061] Al: below 0.10%

[0062] Aluminum (Al) is typically used for deoxidation of steel. However, when the Al content is high, the Ac3 point of the steel substrate increases, thus requiring a higher heating temperature to ensure the hardenability of the steel during hot stamping. Therefore, the Al content is preferably 0.10% or less. More preferably, it is 0.05% or less, and even more preferably 0.01% or less.

[0063] Ti: 0.01~0.10%

[0064] Titanium (Ti) is one of the strength-enhancing elements. If the Ti content is less than 0.01%, the strength and oxidation resistance cannot be fully improved. On the other hand, if the Ti content exceeds 0.10%, carbides and nitrides will form, and the steel may become soft.

[0065] B: 0.0001~0.0100%

[0066] Boron (B) plays a role in increasing strength during quenching. If the B content is less than 0.0001%, the strength-enhancing effect cannot be fully obtained. On the other hand, if the B content exceeds 0.0100%, inclusions will form, the steel substrate will become embrittled, and the fatigue strength may decrease.

[0067] N: below 0.015%

[0068] Nitrogen (N) is an impurity contained in the steel substrate. N in the steel substrate sometimes forms nitrides, which reduces the toughness of the steel substrate. Furthermore, N in the steel substrate sometimes bonds with boron (B), reducing the amount of B dissolved in the substrate and decreasing the effect of B on improving hardenability. The N content is preferably as low as possible. The upper limit of the N content is preferably 0.010%.

[0069] The steel substrate 10 contains one or more of Cr, Mo, Ni, Cu, and Nb. Cr, Mo, Ni, Cu, and Nb are all arbitrary elements. That is, the steel substrate 10 may also not contain any or all of Cr, Mo, Ni, Cu, and Nb.

[0070] Cr: 0–1.0%

[0071] Chromium (Cr) improves hardenability and increases temper softening resistance through carbide formation. It also improves corrosion resistance and is effective in increasing high-temperature strength. Therefore, it can be included as needed. The lower limit of Cr content is preferably 0.01%. On the other hand, even when the content exceeds 1.0%, the effect saturates, leading to increased costs.

[0072] Mo: 0–1.0%

[0073] Molybdenum (Mo) readily forms carbides, increasing the resistance to tempering softening. This effect is amplified by its combination with chromium (Cr). Furthermore, a small amount improves hardenability, raises the grain coarsening temperature, and significantly reduces temper brittleness. Therefore, it can be included as needed. The lower limit for Mo content is preferably 0.01%. On the other hand, even when the content exceeds 1.0%, the effect saturates, leading to increased costs.

[0074] Ni: 0–1.0%

[0075] Nickel (Ni) significantly lowers the Al phase transformation point, improving strength, toughness, and hardenability. Synergistic effects are easily achieved through its combined addition with Cr and Mo. Furthermore, it improves corrosion resistance and inhibits low-temperature embrittlement. Therefore, it can be included as needed. The lower limit for Ni content is preferably 0.01%. On the other hand, even when the content exceeds 1.0%, the effect saturates, leading to increased costs.

[0076] Cu: 0–1.0%

[0077] Copper (Cu) improves hardenability and corrosion resistance. Therefore, it can be included as needed. The lower limit of Cu content is preferably 0.01%. On the other hand, even if the content exceeds 1.0%, the effect saturates, leading to increased costs.

[0078] Nb: 0–1.0%

[0079] Niobium (Nb) improves hardenability. Nb has a larger metallic radius and higher density than Fe, the main component of steel, making it difficult to dissolve in the Fe matrix. It precipitates at the grain boundaries of the steel, thus preventing grain coarsening. It also suppresses temper brittleness. Therefore, it can be included as needed. The lower limit of Nb content is preferably 0.01%. On the other hand, even when the content exceeds 1.0%, the effect saturates, leading to increased costs.

[0080] The chemical composition of the steel substrate 10 consists of Fe and impurities. Impurities, as referred to here, are elements introduced from ores or waste used as raw materials for steel, or from the environment during the manufacturing process. Examples of impurities, in addition to those listed above, include Zn, Co, Sn, V, As, Zr, Ca, and Mg.

[0081] [Manufacturing method of Al-coated steel sheet]

[0082] Next, an example of the manufacturing method of Al-plated steel sheet 1 will be described.

[0083] [Plating Process]

[0084] An Al coating 20 is formed on the surface of a steel substrate 10 using a hot-dip galvanizing method. The preferred temperature of the plating bath is 600–700°C. If the temperature of the plating bath is below 600°C, the plating bath becomes low in viscosity, making it difficult to achieve uniform plating. If the temperature of the plating bath is above 700°C, the composition changes rapidly due to volatilization, making process management more difficult.

[0085] The plating bath contains Si in addition to Al. The Si content in the plating bath is, for example, 1–20% by mass, preferably 5–15% by mass. The plating bath may also contain elements other than Al and Si. Specifically, in addition to Fe, Al, Si, O, and H, the plating bath sometimes contains Be, Mg, Ca, Sr, Ba, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Zr, Y, Nb, Ce, and Ta. The metal composition ratio of the Al coating 20 and the oxide layer 30 varies depending on the chemical composition of the plating bath, but it is acceptable as long as the coating formed in the plating process is primarily Al-based. The lower limit of the Al content in the plating bath is preferably 70% by mass, preferably 80% by mass, more preferably 85% by mass, and even more preferably 88% by mass.

[0086] The steel substrate 10 is immersed in a plating bath after being kept at, for example, 700–800°C. The plating process is preferably carried out entirely in a non-oxidizing atmosphere (including a reducing atmosphere). This is because, in an oxidizing atmosphere, the surface of the steel substrate 10 may oxidize, sometimes resulting in uneven plating, and losses may occur due to oxidation of the plating bath. The thickness of the Al coating 20 can be adjusted by the temperature, viscosity, immersion time, and gas blowing of the plating bath.

[0087] The above describes the formation of a coating using hot-dip plating. However, coatings can also be formed using vapor deposition or spraying instead of hot-dip plating. In this case, an Al alloy can be used, or Al and the additive elements can be vapor-deposited or spray-deposited separately.

[0088] [Pre-oxidation process]

[0089] A pre-oxidation process is performed on the Al-coated steel sheet by heating it under specified conditions. Specifically, the Al-coated steel sheet is heated to a temperature of 120 to 600°C (any temperature within the range of 120°C to 600°C).

[0090] If the pre-oxidation temperature is below 120°C, dehydration from the initial oxide layer proceeds slowly, and sometimes the ratio of the thickness d1 of the hydroxide layer 31 to the thickness d of the oxide layer 30 cannot be sufficiently reduced. On the other hand, if the pre-oxidation temperature exceeds 600°C, it is sometimes difficult to properly control the thickness d of the oxide layer 30. Furthermore, since the Al coating 20 forms an alloy with the steel substrate 10 or the steel substrate 10 deteriorates, its processability sometimes decreases, making it unsuitable as a steel sheet for hot stamping. The lower limit of the pre-oxidation temperature is preferably 150°C, more preferably 180°C. The upper limit of the pre-oxidation temperature is preferably 400°C, more preferably 300°C, and more preferably 250°C.

[0091] The holding time also depends on temperature and other conditions, but ranges from 1 minute to 48 hours. The lower limit of the holding time is preferably 20 minutes, more preferably 40 minutes. The upper limit of the holding time is preferably 24 hours, more preferably 12 hours, more preferably 4 hours, and even more preferably 2 hours.

[0092] The pre-oxidation process is preferably carried out in an oxidizing atmosphere, and from a cost perspective, it is particularly preferred to be carried out in the atmosphere. Of course, the pre-oxidation process can be carried out as long as it is not in an excessively non-oxidizing atmosphere; for example, even under reduced pressure, a reducing atmosphere, an inert gas atmosphere, or a high dew point environment, the desired oxide layer 30 can be formed depending on the conditions. The atmosphere for the pre-oxidation process preferably has a dew point of -70°C or higher, more preferably -30°C or higher, and even more preferably 0°C or higher. The atmosphere for the pre-oxidation process preferably has an oxygen partial pressure of 0.001 MPa or higher, and even more preferably 0.01 MPa or higher.

[0093] The heating method for the pre-oxidation process is arbitrary; for example, a high-temperature furnace or electric heating can be used. The heating and cooling rates are also arbitrary, for example, they can be set to 10–1000℃ / s.

[0094] The pre-oxidation process can be performed at any time after the plating process and before the plating steel sheet is supplied to the hot stamping process. For example, the pre-oxidation process can also be performed at any time during the coiling process after the plating process, during the storage of the coiled coil, and during the unwinding of the coil before the hot stamping process.

[0095] Through the above processes, Al-plated steel sheet 1 is obtained. In Al-plated steel sheet 1, the thickness d of oxide layer 30 is 10-400 nm, and the ratio of the thickness d1 of hydroxide layer 31 to the thickness d of oxide layer 30 is less than 30%. Thus, an Al-plated steel sheet can be obtained, which can suppress hydrogen embrittlement caused by hydrogen intrusion into the steel during hot stamping due to oxidation of the Al coating, and has high reliability.

[0096] Al-plated steel sheet 1 is suitable for use as a hot stamping steel sheet. The hot stamping process performed on the Al-plated steel sheet 1 is not particularly limited, but can be as follows: After shaping the Al-plated steel sheet 1 to the specified dimensions, it is heated. The heating method can be any type of heating, such as a high-temperature furnace or electric heating. The holding temperature is preferably set to 850–950°C, and the holding time is preferably set to 2 minutes or more. After heating, it is formed using a mold while simultaneously being cooled using the mold.

[0097] [Manufacturing method of hot-stamped parts]

[0098] One embodiment of the present invention relates to a method for manufacturing a hot-stamped formed article, which includes a step of hot-stamping an Al-plated steel sheet 1. The Al-plated steel sheet 1 can suppress hydrogen embrittlement caused by hydrogen intruding into the steel during the oxidation of the Al coating during hot stamping. Therefore, according to the method for manufacturing a hot-stamped formed article based on this embodiment, a hot-stamped formed article with suppressed hydrogen embrittlement can be obtained.

[0099] Example

[0100] The present invention will be described in more detail below through examples. However, the present invention is not limited to these examples.

[0101] For steel plates with the chemical composition shown in Table 1, an Al coating was formed on both sides of the steel plate using a hot-dip galvanizing method.

[0102] [Table 1]

[0103] Table 1

[0104]

[0105] The chemical composition of the plating bath is Al-10% by mass, Si-2% by mass, and Fe. The Fe in the plating bath is inevitably supplied from the plating equipment and the steel sheet. The temperature of the plating bath during hot-dip plating is 700℃. After immersing the steel sheet in the plating bath, the adhesion amount is adjusted to 70 g / m² per single side using a gas wiping method. 2 .

[0106] Then, a pre-oxidation process was performed under the conditions shown in Table 2 below to produce Al-plated steel sheets. Multiple Al-plated steel sheets that underwent the pre-oxidation process under the same conditions were produced, and a portion of them were subjected to surface analysis using XPS.

[0107] [Surface Analysis Using XPS]

[0108] XPS measurements were performed using a PHIQuantera SXM manufactured by ULVAC PHI Corporation. The X-ray source used monochromatic Al Kα rays (1486.6 eV), and the X-ray irradiation area was a region with a diameter of approximately 100 μm.

[0109] Repeated short-duration sputtering and XPS measurements were performed to obtain depth-direction curves of the integrated intensity of oxides and hydroxides. Using Ar... + Ion sputtering was performed at an accelerating voltage of 4 kV, with a sputtering area of ​​1 mm × 1 mm. The sputtering rate, converted to SiO2, was 75.1 nm / min.

[0110] Based on the depth-direction curves of the integrated intensity of oxides and hydroxides, the thickness d of oxide layer 30 and the thickness d1 of hydroxide layer 31 are determined using the method described above.

[0111] [Hot stamping process and hydrogen analysis]

[0112] The pre-oxidized Al-coated steel sheet is heated in a resistance furnace at 900°C for 5 minutes. Then, it is cooled in a mold while being formed, resulting in a hot-stamped part.

[0113] The hot-stamped body was stored in liquid nitrogen, and the hydrogen content was quantified by direct temperature-programmed desorption analysis. The cumulative hydrogen content up to 250℃ was calculated. It should be noted that if the hydrogen content exceeds 0.7 ppm by mass, it is considered unsuitable for steel of the 1.5 GPa grade; if it exceeds 0.5 ppm by mass, it is considered unsuitable for steel of the 1.8 GPa grade.

[0114] The conditions of the pre-oxidation process, the results of surface analysis using XPS, and the results of hydrogen analysis are shown in Table 2.

[0115] [Table 2]

[0116]

[0117] As shown in Table 2, the oxide layer thickness d of the Al-plated steel sheets designated A01 to A11 ranges from 10 to 400 nm, and the ratio of the hydroxide layer thickness d1 to the oxide layer thickness d is less than 30%. The hydrogen content of the hot-stamped bodies manufactured from these Al-plated steel sheets is less than 0.2 ppm by mass.

[0118] In contrast, the hydrogen content of hot-stamped bodies made from Al-plated steel sheets designated a1 to a8 is all above 0.5 ppm by mass.

[0119] The Al-plated steel sheet designated a1 is an example of a case where no pre-oxidation process has been performed. In the Al-plated steel sheet designated a1, the oxide layer thickness d is less than 10 nm, and the ratio of the hydroxide layer thickness d1 to the oxide layer thickness d is greater than 30%.

[0120] The thickness d of the oxide layer of Al-plated steel sheets designated a2, a4 and a6 is in the range of 10 to 400 nm, but the ratio of the thickness d1 of the hydroxide layer to the thickness d of the oxide layer is higher than 30%.

[0121] For Al-plated steel sheets designated a3, the ratio of the hydroxide layer thickness d1 to the oxide layer thickness d is less than 30%, but the oxide layer thickness d is greater than 400 nm. For Al-plated steel sheets designated a5 and a7, the ratio of the hydroxide layer thickness d1 to the oxide layer thickness d is less than 30%, but the oxide layer thickness d is less than 10 nm. For Al-plated steel sheets designated a8, the ratio of the hydroxide layer thickness d1 to the oxide layer thickness d is greater than 30%, and the oxide layer thickness d is also less than 10 nm.

[0122] The embodiments of the present invention have been described above, but these embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above embodiments, and can be implemented by appropriate modifications without departing from its spirit.

[0123] Explanation of reference numerals in the attached figures

[0124] 1 Al-coated steel sheet

[0125] 10 Steel substrate

[0126] 20 Al coating

[0127] 30 Oxide Layer

[0128] 31 Hydroxide layer

[0129] 32 Oxide Layer

Claims

1. An Al-coated steel sheet for hot stamping, comprising: Steel substrate; An Al coating, which is a coating comprising Al and Si formed on the steel substrate; and An oxide layer is formed on the Al coating. The chemical composition of the steel substrate, expressed as C (by mass%), is 0.1–0.6%. Si: 0.01~1.50% Mn: 0.10~3.00% P: below 0.05% S: Below 0.020% Al: below 0.10% Ti: 0.01~0.10% B：0.0001～0.0100％、 N: less than 0.015% Cr：0～1.0％、 Mo: 0–1.0% Ni: 0-1.0% Cu: 0–1.0% Nb: 0–1.0% Balance: Fe and impurities, The thickness of the oxide layer is 10–400 nm. The oxide layer comprises a hydroxide layer, wherein the thickness of the hydroxide layer is less than 30% of the thickness of the oxide layer. in, The thickness of the oxide layer is defined as the depth from the surface when the integrated intensity of the oxide is half of its maximum value in depth-direction analysis using X-ray photoelectron spectroscopy, and the thickness of the hydroxide layer is defined as the depth from the surface when the integrated intensity of the hydroxide is half of its maximum value in depth-direction analysis using X-ray photoelectron spectroscopy.

2. An Al-coated steel sheet, comprising: Steel substrate; An Al coating, which is formed on the steel substrate; and An oxide layer is formed on the Al coating. The chemical composition of the steel substrate, expressed in mass percent, is C: 0.1–0.5%. Si: 0.01~1.50% Mn: 0.10~3.00% P: below 0.05% S: Below 0.020% Al: below 0.10% Ti: 0.01~0.10% B：0.0001～0.0100％、 N: less than 0.010% Cr：0～1.0％、 Mo: 0–1.0% Ni: 0-1.0% Nb: 0–1.0% Balance: Fe and impurities, The thickness of the oxide layer is 10–400 nm. The oxide layer comprises a hydroxide layer, wherein the thickness of the hydroxide layer is less than 30% of the thickness of the oxide layer. in, The thickness of the oxide layer is defined as the depth from the surface when the integrated intensity of the oxide is half of its maximum value in depth-direction analysis using X-ray photoelectron spectroscopy, and the thickness of the hydroxide layer is defined as the depth from the surface when the integrated intensity of the hydroxide is half of its maximum value in depth-direction analysis using X-ray photoelectron spectroscopy.

3. A method for manufacturing an Al-plated steel sheet according to claim 1 or 2, comprising a pre-oxidation step of heating the Al-plated steel sheet to a temperature of 120 to 600°C.

4. A method for manufacturing a hot-stamped formed body, comprising a step of hot-stamping the Al-plated steel sheet as described in claim 1 or 2.

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