WO2023286706A1

WO2023286706A1 - Al-plated steel sheet, method for manufacturing al-plated steel sheet, and method for manufacturing hot stamp molded body.

Info

Publication number: WO2023286706A1
Application number: PCT/JP2022/027091
Authority: WO
Inventors: 教史土井; 憲司小林
Original assignee: 日本製鉄株式会社
Priority date: 2021-07-14
Filing date: 2022-07-08
Publication date: 2023-01-19
Also published as: CN117396627A; JPWO2023286706A1; KR20230169265A

Abstract

Provided is an Al-plated steel sheet with which it is possible to suppress hydrogen embrittlement that progresses due to hydrogen entering a steel material with oxidation of Al plating during hot stamp molding. This Al-plated steel sheet (1) comprises a steel substrate having a prescribed chemical composition, an Al plating layer (20) that is formed on the steel substrate and that is a plating layer containing Al and Si, and an oxide layer (30) formed on the Al plating layer (20), the oxide layer (30) having a thickness d of 10-400 nm, the oxide layer (30) containing a hydroxide layer (31), and the ratio of the thickness d1 of the hydroxide layer (31) relative to the thickness d of the oxide layer (30) being 30% or less. The thickness d of the oxide layer (30) is deemed to be the depth from the surface at which, in a depth-direction analysis by X-ray photoelectron spectroscopy, the integrated intensity for oxide is 1/2 of the maximum value. The thickness d1 of the hydroxide layer 31 is deemed to be the depth from the surface at which, in a depth-direction analysis by X-ray photoelectron spectroscopy, the integrated intensity for hydroxide is 1/2 of the maximum value.

Description

Al-plated steel sheet, method for producing Al-plated steel sheet, and method for producing hot-stamped product

TECHNICAL FIELD The present invention relates to an Al-plated steel sheet, a method for producing an Al-plated steel sheet, and a hot stamped product, and more specifically, an Al-plated steel sheet for hot stamping, a method for producing the same, and a method for producing a hot stamped product using the same. Regarding.

The hot stamping method is known as a technique for forming high-strength steel (in particular, ultra-high-strength steel of 1500 MPa or more) with high dimensional accuracy for which it is difficult to ensure formability. In the hot stamping method, the problem of formability is eliminated by forming a steel sheet while it is heated to a high temperature of 800° C. or higher, and a desired high strength is obtained by cooling after forming.

　Because the hot stamping method involves heating in the atmosphere, scale is generated on the surface of the steel sheet, which must be removed in a post-process. As a solution to this problem, a technique is known in which a steel sheet is plated with Al containing Si to suppress oxidation.

Japanese Patent Application Laid-Open No. 2003-193187 describes that crack generation during processing is suppressed by setting the Al content of the Fe—Al coating to 35% or less. International Publication No. 2019/160106 describes an Fe—Al plated hot stamp member having a predetermined structure of the Fe—Al plated layer in order to improve the corrosion resistance of the molded part and the corrosion resistance after painting.

In the hot stamping method, it is said that the susceptibility of steel to hydrogen embrittlement increases because hydrogen penetrates into the steel during heating. In International Publication No. 2012/120692, in order to improve delayed fracture resistance, an Fe—Mn-based composite oxide is generated in a steel plate, and hydrogen is trapped at the interface between the composite oxide and the matrix steel. is stated.

Japanese Patent Application Laid-Open No. 2003-193187 WO2019/160106 WO2012/120692 JP 2016-65312 A JP 2022-31677 A Japanese Patent Publication No. 2022-513595 JP-A-6-272017

When a hot stamping process is performed on an Al-plated steel sheet, hydrogen may enter the steel material as the Al plating is oxidized.

An object of the present invention is to provide a highly reliable Al-plated steel sheet that can suppress hydrogen embrittlement that progresses due to hydrogen entering the steel material accompanying oxidation of the Al plating during hot stamping. and a method for producing the same. Another object of the present invention is to provide a hot-stamped molded article in which hydrogen embrittlement is suppressed.

An Al-plated steel sheet according to one embodiment of the present invention is an Al-plated steel sheet for hot stamping, and includes a steel substrate and an Al plated layer containing Al and Si formed on the steel substrate. It comprises a plating layer and an oxide layer formed on the Al plating layer, and the chemical composition of the steel base material is C: 0.1 to 0.6% and Si: 0.01 to 0.01% by mass. 1.50%, Mn: 0.10-3.00%, P: 0.05% or less, S: 0.020% or less, Al: 0.10% or less, Ti: 0.01-0.10% , B: 0.0001 to 0.0100%, N: 0.015% or less, Cr: 0 to 1.0%, Mo: 0 to 1.0%, Ni: 0 to 1.0%, Cu: 0 ~1.0%, Nb: 0 to 1.0%, the balance: Fe and impurities, the thickness of the oxide layer is 10 to 400 nm, the oxide layer includes a hydroxide layer, and the A ratio of the thickness of the hydroxide layer to the thickness of the oxide layer is 30% or less. Here, the thickness of the oxide layer is defined as the depth from the surface when the integrated intensity of the oxide becomes 1/2 of the maximum value in the depth direction analysis by X-ray photoelectron spectroscopy, and the hydroxide layer The thickness of is defined as the depth from the surface when the integrated intensity of hydroxide becomes half of the maximum value in depth direction analysis by X-ray photoelectron spectroscopy.

A method for manufacturing an Al-plated steel sheet according to one embodiment of the present invention is the above-described method for manufacturing an Al-plated steel sheet, and includes a preliminary oxidation step of heating the Al-plated steel sheet to a temperature of 120 to 600°C.

A hot-stamped compact according to one embodiment of the present invention includes a step of hot-stamping the above Al-plated steel sheet.

According to the present invention, it is possible to obtain a highly reliable Al-plated steel sheet that can suppress hydrogen embrittlement that progresses due to hydrogen entering the steel material accompanying oxidation of the Al plating during hot stamping. According to the present invention, it is also possible to obtain a hot-stamped article in which hydrogen embrittlement is suppressed.

FIG. 1 is a schematic cross-sectional view of an Al-plated steel sheet according to one embodiment of the present invention. FIG. 2 is a schematic cross-sectional view showing an enlarged structure of the oxide layer in FIG. FIG. 3 is an example of an O 1s spectrum measured by XPS. FIG. 4 is an example depth profile of the integrated intensity of oxides and hydroxides.

The inventors studied the relationship between the oxide layer formed on the surface of the Al-plated steel sheet and the amount of hydrogen that penetrates into the steel when the Al-plated steel sheet is subjected to the hot stamping process. In the course of the study, it was found that the amount of hydrogen that penetrates into the steel material is reduced by performing a preliminary oxidation step of heating the Al-plated steel sheet under predetermined conditions and then performing a hot stamping step.

An Al-plated steel sheet has an oxide layer (initial oxide layer) with a thickness of several nanometers on the surface immediately after plating. This initial oxide layer includes, in order from the surface side, a hydroxide layer containing a large amount of hydroxide and an oxide layer containing a small amount of hydroxide. Hydroxides here include OH groups (hydroxyl groups) in their structure, such as Al(OH) ₃ and AlOOH. On the other hand, an oxide is represented by a chemical formula of only metal and oxygen, such as Al ₂ O ₃ and AlFeO ₄ . In the initial oxide layer, the ratio of the thickness of the hydroxide layer to the thickness of the oxide layer is relatively high. When the Al-plated steel sheet is subjected to the preliminary oxidation step, the oxide layer becomes thicker and the ratio of the thickness of the hydroxide layer to the thickness of the oxide layer decreases.

　The hydrogen that enters the steel material during the hot stamping process is assumed to be hydrogen from the hydroxide on the surface of the steel material and hydrogen from the moisture in the atmosphere. It is believed that the pre-oxidation step described above contributes to the reduction of both hydrogens. That is, the preliminary oxidation process promotes the release of hydrogen from the hydroxide on the surface of the steel material, and the oxide layer with reduced hydroxide functions as a protective layer against further oxidation, suppressing the oxidation reaction in the hot stamping process. It is thought that the dissociation reaction of moisture in the atmosphere is suppressed.

The present invention was completed based on the above findings. BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The same reference numerals are given to the same or corresponding parts in the drawings, and the description thereof will not be repeated. The dimensional ratios between the components shown in each drawing do not necessarily represent the actual dimensional ratios.

[Al plated steel sheet]
FIG. 1 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 plating layer 20 formed on the steel substrate 10 , and an oxide layer 30 formed on the Al plating layer 20 . The Al plating layer 20 and the oxide layer 30 may be formed on one side of the steel base 10 or may be formed on both sides of the steel base 10 .

[Oxidation layer]
The oxide layer 30 is a layer formed by oxidizing the surface of the Al plating layer 20, and contains Al oxide and Al hydroxide. Al oxides are, for example, Al ₂ O ₃ and AlFeO ₄ . Al hydroxides are, for example, Al(OH) ₃ and AlOOH. The oxide layer 30 may contain oxides other than Al oxides and hydroxides other than Al hydroxides.

FIG. 2 is a schematic cross-sectional view showing an enlarged configuration of the oxide layer 30. As shown in FIG. The oxide layer 30 includes a hydroxide layer 31 that is a layer containing a large amount of hydroxide and an oxide layer 32 that is a layer containing a small amount of hydroxide. Hydroxide contained in the oxide layer 30 is mostly distributed on the surface side of the oxide layer 30 . Therefore, a hydroxide layer 31 is formed on the surface side and an oxide layer 32 is formed on the substrate side.

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.

The oxide layer 30 functions as a protective layer against further oxidation of the Al-plated steel sheet 1. That is, by forming the oxide layer 30 having a predetermined thickness on the Al-plated steel sheet 1 in advance, the oxidation reaction in the hot stamping process can be suppressed. This can reduce the amount of hydrogen that penetrates into the steel substrate 10 during the hot stamping process. This effect cannot be sufficiently obtained if the thickness d of the oxide layer 30 is less than 10 nm. On the other hand, when the thickness d of the oxide layer 30 exceeds 400 nm, the influence of hydrogen in the hydroxide contained in the oxide layer 30 increases. 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, still more preferably 100 nm, still more preferably 50 nm.

The hydroxide contained in the oxide layer 30 can be a source of hydrogen that penetrates into the steel substrate 10. By setting the ratio of the thickness d1 of the hydroxide layer 31 to the thickness d of the oxide layer 30 to 30% or less, the amount of hydrogen that penetrates into the steel material 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, still more preferably 15% or less.

The thickness d of the oxide layer 30 and the thickness d1 of the hydroxide layer 31 are measured by depth direction analysis by X-ray photoelectron spectroscopy (XPS) as follows.

Fig. 3 is an example of an O 1s spectrum measured by XPS. The O 1s spectrum contains an oxide peak and a hydroxide peak. The background is subtracted from this O 1s spectrum, the oxide peak and the hydroxide peak are waveform-separated, and the integrated intensity of each is obtained. Background processing uses the Shirley method typical of XPS data processing. The integrated intensity ratio obtained here agrees with the existing ratio of oxide and hydroxide at the depth being measured.

Sputtering and XPS measurements are repeated while sputtering with Ar ^{2 +} ions to obtain depth profiles of the integrated intensity of oxides and hydroxides. FIG. 4 is an example depth profile of the integrated intensity of oxides and hydroxides.

Here, the layer boundary between the hydroxide layer 31 and the oxide layer 32 is defined as the position where the integrated intensity of hydroxide is 1/2 of the maximum value. A layer boundary between the oxide layer 32 and the Al plating layer 20 is defined as a position where the integrated intensity of the oxide is 1/2 of the maximum value.

That is, the thickness d1 of the hydroxide layer 31 is the depth from the surface when the integrated intensity of the hydroxide becomes 1/2 of the maximum value. Also, the thickness d of the oxide layer 30 is the depth from the surface when the integrated intensity of the oxide is 1/2 of the maximum value. 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 .

[Al plating layer]
The Al plating layer 20 is a plating layer containing Al and Si. In the Al plating layer 20, an intermetallic compound of Al, Fe and Si may be formed. The chemical composition of the Al plating layer 20 (average composition in the thickness direction, hereinafter the same in this paragraph) is not limited to this, for example, Al: 20 to 100% by mass, Si: 1 to 20% by mass, Fe: 0 to 60% by mass. The lower limit of the Al content of the Al plating layer 20 is preferably 25% by mass. The upper limit of the Al content of the Al plating layer 20 is preferably 95% by mass, more preferably 90% by mass, still more preferably 70% by mass, still more preferably 55% by mass. The lower limit of the Si content of the Al plating layer 20 is preferably 2% by mass, more preferably 5% by mass. The upper limit of the Si content of the Al plating layer 20 is preferably 15% by mass, more preferably 12% by mass. The lower limit of the Fe content of the Al plating layer 20 is preferably 20% by mass. The upper limit of the Fe content of the Al plating layer 20 is preferably 50% by mass, more preferably 40% by mass.

The Al plating layer 20 may contain elements other than Al, Si, and Fe. Specifically, Be, Mg, Ca, Sr, Ba, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Zr, Y, Nb, Ce, and Ta may be added. be. 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, still more preferably 3% by mass or less, and still more preferably 1% by mass. It is below.

Although the thickness of the Al plating layer 20 is not particularly limited, it is, for example, 1 to 100 μm. The lower limit of the thickness of the Al plating layer 20 is preferably 5 μm, more preferably 10 μm, still more preferably 15 μm. The upper limit of the thickness of the Al plating layer 20 is preferably 50 μm, more preferably 40 μm, and still more preferably 30 μm.

[Steel substrate]
The steel base material 10 is, for example, a hot-rolled steel plate or a cold-rolled steel plate. The chemical composition of the steel base material 10 will be described below. In the following description, "%" of element content means % by mass.

C: 0.1-0.6%
Carbon (C) is contained to ensure the desired mechanical strength. If the C content is less than 0.1%, sufficient improvement in mechanical strength cannot be obtained. On the other hand, when the C content exceeds 0.6%, the elongation and reduction of area are likely to decrease. The upper limit of the C content is preferably 0.5%.

Si: 0.01-1.50%
Silicon (Si) is an element that improves mechanical strength, and like C, is contained to ensure the desired 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 influence of Si oxides formed on the surface layer of the steel base material may reduce the wettability during plating, resulting in non-plating. The upper limit of Si content is preferably 0.60%.

Mn: 0.10-3.00%
Manganese (Mn) is one of the strengthening elements that strengthen steel and is also one of the elements that improve hardenability. Mn is also effective in preventing hot shortness due to S, which is one of the impurities. If the Mn content is less than 0.10%, these effects cannot be sufficiently obtained. On the other hand, if the Mn content exceeds 3.00%, the amount of retained austenite may become too large and the strength may decrease. The lower limit of Mn content is preferably 0.50%. The upper limit of the Mn content is preferably 2.00%.

P: 0.05% or less Phosphorus (P) is an impurity contained in the steel base material. P contained in the steel base may segregate at the grain boundaries of the steel base and reduce the toughness of the steel base. It is preferable to reduce the P content as much as possible.

S: 0.020% or less Sulfur (S) is an impurity contained in the steel base material. S contained in the steel substrate may form sulfides and reduce the toughness of the steel substrate. It is preferable to reduce the S content as much as possible.

Al: 0.10% or less Aluminum (Al) is generally used for the purpose of deoxidizing steel. However, when the Al content is high, the Ac ₃ point of the steel substrate increases, so it is necessary to raise the heating temperature necessary to ensure the hardenability of the steel during the hot stamping process. Therefore, the Al content is preferably 0.10% or less. The Al content is more preferably 0.05% or less, still more preferably 0.01% or less.

Ti: 0.01-0.10%
Titanium (Ti) is one of strength enhancing elements. If the Ti content is less than 0.01%, the effect of improving strength and the effect of improving oxidation resistance cannot be sufficiently obtained. On the other hand, when the Ti content exceeds 0.10%, carbides and nitrides are formed, which may soften the steel.

B: 0.0001 to 0.0100%
Boron (B) acts during quenching and has the effect of improving strength. If the B content is less than 0.0001%, a sufficient strength improvement effect cannot be obtained. On the other hand, if the B content exceeds 0.0100%, inclusions may be formed to embrittle the steel base material and reduce the fatigue strength.

N: 0.015% or less Nitrogen (N) is an impurity contained in the steel base material. N contained in the steel substrate may form nitrides and reduce the toughness of the steel substrate. Furthermore, N contained in the steel base material may combine with B to reduce the amount of solid solution B, thereby reducing the hardenability improvement effect of B. It is preferable to reduce the N content as much as possible. The upper limit of the N content is preferably 0.010%.

The steel base material 10 may contain one or more of Cr, Mo, Ni, Cu, and Nb. Cr, Mo, Ni, Cu, and Nb are all optional elements. That is, the steel base material 10 may not contain any or all of Cr, Mo, Ni, Cu, and Nb.

Cr: 0-1.0%
Chromium (Cr) improves hardenability and increases temper softening resistance through carbide formation. It is also effective in improving corrosion resistance and high-temperature strength. Therefore, it may be contained as necessary. The lower limit of Cr content is preferably 0.01%. On the other hand, even if the content exceeds 1.0%, the effect is saturated, leading to an increase in cost.

Mo: 0-1.0%
Molybdenum (Mo) easily forms carbides and increases temper softening resistance. The effect is enhanced by combined addition with Cr. In addition, a small amount of N improves hardenability, raises the grain coarsening temperature, and is highly effective in preventing temper embrittlement. Therefore, it may be contained as necessary. The lower limit of Mo content is preferably 0.01%. On the other hand, even if the content exceeds 1.0%, the effect is saturated, leading to an increase in cost.

Ni: 0-1.0%
Nickel (Ni) significantly lowers the A1 transformation point and improves strength, toughness and hardenability. Combined addition of Cr and Mo tends to produce a synergistic effect. It also improves corrosion resistance and suppresses low-temperature embrittlement. Therefore, it may be contained as necessary. The lower limit of the Ni content is preferably 0.01%. On the other hand, even if the content exceeds 1.0%, the effect is saturated, leading to an increase in cost.

Cu: 0-1.0%
Copper (Cu) improves hardenability and corrosion resistance. Therefore, it may be contained as necessary. The lower limit of Cu content is preferably 0.01%. On the other hand, even if the content exceeds 1.0%, the effect is saturated, leading to an increase in cost.

Nb: 0-1.0%
Niobium (Nb) improves hardenability. Nb has a larger metal radius than Fe, which is the main component of steel, and has a higher density. have. It also suppresses temper embrittlement. Therefore, it may be contained as necessary. The lower limit of the Nb content is preferably 0.01%. On the other hand, even if the content exceeds 1.0%, the effect is saturated, leading to an increase in cost.

The rest of the chemical composition of the steel base 10 is Fe and impurities. The term "impurities" as used herein refers to elements mixed in from ores and scraps used as raw materials for steel, or elements mixed in from the environment during the manufacturing process. Impurities include, for example, Zn, Co, Sn, V, As, Zr, Ca, Mg, etc., in addition to the elements listed above.

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

[Plating process]
An Al plating layer 20 is formed on the surface of the steel base 10 by hot dip plating. The temperature of the plating bath is preferably 600-700°C. If the temperature of the plating bath is lower than 600° C., the viscosity of the plating bath will be low, making uniform plating difficult. If the temperature of the plating bath is higher than 700° C., the components change in a short time due to volatilization, making process control difficult.

The plating bath contains Si in addition to Al. The Si content in the plating bath is, for example, 1-20 mass %, preferably 5-15 mass %. The plating bath may contain elements other than Al and Si. Specifically, in addition to Fe, Al, Si, O, and H, the plating bath contains Be, Mg, Ca, Sr, Ba, Sc, Ti, V, Cr, Mn, Co, Ni, Cu, and Zn. , Zr, Y, Nb, Ce, and Ta may be added. The metal component ratios of the Al plated layer 20 and the oxide layer 30 are subject to change depending on the chemical composition of the plating bath, but are applicable as long as the plated layer formed in the plating process is mainly composed of Al. 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, still more preferably 88% by mass.

After holding the steel substrate 10 at, for example, 700 to 800° C., it is immersed in a plating bath. The plating process is preferably carried out 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, resulting in non-uniform plating, and in addition, loss may occur due to oxidation of the plating bath. The thickness of the Al plating layer 20 can be adjusted by the temperature, viscosity, immersion time, gas spraying, etc. of the plating bath.

Although the case where the plating layer is formed by the hot dipping method has been described above, the plating layer may be formed by vapor deposition or thermal spraying instead of the hot dipping method. In this case, an Al alloy may be used, or Al and the additive element may be vapor-deposited or thermally sprayed separately.

[Preliminary oxidation step]
A preliminary oxidation step is performed in which the Al-plated steel sheet is heated under predetermined conditions. Specifically, the Al-plated steel sheet is heated to a temperature of 120 to 600° C. (an arbitrary temperature within the range of 120° C. to 600° C.).

If the temperature of the preliminary oxidation is lower than 120° C., the progress of dehydration from the initial oxidation layer is slow, and the ratio of the thickness d1 of the hydroxide layer 31 to the thickness d of the oxide layer 30 cannot be sufficiently reduced. There is On the other hand, if the preliminary oxidation temperature exceeds 600° C., it may become difficult to appropriately control the thickness d of the oxide layer 30 . In addition, the Al plating layer 20 may form an alloy with the steel base 10 or the steel base 10 may be deteriorated, resulting in reduced workability and unsuitability 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, still more preferably 250°C.

The holding time depends on the temperature and other conditions, but is for example 1 minute to 48 hours. The lower limit of the retention time is preferably 20 minutes, more preferably 40 minutes. The upper limit of the retention time is preferably 24 hours, more preferably 12 hours, still more preferably 4 hours, still more preferably 2 hours.

The preliminary oxidation step is preferably carried out in an oxidizing atmosphere, and particularly preferably in the air from the viewpoint of cost. However, the preliminary oxidation step can be performed in any atmosphere other than an excessively non-oxidizing atmosphere. , the desired oxide layer 30 can be formed depending on the conditions. The atmosphere of the preliminary oxidation step preferably has a dew point of −70° C. or higher, more preferably −30° C. or higher, further preferably 0° C. or higher. The atmosphere of the preliminary oxidation step preferably has an oxygen partial pressure of 0.001 MPa or more, more preferably 0.01 MPa or more.

The heating method for the preliminary oxidation process is arbitrary, and for example, a high-temperature furnace or electric heating can be used. The temperature rise rate and temperature drop rate are arbitrary, and can be, for example, 10 to 1000° C./s.

The preliminary oxidation process can be performed at any time after the plating process and before the plated steel sheet is subjected to the hot stamping process. For example, the preliminary oxidation process may be performed at any timing during the winding process after the plating process, during roll storage after winding, or during roll development before the hot stamping process.

The Al-plated steel sheet 1 is manufactured through the above steps. In the Al-plated steel sheet 1, 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. As a result, it is possible to obtain a highly reliable Al-plated steel sheet that can suppress hydrogen embrittlement, which progresses due to hydrogen entering the steel material accompanying oxidation of the Al plating during hot stamping.

The Al-plated steel sheet 1 can be suitably used as a steel sheet for hot stamping. Although the hot stamping process applied to the Al-plated steel sheet 1 is not particularly limited, the following is an example. After shaping the Al-plated steel sheet 1 into a predetermined size, it is heated. The heating method may be either a high-temperature furnace or electric heating. The holding temperature is preferably 850 to 950° C., and the holding time is preferably 2 minutes or longer. After heating, it is molded in a mold and cooled in the mold at the same time.

[Manufacturing method of hot stamp molded body]
A method for manufacturing a hot-stamped product according to one embodiment of the present invention includes a step of hot-stamping an Al-plated steel sheet 1 . The Al-plated steel sheet 1 can suppress hydrogen embrittlement that progresses due to hydrogen entering the steel material accompanying oxidation of the Al plating during hot stamping. Therefore, according to the method for manufacturing a hot-stamped article according to the present embodiment, a hot-stamped article in which hydrogen embrittlement is suppressed can be obtained.

The present invention will be described in more detail below with reference to examples. The invention is not limited to these examples.

For the steel sheet having the chemical composition shown in Table 1, Al plating layers were formed on both sides of the steel sheet by hot dip plating.

The chemical composition of the plating bath was Al-10 wt% Si-2 wt% Fe. Fe in the plating bath is inevitably supplied from plating equipment and steel sheets. The temperature of the plating bath during hot-dip plating was 700°C. After the steel sheet was immersed in the plating bath, the coating weight was adjusted to 70 g/m ² per side by gas wiping.

After that, a preliminary oxidation process was performed under the conditions shown in Table 2 below to produce an Al-plated steel sheet. A plurality of Al-plated steel sheets that had undergone the preliminary oxidation process under the same conditions were produced, and a part of them was subjected to surface analysis by XPS.

[Surface analysis by XPS]
PHI Quantera SXM manufactured by ULVAC-PHI, Inc. was used for the XPS measurement. A monochromatic Al Kα ray (1486.6 eV) was used as the X-ray light source, and the X-ray irradiation area was about 100 μm in diameter.

Short-time sputtering and XPS measurements were repeated to obtain the depth profiles of the integrated intensity of oxides and hydroxides. Sputtering with Ar ⁺ ions was performed at an acceleration voltage of 4 kV and a sputtering area of 1 mm×1 mm. The sputtering rate was 75.1 nm/min in terms of _SiO2 .

The thickness d of the oxide layer 30 and the thickness d1 of the hydroxide layer 31 were obtained from the profile of the integrated intensity of the oxide and hydroxide in the depth direction by the method described above.

[Hot stamping process and hydrogen analysis]
The Al-plated steel sheet after the preliminary oxidation step was heated in an electric resistance furnace at a furnace temperature of 900° C. for a soaking time of 5 minutes. After that, it was molded in a mold and simultaneously cooled in the mold to obtain a hot-stamped product.

The hot-stamped compact was stored in liquid nitrogen, and thermal desorption analysis was performed as it was to quantify the amount of hydrogen. A hydrogen amount integrated value up to 250° C. was obtained. It should be noted that if the amount of hydrogen exceeds 0.7 ppm by mass, it is unsuitable for 1.5 GPa class steel, and if it exceeds 0.5 mass ppm, it is considered unsuitable for 1.8 GPa class.

Table 2 shows the conditions of the preliminary oxidation process, the results of surface analysis by XPS, and the results of hydrogen analysis.

As shown in Table 2, in the Al-plated steel sheets of symbols A01 to A11, the thickness d of the oxide layer is within the range of 10 to 400 nm, and the ratio of the thickness d1 of the hydroxide layer to the thickness d of the oxide layer is The ratio was 30% or less. The amount of hydrogen in hot-stamped bodies produced from these Al-plated steel sheets was 0.2 ppm by mass or less.

On the other hand, the amount of hydrogen in the hot-stamped compacts produced from the Al-plated steel sheets of symbols a1 to a8 was 0.5 ppm by mass or more.

The Al-plated steel sheet with symbol a1 is an example in which the preliminary oxidation process was not performed. In the Al-plated steel sheet of symbol a1, the thickness d of the oxide layer was less than 10 nm, and the ratio of the thickness d1 of the hydroxide layer to the thickness d of the oxide layer was higher than 30%.

In the Al-plated steel sheets of symbols a2, a4, and a6, the thickness d of the oxide layer was within 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 was higher than 30%.

In the Al-plated steel sheet of symbol a3, the ratio of the thickness d1 of the hydroxide layer to the thickness d of the oxide layer was 30% or less, but the thickness d of the oxide layer was greater than 400 nm. In the Al-plated steel sheets of symbols a5 and a7, the ratio of the thickness d1 of the hydroxide layer to the thickness d of the oxide layer was 30% or less, but the thickness d of the oxide layer was smaller than 10 nm. In the Al-plated steel sheet of symbol a8, the ratio of the thickness d1 of the hydroxide layer to the thickness d of the oxide layer was higher than 30%, and the thickness d of the oxide layer was also smaller than 10 nm.

Although the embodiments of the present invention have been described above, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and the above-described embodiment can be modified appropriately without departing from the scope of the invention.

1 Al-plated steel sheet 10 Steel base material 20 Al-plated layer 30 Oxide layer 31 Hydroxide layer 32 Oxide layer

Claims

An Al-plated steel sheet for hot stamping,
a steel substrate;
an Al plating layer, which is a plating layer containing Al and Si, formed on the steel base;
An oxide layer formed on the Al plating layer,
The chemical composition of the steel base material is, in mass %,
C: 0.1 to 0.6%,
Si: 0.01 to 1.50%,
Mn: 0.10-3.00%,
P: 0.05% or less,
S: 0.020% or less,
Al: 0.10% or less,
Ti: 0.01 to 0.10%,
B: 0.0001 to 0.0100%,
N: 0.015% or less,
Cr: 0 to 1.0%,
Mo: 0-1.0%,
Ni: 0 to 1.0%,
Cu: 0-1.0%,
Nb: 0 to 1.0%,
Balance: Fe and impurities,
The oxide layer has a thickness of 10 to 400 nm,
The Al-plated steel sheet, wherein the oxide layer includes a hydroxide layer, and the ratio of the thickness of the hydroxide layer to the thickness of the oxide layer is 30% or less.
Here, the thickness of the oxide layer is defined as the depth from the surface when the integrated intensity of the oxide becomes 1/2 of the maximum value in the depth direction analysis by X-ray photoelectron spectroscopy, and the hydroxide layer The thickness of is defined as the depth from the surface when the integrated intensity of hydroxide becomes half of the maximum value in depth direction analysis by X-ray photoelectron spectroscopy.
A method for manufacturing an Al-plated steel sheet according to claim 1,
A method for producing an Al-plated steel sheet, comprising a preliminary oxidation step of heating the Al-plated steel sheet to a temperature of 120 to 600°C.
A method for manufacturing a hot-stamped compact, comprising the step of hot-stamping the Al-plated steel sheet according to claim 1.