Hot stamped molded parts

Abstract

To provide a hot stamp molding capable of improving delayed fracture resistance without damaging coating material adhesion and corrosion resistance after coating.SOLUTION: A hot stamp molding 1 includes a steel base material 10 and an Al coating 20 A formed on the steel base material 10. The Al coating 20 includes an interlayer 21, an intermediate layer 22 and an oxide film layer 23; the intermediate layer 22 includes an Fe-Al-Si phase 22a having a structure having a part of α Fe substituted with Al and Si; the content of Si in the Fe-Al-Si phase 22a is 1-20 mass%; the total content of Ni and Cr is 0.10-5.0 mass%; the ratio of Mg to a component except carbon and oxygen in the oxide film layer 23 is 0.01-80.0 mass%; and (1 0 -2) peak intensity α of α-Al2O3 of the oxide film layer 23 and (1 1 1) peak intensity γ of γ-Al2O3 detected by XRD satisfy the following relational expression, γ / (α+γ)≥0.50.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a hot stamping molded body.

Background Art

[0002] As a method for forming high-strength steel materials with high dimensional accuracy, hot stamping (also called hot pressing, hot press, die quenching, press quenching, etc.) is known. Hot stamping involves heating a steel material such as a steel sheet in the austenite region, performing hot forming, and obtaining desired properties by cooling after forming.

[0003] In hot stamping, scale may be generated on the surface of the steel material during heating, and it is necessary to remove this in a subsequent process. To avoid this, a technique of applying Al plating to the steel material for hot stamping to suppress scale generation is known.

[0004] Japanese Patent Publication No. 63-3929 discloses a method for manufacturing a molten aluminum-plated steel sheet that exhibits a low oxidation weight gain value during high-temperature oxidation. Japanese Patent No. 2943021 discloses a method for manufacturing an austenitic stainless steel sheet strip having a NiAl intermetallic compound on the surface layer, in which Al is coated on the surface of an austenitic stainless steel sheet strip and then diffusion heat treatment is performed in a non-oxidizing atmosphere within a temperature range of 700 to 800°C.

[0005] International Publication No. 2018 / 221738 discloses a hot stamping member having a steel material, an Al-Fe intermetallic compound layer, and an oxide film layer, wherein the oxide film layer contains one or more elements selected from the group consisting of Be, Mg, Ca, Sr, Ba, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn in a ratio of 0.01 atomic% or more and 80 atomic% or less excluding oxygen.

[0006] Japanese Patent Publication No. 2017-536472 discloses a flat steel product for hot forming, comprising a steel substrate and a protective coating mainly composed of Al. The publication states that the protective coating contains at least one alkaline earth metal or transition metal in a total amount of 0.1% to 0.5% by weight, and that oxides of the alkaline earth metal or transition metal are formed on the outer surface of the protective coating during hot forming. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Special Publication No. 63-3929 [Patent Document 2] Patent No. 2943021 [Patent Document 3] International Publication No. 2018 / 221738 [Patent Document 4] Special Publication No. 2017-536472 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] When aluminum-plated steel is hot-stamped, oxygen and moisture from the atmosphere are consumed during the oxidation process of the aluminum. The hydrogen released at this time may be absorbed by the steel, potentially causing delayed fracture.

[0009] The object of the present invention is to provide a hot-stamped molded article with improved delayed fracture resistance without impairing paint adhesion or corrosion resistance after painting. [Means for solving the problem]

[0010] A hot-stamped molded body according to one embodiment of the present invention comprises a steel substrate and an Al coating formed on the steel substrate, wherein the Al coating includes an interface layer formed at the interface with the steel substrate and having a structure in which a portion of αFe is replaced with Al and Si, an intermediate layer formed on the interface layer, and an oxide film layer formed on the intermediate layer, wherein the intermediate layer includes an Fe-Al-Si phase having a structure in which a portion of αFe is replaced with Al and Si, the Fe-Al-Si phase contains at least one of Ni and Cr, the Si content in the Fe-Al-Si phase is 1 to 20 mass%, and the total content of Ni and Cr is 0.10 to 5.0 mass%, the oxide film layer contains Mg, the ratio of Mg in the components excluding carbon and oxygen in the oxide film layer is 0.01 to 80.0 mass%, and the peak intensity α of α-Al2O3 (1 0 -2) and the peak intensity of γ-Al2O3 (1 1) detected by XRD in the oxide film layer The peak intensity γ of 1) satisfies the following relationship. γ / (α+γ)≧0.50 [Effects of the Invention]

[0011] According to the present invention, a hot-stamped molded article can be obtained that has improved delayed fracture resistance without impairing paint adhesion or corrosion resistance after painting. [Brief explanation of the drawing]

[0012] [Figure 1] Figure 1 is a schematic cross-sectional view showing the structure of a hot-stamped molded body according to one embodiment of the present invention. [Figure 2] Figure 2 shows an example of an XRD pattern containing (1 0 -2) of α-Al2O3 and (1 1 1) of γ-Al2O3. [Modes for carrying out the invention]

[0013] The inventor focused on the crystal structure of the oxide film layer formed when hot stamping was performed on an Al-plated steel sheet and conducted a detailed analysis by XRD. Specifically, the analysis was advanced focusing on α-Al2O3 and γ-Al2O3. As a result, it was found that by containing a predetermined amount of Mg and Ni or a predetermined amount of Mg and Cr in the plating layer mainly composed of Al, the formation of α-Al2O3, which should be a stable oxide in the temperature range during hot stamping, is suppressed, and a large amount of γ-Al2O3 is formed. Furthermore, it was found that by making the oxide film layer contain a large amount of γ-Al2O3, the amount of hydrogen absorbed from the atmosphere can be reduced.

[0014] The present invention has been completed based on the above findings. Hereinafter, a hot-stamped molded body according to an embodiment of the present invention will be described in detail.

[0015] FIG. 1 is a cross-sectional view schematically showing the configuration of a hot-stamped molded body 1 according to an embodiment of the present invention. The hot-stamped molded body 1 is a molded body obtained by molding a steel material plated with Al by hot stamping. The hot-stamped molded body 1 includes a steel base material 10 and an Al film 20 formed on the steel base material 10.

[0016] The Al film 20 includes an interface layer 21 formed at the interface with the steel base material 10, an intermediate layer 22 formed on the interface layer 21, and an oxide film layer 23 formed on the intermediate layer 22.

[0017] The Al film 20 of the hot-stamped molded body 1 according to the present embodiment contains at least one of Ni and Mg and Mg. However, these elements do not exist uniformly throughout the Al film 20. Ni and Cr tend to be unevenly distributed in the intermediate layer 22, and Mg tends to be unevenly distributed in the oxide film layer 23. Further, Ni and Cr also tend to be unevenly distributed in the Fe-Al-Si phase 22a, which will be described later, in the intermediate layer 22.

[0018] [Interface layer] The interface layer 21 is formed at the interface with the steel substrate 10. The interface layer 21 has a structure in which part of the bcc structure of αFe is substituted with Al and Si.

[0019] The chemical composition of the interface layer 21 has a distribution that changes along the thickness direction. For example, the chemical composition of the interface layer 21 is, on average in the thickness direction, Fe: 60.0 to 98.0 mass%, Al: 1.0 to 40.0 mass%, and Si: 1.0 to 20.0 mass%. The interface layer 21 may contain a small amount of elements other than Si, Fe, and Al. The upper limit of the total content of elements other than Si, Fe, and Al contained in the interface layer 21 is preferably 5.0 mass%, more preferably 3.0 mass%, and even more preferably 1.5 mass%. Although the boundary between the interface layer 21 and the steel substrate 10 may not be clear, in this case, the range from the interface of the intermediate layer 22 to a depth of 10 μm is regarded as the interface layer 21, and the average chemical composition of this range is determined.

[0020] The chemical composition of the interface layer 21 can be measured by analyzing the cross-section of the Al coating 20 with an electron probe microanalyzer (EPMA) or an energy dispersive spectrometer (EDS) of a scanning electron microscope (SEM).

[0021] The thickness of the interface layer 21 is not particularly limited, but is, for example, 5 to 15 μm.

[0022] [Intermediate layer] The intermediate layer 22 is formed on the interface layer 21. The intermediate layer 22 contains a Fe-Al-Si phase 22a and an Al-Fe phase 22b. The Fe-Al-Si phase 22a has a structure in which part of the bcc structure of αFe is substituted with Al and Si, similar to the interface layer 21. The Al-Fe phase 22b has the structure of Fe4Al 13 and Fe2Al5. In addition, there may be Si dissolved in the Al-Fe phase 22b.

[0023] The Fe-Al-Si phase 22a and the Al-Fe phase 22b can be distinguished by analyzing the chemical composition of a cross-section of the Al coating 20 using EPMA or EDS on a scanning electron microscope (SEM). The Fe-Al-Si phase 22a and the Al-Fe phase 22b can also be distinguished by analyzing the crystal structure using X-ray diffraction or electron diffraction.

[0024] Although the interface layer 21 and the Fe-Al-Si phase 22a have similar structures, they can be distinguished by their location. Specifically, the interface layer 21 is formed at the interface with the steel substrate 10, while the Fe-Al-Si phase 22a is formed at an intermediate position in the thickness direction of the intermediate layer 22.

[0025] The Si content in the Fe-Al-Si phase 22a is 1.0 to 20.0 mass%. If the Si content is too low, the oxide film layer 23 will grow, which may result in an increase in hydrogen absorption by the steel substrate 10. On the other hand, if the Si content is too high, the intermediate layer 22 will not grow sufficiently, which may also result in an increase in hydrogen absorption by the steel substrate 10. The lower limit of the Si content in the Fe-Al-Si phase 22a is more preferably 5.0 mass%, and even more preferably 8.0 mass%. The upper limit of the Si content in the Fe-Al-Si phase 22a is more preferably 18.0 mass%, and even more preferably 16.0 mass%.

[0026] The Fe content in the Fe-Al-Si phase 22a is not particularly limited, but is, for example, 30.0 to 80.0 mass%. The Al content in the Fe-Al-Si phase 22a is not particularly limited, but is, for example, 5.0 to 50.0 mass%.

[0027] Fe-Al-Si phase 22a contains at least one of Ni and Cr in addition to Fe, Al, and Si.

[0028] The total content of Ni and Cr in the Fe-Al-Si phase 22a is 0.10 to 5.0 mass%. If the content of Ni and Cr in the Fe-Al-Si phase 22a is too low, the formation of α-Al2O3 cannot be sufficiently suppressed, and an oxide film layer 23 containing a large amount of γ-Al2O3 may not be formed. On the other hand, if the content of Ni and Cr in the Fe-Al-Si phase 22a is too high, cracks are more likely to occur in the intermediate layer 22, which may reduce paint adhesion and corrosion resistance after painting. The lower limit of the total content of Ni and Cr in the Fe-Al-Si phase 22a is preferably 0.20 mass%, and more preferably 0.30 mass%. The upper limit of the total content of Ni and Cr in the Fe-Al-Si phase 22a is preferably 4.0 mass%, and more preferably 3.0 mass%.

[0029] The Fe-Al-Si phase 22a may contain small amounts of elements other than Si, Fe, Al, Ni, and Cr. The elements that may be contained in the Fe-Al-Si phase 22a are not limited to these, but include, for example, Mn, Zr, Ce, Y, Ta, Cu, Nb, Co, V, and Ti. The upper limit of the total content of elements other than Si, Fe, Al, Ni, and Cr in the Fe-Al-Si phase 22a is preferably 3.0 mass%, more preferably 1.0 mass%, and even more preferably 0.5 mass%.

[0030] The chemical composition of the Fe-Al-Si phase 22a can be measured by analyzing the cross-section of the Al coating 20 using EPMA or SEM EDS, similar to the case of the interface layer 21. Specifically, the elemental content is measured at multiple locations, and the average value is taken as the content of that element. The chemical composition of the Al-Fe phase 22b, which will be described next, can also be measured using a similar method.

[0031] The chemical composition of Al-Fe phase 22b is not particularly limited, but for example, it is Fe: 15.0-70.0 mass%, Al: 30.0-85.0 mass%, and Si: 0-20.0 mass%. As mentioned above, Al-Fe phase 22b is basically Fe4Al 13The structure is Fe2Al5, but dissolved Si may be present. The Al-Fe phase 22b may contain small amounts of elements other than Si, Fe, and Al. The upper limit of the total content of elements other than Si, Fe, and Al in the Al-Fe phase 22b is preferably 3.0% by mass, more preferably 1.0% by mass, and even more preferably 0.5% by mass.

[0032] The area ratio of the Fe-Al-Si phase 22a in the intermediate layer 22 is preferably 5 to 35%. The lower the area ratio of the Fe-Al-Si phase 22a in the intermediate layer 22, the more likely it is that hydrogen absorption by the steel substrate 10 will be suppressed. The upper limit of the area ratio of the Fe-Al-Si phase 22a in the intermediate layer 22 is more preferably 30%, and even more preferably 25%.

[0033] The intermediate layer 22 may contain small amounts of other phases in addition to the Fe-Al-Si phase 22a and the Al-Fe phase 22b. The phase other than the Fe-Al-Si phase 22a and the Al-Fe phase 22b is, for example, the τ phase. The area ratio of the phase other than the Fe-Al-Si phase 22a and the Al-Fe phase 22b in the intermediate layer 22 is preferably 10.0% or less, and more preferably 5.0% or less.

[0034] The thickness of the intermediate layer 22 is preferably 15 μm or more. The thicker the intermediate layer 22, the more likely it is that hydrogen absorption by the steel substrate 10 will be suppressed. Furthermore, the thicker the intermediate layer 22, the more likely it is that paint adhesion and corrosion resistance will also improve. The lower limit of the thickness of the intermediate layer 22 is preferably 20 μm, and more preferably 25 μm. The upper limit of the thickness of the intermediate layer 22 is not particularly limited, but for example, it is 50 μm.

[0035] [Oxide film layer] The oxide film layer 23 is formed on the surface of the Al coating 20. The oxide film layer 23 is a layer mainly composed of Al oxide. The oxide film layer 23 may contain elements other than Al and O. The oxide film layer 23 is not limited to these, but may contain, for example, Si, Fe, Ni, Cr, Mn, Zr, Ce, Y, Ta, Cu, Nb, Co, V, Ti, Be, Ca, Sr, Ba, Sc, and Zn.

[0036] The oxide film layer 23 further contains Mg. It is preferable that the Mg exists in an oxide state.

[0037] The ratio of Mg in the components of the oxide film layer 23, excluding carbon and oxygen (hereinafter simply referred to as "Mg ratio"), is preferably 0.01 to 80.0 mass%. If the Mg ratio is too low, the formation of α-Al2O3 cannot be sufficiently suppressed, and an oxide film layer 23 containing a large amount of γ-Al2O3 may not be formed. On the other hand, if the Mg ratio is too high, cracks are more likely to occur in the intermediate layer 22, which may reduce paint adhesion and corrosion resistance after painting. The lower limit of the Mg ratio is more preferably 1.0 mass%, even more preferably 3.0 mass%, even more preferably 10.0 mass%, and even more preferably 15.0 mass%. The upper limit of the Mg ratio is more preferably 60.0 mass%, even more preferably 40.0 mass%, and even more preferably 35.0 mass%.

[0038] The chemical composition of the oxide film layer 23 can be measured by analyzing the cross-section of the Al coating 20 using EPMA or SEM EDS, similar to the interface layer 21 and the intermediate layer 22. TEM EDS is also possible. The chemical composition of the oxide film layer 23 can also be measured by depth profiling analysis using glow discharge spectroscopy (GDS) or Auger electron spectroscopy (AES).

[0039] The thickness of the oxide film layer 23 is preferably 0.01 to 1.00 μm. The thinner the oxide film layer 23, the more likely it is that hydrogen absorption by the steel substrate 10 will be suppressed. The upper limit of the thickness of the oxide film layer 23 is preferably 0.50 μm, and more preferably 0.30 μm.

[0040] The interface between the oxide film layer 23 and the intermediate layer 22 can be determined by observing the distribution of oxygen concentration. In this embodiment, the interface between the oxide film layer 23 and the intermediate layer 22 is determined to be the position where the oxygen detection intensity decreases to 1 / 6 of the maximum value using GDS.

[0041] In the hot-stamped molded article according to this embodiment, the peak intensity α of α-Al2O3 (1 0 -2) and the peak intensity γ of γ-Al2O3 (1 1 1) detected by XRD in the oxide film layer 23 satisfy the following relationship. γ / (α+γ)≧0.50

[0042] XRD is performed by taking a sample from the surface of the hot-stamped molded body 1 containing the Al coating 20, irradiating the oxide film layer 23 with CoKα radiation, and measuring using the θ-2θ method. A straight line is drawn from the background of each peak, and the height from the peak to the point where the line drawn straight down intersects the background is defined as the peak intensity. If the noise is very large, the data is smoothed using a moving average or the like before determining the peak intensity in the same way. Figure 2 shows an example of an XRD pattern containing (1 0 -2) of α-Al2O3 and (1 1 1) of γ-Al2O3. In this example, γ / (α+γ) is 0.60.

[0043] The higher the proportion of γ-Al2O3 in the oxide film layer 23, that is, the higher the value of γ / (α+γ), the less hydrogen is incorporated into the steel substrate 10, and the more likely it is that delayed fracture is suppressed. The lower limit of γ / (α+γ) is preferably 0.60, more preferably 0.70, and even more preferably 0.80.

[0044] [Steel base material] The steel base material 10 is not particularly limited as long as it is a steel material suitable for hot stamping. Examples of steel base materials applicable to the hot stamped molded body 1 include steel materials with a chemical composition in mass% of C: 0.1~0.6%, Si: 0.01~0.60%, Mn: 0.50~3.00%, P: 0.05% or less, S: 0.020% or less, Al: 0.10% or less, Ti: 0.01~0.10%, B: 0.0001~0.0100%, N: 0.015% or less, Cr: 0~1.0%, Mo: 0~1.0%, Cu: 0~1.0%, Ni: 0~1.0%, and the remainder being Fe and impurities. Examples of the form of the steel base material before hot stamping include steel sheets such as hot-rolled steel sheets and cold-rolled steel sheets. The chemical composition of the steel base material 10 will be described below. In the following explanation, the "%" for elemental content refers to mass percentage.

[0045] C: 0.1~0.6% Carbon (C) is included to ensure the desired mechanical strength. If the C content is too low, a sufficient improvement in mechanical strength may not be achieved. On the other hand, if the C content is too high, elongation and reduction of area tend to decrease. The upper limit of the C content is more preferably 0.4%.

[0046] Si: 0.01~0.60% Silicon (Si) is an element that improves mechanical strength and, like carbon (C), is included to ensure the desired mechanical strength. If the Si content is too low, sufficient improvement in mechanical strength may not be achieved. On the other hand, if the Si content is too high, the Si oxide formed on the surface of the steel substrate may reduce wettability during plating, potentially resulting in unplated areas.

[0047] Mn: 0.50~3.00% Manganese (Mn) is one of the strengthening elements that enhances steel, and it is also one of the elements that improves hardenability. Mn is also effective in preventing hot brittleness caused by sulfur, an impurity. If the Mn content is too low, these effects may not be fully obtained. On the other hand, if the Mn content is too high, there is a risk of excessive retained austenite, which may reduce the strength.

[0048] P:0.05% or less Phosphorus (P) is an impurity contained in steel substrates. P present in steel substrates can segregate at the grain boundaries of the steel substrate, potentially reducing its toughness. It is preferable to minimize the P content as much as possible.

[0049] S: 0.020% or less Sulfur (S) is an impurity contained in steel substrates. S present in steel substrates can form sulfides, which can reduce the toughness of the steel substrate. It is preferable to keep the S content as low as possible.

[0050] Al: 0.10% or less Aluminum (Al) is generally used for deoxidizing steel. However, if the Al content is high, the Ac3 point of the steel substrate rises, requiring an increased heating temperature to ensure the hardenability of the steel during hot stamping. Therefore, the Al content is preferably 0.10% or less. More preferably, the Al content is 0.05% or less, and even more preferably 0.01% or less.

[0051] Ti: 0.01~0.10% Titanium (Ti) is one of the elements that strengthens materials. If the Ti content is too low, the strength-enhancing and oxidation-resistant effects may not be sufficiently obtained. On the other hand, if the Ti content is too high, carbides and nitrides may form, potentially softening the steel.

[0052] B: 0.0001~0.0100% Boron (B) acts during quenching to improve strength. If the B content is too low, the strength-improving effect may not be sufficient. On the other hand, if the B content is too high, inclusions may form, making the steel base material brittle and potentially reducing fatigue strength.

[0053] N: 0.015% or less Nitrogen (N) is an impurity present in steel substrates. N in steel substrates can form nitrides, reducing the toughness of the steel. Furthermore, N in steel substrates can combine with B, reducing the amount of dissolved B and diminishing the hardenability-improving effect of B. It is preferable to minimize the N content as much as possible.

[0054] The steel base material 10 may contain Cr, Mo, Cu, and Ni.

[0055] Cr: 0~1.0% Mo: 0~1.0% Cu: 0~1.0% Ni: 0~1.0% To improve the hardenability of the steel substrate, one or more elements selected from the group consisting of chromium (Cr), molybdenum (Mo), copper (Cu), and nickel (Ni) may be included. The preferred lower limit for the content of each of these elements is 0.01%. On the other hand, excessive content will saturate the effect and lead to increased costs.

[0056] The remainder of the chemical composition of the steel substrate 10 consists of Fe and impurities. These impurities are elements introduced from the ore or scrap used as raw materials for steel, or from the environment during the manufacturing process. Examples of impurities include, in addition to the elements listed above, Zn, Co, Sn, Nb, V, As, Zr, Ca, Mg, etc.

[0057] [Method for manufacturing hot-stamped molded products] Next, an example of a method for manufacturing the hot-stamped molded body 1 will be described. In the manufacturing method described below, an Al coating is applied to a steel material such as a steel plate to create a plated steel material, and an Al coating 20 is formed on the steel substrate 10 by hot stamping the plated steel material. The method described here is just one example and does not limit the method for manufacturing the hot-stamped molded body 1.

[0058] [Plating process] A plating layer is formed on the surface of the steel material by a hot-dip galvanizing method. The temperature of the plating bath is preferably 600 to 700°C. If the temperature of the plating bath is lower than 600°C, the plating bath becomes less viscous, making uniform plating difficult. If the temperature of the plating bath is higher than 700°C, the components change rapidly due to volatilization, making process control difficult.

[0059] Ni and Cr are added to the Al coating 20 by adding them to the plating bath. The total content of Ni and Cr in the plating bath is preferably 0.01 to 1.80% by mass. The higher the content of Ni and Cr in the plating bath, the higher the content of Ni and Cr in the Fe-Al-Si phase 22a tends to be. The lower limit of the total content of Ni and Cr in the plating bath is more preferably 0.02% by mass, and even more preferably 0.05% by mass. The upper limit of the total content of Ni and Cr in the plating bath is more preferably 1.60% by mass, even more preferably 1.50% by mass, and even more preferably 1.00% by mass.

[0060] Mg is added to the Al coating 20 by adding it to the plating bath. The Mg content in the plating bath is preferably 0.01 to 0.60% by mass. The higher the Mg content in the plating bath, the higher the ratio of Mg in the oxide film layer 23 tends to be. The lower limit of the Mg content in the plating bath is more preferably 0.02% by mass, and even more preferably 0.05% by mass. The upper limit of the Mg content in the plating bath is more preferably 0.40% by mass.

[0061] The Si content in the plating bath is, for example, 1.0 to 20.0% by mass. The remainder of the plating bath is mainly Al. The plating bath may also contain small amounts of elements other than Al, Si, Ni, Cr, and Mg. The total content of elements other than Al, Si, Ni, Cr, and Mg in the plating bath is preferably 8.0% by mass or less, more preferably 5.0% by mass or less, even more preferably 3.0% by mass or less, even more preferably 2.0% by mass or less, and even more preferably 1.0% by mass or less.

[0062] The steel material is kept in a hydrogen-reducing atmosphere at 700-800°C before being immersed in the plating bath. It is preferable to carry out the entire process in a non-oxidizing atmosphere. The plating layer thickness is preferably 20-30 μm. The thickness of the plating layer can be adjusted by the temperature and viscosity of the plating bath, the immersion time, gas blowing, etc.

[0063] [Hot stamping process] After shaping the plated steel material to the required size, hot stamping is performed. The heating method can be either a high-temperature furnace or electric heating. The heating rate is, for example, 1 to 50°C / s. The holding temperature is preferably 850 to 950°C, and the holding time is preferably 2 minutes or more. The cooling rate is, for example, 30 to 1000°C / s.

[0064] If the holding time is too long, the crystal structure of the intermediate layer may change. The upper limit of the holding time is preferably 30 minutes, and more preferably 10 minutes.

[0065] The hot-stamped molded body 1 can be manufactured by the above process. Although the above describes a method of hot-stamping plated steel, alternatively, an Al coating layer may be formed by depositing Al or the like onto the surface of a steel substrate 10 by vapor deposition or thermal spraying, and the hot-stamped molded body 1 may be manufactured by hot-stamping the steel substrate 10 having this Al coating layer.

[0066] According to this embodiment, a hot-stamped molded article with excellent resistance to delayed fracture can be obtained. [Examples]

[0067] The present invention will be described more specifically below with reference to examples. The present invention is not limited to these examples.

[0068] For steel sheets having the chemical composition shown in Table 1, Al-Si plating layers were formed on both sides of the steel sheet by a hot-dip galvanizing method.

[0069] [Table 1]

[0070] The plating bath used had the chemical composition shown in Table 2. The temperature of the plating bath during molten plating was 700°C. After immersing the steel plate in the plating bath, the amount of plating applied was 70 g / m² per side using the gas wiping method. 2 I adjusted it to that.

[0071] [Table 2]

[0072] A plated steel sheet was heated in an electric resistance furnace at a furnace temperature of 900°C under a synthetic air stream with a dew point of 20°C for a soaking time of 5 minutes. Subsequently, the sheet was formed in a mold and simultaneously cooled in the mold to obtain a hot-stamped molded body.

[0073] A 15mm x 15mm, 2mm thick sample was taken from the surface containing the Al coating of the obtained hot-stamped molded body. XRD was performed using a Co tube, and the X-rays generated at an output of 40kV-150mA were monochromatized and irradiated onto the oxide film layer, and measured using the θ-2θ method. γ / (α+γ) was determined from the peak intensity α of (1 0 -2) of α-Al2O3 and the peak intensity γ of (1 1 1) of γ-Al2O3.

[0074] Furthermore, the Si, Ni, and Cr content in the Fe-Al-Si phase, the ratio of Mg among the components excluding carbon and oxygen in the oxide film layer, and the thickness of the oxide film layer were investigated. In addition, properties such as hydrogen embrittlement resistance (delayed fracture resistance), paint adhesion, post-coating corrosion resistance, and pitting corrosion resistance were evaluated.

[0075] [Hydrogen embrittlement resistance] The obtained hot-stamped molded products were subjected to hydrogen analysis. Hydrogen analysis was performed using the temperature-induced desorption method, and hydrogen released up to 250°C was defined as diffusible hydrogen. The amount of hydrogen released was scored as follows. 2: Amount of diffusible hydrogen: Less than 0.10 ppm by mass 1: Amount of diffusible hydrogen: 0.10 ppm by mass or more and less than 0.17 ppm by mass 0: Amount of diffusible hydrogen: 0.17 ppm by mass or more

[0076] [Paint adhesion] Paint adhesion was evaluated according to the method described in Japanese Patent Publication No. 4373778. Specifically, after immersing the sample in deionized water at 60°C for 240 hours, 100 grid lines were cut with a cutter at 1 mm intervals, and the area ratio calculated by visually measuring the number of peeled areas in the grid lines was used to assign scores as follows. 3: Peeling area 0% or more and less than 10% 2: Peeling area 10% or more but less than 70% 1: Peeling area 70% to 100%

[0077] [Corrosion resistance after painting] The corrosion resistance after painting was evaluated according to the method specified in JASO M609, established by the Society of Automotive Engineers of Japan. Scratches were made in the paint film with a cutter, and the width of the paint film blister from the cuts after 180 corrosion test cycles (maximum value on one side) was measured and scored as follows. 3: Swelling width: 0mm or more and less than 1.5mm 2: Swelling width 1.5 mm or more but less than 3.0 mm 1: Swelling width of 3.0 mm or more

[0078] [Pitting corrosion resistance] The samples were immersed in Preparen X, a surface conditioner manufactured by Nippon Parkerizing Co., Ltd., at room temperature for 1 minute, and then immersed in Palbond SX35, a paint primer manufactured by the same company, at 35°C for 2 minutes. Subsequently, they were subjected to a combined cycle corrosion test according to the method described in JIS H 8502. A 15 μm thick coating was applied using Power Float 1200 manufactured by Nippon Paint Co., Ltd., and cuts were made with a utility knife as described in JIS H 8502. The amount of thickness reduction of the steel plate after 60 cycles in the cut areas was scored as follows. 5: Plate thickness reduction of less than 0.1 mm 4: Plate thickness reduction of 0.1 mm or more and less than 0.2 mm 3: Plate thickness reduction amount 0.2mm or more and less than 0.3mm 2: Plate thickness reduction amount 0.3mm or more and less than 0.4mm 1: Plate thickness reduction 0.4mm or more

[0079] The results are shown in Table 3.

[0080] [Table 3]

[0081] As shown in Table 3, the hot-stamped molded articles with subscripts A01 to A16 had a γ / (α+γ) ratio of 0.50 or higher. These molded articles exhibited excellent resistance to hydrogen embrittlement (resistance to delayed fracture). These molded articles also showed excellent paint adhesion, corrosion resistance after painting, and pore color resistance.

[0082] The hot-stamped molded parts of substitute labels a3, a5, and a7 exhibited inferior paint adhesion, post-painting corrosion resistance, and pitting corrosion resistance compared to substitute labels A01 to A16. This is thought to be due to excessively high content of Ni, Cr, and Mg in the Al coating.

[0083] The hot-stamped molded articles with substitute symbols a1, a2, a4, a6, and a8 exhibited inferior hydrogen embrittlement resistance (delayed fracture resistance) compared to the hot-stamped molded articles with substitute symbols A01 to A16. This is thought to be because the γ / (α+γ) ratio was less than 0.50. It is believed that these hot-stamped molded articles either did not contain Ni and Mg simultaneously, or did not contain Cr and Mg simultaneously, or if they contained these elements simultaneously, the content was too low, and therefore the formation of α-Al2O3 could not be sufficiently suppressed during hot stamping.

[0084] Although embodiments of the present invention have been described above, the embodiments described above are merely illustrative examples for carrying out the present invention. Therefore, the present invention is not limited to the embodiments described above, and it is possible to carry out the present invention by appropriately modifying the embodiments described above without departing from the spirit of the invention.

Claims

[Claim 1] Steel base material and The steel substrate is formed on an Al coating, The aforementioned Al coating is An interface layer formed at the interface with the steel substrate, having a structure in which a portion of αFe is replaced with Al and Si, An intermediate layer formed on the aforementioned interface layer, The intermediate layer includes an oxide film layer formed on the intermediate layer, The aforementioned intermediate layer includes an Fe-Al-Si phase having a structure in which a portion of αFe is replaced with Al and Si. The Fe-Al-Si phase contains at least one of Ni and Cr, The Si content in the Fe-Al-Si phase is 13 to 17% by mass, and the total content of Ni and Cr is 0.10 to 5.0% by mass. The oxide film layer contains Mg, The ratio of Mg in the components of the oxide film layer, excluding carbon and oxygen, is 10.0 to 35.0% by mass. α-Al of the oxide film layer detected by XRD ２ O ３ The peak intensity α and γ-Al of (1 0 - 2) ２ O ３ A hot-stamped molded body in which the peak intensity γ of (1 1 1) satisfies the following relationship. γ / (α+γ)≧0.50

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