Hot stamped compact

Abstract

Provided is a hot-stamp-molded object having excellent delayed fracture resistance. The hot-stamp-molded object 1 comprises a steel substrate 10 and an Al coating 20 formed on the steel substrate 10. The Al coating 20 is formed at the interface with the steel substrate 10 and includes an interfacial layer 21 having a structure in which αFe is partly substituted with Al and Si, a middle layer 22 formed on the interfacial layer 21, and an oxide film layer formed on the middle layer, wherein: the middle layer 22 includes an Fe-Al-Si phase 22a having a structure in which αFe is partly substituted with Al and Si; the Fe-Al-Si phase 22a contains one or more kinds of elements selected from the group consisting of Zr, Ce, Y, Ta, Ni, Cu, Nb, Cr, Co, V, and Ti; and the oxide film layer 23 contains one or more kinds of elements selected from the group consisting of Be, Mg, Ca, Sr, Ba, Sc, and Zn.

Description

[Technical Field]

[0001] The present invention relates to a hot stamped product. [Background technology]

[0002] Hot stamping (also known as hot pressing, hot pressing, die quenching, press quenching, etc.) is a known method for forming high-strength steel materials with high dimensional accuracy. Hot stamping involves heating a steel material such as a steel plate to the austenite region, forming it in a hot state, and then cooling it after forming to obtain the desired properties.

[0003] In hot stamping, scale may form on the surface of the steel material during heating, which must be removed in a subsequent process. To avoid this, a technology is known in which the steel material to be hot stamped is coated with aluminum to suppress the formation of scale.

[0004] Japanese Patent Publication No. 63-3929 discloses a method for producing a hot-dip aluminized steel sheet that exhibits a low oxidation weight gain during high-temperature oxidation. Japanese Patent Publication No. 2943021 discloses a method for producing an austenitic stainless steel strip having a NiAl intermetallic compound on the surface, in which the surface of the austenitic stainless steel strip is coated with Al and then subjected to a diffusion heat treatment in a non-oxidizing atmosphere at a temperature range of 700 to 800°C.

[0005] International Publication No. 2018 / 221738 discloses a hot-stamped member that has excellent corrosion resistance after painting, etc. This hot-stamped member has a steel material, an Al-Fe intermetallic compound layer, and an oxide film layer, and 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, and Zn in a proportion of 0.01 atomic % to 80 atomic % excluding oxygen.

[0006] JP 2017-536472 A discloses a flat steel product for hot forming, which comprises a steel substrate and a protective coating containing Al as a main component. The publication states that the protective coating contains a total of 0.1 to 0.5% by weight of at least one alkaline earth metal or transition metal, and that an oxide of the alkaline earth metal or transition metal is 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 Summary of the Invention [Problem to be solved by the invention]

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

[0009] An object of the present invention is to provide a hot stamped steel sheet having excellent resistance to delayed fracture. [Means for solving the problem]

[0010] A hot-stamped steel according to one embodiment of the present invention comprises a steel substrate and an Al coating formed on the steel substrate, the Al coating comprising: an interface layer formed at an interface with the steel substrate and having a structure in which a portion of α-Fe is substituted with Al and Si; an intermediate layer formed on the interface layer; and an oxide film layer formed on the intermediate layer, the intermediate layer having a thickness of 15 μm or more; the intermediate layer comprising an Fe—Al—Si phase having a structure in which a portion of α-Fe is substituted with Al and Si; and the Fe—Al—Si phase comprising at least one of Zr, Ce, Y, Ta, Ni, Cu, Nb, Cr, The Fe-Al-Si phase contains one or more elements selected from the group consisting of Co, V, and Ti, the Si content in the Fe-Al-Si phase is 1 to 20 mass%, and the total content of Zr, Ce, Y, Ta, Ni, Cu, Nb, Cr, Co, V, and Ti is 0.10 to 5.0 mass%, and the oxide film layer contains one or more elements selected from the group consisting of Be, Mg, Ca, Sr, Ba, Sc, and Zn, and the total ratio of Be, Mg, Ca, Sr, Ba, Sc, and Zn to the components excluding oxygen in the oxide film layer is 0.01 to 80.0 mass%. [Effects of the Invention]

[0011] According to the present invention, a hot stamped steel sheet having excellent resistance to delayed fracture can be obtained. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a cross-sectional view that schematically shows the structure of a plated steel material before hot stamping. [Figure 2] FIG. 2 is a cross-sectional view that schematically shows the configuration of an example of a hot-stamped product that is formed by hot stamping a plated steel material. [Figure 3] FIG. 3 is a cross-sectional view schematically showing the configuration of a hot-stamped steel according to one embodiment of the present invention. [Figure 4] Figure 4 is a cross-sectional photograph of the Al coating of symbol a1. [Figure 5] Figure 5 is a cross-sectional photograph of the Al coating of the prototype A13. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present inventors have conducted various studies on hydrogen absorption during hot stamping of Al-plated steel material, particularly Al-Si-plated steel material.

[0014] 1 is a cross-sectional view schematically showing the configuration of a plated steel material 1A before hot stamping. The plated steel material 1A includes a steel substrate 10 and an Al-Si plating layer 30. The plated steel material 1A further includes an Al-Si-Fe alloy layer 35 formed between the steel substrate 10 and the Al-Si plating layer 30 by diffusing Fe from the steel substrate 10 into the plating layer.

[0015] 2 is a cross-sectional view that schematically shows the configuration of a hot-stamped body 9, which is an example of a hot-stamped body formed by hot stamping a plated steel material 1 A. The hot-stamped body 9 includes a steel substrate 10 and an Al coating 20 formed on the steel substrate 10.

[0016] The Al coating 20 is composed of multiple layers. Specifically, the Al coating 20 includes an interface layer 21 formed at the interface with the steel substrate 10, an intermediate layer 22 formed on the interface layer 21, and an oxide layer 23 formed on the intermediate layer 22. However, the intermediate layer 22 includes an Fe-Al-Si phase 22a and an Al-Fe phase 22b, which have different structures, and the Fe-Al-Si phase 22a and the Al-Fe phase 22b may be distributed in layers. Furthermore, the above classification is based on how they appear when observed with an optical microscope or a scanning electron microscope (SEM). When observed with a transmission electron microscope (TEM), each layer may be observed as an aggregate of several crystalline phases.

[0017] The interface layer 21 is derived from the Al-Si-Fe alloy layer 35 of the plated steel material 1A (FIG. 1), and has a structure in which a part of the bcc structure of mainly αFe is substituted with Al and Si. The interface layer 21 may contain an intermediate phase such as a τ phase.

[0018] The intermediate layer 22 includes an Fe-Al-Si phase 22a and an Al-Fe phase 22b. The Fe-Al-Si phase 22a has a structure in which a part of the bcc structure of αFe is substituted with Al and Si, similar to the interface layer 21. The Al-Fe phase 22b is composed of Fe4Al 13 and Fe2Al5 structures. As shown in Fig. 2, the Fe-Al-Si phase 22a is often formed in a layer (strip) shape at the middle position in the thickness direction of the intermediate layer 22, sandwiched between the Al-Fe phases 22b on the top and bottom.

[0019] The oxide film layer 23 is a layer mainly made of an oxide of Al.

[0020] The present inventors have found that hydrogen absorption by the steel substrate 10 is suppressed when the Fe-Al-Si phase 22a contains a predetermined amount of one or more specific elements, specifically, Zr, Ce, Y, Ta, Ni, Cu, Nb, Cr, Co, V, and Ti (hereinafter, these 11 elements are referred to as "A group elements").

[0021] 3 is a cross-sectional view schematically illustrating the configuration of a hot-stamped steel 1, which is an example of a hot-stamped steel containing a predetermined amount of A group elements in an Fe—Al—Si phase 22a. In the hot-stamped steel 1, the Fe—Al—Si phase 22a is smaller in size than in the hot-stamped steel 9 (FIG. 2). While the Fe—Al—Si phase 22a is layered (band-shaped) in the hot-stamped steel 9 (FIG. 2), the Fe—Al—Si phase 22a in the hot-stamped steel 1 is discontinuous. Furthermore, the A group elements are selectively distributed in the Fe—Al—Si phase 22a.

[0022] From this, it is considered that the A group elements are selectively distributed to the Fe-Al-Si phase 22a during heating, and have the effect of preventing the stabilization of the Fe-Al-Si phase 22a.

[0023] The stabilization of the Fe-Al-Si phase 22a is hindered, thereby promoting the growth of the Al-Fe phase 22b. The thickening of the Al-Fe phase 22b suppresses the diffusion of Al to the surface of the Al coating 20, thereby suppressing the oxidation of Al. This suppresses the hydrogen absorption of the steel substrate 10. Furthermore, the Al-Fe phase 22b fixes hydrogen atoms, thereby suppressing the hydrogen absorption of the steel substrate 10.

[0024] The present inventors further found that hydrogen absorption by the steel substrate 10 can be further suppressed by including a predetermined amount of one or more elements selected from Be, Mg, Ca, Sr, Ba, Sc, and Zn (hereinafter, these seven elements are referred to as "B group elements") in the oxide film layer 23. The B group elements are more easily oxidized than Al, and have the effect of suppressing hydrogen absorption by the steel substrate 10 by suppressing the oxidation of Al.

[0025] By including a predetermined amount of an A group element in the Fe-Al-Si phase 22a of the intermediate layer 22 and including a predetermined amount of a B group element in the oxide film layer 23, the combined effect of these elements makes it possible to obtain a hot-stamped body 1 having excellent resistance to fracture due to delay.

[0026] The present invention has been completed based on the above findings. Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The same or corresponding parts in the drawings are designated by the same reference numerals, and their description will not be repeated. The dimensional ratios between the components shown in each drawing do not necessarily represent the actual dimensional ratios.

[0027] [Hot stamped compact] Referring again to Fig. 3, the configuration of a hot-stamped steel 1 according to one embodiment of the present invention will be described. The hot-stamped steel 1 is a steel material formed by hot stamping. The hot-stamped steel 1 includes a steel substrate 10 and an Al coating 20 formed on the steel substrate 10.

[0028] The Al coating 20 comprises an interface layer 21 formed at the interface with the steel substrate 10, an intermediate layer 22 formed on the interface layer 21, and an oxide film layer 23 formed on the intermediate layer 22.

[0029] [Interfacial layer] The interface layer 21 is formed at the interface with the steel base material 10. The interface layer 21 has a structure in which a part of the bcc structure of αFe is substituted with Al and Si.

[0030] The chemical composition of the interface layer 21 has a distribution that varies along the thickness direction. The chemical composition of the interface layer 21 is, for example, 60 to 98 mass % Fe, 1 to 40 mass % Al, and 1 to 20 mass % Si, averaged in the thickness direction. The interface layer 21 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 contained in the interface layer 21 is preferably 3.0 mass %, and more preferably 1.5 mass %. In some cases, the boundary between the interface layer 21 and the steel base material 10 is not clear. In such cases, the range from the interface of the intermediate layer 22 to a depth of 10 μm is considered to be the interface layer 21, and the average chemical composition of this range is determined.

[0031] The chemical composition of the interface layer 21 can be measured by analyzing a cross section of the Al coating 20 with an electron probe microanalyzer (EPMA) or an energy-resolved spectrometer (EDS) of an SEM.

[0032] The thickness of the interface layer 21 is not particularly limited, but is, for example, 5 to 15 μm.

[0033] [Middle layer] The intermediate layer 22 is formed on the interface layer 21. The intermediate layer 22 includes an Fe-Al-Si phase 22a and an Al-Fe phase 22b. The Fe-Al-Si phase 22a has a structure in which a part of the bcc structure of αFe is substituted with Al and Si, similar to the interface layer 21. The Al-Fe phase 22b is composed of Fe4Al 13 and Fe2Al5 structure. Note that the Al-Fe phase 22b may also contain dissolved Si.

[0034] 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 of an 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 beam diffraction.

[0035] Although the interface layer 21 and the Fe—Al—Si phase 22a have similar structures, they can be distinguished by their positions. That is, the interface layer 21 is formed at the interface with the steel substrate 10, whereas the Fe—Al—Si phase 22a is formed at the middle position in the thickness direction of the intermediate layer 22.

[0036] The Si content in the Fe—Al—Si phase 22a is 1 to 20 mass%. If the Si content is lower than 1 mass%, the oxide film layer 23 grows, resulting in an increase in the amount of hydrogen absorbed by the steel substrate 10. If the Si content is higher than 20 mass%, the intermediate layer 22 does not grow sufficiently, and the amount of hydrogen absorbed by the steel substrate 10 also increases. The lower limit of the Si content in the Fe—Al—Si phase 22a is preferably 5 mass%, more preferably 8 mass%. The upper limit of the Si content in the Fe—Al—Si phase 22a is preferably 18 mass%, more preferably 16 mass%.

[0037] The Fe content in the Fe—Al—Si phase 22a is not particularly limited, but is, for example, 30 to 80 mass %. The Al content in the Fe—Al—Si phase 22a is not particularly limited, but is, for example, 5 to 50 mass %.

[0038] The Fe-Al-Si phase 22a contains, in addition to Fe, Al, and Si, one or more elements selected from the group consisting of Zr, Ce, Y, Ta, Ni, Cu, Nb, Cr, Co, V, and Ti. Hereinafter, the eleven elements Zr, Ce, Y, Ta, Ni, Cu, Nb, Cr, Co, V, and Ti are referred to as group A elements. Among the group A elements, Zr, Ce, Ni, Cr, Co, V, and Ti are preferred, and Ni and Cr are particularly preferred.

[0039] The total content of the A group elements in the Fe—Al—Si phase 22a is 0.10 to 5.0 mass%. The A group elements prevent the stabilization of the Fe—Al—Si phase 22a, thereby promoting the growth of the Al—Fe phase 22b and suppressing hydrogen absorption by the steel substrate 10. If the total content of the A group elements is less than 0.1 mass%, this effect cannot be sufficiently obtained. On the other hand, if the total content of the A group elements is more than 5.0 mass%, the Fe—Al—Si phase 22a may become enlarged. The lower limit of the total content of the A group elements in the Fe—Al—Si phase 22a is preferably 0.20 mass%, more preferably 0.30 mass%. The upper limit of the content of the A group elements in the Fe—Al—Si phase 22a is preferably 4.0 mass%, more preferably 3.0 mass%.

[0040] The Fe—Al—Si phase 22a may contain small amounts of elements other than Si, Fe, Al, and the A-group elements. The upper limit of the total content of elements other than Si, Fe, Al, and the A-group elements contained in the Fe—Al—Si phase 22a is preferably 1.0 mass %, and more preferably 0.5 mass %.

[0041] The chemical composition of the Fe-Al-Si phase 22a can be measured by analyzing a cross section of the Al coating 20 using an EPMA or EDS of an SEM, as in the case of the interface layer 21. Specifically, the content of an element 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 in a similar manner.

[0042] The chemical composition of the Al-Fe phase 22b is not particularly limited, but may be, for example, 15 to 70 mass % Fe, 30 to 85 mass % Al, and 0 to 20 mass % Si. As described above, the Al-Fe phase 22b is basically composed of Fe4Al 13 and Fe2Al5 structure, but may contain dissolved Si. 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 contained in the Al-Fe phase 22b is preferably 1.0 mass %, and more preferably 0.5 mass %.

[0043] 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 base material 10 is 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%.

[0044] In addition to the Fe-Al-Si phase 22a and the Al-Fe phase 22b, the intermediate layer 22 may contain a small amount of other phases. The phases other than the Fe-Al-Si phase 22a and the Al-Fe phase 22b may be, for example, τ phase. The area ratio of the phases 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.

[0045] The thickness of the intermediate layer 22 is 15 μm or more. The thicker the intermediate layer 22, the more Al is inhibited from diffusing into the surface of the Al coating 20, and the more Al is inhibited from oxidizing. Furthermore, hydrogen is more easily immobilized in the intermediate layer 22. Therefore, the thicker the intermediate layer 22, the more hydrogen absorption by the steel substrate 10 tends to be inhibited. Furthermore, the thicker the intermediate layer 22, the more paint adhesion and corrosion resistance tend to be improved. 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 is, for example, 50 μm.

[0046] [Oxide film layer] The oxide film layer 23 is formed on the intermediate layer 22. The oxide film layer 23 is a layer mainly composed of an oxide of Al. The oxide film layer 23 may contain elements other than Al and O. The oxide film layer 23 may contain, for example, Si, Fe, and the above-mentioned A group elements.

[0047] The oxide film layer 23 further contains one or more elements selected from the group consisting of Be, Mg, Ca, Sr, Ba, Sc, and Zn. Hereinafter, the seven elements of Be, Mg, Ca, Sr, Ba, Sc, and Zn will be referred to as B-group elements. Among the B-group elements, Mg, Ca, Sr, and Ba are preferred, with Mg being particularly preferred. The B-group elements are preferably present in the form of an oxide.

[0048] The total ratio of B group elements to the components excluding oxygen in the oxide film layer 23 (hereinafter simply referred to as the "ratio of B group elements") is 0.01 to 80.0 mass%. B group elements are more easily oxidized than Al, and have the effect of suppressing hydrogen absorption in the steel base material 10 by suppressing the oxidation of Al. If the ratio of B group elements is lower than 0.01 mass%, this effect cannot be sufficiently obtained. If the ratio of B group elements is higher than 80.0 mass%, the oxide film layer 23 may become thicker. The lower limit of the ratio of B group elements is preferably 1.0 mass%, more preferably 3.0 mass%, even more preferably 10 mass%, and even more preferably 15 mass%. The upper limit of the ratio of B group elements is preferably 60.0 mass%, even more preferably 40.0 mass%, and even more preferably 35 mass%.

[0049] The chemical composition of the oxide layer 23 can be measured by analyzing a cross section of the Al coating 20 with an EPMA or EDS of an SEM, as with the interface layer 21 and the intermediate layer 22. EDS of a TEM is also possible. The chemical composition of the oxide layer 23 can also be measured by a depth profile composition analysis method using a glow discharge spectrometer (GDS) or Auger electron spectroscopy (AES).

[0050] 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 base material 10 is suppressed. The upper limit of the thickness of the oxide film layer 23 is preferably 0.50 μm, and more preferably 0.30 μm.

[0051] 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 position where the oxygen detection intensity using GDS drops to 1 / 6 of the maximum value is determined to be the interface between the oxide film layer 23 and the intermediate layer 22.

[0052] As the combination of the A group element and the B group element, a combination of Ni and Mg and a combination of Cr and Mg are particularly preferred. That is, it is particularly preferred that the Fe-Al-Si phase 22a contains one or two elements selected from the group consisting of Ni and Cr, the total content of Ni and Cr in the Fe-Al-Si phase 22a is 0.10 to 5.0 mass%, and the oxide film layer 23 contains Mg, and the ratio of Mg to the components excluding oxygen in the oxide film layer 23 is 0.01 to 80.0 mass%.

[0053] [Steel base material] The steel substrate 10 is not particularly limited as long as it is a steel material suitable for hot stamping. Examples of steel substrates applicable to the hot-stamped body 1 include steel materials having 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: 0.010% or less, Cr: 0-1.0%, Mo: 0-1.0%, Cu: 0-1.0%, Ni: 0-1.0%, and the balance: Fe and impurities. Examples of the form of the steel substrate before hot stamping include steel sheets such as hot-rolled steel sheets and cold-rolled steel sheets. The chemical composition of the steel substrate 10 will be described below. In the following description, "%" in the content of an element means mass %.

[0054] C: 0.1 to 0.6% Carbon (C) is contained to ensure the desired mechanical strength. If the C content is too low, sufficient improvement in mechanical strength may not be obtained. 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%.

[0055] Si: 0.01 to 0.60% Silicon (Si) is an element that improves mechanical strength and, like 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, resulting in bare spots.

[0056] Mn: 0.50 to 3.00% Manganese (Mn) is one of the elements that strengthens steel and also improves hardenability. Mn is also effective in preventing hot embrittlement caused by S, an impurity. If the Mn content is too low, these effects may not be fully achieved. On the other hand, if the Mn content is too high, there is a risk of excessive retained austenite, resulting in a decrease in strength.

[0057] P:0.05% or less Phosphorus (P) is an impurity contained in the steel substrate. P contained in the steel substrate may segregate at the grain boundaries of the steel substrate and reduce the toughness of the steel substrate. It is preferable to keep the P content as low as possible.

[0058] S: 0.020% or less Sulfur (S) is an impurity contained in the steel substrate. S contained in the steel substrate may form sulfides and reduce the toughness of the steel substrate. It is preferable to keep the S content as low as possible.

[0059] Al: 0.10% or less Aluminum (Al) is generally used for the purpose of deoxidizing steel. However, if the Al content is high, the Ac3 point of the steel substrate rises, and therefore the heating temperature required to ensure the hardenability of the steel during hot stamping must be increased. Therefore, the Al content is preferably 0.10% or less. The Al content is more preferably 0.05% or less, and even more preferably 0.01% or less.

[0060] Ti: 0.01 to 0.10% Titanium (Ti) is one of the strength-enhancing elements. If the Ti content is too low, the strength and oxidation resistance may not be sufficiently improved. On the other hand, if the Ti content is too high, carbides and nitrides may be formed, which may soften the steel.

[0061] B: 0.0001 to 0.0100% Boron (B) acts during hardening to improve strength. If the B content is too low, the strength improvement effect may not be sufficient. On the other hand, if the B content is too high, inclusions may be formed, embrittling the steel base material and reducing fatigue strength.

[0062] N: 0.010% or less Nitrogen (N) is an impurity contained in the steel substrate. N contained in the steel substrate may form nitrides, reducing the toughness of the steel substrate. Furthermore, N contained in the steel substrate may combine with B to reduce the amount of solute B, reducing the hardenability-improving effect of B. It is preferable to keep the N content as low as possible.

[0063] The steel substrate 10 may include Cr, Mo, Cu, and Ni.

[0064] Cr: 0 to 1.0% Mo: 0 to 1.0% Cu: 0 to 1.0% Ni: 0 to 1.0% In order 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 added. The preferred lower limit of the content of each of these elements is 0.01%. However, if an excessive amount is added, the effect saturates and costs increase.

[0065] The balance of the chemical composition of the steel substrate 10 is Fe and impurities. The impurities referred to here refer to elements that are mixed in from ores or scrap used as raw materials for steel, or elements that are mixed in from the manufacturing process environment, etc. Examples of impurities include the elements listed above, as well as Zn, Co, Sn, Nb, V, As, Zr, Ca, Mg, etc.

[0066] [Method of manufacturing hot stamped compact] Next, an example of a method for manufacturing the hot-stamped compact 1 will be described. In the manufacturing method described below, a steel material such as a steel sheet is plated with Al to form a plated steel material, and the plated steel material is hot stamped to form an Al coating 20 on the steel substrate 10. The method described here is an example and is not intended to limit the manufacturing method of the hot-stamped compact 1.

[0067] [Plating process] A plating layer is formed on the surface of a steel material by hot dip plating. 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 viscosity of the plating bath becomes low, making it difficult to achieve uniform plating. 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.

[0068] The A group element is added to the plating bath. The content of the A group element in the plating bath is preferably 0.05 to 5.0 mass%. The higher the content of the A group element in the plating bath, the higher the content of the A group element in the Fe-Al-Si phase 22a tends to be. The lower limit of the content of the A group element in the plating bath is more preferably 0.2 mass%, and even more preferably 0.5 mass%. The upper limit of the content of the A group element in the plating bath is more preferably 3.0 mass%, and even more preferably 2.0 mass%.

[0069] The B group element is also added by adding it to the plating bath. When the B group element is added, the content of the B group element in the plating bath is preferably 0.01 to 1.0 mass%. The higher the content of the B group element in the plating bath, the higher the content of the B group element in the oxide film layer 23 tends to be. The lower limit of the content of the B group element in the plating bath is more preferably 0.05 mass%, and even more preferably 0.08 mass%. The upper limit of the content of the B group element in the plating bath is more preferably 0.8 mass%, and even more preferably 0.6 mass%.

[0070] The Si content in the plating bath is, for example, 1.0 to 20.0 mass%. The remainder of the plating bath is mainly Al. The plating bath may contain small amounts of elements other than Al, Si, group A elements, and group B elements. The total content of elements other than Al, Si, group A elements, and group B elements contained in the plating bath is preferably 5.0 mass% or less, more preferably 3.0 mass% or less, even more preferably 2.0 mass% or less, and even more preferably 1.0 mass% or less.

[0071] The steel material is maintained in a hydrogen reducing atmosphere at 700 to 800°C, and then immersed in a plating bath. The entire process is preferably carried out in a non-oxidizing atmosphere. The plating is preferably carried out so that the thickness of the plating layer is 20 to 30 μm. The thickness of the plating layer is adjusted so that the thickness of the intermediate layer 22 after the hot stamping process is 15 μm or more. The thickness of the plating layer can be adjusted by the temperature, viscosity, immersion time, gas spraying, etc. of the plating bath.

[0072] [Hot stamping process] After the plated steel material is shaped to the required size, it is subjected to hot stamping. The heating method may be either a high-temperature furnace or electrical heating. The temperature rise 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 temperature drop (cooling) rate is, for example, 30 to 1000°C / s.

[0073] 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.

[0074] The above steps can produce the hot-stamped body 1. Although the method of hot-stamping a plated steel material has been described above, the hot-stamped body 1 may alternatively be produced by forming an Al coating layer by adhering Al or the like to the surface of the steel substrate 10 by vapor deposition or thermal spraying, and then hot-stamping the steel substrate 10 having this Al coating layer.

[0075] According to this embodiment, a hot stamped steel sheet having excellent delayed fracture resistance can be obtained. [Example]

[0076] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0077] Steel sheets having the chemical compositions shown in Table 1 were coated with Al-Si layers on both sides by hot dip plating.

[0078] [Table 1]

[0079] The plating bath used had the chemical composition shown in Table 2. The temperature of the plating bath during hot dip plating was 700°C. After immersing the steel sheet in the plating bath, the coating weight was reduced to 70 g / m per side using the gas wiping method. 2 was adjusted to.

[0080] [Table 2]

[0081] The plated steel sheet was heated in an electric resistance furnace at a furnace temperature of 900°C under a synthetic air flow with a dew point of 20°C for a soaking time of 5 minutes. Thereafter, the steel sheet was formed in a die and simultaneously cooled in the die to obtain a hot stamped body.

[0082] The obtained hot-stamped compacts were investigated for the Si content in the Fe-Al-Si phase, the content of group A elements in the Fe-Al-Si phase, the area ratio of the Fe-Al-Si phase in the intermediate layer, the thickness of the intermediate layer, the ratio of group B elements in the components excluding oxygen in the oxide layer, and the thickness of the oxide layer.In addition, the properties evaluated were hydrogen embrittlement resistance (delayed fracture resistance), paint adhesion, corrosion resistance after painting, and pitting corrosion resistance.

[0083] [Hydrogen embrittlement resistance] The resulting hot stamped body was subjected to hydrogen analysis by thermal desorption, where hydrogen released up to 250°C was defined as diffusible hydrogen, and the amount was evaluated as follows: 2: Amount of diffusible hydrogen: Less than 0.1 ppm by mass 1: Amount of diffusible hydrogen: 0.1 mass ppm or more and less than 0.2 mass ppm 0: Diffusible hydrogen content 0.2 mass ppm or more

[0084] [Paint adhesion] Paint adhesion was evaluated according to the method described in Japanese Patent No. 4373778. Specifically, after immersing the sample in deionized water at 60°C for 240 hours, 100 grids spaced 1 mm apart were cut with a cutter, and the number of peeled areas in the grids was counted visually to calculate the area ratio, which was then rated as follows: 3: Peeling area: 0% to less than 10% 2: Peeling area 10% or more but less than 70% 1: Peeling area 70% to 100%

[0085] [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. A scratch was made in the coating with a cutter, and the width of the blister from the scratch (maximum value on one side) was measured after 180 cycles of corrosion testing, and the evaluation was graded as follows: 3: Bulge width: 0mm or more and less than 1.5mm 2: Bulge width 1.5mm or more but less than 3.0mm 1: Blister width 3.0mm or more

[0086] [Pitting corrosion resistance] The specimens were immersed in Preparen X, a surface conditioner manufactured by Nihon Parkerizing Co., Ltd., for 1 minute at room temperature, and then in Palbond SX35, a paint primer chemical manufactured by the same company, for 2 minutes at 35°C. They were then subjected to a cyclic corrosion test according to the method described in JIS H 8502. A 15 μm thick coating was applied using Nippon Paint's Power Float 1200, and cut using a cutter knife as described in JIS H 8502. The steel sheet was scored based on the amount of thickness reduction in the cut area after 60 cycles, as follows: 5: Thickness reduction less than 0.1 mm 4: Thickness reduction: 0.1 mm or more but 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

[0087] The results are shown in Table 3.

[0088] [Table 3]

[0089] As shown in Table 3, the hot-stamped steels designated A01 to A17 had a Si content in the Fe-Al-Si phase ranging from 1 to 20 mass% and a group A element content ranging from 0.10 to 5.0 mass%. These hot-stamped steels had a diffusible hydrogen content of less than 0.2 ppm by mass in a hydrogen embrittlement resistance test. These hot-stamped steels also had good paint adhesion, post-painting corrosion resistance, and pitting corrosion resistance. Among these, the hot-stamped steels designated A01 to A05, A10, A12, A13, A15, and A17, which contained group B elements in the oxide film layer, had a diffusible hydrogen content of less than 0.1 ppm by mass in a hydrogen embrittlement resistance test, demonstrating particularly excellent hydrogen embrittlement resistance (delayed fracture resistance).

[0090] The hot stamped compacts with the symbols a1 to a7 had a diffusible hydrogen content of 0.2 mass ppm or more in the hydrogen embrittlement resistance test, and were inferior in hydrogen embrittlement resistance (delayed fracture resistance) compared to the hot stamped compacts with the symbols A01 to A20. This is thought to be because the content of A group elements in the Fe-Al-Si phase in the hot stamped compacts with the symbols a1 to a7 was less than 0.1 mass% or higher than 5.0 mass%.

[0091] Figure 4 is a cross-sectional photograph of the Al coating of sample a1. Figure 5 is a cross-sectional photograph of the Al coating of sample A13. As shown in Figures 4 and 5, the area ratio of the Fe-Al-Si phase was smaller in sample A13 than in sample a1. Furthermore, the Fe-Al-Si phase, which was formed in layers (bands) in sample a1, became discontinuous.

[0092] 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 embodiments, and the above-described embodiments can be appropriately modified and carried out without departing from the spirit of the present invention. [Explanation of symbols]

[0093] 1,9 Hot stamped compact 1A Plated Steel 10 Steel base material 20 Al coating 21 Interface layer 22 Middle Class 22a Fe-Al-Si phase 22b Al-Fe phase 23 Oxide layer 30 Al-Si plating layer 35 Al-Si-Fe alloy layer

Claims

1. A steel substrate; and an Al coating formed on the steel substrate, The Al coating is an interface layer formed at the interface with the steel substrate and having a structure in which a portion of α-Fe is substituted with Al and Si; an intermediate layer formed on the interface layer; an oxide film layer formed on the intermediate layer; The thickness of the intermediate layer is 15 μm or more, the intermediate layer contains an Fe—Al—Si phase having a structure in which a portion of α-Fe is substituted with Al and Si, The Fe—Al—Si phase contains one or more elements selected from the group consisting of Zr, Ce, Y, Ta, Ni, Cu, Nb, Cr, Co, and V; the content of Si in the Fe—Al—Si phase is 1 to 20 mass %, and the total content of Zr, Ce, Y, Ta, Ni, Cu, Nb, Cr, Co, and V is 0.10 to 5.0 mass %, the oxide film layer contains one or more elements selected from the group consisting of Be, Mg, Ca, Sr, Ba, and Sc; a total ratio of Be, Mg, Ca, Sr, Ba, and Sc to components excluding oxygen in the oxide film layer is 0.01 to 80.0 mass%.

2. The hot stamped body according to claim 1, The hot-stamped body, wherein the area ratio of the Fe—Al—Si phase in the intermediate layer is 5 to 35%.

3. The hot stamped product according to claim 1 or 2 The hot-stamped body, wherein the oxide film layer has a thickness of 0.01 to 1.0 μm.

4. The hot stamped body according to claim 1 or 2, the total content of Ni and Cr in the Fe—Al—Si phase is 0.10 to 5.0 mass %, The hot-stamped body, wherein the ratio of Mg in components excluding oxygen in the oxide film layer is 0.01 to 80.0 mass%.

5. The hot stamped body according to claim 3, the total content of Ni and Cr in the Fe—Al—Si phase is 0.10 to 5.0 mass %, The hot-stamped body, wherein the ratio of Mg in components excluding oxygen in the oxide film layer is 0.01 to 80.0 mass%.

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