Hot stamping parts

Abstract

A hot-stamped formed body with excellent resistance to delayed fracture is provided. The hot-stamped formed body (1) comprises a steel substrate (10) and an Al coating (20) formed on the steel substrate (10). The Al coating (20) comprises: an interface layer (21) formed at the interface with the steel substrate (10) having a structure in which a portion of αFe is replaced by Al and Si; an intermediate layer (22) formed on the interface layer (21); and an oxide film layer formed on the intermediate layer. The intermediate layer (22) comprises a Fe-Al-Si phase (22a) having a structure in which a portion of αFe is replaced by Al and Si. The Fe-Al-Si phase (22a) comprises one or more elements selected from the group consisting of Zr, Ce, Y, Ta, Ni, Cu, Nb, Cr, Co, V and Ti. The oxide film layer (23) comprises one or more elements selected from the group consisting of Be, Mg, Ca, Sr, Ba, Sc and Zn.

Description

Technical Field

[0001] This invention relates to a hot stamping formed body. Background Technology

[0002] Hot stamping (also known as hot pressing, hot pressing, die hardening, pressure hardening, etc.) is a known method for forming high-strength steel with high dimensional accuracy. Hot stamping refers to heating steel sheets or other steel materials to the austenitic region, performing hot forming, and obtaining the desired properties through cooling after forming.

[0003] In hot stamping, oxide scale sometimes forms on the surface of steel during heating, which needs to be removed in subsequent processes. To avoid this, it is known to apply Al plating to steel used for hot stamping to suppress oxide scale formation.

[0004] Japanese Patent Publication No. 63-3929 discloses a method for manufacturing hot-dip aluminized steel sheets that exhibit low oxidation increment values ​​during high-temperature oxidation. Japanese Patent Publication No. 2943021 discloses a method for manufacturing austenitic stainless steel strip with a NiAl intermetallic compound on its surface, wherein after covering the surface of the austenitic stainless steel strip with Al, diffusion heat treatment is performed at a temperature range of 700–800°C in a non-oxidizing atmosphere.

[0005] International Publication No. 2018 / 221738 discloses a hot-stamped part with excellent corrosion resistance after coating. This hot-stamped part comprises steel, an Al-Fe intermetallic compound layer, and an oxide film layer. 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, with a content of 0.01 atomic% to 80 atomic% (excluding oxygen).

[0006] Japanese Patent Publication No. 2017-536472 discloses a flat steel product for hot forming, which is composed of 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 total 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.

[0007] Prior art literature

[0008] Patent Literature

[0009] Patent Document 1: Japanese Patent Publication No. 63-3929

[0010] Patent Document 2: Japanese Patent No. 2943021

[0011] Patent Document 3: International Publication No. 2018 / 221738

[0012] Patent Document 4: Japanese Patent Publication No. 2017-536472 Summary of the Invention

[0013] The problem the invention aims to solve

[0014] When hot stamping steel coated with Al, oxygen and atmospheric moisture are consumed during the oxidation of Al. The hydrogen released at this time is sometimes absorbed by the steel, becoming a potential cause of delayed fracture.

[0015] The objective of this invention is to provide a hot-stamped molded body with excellent resistance to delayed fracture.

[0016] Solutions for solving problems

[0017] One embodiment of the present invention relates to a hot-stamped formed body comprising a steel substrate and an Al coating formed on the steel substrate, the Al coating comprising: an interface layer formed at the interface with the steel substrate, having a structure in which a portion of αFe is replaced by 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 a Fe-Al-Si phase having a structure in which a portion of αFe is replaced by Al and Si, the Fe-Al-Si phase comprising Zr, Ce, Y, Ta, Ni, and C. The Fe-Al-Si phase contains one or more elements selected from the group consisting of u, Nb, Cr, Co, V, and Ti, with the Si content being 1-20% by mass and the total content of Zr, Ce, Y, Ta, Ni, Cu, Nb, Cr, Co, V, and Ti being 0.10-5.0% by mass. The oxide film layer contains one or more elements selected from the group consisting of Be, Mg, Ca, Sr, Ba, Sc, and Zn, with the total ratio of Be, Mg, Ca, Sr, Ba, Sc, and Zn in the components other than oxygen being 0.01-80.0% by mass.

[0018] The effects of the invention

[0019] According to the present invention, a hot stamped part with excellent resistance to delayed fracture can be obtained. Attached Figure Description

[0020] Figure 1 A cross-sectional view illustrating the structure of the coated steel before hot stamping.

[0021] Figure 2 A cross-sectional view illustrating an example of the structure of a hot-stamped body formed by hot stamping coated steel.

[0022] Figure 3 A cross-sectional view illustrating the structure of a hot-stamped formed article according to one embodiment of the present invention is shown.

[0023] Figure 4 This is a cross-sectional photograph of the Al coating designated a1.

[0024] Figure 5 This is a cross-sectional photograph of the Al coating, designated A13. Detailed Implementation

[0025] The inventors have conducted various studies on hydrogen absorption during hot stamping of Al-plated steel, particularly Al-Si-plated steel.

[0026] Figure 1 A cross-sectional view schematically showing the structure of the coated steel 1A before hot stamping is shown. The coated steel 1A includes a steel substrate 10 and an Al-Si coating 30. Between the steel substrate 10 and the Al-Si coating 30, the coated steel 1A also includes an Al-Si-Fe alloy layer 35 formed by the diffusion of Fe from the steel substrate 10 into the coating.

[0027] Figure 2 A cross-sectional view is shown to illustrate an example of a hot-stamped formed body 9, which is formed by hot stamping a coated steel material 1A. The hot-stamped formed body 9 includes a steel substrate 10 and an Al coating 20 formed on the steel substrate 10.

[0028] The Al coating 20 consists 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 film layer 23 formed on the intermediate layer 22. The intermediate layer 22 contains Fe-Al-Si phases 22a and Al-Fe phases 22b with different structures; sometimes the Fe-Al-Si phases 22a and Al-Fe phases 22b are distributed in a layered manner. Furthermore, the above classification is based on observations using optical microscopes and scanning electron microscopes (SEM). When observed using transmission electron microscopes (TEM), it is possible to observe that individual layers are aggregates of several crystalline phases.

[0029] Interface layer 21 is derived from plated steel material 1A ( Figure 1 The Al-Si-Fe alloy layer 35 is primarily a portion of the bcc structure with αFe replaced by Al and Si. The interface layer 21 sometimes contains intermediate phases such as the τ phase.

[0030] The intermediate layer 22 comprises a Fe-Al-Si phase 22a and an Al-Fe phase 22b. The Fe-Al-Si phase 22a, like the interface layer 21, has a structure in which a portion of the α-Fe bcc structure is replaced by Al and Si. The Al-Fe phase 22b has Fe₄Al 13 The structure of Fe2Al5. The Fe-Al-Si phase 22a is mostly like... Figure 2 As shown, it is formed in a layered (band-like) state, located at the middle position in the thickness direction of the intermediate layer 22 and sandwiched between the upper and lower Al-Fe phase 22b.

[0031] The oxide film layer 23 is a layer mainly composed of Al oxides.

[0032] The inventors have determined that when the Fe-Al-Si phase 22a contains a specified amount of a specific element, specifically, one or more of the following 11 elements (Zr, Ce, Y, Ta, Ni, Cu, Nb, Cr, Co, V, and Ti, hereinafter referred to as "Group A elements"), the hydrogen absorption of the steel substrate 10 is suppressed.

[0033] Figure 3 A cross-sectional view illustrating the structure of a hot-stamped formed body 1, which schematically shows an example of a hot-stamped formed body containing a specified amount of group A elements in the Fe-Al-Si phase 22a. In hot-stamped formed body 1, compared with hot-stamped formed body 9 (… Figure 2 Compared to the case of ), the size of the Fe-Al-Si phase 22a is smaller. In the hot-stamped body 9 ( Figure 2 In the original Fe-Al-Si phase 22a, it is layered (banded), while in the hot-stamped body 1, the Fe-Al-Si phase 22a becomes discontinuous. Furthermore, group A elements are selectively distributed in the Fe-Al-Si phase 22a.

[0034] Therefore, it can be concluded that the elements in group A are selectively distributed in the Fe-Al-Si phase 22a during heating, and have the effect of hindering the stabilization of the Fe-Al-Si phase 22a.

[0035] The growth of the Al-Fe phase 22b is promoted by hindering the stabilization of the Fe-Al-Si phase 22a. As the Al-Fe phase 22b thickens, the diffusion of Al to the surface of the Al coating 20 is suppressed, and the oxidation of Al is inhibited. Therefore, hydrogen absorption of the steel substrate 10 is suppressed. Furthermore, hydrogen absorption of the steel substrate 10 is suppressed by fixing hydrogen atoms in the Al-Fe phase 22b.

[0036] The inventors further clarified that by including one or more elements selected from Be, Mg, Ca, Sr, Ba, Sc, and Zn (hereinafter referred to as "Group B elements") in a specified amount in the oxide film layer 23, hydrogen absorption of the steel substrate 10 can be further suppressed. Group B elements have the property of being more easily oxidized than Al, and have the effect of suppressing hydrogen absorption of the steel substrate 10 by suppressing the oxidation of Al.

[0037] By making the Fe-Al-Si phase 22a of the intermediate layer 22 contain a specified amount of group A elements and the oxide film layer 23 contain a specified amount of group B elements, the hot stamping molded body 1 with excellent resistance to delayed fracture can be obtained by utilizing their superposition effect.

[0038] This invention is based on the above insights. Hereinafter, embodiments of the invention will be described in detail with reference to the accompanying drawings. Identical or equivalent parts in the drawings are labeled with the same symbols, and their descriptions are not repeated. The dimensional ratios between the constituent parts shown in the figures do not necessarily represent actual dimensional ratios.

[0039] [Hot-stamped formed body]

[0040] Refer again Figure 3 The structure of a hot-stamped formed body 1 according to one embodiment of the present invention will be described. The hot-stamped formed body 1 is a formed body formed by hot stamping a steel billet. The hot-stamped formed body 1 includes a steel substrate 10 and an Al coating 20 formed on the steel substrate 10.

[0041] 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 film layer 23 formed on the intermediate layer 22.

[0042] [Interface Layer]

[0043] Interface layer 21 is formed at the interface with the steel substrate 10. Interface layer 21 has a structure in which a portion of the αFe bcc structure is replaced by Al and Si.

[0044] The chemical composition of the interface layer 21 has a distribution that varies along the thickness direction. For example, the chemical composition of the interface layer 21, on average along the thickness direction, is Fe: 60–98% by mass, Al: 1–40% by mass, and Si: 1–20% by mass. 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% by mass, more preferably 1.5% by mass. Sometimes the boundary between the interface layer 21 and the steel substrate 10 is unclear; however, in such cases, the average chemical composition of the range from the interface of the intermediate layer 22 to a depth of 10 μm is taken as the interface layer 21.

[0045] The chemical composition of the interface layer 21 can be determined by analyzing the cross section of the Al coating 20 using electron beam microscopy (EPMA) and energy dispersive spectroscopy (EDS) of SEM.

[0046] The thickness of the interface layer 21 is not particularly limited, for example, it is 5 to 15 μm.

[0047] [Middle Layer]

[0048] Intermediate layer 22 is formed on interface layer 21. Intermediate layer 22 comprises Fe-Al-Si phase 22a and Al-Fe phase 22b. Fe-Al-Si phase 22a, like interface layer 21, has a structure in which a portion of the αFe bcc structure is replaced by Al and Si. Al-Fe phase 22b has Fe4Al... 13 The structure of Fe2Al5. It should be noted that sometimes Si is also present in solid solution in the Al-Fe phase 22b.

[0049] The Fe-Al-Si phase 22a and Al-Fe phase 22b can be distinguished by analyzing the chemical composition of the cross-section of the Al coating 20 using EPMA and SEM EDS. The Fe-Al-Si phase 22a and Al-Fe phase 22b can also be distinguished by analyzing their crystal structures using X-ray diffraction and electron diffraction.

[0050] It should be noted that 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, while the Fe-Al-Si phase 22a is formed at the middle position in the thickness direction of the intermediate layer 22.

[0051] The Si content in the Fe-Al-Si phase 22a is 1–20% by mass. If the Si content is less than 1% by mass, the oxide film layer 23 grows, resulting in an increase in hydrogen absorption in the steel substrate 10. If the Si content is greater than 20% by mass, the intermediate layer 22 cannot grow sufficiently, and the hydrogen absorption in the steel substrate 10 will still increase. The lower limit of the Si content in the Fe-Al-Si phase 22a is preferably 5% by mass, more preferably 8% by mass. The upper limit of the Si content in the Fe-Al-Si phase 22a is preferably 18% by mass, more preferably 16% by mass.

[0052] The Fe content in the Fe-Al-Si phase 22a is not particularly limited, for example, it is 30 to 80% by mass. The Al content in the Fe-Al-Si phase 22a is not particularly limited, for example, it is 5 to 50% by mass.

[0053] In addition to Fe, Al, and Si, the Fe-Al-Si phase 22a contains one or more elements selected from the group consisting of Zr, Ce, Y, Ta, Ni, Cu, Nb, Cr, Co, V, and Ti. Hereinafter, these 11 elements—Zr, Ce, Y, Ta, Ni, Cu, Nb, Cr, Co, V, and Ti—are referred to as Group A elements. Among Group A elements, Zr, Ce, Ni, Cr, Co, V, and Ti are preferred, with Ni and Cr being particularly preferred.

[0054] The total content of group A elements in the Fe-Al-Si phase 22a is 0.10 to 5.0% by mass. Group A elements promote the growth of Al-Fe phase 22b by hindering the stabilization of Fe-Al-Si phase 22a and inhibiting hydrogen absorption of the steel substrate 10. If the total content of group A elements is less than 0.1% by mass, this effect cannot be sufficiently obtained. On the other hand, if the total content of group A elements is greater than 5.0% by mass, the Fe-Al-Si phase 22a may sometimes become enlarged. The lower limit of the total content of group A elements in Fe-Al-Si phase 22a is preferably 0.20% by mass, and more preferably 0.30% by mass. The upper limit of the content of group A elements in Fe-Al-Si phase 22a is preferably 4.0% by mass, and more preferably 3.0% by mass.

[0055] The Fe-Al-Si phase 22a may contain small amounts of elements other than Si, Fe, Al, and group A elements. The upper limit of the total content of elements other than Si, Fe, Al, and group A elements in the Fe-Al-Si phase 22a is preferably 1.0% by mass, more preferably 0.5% by mass.

[0056] The chemical composition of the Fe-Al-Si phase 22a, similar to that of the interface layer 21, can be determined by EDS analysis of the cross-section of the Al coating 20 using EPMA and SEM. 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 determined using the same method.

[0057] The chemical composition of Al-Fe phase 22b is not particularly limited, but may be, for example, Fe: 15–70 wt%, Al: 30–85 wt%, Si: 0–20 wt%. As described above, Al-Fe phase 22b essentially has the following composition: Fe₄Al. 13 It has the same structure as Fe2Al5, but sometimes contains 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 in the Al-Fe phase 22b is preferably 1.0% by mass, more preferably 0.5% by mass.

[0058] The area fraction of the Fe-Al-Si phase 22a in the intermediate layer 22 is preferably 5% to 35%. A lower area fraction of the Fe-Al-Si phase 22a in the intermediate layer 22 tends to suppress hydrogen absorption in the steel substrate 10 more. The upper limit of the area fraction of the Fe-Al-Si phase 22a in the intermediate layer 22 is further preferably 30%, and more preferably 25%.

[0059] The intermediate layer 22 may contain a small amount of phases other than the Fe-Al-Si phase 22a and the Al-Fe phase 22b. For example, the τ phase may be a phase other than the Fe-Al-Si phase 22a and the Al-Fe phase 22b. The area fraction 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, more preferably 5.0% or less.

[0060] The thickness of the intermediate layer 22 is 15 μm or more. The thicker the intermediate layer 22, the more suppressed the diffusion of Al to the surface of the Al coating 20, and the more suppressed the oxidation of Al. Furthermore, hydrogen is easily immobilized by the intermediate layer 22. Therefore, the thicker the intermediate layer 22, the more suppressed the hydrogen absorption of the steel substrate 10. Furthermore, the thicker the intermediate layer 22, the better the coating adhesion and corrosion resistance. 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, for example, it is 50 μm.

[0061] [Oxide film]

[0062] An oxide film 23 is formed on the intermediate layer 22. The oxide film 23 is primarily composed of Al oxides. The oxide film 23 may contain elements other than Al and O. For example, the oxide film 23 may contain Si, Fe, and the aforementioned A group elements.

[0063] The oxide film layer 23 further comprises one or more elements selected from the group consisting of Be, Mg, Ca, Sr, Ba, Sc, and Zn. Hereinafter, these seven elements—Be, Mg, Ca, Sr, Ba, Sc, and Zn—are referred to as group B elements. Among the group B elements, Mg, Ca, Sr, and Ba are preferred, with Mg being particularly preferred. The group B elements are preferably present in the form of oxides.

[0064] The total percentage of Group B elements (hereinafter referred to as "the percentage of Group B elements") in the oxide film layer 23, excluding oxygen, is 0.01 to 80.0% by mass. Group B elements are more easily oxidized than Al, and thus have the effect of suppressing hydrogen absorption of the steel substrate 10 by inhibiting Al oxidation. If the percentage of Group B elements is less than 0.01% by mass, this effect cannot be fully achieved. If the percentage of Group B elements is greater than 80.0% by mass, the oxide film layer 23 may sometimes become thicker. The lower limit of the percentage of Group B elements is preferably 1.0% by mass, more preferably 3.0% by mass, more preferably 10% by mass, and more preferably 15% by mass. The upper limit of the percentage of Group B elements is preferably 60.0% by mass, more preferably 40.0% by mass, and more preferably 35% by mass.

[0065] The chemical composition of the oxide film layer 23, like that of the interface layer 21 and the intermediate layer 22, can be determined by EDS analysis of the cross-section of the Al coating 20 using EPMA and SEM. TEM EDS is also an option. The chemical composition of the oxide film layer 23 can also be determined by depth-direction composition analysis using glow discharge spectroscopy (GDS) and Auger electron spectroscopy (AES).

[0066] The thickness of the oxide film layer 23 is preferably 0.01 to 1.00 μm. There is a tendency that the thinner the oxide film layer 23, the more the hydrogen absorption of the steel substrate 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.

[0067] 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 by using GDS to reduce the oxygen detection intensity to 1 / 6 of the maximum value.

[0068] It should be noted that, as a combination of elements from group A and group B, the combination of Ni and Mg, and the combination of Cr and Mg are particularly preferred. That is, the Fe-Al-Si phase 22a contains one or two elements selected from the group consisting of Ni and Cr, and the total content of Ni and Cr in the Fe-Al-Si phase 22a is 0.10 to 5.0% by mass. The oxide film layer 23 contains Mg, and the proportion of Mg in the components other than oxygen in the oxide film layer 23 is particularly preferred to be 0.01 to 80.0% by mass.

[0069] [Steel substrate]

[0070] There are no particular restrictions on the steel substrate 10, as long as it is suitable for hot stamping. Examples of steel substrates suitable for use in hot-stamped formed bodies 1 include those with the following chemical composition (in mass percent): 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%, with the balance being Fe and impurities. Examples of the form of the steel substrate before hot stamping include hot-rolled steel sheets and cold-rolled steel sheets. The chemical composition of the steel substrate 10 will be described below. In the following explanation, the percentage of element content refers to mass%.

[0071] C: 0.1–0.6%

[0072] Carbon (C) is included to ensure the target mechanical strength. If the C content is too low, the mechanical strength may not be sufficiently improved. On the other hand, if the C content is too high, the elongation and reduction of area may decrease. The upper limit of the C content is more preferably 0.4%.

[0073] Si: 0.01~0.60%

[0074] Silicon (Si) is an element that enhances mechanical strength, and like carbon, it is included to ensure the target mechanical strength. If the Si content is too low, the improvement in mechanical strength may not be sufficient. On the other hand, if the Si content is too high, the wettability during plating may be reduced due to the Si oxide formed on the surface of the steel substrate, potentially resulting in no plating.

[0075] Mn: 0.50~3.00%

[0076] Manganese (Mn) is one of the strengthening elements in steel and also improves hardenability. Mn can also effectively prevent hot work brittleness caused by sulfur (S), which is 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 will be too much retained austenite, which may reduce strength.

[0077] P: below 0.05%

[0078] Phosphorus (P) is an impurity contained in steel substrates. P in steel substrates can sometimes segregate to the grain boundaries, thus reducing the toughness of the steel substrate. The P content should ideally be as low as possible.

[0079] S: below 0.020%

[0080] Sulfur (S) is an impurity contained in steel substrates. S in steel substrates can sometimes form sulfides, which reduces the toughness of the steel substrate. The S content should ideally be as low as possible.

[0081] Al: below 0.10%

[0082] Aluminum (Al) is typically used for deoxidation of steel. However, when the Al content is high, the Ac3 point of the steel substrate increases, thus requiring a higher heating temperature to ensure the hardenability of the steel during hot stamping. Therefore, the Al content is preferably 0.10% or less. More preferably, it is 0.05% or less, and even more preferably 0.01% or less.

[0083] Ti: 0.01~0.10%

[0084] Titanium (Ti) is one of the strength-enhancing elements. If the Ti content is too low, the strength and oxidation resistance improvements may not be fully achieved. On the other hand, if the Ti content is too high, carbides and nitrides may form, potentially softening the steel.

[0085] B: 0.0001~0.0100%

[0086] Boron (B) plays a role in increasing strength during quenching. If the B content is too low, the strength improvement effect may not be fully achieved. On the other hand, if the B content is too high, inclusions will form, the steel substrate will become brittle, and fatigue strength may decrease.

[0087] N: below 0.010%

[0088] Nitrogen (N) is an impurity contained in steel substrates. N in steel substrates can sometimes form nitrides, thus reducing the toughness of the steel substrate. Furthermore, N in steel substrates can sometimes bond with boron (B), reducing the amount of boron dissolved in the substrate and decreasing the effect of improving the hardenability of boron. The N content should ideally be as low as possible.

[0089] The steel substrate 10 may contain Cr, Mo, Cu and Ni.

[0090] Cr: 0–1.0%

[0091] Mo: 0–1.0%

[0092] Cu: 0–1.0%

[0093] Ni: 0–1.0%

[0094] To improve the hardenability of steel substrates, one or more elements selected from the group consisting of chromium (Cr), molybdenum (Mo), copper (Cu), and nickel (Ni) can be included. The preferred lower limit for the content of these elements is 0.01%. On the other hand, even if they are included in excess, the effect will saturate, leading to increased costs.

[0095] The chemical composition of the steel substrate 10 consists of Fe and impurities. Impurities, as referred to here, are elements introduced from ores or waste used as raw materials for steel, or from the environment during the manufacturing process. Examples of impurities, in addition to those listed above, include Zn, Co, Sn, Nb, V, As, Zr, Ca, and Mg.

[0096] [Manufacturing method of hot-stamped parts]

[0097] Next, an example of a manufacturing method for the hot-stamped formed body 1 will be described. In the manufacturing method described below, an Al coating is applied to a steel billet such as a steel sheet to produce a coated steel material, and the coated steel material is hot-stamped to form an Al coating 20 on the steel substrate 10. The method described here is an example and does not limit the manufacturing method of the hot-stamped formed body 1.

[0098] [Plating Process]

[0099] A coating is formed on the surface of a steel billet using hot-dip galvanizing. The preferred temperature of the galvanizing bath is 600–700°C. If the temperature is below 600°C, the galvanizing bath becomes low-viscosity, making uniform coating difficult. If the temperature is above 700°C, the composition changes rapidly due to volatilization, making process management more difficult.

[0100] The addition of Group A elements is carried out by adding them to the plating bath. The content of Group A elements in the plating bath is preferably 0.05 to 5.0% by mass. There is a tendency that the higher the content of Group A elements in the plating bath, the higher the content of Group A elements in the Fe-Al-Si phase 22a becomes. The lower limit of the content of Group A elements in the plating bath is more preferably 0.2% by mass, and even more preferably 0.5% by mass. The upper limit of the content of Group A elements in the plating bath is more preferably 3.0% by mass, and even more preferably 2.0% by mass.

[0101] The addition of group B elements is also carried out by adding them to the plating bath. When group B elements are added, the content of group B elements in the plating bath is preferably 0.01 to 1.0% by mass. There is a tendency that the higher the content of group B elements in the plating bath, the higher the content of group B elements in the oxide film layer 23 becomes. The lower limit of the content of group B elements in the plating bath is more preferably 0.05% by mass, and even more preferably 0.08% by mass. The upper limit of the content of group B elements in the plating bath is more preferably 0.8% by mass, and even more preferably 0.6% by mass.

[0102] The Si content in the plating bath is, for example, 1.0 to 20.0% by mass. The balance of the plating bath is mainly Al. The plating bath may contain small amounts of elements other than Al, Si, elements from group A, and elements from group B. The total content of elements other than Al, Si, elements from group A, and elements from group B in the plating bath is preferably 5.0% by mass or less, more preferably 3.0% by mass or less, more preferably 2.0% by mass or less, and more preferably 1.0% by mass or less.

[0103] The steel billet is kept in a hydrogen-reducing atmosphere at 700–800°C and then immersed in a plating bath. It is preferable to perform all processes in a non-oxidizing atmosphere. The plating is preferably performed to achieve a coating thickness of 20–30 μm. The coating thickness is adjusted such that the thickness of the intermediate layer 22 after the hot stamping process is 15 μm or more. The coating thickness can be adjusted by adjusting the temperature, viscosity, immersion time, and gas blowing of the plating bath.

[0104] [Hot stamping process]

[0105] After the coated steel is formed to the required size, it is hot-stamped. The heating method can be either a high-temperature furnace or electric heating. The heating rate is, for example, 1 to 50°C / s. It is preferred to hold the temperature at 850 to 950°C for at least 2 minutes. The cooling rate is, for example, 30 to 1000°C / s.

[0106] 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.

[0107] Through the above processes, the hot-stamped formed body 1 can be manufactured. The above describes a method for hot-stamping coated steel. However, alternatively, an Al coating layer can be formed by depositing Al or the like on the surface of the steel substrate 10 using vapor deposition or spraying, and the steel substrate 10 with the Al coating layer can be hot-stamped to manufacture the hot-stamped formed body 1.

[0108] According to this embodiment, a hot stamped part with excellent resistance to delayed fracture can be obtained.

[0109] Example

[0110] The present invention will be described in more detail below through examples. However, the present invention is not limited to these examples.

[0111] For steel plates with the chemical composition shown in Table 1, Al-Si coatings are formed on both sides of the steel plates using a hot-dip galvanizing method.

[0112] [Table 1]

[0113] Table 1

[0114]

[0115] The plating bath used was the one with the chemical composition shown in Table 2. The temperature of the plating bath for hot-dip plating was set to 700℃. The steel sheet was immersed in the plating bath, and then the adhesion was adjusted to 70 g / m² per single side using a gas wiping method. 2 .

[0116] [Table 2]

[0117] Table 2

[0118]

[0119] The coated steel sheet is heated in a resistance furnace at 900°C for 5 minutes under a synthetic air flow with a dew point of 20°C. Then, it is cooled in a mold while being formed, resulting in a hot-stamped part.

[0120] The contents of Si in the Fe-Al-Si phase, the contents 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 oxide film (excluding oxygen), and the thickness of the oxide film were investigated for the obtained hot-stamped parts. In addition, as characteristics, resistance to hydrogen embrittlement (resistance to delayed fracture), coating adhesion, post-coating corrosion resistance, and pitting corrosion resistance were evaluated.

[0121] [Resistance to hydrogen embrittlement]

[0122] Hydrogen analysis was performed on the obtained hot-stamped parts. The hydrogen analysis was carried out by temperature-programmed desorption, and the hydrogen released up to 250°C was defined as diffusible hydrogen, which was scored according to its amount as follows.

[0123] 2: The amount of diffusible hydrogen is less than 0.1 ppm by mass.

[0124] 1: The amount of diffusible hydrogen is greater than or equal to 0.1 ppm by mass and less than 0.2 ppm by mass.

[0125] 0: The amount of diffusible hydrogen is 0.2 ppm or more by mass.

[0126] [Coating Adhesion]

[0127] The adhesion of the coating was evaluated according to the method described in Japanese Patent No. 4373778. That is, after immersing the sample in deionized water at 60°C for 240 hours, 100 checkerboard patterns with a spacing of 1 mm were cut with a cutter. The area ratio was calculated based on the number of peeled parts of the checkerboard pattern by visual measurement, and the following score was given.

[0128] 3: The peeled area is more than 0% and less than 10%.

[0129] 2: The peeled area is more than 10% but less than 70%.

[0130] 1: The peeling area is more than 70% and less than 100%.

[0131] [Corrosion resistance after coating]

[0132] The corrosion resistance evaluation after coating was conducted according to the method specified in JASO M609 established by the China Automotive Technology and Research Center. Scratches were made on the coating with a cutter, and the width of the coating expansion from the scratches (maximum value on one side) was measured after 180 cycles of corrosion testing, and the score was given as follows.

[0133] 3: Expansion width ≥ 0mm and < 1.5mm

[0134] 2: Expansion width ≥ 1.5mm and < 3.0mm

[0135] 1: Expansion width 3.0mm or more

[0136] [Resistance to pitting corrosion]

[0137] The sample was immersed in PREPALENEEX, a surface conditioner manufactured by Nihon Parkerizing Co., Ltd., for 1 minute at room temperature, and then immersed in Palbond SX35, a coating substrate chemical conversion agent manufactured by the same company, for 2 minutes at 35°C. Then, a composite cyclic corrosion test was performed using 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 cut with a cutting tool as described in JIS H 8502. The reduction in sheet thickness of the cut section after 60 cycles was scored as follows.

[0138] 5: The reduction in plate thickness is less than 0.1mm

[0139] 4: The reduction in plate thickness is greater than 0.1mm but less than 0.2mm

[0140] 3: The reduction in plate thickness is greater than 0.2mm but less than 0.3mm.

[0141] 2: The reduction in plate thickness is greater than 0.3mm but less than 0.4mm.

[0142] 1: Plate thickness reduction of 0.4mm or more

[0143] The results are shown in Table 3.

[0144] [Table 3]

[0145]

[0146] As shown in Table 3, in the hot-stamped bodies designated A01 to A17, the Si content in the Fe-Al-Si phase ranges from 1% to 20% by mass, and the content of group A elements ranges from 0.10% to 5.0% by mass. In the hydrogen embrittlement resistance test, the amount of diffusible hydrogen in these hot-stamped bodies is less than 0.2 ppm by mass. These hot-stamped bodies also exhibit good coating adhesion, post-coating corrosion resistance, and pitting corrosion resistance. Among them, the hot-stamped bodies A01–A05, A10, A12, A13, A15, and A17, which contain group B elements in their oxide film layer, show particularly excellent hydrogen embrittlement resistance (resistance to delayed fracture) with a diffusible hydrogen content of less than 0.1 ppm by mass in the hydrogen embrittlement resistance test.

[0147] In the hydrogen embrittlement resistance test, the hot-stamped bodies designated a1 to a7 exhibited a diffusible hydrogen content exceeding 0.2 ppm by mass, indicating poorer hydrogen embrittlement resistance (resistance to delayed fracture) compared to the hot-stamped bodies designated A01 to A20. This can be attributed to the fact that the content of group A elements in the Fe-Al-Si phase of the hot-stamped bodies designated a1 to a7 is less than 0.1% by mass or greater than 5.0% by mass.

[0148] Figure 4 This is a cross-sectional photograph of the Al coating designated a1. Figure 5 This is a cross-sectional photograph of the Al coating, designated A13. Figure 4 and Figure 5 As shown, in designation A13, the area fraction of the Fe-Al-Si phase is smaller compared to designation a1. Furthermore, in designation a1, the layered (banded) Fe-Al-Si phase becomes discontinuous.

[0149] The embodiments of the present invention have been described above, but these embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above embodiments, and can be implemented by appropriate modifications without departing from its spirit.

[0150] Explanation of reference numerals in the attached figures

[0151] 1.9 Hot stamping formed body

[0152] 1A Coated Steel

[0153] 10 Steel substrate

[0154] 20 Al coating

[0155] 21 Interface Layer

[0156] 22 Intermediate Layer

[0157] 22a Fe-Al-Si phase

[0158] 22b Al-Fe phase

[0159] 2. Oxide film layer

[0160] 30 Al-Si coating

[0161] 35 Al-Si-Fe alloy layer

Claims

1. A hot-stamped formed body comprising a steel substrate and an Al coating formed on said steel substrate, The Al coating comprises: An interface layer, formed at the interface with the steel substrate, has a structure in which a portion of αFe is replaced by Al and Si; An intermediate layer is formed on the interface layer; as well as An oxide film layer is formed on the intermediate layer. The thickness of the intermediate layer is 15 μm or more. The intermediate layer comprises a Fe-Al-Si phase with a structure in which a portion of αFe is replaced by 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, V, and Ti. The Si content in the Fe-Al-Si phase is 1–20% by mass, and the total content of Zr, Ce, Y, Ta, Ni, Cu, Nb, Cr, Co, V, and Ti is 0.10–5.0% by mass. The total content of elements other than Si, Fe, Al, Zr, Ce, Y, Ta, Ni, Cu, Nb, Cr, Co, V, and Ti in the Fe-Al-Si phase is less than 0.5% by mass. The oxide film layer contains one or more elements selected from the group consisting of Be, Mg, Ca, Sr, Ba, Sc, and Zn. The total ratio of Be, Mg, Ca, Sr, Ba, Sc and Zn in the oxide film layer, excluding oxygen, is 0.01 to 80.0% by mass.

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

3. The hot-stamped formed body according to claim 1 or 2, wherein, The thickness of the oxide film is 0.01 to 1.0 μm.

4. The hot-stamped formed article according to claim 1 or 2, wherein, The total content of Ni and Cr in the Fe-Al-Si phase is 0.10–5.0% by mass. The proportion of Mg in the oxide film, excluding oxygen, is 0.01–80.0% by mass.

5. The hot-stamped formed article according to claim 3, wherein, The total content of Ni and Cr in the Fe-Al-Si phase is 0.10 to 5.0% by mass, and the proportion of Mg in the oxide film (excluding oxygen) is 0.01 to 80.0% by mass.

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