WO2024172076A1 - ホットスタンプ成形体およびホットスタンプ用アルミめっき鋼板 - Google Patents

ホットスタンプ成形体およびホットスタンプ用アルミめっき鋼板 Download PDF

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Publication number
WO2024172076A1
WO2024172076A1 PCT/JP2024/005045 JP2024005045W WO2024172076A1 WO 2024172076 A1 WO2024172076 A1 WO 2024172076A1 JP 2024005045 W JP2024005045 W JP 2024005045W WO 2024172076 A1 WO2024172076 A1 WO 2024172076A1
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Prior art keywords
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content
steel sheet
diffusion layer
hot
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PCT/JP2024/005045
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English (en)
French (fr)
Japanese (ja)
Inventor
和久 楠見
宗士 藤田
優貴 鈴木
進一郎 田畑
秀昭 入川
晴彦 江口
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Nippon Steel Corp
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Nippon Steel Corp
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Application filed by Nippon Steel Corp filed Critical Nippon Steel Corp
Priority to KR1020257023832A priority Critical patent/KR20250126768A/ko
Priority to CN202480008418.9A priority patent/CN120569505A/zh
Priority to EP24756917.1A priority patent/EP4667600A4/en
Priority to JP2025501182A priority patent/JPWO2024172076A1/ja
Publication of WO2024172076A1 publication Critical patent/WO2024172076A1/ja
Priority to MX2025009335A priority patent/MX2025009335A/es
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B15/00—Layered products comprising a layer of metal
    • B32B15/01—Layered products comprising a layer of metal all layers being exclusively metallic
    • B32B15/012—Layered products comprising a layer of metal all layers being exclusively metallic one layer being formed of an iron alloy or steel, another layer being formed of aluminium or an aluminium alloy
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/62—Quenching devices
    • C21D1/673—Quenching devices for die quenching
    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0257—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment with diffusion of elements, e.g. decarburising, nitriding
    • C—CHEMISTRY; METALLURGY
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C21/00—Alloys based on aluminium
    • C22C21/02—Alloys based on aluminium with silicon as the next major constituent
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    • C22C—ALLOYS
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    • C23C30/00—Coating with metallic material characterised only by the composition of the metallic material, i.e. not characterised by the coating process
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a hot stamped steel and an aluminum-plated steel sheet for hot stamping.
  • This application claims priority based on Japanese Patent Application No. 2023-022724, filed on February 16, 2023, the contents of which are incorporated herein by reference.
  • Hot stamping technology in which steel sheets are heated to high temperatures in the austenite region where they soften before being press-formed, is being promoted.
  • Hot stamping is attracting attention as a technology that achieves both forming into vehicle body parts and ensuring strength by simultaneously performing a quenching process in a die during press forming.
  • Patent Document 1 discloses an aluminum-based plated steel sheet for hot pressing that has excellent heat resistance and corrosion resistance, characterized by having an Al-based metal coating on the surface of the steel that is mainly made of Al and contains 1-15% Si and 0.5-10% Mg.
  • the present invention has been made in consideration of the above-mentioned circumstances.
  • the object of the present invention is to provide a hot stamped body having high strength and excellent corrosion resistance, and an aluminum-plated steel sheet for hot stamping from which this hot stamped body can be manufactured.
  • the gist of the present invention is as follows.
  • a hot stamped product according to one aspect of the present invention A steel plate, A diffusion layer disposed on the steel plate; an Fe—Al alloy layer disposed on the diffusion layer;
  • the chemical composition of the steel sheet is, in mass%, C: 0.050 to 0.250%; Si: 0.010-2.000%, Mn: 0.80-3.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001-0.500%, N: 0.0001 to 0.0150%, O: 0.1000% or less, Ti: 0.005-0.100%, B: 0.0005 to 0.0050%, Cu: 0.050-3.000%, As: 0.0005 to 1.0000%, Cr: 0-1.000%, Mo: 0-1.000%, Ni: 0-3.000%, Co: 0 to 0.5000%, W: 0-3.0000%, Sn: 0-0.5000%, Nb: 0 to 0.1000%, V: 0 to 0.100%, Zr: 0 to 0.100%
  • Cu d represents the Cu concentration in atomic % in the diffusion layer
  • Cu m represents the Cu concentration in atomic % in the Fe-Al phase in the Fe-Al alloy layer
  • Cu 0 represents the Cu concentration in atomic % in the steel sheet.
  • Td represents the average thickness in ⁇ m of the diffusion layer
  • Tm represents the average thickness in ⁇ m of the Fe—Al alloy layer.
  • An aluminum-plated steel sheet for hot stamping comprises: A steel plate, An Fe-Al-Si alloy layer disposed on the steel plate; an Al-based plating layer disposed on the Fe-Al-Si alloy layer; The chemical composition of the steel sheet is
  • Cu i represents the Cu concentration in atomic % in the Fe-Al-Si alloy layer
  • Cu 0 represents the Cu concentration in atomic % in the steel plate.
  • the above-mentioned aspects of the present invention can provide a hot stamped body having high strength and excellent corrosion resistance, as well as an aluminum-plated steel sheet for hot stamping from which this hot stamped body can be manufactured.
  • FIG. 2 is a schematic diagram showing an example of a cross section of a hot stamped body including an Fe—Al alloy layer, a diffusion layer, and a steel plate.
  • FIG. 1 is a schematic diagram showing an example of a cross section of an aluminum-plated steel sheet for hot stamping, the aluminum-based plating layer, an Fe—Al—Si alloy layer, and a steel sheet.
  • the hot stamped body and aluminum-plated steel sheet for hot stamping according to this embodiment will be described in detail below.
  • the hot stamped steel according to this embodiment includes a steel sheet, a diffusion layer disposed on the steel sheet, and an Fe—Al alloy layer disposed on the diffusion layer.
  • the chemical composition of the steel plate constituting the hot stamped product according to this embodiment contains, in mass%, C: 0.050 to 0.250%, Si: 0.010 to 2.000%, Mn: 0.80 to 3.00%, P: 0.100% or less, S: 0.0100% or less, Al: 0.001 to 0.500%, N: 0.0001 to 0.0150%, O: 0.1000% or less, Ti: 0.005 to 0.100%, B: 0.0005 to 0.0050%, Cu: 0.050 to 3.000%, As: 0.0005 to 1.0000%, and the balance: Fe and impurities.
  • C 0.050 to 0.250%
  • Si 0.010 to 2.000%
  • Mn 0.80 to 3.00%
  • P 0.100% or less
  • S 0.0100% or less
  • Al 0.001 to 0.500%
  • N 0.0001 to 0.0150%
  • O 0.1000% or less
  • B: 0.0005 to 0.0050% Cu: 0.050 to
  • C 0.050-0.250%
  • C is an element that greatly affects the strength of a hot stamped steel. If the C content is less than 0.050%, the strength of the hot stamped steel is reduced. Therefore, the C content is set to 0.050%.
  • the C content is preferably 0.070% or more, and more preferably 0.090% or more.
  • the C content exceeds 0.250%, the strength of the hot stamped steel becomes too high, and the steel is prone to fracture when the automobile is crashed. Therefore, the C content is set to 0.250% or less.
  • the C content is preferably less than 0.250%, 0.249% or less, 0.247% or less, 0.245% or less, and more preferably 0.240% or less.
  • Si 0.010-2.000% Silicon has resistance to temper softening and is effective in suppressing the decrease in strength due to auto-tempering during hot stamp quenching. If the silicon content is less than 0.010%, the above effect cannot be obtained, and the hot stamped product The strength decreases. Therefore, the Si content is set to 0.010% or more.
  • the Si content is preferably 0.020% or more, 0.030% or more, 0.150% or more, or 0.200% or more. It is.
  • the Si content exceeds 2.000%, a problem of surface scale occurs. That is, after the scale formed during hot rolling is pickled, a pattern due to surface irregularities occurs, resulting in poor surface appearance.
  • the Si content is set to 2.000% or less.
  • the content is preferably 1.700% or less, 1.500% or less, 1.450% or less, and more preferably 1.000% or less, 0.800% or less.
  • Mn 0.80-3.00%
  • Mn is an element that improves the strength of a hot stamped body and the hardenability of steel. If the Mn content is less than 0.80%, the strength of the hot stamped body decreases.
  • the Mn content is preferably 1.00% or more, 1.20% or more, or 1.40% or more. On the other hand, if the Mn content exceeds 3.00%, the above effects become saturated and the toughness of the hot stamped steel deteriorates. Therefore, the Mn content is set to 3.00% or less. , preferably 2.80% or less, 2.60% or less, and more preferably 2.40% or less, 2.00% or less.
  • P 0.100% or less
  • P is an element that segregates at grain boundaries and reduces the strength of the grain boundaries. If the P content exceeds 0.100%, the strength of the grain boundaries is significantly reduced, and the toughness of the hot stamped steel deteriorates. Therefore, the P content is set to 0.100% or less.
  • the P content is preferably 0.080% or less, 0.050% or less.
  • the lower limit of the P content is not particularly specified, and may be 0%. However, since an excessive reduction in the P content increases the refining cost, the P content may be 0.001% or more.
  • S 0.0100% or less
  • S is an element that affects nonmetallic inclusions in steel and deteriorates the toughness of hot stamped steel. Therefore, the S content is set to 0.0100% or less.
  • the S content is preferably 0.0080% or less, more preferably 0.0050% or less.
  • the lower limit of the S content is not particularly specified, and may be 0%. However, since excessively reducing the S content increases the manufacturing cost of the desulfurization process, the S content may be 0.0001% or more.
  • Al 0.001-0.500%
  • Al is an element used as a deoxidizer for molten steel. If deoxidization is insufficient, the toughness of the hot stamped steel deteriorates due to the excessive oxides produced.
  • the Al content is set to 0.001% or more.
  • the Al content is preferably 0.010% or more, and more preferably 0.030% or more.
  • the Al content is set to 0.500% or less.
  • the Al content is preferably 0.300% or less, 0.200% or less, or 0.100% or less.
  • N 0.0001-0.0150% If the N content exceeds 0.0150%, coarse nitrides are formed in the steel, causing the toughness of the hot stamped steel to deteriorate significantly. Therefore, the N content is set to 0.0150% or less.
  • the N content is preferably 0.0100% or less, and more preferably 0.0080% or less. If the N content is reduced to less than 0.0001%, the cost of denitrification increases significantly, which is economically undesirable. Therefore, the N content is set to 0.0001% or more.
  • the N content is preferably set to 0.0005% or more, 0.0010% or more.
  • the O content is set to 0.1000% or less.
  • the O content is preferably 0.0080% or less or 0.0050% or less.
  • the lower limit of the O content is not particularly specified, but may be 0%.
  • the O content may be 0.0005% or more, or 0.0010% or more.
  • Ti forms carbonitrides in steel and increases the strength of hot stamped steel through precipitation strengthening. In addition, Ti fixes N as nitrides, suppressing the formation of BN and improving the hardenability of B. If the Ti content is less than 0.005%, the above effect cannot be obtained, and the strength of the hot stamped product decreases. Therefore, the Ti content is set to 0.005%.
  • the Ti content is preferably 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, if the Ti content exceeds 0.100%, a large amount of carbonitrides are formed, which deteriorates the toughness of the hot stamped steel. Therefore, the Ti content is set to 0.100% or less. The amount is preferably not more than 0.080% or not more than 0.070%.
  • B 0.0005-0.0050% B improves the hardenability during hot stamping or during cooling after hot stamping, thereby improving the strength of the hot stamped body. If the B content is less than 0.0005%, the above effect cannot be obtained. Therefore, the B content is set to 0.0005% or more, and preferably 0.0007% or more, and more preferably 0.0010% or more. On the other hand, if the B content exceeds 0.0050%, the above effect becomes saturated and cracks may occur during hot rolling. Therefore, the B content is set to 0.0050% or less. is preferably 0.0040% or less or 0.0030% or less.
  • Cu 0.050-3.000% Cu is an element that enhances the corrosion resistance of the steel sheet that constitutes the hot stamped steel.
  • the corrosion resistance can be further enhanced by including a desired amount of Cu in the diffusion layer. If the Cu content is less than 0.050%, it is not possible to allow the diffusion layer to contain a desired amount of Cu, and the corrosion resistance of the hot stamped product is deteriorated. The corrosion resistance of the steel sheet itself deteriorates. Therefore, the Cu content is set to 0.050% or more.
  • the Cu content is preferably set to 0.070% or more, 0.100% or more, 0.120% or more, 0. More than 150%.
  • the Cu content exceeds 3.000%, the carbides present after hot rolling, cold rolling or annealing (including after plating) are stabilized, and the carbides are reduced by heating during hot stamping.
  • the Cu content is preferably 2.000% or less, and the Cu content is preferably 2.000% or less. 800% or less, 2.600% or less.
  • the balance of the chemical composition of the hot stamped body according to this embodiment may be Fe and impurities.
  • impurities include elements that are inevitably mixed in from steel raw materials or scrap and/or during the steelmaking process and are permissible to the extent that they do not impair the properties of the hot stamped body according to this embodiment.
  • the chemical composition of the hot stamped body according to this embodiment may contain the following elements as optional elements in place of a portion of Fe. If the following optional elements are not contained, the content is 0%.
  • Ni: 0.005-3.000% Cr, Mo and Ni are elements that improve the hardenability of steel and have the effect of improving the strength of the hot stamped steel. In addition, these elements have the effect of improving the corrosion resistance of the hot stamped steel.
  • the Cr content is set to 0.020% or more
  • the Mo content is set to 0.020% or more.
  • the Mo content is preferably 0.008% or more, or the Ni content is preferably 0.005% or more.
  • the Cr content is more preferably 0.040% or more, or 0.100% or more.
  • the Ni content is more preferably 0.010% or more, and 0.020% or more.
  • the Ni content is more preferably 0.010% or more, and 0.020% or more.
  • the Cr content or Mo content exceeds 1.000%, or when the Ni content exceeds 3.000%, after hot rolling, cold rolling or annealing (including after plating treatment), This stabilizes the carbides present in Cr and H, which may delay the dissolution of the carbides during heating during hot stamping, resulting in reduced hardenability. As a result, the strength of the hot stamped product may decrease.
  • the Mo content is set to 1.000% or less, and the Ni content is set to 3.000% or less.
  • the Cr and Mo contents are preferably set to 0.700% or less and 0.500% or less, respectively.
  • the Ni content is preferably 2.000% or less, and more preferably 1.500% or less.
  • Co is an element that improves corrosion resistance in a corrosive environment. Therefore, Co may be contained as necessary.
  • the Co content is set to 0.0005% or more.
  • the Co content is preferably 0.0010% or more, more preferably 0.0020% or more.
  • the Co content is set to 0.5000% or less. More preferably, the Co content is It is less than 0.4000%.
  • W 0.0005-3.0000%
  • W is an element that suppresses phase transformation at high temperatures and contributes to improving the strength of hot stamped steel.
  • W has the effect of improving the corrosion resistance of hot stamped steel. Therefore, W may be added as necessary.
  • the W content is preferably 0.0005% or more. More preferably, the W content is 0.0010% or more. , 0.0030% or more.
  • the W content exceeds 3.0000%, the hot workability may deteriorate, leading to a decrease in productivity, or the strength of the hot stamped steel may decrease.
  • the W content is preferably 2.5000% or less, more preferably 2.0000% or less.
  • Sn 0.0005-0.5000%
  • Sn has the effect of improving the corrosion resistance of the hot stamped steel. Therefore, Sn may be contained as necessary.
  • the Sn content is set to 0.0005% or more.
  • the Sn content is more preferably 0.0010% or more, 0.0030% or more, or 0.0050% or more.
  • the Sn content is set to 0.5000% or less.
  • the Sn content is preferably set to 0.4000% or less, more preferably 0.3000% or less. % or less, and 0.2000% or less.
  • Nb 0.0002-0.1000% Nb, as a solid solution element, refines the grain structure and thereby increases the strength of the hot stamped steel. Therefore, Nb may be contained as necessary.
  • the Nb content is preferably 0.0002% or more, more preferably 0.0005% or more, 0.0010% or more, or 0.0030% or more.
  • the Nb content is set to 0.1000% or less.
  • the amount is preferably not more than 0.0800% or not more than 0.0700%.
  • V has the effect of forming carbonitrides in steel and improving the strength of hot stamped steel through precipitation strengthening.
  • V acts as a solid solution element to refine the grain structure, thereby improving the strength of hot stamped steel.
  • V has the effect of improving the strength and bendability of the steel. Therefore, V may be contained as necessary.
  • the V content should be 0.005% or more.
  • the V content is preferably 0.010% or more, more preferably 0.020% or more.
  • the V content exceeds 0.100%, a large amount of carbonitrides are formed, which deteriorates the bendability of the hot stamped steel. Therefore, the V content is set to 0.100% or less.
  • the content is preferably 0.070% or less, and more preferably 0.050% or less.
  • Zr 0.005-0.100%
  • Zr has the effect of forming carbonitrides in steel and improving the strength of hot stamped bodies by precipitation strengthening. Furthermore, it fixes N as nitrides, suppresses the formation of BN, and improves the hardenability of B. Therefore, Zr may be contained as necessary.
  • the Zr content is preferably 0.005% or more. The content is more preferably 0.010% or more, and more preferably 0.020% or more. On the other hand, if the Zr content exceeds 0.100%, a large amount of carbonitrides is generated, and the bendability of the hot stamped steel deteriorates. Therefore, the Zr content is set to 0.100% or less. The content is preferably 0.070% or less, and more preferably 0.050% or less.
  • Ca 0.0005-0.0050%
  • Ca, Mg and REM have the effect of refining inclusions in steel and preventing the occurrence of cracks due to the inclusions during hot stamping. Therefore, one or more of these elements may be added as necessary.
  • the content of at least one of Ca, Mg and REM is 0.0005% or more. Contents of Ca, Mg and REM is more preferably 0.0010% or more.
  • the content of Ca, Mg or REM exceeds 0.0050% the effect of refining inclusions in the steel is saturated and the alloy cost increases.
  • the contents of Ca, Mg and REM are The contents of Ca, Mg and REM are preferably 0.0040% or less, 0.0030% or less and 0.0020% or less, respectively.
  • REM refers to a total of 17 elements consisting of Sc, Y and lanthanoids, and the content of REM refers to the total content of these elements.
  • Sb 0.0005-0.0200%
  • Sb may be added as necessary to suppress decarburization during hot rolling. By adding Sb, decarburization during hot rolling can be suppressed. This effect can be reliably achieved.
  • the Sn content is preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0050% or more.
  • the Sb content is set to 0.0200% or less.
  • the Sb content is preferably set to 0.0150% or less, more preferably 0.0100% or less. The following is the result.
  • Pb 0.0003-0.0100%
  • Pb has the effect of suppressing the growth of austenite grains during the heating process of hot stamping, so it may be contained as necessary.
  • the Pb content is 0.0003
  • the Pb content is preferably 0.0005% or more.
  • the Pb content is set to 0.0100% or less.
  • the amount is preferably not more than 0.0080%.
  • the chemical composition of the hot stamped body may be measured by a general analytical method. For example, it may be measured by using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Note that C and S may be measured by a combustion-infrared absorption method, N by an inert gas fusion-thermal conductivity method, and O by an inert gas fusion-non-dispersive infrared absorption method. The chemical composition is analyzed after removing the diffusion layer and the Fe-Al alloy layer on the surface of the hot stamped body by mechanical grinding.
  • ICP-AES Inductively Coupled Plasma-Atomic Emission Spectrometry
  • the metal structure of the steel plate constituting the hot stamped body according to this embodiment is composed, by area percentage, of 90.0 to 100.0% martensite, 0.0 to 3.0% retained austenite, and a total of 0.0 to 10.0% of one or more of the following: ferrite, pearlite, bainite, and cementite.
  • the metal structure is specified at a position that is 1/4 of the thickness of the steel plate that constitutes the hot stamped body (the region from 1/8 of the thickness of the steel plate to 3/8 of the thickness of the steel plate from the surface). This is because the metal structure at this position represents a representative metal structure of the steel plate that constitutes the hot stamped body.
  • Martensite 90.0 to 100.0% Martensite is a structure that increases the strength of a hot stamped body. If the area ratio of martensite is less than 90.0%, the strength of the hot stamped body decreases. Therefore, the area ratio of martensite is set to 90.0% or more.
  • the area ratio of martensite is preferably 93.0% or more, 95.0% or more, or 97.0% or more. Since the area ratio of martensite is preferably as high as possible, the area ratio of martensite may be 100.0%.
  • Retained austenite 0.0 to 3.0% Although retained austenite is a structure that enhances the ductility of a hot stamped body, if the area fraction is too high, the strength of the hot stamped body decreases. If the area fraction of retained austenite exceeds 3.0%, the decrease in strength of the hot stamped body becomes significant. Therefore, the area fraction of retained austenite is set to 3.0% or less.
  • the area fraction of retained austenite is preferably 2.0% or less, more preferably 1.0% or less, and even more preferably 0.0%.
  • Remaining structure 0.0 to 10.0% in total
  • the remaining structure is one or more of ferrite, pearlite, bainite, and cementite.
  • the area ratio of the remaining structure is set to 10.0% or less in total.
  • the area ratio of the remaining structure is preferably set to 7.0% or less, 5.0% or less, or 3.0% or less in total. Since the area ratio of the remaining structure is preferably as low as possible, the total area ratio of the remaining structure may be 0.0%.
  • the area ratio of each structure is measured by the following method.
  • a sample is cut out from the hot stamped compact so that a cross section perpendicular to the surface and 1/4 of the thickness can be observed (a region from 1/8 of the thickness from the surface to 3/8 of the thickness from the surface).
  • the cross section of the thickness of this sample is polished stepwise using silicon carbide paper from #600 to #1500, and then finished to a mirror surface using a liquid in which diamond powder with a grain size of 1 to 6 ⁇ m is dispersed in a diluted solution such as alcohol or pure water, and then Nital etching is performed.
  • the reagent used is 2% Nital (2% nitric acid ethyl alcohol solution), and the sample is immersed in it for 10 seconds or less at room temperature.
  • a thermal field emission scanning electron microscope (e.g., JSM-7001F manufactured by JEOL Ltd.) is used to take secondary electron images in at least five fields of view at a position 1/4 of the plate thickness.
  • the acceleration voltage is 15 kV and the working distance is 10 mm.
  • An equally spaced grid is drawn on the backscattered electron image, and the structure at the grid points is identified. The number of grid points corresponding to each structure is found and divided by the total number of grid points to obtain the area ratio of each structure. The greater the total number of grid points, the more accurately the area ratio can be obtained.
  • the grid spacing is 2 ⁇ m ⁇ 2 ⁇ m, and the total number of grid points is 1500.
  • the magnification can be selected so that the microstructure can be adequately determined, and a magnification of 2000 times is preferable.
  • the diffusion layer, Fe-Al alloy layer, and steel plate can be distinguished because they have different brightness in the backscattered electron image.
  • the area located at the center of the plate thickness in the backscattered electron image is the steel plate, and the area ratio of each structure is measured for this steel plate.
  • the backscattered electron image can be obtained by observing under the same conditions using the same electron microscope as the secondary electron image and switching the detector.
  • each tissue can be distinguished by the following methods. Ferrite can be identified as a phase that is roughly equiaxed grain and does not contain any internal carbides. The region where cementite precipitates in a lamellar form within the grains is determined to be pearlite. Granular regions with high brightness and a grain size (circle equivalent diameter) of 2 ⁇ m or less are judged to be cementite. The area where intragranular lath-like structure appears by etching is judged to be martensite. The following literature provides examples of martensite structure observation. Masashi Maki and Imao Tamura: Tetsu-to-Haganen, Vol. 67, No. 1, 1987, pp. 852-866.
  • the plate thickness cross section of this sample is polished stepwise using silicon carbide paper from #600 to #1500 in sequence, and then finished to a mirror surface using a dilution liquid such as alcohol or a liquid in which diamond powder with a grain size of 1 to 6 ⁇ m is dispersed in pure water, and the Vickers hardness at the 1/4 position of the plate thickness is measured.
  • the test force is set to 98.07 N, and the average value of the five points is obtained. If the average value H of the Vickers hardness satisfies H(HV) ⁇ 764 ⁇ C+208, it can be determined that the area ratio of martensite is 90.0% or more. In this embodiment, when martensite cannot be clearly identified by image analysis or when there is doubt about the identification of the structure, the area ratio of martensite is determined by Vickers hardness.
  • the area fraction of retained austenite is measured by X-ray diffraction.
  • the integrated intensity of a total of five peaks, ⁇ (200), ⁇ (211), ⁇ (200), ⁇ (220), and ⁇ (311) is determined using a Rigaku RINT-2200 and Mo-K ⁇ ray at a position 1/4 of the surface of the plate thickness cross section perpendicular to the surface (region 1/8 depth of plate thickness from the surface to 3/8 depth of plate thickness from the surface), and the volume fraction of retained austenite is calculated using the intensity averaging method. This volume fraction of retained austenite is regarded as the area fraction of retained austenite.
  • the area ratio of each texture is measured by tissue observation using secondary electron images and X-ray diffraction, so the total of the area ratios of each texture obtained by measurement may not be 100.0%. If the total of the area ratios of each texture obtained by the above method is not 100.0%, the area ratios of each texture are converted so that the total of the area ratios of each texture is 100.0%. For example, if the total of the area ratios of each texture is 103.0%, the area ratio of each texture is multiplied by "100.0/103.0" to obtain the area ratio of each texture.
  • the diffusion layer is disposed between the above-mentioned steel plate and the Fe-Al alloy layer described below.
  • the diffusion layer has a chemical composition, in mass%, of Al: more than 0.5% and less than 20.0%, Fe: more than 80.0% and less than 99.5%, and Si: 0.0 to 15.0%, the Cu concentration in the diffusion layer satisfies the following formulas (1) and (2), and the average thickness of the diffusion layer satisfies the following formula (3).
  • Cu d represents the Cu concentration in atomic % in the diffusion layer
  • Cu m represents the Cu concentration in atomic % in the Fe-Al phase in the Fe-Al alloy layer.
  • Cu 0 indicates the Cu concentration in atomic % in the steel sheet.
  • Td represents the average thickness in ⁇ m of the diffusion layer
  • Tm represents the average thickness in ⁇ m of the Fe—Al alloy layer.
  • the diffusion layer is a layer in which Al is diffused into Fe, and the Al content in the diffusion layer is set to be more than 0.5%, and preferably, the Al content in the diffusion layer is 1.0% or more, and 5.0% or more.
  • the Al content in the diffusion layer is 20.0% or more, the Fe-Al phase and the ⁇ 1 phase are generated in the diffusion layer, and the desired diffusion layer cannot be obtained. Therefore, the Al content in the diffusion layer is set to less than 20.0%.
  • the Fe content in the diffusion layer must be greater than 80.0% and less than 99.5% in relation to the Al content.
  • the Si content in the diffusion layer is 0.0-15.0%. Since Si may or may not be contained in the diffusion layer, the lower limit of the Si content is 0.0%. However, if the Si content in the diffusion layer exceeds 15.0%, Fe-Al phase and ⁇ 1 phase will be generated in the diffusion layer, and the desired diffusion layer cannot be obtained.
  • the chemical composition of the diffusion layer may contain Al, Fe, and Si, as well as elements other than Cu described below, with the remaining elements (Zn, Cu, Na, K, Co, Ni, Mg, etc., and elements contained in the steel components) totaling less than 10.0%.
  • Fig. 1 is a schematic diagram showing an example of a cross section of a hot stamped body including an Fe-Al alloy layer, a diffusion layer, and a steel sheet.
  • the Fe-Al alloy layer contains an Fe-Al phase mainly containing Fe and Al, and a ⁇ 1 phase containing Si.
  • a diffusion layer exists between the Fe-Al alloy layer and the steel sheet.
  • the Fe-Al phase in the Fe-Al alloy layer contains 40 to 60 mass % of Fe and 40 to 60 mass % of Al.
  • the present inventors have obtained the following findings regarding the diffusion layer.
  • a corrosion potential difference occurs between the diffusion layer and the Fe-Al phase in the Fe-Al alloy layer present on the surface side of the diffusion layer, and a sacrificial corrosion protection effect can be obtained. This effect prevents corrosion pits from reaching the steel sheet, and the corrosion depth can be suppressed. As a result, the corrosion resistance of the hot stamped steel is improved.
  • the Cu concentration (Cu d ) in the diffusion layer is set to 0.025 atomic % or more.
  • the Cu concentration (Cu d ) in the diffusion layer is preferably 0.040 atomic % or more, 0.060 atomic % or more, or 0.100 atomic % or more.
  • the Cu concentration (Cu d ) in the diffusion layer is equal to or lower than the Cu concentration (Cu m ) in the Fe—Al phase in the Fe—Al alloy layer, the sacrificial corrosion protection effect of the Fe—Al phase in the Fe—Al alloy layer on the diffusion layer is not obtained. As a result, the corrosion resistance of the hot stamped body is deteriorated. Therefore, the Cu concentration (Cu d ) in the diffusion layer is set to exceed the Cu concentration (Cu m ) in the Fe—Al phase in the Fe—Al alloy layer.
  • the Cu concentration in the diffusion layer (Cu d ) is set to be less than the Cu concentration in the steel sheet (Cu 0 ).
  • the average thickness Td of the diffusion layer is set to 2.0 ⁇ m or more.
  • the average thickness Td of the diffusion layer is preferably 3.0 ⁇ m or more, and more preferably 4.0 ⁇ m or more.
  • the average thickness Td of the diffusion layer exceeds 0.8 ⁇ ( Td + Tm ) ( Td indicates the average thickness of the diffusion layer in ⁇ m, and Tm indicates the average thickness of the Fe-Al alloy layer in ⁇ m), the diffusion layer becomes too thick, making it difficult to generate a corrosion potential difference between the Fe-Al phase in the Fe-Al alloy layer and the diffusion layer, and the sacrificial corrosion protection effect is not fully exhibited. As a result, the corrosion resistance of the hot stamped body deteriorates. Therefore, the average thickness Td of the diffusion layer is set to 0.8 ⁇ ( Td + Tm )( ⁇ m) or less.
  • the average thickness Td of the diffusion layer is preferably 0.7 ⁇ ( Td + Tm )( ⁇ m) or less, more preferably 0.6 ⁇ ( Td + Tm )( ⁇ m) or less.
  • the Fe—Al alloy layer is disposed on the diffusion layer.
  • the Fe—Al alloy layer is not particularly limited.
  • the Fe-Al alloy layer may include an Fe-Al phase mainly containing Fe and Al, and a ⁇ 1 phase containing Si.
  • the Fe-Al phase contains 40 to 60 mass% Fe and 40 to 60 mass% Al.
  • the ⁇ 1 phase contains 50 to 77 mass% Fe, 20 to 40 mass% Al, and 3 to 10 mass% Si.
  • the Fe-Al phase in the Fe-Al alloy layer specifically refers to one or more of the FeAl 2 phase and the Fe 2 Al 5 phase, although there is no need to distinguish between them in this embodiment.
  • the chemical composition of the Fe-Al alloy layer may be Fe: 25-80% by mass, Al: 20-60% by mass, Si: 0-10%, and the balance: less than 5%.
  • the average thickness of the Fe-Al alloy layer is not particularly limited, but may be 5 to 60 ⁇ m.
  • the chemical compositions of the diffusion layer and the Fe—Al alloy layer, and the Cu concentration in the steel sheet are measured by the following methods.
  • a backscattered electron image is taken of a cross section of the plate thickness perpendicular to the surface of the hot stamped body by the same method as that used to measure the structural fraction of the metal structure of the steel plate constituting the hot stamped body.
  • the two layers arranged on the surface side and the steel plate which is the region located at the center of the plate thickness, are distinguished from each other based on the difference in brightness.
  • the layer arranged on the surface side is regarded as an Fe-Al alloy layer
  • the layer arranged between the Fe-Al alloy layer and the steel plate is regarded as a diffusion layer.
  • the element (Al, Fe, Si) is quantitatively analyzed by point analysis using an electron probe microanalyzer (EPMA, for example, "JXA-8530F” manufactured by JEOL Ltd.) for the thickness center part of the part regarded as the diffusion layer from the backscattered electron image.
  • EPMA electron probe microanalyzer
  • a device having a field emission type electron gun for example, "JXA-8530F” manufactured by JEOL Ltd.
  • the quantitative correction is performed using the ZAF correction method.
  • the content and concentration of each element are obtained by performing quantitative analysis for at least five points and calculating the average value from the obtained results.
  • the layer is determined to be a diffusion layer.
  • the Cu concentration is measured by the same method to obtain the Cu concentration (Cu d ) in atomic % in the diffusion layer.
  • the central portion of the portion that is determined to be an Fe-Al alloy layer from the backscattered electron image is subjected to quantitative analysis of each element by the same method as described above.
  • the layer that is determined to be an Fe-Al alloy layer show a chemical composition, in mass %, of Fe: 25-80 mass %, Al: 20-60 mass %, Si: 0-10%, and the balance: less than 5%
  • the layer is determined to be an Fe-Al alloy layer.
  • a region containing 40 to 60 mass % Fe and 40 to 60 mass % Al is identified from the results of the quantitative analysis. This region is identified as the Fe-Al phase.
  • the Cu concentration is measured by the same method as above to obtain the Cu concentration (Cu m ) in atomic % in the Fe-Al phase.
  • the Cu concentration in the central part of the part regarded as the steel sheet in the backscattered electron image (the central part of the steel sheet in the backscattered electron image) is measured by the same method as above to obtain the Cu concentration (Cu 0 ) in atomic % in the steel sheet.
  • the Fe-Al alloy layer, the diffusion layer, and the steel sheet are identified by the same method as that used to measure the chemical compositions of the diffusion layer and the Fe-Al alloy layer of the hot stamped body, and the Cu concentration in the steel sheet.
  • the layer thickness in the sheet thickness direction is measured on the backscattered electron image at 2 ⁇ m intervals in a direction perpendicular to the sheet thickness direction at a total of 25 locations (within a range of 50 ⁇ m in the direction perpendicular to the sheet thickness direction).
  • the average value of the obtained values at the 25 locations is calculated to obtain the average thickness of the diffusion layer.
  • the Fe-Al alloy layer is measured by the same method to obtain the average thickness of the Fe-Al alloy layer.
  • the plate thickness of the hot stamped steel according to this embodiment is not particularly limited, but may be 0.5 to 3.5 mm from the viewpoint of reducing the vehicle body weight.
  • the tensile strength of the hot stamped steel according to this embodiment may satisfy the following criteria depending on the C content of the steel plate.
  • the tensile strength As described above, the effect of reducing the weight of the vehicle body can be further enhanced.
  • There is no particular upper limit to the tensile strength but if the tensile strength is too high, the bendability deteriorates, so the tensile strength may be 1700 MPa or less.
  • the tensile strength is determined by preparing a No. 5 test piece as described in JIS Z 2241: 2022 and following the test method as described in JIS Z 2241: 2022.
  • the tensile test piece is taken from the center position in the sheet width direction, and the direction perpendicular to the rolling direction is the longitudinal direction.
  • any direction may be regarded as the rolling direction, and the direction perpendicular to the rolling direction may be regarded as the sheet width direction. If the hot stamped body is too small or has a complex shape to obtain a No.
  • a rectangular piece having a parallel portion of any width may be obtained, and a tensile test may be performed using the rectangular piece to obtain the tensile strength from the maximum test force and the original cross-sectional area of the parallel portion.
  • the position of the rectangular piece is not particularly limited as long as a rectangular (flat) piece can be obtained, but it is preferable to obtain the rectangular piece at the center of the flat portion where the strain is as small as possible.
  • the direction of the rectangular piece is the longitudinal direction perpendicular to the rolling direction.
  • the hot stamped steel according to the present embodiment is evaluated for corrosion resistance after painting by the method specified in JASO M609-91 established by the Society of Automotive Engineers of Japan.
  • a sample is taken from the hot stamped compact, and a 70 mm long scratch is made with a cutter on the flat surface of the sample to which an electrodeposition coating film of 15 ⁇ m thickness is applied, and the sample is subjected to a cyclic corrosion test. After 120 cycles, the sample is taken out and immersed in a commercially available coating film remover (e.g., "CS-500" manufactured by Neos) for 30 minutes, and then the coating film is peeled off with a brush.
  • a commercially available coating film remover e.g., "CS-500” manufactured by Neos
  • the sample is then immersed in a 10% by volume aqueous solution of ammonium citrate containing an inhibitor for steel sheets (e.g., "Hibiron YA-9R" manufactured by Sugimura Chemical Industry Co., Ltd.), and rust formed on the corroded parts is removed with a brush.
  • the temperature of the liquid is 80 to 90° C., and the immersion time is about 30 to 60 minutes. If rust remains after one immersion, the immersion is repeated until the rust is removed.
  • the center of the 70 mm scratch is used as the boundary, and the maximum thickness reduction from the reference surface is measured every 35 mm of the scratch length.
  • the reference surface is the surface of the uncorroded area after the paint film is peeled off, regardless of whether it is plated or not. The average value of the two maximum thickness reductions obtained is calculated.
  • the obtained average value of the maximum thickness reduction can be evaluated according to the following criteria: When the evaluation is E, it can be determined that the hot stamped steel has particularly excellent corrosion resistance.
  • E Excellent: Less than 0.05 mm V (Very Good): 0.05 mm or more, less than 0.10 mm G (Good): 0.10 mm or more, less than 0.15 mm B (Bad): 0.15 mm or more
  • the aluminum-plated steel sheet for hot stamping comprises a steel sheet, an Fe-Al-Si alloy layer disposed on the steel sheet, and an Al-based plating layer disposed on the Fe-Al-Si alloy layer.
  • the metal structure of the steel sheet that constitutes the aluminum-plated steel sheet for hot stamping consists, by area percentage, of 20-95% ferrite and a total of 5-80% of one or more of the following: martensite, bainite, pearlite, and cementite.
  • the metal structure is specified at a position that is 1/4 of the thickness of the steel sheet that constitutes the aluminum-plated steel sheet for hot stamping (a region from 1/8 of the thickness of the steel sheet from the surface to 3/8 of the thickness of the steel sheet from the surface). This is because the metal structure at this position represents a representative metal structure of the steel sheet that constitutes the aluminum-plated steel sheet for hot stamping.
  • the area ratio of ferrite is set to 20% or more.
  • the area ratio of ferrite is preferably 30% or more, 40% or more, 50% or more, or 60% or more.
  • the area ratio of ferrite is set to 95% or less.
  • the area ratio of ferrite is preferably 90% or less, more preferably 80% or less.
  • Remaining tissue 5-80% in total
  • the remaining structure is one or more of martensite, bainite, pearlite, and cementite.
  • the remaining structure is set to 5 to 80% in total.
  • the area ratio of the remaining structure is preferably set to 70% or less, 60% or less, 50% or less, or 40% or less in total.
  • the area ratio of the remaining structure is preferably set to 10% or more, 20% or more in total.
  • the area ratio of each structure in the steel sheet that constitutes the aluminum-plated steel sheet for hot stamping is measured using the same method as for the hot stamped body.
  • the Al-based plating layer, the Fe-Al-Si alloy layer, and the steel sheet can be distinguished because they have different brightness and morphology in the backscattered electron image.
  • the area located at the center of the sheet thickness in the backscattered electron image is the steel sheet, and the area ratio of each structure is measured for this steel sheet.
  • the Fe-Al-Si alloy layer is disposed between the above-mentioned steel sheet and an Al-based plating layer described later.
  • the Fe-Al-Si alloy layer has a Cu content satisfying the following formula (4).
  • Cui represents the Cu concentration in atomic % in the Fe-Al-Si alloy layer
  • Cu 0 represents the Cu concentration in atomic % in the steel sheet.
  • FIG 2 is a schematic diagram showing an example of a cross section of an aluminum-plated steel sheet for hot stamping comprising an Al-based plating layer, an Fe-Al-Si alloy layer, and a steel sheet.
  • a needle-shaped Si phase is present in a phase mainly composed of Al.
  • an Fe-Al-Si alloy layer is present between the Al-based plating layer and the steel sheet.
  • the present inventors have obtained the following findings regarding the Fe-Al-Si alloy layer.
  • the Fe-Al-Si alloy layer grows by diffusion of Fe from the steel sheet through hot stamping, and becomes the Fe-Al alloy layer in the hot stamped body.
  • the Cu content in the Fe-Al alloy layer is high and there is no difference between the Cu content in the Fe-Al alloy layer and the Cu content in the diffusion layer, the sacrificial corrosion protection effect is not sufficiently obtained. As a result, the corrosion resistance of the hot stamped body deteriorates.
  • the Cu content in the Fe-Al-Si alloy layer that is the source of the Fe-Al alloy layer can be reduced, and the corrosion resistance of the hot stamped body can be improved.
  • the Cu concentration (Cu i ) in the Fe-Al-Si alloy layer exceeds 0.40 ⁇ Cu 0 (Cu 0 is the Cu concentration in atomic % in the steel sheet), the above-mentioned effect cannot be obtained, and the corrosion resistance of the hot stamped body deteriorates. Therefore, the Cu concentration (Cu i ) in the Fe-Al-Si alloy layer is set to 0.40 ⁇ Cu 0 (atomic %) or less.
  • the Cu concentration (Cu i ) in the Fe-Al-Si alloy layer is preferably 0.25 ⁇ Cu 0 (atomic %) or less, 0.15 ⁇ Cu 0 (atomic %) or less, or 0.10 ⁇ Cu 0 (atomic %) or less.
  • the Cu content Cu i in the Fe-Al-Si alloy layer may be 0.015 mass % or less, or 0.030 mass % or less.
  • the Cu content Cu i in the Fe-Al-Si alloy layer is preferably as low as possible, and the lower limit may be set to 0 atomic %.
  • the Fe-Al-Si alloy layer may contain, in mass %, Al: 40 to 70%, Fe: 20 to 50%, and Si: 5 to 20%, in addition to Cu.
  • the chemical composition of the Fe-Al-Si alloy layer is measured by the same method as that for the diffusion layer of the hot stamped body. Specifically, it is measured by the following method. First, a backscattered electron image is taken in the same manner as for the diffusion layer of the hot stamped body.
  • the steel sheet is identified as the region located at the center of the sheet thickness, and the layer disposed on the most surface is regarded as the Al-based plating layer, and the layer disposed between the Al-based plating layer and the steel sheet is regarded as the Fe-Al-Si alloy layer.
  • a quantitative analysis is performed using EPMA on the thickness center portion of the layer regarded as the Fe-Al-Si alloy layer from the backscattered electron image.
  • the measurement result of the layer regarded as the Fe-Al-Si alloy layer shows a chemical composition, in mass %, of Al: 40-70%, Fe: 20-50%, and Si: 5-20%, the layer is determined to be the Fe-Al-Si alloy layer.
  • the average thickness of the Fe-Al-Si alloy layer is not particularly limited, but may be 2 to 10 ⁇ m.
  • the average thickness of the Fe-Al-Si alloy layer is measured using the same method as for the diffusion layer of the hot stamped body.
  • Al-based plating layer The Al-based plating layer is disposed on the above-mentioned Fe-Al-Si alloy layer. As described above, the Al-based plating layer has a needle-shaped Si phase in a phase mainly composed of Al.
  • the overall chemical composition of the Al-based plating layer is, in mass%, Al: 85.0 to 95.0%, Si: 5.0 to 15.0%.
  • the Al content in the Al-based plating layer is less than 85.0%, the corrosion resistance after hot stamping (corrosion resistance in the hot stamped body) deteriorates. Therefore, the Al content is set to 85.0% or more.
  • the Al content is preferably 88.0% or more.
  • the Al content in the Al-based plating layer exceeds 95.0%, the plating adhesion deteriorates. Therefore, the Al content is set to 95.0% or less.
  • the Al content is preferably 92.0% or less.
  • the Si content in the Al-based plating layer is less than 5.0%, the plating adhesion deteriorates. Therefore, the Si content is set to 5.0% or more, and preferably 7.0% or more. On the other hand, if the Si content in the Al-based plating layer exceeds 15.0%, the plating adhesion deteriorates. Therefore, the Si content is set to 15.0% or less. The Si content is preferably 13.0% or less.
  • the chemical composition of the Al-based plating layer may contain elements other than Al and Si, such as 0.1-10.0% Fe, 0.1-45.0% Zn, and the remainder (Cu, Na, K, Co, Ni, Mg, etc.) totaling less than 0.5%.
  • the chemical composition of the Al-based plating layer can be measured using the same method as for the chemical composition of the diffusion layer of the hot stamped body.
  • the average thickness of the Al-based plating layer is not particularly limited, but may be 5 to 50 ⁇ m.
  • the average thickness of the Al-based plating layer is measured using the same method as for the diffusion layer of the hot stamped body.
  • the sheet thickness of the aluminum-plated steel sheet for hot stamping according to this embodiment is not particularly limited, but may be 0.5 to 3.5 mm from the viewpoint of reducing the weight of the vehicle body.
  • the aluminum-plated steel sheet for hot stamping according to the present embodiment can be stably produced by the production method including the following steps.
  • the temperatures described below refer to the surface temperatures of the slab or steel plate.
  • a preferred method for producing the aluminum-plated steel sheet for hot stamping includes the following steps.
  • Rough rolling is performed so that the total interpass time in the temperature range of 1050 to 1150° C. is 120 seconds or less.
  • the steel sheet is immersed in the plating bath for an immersion time t (seconds) that satisfies the following formula (5) and is 8.00 seconds or less, thereby imparting plating to the surface of the steel sheet.
  • t immersion time
  • Rough rolling is performed so that the total interpass time in the temperature range of 1050 to 1150° C. is 120 seconds or less.
  • Cu contained in the steel sheet is unlikely to be contained in the scale, and therefore concentrates near the surface of the steel sheet in the temperature range of 1050 to 1150°C during rough rolling. If plating is applied to a steel sheet with Cu concentrated near the surface, the Cu content of the Fe-Al-Si alloy layer of the aluminum-plated steel sheet for hot stamping will be high. Therefore, the total interpass time in the temperature range of 1050 to 1150°C, where Cu is likely to concentrate near the surface of the steel sheet, is shortened.
  • the interpass time can be adjusted by controlling the shipping speed of the steel sheet.
  • the total interpass time in the temperature range of 1050 to 1150°C is set to 120 seconds or less.
  • the total interpass time is preferably 100 seconds or less, 80 seconds or less, or 60 seconds or less.
  • the conditions for slab heating before rough rolling, finish rolling after rough rolling, cold rolling, etc. are not particularly limited and may be performed according to conventional methods. That is, for example, slab heating is performed in the range of 1100 to 1350°C, and after the above-mentioned rough rolling, finish rolling is performed as hot rolling with 4 to 8 stages, a total reduction of all stages (total reduction) of 70 to 98%, an entry temperature for finish rolling of 900 to 1200°C, and a finish rolling completion temperature of 800 to 1100°C.
  • the steel sheet after hot rolling is pickled, and cold rolling is performed with a reduction of 30 to 85%.
  • the steel sheet is immersed in the plating bath so that the immersion time t (seconds) satisfies the above formula (5). If the immersion time t (seconds) is longer than the right side of the above formula (5), the Cu content in the Fe-Al-Si alloy layer of the aluminum-plated steel sheet for hot stamping becomes high. If the immersion time t (seconds) is shorter than the left side of the above formula (5), the Fe-Al-Si alloy layer is not sufficiently generated, and the adhesion of the Al-based plating layer deteriorates. If the immersion time t (seconds) is longer than 8.00 seconds, the Fe-Al-Si alloy layer grows too much, and powdering of the plating is likely to occur during processing such as blanking.
  • Annealing may be performed before plating, and the conditions are not particularly limited.
  • the plating bath temperature is not particularly limited, but may be, for example, in the temperature range of 620 to 720° C.
  • the plating bath temperature is more preferably in the temperature range of 650 to 690° C.
  • the plating bath composition is not particularly limited, but may have a Si concentration of 5 to 15 mass %, an Fe concentration of 0 to 5 mass %, and the balance being Al and less than 0.5 mass % of impurities.
  • the aluminum-plated steel sheet for hot stamping according to this embodiment can be stably manufactured.
  • a preferred method for producing the hot stamped product according to this embodiment using the aluminum-plated steel sheet for hot stamping described above will be described.
  • the hot stamped body according to this embodiment can be manufactured by hot stamping the above-mentioned aluminum-plated steel sheet for hot stamping.
  • the hot stamped product is preferably produced by heating an aluminum-plated steel sheet for hot stamping to a temperature range of Ac 3 (° C.) or more, quickly transporting it onto a die, and performing hot stamping in a temperature range of Ar 3 (° C.) or more. Thereafter, it is preferable to cool the steel sheet in the die at an average cooling rate of 30° C./s or more by heat transfer between the steel sheet and the die.
  • Ac 3 (° C.) can be calculated by the following formula.
  • the element symbol indicates the content of the element in mass %, and 0 is substituted when the element is not contained.
  • the metal structure of the steel sheet can be sufficiently austenitized.
  • a desired amount of martensite can be obtained by cooling, which will be described later. Therefore, it is preferable that the heating temperature before hot stamping is in a temperature range of Ac 3 (° C.) or more.
  • the holding time in the temperature range of Ac 3 (°C) or higher may be 0.1 minutes (6 seconds) to 30.0 minutes.
  • the hot stamping start temperature (forming start temperature) is set to a temperature range of Ar 3 (° C.) or more.
  • the desired amount of martensite can be obtained by cooling at an average cooling rate of 30°C/s or more after hot stamping. Therefore, it is preferable to cool at an average cooling rate of 30°C/s or more after hot stamping.
  • the manufacturing method described above allows for the stable manufacture of the hot stamped body according to this embodiment.
  • the conditions in the embodiment are merely an example of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example of conditions.
  • Various conditions may be adopted in the present invention as long as they do not deviate from the gist of the present invention and achieve the object of the present invention.
  • the metal structure of the steel sheets, the chemical composition of the Fe-Al-Si alloy layer and the average thickness thereof were analyzed, and the chemical composition of the Al-based plating layer and the average thickness thereof were measured, all by the methods described above.
  • the chemical composition of the Fe-Al-Si alloy layer other than Cu was Al: 40-70%, Fe: 20-50%, Si: 5-20%, the average thickness was 2-10 ⁇ m, and the average thickness of the Al-based plating layer was 5-50 ⁇ m.
  • the coating adhesion was insufficient in the aluminum-plated steel sheet for hot stamping, and the desired Fe-Al-Si alloy layer was not formed.
  • the coating adhesion was evaluated by performing close bending according to the press bending method described in JIS Z 2248 (2014), and visually checking whether the coating peeled off at the bent part. Since the coating peeled off in Production No. 78, it was determined that the coating adhesion was insufficient.
  • hot stamping was performed under the conditions shown in Tables 5A to 5C using the aluminum-plated steel sheet for hot stamping.
  • hot stamping was performed by sandwiching a flat aluminum-plated steel sheet for hot stamping between water-cooled dies and applying pressure of 20 MPa for 10 seconds.
  • the aluminum-plated steel sheet for hot stamping was heated to a temperature range of Ac 3 (°C) or more, quickly transported onto a water-cooled die, sandwiched in a temperature range of Ar 3 (°C) or more, and cooled in the water-cooled die at an average cooling rate of 30°C/s or more.
  • the plate thickness of the obtained hot stamped body was 0.5 to 3.5 mm.
  • the water-cooled die here refers to a die having a water channel in the die and flowing cooling water through the water channel to suppress the temperature rise of the die.
  • the "other" value for the diffusion layer in the table also includes the Cu concentration (mass %) in the diffusion layer.
  • Production Nos. 68 to 71 were hot stamped under the following conditions.
  • Production No. 68 An aluminum-plated steel sheet for hot stamping was heated to 870°C, held for 5 seconds, and then quickly transported to a water-cooled mold, sandwiched at 780°C, and cooled in the water-cooled mold at an average cooling rate of 30°C/s or more.
  • a desired amount of martensite was obtained in the metal structure of the steel sheet of the hot stamped body, but a diffusion layer could not be sufficiently generated because the holding time in the temperature range of Ac 3 point or higher was short.
  • An aluminum-plated steel sheet for hot stamping was heated to 950°C, held for 300 seconds, and then quickly transported to a water-cooled mold, sandwiched at 780°C, and cooled in the water-cooled mold at an average cooling rate of 30°C/s or more.
  • the desired amount of martensite was obtained in the metal structure of the steel sheet of the hot stamped body, but the heating temperature and holding time were inappropriate for generating a diffusion layer of the desired thickness.
  • Production No. 70 The aluminum-plated steel sheet for hot stamping was heated to 750°C, held for 20 seconds, and then quickly transported to a water-cooled mold, sandwiched at 600°C, and cooled in the water-cooled mold at an average cooling rate of 30°C/s or more.
  • the heating temperature and the temperature at which hot stamping was performed were low. In this example, a diffusion layer was not formed, so the diffusion layer could not be analyzed.
  • Production No. 71 The aluminum-plated steel sheet for hot stamping was heated to 950°C, held for 70 seconds, pressed with a normal die in a clamp press at a press speed of 20 spm, and then air-cooled to room temperature. The average cooling rate at this time was less than 30°C/s.
  • Production No. 82 The aluminum-plated steel sheet for hot stamping was heated to 900°C, held for 32 minutes, and then quickly transferred to a water-cooled mold, sandwiched at 780°C, and cooled in the water-cooled mold at an average cooling rate of 30°C/s or more.
  • the diffusion layer was too thick, making it difficult to generate a corrosion potential difference between the Fe-Al phase in the Fe-Al alloy layer and the diffusion layer, and the sacrificial anticorrosion effect was not fully expressed. As a result, the corrosion resistance of the hot stamped body was deteriorated.
  • the hot stamped bodies obtained were subjected to the above-mentioned methods: metal structure observation of the steel plate, analysis of the chemical composition of the diffusion layer and measurement of the average thickness, and analysis of the chemical composition of the Fe-Al alloy layer and measurement of the average thickness. Tensile tests and corrosion resistance evaluations were also performed using the above-mentioned methods.
  • the Fe-Al alloy layer contained an Fe-Al phase containing 40-60 mass% Fe and 40-60 mass% Al, and a ⁇ 1 phase containing 50-77 mass% Fe, 20-40 mass% Al, and 3-10% Si.
  • the Fe-Al alloy layer had an average thickness of 5-60 ⁇ m, and a chemical composition of Fe: 25-80 mass%, Al: 20-60 mass%, Si: 0-10%, and the balance: less than 5%.
  • the Al-based plating layer had an average thickness of 5-50 ⁇ m.
  • the tensile strength was evaluated according to the following criteria.
  • the hot stamped body was judged to have passed as having high strength.
  • the hot stamped body was judged to have not passed as having high strength.
  • the average value of the maximum thickness reduction obtained was evaluated according to the following criteria: When the evaluation was E, the hot stamped steel sheet was judged to have particularly excellent corrosion resistance.
  • E Excellent: Less than 0.05 mm V (Very Good): 0.05 mm or more, less than 0.10 mm G (Good): 0.10 mm or more, less than 0.15 mm B (Bad): 0.15 mm or more
  • the above-mentioned aspects of the present invention can provide a hot stamped body having high strength and excellent corrosion resistance, as well as an aluminum-plated steel sheet for hot stamping from which this hot stamped body can be manufactured.

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