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WO2024190779A1 - Élément en acier, et tôle en acier - Google Patents

Élément en acier, et tôle en acier Download PDF

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Publication number
WO2024190779A1
WO2024190779A1 PCT/JP2024/009561 JP2024009561W WO2024190779A1 WO 2024190779 A1 WO2024190779 A1 WO 2024190779A1 JP 2024009561 W JP2024009561 W JP 2024009561W WO 2024190779 A1 WO2024190779 A1 WO 2024190779A1
Authority
WO
WIPO (PCT)
Prior art keywords
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steel
content
coating
steel member
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
PCT/JP2024/009561
Other languages
English (en)
Japanese (ja)
Inventor
進一郎 田畑
一真 伊藤
和久 楠見
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Nippon Steel Corp
Original Assignee
Nippon Steel Corp
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Nippon Steel Corp filed Critical Nippon Steel Corp
Priority to CN202480018373.3A priority Critical patent/CN120787269A/zh
Priority to KR1020257029860A priority patent/KR20250142439A/ko
Priority to JP2025506869A priority patent/JPWO2024190779A1/ja
Publication of WO2024190779A1 publication Critical patent/WO2024190779A1/fr
Priority to MX2025010598A priority patent/MX2025010598A/es
Anticipated expiration legal-status Critical
Pending legal-status Critical Current

Links

Classifications

    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C18/00—Alloys based on zinc
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/12—Aluminium or alloys based thereon
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/004—Dispersions; Precipitations

Definitions

  • the present invention relates to a steel member and a steel plate.
  • Hot stamping has been adopted as a technique for press forming difficult-to-form materials such as high-strength steel plates, as disclosed in, for example, Patent Documents 1 to 3.
  • Hot stamping is a hot forming technique in which the material to be formed is heated and then formed.
  • the material is heated before it is shaped. Therefore, when shaped, the steel is soft and has good formability. This allows even high-strength steel plates to be shaped with high precision into complex shapes.
  • the steel is quenched at the same time as it is shaped using a press die, so the steel (steel component) has sufficient strength after shaping.
  • Patent Document 1 discloses that hot stamping can impart a tensile strength of 1400 MPa or more to a steel member obtained by forming a steel plate.
  • Patent Document 2 discloses a hot press-formed press-formed product with excellent toughness and a tensile strength of 1.8 GPa or more.
  • Patent Document 3 discloses a steel material with an extremely high tensile strength of 2.0 GPa or more, and also with good toughness and ductility.
  • Patent Document 4 discloses a steel material with a high tensile strength of 1.8 GPa or more, and also with good toughness.
  • Patent Document 5 discloses a steel material with an extremely high tensile strength of 2.0 GPa or more, and also with good toughness.
  • the objective of the present invention is to provide a steel member that has high strength and excellent resistance to hydrogen embrittlement, and a steel plate that is suitable as a material for such a steel member.
  • a steel member according to an embodiment of the present invention has a chemical composition, in mass%, of C: 0.26 to 0.65%, Si: 0 to 2.00%, Mn: 0 to 3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.020% or less, O: 0.010% or less, Mo: 0.10 to 2.00%, Nb: 0 to 0.10%, Ti: 0 to 0.200%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, Cr: 0 to 1.00%, B: 0 to 0.02%.
  • W 0-1.00%
  • V 0-1.00%
  • Ca 0-0.020%
  • Mg 0-0.010%
  • Al 0-1.00%
  • Sn 0-1.00%
  • Sb 0-1.00%
  • Zr 0-1.00%
  • Se 0-1.00%
  • Bi 0-1.00%
  • Re 0-1.00%
  • Os 0-1.00%
  • Ir 0-1.00%
  • Tc 0-1.00%
  • Co 0-1.0 0%
  • REM 0-0.30%
  • Fe and impurities Fe and impurities.
  • the length of the grain boundary where the rotation angle is 2° is L64-72°
  • the length of the grain boundary where the rotation angle is 57 to 63° is L57-63°
  • the length of the grain boundary where the rotation angle is 4 to 12° is L4-12°
  • the ratio of the sum of L49-56° and L64-72° to the sum of L57-63° and L4-12°, that is, (L49-56°+L64-72°)/(L57-63°+L4-12°) is 1.30 or more, and the tensile strength exceeds 1500 MPa.
  • the steel member according to [1] may have a chemical composition, in mass%, of Nb: 0.01 to 0.10%, Nb-based precipitates are present at the 1/4 depth position, and a Mo concentration of the Nb-based precipitates is 4.5 times or more the Mo content of the steel member.
  • the steel member according to [1] or [2] may have a coating on the surface.
  • the coating may be mainly composed of an Fe-Al based alloy or an Fe-Zn based alloy.
  • the chemical composition may be, in mass%, W: more than 0% and less than 0.10%, or Mo: more than 1.00% and less than 2.00%.
  • a steel sheet according to another embodiment of the present invention has a chemical composition, in mass%, of C: 0.26 to 0.65%, Si: 0 to 2.00%, Mn: 0 to 3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.020% or less, O: 0.010% or less, Mo: 0.10 to 2.00%, Nb: 0 to 0.10%, Ti: 0 to 0.200%, Cu: 0-2.00%, Ni: 0-2.00%, Cr: 0-1.00%, B: 0-0.0200%, W: 0-1.00%, V: 0-1.00%, Ca: 0-0.020%, Mg: 0-0.010%, Al: 0-1.00%, Sn: 0-1.00%, Sb: 0-1.00%, Zr: 0-1.00%
  • the steel plate according to [7] may have a chemical composition, in mass%, of Nb: 0.01 to 0.10%, Nb-based precipitates are present at the 1/4 depth position, and the Mo concentration of the Nb-based precipitates is 4.5 times or more the Mo content of the steel plate.
  • the steel sheet according to [7] or [8] may have a coating on the surface.
  • the coating may be an Al-based coating or a Zn-based coating.
  • the steel plate according to any one of [7] to [10] may have a chemical composition, in mass%, of W: more than 0% and less than 0.10%, or Mo: more than 1.00% and less than 2.00%.
  • the above-mentioned aspects of the present invention make it possible to provide a steel member having high strength and excellent resistance to hydrogen embrittlement, and a steel plate suitable as a material for the steel member.
  • a steel member according to an embodiment of the present invention (steel member according to this embodiment) will be described below.
  • the range from 1/8 to 3/8 of the thickness from the surface in the thickness direction, centered at a position of 1/4 of the thickness from the surface will be described as the 1/4 depth position.
  • the steel member according to this embodiment has a predetermined chemical composition, and at the 1/4 depth position, (L49-56°+L64-72°)/(L57-63°+L4-12°) is 1.30 or more, and the tensile strength exceeds 1500 MPa. Each one will be explained below.
  • the chemical composition of the steel member according to this embodiment is, in mass %, C: 0.26 to 0.65%, Si: 0 to 2.00%, Mn: 0 to 3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.020% or less, O: 0.010% or less, Mo: 0.10 to 2.00%, Nb: 0 to 0.10%, Ti: 0 to 0.200%, Cu: 0 to 2.00%, Ni: 0 to 2.00%, Cr: 0 to 1.00%, B: 0 to 0.0200%, W: 0 to 1.00%, V: 0 to 1.00%, and W: 0 to 1.00%.
  • C is an element that enhances the hardenability of steel and improves the strength of the steel member obtained after the steel plate is subjected to hardening such as hot stamping. If the C content is less than 0.26%, the strength of the steel member after hardening (hardening It becomes difficult to ensure sufficient strength (over 1.5 GPa (1500 MPa)) in the steel member (obtained after subjecting to the annealing treatment). Therefore, the C content is set to 0.26% or more. It is preferable that the C content is 0.28% or more, and more preferably 0.31% or more or 0.32% or more. In order to obtain a higher tensile strength, for example, 2300 MPa or more, the C content is 0.
  • the ratio is 45% or more.
  • the C content is set to 0.65% or less.
  • the C content is preferably 0.60% or less, and more preferably 0.55% or less.
  • the Si content is preferably 0.10% or more, more preferably 0.20% or more, and further preferably 0.30% or more.
  • the Si content in the steel exceeds 2.00%, the heating temperature required for austenitic transformation during heat treatment (quenching) becomes significantly high, which increases the cost required for heat treatment and When heated, ferrite may remain, which may reduce the strength of the steel member. Therefore, the Si content is set to 2.00% or less.
  • the Si content is preferably set to 1.80% or less. It is more preferable to set it to 1.50% or less, and further more preferable to set it to 1.10% or less.
  • Mn does not necessarily have to be contained (it may be 0%), but it is an extremely effective element for improving the hardenability of steel and stably securing strength after hardening. Mn further has the following properties: Mn is an element that lowers the Ac3 point and promotes lowering the quenching temperature. Therefore, Mn may be added. To obtain the above effect, the Mn content is preferably 0.05% or more, and 0.15% or more. % or more, more preferably 0.25% or more or 0.30% or more. On the other hand, if the Mn content exceeds 3.00%, the hydrogen embrittlement resistance of the steel member after quenching deteriorates. Therefore, the Mn content is set to 3.00% or less. The Mn content is set to 2.50% or less. It is preferable to set it to 1.80% or less, more preferably to set it to 1.50% or less.
  • P is an element that reduces the hydrogen embrittlement resistance of steel members after quenching. In particular, when the P content exceeds 0.100%, the hydrogen embrittlement resistance is significantly reduced. Therefore, the P content is set to 0.
  • the P content is preferably limited to 0.055% or less, and more preferably limited to 0.020% or less. Since a small P content is preferable, it may be 0%, but from the viewpoint of cost, it may be 0.001% or more.
  • S is an element that reduces the hydrogen embrittlement resistance of steel members after quenching.
  • the S content should be less than 0.
  • the S content is preferably limited to 0.0050% or less, and more preferably limited to 0.0030% or less. Since a small S content is preferable, even 0% is acceptable. However, from the viewpoint of cost, it may be set to 0.0001% or more.
  • N is an element that reduces the hydrogen embrittlement resistance of steel members after quenching.
  • the N content is set to 0.020% or less.
  • the N content is preferably set to 0.015% or less, 0.010% or less, or 0.006% or less.
  • the lower limit of the N content is particularly There is no need to limit the N content, and it may be 0%, but making the N content less than 0.0002% increases the steelmaking cost and is economically undesirable. Therefore, the N content is set to 0.0002% or more. Preferably, it may be 0.0008% or more, or 0.001% or more.
  • O is an element that reduces the hydrogen embrittlement resistance of steel members after quenching.
  • the O content is set to 0.010% or less.
  • the O content is preferably set to 0.007% or less, 0.005% or less, or 0.003% or less.
  • the lower limit of the O content is particularly There is no need to limit the O content, and it may be 0%, but making the O content less than 0.0002% increases the steelmaking cost and is economically undesirable. Therefore, the O content is set to 0.0002% or more. Preferably, it may be 0.0008% or more, or 0.001% or more.
  • Mo is an important element in the steel member according to this embodiment.
  • Mo is an element that segregates at grain boundaries and is an element effective in promoting the development of crystal grain boundaries having the above-mentioned specific rotation angle.
  • Mo is an element that is effective in improving the hardenability of steel and stably securing the strength of steel members after hardening.
  • Mo is an element that improves corrosion resistance in a corrosive environment. . If the Mo content is less than 0.10%, sufficient effects cannot be obtained. Therefore, the Mo content is set to 0.10% or more.
  • the Mo content is preferably set to 0.20% or more, and more preferably 0. It is more preferable that the content is 40% or more, and it may be more than 1.00%.
  • the Mo content is set to 2.00% or less.
  • the Mo content is set to 1.50% or less. It is preferable to set the content at 1.00% or less, and more preferable to set the content at 1.00% or less.
  • Nb is an element that forms fine carbides, nitrides, or carbonitrides in steel, and suppresses Cu hot embrittlement cracking in the hot rolling process due to the grain refining effect of these precipitates.
  • the hydrogen embrittlement resistance of the steel member is improved by concentrating Mo in the Nb-based precipitates (higher than the Mo concentration (Mo content) of the base steel material).
  • the content may be 0%, but Nb may also be contained.
  • the Nb content is preferably 0.01% or more, and more preferably 0.02% or more.
  • the Nb content is set to 0.10% or less.
  • the Nb content is preferably set to 0.08% or less.
  • Ti forms fine carbides and carbonitrides together with Nb in steel, and the grain refining effect of these compounds suppresses Cu hot embrittlement cracking during the hot rolling process and improves the hydrogen embrittlement resistance of steel members.
  • Ti is an element that has the effect of improving the hardness of steel.
  • Ti preferentially bonds with N in steel to form nitrides, suppresses the consumption of solute B due to the precipitation of BN, and improves the hardening effect of B, which will be described later.
  • Ti is also an element that promotes the effect of improving the properties. Therefore, Ti may not be contained, that is, the Ti content may be 0%, but Ti may be contained.
  • the Ti content is preferably 0.005% or more, more preferably 0.010% or more, and even more preferably 0.015% or more.
  • the Ti content exceeds 0.200%, carbonitrides and the like become coarse, and bending straightening cracks are promoted in the continuous casting process. This inhibits the development of grain boundaries, which have Nb, and reduces the hydrogen embrittlement resistance of the steel member.
  • the amount of TiC precipitated increases in addition to Nb carbonitrides and TiN, and C is consumed, so the quenching The strength of the steel member after the addition of Ti decreases. Therefore, the Ti content is set to 0.200% or less.
  • the Ti content is preferably set to 0.080% or less, and more preferably set to 0.050% or less. .
  • Cu is an element effective in improving the hardenability of steel and stably securing the strength of steel members after hardening.
  • Cu is also an element that improves corrosion resistance in a corrosive environment.
  • No Cu content that is, the Cu content may be 0%, but Cu may be contained.
  • the Cu content is preferably 0.10% or more.
  • the Cu content is more preferably 0.10% or more.
  • the content is preferably 0.20% or more.
  • the Cu content is set to 2.00% or less.
  • the Cu content is set to 1.50% or less. It is preferable to set the content of C to 1.00% or less, and more preferable to set the content of C to 0.60% or less.
  • Ni is an effective element for improving the hardenability of steel and stably securing the strength of steel members after hardening. Ni also has the effect of suppressing Cu hot embrittlement cracking in the production of steel sheets. Therefore, Ni may not be contained, that is, the Ni content may be 0%, but Ni may be contained. In order to obtain the above effects, the Ni content is set to 0.10% or more. It is preferable that the content of C be 0.20% or more, and more preferable that the content of C be 0.20% or more. On the other hand, if the Ni content exceeds 2.00%, the above effects are saturated and the cost increases. Therefore, the Ni content is set to 2.00% or less. The Ni content is set to 1.00% or less. It is preferable to keep it at 0.50% or less, more preferable to keep it at 0.20% or less or even more preferable to keep it at 0.10% or less.
  • Cr Cr: 0-1.00%) Cr is an effective element for improving the hardenability of steel and stably securing the strength of the steel member after hardening. Therefore, it is acceptable for Cr not to be contained, that is, the Cr content may be 0%, but Cr may be contained. In order to obtain the above effects, the Cr content is preferably 0.03% or more, and more preferably 0.05% or more. On the other hand, if the Cr content exceeds 1.00%, the above effects are saturated and the cost increases. In addition, Cr has the effect of stabilizing iron carbide. If the Cr content is less than 1.00%, coarse iron carbides may remain undissolved during heat treatment of the steel plate, which may reduce the hydrogen embrittlement resistance of the steel member. Therefore, the Cr content is set to 1.00% or less. The Cr content is set to 0.50% or less. It is preferable to keep it at 0.30% or less, more preferable to keep it at 0.15% or less.
  • W is an extremely effective element for improving the hardenability of steel and stably securing the strength of the steel member after hardening. Therefore, no W is contained, that is, the W content is 0%. However, W may be contained. In order to obtain the above-mentioned effects, the W content is preferably 0.01% or more, more preferably 0.10% or more, or even more preferably 0.20% or more. More preferred. On the other hand, W is an element that has the effect of stabilizing iron carbides. If the W content exceeds 1.00%, coarse iron carbides remain undissolved when the steel plate is heated, and the hydrogen embrittlement resistance of the steel member after quenching is deteriorated. In addition, the cost increases significantly. Therefore, if W is contained, the W content is set to 1.00% or less. The W content is preferably set to 0.80% or less, and more preferably set to 0.10% or less. % or less.
  • V is an element that forms fine carbides in steel, and improves the hydrogen embrittlement resistance of steel members due to the grain refining effect and hydrogen trapping effect of the carbides.
  • the amount of V may be 0%, but V may be contained.
  • the V content is preferably 0.01% or more, and more preferably 0.10% or more.
  • the V content is set to 1.00% or less. It is preferable to set it to 0.50% or less, or 0.20% or less.
  • Ca is an element that has the effect of refining inclusions in steel and enhancing the hydrogen embrittlement resistance of steel members after quenching. Therefore, no Ca content, that is, the Ca content may be 0%, but Ca In order to obtain the above effects, the Ca content is preferably 0.001% or more, and more preferably 0.002% or more. On the other hand, if the Ca content exceeds 0.020%, the effect saturates and the cost increases. Therefore, if Ca is contained, the Ca content is set to 0.020% or less. It is preferable that the content be 0.006% or less, and more preferable that the content be 0.004% or less.
  • Mg is an element that has the effect of refining inclusions in steel and improving hydrogen embrittlement resistance after heat treatment. Therefore, it is acceptable to not include Mg, that is, the Mg content may be 0%, but it is preferable to include Mg.
  • the Mg content is preferably 0.001% or more, and more preferably 0.002% or more.
  • the Mg content is set to 0.010% or less.
  • the Mg content is preferably It is 0.005% or less, and more preferably 0.004% or less.
  • Al 0-1.00%
  • Al is an element that is generally used as a deoxidizer for steel. Therefore, no Al is contained, that is, the Al content is 0%, but Al may be contained. To achieve this, the Al content is preferably 0.01% or more. On the other hand, if the Al content exceeds 1.00%, the above effects become saturated and the economic efficiency decreases. Therefore, when Al is contained, the Al content is set to 1.00% or less.
  • the Al content is as follows: It is preferably 0.20% or less, and may be 0.05% or less.
  • Sn is an element that improves corrosion resistance in a corrosive environment. Therefore, no Sn is contained, that is, the Sn content may be 0%, but Sn may be contained.
  • the Sn content is preferably 0.01% or more.
  • the Sn content is more preferably 0.03% or more, and further preferably 0.05% or more.
  • the Sn content is set to 1.00% or less.
  • the Sn content is preferably 0.30% or less, and may be 0.10% or less.
  • Sb is an element that improves corrosion resistance in a corrosive environment. Therefore, no Sb is contained, that is, the Sb content may be 0%, but Sb may be contained. The amount is preferably 0.01% or more. On the other hand, if the Sb content exceeds 1.00%, the grain boundary strength decreases, and the hydrogen embrittlement resistance of the steel member after quenching decreases. Therefore, if Sb is contained, the Sb content is set to 1.00% or less. The Sb content is preferably 0.30% or less, and may be 0.20% or less.
  • Zr 0-1.00%
  • Zr is an element that improves corrosion resistance in a corrosive environment. Therefore, no Zr is contained, that is, the Zr content may be 0%, but Zr may be contained. The amount is preferably 0.01% or more.
  • the Zr content exceeds 1.00%, the grain boundary strength decreases, and the hydrogen embrittlement resistance of the steel member after quenching decreases. Therefore, if Zr is contained, the Zr content is set to 1.00% or less.
  • the Zr content is preferably 0.35% or less.
  • Se is an element that improves hydrogen embrittlement resistance. Therefore, Se may not be contained, that is, the Se content may be 0%, but Se may be contained. It is preferable that the content of Si is 0.01% or more. On the other hand, if the Se content exceeds 1.00%, the effect is saturated and the cost increases. Therefore, if Se is contained, the Se content is set to 1.00% or less. The Se content is preferably It is less than 0.40%.
  • Bi (Bi: 0-1.00%) Bi is an element that improves hydrogen embrittlement resistance. Therefore, no Bi may be contained, that is, the Bi content may be 0%, but Bi may also be contained. It is preferable that the content of Si is 0.01% or more. On the other hand, if the Bi content exceeds 1.00%, the effect saturates and the cost increases. Therefore, if Bi is contained, the Bi content is set to 1.00% or less. The Bi content is preferably It is less than 0.30%.
  • As is an element that improves hydrogen embrittlement resistance. Therefore, no As content, that is, the As content may be 0%, may be used, but As may be contained. In order to obtain the above effects, the As content It is preferable that the content of Si is 0.01% or more. On the other hand, if the As content exceeds 1.00%, the effect saturates and the cost increases. Therefore, if As is contained, the As content is set to 1.00% or less. The As content is preferably It is less than 0.40%.
  • Ta is an element that improves hydrogen embrittlement resistance. Therefore, Ta may not be contained, that is, the Ta content may be 0%, but Ta may be contained. It is preferable that the content of Si is 0.01% or more. On the other hand, if the Ta content exceeds 1.00%, the effect is saturated and the cost increases. Therefore, if Ta is contained, the Ta content is set to 1.00% or less. The Ta content is preferably It is less than 0.50%.
  • Re (Re: 0-1.00%) Re is an element that improves hydrogen embrittlement resistance. Therefore, Re may not be contained, that is, the Re content may be 0%, but Re may be contained. In order to obtain the above effect, the Re content It is preferable that the content of Si is 0.01% or more. On the other hand, if the Re content exceeds 1.00%, the effect is saturated and the cost increases. Therefore, when Re is contained, the Re content is set to 1.00% or less. The Re content is preferably It is less than 0.40%.
  • Os is an element that improves hydrogen embrittlement resistance. Therefore, no Os is contained, that is, the Os content may be 0%, but Os may be contained. In order to obtain the above effects, the Os content It is preferable that the content of Si is 0.01% or more. On the other hand, if the Os content exceeds 1.00%, the effect saturates and the cost increases. Therefore, if Os is contained, the Os content is set to 1.00% or less. The Os content is preferably It is less than 0.20%.
  • Ir is an element that improves hydrogen embrittlement resistance. Therefore, Ir may not be contained, that is, the Ir content may be 0%, but Ir may be contained. It is preferable that the content of Si is 0.01% or more. On the other hand, if the Ir content exceeds 1.00%, the effect is saturated and the cost increases. Therefore, if Ir is contained, the Ir content is set to 1.00% or less. The Ir content is preferably It is less than 0.30%.
  • Tc is an element that improves hydrogen embrittlement resistance. Therefore, Tc may not be contained, that is, the Tc content may be 0%, but Tc may be contained. It is preferable that the content of Si is 0.01% or more. On the other hand, if the Tc content exceeds 1.00%, the effect is saturated and the cost increases. Therefore, if Tc is contained, the Tc content is set to 1.00% or less.
  • the Tc content is preferably It is 0.40% or less, and more preferably 0.15% or less.
  • Co (Co: 0-1.00%) Co is an element that improves corrosion resistance in a corrosive environment. Therefore, no Co is contained, that is, the Co content may be 0%, but Co may be contained. It is preferable that the content of Si is 0.01% or more. On the other hand, if the Co content exceeds 1.00%, the above effects are saturated and the economic efficiency decreases. Therefore, if Co is contained, the Co content is set to 1.00% or less.
  • the Co content is as follows: The content is preferably 0.40% or less, and more preferably 0.10% or less.
  • REM 0-0.30%) REM, like Ca, is an element that has the effect of refining inclusions in steel and improving the hydrogen embrittlement resistance of steel members after quenching. Therefore, no REM is contained, that is, the REM content is 0%.
  • the REM content is preferably 0.01% or more, and more preferably 0.02% or more.
  • the REM content is set to 0.30% or less. It is preferable to set it to 20% or less.
  • REM refers to a total of 17 elements, including Sc, Y, and lanthanides such as La, Ce, and Nd, and the content of REM means the total content of these elements. Alloys are used to add to the molten steel, and include, for example, Sc, Y, La, Ce, Pr, and Nd.
  • the elements other than those described above, that is, the balance are Fe and impurities.
  • impurities refer to components that are mixed in due to various factors in raw materials such as ores and scraps, and in the manufacturing process, when industrially manufacturing steel sheets, and are acceptable within a range that does not adversely affect the properties of the steel member according to the present embodiment.
  • the industrial manufacturing method is the blast furnace steelmaking method or the electric furnace steelmaking method, and includes the level (impurity level) of contamination when manufactured by either method.
  • the impurities may include Pb.
  • the chemical composition of the steel member can be determined by the following method.
  • C and S which are difficult to measure with ICP-AES, can be measured using the combustion-infrared absorption method, N using the inert gas fusion-thermal conductivity method, and O using the inert gas fusion-non-dispersive infrared absorption method.
  • the chemical composition at the 1/4 depth position does not substantially change during the manufacturing process, when the analytical value of the chemical composition of the molten steel or steel plate is known, the analytical value of the chemical composition of the molten steel or steel plate may be used as the chemical composition of the steel member.
  • the surface that serves as the reference for the 1/4 depth position is the surface of the steel component, but if the steel component has a coating, i.e., if the steel component has a coating on its surface, the surface means the surface of the base steel material excluding the coating.
  • metal structure In the steel member according to this embodiment, the metal structure (microstructure) at the 1/4 depth position is specified as described below.
  • the grain boundaries of crystal grains having a body-centered structure mainly consist of one or more of the following: grain boundaries with a rotation angle of 4 to 12°, grain boundaries with a rotation angle of 49 to 56°, grain boundaries with a rotation angle of 57 to 63°, and grain boundaries with a rotation angle of 64 to 72°.
  • martensite often consists mainly of the above four types of grain boundaries.
  • the sum (L49-56°+L64-72°) of the grain boundary length where the rotation angle is 49 to 56° (L49-56°) and the grain boundary length where the rotation angle is 64 to 72° (L64-72°) with the ⁇ 011> direction as the rotation axis is increased relative to the sum (L57-63°+L4-12°) of the grain boundary length where the rotation angle is 57 to 63° (L57-63°) and the grain boundary length where the rotation angle is 4 to 12° (L4-12°), thereby improving hydrogen embrittlement resistance.
  • (L49-56°+L64-72°)/(L57-63°+L4-12°) is less than 1.30, sufficient improvement in hydrogen embrittlement resistance cannot be obtained.
  • (L49-56°+L64-72°)/(L57-63°+L4-12°) is preferably 1.50 or more, more preferably 1.60 or more. There is no upper limit, but it is substantially 2.80 or less. In order to improve hydrogen embrittlement resistance, it is preferable that (L49-56°+L64-72°)/(L57-63°+L4-12°) is higher.
  • the grain boundary having a rotation angle of A to B degrees with the ⁇ 011> direction as the rotation axis means a grain boundary in which adjacent crystal grains on either side of the grain boundary overlap when rotated by A to B degrees with the ⁇ 011> direction as the rotation axis.
  • (L49-56°+L64-72°)/(L57-63°+L4-12°) can be measured by the following method.
  • a sample is cut out from a position 50 mm or more away from the end of the steel member so that a cross section perpendicular to the surface (plate thickness cross section) can be observed. Although it depends on the measuring device, the length of the sample is set so that the cross section can be observed about 10 mm in the thickness direction.
  • the cut sample is polished.
  • the cross section of the cut sample is polished using waterproof paper from #320 to #1200 or more, and then polished using a diamond suspension with a particle size of 3 to 1 ⁇ m to finish it into a mirror surface.
  • electrolytic polishing is performed to remove the strain introduced into the surface layer of the cross section.
  • EBSD analysis is performed on a measurement area of 50 ⁇ m x 50 ⁇ m at measurement intervals of 0.1 ⁇ m to obtain crystal orientation information.
  • the EBSD analysis is carried out at an analysis speed of 200 to 300 points/second using an apparatus consisting of, for example, a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL).
  • a scanning electron microscope and an EBSD detector having performance equal to or higher than those described above may be used, but those manufactured by JEOL and TSL are preferable.
  • the lengths of the grain boundaries of crystal grains having a body-centered structure where the rotation angle is 49 to 56° with the ⁇ 011> direction as the rotation axis, the lengths of the grain boundaries where the rotation angle is 64 to 72°, the lengths of the grain boundaries where the rotation angle is 57 to 63°, and the lengths of the grain boundaries where the rotation angle is 4 to 12° are determined, and each of these results is used to calculate (L49-56°+L64-72°)/(L57-63°+L4-12°).
  • the length of the grain boundary can be easily calculated, for example, by using the "Inverse Pole Figure Map” and "Axis Angle” functions installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. With these functions, the total length of the grain boundary of a crystal grain having a body-centered structure can be calculated by specifying a specific rotation angle with an arbitrary direction as the rotation axis.
  • the above analysis is performed for all crystal grains included in the measurement area, and the lengths of the above-mentioned four types of grain boundaries can be calculated with the ⁇ 011> direction as the rotation axis.
  • the measurement is performed in five fields of view, and the average value of (L49-56°+L64-72°)/(L57-63°+L4-12°) in each field of view is defined as (L49-56°+L64-72°)/(L57-63°+L4-12°) in this embodiment.
  • the formation of grain boundaries with rotation angles of 57 to 63° and grain boundaries with rotation angles of 4 to 12° with respect to the ⁇ 011> direction as the rotation axis is suppressed by segregating Mo at the grain boundaries.
  • the Mo concentration of the Nb-based precipitates is preferably 8.0 times or more, more preferably 10.0 times or more, the Mo content of the steel member.
  • the size of the Nb-based precipitates is preferably 15 ⁇ m or less.
  • the Nb-based precipitates of interest in this embodiment are precipitates containing 50 mass % or more of Nb, such as Nb carbide, Nb carbonitride, Nb nitride, NbTi carbide, and NbTi carbonitride.
  • the presence or absence of Nb-based precipitates and the Mo concentration (content) of the Nb-based precipitates are determined by the following method.
  • a sample is taken from a position 1/4 of the plate width (short side) from the width direction end of the steel member so that the cross section in the thickness direction of the steel member can be observed.
  • a COMPO image is obtained for this sample using a scanning electron microscope to confirm the presence of Nb-based precipitates. Since Nb-based precipitates contain more Nb, which is a heavier element than Fe, they appear brighter than the iron base.
  • the Mo content contained in the Nb-based precipitates can be obtained by performing spot element analysis (beam diameter: 0.5 ⁇ m) on these bright Nb-based precipitates using an electron probe microanalyzer (EPMA).
  • EPMA electron probe microanalyzer
  • Nb-based precipitates When the number of Nb-based precipitates is less than 10, the average of the Mo contents of the number of Nb-based precipitates is taken as the Mo content of the Nb-based precipitates.
  • Nb-based precipitates may also contain C, N, Ti, Cr, and B.
  • the Nb-based precipitates are those having a size of 0.5 ⁇ m or more and an Nb concentration of 50 mass% or more. Most of the Nb-based precipitates in the steel member of this embodiment have a size of 1.0 to 12.0 ⁇ m.
  • the size of the Nb-based precipitates is defined as the average value of the distance between the parallel lines in the horizontal direction sandwiching the second region (horizontal Feret diameter) and the distance between the parallel lines in the vertical direction sandwiching the second region (vertical Feret diameter).
  • the horizontal direction is the longitudinal direction of the steel member, and the vertical direction is the thickness direction perpendicular to the longitudinal direction.
  • the steel member according to this embodiment preferably contains martensite at an area fraction (area%) of 75% or more.
  • area fraction area% of 75% or more.
  • the area fraction of martensite is high.
  • the martensite fraction is more preferably 90% or more in area fraction.
  • the area fraction of martensite may be 100%.
  • Martensite also includes tempered martensite and auto-tempered martensite.
  • the steel member may contain retained austenite and/or bainite in addition to martensite at the 1/4 depth position. There is no ferrite or pearlite, and the area fractions of ferrite and pearlite are 0%.
  • the total area fraction of martensite, retained austenite and bainite is preferably 98% or more or 99% or more, and more preferably 100%.
  • the area fraction of martensite is preferably 95% or more or 97% or more, and more preferably 98% or more.
  • the upper limit of the area fraction of martensite is 100%.
  • the area fraction of the metal structure of a steel member can be measured by the following method.
  • the area fraction of martensite (including fresh martensite, tempered martensite, and auto-tempered martensite) is measured by a transmission electron microscope (TEM) and an electron beam diffraction device attached to the TEM. Since the structure of the thin film sample in the thickness direction is also observed in the TEM, the structure fraction is essentially a volume fraction.
  • the steel member of this embodiment has a structure mainly composed of martensite, and the size of the structure identified as martensite is much larger than the thickness of the thin film.
  • the area fraction of each structure is calculated from the area ratio of each structure in the TEM photograph (field of view). Specifically, a measurement sample is cut out from the width direction end of the steel member at a position that is 1/4 of the width of the steel member (1/4 width position) and includes a 1/4 depth position of the steel member, and used as a thin film sample for TEM observation.
  • a range of 400 ⁇ m2 or more at the 1/4 depth position of the steel member of this thin film sample is observed with the TEM.
  • the electron diffraction pattern of the thin film sample is used to distinguish between martensite and bainite, which are body-centered cubic lattices, and retained austenite, which is a face-centered cubic lattice.
  • Iron carbide (Fe 3 C) in martensite and bainite, which are body-centered cubic lattices, is found from the diffraction pattern, and the precipitation form is observed to measure the structure fraction of martensite and bainite.
  • the precipitation form is precipitation in three directions, it is judged to be martensite (tempered martensite), and if it is precipitation limited to one direction, it is judged to be bainite. If no precipitation of iron carbide is observed, it is also judged to be martensite (fresh martensite). Carbide is observed to distinguish between martensite and bainite, but in this embodiment, the carbide itself is not included in the area fraction of the structure. The area fraction of the retained austenite is measured using an X-ray diffraction method.
  • a measurement sample is cut out from the end of the width direction of the steel member at a position that is 1/4 of the width of the steel member (position of 1/4 width) to be used as a sample for X-ray diffraction.
  • the cut out sample is chemically polished from the surface to a depth of 1/4 of the thickness using hydrofluoric acid and hydrogen peroxide. If chemical polishing takes a long time, for example, it may be polished from the surface to about 1/8 of the thickness using waterproof paper, and then chemically polished to 1/4 of the thickness.
  • the measurement conditions are a Co tube and 2 ⁇ in the range of 45° to 105°.
  • the diffracted X-ray intensity of the face-centered cubic lattice (retained austenite) contained in the steel member is measured, and the volume fraction of the retained austenite is calculated from the area ratio of the diffraction curve.
  • the volume fraction obtained by X-ray diffraction of the retained austenite is regarded as the area fraction as it is.
  • Measurement of the area fractions of martensite, retained austenite, and bainite is complicated. Instead of measuring these area fractions, the area fractions of ferrite and pearlite may be measured by the following method, and the value obtained by subtracting the total from 100% may be regarded as the total area fractions of martensite, retained austenite, and bainite.
  • ferrite or pearlite When ferrite or pearlite is present, it can be easily confirmed by an optical microscope or a scanning electron microscope. Specifically, a measurement sample including a position of 1/4 width and a position of 1/4 depth of the steel member is cut out to be used as a sample for observation. The cut out sample is mechanically polished and then mirror-finished. Next, the sample is etched with a nital etching solution to reveal ferrite and pearlite, and a scanning electron microscope is used to observe an area of 40,000 ⁇ m2 or more at the position of 1/4 depth of the steel member to confirm the presence of ferrite or pearlite.
  • a structure in which ferrite and cementite are arranged in layers alternately is determined as pearlite, and is distinguished from bainite in which cementite precipitates in granular form.
  • the structure fraction by a scanning electron microscope is an area fraction (area %).
  • area % the above-mentioned Nb-based precipitates themselves are not included in the area fraction of the Nb-based precipitates, but are included in the area fraction of the metal structure surrounding the Nb-based inclusions. Therefore, the presence of the Nb-based precipitates is not taken into consideration when measuring the above-mentioned area fraction.
  • the steel member according to the present embodiment has a tensile strength of more than 1500 MPa (1.5 GPa) in order to contribute to improving both fuel efficiency and crashworthiness when applied to automobile components.
  • the tensile strength is preferably 1800 MPa or more, and more preferably 2300 MPa or more.
  • the tensile strength is preferably 3150 MPa or less, and more preferably 2850 MPa or less.
  • the tensile strength can be measured according to ASTM E8M-22.
  • a tensile test piece of a sub-size test piece (parallel part width: 6.0 ⁇ 0.1 mm, Gauge Length: 25.0 ⁇ 0.1 mm) in Table 1 of ASTM E8M-22 can be taken and subjected to a tensile test to measure the tensile strength (TS).
  • the average hardness in the plate thickness direction is measured by a Vickers hardness test (HV1 with a test force of 9.807 N) in accordance with JIS Z2244-1:2020, and the average hardness in the plate thickness direction obtained is converted to tensile strength by a known hardness conversion table (for example, SAE J417-1983).
  • the value may be regarded as the tensile strength of the steel member according to this embodiment.
  • the shape of the steel member according to this embodiment is not particularly limited. That is, the steel member may be a flat plate, or may be a formed body in which the steel plate is formed into a predetermined shape. Hot-formed steel members are often formed bodies, but in this embodiment, the term "steel member" includes both the formed body and the flat plate.
  • the steel member may also be a part of a tailored property material whose strength varies depending on the location.
  • the tailored property material is a steel member in which the steel member according to this embodiment is combined with a steel member other than this embodiment, and the entire tailored property material does not need to satisfy the above-mentioned chemical composition, metal structure (here, L49-56° + L64-72°) / (L57-63° + L4-12°) and tensile strength. It is sufficient that at least a part of the tailored property material satisfies the above-mentioned chemical composition, metal structure (here, L49-56° + L64-72°) / (L57-63° + L4-12°) and tensile strength, and there is no need to determine the ratio, etc.
  • Tailored property materials may be made by joining steel plates with different chemical compositions, strengths, or thicknesses, or may be made by subjecting a portion of the steel plate to heat treatment.
  • the steel member may have a decarburized layer or a soft layer on part of the surface.
  • the thickness of the steel member (when the steel member is a member obtained by processing a steel plate, it can also be said to be the plate thickness of the steel plate constituting the steel member) is not limited. If the steel member is an automotive steel member manufactured by hot stamping, it may be 0.6 mm or more or 0.8 mm or more from the main plate thickness range in which the steel member is used. For the same reason, the thickness may be 4.0 mm or less or 2.5 mm or less.
  • the steel member according to this embodiment may have a coating on a part or the whole of its surface.
  • the coating may be a coating mainly made of an Fe-Al alloy or a coating mainly made of an Fe-Zn alloy.
  • the coating is also called a film, an alloy plating layer, or an intermetallic compound layer.
  • a coating mainly made of an Fe-Al alloy is a coating containing Fe and Al in a total of 70 mass% or more
  • a coating mainly made of an Fe-Zn alloy is a coating containing Fe and Zn in a total of 70 mass% or more
  • a coating mainly made of an Fe-Al alloy may further contain Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, and REM in addition to Fe and Al, with the remainder being impurities.
  • the coating mainly made of an Fe-Zn alloy may further contain, in addition to Fe and Zn, Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Al, Co, In, Bi, Zr, Se, As, and REM, with the balance being impurities, the total content of which may be 1% or less.
  • the coating has the effect of improving resistance to hydrogen embrittlement when used in automobiles because of its corrosion resistance.
  • the thickness of the coating is preferably 10 to 100 ⁇ m.
  • the chemical composition and thickness of the coating can be determined by cross-sectional scanning electron microscopy. Specifically, a measurement sample is cut out from 1/2 of the longitudinal direction of the steel member (1/2 of the longitudinal direction from the longitudinal end) and 1/4 of the width (1/4 of the width from the width end) and observed. The observation range using a microscope is, for example, 400 times magnification and an area of 40,000 ⁇ m2 or more . The cut sample is mechanically polished and then mirror-finished. Next, the thickness of the coating is measured in any 10 fields of view, and the average value is taken as the coating thickness. When observing using a BSE image (or COMPO image), a clear contrast difference is observed between the coating and the base steel (steel sheet substrate).
  • the thickness of the coating can be measured by measuring the thickness from the outermost surface to the position where the contrast changes. Measurements are made at 20 equally spaced locations in the observation photograph, with the distance between measurement locations being 6.5 ⁇ m. In addition, when making the measurement, observations are made in five fields of view in the manner described above, and the average value is used to determine the coating thickness.
  • the chemical composition of the coating can be determined by performing spot elemental analysis (beam diameter: 0.5 ⁇ m) on the same observation range as above using an electron probe microanalyzer (EPMA) to determine the Fe, Al, and Zn contents contained in the coating. A total of 10 points are analyzed in any 10 fields of view of the coating, and the average value is taken as the Fe, Al, and Zn contents contained in the coating. The same method is used to determine the Fe, Al, and Zn contents in the coating even when elements other than Fe, Al, and Zn are contained.
  • the steel plate according to the present embodiment can be used as a material for the steel member according to the present embodiment by subjecting it to heat treatment.
  • the chemical composition of the steel plate according to this embodiment needs to be set so as to obtain preferable properties as a steel member after heat treatment, but since the chemical composition does not substantially change due to heat treatment, the chemical composition of the steel plate according to this embodiment may be equivalent to the chemical composition of the steel member according to this embodiment.
  • the chemical composition of the steel plate can be measured from a 1/4 depth position (a range of 1/8 to 3/8 of the thickness from the surface in the plate thickness direction) by the same method as that for the steel member. As described above, the chemical composition at the 1/4 depth position does not substantially change during the manufacturing process. Therefore, when the analytical value of the chemical composition in the molten steel is known, the analytical value of the chemical composition of the molten steel may be used as the chemical composition of the steel plate.
  • the surface serving as the reference for the 1/4 depth position is the surface of the steel plate, but in the case where the steel plate has a coating, i.e., where the steel plate has a base steel plate and a coating formed on the surface of the base steel plate, the surface means the surface of the base steel plate excluding the coating.
  • the metal structure at a 1/4 depth position is defined, which is in the range from 1/8 to 3/8 of the thickness in the thickness direction from the surface, with the 1/4 position of the thickness (plate thickness) as the center.
  • the area fraction of the region surrounded by a boundary with a crystal orientation mismatch of 5° or more and with an average crystal orientation mismatch of 0.4 to 3.0° within the boundary 80% or less
  • the area fraction of the region (crystal grain) surrounded by a boundary with a crystal orientation difference of 5° or more at the 1/4 depth position and having an average crystal orientation difference of 0.4 to 3.0° inside the boundary is 80% or less.
  • This region is a base structure from which the metal structure of the steel member described above is obtained in the heat treatment described below, and if the area fraction of this region exceeds 80%, the steel member may have a ratio (L49-56°+L64-72°)/(L57-63°+L4-12°) of less than 1.30.
  • the area fraction of this region is preferably 50% or less, more preferably 35% or less, and even more preferably 30% or less.
  • the area fraction of this region may be 1% or more, or may be 10% or more. As long as the area fraction of the above region is 80% or less, there is no limitation on the metal structure of the other parts.
  • a method for measuring the area fraction of crystal grains having an average crystal orientation misorientation of 0.4 to 3.0° within crystal grains surrounded by crystal boundaries having a crystal orientation misorientation of 5° or more (hereinafter referred to as the "area fraction of a specific structure") will be described.
  • a sample is cut out from a position 50 mm or more away from the end of the steel sheet so that a cross section perpendicular to the surface (sheet thickness cross section) can be observed.
  • sheet thickness cross section the size of the sample depends on the measuring device, the size is set so that a cross section of about 10 mm can be observed in the sheet thickness direction.
  • a measurement area of 100 ⁇ m x 100 ⁇ m is centered at a position of 1/4 of the sheet thickness from the surface, and EBSD analysis is performed at a measurement interval of 0.2 ⁇ m to obtain crystal orientation information.
  • the EBSD analysis is performed at an analysis speed of 200 to 300 points/second using a device consisting of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL).
  • JSM-7001F thermal field emission scanning electron microscope
  • DVC5 type detector manufactured by TSL EBSD detector
  • a scanning electron microscope and an EBSD detector having performance equal to or higher than those described above may be used, but those manufactured by JEOL and TSL are preferable.
  • the area fraction of the specific structure can be easily calculated from the obtained crystal orientation information, for example, by using the "Grain Average Misorientation” function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analysis device. With this function, it is possible to calculate the orientation difference between adjacent measurement points for a crystal grain having a body-centered structure, and then obtain the average value for all measurement points within the crystal grain.
  • the area with an orientation difference of 5° or more is defined as a crystal grain
  • the "Grain Average Misorientation” function is used to calculate the area fraction of the area within the crystal grain where the average crystal orientation difference is 0.4 to 3.0° relative to the observation field, thereby obtaining the area fraction of the specific structure. Measurements are performed in five fields, and the average value of the area fraction of the specific structure in each field is taken as the area fraction of the specific structure in this embodiment.
  • the area fraction of the metal structure of the steel plate according to this embodiment can be determined in the same manner as for the steel member described above.
  • Nb-based precipitates are present, and the Mo concentration of the Nb-based precipitates is 4.5 times or more the Mo content of the steel sheet
  • the Nb-based precipitates in which Mo is dissolved have the effect of dragging the austenite ( ⁇ ) grain boundaries during heat treatment at a temperature of Ac3 point or higher.
  • the Nb-based precipitates partially dissolve in a state in which the ⁇ grain boundaries are dragged by the Nb-based precipitates in which Mo is dissolved, the dissolved Mo segregates at the ⁇ grain boundaries.
  • the grain boundaries in which Mo is segregated promote the formation of lath martensite having a specific crystal orientation during ⁇ ' (martensite) transformation.
  • the laths of lath martensite having a specific crystal orientation collide with each other and merge as they grow, thereby contributing to the formation of the grain boundaries having the above-mentioned specific rotation angle.
  • the presence of Nb-based precipitates in which Mo is concentrated can increase the amount of Mo segregated at the grain boundaries during heat treatment.
  • Nb-based precipitates that dissolve Mo are present at the 1/4 depth position.
  • the Mo concentration of the Nb-based precipitates is 4.5 times or more the Mo content of the steel sheet.
  • the Mo concentration of the Nb-based precipitates is more preferably 8.0 times or more the Mo content of the steel sheet, and further preferably 10.0 times or more.
  • the size of the Nb-based precipitates is preferably 15 ⁇ m or less.
  • the Nb-based precipitates are precipitates containing 50 mass % or more of Nb, and are, for example, Nb carbide, Nb carbonitride, Nb nitride, NbTi carbide, NbTi carbonitride, and the like.
  • the presence or absence of Nb-based precipitates and the Mo concentration of Nb-based precipitates can be determined using the same method as described for steel components.
  • the shape of the steel plate in this embodiment is not particularly limited.
  • the steel plate may be a flat plate, or may be part of the base plate of a tailored property material in which steel plates with different strengths and thicknesses are joined together.
  • the thickness of the steel sheet according to the present embodiment is not limited. If the steel sheet is for hot stamping of automobile parts, the thickness may be 0.6 mm or more or 0.8 mm or more from the main thickness range of the steel sheet. For the same reason, the thickness may be 4.0 mm or less or 2.5 mm or less.
  • the steel sheet according to the present embodiment may have a coating on a part of the surface.
  • the coating may be a coating mainly made of Al (Al-based coating) or a coating mainly made of Zn (Zn-based coating).
  • the coating is also called a film or a plating layer.
  • the coating mainly made of Al is a coating containing 70% by mass or more of Al
  • the coating mainly made of Zn is a coating containing 70% by mass or more of Zn.
  • the coating mainly made of Al may contain Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, and REM in addition to Al, and the balance may be impurities.
  • the Zn-based coating may further contain, in addition to Zn, Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Al, Co, In, Bi, Zr, Se, As, and REM, with the balance being impurities, the total content of which may be 1% or less.
  • the thickness of the coating is preferably 10 to 100 ⁇ m.
  • the chemical composition and thickness of the coating on the steel sheet can be determined by the same method as the above-mentioned method for measuring the chemical composition and thickness of the coating on the steel member.
  • the method for manufacturing the steel plate according to this embodiment which is suitable as the material for the steel member according to this embodiment, is not limited, but it can be manufactured, for example, by using a manufacturing method including the steps described below.
  • a casting step of melting steel having the above-mentioned chemical composition and casting it to produce a slab
  • a hot rolling step in which the obtained slab is heated and then hot rolled to obtain a hot rolled steel sheet (hot rolled steel sheet);
  • ⁇ Casting process> steel having the above-mentioned chemical composition is melted and cast to produce a slab to be subjected to hot rolling.
  • a slab produced by melting molten steel having the above-mentioned chemical composition using a converter or an electric furnace and then performing a continuous casting process can be used.
  • an ingot casting process, a thin slab casting process, or the like may be used.
  • Mo uniformly (macroscopically uniformly) dissolved but Mo is difficult to dissolve because it has a high melting point and is a heavy element.
  • the casting temperature is set to the liquidus temperature + 10°C or higher.
  • the liquidus temperature is determined by the chemical composition of the molten steel and can be obtained by thermodynamic calculation.
  • the method of thermodynamic calculation is not particularly limited, it is recommended to use integrated thermodynamic calculation software: Thermo-Calc.
  • the casting speed (Vc) is slowed down. Specifically, the casting speed is set to 0.9 m/min or less.
  • the steel plate according to this embodiment contains a predetermined amount of Mo or more.
  • This Mo has the effect of suppressing the precipitation of grain boundary ferrite, which segregates at grain boundaries and causes cracks, so that even at this casting speed, there is no problem with cracks.
  • the casting speed is preferably set to 0.5 m/min or less.
  • ⁇ Hot rolling process> In the hot rolling process, the slab is heated and subjected to rough rolling, followed by descaling if necessary, and finally finish rolling.
  • Nb-based precipitates in which Mo is concentrated for example, the Mo concentration is 4.5 times or more the Mo content of the base steel sheet
  • the Nb-based precipitates precipitated in the casting process are coarse and do not contain any enrichment of Mo. In order to obtain Nb-based precipitates with enrichment of Mo, it is necessary to melt them once and then finely precipitate them again by hot rolling.
  • the heating temperature is less than the solution temperature of the Nb-based precipitates + 5°C, the coarse Nb-based precipitates precipitated in the casting process cannot be dissolved.
  • the time from the end of rough rolling to the start of finish rolling is 10 seconds or less.
  • Nb-based precipitates are precipitated in the fine ferrite after finish rolling. If the Nb-based precipitates are finely dispersed, Mo dissolves in the precipitates, increasing the Mo concentration.
  • the steel is usually in the ⁇ region, and if the time from the end of rough rolling to the start of finish rolling exceeds 10 seconds, Nb-based precipitates precipitate in the ⁇ state, and fine Nb-based precipitates with concentrated Mo cannot be obtained. It is more preferable to set the time from the end of rough rolling to the start of finish rolling to 7 seconds or less.
  • the hot-rolled steel sheet after the hot rolling process is coiled at a temperature range of 850° C. or less. If the coiling temperature exceeds 850° C., the steel sheet is coiled before transformation has progressed much, and the transformation progresses in the coil, which may result in a defective coil shape, which is not preferable.
  • annealing may be performed for 5 hours or more at 450 to 950° C. in an atmosphere containing 80% by volume or more of nitrogen or in an air atmosphere, as necessary. Annealing of the hot-rolled steel sheet is preferable because it can soften the hot-rolled steel sheet and reduce the load in the subsequent cold rolling process.
  • the hot rolled steel sheet after the hot rolled sheet annealing process (the hot rolled steel sheet after the coiling process when the hot rolled sheet annealing process is not performed) is descaled and cold rolled to obtain a cold rolled steel sheet.
  • Descaling and cold rolling are not necessarily required, but when cold rolling is performed, the cumulative reduction in cold rolling is preferably 30% or more from the viewpoint of ensuring good flatness. On the other hand, in order to avoid excessive rolling load, the cumulative reduction in cold rolling is preferably 80% or less.
  • the method of descaling is not particularly limited, but pickling is preferred. When pickling is performed, pickling with hydrochloric acid or sulfuric acid is preferably used to remove only iron scale.
  • the hot-rolled steel sheet or the cold-rolled steel sheet is annealed in a temperature range of 700 to 950° C. to obtain an annealed steel sheet.
  • the annealing step is preferable because it can soften the cold-rolled steel sheet and facilitates threading in the subsequent plating step.
  • a coating is formed on the surface of a steel sheet (hot-rolled steel sheet after a coiling process, hot-rolled steel sheet after a hot-rolled sheet annealing process, cold-rolled steel sheet after a cold-rolling process, or annealed steel sheet after an annealing process) to produce a coated steel sheet (if the coating is a plating layer, it is a plated steel sheet).
  • a coating method there is no particular limitation on the coating method, and possible methods include hot-dip plating, electroplating, vacuum deposition, cladding, and thermal spraying. Hot-dip plating is the most widely used method industrially.
  • the coating include an Al-based coating containing Al and a Zn-based coating containing Zn.
  • the plating bath When an Al-based coating is formed by hot-dip plating, the plating bath often contains Fe as an impurity in addition to Al.
  • the plating bath may contain Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, and misch metals, so long as the plating bath contains 70 mass% or more of Al.
  • the annealed steel sheet after the annealing process may be cooled to room temperature and then heated again to perform plating, or may be cooled to 650 to 750 ° C., which is close to the plating bath temperature, after annealing, and hot-dip plating may be performed without cooling to room temperature.
  • pre- and post-treatment for coating, and pre-coating, solvent application, alloying, temper rolling, etc. are possible.
  • alloying treatment for example, annealing at 450 to 800°C is possible.
  • temper rolling is useful for adjusting the shape, and for example, reduction of 0.1 to 0.5% is possible.
  • the method for manufacturing the steel member according to this embodiment is not limited, but the steel member can be manufactured, for example, by using a manufacturing method including the steps shown below for the steel plate according to this embodiment obtained by the method described above.
  • the steel plate according to the present embodiment having a predetermined chemical composition is heat-treated to produce a steel member.
  • the heat treatment is performed under the conditions that the steel plate obtained by the method described below is heated to Ac3 to (Ac3+300)°C at an average heating rate of 1.0 to 1000°C/s, and then cooled to Ms at an average cooling rate equal to or higher than the upper critical cooling rate. If the heating rate is less than 1.0° C./sec, the productivity of the heat treatment decreases, which is not preferable, whereas if the heating rate exceeds 1000° C./sec, a duplex grain structure is formed and the limit hydrogen content decreases, which is not preferable.
  • the upper critical cooling rate is the minimum cooling rate at which austenite is supercooled to form martensite without precipitating ferrite or pearlite in the structure. If cooling is performed at a rate below the upper critical cooling rate, ferrite and pearlite will form, resulting in insufficient strength.
  • the heating temperature may be held within a range of ⁇ 10° C. for 1 to 300 seconds.
  • the steel member may be subjected to a tempering treatment at a temperature in the range of about 100 to 600° C. in order to adjust the strength of the steel member.
  • the Ac3 point, the Ms point and the upper critical cooling rate are measured by the following method.
  • a rectangular test piece having a width of 30 mm and a length of 200 mm is cut out from the steel plate according to this embodiment, and the test piece is heated to 1000°C in a nitrogen atmosphere at a heating rate of 10°C/sec, held at that temperature for 5 minutes, and then cooled to room temperature at various cooling rates.
  • the cooling rate is set at intervals of 10°C/sec from 1°C/sec to 100°C/sec (however, the rate after 1°C/sec is 10°C/sec).
  • the Ac3 point and Ms point are measured by measuring the change in thermal expansion of the test piece during heating and cooling.
  • the minimum cooling rate at which ferrite and pearlite did not precipitate was defined as the upper critical cooling rate.
  • the Ms point obtained from the thermal expansion change when cooled at or above the upper critical cooling rate was defined as the Ms point of the steel member.
  • hot forming such as hot stamping may be performed at the same time.
  • hot forming include bending, drawing, stretching, hole expansion, and flange forming.
  • a forming method other than press forming such as roll forming, may be applied. If the above thermal history is followed, repeated hot forming may be performed. In addition, the above series of heat treatments may be repeated multiple times.
  • the above heat treatment may also be performed on a portion of the steel plate that is the raw material for hot forming or heat treatment. In this case, a steel component with areas of different strength is obtained.
  • heating can be carried out by high-frequency heating, electrical heating, infrared heating, or furnace heating. Cooling can also be carried out by water cooling, mold cooling, or the like.
  • the atmosphere inside the heating furnace can be air, city gas, or nitrogen gas.
  • the dew point inside the heating furnace can also be controlled to suppress hydrogen generation during the heat treatment.
  • the obtained slab was heated to 1260°C, which is 5°C or higher than the solid solution temperature of Nb-based precipitates, hot-rolled, and coiled at a temperature of 850°C or lower and 450°C or higher to form a steel plate (hot-rolled steel plate) with a thickness of 2.7 mm.
  • the hot-rolled steel sheets were pickled and then cold-rolled to obtain steel sheets (cold-rolled steel sheets) having a thickness of 1.6 mm.
  • Some of the steel sheets were annealed by heating to 760°C after cold rolling and holding for 10 seconds, and then immersed in an Al plating bath at 680°C containing 10% Si, 2% Fe, and the balance being impurities to obtain Al-plated steel sheets (coating in Table 2-2: Al). Some of the steel sheets were immersed in a Zn plating bath containing Zn and impurities to obtain Zn-plated steel sheets (coating in Table 2-2: Zn). The thickness of the coating was adjusted to 30 ⁇ m in each case.
  • the area fraction of the region surrounded by a boundary with a crystal orientation difference of 5° or more and having an average crystal orientation difference of 0.4 to 3.0° inside the boundary was evaluated by the above-mentioned method.
  • the evaluation results are shown in Table 2-2.
  • the ratio of the Mo concentration of the Nb-based precipitates to the Mo content of the steel sheets was measured. The results are shown in Table 2-2.
  • invention examples B1 to B14 which satisfy the range of the present invention, produced steel sheets with the specified chemical composition and metal structure.
  • comparative examples b1 to b9 which do not satisfy the range of the present invention, did not satisfy the chemical composition and metal structure.
  • Example 2 The steel plates (B1 to B14 and b1 to b9) produced in Example 1 were subjected to a heat treatment in which they were heated to the heating temperatures shown in Table 3-1 at the heating rate shown in Table 3-1, held at the heating temperature within ⁇ 10°C for 90 seconds, and cooled to a temperature below the Ms point at the average cooling rate shown in Table 3-1, to obtain steel members C1 to C14 and c1 to c9.
  • the area fraction of martensite and (L49-56°+L64-72°)/(L57-63°+L4-12°) of the obtained steel member were measured by the above-mentioned method to confirm the presence or absence of Nb-based inclusions. If Nb-based precipitates were present, the ratio of the Mo concentration to the Mo content in the chemical composition of the steel member (the chemical composition of the base steel plate if coated) was measured by the above-mentioned method. The results are shown in Table 3-2. Although not shown in the table, the metal structure of the inventive examples was, apart from martensite, retained austenite and/or bainite, and the Nb-based precipitates that were present were 0.5 to 15 ⁇ m in size.
  • the tensile strength and the limiting hydrogen content Hc which is an index of hydrogen embrittlement resistance, were evaluated in the following manner.
  • the evaluation results are shown in Table 3-2.
  • the steel members obtained from the steel plate having an Al-based coating had an Fe-Al-based coating on the surface
  • the steel members obtained from the steel plate having a Zn-based coating had an Fe-Zn-based coating on the surface.
  • the Fe-Al-based coating was a coating containing about 10 mass% Si with the remainder being Fe, Al, and 1% or less of impurities.
  • the Fe-Zn-based coating was a coating consisting of an Fe-Zn alloy and 1% or less of impurities.
  • ⁇ Tensile strength> The tensile test was carried out in accordance with the provisions of ASTM standard E8M-22. That is, after the portion of the steel member avoiding the end portion was ground evenly on both sides to a thickness of 1.2 mm, a sub-size tensile test piece (gauge length: 25.0 ⁇ 0.1 mm (parallel portion length: 32.0 mm), parallel portion plate width: 6.0 ⁇ 0.1 mm) in Table 1 of ASTM standard E8M-22 was taken.
  • a strain gauge (gauge length: 5 mm) was attached to the center in the width and length directions of the parallel portion of the test piece, and a room temperature tensile test was carried out at a strain rate of 3 mm/min to measure the tensile strength (TS).
  • TS tensile strength
  • Hydrogen embrittlement resistance was evaluated by performing four-point bending on a test piece in which hydrogen had been absorbed, and measuring the limit of hydrogen amount Hc at which the steel member did not crack. Specifically, a rectangular test piece with a width of 8 mm and a length of 68 mm was cut out, avoiding the ends of the steel member.
  • a strain gauge (gauge length: 5 mm) similar to that used in the tensile test was attached to the center of the width and length of the surface of the test piece, and the test piece was bent with a four-point support jig so that a strain equivalent to a stress of 1/2 the tensile strength obtained in the tensile test was generated on the surface of the test piece (a four-point bending test piece was used).
  • the four-point bending test pieces in which various amounts of hydrogen had been absorbed were observed for cracking, and the limit of hydrogen amount Hc at which the test piece did not crack was determined.
  • the amount of hydrogen absorption was changed by changing the dew point inside the furnace during heat treatment.
  • the test pieces were immersed in ammonium thiocyanate solutions of various concentrations for 72 hours after four-point bending to absorb hydrogen.
  • the hydrogen absorbed in the steel member was measured by temperature rise at 100° C./hr in a temperature rise hydrogen analysis, and the amount of diffusible hydrogen released up to 250° C. was regarded as the amount of hydrogen contained in the steel member.
  • Hc was 0.6 mass ppm or more for a tensile strength of 1500 to less than 2000 MPa, 0.4 mass ppm or more for a tensile strength of 2000 to less than 2500 MPa, and 0.2 mass ppm or more for a tensile strength of 2500 MPa or more, it was evaluated as having excellent hydrogen embrittlement resistance.
  • invention examples C1 to C14 have chemical compositions and metal structures that fall within the ranges of the present invention, and have good results in both tensile strength and hydrogen embrittlement resistance.
  • comparative examples c1 to c9 do not have chemical compositions and metal structures that fall within the ranges of the present invention, and at least one of the strength and hydrogen embrittlement resistance was poor.
  • the steel members according to the present invention are particularly suitable for use as automotive frame components.
  • the steel members according to the present invention have high strength and excellent resistance to hydrogen embrittlement, and therefore contribute to improved fuel economy and collision safety when applied to automotive parts.

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Abstract

L'élément en acier de l'invention présente une composition prédéfinie, et est tel que lorsqu'une position à 1/4 d'épaisseur dans une direction épaisseur depuis une surface constitue un centre, et qu'une plage d'une position à 1/8 à une position à 3/8 de ladite épaisseur dans ladite direction épaisseur depuis ladite surface, constitue une position à 1/4 de profondeur, la direction de ＜０１１＞ constitue un axe de rotation au niveau de ladite position à 1/4 de profondeur parmi des joints de grains de grains cristallins ayant une structure centrée, et lorsque la longueur de joint de grains présentant un angle de rotation compris entre ４９ et ５６° est représentée par Ｌ４９－５６°, la longueur de joint de grains présentant un angle de rotation compris entre ６４ et ７２° est représentée par Ｌ６４－７２°, la longueur de joint de grains présentant un angle de rotation compris entre ５７ et ６３° est représentée par Ｌ５７－６３°, et la longueur de joint de grains présentant un angle de rotation compris entre ４ et １２° est représentée par Ｌ４－１２°, alors （Ｌ４９－５６°＋Ｌ６４－７２°）／（Ｌ５７－６３°＋Ｌ４－１２°） qui constitue le rapport entre la somme desdites Ｌ４９－５６° et Ｌ６４－７２° et la somme desdites Ｌ５７－６３° et Ｌ４－１２°, est supérieur ou égal à １,３０, et la résistance à la traction est supérieure à １５００ＭＰａ.
PCT/JP2024/009561 2023-03-13 2024-03-12 Élément en acier, et tôle en acier Pending WO2024190779A1 (fr)

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MX2025010598A (es) 2025-10-01
MX2025010501A (es) 2025-10-01
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KR20250142411A (ko) 2025-09-30
CN120787269A (zh) 2025-10-14

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