WO2023190867A1 - Élément en acier et feuille d'acier - Google Patents

Élément en acier et feuille d'acier Download PDF

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Publication number
WO2023190867A1
WO2023190867A1 PCT/JP2023/013175 JP2023013175W WO2023190867A1 WO 2023190867 A1 WO2023190867 A1 WO 2023190867A1 JP 2023013175 W JP2023013175 W JP 2023013175W WO 2023190867 A1 WO2023190867 A1 WO 2023190867A1
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Prior art keywords
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steel
content
steel member
depth position
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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.)
Ceased
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PCT/JP2023/013175
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English (en)
Japanese (ja)
Inventor
進一郎 田畑
伸麻 吉川
高志 荒牧
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
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Nippon Steel Corp
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Application filed by Nippon Steel Corp filed Critical Nippon Steel Corp
Priority to KR1020247031722A priority Critical patent/KR20240155913A/ko
Priority to MX2024011522A priority patent/MX2024011522A/es
Priority to EP23780875.3A priority patent/EP4502193A4/fr
Priority to JP2024512794A priority patent/JPWO2023190867A1/ja
Priority to CN202380029343.8A priority patent/CN118922573A/zh
Publication of WO2023190867A1 publication Critical patent/WO2023190867A1/fr
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
    • 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
    • C21D8/0221—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 working steps
    • C21D8/0242—Flattening; Dressing; Flexing
    • 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
    • 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
    • C21D8/0263—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 following hot rolling
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • 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
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
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    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
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    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
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    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • 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
    • 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
    • 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
    • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
    • C21D1/76—Adjusting the composition of the atmosphere
    • 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
    • C21D2211/008—Martensite
    • 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
    • C21D8/0221—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 working steps
    • C21D8/0226—Hot rolling
    • 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
    • C21D8/0221—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 working steps
    • C21D8/0236—Cold rolling
    • 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
    • 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
    • C21D8/0273—Final recrystallisation annealing

Definitions

  • the present invention relates to a steel member and a steel plate. This application claims priority based on Japanese Patent Application No. 2022-055819 filed in Japan on March 30, 2022, the contents of which are incorporated herein.
  • Hot stamping technology is a hot forming technology in which the material to be molded is heated and then molded.
  • the material is heated and then molded. Therefore, during forming, the steel material is soft and has good formability. Thereby, even a high-strength steel plate can be formed into a complex shape with high precision. Further, in the hot stamping technique, since quenching is performed simultaneously with forming using a press die, the steel member after forming has sufficient strength.
  • Patent Document 1 discloses that hot stamping technology makes it possible to impart a tensile strength of 1400 MPa or more to a steel member after forming.
  • automobiles are also required to have collision safety.
  • the collision safety of an automobile is evaluated based on the crushing strength and absorbed energy in a crash test of the entire vehicle body or some parts of the vehicle.
  • crushing strength is highly dependent on material strength
  • the demand for ultra-high strength steel members for automobile parts is increasing dramatically.
  • their fracture toughness and deformability generally decrease.
  • the material may break early or break at a location where deformation is concentrated during impact crushing, and the crushing strength commensurate with the material strength may not be achieved and sufficient absorbed energy may not be obtained.
  • the steel members used are required to have not only material strength but also improved fracture toughness and deformability, that is, improved toughness and bendability. Therefore, in order to apply high-strength steel members with a tensile strength exceeding 1.0 GPa to car bodies, steel members must have toughness and bendability higher than conventional ones, and exhibit sufficient energy absorption even in the event of a collision. A technology that provides this is needed.
  • Patent Document 2 discloses a hot press-formed product having excellent toughness and a tensile strength of 1.8 GPa or more.
  • Patent Document 3 discloses a steel material that has an extremely high tensile strength of 2.0 GPa or more and also has good toughness and ductility.
  • Patent Document 4 discloses a steel material that has a high tensile strength of 1.8 GPa or more and also has good toughness.
  • Patent Document 5 discloses a steel material that has extremely high tensile strength of 2.0 GPa or more and also has good toughness.
  • Patent Documents 1 to 5 do not contain any description regarding bendability, and when using high-strength steel materials with a tensile strength exceeding 1.0 GPa as automobile parts, higher demands may not be met sufficiently. .
  • Patent Documents 1 to 5 have room for further improvement from the viewpoint of hydrogen embrittlement resistance.
  • An object of the present invention is to provide a steel member having high strength and excellent bendability and hydrogen embrittlement resistance (hydrogen embrittlement resistance), and a steel plate suitable as a material for the steel member.
  • the present inventors conducted studies to obtain excellent bendability and hydrogen embrittlement resistance in high-strength steel members obtained by hot stamping steel plates. As a result, it has been found that excellent bendability and hydrogen embrittlement resistance can be obtained by enriching B while forming a decarburized layer on the surface layer of a steel member. We have also found that the above-mentioned steel member can be stably obtained by introducing cracks into the scale formed on the surface of the steel plate (material steel plate) to be subjected to hot stamping.
  • the present invention has been made in view of the above problems.
  • the gist of the invention is as follows.
  • the steel member according to one embodiment of the present invention has, in mass %, C: 0.10 to 0.70%, Si: 3.00% or less, Mn: 0.01 to 3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.020% or less, O: 0.010% or less, B: 0.0002 to 0.0200%, Ti: 0 to 0.200%, Cr: 0-1.00%, Mo: 0-1.00%, Ni: 0-2.00%, Nb: 0-0.10%, Cu: 0-2.00%, V: 0-1 .00%, Ca: 0-0.020%, Mg: 0-0.020%, Al: 0-1.00%, Sn: 0-1.00%, W: 0-2.00%, Sb : 0-1.00%, Zr: 0-1.00%, Se: 0-1.00%, Bi: 0-1.00%, As: 0-1.00%, Ta: 0-1.00%
  • the decarburization depth is 20 ⁇ m.
  • the Vickers hardness at the 1/4 depth position is 310 to 890.
  • the steel member according to [1] has Vickers hardness at the 0.1 mm depth position, when a position 0.1 mm deep from the surface in the thickness direction is defined as a 0.1 mm depth position. The hardness may be 0.95 times or less of the Vickers hardness at the 1/4 depth position.
  • the Vickers hardness at the 1/4 depth position is 310 to 450, and the steel member with a plate thickness of 1.6 mm is deformed to a bending angle of 50 degrees. In this case, the maximum principal plastic strain at the center of the outer bending surface layer may be 0.340 or less.
  • the Vickers hardness at the 1/4 depth position is more than 450 and 530 or less, and the Vickers hardness at the 0.1 mm depth position is the 1/4 depth position.
  • the maximum principal plastic strain at the center of the outer bending surface layer is 0.310. It may be the following.
  • the Vickers hardness at the 1/4 depth position is more than 530 and 700 or less, and the Vickers hardness at the 0.1 mm depth position is the 1/4 depth position.
  • the maximum principal plastic strain at the center of the outer surface layer of the bend is 0.280.
  • the Vickers hardness at the 1/4 depth position is more than 700 and 890 or less, and the Vickers hardness at the 0.1 mm depth position is the 1/4 depth position.
  • the maximum principal plastic strain at the center of the outer bending surface layer is 0.260. It may be the following.
  • the steel member according to any one of [1] to [6] may further have an alloy layer on the surface.
  • the alloy layer may be a Fe--Al alloy layer.
  • the alloy layer may be a Fe--Zn alloy layer.
  • a steel plate according to another aspect of the present invention has a base steel plate and an iron scale formed on the surface of the base steel plate, and the base steel plate has a C:0 in mass%.
  • the steel member includes cases in which the steel member has an alloy layer on its surface, and the steel plate also includes cases in which the steel plate has a plating layer (in the case of a plated steel plate).
  • (A) Steel member Maximum B content in the range from the surface to a depth of 10 ⁇ m in the thickness direction, where the position 1/4 depth from the surface in the thickness direction is defined as the 1/4 depth position is more than 5 times the B content at the 1/4 depth position, and the C content was measured from the surface in the thickness direction using GDS.
  • the decarburization depth is defined as the distance to the position where the C content is obtained, the decarburization depth is 20 ⁇ m or more, and the Vickers hardness at the 1/4 depth position is 310 to 890.
  • the steel member may have a strength gradient.
  • the steel member may have an alloy layer (coating) on the surface.
  • the steel member according to this embodiment has, in mass %, C: 0.10 to 0.70%, Si: 3.00% or less, Mn: 0.01 to 3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.020% or less, O: 0.010% or less, B: 0.0002 to 0.0200%, Ti: 0 to 0.200%, Cr: 0-1.00%, Mo: 0-1.00%, Ni: 0-2.00%, Nb: 0-0.10%, Cu: 0-2.00%, V: 0-1 .00%, Ca: 0-0.020%, Mg: 0-0.020%, Al: 0-1.00%, Sn: 0-1.00%, W: 0-2.00%, Sb : 0-1.00%, Zr: 0-1.00%, Se: 0-1.00%, Bi: 0-1.00%, As: 0-1.00%, Ta: 0-1.00%.
  • the chemical composition of the steel member refers to the chemical composition of a portion excluding the vicinity of the surface (for example, a position 1/4 of the thickness in the thickness direction from the surface of the steel member: 1/4 depth position).
  • % regarding content is mass % unless otherwise specified. The reason for limiting the content of each element is as follows.
  • C 0.10-0.70%
  • C is an element that increases the hardenability of steel and improves the strength of a steel member obtained by hot stamping a steel plate.
  • the C content is set to 0.10% or more.
  • the C content is preferably 0.15% or more.
  • the C content is set to 0.70% or less.
  • the C content is preferably 0.60% or less.
  • Si 3.00% or less
  • Si is an element that is effective in improving the hardenability of steel and stably ensuring strength in steel members. Therefore, it may be included.
  • the Si content is preferably 0.05% or more, more preferably 0.10% or more.
  • the Si content in the steel sheet exceeds 3.00%, the heating temperature required for austenite transformation during heat treatment becomes significantly high. This may increase the cost required for heat treatment.
  • the Si content exceeds 3.00%, the toughness of the hardened portion deteriorates. Therefore, the Si content is set to 3.00% or less.
  • the Si content is preferably 2.00% or less, more preferably 1.80% or less.
  • Mn 0.01-3.00%
  • Mn is a very effective element for improving the hardenability of steel and ensuring stable strength in steel members.
  • Mn is an element that further lowers the Ac3 point and promotes lowering of the quenching temperature. If the Mn content is less than 0.01%, these effects are not sufficient, so the Mn content is set to 0.01% or more.
  • the Mn content is preferably 0.05% or more. On the other hand, if the Mn content exceeds 3.00%, the above effects are saturated, and the toughness, bendability, and hydrogen embrittlement resistance of the quenched part deteriorate. Therefore, the Mn content is set to 3.00% or less.
  • the Mn content is preferably 2.80% or less, more preferably 2.50% or less.
  • P 0.100% or less
  • P is an element that deteriorates the toughness and hydrogen embrittlement resistance of steel members.
  • the P content is limited to 0.100% or less. It is preferable to limit the P content to 0.050% or less. Since it is preferable that the P content be small, it may be 0%, but from the viewpoint of cost it may be 0.001% or more.
  • S 0.0100% or less
  • S is an element that deteriorates the toughness, bendability, and hydrogen embrittlement resistance of steel members.
  • the S content is limited to 0.0100% or less. It is preferable to limit the S content to 0.0050% or less. Since it is preferable that the S content be small, it may be 0%, but from the viewpoint of cost it may be 0.0001% or more.
  • N 0.020% or less
  • N is an element that deteriorates the toughness and hydrogen embrittlement resistance of steel members.
  • the N content is set to 0.020% or less.
  • the lower limit of the N content does not need to be particularly limited and may be 0%, but setting the N content to less than 0.0002% increases steel manufacturing costs and is economically unfavorable. Therefore, the N content may be set to 0.0002% or more, or may be set to 0.0008% or more.
  • O 0.010% or less
  • O is an element that deteriorates the toughness and hydrogen embrittlement resistance of steel members.
  • the O content is set to 0.010% or less.
  • the lower limit of the O content does not need to be particularly limited and may be 0%, but setting the O content to less than 0.0002% increases steel manufacturing costs and is economically unfavorable. Therefore, the O content may be 0.0002% or more, 0.0008% or more, or 0.001% or more.
  • B 0.0002-0.0200%
  • B is an element that has the effect of dramatically increasing the hardenability of steel even in a small amount.
  • B is an element that segregates at grain boundaries to strengthen the grain boundaries and improve toughness and hydrogen embrittlement resistance, and is an element that suppresses austenite grain growth during heating of the steel sheet.
  • B is an effective element for obtaining a fine martensite-based surface structure having strong grain boundaries by concentrating it in the surface layer as described below. If the B content is less than 0.0002%, these effects are not sufficient. Therefore, the B content is set to 0.0002% or more.
  • the B content is preferably 0.0005% or more, more preferably 0.0010% or more.
  • the B content exceeds 0.0200%, many coarse compounds precipitate, and the toughness and hydrogen embrittlement resistance of the steel member deteriorate. Therefore, when B is included, the B content is set to 0.0200% or less.
  • the B content is preferably 0.0100% or less.
  • the steel member according to the present embodiment contains the following elements in addition to the above elements: Ti, Cr, Mo, Ni, Nb, Contains one or more elements selected from Cu, V, Ca, Mg, Al, Sn, W, Sb, Zr, Se, Bi, As, Ta, Re, Os, Ir, Tc, Co, and REM. It's okay. These elements are optional elements and do not necessarily need to be included, so the lower limit is 0%. These elements may be contained as impurities as long as the content is below the upper limit described below.
  • Ti 0-0.200% Ti suppresses recrystallization when a steel plate is heated to a temperature of Ac 3 or higher and undergoes heat treatment, and also forms fine carbides and suppresses grain growth, which has the effect of making austenite grains finer. It is an element that has Therefore, by including Ti, the toughness of the steel member can be greatly improved. Further, Ti is an element that suppresses the consumption of B due to precipitation of BN by preferentially bonding with N in the steel, and promotes the effect of improving hardenability due to B, which will be described later. Therefore, Ti may be contained. In order to fully obtain the above effects, the Ti content is preferably 0.010% or more. The Ti content is more preferably 0.020% or more.
  • the Ti content exceeds 0.200%, the amount of precipitated TiC increases and C is consumed, resulting in a decrease in the strength of the steel member. Therefore, the Ti content is set to 0.200% or less.
  • the Ti content is preferably 0.100% or less.
  • Cr 0-1.00% Cr is an effective element for improving the hardenability of steel and stably ensuring the strength of steel members. Therefore, Cr may be contained.
  • the Cr content is preferably 0.01% or more.
  • the Cr content is more preferably 0.05% or more, and still more preferably 0.08% or more.
  • the Cr content exceeds 1.00%, the above effects are saturated and costs increase.
  • the Cr content should be 1.00% or less.
  • the Cr content is preferably 0.80% or less.
  • Mo 0-1.00%
  • Mo is an effective element for improving the hardenability of steel and stably ensuring the strength of steel members. Therefore, Mo may be included.
  • the Mo content is preferably 0.01% or more.
  • Mo content is more preferably 0.05% or more.
  • Mo content exceeds 1.00%, the above effects are saturated and costs increase.
  • Mo has the effect of stabilizing iron carbides, if the Mo content exceeds 1.00%, coarse iron carbides remain undissolved when the steel plate is heated, and the toughness of the steel member may deteriorate. Therefore, when Mo is included, the Mo content should be 1.00% or less. Mo content is preferably 0.80% or less.
  • Ni is an effective element for improving the hardenability of steel and stably ensuring the strength of steel members. Therefore, Ni may be included.
  • the Ni content is preferably 0.01% or more.
  • the Ni content is more preferably 0.10% or more.
  • the Ni content should be 2.00% or less.
  • Ni content is preferably 1.00% or less.
  • Nb 0-0.10%
  • Nb is an element that forms fine carbides and has the effect of increasing the toughness of steel due to its grain refining effect. Therefore, Nb may be included.
  • the Nb content is 0.02% or more.
  • the Nb content is more preferably 0.03% or more.
  • the Nb content is set to 0.10% or less.
  • the Nb content is preferably 0.08% or less.
  • Cu 0-2.00%
  • Cu is an effective element for improving the hardenability of steel and stably ensuring the strength of steel members. Therefore, Cu may be contained.
  • Cu is an element that has the effect of improving the corrosion resistance of steel members.
  • the Cu content is preferably 0.01% or more.
  • the Cu content is more preferably 0.05% or more.
  • the Cu content is set to 2.00% or less.
  • the Cu content is preferably 1.00% or less.
  • V 0-1.00%
  • V is an element that forms fine carbides and improves the toughness of steel due to its grain refining effect. Therefore, V may be contained.
  • the V content is preferably 0.01% or more.
  • the V content is more preferably 0.10% or more.
  • V content should be 1.00% or less.
  • Ca is an element that has the effect of making inclusions in steel finer and improving the toughness of steel members. Therefore, Ca may be contained. In order to obtain the above effects, it is preferable that the Ca content is 0.001% or more. The Ca content is more preferably 0.002% or more. On the other hand, when the Ca content exceeds 0.020%, the effect is saturated and the cost increases. Therefore, when Ca is contained, the Ca content should be 0.020% or less. The Ca content is preferably 0.010% or less, more preferably 0.005% or less.
  • Mg 0-0.020%
  • Mg is an element that has the effect of making inclusions in steel finer and improving the toughness of steel members. Therefore, Mg may be included.
  • the Mg content is 0.001% or more.
  • the Mg content is more preferably 0.002% or more.
  • the Mg content should be 0.020% or less.
  • the Mg content is preferably 0.010% or less, more preferably 0.005% or less.
  • Al 0-1.00%
  • Al is an element commonly used as a deoxidizing agent for steel. Therefore, Al may be contained.
  • the Al content is 0.01% or more.
  • the Al content should be 1.00% or less.
  • the Al content here is total. This is the Al content.
  • Sn 0-1.00%
  • Sn is an element that improves corrosion resistance in a corrosive environment. Therefore, Sn may be contained.
  • the Sn content is preferably 0.01% or more.
  • the Sn content is set to 1.00% or less.
  • W 0-2.00%
  • W is an element that improves the hardenability of steel and makes it possible to stably ensure the strength of steel members. Therefore, W may be contained. Further, W is an element that improves corrosion resistance in a corrosive environment. In order to obtain the above effects, it is preferable that the W content is 0.01% or more. On the other hand, when the W content exceeds 2.00%, the above-mentioned effects are saturated and economical efficiency decreases. Therefore, when W is included, the W content should be 2.00% or less. The W content is preferably 1.00% or less.
  • Sb 0-1.00%
  • Sb is an element that improves corrosion resistance in a corrosive environment. Therefore, Sb may be contained. In order to obtain the above effects, it is preferable that the Sb content is 0.01% or more. On the other hand, when the Sb content exceeds 1.00%, the grain boundary strength decreases and the toughness of the steel member deteriorates. Therefore, when Sb is included, the Sb content should be 1.00% or less.
  • Zr 0-1.00%
  • Zr is an element that improves corrosion resistance in a corrosive environment. Therefore, Zr may be contained.
  • the Zr content is preferably 0.01% or more.
  • the Zr content should be 1.00% or less.
  • Se 0-1.00% Se is an element that improves hydrogen embrittlement resistance. Therefore, it may be included. In order to obtain the above effects, it is preferable that the Se content is 0.01% or more. On the other hand, if the Se content exceeds 1.00%, the effect will be saturated and the cost will increase. Therefore, when included, the Se content should be 1.00% or less.
  • Bi 0 ⁇ 1.00%
  • Bi is an element that improves hydrogen embrittlement resistance. Therefore, it may be included.
  • the Bi content is 0.01% or more.
  • the Bi content exceeds 1.00%, the effect is saturated and the cost increases. Therefore, when included, the Bi content is set to 1.00% or less.
  • As is an element that improves hydrogen embrittlement resistance. Therefore, it may be included.
  • the As content is preferably 0.01% or more.
  • the As content should be 1.00% or less.
  • Ta 0-1.00%
  • the Ta content is preferably 0.01% or more.
  • the Ta content is set to 1.00% or less.
  • Re 0 ⁇ 1.00%
  • Re is an element that improves hydrogen embrittlement resistance. Therefore, it may be included.
  • the Re content is 0.01% or more.
  • the Re content exceeds 1.00%, the effect is saturated and the cost increases. Therefore, when it is included, the Re content is set to 1.00% or less.
  • Os 0 ⁇ 1.00%
  • Os is an element that improves hydrogen embrittlement resistance. Therefore, it may be included.
  • the Os content is preferably 0.01% or more.
  • the Os content exceeds 1.00%, the effect will be saturated and the cost will increase. Therefore, when included, the Os content should be 1.00% or less.
  • Ir 0-1.00% Ir is an element that improves hydrogen embrittlement resistance. Therefore, it may be included. In order to obtain the above effects, it is preferable that the Ir content is 0.01% or more. On the other hand, if the Ir content exceeds 1.00%, the effect will be saturated and the cost will increase. Therefore, when Ir is included, the Ir content should be 1.00% or less.
  • Tc 0-1.00%
  • Tc is an element that improves hydrogen embrittlement resistance. Therefore, it may be included.
  • the Tc content is preferably 0.01% or more.
  • the Tc content should be 1.00% or less.
  • Co 0-1.00%
  • Co is an element that improves corrosion resistance in a corrosive environment. Therefore, Co may be included.
  • the Co content is 0.01% or more.
  • the Co content should be 1.00% or less.
  • REM 0-0.30% Like Ca, REM is an element that has the effect of making inclusions in steel finer and improving the toughness of steel members. Therefore, REM may be included. In order to obtain the above effects, it is preferable that the REM content is 0.01% or more. The REM content is more preferably 0.02% or more. On the other hand, when the REM content exceeds 0.30%, the effect is saturated and the cost increases. Therefore, when it is included, the REM content should be 0.30% or less. The REM content is preferably 0.20% or less.
  • REM refers to a total of 17 elements such as Sc, Y, and lanthanoids such as La and Nd, and the content of REM refers to the total content of these elements.
  • REM is added to molten steel using, for example, a Fe-Si-REM alloy, which includes, for example, La, Nd, Ce, Pr.
  • the elements other than those mentioned above, that is, the remainder are Fe and impurities.
  • impurities are components that are mixed into raw materials such as ores and scraps and various factors in the manufacturing process when steel sheets are manufactured industrially, and have an adverse effect on the characteristics of the steel member according to this embodiment. It means that it is permissible within the range that does not give.
  • the industrial manufacturing method is a blast furnace steel manufacturing method or an electric furnace steel manufacturing method, and includes the level of contamination (impurity level) when manufactured by either method.
  • the chemical composition of a steel member can be determined by the following method. It is obtained by cutting out an analysis sample from a steel member and performing elemental analysis such as ICP (inductively coupled plasma) emission spectroscopy. C and S may be measured using the combustion-infrared absorption method, N may be measured using the inert gas melting-thermal conductivity method, and O may be measured using the inert gas melting-non-dispersive infrared absorption method. You can measure it using As described in JIS G 0417:1999, the analysis sample was prepared from the surface, avoiding the widthwise edges of the base steel plate, in order to obtain an average chemical composition over the entire thickness of the steel member. An analysis sample is taken from a position of 1/4 of the thickness in the direction.
  • ICP inductively coupled plasma
  • the C content and B content obtained here are the C content and B content at the 1/4 depth position, which will be described later, but in the steel member according to this embodiment, the chemical composition is generally uniform. It can also be said that this is the average chemical composition of the entire steel member, excluding the decarburized layer and the surface layer where B is concentrated.
  • the maximum B content in the range from the surface to a depth of 10 ⁇ m is more than 5 times the B content at the 1/4 depth position, and the decarburization depth is 20 ⁇ m. That's all.
  • the decarburization depth of the steel member is 20 ⁇ m or more, bendability is improved. Therefore, the decarburization depth is set to 20 ⁇ m or more.
  • decarburizing the surface layer to soften it is effective in improving the bendability of steel members.
  • carbon is an element that segregates and strengthens grain boundaries, it is not preferable that carbon at grain boundaries decrease. Since carbon is an element that segregates and strengthens grain boundaries, if the amount of carbon in grain boundaries decreases and grain boundary strength decreases, hydrogen embrittlement resistance and toughness may decrease.
  • it is effective to soften the surface layer by decarburizing it, it is not preferable to make the surface a ferrite-based structure.
  • B is concentrated in the surface layer while decarburizing the surface layer.
  • B is an element that segregates at grain boundaries, strengthens the grain boundaries, and improves hardenability. Therefore, by concentrating B in the surface layer, a martensite-based structure with tough and fine grain boundaries can be obtained.
  • the martensite-based structure in the surface layer has been decarburized, it is softer than the martensite-based structure in the base material, resulting in high levels of bendability, hydrogen embrittlement resistance, and toughness. Specifically, bendability and hydrogen embrittlement resistance are improved when the maximum B content in the range from the surface of the steel member to a depth of 10 ⁇ m is 5 times or more the B content at the 1/4 depth position.
  • the steel member according to the present embodiment has a hardness (Vickers hardness (HV)) of 310 to 890 at the 1/4 depth position. Having this Vickers hardness corresponds to a steel member having a tensile strength of 1.0 GPa to 3.1 GPa, and when applied to automobile parts, it contributes to reducing the weight of automobile bodies and enhancing collision safety. do.
  • the relationship between hardness and tensile strength is described, for example, in SAE J417.
  • the surface layer is decarburized and softened, and that the strength gradient from the surface layer toward the inside is controlled within the following range.
  • bendability is further improved. The reason for this is that during bending deformation, as mentioned above, the maximum strain and stress are applied to the outer bending surface, and cracks tend to propagate from the surface to the inside. However, if an appropriate strength gradient exists, the development of strain on the surface is inhibited. This is thought to be because it has a suppressing effect.
  • the Vickers hardness at the position (0.1 mm depth position) within an appropriate range. Specifically, it is preferable that the Vickers hardness at the 0.1 mm depth position is 0.95 times or less than the Vickers hardness at the 1/4 depth position. Further, it is more preferable to further control the above-mentioned hardness ratio according to the Vickers hardness at the 1/4 depth position, and to control the maximum principal plastic strain at the center of the outer bending surface layer within a predetermined range.
  • the Vickers hardness at the 1/4 depth position is over 700 and 890 or less (equivalent to a tensile strength of over 2.4 GPa and 3.1 GPa or less)
  • the Vickers hardness at the 0.1 mm depth position is It is 0.86 times or less of the Vickers hardness at the 1/4 depth position, and when a steel member with a plate thickness of 1.6 mm is deformed to a bending angle of 50 degrees, the maximum principal plastic strain at the center of the outer surface layer of the bend is 0. It is preferably 260 or less.
  • the plate thickness is not limited, it is preferably 0.6 to 4.0 mm.
  • the Vickers hardness at the 1/4 depth position is over 530 and 700 or less (equivalent to a tensile strength of over 1.8 GPa and 2.4 GPa or less)
  • the Vickers hardness at the 0.1 mm depth position is is less than 0.88 times the Vickers hardness at the 1/4 depth position, and when a steel member with a thickness of 1.6 mm is deformed to a bending angle of 50 degrees, the maximum principal plastic strain at the center of the outer surface layer of the bend is 0. It is preferable that it is .280 or less.
  • the Vickers hardness at the 1/4 depth position is over 450 and 530 or less (equivalent to a tensile strength of over 1.5 GPa and 1.8 GPa or less)
  • the Vickers hardness at the 0.1 mm depth position is is less than 0.90 times the Vickers hardness at the 1/4 depth position, and when a steel member with a thickness of 1.6 mm is deformed to a bending angle of 50 degrees, the maximum principal plastic strain at the center of the outer surface layer of the bend is 0. It is preferable that it is .310 or less.
  • the Vickers hardness at a depth of 1/4 is 310 to 450 (equivalent to a tensile strength of 1.0 to 1.5 GPa)
  • the Vickers hardness at a depth of 0.1 mm is 1/4.
  • the Vickers hardness at the depth position is 0.95 times or less, and when a steel member with a plate thickness of 1.6 mm is deformed to a bending angle of 50 degrees, the maximum principal plastic strain at the center of the outer surface layer of the bend is 0.340 or less. It is preferable that there be.
  • the Vickers hardness at the 0.1 mm depth position is preferably at least 0.70 times the Vickers hardness at the 1/4 depth position in any case.
  • B enrichment ratio the ratio of the maximum B content in the range up to a depth of 10 ⁇ m to the B content at the 1/4 depth position
  • decarburization depth GDS was used. It can be determined using the following method. At a position 1/4 of the plate width (short side) from the end of the steel member in the width direction, the outermost surface of the steel member (the surface of the scale if the surface has scale, the surface of the alloy layer if the surface has an alloy layer) GDS (glow discharge optical emission spectrometry) is performed at a pitch of 50 nm or less from the top surface in the thickness direction to measure the B content, C content, and Fe content.
  • the position where the Fe content first becomes 95% by mass or more is defined as the surface.
  • the maximum value among the B contents in the depth range of 10 ⁇ m from the above-mentioned surface obtained by GDS analysis is defined as the maximum B content.
  • the maximum value of the B content within a depth of 10 ⁇ m from the position where the total of Fe and Al is 70% by mass or more is determined.
  • the maximum B content is determined as the maximum value.
  • the concentration degree of B in the surface layer is calculated from this maximum B content and the B content at the 1/4 depth position determined by the method described above.
  • the position where the C content obtained by the same GDS analysis is equivalent to the C content at the 1/4 depth position described above is the end of the decarburization depth, and the distance from the surface to this end is defined as the end of the decarburization depth. Let be the decarburization depth. However, the above GDS measurement is performed five times at different locations, and the average value of the five measurements is used.
  • the hardness at the 1/4 depth position and the hardness at the 0.1 mm depth position are determined by the following method. A sample for cross-sectional observation was taken from a position 1/4 of the width (short side) from the width direction end of the steel member, and the target depth position (1/4 depth position) was measured according to JIS Z2244-1:2020. , 0.1 mm depth position), the Vickers hardness is measured. The test force is 100gf. Measurement is performed five times at each depth, and the average value is taken as the hardness at that position.
  • the maximum principal plastic strain at the center of the outer bending surface layer when deformed to a predetermined bending angle is determined by the following method.
  • a steel member with a strength gradient on its surface is modeled using the finite element method. With respect to the total plate thickness, the ranges of 50 ⁇ m, 50 to 100 ⁇ m, and 100 to 250 ⁇ m from the surface are set to have the same hardness as the steel member to be evaluated.
  • a test jig for conducting a bending test of VDA238-100 will be modeled. According to the standard, for example, the punch diameter R is 0.4 mm, the roll diameter is 30 mm ⁇ , and the distance between the rolls is 2 ⁇ plate thickness+0.05.
  • the modeled steel member and test jig Using the modeled steel member and test jig, push (simulation) is performed to a predetermined bending angle.
  • the maximum principal plastic strain at a predetermined bending angle is output, and the maximum principal plastic strain at the center of the outer bending surface layer (the location on the bending ridgeline where the maximum principal plastic strain is maximum) is determined.
  • the software for analysis using the finite element method is not limited, but for example, Marc manufactured by MSC Software is used.
  • the mesh size is 5 ⁇ m, and the friction coefficient ⁇ of the contact area between the steel member and the test jig is 0.05.
  • the metal structure (internal structure) at the 1/4 depth position of the steel member according to the present embodiment is not limited, it is preferably a structure containing martensite at a volume fraction of 80% or more.
  • the Vickers hardness at the 1/4 depth position is 310 or more (equivalent to a tensile strength of 1.0 GPa or more). be able to.
  • the volume fraction is 90% or more martensite. More preferably, it is 95% or more. 100% martensite may be used. When the volume fraction of martensite is small, it becomes difficult to obtain a tensile strength of 1.0 GPa or more.
  • the metal structure at the 1/4 depth position of the steel member may contain residual austenite, bainite, ferrite, and/or pearlite as the remainder other than martensite.
  • Martensite includes not only so-called fresh martensite but also tempered martensite and automatically tempered martensite.
  • Automatically tempered martensite is tempered martensite that is generated during cooling during quenching without any heat treatment for tempering. It is produced by tempering.
  • the microstructure fraction in the metal structure at a 1/4 depth position of a steel member can be measured by the following method.
  • the area fraction of martensite (including fresh martensite, tempered martensite, and automatically tempered martensite) is measured using a transmission electron microscope (TEM) and an electron diffraction device attached to the TEM.
  • TEM transmission electron microscope
  • a measurement sample was cut out from the width direction end of the steel member at a position 1/4 of the width of the steel member (1/4 width position) and at a 1/4 depth position of the steel member, and was subjected to TEM analysis.
  • a range of 400 ⁇ m 2 or more including the 1/4 width position and the 1/4 depth position of this thin film sample is observed with a TEM.
  • the electron beam diffraction pattern of the thin film sample distinguishes between martensite and bainite, which have body-centered cubic lattices, and retained austenite, which has face-centered cubic lattice.
  • iron carbides (Fe 3 C) in martensite and bainite are found by diffraction patterns, and the structure fractions of martensite and bainite are measured by observing their precipitation forms. Specifically, if the precipitation form is three-directional precipitation, it is determined to be martensite (tempered martensite), and if the precipitation is limited to one direction, it is determined to be bainite. If no iron carbide precipitation is observed, it is also determined to be martensite (fresh martensite).
  • the structure fractions of martensite and bainite measured by TEM are measured as area percentages, but since the steel member according to this embodiment has an isotropic metal structure, the area fraction values are directly calculated as volume fractions. It can be replaced by a rate. Although carbides are observed to distinguish between martensite and bainite, in this embodiment, carbides are not included in the volume fraction of the structure. If ferrite or pearlite is present as a residual structure, it can be easily confirmed using an optical microscope or a scanning electron microscope. Specifically, a measurement sample including a position of 1/4 width of the steel member and a position of 1/4 depth of the steel member is cut out and used as a sample for observation. The cut samples are mechanically polished and then polished to a mirror finish.
  • the sample is etched with a nital etching solution to reveal ferrite and pearlite, and the presence of ferrite or pearlite is confirmed by observing an area of 40,000 ⁇ m 2 or more using a scanning electron microscope.
  • Pearlite is a structure in which ferrite and cementite are arranged in alternating layers, and it is distinguished from bainite, in which cementite is precipitated in granular form.
  • the steel member according to this embodiment may include an alloy layer on at least a portion of the surface (or the entire surface may be sufficient).
  • the alloy layer may be a Fe--Al alloy layer or a Fe--Zn alloy layer.
  • the Fe-Al alloy layer is an alloy layer containing 70% by mass or more of Fe and Al in total
  • the Fe-Zn alloy layer is an alloy layer containing 70% by mass or more of Fe and Zn in total. .
  • the thickness of the alloy layer is preferably 5 to 100 ⁇ m.
  • the reference surface for each depth position is the surface of the portion (base steel material) excluding the alloy layer.
  • the thickness of the alloy layer can be determined by observing a cross section in the thickness direction using a scanning electron microscope. Specifically, 1/2 part in the longitudinal direction of the steel member (1/2 position in the longitudinal direction from the end in the longitudinal direction) and 1/4 part in the width (1/2 part in the width direction from the end in the width direction) Cut out the measurement sample from the /4 position) and observe it.
  • the observation range using a microscope is, for example, a magnification of 400 times and an area of 40,000 ⁇ m 2 or more.
  • the cut samples are mechanically polished and then polished to a mirror finish.
  • the thickness of the alloy layer in ten arbitrary fields of view is measured, and the average value is taken as the thickness of the alloy layer.
  • the thickness of the alloy layer can be measured by measuring the thickness from the outermost surface to the position where the contrast changes. Measurements were made at 20 equally spaced locations within the observation photograph, with the distance between the measurement locations being 6.5 ⁇ m. In addition, during measurement, five visual fields were observed in the manner described above, and the average value was used to determine the thickness of the alloy layer.
  • spot elemental analysis (beam diameter of 1 ⁇ m or less) was performed using an electron probe microanalyzer (EPMA) in the same observation range as above. Contents of Al, Zn, etc. can be determined. A total of 10 points are analyzed in the alloy layer in 10 arbitrary views, and the average value is taken as the content of Fe, Al, and Zn contained in the alloy layer. Even when elements other than Fe, Al, and Zn are included, the same method is used.
  • 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 molded body formed from a steel plate into a predetermined shape. A hot-formed steel member is often a molded body, but in this embodiment, the term “steel member” includes both a molded body and a flat plate. Further, the steel member may be a tailored property material whose strength differs depending on its location. In this case, it is preferable that at least a part of the steel member has a tensile strength of 1.0 GPa (1000 MPa) or more.
  • the tailored property material may be made by joining steel plates with different chemical compositions, strengths, and thicknesses, or may be made by subjecting a portion of the steel plate to heat treatment.
  • the steel plate according to the present embodiment that is the material of the steel member according to the present embodiment will be explained.
  • a steel member can be obtained by subjecting the steel plate described below to the heat treatment described below.
  • the steel plate according to this embodiment includes a base steel plate and an iron scale formed on the surface of the base steel plate.
  • the steel plate according to this embodiment may have a plating layer on the iron scale (the surface of the iron scale).
  • (B1) Chemical composition of steel plate The range of chemical composition of the base steel plate included in the steel plate according to this embodiment is the same as the chemical composition of the steel member according to this embodiment described above, and the reason for its limitation is also the same.
  • the chemical composition of the base steel plate can be determined, for example, at a representative position (1/4 depth position) from the surface of the base steel plate in the thickness direction. It is obtained by performing elemental analysis using a method.
  • the scale included in the steel plate according to this embodiment has a density of 90 cracks/mm 2 or more.
  • the steel plate according to the present embodiment decarburizes the base steel plate by utilizing scale O formed on the surface of the base steel plate by hot rolling or the like, as will be described later.
  • decarburizing is carried out by annealing in a coil shape (box annealing), the sheets are closely and tightly sealed in the inner part of the coil, so the decarburization reaction, which is the release of CO, does not proceed and the coil is heated in the longitudinal and width directions. Uniform decarburization cannot be achieved.
  • the scale crack density is controlled in order to obtain the above effects.
  • the scale crack density can be determined by SEM observation of the surface. Avoiding the edges of the steel plate (for example, within 50 mm from the edges), collect a sample approximately 15 mm square. At this time, be careful not to peel off the scale, and if necessary, cut out pieces larger than 15 mm square.
  • the surface on which the scale is formed is observed by SEM, a BSE image is obtained, and the crack density is counted using a cutting method. SEM observation is performed at a magnification of 500 times, avoiding the edges of the sample (for example, within 2 mm from the edges) so that the field of view is 50,000 ⁇ m or more per field of view.
  • a BSE image or COMPO image
  • the obtained BSE image (or COMPO image) is cut into 10 pieces vertically and 10 pieces horizontally, and the number of cracks that overlap the cutting line is counted and converted into a number density. Ten arbitrary visual fields are observed, and the average value of the crack density in each visual field is defined as the crack density of the scale.
  • the steel plate according to this embodiment may be provided with a plating layer (coating) on a part of the surface (or the entire surface).
  • the plating layer may be an Al-based plating layer mainly containing Al, or a Zn-based plating layer mainly containing Zn.
  • the Al-based plating layer is a plating layer containing 70% by mass or more of Al
  • the Zn-based plating layer is a plating layer containing 70% by mass or more of Zn.
  • the shape of the steel plate according to this embodiment is not particularly limited. That is, the steel plate may be a flat plate, or may be a tailored property material made by joining steel plates with different strengths and plate thicknesses.
  • the steel plate steel member according to this embodiment can obtain the effect regardless of the manufacturing method as long as it has the above characteristics. This is preferred because it can be produced stably.
  • Winding Step (IV) Light Reduction Step of Lightly Reducing the Hot-Rolled Steel Sheet After the Winding Step
  • each step will be explained. Steps and conditions not described below can be performed by appropriately known methods.
  • a steel billet such as a slab, having a preferable chemical composition of the steel plate according to the present embodiment described above is manufactured.
  • Molten steel adjusted to a predetermined chemical composition under known conditions may be made into steel slabs by continuous casting or the like.
  • Hot rolling process In the hot rolling process, the obtained steel piece is heated and hot rolled to form a hot rolled steel plate. In the hot rolling process, iron scale (hot rolling scale) is formed on the surface of the steel plate.
  • the hot rolling conditions are not particularly limited, and may be appropriately set within a known range of conditions depending on the required characteristics of the steel sheet.
  • (IV) Light Reduction Step the hot rolled steel sheet on which hot rolling scale has been formed is subjected to light reduction by unwinding the coil.
  • the scale cracks described above can be introduced.
  • the upper limit of the rolling reduction rate is not particularly limited, rolling down by 10.0% or more is not preferable because scale may peel off.
  • the reduction ratio is preferably less than 5.0%, or less than 2.0%, or less than 1.0%.
  • shot blasting may be performed on the hot rolled steel sheet together with or in place of light reduction.
  • a steel plate which has a base steel plate having a predetermined chemical composition and iron scale formed on the surface of the base steel plate, and in which the iron scale includes cracks at a predetermined density.
  • the purpose is generally to facilitate pickling. That is, lightly rolling a hot-rolled sheet whose next step is not a pickling step is not normally carried out because it increases the number of steps.
  • a steel plate to be subjected to hot-rolled plate annealing is subjected to light reduction without scale removal (in a so-called black scale state). Moreover, this provides effects that were not previously anticipated.
  • (D) Method for manufacturing steel member The method for manufacturing the steel member according to the present embodiment is not limited, but the steel member described above can be manufactured by using, for example, a manufacturing method including the steps shown below.
  • V A hot rolled sheet annealing step of annealing the hot rolled steel sheet after the light reduction step
  • (VI) A pickling step of pickling the hot rolled steel sheet after the hot rolled sheet annealing step if necessary
  • VII If necessary, a cold rolling step of cold rolling the hot rolled steel sheet after the pickling step to obtain a cold rolled steel sheet
  • VIII If necessary, rolling the hot rolled steel sheet after the cold rolling step.
  • annealing step (IX) of annealing the rolled steel sheet optionally a plating step (X) of plating the cold rolled steel sheet after the annealing step; or (X) the hot rolled steel sheet after the hot rolled sheet annealing step;
  • the steel plate according to the present embodiment (a steel plate on which hot-rolled scale with cracks introduced on the surface is formed) is not subjected to scale removal (so-called black Box annealing (BAF) is performed in the coiled state.
  • BAF black Box annealing
  • the annealing atmosphere is an inert gas atmosphere (N 2 atmosphere, H 2 atmosphere, etc.), and annealing is performed at 650 to 950° C. for 4 to 30 hours.
  • decarburization is performed by annealing a hot-rolled steel sheet with hot-rolled scale, using O in the scale as a decarburization source.
  • decarburization occurs when C in the outermost layer of the base steel plate reacts with O in the scale to become CO gas. Further, the insufficient C is subsequently supplied from the inside of the base steel plate to the outermost layer, and the C becomes CO gas, thereby further progressing the decarburization reaction.
  • the annealing temperature is less than 650°C or the annealing time is less than 4 hours, decarburization will not proceed sufficiently.
  • the annealing temperature exceeds 950° C. or the annealing time exceeds 30 hours, the scale reduction reaction is completed and C continues to be supplied from the inside of the steel sheet to the surface layer, resulting in shallow decarburization.
  • the hot-rolled steel plate to be subjected to the hot-rolled plate annealing process preferably has a thickness of 9 mm or less, a plate width of 2100 mm or less, an outer diameter of 2000 mm or less when made into a coil, and a weight of one coil of 30 tons or less.
  • (VI) Pickling process In the pickling process, the hot rolled steel sheet after the hot rolled sheet annealing process is pickled.
  • the pickling solution known hydrochloric acid or sulfuric acid may be used, but if scale is not removed sufficiently during pickling, shot blasting may be performed before pickling to mechanically promote scale removal. For example, a shot particle size of #60 may be used. The pickling step may be omitted.
  • the cold rolled steel sheet after the pickling process is cold rolled.
  • the rolling reduction is not particularly limited, from the viewpoint of ensuring good flatness, the cumulative rolling reduction in cold rolling is preferably 30% or more.
  • the cumulative reduction rate in cold rolling is preferably 80% or less. The cold rolling step may be omitted.
  • the annealing atmosphere is a moist hydrogen atmosphere with a high dew point due to humidification, and annealing is performed in a temperature range of 700 to 950°C.
  • a temperature range of 700 to 950°C For example, in a two-stage heating furnace with a direct flame burner and a radiant tube, it is heated to 560 to 650°C at an air-fuel ratio of 0.9 to 1.2, an oxygen potential of -1.5 or more, a hydrogen concentration of 1 to 20% by mass, Heat to 700 to 950°C in an atmosphere with a dew point of -10°C to +30°C.
  • the temperature is maintained at 700° C. or higher for 30 seconds or more in a high dew point atmosphere containing hydrogen.
  • the main annealing step may be omitted or may be performed under conditions other than the above.
  • the coating method is not particularly limited, and may include hot-dip plating, electroplating, vacuum evaporation, cladding, thermal spraying, and the like.
  • the most widely used method industrially is hot-dip plating.
  • the coating include Al-based plating containing Al and Zn-based plating containing Zn. The plating step may be omitted.
  • the plating bath When an Al-based plating layer is formed by hot-dip plating, the plating bath often contains Fe as an impurity in addition to Al. In addition, as long as it contains 70% by mass or more of Al, the plating bath may further contain Si, Ni, Mg, Ti, Zn, Sb, Sn, Cu, Co, In, Bi, Ca, misch metal, etc. in addition to the above-mentioned elements. You may let them.
  • the steel plate after the annealing process may be cooled to room temperature and then heated again to perform plating, or after the annealing process is held, the steel plate is cooled to 650 to 750°C near the plating bath temperature (i.e. once heated). Hot-dip plating may be performed without cooling to room temperature).
  • Pre-treatment and post-treatment of plating are not particularly limited, and pre-coating, solvent coating, alloying treatment, temper rolling, etc. are possible.
  • As an alloying treatment it is also possible to maintain the temperature at, for example, 450 to 800°C.
  • temper rolling is useful for shape adjustment, etc., and it is possible to reduce the rolling by, for example, 0.1 to 0.5%.
  • (X) Heat Treatment Step a steel plate having a predetermined chemical composition that has gone through the above steps is heat treated to produce a steel member.
  • a steel plate obtained by the method described below is heated at an average temperature increase rate of 1.0 to 1000 °C/sec to a temperature of Ac3 point to (Ac3 point + 300) °C, and then the upper critical cooling rate is reduced to below the Ms point. This is performed under the conditions of cooling at the above average cooling rate. It is not preferable that the temperature increase rate is less than 1.0° C./sec because the productivity of heat treatment decreases.
  • the temperature increase rate exceeds 1000° C./sec, a mixed grain structure will be formed and the bendability will decrease, which is not preferable. Further, if the heat treatment temperature is less than the Ac3 point (° C.), ferrite remains after cooling, which is not preferable because the strength is insufficient. On the other hand, if the heat treatment temperature exceeds Ac3 point + 300°C, the structure becomes coarse grained and the hydrogen embrittlement resistance decreases, which is not preferable.
  • the upper critical cooling rate is the minimum cooling rate that does not precipitate ferrite or pearlite in the structure and supercools austenite to produce martensite.If cooled below the upper critical cooling rate, ferrite and pearlite will form. However, the strength is insufficient.
  • the heating temperature may be maintained within ⁇ 10° C. for 1 to 300 seconds. Further, after cooling to a temperature below the Ms point, a tempering treatment may be performed in a temperature range of about 100 to 600° C. in order to adjust the strength of the steel member.
  • the Ac3 point, Ms point, and upper critical cooling rate are measured by the following method.
  • a strip-shaped test piece with a width of 30 mm and a length of 200 mm was cut out from the steel plate according to this embodiment, and this test piece was heated in a nitrogen atmosphere to 1000°C at a heating rate of 10°C/sec, and kept at that temperature for 5 minutes. After holding, it is cooled to room temperature at various cooling rates.
  • the cooling rate is set from 1°C/second to 100°C/second at intervals of 10°C/second (however, the rate is 10°C/second after 1°C/second).
  • the Ac3 point and Ms point are determined by measuring the change in thermal expansion of the test piece during heating and cooling.
  • the minimum cooling rate at which precipitation of the ferrite phase did not occur is defined as the upper critical cooling rate.
  • the Ms point obtained from the change in thermal expansion when cooling at a rate equal to or higher than the upper critical cooling rate is defined as the Ms point of the steel member.
  • Hot forming such as stamping may also be performed.
  • hot forming include bending, drawing, stretch forming, hole expansion forming, and flange forming.
  • the present invention may be applied to forming methods other than press forming, such as roll forming, as long as a means for cooling the steel plate at the same time as forming or immediately after forming is provided. Repeated hot forming may be performed if the thermal history described above is followed. Further, the above series of heat treatments may be repeated multiple times.
  • the steel member according to the present embodiment includes both a steel plate that is hot-formed into a molded body and a flat plate that has been subjected to only heat treatment.
  • hot forming or heat treatment may be performed on a portion of a steel plate serving as a raw material to obtain a steel member having regions with different strengths.
  • the series of heat treatments described above can be performed by any method; for example, the heating may be performed by high frequency heating, electrical heating, infrared heating, or furnace heating. Cooling may also be performed by water cooling, mold cooling, or the like. In addition to the atmosphere, city gas or nitrogen gas may be used as the atmosphere inside the heating furnace. Further, in order to suppress hydrogen generation during heat treatment, the dew point in the heating furnace may be controlled.
  • the obtained slab was hot-rolled to form a hot-rolled steel plate with a thickness of 3.2 mm and a width of 1000 mm, and then wound at a temperature of 800°C or less to form a hot-rolled coil with an outer diameter of 1700 mm and a weight of 14 tons. .
  • the obtained hot-rolled coils were subjected to light rolling at the rolling reduction ratios listed in Table 2.
  • a steel plate (blank) of a predetermined size was cut out from the obtained lightly rolled coil, and SEM observation was performed in the manner described above to evaluate the scale crack density.
  • the chemical composition at the 1/4 depth position of the steel plate was measured and found to be similar to the chemical composition of the slab.
  • steel plates B1 to B21 satisfying the scope of the present invention had a base steel plate with a predetermined chemical composition and a scale with a predetermined crack density.
  • the steel plates shown in Table 2 above were subjected to hot-rolled plate annealing under the conditions shown in Table 3A.
  • some of the steel plates (C2 to C30, C2 to C16) after hot-rolled plate annealing were pickled and cold-rolled to obtain cold-rolled steel plates with a thickness of 1.6 mm.
  • some of the cold-rolled steel sheets (C4 to C22, C24 to C30, and C5 to C16) were subjected to hot-dip Al plating or hot-dip Zn plating.
  • Hot-rolled steel sheets after annealing, or cold-rolled steel sheets that have been pickled, cold-rolled, and annealed, or pickled, cold-rolled, and annealed and plated during annealing were heat treated under the conditions shown in Table 3A to obtain steel members.
  • the surface In a steel member in which a plated steel plate is heat-treated, the surface has an Fe-Al alloy layer containing a total of 70% by mass or more of Fe and Al, or an Fe-Zn alloy layer containing a total of 70% by mass or more of Fe and Zn.
  • a system alloy layer was formed with a thickness of 5 to 100 ⁇ m.
  • the obtained steel member was cut out and subjected to GDS (glow discharge optical spectroscopy) in the manner described above to determine the B concentration ratio and decarburization depth.
  • GDS low discharge optical spectroscopy
  • the martensite volume fraction at the 1/4 depth position was measured. The results are shown in Table 3B.
  • ⁇ Vickers hardness> A sample for cross-sectional observation was taken from a position 1/4 of the width (short side) from the width direction end of the steel member, and was measured at a 1/4 depth position and a 0.1 mm depth position in accordance with JIS Z2244-1:2020. Vickers hardness was measured for each. The test force was 100 gf. Measurements were taken five times at each depth, and the average value was taken as the hardness at that position.
  • ⁇ Tensile strength> The tensile test was conducted in accordance with the provisions of ASTM Standard E8. After grinding the soaking part of the steel member to a thickness of 1.2 mm, a half-size plate-shaped test piece of ASTM standard E8 (parallel part length: 32 mm, parallel part plate width: 6.25 mm) was collected. Then, a strain gauge (gauge length: 5 mm, e.g. FLAB-5 manufactured by Tokyo Sokki Kenkyusho Co., Ltd.) was attached to the center of the width and length of the parallel part of the specimen, and a room temperature tensile test was performed at a strain rate of 3 mm/min. , tensile strength (maximum strength) was measured. In this example, a case having a tensile strength of 1000 MPa or more was evaluated as having high strength.
  • ⁇ Bendability> A bending test piece measuring 60 mm parallel to the rolling direction and 30 mm perpendicular to the rolling direction was taken from the soaking area of the steel member, and a bending test was performed on this test piece in accordance with the regulations of DA238-100. During the test, the bending punch was aligned perpendicular to the rolling direction, and the bending angle at the maximum load was measured. Since the bending angle has a correlation with strength, in this example, the bending angle is 70 degrees when the tensile strength is less than 1500 MPa, 55 degrees when the tensile strength is 1500 MPa or more and less than 2100 MPa, and 45 degrees when the tensile strength is 2100 MPa or more. A case in which the bending angle exceeds 100 degrees was evaluated as having better bendability than the conventional technology.
  • Hydrogen embrittlement resistance was evaluated by performing a 4-point bending test on a test piece and determining the amount of hydrogen Hc that can be stored in the test piece to the limit where no cracking occurs. Specifically, as a test piece, a strip-shaped test piece with a width of 8 mm and a length of 68 mm was cut out, avoiding the edge of the steel member to be evaluated. Then, after pasting a strain gauge similar to that used in the tensile test (gauge length: 5 mm, e.g.
  • test piece was bent along the longitudinal direction using a four-point support jig to a strain equivalent to 3/5 of the tensile strength measured in the above-described tensile test for steel members.
  • the jig used had an interval of 10 mm between inner pins (two inner stress points) and a interval of 60 mm between outer pins (two outer fulcrums).
  • a plurality of test pieces having different amounts of hydrogen occluded (hydrogen storage amount) were observed for the presence or absence of cracking, and the maximum (limit) amount of hydrogen Hc (mass ppm) at which no cracking occurs was determined.
  • the hydrogen amount Hc is 0.7 mass ppm or more for steel members whose tensile strength is less than 1500 to 2000 MPa, and 0.5 mass ppm or more for steel members whose tensile strength is 2000 to less than 2500 MPa.
  • the hydrogen embrittlement resistance was evaluated to be excellent if the resistance was 0.3 mass ppm or more.
  • the maximum principal plastic strain at the center of the outer bending surface layer was determined using the finite element method.
  • invention examples C1 to C30 that satisfied the scope of the present invention had good results in both structure and properties, and had high strength and excellent bendability and hydrogen embrittlement resistance.
  • Comparative Examples c1 to c16 which did not satisfy the scope of the present invention, had insufficient chemical composition and structure formation, and were inferior in at least one of strength, bendability, and hydrogen embrittlement resistance.
  • the steel member according to the present invention is particularly suitable for use as a frame part of an automobile.

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Abstract

Élément en acier présentant une composition chimique prédéterminée. Dans l'élément en acier, lorsqu'une position à une profondeur de 1/4 de l'épaisseur dans le sens de l'épaisseur à partir de la surface est définie comme étant une position de profondeur 1/4, la teneur maximale en B dans une plage allant de la surface à une profondeur de 10 µm dans le sens de l'épaisseur est de 5 fois ou plus la teneur en B à la position de profondeur 1/4. Lorsque la teneur en C est mesurée à partir de la surface dans le sens de l'épaisseur à l'aide d'un GDS, et lorsque la distance de la surface à une position où la teneur en C devient d'abord égale à la teneur en C à la position de profondeur 1/4 est définie comme la profondeur de décarbonisation, la profondeur de décarbonisation est de 20 µm ou plus. La dureté Vickers à la position de profondeur 1/4 est comprise entre 310 et 890.
PCT/JP2023/013175 2022-03-30 2023-03-30 Élément en acier et feuille d'acier Ceased WO2023190867A1 (fr)

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MX2024011522A MX2024011522A (es) 2022-03-30 2023-03-30 Miembro de acero y lamina de acero.
EP23780875.3A EP4502193A4 (fr) 2022-03-30 2023-03-30 Élément en acier et feuille d'acier
JP2024512794A JPWO2023190867A1 (fr) 2022-03-30 2023-03-30
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WO2025249571A1 (fr) * 2024-05-30 2025-12-04 日本製鉄株式会社 Élément en acier et tôle d'acier
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KR20240155913A (ko) 2024-10-29
JPWO2023190867A1 (fr) 2023-10-05
CN118922573A (zh) 2024-11-08
MX2024011522A (es) 2024-09-25
EP4502193A4 (fr) 2025-07-30

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