WO2024029145A1 - 鋼板 - Google Patents

鋼板 Download PDF

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Publication number
WO2024029145A1
WO2024029145A1 PCT/JP2023/016439 JP2023016439W WO2024029145A1 WO 2024029145 A1 WO2024029145 A1 WO 2024029145A1 JP 2023016439 W JP2023016439 W JP 2023016439W WO 2024029145 A1 WO2024029145 A1 WO 2024029145A1
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content
steel plate
steel
hydrogen embrittlement
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PCT/JP2023/016439
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English (en)
French (fr)
Japanese (ja)
Inventor
光陽 大賀
健悟 竹田
諭 弘中
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Nippon Steel Corp
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Nippon Steel Corp
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Priority to CN202380054953.3A priority Critical patent/CN119585458A/zh
Priority to KR1020257002988A priority patent/KR20250028466A/ko
Priority to JP2024538824A priority patent/JPWO2024029145A1/ja
Publication of WO2024029145A1 publication Critical patent/WO2024029145A1/ja
Priority to MX2025001221A priority patent/MX2025001221A/es
Anticipated expiration legal-status Critical
Ceased legal-status Critical Current

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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
    • 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
    • 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
    • C22C38/005—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
    • C22C38/02—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
    • C22C38/18—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
    • 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
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • 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
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • 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
    • C22C38/48—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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • 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

Definitions

  • the present invention relates to a steel plate.
  • Hydrogen embrittlement cracking is a phenomenon in which a steel member under high stress during use suddenly breaks due to hydrogen penetrating into the steel from the environment.
  • Patent Document 1 in mass %, C: 0.08 to 0.35%, Si: 0.01 to 3.0%, Mn: 2.0 to 4.0%, P: Contains 0.010% or less (not including 0), S: 0.002% or less (not including 0), Al: 0.01 to 1.50%, B: 0.0005 to 0.010%. , and a steel plate containing one or more selected from Mo: 0.03 to 2.0% and Ti: 0.010 to 0.10%, with the balance consisting of Fe and inevitable impurities; In the range of 300 to 400 ⁇ m from the surface layer in the sheet thickness direction, the total area ratio of martensite and bainite containing carbides is 60 to 100%, and the average grain size of prior austenite is 15 ⁇ m or less.
  • the ratio of the peak height of the Auger electron spectrum of P at a position 5 nm or more away from the prior austenite grain boundary to the peak height of the Auger electron spectrum of P at the prior austenite grain boundary is 0.
  • a high-strength hot-dip galvanized steel sheet having a steel structure of 20 or more and having a hot-dip galvanized layer on the surface of the steel sheet is described.
  • Patent Document 1 teaches that, according to the above structure, it is possible to suppress P segregation to prior austenite grain boundaries and obtain a high-strength galvanized steel sheet with excellent delayed fracture resistance.
  • Patent Document 2 in mass %, C: 0.12 to 0.35%, Si: 0.01 to 3.0%, Mn: 2.0 to 4.0%, P: 0.100% or less ( (does not contain 0), S: 0.02% or less (does not include 0), Al: 0.01 to 1.50%, and the remainder is Fe and unavoidable impurities.
  • the total amount of ferrite and upper bainite is 0 to 15%, the total amount of lower bainite and martensite is 80 to 100%, and the retained austenite is 0 to 10%.
  • a high-strength hot-dip galvanized steel sheet having a hot-dip galvanized layer on the surface of the steel sheet has a ratio of an average C amount of 0.20 to 0.80 at a position 5 ⁇ m from the surface layer in the thickness direction.
  • Patent Document 2 by setting the ratio of the average C amount at a position 5 ⁇ m from the steel sheet surface layer to the average C amount at a position 70 ⁇ m from the steel sheet surface layer in the sheet thickness direction to 0.20 to 0.80. , it is taught that stress near the surface layer is relaxed and excellent delayed fracture resistance can be obtained.
  • Patent Document 3 in mass%, C: 0.15 to 0.30%, Si: 0.01 to 1.8%, Mn: 1.5 to 3.0%, P: 0.05% or less, Contains S: 0.005% or less, Al: 0.005 to 0.05%, N: 0.005% or less, and the remainder consists of Fe and inevitable impurities, and has the following formula (1) Hv (S) / Hv(C) ⁇ 0.8 (Hv(S): hardness of the surface soft part of the steel plate, Hv(C): hardness of the center of the steel plate) and formula (2) 0.10 ⁇ t(S)/t ⁇ 0.
  • Patent Document 4 in weight percent, C: 0.05 to 0.20%, P: 0.001 to 0.030%, S: 0.001 to 0.050%, Al: 0.001 to 0. 100%, N: contained in the range of 0.0002 to 0.0050%, Si: 0.10 to 2.50%, Mn: 0.5 to 3.50%, Cr: 0.10 to 1.5 %, Mo: 0.10 to 1.5%, B: 0.001 to 0.005%, and the remainder: Fe and unavoidable impurities.
  • a high-strength cold-rolled steel sheet is described in which the surface layer portion of 0.010 to 0.20 mm per side from the front and back surfaces of the steel sheet is mainly composed of ferrite, and the inner layer portion is mainly composed of bainite and martensite.
  • the surface layer of the steel sheet has a layer mainly composed of ferrite, and the inner layer has a layer mainly composed of martensite/bainite, so that it has good workability and delayed fracture resistance. It is taught that superior steel sheets can be provided.
  • an object of the present invention is to provide a steel plate with a novel configuration that has high strength and excellent hydrogen embrittlement resistance in the bent portion.
  • the present inventors conducted a Charpy impact test by reducing grain boundary segregation of elements that cause grain boundary embrittlement in high-strength steel sheets having a tensile strength of 1500 MPa or more. controlling the proportion of macroscopically embrittled fracture surfaces within a predetermined range; and softening the surface layer of the steel sheet to increase the upper limit of the amount of hydrogen that does not cause hydrogen embrittlement in the bending section, that is, the limit amount of hydrogen.
  • the present invention was completed based on the discovery that by performing the above steps at the same time, the hydrogen embrittlement resistance of the bent portion can be significantly improved even in a steel plate having a very high tensile strength of 1500 MPa or more.
  • the present invention that achieves the above object is as follows. (1) In mass%, C: 0.15-0.40%, Si: 0.01-2.00%, Mn: 0.80 to 3.50%, P: 0.0001-0.0200%, S: 0.0001-0.0200%, Al: 0.001-1.000%, N: 0.0001-0.020%, O: 0.0001 to 0.0200%, Co: 0 to 0.5000%, Ni: 0-1.000%, Mo: 0-1.000%, Cr: 0-2.000%, Ti: 0-0.500%, B: 0 to 0.0100%, Nb: 0 to 0.500%, V: 0 to 0.500%, Cu: 0-0.500%, W: 0-0.100%, Ta: 0-0.100%, Sn: 0-0.100%, Sb: 0 to 0.100%, As: 0 to 0.100%, Mg: 0 to 0.0500%, Ca: 0-0.050%, Y: 0 to 0.050%, Zr: 0 to 0.050%, La: 0
  • the chemical composition is in mass%; Co: 0.0100-0.5000%, Ni: 0.010-1.000%, Mo: 0.010-1.000%, Cr: 0.001-2.000%, Ti: 0.001 to 0.500%, B: 0.0001 to 0.0100%, Nb: 0.001-0.500%, V: 0.001-0.500%, Cu: 0.001 to 0.500%, W: 0.001-0.100%, Ta: 0.001 to 0.100%, Sn: 0.001 to 0.100%, Sb: 0.001 to 0.100%, As: 0.001 to 0.100%, Mg: 0.0001-0.0500%, Ca: 0.001-0.050%, Y: 0.001-0.050%, Zr: 0.001 to 0.050%, La: 0.001 to 0.050%, and Ce: 0.001 to 0.050%.
  • the steel plate according to (1) above characterized by containing one or more selected from the group consisting of: (3)
  • the microstructure at the 1/2 plate thickness position is expressed as an area ratio, The steel sheet
  • the steel plate according to the embodiment of the present invention has, in mass%, C: 0.15-0.40%, Si: 0.01-2.00%, Mn: 0.80 to 3.50%, P: 0.0001-0.0200%, S: 0.0001-0.0200%, Al: 0.001-1.000%, N: 0.0001-0.020%, O: 0.0001 to 0.0200%, Co: 0 to 0.5000%, Ni: 0-1.000%, Mo: 0-1.000%, Cr: 0-2.000%, Ti: 0 to 0.500%, B: 0 to 0.0100%, Nb: 0 to 0.500%, V: 0 to 0.500%, Cu: 0-0.500%, W: 0-0.100%, Ta: 0-0.100%, Sn: 0-0.100%, Sb: 0 to 0.100%, As: 0 to 0.100%, Mg: 0 to 0.0500%, Ca: 0-0.050%, Y: 0 to 0.050%, Zr: 0 to 0.050%, La: 0
  • the present inventors conducted a study to improve the hydrogen embrittlement resistance, especially in the bent portion, in a steel plate having extremely high tensile strength of 1500 MPa or more.
  • the present inventors succeeded in suppressing the embrittlement of prior austenite grain boundaries, which are the starting point of hydrogen embrittlement cracking in the microstructure, and also determined that the upper limit of the amount of hydrogen that does not cause hydrogen embrittlement in the bending part, that is, the limit hydrogen
  • the present inventors reduced the grain boundary segregation of elements such as P, which embrittle grain boundaries, and reduced the proportion of macroscopically embrittled fracture surfaces in the Charpy impact test at room temperature to 35.0%.
  • the surface layer is softened, and more specifically, the thickness of the steel sheet is increased from at least one surface of the steel sheet.
  • the present invention it is extremely important to simultaneously control the Vickers hardness to 0.95 times or less than the Vickers hardness Hc. According to the present invention, by a specific combination of these technical matters, even though the steel plate has a very high tensile strength of 1500 MPa or more, the hydrogen embrittlement resistance of the bent part can be reliably and significantly improved. It becomes possible to improve the performance. Therefore, according to the present invention, even in applications such as automobiles that require conflicting properties such as excellent bending workability and high strength, it is possible to achieve both of these properties while also achieving excellent hydrogen embrittlement resistance in the bending part. It becomes possible to achieve the following characteristics.
  • % which is the unit of content of each element, means “% by mass” unless otherwise specified.
  • ⁇ indicating a numerical range is used to include the numerical values written before and after it as a lower limit value and an upper limit value, unless otherwise specified.
  • C is an effective element for increasing tensile strength at low cost.
  • the C content is set to 0.15% or more.
  • the C content may be 0.16% or more, 0.18% or more, 0.20% or more, more than 0.20%, 0.21% or more, 0.22% or more, or 0.23% or more.
  • the C content is set to 0.40% or less.
  • the C content may be 0.38% or less, 0.35% or less, 0.30% or less, or 0.25% or less.
  • Si is an element that acts as a deoxidizer and affects the morphology of carbides and retained austenite after heat treatment. If Si is not contained, it may be difficult to suppress the generation of coarse oxides. These coarse oxides act as starting points for cracks, and as these cracks propagate within the steel material, the hydrogen embrittlement resistance may deteriorate. Therefore, the Si content is set to 0.01% or more. The Si content may be 0.05% or more, 0.10% or more, 0.30% or more, or 0.50% or more. On the other hand, if Si is contained excessively, local ductility may decrease and hydrogen embrittlement resistance may deteriorate. Therefore, the Si content is set to 2.00% or less. The Si content may be 1.80% or less, 1.60% or less, 1.40% or less, or 1.20% or less.
  • Mn is an element effective in improving the hardenability of steel and increasing the strength of the steel plate.
  • the Mn content is set to 0.80% or more.
  • the Mn content may be 1.00% or more, 1.20% or more, 1.40% or more, or 1.60% or more.
  • Mn when Mn is contained excessively, Mn not only promotes co-segregation with P and S, but also may deteriorate corrosion resistance and hydrogen embrittlement resistance. Therefore, the Mn content is set to 3.50% or less.
  • Mn content is 3.40% or less, 3.30% or less, 3.20% or less, 3.10% or less, 3.00% or less, 2.90% or less, 2.80% or less, 2.60% It may be less than 2.55%, 2.50% or less, less than 2.50%, 2.45% or less, 2.40% or less, 2.20% or less, or 2.00% or less.
  • P is an element that segregates at prior austenite grain boundaries and promotes embrittlement of the grain boundaries. If too much P is contained, the hydrogen embrittlement resistance will be significantly reduced due to grain boundary embrittlement. Therefore, the P content is set to 0.0200% or less.
  • the P content may be 0.0180% or less, 0.0150% or less, 0.0120% or less, or 0.0100% or less. From the viewpoint of improving hydrogen embrittlement resistance, the P content is preferably as low as possible. However, reducing the P content to less than 0.0001% requires a lot of time for refining, leading to a significant increase in cost. Therefore, the P content is set to 0.0001% or more.
  • the P content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more.
  • S is an element that generates nonmetallic inclusions such as MnS in steel. If S is contained excessively, nonmetallic inclusions that become crack starting points during cold working will be significantly formed. In this case, cracks may occur from the nonmetallic inclusions and propagate within the steel material, resulting in deterioration of the hydrogen embrittlement resistance. Therefore, the S content is set to 0.0200% or less. The S content may be 0.0180% or less, 0.0150% or less, 0.0120% or less, or 0.0100% or less. From the viewpoint of improving hydrogen embrittlement resistance, the smaller the S content, the more preferable.
  • the S content is set to 0.0001% or more.
  • the S content may be 0.0005% or more, 0.0010% or more, or 0.0020% or more.
  • Al is an element that acts as a deoxidizer for steel and stabilizes ferrite.
  • the Al content is set to 0.001% or more.
  • the Al content may be 0.005% or more, 0.010% or more, 0.020% or more, or 0.050% or more.
  • the Al content is set to 1.000% or less.
  • the Al content may be 0.950% or less, 0.900% or less, 0.800% or less, or 0.600% or less.
  • N is an element that forms coarse nitrides in the steel sheet and reduces the hydrogen embrittlement resistance of the steel sheet. Further, N is an element that causes blowholes to occur during welding. Therefore, the N content is preferably as low as possible, and is set to 0.020% or less. The N content may be 0.018% or less, 0.016% or less, 0.012% or less, or 0.010% or less. On the other hand, reducing N to less than 0.0001% results in a significant increase in manufacturing costs. Therefore, the N content is set to 0.0001% or more. The N content may be 0.0005% or more, 0.001% or more, or 0.002% or more.
  • O is an element that forms oxides and deteriorates hydrogen embrittlement resistance.
  • oxides often exist as inclusions, and if they exist on punched edges or cut surfaces, they will form notch-like scratches or coarse dimples on the edges, leading to stress concentration during heavy machining. This may become the starting point for crack formation, resulting in significant deterioration of workability.
  • the O content is set to 0.0200% or less.
  • the O content may be 0.0180% or less, 0.0150% or less, 0.0120% or less, or 0.0100% or less. The lower the O content, the better.
  • reducing the O content to less than 0.0001% results in a significant increase in manufacturing costs. Therefore, the O content is set to 0.0001% or more.
  • the O content may be 0.0005% or more, 0.0010% or more, or 0.0015% or more.
  • the steel plate may contain at least one of the following optional elements in place of a portion of the remaining Fe, if necessary.
  • the steel plate contains Co: 0 to 0.5000%, Ni: 0 to 1.000%, Mo: 0 to 1.000%, Cr: 0 to 2.000%, Ti: 0 to 0.500%, B: 0-0.0100%, Nb: 0-0.500%, V: 0-0.500%, Cu: 0-0.500%, W: 0-0.100% and Ta: 0-0 It may contain at least one selected from the group consisting of .100%.
  • the steel plate may contain at least one selected from the group consisting of Sn: 0 to 0.100%, Sb: 0 to 0.100%, and As: 0 to 0.100%.
  • the steel plate contains Mg: 0 to 0.0500%, Ca: 0 to 0.050%, Y: 0 to 0.050%, Zr: 0 to 0.050%, La: 0 to 0.050%, and Ce: may contain at least one selected from the group consisting of 0 to 0.050%.
  • Co is an element that is effective in controlling the morphology of carbides and increasing the strength of steel sheets.
  • the Co content may be 0%, in order to obtain these effects, the Co content is preferably 0.0010% or more.
  • the Co content may be 0.0100% or more, 0.0200% or more, 0.0500% or more, or 0.1000% or more.
  • the Co content is set to 0.5000% or less.
  • the Co content may be 0.4000% or less, 0.3000% or less, or 0.2000% or less.
  • Ni is an element effective in increasing the strength of steel sheets. Further, Ni is an element that is effective in improving wettability and promoting alloying reactions. Although the Ni content may be 0%, in order to obtain these effects, the Ni content is preferably 0.001% or more. The Ni content may be 0.010% or more, 0.020% or more, 0.050% or more, or 0.100% or more. On the other hand, if Ni is contained excessively, the hydrogen embrittlement resistance may deteriorate. Therefore, the Ni content is set to 1.000% or less. The Ni content may be 0.900% or less, 0.800% or less, 0.600% or less, or 0.300% or less.
  • Mo is an element effective in increasing the strength of steel sheets. Further, Mo is an element that has the effect of suppressing ferrite transformation that occurs during heat treatment in continuous annealing equipment or continuous hot-dip galvanizing equipment. Although the Mo content may be 0%, in order to obtain these effects, the Mo content is preferably 0.001% or more. The Mo content may be 0.010% or more, 0.020% or more, 0.050% or more, or 0.080% or more. On the other hand, even if Mo is contained excessively, the effect of suppressing ferrite transformation may be saturated, or coarse intermetallic compounds and carbides may be formed, resulting in a decrease in hydrogen embrittlement resistance. Therefore, the Mo content is set to 1.000% or less. The Mo content may be 0.900% or less, 0.800% or less, 0.600% or less, or 0.300% or less.
  • Cr is an element that suppresses pearlite transformation and is effective in increasing the strength of steel.
  • the Cr content may be 0%, in order to obtain such an effect, the Cr content is preferably 0.001% or more.
  • the Cr content may be 0.005% or more, 0.010% or more, 0.020% or more, or 0.050% or more.
  • the Cr content is set to 2.000% or less.
  • the Cr content may be 1.800% or less, 1.500% or less, 1.000% or less, or 0.500% or less.
  • Ti is an element that contributes to increasing the strength of steel sheets through precipitate strengthening, fine grain strengthening by suppressing the growth of ferrite crystal grains, and dislocation strengthening through suppressing recrystallization.
  • the Ti content may be 0%, in order to obtain these effects, the Ti content is preferably 0.001% or more.
  • the Ti content may be 0.005% or more, 0.010% or more, or 0.050% or more.
  • the Ti content is set to 0.500% or less.
  • the Ti content may be 0.450% or less, 0.400% or less, 0.300% or less, or 0.100% or less.
  • B is an element that suppresses the formation of ferrite and pearlite and promotes the formation of low-temperature transformed structures such as bainite or martensite in the cooling process from the austenite temperature range. Further, B is an element useful for increasing the strength of steel. Although the B content may be 0%, in order to obtain these effects, the B content is preferably 0.0001% or more. The B content may be 0.0003% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if B is contained excessively, coarse B oxides may be generated in the steel. Since this oxide becomes a starting point for the generation of voids during cold working, hydrogen embrittlement resistance may deteriorate due to the formation of coarse B oxides. Therefore, the B content is set to 0.0100% or less. The B content may be 0.0080% or less, 0.0060% or less, 0.0050% or less, or 0.0020% or less.
  • Nb is an element effective in controlling the morphology of carbides, and is also effective in improving toughness by refining the structure.
  • the Nb content may be 0%, in order to obtain these effects, the Nb content is preferably 0.001% or more.
  • the Nb content may be 0.002% or more, 0.010% or more, or 0.020% or more.
  • the Nb content is set to 0.500% or less.
  • the Nb content may be 0.450% or less, 0.400% or less, 0.300% or less, or 0.100% or less.
  • V is an element that contributes to increasing the strength of the steel sheet by strengthening precipitates, strengthening fine grains by suppressing the growth of ferrite crystal grains, and strengthening dislocations by suppressing recrystallization.
  • the V content may be 0%, in order to obtain these effects, the V content is preferably 0.001% or more.
  • the V content may be 0.002% or more, 0.010% or more, or 0.020% or more.
  • the V content is set to 0.500% or less.
  • the V content may be 0.450% or less, 0.400% or less, 0.300% or less, or 0.100% or less.
  • Cu is an element effective in improving the strength of steel sheets.
  • the Cu content may be 0%, in order to obtain such an effect, the Cu content is preferably 0.001% or more.
  • the Cu content may be 0.002% or more, 0.010% or more, or 0.030% or more.
  • the Cu content is set to 0.500% or less.
  • the Cu content may be 0.450% or less, 0.400% or less, 0.300% or less, or 0.100% or less.
  • W is an element effective in increasing the strength of steel sheets. Further, W forms precipitates and crystallized substances. Since W-containing precipitates and crystallized substances serve as hydrogen trap sites, W is an effective element for improving hydrogen embrittlement resistance. Although the W content may be 0%, in order to obtain these effects, the W content is preferably 0.001% or more. The W content may be 0.002% or more, 0.005% or more, or 0.010% or more. On the other hand, if W is contained excessively, coarse W precipitates or crystallized substances may be generated. These coarse W precipitates or crystallized substances tend to cause cracks, and these cracks may propagate within the steel material with relatively low applied stress.
  • the W content is set to 0.100% or less.
  • the W content may be 0.080% or less, 0.060% or less, 0.050% or less, or 0.030% or less.
  • Ta is an element effective in controlling the morphology of carbides and increasing the strength of steel sheets.
  • the Ta content may be 0%, but in order to obtain these effects, the Ta content is preferably 0.001% or more.
  • the Ta content may be 0.002% or more, 0.005% or more, or 0.010% or more.
  • the Ta content is set to 0.100% or less.
  • the Ta content may be 0.080% or less, 0.060% or less, 0.050% or less, or 0.020% or less.
  • Sn is an element contained in steel when scrap is used as a steel raw material. If the Sn content is high, the hydrogen embrittlement resistance may deteriorate due to grain boundary embrittlement. Therefore, the Sn content is set to 0.100% or less.
  • the Sn content may be 0.060% or less, 0.030% or less, or 0.020% or less.
  • the Sn content is preferably as low as 0%, but reducing the Sn content to less than 0.001% requires a lot of time for refining, leading to a significant increase in cost. Therefore, the Sn content may be 0.001% or more.
  • the Sn content may be 0.002% or more, 0.005% or more, or 0.010% or more.
  • Sb is an element contained when scrap is used as a steel raw material. Further, Sb is an element that strongly segregates at grain boundaries and causes embrittlement of the grain boundaries and a decrease in ductility. Therefore, the Sb content is set to 0.100% or less.
  • the Sb content may be 0.040% or less, 0.030% or less, or 0.020% or less.
  • the Sb content is preferably as low as possible, and may be as low as 0%, but in order to reduce the Sb content to less than 0.001%, a lot of time is required for refining, leading to a significant increase in cost. Therefore, the Sb content may be 0.001% or more.
  • the Sb content may be 0.002% or more, 0.005% or more, or 0.008% or more.
  • the As content is set to 0.100% or less.
  • the As content may be 0.040% or less, 0.030% or less, or 0.020% or less.
  • the As content is preferably as low as possible, and may be as low as 0%, but in order to reduce the As content to less than 0.001%, a lot of time is required for refining, leading to a significant increase in cost. Therefore, the As content may be 0.001% or more.
  • the As content may be 0.002% or more, 0.003% or more, or 0.005% or more.
  • Mg is an element that can control the form of sulfide when contained in a trace amount.
  • the Mg content may be 0%, in order to obtain such an effect, the Mg content is preferably 0.0001% or more.
  • the Mg content may be 0.0005% or more, 0.0010% or more, 0.0015% or more, or 0.0020% or more.
  • the Mg content is set to 0.0500% or less.
  • the Mg content may be 0.0300% or less, 0.0100% or less, 0.0050% or less, or 0.0030% or less.
  • Ca is an element that is useful as a deoxidizing element and is also effective in controlling the form of sulfides.
  • the Ca content may be 0%, in order to obtain these effects, the Ca content is preferably 0.0001% or more.
  • the Ca content may be 0.0005% or more, 0.001% or more, or 0.002% or more.
  • the Ca content is set to 0.050% or less.
  • the Ca content may be 0.030% or less, 0.010% or less, 0.005% or less, or 0.003% or less.
  • Y is an element that can control the form of sulfide when contained in a trace amount.
  • the Y content may be 0%, in order to obtain such an effect, the Y content is preferably 0.0001% or more.
  • the Y content may be 0.0005% or more, 0.001% or more, or 0.002% or more.
  • the Y content is set to 0.050% or less.
  • the Y content may be 0.030% or less, 0.010% or less, 0.005% or less, or 0.003% or less.
  • Zr 0 to 0.050%
  • Zr is an element that can control the form of sulfide when contained in a trace amount.
  • the Zr content may be 0%, in order to obtain such an effect, the Zr content is preferably 0.0001% or more.
  • the Zr content may be 0.0005% or more, 0.001% or more, or 0.002% or more.
  • the Zr content is set to 0.050% or less.
  • the Zr content may be 0.030% or less, 0.010% or less, 0.005% or less, or 0.003% or less.
  • La is an element that can control the form of sulfide when contained in a trace amount.
  • the La content may be 0%, in order to obtain such an effect, the La content is preferably 0.0001% or more.
  • the La content may be 0.0005% or more, 0.001% or more, or 0.002% or more.
  • the La content is set to 0.050% or less.
  • the La content may be 0.030% or less, 0.010% or less, 0.005% or less, or 0.003% or less.
  • Ce is an element that can control the form of sulfide when contained in a trace amount.
  • the Zr content may be 0%, in order to obtain such effects, the Ce content is preferably 0.0001% or more.
  • the Ce content may be 0.0005% or more, 0.001% or more, or 0.002% or more.
  • the Ce content is set to 0.050% or less.
  • the Ce content may be 0.030% or less, 0.010% or less, 0.005% or less, or 0.003% or less.
  • the remainder other than the above elements consists of Fe and impurities.
  • Impurities are components that are mixed in during industrial manufacturing of steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap.
  • the chemical composition of the steel plate according to the embodiment of the present invention may be measured by a general analysis method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) for chips according to JIS G 1201:2014. Specifically, for example, a 35 mm square test piece was obtained from a steel plate, ground to 1/2 the plate thickness, and the ground surface was prepared in advance using a Shimadzu ICPS-8100 or the like (measuring device). It can be identified by measuring under conditions based on a calibration curve.
  • ICP-AES Inductively Coupled Plasma-Atomic Emission Spectrometry
  • C and S which cannot be measured by ICP-AES, should be measured using the combustion-infrared absorption method, N using the inert gas melting-thermal conductivity method, and O using the inert gas melting-non-dispersive infrared absorption method.
  • the chemical composition may be analyzed after removing the coating layer by mechanical grinding.
  • the microstructure of the steel plate according to the embodiment of the present invention may be any microstructure that satisfies the requirement of a tensile strength of 1500 MPa or more. More specifically, as described above, the present invention aims to provide a steel plate that has high strength and excellent hydrogen embrittlement resistance in the bent portion, and has a tensile strength of 1500 MPa or more.
  • the ratio of macroscopic embrittlement fractures in the Charpy impact test at room temperature is controlled to 35.0% or less, and the average Vickers hardness in the area from at least one surface of the steel plate to 10% of the plate thickness is This objective is achieved by controlling the hardness Hs to 0.95 times or less the average Vickers hardness Hc at the 1/2 plate thickness position. Therefore, it is clear that the microstructure of the steel sheet is not an essential technical feature to achieve the object of the present invention.
  • a preferred microstructure for a steel plate having a tensile strength of 1500 MPa or more will be described in detail, but these descriptions are intended to be mere examples of a steel plate having a tensile strength of 1500 MPa or more.
  • the present invention is not intended to be limited to steel sheets having such a specific microstructure.
  • the surface layer portion of the steel sheet is softened, so the microstructure may differ between the surface layer portion and the inner layer portion of the steel sheet. Therefore, in the following, the microstructure at a position of 1/2 of the plate thickness will be defined as representing the portion of the steel plate that has not been softened.
  • the microstructure at the 1/2 plate thickness position of the steel plate is not particularly limited, but may include martensite and tempered martensite: more than 93.0% in total in terms of area ratio.
  • the predetermined chemical composition described above in particular a C content of 0.15% or more, and containing martensite and tempered martensite in an area ratio of more than 93.0% at the 1/2 position of the plate thickness. , it becomes possible to reliably achieve a tensile strength of 1500 MPa or more.
  • the total area ratio of martensite and tempered martensite may be 95.0% or more or 97.0% or more.
  • the upper limit of the total area ratio of martensite and tempered martensite is not particularly limited and may be 100%.
  • the area ratio of martensite and tempered martensite may be 99.0% or less or 98.0% or less.
  • the microstructure at the 1/2 thickness position of the steel plate is not particularly limited, but may contain less than 7.0% ferrite in terms of area ratio. Since ferrite is a soft structure, by limiting the area percentage of the ferrite to less than 7.0% compared to the area percentage of the above-mentioned martensite and tempered martensite, which are hard structures, of more than 93.0%, It becomes possible to reliably achieve a tensile strength of 1500 MPa or more.
  • the area ratio of ferrite may be 5.0% or less or 3.0% or less, for example, 0%.
  • the area ratio of ferrite may be 1.0% or more or 2.0% or more.
  • the area ratio of residual structures other than martensite, tempered martensite, and ferrite may be 0%, but when residual structures exist, the residual structures include at least one type of bainite, pearlite, and retained austenite. It may be.
  • the area ratio of the residual tissue is not particularly limited, but may be, for example, 0 to less than 7.0%, 0 to 5.0%, or 0 to 3.0%.
  • setting the area ratio of the remaining structure to 0% requires sophisticated control in the manufacturing process of the steel plate, which may lead to a decrease in yield. Therefore, the area ratio of the remaining tissue may be 1.0% or more.
  • Identification and calculation of the microstructure at the 1/2 thickness position of the steel plate are performed as follows. First, a sample having a plate thickness cross section parallel to the rolling direction of the steel plate is taken, and the cross section is used as an observation surface. This observation surface is etched with a nital reagent, and a region of 100 ⁇ m ⁇ 100 ⁇ m centered at 1/2 the thickness of the steel sheet from the surface of the etched observation surface is defined as an observation region. This observation area is observed at a magnification of 1000 to 50000 times using an electron channeling contrast image using a field emission scanning electron microscope (FE-SEM).
  • FE-SEM field emission scanning electron microscope
  • Electron channeling contrast imaging is a method of detecting crystal orientation differences within crystal grains as contrast differences in the image, and for martensite and tempered martensite, the total area ratio can be calculated from images taken with this electron channeling contrast. demand. Since these structures are more difficult to etch than ferrite, they exist as convex portions on the structure observation surface. Tempered martensite is a collection of lath-shaped crystal grains, and contains iron-based carbides with a major axis of 20 nm or more inside, and these carbides belong to multiple variants, that is, multiple iron-based carbide groups extending in different directions. be. Further, retained austenite also exists in the form of convex portions on the microstructure observation surface. Therefore, the total area ratio of martensite and tempered martensite can be accurately measured by subtracting the area ratio of the convex portions obtained in the above procedure by the area ratio of retained austenite measured in the procedure described below. It becomes possible.
  • the area ratio of ferrite is similarly determined by observing a region of 100 ⁇ m ⁇ 100 ⁇ m centered at 1/2 the thickness of the steel sheet from the surface of the steel sheet using the electron channeling contrast image described above.
  • the area that can be recognized as a uniform medium brightness (gray) contrast is polygonal ferrite, and this is determined to be ferrite.
  • Such determination of brightness can be carried out as a normal tissue observation by those skilled in the art.
  • this region recognized as a uniform intermediate brightness contrast is a region that is easily corroded by etching, and is a recessed portion in terms of surface morphology.
  • the area ratio of polygonal ferrite in each field of view is calculated using an image analysis method for eight fields of view of an electron channeling contrast image of 35 ⁇ m x 25 ⁇ m, and the average value thereof is taken as the area ratio of ferrite.
  • the area ratio of retained austenite is calculated by measurement using X-rays. First, a portion of the sample from the plate surface to 1/2 of the plate thickness in the thickness direction is removed by mechanical polishing and chemical polishing. Next, diffraction peaks of (200), (211) of the bcc phase and (200), (220), (311) of the fcc phase were obtained using MoK ⁇ rays as characteristic X-rays for the polished sample. The structural fraction of retained austenite is calculated from the integrated intensity ratio of , and this is taken as the area fraction of retained austenite. Furthermore, the area ratio of pearlite and bainite is determined from images taken using the above-mentioned electron channeling contrast.
  • Pearlite is a structure in which plate-shaped carbides and ferrite are lined up.
  • bainite is a collection of lath-shaped crystal grains, and either does not contain iron-based carbides with a major axis of 20 nm or more inside, or contains iron-based carbides with a major axis of 20 nm or more inside, and the carbide is a single variant. In other words, it belongs to a group of iron-based carbides that extend in the same direction.
  • the term "iron-based carbide groups extending in the same direction” refers to iron-based carbide groups in which the difference in elongation direction is within 5 degrees.
  • the Vickers hardness Hs is 0.95 times or less the average Vickers hardness Hc at the 1/2 position of the plate thickness.
  • sufficient bending workability may not be obtained, and there is a problem that hydrogen embrittlement is likely to occur because a relatively large plastic strain is applied at the bending part. .
  • the average Vickers hardness Hs in a region up to 10% of the plate thickness from at least one, preferably both surfaces of the steel plate is equal to the average Vickers hardness Hc at a position of 1/2 the plate thickness.
  • Hs is preferably a smaller value than Hc.
  • Hs is 0.92 times or less, 0.90 times or less, 0.88 times or less, or 0.
  • Hs may be 0.20 times or more or 0.30 times or more than Hc, but from the viewpoint of ensuring higher strength of the steel plate, for example, Hs is 0.40 times or more than Hc, or 0.45 times or more. It may be 0.50 times or more or 0.50 times or more.
  • the "average Vickers hardness Hs in the region from the surface to 10% of the plate thickness” and the “average Vickers hardness Hc at the 1/2 position of the plate thickness” are determined as follows, and the Vickers hardness The test will be conducted in accordance with JIS Z 2244-1:2020. First, a test piece is cut out so that a cross section perpendicular to the surface (plate thickness cross section) can be observed from any position 50 mm or more away from the edge of the steel plate.
  • the Vickers hardness at 10 equally divided depth positions is measured with an indentation load of 10 g, and the average value thereof is determined as the average Vickers hardness Hs in the area from the surface to 10% of the plate thickness.
  • the Vickers hardness at the 1/2 plate thickness position of the test piece was measured with an indentation load of 10 g, and then the same indentation load was applied from that position on a line perpendicular to the plate thickness direction and parallel to the rolling direction.
  • the Vickers hardness is measured at a total of 5 or more points, for example 10 points, with a weight of 10 g, and the average value thereof is determined as the average Vickers hardness Hc at the 1/2 plate thickness position. It is preferable that the distance between each measurement point at the 1/2 plate thickness position be at least four times the distance of the indentation.
  • the distance 4 times or more of the indentation means a distance that is 4 times or more the length of the diagonal line of the rectangular opening of the indentation created by the diamond indenter during the measurement of Vickers hardness.
  • the value of Hs/Hc is determined based on Hs and Hc thus obtained.
  • the carbon concentration Cs is preferably 0.10 to 0.90 times the average carbon concentration Cc of the base material.
  • the average Vickers hardness Hs in the region up to 10% of the thickness can be controlled to be 0.95 times or less the average Vickers hardness Hc at the 1/2 position of the plate thickness. Therefore, in addition to improving hydrogen embrittlement resistance, it is also very effective from the viewpoint of improving bending workability.
  • Cs on both surfaces of the steel plate to 0.90 times or less of the average carbon concentration Cc of the base material, that is, it is preferable to satisfy Cs/Cc ⁇ 0.90 on both surfaces of the steel plate, Naturally, embodiments in which Cs/Cc ⁇ 0.90 is satisfied only on at least one surface of the surface are also included in the present invention.
  • Cs is preferably a smaller value than Cc; for example, Cs is 0.85 times or less, 0.80 times or less, 0.75 times or less, It may be 0.70 times or less, 0.65 times or less, 0.60 times or less, or 0.55 times or less.
  • Cs is preferably 0.10 times or more of Cc, for example, 0.15 times or more, 0.20 times or more, 0.25 times or more, 0. It may be 30 times or more, 0.35 times or more, or 0.40 times or more.
  • the "average carbon concentration Cs in the region from the surface to 10% of the plate thickness" and the “average carbon concentration Cc of the base material” are determined as follows using a high-frequency glow discharge optical spectrometer (GDS). It is determined. Specifically, a method is used in which the surface of the steel plate is placed in an Ar atmosphere, a voltage is applied to generate glow plasma, and the surface of the steel plate is sputtered and analyzed in the depth direction. Then, the element contained in the material is identified from the emission spectrum wavelength unique to the element emitted when atoms are excited in the glow plasma, and the emission intensity of the identified element is estimated. Data in the depth direction can be estimated from the sputtering time.
  • GDS glow discharge optical spectrometer
  • the sputtering time can be converted into the sputtering depth. Therefore, the sputtering depth converted from the sputtering time can be defined as the depth from the surface of the material.
  • the obtained luminescence intensity is converted into mass % by preparing a calibration curve.
  • the average carbon concentration in the area from the surface measured in this way (if a coating layer such as a plating layer is present on the surface of the steel plate, the interface between the coating layer and the steel plate) to 0.1 to 10% of the plate thickness. Determine as Cs.
  • the average value of the luminescence intensity of carbon is calculated in the depth range where the luminescence intensity of carbon is sufficiently stable. This is determined as the average carbon concentration Cc of the base material. Based on Cs and Cc thus obtained, the value of Cs/Cc is determined.
  • the average carbon concentration Cs in the area from the surface of the other side to 10% of the plate thickness can be determined by measuring in the same manner as explained above. Ru.
  • Controlling 0.10 ⁇ Cs/Cc ⁇ 0.90 by decarburization as shown above is only one preferable means for achieving the desired softening of the surface layer portion of the steel sheet, that is, Hs/Hc ⁇ 0.95. do not have.
  • the feature of Hs/Hc ⁇ 0.95 according to embodiments of the present invention can be achieved by any suitable means known to those skilled in the art.
  • the characteristic of Hs/Hc ⁇ 0.95 is that by applying appropriate heat treatment, the structure of the surface layer of the steel sheet is modified to a coarser structure compared to the inside of the steel sheet, and thereby the surface layer of the steel sheet is It is also possible to achieve this by softening the material.
  • the characteristic of Hs/Hc ⁇ 0.95 can also be achieved by configuring the surface layer of the steel sheet to have a softer structure than the inside of the steel sheet.
  • the ratio of macroscopic embrittled fracture surfaces in the Charpy impact test at room temperature is 35.0% or less.
  • the macroscopic embrittlement fracture surface ratio refers to the embrittlement fracture surface ratio in the base metal portion excluding the softened portion of the surface layer of the steel sheet.
  • the hydrogen embrittlement resistance of the steel sheet can be improved. Specifically, by limiting the proportion of such macroscopic embrittlement fracture surfaces in the Charpy impact test at room temperature to 35.0% or less, the hydrogen embrittlement resistance of the steel sheet can be reliably improved. Furthermore, in combination with the predetermined softening of the surface layer of the steel sheet as explained above, it is possible to significantly improve the hydrogen embrittlement resistance of the bent portion, even in steel sheets with extremely high tensile strength of 1500 MPa or more. It becomes possible.
  • the proportion of macroscopic embrittlement fracture surfaces in the Charpy impact test at room temperature be as small as possible, for example, 30.0% or less, 25.0% or less, 20.0% or less, or It may be 15.0% or less.
  • the lower limit is not particularly limited and may be 0%.
  • the percentage of macroscopically embrittled fracture surfaces in a Charpy impact test at room temperature is 0.1% or more, 0.2% or more, 0.4% or more, 0.6% or more, 0.8% or more, 1 It may be .0% or more, 3.0% or more, or 5.0% or more.
  • the ratio of macroscopic embrittlement fracture surfaces in the Charpy impact test at room temperature is determined as follows. First, a V-notch test piece with a length of 55 mm, a width of 10 mm, and a predetermined steel plate thickness as specified in JIS Z 2242:2018 is taken from a steel plate. The softened part of the surface layer of the steel plate (approximately 10% of the thickness per side, or approximately 10% on each surface of the steel plate if both sides of the steel plate are softened) is removed by grinding, and the surface layer of the steel plate is removed.
  • the thickness of the test piece is two sheets stacked and fastened with bolts, and if the thickness of the steel plate is 2.5 mm or less, three sheets are stacked and fastened with bolts.
  • the longitudinal direction of the test piece is the width direction of the plate, and the notch is provided so that fracture propagates in the rolling direction. Based on this V-notch test piece, a Charpy impact test is conducted at room temperature (approximately 25° C.) in accordance with the provisions of JIS Z 2242:2018.
  • the fracture surface of the specimen after the Charpy impact test was observed, and the ratio of the brittle fracture surface area to the total fracture surface area was calculated using image analysis software, and the macroscopic brittle fracture surface in the Charpy impact test at room temperature was calculated. Determine as a percentage of the surface.
  • P segregation amount at prior austenite grain boundaries from the 1/8th position to the 1/2th position of the plate thickness: 3.5 at % or less P is an element that segregates at prior austenite grain boundaries and promotes embrittlement of the grain boundaries, and is therefore a very important element in controlling the ratio of the above-mentioned macroscopically embrittled fracture surfaces.
  • the present inventors have succeeded in reducing the above-mentioned macroscopic embrittlement by reducing the amount of P segregation in prior austenite grain boundaries from 1/8 to 1/2 of the thickness of a steel sheet to 3.5 at% or less.
  • the ratio of the cracked surface can be reliably controlled to 35.0% or less, and as a result, the hydrogen embrittlement resistance of the steel sheet can be significantly improved.
  • the amount of P segregation in the prior austenite grain boundaries is preferably as low as possible, for example, 3.2 at% or less, 3.0 at% or less, 2.5 at% or less, or 2.0 at% or less. It may be less than atomic percent.
  • the lower limit is not particularly limited and is ideally 0 atomic %.
  • the amount of P segregation may be, for example, 0.1 atomic % or more, 0.2 atomic % or more, 0.3 atomic % or more, 0.5 atomic % or more, or 1.0 atomic % or more.
  • the amount of P segregation at the prior austenite grain boundary from the 1/8th position to the 1/2th position of the steel plate thickness is determined as follows. First, a test piece is taken from a position 50 mm or more away from the end face of the steel plate. At this time, the front and back surfaces of the test piece are finished by mechanical grinding. Furthermore, if the steel plate has a coating layer (for example, a plating layer) on its surface, the coating layer is removed and the front and back surfaces of the steel plate test piece are finished by mechanical grinding.
  • a coating layer for example, a plating layer
  • the thickness is not specified, but the front and back sides of the test piece should be measured in equal amounts so that the thickness is 1.2 mm. It may also be removed by mechanical grinding.
  • a test piece with a length of 20 mm and a width of 3.2 mm is processed, and a V-notch with an angle of 45° is inserted at a length of 11.5 mm.
  • the specimen is immersed in a 20% ammonium thiocyanate solution.
  • the immersion time is not particularly limited, and may be any condition that allows prior austenite grain boundaries to be exposed when the sample is set in an Auger electron emission spectrometer and broken, and may be, for example, 48 hours.
  • the test piece Galvanize the front and back surfaces of the test piece within 10 minutes after completion of immersion.
  • the specimen is subjected to Auger electron emission spectroscopy and destroyed.
  • the time from plating to destruction of the test piece is preferably within 1.5 hours, more preferably within 0.5 hours.
  • the test piece is set in an Auger electron emission spectrometer, and the notch portion of the test piece is broken to expose the prior austenite grain boundaries.
  • the device may be a field emission type Auger electron spectrometer, and the model is not particularly limited, but PHI680 manufactured by ULVAC-PHI may be used, and the measurement conditions are an accelerating voltage of 10 keV and an irradiation current of 10 nA. Good too.
  • the exposed prior austenite grain boundaries are irradiated with an electron beam at an accelerating voltage of 1 to 30 kV, and the atomic percent of P in the grain boundaries is measured. Measurements are carried out at 10 prior austenite grain boundaries from the surface at a position of 1/8 of the plate thickness to a position of 1/2 of the plate thickness. In order to prevent grain boundary contamination, it is preferable to complete the measurement quickly after fracture, and the measurement may be completed within 30 minutes.
  • the average value of the obtained atomic % of P is calculated and determined as the amount of P segregation in the prior austenite grain boundaries from the 1/8th thickness position to the 1/2th thickness position of the steel sheet.
  • Steel plates according to embodiments of the present invention generally have a thickness of 0.6 to 6.0 mm.
  • the plate thickness may be 1.0 mm or more, 1.2 mm or more, or 1.4 mm or more, and/or 5.0 mm or less, 4.0 mm or less, 3.0 mm or less, or 2.5 mm or less. It may be.
  • the steel plate according to the embodiment of the present invention may further include a coating layer, such as a plating layer, on at least one surface, preferably both surfaces, for the purpose of improving corrosion resistance.
  • a coating layer such as a plating layer
  • the coating layer includes, for example, at least one member selected from the group consisting of zinc, aluminum, magnesium, and alloys thereof. More specifically, the covering layer may be a hot dipping layer or an electroplating layer.
  • the hot-dip plating layer examples include a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, a hot-dip aluminum plating layer, a hot-dip Zn-Al alloy plating layer, a hot-dip Zn-Al-Mg alloy plating layer, and a hot-dip Zn-Al-Mg-Si. Including alloy plating layer etc.
  • the electroplating layer includes, for example, an electrogalvanizing layer, an electrolytic Zn--Ni alloy plating layer, and the like.
  • the coating layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electrogalvanized layer.
  • the amount of the coating layer to be deposited is not particularly limited and may be a general amount to be deposited.
  • the steel plate according to the embodiment of the present invention has a tensile strength of 1500 MPa or more.
  • the tensile strength is preferably 1600 MPa or more, 1700 MPa or more, or 1800 MPa or more.
  • the steel sheet according to the embodiment of the present invention despite having such a very high tensile strength, it is possible to appropriately control the proportion of the macroscopic embrittled fracture surface described above and to In combination with predetermined softening, the hydrogen embrittlement resistance of the bent portion can be significantly improved.
  • the upper limit of the tensile strength is not particularly limited, for example, the tensile strength may be 2300 MPa or less, 2200 MPa or less, or 2100 MPa or less.
  • a total elongation of 4.0% or more can be achieved.
  • the total elongation is preferably 5.0% or more, more preferably 6.0% or more or 7.0% or more.
  • the upper limit is not particularly limited, for example, the total elongation may be 12.0% or less or 10.0% or less.
  • Tensile strength and total elongation are measured by conducting a tensile test in accordance with JIS Z 2241:2011 based on a JIS No. 5 test piece taken from a direction in which the longitudinal direction of the test piece is parallel to the rolling direction of the steel plate. Ru.
  • the steel plate according to the embodiment of the present invention has excellent hydrogen embrittlement resistance and also excellent bending workability, even though it has a very high tensile strength of 1500 MPa or more. It is extremely useful for use as automobile frame members, bumpers, and other structural and reinforcing members that require strength, and is also extremely useful for applications that require high bending workability in addition to high strength. Are suitable.
  • the method for manufacturing a steel plate according to an embodiment of the present invention includes: A slab having the chemical composition described above in connection with the steel plate is heated to a temperature of 1100 to 1300°C, then finish rolled, and the finish rolled steel plate is heated to a temperature of 500°C or less at an average cooling rate of 20°C/s or more.
  • a hot rolling process including cooling and winding, the finishing temperature of the finish rolling being 850 to 1050°C;
  • a pickling process of pickling the obtained hot rolled steel sheet A cold rolling process in which pickled hot rolled steel sheets are cold rolled at a reduction rate of 35 to 80%;
  • a step of annealing the obtained cold-rolled steel sheet wherein the annealing is performed by annealing the cold-rolled steel sheet in an atmosphere with a dew point of -30.0 to 10.0°C and a hydrogen concentration of 1.0 to 10.0%.
  • An annealing step comprising heating to a temperature of 830 to 900 °C or higher for 20 to 150 seconds, and then limiting the residence time in the temperature range of 400 to 600 °C to 2 to 350 seconds.
  • a slab having the chemical composition described above in connection with steel plate is heated.
  • the slab used is preferably cast by a continuous casting method from the viewpoint of productivity, but may be manufactured by an ingot casting method or a thin slab casting method.
  • the slabs used contain relatively high amounts of alloying elements in order to obtain high strength steel sheets. For this reason, it is necessary to heat the slab to dissolve the alloying elements in the slab before hot rolling. If the heating temperature is less than 1100° C., the alloying elements will not be fully dissolved in the slab, leaving coarse alloy carbides, which may cause embrittlement cracking during hot rolling. For this reason, the heating temperature is preferably 1100°C or higher.
  • the upper limit of the heating temperature is not particularly limited, but is preferably 1300° C. or lower from the viewpoint of the capacity of the heating equipment and productivity.
  • the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness or the like.
  • the conditions for rough rolling are not particularly limited as long as the desired sheet bar dimensions can be ensured.
  • the heated slab, or the optionally rough rolled slab is then subjected to finish rolling. Since the slab used as described above contains a relatively large amount of alloying elements, it is necessary to increase the rolling load during hot rolling. For this reason, hot rolling is preferably performed at a high temperature.
  • the finishing temperature of finish rolling is important in terms of controlling the metallographic structure of the steel sheet. If the finishing temperature of finish rolling is low, the metal structure may become non-uniform and formability may deteriorate. For this reason, it is preferable that the finishing temperature of finish rolling is 850° C. or higher. On the other hand, in order to suppress coarsening of austenite, the finishing temperature of finish rolling is preferably 1050° C. or lower.
  • the finish-rolled steel plate is cooled to 500° C. or less at an average cooling rate of 20° C./sec or more and wound up. If the average cooling rate is less than 20°C/sec or the coiling temperature is more than 500°C, P segregation will occur in the hot rolling process, making the hot rolled steel sheet brittle and making subsequent cold rolling difficult. It may happen.
  • the average cooling rate is preferably 25°C/second or more, and the winding temperature is preferably 480°C or less.
  • the obtained hot-rolled steel sheet is pickled to remove oxidized scale formed on the surface of the hot-rolled steel sheet.
  • Pickling may be carried out under conditions suitable for removing oxide scale, and may be carried out once or in multiple steps to ensure removal of oxide scale.
  • the pickled hot rolled steel sheet is cold rolled at a rolling reduction of 35 to 80% in a cold rolling process.
  • the reduction ratio of cold rolling is preferably 50% or more.
  • the reduction ratio in cold rolling is preferably 70% or less.
  • the number of rolling passes and the rolling reduction rate for each pass are not particularly limited, and may be appropriately set so that the rolling reduction rate of the entire cold rolling falls within the above range.
  • the average carbon concentration Cs in the region up to 10% of the plate thickness from at least one, especially both surfaces of the finally obtained steel plate is 0.10 to 0.90 times the average carbon concentration Cc of the base material.
  • the average Vickers hardness Hs in a region up to 10% of the thickness can be 0.95 times or less (Hs/Hc ⁇ 0.95) the average Vickers hardness Hc at the 1/2 position of the plate thickness. If the dew point is less than -30.0°C, the hydrogen concentration is more than 10.0%, and/or the heating temperature is less than 700°C, decarburization of the surface layer cannot proceed sufficiently.
  • the dew point is over 10.0°C and/or the hydrogen concentration is less than 1.0%, decarburization may proceed too much depending on the C content, resulting in It becomes impossible to achieve the desired tensile strength of 1500 MPa or more.
  • the dew point is -20.0 to 8.0°C
  • the hydrogen concentration is 1.5 to 10.0%
  • the heating temperature is 750°C or higher. It is.
  • Hs/Hc ⁇ 0.95 only in one surface layer of the steel plate, and as a specific method to achieve this, for example, two cold-rolled steel plates are It is also possible to decarburize and soften only one surface layer portion of the steel plate by stacking them and annealing them under the conditions described above.
  • the cold rolled steel sheet is further heated to a maximum heating temperature of 830 to 900° C. in an atmosphere having the dew point and hydrogen concentration described above, and held at the maximum heating temperature for 20 to 150 seconds.
  • This promotes austenitization and obtains a desired hard structure composed of martensite or the like in the subsequent cooling step, thereby making it possible to reliably achieve a tensile strength of 1500 MPa or more.
  • the temperature is high, P diffusion is relatively fast, and segregation of P to prior austenite grain boundaries in the finally obtained steel sheet can be suppressed or reduced.
  • the amount of P segregation at the prior austenite grain boundary from the 1/8th position to the 1/2th position of the steel sheet can be controlled to 3.5 at. It becomes possible to reduce the ratio of macroscopically embrittled fracture surfaces to 35.0% or less.
  • the surface layer of the steel plate can be sufficiently decarburized, so that the final steel plate has an Hs/Hc of 0.95 or less. Desired surface layer softening can be achieved.
  • the maximum heating temperature is less than 830°C, austenitization will be insufficient, and a hard structure such as martensite will not be sufficiently obtained in the final steel plate, making it difficult to achieve a tensile strength of 1500 MPa or more. become unable to do so.
  • grain boundary segregation of P in the final structure may not be sufficiently suppressed. It becomes impossible to control the amount below .0%.
  • the maximum heating temperature exceeds 900°C, martensite becomes coarse in the final structure and the grain boundary area decreases, so even if the overall P segregation amount is the same, the prior austenite grain boundaries are The amount of P segregation will increase. As a result, it becomes impossible to control the ratio of the macroscopically embrittled fracture surface to 35.0% or less.
  • the maximum heating temperature is 835-896°C.
  • the holding time at the maximum heating temperature is less than 20 seconds, austenitization will be insufficient and a hard structure such as martensite will not be sufficiently obtained in the final structure, making it impossible to achieve a tensile strength of 1500 MPa or more. be unable to do so.
  • the surface layer portion of the steel sheet cannot be sufficiently decarburized, it becomes impossible to achieve the desired surface softening with Hs/Hc of 0.95 or less.
  • the holding time at the maximum heating temperature exceeds 150 seconds, even if the temperature is within the range of 830 to 900°C, the time is too long and segregation of P to grain boundaries may increase.
  • the holding time at high temperature becomes longer, decarburization progresses excessively, making it impossible to achieve the desired tensile strength of 1500 MPa or more in the final steel plate.
  • the holding time is between 23 and 146 seconds.
  • the residence time is preferably 300 seconds or less. From the viewpoint of suppressing grain boundary segregation of P, the shorter the residence time is, the more preferable it is, but it is practically impossible to reduce the residence time to around 0 seconds, so the lower limit of the residence time is set to 2 seconds.
  • the cold rolled steel sheet after the annealing step is cooled to room temperature at an average cooling rate of 20° C./sec or more in the next cooling step.
  • the final structure a hard structure mainly consisting of martensite, it is possible to ensure the achievement of a tensile strength of 1500 MPa or more.
  • the average cooling rate is less than 20° C./sec, a relatively large amount of bainite will be formed during the cooling process, making it impossible to achieve sufficient tensile strength.
  • the surface of the cold rolled steel sheet may be coated.
  • the coating treatment may be hot-dip plating, alloying hot-dip plating, electroplating, or the like.
  • the steel plate may be subjected to hot-dip galvanizing treatment as a coating treatment, or alloying treatment may be performed after hot-dip galvanizing treatment.
  • the coating layer includes, for example, at least one member selected from the group consisting of zinc, aluminum, magnesium, and alloys thereof. More specifically, the covering layer may be a hot dipping layer or an electroplating layer.
  • the hot-dip plating layer examples include a hot-dip galvanized (GI) layer, an alloyed hot-dip galvanized (GA) layer, a hot-dip aluminum plating layer, a hot-dip Zn-Al alloy plating layer, a hot-dip Zn-Al-Mg alloy plating layer, and a hot-dip Zn alloy plating layer.
  • GI hot-dip galvanized
  • GA alloyed hot-dip galvanized
  • the electroplating layer includes, for example, an electrogalvanizing layer, an electrolytic Zn--Ni alloy plating layer, and the like.
  • the coating layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electrogalvanized layer.
  • the specific conditions for the coating treatment and the alloying treatment are not particularly limited, and may be any suitable conditions known to those skilled in the art.
  • steel plates according to embodiments of the present invention were manufactured under various conditions, and the tensile strength, total elongation, and hydrogen embrittlement resistance of the obtained steel plates were investigated.
  • Example A molten steel was cast by a continuous casting method to form slabs having various chemical compositions shown in Table 1, and these slabs were heated to 1230° C. and hot rolled. Hot rolling was carried out by performing rough rolling and finish rolling, and the finishing temperature of finish rolling was 940°C. Next, the finish-rolled steel plate was cooled to 420° C. at an average cooling rate of 30° C./sec and wound. Next, the obtained hot rolled steel plate having a thickness of 2.6 mm was pickled and then cold rolled at a rolling reduction ratio of 45% to obtain a cold rolled steel plate having a thickness of 1.4 mm.
  • the obtained cold-rolled steel sheet was heated to a temperature of 700°C or higher in an atmosphere with a dew point of -2.0°C and 6.0% hydrogen (nitrogen balance), and heated to a maximum heating temperature of 860°C for 90°C.
  • the surface layer portions on both sides of the steel plate were sufficiently decarburized by holding the steel plate for seconds, and then the residence time in the temperature range of 400 to 600°C was adjusted to 180 seconds.
  • the cold-rolled steel sheet was cooled to room temperature at an average cooling rate of 30° C./sec, and then hot-dip galvanizing (GI) or galvannealing (GA) was appropriately applied as a coating treatment.
  • GI hot-dip galvanizing
  • GA galvannealing
  • the properties of the obtained steel plate were measured and evaluated by the following methods.
  • Tensile strength (TS) and total elongation (t-El) Tensile strength (TS) and total elongation (t-El) were measured in accordance with JIS Z 2241:2011 based on a JIS No. 5 test piece taken from a direction in which the longitudinal direction of the test piece was parallel to the rolling direction of the steel plate. It was measured by performing a tensile test.
  • the hydrogen embrittlement resistance of the bent portion of the obtained steel plate was evaluated by the following method. Specifically, first, a steel plate was sheared with a clearance of 12.5%, and then a U-bending test was conducted at 8R. Next, a strain gauge was attached to the center of the obtained test piece, and stress was applied by tightening both ends of the test piece with bolts. The applied stress was calculated from the strain measured by the strain gauge. The applied stress was a stress corresponding to 80% of the tensile strength (TS). This is because the residual stress introduced during forming is considered to correspond to the tensile strength of the steel plate.
  • TS tensile strength
  • the obtained U-bending test piece was immersed in an aqueous HCl solution with a pH of 2 at a temperature of 35° C., and then held for 72 hours to examine the presence or absence of cracks.
  • a steel plate with a tensile strength (TS) of 1500 MPa or more and an OK evaluation of hydrogen embrittlement resistance was evaluated as a steel plate with high strength and excellent hydrogen embrittlement resistance in the bent portion.
  • the results are shown in Table 2.
  • Table 2 and Table 4 shown later the values of Hs/Hc and Cs/Cc are calculated based on the average Vickers hardness Hs and average carbon concentration Cs in the area from one surface of the steel plate to 10% of the plate thickness. Only the calculated values are shown. However, since all steel plates are manufactured with the same treatment on both sides, the values of Hs and Cs are essentially the same on both sides of the steel plate, and in fact in some steel plates these values are I confirmed that it was the same on both sides. Further, in Table 2 and Table 4 shown later, the remaining structure was bainite, pearlite, and/or retained austenite.
  • Example X-1 had a TS of less than 1500 MPa due to the low C content.
  • Example Y-1 since the C content was high, coarse grain boundary carbides were formed, and the hydrogen embrittlement resistance of the bent portion deteriorated.
  • Example Z-1 the hydrogen embrittlement resistance of the bent portion deteriorated due to the high Si content.
  • Example AA-1 the hardenability decreased due to the low Mn content, and the desired hard structure could not be obtained, resulting in a TS of less than 1500 MPa.
  • Example AB-1 the hydrogen embrittlement resistance of the bent portion deteriorated due to the high Mn content.
  • Example AC-1 since the P content was high, it was not possible to control the ratio of macroscopic embrittlement fracture surfaces to 35.0% or less, and as a result, the hydrogen embrittlement resistance of the bent portion was reduced.
  • Examples AD-1 to AT-1 had good TS, but S, Al, N, O, Co, Ni, Mo, Cr, Ti, B, Cu, W, Sn, Sb, Ca, Y Also, because the Zr content was high, it was not possible to achieve sufficient hydrogen embrittlement resistance in the bent portion.
  • all the steel plates according to the invention have a predetermined chemical composition, Hs/Hc is controlled to 0.95 or less, and the ratio of macroscopically embrittled fracture surfaces in the Charpy impact test is 35. By controlling it to .0% or less, it was possible to significantly improve the hydrogen embrittlement resistance of the bent portion despite having a very high TS of 1500 MPa or more.
  • Example B steel plates having a thickness of 1.4 mm were manufactured using the steel types A to W, which were found to have excellent properties in Table 2, under the manufacturing conditions shown in Table 3. Conditions other than the manufacturing conditions shown in Table 3 were the same as in Example A, and the properties of the obtained steel plate were measured and evaluated in the same manner as in Example A. The results are shown in Table 4.
  • Example H-3 because the dew point of the annealing step was low, decarburization of the surface layer portion could not proceed sufficiently, and the desired surface layer softening, that is, Hs/Hc ⁇ 0. I could't reach 95. As a result, the hydrogen embrittlement resistance of the bent portion decreased.
  • Example J-3 since the hydrogen concentration in the annealing step was low, decarburization of the surface layer progressed too much, and it was not possible to achieve a TS of 1500 MPa or more.
  • Example K-3 since the hydrogen concentration in the annealing step was high, decarburization of the surface layer portion could not proceed sufficiently, and Hs/Hc ⁇ 0.95 could not be achieved.
  • Example L-3 since the maximum heating temperature in the annealing step was low, austenitization was insufficient and the desired hard structure could not be obtained in the final steel plate. As a result, it was not possible to achieve a TS of 1500 MPa or more.
  • Example M-3 because the maximum heating temperature in the annealing process was high, martensite became coarse in the final structure, and the accompanying decrease in grain boundary area resulted in a relative increase in the amount of P segregation at prior austenite grain boundaries. Conceivable.
  • Example N-3 since the holding time at the maximum heating temperature in the annealing step was short, austenitization was insufficient and the desired hard structure could not be obtained in the final steel plate. As a result, it was not possible to achieve a TS of 1500 MPa or more. In Example O-3, the holding time at the maximum heating temperature in the annealing step was long, so the segregation of P to the grain boundaries increased. As a result, the ratio of macroscopically embrittled fracture surfaces could not be controlled to 35.0% or less, and the hydrogen embrittlement resistance of the bent portion deteriorated.
  • Example P-3 the residence time in the temperature range of 400 to 600° C. during the annealing process was long, so the segregation of P to grain boundaries increased. As a result, the ratio of macroscopically embrittled fracture surfaces could not be controlled to 35.0% or less, and the hydrogen embrittlement resistance of the bent portion deteriorated.
  • Example Q-3 the desired hard structure could not be obtained in the final steel plate because the average cooling rate in the cooling process was low. As a result, it was not possible to achieve a TS of 1500 MPa or more. As is clear from these comparative examples, even if only one of "Hs/Hc ⁇ 0.95" and "macro embrittlement fracture surface ratio: 35.0% or less" is satisfied, sufficient bending process can be achieved. It was not possible to achieve hydrogen embrittlement resistance.
  • slabs with a given chemical composition are used, in particular the dew point of the annealing step, the hydrogen concentration, the maximum heating temperature and the holding time at that temperature, from 400 to
  • the residence time in the 600°C temperature range and the average cooling rate of the cooling process we achieved "Hs/Hc ⁇ 0.95" and "ratio of macroscopically embrittled fracture surfaces: 35.0% or less.
  • the hydrogen embrittlement resistance of the bent portion could be significantly improved.
  • Example I-3 the dew point in the annealing process was high, but since the original C content was relatively high, the surface layer did not become excessively decarburized, and the final steel plate had a pressure of 1500 MPa or more. We were able to achieve the TS.

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JP7786653B1 (ja) * 2024-06-11 2025-12-16 Ｊｆｅスチール株式会社 溶融亜鉛めっき鋼板及びその製造方法
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