WO2023189175A1 - Steel sheet for hot stamping and hot stamp molded body - Google Patents

Steel sheet for hot stamping and hot stamp molded body Download PDF

Info

Publication number
WO2023189175A1
WO2023189175A1 PCT/JP2023/007831 JP2023007831W WO2023189175A1 WO 2023189175 A1 WO2023189175 A1 WO 2023189175A1 JP 2023007831 W JP2023007831 W JP 2023007831W WO 2023189175 A1 WO2023189175 A1 WO 2023189175A1
Authority
WO
WIPO (PCT)
Prior art keywords
less
hot
pearlite
content
stamped
Prior art date
Application number
PCT/JP2023/007831
Other languages
French (fr)
Japanese (ja)
Inventor
祐馬 浅田
由梨 戸田
靖之 荻巣
環輝 鈴木
Original Assignee
日本製鉄株式会社
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by 日本製鉄株式会社 filed Critical 日本製鉄株式会社
Priority to JP2024511544A priority Critical patent/JPWO2023189175A1/ja
Publication of WO2023189175A1 publication Critical patent/WO2023189175A1/en

Links

Classifications

    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • 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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur

Definitions

  • the present invention relates to a hot stamping steel plate and a hot stamping molded body manufactured using the same.
  • Patent Document 1 discloses that the metal structure has a predetermined chemical composition, is polygonal ferrite in an area ratio of 40.0% or more and less than 60.0%, and bainitic ferrite as 30.0% or more and less than 60.0%.
  • retained austenite is 10.0% or more and 25.0% or less, and martensite is 15.0% or less, and the aspect ratio of the retained austenite is 2.0 or less, and the length of the major axis is is 1.0 ⁇ m or less and the short axis length is 1.0 ⁇ m or less, the proportion of retained austenite is 80.0% or more, the aspect ratio of the bainitic ferrite is 1.7 or less, and , the percentage of bainitic ferrite in which the average value of the crystal orientation difference in a region surrounded by grain boundaries with a crystal orientation difference of 15° or more is 0.5° or more and less than 3.0° is 80.0% or more.
  • Patent Document 1 discloses that the above configuration has excellent punching fatigue properties, elongation, and hole expandability, with a tensile strength of 980 MPa or more and a 0.2% yield strength of 600 MPa or more, suitable for structural members of automobiles, etc. It is stated that high-strength cold-rolled steel sheets can be provided.
  • Hot stamping is known as a technique for press forming materials that are difficult to form, such as high-strength steel plates.
  • Hot stamping is a hot forming technique in which the material to be formed is heated and then formed. In this technique, the material is heated and then molded, so the steel material is soft and has good formability during molding. Therefore, even high-strength steel materials can be formed into complex shapes with high precision.Also, since the press mold is used to quench the material at the same time as forming, the steel material after forming has sufficient strength. It has been known.
  • 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. It is generally known that hydrogen embrittlement cracking occurs more easily as the strength of steel increases.
  • the automobile industry and the like there is a demand for further weight reduction of steel materials, and in order to achieve such weight reduction, it is necessary to make steel materials higher in strength than ever before. Therefore, there is a strong need for steel materials that can solve the problem of hydrogen embrittlement even when the strength is increased to be equal to or higher than conventional steel materials, and more specifically, for hot-stamped molded products.
  • an object of the present invention is to provide a hot-stamped molded article with a novel configuration that has high strength and can suppress hydrogen embrittlement, and a hot-stamped steel plate for producing such a hot-stamped molded article. do.
  • the present inventors conducted a study focusing on the metal structure of each of the steel plate before hot stamping and the hot stamped body after hot stamping.
  • the present inventors discovered that by uniformly dispersing pearlite, which is the starting point of austenite grains, during heating during hot stamping in a steel sheet before hot stamping, prior austenite was formed in the final hot stamped product. It has been found that the grains are made uniform, thereby reducing variations in the hardness of prior austenite grains in the metallographic structure of the hot-stamped compact.
  • the present inventors have found that by reducing the variation in hardness of prior austenite grains in the metallographic structure of a hot-stamped compact, it is possible to suppress local increases in hardness, resulting in high tensile strength.
  • the present inventors have discovered that the hydrogen embrittlement resistance can be significantly improved despite the above characteristics, and have completed the present invention.
  • the present invention that achieves the above object is as follows. (1) In mass%, C: 0.40-0.70%, Si: 0.010-1.300%, Mn: 0.60-3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.0010-0.5000%, Nb: 0.0010 to 0.100%, Ti: 0.010-0.200%, B: 0.0005-0.0200%, Cr: 0.010-0.80%, Mo: 0.0010-1.000%, Co: 0-2.00%, Ni: 0-3.00%, Cu: 0 to 1.00%, V: 0 to 1.00%, W: 0-1.000%, Ca: 0-0.010%, Mg: 0-1.000%, REM: 0-1.000%, Sb: 0 to 1.000%, Zr: 0 to 1.000%, Sn: 0-1.000%, Has a chemical composition consisting of As: 0 to 0.100%, and the balance: Fe and impur
  • the chemical composition is in mass%; Co: 0.001 to 2.00%, Ni: 0.001 to 3.00%, Cu: 0.001 to 1.00%, V: 0.001 to 1.00%, W: 0.001-1.000%, Ca: 0.0001-0.010%, Mg: 0.0001-1.000%, REM: 0.0001-1.000%, Sb: 0.001 to 1.000%, Zr: 0.001 to 1.000%, Sn: 0.001 to 1.000%, and As: 0.001 to 0.100%
  • the present invention it is possible to provide a hot-stamped molded product that has high strength and can suppress hydrogen embrittlement, and a hot-stamped steel plate for producing such a hot-stamped molded product.
  • the hot stamping steel plate according to the embodiment of the present invention has, in mass%, C: 0.40-0.70%, Si: 0.010-1.300%, Mn: 0.60-3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.0010-0.5000%, Nb: 0.0010 to 0.100%, Ti: 0.010-0.200%, B: 0.0005-0.0200%, Cr: 0.010-0.80%, Mo: 0.0010-1.000%, Co: 0-2.00%, Ni: 0-3.00%, Cu: 0 to 1.00%, V: 0 to 1.00%, W: 0-1.000%, Ca: 0-0.010%, Mg: 0-1.000%, REM: 0-1.000%, Sb: 0 to 1.000%, Zr: 0 to 1.000%, Sn: 0-1.000%, Has a chemical composition consisting of As: 0 to
  • the present inventors have developed a method for reducing or suppressing the area of the steel sheet before hot stamping and the hot stamped body after hot stamping.
  • a study was conducted focusing on the metal structure of each. More specifically, the present inventors first found that when the variation in prior austenite grain size is large in the metallographic structure of a hot-stamped compact, the hardness increases in the region where the prior austenite grain size is smaller; We found that localized high hardness regions can be the starting point for hydrogen embrittlement cracking.
  • the present inventors aimed to reduce the variation in prior austenite grain size and, as a result, reduce the variation in hardness in prior austenite grains, and more specifically, to improve the hardness distribution of prior austenite grains. It has been found that such local increases in hardness can be reliably suppressed by controlling the standard deviation to 150 Hv or less.
  • austenite grains having a larger grain size have lower hardness due to a higher initiation temperature of martensitic transformation than austenite grains having a smaller grain size.
  • Austenite grains with smaller grain sizes undergo martensitic transformation at lower temperatures than larger grains, resulting in increased hardness. Therefore, in order to suppress or reduce such a local increase in hardness, it is important to reduce variations in austenite grain size before martensitic transformation.
  • the present inventors focused on the metallographic structure of a steel sheet before hot stamping, for example, a hot rolled steel sheet, and the method of manufacturing a hot stamping steel sheet will be explained in detail later.
  • a hot stamping steel sheet By uniformly dispersing pearlite in the hot-stamped compact, it is possible to reduce the variation in the prior austenite grain size in the final metallographic structure of the hot-stamped compact, and in this regard, the standard deviation of the hardness distribution of the prior austenite grains can be reduced. It has been found that it can be controlled to 150 Hv or less. As the strength of steel materials increases, relatively large amounts of Mn are sometimes added to improve the hardenability of steel materials.
  • the Mn content is 0.60% by mass or more
  • pearlite is relatively easily generated, and therefore it is very difficult to uniformly disperse pearlite, which is generated in large quantities in the metallographic structure of the hot rolled steel sheet, compared to the case where the Mn content is low.
  • the variation in prior austenite grain size in the metal structure after hot stamping became large.
  • the present inventors have solved this problem by applying a relatively high rolling reduction in the final stage of finish rolling and by appropriately controlling the subsequent cooling to achieve uniform dispersion of pearlite in the metallographic structure of hot rolled steel sheets.
  • the present inventors determined that the number of A/B boundaries (boundaries between ferrite phase and pearlite phase) in an electron microscope image of the metal structure of a hot stamping steel sheet mainly composed of ferrite and pearlite, Total number of A/A boundaries (boundaries between ferrite phase and ferrite phase), number of B/B boundaries (boundaries between pearlite phase and pearlite phase), and number of A/B boundaries (boundaries between ferrite phase and pearlite phase) It is important to control the dispersion index of pearlite obtained by dividing by 0.5 or more, and by controlling the dispersion index of pearlite to 0.5 or more, It has been found that it is possible to achieve a standard deviation of the hardness distribution of prior austenite grains of 150 Hv or less
  • % 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 element that improves the strength of the hot stamp molded product. If the C content is less than 0.40%, the desired strength cannot be obtained in the hot-stamped molded product. Therefore, the C content is set to 0.40% or more.
  • the C content is preferably more than 0.40%, 0.42% or more, 0.43% or more, 0.44% or more, 0.45% or more, or 0.46% or more.
  • the C content is set to 0.70% or less.
  • the C content is 0.68% or less, 0.67% or less, 0.65% or less or 0.60% or less.
  • Si is an element that improves the strength of the hot stamp molded product through solid solution strengthening. If the Si content is less than 0.010%, desired strength cannot be obtained. Therefore, the Si content is set to 0.010% or more.
  • the Si content is preferably 0.050% or more, 0.100% or more, 0.200% or more, more than 0.250%, 0.255% or more, 0.260% or more, 0.270% or more, 0 .280% or more, 0.300% or more, or 0.400% or more.
  • the Si content exceeds 1.300%, the amount of ferrite increases in the steel sheet for hot stamping, and a desired metal structure may not be obtained. Therefore, the Si content is set to 1.300% or less.
  • the Si content is preferably 1.200% or less, 1.000% or less, 0.800% or less, 0.600% or less, or 0.500% or less.
  • Mn promotes the transformation from austenite to pearlite in hot-rolled steel sheets during the manufacturing process of hot-stamped compacts, and improves the dispersion index of pearlite in hot-stamped steel sheets and the hardness distribution of prior austenite grains in hot-stamped compacts. It is an element that contributes to control.
  • the Mn content is set to 0.60% or more. The Mn content is preferably more than 0.60%, 0.70% or more, 0.80% or more, 1.00% or more, or 1.30% or more.
  • the Mn content is set to 3.00% or less.
  • the Mn content is 2.90% or less, 2.70% or less, 2.50% or less, 2.30% or less or 2.00% or less.
  • P is an impurity element that segregates at grain boundaries and deteriorates hydrogen embrittlement resistance. Therefore, the P content is set to 0.100% or less.
  • the P content is preferably 0.070% or less, 0.050% or less, or 0.010% or less.
  • the lower limit of the P content is not particularly limited, but if it is reduced to less than 0.0001%, the cost for removing P will increase significantly, which is economically unfavorable. Therefore, the P content may be set to 0.0001% or more.
  • S is an impurity element and forms inclusions in steel. Since these inclusions deteriorate the hydrogen embrittlement resistance, the S content is set to 0.0100% or less.
  • the S content is preferably 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.
  • the lower limit of the S content is not particularly limited, but if it is reduced to less than 0.0001%, the cost for removing S will increase significantly, which is economically unfavorable. Therefore, the S content may be set to 0.0001% or more.
  • N is an impurity element and forms nitrides in steel. Since this nitride deteriorates hydrogen embrittlement resistance, the N content is set to 0.0200% or less.
  • the N content is preferably 0.0180% or less, 0.0150% or less, 0.0100% or less, 0.0060% or less, or 0.0040% or less.
  • the lower limit of the N content is not particularly limited, but if it is reduced to less than 0.0001%, the cost for removing N will increase significantly, which is economically unfavorable. Therefore, the N content may be set to 0.0001% or more.
  • the O content is set to 0.0200% or less.
  • the O content is preferably 0.0150% or less, 0.0100% or less, 0.0070% or less, or 0.0040% or less. From the viewpoint of reducing refining costs, the O content may be 0.0001% or more. In order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be set to 0.0005% or more.
  • Al is an element that has the effect of deoxidizing molten steel and making the steel sound.
  • the Al content is set to 0.0010% or more.
  • the Al content is preferably 0.0030% or more, 0.0050% or more, 0.0100% or more, or 0.0300% or more.
  • the Al content is set to 0.5000% or less.
  • the Al content is preferably 0.4000% or less, 0.3000% or less, 0.2000% or less, 0.1500% or less, or 0.1000% or less.
  • Nb is an element that forms carbonitrides in steel and improves the strength of hot stamped products through precipitation strengthening. If the Nb content is less than 0.0010%, desired strength cannot be obtained. Therefore, the Nb content is set to 0.0010% or more. The Nb content is preferably 0.005% or more, 0.009% or more, or 0.015% or more. On the other hand, if the Nb content exceeds 0.100%, a large amount of carbonitrides will be generated in the steel, resulting in a decrease in the hydrogen embrittlement resistance of the hot stamped product. Therefore, the Nb content is set to 0.100% or less. The Nb content is preferably 0.080% or less, 0.060% or less, or 0.050% or less.
  • Ti is an element that forms carbonitrides in steel and improves the strength of hot stamped products through precipitation strengthening. If the Ti content is less than 0.010%, desired strength cannot be obtained. Therefore, the Ti content is set to 0.010% or more. The Ti content is preferably 0.015% or more, 0.020% or more, or 0.025% or more. On the other hand, if the Ti content exceeds 0.200%, a large amount of carbonitrides will be generated in the steel, resulting in a decrease in the hydrogen embrittlement resistance of the hot stamped body. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably 0.180% or less, 0.150% or less, 0.100% or less, 0.080% or less, 0.060% or less, or 0.050% or less.
  • B is an element that improves the hardenability of steel. If the B content is less than 0.0005%, desired strength cannot be obtained. Therefore, the B content is set to 0.0005% or more.
  • the B content is preferably 0.0010% or more, 0.0015% or more, or 0.0020% or more.
  • the B content is set to 0.0200% or less.
  • the B content is preferably 0.0150% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0040% or less.
  • Cr is an element that increases the strength of the hot-stamped molded product by forming a solid solution in the prior austenite grains during heating before hot-stamping. If the Cr content is less than 0.010%, desired strength cannot be obtained. Therefore, the Cr content is set to 0.010% or more.
  • the Cr content is preferably 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more.
  • the Cr content is set to 0.80% or less.
  • the Cr content is preferably 0.70% or less, 0.60% or less, 0.50% or less, or 0.40% or less.
  • Mo is an element that improves the hardenability of steel. If the Mo content is less than 0.0010%, desired strength cannot be obtained. Therefore, the Mo content is set to 0.0010% or more. Mo content is preferably 0.005% or more, 0.010% or more, 0.050% or more, or 0.100% or more. On the other hand, if the Mo content exceeds 1.000%, coarse intermetallic compounds will be formed in the hot stamped body, and the hydrogen embrittlement resistance of the hot stamped body will deteriorate. Therefore, the Mo content is set to 1.000% or less. Mo content is preferably 0.800% or less, 0.600% or less, 0.500% or less, or 0.300% or less.
  • the hot stamping steel plate may contain at least one of the following optional elements in place of a portion of the remaining Fe, if necessary.
  • a steel plate for hot stamping includes Co: 0 to 2.00%, Ni: 0 to 3.00%, Cu: 0 to 1.00%, V: 0 to 1.00%, and W: 0 to 1.0%. It may contain at least one selected from the group consisting of 000%.
  • the hot stamping steel plate may contain at least one selected from the group consisting of Ca: 0 to 0.010%, Mg: 0 to 1.000%, and REM: 0 to 1.000%. .
  • the hot stamping steel plate may contain at least one selected from the group consisting of Sb: 0 to 1.000%, Zr: 0 to 1.000%, and Sn: 0 to 1.000%. . Further, the hot stamping steel plate may contain As: 0 to 0.100%.
  • Co is an element that improves the strength of the hot stamp molded product through solid solution strengthening.
  • the Co content may be 0.001% or more, but to ensure this effect, the Co content is preferably 0.01% or more or 0.05% or more.
  • the Co content is preferably 2.00% or less.
  • the Co content may be 1.80% or less, 1.50% or less, 1.00% or less, 0.80% or less, or 0.60% or less.
  • Ni has the effect of increasing the strength of the hot stamp molded product by solidly dissolving in the austenite grains during heating in the hot stamp molding process.
  • the Ni content may be 0.001% or more, but in order to ensure this effect, the Ni content is preferably 0.01% or more.
  • the Ni content is 3.00% or less.
  • the Ni content may be less than 3.00%, 2.80% or less, 2.50% or less, 2.00% or less, 1.50% or less, 1.00% or less, or 0.80% or less. .
  • Cu has the effect of increasing the strength of the hot stamp molded product by solidly dissolving in the austenite grains during heating in the hot stamp molding process.
  • the Cu content may be 0.001% or more, but to ensure this effect, the Cu content is preferably 0.01% or more or 0.05% or more.
  • the Cu content is 1.00% or less.
  • the Cu content may be 0.80% or less, 0.60% or less, 0.50% or less, or 0.30% or less.
  • V has the effect of forming carbonitrides in the steel and improving the strength of the hot stamped product through precipitation strengthening.
  • the V content may be 0.001% or more, but in order to ensure this effect, the V content is preferably 0.01% or more or 0.05% or more.
  • the V content is 1.00% or less.
  • the V content may be 0.80% or less, 0.60% or less, 0.50% or less, or 0.30% or less.
  • W is an element that improves the hardenability of steel.
  • the W content may be 0.001% or more, but to ensure this effect, the W content is preferably 0.005% or more or 0.010% or more.
  • the W content is 1.000% or less.
  • the W content may be 0.800% or less, 0.600% or less, 0.500% or less, or 0.300% or less.
  • Ca is an element that suppresses the formation of oxides.
  • the Ca content may be 0.0001% or more, but to ensure this effect, the Ca content is preferably 0.0005% or more or 0.001% or more.
  • the Ca content is preferably 0.010% or less.
  • the Ca content may be 0.008% or less, 0.006% or less, 0.004% or less, 0.003% or less, or 0.002% or less.
  • Mg forms oxides and sulfides in molten steel, suppresses the formation of coarse MnS, disperses many fine oxides, and contributes to refinement of the metal structure.
  • the Mg content may be 0.0001% or more, but in order to ensure these effects, the Mg content is preferably 0.0005% or more or 0.001% or more. On the other hand, since the above effect is saturated even if it is contained in a large amount, it is preferable that the Mg content is 1.000% or less.
  • the Mg content may be 0.500% or less, 0.100% or less, 0.050% or less, 0.010% or less, 0.005% or less, or 0.002% or less.
  • REM 0-1.000%
  • the REM content may be 0.0001% or more, but to ensure this effect, the REM content is preferably 0.0005% or more or 0.001% or more. On the other hand, since the above effect is saturated even if it is contained in a large amount, it is preferable that the REM content is 1.000% or less.
  • the REM content may be 0.500% or less, 0.100% or less, 0.050% or less, 0.010% or less, 0.005% or less or 0.002% or less.
  • REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids such as lanthanum (La) with atomic number 57 to lutetium with atomic number 71.
  • Sc scandium
  • Y yttrium
  • La lanthanoids
  • La lanthanum
  • Lu is a general term for 17 elements, and the REM content is the total content of these elements.
  • Sb is an element that suppresses the formation of oxides.
  • the Sb content is preferably 0.001% or more.
  • the Sb content is 1.000% or less.
  • the Sb content may be 0.800% or less, 0.500% or less, 0.200% or less, or 0.100% or less.
  • Zr is an element that suppresses the formation of oxides.
  • the Zr content is preferably 0.001% or more.
  • the Zr content is 1.000% or less.
  • the Zr content may be 0.800% or less, 0.500% or less, 0.200% or less, or 0.100% or less.
  • Sn is an element that suppresses the formation of oxides.
  • the Sn content is preferably 0.001% or more.
  • the Sn content is 1.000% or less.
  • the Sn content may be 0.800% or less, 0.500% or less, 0.200% or less, or 0.100% or less.
  • the As content is preferably 0.001% or more.
  • the As content is preferably 0.100% or less.
  • the As content may be 0.080% or less, 0.050% or less, 0.020% or less, or 0.010% or less.
  • the remainder other than the above elements consists of Fe and impurities.
  • Impurities are components that are mixed in during industrial production of hot stamping steel sheets due to various factors in the production process, including raw materials such as ores and scraps.
  • the chemical composition of the hot stamping steel sheet described above may be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Note that C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas melting-thermal conductivity method, and O may be measured using an inert gas melting-non-dispersive infrared absorption method. When a plating layer is provided on the surface of a steel plate for hot stamping, the chemical composition may be analyzed after removing the plating layer by mechanical grinding.
  • ICP-AES Inductively Coupled Plasma-Atomic Emission Spectrometry
  • the metal structure of the hot stamping steel sheet according to the embodiment of the present invention contains ferrite: 10% or more and pearlite: 10% or more, and the total of ferrite and pearlite is 80% or more in terms of area ratio.
  • Pearlite serves as a starting point for austenite grains during heating during hot stamping, and therefore needs to be present in the metal structure at an area ratio of 10% or more. Further, in this embodiment, by including pearlite in combination with ferrite, it is possible to uniformly disperse pearlite.
  • the area ratios of ferrite and pearlite may each be any value of 10% or more within the range where their total is 80% or more, for example, each independently 20% or more, 30% or more, 40%, 50% or more. % or more or 60% or more.
  • the area ratios of ferrite and pearlite may be, for example, independently 85% or less, 80% or less, or 70% or less.
  • the total area ratio of ferrite and pearlite may be 85% or more, 90% or more, or 95% or more.
  • the total area ratio of ferrite and pearlite may be 100%, for example, 99% or less or 98% or less.
  • the residual structure is not particularly limited, but may include at least one of bainite, martensite, retained austenite, and carbide.
  • the carbide is, for example, Fe carbide, and may be generated in small amounts at the interface between ferrite phases.
  • the area ratio of the residual tissue is 20% or less, and may be, for example, 17% or less, 15% or less, 12% or less, 10% or less, 8% or less, 5% or less, or 3% or less.
  • Dispersion index of pearlite 0.50 or more
  • the dispersion index of pearlite is calculated by calculating the number of A/B boundaries (boundary between ferrite phase and pearlite phase) in an electron microscope image of the metal structure of a hot stamping steel sheet mainly composed of ferrite and pearlite.
  • a high dispersion index of pearlite means that the ratio of A/B boundaries is high in a metal structure mainly composed of ferrite and pearlite, that is, there are many boundaries between ferrite and pearlite phases. Therefore, by controlling the dispersion index of pearlite to a high value, it is possible to further reduce the portion where pearlite exists in a connected manner, and it is possible to achieve a more uniform dispersion of pearlite.
  • prior austenite grains of 150 Hv or less are reduced in the final metal structure of the hot-stamped compact due to the uniform dispersion of pearlite. It becomes possible to achieve a standard deviation of the hardness distribution. As a result, it is possible to reduce variations in the hardness of prior austenite grains in the metallographic structure of the hot stamped compact, and in turn, it is possible to significantly suppress local increases in hardness, resulting in high tensile strength and For example, despite having a high tensile strength of 2200 MPa or more, it is possible to significantly improve hydrogen embrittlement resistance.
  • the dispersion index of pearlite is preferably higher, and may be, for example, 0.52 or more, 0.55 or more, 0.58 or more, 0.60 or more, 0.62 or more, or 0.65 or more. Although the upper limit is not particularly limited, the dispersion index of pearlite may be, for example, 0.80 or less, 0.75 or less, or 0.70 or less.
  • the dispersion index of pearlite is determined as follows. First, using a scanning electron microscope, an electron channeling contrast image was obtained in a cross section perpendicular to the surface in a range of 35 ⁇ m in the direction perpendicular to the thickness direction and 10 ⁇ m in the thickness direction centered on the 1/4 position of the sheet thickness. obtain. Specifically, for this measurement, an EBSD analysis device consisting of a thermal field emission scanning electron microscope and an EBSD detector may be used. An EBSD analysis device may be used. Next, in the obtained electron channeling contrast image, ten straight lines perpendicular to the plate thickness direction are drawn at intervals of 1 ⁇ m.
  • phase boundaries that intersect with these straight lines are defined as A/A boundary (boundary between ferrite phase and ferrite phase), B/B boundary (boundary between pearlite phase and pearlite phase), and A/B boundary (boundary between ferrite phase and pearlite phase). boundaries) and calculate the intersection of each boundary.
  • A/A boundary boundary between ferrite phase and ferrite phase
  • B/B boundary boundary between pearlite phase and pearlite phase
  • A/B boundary boundary between ferrite phase and pearlite phase. boundaries
  • the steel plate for hot stamping has a thickness of, for example, 0.1 to 4.0 mm, although it is not particularly limited.
  • the plate thickness may be 0.2 mm or more, 0.4 mm or more, 0.6 mm or more, 0.8 mm or more, or 1.0 mm or more.
  • the plate thickness may be 3.6 mm or less, 3.2 mm or less, 2.8 mm or less, 2.4 mm or less, or 2.0 mm or less.
  • the plate thickness may be, for example, 1.0 to 4.0 mm.
  • the steel plate for hot stamping is a cold-rolled steel plate
  • the plate thickness may be, for example, 0.1 to 2.0 mm.
  • the hot stamping steel plate according to the embodiment of the present invention may have a plating layer on the surface.
  • Plating layers include aluminum plating layer, aluminum-zinc plating layer, aluminum-silicon plating layer, hot-dip galvanizing layer, electrolytic galvanizing layer, alloyed hot-dip galvanizing layer, zinc-nickel plating layer, aluminum-magnesium-zinc system. Examples include a plating layer.
  • the present invention further provides a hot stamped molded article manufactured using the steel plate for hot stamping. Therefore, the hot stamp molded article according to the embodiment of the present invention will be described in more detail below.
  • the hot stamp molded body has a mass percentage of C: 0.40-0.70%, Si: 0.010-1.300%, Mn: 0.60-3.00%, P: 0.100% or less, S: 0.0100% or less, N: 0.0200% or less, O: 0.0200% or less, Al: 0.0010-0.5000%, Nb: 0.0010 to 0.100%, Ti: 0.010-0.200%, B: 0.0005-0.0200%, Cr: 0.010-0.80%, Mo: 0.0010-1.000%, Co: 0-2.00%, Ni: 0-3.00%, Cu: 0 to 1.00%, V: 0 to 1.00%, W: 0-1.000%, Ca: 0-0.010%, Mg: 0-1.000%, REM: 0-1.000%, Sb: 0 to 1.000%, Zr: 0 to 1.000%, Sn: 0-1.000%, Has a chemical composition consisting of As: 0 to 0.100%, and the balance: Fe and impurities, Contains at least
  • the metal structure of the hot-stamped compact contains at least 90% in total of at least one of martensite, bainite, and tempered martensite in terms of area percentage.
  • the residual structure is not particularly limited, but may consist of 10% or less of at least one of ferrite, retained austenite, and pearlite. Martensite, bainite, and tempered martensite are very hard structures. Therefore, by including at least one of martensite, bainite, and tempered martensite in a total area ratio of 90% or more in the hot stamped body, It becomes possible to achieve high tensile strength, specifically, a tensile strength of 2200 MPa or more.
  • the total area ratio of at least one of martensite, bainite, and tempered martensite is preferably 92% or more or 94% or more, and more preferably 95% or more or 97% or more.
  • the upper limit of the total area ratio of at least one of martensite, bainite, and tempered martensite is not particularly limited, and may be 100%.
  • Identification of the metal structure and calculation of area ratio in the hot-stamped molded body and the hot-stamped steel plate described above are performed as follows. First, a sample is cut out from an arbitrary position 50 mm or more away from the end face of the steel material (if the sample cannot be taken from this position, avoid the end part) so that the thickness cross section perpendicular to the surface can be observed. Although the size of the sample depends on the measuring device, it should be large enough to allow observation of about 10 mm in the direction perpendicular to the plate thickness direction.
  • an EBSD analysis device consisting of a JEOL JSM-7001F and a TSL DVC5 type detector may be used. Just use it.
  • the degree of vacuum in the EBSD analyzer may be 9.6 ⁇ 10 ⁇ 5 Pa or less
  • the acceleration voltage may be 15 kV
  • the irradiation current level may be 13.
  • the remaining area (the area where "Grain Average Misorientation" exceeds 0.5°) is defined as the total area ratio of martensite, tempered martensite, and bainite.
  • Carbides are determined to be carbides if they have a granular shape in areas with bright contrast in a secondary electron image taken using a scanning electron microscope in the same field of view as the EBSD observation, and the area ratio of the corresponding area is calculated. By this, the area ratio of carbide is obtained.
  • the area ratio of pearlite is calculated by subtracting the area ratio of retained austenite, the area ratio of bainite, tempered martensite, martensite, and ferrite, and the area ratio of carbide from 100%.
  • the standard deviation of the hardness distribution of prior austenite grains at the 1/4 plate thickness position of the hot-stamped molded body is 150 Hv or less. Large variations in the hardness of prior austenite grains may lead to a local increase in hardness and cause hydrogen embrittlement cracking. According to an embodiment of the present invention, the standard deviation of the hardness distribution of the prior austenite grains at the 1/4 plate thickness position of the hot-stamped molded body is controlled to be 150 Hv or less to reduce the variation in the hardness of the prior austenite grains.
  • the standard deviation is 140 Hv or less, 130 Hv or less, 120 Hv or less, or 110 Hv or less.
  • the standard deviation of the hardness distribution of prior austenite grains at the 1/4 plate thickness position of the hot stamp molded body may be, for example, 50 Hv or more, 60 Hv or more, or 80 Hv or more.
  • the hardness distribution of prior austenite grains at the 1/4 position of the plate thickness of the hot stamped body (not the hardness distribution of the entire metal structure at the 1/4 position of the plate thickness) It is important to reduce the variation in the hardness of prior austenite grains by controlling the standard deviation of the hardness distribution of prior austenite grains existing in the metal structure to 150Hv or less, and therefore the hardness of prior austenite grains itself There is no need to control it within a specific range. Therefore, the hardness of the prior austenite grains at the 1/4 plate thickness position of the hot-stamped molded body is not particularly limited, but may be, for example, 500 Hv or more and/or 1000 Hv or less.
  • the hardness of the prior austenite grains at the 1/4 plate thickness position of the hot-stamped compact refers to the average of all hardness measurements measured by the method for determining the standard deviation of the hardness distribution of the prior austenite grains described below. It is something.
  • the standard deviation of the hardness distribution of prior austenite grains is determined as follows. First, a sample is cut out so that a cross section perpendicular to the surface (thickness cross section) can be observed from an arbitrary position 50 mm or more away from the end surface of the hot stamp molded body. The size of the sample is such that it can be observed at a distance of 10 mm in the direction perpendicular to the thickness direction, although it depends on the measuring device. After polishing the cross section of the sample using #600 to #1500 silicon carbide paper, it is finished to a mirror surface using a liquid in which diamond powder with a particle size of 1 to 6 ⁇ m is dispersed in diluted liquid such as alcohol and pure water.
  • the standard deviation obtained based on the measured values of Vickers hardness for the extracted 20 or more different prior austenite grains is calculated as the hardness distribution of prior austenite grains at the 1/4 plate thickness position of the hot stamped compact.
  • an EBSD analysis device consisting of a thermal field emission scanning electron microscope and an EBSD detector may be used.
  • an EBSD analysis device consisting of a JEOL JSM-7001F and a TSL DVC5 type detector may be used. You can use At this time, the degree of vacuum in the EBSD analyzer may be 9.6 ⁇ 10 ⁇ 5 Pa or less, the acceleration voltage may be 15 kV, and the irradiation current level may be 13.
  • a crystal orientation map of prior austenite grains is created by the method described in Acta Materialia, 58 (2010), 6393-6403, and the prior austenite grains are identified based on the crystal orientation map.
  • the hot stamp molded article according to this embodiment may have a plating layer on the surface. Corrosion resistance can be improved by having a plating layer on the surface.
  • Plating layers include aluminum plating layer, aluminum-zinc plating layer, aluminum-silicon plating layer, hot-dip galvanizing layer, electrolytic galvanizing layer, alloyed hot-dip galvanizing layer, zinc-nickel plating layer, aluminum-magnesium-zinc system. Examples include a plating layer.
  • the hot-stamped molded article According to the hot-stamped molded article according to the embodiment of the present invention, it is possible to achieve excellent mechanical properties, for example, a tensile strength of 2200 MPa or more.
  • the tensile strength is preferably 2300 MPa or more, more preferably 2400 MPa or more, and most preferably 2500 MPa or more or 2600 MPa or more.
  • the upper limit is not particularly limited, for example, the tensile strength may be 3500 MPa or less, 3300 MPa or less, or 3000 MPa or less.
  • the tensile strength of the hot stamp molded product is measured by preparing a No. 5 test piece and conducting a tensile test in accordance with JIS Z 2241:2011.
  • the hot-stamped molded product according to the embodiment of the present invention has excellent hydrogen embrittlement resistance despite having a high tensile strength of, for example, 2200 MPa or more. It is very useful for use as other structural members and reinforcing members that require strength.
  • a preferred method for manufacturing a hot stamping steel plate and a hot stamping molded body according to an embodiment of the present invention will be described.
  • the following description is intended to illustrate a characteristic method for producing a hot stamping steel plate and a hot stamping molded article according to an embodiment of the present invention, and includes the hot stamping steel sheet and hot stamping molded article. is not intended to be limited to those manufactured by the manufacturing method described below.
  • the method for manufacturing a hot stamping steel plate according to an embodiment of the present invention includes: A process of hot rolling a slab having the chemical composition described above in connection with a steel plate for hot stamping, the process comprising heating said slab and then finishing rolling, the reduction rate of the final stage in said finishing rolling. is 40% or more (hot rolling process), A step of rapidly cooling the obtained hot rolled steel sheet within 1.0 seconds after finish rolling and then cooling it at an average cooling rate of 90° C./second or more (cooling step), and cooling the hot rolled steel sheet at 500 to 700° C. Winding process at temperature (winding process) It is characterized by including. Each step will be explained in detail below.
  • a slab having the chemical composition described above in connection with hot stamping steel sheets is heated.
  • the method for casting molten steel is not particularly limited, and it may be manufactured by a continuous casting method, an ingot casting method, or a thin slab casting method.
  • Heating before hot rolling is not particularly limited, but since the slab used contains a relatively large amount of alloying elements in order to obtain high-strength steel sheets, the slab is heated before hot rolling to form an alloy.
  • the heating temperature may be 1100° C. or higher for the purpose of dissolving the elements in the slab.
  • 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 rolling reduction ratio of the final stage in finish rolling is 45% or more or 50% or more.
  • the rolling reduction ratio in the final stage of finish rolling is not particularly limited.
  • pearlite can be sufficiently dispersed and arranged in the metallographic structure of hot-rolled steel sheets, especially by appropriately controlling the reduction rate in the final stage of finish rolling. Therefore, it is possible to reduce the variation in prior austenite grain size and suppress a local increase in hardness.
  • the morphology of such a metallographic structure is determined by the reduction rate in the final stage in finish rolling, the average cooling rate in the subsequent cooling process, and the coiling temperature in the coiling process. It is not particularly affected by subsequent annealing, etc. This is because if a hot-rolled steel plate is formed with a reduction ratio of 40% or more in the final stage of finish rolling, even if the hot-rolled steel plate is cold rolled and then annealed at a relatively high temperature, This is because after cooling, there is a high tendency for a metal structure in which carbides, grain boundaries, and retained austenite, which are the starting points of austenite, are dispersed to be generated.
  • the finish-rolled hot-rolled steel sheet is rapidly cooled within 1.0 seconds after finishing the finish rolling. Since ferrite is generally generated from grain boundaries of austenite grains, as austenite grains become larger, the number of grain boundaries that serve as starting points for ferrite decreases. In such a case, it becomes difficult to prevent pearlite from linking and to disperse it uniformly. Therefore, by rapidly cooling the hot rolled steel sheet immediately after finish rolling, specifically within 1.0 seconds, preferably within 0.8 seconds after finish rolling, the austenite grains can be reduced. Suppression of growth is extremely important for uniformly dispersing and producing pearlite in hot rolled steel sheets.
  • the average cooling rate and cooling time during rapid cooling are not particularly limited, but, for example, the average cooling rate is preferably 200 to 1000° C./second, and the cooling time is preferably 0.2 to 2.0 seconds.
  • the hot-rolled steel sheet after quenching is cooled at an average cooling rate of 90° C./second or more. If the cooling rate is slow, a large amount of bainite is generated or austenite remains as retained austenite, making it impossible to form a metal structure mainly composed of ferrite and pearlite. As a result, it becomes difficult to control the arrangement of ferrite and pearlite, and in particular it becomes difficult to achieve a uniform distribution of pearlite.
  • by cooling at an average cooling rate of 90°C/sec or more it is possible to form a metal structure mainly composed of ferrite and pearlite, and more specifically, the total area ratio of ferrite and pearlite is 80°C. % or more of metal structure can be formed.
  • the average cooling rate is preferably 95°C/sec or more. Although the upper limit is not particularly limited, the average cooling rate may be, for example, 200° C./second or less or 150° C./second or less.
  • the finish-rolled hot rolled steel sheet is wound up at a temperature of 500 to 700°C. If the winding temperature is high, grain growth may occur and uniform dispersion of pearlite may be inhibited. On the other hand, if the winding temperature is low, bainite and martensite are generated, making it impossible to form a metal structure mainly composed of ferrite and pearlite. On the other hand, by controlling the coiling temperature to 500 to 700°C, it is possible to suppress grain growth and prevent the ferrite from being connected and arranged in the hot-rolled steel sheet after rolling, thereby making the pearlite uniform. can be dispersed into Preferably, the winding temperature is 505-650°C or 550-650°C. Further, for the purpose of softening the hot rolled steel sheet, the coil after winding may be subjected to a softening heat treatment. The method of softening heat treatment is not particularly limited, and general conditions may be used.
  • pickling After the winding process and before the optional cold rolling process, pickling may be performed to remove oxide 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.
  • Cold rolling After the winding step, cold rolling may optionally be performed.
  • Cold rolling is not particularly limited and may be carried out under any appropriate conditions.
  • the reduction ratio of cold rolling may be 30 to 80%.
  • 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.
  • an optional annealing may be performed to adjust the metallographic structure and/or properties.
  • the heating temperature in the annealing step is not particularly limited, but may be, for example, 800° C. or lower.
  • a plating treatment may be performed on the surface of a hot rolled steel sheet or a cold rolled steel sheet.
  • the plating process may be hot-dip plating, alloyed hot-dip plating, electroplating, or the like.
  • the steel plate may be subjected to hot-dip galvanizing treatment, or alloying treatment may be performed after hot-dip galvanizing treatment.
  • Plating layers include aluminum plating layer, aluminum-zinc plating layer, aluminum-silicon plating layer, hot-dip galvanizing layer, electrolytic galvanizing layer, alloyed hot-dip galvanizing layer, zinc-nickel plating layer, aluminum-magnesium-zinc system. Examples include a plating layer. Specific conditions for the plating treatment and alloying treatment are not particularly limited, and may be any suitable conditions known to those skilled in the art.
  • the steel plate may be subjected to temper rolling, for example, after the annealing process or the plating process.
  • the manufacturing method is a step of hot stamping a hot stamping steel sheet obtained by the method for manufacturing a hot stamping steel sheet described above, the hot stamping steel sheet being heated at 800°C to 1000°C. It is characterized in that it includes a step of heating to a temperature range of 100 to 1000 and then holding for 60 to 600 seconds.
  • a hot stamping steel plate is hot stamped in a hot stamping process to produce a hot stamped body having a desired chemical composition and metal structure.
  • austenite is generated starting from pearlite that is uniformly dispersed in the metallographic structure of the steel sheet during heating during hot stamping, and through subsequent forming and cooling operations, a desired hard structure is formed and variations are eliminated.
  • a hot-stamped compact is produced having a metallographic structure with a reduced desired prior austenite grain size distribution and therefore reduced variation in the hardness of the prior austenite grains.
  • the steel plate for hot stamping is heated to a temperature range of 800°C to 1000°C and held in this temperature range for 60 to 600 seconds. is preferred. If the heating temperature is less than 800°C, austenitization will be insufficient, the desired area ratio of hard structure (at least one of martensite, bainite, and tempered martensite) cannot be obtained, and the tensile strength will deteriorate. There is. On the other hand, when the heating temperature exceeds 1000°C, austenite grains grow excessively, making it impossible to obtain the desired prior austenite grain size distribution, and as a result, it is difficult to obtain the desired prior austenite grain hardness distribution.
  • the holding time is less than 60 seconds, as in the case where the heating temperature is less than 800°C, austenitization will be insufficient, and the area ratio of the desired hard structure (at least one of martensite, bainite, and tempered martensite) will be reduced. However, the tensile strength may deteriorate. If the holding time exceeds 600 seconds, austenite grains will grow excessively, making it impossible to obtain the desired prior austenite grain size distribution and, as a result, failing to obtain the desired prior austenite grain hardness distribution. First, the hydrogen embrittlement resistance may deteriorate.
  • the heating atmosphere is not particularly limited and may be under normal conditions, such as the atmosphere, a gas combustion atmosphere with a controlled ratio of air and fuel, or a nitrogen atmosphere, and the dew point of these gases may be controlled. .
  • hot stamp molding is performed. After hot stamp molding, cooling may be performed to a temperature range of 250° C. or lower at an average cooling rate of 20° C./second or higher.
  • heating methods before hot stamping include furnace heating using an electric furnace, gas furnace, etc., flame heating, electrical heating, high frequency heating, induction heating, and the like.
  • a hot stamp molded article according to the present embodiment is obtained.
  • tempering treatment at 130 to 600°C or baking hardening treatment after painting may be performed.
  • a portion of the hot stamp molded body may be tempered by laser irradiation or the like to provide a partially softened region.
  • hot-stamped molded bodies according to embodiments of the present invention were manufactured under various conditions, and the tensile strength and hydrogen embrittlement resistance of the obtained hot-stamped molded bodies were investigated.
  • molten steel having the chemical composition shown in Table 1 was cast by a continuous casting method to produce a slab.
  • the remainder other than the components shown in Table 1 is Fe and impurities.
  • These slabs were heated to a temperature of 1100° C. or higher and rough rolled under predetermined conditions, and then finished rolled, cooled, and wound up under the conditions shown in Table 2.
  • the average cooling rate during quenching after finish rolling was within the range of 200 to 1000°C/sec, and the cooling time was within the range of 0.2 to 2.0 seconds.
  • the obtained hot rolled steel sheet was cold rolled at a predetermined rolling reduction of 30 to 80%.
  • some of the steel plates were annealed, plated, or temper rolled under predetermined conditions.
  • the heating atmosphere and heating method in the hot stamp molding process were a gas combustion atmosphere (air-fuel ratio 0.85) and furnace heating, unless otherwise specified. After hot stamp molding, some of the hot stamp molded bodies were subjected to tempering treatment or partial softening treatment.
  • the properties of the obtained hot-stamped molded product were measured and evaluated by the following methods.
  • the tensile strength of the hot-stamped molded product was obtained by preparing a No. 5 test piece from any position of the hot-stamped molded product in accordance with JIS Z 2241:2011 and conducting a tensile test. Note that the crosshead speed was 1 mm/min.
  • Table 3 shows the area ratio of ferrite and pearlite and the dispersion index of pearlite in the hot stamping steel sheet after the winding process.
  • the remaining structures other than ferrite and pearlite were bainite, martensite, retained austenite, and/or a trace amount of carbide.
  • Table 3 shows the area ratio of hard structures in the hot-stamped compact and the standard deviation of the hardness distribution of prior austenite grains (prior ⁇ grains) at the 1/4 position of the plate thickness.
  • the area ratio of hard structure means the sum of the area ratios of martensite, bainite, and tempered martensite. Further, the remaining structure other than the hard structure was ferrite, retained austenite, and/or pearlite.
  • Comparative Example 1 the tensile strength decreased because the C content was low. In Comparative Example 13, the strength became too high due to the high C content, and the hydrogen embrittlement resistance deteriorated. In Comparative Example 14, the tensile strength decreased because the Si content was low. In Comparative Example 25, the amount of ferrite increased in the hot stamping steel sheet due to the high Si content, the desired metal structure could not be obtained, and the pearlite dispersion index was less than 0.50. As a result, the standard deviation of the hardness distribution of prior austenite grains in the hot-stamped compact could not be controlled within a desired range, and the hydrogen embrittlement resistance deteriorated.
  • Comparative Example 26 because the Mn content was low, the dispersion index of pearlite in the hot stamping steel sheet and the standard deviation of the hardness distribution of prior austenite grains in the hot stamping compact could not be controlled within the desired range, Hydrogen embrittlement resistance decreased. It is considered that in Comparative Example 41, the transformation from austenite to pearlite was promoted too much in the hot rolled steel sheet due to the high Mn content. As a result, the dispersion index of pearlite in the hot-stamped steel sheet and the standard deviation of the hardness distribution of prior austenite grains in the hot-stamped molded product could not be controlled within desired ranges, and the hydrogen embrittlement resistance deteriorated.
  • Comparative Examples 50, 59, 68, 77, 78, and 90 the hydrogen embrittlement resistance deteriorated because the P, S, N, O, or Al content was not appropriate.
  • Comparative Examples 91, 103, 115, 127, and 139 the strength could not be sufficiently improved because the Nb, Ti, B, Cr, and Mo contents were low, and the tensile strength decreased.
  • Comparative Examples 102, 114, 126, 138, and 150 a large amount of carbonitrides or coarse intermetallic compounds were generated in the steel because the Nb, Ti, B, Cr, and Mo contents were high, respectively. As a result, the hydrogen embrittlement resistance decreased.
  • Comparative Example 259 it is considered that pearlite could not be uniformly dispersed in the hot-rolled steel sheet after rolling because the rolling reduction ratio in the final stage of finish rolling in the hot rolling process was low.
  • the dispersion index of pearlite in the hot-stamped steel sheet and the standard deviation of the hardness distribution of prior austenite grains in the hot-stamped molded product could not be controlled within desired ranges, and the hydrogen embrittlement resistance deteriorated.
  • Comparative Example 270 the grain growth of austenite grains could not be sufficiently suppressed because the time from the end of finish rolling to the start of quenching was long, so ferrite was arranged in a connected manner and pearlite could not be uniformly dispersed.
  • Comparative Example 276 the desired metal structure could not be formed in the hot stamping steel sheet because the winding temperature was low. As a result, the standard deviation of the hardness distribution of prior austenite grains in the hot-stamped compact could not be controlled within a desired range, and the hydrogen embrittlement resistance deteriorated.
  • Comparative Example 284 it is considered that the high winding temperature caused grain growth and inhibited the uniform dispersion of pearlite. As a result, the dispersion index of pearlite in the hot-stamped steel sheet and the standard deviation of the hardness distribution of prior austenite grains in the hot-stamped molded product could not be controlled within desired ranges, and the hydrogen embrittlement resistance deteriorated.
  • all of the hot stamping steel sheets and hot stamping molded bodies according to the invention examples have a predetermined chemical composition and metal structure, and the pearlite dispersion index in the hot stamping steel sheets is 0.50 or more.
  • the standard deviation of the hardness distribution of prior austenite grains in the hot-stamped molded product is 150Hv or less, hydrogen embrittlement is ensured despite having a high tensile strength of 2200MPa or more. was able to be suppressed.
  • the hardness of prior austenite grains at the 1/4 plate thickness position is controlled within the range of 500 to 1000 Hv. Ta.

Landscapes

  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)

Abstract

The present invention provides a steel sheet for hot stamping, the steel sheet having a specific chemical composition, while having a metal structure that contains, in area ratios, 10% or more of ferrite and 10% or more of pearlite, with the sum of ferrite and pearlite being 80% or more, and has a dispersion index of pearlite of 0.50 or more. The present invention also provides a hot stamp molded body which has a specific chemical composition, while having a metal structure that contains, in an area ratio, a total of 90% or more of at least one of martensite, bainite and tempered martensite, wherein the standard deviation of the hardness distribution of prior austenite grains at a position corresponding to 1/4 the sheet thickness is 150 Hv or less.

Description

ホットスタンプ用鋼板及びホットスタンプ成形体Steel plates for hot stamping and hot stamping molded bodies
　本発明は、ホットスタンプ用鋼板及びそれを用いて製造されるホットスタンプ成形体に関する。 The present invention relates to a hot stamping steel plate and a hot stamping molded body manufactured using the same.
　近年、自動車業界では、燃費向上の観点から車体の軽量化が求められている。車体の軽量化と衝突安全性を両立するためには、使用する鋼板の高強度化が有効な方法の一つであり、このような背景から高強度鋼板の開発が進められている。一方で、鋼板を高強度化すると成形性が低下するため、鋼板において強度と成形性の両立を図ることは一般に困難である。 In recent years, in the automobile industry, there has been a demand for lighter vehicle bodies from the perspective of improving fuel efficiency. Increasing the strength of the steel plates used is one effective way to achieve both weight reduction and collision safety for vehicle bodies, and it is against this background that the development of high-strength steel plates is progressing. On the other hand, increasing the strength of a steel plate reduces formability, so it is generally difficult to achieve both strength and formability in a steel plate.
　これに関連して、特許文献１では、所定の化学組成を有し、金属組織が、面積率でポリゴナルフェライトを４０．０％以上、６０．０％未満、ベイニティックフェライトを３０．０％以上、残留オーステナイトを１０．０％以上、２５．０％以下、マルテンサイトを１５．０％以下含有し、前記残留オーステナイトのうち、アスペクト比が２．０以下であり、長軸の長さが１．０μｍ以下かつ短軸の長さが１．０μｍ以下である残留オーステナイトの割合が８０．０％以上であり、前記ベイニティックフェライトのうち、アスペクト比が１．７以下であり、かつ、結晶方位差が１５°以上の粒界に囲まれた領域の結晶方位差の平均値が０．５°以上、３．０°未満であるベイニティックフェライトの割合が８０．０％以上であり、前記マルテンサイトと前記ベイニティックフェライトと前記残留オーステナイトとの連結性Ｄ値が０．７０以下である冷延鋼板が記載されている。また、特許文献１では、上記の構成によれば、自動車等の構造部材として好適な、引張強度が９８０ＭＰａ以上、０．２％耐力が６００ＭＰａ以上の、打ち抜き疲労特性、伸び及び穴拡げ性に優れた高強度冷延鋼板を提供することができると記載されている。 In relation to this, Patent Document 1 discloses that the metal structure has a predetermined chemical composition, is polygonal ferrite in an area ratio of 40.0% or more and less than 60.0%, and bainitic ferrite as 30.0% or more and less than 60.0%. % or more, retained austenite is 10.0% or more and 25.0% or less, and martensite is 15.0% or less, and the aspect ratio of the retained austenite is 2.0 or less, and the length of the major axis is is 1.0 μm or less and the short axis length is 1.0 μm or less, the proportion of retained austenite is 80.0% or more, the aspect ratio of the bainitic ferrite is 1.7 or less, and , the percentage of bainitic ferrite in which the average value of the crystal orientation difference in a region surrounded by grain boundaries with a crystal orientation difference of 15° or more is 0.5° or more and less than 3.0° is 80.0% or more. There is described a cold-rolled steel sheet in which the connectivity D value between the martensite, the bainitic ferrite, and the retained austenite is 0.70 or less. Further, Patent Document 1 discloses that the above configuration has excellent punching fatigue properties, elongation, and hole expandability, with a tensile strength of 980 MPa or more and a 0.2% yield strength of 600 MPa or more, suitable for structural members of automobiles, etc. It is stated that high-strength cold-rolled steel sheets can be provided.
国際公開第２０１６／１３６８１０号International Publication No. 2016/136810
　高強度鋼板のような成形が困難な材料をプレス成形する技術としてホットスタンプ（熱間プレス）が知られている。ホットスタンプは、成形に供される材料を加熱してから成形する熱間成形技術である。この技術では、材料を加熱してから成形するため、成形時には鋼材が軟質で良好な成形性を有する。したがって、高強度の鋼材であっても複雑な形状に精度よく成形することが可能であり、また、プレス金型によって成形と同時に焼き入れを行うため、成形後の鋼材は十分な強度を有することが知られている。 Hot stamping (hot pressing) is known as a technique for press forming materials that are difficult to form, such as high-strength steel plates. Hot stamping is a hot forming technique in which the material to be formed is heated and then formed. In this technique, the material is heated and then molded, so the steel material is soft and has good formability during molding. Therefore, even high-strength steel materials can be formed into complex shapes with high precision.Also, since the press mold is used to quench the material at the same time as forming, the steel material after forming has sufficient strength. It has been known.
　このような高強度を有するホットスタンプ成形体においては、水素脆化割れ（遅れ破壊などともいう）が問題となる場合がある。水素脆化割れとは、使用状況下において高い応力が作用している鋼部材が、環境から鋼中に侵入した水素に起因して突然破壊する現象である。一般に、水素脆化割れは、鋼材の強度が高くなるほど発生しやすくなることが知られている。一方で、自動車業界等では、鋼材のさらなる軽量化も求められており、このような軽量化を達成するためには、鋼材をこれまで以上に高強度化する必要が生じる。したがって、従来と同等又はそれ以上の高強度化を行った場合においても水素脆化の課題を解決し得る鋼材、より具体的にはホットスタンプ成形体に対して高いニーズがある。 In hot-stamped molded bodies having such high strength, hydrogen embrittlement cracking (also referred to as delayed fracture) may become a problem. 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. It is generally known that hydrogen embrittlement cracking occurs more easily as the strength of steel increases. On the other hand, in the automobile industry and the like, there is a demand for further weight reduction of steel materials, and in order to achieve such weight reduction, it is necessary to make steel materials higher in strength than ever before. Therefore, there is a strong need for steel materials that can solve the problem of hydrogen embrittlement even when the strength is increased to be equal to or higher than conventional steel materials, and more specifically, for hot-stamped molded products.
　そこで、本発明は、新規な構成により、高強度でかつ水素脆化を抑制可能なホットスタンプ成形体及びこのようなホットスタンプ成形体を製造するためのホットスタンプ用鋼板を提供することを目的とする。 SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a hot-stamped molded article with a novel configuration that has high strength and can suppress hydrogen embrittlement, and a hot-stamped steel plate for producing such a hot-stamped molded article. do.
　本発明者らは、上記目的を達成するために、ホットスタンプ成形前の鋼板及びホットスタンプ成形後のホットスタンプ成形体のそれぞれにおける金属組織に着目して検討を行った。その結果、本発明者らは、ホットスタンプ成形前の鋼板において、ホットスタンプ成形の加熱時にオーステナイト粒の起点となるパーライトを均一に分散させることにより、最終的に得られるホットスタンプ成形体において旧オーステナイト粒が均一化され、それによって当該ホットスタンプ成形体の金属組織における旧オーステナイト粒の硬さのばらつきを低減することができることを見出した。さらに、本発明者らは、ホットスタンプ成形体の金属組織における旧オーステナイト粒の硬さのばらつきを低減することで、局所的な硬さの上昇を抑制することができ、その結果として高い引張強さを有するにもかかわらず、耐水素脆化特性を顕著に向上させることができることを見出し、本発明を完成させた。 In order to achieve the above object, the present inventors conducted a study focusing on the metal structure of each of the steel plate before hot stamping and the hot stamped body after hot stamping. As a result, the present inventors discovered that by uniformly dispersing pearlite, which is the starting point of austenite grains, during heating during hot stamping in a steel sheet before hot stamping, prior austenite was formed in the final hot stamped product. It has been found that the grains are made uniform, thereby reducing variations in the hardness of prior austenite grains in the metallographic structure of the hot-stamped compact. Furthermore, the present inventors have found that by reducing the variation in hardness of prior austenite grains in the metallographic structure of a hot-stamped compact, it is possible to suppress local increases in hardness, resulting in high tensile strength. The present inventors have discovered that the hydrogen embrittlement resistance can be significantly improved despite the above characteristics, and have completed the present invention.
　上記目的を達成し得た本発明は下記のとおりである。
　（１）質量％で、
　Ｃ　：０．４０～０．７０％、
　Ｓｉ：０．０１０～１．３００％、
　Ｍｎ：０．６０～３．００％、
　Ｐ　：０．１００％以下、
　Ｓ　：０．０１００％以下、
　Ｎ　：０．０２００％以下、
　Ｏ　：０．０２００％以下、
　Ａｌ：０．００１０～０．５０００％、
　Ｎｂ：０．００１０～０．１００％、
　Ｔｉ：０．０１０～０．２００％、
　Ｂ　：０．０００５～０．０２００％、
　Ｃｒ：０．０１０～０．８０％、
　Ｍｏ：０．００１０～１．０００％、
　Ｃｏ：０～２．００％、
　Ｎｉ：０～３．００％、
　Ｃｕ：０～１．００％、
　Ｖ　：０～１．００％、
　Ｗ　：０～１．０００％、
　Ｃａ：０～０．０１０％、
　Ｍｇ：０～１．０００％、
　ＲＥＭ：０～１．０００％、
　Ｓｂ：０～１．０００％、
　Ｚｒ：０～１．０００％、
　Ｓｎ：０～１．０００％、
　Ａｓ：０～０．１００％、並びに
　残部：Ｆｅ及び不純物からなる化学組成を有し、
　面積率で、
　フェライト：１０％以上、及び
　パーライト：１０％以上
を含み、フェライトとパーライトの合計が８０％以上であり、
　パーライトの分散指標が０．５０以上である金属組織を有する、ホットスタンプ用鋼板。
　（２）前記化学組成が、質量％で、
　Ｃｏ：０．００１～２．００％、
　Ｎｉ：０．００１～３．００％、
　Ｃｕ：０．００１～１．００％、
　Ｖ　：０．００１～１．００％、
　Ｗ　：０．００１～１．０００％、
　Ｃａ：０．０００１～０．０１０％、
　Ｍｇ：０．０００１～１．０００％、
　ＲＥＭ：０．０００１～１．０００％、
　Ｓｂ：０．００１～１．０００％、
　Ｚｒ：０．００１～１．０００％、
　Ｓｎ：０．００１～１．０００％、及び
　Ａｓ：０．００１～０．１００％
からなる群から選択される１種又は２種以上を含む、上記（１）に記載のホットスタンプ用鋼板。
　（３）質量％で、
　Ｃ　：０．４０～０．７０％、
　Ｓｉ：０．０１０～１．３００％、
　Ｍｎ：０．６０～３．００％、
　Ｐ　：０．１００％以下、
　Ｓ　：０．０１００％以下、
　Ｎ　：０．０２００％以下、
　Ｏ　：０．０２００％以下、
　Ａｌ：０．００１０～０．５０００％、
　Ｎｂ：０．００１０～０．１００％、
　Ｔｉ：０．０１０～０．２００％、
　Ｂ　：０．０００５～０．０２００％、
　Ｃｒ：０．０１０～０．８０％、
　Ｍｏ：０．００１０～１．０００％、
　Ｃｏ：０～２．００％、
　Ｎｉ：０～３．００％、
　Ｃｕ：０～１．００％、
　Ｖ　：０～１．００％、
　Ｗ　：０～１．０００％、
　Ｃａ：０～０．０１０％、
　Ｍｇ：０～１．０００％、
　ＲＥＭ：０～１．０００％、
　Ｓｂ：０～１．０００％、
　Ｚｒ：０～１．０００％、
　Ｓｎ：０～１．０００％、
　Ａｓ：０～０．１００％、並びに
　残部：Ｆｅ及び不純物からなる化学組成を有し、
　面積率で、マルテンサイト、ベイナイト及び焼き戻しマルテンサイトの少なくとも１種：合計で９０％以上を含み、
　板厚１／４位置における旧オーステナイト粒の硬さ分布の標準偏差が１５０Ｈｖ以下である金属組織を有する、ホットスタンプ成形体。
　（４）前記化学組成が、質量％で、
　Ｃｏ：０．００１～２．００％、
　Ｎｉ：０．００１～３．００％、
　Ｃｕ：０．００１～１．００％、
　Ｖ　：０．００１～１．００％、
　Ｗ　：０．００１～１．０００％、
　Ｃａ：０．０００１～０．０１０％、
　Ｍｇ：０．０００１～１．０００％、
　ＲＥＭ：０．０００１～１．０００％、
　Ｓｂ：０．００１～１．０００％、
　Ｚｒ：０．００１～１．０００％、
　Ｓｎ：０．００１～１．０００％、及び
　Ａｓ：０．００１～０．１００％
からなる群から選択される１種又は２種以上を含む、上記（３）に記載のホットスタンプ成形体。 The present invention that achieves the above object is as follows.
(1) In mass%,
C: 0.40-0.70%,
Si: 0.010-1.300%,
Mn: 0.60-3.00%,
P: 0.100% or less,
S: 0.0100% or less,
N: 0.0200% or less,
O: 0.0200% or less,
Al: 0.0010-0.5000%,
Nb: 0.0010 to 0.100%,
Ti: 0.010-0.200%,
B: 0.0005-0.0200%,
Cr: 0.010-0.80%,
Mo: 0.0010-1.000%,
Co: 0-2.00%,
Ni: 0-3.00%,
Cu: 0 to 1.00%,
V: 0 to 1.00%,
W: 0-1.000%,
Ca: 0-0.010%,
Mg: 0-1.000%,
REM: 0-1.000%,
Sb: 0 to 1.000%,
Zr: 0 to 1.000%,
Sn: 0-1.000%,
Has a chemical composition consisting of As: 0 to 0.100%, and the balance: Fe and impurities,
In area ratio,
ferrite: 10% or more, and pearlite: 10% or more, the total of ferrite and pearlite is 80% or more,
A steel plate for hot stamping, which has a metal structure in which a pearlite dispersion index is 0.50 or more.
(2) the chemical composition is in mass%;
Co: 0.001 to 2.00%,
Ni: 0.001 to 3.00%,
Cu: 0.001 to 1.00%,
V: 0.001 to 1.00%,
W: 0.001-1.000%,
Ca: 0.0001-0.010%,
Mg: 0.0001-1.000%,
REM: 0.0001-1.000%,
Sb: 0.001 to 1.000%,
Zr: 0.001 to 1.000%,
Sn: 0.001 to 1.000%, and As: 0.001 to 0.100%
The steel plate for hot stamping according to (1) above, comprising one or more selected from the group consisting of:
(3) mass%,
C: 0.40-0.70%,
Si: 0.010-1.300%,
Mn: 0.60-3.00%,
P: 0.100% or less,
S: 0.0100% or less,
N: 0.0200% or less,
O: 0.0200% or less,
Al: 0.0010-0.5000%,
Nb: 0.0010 to 0.100%,
Ti: 0.010-0.200%,
B: 0.0005-0.0200%,
Cr: 0.010-0.80%,
Mo: 0.0010-1.000%,
Co: 0-2.00%,
Ni: 0-3.00%,
Cu: 0 to 1.00%,
V: 0 to 1.00%,
W: 0-1.000%,
Ca: 0-0.010%,
Mg: 0-1.000%,
REM: 0-1.000%,
Sb: 0 to 1.000%,
Zr: 0 to 1.000%,
Sn: 0-1.000%,
Has a chemical composition consisting of As: 0 to 0.100%, and the balance: Fe and impurities,
Contains at least one of martensite, bainite and tempered martensite: 90% or more in total in terms of area percentage,
A hot-stamped molded product having a metal structure in which the standard deviation of the hardness distribution of prior austenite grains at a position of 1/4 of the plate thickness is 150 Hv or less.
(4) the chemical composition is in mass%;
Co: 0.001 to 2.00%,
Ni: 0.001 to 3.00%,
Cu: 0.001 to 1.00%,
V: 0.001 to 1.00%,
W: 0.001-1.000%,
Ca: 0.0001-0.010%,
Mg: 0.0001-1.000%,
REM: 0.0001-1.000%,
Sb: 0.001 to 1.000%,
Zr: 0.001 to 1.000%,
Sn: 0.001 to 1.000%, and As: 0.001 to 0.100%
The hot-stamped molded article according to (3) above, comprising one or more selected from the group consisting of:
　本発明によれば、高強度でかつ水素脆化を抑制可能なホットスタンプ成形体及びこのようなホットスタンプ成形体を製造するためのホットスタンプ用鋼板を提供することができる。 According to the present invention, it is possible to provide a hot-stamped molded product that has high strength and can suppress hydrogen embrittlement, and a hot-stamped steel plate for producing such a hot-stamped molded product.
＜ホットスタンプ用鋼板＞
　本発明の実施形態に係るホットスタンプ用鋼板は、質量％で、
　Ｃ　：０．４０～０．７０％、
　Ｓｉ：０．０１０～１．３００％、
　Ｍｎ：０．６０～３．００％、
　Ｐ　：０．１００％以下、
　Ｓ　：０．０１００％以下、
　Ｎ　：０．０２００％以下、
　Ｏ　：０．０２００％以下、
　Ａｌ：０．００１０～０．５０００％、
　Ｎｂ：０．００１０～０．１００％、
　Ｔｉ：０．０１０～０．２００％、
　Ｂ　：０．０００５～０．０２００％、
　Ｃｒ：０．０１０～０．８０％、
　Ｍｏ：０．００１０～１．０００％、
　Ｃｏ：０～２．００％、
　Ｎｉ：０～３．００％、
　Ｃｕ：０～１．００％、
　Ｖ　：０～１．００％、
　Ｗ　：０～１．０００％、
　Ｃａ：０～０．０１０％、
　Ｍｇ：０～１．０００％、
　ＲＥＭ：０～１．０００％、
　Ｓｂ：０～１．０００％、
　Ｚｒ：０～１．０００％、
　Ｓｎ：０～１．０００％、
　Ａｓ：０～０．１００％、並びに
　残部：Ｆｅ及び不純物からなる化学組成を有し、
　面積率で、
　フェライト：１０％以上、及び
　パーライト：１０％以上
を含み、フェライトとパーライトの合計が８０％以上であり、
　パーライトの分散指標が０．５０以上である金属組織を有することを特徴としている。 <Steel plate for hot stamping>
The hot stamping steel plate according to the embodiment of the present invention has, in mass%,
C: 0.40-0.70%,
Si: 0.010-1.300%,
Mn: 0.60-3.00%,
P: 0.100% or less,
S: 0.0100% or less,
N: 0.0200% or less,
O: 0.0200% or less,
Al: 0.0010-0.5000%,
Nb: 0.0010 to 0.100%,
Ti: 0.010-0.200%,
B: 0.0005-0.0200%,
Cr: 0.010-0.80%,
Mo: 0.0010-1.000%,
Co: 0-2.00%,
Ni: 0-3.00%,
Cu: 0 to 1.00%,
V: 0 to 1.00%,
W: 0-1.000%,
Ca: 0-0.010%,
Mg: 0-1.000%,
REM: 0-1.000%,
Sb: 0 to 1.000%,
Zr: 0 to 1.000%,
Sn: 0-1.000%,
Has a chemical composition consisting of As: 0 to 0.100%, and the balance: Fe and impurities,
In area ratio,
ferrite: 10% or more, and pearlite: 10% or more, the total of ferrite and pearlite is 80% or more,
It is characterized by having a metal structure with a pearlite dispersion index of 0.50 or more.
　先に述べたとおり、水素脆化割れは、鋼材の強度が高くなるほど発生しやすくなることが知られている。そこで、本発明者らは、このような高強度鋼材において水素脆化割れの起点となり得る領域を低減又は抑制するという観点から、ホットスタンプ成形前の鋼板及びホットスタンプ成形後のホットスタンプ成形体のそれぞれにおける金属組織に着目して検討を行った。より具体的には、本発明者らは、まず、ホットスタンプ成形体の金属組織において旧オーステナイト粒径のばらつきが大きいと、旧オーステナイト粒径がより小さい領域において硬さが高くなり、このような局所的な高硬度領域が水素脆化割れの起点となり得ることを見出した。これに対し、本発明者らは、旧オーステナイト粒径のばらつきを低減して、その結果として旧オーステナイト粒における硬さのばらつきを低減すること、より具体的には旧オーステナイト粒の硬さ分布の標準偏差を１５０Ｈｖ以下に制御することで、このような局所的な硬さの上昇を確実に抑制することができることを見出した。 As mentioned above, it is known that hydrogen embrittlement cracking occurs more easily as the strength of the steel increases. Therefore, from the viewpoint of reducing or suppressing the area that can be the starting point of hydrogen embrittlement cracking in such high-strength steel materials, the present inventors have developed a method for reducing or suppressing the area of the steel sheet before hot stamping and the hot stamped body after hot stamping. A study was conducted focusing on the metal structure of each. More specifically, the present inventors first found that when the variation in prior austenite grain size is large in the metallographic structure of a hot-stamped compact, the hardness increases in the region where the prior austenite grain size is smaller; We found that localized high hardness regions can be the starting point for hydrogen embrittlement cracking. On the other hand, the present inventors aimed to reduce the variation in prior austenite grain size and, as a result, reduce the variation in hardness in prior austenite grains, and more specifically, to improve the hardness distribution of prior austenite grains. It has been found that such local increases in hardness can be reliably suppressed by controlling the standard deviation to 150 Hv or less.
　何ら特定の理論に束縛されることを意図するものではないが、ホットスタンプ成形の際、オーステナイト粒の粒径に応じてマルテンサイト変態の開始温度が変化するものと考えられる。より詳しく説明すると、より大きな粒径を有するオーステナイト粒は、より小さな粒径を有するオーステナイト粒と比べてマルテンサイト変態の開始温度が高いために硬さが低くなると考えられる。より小さな粒径を有するオーステナイト粒は、大きな粒よりも低い温度でマルテンサイト変態するため硬さが上昇する。したがって、このような局所的な硬さの上昇を抑制又は低減するためには、マルテンサイト変態前のオーステナイト粒径におけるばらつきを低減することが重要となる。言い換えると、マルテンサイト変態前のオーステナイト粒径におけるばらつきを低減することで、マルテンサイト変態後の旧オーステナイト粒径におけるばらつきを低減することができ、その結果としてホットスタンプ成形体の金属組織において旧オーステナイト粒の硬さのばらつきを低減することができると考えられる。このような理由から、ホットスタンプ成形体の金属組織における旧オーステナイト粒の硬さ分布の標準偏差を１５０Ｈｖ以下に制御して旧オーステナイト粒の硬さのばらつきを低減することにより、マルテンサイト変態のタイミングの相違に基づく局所的な硬さの上昇を顕著に抑制することが可能になるものと考えられる。局所的に硬度が高い領域があると、特に硬さの差がある旧オーステナイト粒の界面で水素脆化割れを引き起こす可能性が高いと考えられることから、ホットスタンプ成形体の金属組織において旧オーステナイト粒の硬さのばらつきを低減することは耐水素脆化特性を向上させる上で非常に有効である。 Although not intending to be bound by any particular theory, it is believed that during hot stamping, the starting temperature of martensitic transformation changes depending on the particle size of the austenite grains. More specifically, it is believed that austenite grains having a larger grain size have lower hardness due to a higher initiation temperature of martensitic transformation than austenite grains having a smaller grain size. Austenite grains with smaller grain sizes undergo martensitic transformation at lower temperatures than larger grains, resulting in increased hardness. Therefore, in order to suppress or reduce such a local increase in hardness, it is important to reduce variations in austenite grain size before martensitic transformation. In other words, by reducing the variation in the austenite grain size before martensitic transformation, it is possible to reduce the variation in the prior austenite grain size after martensitic transformation, and as a result, the prior austenite grain size in the metallographic structure of the hot stamped compact is reduced. It is thought that variations in grain hardness can be reduced. For these reasons, by controlling the standard deviation of the hardness distribution of prior austenite grains in the metallographic structure of the hot-stamped compact to 150 Hv or less and reducing the variation in hardness of prior austenite grains, the timing of martensitic transformation can be improved. It is considered that it becomes possible to significantly suppress the local increase in hardness due to the difference in hardness. It is thought that if there is a locally high hardness region, hydrogen embrittlement cracking is likely to occur, especially at the interface of prior austenite grains where there is a difference in hardness. Reducing the variation in grain hardness is very effective in improving hydrogen embrittlement resistance.
　これに関連して、ホットスタンプ用鋼板の製造方法について後で詳しく説明されるように、本発明者らは、ホットスタンプ成形前の鋼板、例えば熱延鋼板の金属組織に着目し、当該金属組織においてパーライトを均一に分散させることにより、ホットスタンプ成形体の最終的な金属組織において旧オーステナイト粒径のばらつきを低減することができ、これに関連して旧オーステナイト粒の硬さ分布の標準偏差を１５０Ｈｖ以下に制御することができることを見出した。鋼材の高強度化に伴い、鋼材の焼き入れ性を改善するために比較的多量のＭｎが添加される場合があるが、今回、本発明者らによる研究で、このような高Ｍｎ含有量（例えば０．６０質量％以上）では、パーライトが比較的生成しやすく、それゆえ低Ｍｎ含有量の場合と比較して熱延鋼板の金属組織において多く生成されるパーライトを均一に分散させることが非常に困難となること、その結果としてホットスタンプ成形後の金属組織において旧オーステナイト粒径のばらつきが大きくなることがわかった。しかしながら、本発明者らは、このような問題に対し、仕上げ圧延の最終段で比較的高い圧下を施し、さらにその後の冷却を適切に制御することにより熱延鋼板の金属組織においてパーライトの均一分散を可能にすることができ、その結果としてホットスタンプ成形体の最終的な金属組織において旧オーステナイト粒径のばらつきを低減し、それによって旧オーステナイト粒の硬さ分布のばらつきを顕著に低減することができることを見出した。より具体的には、本発明者らは、主としてフェライトとパーライトから構成されるホットスタンプ用鋼板の金属組織の電子顕微鏡像において、Ａ／Ｂ境界（フェライト相とパーライト相の境界）の数を、Ａ／Ａ境界（フェライト相とフェライト相の境界）の数、Ｂ／Ｂ境界（パーライト相とパーライト相の境界）の数、及びＡ／Ｂ境界（フェライト相とパーライト相の境界）の数の合計で除することにより得られるパーライトの分散指標を０．５以上に制御することが重要であること、そして当該パーライトの分散指標を０．５以上に制御することで、パーライトの均一分散に起因してホットスタンプ成形体の最終的な金属組織において１５０Ｈｖ以下の旧オーステナイト粒の硬さ分布の標準偏差を達成することができることを見出した。 In this regard, the present inventors focused on the metallographic structure of a steel sheet before hot stamping, for example, a hot rolled steel sheet, and the method of manufacturing a hot stamping steel sheet will be explained in detail later. By uniformly dispersing pearlite in the hot-stamped compact, it is possible to reduce the variation in the prior austenite grain size in the final metallographic structure of the hot-stamped compact, and in this regard, the standard deviation of the hardness distribution of the prior austenite grains can be reduced. It has been found that it can be controlled to 150 Hv or less. As the strength of steel materials increases, relatively large amounts of Mn are sometimes added to improve the hardenability of steel materials. For example, when the Mn content is 0.60% by mass or more, pearlite is relatively easily generated, and therefore it is very difficult to uniformly disperse pearlite, which is generated in large quantities in the metallographic structure of the hot rolled steel sheet, compared to the case where the Mn content is low. As a result, it was found that the variation in prior austenite grain size in the metal structure after hot stamping became large. However, the present inventors have solved this problem by applying a relatively high rolling reduction in the final stage of finish rolling and by appropriately controlling the subsequent cooling to achieve uniform dispersion of pearlite in the metallographic structure of hot rolled steel sheets. As a result, it is possible to reduce the variation in the prior austenite grain size in the final metallographic structure of the hot stamped compact, thereby significantly reducing the variation in the hardness distribution of the prior austenite grains. I found out what I can do. More specifically, the present inventors determined that the number of A/B boundaries (boundaries between ferrite phase and pearlite phase) in an electron microscope image of the metal structure of a hot stamping steel sheet mainly composed of ferrite and pearlite, Total number of A/A boundaries (boundaries between ferrite phase and ferrite phase), number of B/B boundaries (boundaries between pearlite phase and pearlite phase), and number of A/B boundaries (boundaries between ferrite phase and pearlite phase) It is important to control the dispersion index of pearlite obtained by dividing by 0.5 or more, and by controlling the dispersion index of pearlite to 0.5 or more, It has been found that it is possible to achieve a standard deviation of the hardness distribution of prior austenite grains of 150 Hv or less in the final metallographic structure of a hot-stamped compact.
　パーライトが所定の範囲内に均一に分散されたホットスタンプ用鋼板をホットスタンプ成形することで、得られるホットスタンプ成形体の金属組織において旧オーステナイト粒の硬さ分布のばらつきを所定の範囲内に制御することができるという事実は、今回、本発明者らによって初めて明らかにされたことである。加えて、本発明の実施形態に係るホットスタンプ成形体によれば、旧オーステナイト粒の硬さ分布のばらつきを所定の範囲内に制御することで、局所的な硬さの上昇を顕著に抑制することができるので、高い引張強さ、例えば２２００ＭＰａ以上の高い引張強さを有するにもかかわらず、耐水素脆化特性を顕著に向上させることが可能となる。 By hot-stamping a hot-stamping steel plate in which pearlite is uniformly dispersed within a predetermined range, variations in the hardness distribution of prior austenite grains in the metal structure of the resulting hot-stamped product can be controlled within a predetermined range. The fact that it is possible to do this was revealed for the first time by the present inventors. In addition, according to the hot-stamped molded article according to the embodiment of the present invention, by controlling the variation in hardness distribution of prior austenite grains within a predetermined range, local increases in hardness are significantly suppressed. Therefore, it is possible to significantly improve hydrogen embrittlement resistance despite having a high tensile strength, for example, 2200 MPa or more.
　以下、本発明の実施形態に係るホットスタンプ用鋼板についてより詳しく説明する。以下の説明において、各元素の含有量の単位である「％」は、特に断りがない限り「質量％」を意味するものである。また、本明細書において、数値範囲を示す「～」とは、特に断りがない場合、その前後に記載される数値を下限値及び上限値として含む意味で使用される。 Hereinafter, the hot stamping steel plate according to the embodiment of the present invention will be explained in more detail. In the following description, "%", which is the unit of content of each element, means "% by mass" unless otherwise specified. In addition, in this specification, "~" 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：０．４０～０．７０％］
　Ｃは、ホットスタンプ成形体の強度を向上させる元素である。Ｃ含有量が０．４０％未満では、ホットスタンプ成形体において所望の強度を得ることができない。そのため、Ｃ含有量は０．４０％以上とする。Ｃ含有量は、好ましくは０．４０％超、０．４２％以上、０．４３％以上、０．４４％以上、０．４５％以上又は０．４６％以上である。
　一方、Ｃ含有量が０．７０％超では、強度が高くなりすぎてしまい、優れた耐水素脆化特性を得ることができない場合がある。そのため、Ｃ含有量は０．７０％以下とする。好ましくは、Ｃ含有量は、０．６８％以下、０．６７％以下、０．６５％以下又は０．６０％以下である。 [C:0.40-0.70%]
C is an element that improves the strength of the hot stamp molded product. If the C content is less than 0.40%, the desired strength cannot be obtained in the hot-stamped molded product. Therefore, the C content is set to 0.40% or more. The C content is preferably more than 0.40%, 0.42% or more, 0.43% or more, 0.44% or more, 0.45% or more, or 0.46% or more.
On the other hand, if the C content exceeds 0.70%, the strength becomes too high and excellent hydrogen embrittlement resistance may not be obtained. Therefore, the C content is set to 0.70% or less. Preferably, the C content is 0.68% or less, 0.67% or less, 0.65% or less or 0.60% or less.
［Ｓｉ：０．０１０～１．３００％］
　Ｓｉは、固溶強化により、ホットスタンプ成形体の強度を向上する元素である。Ｓｉ含有量が０．０１０％未満では、所望の強度を得ることができない。そのため、Ｓｉ含有量は０．０１０％以上とする。Ｓｉ含有量は、好ましくは０．０５０％以上、０．１００％以上、０．２００％以上、０．２５０％超、０．２５５％以上、０．２６０％以上、０．２７０％以上、０．２８０％以上、０．３００％以上又は０．４００％以上である。
　一方、Ｓｉ含有量が１．３００％超では、ホットスタンプ用鋼板においてフェライト量が増加し、所望の金属組織を得ることができない場合がある。そのため、Ｓｉ含有量は１．３００％以下とする。Ｓｉ含有量は、好ましくは１．２００％以下、１．０００％以下、０．８００％以下、０．６００％以下又は０．５００％以下である。 [Si: 0.010-1.300%]
Si is an element that improves the strength of the hot stamp molded product through solid solution strengthening. If the Si content is less than 0.010%, desired strength cannot be obtained. Therefore, the Si content is set to 0.010% or more. The Si content is preferably 0.050% or more, 0.100% or more, 0.200% or more, more than 0.250%, 0.255% or more, 0.260% or more, 0.270% or more, 0 .280% or more, 0.300% or more, or 0.400% or more.
On the other hand, if the Si content exceeds 1.300%, the amount of ferrite increases in the steel sheet for hot stamping, and a desired metal structure may not be obtained. Therefore, the Si content is set to 1.300% or less. The Si content is preferably 1.200% or less, 1.000% or less, 0.800% or less, 0.600% or less, or 0.500% or less.
［Ｍｎ：０．６０～３．００％］
　Ｍｎは、ホットスタンプ成形体の製造過程で熱延鋼板においてオーステナイトからパーライトへの変態を促進させ、ホットスタンプ用鋼板におけるパーライトの分散指標、さらにはホットスタンプ成形体における旧オーステナイト粒の硬さ分布の制御に寄与する元素である。パーライトの分散指標及び旧オーステナイト粒の硬さ分布の標準偏差を所望の範囲とするために、Ｍｎ含有量は０．６０％以上とする。Ｍｎ含有量は、好ましくは０．６０％超、０．７０％以上、０．８０％以上、１．００％以上又は１．３０％以上である。
　一方、Ｍｎ含有量が３．００％超であると、熱延鋼板においてオーステナイトからパーライトへの変態が促進されすぎてしまい、パーライトの分散指標及び旧オーステナイト粒の硬さ分布の標準偏差を所望の範囲とすることができない。そのため、Ｍｎ含有量は３．００％以下とする。好ましくは、Ｍｎ含有量は２．９０％以下、２．７０％以下、２．５０％以下、２．３０％以下又は２．００％以下である。 [Mn: 0.60-3.00%]
Mn promotes the transformation from austenite to pearlite in hot-rolled steel sheets during the manufacturing process of hot-stamped compacts, and improves the dispersion index of pearlite in hot-stamped steel sheets and the hardness distribution of prior austenite grains in hot-stamped compacts. It is an element that contributes to control. In order to keep the dispersion index of pearlite and the standard deviation of the hardness distribution of prior austenite grains within desired ranges, the Mn content is set to 0.60% or more. The Mn content is preferably more than 0.60%, 0.70% or more, 0.80% or more, 1.00% or more, or 1.30% or more.
On the other hand, if the Mn content exceeds 3.00%, the transformation from austenite to pearlite in the hot rolled steel sheet will be promoted too much, and the dispersion index of pearlite and the standard deviation of the hardness distribution of prior austenite grains will be reduced to the desired level. It cannot be a range. Therefore, the Mn content is set to 3.00% or less. Preferably, the Mn content is 2.90% or less, 2.70% or less, 2.50% or less, 2.30% or less or 2.00% or less.
［Ｐ：０．１００％以下］
　Ｐは、不純物元素であり、粒界に偏析して耐水素脆化特性を劣化させる。そのため、Ｐ含有量は０．１００％以下とする。Ｐ含有量は、好ましくは０．０７０％以下、０．０５０％以下又は０．０１０％以下である。
　Ｐ含有量の下限は特に限定しないが、０．０００１％未満に低減すると、脱Ｐコストが大幅に上昇し、経済的に好ましくない。そのため、Ｐ含有量は０．０００１％以上としてもよい。 [P: 0.100% or less]
P is an impurity element that segregates at grain boundaries and deteriorates hydrogen embrittlement resistance. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.070% or less, 0.050% or less, or 0.010% or less.
The lower limit of the P content is not particularly limited, but if it is reduced to less than 0.0001%, the cost for removing P will increase significantly, which is economically unfavorable. Therefore, the P content may be set to 0.0001% or more.
［Ｓ：０．０１００％以下］
　Ｓは、不純物元素であり、鋼中に介在物を形成する。この介在物は耐水素脆化特性を劣化させるため、Ｓ含有量は０．０１００％以下とする。Ｓ含有量は、好ましくは０．００８０％以下、０．００５０％以下、０．００３０％以下又は０．００２０％以下である。
　Ｓ含有量の下限は特に限定しないが、０．０００１％未満に低減すると、脱Ｓコストが大幅に上昇し、経済的に好ましくない。そのため、Ｓ含有量は０．０００１％以上としてもよい。 [S: 0.0100% or less]
S is an impurity element and forms inclusions in steel. Since these inclusions deteriorate the hydrogen embrittlement resistance, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.
The lower limit of the S content is not particularly limited, but if it is reduced to less than 0.0001%, the cost for removing S will increase significantly, which is economically unfavorable. Therefore, the S content may be set to 0.0001% or more.
［Ｎ：０．０２００％以下］
　Ｎは、不純物元素であり、鋼中に窒化物を形成する。この窒化物は耐水素脆化特性を劣化させるため、Ｎ含有量は０．０２００％以下とする。Ｎ含有量は、好ましくは０．０１８０％以下、０．０１５０％以下、０．０１００％以下、０．００６０％以下又は０．００４０％以下である。
　Ｎ含有量の下限は特に限定しないが、０．０００１％未満に低減すると、脱Ｎコストが大幅に上昇し、経済的に好ましくない。そのため、Ｎ含有量は０．０００１％以上としてもよい。 [N: 0.0200% or less]
N is an impurity element and forms nitrides in steel. Since this nitride deteriorates hydrogen embrittlement resistance, the N content is set to 0.0200% or less. The N content is preferably 0.0180% or less, 0.0150% or less, 0.0100% or less, 0.0060% or less, or 0.0040% or less.
The lower limit of the N content is not particularly limited, but if it is reduced to less than 0.0001%, the cost for removing N will increase significantly, which is economically unfavorable. Therefore, the N content may be set to 0.0001% or more.
［Ｏ：０．０２００％以下］
　Ｏは、鋼中に多く含まれると粗大な酸化物を形成し、耐水素脆化特性を劣化させる。そのため、Ｏ含有量は０．０２００％以下とする。Ｏ含有量は、０．０１５０％以下、０．０１００％以下、０．００７０％以下又は０．００４０％以下とすることが好ましい。
　精錬コスト低減の観点から、Ｏ含有量は０．０００１％以上としてもよい。溶鋼の脱酸時に微細な酸化物を多数分散させるために、Ｏ含有量は０．０００５％以上としてもよい。 [O: 0.0200% or less]
When O is contained in a large amount in steel, it forms coarse oxides and deteriorates the hydrogen embrittlement resistance. Therefore, the O content is set to 0.0200% or less. The O content is preferably 0.0150% or less, 0.0100% or less, 0.0070% or less, or 0.0040% or less.
From the viewpoint of reducing refining costs, the O content may be 0.0001% or more. In order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be set to 0.0005% or more.
［Ａｌ：０．００１０～０．５０００％］
　Ａｌは、溶鋼を脱酸して鋼を健全化する作用を有する元素である。Ａｌ含有量が０．００１０％未満では、脱酸が十分に行われず、粗大な酸化物が生成して耐水素脆化特性を劣化させる。そのため、Ａｌ含有量は０．００１０％以上とする。Ａｌ含有量は、好ましくは０．００３０％以上、０．００５０％以上、０．０１００％以上又は０．０３００％以上である。
　一方、Ａｌ含有量が０．５０００％超であると、鋼中に粗大な酸化物が生成し、ホットスタンプ成形体の耐水素脆化特性が低下する。そのため、Ａｌ含有量は０．５０００％以下とする。Ａｌ含有量は、好ましくは０．４０００％以下、０．３０００％以下、０．２０００％以下、０．１５００％以下又は０．１０００％以下である。 [Al: 0.0010-0.5000%]
Al is an element that has the effect of deoxidizing molten steel and making the steel sound. When the Al content is less than 0.0010%, deoxidation is not performed sufficiently, and coarse oxides are generated, which deteriorates the hydrogen embrittlement resistance. Therefore, the Al content is set to 0.0010% or more. The Al content is preferably 0.0030% or more, 0.0050% or more, 0.0100% or more, or 0.0300% or more.
On the other hand, if the Al content exceeds 0.5000%, coarse oxides will be generated in the steel, and the hydrogen embrittlement resistance of the hot stamped product will deteriorate. Therefore, the Al content is set to 0.5000% or less. The Al content is preferably 0.4000% or less, 0.3000% or less, 0.2000% or less, 0.1500% or less, or 0.1000% or less.
［Ｎｂ：０．００１０～０．１００％］
　Ｎｂは、鋼中に炭窒化物を形成して、析出強化によりホットスタンプ成形体の強度を向上する元素である。Ｎｂ含有量が０．００１０％未満であると、所望の強度を得ることができない。そのため、Ｎｂ含有量は０．００１０％以上とする。Ｎｂ含有量は、好ましくは０．００５％以上、０．００９％以上又は０．０１５％以上である。
　一方、Ｎｂ含有量が０．１００％超であると、鋼中に多量に炭窒化物が生成してホットスタンプ成形体の耐水素脆化特性が低下する。そのため、Ｎｂ含有量は０．１００％以下とする。Ｎｂ含有量は、好ましくは０．０８０％以下、０．０６０％以下又は０．０５０％以下である。 [Nb: 0.0010 to 0.100%]
Nb is an element that forms carbonitrides in steel and improves the strength of hot stamped products through precipitation strengthening. If the Nb content is less than 0.0010%, desired strength cannot be obtained. Therefore, the Nb content is set to 0.0010% or more. The Nb content is preferably 0.005% or more, 0.009% or more, or 0.015% or more.
On the other hand, if the Nb content exceeds 0.100%, a large amount of carbonitrides will be generated in the steel, resulting in a decrease in the hydrogen embrittlement resistance of the hot stamped product. Therefore, the Nb content is set to 0.100% or less. The Nb content is preferably 0.080% or less, 0.060% or less, or 0.050% or less.
［Ｔｉ：０．０１０～０．２００％］
　Ｔｉは、鋼中に炭窒化物を形成して、析出強化によりホットスタンプ成形体の強度を向上する元素である。Ｔｉ含有量が０．０１０％未満であると、所望の強度を得ることができない。そのため、Ｔｉ含有量は０．０１０％以上とする。Ｔｉ含有量は、好ましくは０．０１５％以上、０．０２０％以上又は０．０２５％以上である。
　一方、Ｔｉ含有量が０．２００％超であると、鋼中に多量に炭窒化物が生成してホットスタンプ成形体の耐水素脆化特性が低下する。そのため、Ｔｉ含有量は０．２００％以下とする。Ｔｉ含有量は、好ましくは０．１８０％以下、０．１５０％以下、０．１００％以下、０．０８０％以下、０．０６０％以下又は０．０５０％以下である。 [Ti: 0.010 to 0.200%]
Ti is an element that forms carbonitrides in steel and improves the strength of hot stamped products through precipitation strengthening. If the Ti content is less than 0.010%, desired strength cannot be obtained. Therefore, the Ti content is set to 0.010% or more. The Ti content is preferably 0.015% or more, 0.020% or more, or 0.025% or more.
On the other hand, if the Ti content exceeds 0.200%, a large amount of carbonitrides will be generated in the steel, resulting in a decrease in the hydrogen embrittlement resistance of the hot stamped body. Therefore, the Ti content is set to 0.200% or less. The Ti content is preferably 0.180% or less, 0.150% or less, 0.100% or less, 0.080% or less, 0.060% or less, or 0.050% or less.
［Ｂ：０．０００５～０．０２００％］
　Ｂは、鋼の焼き入れ性を向上させる元素である。Ｂ含有量が０．０００５％未満であると、所望の強度を得ることができない。そのため、Ｂ含有量は０．０００５％以上とする。Ｂ含有量は、好ましくは０．００１０％以上、０．００１５％以上又は０．００２０％以上である。
　一方、Ｂ含有量が０．０２００％超であると、ホットスタンプ成形体において粗大な金属間化合物が形成され、ホットスタンプ成形体の耐水素脆化特性が低下する。そのため、Ｂ含有量は０．０２００％以下とする。Ｂ含有量は、好ましくは０．０１５０％以下、０．０１００％以下、０．００８０％以下、０．００６０％以下又は０．００４０％以下である。 [B:0.0005-0.0200%]
B is an element that improves the hardenability of steel. If the B content is less than 0.0005%, desired strength cannot be obtained. Therefore, the B content is set to 0.0005% or more. The B content is preferably 0.0010% or more, 0.0015% or more, or 0.0020% or more.
On the other hand, if the B content exceeds 0.0200%, coarse intermetallic compounds will be formed in the hot stamped body, and the hydrogen embrittlement resistance of the hot stamped body will deteriorate. Therefore, the B content is set to 0.0200% or less. The B content is preferably 0.0150% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0040% or less.
［Ｃｒ：０．０１０～０．８０％］
　Ｃｒは、ホットスタンプ前の加熱時に旧オーステナイト粒に固溶することで、ホットスタンプ成形体の強度を高める元素である。Ｃｒ含有量が０．０１０％未満であると、所望の強度を得ることができない。そのため、Ｃｒ含有量は０．０１０％以上とする。Ｃｒ含有量は、好ましくは０．０５％以上、０．１０％以上、０．１５％以上又は０．２０％以上である。
　一方、Ｃｒ含有量が０．８０％超であると、ホットスタンプ成形体において粗大な金属間化合物が形成され、ホットスタンプ成形体の耐水素脆化特性が劣化する。そのため、Ｃｒ含有量は０．８０％以下とする。Ｃｒ含有量は、好ましくは０．７０％以下、０．６０％以下、０．５０％以下又は０．４０％以下である。 [Cr:0.010-0.80%]
Cr is an element that increases the strength of the hot-stamped molded product by forming a solid solution in the prior austenite grains during heating before hot-stamping. If the Cr content is less than 0.010%, desired strength cannot be obtained. Therefore, the Cr content is set to 0.010% or more. The Cr content is preferably 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more.
On the other hand, if the Cr content exceeds 0.80%, coarse intermetallic compounds are formed in the hot-stamped body, and the hydrogen embrittlement resistance of the hot-stamped body deteriorates. Therefore, the Cr content is set to 0.80% or less. The Cr content is preferably 0.70% or less, 0.60% or less, 0.50% or less, or 0.40% or less.
［Ｍｏ：０．００１０～１．０００％］
　Ｍｏは、鋼の焼き入れ性を向上させる元素である。Ｍｏ含有量が０．００１０％未満であると、所望の強度を得ることができない。そのため、Ｍｏ含有量は０．００１０％以上とする。Ｍｏ含有量は、好ましくは０．００５％以上、０．０１０％以上、０．０５０％以上又は０．１００％以上である。
　一方、Ｍｏ含有量が１．０００％超であると、ホットスタンプ成形体において粗大な金属間化合物が形成され、ホットスタンプ成形体の耐水素脆化特性が劣化する。そのため、Ｍｏ含有量は１．０００％以下とする。Ｍｏ含有量は、好ましくは０．８００％以下、０．６００％以下、０．５００％以下又は０．３００％以下である。 [Mo: 0.0010-1.000%]
Mo is an element that improves the hardenability of steel. If the Mo content is less than 0.0010%, desired strength cannot be obtained. Therefore, the Mo content is set to 0.0010% or more. Mo content is preferably 0.005% or more, 0.010% or more, 0.050% or more, or 0.100% or more.
On the other hand, if the Mo content exceeds 1.000%, coarse intermetallic compounds will be formed in the hot stamped body, and the hydrogen embrittlement resistance of the hot stamped body will deteriorate. Therefore, the Mo content is set to 1.000% or less. Mo content is preferably 0.800% or less, 0.600% or less, 0.500% or less, or 0.300% or less.
　本発明の実施形態に係るホットスタンプ用鋼板の基本化学組成は上記のとおりである。さらに、当該ホットスタンプ用鋼板は、必要に応じて、残部のＦｅの一部に替えて以下の任意選択元素のうち少なくとも１種を含有してもよい。例えば、ホットスタンプ用鋼板は、Ｃｏ：０～２．００％、Ｎｉ：０～３．００％、Ｃｕ：０～１．００％、Ｖ：０～１．００％及びＷ：０～１．０００％からなる群より選択される少なくとも１種を含有してもよい。また、ホットスタンプ用鋼板は、Ｃａ：０～０．０１０％、Ｍｇ：０～１．０００％及びＲＥＭ：０～１．０００％からなる群より選択される少なくとも１種を含有してもよい。また、ホットスタンプ用鋼板は、Ｓｂ：０～１．０００％、Ｚｒ：０～１．０００％及びＳｎ：０～１．０００％からなる群より選択される少なくとも１種を含有してもよい。また、ホットスタンプ用鋼板は、Ａｓ：０～０．１００％を含有してもよい。以下、これらの任意選択元素について詳しく説明する。 The basic chemical composition of the hot stamping steel plate according to the embodiment of the present invention is as described above. Furthermore, the hot stamping steel plate may contain at least one of the following optional elements in place of a portion of the remaining Fe, if necessary. For example, a steel plate for hot stamping includes Co: 0 to 2.00%, Ni: 0 to 3.00%, Cu: 0 to 1.00%, V: 0 to 1.00%, and W: 0 to 1.0%. It may contain at least one selected from the group consisting of 000%. Further, the hot stamping steel plate may contain at least one selected from the group consisting of Ca: 0 to 0.010%, Mg: 0 to 1.000%, and REM: 0 to 1.000%. . Further, the hot stamping steel plate may contain at least one selected from the group consisting of Sb: 0 to 1.000%, Zr: 0 to 1.000%, and Sn: 0 to 1.000%. . Further, the hot stamping steel plate may contain As: 0 to 0.100%. These optional elements will be explained in detail below.
［Ｃｏ：０～２．００％］
　Ｃｏは、固溶強化により、ホットスタンプ成形体の強度を向上させる元素である。Ｃｏ含有量は０．００１％以上であってもよいが、この効果を確実に得る場合、Ｃｏ含有量は０．０１％以上又は０．０５％以上とすることが好ましい。
　一方、多量に含有させても上記効果は飽和するため、Ｃｏ含有量は２．００％以下とすることが好ましい。Ｃｏ含有量は１．８０％以下、１．５０％以下、１．００％以下、０．８０％以下又は０．６０％以下であってもよい。 [Co: 0-2.00%]
Co is an element that improves the strength of the hot stamp molded product through solid solution strengthening. The Co content may be 0.001% or more, but to ensure this effect, the Co content is preferably 0.01% or more or 0.05% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, the Co content is preferably 2.00% or less. The Co content may be 1.80% or less, 1.50% or less, 1.00% or less, 0.80% or less, or 0.60% or less.
［Ｎｉ：０～３．００％］
　Ｎｉは、ホットスタンプ成形工程における加熱時にオーステナイト粒に固溶することで、ホットスタンプ成形体の強度を高める作用を有する。Ｎｉ含有量は０．００１％以上であってもよいが、この効果を確実に得る場合、Ｎｉ含有量は０．０１％以上とすることが好ましい。
　一方、多量に含有させても上記効果は飽和するため、Ｎｉ含有量は３．００％以下とすることが好ましい。Ｎｉ含有量は３．００％未満、２．８０％以下、２．５０％以下、２．００％以下、１．５０％以下、１．００％以下又は０．８０％以下であってもよい。 [Ni: 0-3.00%]
Ni has the effect of increasing the strength of the hot stamp molded product by solidly dissolving in the austenite grains during heating in the hot stamp molding process. The Ni content may be 0.001% or more, but in order to ensure this effect, the Ni content is preferably 0.01% or more.
On the other hand, since the above effect is saturated even if Ni is contained in a large amount, it is preferable that the Ni content is 3.00% or less. The Ni content may be less than 3.00%, 2.80% or less, 2.50% or less, 2.00% or less, 1.50% or less, 1.00% or less, or 0.80% or less. .
［Ｃｕ：０～１．００％］
　Ｃｕは、ホットスタンプ成形工程における加熱時にオーステナイト粒に固溶することで、ホットスタンプ成形体の強度を高める作用を有する。Ｃｕ含有量は０．００１％以上であってもよいが、この効果を確実に得る場合、Ｃｕ含有量は０．０１％以上又は０．０５％以上とすることが好ましい。
　一方、多量に含有させても上記効果は飽和するため、Ｃｕ含有量は１．００％以下とすることが好ましい。Ｃｕ含有量は０．８０％以下、０．６０％以下、０．５０％以下又は０．３０％以下であってもよい。 [Cu: 0-1.00%]
Cu has the effect of increasing the strength of the hot stamp molded product by solidly dissolving in the austenite grains during heating in the hot stamp molding process. The Cu content may be 0.001% or more, but to ensure this effect, the Cu content is preferably 0.01% or more or 0.05% or more.
On the other hand, since the above effect is saturated even if Cu is contained in a large amount, it is preferable that the Cu content is 1.00% or less. The Cu content may be 0.80% or less, 0.60% or less, 0.50% or less, or 0.30% or less.
［Ｖ：０～１．００％］
　Ｖは、鋼中に炭窒化物を形成して、析出強化によりホットスタンプ成形体の強度を向上する効果を有する。Ｖ含有量は０．００１％以上であってもよいが、この効果を確実に得る場合、Ｖ含有量は０．０１％以上又は０．０５％以上とすることが好ましい。
　一方、多量に含有させても上記効果は飽和するため、Ｖ含有量は１．００％以下とすることが好ましい。Ｖ含有量は０．８０％以下、０．６０％以下、０．５０％以下又は０．３０％以下であってもよい。 [V: 0-1.00%]
V has the effect of forming carbonitrides in the steel and improving the strength of the hot stamped product through precipitation strengthening. The V content may be 0.001% or more, but in order to ensure this effect, the V content is preferably 0.01% or more or 0.05% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, it is preferable that the V content is 1.00% or less. The V content may be 0.80% or less, 0.60% or less, 0.50% or less, or 0.30% or less.
［Ｗ：０～１．０００％］
　Ｗは、鋼の焼き入れ性を向上させる元素である。Ｗ含有量は０．００１％以上であってもよいが、この効果を確実に得る場合、Ｗ含有量は０．００５％以上又は０．０１０％以上とすることが好ましい。
　一方、多量に含有させても上記効果は飽和するため、Ｗ含有量は１．０００％以下とすることが好ましい。Ｗ含有量は、０．８００％以下、０．６００％以下、０．５００％以下又は０．３００％以下であってもよい。 [W: 0-1.000%]
W is an element that improves the hardenability of steel. The W content may be 0.001% or more, but to ensure this effect, the W content is preferably 0.005% or more or 0.010% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, it is preferable that the W content is 1.000% or less. The W content may be 0.800% or less, 0.600% or less, 0.500% or less, or 0.300% or less.
［Ｃａ：０～０．０１０％］
　Ｃａは、酸化物の生成を抑制する元素である。Ｃａ含有量は０．０００１％以上であってもよいが、この効果を確実に得る場合、Ｃａ含有量は０．０００５％以上又は０．００１％以上とすることが好ましい。
　一方、多量に含有させても上記効果は飽和するため、Ｃａ含有量は０．０１０％以下とすることが好ましい。Ｃａ含有量は０．００８％以下、０．００６％以下、０．００４％以下、０．００３％以下又は０．００２％以下であってもよい。 [Ca: 0-0.010%]
Ca is an element that suppresses the formation of oxides. The Ca content may be 0.0001% or more, but to ensure this effect, the Ca content is preferably 0.0005% or more or 0.001% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, the Ca content is preferably 0.010% or less. The Ca content may be 0.008% or less, 0.006% or less, 0.004% or less, 0.003% or less, or 0.002% or less.
［Ｍｇ：０～１．０００％］
　Ｍｇは、溶鋼中に酸化物や硫化物を形成して、粗大なＭｎＳの形成を抑制し、微細な酸化物を多数分散させ、金属組織の微細化に寄与する。Ｍｇ含有量は０．０００１％以上であってもよいが、これらの効果を確実に得る場合、Ｍｇ含有量は０．０００５％以上又は０．００１％以上とすることが好ましい。
　一方、多量に含有させても上記効果は飽和するため、Ｍｇ含有量は１．０００％以下とすることが好ましい。Ｍｇ含有量は０．５００％以下、０．１００％以下、０．０５０％以下、０．０１０％以下、０．００５％以下又は０．００２％以下であってもよい。 [Mg: 0-1.000%]
Mg forms oxides and sulfides in molten steel, suppresses the formation of coarse MnS, disperses many fine oxides, and contributes to refinement of the metal structure. The Mg content may be 0.0001% or more, but in order to ensure these effects, the Mg content is preferably 0.0005% or more or 0.001% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, it is preferable that the Mg content is 1.000% or less. The Mg content may be 0.500% or less, 0.100% or less, 0.050% or less, 0.010% or less, 0.005% or less, or 0.002% or less.
［ＲＥＭ：０～１．０００％］
　ＲＥＭは、酸化物の生成を抑制する元素である。ＲＥＭ含有量は０．０００１％以上であってもよいが、この効果を確実に得る場合、ＲＥＭ含有量は０．０００５％以上又は０．００１％以上とすることが好ましい。
　一方、多量に含有させても上記効果は飽和するため、ＲＥＭ含有量は１．０００％以下とすることが好ましい。ＲＥＭ含有量は０．５００％以下、０．１００％以下、０．０５０％以下、０．０１０％以下、０．００５％以下又は０．００２％以下であってもよい。
　なお、本実施形態においてＲＥＭとは、原子番号２１番のスカンジウム（Ｓｃ）、原子番号３９番のイットリウム（Ｙ）、及びランタノイドである原子番号５７番のランタン（Ｌａ）～原子番号７１番のルテチウム（Ｌｕ）の１７元素の総称であり、ＲＥＭ含有量はこれら元素の合計含有量である。 [REM: 0-1.000%]
REM is an element that suppresses the formation of oxides. The REM content may be 0.0001% or more, but to ensure this effect, the REM content is preferably 0.0005% or more or 0.001% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, it is preferable that the REM content is 1.000% or less. The REM content may be 0.500% or less, 0.100% or less, 0.050% or less, 0.010% or less, 0.005% or less or 0.002% or less.
In this embodiment, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanoids such as lanthanum (La) with atomic number 57 to lutetium with atomic number 71. (Lu) is a general term for 17 elements, and the REM content is the total content of these elements.
［Ｓｂ：０～１．０００％］
　Ｓｂは、酸化物の生成を抑制する元素である。この効果を確実に得る場合、Ｓｂ含有量は０．００１％以上とすることが好ましい。
　一方、多量に含有させても上記効果は飽和するため、Ｓｂ含有量は１．０００％以下とすることが好ましい。Ｓｂ含有量は０．８００％以下、０．５００％以下、０．２００％以下又は０．１００％以下であってもよい。 [Sb: 0 to 1.000%]
Sb is an element that suppresses the formation of oxides. To ensure this effect, the Sb content is preferably 0.001% or more.
On the other hand, since the above effects are saturated even if Sb is contained in a large amount, it is preferable that the Sb content is 1.000% or less. The Sb content may be 0.800% or less, 0.500% or less, 0.200% or less, or 0.100% or less.
［Ｚｒ：０～１．０００％］
　Ｚｒは、酸化物の生成を抑制する元素である。この効果を確実に得る場合、Ｚｒ含有量は０．００１％以上とすることが好ましい。
　一方、多量に含有させても上記効果は飽和するため、Ｚｒ含有量は１．０００％以下とすることが好ましい。Ｚｒ含有量は０．８００％以下、０．５００％以下、０．２００％以下又は０．１００％以下であってもよい。 [Zr: 0 to 1.000%]
Zr is an element that suppresses the formation of oxides. To ensure this effect, the Zr content is preferably 0.001% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, it is preferable that the Zr content is 1.000% or less. The Zr content may be 0.800% or less, 0.500% or less, 0.200% or less, or 0.100% or less.
［Ｓｎ：０～１．０００％］
　Ｓｎは、酸化物の生成を抑制する元素である。この効果を確実に得る場合、Ｓｎ含有量は０．００１％以上とすることが好ましい。
　一方、多量に含有させても上記効果は飽和するため、Ｓｎ含有量は１．０００％以下とすることが好ましい。Ｓｎ含有量は０．８００％以下、０．５００％以下、０．２００％以下又は０．１００％以下であってもよい。 [Sn: 0 to 1.000%]
Sn is an element that suppresses the formation of oxides. To ensure this effect, the Sn content is preferably 0.001% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, it is preferable that the Sn content is 1.000% or less. The Sn content may be 0.800% or less, 0.500% or less, 0.200% or less, or 0.100% or less.
［Ａｓ：０～０．１００％］
　Ａｓは、オーステナイト単相化温度を低下させることにより、旧オーステナイト粒の細粒化に寄与する。この効果を確実に得る場合、Ａｓ含有量は０．００１％以上とすることが好ましい。
　一方、多量に含有させても上記効果は飽和するため、Ａｓ含有量は０．１００％以下とすることが好ましい。Ａｓ含有量は０．０８０％以下、０．０５０％以下、０．０２０％以下又は０．０１０％以下であってもよい。 [As: 0 to 0.100%]
As contributes to the refinement of prior austenite grains by lowering the austenite single-phase temperature. To ensure this effect, the As content is preferably 0.001% or more.
On the other hand, since the above effect is saturated even if it is contained in a large amount, the As content is preferably 0.100% or less. The As content may be 0.080% or less, 0.050% or less, 0.020% or less, or 0.010% or less.
　本発明の実施形態に係るホットスタンプ用鋼板において、上記の元素以外の残部は、Ｆｅ及び不純物からなる。不純物とは、ホットスタンプ用鋼板を工業的に製造する際に、鉱石やスクラップ等のような原料を始めとして、製造工程の種々の要因によって混入する成分等である。 In the hot stamping steel sheet according to the embodiment of the present invention, the remainder other than the above elements consists of Fe and impurities. Impurities are components that are mixed in during industrial production of hot stamping steel sheets due to various factors in the production process, including raw materials such as ores and scraps.
　上述したホットスタンプ用鋼板の化学組成は、一般的な分析方法によって測定すればよい。例えば、ＩＣＰ－ＡＥＳ（Ｉｎｄｕｃｔｉｖｅｌｙ　Ｃｏｕｐｌｅｄ　Ｐｌａｓｍａ－Ａｔｏｍｉｃ　Ｅｍｉｓｓｉｏｎ　Ｓｐｅｃｔｒｏｍｅｔｒｙ）を用いて測定すればよい。なお、Ｃ及びＳは燃焼－赤外線吸収法を用い、Ｎは不活性ガス融解－熱伝導度法を用い、Ｏは不活性ガス融解－非分散型赤外線吸収法を用いて測定すればよい。
　ホットスタンプ用鋼板の表面にめっき層を備える場合は、機械研削によりめっき層を除去してから化学組成の分析を行えばよい。 The chemical composition of the hot stamping steel sheet described above may be measured by a general analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Note that C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas melting-thermal conductivity method, and O may be measured using an inert gas melting-non-dispersive infrared absorption method.
When a plating layer is provided on the surface of a steel plate for hot stamping, the chemical composition may be analyzed after removing the plating layer by mechanical grinding.
［フェライト：１０％以上、パーライト：１０％以上、フェライトとパーライトの合計：８０％以上］
　本発明の実施形態に係るホットスタンプ用鋼板の金属組織は、面積率で、フェライト：１０％以上及びパーライト：１０％以上を含み、フェライトとパーライトの合計が８０％以上である。パーライトは、ホットスタンプ成形の加熱時にオーステナイト粒の起点となるため、１０％以上の面積率で金属組織中に存在させる必要がある。また、本実施形態では、パーライトをフェライトと組み合わせて含むことでパーライトの均一分散を可能としている。フェライト及びパーライトの面積率は、それらの合計が８０％以上となる範囲において、それぞれ１０％以上の任意の値であってよく、例えばそれぞれ独立して２０％以上、３０％以上、４０％、５０％以上又は６０％以上であってもよい。上限は特に限定されないが、フェライト及びパーライトの面積率は、例えばそれぞれ独立して８５％以下、８０％以下又は７０％以下であってもよい。フェライトとパーライトの面積率の合計は、８５％以上、９０％以上又は９５％以上であってもよい。上限は特に限定されないが、フェライトとパーライトの面積率の合計は１００％であってもよく、例えば９９％以下又は９８％以下であってもよい。残部組織は、特に限定されないが、ベイナイト、マルテンサイト、残留オーステナイト、及び炭化物の少なくとも１種からなっていてもよい。炭化物は、例えばＦｅ炭化物であり、フェライト相とフェライト相の界面に微量生成される場合がある。残部組織の面積率は２０％以下であり、例えば、１７％以下、１５％以下、１２％以下、１０％以下、８％以下、５％以下又は３％以下であってよい。 [Ferrite: 10% or more, Pearlite: 10% or more, Total of ferrite and pearlite: 80% or more]
The metal structure of the hot stamping steel sheet according to the embodiment of the present invention contains ferrite: 10% or more and pearlite: 10% or more, and the total of ferrite and pearlite is 80% or more in terms of area ratio. Pearlite serves as a starting point for austenite grains during heating during hot stamping, and therefore needs to be present in the metal structure at an area ratio of 10% or more. Further, in this embodiment, by including pearlite in combination with ferrite, it is possible to uniformly disperse pearlite. The area ratios of ferrite and pearlite may each be any value of 10% or more within the range where their total is 80% or more, for example, each independently 20% or more, 30% or more, 40%, 50% or more. % or more or 60% or more. Although the upper limit is not particularly limited, the area ratios of ferrite and pearlite may be, for example, independently 85% or less, 80% or less, or 70% or less. The total area ratio of ferrite and pearlite may be 85% or more, 90% or more, or 95% or more. Although the upper limit is not particularly limited, the total area ratio of ferrite and pearlite may be 100%, for example, 99% or less or 98% or less. The residual structure is not particularly limited, but may include at least one of bainite, martensite, retained austenite, and carbide. The carbide is, for example, Fe carbide, and may be generated in small amounts at the interface between ferrite phases. The area ratio of the residual tissue is 20% or less, and may be, for example, 17% or less, 15% or less, 12% or less, 10% or less, 8% or less, 5% or less, or 3% or less.
［パーライトの分散指標：０．５０以上］
　本発明の実施形態に係るホットスタンプ用鋼板の金属組織では、パーライトが均一に分散されていることが必要であり、本実施形態では、このようなパーライトの均一分散をパーライトの分散指標が０．５０以上となるように制御することで達成している。パーライトの分散指標は、主としてフェライトとパーライトから構成されるホットスタンプ用鋼板の金属組織の電子顕微鏡像において、Ａ／Ｂ境界（フェライト相とパーライト相の境界）の数を、Ａ／Ａ境界（フェライト相とフェライト相の境界）の数、Ｂ／Ｂ境界（パーライト相とパーライト相の境界）の数、及びＡ／Ｂ境界（フェライト相とパーライト相の境界）の数の合計で除することにより得られる。パーライトの分散指標が高いことは、主としてフェライトとパーライトから構成される金属組織においてＡ／Ｂ境界の割合が高いこと、すなわちフェライト相とパーライト相の境界の数が多いことを意味する。したがって、パーライトの分散指標を高い値に制御することで、パーライトが連結して存在する部分をより少なくすることができ、より均一なパーライトの分散を達成することができる。本実施形態によれば、パーライトの分散指標を０．５０以上に制御することで、このようなパーライトの均一分散に起因してホットスタンプ成形体の最終的な金属組織において１５０Ｈｖ以下の旧オーステナイト粒の硬さ分布の標準偏差を達成することが可能となる。その結果として、ホットスタンプ成形体の金属組織における旧オーステナイト粒の硬さのばらつきを低減することができ、ひいては局所的な硬さの上昇を顕著に抑制することができるので、高い引張強さ、例えば２２００ＭＰａ以上の高い引張強さを有するにもかかわらず、耐水素脆化特性を顕著に向上させることが可能となる。パーライトの分散指標は高いほど好ましく、例えば０．５２以上、０．５５以上、０．５８以上、０．６０以上、０．６２以上又は０．６５以上であってもよい。上限は特に限定されないが、パーライトの分散指標は、例えば０．８０以下、０．７５以下又は０．７０以下であってもよい。 [Dispersion index of pearlite: 0.50 or more]
In the metallographic structure of the steel sheet for hot stamping according to the embodiment of the present invention, it is necessary that pearlite is uniformly dispersed, and in this embodiment, such uniform dispersion of pearlite is achieved by setting the pearlite dispersion index to 0. This is achieved by controlling the number to be 50 or more. The dispersion index of pearlite is calculated by calculating the number of A/B boundaries (boundary between ferrite phase and pearlite phase) in an electron microscope image of the metal structure of a hot stamping steel sheet mainly composed of ferrite and pearlite. obtained by dividing the number by the sum of the number of B/B boundaries (boundaries between pearlite phase and pearlite phase), and the number of A/B boundaries (boundaries between ferrite phase and pearlite phase). It will be done. A high dispersion index of pearlite means that the ratio of A/B boundaries is high in a metal structure mainly composed of ferrite and pearlite, that is, there are many boundaries between ferrite and pearlite phases. Therefore, by controlling the dispersion index of pearlite to a high value, it is possible to further reduce the portion where pearlite exists in a connected manner, and it is possible to achieve a more uniform dispersion of pearlite. According to this embodiment, by controlling the dispersion index of pearlite to 0.50 or more, prior austenite grains of 150 Hv or less are reduced in the final metal structure of the hot-stamped compact due to the uniform dispersion of pearlite. It becomes possible to achieve a standard deviation of the hardness distribution. As a result, it is possible to reduce variations in the hardness of prior austenite grains in the metallographic structure of the hot stamped compact, and in turn, it is possible to significantly suppress local increases in hardness, resulting in high tensile strength and For example, despite having a high tensile strength of 2200 MPa or more, it is possible to significantly improve hydrogen embrittlement resistance. The dispersion index of pearlite is preferably higher, and may be, for example, 0.52 or more, 0.55 or more, 0.58 or more, 0.60 or more, 0.62 or more, or 0.65 or more. Although the upper limit is not particularly limited, the dispersion index of pearlite may be, for example, 0.80 or less, 0.75 or less, or 0.70 or less.
　パーライトの分散指標は、以下のようにして決定される。まず、走査型電子顕微鏡を用いて、表面に垂直な断面において板厚１／４位置を中心として板厚方向と垂直な方向に３５μｍでかつ板厚方向に１０μｍの範囲の電子チャンネリングコントラスト像を得る。この測定には、具体的にはサーマル電界放射型走査電子顕微鏡とＥＢＳＤ検出器とで構成されるＥＢＳＤ解析装置を用いればよく、例えばＪＥＯＬ製ＪＳＭ－７００１ＦとＴＳＬ製ＤＶＣ５型検出器とで構成されたＥＢＳＤ解析装置を用いればよい。次に、得られた電子チャンネリングコントラスト像において板厚方向と垂直な直線を１μｍ間隔で１０本描く。次に、これらの直線と交わる相境界をＡ／Ａ境界（フェライト相とフェライト相の境界）、Ｂ／Ｂ境界（パーライト相とパーライト相の境界）及びＡ／Ｂ境界（フェライト相とパーライト相の境界）に分類して各境界の交点を算出する。次に、Ａ／Ｂ境界の交点数を総交点数すなわちＡ／Ａ境界の交点数、Ｂ／Ｂ境界の交点数及びＡ／Ｂ境界の交点数の合計で除することにより、その視野におけるＡ／Ｂ境界の割合が得られる。同じ手順を同一試料で５視野実施し、５視野におけるＡ／Ｂ境界の割合の平均値をパーライトの分散指標として決定する。 The dispersion index of pearlite is determined as follows. First, using a scanning electron microscope, an electron channeling contrast image was obtained in a cross section perpendicular to the surface in a range of 35 μm in the direction perpendicular to the thickness direction and 10 μm in the thickness direction centered on the 1/4 position of the sheet thickness. obtain. Specifically, for this measurement, an EBSD analysis device consisting of a thermal field emission scanning electron microscope and an EBSD detector may be used. An EBSD analysis device may be used. Next, in the obtained electron channeling contrast image, ten straight lines perpendicular to the plate thickness direction are drawn at intervals of 1 μm. Next, phase boundaries that intersect with these straight lines are defined as A/A boundary (boundary between ferrite phase and ferrite phase), B/B boundary (boundary between pearlite phase and pearlite phase), and A/B boundary (boundary between ferrite phase and pearlite phase). boundaries) and calculate the intersection of each boundary. Next, by dividing the number of intersections of A/B boundaries by the total number of intersections, that is, the sum of the number of intersections of A/A boundaries, the number of intersections of B/B boundaries, and the number of intersections of A/B boundaries, /B boundary ratio is obtained. The same procedure is performed for 5 fields of view on the same sample, and the average value of the ratio of A/B boundaries in the 5 fields of view is determined as a dispersion index of pearlite.
［板厚］
　本発明の実施形態に係るホットスタンプ用鋼板は、特に限定されないが、例えば０．１～４．０ｍｍの板厚を有する。板厚は０．２ｍｍ以上、０．４ｍｍ以上、０．６ｍｍ以上、０．８ｍｍ以上又は１．０ｍｍ以上であってもよい。同様に、板厚は３．６ｍｍ以下、３．２ｍｍ以下、２．８ｍｍ以下、２．４ｍｍ以下又は２．０ｍｍ以下であってもよい。ホットスタンプ用鋼板が熱延鋼板の場合には、板厚は、例えば１．０～４．０ｍｍであってもよい。一方、ホットスタンプ用鋼板が冷延鋼板の場合には、板厚は、例えば０．１～２．０ｍｍであってもよい。 [Plate thickness]
The steel plate for hot stamping according to the embodiment of the present invention has a thickness of, for example, 0.1 to 4.0 mm, although it is not particularly limited. The plate thickness may be 0.2 mm or more, 0.4 mm or more, 0.6 mm or more, 0.8 mm or more, or 1.0 mm or more. Similarly, the plate thickness may be 3.6 mm or less, 3.2 mm or less, 2.8 mm or less, 2.4 mm or less, or 2.0 mm or less. When the steel plate for hot stamping is a hot rolled steel plate, the plate thickness may be, for example, 1.0 to 4.0 mm. On the other hand, when the steel plate for hot stamping is a cold-rolled steel plate, the plate thickness may be, for example, 0.1 to 2.0 mm.
［めっき］
　本発明の実施形態に係るホットスタンプ用鋼板は、表面にめっき層を有していてもよい。表面にめっき層を有することで、ホットスタンプ後において、耐食性を向上させることができる。めっき層としては、アルミめっき層、アルミ－亜鉛めっき層、アルミ－珪素めっき層、溶融亜鉛めっき層、電気亜鉛めっき層、合金化溶融亜鉛めっき層、亜鉛－ニッケルめっき層、アルミ－マグネシウム－亜鉛系めっき層などが例示される。 [Plating]
The hot stamping steel plate according to the embodiment of the present invention may have a plating layer on the surface. By having a plating layer on the surface, corrosion resistance can be improved after hot stamping. Plating layers include aluminum plating layer, aluminum-zinc plating layer, aluminum-silicon plating layer, hot-dip galvanizing layer, electrolytic galvanizing layer, alloyed hot-dip galvanizing layer, zinc-nickel plating layer, aluminum-magnesium-zinc system. Examples include a plating layer.
＜ホットスタンプ成形体＞
　本発明においては、上記のホットスタンプ用鋼板に加え、当該ホットスタンプ用鋼板を用いて製造されるホットスタンプ成形体がさらに提供される。そこで、以下、本発明の実施形態に係るホットスタンプ成形体についてより詳しく説明する。当該ホットスタンプ成形体は、質量％で、
　Ｃ　：０．４０～０．７０％、
　Ｓｉ：０．０１０～１．３００％、
　Ｍｎ：０．６０～３．００％、
　Ｐ　：０．１００％以下、
　Ｓ　：０．０１００％以下、
　Ｎ　：０．０２００％以下、
　Ｏ　：０．０２００％以下、
　Ａｌ：０．００１０～０．５０００％、
　Ｎｂ：０．００１０～０．１００％、
　Ｔｉ：０．０１０～０．２００％、
　Ｂ　：０．０００５～０．０２００％、
　Ｃｒ：０．０１０～０．８０％、
　Ｍｏ：０．００１０～１．０００％、
　Ｃｏ：０～２．００％、
　Ｎｉ：０～３．００％、
　Ｃｕ：０～１．００％、
　Ｖ　：０～１．００％、
　Ｗ　：０～１．０００％、
　Ｃａ：０～０．０１０％、
　Ｍｇ：０～１．０００％、
　ＲＥＭ：０～１．０００％、
　Ｓｂ：０～１．０００％、
　Ｚｒ：０～１．０００％、
　Ｓｎ：０～１．０００％、
　Ａｓ：０～０．１００％、並びに
　残部：Ｆｅ及び不純物からなる化学組成を有し、
　面積率で、マルテンサイト、ベイナイト及び焼き戻しマルテンサイトの少なくとも１種：合計で９０％以上を含み、
　板厚１／４位置における旧オーステナイト粒の硬さ分布の標準偏差が１５０Ｈｖ以下である金属組織を有することを特徴としている。 <Hot stamp molded body>
In addition to the above-mentioned steel plate for hot stamping, the present invention further provides a hot stamped molded article manufactured using the steel plate for hot stamping. Therefore, the hot stamp molded article according to the embodiment of the present invention will be described in more detail below. The hot stamp molded body has a mass percentage of
C: 0.40-0.70%,
Si: 0.010-1.300%,
Mn: 0.60-3.00%,
P: 0.100% or less,
S: 0.0100% or less,
N: 0.0200% or less,
O: 0.0200% or less,
Al: 0.0010-0.5000%,
Nb: 0.0010 to 0.100%,
Ti: 0.010-0.200%,
B: 0.0005-0.0200%,
Cr: 0.010-0.80%,
Mo: 0.0010-1.000%,
Co: 0-2.00%,
Ni: 0-3.00%,
Cu: 0 to 1.00%,
V: 0 to 1.00%,
W: 0-1.000%,
Ca: 0-0.010%,
Mg: 0-1.000%,
REM: 0-1.000%,
Sb: 0 to 1.000%,
Zr: 0 to 1.000%,
Sn: 0-1.000%,
Has a chemical composition consisting of As: 0 to 0.100%, and the balance: Fe and impurities,
Contains at least one of martensite, bainite and tempered martensite: 90% or more in total in terms of area percentage,
It is characterized by having a metal structure in which the standard deviation of the hardness distribution of prior austenite grains at the 1/4 position of the plate thickness is 150 Hv or less.
［ホットスタンプ成形体の化学組成］
　ホットスタンプ成形によって化学組成は実質的に変化しないため、ホットスタンプ成形体の化学組成は、先に記載したホットスタンプ用鋼板と同じ化学組成を有する。したがって、先に記載したホットスタンプ用鋼板の化学組成に関連する各元素及び残部の説明は、ホットスタンプ用鋼板だけでなくホットスタンプ成形体においても適用するものとする。 [Chemical composition of hot stamp molded product]
Since the chemical composition is not substantially changed by hot stamping, the hot stamping molded body has the same chemical composition as the hot stamping steel sheet described above. Therefore, the explanation of each element and the remainder related to the chemical composition of the hot stamping steel sheet described above applies not only to the hot stamping steel sheet but also to the hot stamping molded product.
［マルテンサイト、ベイナイト及び焼き戻しマルテンサイトの少なくとも１種：合計で９０％以上］
　ホットスタンプ成形体の金属組織は、面積率で、マルテンサイト、ベイナイト及び焼き戻しマルテンサイトの少なくとも１種を合計で９０％以上含む。残部組織は、特に限定されないが、１０％以下のフェライト、残留オーステナイト及びパーライトの少なくとも１種からなっていてもよい。マルテンサイト、ベイナイト及び焼き戻しマルテンサイトは非常に硬質な組織であり、それゆえホットスタンプ成形体においてマルテンサイト、ベイナイト及び焼き戻しマルテンサイトの少なくとも１種を面積率で合計９０％以上含むことで、高い引張強さ、具体的には２２００ＭＰａ以上の引張強さを達成することが可能となる。マルテンサイト、ベイナイト及び焼き戻しマルテンサイトの少なくとも１種の面積率の合計は好ましくは９２％以上又は９４％以上、さらに好ましくは９５％以上又は９７％以上であってもよい。マルテンサイト、ベイナイト及び焼き戻しマルテンサイトの少なくとも１種の面積率の合計の上限は、特に限定されず１００％であってもよい。 [At least one of martensite, bainite, and tempered martensite: 90% or more in total]
The metal structure of the hot-stamped compact contains at least 90% in total of at least one of martensite, bainite, and tempered martensite in terms of area percentage. The residual structure is not particularly limited, but may consist of 10% or less of at least one of ferrite, retained austenite, and pearlite. Martensite, bainite, and tempered martensite are very hard structures. Therefore, by including at least one of martensite, bainite, and tempered martensite in a total area ratio of 90% or more in the hot stamped body, It becomes possible to achieve high tensile strength, specifically, a tensile strength of 2200 MPa or more. The total area ratio of at least one of martensite, bainite, and tempered martensite is preferably 92% or more or 94% or more, and more preferably 95% or more or 97% or more. The upper limit of the total area ratio of at least one of martensite, bainite, and tempered martensite is not particularly limited, and may be 100%.
［金属組織の同定及び面積率の算出］
　ホットスタンプ成形体及び先に説明したホットスタンプ用鋼板における金属組織の同定及び面積率の算出は以下のようにして行われる。まず、鋼材の端面から５０ｍｍ以上離れた任意の位置（この位置からサンプルを採取できない場合は、端部を避けた位置）から表面に垂直な板厚断面が観察できるようにサンプルを切り出す。サンプルの大きさは、測定装置にもよるが、板厚方向と垂直な方向に１０ｍｍ程度観察できる大きさとする。 [Identification of metallographic structure and calculation of area ratio]
Identification of the metal structure and calculation of area ratio in the hot-stamped molded body and the hot-stamped steel plate described above are performed as follows. First, a sample is cut out from an arbitrary position 50 mm or more away from the end face of the steel material (if the sample cannot be taken from this position, avoid the end part) so that the thickness cross section perpendicular to the surface can be observed. Although the size of the sample depends on the measuring device, it should be large enough to allow observation of about 10 mm in the direction perpendicular to the plate thickness direction.
　上記サンプルの断面を＃６００から＃１５００の炭化珪素ペーパーを使用して研磨した後、粒度１～６μｍのダイヤモンドパウダーをアルコール等の希釈液や純水に分散させた液体を使用して鏡面に仕上げる。次に、電解研磨により観察面を仕上げる。サンプル断面の長手方向の任意の位置で板厚の１／４深さ位置において、長さ５０μｍ、板厚方向に５０μｍの領域を、０．１μｍの測定間隔で電子後方散乱回折法により測定して結晶方位情報を得る。測定には、サーマル電界放射型走査電子顕微鏡とＥＢＳＤ検出器とで構成されるＥＢＳＤ解析装置を用いればよく、例えばＪＥＯＬ製ＪＳＭ－７００１ＦとＴＳＬ製ＤＶＣ５型検出器とで構成されたＥＢＳＤ解析装置を用いればよい。この際、ＥＢＳＤ解析装置内の真空度は９．６×１０-5Ｐａ以下、加速電圧は１５ｋＶ、照射電流レベルは１３としてもよい。 After polishing the cross section of the sample above using #600 to #1500 silicon carbide paper, polish it to a mirror surface using a liquid made by dispersing diamond powder with a particle size of 1 to 6 μm in diluted liquid such as alcohol or pure water. . Next, the observation surface is finished by electrolytic polishing. An area of 50 μm in length and 50 μm in the thickness direction was measured at an arbitrary position in the longitudinal direction of the sample cross section at a depth of 1/4 of the plate thickness using an electron backscatter diffraction method at a measurement interval of 0.1 μm. Obtain crystal orientation information. For measurement, an EBSD analysis device consisting of a thermal field emission scanning electron microscope and an EBSD detector may be used. For example, an EBSD analysis device consisting of a JEOL JSM-7001F and a TSL DVC5 type detector may be used. Just use it. At this time, the degree of vacuum in the EBSD analyzer may be 9.6×10 −5 Pa or less, the acceleration voltage may be 15 kV, and the irradiation current level may be 13.
　得られた結晶方位情報をＥＢＳＤ解析装置に付属のソフトウェア「ＯＩＭ　Ａｎａｌｙｓｉｓ（登録商標）」に搭載された「Ｐｈａｓｅ　Ｍａｐ」機能を用いて、結晶構造がｆｃｃであるものを残留オーステナイトと判断する。この残留オーステナイトの面積率を算出することで、残留オーステナイトの面積率を得る。次に、結晶構造がｂｃｃである領域をベイナイト、焼き戻しマルテンサイト、マルテンサイト及びフェライトと判断する。これらの領域について、ＥＢＳＤ解析装置に付属のソフトウェア「ＯＩＭ　Ａｎａｌｙｓｉｓ（登録商標）」に搭載された「Ｇｒａｉｎ　Ａｖｅｒａｇｅ　Ｍｉｓｏｒｉｅｎｔａｔｉｏｎ」機能を用いて、５°粒界を結晶粒界とみなす条件下で、「Ｇｒａｉｎ　Ａｖｅｒａｇｅ　Ｍｉｓｏｒｉｅｎｔａｔｉｏｎ」が０．５°以下の領域をフェライトとして抽出する。抽出したフェライトの面積率を算出することで、フェライトの面積率を得る。 Using the obtained crystal orientation information and the "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer, those whose crystal structure is fcc are determined to be retained austenite. By calculating the area ratio of this retained austenite, the area ratio of retained austenite is obtained. Next, regions having a BCC crystal structure are determined to be bainite, tempered martensite, martensite, and ferrite. Regarding these regions, using the "Grain Average Misorientation" function installed in the software "OIM Analysis (registered trademark)" included with the EBSD analyzer, "Grain Average Misorientation" is performed under conditions where 5° grain boundaries are regarded as grain boundaries. A region where "Average Misorientation" is 0.5° or less is extracted as ferrite. The area ratio of ferrite is obtained by calculating the area ratio of the extracted ferrite.
　続いて、残部領域（「Ｇｒａｉｎ　Ａｖｅｒａｇｅ　Ｍｉｓｏｒｉｅｎｔａｔｉｏｎ」が０．５°超の領域）をマルテンサイト、焼き戻しマルテンサイト及びベイナイトの合計の面積率とする。炭化物は、ＥＢＳＤ観察と同一視野に関して、走査型電子顕微鏡を用いて撮影した二次電子像においてコントラストが明るい領域で粒状な形状を有するものを炭化物と判断し、該当する領域の面積率を算出することで、炭化物の面積率を得る。パーライトの面積率は、１００％から残留オーステナイトの面積率と、ベイナイト、焼き戻しマルテンサイト、マルテンサイト及びフェライトの面積率と、炭化物の面積率とを引き算することで算出される。 Subsequently, the remaining area (the area where "Grain Average Misorientation" exceeds 0.5°) is defined as the total area ratio of martensite, tempered martensite, and bainite. Carbides are determined to be carbides if they have a granular shape in areas with bright contrast in a secondary electron image taken using a scanning electron microscope in the same field of view as the EBSD observation, and the area ratio of the corresponding area is calculated. By this, the area ratio of carbide is obtained. The area ratio of pearlite is calculated by subtracting the area ratio of retained austenite, the area ratio of bainite, tempered martensite, martensite, and ferrite, and the area ratio of carbide from 100%.
［板厚１／４位置における旧オーステナイト粒の硬さ分布の標準偏差：１５０Ｈｖ以下］
　本発明の実施形態においては、ホットスタンプ成形体の板厚１／４位置における旧オーステナイト粒の硬さ分布の標準偏差は１５０Ｈｖ以下である。旧オーステナイト粒の硬さのばらつきが大きいと、局所的な硬さの上昇を招いて水素脆化割れを引き起こす場合がある。本発明の実施形態によれば、ホットスタンプ成形体の板厚１／４位置における旧オーステナイト粒の硬さ分布の標準偏差を１５０Ｈｖ以下に制御して旧オーステナイト粒の硬さのばらつきを低減することにより、水素脆化割れの起点となる局所的な硬さの上昇を確実に抑制することが可能となる。好ましくは、当該標準偏差は１４０Ｈｖ以下、１３０Ｈｖ以下、１２０Ｈｖ以下又は１１０Ｈｖ以下である。下限は特に限定されないが、ホットスタンプ成形体の板厚１／４位置における旧オーステナイト粒の硬さ分布の標準偏差は、例えば５０Ｈｖ以上、６０Ｈｖ以上又は８０Ｈｖ以上であってもよい。 [Standard deviation of hardness distribution of prior austenite grains at 1/4 plate thickness position: 150Hv or less]
In the embodiment of the present invention, the standard deviation of the hardness distribution of prior austenite grains at the 1/4 plate thickness position of the hot-stamped molded body is 150 Hv or less. Large variations in the hardness of prior austenite grains may lead to a local increase in hardness and cause hydrogen embrittlement cracking. According to an embodiment of the present invention, the standard deviation of the hardness distribution of the prior austenite grains at the 1/4 plate thickness position of the hot-stamped molded body is controlled to be 150 Hv or less to reduce the variation in the hardness of the prior austenite grains. This makes it possible to reliably suppress the local increase in hardness that becomes the starting point of hydrogen embrittlement cracking. Preferably, the standard deviation is 140 Hv or less, 130 Hv or less, 120 Hv or less, or 110 Hv or less. Although the lower limit is not particularly limited, the standard deviation of the hardness distribution of prior austenite grains at the 1/4 plate thickness position of the hot stamp molded body may be, for example, 50 Hv or more, 60 Hv or more, or 80 Hv or more.
　本発明の実施形態においては、上記のとおり、ホットスタンプ成形体の板厚１／４位置における旧オーステナイト粒の硬さ分布（板厚１／４位置における金属組織全体の硬さ分布ではなく、当該金属組織中に存在する旧オーステナイト粒の硬さ分布）の標準偏差を１５０Ｈｖ以下に制御して旧オーステナイト粒の硬さのばらつきを低減することが重要であり、それゆえ旧オーステナイト粒の硬さ自体を特定の範囲に制御する必要はない。したがって、ホットスタンプ成形体の板厚１／４位置における旧オーステナイト粒の硬さは特に限定されないが、例えば５００Ｈｖ以上であってもよく、及び／又は１０００Ｈｖ以下であってもよい。ホットスタンプ成形体の板厚１／４位置における旧オーステナイト粒の硬さとは、以下で説明する旧オーステナイト粒の硬さ分布の標準偏差の決定方法において測定される全硬さ測定値の平均をいうものである。 In the embodiment of the present invention, as described above, the hardness distribution of prior austenite grains at the 1/4 position of the plate thickness of the hot stamped body (not the hardness distribution of the entire metal structure at the 1/4 position of the plate thickness) It is important to reduce the variation in the hardness of prior austenite grains by controlling the standard deviation of the hardness distribution of prior austenite grains existing in the metal structure to 150Hv or less, and therefore the hardness of prior austenite grains itself There is no need to control it within a specific range. Therefore, the hardness of the prior austenite grains at the 1/4 plate thickness position of the hot-stamped molded body is not particularly limited, but may be, for example, 500 Hv or more and/or 1000 Hv or less. The hardness of the prior austenite grains at the 1/4 plate thickness position of the hot-stamped compact refers to the average of all hardness measurements measured by the method for determining the standard deviation of the hardness distribution of the prior austenite grains described below. It is something.
　旧オーステナイト粒の硬さ分布の標準偏差は、以下のようにして決定される。まず、ホットスタンプ成形体の端面から５０ｍｍ以上離れた任意の位置から表面に垂直な断面（板厚断面）が観察できるようにサンプルを切り出す。サンプルは、測定装置にもよるが、板厚方向と垂直な方向に１０ｍｍ観察できる大きさとする。サンプルの断面を＃６００から＃１５００の炭化珪素ペーパーを使用して研磨した後、粒度１～６μｍのダイヤモンドパウダーをアルコール等の希釈液及び純水に分散させた液体を使用して鏡面に仕上げる。鏡面に仕上げた断面に対し、表面から板厚の１／４深さ位置において、マイクロビッカース硬さ試験機を用いて、板面と平行な方向に、荷重１ｇｆで、圧痕の３倍以上の間隔でビッカース硬さを測定し、合計で１００点以上の測定値を得る。次に、ＥＢＳＤ解析装置を用いて同じサンプルを測定し、得られた組織解析結果を参照して、旧オーステナイト粒内に圧痕がある（すなわち圧痕が粒界に被っていない）測定点のみを抽出する。最後に、抽出された２０個以上の異なる旧オーステナイト粒に関するビッカース硬さの測定値に基づいて得られた標準偏差を、ホットスタンプ成形体の板厚１／４位置における旧オーステナイト粒の硬さ分布の標準偏差として決定する。ＥＢＳＤ測定には、サーマル電界放射型走査電子顕微鏡とＥＢＳＤ検出器とで構成されるＥＢＳＤ解析装置を用いればよく、例えばＪＥＯＬ製ＪＳＭ－７００１ＦとＴＳＬ製ＤＶＣ５型検出器とで構成されたＥＢＳＤ解析装置を用いればよい。この際、ＥＢＳＤ解析装置内の真空度は９．６×１０-5Ｐａ以下、加速電圧は１５ｋＶ、照射電流レベルは１３としてもよい。Ａｃｔａ　Ｍａｔｅｒｉａｌｉａ、５８（２０１０）、６３９３－６４０３に記載の方法で旧オーステナイト粒の結晶方位マップを作成し、当該結晶方位マップに基づいて旧オーステナイト粒を識別する。 The standard deviation of the hardness distribution of prior austenite grains is determined as follows. First, a sample is cut out so that a cross section perpendicular to the surface (thickness cross section) can be observed from an arbitrary position 50 mm or more away from the end surface of the hot stamp molded body. The size of the sample is such that it can be observed at a distance of 10 mm in the direction perpendicular to the thickness direction, although it depends on the measuring device. After polishing the cross section of the sample using #600 to #1500 silicon carbide paper, it is finished to a mirror surface using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in diluted liquid such as alcohol and pure water. Using a micro Vickers hardness tester at a depth of 1/4 of the plate thickness from the surface of a mirror-finished cross-section, a load of 1 gf was applied in the direction parallel to the plate surface, and the distance was at least three times the indentation. The Vickers hardness is measured with a total of 100 points or more. Next, measure the same sample using an EBSD analyzer, and refer to the obtained microstructure analysis results to extract only the measurement points where there are indentations within the prior austenite grains (that is, the indentations do not cover the grain boundaries). do. Finally, the standard deviation obtained based on the measured values of Vickers hardness for the extracted 20 or more different prior austenite grains is calculated as the hardness distribution of prior austenite grains at the 1/4 plate thickness position of the hot stamped compact. Determine as the standard deviation of For EBSD measurement, an EBSD analysis device consisting of a thermal field emission scanning electron microscope and an EBSD detector may be used. For example, an EBSD analysis device consisting of a JEOL JSM-7001F and a TSL DVC5 type detector may be used. You can use At this time, the degree of vacuum in the EBSD analyzer may be 9.6×10 −5 Pa or less, the acceleration voltage may be 15 kV, and the irradiation current level may be 13. A crystal orientation map of prior austenite grains is created by the method described in Acta Materialia, 58 (2010), 6393-6403, and the prior austenite grains are identified based on the crystal orientation map.
［めっき］
　本実施形態に係るホットスタンプ成形体は、表面にめっき層を有していてもよい。表面にめっき層を有することで耐食性を向上することができる。めっき層としては、アルミめっき層、アルミ－亜鉛めっき層、アルミ－珪素めっき層、溶融亜鉛めっき層、電気亜鉛めっき層、合金化溶融亜鉛めっき層、亜鉛－ニッケルめっき層、アルミ－マグネシウム－亜鉛系めっき層などが例示される。 [Plating]
The hot stamp molded article according to this embodiment may have a plating layer on the surface. Corrosion resistance can be improved by having a plating layer on the surface. Plating layers include aluminum plating layer, aluminum-zinc plating layer, aluminum-silicon plating layer, hot-dip galvanizing layer, electrolytic galvanizing layer, alloyed hot-dip galvanizing layer, zinc-nickel plating layer, aluminum-magnesium-zinc system. Examples include a plating layer.
［機械特性］
　本発明の実施形態に係るホットスタンプ成形体によれば、優れた機械特性、例えば２２００ＭＰａ以上の引張強さを達成することができる。引張強さは、好ましくは２３００ＭＰａ以上であり、より好ましくは２４００ＭＰａ以上であり、最も好ましくは２５００ＭＰａ以上又は２６００ＭＰａ以上である。上限は特に限定されないが、例えば、引張強さは３５００ＭＰａ以下、３３００ＭＰａ以下又は３０００ＭＰａ以下であってもよい。ホットスタンプ成形体の引張強さはＪＩＳ　Ｚ　２２４１：２０１１に準拠して、５号試験片を作製し、引張試験を行うことで測定される。 [Mechanical properties]
According to the hot-stamped molded article according to the embodiment of the present invention, it is possible to achieve excellent mechanical properties, for example, a tensile strength of 2200 MPa or more. The tensile strength is preferably 2300 MPa or more, more preferably 2400 MPa or more, and most preferably 2500 MPa or more or 2600 MPa or more. Although the upper limit is not particularly limited, for example, the tensile strength may be 3500 MPa or less, 3300 MPa or less, or 3000 MPa or less. The tensile strength of the hot stamp molded product is measured by preparing a No. 5 test piece and conducting a tensile test in accordance with JIS Z 2241:2011.
　本発明の実施形態に係るホットスタンプ成形体は、上記のように、例えば２２００ＭＰａ以上の高い引張強さを有するにもかかわらず、耐水素脆化特性に優れるため、例えば自動車の骨格部材や、バンパー、その他、強度が必要な他の構造部材及び補強部材としての使用に非常に有用である。 As described above, the hot-stamped molded product according to the embodiment of the present invention has excellent hydrogen embrittlement resistance despite having a high tensile strength of, for example, 2200 MPa or more. It is very useful for use as other structural members and reinforcing members that require strength.
＜製造方法＞
　次に、本発明の実施形態に係るホットスタンプ用鋼板及びホットスタンプ成形体の好ましい製造方法について説明する。以下の説明は、本発明の実施形態に係るホットスタンプ用鋼板及びホットスタンプ成形体を製造するための特徴的な方法の例示を意図するものであって、当該ホットスタンプ用鋼板及びホットスタンプ成形体を以下に説明するような製造方法によって製造されるものに限定することを意図するものではない。 <Manufacturing method>
Next, a preferred method for manufacturing a hot stamping steel plate and a hot stamping molded body according to an embodiment of the present invention will be described. The following description is intended to illustrate a characteristic method for producing a hot stamping steel plate and a hot stamping molded article according to an embodiment of the present invention, and includes the hot stamping steel sheet and hot stamping molded article. is not intended to be limited to those manufactured by the manufacturing method described below.
＜ホットスタンプ用鋼板の製造方法＞
　本発明の実施形態に係るホットスタンプ用鋼板の製造方法は、特に、仕上げ圧延条件及びその後の冷却条件を制御することが効果的である。具体的には、本発明の実施形態に係るホットスタンプ用鋼板の製造方法は、
　ホットスタンプ用鋼板に関連して上で説明した化学組成を有するスラブを熱間圧延する工程であって、前記スラブを加熱し、次いで仕上げ圧延することを含み、前記仕上げ圧延における最終段の圧下率が４０％以上である工程（熱間圧延工程）、
　得られた熱延鋼板を仕上げ圧延終了後１．０秒以内に急冷し、次いで９０℃／秒以上の平均冷却速度で冷却する工程（冷却工程）、及び
　前記熱延鋼板を５００～７００℃の温度で巻き取る工程（巻取工程）
を含むことを特徴としている。以下、各工程について詳しく説明する。 <Method for manufacturing steel plate for hot stamping>
In the method for manufacturing a hot stamping steel plate according to the embodiment of the present invention, it is particularly effective to control the finish rolling conditions and the subsequent cooling conditions. Specifically, the method for manufacturing a hot stamping steel plate according to an embodiment of the present invention includes:
A process of hot rolling a slab having the chemical composition described above in connection with a steel plate for hot stamping, the process comprising heating said slab and then finishing rolling, the reduction rate of the final stage in said finishing rolling. is 40% or more (hot rolling process),
A step of rapidly cooling the obtained hot rolled steel sheet within 1.0 seconds after finish rolling and then cooling it at an average cooling rate of 90° C./second or more (cooling step), and cooling the hot rolled steel sheet at 500 to 700° C. Winding process at temperature (winding process)
It is characterized by including. Each step will be explained in detail below.
［熱間圧延工程］
［スラブの加熱］
　まず、ホットスタンプ用鋼板に関連して上で説明した化学組成を有するスラブが加熱される。溶鋼の鋳造方法は特に限定されず、連続鋳造法、造塊法又は薄スラブ鋳造法によって製造してもよい。熱間圧延前の加熱は特に限定されないが、使用されるスラブは、高強度鋼板を得るために合金元素を比較的多く含有しているため、スラブを熱間圧延に供する前に加熱して合金元素をスラブ中に固溶させる目的で加熱温度は１１００℃以上であってもよい。 [Hot rolling process]
[Heating the slab]
First, a slab having the chemical composition described above in connection with hot stamping steel sheets is heated. The method for casting molten steel is not particularly limited, and it may be manufactured by a continuous casting method, an ingot casting method, or a thin slab casting method. Heating before hot rolling is not particularly limited, but since the slab used contains a relatively large amount of alloying elements in order to obtain high-strength steel sheets, the slab is heated before hot rolling to form an alloy. The heating temperature may be 1100° C. or higher for the purpose of dissolving the elements in the slab.
［粗圧延］
　本方法では、例えば、加熱されたスラブに対し、板厚調整等のために、仕上げ圧延の前に粗圧延を施してもよい。粗圧延は、所望のシートバー寸法が確保できればよく、その条件は特に限定されない。 [Rough rolling]
In this method, for example, 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.
［仕上げ圧延］
　加熱されたスラブ又はそれに加えて必要に応じて粗圧延されたスラブは、次に仕上げ圧延を施される。本方法では、仕上げ圧延における最終段の圧下率を４０％以上とすることが重要である。仕上げ圧延における最終段の圧下率を４０％以上とすることで、圧延後の熱延鋼板においてパーライトが均一に分散する。このパーライトは、後でホットスタンプ成形体の製造方法に関連して詳しく説明するホットスタンプ成形工程における加熱時にオーステナイトの起点となる。そのため、パーライトが均一に分散すると、ホットスタンプ成形体において旧オーステナイト粒径のばらつきを低減することができる。その結果として、ホットスタンプ成形体の金属組織における旧オーステナイト粒の硬さのばらつきを低減することができ、ひいては局所的な硬さの上昇を顕著に抑制することができる。したがって、高い引張強さ、例えば２２００ＭＰａ以上の高い引張強さを有するにもかかわらず、耐水素脆化特性を顕著に向上させることが可能となる。より好ましくは、仕上げ圧延における最終段の圧下率は４５％以上又は５０％以上である。 [Finish rolling]
The heated slab, or the optionally rough rolled slab, is then subjected to finish rolling. In this method, it is important that the rolling reduction ratio in the final stage in finish rolling is 40% or more. By setting the rolling reduction ratio in the final stage of finish rolling to 40% or more, pearlite is uniformly dispersed in the hot rolled steel sheet after rolling. This pearlite becomes the starting point of austenite during heating in the hot stamping process, which will be explained in detail later in connection with the method for producing a hot stamped molded body. Therefore, when pearlite is uniformly dispersed, it is possible to reduce variations in the prior austenite grain size in the hot-stamped molded body. As a result, variations in the hardness of prior austenite grains in the metallographic structure of the hot-stamped molded body can be reduced, and local increases in hardness can be significantly suppressed. Therefore, despite having a high tensile strength, for example, 2200 MPa or more, it is possible to significantly improve the hydrogen embrittlement resistance. More preferably, the rolling reduction ratio of the final stage in finish rolling is 45% or more or 50% or more.
　ホットスタンプ用鋼板では、高い焼き入れ性を確保する目的でＭｎの添加量を高める傾向があり、例えば０．６０％以上のＭｎが添加されている。これに関連して、今回、本発明者らによる研究で、このような高Ｍｎ含有量では、熱延鋼板においてパーライトが比較的連結して配置する傾向があり、それゆえ低Ｍｎ含有量の場合と比較して熱延鋼板の金属組織においてパーライトを均一に分散させることが非常に困難であることがわかった。したがって、このような高Ｍｎ含有量の鋼材を４０％未満の比較的低い圧下率で仕上げ圧延した場合には、金属組織においてパーライトが連結した部分の存在が特に顕著なものになると考えられる。 In steel sheets for hot stamping, there is a tendency to increase the amount of Mn added in order to ensure high hardenability, and for example, 0.60% or more of Mn is added. In this regard, research by the present inventors has shown that at such high Mn contents, pearlite tends to be relatively connected and arranged in hot-rolled steel sheets, and therefore, at low Mn contents, pearlite tends to be relatively connected and arranged. It was found that it is extremely difficult to uniformly disperse pearlite in the metal structure of hot rolled steel sheets. Therefore, when a steel material with such a high Mn content is finish-rolled at a relatively low rolling reduction of less than 40%, it is thought that the presence of a portion in which pearlite is connected becomes particularly noticeable in the metal structure.
　しかしながら、仕上げ圧延における最終段の圧下率を４０％以上とすることで、０．６０％以上の高Ｍｎ含有量にもかかわらず、熱間圧延工程並びに以降の冷却工程及び巻取工程後の熱延鋼板においてパーライトを十分に分散して配置することが可能となる。したがって、このような圧下を行った熱延鋼板の金属組織では、パーライトが連結して存在する部分がないか又は十分に低減されているために、ホットスタンプ成形後の組織において旧オーステナイト粒径のばらつきを低減することができる。その結果として、ホットスタンプ成形体の金属組織における旧オーステナイト粒の硬さのばらつきを低減することができる。仕上げ圧延における最終段の圧下率の上限は特に限定されない。このように高Ｍｎ含有量を有する鋼材であっても、特に仕上げ圧延における最終段の圧下率を適切に制御することにより、熱延鋼板の金属組織においてパーライトを十分に分散して配置することができ、ひいては旧オーステナイト粒径のばらつきを低減して局所的な硬さの上昇を抑制することができる。 However, by setting the rolling reduction ratio in the final stage of finish rolling to 40% or more, despite the high Mn content of 0.60% or more, the heat after the hot rolling process and subsequent cooling process and coiling process It becomes possible to sufficiently disperse and arrange pearlite in the rolled steel plate. Therefore, in the metallographic structure of hot-rolled steel sheets that have been subjected to such reduction, there is no connected pearlite, or the pearlite is sufficiently reduced, so that the prior austenite grain size in the structure after hot stamping is reduced. Variations can be reduced. As a result, variations in the hardness of prior austenite grains in the metallographic structure of the hot-stamped compact can be reduced. The upper limit of the rolling reduction ratio of the final stage in finish rolling is not particularly limited. Even for steel materials with such a high Mn content, pearlite can be sufficiently dispersed and arranged in the metallographic structure of hot-rolled steel sheets, especially by appropriately controlling the reduction rate in the final stage of finish rolling. Therefore, it is possible to reduce the variation in prior austenite grain size and suppress a local increase in hardness.
　このような金属組織の形態は、仕上げ圧延における最終段の圧下率とその後の冷却工程における平均冷却速度及び巻取工程における巻取温度が支配的な因子であり、例えば、任意選択の冷間圧延やその後の焼鈍等によっても特に大きな影響は受けない。というのも、仕上げ圧延における最終段の圧下率を４０％以上として熱延鋼板を形成していれば、仮に当該熱延鋼板が冷間圧延され、次いで比較的高温下で焼鈍されたとしても、冷却後にはオーステナイトの起点となる炭化物、粒界、残留オーステナイトが分散して配置された金属組織が生成する傾向が高いためである。一般的には、仕上げ圧延における最終段の圧下率を高くしすぎると、圧延時における鋼板の割れが懸念される。とりわけ、Ｃ含有量が０．４０％以上であるような高強度鋼板の場合、最終段の圧下率を高くしすぎると、このような鋼板の割れに対する懸念に加えて、圧延機の圧延負荷も顕著に高まる。このため、本発明の実施形態に係るホットスタンプ用鋼板と同様の化学組成を有する鋼材において、最終段の圧下率が４０％以上となるような仕上げ圧延は従来行われていない。したがって、仕上げ圧延における最終段の圧下率を４０％以上とし、さらにその後の冷却工程における平均冷却速度及び巻取工程における巻取温度を適切に制御しそれらを組み合わせることで、ホットスタンプ成形の加熱時にオーステナイト粒の起点となるパーライトを均一に分散させることができるという事実は従来知られていない。したがって、当然ながら、これに関連して、最終的に得られるホットスタンプ成形体において旧オーステナイト粒が均一化され、それによって当該ホットスタンプ成形体の金属組織における旧オーステナイト粒の硬さのばらつきを低減することができるという事実についても従来知られておらず、これらの事実は、今回、本発明者らによって初めて明らかにされたことである。 The morphology of such a metallographic structure is determined by the reduction rate in the final stage in finish rolling, the average cooling rate in the subsequent cooling process, and the coiling temperature in the coiling process. It is not particularly affected by subsequent annealing, etc. This is because if a hot-rolled steel plate is formed with a reduction ratio of 40% or more in the final stage of finish rolling, even if the hot-rolled steel plate is cold rolled and then annealed at a relatively high temperature, This is because after cooling, there is a high tendency for a metal structure in which carbides, grain boundaries, and retained austenite, which are the starting points of austenite, are dispersed to be generated. Generally, if the rolling reduction ratio in the final stage of finish rolling is made too high, there is a concern that the steel plate will crack during rolling. In particular, in the case of high-strength steel sheets with a C content of 0.40% or more, if the rolling reduction in the final stage is too high, in addition to concerns about cracking of the steel sheet, the rolling load of the rolling mill will also increase. Noticeably increases. For this reason, in steel materials having the same chemical composition as the hot stamping steel sheet according to the embodiment of the present invention, finish rolling has not been conventionally performed so that the rolling reduction in the final stage is 40% or more. Therefore, by setting the rolling reduction ratio in the final stage of finish rolling to 40% or more, and by appropriately controlling and combining the average cooling rate in the subsequent cooling process and the coiling temperature in the coiling process, it is possible to The fact that pearlite, which is the starting point of austenite grains, can be uniformly dispersed has not been previously known. Therefore, naturally, related to this, the prior austenite grains are made uniform in the finally obtained hot stamped compact, thereby reducing the variation in the hardness of the prior austenite grains in the metallographic structure of the hot stamped compact. The fact that it is possible to do this has not been previously known, and these facts have now been revealed for the first time by the present inventors.
［冷却工程］
　次に、仕上げ圧延された熱延鋼板は、仕上げ圧延終了後１．０秒以内に急冷される。フェライトは、オーステナイト粒の粒界から一般に生成するため、オーステナイト粒が大きくなると、フェライトの起点となる粒界の数が減少してしまうことになる。このような場合には、パーライトの連結を防いで均一に分散させることが難しくなる。したがって、仕上げ圧延終了後直ちに熱延鋼板を急冷すること、具体的には仕上げ圧延終了後１．０秒以内、好ましくは０．８秒以内に熱延鋼板を急冷することにより、オーステナイト粒の粒成長を抑制することが熱延鋼板においてパーライトを均一に分散して生成させる上で極めて重要となる。急冷時の平均冷却速度及び冷却時間は特に限定されないが、例えば、平均冷却速度は２００～１０００℃／秒であることが好ましく、冷却時間は０．２～２．０秒であることが好ましい。 [Cooling process]
Next, the finish-rolled hot-rolled steel sheet is rapidly cooled within 1.0 seconds after finishing the finish rolling. Since ferrite is generally generated from grain boundaries of austenite grains, as austenite grains become larger, the number of grain boundaries that serve as starting points for ferrite decreases. In such a case, it becomes difficult to prevent pearlite from linking and to disperse it uniformly. Therefore, by rapidly cooling the hot rolled steel sheet immediately after finish rolling, specifically within 1.0 seconds, preferably within 0.8 seconds after finish rolling, the austenite grains can be reduced. Suppression of growth is extremely important for uniformly dispersing and producing pearlite in hot rolled steel sheets. The average cooling rate and cooling time during rapid cooling are not particularly limited, but, for example, the average cooling rate is preferably 200 to 1000° C./second, and the cooling time is preferably 0.2 to 2.0 seconds.
　次に、急冷後の熱延鋼板は９０℃／秒以上の平均冷却速度で冷却される。冷却速度が遅いと、ベイナイトが多く生成したり、オーステナイトが残留オーステナイトとして残ったりして、主としてフェライトとパーライトから構成される金属組織を形成することができなくなる。その結果として、フェライトとパーライトの配置を制御することが難しくなり、とりわけパーライトの均一分散を達成することが難しくなる。一方で、９０℃／秒以上の平均冷却速度で冷却することで、主としてフェライトとパーライトから構成される金属組織を形成することができ、より具体的にはフェライトとパーライトの面積率の合計が８０％以上の金属組織を形成することができる。平均冷却速度は好ましくは９５℃／秒以上である。上限は特に限定されないが、平均冷却速度は、例えば２００℃／秒以下又は１５０℃／秒以下であってもよい。 Next, the hot-rolled steel sheet after quenching is cooled at an average cooling rate of 90° C./second or more. If the cooling rate is slow, a large amount of bainite is generated or austenite remains as retained austenite, making it impossible to form a metal structure mainly composed of ferrite and pearlite. As a result, it becomes difficult to control the arrangement of ferrite and pearlite, and in particular it becomes difficult to achieve a uniform distribution of pearlite. On the other hand, by cooling at an average cooling rate of 90°C/sec or more, it is possible to form a metal structure mainly composed of ferrite and pearlite, and more specifically, the total area ratio of ferrite and pearlite is 80°C. % or more of metal structure can be formed. The average cooling rate is preferably 95°C/sec or more. Although the upper limit is not particularly limited, the average cooling rate may be, for example, 200° C./second or less or 150° C./second or less.
［巻取工程］
　次に、仕上げ圧延された熱延鋼板は、５００～７００℃の温度で巻き取られる。巻取温度が高いと、粒成長が生じてパーライトの均一な分散が阻害される場合がある。一方で、巻取温度が低いと、ベイナイトやマルテンサイトが生成して主としてフェライトとパーライトから構成される金属組織を形成することができなくなる。これに対し、巻取温度を５００～７００℃に制御することで、粒成長を抑制するとともに、圧延後の熱延鋼板においてフェライトが連結して配置することを抑制することができ、パーライトを均一に分散させることができる。好ましくは、巻取温度は５０５～６５０℃又は５５０～６５０℃である。また、熱延鋼板の軟質化を目的として、巻取後のコイルに軟質化熱処理を施してもよい。軟質化熱処理の方法は特に限定されず、一般的な条件とすればよい。 [Winding process]
Next, the finish-rolled hot rolled steel sheet is wound up at a temperature of 500 to 700°C. If the winding temperature is high, grain growth may occur and uniform dispersion of pearlite may be inhibited. On the other hand, if the winding temperature is low, bainite and martensite are generated, making it impossible to form a metal structure mainly composed of ferrite and pearlite. On the other hand, by controlling the coiling temperature to 500 to 700°C, it is possible to suppress grain growth and prevent the ferrite from being connected and arranged in the hot-rolled steel sheet after rolling, thereby making the pearlite uniform. can be dispersed into Preferably, the winding temperature is 505-650°C or 550-650°C. Further, for the purpose of softening the hot rolled steel sheet, the coil after winding may be subjected to a softening heat treatment. The method of softening heat treatment is not particularly limited, and general conditions may be used.
［酸洗工程］
　巻取工程後、任意選択の冷間圧延工程前に、熱延鋼板の表面に形成された酸化スケールを除去するために酸洗を実施してもよい。酸洗は、酸化スケールを除去するのに適切な条件下で実施すればよく、一回でもよいし、又は酸化スケールを確実に取り除くために複数回に分けて実施してもよい。 [Acid washing process]
After the winding process and before the optional cold rolling process, pickling may be performed to remove oxide 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.
［冷間圧延工程］
　巻取工程後、任意選択で冷間圧延を実施してもよい。冷間圧延は特に限定されず、任意の適切な条件下で実施すればよい。例えば、冷間圧延の圧下率は３０～８０％であってよい。圧延パスの回数及びパス毎の圧下率は特に限定されず、冷間圧延全体の圧下率が上記範囲となるように適宜設定すればよい。 [Cold rolling process]
After the winding step, cold rolling may optionally be performed. Cold rolling is not particularly limited and may be carried out under any appropriate conditions. For example, the reduction ratio of cold rolling may be 30 to 80%. 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.
［焼鈍工程］
　例えば、冷間圧延工程後に、金属組織及び／又は特性を調整するために任意選択で焼鈍を実施してもよい。焼鈍工程の加熱温度は特に限定されないが、例えば８００℃以下であってよい。 [Annealing process]
For example, after the cold rolling process, an optional annealing may be performed to adjust the metallographic structure and/or properties. The heating temperature in the annealing step is not particularly limited, but may be, for example, 800° C. or lower.
［めっき工程］
　耐食性の向上等を目的として、熱延鋼板又は冷延鋼板の表面にめっき処理を施してもよい。めっき処理は、溶融めっき、合金化溶融めっき、電気めっき等の処理であってよい。例えば、めっき処理として鋼板に溶融亜鉛めっき処理を行ってもよく、溶融亜鉛めっき処理後に合金化処理を行ってもよい。めっき層としては、アルミめっき層、アルミ－亜鉛めっき層、アルミ－珪素めっき層、溶融亜鉛めっき層、電気亜鉛めっき層、合金化溶融亜鉛めっき層、亜鉛－ニッケルめっき層、アルミ－マグネシウム－亜鉛系めっき層などが例示される。めっき処理及び合金化処理の具体的な条件は特に限定されず、当業者に公知の任意の適切な条件であってよい。 [Plating process]
For the purpose of improving corrosion resistance, etc., a plating treatment may be performed on the surface of a hot rolled steel sheet or a cold rolled steel sheet. The plating process may be hot-dip plating, alloyed hot-dip plating, electroplating, or the like. For example, the steel plate may be subjected to hot-dip galvanizing treatment, or alloying treatment may be performed after hot-dip galvanizing treatment. Plating layers include aluminum plating layer, aluminum-zinc plating layer, aluminum-silicon plating layer, hot-dip galvanizing layer, electrolytic galvanizing layer, alloyed hot-dip galvanizing layer, zinc-nickel plating layer, aluminum-magnesium-zinc system. Examples include a plating layer. Specific conditions for the plating treatment and alloying treatment are not particularly limited, and may be any suitable conditions known to those skilled in the art.
［調質圧延工程］
　鋼板の形状矯正や表面粗さの調整等を目的として、例えば焼鈍工程後又はめっき工程後の鋼板に調質圧延を施してもよい。 [Temper rolling process]
For the purpose of correcting the shape of the steel plate, adjusting the surface roughness, etc., the steel plate may be subjected to temper rolling, for example, after the annealing process or the plating process.
＜ホットスタンプ成形体の製造方法＞
　次に、本発明の実施形態に係るホットスタンプ成形体の製造方法を説明する。具体的には、当該製造方法は、上で説明したホットスタンプ用鋼板の製造方法によって得られたホットスタンプ用鋼板をホットスタンプ成形する工程であって、前記ホットスタンプ用鋼板を８００℃～１０００℃の温度域に加熱し、次いで６０～６００秒間保持することを含む工程を含むことを特徴としている。 <Method for producing hot stamp molded body>
Next, a method for manufacturing a hot stamp molded article according to an embodiment of the present invention will be described. Specifically, the manufacturing method is a step of hot stamping a hot stamping steel sheet obtained by the method for manufacturing a hot stamping steel sheet described above, the hot stamping steel sheet being heated at 800°C to 1000°C. It is characterized in that it includes a step of heating to a temperature range of 100 to 1000 and then holding for 60 to 600 seconds.
［ホットスタンプ成形工程］
　ホットスタンプ用鋼板は、ホットスタンプ成形工程においてホットスタンプ成形されて所望の化学組成及び金属組織を有するホットスタンプ成形体が製造される。本実施形態においては、ホットスタンプ成形の加熱時に鋼板の金属組織において均一に分散されたパーライトを起点としてオーステナイトが生成し、その後の成形及び冷却操作により、所望の硬質組織を形成するとともに、ばらつきが低減された所望の旧オーステナイト粒径分布を有し、それゆえ旧オーステナイト粒の硬さのばらつきが低減された金属組織を有するホットスタンプ成形体が製造される。このような所望の硬質組織及び旧オーステナイト粒の硬さ分布を得る観点からは、ホットスタンプ用鋼板を８００℃～１０００℃の温度域に加熱し、この温度域にて６０～６００秒間保持することが好ましい。加熱温度が８００℃未満ではオーステナイト化が不十分となり、所望の硬質組織（マルテンサイト、ベイナイト及び焼き戻しマルテンサイトの少なくとも１種）の面積率を得ることができず、引張強さが劣化する場合がある。一方、加熱温度が１０００℃を超えると、オーステナイトが過度に粒成長してしまい、所望の旧オーステナイト粒径分布を得ることができず、その結果として所望の旧オーステナイト粒の硬さ分布を得ることができず、耐水素脆化特性が劣化する場合がある。保持時間が６０秒未満では、加熱温度が８００℃未満の場合と同様に、オーステナイト化が不十分となり、所望の硬質組織（マルテンサイト、ベイナイト及び焼き戻しマルテンサイトの少なくとも１種）の面積率を得ることができず、引張強さが劣化する場合がある。保持時間が６００秒を超えると、オーステナイトが過度に粒成長してしまい、所望の旧オーステナイト粒径分布を得ることができず、その結果として所望の旧オーステナイト粒の硬さ分布を得ることができず、耐水素脆化特性が劣化する場合がある。 [Hot stamp molding process]
A hot stamping steel plate is hot stamped in a hot stamping process to produce a hot stamped body having a desired chemical composition and metal structure. In this embodiment, austenite is generated starting from pearlite that is uniformly dispersed in the metallographic structure of the steel sheet during heating during hot stamping, and through subsequent forming and cooling operations, a desired hard structure is formed and variations are eliminated. A hot-stamped compact is produced having a metallographic structure with a reduced desired prior austenite grain size distribution and therefore reduced variation in the hardness of the prior austenite grains. From the viewpoint of obtaining such a desired hard structure and hardness distribution of prior austenite grains, the steel plate for hot stamping is heated to a temperature range of 800°C to 1000°C and held in this temperature range for 60 to 600 seconds. is preferred. If the heating temperature is less than 800°C, austenitization will be insufficient, the desired area ratio of hard structure (at least one of martensite, bainite, and tempered martensite) cannot be obtained, and the tensile strength will deteriorate. There is. On the other hand, when the heating temperature exceeds 1000°C, austenite grains grow excessively, making it impossible to obtain the desired prior austenite grain size distribution, and as a result, it is difficult to obtain the desired prior austenite grain hardness distribution. This may result in deterioration of hydrogen embrittlement resistance. If the holding time is less than 60 seconds, as in the case where the heating temperature is less than 800°C, austenitization will be insufficient, and the area ratio of the desired hard structure (at least one of martensite, bainite, and tempered martensite) will be reduced. However, the tensile strength may deteriorate. If the holding time exceeds 600 seconds, austenite grains will grow excessively, making it impossible to obtain the desired prior austenite grain size distribution and, as a result, failing to obtain the desired prior austenite grain hardness distribution. First, the hydrogen embrittlement resistance may deteriorate.
　加熱雰囲気は特に限定されず、通常の条件でよく、例えば、大気中や、空気と燃料の比率を制御したガス燃焼雰囲気や、窒素雰囲気であればよく、これらガスにおいて露点を制御してもよい。８００℃～１０００℃の温度域で保持してから、ホットスタンプ成形する。ホットスタンプ成形後には、２５０℃以下の温度域まで２０℃／秒以上の平均冷却速度で冷却すればよい。 The heating atmosphere is not particularly limited and may be under normal conditions, such as the atmosphere, a gas combustion atmosphere with a controlled ratio of air and fuel, or a nitrogen atmosphere, and the dew point of these gases may be controlled. . After maintaining the temperature in a temperature range of 800°C to 1000°C, hot stamp molding is performed. After hot stamp molding, cooling may be performed to a temperature range of 250° C. or lower at an average cooling rate of 20° C./second or higher.
　ホットスタンプ前の加熱方法としては、例えば、電気炉やガス炉等による炉加熱、火炎加熱、通電加熱、高周波加熱、誘導加熱等が挙げられる。 Examples of heating methods before hot stamping include furnace heating using an electric furnace, gas furnace, etc., flame heating, electrical heating, high frequency heating, induction heating, and the like.
　以上の方法により、本実施形態に係るホットスタンプ成形体を得る。ホットスタンプ成形後に１３０～６００℃で焼き戻し処理や塗装後の焼き付け硬化処理を行ってもよい。また、ホットスタンプ成形体の一部をレーザー照射等により焼き戻しして部分的に軟化領域を設けてもよい。 By the above method, a hot stamp molded article according to the present embodiment is obtained. After hot stamp molding, tempering treatment at 130 to 600°C or baking hardening treatment after painting may be performed. Further, a portion of the hot stamp molded body may be tempered by laser irradiation or the like to provide a partially softened region.
　以下、実施例によって本発明をより詳細に説明するが、本発明はこれらの実施例に何ら限定されるものではない。 Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited to these Examples in any way.
　以下の実施例では、本発明の実施形態に係るホットスタンプ成形体を種々の条件下で製造し、得られたホットスタンプ成形体の引張強さ及び耐水素脆化特性について調べた。 In the following examples, hot-stamped molded bodies according to embodiments of the present invention were manufactured under various conditions, and the tensile strength and hydrogen embrittlement resistance of the obtained hot-stamped molded bodies were investigated.
　まず、表１に示す化学組成を有する溶鋼を連続鋳造法にて鋳造し、スラブを作製した。表１に示す成分以外の残部はＦｅ及び不純物である。これらのスラブを１１００℃以上の温度に加熱して所定の条件下で粗圧延し、次いで表２に示す条件で仕上げ圧延、冷却及び巻き取りを実施した。全ての発明例及び比較例において、仕上げ圧延終了後における急冷時の平均冷却速度は２００～１０００℃／秒の範囲内であり、冷却時間は０．２～２．０秒の範囲内であった。次いで、得られた熱延鋼板を３０～８０％の所定の圧下率にて冷間圧延を行った。次に、一部の鋼板については所定の条件下で焼鈍、めっき又は調質圧延を施した。次に、得られた鋼板を表２に示す条件でホットスタンプ成形を行った。ホットスタンプ成形工程における加熱雰囲気及び加熱方法は特段の明示のあるものを除いてガス燃焼雰囲気（空燃比０．８５）及び炉加熱であった。ホットスタンプ成形後、一部のホットスタンプ成形体に焼き戻し処理又は部分軟化処理を行った。 First, molten steel having the chemical composition shown in Table 1 was cast by a continuous casting method to produce a slab. The remainder other than the components shown in Table 1 is Fe and impurities. These slabs were heated to a temperature of 1100° C. or higher and rough rolled under predetermined conditions, and then finished rolled, cooled, and wound up under the conditions shown in Table 2. In all invention examples and comparative examples, the average cooling rate during quenching after finish rolling was within the range of 200 to 1000°C/sec, and the cooling time was within the range of 0.2 to 2.0 seconds. . Next, the obtained hot rolled steel sheet was cold rolled at a predetermined rolling reduction of 30 to 80%. Next, some of the steel plates were annealed, plated, or temper rolled under predetermined conditions. Next, the obtained steel plate was hot stamped under the conditions shown in Table 2. The heating atmosphere and heating method in the hot stamp molding process were a gas combustion atmosphere (air-fuel ratio 0.85) and furnace heating, unless otherwise specified. After hot stamp molding, some of the hot stamp molded bodies were subjected to tempering treatment or partial softening treatment.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000002
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000003
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000004
Figure JPOXMLDOC01-appb-T000005
Figure JPOXMLDOC01-appb-T000005
Figure JPOXMLDOC01-appb-T000006
Figure JPOXMLDOC01-appb-T000006
Figure JPOXMLDOC01-appb-T000007
Figure JPOXMLDOC01-appb-T000007
Figure JPOXMLDOC01-appb-T000008
Figure JPOXMLDOC01-appb-T000008
Figure JPOXMLDOC01-appb-T000009
Figure JPOXMLDOC01-appb-T000009
Figure JPOXMLDOC01-appb-T000010
Figure JPOXMLDOC01-appb-T000010
Figure JPOXMLDOC01-appb-T000011
Figure JPOXMLDOC01-appb-T000011
Figure JPOXMLDOC01-appb-T000012
Figure JPOXMLDOC01-appb-T000012
Figure JPOXMLDOC01-appb-T000013
Figure JPOXMLDOC01-appb-T000013
Figure JPOXMLDOC01-appb-T000014
Figure JPOXMLDOC01-appb-T000014
　得られたホットスタンプ成形体の特性は以下の方法によって測定及び評価した。 The properties of the obtained hot-stamped molded product were measured and evaluated by the following methods.
［引張強さ］
　ホットスタンプ成形体の引張強さは、ホットスタンプ成形体の任意の位置からＪＩＳ　Ｚ　２２４１：２０１１に準拠して、５号試験片を作製し、引張試験を行うことで得た。なお、クロスヘッド速度は１ｍｍ／ｍｉｎとした。 [Tensile strength]
The tensile strength of the hot-stamped molded product was obtained by preparing a No. 5 test piece from any position of the hot-stamped molded product in accordance with JIS Z 2241:2011 and conducting a tensile test. Note that the crosshead speed was 1 mm/min.
［耐水素脆化特性］
　ホットスタンプ成形体の耐水素脆化特性は、以下のようにして評価した。まず、１．２ｔ×７．０Ｗ×６８Ｌ（ｍｍ）の試験片を作製し、４点曲げジグにより試験片に種々の歪み（応力）を付与し、次いで塩酸（室温、ｐＨ＝４）に４８時間浸漬し、割れが発生する限界歪み量を調査した。限界歪み量が０．６％以上の場合を合格（〇）、限界歪み量が０．６％未満の場合を不合格（×）として評価した。 [Hydrogen embrittlement resistance]
The hydrogen embrittlement resistance of the hot-stamped molded product was evaluated as follows. First, a test piece of 1.2t x 7.0W x 68L (mm) was prepared, various strains (stresses) were applied to the test piece using a 4-point bending jig, and then immersed in hydrochloric acid (room temperature, pH = 4) for 48 hours. The material was immersed for a period of time to investigate the critical amount of strain at which cracks would occur. A case where the limit strain amount was 0.6% or more was evaluated as a pass (○), and a case where the limit strain amount was less than 0.6% was evaluated as a fail (×).
　引張強さが２２００ＭＰａ以上でかつ耐水素脆化特性の評価が合格の場合を、高強度でかつ水素脆化を抑制可能なホットスタンプ成形体として評価した。その結果を表３に示す。表３では、巻取工程後のホットスタンプ用鋼板におけるフェライト及びパーライトの面積率並びにパーライトの分散指標を示している。フェライト及びパーライト以外の残部組織は、ベイナイト、マルテンサイト、残留オーステナイト、及び／又は微量の炭化物であった。同様に、表３では、ホットスタンプ成形体における硬質組織の面積率並びに板厚１／４位置における旧オーステナイト粒（旧γ粒）の硬さ分布の標準偏差を示している。硬質組織の面積率は、マルテンサイト、ベイナイト及び焼き戻しマルテンサイトの面積率の合計を意味する。また、当該硬質組織以外の残部組織は、フェライト、残留オーステナイト及び／又はパーライトであった。 If the tensile strength was 2200 MPa or more and the evaluation of hydrogen embrittlement resistance passed, it was evaluated as a hot-stamped molded article with high strength and capable of suppressing hydrogen embrittlement. The results are shown in Table 3. Table 3 shows the area ratio of ferrite and pearlite and the dispersion index of pearlite in the hot stamping steel sheet after the winding process. The remaining structures other than ferrite and pearlite were bainite, martensite, retained austenite, and/or a trace amount of carbide. Similarly, Table 3 shows the area ratio of hard structures in the hot-stamped compact and the standard deviation of the hardness distribution of prior austenite grains (prior γ grains) at the 1/4 position of the plate thickness. The area ratio of hard structure means the sum of the area ratios of martensite, bainite, and tempered martensite. Further, the remaining structure other than the hard structure was ferrite, retained austenite, and/or pearlite.
Figure JPOXMLDOC01-appb-T000015
Figure JPOXMLDOC01-appb-T000015
Figure JPOXMLDOC01-appb-T000016
Figure JPOXMLDOC01-appb-T000016
Figure JPOXMLDOC01-appb-T000017
Figure JPOXMLDOC01-appb-T000017
Figure JPOXMLDOC01-appb-T000018
Figure JPOXMLDOC01-appb-T000018
Figure JPOXMLDOC01-appb-T000019
Figure JPOXMLDOC01-appb-T000019
Figure JPOXMLDOC01-appb-T000020
Figure JPOXMLDOC01-appb-T000020
　表３を参照すると、比較例１では、Ｃ含有量が低かったために引張強さが低下した。比較例１３では、Ｃ含有量が高かったために強度が高くなりすぎてしまい、耐水素脆化特性が低下した。比較例１４では、Ｓｉ含有量が低かったために引張強さが低下した。比較例２５では、Ｓｉ含有量が高かったためにホットスタンプ用鋼板においてフェライト量が増加し、所望の金属組織が得られず、パーライトの分散指標が０．５０未満となった。その結果としてホットスタンプ成形体において旧オーステナイト粒の硬さ分布の標準偏差を所望の範囲に制御することができず、耐水素脆化特性が低下した。比較例２６では、Ｍｎ含有量が低かったためにホットスタンプ用鋼板におけるパーライトの分散指標及びホットスタンプ成形体における旧オーステナイト粒の硬さ分布の標準偏差を所望の範囲内に制御することができず、耐水素脆化特性が低下した。比較例４１では、Ｍｎ含有量が高かったために熱延鋼板においてオーステナイトからパーライトへの変態が促進されすぎてしまったと考えられる。その結果としてホットスタンプ用鋼板におけるパーライトの分散指標及びホットスタンプ成形体における旧オーステナイト粒の硬さ分布の標準偏差を所望の範囲内に制御することができず、耐水素脆化特性が低下した。比較例５０、５９、６８、７７、７８及び９０はＰ、Ｓ、Ｎ、Ｏ又はＡｌ含有量が適切でなかったために耐水素脆化特性が低下した。比較例９１、１０３、１１５、１２７及び１３９では、それぞれＮｂ、Ｔｉ、Ｂ、Ｃｒ及びＭｏ含有量が低かったために強度を十分に向上させることができず、引張強さが低下した。比較例１０２、１１４、１２６、１３８及び１５０では、それぞれＮｂ、Ｔｉ、Ｂ、Ｃｒ及びＭｏ含有量が高かったために鋼中に多量に炭窒化物が生成し又は粗大な金属間化合物が生成したと考えられ、結果として耐水素脆化特性が低下した。 Referring to Table 3, in Comparative Example 1, the tensile strength decreased because the C content was low. In Comparative Example 13, the strength became too high due to the high C content, and the hydrogen embrittlement resistance deteriorated. In Comparative Example 14, the tensile strength decreased because the Si content was low. In Comparative Example 25, the amount of ferrite increased in the hot stamping steel sheet due to the high Si content, the desired metal structure could not be obtained, and the pearlite dispersion index was less than 0.50. As a result, the standard deviation of the hardness distribution of prior austenite grains in the hot-stamped compact could not be controlled within a desired range, and the hydrogen embrittlement resistance deteriorated. In Comparative Example 26, because the Mn content was low, the dispersion index of pearlite in the hot stamping steel sheet and the standard deviation of the hardness distribution of prior austenite grains in the hot stamping compact could not be controlled within the desired range, Hydrogen embrittlement resistance decreased. It is considered that in Comparative Example 41, the transformation from austenite to pearlite was promoted too much in the hot rolled steel sheet due to the high Mn content. As a result, the dispersion index of pearlite in the hot-stamped steel sheet and the standard deviation of the hardness distribution of prior austenite grains in the hot-stamped molded product could not be controlled within desired ranges, and the hydrogen embrittlement resistance deteriorated. In Comparative Examples 50, 59, 68, 77, 78, and 90, the hydrogen embrittlement resistance deteriorated because the P, S, N, O, or Al content was not appropriate. In Comparative Examples 91, 103, 115, 127, and 139, the strength could not be sufficiently improved because the Nb, Ti, B, Cr, and Mo contents were low, and the tensile strength decreased. In Comparative Examples 102, 114, 126, 138, and 150, a large amount of carbonitrides or coarse intermetallic compounds were generated in the steel because the Nb, Ti, B, Cr, and Mo contents were high, respectively. As a result, the hydrogen embrittlement resistance decreased.
　比較例２５９では、熱間圧延工程の仕上げ圧延における最終段の圧下率が低かったために圧延後の熱延鋼板においてパーライトを均一に分散させることができなかったと考えられる。その結果としてホットスタンプ用鋼板におけるパーライトの分散指標及びホットスタンプ成形体における旧オーステナイト粒の硬さ分布の標準偏差を所望の範囲内に制御することができず、耐水素脆化特性が低下した。比較例２７０では、仕上げ圧延終了後から急冷開始までの時間が長かったためにオーステナイト粒の粒成長を十分に抑制することができず、フェライトが連結して配置し、パーライトを均一に分散させることができなかったと考えられる。その結果としてホットスタンプ用鋼板におけるパーライトの分散指標及びホットスタンプ成形体における旧オーステナイト粒の硬さ分布の標準偏差を所望の範囲内に制御することができず、耐水素脆化特性が低下した。比較例２７１では、冷却工程における急冷後の平均冷却速度が遅かったために、ホットスタンプ用鋼板において所望の金属組織を形成することができなかった。その結果としてホットスタンプ用鋼板におけるパーライトの分散指標及びホットスタンプ成形体における旧オーステナイト粒の硬さ分布の標準偏差を所望の範囲内に制御することができず、耐水素脆化特性が低下した。比較例２７６では、巻取温度が低かったために、ホットスタンプ用鋼板において所望の金属組織を形成することができなかった。その結果としてホットスタンプ成形体における旧オーステナイト粒の硬さ分布の標準偏差を所望の範囲内に制御することができず、耐水素脆化特性が低下した。比較例２８４では、巻取温度が高かったために、粒成長が生じてパーライトの均一な分散が阻害されたものと考えられる。その結果としてホットスタンプ用鋼板におけるパーライトの分散指標及びホットスタンプ成形体における旧オーステナイト粒の硬さ分布の標準偏差を所望の範囲内に制御することができず、耐水素脆化特性が低下した。 In Comparative Example 259, it is considered that pearlite could not be uniformly dispersed in the hot-rolled steel sheet after rolling because the rolling reduction ratio in the final stage of finish rolling in the hot rolling process was low. As a result, the dispersion index of pearlite in the hot-stamped steel sheet and the standard deviation of the hardness distribution of prior austenite grains in the hot-stamped molded product could not be controlled within desired ranges, and the hydrogen embrittlement resistance deteriorated. In Comparative Example 270, the grain growth of austenite grains could not be sufficiently suppressed because the time from the end of finish rolling to the start of quenching was long, so ferrite was arranged in a connected manner and pearlite could not be uniformly dispersed. It is considered that it could not be done. As a result, the dispersion index of pearlite in the hot-stamped steel sheet and the standard deviation of the hardness distribution of prior austenite grains in the hot-stamped molded product could not be controlled within desired ranges, and the hydrogen embrittlement resistance deteriorated. In Comparative Example 271, the average cooling rate after quenching in the cooling process was slow, so the desired metal structure could not be formed in the hot stamping steel sheet. As a result, the dispersion index of pearlite in the hot-stamped steel sheet and the standard deviation of the hardness distribution of prior austenite grains in the hot-stamped molded product could not be controlled within desired ranges, and the hydrogen embrittlement resistance deteriorated. In Comparative Example 276, the desired metal structure could not be formed in the hot stamping steel sheet because the winding temperature was low. As a result, the standard deviation of the hardness distribution of prior austenite grains in the hot-stamped compact could not be controlled within a desired range, and the hydrogen embrittlement resistance deteriorated. In Comparative Example 284, it is considered that the high winding temperature caused grain growth and inhibited the uniform dispersion of pearlite. As a result, the dispersion index of pearlite in the hot-stamped steel sheet and the standard deviation of the hardness distribution of prior austenite grains in the hot-stamped molded product could not be controlled within desired ranges, and the hydrogen embrittlement resistance deteriorated.
　これとは対照的に、全ての発明例に係るホットスタンプ用鋼板及びホットスタンプ成形体において所定の化学組成及び金属組織を有し、ホットスタンプ用鋼板におけるパーライトの分散指標を０．５０以上となるよう制御するとともに、ホットスタンプ成形体において旧オーステナイト粒の硬さ分布の標準偏差を１５０Ｈｖ以下となるよう制御することで、２２００ＭＰａ以上の高い引張強さを有するにもかかわらず、水素脆化を確実に抑制することができた。また、全ての発明例に係るホットスタンプ成形体において、板厚１／４位置における旧オーステナイト粒の硬さ（各発明例における全硬さ測定値の平均）は５００～１０００Ｈｖの範囲に制御されていた。 In contrast, all of the hot stamping steel sheets and hot stamping molded bodies according to the invention examples have a predetermined chemical composition and metal structure, and the pearlite dispersion index in the hot stamping steel sheets is 0.50 or more. By controlling the standard deviation of the hardness distribution of prior austenite grains in the hot-stamped molded product to be 150Hv or less, hydrogen embrittlement is ensured despite having a high tensile strength of 2200MPa or more. was able to be suppressed. In addition, in the hot-stamped molded products according to all invention examples, the hardness of prior austenite grains at the 1/4 plate thickness position (average of all hardness measurements in each invention example) is controlled within the range of 500 to 1000 Hv. Ta.

Claims (4)

  1. 　質量％で、
    　Ｃ　：０．４０～０．７０％、
    　Ｓｉ：０．０１０～１．３００％、
    　Ｍｎ：０．６０～３．００％、
    　Ｐ　：０．１００％以下、
    　Ｓ　：０．０１００％以下、
    　Ｎ　：０．０２００％以下、
    　Ｏ　：０．０２００％以下、
    　Ａｌ：０．００１０～０．５０００％、
    　Ｎｂ：０．００１０～０．１００％、
    　Ｔｉ：０．０１０～０．２００％、
    　Ｂ　：０．０００５～０．０２００％、
    　Ｃｒ：０．０１０～０．８０％、
    　Ｍｏ：０．００１０～１．０００％、
    　Ｃｏ：０～２．００％、
    　Ｎｉ：０～３．００％、
    　Ｃｕ：０～１．００％、
    　Ｖ　：０～１．００％、
    　Ｗ　：０～１．０００％、
    　Ｃａ：０～０．０１０％、
    　Ｍｇ：０～１．０００％、
    　ＲＥＭ：０～１．０００％、
    　Ｓｂ：０～１．０００％、
    　Ｚｒ：０～１．０００％、
    　Ｓｎ：０～１．０００％、
    　Ａｓ：０～０．１００％、並びに
    　残部：Ｆｅ及び不純物からなる化学組成を有し、
    　面積率で、
    　フェライト：１０％以上、及び
    　パーライト：１０％以上
    を含み、フェライトとパーライトの合計が８０％以上であり、
    　パーライトの分散指標が０．５０以上である金属組織を有する、ホットスタンプ用鋼板。     In mass%,
    C: 0.40-0.70%,
    Si: 0.010-1.300%,
    Mn: 0.60-3.00%,
    P: 0.100% or less,
    S: 0.0100% or less,
    N: 0.0200% or less,
    O: 0.0200% or less,
    Al: 0.0010-0.5000%,
    Nb: 0.0010 to 0.100%,
    Ti: 0.010-0.200%,
    B: 0.0005-0.0200%,
    Cr: 0.010-0.80%,
    Mo: 0.0010-1.000%,
    Co: 0-2.00%,
    Ni: 0-3.00%,
    Cu: 0 to 1.00%,
    V: 0 to 1.00%,
    W: 0-1.000%,
    Ca: 0-0.010%,
    Mg: 0-1.000%,
    REM: 0-1.000%,
    Sb: 0 to 1.000%,
    Zr: 0 to 1.000%,
    Sn: 0-1.000%,
    Has a chemical composition consisting of As: 0 to 0.100%, and the balance: Fe and impurities,
    In area ratio,
    ferrite: 10% or more, and pearlite: 10% or more, the total of ferrite and pearlite is 80% or more,
    A steel plate for hot stamping, which has a metal structure in which a pearlite dispersion index is 0.50 or more.
  2. 　前記化学組成が、質量％で、
    　Ｃｏ：０．００１～２．００％、
    　Ｎｉ：０．００１～３．００％、
    　Ｃｕ：０．００１～１．００％、
    　Ｖ　：０．００１～１．００％、
    　Ｗ　：０．００１～１．０００％、
    　Ｃａ：０．０００１～０．０１０％、
    　Ｍｇ：０．０００１～１．０００％、
    　ＲＥＭ：０．０００１～１．０００％、
    　Ｓｂ：０．００１～１．０００％、
    　Ｚｒ：０．００１～１．０００％、
    　Ｓｎ：０．００１～１．０００％、及び
    　Ａｓ：０．００１～０．１００％
    からなる群から選択される１種又は２種以上を含む、請求項１に記載のホットスタンプ用鋼板。     The chemical composition is in mass%,
    Co: 0.001 to 2.00%,
    Ni: 0.001 to 3.00%,
    Cu: 0.001 to 1.00%,
    V: 0.001 to 1.00%,
    W: 0.001-1.000%,
    Ca: 0.0001-0.010%,
    Mg: 0.0001-1.000%,
    REM: 0.0001-1.000%,
    Sb: 0.001 to 1.000%,
    Zr: 0.001 to 1.000%,
    Sn: 0.001 to 1.000%, and As: 0.001 to 0.100%
    The hot stamping steel plate according to claim 1, comprising one or more selected from the group consisting of:
  3. 　質量％で、
    　Ｃ　：０．４０～０．７０％、
    　Ｓｉ：０．０１０～１．３００％、
    　Ｍｎ：０．６０～３．００％、
    　Ｐ　：０．１００％以下、
    　Ｓ　：０．０１００％以下、
    　Ｎ　：０．０２００％以下、
    　Ｏ　：０．０２００％以下、
    　Ａｌ：０．００１０～０．５０００％、
    　Ｎｂ：０．００１０～０．１００％、
    　Ｔｉ：０．０１０～０．２００％、
    　Ｂ　：０．０００５～０．０２００％、
    　Ｃｒ：０．０１０～０．８０％、
    　Ｍｏ：０．００１０～１．０００％、
    　Ｃｏ：０～２．００％、
    　Ｎｉ：０～３．００％、
    　Ｃｕ：０～１．００％、
    　Ｖ　：０～１．００％、
    　Ｗ　：０～１．０００％、
    　Ｃａ：０～０．０１０％、
    　Ｍｇ：０～１．０００％、
    　ＲＥＭ：０～１．０００％、
    　Ｓｂ：０～１．０００％、
    　Ｚｒ：０～１．０００％、
    　Ｓｎ：０～１．０００％、
    　Ａｓ：０～０．１００％、並びに
    　残部：Ｆｅ及び不純物からなる化学組成を有し、
    　面積率で、マルテンサイト、ベイナイト及び焼き戻しマルテンサイトの少なくとも１種：合計で９０％以上を含み、
    　板厚１／４位置における旧オーステナイト粒の硬さ分布の標準偏差が１５０Ｈｖ以下である金属組織を有する、ホットスタンプ成形体。     In mass%,
    C: 0.40-0.70%,
    Si: 0.010-1.300%,
    Mn: 0.60-3.00%,
    P: 0.100% or less,
    S: 0.0100% or less,
    N: 0.0200% or less,
    O: 0.0200% or less,
    Al: 0.0010-0.5000%,
    Nb: 0.0010 to 0.100%,
    Ti: 0.010-0.200%,
    B: 0.0005-0.0200%,
    Cr: 0.010-0.80%,
    Mo: 0.0010-1.000%,
    Co: 0-2.00%,
    Ni: 0-3.00%,
    Cu: 0 to 1.00%,
    V: 0 to 1.00%,
    W: 0-1.000%,
    Ca: 0-0.010%,
    Mg: 0-1.000%,
    REM: 0-1.000%,
    Sb: 0 to 1.000%,
    Zr: 0 to 1.000%,
    Sn: 0-1.000%,
    Has a chemical composition consisting of As: 0 to 0.100%, and the balance: Fe and impurities,
    Contains at least one of martensite, bainite and tempered martensite: 90% or more in total in terms of area percentage,
    A hot-stamped molded product having a metal structure in which the standard deviation of the hardness distribution of prior austenite grains at a position of 1/4 of the plate thickness is 150 Hv or less.
  4. 　前記化学組成が、質量％で、
    　Ｃｏ：０．００１～２．００％、
    　Ｎｉ：０．００１～３．００％、
    　Ｃｕ：０．００１～１．００％、
    　Ｖ　：０．００１～１．００％、
    　Ｗ　：０．００１～１．０００％、
    　Ｃａ：０．０００１～０．０１０％、
    　Ｍｇ：０．０００１～１．０００％、
    　ＲＥＭ：０．０００１～１．０００％、
    　Ｓｂ：０．００１～１．０００％、
    　Ｚｒ：０．００１～１．０００％、
    　Ｓｎ：０．００１～１．０００％、及び
    　Ａｓ：０．００１～０．１００％
    からなる群から選択される１種又は２種以上を含む、請求項３に記載のホットスタンプ成形体。     The chemical composition is in mass%,
    Co: 0.001 to 2.00%,
    Ni: 0.001 to 3.00%,
    Cu: 0.001 to 1.00%,
    V: 0.001 to 1.00%,
    W: 0.001-1.000%,
    Ca: 0.0001-0.010%,
    Mg: 0.0001-1.000%,
    REM: 0.0001-1.000%,
    Sb: 0.001 to 1.000%,
    Zr: 0.001 to 1.000%,
    Sn: 0.001 to 1.000%, and As: 0.001 to 0.100%
    The hot-stamped molded article according to claim 3, comprising one or more selected from the group consisting of:
PCT/JP2023/007831 2022-03-31 2023-03-02 Steel sheet for hot stamping and hot stamp molded body WO2023189175A1 (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
JP2024511544A JPWO2023189175A1 (en) 2022-03-31 2023-03-02

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
JP2022060634 2022-03-31
JP2022-060634 2022-03-31

Publications (1)

Publication Number Publication Date
WO2023189175A1 true WO2023189175A1 (en) 2023-10-05

Family

ID=88201207

Family Applications (1)

Application Number Title Priority Date Filing Date
PCT/JP2023/007831 WO2023189175A1 (en) 2022-03-31 2023-03-02 Steel sheet for hot stamping and hot stamp molded body

Country Status (2)

Country Link
JP (1) JPWO2023189175A1 (en)
WO (1) WO2023189175A1 (en)

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2020162509A1 (en) * 2019-02-05 2020-08-13 日本製鉄株式会社 Steel member, steel sheet, and methods for producing same
JP2021155793A (en) * 2020-03-26 2021-10-07 日本製鉄株式会社 Steel plate for hot-stamping component and method for manufacturing the same
WO2022239731A1 (en) * 2021-05-13 2022-11-17 日本製鉄株式会社 Steel sheet for hot stamping and hot stamping molded body

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2020162509A1 (en) * 2019-02-05 2020-08-13 日本製鉄株式会社 Steel member, steel sheet, and methods for producing same
JP2021155793A (en) * 2020-03-26 2021-10-07 日本製鉄株式会社 Steel plate for hot-stamping component and method for manufacturing the same
WO2022239731A1 (en) * 2021-05-13 2022-11-17 日本製鉄株式会社 Steel sheet for hot stamping and hot stamping molded body

Non-Patent Citations (1)

* Cited by examiner, † Cited by third party
Title
ACTA MATERIALIA, vol. 58, 2010, pages 6393 - 6403

Also Published As

Publication number Publication date
JPWO2023189175A1 (en) 2023-10-05

Similar Documents

Publication Publication Date Title
RU2557035C1 (en) High-strength cold-rolled sheet steel and method of its production
KR101420035B1 (en) Pressed member and method for producing same
EP3214199B1 (en) High-strength steel sheet, high-strength hot-dip galvanized steel sheet, high-strength hot-dip aluminum-coated steel sheet, and high-strength electrogalvanized steel sheet, and methods for manufacturing same
KR101509362B1 (en) Method for manufacturing hot stamped body having vertical wall, and hot stamped body having vertical wall
KR101479391B1 (en) Cold rolled steel sheet having excellent shape fixability and method for manufacturing the same
JP6795042B2 (en) Hot stamp molded product and its manufacturing method
US20120040203A1 (en) High strength galvanized steel sheet having excellent formability, weldability, and fatigue properties and method for manufacturing the same
EP2799568A1 (en) High-strength steel sheet and method for manufacturing same
WO2018033960A1 (en) Hot press-formed member
KR20180119616A (en) Steel plate for hot press, method of manufacturing the same, and hot press member and manufacturing method thereof
EP2896710B1 (en) Hot-rolled steel sheet and method for manufacturing the same
JP7215646B1 (en) High-strength steel plate and its manufacturing method
KR102477508B1 (en) Hot-dip galvanized steel and alloyed hot-dip galvanized steel
JP6841383B2 (en) Steel plate and its manufacturing method
JP7350057B2 (en) hot stamp molded body
EP3875616B1 (en) Steel sheet, member, and methods for producing them
JP7216933B2 (en) Steel plate and its manufacturing method
CN115087755A (en) Hot press molded article
CN115461482A (en) Steel sheet, component and method for producing same
JP6098537B2 (en) High-strength cold-rolled steel sheet and manufacturing method thereof
WO2023095920A1 (en) Steel member and steel sheet
WO2023002910A1 (en) Cold-rolled steel sheet and manufacturing method thereof
KR20220129615A (en) Steel plate, member and manufacturing method thereof
KR20220129616A (en) Steel plate, member and manufacturing method thereof
WO2023189175A1 (en) Steel sheet for hot stamping and hot stamp molded body

Legal Events

Date Code Title Description
121 Ep: the epo has been informed by wipo that ep was designated in this application

Ref document number: 23779204

Country of ref document: EP

Kind code of ref document: A1

ENP Entry into the national phase

Ref document number: 2024511544

Country of ref document: JP

Kind code of ref document: A

WWE Wipo information: entry into national phase

Ref document number: 1020247031688

Country of ref document: KR