US20250101557A1 - Hot-stamping formed body - Google Patents

Hot-stamping formed body Download PDF

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Publication number
US20250101557A1
US20250101557A1 US18/832,024 US202318832024A US2025101557A1 US 20250101557 A1 US20250101557 A1 US 20250101557A1 US 202318832024 A US202318832024 A US 202318832024A US 2025101557 A1 US2025101557 A1 US 2025101557A1
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steel
present
hot
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content
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Yuri Toda
Shingo FUJINAKA
Jun Haga
Yuma Asada
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Nippon Steel Corp
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Nippon Steel Corp
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Assigned to NIPPON STEEL CORPORATION reassignment NIPPON STEEL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: ASADA, YUMA, FUJINAKA, Shingo, HAGA, JUN, TODA, Yuri
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
    • B21D22/02—Stamping using rigid devices or tools
    • B21D22/022—Stamping using rigid devices or tools by heating the blank or stamping associated with heat treatment
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
    • B21D22/20—Deep-drawing
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
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    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/74—Methods of treatment in inert gas, controlled atmosphere, vacuum or pulverulent material
    • C21D1/76—Adjusting the composition of the atmosphere
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
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Definitions

  • the present invention relates to a hot-stamping formed body.
  • Hot stamping is attracting attention as a technique that achieves both the formability of a steel sheet into a vehicle member and strength of a vehicle member by performing hardening of the steel sheet in a die at the same time as press working.
  • Patent Document 1 discloses an electrolytic zinc-based plated steel sheet having a high yield ratio and excellent bendability, in which the critical hydrogen amount in the steel is 0.20 mass ppm or less.
  • Hydrogen embrittlement cracking is a phenomenon in which a steel member, to which high stress is applied in use, suddenly fractures due to hydrogen which is irrupted into the steel from an external environment. This phenomenon is also called delayed fracture due to the mode of the occurrence of fracture. It is generally known that hydrogen embrittlement cracking is more likely to occur in the steel sheet as tensile strength of the steel sheet increases. It is considered that this is because the higher tensile strength of the steel sheet, the greater residual stress in the steel sheet after a component is formed. This susceptibility to hydrogen embrittlement cracking (delayed fracture) is called hydrogen embrittlement resistance.
  • Early fracture is a phenomenon in which fracture occurs at a stress lower than tensile strength estimated from the hardness of the steel member. This susceptibility to early fracture is called early fracture resistance:
  • Patent Document 1 bendability is considered, but hydrogen embrittlement resistance and early fracture resistance are not considered.
  • An object of the present invention is to provide a hot-stamping formed body having high strength, and excellent hydrogen embrittlement resistance and early fracture resistance.
  • the gist of the present invention is as follows.
  • a hot-stamping formed body comprising, as a chemical composition, by mass %;
  • the hot-stamping formed body according to [ 1 ] may comprise, as the chemical composition, by mass %, one or more selected from the group consisting of:
  • FIG. 1 A FIGURE explaining the method to obtain a deboronization index.
  • the present inventors found that by reducing the standard deviation of grain sizes of prior austenite grains in the interior region, hydrogen embrittlement resistance and early fracture resistance of the hot-stamping formed body can be improved.
  • the present inventors found that in the surface layer region, by generating a desired amount of bainite, by creating the texture with a desired crystal orientation, and by achieving a desired deboronization index, hydrogen embrittlement resistance can be further improved.
  • the present inventors found that in order to obtain a hot-stamping formed body having the above features, it is particularly effective to perform finish rolling and annealing under desired conditions during manufacturing of a steel sheet for hot stamping.
  • the hot-stamping formed body according to the present embodiment will be described in detail. First, the reason the chemical composition of the hot-stamping formed body according to the present embodiment is limited will be described.
  • a limited numerical range described using “to” described below includes a lower limit and an upper limit. Numerical values represented using “less than” or “more than” are not included in a numerical range. All percentages (%) related to the chemical composition mean mass %.
  • the hot-stamped formed body comprises, as a chemical composition, by mass %, C: more than 0.40% and 0.70% or less, Si: 0.010% to 3.00%, Mn: 0.60% to 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% to 0.5000%, Nb: 0.0010% to 0.100%, Ti: 0.010% to 0.200%, Cr: 0.01% to 0.80%, Mo: 0.0010% to 1.000%, B: 0.0005% to 0.0200%, and a remainder: Fe and impurities.
  • C by mass %
  • C more than 0.40% and 0.70% or less
  • Mn 0.60% to 3.00%
  • P 0.100% or less
  • S 0.100% or less
  • S 0.100% or less
  • S 0.100% or less
  • N: 0.0200% or less O: 0.0200
  • C is an element that improves the strength of the hot-stamping formed body.
  • the C content is set to more than 0.40%.
  • the C content is preferably 0.42% or more or 0.44% or more.
  • the C content is set to 0.70% or less.
  • the C content is preferably 0.65% or less or 0.60% or less.
  • Si is an element that improves strength of the hot-stamping formed body by solid-solution strengthening.
  • the Si content is set to 0.010% or more.
  • the Si content is preferably 0.05% or more, 0.10% or more or 0.15% or more.
  • the Si content is set to 3.00% or less.
  • the Si content is preferably 2.00% or less, 1.00% or less or 0.70% or less.
  • Mn promotes the transformation from prior austenite to pearlite in a hot-rolled steel sheet having the chemical composition of the present embodiment, and contributes to control of grain size distribution of prior austenite of the hot-stamping formed body.
  • the Mn content is set to 0.60% or more.
  • the Mn content is preferably 0.70% or more or 1.00% or more.
  • the Mn content is set to 3.00% or less.
  • the Mn content is preferably 2.50% or less or 2.30% or less.
  • the P is an impurity element, and by segregating in the grain boundaries, it becomes a starting point for fracture and deteriorates early fracture resistance. For this reason, the P content is set to 0.100% or less.
  • the P content is preferably 0.050% or less or 0.010% or less.
  • the lower limit of the P content is not particularly limited, but may be 0%. However, when the P content is reduced to less than 0.0001%, the dephosphorization cost increases significantly, which is not preferable economically. For this reason, the P content may be set to 0.0001% or more, 0.001% or more or 0.005% or more.
  • S is an impurity element, and forms inclusions in steel.
  • the inclusions become starting points for fracture and deteriorate early fracture resistance. For this reason, the S content is set to 0.0100% or less.
  • the S content is preferably 0.0080% or less, 0.0050% or less or 0.0030% or less.
  • the lower limit of the S content is not particularly limited, but may be 0%. However, when the S content is reduced to less than 0.0001%, the desulfurization cost increases significantly, which is not preferable economically. For this reason, the S content may be set to 0.0001% or more, 0.0002% or more, 0.0003% or more or 0.0010% or more.
  • N is an impurity element, and forms nitrides in steel.
  • the nitrides become starting points for fracture and deteriorate early fracture resistance.
  • the N content is set to 0.0200% or less.
  • the N content is preferably 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 may be 0%. However, when the N content is reduced to less than 0.0001%, the denitrification cost increases significantly, which is not preferable economically. For this reason, the N content may be set to 0.0001% or more or 0.0010% or more.
  • the O content is set to 0.0200% or less.
  • the O content is preferably 0.0100% or less, 0.0070% or less or 0.0040% or less.
  • the O content may be 0%, in order to disperse many oxides during deoxidizing of molten steel, the O content may be set to 0.0005% or more or 0.0010% or more.
  • Al is an element having an effect of deoxidizing molten steel and achieving soundness of the steel.
  • the Al content is set to 0.0010% or more.
  • the Al content is preferably 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, or 0.2000% or less or 0.1000% or less.
  • Nb is an element that forms carbonitride in steel and improves strength of the hot-stamping formed body by precipitation strengthening.
  • 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.
  • the Nb content is set to 0.100% or less.
  • the Nb content is preferably 0.080% or less or 0.060% or less.
  • Ti is an element that forms carbonitride in steel and improves strength of the hot-stamping formed body by precipitation strengthening.
  • the Ti content is set to 0.010% or more.
  • the Ti content is preferably 0.020% or more or 0.025% or more.
  • the Ti content is set to 0.200% or less.
  • the Ti content is preferably 0.150% or less, 0.100% or less, 0.080% or less, 0.060% or less or 0.050% or less.
  • Cr is an element that increases strength of the hot-stamping formed body by dissolving in prior austenite grains during heating before hot stamping.
  • the Cr content is set to 0.01% or more.
  • the Cr content is preferably 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, 0.50% or less or 0.40% or less.
  • Mo is an element that increases strength of the hot-stamping formed body by dissolving in prior austenite grains during heating before hot stamping.
  • Mo content is set to 0.0010% or more.
  • the Mo content is preferably 0.010% or more, 0.050% or more or 0.100% or more.
  • the Mo content is set to 1.000% or less.
  • the Mo content is preferably 0.800% or less, 0.600% or less or 0.400% or less.
  • B is an element that improves the hardenability of steel.
  • the B content is set to 0.0005% or more.
  • the B content is preferably 0.0010% or more or 0.0015% 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, 0.0040% or less or 0.0030% or less.
  • the remainder of the chemical composition of the hot-stamping formed body may be Fe and impurities.
  • Elements which are unavoidably mixed from a steel raw material or scrap and/or during the manufacture of steel and are allowed in a range where the properties of the hot-stamping formed body according to the present embodiment do not deteriorate are exemplary examples of the impurities.
  • the hot-stamping formed body may comprise the following elements as: optional elements.
  • the content of the following optional elements obtained in a case where the following optional elements are not contained is 0%.
  • Co is an element that improves strength of the hot-stamping formed body by solid-solution strengthening. In order to reliably obtain the effect, it is preferable that the Co content be set to 0.01% or more. The Co content is more preferably set to 0.05% or more.
  • the Co content is set to 4.00% or less. If necessary, the upper limit of Co content may be set to 1.00%, 0.50%, 0.10%, 0.05% or 0.02%.
  • the Ni content is preferably set to 3.00% or less. If necessary, the upper limit of Ni content may be set to 1.50%, 1.00%, 0.50%, 0.10%, 0.05% or 0.02%.
  • the Cu has an effect of increasing strength of the hot-stamping formed body by dissolving in prior austenite grains during heating before hot stamping.
  • the Cu content is preferably set to 0.01% or more.
  • the Cu content is more preferably set to 0.05% or more.
  • the Cu content is preferably set to 3.00% or less. If necessary, the upper limit of Cu content may be set to 1.50%, 1.00%, 0.50%, 0.10%, 0.05% or 0.02%.
  • V 0% to 3.00%
  • V has an effect of forming carbonitride in steel and improves strength of the hot-stamping formed body by precipitation strengthening.
  • the V content is preferably set to 0.01% or more.
  • the V content is more preferably set to 0.05% or more.
  • the V content is set to 3.00% or less. If necessary, the upper limit of V content may be set to 1.50%, 1.00%, 0.50%, 0.10%, 0.05% or 0.02%.
  • the W has an effect of improving strength of the hot-stamping formed body.
  • the W content is preferably set to 0.01% or more.
  • the W content is preferably set to 0.05% or more.
  • the W content is preferably set to 3.00% or less. If necessary, the upper limit of W content may be set to 1.50%, 1.00%, 0.50%, 0.10%, 0.05% or 0.02%.
  • Ca is an element that suppresses generation of carbides that become starting points for fracture, and contributes to improvement of early fracture resistance.
  • the Ca content is preferably set to 0.001% or more.
  • the Ca content is set to 1.000% or less. If necessary, the upper limit of Ca content may be set to 0.100%, 0.010%, 0.005%, 0.001%, 0.0005% or 0.0002%.
  • Mg forms oxides and sulfides in molten steel, suppresses formation of a coarse MnS, disperses a lot of fine oxides, miniaturizes the microstructure, and contributes to improvement of early fracture resistance.
  • the Mg content is preferably set to 0.001% or more.
  • the Mg content is set to 1.000% or less. If necessary, the upper limit of Mg content may be set to 0.100%, 0.010%, 0.005%, 0.001%, 0.0005% or 0.0002%.
  • the REM suppresses generation of oxides that become starting points of fracture and contributes to improvement of early fracture resistance.
  • the REM content is preferably set to 0.001% or more.
  • the REM content is set to 1.000% or less. If necessary, the upper limit of REM content may be set to 0.100%, 0.010%, 0.005%, 0.001%, 0.0005% or 0.0002%.
  • REM refers to a total of 17 elements that are composed of Sc, Y and lanthanoid, and the REM content refers to the total content of these elements,
  • the Sb suppresses generation of oxides that become starting points of fracture and contributes to improvement of early fracture resistance.
  • the Sb content is preferably set to 0.001% or more.
  • the Sb content is set to 1.000% or less. If necessary, the upper limit of Sb content may be set to 0.100%, 0.050%, 0.020%, 0,010%, 0.005% or 0.002%,
  • the Sn suppresses generation of oxides that become starting points of fracture and contributes to improvement of early fracture resistance.
  • the Sn content is preferably set to 0.001% or more.
  • the Sn content is set to 1.000% or less. If necessary, the upper limit of Sn content may be set to 0.100%, 0.050%, 0.020%, 0.010%, 0.005% or 0.002%.
  • the Zr suppresses generation of oxides that become starting points of fracture and contributes to improvement of early fracture resistance.
  • the Zr content is preferably set to 0.001% or more.
  • the Zr content is set to 1.000% or less. If necessary, the upper limit of Zr content may be set to 0.100%, 0.050%, 0.020%, 0.010%, 0.005% or 0.002%.
  • the As content is preferably set to 0.001% or more.
  • the As content is set to 0.100% or less. If necessary, the upper limit of As content may be set to 0.100%, 0.050%, 0.020%, 0,010%, 0.005% or 0.002%.
  • the above-mentioned chemical composition of the hot-stamping formed body may be measured by a standard analysis method.
  • the chemical composition may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES).
  • ICP-AES inductively coupled plasma-atomic emission spectrometry
  • C and S may be measured using a combustion-infrared absorption method
  • N may be measured using an inert gas fusion-thermal conductivity method
  • O may be measured using an inert gas fusion-nondispersive infrared absorption method.
  • the chemical composition is analyzed after the plating layer or the coating film is removed by mechanical grinding.
  • the standard deviation of grain sizes of prior austenite grains is 5.0 ⁇ m or less; in the surface layer region, which is a region between the surface and 1/25 depth of the sheet thickness from the surface, the area ratio of bainite is more than 10%, the maximum value of pole density of the texture is 4.0 or less, and the deboronization index is 0.05 or more.
  • the interior region in the present embodiment refers to a region between 4/16 depth of the sheet thickness from the surface of the hot-stamping formed body and 5/16 depth of the sheet thickness from the surface.
  • the “surface” refers to the interface of the plating layer or the coating film and the base steel sheet, and for convenience, the plating layer or the coating film is excluded from the hot-stamping formed body.
  • the plating layer or the coating film is excluded from the hot-stamping formed body, when the thickness of the plating layer or the coating film is very small compared to the sheet thickness (thickness) of the hot-stamping formed body and can be ignored (however, when only the plating layer is formed, the thickness of the plating layer is often very small and can be ignored in most cases), when measuring the sheet thickness (thickness) of the hot-stamping formed body, the sheet thickness (thickness) of the hot-stamping formed body may be regarded as the sheet thickness (thickness) including the plating layer or the coating film.
  • the standard deviation of grain sizes of prior austenite grains is set to 5.0 ⁇ m or less, preferably 4.0 ⁇ m or less, 3.0 ⁇ m or less or 2.5 ⁇ m or less.
  • the lower limit of the standard deviation of grain sizes of prior austenite grains is not particularly limited, but may be set to 0.1 ⁇ m. 0.5 ⁇ m, 1.0 ⁇ m or 1.5 ⁇ m.
  • the standard deviation of grain sizes of prior austenite grains is obtained by the following method.
  • a sample is cut out from an arbitrary position away from an end surface of the hot-stamping formed body by a distance of 50 mm or more (a position that avoids an end portion in a case where the sample cannot be collected at this position) so that a sheet thickness cross section parallel to a rolling direction can be observed.
  • the size of the sample depends on a measurement device, but is set to a size that can be observed by about 10 mm in the rolling direction.
  • the cross section of the sample is mirror-finished using liquid in which diamond powder having a grain size in the range of 1 ⁇ m to 6 ⁇ m is dispersed in a diluted solution of alcohol or the like or pure water.
  • the observation surface is finished by electrolytic polishing.
  • a region which has a length of 50 ⁇ m and is present between 4/16 depth of the sheet thickness from the surface and 5/16 depth of the sheet thickness from the surface is measured at a measurement interval of 0.1 ⁇ m by an electron backscatter diffraction method, and thus, crystal orientation information is obtained.
  • An EBSD analyzer composed of a Schottky emission scanning electron microscope and an EBSD detector may be used for measurement, for example, an EBSD analyzer composed of JSM-7001F manufactured by JEOL Ltd. and DVC 5-type detector manufactured by TSL Solutions may be used for measurement.
  • the degree of vacuum in the EBSD analyzer may be set to 9.6 ⁇ 10 ⁇ 5 Pa or less, an accelerating voltage may be set to 15 kV, and an irradiation current level may be set to 13.
  • the crystal orientation of prior austenite grains is calculated from the crystal orientation relationship between general prior austenite grains and grains having a body-centered structure after transformation, and after calculating the average grain size of prior austenite grains using the crystal orientation, the standard deviation is calculated.
  • the method for calculating the crystal orientation of prior austenite grains is the following method.
  • the crystal orientation map of the prior austenite grains is created by the method described in Non-Patent Document 1.
  • an average value of a shortest diameter and a longest diameter is calculated, and the average value is regarded as the grain size of the prior austenite grain.
  • the above operation is performed on all prior austenite grains except for the prior austenite grains which are not entirely included in the photographed visual fields, such as grains in an end portion of the photographed visual field, and the grain sizes of all the prior austenite grains in the photographed visual fields are obtained.
  • the standard deviation of grain sizes of austenite grains is obtained.
  • the rolling direction of the hot-stamping formed body is determined by the following method.
  • a sample is cut out from an arbitrary position away from an end surface of the hot-stamping formed body by a distance of 50 mm or more so that a sheet thickness cross section parallel to a rolling direction can be observed.
  • observations with an optical microscope at 100, 200, 500, and 1000 magnifications are performed respectively.
  • an observation result with an appropriate magnification that the size of the inclusion can be measured is selected.
  • the observation area is width of 500 ⁇ m or more and full of the sheet thickness, and the areas with low brightness are determined to be inclusions. The observation may be performed at multiple fields when observing.
  • the cross-sectional observation of the plane parallel to the plane rotated in 5° increments is performed in the same way as the above method.
  • the average values of the lengths of the long axes of the plurality of inclusions in each cross section are calculated respectively.
  • the cross section in which the obtained average value of the length of the long axes of the inclusions is maximum is specified.
  • a direction parallel to the longitudinal direction of the inclusion in the cross section is determined as the rolling direction.
  • the microstructure of the interior region is not particularly limited as long as the desired strength, hydrogen embrittlement resistance and early fracture resistance can be obtained, for example, in area %, the microstructure may consist of martensite and bainite of 90% to 100% (90% or more and 100% or less) in total, and ferrite and residual austenite of 0% to 10% (0% or more and 10% or less) in total.
  • Martensite in the present embodiment includes untempered martensite (fresh martensite) and tempered martensite.
  • the microstructure of the hot-stamping formed body is measured by the following method.
  • a sample is cut out from an arbitrary position away from an end surface of the hot-stamping formed body by a distance of 50 mm or more (a position that avoids an end portion in a case where the sample cannot be collected at this position) so that a sheet thickness cross section parallel to the rolling direction can be observed.
  • the size of the sample depends on a measurement device, but is set to a size that can be observed by about 10 mm in the rolling direction.
  • the cross section of the sample After polishing the cross section of the sample using silicon carbide paper of #600 to #1500, the cross section is mirror-finished using liquid in which diamond powder having a grain size in the range of 1 ⁇ m to 6 ⁇ m is dispersed in a diluted solution of alcohol or the like or pure water.
  • the observation surface is finished by electrolytic polishing.
  • a region which has a length of 50 ⁇ m and is present between 4/16 depth of the sheet thickness from the surface and 5/16 depth of the sheet thickness from the surface is measured at a measurement interval of 0.1 ⁇ m by an electron backscatter diffraction method, and thus, crystal orientation information is obtained.
  • An EBSD analyzer composed of a Schottky emission scanning electron microscope and an EBSD detector may be used for measurement, for example, an EBSD analyzer composed of JSM-7001F manufactured by JEOL Ltd. and DVC 5-type detector manufactured by TSL Solutions may be used for measurement.
  • the degree of vacuum in the EBSD analyzer may be set to 9.6 ⁇ 10:5 Pa or less, an accelerating voltage may be set to 15 kV, and an irradiation current level may be set to 13.
  • the area ratio of the remaining region (the region where “Grain Average Misorientation” is more than 0.5°) is calculated, and this area ratio is determined as the total area ratio of martensite and bainite.
  • bainite in the surface layer region By generating bainite in the surface layer region, dislocation density of the surface layer region can be decreased. As a result, irruption of hydrogen from the external environment can be suppressed, and hydrogen embrittlement resistance of the hot-stamping formed body can be improved. Furthermore, by generating bainite in the surface layer region, since excessive softening of the surface layer can be suppressed, hydrogen embrittlement resistance can be improved while maintaining a load bearing of the member. When the area ratio of bainite in the surface layer region is 10% or less, hydrogen embrittlement resistance deteriorates. For this reason, the area ratio of bainite is set to more than 10%, preferably 20% or more, 40% or more or 60% or more. The upper limit of the area ratio of bainite is not particularly limited, but may be set to 100%, 90% or 80%.
  • martensite 0% to 90% (0% or more and 90% or less)
  • ferrite and residual austenite 0% to 65% (0% or more and 65% or less) may be included.
  • the area ratio of the microstructure is calculated for the surface layer region (the region between the surface and 1/25 depth of the sheet thickness from the surface) by the following method.
  • a sample is cut out from an arbitrary position away from an end surface of the hot-stamping formed body by a distance of 50 mm or more (a position that avoids an end portion in a case where a sample cannot be collected at this position) so that a sheet thickness cross section parallel to the rolling direction can be observed.
  • the size of the sample depends on a measurement device, but is set to a size that can be observed by about 10 mm in the rolling direction.
  • the cross section of the sample After polishing the cross section of the sample using silicon carbide paper of #600 to #1500, the cross section is mirror-finished using liquid in which diamond powder having a grain size in the range of 1 ⁇ m to 6 ⁇ m is dispersed in a diluted solution of alcohol or the like or pure water, Next, the observation surface is finished by electrolytic polishing. At an arbitrary position on the cross section of the sample in a longitudinal direction, a region which has a length of 50 ⁇ m and is present between the surface of the hot-stamping formed body and 1/25 depth of the sheet thickness from the surface is measured at a measurement interval of 0.1 ⁇ m by an electron backscatter diffraction method, and thus, crystal orientation information is obtained.
  • An EBSD analyzer composed of a Schottky emission scanning electron microscope and an EBSD detector may be used for measurement, for example, an EBSD analyzer composed of JSM-7001F manufactured by JEOL Ltd. and DVC 5-type detector manufactured by TSL Solutions may be used for measurement.
  • the degree of vacuum in the EBSD analyzer may be set to 9.6 ⁇ 10 ⁇ 5 Pa or less, an accelerating voltage may be set to 15 kV, and an irradiation current level may be set to 13.
  • the region where “Grain Average Misorientation” is 0.5° or lower is extracted as ferrite.
  • the area ratio of ferrite is obtained.
  • the remaining region (the region where “Grain Average Misorientation” is more than) 0.75° is extracted as martensite, and the area ratio thereof is calculated, thereby the area ratio of martensite is obtained.
  • the maximum value of pole density of the texture in the surface layer region is more than 4.0, hydrogen embrittlement resistance of the hot-stamping formed body deteriorates.
  • the maximum value of pole density of the texture in the surface layer region is set to 4.0 or less, preferably 3.5 or less, 3.0 or less or 2.5 or less.
  • the lower limit of the pole density of the texture in the surface layer region is not particularly limited, but may be set to 1.0 or 1.2.
  • the texture in the surface layer region is obtained by the following method.
  • a sample is cut out from an arbitrary position away from an end surface of the hot-stamping formed body by a distance of 50 mm or more (a position that avoids an end portion in a case where the sample cannot be collected at this position) so that a sheet thickness cross section parallel to a rolling direction can be observed.
  • the size of the sample depends on a measurement device, but is set to a size that can be observed by about 10 mm in the rolling direction.
  • the cross section of the sample After polishing the cross section of the sample using silicon carbide paper of #600 to #1500, the cross section of the sample is mirror-finished using liquid in which diamond powder having a grain size in the range of 1 ⁇ m to 6 ⁇ m is dispersed in a diluted solution of alcohol or the like or pure water.
  • the observation surface is finished by electrolytic polishing.
  • a region which has a length of 1000 ⁇ m and is present between the surface and 1/25 depth of the sheet thickness from the surface is measured at a measurement interval of 5.0 ⁇ m by an electron backscatter diffraction method, and thus, crystal orientation information is obtained.
  • An EBSD analyzer composed of a Schottky emission scanning electron microscope and an EBSD detector may be used for measurement, for example, an EBSD analyzer composed of JSM-7001F manufactured by JEOL Ltd. and DVC 5-type detector manufactured by TSL. Solutions may be used for measurement.
  • the degree of vacuum in the EBSD analyzer may be set to 9.6 ⁇ 10 ⁇ 5 Pa or less, an accelerating voltage may be set to 15 kV, and an irradiation current level may be set to 13.
  • the deboronization index is an index that quantitatively represents the amount of decrease of the B concentration in the surface layer region. By decreasing the B concentration in the surface layer region, deformability of prior austenite grain is improved by reducing strength of prior austenite before transformation, and the generation of grains having random orientation is facilitated in the surface layer region.
  • the deboronization index in the surface layer region is less than 0.05, grains having a desired texture cannot be obtained in the surface layer region. For this reason, the deboronization index is set to 0.05 or more, preferably 0.20 or more, 0.30 or more or 0.35 or more.
  • the upper limit of the deboronization index is not particularly limited, but may be set to 1.00, 0.80 or 0.60.
  • the deboronization index in the surface layer region is obtained by the following method.
  • An element concentration distribution in the sheet thickness direction in the hot-stamping formed body is measured using glow discharge optical emission spectrometry (GD-OES: Manufactured by Horiba, Ltd., Marcus type high-frequency glow discharge optical emission spectrometer, GD-PROFILER-HR).
  • the measurement conditions are an analysis diameter of 4 mm, a sputtering rate of 4 ⁇ m/min, an argon pressure of 600 Pa, an RF output of 35 W, and a measurement interval of 0.02 ⁇ m or less. All elements that are comprised in the hot-stamping formed body are measured.
  • the “surface” refers to the interface of the plating layer and the base steel sheet.
  • GD-OES measurement is performed after removing a part or all of the plating layer or the coating film by mechanical polishing or chemical polishing such that measurement to 200 ⁇ m depth from the surface of the base steel sheet (the interface of the plating layer and the base steel sheet) can be performed.
  • a measuring point where the Fe concentration becomes 90 mass % is regarded as the surface of the hot-stamping formed body.
  • the hot-stamping formed body may be referred to as a base steel sheet.
  • B concentrations from the surface of the hot-stamping formed body to at least 100 ⁇ m depth from the surface are measured.
  • the measurement in the depth direction of the B concentration is finished at the position of 100 ⁇ m depth from the surface.
  • the measurement of the B concentration in the depth direction is continued. Then, each time a new B concentration measurement value is obtained in the depth direction, the average value of the B concentration in the region between the deepest part and 20 ⁇ m from the deepest part to the surface side is calculated.
  • the absolute value of the difference between the average value of the B concentration in the region between the deepest part and 20 ⁇ m from the deepest part to the surface side and the maximum value of the measured value of the B concentration in the region between the deepest part and 20 ⁇ m from the deepest part to the surface side is 0.0006 mass % or less
  • the absolute value of the difference between the average value of the B concentration in the region between the deepest part and 20 ⁇ m from the deepest part to the surface side and the minimum value of the measured value of the B concentration in the region between the deepest part and 20 ⁇ m from the deepest part to the surface side is 0.0006 mass % or less
  • the measurement of the B concentration in the depth direction is finished at the position of 150 ⁇ m depth from the surface.
  • the measurement of the B concentration in the depth direction is finished when the measurement of the B concentration at the position of 200 ⁇ m depth from the surface is completed. Then, at the time when the measurement of the B concentration in the depth direction is finished, the average value of the B concentration in the region between the deepest part (the deepest position where the B concentration used for calculating the deboronization index was obtained) and the position of 20 ⁇ m from the deepest part to the surface side is used for the below calculation of the deboronization index (hereinafter, the average value of the B concentration in the region will be referred to as the average B concentration at the deepest part of 20 ⁇ m).
  • the shallowest depth position that satisfies the ending condition for the B concentration measurement in the depth direction is searched for, and in a case where the depth position is found, the deboronization index may be calculated without using the measurement results of the B concentration at the position deeper than the shallowest depth position.
  • the B concentration may be measured from the surface to 200 ⁇ m depth from the surface, in this case, in a case where a shallowest depth position that satisfies the ending condition for B concentration measurement in the depth direction exists in a region of 100 ⁇ m or more depth from the surface, the measurement is regarded as ending at the depth position, and the deboronization index is calculated.
  • the amount of decrease in the B concentration per unit depth (the value obtained by subtracting the B concentration at each measurement point from the average B concentration at the deepest part of 20 ⁇ m) is calculated, the integrated value of the product of the unit depth and the amount of decrease in the B concentration is calculated and determined as the area of the B-depletion region (area of region A in FIG. 1 ).
  • the value obtained by subtracting the B concentration at each measurement point from the average B concentration at the deepest part of 20 ⁇ m is negative, it is integrated as 0 (due to the B removal phenomenon near the surface, the B concentration at each measurement point is in most cases lower than the average B concentration at the deepest part of 20 ⁇ m, and the integrated value becomes positive).
  • the product of the average B concentration at the deepest part of 20 ⁇ m and the length of 200 ⁇ m is calculated as a reference area (area of rectangular region B in FIG. 1 ).
  • the value obtained by dividing a B-depletion area (area of region A) by the reference area (area of region B) is defined as the deboronization index (area of region A/area of region B).
  • the reference area (area of region B) is calculated by assuming that the length by which the average B concentration at the deepest part of 20 ⁇ m is multiplied is 200 ⁇ m.
  • the hot-stamping formed body may have a plating layer on the surface.
  • the plating layer By having the plating layer on the surface, corrosion resistance can be improved after hot stamping.
  • the plating layer include an aluminum plating layer, aluminum-galvanized layer, aluminum-silicon plating layer, hot-dip galvanized layer, electrogalvanized layer, galvannealed layer, zinc-nickel plating layer, aluminum-magnesium-zinc-based plating layer.
  • the steel sheet for hot stamping has the above-described chemical composition.
  • the microstructure of the steel sheet for hot stamping is not particularly limited as long as a desired strength, hydrogen embrittlement resistance and early fracture resistance are obtained after hot stamping, for example, in area %, the microstructure may consist of ferrite: 5% to 90%, bainite and martensite: 0% to 100%, pearlite: 10% to 95%, and residual austenite: 0% to 5%.
  • iron carbides, alloy carbides, intermetallic compounds, and inclusions may be included.
  • the steel sheet for hot stamping may have a plating layer on the surface.
  • the plating layer By having the plating layer on the surface, corrosion resistance can be improved after hot stamping.
  • the plating layer include an aluminum plating layer, aluminum-galvanized layer, aluminum-silicon plating layer, hot-dip galvanized layer, electrogalvanized layer, galvannealed layer, zinc-nickel plating layer, aluminum-magnesium-zinc-based plating layer.
  • a manufacturing method to obtain the steel sheet for hot stamping for obtaining the hot-stamping formed body according to the present embodiment will be described.
  • it is particularly effective to control the finish rolling condition and the annealing condition in the manufacturing method of the steel sheet for hot stamping.
  • the rolling reduction of the final pass (final rolling reduction) to 20% or more.
  • the final rolling reduction can be expressed as ⁇ (t 0 ⁇ t 1 )/t 0 ⁇ 100(%), where to is the sheet thickness before rolling of the final pass, and t 1 is the sheet thickness after rolling of the final pass.
  • the Mn content is 0.60% or more
  • the casting method of molten steel the conditions of heating before hot rolling, rough rolling, coiling, and cold rolling are not particularly limited, and may be standard conditions.
  • the coiling temperature may be set to 750° C. or lower. By setting the coiling temperature to 750° C. or lower, it is possible to suppress ferrite from being connected and arranged in the hot-rolled steel sheet after rolling, and pearlite is uniformly dispersed. This pearlite becomes a reverse transformation site of prior austenite during heating of hot stamping. For this reason, when pearlite is uniformly dispersed, the standard deviation of the grain sizes of prior austenite grains in the hot-stamping formed body becomes small. As a result, early fracture resistance of the hot-stamping formed body can be improved.
  • a softening heat treatment may be performed on the coil after coiling.
  • the softening heat treatment method is not particularly limited, and standard conditions may be adopted.
  • annealing After cold rolling, it is preferable to perform annealing to heat for 15 seconds or more in an oxidizing atmosphere. Generally, it is preferable to perform annealing in a reducing atmosphere in order to suppress formation of scale.
  • formation of scale on the steel sheet surface is promoted by performing annealing in the oxidizing atmosphere.
  • the scale formed on the steel sheet surface becomes an oxidation source, and C and B in the surface layer region are oxidized. Since oxidized C and B leave the surface layer of the steel sheet, the amounts of C and B are reduced in the surface layer region.
  • the strength of the prior austenite grains decreases and they become easily deformed, and grains having random orientation are likely to be generated. Thereby, grains having a desired texture can be generated in the surface layer region.
  • the heating temperature during annealing may be set to a temperature range of 730° C. to 900° C., and by staying in this heating temperature range for 15 seconds or more, formation of scale can be promoted while suppressing peeling of scale.
  • the time for annealing is preferably 100 seconds or more, more preferably 200 seconds or more, and even more preferably 300 seconds or more.
  • annealing for more than 3600 seconds is not preferable since the prior austenite grain sizes become coarser, the grain boundary diffusion rate of B decreases, removal of B does not proceed, and the deboronization index cannot be 0.05 or more. For this reason, the annealing time is preferably 3600 seconds or less.
  • the annealing step may be performed again in an oxidizing atmosphere or a non-oxidizing atmosphere unless a treatment for removing oxide scale (for example, pickling) is performed.
  • the oxidizing atmosphere may be any heating atmosphere that generates oxide scale on the surface layer of the steel sheet, and may be a standard condition.
  • a gas combustion atmosphere it is preferable to create an atmosphere in which the mixture ratio of air and fuel (air-fuel ratio) is controlled to 0.80 or more, and more preferably controlled to exceed 1.00. It is preferable to generate an oxide scale of 15 ⁇ m or more on the steel sheet surface by annealing in the oxidizing atmosphere.
  • oxide scale on the steel sheet surface remain in subsequent processes. That is, it is preferable to perform hot stamping, which will be described later, with the oxide scale remaining. Oxide scale is removed by shot blasting after hot stamping.
  • oxide scale remains at the interface between the base steel sheet and the plating layer.
  • the oxide scale disappears after hot stamping due to an alloying reaction during heating before hot stamping.
  • a hot-stamping formed body according to the present embodiment is obtained by hot stamping the steel sheet for hot stamping manufactured by the above-described method.
  • the hot stamping conditions are not particularly limited. However, for example, it is preferable to heat the steel sheet for hot stamping to a temperature range of 800° C. to 1000° C. and hold in this temperature range for 60 to 600 seconds. When the heating temperature is lower than 800° C., austenitization becomes insufficient, a desired distribution of prior austenite grain sizes cannot be obtained, and early fracture resistance may deteriorate. On the other hand, when the heating temperature is higher than 1000° C., the grains of prior austenite grow excessively, a desired distribution of prior austenite grain sizes cannot be obtained, and early fracture resistance may deteriorate.
  • a heating atmosphere is not particularly limited, and may be standard conditions, for example, 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 stamping After holding in the temperature range, hot stamping is performed. After hot stamping, cooling may be performed to a temperature range of 250° C. or lower at an average cooling rate of 20° C./s or faster.
  • heating methods before hot stamping include heating using an electric furnace and gas furnace, flame heating, electrical heating, high-frequency heating, and induction heating.
  • the hot-stamping formed body according to the present embodiment is obtained.
  • a tempering treatment at 130° C. to 600° C. may be performed after hot stamping, or a baking hardening treatment after painting may be performed.
  • a portion of the hot-stamping formed body may be tempered by laser irradiation or the like to provide a partially softened region.
  • the obtained steel sheets for hot stamping were heated to a temperature range of higher than 800° C. in a furnace continuously supplied with nitrogen gas (hot stamp heating), held in the temperature range, subjected to hot stamping, and then cooled to 250° C. or lower at an average cooling rate of 20° C./s or faster.
  • hot stamp heating nitrogen gas
  • the hot-stamping formed bodies shown in Tables 3A to 3H were obtained.
  • a gas combustion atmosphere was used in which the mixture ratio of air and fuel (air-fuel ratio) was controlled to 0.85.
  • Measurements of the microstructure including the standard deviation of the grain sizes of austenite grains), deboronization index, and pole density of the texture of the hot-stamping formed body were performed by the above-described methods.
  • the mechanical properties of the hot-stamping formed body were evaluated by the following methods.
  • the tensile (maximum) strength TS of the hot-stamping formed body was obtained, in accordance with JIS Z 2241:2011, by preparing a No. 5 test piece from an arbitrary position of the hot-stamping formed body and conducting a tensile test.
  • the crosshead speed was set to 1 mm/min.
  • the tensile strength TS was 2200 MPa or more, it was determined as having high strength and successful, and when the tensile strength TS was less than 2200 MPa, it was determined as not having high strength and not successful.
  • Hydrogen embrittlement resistance of the hot-stamping formed body was evaluated by the following method, A test piece with a length of 68 mm and a width of 6 mm was taken from an arbitrary position of the hot-stamping formed body, and the edges of the test piece were polished using silicon carbide paper of #200 to #1500, and then mirror finishing was performed using a liquid in which diamond powder with a particle size of 1 ⁇ m to 6 ⁇ m was dispersed in a diluent such as alcohol and pure water. Furthermore, the corners of the test piece were chamfered using silicon carbide paper of #200 to #1500.
  • a stress of 800 MPa or more was applied to the test piece, the test piece was immersed in a liter of hydrochloric acid adjusted to pH 4 at room temperature for 48 hours, and the presence or absence of cracks was determined.
  • no crack occurred under the load stress of 800 MPa or more it was determined as successful.
  • an evaluation of “Fair” was used in the tables
  • no crack occurred at 900 MPa an evaluation of “Good” was used in the tables
  • no crack occurred at 1000 MPa an evaluation of “Very Good” was used in the tables
  • no crack occurred at 1100 MPa or higher an evaluation of “Excellent” was used in the tables.
  • a crack occurred at a load stress of 800 MPa it was determined as not successful and “Bad” was described in the tables.
  • the early fracture resistance was evaluated by the value calculated by dividing the tensile strength of the hot-stamping formed body, which was obtained by the above method, by the value obtained by multiplying the Vickers hardness, which was obtained by the following method, by 3.3 (tensile strength/(Vickers hardness ⁇ 3.3)). When the value was 0.60 or more, it was determined as having excellent early fracture resistance and successful, and when the value was less than 0.60, it was determined as not successful.
  • the value obtained by multiplying the Vickers hardness by 3.3 is the tensile strength estimated from the hardness, and when the measured value of the tensile strength is 0.60 times or more of the estimated tensile strength, then it can be determined as having excellent early fracture resistance.
  • the Vickers hardness used for evaluation of early fracture resistance was obtained by the following method. First, from an arbitrary position 50 mm or more away from the end surface of the hot-stamping formed body, a sample was cut out so that a cross section perpendicular to the surface (sheet thickness cross section) could be observed. The size of the sample depended on the measuring device, but was set to a size that could be observed by 10 mm in the rolling direction. A cross section of the sample was polished using silicon carbide paper of #600 to #1500, and then mirror finishing was performed using a liquid in which diamond powder with a particle size of 1 ⁇ m to 6 ⁇ m was dispersed in a diluent such as alcohol and pure water.
  • a diluent such as alcohol and pure water.

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