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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
• • 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
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/62—Quenching devices
  • C21D1/673—Quenching devices for die quenching
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D7/00—Modifying the physical properties of iron or steel by deformation
  • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
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  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0236—Cold rolling
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • 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
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  • C22C—ALLOYS
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  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
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  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
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  • 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
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
• • C—CHEMISTRY; METALLURGY
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
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  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/30—Ferrous alloys, e.g. steel alloys containing chromium with cobalt
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • C—CHEMISTRY; METALLURGY
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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  • 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
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/009—Pearlite

Definitions

• the present invention relates to a steel sheet for hot stamping and a hot-stamping formed body.
• Patent Document 1 discloses a steel sheet which is suitable for obtaining components such as gears by improving hardenability and material formability and in particular, performing forming by cold forging such as wall thickness increase or the like and a manufacturing method thereof.
• the present inventors found that, in a vehicle member having an improved tensile strength, it is necessary to further improve the bendability in order to obtain a higher effect of reducing the weight of a vehicle body.
• Patent Document 1 PCT International Publication No. WO2016/190396
• Non-Patent Document 1 Acta Materialia, 58 (2010), 6393-6403
• An object of the present invention is to provide a hot-stamping formed body having high strength and excellent bendability, and a steel sheet for hot stamping capable of manufacturing this hot-stamping formed body.
• the gist of the present invention is as follows.
• the present inventors examined bendability of a hot-stamping formed body. As a result, the present inventors found that in a microstructure of the hot-stamping formed body, the bendability deteriorates when a large amount of fine prior austenite grains are present. In addition, the present inventors found that, in the microstructure of the hot-stamping formed body, when prior austenite grains are set to a desired size and unevenness in the size of the prior austenite grains is suppressed, that is, the prior austenite grains are grain-sized, the bendability of the hot-stamping formed body can be further improved.
• the present inventors examined a method for obtaining the above-described hot-stamping formed body.
• the present inventors found that when a Mn content in a chemical composition of a steel sheet for hot stamping is set to 0.60% or less, and in a microstructure, when pole densities of ferrite in an orientation group consisting of ⁇ 100 ⁇ 011> to ⁇ 223 ⁇ 110> are reduced and the number proportion of the ferrite containing a carbide in grains increase, the above-described hot-stamping formed body can be obtained.
• a limited numerical range described using “to” to be 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 steel sheet for hot stamping includes, as a chemical composition, by mass%, C: more than 0.40% and 0.70% or less, Si: 0.010% to 1.30%, Mn: 0.10% to 0.60%, P: 0.100% or less, S: 0.0100% or less, N: 0.0140% or less, O: 0.0200% or less, Al: 0.0010% to 0.500%, Cr: 0.010% to 0.80%, and a remainder including Fe and impurities.
• C by mass%
• C more than 0.40% and 0.70% or less
• Si 0.010% to 1.30%
• Mn 0.10% to 0.60%
• P 0.100% or less
• S 0.100% or less
• S 0.0100% or less
• N 0.0140% or less
• O 0.0200% or less
• Al 0.0010% to 0.500%
• Cr 0.010% to 0.80%
• a remainder including Fe and impurities each element will be described below.
• the C greatly contributes to improvement in the strength of the hot-stamping formed body.
• the C content is 0.40% or less, it becomes difficult to obtain sufficient strength in the hot-stamping formed body. For this reason, the C content is set to more than 0.40%.
• the C content is preferably 0.42% or more, more preferably 0.45% or more, and still more preferably 0.47% or more.
• the C content is set to 0.70% or less.
• the C content is preferably 0.65% or less and more preferably 0.60% or less.
• Si is an element that improves distortion capability of the hot-stamping formed body by suppressing the formation of an oxide which is combined with oxygen and becomes an origin of fracture.
• the Si content is set to 0.010% or more.
• the Si content is preferably 0.05% or more and more preferably 0.10% or more.
• the Si content is set to 1.30% or less.
• the Si content is preferably less than 1.00% and more preferably 0.50% or less.
• Mn stabilizes austenite and improves hardenability of a steel sheet.
• the Mn content is set to 0.10% or more.
• the Mn content is preferably 0.20% or more and more preferably 0.30% or more.
• the Mn content is set to 0.60% or less.
• the Mn content is preferably 0.55% or less and more preferably 0.50% or less.
• the lower P content is more preferable.
• the P content is set to 0.100% or less.
• the P content is preferably 0.080% or less and more preferably 0.020% or less.
• the lower limit of the P content is not particularly limited and 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.
• the lower S content is more preferable.
• the S content is set to 0.0100% or less.
• the S content is preferably 0.0050% or less and more preferably 0.0010% or less.
• the lower limit of the S content is not particularly limited and 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.
• the N forms a coarse nitride and deteriorates the bendability of the hot-stamping formed body. Therefore, the lower N content is more preferable. In particular, when the N content is more than 0.0140%, the formability of the steel sheet significantly deteriorates. Therefore, the N content is set to 0.0140% or less.
• the C content is preferably 0.0100% or less or 0.0070% or less and more preferably 0.0040% or less.
• the lower limit of the N content is not particularly limited and 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.
• the O content is set to 0.0200% or less.
• the O content is preferably 0.0150% or less, more preferably 0.0100% or less, and still more preferably 0.0060% or less.
• the lower limit of the O content is not particularly limited and may be 0%. However, when the O content is reduced to less than 0.0001%, the manufacturing cost increases significantly, which is not preferable economically. Therefore, the O content may be set to 0.0001% or more.
• Al is an element that improves the distortion capability by deoxidizing molten steel to suppress the formation of oxide which becomes the origin of fracture and improves the bendability of the hot-stamping formed body.
• the Al content is set to 0.0010% or more.
• the Al content is preferably 0.010% or more and more preferably 0.030% or more.
• the Al content is set to 0.500% or less.
• the Al content is preferably 0.450% or less and more preferably 0.350% or less.
• the Cr increases the strength of the hot-stamping formed body by dissolving in prior austenite grains during heating at the time of hot stamping.
• the Cr content is set to 0.010% or more.
• the Cr content is preferably 0.10% or more and more preferably 0.20% or more.
• the Cr content is set to 0.80% or less.
• the Cr content is preferably 0.60% or less and more preferably 0.40% or less.
• the remainder of the chemical composition of the steel sheet for hot stamping according to the present embodiment may be Fe and impurities.
• An example of the impurities includes an element that is unavoidably incorporated from a steel raw material or scrap and/or during a steelmaking process and is allowed in a range in which properties of the hot-stamping formed body according to the present embodiment are not inhibited.
• the steel sheet for hot stamping according to the present embodiment may contain the following elements as arbitrary elements instead of a part of Fe.
• the contents of the following arbitrary elements, which are obtained in a case where the following arbitrary elements are not contained, are 0%.
• Nb forms carbonitride in steel to improve the strength of the hot-stamping formed body by precipitation hardening.
• the Nb content is preferably set to 0.001% or more.
• the Nb content is set to 0.100% or less.
• Ti forms carbonitride in steel to improve the strength of the hot-stamping formed body by precipitation hardening.
• a Ti content is preferably set to 0.010% or more.
• the Ti content is more than 0.100%, a large amount of carbonitride is formed in steel, and the bendability of the hot-stamping formed body deteriorates. For this reason, the Ti content is set to 0.100% or less.
• the B improves the hardenability of the steel and improves the strength of the hot-stamping formed body.
• the B content is preferably set to 0.0015% or more.
• the B content is set to 0.0100% or less.
• Mo improves the hardenability of the steel sheet and improves the strength of the hot-stamping formed body.
• the Mo content is preferably set to 0.05% or more.
• the Mo content is set to be 1.00% or less.
• Co improves the hardenability of the steel sheet and improves the strength of the hot-stamping formed body.
• the Co content is set to 0.05% or more.
• the Co content is set to 2.00% or less.
• Ni 0% or more and less than 3.00%
• Ni improves the hardenability of the steel sheet and improves the strength of the hot-stamping formed body.
• the Ni content is preferably set to 0.01% or more.
• the Ni content is set to less than 3.00%.
• Cu improves the hardenability of the steel sheet and improves the strength of the hot-stamping formed body.
• the Cu content is preferably set to 0.01% or more.
• the Cu content is set to 1.00% or less.
• V 0% to 1.00%
• V improves the hardenability of the steel sheet and improves the strength of the hot-stamping formed body.
• the V content is preferably set to 0.01% or more.
• the V content is set to 1.00% or less.
• the W improves the hardenability of the steel sheet and improves the strength of the hot-stamping formed body.
• the W content is preferably set to 0.001 % or more.
• the W content is set to 1.000% or less.
• the Ca improves the distortion capability by suppressing the formation of an oxide which becomes the origin of fracture and improve the bendability of the hot-stamping formed body.
• the Ca content is preferably set to 0.001% or more.
• the Ca content is set to 0.010% or less.
• Mg improves the distortion capability by suppressing the formation of an oxide which becomes the origin of fracture and improves the bendability of the hot-stamping formed body.
• the Mg content is preferably set to 0.001% or more.
• the Mg content is set to 1.000% or less.
• the REM improves the distortion capability by suppressing the formation of an oxide which becomes the origin of fracture and improves the bendability of the hot-stamping formed body.
• the REM content is preferably set to 0.001% or more.
• the REM content is set to 1.000% or less.
• 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 improves the distortion capability by suppressing the formation of an oxide which becomes the origin of fracture and improves the bendability of the hot-stamping formed body.
• the Sb content is preferably set to 0.005% or more.
• the Sb content is set to 1.000% or less.
• the Zr improves the distortion capability by suppressing the formation of an oxide which becomes the origin of fracture and improves the bendability of the hot-stamping formed body.
• the Zr content is preferably set to 0.001% or more.
• the Zr content is set to 1.000% or less.
• the Sn improves the distortion capability by suppressing the formation of an oxide which becomes the origin of fracture and improves the bendability of the hot-stamping formed body.
• the Sn content is preferably set to 0.001% or more.
• the Sn content is set to 1.000% or less.
• the As content is preferably set to 0.001% or more.
• the As content is set to 0.100% or less.
• the above-mentioned chemical composition of the steel sheet for hot stamping may be measured by an ordinary analysis method.
• the chemical composition of the steel sheet for hot stamping may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES).
• 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 fusionnondispersive infrared absorption method.
• the chemical composition may be analyzed after the plating layer is removed by mechanical grinding.
• the steel sheet for hot stamping has a microstructure in which an average value of pole densities of ferrite in an orientation group consisting of ⁇ 100 ⁇ 011> to ⁇ 223 ⁇ 110> is 10.0 or less, in entire ferrite, a number proportion of ferrite containing a carbide having an equivalent circle diameter of 0.2 ⁇ m or more in grains is 20% or more, and an area ratio of pearlite is 10% to 90% and an area ratio of ferrite is 10% to 90%.
• the microstructure in a sheet thickness cross section parallel to a rolling direction, is specified at a 1/4 depth position of the sheet thickness from the surface (in a region from a 1/8 depth of the sheet thickness from the surface to a 3/8 depth of the sheet thickness from the surface). The reason therefor is that the microstructure at this position indicates a typical microstructure of the steel sheet.
• Average value of pole densities of ferrite in orientation group consisting of ⁇ 100 ⁇ 011> to ⁇ 223 ⁇ 110> is 10.0 or less
• the average value of the pole densities of ferrite in the orientation group consisting of ⁇ 100 ⁇ 011 > to ⁇ 223 ⁇ 110> is more than 10.0, the average grain size of the prior austenite in the hot-stamping formed body cannot be controlled to a predetermined value, and a hot-stamping formed body having excellent bendability cannot be obtained.
• the average value of the pole densities of ferrite in the orientation group consisting of ⁇ 100 ⁇ 011> to ⁇ 223 ⁇ 110> is preferably 9.0 or less, more preferably 7.0 or less, still more preferably 6.0 or less, and even more preferably 5.0 or less.
• a lower limit of the pole density of ferrite in the orientation group consisting of ⁇ 100 ⁇ 011> to ⁇ 223 ⁇ 110> is not particularly limited and may be 0.1 or more.
• crystal orientations of ⁇ 100 ⁇ 011>, ⁇ 116 ⁇ 110>, ⁇ 114 ⁇ 110>, ⁇ 112 ⁇ 110>, and ⁇ 223 ⁇ 110> are included.
• the pole densities of ferrite in the orientation group consisting of t 100 ⁇ 011> to ⁇ 223 ⁇ ⁇ 110> can be obtained from an orientation distribution function (ODF) that displays a three-dimensional texture calculated by computing, using spherical harmonics, an orientation data measured by an electron backscattering diffraction (EBSD) method using a device in which a scanning electron microscope and an EBSD analyzer are combined and OlM Analysis (registered trademark) manufactured by TSL Solutions.
• ODF orientation distribution function
• EBSD electron backscattering diffraction
• a measurement region is set to the region from the 1/8 position of the sheet thickness from the surface to the 3/8 position of the sheet thickness from the surface so that the 1/4 depth position of the sheet thickness from the surface can be observed.
• Measurement pitches are set to 5 ⁇ m/step.
• ⁇ hkl ⁇ indicates a crystal plane parallel to a rolled surface and ⁇ uvw> indicates a crystal direction parallel to a rolling direction. That is, ⁇ hkl ⁇ ⁇ uvw> indicates a crystal in which ⁇ hkl ⁇ is oriented in a sheet surface normal direction and ⁇ uvw> is oriented in the rolling direction.
• the prior austenite grains can be grain-sized in the hot-stamping formed body. As a result, it is not possible to obtain a hot-stamping formed body having excellent bendability.
• the carbide in the grain function preferably as the origin of the prior austenite grains during the heating before the hot stamping.
• the prior austenite grains are uniformly dispersed and grain-sized in the microstructure of the hot-stamping formed body.
• the number proportion of the ferrite containing a carbide having the equivalent circle diameter of 0.2 ⁇ m or more in the grains is preferably 40% or more, more preferably 50% or more, and still more preferably 60% or more.
• An upper limit of the number proportion of ferrite containing a carbide having the equivalent circle diameter of 0.2 ⁇ m or more in the grains in entire ferrite is not particularly specified, but may be set to 90% or less.
• a sample is collected from an arbitrary position (a position that avoids an end portion in a case where the sample cannot be collected at this position) away from an end surface of the steel sheet for hot stamping by a distance of 50 mm or more so that a sheet thickness cross section parallel to a rolling direction can be an observed section.
• the observed section is finished by electropolishing. After that, the region from the 1/8 depth of the sheet thickness from the surface to the 3/8 depth of the sheet thickness from the surface is observed at 10 or more visual fields at a magnification of 20,000 times so that the 1/4 depth position of the sheet thickness from the surface can be observed.
• the equivalent circle diameter of each carbide is obtained from the area of each carbide observed in the grain of ferrite by image analysis.
• the number of ferrite grains including a carbide having the equivalent circle diameter of 0.2 ⁇ m or more in all grains of the observed ferrite is calculated.
• the obtained value is divided by the number of all grains of ferrite and multiplied by 100, thereby obtaining the number proportion of ferrite containing a carbide having an equivalent circle diameter of 0.2 ⁇ m or more in grains.
• particles having the equivalent circle diameter of 0.2 to 30 ⁇ m are regarded as the carbide.
• the area ratio of ferrite is set to 10% or more, and the area ratio of pearlite is set to 90% or less.
• the area ratio of ferrite is preferably 20% or more and more preferably 40% or more.
• the area ratio of pearlite is preferably 80% or less and more preferably 60% or less.
• the area ratio of ferrite is set to 90% or less and the area ratio of pearlite is set to 10% or more.
• the area ratio of ferrite is preferably 70% or less and more preferably 60% or less.
• the area ratio of pearlite is preferably 30% or more and more preferably 40% or more.
• the remainder in microstructure is one or more of martensite, lower bainite, residual austenite, and tempered martensite.
• An area ratio of the remainder in microstructure may be set to 20% or less.
• a sample is cut out from an arbitrary position (a position that avoids an end portion in a case where the sample cannot be collected at this position) away from an end surface of the steel sheet for hot stamping by a distance of 50 mm or more so that a sheet thickness cross section parallel to a rolling direction can be observed.
• the size of the sample also 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 polished using silicon carbide paper having a grit of #600 to #1500, then, is finished as a mirror surface using liquid in which diamond powder having a grain size of 1 to 6 ⁇ m is dispersed in diluted solution of alcohol or the like or pure water and finish-polished by electrolytic polishing.
• the structure is observed using a device including a thermal field emission type scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.) and an EBSD detector (DVC5-type detector manufactured by TSL Solutions).
• the scanning electron microscope used is equipped with a secondary electron detector. In a vacuum of 9.6 ⁇ 10 -5 Pa or less, the sample is irradiated with an electron beam at an acceleration voltage of 15 kV and an irradiation current level of 13, and a secondary electron image is photographed with the scanning electron microscope.
• a region where cementite is precipitated in a lamellar shape in the grains is determined as pearlite.
• the area ratio of the pearlite is obtained by calculating the area ratio of the region determined to be pearlite.
• Lath-shaped grains are determined as lower bainite, martensite, and tempered martensite.
• EBSD analysis is performed on the same visual field at an analysis speed of 200 to 300 points/sec using an EBSD analyzer.
• the area ratio of ferrite is calculated using the "Grain Average Misorientation" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer.
• the steel sheet for hot stamping may have a plating layer formed on the surface for the purpose of improving corrosion resistance after hot stamping.
• the plating layer may be any of an electroplating layer and a hot-dip plating layer.
• the electroplating layer includes, for example, an electrogalvanized layer, an electrolytic Zn-Ni alloy plating layer, and the like.
• the hot-dip plating layer includes, for example, a hot-dip galvanized layer, a hot-dip galvannealed layer, a hot-dip aluminum plating layer, a hot-dip Zn-Al alloy plating layer, a hot-dip Zn-Al-Mg alloy plating layer, a hot-dip Zn-Al-Mg-Si alloy plating layer, and the like.
• An adhesion amount of a plating layer is not particularly limited and may be a general adhesion amount.
• the sheet thickness of the steel sheet for hot stamping according to the present embodiment is not particularly limited, but is preferably 0.5 to 3.5 mm from the viewpoint of a reduction in the weight of the vehicle body or the like.
• the hot-stamping formed body according to the present embodiment has the same chemical composition as the above-described steel sheet for hot stamping.
• a measurement method of the chemical composition may be the same as that for the steel sheet for hot stamping.
• the prior austenite grains are grain-sized in the microstructure.
• the hot-stamping formed body according to the present embodiment has a microstructure in which the average grain size of the prior austenite grains is 5 to 25 ⁇ m and the standard deviation of the grain sizes of the prior austenite grains is 0.1 to 2.0 ⁇ m.
• the microstructure is specified at the 1/4 depth position (the region from the 1/8 depth of the sheet thickness from the surface to the 3/8 depth of the sheet thickness from the surface) of the sheet thickness from the surface of the cross section perpendicular to the sheet surface.
• the microstructure at this position indicates a typical microstructure of the hot-stamping formed body.
• the microstructure will be described.
• the bendability of the hot-stamping formed body can be improved.
• the average grain size of the prior austenite grains or the standard deviation of the grain sizes of the prior austenite grains is outside the above range, it is not possible to obtain excellent bendability in the hot-stamping formed body.
• the average grain size of the prior austenite grains is preferably 10 ⁇ m or more and more preferably 15 ⁇ m or more.
• the average grain size of the prior austenite grains is preferably 20 ⁇ m or less.
• the standard deviation of the grain sizes of the prior austenite grains is set to 2.0 ⁇ m or less.
• the standard deviation is preferably 1.2 ⁇ m or less, more preferably 1.1 ⁇ m or less, and still more preferably 0.4 ⁇ m or less.
• the substantial lower limit is set to 0.1 ⁇ m or more.
• the area ratio of the prior austenite grains having the average grain size of 0.5 to 3.0 ⁇ m is 60% or less, more excellent bendability can be obtained in the hot-stamping formed body. Therefore, the area ratio of the prior austenite grains having the average grain size of 0.5 to 3.0 ⁇ m may be set to 60% or less. The area ratio is more preferably 50% or less and still more preferably 40% or less.
• a sample is cut out from an arbitrary position (a position that avoids an end portion in a case where the sample cannot be collected at this 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.
• the size of the sample also 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 polished using silicon carbide paper having a grit of #600 to #1500, then, is finished as a mirror surface using liquid in which diamond powder having a grain size of 1 to 6 ⁇ m is dispersed in diluted solution of alcohol or the like or pure water and finish-polished by electrolytic polishing.
• a sample is irradiated with an electron beam at an acceleration voltage of 15 kV and an irradiation current level of 13 in a vacuum of 9.6 ⁇ 10 -5 Pa or less using the device including a thermal field emission type scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.) and an EBSD detector (DVC5-type detector manufactured by TSL Solutions), and the EBSD analysis is performed at an analysis speed of 200 to 300 points/sec.
• JSM-7001F thermal field emission type scanning electron microscope
• EBSD detector DVC5-type detector manufactured by TSL Solutions
• the crystal orientation of the prior austenite grains is calculated from a crystal orientation relationship between the general prior austenite grains and grains having a body-centered structure after transformation, and the average grain size of the prior austenite grains is calculated using the calculated crystal orientation.
• the method for calculating the crystal orientation of the prior austenite grains is not particularly limited, and for example, the calculation may be performed using the following method.
• the crystal orientation of the prior austenite grains is calculated by the method described in Non-Patent Document 1, and the crystal orientation of the prior austenite in each coordinate of the EBSD-measured region is specified.
• a crystal orientation map of the prior austenite grain is created using the "Inverse Pole Figure” function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer.
• an average value of a shortest diameter and a longest diameter is calculated, and the average value is used as the grain size of the prior austenite grains.
• the above operation is performed on all the 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 average grain size of the prior austenite grains in the photographed visual fields is obtained by calculating a value obtained by dividing the sum of the obtained grain sizes of the prior austenite grains by the total number of prior austenite grains of which grain sizes are measured. This operation is performed on all the photographed visual fields, and the average grain size of the prior austenite grains of all the photographed visual fields is calculated, thereby obtaining the average grain size of the prior austenite grains.
• the standard deviation of the grain sizes of the prior austenite grains is obtained.
• the standard deviation is calculated by excluding the minimum value and the maximum value of the prior austenite grain sizes.
• the area ratio of the prior austenite grains having an average grain size of 0.5 to 3.0 ⁇ m is obtained.
• the microstructure of the hot-stamping formed body is not particularly limited as long as desired strength and desired bendability can be obtained after hot stamping.
• the microstructure may include, for example, by area%, ferrite: 0% to 50%, bainite and martensite: 0% to 100%, pearlite: 0% to 30%, and residual austenite: 0% to 5%.
• the microstructure of the hot-stamping formed body may be measured by the following method.
• a sample is cut out from an arbitrary position (a position that avoids an end portion in a case where the sample cannot be collected at this position) away from an end surface of the hot-stamping formed body by a distance of 50 mm or more so that the cross section perpendicular to the sheet surface can be observed.
• the cross section of the sample is polished using silicon carbide paper having a grit of #600 to #1500, then, is finished as a mirror surface using liquid in which diamond powder having a grain size of 1 to 6 ⁇ m is dispersed in diluted solution of alcohol or the like or pure water and is performed on Nital etching.
• a region where cementite is precipitated in a lamellar shape in the grains is determined as pearlite.
• a region in which brightness is low and no sub-microstructure is observed is determined as ferrite.
• a region in which the brightness is high and the sub-microstructure is not exposed by etching is determined as "martensite or residual austenite".
• a region that does not correspond to any of the above-described microstructures is determined as bainite.
• the area ratio of martensite is obtained by subtracting the area ratio of residual austenite obtained by EBSD analysis described later from the area ratio of martensite and residual austenite obtained from the taken photographs.
• the area ratio of residual austenite is measured using an electron backscatter diffraction method (EBSD).
• EBSD electron backscatter diffraction method
• a sample collected at the same sample collection position as in the measurement using the above-described taken photograph is used, and the analysis is performed on the region between the 1/8 depth of the sheet thickness from the surface and the 3/8 depth of the sheet thickness from the surface.
• the sample is polished using silicon carbide paper having a grit of #600 to #1500, then, finished into a mirror surface using liquid in which diamond powder having a grain size of 1 to 6 ⁇ m is dispersed in diluted solution of alcohol or the like or pure water, and then finished by electrolytic polishing for the purpose of sufficiently removing strain in a cross section to be measured.
• the sample in order to remove mechanical polishing strain on the observed section, the sample may be polished a minimum of 20 ⁇ m and polished a maximum of 50 ⁇ m.
• the sample is preferably polished 30 ⁇ m or less in consideration of rollover at the end portion.
• an acceleration voltage is set to 15 to 25 kV, the measurement is performed at intervals of at least 0.25 ⁇ m or less, and the crystal orientation information about each measurement point in a range of 150 ⁇ m or more in the sheet thickness direction and 250 ⁇ m or more in the rolling direction is obtained.
• a measurement point at which a crystal structure is fcc is determined as residual austenite using "Phase Map" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. The ratio of measurement points determined as the residual austenite is obtained, thereby obtaining the area ratio of the residual austenite.
• the measurement intervals are narrow and the measurement range is wide.
• the measurement interval is set to 0.01 ⁇ m or more.
• the measurement range may be set to 200 ⁇ m in the sheet thickness direction and 400 ⁇ m in the sheet width direction at a maximum.
• An EBSD device including a thermal field emission type scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.) and an EBSD detector (DVC5-type detector manufactured by TSL Solutions) is used for measurement. In this case, a degree of vacuum in the device is set to 9.6 ⁇ 10 -5 Pa or less, the irradiation current level is set to 13, and the irradiation level of the electron beam is set to 62.
• the hot-stamping formed body according to the present embodiment may have a plating layer formed on the surface for the purpose of improving corrosion resistance after the hot stamping or the like.
• the plating layer may be any of an electroplating layer and a hot-dip plating layer.
• the electroplating layer includes, for example, an electrogalvanized layer, an electrolytic Zn-Ni alloy plating layer, and the like.
• the hot-dip plating layer includes, for example, a hot-dip galvanized layer, a hot-dip galvannealed layer, a hot-dip aluminum plating layer, a hot-dip Zn-Al alloy plating layer, a hot-dip Zn-Al-Mg alloy plating layer, a hot-dip Zn-Al-Mg-Si alloy plating layer, and the like.
• An adhesion amount of a plating layer is not particularly limited and may be a general adhesion amount.
• the sheet thickness of the hot-stamping formed body according to the present embodiment is not particularly limited. However, in terms of reducing the weight of a vehicle body or the like, it is preferable that the sheet thickness of the hot-stamping formed body according to the present embodiment is set to 0.5 to 3.5 mm.
• the hot-stamping formed body according to the present embodiment has a tensile (maximum) strength of 2,200 MPa or more.
• the tensile strength is preferably 2,400 MPa or more, and more preferably 2,550 MPa or more.
• the tensile strength is obtained according to the test method described in JIS Z 2241 :2011 by producing a No. 5 test piece described in JIS Z 2241:2011 from a position as flat as possible in the hot-stamping formed body.
• the maximum bending angle that is obtained by a bending test based on the VDA standard (VDA238-100) specified by the German Association of the Automotive Industry is preferably 20° or more.
• the conditions in the bending test were as described below.
• the rolling reduction in the rolling one pass before the final pass of finish rolling in the hot rolling is set high in order to obtain the steel sheet for hot stamping having the above-described microstructure.
• a steel piece (steel material) to be subjected to hot rolling may be a steel piece manufactured by an ordinary method, and may be, for example, a steel piece manufactured by a general method such as a continuous cast slab or a thin slab caster.
• the hot rolling rough rolling and finish rolling are performed.
• the slab after the rough rolling is rolled by a plurality of finishing mills.
• the rolling one pass before the final pass of finish rolling is performed in a temperature range of 900°C to 1,050°C at a rolling reduction of 10% to 25%.
• the final pass is performed in a temperature range of 850°C or higher and lower than 1,000°C at a rolling reduction (final rolling reduction) of 6% or more.
• the rolling reduction in the rolling one pass before the final pass can be represented by ⁇ (T0 - t1)/t0 ⁇ ⁇ 100 (%).
• the final rolling reduction can be represented by ⁇ (t1 - t2)/t1 ⁇ ⁇ 100 (%).
• dislocation in austenite is reduced, and by setting the rolling reduction (final rolling reduction) of the subsequent final pass to 6% or more, a small amount of dislocation can be introduced into the austenite grains.
• the dislocations introduced into the austenite grains function as the precipitation origins of carbides, and thus, as a result, a desired amount of ferrite containing the carbides can be formed in the grains. Since dislocations in austenite before the final rolling are combined with dislocations introduced in the final pass and disappear, it is presumed that unless the rolling reduction in the rolling one pass before the final pass is controlled within the above range, the precipitation origin of carbides decreases.
• the rolling is performed by gradually reducing the rolling reduction in each pass.
• the rolling in the rolling one pass before the final pass of the finish rolling, the rolling is performed at the above-described rolling reduction with a rolling reduction higher than that of a pass (two passes before the final pass) before that. Accordingly, a desired microstructure can be obtained.
• the rolling reduction in the rolling one pass before the final pass is less than 10% or more than 25%, the recrystallization of austenite in the final pass is suppressed, and a desired texture cannot be obtained.
• the rolling reduction in the rolling one pass before the final pass is preferably 13% or more, more preferably 16% or more, and still more preferably 18% or more.
• the rolling temperature one pass before the final pass is lower than 900°C, the recrystallization of austenite in the final pass is suppressed, and a desired texture cannot be obtained.
• the rolling temperature one pass before the final pass is preferably 910°C or higher, and more preferably 930°C or higher.
• the rolling temperature one pass before the final pass is higher than 1 ,050°C, the austenite grains become coarse and ferritic transformation is suppressed, so that a predetermined amount of ferrite cannot be obtained in the steel sheet for hot stamping.
• the rolling temperature one pass before the final pass is preferably 1,040°C or lower and more preferably 1,020°C or lower.
• the rolling reduction of the final pass (final rolling reduction) is less than 6%, the number of dislocations that are introduced decreases, and the number proportion of the ferrite containing the carbide having the equivalent circle diameter of 0.2 ⁇ m or more in the grains cannot be controlled to a predetermined amount.
• the final rolling reduction is preferably 8% or more, more preferably 10% or more, and still more preferably 12% or more.
• the upper limit of the final rolling reduction is not particularly specified and may be set to less than 40%.
• the rolling temperature of the final pass is preferably 860°C or higher and more preferably 870°C or higher.
• the rolling temperature of the final pass is preferably 980°C or lower and more preferably 960°C or lower.
• the heating temperature and holding time of the steel piece before hot rolling are not particularly limited, but it is preferable that the steel piece is held in a temperature range of 1200°C or higher for 20 minutes or longer.
• the steel sheet is preferably coiled in the temperature range of 400°C to 750°C.
• the coiling temperature is lower than 400°C, the area ratio of pearlite is more than 90% and the area ratio of ferrite is less than 10% in the steel sheet for hot stamping.
• the coiling temperature is preferably 450°C or higher and more preferably 530°C or higher.
• the coiling temperature is higher than 750°C, the area ratio of pearlite is less than 10% and the area ratio of ferrite is more than 90% in the steel sheet for hot stamping.
• the coiling temperature is preferably 700°C or lower and more preferably 660°C or lower.
• cold rolling may be performed as necessary.
• the above-mentioned plating may be formed after finish rolling or after cold rolling.
• Pickling may be performed between the hot rolling and the cold rolling.
• a normal cumulative rolling reduction for example, 30% to 90% may be set.
• temper rolling may be performed under normal conditions.
• hot-rolled sheet annealing in which the hot-rolled steel sheet is heated to a temperature range of 730°C or lower may be performed.
• the steel sheet for hot stamping according to the present embodiment can be manufactured by the above method. Next, a manufacturing method of the hot-stamping formed body according to the present embodiment that can be manufactured using the above-described steel sheet for hot stamping will be described.
• the manufacturing method of the hot-stamping formed body according to the present embodiment is not particularly limited, and for example, the following manufacturing method may be used.
• the above-mentioned steel sheet for hot stamping is heated in a temperature range of 800°C or higher.
• the heating temperature is preferably 820°C or higher and more preferably 860°C or higher.
• the upper limit of the heating temperature is not particularly limited. However, when the heating temperature is too high, decarburization is promoted in the surface layer of the steel sheet, and the strength of the hot-stamping formed body decreases. Therefore, the heating temperature is preferably 1,000°C or lower, more preferably 960°C or lower, and even more preferably 930°C or lower.
• the holding time at the heating temperature is preferably 1.0 to 10.0 minutes.
• the holding time is shorter than 1.0 minutes, there are cases where coarse carbides remain and the bendability of the hot-stamping formed body decreases. Meanwhile, when the holding time is more than 10.0 minutes, decarburization is promoted in the surface layer of the steel sheet, and the strength of the hot-stamping formed body may decrease.
• the average heating rate up to the heating temperature is preferably set to 1.0 °C/s or faster.
• the average heating rate is slower than 1.0 °C/s, decarburization is promoted in the surface layer of the steel sheet, and the strength of the hot-stamping formed body decreases.
• the upper limit of the average heating rate is not particularly determined, since it is difficult to set the upper limit to faster than 1,000 °C/s in actual operation, the actual upper limit is 1,000 °Cls or slower.
• Hot stamping is performed after the heating and the holding described above. After the hot stamping, it is preferable to perform cooling to a temperature range of, for example, 300°C or lower at an average cooling rate of 10 °C/s or faster. When the average cooling rate is slower than 10 °C/s, the strength may be insufficient. Although the upper limit of the average heating rate is not particularly determined, since it is difficult to set the upper limit to faster than 1,000 °C/s in actual operation, the actual upper limit is 1,000 °C/s or slower.
• preheating that is, to perform two-stage heating.
• the segregation region of carbon in the grain boundaries created in the stage of the steel sheet for hot stamping is eliminated, it is not possible to uniformly disperse and form the prior austenite grains, and as a result, the standard deviation of the prior austenite grains cannot be controlled within a desired range.
• the hot-stamping formed body according to the present embodiment can be obtained by the preferable manufacturing method described above. After the hot stamping, a tempering treatment may be performed at 150°C to 600°C. In addition, a part of the hot-stamping formed body may be tempered by laser irradiation or the like to partially provide a softened region. Weldability improves in the softened region. For example, when spot welding is performed after softening the end portion of the hot-stamping formed body, it is possible to reduce a difference in strength between the softened end portion and the spot-welding portion of the end portion, and thus, the fracture from the interface between the end portion and the spot-welding portion can be suppressed.
• the hot-stamping formed body is applied to a high strength member of an automobile, it is possible to control a fracture or deformation mode of the high strength member in the time of a collision by providing a softened region in a part of the high strength member.
• Conditions in the examples are one condition example that is employed to confirm the feasibility and effects of the present invention, and the present invention is not limited to this condition example.
• the present invention may employ various conditions to achieve the object of the present invention without departing from the scope of the present invention.
• a steel piece manufactured by casting molten steel having a chemical composition shown in Tables 1A to 1D was heated, held in a temperature range of 1,200°C or higher for 20 minutes or longer, and then subjected to hot rolling and coiling under conditions shown in Tables 2A to 2G, and subjected to cold rolling, hot-rolled sheet annealing, pickling, and plating as necessary.
• steel sheets for hot stamping shown in Tables 2A to 2G were obtained.
• the finish rolling except for No. 195 marked with "*"
• the rolling was performed with a higher rolling reduction than the pass (two passes before the final pass) before that.
• Steel sheet No. 149 was subjected to the hot-rolled sheet annealing of heating and holding in a temperature range of 730°C or lower.
• the cold rolling was not performed on Steel sheet No. 150.
• An electrogalvanized layer was formed on the surface of Steel sheet No. 151.
• An electrolytic Zn-Ni alloy plating layer was formed on the surface of Steel sheet No. 152.
• a hot-dip galvanized layer was formed on the surface of Steel sheet No. 153.
• a hot-dip galvannealed layer was formed on the surface of Steel sheet No. 154.
• a hot-dip aluminum plating layer was formed on the surface of Steel sheet No. 155.
• a hot-dip Zn-Al alloy plating layer was formed on the surface of Steel sheet No. 156.
• a hot-dip Zn-Al-Mg alloy plating layer was formed on the surface of Steel sheet No. 157.
• a hot-dip Zn-Al-Mg-Si alloy plating layer was formed on the surface of Steel sheet No. 158.
• a "pole density” indicates the "average value of the pole densities of ferrite in the orientation group consisting of ⁇ 100 ⁇ 011> to ⁇ 223 ⁇ 110>
• a "number proportion of ferrite including carbide” indicates the “number proportion of ferrite including a carbide having an equivalent circle diameter of 0.2 ⁇ m or more in grains in entire ferrite”.
• the obtained steel sheets for hot stamping were subjected to hot stamping under the conditions shown in Tables 3A to 3G to obtain hot-stamping formed bodies shown in Tables 3A to 3G.
• a tempering treatment was performed at 150°C to 600°C after hot stamping.
• a partially softened region was formed by irradiating a portion of the hot-stamping formed body with a laser and tempering the portion.
• Manufacturing No. 188 was heated to a heating temperature shown in Table 3G, Manufacturing No. 188 was cooled to a temperature range of 250°C or lower. Thereafter, Manufacturing No. 188 was heated to 900°C and hot-stamped, and then cooled at the average cooling rate in Table 3G.
• the microstructures included, by area%, ferrite: 0% to 50%, bainite and martensite: 0% to 100%, pearlite: 0% to 30%, and residual austenite: 0% to 5%.
• a method for measuring the microstructure of the steel sheet for hot stamping and a method for measuring the microstructure and mechanical properties of the hot-stamping formed body were as described above.
• the hot-stamping formed body was determined to be acceptable for having high strength, and, in a case where the tensile strength of the hot-stamping formed body was less than 2,200 MPa, the hot-stamping formed body was determined to be unacceptable for not having high strength.

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