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EP3584345A1 - Corps moulé par estampage à chaud - Google Patents

Corps moulé par estampage à chaud Download PDF

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Publication number
EP3584345A1
EP3584345A1 EP18754731.0A EP18754731A EP3584345A1 EP 3584345 A1 EP3584345 A1 EP 3584345A1 EP 18754731 A EP18754731 A EP 18754731A EP 3584345 A1 EP3584345 A1 EP 3584345A1
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EP
European Patent Office
Prior art keywords
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none
sheet thickness
hot stamped
good
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP18754731.0A
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German (de)
English (en)
Inventor
Yuri TODA
Genki ABUKAWA
Daisuke Maeda
Kazuo HIKIDA
Shingo FUJINAKA
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Nippon Steel Corp
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Nippon Steel Corp
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Filing date
Publication date
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Publication of EP3584345A1 publication Critical patent/EP3584345A1/fr
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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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • 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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • 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
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals

Definitions

  • the present invention relates to a hot stamped body used for structural members or reinforcing members of automobiles or structures where strength is required, in particular a hot stamped body excellent in strength, impact resistance, ductility, and hydrogen embrittlement resistance after hot stamping and small in scattering in hardness.
  • hot stamping where the steel sheet is heated to a high temperature of the austenite region, then is press-formed, is increasingly being applied. Since hot stamping performs press-forming and simultaneously quenching in the die, it is possible to obtain a strength corresponding to the C amount of the steel sheet. This is being taken note of as a technique achieving both formation of a material into an automobile member and securing strength.
  • a member of a hot stamped body is affected not only by the strength of the member, but also the ease of buckling. If the ductility of the steel sheet is high, in the state of a member formed into a certain shape, it becomes harder for localization of the deformation region to occur. That is, the member becomes resistant to buckling.
  • the way of contact with the die is not necessarily uniform.
  • the cooling rate easily falls.
  • steel sheet is sometimes locally formed with regions with low hardnesses. Deformation concentrates in a local soft part at the time of collision and becomes a cause of cracking, so a small scattering in hardness of the body, that is, securing stable strength, is important in securing impact resistance.
  • ductility is important, but in general the ductility of martensite is low. Further, the density of lattice defects of the surface layer of the steel sheet is high, so there is the problem that penetration by hydrogen is promoted and the part becomes poor in hydrogen embrittlement resistance. Due to such reasons, hot stamped parts produced by press hardening have been limited in locations of use in auto parts.
  • PTL 1 discloses making the hardness of the middle in sheet thickness of a hot pressed part 400Hv or more and forming a softened layer with a thickness of 20 ⁇ m to 200 ⁇ m and a hardness of 300Hv or less on a surface layer so as to secure a strength of a tensile strength of 1300 MPa or more while suppressing cracking at the time of automobile collision.
  • PTL 2 discloses controlling the concentration of carbon at a surface layer in sheet thickness to 1/5 or less of the concentration of carbon of the middle part in sheet thickness so as to reduce the density of lattice defects of the surface layer and improve the hydrogen embrittlement resistance.
  • PTL 3 discloses to make the middle part in sheet thickness a dual phase structure of ferrite and martensite and raise the structural fraction of ferrite of a surface layer portion so as to ease the stress even if the surface layer part receives severe bending deformation.
  • the present invention in consideration of the technical issues in the prior art, has as its object to provide a hot stamped body achieving both a high bendability and high ductility for realizing impact resistance and hydrogen embrittlement resistance and keeping down the scattering in hardness.
  • the inventors engaged in an in-depth study of a method for solving the above technical issues. As a result, to improve the hydrogen embrittlement resistance, it is effective to reduce the density of lattice defects at the surface layer in sheet thickness. For this reason, it is necessary to form soft structures at the surface layer. On the other hand, to secure a 1500 MPa or more tensile strength, it is necessary to form the middle part in sheet thickness by only hard structures.
  • the inventors thought that if forming the surface layer in sheet thickness by soft structures and forming the middle part in sheet thickness by hard structures, if it were possible to reduce the sharp gradient of hardness in the sheet thickness direction occurring near the boundary of the hard structures and soft structures, a strength of a tensile strength of 1500 MPa or more and excellent hydrogen embrittlement resistance could be secured while excellent bendability could be obtained.
  • the metal structures forming the surface layer should be comprised of crystal grains with a maximum crystal orientation difference inside the crystal grains of 1° or less and crystal grains with a maximum crystal orientation difference inside the crystal grains of 8° to 15° when a region surrounded by grain boundaries having an orientation difference of 15° or more in the sheet thickness cross-section is defined as a "crystal grain". These measurements were performed in the region from a position of a depth of 20 ⁇ m below the surface of the surface layer to a position of a depth of 1/2 of the thickness of the surface layer (center of surface layer). It was discovered that the effects of the surface properties of the hot stamped body and the effects of the transitional part from the middle part in sheet thickness to the surface layer can be eliminated by such metal structures.
  • the inventors raised the ductility and raised the hardenability to stably secure high strength. As a result, it is possible to keep down the occurrence of cracking at the time of bending deformation.
  • the inventors succeeded in securing a 1500 MPa or more tensile strength and good hydrogen embrittlement resistance while realizing excellent bendability, ductility, and stability of strength and were able to obtain a hot stamped body excellent in impact resistance and hydrogen embrittlement resistance.
  • the present invention was completed based on the above discovery and has as its gist the following:
  • the present invention it is possible to provide a hot stamped body excellent in bendability, ductility, impact resistance, and hydrogen embrittlement resistance and with small scattering in hardness.
  • the hot stamped body according to the present invention is a structure with a softened layer arranged on the surface at both sides or one side.
  • the softened layer has a region having a hardness 10Hv or more lower than the hardness of the middle part in sheet thickness.
  • the middle part in sheet thickness of the hot stamped body according to the present invention must have a hardness of 500Hv to 800Hv.
  • the reasons for limiting the composition of constituents at the middle part in sheet thickness to make the hardness of the middle part in sheet thickness the above-mentioned range are explained below.
  • the % relating to the component of constituents means mass%.
  • C is an important element for obtaining a 500Hv to 800Hv hardness at the middle part in sheet thickness.
  • C is 0.20% or more.
  • it is 0.30% or more.
  • C is 0.50% or less.
  • Si is an element contributing to improvement of strength by solution strengthening.
  • the amount of addition of Si for obtaining the effect of improvement of strength of the steel sheet by formation of a solid solution of Si in the metal structures is preferably 0.30% or more, but even if adding more than 0.5% of Si, the effect becomes saturated.
  • Si also has the effect of causing the formation of residual austenite and raising the ductility. To obtain this effect, addition of more than 0.50% is at least necessary. On the other hand, even if adding more than 3.00%, the effect becomes saturated, and therefore the amount of addition of Si is one with an upper limit of less than 3.00%. Preferably, the amount is less than 2.0%.
  • Mn is an element contributing to improvement of strength by solution strengthening.
  • the effect of improving the strength of the steel sheet by solid solution of Mn in the metal structures cannot be obtained with an amount of addition of less than 0.20%, so 0.20% or more is added.
  • the content is 0.70% or more.
  • the effect becomes saturated.
  • Mn further, has the effect of raising the hardenability.
  • the preferable amount of addition for obtaining the effect of raising the hardenability is 1.70% or more. Even if adding 3.00% or more, the effect becomes saturated, and therefore the upper limit of the amount of addition of Mn is 3.00%.
  • the content is less than 2.00%.
  • P is an element segregating at the grain boundaries and impairing the strength of the grain boundaries. If more than 0.10%, the strength of the grain boundaries remarkably falls and the hydrogen embrittlement resistance and bendability fall, and therefore P is 0.10% or less. Preferably, it is 0.05% or less.
  • the lower limit is not particularly prescribed, but if reducing this to less than 0.0001%, the dephosphorizing cost greatly rises and the result becomes economically disadvantageous, so in practical steel sheet, 0.0001% is the substantive lower limit.
  • S is an element forming inclusions. If more than 0.10%, inclusions are formed and the hydrogen embrittlement resistance and bendability fall, and therefore S is 0.10% or less. Preferably, it is 0.0025% or less.
  • the lower limit is not particularly prescribed, but if reducing this to less than 0.0015%, the desulfurizing cost greatly rises and the result becomes economically disadvantageous, so in practical steel sheet, 0.0001% is the substantive lower limit.
  • Al is an element acting to deoxidize the molten steel and make the steel sounder.
  • the range of content of not all of the Al contained in the steel but the content of so-called "acid soluble aluminum" (sol. Al) is prescribed.
  • sol. Al content of less than 0.0002%, the deoxidizing is insufficient, and therefore sol. Al is 0.0002% or more.
  • the content is 0.0010% or more.
  • the effect becomes saturated, and therefore the content is 3.0000% or less.
  • N is an impurity element and is an element which forms nitrides and impairs bendability. If more than 0.01%, coarse nitrides are formed and the bendability remarkably falls, and therefore N is 0.01% or less. Preferably the content is 0.0075% or less.
  • the lower limit is not particularly prescribed, but if reducing this to less than 0.0001%, the denitriding cost greatly rises and the result becomes economically disadvantageous, so in practical steel sheet, 0.0001% is the substantive lower limit.
  • Ni is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.010%, the effect is not obtained, so 0.010% or more is added. Preferably, the content is 0.5% or more. On the other hand, even if added in more than 3.00%, the effect becomes saturated, and therefore the content is 3.00% or less. Preferably, the content is 2.50% or less.
  • Nb is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.010%, the effect is not obtained, so 0.010% or more is added. Preferably, the content is 0.035% or more. On the other hand, even if added in more than 0.150%, the effect becomes saturated, and therefore the content is 0.150% or less. Preferably, the content is 0.120% or less.
  • Ti is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.010%, the effect is not obtained, and therefore the content is 0.010% or more. Preferably, the content is 0.020%. On the other hand, even if added in more than 0.150%, the effect becomes saturated, and therefore the content is 0.150% or less. Preferably, the content is 0.120% or less.
  • Mo is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.005%, the effect is not obtained, and therefore the content is 0.005% or more. Preferably, the content is 0.0100% or more. On the other hand, even if added in more than 1.000%, the effect becomes saturated, and therefore the content is 1.000% or less. Preferably, the content is 0.800% or less.
  • the B is an element segregating at the grain boundaries and improving the strength of the grain boundaries, so may be added as needed. With less than 0.0005%, the effect of addition is not sufficiently obtained, so 0.0005% or more is added. Preferably, the content is 0.0010% or more. On the other hand, even if added in more than 0.0100%, the effect becomes saturated, and therefore the content is 0.0100% or less. Preferably, the content is 0.0075% or less.
  • the balance of the composition of constituents of the middle part in sheet thickness consists of Fe and unavoidable impurities.
  • the unavoidable impurities are elements which unavoidably enter from the steel raw materials and/or in the steelmaking process and are allowed in ranges not impairing the characteristics of the hot stamped body of the present invention.
  • the hardness of the middle part in sheet thickness is 500Hv or more, as the tensile strength of the hot stamped body of the present invention, 1500 MPa or more can be secured. Preferably, it is 600Hv or more.
  • the hardness of the middle part in sheet thickness is more than 800Hv, since the difference in hardness with the softened layer becomes too large and deterioration of the bendability is invited, 800Hv is the upper limit.
  • the hardness is 720Hv or less.
  • the method of measurement of the hardness of the middle part in sheet thickness is as follows: A cross-section vertical to the sheet surface of the hot stamped body is taken to prepare a sample of the measurement surface. This is supplied to a hardness test.
  • the method of preparing the measurement surface may be based on JIS Z 2244. For example, #600 to #1500 silicon carbide paper may be used to polish the measurement surface, then a solution of particle size 1 ⁇ m to 6 ⁇ m diamond powder dispersed in alcohol or another diluent or pure water may be used to finish the sample to a mirror surface.
  • the hardness test may be performed by the method described in JIS Z 2244.
  • a micro-Vickers hardness tester is used to measure 10 points at the 1/2 position of thickness of the hot stamped body by a load of 1 kgf and intervals of 3 times or more of the dents. The average value was defined as the hardness of the middle part in sheet thickness.
  • the middle part in sheet thickness can be improved in ductility by including residual austenite in an area percent of 1% or more.
  • the area percent of residual austenite at the middle part in sheet thickness is preferably 2% or more.
  • the upper limit is less than 5.0%.
  • the fraction is less than 4.5%.
  • the area percent of the residual austenite can be measured by the following method. A sample is taken from a hot stamped member and ground down at its surface to a depth of 1/2 of the sheet thickness from the normal direction of the rolling surface. The ground down surface is used for X-ray diffraction measurement.
  • ⁇ (211) is the X-ray diffraction intensity at the (211) face of ferrite
  • ⁇ (220) is the X-ray diffraction intensity at the (220) face of austenite
  • ⁇ (311) is the X-ray diffraction intensity at the (311) face of austenite.
  • the "softened layer” is the region in the sheet thickness direction of the cross-section of sheet thickness of the hot stamped body from the position where the hardness falls by 10Hv or more from hardness of the middle part in sheet thickness (hardness at position of 1/2 of sheet thickness) to the surface of the stamped body.
  • the inventors engaged in intensive studies and as a result discovered that from the viewpoint of the bendability and other effects, the fractions of structures from a position of 20 ⁇ m from the surface of the softened layer to a position of a depth of 1/2 of the thickness of the softened layer are important.
  • the effects of the surface properties of the hot stamped body and the effects of the transitional part from the middle part in sheet thickness to the softened layer can be eliminated by such metal structures.
  • the area rate of the total of crystal grains with a maximum crystal orientation difference inside the crystal grains of 1° or less and crystal grains with a maximum crystal orientation difference inside the crystal grains of 8° to 15° should be 50% or more, more preferably 55% or more.
  • the area rate of the total of the metal structures of the softened layer of 85% or more the difference in hardness of the softened layer and the middle part in sheet thickness becomes too great and the effect of reduction of the sharp gradient of hardness in the sheet thickness direction occurring at the time of bending deformation cannot be obtained, and therefore the area rate is less than 85%. More preferably, it is 80% or less.
  • a cross-section vertical to the surface of the hot stamped body being measured (cross-section of sheet thickness) is taken to prepare a sample of the measurement surface. This is used for a hardness test.
  • the method of preparing the measurement surface may be based on JIS Z 2244. For example, #600 to #1500 silicon carbide paper may be used to polish the measurement surface, then a solution of particle size 1 ⁇ m to 6 ⁇ m diamond powder dispersed in alcohol or another diluent or pure water may be used to finish the sample to a mirror surface.
  • the sample with the prepared measurement surface is measured two times based on the method described in JIS Z 2244 using a micro Vickers hardness tester.
  • the first time measures the hardness from the region within 20 ⁇ m from the surface of the hot stamped body in the sheet thickness direction to the middle part in sheet thickness (position of 1/2 of sheet thickness) in the direction vertical to the surface (sheet thickness direction) by a load of 0.3 kgf at intervals of 3 times or more the dents.
  • this is measured from the region within 20 ⁇ m right under the plating or coating or the alloy layer of the plating or coating and material of the softened layer.
  • the position where the hardness starts to drop by 10Hv or more from the hardness of the middle part in sheet thickness is determined and the layer from that sheet thickness position to the surface of the hot stamped body is defined as the "softened layer". If the softened layer is present at both surfaces, the second measurement is performed at the surface at the opposite side to the first one (back surface) by a similar method to determine the position where the hardness starts to drop by 10Hv or more from the hardness of the middle part in sheet thickness.
  • a sample is cut out from a hot stamped body to enable examination of a cross-section vertical to its surface (sheet thickness direction).
  • the length of the sample depends on the measuring device, but may be about 50 ⁇ m.
  • the region in the sheet thickness direction of the sample from the surface of the softened layer to the position of 1/2 of the thickness of the softened layer (center of softened layer) is analyzed at 0.2 ⁇ m measurement intervals by EBSD to obtain information on the crystal orientation.
  • this EBSD analysis is performed using an apparatus comprised of a thermal field emission type scan electron microscope (JSM-7001F made by JEOL) and EBSD detector (DVC5 type detector made by TSL) at an analysis speed of 200 to 300 points/second.
  • JSM-7001F thermal field emission type scan electron microscope
  • DVC5 type detector made by TSL
  • a region surrounded by grain boundaries having an orientation difference of 15° or more is defined as one crystal grain and a crystal orientation map in the sheet surface direction is prepared.
  • the obtained crystal orientation map is used to find the crossing points of the long axis of one crystal grain and the crystal grain boundaries. Among the two crossing points, one is designated as the starting point and the other is designated as the end point and the difference in orientation among all measurement points contained on the long axis of the crystal grain is calculated.
  • the maximum value of the orientation difference obtained was defined as the maximum crystal orientation difference at that crystal grain.
  • the above analysis was performed for all crystal grains included in the measurement region, then the average of these values was defined as the maximum crystal orientation difference inside a region surrounded by grain boundaries of 15° or more.
  • the above-defined maximum crystal orientation difference can be simply calculated, for example, if using the "Inverse Pole Figure Map” and “Profile Vector” functions included in the software (OIM Analysis®) attached to the EBSD analysis system.
  • the "Inverse Pole Figure Map” function it is possible to draw grain boundaries having slants of 15° or more as large angle grain boundaries and further possible to prepare a crystal orientation map in the sheet surface direction.
  • the “Profile Vector” function it is possible to calculate the misorientation angle (difference in crystal orientations) between all measurement points included on any line.
  • crystal grains at end parts of measurement region are analyzed as explained above and the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° to 15° is calculated. If the softened layer is formed on both surfaces, the above procedure is performed at the back surface side of the hot stamped body as well and the average value of the area rates obtained from the front surface side and the back surface side is employed.
  • the composition of the softened layer is not particularly limited other than regarding the unavoidable impurity elements of P, S, and N impairing the strength and/or bendability, but the layer is preferably the following composition so as to secure the strength of the hot stamped body and steel exhibiting excellent bendability.
  • one or more of the C content, Si content, and Mn content are preferably respectively 0.6 time the corresponding contents of elements of the middle part in sheet thickness.
  • the preferable ranges of the constituents in this case are as follows:
  • the C may be added in 0.05% or more so as to raise the strength.
  • the content is 0.10% or more.
  • the preferable C content of the softened layer is less than 0.42%.
  • the content is 0.35% or less.
  • Si is an element contributing to improvement of strength by solution strengthening, so is added for raising the strength.
  • the hardness of the softened layer lower than the hardness of the middle part in sheet thickness, it is preferable to make this smaller in content than the middle part in sheet thickness.
  • the preferable Si content of the softened layer is 0.30% or less, more preferably 0.20% or less. Further, if the Si content of the middle part in sheet thickness is more than 0.50% and less than 3.00%, the preferable Si content of the softened layer is less than 2.00%, more preferably 1.50% or less.
  • Mn is an element contributing to improvement of strength by solution strengthening, so may be added in 0.12% or more for raising the strength.
  • Mn is an element contributing to improvement of strength by solution strengthening, so may be added in 0.12% or more for raising the strength.
  • it is preferably smaller in content than the middle part in sheet thickness.
  • the preferable Mn content of the softened layer is less than 0.90%, more preferably is 0.70% or less. Further, if the Mn content of the middle part in sheet thickness is 1.50% to less than 3.00%, the preferable Mn content of the softened layer is less than 1.80%, preferably 1.40% or less.
  • P is an element segregating at the grain boundaries and impairing the strength of the grain boundaries. If more than 0.10%, the strength of the grain boundaries remarkably falls and the hydrogen embrittlement resistance and bendability fall, and therefore P is 0.1% or less. Preferably, it is 0.05% or less.
  • the lower limit is not particularly prescribed, but if reducing this to less than 0.0001%, the dephosphorizing cost greatly rises and the result becomes economically disadvantageous, so in practical steel sheet, 0.0001% is the substantive lower limit.
  • S is an element forming inclusions. If more than 0.10%, inclusions are formed and the hydrogen embrittlement resistance and bendability fall, and therefore S is 0.10% or less. Preferably, it is 0.0025% or less.
  • the lower limit is not particularly prescribed, but if reducing this to less than 0.0015%, the desulfurizing cost greatly rises and the result becomes economically disadvantageous, so in practical steel sheet, 0.0001% is the substantive lower limit.
  • Al is an element acting to deoxidize the molten steel and make the steel sounder.
  • the range of content of not all of the Al contained in the steel but the so-called "acid soluble aluminum" (sol. Al) is prescribed.
  • sol. Al content of less than 0.0002%, the deoxidizing is insufficient, and therefore the sol. Al is preferably 0.0002% or more. More preferably the content is 0.0010% or more.
  • the content is 3.0000% or less.
  • N is an impurity element and is an element which forms nitrides and impairs bendability. If more than 0.01%, coarse nitrides are formed and the bendability remarkably falls, and therefore N is 0.01% or less. Preferably the content is 0.0075% or less.
  • the lower limit is not particularly prescribed, but if reducing this to less than 0.0001%, the denitriding cost greatly rises and the result becomes economically disadvantageous, so in practical steel sheet, 0.0001% is the substantive lower limit.
  • the constituents of the softened layer one or more of the C content, Si content, and Mn content are preferably respectively 0.6 time or less the C content, Si content, and Mn content of the middle part in sheet thickness.
  • the other constituents are not particularly limited.
  • the softened layer may optionally and selectively include one or more of the following constituents besides C, Si, and Mn.
  • Ni is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.01%, the effect is not obtained, and therefoere preferably 0.01% or more is added. More preferably, the content is 0.50% or more. On the other hand, even if added in more than 3.00%, the effect becomes saturated, and therefore the content is 3.00% or less. Preferably, the content is 2.50% or less.
  • Nb is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.010%, the effect is not obtained, so preferably 0.010% or more is added. More preferably, the content is 0.035% or more. On the other hand, even if added in more than 0.150%, the effect becomes saturated, and therefore the content is 0.150% or less. Preferably, the content is 0.120% or less.
  • Ti is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.010%, the effect is not obtained, and therefore preferably the content is 0.010% or more. More preferably, the content is 0.020%. On the other hand, even if added in more than 0.150%, the effect becomes saturated, and therefore the content is 0.150% or less. Preferably, the content is 0.120% or less.
  • Mo is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.005%, the effect is not obtained, and therefore preferably the content is 0.005% or more. More preferably, the content is 0.010% or more. On the other hand, even if added in more than 1.000%, the effect becomes saturated, and therefore the content is 1.000% or less. Preferably, the content is 0.800% or less.
  • B is an element segregating at the grain boundaries and improving the strength of the grain boundaries, so may be added as needed. With less than 0.0005%, the effect of addition is not sufficiently obtained, and therefore preferably 0.0005% or more is added. More preferably, the content is 0.0010% or more. On the other hand, even if added in more than 0.0100%, since the effect becomes saturated, the content is 0.0100% or less. Preferably, the content is 0.0075% or less.
  • the distribution of hardness at the middle part in sheet thickness is preferably uniform with no scattering.
  • contact with the die is difficult and the cooling rate becomes low, so sometimes the hardness falls. If there is a region where the hardness falls by 100Hv or more from the average hardness of the cross-section vertical to the longitudinal direction of the hat-shaped member, at the time of impact, the deformation will concentrate at the softened part and the part will fracture early, so a high impact resistance cannot be obtained.
  • the distribution of hardness at the cross-section and the average hardness of the cross-section are obtained by obtaining a cross-section vertical to the longitudinal direction of a long hot stamped body at any position in the longitudinal direction and measuring the Vickers hardness between the end parts of the cross-section at equal intervals of 1 mm pitch or less at the middle position of sheet thickness of the entire cross-sectional region including the vertical walls using a Vickers hardness tester (load of 1 kgf).
  • the surface of the softened layer may be formed with a plated layer for the purpose of improving the corrosion resistance.
  • the plated layer may be either an electroplated layer or a hot dip coated layer.
  • An electroplated layer includes, for example, an electrogalvanized layer, electro Zn-Ni alloy plated layer, etc.
  • a hot dip coated layer a hot dip galvanized layer, a hot dip galvannealed layer, a hot dip aluminum coated layer, a hot dip Zn-Al alloy coated layer, a hot dip Zn-Al-Mg alloy coated layer, a hot dip Zn-Al-Mg-Si alloy coated layer, etc.
  • the amount of deposition of the layer is not particularly limited and may be a general amount of deposition.
  • the present invention is not limited to the form of the double layer steel sheet explained below.
  • a steel sheet satisfying the requirements of the composition of constituents of the middle part in sheet thickness explained above is ground down at its front surface and/or back surface to remove surface oxides, then a steel sheet for softened layer formation use (below, referred to as a "steel sheet for surface layer”) is superposed on each ground down surface side.
  • the method of joining the steel sheet for surface layer and the steel sheet for sheet thickness middle part is not particularly limited, but they may be joined by arc welding.
  • a steel sheet for surface layer wherein one or more of the C content, Si content, and Mn content are 0.6 time or less the content of the corresponding element of the steel sheet for sheet thickness middle part is preferably superposed.
  • Mn raises the yield strength of austenite to thereby affect the behavior in formation of grain boundaries in the transformed structures, so when defining a region surrounded with grain boundaries having orientation differences of 15° or more as a "crystal grain", it has the effect of promoting the formation of crystal grains with a maximum crystal orientation difference inside the crystal grains of 8° to 15°. For this reason, it is also possible to control the casting rate to 6 ton/min or more in the continuous casting process of steel sheet for surface layer for the purpose of promoting the formation of the above micro structures.
  • a double layer steel sheet fabricated by the above method is preferably held at 1100°C or more and 1350°C or less in temperature for 20 minutes to less than 60 minutes.
  • the held sheet is preferably used as the steel sheet for hot stamped body according to the present invention.
  • the multilayer member produced by the above method of production (double layer steel sheet) can be treated by hot rolling, cold rolling, hot stamping, continuous hot dip coating, etc., to obtain the hot stamped body according to the present invention.
  • the hot rolling may be hot rolling performed under usual conditions.
  • the finishing temperature may also be in the temperature range of 810°C or more.
  • the subsequent following cooling conditions also do not have to be particularly prescribed.
  • the steel sheet is coiled in the temperature region of 750°C or less. Further, it may be reheated for the purpose of softening the double layer steel sheet after hot rolling.
  • the hot rolling after the above heat treatment of the double layer steel sheet preferably includes rough rolling and finish rolling with the rough rolling being performed twice under conditions of a temperature of 1100°C or more, a sheet thickness reduction rate per pass of 5% or more and less than 50%, and a time between passes of 3 seconds or more.
  • the concentrations of alloy elements, in particular C atoms have to be controlled to become more moderately distributed.
  • the distribution of concentration of C is obtained by diffusion of C atoms.
  • the diffusion frequency of C atoms increases the higher the temperature. Therefore, to control the C concentration, control in the rough rolling from the hot rolling heating becomes important.
  • the heating temperature has to be high. Preferably, it is 1100°C or more and 1350°C or less, more preferably more than 1150°C and 1350°C or less.
  • (i) shows the diffusion of C atoms from the middle part in sheet thickness to the surface layer, while (ii) shows the decarburization reaction of C being desorbed from the surface layer to the outside.
  • a distribution occurs in the concentration of C due to the balance between this diffusion of C atoms and the desorption reaction of (i) and (ii).
  • the reaction of (i) With less than 1100°C, the reaction of (i) is insufficient, so the preferable distribution of the concentration of C cannot be obtained.
  • the reaction of (ii) excessively occurs, so similarly a preferable distribution of concentration cannot be obtained.
  • pass control in rough rolling becomes extremely important.
  • Rough rolling is performed two times or more under conditions of a rough rolling temperature of 1100°C or more, a sheet thickness reduction rate per pass of 5% or more and less than 50%, and a time between passes of 3 seconds or more. This is so as to promote the diffusion of C atoms of (i) in FIG. 1 by the strain introduced in the rough rolling. Even if using an ordinary method to rough roll and finish roll a slab controlled in concentration of C to a preferable state by hot rolling heating, the sheet thickness will be reduced without the C atoms sufficiently diffusing in the surface layer.
  • curve 2 shows the change in the dislocation density after a rolling pass in the case where the sheet thickness reduction rate per pass in the rough rolling is small. It will be understood that strain remains over a long time period. By causing strain to remain at the surface layer over a long time period in this way, C atoms sufficiently diffuse in the surface layer and the optimum distribution of concentration of C can be obtained.
  • curve 2 shows the change in dislocation density in the case where the sheet thickness reduction rate is large. If the amount of strain introduced by the rolling rises, recovery is easily promoted and the dislocation density rapidly falls. For this reason, to obtain the optimal distribution of concentration of C, it is necessary to prevent the occurrence of a change in dislocation density like the curve 2.
  • the upper limit of the sheet thickness reduction rate per pass becomes less than 50%.
  • the lower limit of the sheet thickness reduction rate becomes 5%.
  • the time between passes 3 seconds or more has to be secured.
  • the cold rolling may be cold rolling performed by a usual rolling reduction, for example, 30 to 90%.
  • the hot rolled steel sheet and the cold rolled steel sheet include steel sheets as hot rolled and cold rolled and also steel sheets obtained by recrystallization annealing hot rolled steel sheet or cold rolled steel sheet under usual conditions and steel sheets obtained by skin pass rolling under usual conditions.
  • the heating, shaping, and cooling steps at the time of hot stamping may also be performed under usual conditions.
  • hot rolled steel sheet obtained by uncoiling hot rolled steel sheet coiled in the hot rolling step cold rolled steel sheet obtained by uncoiling and cold rolling coiled hot rolled steel sheet, or steel sheet obtained by plating or coating cold rolled steel sheet, heating this by a 0.1°C/s to 200°C/s heating rate up to 810°C or more and 1000°C or less in temperature, and holding it at this temperature is formed into the required shape by the usual hot stamping.
  • the holding time may be set according to the mode of forming, so is not particularly limited. For example, if 30 seconds or more and 600 seconds or less, a good hot stamped body is cooled to room temperature.
  • the cooling rate may also be set to a usual condition.
  • the average cooling rate in the temperature region from the heating temperature to more than 400°C may be 50°C/s or more.
  • steel sheet with an Si content at the middle part in sheet thickness of more than 0.50% and less than 3.00% and an Mn content at the middle part in sheet thickness of 0.20% or more and less than 1.50% and steel sheet with an Si content at the middle part in sheet thickness of more than 0.50% and less than 3.00% and an Mn content at the middle part in sheet thickness of 1.50% or more and less than 3.00% for the purpose of increasing the amount of formation of residual austenite to improve the ductility, it is preferable to control the average cooling rate at the cooling after heating and holding at the 200°C to 400°C temperature region to less than 50°C/s.
  • the middle part in sheet thickness and the softened layer were configured by separate steel sheets.
  • the hot stamped body of the present invention is not limited to double layer steel sheet comprised of two of the above-mentioned steel sheets superposed.
  • the middle part in sheet thickness and the softened layer may be formed inside a single material steel sheet. For example, it is possible to treat a single layer steel sheet to decarburize it and soften the surface layer part to thereby produce high strength steel sheet comprised of a softened layer and a middle part in sheet thickness.
  • the Nos.1 to 18 steel sheets for sheet thickness middle part having the chemical compositions shown in Table A-1-1 were ground down at their surfaces to remove the surface oxides.
  • the respective steel sheets for sheet thickness middle part were welded with steel sheets for surface layer having the chemical compositions shown in Table A-1-2 at both surfaces or single surfaces by arc welding to fabricate the Nos. 1 to 43 multilayer steel sheets for hot stamped body.
  • the total of the sheet thicknesses of the steel sheet for surface layer and the steel sheet for sheet thickness middle part after arc welding is 200 mm to 300 mm and the thickness of the steel sheet for surface layer is 1/3 or so of the thickness of the steel sheet for sheet thickness middle part (1/4 or so in case of single side).
  • multilayer steel sheet is steel with the steel sheet for surface layer welded to only one surface.
  • Nos.1 to 43 multilayer steel sheets of Table A-1-1 to Table A-1-2 ones with a steel sheet for sheet thickness middle part not satisfying the requirement for composition of the middle part in sheet thickness of the hot stamped body according to the present invention are indicated as "comparative steel” in the remarks column.
  • the Nos. 1 to 43 multilayer steel sheets were respectively treated under the conditions of the Nos. 1 to 43 manufacturing conditions shown in Table A-2-1 to Table A-2-2 by heat treatment before hot rolling, rough rolling, hot rolling, and cold rolling to obtain steel sheets.
  • the steel sheets were heat treated as shown in Table A-2-1 and Table A-2-2 (in the tables, "heat treatment of hot stamped body") for hot stamping to manufacture the Nos. 1A to 43A hot stamped bodies ("stamped bodies" of Table A-3).
  • the Nos. 35A and 36A hot stamped bodies were coated on a hot dip coating line at the surfaces with 120 to 160 g/m 2 amounts of aluminum.
  • the item “sheet thickness reduction rate” of the “rough rolling” means the sheet thickness reduction rate per pass of the rough rolling.
  • the item “number of rolling operations” means the number of rolling operations under the conditions of a time between passes of 3 seconds or more.
  • the item in the tables of “heating rate (°C/s)” means the rate of temperature rise until reaching the heating temperature of the "heat treatment at the time of hot stamping" after the cold rolling process.
  • the item “heating temperature (°C)" of the “heat treatment at the time of hot stamping” is the temperature at the time of hot stamping
  • the “average cooling rate (°C/s) (more than 400°C)” means the average cooling rate (°C/s) in the temperature region from the heating temperature to more than 400°C
  • the “average cooling rate (°C/s) (400°C or less)” means the average cooling rate (°C/s) in the temperature region from 200°C to 400°C.
  • the fields with the notations "-" indicate no corresponding treatment performed.
  • Table A-3 shows the metal structures and characteristics of the Nos. 1A to 43A hot stamped bodies.
  • the constituents obtained by analyzing the positions of 1/2 of the sheet thicknesses of the samples taken from the hot stamped bodies and positions of 20 ⁇ m from the surfaces of the softened layers were equivalent to the constituents of the steel sheets for sheet thickness middle part and steel sheets for surface layer of the Nos. 1 to 43 multilayer steel sheets of Table A-1-1 to Table A-1-2.
  • the metal structures of the hot stamped steel sheets were measured by the above-mentioned method.
  • the hardness of the steel sheet for sheet thickness middle part forming the middle part in sheet thickness and the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° to 15° in the metal structures from the surface of the steel sheet for surface layer forming the softened layer to 1/2 of the thickness were calculated.
  • the calculated values of the area rate are shown in the item "area rate (%) of total of crystal grains with maximum crystal orientation difference inside large angle grain boundaries of 1° or less and crystal grains with maximum crystal orientation difference of 8° to 15°" of Table A-3.
  • the results are shown in Table A-3.
  • the tensile test was performed by preparing a No. 5 test piece described in JIS Z 2201 and following the test method described in JIS Z 2241.
  • the hydrogen embrittlement resistance of the hot stamped body was evaluated using a test piece cut out from the stamped body.
  • a hot stamped body is joined with other parts using spot welding or another joining method.
  • the hot stamped body will be subjected to twisting and stress will be applied.
  • the stress differs depending on the position of the part. Accurately calculating this is difficult, but if there is no delayed fracture at the yield stress, it is believed there is no problem in practical use.
  • the body was evaluated based on the VDA standard (VDA238-100) prescribed by the German Association of the Automotive Industry under the following measurement conditions.
  • VDA238-100 the displacement at the time of maximum load obtained in the bending test was converted to angle by the VDA standard to find maximum bending angle and thereby evaluate the impact resistance of the hot stamped body.
  • the tensile strength is 1500 MPa or more, the maximum bending angle (°) was 70(°) or more, and the hydrogen embrittlement resistance was a passing level, it was judged that the impact resistance and hydrogen embrittlement resistance were excellent and the case was indicated as an "invention example”. If even one of the three aspects of performance is not satisfied, the case was indicated as a "comparative example".
  • each hot stamped body of the invention examples the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° to 15° in the metal structures from the surface of the steel sheet for surface layer to 1/2 of the thickness was 50% to less than 85%. Further, each hot stamped body of the invention examples was excellent in tensile strength, bendability, and hydrogen embrittlement resistance.
  • the No. 5A hot stamped body was low in carbon content of the steel sheet for sheet thickness middle part, so the hardness of the middle part in sheet thickness became insufficient and the tensile strength became insufficient.
  • the No. 9A hot stamped body was excessive in carbon content of the steel sheet for sheet thickness middle part, so the hardness of the middle part in sheet thickness became excessive and the targeted bendability could not be obtained.
  • the No. 11A hot stamped body was low in Mn content at the steel sheet for sheet thickness middle part, so the hardness of the middle part in sheet thickness became insufficient and the tensile strength became insufficient.
  • the Nos. 30A to 32A hot stamped bodies are comparative examples produced using the multilayer steel sheets for hot stamped body to which the desirable heat treatment had not been applied before the hot stamping process.
  • the No. 30A hot stamped body was low in heat treatment temperature before the hot stamping process, while the No. 31A hot stamped body was short in heat treatment time before the hot stamping process, so in the metal structures from the surface of the softened layer to 1/2 of the thickness, the soft structures and metal structures with intermediate hardnesses insufficiently grew and the target bendability could not be obtained.
  • the No. 32A hot stamped body was excessively high in heat treatment temperature before the hot stamping process, so the effect of reduction of the sharp gradient in hardness in the sheet thickness direction occurring at the time of bending deformation could not be obtained.
  • the No. 41A hot stamped body was low in rolling temperature of the rough rolling. Further, the No. 42A hot stamped body was low in sheet thickness reduction rate of the rough rolling. Further, the No. 43A hot stamped body was low in number of rolling operations under conditions of a time between passes of 3 seconds or more. These hot stamped bodies were not manufactured under the suitable rough rolling conditions, so the soft structures and metal structures with intermediate hardnesses insufficiently grew, it was not possible to ease the strain occurring due to bending deformation, and the targeted bendability could not be obtained.
  • the No. 44 hot stamped body is a steel sheet controlled in casting rate to 6 ton/min or more in the continuous casting process of steel sheet for surface layer. It can raise the area rate (%) of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° to 15° in the metal structures from the surface of the steel sheet for surface layer to 1/2 of the thickness and is excellent in bendability.
  • [Table A-1-1] Multilayer steel sheet no. Chemical constituents of steel sheet for sheet thickness middle part (mass%) Remarks Steel no.
  • the Nos. 1 to 18 steel sheets for sheet thickness middle part having the chemical compositions shown in Table B-1-1 (“Steel Nos. 1 to 18" in Table B-1-1) were ground down at their surfaces to remove the surface oxides. After that, the respective steel sheets for sheet thickness middle part were welded with steel sheets for surface layer having the chemical compositions shown in Table B-1-2 at both surfaces or single surfaces by arc welding to fabricate the Nos. 1 to 41 multilayer steel sheets for hot stamped body.
  • the sheet thickness of the total of the steel sheet for surface layer and the steel sheet for sheet thickness middle part after arc welding was 200 mm to 300 mm and the thickness of the steel sheet for surface layer was 1/3 or so of the thickness of the steel sheet for sheet thickness middle part (in case of single side, 1/4 or so).
  • multilayer steel sheet was steel with steel sheet for surface layer welded to only one side.
  • the multilayer steel sheets other than No. 37 respectively had steel sheets for surface layer welded to both sides of the steel sheet for sheet thickness middle part.
  • Nos. 1 to 41 multilayer steel sheets of Table B-1-3 ones where the steel sheet for sheet thickness middle part did not satisfy the requirements of composition of the middle part in sheet thickness of the hot stamped body according to the present invention are indicated as "comparative steels" in the remarks columns.
  • the Nos. 1 to 41 multilayer steel sheets were respectively treated under the conditions of the Nos. 1 to 41 manufacturing conditions shown in Table B-2-1 to Table B-2-2 by heat treatment before hot rolling, rough rolling, hot rolling, and cold rolling to obtain steel sheets.
  • the steel sheets were heat treated as shown in Table B-2-1 and Table B-2-2 (in the tables, "heat treatment of hot stamped body") for hot stamping to manufacture the Nos. 1B to 41B hot stamped bodies ("stamped bodies" of Table B-3-1 and Table B-3-2).
  • the Nos. 35B and 36B hot stamped bodies were coated on a hot dip coating line at their surfaces with 120 to 160 g/m 2 amounts of aluminum.
  • the items in Table B-2-1 to Table B-2-2 correspond to the items in Table A-2-1 to Table A-2-2.
  • the fields with the notations "-" indicate no corresponding treatment performed.
  • Table B-3-1 and Table B-3-2 show the metal structures and characteristics of the Nos. 1B to 41B hot stamped bodies.
  • the constituents obtained by analyzing the positions of 1/2 of the sheet thicknesses of the samples taken from the hot stamped bodies (middle parts in sheet thickness) and positions of 20 ⁇ m from the surfaces of the softened layers were equivalent to the constituents of the steel sheets for sheet thickness middle part and steel sheets for surface layer of the Nos. 1 to 41 multilayer steel sheets of Table B-1-1 to Table B-1-3.
  • the metal structures of the hot stamped steel sheets were measured by the above-mentioned method.
  • the hardness of the steel sheet for sheet thickness middle part forming the middle part in sheet thickness and the area rate (%) of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° to 15° in the metal structures from the surface of the steel sheet for surface layer forming the softened layer to 1/2 of the thickness of that softened layer were calculated.
  • the calculated values of the area rate are shown in the items "area rate (%) of total of crystal grains with maximum crystal orientation difference inside large angle grain boundaries of 1° or less and crystal grains with maximum crystal orientation difference of 8° to 15°" of Tables B-3-1 to Table B-3-2.
  • Nos. 1B to 41B hot stamped bodies were respectively measured for average hardness (HV) and minimum hardness (HV) at the middle part in sheet thickness (position of 1/2 of sheet thickness) by the above method.
  • the measurement results are shown in Table B-3-1 to Table B-3-2.
  • the Nos. 1B to 41B hot stamped bodies had differences of the average hardness (HV) and minimum hardness (HV) shown in the "scattering in cross-sectional hardness" of Table B-3-1 to Table B-3-2. Further, cases with a scattering in cross-sectional hardness of 100HV or more were indicated as failing.
  • the hot stamped bodies were subjected to tensile tests. The results are shown in Table B-3-1 to Table B-3-2.
  • the tensile tests were performed by fabricating No. 5 test pieces described in JIS Z 2201 and testing them by the method described in JIS Z 2241.
  • the hydrogen embrittlement resistance of the hot stamped body in the same way as Manufacturing Example A, was evaluated using a test piece cut out from the stamped body. That is, a test piece of a sheet thickness of 1.2 mm ⁇ width 6 mm ⁇ length 68 mm was cut out from the stamped body, given a strain corresponding to the yield stress in a four-point bending test, then immersed in pH3 hydrochloric acid for 100 hours and evaluated for hydrogen embrittlement resistance by the presence of any cracks. The case of no cracks was indicated as passing (“Good”) and the case of cracks was evaluated as failing ("Poor").
  • the body was evaluated based on the VDA standard (VDA238-100) prescribed by the German Association of the Automotive Industry under the same measurement conditions as Manufacturing Example A.
  • VDA238-100 the displacement at the time of maximum load obtained in the bending test was converted to angle by the VDA standard to find maximum bending angle and thereby evaluate the impact resistance of the hot stamped body.
  • the tensile strength is 1500 MPa or more, the maximum bending angle (°) was 70(°) or more, and the hydrogen embrittlement resistance was a passing level, it was judged that the impact resistance and hydrogen embrittlement resistance were excellent and the case was indicated as an "invention example”. If even one of the three aspects of performance is not satisfied, the case was indicated as a "comparative example".
  • each hot stamped body of the invention examples the area rate (%) of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° to 15° in the metal structures from the surface of the steel sheet for surface layer to 1/2 of the thickness was 50% to less than 85%. Further, each hot stamped body of the invention examples was excellent in tensile strength, bendability, and hydrogen embrittlement resistance.
  • the No. 5B hot stamped body was low in carbon content at the steel sheet for sheet thickness middle part, so the hardness of the middle part in sheet thickness became insufficient and the tensile strength became insufficient.
  • the No. 9B hot stamped body was excessive in carbon content of steel sheet for sheet thickness middle part, so the hardness of the middle part in sheet thickness also became excessive and the targeted bendability could not be obtained.
  • the No. 11B hot stamped body was sparse in Mn content at the steel sheet for sheet thickness middle part, so the hardness of the middle part in sheet thickness became insufficient and the tensile strength became insufficient.
  • the Nos. 30B to 32B hot stamped bodies are comparative examples produced using the multilayer steel sheets for hot stamped body to which the desirable heat treatment had not been applied before the hot stamping process.
  • the No. 30B hot stamped body was low in heat treatment temperature before the hot stamping process, while the No. 31B hot stamped body was short in heat treatment time before the hot stamping process, so in the metal structures of the softened layer from the surface of the softened layer to 1/2 of the thickness, the soft structures and metal structures with intermediate hardnesses insufficiently grew and the target bendability could not be obtained. Further, the No. 32B hot stamped body was excessively high in heat treatment temperature before the hot stamping process, so the effect of reduction of the sharp gradient in hardness in the sheet thickness direction occurring at the time of bending deformation could not be obtained.
  • the No. 38B hot stamped body was low in rolling temperature of the rough rolling. Further, the No. 39B hot stamped body was low in sheet thickness reduction rate of the rough rolling. Further, the No. 40B hot stamped body was low in number of rolling operations under conditions of a time between passes of 3 seconds or more. These hot stamped bodies were not manufactured under the suitable rough rolling conditions, so the soft structures and metal structures with intermediate hardnesses insufficiently grew, it was not possible to ease the strain occurring due to bending deformation, and the targeted bendability could not be obtained.
  • the No. 41B hot stamped body is a steel sheet controlled in casting rate to 6 ton/min or more in the continuous casting process of steel sheet for surface layer. It can raise the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° to 15° in the metal structures from the surface of the steel sheet for surface layer to 1/2 of the thickness and is excellent in bendability.
  • [Table B-1-1] Multilayer steel sheet no. Composition of constituents of steel sheet for sheet thickness middle part (mass%) Steel no.
  • Multilayer steel sheet no. Composition of constituents of steel sheet for surface layer (mass%) C Si Mn P S sol.Al N Ni Nb Ti Mo B 1 0.10 0.10 0.48 0.005 0.0018 0.030 0.0036 2 0.13 0.10 0.60 0.016 0.0008 0.033 0.0030 3 0.11 0.07 0.75 0.015 0.0010 0.041 0.0029 4 0.22 0.09 0.70 0.006 0.0020 0.029 0.0031 5 0.10 0.07 0.66 0.006 0.0019 0.028 0.0041 6 0.14 0.08 0.56 0.014 0.0019 0.033 0.0028 7 0.11 0.05 0.58 0.008 0.0016 0.030 0.0040 8 0.12 0.06 0.51 0.006 0.0012 0.033 0.0027 9 0.36 0.06 0.52 0.018 0.0017 0.036 0.0032 10 0.15 0.22 0.70 0.018 0.0005 0.033 0.0042 11 0.15 0.12 0.03 0.013 0.0008 0.027 0.0044 12 0.
  • Multilayer steel sheet no. Manufacturing condition no. Heat treatment before hot rolling Rough rolling Hot rolling Cold rolling Heat treatment at hot stamping Plating Sheet thickness (mm) Heating temp. (°C) Holding time (min) Rolling temp. (°C) Rate of reduction of sheet thickness (%) No.
  • 29B 29 29 29 780 78 2271 74.5 Good 734 721 13 Inv. ex. 30B 30 30 657 15 1975 61.6 Poor 627 605 22 Comp. ex. 31B 31 31 647 18 1931 62.4 Poor 620 600 20 Comp. ex. 32B 32 32 644 90 1957 65.1 Good 611 599 12 Comp. ex. 33B 33 33 645 84 1930 75.8 Good 605 551 54 Inv. ex. 34B 34 34 745 64 2180 78.5 Good 717 702 15 Inv. ex. 35B 35 35 740 75 2260 79.3 Good 726 707 19 Inv. ex. 36B 36 36 647 71 1941 77.7 Good 611 581 30 Inv. ex.
  • Steel sheets for sheet thickness middle part having the chemical compositions shown in Table C-1-1 to Table C-1-2 were ground down at their surfaces to remove the surface oxides. After that, the respective steel sheets for sheet thickness middle part were welded with steel sheets for surface layer having the chemical compositions shown in Table C-1-3 to Table C-1-4 at both surfaces or single surfaces by arc welding to fabricate the Nos. 1 to 49 multilayer steel sheets for hot stamped body.
  • the sheet thickness of the total of the steel sheet for surface layer and the steel sheet for sheet thickness middle part after arc welding was 200 mm to 300 mm and the thickness of the steel sheet for surface layer was 1/3 or so of the thickness of the steel sheet for sheet thickness middle part (in case of single side, 1/4 or so).
  • 31 multilayer steel sheet was steel with steel sheet for surface layer welded to only one side.
  • Nos. 1 to 53 multilayer steel sheets of Table C-1-1 to Table C-1-4 ones where the steel sheet for sheet thickness middle part did not satisfy the requirements of composition of the middle part in sheet thickness of the hot stamped body according to the present invention are indicated as "comparative steels" in the remarks columns.
  • the "ratio of C, Si, and Mn contents of steel sheet for surface layer to steel sheet for sheet thickness middle part" of Table C-1-3 to Table C-1-4 show the ratios of C, Si, and Mn contents of steel sheet for surface layer to the C, Si, and Mn contents of steel sheet for sheet thickness middle part in the Nos. 1 to 53 multilayer steel sheets for hot stamped body.
  • the Nos. 1 to 53 multilayer steel sheets were respectively treated under the conditions of the Nos. 1 to 53 manufacturing conditions shown in Table C-2-1 to Table C-2-2 by heat treatment before hot rolling, rough rolling, hot rolling, and cold rolling to obtain steel sheets.
  • the steel sheets were heat treated as shown in Table C-2-1 to Table C-2-2 (in the tables, "heat treatment of hot stamped body") for hot stamping to manufacture the Nos. 1C to 53C hot stamped bodies ("stamped bodies" of Table C-3-1 to Table C-3-2).
  • the No. 30C hot stamped body was coated on a hot dip coating line at the surface with a 120 to 160 g/m 2 amount of aluminum.
  • the items in Table C-2-1 to Table C-2-2 correspond to the items in Table A-2-1 to Table A-2-2.
  • the fields with the notations "-" indicate no corresponding treatment performed.
  • Table C-3-1 to Table C-3-2 show the metal structures and characteristics of the Nos. 1C to 53C hot stamped bodies.
  • the constituents obtained by analyzing the positions of 1/2 of the sheet thicknesses of the samples taken from the hot stamped bodies (middle parts in sheet thickness) and positions of 20 ⁇ m from the surfaces of the softened layers were equivalent to the constituents of the steel sheets for sheet thickness middle part and the steel sheets for surface layer of the Nos.1 to 53 multilayer steel sheets of Table C-1-1 to Table C-1-4.
  • the metal structures of the hot stamped steel sheets were measured by the above-mentioned method.
  • the hardness of the steel sheet for sheet thickness middle part forming the middle part in sheet thickness and the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° to 15° in the metal structures from the surface of the steel sheet for surface layer forming the softened layer to 1/2 of the thickness of that softened layer were calculated.
  • the calculated values of the area rate are shown in the items "area rate (%) of total of crystal grains with maximum crystal orientation difference inside large angle grain boundaries of 1° or less and crystal grains with maximum crystal orientation difference of 8° to 15°" of Tables C-3-1 to C-3-2.
  • the hot stamped bodies were subjected to tensile tests. The results are shown in Table C-3-1 to Table C-3-2.
  • the tensile tests were performed by fabricating No. 5 test pieces described in JIS Z 2201 and testing them by the method described in JIS Z 2241.
  • the hydrogen embrittlement resistance of the hot stamped body in the same way as Manufacturing Example A, was evaluated using a test piece cut out from the stamped body. That is, a test piece of a sheet thickness of 1.2 mm ⁇ width 6 mm ⁇ length 68 mm was cut out from the stamped body, given a strain corresponding to the yield stress in a four-point bending test, then immersed in pH3 hydrochloric acid for 100 hours and evaluated for hydrogen embrittlement resistance by the presence of any cracks. The case of no cracks was indicated as passing (“Good”) and the case of cracks was evaluated as failing ("Poor").
  • the body was evaluated based on the VDA standard (VDA238-100) prescribed by the German Association of the Automotive Industry under the same measurement conditions as Manufacturing Example A.
  • VDA238-100 the displacement at the time of maximum load obtained in the bending test was converted to angle by the VDA standard to find maximum bending angle and thereby evaluate the impact resistance of the hot stamped body.
  • the tensile strength is 1500 MPa or more, the maximum bending angle (°) was 70(°) or more, the uniform elongation was 5% or more, and the hydrogen embrittlement resistance was a passing level, it was judged that the impact resistance, hydrogen embrittlement resistance, and ductility were excellent and the case was indicated as an "invention example”. If even one of the three aspects of performance is not satisfied, the case was indicated as a "comparative example".
  • each hot stamped body of the invention examples the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° to 15° in the metal structures from the surface of the steel sheet for surface layer to 1/2 of the thickness of the steel sheet for surface layer was 50% to less than 85%. Further, each hot stamped body of the invention examples was excellent in tensile strength, bendability, and hydrogen embrittlement resistance.
  • the No. 5C hot stamped body was low in carbon content of the steel sheet for sheet thickness middle part, so became insufficient in hardness of the middle part in sheet thickness and became insufficient in tensile strength.
  • the No. 9C hot stamped body was excessive in carbon content of the steel sheet for sheet thickness middle part, so also became excessive in hardness of the middle part in sheet thickness and could not be given the targeted bendability.
  • the No. 11C hot stamped body was low in Si content of the steel sheet for sheet thickness middle part, so the area percent of the residual austenite of the metal structures at the middle part in sheet thickness was less than 1.0% and the uniform elongation was low.
  • the Nos. 25C to 27C and 49C hot stamped bodies are comparative examples manufactured using the multilayer steel sheets for hot stamped body to which the preferable heat treatment is not applied before the hot stamping process.
  • the No. 25C hot stamped body is too low in heat treatment temperature before the hot stamping process, so the soft structures and metal structures with intermediate hardnesses insufficiently grew, the effect of surface properties of the hot stamped body and effect of the transitional part from the middle part in sheet thickness to the softened layer could not be eliminated, and excellent bendability could not be obtained.
  • the No. 26C hot stamped body was excessively high in heat treatment time before the hot stamping process, so the soft structures and metal structures with intermediate hardnesses excessively grew, the difference in hardness between the softened layer and the middle part in sheet thickness became too large, and the effect of reducing the sharp gradient of hardness in the sheet thickness direction occurring at the time of bending deformation could not be obtained. For this reason, the No. 26C hot stamped body could not be given the targeted bendability.
  • the Nos. 27C and 49C hot stamped bodies were too long in heat treatment time before the hot stamping process, the difference in hardness between the softened layer and the middle part in sheet thickness become too great. Further, the heat treatment temperature was excessively high, so the effect of reducing the sharp gradient of hardness in the sheet thickness direction occurring at the time of bending deformation could not be obtained. For this reason, the Nos. 27C and 49C hot stamped bodies could not be given excellent bendability.
  • the No. 50C hot stamped body was low in rolling temperature of the rough rolling. Further, the No. 51C hot stamped body was low in sheet thickness reduction rate of the rough rolling. Further, the No. 52C hot stamped body was low in number of rolling operations under conditions of a time between passes of 3 seconds or more. These hot stamped bodies were not manufactured under the suitable rough rolling conditions, so the soft structures and metal structures with intermediate hardnesses insufficiently grew, it was not possible to ease the strain occurring due to bending deformation, and the targeted bendability could not be obtained.
  • the No. 53C hot stamped body is a steel sheet controlled in casting rate to 6 ton/min or more in the continuous casting process of steel sheet for surface layer. It can raise the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° to 15° in the metal structures from the surface of the steel sheet for surface layer to 1/2 of the thickness and is excellent in bendability.
  • [Table C-1-1] Multilayer steel sheet no.
  • Multilayer steel sheet no. Composition of constituents of steel sheet for surface layer (mass%) Thickness of steel sheet for surface layer (mm) Remarks C Si Mn P S sol.Al N Ni Nb Ti Mo B 1 0.106 0.735 0.517 0.006 0.0024 0.048 0.0046 0 0 0 0 0 96 2 0.173 1.030 0.360 0.006 0.0065 0.036 0.0016 0 0 0 0 0 91 3 0.099 0.847 0.399 0.006 0.0032 0.036 0.0035 0 0 0 0 0 95 4 0.294 0.479 0.414 0.006 0.0056 0.039 0.0070 0 0 0 0 0 96 5 0.082 1.115 0.592 0.003 0.0070 0.034 0.0020 0 0 0 0 0 78 Comp.
  • Steel sheets for sheet thickness middle part having the Nos. 1 to 37 chemical compositions shown in Table D-1-1 to Table D-1-2 were ground down at their surfaces to remove the surface oxides.
  • the respective steel sheets for sheet thickness middle part were welded with steel sheets for surface layer having the chemical compositions shown in Table D-1-3 to Table D-1-4 at both surfaces or single surfaces by arc welding to fabricate the Nos. 1 to 60 multilayer steel sheets for hot stamped body.
  • the sheet thickness of the total of the steel sheet for surface layer and the steel sheet for sheet thickness middle part after arc welding was 200 mm to 300 mm and the thickness of the steel sheet for surface layer was 1/3 or so of the thickness of the steel sheet for sheet thickness middle part (in case of single side, 1/4 or so).
  • the No. 37 multilayer steel sheet is steel with the steel sheet for surface layer welded to only one surface.
  • the multilayer steel sheets other than No. 37 have steel sheets for surface layer welded to both surfaces of the steel sheet for sheet thickness middle part. In the Nos.
  • the Nos. 1 to 60 multilayer steel sheets were treated under the conditions of the Nos. 1 to 60 manufacturing conditions shown in Table D-2-1 to Table D-2-3 by heat treatment before hot rolling, rough rolling, hot rolling, and cold rolling to obtain steel sheets.
  • the steel sheets were heat treated as shown in Table D-2-1 to Table D-2-3 (in the tables, "heat treatment of hot stamped bodies") for hot stamping to produce the Nos. 1D to 60D hot stamped bodies ("stamped bodies" of Tables D-3-1 to D-3-3).
  • the Nos. 38D and 39D hot stamped bodies were coated on a hot dip coating line at the surfaces with 120 to 160 g/m 2 amounts of aluminum.
  • the items of Table D-2-1 to Table D-2-3 correspond to the items of Table A-2-1 to Table A-2-2.
  • the fields with the notations "-" indicate no corresponding treatment performed.
  • Tables D-3-1 to D-3-3 show the metal structures and characteristics of the Nos. 1D to 60D hot stamped bodies.
  • the constituents obtained by analyzing the positions of 1/2 of the sheet thicknesses of the samples taken from hot stamped bodies (middle parts in sheet thickness) and positions of 20 ⁇ m from the surfaces of the softened layers were equivalent to the constituents of the steel sheets for sheet thickness middle part and the steel sheets for surface layer of the Nos.1 to 60 multilayer steel sheets of Table D-1-1 to Table D-1-3.
  • the metal structures of the hot stamped steel sheets were measured by the above-mentioned method.
  • the hardness of the steel sheet for sheet thickness middle part forming the middle part in sheet thickness and the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° to 15° in the metal structures from the surface of the steel sheet for surface layer forming the softened layer to 1/2 of the thickness of that softened layer were calculated.
  • the calculated values of the area rate are shown in the items "area rate (%) of total of crystal grains with maximum crystal orientation difference inside large angle grain boundaries of 1° or less and crystal grains with maximum crystal orientation difference of 8° to 15°" of Tables D-3-1 to D-3-3.
  • the Nos. 1D to 60D hot stamped bodies were subjected to tensile tests. The results are shown in Tables D-3-1 to D-3-3.
  • the tensile tests were performed by fabricating No. 5 test pieces described in JIS Z 2201 and testing them by the method described in JIS Z 2241.
  • the hot stamped bodies were evaluated for hydrogen embrittlement resistance in the same way as Manufacturing Example A using test pieces cut out from the stamped bodies. That is, test pieces of a sheet thickness of 1.2 mm ⁇ width 6 mm ⁇ length 68 mm were cut out from the stamped bodies, given strain corresponding to the yield stress in four-point bending tests, then immersed in pH3 hydrochloric acid for 100 hours and evaluated for hydrogen embrittlement resistance by the presence of any cracks. Cases of no fracture were evaluated as passing ("good”) and cases of fracture were evaluated as failing ("Poor").
  • the body was evaluated based on the VDA standard (VDA238-100) prescribed by the German Association of the Automotive Industry under the same measurement conditions as Manufacturing Example A.
  • VDA238-100 the displacement at the time of maximum load obtained in the bending test was converted to angle by the VDA standard to find maximum bending angle and thereby evaluate the impact resistance of the hot stamped body.
  • the hot stamped bodies were also evaluated for impact resistance from the viewpoint of ductility. Specifically, the hot stamped steel sheets were subjected to tensile tests to find the uniform elongations of the steel sheet to evaluate the impact resistance. The tensile tests were performed by fabricating No. 5 test pieces described in JIS Z 2201 and testing them by the method described in J1S Z 2241. The elongations where the maximum tensile loads were obtained were defined as the uniform elongations.
  • the impact resistance of a hot stamped body was also evaluated from the viewpoint of the scattering in hardness.
  • a cross-section vertical to the longitudinal direction of a long hot stamped body was taken at any position in that longitudinal direction and measured for hardness at the middle position in sheet thickness at the entire cross-sectional region including the vertical walls.
  • the measurement load was 1 kgf, 10 points were measured, and the measurement interval was 1 mm.
  • the area rate of the total of crystal grains with a maximum crystal orientation difference inside regions surrounded by grain boundaries of 15° or higher of 1° or less and crystal grains with a crystal orientation difference of 8° to 15° in the metal structures from the surface of the steel sheet for surface layer to 1/2 of the thickness was 50% to less than 85%. Further, in each of the hot stamped bodies of the invention examples, the tensile strength, bendability, and hydrogen embrittlement resistance were excellent.
  • the No. 5D hot stamped body was low in carbon content of the steel sheet for sheet thickness middle part, so became insufficient in hardness of the middle part in sheet thickness and became insufficient in tensile strength.
  • the No. 9D hot stamped body was excessive in carbon content of the steel sheet for sheet thickness middle part, so became excessive in hardness of the middle part in sheet thickness as well and could not be given the targeted bendability.
  • the Nos. 10D and 11D hot stamped bodies were sparse in Si content of the steel sheet for sheet thickness middle part, so were insufficient in uniform elongation.
  • the No. 12D hot stamped body was insufficient in Mn content, so became insufficient in hardness of the middle part in sheet thickness and were insufficient in tensile strength.
  • the No. 9D hot stamped body was excessive in carbon content of the steel sheet for sheet thickness middle part, so became excessive in hardness of the middle part in sheet thickness as well and could not be given the targeted bendability.
  • the Nos. 10D and 11D hot stamped bodies were sparse in Si content
  • the Nos. 33D to 35D hot stamped bodies are comparative examples produced using multilayer steel sheets for hot stamped body which were not subjected to the desirable heat treatment before the hot stamping process.
  • the No. 33D hot stamped body was low in heat treatment temperature before the hot stamping process, so became insufficient in growth of soft structures and metal structures of intermediate hardnesses in the metal structures of the softened layer from the surface of the softened layer to 1/2 of the thickness and was not able to be given the targeted bendability.
  • the No. 34D hot stamped body was excessively high in heat treatment temperature before the hot stamping process, so the fraction of structures from a position of 20 ⁇ m from the surface of the softened layer to a position of a depth of 1/2 of the thickness of the softened layer exceeded 85%.
  • the No. 34D hot stamped body the difference in hardness between the softened layer and the middle part in sheet thickness became too large, and the effect of reduction of the sharp gradient in hardness in the sheet thickness direction occurring at the time of bending deformation could not be obtained. Further, the No. 35D hot stamped body was short in heat treatment time before the hot stamping process, so in the metal structures from the surface of the softened layer to 1/2 of the thickness, the soft structures and metal structures with intermediate hardnesses insufficiently grew and the target bendability could not be obtained.
  • the No. 40D hot stamped body was excessive in Si content, so residual austenite was excessively produced exceeding an area percent of 5%. For this reason, the No. 40D hot stamped body was inferior in bendability.
  • the No. 41D hot stamped body was excessive in Mn content, so became the greatest in tensile strength among the Nos. 1D to 56D hot stamped bodies and was inferior in bendability.
  • the No. 42D hot stamped body was poor in content of acid soluble aluminum, so inclusions containing oxygen were excessively produced and bendability was inferior. Further, the No. 45D hot stamped body included excessive aluminum, so oxides mainly comprised of aluminum were excessively produced and bendability was inferior.
  • the No. 57D hot stamped body was low in rolling temperature of the rough rolling. Further, the No. 58D hot stamped body was low in sheet thickness reduction rate of the rough rolling. Further, the No. 59D hot stamped body was low in number of rolling operations under conditions of a time between passes of 3 seconds or more. These hot stamped bodies were not produced under optimal rough rolling conditions, so were insufficient in growth of soft structures and metal structures of intermediate hardnesses, were not able to be eased in strain caused by bending deformation, and were not able to be given the targeted bendability.
  • the No. 60D hot stamped body is steel sheet with a casting rate controlled to 6 ton/min or more in a continuous casting process of steel sheet for surface layer. It can raise the area rate of the total of crystal grains with a maximum crystal orientation difference inside regions surrounded by grain boundaries of 15° or higher of 1° or less and crystal grains with a crystal orientation difference of 8° to 15° in the metal structures from the surface of the steel sheet for surface layer to 1/2 of the thickness and is excellent in bendability.
  • [Table D-1-1] Multilayer steel sheet no. Chemical constituents of steel sheet for sheet middle part (mass%) Remarks Steel no.
  • Multilayer steel sheet no. Chemical constituents of steel sheet for surface layer (mass%) Remarks C Si Mn P S sol.Al N Ni Nb Ti Mo B 31 0.24 1.26 1.02 0.003 0.0009 0.029 0.0021 0 0 0 0 0 32 0.27 0.84 1.68 0.003 0.0009 0.030 0.0024 0 0 0 0 0 33 0.13 0.62 1.01 0.010 0.0009 0.039 0.0032 0 0 0 0 34 0.16 0.62 0.83 0.007 0.0010 0.041 0.0031 0 0 0 0 35 0.12 0.63 0.96 0.007 0.0010 0.040 0.0033 0 0 0 0 0 36 0.15 0.66 0.85 0.011 0.0009 0.040 0.0031 0 0 0 0 0 37 0.3 0.59 1.03 0.006 0.0003 0.053 0.0027 0 0 0 0 0 38 0.32 0.74 0.98 0.00
  • the hot stamped body of the present invention is excellent in strength, ductility, bendability, impact resistance, and hydrogen embrittlement resistance and is small in scattering in hardness, so can be suitably used for structural members or reinforcing members for automobiles or structures requiring strength.

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