Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/18—Hardening; Quenching with or without subsequent tempering
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite

Definitions

• the present invention relates to a steel sheet for hot stamping and a hot stamp molded body.
• a tailored blank is a single steel plate made by joining a plurality of steel plates having different plate thicknesses, chemical compositions, metal structures, etc. by welding.
• the properties in a single joined steel sheet can be partially changed. For example, by giving a certain part a high strength, deformation in that part can be suppressed, and by giving another part a low strength, the part can be deformed and an impact can be absorbed.
• the low-strength portion is required to have excellent ductility so as to suppress breakage during deformation.
• a technique for applying a tailored blank to the hot stamping method there is a technique for using a tailored blank in which a steel plate having low strength after hot stamping and a steel plate having high strength after hot stamping are joined by welding.
• a steel sheet having high strength after hot stamping for example, a steel sheet as disclosed in Patent Document 1 can be used.
• the steel sheet having low strength after hot stamping the chemical composition of the steel may be adjusted so as to have low strength after cooling the mold in the hot stamping.
• Low carbon steel is one of the steel types applied to tailored blanks. Since low carbon steel has a low carbon content, it has the characteristic that it is difficult to increase its strength even if it is rapidly cooled after heating.
• Patent Document 2 discloses that ultra-low carbon steel is used as a low-strength material in the hot stamping method. Patent Document 2 discloses a technique for improving local deformability by heating a steel sheet to a temperature of 3 points or more and then hot stamping it to form a metal structure having bainite and bainitic ferrite as main phases. ing. Patent Document 2 discloses that this technique makes it difficult for fracture to occur when a vehicle body part is deformed in a bending mode at the time of a collision, and is excellent in impact absorption ability due to plastic deformation.
• a hot stamp molded body having a tensile strength of less than 1500 MPa has been attracting attention.
• Such a hot stamped body is required to have a desired strength and a higher ductility after hot stamping so as to sufficiently suppress fracture during deformation.
• the present invention has been made in view of the above circumstances, and an object of the present invention is to provide a hot stamped molded product having high strength and excellent ductility, and a steel plate for hot stamping capable of producing this hot stamped molded product.
• the present inventors have studied a method for improving the ductility of a hot stamp molded product. As a result, it was found that the ductility of the hot stamped body can be improved by increasing the number ratio of the ferrites containing the hard phase having a high dislocation density in the ferrite grains in the metal structure of the hot stamped body.
• the hot stamped product can be obtained by preferably controlling the chemical composition of the hot stamping steel sheet and increasing the ratio of the number of ferrites containing a hard phase in the ferrite grains. I found out.
• the present invention has been obtained based on the above findings, and the gist of the present invention is as follows.
• the steel sheet for hot stamping according to one aspect of the present invention has a chemical composition of% by mass.
• C 0.060 to 0.200%, Si: 0.010 to 1.000%, Mn: 0.80 to 2.00%, Al: 0.010 to 0.500%, Nb: 0.020 to 0.100%, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, Ti: 0 to 0.10%, Cr: 0 to 0.50%, B: 0 to 0.0100%, Mo: 0 to 1.00%, Co: 0 to 2.00%, Ni: 0 to 0.50%, V: 0 to 0.10%, Ca: 0-0.0100%, Mg: 0 to 0.0100%, and REM: 0 to 0.0100%,
• the rest consists of Fe and impurities In the metallographic structure
• the steel sheet for hot stamping according to (1) above has a chemical composition of% by mass.
• the hot stamp molded product according to another aspect of the present invention has a chemical composition of% by mass.
• the rest consists of Fe and impurities In the metallographic structure In terms of area ratio, martensite is 20% or more, Of all the ferrites, the ratio of the number of ferrites containing a hard phase having a GAIQ value of
• the hot stamp molded product according to (3) above has a chemical composition of% by mass.
• the steel plate for hot stamping and the hot stamp molded body according to the present embodiment will be described in detail.
• the reason for limiting the chemical composition of the hot stamping steel sheet according to the present embodiment will be described.
• the lower limit value and the upper limit value are included in the numerical limitation range described with " ⁇ " in between. Numerical values indicated as “less than” and “greater than” do not include the value in the numerical range.
• % of the chemical composition means mass%.
• the hot stamped body according to the present embodiment has a chemical composition of% by mass, C: 0.060 to 0.200%, Si: 0.010 to 1.000%, Mn: 0.80 to 2.00. %, Al: 0.010 to 0.500%, Nb: 0.020 to 0.100%, P: 0.100% or less, S: 0.0100% or less, N: 0.0100% or less, and Remaining: Contains Fe and impurities.
• C 0.060 to 0.200%
• Si 0.010 to 1.000%
• Mn 0.80 to 2.00.
• Al 0.010 to 0.500%
• Nb 0.020 to 0.100%
• P 0.100% or less
• S 0.0100% or less
• N 0.0100% or less
• Remaining Contains Fe and impurities.
• C 0.060 to 0.200% C is an element that greatly affects the strength and ductility of the hot stamped product. If the C content is too low, the martensitic transformation is not promoted, the strength of the hot stamped compact is lowered, and fracture is likely to occur due to insufficient strength. Therefore, the C content is set to 0.060% or more. Preferably, it is 0.080% or more, 0.100% or more, or 0.120% or more. On the other hand, if the C content is too high, the transformation from austenite to ferrite is inhibited, a desired amount of ferrite cannot be obtained, and the ductility of the hot stamped product is lowered. Therefore, the C content is set to 0.200% or less. Preferably, it is 0.170% or less or 0.150% or less.
• Si 0.010 to 1.000%
• Si is an element having a solid solution strengthening ability and is an element necessary for obtaining the strength of a hot stamp molded product. If the Si content is too low, the desired strength cannot be obtained in the hot stamped body. Therefore, the Si content is 0.010% or more. Preferably, it is 0.100% or more, more than 0.200%, 0.250% or more, 0.300% or more, 0.400% or more, or 0.500% or more. On the other hand, if the Si content is too high, the ferrite transformation proceeds excessively, and it becomes impossible to obtain a desired amount of martensite in the hot stamp molded product. Therefore, the Si content is 1.000% or less. It is preferably 0.900% or less or 0.800% or less.
• Mn 0.80 to 2.00%
• Mn is an element having a solid solution strengthening ability, and is contained in order to obtain the strength of the hot stamp molded product. If the Mn content is too low, the ferrite transformation proceeds too much and martensite is difficult to be generated, and the desired strength cannot be obtained in the hot stamped body. Therefore, the Mn content is set to 0.80% or more. It is preferably 1.00% or more or 1.20% or more. On the other hand, if the Mn content is too high, the hardenability of the steel becomes high, and the formation of ferrite in air cooling after heating during hot stamping is suppressed, so that the ductility of the hot stamped product is lowered. Therefore, the Mn content is set to 2.00% or less. It is preferably 1.80% or less or 1.60% or less.
• Al 0.010 to 0.500%
• Al is an important element for promoting ferrite transformation. If the Al content is too low, the ferrite transformation is less likely to proceed, and a desired amount of ferrite cannot be obtained in the hot stamp molded product. Therefore, the Al content is 0.010% or more. Preferably, it is 0.020% or more or 0.030% or more. On the other hand, if the Al content is too high, the transformation to ferrite proceeds excessively, and a desired amount of martensite cannot be obtained in the hot stamped body. Therefore, the Al content is set to 0.500% or less. Preferably, it is 0.450% or less or 0.400% or less.
• Nb 0.020 to 0.100%
• Nb is an element that suppresses the grain growth of austenite, atomizes the austenite grains, and promotes the transformation to ferrite. If the Nb content is too low, a desired amount of ferrite cannot be obtained in the hot stamped body. Therefore, the Nb content is 0.020% or more. Preferably, it is 0.030% or more or 0.040% or more. On the other hand, if the Nb content is too high, the above effects are saturated and the cost increases. Therefore, the Nb content is set to 0.100% or less. Preferably, it is 0.090% or less or 0.080% or less.
• P 0.100% or less
• P is an element that has a solid solution strengthening ability and is effective for obtaining a desired strength in a hot stamped article.
• the P content is set to 0.100% or less.
• it is 0.080% or less, 0.060% or less, or 0.050% or less.
• the lower limit of the P content is not particularly specified, but from the viewpoint of ensuring the strength by P, the P content may be 0.001% or more or 0.005% or more.
• S 0.0100% or less
• S is an element contained in steel as an impurity and embrittles the steel. Therefore, the smaller the S content, the more preferable.
• the S content shall be 0.0100% or less. Preferably, it is 0.0080% or less, 0.0060% or less, or 0.0040% or less.
• the lower limit of the S content is not particularly specified, but the S content may be 0.0005% or more or 0.0010% or more because the cost in the desulfurization step increases if the S content is excessively reduced.
• N is an impurity element, which is an element that forms a nitride in steel and deteriorates the ductility of the hot stamped body. If the N content is too high, the nitride in the steel becomes coarse and the ductility of the hot stamped body deteriorates. Therefore, the N content is 0.0100% or less. Preferably, it is 0.0080% or less or 0.0060% or less. The lower limit of the N content is not particularly specified, but the N content may be 0.0010% or more because the cost in the steelmaking process increases if the N content is excessively reduced.
• the steel sheet for hot stamping according to the present embodiment may contain the above elements, and the balance may consist of Fe and impurities.
• impurities include those that are inevitably mixed from the steel raw material or scrap and / or in the steelmaking process, or elements that are allowed as long as they do not impair the characteristics of the hot stamped product according to the present embodiment.
• the hot stamping steel sheet according to the present embodiment may contain an optional element shown below in place of a part of Fe in order to improve various properties. Since it is not necessary to intentionally contain these arbitrary elements in the steel in order to reduce the alloy cost, the lower limit of the content of these optional elements is 0%.
• Ti 0.01-0.10%
• Ti is an element that suppresses the grain growth of austenite, atomizes the austenite grains, and promotes the transformation to ferrite.
• the Ti content is preferably 0.01% or more.
• the Ti content is set to 0.10% or less.
• Cr 0.01-0.50% Cr is an element effective for enhancing the hardenability of steel, promoting the formation of martensite, and increasing the strength of the hot stamped compact.
• the Cr content is preferably 0.01% or more.
• the Cr content is set to 0.50% or less.
• B 0.0001 to 0.0100%
• B is an element that segregates into the old austenite grain boundaries, has the effect of suppressing ferrite transformation, and contributes to the improvement of the strength of the hot stamped compact.
• the B content is preferably 0.0001% or more.
• the B content is 0.0100% or less.
• Mo 0.01-1.00% Mo forms carbides in the steel and improves the strength of the hot stamped body by strengthening precipitation. In order to surely obtain this effect, the Mo content is preferably 0.01% or more. On the other hand, if the Mo content is too high, the ductility of the hot stamped product will decrease. Therefore, the Mo content is set to 1.00% or less.
• Co 0.01-2.00% Co improves the strength of the hot stamped body by strengthening the solid solution.
• the Co content is preferably 0.01% or more.
• the Co content is 2.00% or less.
• Ni 0.01-0.50% Ni improves the strength of the hot stamped body. In order to surely obtain this effect, the Ni content is preferably 0.01% or more. On the other hand, if the Ni content is too high, the castability may deteriorate. Therefore, the Ni content is set to 0.50% or less.
• V 0.01-0.10% V improves the strength of the hot stamped compact by strengthening with precipitates and atomizing ferrite grains.
• the V content is preferably 0.01% or more.
• the V content is set to 0.10% or less.
• Ca 0.0005-0.0100%
• Ca is an element having an action of deoxidizing molten steel to make the steel sound (suppressing the occurrence of defects such as blow holes in the steel).
• the Ca content is preferably 0.0005% or more.
• the Ca content is preferably 0.0100% or less.
• Mg 0.0005-0.0100%
• Mg is an element having an action of deoxidizing molten steel to make the steel sound.
• the Mg content is preferably 0.0005% or more.
• the Mg content is preferably 0.0100% or less.
• REM 0.0005-0.0100% REM is an element having an action of deoxidizing molten steel to make the steel sound.
• the REM content is preferably 0.0005% or more.
• the REM content is preferably 0.0100% or less.
• REM refers to a total of 17 elements consisting of Sc, Y and lanthanoids.
• the REM content refers to the total content of these elements.
• the above-mentioned chemical composition may be measured by a general analysis method.
• ICP-AES Inductively Coupled Plasma-Atomic Emission Spectroscopy
• C and S may be measured by using the combustion-infrared absorption method
• N may be measured by using the inert gas melting-thermal conductivity method.
• the steel sheet for hot stamping according to the present embodiment has a metal structure having a polar density of more than 4.0 in the ⁇ 112 ⁇ ⁇ 110> direction at the center of the plate thickness and an area ratio of 50% or more of ferrite.
• the number ratio of the ferrites containing a hard phase in the ferrite grains is 65% or more.
• each regulation will be described in detail.
• the area ratio of the ferrite and the ferrite Specify the number ratio.
• the extreme density of the ⁇ 112 ⁇ ⁇ 110> orientation at the center of the plate thickness is 4.0 or less. Cannot obtain the metallographic structure of. Therefore, the extreme density in the ⁇ 112 ⁇ ⁇ 110> direction at the center of the plate thickness is set to exceed 4.0. It is preferably 4.5 or more or 5.0 or more. The upper limit is not particularly limited, but may be 10.0 or less.
• the central portion of the plate thickness means a region from the surface to a depth of 1/4 of the plate thickness to a region from the surface to a depth of 3/4 of the plate thickness.
• the extreme density of the ⁇ 112 ⁇ ⁇ 110> orientation at the center of the plate thickness is obtained by the following method.
• a device combining a scanning electron microscope and an EBSD analysis device and OIM Analysis (registered trademark) manufactured by TSL Co., Ltd. are used.
• OIM Analysis registered trademark
• ⁇ 112 ⁇ ⁇ 110> Obtain the extreme density of the orientation.
• the measurement range is from the surface to the depth of 1/4 of the plate thickness to the region of the surface to the depth of 3/4 of the plate thickness.
• the measurement pitch is 5 ⁇ m / step.
• ⁇ hkl ⁇ represents a crystal plane parallel to the rolling plane
• ⁇ uvw> represents a crystal plane parallel to the rolling direction. That is, ⁇ hkl ⁇ ⁇ uvw> indicates a crystal in which ⁇ hkl ⁇ is oriented in the plate normal direction and ⁇ uvw> is oriented in the rolling direction.
• the area ratio of ferrite is set to 50% or more. It is preferably 60% or more, 70% or more, or 80% or more.
• the upper limit of the area ratio of ferrite is not particularly limited, but may be 97% or less, 95% or less, or 90% or less.
• the remnant structure other than ferrite is a hard phase consisting of one or more of martensite, bainite and pearlite.
• the area ratio of the hard phase is preferably 5% or more in total. It is preferably 10% or more.
• the upper limit of the area ratio of the hard phase is not particularly limited, but may be 50% or less, 40% or less, 30% or less, or 20% or less in total.
• Method for measuring the area ratio of the metallographic structure A sample is taken from a position 10 mm or more away from the end face of the hot stamping steel plate so that the thick cross section perpendicular to the surface becomes the observation surface. After polishing the observation surface, it corrodes with nital, and using an optical microscope and a scanning electron microscope (SEM), the plate thickness is 1/4 position from the surface (1/8 depth from the surface to the plate thickness from the surface). Observe at least 3 regions of 30 ⁇ m ⁇ 30 ⁇ m in the region of 3/8 depth). By performing image analysis on the microstructure photograph obtained by this microstructure observation, the area ratios of ferrite, pearlite and bainite are obtained. Then, after the repeller corrodes to the same observation position, the tissue is observed using an optical microscope and a scanning electron microscope, and the obtained tissue photograph is image-analyzed to obtain the area ratio of martensite. Is calculated.
• each tissue is identified by the following method. Since martensite has a high dislocation density and has substructures such as blocks and packets in the grain, it can be distinguished from other metal structures by the electron channeling contrast image using a scanning electron microscope. It is possible. It is a collection of lath-shaped crystal grains, and has a structure that is not martensite among structures that do not contain Fe-based carbides with a major axis of 20 nm or more inside the structure, and Fe-based carbides that contain Fe-based carbides with a major axis of 20 nm or more inside the structure.
• a structure in which the carbide has a single variant, that is, an Fe carbide extending in the same direction, is considered bainite.
• the Fe-based carbide elongated in the same direction means that the difference in the elongation direction of the Fe-based carbide is within 5 °.
• a structure that is a lumpy crystal grain and does not contain a substructure such as a lath inside the structure is regarded as ferrite.
• a structure in which plate-shaped ferrite and Fe-based carbide are layered is regarded as pearlite.
• Ratio of the number of ferrites containing a hard phase in the ferrite grains 65% or more
• the number ratio of ferrites containing a hard phase in the ferrite grains is less than 65% of all ferrites, the metal structure of the hot stamped body will be affected.
• the ratio of the number of ferrite grains containing the hard phase becomes low, and as a result, excellent ductility cannot be obtained. Therefore, the ratio of the number of ferrites containing a hard phase in the ferrite grains is 65% or more. It is preferably 70% or more, 75% or more, or 80% or more.
• the upper limit of the number ratio of ferrites containing a hard phase in the ferrite grains is not particularly limited, but may be 100% or less, 90% or less, or 85% or less.
• the hard phase referred to here is the above-mentioned residual structure, and refers to one or more of martensite, bainite and pearlite.
• the steel sheet for hot stamping according to this embodiment may have a plating layer on one side or both sides. Having a plating layer on the surface is preferable because the corrosion resistance of the hot stamped molded product after hot stamping is improved.
• the plating to be applied include aluminum plating, aluminum-zinc plating, aluminum-silicon plating, hot-dip galvanizing, electrozinc plating, and alloyed hot-dip galvanizing.
• the thickness of the steel plate for hot stamping is not particularly limited, but it is preferably 0.5 to 3.5 mm from the viewpoint of reducing the weight of the vehicle body.
• the hot stamping compact according to the present embodiment which is obtained by hot stamping the above-mentioned hot stamping steel sheet, will be described. Since the chemical composition of the hot stamping compact according to the present embodiment can be regarded as the same as the chemical composition of the above-mentioned steel sheet for hot stamping, the description of the chemical composition will be omitted.
• the hot stamped body according to the present embodiment has a metal structure in which martensite is 20% or more in terms of area ratio, and among all ferrites, ferrites containing a hard phase having a GAIQ value of 26000 or less in the ferrite grains are used.
• the number ratio is 50% or more.
• the area ratio of the martensite and the ferrite at the position of 1/4 of the plate thickness from the surface (the region from the depth of 1/8 of the plate thickness to the depth of 3/8 of the plate thickness from the surface). Specify the number ratio of.
• the area ratio of martensite 20% or more If the area ratio of martensite is less than 20%, the desired strength cannot be obtained in the hot stamp molded product. Therefore, the area ratio of martensite is set to 20% or more. Preferably, it is 30% or more, 40% or more, or 50% or more.
• the upper limit of the area ratio of martensite is not particularly limited, but may be 95% or less, 90% or less, 85% or less, or 80% or less.
• the remaining structure other than martensite is one or more of ferrite, bainite and pearlite. If the area ratio of ferrite is less than 5%, excellent ductility may not be obtained. Therefore, the area ratio of ferrite may be 5% or more. More preferably, it is 10% or more, 20% or more, or 30% or more. The total area ratio of bainite and pearlite may be 50% or less, 40% or less, or 30% or less.
• Percentage of ferrites containing a hard phase with a GAIQ value of 26000 or less in the ferrite grains 50% or more The higher the GAIQ value, the lower the dislocation density, and the lower the GAIQ value, the higher the dislocation density. .. Therefore, the GAIQ value is a parameter that can reflect the dislocation density of the crystal grains.
• the number ratio of ferrites containing a hard phase having a GAIQ value of 26000 or less in the ferrite grains is less than 50% of all ferrites, excellent ductility cannot be obtained. Therefore, the number ratio of ferrites containing a hard phase having a GAIQ value of 26000 or less in the ferrite grains is 50% or more. It is preferably 55% or more, 60% or more or 70% or more.
• the upper limit of the number ratio of ferrites containing a hard phase having a GAIQ value of 26000 or less in the ferrite grains is not particularly limited, but may be 100% or less or 95% or less.
• the hard phase having a GAIQ value of 26000 or less includes martensite and bainite.
• one or both of martensite and bainite may be contained as the hard phase having a GAIQ value of 26000 or less.
• Method for measuring the area ratio of the metal structure and the number ratio of ferrites containing a hard phase having a GAIQ value of 26000 or less in the ferrite grains From a position 10 mm or more away from the end face of the hot stamped body (or a position avoiding the end) Cut out a sample so that the cross section of the plate thickness can be observed. After polishing the plate thickness section of this sample with # 600 to # 1500 silicon carbide paper, a mirror surface is used with a diluted solution such as alcohol or a liquid in which diamond powder having a particle size of 1 to 6 ⁇ m is dispersed in pure water. Finish to. Next, the strain introduced into the surface layer of the sample is removed by polishing at room temperature with colloidal silica containing no alkaline solution for 8 minutes.
• Crystal orientation information is obtained by electron backscatter diffraction method.
• an EBSD device composed of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used.
• the degree of vacuum in the EBSD device is 9.6 ⁇ 10 -5 Pa or less
• the acceleration voltage is 15 kV
• the irradiation current level is 13
• the irradiation level of the electron beam is 62.
• the number of all ferrites and the number of ferrites containing a hard phase having a GAIQ value of 26000 or less inside the ferrite grains are measured.
• the ratio of the number of ferrites containing a hard phase having a GAIQ value of 26000 or less in the ferrite grains to the total number of ferrites. ((Number of ferrites including a hard phase having a GAIQ value of 26000 or less inside the ferrite grains / Number of all ferrites) ⁇ 100) is obtained.
• a sample is taken from a position 10 mm or more away from the end face of the hot stamp molded body (or a position avoiding the end portion) so that the plate thickness cross section perpendicular to the surface becomes the observation surface.
• the observation surface After polishing the observation surface, it corrodes with nital, and using an optical microscope and a scanning electron microscope (SEM), the plate thickness is 1/4 position from the surface (1/8 depth from the surface to the plate thickness from the surface). Observe at least 3 regions of 30 ⁇ m ⁇ 30 ⁇ m in the region of 3/8 depth).
• each structure is identified by the same method as for the hot stamping steel sheet.
• the hot stamp molded product according to the present embodiment may have a plating layer on one side or both sides. Having a plating layer on the surface is preferable because the corrosion resistance of the hot stamped molded product is improved.
• the plating to be applied include aluminum plating, aluminum-zinc plating, aluminum-silicon plating, hot-dip galvanizing, electrozinc plating, and alloyed hot-dip galvanizing.
• the plate thickness of the hot stamp molded product is not particularly limited, but is preferably 0.5 to 3.5 mm from the viewpoint of reducing the weight of the vehicle body.
• the tensile (maximum) strength of the hot stamp molded product according to this embodiment may be 590 to 980 MPa. Further, the total elongation of the hot stamp molded product according to the present embodiment may be 10.0% or more. Further, in the hot stamp molded product according to the present embodiment, the product (TS ⁇ El) of the tensile strength and the total elongation may be 12000 MPa ⁇ % or more. Tensile strength and total elongation are obtained by taking a JIS No. 5 test piece from a hot stamped body and performing a tensile test in accordance with JIS Z 2241: 2011.
• a preferred method for manufacturing a steel sheet for hot stamping according to the present embodiment includes the following steps.
• a slab is obtained by setting the casting speed to 0.80 m / min or more.
• a hot-rolled steel sheet is obtained by hot rolling with the winding temperature in the temperature range of 500 to 700 ° C.
• the cold-rolled steel sheet is heated and held in a temperature range of 750 to Ac 3 points (first holding), and then the average cooling rate in the temperature range of 600 to 700 ° C. Cool to 15 ° C./s or less. Then, it is rapidly cooled to a temperature range of 100 ° C.
• quenching means cooling having an average cooling rate of more than 15 ° C./s.
• Casting speed 0.80 m / min or more By manufacturing the slab at a casting speed of 0.80 m / min or more, Mn segregation in steel can be promoted.
• the casting speed may be 3.00 m / min or less from the viewpoint of suppressing slab cracking.
• Winding temperature 500-700 ° C
• Mn can be concentrated in the carbide.
• the other conditions for hot rolling are not particularly limited and may be general conditions.
• the conditions for cold rolling may also be general, and the cumulative rolling reduction may be 30 to 70%.
• the cold-rolled steel sheet After the first holding, cool so that the average cooling rate is 15 ° C / s or less.
• the cold-rolled steel sheet After cold rolling, the cold-rolled steel sheet is heated and held in the two-phase range, that is, in the temperature range of 750 to Ac 3 points (first holding). ), Then cooling is performed so that the average cooling rate in the temperature range of 600 to 700 ° C. is 15 ° C./s or less.
• carbides enriched with Mn can remain inside the ferrite grains.
• the carbide in which Mn is not concentrated is transformed into ferrite, but the carbide in which Mn is concentrated has a lowered transformation point, so that the carbide remains as a carbide without ferrite transformation.
• the holding time in the first holding may be 10 to 300 seconds.
• the average cooling rate is a value obtained by dividing the temperature difference between the surface temperature at the start of cooling and the surface temperature at the stop of cooling by the time difference from the start of cooling to the stop of cooling.
• Ac 3 points can be obtained by the following formula.
• the average cooling rate in the temperature range of 600 to 700 ° C. is 15 ° C./s or less, and then rapidly cooled to the temperature range of 100 ° C. or lower.
• it may be rapidly cooled to a temperature range of 300 to 500 ° C., held in that temperature range (held for the second time), and then rapidly cooled to a temperature range of 100 ° C. or lower.
• the carbide remaining in the ferrite grains can be transformed into a hard phase.
• the holding time in the second holding may be 10 to 600 seconds.
• the steel sheet for hot stamping according to the present embodiment can be stably manufactured.
• a step of forming a plating layer on one side or both sides of the hot stamping steel sheet may be provided.
• the method for manufacturing a hot stamp molded product according to the present embodiment includes the following steps.
• the steel sheet for hot stamping is heated to a temperature range of 3 points or more and held. After holding, it is cooled to 700 ° C. so that the average cooling rate is 5 to 20 ° C./s. Cool to a temperature range of 100 ° C. or lower so that the average cooling rate is 30 ° C./s or higher.
• each step will be described.
• Heating temperature and holding temperature Ac 3 points or more
• austenite By heating and holding the above-mentioned hot stamping steel sheet in a temperature range of Ac 3 points or more, austenite can be sufficiently formed.
• the holding time in the temperature range of Ac 3 points or more is not particularly limited, but may be, for example, 10 to 300 seconds.
• Average cooling rate up to 700 ° C 5-20 ° C / s After the above-mentioned holding, cooling is performed so that the average cooling rate up to 700 ° C. is 5 to 20 ° C./s, whereby a desired amount of ferrite can be obtained. Cooling with an average cooling rate of 5 to 20 ° C./s may be performed by air cooling. After cooling to 700 ° C., hot stamping is performed.
• Average cooling rate up to a temperature range of 100 ° C. or lower 30 ° C./s or more
• a desired amount of hard phase is obtained. be able to.
• it is possible to increase the number ratio of ferrites containing a hard phase having a GAIQ value of 26000 or less in the ferrite grains. Cooling to a temperature range of 100 ° C. or lower may be performed by contact with a mold.
• the hot stamp molded product according to the present embodiment can be obtained. Since the steel sheet for hot stamping according to the present embodiment has relatively low strength, it is joined to a steel sheet having high strength after hot stamping to form a tailored blank, which is then hot stamped and formed into a vehicle body part. Since this vehicle body part is manufactured by hot-stamping a tailored blank made of a low-strength material and a high-strength material, it has a low-strength portion and a high-strength portion.
• the welding method is not particularly limited.
• the high-strength material (steel plate having high strength after hot stamping) used together with the low-strength material (steel plate for hot stamping according to the present embodiment) is not particularly limited. These may be selected appropriately for each vehicle body part to be manufactured.
• the conditions in the examples are one condition example adopted for confirming the feasibility and effect of the present invention, and the present invention is based on this one condition example. Not limited.
• the present invention can adopt various conditions as long as the gist of the present invention is not deviated and the object of the present invention is achieved.
• the hot stamping steel sheets shown in Tables 2A and 2B were manufactured under the conditions shown in Tables 2A and 2B.
• the hot stamped bodies shown in Table 3A and Table 3B were obtained under the conditions shown in Table 3A and Table 3B.
• the slab was manufactured at the casting speeds shown in Tables 2A and 2B.
• the cumulative rolling reduction was set to 30 to 70%.
• the holding time in the first holding was 10 to 300 seconds, and the holding time in the second holding was 10 to 600 seconds.
• the mixture was rapidly cooled to the second holding temperature. After the second holding, it was rapidly cooled to a temperature range of 100 ° C. or lower.
• the steel plate No. 56 was cooled so that the average cooling rate in the temperature range of 600 to 700 ° C. would be the average cooling rate shown in Table 2B, and then rapidly cooled to 25 ° C. without performing the second holding.
• the holding time was set to 10 to 300 seconds.
• the metallographic structure of the hot stamped steel sheet, the metallic structure of the hot stamped body, and the mechanical properties were measured. Examples having a tensile strength of 590 to 980 MPa were judged to be acceptable because they had high strength. On the other hand, an example in which the tensile strength was less than 590 MPa or more than 980 MPa was judged to be unacceptable. Further, an example in which the total elongation was 10.0% or more and the product of the tensile strength and the total elongation (TS ⁇ El) was 12000 MPa ⁇ % or more was judged to be acceptable because of its excellent ductility.
• the hot stamp molded product according to the example of the present invention has high strength and excellent ductility.
• the hot stamped product according to the comparative example does not have high strength and / or excellent ductility.

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